Dianionic Tridentate Research Articles

N‐(Aryl)picolinamide Complexes of Ruthenium: Usual Coordination and Strategic Cyclometalation

AbstractReaction of five N‐(4‐R‐phenyl)picolinamides (R = OCH3, CH3, H, Cl, and NO2) with [Ru(PPh3)2(CO)2Cl2] in refluxing 2‐methoxyethanol in the presence of a base (NEt3) affords two geometrical isomers of a group of complexes (1‐R and 2‐R), each of which contains an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphanes, a carbonyl, and a hydride. Similar reaction of N‐(naphthyl)picolinamide with [Ru(PPh3)2(CO)2Cl2] affords an organometallic complex, 3, in which the amide ligand is coordinated to the metal center, by C–H activation of the naphthyl ring at the 8‐position, as a dianionic tridentate N,N,C donor along with two triphenylphosphanes and one carbonyl. Structures of the 1‐OCH3, 2‐CH3, and 3 complexes have been determined by X‐ray crystallography. In all the complexes the two triphenylphosphanes are trans. In the 1‐R complexes the hydride is trans to the pyridine nitrogen and in the 2‐R complexes it is trans to the amide‐nitrogen. All the complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a RuII–RuIII oxidation within 0.71–0.93 V versus SCE. An oxidation and a reduction of the coordinated amide ligand are also observed within 1.29–1.69 V versus SCE and –1.02 to –1.21 V versus SCE respectively.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Synthesis, structure and spectroscopic properties of some thiosemicarbazone complexes of platinum

Reaction of salicyldehyde thosemicarbazone (H2L1), 2-hydroxyacetophenone thiosemicarbazone (H2L2) and 2-hydroxynapthaldehyde thiosemicarbazone (H2L3) (general abbreviation H2L, where H2 stands for the two dissociable protons, one phenolic proton and one hydrazinic proton) with K2[PtCl4] afforded a family of polymeric complexes of type [{Pt(L)}n]. Reaction of the polymeric species with two monodentate ligands (D), viz. triphenylphosphine (PPh3) and 4-picoline (pic), yielded complexes of the type [Pt(L)(D)]. These mixed-ligand complexes were also obtained from the reaction of the thiosemicarbazones with [Pt(PPh3)2Cl2] and [Pt(pic)2Cl2]. The crystal structure of [Pt(PPh3)(L2)] has been determined. The thiosemicarbazone ligands are coordinated, via dissociation of the two protons, as dianionic tridentate O,N,S-donors. The [Pt(L)(D)] complexes show characteristic 1H NMR spectra and intense absorptions in the visible and ultraviolet region. They also fluoresce in the visible region at ambient temperature.

First example of pyrrole-2-carbaldehyde thiosemicarbazone as tridentate dianion in [Pd(η3-N4,N3,S-ptsc)(PPh3)] complex

Reaction of pyrrole-2-carbaldehyde thiosemicarbazone (H2ptsc) with PdCl2(PPh3)2 in 1:1mol ratio in presence of Et3N leads to complete dechlorination to generate square planar complex, [Pd(η3-N4,N3,S-ptsc)(PPh3)] (1), existing as three independent molecules in the same unit cell, and H2ptsc behaves in an unusual dinegative tridentate mode. Salicylaldehyde thiosemicarbazone (H2stsc) formed similar compound, [Pd(η3-O,N3,S-stsc)(PPh3)] (3).

