Biomedical subjects

Dennis L Lichtenberger

Publications and source records attributed to Dennis L Lichtenberger.

12 recordsLinked to original sources

PubMed2006

Iron-only hydrogenase mimics. Thermodynamic aspects of the use of electrochemistry to evaluate catalytic efficiency for hydrogen generation.

Voltammetry is widely used for the evaluation of iron-only hydrogenase mimics and other potential catalysts for hydrogen generation using various dipolar aprotic solvents. Effective catalysts show enhanced current in the presence of a proton donor at the potential where the catalyst is reduced. To facilitate the comparison of catalytic efficiencies, this paper provides a simple means of calculating the standard potential for reduction of the acid, HA, according to the half reaction 2HA + 2e- <==> H2 + 2A-. This standard potential depends on the pKa of HA in the solvent being used. It is thermodynamically impossible for reduction of HA to occur at less negative potentials than the standard potential, and the most effective catalysts will operate at potentials as close as possible to the standard potential. In addition, direct reduction of HA at the electrode will compete with the catalyzed reduction, thus complicating evaluation of the rate of the catalyzed reaction. Glassy carbon electrodes, commonly used in such evaluations, show a quite large overpotential for direct reduction of HA so that the necessary corrections are small. However, catalysis at very negative potentials will be contaminated by significant direct reduction of HA at glassy carbon. It is demonstrated that direct reduction can be almost completely suppressed by using a mercury or amalgamated gold electrode, even at very negative potentials.

Acids↗

PubMed2006

Photoelectron spectroscopy and DFT calculations of easily ionized quadruply bonded Mo2(4+) compounds and their bicyclic guanidinate precursors.

A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo2(4+) units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo2(TEhpp)4 complex which has a broad first ionization band centered around 4.27 +/- 0.03 eV and an ionization onset at the very low energy of 3.93 +/- 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo-Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (eta5-C9Me7)2Co and (eta5-C5Me5)2Cr.

Journal Article↗

PubMed2006

NMR and EPR studies of chloroiron(III) tetraphenyl-chlorin and its complexes with imidazoles and pyridines of widely differing basicities.

The NMR and EPR spectra of two bisimidazole and three bispyridine complexes of tetraphenylchlorinatoiron(III), [(TPC)Fe(L)2]+ (L = Im-d4, 2-MeHIm, 4-Me2NPy, Py, and 4-CNPy), have been investigated. The full resonance assignments of the [(TPC)Fe(L)2]+ complexes of this study have been made from correlation spectroscopy (COSY) and nuclear Overhauser enhancement spectroscopy (NOESY) experiments and Amsterdam density functional (ADF) calculations. Unlike the [(OEC)Fe(L)2]+ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44, 1890-1903), the NMR data for the [(TPC)Fe(L)2]+ complexes of this study indicate that the ground state is S = 1/2 for each bisligand complex, whereas a higher spin state was present at NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all [TPCFe(L)2]+ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons that are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases with decreasing donor ability of the substituted pyridine ligands, with the nonhindered imidazole ligand having slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower (4-Me2NPyH+ pKa = 9.7, H2Im+ pKa = 6.65 (adjusted for the statistical factor of 2 protons)). The temperature dependence of the chemical shifts of all but the 4-Me2NPy bisligand complexes studied over the temperature range of the NMR investigations shows that they have mixed (dxy)2(dxz,dyz)3/(dxzdyz)4(dxy)1 electron configurations that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with an S = 3/2 excited state in each case that in most cases lies at more than kT at room temperature above the ground state. The observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicate that it has a pure (dxzdyz)4(dxy)1 ground state and that it is ruffled, because ruffling mixes the a(2u)(pi)-like orbital of the chlorin into the singly occupied molecular orbital (SOMO). This mixing accounts for the negative chemical shift of the pyrroline-H (-6.5 ppm at -40 degrees C) and thus the negative spin density at the pyrroline-alpha-carbons, but the mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)2]+, whose pyrroline-H chemical shift is -36 ppm at 25 degrees C (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct.

Electron Spin Resonance Spectroscopy↗

PubMed2006

Competition between metal-amido and metal-imido chemistries in the alkaline earth series: an experimental and theoretical study of BaNH.

