Activation of B–H, Si–H, and C–F Bonds with Tp′Rh(PMe3) Complexes: Kinetics, Mechanism, and Selectivity

The photochemical reactions of Tp′Rh­(PMe3)­H2 (1) and thermal reactions of Tp′Rh­(PMe3)­(CH3)H (1a, Tp′ = tris­(3,5-dimethylpyrazolyl)­borate) with substrates containing B–H, Si–H, C–F, and C–H bonds are reported. Complexes 1 and 1a are known activators of C–H bonds, including those of alkanes. Kinetic studies of reactions with HBpin and PhSiH3 show that photodissociation of H2 from 1 occurs prior to substrate attack, whereas thermal reaction of 1a proceeds by bi­molecular reaction with the substrate. Complete intra­molecular selectivity for B–H over C–H activation of HBpin (pin = pinacolate) leading to Tp′Rh­(PMe3)­(Bpin)H is observed. Similarly, the reaction with Et2SiH2 shows a strong preference for Si–H over C–H activation, generating Tp′Rh­(PMe3)­(SiEt2H)­H. The Rh­(Bpin)H and Rh­(SiEt2H)H products were stable to heating in benzene in accord with DFT calculations that showed that reaction with benzene is endoergic. The intra­molecular competition with PhSiH3 yields a ∼1:4 mixture of Tp′Rh­(PMe3)­(C6H4SiH3)H and Tp′Rh­(PMe3)­(SiPhH2)­H, respectively. Reaction with pentafluoro­pyridine generates Tp′Rh­(PMe3)­(C5NF4)­F, while reaction with 2,3,5,6-tetrafluoro­pyridine yields a mixture of C–H and C–F activated products. Hexafluoro­benzene proves unreactive. Crystal structures are reported for B–H, Si–H, and C–F activated products, but in the latter case a bifluoride complex Tp′Rh­(PMe3)­(C5NF4)­(FHF) was crystallized. Inter­molecular competition reactions were studied by photoreaction of 1 in C6F6 with benzene and another substrate (HBpin, PhSiH3, or pentafluoro­pyridine) employing in situ laser photolysis in the NMR probe, resulting in a wide-ranging map of kinetic selectivities. The mechanisms of intra­molecular and inter­molecular selection are analyzed.