Synthesis, Crystal Structure, Spectral Studies, and Catechol Oxidase Activity of Trigonal Bipyramidal Cu(II) Complexes Derived from a Tetradentate Diamide Bisbenzimidazole Ligand

Abstract

A new benzimidazole-based diamide ligand-N,N'-bis(glycine-2- benzimidazolyl)hexanediamide (GBHA)-has been synthesized and utilized to prepare Cu(II) complexes of general composition [Cu(GBHA)X]X, where X is an exogenous anionic ligand (X = Cl(-), NO(3)(-), SCN(-)). The X-ray structure of one of the complexes, [Cu(GBHA)Cl]Cl.H(2)O.CH(3)OH, has been obtained. The compound crystallizes in the monoclinic space group C2/c with unit cell dimensions a = 26.464(3) A, b = 10.2210(8) A, c = 20.444(2) A, alpha = 90 degrees, beta = 106.554(7) degrees, gamma = 90 degrees, V= 5300.7(9) A(3), and Z = 8. To the best of our knowledge, the [Cu(GBHA)Cl]Cl.H(2)O.CH(3)OH complex is the first structurally characterized mononuclear trigonal bipyramidal copper(II) bisbenzimidazole diamide complex having coordinated amide carbonyl oxygen. The coordination geometry around the Cu(II) ion is distorted trigonal bipyramidal (tau = 0.59). Two carbonyl oxygen atoms and a chlorine atom form the equatorial plane, while the two benzimidazole imine nitrogen atoms occupy the axial positions. The geometry of the Cu(II) center in the solid state is not preserved in DMSO solution, changing to square pyramidal, as suggested by the low-temperature EPR data g( parallel) > g( perpendicular) > 2.0023. All the complexes display a quasi-reversible redox wave due to the Cu(II)/Cu(I) reduction process. E(1/2) values shift anodically from Cl(-) < NO(3)(-) < SCN(-), indicating that the bound Cl(-) ion stabilizes the Cu(II) ion while the N-bonded SCN(-) ion destabilizes the Cu(II) state in the complex. When calculated against NHE, the redox potentials turn out to be quite positive as compared to other copper(II) benzimidazole bound complexes (Nakao, Y.; Onoda, M.; Sakurai, T.; Nakahara, A.; Kinoshita, L.; Ooi, S. Inorg. Chim. Acta 1988, 151, 55. Addison, A. W.; Hendricks, H. M. J.; Reedijk, J.; Thompson, L. K. Inorg. Chem. 1981, 20 (1), 103. Sivagnanam, U.; Palaniandavar, M. J. Chem. Soc., Dalton Trans. 1994, 2277. Palaniandavar, M.; Pandiyan, T.; Laxminarayan, M.; Manohar, H. J. Chem. Soc., Dalton Trans. 1995, 457. Sakurai, T.; Oi, H.; Nakahara, A. Inorg. Chim. Acta 1984, 92, 131). It is therefore concluded that binding of amide carbonyl oxygen destabilizes the Cu(II) state. The complex [Cu(II)(GBHA)(NO(3))](NO(3)) could be successfully reduced by the addition of dihydroxybenzenes to the corresponding [Cu(I)(GBHA)](NO(3)). (1)H NMR of the reduced complex shows slightly broadened and shifted (1)H signals. The reduction of the Cu(II) complex presumably occurs with the corresponding 2e(-) oxidation of the quinol to quinone. Such a conversion is reminiscent of the functioning of a copper-containing catechol oxidase from sweet potatoes and the met form of the enzyme tyrosinase.

Synthesis, structural characterization and thermal properties of three copper(II) complexes based on aryl hydrazide ligands

“…The broad shape of this band suggests the existence of H-bonds. The intense bands in the region of 1,637-1,518 cm -1 can be assigned to m s (C-O), r(NH), m s (C=N) and m as (C=N) which indicate the deprotonation through thioenolization on coordination [32], plus the pyridine ring vibrations. Coupled vibrations among the thioamide bands are distributed around 1,437 and 1,361 cm -1 with two equal bands.…”

Azido bridge mediated catecholase activity, electrochemistry and magnetic behavior of a dinuclear copper(II) complex of a phenol based “end-off” compartmental ligand

“…Dinuclear copper complexes with Cu-Cu separation of $3 Å are interesting as corroborative model of the active site of catechol oxidase to elucidate the functional mechanism of the native enzyme and to develop bio-inspired catalytic systems [1][2][3][4][5][6][7][8][9]. When that separation is associated with bridging azido ligand the species becomes of particularly interesting for gaining deeper insight into magneto-structural correlation, a study very much essential for developing new functional molecule based materials [10][11][12][13][14].…”

Oxidation of electron deficient olefins using a copper(II) complex based on a bis-benzimidazole diamide ligand

“…We believe that a similar mechanism may be operative in our case also. Earlier we have found that similar, bis-benzimidazole copper(II) complexes, also catalyze reactions by the hydrogen abstraction mechanism [11,13].…”

“…of commercially available TBHP. Earlier, we have initiated a study of copper(II) complexes utilizing bis benzimidazole diamide ligands, as oxidants towards a variety of substrates [11][12][13].…”