Synthesis, solid-state (X-ray) and solution (nuclear magnetic resonance) studies of the hydridocarbido carbonyl cluster anion [re7H2C(CO)21]

The protonation of [Re7H(C)(CO)21]2- with CF3SO3H in dichloromethane solution produces [Re7H2C(CO)21]- which behaves as a strong acid, being completely dissociated in basic solvents. In acetonitrile the compound loses the capping Re(CO)3 unit, giving the octahedral dianion [Re6H2C(CO)18]2-. The [NEt4]+ salt crystallizes in the monoclinic system, space group P21/c (no. 14), with a = 9.394(3), b = 15.669(3), c = 27.983(6) \uc5, \u3b2 = 90.87(3)\ub0, and Z = 4. The structure of the anion is closely related to that of the parent compounds [Re7H(C)(CO)21]2- and [Re7C(CO)21]3-, where a carbon-centred monocapped octahedron of rhenium atoms is surrounded by 21 terminal carbonyl ligands, three for each metal. The two hydride ligands have been indirectly located on the basis of potential-energy computations as double bridging, one on an edge of the basal triangle and the other one on an inter-layer edge. The 1H n.m.r. spectra at low temperature show the presence in solution of three isomers (ratio ca. 1:1.5:10) rapidly exchanging at room temperature. The possible locations of the hydrides in these species are also discussed in the light of the variable-temperature 13C n.m.r. spectra