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 representat ive phosphine bases and pyridine as a representative nitrogen base have bee n 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 latt er 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 o ne the phosphine is adjacent to the BE face, and in the minor one it is adj acent to the Mg face. Reaction with pyridine leads to three products, two o f which have been characterized by X-ray structure determinations. Formatio n of {(Cp*Ru(CO)}{Co(CO)(2)}(mu -CO){Cp*Ru}B2H2(NC4H4C), 4, demonstrates bo rane fragment displacement plus pyridine addition and subsequent orthometal ation. Formation of {Cp*Ru}(2)}{Co(CO)(2)}(mu -CO)(2)B2H2B(NC5H5), 5, estab lishes a parallel pathway involving H substitution on boron accompanied by hydrogen elimination.