Silvia E. Castillo-Blum

Coordination chemistry of some biologically active ligands

Abstract We have been interested in the coordination behaviour of biologically active ligands containing N and O donor atoms towards metal ions and boron. Structural and spectroscopic studies have been carried out. Also, the stability, chemical structure and behaviour of some of the coordination compounds in solution have been studied and their effect on photosynthetic activities investigated.

Synthesis, X-ray and spectroscopic characterisation of chromium(III) coordination compounds with benzimidazolic ligands

Abstract The syntheses and characterisation of chromium(III) complexes with tris(2-benzimidazolylmethyl)amine (ntb), 2-guanidinobenzimidazole (2gb), 2,6-bis(2-benzimidazolyl)pyridine (bbimpy) and 2-(2-pyridyl)benzimidazole (2pb), of composition [Cr(ntb)Cl2]Cl·4.5H2O (1), [Cr(2gb)3]Cl[ZnCl4]·CH3OH (2), [Cr(2gb)3]Cl3·4H2O (3), [Cr2(2gb)4(μ-OH)2](ClO4)4·5H2O (4), [Cr(bbimpy)Cl3] (5) and [Cr(2pb)2Cl2]Cl·C2H5OH·0.5H2O (6) are presented. The compounds are obtained from Cr(III) in alcoholic solution, or a Cr(II) aqueous acidic solution. The latter were carried out under air-free conditions. The reaction of 2gb with Cr(II) in aqueous solution yielded a dinuclear compound. The X-ray crystal structures of 1 and 4 are discussed.

Supramolecular structures of metronidazole and its copper(II), cobalt(II) and zinc(II) coordination compounds

In this work we present the synthesis and structural and spectroscopic characterization of Cu(II), Co(II) and Zn(II) coordination compounds with the antibiotic metronidazole ([double bond]emni). Coordination to metal ions is through its imidazolic nitrogen, while the hydroxyethyl and nitro groups act as supramolecular synthons. [Co(emni)(2)Br(2)], and [Zn(emni)(2)X(2)] (X(-)=Cl, Br) stabilize zig-zag chains, and a 2D supramolecular structure is formed by inter-chain contacts through inter-molecular hydrogen-bonding. Pleated sheet or layers are formed by [Co(emni)(2)Cl(2)] and [Cu(emni)(2)Cl(H(2)O)](2)Cl(2), respectively. The dinuclear Cu(II) compound [Cu(emni)mu(O(2)CMe)(2)](2) gives a one-dimensional zig-zag arrangement. The contribution of metal ions in metronidazole coordination compounds is shown in the stabilization of the different aggregate structures.

Coordination behaviour of 2-guanidinobenzimidazole towards cobalt(II), nickel(II), copper(II) and zinc(II). An experimental and theoretical study

SummaryThe following coordination compounds derived from 2-guanidinobenzimidazole (2GB) (1); [Ni(2GB)2]Cl2· H2O, (2); [Ni(2GB)2]Br2·3H2O, (3); [Ni(2GB)2-(NO3)2, (4); [Ni(2GB)2](OAc)2, (5); [Cu(2GB)Cl2], (6); [Cu(2GB)Br2], (7); [Cu(2GB)2]Br2·2H2O, (8); [Cu(2GB)2](NO3)2·H2O, (9); [Cu(2GB)2](OAc)2· H2O, (10); [Zn(2GB)Cl2]·H2O, (11); [Zn(2GB)Br2]·H2O, (12); [Co(2GB)Cl2(H2O)2]·5H2O, (13); [Co-(2GB)2Cl2]·3H2O, (14); [Co(2GB)2(H2O)2](NO3)2· 4H2O, (15); and [Co(2GB)2(H2O)2](OAc)2, (16) have been synthesized and characterized by i.r. and electronic spectroscopy. In addition (6)–(10) were analysed by e.p.r. The X-ray diffraction structure of compound (4) was obtained. It crystallizes in the monoclinic system, C2/c (a = 22.511(7), b = 6.735(6) and c= 15.345(5)Å, β =115.31(3)°, Z = 4, final R = 0.0360 and Rw = 0.0388 for 1167 observed independent reflections). The nickel(II) atom coordinates two ligands in a square-planar geometry through the imidazolic N(3) and the guanidino N(12).The probable ligand isomers involved in the coordination were determined by theoretical calculations, and the possible structures of the coordination compounds were investigated in order to verify that the experimentally proposed structures were stable. Two different types of coordination compounds were found. One, where the ligand is chelating through the imidazolic N(3) and the guanidino N(12), which is the case for most of the complexes [(2)–(13)]. With only one ligand in the coordination sphere, the structure was either tetrahedral (copper and zinc chloride and bromide complexes) or octahedral (cobalt). With two chelating 2GB units a square-planar geometry was stabilized [(2)–(5) and (8)–(10)]. The second type of coordination behaviour was observed in the cobalt compounds [(14)–(16)]. Here the ligand coordinates monodentate through the imidazolic N(3); the structure is tetrahedral.

