Synthesis and Characterization of Pseudotetrahedral Copper(II) Dihalide Complexes with (s)-(−)-Nicotine

Blue copper proteins (Type I) exhibit unusual spectral and chemical properties such as higher reduction potentials and small hyperfine coupling constants A|| in the EPR spectrum in comparison with other copper(II) complexes. Such unusual properties are justified partly by the tetralhedral geometry around the copper(II) site and partly by the CuN2S2 chromophores in which the soft S atoms coordinate with the copper ions to stabilize the copper(I) state in the proteins. Recently we prepared neutral alkaloid (−)-l-sparteine complexes with copper(II) malonitrilodithiolate and with coper(II) oxyanion complexes, and suggested them as model compounds for the type I copper(II) site of blue copper proteins since their physicochemical properties mimic interesting spectral and redox behaviors in the blue copper centers. In the former compound, the copper atom adopted a distorted tetrahedral geometry with a dihedral angle of 68.07 between the CuN2 and CuS2 planes, and the distortion was caused from the steric hindrance of the bulky (−)-l-sparteine ligand in the reported Cu(sp)(mnt) complex. We focussed on the copper coordination sphere as a first step in model studies on the copper proteins. Herein we prepared (s)-(−)-nicotine complexes with copper(II) halides since (s)-(−)-nicotine is another bulky ligand forming a distorted tetrahedral geometry around the copper(II) site. Furthermore, (s)-(−)-nicotine is a basic ligand possessing methylated pyrrolidine nitrogen and pyridine nitrogen to form a stable chelate with the copper(II) ion. The prepared compounds were characterized using spectroscopic (optical, EPR), electrochemical methods and magnetic susceptibility measurements, and the examined properties are discussed in relation to the geometry around the copper site.