6-Halogenopyridin-2-olato complexes with the diruthenium(2+) core: Equilibria between head-to-head and head-to-tail structures

Abstract

The dinuclear bis(6-X-pyridin-2-olato) ruthenium complexes [Ru 2(μ-XpyO) 2(CO) 4(PPh 3) 2] (X = Cl ( 4B) and Br ( 5B)), [Ru 2(μ-XpyO) 2(CO) 4(CH 3CN) 2] (X = Cl ( 6B), Br ( 7B) and F (8B)) and [Ru 2(μ-ClpyO) 2(CO) 4(PhCN) 2] ( 9B) were prepared from the corresponding tetranuclear coordination dimers [Ru 2(μ-XpyO) 2(CO) 4] 2 ( 1: X = Cl; 2: X = Br) and [Ru 2(μ-FpyO) 2(CO) 6] 2 ( 3) by treatment with an excess of triphenylphosphane, acetonitrile and benzonitrile, respectively. In the solid state, complexes 4B– 9B all have a head-to-tail arrangement of the two pyridonate ligands, as evidenced by X-ray crystal structure analyses of 4B, 6B and 9B, in contrast to the head-to-head arrangement in the precursors 1– 3. A temperature- and solvent-dependent equilibrium between the yellow head-to-tail complexes and the red head-to-head complexes 4A– 7A and 9A, bearing an axial ligand only at the O, O-substituted ruthenium atom, exists in solution and was studied by NMR spectroscopy. Full 1H and 13C NMR assignments were made in each case. Treatment of 1 and 2 with the N-heterocyclic carbene (NHC) 1-butyl-3-methylimidazolin-2-ylidene provided the complexes [Ru 2(μ-XpyO) 2(CO) 4(NHC)], X = Cl ( 11A) or Br ( 12A). An XRD analysis revealed the head-to-head arrangement of the pyridonate ligands and axial coordination of the carbene ligand at the O, O-substituted ruthenium atom. The conversion of 11A and 12A into the corresponding head-to-tail complexes was not possible.