Insights into the Photochemical Disproportionation of Transition Metal Dimers on the Picosecond Time Scale Justin P. Lomont, Son C. Nguyen, and Charles B. Harris* Department of Chemistry, University of California, Berkeley, California 94720, United States United States Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States * S Supporting Information ABSTRACT: The reactivity of five transition metal dimers toward photochemical, in-solvent-cage disproportionation has been inves- tigated using picosecond time-resolved infrared spectroscopy. Previous ultrafast studies on [CpW(CO) 3 ] 2 established the role of an in-cage disproportionation mechanism involving electron transfer between 17- and 19-electron radicals prior to diffusion out of the solvent cage. New results from time-resolved infrared studies reveal that the identity of the transition metal complex dictates whether the in-cage disproportionation mechanism can take place, as well as the more fundamental issue of whether 19-electron intermediates are able to form on the picosecond time scale. Significantly, the in-cage disproportionation mechanism observed previously for the tungsten dimer does not characterize the reactivity of four out of the five transition metal dimers in this study. The differences in the ability to form 19-electron intermediates are interpreted either in terms of differences in the 17/19-electron equilibrium or of differences in an energetic barrier to associative coordination of a Lewis base, whereas the case for the in-cage vs diffusive disproportionation mechanisms depends on whether the 19-electron reducing agent is genuinely characterized by 19-electron configuration at the metal center or if it is better described as an 18 + δ complex. These results help to better understand the factors that dictate mechanisms of radical disproportionation and carry implications for radical chain mechanisms. I. INTRODUCTION While the majority of organometallic complexes contain an even number of valence electrons, the importance of odd electron species has long been known. 1-3 Among odd electron complexes, the most commonly encountered are 17-electron (17e) and 19-electron (19e) species. Electron rich 19e complexes have been implicated in electron transfer reactions as well as catalytic processes. 3 In the present study, we focus our attention on the reactivity of 17e and 19e complexes toward radical disproportionation on the picosecond time scale. In the reactions studied in this paper, photochemical cleavage of the metal-metal bond in a transition metal dimer leads to the initial formation of a 17e radical pair, and these 17e radicals may coordinate a Lewis base to form a 19e adduct. A 17e and 19e radical pair may then undergo disproportionation, transferring an electron to yield two 18e complexes. Previous picosecond studies on the photolysis of [CpW(CO) 3 ] 2 in the presence of Lewis bases have demonstrated that radical disproportionation between 17e and 19e species can occur prior to diffusion out of the solvent cage. 4 Here we investigate the role of the identity of the transition metal complex in these reactions, and we find that different transition metal complexes exhibit markedly different reactivity with regard to picosecond disproportionation and the formation of 19e adducts. For 19e complexes, a distinction is often made between “true” 19e complexes, in which the 19th electron is localized at the metal center, and 18 + δ complexes, in which the 19th electron localizes on a ligand away from the metal center. 1,3,5 For example, we recently discovered that the 19e adduct CpRu(CO) 2 P(OMe) 3 • is an 18 + δ complex containing a bent CO ligand. 6 There also exists evidence that CpFe(CO) 2 P- (OMe) 3 • is likely to be an 18 + δ complex with an η 4 Cp ligand. 7,8 Tyler et al. studied the photochemically initiated radical disproportionation of [CpM(CO) 3 ] 2 (M = Cr, Mo, W) into CpM(CO) 3 - and CpM(CO) 3 L + (L = phosphine, phosphite), and their originally proposed mechanism is shown in Scheme 1, panel C. 9 More recent time-resolved infrared (TRIR) studies 4,10 on [CpW(CO) 3 ] 2 in phosphite and phosphine solutions ranging from picosecond through microsecond time scales led to the proposal of the two additional mechanisms, shown in Scheme 1, panels A and B. The disproportionation mechanism in panel A occurs on the picosecond time scale, prior to diffusion of the 17e and 19e radicals out of the solvent cage. This will be referred to as the “in-cage” mechanism. It is also possible that a pair of 17e radicals diffuses apart prior to formation of a 19e intermediate, or that a 17e and 19e radical pair do not form disproportionation products prior to diffusion out of the solvent cage. In this case, the 17e and 19e species may Received: February 28, 2013 Revised: April 5, 2013 Published: April 15, 2013 Article pubs.acs.org/JPCA © 2013 American Chemical Society 3777 dx.doi.org/10.1021/jp4021036 | J. Phys. Chem. A 2013, 117, 3777-3785