Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex

Abstract

In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O-2-activating iron enzymes. This work details an electronic-structure investigation of [Fe-IV(O)(L-NHC)(NCMe)](2+) (L-NHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based-multireferenee calculations. The IRPD spectrum of complex 1 reveals the Fe-O- stretching vibration at 832 +/- 3 cm(-1). By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 +/- 10 cm(-1) in the E(d(xy) -> d(xz,yz)) excited state. Both values are similar to those measured for [Fe-IV(O)(TMC)(NCMe)](2+) (TMC = 1,4,8,11-tetramethy1-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm(-1). We can unequivocally assign them to the ligand field transitions of d(xy) -> d(xz,yz), d(xz,yz) -> d(z2), and d(xz,yz) -> d(x2-y2), respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [Fe-IV(0) (SR-TPA)(NCMe)](2+) (SR-TPA = tris(3,5-dirnethy14-methoxyp-yridy1-2-methy)amine), the excitations within the (FeO)(2+) core of complex 1 have similar transition energies, whereas the excitation energy for d(xz,yz) -> d(x2-y2) is significantly higher (similar to 12 000 cm-1 for [Fe-IV(O)(SR-TPA)(NCMe)](2+)). Our results thus substantiate that the tetracarbene ligand (L-NHC) of complex 1 does not significantly affect the bonding in the (FeO)(2+) unit but strongly destabilizes the d(x2-y2) orbital to eventually lift it above d(z2). As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed: