Non-coordinated Oxygen Research Articles

Computational and crystallographic study of hydrogen bonds in the second coordination sphere of chelated amino acids with a free water molecule: Influence of complex charge and metal ion

Hydrogen bonds of glycine complexes were calculated using quantum chemistry calculations at M06L-GD3/def2-TZVPP level and by analyzing the crystal structures from the Cambridge Structural Database (CSD). One hydrogen bond where amino acid plays the role of the H-donor (NH/O), and two where it plays the role of the H-acceptor (O1/HO, O1 is a coordinated oxygen atom, and, O2/HO, O2 is a non-coordinated oxygen atom) were investigated. The calculations were done on octahedral nickel(II), square pyramidal copper(II), square planar copper(II), palladium(II), and platinum(II) glycine complexes with different charges adjusted using water(s) and/or chlorine ion(s) as the remaining ligands. For NH/O hydrogen bond, interaction energies of neutral complexes are the weakest, from −5.2 to −7.2 kcal/mol for neutral, stronger for singly positive, from −8.3 to −12.1 kcal/mol, and the strongest for doubly positive complex, −16.9 kcal/mol. For O1/HO and O2/HO interactions, neutral complexes have weaker interaction energies (from −2.2 to −5.1 kcal/mol for O1/HO, and from −3.7 to −5.0 kcal/mol for O2/HO), than for singly negative complexes (from −6.9 to −8.2 kcal/mol for O1/HO, and from −8.0 to −9.0 kcal/mol for O2/HO). Additionally to the complex charge, metal oxidation number, coordination number, and metal atomic number also influence the hydrogen bond strength, however, the influence is smaller.

Synthesis, crystal structure, Hirshfeld surface, and DFT studies of a Copper(II) complex of 5,5′-dimethyl-2,2′-bipyridine and 1,2,2-trimethylcyclopentane-1,3-dicarboxylic acid

A new metal-organic hybrid complex [Cu(5,5′-dmbipy) (D-cam) (H2O)]n (1), (5,5′-dmbipy = 5,5′-dimethyl-2,2′-bipyridine, D-cam = D-camphoric acid anion) was hydrothermally synthesized. This complex was characterized by FTIR spectroscopy, TGA, and single-crystal X-ray diffraction. Crystallographic studies show that the title complex 1 crystallizes in an orthorhombic system with a P212121 space group with a = 06.9518(05) Ǻ, b = 13.5516(13) Ǻ, c = 22.6380(02)Ǻ; V = 2132.7(3) Ǻ3. The title CuII complex adopts a square pyramidal configuration. DFT study and Hirshfeld topology analysis of complex 1 was also done. The crystal achieves its three-dimensional structure and stability through polymeric chains having helical motifs of arrangement in between moieties and interconnected through hydrogen bonding interactions between the apical water molecule and non-coordinated oxygen atoms of the D-cam2- ligands. TGA, DFT calculations and Hirshfeld topology analysis revealed that the title complex 1 was stable.

Cu(II) complexes with coordinated pyrazine-dioxide: pyrazine-bridged chains

Two new pyrazine-bridged, linear chain complexes of Cu(II) of formula [Cu(pzdo)2(H2O)2(pz)](A)2·nH2O [pzdo = pyrazine-N,N′-dioxide; pz = pyrazine; A = BF4 − (1), ClO4 − (2)] have been prepared. Single-crystal X-ray diffraction shows the Cu ions to be six-coordinate, pyrazine-bridged chains with trans-pairs of ancillary ligands. The pzdo molecules are coordinated through their oxygen atoms. The ClO4 − and BF4 − anions are each two-site disordered in the lattice. Further, there are partial occupancy water molecules in the lattice which are very weakly bound. The structures are stabilized by hydrogen bonds between the coordinated water molecules and the non-coordinated pzdo oxygen atoms as well as the anions. Variable temperature magnetic susceptibility data show antiferromagnetic interactions and the data were fit to the uniform chain model yielding J/kB = −13.0(2) K and −11.8(2) K for 1 and 2, respectively []. In addition, the structure of the serendipitously prepared compound [Cu(pz)(pzdo)(H2O)2](ClO4)2 (3) is described. The compound crystallizes as rectangular layers of Cu(II) ions bridged by pzdo (parallel to the a-axis) and pyrazine (parallel to the b-axis) with water molecules coordinated in the axial sites.

