Pavel A Dub

Natalia V Belkova
email: nataliabelk@ineos.ac.ru

Konstantin A Lyssenko

Gleb A Silantyev

Lina M Epstein

Elena S Shubina

Jean-Claude Daran

Rinaldo Poli

Synthesis and Protonation Studies of Cp*Os(dppe)H: Kinetic versus Thermodynamic Control

Keywords: osmium, hexahaloosmate(IV), pentamethylcyclopentadienyl ligand, hydrido complexes, protonation

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

Considerable amount of information has been accumulated since the 80's on proton transfer to transition metal hydrides. Classical polyhydrides may be obtained indirectly through initial, kinetically favored proton transfer at the hydride site, followed by rearrangement of the dihydrogen intermediate to the thermodynamically more stable polyhydride. 1 However, the question of how the nature of the metal center and the electronic and steric properties of the ligands in its coordination sphere affect the choice of the kinetic protonation site (metal or hydride) is not yet fully understood.

Thus, for the recently studied Cp*M(dppe)H complexes (M = Fe, 2 Ru 3 ) proton transfer yields initially ( 2 -H2)-complexes, which convert quantitatively into the corresponding trans-dihydrides upon warming. This isomerization process involves ligand reorganization without intermediates. 4 Notably, no stationary point was found corresponding to a putative cis-dihydride isomer.

From this point of view the protonation of the yet unreported Cp*Os(dppe)H complex would be of special interest because it would complete the series of Group 8 Cp*M(dppe)H complexes and because of the reported 5 ability of some CpOs(P-P)H complexes [P-P = dppm, dppe, dppp] to form cis-dihydrides [CpOs(P-P)(H)2] + upon protonation by HBF4•Et2O at -78 °C in CD2Cl2. Herein, we report the preparation of Cp*Os(dppe)H (1) through Cp*Os(COD)X (X = Cl, Br) intermediates and the results of its protonation by HBF4•Et2O in CD2Cl2. An improved preparation of the K2OsX6 precursors is also described.

(a) Synthesis of Cp*Os(dppe)H. An apparently easy synthetic route ("one-pot" procedure, use of non-toxic potassium osmate, K2OsO2(OH)4, and aqueous ethanol, higher yield) was reported recently 6 for the synthesis of some C5R5-containing (R = H, Me) osmium complexes (Scheme 1, X = Cl). However, the presumed identity of intermediates A and B in Scheme 1 was not confirmed.

Upon repeating this procedure, we found that the addition of EtOH allows to reduce the reflux time to 20 min for the complete conversion of K2OsO2(OH)4 to species A. 7 In addition, we have confirmed the identity of intermediate A as K2OsCl6 8 and B as Cp*Os(COD)Cl, 2 9 as well as structurally characterized the later by X-ray diffraction (Figure 1). However, compound 2 could not be obtained in acceptable yields in our hands. A significant amount of K2OsCl6, insoluble in EtOH, often remained unreacted. The application of the same strategy to the preparation of the bromide compound, Cp*Os(COD)Br

(3), by using HBr instead of HCl (Scheme 1, X = Br) turned to be quite straightforward, faster (ca. 3 h for Br vs 30 h for Cl) and reproducible. The brown-black residue obtained from potassium osmate and conc. HBr corresponds to K2OsBr6. This procedure yields the hexabromoosmate(IV) salt in good yields, being considerably more convenient than those reported in the literature. 7,10 However the overall route suffers from quite low yield of compound 3.

Thus, despite the appealing use of the non-toxic potassium osmate as a starting material, both chloro-and bromo-synthetic pathways to Cp*Os(dppe)X complexes do not appear sufficiently efficient, therefore the original Girolami's protocol 11,12 (Scheme 2) was used in our further work.

However, our improved syntheses of K2OsX6 (X = Cl, Br) may become advantageous for other synthetic applications. (b) X-ray analyses. The geometry of Cp*Os(COD)X (X = Cl, 2; Br, 3) is identical to that of the Cp*Ru(COD)Cl analogue, 15 with the halogen atom and the centers of the two COD double bonds occupying the basal position of a "three-legged piano stool" (Figure 1). Whereas the M-CNT distance is statistically longer for the Os-Cl compound, the M-Cl and M-C (COD) distances are statistically shorter for osmium relative to the Ru analogue. 8 The reported covalent radii are sensibly similar (Os, 1.28 Å; Ru, 1.26 Å). 16 The longer C-C distances for the coordinated COD double bonds (averages of 1.417(17) Å for Os and 1.388(8) Å for Ru) suggest that Os is capable of stronger M-COD  back-bonding. Compound Cp*Os(dppe)H (1) 17 (Figure 2) also adopts a three-legged piano stool geometry, the H and dppe ligands taking the place of the halogen and COD ligands in the above described structures.

Curiously, except for the P-Os-P angle, which is much narrower (by almost 6°), the bond distances and angles are almost identical to those of the previously reported Cp*Ru(dppe)H compound. 18,8 Another feature of this structure is a short contact (2.32 Å) between Os-H and one ortho-CH proton (bonded to atom C4), 19 which gives rise to the negative 1 H NOE and the cross-peak in the 2D ( 1 H-1 H) ROESY spectra in solution. 8 triplet resonance at -13.34, which becomes much better resolved at 233 K with 2 JH-P = 10 Hz (Figure 3). 8 Its 2 JH-P value, together with the T1min value (186 ms at 220 K and 500 MHz), the overall resonance shape, and temperature-dependent behavior 8 similar to that reported for the related [CpOs(P-P)H2] + and [Cp*Ir(dmpm)H2] 2+ complexes, 5,20 The protonation of 1 by 98% CF3SO3D at 200 K led to the appearance of a new triplet resonance at -13.41 ppm ( 2 JH-P = 10 Hz) attributed to the cis-[Cp*Os(dppe

)(H)(D)] + CF3SO3 -(cis-1D + ).
Interestingly, besides this resonance the signal of cis-1H + was also observed. 8 Correspondingly, two singlets appear in the 31 P{ 1 H} specrum at 41.8 and 41.7 ppm, which were observed as doublet and triplet (JP-H = 10 Hz) in non-coupled 31 P spectra. The chemical shift difference between the hydride signals of two isotopomers is quite high (= -0.08 ppm), whereas the resonances of other hydrogen atoms are the same for both complexes. The 2 H NMR spectrum of the above-mentioned mixture features a single resonance at -13.18 ppm ( = 15 Hz). The 3:1 ratio cis-1D + :cis-1H + was determined from inverse-gated 31 P{ 1 H} spectra. 23 The observation of a simple triplet for the HH'PP' spin system could be rationalized in several different ways: on the basis of a rapid exchange of the two hydride environments; the JHP = JH'P' and JHP' = JH'P could be accidentally degenerate; the JPP could be very large, leading to a virtual triplet; or JHH' could be very large because of quantum mechanical exchange coupling (QMEC) between the two hydride ligands. 24 The temperature dependent chemical shift and line broadening for this When the temperature was raised to 293 K, the above described hydride signals of cis-1H + disappeared and were replaced by a new hydride triplet at  -13.15 ( 2 JH-P = 35Hz), see Figure 3. The signal of trans-1D + is only slightly shifted to strong field ( = -0.01 ppm). 8 The spectral properties allow the new product to be formulated as trans-[Cp*Os(dppe)(H)2]BF4, (trans-1H + ) (Table 1). [START_REF]IR spectrum of trans-1H + (CH2Cl2) at 200 K features two νOsH bands at 2053 (ε/ L mol -1 cm -1 = 25) and 2102 (ε = 41) cm[END_REF] The relatively large 2 JH-P coupling constant is very similar to those observed for trans-[CpOs(P-P)(H)2] + 5 and for trans-[CpOs(LL')H2] + (LL' = (PR3)2, (CO)(PiPr3)). 26 The T1min of trans-[Cp*Os(dppe)(H)2] + (530 ms at 500 MHz) may be compared with that measured for complex trans-[CpOs(dppe)(H)2] + (610 ms at 300 MHz). 5 Cooling the sample down again to 233 K does not show any further change, showing that the cis-trans isomerization is irreversible. When the HBF4•Et2O addition (ca. 1 equiv) was carried out at room temperature, the nearly quantitative formation of the trans isomer was immediately observed; the traces of cis-1H + completely disappeared within 10 min. [START_REF]An 1 H NMR monitoring of the isomerization process at 250 K gives rates of (3.15±0[END_REF] Thus, the protonation of compound 1 by HBF4•Et2O occurs as shown in Scheme 3. Complex cis-1H + is the kinetic product formed selectively at low temperatures (193 K) and isomerizes quantitatively to trans-1H + at higher temperature. We cannot exclude that the minor amount of trans-1H + observed in the low-temperature protonation process derives from a direct protonation at the metal site, but we consider it more likely that this isomer derives from the isomerization of the cis kinetic product due to the occasional warming of the sample during the transfer into the NMR probe. This amount, in fact, was not perfectly reproducible in different experiments but always remained < 2%, see Figure 3. For other related protonation studies, a greater amount of trans isomer is obtained directly by low-temperature protonation: the cis/trans ratios observed for protonation at -78°C were 24:1 for [CpOs(dppm)H2] + , 1:3 for [CpOs(dppe)H2] + , 1:3 for [CpOs(dppp)H2] + (changing to 10:1, 1:70 and to 100% trans, respectively, upon warming to room temperature). 5 The 10:1 ratio for the dppm derivative at room temperature also obtained upon generating the complex by a different route (reaction between CpOs(dppm)Br with Na[BAr F *4] under H2, where

Ar F * = 3,5-(CF3)2C6H3), [START_REF] Egbert | [END_REF] indicating the presence of a reversible equilibrium in that case. Thus, the Cp* system differs from the Cp analogues in two ways: protonation of the monohydride complex yields the cis isomer selectively at low temperature, and the higher temperature rearrangement to the trans isomer is quantitative. Our previous results on the kinetics of the ( 2 -H2) → (H)2 isomerization for the Cp*M(dppe)H2 + (M = Fe, Ru) systems 3,4 revealed that the high activation barrier renders this process too slow at temperatures close to 200 K. At the same time, the proton transfer reaction has a very low activation barrier so the competitive direct proton attack at the metal site can occur at low temperatures to give trans-species for the less sterically encumbered complexes. The sterically more crowded coordination sphere, like that in Cp*M(dppe)H, blocks quite efficiently the direct access of the metal site as was shown by calculations of different models for Cp*Fe(dppe)H. 29 On the other hand, the trans-form is energetically stabilized relative to the cis isomer to a greater extent for the Cp* system than for Cp one, for either steric or electronic reasons or both, rendering the isomerization process irreversible like for the Fe and Ru analogues. For all the previously reported examples of [CpOs(P-P)H2] + complexes, as well as for the [Cp*Os(dppe)H2] + complex described here, a classical cis dihydride structure is generated without the observation of a nonclassical isomer (dihydrogen complex). This behavior is different from that of the Fe and Ru analogues, whose protonation systematically affords dihydrogen complexes as intermediates and trans-dihydride isomers as thermodynamically stable products, without the observation of intermediate cis-dihydride isomers. 1d,2,3,29,30 The increase of the relative energy of the metal d electrons upon descending the group destabilizes a dihydrogen structure M( 2 -H2), through a better back-donation to the * orbital of the dihydrogen ligand, relative to the classical cisdihydride isomer, and also helps the direct transfer of the proton to the metal.

