Molecular and electronic structures of one-electron oxidized Ni(II)-(dithiosalicylidenediamine) complexes: Ni(III)-thiolate versus Ni(II)-thiyl radical states

The dithiosalicylidenediamine Ni II complexes [Ni(L)] (R=tBu, R'=CH2C(CH3)2CH2 1, R'=C6H4 2; R=H, R'=CH2C(CH3)2CH2 3, R'=C6H4 4) have been prepared by transmetallation of the tetrahedral complexes [Zn(L)] (R=tBu, R'=CH2C(CH3)2CH2 7, R'=C6H4 8; R=H, R'=CH2C(CH3)2CH2 9, R'=C6H4 10) formed by condensation of 2,4-di-R-thiosalicylaldehyde with diamines H2N-R'-NH2 in the presence of Zn II salts. The diamagnetic mononuclear complexes [Ni(L)] show a distorted square-planar N2S2 coordination environment and have been characterized by 1H- and 13C NMR and UV/Vis spectroscopies and by single-crystal X-ray crystallography. Cyclic voltammetry and coulombic measurements have established that complexes 1 and 2, incorporating tBu functionalities on the thiophenolate ligands, undergo reversible one-electron oxidation processes, whereas the analogous redox processes for complexes 3 and 4 are not reversible. The one-electron oxidized species, 1+ and 2+, can be generated quantitatively either electrochemically or chemically with 70 % HClO4. EPR and UV/Vis spectroscopic studies and supporting DFT calculations suggest that the SOMOs of 1+ and 2+ possess thiyl radical character, whereas those of 1(py)2 + and 2(py)2 + possess formal Ni III centers. Species 2+ dimerizes at low temperature, and an X-ray crystallographic determination of the dimer [(2)2](ClO4)2.2 CH2Cl2 confirms that this dimerization involves the formation of a S-S bond (S...S=2.202(5) A).     

One-electron oxidized nickel(II)-(disalicylidene)diamine complex: temperature-dependent tautomerism between Ni(III)-phenolate and Ni(II)-phenoxyl radical states

The one-electron oxidized species of a Ni(II)-phenolate complex has been shown to be in the Ni(II)-phenoxyl radical state at room temperature and the Ni(III)-phenolate state at < -120 degrees C, indicating that the oxidation state is temperature dependent.     

Homogeneous and heterogeneous olefin epoxidation catalyzed by a binuclear Mn(II)Mn(III) complex

The synthesis and characterization of a binuclear carboxylated bridged manganese complex containing the heptadentate ligand N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-2-ol-1,3-propanediamine (H₃bbppnol) is reported. This complex was characterized by elemental analysis; infrared, electronic (UV–vis) and EPR spectroscopy; and conductivity measurements. The complex was immobilized on silica by either adsorption or entrapment via a sol–gel route. The obtained solids were characterized by thermogravimetric analyses (TG and DSC), UV–vis and infrared spectroscopy, and X-ray diffraction. The catalytic performance of the binuclear manganese complex in epoxidation reactions was evaluated for both homogeneous and heterogeneous systems. The catalytic investigation revealed that the complex performs well as an epoxidation catalyst for the substrates cyclohexene (26–39%) and cyclooctene (29–74%). The solids containing the immobilized complex can be recovered from the reaction medium and reused, maintaining good catalytic activity.     

