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Keywords:

  • cluster compounds;
  • N-heterocyclic carbenes;
  • pyrimidylidenes;
  • reductive dimerization;
  • ruthenium

Abstract

The methylation of the uncoordinated nitrogen atom of the cyclometalated triruthenium cluster complexes [Ru3(μ-H)(μ-κ2N1,C6-2-Mepyr)(CO)10] (1; 2-MepyrH=2-methylpyrimidine) and [Ru3(μ-H)(μ-κ2N1,C6-4-Mepyr)(CO)10] (9; 4-MepyrH=4-methylpyrimidine) gives two similar cationic complexes, [Ru3(μ-H)(μ-κ2N1,C6-2,3-Me2pyr)(CO)10]+(2+) and [Ru3(μ-H)(μ-κ2N1,C6-3,4-Me2pyr)(CO)10]+ (9+), respectively, whose heterocyclic ligands belong to a novel type of N-heterocyclic carbenes (NHCs) that have the Ccarbene atom in 6-position of a pyrimidine framework. The position of the C-methyl group in the ligands of complexes 2+ (on C2) and 9+ (on C4) is of key importance for the outcome of their reactions with K[N(SiMe3)2], K-selectride, and cobaltocene. Although these reagents react with 2+ to give [Ru3(μ-H)(μ-κ2N1,C6-2-CH2-3-Mepyr)(CO)10] (3; deprotonation of the C2-Me group), [Ru3(μ-H)(μ33N1,C5,C6-4-H-2,3-Me2pyr)(CO)9] (4; hydride addition at C4), and [Ru6(μ-H)266N1,N1′,C5,C5′,C6,C6′-4,4′-bis(2,3-Me2pyr)}(CO)18] (5; reductive dimerization at C4), respectively, similar reactions with 9+ have only allowed the isolation of [Ru3(μ-H)(μ32N1,C6-2-H-3,4-Me2pyr)(CO)9] (11; hydride addition at C2). 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.