Though formation of cis-dihydride can be formally regarded as a result of proton transfer to the metal atom, it is more likely that the initial proton attack takes place at the hydride site, as it does in the case of ( 2 -H2)-complexes formation for the corresponding Fe and Ru complexes. 1d,2,3 The nonclassical ( 2 -H2)-like structure could be a transition state for this proton transfer reaction or be generated as a short-lived intermediate, then collapsing to the classical cis-dihydride with a very low activation barrier. The latter scenario was found by DFT calculations for the Cp*Mo(dppe)H3

protonation, although a nonclassical intermediate could not be observed under any experimental conditions. 31 The former one was suggested for the Cp*W(dppe)H3 protonation, where the initial Hbonding interaction involves an important contribution from the metal atom. 32 The studies of hydrogen-bonding and proton transfer from weaker acids to Cp*Os(dppe)H are underway aiming to shed more light on the ground state properties of this hydride and the details of the proton transfer mechanism.

In summary, the protonation of the new osmium hydride Cp*Os(dppe)H (1) by HBF4 13 The infrared spectrum (Nujol mull) of 1 contains a strong band at 2008 cm -1 due to the Os-H stretch, this frequency being considerably higher than those of the Ru and Fe analogues (RuH = 1914 cm -1 , FeH = 1839 cm -1 ).

14 The rate of this exchange is essentially identical to that of the Ru analogue (about 16% in 1 h at 300 K for Os, vs. 15% for Ru), the pseudo-first order rate constant being 7.7•10 -5 s -1 at T = 300 K. 19 With normalization of the C-H length to the ideal value of 1.08 Å. This r(H…H) value is certainly overestimated since M-H bond lengths are usually underestimated in X-ray. 20 Pons, V.; Heinekey, D. M. J. Am. Chem. Soc., 2003, 125, 8428-8429. 21 IR spectra (CH2Cl2) at 200 K show full disappearance of the νOsH band at 2035 cm -1 (ε/ L mol -1 cm -1 = 84) of the starting hydride 1 and appearance of two new bands at 2126 (ε = 29) and 2177 (ε = 16) cm -1 of cis-1H + .

22 (a) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J., J. Am. Chem. Soc. 1991, 113, 4173-4184. (b) Bayse, C. A.; Luck, R. L.; Schelter, E. J., Inorg. Chem. 2001, 40, 3463-3467. 23 The income of the extra proton could be explained by the exchange of o-CH of the phenyl ring of the dppe ligand and the osmium bound deuterium via an electrophilic substitution on the arene ring (the cationic dihydride serves as acid).

SYNOPSIS TOC

Synthesis and Protonation

Figure 1 .

 1 Figure 1. Molecular structures of Cp*Os(COD)Cl (2) and Cp*Os(COD)Br (3) (30% probability

Figure 2 .

 2 Figure 2. Molecular structure of Cp*Os(dppe)H (1) (30% probability level). Key bond lengths

( c )

 c Protonation of Cp*Os(dppe)H. The protonation of Cp*Os(dppe)H by strong acids was monitored in CD2Cl2 by 1 H and 31 P{ 1 H} NMR spectroscopy at low temperatures, where no H/Cl exchange with the solvent occurs. The addition of 1 equiv HBF4•Et2O at 193 K led to the complete disappearance of the starting material and to the quantitative appearance of a new poorly resolved

Figure 3 .

 3 Figure 3. 1 H NMR (500.33 MHz) spectra (hydride region) of [Cp*OsH2(dppe)]BF4; 233K,

  Studies of Cp*Os(dppe)H: kinetic vs thermodynamic control. Pavel A. Dub, Natalia V. Belkova,* Konstantin A. Lyssenko, Gleb A. Silantyev, Lina M. Epstein, Elena S. Shubina, Jean-Claude Daran, Rinaldo Poli* The previously undescribed complex Cp*Os(dppe)H was synthesized starting from either K2OsX6 salts or H2OsBr6 and characterized together with its precursors Cp*Os(COD)X (X = Cl, Br). Protonation of 1 by HBF4•Et2O in CD2Cl2 at 193K leads exclusively to cis-[Cp*Os(dppe)(H)2] + BF4 -, which irreversibly transforms into trans-[Cp*Os(dppe)(H)2] + BF4 -above 230 K.

Table 1 .

 1 Selected NMR ( 1 H 500.33 MHz, 31 P{ 1 H} 202.5 MHz) data for the Cp*OsH(dppe), cisand trans-[Cp*OsH2(dppe)]BF4 generated in situ in CD2Cl2 at 193K.

		1	cis-1H +	trans-1H +
	1 H	-17.19 (t, H, Os-H) 2 J H-P = 27 Hz, 1	-13.34 (bt, 2 JH-P = 10 Hz, 2H, Os-H)	-13.32 (t, 2 JH-P = 35 Hz, 2H, Os-H)
	1 H	1.92 (s, 15 H, Cp*)	1.51 (s, 15H, C5Me5)	1.72 (s, 15H, C5Me5)
	1 H	1.89-2.06 (2 H, -CH CH A -), 2.08-2.26 (2 H, -A -CH B -CH B -)	2.02-2.25 (m, 2H, -CH A -CH A -), CH B -) 2.65-2.88 (m, 2H, -CH B -	2.32 (vd, 2 JH-P = 10 Hz, 4H, -CH2-CH2-)
	P{ 1 H}	51.9 (s)	41.8 (s)	32.2 (s)

  BF4 -above 230K. The comparison with the previously reported protonation of less sterically hindered Cp analogues suggests an important difference: a competitive direct proton attack at the metal center, yielding the trans-dihydride species, is possible for the sterically less hindered Cp system, but is blocked for the Cp* system.

	9	Liles, D.C.; Shaver, A.; Singleton, E.; Wiege, M.B. J. Organomet. Chem., 1985, 288, C33-
	C36
	10 Lott, K. A. K.; Symons, M. C. R. J. Chem. Soc., 1960, 973-976		•Et2O in
	CD2Cl2 was found to lead at low temperatures essentially exclusively to
	which Gross, C. L.; Girolami, G. S. Organometallics 1996, 15, 5359-5367. then quantitatively transforms cis-[Cp*Os(dppe)(H)2] + BF4 -, 11	into	trans-
	[Cp*Os(dppe)(H)2] The same is true for the OsO4 reduction by concentrated HCl, which is usually done in the 12 Gross, C. L.; Brumaghim, J. L.; Girolami, G. S. Organometallics, 2007, 26, 2258-2265.
	presence of Fe(II) [Dwyer, F.P.; Hogarth, J.W. Inorg. Synth. 1957, 5, 206] but could be facilitated
	by addition of EtOH [(a) Gilchrist, R., Bur. Standards J. Research 9, 282 (1932); (b) Turner, A. G.;
	Clifford, A. F.; Ramachandra Rao, C. N. Anal. Chem. 1958, 30, 1708-1709].
	8 See Supplementary Information for details
				14

+

Acknowledgement. We thank the CNRS and the RFBR (05-03-22001) for support through a France-Russia bilateral PICS grant. Support from the IUF (France), the RFBR (08-03-00464) and Division of Chemistry and Material Sciences of RAS (Russia) is also gratefully acknowledged.

Thanks are expressed to Dr. Y. Coppel (LCC CNRS) for assistance with the low temperature NMR measurements.

Supporting Information Available: Synthetic, spectroscopic, crystallographic, and analytical data, variable temperature NMR spectra, and X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http:/pubs.acs.org.


Joffrey Wolf

Agnès Labande
email: agnes.labande@lcc-toulouse.fr

Jean-Claude Daran

Rinaldo Poli

Reactivity of phosphane-imidazolium salts toward [Ir(COD)Cl]2: preparation of new hydridoiridium(III) complexes bearing abnormal carbenes. §

Keywords: Carbenes, Phosphane ligands, Iridium, Hydrides, C-H activation Carbenes, Phosphane ligands, Iridium, Hydrides, C-H activation Key Topic: Iridium complexes of abnormal carbenes

The unusual reactivity of chelating phosphine-imidazolium salts MesImEtPPh2 + Br -, DIPP-ImEtPPh2 + Brand MesImEtPPh2 + BF4toward low oxidation state iridium complex [Ir(COD)(-Cl)]2 was studied. In the absence of a base, the C-H insertion at the C5 position of the imidazolium ring is the only reaction occurring, with no normal NHC observed, and lead to iridium(III) hydride complexes. This reactivity is independent of the nature of the imidazolium counteranion and of the substitution pattern of the aryl group.

Introduction

N-heterocyclic carbene (NHC) complexes are now widely used for catalytic applications [1] . This interest is particularly due to the production of more robust catalysts by coordination of NHCs on transition metals such as palladium, nickel, rhodium, ruthenium and iridium [START_REF] Hillier | [END_REF] . Over the last fifteen years, a number of bidentate as well as polydentate ligands incorporating NHCs [3] and other coordinating atoms [4][5][6], [7,8] have been developed and used successfully in catalysis.