Oxamato-bridged trinuclear Ni(II)Cu(II)Ni(II) complexes with irregular spin state structures and a binuclear Ni(II)Cu(II) complex with an unusual supramolecular structure: crystal structure and magnetic properties

Four oxamato-bridged heterotrinuclear Ni(II)Cu(II)Ni(II) complexes of formula ([Ni(bispictn)](2)Cu(pba))(ClO(4))(2).2.5H(2)O (1), ([Ni(bispictn)](2)Cu(pbaOH))(ClO(4))(2).H(2)O (2), ([Ni(cth)](2)Cu(pba))(ClO(4))(2) (3), and ([Ni(cth)](2)Cu(opba))(ClO(4))(2).H(2)O (4) and a binuclear Ni(II)Cu(II) complex of formula [Cu(opba)Ni(cth)].CH(3)OH (5) have been synthesized and characterized by means of elemental analysis, IR, ESR, and electronic spectra, where pba = 1,3-propylenebis(oxamato), pbaOH = 2-hydroxyl-1,3-propylenebis(oxamato), opba = o-phenylenebis(oxamato), bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, and cth = rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. The crystal structures of 1, 3, and 5 have been determined. The structures of complexes 1 and 3 consist of trinuclear cations and perchlorate anions, and that of 5 consists of neutral binuclear molecules which are connected by hydrogen bonds and pi-pi interactions to produce a unique supramolecular "double" sheet. In the three complexes, the copper atom in a square-planar or axially elongated octahedral environment and the nickel atom in a distorted octahedral environment are bridged by the oxamato groups, with Cu.Ni separations between 5.29 and 5.33 A. The magnetic properties of all five complexes have been investigated. The chi(M)T versus T plots for 1-4 exhibit the minimum characteristic of antiferromagnetically coupled NiCuNi species with an irregular spin state structure and a spin-quartet ground state. The chi(M)T versus T plot for 5 is typical of an antiferromagnetically coupled NiCu pair with a spin-doublet ground state. The Ni(II)-Cu(II) isotropic interaction parameters for the five complexes were evaluated and are between 102 and 108 cm(-)(1) (H = -JS(Cu).S(Ni)).     

Crystal structure of a binuclear polymeric self-assembled lead(II) complex.

A polymeric self-assembled complex [[Pb(pydc)(pydc-H2)(H2O)2]2]n is prepared from the complexation of a novel pyridine containing self-assembling system, LH2, [pyda-H2]2+[pydc]2- (pyda = 2,6-pyridindiamine and pydc-H2 = 2,6-pyridinedicarboxilic acid) and lead(II) nitrate in 84% yield. The characterization was performed using X-ray crystallography. The crystal system is triclinic with space group P1 and two molecules per unit cell. The unit cell dimensions are a = 6.913(2) A, b = 10.687(4) A and c = 11.182(4) A with alpha = 92.805(6) degrees, beta = 101.821(6) degrees and gamma = 95.688(6) degrees. The final R value is 0.0373 for 4633 reflections measured. This compound is a nine-coordinate binuclear complex with two metal fragments linked via the central four-membered Pb2O2 ring. The crystal also contains a neutral [pydc-H2] molecule, that form hydrogen and coordination bonds that dominate the crystal packing, by forming layers of molecules.     

Magnetic studied of a binuclear Cu(II) complex constructed from Cu(II) macrocyclic complex

A new binuclear copper(II) compound [(CuL)₂(Tpha)](ClO₄)₂ · 4H₂O (I), where L = 1,3,6,9,11,14-hexaazatricyclo[12.2.1.1^6,9]octadecane, Tpha = terephthalate dianion, has been constructed and structurally characterized by X-ray crystallography. Crystal analysis denotes that complex I crystallizes in the monoclinic system, space group P2₁/c with a = 11.115(2), b = 13.185(3), c = 16.184(3) Å and β = 105.68(3)°. Magnetic measurements confirm that I present an antiferromagnetic interaction between the paramagnetic ions.     

