Ruthenium Bidentate Phosphine Complexes for the Coordination and Catalytic Dehydrogenation of Amine– and Phosphine–Boranes

Abstract

Addition of the amine-boranes H(3)B⋅NH(2)tBu, H(3)B⋅NHMe(2) and H(3)B⋅NH(3) to the cationic ruthenium fragment [Ru(xantphos)(PPh(3))(OH(2))H][BAr(F)(4)] (2; xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; BAr(F)(4)=[B{3,5-(CF(3))(2)C(6)H(3)}(4)](-)) affords the η(1)-B-H bound amine-borane complexes [Ru(xantphos)(PPh(3))(H(3)B⋅NH(2)tBu)H][BAr(F)(4)] (5), [Ru(xantphos)(PPh(3))(H(3) B⋅NHMe(2))H][BAr(F)(4)] (6) and [Ru(xantphos)(PPh(3))(H(3)B⋅NH(3))H][BAr(F)(4)] (7). The X-ray crystal structures of 5 and 7 have been determined with [BAr(F)(4)] and [BPh(4)] anions, respectively. Treatment of 2 with H(3)B⋅PHPh(2) resulted in quite different behaviour, with cleavage of the B-P interaction taking place to generate the structurally characterised bis-secondary phosphine complex [Ru(xantphos)(PHPh(2))(2)H][BPh(4)] (9). The xantphos complexes 2, 5 and 9 proved to be poor precursors for the catalytic dehydrogenation of H(3)B⋅NHMe(2). While the dppf species (dppf=1,1'-bis(diphenylphosphino)ferrocene) [Ru(dppf)(PPh(3))HCl] (3) and [Ru(dppf)(η(6)-C(6)H(5)PPh(2))H][BAr(F)(4)] (4) showed better, but still moderate activity, the agostic-stabilised N-heterocyclic carbene derivative [Ru(dppf)(ICy)HCl] (12; ICy=1,3-dicyclohexylimidazol-2-ylidene) proved to be the most efficient catalyst with a turnover number of 76 h(-1) at room temperature.

Borane-Bridged Ruthenium Complex Bearing a PNP Ligand: Synthesis and Structural Characterization

“…The Ru···B distance of 2.458(2) Å is significantly longer than the sum of the covalent radii (2.09 Å) 17 and longer than those found in other bis(σ-B−H) borane ruthenium complexes. 18 19 The Ru···B distance in 4 appears to reflect the geometric demands of the coordination mode of the BH 3 unit to the Ru−N function in the complex rather than additional attractive interactions between the Ru and the B centers.…”

The Synthesis, Characterization and Dehydrogenation of Sigma‐Complexes of BN‐Cyclohexanes

“…[18] Amine–boranes acting in a monodentate bonding mode through a single B–H bond, or as B–H activated boryls, show significantly longer (ca. 2.6 Å or longer) [34, 35] and shorter M–B distances (less than 2.1 Å) respectively. [36, 37] The cyclic amine–borane adopts a chair conformation, Figure 1B, meaning there is no plane of symmetry in the molecule in the solid–state.…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…An early example of a σ-complex was reported in 1984 in which zinc is complexed with diphosphine-diborane(4) 4 ]. This shows the relative weakness of the P-B bond compared to the N-B bond in amine-boranes and suggests P-B cleavage is likely to play a role in metal-catalysed dehydrocoupling of phosphine-boranes [63].…”