Modeling dioxygen-activating centers in non-heme diiron enzymes: carboxylate shifts in diiron(II) complexes supported by sterically hindered carboxylate ligands

General synthetic routes are described for a series of diiron(II) complexes supported by sterically demanding carboxylate ligands 2,6-di(p-tolyl)benzoate (Ar(Tol)CO(2)(-)) and 2,6-di(4-fluorophenyl)benzoate (Ar(4-FPh)CO(2)(-)). The interlocking nature of the m-terphenyl units in self-assembled [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (L = C(5)H(5)N (4); 1-MeIm (5)) promotes the formation of coordination geometries analogous to those of the non-heme diiron cores in the enzymes RNR-R2 and Delta 9D. Magnetic susceptibility and M?ssbauer studies of 4 and 5 revealed properties consistent with weak antiferromagnetic coupling between the high-spin iron(II) centers. Structural studies of several derivatives obtained by ligand substitution reactions demonstrated that the [Fe(2)(O(2)CAr')(4)L(2)] (Ar' = Ar(Tol); Ar(4-FPh)) module is geometrically flexible. Details of ligand migration within the tetracarboxylate diiron core, facilitated by carboxylate shifts, were probed by solution variable-temperature (19)F NMR spectroscopic studies of [Fe(2)(mu-O(2)CAr(4-FPh))(2)-(O(2)CAr(4-FPh))(2)(THF)(2)] (8) and [Fe(2)(mu-O(2)CAr(4-FPh))(4)(4-(t)BuC(5)H(4)N)(2)] (12). Dynamic motion in the primary coordination sphere controls the positioning of open sites and regulates the access of exogenous ligands, processes that also occur in non-heme diiron enzymes during catalysis.     

Triarylboron-functionalized Cu(II) carboxylate paddlewheel complexes

The assembly of two copper(II)-carboxylate dimer complexes appended with four peripheral triarylborane functionalities has been achieved. Complex stabilities in the presence of fluoride are examined.     

Role of carboxylate bridges in modulating nonheme diiron(II)/O(2) reactivity

A series of diiron(II) complexes of the dinucleating ligand HPTP (N,N,N',N'-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-diaminopropane) with one or two supporting carboxylate bridges has been synthesized and characterized. The crystal structure of one member of each subset has been obtained to reveal for subset A a (micro-alkoxo)(micro-carboxylato)diiron(II) center with one five- and one six-coordinate metal ion and for subset B a coordinatively saturated (micro-alkoxo)bis(micro-carboxylato)diiron(II) center. These complexes react with O(2) in second-order processes to form adducts characterized as (micro-1,2-peroxo)diiron(III) complexes. Stopped-flow kinetic studies show that the oxygenation step is sensitive to the availability of an O(2) binding site on the diiron(II) center, as subset B reacts more slowly by an order of magnitude. The lifetimes of the O(2) adducts are also distinct and can be modulated by the addition of oxygen donor ligands. The O(2) adduct of a monocarboxylate complex decays by a fast second-order process that must be monitored by stopped-flow methods, but becomes stabilized in CH(2)Cl(2)/DMSO (9:1 v/v) and decomposes by a much slower first-order process. The O(2) adduct of a dicarboxylate complex is even more stable in pure CH(2)Cl(2) and decays by a first-order process. These differences in adduct stability are reflected in the observation that only the O(2) adducts of monocarboxylate complexes can oxidize substrates, and only those substrates that can bind to the diiron center. Thus, the much greater stability of the O(2) adducts of dicarboxylate complexes can be rationalized by the formation of a (micro-alkoxo)(micro-1,2-peroxo)diiron(III) complex wherein the carboxylate bridges in the diiron(II) complex become terminal ligands in the O(2) adduct, occupy the remaining coordination sites on the diiron center, and prevent binding of potential substrates. Implications for the oxidation mechanisms of nonheme diiron enzymes are discussed.     