Ligand substitution reaction at a binuclear organoplatinum(II) complex

The ligand substitution reactions of the N-donor ligand in the binuclear dimethylplatinum(II) complex of formula cis,cis-[Me₂Pt(μ-NN)(μ-dppm)PtMe₂], 1, in which dppm = bis(diphenylphosphino)methane and NN = phthalazine, by different nucleophilic phosphorous-donors L, L = P(O-ⁱPr)₃ or PPh₃ and L₂ = dppm, to form the dinuclear complexes 2, cis,cis-[Me₂LPt(μ-dppm)PtLMe₂] and cis,cis-[Me₂Pt(μ-dppm)₂PtMe₂], respectively, are studied. Complex 1 has a MLCT band in the visible region which was used to easily follow the kinetics of its ligand substitution reactions. These reactions which involve diplatinum(II) complex 1 containing cis Pt-C bonds, proceeded by the normal associative mechanism. In associative reactions of the present work, as expected, the rate of the reactions was depended on the concentration and the nature of the entering group. The nucleophilicity of PPh₃ is stronger than P(O-ⁱPr)₃ on the basis of its stronger σ-donor ability and its lower solvation and is responsible for the observed 3-fold increase of its rate as compared to that of P(O-ⁱPr)₃. Also, the solvation energy involved is suggested to be responsible for the observation of higher rates in benzene than in acetone. The ΔH^‡/ΔS^‡ compensation plot gives a straight line which suggests the operation of the same mechanism for all entering nucleophiles.     

A binuclear tantalum(III) complex with a bridging carboxylato ligand

Ta₂Cl₆(SMe₂)₃ reacts with one equivalent of C₄H₉CO₂Li to give a complex with a bridging carboxylate ligand, Ta₂Cl₅(O₂CC₄H₉)(SMe₂)(THF)₂. The product was isolated in two crystalline forms, 1 and 2, from a THF/hexane and benzene/hexane solvent mixture, respectively. The following are the unit cell parameters, for 1: monoclinic (P2₁/n), a = 10.537(5) Å, b = 22.015(4) Å, c = 11.663(4) Å, β = 107.80(3)°, V = 2576(3) ų, and Z = 4; for 2: monoclinic (P2₁/c), a = 15.584(4) Å, b = 15.647(4) Å, c = 11.275(3) Å, β = 106.04(5)°, V = 2642(3) ų and Z = 4. The complex is a dimer with a distorted octahedra-sharing-an-edge geometry. The TaTa distances in 1 and 2 were 2.766(1) Å and 2.779(1) Å, respectively, which is somewhat longer than in previously reported Nb(III) and Ta(III) dinuclear compounds. Diamagnetism of the complex is shown by NMR. Fenske—Hall calculations, which correctly predict an electronic transition at about 16,000 cm^?1, indicate a double TaTa bond. The observed elongation of the metalmetal bond is attributed mainly to steric crowding. The complex is the first proven low-valent Ta species with a coordinated carboxylate ion.     

A sugar discriminating binuclear copper(II) complex

We investigated the complex formation between various underivatized carbohydrates and the binuclear copper(II) complex 1, Cu(2)(bpdpo). A combined approach of UV/vis and CD spectroscopic investigations shows a large discrimination ability of 1 for structurally closely related monosaccharides in alkaline solution. The dominating form of the binuclear copper(II) complex consists of a [Cu(2)L(-)(H)(OH)(2)](+) species between pH 11 and 13, as determined from pH-dependent spectrophotometric titration experiments. The binding strengths of the 1:1 sugar-1 complexes, derived from the biologically important monosaccharides d-mannose (3) and d-glucose (5), is about 1.5 orders of magnitude different at pH 12.40. Moreover, a blue- or a red-shift of the absorption maximum of 1 accompanies the sugar binding and highlights the ability of 1 to discriminate carbohydrates. This phenomenon is due to the number of hydroxyl groups of the particular monosaccharide involved in chelation to the binuclear metal complex.     

