Synthesis and Reactivity of Cationic Triruthenium Clusters Derived from 2‐Methyl‐ and 4‐Methylpyrimidines: From Conventional Cyclometalated Ligands to Novel Types of N‐Heterocyclic Carbenes

Abstract

The methylation of the uncoordinated nitrogen atom of the cyclometalated triruthenium cluster complexes [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-2-Mepyr)(CO)(10)] (1; 2-MepyrH = 2-methylpyrimidine) and [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-4-Mepyr)(CO)(10)] (9; 4-MepyrH = 4-methylpyrimidine) gives two similar cationic complexes, [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-2,3-Me(2)pyr)(CO)(10)](+) (2(+)) and [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-3,4-Me(2) pyr)(CO)(10)](+) (9(+)), respectively, whose heterocyclic ligands belong to a novel type of N-heterocyclic carbenes (NHCs) that have the C(carbene) atom in 6-position of a pyrimidine framework. The position of the C-methyl group in the ligands of complexes 2(+) (on C(2)) and 9(+) (on C(4)) is of key importance for the outcome of their reactions with K[N(SiMe(3))(2)], K-selectride, and cobaltocene. Although these reagents react with 2(+) to give [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-2-CH(2)-3-Mepyr)(CO)(10)] (3; deprotonation of the C(2)-Me group), [Ru(3)(μ-H)(μ(3)-κ(3)N(1),C(5),C(6)-4-H-2,3-Me(2)pyr)(CO)(9)] (4; hydride addition at C(4)), and [Ru(6)(μ-H)(2){μ(6)-κ(6) N(1),N(1'),C(5),C(5'),C(6),C(6')-4,4'-bis(2,3-Me(2)pyr)}(CO)(18)] (5; reductive dimerization at C(4)), respectively, similar reactions with 9(+) have only allowed the isolation of [Ru(3)(μ-H)(μ(3)-κ(2)N(1),C(6)-2-H-3,4-Me(2)pyr)(CO)(9)] (11; hydride addition at C(2)). Compounds 3 and 11 also contain novel six-membered ring NHC ligands. Theoretical studies have established that the deprotonation of 2(+) and 9(+) (that have ligand-based LUMOs) are charge-controlled processes and that both the composition of the LUMOs of these cationic complexes and the steric protection of their ligand ring atoms govern the regioselectivity of their nucleophilic addition and reduction reactions.