Remaining Coordination Sites Research Articles

Synthesis, Structure, and Catalase-Like Activity of Dimanganese(III) Complexes of 1,5-Bis[(2-hydroxy-5-X-benzyl)(2-pyridylmethyl)amino]pentan-3-ol (X = H, Br, OCH3)

New diMn(III) complexes of general formula [Mn(2)L(mu-OR)(mu-OAc)]BPh(4) (H(3)L = 1,5-bis[(2-hydroxy-5-X-benzyl)(2-pyridylmethyl)amino]pentan-3-ol, 1: X = H, R = Me, 2: X = OMe, R = Me, 3: X = Br, R = Me, 4: X = Br, R = Et) have been prepared and structurally characterized. The synthesized complexes possess a triply bridged (mu-alkoxo)(2)(mu-acetato)Mn(2)(3+) core, a short intermetallic distance of 2.95/6 A modulated by the aliphatic spacers between the central alcoholato and N-amino donor sites, and the remaining coordination sites of the two Mn(III) centers occupied by the six donor atoms of the polydentate ligand. In dimethylformamide, complexes 1-3 are able to disproportionate more than 1500 equiv of H(2)O(2) without significant decomposition, with first-order dependence on catalyst and saturation kinetic on [H(2)O(2)]. Spectroscopic monitoring of the reaction mixtures revealed that the catalyst converts into [Mn(2)(III)(mu-O)(mu-OAc)L], which is the major active form during cycling. Overall, kinetics and spectroscopic studies of H(2)O(2) dismutation by these complexes converge at a catalytic cycle between Mn(III)(2) and Mn(II)(2) oxidation levels. Comparison to other alkoxo-bridged complexes suggests that the binding mode of peroxide to the metal center of the Mn(III)(2) form of the catalyst is a key factor for tuning the Mn oxidation states involved in the H(2)O(2) dismutation mechanism.

Synthesis, structure, spectra and reactivity of iron(iii) complexes of imidazole and pyrazole containing ligands as functional models for catechol dioxygenases

A series of new 1 : 1 iron(iii) complexes of the type [Fe()Cl(3)], where is a tridentate 3N donor ligand, has been isolated and studied as functional models for catechol dioxygenases. The ligands (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethyl-amine (), N,N-dimethyl-N'-(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine () and N-(1-methyl-1H-imidazol-2-ylmethyl)-N'-phenylethane-1,2-diamine () are linear while the ligands tris(1-pyrazolyl)methane (), tris(3,5-dimethyl-1-pyrazolyl)methane () and tris(3-iso-propylpyrazolyl)methane () are tripodal ones. All the complexes have been characterized by spectral and electrochemical methods. The X-ray crystal structure of the dinuclear catecholate adduct [Fe()(TCC)](2)O, where TCC(2-) is a tetrachlorocatecholate dianion, has been successfully determined. In this complex both the iron(iii) atoms are bridged by a mu-oxo group and each iron(iii) center possesses a distorted octahedral coordination geometry in which the ligand is facially coordinated and the remaining coordination sites are occupied by the TCC(2-) dianion. Spectral studies suggest that addition of a base like Et(3)N induces the mononuclear complex species [Fe()(TCC)Cl] to dimerize forming a mu-oxo-bridged complex. The spectral and electrochemical properties of the catecholate adducts of the complexes generated in situ reveal that a systematic variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(iii) center and hence the interaction of the complexes with simple and substituted catechols. The 3,5-di-tert-butylcatecholate (DBC(2-)) adducts of the type [Fe()(DBC)Cl], where is a linear tridentate ligand (), undergo mainly oxidative intradiol cleavage of the catechol in the presence of dioxygen. Also, the extradiol-to-intradiol product selectivity (E : I) is enhanced upon removal of the coordinated chloride ion in these adducts to obtain [Fe()(DBC)(Sol)](+) and upon incorporating coordinated N-methylimidazolyl nitrogen in them. In contrast to the iron(iii) complexes of imidazole-based ligands, those of the tripodal pyrazole-based ligands yield major amounts of the oxidized product benzoquinone and small amounts of both intra- and extradiol products. One of the pyrazole arms coordinated in the equatorial plane of these sterically constrained complexes is substituted by a solvent molecule upon adduct formation with DBC(2-), which encourages molecular oxygen to attack this site leading to benzoquinone formation. The DBSQ/DBC(2-) redox potentials of both the imidazole- and pyrazole-based complexes fall in the narrow range of -0.186 to -0.214 V supporting this proposal.

