Bis Imidazolyl Ligands Research Articles

Structural diversity of Zn(ii) coordination polymers based on bis-imidazolyl ligands and 5-R-1,3-benzenedicarboxylate and their photocatalytic properties

Eight Zn(II) coordination polymers with 5-R-1,3-benzenedicarboxylate (R = H, Br) ligands and bis-imidazolyl ligands, {[Zn(1,3-BDC)(abimb)]·H2O}n (1), {[Zn(5-Br-1,3-BDC)(abimb)]2·0.5H2O}n (2), [Zn(1,3-BDC)(nbimb)]n (3), [Zn(5-Br-1,3-BDC)(nbimb)]n (4), {[Zn(1,3-BDC)(bimpa)]·2H2O}n (5), {[Zn(5-Br-1,3-BDC)(bimpa)]·2H2O}n (6), [Zn(1,3-BDC)(bimcz)]n (7) and [Zn(5-Br-1,3-BDC)(bimcz)]n (8) (1,3-H2BDC = 1,3-benzenedicarboxylate; 5-Br-1,3-H2BDC = 5-bromo-1,3-benzenedicarboxylate; abimb = 2-amine-4,4′-bis(1-imidazolyl)-bibenzene; nbimb = 2-nitro-4,4′-bis(1-imidazolyl)-bibenzene; bimpa = bis-(4-imidazol-1-yl-phenyl)-amine; bimcz = 3,6-bis(1-imidazolyl)-carbazole), have been synthesized and structurally characterized by elemental analysis, IR and X-ray single crystal diffraction. 1 and 6 exhibit 4-connected 3D frameworks with the Schlafli symbol (65·8). 2, 7 and 8 display 3D polythreaded networks based on 2D wrinkled layers. 3 and 4 show 3D polycatenated arrays based on 2D puckered layers. 5 has a 3D supramolecular structure consisting of two-fold parallel interwoven layers through hydrogen bonding and π⋯π stacking interactions. The solid state photoluminescence and photocatalytic properties of 1–8 were also investigated.

Bidentate ligation of heme analogues; novel biomimetics of peroxidase active site.

The multifunctional nature of proteins that have iron-heme cofactors with noncovalent histidine linkage to the protein is controlled by the heme environment. Previous studies of these active-site structures show that the primary difference is the length of the iron-proximal histidine bond, which can be controlled by the degree of H-bonding to this histidine. Great efforts to mimic these functions with synthetic analogues have been made for more than two decades. The peroxidase models resulted in several catalytic systems capable of a large range of oxidative transformations. Most of these model systems modified the porphyrin ring covalently by directly binding auxiliary elements that control and facilitate reactivity; for example, electron-donating or -withdrawing substituents. A biomimetic approach to enzyme mimicking would have taken a different route, by attempting to keep the porphyrin ring system unaltered, as close as possible to its native form, and introducing all modifications at or close to the axial coordination sites. Such a model system would be less demanding synthetically, would make it easy to study the effect of a single structural modification, and might even provide a way to probe effects resulting from porphyrin exchange. We introduce here an alternative model system based on these principles. It consists of a two component system: a bis-imidazolyl ligand and an iron-porphyrin (readily substituted by a hemin). All modifications were introduced only to the ligand that engulfs the porphyrin and binds to the iron's fifth and sixth coordination sites. We describe the design, synthesis, and characterization of nine different model compounds with increased complexity. The primary tool for characterizing the environment of each complex Fe(III) center was the Extended X-ray Absorption Fine Structure (EXAFS) measurements, supported by UV/Vis, IR, and NMR spectroscopy and by molecular modeling. Introduction of asymmetry, by attaching different imidazoles as head groups, led to the formation of two axial bonds of different length. Addition of H-bonds to one of the imidazoles in an advanced model increased this differentiation and expanded the porphyrin ring. These complexes were found to be almost identical in structure to peroxidase active sites. Similarly to the peroxidases and other synthetic models, these compounds stabilize the green, compound I-like intermediate, and catalyze the oxidation of organic substrates.

A cofactor approach to copper-dependent catalytic antibodies.

