Coordination and Hydroboration of Ru(II)-Borate Complexes: Dihydridoborate vs. Bis(dihydridoborate)

Treatment of [Cp*RuCl2](2), 1, [(COD)IrCl](2), 2 or [(p-cymene)RuCl2](2), 3 (Cp*=eta(5)-C5Me5, COD= 1,5-cyclooctadiene and p-cymene=eta(6)-(PrC6H4Me)-Pr-i) with heterocyclic borate ligands [Na[(H3B)L], L-1 and L-2 (L-1: L=amt, L-2: L= mp; amt=2-amino-5-mercapto-1,3,4-thiadiazole, mp= 2-mercaptopyridine) led to the formation of borate complexes having uncommon coordination. For example, complexes 1 and 2 on reaction with L-1 and L-2 afforded dihydridoborate species [(LM)-M-A(mu-H)(2)BHL] 4-6 (4: L-A=Cp*, M=Ru, L=amt; 5: L-A= Cp*, M=Ru, L= mp; 6: L-A=COD, M=Ir, L=mp). On the other hand, treatment of 3 with L-2 yielded cis- and trans-bis(dihydridoborate) species, [Ru{(mu-H)(2)BH(mp)}(2)], cis-7 and trans-7. The isolation and structural characterization of fac- and mer-[Ru{(mu-H)(2)BH(mp)}{(mu-H)BH(mp)(2)}], 8 from the same reaction offered an insight into the behaviour of these dihydridoborate species in solution. Fascinatingly, despite having reduced natural charges on Ru centres both at cis-and trans-7, they underwent hydroboration reaction with alkynes that yielded both Markovnikov and anti-Markovnikov addition products, 10a-d.