NITROGEN FIXATION VIA A TERMINAL IRON(IV) NITRIDE
5.2 Results and Discussion
5.2.7 Summary and Implications for Catalytic Nitrogen Fixation
To summarize our results, we have leveraged a variety of spectrocopic and quantum- chemical techniques to characterize the electronic structures of the [(P3B)Fe(NNMe2)]+/0/−
redox series. While these studies reveal the complex, multiconfigurational nature of these species, a simple, qualitative, model that captures their essential features is that of intermedi- ate spin Fe centers coupled antiferromagnetically to [NNMe2]•−and [R3B]•−ligands (Figure 5.14). As the Fe center is reduced from formally Fe(III) in [(P3B)Fe(NNMe2)]+, to Fe(II) in (P3B)Fe(NNMe2), and finally to Fe(I) in [(P3B)Fe(NNMe2)]−, this produces ground states with Stot = 1/2, Stot = 0, and Stot = 1/2. For (P3B)Fe(NNMe2) and [(P3B)Fe(NNMe2)]+, additional evidence for these intermediate spin configurations comes from the presence of
low-lying (∆E ≤ 5 kcal mol−1) excited states with Stot = 1 and Stot = 3/2 respectively, corresponding to the high-spin configuration of the Fe(III) and Fe(II) centers. Given its Fe(I) configuration, [(P3B)Fe(NNMe2)]−does not possess low-lying excited multiplets with Stot = 1/2, which is reflected by its lowg-anisotropy. As revealed by magnetic resonance spectroscopies and correlated ab initio calculations, the strength of the magnetic coupling between the Fe and its ligands decreases monotonically upon reduction, and also in the thermally-populated excited states of (P3B)Fe(NNMe2) and [(P3B)Fe(NNMe2)]+ (Figure 5.15).
Figure 5.14: Simple model of the electronic structures of [(P3B)Fe(NNMe2)]+/0/−. Here, we assume an approximately Td ligand field splitting (with the principle axis along the Fe–Nα bond), and ignore covalency, which determines the actual metal-ligand coupling strength. The arrows in the lower-left indicate the trends in coupling strength (See Figure 5.15 for a quantitative measure).
Combined with our previous work,17,18the studies presented here demonstrate that the N-alkylated complexes [(P3E)Fe(NNMe2)]n serve as faithful electronic models of theirN- protonated congeners, as judged by EPR, Mössbauer, NMR, UV-vis, and X-ray absorption spectroscopies. Moreover, when making isoelectronic comparisons, the (P3B)Fe complexes do not appear to differ meaningfully from those supported by the P3Siligand, leading us to propose analogous electronic structures. So, the question naturally arises: Do the electronic structures of title compounds rationalize the differential reactivity of the isoelectronic complexes (P3B)Fe(NNH2) and [(P3Si)Fe(NNH2)]+in the context of nitrogen fixation?
154
- 0
+
n 0.6
0.8 1 1.2 1.4 1.6 1.8
N D
2 +,0 4
+,0
1 0,0 3
0,0
2 -,0
Figure 5.15: Number of effectively unpaired electrons (measured byND) within the 3dz2 ± B 2pzand 3dyz±π*NNinteractions as a function of charge state (n) and multiplicity (2S+1), which can be taken as a measure of the antiferromagnetic coupling strength. Qualitatively similar plots are obtained usingNU orY, and by extending this analysis over the complete active space and subtracting 2S.
