Synthesis, solid-state (X-ray) and solution (nuclear magnetic resonance) studies of the hydridocarbido carbonyl cluster anion [Re7H2C(CO)21]–

Abstract

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)Å, β= 90.87(3)°, 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.