Effects of transition metal dopants on the electronic structure of potassium niobate

Potassium niobate (KNbO3) is an important ferroelectric and photorefractive material that finds multiple nonlinear optical and photorefractive applications such as two-beam coupling and dynamic holography. We employ standard Density Functional Theory to examine two defect complexes in KNbO3 doped with 3-d transition metals. These defect complexes involve the substitution of a Nb with a 3-d transition metal element and (1) a coordinating oxygen vacancy induced in the nearest-neighbor oxygen shell or (2) a non-coordinating oxygen vacancy induced in the supercell as far away as possible from substitution impurity. With the exception of Ti and V, the 3-d transition metal dopants studied here may represent viable dopants for purposes of extending the photorefractive spectral response of KNbO3. Our results indicate that each of these complexes introduces defect states into the gap of KNbO3. Furthermore, we notice that the distribution of impurity states changes quite systematically across the 3-d transition metal row, and we note the effects of the disruption of the crystal field in the case of the coordinating O vacancy on the position and ordering of defect states within the gap. Substituting KNbO3 with transition metal ions can, therefore, significantly affect its optical and infrared properties, exemplifying how a detailed understanding of the electronic structure can be an important tool in tailoring the properties of this material for ferroelectric and photorefractive applications.

Bis(3,5-di-nitro-benzoato-κO)bis-(ethane-1,2-di-amine-κ2 N,N')cadmium(II).

During systematic investigations of bioavailability and biological action enhancement of well known compounds with low bioactivity, a new mixed-ligand metal complex, [Cd(DNBA)2(en)2)] (DNBA = 3,5-di-nitro-benzoate, C7H3N2O6; en = ethyl-endi-amine, C2H8N2), has been synthesized. The complex mol-ecules are located on inversion centers. Two DNBA anions monodentately coordinate the CdII atom through an oxygen atom of the carboxyl-ate group while two en mol-ecules coordinate in a chelate fashion, resulting in a distorted O2N4 coordination set. There is a weak intra-molecular hydrogen bond of 3.099 (4) Å between the non-coordinating oxygen atom of the carboxyl-ate group and one of the en amine groups. Three relatively weak inter-molecular N-H⋯O hydrogen bonds associate complex mol-ecules into sheets extending parallel to (01), which are further stabilized by π-π inter-actions. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions to the crystal packing are from H⋯O/O⋯H (50.2%) and H⋯H (21.1%) inter-actions.

Gallium(III) chelates of mixed phosphonate-carboxylate triazamacrocyclic ligands relevant to nuclear medicine: Structural, stability and in vivo studies

Three triaza macrocyclic ligands, H6NOTP (1,4,7-triazacyclononane-N,N′,N″-trimethylene phosphonic acid), H4NO2AP (1,4,7-triazacyclononane-N-methylenephosphonic acid-N′,N″-dimethylenecarboxylic acid), and H5NOA2P (1,4,7-triazacyclononane-N,N′-bis(methylenephosphonic acid)-N″-methylene carboxylic acid), and their gallium(III) chelates were studied in view of their potential interest as scintigraphic and PET (Positron Emission Tomography) imaging agents. A 1H, 31P and 71Ga multinuclear NMR study gave an insight on the structure, internal dynamics and stability of the chelates in aqueous solution. In particular, the analysis of 71Ga NMR spectra gave information on the symmetry of the Ga3+ coordination sphere and the stability of the chelates towards hydrolysis. The 31P NMR spectra afforded information on the protonation of the non-coordinated oxygen atoms from the pendant phosphonate groups and on the number of species in solution. The 1H NMR spectra allowed the analysis of the structure and the number of species in solution.31P and 1H NMR titrations combined with potentiometry afforded the measurement of the protonation constants (log KHi) and the microscopic protonation scheme of the triaza macrocyclic ligands. The remarkably high thermodynamic stability constant (log KGaL=34.44 (0.04) and stepwise protonation constants of Ga(NOA2P)2− were determined by potentiometry and 69Ga and 31P NMR titrations. Biodistribution and gamma imaging studies have been performed on Wistar rats using the radiolabeled 67Ga(NO2AP)− and 67Ga(NOA2P)2−chelates, having both demonstrated to have renal excretion. The correlation of the molecular properties of the chelates with their pharmacokinetic properties has been analysed.

