Factors affecting the structure of substituted tris(pyrazolyl)borate rhodium dicarbonyl complexes

Abstract

Predictions by density functional calculations of the structure and relative energy of various isomers of the hydridotris(pyrazol-1-yl)borate ligand in Tp 3R,5R rhodium(I) dicarbonyl complexes (R=H, Me) and their IR and 11B NMR spectra are compared to experimental observations. The lowest energy structure of Tp 3,5-Me-, Tp 3-Me-, and TpRh(CO) 2 is a non-classical square pyramidal (SPy) structure with a long metal apical ligand distance in rapid exchange with an equivalent SPy structure through a low energy trigonal bipyramid (TBP) transition state (a ‘reverse’ Berry pseudorotation). A second higher energy minimum, a pseudo square-planar complex with the third uncoordinated pyrazolyl arm rotated approximately parallel with the metal ligand pseudo-plane (SP1), is accessed through a second low energy transition state. Another pseudo square-planar minimum structure (SP2) is produced by a transition state, which lengthens the rhodium-apical nitrogen (of the third pyrazolyl arm) bond distance. The relative stability of SP2 depends on the degree of tris(pyrazolyl)borate (Tp) substitution, where 5-substituents larger than hydrogen disfavor SP2 because of steric interactions. The previously reported empirical correlation between 11B NMR chemical shifts, ν BH stretching frequencies, and the crystallographic Tp ligand denticity is reproduced by our calculations. The variety of structures observed by experiment can be explained by the calculated relative energies of the structures, the bulk dielectric of the solvents when in solution, specific interaction by certain solvents, and conditions of crystallization when in the solid-state.