Bonding properties of Cu(II)N chromophores: preparation, structure and spectroscopic properties of mixed ligand 2,2′:6′,2″-terpyridine copper(II) complexes. Molecular structures of [Cu(terpyridine)(4-methylimidazole)(H 2O)(BF 4)] (BF 4) and [Cu(terpyridine)(3-methylpyridine)(BF 4) 2)

Abstract

Mixed ligand 2,2′:6′,2″-terpyridine copper(II) complexes of the Cu(terpy)L(BF 4) 2·nH 2O type, where terpy stands for terpyridine, and L for imidazole ( n = 1), N-methylimidazole ( n = 1), 4-methylimidazole ( n = 1), benzimidazole ( n = 1), pyridine ( n = 0), 3-methylpyridine ( n = 0), 4-methylpyridine ( n = 1), and NH 3 ( n = 0), have been synthesized and characterized by elemental analyses, and electronic, vibrational, and EPR spectroscopic measurements. The X-ray crystal structures of [Cu(terpy)(4MImH)(H 2O) (BF 4)](BF 4) ( 1) and [Cu(terpy)(3Mpy)(BF 4)] ( 2) (4MImH = 4-methylimidazole; 3Mpy = 3-methylpyridine) have been determined from three-dimensional X-ray diffraction data. Both complexes comprise [4+1 + 1] copper units where the terpy and the unidentate N-donor ligand bind to the central copper ion forming a square plane. The CuN bond lengths along the unidentate ligand are considerably shorter than the others. The dihedral angle between the CuN 4 coordination plane and the imidazole nucleus in 1 is 85.3°; the corresponding angle between the CuN 4 plane and the pyridine nucleus in 2 is 66.2°. Similar structures with a CuN 4 coordination plane are proposed for other complexes based on spectroscopic data. The sequence of the d orbitals has been deduced from Gausian analysis of the LF spectra as d x 2−y 2 > d z 2 > d xy > d yz > d xz, where x axis is along the CuN(unidentate) on the CuN 4 plane. The bonding properties of these complexes are elucidated and discussed with reference to the electronic structures.