New Insights on the Electronic and Molecular Structure of Cyanide-Ligated Iron(III) Porphyrinates

Abstract

The preparation and characterization of several new cyanoligated six-coordinate low-spin iron(III) porphyrinates are reported. The synthesis and structure of the new bis(cyanide) derivative K(222)] [Fe(TMP)(CN) 2 ] (TMP= tetramesitylporphyrinate) is described. Three mixed-ligand species of the general form [Fe(Porph)(CN)(L)], where L = 1-methylimidazole or pyridine, have also been prepared and structurally characterized. All complexes have been studied with EPR spectroscopy in frozen solution and in microcrystalline form. In some cases, especially those of the bis(cyanide) derivative above and the previously reported [Fe(TPP)(CN) 2 ] − , there are significant differences in the EPR spectra as a result of the state change. These spectral differences can be correlated with changes in the electron configuration that are the likely result of a differing environment of the coordinated cyanide ligands; core conformation and electronic structure of the porphyrin ligand are unlikely to play a role. All four new complexes and [Fe(TPP)(CN) 2 ] − have been studied by Mössbauer spectroscopy with variable temperature and applied magnetic field measurements. The sign of the quadrupole splitting value has been established as negative. These measurements have allowed us to give estimates of the energy difference between the two close-lying d π (d xz and d yz ) orbitals. These splitting values range from ∼267 cm −1 for [Fe(TPP)(CN) 2 ] − to ∼614 cm −1 for [Fe(TPP)(CN)(Py)].

Correlations between the Electronic Properties of Shewanella oneidensis Cytochrome c Nitrite Reductase (ccNiR) and Its Structure: Effects of Heme Oxidation State and Active Site Ligation

“…bisecting the N-Fe-N right-angle (32) ). In addition, HALS-type signals have also been observed for low-spin ferric heme cyanide model compounds (33) . The S. oneidensis ccNiR crystal structure reveals two hemes with imidazoles roughly perpendicular (hemes 4 and 5, Fig.…”

Bis(cyano) Iron(III) Porphyrinates: What Is the Ground State?

“…Note the agreement with the observed solid-state structure of two K⋯N interactions in this crystalline form. Interestingly, we had previously observed 16 Δ E q = 1.15 mm/s and an isomer shift of 0.16 mm/s for [K(18-C-6)][Fe(TMP)(CN) 2 ], values intermediate to the pair above. This derivative has only one of the two cyanide ligands involved in a K⋯N interaction again pointing out the importance of cyanide ligand environment on the values of the Mössbauer spectrum.…”

“…The values for the first six entries in the table are from this work; the next two have been reported previously from work in these laboratories. 16 To our knowledge, these Mössbauer spectra are the only ones observed from complexes of known solid-state structure.…”

“…As expected, these values are in accord with our previous results, and which confirms again that bis(cyano) complexes show lower frequencies compared to mixed-ligand complexes because of the change in the σ -acceptor behavior of iron(III). 16 The C–N frequencies for all iron(II) derivatives are found at lower frequencies (Table 5). …”

“…16, 40–42 This work has led to our further interest in this topic, especially for the bis(cyano) species. Here we report six new bis(cyano) porphyrinate complexes which includes four [Fe(TPP)(CN) 2 ] − structures, two each with [K(18-C-6)] + or [K(222)] + cations, and [Fe(TMP)(CN) 2 ] − and [Fe(TTP)(CN) 2 ] − structures with [K(222)] + cations.…”