Abdelhak Hidouri
email: abdelhakhdr@gmail.com

Mohamed Hadded
email: mohamed.elhadad@vedecom.fr

Nasreddine Hajlaoui
email: nasreddine.hajlaoui@fsg.rnu.tn

Haifa Touati
email: haifa.touati@cristal.rnu.tn

Paul Mühlethaler

Hatem Bettaher

Nasreddine Hajlaoui Hatem

Haifa Touati Hatem

Paul Muhlethaler
email: paul.muhlethaler@inria.fr

Cache Pollution Attacks in the NDN Architecture: Impact and Analysis

Keywords: Named Data Networks, Security, Cache pollution attack, NDNSim

I. INTRODUCTION AND MOTIVATION

Since its proposal in 2010, the Named Data Networking (NDN) architecture has attracted significant attention from the research community that is continuously exploring its potential in various network environments and applications such as WSN [START_REF] Aboud | Geographic Interest Forwarding in NDN-Based Wireless Sensor Networks[END_REF], IoT [START_REF] Aboud | Efficient forwarding strategy in a NDN-based internet of things[END_REF], [START_REF] Wang | Secure healthcare monitoring framework integrating NDN-based IoT with edge cloud[END_REF], VANETs [START_REF] Kardi | NDVN : Named Data For Vehicular Networking[END_REF], Web [START_REF] Nan | Design and Implementation: the Native Web Browser and Server for Content-Centric Networking[END_REF], [START_REF] Qiaoa | Session persistence for dynamic web applications in Named Data Networking[END_REF], Big Data delivery [START_REF] Mejri | Are NDN Congestion Control Solutions Compatible with Big Data Traffic?[END_REF] and edge and cloud computing [START_REF] Ullah | Design and Implementation of an Open Source Framework and Prototype For Named Data Networking-Based Edge Cloud Computing System[END_REF]. Moreover, over the last 5 years, several trial deployments have been launched to boost the adoption of NDN by industry, examples being Genomics datasets access based on NDN and the N-DISE project to deploy an NDN petascale data distribution system to serve major science programs [START_REF] Touati | Fair hop-by-hop interest rate control to mitigate congestion in named data networks[END_REF].

All these applications deal with the exchange of huge and sensitive data, which raises the need for a secure and efficient communication model. However, although NDN is resilient to most traditional security attacks in the TCP/IP model by enforcing the producer to sign each content item before spreading it in the network [START_REF] Mejri | Preventing unnecessary interests retransmission in named data networking[END_REF], this architecture remains vulnerable to new types of attacks. With the remarkable increase in security threats and the growing interest in the NDN communication model, securing this architecture against attacks is therefore vital, since fake messages or malicious node behaviour may affect data privacy and disrupt the caching process, which is a core pillar of NDN. Thus, studying the impact of attacks in order to find out how much they can affect NDN operation is essential before forming an appropriate mitigation solution.

This paper outlines the potential threats that could affect the NDN architecture. New vulnerabilities, have been illustrated in this work. Most of these vulnerabilities result from the availability of the data in the cache of intermediate routers and threaten the caching process of the NDN architecture. The most important of these attacks are the Cache Poisoning Attack, the Cache Privacy Attack and the Cache Pollution Attack, in which a malicious node could fake the popularity of the content residing in the cache, deplete the main resources of the legitimate nodes and exhaust them. In a second step of this work, we assess the impact of the Cache Pollution Attack on the efficiency of the caching functionality of NDN in terms of Cache Hit Ratio, Hit Damage Ratio and Data Retrieval Delay. The remainder of this paper is organized as follows: Section II presents the basic concepts and architecture of NDN. In Section III, we describe the security vulnerabilities that can affect the operation of Named Data Networking. Then, in Section IV, we present the simulation results and analyze the impact on NDN performance of the Cache Pollution Attack. Finally, in Section V, we conclude the paper and highlight some open issues.

II. OVERVIEW OF THE NAMED DATA NETWORKING ARCHITECTURE

In this section, we briefly describe the main building blocks of the NDN communication model.

A Named Data Network (NDN) is a recently proposed Internet architecture that shifts the Internet communication paradigm from the IP address-based model to a content namebased model where consumers request the desired data by names rather than using the IP addresses of the receivers. As explained in RFC 7927 [START_REF]Information-Centric Networking (ICN) Research Challenges[END_REF], users insert the name of the requested content into a request packet, called an "Interest", and the nearest node that has previously stored this content, or "Data" packet, must deliver it. The node that sends the Interest is called the Consumer and the original Data source is called the Producer.

To manage the Interest/Data communication process, each NDN node implements three data structures: (i) the Pending Interest Table (PIT), which tracks the outstanding Interests as well as the interface from which they come, in order to deliver the Data back to the Consumer on the reverse path of the Interest, (ii) the Content Store (CS), which stores the received Data packets in order to serve the upcoming Interests that request the same content and (iii) the Forwarding Information Base (FIB), which stores the Data name prefixes and the interfaces to which the Interest packets [START_REF] Mejri | Hop-by-Hop Congestion Control for Named Data Networks[END_REF] should be forwarded.

To disseminate Interest and Data packets, each NDN node performs the following process: when an NDN router receives an Interest packet, it searches for the requested Data in its Content Store; if it exists, the Data will be returned through the same interface from which the Interest was received. Otherwise, the router performs a lookup in the PIT. If an entry is found, the router concatenates the interface which received the interest to the PIT entry, otherwise, i.e. if no match is found in the PIT, the router transmits the interest using the information contained in the FIB. When an NDN router receives a Data packet, it checks the PIT table. If a matching is found, the router forwards the data packet to all interfaces listed in the PIT entry and possibly caches the data in the Content Store, otherwise the received Data packet is rejected [START_REF] Mejri | Hop-by-hop interest rate notification and adjustment in named data networks[END_REF].

III. SECURITY VULNERABILITIES IN NAMED DATA NETWORKS

One key feature of an NDN is that security and privacy are built into the architecture. NDN introduces new mechanisms of signature and cryptography that protect the data from being intercepted by malicious users. Each Data packet is cryptographically signed by its producer in order to build a secure data transmission [START_REF] Zhang | Named data networking[END_REF]. With this signature field, consumers and neighbour nodes can verify data integrity and provenance.

Another key feature of NDN is Data caching. As explained in Section II, the Content Store (CS) essentially caches the data received for future requests by the same consumer or even other neighbour nodes, so that the data can be accessed more quickly. Caching in NDN is managed by different policies such as: Least Recently Used (LRU), Least Frequently Used (LFU), First In First Out (FIFO), Random. However, since no pre-existing mechanism is defined to detect the access of different malicious nodes into the Content Store (CS) [START_REF] Kumar | Security Attacks in Named Data Networking: A Review and Research Directions[END_REF], this "secured-by design" architecture is vulnerable to attacks that mainly aim to abuse the Content Caching process, disturb the availability of the data, maximize the data retrieval delay and manipulate the privacy of data. These attacks include Cache Privacy Attacks, Content Poisoning Attacks, Interest Flooding Attacks and Cache Pollution Attacks. The main principles of these attacks are described in the following subsections.

A. Cache Privacy Attack

In a Cache Privacy Attack, the attacker tries to discover the content items existing in the cache and enumerate them. This step is called an Object Discovery Attack (ODA) [START_REF] Lauinger | Privacy Implications of Ubiquitous Caching in Named Data Networking Architectures[END_REF]. After that, the attacker starts requesting them consecutively. Another type of Cache Privacy Attack is called the Data Flow Cloning Attack (DFCA) [START_REF] Lauinger | Privacy Implications of Ubiquitous Caching in Named Data Networking Architectures[END_REF] where the attackers have knowledge about the content existing in the cache, either by using ODA or by guessing the structure of the name space of the on-going data exchange such as for Voice-over-CCN. Then the attacker tries to clone and predict the next requests.

B. Content Poisoning Attack

In a Content Poisoning Attack, the attacker, presented in most cases as a malicious router, many compromised routers or a malicious producer, has the role of spreading malicious or corrupted data into the neighbour routers which leads to reserving most of the content cache space, and every time a consumer requests such data it spreads through neighbour routers. Although the routers can not check the validity of these malicious data due to their limited resources and time, as a result, only the corrupted content will stay in the cache of the CS, which causes a high delay in retrieving certain content and legitimate content will not be served from the cache. Content Poisoning Attack affects consumer applications and routers [START_REF] Kumar | Security Attacks in Named Data Networking: A Review and Research Directions[END_REF].

C. Interest Flooding Attack

The fundamental goal of the Interest Flooding Attack is to deplete the resources of the routers. In this type of attack, the attackers send a massive amount of Interests into the targeted node, which prevents legitimate consumers from allocating an entry in the Pending Interest Table (PIT) since the attackers have reserved all the entries in the PIT of the router. This results in Interest timeout and unnecessary retransmissions. There are three types of Interest Flooding Attack suggested by Signorello et al. [START_REF] Signorello | Advanced interest flooding attacks in Named-Data Networking[END_REF] and Salah et al. [START_REF] Salah | Evaluating and mitigating a collusive version of the interest flooding attack in NDN[END_REF] in which they use either existing content, or non-existing content (i.e. to confuse the router from detecting the attack), or both types of content.

D. Cache Pollution Attack (CPA)

In a Cache Pollution Attack (CPA), the attackers try to deplete the size of the cache presented in the Content Store (CS) and deny other legitimate consumers from getting the desired content from the cache. The attackers start requesting unpopular content from a malicious producer that serves these requests. These content items reserve spaces in the content cache for subsequent requests, and they keep requesting the same content, which consumes the total space of the cache and turns these unpopular content items into popular content items that remain in the cache for a longer duration. As a result, they confuse the caching policy and lead to a decrease in the Cache Hit Ratio (CHR) for the targeted routers and increase the data retrieval delay for the legitimate consumers. This type of attack is easy to perform but hard to detect as it varies the content served from the cache and the time to perform the attack is unstable, which, in some cases, exhausts the router and depletes most of the Content Store (CS) resources.

To study the security level of the NDN architecture, in this work we focus on the CPA attack, and we inject it into the NDN architecture to study its impact. The simulation results are detailed in Section IV.

IV. SIMULATION SCENARIOS AND IMPACT ANALYSIS OF THE CACHE POLLUTION ATTACK

A. CPA simulation setup

To evaluate the impact of the CPA attack described in the previous section, we developed several attack scenarios by injecting malicious behavior into the network. We simulated these attack models using the official NDN simulation module for the Network Simulator-3 (ndnSIM [START_REF] Mastorakis | On the Evolution of ndnSIM: an Open-Source Simulator for NDN Experimentation[END_REF]). Two topologies are considered in this study: i a Simple Network topology in which 3 legitimate consumers, LConsumer, request Interests from a legitimate producer, LProducer, while an attacker issues Interests to retrieve Data from a malicious producer, MProducer, in order to disrupt the cache locality of intermediate routers, R1 and R2 (see Figure 1). ii the German Research Network (DFN) [START_REF]Deutsches forschungsnetz[END_REF] which is a German national research and education network (NREN) used for academic and research purposes. The DFN topology allows us to provide a more realistic assessment of the impact of a CPA attack, (see Figure 2). In all the simulations, the consumers' and attackers' requests follow a Zipf-like distribution. The simulation parameters used in these experiments are summarized in Table I and Table II. To evaluate the performance of the NDNs under the CPA scenarios, we used the following metrics:

• the Cache Hit Ratio (CHR) is defined as the ratio of cache hits to the total number of received requests, and it is calculated as follows:

CHR = ( CacheHits CacheHits + CacheM isses ) × 100

• the Hit Damage Ratio (HDR) is defined in [START_REF] Karami | An ANFIS-based cache replacement method for mitigating cache pollution attacks in Named Data Networking[END_REF] as the key measure of the effectiveness of a caching attack and is calculated as follows :

HDR = 1-CHR(without attack) -CHR(under attack) CHR(without attack)

When the HDR becomes higher, the performance of the attack increases and when it gets lower the efficiency of the attack decreases, in other words when the HDR is close to 1, the cache is completely under attack and when it is near 0, the attack is completely ineffective. • the Average Retrieval Delay (ARD) is defined as the average delay required for a given LConsumer to receive a requested data packet and is calculated as follows:

ARD k = N b P ackets k i=1 (DRT k (i) -IST k (i)) N b P ackets k
Where N b packets k is the total number of data packets received by LConsumer k, IST k (i) is the Interest Sending T ime and it refers to the time when LConsumer k sends the i th Interest packet and DRT k (i), Data Reception T ime, is the time when LConsumer k receives the i th requested data packet.

