Synthesis and structure of novel RuII–N≡C–Me complexes and their activity towards nitrile hydrolysis: An examination of ligand effects

The synthesis and isolation of new Ru^II-acetonitrile complexes, of general formula trans,fac-[Ru(bpea)(B)(MeCN)](BF₄)₂ (bpea = N,N-bis(2-pyridylmethyl)ethylamine; B = bpy, 2,2′-bipyridine, 4; B = dppe, 1,2-bis(diphenylphosphino)ethane, 5), together with a synthetic intermediate trans,fac-[Ru(NO₃)(bpea)(dppe)](BF₄), 6, are described. Ru(bpea)Cl₃, 1, is used as the starting material for the synthesis of all complexes 2-6 presented in this paper, which are characterized by analytical, spectroscopic (IR, UV/Vis, 1D and 2D NMR), and electrochemical techniques (cyclic voltammetry). Furthermore, complexes 4, 5, and 6 have also been characterized in the solid state by single crystal X-ray diffraction analysis. Their structures show a distorted octahedral geometry where the bpea ligand binds in a facial mode, the bidentate ligands bpy and dppe bind in a chelate manner, and finally the MeCN or the NO₃ – ligand occupy the sixth position of the octahedral Ru metal centre. The kinetics of the basic hydrolysis of the coordinated MeCN ligand for complexes 4 and 5 and for the related complex [Ru(phen)(MeCN)([9]aneS₃)](BF₄)₂, 7, which contains the 1,4,7-trithiacyclonane ligand ([9]aneS₃) and 1,10-phenanthroline (phen) is also described. Second-order rate constants for acetonitrile hydrolysis measured at 25°C of k = 1.01 × 10^-3 M^-1 s^-1 for 4, 1.08 × 10^-4 M^-1 s^-1 for 5, and 6.8 × 10^-3 M^-1 s^-1 for 7, have been obtained through UV-vis spectroscopy. Activation parameters have also been determined over the temperature range 25.0-45.0°C and agree with a mechanism that involves an associative rate-determining step. Finally the electronic and steric influence of the auxiliary ligands on this reaction for the above and related complexes is discussed.