NMR Observation of Three Interconverting Structures in Tris(3-Phenyl-5 methylpyrazolyl)Borate Cobalt(II) Nitrate

Abstract

The complex tris(3-phenyl-5methylpyrazolyl)boratocobalt nitrate, TpPh,MeCo(NO₃), is reported for the first time. An extensive temperature-dependent ¹H NMR study of Tp^Ph,MeCo(NO₃) in a non-interacting, non-polar organic solvent (toluene), from 187 to 357 K is presented. The protons of the Tp^Ph,Me ligand indicate the coordinated nitrate anion is surprisingly dynamic. The complex transitions near 247 K, from a time-average four-coordinate structure at elevated temperature, to three limiting structures in low temperature solution. The low temperature data are consistent with the persistence of a symmetric four-coordinate structure, and two new asymmetric five-coordinate structures. The three species appear in near equimolar amounts at the lowest temperatures, with the five-coordinate species decreasing, and the four-coordinate species increasing, as the temperature is raised. Relaxation data are presented that confirm the NMR assignments, and offer insight into the speciation. An X-ray structure of the title compound is also presented, and used as the starting point for DFT and SCF calculations, which show the nitrate ion need only rotate ~45° about the Co⋅⋅N_nitrate direction to interconvert the trigonal-bipyramid favored by DFT and the square-pyramid.