Terminal Ni^II-OH/-OH₂ complexes in trigonal bipyramidal geometries derived from H₂O

The preparation and characterization of two Ni^II complexes are described, a terminal Ni^II-OH complex with the tripodal ligand tris[(N)-tertbutylureaylato)-N-ethyl)]aminato ([H₃buea]^3-) and a terminal Ni ^II-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[Ni^IIH₃buea(OH)] and NMe₄[Ni^IIMST(OH₂)] were characterized using perpendicular-mode X-band electronic paramagnetic resonance, Fourier transform infrared, UV-visible 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 Ni^II complexes. Moreover, the Ni^II-OH and Ni^II-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 Ni^II compounds with one-electron oxidants. Species were formed with S = 1/2 spin ground states that are consistent with formation of monomeric Ni^III species. While the formation of Ni^III-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.