A Novel Ligand Modification and Diamond-core Molybdenum(IV) 2,6-Bis(2,2-diphenyl-2-thioethyl)pyridinate(2-) Complex

The reaction of Mo(VI)O(2)(L-NS(2)) [L-NS(2) = 2,6-bis(2,2-diphenyl-2-thioethyl)pyridinate(2-)] or Mo(V)(2)O(3)(L-NS(2))(2) with excess PPh(3) in N,N-dimethylformamide at 70 degrees C results in the formation of gray-green (L-NOS)Mo(IV)(mu-O)(mu-S)Mo(IV)(L-NS(2)) [L-NOS = 2-(2,2-diphenyl-2-thioethyl)-6-(2,2-diphenyl-2-oxoethyl)pyridinate(2-)] (1). The crystal structure of 1 revealed a dinuclear complex comprised of two trigonal bipyramidal Mo centers bridged along an axial-equatorial edge (the mu-O-mu-S vector) such that the Mo-N bonds are trans to the bridging atoms and are anti with respect to the Mo-Mo bond (d(Mo-Mo) = 2.5535(5) A); the remaining coordination sites are occupied by the S- and O-donor atoms of the L-NOS and L-NS(2) ligands. The diamond core is asymmetric, with Mo(1/2)-O(1) distances of 1.845(2) and 2.009(2) A and Mo(1/2)-S(1) distances of 2.374(1) and 2.230(1) A. Compound 1 is unique in possessing a planar, diamond-core unit devoid of terminal oxo ligation and a new tridentate L-NOS ligand formed via a novel intramolecular modification of the original L-NS(2) ligand.

Spin-polarization in 1,3,5-trihydroxybenzene-bridged first-row transition metal complexes

Abstract The late Olivier Kahn formulated a research objective for molecular magnetism: ‘The normal trend for the molecular state is the pairing of electrons […] with a cancellation of the electron spins. The design of a molecule-based magnet requires that this trend be successfully opposed.’ One strategy for enforcing ferromagnetic interactions is the spin-polarization mechanism. While this mechanism is almost always working in organic chemistry, the application to transition metal complexes is not straight-forward. We have established a structurally related series of trinuclear complexes bridged by modified 1,3,5-trihydroxybenzene (phloroglucinol) ligands. The trinuclear Cu II complexes all exhibit weak, but ferromagnetic interactions. The trinuclear V IV complex exhibits even smaller ferromagnetic interactions, while a trinuclear Mn complex exhibits antiferromagnetic interactions. The correlation between structural and magnetic parameters in the series of Cu complexes gives experimental insight into the spin-polarization mechanism. The crucial parameter for an efficient spin-polarization mechanism through the bridging benzene unit seems to be the amount of spin density in the p z π orbitals of the phenolic oxygen atoms. This spin density crucially depends on the remaining coordination sites and on the ligand folding at the central Cu–phenolate bond. The spin transfer from the metal to the phenolate oxygen atom occurs by two different mechanisms, namely spin-polarization and spin-delocalization, which can provide opposing contributions. The main conclusion of this study is that for a more efficient spin-polarization through the central benzene ring the spin density in the phenolate O p z π orbital must be maximized, which can mainly be achieved by increasing the covalency of the metal–phenolate bond.

