Rinaldo Poli
email: poli@lcc-toulouse.fr

Prof Rinaldo Poli

Relationship between one-electron transition-metal reactivity and radical polymerization processes

Keywords: transition metals, controlled radical polymerization, atom transfer, homolytic bond dissociation energy, one-electron oxidative addition, catalytic chain transfer

The achievement of controlled radical polymerization (CRP) has been one of the most spectacular advances of polymer chemistry in recent years, because materials where the architecture, average molecular weight, molecular weight distribution, and nature of chainend functions are well defined have now become accessible by the use of a wide variety of polar monomers and "friendlier" polymerization techniques (e.g. unpurified organic solvents, emulsions, etc.). The key requirement to achieve this level of control is the ability to reduce to negligible levels the chances for two propagating radicals to encounter each other, leading to irreversible bimolecular terminations. There are many ways in which this condition has been achieved, but the method that has attracted the greatest attention is Atom Transfer Radical Polymerization (ATRP), following its discovery in 1995 in the laboratories of Matyjaszewski [1] and Sawamoto. [2] The main interest and potential practical advantage of ATRP lies in the fact that a transition metal complex acts as a regulator of the radical concentration by reversibly transferring a halogen atom to the active radical, thereby transforming it into a halogenterminated dormant species, without being incorporated itself into the polymer chain.

Therefore, the metal complex acts merely as a catalyst and can potentially be recovered and recycled. The overall process boils down to a catalyzed sequential insertion of a certain (controlled) amount of monomer units into the carbon-halogen bond of a suitable alkyl halide initiator, see Scheme 1 (m = monomer), and can be considered as a natural extension of the metal-catalyzed atom transfer radical addition (ATRA). [3] The catalyst, LnM, activates the dormant species, R(m)x-Y, by accepting its terminal halogen atom (Y) and generates the active radical, R(m)x • , plus an oxidized complex, LnM-Y, the latter being related to the catalyst by an atom transfer, where the metal electronic configuration changes by one electron.

<Scheme 1>

Before the basic principles of ATRP were uncovered, however, transition metal complexes had already been shown capable to control the concentration of radical species and thereby to allow controlled chain growth for radical polymerization. The method is a particular example of the general concept of reversible addition-cleavage, see Scheme 2. In this scheme, I is a radical initiator producing R • as primary radical, like in classical free radical polymerization, while T is a reversible spin trap, namely a species capable to form relatively labile bonds with the propagating radical. Since T remains bound to the chain end in the polymer product, this is not a catalytic species. This procedure was first introduced by Otsu using the trityl radical as spin trap [4] and subsequently developed with nitroxide radicals (nitroxide-mediated polymerization, or NMP). A stable but labile T-R compound may also be used as initiator (for instance, an alkoxylamine for NMP). However, Wayland extended this method to transition metal complexes as spin traps, demonstrating the ability of Co(porphyrin) complexes to control the polymerization of acrylates. [5] In this case, the radical concentration is controlled by the reversible homolytic cleavage of the metal-carbon bond, a process closely related to the mechanism of action of vitamin B12. [6] This, like the atom transfer step in ATRP, is also a one-electron process. It is to be mentioned that this "reversible addition-cleavage" methodology has also been termed "Stable Free Radical Polymerization" (SFRP), on the basis on the notion that the spin trap T is often a stable radical, e.g. a nitroxide radical or a metallaradical. However, this is not strictly a requirement and, as we shall see later, other species (diamagnetic complexes, but also complexes whose spin state is triplet, quartet, etc.) may equally well assure this function. The term NMP, on the other hand, is specific for the reversible cleavage where T = organic nitroxide. Other specially coined terms have also been used for specific reversible addition-cleavage processes (e.g. CMRP for "cobalt-mediated radical polymerization"). [7] In this Minireview, we focus our attention on the reversible addition-cleavage phenomenon where T as a generic metal complex. We shall therefore use the term "organometallic radical polymerization" (OMRP), because it involves the reversible cleavage of a metal-carbon bond. We should also mention another important method for controlling radical polymerization, termed "reversible additionfragmentation chain transfer" (RAFT), where the trapping function is exerted by dithioesters. [8,9] The latter methodology, however, combines the mechanistic paradigm of reversible addition-cleavage with a second one, namely that of degenerative transfer. [10] It is important to underline that all the above mentioned methods differ in the nature of the "moderator" and the mechanism with which a dormant species is produced, but the monomer consumption always occurs via the same radical intermediates.

<Scheme 2>

This Minireview analyzes these two basic one-electron processes involving transition metals, ATRP (Scheme 1) and OMRP (Scheme 2, for T = transition metal complex), as well as other one-electron processes, as they relate to controlled radical polymerization. It will bring together very old knowledge from transition metal coordination chemistry and the more recent notions of metal-mediated controlled radical polymerization.

One-electron reactivity of transition metal complexes

In transition metal chemistry (especially containing metal-carbon bonds, i.e. organometallic chemistry), studies of chemical reactivity are mostly focused on two-electron processes such as ligand exchange, migratory insertion, oxidative addition, and so on, because of their close relevance to a variety of catalytic processes. However, one-electron reactivity also plays a crucial role in many catalytic and stoichiometric reactions. [11] The ubiquitous process, of prime relevance to this Minireview, is the one-electron oxidative addition of alkyl halides (Scheme 3), whereby one molecule of substrate splits homolytically to deliver the halogen atom to one metal complex molecule, and the alkyl radical to a second one. This process is favoured when the metal has the ability to expand its electron count and coordination number by only one unit.

<Scheme 3>

A seminal example involves the tetracyanocobaltate(II) ion, [Co(CN)5] 3-, yielding a 1:1 mixture of [Y-Co(CN)5] 3-and [R-Co(CN)5] 3-(Scheme 4a). [12] A very early and relevant example is also the reaction of alkyl halides with aqueous Cr 2+ (Scheme 4b), providing access to [R-Cr(H2O)5] 2+ , one of the earliest examples of stable alkyl derivatives of an open-shell transition metal, together with [X-Cr(H2O)5] 2+ . [13] This reaction is quite general for complexes of Cr II and has become the key part of one the workhorses of modern sophisticated organic synthesis, the Nozaki-Hiyama-Kishi reaction, [14,15] which has been rendered catalytic in chromium by Fürstner. [16] The reaction shown in Scheme 4c is also an example of oneelectron oxidative addition, although its intimate mechanism seems to involve two-electron steps. [17] The [CpFe(CO)2]2 complex does not react spontaneously with alkyl halides, but this process may also be relevant in controlled radical polymerization (vide infra). The mechanism of the one-electron oxidative addition reaction of alkyl halides has been quite firmly established as involving first the halogen atom abstraction step, followed by capture of the residual organic radical by the second transition metal complex molecule, according to Scheme 3. [18,19] The reactivity of organic radical with transition metal complexes has also been extensively investigated by Kochi, [20] Espenson [21,22] and Meyerstein. [23,24] The reason for the first event to be the M-Y formation, rather than the M-R formation, is related to the greater bond homolytic strength of the M-Y bond relative to the M-R bond. Thus, if we consider the process to ideally take place by homolytic rupture of the R-Y bond to yield R • and Y • , the unsaturated metal complex finds a greater thermochemical advantage to form a bond with Y first. Note that the two steps of this process correspond to the two basic equilibria needed to accomplish a metal-mediated radical polymerization (ATRP and OMRP, respectively).

<Scheme 4>

It is quite ironic that the coordination chemists, who have known and understood the mechanism of this reaction for such a long time, have not realized its potential for controlled organic synthesis (ATRA) or polymerization (ATRP and/or OMRP). Conversely, in the opinion of this Author, the polymer scientists who have exploited ATRP to such a sophisticated degree have not yet fully explored its limitations in terms of the constraints imposed by the one-electron oxidative addition process. We shall explore these limits in this Minireview.

A bond dissociation energy approach to the study of one-electron processes

The concepts developed in this section are a logical deduction from generally accepted considerations and reasonable assumptions. They allow the rationalization of a wealth of experimental facts, as will be shown in the remainder of the article. We start by observing, from the phenomenological point of view, that an OMRP process where the organometallic complex is devoid of halogen atoms cannot enter the ATRP equilibrium. This is the case, for instance, for the Co(porphyrin) system. Unless halogen atoms are present in the coordination sphere (vide infra), an OMRP is therefore uncontaminated by ATRP activation/deactivation equilibria. For systems that are set up under ATRP conditions, however, the situation may be more complicated. An analysis may be carried out using a variant of Scheme 3, where the active radical has already inserted a certain number (x) of monomer molecules, see Scheme 5.

Once MLn has accepted the halogen atom and generated the active radical R(m)x • (Scheme 5, first equilibrium), the latter may go back to the R(m)x-Y dormant species upon encountering the Y-MLn spin trap. On the other hand, the propagating radical may also encounter a second MLn molecule, leading to R(m)x-MLn (the OMRP dormant species), according to Scheme 5, second equilibrium. Thus, an ATRP process may be contaminated by the OMRP activation/deactivation equilibrium and ultimately by the one-electron oxidative addition process.

<Scheme 5>

In order to deepen this analysis and understand all possibilities and limitations, it is useful to consider the thermochemistry of the two equilibria in terms of the homolytic bond dissociation energies. The OMRP equilibrium is simply the homolytic rupture of the metal-

HATRP = BDE[R(m)x-Y]-BDE(LnM-Y).
From the point of view of an OMRP system, since this is uncontaminated by ATRP, its capability to control the radical polymerization is simply related to BDE[LnM-(m)xR]. Thus, if the metal-carbon bond is too weak, the MLn species has an insufficient ability to trap the radicals and the resulting OMRP equilibrium leads to too high a radical concentration, which translates into a significant proportion of bimolecular terminations and to an uncontrolled process. If, at the other extreme, the R(m)x-MLn bond is too strong (namely, the equilibrium is too displaced toward the dormant species), no significant amount of radicals will be generated and there will be no polymerization. Finally, when the bond strength is "just right", a controlled polymerization will take place, see Figure 1. All of this is valid, of course, when there are no halogens in the system, thus no radical trapping by the ATRP equilibrium may occur (but see more about this further down).

<Figure 1>

A brief parenthesis is in order, to comment on the meaning of "just right". Obviously, more than a single value of the metal-carbon BDE will be suitable for a controlled radical polymerization process. Intuitively, a stronger bond will lead to a lower equilibrium radical concentration and therefore to a slower polymerization, but at the same time to a better control. However, the so-called "persistent radical effect" [25] insures that a controlled process will also occur when the system generates a rather high amount of free radicals initially. This is because the spin trap (MLn) is stable and will accumulate in the system following the initial disappearance of the reactive radicals caused by the irreversible terminations. The result is a self-regulation of the OMRP equilibrium. This observation also serves to alert us about the importance of the dynamic radical concentration, i.e. the equilibrium position, which depends on the free energy and not on the enthalpy. Thus, looking merely at the thermochemistry is insufficient. In fact, the concentration of other species (e.g. MLn which accumulates because of the irreversible terminations) is not the only factor playing an important role; the reaction entropy also affects the system ability to control the polymerization. Since the OMRP spin trapping reaction is obviously characterized by a negative reaction entropy, the BDE[R(m)x-MLn] value overestimates the stability of the OMRP dormant species.

