Synthesis and characterization of ruthenium complexes which utilize a new family of terdentate ligands based upon 2,6-bis(pyrazol-1-yl)pyridine

To demonstrate the synthetic utility of a new family of terdentate ligands based on 2,6-bis(pyrazol-1-yl)pyridine (bpp), reaction conditions were developed to generate a variety of [RuL(NO₂)(PMe₃)₂]⁺ complexes [L = bpp, 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine(bdmpp), 2,6-bis(3-phenylpyrazol-1-yl)pyridine (bppp) or 2,6-bis(3-p-chlorophenylpyrazol-1-yl)pyridine (bcppp)]. These complexes were characterized by elemental analysis, ¹H and ¹³C NMR, infrared and UV/VIS spectroscopies, cyclic voltammetry, and single-crystal X-ray diffraction studies. The substituents of the terdentate bpp ligands sterically affected the Ru-N(pyrazole) bond lengths, the displacement of the nitrogen atoms of the nitro ligands from the RuL plane, and the twisting of the N-O vectors of the nitro ligand from that plane. Also the substituents affected the potentials and peak-current ratios of the Ru^III-Ru^II couples. The log (i_pc/i_pa) values (i_pc = cathodic peak current, i_pa = anodic peak current) are linearly correlated with the steric size of the substituents as estimated by Tolman's cone angles and with the distance of the nitro ligand out of the RuL plane. A linear correlation was also found between the differences in infrared absorbances due to the N-O symmetric and asymmetric stretches and the ratio of the N-O bond distances observed from the four crystal structures.