1D slide-fastener-like coordination polymers of Mn(II) derived from pyrazine-2,3,5,6-tetracarboxylic acid

Two new 1D slide-fastener-like coordination polymers {[Mn 2(pztc)(phen) 2(H 2O) 2]·4H 2O} n ( 1) and {[Mn 2(pztc)(bpy) 2(H 2O) 2]·2H 2O} n ( 2) have been synthesized by the reactions of pyrazine-2,3,5,6-tetracarboxylic acid (pztcH 4) and Mn(OAc) 2·4H 2O in the presence of 1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy) under hydrothermal conditions and characterized by elemental analyses, FT-IR, TGA and X-ray diffraction. In complex 1 and 2, metal ions are bridged by pyrazine-2,3,5,6-tetracarboxylate, coordinating in a hexadentate manner, so forming 1D polymeric chains. The remaining coordination sites of the metal ions are occupied by one O atom of water molecule and two N atoms of the terminal ligand (phen or bpy). IR spectra and thermal analysis data are in agreement with the crystal structure.

The Synthesis and Crystal Structure of [BiI2(tpy)2][Bi2I7(tpy)]: A New Metal Halide Material

[BiI2(tpy)2][Bi2I7(tpy)] (1) (tpy = 2,2′:6′2′′-terpyridine), a new compound containing a discrete bismuth halide anion, has been synthesized solvothermally and characterized by single crystal X-ray diffraction. The solvothermal reaction between BiI3, Zn(NO3)2 · 6H2O, and 2,2′:6′2′′-terpyridine in an ethanol/water solvent mixture resulted in the title compound. The anion and cation of this compound both contain bismuth, iodide ligands, and the organic ligand. The anion is composed of two face-sharing bismuth polyhedra, one of which is coordinated in a tridentate fashion to a disordered tpy ligand; and the remaining coordination sites at each Bi center are occupied by iodide ligands. The cation of the compound is a distorted, eight-coordinate polyhedron with a bismuth center, two iodide ligands, and six nitrogen atoms from two, tridentate tpy ligands. [BiI2(tpy)2][Bi2I7(tpy)] crystallizes in the triclinic space group P $$ \overline{1} $$ with the following lattice parameters: a = 10.0744(5) A, b = 16.6115(8) A, c = 18.5333(9) A, α = 102.6490(10)°, β = 101.0850(10)°, and γ = 93.9160(10)°. The title compound, [BiI2(tpy)2][Bi2I7(tpy)], was synthesized solvothermally by the reaction between BiI3 and 2,2′:6′2′′-terpyridine. Single crystal X-ray diffraction analysis reveals that the molecular structure contains one anion and one cation, both of which contain bismuth, iodine, and the tpy ligand, a situation not typically observed in these types of compounds.

Catena-Poly[[triaqua-cadmium(II)]-μ-pyridine-2,3-dicarboxyl-ato-κN,O:O

The title polymeric compound, [Cd(C7H3NO4)(H2O)3]n or [Cd(py-2,3-dc)(H2O)3]n, where py-2,3-dcH2 is pyridine-2,3-dicarboxylic acid, was obtained by the reaction of cadmium(II) nitrate hexa­hydrate with (pipzH2)(py-2,3-dc) as a proton-transfer compound in aqueous solution (pipz is piperazine). The mol­ecular structure shows that only the anionic fragment of the starting proton-transfer compound is present in the complex, while the (pipzH2)2+ dication has been lost. Each (py-2,3-dc)2− ligand bridges two CdII atoms in two different coordination modes, i.e. one end acts as a monodentate and the other end as a bidentate ligand. The three remaining coordination sites on the metal center are occupied by water mol­ecules. The geometric arrangement of the six donor atoms around the CdII atom is distorted octa­hedral. In the crystal structure, O—H⋯O and C—H⋯O hydrogen bonds play an important role in stabilizing the supra­molecular structure.

