3.2 Results and Discussion
3.2.3 Targeting [PhBP R 3 ]Fe(N) Complexes
-2.3 -2.1
-1.9 -1.7
-1.5 -1.3
-1.1
V (vs Fc/Fc+) 3.7
3.8
Figure 3.3. Cyclic voltammograms of [PhBPter3]FeCl (3.7) and [PhBPCH2Cy3]FeCl (3.8) recorded at scan rates of 10 and 50 mV/s, respectively, in THF with 0.35 M [nNBu4][PF6] as the supporting electrolyte. Potentials are internally referenced to Fc/Fc+.
Similar to the [PhBPiPr3]Fe system, [PhBPCH2Cy3]FeCl reacts with Li(dbabh) to give cleanly the iron(IV) nitride, [PhBPCH2Cy3]Fe(N) (3.9), and one equivalent of anthracene (1H and 31P NMR, Scheme 3.4). However, [PhBPCH2Cy3]FeCl converts directly to the nitride 3.9 at -50 °C without the observation of the intermediate iron amide, “[PhBPCH2Cy3]Fe(dbabh).” Steric repulsion between the bulkier [PhBPCH2Cy3] ligand and dbabh may accelerate the rate of anthracene expulsion.
The formulation of 3.9 as a terminal nitride was confirmed by a peak at 929 ppm in the 15N NMR spectrum of 15N-3.9. The Mössbauer spectrum of 3.9 contains two doublets (Figure 3.4), revealing a mixture of 3.9 (~75%) and an unknown diamagnetic impurity (~25%).15 The larger doublet for 3.9 has an isomer shift (δ) of -0.34(1) mm/s and a quadrupole splitting (ΔEQ) of 6.01(1) mm/s. The low isomer shift corroborates the assignment of a high valent iron center, while the quadrupole splitting indicates a non- spherical distribution of nuclear electron density. These values are identical to those observed for [PhBPiPr3]Fe(N), although the spectrum only contained 35% of the desired species and was complicated by the presence of [PhBPiPr3]FeCl, [PhBPiPr3]Fe(dbabh), and {[PhBPiPr3]Fe}2(μ-N2) (Figure 3.4).
-4 0 4 Velocity (mm/s) 0.0
1.0
A
0.0
2.0
4.0
B
4.2 K
Absorbtion (%)
Figure 3.4. Mössbauer spectra of 40 mM (A) [PhBPiPr3]Fe≡N in THF and (B) 35 mM [PhBPCH2Cy3]Fe≡N (3.9) in THF recorded at 4.2 K with an applied field of 45 mT. The vertical lines are the experimental data and the solid lines are fits using the following parameters: (A) δ = -0.34 mm/s, ΔEQ = 6.0 mm/s (35%); and (B) δ
= -0.34(1) mm/s, ΔEQ = 6.01(1) mm/s (75%). The other species in (B) has the following values: δ = 0.15 mm/s, ΔEQ =1.61 mm/s (25%).
The Mössbauer parameters for several iron(IV) complexes are presented in Table 3.2 for comparison. Iron(IV), though uncommon, is an important oxidation state because it is believed to function in metalloenzymes.16 Many well-defined iron(IV) compounds feature a bridging or terminal oxo ligand, although there are several exceptions including (cyclam-acetato)Fe(N3),17 [NEt4][(L4)FeCl]18 (where L4 is a tetraanionic macrocyclic ligand), (L4)Fe(CNtBu)2,19 Fe(1-norbornyl)4,20 (triamido(amine))Fe(CN),21 [PhBPiPr3]Fe(H)3(PMe3),22 and the nitride compounds described here. The δ values for the majority of these iron(IV) species are near zero. The δ values of -0.34 mm/s for the iron nitrides is slightly different, but they are consistent with high valent iron and similar to (triamido(amine))Fe(CN). Unlike the isomer shift, no consistent trend is observable for the quadrupole splitting, which ranges from 0.6 to 6 mm/s. Notably, the ΔEQ values for the iron nitrides are by far the largest.
Table 3.2. Mössbauer parameters for various iron(IV) species.
Iron(IV) Species a S δ (mm/s) ΔEQ (mm/s)
[(tris(2-pyridylmethyl)amine)Fe=O]2+ 23 2 0.01(2) 0.92(2)
[(L4)FeCl]- 2 -0.04(2) 0.89(2)
[(Me4-cyclam)Fe=O(CH3CN)]2+ 24 1 0.17(1) 1.24(1)
[(cyclam-acetato)Fe=O]+ 3b 1 0.01 1.39
(L4)Fe(CNtBu)2 1 -0.04 3.38
[(cyclam-acetato)Fe(N3)]2+ 1 0.11 1.92
(triamido(amine))Fe(CN) 1 -0.22 3.28
[PhBPiPr3]Fe(H) 3(PMe3) 0 0.01(2) 0.58(2)
[PhBPiPr3]Fe≡N 0 -0.34(1) 6.01(1)
[PhBPMeCy3]Fe≡N (3.9) 0 -0.34(1) 6.01(1)
a References are provided in the main text if not given here.
There are only a handful of iron complexes with uncharacteristically large quadrupole splittings, but even so, their values are on the order of 4 mm/s.25 Most of these examples are of the type [(porphyrin)FeX]- (where X = halide, OR, OCOR) that feature a five-coordinate iron(II) center (Figure 3.5).25a-e One exception is an iron(II) complex of a macrocyclic tetrathioether ligand reported by Silver and coworkers.25f Although two iodides are located in the axial positions above and below the square plane of the ligand, the Fe-I bond distances are extremely long at 2.8896(2) Å. Moreover, the complex shows no proclivity to bind CH3CN. Based on these observations, Silver and coworkers report this species as an unusual example of square-planar iron(II).
Figure 3.5. Representative examples of iron complexes with unusually large quadrupole splittings.
Comparing the quadrupole splittings featured in Figure 3.5, the ΔEQ value of 6.0 mm/s observed for the iron(IV) nitrides is the largest quadrupole splitting reported for any iron complex. One explanation for the exceptionally large quadrupole splitting is due to a very aspherical nuclear electron density. The disposition of the trianionic nitride and the monoanionic borate moiety on the pseudo three-fold axis creates a highly anisotropic
field gradient. The charges strongly interact with the iron nucleus and thus are responsible for the large asymmetry.
Compound 3.9 is stable at -50 ºC for at least 8 h. However, it is thermally unstable and begins to decompose at -40 ºC. The decomposition of 3.9 was monitored by NMR spectroscopy at -40 ºC. After 10 h, an ill-defined mixture of several species was observed. The decreased thermal stability of 3.9 relative to [PhBPiPr3]Fe(N) was surprising, as the bulkier ligand was expected to hinder dimerization. However, subsequent coordination chemistry of [PhBPCH2Cy3]Fe reveals that the methylcyclohexyl groups impart more flexibility than the isopropyl groups, and hence, may allow dimerization processes to occur more readily (vide infra). Moreover, other decomposition pathyways may also be operative and need to be considered.