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

Abstract

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.

Paramagnetic NMR Shifts for Saddle‐Shaped Five‐Coordinate Iron(III) Porphyrin Complexes with Intermediate‐Spin Structure

“…Normally, the disappearance of bonding interactions was found when the coordination number was altered as, for example, from a five-to a six-coordinate iron(III) porphyrin complex. [13] However, although the crystal structure of Fe(OETPP)ClO 4 still shows the five-coordinate saddled shape with a C 2v local symmetry which resembles Fe(OETPP)Cl, obviously the d z 2 -a 2u interaction is not sufficient to simultaneously interpret both saddled complexes. [6] This difference between Fe(OETPP)Cl and Fe(OETPP) ClO 4 has not been observed for five-coordinate planar iron(III) …”

“…[4,6] Generally, the properties determined by Mçssbauer spectroscopy, EPR and magnetic susceptibility measurements in the series of Fe(OETPP)X (X = Cl, Br, I, ClO 4 ) complexes are similar to planar five-coordinate iron-(III) porphyrin complexes. [6][7][8] However, with careful examination of their paramagnetic NMR shifts, they exhibit a distinct difference in the 13 C NMR shifts at the meso-C position, which is important for analyzing the bonding interactions between a metal center and a porphyrin macrocycle. [7][8][9][10] For planar porphyrin ligands, such as high-spin FeTPPCl (or FeOEPCl), the meso-C signal appears at a downfield position of 500 ppm (or 380 ppm), and 368 ppm (or 246 ppm) for the admixed (5/2, 3/2) FeTPPClO 4 (or FeOEPClO 4 ), respectively (see Table 1; TPP = dianion of 5,10,15,20-tetraphenylporphyrin and OEP = dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin).…”

“…[11,12] These downfield shifts at the meso-C atoms have been attributed by Cheng et al to interactions between the iron(III) d z 2 and the porphyrin a 2u orbitals. [13] However, for saddled porphyrin ligands, considering the main skeleton of the macrocycle, the five-coordinate saddleshaped Fe(OETPP)Cl has a local C 2v symmetry, which is lower than the C 4v symmetry of five-coordinate planar iron(III) porphyrin complexes. Theoretically, Fe(OETPP)Cl shall have similar bonding interactions as found for a C 4v symmetry.…”

“…To clarify how these bonding interactions affect the chemical shifts, a detail analysis of the spin distribution on the macrocycle may reveal the relationship between decisive Fe-porphyrin bonding interactions and directions of 13 C NMR shifts for the significant carbon atoms of the macrocycle, and most importantly this analysis gives the chance to elucidate all components of the paramagnetic NMR shifts. The well-known shift terms of paramagnetic NMR shifts for S = 5/2 have been defined by the following Equations (1-7) for 13 C NMR shifts, [14,17,18] …”

“…The well-known shift terms of paramagnetic NMR shifts for S = 5/2 have been defined by the following Equations (1-7) for 13 C NMR shifts, [14,17,18] …”