The pure rotational spectrum of BaNH in its X(1)Sigma(+) ground electronic state has been recorded using millimeter/submillimeter direct absorption methods; data for the deuterium and barium 137 isotopomers have been measured as well. The molecules were produced by the reaction of ammonia or ND(3) and barium vapor in the presence of a dc discharge. Transitions arising from the ground vibrational state and the excited vibrational bending (01(1)0) and heavy atom stretching (100) modes were measured. The rotational spectrum indicates a linear structure, with B(0)(BaNH) = 7984.549 MHz and B(0)(BaND) = 7060.446 MHz. An r(m)((1)) structure has been determined, yielding r(BaN) = 2.077 +/- 0.002 Angstroms and r(NH) = 1.0116 +/- 0.0006 Angstroms. Density functional calculations using an extensive Slater-type basis set with inclusion of scalar relativistic effects gives geometrical parameters and vibrational frequencies for BaNH in excellent agreement with those determined by experiment. The molecular orbital and natural bond order analyses of the BaNH wave function show Ba-N pi bonds formed by electron donation from the formally filled N 2p orbitals of the imido group to the empty Ba 5d orbitals. The multiple bonding between Ba and N stabilizes the linear geometry and, along with the relative ease of oxidation of the Ba atom, favors formation of the metal-imido species over that of the metal-amido species that have been found from similar studies with Mg, Ca, and Sr atoms in this group.

Journal Article↗

PubMed2006

Facilitating access to the most easily ionized molecule: an improved synthesis of the key intermediate, W2(hpp)4Cl2, and related compounds.

A far superior synthesis is reported for W(2)(hpp)(4)Cl(2), a key intermediate in the synthesis of the most easily ionized closed-shell molecule W(2)(hpp)(4) (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). At 200 degrees C, the one-pot reaction of the air-stable and commercially available compounds W(CO)(6) and Hhpp in o-dichlorobenzene produces W(2)(hpp)(4)Cl(2) in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)(2)(CO)(4) or W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2), which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W(2)(hpp)(4)Cl(2) and W(2)(hpp)(4). Two additional derivatives, W(2)(hpp)(4)X(2) where X is PF(6)(-) or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2) and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W(2)(hpp)(4)(TFPB)(2) in THF shows two reduction processes with an E(1/2) of -0.97 V for the first and -1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W(2)(hpp)(4)(2+) units in W(2)(hpp)(4)X(2) compounds show that the metal-metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W-W distances. The low-energy gas-phase ionizations of W(2)(hpp)(4) are also reported and discussed.

Chlorides↗

PubMed2005

Expeditious access to the most easily ionized closed-shell molecule, W2(hpp)4.

A new synthetic path, far superior to either of those previously available, to the W2(hpp)4 molecule (Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) is reported. The reaction of W(CO)6 with Hhpp in o-dichlorobenzene at 200 degrees C produces W2(hpp)4Cl2 in a one-pot reaction in over 90% yield. This compound is stable and easily stored for further use, and it can be efficiently reduced in a one-step reaction to the title compound W2(hpp)4.

Electrochemistry↗

PubMed2005

NMR and EPR studies of the bis(pyridine) and bis(tert-butyl isocyanide) complexes of iron(III) octaethylchlorin.

The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L)2]+ (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (d(xy))2 (d(xz)d(yz))3 to (d(xz)d(yz))4 (d(xy))1. The NMR data from 303 to 183 K show that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)2]+ complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large pi spin density on the adjacent carbons which are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Hückel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(II) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)2]+ looks somewhat like that of [(OEC)Fe(4-Me2NPy)2]+, while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)Fe(t-BuNC)2]+. The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)2]+ shows that it has a mixed (d(xz)d(yz))4 (d(xy))1 and (d(xy))2 (d(xz),d(yz))3 electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the d(yz) orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the d(xy) orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm(-1), explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the alpha-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)2]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both sigma and pi spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring.

Crystallography, X-Ray↗

PubMed2004

Inner-sphere electron-transfer reorganization energies of zinc porphyrins.

Inner-sphere electron-transfer reorganization energies of Zn(protoporphyrin IX) and Zn(octaethylporphyrin) are determined from band-shape analyses of the first ionization obtained by gas-phase valence photoelectron spectroscopy. The experimentally determined total inner-sphere reorganization energies for self-exchange (120-140 meV) indicate that structural changes upon oxidation are largely confined to the porphyrin ring, and substituents on the ring or solvent and other environmental factors make smaller contributions. Computational estimates by different models vary over a wide range and are sensitive to numerical precision factors for these low reorganization energies. Of current computational models that are widely available and practical for molecules of this size, functionals that contain a mixture of Hartree-Fock exchange and DFT exchange-correlation appear to be the most applicable.

Metalloporphyrins↗

PubMed2004

Pentadienyls vs cyclopentadienyls and reversal of metal-ligand bonding affinity with metal oxidation state: synthesis, molecular structures, and electronic structures of high-valent zirconium pentadienyl complexes.

