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

Treatment of [Cp*RuCl 2 ] 2 , 1, [(COD)IrCl] 2 , 2 or [(p-cymene)RuCl 2 ] 2, 3 (Cp*=η 5 -C 5 Me 5, COD= 1,5-cyclooctadiene and p-cymene=η 6 - i PrC 6 H 4 Me) with heterocyclic borate ligands [Na[(H 3 B)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 [L A M(μ-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{(μ-H) 2 BH(mp)} 2 ], cis-7 and trans-7. The isolation and structural characterization of fac- and mer-[Ru{(μ-H) 2 BH(mp)}{(μ-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, 10 a-d.