New Metallacarborane Chemistry:  Molybdenum and Tungsten Carbonyl Multidecker Sandwiches. Cluster Dimers Linked via Metal−Metal Bonds¹

This article reports the synthesis, characterization, and chemistry of the first molybdenum and tungsten complexes bearing small carborane ligands, including novel M−M-linked dicluster species. The reaction of the nido-2,3-Et₂C₂B₄H₄^2- dianion with (RCN)₃M(CO)₃ reagents (M = Mo, W; R = Me, Et) gave the (Et₂C₂B₄H₄)M(CO)₃^2- anions, which were isolated as lithium (12-crown-4) salts 1b (M = Mo) and 2b (M = W). Treatment of 1b and 2b with Ph₄PX (X = Cl, Br, I) followed by triflic acid afforded the dimeric products [(Et₂C₂B₄H₄)Mo(CO)₂]₂(μ-X)₂ (X = Cl (3a), Br (3b), I (3c)) and the corresponding tungsten dimers 4a−c, all of which are red or orange air-stable crystalline solids. X-ray crystallography on 3b revealed a geometry without precedent in small metallacarborane dimers, in which two MC₂B₄ pentagonal pyramidal clusters are linked via an intercluster metal−metal bond. From NMR and other spectroscopic evidence, the remaining dimeric products 3a, 3c, and 4a−c are proposed to have structures similar to 3b. Decapitation of 3b with HCl/EtOAc in THF gave red solid [(Et₂C₂B₃H₅)Mo(CO)₂]₂(μ-Br)₂ (5), which is proposed to have two open planar C₂B₃ end rings and, hence, is a potential building block for linear polymers via deprotonation and metal-ion-stacking reactions. Compound 3b was also generated in a different reaction, involving the nido-Et₂C₂B₄H₅^- anion and [Mo(CO)₄Br]₂[μ-Br]₂ which also produced the bis(carboranyl)molybdenum complex (Et₂C₂B₄H₄)₂Mo(CO)₂ (6), a nearly colorless solid for which a bent sandwich structure is postulated. The reaction of the nido-cobaltacarborane anion Cp*Co(Et₂C₂B₃H₄)^- with [Mo(CO)₄Cl]₂[μ-Cl]₂ in toluene gave two isolable products, red Cp*Co(Et₂C₂B₃H₃)Mo(CO)₄ (7) and dark red [Cp*Co(Et₂C₂B₃H₃)]₂Mo(CO)₂ (8), which have, respectively, triple-decker and bent-tetradecker sandwich structures based on X-ray crystallographic structure determinations. The reaction of Cp*Co(Et₂C₂B₃H₄)^- with [Mo(CO)₄Br]₂[μ-Br]₂ in toluene gave 8 and a known fused-cluster complex Cp*₂Co₂(Et₄C₄B₆H₆) (9). Treatment of the Cp*Co(Et₂C₂B₃H₄)^- anion with [W(CO)₄Br]₂[μ-Br]₂ afforded red crystalline Cp*Co(Et₂C₂B₃H₃)W(CO)₄ (10), which on reaction with PhLi followed by Me₃OBF₄ gave 11, a B(5)−benzyl derivative of 10. X-ray diffraction analyses established triple-decker sandwich structures for both compounds. The new products were characterized by multinuclear NMR, IR, UV-vis, and mass spectroscopy, and elemental analysis, supplemented by electrochemical data on 3b, 8, and 11.