Imide- and amide-supported Group 5 and 6 metallacarboranes

This account summarises recent results in the synthesis, reactivity and spectroscopic and structural characterisation of high oxidation state icosahedral closo-metallacarboranes of niobium, tantalum and tungsten, with an emphasis on the relationship between these metallacarboranes and analogous half-sandwich and metallocene complexes. Group 5 M(NMe2)(5) reagents (M = Nb, Ta) react with nido-C2B9H13 to yield 3,3,3-(NMe2)(3)-closo-3,1,2-MC2B9H11. This reacts further by insertion of the metal-amide bonds with polar multiple bonds in CO2 and CS2, and with nitriles to give rare examples of the extensively delocalised strong pi-donor N,N-dimethyl amidinate ligands. The amide bonds are cleaved by protic reagents. The isomeric 2,1,12, and 2,1,7 metallacarboranes are obtained from nido-2,9-C2B9H12 and (Me3NH)(nido-7,9-C2B9H12). Comparison of B-11-NMR data of the tantalum complexes reveals the influence on B-11-NMR chemical shifts of the cluster on replacing a {BH} vertex by a {Ta(NMe2)(3)} fragment. The base-sensitive cage-alkylated carborane nido-11-Me-2,7-C2B9H12 is metallated to 4,4,4-(NMe2)-3-Me-4,1,2-closo-TaC2B9H10, and the isomeric metallacarboranes 3,3,3-(NMe2)(3)-4-Me-3,1,2-closo-TaC2B9H10 and 2,2,2-(NMe2)(3)-3-Me-2,1,7-closo-TaC2B9H10 can be obtained from (Me3NH)(nido-9-Me-7,8-C2B9H11) and (Me3NH)(nido-8Me-7,9-C2B9H11). Amine elimination reaction from W(N' Bu)(2)(NH' Bu)(2) yields 3-N' Bu-3,3-(NH' Bu)(2)-closo-3,1,2-WC2B9H11 of which one of the remaining amide ligands can be substituted by 2,6-dimethylphenol, water or Me3SiCl. Acetonitrile inserts into the tungsten amide bond to give the N,N-dimethyl acetamidine adduct of W(N' Bu)(2)(C2B9H11), containing a hydrogen bond between the amidine and a bent imide. Structural studies reveal that the imide is the dominant pi-donor in these complexes. (C) 2002 Elsevier Science B.V. All rights reserved.