Theoretical Investigations of the Electronic Structure and Spectroscopy of Mononuclear, Non-Heme {Fe−NO}⁶ Complexes

Abstract

The unusual metal coordination and spin-state of the Fe(III) center in nitrile hydratase (NHase) has stimulated the synthesis of numerous model complexes in efforts to understand the reactivity and spectroscopic properties of the enzyme. A particular problem has been the development of model Fe(III) complexes that exhibit reversible, photolabile binding to nitric oxide (NO) in a manner similar to that observed for the NHase metal center. We now report a detailed NBO analysis of the ground-state chemical bonding in three [Fe-NO](6) complexes that exhibit different responses to irradiation, together with investigations of their spectroscopic properties using semiempirical INDO/S CI singles calculations. Our computational studies reveal a correlation between the photolability of these complexes and the existence of low-energy transitions that promote an electron into the Fe-NO pi(*) antibonding molecular orbital. In addition to providing detailed insights into how the ligand field influences the spectroscopy of these mononuclear complexes, these studies strengthen our previous conclusions regarding the role of post-translational cysteine modification in modulating the photoreactivity of the inactive, NO complex of NHase.

Formation of a triply bridged µ-oxo diiron(iii) core stabilized by two deprotonated carboxamide groups upon photorelease of NO from a {Fe–NO}6 iron nitrosyl

“…Theoretical investigations into the electronic structure of 1 reveal the existence of a LMCT transition in the visible range that corresponds to an electron promoted from a MO composed primarily of nonbonding p orbitals associated with the oxygen and nitrogen atoms of the carboxamido moiety into the Fe-NO p* antibonding MO. 10 Our results so far strongly suggest that the NO photolability of 1 is critically connected to the deprotonated carboxamido moiety. When the carboxamido nitrogen is replaced by an imine nitrogen in the analogous Schiff base ligand SBPy 3 , there is no reaction between the Fe(III) complex [(SBPy 3 )Fe(DMF)](ClO 4 ) 3 and NO.…”

Ruthenium Nitrosyls Derived from Tetradentate Ligands Containing Carboxamido-N and Phenolato-O Donors: Syntheses, Structures, Photolability, and Time Dependent Density Functional Theory Studies

“…From this singlet excited state, relatively efficient interconversion and intersystem crossing to lower energy triplet excited states can occur. Rapid solvation of such triplet species could be competitive with other deactivation pathways and account for NO photolability 20,40. Optimization of the lowest energy triplet excited states of 1 , 2 and 3 reveal elongated Ru-NO bond lengths (1.936, 1.937, and 1.926 Å respectively) compared to those observed in the calculated singlet states (1.726, 1.732, and 1.742 Å respectively).…”

Sulfur K-Edge XAS and DFT Calculations on Nitrile Hydratase: Geometric and Electronic Structure of the Non-heme Iron Active Site

“…63 However, the electronic properties of these complexes, investigated by recent DFT calculations, have suggested that this unit can be described as a low-spin Fe 2+ coordinated to a NO + unit. 60,64 On photolysis, a low-spin Fe III center is produced which is well-characterized by EPR, Mössbauer, and resonance Raman spectroscopy. 1,6 Note that, from the XAS near-edge transitions, we know that the chemical nature of the CysS − does not change upon photolysis, and hence it serves as a spectator of the change in Z eff of the Fe ion in this process.…”