B. Impact analysis of the CPA on the Cache Hit Ratio

To evaluate the impact of the attack in terms of the CHR, we use the following three main scenarios:

-CHR in the simple topology using LRU:In this scenario, we run simulations for 20s and then for longer period, i.e. 100s, while increasing the Interest sending from 40 to 400 Interest/s. As shown in Figure 3, without attack, every time we raise the frequency of the Interest packets, the CHR rises. For instance, when the LConsumers send 40 interests per second, the CHR reaches 42%, and it keeps increasing, up to 74.41% when the consumers send 400 interest/s. Fig. 3. Cache Hit Ratio using LRU in the simple network topology Figure 3 also highlights the effectiveness of CPA on the CHR. When the simulation runs for 20s, the CPA, even with only one attacker, drops the CHR from 42% to 5.2% when the interest sending rate is set to 40 interest/s and from 74.41% to 8.7% when it is set to 400 interest/s. The CHR decrease is also confirmed for longer simulation time, i.e. 100s. For example, when the sending rate is set to 400 interest/s, the CHR drops from 56.33% to 26.39%.

-CHR in the simple topology for different caching policies: In this scenario we varied the caching policies to discover whether a CPA is still effective. We observe (Figure 4) that no matter which caching policy is adopted, the CHR is still affected by the attack. Using LFU, the value of the CHR drops from 54.96% to 34.84% when sending 40 Interest/s. With FIFO, the CPA leads to a decrease from 53.49% to 25.55% when sending 240 interest/s and by using the Random policy, the CHR decreases from 88.37% to 35.5%. Results show that as soon as the CPA starts, i.e. at time t = 5s, the CHR takes only one second, i.e. t = 6s, to be severely effected by the attack. For example, the CHR of routers R3 and R7 drops from 41.86% and 28.57%, respectively to about 0%, which means that attackers quickly succeed to completely deprive legitimate consumers from the caching feature benefit. As a result, we can conclude that a CPA has a huge impact on the CHR in both simple topologies and much larger and more complex real-world topologies, which shows the highly negative impact that this attack can have on the availability of the legitimate content in the Content Store.

C. Impact analysis of CPA on the Hit Damage Ratio

In this subsection, we analyse the impact of CPA on the HDR, as an essential metric to evaluate the criticality of the attack,

In the simple topology, Figure 6 reports the results obtained for a simulation time set to 20s and 100s. We observe that the HDR reaches the maximum thresh value of 0.365 while in Using different cache policies in the simple topology, as shown in Figure 7, we observe that the HDR is still high for each of the caching policies. For example, with LFU, the HDR increases and reaches the thresh of 0, 78137, which demonstrates the huge impact of a CPA on the data served from the Content Cache in a simple topology. In the DFN topology, results plotted in Figure 8 show that by only launching the attack from 5s to 15s the HDR reaches the thresh value of 0.3 in router R7 and 0.73 in router R9. To summarize, in each of these three main scenarios we observe through the evaluation of the HDR how effective a CPA can be on the content cache, and despite varying the caching policies and the topologies, the attack still has a huge impact on the availability and the popularity of legitimate data in the cache, resulting in diminished performance in Data caching in each of the router nodes.

D. Impact analysis of CPA on Data Retrieval Delay

In this subsection, we evaluate the impact of CPA on the Average Retrieval Delay (ARD), since such an attack also has a negative effect on the legitimate consumers, essentially concerning the delay to retrieve content.

In a simple topology, Figure 9 demonstrates the negative impact of CPA on ARD by measuring it in each of the three legitimate consumers, while varying the simulation time. When the simulation time is set to 20s, the CPA increases the ARD from 14.88ms to 16.34ms for LConsumer 1, from 14.33ms to 15.722ms for LConsumer2 and from 14.3ms to 15.9ms for LConsumer 3. Similar results are obtained for longer simulation time, i.e. 100s. Hence a delay of about 1.5ms, i.e. an increase of 10% of the average data retrieval delay, is induced by only one cache pollution attacker that is sending 120 interest/s.

In the DFN topology, Figure 10 shows the impact of a CPA on the ARD of 5 legitimate Consumers. We observe that the ARD increases from 15.795ms to 21.19ms for LConsumer 1 and from 15.719ms to 17.84ms for LConsumer 8. In other words, for LConsumer1, the CPA increases the ARD by about 5.48ms, i.e. 34.88%. Hence we can conclude that the impact of the attack is more severe when the topology gets larger.

To summarize, in both topologies, although the small number of attacker nodes compared to the number of LConsumers, the ARD still becomes higher and higher and this can result in a severe decrease of the perceived throughput of the legitimate consumers, a decrease in the performances of the NDN routers and wastage of the network bandwidth usage.

V. CONCLUSION AND FUTURE WORKS

The success of NDN has always been based on its security and its high performances, ensuring it a promising future in the network revolution. However, since NDN is not vulnerable to a range of basic attacks that are effective on the TCP/IP model, a In this paper, we have analysed one of the most effective attack of these attacks, the CPA, and we have studied its impact on NDN through ndnSim simulations. Using different scenarios in simple as well as complex and realistic topologies, we have shown the impact of a CPA on the caching efficiency More specifically, our results reveal that a CPA decreases the CHR to almost 0% in several scenarios and increases the ARD to around 20% while the HDR reaches 0.6, which confirms the highly negative impact of this form of attack. In future work, we intend to exploit the results of this investigation to design a solution for detecting and mitigating the Cache Pollution Attack. In particular, we will develop an intelligent mechanism that computes the illegibility of each cached data packet and uses this parameter to improve the caching policy.

Fig. 4 .

 4 Fig. 4. Cache Hit Ratio using different caching policies in the Simple network topology

Fig. 5 .

 5 Fig. 5. Cache Hit Ratio of each edge router per time of simulation in the DFN topology

  100s simulation time it gets to the thresh value of 0.5205 by sending only 40 Interest/s.

Fig. 6 .

 6 Fig. 6. Hit Damage Ratio in the Simple network topology

Fig. 7 .

 7 Fig. 7. Hit Damage Ratio for different caching policies in the Simple network topology

Fig. 8 .

 8 Fig. 8. Hit Damage Ratio in the DFN topology

Fig. 10 .

 10 Fig. 10. Average retrieval delay in the DFN topology

Fig. 9 .

 9 Fig. 9. Average retrieval delay of each legitimate consumer in the simple network topology

TABLE I

 I SIMULATION PARAMETERS FOR THE SIMPLE NETWORK TOPOLOGY

	Simulation time	20s, 100s
	Number of Legitimate Consumers 3
	Number of Attackers	1
	Consumer type	ConsumerZipfMandelbrot
	Interest rate	40, 80, 120, 160, 200, 240, 280,
		320, 360, 400 Interest/s
	Routers CS size	50
	Cache policy	LRU, LFU, Random, FIFO

TABLE II SIMULATION

 II PARAMETERS FOR THE DFN TOPOLOGY

	Fig. 2. DFN topology
	Simulation time	20s
	Number of Legitimate Consumers 6
	Number of Attackers	5
	Number of Producers	2
	Consumer type	ConsumerZipfMandelbrot
	Interest rate	120 Interest/s
	Attack Time	from t = 5s to t = 15s
	Routers CS size	50
	Cache policy	LRU
	Fig. 1. Simple network topology


Rinaldo Poli
email: rinaldo.poli@u-bourgogne.fr

Kevin M Smith

Spin State and Ligand Dissociation in [CpCoL2] Complexes (L = PH3, H2C=CH2): a Computational Study

Keywords: cobalt, density functional calculations, electronic structure, ligand effects, spin state

The relative energies of [CpCoL] compounds (L = PH3, H2C=CH2) were calculated at the DFT/B3LYP level of theory. The triplet spin state was found to be favored over the singlet by between 33.0 and 21.0 kcal mol -1 for both fixed and optimized geometries. The basis set size was found to be important for the energy calculations, particularly when the energetics of ligand dissociation was examined. The role of the triplet spin state in facilitating the ligand dissociation process is discussed.

Introduction

The role of spin state changes in the reactivity of [CpCoL2] species remains under dispute.

Several kinetic, spectroscopic, and theoretical studies have investigated this issue. Bergman et al. examined the mechanism of phosphine substitution in [CpCo(PPh3)2] in order to determine if interconverting between singlet and triplet species resulted in an activation barrier for the dissociative pathway. The first step in the ligand replacement reaction was found to be the rapid, reversible formation of [CpCo(PPh3)]. [1] Extended Hückel molecular orbital theory calculations by Hofmann and Padmanabhan suggested that although linear d 8 [CpML] compounds are symmetry-required to have a triplet ground spin state, a "bent" geometry for [CpCo(PPh3)] (i.e.

Cp-Co-P angle < 180°) might allow the substitution reaction to proceed via the singlet spin surface, thereby accounting for the observed lack of a "spin flip barrier". [2] Later studies by Bergman, Moore, and co-workers demonstrated that photogenerated [CpCo(CO)] and [Cp*Co(CO)] did not form adducts with alkanes or noble gases, although these unsaturated species did react very rapidly with CO. [3] Using a variety of computational techniques including density functional theory (DFT), Siegbahn calculated that [CpCo(CO)] possesses a triplet ground spin state, and attributed the difference in reactivity of the 16-electron species with alkanes and CO to the varying ability of the incoming ligand to induce triplet-to-singlet spin crossover. [4] In contrast, Brookhart et. al. have recently reported C-H activation reactions of [Cp*Co(η 2 -H2C=CHSiMe3)2] that proceed via oxidative addition of benzene or aldehydes to unsaturated [Cp*Co(η 2 -H2C=CHSiMe3)]. [5] As part of an ongoing investigation of spin state and reactivity, [6] we have recently used DFT techniques to examine how ligands can affect the relative energy of the singlet and triplet states (ΔES-T) of 16-electron [CpM(NO)]-containing complexes of Cr, Mo, and W. [7] Ultrafast IR studies have implicated triplet [CpMn(CO)2] in the photolysis of [CpMn(CO)3] in Et3SiH. [8] Spin state changes have also been proposed to play a critical role in the reactivity of cytochrome P450. [9] The computational study described in this paper was performed to address the following questions. ( 1

Results and Discussion

Calculated Geometries and Energies

The saturated [CpCoL2] (L = PH3, H2C=CH2) complexes were studied with DFT using the three-parameter form of the Becke, Lee, Yang, and Parr functional (B3LYP) [10] and two basis sets, following the methodology recently developed by Siegbahn and co-workers. [11] The geometries were optimized without constraints (C1 symmetry) using the LanL2DZ basis set; [12] the structural parameters for both complexes are listed in Table 1. Since energy calculations are more sensitive to basis set size than geometry optimizations, [13] the energies of the LanL2DZ structures were also calculated using the larger 6-311+G(2d,2p) basis set. Parameter [b] [ The bond lengths obtained for the optimized [CpCoL2] geometries are marginally longer than those reported for the solid-state molecular structures of [CpCo(PEt3)2] [14b] and [Cp*Co(η 2 -H2C=CHSiMe3)2]. [5c] For instance, the average calculated Co-C(Cp) bond lengths are 0.09 Å longer than those found experimentally, and the theoretical Co-L(average) distances are 0.015 Å and 0.035 Å longer than those found in the corresponding X-ray molecular structures for the phosphine and alkene complexes, respectively. The alkene orientation of [CpCo(η 2 -H2C=CH2)2] corresponds to the alignment observed experimentally, namely parallel with the Cp plane. [5] The calculated alkene C-C bond lengths in [CpCo(η 2 -H2C=CH2)2] (1.417 and 1.414 Å) are almost identical to those reported for [Cp*Co(η 2 -H2C=CHSiMe3)2] (1.414 and 1.404 Å). The slight distortions in C-C bond lengths in the Cp ligand also correspond well with those previously discussed for related [CpCoL2] derivatives. [14] As noted in the introduction, Hofmann and Padmanabhan have previously suggested that ligand dissociation from d 8 [CpML2] complexes may remain on the singlet spin hypersurface if the unsaturated [CpML] intermediate adopts a bent geometry. [2] To test this hypothesis, the energies of bent [CpCoL] compounds were calculated in both singlet and triplet spin states, using the optimized [CpCoL2] geometries with one L group removed and leaving the other structural parameters unchanged. For both [CpCo(PH3)] and [CpCo(η 2 -H2C=CH2)] in this bent conformation (Cp-Co-L < 132°), the triplet state is greatly favored over the singlet. The value of ΔES-T decreases only slightly when the larger basis set is employed, from -31.0 to -28.1 kcal mol -1 for the phosphine system and from -25.9 to -23.9 kcal mol -1 for the alkene system. Overall, only a minor difference in ΔES-T is observed upon changing the L group in [CpCoL].