Interactions between Co(III) and Ni(II) complex ions and polyelectrolytes

An investigation has been carried out of the interaction of optically active tris(ethylenediamine)–Co(III) and of tris(o-phenanthroline)–Ni(II) complex ions with different polyelectrolytes in dilute aqueous solution. Optical rotatory dispersion measurements reveal that binding of the Co(III) complex ions occurs with a noticeable degree of specificity which in addition to depending on the nature of the fixed charges is also affected by the chemical constitution of the polyelectrolytes chain backbones. The dependence of tris(ethylenediamine)–Co(III) optical activity on polyelectrolyte concentration also exhibits interesting features. Polarimetric measurements of the rate of racemization of tris(o-phenanthroline)–Ni(II) ions in the presence of different polyelectrolytes lead to similar conclusions. Sodium dextran sulfate is shown to interact selectively with one of the antipodes of tris(phenanthroline)–Ni(II) complex.     

Solvent effects on binuclear complex formation between aquopentacyanoferrate(II) and tetraamminepyrazinecarboxylatocobalt(III) in binary aqueous mixtures

The substitution reaction between aquopentacyanoferrate(II) and tetraamminepirazynecarboxylatocobalt(III) to form a binuclear species is studied in various binary aqueous mixtures at different temperatures. Values of activation parameters seem to indicate no change in the mechanism of the reaction in the different mixtures. Medium effects have been rationalized by using a multiparameter regression of solvent parameters. Results show that the reaction follows a dissociative mechanism. © 1993 John Wiley & Sons, Inc.     

Kinetics and mechanism of the reduction of a macrocyclic Rh(III) complex by chromium(II) ions: pH-controlled selectivity to rhodium(II) vs. rhodium(III) hydride

Aqueous chromium(II) ions reduce a macrocyclic Rh(III) complex L(1)(H(2)O)(2)Rh(3+) (L(1) = 1,4,8,11-tetraazacyclotetradecane) to the hydride L(1)(H(2)O)RhH(2+) in two discrete, one-electron steps. The first step generates L(1)(H(2)O)Rh(2+) with kinetics that are first order in each rhodium(III) complex and Cr(H(2)O)(6)(2+), and inverse in [H(+)], k/M(-1) s(-1) = 0.065/(0.0031 + [H(+)]). Further reduction of L(1)(H(2)O)Rh(2+) to L(1)(H(2)O)RhH(2+) is kinetically independent of [H(+)], k/M(-1) s(-1) = 0.30. The difference in [H(+)] dependence allows relative rates of the two steps to be manipulated to generate either L(1)(H(2)O)Rh(2+) or L(1)(H(2)O)RhH(2+) as the final product.     

A binuclear copper(II) complex with unusual magnetism

A binuclear copper(II) complex [Cu₂ (μ-pyo)₂Br₄] _n (where pyo = pyridine N-oxide) has been synthesized and its structure determined by X-ray crystallography. This complex crystallizes in monoclinic, space group P2₁/c, with unit cell dimensions a = 11.020(3) Å, b = 10.049(3) Å, c = 7.905(2) Å, β = 110.609(3)°, and Z = 2. The structure was refined to final R = 0.0311 and wR = 0.0721 for 1302 observed reflections (I > 2σ(I)). In the complex, two Cu(II) ions are bridged by two pyo ligands and four bromides coordinate the Cu(II); the distance between the bridged Cu(II) ions is 3.261 Å. The variable-temperature (4–300 K) magnetic susceptibility data show that the magnetic moment is zero. Thus, there exists very strong anti-ferromagnetic coupling between the bridged binuclear Cu(II) ions. Density functional calculations yield a singlet-triplet splitting 2J = ?1355 cm^?1.     

Magnetic anisotropy in Ni(II)-Y(III) binuclear complexes: on the importance of both the first coordination sphere of the Ni(II) ion and the Y(III) ion belonging to the second coordination sphere

