Trimolybdenum Cluster Compounds with Two Capping Ethylidyne Groups

Three compounds containing equilateral-triangular trimolybdenum clusters capped on both sides by ethylidyne (CCH₃) groups are described: [Mo₃(CCH₃)₂(0₂CCH₃)₆(H₂0)₃]SbF₆-3H₂0 (1), [Mo₃(CCH₃)₂(0₂CCH₃)₆(H₂0)₃](CF₃S0₃)₂ (2), and [Mo₃(CCH₃)₂(O₂CCH₃)₆(H₂O)₃](p-CH₃C₆H₄SO₃)₂-10H₂O (3). All three are obtained by suitable workup of the reaction mixture after refluxing Mo(CO)₆ with a 10:1 v/v mixture of acetic acid and acetic anhydride. For the last two permanganate oxidation is used. The crystal structures of the three compounds have been solved and refined. In 1, where there are five electrons for the Mo₃ cluster, the Mo-Mo bond orders are 5/6 and the mean Mo-Mo distance is 2.814 (5) Å; this is about 0.06 Å longer than that in a closely related cluster with Mo-Mo bond orders of 1.0. In 2 and 3, which each contain a bicapped Mo₃ cluster with only four electrons, and hence a bond order of 12/3, the mean Mo-Mo distances are 2.883 (1) Å and 2.892 (1) Å, i.e., about 0.13 Å longer than for the Mo-Mo bonds of order 1.0. The cation in 1 has one unpaired electron showing an essentially constant g factor slightly above 2.0 in the range 4-300 K according to bulk susceptibility measurements. It shows no EPR spectrum but has a sharp 360-MHz ¹H NMR spectrum in which the resonance of the 0₂CCH₃ protons is shifted to 14 ppm downfield while that of the CCH₃ protons is broadened and shifted 85 ppm downfield. The cations in 2 and 3 appear to be paramagnetic, but the temperature dependence is unusual and still under study. The cation gives an ¹H NMR spectrum with lines at 15.6 and 27.1 ppm with relative intensities of 3.3 to 1.0.