Complex Reduction Chemistry of (abpy)PtCl2, abpy = 2,2‘-Azobispyridine: Formation of Cyclic [(μ,η2:η1-abpy)PtCl]22+with a New Coordination Mode for abpy and a Near-Infrared Ligand-to-Ligand Intervalence Charge Transfer Absorption of the One-Electron Reduced State

Photo-/Electroinduced Irreversible Isomerization of 2,2'-Azobispyridine Ligands in Arene Ruthenium(II) Complexes

Novel arene Ru^II complexes containing 2,2'-azobispyridine ligands were synthesized and characterized by using ¹ H and ¹³ C NMR spectroscopy, UV/vis spectroscopy, electrochemistry, DFT calculations and single-crystal X-ray diffraction. Z-configured complexes featuring unprecedented seven-membered chelate rings involving the nitrogen atom of both pyridines were isolated and were shown to undergo irreversible isomerization to the corresponding E-configured five-membered chelate complexes in response to light or electrochemical stimulus.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl₂ (1a) along with the two hydrogenated complexes of 1a, ZnH₂LCl₂ (1b), and ZnH₄LCl₂ (1c) are reported. It has been argued that the reversible azo-hydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O₂ and para-benzoquinone to H₂O₂ and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.

Syntheses, crystal structures, DNA binding, DNA cleavage and DFT study of Co(iii) complexes involving azo-appended Schiff base ligands

Herein, we have reported three new Co(iii) complexes involving azo-appended Schiff base ligands.

Redox Noninnocent Azo-Aromatic Pincers and Their Iron Complexes. Isolation, Characterization, and Catalytic Alcohol Oxidation

The new redox-noninnocent azoaromatic pincers 2-(arylazo)-1,10-phenanthroline (L¹) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L²) are reported. The ligand L¹ is a tridentate pincer having NNN donor atoms, whereas L² is tetradentate having two azo-N donors and two N-donor atoms from the 1,10-phenanthroline moiety. Reaction of FeCl₂ with L¹ or L² produced the pentacoordinated mixed-ligand Fe(II) complexes FeL¹Cl₂ (1) and FeL²Cl₂ (2), respectively. Homoleptic octahedral Fe(II) complexes, mer-[Fe(L¹)₂](ClO₄)₂ [3](ClO₄)₂ and mer-[Fe(L²)₂](ClO₄)₂ [4](ClO₄)₂, have been synthesized from the reaction of hydrated Fe(ClO₄)₂ and L¹ or L². The ligand L², although having four donor sites available for coordination, binds the iron center in a tridentate fashion with one uncoordinated pendant azo function. Molecular and electronic structures of the isolated complexes have been scrutinized thoroughly by various spectroscopic techniques, single-crystal X-ray crystallography, and density functional theory. Beyond mere characterization, complexes 1 and 2 were successfully used as catalysts for the aerobic oxidation of primary and secondary benzylic alcohols. A wide variety of substituted benzyl alcohols were found to be converted to the corresponding carbonyl compounds in high yields, catalyzed by complex 1. Several control reactions were carried out to understand the mechanism of this alcohol oxidation reactions.

Azobenzene-functionalized iridium(iii) triscyclometalated complexes

Photochromic triscyclometalated iridium(iii) organometallic complexes based on azobenzene-containing phenylpyridyl-type ligands.

Light-responsive three-dimensional microstructures composed of azobenzene-based palladium complexes

We describe not only fleeting assembly of photoisomerizable azobenzene-based palladium complexes into microstructured crystalline architectures but also their light-responsive functions. A transformation in the crystalline morphology from two-dimensional (2D) parallelogram-like sheets to three-dimensional (3D) cuboid- or rhombus-like structures was achieved by changing the solvent from tetrahydrofuran (THF) to acetone and N,N-dimethylformamide (DMF). The sizes of the structures, ranging from a few hundred nanometers to several hundred micrometers, were also modified by varying the complex concentration. In stark contrast to the very stable 2D sheets in the THF-H2O suspensions, exposure of 3D structures in polar DMF-H2O suspensions to ultraviolet (UV) light led to fast disassembly of the structures into isolated metal complexes and further dissociation of free azobenzene ligands from the complexes. In acetone-H2O suspensions, interestingly, disassembly of 3D cuboid-like structures into isolated complex components occurred upon exposure to UV light without further dissociation of azobenzene ligands from the palladium complexes. Considering the photoisomerization ability of the azobenzene-based palladium complex in common organic solvents, the π-stacking interactions that support 3D structures are likely to be sufficiently weak that they might be broken by the UV-induced trans-to-cis isomerization in more polar solvent mixtures. As a consequence, disassembly proceeded under UV light irradiation. Moreover, the effect of solvent polarity on the UV-assisted dissociation (in DMF-H2O) may be associated with the coordination ability of solvent molecules with the metal center.