Analogous results are obtained when the [CpCoL] geometries are optimized using B3LYP/LanL2DZ. Optimized triplet [CpCo(PH3)] was found to be -33.0 kcal mol -1 more stable using the LanL2DZ basis set, and -27.1 kcal mol -1 more stable at the B3LYP/6-311+G(2p,2d) level. For the alkene system, triplet [CpCo(η 2 -H2C=CH2)] was calculated to be -26.5 kcal mol - 1 more stable than the singlet at the B3LYP/LanL2DZ level, and -21.0 kcal mol -1 more stable using the larger basis set. By comparison, Siegbahn's B3LYP calculation of [CpCo(CO)] gave a ΔES-T value of -27.8 kcal mol -1 , which falls between the results obtained here for the phosphine and alkene derivatives. [4] The structural parameters of the B3LYP/LanL2DZ optimized [CpCoL] geometries are listed in Table 2, and the relative energies are illustrated in Figure 1.

Table 2. Optimized geometries for [CpCo(PH3)] and [CpCo(C2H4)] [a]

Parameter [b] [ [a] Singlet bond lengths in A ° and angles in °, with triplet parameters indicated in parentheses. [b] CNT = center of gravity of the Cp ring, X = P for [CpCo(PH3)] or center of C-C bond for [CpCo(C2H4)].

An indication of the energetics of the ligand dissociation process can be attained by comparing the energy of [CpCoL2] complexes with the sum of the energies of optimized [CpCoL] and the free L ligand, as shown in Table 3. For these relative energies, the difference between the values obtained with the two different basis sets is pronounced. In fact, at the B3LYP/LanL2DZ level, [CpCo(PH3)2] is calculated to be 4.3 kcal mol -1 less stable than triplet [CpCo(PH3)] and free PH3, contrary to experimental observations. This erroneous result is presumably due to the previously observed correlation between basis set size and bond strength. [4] Thus, while the relative energies calculated using the small basis set are skewed in favor of the dissociated species, employing the larger 6-311+G(2d,2p) basis set results in the saturated complex being 7.3 kcal mol -1 more stable, in accord with experimental studies of the [CpCo(PPh3)2] system. [1] [a] ΔE (in kcal mol -1 calculated at the B3LYP/6-3111G(2d,2p) level of theory, with B3LYP/LanL2DZ values indicated in parentheses.

Implications for Reactivity

It has been previously proposed that the lack of a high activation barrier for ligand exchange reactions in d 8 [CpML2] compounds might be due to the bent geometry of unsaturated [CpML] species, allowing the singlet spin state to be maintained thoughout the dissociative substitution process. [START_REF]For a detailed analysis of the orbital mixing and energetic changes that accompany variation of the Cp(M(L angle[END_REF] Although Extended Hückel molecular orbital calculations did not permit the relative energies of the singlet and triplet [CpML] species to be ascertained, Hofmann and co-workers recognized that this possibility was more likely for Rh and Ir than for Co. [2] Our results at the DFT/B3LYP level indicate that even in the bent conformation, the triplet spin state is considerably lower in energy for [CpCo(PH3)] and [CpCo(η 2 -H2C=CH2)] than the corresponding singlet electronic configuration.

It should be observed that the crossover from the singlet to the triplet surface upon phosphine dissociation from [CpCo(PPh3)2] will in fact lower the enthalpic barrier to the phosphine exchange process if the crossing point is located below the singlet [CpCo(PPh3)] species. A change in spin state would only translate into an entropic contribution to the activation free energy. Thus, the facile phosphine exchange process observed for [CpCo(PPh3)2] is in fact quite nicely rationalized by the large energetic stabilization of the [CpCo(PPh3)] intermediate upon changing the spin state, accompanied by a low-energy crossing point and lack of a "spin flip barrier". [START_REF] Detrich | [END_REF] Differences in ground spin state between [CpCo(CO)] and the heavier Rh and Ir congeners were utilized by Siegbahn to explain the experimentally observed differences in alkane binding and C-H bond activation reactivity of these species. [4] With this precedent, it seemed conceivable that the H/D exchange reactivity with C6D6 and the aldehyde C-H activation reactions observed by Brookhart and co-workers might be attributable to a low-lying or ground singlet spin state for [Cp*Co(η 2 -H2C=CHSiMe3)] intermediates. [5] Although the present study indicates that ΔES-T is slightly more favorable for the singlet alkene species compared to the analogous phosphine system, the triplet state is still the ground state by over 20 kcal mol -1 for [CpCo(η 2 -H2C=CH2)]. However, it has previously been shown that photogenerated [CpCo(CO)] does form a solvated complex with benzene [17] and forms alkene adducts at diffusion limited rates. [17][18] Therefore, it is possible that the discrepancy in reactivity between [Cp*Co(CO)] and [Cp*Co(η 2 -H2C=CHSiMe3)] is due to substrate effects; i.e. that the aromatic C6D6 solvent or the C=O bond of the aldehyde may help to facilitate the spin state change from triplet to singlet in the 16-electron alkene intermediate, whereas alkanes do not induce this process for [Cp*Co(CO)]. MP2 calculations have indicated that a similar difference in the ability of the incoming ligand to induce spin-crossover was responsible for the experimentally-observed variation between the rate of addition of CO and N2 to triplet Cp*MoCl(PMe3)2. [19]

Conclusion

The triplet spin state is calculated to be much more stable than the singlet state for [CpCoL] species, even in "bent" structures derived directly from the saturated [CpCoL2] precursors. Varying the ligand in [CpCoL] exerts only a minor influence on the energy difference between singlet and triplet species: our present results for the PH3 and H2C=CH2 systems lie on either side of the ΔES-T value obtained by Siegbahn for the CO complex. The C-H bond activation reactions reported for [Cp*Co(η 2 -H2C=CHSiMe3)] may thus be attributable to substrate effects rather than to a difference in ground spin state of the unsaturated

Experimental Section

General Remarks: All calculations were performed using GAUSSIAN 94 [START_REF] Frisch | Gaussian 94[END_REF] on an SGI Origin 200 workstation. The geometries of the [CpCoL2] and [CpCoL] species and the free L ligands were optimized using B3LYP, [10] a DFT type of calculation, employing the LanL2DZ basis set.

The LanL2DZ basis set includes both Dunning and Hay's D95 sets for H and C and the core potential sets of Hay and Wadt for the heavy atoms. [12] Electrons outside the core were all those for H and C, the 3s and 3p electrons for P, and the 3s, 3p, 3d and 4s electrons for Co. All geometries were optimized at the B3LYP/LanL2DZ level without constraints, with the exception of triplet CpCo(PH3), which failed to achieve geometric convergence despite several attempts from various initial geometries. At the suggestion of a reviewer, the complex was optimized with imposed Cs symmetry starting from both possible orientations of the Cp ligand.

Convergence was attained in both cases, and the configuration with the C-H group opposite to the P was found to be slightly less stable than the other conformation, with an energy difference of less than 0.1 kcal mol -1 . The fixed, bent [CpCoL] geometries were obtained from the [CpCoL2] structures by removing one L ligand and keeping all other parameters unchanged.

The B3LYP energy calculations were performed separately with both the LanL2DZ basis set and the larger 6-311+G(2d,2p) basis set, which has two sets of polarization functions on all atoms including two f-sets on the Co, and also diffuse functions. [11] The mean value of the first-order electronic wavefunction, which is not an exact eigenstate of S 2 for unrestricted calculations on the triplet systems, was considered suitable for unambiguous identification of the spin state. Spin contamination was carefully monitored and the value of <S 2 > for the unrestricted B3LYP (UB3LYP) calculations on the triplet [CpCoL] species using the 6-311+G(2d,2p) basis set (LanL2DZ results shown in parentheses) [L = bent PH3, 2.0261 (2.0228); optimized PH3, 2.0234 (2.0205); bent H2C=CH2, 2.0499 (2.0463); optimized H2C=CH2, 2.0531 (2.0484)] indicated minor spin contamination. The energies shown in the Results and Discussion section correspond to UB3LYP calculations for the triplet [CpCoL] compounds, and to restricted B3LYP calculations for the singlet [CpCoL] and [CpCoL2] complexes. [21]

  ) What is the ground spin state of bent [CpCoL] species (L = PH3, H2C=CH2)? (2) Is ΔES-T substantially altered for [CpCoL] species by variation of L? (3) Can the observed C-H bond activation reactivity of [Cp*Co(η 2 -H2C=CHSiMe3)] be attributed to a difference in ground spin state compared to triplet [CpCo(CO)]?

  [a] Bond lengths in A ° and angles in °.[b] CNT = center of gravity of the Cp ring, X = P for [CpCo(PH3)2] or center of C-C bond for [CpCo(C2H4)2].

Figure 1 .

 1 Figure 1. Relative energies of singlet and triplet [CpCo(PH3)] and [CpCo(C2H4)]; ΔES-T calculated at the B3LYP/6-3111G(2d,2p) level of theory, with B3LYP/LanL2DZ values indicated in parentheses.

  intermediate. The comparatively high energy of the singlet [CpCoL] complexes suggest that the ligand dissociation process from [CpCoL2] may be facilitated by spin crossover to the lower energy triplet species. Although the ΔES-T values are relatively basis-set independent, the energies of the [CpCoL] species compared to the saturated [CpCoL2] complexes shows a marked dependence on the size of the basis set used.

Table 1 .

 1 Optimized geometries for [CpCo(PH3)2] and [CpCo(C2H4)2][a]

Table 3 .

 3 Relative energies of [CpCoL2] and [CpCoL] + L[a]

		[CpCo(PH3)]	[CpCo(C2H4)]
	Singlet	-34.4 (-28.8)	-24.6 (-25.1)
	Triplet	-7.3 (+4.3)	-3.6 (+1.4)

Acknowledgments

R. P. thanks the Région Bourgogne for supporting this research, and K. M. S. thanks the European Commission for a TMR Marie Curie Postdoctoral Fellowship.


Erwan Le Grognec

Rinaldo Poli
email: poli@u-bourgogne.fr

Li-Sheng Wang

Prof Rinaldo Poli

Stable 17-electron Mo(III) complexes containing alkyl ligands

Keywords: Half-sandwich complexes, 17-electron compounds, Mo(III)-alkyl bonds, Alkylation reactions

The alkylation of the half-sandwich complex CpMoCl2(η 4 -diene) (diene = C4H6, 2,3-Me2C4H4) affords the first thermally stable 17-electron compounds containing Mo(III)-alkyl bonds.

Mononuclear organometallic complexes of Mo(III) are paramagnetic and quite reactive.

The most previously described stable species contain metal-carbon bonds to π ligands only.

