New 13-vertex metallacarborane sandwich compounds; synthetic and structural studies

Reduction of 1,12-closo-C(2)B(10)H(12) followed by reaction with the appropriate metal halide and metathesis with either [K(18-crown-6)]Br or [BTMA] Cl ([BTMA] = [C(6)H(5)CH(2)N(CH(3))(3)](+)) affords isolable salts of the supraicosahedral metallacarborane sandwich anions [4,4'-M-(1,10-closo-C(2)B(10)H(12))(2)](n-) in moderate to good yield. Compounds prepared are [BTMA][4,4'-Co-(1,10-closo-C(2)B(10)H(12))(2)] (1), [K(18-crown-6)][4,4'-Co-(1,10-closo-C(2)B(10)H(12))(2)] (2), [K(18-crown-6)](2)[4,4'-Ni-(1,10-closo-C(2)B(10)H(12))(2)] (3), [K(18-crown-6)](2)[4,4'-Fe-(1,10-closo-C(2)B(10)H(12))(2)] (4), [BTMA] 2[4,4'-Fe-(1,10-closo-C(2)B(10)H(12))(2)] (5) and [K(18-crown-6)](2)[4,4'-Ti-(1,10-closo-C(2)B(10)H(12))(2)] (6). Oxidation of the iron(II) species 4 and 5 with FeCl(3) in THF generates the iron(III) analogues [K(18-crown-6)][4,4'-Fe-(1,10-closo-C(2)B(10)H(12))(2)] (7) and [BTMA][4,4'-Fe-(1,10-closo-C(2)B(10)H(12))(2)] (8), respectively. All diamagnetic compounds were characterised spectroscopically and the structures of 1, 3, 4, 6, 7 and 8 were established by single crystal X-ray diffraction. All anions have the anticipated cluster structures with two docosahedral 13-vertex cages joined at the central metal atom (the common degree-six vertex 4). Carbon atoms occupy the degree-four vertex 1 and the degree-five vertex 10. (11)B NMR spectroscopy suggests the anions have, on the NMR timescale, C(2h) symmetry in solution at room temperature, consistent with free rotation, or at least substantial libration, of cage units about the long molecular axis. In the solid state the relative conformations of the two cages may be rationalised by simple bonding arguments, the single exception being the conformation of 4, in which both cages are subject to directional B-H center dot center dot center dot K(+) interactions to the [K(18-crown-6)](+) counterion. The salts 3, 6 and 7 also show B-H center dot center dot center dot K(+) interactions but involving one cage only.