Looking now at a system which is set up under ATRP conditions, the outcome depends first of all on the value of HATRP: if this enthalpy difference is too small, too large, or just right, the amount of free radicals generated by the atom transfer process will correspondingly be too large, too small, or just right (note that the ATRP equilibrium involves the same number of molecules on either side and thus SATRP  0 or GATRP  HATRP). In this case we also need to consider, however, the effect of the coupled OMRP equilibrium. Since the ATRP equilibrium will generally be shifted toward the combination of ATRP catalyst (MLn, which is also the OMRP spin trap) and dormant species (R-Y), a fair concentration of OMRP spin trap will be available. The simplest case to examine is that in which HATRP is too large (Figure 2). Since the equilibrium amount of radicals will be too small, there will be essentially no polymerization. Under these conditions, the value of HOMRP is irrelevant.

<Figure 2>

Under the opposite circumstances (HATRP too small), the free radicals will not be efficiently trapped and the only possible regulation of their concentration may be provided by the OMRP equilibrium, see Figure 3. If HOMRP is also too small (case a), no efficient trapping is possible and the polymerization will be uncontrolled. If, at the opposite extreme (case c), HOMRP is too large, all radicals generated by the ATRP equilibrium will be irreversibly trapped by the OMRP equilibrium. The overall result is an irreversible oneelectron oxidative addition and no polymer is formed. Finally, if HOMRP is just right (case b), a controlled polymerization will take place. Note, however, that the control is insured by the OMRP equilibrium, even though the system is set up under ATRP conditions. The thermochemistry of the overall one-electron oxidative addition (HOx.Add., Scheme 5) favors the combination of oxidized complexes. Therefore, when the ATRP process is set up starting from a 1:1 MLn/R-Y ratio (which is often the chosen stoichiometry), the resulting polymer will contain a 1:1 mixture of MLn and Y end-functionalized chains. However, only one half of the MLn complex exerts its regulating function (the polymer chain capping complex, by the OMRP mechanism), because the other half is sequestered in the LnM-Y complex, a poor spin trap for the ATRP mechanism.

<Figure 3>

Finally, we need to examine the situation in which HATRP is just right, see Figure 4.

The meaning of "just right" is identical here to the OMRP situation examined above (the persistent radical effect operates in this case too). Under these circumstances, in the limiting situation where HOMRP is too small (case a) there is no efficient trapping of the active radical by the ATRP catalyst to yield the organometallic dormant species, but trapping still takes place by the oxidized LnM-Y complex to yield the halogen-capped dormant species R(m)x-Y, therefore a controlled polymerization process takes place. This is a pure ATRP. When HOMRP is also just right (case b), both trapping events will occur simultaneously. In this case, the polymerization will be controlled and the two mechanisms, ATRP and OMRP, will cooperate. A better control (and a slower polymerization) is expected relative to the case where HOMRP is too small, because the OMRP equilibrium contributes to further lower the concentration of propagating radicals. Finally, when HOMRP is too large (case c), trapping by MLn will be irreversible. Therefore, even though the ATRP equilibrium is suitably placed to provide a controlled polymerization process, no polymerization will be possible and the reagents will lead to the products of the one-electron oxidative addition quantitatively.

<Figure 4>

The bond energy criteria elaborated above illustrates the thermochemical conditions to insure the possibility to achieve a controlled radical polymerization by either (or both) mechanisms. However, before going on to the next section, it is important to underline that these are necessary but not sufficient conditions to insure that the polymerization is well controlled. We must also add a crucial kinetic condition: the trapping rates must be fast relative to the propagation rates (low energy for the transition states leading to the dormant species).

Relevance to controlled radical polymerizations: interplay of ATRP and OMRP

The conditions examined in Figure 4 are quite important to all researchers practicing radical polymerizations under ATRP conditions. Generally, situation a is implicitly assumed since the vast majority of published ATRP studies reporting new catalysts only make reference to a mechanistic scheme such as that of Scheme 1. Very few studies have explicitly challenged this dogma, asking the specific question of whether the ATRP catalyst could generate an organometallic dormant species. One such example is the interaction of Cu I complexes with the growing radical chains of polystyrene and poly(methyl acrylate). It was found that the CuBr/(4,4'-di-5-nonyl-2,2'-bipyridine) and CuOTf/(4,4-di-tert-butyl-2,2bipyridine) systems have no effect on the rate of styrene polymerization (e.g. initiated by the AIBN thermal decomposition) but significantly slow down the rate of MA polymerization. [26] This shows that there is a reversible interaction between the propagating methyl acrylatederived radicals (but not the styrene-derived radicals) and the Cu I metal center. However, the authors note that the interactions are very weak and neither improve the molecular weight and polydispersity control, nor affect tacticities which are the same as in conventional free radical polymerization without copper complexes. They conclude that "control in ATRP does not originate in interactions of growing radicals with copper complexes but in the reversible halogen atom transfer". However, the same situation may not be valid in other cases. We were the first to report a system that is capable to control radical polymerization (in the case of styrene) under both ATRP and OMRP conditions. The system in question is CpMoCl2L2, a cyclopentadienylmolybdenum(III) system where L2 could be bis(diphenylphosphino)ethane, dppe, of (PMe3)2, or  4 -butadiene. [27] Later, we found the same phenomenon when L2 is ArN=CH-CH=NAr (Ar = 2,6-diisopropylphenyl). [28] Since the Mo III complex is capable of reversibly trapping the propagating radical under OMRP conditions and since its concentration remains large under ATRP conditions, it is evident that both trapping mechanisms occur simultaneously, as illustrated by case b of Figure 4.

It is important to underline that the CpMoCl2L2 complexes contain halogen atoms in the coordination sphere. Therefore, after trapping the propagating radical to form a CpL2Cl2Mo-(m)xR dormant species, the latter could act, in turn, as an ATRP spin trap. If this happens, a mixture of R(m)x-Cl and a new molecule, a Mo III -capped polymer chain, R(m)x-MoClL2Cp, would form. This means that the OMRP system would leak over to the ATRP mechanism. However, polymer analyses by 1 H NMR and MALDI-TOF revealed, in that particular case, no chlorinated end-functions. [27] It can be imagined that a Cl atom transfer from a sterically encumbered R(m)x-MoClL2Cp complex to the propagating radical (itself a sterically hindered molecule) may be an unfavorable process relative to trapping by the less encumbered and more mobile CpMoCl2L2. Whether this phenomenon is true in general, however, is not known and the possible contamination by ATRP for a system set up under OMRP conditions, when the organometallic dormant species contains halogen, should always be considered as a possibility.

It is now pertinent to question whether other metal catalysts might display the same phenomenon of dual (ATRP/OMRP) control. A wide range of transition metal complexes have been proven capable of controlling the radical polymerization of various monomers under typical ATRP conditions. These include Ti III X3 (X = Cl, Br), [29] ReO2I(PPh3)2, [30] a variety of Fe II coordination compounds, [31][32][33][34] , RuCl2(PPh3)3, [2] OsCl2(PPh3)3, [35] Co(O2C7F15)2, [36] RhCl(PPh3)3, [37] Ni(PPh3)4, [38] Ni[o,o'(CH2NMe2)2C6H3]Br, [39] Pd(OAc)2/PPh3, [40] and SmCl2/lactic acid. [41] For the particular case of Cp2TiCl, it has been proposed that both ATRP and OMRP processes take place contemporarily (the reactions are carried out under "reverse" ATRP conditions, thermally decomposing AIBN in the presence of Cp2TiCl2). [42,43] A related system, involving a Cp2TiCl-induced oxirane ring opening as initiation process, also seems to involve the reversible formation of a Ti IV -capped dormant species. [44,45] The latter system is rather interesting because the initiation step is an ATRPlike activation, yielding Cp2(Cl)Ti-OCH2CH2 • , which is however irreversible because of the high homolytic strength of the Ti IV -O bond, due to the titanium oxophilic character. The controlling function is assured by an OMRP-type trapping process by additional Cp2TiCl.

Amongst the numerous reports of ATRP using Fe II systems, none to the best of our knowledge addresses the possibility of simultaneous OMRP control through the formation of organometallic Fe III dormant species. On the other hand, alkyl derivatives of Fe III have been described in the literature, though these are relatively unstable compounds. For instance, Kochi has shown that the one-electron oxidation of stable Et2Fe(bipy)2 yields [Et2Fe(bipy)2] + , which then decomposes in a variety of ways including Fe-C bond homolysis. [46] Also, (porphyrin)Fe III (alkyl) systems [47,48] were shown to readily undergo Fe-C bond homolysis. [49] The generation of Fe III alkyl species has been recently suggested as an event leading to catalytic chain transfer (CCT). [50] This phenomenon will be further discussed in a later section. The radical trapping capability of RuCl2(PPh3)3 has also not been considered, to the best of our knowledge. [51] Quite recently, the osmium complex OsCl2(PPh3)3 was shown to effectively control, like our CpMoCl2L2 system, the styrene radical polymerization under both ATRP and OMRP conditions. [35] Moreover, the reactions carried out under OMRP conditions did not reveal any chlorinated end-groups, whereas those carried out under ATRP conditions did not reveal any osmium-containing end-groups. Like with our CpMoCl2L2 complexes, the OsCl2(PPh3)3-controlled polymerization set up under OMRP conditions does not appear to be not contaminated by the ATRP process, whereas the polymerization set up under ATRP conditions must necessarily feature the OMRP trapping process. However, the system is energetically more stable as the sum of the Os II catalyst plus the halogen-capped dormant species, leading to a polymer product capped only by halogen atoms under ATRP conditions.

A given Os III -C bond is certainly expected to be stronger, in a homolytic sense, than the corresponding Ru III -C bond, but it would certainly be interesting to establish whether Ru II complexes display any efficient trapping capability toward propagating radicals.

