Excited states from UV-vis and VT NMR

NITROGEN FIXATION VIA A TERMINAL IRON(IV) NITRIDE

5.2 Results and Discussion

5.2.4.1 Excited states from UV-vis and VT NMR

The optical spectra of (P₃^B)Fe(NNMe₂), [(P₃^Si)Fe(NNH₂)]⁺, and [(P₃^Si)Fe(NNMe₂)]⁺ all exhibit resonances in the range from 12,500 to 13,500 and from 18,000 to 19,000 cm⁻¹, which we hypothesize are due to transitions involving the common Fe–NNR₂core (vide in- fra). To investigate this in greater detail we have collected UV-vis spectra of (P₃^B)Fe(NAd), (P₃^B)Fe(NN[Si₂]), and (P₃^B)Fe(NNMe₂), which are shown in Figure 5.6, A through C, with a Gaussian spectral deconvolution. (P₃^B)Fe(NAd) features just a single resolved optical res- onance (Figure 5.6, A), which we assign as the transition from the filled, quasi-degenerate e orbital set (of 3d_xyand 3d_x2−y²parentage) into the empty, quasi-degenerate e orbital set of π*-symmetry (of 3dxz and 3d_yz parentage) expected for a pseudo-tetrahedral, terminal Fe imide underC₃symmetry.⁴¹This assignment is corroborated by TD-DFT calculations.

In the optical spectrum of (P₃^B)Fe(NN[Si₂]) (Figure 5.6, B), these transitions are preserved, but the degeneracy of the excited states is lifted slightly, presumably due to donation of the Nβlone pair into the “in-plane”π*-symmetry interaction (3d_yz −π). Using the UV-vis transitions as a proxy for a one-particle spectrum based on the MO diagram of Figure 5.2, A, we can estimate ∆π ≈ 1,740 cm⁻¹. Moving to the alkylated complex (P₃^B)Fe(NNMe₂), the splitting in these transitions is even more dramatic (Figure 5.6, C); in fact, three resonances are now required to adequately fit the experimental spectrum. From this spectrum, we estimate∆π ≈5,570 cm⁻¹, indicating a significant Nα–Nβπinteraction, in agreement with the crystallographic analysis. We note that the Fe K-edge XANES spectrum of (P₃^B)Fe(NNMe₂) also exhibits two resolved resonances, which likely arise from similar acceptor states to those observed optically;⁶⁵moreover, the XANES spectrum of the protonated analogue, (P₃^B)Fe(NNH₂), is nearly identical to that of (P₃^B)Fe(NNMe₂).¹⁷

Although (P₃^B)Fe(NNMe₂) is a diamagnet in its ground state, preliminary VT NMR and DFT studies have evidenced the presence of a low-lying,S_tot =1 paramagnetic excited

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Figure 5.6: (A) UV-vis spectrum of (P₃^B)Fe(NAd) (THF, 298 K). (B) UV-vis spectrum of (P₃^B)Fe(NN[Si₂]) (2-MeTHF, 153 K). (C) UV-vis spectrum of (P₃^B)Fe(NNMe₂) (2- MeTHF, 153 K). For A through C, raw data are plotted as a solid line, with a Gaus- sian spectral deconvolution shown in dotted lines. (D) Ab initio electronic spectrum of (P₃^B)Fe(NNMe₂) computed from an NEVPT2 calculation on top of a SA-CASSCF(10,10) reference including 10 singlet roots. Contributions from individual states are shown in dotted lines.

state.¹⁷Similar behavior was observed for the isoelectronic complex [(P₃^Si)Fe(NNMe₂)]⁺.¹⁸ In both of these cases, fitting the VT NMR data to a simple two-state, Boltzmann-weighted magnetization function showed that these triplet states lie only 3.7 ±0.1 and 6.7± 0.3 kcal mol⁻¹(1300±30 and 2300±100 cm⁻¹) above the diamagnetic ground states, respec- tively.^17,18It is noteworthy that the entropic contributions to these energy differences appear to be small, and we have obtained a more precise estimate of the adiabatic singlet–triplet gap of (P₃^B)Fe(NNMe₂) of 1266±7 cm⁻¹assuming∆G ≈∆H.

While the atoms directly coordinated to the Fe center of (P₃^B)Fe(NNMe₂) are ex- pected to experience large magnetization in this excited state, VT ¹⁵N NMR studies of (P₃^B)Fe(¹⁵N¹⁵NMe₂) reveal that both Nαand Nβaccumulate significant spin density in the excited state (Figure 5.7, A). Moreover, an examination of the VT ¹H NMR data shows

132 that the magnetization experienced by the Nβ-CH₃ protons is roughly an order of mag- nitude greater than that of any of the protons on the P₃^B ligand (Figure 5.7, B). This is most consistent with significant spin delocalization onto the entire “NNMe₂” moiety in the excited state, as opposed to only spin polarization by Fe-centered electrons. For example, if one assumes that the N_β-CH₃ protons can be treated as point-dipoles, which would be reasonable for a largely Fe-centered spin given that these protons are, on average, > 4 Å from the Fe ion based on the DFT-optimzed triplet geometry, one would estimate that

|a_iso| ≈50 MHz. Given thata_iso(¹Hγ) = 19.4 and 39.8 MHz for theN,N-dimethylhydrazyl radical and theN,N-dimethylhydrazyl radical cation,^37,39respectively, this value appears to be unreasonably large. These data belie a more complex electronic structure, in which the both the Fermi- and pseudo-contact contributions to the paramagnetic ¹H NMR shift are large.

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Figure 5.7: (A) Variable temperature ¹⁵N NMR for (P₃^B)Fe(¹⁵N¹⁵NMe₂). (B) Variable temperature¹H NMR for (P₃^B)Fe(NNMe₂). The resonance due to the Nβ-CH₃protons is plotted in red.

Under a point-dipole approximation, this would require largeg-anisotropy in the excited state,⁶⁶ and, hence, manifold low-lying Stot = 1 states, which is unusual for Fe centers in non-Kramer’s spin states.^67–70 Alternatively, significant spin delocaliztion onto the entire

“NNMe₂” moiety would invalidate the point-dipole approximation altogether. This latter interpretation is more consistent with the VT ¹⁵N NMR data, and points to an electronic structure of the “NNMe₂” moiety with open-shell character in the excited state. This

could be explained, for example, in terms of a S= 1/2 [NNMe₂]^•− ligand either coupled ferromagnetically to an S =1/2 or antiferromagnetically to anS = 3/2 Fe center. In turn, this suggests that the Stot = 0 ground state of (P₃^B)Fe(NNMe₂) may possesses open-shell singlet character.