Nimma Rajaiah Sangeetha

SYNTHESIS, STRUCTURE AND PROPERTIES OF A DICOPPER(II) COMPLEX

Abstract A dinuclear copper(II) complex, [Cu2(bhsNO2)2(H2O)2] was isolated by reacting Cu(O2CCH3)2·H2O and 2-hydroxy-5-nitrobenzaldehyde benzoylhydrazone (H2bhsNO2) in methanol. The X-ray structure of the complex has been determined. The dinegative ligands bind the Cu(II) ions through the deprotonated amide–O, imine–N and phenolate–O atoms. The pair of Cu(II) ions are linked by the two phenolate–O atoms. The coordination geometry around each Cu(II) is square pyramidal. The imine–N, amide–O and the bridging phenolate–O atoms form the square plane and the oxygen atom of a water molecule occupies the axial coordination site. The complex is redox active. Variable temperature (25–300 K) magnetic susceptibility measurements in the solid state are consistent with an antiferromagnetic interaction between the two metal centres. The coupling constant J is found to be −186(5) cm−1 by least-squares fitting of the magnetic susceptibility data using an expression derived from the isotropic spin Hamiltonian, H=−2JS1·S2 (S1=S2=1/2).

Dimeric and polymeric square-pyramidal copper(II) complexes containing equatorial–apical chloride or acetate bridges

Syntheses, crystal structures and physical properties of four new copper(II) complexes of deprotonated aroylhydrazones of 2-pyridine-carboxaldehyde are reported. Two of the complexes contain chloride as a coligand and the other two contain acetate as a coligand. In each complex, the planar tridentate monoanionic ligand binds the metal ion via the pyridine-N, the imine-N and the amide-O atoms. The fourth site is satisfied by the chloride or the acetate-O to form a square-plane. In the solid state, both complexes containing the acetate as the coligand exist as centrosymmetric dimeric species with the metal ion in a distorted square-pyramidal coordination sphere and the acetate group acting as a monoatomic equatorial–apical bridge. One of the complexes containing chloride is a similar centrosymmetric dichloro-bridged dinuclear species. However, the other one exists as a polymeric chain species via equatorial–apical chloride bridges. The electronic spectra of the complexes in methanol solutions display a ligand-field band in the visible region (687–713 nm). The complexes are redox active and, in each case, two reduction responses are observed in the potential range −0.06 to −0.29 and −0.60 to −0.70 V (vs. Ag/AgCl) in methanol solutions. Cryomagnetic studies revealed that no significant spin–spin interaction is operative between the metal ions in any of these four complexes.

Vanadium(V) Complexes of O,N,O-Donor Tridentate Ligands Containing the {VVO(OMe)}2+ Unit: Syntheses, Structures and Properties

Abstract Syntheses, characterisation and properties of two complexes containing the oxovanadium(V) methoxide unit have been described. Deprotonated benzoylhydrazones of 2–hydroxy–5–methoxy‐benzaldehyde (H2bhsOMe) and 2–hydroxy–5–chlorobenzaldehyde (H2bhsCl) were used as coligands. Crystal structures of both the complexes were determined. In solid state one of them is a dinuclear species [VO(bhsOMe)(OMe)]2 (1) whereas the other one is a mononuclear complex [VO(bhsCl)(OMe)(HOMe)](2). The dinegative ligands coordinate the metal ions via phenolate–O, imine–N and deprotonated amide–O atoms. In 1, the metal ions of two square pyramidal VO(bhsOMe)(OMe) units share the methoxide groups to form a dinuclear species. The oxygen of a methanol molecule completes the hexacoordination of the metal centre in 2. In each of the two distorted octahedral VO5N moieties of 1 the bridging methoxide oxygen and in that of 2 the methanol oxygen is trans to the corresponding oxo group. Both the complexes are redox active. The VO3+ to VO2+ reduction potentials (vs Ag/AgCl) of 1 and 2 are observed at −0.25 and −0.04 V, respectively. The band positions in the electronic spectra and the redox potentials reflect the influence of the substituents present on the ligands.

