SYNTHESIS, STRUCTURE AND PROPERTIES OF 5-COORDINATE COPPER(II) COMPLEXES OF PENTADENTATE LIGANDS WITH PYRIDYL PENDANT ARMS

The syntheses and characterization of five-coordinate Cu(II) complexes of the pentadentate ligands trenimpy 2-pyridyl)-3-aza-3-butenyl]-1,4, 7-,triazaheptane), trenpy (4-[4-(2-pyridyl)-3-azabutyl]-1,4,7-triazahe ptane) and dmptacn (1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane) are described. X-ray diffraction studies have established the structur es of [Cu(trenimpy)](ClO4)(2) and [Cu(trenpy)](ClO4)(2). [Cu(trenimpy) ](ClO4)(2) crystallizes in the monoclinic space group P2(1)/n; with a = 12.940(5) Angstrom, b = 15.418(2) Angstrom, c = 9.853(3) Angstrom, b eta = 96.59(2)degrees, and Z = 4. Refinement gave final R and R(W) val ues of 0.066 and 0.070, respectively, for 2170 observed reflections. [ Cu(trenpy)](ClO4)(2) crystallizes in the monoclinic space group P2(1), with a = 16.367(2) Angstrom, b = 15.191(1) Angstrom, c = 8.132(1) Ang strom, beta = 96.21(2)degrees, and Z = 4. Refinement gave final R and R(W) values of 0.065 and 0.063, respectively, for 2796 observed reflec tions. Two slightly different cations were present in the unit cell. T he geometry of Cu(II) in the [Cu(trenimpy)](2+) cation is close to squ are pyramidal while that of Cu(II) in [Cu(trenpy)](2+) is intermediate between square pyramidal (SP) and trigonal bipyramidal (TBP). Visible spectra recorded in Nujol mulls were consistent with these geometries but the spectra recorded in various solvent indicated a shift in ster eochemistry toward TBP for both complexes. This was not apparent in th e case of [Cu(dmptacn)](ClO4)(2) were the mull and solution spectra we re very similar and indicated a geometry close to SP. Frozen solution ESR spectra indicate that the geometry is close to SP for all three co mplexes. Significant changes in stereochemistry are induced by changes in the environment around the cations. Cyclic voltammetry of the comp lexes in acetonitrile solution revealed a quasi-reversible redox wave, corresponding to the Cu(II)-Cu(I) couple, at -0.80, -0.94, and -0.81 V for [Cu(trenimpy)](2+), [Cu(trenpy)](2+), and [Cu(dmptacn)](2+), res pectively. Thus, some stabilization of the Cu(I) state results from th e greater ability of trenimpy and dmptacn (cf. trenpy) to accept elect ron density from Cu(I).