Coordination chemistry of mono- and di-nitriles in [Ru 3(CO) 12− n(RCN) n] ( n = 1–3): influence of the CO/nitrile ratio on fluxionality

Abstract

The clusters [Ru 3(CO) 12− n (RCN) n ] ( n = 1–3; RCN = acetonitrile, adiponitrile, phenylenediacetonitrile) have been prepared in dichloromethane solution by reaction of [Ru 3(CO) 12] with Me 3NO in the presence of the desired nitrile. The number of coordinated nitriles depends on the nitrile concentration and to a lesser extend on the Me 3NO/Ru 3 ration. The new cluster [Ru 3(CO) 9(MeCN) 3] is prepared and isolated in high yield. Spectroscopic IR and 13C NMR investigations in solution indicate that the dinitriles remain monocoordinated but only in a concentrated medium (1M <[RCN] < 4M). Spectroscopic data suggest axial coordination of the nitriles, one on each ruthenium atom, and equatorial bridging carbonyl groups. For [Ru 3(CO) 7( μ 2−CO) 3(MeCN) 2], the two nitriles are located on each side of the triruthenium plane, whereas for [Ru 3(CO) 6( μ 2−CO) 3(MeCN) 3], two MeCN are located on one side of the triruthenium plane with the third lying on the other side. Variable-temperature 13C NMR study reveals a fluxionality of the carbonyls around the trimetal core. The first fluxional motion involves in-plane carbonyl exchange with activation energies depending on the degree of substitution n. The influence of ligand substitution in the axial position on the mobility of the carbonyls is attributed to an increase of the π back-donation from the cluster to each of the remaining carbonyls with increasing substitution.