One-Electron Transformations of Paramagnetic Cobalt Complexes. Synthesis and Structure of Cobalt(II) Amidodiphosphine Halide and Alkyl Complexes and Their Reaction with Alkyl Halides Michael D. Fryzuk,* Daniel B. Leznoff, Robert C. Thompson, and Steven J. Rettig ‡ Contribution from the Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer, B.C., Canada V6T 1Z1 ReceiVed June 30, 1997 Abstract: Complexes of the type CoX[N(SiMe 2 CH 2 PPh 2 ) 2 ], where X ) Cl, Br, or I, can be prepared via reaction of CoX 2 with LiN(SiMe 2 CH 2 PPh 2 ) 2 ; these derivatives are tetrahedral high-spin d 7 systems. Reaction of these halide complexes with organolithium, sodium, or potassium reagents generates square-planar, low- spin hydrocarbyl complexes of the formula CoR[N(SiMe 2 CH 2 PPh 2 ) 2 ] (R ) Me, CH 2 Ph, CH 2 SiMe 3 ,C 5 H 5 ). One-electron oxidations have been carried out; only the product of halide abstraction is observed. For example, addition of PhCH 2 X to the halide derivatives CoX[N(SiMe 2 CH 2 PPh 2 ) 2 ] generates trivalent, paramagnetic complexes, CoX 2 [N(SiMe 2 CH 2 PPh 2 ) 2 ]; these derivatives show variable-temperature magnetic susceptibility data that are consistent with zero-field splitting of the S ) 1 state. Addition of methyl bromide or methyl iodide to low-spin CoMe[N(SiMe 2 CH 2 PPh 2 ) 2 ] results in the formation of the Co(II) halide derivatives CoX- [N(SiMe 2 CH 2 PPh 2 ) 2 ] along with methane and bibenzyl. It is proposed that the Co(III) methyl halide complex CoMe(X)[N(SiMe 2 CH 2 PPh 2 ) 2 ] is unstable and loses methyl radical homolytically to generate the Co(II) halide derivative; the methyl subsequently reacts with the toluene solvent to produce methane and bibenzyl. Addition of excess benzyl halides has also been found to generate the Co(II) halide complexes initially, followed by a one-electron oxidation to the Co(III) dihalide derivatives. In much of the one-electron chemistry of the Co(II) derivatives incorporating the amidodiphosphine ligand, the decomposition of the putative but unstable Co(III) alkyl halide derivative CoRX[N(SiMe 2 CH 2 PPh 2 ) 2 ] is proposed as a recurring event. Introduction Electron transfer and radical processes figure prominently in many different areas of chemistry. 1-5 In organic chemistry, convenient and versatile sources of alkyl radicals are in demand to initiate a wide variety of reactions. 6,7 Polymerization with free radicals is arguably one of the most important industrial processes for high molecular weight polymers. 6 In transition metal chemistry, the discovery that the active site of vitamin B 12 contains a readily homolyzable Co(III)-carbon bond 8,9 has fueled research into the preparation and reactivity of cobalt complexes, 10-16 particularly those species that can mimic coenzyme bioinorganic functions. That paramagnetic transition metal complexes provide a unifying theme to all of the above areas is not accidental; metal complexes with unpaired electrons are part of the fabric of coordination chemistry and are now becoming more prevalent in organometallic chemistry. The reaction of alkyl halides with paramagnetic metal complexes has been the focus of numerous studies. Not only do these processes have relevance to radical-based organic transformations but they are probably one of the most common methods for the synthesis of vitamin B 12 models, although other reagent types, including direct addition of in situ generated radicals to Co(II) systems, 17 have been described. Although most free radical polymerizations do not involve metal com- plexes, there is the recent discovery that living radical poly- merizations can be achieved in the presence of Cu(II) deriva- tives. With regard to cobalt complexes, both one- and two- electron processes involving alkyl halides have been reported; 18-20 for example, the two-electron oxidative addition of alkyl halides to Co(I) derivatives (eq 1) is a classic route to Co(III) species: 21,22 Of particular significance is the addition of alkyl halides ‡ Professional Officer: UBC X-ray Structural Laboratory. (1) Fossey, J.; Lefort, D.; Sorba, J. Free Radicals in Organic Chemistry; John Wiley & Sons: New York, 1995. (2) Eberson, L. Electron-Transfer Reactions in Organic Chemistry; Springer-Verlag: Berlin, 1987. (3) Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, J. R., Mataga, N., McLendon, G. L., Eds.; American Chemical Society: Washington, DC, 1991; Vol. 228. (4) Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier: New York, 1990. (5) Macromol. Symp. 1987, 10-11. (6) Matyjaszewski, K.; Patten, T. E.; Xia, J. J. Am. Chem. Soc. 1997, 119, 674. (7) Curran, D. P. Synthesis 1988, 417. 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