Surface-enhanced Raman scattering of trans-1,2-bis (4-pyridyl)-ethylene on silver by theory calculations

Surface-enhanced Raman spectra of trans-1,2-bis (4-pyridyl)-ethylene (t-BPE) on silver foil were detected at laser line of 514.5, 633, 785 and 1064 nm, respectively. The structure of Ag-t-BPE, Ag4-t-BPE, Ag6-t-BPE, Ag10-t-BPE and Ag20-t-BPE complexes has been calculated using a local version of the Amsterdam density functional program package. The Raman spectra and electronic polarizability of t-BPE-Ag at 514.5, 633, 785 and 1064 nm excitation lines were calculated. The Raman bands of t-BPE were assigned according to the calculation of potential energy distribution. The experimental and calculated Raman spectra of t-BPE-Ag at 514.5, 633, 785 and 1064 nm were compared. The relative Raman intensities change at different excitation lines were discussed based on the Raman enhanced mechanism and surface selection rules.

2,3-Bis(1-methylimidazol-2-yl)quinoxaline (bmiq), a new ligand with decoupled electron transfer and metal coordination sites: the very different redox behaviour of isoelectronic complexes with [PtCl2] and [AuCl2]+

The new, potentially ambidentate heterocyclic ligand 2,3-bis(1-methylimidazol-2-yl)quinoxaline (bmiq) was obtained from 2,3-bis(1-methylimidazol-2-yl)glyoxal and 1,2-diaminobenzene. Its coordination to PtCl(2) and to the isoelectronic [AuCl(2)](+) in [AuCl(2)(bmiq)](AuCl(4)) occurs via the imine N donors of the imidazolyl groups, leading to the formation of seven-membered chelate rings with boat conformation. According to the spectroelectrochemistry (UV-vis-NIR, EPR), the reversible electron addition to the [PtCl(2)(bmiq)] and the free ligand takes place in the (non-coordinated) quinoxaline part of the molecule, similarly as for related complexes of dipyrido[3,2-a:2',3'-c]phenazines (dppz), 2,3-bis(2-pyridyl)quinoxalines (bpq) and 2,3-bis(dialkylphosphino)quinoxalines (QuinoxP). DFT calculations confirm the experimental results (structures, spectroscopy) and also point to the coordination potential of the quinoxaline N atoms. The electron addition to [AuCl(2)(bmiq)](+) takes place not at the ligand but at the metal site, according to experimental and DFT results.

Syntheses and Structures of [Tb(C6NO2H5)3(H2O)2]2n(3nHgCl4)·(4nH2O) and [La(C6NO2H5)2(C6NO2H4)(H2O)2]n(nZnCl4)·(2nH2O)

Two new transition metal-lanthanide heterometallic complexes with isonicotinic acid and nicotinic acid as ligands, [Tb(C6NO2H5)3(H2O)2]2 n(3 nHgCl4)·(4 nH2O) and [La(C6NO2H5)2(C6NO2H4)(H2O)2] n( nZnCl4)·(2 nH2O), have been synthesised via hydrothermal reactions and structurally characterised by single-crystal X-ray diffraction. Both complexes feature an infinite 1-D polycationic chain-like structure.

Detailed evaluation of the geometric and electronic structures of one-electron oxidized group 10 (Ni, Pd, and Pt) metal(II)-(disalicylidene)diamine complexes

The geometric and electronic structures of a series of one-electron oxidized group 10 metal salens (Ni, Pd, Pt) have been investigated in solution and in the solid state. Ni (1) and Pd (2) complexes of the tetradentate salen ligand N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine (H(2)Salcn) have been examined along with the Pt (3) complex of the salen ligand N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediamine (H(2)Salen). All three oxidized compounds exist as ligand radical species in solution and in the solid state. The solid state structures of [1](+) and [3](+) exhibit a symmetric coordination sphere contraction relative to the neutral forms. By contrast, the coordination sphere of the Pd derivative [2](+) exhibits a pronounced asymmetry in the solid state. In solution, the oxidized derivatives display intense low-energy NIR transitions consistent with their classification as ligand radical compounds. Interestingly, the degree of communication between the phenolate moieties depends strongly on the central metal ion, within the Ni, Pd, and Pt series. Electrochemical measurements and UV-vis-NIR spectroscopy, in conjunction with density functional theory calculations provide insights into the degree of delocalization of the one-electron hole in these systems. The Pd complex [2](+) is the least delocalized and is best described as a borderline Class II/III intervalence complex based on the Robin-Day classification system. The Ni [1](+) and Pt [3](+) analogues are Class III (fully delocalized) intervalence compounds. Delocalization is dependent on the electronic coupling between the redox-active phenolate ligands, mediated by overlap between the formally filled metal d(xz) orbital and the appropriate ligand molecular orbital. The degree of coupling increases in the order Pd < Ni < Pt for the one-electron oxidized group 10 metal salens.

