New Metallacarborane Chemistry: Molybdenum and Tungsten Carbonyl Multidecker Sandwiches. Cluster Dimers Linked via Metal-Metal Bonds(1).

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-Et(2)C(2)B(4)H(4)(2)(-) dianion with (RCN)(3)M(CO)(3) reagents (M = Mo, W; R = Me, Et) gave the (Et(2)C(2)B(4)H(4))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 Ph(4)PX (X = Cl, Br, I) followed by triflic acid afforded the dimeric products (Et(2)C(2)B(4)H(4))Mo(CO)(2)(&mgr;-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 MC(2)B(4) 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 (Et(2)C(2)B(3)H(5))Mo(CO)(2)(&mgr;-Br)(2) (5), which is proposed to have two open planar C(2)B(3) 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-Et(2)C(2)B(4)H(5)(-) anion and Mo(CO)(4)Br&mgr;-Br which also produced the bis(carboranyl)molybdenum complex (Et(2)C(2)B(4)H(4))(2)Mo(CO)(2) (6), a nearly colorless solid for which a bent sandwich structure is postulated. The reaction of the nido-cobaltacarborane anion CpCo(Et(2)C(2)B(3)H(4))(-) with Mo(CO)(4)Cl&mgr;-Cl in toluene gave two isolable products, red CpCo(Et(2)C(2)B(3)H(3))Mo(CO)(4) (7) and dark red CpCo(Et(2)C(2)B(3)H(3))Mo(CO)(2) (8), which have, respectively, triple-decker and bent-tetradecker sandwich structures based on X-ray crystallographic structure determinations. The reaction of CpCo(Et(2)C(2)B(3)H(4))(-) with Mo(CO)(4)Br&mgr;-Br in toluene gave 8 and a known fused-cluster complex Cp(2)Co(2)(Et(4)C(4)B(6)H(6)) (9). Treatment of the CpCo(Et(2)C(2)B(3)H(4))(-) anion with W(CO)(4)Br&mgr;-Br afforded red crystalline CpCo(Et(2)C(2)B(3)H(3))W(CO)(4) (10), which on reaction with PhLi followed by Me(3)OBF(4) 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.