Time-resolved IR studies on the mechanism for the functionalization of primary C-H bonds by photoactivated Cp*W(CO)3(Bpin).

Recently, transition-metal-boryl compounds have been reported that selectively functionalize primary C-H bonds in alkanes in high yield. We have investigated this process with one of the well-defined systems that reacts under photochemical conditions using both density functional theory calculations and pico- through microsecond time-resolved IR spectroscopy. UV irradiation of CpW(CO)(3)(Bpin) (Cp = C(5)(CH(3))(5); pin = 1,2-O(2)C(2)-(CH(3))(4)) in neat pentane solution primarily results in dissociation of a single CO ligand and solvation of the metal by a pentane molecule from the bath within 2 ps. The spectroscopic data imply that the resulting complex, cis-CpW(CO)(2)(Bpin)(pentane), undergoes C-H bond activation by a sigma-bond metathesis mechanism--in 16 micros, a terminal hydrogen on pentane appears to migrate to the Bpin ligand to form a sigma-borane complex, CpW(CO)(2)(H-Bpin)(C(5)H(11)). Our data imply that the borane ligand rotates until the boron is directly adjacent to the C(5)H(11) ligand. In this configuration, the B-H sigma-bond is broken in favor of a B-C sigma-bond, forming CpW(CO)(2)(H)(C(5)H(11)-Bpin), a tungsten-hydride complex containing a weakly bound alkylboronate ester. The ester is then eliminated to form CpW(CO)(2)(H) in approximately 170 micros. We also identify two side reactions that limit the total yield of bond activation products and explain the 72% yield previously reported for this complex.