Using the Singly Deprotonated Triethanolamine to Prepare Dinuclear Lanthanide(III) Complexes: Synthesis, Structural Characterization and Magnetic Studies

The 1:1 reactions between hydrated lanthanide(III) nitrates and triethanolamine (teaH3) in MeOH, in the absence of external bases, have provided access to the dinuclear complexes [Ln2(NO3)4(teaH2)2] (Ln = Pr, 1; Ln = Gd, 2; Ln = Tb, 3; Ln = Dy, 4; Ln = Ho, 5) containing the singly deprotonated form of the ligand. Use of excess of the ligand in the same solvent gives mononuclear complexes containing the neutral ligand and the representative compound [Pr(NO3)(teaH3)2](NO3)2 (6) was characterized. The structures of the isomorphous complexes 1∙2MeOH, 2∙2MeOH and 4∙2MeOH were solved by single-crystal X-ray crystallography; the other two dinuclear complexes are proposed to be isostructural with 1, 2 and 4 based on elemental analyses, IR spectra and powder XRD patterns. The IR spectra of 1–6 are discussed in terms of structural features of the complexes. The two LnIII atoms in centrosymmetric 1∙2MeOH, 2∙2MeOH and 4∙2MeOH are doubly bridged by the deprotonated oxygen atoms of the two η1:η1:η1:η2:μ2 teaH2− ligands. The teaH2− nitrogen atom and six terminal oxygen atoms (two from the neutral hydroxyl groups of teaH2− and four from two slightly anisobidentate chelating nitrato groups) complete 9-coordination at each 4f-metal center. The coordination geometries of the metal ions are spherical-relaxed capped cubic (1∙2MeOH), Johnson tricapped trigonal prismatic (2∙2MeOH) and spherical capped square antiprismatic (4·2MeOH). O–H∙∙∙O H bonds create chains parallel to the a axis. The cation of 6 has crystallographic two fold symmetry and the rotation axis passes through the PrIII atom, the nitrogen atom of the coordinated nitrato group and the non-coordinated oxygen atom of the nitrato ligand. The metal ion is bound to the two η1:η1:η1:η1 teaH3 ligands and to one bidentate chelating nitrato group. The 10-coordinate PrIII atom has a sphenocoronal coordination geometry. Several H bonds are responsible for the formation of a 3D architecture in the crystal structure of 6. Complexes 1–6 are new members of a small family of homometallic LnIII complexes containing various forms of triethanolamine as ligands. Dc magnetic susceptibility studies in the 2–300 K range reveal the presence of a weak to moderate intramolecular antiferromagnetic exchange interaction (J = −0.30(2) cm−1 based on the spin Hamiltonian H ^ = - J ( S ^ Gd 1 ⋅ S ^ Gd 1 ′ ) ) for 2 and probably weak antiferromagnetic exchange interactions within the molecules of 3–5. The antiferromagnetic GdIII∙∙∙GdIII interaction in 2 is discussed in terms of known magnetostructural correlations for complexes possessing the {Gd2(μ2-OR)2}4+ core. Ac magnetic susceptibility measurements in zero dc field for 3–5 do not show frequency dependent out-of-phase signals; this experimental fact is discussed and rationalized for complex 4 in terms of the magnetic anisotropy axis for each DyIII center and the oblate electron density of the metal ion.

