Trapping of pyrid-2-ylidenes by [Ru3(CO)12]: Orthometalated pyrid-2-ylidenes in triruthenium clusters

Abstract

Six-membered-ring N-heterocyclic carbenes having only one N atom (NHC1N-R6 carbenes) can be easily prepared by simple deprotonation of readily available N-methylpyridinium-type cations with K[N(SiMe3)2]. They have been trapped in solution with [Ru3(CO)12] to give novel triruthenium clusters that contain orthometalated NHC1N-R6 carbene ligands in a face-capping coordination mode. Four NHC1N-R6-derived compounds of general formula [Ru3(μ-H)(μ3-κ2C2,C3-L)(CO)9] (L = N-methylpyrid-3-yl-2-ylidene (1; from N-methylpyridinium), N-methyl-6-picolin-3-yl-2-ylidene (2; from N-methyl-2-picolinium), N-methyl-4-picolin-3-yl-2-ylidene (3; from N-methyl-4-picolinium), and N-methylquinolin-3-yl-2-ylidene (4; from N-methylquinolinium)) have been isolated and characterized. The mechanism of the reaction that leads to compound 1 from N-methylpyrid-2-ylidene and [Ru3(CO)12], as a case study, has been modeled using DFT calculations. The high basicity of the NHC1N-R6 carbenes, the close proximity of a carbene ligand ortho-C−H bond to a cluster metal atom, and the irreversibility of the CO elimination steps trigger the orthometalation of the NHC1N-R6 carbenes and the subsequent face-capping coordination of the metalated ligands. The N-methyl-4-picolinium salts of two polynuclear carbonyl dianions, the previously known [Ru6(μ3-CO)2(μ-CO)2(CO)14]2− (52−) and the novel [Ru5(μ4-O)(μ-CO)2(μ3-CO)2(CO)10]2− (62−), have also been isolated from [Ru3(CO)12] and N-methyl-4-picolin-2-ylidene in the presence of adventitious moisture. The pentanuclear dianion 62− contains a μ4-oxo ligand in a hitherto unknown square face-capping coordination mode.