5], and

allyl complexes such as CpMo(η 3 -C3H5)2 [6,7], and [CpMo(η 3 -C3H5)(η 4 -C4H6)] + [5]. Single (σ) Mo(III)-C bonds to aryl ligands are extremely rare, two examples being Li3MoAr6•3Et2O (Ar = Ph, p-Tol) [8,9]. To the best of our knowledge, compounds containing a single bond to an alkyl ligand have not been previously reported, although dinuclear, diamagnetic alkylmolybdenum(III) compounds containing metal-metal bonds are known [10,11,12]. A few years ago, we attempted to produce the first such example by alkylation of CpMoCl2(PR3)2 complexes. When PR3 = PMe3, spectroscopic (EPR) evidence for the formation of the desired alkyl-Mo(III) product was obtained at low temperature, but a facile decomposition occurred upon warming to room temperature [13]. Trapping experiments indicated that this decomposition involves Mo(III)-CH3 bond homolysis, the formation of CH4 and CpMo(CH3)(PMe3)3 being observed when the reaction was conducted in THF in the presence of excess PMe3. When PR3 = PMe2Ph, the same reaction led to the ortho-metallated compound CpMo(C6H4PMe2)(PMe2Ph)2 [13].

We now wish to report the formation and stability of mononuclear, paramagnetic alkylcontaining cyclopentadienyl-diene complexes of Mo(III). Complex CpMo(CH3)2(η 4 -C4H6), 1, is obtained by low temperature alkylation of CpMoCl2(η 4 -C4H6) [5] with MeMgBr (see Scheme 1). The use of one equivalent of the methyl Grignard reagent allows the isolation of the mixed chloromethyl complex, CpMoCl(CH3)(η 4 -C4H6) (2) 2 . The two complexes show EPR resonances with distinctive (in dilute solutions) hyperfine splittings caused by the syn and anti external diene H atoms and by the alkyl H atoms, whereas no discernible coupling is observed for the internal diene and Cp protons (Figure 1). The hyperfine coupling constants of the three types of H atoms for compound 1 and 2 are accidentally degenerate. This is shown by the successful simulation of the spectra as a binomial undecet (for 1) and octet (for 2) resulting from coupling to 10 or 7, respectively, accidentally equivalent H nuclei. Complex CpMo(CD3)2(η 4 -C4H6) (1-d6), which was synthesized from CpMoCl2(η 4 -C4H6) and CD3MgI (see footnote 1) displays a quintet resonance with g value and Mo and H hyperfine constants identical with those of 1 (Figure 1b). An analogous reaction between CpMoCl2(η 4 -C4H6) and Me3SiCH2MgCl (2.2 equiv) yielded CpMo(CH2SiMe3)2(η 4 -C4H6) (3), while treatment of CpMoCl2(η 4 -2,3-Me2C4H4) with MeMgBr (2 equiv) allowed access to the sterically more encumbered CpMoMe2(η 4 -2,3-Me2C4H4) (4) 3 .

Scheme 1

Compounds 1-4 are stable under an inert atmosphere at room temperature and in the dark.

Less than 50 % decomposition is observed over 22 h upon warming compound 1 to 70°C in toluene in the dark as judged by EPR spectroscopy. The unexpected thermal stability of this material, especially when compared with the ease of bond homolysis in the isoelectronic compound CpMo(CH3)2(PMe3)2 [13], does not have an obvious explanation. A faster decomposition process, resulting in the precipitation of a black intractable material, occurs at room temperature upon exposure to light. Pentane solutions of compound 2 exposed to the laboratory light, on the other hand, led to the formation of crystals of compound [CpMo(µ-Cl)(η 4 -C4H6)]2 [5], upon standing at room temperature over > 1 week, presumably by Mo(III)-CH3 bond homolysis. The susceptibility of Mo(III)-alkyl compounds toward Mo-R bond homolysis as a function of the ancillary ligands will be a topic of further investigations in our laboratory.

Preliminary studies show that compound 1 is quite unreactive toward Lewis bases. The interaction with either CO (1 atm) or PMe3 over 2 weeks at room temperature, or with PEt3 over 24 h at 65°C in THF gave only a slight decrease of EPR intensity for the starting material and no evidence for the formation of new soluble products by EPR, NMR or IR spectroscopy. This is again quite surprising, since the CO insertion process into metal-alkyl bonds is generally thought to be a thermodynamically favorable and facile process for 17-electron compounds [14,[START_REF] Trogler | Organometallic Radical Processes[END_REF]. Also, the lack of reactivity with PR3 (R = Me, Et) contrasts with the easy decomposition of CpMo(CH3)2(PMe3)2, leading to CpMo(CH3)(PMe3)3 in the presence of PMe3 [13].

The addition of protons, on the other hand, results in a facile heterolytic cleavage of the Mo-CH3 bond: reaction with CF3CO2D leads to the evolution of CH3D. These combined reactivity essays indicate that the complex, in spite of its electronically unsaturated configuration, is rather electron-rich. The electron-richness of this system is confirmed by cyclic voltammetric studies, which show a reversible one-electron reduction process at quite negative potentials (E1/2 = -2.09 V for 1, -1.97 V for 3, and -2.14 V for 4, all versus. the ferrocene standard in THF solution), namely ca. 1 V more negative with respect of the same reduction process for CpMoCl2(η 4 -C4H6) [4]. The mixed complex 2 correspondingly shows a reversible reduction process at E1/2 = -1.62 V, midway between those of 1 and the dichloride precursor.

All these compounds also show irreversible oxidation waves. Additional reactivity studies with Lewis-acidic reagents and with oxidizing agents are under investigation and will be the subject of a future full report.

Fig. 1 .

 1 Fig. 1. EPR spectra for (a) CpMo(CH3)2(η 4 -C4H6), 1, at -80°C in toluene. (b) CpMo(CD3)2(η 4 -C4H6), 1-d 6 , at -113°C in Et2O. (c) CpMoCl(CH3)(η 4 -C4H6), 2, at room temperature in toluene.

Acknowledgments

This work was supported by the Conseil Régional de Bourgogne and by the CNRS (Programme Catalyse pour l'Industrie et l'Environnement). E. L. G. thanks the MENRT for a doctoral fellowship.


Edmond Collange

David Duret

Rinaldo Poli
email: poli@u-bourgogne.fr

Prof Rinaldo Poli

Experimental and computational investigations of phosphine exchange in 15-electron [CrCpCl2(PR3)] systems by stopped-flow and density functional calculations: a single-state SN2 mechanism

The exchange of the phosphine ligand on the half-sandwich 15-electron, spin quartet [CrCpCl2L] system has been investigated experimentally by stopped-flow kinetics with visible detection and theoretically by calculations with DFT methods on the PH3 self-exchange model system. The exchange of PMePh2 with PMe3 follows clean second-order kinetics with the activation parameters ΔH ‡ = 7.0(2) kcal mol -1 and ΔS ‡ = -24.3(8) cal K -1 mol -1 , consistent with an associative exchange. The rate constant for the exchange of L with PMe3 in [CrCpCl2L] at room temperature varies only within a factor of 8 for the series of complexes with L = PPh3, PMePh2, PMe2Ph, PEt3, or η 1 -dppe. The computational work showed that the PH3 selfexchange process occurs via a symmetric transition state along the spin quartet hypersurface, without crossover to the spin doublet state. The optimized transition state corresponds to a firstorder saddle point with Cr-P distances of 3.190 and 3.174 Å, located 7.6 kcal mol -1 above the [CrCpCl2(PH3)] (spin quartet) + PH3 combination, or 13.6 kcal mol -1 below the [CrCpCl2(PH3)2] doublet minimum. Thus, the phosphine exchange reaction can be classified as a classical SN2 process.

Introduction

The organometallic chemistry of Cr III has boomed in recent years, in large part because of its implication in the Phillips process for ethylene polymerization. 1 The first reported chromium(III) organometallic complex was the pseudooctahedral complex [CrEt(H2O)5] 2+ , 2 followed only a few years later by half-sandwich derivatives of types CrCpX2L and CrCpX3 - (X = Cl, Br or I; L = py, THF or PPh3). 3 Subsequently, many other similar half-sandwich complexes containing alkyl groups, phosphines, Cp*, and other ligands were described by several groups including ours. 1,[4][5][6][7][8][9][10][11][12][13][14] The dicationic aqua complexes [CrCp(H2O)3] 2+ and [CrCp*(H2O)3] 2+ have also been described. 15,16 Apart from particular cases where the formation of dinuclear compounds with metal-metal bonds is favored, [17][18][19] these systems are overwhelmingly seen to adopt an open shell, spin quartet, 15-electron configuration which can be related to the ubiquitous octahedral configuration of classical Werner-type complexes. They strongly resist co-ordination by another 2-electron donor which would bring the electron count to 17, i.e. closer to the closed-shell configuration, but also necessarily involve an energetically quite costly (for chromium) pairing of electrons. This behavior is opposite from that of the neighbouring molybdenum, which prefers a spin-paired 17-electron configuration in view of stronger metal-ligand bonds and reduced electron pairing energies. These considerations, which have been backed-up by computational studies at both the ab initio and density functional levels, 20,21 may be generalized to all open-shell organometallic systems. 22,23 In the absence of steric effects, the relative stability of 15-and 17-electron systems for Cr III and Mo III relative to the association/dissociation of a 2-electron donor ligand may be qualitatively summarized as illustrated in the reaction coordinates (a) through (d) of Scheme 1, 23 corresponding to an increasing degree of electron pairing energy.

Scheme 1

Bulky ligands and/or ligands that establish relatively weak bonds lead to the stabilization of even less saturated (13-electron) 26 Another 17-electron complex of Cr III , albeit stable only at low temperatures, is [CrCp(η 3 -C3H5)2]. 27 On the basis of all the above it is clear that the chromium(III) center in organometallic systems has the ability to accommodate a variety of different coordination spheres and electron counts.

In view of the relevance that the half-sandwich chromium(III) system has to the olefin polymerization process, we considered it of interest to probe the mechanism of fundamental chemical transformations on model compounds in this electronic configuration. The reaction that we have chosen for our initial studies is the exchange of a phosphine ligand on the [CrCpCl2(PR3)] system. A dissociative exchange process would parallel the dissociation of L from [CrCp*RL2] + (L = THF or Et2O) to generate the proposed active species in ethylene polymerization catalysis. An associative process, on the other hand, could generate a species having the same configuration as the stable 17-electron [MoCpCl2(PR3)2] or the cyanosubstituted chromium(III) derivatives mentioned above. The latter process could involve a spin flip from a quartet state in the starting material to a doublet in the intermediate, and then back to a quartet in the product, thus representing a new case of the so-called two-state reactivity (TSR), 28,29 namely a reaction occurring on two different spin surfaces. [30][31][32] The phosphine exchange process on the 17-electron molybdenum(III) complexes [Mo(ring)Cl2(PMe3)2] (ring = Cp or Cp*) was conveniently investigated by monitoring the growth of the 31 P NMR resonance of free PMe3 upon treatment with a large excess of PMe3-d 9 (t1/2 of the order of hours at room temperatures), allowing the establishment of a first order (dissociative) process. Preliminary studies of the analogous exchange for the [CrCpCl2(PMe3)] system have shown that the reaction is complete within the time necessary to record the first NMR spectrum, thus the reaction is too fast for classical monitoring kinetics studies. 33 On the other hand, the addition of PMe3 to a solution of [CrCpCl2(PMe3)] does not significantly affect the line shape of the resonances of free and co-ordinated PMe3, 33 thus the reaction is too slow for the application of NMR line broadening kinetic studies. Consequently, we turned to nondegenerate, quantitative phosphine exchange reactions and to the use of the stopped-flow kinetics methodology. We report here our experimental investigations of various [CrCpCl2(PR3)] + PMe3 reactions, and a parallel computational study along the reaction coordinate for the model [CrCpCl2(PH3)] + PH3 system.

Experimental

All operations were carried out under an atmosphere of dinitrogen. Solvents were dehydrated by conventional methods and distilled directly from the dehydrating agent prior to use (THF from sodium-benzophenone and toluene from Na). Compounds [CrCpCl2L] (L = PEt3, PMe2Ph, PMePh2, PPh3, or η 1 -dppe) were prepared as previously described. 5,12,13 Solutions of PMe3 [1 M in THF and 1 M in toluene (Aldrich)] were used as received.

