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

Abstract

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 2,6-bis(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 ν(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 π-π* 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.