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

Abstract

Reduction of 1,12-closo-C₂B₁₀H₁₂ followed by reaction with the appropriate metal halide and metathesis with either [K(18-crown-6)]Br or [BTMA]Cl ([BTMA] = [C₆H₅CH₂N(CH₃)₃]⁺) affords isolable salts of the supraicosahedral metallacarborane sandwich anions [4,4′-M-(1,10-closo-C₂B₁₀H₁₂)₂]ⁿ⁻ in moderate to good yield. Compounds prepared are [BTMA][4,4′-Co-(1,10-closo-C₂B₁₀H₁₂)₂] (1), [K(18-crown-6)][4,4′-Co-(1,10-closo-C₂B₁₀H₁₂)₂] (2), [K(18-crown-6)]₂[4,4′-Ni-(1,10-closo-C₂B₁₀H₁₂)₂] (3), [K(18-crown-6)]₂[4,4′-Fe-(1,10-closo-C₂B₁₀H₁₂)₂] (4), [BTMA]₂[4,4′-Fe-(1,10-closo-C₂B₁₀H₁₂)₂] (5) and [K(18-crown-6)]₂[4,4′-Ti-(1,10-closo-C₂B₁₀H₁₂)₂] (6). Oxidation of the iron(II) species 4 and 5 with FeCl₃ in THF generates the iron(III) analogues [K(18-crown-6)][4,4′-Fe-(1,10-closo-C₂B₁₀H₁₂)₂] (7) and [BTMA][4,4′-Fe-(1,10-closo-C₂B₁₀H₁₂)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. ¹¹B 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⋯K⁺ interactions to the [K(18-crown-6)]⁺ counterion. The salts 3, 6 and 7 also show B–H⋯K⁺ interactions but involving one cage only.