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The structure of an Fe(III) complex of reduced Schiff base is reported. The title compound, C21H30ClFeN4O6 (I), crystallizes in the monoclinic space group P21/n with cell constants: a = 9.988(2) A, b = 20.430(5) A, c = 11.415(3) A, β = 105.480(4)°. It contains a six-coordinate FeN4O2 cation where the ligand is a reduced Schiff base resulting from the NaBH4 reduction of the condensation product between salicylaldehyde and 1,4,8,11-tetraazaundecane. Due to the increased flexibility of the saturated backbone of the ligand compared to the Schiff base from which it was synthesized, the complex adopts a trans-FeN4O2 conformation. There is extensive hydrogen bonding between the amine H atoms and the anion O atoms. The structure of an Fe(III) complex of reduced Schiff base is reported which adopts a trans-FeN4O2 conformation where the ligand is a reduced hexadentate Schiff base resulting from the NaBH4 reduction of the condensation product between salicylaldehyde and 1,4,8,11-tetraazaundecane.

Cysteinyldopas are naturally occurring conjugates of cysteine and dopa (3,4-dihydroxy-L-phenylalanine) that are precursors to red pheomelanin pigments. Metal ions are known to influence pheomelanogenesis in vitro and may be regulatory factors in vivo. Cydo (3-[(2-amino-ethyl)sulfanyl]-4,6-di-tert-butylbenzene-1,2-diol) and CarboxyCydo (2-amino-3-(4,6-di-tert-butyl-2,3-dihydroxyphenylsulfanyl)-propionic acid) are model compounds of cysteinyldopa that retain its metal-binding functionalities but cannot polymerize due to the presence of blocking tert-butyl groups. Cydo reacts readily with zinc(II) acetate or nickel(ll) acetate to form a cyclized 1,4-benzothiazine (zine) intermediate that undergoes ring contraction to form benzothiazole (zole) unless it is stabilized by coordination to a metal ion. The crystal structure of [Ni(zine) 2 ] is reported. The acetate counteranion is required for the zinc-promoted reactivity, as neither zinc(ΙI) sulfate nor zinc(ll) chloride alone promotes the transformation. The counterion is less important for redox-active copper and iron, which both readily promote the oxidation of Cydo to zine and zole species; Cu(ΙΙ) complexes of both zine and zole have been characterized by X-ray crystallography. In the case of CarboxyCydo, a 3-carboxy-1,4-benzothiazine intermediate decarboxylates to form [Cu(zine) 2 ] under basic conditions, but in the absence of base forms a mixture of products that includes the carboxylated dimer 2,2'-bibenzothiazine (bi-zine). These products are consistent with species implicated in the pheomelanogenesis biosynthetic pathway and emphasize how metal ions, their counteranions, and reaction conditions can alter pheomelanin product distribution.

The crystal structures of [Mn(bpa)2](ClO4)2 (1), [bpa = bis(2-pyridylmethyl)amine], and Mn(6-Mebpa)2(ClO4)2 (2), [6-Mebpa = (6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine] have been determined. In 1, two facial [Mn(bpa)2]2+ isomers are observed in the same unit cell, one with C i (1a) and the other with C2 (1b) symmetries. In 2, only the isomer with C2 symmetry is observed. The structure of [Mn(bpa)2]2+ with only C2 symmetry has been reported previously (Inorg. Chem., 31, 4611 (1992)). The bond length order Mn–Namine > Mn–Npyridyl, observed in the C2 and the C i isomers in the crystals of 1, is the reverse of the order observed in the structure of [Mn(bpa)2](ClO4)2 which contains only the C2 isomer in the unit cell. The structure of 2 in which only the C2 isomer is found, also shows the bond length order Mn–Npyridyl > Mn–Namine. In cyclic voltammetric experiments in acetonitrile solutions, 1 and 2 show irreversible anodic peaks at E p = 1.60 and 1.90 V respectively, (vs. Ag/AgCl), assigned to the oxidat...