Crystal structure and physical properties of 1-methyl-3-(carboxymethyl)benzimidazolium betaine·CuBr2 in crystal and water solution

Planar CuO2Br2 complex exists in a triclinic unit cell. Single crystal EPR and MO-theory give electron spin density delocalization parameters.

An oxygen-17 dynamic NMR study of the Pr–DOTA complex

The complex between (17)O-enriched DOTA (tetraazacyclododecanetetraacetic acid) and praseodymium(III) (Pr(3+)) was studied in aqueous solution by variable-temperature (17)O NMR at 14.1 T. pH effects as well as the influence of metal ions free in solution were investigated. At low temperature, the so-called TSAP and SAP conformations give rise to distinct signals for the oxygen atoms coordinated to the metal ion (O2); coalescence occurs between 20 and 30 °C. In contrast, a single signal was detected for the noncoordinated oxygen atoms (O1) in the entire investigated temperature range, i.e. between -3 and 135 °C. At high temperature, the spectra exhibit signal broadening that reveals the interchange of the O1 and O2 oxygen atoms of the carboxylate groups. The linewidths measured for O1 were deconvolved into contributions from quadrupole relaxation and chemical exchange, allowing the corresponding activation barriers to be determined. The present (17)O dynamic NMR study provides the first quantitative experimental data characterizing the interchange of the oxygen atoms in a DOTA chelate of a lanthanide metal ion. The activation entropy of this process is negligible and the activation enthalpy is found to range between 66 and 77 kJ mol(-1), depending on the pH and the presence of free Pr(3+) ions in solution. These data support the results of a previous computational study according to which the exchange mechanism involves the internal rotation of the carboxylate groups.

Regional Stacking in Supramolecular Architecture of Two New Coordination Compounds of 2-propyl-4,5- imidazoledicarboxylate Acid: Synthesis, Structure and Characterization

Two coordination compounds of the 2-propyl-4,5-imidazoledi- carboxylate anion (H2pimdc), [Mn(H2pimdc)2(H2O)2]·4H2O (1) and [Co(H2pimdc)2(H2O)2]·3H2O (2), were successfully synthesized under the conventional synthesis conditions and characterized by elemental analysis, IR and UV-vis spectroscopy, thermal analysis, and single-crystal X-ray diffraction technique. The structural analysis results reveal that in 1 and 2 the metal ions are coordinated by two H2pimdc ligands in single-chelating mode and two water molecules in a cis-pattern, forming a neutral coordination molecule with a cis-octahedral geometry. The stacking of these neutral molecules results in a hydrophobic region via the propyl groups of H2pimdc ligands and a hydrophilic region via the crystallization and coordination water molecules as well as the noncoordinated oxygen atoms of the carboxyls. Their 3D supramolecular structures are formed through extensive hydrogen bonds, π-π interaction and intermolecular forces. Compounds 1 and 2 exhibit paramagnetic property. Supplemental materials are available for this article. Go to the publisher's online edition of Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry to view the supplemental file.

Syntheses, crystal structures and copper-binding capabilities of amidate-hanging platinum mononuclear complexes containing alkylamine moieties

As analogs of cis-[Pt(piam)2(NH3)2]·2H2O (1, piam=pivalamidate), two kinds of platinum mononuclear complexes, cis-[Pt(piam)2(NH2C3H7)2]·H2O (2) and cis-[Pt(piam)2(NH2C4H9)2] (3), have been synthesized and characterized by single-crystal X-ray analyses. The square-planar complexes 2 and 3 contain propyl and butyl moieties, respectively, enhancing the hydrophobicity. Both compounds have non-coordinated oxygen atoms in the piam ligands that can bind to a second metal, such as the Cu2+ ion, to afford trinuclear Pt–Cu–Pt complexes. UV–Vis and electron paramagnetic resonance spectra of the reaction of 2 and 3 with the Cu2+ ion show that both compounds form dinuclear Pt–Cu complexes as intermediates.

