High-yield syntheses of [Rh7(CO)16]3− and [Rh14(CO)25]4− working in ethylene glycol solution under 1 atm of CO

The anionic rhodium carbonyl clusters [Rh7(CO)16]3− and [Rh14(CO)25]4− can be easily prepared by a new simple and high yield one-pot synthesis starting from RhCl3·nH2O dissolved in ethylene glycol and involving two steps: (i) treatment of RhCl3·nH2O under 1 atm of CO at 50 °C to give [Rh(CO)2Cl2]−; (ii) addition of a base (CH3CO2Na or Na2CO3) followed by reductive carbonylation under 1 atm of CO at an adequate temperature (50 °C for [Rh7(CO)16]3−; 150 °C for [Rh14(CO)25]4−). These new syntheses are more convenient than those previously reported, especially since such clusters are not accessible via silica surface-mediated reactions. This different behavior is due to the particular stabilization on the silica surface and under 1 atm of CO of an anionic carbonyl cluster, called A, which does not allow the formation of a higher nuclearity carbonyl cluster, called B, which was shown to be the key-intermediate in the synthesis of [Rh14(CO)25]4− working in ethylene glycol solution. Although it was not possible to isolate crystals of A and B suitable for X-ray structural determination, a combination of cyclovoltammetry, one of the few examples so far available of the use of this technique for anionic rhodium carbonyl clusters, infrared spectroscopy and elemental analyses suggest that A and B are probably the never reported [Rh7(CO)14]− and [Rh15(CO)28]3− clusters, respectively. In particular the tentative formulation of the two clusters was carried out by a non-conventional method based on the existence of a linear correlation between carbonyl frequencies of the main band and the [(charge/Rh atoms)/CO number] ratio.