Targeted self-assembly and quantum Monte Carlo magnetic study of an alternating nickel(ii) 1D coordination polymer composed of highly preorganized binuclear tectons

A pyrazolate-based binucleating ligand HL with pyridyl groups in the chelate arms has been used to synthesize the dinickel(II) complex [LNi2(N3)(MeOH)2](ClO4)2 (1) and the corresponding 1D polymeric [LNi2(mu-N3)2]n(NO3)n (2) depending on the amount of NaN3 added. X-Ray crystallography shows that structural parameters of the (LNi2(N3)) units are very similar in both compounds. This is ascribed to pi-pi stacking between the pyridyl rings that leads to rigidification of the framework and a fixed cis-orientation of the remaining coordination sites, which are filled by MeOH molecules in or by a mu1,3-bridging azide that connects the subunits in 2. Variable-temperature magnetic measurements reveal strong antiferromagnetic coupling with parameters g = 2.21 and J = -60.7 cm(-1) for compound 1. Magnetic data for the extended chain 2 have been analyzed by Quantum Monte Carlo (QMC) simulations to give g = 2.34, J1 = -55 cm(-1), and J2 = -12 cm(-1). The known J value for finally allows unambiguous assignment of J1 to the intrasubunit coupling within each bimetallic chain constituent. The alternation ratio gamma = J2/J1 signifies a singlet-dimer ground state of the new 1D polymer 2.

Synthesis, structure, spectra and reactivity of iron(iii) complexes of facially coordinating and sterically hindering 3N ligands as models for catechol dioxygenases

A series of 1 : 1 iron(III) complexes of sterically hindered and systematically modified tridentate 3N donor ligands have been isolated and studied as functional models for extradiol-cleaving catechol dioxygenases. All of them are of the type [Fe(L)Cl(3)], where L is N-methyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L1), N-ethyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L2), N-benzyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L3), N,N-dimethyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L4), N'-methyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L5), N'-ethyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L6) and N'-benzyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L7). They have been characterized by elemental analysis and spectral and electrochemical methods. The X-ray crystal structures of the complexes [Fe(L2)Cl(3)] 2, [Fe(L3)Cl(3)] 3 and [Fe(L7)Cl(3)] 7 have been successfully determined. All the three complexes possess a distorted octahedral coordination geometry in which the ligand is facially coordinated to iron(III) and the chloride ions occupy the remaining coordination sites. Upon replacing the N-ethyl group on the terminal nitrogen donor in 2 by the bulky N-benzyl group as in 3, the terminal Fe-N bond distance increases slightly from 2.229(5) A to 2.244(5) A. Upon incorporating the sterically demanding N-benzyl group on the central nitrogen donor in 4 to obtain 7, the central Fe-N(amine) bond distance increases from 2.181(5) A to 2.299(2) A. The catecholate adducts [Fe(L)(DBC)(Cl)] and [Fe(L)(DBC)(Sol)](+), where H(2)DBC is 3,5-di-tert-butylcatechol and Sol = solvent (H(2)O/DMF), have been generated in situ and their spectral and redox properties and dioxygenase activities have been studied in N,N-dimethylformamide and dichloromethane solutions. The adducts [Fe(L)(DBC)(Sol)](+) undergo cleavage of DBC(2-) in the presence of molecular oxygen to afford both intra- and extradiol cleavage products. The extradiol products are higher in dichloromethane than in DMF solution and the extradiol to intradiol product selectivity (E/I, 7.2 : 1-18.5 : 1) observed decreases upon increasing the steric bulk of N-alkyl substituent on the terminal nitrogen atom and upon incorporating an N-alkyl substituent on the central nitrogen atom. The plot of log(k(O(2))) vs. energy of the low energy catecholate-iron(III) LMCT band is linear, which is consistent with the proposal that the LMCT band energy corresponds to the energy needed for a spin-inversion process at the iron center upon dioxygen attack. Also, the rate of oxygenation is dictated by the solvent as well as the Lewis acidity of the iron(III) center, as shown by the linear plot of log(k(O(2))) vs.E(1/2) of the Fe(III)/Fe(II) redox potentials of the [Fe(L)(Sol)(3)](3+) complexes.

