Reaction of phosphines and pyridine with a heterometal metallaborane. Ligand substitution on boron vs metal sites, borane displacement, and orthometalation of a boron-coordinated pyridine

The reactions of the mixed-metal metallaborane {Cp*Ru}{Cp*Ru(CO)}{Co(CO)(2)}(mu -CO)-B3H6, 1, Cp* = eta (5)-C5Me5, with PPh3 and PMe2Ph as representative phosphine bases and pyridine as a representative nitrogen base have been studied. Phosphines lead to substitution of CO on Co and the formation of {Cp*Ru}(2){Co(CO)L}(mu -CO)(2)B3H4, L = PPh3 (2) and PMe2-Ph (3): The latter has been structurally characterized in the solid state and exhibits the fac isomeric form of an octahedral M3B3 cluster. NMR data demonstrate that 3 is fluxional in solution and exists in two isomeric forms: in the major one the phosphine is adjacent to the BE face, and in the minor one it is adjacent to the Mg face. Reaction with pyridine leads to three products, two of which have been characterized by X-ray structure determinations. Formation of {(Cp*Ru(CO)}{Co(CO)(2)}(mu -CO){Cp*Ru}B2H2(NC4H4C), 4, demonstrates borane fragment displacement plus pyridine addition and subsequent orthometalation. Formation of {Cp*Ru}(2)}{Co(CO)(2)}(mu -CO)(2)B2H2B(NC5H5), 5, establishes a parallel pathway involving H substitution on boron accompanied by hydrogen elimination.