Is the corrolate macrocycle innocent or noninnocent? Magnetic susceptibility, Mössbauer, 1H NMR, and DFT investigations of chloro- and phenyliron corrolates

Symmetry and bonding in metalloporphyrins. A modern implementation for the bonding analyses of five- and six-coordinate high-spin iron(III)-porphyrin complexes through density functional calculation and NMR spectroscopy

Bonding interactions between the iron and the porphyrin macrocycle of five- and six-coordinate high-spin iron(III)-porphyrin complexes are analyzed within the framework of approximate density functional theory with the use of the quantitative energy decomposition scheme in combination with removal of the vacant pi orbitals of the porphyrin from the valence space. Although the relative extent of the iron-porphyrin interactions can be evaluated qualitatively through the spin population and orbital contribution analyses, the bond strengths corresponding to different symmetry representations can be only approximated quantitatively by the orbital interaction energies. In contrast to previous suggestions, there are only limited Fe --> P pi back-bonding interactions in high-spin iron(III)-porphyrin complexes. It is the symmetry-allowed bonding interaction between d(z)2 and a(2u) orbitals that is responsible for the positive pi spin densities at the meso-carbons of five-coordinate iron(III)-porphyrin complexes. Both five- and six-coordinate complexes show significant P --> Fe pi donation, which is further enhanced by the movement of the metal toward the in-plane position for six-coordinate complexes. These bonding characteristics correlate very well with the NMR data reported experimentally. The extraordinary bonding interaction between d(z)2 and a(2u) orbitals in five-coordinate iron(III)-porphyrin complexes offers a novel symmetry-controlled mechanism for spin transfer between the axial ligand sigma system and the porphyrin pi system and may be critical to the electron transfer pathways mediated by hemoproteins.

Ab initio multiconfiguration reference perturbation theory calculations on the energetics of low-energy spin states of iron(III) porphyrins

Although a major goal of inorganic spectroscopy is to determine the energetics of the low-lying spin states of transition metal complexes, surprisingly little has been accomplished in this respect by means of accurate ab initio calculations. Against this context, we present ab initio multiconfiguration reference perturbation theory (CASPT2) calculations with large basis sets on the low-lying spin states of Fe(III)(P)Cl and [Fe(P)Cl](+) (P(2-)=porphinato). The CASPT2 results on the energetics of various low-lying spin states studied differ significantly, sometimes even dramatically, from those obtained from density functional theory calculations.

High-level ab initio calculations on the energetics of low-lying spin states of biologically relevant transition metal complexes: a first progress report

Although DFT is the unrivaled method of choice for quantum chemical studies of bioinorganic problems, little is known about its ability to predict the energetics of the low-lying electronic states of transition metal complexes. The first high-level ab initio calculations aimed at calibrating DFT vis-a-vis this issue indicate that, despite its many successes, DFT is far from infallible. In the short term, additional calibration of DFT against more elaborate ab initio methods remains an important goal for computational bioinorganic researchers. In the longer term, we are optimistic that high-level ab initio methods such as CASPT2 and CCSD(T) will be regularly used to study realistic molecules of genuine biochemical interest.

Quantum chemical calculations of spectroscopic properties of metalloproteins and model compounds: EPR and Mössbauer properties

Recently developed theoretical methods to predict EPR and Mössbauer parameters open the way for close interactions between theorists and experimentalists to elucidate the geometric and electronic structures of metalloenzymes and model complexes and to obtain insight into their reactive properties. Spectral calculations (g-values, hyperfine couplings, zero-field splittings, isomer shifts and quadrupole splittings) are also a means to validate theoretical approaches and therefore complement the prediction of geometries, reaction energies and transition states.

High-valent transition metal centers versus noninnocent ligands in metallocorroles: insights from electrochemistry and implications for high-valent heme protein intermediates

For relatively electron-rich corrole ligands, the halfwave potentials for oxidation of Cu(III), Sn(IV)Ph, Fe(IV)Ph, and Fe(IV)-O-Fe(IV) complexes are significantly lower than those of Sn(IV)Cl, Fe(IV)Cl, Mn(IV)Cl, and Cr(V)(O) complexes, suggesting that the corrole ligand is relatively electron-rich or 'innocent' in the former group of complexes and that it is relatively electron-deficient or 'noninnocent' in the latter group. Both the formal charge of the central metal ion and the nature of the axial ligand, if any, appear to be key determinants of the electronic character of the corrole ligand in metallocorrole complexes, a theme that has interesting resonances with recent findings on high-valent heme protein intermediates. However, for very strongly electron-deficient ligands such as meso-tris(pentafluorophenyl)corrole (TPFPC) and beta-octabromo-meso-tris(pentafluorophenyl)corrole (Br(8)TPFPC), which cannot sustain significant radical character, the various metal complexes all exhibit comparable halfwave potentials for oxidation and the ligand may be considered to be relatively innocent.