Thomas G Ribelli

Kyle F Augustine

Marco Fantin

Pawel Krys

Rinaldo Poli

Krzysztof Matyjaszewski

Disproportionation or Combination? The Termination of Acrylate Radicals in ATRP

The termination of acrylate radicals in atom transfer radical polymerization (ATRP) can involve either conventional bimolecular radical termination (RT) or catalytic radical termination (CRT). These processes were investigated using a poly(methyl acrylate)-Br macroinitiator under different initial conditions tuned to change the RT/CRT ratio. The polymers, obtained from alkyl halide chain-end activation by [Cu I (L)] + (L = tris[2-(dimethylamino)ethyl]amine (Me6TREN), tris(2-pyridylmethyl)amine (TPMA), or tris(3,5dimethyl-4-methoxy-2-pyridylmethyl)amine (TPMA* 3 )) in the absence of monomer, were

analyzed by size exclusion chromatography (SEC). RT-promoting conditions resulted in the increase of a shoulder with double molecular weight (MW) relative to the macroinitiator distribution, indicating that RT occurred predominantly via radical combination. Conversely, when CRT was promoted, the macroinitiator distribution did not shift, indicating a disproportionation-like pathway. The termination reactions for the TPMA system were further analyzed via PREDICI simulations, which showed the significant impact of mid-chain radicals, arising from backbiting, on the overall termination profile. In all cases, CRT and cross termination between secondary chain-end and tertiary mid-chain radicals contributed the most to the overall amount of terminated chains.

INTRODUCTION 2

Conventional radical polymerization (RP) and the recently developed reversibledeactivation radical polymerization (RDRP) methods have achieved tremendous success. [START_REF] Nicolas | Nitroxidemediated polymerization[END_REF][START_REF] Moad | Living Radical Polymerization by the RAFT Process[END_REF][START_REF] Matyjaszewski | Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives[END_REF][START_REF] Matyjaszewski | Atom Transfer Radical Polymerization[END_REF][START_REF] Matyjaszewski | Macromolecular Engineering by Atom Transfer Radical Polymerization[END_REF] Versatility of the radical-based processes originates from facile experimental setup, wide range of reaction temperatures, and tolerance to functional groups, solvents, and impurities. Throughout the years, elementary reactions occurring in radical polymerizations have been identified and meticulously studied. [START_REF] Krys | Kinetics of Atom Transfer Radical Polymerization[END_REF][START_REF] Barner-Kowollik | Critically evaluated termination rate coefficients for free-radical polymerization: Experimental methods[END_REF][START_REF] Moad | 1 -Introduction[END_REF][START_REF] Buback | Critically Evaluated Termination Rate Coefficients for Free-Radical Polymerization, 1. The Current Situation[END_REF][START_REF] Beuermann | Critically Evaluated Rate Coefficients for Free-Radical Polymerization, 4[END_REF][START_REF] Beuermann | Critically evaluated rate coefficients for free-radical polymerization, 2.. Propagation rate coefficients for methyl methacrylate[END_REF] This also led to development of sophisticated techniques allowing for the precise determination of rate coefficients. [START_REF] Barner-Kowollik | Critically evaluated termination rate coefficients for free-radical polymerization: Experimental methods[END_REF][START_REF] Fantin | ATRP in Water: Kinetic Analysis of Active and Super-Active Catalysts for Enhanced Polymerization Control[END_REF][START_REF] Tang | Effect of Ligand Structure on Activation Rate Constants in ATRP[END_REF] In conventional RP, propagation and termination rate coefficients can be accurately measured using time-resolved electron paramagnetic resonance (EPR) spectroscopy coupled with single pulse pulsed laser polymerization technique (SP-PLP). [START_REF] Kattner | Detailed Investigations into Radical Polymerization Kinetics by Highly Time-Resolved SP-PLP-EPR[END_REF][START_REF] Buback | Detailed Kinetic and Mechanistic Insight into Radical Polymerization by Spectroscopic Techniques[END_REF][START_REF] Barth | SP-PLP-EPR-A Novel Method for Detailed Studies into the Termination Kinetics of Radical Polymerization[END_REF][START_REF] Buback | Free-Radical Termination Kinetics Studied Using a Novel SP-PLP-ESR Technique[END_REF][START_REF] Olaj | The laser-flash-initiated polymerization as a tool of evaluating (individual) kinetic constants of free-radical polymerization, 2. The direct determination of the rate of constant of chain propagation[END_REF][START_REF] Buback | Consistent values of rate parameters in free radical polymerization systems. II. Outstanding dilemmas and recommendations[END_REF][START_REF] Beuermann | Rate coefficients of free-radical polymerization deduced from pulsed laser experiments[END_REF] Although the kinetics of termination has been well studied, the mechanism of bimolecular termination is still a topic of some debate.

As shown in Scheme 1, bimolecular radical termination can occur via combination (Comb) or disproportionation (Disp), resulting in one chain with doubled molecular weight or two chains with a saturated and unsaturated chain end, respectively. The relative extent of these reactions depends on the nature of the radicals. Styrenes [START_REF] Bamford | Termination Reaction in Vinyl Polymerization: Preparation of Block Copolymers[END_REF][START_REF] Nakamura | Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene[END_REF] and acrylonitrile [START_REF] Bamford | Network formation IV. The nature of the termination reaction in free-radical polymerization[END_REF] were suggested to primarily undergo coupling, while methacrylates undergo both Disp and Comb [START_REF] Nakamura | Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene[END_REF][START_REF] Buback | Determination of the Mode of Termination in Radical Polymerization via Mass Spectrometry[END_REF] . Acrylates, however, present a more complex, debated, and yet unresolved case. [START_REF] Bamford | Termination Reaction in Vinyl Polymerization: Preparation of Block Copolymers[END_REF][START_REF] Ayrey | Radiochemical studies of free-radical vinyl polymerizations: 7. Polymerization of methyl acrylate[END_REF][START_REF] Szablan | Mapping Photolysis Product Radical Reactivities via Soft Ionization Mass Spectrometry in Acrylate, Methacrylate, and Itaconate Systems[END_REF] Recently, using radicals photo-generated from organotellurium macroinitiators, Yamago et al. suggested that acrylate radicals terminate predominantly (99%) by disproportionation at room temperature. [START_REF] Nakamura | Termination Mechanism of the Radical Polymerization of Acrylates[END_REF] Most recently, Yamago et al has published on a surprising temperature and viscosity dependence on the termination mechanism of macro and small molecule RTe-(p)MMA and RTe-(p)St compounds with higher temperature increasing the fraction of combination and higher viscosities promoting combination. [START_REF] Nakamura | Control of the Termination Mechanism in Radical Polymerization by Viscosity: Selective Disproportionation in Viscous Media[END_REF] Because of this, we recently reported on a newly proposed tellanyl catalyzed disproportionation [START_REF] Ribelli | Catalyzed Radical Termination in the Presence of Tellanyl Radicals[END_REF] to rationalize their results. Asua et al. [START_REF] Ballard | On the Termination Mechanism in the Radical Polymerization of Acrylates[END_REF] also suggested that acrylate radicals terminate via combination, but transfer reactions can also explain the surprising results reported by Yamago. Due to the relatively high reactivity of the acrylate radicals, both intermolecular [START_REF] Plessis | Modeling of Seeded Semibatch Emulsion Polymerization of n-BA[END_REF] and intramolecular (backbiting) [START_REF] Buback | PLP Labeling in ESR Spectroscopic Analysis of Secondary and Tertiary Acrylate Propagating Radicals[END_REF][START_REF] Nikitin | Determination of Intramolecular Chain Transfer and Midchain Radical Propagation Rate Coefficients for Butyl Acrylate by Pulsed Laser Polymerization[END_REF] transfer to polymer can occur. [START_REF] Yu-Su | Molecular Imaging and Analysis of Branching Topology in Polyacrylates by Atomic Force Microscopy[END_REF][START_REF] Ahmad | Chain Transfer to Polymer and Branching in Controlled Radical Polymerizations of n-Butyl Acrylate[END_REF][START_REF] Konkolewicz | Origin of the Difference between Branching in Acrylates Polymerization under Controlled and Free Radical Conditions: A Computational Study of Competitive Processes[END_REF][START_REF] Konkolewicz | Explaining Unexpected Data via Competitive Equilibria and Processes in Radical Reactions with Reversible Deactivation[END_REF] Such reactions lead to tertiary mid-chain radicals (MCR), which can terminate with another secondary propagating radical (SPR) or MCR. [START_REF] Hamzehlou | Analyzing the discrepancies in the activation energies of the backbiting and [small beta]-scission reactions in the radical polymerization of n-butyl acrylate[END_REF] At higher temperatures, MCRs undergo radical migration [START_REF] Vandenbergh | Macromonomers from AGET Activation of Poly(n-butyl acrylate) Precursors: Radical Transfer Pathways and Midchain Radical Migration[END_REF][START_REF] Van Steenberge | Kinetic Monte Carlo Generation of Complete Electron Spray Ionization Mass Spectra for Acrylate Macromonomer Synthesis[END_REF] and β-scission leading to macromonomers. [START_REF] Chiefari | Chain Transfer to Polymer: A Convenient Route to Macromonomers[END_REF][START_REF] Junkers | Optimum Reaction Conditions for the Synthesis of Macromonomers Via the High-Temperature Polymerization of Acrylates[END_REF][START_REF] Peck | Secondary Reactions in the High-Temperature Free Radical Polymerization of Butyl Acrylate[END_REF] Undoubtedly, transfer reactions are important for in-depth understanding of the termination mechanism of acrylates. This also holds true in RDRP techniques, where radical termination is suppressed. Scheme 1 (Top) Pathways of bimolecular radical termination of two chain-end acrylate radicals (P2•), proceeding either via disproportionation (blue) or combination (red); (bottom) formation of tertiary mid-chain radicals (P3•) via backbiting and subsequent termination with another mid-chain radical or secondary chain-end radical, both resulting in disproportionated chains. [START_REF] Hamzehlou | Analyzing the discrepancies in the activation energies of the backbiting and [small beta]-scission reactions in the radical polymerization of n-butyl acrylate[END_REF] RDRP methods such as reversible addition-fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP) can successfully control the polymerization of acrylate-based monomers. [START_REF] Ahmad | Chain Transfer to Polymer and Branching in Controlled Radical Polymerizations of n-Butyl Acrylate[END_REF][START_REF] Konkolewicz | Origin of the Difference between Branching in Acrylates Polymerization under Controlled and Free Radical Conditions: A Computational Study of Competitive Processes[END_REF][START_REF] Jakubowski | Activators Regenerated by Electron Transfer for Atom-Transfer Radical Polymerization of (Meth)acrylates and Related Block Copolymers[END_REF][START_REF] Xia | Controlled/"Living" Radical Polymerization. Atom Transfer Radical Polymerization of Acrylates at Ambient Temperature[END_REF][START_REF] Chiefari | Living Free-Radical Polymerization by Reversible Addition-Fragmentation Chain Transfer: The RAFT Process[END_REF][START_REF] Chong | Thiocarbonylthio Compounds [SC(Ph)S-R] in Free Radical Polymerization with Reversible Addition-Fragmentation Chain Transfer (RAFT Polymerization). Role of the Free-Radical Leaving Group (R)[END_REF] The reactions can be conducted under mild conditions and are typically fast and efficient. However, ATRP systems with highly active catalysts [START_REF] Schröder | Substituted Tris(2-pyridylmethyl)amine Ligands for Highly Active ATRP Catalysts[END_REF] present an additional side reaction in which acrylate radicals can reversibly coordinate to L/Cu I resulting in formation of a L/Cu II -Pn complex as shown in Scheme 2. [START_REF] Schröder | Formation and Possible Reactions of Organometallic Intermediates with Active Copper(I) Catalysts in ATRP[END_REF][START_REF] Wang | Improving the "Livingness" of ATRP by Reducing Cu Catalyst Concentration[END_REF][START_REF] Matyjaszewski | Interaction of Propagating Radicals with Copper(I) and Copper(II) Species[END_REF] This is related to organometallic mediated radical polymerization (OMRP) systems [START_REF] Poli | Relationship between One-Electron Transition-Metal Reactivity and Radical Polymerization Processes[END_REF] , most common with cobalt complexes. [START_REF] Allan | Organometallic mediated radical polymerization[END_REF][START_REF] Wayland | Degenerative Transfer and Reversible Termination Mechanisms for Living Radical Polymerizations Mediated by Cobalt Porphyrins[END_REF][START_REF] Wang | The Mechanism and Thermodynamic Studies of CMRP: Different Control Mechanisms Demonstrated by CoII(TMP), CoII(salen*), and CoII(acac)2 Mediated Polymerization, and the Correlation of Reduction Potential, Equilibrium Constant, and Control Mechanism[END_REF][START_REF] Debuigne | Overview of cobaltmediated radical polymerization: Roots, state of the art and future prospects[END_REF] The L/Cu II -Pn species can then react with a second radical, leading to catalytic radical termination (CRT). [START_REF] Soerensen | SP-PLP-EPR Measurement of CuII-Mediated ATRP Deactivation and CuI-Mediated Organometallic Reactions in Butyl Acrylate Polymerization[END_REF][START_REF] Ribelli | Effect of Ligand Structure on the CuII-R OMRP Dormant Species and Its Consequences for Catalytic Radical Termination in ATRP[END_REF] Indeed, CRT has recently been shown to be the dominant mode of termination in ATRP of acrylate monomers. [START_REF] Wang | Improving the "Livingness" of ATRP by Reducing Cu Catalyst Concentration[END_REF][START_REF] Pan | Photomediated controlled radical polymerization[END_REF] Despite the low overall amount of terminated chains (<3%) in these reactions, the majority (≈90%) of these dead chains originated from CRT reactions, even when low amounts of L/Cu I were used. Scheme 2 Catalytic radical termination (CRT) of acrylates in the presence of L/Cu I complexes.