The stopped-flow kinetic investigations were carried out with a Hi-Tech DX2 apparatus, equipped with a xenon lamp (75 W) and a KinetaScan diode array UV-visible detector. The data were analysed with the SPECFIT global analysis package [START_REF] Binstead | Spectrum Software Associates[END_REF] on a Pentium PC.

Theoretical calculations were carried out by GAUSSIAN 94 [START_REF] Frisch | GAUSSIAN 94[END_REF] on a SGI Origin 200 workstation. The three-parameter form of the Becke, Lee, Yang and Parr functional (B3LYP) [START_REF] Becke | [END_REF] was employed. The LanL2DZ basis set includes both Dunning and Hay's D95 sets for H and C [START_REF] Dunning | Modern Theoretical Chemistry[END_REF] and the relativistic electron core potential (ECP) sets of Hay and Wadt [START_REF] Hay | [END_REF][39][40] for the heavy atoms. Electrons outside the core were all those of H and C atoms, the 3s, 3p electrons in Cl and P, and the 3s, 3p, 3d and 4s electrons in Cr. A Cs symmetry arrangement was imposed for the [CrCpCl2(PH3)n] systems (n = 1 or 2) at each fixed Cr•••P distance. The transition state calculation was carried out with a Synchronous Transit-Guided Quasi-Newton (QST2) algorithm. The energies shown in the Results section correspond to unrestricted B3LYP (UB3LYP) calculations. The value of <S 2 > at convergence was in the range 0.7503-0.7504 for all spin doublet calculations and in the range 3.7527-3.7630 for all spin quartet calculations.

Results and discussion

The exchange reaction (1) (PR3 = PPh3, PPh2Me, PPhMe2, PEt3 or η 1 -dppe) is quite rapid and quantitative at room temperature and is therefore amenable to kinetics investigations by the stopped-flow technique. The thermodynamic drive for the reaction is provided by the stronger bonds established by PMe3 with the chromium(III) center relative to the other phosphine ligands, presumably because of a combination of the stronger donating ability and the less stringent steric requirements of the PMe3 ligand. 41 A similar trend of relative stability was observed for the related half-sandwich complexes of Mo III . 31,42 [CrCpCl2(PR3)] + PMe3  [CrCpCl2(PMe3)] + PR3

(1)

The PMePh2 derivative was selected for carrying out detailed rate law and activation Indeed, the investigation in toluene afforded clean single exponential decays, leading to the pseudo-first-order rate constants in Table 1. The graphic representation of the constants obtained at 25 °C as a function of the phosphine concentration (runs 1-4, see Fig. 1) immediately establishes a second-order rate law for this phosphine exchange reaction. From the slope of the straight line, the second-order rate constant k = 223 ± 5 M -1 s -1 is obtained at this temperature. (1). These were only determined at a single [PMe3] : [Cr] ratio and at a single temperature. The second order rate constants (see Table 2) were derived under the reasonable assumption that all these reactions occur by the same mechanism as the PMePh2/PMe3 exchange. The results are themselves further evidence in favor of a common associative mechanism for all these exchange reactions. For a pure dissociative process, the Cr-PR3 bond would be broken in the slow step of the reaction. Consequently, the rate contants should be very sensitive to the nature of the phosphine ligand, the more weakly bonded phosphine ligands leading to faster exchange processes. Other systems that have previously been established to follow a dissociative ligand exchange, such as the 18-electron complex cis-[Mo(CO)4L2], afford first order rate constants over 3 orders of magnitude smaller for PMe2Ph relative to PPh3. 43 On the other hand, a pure associative process would involve the formation of the new Cr-PMe3 bond, which is essentially the same for all systems. Consequently, the rate should only experience a slight dependence on the nature of the outgoing phosphine, the faster rates being expected for the least sterically encumbering and/or least electron donating phosphines. For the present system, the data in Table 2 show that the pseudo-first-order rate constant varies over a factor of only 8 on going from PPh3 to PMe2Ph. In addition, the slower reaction is given by the PPh3 complex, consistent with a sterically controlled associative process and in strong disagreement with a dissociative process.

Table 2. Rate constants for the [CrCpCl2(PMePh2)] + PMe3 reaction in toluene. a

PR3

kobs/s -1 10 -2 k/M -1 s -1 PPh3 1.03(4) 1.03(4) η 1 -dppe 17(1) 17(1) PMe2Ph 9(1) 9(1) PEt3

1.8(1) 1.8(1) a Complex concentration = 5.00•10 -4 M, [PMe3] = 1.00•10 -2 M Fig. 3 shows the correlation between the second-order rate constants and Tolman's electronic and steric parameters. 41 There is a poor correlation with both parameters for the entire series of compounds investigated. The three systems based on the homologous PMenPh3-n series (n = 0, 1 or 2) show a linear correlation with both parameters, but the electronic parameters would be expected to lead to the opposite trend of reactivity (PPh3 > PMePh2 > PMe2Ph). Thus, these phosphine ligands influence the exchange rate mostly by virtue of their steric encumbrance. The PEt3 ligand shows an unusually slow rate when its steric requirements are compared with those of the PMenPh3-n systems. The reason for this discrepancy may be either an important contribution of the electronic factor [PEt3 is a much stronger donor than the PMenPh3-n phosphines, see Fig. 3(a)], or by the gross underestimation of the actual steric encumbrance of PEt3 by Tolman's cone angle, 44,45 or a combination of the two effects. A discrepancy that cannot be easily rationalized is the relatively fast exchange rate for the η 1 -dppe complex. This ligand, when co-ordinated in a monodentate fashion, has electronic and steric parameters identical with those of PMePh2, yet it leads to an exchange rate constant ca. 7.5 times bigger than that furnished by PMePh2. Although the rate law and the activation entropy strongly suggest an associative mechanism, these experimental data cannot distinguish between the formation of a distinct 17electron intermediate and an interchange transition state, nor whether a spin state change occurs along the reaction coordinate, namely whether this is a one-state or a two-state reaction. To help clarify this point, we have complemented our experimental studies with theoretical calculations.

Previous calculations with full geometry optimization on [CrCpCl2(PH3)] (in the doublet and quartet states) and [CrCpCl2(PH3)2] (doublet state) at various levels of theory indicated that the combination of quartet [CrCpCl2(PH3)] and free PH3 is at least 13.6 kcal mol -1 more stable than doublet [CrCpCl2(PH3)2]. 20,21 This is a substantially greater number than the measured activation enthalpy for the [CrCpCl2(PMePh2)] + PMe3 system, thus suggesting that the formation of a 17-electron intermediate having a doublet configuration may not occur. An analogous study along the quartet curve revealed the expected repulsive interaction upon approach of the incoming PH3 ligand, after an initial slight energy decrease. However, the energy of the system remained low relative to the bound doublet minimum and an elongation of the bond between Cr and the already co-ordinated PH3 ligand accompanied the approach of the incoming PH3 ligand. An attempt to optimize a structure with Cr•••P < 3 Å led to the expulsion of the other PH3 ligand. A transition state calculation for the PH3 exchange provided a relatively symmetric geometry with Cr-P distances of 3.190 and 3.174 Å, at an energy only 0.0122 hartree (7.6 kcal mol -1 ) above the [CrCpCl2(PH3)] (spin quartet) + PH3 combination, or 13.6 kcal mol -1 below the bound doublet minimum. A frequency calculation confirms that this geometry corresponds to a first-order saddle point. The single imaginary frequency corresponds to the normal mode illustrated in Fig. 5 These results may be relevant to the intimate mechanism of CpCr III -catalysed olefin polymerization.

  configurations. Examples are [Na(OEt2)2][Na(OEt2)-(THF)][CrPh5] 24 or, for half-sandwich complexes, [CrCp*Cl(CH2SiMe3)] and [CrCp*-(CH2SiMe3)2]. 1,25 Also, the activity of [CrCp*R(THF)2] + , [CrCp*R(OEt2)2] + , and [CrCp*-(CH2Ph)3Li] as polymerization catalysts has been attributed to dissociation equilibria with 13electron species which can bind and activate the olefin substrate. 1 On the other hand, we have recently shown that the use of the strongly bonding and highly nephelauxetic CN -ligand allows the stabilization, in solution, of the spin-paired, 17-electron complexes [Cr(ring)(CN)4] 2-(ring = Cp or Cp*) and [CrCp(CN)2L2] (L = tertiary phosphine).

  parameter investigations. Although both starting and final complexes are blue, the visible absorption spectra differ sufficiently, especially in the 450-700 nm region, to allow an accurate determination of kinetic parameters on the basis of variations up to 0.1 absorbance unit. The instrument background noise (and the residuals of the final data fittings) are below 1 milliabsorbance unit. Various kinetic runs were carried out under pseudo-first order conditions. Initial investigations were carried out in THF. The global analysis of the data obtained in this solvent, however, did not yield a satisfactory fit to a common single exponential for all absorption frequencies. An acceptable fit was obtained instead for an A  B  C model. The absorption spectra calculated with the SPECFIT global analysis program for A, B and C were qualitatively similar and suggest that all species are 15-electron half-sandwich derivatives of Cr III . The most reasonable structural assignment of the different species appears to be the product of phosphine exchange, [CrCpCl2(PMe3)], to B and the ionic product [CrCpCl(PMe3)2] + Cl -, derived from substitution of a chloride ligand by a second PMe3 molecule, to C. Under the above assumption, we reasoned that the second exchange process could be strongly retarded or completely suppressed upon carrying out the kinetic study in a less polar solvent, e.g. toluene.

Fig. 1

 1 Fig. 1 Plot of kobs vs. [PMe3] for the reaction between [CrCpCl2(PMePh2)] and PMe3 in toluene at 25 °C.

Fig. 2

 2 Fig. 2 Eyring plot for the reaction between [CrCpCl2(PMePh2)] and PMe3 in toluene.

Fig. 3

 3 Fig. 3 Correlation between the second-order rate constant of the [CrCpCl2(PR3)] + PMe3 reaction and Tolman's electronic and steric parameters.

  Additional calculations on the spin doublet curve at various fixed distances between the Cr and the P atom of the entering PH3 ligand, with complete optimization of all other parameters, yield the results shown in Fig.4. The minimum of this curve corresponds to the previously fully optimized spin doublet adduct which is located at 0.03375 hartree (21.2 kcal mol -1 ) above the [CrCpCl2(PH3)] (spin quartet) + PH3 combination at the chosen level of theory.

Fig. 4

 4 Fig. 4 Energy of the [CrCpCl2(PH3)] + PH3 system in the doublet spin state at various fixed distances between the Cr atom and the P atom of the incoming PH3 ligand. The energy (kcal mol -1 ) is relative to the [CrCpCl2(PH3)] (S = 3/2) + PH3 system at infinite distance.

  , featuring the motion of one P atom toward the Cr atom and the other one away from it. The translation movement of the two PH3 ligands is accompanied by a slight rocking motion, tilting the C3v symmetry axis of the PH3 ligands from a direction collinear with the Cr-P bond in the bonded geometry toward a direction collinear with the Cr-Cp axis in the non-bonded geometry.

Fig. 5

 5 Fig. 5 Normal mode corresponding to the imaginary frequency of the first-order saddle point for the PH3 exchange reaction coordinate of the [CrCpCl2(PH3)] + PH3 system.

Table 1 .

 1 Rate constants for the [CrCpCl2(PMePh2)] + PMe3 reaction in toluene. a

	Run	10 -2 [PMe3]/M	T/K	kobs/s -1	10 -2 k/M -1 s -1
	1	1.00	298	2.075(8)
	2	2.50	298	5.9(1)
	3	4.00	298	8.9(1)
	4	4.70	298	10.39(7)
	5	2.10	278	1.88(3)	0.89(1)
	6	2.10	288	3.09(3)	1.47(1)
	7	2.10	298	4.66(4)	2.22(2)
	8	2.10	308	7.29(6)	3.47(3)

a Complex concentration = 1.23•10 -3 M

Acknowledgments

We are grateful to the Ministère de l'Education Nationale de la Recherche et de le Technologìe and the Centre National de la Recherche Scientifique for support of this work. We also thank the Région Bourgogne for equipment funding (Accueil de Nouvelles Equipes) and for providing part of the funds necessary for the purchase of the stopped-flow apparatus.


