Hyperfine Splitting In ESR Spectra Research Articles

Compartmental unsymmetrical lateral macrobicyclic ligands with an aromatic backbone providing a phenoxo bridge: synthetic, spectral, magnetic, electrochemical and kinetic features of mono and binuclear copper(II) complexes

From macrobicyclic ligands having different compartments originated from their corresponding precursor compounds (PC-1: 3,4:9,10-dibenzo-1,12[ N, N ′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,8-dioxacyclotetradecane and PC-2: 3,4:9,10-dibenzo-1,12[ N, N ′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,8-dioxacyclopentadecane) several mono and binuclear copper(II) complexes denoted as [CuL](ClO) 4 and [Cu 2L(ClO 4)](ClO 4) have been synthesized. Electrochemical studies vindicate that one quasireversible reduction wave ( E pc=−0.80 to −0.88 V) for the mononuclear complexes and two quasireversible one electron transfer reduction waves ( E pc 1=−0.84 to −0.94 V, E pc 2=−1.25 to −1.40 V) for the binuclear complexes are obtained in the cathodic region. The probe of room temperature magnetic moment studies depicts the presence of an antiferromagnetic interaction in the binuclear complexes ( μ eff of 1.35–1.42 B.M.), which is also observed from the broad ESR spectra with a g value of 2.10 or 2.11. Whereas, hyperfine splitting in ESR spectra is observed for mononuclear complexes and the magnetic moment value is found to be close to the spin only value ( μ eff of 1.69–1.71 B.M.). A variable temperature magnetic susceptibility study of the complex was carried out and the calculated −2 J values for the binuclear complexes [Cu 2L 1a(ClO 4)](ClO 4) and [Cu 2L 1b(ClO 4)](ClO 4) are 240 and 219 cm −1, respectively. The initial rate constant values of catechol oxidation using the complexes as catalysts have been found to span a domain ranging from 5.06 × 10 −3 to 2.56 × 10 −2 min −1 and the values are found to be higher for binuclear complexes than the corresponding mononuclear complexes. Spectral, electrochemical and catalytic studies support the distortion of the copper ion geometry that arises as the macrocyclic ring size increases.

Synthesis of lateral macrobicyclic compartmental ligands: structural, magnetic, electrochemical, and catalytic studies of mono- and binuclear copper(II) complexes.

A series of putative mono- and binuclear copper(II) complexes, of general formulas [CuL](ClO(4)) and [Cu(2)L](ClO(4))(2), respectively, have been synthesized from lateral macrocyclic ligands that have different compartments, originated from their corresponding precursor compounds (PC-1, 3,4:9,10-dibenzo-1,12-[N,N'-bis[(3-formyl-2-hydroxy-5-methyl)benzyl]diaza]-5,8-dioxacyclotetradecane; and PC-2, 3,4:9,10-dibenzo-1,12-[N,N'-bis[(3-formyl-2-hydroxy-5-methyl)benzyl]diaza]-5,8-dioxacyclopentadecane). The precursor compound PC-1 crystallized in the triclinic system with space group P(-)1. The mononuclear copper(II) complex [CuL(1a)](ClO(4)) is crystallized in the monoclinic system with space group P2(1)/c. The binuclear copper(II) complex [Cu(2)L(2c)](ClO(4))(2) is crystallized in the triclinic system with space group P(-)1; the two Cu ions have two different geometries. Electrochemical studies evidenced that one quasi-reversible reduction wave (E(pc) = -0.78 to -0.87 V) for mononuclear complexes and two quasi-reversible one-electron-transfer reduction waves (E(1)(pc) = -0.83 to -0.92 V, E(2)(pc) = -1.07 to -1.38 V) for binuclear complexes are obtained in the cathodic region. Room-temperature magnetic-moment studies convey the presence of antiferromagnetic coupling in binuclear complexes [mu(eff) = (1.45-1.55)mu(B)], which is also suggested from the broad ESR spectra with g = 2.10-2.11, whereas mononuclear complexes show hyperfine splitting in ESR spectra and they have magnetic-moment values that are similar to the spin-only value [mu(eff) = (1.69-1.72)mu(B)]. Variable-temperature magnetic susceptibility study of the complex shows that the observed -2J value for the binuclear complex [Cu(2)L(1b)](ClO(4))(2) is 214 cm(-1). The observed initial rate-constant values of catechol oxidation, using complexes as catalysts, range from 4.89 x 10(-3) to 5.32 x 10(-2) min(-1) and the values are found to be higher for binuclear complexes than for the corresponding mononuclear complexes.

