Copper(I) complexes of tripodal tris(imidazolyl) ligands: potential mimics of the Cu(A) site of hydroxylase enzymes

Copper(I) complexes of tripodal tris(N-methyl-4,5-diphenyl-imidazolyl)methane ligands, N3CR (1a-c, R = OH, OMe, H), have been prepared as models for the Cu(A) site of copper hydroxylase enzymes. In the absence of additional donors, the ligands 1 react with [Cu(CH3CN)4]PF6 (2) to produce dinuclear complexes [(N3CR)2Cu2](PF6)2 (3) in which the tripodal ligands bridge two trigonal Cu centers; the structures of 3b and 3c are established by X-ray diffraction. Mononuclear adducts [(N3CR)CuL]Z are produced with L = acetonitrile (4), carbon monoxide (5), and t-BuNC (6, 7). The carbonyl complexes 5 are in dynamic equilibrium with the dimeric complexes 3, but 5c (R = H) can be isolated. The structures of the isocyanide derivatives depend critically on the tripod methane substituent, R. Thus, the X-ray structures of 6 (R = OMe) and 7 (R = H) show trigonal and tetrahedral geometries, respectively, with bi- or tridentate coordination of the tripod. A trinuclear complex [Cu3(N3COH)2(t-BuNC)2](PF6)3 (8) is formed from N3COH (1a) which features both three-coordinate and two-coordinate Cu atoms and bidentate tripod coordination. Reactions of dioxygen with dinuclear 3c or mononuclear [(N3CR)CuL]Z are sluggish, producing from the latter in acetone [(N3CH)CuII(L)(L')](PF6)2 (9, L = acetone, L' = H2O).     

Synthesis of sterically hindered tris(4-imidazolyl)carbinol ligands and their copper(I) complexes related to metalloenzymes

Tris(4-imidazolyl)carbinol, which has close coordination environment to the active site of metalloenzymes, has not been utilized as a biomimetic ligand because of its instability. We have synthesized stable tris(4-imidazolyl)carbinol derivatives having a methyl group as the NH protective group and a bulky substituent on the imidazole ring for stabilizing reactive species bound to the metal center. These ligands provide stable monomeric copper(I) complexes whose coordination environment are very close to the active site of metalloenzymes.     

Synthesis and spectroscopy of micro-oxo (O(2)(-))-bridged heme/non-heme diiron complexes: models for the active site of nitric oxide reductase

In this paper, we describe the synthesis and study of a series of heme/non-heme Fe-O-Fe' complexes supported by a porphyrin and the tripodal nitrogen ligand TMPA [TMPA = tris(2-pyridylmethyl)amine]. The complete synthesis of [((6)L)Fe-O-Fe(X)](+) (1) (X = OMe(-) or Cl(-), 69:31 ratio), where (6)L is the dianion of 5-(o-O-[(N,N-bis(2-pyridylmethyl)-2-(6-methoxyl)pyridinemethanamine)phenyl]-10,15,20-tris(2,6-difluorophenyl)porphine, is reported. The crystal structure for 1.PF(6) reveals an intramolecular heme/non-heme diferric complex bridged by an Fe-O-Fe' moiety; 90 degree angle (Fe-O-Fe') = 166.7(3) degrees, and d(Fe.Fe') = 3.556 A. Crystal data for C(70)H(57)ClF(12)Fe(2)N(8)O(3)P (1.PF(6)): triclinic, Ponemacr;, a = 13.185(3) A, b = 14.590 (3) A, c = 16.885(4) A, alpha = 104.219(4) degrees, beta = 91.572(4) degrees, gamma = 107.907(4) degrees, V = 2977.3(11) A(3), Z = 2, T = 150(2) K. Complex 1 (where X = Cl(-)) is further characterized by UV-vis (lambda(max) = 328, 416 (Soret), 569 nm), (1)H NMR (delta 27-24 [TMPA -CH(2)-], 16.1 [pyrrole-H], 15.2-10.5 [PY-3H, PY-5H], 7.9-7.2 [m- and p-phenyl-H], 6.9-5.8 [PY-4H] ppm), resonance Raman (nu(as)(Fe-O-Fe') 844 cm(-)(1)), and M?ssbauer (delta(Fe) = 0.47, 0.41 mm/s; deltaE(A) = 1.59, 0.55 mm/s; 80 K) spectroscopies, MALDI-TOF mass spectrometry (m/z 1202), and SQUID susceptometry (J = - 114.82 cm(-)(1), S = 0). We have also synthesized a series of 3-, 4-, and 5-methyl-substituted as well as selectively deuterated TMPA(Fe') complexes and condensed these with the hydroxo complex (F(8))FeOH or (F(8)-d(8))FeOH to yield "untethered" Fe-O-Fe' analogues. Along with selective deuteration of the methylene hydrogens in TMPA, complete (1)H NMR spectroscopic assignments for 1 have been accomplished. The magnetic properties of several of the untethered complexes and a comparison to those of 1 are also presented. Complex 1 and related species represent good structural and spectroscopic models for the heme/non-heme diiron active site in the enzyme nitric oxide reductase.     

