Effects of nitrogen substitution in poly(pyrazolyl)borato ligands: From orbital energy levels to C-H center dot center dot center dot O hydrogen bonding

The electronic effect of substituting CH with N in poly(pyrazoiyl)borato ligands and their transition-metal complexes is shown to be a decrease in energy of the filled metal and ligand orbitals. This conclusion is based on the cyclovoltammograms and photoelectron spectra of bis(hydrotris(azolyl)borato)iron(II) and -cobalt(II) complexes (azolyl = pyrazolyl and 1,2,4-triazolyl) and on MO calculations. Fe-57 Mossbauerspectra of bis(hydrotris(1,2,4-triazolyl)borato)iron(II) show that there is a fine-tuning of the HOMO-LUMO gap by a shift in transition temperature for the spin equilibrium. N-15 NMR spectroscopy supports the assignment of a higher negative charge to the exodentate N-4 nitrogen than to the endodentate N-2 or N-3 positions in the poly(azolyl)borato anions, where azolyl is 1,2,4-triazolyl or tetrazolyl. The electron withdrawing effect of the additional nitrogen atoms and the incorporation of water of crystallization by O-H...N bonding both assist in the formation of (azolyl)C-H...O bonds.