Spectroscopic Characterization and Reactivity of Triplet and Quintet Iron(IV) Oxo Complexes in the Gas Phase

Abstract

Closely structurally related triplet and quintet iron(IV) oxo complexes with a tetradentate aminopyridine ligand were generated in the gas phase, spectroscopically characterized, and their reactivities in hydrogen-transfer and oxygen-transfer reactions were compared. The spin states were unambiguously assigned based on helium tagging infrared photodissociation (IRPD) spectra of the mass-selected iron complexes. It is shown that the stretching vibrations of the nitrate counterion can be used as a spectral marker of the central iron spin state.

Keywords: C−H activation; IR spectroscopy; iron complexes; mass spectrometry; spin state.

Scheme 1

Generation of [(PyTACN)Fe(O)(NO₃)]⁺ by oxidation of the iron(II) precursor in solution or by in‐source CID during electrospray ionization.

Figure 1

a) IRPD spectra of [(PyTACN)Fe(O)(NO₃)]⁺ (m/z 382) generated by oxidation in solution (blue trace, shifted on the y‐scale by 0.4 for clarity) or by in‐source collisional activation during ESI (red trace) G.P.=gas phase. b) Top panel: Two‐color IRPD experiment with the ions generated in the gas phase, one optical paramagnetic oscillator (OPO) was set to 1603 cm⁻¹, the second OPO was scanned (black trace). Lower panel shows one‐color IRPD spectra from (a) in detail. c) Separated spectra of the two complexes contributing to the IRPD spectra shown in (a). The spectra are normalized to the original N ₀. d) Theoretical IR spectra of ³ 1, ⁵ 1, ³ 2, and ⁵ 2 calculated with B3LYP‐D3/6‐311++G**. e) The most stable structures of ³ 1 and ⁵ 2. The red arrows denote the NO and FeO stretching vibrations responsible for the bands at around 900 cm⁻¹ and the blue arrows show the antisymmetric stretch of the NO₂ group, located at about 1600 cm⁻¹.

Figure 2

Results of DFT calculations (the functionals are specified at the bottom of the data, the basis set was always 6‐311++G**, for the OPBE functional also def2‐TZVP was tested—denoted by asterisk). The red and blue lines denote relative energies of the isomers (left axis); the pink and light blue lines show the unscaled antisymmetric NO₂ stretch for quintet and triplet states, respectively (dashed lines are for the 1 isomers and solid lines are for the 2 isomers).

Figure 3

Mass spectra corresponding to the reaction of [(PyTACN)Fe(O)(NO₃)]⁺ (m/z 382, these ions were generated by in‐source fragmentation) with a) 1,4‐cyclohexadiene and b) 1,4‐cyclohexadiene‐[D₆] (pressure of the neutral reactant was 0.2 mTorr). c) Collision‐energy dependence (center‐of‐mass) of the cross‐sections σ, and d) pressure dependence of the relative cross section A of HAT, DAT, and OAT in the reaction with 1,4‐cyclohexadiene‐[D₆] (A _i is defined as −ln(1−ΣI _n/(ΣI _n+I _P))(I _i/ΣI _n), where I _n and I _P are intensities of the fragments and the parent, respectively). DAT=deuterium‐atom transfer.