Dolores Morales

Rinaldo Poli
email: poli@u-bourgogne.fr

Philippe Richard

Jacques Andrieu

Edmond Collange

Prof Rinaldo Poli

Half-sandwich molybdenum compounds with phosphine-alkylthiolate and phosphine-thioether ligands. Crystal structure of [CpMo(SCH2CH2PPh2)2][BPh4]

The reaction of CpMoCl2 with Ph2PCH2CH2SR (R = H, CH3) yields the corresponding addition products CpMoCl2(Ph2PCH2CH2SR), but only the derivative with R = CH3 (compound 5) is sufficiently stable to be isolated as a crystalline solid. The derivative with R = H evolves rapidly to afford a mixture of compounds [CpMo(SCH2CH2PPh2)2] + Cl -, 1, and [CpMoCl(SCH2CH2-PPh2)]2, 2, the former being favored by a larger ligand/Mo ratio. Compound 1 undergoes metathesis with NaBPh4 to afford [CpMo(SCH2CH2PPh2)2] + BPh4 -, 3, which has been characterized by X-ray crystallography. The reaction of CpMoCl2 with 2 equiv of Ph2PCH2CH2S -Li + affords the paramagnetic complex CpMo(SCH2CH2PPh2)2, 4, which is readily oxidized by Cp2Fe 1 or by H + to the corresponding cation. The salts 1 and 3, in turn, may be reduced by Na amalgam, MeLi, or Bu t OK to compound 4. The reversible redox process interconverting 4 and its cation occurs at E1/2 = -1.23 V relative to the ferrocene standard, while compound 5 shows a reversible oxidation process at E1/2 = 0.12 V by cyclic voltammetry. The comparison between these potentials and that previously reported for CpMoCl2(dppe) indicates relative donor abilities in the order Ph2P > MeS and RS -> Cl -. Compound 5 can also be synthesized by Na amalgam or Zn reduction of CpMoCl4(Ph2PCH2CH2SCH3) 6, which is obtained by addition of the ligand to CpMoCl4.

Introduction

Molybdenum complexes with sulfide, thiolate, or thioether ligands are extensively used models for understanding the mechanism of action of fossil fuel hydrotreating catalysts and metalloenzymes involved in the nitrogen cycle. [START_REF] Chianelli | [END_REF][2][3][START_REF]Molybdenum Enzymes, Cofactors, and Model Systems[END_REF][START_REF] Stiefel | [END_REF][6][7][8][9][10] In both areas, useful information has been obtained from fundamental investigations of the effect of the coordination environment on the stability, redox properties, and reactivity. A great many studies have been devoted to dinuclear half-sandwich complexes of Mo(III) and Mo(IV), 6,7,10 generally containing only anionic ligands (halides, thiolates, sulfide) or a combination of these and neutral p-acceptor ligands (carbonyl, isocyanides, thioethers). In our laboratory, we have investigated in detail reactivity and redox properties as a function of the ligands for a class of mononuclear complexes of formula CpMoX2L2 where X is a halide ligand and L is typically a tertiary phosphine. [11][12][13][14][15][16][17][18] These are stable paramagnetic compounds characterized by a 17-electron configuration and sharp room temperature isotropic EPR resonances. We extend in this contribution the above class to thiolate and thioether derivatives.

Results

Reactions with the Ph2PCH2CH2SH ligand.

The reaction between CpMoCl2 and the bifunctional ligand Ph2PCH2CH2SH in a 1:1 or 1:2 ratio produces compounds [CpMo(SCH2CH2PPh2)2]Cl, 1, and [CpMoCl(SCH2CH2PPh2)]2, 2 (eq 1). The interaction initially affords an EPR active intermediate which disappears within a few minutes. The EPR properties of this intermediate indicate its probable composition as the addition product, CpMoCl2(Ph2PCH2CH2SH), by comparison with those of the stable thioether analogue CpMoCl2(Ph2PCH2CH2SCH3), compound 5, see below. When a larger excess of the ligand was used (3.5 equivalents), however, compound 1 was recovered in a greater yield (57 % relative to 40 % when 2 equiv were used) and product 2 was absent. The yield of compound 1 was even lower (20 %) when using a 1:1 Mo:ligand ratio.

CpMoCl2 + Ph2CH2CH2SH  [CpMo(SCH2CH2PPh2)2]Cl + [CpMoCl(SCH2CH2PPh2)]2 (1) 1 2
Compound 1, which is obtained as a yellow precipitate from the reaction mixture, is insoluble in all common solvents and only slightly soluble in MeOH. A 31 P{ 1 H}-NMR spectrum in MeOH shows a single resonance at δ 85.1, indicating its diamagnetic nature.

Metathesis of 1 with NaBPh4 yields a more soluble tetraphenylborate salt, 3, which was amenable to a more detailed characterization. The 31 P{ 1 H} NMR resonance of 3 compares with that of 1, while the triplet [START_REF] Chianelli | [END_REF] H NMR resonance for the Cp ring at δ 4.40 (JHP = 1.83 Hz) is direct evidence for the presence of two ligands per metal atom. These spectral data suggest a fourlegged piano stool structure for the cation, leaving uncertain the stereochemistry (cis vs. trans).

The trans configuration is shown by the X-ray structural characterization (see below).

Compound 2 is obtained as a microcrystalline brown solid by diffusion of pentane into the CH2Cl2 solution after separation of compound 1. This compound is insoluble in hydrocarbon solvents but soluble in CH2Cl2, CHCl3 and THF. Elemental analyses (C, H, S) and NMR investigations ( 1 H and 31 P) help in the structural assignment of the compound. The doublet Cp resonance in the 1 H NMR spectrum indicates the presence of only one ligand per metal atom and the diamagnetism requires a dimeric formulation, since mononuclear half-sandwich Mo(III) complexes are paramagnetic and EPR active. 17 A saturated electronic configuration can be reached by adopting a bridged structure with a metal-metal bond, as described for the isoelectronic complex {[CpMo(µ-SBu t )(CO)2]2} 2+ . 19 In principle, either the two chloride ligands or the two thiolate functions may occupy the bridging positions. The superior bridging ability of thiolates relative to halides lead us to propose structure I for compound 2. The isoelectronic Mo(III) complexes [CpM(SMe)X(CO)]2 (M = Mo, W; X = Cl, Br) have also been proposed to adopt a thiolato-bridged structure with terminal halide ligands, 20 and complexes with bridging thiolato or hydrosulfide and terminal chlorides are known for other metals. 21,22 The single 31 P{ 1 H} NMR resonance at δ 78.5 indicates the equivalence of the two phosphorus atoms and the chelating nature of the ligand, but cannot distinguish between the P:S and the P:(µ-S) binding modes. Unfortunately, suitable crystals for an X-ray investigation could not be obtained.

The reaction between CpMoCl2 and one equivalent of Ph2PCH2CH2S -Li + leads to a mixture of unidentified paramagnetic products. However, the reaction with two equivalents of the same reagent produces a stable paramagnetic Mo(III) compound, namely CpMo(SCH2CH2PPh2)2, 4, see equation 2. Compound 4 also forms upon treatment of compound 2 with Ph2PCH2CH2S -Li + , see equation 3. The identity of this complex as a 17electron monomer is confirmed by the EPR spectrum which shows a binomial triplet (g = 1.987, aP= 3.9 G, aMo= 32.3 G) in hexane (other solvents yield a broader resonance which does not permit the observation of the phosphorus coupling). It is interesting to remark the unusually small phosphorus hyperfine coupling. In previously reported bis-phosphine dichloro complexes of half-sandwich Mo(III), the aP values are smaller when the two P donors adopt a relative trans configuration (in the 9-16 G range) than when they are located cis to each other (greater than 23 G). 12,14,23 A trans geometry seems therefore most reasonable for compound 4.

CpMoCl2 + 2 Ph2PCH2CH2S -Li +  CpMo(SCH2CH2PPh2)2 + 2 LiCl (2)
This compound can be also synthetized by reduction of compounds 1 and 3, eq. 4. The reduction of the THF-soluble 3 can be easily accomplished with sodium, while the reduction of the insoluble 1 can conveniently be carried out by the use of THF-soluble reducing agents. One such reagent is MeLi, which is known to display single electron transfer properties. 24 The reduction process with this reagent is instantaneous in THF. Somewhat surprisingly, a clean reduction process also occurs, albeit more slowly (12 h), with Bu t OK. Conversely, compound (equation 4)

The reversibility of the redox process in eq. 4 can also be witnessed by electrochemical investigations. The cyclic voltammogram of compound 4 shows a reversible oxidation wave at E1/2 = -1.23 V, which is quite close to the potential of the reversible reduction wave measured for compound 3 (E1/2 = -1.25 V). This value is an indicator of the electron richness of this system relative to the CpMoCl2L2 complexes (L = tertiary phosphine), whose oxidation potentials are in the range -0.33 V (for L2 = dppe) to -0.63 V (for L = PPr n 3). 14 This illustrates the greater donor capability of a thiolate ligand relative to a chloride. Since compound 3 is shown by the X-ray analysis to adopt a trans geometry, the reversibility of the redox process indicates the same relative configuration for compound 4, confirming the prediction previously made on the basis of the aP value in the EPR spectrum.

A view of the cation of compound 3 is shown in Figure 1, and bond lengths and angles in Table 1. The geometry can be described as a four-legged piano stool, with the pairs of sulfur and phosphorus atoms occupying relative trans positions to yield an approximate C2 local symmetry. The CNT-Mo-L angles (CNT = center of gravity of the Cp ring) are larger for the thiolate donors than for the phosphorus donors, as predicted on the basis of the π-donor/acceptor properties of these ligands and the diamagnetic configuration of the complex. 25 The geometry and metric parameters can be best compared with those of the isoelectronic oxo compound MoO(SCH2CH2PPh2)2. 26 Both are compounds of Mo(IV) with the same bifunctional ligand, the O 2-and Cp -ligands being both capable of establishing three bonding interactions (σ+2π) and, therefore, being able to donate 6 electrons to the metal center. The Mo-P distances are comparable in the two compounds, whereas the Mo-S distances are significantly shorter in the cation of 3 relative to the oxo analogue (2.372(4) Å and 2.348(4) Å). 26 This difference can be rationalized by the stronger electron donating capability of the O 2-ligand relative to the Cp -, as also indirectly established from other comparisons. 27 Reactions with the Ph2PCH2CH2SCH3 ligand.

The reaction between CpMoCl2 and the phosphine-thioether ligand Ph2PCH2CH2SCH3 leads to the addition product 5, see Scheme 1, which was isolated as a microcrystalline red solid and characterized by elemental analysis, EPR spectroscopy, and cyclic voltammetry. The compound is indefinitely stable under an argon atmosphere at -80°C, but decomposes at room temperature rather quickly even under an inert atmosphere. The EPR spectrum shows the expected doublet due to coupling with a phosphorus atom (g = 1.973, aP= 24.7 G, aMo = 35.9 G). These properties compare quite well with those of the diphosphine derivatives CpMoCl2(L-L) (L-L = dppe, dmpe) for which a four-legged piano-stool structure with a cis arrangement of the two phosphorus donors was demostrated. 12,28 An analogous structure is therefore proposed also for compound 5. In particular, the phosphorus hyperfine coupling constant of 5 is only slightly smaller than those of the diphosphine analogues (26 G for the dppe complex and 28 G for the dmpe complex) and the Mo hyperfine coupling constant is correspondingly slightly greater (29 and 33 G for dppe and dmpe analogues, respectively). The alternative trans arrangement, observed for the bulkier Cp*MoCl2(dppe) derivative, leads to completely different spectral parameters. 23 The g factor was shown to be highly dependent on the nature of the halide ligands but rather independent on the phosphine substituents (in the 1.978-1.994 range for the dichloride complexes). The g value recorded for compound 5 is at the low end of this range, and the direct comparison with the value for CpMoCl2(dppe) (g = 1.986) 12 indicates a rather small low-field shift caused by the replacement of a PPh2 donor with a SMe donor.

