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

Abstract

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 Nheterocyclic 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 Ag 2 O to form intermediate silver carbene complexes and subsequent transmetalation with [Ru(η 6 -p-cymene)Cl 2 ] 2 . The cationic ruthenium complexes [Ru(η 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 (1D). The bidentate coordination of the ligand precursors 1 was followed by multinuclear NMR spectroscopy, revealing significant changes in chemical shifts for triazole C-5, pyridine nitrogen atoms, and the neighboring α-proton (H-6 pyridyl ) in 13 C, 15 N, and 1 H 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(η 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.

Synthesis and NMR Analysis of 1,4‐Disubstituted 1,2,3‐Triazoles Tethered to Pyridine, Pyrimidine, and Pyrazine Rings

“…[12,37] In compound series 3, the 15 (Table 5, entries [12][13][14][15][16][17]. Unexpectedly, in contrast to compounds 3, the triazole N-1 and N-3 atoms in compounds 4 are more shielded than N-2 (compare Table 5, entries 2, 3, and 6 with entries 13, 14, and 17, respectively).…”

“…This is interpreted as being due to a response by the pyridine ring to the π-donation of the triazole N-1 nitrogen atom, and to a lesser extent to the weakly π-excessive nature [41] of the triazole ring. The effect of the latter is seen in the 4-(2-pyridyl) triazole series 4 (δ = 302 ppm, Table 5, entries [12][13][14][15][16][17], where the triazole C-4 atom is bound to the pyridine C-2Ј atom, resulting in an intermediate shielding effect of ca. 8 ppm.…”

“…These compounds will serve as ligands or ligand precursors for the synthesis of five-and six-membered metallacycles with bidentate N,N and N,tzNHC [12] coordination environments. The compounds will have tuneable properties so will be useful in research towards new homogeneous catalysts and metal-based pharmacologically relevant agents.…”

Solvent Effects on Nitrogen Chemical Shifts

“…Considerable care must be taken at solvent selection as polar, protic solvents may form hydrogen bonds with the free ligand (N⋯H) diminishing their complexation ability, whereas Lewis basic solvents may compete for metal coordination, and should therefore be avoided. Moreover, various solvents may indirectly influence the Δ 15N coord differently due to their varying ability to solvate the counterions of charged complexes [78,79]. The absolute value of Δ 15N coord may thus be applied for comparison of the interaction strength of the coordination bonds of structurally closely related complexes that encompass identical transition metal, or halogen(I), provided the same solvent is used for both the measurements of δ 15N lig and δ 15N compl .…”

“…28) [79]. Upon Ru(II) complexation in CDCl 3 solution, significant shielding and large absolute 15 N coordination shifts (60-77 ppm) are seen for the pyridinyl nitrogens of these ligands.…”