Arsacarborane chemistry. The 7,8,9,11-, 7,9,8,10- and 7,8,9,10-isomers of nido-As2C2B7H9 and some of their halogenated derivatives

The reaction between arachno-4,6-C2B7H13(1) and AsCl3 or AsI3 in CH2Cl2 in the presence of PS (PS =proton sponge, 1,8-bis(dimethylamino)naphthalene) resulted in the isolation of the parent, isomeric eleven-vertex nido diarsadicarbaboranes 7,8,9,11-As2C2B7H9(2) and 7,9,8,10-As2C2B7H9(3). Isolated were also halogenated derivatives of 2, 3-Cl-7,8,9,11-As2C2B7H8(3-Cl-2) and 3-I-7,8,9,11-As2C2B7H8(3-I-2). The presence of Et3N·BH3 in the reaction mixture increased the yield of unsubstituted compounds and suppressed the formation of halogenated species. The reaction between the isomeric carborane arachno-4,5-C2B7H13(4) and AsCl3 in CH2Cl2 in the presence of PS gave another isomer, nido-7,8,9,10-As2C2B7H9(5), which was accompanied by small amounts of its chloroderivative 3-Cl-7,8,9,10-As2C2B7H8(3-Cl-5) and compound 3. The structure of the iodinated derivative 3-I-2 was determined by an X-ray diffraction analysis. Moreover, the structures of all parent compounds and chloroderivatives, inclusive of that of the still missing nido-7,10,8,9-As2C2B7H9(6) isomer, were geometry-optimised at the DFT-B3LYP/6-31G* level.