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(PPh₃)₂Cl] (1) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; PPh₃ = triphenylphosphine) with [BH₃·THF] (THF = tetrahydrofuran) at 60 °C led to the formation of the hydrogen-rich ruthena-octahydrotetraborane arachno-[2-{Cp*Ru(PPh₃)B₃H₈}] (2). The chemistry of 2 is explored with [Fe₂(CO)₉] at room temperature, which resulted in the formation of a metal-stabilized triborane species, [{Cp*Ru(PPh₃)}(μ₃-η¹:η²:η²-B₃H₆){Fe₂(CO)₇}] (3). Compound 3 can be considered as a triborane analogue [B₃H₆]^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(B₃H₇)] (4) with [M(CO)₅·THF] (M = Mo and W) afforded an arachno-[1,2-(Cp*Ru)(Cp*RuCO)(μ-H)(B₃H₈)] (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)(μ₃-H)(B₃H₆)] (6) having two ruthenium atoms at the basal position. Irradiation of arachno-5 with an intermediate generated from CS₂ and [LiBH₄·THF] in THF afforded the diborane(5) species [(Cp*RuCO)(Cp*Ru)(μ-H)(μ-η¹:η²-B₂H₄)(CS₂H)] (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.