Compound 5 shows a reversible oxidation wave at E1/2 = 0.12 V in the cyclic voltammogram. This is 0.45 V more positive relative to the oxidation process of the analogous CpMoCl2(dppe) complex, 14 indicating that the SMe group is a weaker electron donor relative to the PPh2 group, as expected from the different electronegativity of the two donor elements.

Scheme 1

Compound 5 has also been prepared by an alternative procedure (Scheme 1). The reaction between CpMoCl4 and one equivalent of the ligand in CH2Cl2 affords the corresponding addition product CpMoCl4(Ph2PCH2CH2SCH3), 6, in good yields. Reduction of compound 6 in THF with either two equivalents of sodium amalgam or one equivalent of Zn gives the Mo(III) complex 5. The EPR spectrum of compound 6 does not show coupling to the phosphorus atom (singlet at g = 1.954, aMo = 51.8 G). Previously reported phosphine adducts of alkyl-substituted cyclopentadienyl derivatives of Mo(V) display rather large phosphorus hyperfine constants (greater than 24 G), [29][30][31] thus indicating the possibility that the bifunctional ligand binds the metal in a monodentate fashion via the sulfur donor in compound 6. However, we find that the addition of the electronically similar PMePh2 ligand to CpMoCl4 affords an EPR spectrum which consists of a single resonance with no observable hyperfine coupling to the P nucleus.

The g value and aMo hyperfine coupling of this spectrum are very similar to those of the Ph2PCH2CH2SMe adduct (see Experimental section). Therefore, phosphorus coordination to the metal center remains a structural possibility. We have also considered the possibility of a bidentate coordination mode for the phosphine-thioether ligand. This alternative arrangement would likely induce the displacement of a chloro ligand, to afford an ionic isomer, [CpMoCl3(η 2 -Ph2PCH2CH2SMe-P,S)] + Cl -. Electrical conductivity measurements indicate the presence of ionic species (Λꝏ = 9.3 and 90.5 S cm 2 mol -1 in THF and MeCN solutions, respectively). The values measured, however, are slightly smaller than those typically observed for fully dissociated 1:1 salts. 32 A possible rationalization of this result is the existence of an equilibrium between ionic and neutral isomeric forms.

An attempt was made to methylate compound 5 with methyllithium. The reaction with one equivalent of MeLi carried out at -80°C in THF led to the disappearance of the starting material without the appearance of new EPR-active species. The 31 P{ 1 H} NMR spectrum showed the formation of a complex mixture of several products, which was not further investigated. When two equivalents of MeLi were used, the immediately recorded EPR spectrum showed a new doublet resonance (g = 1.987, aP = 24.3 G, aMo = 38.6 G), which we tentatively assign to the dimethyl product CpMo(CH3)2(Ph2PCH2CH2SMe), but the signal disappeared after ca. 1/2 h at room temperature. The 31 P{ 1 H} NMR spectrum showed the release of the free ligand Ph2PCH2CH2SMe. An analogous alkylation attempts had been carried out previously for compound CpMoCl2(PMe3)2, also resulting in decomposition of the alkylation product, 13 whereas the alkylation of CpMoCl2(η 4 -C4H6) affords thermally stable dialkyl derivatives. 33 The positive shift of the g value upon methylation of compound 5 (from 1.973 to 1.987) parallels those observed upon methylation of CpMoCl2(PMe3)2 (from 1.982 to 2.003) and CpMoCl2(η 4 -C4H6) (from 1.994 to 2.012).

Discussion

The addition of neutral ligands to CpMoCl2 to form 17-electron CpMoCl2L2 adducts had previously been established when L = tertiary phosphine 11,14 or L2 = diphosphine 12 and diene. 34 In this contribution, we have analyzed the results of the addition of the bifunctional ligands Ph2PCH2CH2SR (R = H, CH3). An isolable addition product (compound 5) is only obtained for R = CH3. When R = H, the addition intermediate (which can be spectroscopically observed) rapidly evolves to lead to the isolation of two different products, 1 and 2, in a relative ratio that depends on the amount of ligand used. The results of the electrochemical investigations help us formulate a mechanism for the formation of these compounds (see Scheme 2).

Scheme 2

The difference between the two addition products is likely due to the acidity of the SH function, especially once this is coordinated to the metal center. Thus, deprotonation of the intermediate II and loss of chloride would lead to the unsaturated complex III. A related deprotonation process has been described for a very similar reaction, namely the addition of HS(CH2)nSH (n = 2, 3) to Cp2Mo2(µ-SMe)3(µ-Cl), whereby the dinuclear product Cp2Mo2(µ-SMe)3[µ-S(CH2)nSH] is obtained with elimination of HCl. 35 Compound CpMoCl2 is also a dinuclear compound with four bridging chloro ligands. [36][37][38] Intermediate III can evolve to a saturated product either by dimerization, leading directly to the observed product 2, or by addition of a second molecule of the ligand Ph2PCH2CH2SH. Further deprotonation and chloride loss would afford compound 4, but the redox properties of this 17-electron Mo(III) product make it susceptible to oxidation by the available protons, as it has independently been verified, to afford the observed Mo(IV) product 1. The essential features of this proposed mechanism are consistent with the observation that the use of an increased amount of the ligand Ph2PCH2CH2SH increases the yield of 1 and decreases the yield of 2. In addition, compound 2 converts into compound 4 upon treatment with Ph2PCH2CH2S -.

It is notable to observe that intermediate II has the same electronic configuration as compound 4, but is not oxidized under the reaction conditions that lead to products 1 and 2.

The electrochemical investigation of the analogous thioether complex 5 indicates that the potential at which its oxidation would occur is much more positive relative to that of compound 4 (ca. 1.3 V more positive), revealing a dramatic effect of the ligands nature on the redox properties in this system. A comparison of the redox potentials for the Mo(III)/Mo(IV) processes in compounds 3/4 and 5 with that already reported in the literature for compound CpMoCl2(dppe) (-0.33 V) shows trends in ligand donor properties in the order RS -> Cl -and RSMe < RPPh2. Both these two effects contribute to render compound 4 much more easily oxidized relative to compound 5.

Conclusions

The present study is relevant in comparison with previous investigations of electron-poorer half-sandwich Mo(III) complexes, which by and large prefer to adopt a dinuclear structure with a metal-metal bond. The combination of a sulfur ligand (alkylsulfido or thioether) with a phosphorus donor in a chelating bifunctional ligand leads to stable mononuclear, electron-richer compounds. This greater electron-richness is clearly manifested in the oxidation of Mo(III) to Mo(IV) by the protons generated from coordinated mercaptans, to yield H2. Similar oxidation of dinuclear, thiolate-bridged cyclopentadienyl derivatives of Mo(III) by the proton have been reported. 7

Experimental

All reactions were carried out in dry solvents under a dinitrogen or argon atmosphere by the use of Schlenk line or glove-box techniques. The solvents were dried by conventional methods (CH2Cl2 from CaH2, THF from Na/K, pentane and toluene from Na/benzophenone) and distilled under nitrogen prior to use. Deuterated solvents were dried over molecular sieves and degassed by 3 freeze-pump-thaw cycles prior to use. 1 H and 31 P{ 1 H}-NMR measurements were carried out on a Bruker AC200 spectrometer. The peak positions are reported with positive shifts downfield of TMS as calculated from the residual solvent peaks ( 1 H) or downfield of external 85% H3PO4 ( 31 P). EPR measurements were carried out at the X-band microwave frequency on a Bruker ESP300 spectrometer. The spectrometer frequency was calibrated with DPPH (g = 2.0037). Cyclic voltammograms were carried out at room temperature with a Radiometer digital electrochemical analyzer (model DEA332). The electrochemical cell was fitted with an SCE reference electrode, a platinum disk working electrode and a Pt wire counterelectrode. Bu4NPF6 (ca 0.1 M) was used as supporting electrolyte. All potentials are reported relative to the ferrocene standard, which was added to each solution and measured at the end of the experiments. The solution conductivity measurements were carried out at 25°C with a Tacussel type CD6 N conductimeter equipped with an XE 110 cell which had been calibrated with a 0.1 M KCl solution. The elemental analyses were carried out by the analytical service of the Laboratoire de Synthèse et d'Electrosynthèse Organométalliques. NaBPh4, HBF4•OEt2, MeLi (1 M solution in diethyl ether), Bu n Li (1.6 M solution in hexanes), Bu t OK and Zn powder were used as received, without further purification. CpMoCl4, 39 CpMoCl2, 40 Ph2PCH2CH2SH, 26 and [Cp2Fe]BF4, 41 were prepared according to literature procedures.

Ph2PCH2CH2SCH3 was prepared by a slight modification of the method described in the literature: 42 to a solution of Ph2PCH2CH2SH (1.057 mL, 4.57 mmol) in 20 mL of THF at 0°C was added a solution of 1.6 M Bu n Li (2.85 mL, 4.57 mmol) and MeI (284 ml, 4.57 mmol). The mixture was stirred overnight. The solvent was evaporated in vacuo, the residue was extracted with pentane, filtered through Celite, and concentrated in vacuo to ca. 20 mL. Cooling to -80°C for 24 h afforded white crystals of Ph2PCH2CH2SCH3. The 1 H and 31 P NMR properties of this product are identical with those previously reported. 42 Yield: 0.845g, 71%.

Synthesis of [CpMo(SCH2CH2PPh2)2]Cl (1)

A solution of Ph2PCH2CH2SH (474 mL, 2.17 mmol) in 5 mL of CH2Cl2 was added to a suspension of CpMoCl2 (0.143 g, 0.62 mmol) in 10 mL of CH2Cl2 and the mixture was stirred overnight at room temperature. An immediate analysis of the supernatant solution by EPR spectroscopy revealed a doublet resonance (aP = 26.20 G) at g = 1.982. After a few minutes this EPR signal was no longer present. Compound 1 precipitated as a very insoluble yellow solid, which was collected on a filter, washed with CH2Cl2 (4 x 5mL) and dried in vacuo. Yield: 0.244 g, 57%. Anal. Calcd for C33H33ClMoP2S2: C, 57.69; H, 4.84; S, 9.33. Found C, 57.27; H, 4.87; S, 8.96. 31 P{ 1 H}-NMR (CH3OH, with external D2O capillary): 85.1 ppm.

Synthesis of [CpMoCl(SCH2CH2PPh2)]2 (2)

A solution of Ph2PCH2CH2SH (628 mL, 2.88 mmol) in 5 mL of CH2Cl2 was added to a suspension of CpMoCl2 (0.668 g, 2.88 mmol) in 20 mL of CH2Cl2 and the mixture was stirred overnight at room temperature. A yellow microcrystalline precipitate corresponding to compound 1 which had formed was filtered (yield: 0.402 g, 20%). The solution filtered again through Celite, concentrated in vacuo ca. 5 mL, layered with pentane (20 mL) and kept in a refrigerator at -20°C for several days. When the diffusion was complete compound 2 was obtained as a microcrystalline brown solid. Yield: 0.589 g, 46%. Anal. Calcd for C19H19ClMoPS: C, 51.66; H, 4.33; S, 7.26. Found C, 51.28; H, 4.69; S, 6.97. 31

Synthesis of [CpMo(SCH2CH2PPh2)2]BPh4 (3)

To a suspension of [CpMo(SCH2CH2PPh2)2]Cl (0.034 g, 0.05 mmol) in 10 mL of THF was added NaBPh4 (0.016 g, 0.05 mmol) and the mixture was stirred overnight at room temperature, resulting in the solubilization of the yellow starting material to yield an orange solution. After evaporation, the residue was redissolved in 10 mL of CH2Cl2. The resulting orange solution was filtered through Celite and concentrated under reduced pressure to ca. 5 mL. Slow diffusion of pentane into this solution at -20°C produced orange crystals after 3 days. Yield: 0.038 g, 80%. A suitable crystal obtained in this way was used for the X-ray analysis. Anal. Calcd for

Synthesis of CpMo(SCH2CH2PPh2)2 (4)

A) From CpMoCl2 and two equiv of Ph2PCH2CH2S -Li + . A solution of Ph2PCH2CH2S -Li + , prepared in situ from Ph2PCH2CH2SH (496 mL, 2.15 mmol) and MeLi (2.15 mL, 2.15 mmol) in 10 mL of THF, was added to a suspension of CpMoCl2 (0.255 g, 1.07 mmol) in 40 mL of THF. The mixture was stirred overnight at room temperature. The brown-red solution was evaporated under reduced pressure. The residue was extracted in toluene and filtered through Celite. The solvent was evaporated in vacuo, the residue was washed with cold pentane (3 x 5mL) and dried under vacuum. Yield: 0.382 g, 55%. EPR (hexane): g = 1.987 (triplet with Mo satellites, aP = 3.94 G, aMo = 32.39 G). Cyclic voltammetry (THF, room temperature): reversible oxidation at E1/2 = -1.23 V.

