Group 6 metallacarborane chemistry : synthesis, structure, and redox reaction study

Abstract

The synthesis and structural analysis of two novel families of three-component reversible redox cycles $[(C_2B_9H_11)M(μ-SPh)_2]_2^{n}PPN_{n}$ (M = Mo, n = 2-, 2; 1-, 3; 0, 4 and M = W, n = 2-, 6; 1-, 7; 0, 8), where the cleavage and reformation of the carborane cage C-C bond is observed during the redox reaction, are reported. Electronic saturation of the metal center (18e center) and the lack of bulky substituents on the carborane cage suggest that the deformed carborane cages in $2ㆍPPN_2$, $6ㆍPPN_2$ and 7ㆍPPN invoke a new kind of deformed cage ("semicloso" framework). The XPS results show that the unprecedented competition for electron density between the metal center and the carborane cage is involved in the cleavage and formation of the carborane C-C bond. The synthesis and structural analysis of $[Cr(CO)_3(μ-H-η^{5} -C_2B_9H_10)CuCl](NMe_4)_2$ $(3ㆍ(NMe_4)_2)$ and $[M(CO)_3((μ-H-η^{5} -C_2B_9H_10)Cu]_2PPN_2$ $(M = Cr for 4ㆍPPN_2, Mo for 7ㆍPPN_2, and W for 10ㆍPPN_2)$ are reported. All products show dative bonding from the 18 electron saturated group 6 metals to the electronically unsaturated Cu atom, which is supported by a B-H→Cu interaction. The dimetallic 3 shows short Cr-Cu distance, (2.494 Å) suggesting the presence of multibonding character. The structural characterizations of 4, 7, and 10 show parallelograms of the tetrametallic $M_2Cu_2$ core, where extremely short Cu-Cu distances of 2.3446, 2.3894, and 2.3883Å, are observed, respectively. The Cu-Cu distance in 4 sets a new lower limit of Cu-Cu distance. The synthesis and the reactivity study of $(C_2B_9H_11)Mo(CO)_3(MeCN)$ (2) is reported. 2 reacts with THF to give charge-compensated $(C_2B_9H_10-10-O(CH_2)_4)Mo(CO)_3)$ (3). Substitution reactions of 2 with $PPh_3$ gave $(C_2B_9H_11)Mo(CO)_3(PPh_3)$ (4) and $(C_2B_9H_11)Mo(CO)_2(PPh_3)(MeCN)$ (5) or $(C_2B_9H_11)Mo(CO)(PPh_3)_2(MeCN)$ (6}) when reacted in $CH_2Cl_2$ or MeCN, respectively, showing that the two ligands, CO and MeCN, can be substitu...