Copper(II)-activated transformation of azomethine to imidate: synthetic and structural studies

Abstract The Schiff base 2-pyridine–carboxaldehyde 4-dimethylaminobenzoylhydrazone (HL, 1) was prepared by reacting 2-pyridine–carboxaldehyde and 4-dimethylaminobenzoylhydrazine in a 1:1 molar ratio in methanol. Reaction of HL (1) with Cu(O2CCH3)2·H2O (in a 1:1 molar ratio) in methanol afforded a dinuclear copper(II) complex, [Cu2(μ-O2CCH3)2L′2]·2H2O (2). The azomethine functionality (CHN) of 1 is converted to imidate (C(OMe)N) in the complexed ligand L′−. Molecular structures of both 1 and 2 were determined by X-ray crystallographic studies. The dinuclear molecule of 2 is centrosymmetric and contains two monoatomic bridging acetate groups. Each copper(II) centre is in a square-pyramidal N2O3 coordination sphere. The ligand, L′− coordinates the metal ion via the pyridine-N, the imidate-N, and the deprotonated amide-O atoms. One of the acetate oxygen atoms completes the N2O2 square-plane. The oxygen of the symmetry-related acetate fills the apical coordination site. Structural parameters are consistent with both copper ions being in a +2 oxidation state. The room temperature magnetic moment is 1.89 μB (per Cu). In powder phase the complex displays an axial EPR spectrum at 298 K. The complex is nonconducting in methanol solution. The electronic spectrum shows a ligand field absorption at 680 nm and charge transfer bands in the range 426–215 nm.

A one-dimensional assembly of copper(II) polyhedra via dual use of hydrogen-bonding and π–π interaction

Abstract A copper(II) complex [Cu(Hpadh)Cl2] with pyridine-N, imine-N and amide-O coordinating 2-pyridine-carboxaldehyde 4-dimethylaminobenzoylhydrazone (Hpadh) has been synthesized and structurally characterized. In the solid state, each square–pyramidal Cu(Hpadh)Cl2 unit is connected to one neighbour by a pair of hydrogen bonds between the amide-H and the apical chloride and to the other by a couple of π–π interactions between the aromatic rings of the coordinated ligands forming a novel chain-like arrangement of copper(II) centres with successive long and short Cu···Cu distances. Variable temperature (300–18 K) magnetic susceptibility measurements showed that the complex is essentially Curie paramagnetic.

Syntheses and structures of square-pyramidal acetonitrile coordinated copper(II) complexes with N-(aroyl)-N′-(5-nitrosalicylidene)hydrazines

The syntheses and structures of two square-pyramidal acetonitrile coordinated copper(II) complexes, [Cu(HbhsNO2)(CH3CN)2]ClO4 (1) and [Cu(HahsNO2)(CH3CN)(ClO4)] (2), are described. The mononegative ligands are obtained by deprotonation of the phenolic-OH of N-(benzoyl)-N′-(5-nitrosalicylidene)hydrazine (H2bhsNO2) and of N-(anisoyl)-N′-(5-nitrosalicylidene)hydrazine (H2ahsNO2). Crystal data for 1: monoclinic, P21/c (#14), a = 9.7245(14), b = 20.23(4), c = 11.042(3) Å, β = 91.86(2)°, V = 2171.0(7) Å3, and Z = 4 and for 2: triclinic, P1¯ (#2), a = 9.7710(19), b = 9.9687(15), c = 11.062(5) Å, α = 73.10(3), β = 88.77(3), γ = 88.379(14)°, V = 1030.4(5) Å3, and Z = 2. In each complex the planar ligand binds the metal ion via phenolate-O, imine-N, and amide-O. The nitrogen of an acetonitrile molecule satisfies the fourth site of the square-plane containing the metal ion. In 1 the axial coordination is provided by the nitrogen of a second acetonitrile molecule, whereas in 2 one of the oxygen atoms of the perchlorate occupies the fifth axial site. The axial acetonitrile molecule of 1 is bound to the metal center in a bent mode. The other noticeable difference between the two structures is in the geometry at the metal center. A large displacement (0.23 Å) of the metal ion from the ONON basal plane towards the axial coordinating atom is observed in 1. On the other hand, there is no displacement of the metal center from the same ONON basal plane in 2. The EPR and electronic spectral features in acetonitrile solutions are consistent with the solid state structures.

A FeIIIO4N2 co-ordination sphere assembled via an enolate–imine–amide ligand. Effect of amide protonation on the redox behaviour and stereochemistry of the iron(III) centre

The reaction of anhydrous FeCl3, acetylacetone benzoylhydrazone (H2L) and KOH (1 : 2 : 3 mole ratio) in methanol produced an iron(III) complex, [FeL(HL)]1, the crystal structure of which was determined. Each ligand binds through enolate O, imine N and amide O atoms in meridional fashion. In the dianionic L both the enolic OH and the amide protons are dissociated, whereas in the monoanionic HL only the enolic OH is deprotonated. Addition of 1 equivalent of HClO4 to 1 in MeOH gave [Fe(HL)2]ClO42. Similarly reaction of 1 equivalent of KOH with 1 yielded K[FeL2]3. All the complexes were characterized by analytical, spectroscopic, electrochemical and magnetic measurements. The metal centre in 1 is redox inactive. However, in cyclic voltammetric experiments 2 displayed FeIII→ FeII reduction at –0.21 V and for 3 an oxidation at 0.40 V (vs. saturated calomel electrode) was observed due to FeIII→ FeIV oxidation. Magnetic moments (at 298 K) of the three complexes reflect a S= 5/2 spin state in each. The ESR spectra (at 298 K) of polycrystalline 1 and 2 are rhombic. On the other hand, an ideal axial spectrum was observed for 3.