Dichloridobis(7-amino-2,4-dimethyl-1,8-naphthyridine-κ2N,N′)cobalt(II) methanol disolvate

The title compound, [CoCl2(C10H11N3)2]·2CH3OH, crystallizes with an elongated Co coordination polyhedron in a very distorted octa­hedral geometry. Both naphthyridine ligands coordinate to the Co atom via two N atoms in a bidentate chelating mode. The remaining coordination sites are occupied by two Cl atoms. Two uncoordinated solvent methanol mol­ecules are hydrogen bonded to the Cl atoms. Additional N—H⋯O, C—H⋯Cl and N—H⋯Cl hydrogen bonds, and π–π stacking inter­actions [centroid–centroid distance 3.664 (4) Å], give rise to a three-dimensional network structure.

Tetrakis(μ-anthracene-9-carboxylato)bis[(anthracene-9-carboxylato)(2,2′-bipyridyl)lanthanum(III)

The title complex, [La2(C15H9O2)6(C10H8N2)2], has a centrosymmetric binuclear cage structure in which two LaIII atoms are both nine-coordinated and bridged by four anthracene-9-carboxyl­ate ligands, with an La⋯La separation of 4.0880 (4) Å. The remaining coordination sites are occupied by two N atoms of a 2,2′-bipyridine (bipy) and two O atoms of an anthracene-9-carboxyl­ate ligand. The six anthracene-9-carboxyl­ate groups coordinate each LaIII atom in three different ways. Adjacent discrete dinuclear units are arranged into a one-dimensional chain along the [111] direction by inter­molecular π–π stacking inter­actions, with a centroid–centroid separation of 3.704 (7) Å.

Bis(7-amino-2,4-dimethyl-1,8-naphthyridine)dinitratocadmium(II)

In the title compound, [Cd(NO3)2(C10H11N3)2], two naph­thyridine ring systems are coordinated to the Cd ion through the two N atoms in a bidentate chelating mode, whereas the remaining coordination sites are occupied by two O atoms from two different nitrate groups to complete the octahedral geometry. Inter­moleular N—H⋯O hydrogen bonds link the mol­ecules to form a one-dimensionnal sheet parallel to the ac plane. Weak slipped π–π stacking involving the naphthyridine ring systems stabilizes the structure.

Iron(III) Complexes of Tridentate 3N Ligands as Functional Models for Catechol Dioxygenases: The Role of Ligand N-alkyl Substitution and Solvent on Reaction Rate and Product Selectivity

