Makoto Chikira

ESR studies of the spin-exchange interaction within parallel planar copper (II) dimers in frozen solutions

Abstract The temperature variations of the ESR spectral intensity of the triplet dimers over the range 1.6 to 4.2 K indicated that the spin-exchange interaction within the parallel planar dimers, which had been reported to be ferromagnetic in crystal, is antiferromagnetic (1.5–2.5 cm −1 ) in frozen solutions.

An improved insertion‐type liquid‐helium Dewar for X‐band ESR spectroscopy

A liquid‐helium Dewar in conjunction with the X‐band room‐temperature cavity is reported. The Dewar consists of a fourfold silvered Pyrex glass tube graded to fourfold unsilvered clear fused quartz fingers. A full charge (250 ml) of liquid helium has given about 10 h of operation.

Evidence for dimer formation of nitrosyl(meso-2,3,7,8,12,13,17,18-octaethyl-5-nitroporphyrinato)iron(II) and its implication in the interpretation of the electron spin resonance spectrum of the nitrosyl-haemoglobin–salicylate system

Dimer formation of the title complex in toluene has been demonstrated by an electron spin resonance study of this complex at a high concentration at 9 K, and the approximate dimer structure has been determined by spectral simulation. The observation enables us to interpret the hitherto unexplained four resonance absorptions which are known to appear in nitric oxide ligated haemoglobin (NO-haem) as a result of interaction with salicylate, on the basis of similar dimer formation of the NO-haem prosthetic groups. The event appears to take place intramolecularly by association of NO-haem molecules coming out of the haem pockets, and the complex is stabilized by the globin structure.

The high-spin ground state of quadridentate salicylaldehyde Schiff-base complexes of cobalt(II) in N-heterocyclic solvents

Formation of species with a high-spin ground state (S=) has been observed by e.s.r., n.m.r., and magnetic susceptibility measurements for quadridentate salicylaldehyde Schiff-base complexes of cobalt(II) in solution containing N-heterocyclic bases. It was found that a bidentate base, 1,10-phenanthroline, also gave similar high-spin species. The high-spin species were shown to be the base adducts having a six-co-ordinate structure. Magnetic susceptibility measurements revealed that 22% of NN′- ethylenebis(salicylideneiminato)cobalt(II) exists as the high-spin base di-adduct in pyridine at 295 K. The thermodynamic parameter ΔH⊖ for the formation of this high-spin species was found to be –4.25 × 103J mol–1. The amount of the high-spin species increased as the temperature decreased. The analysis of the isotropic shifts in 1H n.m.r. Revealed that the unpaired electron spins in the high-spin species mainly delocalize through σ orbitals.

Electron spin resonance study of dimers of copper(II) octaethylporphyrin, meso-nitro-octaethylporphyrin, and meso-dinitro-octaethylporphyrin in solution

The dimer e.s.r. spectra of copper(II)meso-nitro-octaethylporphyrin (1), copper(II)α,β-meso-dinitro-octaethylporphyrin (2), copper(II)α,γ-meso-dinitro-octaethylporphyrin (3), and copper(II) octaethylporphyrin (4) observed in a frozen toluene solution at 77 K are described. The equilibrium constants (K) for dimer formation have been obtained by fitting the observed monomer intensity as a function of the total concentration to a simple monomer–dimer scheme. For (1) and (2) the K values are (in 104 dm3 mol–1) 27 and 18, respectively, and there is an indication of a higher-order association, while the data for (3) and (4) can be explained only by dimer formation with smaller K values (0.15 × 104 and 0.68 × 104 dm3 mol–1, respectively). A structural analysis based on the point-dipole approximation shows that the Cu–Cu distance (4.16–4.19 A) and the lateral shift angle (13–19°) are roughly the same in (1), (3), and (4). Complex (2) is unique in having a large angle (40°) and shorter interplane distance (3.2 A). The effect of the electron-withdrawing nitro-group on dimerization is clearly demonstrated.

Electron spin resonance study of the formation of two dimeric species in bis(1-phenylbutane-1,3-dionato)- and bis(ethylacetoacetato)-copper(II)

From the e.s r. spectra of the title complexes in frozen toluene solutions, it has been found that both form two dimeric species. The spin-exchange interaction energies of the dimers have been determined by measuring the temperature variation of the spectral intensity of the ΔM= 2 transitions over the range 1.6–4.2 K. The drmeric structures have been examined by computer simulation of the dimer e.s.r. spectra and are discussed in relation to two well known types of molecular packing in crystals of planar metal complexes and to the polymorphism drsplayed by many metal complexes.

pH Dependence of the formation of simple imidazolate-bridged binuclear copper(II) complexes

It is revealed by e.s.r. spectroscopy that the formation of many imidazolate-bridged binuclear copper(II) complexes of aminocarboxylates is pH-dependent with an apparent pKa of 8.3; this closely resembles an unusual pH-dependent property of zinc-free bovine erythrocyte superoxide dismutase.

Electron spin resonance evidence for dimer formation in high spin iron(III)-octaethylporphyrin and iron(III)-meso-nitro-octaethylporphyrin in solution

E.s.r. signals representing a dimeric species and another form of aggregation in high-spin FeIIICl-meso-nitro-octaethylporphyrin (OEP)(1) are presented, and a spectral assignment is made for the dimer signal to a transition within the lowest triplet levels.

High-spin quadridentate salicylaldehyde Schiff base complexes of cobalt(II) in N-heterocyclic solvents

E.s.r. and magnetic susceptibility studies of the title complexes show that an appreciable amount of the high-spin base di-adduct is formed in N-heterocyclic solvents such as pyridine and its methyl derivatives.

Electron spin resonance study of dimer formation in some bis(N-alkylsalicylideneiminato)copper(II) complexes in inert solvents

The complexes [Cu(N-R-sal)2](R = Et, Prn, Pri, Bun, n-hexyl, cyclohexyl, and n-octyl) show well defined triplet-state e.s.r. spectra in methylcyclohexene for both ΔM= 1 and 2 transitions with the zero-field splitting due to dipolar interaction between the copper(II) pairs. Computer analysis of the spectra has revealed that these complexes adopt various dimeric structures according to R which do not require apical co-ordination of the phenolic oxygen atoms, and that the structures in methylcyclohexane are different from those in toluene. Intradimer spin-exchange interactions for these dimers are weakly antiferromagnetic (1.1–4.5 cm–1 from the temperature dependence of intensity for ΔM= 2 transitions in the range 1.57–4.2 K). Thermodynamic constants for dimer formation in methylcyclohexane have been estimated as ΔH≈–3 kcal mol–1 and ΔS≈–9 cal K–1 mol–1 and are discussed.