Spectroscopic Properties and Electronic Structure of Five- and Six-Coordinate Iron(II) Porphyrin NO Complexes: Effect of the Axial N-Donor Ligand

In this paper, the differences in the spectroscopic properties and electronic structures of five- and six-coordinate
iron(II) porphyrin NO complexes are explored using [Fe(TPP)(NO)] (1; TPP = tetraphenylporphyrin) and [Fe(TPP)(MI)(NO)] (2; MI = 1-methylimidazole) type systems. Binding of N-donor ligands in axial position trans to NO to
five-coordinate complexes of type 1 is investigated using UV−vis absorption and 1H NMR spectroscopies. This
way, the corresponding binding constants Keq are determined and the 1H NMR spectra of 1 and 2 are assigned for
the first time. In addition, 1H NMR allows for the determination of the degree of denitrosylation in solutions of 1 with
excess base. The influence of the axial ligand on the properties of the coordinated NO is then investigated. Vibrational
spectra (IR and Raman) of 1 and 2 are presented and assigned using isotope substitution and normal-coordinate
analysis. Obtained force constants are 12.53 (N−O) and 2.98 mdyn/Å (Fe−NO) for 1 compared to 11.55 (N−O)
and 2.55 mdyn/Å (Fe−NO) for 2. Together with the NMR results, this provides experimental evidence that binding
of the trans ligand weakens the Fe−NO bond. The principal bonding schemes of 1 and 2 are very similar. In both
cases, the Fe−N−O subunit is strongly bent. Donation from the singly occupied π* orbital of NO into dz2 of iron(II)
leads to the formation of an Fe−NO σ bond. In addition, a medium-strong π back-bond is present in these complexes.
The most important difference in the electronic structures of 1 and 2 occurs for the Fe−NO σ bond, which is
distinctively stronger for 1 in agreement with the experimental force constants. The increased σ donation from NO
in 1 also leads to a significant transfer of spin density from NO to iron, as has been shown by magnetic circular
dichroism (MCD) spectroscopy in a preceding Communication (Praneeth, V. K. K.; Neese, F.; Lehnert, N. Inorg.
Chem. 2005, 44, 2570−2572). This is confirmed by the 1H NMR results presented here. Hence, further experimental
and computational evidence is provided that complex 1 has noticeable FeINO+ character relative to 2, which is an
FeIINO(radical) complex. Finally, using MCD theory and quantum chemical calculations, the absorption and MCD
C-term spectra of 1 and 2 are assigned for the first time.