Resonance Raman enhancement of FeIVO stretch in high-valent iron porphyrins: An insight from TD-DFT calculations
Introduction
Iron porphyrins [1] play an important role in many biological systems where they form active sites capable of binding oxygen and perform catalytic reactions. Such iron porphyrin mediated reactions usually involve activation and subsequent cleavage of the O–O bond resulting in the formation of a high-valent iron intermediates, which are powerful reactive species capable of carrying out a variety of chemical reactions [2], [3], [4]. The cleavage of the O–O bond can be, in principle, heterolytic or homolytic (Scheme 1), though the former mode is preferable in biological systems since eliminates formation of hazardous OH radicals. The heterolytic cleavage of the O–O bond can be formally associated with the FeVO moiety, which has iron in oxidation state +5. However the iron can not sustain such a high oxidation state and the more stable configuration corresponds to the FeIVO moiety with a radical located on the porphyrin ring (Por). Consequently, the resulting intermediate upon heterolytic cleavage is best characterized as [L–(Por)Fe IVO]+ which has two unpaired electrons associated with the FeIVO unit and one delocalized over the porphyrin ligand, commonly termed as Compound I (Cpd I). Addition of an electron leads to formation of the so-called Compound II (Cpd II), in which the hole in the porphyrin ligand is filled by an additional electron. Alternatively the L–(Por)FeIVO intermediate can be generated by the homolytic cleavage of the O–O bond (Scheme 1).
A variety of biochemical and biophysical methods have been used to investigate high-valent iron porphyrins. For many heme-based enzymes including horseradish peroxidase [5] cytochrome c peroxidase [6] catalase [7] or cytochrome P450 [8] high-valent intermediates have been structurally characterized by X-ray crystallography. Extensive efforts have been devoted to prepare and characterize synthetic models of enzymatic Cpd I and Cpd II species [9]. A variety of spectroscopic techniques including electronic absorption, NMR, EPR, Mössbauer, ENDOR, EXAFS and Resonance Raman (RR) have been also applied to characterized their structural and electronic properties. In particular, absorption spectroscopy coupled with RR spectroscopy has been recognized as excellent tool for direct detection and characterization of the FeIVO unit in both Cpd I and II species as well as synthetic models [10], [11]. The FeIVO stretch is typically observed in the range of 800–850 cm−1when laser excitation wavelength is tuned into the Soret maximum absorption band. The 18O isotopic labeling, which produces shift around 40 cm−1, has been used routinely to identify oxo-ferryl stretch in iron–porphyrins and heme proteins. Furthermore, it appears that the FeIVO stretch does not mix with other modes and behaves like an isolated harmonic oscillator, thus it can be used as a sensitive probe of local environment.
In order to fully characterize properties associated with the Fe IVO vibration two issues need to be addressed (i) environmental factors which influence vibrational changes associated with the FeIVO unit, and (ii) electronic properties that influence resonance enhancement. The dependence of both of these factors can be investigated with use of density functional theory (DFT). It has already been demonstrated that DFT can be successfully applied to obtain reliable vibrational force field of metalloporphyrins [12], [13], [14], [15], [16], [17]. The electronic and structural properties of models of Cpd I and II have been also investigated computationally using DFT [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. The focus of the present work is the electronic excited states of high-valent iron complexes emphasizing the information that can be obtained from time-dependent DFT (TD-DFT). While DFT has been extensively applied to study electronic properties of high-valent iron porphyrins (see Refs. [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]) relatively less attention has been placed to elucidate their electronic excited states. Due to the size of systems and their complexity methods like CASSCF or CI can be only applied to analyze a small number of low-lying excited states. Thus, the TD-DFT modeling framework, which takes into account only single-electron excitations, is currently only practical tool that can be applied to study excited state of complex systems such as high-valent iron porphyrins.
Section snippets
Computational details
To explore the mechanism of resonance Raman enhancement associated with the FeIVO stretch, the manifold of electronically excited states was computed for a structural model of iron(IV)oxo porphyrin. The model selected for computational analysis, denoted as Im–(Por)Fe IVO, consists of porphine (Por) simplified with respect to substituents and imidazole (Im) as an axial base. The geometry of the Im–(Por)FeIVO complex corresponding to the triplet electronic ground state was optimized at the
Summary and conclusions
The resonance Raman enhancement of the FeIVO stretch observed when laser excitation wavelength is tuned into the Soret maximum absorption band provides direct insight into the electronic and vibrational properties of iron(IV)oxo porphyrins. The TD-DFT calculations shows that the Soret band contains the dominant (a1u → ∗eg) and (a2u → ∗eg) excitations in accord with the Gouterman model. Consequently, the resonance with the Soret band, which is of π → π∗ type, primarily enhances totally symmetric
Acknowledgments
This work was partially supported by JSPS Fellowship to P.M.K.
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Present address: Department of Chemistry, Stanford University, Stanford, CA 94305, USA.