More recently a different binding mode for NHCs has been described by Crabtree and co-workers: [9] instead of the normal C2 coordination, the metal can be bound to the C4 or C5 atom of an imidazolium salt. Since this discovery, only few examples of "abnormal" carbenes have been described, for example with iridium, [10][11][12] ruthenium, [13] osmium [14] and several other transition metals. [15] In the case of Crabtree's work with iridium precursor IrH5(PPh3)2, it has been shown that the formation of classical or abnormal NHCs was anion-dependent: imidazolium salts bearing a halogen anion led preferentially to classical NHCs, whereas non-coordinating anions BF4 -or PF6 -gave mostly abnormal NHCs. [12] Monodentate ligands have then been used instead of bidentate NHCs, with similar results, showing no chelate effect. [11] Very recently, the group of Li has reported iridium III hydride complexes bearing chelating phosphine-(abnormal)NHC ligands. [16] They did not observe any influence of the counter-anion on the product selectivity.

We are interested in the preparation of bifunctional imidazolium salts containing a coordinating heteroatom such as phosphorus or sulfur, and their use as ligands or ligand precursors for transition metals. [4][5][6] More particularly, we have recently described the use of phosphine-imidazolium salts to prepare zwitterionic Ni II complexes and shown their excellent activities for Kumada-Tamao-Corriu cross coupling reactions. [4] We now report the unusual reactivity of these salts with [IrCl(COD)]2. These results were obtained during attempts to synthesise new Ir I pre-catalysts, following the same procedure that allowed us to access Ni II and Pd II analogues. [4,5]

Results and discussion

The phosphine-imidazolium salts were prepared according to previously reported procedures (Scheme 1). [4] The reaction of 1-(2-diphenylphosphinoethyl)-3- (2,4,6trimethylphenyl) We attribute this behavior to halogen exchange, induced by the bromide anion of 1. This hypothesis is confirmed by the observation that using MesImC2H4PPh2 + BF4 -3 in place of 1 leads to a single product 5, whose 31 P and 1 H (hydride) resonances match with those of 4a (Scheme 3, figure 1b). Thus, both products 4a and 5 contain a chloride ligand and a different counterion (Br -for 4a and BF4 -for 5), whereas the chloride and bromide ions are exchanged in 4b.

-15.00 -14.50 approximately matches with that of 4a, the downfield one is quite anion dependent.

/ppm (a) (b) (c)
Furthermore, 1:1 13 C resonances for quaternary carbons are observed at  119.9 and 121.1, which is a typical region for aryl substituents and not for NHC ligands. These results indicate that the products are Ir III complexes coordinated by a bidentate phosphineabnormal carbene ligand (scheme 2). The shift of the downfield 1 H resonance is attributed to differences in H-bonding between the acidic imidazolium proton and the different counterion. Related iridium(III) hydride complexes, also featuring an abnormal carbene coordination, have been recently reported by Li et al. and show similar NMR features (complex A, figure 3). [16] Danopoulos et al. also obtained and crystallographically characterised a similar complex, Ir(COD)(Br){ 2 :P,C5-Ph2PCH2CH2(N2C3HMes)} (B in figure 3), from the reaction of 1 with [Ir(COD)(HCl)(-Cl)2]2 . It is related to 4b by elimination of HCl. [8] /ppm [16] and Danopoulos et al.

(B) [8] .

Complex 5 has also been characterised by X-ray crystallography (figure 4, table 1).

The Ir-C( 2) distance [2.068( 5) Å] and the Ir(1)-Cl (1) distance are in agreement with the data given by Li et al. for (A) (table 2). [16] The Ir(1)-P(1) distance, however, appears much shorter than in Li's compound, but is very close to the distance measured in Danopoulos' compound (B), [8] where the N(2) atom is also substituted with a bulky aryl group. The C(2)-Ir(1)-P(1) bite angle is also very close to that of Li's complex, with a large value of 92.70( 13 [16] and Danopoulos et al. (B) [8] .

(A) (B)

Bond distances (Å) 068(5) 2.065(6) 2.018(9) Ir(1)-P(1) 2.2972(16) 2.585(1) 2.283(2) Ir(1)-Cl(1) 2.4845(15) 2 a Labelled as C( 9) in Li's complex (A). [16] A 31 P NMR monitoring of the [Ir(COD)(μ-Cl)]2/3 reaction at room temperature revealed additional features. After five minutes, the ligand was completely consumed and two products were visible: the final product at 1.1 ppm, and an intermediate compound at 19.4 ppm which quickly disappeared (Scheme 3). Our hypothesis is that, in a first step, the phosphine coordinates to the iridium center, splitting the dimer. Subsequently, the CH insertion occurs at the C5 position of the imidazolium ring, possibly assisted by a chelating effect. This hypothesis is supported by the recent results obtained by Li et al. [16] However, our observed reactivity is at variance with the results described by Danopoulos with a similar ligand, where the mesityl substituent is replaced by DIPP (2,6diisopropylphenyl, compound 2). [8] That contribution indicated that the interaction of 6.35 and 6.28, with integration values of 0.5 each, for the mixture of 6a and 6b. We can also observe two equivalent 1 H NMR hydride doublets at  -15.07 and -14.41 (figure 1c)

Ir(1)-C(2) a 2.
and two signals at  -2.04 and -3.08 in the 31 P NMR spectrum, corresponding to the two isomers. Moreover, the mass spectrum clearly shows two peaks at m/z 777.9 and 821.8, corresponding respectively to cations 6a + and 6b + .

Conclusions

In summary, we have described an unusual reactivity of chelating phosphineimidazolium salts toward low oxidation state iridium complexes. In the absence of a base, we have shown that the C-H insertion at the C5 position of the imidazolium ring is the only occurring reaction, with no normal NHC observed. Contrary to related iridium chemistry previously reported by the Crabtree group, [12] this reactivity is independent of the nature of the imidazolium counteranion. It is also independent of the substitution pattern of the aryl group. In fact, the recent paper by Li et al. [16] shows identical chemistry for ligands bearing alkyl substituents (Me, iPr) on the 3-N atom of the imidazolium ring.

Thus, this reactivity pattern appears to be quite general.

Experimental Section

All reactions were carried out under a dry argon atmosphere using Schlenk glassware and vacuum line techniques. Solvents for syntheses were dried and degassed by standard methods before use. Elemental analyses were carried out by the analytical service of the "Laboratoire de Chimie de Coordination" in Toulouse. 1 H data were recorded on a Bruker AV-500

spectrometer, operating at 500 MHz. 13 C{H, P} and 31 P{H} NMR data were recorded on a

Bruker AV-500 instrument, operating at 125.8 and 202.5 MHz, respectively. 19 F NMR data were recorded on a Bruker AC-200 instrument, operating at 188 MHz. The spectra were referenced internally using the signal from the residual protiosolvent ( 1 H) or the solvent signals ( 13C), the signal of CF3CO2H (10%) in C6D6 ( 19 F) and externally using 85% H3PO4 for 31 P. Mass spectra were obtained from acetonitrile or methanol solutions on a TSQ7000 instrument from ThermoElectron (electrospray ionisation), and from DMSO or DMF solutions on a Nermag R10-10 instrument (FAB). [Ir(COD)(-Cl)]2 was from Strem and used as received. MesImEtPPh2 + Br -1, DIPP-ImEtPPh2 + Br -2 and MesImEtPPh2 + BF4 -3 were prepared as previously described. [4] General procedure for the synthesis of Ir III complexes

In a Schlenk tube, CH2Cl2 was added to a phosphine-imidazolium salt and [Ir(COD)Cl)]2 mixture and stirred for 30 minutes at room temperature. The solvent was removed under vacuum, the residue washed with diethyl ether until the ethereal phase remains colourless, then dried under vacuum.

These reactions can also be done in THF without any change in the results.

Iridium chloro-1,4-cyclooctadienyl-1-(2-(diphenylphosphinoethyl) -3-(2,4,6-trimethylphe nyl) imidazol-5-ylidene bromide (4a+4b)

CH2Cl2 (3 mL), MesImEtPPh2

Crystallographic Data

A single crystal was mounted under inert perfluoropolyether on the tip of a glass fibre and cooled in the cryostream of an Bruker APEX2 diffactometer. Data were collected using the monochromatic Mo Kα radiation (λ = 0.71073). The final unit cells parameters were obtained by the least-squares refinement of a large number of selected reflections. The structures were solved by direct methods (SIR97 [17] ) and refined by least-squares procedures on F2 with the SHELXL-97 program [18] using the integrated system WINGX(1.63). [19] Hydrogen atoms attached to carbon atoms were introduced at calculated positions and treated as riding on their parent atoms [d(CH) = 0.96-0.98 Å] with a displacement parameter equal to 1.2 (C6H5, CH2) or 1.5 (CH3) times that of the parent atom. The hydride atom was located in difference Fourier syntheses and its coordinates were refined using a Ir-H restraint of 1.60(2) Å. The BF4 anion was partially disordered by rotation around one B-F axis, this disorder was treated using the tools available in SHELX-97 [17] . Some residual electron densities were difficult to modelise and therefore, the SQUEEZE function of PLATON [20] was used to eliminate the contribution of the electron density in the solvent region from the intensity data, and the solvent-free model was employed for the final refinement. There are two cavities of 127 Å 3 per unit cell.

PLATON estimated that each cavity contains 62 electrons which may correspond to a mixture of dichloromethane and pentane solvent molecules. The molecular views were realised with the help of ORTEP-III. [START_REF] Burnett | ORTEP-III_Report ORNL-6895[END_REF], [START_REF] Farrugia | [END_REF] CCDC 681170 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/datarequest/cif.

1 .

 1 Scheme 1. Synthesis of phosphine-imidazolium salts. Mes = mesityl or 2,4,6trimethylphenyl; DIPP = 2,6-diisopropylphenyl.

2 .

 2 Scheme 2. Mixture of isomers 4a+4b from the reaction of ligand 1, or isomers 6a+6b from the reaction of ligand 2, with [Ir(COD)(-Cl)]2.

Figure 1 . 1 H 5 (3 3 .

 1153 Figure 1. 1 H NMR spectra (500 MHz, 20°C) of complexes a) 4a/4b (CDCl3), b) 5 (CD2Cl2), c) 6a/6b (CD2Cl2) (hydride resonances).

Figure 2 . 1 H 5 (

 215 Figure 2. 1 H NMR spectra (500 MHz, 20°C) of complexes a) 4a/4b (CDCl3), b) 5 (CD2Cl2), c) 6a/6b (CD2Cl2) (imidazolium resonances).

