Hydroboration of alkynes with zwitterionic ruthenium-borate complexes: novel vinylborane complexes

Abstract

Building upon previous studies on the synthesis of bis(sigma)borate and agostic complexes of ruthenium, the chemistry of nido-[(Cp*Ru)₂B₃H₉] (1) with other ligand systems was explored. In this regard, mild thermolysis of nido-1 with 2-mercaptobenzothiazole (2-mbzt), 2-mercaptobenzoxazole (2-mbzo) and 2-mercaptobenzimidazole (2-mbzi) ligands were performed which led to the isolation of bis(sigma)borate complexes [Cp*RuBH₃L] (2 a–c) and β-agostic complexes [Cp*RuBH₂L₂] (3 a–c; 2 a, 3 a: L=C₇H₄NS₂; 2 b, 3 b: L=C₇H₄NSO; 2 c, 3 c: L=C₇H₅N₂S). Further, the chemistry of these novel complexes towards various diphosphine ligands was investigated. Room temperature treatment of 3 a with [PPh₂(CH₂)nPPh₂] (n=1–3) yielded [Cp*Ru(PPh₂ (CH₂)nPPh₂)-BH₂(L₂)] (4 a–c; 4 a: n=1; 4 b: n=2; 4 c: n=3; L=C₇H₄NS₂). Mild thermolysis of 2 a with [PPh₂(CH₂)nPPh₂] (n=1–3) led to the isolation of [Cp*Ru(PPh₂(CH₂)nPPh₂)(L)] (L=C₇H₄NS₂ 5 a–c; 5 a: n=1; 5 b: n=2; 5 c: n=3). Treatment of 4 a with terminal alkynes causes a hydroboration reaction to generate vinylborane complexes [Cp*Ru(R[BOND]C[DOUBLE BOND]CH₂)BH(L₂)] (6 and 7; 6: R=Ph; 7: R=COOCH₃; L=C₇H₄NS₂). Complexes 6 and 7 can also be viewed as η-alkene complexes of ruthenium that feature a dative bond to the ruthenium centre from the vinylinic double bond. In addition, DFT computations were performed to shed light on the bonding and electronic structures of the new compounds.