Until recently, Co(II) complexes had been proven effective only as OMRP spin traps and as chain transfer catalysts (see below). However, a recent report shows the ability of cobalt(II) perfluorooctanoate to control the polymerization of styrene initiated by (1bromoethyl)benzene (BEB)typical ATRP conditions. [36] The same report shows the efficiency of the same system under typical OMRP conditions (thermal AIBN decomposition), although the authors incorrectly term this process "reverse ATRP" (no halogen is present, other than that of the CoCl2 precursor, transformed to Co(O2CC7F15)2 in situ by reaction with sodium perfluorooctanoate). A question remains, therefore, as to whether the Co(O2CC7F15)2/BEB process involves a combination of ATRP and OMRP (case b of Figure 4) or is a pure OMRP (case b of Figure 3). The authors indicate the presence of halogen end-groups (by NMR) and the absence of cobalt (by atomic absorption spectroscopy), suggesting formation of the ATRP dormant species (apparently, even for the polymer obtained under OMRP conditions!), but the evidence is not compelling since the polymer was treated in HCl-containing methanol before analysis. Another recent report shows the ability of CoCl2/Me6TREN to control the styrene and MMA polymerizations under typical ATRP conditions (2-EiBBr initiator), although the resulting polymers show high polydispersities (1.6-1.8). [52] The possible OMRP trapping is not mentioned in this report and the characterization of the chain-end functions (again, only by NMR) is not unambiguous.

Interestingly, the control of these processes is improved by addition of a few percent of FeBr3 or CuBr2. Under these conditions, obviously, a Fe II /Fe III or Cu I /Cu II -based ATRP mechanism, becomes operational. The authors propose that the higher oxidation state Fe or Cu complex traps more efficiently the growing radical chain and then is reoxidized by the Co III /Me6TREN species. Thus, the process would remain controlled by the Co II /Co III pair through an ATRP mechanism. However, amine-supported low-spin Co(III) complex are not strong oxidants [START_REF] Huheey | Inorganic Chemistry. Principles of Structure and Reactivity[END_REF][START_REF] Rock | [END_REF] and the redox equilibria shown in Scheme 6 are likely shifted to the side of the Co III species and the more active Fe II or Cu I catalyst. It then appears that the improved control upon addition of FeBr3 or CuBr2 likely originates from a switch to a Fe II /Fe III or Cu I /Cu II ATRP mechanism.

<Scheme 6>

The effect of irreversible radical trapping on initiator efficiencies

The situation corresponding to case c of Figure 3 and Figure 4 merits further analysis.

The considerations that will be developed here are in fact valid for any process where the propagating radical is trapped irreversibly by any reaction, not necessarily consisting of the irreversible formation of a metal-carbon bond. Namely, the propagating radical may also bind irreversibly to one of the ligands. Under these circumstances, as we concluded in a previous section, no polymerization should result. This is true, however, provided that the reaction products of the radical trapping reaction display no ability to get themselves involved in radical controlling equilibria. Note that the one-electron oxidative addition reaction is characterized by a MLn/R-Y stoichiometry of 2:1 (Scheme 3). However, most ATRP processes are set up with a 1:1 stoichiometry, under the assumption that only the atom transfer equilibrium of Scheme 1 is operative (i.e., the first step of Scheme 3). Under these conditions, and if the reaction leads to the oxidative addition products LnM-Y + LnM-R (or any other product of radical trapping), then 50% of the alkyl halide that was initially present remains unconsumed. At this point, a controlled polymerization may still take place, if the LnM-Y molecule, or the LnM-R molecule, or both, has the ability to function as ATRP catalyst. However, the initiator efficiency will be reduced, e.g. to 0.5 for this particular example of the 1:1 stoichiometry. The initiator efficiency is readily determined experimentally from the ratio of the theoretical and experimental molecular weights. For instance, when only half of the initiator molecules are able to give rise to growing polymer chains, the number average molecular weight for the polymer isolated at any given monomer conversion will be twice that calculated if the initiator had 100% efficiency.

There are several reports in the literature of unusually low initiator efficiencies, with no satisfactory interpretation being offered. In some cases, initiator efficiency values are not mentioned at all, but low values may be calculated as the ratio between the expected and the experimentally observed number-average molecular weights. We have reported ourselves a few systems that are characterized by quite low initiator efficiencies, for instance f = 0.55 for the styrene polymerization process shown in Figure 5. [55] A similar behavior is observed for the analogous polymerization of methyl acrylate using the same catalyst (f = 0.6). Two main reasons are recognized as responsible for low initiator efficiencies. The first one is related to a slow activation rate relative to the propagation rate. Under these circumstances, only a fraction of initiator molecules will be activated at a particular time and will grow at regular rate, while another fraction has not yet been activated. This results in a broad polydispersity.

However, as time goes on, all initiator molecules will eventually be activated and the efficiency factor will tend to one, see Figure 6(a). A second reason leading to low initiator efficiencies is a slow deactivation of the propagating radicals, e.g. a small value of kd in Scheme 1. In this case, the propagating radical concentration builds up to higher levels, inducing a significant amount of bimolecular terminations. This also results in a broad molecular weight distribution. In this case, the corresponding amount of initiator molecules is lost irreversibly and the initiator efficiency factor remains low throughout the polymerization process. However, the persistent radical effect will set in sooner or later, bringing back the process under control and the polydispersity will decrease with an increase of conversion, see The hypothesis of an irreversible one-electron oxidative addition as the factor reducing the initiator efficiency when using CpMoCl2(iPrNCHCHNiPr) as ATRP catalyst is proven by the following observations: [56] (a) the MA/CpMoCl2(iPrNCHCHNiPr)/AIBN system -typical OMRP conditions -leads to no polymer whatsoever, proving that all radicals generated by the initiator are irreversibly trapped by the metal complex; (b) whereas the MA/CpMoCl2(iPrNCHCHNiPr)/CH3CHICOOEt system (140:1:1; 30% in toluene)typical ATRP conditionsyields PMA with an initiator efficiency of 0.6 (vide supra), the corresponding system with a 154:1:0.5 stoichiometry does not yield any polymer. However, after the addition of one more equivalent of CH3CHICOOEt (1.5 equivalents overall), the polymerization starts in a controlled fashion and the experimental molecular weights are close to, in fact slightly smaller than, the theoretical ones (calculated on the basis of only the equivalent of initiator added later), see Figure 7. The fact that a polymerization still takes place, after one half of the initiator is irreversibly neutralized by the CpMoCl2(iPrNCHCHNiPr), means that the products of this irreversible reaction must themselves act as ATRP catalysts, generating polymer by the activation of the residual initiator.

<Figure 7>

The notion that the irreversible radical trapping may involve a ligand, rather than the metal center itself, is illustrated by a recent report of the cobaltocene (Cp2Co) activity in the ATRP of MMA, initiated by (CH3)2CBrCOOEt. [57] The authors propose, on the basis of well established organometallic chemistry, the transformations described in Scheme 7. However, they report an efficiency factor as low as 0.25 for a Co/RBr ratio of 1:1. Such a low value cannot be consistent with the suite of events described in Scheme 7, for which a minimum value of 0.5 should be observed for this particular stoichiometry (and, still, under the assumption that one of the two products are active ATRP catalysts). In addition, this schemes includes the hypothesis that the outer-sphere oxidation product, [Cp2Co] + Br -, is an efficient radical trap. Other work, however, shows that outer sphere complexes are less efficient radical traps in ATRP. [58,59] On the basis of other known organometallic chemistry of cobalt, is seems possible to rationalize this very low efficiency factor in another way. It is possible to imagine that an additional fraction of available initiator further oxidatively add to the Co I product of Scheme 7, yielding Co II and/or Co III products that could then operate as ATRP catalysts. Indeed, [CpCo(-Y)]2 and [CpCoX(-Y)]2 complexes have been described. [60] The [Cp2Co] + Br -system, on the other hand, may well be a dead end, inactive complex.

Various tests could further probe the mechanism of this reaction (for instance, by use of [Cp2Co] + Br -under "reverse" ATRP conditions).

<Scheme 7>

Extension to "difficult" monomers.

A current challenge of CRP is the possibility to control the radical polymerization of monomers that generate more reactive radicals (e.g. vinyl acetate, vinyl chloride, vinylidene chloride, ethylene, …). Much research effort has been devoted to the ATRP method, but success has remained very limited. The reason why these monomers are not easily controlled by ATRP is evident on the basis of the thermochemical considerations developed above and from the BDE values obtained for model R-Y molecules, see Table 1. [61] These molecules may be considered as models of propagating radicals derived from the monomers indicated in the last column. Thus, relatively stabilized radicals such as those of styrene and methyl acrylate form weaker bonds with the halogen atoms, whereas more reactive radicals such as those of vinyl chloride, vinyl acetate, or ethylene form stronger bonds. Since HATRP = BDE(R-Y) -BDE(LnM-Y) and the second term is constant for a given catalyst/halogen combination, the more reactive radicals will lead to more difficult activations for a given catalyst. The difference between styrene and vinyl acetate, for instance, is close to 10 kcal/mol for both Cl and Br derivatives. Clearly, these two monomers cannot be controlled by the same metal system. The BDEs of alkyl bromides are 8-10 kcal/mol weaker than those of the corresponding alkyl chlorides, but a similar differential is expected for the LnM-Y bonds, thus a change of halogen is not expected to provide a great variation of HATRP.

<Table 1>

It is possible to imagine that a suitable solution may be found by changing other factors in the catalytic metal complex: the ligands, the metal oxidation state, or the type of metal.

DFT calculations on model complexes have shown that the M-Y BDE decreases upon increasing the oxidation state, whereas it is much less dependent on the nature of two-electron (L-type) ligands. [27] Therefore, it may be predicted that the radical polymerization of more difficult monomers may become controllable by using lower oxidation state catalysts. In this respect, the ubiquitous Cu I system is rather limited, whereas more possibilities may exist for other metals, e.g. Ti, Mo, Re, … However, the effect of the nature of one-electron (X-type) ligands, coordination geometry, electronic configuration and spin state has not yet been studies in detail. Much useful information may be anticipated by systematic studies in this area. Another useful argument is that BDE's usually increase within a group in the order 3d < 4d < 5d. Thus, more difficult monomers may be controlled by changing the metal to a heavier congener (e.g. Mo to W, or Ru to Os). Systematic studies in this respect are also lacking.

There is, however, a potential problem with this approach. As M is changed in order to strengthen the M-Y bond, the M-R bond will strengthen too. Therefore, while the HATRP is displaced toward the suitable range, HOMRP will correspondingly increase and this increase may be such to render the radical trapping process irreversible and lead to the one-electron oxidative addition products. This negative effect may be somewhat attenuated by playing on the ligand steric properties. Indeed, since propagating radicals occupy a larger cone angle in the metal coordination sphere relative to the halogen atoms, the LnM-(m)xR BDE is expected to be more sensitive than the LnM-Y BDE to the ligand steric bulk. Thus, the selection of a sterically more encumbering ligand shell, perhaps in combination with lower oxidation states or with heavier metals, may hold some promise for matching the required HATRP while maintaining a relatively low HOMRP when using reactive propagating radicals.

There is an interesting report that must be mentioned and commented at this point.