Oxygen activation by nonheme iron(II) complexes: alpha-keto carboxylate versus carboxylate

Mononuclear iron(II) alpha-keto carboxylate and carboxylate compounds of the sterically hindered tridentate face-capping ligand Tp(Ph2) (Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate) were prepared as models for the active sites of nonheme iron oxygenases. The structures of an aliphatic alpha-keto carboxylate complex, [Fe(II)(Tp(Ph2))(O(2)CC(O)CH(3))], and the carboxylate complexes [Fe(II)(Tp(Ph2))(OBz)] and [Fe(II)(Tp(Ph2))(OAc)(3,5-Ph(2)pzH)] were determined by single-crystal X-ray diffraction, all of which have five-coordinate iron centers. Both the alpha-keto carboxylate and the carboxylate compounds react with dioxygen resulting in the hydroxylation of a single ortho phenyl position of the Tp(Ph2) ligand. The oxygenation products were characterized spectroscopically, and the structure of the octahedral iron(III) phenolate product [Fe(III)(Tp(Ph2))(OAc)(3,5-Ph(2)pzH)] was established by X-ray diffraction. The reaction of the alpha-keto carboxylate model compounds with oxygen to produce the phenolate product occurs with concomitant oxidative decarboxylation of the alpha-keto acid. Isotope labeling studies show that (18)O(2) ends up in the Tp(Ph2) phenolate oxygen and the carboxylate derived from the alpha-keto acid. The isotope incorporation mirrors the dioxygenase nature of the enzymatic systems. Parallel studies on the carboxylate complexes demonstrate that the oxygen in the hydroxylated ligand is also derived from molecular oxygen. The oxygenation of the benzoylformate complex is demonstrated to be first order in metal complex and dioxygen, with activation parameters DeltaH++ = 25 +/- 2 kJ mol(-1) and DeltaS++ = -179 +/- 6 J mol(-1) K(-1). The rate of appearance of the iron(III) phenolate product is sensitive to the nature of the substituent on the benzoylformate ligand, exhibiting a Hammett rho value of +1.3 indicative of a nucleophilic mechanism. The proposed reaction mechanism involves dioxygen binding to produce an iron(III) superoxide species, nucleophilic attack of the superoxide at the alpha-keto functionality, and oxidative decarboxylation of the adduct to afford the oxidizing species that attacks the Tp(Ph2) phenyl ring. Interestingly, the alpha-keto carboxylate complexes react 2 orders of magnitude faster than the carboxylate complexes, thus emphasizing the key role that the alpha-keto functionality plays in oxygen activation by alpha-keto acid-dependent iron enzymes.     

Nitric oxide reactivity of fluorophore coordinated carboxylate-bridged diiron(II) and dicobalt(II) complexes

The synthesis, structural characterization, and NO reactivity of carboxylate-bridged dimetallic complexes were investigated. The diiron(II) complex [Fe(2)(mu-O(2)CAr(Tol))(4)(Ds-pip)(2)] (1), where O(2)CAr(Tol) = 2,6-di(p-tolyl)benzoate and Ds-pip = dansyl-piperazine, was prepared and determined by X-ray crystallography to have a paddlewheel geometry. This complex reacts with NO within 1 min with a concomitant 4-fold increase in fluorescence emission intensity ascribed to displacement of Ds-pip. Although the diiron complex reacts with NO, as revealed by infrared spectroscopic studies, its sensitivity to dioxygen renders it unsuitable as an atmospheric NO sensor. The air-stable dicobalt(II) analogue was also synthesized and its reactivity investigated. In solution, the dicobalt(II) complex exists as an equilibrium between paddlewheel [Co(2)(mu-O(2)CAr(Tol))(4)(Ds-pip)(2)] (2) and windmill [Co(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(Ds-pip)(2)] (3) geometric isomers. Conditions for crystallizing pure samples of each of these isomers are described. Reaction of 2 with excess NO proceeds by reductive nitrosylation giving [Co(mu-O(2)CAr(Tol))(2)(NO)(4)] (5), which is accompanied by release of the Ds-pip fluorophore that is N-nitrosated in the process. This reaction affords an overall 9.6-fold increase in fluorescence emission intensity, further demonstrating the potential utility of ligand dissociation as a strategy for designing fluorescence-based sensors to detect nitric oxide in a variety of contexts.     

