Kinetic study of the reaction of [Rh6(CO)16] with NO2−: insertion of the nitrogen atom into a Rh6 cluster core

Abstract

The cluster [Rh₆(CO)₁₆] reacts with (PPN)NO₂ [PPN = N(PPh₃)₂] in dichloromethane to form the known cluster [Rh₆N(CO)₁₅]⁻ in which the nitrogen atom is encapsulated within a trigonal prismatic Rh₆ cluster. A kinetic study shows that the reactions proceed in six kinetically distinguishable steps followed by another four steps that are kinetically unidentifiable but that can nevertheless be identified speculatively on the basis of analogies to other reactions of carbonyl clusters. Stopped-flow studies show that the initial step involves bimolecular nucleophilic attack on a carbon atom of a CO ligand by an oxygen atom in the NO₂⁻ ion. This is followed by CO loss (via a [CO]-dependent and a [CO]-independent path) with formation of [Rh₆(CO)₁₄(OOO)]⁻. Activation parameters have been obtained for all three steps. [Rh₆(CO)₁₄(OOO)]⁻ has been characterized spectroscopically and has the C and N atoms coordinated to one Rh atom. Reaction of this cluster with CO leads to the fairly rapid establishment of an equilibrium with the adduct [Rh₆(CO)_n(OOO)]⁻, where n is most probably 15. Both participants in this equilibrium react more slowly with loss of CO₂, and these reactions show negligible dependence on temperature and are therefore largely entropy controlled. The overall reaction is very solvent dependent and numerous side reactions can be identified if the conditions of reaction are not closely controlled.