Synthesis, structure and electrochemical properties of some thiosemicarbazone complexes of iridium

Reaction of thiosemicarbazones of salicylaldehyde and 2-hydroxyacetophenone (H2L1 and H2L2) with [Ir(PPh3)3Cl] affords complexes of type [Ir(PPh3)2(L)(H)] (L = L1 or L2) in ethanol. A similar reaction carried out in toluene affords the [Ir(PPh3)2(L)(H)] complexes along with complexes of type [Ir(PPh3)2(L)Cl], where a chloride is coordinated to iridium instead of the hydride. The structure of the [Ir(PPh3)2(L2)(H)] and [Ir(PPh3)2(L2)Cl] complexes has been determined by X-ray crystallography. Crystal data for [Ir(PPh3)2(L2)(H)]: space group, P21/c; crystal system, monoclinic; a=12.110(2) A, b=17.983(4) A, c=18.437(4) A, β=103.42(3)°, Z=4; R1=0.0591, wR2=0.1107. Crystal data for [Ir(PPh3)2(L2)Cl]: space group, P21/c; crystal system, monoclinic; a=17.9374(11) A, b=19.2570(10) A, c=24.9135(16) A, β=108.145(5)°, Z=4; R1=0.0463, wR2=0.0901. In all the complexes the thiosemicarbazones are coordinated to the metal center as dianionic tridentate O, N, S-donors and the two triphenylphosphines are trans. The complexes are diamagnetic (low-spin d 6, S=0) and show intense MLCT transitions in the visible region. Cyclic voltammetry on all the [Ir(PPh3)2(L)(H)] and [Ir(PPh3)2(L)Cl] complexes shows a quasi-reversible Ir(III)–Ir(IV) oxidation within 0.55–0.78 V vs. SCE followed by an irreversible oxidation of the thiosemicarbazone within 0.91–1.27 V vs. SCE. An irreversible reduction of the thiosemicarbazone is also observed within −1.10 to −1.23 V vs. SCE.

Extraction Behavior of Copper(II) Cation Using Sulfonamide-type Polyanionic Chelators Having Ethylene Bridges

To research possible use of sulfonamide-type polyanionic chelators having ethylene bridges as extractants, extraction (cation-exchange) behavior of Cu2+ with several chelators was investigated. Tripod sulfonamide-type chelator, tris [2- (p-toluenesulfonamido) ethyl] amine, showed higher extractability than its monosulfonamide analog, 2- (p-toluenesulfonamido) ethylamine. Although the former can be generally considered as trianionic tetradentate chelator, on extraction of Cu2+, it acted as dianionic tridentate ligand and formed neutral 1: 1 complex having one coordinating H2O. The formed complex seems to be stabilized by chelate effect of the ligand and shielding effect of the coordinating sulfonamido group.

Synthesis and characterization of four new manganese(III) complexes and amino acid (L-aspartic acid, L-asparagine, L-glutamic acid, L-glutamine) Schiff bases

New series of manganese(III) complexes and amino acid Schiff bases have been prepared from 2-hydroxy-1-naphthaldehyde and α-amino acids [L-aspartic acid (Asp), L-asparagine (Asn), L-glutamic acid (Glu) and L-glutamine (Gln)]. The structures of the ligands and manganese complexes were identified using elemental analyses, i.r, electronic spectra, 1H-n.m.r spectra, magnetic moment measurements and thermogravimetric analyses (t.g.a). The results suggest that H2L1: [N-(2-hydroxy-1-naphthylidene) aspartic acid] and H2L3: [N-(2-hydroxy-1-naphthylidene)glutamic acid] Schiff bases behave as trianionic tetradentate species and coordinate to Mn(III) ion according to the general formula [MnL] · xH2O complexes. But, H2L2: [N-(2-hydroxy-1-naphthylidene) asparagine] and H2L4: [N-(2-hydroxy-1-naphthylidene)glutamine] Schiff bases behave as dianionic tridentate and coordinate to Mn(III) ion in the general formula for [MnL(OOCH3)] · xH2O complexes.

