Diborane and Triborane Species in the Coordination Sphere of Group-8 Transition Metals.

The synthesis and structural elucidation of a series of ruthenium diborane and triborane compounds are described. Treatment of [Cp*Ru(PPh 3 ) 2 Cl] ( 1 ) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; PPh 3 = triphenylphosphine) with [BH 3 ·THF] (THF = tetrahydrofuran) at 60 °C led to the formation of the hydrogen-rich ruthena-octahydrotetraborane arachno -[2-{Cp*Ru(PPh 3 )B 3 H 8 }] ( 2 ). The chemistry of 2 is explored with [Fe 2 (CO) 9 ] at room temperature, which resulted in the formation of a metal-stabilized triborane species, [{Cp*Ru(PPh 3 )}(μ 3 -η 1 :η 2 :η 2 -B 3 H 6 ){Fe 2 (CO) 7 }] ( 3 ). Compound 3 can be considered as a triborane analogue [B 3 H 6 ] 3- that stabilizes in the coordination sphere of two iron and one ruthenium atoms. Further, the photolysis of nido -[1,2-(Cp*Ru) 2 (μ-H) 2 (B 3 H 7 )] ( 4 ) with [M(CO) 5 ·THF] (M = Mo and W) afforded an arachno -[1,2-(Cp*Ru)(Cp*RuCO)(μ-H)(B 3 H 8 )] ( 5 ), in which the [M(CO) 5 ·THF] acted as a CO source. In an attempt to convert arachno - 5 into a closo or nido species, we have pyrolyzed arachno - 5 in toluene at 90 °C for 20 h that afforded nido -[2,3-(Cp*Ru) 2 (μ-CO)(μ 3 -H)(B 3 H 6 )] ( 6 ) having two ruthenium atoms at the basal position. Irradiation of arachno - 5 with an intermediate generated from CS 2 and [LiBH 4 ·THF] in THF afforded the diborane(5) species [(Cp*RuCO)(Cp*Ru)(μ-H)(μ-η 1 :η 2 -B 2 H 4 )(CS 2 H)] ( 7 ) in which a dithioformato ligand (SHC═S) is attached to one Ru-B bond. Compound 7 can be considered as a diborane(5) species, which is stabilized by a dithioformato ligand. All the synthesized compounds have been characterized by employing electrospray ionization-mass spectrometry, multinuclear NMR, and IR spectroscopy techniques. The single-crystal X-ray diffraction studies of compounds 2 , 3 , 6 , and 7 helped to establish the structural integrity of these compounds. Further, density functional theory studies were performed to provide insight into the bonding of these metal-stabilized diborane and triborane species.