Assigning an [NNH2]•−oxidation state to the “NNH2” ligand of (P3B)Fe(NNH2) may seem at odds with its apparent susceptibilty toward protonation at Nβ to yield NH3 and [(P3B)Fe≡N]+,17 although it should be noted that the dominant resonance structure of [NNH2]•− results in lone-pair character on Nβ. Moreover, in (P3B)Fe(NNH2), additional reducing equivalents are harbored in the redox-active Fe–B interaction. Thus, as Nβ be- comes protonated, the [R3B]•−ligand can facilitate N–H bond formation by simultaneously reducing the [NNH2]•− ligand. From this perspective, the protonation reaction could be viewed as a form of “intramolecular” CPET. This flow of electrons is consistent with the XAS/TD-DFT characterization of [(P3B)Fe≡N]+, which shows that the LUMO of the system is an a1-symmetry orbital of dominant (3dz2 + B 2pz) character, i.e., the B atom becomes oxidized in the transformation shown in Figure 5.16, A. Our proposal here is consistent with previous observations made by us,17 and a similar conclusion was reached in a recent computational study.82
This behavior helps to explain why a similar N–N bond cleavage reaction is not observed for [(P3Si)Fe(NNH2)]+. From a coarse-grained perspective, the stability of this
Figure 5.16: (A) Proposed role of ligand radical character in the N–N bond cleavage reaction of (P3B)Fe(NNH2). (B) Proposed role of ligand radical character in the N–H bond forming reactivity of (P3Si)Fe(NNH2).
species toward protonation can be rationalized in terms of electrostatic effects. That is, owing to its cationic nature, one would expect the Nβ atom of [(P3Si)Fe(NNH2)]+ to be less basic than that of (P3B)Fe(NNH2).83 With an understanding of the electronic structure of [(P3Si)Fe(NNH2)]+, we can reformulate this statement in terms of the reducing power of the silyl ligand. That is, protonation at Nβ and N–N bond cleavage would produce an Fe(IV) nitrido in which the silyl ligand becomes oxidized to [R3Si]+(Figure 5.16, B). As the [R3Si]• −e
−
−−−*
)−−−[R3Si]+couple should occur at much higher potentials than the corresponding [R3B]•− −e
−
−−−*
)−−− [R3B] couple, one would expect this transformation to be more challenging for the P3Siligand when compared with the P3Bligand. If the Fe–Si bonding is substantially more covalent than the Fe–B bonding, then the Fe center would have to provide the reducing equivalents necessary for N–N bond cleavage, producing a Fe(V) or Fe(VI) oxidation state, which should occur at similarly high potentials.
Although [(P3Si)Fe(NNH2)]+ is thus stable to protonation, it can be reduced in the
156 presence of CoCp*2 (Figure 5.16, B). In the Fe(I) (or Fe(II) in the case of covalent Fe–
Si bonding) state, we expect the antiferromagnetic coupling between the Fe center and the [NNH2]•−ligand to weaken substantially, resulting in significant spin density delocalization onto both N atoms. Based on EPR studies of [(P3B)Fe(NNMe2)]−, the distribution of ligand-centered spin should be weighted in favor of Nα (|t(Nα)/t(Nβ)| ≈ 5), which would explain preferential Nα functionalization if N–H bond formation occurs via HAT. Either [(P3Si)Fe(NNH2)]+ or (P3Si)Fe(NNH2) could serve as suitable HAT donors, given the extremely weak N–H bonds of these species (BDEs estimated to be 49 and 37 kcal mol−1, respectively).84 It should also be noted that the bending of the Fe–Nα–Nβis accompanied by greater lone-pair character at Nα (vide supra), and N–H bond formation via a proton transfer or CPET mechanism is thus also conceivable.
Given the low potential of the [(P3B)Fe(NNMe2)]0/− couple (ca. −2.7 V), we do not expect [(P3B)Fe(NNH2)]− to be a relevant oxidation state in catalytic nitrogen fixation by [(P3B)Fe(N2)]−using our most efficient conditions in which CoCp*2is the reductant (E◦0 =
−2.1 V).23However, based on the results above, the [(P3B)Fe(NNH2)]+and (P3B)Fe(NNH2) redox states should possess low-lying excited multiplets that are characterized by weakened antiferromagnetic metal-ligand coupling. Indeed, the similarity between these excited states and the reduced Fe(I) state, at least from the perspective of the [NNH2]•−ligand, suggests that population of these states under catalytic conditions could engender “(P3Si)Fe-like”
reactivity—that is, a preference for a distal-to-hybrid mechanism producing N2H4,18rather than a purely distal mechanism producing NH3.17While direct thermal population of these states seems unlikely at the low temperatures relevant to catalysis (−78 ◦C), it may be possible to access similar electronic structures photochemically (cf. Figure 5.6, C and D).
Studies evaluating this hypothesis are currently underway.