The copper(II) coordination chemistry of alkyl substituted bispyrazole pyridine ligands: Structure and spectral properties

Tridentate, planar nitrogen containing ligands based on pyridine and pyrazoles were synthesized by the reaction of 2,6-dibromopyridine with the sodium salts of 3,5-dimethylpyrazole and 3,5-diisopropylpyrazole to yield 2,6bis(3,5-dimethyl-N-pyrazolyl)pyridine (L0py) and 2,6-bis(3,5-diisopropyl-N-pyrazolyl)pyridine (L1py), respectively. The conformation of the pyrazole rings in the solid-state structures of both L0py and L1py were anti due to relatively low repulsion between the alkyl substituents. Both copper(II) chlorido complexes [CuCl2(L0py)] and [CuCl2(L1py)] were five-coordinate with solid-state structures best described as distorted square-pyramidal geometry involving N3Cl2 donor sets. The nu(Cu-Cl) energies and Cu-Cl bond lengths were clearly different due to the different donating properties of the two ligands. By comparison, the structure of copper(II) trifluoromethanesulfonato complex [Cu(OTf)2(H2O)(L1py)] was six-coordinate with highly distorted octahedral geometry involving an N3O3 donor set. The solution UV-Vis spectra of [CuCl2(L0py)], [CuCl2(L1py)], and [Cu (OTf)2(H2O)(L1py)] revealed pi-pi* transition bands red-shifted by -30 nm on complexation. The d-d transition bands of all three copper(II) complexes were observed at almost the same energy of -750 nm, while the corresponding solid-state UV-Vis spectra exhibited absorptions at -1010 nm and 674 nm due to differences in coordination geometry. These structural changes in the solution-state were supported by results from EPR spectroscopy.