A series of iron(III) complexes of the type [Fe(L)Cl3], where L is the variously N-alkyl-substituted bis(pyrid-2-ylmethyl)amine ligand such as bis(pyrid-2-ylmethyl)amine (L1), N,N-bis(pyrid-2-ylmethyl)methylamine (L2), N,N-bis(pyrid-2-ylmethyl)-n-propylamine (L3), N,N-bis(pyrid-2-ylmethyl)-iso-butylamine (L4), N,N-bis(pyrid-2-ylmethyl)-iso-propylamine (L5), N,N-bis(pyrid-2-ylmethyl)cyclohexylamine (L6), and N,N-bis(pyrid-2-ylmethyl)-tert-butylamine (L7), have been isolated and characterized by elemental analysis and spectral and electrochemical methods. The crystal structures of the complexes [Fe(L2)Cl3] 2, [Fe(L3)Cl3] 3, and the complex-substrate adduct [Fe(L5)(TCC)(NO3)] 5a, where TCC2- is the tetrachlorocatecholate dianion, have been determined by single-crystal X-ray crystallography. The complexes [Fe(L2)Cl3] 2 and [Fe(L3)Cl3] 3 possess a distorted octahedral geometry, in which the linear tridentate 3N ligands are cis-facially coordinated to the iron(III) center, and three chloride ions occupy the remaining coordination sites. The replacement of the N-methyl group in 2 by N-n-propyl group as in 3 leads to the formation of the Fe-Npy bonds and also the Fe-Cl bonds located trans to them of different lengths. The catecholate adduct 5a also possesses a distorted octahedral geometry, in which the ligand is cis-facially coordinated to iron(III) center, TCC2- is asymmetrically chelated trans to the two pyridyl moieties of the ligand, and one of the oxygen atoms of the nitrate ion occupies the sixth coordination site. All of the present complexes have been interacted with simple and substituted catechols. The catecholate adducts [Fe(L)(DBC)Cl] and [Fe(L)(DBC)(Sol)]+, where H2DBC is 3,5-di-tert-butylcatechol and Sol=H2O/CH3CN, have been generated in situ, and their spectral and redox properties and dioxygenase activities have been studied in dimethylformamide and dichloromethane solutions. All of the complexes catalyze the cleavage of H2DBC using molecular oxygen to afford both intra- and extradiol cleavage products. The formation of extradiol cleavage products is facilitated by cis-facial coordination of the 3N ligands and availability of vacant coordination site on iron(III) center for dioxygen binding. It is remarkable that the nature of the N-alkyl substituent in 3N ligands controls the regioselectivity of cleavage, with the n-propyl, iso-butyl, iso-propyl, and cyclohexyl groups enhancing the yield of extradiol products (46-68%) in dichloromethane. The rate of oxygenation depends upon the solvent and the Lewis acidity of iron(III) center as modified by the sterically demanding N-alkyl groups-length and degree of substitution. The plot of log (kO2) versus energy of the low-energy DBC2--to-iron(III) LMCT band is linear, demonstrating the importance of the Lewis acidity of the iron(III) center in dictating the rate of the dioxygenase reaction.

Synthesis, X-ray crystal structure and magnetic study of a dicyanamido bridged 1D chain nickel(II) complex

Reaction of Ni(NO 3) 2 · 6H 2O and sodium dicyanamide (Nadca) yields a 1D infinite chain complex {[Ni(dien)(μ 1,5-dca)(H 2O)](NO 3)} n ( 1) (where dien = diethylenetriamine). The coordination environment in complex 1 around the nickel(II) ions is distorted octahedron. Three nitrogen atoms of the ligand diethylenetriamine and an oxygen atom of H 2O molecule constitute the four coordination sites of the basal plane of the octahedron. Of two axial positions of the octahedron, one position is occupied by the nitrogen atom of a μ 1,5-dca anion the remaining coordination site is occupied by a nitrogen atom of another end-to-end bridging dca from an adjacent [Ni(dien)(μ 1,5-dca)(H 2O)] moiety, yielding 1D infinite chains which propagate parallel to crystallographic a-axis. No measurable magnetic interaction was evidenced through variable temperature magnetic susceptibility measurements (4–300 K). However, the magnetic susceptibility of the compound can be explained in terms of single-ion anisotropic model with zero-field splitting for nickel(II) ions.

Diaqua(5-methylpyrazine-2-carboxylato-κ2 N,O)cobalt(II)

In the title complex, [Co(C6H5N2O2)2(H2O)2], the Co2+ cation lies on an inversion centre and the coordination geometry is distorted octahedral, with two N atoms and two O atoms from the 5-methylpyrazine-2-carboxylate ligands in the equatorial plane. The two remaining coordination sites are occupied by two water molecules. The crystal structure is stabilized by a network of O—H...O hydrogen-bonding interactions, forming a two-dimensional supramolecular structure.

