Imide- and amide-supported Group 5 and 6 metallacarboranes

Abstract

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(NMe 2) 5 reagents (M=Nb, Ta) react with nido-C 2B 9H 13 to yield 3,3,3-(NMe 2) 3- closo-3,1,2-MC 2B 9H 11. This reacts further by insertion of the metal–amide bonds with polar multiple bonds in CO 2 and CS 2, and with nitriles to give rare examples of the extensively delocalised strong π-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-C 2B 9H 13 and (Me 3NH)( nido-7,9-C 2B 9H 12). Comparison of 11B-NMR data of the tantalum complexes reveals the influence on 11B-NMR chemical shifts of the cluster on replacing a {BH} vertex by a {Ta(NMe 2) 3} fragment. The base-sensitive cage-alkylated carborane nido-11-Me-2,7-C 2B 9H 12 is metallated to 4,4,4-(NMe 2) 3-3-Me-4,1,2- closo-TaC 2B 9H 10, and the isomeric metallacarboranes 3,3,3-(NMe 2) 3-4-Me-3,1,2- closo-TaC 2B 9H 10 and 2,2,2-(NMe 2) 3-3-Me-2,1,7- closo-TaC 2B 9H 10 can be obtained from (Me 3NH)( nido-9-Me-7,8-C 2B 9H 11) and (Me 3NH)( nido-8-Me-7,9-C 2B 9H 11). Amine elimination reaction from W(N t Bu) 2(NH t Bu) 2 yields 3-N t Bu-3,3-(NH t Bu) 2- closo-3,1,2-WC 2B 9H 11 of which one of the remaining amide ligands can be substituted by 2,6-dimethylphenol, water or Me 3SiCl. Acetonitrile inserts into the tungsten amide bond to give the N, N-dimethyl acetamidine adduct of W(N t Bu) 2(C 2B 9H 11), containing a hydrogen bond between the amidine and a bent imide. Structural studies reveal that the imide is the dominant π-donor in these complexes.