A detailed investigation into the electronic structures of macrocyclic iron(II)-nitrosyl compounds and their similarities to ferrous heme-nitrosyls

作者：Sulok Corinne D; Bauer Jonathan L; Speelman Amy L; Weber Birgit; Lehnert Nicolai^*

来源：Inorganica Chimica Acta, 2012, 380: 148-160.

DOI：10.1016/j.ica.2011.09.039

• Nitric oxide
• Iron complexes
• MCD spectroscopy
• DFT calculations
• Model complexes

This paper presents vibrational (IR and FT-Raman) and electronic (UV-Vis absorption and magnetic circular dichroism (MCD)) spectra of the five-coordinate (5C) low-spin ferrous iron-nitrosyl model complexes [Fe(L1)(NO)] (1), [Fe(L2)(NO)], (2) and [Fe(L3)(NO)] (3) that contain Schiff base type tetradentate macrocyclic ligands. These complexes show structural and vibrational data that are very similar to corresponding 5C ferrous heme-nitrosyls. For example, complexes 1-3 exhibit N-O and Fe-NO stretching frequencies of 1630-1680 and 520-595 cm(-1), respectively, which compares well with v(N-O) and v(Fe-NO) of [Fe(TPP)(NO)] at 1697 and 532 cm(-1). In complexes 1-3, a strong Fe-NO sigma bond, mediated by the singly-occupied pi* orbital of NO and d(z2) of iron, and a medium strong Fe-NO pi backbond are present. This is again in close agreement with the electronic structures of 5C ferrous heme-nitrosyls. Interestingly, a stepwise increase in the strength of the Fe-NO sigma bond is observed in the order 1 %26lt; 2 %26lt; 3, as reflected by the direct correlation of the Fe-NO and N-O stretching frequencies in these complexes. Compared to the ferrous heme-nitrosyl model complex [Fe(TPP)(NO)], complexes 1-3 show a somewhat weaker Fe-NO sigma bond and in particular, a stronger Fe-NO pi backbond. The latter finding is reflected by the trend in v(N-O) and v(Fe-NO) vibrational frequencies between 3 and [Fe(TPP)(NO)]. In summary, complexes 1-3 serve as good spectroscopic models for 5C ferrous heme-nitrosyls. Correspondingly, these complexes can be used to locate d -%26gt; pi*(NO) CT transitions, which would then provide evidence where such transitions could be expected for the heme-nitrosyls. In the heme complexes, the identification of CT transitions between Fe and NO is hampered by the very intense pi -%26gt; pi* transitions of the heme macrocycle. In contrast, the ligands applied in 1-3 are not aromatic, and hence, show greatly reduced extinction coefficients for their pi -%26gt; pi* transitions. Unfortunately, no d -%26gt;pi*(NO) CT bands could be identified in the optical spectra of 1-3. The TD-DFT calculations predict that these features should be observed around 350 nm, which would be to higher energy of the Soret band in 5C ferrous heme-nitrosyls.

• 出版日期2012-1-15