Figure 3 .

 3 Figure 3. Related iridium complexes reported by Li et al. (A)[16] andDanopoulos et al.

  )°. The structure confirms the binding of the imidazole moiety through C5 (labeled as {C2} in the structure). It also shows the presence of a hydride trans to the chloride, with a Ir(1)-H(100) bond length of1.586(19) Å, and a relative angle to the Cl atom of 164(2)°. The coordination is distorted octahedral at the iridium(III) center. All other characterisations (elemental analysis and mass spectrometry) confirmed the nature of these products.

Figure 4 .

 4 Figure 4. An ORTEP view of compound 5. Ellipsoids are represented at the 30% probability level. Hydrogen atoms, except H100, cocrystallised solvent molecules and BF4 -are omitted for clarity.

[

  Ir(COD)(-Cl)]2 with 2, under unspecified reaction conditions, affords the simple addition product where the chloride ligand has however been exchanged with bromide, [Ir(COD)(Br)(PPh2CH2CH2Im DIPP ), similar to the proposed intermediate of our reaction leading to 5. In the interest of checking whether a simple change of aryl group would enforce such a big reactivity change, we decided to repeat the same reaction under conditions similar to those leading to 4a/4b. The reactions were carried out in two different solvents (CH2Cl2 or THF), always leading to complete conversion in 20 minutes at room temperature and leading to iridium complex bearing an abnormal carbene (mixture of isomers, 6a+6b, Scheme 2). These compounds have been fully characterised by NMR and mass spectrometry, leaving no doubt about their nature. Indeed, the NMR spectra present many similar features to those of complexes 4a and 4b: the 1 H NMR spectrum shows four signals corresponding to the imidazole ring protons at  9.73, 9.71,

  CH2(COD); o-CH3), -15.12 (d,[START_REF] Hillier | [END_REF] JP-H = 8.3 Hz, 1 H, Ir-H).13 C NMR (125.8 MHz, CD2Cl2, 20°C): δ = 140.6 (s, p-C(Mes)), 135.9 (s, NCHN), 134.8; 134.4 (s, o-C(Mes)), 134.3 (d, 2 JC-P = 7.3 Hz, o-CH(Ph 2 )), 132.8 (d, 2 JC-P = 8 Hz, o-CH(Ph 1 )), 132.6 (d, 4 JC-P = 2.3 Hz, p-CH(Ph 2 )), 131.4 (d, 4 JC-P = 2.6 Hz, p-CH(Ph 1 )), 131.3 (s, NC(Mes)), 129.5 (d, 3 JC-P = 10.6 Hz, m-CH(Ph 2 )), 129.4 (br s, CH(Mes)), 128.8 (d, 1 JC-P = 58.7 Hz, ipso-C(Ph 1 )), 128.6 (d, 1 JC-P = 58.7 Hz, ipso-C(Ph 2 )), 128.1 (d, 3 JC-P = 10.5 Hz, m-CH(Ph 1 )), 124.5 (s, MesNCH=), 121.4 (d, 2 JC-P = 9.2 Hz, Ir-C), 98.7 (d, 2 JC-P = 15.6 Hz, CH(COD)), 94.6 (d, 3 JC-P = 9.4 Hz, CH(COD)), 94.1; 84.4 (s, CH(COD)), 45.7 (s, NCH2), 35.6; 30.7 (s, CH2(COD)), 29.6 (d, 3 JC-P = 2.1 Hz, CH2(COD)), 28.0 (d, 3 JC-P = 3.8 Hz, CH2(COD)), 25.1 (d, 1 JC-P = 39.7 Hz, CH2P), 20.8 (s, p-CH3(Mes)), 17.1; 17.0 (s, o-CH3(Mes)). 31 P NMR (202.5 MHz, CD2Cl2, 20°C): δ = 2.42 (s, PPh2). 19 F NMR (188 MHz, CD2Cl2, 20°C): δ = -76.2 (s, BF4). MS (ESI, positive mode) m/z (%) = 627.1 (10) [C26H28ClIrN2P + ], 735.1 (100) [C34H40ClIrN2P + ]. MS (ESI, negative mode) m/z (%) = 87 (100) [BF4 -]. Iridium chloro-1,4-cyclooctadienyl-1-(2-(diphenylphosphinoethyl) -3-(2,6-diisopropylphe nyl) imidazol-5-ylidene bromide (6a+6b) CH2Cl2 (3 mL), DIPP-ImEtPPh2 + Br -

137. 4

 4 (s, NCHN), 134.6 (d, 2 JP-C = 10.5 Hz, o-CH(PPh2)), 133.0 (s, 2 JP-C = 6.7 Hz, o-CH(PPh2)), 132.5 (d, 4 JP-C = 2.6 Hz, p-CH(PPh2)), 131.4 (d, 4 JP-C = 2.7 Hz, p-CH(PPh2)), 131.2 (s, NC(Mes)), 131.0 (s, p-CH(DIPP)), 129.4 (d, 3 JP-C = 10.5 Hz, m-CH(PPh2)), 127.9 (s d, 3 JP-C = 10.5 Hz, m-CH(PPh2)), 125.5 (d, 3 JC-P = 8.5 Hz, (DIPP)NCH=), 124.3; 124.1 (s, m-CH(DIPP)), 120.1 (d, 2 JC-P = 8.4 Hz, Ir-C), 96.8 (d, 2 JP-C = 16.2 Hz, CH(COD)), 93.8 (d, 2 JP-C = 9.0 Hz, CH(COD)), 91.7; 82.7 (s, CH(COD), 45. 4 (s, NCH2), 33.7 (d, 3 JP-C = 2.1 Hz, CH2(COD)), 31.9 (s, CH2(COD)), 29.9 (d, 3 JP-C = 4.3 Hz, CH2(COD)), 29.7 (d, 3 JP-C = 2.2 Hz, CH2(COD)), 28.5; 28.4 (s, CH(CH3)2), 25.0 (d, 1

Table 1 .

 1 Crystallographic data for compound 5.

		5
	Empirical formula	C36H44BCl5F4IrN2P
	Formula weight	991.96
	Temperature (K)	180(2)
	Crystal system	triclinic
	Space group	P -1
	a (Å)	10.071(5)
	b (Å)	15.126(5)
	c (Å)	15.988(5)
	α (°)	68.528(5)
	β (°)	74.532(5)
	γ (°)	78.030(5)
	Volume (Å 3 )	2168.0(15)
	Z	2
	Density	1.520
	Absorption coefficient. (mm -1 )	3.468
	F(0 0 0)	984
	θ range for data collection	30.5> θ >1.4
	Reflections collected	57426
	Independent reflections	13227 [R(int) = 0.0612]
	Godness-of-fit on F²	1.087
	Final R indices [I>2σ(I)]	R1 = 0.0481, wR2 = 0.1217
	R indices for all data	R1 = 0.0628, wR2 = 0.1267

Table 2 .

 2 Comparison of the main structural parameters of 5 and of the iridium complexes described by Li et al. (A)

  2 (82 mg, 157 μmol) and [Ir(COD)Cl)]2 (53 mg, 79 μmol). A pale yellow, air-sensitive product was obtained (118 mg, 87%).

	C37H46BrClIrN2P (857.33). Calcd for C37H46BrClIrN2P.CH2Cl2: C 48.44, H 5.13, N 2.97;
	found C 48.41, H 5.13, N 2.92.
	First isomer:
	1 H NMR (500 MHz, CD2Cl2, 20°C): δ = 9.73 (d, 4 J = 1.1 Hz, 1 H, NCHN), 8.02-8.05 (m, 2
	H, o-CH(Ph 2 )), 7.63-7.69 (m, 3 H, m,p-CH(Ph 2 )), 7.39-7.55 (m, 5 H, o,m,p-CH(Ph 1 )), 7.32 (d,
	3 J = 6.5 Hz, 1 H, m-CH(DIPP)), 7.31 (d, 3 J = 6.6 Hz, 1 H, m-CH(DIPP)), 6.28 (d, 3 J = 1.6 Hz,
	1 H, (DIPP)NCH=), 5.70-5.78 (m, 1 H, NCH2), 5.31-5.36 (m, 1 H, CH(COD)), 5.16-5.20 (m,
	1 H, CH(COD), 4.67-4.75 (m, 1 H, CH(COD)), 3.89-3.97 (m, 1 H, NCH2), 3.66-3.74 (m, 1 H,
	CH(COD)), 3.54-3.61 (m, 1 H, CH2P), 3.00-3.09 (m, 1 H, CH2(COD)), 2.66-2.91 (m, 3 H,
	CH2(COD); CH2P), 2.54-2.63 (m, 1 H, CH2(COD)), 2.32-2.52 (m, 4 H, CH2(COD);
	CH(CH3)2), 1.90-2.12 (m, 1 H, CH2(COD)), 1.16-1.26 (m, 12 H,CH(CH3)2), -14.41 (d, 2 JP-H =
	8.2 Hz, 1 H, Ir-H). 13 C NMR (125.8 MHz, CD2Cl2, 20°C): δ = 146.0; 145.3 (s, o-C(DIPP)),

  PPh2)), 132.5 (d, 4 JP-C = 2.4 Hz, p-CH(PPh2)), 131.4 (d, 4 JP-C = 2.5 Hz, p-CH(PPh2)), 131.1 (s, NC(Mes)), 131.0 (s, p-CH(DIPP)), 129.4 (d, 3 JP-C = 10.5 Hz, m-CH(PPh2)), 128.1 (s d, 3 JP-C = 10.6 Hz, m-CH(PPh2)), 125.3 (d, 3 JC-P = 8.5 Hz, (DIPP)NCH=), 124.3; 124.2 (s, m-CH(DIPP)), 120.2 (d, 2 JC-P = 8.6 Hz, Ir-C), 98.7 (d, 2 JP-C = 15.7 Hz, CH(COD)), 94.2 (d, 2 JP-C JP-C = 3.9 Hz, CH2(COD)), 25.1 (d, 1 JC-P = 39.6 Hz, CH2P), 24.3; 24.2; 24.1; 24.0 (s, CH(CH3)2). 31 P NMR (202.5 MHz, CD2Cl2, 20°C): δ = -2.04 (s, PPh2).