Heme/non-heme diiron(II) complexes and O2, CO, and NO adducts as reduced and substrate-bound models for the active site of bacterial nitric oxide reductase

As a first generation model for the reactive reduced active-site form of bacterial nitric oxide reductase, a heme/non-heme diiron(II) complex [(6L)Fe(II)...Fe(II)-(Cl)]+ (2) {where 6L = partially fluorinated tetraphenylporphyrin with a tethered tetradentate TMPA chelate; TMPA = tris(2-pyridyl)amine} was generated by reduction of the corresponding mu-oxo diferric compound [(6L)Fe(III)-O-Fe(III)-Cl]+ (1). Coordination chemistry models for reactions of reduced NOR with O2, CO, and NO were also developed. With O2 and CO, adducts are formed, [(6L)Fe(III)(O2-))(thf)...Fe(II)-Cl]B(C6F5)4 (2a x O2) {lambda(max) 418 (Soret), 536 nm; nu(O-O) = 1176 cm(-1), nu(Fe-O) = 574 cm(-1) and [(6L)Fe(II)(CO)(thf)Fe(II)-Cl]B(C6F5)4 (2a x CO) {nu(CO) 1969 cm(-1)}, respectively. Reaction of purified nitric oxide with 2 leads to the dinitrosyl complex [(6L)Fe(NO)Fe(NO)-Cl]B(C6F5)4 (2a x (NO)2) with nu(NO) absorptions at 1798 cm(-1) (non-heme Fe-NO) and 1689 cm(-1) (heme-NO).     

Synthesis and structure of electron-deficient trimethylacetatocobalt(II) 3,5-dimethylpyrazole complexes, which are structural analogs of the active site of metalloenzymes

The formation laws of [Co₂(μ-OOCR)₂L₄]²⁺ binuclear complexes, which model the active site of metalloenzymes, are studied.     

Synthesis and characterization of two binuclear iron(III) complexes of aminoethanol derivatives of aminophenol as models for non-heme iron enzymes active sites

Two aminoethanol derivatives of aminophenol ligands were synthesized and characterized by IR and ¹H NMR spectroscopies. The binuclear iron(III) complexes of these ligands have been prepared and characterized by IR, ¹H NMR and UV-Vis spectroscopic techniques, cyclic voltammetry, single crystal X-ray diffraction and magnetic susceptibility studies. X-ray analysis revealed binuclear complexes, Fe₂(L₂), in which Fe(III) centers are surrounded by two phenolate and hydroxyl oxygen atoms, and amine nitrogens of the ligands. The metal active sites of both complexes are held together by the two above mentioned hydroxyl bridges. Variable temperature magnetic susceptibility indicates antiferromagnetic coupling between the iron centers of both complexes. This exchange coupling is stronger for Fe₂(L^ae)₂, such that it shows a room temperature strong coupling between the two iron centers. The investigated complexes undergo irreversible electrochemical oxidation and reduction.     