The synthesis, structure and characterization of the dinuclear Mn(II) complex [Mn2(LO)(μ-OAc)2](ClO4) (1) where LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl)}-4-methylphenol are reported. The reaction of Mn(ClO4)2 · 6H2O with the dinucleating ligand LOH and H3CCOONa in the presence of NEt3 in dry, degassed methanol and under an argon atmosphere, yields 1 as a colorless powder. The crystal structure of 1, determined by X-ray diffraction methods, shows a dinuclear Mn(II) complex in which two Mn(II) ions, each in six-coordinate approximate octahedral coordination, are bridged by the phenolate oxygen of LO− and by two acetate ions in a syn,syn-1,3-bridging mode. The Mn–Mn distance is 3.557(1) A and Mn–Ophenolate–Mn angle is 112.50(9)°. Cyclic voltammetry of 1 in acetonitrile solution shows a quasi-reversible wave at E1/2 = 0.65 V, for the Mn2(II,II)/Mn2(II,III) redox process, and an irreversible oxidation peak at Ep,c = 1.22 V versus Ag/AgCl for the Mn2(II,III) to Mn2(III,III) oxidation process. Controlled potential electrolysis of 1 in acetonitrile solution at 0.85 V (versus Ag/AgCl) takes up 1 F of charge per mole of 1 to yield a brown solution of the Mn2(II,III) state of the complex, which, however, is unstable and reverts back to the Mn2(II,II) state in solution at room temperature. Least square fitting of the variable temperature magnetic susceptibility measurements on powdered sample of 1 is obtained with g = 1.888, J = −2.75 cm−1, Par = 0.008, TIP = 0. The low −J value and the room temperature calculated magnetic moment of the complex (5.30 BM per Mn(II)), which is less than the spin-only moment of Mn(II), show that the two Mn(II) ions are weakly antiferromagnetically coupled.

The dinucleating ligand, 2,6-bis{[(2-(2-pyridyl)ethyl)(2-pyridylmethyl)-amino]-methyl}-4-methylphenol) (L1OH) reacts with Mn(ClO4)2.6H2O to form the dinuclear complex [Mn2(II,II)(L1O)(mu-OOCCH3)2]ClO4 (1). The electrolytic oxidation of 1 at 0.7 V (vs Ag/AgCl) produces the mixed valent complex [Mn2(II,III)(L1O)(mu-OOCCH3)2](ClO4)2 (1ox) quantitatively, while electrolysis at 0.20 V converts 1ox back to 1. X-ray crystallographic structures show that both 1 and 1ox are dinuclear complexes in which the two manganese ions are each in distorted octahedral coordination environments bridged by the phenoxo oxygen and two acetate ions. The structural changes that occur upon the oxidation 1 to 1ox suggest an extended pi-bonding system involving the phenoxo ring C-O(phenoxo)-Mn(II)-N(pyridyl) chain. In addition, as 1 is oxidized to 1ox, the rearrangements in the coordination sphere resulting from the oxidation of one Mn(II) ion to Mn(III) are transmitted via the bridging Mn-O(phenoxo) bonds and cause structural changes that render the site of the second manganese ion unfit for the +3 state and hence unstable to reduction. Thus the electrolytic oxidation of 1ox in acetonitrile at 1.20 V takes up slightly greater than 1 F of charge/mol of 1ox, but the starting complex, 1ox, is recovered, showing the instability of the Mn2(III,III) state that is formed with respect to reduction to 1ox. Variable-temperature magnetic susceptibility measurements of 1 and 1ox over the temperature range from 1.8 to 300 K can be modeled with magnetic coupling constants J = -4.3 and -4.1 cm(-1), respectively showing the weak antiferromagnetic coupling between the two manganese ions in each dinuclear complex, which is commonly observed among similar phenoxo- and bis-1,3-carboxylato-bridged dinuclear Mn2(II,II) and Mn2(II,III) complexes.

In the title salt, C15H17N2+·ClO4−, the isoindoline N atom is protonated and an intramolecular N—H...N hydrogen bond occurs. In the crystal, N—H...O and numerous weak C—H...O interactions occur between the cation and anion. The O atoms of the perchlorate anion are disordered over four sets of sites with occupancies of 0.438 (4), 0.270 (9), 0.155 (8) and 0.138 (5).

