Terminal NiII−OH/−OH2 complexes in trigonal bipyramidal geometries derived from H2O

Abstract

The preparation and characterization of two NiII complexes are described, a terminal NiII–OH complex with the tripodal ligand tris[(N)-tert-butylureaylato)-N-ethyl)]aminato ([H3buea]3−) and a terminal NiII–OH2 complex with the tripodal ligand N,N′,N″-[2,2′,2″-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3−). For both complexes, the source of the –OH and –OH2 ligand is water. The salts K2[NiIIH3buea(OH)] and NMe4[NiIIMST(OH2)] were characterized using perpendicular-mode X-band electronic paramagnetic resonance, Fourier transform infrared, UV–vis spectroscopies, and its electrochemical properties were evaluated using cyclic voltammetry. The solid state structures of these complexes determined by X-ray diffraction methods reveal that they adopt a distorted trigonal bipyramidal geometry, an unusual structure for 5-coordinate NiII complexes. Moreover, the NiII–OH and NiII–OH2 units form intramolecular hydrogen bonding networks with the [H3buea]3− and [MST]3− ligands. The oxidation chemistry of these complexes was explored by treating the high-spin NiII compounds with one-electron oxidants. Species were formed with S=1/2 spin ground states that are consistent with formation of monomeric NiIII species. While the formation of NiIII–OH complexes cannot be ruled out, the lack of observable O–H vibrations from the putative Ni–OH units suggest the possibility that other high valent Ni species are formed.