Insight into the Dynamics of Lanthanide-DTPA Complexes As Revealed by Oxygen-17 NMR

DTPA chelates of various diamagnetic and paramagnetic lanthanide(III) metal ions, as well as the chemically similar DTPA chelate of Y(3+), were studied in aqueous solution by variable temperature (17)O NMR with the aim of characterizing their internal dynamics. As a consequence of poor chemical shift dispersion and fast quadrupole relaxation, no dynamic exchange process could be detected for the diamagnetic complexes nor for the Sm-DTPA complex. In contrast, the spectra recorded for the Eu-DTPA complex show chemical exchange due to the well-known racemization process and, at high temperature, feature signal broadening that reveals a fluxional process involving the interchange of the coordinated and noncoordinated oxygen atoms of the carboxylate groups. The spectra recorded for the Pr-DTPA complex feature coalescence events due to such a fluxional process, which is ascribable to the rotation of the carboxylate groups. The activation free energy barriers determined experimentally are remarkably lower than the calculated activation barriers recently reported for the rotation of the carboxylate groups of various Ln-DOTA complexes. Furthermore, the smallest activation free energy measured for the Pr-DTPA complex, about 45 kJ mol(-1), is significantly lower than the activation free energy characterizing the racemization process. The fluxional behavior of the carboxylate groups is, however, not expected to significantly affect the residence time of the water molecule coordinated to the metal ion.

Supramolecular organotin tris-carboxylates: crystal and molecular structure of [Cy2NH2]2[1-Me3(H2O)SnOCO-3,5-(OOC)2C6H3]·EtOH

The crystal and molecular structure of [Cy2NH2]2[C6H3(CO2)3SnMe3·H2O]·CH3CH2OH (1) has been determined by single crystal X-ray diffraction analysis. In this compound, the tin atom is pentacoordinated by three methyl groups, one oxygen atom of a water molecule and one oxygen atom deriving from the tris-carboxylato ligand, in a trans-coordinated O2SnC3 environment. The dicyclohexylammonium cations are involved in intermolecular hydrogen bonds with the non-coordinated oxygen atoms of the carboxylate, while, along with hydrogen bonds involving water and ethanol, gives rise to a complex 3D lattice structure.

Synthesis, characterization, and crystal structure of a 2-D metal diphosphonate: Cu3(H2O)2[HO3PCH2N(CH3)CH2PO3]2 · 2H2O

Hydrothermal reaction of N-methyl-iminobis(methylenephosphonic acid), CH3N(CH2PO3H2)2, (H4L) with copper(II) acetate afforded a new layered Cu(II) amino diphosphonate, Cu3(H2O)2(HL)2 · 2H2O (1). Compound 1 was studied by IR spectroscopy, TGA/DTA data, and X-ray diffraction (XRD) techniques. The XRD patterns are the same for the hydrated and the dehydrated complexes. A single-crystal X-ray crystallographic determination reveals copper in two different coordination environments. Cu1 has a distorted elongated tetragonal octahedral geometry, whereas Cu2 has a square-pyramidal distorted geometry. The HL trianion is a pentadentate ligand with a deprotonated nitrogen atom and two oxygen atoms of each phosphonate binding to copper. Hydrogen bonds between lattice water molecules in interlayer spaces and the non-coordinated phosphonate oxygen atoms as well as water ligands leads to a 3-D supramolecular network structure.

Sulfate binding in zinc(II) complexes of a monopyridylurea ligand N-(1-naphthyl)-N′-(3-pyridyl)urea

Abstract Two zinc(II) complexes of a urea-functionalized pyridyl ligand, [Zn(SO4)L3]·CH3OH ( 1 ) and {[Zn(μ2-SO4)L2]·0.5CH3OH}n ( 2 ) (L = N-(1-naphthyl)-N′-(3-pyridyl)urea), have been synthesized by the reaction of L with ZnSO4·7H2O under different conditions. Complex 1 is a hydrogen-bonded 2D grid structure in which the sulfate anion not only coordinates to the ZnII ion as a monodentate ligand but also forms multiple hydrogen bonds with the urea groups of adjacent molecules. In complex 2 the sulfate ion serves as a μ2-bridging ligand, connecting the ZnII ions to form an infinite 1D organic–inorganic hybrid framework. Meanwhile, the sulfate anion is also bound by neighboring ligands through hydrogen bonds between urea groups and the non-coordinated oxygen atoms. The solid-state emission spectra of 1 and 2 show a red shift of the fluorescence emission of ligand L.