Intramolecular Apical C–H···M Interactions in Square‐Planar Nickel(II) Complexes with Dianionic Tridentate Ligands and 2‐Phenylimidazole

AbstractTwo square‐planar ternary nickel(II) complexes [Ni(bhac)(phim)] (1) and [Ni(ahac)(phim)] (2) with O,N,O‐donor Schiff bases [acetylacetone benzoylhydrazone (H2bhac) and acetylacetone acetylhydrazone (H2ahac)] and sp2 N‐donor 2‐phenylimidazole (phim) as the ancillary ligand have been synthesised and characterised by analytical, spectroscopic, magnetic and electrochemical methods. The solid‐state structures of both complexes have been determined by X‐ray crystallography. The asymmetric unit of 1 contains a single complex molecule while that of 2 contains four complex molecules with different conformations. The molecular structures of both complexes reveal that one of the two ortho C–H groups of the pendant phenyl ring of the phim ligand is very close to the metal centre at the apical site indicating the presence of an intramolecular C–H···Ni interaction. In the structures of 1 and 2, the shape of the C–H···Ni interaction is determined by the twisting of the phim ligand and the extent of orthogonality between the planes containing the chelate rings formed by the tridentate ligand and the imidazole ring. A survey of the reported X‐ray structures of similar d8 metal ion complexes containing intramolecular C–H···M interactions has been performed for comparison with the structural features of 1 and 2. The optimised molecular structures of 1 and 2 generated by calculations based on the density functional method are compared with the experimental structures. Calculations also reveal that the four molecules present in the asymmetric unit of 2 are very close in energy. Theoretical studies and the down‐field shift of the proton resonance on cooling in the NMR spectra suggest three‐centre four‐electron hydrogen‐bond character of the observed apical C–H···Ni interaction in 1 and 2. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

(5-Methoxy-2-oxidoacetophenone 4-methylthiosemicarbazonato-κ3O,N,S)pyridinenickel(II)

In the title compound, [Ni(C11H13N3O2S)(C5H5N)], the NiII ion is in a distorted square-planar geometry, with the C11H13N3O2S ligand coordinated in a dianionic tridentate fashion; the pyridine mol­ecule occupies the fourth position. The mol­ecules are linked by N—H⋯S inter­actions about an inversion center.

Synthesis, structure and catalytic activity of cycloruthenated carbonyl complexes containing arylazo phenolate ligands

A simple and convenient synthesis of a new class of air-stable mononuclear cyclometallated ruthenium(II) carbonyl 2-(arylazo)phenolate complexes bearing triphenylarsine [Ru(ap-R)(AsPh 3) 2(CO)] has been described. The 2-(arylazo)phenolate ligands behave as dianionic tridentate ligands and are coordinated to ruthenium through C, N and O by dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring forming two five-membered chelate rings. These complexes have been characterized by elemental analysis, and FT-IR, 1H NMR and UV–vis spectroscopies. In dichloromethane solution all the metal complexes exhibit characteristic metal-to-ligand charge transfer (MLCT) absorption and emission bands in the visible region. One of the complexes [Ru(ap-Cl)(AsPh 3) 2(CO)] was successfully characterized by X-ray crystallography. Cyclic voltammetric data of all the complexes show Ru III/Ru II oxidation and Ru II/Ru I reduction within the range of +0.75 to +0.86 V and −0.50 to −0.57 V respect to Ag/AgCl, respectively. The potentials are observed with respect to the electronic nature of substituents (R) in the 2-(arylazo)phenolate ligands. Further, a complex ( 2) was tested as a new catalyst in the oxidation of primary and secondary alcohols in the presence of NMO as a more viable oxidant with moderate to high conversion. The formation of high valent Ru IV = O species as a catalytic intermediate is proposed for the catalytic process.

Rhodium assisted C [sbnd]H activation of N-(2′-hydroxyphenyl)benzaldimines. Synthesis, structure and electrochemical properties of a group of organorhodium complexes

Reaction of a group of N-(2′-hydroxyphenyl)benzaldimines, derived from 2-aminophenol and five para-substituted benzaldehydes (the para substituents are OCH 3, CH 3, H, Cl and NO 2), with [Rh(PPh 3) 3Cl] in refluxing toluene in the presence of a base (NEt 3) afforded a family of organometallic complexes of rhodium(III). The crystal structure of one complex has been determined by X-ray crystallography. In these complexes the benzaldimine ligands are coordinated to the metal center, via dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring in the imine fragment, as dianionic tridentate C,N,O-donors, and the two PPh 3 ligands are trans. The complexes are diamagnetic (low-spin d 6, S = 0) and show intense MLCT transitions in the visible region. Cyclic voltammetry shows a Rh(III) Rh(IV) oxidation within 0.63–0.93 V vs SCE followed by an oxidation of the coordinated benzaldimine ligand. A reduction of the coordinated benzaldimine is also observed within −0.96 to −1.04 V vs SCE. Potential of the Rh(III) Rh(IV) oxidation is found to be sensitive to the nature of the para-substituent.