Previously, we have reported that CRT gives disproportionation-like products with a tris(2pyridylmethyl)amine (TPMA) based system. [START_REF] Wang | Improving the "Livingness" of ATRP by Reducing Cu Catalyst Concentration[END_REF] One method of studying termination mechanisms is by generating chain-end radicals via activation of "living" macroinitiators. Using various initial conditions and catalytic systems, we were able to kinetically promote different contributions of conventional radical termination of chain-end radicals (RT) and catalytic radical termination (CRT). By analyzing the kinetics of termination and molecular weights of the resulting polymers, valuable mechanistic insights were obtained. According to Eq. 1, to increase the fraction of RT relative to CRT one can: a) decrease [L/Cu II -X]0 or b) increase the ATRP equilibrium constant (KATRP):

𝑅𝑎𝑡𝑒 RT 𝑅𝑎𝑡𝑒 CRT = 2𝑘 t [P n •] 2 2𝑘 CRT [P n •][L/Cu I ] = 𝑘 t [P n •] 𝑘 CRT [L/Cu I ] = 𝑘 t [P n X] 𝑘 CRT [L/Cu II -X] 𝐾 ATRP (1)
Indeed, PREDICI simulations carried out at three different initial conditions confirmed the predictions from Eq 1. As shown in Figure 1, when using the same catalyst (i.e. same KATRP), RT22/CRT is higher for a) lower [PMA-Br]:[L/Cu I ] ratio (1:1) and b) lower [L/Cu II ].

Note that RT22 refers to termination between secondary radicals. Figure 1 also shows the large variation of the RT22/CRT ratio with time. RT22/CRT is largest only at the very first instants when radical concentration is highest. Since the bimolecular termination of chain-end radicals depends on [R•] 2 , RT22 dominates only during the very first milliseconds. Once the L/Cu II -X deactivator builds-up via the persistent radical effect (PRE), the [R•] is suppressed, thus significantly decreasing the rate of RT22. After this initial "influx" of radicals, CRT dominates since [Cu I ] >> [R•] and thus a radical will preferentially coordinate to L/Cu I before terminating with a second radical. This is further shown under the most CRT-inducing conditions (green line), where additional deactivator is present from the beginning and a very small RT/CRT is observed, even at the onset of the reaction. This is because the initial influx of radicals can be quickly deactivated before termination. Therefore, since PREDICI confirmed that the RT/CRT ratio can be kinetically controlled by changing the initial conditions, a pMA-Br ATRP macroinitiator with 99% chain end functionality (CEF; from 1 H NMR, Figure S1) was synthesized via Ag 0 ATRP. [START_REF] Williams | A Silver Bullet: Elemental Silver as an Efficient Reducing Agent for Atom Transfer Radical Polymerization of Acrylates[END_REF] By changing the RT22/CRT ratio, one can analyze the resulting polymer products via SEC to determine the proportion of high MW to low MW polymer and thus obtain invaluable mechanistic information about the products of RT and CRT. S2.

In order to reassess previous contributions, experiments were first conducted under conditions similar to those reported by Yamago et al. [START_REF] Nakamura | Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene[END_REF] In order to eliminate assumptions which were not accounted for by Yamago, we slightly altered the experimental setup.

RESULTS AND DISCUSSION

Termination in the Presence of Me6TREN/Cu I Complex

Termination experiments were conducted in the absence of monomer and termination extent could be determined from the increase in [L/Cu II -Br], according to the principle of halogen conservation. [START_REF] Wang | Halogen Conservation in Atom Transfer Radical Polymerization[END_REF] This allowed the amount of termination to be related to the SEC traces of the resulting terminated polymer (Figure 2). Using the same [pMA-Br]0:[Me6TREN/Cu I ]0 = 1:5 as previously reported [START_REF] Nakamura | Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene[END_REF] , a high molecular weight shoulder accounting for 27% of chains was observed for the polymer recovered after quantitative (>99 %) termination. This is in very good agreement with the Yamago contribution, where 25% of chains were reported to have doubled molecular weight. As predicted by Eq 1, upon addition of [Me6TREN/Cu II -Br] + deactivator, the amount of conventional radical termination was kinetically suppressed relative to CRT. Upon addition of 0.5 or 1 equivalents of [Me6TREN/Cu II -Br] + relative to [pMA-Br]0, the fraction of high molecular weight polymer decreased from 27% to 9% and 7%, respectively (in all cases, the termination was quantitative after 1 hour according to the [Me6TREN/Cu II ] analysis).

These results would indicate that the high molecular weight peak previously attributed to CRT coupling [START_REF] Nakamura | Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene[END_REF] is actually the result of non-catalyzed termination (RT), due to the high radical concentration triggered by fast pMA-Br chain-end activation by the [Cu I (Me6TREN)] + catalyst. Due to the high activity of the [Me6TREN/Cu I ] activator complex, the initial concentration of radicals could not be suppressed efficiently to completely eliminate RT, as evidenced by the small high molecular weight fraction even in the presence of 1 eq. of deactivator complex.

Termination in the Presence of Cu I /TPMA* 3 Complex

As noted earlier, the ratio of RT to CRT can be tuned by changing KATRP (Eq 1). One way to increase KATRP is by using more active L/Cu I catalysts. [START_REF] Schröder | Substituted Tris(2-pyridylmethyl)amine Ligands for Highly Active ATRP Catalysts[END_REF][START_REF] Kaur | Properties and ATRP Activity of Copper Complexes with Substituted Tris(2-pyridylmethyl)amine-Based Ligands[END_REF] In fact, the use of Me6TREN An interesting question arises as to why the TPMA* 3 system did not show as much of a high molecular weight peak as the Me6TREN system. This is most likely due to much faster association of the pMA radicals to TPMA* 3 /Cu I relative to Me6TREN/Cu I . In fact, UV-Vis analysis shows the formation of both the TPMA* 3 /Cu II -Br and the TPMA* 3 /Cu II -pMA organometallic species (Figure S5). Since this organometallic species was observed on the time scale of the reaction, this means that TPMA* 3 /Cu II -pMA is more thermodynamically stable (larger value of KOMRP) [START_REF] Ribelli | Effect of Ligand Structure on the CuII-R OMRP Dormant Species and Its Consequences for Catalytic Radical Termination in ATRP[END_REF] than Me6TREN/Cu II -pMA. Formation of TPMA* 3 /Cu II -pMA decreases the radical concentration, thus suppressing the rate of bimolecular termination and therefore decreasing the fraction of high molecular weight product in SEC.

The summary of all termination reactions (Table 1) clearly shows that the RT/CRT ratio can be kinetically altered by changing the [pMA-Br]:[L/Cu I ]:[L/Cu II -Br] ratios but also by changing KATRP. As the RT/CRT ratio was kinetically decreased, the relative fraction of high molecular polymer (Comb/Disp) also decreased. These results indicate that bimolecular radical termination of acrylates operates predominantly via combination while CRT gives predominantly chains with the same molecular weight as the pMA-Br initiator.

PREDICI Simulations

To further quantify the termination reactions, PREDICI simulations were conducted. In order to obtain useful mechanistic information, the kinetic model in use must first be validated by comparing to experimental results. In this case, experimental [L/Cu II -Br] was conveniently monitored by UV-Vis spectroscopy and then compared to the PREDICI simulations.

One major area of debate in the termination of acrylate radicals is the extent of backbiting for which reported rate coefficients, kbb, span 4 orders of magnitude at the same temperature. [START_REF] Hamzehlou | Analyzing the discrepancies in the activation energies of the backbiting and [small beta]-scission reactions in the radical polymerization of n-butyl acrylate[END_REF] However, backbiting is an important feature of acrylate polymerization and cannot be neglected. [START_REF] Buback | PLP Labeling in ESR Spectroscopic Analysis of Secondary and Tertiary Acrylate Propagating Radicals[END_REF] In fact, setting kbb = 0 in the simulation (Figure 6A) caused a significant underestimation of the experimental [L/Cu II -Br]. Therefore, backbiting was included in the simulation, taking into account the most recent and accurate kbb value at room temperature reported by Buback et al. [START_REF] Barth | SP-PLP-EPR-A Novel Method for Detailed Studies into the Termination Kinetics of Radical Polymerization[END_REF][START_REF] Nikitin | Determination of Intramolecular Chain Transfer and Midchain Radical Propagation Rate Coefficients for Butyl Acrylate by Pulsed Laser Polymerization[END_REF] using the PLP-SEC or PLP-EPR methods (40 < kbb < 250) and Asua et al [START_REF] Hamzehlou | Analyzing the discrepancies in the activation energies of the backbiting and [small beta]-scission reactions in the radical polymerization of n-butyl acrylate[END_REF] Figure 6C shows the fraction of disproportionated chains to total terminated chains vs.

time under two different scenarios. In the bold lines, it was assumed that termination between two chain-end radicals (RT22) gives 100% Comb while CRT gives 100% Disp.

On the other hand, in the dashed lines, the assumptions were reversed, in which case RT22 gives 100% disproportionation and CRT gives 100% combination, as has been proposed by Yamago. 22, 27 The extent of Disp/Comb for the mid-chain radical is not precisely known and has only been taken as the average of RT22 and RT33. 66 In light of this and due to the best fitting between experimental and PREDICI, it was assumed that any termination involving tertiary mid-chain radicals (RT23 and RT33) gives 100% disproportionation. 36, 38 In fact, Asua et al [START_REF] Ballard | On the Termination Mechanism in the Radical Polymerization of Acrylates[END_REF] has proposed that the study conducted by Yamago et al [START_REF] Nakamura | Termination Mechanism of the Radical Polymerization of Acrylates[END_REF] more accurately models how tertiary mid-chain radicals terminate as opposed to chain-end radicals due

to [P3•] > [P2•].
This gives further support of bimolecular termination involving mid-chain radicals terminating by disproportionation at room temperature. The experimental results can only be explained if one assumes that RT22 operates via combination while CRT gives products of disproportionation. This further validates the experiments conducted above, in which reactions that were kinetically tuned to promote RT vs. CRT gave a higher molecular weight shoulder.

To quantify the contribution of each reaction toward the decrease in chain-end functionality, the evolution of all products of termination were simulated with respect to time. In the present model, there are four pathways in which radicals can terminate: 1)

CRT, the reaction between TPMA/Cu II -P2 and a chain-end radical, P2 • ; 2) RT22, the reaction between two chain-end radicals; 3) RT23, the "cross-termination" between a chain-end radical and a mid-chain radical; 4) RT33, the reaction between two mid-chain radicals. Based on the good fit between experimental and simulations in Figure 6C, the three reactions CRT, RT23 and RT33 were simulated to give disproportionated chains and RT22 was set to give combined chains. Figure 7 shows the distribution of products obtained by these four termination reactions. S2.

For The high abundance of mid-chain radicals in acrylate polymerization has previously been observed via EPR even at room temperature. [START_REF] Buback | PLP Labeling in ESR Spectroscopic Analysis of Secondary and Tertiary Acrylate Propagating Radicals[END_REF] PREDICI simulations showed that after one hour, 50% of all terminated chains were terminated via RT23 while surprisingly only 39% were terminated via CRT. Only within the first millisecond of the reaction, when [P2 • ] was highest (Figure S5A), did RT22 dominate RT23, as shown in the insert in Figure 7A.

