Exploring the Scope of Pyridyl- and Picolyl-Functionalized 1,2,3-Triazol-5-ylidenes in Bidentate Coordination to Ruthenium(II) Cymene Chloride Complexes

1-(2-Pyridyl)-, 4-(2-pyridyl)-, 1-(2-picolyl)-, and 4-(2-picolyl)-functionalized 1,3,4-trisubstituted 1,2,3-triazolium salts (1A-D, respectively) were investigated as N-heterocyclic carbene (trzNHC) precursors for bidentate coordination to ruthenium(II) through the C-NHc and N-pyridyl donors. In addition to the pyridyl and picolyl pendant groups, a variety of para-substituted phenyl rings were attached to the 1,2,3-triazolylidene via carbon or nitrogen atoms. The ruthenation was accomplished by metalation with Ag2O to form intermediate silver carbene complexes and subsequent transmetalation with [Ru(eta(6)-p-cymene)Cl-2](2). The cationic ruthenium complexes [Ru(eta(6)-p-cymene)(trzNHC)Cl](+) (3A-C) were readily obtained with 1-(2-pyridyl)-, 4-(2-pyridyl)-, and 1-(2-picolyl)-1,2,3-triazolium salts (1A-C) but not with the 4-picolyl analogue (ID). The bidentate coordination of the ligand precursors I was followed by multinuclear NMR spectroscopy, revealing significant changes in chemical shifts for triazole C-5, pyridine nitrogen atoms, and the neighboring alpha-proton (H-6(Pyridyl)) in C-13, N-15, and H-1 NMR spectra. The molecular composition of complexes 3A-C was confirmed by elemental analysis and positive ion electrospray ionization (ESI+) mass spectra, the latter showing ions corresponding to [Ru(eta(6)-p-cymene)(trzNHC)Cl](+). The solid-state structures of the three representative complexes were confirmed by single-crystal X-ray analyses; all complexes displayed a typical piano-stool type configuration. Preliminary catalytic activity screening of 3A-C in the oxidation of selected primary and secondary alcohols with tert-butyl hydroperoxide (TBHP) to give carbonyl compounds is also discussed.