Synthesis of new macrobicyclic tricompartmental ligands and site preference of metal ion: structural and electrochemical properties of mononuclear copper(II) complexes

New macrobicyclic tricompartmental ligands and their mononuclear copper(II) complexes of type [CuH 2L](ClO 4) 2 have been synthesized from the precursor compound 3,4:10,11-dibenzo-1,13[ N, N ′{bis(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,9-dioxa-cycloheptadecane with diamines like en, pn and bn by template method. These complexes have been characterized by IR, UV spectral and X-ray studies. Complex [CuH 2L 3](ClO 4) 2 (in L 3 the diamine used is bn) crystallized as triclinic system with space group p-1. The crystal structure explains that the geometry around copper is distorted square planar. Among the three compartments, it is interesting to observe that the metal ion prefers to be encompassed by the compartment containing phenolic oxygens and imine nitrogens (N 2O 2). From X-ray studies the presence of perchlorate anions, acetonitrile solvent molecules and the protonation of tertiary nitrogen atoms were well understood. All the complexes show hyperfine splittings in ESR spectra. The μ eff values observed in the range 1.69–1.72 BM are close to the spin only value of single copper(II) ion. Cyclic voltammetry studies of these complexes depict one electron quasireversible reduction in the range −0.80 to −0.90 V at negative potential. A red shift in UV–Visible spectra and shifting of reduction potential towards anodic in cyclic voltammetry were witnessing the distortion of the geometry around metal center from the planarity as the macrocyclic chain length in imine nitrogen compartment increases.

Reduction and Oxidation of the Arachno Clusters Fe2(CO)6(μ-S)2PtL2: Characterization of Distinct One- and Two-Electron Transfer Sites in Heteropolynuclear Compounds

The reduction and oxidation properties of the arachno-heteropolynuclear metal clusters Fe2(CO)6(μ3-S)2PtL2, where L2 = bipyridine (1), phenanthroline (2), diphenylphosphinoethane (3), diphenylphosphinoferrocene (4), or cyclooctadiene (5), have been examined by electrochemistry, IR spectroscopy, and ESR spectroscopy. An electron-transfer series involving up to five electrons is observed. The common characteristics are a fully reversible one-electron oxidation process to a monocation (in the range 0.13−0.38 V vs ferrocene) and a chemically reversible but electrochemically quasi-reversible two-electron reduction process (range −1.9 to −2.4 V). Complexes 1 and 2 also display a fully reversible one-electron reduction at a potential less negative (ca. −1.8 V) than the subsequent two-electron reduction. The LUMOs of 1 and 2 are predicted by EHMO calculations to be primarily π*-diimine based. Spectral data from the monoanions 1- and 2- are consistent with this orbital assignment: very small shifts of the metal−carbonyl IR frequencies (ca. −13 cm-1) from those of the neutral complexes are observed, and the 195Pt hyperfine splitting in ESR spectra is very small compared to that expected for a metal-localized radical. The two-electron reductions forming, e.g., 13- or 32-, appear to involve the Fe2S2 moiety and most likely are concomitant with breaking and making of an Fe−Fe bond. Carbonyl IR energies are lowered in excess of 100 cm-1 in the two-electron reduction. The oxidation processes appear to be localized on the Fe2(CO)6S2 fragment, with IR shifts of 46−76 cm-1 for 1+−5+ and small 195Pt hyperfine splittings. X-ray crystal structures are reported for 2 and 5.

Conformational Composition and Spin Properties of β-Phosphone Nitroxyl Radicals in Gaseous Phase and Solutions

As part of our studies of the extremely high sensitivity to substitutients A,B,R, temperature and solvents of phosphorus hyperfine splitting in ESR spectra of β-phosphone nitroxyl radicals ABP(O)CR2N(Î)Bu1, we have studied them theoretically. The conformational composition and spin delocalization in H2(O)CH2N(Î)Í, (ÌåÌ)2P(O)CÌå2N(Î)Ìå and (ÍÎ)2P(O)CÌ˚2N(Î)Bu' has been studied by UHF/6-31G& and MNDO (GAMESS). Solvent effects on conformational equilibrium, rotational and inversion barriers, charge distributions and spin properties of β-phosphone nitroxyl were calculated by SCRF UHF/6-31G* method.