B) By reduction of compound 1.

By MeLi. To a suspension of compound 1 (0.091 g, 0.132 mmol) in 20 mL of toluene was added dropwise 264 ml (0.264 mmol) of a MeLi solution (1 M in diethyl ether) causing the dissolution of the yellow starting material within 30 min and formation of a brown-red solution. The solution was evaporated under reduced pressure to ca. 10 mL and filtered through Celite. The solvent was further evaporated until dryness and the residue was washed with cold pentane (3 x 5 mL) and dried under reduced pressure. Yield: 0.060 g, 69.83%. This product had spectroscopic (EPR) and electrochemical properties identical those of the material obtained by method A.

By ButOK. To a suspension of compound 1 (0.084 g, 0.122 mmol) in 20 mL of THF was added Bu t OK (0.027 g, 0.244 mmol). After 24 h of stirring at room temperature the solvent was removed in vacuo. The residue was extracted with toluene and filtered through Celite, the solution was evaporated to dryness under reduced pressure and the residue was washed with cold pentane (3 x 5 mL) and dried under vacuum. Yield: 0.185 g, 72.88%. Anal. Calcd for C33H33MoP2S2: C, 60.83; H, 5.10; S, 9.84. Found C, 61.15; H, 5.36; S, 9.31.

C

) By reduction of compound 3. To a solution of [CpMo(SCH2CH2PPh2)2]BPh4 (0.010 g, 0.01 mmol) in 2 mL of THF was added Na (0.023 g, 0.01 mmol) and the mixture was stirred at room temperature for 45 min. During this time, the yellow solution became red and the EPR spectrum showed the signal corresponding to compound 4. The EPR properties matched those described above for the product of method A. D) From compound 2 and two equiv of Ph2PCH2CH2S -Li + . To a solution of compound 2 (0.007 g, 0.009 mmol) in 1 mL of THF was added a solution of Ph2PCH2CH2S -Li + , prepared in situ from Ph2PCH2CH2SH (5 mL, 0.018 mmol) and MeLi (18 mL, 0.018 mmol) in 1 mL of THF. The brown solution became red. The EPR spectrum shows the signal corresponding to compound 4.

NMR study of the chemical oxidation of compound 4

A) By ferrocenium. To a solution of compound 4 (0.006 g, 0.01 mmol) in 5 mL of THF was added [Cp2Fe]BF4 (0.027 g, 0.01 mmol) and the mixture was stirred for 15 min. The red solution changes to yellow and it becomes EPR silent. The 31 P{ 1 H}-NMR (THF) shows one sharp peak at 84.7 ppm attributed to [CpMo(SCH2CH2PPh2)2]BF4. B) By HBF4. To a solution of compound 4 (41.65 g, 0.064 mmol) in 1 mL of C6D6 was added HBF4•OEt2 (0.064 mmol, 69 mL) at 0°C. Gas evolution was immediately observed, discharging the red colour of the solution and yielding a yellow precipitate. 1 H-NMR (C6D6, δ): 4.50 (s, H2).

The suspension was filtered and the yellow solid was dissolved in THF. 31 P{ 1 H}-NMR (THF, δ): 84.7 ppm.

Synthesis of CpMoCl2(CH3SCH2CH2PPh2) (5) from CpMoCl2 and Ph2PCH2CH2SCH3

To a suspension of CpMoCl2 (0.185 g, 0.8 mmol) in 15 mL of CH2Cl2 was added a solution of Ph2PCH2CH2SCH3 (0.209 g, 0.8 mmol) in 5 mL of CH2Cl2 at room temperature. The mixture was stirred for 2 h, filtered through Celite and concentrated under reduced pressure to ca. 5 mL.

Addition of pentane (20 mL) gave 6 as a brown-red microcrystalline solid. Yield: 0.324 g, 82%.

Anal. Calcd for C20H22Cl2MoPS: C, 48.80; H, 4.50; S, 6.51. Found C, 48.76; H, 4.87; S, 5.91. EPR (hexane): g = 1.973 (doublet, with Mo satellites, aP = 24.7 G, aMo = 35.9 G). Cyclic voltammetry (CH2Cl2, room temperature): reversible oxidation at E1/2 = 0.12 V.

Synthesis of CpMoCl4(CH3SCH2CH2PPh2) (6)

A solution of Ph2PCH2CH2SCH3 (0.551 g, 2.11 mmol) in 5 mL of CH2Cl2 was added to a suspension of CpMoCl4 (0.669 g, 2.21 mmol) in 20 mL of CH2Cl2 at -80°C. The solution was warmed to room temperature, stirred for 3 h and filtered through Celite. The filtrate was concentrated under reduced pressure to ca. 5 mL. Addition of pentane (20 mL) gave 6 as a brown-red microcrystalline solid. Yield: 1.152 g, 96%. Anal. Calcd for C20H22Cl4MoPS: C, 42.65; H, 3.94; S, 5.69. Found C, 42.28; H, 3.82; S, 5.65. EPR (CH2Cl2): g = 1.987 (s, with Mo satellites, aMo = 51.82 G). Molar conductivity (Λ, S cm 2 mol -1 ); in THF: 2.6 (8.9x10 -3 M), 7.2 (8.9x10 -4 M), Λꝏ = 9.3 S cm 2 mol -1 ; in MeCN: 48.9 (9.05x10 -3 M), 75.7 (9.05x10 -4 M), Λꝏ = 90.5 S cm 2 mol -1 .

Reduction of compound 6 to compound 5

A) By sodium amalgam. Compound 6 (0.400 g, 0.71 mmol) was dissolved in THF (20 mL) and the solution was cooled to 0°C. Freshly prepared sodium amalgam (1% ww, 0.032 g, 1.42 mmol) was added and the mixture was stirred for 45 min. The mixture was decanted and the solution was filtered (via cannula). The solvent was removed in vacuo and the solid residue was extracted with CH2Cl2 and filtered through Celite. The solution was evaporated and the precipitate was washed with pentane and dried under vacuum. Yield: 0.162 g, 46%. This product had spectroscopic (EPR) properties identical those of the material obtained from CpMoCl2 and Ph2PCH2CH2SMe as described above.

B). By Zn.

To a solution of compound 6 (0.010 g, 0.017 mmol) in 5 mL of THF was cooled to 0°C and then Zn powder (0.001 g, 0.025 mmol) was added. The mixture was stirred for 3 h. An EPR investigation of this solution showed an identical spectrum as observed for the compound 5 as obtained by the two methods described above.

Reactions of compound 5 with MeLi

A) With one equivalent. A solution of compound 5 (0.030 g, 0.11 mmol) in 10 mL of THF was cooled to -80°C and then a solution of MeLi (109 mL, 0.109 mmol) was added dropwise via a microsyringe. The EPR spectrum showed a decrease in intensity of the EPR signal of the starting material until 5 was completely consumed, but no EPR-active products were observed.

B) With two equivalents.

To a cooled (-80°C) solution of compound 5 (0.046 g, 0.16 mmol) in 10 mL of THF was added dropwise a solution of MeLi (320 mL, 0.32 mmol). The EPR spectrum recorded immediately showed a new doublet resonance with Mo satellites, g = 1.987, aP = 24.3 G, aMo = 38.6 G.

Reaction between CpMoCl4 and Ph2PMe.

To a suspension of CpMoCl4 (0.028 g, 0.092 mmol) in 10 mL of THF cooled to 0°C was added The data were corrected for absorption (psi-scan). [START_REF] Fair | An Interactive Intelligent System For Crystal Structure Analysis[END_REF] No decay was observed. The structure was solved via a Patterson search program [START_REF] Sheldrick | SHELXS and SHELXL97[END_REF] and refined (space group P21/c) with full-matrix leastsquares methods [START_REF] Sheldrick | SHELXS and SHELXL97[END_REF] based on |F 2 |. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of the complex were included in their calculated positions and refined with a riding model. The cyclopentadienyl ligand was found to be disordered around its geometrical center and was modelled as lying in two positions with occupancies m1 = 0.624 and m2 = 1-m1 = 0.376. The cyclopentadienyl rings were refined as variable metric groups (the shape is retained but the group may shrink or expand uniformly). The final agreement indices are Rw(F 2 ) = 0.0885 and R(F) = 0.0987 for all data and 600 parameters; R(F) = 0.0336 for 5311 data with I > 2σ(I); goodness of fit = 1.043. The final Fourier difference map is featureless: Δρ = 0.335 and -0.273 e Å -3 . CCDC reference number 186/1324. See http://www.rsc.org/suppdata/dt/1999/867/ for crystallographic files in .cif format.

4

  can be chemically oxidized to the corresponding cation by a ferricinium salt, [Cp2Fe]BF4 or by the proton of HBF4•OEt2, as shown by EPR and NMR spectroscopic monitorings. The oxidation with HBF4•OEt2 in C6D6 led to the immediate evolution of H2, which was identified by the characteristic NMR resonance at δ 4.5.

Figure 1 .Table 1 .

 11 Figure 1. An ORTEP view of the cation of compound 3 with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

  P{ 1 H}-NMR (CDCl3, δ): 78.5 ppm. 1 H-NMR (CDCl3, δ): 7.88-7.32 (m, 10H, Ph), 4.92 (d, JP-H = 2.20 Hz, 5H, Cp), 4.08-3.07 (m, 4H, SCH2CH2P).

  C57H53BMoP2S2: C, 70.52; H, 5.50; S, 6.60. Found C, 70.37; H, 5.40; S, 6.31. 31 P{ 1 H}-NMR (CD2Cl2, δ): 83.4 ppm. 1 H-NMR (CD2Cl2, δ): 7.48-6.79 (m, 40H, Ph), 4.40 (t, JP-H = 1.83 Hz, 5H, Cp), 3.68-2.90 (m, 8H, SCH2CH2P). Cyclic Voltammetry (THF, room temperature): reversible reduction at E1/2 = -1.25 V.

Ph2PMe ( 17 .

 17 20 ml, 0.092 mmol) via a microsyringe. The red starting compound completely dissolved to yield a brown solution. The EPR spectrum showed a new resonance with Mo satellites at g = 1.947, aMo = 51.5 G. The EPR signal decreased in intensity and after 3 h the solution became EPR silent. Crystal structure determination of compound 3 Crystal data. C57H53BMoP2S2, M = 970.80, monoclinic, a = 18.943(2), b = 12.397(1), c = 20.508(2) Å, β =91.138(7) °, U = 4815.1(8) Å 3 , T = 293(2) K, space group = P21/c (no. 14), Z = 4, µ = 0.463 mm -1 , 8365 reflections measured up to sinθ/λ = 0.59 Å -1 , 8108 unique (Rint = 0.0302), which were used in all the calculations.

Acknowledgments

We are grateful to the Conseil Régional de Bourgogne, the MENRT and the CNRS for support of this work. We also thank the Conseil Régional de Bourgogne and the II Plan Regional de Investigation del Principado de Asturias (Spain) for postoctoral fellowships to D. M.