Ligand Effects on Reactivity of Cobalt Acyl Complexes

Hydrogenolysis reactions of cobalt acyl complexes, [Co­(CO)3(L)­(COR)] (L = phosphine, R = Me, nPr) have been monitored using in situ IR spectroscopy at moderate temperatures (n-pentyl-9-phosphabicyclo­[4.2.1]­nonane (a5-PhobPC5). The observed dependence of rate on H2 and CO pressure is consistent with a mechanism involving initial CO dissociation, followed by reaction of [Co­(CO)2(L)­(COR)] with H2. Isotopic exchange experiments, monitored by IR spectroscopy, demonstrate that both terminal and acetyl carbonyls of [Co­(CO)3(L)­(COR)] exchange with free 13CO. Kinetic data are also reported for reactions of [Co­(CO)3(L)­(COR)] with triphenyltinhydride. A zero-order dependence on [Ph3SnH] (at large excess) and positive values of ΔS‡ demonstrate rate-determining CO dissociation. For a series of less bulky, symmetrical phosphines, the rates follow the sequence PEt3 2Ph ∼ PEtPh2 3 6H4)3, in accord with their electron-donating strength. Higher rates are found for more bulky phosphines, and the fastest rate is again found for L = a5-PhobPC5. Calculations using density functional theory indicate that the CO dissociation energy for [Co­(CO)3(L)­(COMe)] is influenced by the stereoelectronic properties of L, with steric bulk having a substantial effect. X-ray crystal structures are reported for [Co­(CO)3(PEtPh2)­(COMe)], [Co­(CO)3(s-PhobPC5)]2, and [Co­(CO)3(a5-PhobPC5)]2.