Diaqua(5-methylpyrazine-2-carboxylato-κ2 N,O)zinc(II)

In the title complex, [Zn(C6H5N2O2)2(H2O)2], the Zn2+ cation lies on an inversion centre and the coordination geometry is distorted octahedral, with two N atoms and two O atoms from the 5-methylpyrazine-2-carboxylate ligand in the equatorial plane. The two remaining coordination sites are occupied by two water molecules. The crystal structure is stabilized by a network of O—H...O hydrogen-bonding interactions, forming a two-dimensional supramolecular structure.

Dichloro[2,2′-(methylenedithio)bi-1,3-thiazole-κ2 N,N′]cobalt(II)

In the title complex, [CoCl2(C7H6N2S4)], 2,2′-(methylenedithio)bi-1,3-thiazole acts as a bidentate ligand and chelates one CoII ion through the two N atoms to give an eight-membered symmetric unit. The CoII ion is located on a twofold axis and displays a slightly distorted tetrahedral coordination environment, where the two Cl− anions occupy the remaining coordination sites. The N—Co—N and Cl—Co—Cl planes are nearly perpendicular, with a dihedral angle of 88.0 (3)°.

The Structural Basis of Signal Transduction for the Response Regulator PrrA from Mycobacterium tuberculosis

The structure of the two-domain response regulator PrrA from Mycobacterium tuberculosis shows a compact structure in the crystal with a well defined interdomain interface. The interface, which does not include the interdomain linker, makes the recognition helix and the trans-activation loop of the effector domain inaccessible for interaction with DNA. Part of the interface involves hydrogen-bonding interactions of a tyrosine residue in the receiver domain that is believed to be involved in signal transduction, which, if disrupted, would destabilize the interdomain interface, allowing a more extended conformation of the molecule, which would in turn allow access to the recognition helix. In solution, there is evidence for an equilibrium between compact and extended forms of the protein that is far toward the compact form when the protein is inactivated but moves toward a more extended form when activated by the cognate sensor kinase PrrB.

Models of hypoxia activated prodrugs: Co(iii) complexes of hydroxamic acids

Co(III) complexes of simple hydroxamic acids have been evaluated as models of hypoxia activated prodrugs containing MMP inhibitors. The complexes are based upon a proposed carrier system comprising the tripodal tetradentate ligand tris(2-methylpyridyl)amine (tpa) with the hydroxamate functionality occupying the remaining coordination sites of the Co centre. Acetohydroxamato (aha), propionhydroxamato (pha), and benzohydroxamato (bha) complexes were synthesised and characterised by single crystal X-ray diffraction. For aha and pha both the hydroxamato and hydroximato (deprotonated) forms were obtained and were readily interconverted by pH manipulation; for bha only the hydroximato complex was obtained as a stable species. Electrochemical analysis was used to probe the redox chemistry of the complexes and assess their ease of reduction. All of the complexes displayed irreversible reduction and had low cathodic peak potentials. This suggests that the Co-tpa carrier system would provide a suitably inert framework to deliver the drugs to target sites intact yet would release the ligands upon reduction to the more labile Co(II) oxidation state.

Dichloro[(6R,7S,8S,14S)-(–)-sparteine-κ2N,N′]mercury(II)

In the title compound, [HgCl2(C15H26N2)], the chiral alkaloid (6R,7S,8S,14S)-(-)-L-sparteine acts as a bidentate ligand, with two Cl- ligands occupying the remaining coordination sites, producing a distorted tetrahedron. The N-Hg-N plane is twisted by 81.1 (2) degrees from the Cl-Hg-Cl plane. The mid-point of the N...N line does not lie exactly on the Cl-Hg-Cl plane but is tilted towards one of the N atoms by 0.346 angstroms. Similarly, the mid-point of the Cl...Cl line is tilted toward one of the Cl atoms by 0.163 angstroms. The packing structure shows that the complex is stabilized by two interatomic Cl...H contacts involving both Cl atoms and the methylene or methine H atoms of the (-)-sparteine ligand.