Combined Experimental and Theoretical Investigations of Group 6 Dimetallaboranes [(Cp∗M)<inf>2</inf>B<inf>4</inf>H<inf>10</inf>] (M = Mo and W)

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.