Imidazolate bridged Cu(ii)–Cu(ii) and Cu(ii)–Zn(ii) complexes of a terpyridinophane azamacrocycle: a solution and solid state study

Abstract

The dinuclear Cu2+ and Zn2+ as well as the mixed Cu2+-Zn2+ complexes of a 5,5''-pentaazaterpyridinophane ligand (L) are able to incorporate imidazolate (Im-) as a bridging ligand. The crystal structure of [Cu(2)L(Im)(Br)(H2O)](CF(3)SO(3))(2).3H2O (1) shows one copper coordinated by the three pyridine nitrogens of the terpyridine unit, one nitrogen of the imidazolate bridge (Im-) and one bromide anion occupying the axial position of a distorted square pyramid. The second copper atom is coordinated by the remaining imidazolate nitrogen, the three secondary nitrogens at the centre of the polyamine bridge and one water molecule that occupies the axial position. Magnetic measurements have been performed in the 2.0-300.0 K temperature range. Experimental data could be satisfactorily reproduced by using an isotropic exchange model H = -JS(1)S(2) with J = -52.3 cm(-1) and g = 2.09. Potentiometric studies have provided details of the speciation and stability constants for the mixed Cu2+-L-HIm, Zn2+-L-HIm (HIm = imidazole) and Cu2+-Zn2+-L-HIm systems. The apparent stability constant obtained at pH = 9 for the addition of imidazole to the dinuclear Cu2+ complexes is one of the highest so far reported (log K = 7.5). UV-Vis spectroscopy and paramagnetic NMR data show that imidazole coordinates to the Cu2+ ions as a bridging imidazolate ligand from pH 5 to 10. Electrochemical reduction of the Cu2+-Zn2+-L complex occurs in two successive one-electron per copper ion quasi-reversible steps. The formal potential of the Cu2+-Zn2+-L/Cu+-Zn2+-L couple is close to that of SOD. The IC50 values measured at pH 7.8 by means of the nitro blue tetrazolium method show significant SOD activity for the dinuclear Cu2+ complexes (IC50 = 2.5 microM) and moderate activity for the Cu2+-Zn2+ mixed systems (IC50 = 30 microM).

Imidazolate-bridged dinuclear copper(II) complex with new macrocyclic ligand bearing two 1H-imidazol-4-yl-pendants

“…The distance is shorter than the one in Cu 2 Zn 2 -SOD (Cu⋅⋅⋅Zn = 6.3 Å) but is close to those in other reported model complexes [9,10,12]. Both Cu(II) atoms are five -coordinated by four nitrogen atoms from L ligand and one nitrogen atom from imidazolate bridge, and all the bond lengths and bond angles are in the normal range [9][10][11][12]. Notably, the coordination geometry around Cu1 is a distorted trigonal-bipyramid as indicated by an index τ value of 0.81, while that of Cu2 is a distorted square-pyramid since the τ value is 0.46 [17].…”

“…As an extension of our work, we designed a new macrocyclic ligand with 1H-imidazol-4-yl-pendants, namely 6, 20-bis (1H-imidazol-4-ylmethyl)-3, 6, 9, 17, 20, 23-hexaazatricyclo [24.3.1.1 11,15 ]hentriaconta-1(29), 11, 13, 15(31), 26(30), 27-hexaene (L, Scheme 1). Herein we report a imidazolate-bridged dinuclear Cu (II) complex [Cu 2 (L)(Im)](ClO 4 ) 3 ·H 2 O (1) obtained by reaction of ligand L and imidazole with Cu(ClO 4 ) 2⋅ 6H 2 O [14,15].…”

“…Therefore, the rational design and synthesis of imidazolate-bridged homo/heterodinuclear complexes to mimic the active site of Cu 2 Zn 2 -SOD have attracted great attention, and the reported studies have provided valuable insights into the structure and activity of the Cu 2 Zn 2 -SOD [5][6][7][8][9]. Among the reported model compounds, macrocyclic ligands, especially those bearing flexible pendants, have been extensively used to construct the imidazolatebridged dinuclear complexes for their enhanced stability, and the obtained complexes have been proved to be successful in mimicking the active site of the enzyme [9][10][11]. However, macrocyclic ligands with imidazole-pendants were less documented to now [12].…”

Imidazolate-bridged dicopper(II) and copper(II)–zinc(II) complexes of macrocyclic ligand with methylimidazol pendants: Model study of copper(II)–zinc(II) superoxide dismutase

“…The SOD activities were evaluated according to the nitro blue tetrazolium (NBT) assay [12,19,20]. The superoxide anions are generated from the riboflavin-methionine reaction and the NBT is utilized to react with the O ÅÀ 2 to generate blue formazan.…”

“…Ever since the X-ray crystal structure analysis revealed the existence of imidazolate-bridged copper-zinc heterodinuclear structure in the active center of the bovine erythrocyte Cu 2 Zn 2 -SOD, a number of imidazolate-bridged dinuclear complexes have been synthesized [8][9][10][11][12]. Comparison between [Cu 2 (TMDT) 2 (im)] 3+ (TMDT = 1, 1, 7, 7-tetramethyldiethylenetriamine) and [Cu 2 (bpim)] 3+ (bpim À = 4,5-bis[2-(2-pyridyl)-ethyliminomethyl]imidazolate) revealed that the imidazolate bridge in the former is stable only over a narrow pH range while the one in the latter is quite stable [13,14].…”