The synthesis of a new Ni(II)-Y(III) binuclear complex with a marked elongation axis in the first coordination sphere of the Ni(II) ion is presented. Its zero-field splitting (ZFS) is studied by means of magnetic data and state-of-the-art ab initio calculations. A good agreement between the experimental and theoretical ZFS parameter values is encountered, validating the whole approach. The magnetic anisotropy axes are extracted from the ab initio calculations, showing that the elongation axis around the Ni(II) ion corresponds to the hard axis of magnetization and that the sign of the axial D parameter is imposed by this axis. The Ni-Y axis is found to be an easy axis of magnetization, which is, however, not significant according to the sign of D. The already reported [(H(2)O)Ni(ovan)(2)(μ-NO(3))Y(ovan)(NO(3))]·H(2)O (ovan = o-vanillin) complex is then revisited. In this case, the elongation axis in the Ni(II) coordination sphere is less marked and the ZFS is dominated by the effect of the Y(III) ion belonging to the second coordination sphere. As a consequence, the D parameter is negative and the low-temperature behavior is dominated by the Ni-Y easy axis of magnetization. A competition between the first coordination sphere of the Ni(II) ion and the electrostatic effect of the Y(III) ion belonging to the second coordination sphere is then evidenced in both complexes, and the positive and negative D parameters are then linked to the relative importance of both effects in each complex.     

Identification of Cu(II)/Cu(III) couples in binuclear copper(II) complexes

A new series of binuclear copper(II) complexes were synthesised and studied by magnetic, spectral, ESR and cyclic voltammetry methods. The μ_eff values per copper atom correspond to the values observed for mononuclear copper(II) complexes. ESR spectral data in solution indicate weak interactions resulting from the electron delocalisation through the ligand system. Two nearly reversible red-ox couples are identified at $\text{+}\text{?}$0.50 V and $\text{+}\text{?}$0.75 V vs SCE. They correspond to Cu(II)αCu(III) red-ox processes, successively occurring at the two copper sites in the binuclear complexes.     

Redox chemistry of mononuclear manganese(II), binuclear manganese(III) and binuclear mixed manganese(II)-manganese(III) complexes with 3-aminopyrazine-2-carboxylate in dimethylsulphoxide

Summary Mn^II forms a yellow mononuclear species with the title ligand having a 12 stoichiometry and whose conditional stability constant is 8.9 × 10¹⁰ m ^–2. The c.v. of this complex shows an oxidation at +0.78V versus s.c.e. Controlled-potential electrolysis at +0.80V versus s.c.e. yields a binuclear species of Mn^III with a 12 metal:ligand stoichiometry.The addition of Mn^III(urea)₆(ClO₄)₃ to a solution of the ligand produces a mononuclear complex of Mn^III if the concentration of the metal ion is less than 1 mM. At higher concentrations a binuclear species is obtained. The latter is reduced in two steps, at +0.24 and –0.58 V versus s.c.e. Controlled-potential electrolysis at 0.0 V produces a dark green complex after the transfer of 0.5 equivalents of charge per mole of Mn. This binuclear L₂Mn^II-Mn^IIIL₂ mixed-valence complex can be obtained only by electrolysis of the binuclear L₂Mn^IIIMn^IIIL₂ species. Attempts to prepare the complex chemically were unsuccessful - the binuclear Mn^III species was obtained in every case.Author to whom all correspondence should be directed.     

A dinuclear Ni(II) complex with two types of intramolecular magnetic couplings: Ni(II)-Ni(II) and Ni(II)-TTF*+

A dinuclear Ni(II) complex involving tetrathiafulvalene (TTF) radicals as ligands has been prepared and characterized, [Ni2(mu-Cl)2(L*+)2(I3)4(I2)3.(H2O)2.(C4H8O)3 (1), L = 4,5-bis(2-pyridylmethylsulfanyl)-4',5'-ethylenedithiotetrathiafulvalene. There are two types of intramolecular magnetic exchange interactions, namely one ferromagnetic Ni(II)-Ni(II) and one antiferromagnetic Ni(II)-TTF*+. This study is new in the respect of revealing a magnetic exchange interaction between a TTF*+ radical and a paramagnetic transition metal ion. This is due to the fact of a direct binding of the transition metal ion to the skeleton of the TTF*+ radical.