Sawamoto has shown that the radical polymerization of vinyl acetate (VAc) can be controlled by [CpFe(CO)2]2 in the presence of the iodide initiators (CH3)2C(I)COOEt or CH3CH(I)OCOCH3, and the Al(OiPr)3 co-catalyst in anisole at 60°C. [62] This is a Fe I catalyst and would therefore be expected to form stronger bonds with I (leading to Fe II -I) relative to, say, Fe II (leading to Fe III -I). It is also a relatively unencumbered system, for which it is possible to envisage the formation of relatively strong Fe II -alkyl bonds. In addition, the metal-metal bonded dinuclear nature of the catalyst imposes the formation of the 17-electron and highly reactive CpFe(CO)2 metallaradical, right in the proximity of the propagating radical. It seems logical that a one-electron oxidative addition process should occurs to yield a mixture of Cp(CO)2FeI and Cp(CO)2Fe-(VAc)xR, see Scheme 8. Indeed, stable alkyl derivatives such as CpFe(CO)2Et are well known. [63,64] The experimental evidence that the initiator efficiency factor is close to 1 and that the isolated polymer contains iodo end groups shows that the one-electron oxidative addition equilibrium lies on the side of [CpFe(CO)2]2 and R(VAc)xI. This is as expected since oxidative addition of alkyl halides to [CpFe(CO)2]2

have not been reported. However, the control mechanism could well involve the active participation of the OMRP-type trapping process (case b of Figure 4). This could be easily checked by carrying out a radical polymerization under OMRP conditions, for instance from a suitable pre-synthesized CpFe(CO)2R complex such as CpFe(CO)2[CH(Ph)CH3], [65,66] of from [CpFe(CO)2]2 and a radical initiator.

<Scheme 8>

The ultimate way to regulate the concentration of free radicals for highly reactive propagating radicals, however, is through the OMRP equilibrium. As discussed above, the OMRP activation relies only on the BDE of the metal-carbon bond in the organometallic dormant species. It is always possible in principle to design a metal complex, through an intelligent selection of ligands, metal centers, oxidation state, etc., leading to an organometallic dormant species with BDE[LnM-(m)xR] = HOMRP in a suitable range for any monomer, since the OMRP equilibrium is not contaminated by other coupled equilibria (at least not in the absence of halogen atoms in the coordination sphere). In this respect, it is pertinent to underline the recent success in the controlled polymerization of vinyl acetate by this technique using Co(acac)2 as spin trap. [7] This is in contrast to other Co II systems (e.g.

with porphyrines of Schiff bases as ligands), which irreversibly trap the propagating radicals of VAc (HOMRP too high), but control the polymerization of acrylates. Conversely, while the HOMRP (acac)2Co-(m)xR is just right for m = VAc, it is too small for m = MA, leading to an uncontrolled process. There is, therefore, in the opinion of this Author, greater promise for the development of new controlled radical processes of the "difficult" monomers in OMRP than in ATRP. The difficulty associated to the formation of metal-capped polymers may be surmounted by subsequent transformations, which may allow the recovery and recycling of the metal complex. For instance, this is the case for the Co(acac)2-capped PVAc, which may be converted to a R2NO-capped polymer and Co(acac)2 by treatment with stable nitroxides. [67] The intervention of catalytic chain transfer (CCT)

Up until this point, we have neglected the catalyzed chain transfer, another phenomenon intimately related to the presence of transition metals capable of undergoing one-electron processes. Chain transfer can be catalyzed through the abstraction by MLn of a -hydrogen atom from the propagating radical, to yield a hydride complex, LnM-H, and a dead polymer chain with an unsaturated end-group, see Scheme 9. [68] <Scheme 9>

This phenomenon has its own practical relevance because it gives access to macromonomers with a controlled average molecular weight. Thus, considerable effort is devoted to a search of MLn systems that exhibit high transfer rate constants (ktr in Scheme 9)

for a number of industrial applications. [69] However, this phenomenon interferes with the chain growth process when "pseudo-living" polymerizations are sought via ATRP or OMRP methods. To resume, a LnM complex apt to increase both its electron count and its coordination number by one unit has the potential to simultaneously exert three functions:

ATRP catalyst, OMRP spin trap, and radical chain transfer catalyst. Our above mentioned study of the CpMoCl2L2 systems in the controlled polymerization of styrene has highlighted for the first time the interplay of these three functions. [27] This phenomenon can be integrated in a general scheme which encompasses the metal-mediated radical generation and concentration regulation, the living chain grow, the chain transfer, and the one-electron oxidative addition, as depicted in Scheme 10.

<Scheme 10>

A first interesting consideration concerns which conditions are most appropriate for the manifestation of CCT. On the basis of the general energetic schemes in Figure 1 and Figure 4, the equilibrium concentration of the potential chain transfer catalyst will be low under OMRP conditions (the equilibrium is largely shifted toward the organometallic dormant species), whereas it will be high under ATRP conditions (the equilibrium is largely shifted toward MLn + halogenated dormant species). The latter statement is correct, obviously, provided that the OMRP trapping process does not predominate. Thus, we can readily conclude that CCT is favored under two circumstances: (a) under OMRP conditions, when HOMRP is too small, (case a of Figure 1); (b) under ATRP conditions, provided that the OMRP trapping process is not too important (e.g. case a of Figure 4). The most efficient catalytic processes are those carried out under OMRP conditions, with MLn complexes that form very weak metal-carbon bonds. This is because both free radical and catalyst concentrations may be relative high at the same time. Under ATRP conditions, on the other hand, the MLn concentration may be high, but the free radical concentration is kept low by the ATRP (and possibly also the OMRP) activation/deactivation equilibrium. The most active catalysts appear to be those based on cobaloximes, containing no halogen ligands in the system. [START_REF] Gridnev | Vysokomolekulyarnye Soedineniya[END_REF]71] Catalytic activity, however, has also been shown for other systems such as (C5R5)Cr(CO)3 (R = H, Me, Ph), Cp*Fe(CO)2, heavier analogues, the tris(pyrazolyl)borato derivatives TpMo(CO)3 and Tp*Mo(CO)3, [72][73][74] and certain (diimine)Fe systems. [75] The relative ability of a transition metal complex to abstract a -H atom and to form a M-C bond obviously depends on the nature of the propagating radical. Certain radicals have a greater propensity than others to transfer a hydrogen atom. For a given radical, on the other hand, the type of metal complex affects the relative trapping/transfer ratio in ways that are not yet completely understood. Steric effects appear to be quite important to the extent that they have a greater effect on the formation of the organometallic dormant species than on the hydrogen atom transfer process yielding the hydride intermediate. Another potential side reaction consuming the chain transfer catalyst and equally disfavored by a greater steric encumbrance is the dimerization of MLn. Recent work by Norton on (C5R5)Cr(CO)3-type catalysts has shown that steric bulk enhances CCT activity by disfavouring the dimerization to M-M and the formation of M-R, in favor of the H-atom transfer reaction. [74] A recent study in our laboratory has shown that the complexes MoX3(PMe3)3 (X = Cl, I) have a different trapping ability for free radicals generated under OMRP conditions: the trichloride complex significantly slows down the AIBN-initiated radical polymerization of styrene (though the observed molecular weight and n w M M / are typical of an uncontrolled process), whereas the triiodide complex has essentially no effect. [76] This is attributed to the greater steric bulk of the three iodide ligands relative to the corresponding number of chlorides. Consequently, when the polymerization is carried out under ATRP conditions [MoX3(PMe3)3 + alkyl halide initiator], the equilibrium concentration of MoX3(PMe3)3 during the polymerization process will be greater for X = I than for X = Cl. This difference can rationalize the experimental observation of "living" growth in the trichloride case and CCT in the triiodide case. Incidentally, the MoX3(PMe3)3 complexes are not "stable free radicals", but rather contain three unpaired electrons (spin quartet state).

A recent report proposes an intriguing new pathway for CCT, apparently associated to changes of spin state: four-coordinate Fe II complexes of type FeCl2(RN=CHCH=NR) control the polymerization of styrene under ATRP conditions. However, whereas derivatives with R = alkyl yield "living" chain growth, those with R = aryl yield CCT. [77,78] This phenomenon correlates with the spin state of the atom transfer product, the Fe III complex FeCl3(RN=CHCH=NR) (S = 5/2 for R = alkyl, 3/2 for R = aryl). The authors propose that the same relative spin state preference holds for the organometallic dormant species, whose formation would then be preferentially observed for the aryl-substituted complexes. Both Hartree-Fock calculations and the experimentally observed greater stability (higher decomposition temperature) of the aryl-substituted FeCl2(CH2Ph)(RN=CHCH=NR) complexes are in support with this view. The organometallic dormant species would then lead to chain transfer via a -hydride elimination process, as shown in Scheme 11. Thus, this particular metal complex system appears to function by a combination of one-electron and two-electron processes, suitably orchestrated by the spin state. In this interesting variant of CCT mechanism, the catalytic ability is not related to the weakness of the M-R bond but In a previous study, we found that complexes CpMoCl2(PMe3)2 and CpMoCl2(dppe) lead to a "living" polystyrene chain growth under ATRP conditions, whereas the very similar complex CpMoCl2( 4 -C4H6), under identical conditions, leads to CCT. [27] However, in this case we observed a "living" polymer growth (no CCT) when working under OMRP conditions, for all the above three complexes. This could be explained by invoking a lower concentration of the chain transfer catalyst under OMRP conditions. However, the reason for the different outcome of the different systems under ATRP conditions may not be so straightforward in this case. The attempted rationalization advanced in our original report was based on a greater steric impediment in the -H transfer process for the more encumbered phosphine complexes. However, this impediment should be even stronger for the formation of the organometallic dormant species, yet there is control in all cases under OMRP conditions, showing that the metal-carbon bond is strong enough and forms rapidly for all systems. If the steric factor is important for the generation of the organometallic dormant species, then HOMRP should be smaller for the bulkier phosphine system, resulting in a greater concentration of chain transfer catalyst, which is inconsistent with the experimental result. A CCT mechanism involving -hydride elimination, like for the above-mentioned Fe system (Scheme 11) also appears unsuitable, because in that case more CCT should be observed under OMRP condition, contrary to the experimental result. Clearly, we have not yet achieved a complete understanding of the key parameters that regulate chain transfer vs. radical trapping.