Active transition metal oxo and hydroxo moieties in nature's redox,enzymes and their synthetic models: Structure and reactivity relationships

The high oxidation state transition metal oxo moieties in redox enzymes and their models are generally recognized to serve as the key active intermediates in a series of hydrogen abstraction, oxygen transfer, and electron transfer processes. New evidence suggests that certain transition metal hydroxo moieties also play key roles in oxidative processes in biological and chemical systems. Clarifying the structure and reactivity similarities and differences between the metal oxo functionality and its corresponding metal hydroxo form will help promote understanding of their complementary roles in oxidation processes and aid in the rational design of selective oxidation catalysts to match different requirements. This review summarizes the structure and reactivity similarities and differences of the reported redox enzymes and their models in which the metal oxo and/or corresponding metal hydroxo moieties have demonstrated their activity in oxidation processes. Those enzymes include heme enzymes, lipoxygenases, sulfite oxidases and xanthine oxidases, because the heme enzymes and lipoxygenases would provide the platform to compare the iron oxo with its corresponding hydroxo species, and the sulfite oxidases and xanthine oxidases provide the platform for molybdenum oxo and hydroxo species.     

Models for the active site in galactose oxidase: Structure, spectra and redox of copper(II) complexes of certain phenolate ligands

Galactose oxidase (GOase) is a fungal enzyme which is unusual among metalloenzymes in appearing to catalyse the two electron oxidation of primary alcohols to aldehydes and H₂O₂. The crystal structure of the enzyme reveals that the coordination geometry of mononuclear copper(II) ion is square pyramidal, with two histidine imidazoles, a tyrosinate, and either H₂O (pH 7.0) or acetate (from buffer,pH 4-5) in the equatorial sites and a tyrosinate ligand weakly bound in the axial position. This paper summarizes the results of our studies on the structure, spectral and redox properties of certain novel models for the active site of the inactive form of GOase. The monophenolato Cu(II) complexes of the type [Cu(L1)X][H(L1) = 2-(bis(pyrid-2-ylmethyl)aminomethyl)-4-nitrophenol and X⁻ = Cl⁻ 1, NCS⁻ 2, CH₃COO⁻ 3, ClO₄ ⁻ 4] reveal a distorted square pyramidal geometry around Cu(II) with an unusual axial coordination of phenolate moiety. The coordination geometry of 3 is reminiscent of the active site of GOase with an axial phenolate and equatorial CH₃COO⁻ ligands. All the present complexes exhibit several electronic and EPR spectral features which are also similar to the enzyme. Further, to establish the structural and spectroscopic consequences of the coordination of two tyrosinates in GOase enzyme, we studied the monomeric copper(II) complexes containing two phenolates and imidazole/pyridine donors as closer structural models for GOase. N,N-dimethylethylenediamine and N,N’-dimethylethylenediamine have been used as starting materials to obtain a variety of 2,4-disubstituted phenolate ligands. The X-ray crystal structures of the complexes [Cu(L5)(py)], (8) [H₂(L5) = N,N-dimethyl-N’,N’-bis(2-hydroxy-4-nitrobenzyl) ethylenediamine, py = pyridine] and [Cu(L8)(H₂O)] (11), [H₂(L8) = N,N’-dimethyl-N,N’-bis(2-hydroxy-4-nitrobenzyl)ethylenediamine] reveal distorted square pyramidal geometries around Cu(II) with the axial tertiary amine nitrogen and water coordination respectively. Interestingly, for the latter complex there are two different molecules present in the same unit cell containing the methyl groups of the ethylenediamine fragmentcis to each other in one molecule andtrans to each other in the other. The ligand field and EPR spectra of the model complexes reveal square-based geometries even in solution. The electrochemical and chemical means of generating novel radical species of the model complexes, analogous to the active form of the enzyme is presently under investigation.     

Electronic property and molecule design for luminescent metal complexes of tris(8-hydroxyquinoline) gallium

By means of ab initio HF and DFT B3LYP methods, the structure of Gaq3 (q = 8-hydroxyquinoline) was optimized. The frontier molecular orbital characteristics and energy levels of Gaq3 have been analyzed systematically in order to study the electronic transition mechanism in Gaq3. Three derivatives of Gaq3 and their polymers were designed and the possibilities that they were employed as luminescent materials were discussed. The regularities and characteristic of energy bands of Gaq3 and its derivatives were also investigated. The results show that the electronic π-π* transitions in Gaq3 are localized on the quinolate ligands. The emission of Gaq3 is due to the electron transitions from a phenoxide donor to a pyridyl acceptor. Two possible electron transfer pathways are presented, one by carbon atoms, and the other via metal cation Ga3 . The derivatives of Gaq3 may possess high luminescence efficiency.     

