Ruthenium−Carbene Functionality Bonded to Dibenzotetramethyltetraaza[14]annulene: Metal-to-Macrocycle Ligand-Induced Carbene Migration

This report contains the chemistry of the ruthenium−carbene functionality bonded to the dibenzotetramethyltetraaza[14]annulene dianion, tmtaa. [{Ru(tmtaa)} 2 (μ-C 8 H 12 )], 1, was the most appropriate starting material, as it contains a labile olefin, which can be displaced by a number of ligands to give [Ru(tmtaa)(L)(L‘)] [L = L‘ = Py, 2; L = CO, L‘ = thf, 3; L = Bu t NC, L‘ = thf, 4; L = L‘ = Bu t NC, 5 ]. Reaction of 1 with diazoalkanes led to the displacement of the olefin and gave the corresponding carbene derivatives [Ru(tmtaa)(CRR‘)] [R = R‘ = Ph, 6; R = Ph, R‘ = H, 7; R = Ph, R‘ = COOMe, 8 ], which have square-pyramidal structures. Complexes 7 and 8 underwent a remarkable labilization of the carbene functionality in the reaction with carbon monoxide, while 6 was only converted to the corresponding carbonyl [Ru(tmtaa)(CPh 2 )(CO)], 10, by CO. The reaction of 7 with both CO and Bu t NC led to the migration of the carbene to one of the metalladiimino rings of the ligands in 12 and 13, while the reaction of 8 with both CO and Bu t NC enabled us to intercept the preliminary product of a free-radical-type migration of the carbene to the tmtaa ligand, with the isolation of 11 and 14 . The ligand (CO or Bu t NC)-induced migration of the CR 2 fragment to the macrocycle seemed to proceed via a carbene mechanism in the case of 7, while 8 seemed to prefer a free-radical-like pathway. This assumption was further supported by extended Hückel calculations and by the reaction of 6 − 8 with ligands of different σ/π donor/acceptor capability. The pyridine converted 6 − 8 to the corresponding coordinate adducts, 15 − 17, while the reaction of 7 with PMe 3 led to the formation of [Ru(tmtaa)(PMe 3 ) 2 ] and stilbenes, via a plausible carbene mechanism. The reaction of 8 with PMe 3 proceeded through a free-radical pathway leading to 19, which has a structure similar to 11 and 14 . The ligand-induced labilization of the metallacarbene functionality in the macrocyclic environment was analyzed using the extended Hückel calculations.