Conclusions and outlook

A thorough understanding of one-electron transition metal reactivity is of fundamental importance for the correct interpretation of experimental results in metal-mediated radical polymerization processes. As we have shown in this Minireview, a variety of situations may result, depending on the thermodynamic parameters of the two fundamental steps of the oneelectron oxidative addition process (Scheme 5) and on the kinetics of the catalyzed chain transfer (Scheme 9). The two fundamental steps of the one-electron oxidative addition correspond to the activation/deactivation equilibria of the two different metal-mediated controlled radical polymerization processes, ATRP and OMRP. These depend, from the thermochemical point of view, on the homolytic bond dissociation energies of three bonds:

the LnM-Y bond of the ATRP spin trap, the R(m)x-Y bond of the ATRP dormant species, and the R(m)x-MLn bond of the OMRP dormant species. Knowledge of these three bond dissociation energies for a given system is essential to predict whether it will lead to an uncontrolled polymerization, to an irreversible one-electron oxidative addition, to no polymerization at all, or finally to a controlled polymerization and, in the latter case, whether the control will be insured solely by the ATRP activation/deactivation equilibrium, or solely by the analogous OMRP equilibrium, or by a combination of both. We have shown that the interplay of the two equilibria, accompanied by an irreversibility of the OMRP-type radical trapping process, may lead to low initiator efficiency factors, provided the oxidative addition products are themselves active ATRP catalysts. We have explored the necessary conditions that must be met in order to achieve the controlled polymerization of monomers producing more reactive radicals. As we have argued, greater flexibility in terms of the radical controlling equilibrium position is provided under OMRP conditions by systems devoid of halogen atoms, although a drawback of this method is the necessary post-polymerization treatment to remove the metal from the chain ends. Finally, we have explored the conditions that regulate the extent of metal-catalyzed chain transfer, although a complete understanding of the parameters that regulate the relative extent of OMRP trapping and chain transfer events is still lacking.

Radical processes mediated by transition metal complexes will certainly continue to be heavily investigated in the future. These processes need complexes that are capable to increase both their electronic configuration and their coordination number by one unit, a condition that, given an appropriate coordination sphere (steric and electronic properties of the ligands, coordination geometry and spin state), can be satisfied for many different metals in a variety of different oxidation states. Further advance in this area will require a better understanding of how the above mentioned parameters (ligand properties, etc.) independently affect the three basic bond dissociation energies and the rate of -H transfer. A key role in this game may be played by computational work. ) is also shown as triangles. Data are taken from ref. [55] . ) and CH3CHICOOEt as initiator: 0.5 equiv initially, then 1 additional equivalent after ca. 30 h. The straight line represents the theoretical Mn for a living process (f = 1). [56] Scheme 1. Mechanism of action in ATRP a] Values taken from reference [61].

Captions for Figures and Schemes

Figures and Schemes

  carbon bond, thus HOMRP = BDE[LnM-(m)xR]. The ATRP equilibrium formally involves the hemolytic rupture of the R(m)x-Y bond and the formation of the Y-MLn bond, therefore

Figure 6 ( 2 )

 62 Figure 6(b). Clearly, the process represented in Figure5does not belong to either of these

  rather to the availability of open coordination sites cis to the M-R function, capable to induce the -hydride elimination process. It is worth to underline that the -hydride elimination is mechanistically related to the olefin insertion into M-H and M-R bonds, i.e. the key step of olefin coordination polymerization. The accomplishment of both olefin coordination/insertion and homolytic bond cleavage using the same metal complex might open up new doors in polymer chemistry. <Scheme 11>

Figure 1 .

 1 Figure 1. Possibilities in OMRP, depending on the HOMRP = BDE[R(m)x-MLn].

Figure 2 .

 2 Figure 2. Possibilities in ATRP, when HATRP is too large.

Figure 3 .

 3 Figure 3. Possibilities in ATRP, when HATRP is too small.

Figure 4 .

 4 Figure 4. Possibilities in ATRP, when HATRP is just right.

Figure 5 .

 5 Figure 5. Experimental (squares) and theoretical (solid line) number average molecular weights (Mn) for the polymerization of styrene under ATRP conditions (CpMoCl2(iPrNCHCHNiPr)/Al(OiPr)3/CH3CHICOOEt) in toluene solution (30% v/v) at 90 °C. The polydispersity index ( n w M M /) is also shown as triangles. Data are taken from ref.[55] .

Figure 6 .

 6 Figure 6. Typical evolution of molecular weight (triangles) and polydispersity (hexagons) in the presence of low initiator efficiency: (a) slow initiation rate; (b) slow deactivation rate. The straight line represents the theoretical Mn for a living process (f = 1).

Figure 7 .

 7 Figure 7. (a) kinetics; (b) evolution of the molecular weight (squares) and polydispersity index (triangles) as a function of conversion, for a PMA obtained in toluene (30% v/v) at 80°C with MA/CpMoCl2(iPrNCHCHNiPr)/Al(OiPr)3 (MA/Mo/Al = 154:1:1) and CH3CHICOOEt as initiator: 0.5 equiv initially, then 1 additional equivalent after ca. 30 h. The straight line represents the theoretical Mn for a living process (f = 1).[56]

Scheme 2 .Scheme 3 .Scheme 4 .Scheme 5 .Scheme 6 .

 23456 Scheme 2. Mechanism of action in reversible addition-cleavage polymerization

[

  Figure 1

2 RR 4 - 5 FeBr 2 /

 2452 Figure 3

Table 1 .

 1 Computed (BP86/6-31G**) Bond Dissociation Energies (kcal/mol) of representative R-Y molecules as propagating radical models for the monomers shown.[a]

	R	Y = Cl	Y = Br monomer
	CH3CH2	83.1	73.1	ethylene
	CH3CH(OCH3)	82.2	73.3	vinyl ether
	CH3CH(O2CCH3)	79.5	69.5	vinyl acetate
	CH3CH(Cl)	76.0	66.0	vinyl chloride
	CH3CH(COCH3)	71.0	62.9	vinyl ketone
	CH3CH(CO2CH3)	69.9	61.4	methyl acrylate
	CH3CH(Ph)	68.5	59.9	styrene
	(CH3)2C(CO2CH3)	66.9	58.9	methyl metacrylate
	CH3CH(CN)	65.9	57.2	acrylonitrile

Acknowledgment

I am grateful to the polymer chemists who have sparked my interest in controlled radical polymerization (Yves Gnanou and Jerome Claverie) and those with whom I have had fruitful collaborations (Krzysztof Matyjaszewski and Dave Haddleton), as well as the students and post-doctoral fellows who have invested their time in my laboratory in this research area (Erwan Le Grognec, François Stoffelbach, Sébastien Maria, José Mata). Funding provided by the CNRS, the Conseil Régional de Bourgogne, and the ANR (Projet n° NT05-2_42140) is also gratefully acknowledged. Theoretical calculations carried out in our group were made possible by grants of computing time by CINES (Montpellier) CALMIP (Toulouse).

Scheme 7. Proposed mechanism for the cobaltocene-catalyzed ATRP of MMA.

Scheme 8.

Oxidative addition of alkyl halides to [CpFe(CO)2]2.

Scheme 9.

Mechanism of catalyzed chain transfer (CCT).

Scheme 10. A global scheme for metal-mediated radical generation and one-electron processes.

Scheme 11. A proposed CCT pathway involving -hydride elimination.

Text for


José A Mata

Sébastien Maria

Jean-Claude Daran

Rinaldo Poli
email: poli@lcc-toulouse.fr

Synthesis and characterization of MoOI2(PMe3)3, and use of MoOX2(PMe3)3 (X = Cl, I) in controlled radical polymerization

Keywords: Molybdenum, oxo ligands, phosphine ligands, halide exchange, atom transfer radical polymerization

Complex MoOCl2(PMe3)3 smoothly reacts with NaI in acetone to produce MoOI2(PMe3)3 in good yields. The geometry of the compound is mer,cis octahedral, i.e. identical to that of the dichloride precursor, as shown by NMR spectroscopy and by an Xray crystallographic study. Electrochemical investigations of MoOX2(PMe3)3 show irreversible oxidation waves at Ep,a = +0.18 and +0.39 V for X = Cl and I, respectively.

A study of the halide exchange between MoOCl2(PMe3)3 and NaI, or between MoOI2(PMe3)3 and Bu4NCl, shows two equilibrated isomers for the mixed halide intermediate MoOICl(PMe3)3. The diiodide complex exchanges rapidly the iodide ligands with chlorides upon dissolution in chloroform at room temperature, and with bromide from 1-phenyl-1-bromoethane (BEB) under more forcing conditions. The equilibrium favors the softer halide (I) on carbon and the harder one (Cl or Br) on Mo IV .

Both oxo compounds catalyze the atom transfer radical polymerization (ATRP) of styrene in combination with the BEB initiator, yielding polymers with quite narrow molecular weight distributions (down to 1.11). The apparent polymerization rate constant is ca. doubled in the presence of 1 equivalent of the Al(OiPr)3 cocatalyst. On the other hand, the system is not capable to efficiently control the radical chain growth for methyl acrylate polymerization.

Introduction

In recent years, a number of coordination and organometallic compounds based on different transition metals have been proven capable to control the radical polymerization of activated olefins. The phenomenon is accomplished by maintaining the active radical concentration at very low levels though a fast reversible equilibrium with a dormant species, thereby reducing the incidence of bimolecular terminations. The equilibrium may implicate a halogen atom transfer (see Scheme 1), thus the term Atom Transfer Radical Polymerization (ATRP). [1,2] In this respect, the process corresponds to a transition metal catalyzed multiple insertion of olefins into the carbon-halogen bond of the initiator molecule. A metal complex, however, may also yield a dormant species by reversibly forming a metal-carbon bond (see Scheme 2). This process is termed, somewhat inappropriately, Stable Free Radical Polymerization (SFRP). Indeed, the stable transition metal complex acting as a spin trap need not possess a radical character.

We have recently proposed a new term for this process, Organometallic Radical Polymerization (OMRP), because it involves the reversible formation of a metal-carbon bond. [3] <Scheme 1 and Scheme 2>

The various metals so far shown to be effective include Ti III , [4] Mo III , [5,6] Re V , [7] Fe II , [8,9] Ru II , [10] Ni II , [11] Ni 0 , [12] and Cu I . [13] Although no system currently appears to parallel the activities and practical advantages of certain Cu I complexes, the investigation of other metal systems is useful in order to shine additional light on the mechanistic details of the process. For instance, by using CpMoCl2L2 complexes (L = tertiary phosphines or L2 = diene), we have demonstrated how the two trapping processes in Scheme 1 and Scheme 2 may simultaneously take place when the polymerization is carried out under ATRP conditions. [5] Furthermore, the use of MoX3(PMe3)3 catalysts (X = Cl, Br, I) has revealed that the oxidized metal complex (ATRP spin trap) need not be a thermodynamically stable species in order for the catalyst to ensure a good control. [14] In fact, whereas the Mo IV complexes MoX3Y(PMe3)3 (X, Y = Cl, Br) could be isolated and fully characterized, no such complex containing an iodide ligand was shown to exist, because it would decompose by an internal redox process to a Mo(III) complex and I2.