Density functional theory study on surface-enhanced Raman scattering of 4,4'-azopyridine on silver

Surface-enhanced Raman scattering (SERS) of 4,4'-azopyridine (AZPY) on silver foil substrate was measured under 1064nm excitation lines. Density-functional theory (DFT) methods were used to calculate the structure and vibrational spectra of models such as Ag-AZPY, Ag(4)-AZPY and Ag(6)-AZPY complexes with B3LYP/6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set. The Raman bands of AZPY were identified on the ground of analog computation of potential energy distribution. The calculated spectra of Ag(4)-AZPY and Ag(6)-AZPY models were much approximated to the experimental results than that of Ag-AZPY model. The DFT results showed that the angles between two pyridyl rings keep 0 degrees from AZPY to Ag-AZPY, Ag(4)-AZPY and Ag(6)-AZPY model. The energy gaps between the HOMO and LUMO changed from 363 to 1140nm for AZPY-Ag complexes according to the DFT results. An conclusion was conceived that chemical enhancement mechanism may play an important role in the SERS of AZPY on silver substrate.

Synthesis, structure, semiconductive and photoluminescent properties of [{Eu(NC5H4COOH)3(H2O)2}(1.5ZnCl4)·(2H2O)]n

A novel bimetallic 4f-3d metal-isonicotinic acid inorganic-organic hybrid complex [{Eu(NC5H4COOH)3(H2O)2}(1.5ZnCl4)?(2H2O)]n (1) was synthesized via a hydrothermal reaction and structurally characterized by single- crystal X-ray diffraction. Complex 1 crystallizes in the space group C2/c of the monoclinic system with eight formula units in a cell: a = 23.878(8) ?, b = 20.573(6) ?, c = 15.358(5) ?, ? = 127.276(5)?, V = 6003(3) ?3, C18H23Cl6EuN3O10Zn1.5, Mr = 904.11 g/mol, ? = 2.001 g/cm3, S = 1.077, ?(MoK?) = 3.846 mm-1, F(000) = 3536, R = 0.0270 and wR = 0.0672. Complex 1 has a characteristic, one-dimensional polycationic chain-like structure. A photoluminescent investigation revealed that the title complex displays intense emissions in the orange and red regions. The luminescence spectra show that the red emission is stronger than the orange emission. Optical absorption spectra of 1 revealed the presence of an optical gap of 3.56 eV.

Synthesis, Crystal Structure and Properties of [Gd(C6NO2H5)3(H2O)2]n (1.5nZnCl4)·nH2O with Unprecedented Poly-Cationic Chains

A new heterometallic 4f-3d inorganic-organic metal-nicotinic acid complex [Gd(C6NO2H5)3(H2O)2] n(1.5 nZnCl4) nH2O, (1), has been synthesised via hydrothermal reaction and structurally characterised. Complex 1 crystallises in the space group P21/ c of the monoclinic system with four formula units in a cell: a = 9.524(2), b = 20.888(4), c = 16.078(3) Å β = 104.745(2)°, V= 3093.2(9) Å3, C18H21Cl6GdN3O9Zn1.50, M r=891.38 g/mol, D c= 1.914 g/cm3, S = 0.997, μ(MoKα) = 3.844 mm1, F(000) = 1732, R = 0.0493 and wR = 0.1350. Complex 1 is characteristic of a one-dimensional polycationic chain-like structure. Photoluminescent investigation shows that the title complex displays strong emission in the blue region, which is attributed to the intraligand π–π* transition of nicotinic ligands. Optical absorption spectra of 1 reveal the presence of a wide optical bandgap of 3.73 eV.

