D. Srinivas

Copper(II) diethylenetriamine perchlorato complexes bridged through varying length dicarboxylato spacers: synthesis, characterization and EPR studies

Abstract The synthesis and spectral characterization of dinuclear copper(II) complexes of the composition [(dien)CuLCu(dien)](ClO4)2, where dien = diethylenetriamine and L2 = oxalato, malonato, succinato, phthalato, isophthalato and terephthalato, are reported. IR spectra reveal that the mode of carboxylato coordination is anti-anti for oxalato complexes while it adopts chelate or polymeric syn-anti coordination for the rest of the complexes. The stereochemistry around copper is distorted square pyramidal. Oxalato forms two types of complexes I and II. EPR for I is characterized by a rhombic g tensor with g3 g⊥) suggesting the occupancy of unpaired electron in a dx2−y2 orbital. Frozen solution EPR spectra at 77 K indicate that the solvent molecule coordinates with the metal ion. Magnetic exchange in these complexes is intramolecular and both conjugated and unconjugated spacer dicarboxylato ligands propagate the exchange between the metal ions.

Spin crossover in substituted N,N′-ethylenebis(salicylidenamine) iron(III) complexes : variable temperature EPR and X-ray structures of [Fe(5-CH3O-SALEN)(Im)2]Y, where Y=ClO−4 and Cl−

Abstract The spin state of iron(III) in substituted N,N′-ethylenebis(salicylidenaminato) complexes of type [Fe(5-X-salen)L 2 ]Y, where X = CH 3 O, H and Cl, L = Im and 2-MeIm and Y = Cl − , ClO − 4 and BPh − 4 was determined using X-ray structures, magnetic susceptibility and variable temperature EPR studies. X-ray data of [Fe(5-CH 3 O-salen)(Im) 2 ]Y, where Y = ClO − 4 (1) and Cl − (2) are as follows : 1, C 24 H 26 N 6 O 8 ClFe, monoclinic P 2 1 \ n , a = 10.516(4) A, b = 13.752(3) A, c = 19.227(3) A, β = 102.14(2)°, V = 2718(11) A 3 , Z = 4, R = 0.097 ( R w = 0.084) ; 2, C 24 H 26 N 6 O 4 ClFe, monoclinic P 2 1 \ n , a = 11.709(3) A, b = 12.746(2) A, c = 17.388(4) A, β = 106.15(2)°, V = 2493(15) A 3 , Z = 4, R = 0.062 ( R w = 0.065). Metal-ligand bond distances are consistent with a HS state for iron(III) which has a tetragonally distorted octahedral geometry. Imidazole planes are coplanar and bisect the N—Fe—O bond angle in 2 while they are oriented by 77.3° apart in 1 almost coinciding with the N—Fe—O direction. Counterions showed significant effect on the overall molecular geometry and FeN 2 C 2 ring. Methanolic solutions of all the complexes showed spin crossover whereas only salen and 5-Cl-salen complexes exhibited this phenomenon in solid state. EPR investigations revealed, for the first time, two types of interconvertible low spin species (LS1 and LS2) in solution ; the former was characterized by an almost axial g tensor with lower g anisotropy (2.238–1.937) while the latter with a rhombic tensor with larger g anisotropy (2.382–1.911). Ground state wave functions and distortion parameters were calculated. Structure-spin state-reactivity correlations are proposed.

Molecular association, chelate conformation and reactivity correlations in substituted o-phenylenebis(salicylidenato)copper(II) complexes: UV-visible, EPR and X-ray structural investigations

Abstract Substituted o -phenylenesbis(salicylidenaminato) complexes of copper(II), Cu(5-X-saloph), where X = H ( 1 ), CH 3 O ( 2 ) and Cl ( 3 ) have been synthesized and characterized. Complex 1 , C 20 H 14 N 2 O 2 Cu, crystallizes in the orthorhombic space group P 2 1 2 1 2 1 with two molecules (A and B) in the assymetric unit related by a non-crystallographic centre of inversion and the nitrogen atom of A coming closer to the metal ion of B and vice versa . The geometry around copper is essentially square planar in both A and B with average CuN and CuO bond distances being 1.89(2) and 1.95(2) →ing chelate conformation is also planar with a moderate “twist” at the “sal” groups. EPR spectra of the complexes are characterized by axial or rhombic g and A Cu tensors and indicate the occupancy of an unpaired electron in a “formal” d xy orbital. Solutions at 298 K showed superhyperfine features due to interaction of transferred unpaired spin with the nuclear spin of two magnetically equivalent 14 N nuclei and the protons attached to adjacent methylene carbon atoms. The complexes exhibit solvatochromism. Substitution and δ-donor solvents show marked effects on the UV-visible and EPR spectra. MO calculations reveal that the in-plane δ and π bondings are covalent in nature. The correlation between conformational flexibility and reactivity is discussed.