As a further example, a comparison of the rates of halide exchange between various ATRP catalysts [i.e. MoX3(PMe3)3, CpMoCl2(RN=CHCH=NR) (R = iPr), RuCl2(PPh3)3] and initiator molecules, in the presence and in the absence of Al(OiPr)3, has revealed two possible mechanisms for this exchange process. [15,16] The results of DFT calculations have shown that the Mo-X and Mo-R bond strengths decrease upon raising the metal oxidation state, the computed values suggesting that the Mo III/IV redox couple is ideally placed to insure appropriate equilibrium positions for both ATRP and SFRP processes. Indeed, complex CpMo(CH2SiMe3)2( 4 -C4H6) does not yield radical polymerization, showing that this particular Mo III -R bond is too strong. [5] On the other hand, preliminary evidence, which will be reported separately, has indicated the possible implication of Mo IV/V couples in ATRP. For this reason, we have decided to test some typical Mo(IV) coordination compounds as ATRP catalysts. In this paper, we report our results obtained by using a well known Mo(IV) coordination compound, MoOCl2(PMe3)3. In addition, we also report the results obtained with the related compound MoOI2(PMe3)3, whose synthesis and characterization is reported here for the first time. Our interest in this compound was twofold. From the point of view of ATRP catalysis, we have shown that some iodo derivatives of certain Mo(III) ATRP catalysts, e.g. CpMoI2(RN=CHCH=NR) (R = iPr) are more active than the corresponding chloro derivatives. [15] From the coordination chemistry point of view, we have demonstrated the incompatibility between iodo ligands and Mo(IV) in complexes of type MoXnY4-n(PMe3)3 (vide supra). [14] Yet, stable iodide derivatives of molybdenum in higher oxidation states do exist when the metal electronic deficiency is satisfied by strongly donor ligands (e.g. Cp*Mo IV I4 -and Mo V OI4 -). [17,18] Therefore, it was of interest to see whether compound MoOI2(PMe3)3 would be stable, to investigate its redox properties, and to test its catalytic properties in ATRP.

Results and Discussion

(a) Syntheses and characterization

The new complex MoOI2(PMe3)3 was obtained by halide exchange from the dichloride precursor as described in Scheme 3. The reaction is driven by the precipitation of the less soluble NaCl and is relatively rapid at room temperature. The spectroscopic properties of the isolated compound indicate an identical structure to that of the precursor. In particular, the 31 P NMR spectrum shows two types of phosphine ligands in a 2:1 ratio, whereas the virtual triplet pattern for the 1 H NMR resonance of the two equivalent PMe3 ligands establishes their mutual trans arrangement (the spectral parameters are collected in Table 1). This clearly identifies the mer arrangement of the Mo(PMe3)3 moiety. The 31 P NMR spectrum is qualitatively different in acetone-d6 and in C6D6. Whereas the doublet-triplet pattern expected for an AX2 spin system is relatively unperturbed in the former solvent, a more complex pattern, which is indicative of second order coupling for an AB2 system, is evident in C6D6, see Figure 1. This difference is caused by the closer proximity of the two 31 P resonances in C6D6. The values reported in Table 1 result from the successful simulation of the spectrum.

<Figure 1>

The relative cis arrangement of the two iodide ligands is unambiguously confirmed by the single crystal X-ray diffraction study, but is also indicated by the NMR monitoring of the halide exchange process, notably by the presence of two different stereoisomers for the mixed halide complex (vide infra). When using a sufficiently large excess of NaI (> 5 equiv), the isolated product is devoid of any mixed halide impurity. It is also devoid of the known [19] Mo III complex MoI3(PMe3)3, a problem that often affects its dichloride precursor MoOCl2(PMe3)3. [20] Indeed, the latter is usually obtained from MoCl4(THF)2 and PMe3 in the presence of water, [21] giving a product whose color (varying from blue to green) depends on the level of contamination by yellow MoCl3(PMe3)3. By replacing the tetrachloride Mo IV precursor with the bis(ether) complex, MoCl4(Et2O)2, [22] we have obtained a blue material that is also contaminated by minor amounts of the trichloride Mo III complex, as evidence by 1 H NMR. A better synthesis of pure MoOCl2(PMe3)3 is that reported by Galindo et al., from MoO2Cl2(MeOCH2CH2OMe) and PMe3. [23] Our synthesis of MoOI2(PMe3)3 was carried out from a dichloride precursor obtained by the latter procedure. <Scheme 3 and Table 1> A view of the molecule resulting from the X-ray diffraction study is shown in Figure 2. The relevant geometric parameters are collected in Table 2. The thermal ellipsoids are well behaved, showing no evidence for positional disorder, in agreement with the NMR evidence for the absence of MoI3(PMe3)3. In addition, the presence of iodide in the oxo position would have a notable effect on the refined Mo=O distance, whereas the experimental value of 1.685(4) Å is quite short. Moreover, this distance is identical within experimental error to the distance reported for the corresponding complex cis-mer-MoOCl2(PMe2Ph)3 [1.675(3) Å]. [20] Therefore, these results show that there is no compositional I, O disorder, contrary to previous reports for the dichloro analogues. [20] The Mo-P distances [2.494-2.513 Å] are within the normal range for related compounds. <Figure 2 and Table 2> From a benzene/n-pentane solution of MoOCl2(PMe3)3 set aside for several days, crystals of compound [MoOCl(PMe3)(OPMe3)]2(-O) were obtained, possibly by adventitious contact with air and moisture. Since the resulting structure is more accurately determined and is in a different crystal system than one reported previously for the same compound, [24] we report it here but do not comment on it any further. No analogous compound was obtained from the corresponding oxo diiodide complex.

The existence and stability of the MoOI2(PMe3)3 compound [cf. the non existence of MoI4(PMe3)3] [14] indicates that the oxo ligand is a much stronger electron donor than two iodide ligands toward the Mo center, raising the energy of the LUMO and thereby making the metal center a less powerful oxidizing agent. Indeed, qualitative MO considerations lead to the expectation of an interaction as shown in Scheme 4, where the energy of the dxz and dyz orbitals of the pseudo-t2g set (octahedral coordination environment) is raised through the -donation mechanism.

<Scheme 4>

An electrochemical investigation of compounds MoOX2(PMe3)3 yields irreversible oxidation processes at Ep,a = +0.18 and +0.39 V for X = Cl and I, respectively, see Figure 3. The lack of reversibility indicates the occurrence of a fast chemical process after oxidation to the cationic Mo V complex [MoOX2(PMe3)3] + , probably involving solvent coordination. The voltammogram recorded on the chloride complex shows that a new and broad cathodic peak is generated after the oxidation peak (this feature is not observed during the first cathodic scan prior to the oxidation peak). In the presence of chloride ions, the oxidation peak shifted slightly toward less positive potentials, but the overall shape of the voltammogram was otherwise unaffected. This result can be rationalized by a follow-up coordination of Cl -to [MoOCl2(PMe3)3] + , with generation of the putative ATRP spin trap, MoOCl3(PMe3)3. The same phenomenon was previously observed by us for the oxidation of MoX3(PMe3)3 in the presence of X -(X = Cl, Br) and verified by an independent study of the isolated MoX4(PMe3)3 products. [14] In the present case, a similar verification is impossible because, to the best of our knowledge, complex MoOCl3(PMe3)3 is non existent. Indeed, the voltammetric response in the presence of Cl - shows that such compound is unstable under the condition of the CV experiment (THF solution) and decomposes in a similar way to that of the [MoOCl2(PMe3)3] + complex.

<Figure 3>

Concerning the voltammogram of the diiodide complex, the greater oxidation potential relative to the dichloride analogue parallels the corresponding shift for the Mo III complexes MoX3(PMe3)3 upon going from X = Cl to I. [14] Note the presence of a small reversible wave at ca. -0.1 V in this voltammogram. This corresponds to the oxidation of I -to I3 -(as verified by the deliberate addition of I -to the solution) and grows with the time lapse between the solution preparation and the voltammetric study. Even upon rapidly recording the voltammogram (as in the example shown in Figure 3), this feature could not be completely eliminated, suggesting the establishment of a slow ligand substitution equilibrium between iodide and THF. The other important observation is that the oxidation of I -occurs at a less positive potential relative to the oxidation of the Mo IV oxo complex. This has implications for the ATRP process (vide infra).

It is interesting to compare the oxidation potentials of MoOX2(PMe3)3 with those of the corresponding MoX4(PMe3)3. Because of its non-existence, the E1/2 for the oxidation of MoI4(PMe3)3 could not be established, but those of the analogous MoX4(PMe3)3 complexes are -0.15 V (X = Cl) and -0.06 V (X = Br), allowing to extrapolate a value of ca. +0.05 for X = I. Thus, we can observe that the replacement of two X ligands by one O ligand has the effect of raising the oxidation potential by ca. 0.3 V. This change can be related to a lower energy of the HOMO for the oxo compound (Scheme 4), resulting from the greater electronegativity of the oxygen atom. In other words, there is less electron donation by one O relative to two I atoms via the  bonding mechanism.

(b) NMR study of the halide exchange reactions

The conversion of the dichloride to the diiodide complex was followed by 1 A preferential formation of A would seem consistent with a kinetic control of the halide exchange, since the oxo ligand should exert a stronger trans effect relative to the PMe3 ligand. However, the conversion in the opposite direction, which occurs in the presence of Bu4N + Cl -in acetone-d6, yields again isomer A as the major intermediate. If the exchange process had been under kinetic control, the prevalence of isomer B was expected.

Therefore, we conclude that the halide exchange process is under thermodynamic control and that the position trans to the oxo ligand is preferentially occupied by the softer iodide ligand.

In order to gain further insight on this issue, we have also carried out a reaction between MoOI2(PMe3)3 and NaBr in acetone-d 6 . The reaction qualitatively follows the same pathway (see Scheme 6). The use of a small excess amount of NaBr (ca. 1. and B (very minor), though the exchange is eventually complete. In dichloromethane, this exchange is much slower, since no significant amount of mixed halide species (A) is formed after 20 h stirring at room temperature. This phenomenon could either take place by an internal nucleophilic substitution assisted by solvent coordination to the metal center, as we have recently pointed out for other transition metal complexes, [16] or by reversible atom transfer. In the latter case, the phenomenon would suggest that this system could serve as a good catalyst in atom transfer radical polymerization. This has indeed been verified experimentally (vide infra).

Since MoOI2(PMe3)3 rapidly exchanges its halogen atoms with chloroform, we have also tested the halogen exchange with the initiator molecule used in ATRP, namely 1-bromethylbenzene, PhCH(Br)CH3 (BEB). The reaction (Scheme 7) was followed by 1 H NMR, since the methyne proton is very sensitive to the nature of the halogen atom.