2-Oxido-1-naphthaldehyde (2-hydroxybenzoyl)hydrazonato]pyridinecopper(II)

The approximately planar complex molecule, [Cu(C18H12N2O3)(C5H5N)], contains one L 2− ligand (L 2− is the dianion of 2-hydroxy-1-naphthaldehyde 2-hydroxybenzoylhydrazone), one Cu atom and one pyridine molecule. The Cu centre shows square-planar N2O2Cu coordination. The tridentate dianion has an intramolecular N...H—O hydrogen bond. Each pair of adjacent molecules is linked together by π–π stacking and Cu...N interactions, which lead to the existence of a dimer. Owing to C—H...O hydrogen bonds, these dimers are further assembled into a two-dimensional framework.

Synthesis and Structural Characterization of Neutral Higher‐Coordinate Silicon(iv) Complexes with Tridentate Dianionic Chelate Ligands

The neutral pentacoordinate silicon(IV) complexes 8 and 9 with an SiO2N3 skeleton and the neutral hexacoordinate silicon(IV) complex 10.1/2 CH3CN with an SiO4N2 skeleton were synthesized, starting from tetra(cyanato-N)silane or tetra(thiocyanato-N)silane. Compounds 8 and 9 contain one tridentate dianionic ligand derived from 4-[(2-hydroxyphenyl)amino]pent-3-en-2-one and two monodentate singly charged cyanato-N or thiocyanato-N ligands bound to the silicon(IV) coordination center, whereas the silicon(IV) center of 10 is coordinated by two of these tridentate dianionic ligands. All compounds were characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of the compounds studied, the experimental investigations were complemented by computational studies.

Synthesis and structure of cycloruthenated carbonyl complexes and their emission, redox and biological properties

An interesting series of mononuclear organoruthenium complexes of formulation [Ru(CO)(PPh 3) 2(ap-R)] (where ap-R = –H, –Cl, –Me, –OMe, –OEt) have been synthesized from the reaction of five 2-(arylazo)phenol ligands with ruthenium(II) precursor [RuH(Cl)(CO)(PPh 3) 3] in benzene under reflux. The 2-(arylazo)phenolate ligands behave as dianionic tridentate ligand and are coordinated to ruthenium through C, N and O by dissociation of the phenolic and phenyl proton at the ortho position of the phenyl ring forming two five-membered chelate rings. These complexes have been characterized by elemental analysis, FT-IR, 1H NMR and UV–visible spectroscopy. In dichloromethane solution all the metal complexes exhibit characteristic metal-to-ligand charge transfer (MLCT) absorption and emission bands in the visible region. The structures of [Ru(CO)(PPh 3) 2(ap-H)] and [Ru(CO)(PPh 3) 2(ap-Cl)] have been determined by X-ray crystallography. Cyclic voltammetric data of all the complexes show a Ru III/Ru II oxidation and reduction Ru II/Ru I within the range 0.74–0.84 V and −0.38 to −0.50 V vs saturated calomel electrode (SCE) respectively. The potentials are observed with respect to the electronic nature of substituents (R) in the 2-(arylazo)phenolate ligands. Further, the free ligands and their ruthenium complexes have also been screened for their antibacterial and antifungal activities, which have shown great promise in inhibiting the growth of both Gram +ve and Gram −ve bacteria Staphylococcus aureus and Escherichia coli and fungus Candida albicans and Aspergillus niger. These results made it desirable to delineate a comparison between free ligands and their complexes.