Visualising the carboxylate shift at a bioinspired diiron(II) site in the solid state

A novel pyrazolate-based diiron(II) complex shows five different binding modes of exogenous carboxylate ligands in a single crystal structure. Temperature dependent X-ray data reveal thermally induced disorder due to carboxylate dynamics that resemble the carboxylate shift, as it is known from various diiron enzyme active sites.     

Functional mimic of dioxygen-activating centers in non-heme diiron enzymes: mechanistic implications of paramagnetic intermediates in the reactions between diiron(II) complexes and dioxygen

Two tetracarboxylate diiron(II) complexes, [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(C(5)H(5)N)(2)] (1a) and [Fe(2)(mu-O(2)CAr(Tol))(4)(4-(t)BuC(5)H(4)N)(2)] (2a), where Ar(Tol)CO(2)(-) = 2,6-di(p-tolyl)benzoate, react with O(2) in CH(2)Cl(2) at -78 degrees C to afford dark green intermediates 1b (lambda(max) congruent with 660 nm; epsilon = 1600 M(-1) cm(-1)) and 2b (lambda(max) congruent with 670 nm; epsilon = 1700 M(-1) cm(-1)), respectively. Upon warming to room temperature, the solutions turn yellow, ultimately converting to isolable diiron(III) compounds [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (L = C(5)H(5)N (1c), 4-(t)BuC(5)H(4)N (2c)). EPR and M?ssbauer spectroscopic studies revealed the presence of equimolar amounts of valence-delocalized Fe(II)Fe(III) and valence-trapped Fe(III)Fe(IV) species as major components of solution 2b. The spectroscopic and reactivity properties of the Fe(III)Fe(IV) species are similar to those of the intermediate X in the RNR-R2 catalytic cycle. EPR kinetic studies revealed that the processes leading to the formation of these two distinctive paramagnetic components are coupled to one another. A mechanism for this reaction is proposed and compared with those of other synthetic and biological systems, in which electron transfer occurs from a low-valent starting material to putative high-valent dioxygen adduct(s).     

Substitution reactions of thorium(IV) acetate to synthesize nano-sized carboxylate complexes

Some mixed-ligand thorium(IV) complexes with the general formula [Th(OOCCH₃)_4?nL_n] (L = anions of myristic, palmitic or stearic acid and n = 1–4) have been synthesized by the stepwise substitution of acetate ions of thorium(IV) acetate with straight chain carboxylic acids in toluene under reflux. The complexes were characterized by elemental analyses, spectral (electronic, infrared, NMR and powder XRD) studies, electrical conductance and magnetic susceptibility measurements. Doubly and triply bridged coordination modes of the ligands were established by their infrared spectra and nano-size of the complexes by powder XRD. Room temperature magnetic susceptibility measurements revealed diamagnetic nature of the complexes. Electronic absorption spectra of the complexes showed π → π*, n → π* and charge transfer transitions. Molar conductance values indicated the complex to be non-electrolytes. These are a new type of mixed-ligand thorium(IV) complexes for which a nano-sized, oxygen bridged polymeric structure has been established on the basis of physico-chemical studies.     

A sulfido-bridged diiron(II) compound and its reactions with nitrogenase-relevant substrates

The active site iron-molybdenum cofactor of nitrogenase has sulfide-bridged pairs of redox-active, trigonal pyramidal iron atoms that are postulated to be the site of N2 transformation. A synthetic compound is described in which two three-coordinate iron(II) ions are bridged similarly by sulfide. The compound binds nitrogen donors to become trigonal pyramidal and cleaves the N-N bond of phenylhydrazine with oxidation of iron(II) to iron(III).     

