PubMed Health⌕ Search

Biomedical subjects

Nizamuddin Shaikh

Publications and source records attributed to Nizamuddin Shaikh.

5 recordsLinked to original sources
PubMed2006

Consistent simulation of X- and Q-band EPR spectra of an unsymmetric dinuclear Mn2(II,III) complex.

Simulation of X- and Q-band electron paramagnetic resonance (EPR) spectra of an unsymmetric dinuclear [Mn(2)(II,III)L(mu-OAc)(2)]ClO(4) complex (1), (L is the dianion of 2-{[N,N-bis(2-pyridylmethyl)amino]methyl}-6-{[N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N-(2-pyridylmethyl)amino]methyl}-4-methylphenol) was performed using one consistent set of simulation parameters. Rhombic g-tensors and hyperfine tensors were necessary to obtain satisfactory simulation of the EPR spectra. The anisotropy of the effective hyperfine tensors of each individual (55)Mn ion was further analyzed in terms of intrinsic hyperfine tensors. Detailed analysis shows that the hyperfine anisotropy of the Mn(III) ion is a result of the Jahn-Teller effect and thus an inherent character. In contrast, the anomalous hyperfine anisotropy of the Mn(II) ion is attributed as being transferred from the Mn(III) ion through the spin exchange interaction. The anisotropy parameter for the Mn(II) is deduced as D(II)=-1.26+/-0.2cm(-1). This is the first reported D(II) value for a Mn(II) ion in a weakly exchange coupled mixed-valence Mn(2)(II,III) complex with a bis-mu-acetato-bridge. The [see text] electronic configuration of the Mn(III) ion in 1 is revealed by the negative sign of its intrinsic hyperfine tensor anisotropy, Deltaa(III)=a(z)-a(x,y)=-46cm(-1). Lower spectral resolution of the Q-band EPR spectrum as compared to the X-band EPR spectrum is associated to large line width broadening of the x- and y-components in contrast to the z-component. The origins of the unequal distribution of line width between the z- and x-, y-components are discussed.

Electron Spin Resonance Spectroscopy↗
PubMed2005

Syntheses, crystal structures, and spectroscopic and magnetic properties of [Mn2III(H2L1)(Cl4Cat)4.2H2O] and [Mn2III(H2L2)(Cl4Cat)4.2CH3CN.2H2O]infinity: temperature-dependent valence tautomerism in solution.

The synthesis, X-ray data, and electronic structures of two manganese(III) 1D polymers ligated by tetrachlorocatechol, [Mn(2)(III)(H(2)L(1))(Cl(4)Cat)(4).2H(2)O](infinity) (1) and [Mn(2)(III)(H(2)L(2))(Cl(4)Cat)(4).2CH(3)CN.2H(2)O](infinity) (2), are reported. The electronic structures of the complexes have been determined by UV-vis-near-IR, IR, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements. Both 1 and 2 are air stable in the solid state and in solution, unlike most of the previously reported o-quinone-chelated transition-metal complexes. Electronic spectroscopy exhibits a strong near-IR band near 1900 nm for both, suggesting the presence of a mixed-valence semiquinone-catecholate oxidation state of the catechol ligands, Mn(2)(III)(Cl(4)Cat)(2)(Cl(4)SQ)(2), together with the pure catecholate forms. The presence of this isomer was further supported by EPR and magnetic susceptibility measurements. The complexes undergo intramolecular electron transfer (valence tautomerism) upon an increase of the temperature involving the equilibrium Mn(2)(III)(Cl(4)Cat)(2)(Cl(4)SQ)(2) <==> Mn(2)(II)(Cl(4)SQ)(4). This phenomenon is reversible and is studied in solution using UV-vis-near-IR spectroscopy.

Journal Article↗
PubMed2005

Light induced manganese oxidation and long-lived charge separation in a Mn2II,II-RuII(bpy)3-acceptor triad.

The photoinduced electron-transfer reactions in a Mn2II,II-RuII-NDI triad (1) ([Mn2(bpmp)(OAc)2]+, bpmp = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenolate and OAc = acetate, RuII = tris-bipyridine ruthenium(II), and NDI = naphthalenediimide) have been studied by time-resolved optical and EPR spectroscopy. Complex 1 is the first synthetically linked electron donor-sensitizer-acceptor triad in which a manganese complex plays the role of the donor. EPR spectroscopy was used to directly demonstrate the light induced formation of both products: the oxidized manganese dimer complex (Mn2II,III) and the reduced naphthalenediimide (NDI*-) acceptor moieties, while optical spectroscopy was used to follow the kinetic evolution of the [Ru(bpy)3]2+ intermediate states and the NDI*- radical in a wide temperature range. The average lifetime of the NDI*- radical is ca. 600 micros at room temperature, which is at least 2 orders of magnitude longer than that for previously reported triads based on a [Ru(bpy)3]2+ photosensitizer. At 140 K, this intramolecular recombination was dramatically slowed, displaying a lifetime of 0.1-1 s, which is comparable to many of the naturally occurring charge-separated states in photosynthetic reaction centra. It was found that the long recombination lifetime could be explained by an unusually large reorganization energy (lambda approximately 2.0 eV), due to a large inner reorganization of the manganese complex. This makes the recombination reaction strongly activated despite the large driving force (Delta-G degrees = 1.07 eV). Thus, the intrinsic properties of the manganese complex are favorable for creating a long-lived charge separation in the "Marcus normal region" also when the charge separated state energy is high.

Journal Article↗
PubMed2004

New route to the mixed valence semiquinone-catecholate based mononuclear FeIII and catecholate based dinuclear MnIII complexes: first experimental evidence of valence tautomerism in an iron complex.

The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] x DMF (1), and catecholate based (H2bispictn)[Mn2III(Cl4Cat)4(DMF)2] (2) (bispicen = N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl4Cat = tetrachlorocatecholate dianion, and Cl4SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffraction study. The electronic structures have been elucidated by UV-vis-NIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramolecular electron transfer, a phenomenon known as valence tautomerism (VT), has been followed by electronic absorption spectroscopy. For 1, the isomeric form [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] is favored at low temperature, while at an elevated temperature, the [FeII(bispicen)(Cl4SQ)2] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the MnIII(Cat)2- moiety is admixed with its mixed valence semiquinone-catecholate isomer MnII(SQ)(Cat)-, and the electronic absorption spectrum is dominated by the mixed charged species. The origin of the intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form, MnII(Cl4Cat)(Cl4SQ)-. The observation of VT in complex 1 is the first example where a mixed valence semiquinone-catecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes.

Journal Article↗