Finally, due to the slower reaction between two sterically hindered mid-chain radicals, RT33 accounted for only 1% of all terminated chains.

The simulations under the more CRT promoting conditions of [pMA-Br]0:[TPMA/Cu I ]0:[TPMA/Cu II -Br]0 = 1:2:0 were similar to those using the 1:1:0 ratio, since the initial ratio of RT and CRT rates does not depend on [TPMA/Cu II -Br]0 (Equation 1).

However, relatively more chains were terminated via CRT. This is due to a two-fold effect of the excess TPMA/Cu I on the overall rate of termination. Firstly, the faster P2 • trapping by TPMA/Cu I suppressed chain-end radical concentration thus slowing down the backbiting reaction (Figure S6) and suppressing the rate of cross-termination, RT23, by an order of magnitude. Secondly, this caused a slight initial increase in [TPMA/Cu II -P],

which resulted in a faster increase of the CRT rate as shown in Figure S6D. Under these initial conditions, CRT accounted for 57% of all terminated chains compared to 39% for the 1:1:0 ratio. Furthermore, at this 1:2:0 ratio, 7% of chains were terminated via RT22 compared to 10% under the 1:1:0 ratio, as expected according to and RT23 were 46% and 48%, respectively. Under these "most CRT promoting conditions," one would have expected the most amount of CRT according to Eq 1. This can be rationalized on the basis that the initial rate of CRT is suppressed by a factor of 50 due to the suppression of [P2 • ] by the initially added TPMA/Cu II -Br deactivator. Since CRT requires the reaction between TPMA/Cu II -P2 and [P2 • ], the suppression of the latter will undoubtedly slow down the rate of CRT. Although the "most CRT-promoting conditions" did in fact lead to the most amount of disproportionation, they did not result in the most amount of chains being terminated via CRT. Thus, Eq 1 would be better referenced as a way to kinetically suppress RT22 as opposed to kinetically promote CRT.

The summary of the distribution of terminated polymer according to our simulations are summarized in Table 2.

It should be noted that in the case of a polymerization reaction, the mid-chain radicals can also add to acrylate monomer leading to a branching point and regenerate the secondary chain-end radical. This could significantly decrease the concentration of midchain radicals and increase the concentration of chain-end radicals compared to this study. This would presumably decrease the contribution of RT23 and RT33 while increasing the contribution of RT22 and CRT. Nonetheless, in "termination reactions" the lack of monomer allows for a build-up of MCRs and thus bimolecular radical termination involving MCRs cannot be neglected.

Table 2 Product distribution of terminated polymer chains. The results presented here indicate that RT22 operates predominately via combination whereas CRT gives products with the same molecular weight as the living chain. This is in direct contrast to the previous contribution by Yamago et al., which concluded that acrylate chain-end termination occurs predominately (>99%) via disproportionation [START_REF] Nakamura | Termination Mechanism of the Radical Polymerization of Acrylates[END_REF] while CRT gives coupling. [START_REF] Nakamura | Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene[END_REF] In order to rationalize the results observed with the organotellurium systems studied by Yamago, we have disclosed •TeR radical catalyzed disproportionation via an H-TeR intermediate. [START_REF] Ribelli | Catalyzed Radical Termination in the Presence of Tellanyl Radicals[END_REF] Due to the incorrect assumption that acrylate radicals terminate exclusively via disproportionation at room temperature, it seems that Yamago et al have erroneously assigned the formation of the observed combination products to copper catalyzed termination. The high molecular weight in that study [START_REF] Nakamura | Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene[END_REF] is actually the result of conventional radical termination via combination.

CONCLUSIONS

Termination reactions using a pMA-Br ATRP macroinitiator were utilized to shed light on the termination mechanism of polyacrylates in ATRP. Three different catalysts were used under various conditions to change the fraction of bimolecular chain-end radical termination (RT) to copper-catalyzed radical termination (CRT). Wherever the RT/CRT ratio was largest, a high molecular weight shoulder became apparent in the SEC traces.

As this ratio was gradually decreased, the relative fraction of high molecular weight polymer decreased significantly. This indicates that the bimolecular radical termination of acrylates operates via combination while CRT gives terminated polymers of unchanged molecular weight. These results are in stark contrast to recent reports; therefore, PREDICI simulations were conducted to support our experimental findings. In fact, very similar results were observed for the experimental and simulated data for the TPMAbased system. It was found that due to the high initial radical concentration, bimolecular termination dominated only at the very first instants. After this initial influx of radicals, catalytic radical termination and backbiting dominated the fate of the secondary chainend radical. Due to the relatively high concentration of mid-chain radicals relative to chain-end radicals, the amount of cross termination between mid-chain and chain-end radicals (RT23) was found to be more dominant than termination via two chain-ends (RT22) and, in some cases, even more kinetically significant than CRT.

  Interestingly, Yamago et al. recently proposed 22 that using the tris[2-(dimethylamino)ethyl]amine (Me6TREN)-based system, termination gave products of doubled molecular weight via a proposed Me6TREN/Cu III -(pMA)2 intermediate. Due to the ongoing debate involving the mechanism of bimolecular radical termination and contradictory results of CRT, we sought to obtain a better understanding of these two mechanisms of termination. Using three different L/Cu catalytic systems, L = Me6TREN, TPMA and tris(3,5-dimethyl-4-methoxy-2pyridylmethyl)amine (TPMA* 3 ), under controlled conditions, it is possible to reassess the products of CRT as well as obtain valuable information about the mechanism of conventional bimolecular radical termination.

Figure 1 .

 1 Figure 1. PREDICI simulations of termination of radicals generated from a pMA-Br macroinitiator. RT22/CRT ratio vs. time under three different initial conditions [pMA-Br]0:[L/Cu I ]0:[L/Cu II -Br]0 = 1:1:0 (red), 1:2:0 (blue) and 1:2:1 (green) where L = TPMA. The reaction model and rate coefficients used for simulations are presented in TableS2.

  One significant difference is the choice of copper salt and solvent. [Cu I (MeCN)4][PF6] in acetonitrile was used throughout this study, while Yamago et al. used Cu I Br in toluene. In our study, the use of [Cu I (MeCN)4][PF6] results in the formation of a discrete [Cu I (Me6TREN)] + catalyst in situ, while the use of Cu I Br has been shown to form a mixture of [Cu I (Me6TREN)] + , [Cu I (Me6TREN)Br], and Cu I Br2 -, which have different activities in ATRP 61 and thus could have skewed previously reported conclusions. Secondly, to make sure there was no disproportionation of the L/Cu I species, MeCN was chosen as the solvent instead of toluene. Finally, Yamago et al. conducted all but one of their termination experiments in the presence of Cu 0 powder, which has recently 62 been shown to also catalyze the termination of acrylate radicals and would be indistinguishable from Cu I CRT. In addition, avoiding the use of Cu 0 allows facile monitoring of the extent of termination by the determination of [L/Cu II ], since Cu 0 is able to comproportionate with Cu II to regenerate Cu I and would not allow for efficient monitoring of [L/Cu II ].

Figure 2

 2 Figure 2 (A) SEC traces of the termination products from the pMA-Br macroinitiator recovered after 30 min at different RT/CRT ratios and (B) deconvolution of the product of reaction [pMA-Br]:[Me6TREN/Cu I ] = 1:5. Conditions: [pMA-Br]:[Me6TREN/Cu I ]:[Me6TREN/Cu II -Br] = 1:5:0-1 in anhydrous MeCN at room temperature; [pMA-Br]0 = 1.6 mM.

Figure 3 Figure 4

 34 Figure 3 (A) evolution of fraction of terminated chains with time and (B) SEC traces of the termination product of pMA-Br macroinitiator recovered after 30 min at different RT/CRT ratios. Reaction conditions: [pMA-Br]0:[Me6TREN/Cu I ]0:[Me6TREN/Cu II -Br]0 = 1:1-2:0-1, in anhydrous MeCN at room temperature; [pMA-Br]0 = 1.6 mM.

by 3 . 1 (Figure 5 )ElutionFigure 5

 3155 Figure 5 (A) Evolution of fraction of terminated chains with time and (B) SEC traces of the termination product of pMA-Br macroinitiator recovered after 1 h at different RT/CRT ratios. Reaction conditions: [pMA-Br]0:[TPMA* 3 /Cu I ]0:[TPMA* 3 /Cu II -Br]0 = 1:1-2:0-1, in anhydrous MeCN at room temperature; [pMA-Br]0 = 2.5 mM).

  ) or 1 h (TPMA and Me6TREN); Relative to [pMA-Br]0 unless otherwise noted. a Calculated based on the increase of [L/Cu II -Br]; b The High MW from the deconvoluted SEC traces; c Calculated from the Low MW fraction of the deconvoluted SEC traces as Low MW = Living + Disp; d Calculated as the amount of chains terminated by disproportionation related to all terminated chains.

Figure 6 A

 6 Figure 6 A) Evolution of [TPMA/Cu II -Br] vs. time for different values of kbb (in s -1); B) experimental (squares) and simulated (lines) evolution of TPMA/Cu II -Br vs. time; C) fraction of chains terminated by disproportionation relative to total terminated chains for two cases, where RT and CRT give combination and disproportionation, respectively (bold lines) and vice versa (dashed lines). All squares are experimental data obtained either via UV-Vis or SEC under the specified initial ratio with [pMA-Br]0 = 2 mM in anhydrous MeCN at room temperature.

Figure 7

 7 Figure 7Simulations of the contribution of CRT (black), RT23 (red), RT22 (green) and RT33 (blue) to total termination vs. time under the initial [pMA-Br]0:[TPMA/Cu I ]0: [TPMA/Cu II -Br]0 ratios of (A) 1:1:0 (B) 1:2:0 and (C) 1:2:1 at room temperature. The reaction model and rate coefficients used for simulations are presented in TableS2.

  the ratio [pMA-Br]0:[TPMA/Cu I ]0:[TPMA/Cu II -Br]0 = 1:1:0 (Figure7A), a surprising result is the dominance of the bimolecular "cross termination" between chain-end and mid-chain radicals (RT23). So much, in fact, that after approximately 25 minutes, the majority of terminated chains underwent termination via cross termination. The reasoning becomes apparent when looking at FigureS5Bwhich shows that the concentration of tertiary mid-chain radicals (P3 • ) was greater than that of secondary chain-end radicals (P2 • ) after only 5 minutes. This relatively higher concentration of mid-chain radicals results from the fast rate of backbiting (BB) as shown in FigureS5D. In fact, backbiting was between 10 and 100 times faster than RT22 due to the unimolecular nature of the reaction.

Figure 1 .

 1 Nonetheless, RT23 still dominated all bimolecular radical-based termination events, accounting for 34% of all terminated chains. Using the initial ratio [pMA-Br]0:[TPMA/Cu I ]0:[TPMA/Cu II -Br]0 = 1:2:1, only 4% of chains were terminated via RT22. This is because the concentration of chain-end radicals was significantly suppressed by the quick and efficient trapping by the TPMA/Cu II -Br deactivator present from the beginning. As shown in Figure S7C, after one second, the concentration of chain-end radicals was [P2 • ] ≈ 7 x 10 -8 M in the presence of the deactivator. In comparison, after one second but in the absence of initially added deactivator, [P2 • ] ≈ 3 x 10 -7 M as shown in Figure S6A. Interestingly, the amount of CRT

  calculated based on total terminated chains Termtot= [CRT]+[RT23]+[RT33]+2*[RT22] since chains undergoing combination comprise two initial pMA-Br macroinitiators. For all cases L = TPMA.

Table 1

 1 Experimental product distribution of the termination reactions.

		[pMA-Br]:
		[L/Cu I ]:[L/Cu II -	Terminated	Living	Combined	Disp	% Disp. Of
	Ligand	Br]	(%) a	(%)	(%) b	(%) c	All Term d
		1:1:0	81	19	38	43	53
		1:2:0.5	91	9	18	73	80
	Me6TREN	1:2:1	>99	<1	13	86	86
		1:5:0	>99	<1	27	73	73
		1:5:0.5	>99	<1	9	91	91
		1:5:1	>99	<1	7	93	93
		1:1:0	67	33	7	60	90
	TPMA	1:2:0	98	2	5	93	95
		1:2:1	71	29	2	69	97
		1:1:0	98	2	24	74	76
	TPMA* 3	1:2:0	>99	<1	12	88	88
		1:2:0.5	>99	<1	7	93	93
		1:2:1	>99	<1	4	96	96
	All percentages were calculated at t = 30 min (TPMA* 3

ACKNOWLEDGEMENTS

Support from the NSF (CHE 1707490) is acknowledged. We thank the Centre National de la Recherche Scientifique (CNRS) for support through the PICS06782 grant and through the Laboratoire International Associé (LIA) "Laboratory of Coordination Chemistry for Controlled Radical Polymerization". We are also grateful to the French Embassy in Washington D.C. for a Chateaubriand Fellowship to T.R. P.K. acknowledges Dr. Konrad M. Weis Fellowship in Chemistry. Finally we would like to acknowledge Prof. Michael Buback for fruitful discussion and seminal ideas for this work.

