Copper(II) complexes of bis(1,4,7-triazacyclononane) ligands with polymethylene bridging groups: An equilibrium and structural study

Copper(II) complexation by a series of Ligands containing two 1,4,7-triazacyclononane, [9]aneN(3), groups conjoined by polymethylene chains two to six carbons in length is described. Equilibrium modeling studies in aqueous solution using pH-potentiometry indicate that the smallest homologue of the series, EM2, forms only Cu(EM2)(2+) in dilute aqueous solutions. All other ligands of the series form stable 1:1 (protonated and nonprotonated) and 2:1 dicopper(II) (hydroxo and non-hydroxo) complexes. Those ligands that contain bridging chains of four or more carbon atoms likely form dimeric or oligomeric complex species in solution. The EM ligands with the shortest polymethylene bridging groups form the most stable 1:1 species. There is little difference among the ligands (n = 3-6) in complex stability of the protonated, CuH2(EMn)(4+), and dicopper(II), Cu-2(EMn)(4+), species. W-vis spectroscopic continuous variation studies at pH 4.0 and 7.5 are interpreted on the basis of the principal equilibrium species obtained from the equilibrium models. Single-crystal X-ray diffraction studies on four complexes ([Cu(EM2)]SO4 . 6H(2)O (1), [Cu-2(EM2)Cl-4]. 2H(2)O (2), [Cu-2(EM6)Cl-4] (3), and [Cu(EM3)][ZnBr4]. H2O (4)) characterize structural features of several 1:1 monomeric and dicopper(II) complexes in the crystalline solid. The monomeric compounds contain CuN6 chromophores while the dicopper(IT) compounds contain square pyramidal CuN3Cl2 coordination geometry. Compound 1 crystallizes in space group <P(1)over bar> with a = 7.849(2) Angstrom, b = 9.783(2) Angstrom, c = 16.919(5) Angstrom, alpha = 78.42(3)degrees, beta = 85.76(3)degrees, gamma = 73.06(3)degrees, and Z = 2. 2: P2(1)/n with a = 9.689(3) Angstrom, b = 11.733(3) Angstrom, c = 10.124(3) Angstrom, beta = 98.20(2)degrees, and Z = 2. 3: P2(1)/n with a = 7.278(2) Angstrom, b = 12.416(3) Angstrom, c = 13.781(2) Angstrom, beta = 90.15(2)degrees, and Z = 2. 4: P2(1)/c with a = 9.295(3) Angstrom, b = 16.233(4) Angstrom, c = 16.544(5) Angstrom, beta = 92.62(2)degrees, and Z = 4. Cyclic voltammograms of aqueous solutions prepared by dissolving [Cu-2(EM2)Cl-4]. 2H(2)O confirm its dissociation to Cu(EM2)(2+). Aqueous solutions containing 1:1 molar ratios of Cu(II) and EM2 in 0.1 mol dm(-3) KCl at 25 degrees C show a one-electron chemically reversible reduction at scan rates of 500 mV s(-1) with E-1/2 (Cu(II)-Cu(I)) = -868 mV relative to SCE. EPR (X- and Q- band) spectra of frozen solutions (1:1 DMSO/H2O and glycerol/H2O) of Cu(EM2)(2+) at 100 K are typical of axial copper(II) features (X-band parameters: g(parallel to) = 2.225 (A(parallel to) = 164 x 10(-4)) and g(perpendicular to) = 2.045).