Structural disorder in alkaline earth metal doped BaxMn[Fe(CN)6]2(x+1)/3·zH2O molecular magnets: a reverse Monte Carlo study

We report a detailed structural disorder study of the Ba(x)Mn[Fe(CN)(6)](2(x+1)/3)·zH(2)O (x = 0 and 0.3) molecular magnets by carrying out Reverse Monte Carlo (RMC) simulations of neutron diffraction data. Both samples have also been investigated using X-ray diffraction, infrared spectroscopy and dc magnetization techniques. Rietveld refinement of X-ray and neutron diffraction patterns confirmed the single phase formation for both compounds in a face-centered cubic structure. IR spectral study establishes the presence of cyanide flipping in both compounds, thereby revealing inherent structural disorder in the compounds. A ferrimagnetic coupling of Mn(2+) (S = 5/2) spins and the Fe(3+) (S = 1/2) spins is found for both compounds. Results of RMC simulations of neutron diffraction data for both compounds show that: (i) around the coordinated oxygen atoms (located at the 24e crystallographic sites with [Fe(CN)(6)] vacancies), there are formations of small clusters of non-coordinated oxygen atoms; and (ii) Ba substitution leads to a reduction in this structural disorder. The role of reduced water content as well as vacancies of [Fe(CN)(6)] towards the observed reduction in the structural disorder is discussed.

The first report on specific binding mode of diethylmalonate acting as a bridging ligand between ruthenium (II) ions stabilized by intramolecular hydrogen bonds

Typically 2,2-diethylmalonate (dem) acts as a chelating ligand and binds to the metal in a η 2 (dem-O, O′) mode. However, when cis,fac-[RuCl 2(TMSO-S) 4] is treated with K 2(dem), it prefers to bind in an unusual bridging mode (μ-dem-O, O′) with the ruthenium (II) cation containing coordinated water, forming a strong hydrogen bond with the non-coordinated oxygen atoms of the 2,2-diethylmalonate ligand. The reaction products of cis, fac-[RuCl 2(TMSO-S) 4] ( 1) and cis, fac-[RuCl 2(DMSO-S) 3(DMSO-O)] ( 2) with dem are the dinuclear species with two bridging dem units, fac-[Ru(TMSO-S) 3(H 2O)(μ 2-dem-O, O′)] 2 ( 3) and fac-[Ru(DMSO-S) 3(H 2O)(μ 2-dem-O, O′)] 2 ( 4), respectively. The complex 3 was characterized by X-ray crystallography in which water ligands occupy anti positions with respect to each other. The NMR and X-ray study support each other with respect to dinuclear structure of 3 and 4, indicating that the dinuclear structure observed in the solid state is preserved in solution as well. The mononuclear anionic complex with chelating dem unit, K{ fac-[RuCl(η 2-dem-O, O′)(TMSO-S) 3} ( 5), was also isolated from the reaction of 1 and K 2(dem) demonstrating that 5 is an intermediate in the formation of 3.

XANES Study of the Carboxylate Binding Mode in Two Pterin Hydroxylases

The 2-His-1-carboxylate triad is an Fe(II)-binding motif common to several enzyme families. Within the catalytic cycle, the metal ion seems to alter between hexa- and pentacoordination, providing an open space in the Fe moiety for dioxygen binding. Anyhow, based on crystallographic studies, the picture is not fully consistent as different coordination numbers are reported for similar states. Moreover, the orientation of the metal-coordinating carboxylate varies in these studies. These differences are reflected in the XANES spectra analyzed in this paper. For isolated tyrosine and phenylalanine hydroxylase, the different active-site structures postulated by protein crystallography and further improved using spectroscopic data result in calculated XANES pattern that resemble very well the experiments. This work shows that structural differences in the non-coordinated oxygen of the carboxylate have no effect in the EXAFS but cause a change in the white-line intensity that can be successfully modeled within the muffin-tin approximation in small clusters. Thus, the study shows a way to analyze the 'carboxylate shift'. This highlights the potential of XANES analysis and clearly shows that the mentioned structural differences are present in solution as well, and does neither reflect the crystallization artifacts nor result from radiation damage or lacking resolution.

