Structural and spectroscopic studies of tetrachlorophosphonium tetrachloroferrate(III), [PCl4][FeCl4]

K- and L-edge X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) Determination of Differential Orbital Covalency (DOC) of Transition Metal Sites

Continual advancements in the development of synchrotron radiation sources have resulted in X-ray based spectroscopic techniques capable of probing the electronic and structural properties of numerous systems. This review gives an overview of the application of metal K-edge and L-edge X-ray absorption spectroscopy (XAS), as well as K resonant inelastic X-ray scattering (RIXS), to the study of electronic structure in transition metal sites with emphasis on experimentally quantifying 3d orbital covalency. The specific sensitivities of K-edge XAS, L-edge XAS, and RIXS are discussed emphasizing the complementary nature of the methods. L-edge XAS and RIXS are sensitive to mixing between 3d orbitals and ligand valence orbitals, and to the differential orbital covalency (DOC), that is, the difference in the covalencies for different symmetry sets of the d orbitals. Both L-edge XAS and RIXS are highly sensitive to and enable separation of and donor bonding and back bonding contributions to bonding. Applying ligand field multiplet simulations, including charge transfer via valence bond configuration interactions, DOC can be obtained for direct comparison with density functional theory calculations and to understand chemical trends. The application of RIXS as a probe of frontier molecular orbitals in a heme enzyme demonstrates the potential of this method for the study of metal sites in highly covalent coordination sites in bioinorganic chemistry.

Prediction of high-valent iron K-edge absorption spectra by time-dependent density functional theory

In recent years, a number of high-valent iron intermediates have been identified as reactive species in iron-containing metalloproteins. Inspired by the interest in these highly reactive species, chemists have synthesized Fe(IV) and Fe(V) model complexes with terminal oxo or nitrido groups, as well as a rare example of an Fe(VI)-nitrido species. In all these cases, X-ray absorption spectroscopy has played a key role in the identification and characterization of these species, with both the energy and intensity of the pre-edge features providing spectroscopic signatures for both the oxidation state and the local site geometry. Here we build on a time-dependent DFT methodology for the prediction of Fe K- pre-edge features, previously applied to ferrous and ferric complexes, and extend it to a range of Fe(IV), Fe(V) and Fe(VI) complexes. The contributions of oxidation state, coordination environment and spin state to the spectral features are discussed. These methods are then extended to calculate the spectra of the heme active site of P450 Compound II and the non-heme active site of TauD. The potential for using these methods in a predictive manner is highlighted.

Probing valence orbital composition with iron Kbeta X-ray emission spectroscopy

A systematic study of 12 ferric and ferrous Kbeta X-ray emission spectra (XES) is presented. The factors contributing to the Kbeta main line and the valence to core region of the spectra are experimentally assessed and quantitatively evaluated. While the Kbeta main line spectra are dominated by spin state contributions, the valence to core region is shown to have greater sensitivity to changes in the chemical environment. A density functional theory (DFT) based approach is used to calculate the experimental valence spectra and to evaluate the contributions to experimental intensities and energies. The spectra are found to be dominated by iron np to 1s electric dipole allowed transitions, with pronounced sensitivity to spin state, ligand identity, ligand ionization state, hybridization state, and metal-ligand bond lengths. These findings serve as an important calibration for future applications to iron active sites in biological and chemical catalysis. Potential applications to Compound II heme derivatives are highlighted.

New synthetic procedures to catena-phosphorus cations: preparation and dissociation of the first cyclo-phosphino-halophosphonium salts

Chlorination of 1,2,3,4-tetracyclohexyl-cyclo-tetraphosphine (2) by PhICl(2) or PCl(5) in the presence of Me(3)SiOTf or GaCl(3) provides a stepwise approach to salts of the first cyclo-phosphino-chlorophosphonium cations [Cy(4)P(4)Cl](+) ([19](+)) and [Cy(4)P(4)Cl(2)](2+) ([20](2+)). The analogous iodo derivative [Cy(4)P(4)I](+) ([17](+)) is obtained as the tetraiodogallate salt from reaction of 2 with I(2) in the presence of GaI(3). Reactions of the dication [20](2+) with PMe(3) or dmpe effect a dissociation of the cyclic framework resulting in the formation of salts containing [Me(3)PPCyPCyPMe(3)](2+) ([27](2+)), [dmpeCyP](2+) ([29](2+)), and [dmpeCyPCyP](2+) ([30](2+)), respectively. The new cations represent phosphine complexes of the [PCy](2+) and [P(2)Cy(2)](2+) cationic fragments from [20](2+), demonstrating the coordinate nature of the phosphinophosphonium bonds in cyclo-phosphino-halophosphonium cations. The compounds have been characterized by NMR spectroscopy, single crystal X-ray crystallography, and Raman spectroscopy.