Synthesis, X-ray and NMR characterisation of cobalt(III) coordination compounds with 2-guanidinobenzimidazole

Abstract The synthesis and characterisation of cobalt(III) coordination compounds with 2-guanidinobenzimidazole (2gb), namely [Co(2gb)3]Cl2(NO3)·H2O, [Co(2gb)3]Cl(NO3)2·H2O, [Co(2gb)3]Cl3·3H2O and [Co(2gb)3](Cl3CCOO)3·H2O, are presented. The compounds with the 2-guanidinobenzimidazole ligand were synthesised from [Co(NH3)4CO3]NO3 or Na3[Co(CO3)3]·3H2O or from cobalt(II) chloride. 1H, 13C and 2D NMR spectra together with the X-ray diffraction data for the distorted octahedral [Co(2gb)3]3+ cation showed that only the mer isomer was formed.

NMR study of the coordinating behavior of 2,6‐bis(benzimidazol‐2′‐yl)pyridine

The coordination sites of 2,6- bis(benzimidazol-2-yl)pyridine (1) toward protons and the diamagnetic metal ions Li ,N a , and Co 3 were investigated by NMR spectroscopy. Variable tem- perature 1 H and 13 C NMR experiments were performed on 1 in order to evaluate the tautomeric equilibrium and hydrogen bonding. Imidazole dicoordinated aro- matic nitrogen atoms were protonated by trichloroa- cetic acid and the three N-dicoordinated atoms by fuming H2SO4. Reactions of the ligand 1 and benzim- idazole 2 with metallic sodium or LiH afforded an- ionic species; the alkali metal ions appeared solvated by THF, but not by the ligands 1 or 2. In contrast, reaction of 1 with Co(III) produces the stable cation (Co(1-H)2) with cobalt ion coordinated by two mol- ecules of the monodeprotonated ligand. 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:392-398, 2000

A new isomer of the cis-[Co(cyclen)Cl2]+ cation (cyclen = 1,4,7,10-tetraazacyclododecane)

Abstract This work describes the preparation of a new complex, 1, and its perchlorate salt 2 when cyclen (1,4,7,10-tetraazacyclododecane) and CoCl2·6H2O are reacted in anhydrous methanol. 1H and 13C NMR spectroscopies were used to determine the absolute configuration of the new coordination compound.

Substitution inertness of [Ir(H2O)6]3+

Ligand substitution properties of the recently characterized pale-yellow [Ir(H2O)6]3+ ion in perchlorate solutions, have been investigated. At 120°C in sealed tubes observations on the exchange with H218O are impaired by a spurious oxidation to purple Ir(IV). Over extended periods at 40°C the ion has been shown to be extremely inert, and the rate constants for substitution of Cl− into [Ir(H2O)6]3+ is estimated to be < 2 × 10−9 M−1 S−1.

Coordination compounds quinic acid as decouplers on photosynthesis

Abstract We found that quinic acid ( 1 ) inhibits ATP-synthesis and the light dependent H + -uptake, it also enhances the basal electron transport rate from water to methylviologen in spinach chloroplasts. These results indicate that it behaves as non-classical decoupler on photophosphorylation. The effect on photosynthetic activities of M(quin) X H 2 O (M 2+ = Co, Ni, X − = Cl; and M(quin) 2 (M 2+ = CO, Zn, Cd) was studied. The compounds [Co(quin)Cl(H 2 O)] · H 2 O ( 2 ), Co(quin) 2 ( 3 ) [Ni(quin)Cl(H 2 O)] · (H 2 O) 2 ( 4 ), [Zn(quin) 2 ] ( 5 ) and [Cd(quin) 2 ] ( 6 ) inhibit ATP-synthesis, dissipate H + -uptake and accelerate electron transport, behaving as non-classical decouplers. An electronic spectroscopy study of these complexes indicates that they are stable both in water and buffer solutions. The effect of metal salts on photosynthesis was assayed: cobalt chloride has a very mild effect as decoupler, the other metal chlorides (nickel, zinc and cadmium) do not behave as decouplers in freshly lysed chloroplasts.

Cobalt(II) Coordination Compounds of Ethyl 4-Methyl-5-Imidazolecarboxylate: Chemical and Biochemical Characterization on Photosynthesis and Seed Germination

In this work we present the synthesis, structural and spectroscopic characterization of Co2+ coordination compounds with ethyl 4-methyl-5-imidazolecarboxylate (emizco). The effects of emizco, the metal salts CoCl2.6H2O, CoBr2, Co(NO3)2.6H2O and their metal coordination compounds [Co(emizco)2Cl2], [Co(emizco)2 Br2].H2O, [Co(emizco)2 (H2O)2(NO2)2.2H2O were evaluated on photosynthesis in spinach chloroplasts. Seed germination and seedling growth of the monocotyledonous species Lolium multiflorum and Triticum aestivum and the dicotyledonous species Trifolium alexandrinum and Physalis ixocarpa were also assayed under the effect of the compounds and salts. The results showed that cobalt(II) salts and their emizco coordination compounds inhibit photosynthetic electron flow and ATP-synthesis, behaving as Hill reaction inhibitors. Coordination compounds are more potent inhibitors than the salts. It was found that the salts target is at the b6f level while the complexes targets are at QB(D1)-protein and b6f level. The QB inhibition site was confirmed by variable chlorophyll a fluorescence yield. On the other hand, emizco inhibits seed germination, root and shoot development, in both weed and crop species. Cobalt(II) coordination compounds are the most effective photosynthesis inhibitors, but they are less potent than emizco in germination and seedling growth, while the metal salts are the least active of all.