Cobalt tris(mercaptoimidazolyl)borate complexes: synthetic studies and the structure of the first cobaltaboratrane

The paramagnetic complexes (TmtBu)CoX (X = Cl, Br, I) have been readily prepared and structurally characterized and provide a convenient entry into cobalt(II) tris(mercaptoimidazolyl)borate chemistry. A number of derivatives, including mononuclear triphenylphosphine adducts [(TmtBu)Co(PPh3)]X and dinuclear compounds [Co2(TmtBu)2X]Y, have been prepared in order to ascertain whether cobalt is a reliable surrogate for zinc in biological systems, particularly in sulfur-rich coordination environments. The structure of the first cobaltaboratrane is also reported.     

A spectroscopic study of some substituted tris(diimine) complexes of ruthenium(II) and their reduction products

The electronic absorption and resonance Raman spectra of the parent and one- to six-electron reduction products of tris(5,5′-bis(ethoxycarbonyl)-2,2′-bipyridine)ruthenium(II) and tris(5,5′-bis(phenyl)- 2,2′-bipyridine)ruthenium(II) indicate that the redox orbitals are single-ring localized throughout the reduction series. The analogous 4,4′-complexes exhibit extensive shifts of both electronic absorption bands and vibrational bands as electrons are added. The shifts are rationalized within the localized redox orbital model. The occurrence of backbonding between the metal and unreduced ligands can successfully account for the observed shifts in the vibrational spectra, while electrostatic interaction between redox orbitals is consistent with the observed shifts in the electronic spectra.     

Synthesis and structural characterization of cross-linked histidine-phenol Cu(ii) complexes as cytochrome c oxidase active site models

Tridentate cross-linked histidine-phenol Cu(ii) ether and ester complexes, chemical analogs of the active site of several heme-copper oxidases, have been synthesized and crystallized.     

Structural and spectroscopic characterization of (mu-hydroxo or mu-oxo)(mu-peroxo)diiron(III) complexes: models for peroxo intermediates of non-heme diiron proteins

(mu-Hydroxo or oxo)(mu-1,2-peroxo)diiron(III) complexes having a tetradentate tripodal ligand (L) containing a carboxylate sidearm [Fe2(mu-OH or mu-O)(mu-O2)(L)2]n+ were synthesized as models for peroxo-intermediates of non-heme diiron proteins and characterized by various physicochemical measurements including X-ray analysis, which provide fundamental structural and spectroscopic insights into the peroxodiiron(III) complexes.     

Iron(III) complexes of sterically hindered tetradentate monophenolate ligands as functional models for catechol 1,2-dioxygenases: the role of ligand stereoelectronic properties