Low-valence triruthenium compounds via substitution of a bridging acetate in the parent Ru(3)O(OAc)(6) cluster core by 2,2'-azobispyridine (abpy) or 2,2'-azobis(5-chloropyrimidine) (abcp)

Reaction of oxo-centered Ru(3)(III,III,III) precursor [Ru(3)O(OAc)(6)(py)(2)(CH(3)OH)](PF(6)) (1) with 1 equiv of 2,2'-azobispyridine (abpy) or 2,2'-azobis(5-chloropyrimidine) (abcp) induced the formation of stable Ru(3)(III,III,II) derivatives [Ru(3)O(OAc)(5){mu-eta(1)(N),eta(2)(N,N)-L}(py)(2)](PF(6)) (L = abpy (2), abcp (3)). As established in the structure of 3 by X-ray crystallography, 2 or 3 is derived from 1 by substitution of the axial methanol and one of the bridging acetates in the parent Ru(3)O(OAc)(6) cluster core with abpy or abcp in an mu-eta(1)(N),eta(2)(N,N) bonding mode. Reduction of 3 by hydrazine induces isolation of one-electron reduced neutral Ru(3)(III,II,II) product Ru(3)O(OAc)(5){mu-eta(1)(N),eta(2)(N,N)-abcp}(py)(2) (3a). As revealed by electrochemical and spectroscopic studies, substituting one of the bridging acetates in the parent Ru(3)O(OAc)(6) cluster core by abcp or abpy modifies dramatically the electronic and redox characteristics in the triruthenium derivatives. Relative to that for the parent compound [Ru(3)O(OAc)(6)(py)(3)](PF(6)) (E(1/2) = -0.46 V), triruthenium-based redox potential in the redox process Ru(3)O(III,III,III)/Ru(3)O(III,III,II) is significantly anodic-shifted to E(1/2) = +0.36 V for 2 and E(1/2) = +0.53 V for 3. Furthermore, the anodic shifts of redox potentials are progressively enhanced with a decrease of the formal oxidation states in the triruthenium cluster cores. As a consequence of remarkable positive shifts for redox potentials, the low-valence Ru(3)(III,III,II) and Ru(3)(III,II,II) species are stabilized and accessible.

Reversibly Reducible cis-Dichloroplatinum(II) and cis-Dichloropalladium(II) Complexes of Bis(1-methylimidazol-2-yl)glyoxal

Complexes cis-MCl2(big), big=bis(1-methylimidazol-2-yl)glyoxal, M=Pt, Pd, were prepared and characterized through electrochemistry, spectroscopy, and for M=Pt, by X-ray structure analysis. The seven-membered chelate ring formed through N,N' coordination of the ligand big shows a boat conformation in agreement with density functional theory (DFT) calculation results. No significant intermolecular interactions were observed for the platinum compound. Both the PdII and PtII complexes undergo reversible one-electron reduction in CH2Cl2/ 0.1 M Bu4NPF6; the reduced palladium compound disintegrates above -30 degrees C. Electron paramagnetic resonance (EPR), UV-vis, and IR spectroelectrochemistry studies were employed to study the monoanions. The anion radical complex [cis-PtCl2(big)]*- exhibits a well-resolved EPR spectrum with small but well-detectable g anisotropy and an isotropic 195Pt hyperfine coupling of 12.2 G. DFT calculations confirm the spin concentration in the alpha-semidione part of the radical complex with small delocalization to the bis(imidazolyl)metal section. The results show that EPR and electroactive moieties can be linked to the cis-dichloroplatinum(II) group via imidazole coordination.

Surface-enhanced Raman spectroscopy and density functional theory study on 4,4'-bipyridine molecule

The molecular geometry and vibrational frequencies of 4,4'-bipyridine (BPE) in the ground state were calculated using density functional theory (DFT) methods (B3LYP) with 6-31++G(d,p) basis set. The optimized geometric bond lengths and bond angles are obtained by DFT employing the hybrid of Beckes non-local three parameter exchange and correlation functional and Lee-Yang-Parr correlation functional. Fourier transform infrared (FT-IR), Fourier transform Raman (FT-Raman) and near-infrared surface-enhanced Raman scattering (NIR-SERS) spectra of BPE on the silver foil substrate have been recorded. All FT-IR, FT-Raman and NIR-SERS band were assigned on the basis of the B3LYP/6-31++G(d,p) method. The vibrational frequencies obtained by DFT(3LYP) are in good agreement with observed results. The NIR-SERS of BPE excited by 1064nm laser line is little difference with that excited by visible laser line. This phenomenon is result to the increase of the contribution of CHEM enhancement effect. Surface selection rules derived from the electromagnetic enhancement model were employed to infer the orientations of BPE on the silver foil substrate surface. Some vibrational frequency which are sensitive to the planar or non-planar structure of BPE, and to the dihedral angle were concluded.