Reversible (de)protonation-induced valence inversion in mixed-valent diiron(II,III) complexes

The coupling of electron and proton transfers is currently under intense scrutiny. This Communication reports a new kind of proton-coupled electron transfer within a homodinuclear first-row transition-metal complex. The triply-bridged complex [Fe(III)(μ-OPh)(μ(2)-mpdp)Fe(II)(NH(2)Bn)] (1; mpdp(2-) = m-phenylenedipropionate) bearing a terminal aminobenzyl ligand can be reversibly deprotonated to the anilinate complex 2 whose core [Fe(II)(μ-OPh)(μ(2)-mpdp)Fe(III)(NHBn)] features an inversion of the iron valences. This observation is supported by a combination of UV-visible, (1)H NMR, and M?ssbauer spectroscopic studies.     

Latent cationic palladium(II) phosphine carboxylate complexes for norbornene polymerization

The catalytic efficacy of trans‐[(R₃P)₂Pd(O₂CR′)(LB)][B(C₆F₅)₄] ( 1 ) (LB = Lewis base) and [(R₃P)₂Pd(κ²‐O,O‐O₂CR′)][B(C₆F₅)₄] ( 2 ) for mass polymerization of 5‐n‐butyl‐2‐norbornene (Butyl‐NB) was investigated. The nature of PR₃ and LB in 1 and 2 are the most critical components influencing catalytic activity/latency for the mass polymerization of Butyl‐NB. Further, it was shown that 1 is in general more latent than 2 in mass polymerization of Butyl‐NB. 5‐n‐Decyl‐2‐norbornene (Decyl‐NB) was subjected to solution polymerization in toluene at 63(±3) °C in the presence of several of the aforementioned palladium complexes as catalysts and the polymers obtained were characterized by gel permeation chromatography. Cationic trans‐[(R₃P)₂PdMe(MeCN)][B(C₆F₅)₄] [R = Cy ( 3a ), and ⁱPr ( 3b )] and trans‐[(R₃P)₂PdH (MeCN)][B(C₆F₅)₄] [R = Cy ( 4a ), and ⁱPr ( 4b )], possible products from thermolysis of trans‐[(R₃P)₂Pd(O₂CMe)(MeCN)][B(C₆F₅)₄] [R = Cy ( 1a ) and ⁱPr ( 1g )], as well as trans‐[(R₃P)₂Pd(η³‐C₃H₅)][B(C₆F₅)₄] [R = Cy ( 5a ), and ⁱPr ( 5b )], were also examined as catalysts for solution polymerization of Decyl‐NB. A maximum activity of 5360 kg/(molPd h) of 2a was achieved at a Decyl‐NB/Pd: 26,700 ratio which is slightly better than that achieved with 5a [activity: 5030 kg/(molPd h)] but far less compared with 4a [activity: 6110 kg/(molPd h)]. Polydispersity values indicate a single highly homogeneous character of the active catalyst species. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 103–110, 2009     

Heterogeneous reactions of solid nickel(II) complexes

The influence of different modes of preparation on the stoichiometry of thermal decomposition of isothiocyanatonickel(II) complexes with ammonia was studied. It was found that the complex Ni(NCS)₂(NH₃)₄ (I) prepared by heterogeneous reaction undergoes decomposition in two steps (–2 NH₃, –2 NH₃), while for complex II, of the same composition but prepared by homogeneous reaction from solution, the decomposition proceeds in three steps (–1 NH₃, –1 NH₃, –2NH₃). Electronic and infrared absorption spectra were used for the study of the spectral properties of the starting complex. It was found that the different stoichiometries of thermal decompositions of complexes I and II do not cause differences in the bonding strength of the ammonia molecules (chemical factor); this effect is rather brought about by physical factors such as different imperfections of the crystal lattice.