	= 9.3 Hz, CH(COD)), 93.9; 84.2 (s, CH(COD), 45.8 (s, NCH2), 35.2 (d, 3 JP-C = 2.1 Hz,
	CH2(COD)), 32.2 (s, CH2(COD)), 29.4 (d, 3 JP-C = 2.1 Hz, CH2(COD)), 28.6; 28.5 (s,
	CH(CH3)2), 28.3 (d, 3 MS (ESI, positive mode) m/z (%) = 777.9 (100) [C37H46ClIrN2P + ], 821.8 (33)
	[C37H46BrIrN2P + ].
	JC-P = 40.3 Hz, CH2P), 24.3; 24.2; 24.1; 24.0
	(s, CH (CH3)2). 31 P NMR (202.5 MHz, CD2Cl2, 20°C): δ = -3.08 (s, PPh2).
	Second isomer:
	1 H NMR (500 MHz, CD2Cl2, 20°C): δ = 9.71 (d, 4 J = 1.2 Hz, 1 H, NCHN), 7.98-8.01 (m, 2
	H, o-CH(Ph 2 )), 7.63-7.69 (m, 3 H, m,p-CH(Ph 2 )), 7.39-7.55 (m, 5 H, o,m,p-CH(Ph 1 )), 7.36 (d,
	3 J= 7.9 Hz, 1 H, m-CH(DIPP)), 7.35 (d, 3 J = 7.9 Hz, 1 H, m-CH(DIPP)), 6.35 (d, 3 J = 1.5 Hz,
	1 H, (DIPP)NCH=), 5.70-5.78 (m, 1 H, NCH2), 5.37-5.43 (m, 1 H, CH(COD)), 5.10-5.17 (m,
	1 H, CH(COD), 4.67-4.75 (m, 1 H, CH(COD)), 4.20-4.27 (m, 1 H, NCH2), 3.76-3.83 (m, 1 H,
	CH(COD)), 3.38-3.43 (m, 1 H, CH2P), 3.00-3.09 (m, 1 H, CH2(COD)), 2.66-2.91 (m, 3 H,
	CH2(COD); CH2P), 2.54-2.63 (m, 1 H, CH2(COD)), 2.32-2.52 (m, 4 H, CH2(COD);

CH(CH3)2), 1.90-2.12 (m, 1 H, CH2(COD)),

1.16-1.26 (m, 12 H,CH(CH3)

2), -15.07 (d,

[START_REF] Hillier | [END_REF]

JP-H = 8.1 Hz, 1 H, Ir-H).

13

C NMR (125.8 MHz, CD2Cl2, 20°C): δ = 145.9; 145.4 (s, o-C(DIPP)), 137.6 (s, NCHN), 134.4 (d, 2 JP-C = 10.5 Hz, o-CH(PPh2)), 132.9 (s, 2 JP-C = 8.0 Hz, o-CH(

Acknowledgements

We thank the Centre National de la Recherche Scientifique (C.N.R.S.) for support of this work, the Fonds Social Européen (F.S.E.) for a Ph.D. fellowship to Joffrey Wolf, and Yannick Coppel for N.M.R. analysis of the iridium complexes.

Graphical abstract 1

Phosphine-imidazolium salts add to iridium complex [Ir(COD)(-Cl)]2 without the need of a base and lead to iridium(III) hydride complexes containing "abnormal" carbene ligands, through C-H insertion at the C5 position of the imidazolium ring.


Ulrich Baisch

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr

Prof Rinaldo Poli

Copper (I/II) and Cobalt (II) Coordination Chemistry of Relevance to Controlled Radical Polymerization Processes

Keywords: Coordination Chemistry, Cobalt, Copper, Me6TREN ligand, Controlled Radical Polymerization

The reaction of various alkyl halides (1-bromoethylbenzene, BEB; 1-chloroethylbenzene, CEB; methyl iodide; 1-bromoethylacetate, BEA; and ethyl-2-bromopropionate, EBP with CuBr/L (L = bipy, Me6TREN) has been investigated in the presence of [Co(acac)2] or [Co(acac)2]/py, with the goal of generating radicals by halogen atom transfer to Cu I for subsequence trapping by Co II and generation of an alkylcobalt(III) product. No evidence for the formation of the target product has been obtained. The various reactions, carried out under different conditions, have led to the crystallization of the following compounds: {[Cu(bipy)2Br] + }2{[Cu2Br4] 2-}, 1; [Cu(Me6TREN)Br]Br; [Cu(Me6TREN)Br0.37Cl0.63]Br, 2; and trans-[Co(acac)2(THF)2], 3; [Co(Me6TREN)Br]Br, 4; [Cu(Me7TREN)I]{[Cu2I2-(Br0.628I0.372)2]0.9[I]0.2}, 5; [C6H8N]2[CoI4], 6; [Co(acac)3]; and [Co(acac)2(THF)2CoBr2], 7. The structures of 1-7 have been determined by single-crystal X-ray diffraction.

INTRODUCTION

Controlled radical polymerization (CRP) is the fastest developing field in modern polymer chemistry [START_REF] Matyjaszewski | Macromolecular Engineering: Precise Synthesis, Materials Properties, Applications[END_REF]. The scope and flexibility of this technique to access polymers with well-defined structure, chain length, and composition is unmatched by any other polymerization method.

Although non-metal-based methods are also known, the coordination and organometallic chemistry of the transition metals involving one-electron processes (homolytic bond cleavage, atom transfer reactions) has been shown to be of fundamental importance in CRP [START_REF] Poli | [END_REF].

There are three ways in which a transition metal complex can mediate CRP, all based on one-electron processes: (i) reversible halogen atom transfer to a reduced metal catalyst, termed atom transfer radical polymerization (ATRP) [3,4]; (ii) homolytic cleavage of metal-carbon bonds, termed organometallic radical polymerization (OMRP) [5,6]; (iii) degenerative transfer (DT), involving a rapid associative exchange between the active and dormant chains on a transition metal complex [7,[START_REF] Wayland | [END_REF]. Our group investigates the intimate details of coordination chemistry related to transition metal mediated CRP. We have developed a family of compounds capable of simultaneous control by ATRP and OMRP [9][10][11], rationalized the effect of certain Lewis acid promoters [12,13] and explained reason for the observation of low initiator efficiencies under certain conditions [14]. The possible interplay of various one-electron processes in CRP has been recently outlined in a review [START_REF] Poli | [END_REF].

Of current interest is the unique ability of compound Co(acac)2 to mediate the CRP of vinyl acetate (VAc), a challenging monomer for which previous attempts by ATRP and OMRP methods had failed. This compound was recently reported to provide excellent control by a proposed OMRP mechanism for a bulk polymerization (i.e. without solvent or additives) [15,16], but our studies have in fact demonstrated its action through a DT mechanism under these conditions, though the mechanism switches to OMRP in the presence of neutral donors [17].

The reasons for this behavior are summarized in Scheme 1. Whereas the Co-C bond in the organometallic dormant chain is not weak enough to afford a significant free radical concentration in the absence of ligand, stabilization of the Co II radical trap by further coordination of L to give 6-coordinate [Co(acac)2L2] shifts the radical formation equilibrium and allows the CRP to be sustained by the OMRP mechanism. The 6-coordinate, L-bonded Co III dormant chain cannot undergo DT because it is coordinatively saturated, whereas the 5coordinate ligand-free chain, [(acac)2Co-(m)xR], is able to promote DT. In a more recent contribution, we have carried out computational investigations on the nature of the Co III chainend [18]. However, no simple alkylcobalt(III) compound that could represent a model of the OMRP dormant chain or DT transfer agent has so far been isolated and characterized with the acetylacetonate ligand environment, [RCo(acac)2] or [RCo(acac)2L]. This is in contrast with other alkylcobalt(III) species, containing for instance a porphyrin ligand environment [19][20][21]. We have therefore set out to prepare one such compound. This particular synthetic target does not appear straightforward. With supporting ligands such as porphyrin of Schiff bases, organylcobalt(III) products are easily accessed by first reducing Co II L to [Co I L] -, followed by reaction with an alkyl halide [19,20,22]. The hard acac ligand, however, does not provide sufficient stability to the putative [Co(acac)2] -anion. A few derivatives have also been prepared by alkylation of halocobalt(III) reagents [23]. However, [Co III (acac)2Y] (Y = halogen) are nonexisting compounds. A final method leading to organylcobalt(III) compounds is one-electron oxidative addition of alkyl halides to Co II reagents [23], see Scheme 2, a drawback being that half of the Co II reagent is sacrificed for making the Co III -Y by-product.

2[Co II L n ] + R-Y [L n Co III -R] + [L n Co III -Y]
Scheme 2. Generation of R-Co III products by one-electron oxidative addition or R-Y to Co II precursors.

However, we find that no reaction takes place between [Co(acac)2] and BEB, which is a quite active alkyl halide (weak C-Y bonds). The reason for this lack of reactivity may be thermodynamic: cobalt prefers to remain in the +II oxidation state with hard ligands such as acac, and the co-product [Co(acac)2Y] (which is non-existent as already mentioned) does not provide sufficient thermodynamic help to achieve the oxidative addition process. A possible key to alleviate this problem is therefore to use a stronger halogen acceptor. We have considered the use of the Cu I system CuBr/L (L = N-based ligand such as hexamethyl-tris(aminoethyl)amine (Me6TREN) or 2,2'-bipyridine (bipy)). It has already been reported that CuBr/L systems abstract efficiently halide radicals from alkyl halides, forming the related Cu II products [24][25][26][27], while Cu I does not have sufficient affinity for carbon-based radicals [28]. Indeed, this is the basis for the high efficienty of this family of Cu I systems as ATRP catalysts [3]. Some of these complexes, especially the in situ made [(Me6TREN)CuBr], are very rapid under very mild conditions (room temperature or below) [29]. In this manner, we were hoping that alkyl radicals could be efficiently trapped by [Co(acac)2]. Scheme 3 shows the reaction pathway envisaged for this transformation, which could be carried out in the presence of an additional monodentate ligand L, such as pyridine. Indeed, the organylcobalt(III) product should be stabilized by addition of a neutral ligand, with formation of a 6-coordinate [Co(R)(acac)2(L)] complex, because low-spin Co III is known to strongly prefer an octahedral coordination environment. In this contribution, we present the results of synthetic efforts along these lines.