AUTHOR INFORMATION

Corresponding Authors: *(R.P.) Email: rinaldo.poli@lcc-toulouse.fr *(K.M.) Email: km3b@andrew.cmu.edu

Notes

The authors declare no competing financial interest.


Human and conifer demographic expansions from linked microsatellites

We extended the method for demographic inference based on the distribution of pairwise differences (i.e. mismatch distribution) between individuals of a sample (Schneider & Excoffier, 1999) to linked microsatellites by assuming the stepwise mutation model. The proposed method allows for the estimation of initial and final population sizes and time of expansion by a maximum pseudolikelihood approach. Confidence intervals have been estimated by parametric bootstrap.

The interest on linked microsatellites comes from their application in chloroplast and mammal Y-chromosome genetic evolutionary studies. The application of the method is illustrated with microsatellite data of the chloroplast genome from the Canary Island pine (Pinus canariensis) and of the human Y-chromosome. In both cases the distribution of the pairwise differences are unimodal, as expected under a demographic expansion (Figures 2 and3). Canary Island pine populations (Figure 4) have been analysed with six microsatellite loci (Pt15169, Pt30204, Pt71936, Pt87268, Pt26081 and Pt36480). Expansion time estimates (Table 2) are consistent with the geological history of the archipelago and have the same order of magnitude than the estimated colonization time of Brachyderes rugatus (specific parasite weevil of the Canary Island pine). ) populations have been grouped according the geographical areas identified by Li et al., 2008 (Figure 1). Estimates for expansion time (Table 1) are comparable with estimates from other studies using Bayesian statistics: 18000 (7000-41000) years in Pritchard et al., 1999;7400 (4300-14000)

1

  Figure 2

  year and locus, µ = 10 -5 (Provan et al., 2000) b Juan et al., 2000 c Emerson et al.,

  Includes samples not classified in previous groups b mutation rate per generation over the five loci, µ = 0.89 x10 -2 (www.ystr.org); generation time: 25 years Human Y-chromosome population data for five microsatellite loci (DYS19, DYS390, DYS391, DYS392 have been obtained from published works. For the estimates of the time of expansion (

  Figure1

Table 2 Figure 4

 24

ACKNOWLEDGMENTS

We are grateful to Olivier Hardy and Frantz Depaulis for useful discussions and suggestions. C.B. has been funded by a postdoctoral grant of the Mairie de Paris and M.N. by a ESF ConGen Exchange Grant 1142.


Iana S Sinopalnikova

Tatyana A Peganova

Valentin V Novikov

Ivan V Fedyanin

Oleg A Filippov

Natalia V Belkova

Elena S Shubina

Rinaldo Poli

Alexander M Kalsin

] Tat'yana

A Peganova

A Tat'yana

Valentin V Peganova

Ivan V Novikov

Oleg A Fedyanin

Natalia V Filippov

Elena S Belkova

Rinaldo Shubina

Alexander M Poli

Kalsin

Coordinatively labile 18-electron arene ruthenium iminophosphonamide complexes

Keywords: ruthenium iminophosphonamides, electron-deficient complexes, carbonyl complexes, CO insertion, 2D EXSY NMR Arene ruthenium complexes, iminophosphonamides, electron-deficient compounds, N ligands, organometallic chemistry

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Coordinatively unsaturated complexes are often considered as intermediates in various metal-catalyzed organic transformations. Generally the stability of these species correlates with their activity in catalysis, i.e. the more unsaturated species are more reactive, however at the expense of selectivity. Ruthenium complexes are known to catalyze numerous organic reactions [1] of substrates bearing various functional groups, to which a catalyst must be tolerant. The reactivity of the unsaturated species thus should be fine-tuned by reducing the electrophilicity of the ruthenium center with the ligand environment. Ligands having extra lone pair at the coordinated heteroatom are capable to partially compensate the electron deficiency of the metal via πdonation. [2] O-and N-ligands usually provide significantly greater stabilization than halides, e.g. following the order OSiMe2Ph > NHPh > OSiPh3 > OCH2CF3 >> Cl > Br > I for half-sandwich [Cp*Ru(X)(PR3)] complexes. [3] Among the chelating anionic κ 2 -N,N-ligands, strongly electron-donating β-diketiminate and zwitterionic bis(imidazoline-2-imine) ligands were found to generate either 18ē complexes with weakened Ru-Cl bonds, [4][5][6] or very stable 16ē half-sandwich ruthenium complexes that do not coordinate the chloride ion at all. [7,8] The π-donation of the lone electronic pairs at the nitrogen atoms in ruthenium amidinates is limited because of symmetry reasons; it occurs only in the unfavorable conformation with a bent metallacycle and thus the stabilization effect is less pronounced. [9][10][11][12][13] We have recently proposed that structurally similar iminophosphonamide ligands [R2P(NR')2] -can efficiently stabilize the coordinatively unsaturated ruthenium complexes, thanks to their zwitterionic structure [14] and therefore to the absence of symmetry restrictions for πdonation. [15] The observation of elongated Ru-Cl bonds (2.44-2.45 Å) in 18ē complexes [(C6Me6)RuCl{(RN)2PR'2}] and shortened Ru-N bonds (average 2.01-2.04 Å) in the corresponding 16ē complexes [(C6Me6)Ru{(RN)2PR'2}] + (X -) supports this hypothesis. [15] In 1998 Parsons et al. reported the first 16ē ruthenium iminophosphonamide complex [(p-Cymene)Ru{( i PrN)2PPh(NH i Pr)}](BPh4) to be extremely stable and inert to the addition of Cl -, PPh3 and P(OEt)3; even the CO adduct could not be isolated due to intrinsic instability. [16] Since then, no 18ē cationic NPN complexes [(arene)Ru(L){(RN)2PR'2}] + could be isolated, suggesting that the σ,π-donor character of the NPN ligand is too strong to allow stability for such adducts.

Here we report a comprehensive quantitative exploration of reversible coordination of neutral ligands with various π-acceptor capabilities (MeCN, pyridine and CO) to cationic 16ē arene ruthenium iminophosphonamides 2a-d, and the chloride dissociation from the 18ē complexes 1a-d (Chart 1), aiming to determine the influence of the arene nature and of the N-, Psubstituents in the NPN-ligand on the thermodynamics of ligand coordination/dissociation. These data on the coordination behavior of ruthenium iminophosphonamides are particularly important in view of their potential use in catalysis.

Chart 1.

[a]

Ms. Iana S. Sinopalnikova, Dr. Tat'yana A. Peganova

Results and Discussion

Synthesis and characterization of the complexes 1d and 2d.

The new p-cymene ruthenium NPN-complexes (1d, 2d) were synthesized similarly to their hexamethylbenzene analogues 1a and 2a, [15] i.e. by reacting [(p-cymene)RuCl2]2 with iminophosphonamide A after deprotonation with 1 equiv. of NaHMDS to obtain 1d and further abstraction of the chloride ligand with AgPF6 to yield the corresponding 16ē cationic complex 2d (Scheme 1). The isolated products were fully characterized by NMR spectroscopy and elemental analysis, and their molecular structures were confirmed by single crystal X-ray diffraction studies (Figures 1 and2, Table S6 in ESI). The 18ē complex 1d exhibits a three-legged piano stool geometry with a pseudo octahedral configuration of the ligands around the ruthenium atom. The structural parameters of 1d are similar to those of its C6Me6 analogues 1a-c, except for the Ru-N and Ru-Cl distances that are about 0.02 Å shorter (the average Ru-N and Ru-Cl bond lengths in 1a-c are 2.148-2.156(4) Å and 2.437-2.445(4) Å, respectively). [15] The p-cymene ligand is a weaker donor and has lower steric requirements than C6Me6, hence the lack of electron density on the ruthenium atom in 1d is compensated by shortening the bonds with the NPN and Cl ligands. The pyramidalization of the nitrogen atoms, as indicated by Σ(N), is small like for the complexes 1a,b. This is a result of the ability of the N-tolyl substituents in 1a,b,d to delocalize the unshared electron density of the nitrogen atoms, in sharp contrast with the severe pyramidalization of one of the nitrogen atoms in 1c (Σ(N1) = 344.4°), for which such delocalization is impossible.

The 16ē cationic complex 2d exhibits a two-legged pianostool geometry with the chelating NPN-ligand positioned nearly perpendicular to the p-cymene ligand. Most of the structural parameters of 2d are similar to those of its C6Me6 counterparts 2a,b, besides less significant distortion of the arene ligand. The p-cymene ligand in 2d is almost planar (Ru-C(arene) bonds in the 2.159-2.204(3) Å range) compared to the C6Me6 ring in 2a and 2b, which is bent towards a boat conformation with two considerably longer Ru-C(arene) bonds (trans to Ru-N; 2.228-2.275(2) Å) vs. the other four (2.143-2.201(2) Å). [15] This structural peculiarity may result from weaker back-bonding from the ruthenium atom to the arene ligand in 2d.

The geometrical differences observed for the C6Me6 and pcymene complexes were reproduced by the density functional theory (DFT) calculation performed for 1a, 1d, 2a and 2d in the gas phase with PBE functional and def2-TZVP basis set. The optimized structures differ from the experimentally determined ones by <0.03 Å for the distances and <2° for the bond angles (ESI, Tables S7 andS8). It is noteworthy that the DFT calculations also predict the arene orientation relative to the Cl,N,N atoms in 1a and 1d (the dihedral angle Cl-Ru-Centroid-C5 differs by <3°). Coordination of the arene ligand in the 16ē complexes 2a and 2d has a well-pronounced η 2 -η 2 character, with the Ru-C1 and Ru-C4 bonds being ca. 0.04 Å (2d) or 0.09 Å (2a) longer than the other four Ru-C bonds, which also indicates a stronger distortion of the planar arene ring towards a boat conformation for 2a than for 2d. This pattern can be explained by the symmetry of the ruthenium orbitals interacting with the arene ring (see below).

The 16ē complexes 2a-c exhibit a moderate intensity band in the UV-vis spectra centered at 540-550 nm, [15] which was supposed to belong to a d-d* transition. The UV-Vis spectra of 2d exhibit a similar band, although surprisingly shifted to longer wavelengths (λmax = 590 nm). According to the calculated orbital pattern of 2a and 2d, the highest occupied molecular orbitals (HOMO) (ESI, Fig. S10) of both cations are assigned to the antibonding combination of the ruthenium dxy orbital with the A2symmetric group orbital of the NPN ligand (linear combination of the nitrogen atoms py orbitals) and are located at very close energies (-7.4 eV in the gas phase and -5.1 eV in CH2Cl2 solution, Table 1). In both 2a and 2d, the lowest unoccupied molecular orbital (LUMO) (ESI, Fig. S11) is a combination of the ruthenium unoccupied dyz orbital and occupied B2 π-orbital of the arene ligand. Lowering the energy of the corresponding bonding orbital (HOMO-14, see ESI, Fig. S12) is responsible for the elongation of two C-C bonds (C2-C3 and C5-C6) of the η 6 -arene ligand. The greater electron-donating ability of C6Me6 leads to a higher energy LUMO by 0.4 eV in 2a (Table 1). Hence the bands observed at 540 nm (2a) and 590 nm (2d) in the UV-vis spectra appear consistent with a ligand-to-metal charge-transfer transition, as in the 16ē arene ruthenium dithiolate complexes. [17] The HOMO-LUMO gap for 2a and 2d are calculated in CH2Cl2 as 2.05 and 1.65 eV (Table 1), respectively, thus reflecting a significant red shift for the absorption of 2d compared to 2a. It should be noted that the LUMO in complexes 2 is the orbital available to accept the electron pair donated by the external ligand (L or Cl) upon formation of the 18ē adduct. Hence, the lower energy of the LUMO in 2d is responsible for the stronger bonding of chlorine atom in the p-cymene complex vs those containing C6Me6, as reflected by the computed and measured Ru-Cl bond lengths (Tables S7 andS8).