The iron(III) complexes of the monophenolate ligands 2-(bis(pyrid-2-ylmethyl)aminomethyl)-4-nitrophenol [H(L1)], N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxy-4-nitrobenzyl)ethylenediamine [H(L2)], N,N-dimethyl-N'-(6-methyl-pyrid-2-ylmethyl)-N'-(2-hydroxy-4-nitrobenzyl)ethylenediamine [H(L3)], and N,N-dimethyl-N'-(1-methylimidazole-2-ylmethyl)-N'-(2-hydroxy-4-nitrobenzyl)ethylenediamine [H(L4)] have been obtained and studied as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The complexes [Fe(L1)Cl(2)].CH(3)CN (1), [Fe(L2)Cl(2)] (2), [Fe(L3)Cl(2)] (3), and [Fe(L4)Cl(2)] (4) have been characterized using absorption spectral and electrochemical methods. The single crystal X-ray crystal structures of 1 and 2 have been successfully determined. Both the complexes possess a rhombically distorted octahedral coordination geometry for the FeN(3)OCl(2) chromophore. In 2, the phenolate oxygen, the pyridine nitrogen, an amine nitrogen, and a chloride ion are located on the corners of a square plane with the nitrogen atom of a -NMe(2) group and the other chloride ion occupying the axial positions. In 1, also the equatorial plane is constituted by the phenolate oxygen, the pyridine nitrogen, an amine nitrogen atom, and a chloride ion; however, the axial positions are occupied by the second pyridine nitrogen and the second chloride ion. Interestingly, the Fe-O-C angle of 136.1 degrees observed for 2 is higher than that (128.5 degrees ) in 1; however, the Fe-O(phenolate) bond distances in both the complexes are the same (1.929 A). This illustrates the importance of the nearby sterically demanding coordinated -NMe(2) group and implies similar stereochemical constraints from the other ligated amino acid moieties in the 3,4-PCD enzymes, the enzyme activity of which is traced to the difference in the equatorial and axial Fe-O(tyrosinate) bonds (Fe-O-C, 133 degrees, 148 degrees ). The nature of heterocyclic rings of the ligands and the methyl substituents on them regulates the electronic spectral features, Fe(III)/Fe(II) redox potentials, and catechol cleavage activity of the complexes. Upon interacting the complexes with catecholate anions, two catecholate to iron(III) charge transfer bands appear, and the low energy band is similar to that of catechol dioxygenase-substrate complex. Complexes 1 and 3 fail to catalyze the oxidative intradiol cleavage of 3,5-di-tert-butylcatechol (H(2)DBC). However, interestingly, the replacement of pyridine pendant in 1 by the -NMe(2) group to obtain 2 restores the dioxygenase activity, which is consistent with its higher Fe-O-C bond angle. Remarkably, the more basic N-methylimidazole ring in 4 facilitates the rate-determining product releasing phase of the catalytic reaction, leading to enhancement in reaction rate and efficient conversion (77.1%) of the substrate to intradiol cleavage products as well. All these observations provide support to the novel substrate activation mechanism proposed for the intradiol-cleavage pathway.     

Different synthetic pathways of nanoparticle‐cored dendrimers (NCDs): Effects on the properties and their application as redox active centers

The synthesis of nanoparticle‐cored dendrimers (NCDs) through surface functionalization of iron oxide nanoparticles (γ‐Fe₂O₃) by combining the conventional silane coupling agent 3‐(aminopropyl) triethoxysilane (APS) with dendritic moieties is studied and presented. Much emphasis has been put on the role played by each modifier and how they interact not only between themselves, but also with the dispersing media wherein the resulting NCDs are. As a part of the functionalization, redox‐active nitro groups were introduced onto the surface of each synthesized NCD thus making them electrochemically active. Then, the obtained NCDs were immobilized onto glassy carbon electrodes. Both the NCDs and modified electrodes are expected to be eventually exploited in analytical and sensing applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3185–3197     

The co-ordination chemistry of tris(3,5-dimethylpyrazolyl)methane manganese carbonyl complexes: synthetic, electrochemical and DFT studies

The tricarbonyl [Mn(CO)(3){HC(pz')(3)}][PF(6)] 1(+)[PF(6)](-) (pz' = 3,5-dimethylpyrazolyl) reacts with a range of P-, N- and C-donor ligands, L, in the presence of trimethylamine oxide to give [Mn(CO)(2)L{HC(pz')(3)}](+) {L = PEt(3)3(+), P(OEt)(3)4(+), P(OCH(2))(3)CEt 5(+), py 6(+), MeCN 7(+), CNBu(t)8(+) and CNXyl 9(+)}. The complex [Mn(CO)(2)(PMe(3)){HC(pz')(3)}](+)2(+) is formed by reaction of 7(+) with PMe(3). Complexes 2(+) and 6(+) were structurally characterised by X-ray diffraction methods. Reaction of 7(+) with half a molar equivalent of 4,4'-bipyridine gives a purple compound assumed to be the bridged dimer [{HC(pz')(3)}Mn(CO)(2)(μ-4,4'-bipy)Mn(CO)(2){HC(pz')(3)}](2+)10(2+). The relative electron donating ability of HC(pz')(3) has been established by comparison with the cyclopentadienyl and tris(pyrazolyl)borate analogues. Cyclic voltammetry shows that each of the complexes undergoes an irreversible oxidation. The correlation between the average carbonyl stretching frequency and the oxidation potential for complexes of P- and C-donor ligands is coincident with the correlation observed for [Mn(CO)(3-m)L(m)(η-C(5)H(5-n)Me(n))]. The data for complexes of N-donor ligands, however, are not coincident due to the presence of a node (and phase change) between the metal and the N-donor in the HOMO of the complex as suggested by preliminary DFT calculations.     