Molecules of the form Cp(6,6-dmch)ZrX(2) (Cp = eta(5)-cyclopentadienyl, X = Cl, Br, I; 6,6-dmch = eta(5)-6,6-dimethylcyclohexadienyl) have been synthesized, and the molecular and electronic structures have been investigated. These molecules allow direct comparison of the bonding and properties of pentadienyl and cyclopentadienyl ligands in the same high-oxidation-state metal complexes. Unlike the well-known Cp(2)ZrX(2) analogues, these Cp(6,6-dmch)ZrX(2) molecules are intensely colored, indicating significantly different relative energies of the frontier orbitals. Also unusual, the average Zr-C distances to the 6,6-dmch pentadienyl ligand are about 0.1 A longer than the average Zr-C distances to the cyclopentadienyl ligand for these Zr(IV) complexes, opposite of what is observed for the Zr(II) complex Cp(2,6,6-tmch)Zr(PMe(3))(2) (tmch = eta(5)-2,6,6-trimethylcyclohexadienyl), reflecting a dramatic reversal in the favorability of the bonding depending on the metal oxidation state. The experimental and computational results indicate that the color of the Cp(6,6-dmch)ZrX(2) complexes is due to a 6,6-dmch ligand-to-metal charge-transfer band. Compared to the Cp(2)ZrX(2) analogues, the Cp(6,6-dmch)ZrX(2) molecules have a considerably less stable HOMO that is pentadienyl-based and an essentially unchanged metal-based LUMO. Also, the lowest unoccupied orbital of pentadienyl is stabilized relative to cyclopentadienyl and becomes a better potential delta electron acceptor, thus contributing to the differences in structure and reactivity of the low-valent and high-valent metal complexes.

Journal Article↗

PubMed2003

Investigation of metal-dithiolate fold angle effects: implications for molybdenum and tungsten enzymes.

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the interactions between the sulfur pi-orbitals of arene dithiolates and high-valent transition metals as minimum molecular models of the active site features of pyranopterin MoW enzymes. The compounds (Tp*)MoO(bdt) (compound 1), Cp(2)Mo(bdt) (compound 2), and Cp(2)Ti(bdt) (compound 3) [where Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate, bdt is 1,2-benzenedithiolate, and Cp is eta(5)- cyclopentadienyl] provide access to three different electronic configurations of the metal, formally d(1), d(2), and d(0), respectively. The gas-phase photoelectron spectra show that ionizations from occupied metal and sulfur based valence orbitals are more clearly observed in compounds 2 and 3 than in compound 1. The observed ionization energies and characters compare very well with those calculated by density functional theory. A "dithiolate-folding-effect" involving an interaction of the metal in-plane and sulfur-pi orbitals is proposed to be a factor in the electron transfer reactions that regenerate the active sites of molybdenum and tungsten enzymes.

Aldehyde Oxidase↗

PubMed2002

Closed-shell molecules that ionize more readily than cesium.

We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies.

Journal Article↗

PubMed2002

Vibrational progressions in the valence ionizations of transition metal hydrides: evaluation of metal-hydride bonding and vibrations in (eta(5)-C(5)R(5))Re(NO)(CO)H [R = H, CH(3)].

The first examples of vibrational structure in metal-ligand sigma-bond ionizations are observed in the gas-phase photoelectron spectra of CpRe(NO)(CO)H and CpRe(NO)(CO)H [Cp = eta(5)-C(5)H(5), Cp = eta(5)-C(5)(CH(3))(5)]. The vibrational progressions are due to the Re-H stretch in the ion states formed by removal of an electron from the predominantly Re-H sigma-bonding orbitals. A vibrational progression is also observed in the corresponding ionization of the deuterium analogue, CpRe(NO)(CO)D, but with lower vibrational energy spacing as expected from the reduced mass effect. The vibrational progressions in these valence ionizations are directly informative about the nature of the metal-hydride bonding and electronic structure in these molecules. Franck-Condon analysis shows that for these molecules the Re-H or Re-D bond lengthens by 0.25(1) A when an electron is removed from the Re-H or Re-D sigma-bond orbital. This bond lengthening is comparable to that of H(2) upon ionization. Removal of an electron from the Re-H or Re-D bonds leads to a quantum-mechanical inner sphere reorganization energy (lambda(QM)) of 0.34(1) eV. These observations suggest that even in these low symmetry molecules the orbital corresponding to the Re-H sigma bond and the Re-H vibrational mode is very localized. Theoretical calculations of the electronic structure and normal vibrational modes of CpRe(NO)(CO)H support a localized two-electron valence bond description of the Re-H interaction.

Journal Article↗