Coordination lability of complexes 1a-d

Despite the low symmetry (C1) of 1d in the solid state, only two doublets at δ 5.05 and 4.87 (C6D6) were observed for the coordinated arene in the 1 H NMR spectrum, as well as two singlets at δ 79.4, 80.7 in the 13 C NMR spectrum, because of facile rotation of the p-cymene ring around the Ru-Arene axis yielding an effective Cs symmetry in solution. The NMR spectral feature of 2d are equally in accord with Cs-symmetry. On the other hand, the 1 H and 13 C NMR spectra for the P-phenyl substituents of 1d exhibit the expected two sets of signals in C6D6 but only one in CDCl3, suggesting an even higher (effective C2v) point group symmetry in the latter solvent. This can be rationalized, in addition to the rapid p-cymene ring rotation around the Ru-Arene axis, by the reversible dissociation of the Ru-Cl bond that yields an equilibrium mixture of the effectively C2v-symmetric intermediate [2] + Cl -and undissociated 1, as has been previously proposed for 1c. [15] However, whether this process may occur in apolar solvents and what is the dissociation enthalpy as a function of solvent, NPN substituents and type of arene ligand is still open to question. The additional studies detailed below have brought light on this point.

UV-vis spectroscopic studies of 1b and 1c in polar solvents.

As we have previously reported, [15] complex 1c bearing the most electron-releasing N-Me groups partially dissociates in the relatively polar solutions to give the corresponding cationic species [2c] + Cl -(Scheme 2), as revealed by the strong downfield shift of its 31 P NMR resonance in dichloromethane compared to C6D6. The detailed study by means of UV-vis spectroscopy revealed the distinct absorption bands in toluene for 1c (at 430 nm) and 2c (at 550 nm; ESI, Fig. S1A,B). The spectrum of 1c in CH2Cl2 shows a very broad absorption band located in-between these bands (at 520 nm; Fig. 3), thus suggesting the coexistence of the dissociated and undissociated forms. The spectra are temperature dependent in the range 190-290 K, showing reversible changes with an isosbestic point at λ = 480 nm (Fig. 3). The standard enthalpy and entropy for the dissociation process (Scheme 2) were calculated from the Van't Hoff equation using the T-dependent molar extinction coefficient of 2c at λ = 600 nm. The dissociation process is exothermic (ΔHd = -5.0±0.2 kcal/mol), with the cationic form dominating at low temperatures. The unexpectedly large negative entropy change (ΔSd = -27.0±0.7 cal/(mol•K)) for the dissociation process is attributed to the need to re-organize the solvent dipoles around the charged species [18] formed upon dissociation of 1c. Scheme 2. Chloride dissociation from complex 1c. Complex 1b, bearing an NPN-ligand with the less electronreleasing p-tolyl substituents on the nitrogen atoms, does not show any evidence of chloride dissociation in CH2Cl2. The UV-vis spectrum is T-independent and only the undissociated 18ē complexes is present in solution as shown by the band centered at 450 nm (ESI, Fig. S2). However, the compound predominantly exists as cationic complex [2b] + Cl -in solution of the highly polar MeNO2 solvent, exhibiting a band with λmax = 515 nm (Fig. 4). Analysis of the temperature dependence of this band in the range 250-310 K gave ΔHd = 3.5±0.2 kcal/mol and ΔSd = +0.3±0.6 cal/(mol•K). Compared to the dissociation of 1c in CH2Cl2, the more positive dissociation entropy of 1b in MeNO2 is attributed to a more efficient solvation of the charged species by MeNO2. On the other hand, both 1a and 1d, which contain the least donating NPN-ligand, do not noticeably dissociate even in MeNO2 as suggested by the absorption band at 450 nm in their visible spectra, even though the insufficient solubility of both compounds did not allow verification of the temperature independence of these spectra.

NMR study of 1a-d in apolar solvents. The chloride complexes 1a-c show two inequivalent P-Ph groups in their RT-NMR spectra in apolar solvents (benzene, toluene), meaning they exist as Cssymmetric 18ē complexes on the NMR timescale. [15] We now report that heating the solution of 1c in C6D6 to 338 K gives one set of signals for these groups, which is indicative of exchange. The coalescence of the two ortho-protons signals at δ 7.70 and 7.98 is observed at 323±5 K (Tc) (ESI, Fig. S3). From this temperature and from the chemical shift difference, the rate constant (kex = 380 s -1 ) and activation free energy (ΔGex ≠ = 15.2±0.3 kcal/mol) for the degenerative exchange process can be calculated. [19] For complex 1b with a less donor NPN-ligand, coalescence for the P-Et methyl resonances at δ 0.40 and 1.39 could not be reached below the boiling point of toluene-d8 (ESI, Fig. S4). However, we observed slow exchange of the ethyl groups for this compound in C6D6 and in toluene-d8 by the 2D EXSY 1 H NMR method (Fig. 5), [20] yielding the rate constants of 1.8 s -1 and 1.4 s -1 at 293 K, respectively (Table 2, lines 3,7). The activation parameters for the exchange process were obtained from the temperature dependence of the exchange rate constants (kex) in toluene-d8 in the range 230-315 K: ΔHex ≠ = 8.4±0.2 kcal/mol and ΔSex ≠ = -29.2±0.7 cal/(mol•K) (ESI, Fig. S5, Table S1). A large negative value of the activation entropy evidences a highly ordered transition state in the exchange process. The exchange between R'a and R'b substituents occurs via a configuration inversion at the Ru atom, which can most easily be envisaged by moving the chloride ligand from one coordination side to the opposite one by ligand exchange processes. Generally, such ligand exchange may proceed either via a dissociative mechanism with a cationic 16ē intermediate (pathway A in Fig. 6A) or via a SN2-like associative mechanism through a 20e transition state (pathway B in Fig. 6B). In the latter case, however, configuration inversion with exchange of the phosphorus R'a and R'b substituents could take place only when the entering ligand is Cl -. Given that the NMR and UV studies outlined above were carried out in non-coordinating solvents and coordinating ligands L are not present, the exchange in 1b most likely takes place via the dissociated complex [2b] + Cl -, which requires an entropically unfavorable solvation of the chloride-ion by apolar molecules (Fig. 6A), similarly to the dissociation of 1c in CH2Cl2 described above. We have further confirmed that the exchange rates are independent on the concentration of 1b (1.6 s -1 in both 40 mM and 8 mM toluene-d8 solutions at 295 K, Table 2, lines 5,6). The Psubstituent exchange for complexes 1a and 1d, which bear the least electron-donating NPN-ligand, is not observed by 2D EXSY 1 H NMR even at tm = 1 s, hence the rates are slower than 0.1 s -1 and the activation free energies (ΔGex ≠ ) are greater than 18.5 kcal/mol. In principle, addition of Cl -should either slow down or accelerate the exchange depending on whether the mechanism is A or B, respectively. Addition of 0.07 equiv. of Et4N + Cl -to 1d strongly enhances the exchange process (kex > 11 s -1 ). This result strongly suggests that the exchange occurs by the associative mechanism B under these conditions (Table 2, line 11, and ESI, Fig. S6). Therefore, it seems that both mechanisms A and B are possible, the former in the absence of added Cl -and the latter in its presence. Assistance by an external ligand L may also result in Cl -release. While the subsequent attack of the same Ru-L intermediate by Cl - would not result in any P substituent exchange, attack of another Ru-Cl complex achieves a degenerative self-exchange with configuration inversion and P substituent exchange (Fig. 7). Thus, the P-phenyl group exchange for these compounds becomes observable after adding water to the C6D6 solutions (cf. lines 1,2 for 1a and lines 8,9 for 1d) and the rate constants are found to increase with the increase of the water content (Table 2, lines 9,10; ESI, Fig. S7). For the 1d sample with [H2O] = 0.7 mM (0.07 equiv.), the activation parameters could be derived from the temperature dependence (291-335 K) of kex (ESI, Fig. S8, Table S2). The calculated activation enthalpy [START_REF]Although the Van't Hoff plot for 1d allowed us to find the entropy term ΔS = -18.7±0[END_REF] ΔHex ≠ = 11.8±0.2 kcal/mol of the associative exchange can be used as a lower limit for the ΔHex ≠ in dissociative exchange of 1d in the absence of external ligands. It is pertinent to underline that water may have a double role in the promotion of the associative chloride exchange: as a ligand to replace Cl -in the coordination sphere of the Ru atom, and as a proton donor to stabilize the dissociated Cl -by Hbonding. Interestingly, the effect of water for 1a is much smaller; a rate constant of 0.79 s -1 is achieved only in the presence of an equimolar amount of H2O in C6D6. Perhaps the sterically bulky C6Me6 ligand hampers the Cl -or external ligand coordination thus increasing the activation free energy. We would like to underline that the activation enthalpy ΔHex ≠ for the exchange process in dissociative mechanism A should be very close to the Ru-Cl bond dissociation enthalpy, ΔHd. The latter depends predominantly on the Ru-Cl bond strength and on the solvent polarity. Hence, the ΔHd value for complex 1c may be estimated from its ΔG ≠ at 323 K as ΔHd ~ ΔHex ≠ = ΔGex ≠ + TΔSex ≠ , where ΔSex ≠ can be assumed to be close to that for 1b (-29.2 cal/(mol•K)). This gives an estimate for the ΔHd of 1c as ca. 5.7 kcal/mol. Analogously, the ΔHd of 1a and 1d can be estimated as >10 kcal/mol. Thus, we have proved that the Ru-Cl bond in 18ē arene ruthenium iminophosphonamide complexes can dissociate even in apolar solvents. The ΔHd for 1a-d expectedly lowers with an increase of the ligand donating ability: over 10 kcal/mol (1a, 1d) > 8.4 kcal/mol (1b) > 5.7 kcal/mol (1c).

Coordination of ligands to the 16e cationic complexes 2

We have previously mentioned that the 16ē cationic complex 2a interacts with MeCN and CO, but failed to isolate these unstable adducts. [15] We have now studied the activity of the 16ē complexes 2 towards coordination of external ligands L (MeCN, pyridine, CO) in detail by means of UV-Vis and NMR spectroscopies (Scheme 3) to determine the thermodynamics of the process and with regard to the conditions allowing isolation. Since 1a is, among the 18ē chloride complexes 1, the least prone to chloride dissociation, the corresponding 16ē complex 2a is expected to display the highest affinity toward saturation by coordination of L ligands. Hence, the coordination of relatively weak donors MeCN and Py was studied for 2a only.

Scheme 3.

