Juliusz A. Wolny

Vibrational spectrum of the spin crossover complex [Fe(phen)2(NCS)2] studied by IR and Raman spectroscopy, nuclear inelastic scattering and DFT calculations

The vibrational modes of the low-spin and high-spin isomers of the spin crossover complex [Fe(phen)(2)(NCS)(2)] (phen = 1,10-phenanthroline) have been measured by IR and Raman spectroscopy and by nuclear inelastic scattering. The vibrational frequencies and normal modes and the IR and Raman intensities have been calculated by density functional methods. The vibrational entropy difference between the two isomers, DeltaS(vib), which is--together with the electronic entropy difference DeltaS(el)--the driving force for the spin-transition, has been determined from the measured and from the calculated frequencies. The calculated difference (DeltaS(vib) = 57-70 J mol(-1) K(-1), depending on the method) is in qualitative agreement with experimental values (20-36 J mol(-1) K(-1)). Only the low energy vibrational modes (20% of the 147 modes of the free molecule) contribute to the entropy difference and about three quarters of the vibrational entropy difference are due to the 15 modes of the central FeN(6) octahedron.

Biosynthesis of isoprene units: Mössbauer spectroscopy of substrate and inhibitor binding to the [4Fe-4S] cluster of the LytB/IspH enzyme.

The biosynthesis of isoprenoids in many bacteria and in the malaria parasite Plasmodium falciparum occurs according to the methylerythritol phosphate (MEP) pathway,[1] an alternative to the mevalonate pathway.[2] The MEP pathway is a valuable target for the development of new antimicrobial agents as it is essential for microorganisms, and absent in humans.[3] In the last step of this biosynthetic route, HMBPP 1 is converted into a mixture of IPP and DMAPP, which are both precursors of isoprenoids (Scheme 1). This reaction is catalyzed by a peculiar [4Fe-4S] center of the LytB/IspH protein. [4]

Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition

We discover that hcp phases of Fe and Fe(0.9)Ni(0.1) undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mössbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.

Two-step spin transition in a 1D FeII 1,2,4-triazole chain compound

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(βAlatrz)3](BF4)2⋅2 H2O (1⋅2 H2O), whose precursor βAlatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a β-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), (57)Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two-step spin crossover (T1/2(↓) = 230 K and T1/2(↑) = 235 K, and T1/2(↓) = 172 K and T1/2(↑) = 188 K, respectively) is registered for the first time for a 1,2,4-triazole-based Fe(II) 1D coordination polymer. The two-step SCO configuration is observed in a 1:2 ratio of low-spin/high-spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1⋅2 H2O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low-spin and high-spin states of 1⋅2 H2O. Vibrational spectra of 1⋅2 H2O reveal that the BF4(-) anions and water molecules play no significant role on the vibrational properties of the [Fe(βAlatrz)3](2+) polymeric chains, although non-coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(βAlatrz)3](BF4)2 (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated.

Nuclear Inelastic Scattering and Mössbauer Spectroscopy as Local Probes for Ligand Binding Modes and Electronic Properties in Proteins: Vibrational Behavior of a Ferriheme Center Inside a β-Barrel Protein

In this work, we present a study of the influence of the protein matrix on its ability to tune the binding of small ligands such as NO, cyanide (CN(-)), and histamine to the ferric heme iron center in the NO-storage and -transport protein Nitrophorin 2 (NP2) from the salivary glands of the blood-sucking insect Rhodnius prolixus. Conventional Mössbauer spectroscopy shows a diamagnetic ground state of the NP2-NO complex and Type I and II electronic ground states of the NP2-CN(-) and NP2-histamine complex, respectively. The change in the vibrational signature of the protein upon ligand binding has been monitored by Nuclear Inelastic Scattering (NIS), also called Nuclear Resonant Vibrational Spectroscopy (NRVS). The NIS data thus obtained have also been calculated by quantum mechanical (QM) density functional theory (DFT) coupled with molecular mechanics (MM) methods. The calculations presented here show that the heme ruffling in NP2 is a consequence of the interaction with the protein matrix. Structure optimizations of the heme and its ligands with DFT retain the characteristic saddling and ruffling only if the protein matrix is taken into account. Furthermore, simulations of the NIS data by QM/MM calculations suggest that the pH dependence of the binding of NO, but not of CN(-) and histamine, might be a consequence of the protonation state of the heme carboxyls.