A photosensitive caged copper complex has been prepared from a tetradentate ligand (H2cage) composed of two pyridyl-amide arms connected by a photoreactive nitrophenyl group. H2cage binds Cu2+ in aqueous solution with a stability constant (log beta) of 10.8, which corresponds to a KD of 16 pM at pH 7.4. The neutral Cu2+ complex, [Cu(OH2)(cage)], crystallizes as a distorted trigonal bipyramid coordinated by two amide and two pyridyl N atoms, with a water molecule bound in the trigonal plane. Photolysis with 350 nm UV light cleaves the ligand backbone to release photoproducts with significantly diminished affinity for Cu2+, thereby uncaging the metal ion. When coordinated as the caged complex, copper has diminished reactivity to produce hydroxyl radicals from Fenton-like reaction mixtures containing hydrogen peroxide and ascorbic acid. Postphotolysis, uncaged copper promotes hydroxyl radical formation under the same conditions. The strategy of caging copper is promising for applications where light could be used to trigger release of copper as a pro-oxidant to increase oxidative stress or as a tool to release copper intracellularly to study mechanisms of copper trafficking.

A new cysteinyldopa model ligand Cydo {3-[(2-aminoethyl)sulfanyl]-4,6-di-tert-butylbenzene-1,2-diol} was prepared and its reactivity with Cu(II) explored. Under anaerobic conditions, tetranuclear [Cu4(Cydo)4] is isolated, but in the presence of O2, a benzothiazine intermediate accumulates that is trapped as the Cu(II) complex [Cu(zine)2]. Under slightly different reaction conditions, the benzothiazine further oxidizes to benzothiazole (zole). All three compounds were characterized by X-ray crystallography, and the reactions were monitored spectrophotometrically.

The syntheses and structural, spectral, and electrochemical characterization of the dioxo-bridged dinuclear Mn(III) complexes [LMn(mo-O)(2)MnL](ClO(4))(2), of the tripodal ligands tris(6-methyl-2-pyridylmethyl)amine (L(1)) and bis(6-methyl-2-pyridylmethyl)(2-(2-pyridyl)ethyl)amine (L(2)), and the Mn(II) complex of bis(2-(2-pyridyl)ethyl)(6-methyl-2-pyridylmethyl)amine (L(3)) are described. Addition of aqueous H(2)O(2) to methanol solutions of the Mn(II) complexes of L(1) and L(2) produced green solutions in a fast reaction from which subsequently precipitated brown solids of the dioxo-bridged dinuclear complexes 1 and 2, respectively, which have the general formula [LMn(III)(mu-O)(2)Mn(III)L](ClO(4))(2). Addition of 30% aqueous H(2)O(2) to the methanol solution of the Mn(II) complex of L(3) ([Mn(II)L(3)(CH(3)CN)(H(2)O)](ClO(4))(2) (3)) showed a very sluggish change gradually precipitating an insoluble black gummy solid, but no dioxo-bridged manganese complex is produced. By contrast, the Mn(II) complex of the ligand bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine (L(3a)) has been reported to react with aqueous H(2)O(2) to form the dioxo-bridged Mn(III)Mn(IV) complex. In cyclic voltammetric experiments in acetonitrile solution, complex 1 shows two reversible peaks at E(1/2) = 0.87 and 1.70 V (vs Ag/AgCl) assigned to the Mn(III)(2) Mn(III)Mn(IV) and the Mn(III)Mn(IV) Mn(IV)(2) processes, respectively. Complex 2 also shows two reversible peaks, one at E(1/2) = 0.78 V and a second peak at E(1/2) = 1.58 V (vs Ag/AgCl) assigned to the Mn(III)(2) Mn(III)Mn(IV) and Mn(III)Mn(IV) Mn(IV)(2) redox processes, respectively. These potentials are the highest so far observed for the dioxo-bridged dinuclear manganese complexes of the type of tripodal ligands used here. The bulk electrolytic oxidation of complexes 1 and 2, at a controlled anodic potential of 1.98 V (vs Ag/AgCl), produced the green Mn(IV)(2) complexes that have been spectrally characterized. The Mn(II) complex of L(3) shows a quasi reversible peak at an anodic potential of E(p,a) of 1.96 V (vs Ag/AgCl) assigned to the oxidation Mn(II) to Mn(III) complex. It is about 0.17 V higher than the E(p,a) of the Mn(II) complex of L(3a). The higher oxidation potential is attributable to the steric effect of the methyl substituent at the 6-position of the pyridyl donor of L(3).