Structural studies of the [tris(imidazolyl)phosphine]metal nitrate complexes [[PimPrl,But]M(NO3)]+ (M = Co,Cu, Zn,Cd, Hg): comparison of nitrate-binding modes in synthetic analogues of carbonic anhydrase

X-ray diffraction studies on a series of cationic divalent metal nitrate complexes supported by the tris(1-isopropyl-4-tert-butylimidazolyl)phosphine ligand, [[PimPri,But]M(NO3)]+ (M = Co, Cu, Zn, Cd, Hg), demonstrate that the nitrate ligand coordination mode is strongly dependent upon the metal. With the exception of that for the HgII derivative, the nitrate ligand coordination modes correlate with the activities of metal-substituted carbonic anhydrases, such that the only MII-carbonic anhydrases which exhibit significant activity, i.e., the Zn and Co species, are those for which the [[PimPri,But]M(NO3)]+ complexes possess strongly asymmetric nitrate ligands. This trend supports the notion that access to a unidentate, rather than a bidentate, bicarbonate intermediate may be a critical requirement for significant carbonic anhydrase activity. Interestingly, the nitrate coordination modes in the series of group 12 complexes, [[PimPri,But]M(NO3)]+ (M = Zn, Cd, Hg), do not exhibit a monotonic periodic trend: the bidenticity is greater for the cadmium complex than for either the zinc or mercury complexes. Since HgII-carbonic anhydrase is inactive, the correlation between nitrate coordination mode and enzyme activity is anomalous for the mercury complex. Therefore, it is suggested that the inactivity of HgII-carbonic anhydrase may be consequence of the reduced tendency of the mercury center in HgII-carbonic anhydrase to bind water.     

Monometallic,homo- and hetero-bimetallic complexes based on redox active tris(3,5-dimethylpyrazolyl)boratomolybdenum and tungsten nitrosyls. Part V.Para-substituent effects on the reduction potentials of arylamide complexes

Summary Two series of complexes having the formula [M{HB(3,5-Me₂C₃N₂H)₃}(NO)Cl(NHC₆H₄Z-p)]; in which M=Mo and Z=F, Cl, Br, OMe, SMe, CN, CO₂Me or NO₂ and M=W and Z=Br, OMe, CN, CO₂Me or NO₂, have been prepared. The reduction potentials of these new complexes were measured by cyclic voltammetry and, in combination with previously reported data for related species and for [Mo{HB(3,5-Me₂C₃N₂H)₃} (NO)I-(NHC₆H₄Z-p], were used to determine reaction constants for the reduction of [M{HB(3,5-Me₂C₃N₂H)₃}(NO)X(NHC₆H₄Z)]M=Mo and X=I or Cl; M=W, X=Cl.Part IV. N. Al Obaidi, M. Chaudhury, D. Clague, C. J. Jones, J. C. Pearson, J. A. McCleverty and S. S. Salam, submitted toJ. Chem. Soc., Dalton Trans., Paper 6/869.     

Studies of artificial hydrolytic metalloenzymes: The catalyzed carboxyester hydrolysis by copper(II), zinc(II) and cobalt(II) complexes of the tripod ligand tris(2-benzimidazylmethyl)amine

The hydrolysis kinetics of p-nitrophenyl acetate (NA) catalyzed by Cu^II, Zn^II and Co^II complexes of tris(2-benzimidazylmethyl)amine (NBT) have been studied. The hydrolysis rate is first-order in both metal(II) complex and NA. The second-order rate constants, k_cat are 0.083, 0.241 and 0.285mol^–1Ls^–1 (298K, I = 0.10molL^–1 KNO₃, 0.02molL^–1 tris buffer, 40% MeCN aqueous solution) for Zn–NBT, Co–NBT and Cu–NBT complexes, respectively. The result indicates that the hydrolytic metalloenzyme activity of different metal complexes increases with the electrophilicity of the metal ions and that the complexes, in this paper, constitute that most efficient hydrolytic metalloenzyme models reported to date. An increase in MeCN content in the solution greatly reduces the hydrolytic activity of the nucleophiles.