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Hydroboration of Alkynes with Zwitterionic Ruthenium-Borate Complexes: Novel Vinylborane Complexes
Date Issued
01-08-2015
Author(s)
Anju, R. S.
Mondal, Bijan
Saha, Koushik
Panja, Subir
Varghese, Babu
Indian Institute of Technology, Madras
Abstract
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.
Volume
21