---

Zusammenfassung Der Einfluß der Herstellung auf die Stöchiometrie der thermischen Zersetzung von Amin-Komplexen des Isothiocyanatonickels wurde studiert. Der Zerfall des Komplexes Ni(NCS)₂(NH₃)₄ (I), hergestellt durch heterogene Reaktion, verlief in zwei Etappen (–2 NH₃, –2 NH₃). Beim selben Komplex (II), hergestellt durch homogene Reaktion in Lösung, wurden jedoch drei Etappen beobachtet (–1 NH₃, –1 NH₃, –2 NH₃). Die Eigenschaften des Ausgangsproduktes wurden durch Elektronen- und IR-Spektroskopie untersucht. Das unterschiedliche thermische Verhalten ist nicht auf eine verschiedene Bindungsstärke der NH₃ Moleküle, sondern auf physikalische Ursachen, wie z. B. Unregelmässigkeiten im Kristallgitter zurückzuführen.

---

Résumé On a étudié l'influence des diverses méthodes de préparation sur la stoechiométrie de la réaction de décomposition thermique des complexes de l'isothiocyanate de nickel(II) avec l'ammoniac. On a établi que le complexe Ni(SCN)₂(NH₃)₄, (I), préparé rar réaction hétérogène, subit une décomposition en deux étapes (–2NH₃, –2NH₃), tandis que le complexe (II), de même composition, mais préparé par réaction homogène, à partir de solutions, se décompose en trois étapes (–1NH₃,–1NH₃, –2NH₃). Les propriétés spectrales du complexe initial ont été étudiées par absorption électronique et infrarouge. On a trouvé que les différences de stoechiométrie des réactions de décomposition thermique des complexes (I) et (II) n'apportaient pas de différences entre les énergies de liaison des molécules d'ammoniac (facteur chimique). C'est plutôt à des facteurs physiques, comme les différentes imperfections du réseau cristallin, que cet effet serait dû.

---

(II) . , Ni(NCS)₂(NH₃)₄ (I), , (–2NH₃,–2NH₃). , II , , (–1 NH₃,–1 NH₃ –2NH₃). . , I II ( ), , .

     

Decomposition reactions of square-planar bis-aryl-cobalt(II) complexes

The organocobalt complexes [CoR₂L₂], with (a) L = PEtPh₂ and R = 2,3,5,6- C₆HCl₄, 2,4,6-C₆H₂Cl₃ and 2,6-C₆H₃Cl₂; and (b) R = 2,4,6-C₆H₂Cl₃ and L = PEt₃, PEt₂Ph, $\text{1}\text{2}$ dpe, 3,5-lut and $\text{1}\text{2}$ bipy, have been obtained by reaction of RMgX with [CoCl₂L₂] or by ligand exchange from [CoR₂(PEtPh₂)₂]. The decompositions in benzene and carbon tetrachloride, and under oxidative conditions have been studied. In benzene solutions, the stability decreases with decrease in the number of chlorine atoms in R. A mixture of RH and RR is obtained in a ratio which depends on the nature of L, the configuration of the complex, and the presence of oxidants. The thermal decomposition takes place through a tricoordinate intermediate “CoR₂L”, when L = phosphine, or directly from [CoR₂L₂] when L = amine. The oxidatively induced decomposition takes place through a cobalt(III) intermediate, which gives RR when L = phosphine or RX (X = H, Br) when L = amine. The process is intramolecular in all cases.     

Cobalt(II) and nickel(II) complexes of isoquinoline‐1‐carboxylate

trans‐Di­aqua­bis­(iso­quinoline‐1‐carboxyl­ato‐κ²N,O)­cobalt(II) dihydrate, [Co(C₁₀H₆NO₂)₂(H₂O)₂]·2H₂O, and trans‐di­aqua­bis­(iso­quinoline‐1‐carboxyl­ato‐κ²N,O)­nickel(II) dihydrate, [Ni(C₁₀H₆NO₂)₂(H₂O)₂]·2H₂O, contain the same isoquinoline ligand, with both metal atoms residing on a centre of symmetry and having the same distorted octahedral coordination. In the former complex, the Co—O(water) bond length in the axial direction is 2.167 (2) Å, which is longer than the Co—O(carboxylate) and Co—N bond lengths in the equatorial plane [2.055 (2) and 2.096 (2) Å, respectively]. In the latter complex, the corresponding bond lengths for Ni—O(water), Ni—O(carboxylate) and Ni—N are 2.127 (2), 2.036 (2) and 2.039 (3) Å, respectively. Both crystals are stabilized by similar stacking interactions of the ligand, and also by hydrogen bonds between the hydrate and coordinated water molecules.     

