Structural variation in the copper(II) complexes with a tetradentate bis-6-methylpyridine-substituted bispidine ligand

Abstract Four single crystal X-ray structures of the complex cation [Cu II (L)(X)] n + (L = 3,7-dimethyl-2,4-di-(6-methyl-2-pyridyl)-9-diol-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate dimethylester, X = NCCH 3 , OH 2 , Cl – , NO 3 – ) are discussed together with their spectroscopic properties. The bispidine ligand L enforces a square pyramidal or cis -octahedral coordination geometry with two cis -disposed tertiary amines (N3 and N7), two trans -disposed pyridines which are co-planar and in-plane with the metal center and one of the two amines (N3), and one or two co-ligands trans to the amine donors (N3 or N7). Three structural types have been found: ( i ) X = NCCH 3 , square pyramidal, X trans to N3, in-plane with the pyridine donors, elongation of the Cu–N7 bond; ( ii ) X = Cl – , OH 2 , square pyramidal, X trans to N7, perpendicular to the pyridine planes, slight elongation of the Cu–N3 bond; ( iii ) X = NO 3 – , octahedral, bidentate NO 3 – , elongation of the Cu–N pyridine bonds. The three isomeric chromophores (Cu(L)-fragments, the three possible minima of the ‘Mexican hat’ Jahn–Teller potential energy surface) are discussed on the basis of the experimental structural data, the steric contributions to the stabilization/destabilization of the three energetically similar minima are discussed on the basis of empirical force field calculations, and the results of DFT model calculations are used for a qualitative interpretation of electronic effects. Solid state and solution electronic spectra and frozen solution EPR spectra are used for a qualitative analysis of the three electronic ground states and a possible equilibration of the three isomeric forms, and this is supported by a preliminary ligand field analysis, based on AOM model calculations. To cite this article: P. Comba et al., C R Chimie 8 (2005) .