Hypoelectronic Dimetallaheteroboranes of Group 6 Transition Metals Containing Heavier Chalcogen Elements

We
have synthesized and structurally characterized several dimetallaheteroborane
clusters, namely, nido-[(Cp*Mo)2B4SH6], 1; nido-[(Cp*Mo)2B4SeH6], 2; nido-[(Cp*Mo)2B4TeClH5], 3; [(Cp*Mo)2B5SeH7], 4; [(Cp*Mo)2B6SeH8], 5; and [(CpW)2B5Te2H5], 6 (Cp* = η5-C5Me5,
Cp = η5-C5H5). In parallel
to the formation of 1–6, known [(CpM)2B5H9], [(Cp*M)2B5H9], (M = Mo, W) and nido-[(Cp*M)2B4E2H4] compounds (when M
= Mo; E = S, Se, Te; M = W, E = S) were isolated as major products.
Cluster 6 is the first example of tungstaborane containing
a heavier chalcogen (Te) atom. A combined theoretical and experimental
study shows that clusters 1–3 with
their open face are excellent precursors for cluster growth reactions.
As a result, the reaction of 1 and 2 with
[Co2(CO)8] yielded clusters [(Cp*Mo)2B4H4E­(μ3-CO)­Co2(CO)4], 7–8 (7: E = S, 8: E = Se) and [(Cp*Mo)2B3H3E­(μ-CO)3Co2(CO)3], 9–10 (9: E = S, 10: E = Se). In contrast, compound 3 under the
similar reaction conditions yielded a novel 24-valence electron triple-decker
sandwich complex, [(Cp*Mo)2{μ-η6:η6-B3H3TeCo2(CO)5}], 11. Cluster 11 represents an
unprecedented metal sandwich cluster in which the middle deck is composed
of B, Co, and Te. All the new compounds have been characterized by
elemental analysis, IR, 1H, 11B, 13C NMR spectroscopy, and the geometric structures were unequivocally
established by X-ray diffraction analysis of 1, 2, 4–7, and 9–11. Furthermore, geometries obtained from the
electronic structure calculations employing density functional theory
(DFT) are in close agreement with the solid state structure determinations.
We have analyzed the discrepancy in reactivity of the chalcogenato
metallaborane clusters in comparison to their parent metallaboranes
with the help of a density functional theory (DFT) study.