Copper(II) complexes containing a CuN4O2 coordination sphere assembled via pyridine-imine-amide coordination: Synthesis, structure and properties

Copper(II) complexes [CuL2] of pyridine-N, imine-N and amide-O donor Schiff bases (HL) are reported. The Schiff bases Hpabh, Hpamh and Hpadh were prepared by condensation of 2-pyridinecarboxaldehyde with benzhydrazide, 4-methoxybenzhydrazide and 4-dimethylaminobenzhydrazide, respectively. The complexes were synthesised in MeOH media by reacting Cu(OAc)2 · H2O with HL in a 1:2 mole ratio. The X-ray structure of [Cu(pabh)2] was determined. The rhombically distorted octahedral CuN4O2 coordination sphere is formed by two mononegative meridionally spanning pyridine-N, imine-N and deprotonated amide-O donor ligands. Electronic spectra of the complexes display two d–d bands at ca. 700 and ca. 1300 nm and two strong absorptions due to charge transfer transitions in the 450–268 nm range. Room temperature (298 K) solid state magnetic moments (1.90–2.08 μB) suggest an S = 1/2 spin state in each complex. The powder e.p.r. spectra of [Cu(pabh)2] and [Cu(pamh)2] at room temperature are consistent with a rhombic distortion of the N4O2 octahedron around the metal centre. However the spectrum of [Cu(padh)2] suggests a tetragonal elongation of the CuN4O2 octahedron.

SYNTHESIS, STRUCTURE AND PROPERTIES OF MANGANESE(II) COMPLEXES WITH AROYLHYDRAZONES OF 2-PYRIDINE-CARBOXALDEHYDE

Abstract Manganese(II) complexes of terdentate N,N,0 donor Schiff bases (HL) are described. The ligands Hpabh, Hpamh and Hpadh are obtained by condensing 2-pyridine-carboxaldehyde with benzhydrazide, 4-methoxybenzhydrazide and 4-dimethylaminobenzhydrazide, respectively. The reaction of HL with manganese(II) acetate tetrahydrate affords [MnL2]. The crystal structure determination of [Mn(pabh)2] was performed. The complex crystallises in the space group P21/n with a = 9.836(3), b = 23.994(7), c = 10.222(3)A, β = 104.14(3)°, V = 2339.5(13)A3 and Z = 4. In the distorted octahedral MnN4O2 coordination sphere each ligand acts as a meridional N,N,0 donor utilising pyridine-N, imine-N and amide-O atoms. Electronic spectra of the complexes display charge transfer bands in the range 404–298 nm. Room temperature solid state magnetic moments (5.88–6.12βB) of the complexes are consistent with a high-spin d5 system. EPR spectra of the complexes suggest a similar distorted octahedral N4O2 coordination sphere around Mn(I...

NICKEL(II) COMPLEXES OF TRIDENTATE N,N,O-DONOR LIGANDS: SYNTHESES, STRUCTURES AND REDOX PROPERTIES

Abstract Nickel(II) complexes ([NiL2]) of tridentate Schiff bases (HL) containing amide functionality are described. The Schiff bases, Hpabh and Hpamh (H refers to the dissociable amide proton), are derived from 2-pyridinecarboxaldehyde and benzhydrazide, and 2-pyridinecarboxaldehyde and 4-methoxybenzhydrazide, respectively. The reaction of two equivalents of HL and one equivalent of Ni(O2CCH3)2 · 4H2O in methanol affords [NiL2] in high yield. The complexes are characterised by analytical, spectroscopic, magnetic and electrochemical techniques. The structures of both complexes have been determined by X-ray crystallography. The distorted octahedral NiN4O2 sphere in each complex is assembled by the two meridional N,N,O-donor ligands. Each ligand binds the metal ion via the pyridine-N, imine-N and deprotonated amine-O atoms. The solid state room temperature (298 K) magnetic moments are consistent with a d 8 (S = 1) ground state electronic configuration. Electronic spectra of the complexes in CH3CN solutions display the v 1 band at ∼ 850 nm followed by charge transfer bands in the range 381–241 nm. The [NiIIIL2]+-[NiIIL2] couple was observed in the cyclic voltammograms of both complexes. The potentials are 0.97 and 0.91 V (versus Ag-AgCl) for [Ni(pabh)2] and [Ni(pamh)2], respectively.