Estimate of the vibrational contribution to the entropy change associated with the spin transition in the d4 systems [MnIII(pyrol)3tren] and [CrII(depe)2I2]

The vibrational contribution to DeltaS of the low-spin ((3)T(1)) to high-spin ((5)E) spin transition in two 3d(4) octahedral systems [Mn(III)(pyrol)(3)tren] and [Cr(depe)(2)I(2)] have been estimated by means of DFT calculations (B3LYP/CEP-31G) of the vibrational normal-modes frequencies. The obtained value at the transition temperature for the Mn(iii) complex is DeltaS(vib)(44 K) = 6.3 J K(-1) mol(-1), which is comparable with the proposed Jahn-Teller contribution of R ln3 = 9.1 J K(-1) mol(-1) and which is approximately half of the experimentally determined 13.8 J K(-1) mol(-1). The corresponding value for the Cr(ii) complex is DeltaS(vib)(171.45 K) = 46.5 J K(-1) mol(-1), as compared to the experimental value of 39.45 J K(-1) mol(-1). The analysis of the vibrational normal modes reveals that for the d(4) systems under study, contrary to Fe(ii) d(6) systems, not all metal-ligand stretching vibrations make a contribution. For the Mn(iii) complex, the only vibration that contributes to DeltaS(vib) involve the nitrogens occupying the Jahn-Teller axis, while in the case of Cr(ii) the contributing vibrations involve the Cr-I bonds. Low-frequency modes due to ring vibrations, metal-ligand bending and movement of the molecule as a whole also contribute to the vibrational entropy associated with the spin transition.

Spin relaxation in antiferromagnetic Fe–Fe dimers slowed down by anisotropic DyIII ions

Summary By using Mössbauer spectroscopy in combination with susceptibility measurements it was possible to identify the supertransferred hyperfine field through the oxygen bridges between DyIII and FeIII in a {Fe4Dy2} coordination cluster. The presence of the dysprosium ions provides enough magnetic anisotropy to “block” the hyperfine field that is experienced by the iron nuclei. This has resulted in magnetic spectra with internal hyperfine fields of the iron nuclei of about 23 T. The set of data permitted us to conclude that the direction of the anisotropy in lanthanide nanosize molecular clusters is associated with the single ion and crystal field contributions and 57Fe Mössbauer spectroscopy may be informative with regard to the the anisotropy not only of the studied isotope, but also of elements interacting with this isotope.

Cobalt(II) triazene 1-oxide bis(chelates). A case of planar (low spin)–tetrahedral (high spin) isomerism

High- and low-spin cobalt(II) trizene 1-oxide bis(chelates) have been isolated. The low-spin complexes possess square-planar structure whilst the high-spin complexes are tetrahedral. The molecular structure of high-spin [Co(OMeN3C6H4Me-4)2] has been determined: triclinic, space group P, Z= 2, a= 7.970(5), b= 10.174(5), c= 11.676(5)A, α= 87.18(4), β= 74.31(4) and γ= 74.06(4)°. For some complexes the isolation of both planar (low-spin) and tetrahedral (high-spin) isomers or of their conglomerates is possible depending on the synthesis conditions. The crystal structure of square-planar [Ni(OMeN3C6H4-Me-4)2], which is isomorphous with the low-spin isomer of [Co(OMeN3C6H4Me-4)2], has been determined: triclinic, space group P, Z= 1, with a= 7.495(2), b= 7.694(5), c= 8.612(3)A, α= 64.64(5), β= 87.84(2) and γ= 78.64(3)°. The complexes exhibit a planar-tetrahedral equilibrium in non-co-ordinating solvents, the ΔH⊖ and ΔS⊖ values of which, determined from solution magnetic susceptibility measurements, are in the range 1–15 kJ mol–1 and 5–30 J K–1 mol–1, respectively. The electrochemical properties of the complexes are given.

ESR studies of low-spin [N1,N4-bis(salicylidene)-S-alkyl-isothiosemicarbazonato]cobalt(II) complexes doped into the lattices of corresponding zinc(II) chelates

Abstract Direct template reaction of salicylaldehyde S-alkyl-isothiosemicarbazone with salicylaldehyde and its derivatives, in the presence of cobalt(II) [or zinc(II)] acetate, triethylamine and triethyl orthoformate yields the aqua-adducts of the corresponding N 1 ,N 4 -bis(salicylidene)-S-alkyl-isothiosemicarbazone complexes. ESR measurements of cobalt doped zinc complexes were performed. The spectra are typical for the z 2 > 1 ground state. The energies of the two excited doublet states were estimated on the basis of spin Hamiltonian parameters. A six-coordinate structure with coordination of the phenolic oxygen of an adjacent molecule has been proposed for the CoL·H 2 O complexes. The formation of low-spin pyridine adducts of the title complexes in the solid was shown.

X-ray and electrochemical characterization of high-spin cobalt(II) triazene-1-oxide complexes. Molecular structure of bis[3-(2-methoxyphenyl)-1-ethyltriazen-1-olato]cobalt(II)

Abstract The X-ray crystal structure of the title compound was determined. The complex displays strongly distorted octahedral geometry with weak bonds between the methoxy group oxygens and the central ion. The related complex with ortho-chlorine substituted ligands was shown to undergo conversion to the low-spin, square-planar isomer upon doping to the lattice of its nickel(II) analogue. Cyclic voltammery measurements showed that the oxidation potentials of the high-spin, pseudo-octahedral cobalt(II) complexes of triazene-l-oxides are ca 0.1–0.2 V higher than those found for low-spin, square-planar cobalt(II) bis-chelates thereof.