Metal complexes derived from hydrazoneoxime ligands: II. Synthesis, electrospray mass spectra and magnetochemical studies of some copper(II) complexes derived from p-substituted aroylhydrazoneoximes

Mono- and binuclear copper(II) complexes derived from substituted aroylhydrazoneoximes of the general formulae [Cu(H2LR)Cl2]·nH2O, [Cu(HLR)Cl], [{Cu(HLR)}2]·2NO3·nH2O and [{Cu(LR)}2]·nH2O have been prepared and characterized, where H2LR, HLR and LR refer, respectively, to the neutral, monoanionic and dianionic ONN tridentate aroylhydrazoneoxime ligands. Electrospray ionization (ESI) mass spectra revealed the formation of tri- and tetranuclear copper(II) complexes in dimethylformamide (DMF) or dimethysulphoxide (DMSO) solutions. The effect of substitution in the aroylhydrazone residue on the degree of deprotonation of the ligand and the energies of d–d transitions of the copper(II) complexes have been studied. Tuning of the antiferromagnetic exchange coupling by different substituents in [{Cu(HLR)}2]·2NO3·nH2O and [{Cu(LR)}2]·nH2O complexes have been discussed.

Synthesis, structure, catalytic transfer hydrogenation and biological activity of cyclometallated ruthenium(III)2-(arylazo)phenolate complexes

A series of mononuclear organoruthenium complexes of the type [RuX(PPh 3) 2(L)] (X = Cl or Br; L = 2-(arylazo)phenolate ligand) have been synthesized from the reaction of five 2-(arylazo)phenol ligands with ruthenium(III) precursors, viz. [RuCl 3(PPh 3) 3] and [RuBr 3(PPh 3) 2(CH 3OH)] in benzene under reflux. In all these reactions, the 2-(arylazo)phenolate ligand replaces one triphenylphosphine molecule, two chlorides or bromides and one methanol from the precursors leading to five-membered cyclometallated species. The 2-(arylazo)phenol ligands behave as dianionic tridentate C, N, O donors and coordinated to ruthenium by dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring. The compositions of the complexes have been established by elemental analysis, magnetic susceptibility measurement, FT-IR, UV–Vis and EPR spectral data. These complexes are paramagnetic and shows intense d–d and charge transfer transitions in chloroform. The solution EPR spectrum of the complex 7 in dichloromethane at 77 K shows rhombic distortion around the ruthenium ion. The structural conformation of the complex 1 has been carried out by X-ray crystallography. The redox behavior of the complexes has been investigated by cyclic voltammetry and the potentials are observed with respect to the electronic nature of substituents (R) in the 2-(arylazo) phenolate ligands. These complexes catalyze transfer hydrogenation of benzophenone to benzhydrol with up to 99.5% in the presence of i-prOH/KOH. Further, these complexes have shown great promise in inhibiting the growth of both Gram +ve and Gram −ve bacteria, viz. Staphylococcus aureus NCIM 2079 and Escherichia coli NCIM 2065 and fungus Candida albicans NCIM 3102.

Iridium mediated methyl and phenyl C–H activation of 2-(arylazo)phenols. Synthesis, structure, and spectral and electrochemical properties of some organoiridium complexes

Reaction of 2-(2′,6′-dimethylphenylazo)-4-methylphenol with [Ir(PPh 3) 3Cl] in refluxing ethanol in the presence of a base (NEt 3) affords an organoiridium complex 5, where the 2-(2′,6′-dimethylphenylazo)-4-methylphenol is coordinated to iridium, via C–H activation of a methyl group, as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. A similar reaction carried out in toluene affords complex 5 along with a similar complex 7, where a chloride is coordinated to iridium instead of the hydride. Reaction of 2-(2′-methylphenylazo)-4-methylphenol with [Ir(PPh 3) 3Cl] in refluxing ethanol in the presence of a base (NEt 3) affords an organoiridium complex 12, where the 2-(2′-methylphenylazo)-4-methylphenol is coordinated to iridium, via C–H activation at the ortho position of the phenyl group in the 2′-methylphenylazo fragment, as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. A similar reaction carried out in toluene affords a complex 12 along with a similar complex 13, where a chloride is coordinated to iridium instead of the hydride. Structures of complexes 5, 12 and 13 have been determined by X-ray crystallography. In all these complexes, the two triphenylphosphines are trans. All these complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows an Ir(III)–Ir(IV) oxidation within 0.60–0.73 V vs. SCE, followed by an oxidation of the coordinated 2-(arylazo)phenolate ligand within 1.08–1.39 V vs. SCE. A reduction of the coordinated 2-(arylazo)phenolate ligand is observed within −1.10 to −1.26 V vs. SCE.