[L n Cu]Br + R-Y [L n Cu-Br]Br + R [Co(acac) 2 ] + R (+ L) [(acac) 2 Co-R] (or [Co(R)(acac) 2 (L)]) [L n Cu-Br]Br + [(acac) 2 Co-R] [L n Cu]Br + [Co(acac) 2 ] + R-Y (+ L) (or [Co(R)(acac) 2 (L)])
Unfortunately, we have not attained our desired goal, because many side reactions take place as will be discussed in detail in the Results and Discussion section. However, the isolated and characterized compounds present some novelty in Cu I/II and Co II coordination chemistry and may therefore represent useful reference for future work in CRP by the use of these metal systems.

RESULTS AND DISCUSSION

Reaction of [Cu(bipy)2]Br and 1-bromoethylbenzene

We started our studies with the CuBr/bipy system, since the ligand is easily available. This is known to form the salt-like compound [Cu(bipy)2] + Br -in solution [30]. However, this compound is not one of the fastest activators for ATRP (i.e. capable of generating free radicals from an alkyl halide at a high rate under mild conditions) [29]. The rate of radical generation depends on the C-Y bond strength of the halide compound used [31], therefore we restricted our studies with this copper complex to the relatively active initiator 1-bromoethylbenzene (BEB).

The reaction was carried out in MeOH at room temperature, leading to the immediate formation of a red precipitate after the addition of CuBr, signaling the formation of a Cu(II) product. This compound was crystallized and characterized by X-ray diffraction.

The X-ray determination revealed the formation of the mixed valence Cu(I)/Cu(II) salt {[Cu(bipy)2Br] + }2{[Cu2Br4] 2-} (1) (Figure 1). Selected bond lengths and distances are shown in [32], as well as with other anions [33][34][35][36] and the [Cu2Br6] 2-dianion has also been crystallographically characterized in other compounds [37][38][39][40][41][42][43][44][45]. All bond distances are in good agreement with those reported in the above cited structures. The formation of compound 1 suggests that CH3(Ph)CH • radicals are being produced, therefore this reaction was repeated in the presence of [Co(acac)2]. However, the course of the reaction did not change and there was no spectroscopic evidence for the formation of an alkylcobalt(III) derivative. It is possible that the Co-CH(CH3)Ph bond is too weak to be formed in an irreversible fashion at room temperature. Therefore, we have tried to generate these radicals at lower temperatures with a more active Cu complex.

Reaction of CuBr(Me6TREN) and 1-bromoethylbenzene or 1-chloroethylbenzene

Me6TREN, in combination with CuBr, provides one of the fastest initiating systems in copper mediated CRP. In order to insure sufficient solubility of all materials at low temperature, we switched solvent to THF. As already described [47], complexes [Cu(Me6TREN)X] (X = Cl, Br, I) that form by combination of CuX and Me6TREN are not stable at room temperature, yielding fast disproportionation into Cu 0 and [Cu II (Me6TREN)X]X and are also quite sensitive to aerial oxidation. This has been confirmed in our study: the disproportionation can be easily observed by the development of a green color, characteristic of the Cu II system, from the colorless solution of the Cu I complex. At lower temperatures and under strict exclusion of air and moisture the solution remains colorless.

Addition of BEB (1 equiv.) and [Co(acac)2] led to the isolation of light green singlecrystals, which were identified as the already reported [48] [Cu(Me6TREN)Br]Br. The same reaction was worked out using 1-chloroethylbenzene (CEB) instead of BEB, with similar results.

Single-crystal structure determination of the green crystals revealed the formation of The formation of these Cu II complexes confirms the fast and effective oxidation of Cu I by halogen atom transfer from the organic halide molecule, therefore implying the formation of CH3(Ph)CH • radicals in solution. The above reactions yielded a pink/red THF solution as the mother solution of the isolated crystals. There was no indications from spectroscopic analyses that the formation of an alkylcobalt(III) compound had taken place, while attempts to crystallize the pink/red component soluble in THF were not successful. We conclude that, even at lower temperatures, the Co III -CH(CH3)Ph bond is probably too weak to withstand reversible homolytic rupture and ultimately leads to decomposition with formation of the typical radical decomposition products (coupling to PhCH(CH3)CH(CH3)Ph and disproportionation to ethylbenzene and styrene).

Reaction of CuBr(Me6TREN) and ethyl-2-bromopropionate

Using this organic halide, no apparent reaction occurred at low temperature, indicating that the C-Br bond is less susceptible to atom transfer than that of BEB. Warming the solution to room temperature led to the development of a green color, which yielded crystals of Co(acac)3.

When the same reaction was repeated and maintained at low temperature for an extended period, red crystals were isolated, corresponding to the THF solvate complex [Co(acac)2(THF)2], 3.

Compound 3 is only stable at low temperature and decomposes by solvent loss upon standing for longer than 5 min under mineral oil at ambient temperature. The Co II ion has an octahedral coordination with the two acac ligands in equatorial positions and THF in the axial positions. A two-fold axis is situated in the metal centre of the molecule (Figure 3). Many Xray structures of trans-[Co(acac)2(L)2] derivatives have been previously published (L = py, H2O, CH3OH, imidazole, etc.) [49][50][51][52][53][54], but that with THF had not yet been reported. The THF molecules show disorder with half of the molecules oriented in two different envelope conformations. Selected bond distances and angles are shown in Table 3. The Co-O(acac) bond lengths are in good agreement with those reported for the above mentioned other solvent adducts, as well as with those of solvent-free [Co(acac)2] [51,55], where in fact the coordination environment is pseudo-octahedral due to skewed stacking with neighboring molecules. The Co-O(THF) distances are significantly longer than the Co-O(acac) bonds. This result is quite logical considering that a neutral solvent molecule should be more weakely bonded to the metal centre compared to the anionic acac ligand.

a (i) -x, -y, 1-z
The isolation of compound 3 is not to be considered surprising, since the solution contains

[Co(acac)2] in THF, thus the bis-adduct is certainly present, at least in equilibrium amounts, especially at low temperatures. The recovery of 3 gives no indication as to whether or not the Cu I reagent has abstracted a halogen atom.

Reaction of CuBr(Me6TREN) and methyl iodide

We have next turned our attention to an alkyl halide that would yield a stronger Co-C bond. The homolytic bond dissociation energy of R-X bonds for a variety of R groups and for X = Cl, Br, I has been the subject of a computational study [18]. Methyl is the alkyl group at the top of the list.

Although not yet supported by an extensive computational investigation on an homologous series of organometallic compounds, it may be anticipated that the strength of M-R bonds should roughly parallel that of R-X bonds, thus the (acac)2Co-CH3 bond is expected to have a reasonable strength. Indeed, recent calculations have shown that (acac)2Co-R bonds with R = CH3(X)CH • (X = CH3O, CH3COO and CH3OOC) are weaker than with R = CH3 [18]. On the other hand, the strength of CH3-X increases in the order X = I < Br < Cl. Thus, Me3Cl should be difficult to activate. We have chosen to work with CH3I, also in consideration of the fact that this compound is a volatile liquid and therefore easier to handle. Several reactions were carried out at different temperatures and with different order of reagent addition, but no alkylcobalt(III) compound was crystallized under any circumstances. The products that were crystallized are summarized in Scheme 4. well known for its ability to change the color of a correct structure significantly [START_REF] Kleber | Einführung in die Kristallographie[END_REF]. Selected distances and angles for this compound are listed in Table 5. ions. One curious observation is that compound 4 contains only bromide and no iodide, including as free anion, even though it is obtained under very similar conditions as compound 5

where plenty of iodide enters the coordination sphere of the metal in both cation and anion.

Reaction of CuBr(Me6TREN) and 1-bromoethylacetate

Until now, we have found that the CH3(Ph)CH • radicals are too stabilized to afford sufficiently strong R-Co III bonds, whereas MeI, which should lead in principle to a stable Me-Co III derivative, prefers to yield complex products of nucleophilic substitution at carbon and subsequent halide and neutral ligand exchange processes between copper and cobalt. In light of these results, our last attempt involved a halide molecule that is precursor of a radical of Using this organic halide, no apparent reaction occurred at low temperature, whereas only compound Co(acac)3 was identified amongst the product obtained after warming the solution to room temperature, as already observed for the reaction with BEP. If the reaction solution was not allowed to warm up but rather kept at -80 °C, the formation of blue crystals was observed over a period of 10 days. This product was identified as a new binuclear Co(II) complex

[Co(acac)2(THF)2CoBr2], 7. The molecular structure of this compound is depicted in Figure 7 and selected bond lengths and distances are listed in Table 7. The corresponding dichloride compound, [Co(acac)2(THF)2CoCl2], displaying the same coordination geometry, has previously been described [74]. have not met with success. We cannot exclude that such a species is formed, but we do not have any evidence to support this possibility at the moment.

CONCLUSION

[Co(acac)2] has so far resisted the synthesis of a simple alkyl derivative, [RCo III (acac)2] or [RCo III (acac)2(L)]. All attempts made in our laboratory and reported herein have resulted in side reactions that reveal the limitations of this synthetic strategy and the challenge of this synthetic target: the R-Co III is weak for the unreactive radicals, namely those that can be generated most easily; R-X precursors with reactive radicals engage more readily in other reaction pathways (nucleophilic halide displacement by neutral ligands) whereas the simple oneelectron oxidative addition is thermodynamically unfavorable. However, we still believe that it may be possible, given an appropriate method of rapidly generating a large amount of reactive radicals under very mild conditions, to synthesize a sufficiently stable organylcobalt(III) complex supported by two acetylacetonate groups, which would represent a model for the chain end in the dormant form of PVAc grown by [Co(acac)2]-mediated CRP.

EXPERIMENTAL SECTION

General. All manipulations were carried out under strict oxygen-free and dry argon atmosphere. Reaction vessels were flame-dried under vacuum before use and standard Schlenk techniques were used to carry out all reactions. Toluene and THF were purified by distillation under argon after drying over sodium/benzophenone ketyl. n-Pentane and CH2Cl2 were dried over LiAlH4 and P4O10 and then freshly distilled under argon prior to use. MeI, ethyl-2bromoproprionate (EBP), 1-bromoethylbenzene (BEB), and 1-chloroethylbenzene (CEB) were dried over molecular sieve and distilled under reduced pressure before use. Me6TREN [75] and 1-bromoethylacetate (BEA) [76,[START_REF] Cho | Application: KR KR[END_REF] were synthesized according to literature methods.

