Influence of the bridging group of cross-conjugated nitrogenous bases on the spectra and structure of solvatochromic mixed-ligand copper(II) chelates containing β-ketoenols

The concomitant reaction of copper(II) with a heterocyclic nitrogenous cross-conjugated Lewis base (2,2'-dipyridyl ketone, dpk, or 2,2'-dipyridylamine, dpamH) and the anion of a 2-substituted 1,3-dione, beta(-), e.g. X-acac(-) (X = H, Me, Cl, CN, NO2) affords mixed-ligand chelates. The composition and structure of the chelates depend on the group linking the pyridyl rings. Chelation renders the 2,2'-dipyridyl ketone susceptible to nucleophilic attack by protic molecules. It also depends on the group attached to the beta-dionato moiety since electron attracting substituents facilitate ligation of a water molecule when the 2,2'-dipyridylamine is present. Spectroscopic observations indicated square pyramidal or distorted tetragonal stereochemistries of the ensuing mixed-ligand chelates with the carbonyl oxygens and the pyridyl nitrogens forming the basal plane. Confirmation was acquired by X-ray structure determination of representative chelates. The compound [Cu(NC-acac)dpamH(H2O) OClO3] crystallizes in the Pnma space group with Z = 8 (i.e. four molecules per cell), a = 17.996(1) Angstrom, b = 13.972(1) Angstrom,-c = 7.801(1) Angstrom. The copper atom is 2.401(3) Angstrom from the oxygen of the water molecule and 2.60(2) Angstrom from an oxygen atom of the ClO4- group. The chelate [Cu(NC-acac)dpC(OH)OCH3(OClO3), resulting from the addition of methanol to the carbon atom bridging the pyridyl rings, crystallizes in the P2(1)/n space group with a = 10.661(1) Angstrom, b = 14.987(2) Angstrom, c = 13.963(1) Angstrom, beta = 110.57(3)degrees, Z = 4. An oxygen of the ClO4- group is in apical position, and it is 2.634(8) Angstrom from the copper atom. The etheric oxygen is distanced 2.615(2) Angstrom from the copper atom, and the O-Cu-O angle of these weak bonds is only 154.4(2)degrees. In these chelates the nitrogenous bases adopt the boat conformation with the pyridyl rings forming dihedral angles of approximately 33 degrees.