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-tr iazacyclononane, [9]aneN(3), groups conjoined by polymethylene chains two to six carbons in length is described. Equilibrium modeling studie s in aqueous solution using pH-potentiometry indicate that the smalles t homologue of the series, EM2, forms only Cu(EM2)(2+) in dilute aqueo us solutions. All other ligands of the series form stable 1:1 (protona ted 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 solu tion. The EM ligands with the shortest polymethylene bridging groups f orm 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 conti nuous variation studies at pH 4.0 and 7.5 are interpreted on the basis of the principal equilibrium species obtained from the equilibrium mo dels. 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 so lid. The monomeric compounds contain CuN6 chromophores while the dicop per(IT) compounds contain square pyramidal CuN3Cl2 coordination geomet ry. Compound 1 crystallizes in space group <P(1)over bar> with a = 7.8 49(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) Ang strom, 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 = 92.6 2(2)degrees, and Z = 4. Cyclic voltammograms of aqueous solutions prep ared 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 chem ically 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(p arallel to) = 2.225 (A(parallel to) = 164 x 10(-4)) and g(perpendicula r to) = 2.045).