Prediction of iron K-edge absorption spectra using time-dependent density functional theory

Iron K-edge X-ray absorption pre-edge features have been calculated using a time-dependent density functional approach. The influence of functional, solvation, and relativistic effects on the calculated energies and intensities has been examined by correlation of the calculated parameters to experimental data on a series of 10 iron model complexes, which span a range of high-spin and low-spin ferrous and ferric complexes in O(h) to T(d) geometries. Both quadrupole and dipole contributions to the spectra have been calculated. We find that good agreement between theory and experiment is obtained by using the BP86 functional with the CP(PPP) basis set on the Fe and TZVP one of the remaining atoms. Inclusion of solvation yields a small improvement in the calculated energies. However, the inclusion of scalar relativistic effects did not yield any improved correlation with experiment. The use of these methods to uniquely assign individual spectral transitions and to examine experimental contributions to backbonding is discussed.

Normal-mode analysis of FeCl4- and Fe2S2Cl42- via vibrational mössbauer, resonance Raman, and FT-IR spectroscopies

[NEt(4)][FeCl(4)], [P(C(6)H(5))(4)][FeCl(4)], and [NEt(4)](2)[Fe(2)S(2)Cl(4)] have been examined using (57)Fe nuclear resonance vibrational spectroscopy (NRVS). These complexes serve as simple models for Fe-S clusters in metalloproteins. The (57)Fe partial vibrational density of states (PVDOS) spectra were interpreted by computation of the normal modes assuming Urey-Bradley force fields, using additional information from infrared and Raman spectra. Previously published force constants were used as initial values; the new constraints from NRVS frequencies and amplitudes were then used to refine the force field parameters in a nonlinear least-squares analysis. The normal-mode calculations were able to quantitatively reproduce both the frequencies and the amplitudes of the intramolecular-mode (57)Fe PVDOS. The optimized force constants for bending, stretching, and nonbonded interactions agree well with previously reported values. In addition, the NRVS technique also allowed clear observation of anion-cation lattice modes below 100 cm(-1) that are nontrivial to observe by conventional spectroscopies. These features were successfully reproduced, either by assuming whole-body motions of point-mass anions and cations or by simulations using all of the atoms in the unit cell. The advantages of a combined NRVS, Raman, and IR approach to characterization of Fe-S complexes are discussed.

L-edge X-ray Absorption Spectroscopy of Non-Heme Iron Sites: Experimental Determination of Differential Orbital Covalency

X-ray absorption spectroscopy has been utilized to obtain the L-edge multiplet spectra for a series of non-heme ferric and ferrous complexes. Using these data, a methodology for determining the total covalency and the differential orbital covalency (DOC), that is, differences in covalency in the different symmetry sets of the d orbitals, has been developed. The integrated L-edge intensity is proportional to the number of one-electron transition pathways to the unoccupied molecular orbitals as well as to the covalency of the iron site, which reduces the total L-edge intensity and redistributes intensity, producing shake-up satellites. Furthermore, differential orbital covalency leads to differences in intensity for the different symmetry sets of orbitals and, thus, further modifies the experimental spectra. The ligand field multiplet model commonly used to simulate L-edge spectra does not adequately reproduce the spectral features, especially the charge transfer satellites. The inclusion of charge transfer states with differences in covalency gives excellent fits to the data and experimental estimates of the different contributions of charge transfer shake-up pathways to the t(2g) and e(g) symmetry orbitals. The resulting experimentally determined DOC is compared to values calculated from density functional theory and used to understand chemical trends in high- and low-spin ferrous and ferric complexes with different covalent environments. The utility of this method toward problems in bioinorganic chemistry is discussed.

Crystal structure of lignocaine tetrachloroferrate(III) complex

The crystal structure of lignocaine tetrachloroferrate(III) complex, FeC14H22N2OCl4, has been determined by X-ray diffraction using CuKα radiation. The compound crystallizes in triclinic space group P[unk] with unit cell parameters a = 8.7151(2) Å, b = 10.147(2) Å, c = 12.215(3) Å, α = 107.19(2)°, β = 106.29(2)°, γ = 92.69(2)° with V = 980.5(4) Å3, Z = 2, Dm = 1.441 Mg · m−3, Dc = 1.463 Mg · m−3, μ = 22.75 mm−1. The structure was solved using 2728 reflections [I > 2.5σ(I)] out of 3230 reflections by MULTAN. The final residuals are Rf = 0.085 (Rw = 0.106).