The titration of 2a with MeCN in CH2Cl2 at 296 K with UV-Vis monitoring shows that the intensity of the 535 nm band does not significantly change until >100 equiv. of MeCN are added (ESI, Fig. S9A), while a similar amount of pyridine significantly extinguishes this band to give rise a new band of the pyridine adduct at 445 nm (ESI, Fig S9B). The estimated equilibrium constants Kc are <2 M -1 for 2a•MeCN and 32±7 M -1 (from UV-vis), 37±1 M -1 (from NMR, ESI, Fig. S9C) for 2a•Py, (ESI, Tables S3-S5). The UV-vis measurements carried out for the 1:2 mixture of 2a:Py in the temperature range 200-296 K show that 2a dominates at room temperature, while the equilibrium shifts towards formation of the 18ē adduct 2a•Py at lower temperatures. The conversion becomes essentially complete below 220 K (Fig. 8). The analysis of the Van't Hoff's plot yields ΔHc = -12.4±0.5 kcal/mol and ΔSc = -36±2 cal/(mol•K). Thus, pyridine coordination is moderately exothermic, but a large excess of pyridine is required to observe coordination at room temperature because of high association entropy. This is in sharp contrast with the previously reported irreversible coordination of pyridine to the 16ē arene ruthenium amidinate complex [(C6H6)Ru( t BuN-C(Ph)-N t Bu}](BAr F 4). 10 In comparison with acetonitrile and pyridine, carbon monoxide is a stronger π-acceptor and reacts readily with 2a to give the CO adduct 3a. However, the reaction is reversible and facile CO decoordination occurs upon solvent evaporation to give back 2a. Nevertheless, 3a could be isolated by precipitation from its CH2Cl2 solution upon addition of excess diethyl ether. Analogously, the reaction of 2d with CO resulted in formation of 3d, which is stable only under a CO atmosphere. This product was isolated, like 3a, by using CO-saturated CH2Cl2 solution and diethyl ether. Although 3a and 3d slowly evolve CO in the solid state under vacuum, satisfactory elemental analyses were obtained for both. It should be noted that 3a is stable in a CO-saturated solution, whereas 3d degrades within days to unknown arene-free carbonyl complexes, perhaps similar to the previously observed ones resulting from the decomposition of [(p-Cymene)Ru(CO)(SXyl)2]. [START_REF] Mashima | [END_REF] The molecular structures of 3a,d were determined by single crystal X-ray diffraction (ESI, Table S6). Selected structural parameters and views of the molecules are shown in Figures 9 and10. Complexes 3a and 3d are the first crystallographically characterized arene ruthenium carbonyl derivatives with any κ 2 -N,N-anionic chelate ligand. The asymmetric unit of 3d contains two independent molecules, which exhibit identical metric parameters within experimental error. Both complexes exhibit a three-legged piano stool geometry with a pseudo octahedral configuration of the ligands around the ruthenium atom. The coordinated arene adopts a nearly staggered (3a) or eclipsed (3d) conformation relative to the Ru-CO axis, with C4-Arene(centroid)-Ru-CO angles of 25.7° and 5.9-6.4°, respectively. The presence of the CO ligand significantly increases the Ru-Arene(centroid) distance by ~0.1 Å in both 3a and 3d relative to 2a 7) Å) distances, as well as elongations for the two Ru-C(arene) bonds trans to CO (2.389-2.396 Å) were previously reported for the arene ruthenium dithiolate carbonyl complex [(η 6 -C6Me6)Ru(CO)(S2C6H4)] ( 4). [17] The Ru-N bonds are shorter than those in the neutral 18ē complexes 1a-d by ca. 0.04 Å, reflecting the presence of the positive charge, but longer (by 0.05-0.08 Å) than in the 16ē complexes 2a-d, in which they are strengthened by π-donation from the NPN-ligand. The chelate N1-Ru-N2 angles in 3a and 3d are similar to those of the other 18ē NPN-complexes 1a-d. However, unlike in the related chloride complexes 1a and 1d, the Ru-N1-P-N2 metallacycles in 3a and 3d are folded by 13-15.6° from planarity at the N1•••N2 hinge and the nitrogen atoms (N1 in 3a; N1 and N2 in 3d) are noticeably pyramidalized as shown by the Σ(N) parameter. There are several close H•••F contacts between the cation and the PF6 -anion in both structures, although none of these is shorter than the sum of the van der Waals radii of H and F (2.56 Å). [23] Complexes 3a and 3d are sufficiently stable in concentrated solutions to allow recording their NMR spectra, although the fraction of the corresponding 16ē complexes increases with time. The 31 P NMR resonance (δ 61.7 for 3a and δ 60.2 for 3d) is downfield shifted by ca. 10 ppm relative to the corresponding 16ē cationic complexes 2a and 2d. In the 1 H and 13 C{ 1 H} NMR spectra recorded in CDCl3 there are two sets of signals for the magnetically inequivalent phenyl groups at the phosphorus atom, as expected for the Cs-symmetric complexes, hence the decoordination of CO is a slow process with a relatively high activation barrier. The resonance of the coordinated CO ligand in the 13 C NMR spectra is observed at high field (δ 199.2, 193.8), as is typical of terminal linear coordinated CO ligands (cf. to δ 197.8 for 4 [17] and δ 203-208 reported for the cyclopentadienyl ruthenium complexes with N,N-ligands). [9,24] Both carbonyl adducts 3a and 3d exhibit strong CO stretching vibration band in the IR spectrum at 1984 cm -1 (3a) and 2012 cm - 1 (3d). The higher νCO in 3d evidences weaker Ru-CO π-backbonding relative to 3a that possesses the electron-richer C6Me6 arene. These frequencies are substantially lower than those of the more electron-deficient arene ruthenium amidinate carbonylic complex [(η 6 -C6H6)Ru(CO){PhC(N t Bu)2}](BAr F 4) (νCO = 2050 cm - 1 ), [10] but closer to those of the arene ruthenium complex with dianionic ditiolate ligands 4 (νCO = 1951 cm -1 ), [17] [(η 6 -C6Me6)Ru(CO)(SXyl)2] (νCO = 1965 cm -1 ) [START_REF] Mashima | [END_REF] and of the cyclopentadienyl complex with neutral κ 2 -N,N-ligand [(η 5 -C5H5)Ru(CO)(TMEDA)](BAr4) (νCO = 1968 cm -1 ). [24] Surprisingly, bubbling CO into the solution of 2c yielded a stable product, 3c, resulting from the addition of two CO molecules, one of which has inserted into one of the two Ru-N bonds. The product has been fully characterized, including by single crystal X-ray diffractometry (Figure 11, Table S6 in ESI). 5) Å). Similarly to 3a and 3d, the terminal carbonyl group is linear. The carbamoyl C=O and C-N bonds have similar length to or are slightly longer than those in organic amides. The carbamoyl N2 atom is planar, while N1 is considerably pyramidalized as in the corresponding 18ē chloride complex 1c. There are two intermolecular close contacts in the structure of 3c that fall below the sum of the van der Waals radii, [23] implicating hydrogen atoms of the C6Me6 ligand and either a PF6 -F atom or the carbamoyl oxygen atom, H(8A

)•••F (2.269 Å) and H(10B)•••O(2)C (2.418 Å), respectively.
The 13 C{ 1 H} NMR spectrum of 3c reveals two signals corresponding to carbonyl C nuclei: a singlet at δ 198.2 and a doublet at δ 192.8 ( 2 JСР = 19.6 Hz). All the N-and P-substituents in 3c are inequivalent and give rise to two sets of signals for both methyl and phenyl groups in the 1 H and 13 C{ 1 H} spectra. In the IR spectra 3c shows two strong carbonyl bands νCO at 1984 cm -1 (metal bound CO) and at 1644 cm -1 (carbamoyl C=O), the latter frequency being typical of organic amides.

Apparently, the carbonylation of 2c is a two-step reaction involving: 1) coordination of a first CO molecule to give an intermediate C, which is structurally similar to 3a,d; and 2) insertion of the terminal CO into a Ru-N bond and subsequent occupation of the resulting vacant site at the ruthenium atom by a second CO molecule (Scheme 4). This conclusion is further supported by the results of an IR spectroscopic monitoring at low temperatures (Fig. 12). Bubbling CO at -40°C for 10 s fully converts 2c to an orange complex characterized by a strong CO band at 1975 cm -1 , which can be attributed to the mono-adduct C.

The latter is unstable: even at -40°C and in the absence of additional CO it slowly disappears to be replaced by 3c. The coordination of the first CO molecule is reversible, hence dissociation from the mono-adduct C provides the needed CO for the second carbonylation step. The rearrangement is complete within a few minutes at low temperatures and yields a 1:1 mixture of 2c and 3c, which were isolated upon evaporation of the solvent. The peculiarity of the mono-carbonyl intermediate C is the high nucleophilicity of the nitrogen atoms, which is related to the presence of more electron-donating alkyl substituents. Although facile CO-insertion into M-N bonds is a known phenomenon for certain metal aminopyridinates, [25] triazenides, [26] amidinates, [27][28][29][30] and aminophosphines [31] this reaction has not been previously reported to the best of our knowledge for arene ruthenium complexes. The process that most resembles the formation of 3c involves a carbene insertion into a Ru-N bond in the arene ruthenium amidinate complex [(C6Me6)Ru{( i PrN)2CMe}](PF6), followed by CO coordination to give [(C6Me6)Ru(CO){( i PrN)-CMe(N i Pr)CHSiMe3}](PF6) (5). [32] The spectral and structural data of 5 are similar to those of 3c: the νCO is observed at 1963 cm -1 and the carbonyl resonance appears at δ 203 in the 13 C NMR spectrum; the Ru-Arene(centroid) distance is 1.828 Å with the elongation of the Ru-C(arene) bonds trans to CO and to the inserted carbene to 2.320-2.362 Å; the CO ligand is linear (Ru-C-O is 170.2°), and the Ru-CO and C-O bond lengths are 1.836 Å and 1.159 Å, respectively.

Conclusions

Our detailed spectroscopic study has allowed to investigate the stabilization of the coordinative unsaturation in half-sandwich ruthenium iminophosphonamide complexes depending on the N-,P-substituents, arene and other co-ligands. By 2D EXSY 1 H NMR spectroscopy we have proven that the dynamic exchange process observed for chlorides 1a-d is attributed to the dissociation of the chloride ligand via a 16ē intermediate in both polar and apolar solvents. The chloride dissociation enthalpy (ΔHd) determined for the 18ē chloride complexes 1a-d and the Cl - coordination enthalpy (ΔHc) to the 16ē complex 2a quantitatively estimate the stabilization effect of the NPN ligand, which is mostly provided by highly localized negative charges at the nitrogen atoms via additional π-donation. Hence the NPN ligands with Nalkyl groups destabilize the 18ē complexes and compensate the metal electron deficiency much more effectively than the NPN ligands with N-aryl groups. Thus, the N-Me chloride complex 1c dissociates exothermically (ΔHd = -5.0 kcal/mol) even in CH2Cl2, whereas the dissociation is always endothermic for the N-Tol complexes (1a, 1b and 2d), even in polar nitromethane. On the other hand, the less donating arene ligand (p-cymene vs C6Me6) affects the stability of the 18ē complexes counterintuitively: it destabilizes the carbonyl adducts due to the lack of electron density provided for π-backdonation (3d vs 3a), but enhances the stability of the chloride complex (1d vs 1a) due to lower energy of the orbital corresponding to Ru-Cl bond. The coordination of ligands to 16ē complexes 2 appears to be unambiguously exothermic, while the highly negative association entropy makes the 18ē complexes stable only at low temperatures. Nevertheless, stronger π-acceptors (CO > pyridine > MeCN) give more stable adducts due to significant contribution of π-backdonation to Ru-L bonding; this fact allowed us to isolate and structurally characterize complexes 3a and 3d, which are rare examples of cationic 18ē arene ruthenium carbonyl complexes. The unexpectedly facile insertion of CO into the Ru-N bond observed in the 18ē adduct 2c•CO to give the carbonyl-carbamoyl complex 3c proves that arene ruthenium iminophosphonamide complexes can react in diverse manners and suggests that they have potential for application in organic synthesis and (bifunctional) catalysis. Further investigation of these complexes in catalytic transfer hydrogenation is in progress and will be reported soon.

Experimental Section

General procedures. All manipulations were carried out using standard Schlenk techniques under an atmosphere of dry argon. Absolute solvents were used for both synthesis and spectroscopic studies; solvents were purified by standard methods and distilled prior to use. The 1 H, 31 P and 13 C NMR spectra were obtained on Bruker Avance 600 or Bruker Avance 400 spectrometers and referenced to the residual signals of deuterated solvent ( 1 H and 13 C), and to 85% H3PO4 ( 31 P, external standard). The UV spectra were recorded on a Varian Cary 50 WinUV spectrometer in quartz cells (l = 2.2 mm or 10 mm). The IR spectra were obtained on a Fourier spectrometer Nicolet 6700 in KBr cells (l = 0.514 mm). The elemental analyses were carried out on a Carlo Erba 1106 CHN analyzer. The following compounds were prepared according to described procedures: [(η 6 -p-Cymene)RuCl2]2, [33] Ph2P(N-p-Tol)(NH-p-Tol) (A), [15] 1a-c, 2a-c. [15] Synthesis of [(η 6 -p-Cymene)RuCl{Ph2P(N-p-Tol)2}] (1d). To a solution of A (0.79 g, 2.00 mmol) in benzene (60 mL) a 2.0 M solution of NaHMDS in THF (1.10 mL, 2.20 mmol) was added and the resulting solution was stirred for 2 h. Then solid [(η 6 -p-Cymene)RuCl2]2 (0.61 g, 1.00 mmol) was added and the reaction mixture was stirred overnight. The reaction mixture was filtered and the solvent from the filtrate was removed under reduced pressure. The residue was washed with hexane (2x10 mL), Et2O (2x5 mL), and then recrystallized from hot benzene (20 ml). The dark-red crystalline was filtered off, washed with Et2O (5 mL) and dried in vacuo. Yield 1.00 g (88%). Anal. calcd for C36H38ClN2PRu: C, 64.90; H, 5.75%. Found: C, 64.85; H, 5.84%. 31

Synthesis of [(η 6 -p-Cymene)Ru{Ph2P(N-p-Tol)2}](PF6) (2d).