The capto-dative substituent effect on benzylic radicals ESR hyperfine splittings. An evaluation of the extra stabilization based upon spin delocalization

Appropriate hyperfine splittings in ESR spectra of disubstituted (cyano/methoxy) benzylic radicals were compared with the corresponding values of monosubstituted ones. On the basis of the dependence of radical stabilization on spin delocalization, it is concluded that captodative substitution implies a synergetic substituent effect on radical stabilization. On the contrary, an antagonistic effect is observed in the case of two identical substituents.

The 13C hyperfine splittings in ESR spectra of prop-2-ynyl and 1-(trimethylsiloxy)prop-2-ynyl radicals

The 13C satellite lines were observed in the 9.4 GHz ESR spectra of prop-2-ynyl and 1-trimethylsiloxyprop-2-ynyl radicals generated from precursors containing 13C in natural abundance. The 13C hfs indicate that prop-2-ynyl radicals adopt the π-delocalized structure, not the σ-allenyl form. The hfs of 1-trimethylsiloxyprop-2-ynyl radicals indicate that there is appreciable delocalization of spin onto the trimethylsiloxy group and that the radical is essentially planar at C(1). A linear correlation of a(C) with a(H) is shown to hold for a number of neutral hydrocarbon π-delocalized radicals. The experimental hfs are compared with spin densities calculated by a variety of semiempirical and ab initio SCF MO methods.

Some remarks on the calculations of hyperfine splittings in the ESR spectra of conjugated radicals

Some consequences of non-orthogonality of the basic functions of LCAO approximation, on the relations used for calculations of hyperfine splittings in ESR spectra of conjugated radicals, are discussed. Special attention has been paid on the problem of transferability of the hyperfine coupling constants. The discussion is illustrated by some calculations of the hyperfine splittings for radical-ions of naphthalene, and tetracene.

Valence-Bond Studies of the Dependence upon Substituents of C13–H and Si29–H Coupling

An interpretation is presented for the additivity of substituent effects on the C13–H coupling constant, which has been observed previously in the high-resolution NMR spectra of substituted methanes. Each atom or group X is assigned a characteristic ``affinity'' for s character in the carbon hybrid orbital of the C—X bond. Distribution of s character among the carbon orbitals in accord with the relative s affinities of the four substituents leads to the observed additivity relation provided that the total s character is conserved. The valence-bond approach used with this model gives a linear relation between the s character of the carbon hybrid orbital involved in the C–H bond (αH2) and the observed C13–H coupling constant (JCH=500 αH2). Also, it allows the determination of the s character of the other carbon orbitals. The dependence of the s character of the C—X bond on the electronegativity of X is discussed in terms of electron spin and charge correlation. It is noted that the hybridization changes should affect not only JCH but also JHH, which is consistent with the observed proportionality between JCH(CH3X) and the cis and trans H–H coupling constants in CH2=C13HX. The treatment developed for methanes has been extended to JCH in substituted ethylenes and to the Si29–H coupling in silanes. For the former, introduction of the s-electron affinities, obtained from the values of JCH(CH3X) observed for the substituted methanes, leads to the result that JCH(CH2=C13HX) = JCH(CH2=CH2)+43 [JCH(CH3 X) – JCH(CH4)]. Values predicted in this manner are systematically larger than those observed, which implies that there is a small, negative π-electron contribution of 5 to 10 cps to JCH(CH2=C13HX). Such a contribution is compatible with current theories for proton and C13 hyperfine splittings in ESR spectra of free radicals. The Si29–H coupling constants observed in substituted silanes exhibit large, systematic deviations from the simple additivity found in the methanes. These deviations are explicable qualitatively in terms of changes in the Si–H-bond polarity.

Orientation Dependence of Magnetic Hyperfine Structure in Free Radicals

The anisotropic part (B term) of the spin Hamiltonian for a free radical is averaged to zero when there is rapid molecular tumbling. Otherwise, it combines with the isotropic part (A term) to produce orientation-dependent hyperfine splittings in ESR spectra. In the high-field case, the electron is strongly coupled to the external field. The nucleus, however, is effectively coupled to the electron's tensor field. Consequently, the two spins have, in general, different axes of quantization. Applying first-order perturbation theory, expressions are derived for the orientation dependence of the resonance in a single crystal and for the line shape in a powder.