Combined Experimental and Theoretical Investigations of Group 6 Dimetallaboranes [(Cp*M)2B4H10] (M = Mo and W)

Abstract

Thermolysis of mono metal carbonyl fragment, [M′(CO)5·thf, M′ = Mo and W, thf = tetrahydrofuran] with an in situ generated intermediate, obtained from the reaction of [Cp*MCl4] (M = Mo and W, Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) with [LiBH4·thf], yielded dimetallaboranes, 1 and 2. Isolations of [{Cp*M(CO)}2B4H6] (M = Mo (1) and W(2)) provide direct evidence for the existence of saturated molybdaborane and tungstaborane clusters, [(Cp*M)2B4H10]. Our extensive theoretical studies together with the experimental observation suggests that the intermediate may be a saturated cluster [(Cp#M)2B4H10], not unsaturated [(Cp#M)2B4H8] (Cp# = Cp or Cp*), which was proposed earlier by Fehlner. Furthermore, in order to concrete our findings, we isolated and structurally characterized analogous clusters [(Cp*Mo)2(CO)(μ-Cl)B3H4W(CO)4] (3) and [(Cp*WCO)2(μ-H)2B3H3W(CO)4] (4). All the compounds have been characterized by solution-state 1H, 11B, IR, and 13C NMR spectroscopy, mass spectrometry, and the structural architectures of 1, 3, and 4 were unequivocally established by X-ray crystallographic analysis. The density functional theory calculations yielded geometries that are in close agreement with the observed structures. Both the Fenske–Hall and Kohn–Sham molecular orbital analyses showed an increased thermodynamic stability for [(Cp#M)2B4H10] compared to [(Cp#M)2B4H8]. Furthermore, large HOMO–LUMO gap and significant cross cluster M–M bonding have been observed for clusters 1–4.