At 90°C in toluene-d8 and using a stoichiometric BEB/Mo ratio (same conditions used for the ATRP experiments, vide infra), the exchange process is essentially complete in 40 minutes, see Figure 5. A slower exchange is also observed at room temperature. 31 P NMR monitoring shows that the reaction proceeds through the same intermediates seen when using NaBr, namely MoOBrI(PMe3)3 (A' and B', the former being predominant).

The quantitative nature of the exchange illustrates the thermodynamic preference for placing the heavier halogen on the carbon atom and the lighter one on the metal. On the basis of Pearson's HSAB theory, [START_REF] Huheey | Inorganic Chemistry. Principles of Structure and Reactivity[END_REF] this result may be interpreted in terms of a harder character for the Mo IV center with respect to carbon. 11 metals, including CpMoCl2(iPrN=CH-CH=NiPr), [6] MoX3(PMe3)3, [14] ReIO2(PPh3)2, [7] CpFeX(CO)2 (X = Br, I), [8] RuCl2(PPh3)3, [10] NiBr2(PPh3)2, [11] Ni(PPh3)4, [12] and CuBr/bipy. [13] The proposed mode of action of this cocatalyst consists of a shift of the atom transfer equilibrium, caused by the stronger interaction of the Al Lewis acid with the halogen lone pairs in the more polarized Mo-X bond, relative to the C-X bond of the initiator/dormant species. [16] Table 3 shows that Al(OiPr)3 is also effective in conjunction with the MoOX2(PMe3)3 catalysts, although by a smaller margin. In each case, the apparent polymerization rate approximately doubles when one equivalent of Al(OiPr)3 is added. Note also that the diiodide system is about twice as fast as the dichloride system.

<Table 3>

For each dihalide catalyst and in the absence of Al(OiPr)3, the experimentally determined number-average molecular weights (Mn) of the polymer grows linearly with the monomer conversion and corresponds closely to the theoretical molecular weights on the basis of the monomer/initiator ratio, see Figure 6 (a andc). These characteristics are as expected for a controlled radical polymerization process. Furthermore, the polydispersity indexes (PDI = n w M M /

) are quite low, down to 1.11. These are the lowest PDI's reported to date for a Mo-based catalyst, underlining the excellent level of control exerted by these oxomolybdenum(IV) complexes. At high conversions, the experimental Mn is slightly lower than the theory for the dichloride system, whereas it is slightly higher for the iodide system. The former effect could result from the slight intervention of chain transfer processes, whereas the latter one could find a rationalization in an initiator efficiency slightly lower than 100% (f < 1). In the presence of Al(OiPr)3, the dichloride catalyst appears to insure a less effective control, since the PDI's are higher (ca. 1.3 at 75% conversion), although Mn still grows linearly with conversion, see Figure 6(b), and the monomer consumption follows first order kinetics. In addition, the experimental Mn is ca. two times greater than the calculated value (i.e. f = ca. 0.5). The diiodide catalyst also controls the polymerization a little less effectively in the presence of Al(OiPr)3, but f remains close to 1, see Figure 6(d). At the moment, we are not able to rationalize why the nature of the halogen has such a large effect on the initiator efficiency, and only for the experiments carried out in the presence of Al(OiPr)3.

<Figure 6>

The putative coordination compounds produced by the atom transfer process could not be characterized. From the electrochemical study, we have established that the oxidation potentials of complex MoOCl2(PMe3)3 is less positive that those of the free Cl - or Br -ions. [14] Therefore, the oxidized complex [MoOCl2(PMe3)3] + is compatible with the presence of these ions or, in other words, complex MoOCl2X(PMe3)3 (X = Cl, Br) is predicted to be a stable system from the redox point of view. Nonetheless, the electrochemical study indicates instability for MoOCl3(PMe3)3, at least in the THF solvent used for the cyclic voltammetric investigation. On the other hand, the free iodide ion is oxidized at a less positive potential than both complexes MoOX2(PMe3)3 (X = Cl, I). This means that the putative ATRP spin trap, namely a Mo V oxo complex of type MoOX2Y(PMe3)3, is thermodynamically unstable when either X or Y is an iodine atom, relative to an internal redox process leading to the reduction of Mo V to Mo IV and oxidation of I -. This observation might seem inconsistent with the ability of MoOI2(PMe3)3, in combination with a bromide initiator, to catalyze the ATRP of styrene with a controlled chain growth. However, our recently published investigation of the MoI3(PMe3)3 ATRP catalyst, for which the same apparent contradiction was observed, provides the basis to rationalize this phenomenon. [14] In essence, the oxidized MoOI2Br(PMe3)3 complex is capable to efficiently act as a spin trap for the growing radical chain for kinetic reasons, its decomposition being a bimolecular reaction and involving a greater activation barrier than the radical trapping process.

The polymerization of methyl acrylate was not well controlled by these Mo IV oxo complexes. experiments carried out with each dihalide complex at 90° with ethyl 2-iodopropionate as an initiator gave very fast polymerizations, with greater than 90% conversion after the first 300 minutes. The reaction was therefore repeated at a lower temperature (35°C). Even under these conditions, however, the polymerization process remained quite fast. With complex MoOCl2(PMe3)3 the conversion was initially high and slowed down at later times (the monomer decay did not follow a 1 st order rate law), reaching a ca. 60% conversion in 6 days. With complex MoOI2(PMe3)3, the process was even faster, reaching 100% conversion in ca. 18 h. In both cases, the molecular weights were much greater than expected on the basis of a controlled process, and the PDI's were quite high (> 1.5), see for instance Figure 7 for the chloride system. This behavior is consistent with an atom transfer process that is not sufficiently shifted toward the dormant species. Further studies under different conditions (nature of the initiator, different co-catalysts) will need to be carried out in order to optimize this process.

<Figure 7>

In order to check whether the OMRP equilibrium also plays a role in the ATRP catalyzed by MoOX2(PMe3)3, the polymerization of styrene was also carried out using AIBN as a thermal initiator in the presence of these compounds. After heating at 100°C for 30 minutes to decompose most of the initiator, the temperature was set to 80°C for the rest of the experiment. The polymerizations were not controlled, with conversions reaching 30% within the first 4 h and then increasing slowly to 65% over the next 4 days.

There was no first order monomer consumption, and the PDI's remained consistently high throughout the experiment. These results are typical of an uncontrolled free radical process. Only a minor retardation effect was noted in the presence of the metal complexes, e.g. see Figure 8. This suggest the possibility of a radical trapping process by the MoOX2(PMe3)3 complex, but the stability of the adduct is insufficient. In conclusion, the MoOX2(PMe3)3 complexes are not capable to efficiently trap the reactive radical of polystyrene and form sufficiently persistent dormant species.

<Figure 8>

Conclusions

The current investigation has shown, for the first time, the ability of Mo IV complexes to function as ATRP catalysts. Both the well known dichloro complex and its newly developed diiodo analogue are equally capable to produce polystyrenes with rather narrow polydispersities. The diiodide catalyst gives higher apparent polymerization rates, and the rate is further doubled in the presence of one equivalent of Al(OiPr)3 for each catalyst. We could not isolate and characterize the metallic spin traps, which are expected to be Mo V complexes of formula MoOX2Y(PMe3)3 (X, Y = Cl, Br, I). The electrochemical studies indicate instability for these complexes, at least in THF. In addition, the iodide-containing system is thermodynamically unstable with respect to an intramolecular redox process. However, as we have recently demonstrated for a related catalyst, [14] it is not necessary for the spin trap to be a thermodynamically stable compound. It remains competent to perform the radical trapping function, thereby regulating the concentration of the active organic radical, for kinetic reasons. We are now interested in testing these oxomolybdenum(IV) complexes as well as other similar compounds in the polymerization (under ATRP and SFRP conditions) of other monomers.

Experimental Section

All the manipulations were carried out under an atmosphere of dry and oxygen-free argon with standard Schlenk techniques. Styrene was washed with an aqueous NaOH (10%), neutralized with water, dried over MgSO4 and then distilled at 25ºC under reduced pressure. Toluene and pentane were purified by distillation under argon after drying over sodium benzophenone ketyl. CH2Cl2 was purified from P4O10 and distilled under argon. 1-Phenyl-1-bromoethane (or 1-bromoethylbenzene, BEB) and Al(OiPr)3

were purchased from Aldrich Chemical Co, and used as received. Complex MoOCl2(PMe3)3 was prepared according to the literature. [21,23] A slight modification of the procedure described in ref. [21] , with use of complex MoCl4(EtO)2 instead of MoCl4(THF)2 as starting material, gave the product in the form of a blue solid containing a few % of the MoCl3(PMe3)3 by-product, as shown by 1 H NMR. [20] The procedure described in ref. [23] afforded a pure product (deep blue solid) in 70% yields.

Physical Measurements. The cyclic voltammograms were obtained with an EG&G 362 potentiostat connected to a Macintosh computer through MacLab hardware/software. The electrochemical cell was fitted with a Ag-AgCl reference electrode, a platinum disk working electrode and a platinum wire counterelectrode.

[Bu4N]PF6 (ca. 0.1 M) was used as supporting electrolyte. The ferrocene standard was added to each solution and measured at the end of the experiments. Its potential is E1/2 = +0.50 V under our experimental conditions. NMR spectra were recorded on a Bruker DRX 400 MHz spectrometer. The peaks positions are reported with positive shifts in ppm downfield of TMS, as calculated from residual solvent peaks. Digital simulation of the 31 P NMR spectrum was carried out with MestReC. [START_REF] Mestrec | Mestrelab Research[END_REF] The molecular weight distribution, The solution was then stirred at 50 ºC for two hours to achieve completion although after ten minutes it is possible to observe the formation of a white precipitate of NaCl. After removing the volatiles under reduce pressure the crude was redissolved in toluene (15 mL) to eliminate the insoluble salts of NaI and NaCl. Then pentane (25 mL) was added to the toluene solution to precipitate complex MoOI2(PMe3)3 as an emerald-green solid, which was filtered and washed twice with pentane (2 x 10mL). Yield 314 g, 50%. Single crystals for X-ray diffraction were obtained by slow diffusion of pentane into a concentrated solution of the product in toluene. The NMR properties are collected in direct methods (SIR97) [START_REF] Altomare | [END_REF] and refined by least-squares procedures on F 2 using SHELXL-97. [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF] All H atoms attached to carbon were introduced in idealised positions and treated as riding models. The absolute structure for MoOI2(PMe3)3 was confirmed by the refinement of the Flack's enantiopole parameter [START_REF] Flack | [END_REF]30] and by careful examination of the sensitive reflexions. The drawing of the molecules was realised with the help of ORTEP32. [31] Crystal data and refinement parameters are shown in Table 4.

Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 294667 and 294668. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Controlled Radical Polymerizations.