[Co(acac)2] was purchased from Alfa Aesar and sublimed under reduced pressure and filled into glass ampoules under argon atmosphere. CuBr (99.9%) was purchased from Alfa Aesar. All other reagents used were of commercially available reagent-grade quality and were used without further purification.

Syntheses.

Formation of compound {[Cu(bipy)2Br] + }2{[Cu2Br4] 2-}, 1.

A Schlenk tube equipped with a magnetic stirrer was charged with 2 mL of freshly distilled methanol, 0.043 2,2'bipyridine (0.272 mmol), and 35µL BEB (0.262 mmol). CuBr (0.019 g; 0.13 mmol) was added subsequently at room temperature, leading to an immediate color change to intense brown-red.

Stirring overnight yielded a red precipitate, which was isolated and dissolved in acetone. Singlecrystals of 1 were produced upon storing the solution at 4 °C for 2 days.

Formation of [Cu(Me6TREN

)Br]Br. CuBr (0.05 g, 0.35 mmol) was placed into a Schlenk tube equipped with a magnetic stirrer and 3 mL of freshly distilled toluene was added.

Three freeze-pump-thaw cycles guaranteed an oxygen free atmosphere. 124 µL Me6TREN (0.52 mmol) were added at room temperature and the mixture was stirred until a colorless solution was obtained. The solution was then cooled to -90 °C using acetone and liquid nitrogen.

Subsequently, BEB (71 µL, 0.052 mmol) was added and the solution was stirred for another was dissolved in 2 mL of THF. Equimolar amounts of BEA (0.072 mL) and acetonitrile (0.060 mL) were subsequently added. The two solutions were combined at -80 °C using a cannula. The resulting brown-red solution was stirred for 3 h and than stored at -80 °C. After 5 weeks, blue crystals of compound 7 had formed on the bottom of the Schlenk tube.

X-ray Structural Studies.

A single-crystal of each compound was mounted under inert perfluoropolyether on the tip of a cactus spike and cooled immediately in the cryostream of an Oxford-Diffraction XCALIBUR CCD diffractometer. The structures were solved by direct methods (SHELXS-97 [START_REF] Sheldrick | SHELXS97. Program for Crystal Structure solution[END_REF]) and refined by least-square procedures on F 2 using SHELXL-97 [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF]. All H atoms attached to carbon atoms were introduced in ideal positions and treated as riding models. Absolute structures were confirmed by the refinement of Flack's enantiopole parameter [START_REF] Flack | [END_REF] and by careful examination of the sensitive reflections. Crystal data and refinement parameters are shown in

[Scheme 1 .

 1 Scheme 1. Influence of ligand coordination to the mechanism of [Co(acac)2]-mediated CPR ofVAc.

Scheme 3 .

 3 Scheme 3. Possible reaction pathway for alkyl halides in the presence of [{Me6TREN}Cu]Br and [Co(acac)2].

Figure 1 .

 1 Figure 1. Molecular structure of 1 at 180 K (50% probability ellipsoids).

[Table 2 .Figure 2 .

 22 Figure 2. Molecular structure of 2 at 180 K (50% probability ellipsoids).

Figure 3 .

 3 Figure 3. Molecular structure of 7 at 160 K (50% probability ellipsoids).

6 (I) 2 Scheme 4 . 4 Figure 4 .

 62444 Scheme 4. Products generated by the reaction of [Co(acac)2] with MeI in the presence of [Cu(Me6TREN)Br].

Figure 4 . 5

 45 Figure 4. Molecular structure of 4 at 150 K (50% probability ellipsoids).

Figure 5 .

 5 Figure 5. Molecular structure of 5 at 160 K (50% probability ellipsoids).

  II is very close to ideal tetrahedral, with all I-Co-I angles laying between 107.3 and 112.8°. The Co-I bond distances of 2.583 -2.608 Å coincide very well with those found in other compounds containing the same dianion [70-73].

Figure 6 .Table 6 . 18 ]

 6618 Figure 6. Molecular structure of 6 at 180 K (50% probability ellipsoids).

Scheme 5 .

 5 Scheme 5. Proposed structure of the dormant chain for the [Co(acac)2]-controlled polymerization of VAc.

  The molecule is composed of two bivalent Co atoms, connected to each other by two bridging oxygen atoms O11 and O22. Atom Co1 adopts an octahedral coordination mode, with two chelating acetylacetonate ligands and two trans THF molecules, whereas Co2 has a tetrahedral coordination environment, defined by the bridging atoms O11 and O22 of two different acetylacetonato ligands and two Br atoms. Due to the bridging oxygen atoms O11 an O22 which impose a small O11-Co2-O22 angle, the angle Br-Co-Br is widened relative to the tetrahedral value. The Co1-O bonds to the non-bridging atoms O12 and O21 are shorter than those to the bridging ones. The Co2-O bonds are even shorter, as expected from the lower coordination. All bond distances and angles involving the O atoms correspond very closely to those reported for the chloride analogue [74]. The Co-Br bonds agree well with values reported in the literature for other tetrahedrally coordinated Co II bromides .

Figure 7 .

 7 Figure 7. Molecular structure of 7 at 180 K (50% probability ellipsoids).

6 .

 6 30 min. In a separate Schlenk tube [Co(acac)2] (0.09 g, 0.35 mmol) was dissolved in 2 mL of THF and the solution was cooled to -90 °C. Three freeze-pump-thaw procedures guaranteed an oxygen-free atmosphere. While cooling down, the violet [Co(acac)2] solution became red. Then, the [Co(acac)2] solution was added to the other solution at -90 °C using a cannula and the reaction was stirred for another 2 hours and then placed in a -20°C freezer. Crystallization took place in 3 days. The crystalline green solid was redissolved in CH2Cl2 and crystallized placing the solution again into the freezer (-20°C). Suitable light green single-crystals were obtained, which were identified as the already reported[48] [Cu(Me6TREN)Br]Br by comparison of the unit cell parameters.Formation of compound [Cu(Me6TREN) Br0.37Cl0.63]Br, 2. CuBr (0.134 g, 0.93 mmol) was placed into a Schlenk tube equipped with a magnetic stirrer and 3 mL of freshly distilled THF was added. Three freeze-pump-thaw cycles guaranteed an oxygen free atmosphere.Me6TREN (220 µL, 0.93 mmol) was added at room temperature and the mixture was stirred until a colorless solution was obtained. The solution was then cooled down to -90 °C using acetone and liquid nitrogen. Subsequently, CEB (97 µL, 0.93 mmol) was added and the solution was stirred for another 30 min. In a separate Schlenk tube [Co(acac)2] (0.09 g, 0.35 mmol) was dissolved in 2 mL of THF and the solution was cooled to -90 °C. Three freeze-pump-thaw procedures guaranteed an oxygen-free atmosphere. While cooling down, the violet [Co(acac)2] solution became red. Then, the [Co(acac)2] solution was slowly added to the other solution at -90 °C using a cannula and the reaction was stirred for another 7 hours. Crystallization took place after addition of 0.5 mL n-pentane and yielded tetrahedral shaped dark green single-crystals of 2.Reaction of CuBr, Me6TREN, [Co(acac)2] and EBP. Formation of compound [Co(acac)3].[{Me6TREN}CuBr] was prepared freshly at -90 °C in THF by addition of Me6TREN (139 L, 0.6 mmol) to a mixture of CuBr (0.086 g, 0.6 mmol) in 3 mL THF. Addition of equimolar amounts of [Co(acac)2] (0.152 g), EBP (0.072 mL) and acetonitrile (0.060 mL) took place subsequently. After addition of [Co(acac)2] the solution turned red and after addition of EBP no change of color was observed. The solution was warmed up slowly to room temperature, leading to a green solution. This was evaporated to dryness and the residue was redissolved in acetone. The solution was placed into the fridge at 4°C and dark green crystals were obtained after two weeks. X-ray diffraction experiments identified this compound as [Co(acac)3] by comparison of the unit cell parameters with those already available in the literature[START_REF] Hon | [END_REF]79]. Formation of compound [Co(acac)2(THF)2], 3. [{Me6TREN}CuBr] was prepared freshly at -90 °C in THF by addition of Me6TREN (90 µL, 0.35 mmol) to a mixture of CuBr (0.050 g, 0.35 mmol) in 3 mL THF. In a separate Schlenk tube [Co(acac)2] (0.100 g, 0.39 mmol)was dissolved in 3 mL of THF and EBP (0.038 mL, 0.35 mmol) was added dropwise to the pink solution. The two solutions were combined at -80°C using a cannula. The resulting intense red solution was kept stirring for 3 h and than stored at -80 °C. After 1 week a blue solution had formed and red crystals of 3 had formed at the bottom of the Schlenk tube.Formation of compound [{Me6TREN}CoBr]Br, 4. CuBr (0.066 g, 0.46 mmol) was placed in a Schlenk tube equipped with a magnetic stirrer and 2 mL of freshly distilled THF was added. Three freeze-pump-thaw cycles guaranteed an oxygen free atmosphere. Me6TREN (107 µL, 0.46 mmol) were added at room temperature and the mixture was stirred until a colorless solution was obtained. The solution was then cooled down to -90 °C. Subsequently, methyl iodide (62 µL, 0.46 mmol) and immediately afterwards [Co(acac)2] (0.118 g, 0.46 mmol) were added to the solution. Warming up the reaction mixture to room temperature led to a brown solution over a large amount of amorphous white precipitate. The solid was separated and washed with a mixture of toluene/acetone. All toluene/acetone fractions were united and left standing on a vibration-free place. After one week both a white precipitate and violet crystals had formed at the bottom of the Schlenk tube. The crystals were separated and redissolved in CH2Cl2. Cooling the solution to 4 °C yielded tetrahedral shaped violet crystals of compound 4. Formation of compound [{Me7TREN}CuI]2[Cu1.80Br1.13I2.67], 5. CuBr (0.1 g, 0.70 mmol) was placed into a Schlenk tube equipped with a magnetic stirrer and 3 mL of freshly distilled THF was added. Three freeze-pump-thaw cycles guaranteed an oxygen free atmosphere. Me6TREN (165 µL, 0.70 mmol) were added at room temperature and the mixture was stirred until a colorless solution was obtained. The solution was then cooled to -90 °C. Subsequently, methyl iodide (100 µL, 1.2 mmol) and immediately afterwards [Co(acac)2] (0.02 g, 0.08 mmol) were added to the solution. Warming up the reaction mixture to -40 °C led to a green solution over a colorless precipitate. The solution was separated from the solid and evaporated to dryness under reduced pressure. The solid was dissolved in dry acetone, yielding a green solution. Addition of n-pentane and crystallization at -80 °C with pentane layered on top of the CH2Cl2 solution yielded almost colorless (slightly greenish) air and moisture sensitive crystals of compound 5 at the bottom of the Schlenk tube. Formation of compound [C5H5NMe]2[CoI4], 6. CuBr (0.021 g, 0.15 mmol) was placed in a Schlenk tube equipped with a magnetic stirrer and 3 mL of freshly distilled THF was added. Three freeze-pump-thaw cycles guaranteed an oxygen free atmosphere. Me6TREN (35 µL, 0.15 mmol) were added at room temperature and the mixture was stirred until a colorless solution was obtained. The solution was then cooled to -90 °C. [Co(acac)2] (0.080 g, 0.31 mmol) were added and dissolved, followed by methyl iodide (39 µL, 0.62 mmol) and pyridine (25 µL, 0.31 mmol). Storing the resulting red solution at -80 °C for one week yielded green plate-like crystals of compound Formation of compound [Co(acac)2(THF)2CoBr2], 7. [{Me6TREN}CuBr] was prepared freshly at -90 °C in THF by addition of Me6TREN (135 L, 0.58 mmol) to a mixture of CuBr (0.080 g, 0.56 mmol) in 3 mL THF. In a separate Schlenk tube [Co(acac)2] (0.150 g, 0.58 mmol)