Syntheses and crystal structures of a tridentate enaminone and its copper complex

Reaction of ferrocenoylacetone [C5H5FeC5H4C(O)CH2C(O)CH3] and 2-aminoethanol yields a new ferrocene-containing ligand (H 2 L) that exists in the form of an enaminone formulated as [C5H5FeC5H4C(O)CH=C(NHCH2CH2OH)CH3]. Reaction of the enaminone H 2 L with copper acetate affords complex 1. X-ray crystallography of 1 reveals that it has a tetrameric structure composed of four CuL subunits linked by bridging alkoxide oxygen atoms in which the enaminone acts as a dianionic tridentate ligand and each of the four copper atoms is in a distorted square-pyramidal coordination environment.

Coordination Modes between Copper(II) and N-Acetylneuraminic (Sialic) Acid from a 2D-Simulation Analysis of EPR Spectra. Implications for Copper Mediation of Sialoglycoconjugate Chemistry Relevant to Human Biology

The equilibrium distribution of species formed between Cu(II) and N-acetylneuraminic (sialic) acid (I, LH) at 298 K has been determined using a two-dimensional (2D) simulation analysis of electron paramagnetic resonance (EPR) spectra. In acidic solutions (pH values < 4), the major species present are Cu(2+), [CuL]+ [logbeta = 1.64(4)], and [CuL2] [logbeta = 2.77(5)]. At intermediate pH values (4.0 < pH < 7.5), [CuL2H-1]- [logbeta = -2.72(7)] and two isomers of [CuLH-1] [logbeta (overall) = -3.37(2)] are present. At alkaline pH values (7.5 < pH < 11), the major species present is [CuL2H-2]2-, modeled as three isomers with unique giso and Aiso values [logbeta (overall) = -8.68(3)]. Two further species ([CuLH-3]2- and [CuL2H-3]3-) appear at pH values > 11. It is proposed that [CuL]+ most likely features I coordinated via the deprotonated carboxylic acid group (O1) and the endocyclic oxygen atom (OR) forming a five-membered chelate ring. Select Cu(II)-I species of the form [CuLH-1] may feature I acting as a dianionic tridentate chelate, via oxygen atoms derived from O1, OR, and one deprotonated hydroxy group (O7 or O8) from the glycerol tail. Alternatively, I may coordinate Cu(II) in a bidentate fashion as the tert-2-hydroxycarboxylato (O1,O2) dianion. Spectra predicted for Cu(II)-I complexes in which I is coordinated in either a O1,OR {I1-} or O1,O2 {I2-} bidentate fashion {e.g., [CuL]+ (O1,O R), [CuL2] (bis-O1,O R), [CuLH-1] (isomer: O1, O2), [CuL2H-1]- (O1, O R; O1, O2), and [CuL2H-2]2- (isomer: bis-O1, O2)} have "irregular" EPR spectra that are ascribed to the existence of Cu(II)-I(monomer) <==> Cu(II)-I(polymer) equilibria. The formation of polymeric Cu(II)-I species will be favored in these complexes because the glycerol-derived hydroxyl groups at the complex periphery (O, 7O, 8O9) are available for further Cu(II) binding. The presence of polymeric Cu(II)-I species is supported by EPR spectral data from solutions of Cu(II) and the homopolymer of I, colominic acid (Ipoly). Conversely, spectra predicted for Cu(II)-I complexes where I is coordinated in a {I2-} tridentate {e.g., [CuLH-1] (isomer: O1, O R, O7, or O8) and [CuL2H-2]2- (isomer: bis-O1,O R,O7, or O8)} or tetradentate fashion {I3-} {e.g., [CuLH-3]2- (O1, O R, O, 8O9)} are typical for mononuclear tetragonally elongated Cu(II) octahedra. In this latter series of complexes, the tendency toward the formation of polymeric Cu(II)-I analogues is small because the polydentate I effectively wraps up the mononuclear Cu(II) center. This work shows that Cu(II) could potentially mediate the chemistry of sialoglycoconjugate-containing proteins in human biology, such as the sialylated amyloid precursor protein of relevance to Alzheimer's disease.

