[Rh10P(CO)22]3-. A transition-metal carbonyl cluster with a metal polyhedron based on the bicapped square antiprism as illustrated by the structural study of the benzyltriethylammonium salt

Article abstract of DOI:10.1021/ic50215a047

The reaction of Rh(CO)₂acac with triphenylphosphine in the presence of cesium benzoate and cesium borohydride in tetraethylene glycol dimethyl ether solution resulted in the selective formation of [Rh₁₀P(CO)₂₂]^3- (76% yield) after 4 h of contact time under 400 atm of carbon monoxide and hydrogen (CO:H₂ = 1) at 140-160°C. The cluster has been isolated as the cesium and benzyltriethylammonium salts, both of which are stable to moisture and oxygen. These salts are soluble in polar organic solvents, e.g., acetone, but insoluble in solvents of lower polarity. The [C₆H₅CH₂N(C₂H₅) ₃]₃[Rh₁₀P(CO)₂₂] complex has been characterized via a complete three-dimensional X-ray diffraction study. The complex crystallizes in the space group Cc with a = 25.011 (3) A?, b = 16.348 (4) A?, c = 18.267 (4) A?, α = γ = 90.0 (1)°, β = 101.47 (1)°, V = 7319.7 A?³, and p(calcd) = 2.045 g cm^-3, for a molecular weight of 2253.24 and Z = 4. Diffraction data were collected with an Enraf-Nonius CAD 4 automated spectrometer using graphite-monochromatized Mo Kα radiation. The structure was solved by direct methods and refined by difference-Fourier and least-squares techniques. The rhodium cluster was found to have a 50:50 disorder when rotated 45° about the noncrystallographic fourfold axis in the cluster. All nonhydrogen atoms have been located and refined: final discrepancy indices are R_F = 6.4% and R_wF = 7.7% for 2700 reflections in the range of 0.5° < 20 < 45°. The anion's structure shows 10 rhodium atoms on the corners of a bicapped square antiprism with the phosphorus atom placed on the center of the cluster. This is the first instance in which this polyhedron has been reported for a transition-metal cluster. Average bonding distances for the anion are in the ranges Rh-Rh = 2.864-3007 A? and Rh-P = 2.446-3.012 A? with Rh-C and C-O distances similar to those usually observed for these complexes. ¹³C and ³¹P NMR results are interpreted as indicative of the fluxionality of the carbon monoxide and rhodium atoms cores, respectively. The reversibility of the reaction [Rh₁₀P(CO)₂₂]^3- + 3CO ? [Rh₉P(CO)₂₁]^2- + [Rh(CO)₄]^- has been established under ambient- and high-pressure conditions.