The crystal structure of the title compound, [Ni(2)(C(27)H(24)Br(3)N(4)O(3))(CH(3)CO(2))(CH(3)OH)(H(2)O)]·2CH(3)OH·H(2)O contains [L(OAc){(CH(3)OH)Ni}{(H(2)O)Ni}] mol-ecules {H(3)L = 2-(5-bromo-2-hy-droxy-phen-yl)-1,3-bis-[4-(5-bromo-2-hy-droxy-phen-yl)-3-aza-but-3-en-yl]-1,3-imidazolidine} with additional water and two methanol solvent mol-ecules. In this instance, one of the two Ni atoms is coordinated to a water and the other to a methanol mol-ecule. The Ni-O and Ni-N distances, as well as the angles about the metal atoms, show quite regular octa-hedra around the central ions. The Ni-O(phenol)-Ni and Ni-O(acetate)-Ni angles are not similar [95.26 (13) and 97.34 (13)°, respectively], indicating that this subtle solvate exchange induces significant differences in the conformation adopted. The coordinated methanol ligand is involved in an intra-molecular hydrogen bond to the uncoordinated O atom of the bridging acetate ligand, while the coordinated water mol-ecule forms a hydrogen bond with the one of the methanol solvent mol-ecules. The water solvent mol-ecule forms strong hydrogen bonds to both phenolate O atoms. The remaining methanol solvent mol-ecule also forms a hydrogen bond with this solvent water mol-ecule.

The centrosymmetric binuclear complex, [Cu(2)(C(13)H(13)N(2)O)(2)(NCS)(2)], formed via phenolate oxygen bridges, involves the Cu(II) atoms in a distorted square-pyramidal coordination [τ = 0.197 (1)]. A Cu⋯Cu separation of 3.2281 (3) Å is observed. The in-plane Cu-O(phenolate) distance [1.9342 (8) Å] is shorter than the axial distance [2.252 (8) Å]. The Cu-N(amine) and Cu-N(py) distances are similar [2.0095 (10) and 2.0192 (10) Å, respectively]. The Cu-N(thio-cyanate) distance [1.9678 (11) Å] is in the range found for Cu-N distances in previously determined structures containing coordinated thio-cyanate anions. There is an inter-molecular hydrogen bond between the amine H atom and the S atom of a coordinated thio-cyanate anion.

In the crystal of the dinuclear title compound, [Cu(2)(C(13)H(13)N(2)O)(2)(ClO(4))(2)]·2CH(3)CN, the two bridging perchlorate ions chelate to the two Cu(II) atoms in a μ-O:O' fashion on opposite sides of the equatorial plane. The Cu(II) ions display a distorted octa-hedral coordination geometry (in the usual 4 + 2 Jahn-Teller arrangement), each being coordinated by two O atoms from the two perchlorate ligands, and two N and O atoms from the reduced Schiff base ligand. The asymmetric unit contains two acetonitrile solvent mol-ecules. In the crystal structure, in addition to N-H⋯O hydrogen bonds, there are weak C-H⋯O inter-actions between the perchlorate O atoms and the reduced Schiff base ligand. C-H⋯N inter-actions are also present.

In the title compound, [Cu(ClO(4))(2)(C(12)H(13)N(3))(C(2)H(3)N)], the Cu(II) atom is six-coordinate in a Jahn-Teller distorted octahedral geometry, with coordination by the tridentate chelating ligand, an acetonitrile mol-ecule, and two axial perchlorate anions. The tridentate ligand bis-(2-pyridylmeth-yl)amine chelates meridionally and equatorially while an acetonitrile mol-ecule is coordinated at the fourth equatorial site. The two perchlorate anions are disordered with site occupancy factors of 0.72/0.28. The amine H is involved in intra-molecular hydrogen bonding to the perchlorate O atoms and there are extensive but weak inter-molecular C-H⋯O inter-actions.

In the title compound, C24H32N42+·2ClO4−, the two N atoms of the amine are protonated. The H atoms of the protonated amine form hydrogen bonds with the pyridyl N atoms (intramolecular) and O atoms of adjacent perchlorate anions. In addition, there are weak C—H⋯O inter­actions which link the formula units into chains along [101]. Each perchlorate anion is disordered over three conformations, with occupancy factors of 0.482 (2), 0.331 (2) and 0.187 (4), and 0.532 (8), 0.324 (4) and 0.133 (7).