Rhodium and iridium complexes of N-(2′-hydroxyphenyl)pyrrole-2-aldimine: Synthesis, structure, and spectral and electrochemical properties

Reaction of N-(2′-hydroxyphenyl)pyrrole-2-aldimine (H2L) with [M(PPh3)3Cl] (M = Rh, Ir) affords complexes of type [M(PPh3)2(L)Cl]. Structures of both complexes have been determined by X-ray crystallography. In both complexes, the N-(2′-hydroxyphenyl)pyrrole-2-aldimine ligand is coordinated to the metal centre, via dissociation of the acidic protons, as a dianionic tridentate N,N,O-donor. The M(L)Cl fragment constitutes an equatorial plane with the metal at the centre and the two PPh3 ligands occupying axial positions. The complexes are diamagnetic (low-spin d6, S = 0) and show intense MLCT transitions in the visible region. Cyclic voltammetry of the [M(PPh3)2(L)Cl] complexes shows reversible M(III)-M(IV) oxidation near 0·5 V vs SCE and quasi-reversible ligand-centred oxidative response near 1·0 V vs SCE.

Synthesis, characterization and magnetochemical studies of some imidazole and imidazolate copper(II) complexes derived from N-salicylideneacetylhydrazine: X-ray crystal and molecular structures of [imidazole-(N-salicylidenatoacetylhydrazinato)-ONO′ (-2)]copper(II) and potassium [mono(μ-imidazolate) bis(N-salicylidenatoacetylhydrazinato)] dicopper(II)

Three new copper(II) complexes, [Cu(Sah)HIm], [Cu(HSah)Cl·HIm] and K[{Cu(Sah)}2Im], have been prepared and characterized by elemental analyses, infrared and ultraviolet–visible spectroscopy (HSah and Sah refer to mono- and dinegative N-salicylideneacetylhydrazine anions, respectively, while HIm and Im refer to neutral imidazole and mononegative imidazolate anion). The X-ray crystal and molecular structures of [Cu(Sah)HIm] and K[{Cu(Sah)}2Im] have been determined. In [Cu(Sah)HIm], the copper(II) atom is in a distorted square-planar environment and coordinated to the hydrazone ligand via phenoxy oxygen, imine nitrogen and deprotonated enolimine oxygen. The fourth coordination site is occupied by the imidazole nitrogen. The imidazole N–H is involved in an intermolecular hydrogen bond with the hydrazide nitrogen forming a 1-D supramolecular structure. In the (μ-imidazolate) dicopper(II) complex K[{Cu(Sah)}2Im], each copper(II) atom is in a distorted square-planar environment where the hydrazone ligand acts as an ONO tridentate dianion. The imidazolate anion acts as a bridged coligand connecting the two tricoordinated copper(II) units. The K+ is six coordinate, coordinated to four (μ-imidazolate) dicopper(II) units giving rise to a supramolecular network. Both [Cu(Sah)HIm] and [Cu(HSah)Cl·HIm] show normal magnetic moments of 1.82 and 1.85 µβ respectively. The dicopper complex K[{Cu(sah)}2Im] shows antiferromagnetic spin–spin coupling mediated via the μ-imidazolate with −J = 17.0 cm−1.