Reactivity of Cationic Triruthenium Carbonyl Clusters: From Pyrimidinium Ligands to N-Heterocyclic Carbenes

The reaction of [Ru3(CO)12] with pyrimidine (Hpym) leads to the isomeric cyclometalated derivatives [Ru3(μ-H)(μ-κ2N1,C6-pym)(CO)10] (1) and [Ru3(μ-H)(μ-κ2N1,C2-pym)(CO)10] (2), which have been treated with [Me3O][BF4] to give the corresponding N-methylpyrimidinium derivatives [Ru3(μ-H)(μ-κ2N1,C6-pymMe)(CO)10][BF4] (3[BF4]) and [Ru3(μ-H)(μ-κ2N1,C2-pymMe)(CO)10][BF4] (4[BF4]). The reaction of 4[BF4] with K-Selectride renders a separable mixture of two neutral trinuclear products, [Ru3(μ-H){μ-κ2N1,C2-(6-HpymMe)}(CO)9] (5) and [Ru3(μ-H){μ-κ2N1,C2-(4-HpymMe)}(CO)9] (6), which arise from the nucleophilic attack of a hydride at the ligand C6 and C4 carbon atoms of 4+. This cationic complex can be reduced to the short-lived radical species 4•, which spontaneously dimerizes by forming an intermolecular C−C bond between the ligands of two such radicals. Three hexanuclear derivatives, [Ru6(μ-H)2{μ6-κ4N1,N1′,C2,C2′-(4,6′-bipymMe2)}(CO)18] (isomers 7 and 8) and [Ru6(μ-H)2{μ6-κ4N1,N1′,C2,C2′-(4,4′-bipymMe2)}(CO)18] (isomer 9), have been isolated by reducing 4[BF4] with cobaltocene. Molecular orbital calculations (at the DFT level) and electrochemical studies have helped rationalize the experimental results. The ligand-based character of the LUMO of 4+ and its atomic orbital composition account for the regiochemistry observed in the reactions of this cationic complex. The N-heterocyclic ligands of 5−9, which have two N atoms in a six-membered ring and a doubly metalated N atom, constitute novel types of N-heterocyclic carbenes.