To a solution of 1d (0.39 g, 0.59 mmol) in CH2Cl2 (15 ml), solid AgPF6 (0.16 g, 0.62 mmol) was added, causing the color to immediately change from red to deep violet. The reaction mixture was stirred for 2 h and then filtered through a bed of Celite. The solvent was removed under reduced pressure, and the residue was dried in vacuo to give violet-black 2d. Yield 0.41 g (91%). Anal. calcd for C36H38F6N2P2Ru: C, 55.74; H, 4.94%. Found: C, 55.47; H, 4.99%. 31 Reaction of 2a,2c,2d with CO. General procedure. A stream of CO was slowly bubbled through a stirred solution of 2a (0.12 g, 0.15 mmol) in CH2Cl2 (5 mL) for 10 min. The color quickly changed from deep violet to orange. The product was precipitated with Et2O (20 ml) as a yellow-orange crystalline solid, which was filtered off and dried in vacuo for 1 hr. Yield 0.12 g (96%). Anal. calcd for C39H42F6N2OP2Ru: C, 56.32; H, 5.09%. Found: C, 56.34; H, 4.95%. 31 P NMR (СD2Cl2): δ 61.7 (s, PPh2), -144.5 (sept, 1 JPF = 713, PF6 -). 1 H NMR (СD2Cl2): Analogously, from 2c (0.10 g, 0.15 mmol) in CH2Cl2 (3 ml), complex 3c was obtained as a stable yellow crystalline solid. Yield 0.10 g (95%). Anal. calcd for C28H34F6N2O2P2Ru•H2O: C, 46.35; H, 5.00%. Found: C, 46.49; H, 5.00%. 31 P NMR (CD2Cl2): δ 55.2 (s, PPh2), -144.5 (sept, 1 JPF = 713, PF6 -). 1 Analogously, carbonylation of 2d (0.11 g, 0.14 mmol) gave a red solution of 3d. The product was precipitated by Et2O (20 ml) saturated with CO, filtered and dried in vacuo for 30 min. Yield 0.09 g (82%). Anal. calcd for C37H38F6N2OP2Ru: C, 55.29; H, 4.77%. Found: C, 55.05; H, 4.86%. 31 P NMR (CDCl3): δ 60.2 (s, PPh2), -144.2 (sept, 1 JPF = 713, PF6 -). 1

Scheme 1 .

 1 Scheme 1. Synthesis of the complexes 1d, 2d.

Figure 1 .

 1 Figure 1. ORTEP diagram of 1d. Ellipsoids are shown at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Ru•••Arene(centroid) 1.667(1), Ru-Cl 2.415(1), Ru-N1 2.145(2), Ru-N2 2.126(3), N1-Ru-N2 68.24(9), Ru-N1-P-N2 178.43(16), Σ(N1) 357.2(6), Σ(N2) 358.2(6).

Figure 2 .

 2 Figure 2. ORTEP diagram of the cation 2d. Ellipsoids are shown at the 50% probability level; hydrogen atoms and the anion are omitted for clarity. Selected bond lengths (Å) and angles (°): Ru•••Arene(centroid) 1.659(1), Ru-N1 2.031(2), Ru-N2 2.017(3), N1-Ru-N2 72.42(10), Ru-N1-P-N2 174.27(15), Σ(N1) 360.0(6), Σ(N2) 358.9(5).

Figure 3 .

 3 Figure 3. UV-vis spectra of 1c in CH2Cl2 at 190-298 K. The insert shows the Van't Hoff plot of RlnKd vs 1/T.

Figure 4 .

 4 Figure 4. UV-vis spectra of 1b in MeNO2 at 250-310 K. The insert shows the Van't Hoff plot of RlnKd vs 1/T.

Figure 5 .

 5 Figure 5. 2D EXSY 1 H NMR of 1b in toluene-d8 at 291K and the mixing time of 200 ms.

Figure 6 .

 6 Figure 6. Dissociative (A) and associative (B) mechanisms for the exchange of the R'a and R'b groups in 1.

Figure 7 .

 7 Figure 7. Associative exchange promoted by water-induced chloride dissociation.

Figure 8 .

 8 Figure 8. UV-vis spectra of reversible coordination of pyridine to 2a in CH2Cl2 at 200-296K. The insert shows the Van't Hoff dependence of RlnKc vs 1/T.

Figure 9 .

 9 Figure 9. ORTEP diagram of the cation 3a. Ellipsoids are shown at the 50% probability level. The hydrogen atoms and the anion are omitted for clarity. Selected bond lengths (Å) and angles (°): Ru•••Arene(centroid) 1.769(1), Ru-C(O) 1.871(3), Ru-N1 2.112(2), Ru-N2 2.116(2), C-O 1.145(3), N1-Ru-N2 69.20(9), Ru-N1-P-N2 164.60(15), Σ(N1) 348.3(5), Σ(N2) 359.3(5).

Figure 10 .

 10 Figure 10. ORTEP diagram of the cation 3d. Ellipsoids are shown at the 50% probability level. The hydrogen atoms and the anion are omitted for clarity. Selected averaged bond lengths (Å) and angles (°): Ru•••Arene(centroid) 1.768(3), Ru-C(O) 1.879(9), Ru-N1 2.095(7), Ru-N2 2.120(6), C-O 1.145(10), N1-Ru-N2 68.9(3), Ru-N1-P-N2 165.8(4), Σ(N1) 353.7(13), Σ(N2) 355.3(14).

  (1.662(2) Å) and 2d. The Ru-C(arene) distances vary in the range 2.208-2.333(3) Å with the longest bonds being trans to the CO ligand. The C-C bonds length (1.389(12)-1.452(4) Å) in the coordinated arene slightly alternate with the shortest C-C bond (1.389-1.403 Å) located trans to the CO ligand. A similar Ru-Arene(centroid) (1.810 Å), Ru-C(O) (1.846(4) Å) and carbonyl C-O (1.133(

Figure 11 .

 11 Figure 11. ORTEP diagram of the cation 3c. Ellipsoids are shown at 30% probability level. The hydrogen atoms and the anion are omitted for clarity. Selected bond lengths (Å) and angles (°): Ru•••Arene(centroid) 1.828(1), Ru-N1 2.107(2), Ru-C(O)N 2.042(3), C=O 1.211(4), N-C(O) 1.414(3), Ru-C(O) 1.859(3), C-O 1.143(4), N1-Ru-C(O) 83.15(10), Σ(N1) 349.7(5), Σ(N2) 359.3(6).

  Scheme 4.

Figure 12 .

 12 Figure 12. The reaction of 2c with CO monitored by IR. The sample was dissolved in CH2Cl2 at -40°C and CO was bubbled for 10 s. The spectra were recorded (a) immediately after the preparation, (b) after 40 s at room temperature, (c) after 5 min of keeping (a) at -40°C, (d) after evaporation of (c) and redissolution in CH2Cl2.

1 JCP

 1 P NMR (CDCl3): δ 43.8 (s, PPh2).1 H NMR (CDCl3): δ 7.87 (dd, 3 JHP = 10.8, 3 JHH = 8.0, 4H, o-HPh), 7.50 (m, 2H, p-HPh), 7.39 (m, 4H, m-HPh), 6.90 (d, 3 JHH = 8.0, 4H, C6H4(Tol)), 6.82 (d, 3 JHH = 8.4, 4H, C6H4(Tol)), 6.31 (d, 3 JHH = 5.6, 2H, C6H4(Cym)), 5.41 (d, 3 JHH = 5.6, 2H, C6H4(Cym)), 2.89 (sept, 3 JHH = 6.8, 1H, CHMe2), 2.19 (s, 6H, MeTol), 2.11 (s, 3H, MeCym), 1.21 (d, 3 JHH = 6.8, 6H, CHMe2).31 P NMR (C6D6): δ 42.9. 1 H NMR (C6D6): δ 8.03 (m, 4H, o-HPh), 7.27 (d, 3 JHH = 7.8, 4H, C6H4(Tol)), 7.22 (m, 3H, (m+p)-HPh), 6.93 (d, 3 JHH = 7.8, 4H, C6H4(Tol)), 6.75 (m, 3H, (m+p)-HPh'), 5.05 (d, 3 JHH = 6.0, 2H, C6H4(Cym)), 4.87 (d, 3 JHH = 6.0, 2H, C6H4(Cym)), 2.88 (sept, 3 JHH = 6.6, 1H, CHMe2), 2.15 (s, 6H, MeTol), 1.86 (s, 3H, MeCym), 1.05 (d, 3 JHH = 6.8, 6H, CHMe2).13 C NMR (CDCl3): δ 145.8 (d, 2 JСР = 4.4, i-CTol(N)), 133.4 (br.s, о-CPh), 131.9 (d, 4 JСР = 2.7, p-CPh), 129.0 (s, β-CHTol), 128.2 (d, 3 JСР = 11.2, m-CPh), 127.5 (s, i-CTol(Me)), 122.9 (d, 3 JСР = 9.7, α-CHTol), 102.0 (s, i-CCym), 94.8 (s, i-CCym), 80.5 (s, CHCym), 79.6 (s, CHCym), 30.9 (s, CHMe2), 22.5 (s, СHMe2), 20.5 (s, MeTol), 18.9 (s, MeCym).13 C NMR (C6D6): δ 146.9 (s, 2 JСР = 4.2, i-CTol(N)), 136.5 (d, 1 JCP = 95.1, i-CPh), 135.0 (d, 2 JCP = 11.0, o-CPh), 132.6 (d, 2 JСР = 9.8, o-CPh'), 132.0 (d, 4 JСР = 2.7, p-CPh), 131.8 (d, 4 JСР = 2.9, p-CPh'), 130.2 (d, = 84.0, i-CPh'), 129.4 (s, β-CHTol), 128.6 (d, 3 JСР = 12.5, m-CPh), 128.5 (s, i-CTol(Me)), ~127.7 (overlapped, m-CPh'), 123.9 (d, 3 JСР = 9.7, α-CHTol), 102.4 (s, i-CCym), 94.8 (s, i-CCym), 80.7 (s, CHCym), 79.4 (s, CHCym), 31.2 (s, CHMe2), 22.5 (s, СHMe2), 20.7 (s, MeTol), 18.8 (s, MeCym). UV-vis (CH2Cl2; λmax, nm; ε, M -1 cm -1 ): 450 (450, shoulder).

4 JHP = 1 . 2 ,

 412 P NMR (CDCl3): δ 71.3 (s, PPh2), -144.1 (sept, JPF = 712, PF6 -). 1 H NMR (CDCl3): δ 7.65 (m, 2H, p-HPh), 7.52 (m, 4H, m-HPh), 7.47 (m, 4H, o-HPh), 7.02 (d, 3 JHH = 8.0, 4H, C6H4), 6.84 (dd, 3 JHH = 8.0, 4H, C6H4), 5.88 (d, 3 JHH = 6.8, 2H, C6H4(Cym)), 5.84 (d, 3 JHH = 6.8, 2H, C6H4(Cym)), 2.68 (sept, 3 JHH = 6.8, 1H, CHMe2), 2.26 (s, 3H, MeCym), 2.25 (s, 6H, MeTol), 1.31 (d, 3 JHH = 6.8, 6H, CHMe2). 13 C NMR (CDCl3): δ 144.1 (d, 2 JСР = 3.6, i-CTol(N)), 134.4 (d, 4 JCP = 2.2, p-CPh), 134.1 (s, i-CTol(Me)), 132.4 (d, 2 JCP = 10.3, о-CPh), 130.2 (s, β-CHTol), 129.5 (d, 3 JСР = 12.5, m-CPh), 124.7 (d, 1 JCP = 89.4, i-CPh), 123.7 (d, 3 JСР = 8.0, α-CHTol), 99.6 (s, i-CCym), 89.6 (s, i-CCym), 81.0 (s, CHCym), 78.7 (s, CHCym), 31.8 (s, CHMe2), 22.8 (s, СHMe2), 20.9 (s, MeTol), 19.7 (s, MeCym). UV-vis (CH2Cl2; λmax, nm; ε, M -1 cm -1 ): 590 (1880).

2 JСР = 9 . 3 ,

 293 δ 7.84 (t, 3 JHH = 8.6, 1H, p-HPh), 7.81 (dd, 3 JHP = 12.8, 3 JHH = 8.0, 2H, o-HPh), 7.69 (dt, 3 JHH = 7.2, 4 JHP = 3.2, 2H, m-HPh), 7.48 (dt, 3 JHH = 7.2, 5 JHP = 1.6, 1H, p-HPh'), 7.24 (dt, 3 JHH = 8.0, 4 JHP = 3.2, 2H, m-HPh'), 7.19 (dd, 3 JHP = 10.8, 3 JHH = 8.0, 2H, o-HPh'), 6.90 (d, 3 JHH = 8.0, 4H, C6H4(Tol)), 6.46 (dd, 3 JHH = 8.0, 4 JHP = 2.0, 4H, C6H4(Tol)), 2.21 (s, 6H, MeTol), 2.11 (s, 18H, C6Me6). 13 C NMR (СD2Cl2): δ 199.2 (s, Ru-CO), 140.8 (d, 2 JСР = 2.2, i-CTol(N)), 135.8 (d, 1 JСР = 97.2, i-CPh), 134.9 (d, 4 JСР = 2.6, p-CPh), 133.6 (d, 4 JСР = 2.7, p-CPh'), 132.9 (d, o-CPh), 132.7 (d, 2 JСР = 11.0, o-CPh'), 132.8 (s, i-CTol(Me)), 130.4 (d, 4 JСР = 1.3, β-CHTol), 130.0 (d, 3 JСР = 12.1, m-CPh), 128.9 (d, 3 JСР = 11.4, m-CPh'), 128.3 (d, 1 JСР = 83.3, i-CPh'), 126.0 (d, 3 JСР = 7.0, α-CHTol), 110.3 (s, C6Me6), 20.8 (s, MeTol), 17.1 (s, C6Me6). UV-vis (CH2Cl2; λmax, nm; ε, M - 1 cm -1 ): 450 (100, shoulder). IR (CH2Cl2, ν, cm -1 ): 1984 (RuСO).