SYNTHESIS, SPECTRAL CHARACTERIZATION AND MAGNETIC PROPERTIES OF DICARBOXYLATO-BRIDGED DINUCLEAR COPPER(II) COMPLEXES WITH N-PYRIDYLSALICYLIDENAMINATO LIGAND

Abstract The synthesis and characterization of mixed-ligand copper(II) complexes with bridged dicarboxylato groups, of the composition [(PS)Cu] 2 L· n H 2 O, where PS = N-pyridylsalicylidenaminato, L = malonato, phthalato and terephathalato, and n = 8,5,2,respectively, are reported. PS coordinates as a monoanionic bidentate ligand, while carboxylate coordinates as a monodentate ligand. The stereochemistry around copper is distorted octahedral for malonato, square pyramidal for phthalato and square planar for terephthalato complexes. EPR spectra for polycrystalline complexes at room temperature were characterized by axial symmetry ( g ∥ > g ⊥ ) and correspond to non-interacting dimeric units. However, the anomalous magnetic moment data suggest intermolecular interactions as an efficient exchange pathway.

Ruthenium(III) Schiff base chloro and carbonyl complexes: Synthesis, characterization and EPR studies

Abstract The synthesis and characterization of stable ruthenium(III) Schiff base chloro and carbonyl complexes with the axial ligands chloro (Cl), imidazole (Im) and 2-methylimidazole (2-MeIm) are reported. The Schiff bases were synthesized by the interaction of naphthaldehyde with amines such as o-phenylenediamine, ethylenediamine, propylenediamine and diethylenetriamine. The geometry around ruthenium in these complexes is psuedooctahedral. The complexes are low-spin 4d5 (S = 1 2 ) and display characteristic EPR spectra in the powder samples and in frozen solutions at 298 and 77 K. The EPR data and EHMO calculations on model complexes indicate that the dxy orbital lies above dxz and dyz orbitals.

Dioxygen affinities of some ruthenium(III) schiff base complexes

Abstract The synthesis and dioxygen affinities of some ruthenium(III) Schiff base complexes in DMF solution in the presence of different axial bases are reported. The ligands used are bis(salicylaldehyde)ethylenediimine (salen), bis(salicylaldehyde)diethylenetriimine (saldien), bis(picolinaldehyde)- o -phenylenediimine (picoph), bis(picolinaidehyde)ethylenediimine (picen) and bis(picolinaldehyde)diethylenetriimine (picdien). The axial ligands employed are chloride (Cl − ), imidazole (Im) and 2-methylimidazole (2-MeIm). From the oxygenation constants it is found that electron donating substituents on the Schiff bases increase the affinity for dioxygen. Equilibrium dioxygen uptake measurements at 278, 288 and 303 K provide values of Δ H ° and Δ S ° of oxygenation that fall in the range − 6.1 to −13.3 kcal mol − 1 for Δ H ° and − 10 to − 31 cal deg − 1 mol − 1 for Δ S °. The dioxygen adducts of Ru III were characterized by electrochemistry, UV–vis, IR and EPR techniques as Ru IV superoxo complexes.

Labile Electronic Ground State of the π-Cation Radical of Zinc(II) Tetraphenylporphyrin: a Variable-temperature ESR Investigation

The UV-vis and variable-temperature X-band EPR spectral studies (298–77 K) on π-cation of zinc(II) tetraphenylporphyrin (ZnTPP) have revealed for the first time a labile electronic ground state. The electronic structure changes from 2A2u to 2A1u as the temperature is lowered from 298 to 150 K. Axial ligands and σ-donor solvents show a significant effect on the spectral data. The lability in the ground state electronic configuration is attributed to the flexibility of the porphyrin moiety in changing from a ‘ruffled’ to a lower symmetry.

Effect of substitution on the redox behaviour and EPR spectra of zinc(II) substituted tetraphenylporphyrin cation radicals

Substituted tetraphenylporphyrinatozinc(II) complexes have been synthesized and characterized by various physicochemical techniques. The effects of substitution and counterions on the redox behaviour and EPR spectra have been examined. The EPR spectra revealed that chemical oxidation with bromine yields ZnT(X-P)P+Br- (species I) and a brominated tetraphenylporphyrin cation radical (species II). The former exhibits a labile electronic ground state (with 2A2u ground state transforming into 2A1u at 175 K), while the latter is characterized by a 2A1u state. The lability in the electronic ground state for species I is discussed in terms of the change in molecular geometry as the temperature is lowered. The EPR spectra also revealed for the first time the formation of dimeric agglomerates of TPP cation radicals in low concentration below the electronic transition (i.e. near 170 K) wherein the paramagnetic sites are antiferromagnetically coupled.