Hydroboration of Alkynes with Zwitterionic Ruthenium-Borate Complexes: Novel Vinylborane Complexes

Building upon previous studies on the synthesis of bis(sigma)borate and agostic complexes of ruthenium, the chemistry of nido-[(Cp Ru)<inf>2</inf>B<inf>3</inf>H<inf>9</inf>] (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 RuBH<inf>3</inf>L] (2a-c) and β-agostic complexes [Cp RuBH<inf>2</inf>L<inf>2</inf>] (3a-c; 2a, 3a: L=C<inf>7</inf>H<inf>4</inf>NS<inf>2</inf>; 2b, 3b: L=C<inf>7</inf>H<inf>4</inf>NSO; 2c, 3c: L=C<inf>7</inf>H<inf>5</inf>N<inf>2</inf>S). Further, the chemistry of these novel complexes towards various diphosphine ligands was investigated. Room temperature treatment of 3a with [PPh<inf>2</inf>(CH<inf>2</inf>)<inf>n</inf>PPh<inf>2</inf>] (n=1-3) yielded [Cp Ru(PPh<inf>2</inf>(CH<inf>2</inf>)<inf>n</inf>PPh<inf>2</inf>)-BH<inf>2</inf>(L<inf>2</inf>)] (4a-c; 4a: n=1; 4b: n=2; 4c: n=3; L=C<inf>7</inf>H<inf>4</inf>NS<inf>2</inf>). Mild thermolysis of 2a with [PPh<inf>2</inf>(CH<inf>2</inf>)<inf>n</inf>PPh<inf>2</inf>] (n=1-3) led to the isolation of [Cp Ru(PPh<inf>2</inf>(CH<inf>2</inf>)<inf>n</inf>PPh<inf>2</inf>)(L)] (L=C<inf>7</inf>H<inf>4</inf>NS<inf>2</inf> 5a-c; 5a: n=1; 5b: n=2; 5c: n=3). Treatment of 4a with terminal alkynes causes a hydroboration reaction to generate vinylborane complexes [Cp Ru(R-C=CH<inf>2</inf>)BH(L<inf>2</inf>)] (6 and 7; 6: R=Ph; 7: R=COOCH<inf>3</inf>; L=C<inf>7</inf>H<inf>4</inf>NS<inf>2</inf>). 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.