Polymeric nickel(II) squarate complexes with imidazole derivatives: syntheses, crystal structures, spectroscopic and voltammetric studies

Two squarate complexes of nickel(II), [Ni(C4O4)(4-Meim)2(H2O)2] n (1) (4-Meim: 4-methylimidazole) and [Ni(C4O4)(his)2] n · nH2O (2) (his: histamine or 2-(4-imidazolyl)ethylamine), have been synthesized and characterized by X-ray diffraction, IR and UV–Vis techniques. The electrochemical properties of the nickel(II) complexes are investigated by cyclic voltammetry. In both complexes the Ni(II) ions have distorted octahedral geometry. Both structures comprise infinite chains parallel to [010] for 1, for 2 built up of bis(monodentate) squarate-O1,O3 (μ-1,3) anions bridging [Ni(4-Meim)2(H2O)2] (1) and [Ni(his)2] (2) units. These chains are held together by hydrogen bonds between the noncoordinated oxygens of the squarate anions and the water molecules, forming a two-dimensional sheet of Ni(II) ions. These sheets are held together by hydrogen bonds between the imidazole (1) or amine (2) hydrogens and noncoordinated squarate oxygens (1 and 2), or water (2) molecules. IR and UV–Vis spectra of both compounds agree with the observed crystal structure. The voltammetric behaviors of 1 and 2 were investigated in DMSO (dimethylsulfoxide) solution by cyclic voltammetry using n-Bu4NClO4 supporting electrolyte. Both complexes exhibit only ligand () centred electroactivity in the potential −0.2–1.4 V versus Ag/AgCl reference electrode. The squarate anion is oxidized in two consecutive steps to the corresponding radical monoanion and neutral form.

CD and MCD of CytC3 and Taurine Dioxygenase: Role of the Facial Triad in α-KG-Dependent Oxygenases

The alpha-ketoglutarate (alpha-KG)-dependent oxygenases are a large and diverse class of mononuclear non-heme iron enzymes that require FeII, alpha-KG, and dioxygen for catalysis with the alpha-KG cosubstrate supplying the additional reducing equivalents for oxygen activation. While these systems exhibit a diverse array of reactivities (i.e., hydroxylation, desaturation, ring closure, etc.), they all share a common structural motif at the FeII active site, termed the 2-His-1-carboxylate facial triad. Recently, a new subclass of alpha-KG-dependent oxygenases has been identified that exhibits novel reactivity, the oxidative halogenation of unactivated carbon centers. These enzymes are also structurally unique in that they do not contain the standard facial triad, as a Cl- ligand is coordinated in place of the carboxylate. An FeII methodology involving CD, MCD, and VTVH MCD spectroscopies was applied to CytC3 to elucidate the active-site structural effects of this perturbation of the coordination sphere. A significant decrease in the affinity of FeII for apo-CytC3 was observed, supporting the necessity of the facial triad for iron coordination to form the resting site. In addition, interesting differences observed in the FeII/alpha-KG complex relative to the cognate complex in other alpha-KG-dependent oxygenases indicate the presence of a distorted 6C site with a weak water ligand. Combined with parallel studies of taurine dioxygenase and past studies of clavaminate synthase, these results define a role of the carboxylate ligand of the facial triad in stabilizing water coordination via a H-bonding interaction between the noncoordinating oxygen of the carboxylate and the coordinated water. These studies provide initial insight into the active-site features that favor chlorination by CytC3 over the hydroxylation reactions occurring in related enzymes.