Three new manganese(II) coordination complexes based on tetrazole carboxylate ligands

Reactions of three tetrazole carboxylate ligands, namely 5-(4-pyridyl)tetrazole-2-acetic acid (Hpytza), 1,3,5-tris(tetrazol-5-yl)benzene-N2,N2′,N2″-triacetic acid (H₃tzpha) and 5-aminotetrazole-1-propanoic acid (Hatzpa) with Mn(NO₃)₂·6H₂O in the presence of KOH afforded three new complexes, [Mn(pytza)₂] (1), [Mn₃(tzpha)₂(H₂O)₁₂]·2CH₃OH·10H₂O (2) and [Mn(atzpa)₂(H₂O)₂] (3), respectively. These complexes were characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. Complex 1 displays a three-dimensional network while 2 and 3 show one-dimensional chains. Furthermore, the luminescence properties of these complexes were investigated at room temperature in the solid state.     

Synthesis and characterization of Cu(II) paddlewheel complexes possessing fluorinated carboxylate ligands

The goal of this study was to establish the relationship between the ¹⁹F NMR line broadening and the varying distance between the ¹⁹F nucleus and copper(II) ion, with the aim of gathering data that can be used to interpret ¹⁹F NMR spectra of subsequent fluorine-labeled, copper-binding proteins. Fluorinated alkyl and aryl copper(II) carboxylates were synthesized from fluorinated carboxylic acids and Cu(OH)₂. The copper(II) carboxylates were characterized using ¹⁹F NMR, IR, and single crystal X-ray diffraction. In the alkyl carboxylate compounds, the line broadening and chemical shift lessened with increased distance between the fluorine atom and the copper ions; however, in the aryl carboxylate derivatives, increased distance was not a factor in the amount of line broadening or change in chemical shift between the acid and metal salt. The compound, bis(3-(trifluoromethyl)butyrate) copper(II) (5) was found to possess the optimum combination of decreased line broadening and increased chemical shift sensitivity in ¹⁹F NMR. The crystal structures obtained for compounds 1, 2, 4, and 6 were analogous to previous copper(II) carboxylate complexes, though it is noted that compound 6, bis(5,5,5-trifluoropentanoate) copper(II) assumes a tetrameric structure lacking apical ligands, and thus enables the formation of an extended network of near-neighbor copper(II) ions.     

Thermal behaviour of ZINC(II) carboxylate complexes with methyl-3-pyridyl carbamate

Four new complex compounds were prepared by reaction of zinc carboxylate and methyl-3-pyridyl carbamate. The synthesized complex compounds of the general formula (RCOO)₂ZnL₂ (RCOO^-= HCOO^- (form), CH₃COO^- (ac), CH₃CH₂CH₂COO^- (but), (CH₃)₂CHCOO^- (isobut), L= methyl-3-pyridyl carbamate (mpc)) were characterized by chemical analysis, IR spectroscopy and studied by methods of thermal analysis (TG/DTG, DTA). CH₂O, CO₂, (CH₃)₂CO, (C₃H₇)₂CO were found as volatile products of thermal decomposition. ZnO was found as final product of thermal decomposition of the prepared complexes heated up to 700°C. Mass spectroscopy, X-ray powder diffraction method, IR spectra and chemical analysis were used for the determination of the thermal decomposition products. This revised version was published online in July 2006 with corrections to the Cover Date.