Effect of substitution and the counterion on the structural and spectroscopic properties of CuII complexes of methylated pyrazoles

The coordination chemistry of pyrazole and three of its methyl derivatives with the chloride and nitrate salts of copper(II) under strictly controlled reaction conditions is systematically explored to gain a better understanding of the effect of counterion coordination strength and ligand identity on the structure and electronic absorption spectra of their resulting complexes. Despite the initial 2 : 1 ligand to metal ratio in water, copper(II) nitrate forms exclusively 4 : 1 ligand to metal complexes while copper(II) chloride forms a 4 : 1 ligand to metal complex only with pyrazole, with the methyl derivatives forming 2 : 1 ligand to metal complexes, as determined by single-crystal X-ray diffraction (XRD). This is attributed to a combination of ligand sterics and stronger coordination of chloride relative to nitrate. Electronic absorption spectroscopy in both water and methanol reveals a surprisingly strong effect of the pyrazole methyl position on the Cu^II d–d transition, with 4-methylpyrazole producing a higher energy d–d transition relative to the other ligands studied. In addition, the number of methyl groups plays a determining role in the energy of the pz π→Cu^II d_xy LMCT band, lowering the transition energy as more methyl groups are added.