All ATRP polymerization reactions were conducted following the same experimental procedure. A typical procedure is described as a representative example. MoOCl2(PMe3)3 (54 mg, 0,13 mmol) was added to a 25 mL Schlenk tube equipped with a stirring bar. Styrene (3 mL, 26 mmol), toluene (6 mL) and

(1-bromoethyl)benzene (17,7 μL, 0.13 mmol) were added to the reaction flask by a syringe after a 20 min Ar purge. The Schlenk tube was immersed in an oil bath heated at 90°C. Aliquots were withdrawn periodically for monitoring.

All OMRP polymerization reactions were conducted following the same experimental procedure. A typical procedure is described as a representative example.

MoOI2(PMe3)3 (21 mg, 0,035 mmol) and AIBN (2.9 mg, 0.018 mmol) were added to a 15 mL Schlenk tube equipped with a stirring bar. Styrene (1 mL, 8.7 mmol), and toluene (2 ml) were added to the reaction flask by a syringe after a 20 min Ar purge. The Schlenk tube was immersed during 30 min in an oil bath heated at 100°C and at 80°C for the remainder time of the reaction. Aliquots were withdrawn periodically for monitoring.

The yield of polymer, and thus the % conversion, for each withdrawn aliquot was calculated by the mass ratio before and after the complete removal of solvent and residual monomer by evaporation, until achievement of constant weight.

H and 31 P

 31 NMR in the presence of NaI in acetone-d6.

Figure 4

 4 shows representative 31 P{ 1 H} NMR spectra recorded during the transformation. The resonance values for the different compounds are collected in Table 1. Given the mer-cis coordination, there are two possible stereoisomers for the mixed-halide intermediate, A and B (see Scheme 5). A mer-trans or a fac arrangement would only allow a single stereoisomer. The monitoring clearly shows the preferential formation of isomer A. The structural assignment of A and B as shown in Scheme 5 is strongly suggested by the P resonance pattern, especially the chemical shift of the unique P nucleus, which is expected to be most sensitive to the nature of the trans halide ligand. This resonance shifts very little during the first substitution for isomer A, whereas it shifts by a large amount for isomer B, to a position close to that observed for the final product. The two mutually trans P nuclei (cis to both halide coordination positions), on the other hand, shift gradually with the halide substitution. <Figure 4 and Scheme 5>

  3 equiv) leads to rapid equilibration (4 h at room temperature) with the two mixed halide intermediates having structures A' and B', the former being predominant, plus a small amounts of the corresponding dibromide. Treatment with an excess of NaBr increases the amount of dibromide product. The 31 P resonance of the unique PMe3 ligand shifts from  -12.5 from the diodide complex to -3.6 to A' and only to  -10.3 for B'. The same nucleus resonates at  -2.8 for the dibromide complex. Once again, the trans position to the oxo ligand shows a thermodynamic preference for the softer iodo ligand.<Scheme 6>It is interesting to note that complex MoOI2(PMe3)3 rapidly yields halide exchange when dissolved in chloroform (ca. 50% in 30 min at room temperature). The NMR monitoring indicates the intermediate formation of the mixed halide isomers A (major)

<Scheme 7

 7 and Figure 5> (c) Controlled Radical PolymerizationsComplexes MoOX2(PMe3)3 were tested as catalysts for the controlled radical polymerization of styrene and methyl acrylate under ATRP conditions. The experimental conditions and apparent polymerization rates for the styrene polymerization are summarized in Table3. In each case, the first order monomer consumption rate was verified by the linearity of the ln([M]0/[M]) vs. time plot, indicating that the concentration of active radical remains constant throughout the polymerization process, as expected for a controlled process. The polymerization was carried out both in the absence and in the presence of an equivalent amount of the Al(OiPr)3 cocatalyst, whose effectiveness in ATRP seems quite general. In fact, it has been demonstrated that the apparent polymerization rates increase by factors up to 10 or more when one equivalent of this compound is used in conjunction with a variety of different catalysts based on Group 6-

Synthesis of MoOI2(PMe3) 3 .

 3 were measured by size-exclusion chromatography (SEC) using THF as eluent (1mL/min) at room temperature on a 300x7,5 mm PLgel 5m Mixed-D column (Polymer Laboratories), equipped with multiangle light scattering (miniDawn Tristar, Wyatt Technology Corp.) and refractive index (RI2000, Soparès) detectors. To a solution of MoOCl2(PMe3)3 (435mg, 1.06 mmol) in THF (15 mL) freshly distilled was added an excess of NaI (795 mg, 5.3 mmol).

Figure 1 .

 1 Figure 1. Experimental (a) and simulated (b) 31 P-NMR spectrum of compound MoOI2(PMe3)3 in C6D6.

Figure 2 .

 2 Figure 2. An ORTEP view of complex MoOI2(PMe3)3. The ellipsoids are drawn at the 50% probability level.

Figure 3 .Figure 4 .

 34 Figure 3. Cyclic voltammogram of compounds MoOX2(PMe3)3 in THF. Thinner curve: X = Cl; thicker curve: X = I. CMo = 4.10 -3 M. Scan rate = 100 mV s -1 .

Figure 5 . 1 HFigure 6 .

 516 Figure 5. 1 H NMR monitoring of the halogen exchange between MoOI2(PMe3)3 and BEB in toluene-d8 at 90°C ([Mo] = [BEB] = ca. 0.1 mol l -1 ). (a) t = 0. (b) t = 20 min. (c) t = 40 min.

Figure 7 .Figure 8 .

 78 Figure 7. Polymerization of methyl acrylate catalyzed by MoOCl2(PMe3)3 at 35°C. Conditions: Mo:I:Al:MA = 1:1:1:250 (MA = methyl acrylate; I (initiator) = ethyl 2-iodopropionate; Al = Al(OiPr)3). (a) Kinetics plot. (b) Mn vs. convertion and PDI for the Squares: experimental Mn; circles: PDI; straight line: theoretical Mn.

table 1

 1

	. Elemental analysis for C9H27I2MoOP3 (593.98): calcd. C, 18.20; H, 4.58; found
	C, 18.30; H, 4.28. The compound is soluble and stable in THF, acetone and aromatic
	hydrocarbons. It dissolves in CDCl3 but gives rise to rapid halogen exchange in this
	solvent.

Table 1 .

 1 1 H and 31 P{ 1 H} NMR parameters for compounds MoOXY(PMe3)3 (X, Y = Cl or I). Cl trans to O (see Scheme 6). d not identified.

	Compound	Solvent	H(JHP/Hz) a	P(JPP/Hz)
	MoOCl2(PMe3)3	acetone-d6	1.68(d, 8.4) 1.63 (t, 3.9)	2.0 (t, 22) -5.2 (d, 22)
	MoOBr2(PMe3)3	acetone-d6	1.73(d, 8.7) 1.73(t, 4.0)	-2.8 (t, 22) -9.4 (d, 22)
	MoOI2(PMe3)3	acetone-d6	1.91(t, 4.0) 1.80(d, 8.7)	-12.5 (t, 20) -15.7 (d, 20)
	MoOI2(PMe3)3	C6D6	1.73 (t, 4.1) 1.36 (d, 8.4)	-14.98 (t, 22.2) -16.40 (d, 22.2)
	MoOClI(PMe3)3, A b	acetone-d6	1.71(d, 8.4) 1.80(t, 3.9)	2.2 (t, 20) -8.4 (d, 20)
	MoOClI(PMe3)3, B b	acetone-d6	n.i. d n.i. d	-9.7 (t, 20) -12.0 (d, 20)
	MoOBrI(PMe3)3, A' c acetone-d6	1.84(t, 4.0) 1.74(d, 8.7)	-3.6 (t, 23) -11.0 (d, 23)
	MoOBrI(PMe3)3, B' c acetone-d6	n.i. d n.i.	-10.3 (t, 22)

d -13.8 (d, 22) a vt = virtual triplet. b A: I trans to O; B: Cl trans to O (see Scheme 5). c A': I trans to O; B':

Table 3 .

 3 ATRP of styrene using MoOX2(PMe3)3 (X = Cl, I) catalysts. a

	Entry	X Mo:I:Al:S b kapp/min -1
	1	Cl	1:1:-:200	2.18E-04
	2	Cl	1:1:1:200	4.38E-04
	3	I	1:1:-:200	4.26E-04
	4	I	1:1:1:200	9.54E-04

a All polymerizations were carried out in toluene at 90 ºC. b I = initiator (BEB); Al = Al(OiPr)3 co-catalyst; S = styrene.

Table 4 .

 4 Crystal data and structure refinement.

	Compound				MoOI2(PMe3)3	[MoCl2O(PMe3)(OPMe3)]2O
	Empirical formula			C9H27I2MoOP3	C12H36Cl4Mo2O5P4
	Formula weight				593.96	717.97
	Temperature, K				180(2)	180(2) K
	Wavelength, Å				0.71073	0.71073 Å
	Crystal system				Orthorhombic	Monoclinic
	Space group				P 21 21 21	P 21/c
	a, Å				9.9624(8)	16.6058(18)
	b, Å				10.6370(10)	11.8087(12)
	c, Å				18.8084(19)	16.099(2)
	, °				113.384(11)
	Volume, Å 3				1993.1(3)	2897.7(6)
	Z				4	4
	Density (calc.), Mg/m	3	1.979	1.646
	µ, mm -1				3.982	1.474
	F(000)				1128	1440
	Crystal size, mm	3			0.38 x 0.32 x 0.28	0.14 x 0.09 x 0.08
	Theta range, °				2.17 to 26.09	2.85 to 26.37
	Reflections collected			19690	21350
	Unique refl. (Rint)			3874 (0.0455)	5900 (0.0663)
	Completeness to theta, %	98.1 (26.09°)	99.8 (26.37°)
	Absorption correction		Semi-empirical	Semi-empirical
	Max. and min. trans.			0.4148 and 0.2387	0.8737 and 0.8359
	Refinement method			Full-matrix L.S. on F 2	Full-matrix L.S. on F 2
	Data /restraints/parameters	3874 / 0 / 154	5900 / 0 / 256
	Goodness-of-fit on F 2		1.087	0.782
	Final R indices [I>2(I)]	R1= 0.0287, wR2= 0.0816	R1= 0.0379, wR2= 0.0554
	R indices (all data)			R1= 0.0295, wR2 = 0.0822	R1= 0.0899, wR2= 0.0648
	Absolute structure parameter	0.00(2)
	Residual density, e.Å	-3	0.951 and -1.437	0.557 and -0.470

Acknowledgements. We are grateful to the CNRS for support of this work and to Sandrine Vincendeau for technical assistance. JAM thanks the Ministerio de Educación y Ciencia of Spain for a post-doctoral fellowship. SM thanks the Conseil Régional de Bourgogne and the CNRS for a BDI fellowship.