Table 1

 1

	. The structure of 1 features a centrosymmetric [Cu2Br6] 2-ion, which sits on twofold
	axis, and two symmetry-related [Cu(bipy)2Br] + cations. The Cu(II) cation is in a distorted
	trigonal bipyramidal coordination, with axial Cu-N bonds of 1.981(2) and 1.987(2) Å while
	longer Cu-N bonds of 2.070(2) and 2.083(3) Å occur in the equatorial positions as well as the
	Cu-Br bond with 2.4936(9) Å. The monovalent Cu2 site adopts a trigonal planar CuBr3
	coordination, with Cu2-Br2 (bridging) bonds of 2.4689(10) Å and Cu2-Br3 (terminal) bonds of
	2.3215(12) Å. The structure of the cation has already been reported in the simpler
	[Cu(bipy)2Br]Br salt

Table 1 .

 1 Selected

	2-. Thus, not all CuBr has reacted with bipy,

Table 3 .

 3 Selected

	Co1-O2 2.012(2) O2-Co1-O3 i 90.38(9)	O2 i -Co1-O1 i 90.6(1)
	Co1-O3 2.025(2) O2 i -Co1-O3 i 89.62(9)	O3 i -Co1-O1 i 90.16(9)
	O1-C6	1.414(4) O2-Co1-O3 89.62(9)	O3-Co1-O1 i 89.84(9)
	O1-C9A 1.452(14) O2 i -Co1-O3 90.38(9)	C6-O1-Co1	126.4(2)
	O1-C9B 1.440(14) O2-Co1-O1 90.6(1)	C9B-O1-Co1 120.1(6)
	O2-C3	1.253(4) O2 i -Co1-O1 89.4(1)	C9A-O1-Co1 122.5(6)
	O3-C1	1.258(3) O3 i -Co1-O1 89.84(9)	C3-O2-Co1	126.5(2)
		O3-Co1-O1 90.16(9)	C1-O3-Co1	125.58(19)

bond distances (Å) and angles (deg) of [Co(acac)2THF2], 3. a Co1-O1 2.163(2) O2-Co1-O2 i 180.00(14) O2-Co1-O1 i 89.4(1)

Table 4 .

 4 Selected

bond distances (Å) and angles (deg) of [Co(Me6TREN)Br]Br, 4. a Co1-N1 2.137(2) N1-Co1-Br2 98.83(6)

C3-N2-Co1 108.15

(17)

Table 5 .

 5 Selected

	Cu1-N3	2.123(9)	N3-Cu1-N2	119.9(4)	I3 i -Cu2-Cu2 i 58.99(7)
	Cu1-N2	2.131(9)	N3-Cu1-N1	83.7(4)	Cu2-I3-Cu2 i 61.80(7)

bond distances (Å) and angles (deg) of [Cu(Me7TREN)I]2[Cu1.8Br1.13I2.67)], 5. a

Table 6 .

 6 The geometry around Co

Table 8 .

 8 Crystallographic information files (CIF) were also deposited with the Cambridge Crystallographic Data Centre. CCDC 674118-674124 contains the supplementary crystallographic data for compounds 1-7. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.ACKNOWLEDGMENTWe thank the "Agence National de la Recherche" (Contract No. NT05-2_42140) for support of this work.

Table 8 .

 8 Crystallographic data for all compounds.

	Compound	1	2	3	4	5	6	7
	Empirical formula	C20H16Br3Cu2N4 C12H30Br1.37Cl0.63CuN4 C18H30CoO6	C12H30Br2CoN4	C26H66Br1.13Cu3.80I4.67N8 C18H30Br2Co2O6 C18H30Br2Co2O6
	Formula Mass [g mol -1 ]	679.18	425.53	401.35	449.15	1415.17	754.80	620.10
	Temperature T [K]	180	180	160	150	160	180	180
	Wavelength  [Å]				0.71073 (Mo-K)
	Crystal system	triclinic	cubic	monoclinic	cubic	triclinic	orthorhombic	orthorhombic
	Space group	P1	P21/3	C2/c	P21/3	P1	C2/c	P212121
	a (Å)	8.7884(18)	11.9666(6)	15.485(2)	12.0768(3)	8.2697(10)	28.376(4)	9.5934(7)
	b (Å)	11.449(2)		11.1151(14)		8.5175(10)	9.2662(9)	14.5601(11)
	c (Å)	12.346(3)		12.9790(13)		18.607(2)	16.5472(14)	17.1601(13)
	 (deg)	64.95(3)				77.337(10)
	 (deg)	84.89(3)		116.091(10)		86.862(15)	109.953(9)
	 (deg)	77.64(3)				66.140(11)
	Cell volume (Å 3 )	1099.3(5)	1713.61(15)	2006.3(4)	1761.39(8)	1168.6(3)	4089.7(8)	2396.9(3)
	Formula units / cell Z	2	4	4	4	1	8	4
	calc. (g cm -3 )	2.052	1.649	1.329	1.694	2.011	2.452	1.718
	 (mm -1 )	7.401	4.551	0.883	5.506	5.783	6.869	4.746
	F(000)	654	870	852	908	675	2728	1240
	crystal size (mm 3 )	0.380.290.24 0.340.310.30	0.240.120.09	0.290.260.25	0.810.470.40	0.190.180.04 0.120.110.06
	Diffractometer device			Oxford-Diffraction XCALIBUR
	Scan type				omega scan
	2 range (deg)	3.43 -32.11	2.41 -27.45	2.51 -25.00	2.92 -32.04	2.67 -27.50	2.62 -32.02	2.76 -27.50
	Reflections connected	11900	14609	5996	19109	10011	15764	18634
	Unique reflections (Rint)	6967 (0.0230)	1314 (0.0417)	1774 (0.0332)	2007 (0.0263)	5337 (0.0697)	6264 (0.0336)	5518 (0.0722)
	Reflctns with F0 2 ≥ 2σ (F0 2 )	5230	1253	1275	1890	1854	3471	3311
	Completeness to  (%)	90.5 (32.11°)	100 (27.45°)	99.9 (25.00°)	98.4 (32.04°)	99.6 (27.5°)	87.9 (32.02°)	99.8 (27.50°)
	Absorption correction				semiempirical
	Min. -max. trans.	0.086, 0.1708	0.189, 0.257	0.7871, 0.9200	0.1961, 0.2500	0.5411, 0.7952	0.3384, 0.7900	0.6085, 0.7741
	Refinement method				full-matrix least squares on F 2
	Data/restrains/parameters	6967/0/262	1314/0/60	1774/0/125	2007/0/59	5337/0/201	6264/0/173	5518/0/254
	Goodness-of-fit on F 2	1.067	1.315	1.038	1.374	0.781	0.901	0.922
	R1 [I > 2(I)]	0.0366	0.0307	0.0379	0.0253	0.0446	0.0267	0.0392
	wR2	0.0850	0.0861	0.1117	0.0626	0.0925	0.0677	0.0763
	Absolute structure param.		-0.05(2)		0.008(14)
	Max, min resid. density (eÅ -	1.043, -0.960	1.618, -0.487	0.465, -0.302	2.746, -0.695	1.429, -0.975	0.983, -1.121	0.830, -0.947
	3 )

Compound 7 forms under similar operating conditions that led to the isolation of 3 by using the reagent BEP. Both products are coordination compounds of Co II . However, whereas compound 3 may be obtained by direct interaction of [Co(acac)2] and the solvent, without the intervention of the halide reagent, the incorporation of bromine atoms in compound 7 requires the intervention of the BEA reagent, therefore giving indirect evidence of the production of CH3(CH3COO)CH • radicals. Recall that no disproportionation occurs for [Cu(Me6TREN)Br] when this is kept at low temperatures. The formation of compound 7 may be envisaged as follows. Bromine atom transfer to [CuBr(Me6TREN)] affords [Cu(Me6TREN)Br] + Br -and CH3(CH3COO)CH • . Ligand exchange between [Co(acac)2] and the bromide ion generates "Co(acac)Br", which may dimerize and undergo ligand redistribution, to yield 7 with the help of further coordination of two THF molecules. According to our previous findings [17,18], the CH3(CH3COO)CH • radicals should be efficiently trapped by [Co(acac)2]. However, our best efforts to identify an organometallic species by 1 H NMR in the mother solution of the crystals

Material for the Table of Contents Text

The reaction between CuBr/Ln (Ln = 2 bipy or Me6TREN) and a variety of alkyl halides in the presence of [Co(acac)2] leads to a variety of products originating from atom transfer and ligand redistribution reactions. The formation of a stable alkylcobalt(III) derivative was not observed.

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