A strategy for the preparation of semisynthetic copper(II)-based catalytic metalloproteins is described in which a metal-binding bis-imidazole cofactor is incorporated into the combining site of the aldolase antibody 38C2. Antibody 38C2 features a large hydrophobic-combining site pocket with a highly nucleophilic lysine residue, Lys(H93), that can be covalently modified. A comparison of several lactone and anhydride reagents shows that the latter are the most effective and general derivatizing agents for the 38C2 Lys residue. A bis-imidazole anhydride (5) was efficiently prepared from N-methyl imidazole. The 38C2-5-Cu conjugate was prepared by either (i) initial derivatization of 38C2 with 5 followed by metallation with CuCl2, or (ii) precoordination of 5 with CuCl2 followed by conjugation with 38C2. The resulting 38C2-5-Cu conjugate was an active catalyst for the hydrolysis of the coordinating picolinate ester 11, following Michaelis-Menten kinetics [kcat(11) = 2.3 min(-1) and Km(11) 2.2 mM] with a rate enhancement [kcat(11)k(uncat)(11)] of 2.1 x 10(5). Comparison of the second-order rate constants of the modified 38C2 and the Cu(II)-bis-imidazolyl complex k(6-CuCl2) gives a rate enhancement of 3.5 x 10(4) in favor of the antibody complex with an effective molarity of 76.7 M, revealing a significant catalytic benefit to the binding of the bis-imidazolyl ligand into 38C2.

Cooperativity of binuclear Zn(II) complexes of bisimidazolyl ligands in the hydrolysis of bis(2,4-dinitrophenyl) phosphate in aqueous solution

Hydrolysis of bis(2,4-dinitrophenyl) phosphate (BDNPP) by Zn(II) with ligands bearing plural bis(imidazol-2-yl) groups was kinetically studied in aqueous solution. The binuclear Zn(II) complex of 1,3-bis(diimidazol-2-ylhydroxymethyl)benzene (2a) was found to be most effective for hydrolysis of BDNPP (1.2 × 103-fold). ESI-MS study revealed formation of 2a·Zn(II)2 in MeOH–H2O, suggesting that the rate acceleration is due to so called double Lewis acid activation. The pH–rate profile showed that the pKa of the active species is around 7.5. The rate accelerations by the complexes of 2a with other metal ions were 4.3-fold for Ni(II), 7.1-fold for Co(II), and 1.9 × 102-fold for La(III).

Synthesis, structure and properties of a N 3 tridentate bis-imidazolyl ligand with copper(II)

The synthesis of a new tridentate ligand, bis[2-(1-methylimidazolyl)methyl] amine, [B-MIMA], is described. The compound has been characterized by 1H and 13C NMR spectroscopy and mass spectrometry, and forms complexes with various copper(II) salts. Crystallographic studies have been completed on the blue product from the reaction of B-MIMA with copper(II) acetate, [Cu(B-MIMA)(CH 3COO)](ClO 4) ( 6). Crystal data for 6 are: triclinic, P1, a = 5.176(2), b = 8.489(3), c = 10.066(3) Å, α = 95.09(5), β = 95.72(3), γ = 102.37(4), V = 427.1 Å and Z = 1. The structure was solved using the Patterson method which revealed the position of the copper atom and successive difference Fourier syntheses which revealed the location of the remaining atoms. The atom positions were refined by least-squares methods producing a final agreement factor of R = 0.021 and R w = 0.029 for 1897 observed reflections. The structure consists of a polymeric acetate bridged network of distorted square-pyramidal copper(II) ions. The five coordinate geometry around each metal ion is a result of the bonding of three nitrogen atoms from the B-MIMA ligand and two oxygen atoms from different acetate ions. The acetate ions act as bidentate ligands bridging different copper ions and the resulting extended bridging network extends out the crystallograhic a axis. Spectroscopic, magnetic and electrochemical properties of the complex are described and compared with related compounds. The solution spectroscopic properties of 6 suggest that the square-pyramidal geometry of the metal ion in the solid-state is not retained in solution.