  H NMR (CD2Cl2): δ 7.83 (ttd, 3 JHH = 7.8, 4 JHH = 1.8, 5 JHP = 1.2, 1H, p-HPh), 7.75 (ttd, 3 JHH = 7.8, 4 JHH = 1.2, 5 JHP = 1.2, 1H, p-HPh'), 7.71 (dt, 3 JHH = 7.8, 4 JHP = 3.6, 2H, m-HPh), 7.63 (ddd, 3 JHP = 12.6, 3 JHH = 7.8, 4 JHH = 1.2, 2H, o-HPh), 7.60 (dt, 3 JHH = 7.8, 4 JHP = 3.6, 2H, m-HPh'), 7.42 (ddd, 3 JHP = 12.6, 3 JHH = 7.8, 4 JHH = 1.2, 2H, o-HPh'), 2.73 (d, 3 JHP = 8.4, 3H, NMe(CO)), 2.64 (d, 3 JHP = 17.4, 3H, NMe), 2.24 (s, 18H, C6Me6). 13 C NMR (CD2Cl2): δ 198.2 (s, Ru-CO), 192.8 (d, 2 JСР = 19.6, NC=O), 135.1 (d, 4 JСР = 2.4, p-CPh), 135.0 (d, 4 JСР = 2.4, p-CPh'), 133.6 (d, 2 JСР = 10.6, o-CPh'), 132.6 (d, 2 JСР = 10.6, o-CPh), 130.3 (d, 3 JCP = 12.8, m-CPh), 130.2 (d, 3 JСР = 12.6, m-CPh'), 124.2 (d, 1 JCP = 112.2, i-CPh), 122.2 (d, 1 JCP = 98.6, i-CPh'), 113.4 (s, i-C6Me6), 39.3 (d, 3 JСР = 4.2, NMe), 29.3 (d, 3 JСР = 8.4, NMe(CO)), 17.0 (s, C6Me6),. IR (CH2Cl2, ν, cm -1 ): 1983 (RuCO), 1644 (RuC(O)N).

  H NMR (CDCl3): δ 7.93 (dd, 3 JHP = 12.4, 3 JHP = 7.2 , 2H, o-HPh), 7.80 (t, 3 JHH = 7.2,

Table 1 . The HOMO, LUMO energies and the HOMO-LUMO gap for 2a and 2d calculated in the gas phase and CH2Cl2 solution in comparison to the energy of the experimentally observed band (all in eV). 2a 2d

 1

		Gas phase	CH2Cl2	Gas phase	CH2Cl2
	HOMO	-7.45	-5.06	-7.37	-5.05
	LUMO	-5.50	-3.01	-5.92	-3.40
	Gap	1.95	2.05	1.45	1.65
	Expt		2.32		2.10

Table 2 .

 2 Exchange rate constant kex (s -1 ) and activation free energy ΔGex ≠ (kcal/mol) for the P-bound substituent exchange in 1a, 1b and 1d from the 2D EXSY 1 H NMR investigations.

		Complex	Solvent	Additives	C(Ru), mM	C(add), mM	T, K	kex, s -1	ΔGex ≠ , kcal/mol
	1	1a	C6D6	-	10		293	<0.1	>18.5
	2	1a	C6D6	H2O	10	10	294	0.79	17.4
	3	1b	C6D6	-	20		293	1.8	16.8
	4	1b	C6D6	H2O	20	5	295	>15	<15.7
	5	1b	Toluene	-	40		295	1.6	17.0
	6	1b	Toluene	-	8		295	1.6	17.0
	7	1b	Toluene	-	46		293	1.4	17.0
	8	1d	C6D6		10		293	<0.1	>18.5
	9	1d	C6D6	H2O	10	0.7	293	0.72	17.3
	10	1d	C6D6	H2O	10	2.4	293	2.6	16.6
	11	1d	C6D6	Et4N + Cl -	10	0.7	292	>11	<15.7

Table 3 .

 3 Crystal data and structure refinement parameters for 1d

, 2d, 3a, 3c, 3d.

		1d	2d	3a	3c	3d
	Formula	C36.50H39Cl2N2PRu	C36H38F6N2P2Ru	C39H42F6N2OP2Ru	C28H34F6N2O2P2Ru	C37H38F6N2OP2Ru
	Formula weight	708.63	775.69	831.75	707.58	803.70
	T, K	120	120	100	120	120
	Crystal system	monoclinic	monoclinic	monoclinic	monoclinic	monoclinic
	Space group	P21/c	P21/c	P21/n	P21/n	Pn
	Z / Z'	4 / 1	4 / 1	4 / 1	4 / 1	4 / 2
	a, Å	9.4575(3)	9.4269(5)	9.6577(7)	14.1458(9)	10.2091(7)
	b, Å	23.0562(8)	19.0924(11)	20.7348(15)	12.0157(8)	10.0913(8)
	c, Å	14.5931(5)	19.2392(11)	18.7465(14)	18.3241(12)	34.401(3)
	β, °	98.659(2)	92.5880(10)	94.0820(16)	99.2530(10)	96.029(2)
	V, Å 3	3145.81(18)	3459.2(3)	3744.5(5)	3074.1(3)	3524.5(4)
	dcalc, g cm -3	1.496	1.489	1.475	1.529	1.515
	μ, cm -1	7.49	6.07	5.68	6.79	6.00
	2θmax, °	60	54	60	60	58
	Reflns. collected / independent	64054 / 9170	47949 / 7492	38229 / 10895	38560 / 8962	27738 / 14871
	Observed reflections [I>2σ(I)]	6614	6101	7163	7722	11444
	R1	0.0461	0.0455	0.0475	0.0438	0.0514
	wR2	0.1077	0.0918	0.1039	0.1182	0.0829
	GOF	1.052	1.085	1.015	1.079	1.014
	Residual density, e Å -3 (dmax/dmin ) 0.515/-0.916	0.880/-0.646	0.845/-0.563	1.593/-0.750	0.724/-0.695

Acknowledgements

The authors thank the Russian Science Foundation (grant no. 14-13-00801) for financial support. I.S. thanks the French Ministry of Foreign Affairs for a Ph.D. scholarship through the French Embassy in Moscow.

1H, p-HPh), 7.73 (dt, 3 JHH = 7.6, 4 JHP = 2.8, 2H, m-HPh), 7.50 (t, 3 JHH = 7.2, 1H, p-HPh'), 7.35 (dd, 3 JHP = 10.2, 3 JHH = 7.2, 2H, o-HPh'), 7.30 (dt, 3 JHH = 7.6, 3 JHH = 3.2, 2H, m-HPh'), 6.87 (d, 3 JHH = 8.0, 4H, C6H4(Tol)), 6.51 (d, 3 JHH = 8.0, 4H, C6H4(Tol)), 6.29 (br. s, 4H, C6H4(Cym)), 2.65 (sept, 3 JHH = 6.8, 1H, CHMe2), 2.18 (s, 6H, MeTol), 2.11 (s, 3H, MeCym), 1.22 (d, 3 JHH = 6.8, 6H, CHMe2). 13 EXSY 1 H NMR. 2D 1 H-1 H EXSY spectra were collected on a Bruker Avance 600 spectrometer at 25°С using the standard Bruker library noesygpph pulse program. At least three experiments with different values of the mixing time tm were performed to find an optimum mixing time, resulting in sufficiently large exchange cross-peaks without significant relaxation contribution. The rate constants k = kAB + kBA for the A ↔ B exchange reactions were calculated using simple two-site model [19] using Equations ( 1), (2), where kAB and kBA are the rate constants of the direct and inverse reactions, IAB, IBA and IAA, IBB are the cross-peak and the diagonal peak integral intensities, respectively. In case of exchange between equally populated states (kAB = kBA), the exchange rate constant

The activation free energy ΔG ≠ was calculated from the Eyring equation ( 3) and the activation enthalpy ΔH ≠ and entropy ΔS ≠ were derived by linear fitting of Rln(kex/T) plotted vs. 1/T according to the equation (4).

UV-Vis spectroscopic study. The UV-vis monitored titrations were performed in a 10 mm quartz cell, while the spectra at low temperatures were recorded in a 2.2 mm quartz cell. In most experiments the initial concentrations of ruthenium complex (c 0 Ru) were 2.5×10 -3 M, while in other cases c 0 Ru was in the range 5-8×10 -4 M. Neat MeCN and pyridine were used for the titrations. To calculate the dissociation constants Kd for 1b,c (Eq. 5) and the constant Kc for the ligand coordination to 2a (Eq. 6), the equilibrium concentrations of the 16ē (𝒄 𝟏𝟔ē ) and 18ē (𝒄 𝟏𝟖ē ) complexes were obtained from the UV-vis spectra according to the Beer-Lambert law. The absorption of the equilibrium mixture D(λi) was measured at a wavelength λi (Eq. 7), at which the absorption coefficient 𝜺 𝟏𝟖ē for 18ē complexes (1 or 2a(L)) is close to zero, while the 𝜺 𝟏𝟔ē for 16ē species (2a-c) is still high enough. Particularly, at the chosen λi = 600 nm, 𝜺 𝟏𝟔ē is in the range 550 -650 M -1 cm -1 , whereas 𝜺 𝟏𝟖ē is < 30 M -1 cm -1 . Therefore, the equilibrium concentration 𝒄 𝟏𝟔ē can be approximated as in Equation (8).

In the titration of 2a with L, the initial concentrations c 0 16ē of 2a for every ititration point was corrected for the dilution factor (𝒄 𝟏𝟔ē 𝟎 (𝒊)) according to Equation (9).

where 𝑽 𝟏𝟔ē 𝟎 and 𝑽 𝑳 are the initial volume of 2a solution and the added volume of L, correspondingly.

Computational details. The geometry optimizations were carried out with the PBE functional and def2-TZVP [34] basis set for all atoms without any symmetry restrictions in the gas phase using the Gaussian09 package. [START_REF] Frisch | Gaussian 09, Revision D.01[END_REF] The obtained stationary points were confirmed to have no imaginary frequencies. The orbital energies were computed with the SMD [START_REF] Marenich | [END_REF] solvent model using the gas phase optimized geometry. X-ray crystal structure determination. Single crystals 1d, 2d, 3a, 3c, 3d were obtained by slow diffusion of Et2O into CH2Cl2 solutions; in the case of 3d the solution was saturated with CO. The data collection for samples 1d and 2d were performed on a Bruker APEX DUO diffractometer, and those for 3a, 3c and 3d on a Bruker SMART APEX II diffractometer, both equipped with an Apex II CCD detector and operating with MoKα radiation (λ = 0.71073 Å). Frames were integrated using the Bruker SAINT software package [START_REF]SAINT v8.34A[END_REF] by a narrow-frame algorithm. A semiempirical absorption correction was applied with the SADABS [START_REF] Sheldrick | Bruker/Siemens Area Detector Absorption Correction Program[END_REF] program using the intensity data of equivalent reflections. The structures were solved with direct methods and refined by the full-matrix least-squares technique against F 2 hkl in anisotropic approximation with the SHELX [START_REF] Sheldrick | [END_REF] software package. The positions of the hydrogen atoms were calculated, and all hydrogen atoms were refined using the riding model with 1.5Ueq(Cm) and 1.2Ueq(Ci), where Ueq(Cm) and 1.2Ueq(Ci) are respectively the equivalent thermal parameters of methyl and all other carbon atoms to which corresponding H atoms are bonded. The structure of 1d contains an interstitial CH2Cl2 molecule, which is disordered about an inversion center. In the structure of 3d two crystallographically independent cations and anions are related by an approximate non-crystallographic inversion center. Detailed crystallographic information is given in Table 4

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The lone pair of the highly basic iminophosphonamide N-atoms in arene ruthenium complexes can both stabilize 16ē species and attack an external ligand in 18ē species.