Iron Complexes of a Proton-Responsive SCS Pincer Ligand with Sensitive Electronic Structure

SCS pincer ligands have an interesting combination of strong-field and weak-field donors that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a protonresponsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex. The N–H bond dissociation free energy can be quantitated as 56 ± 2 kcal/mol. EPR spectroscopy and SQUID magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with large zero field splitting, and X-ray absorption spectra show high Fe–S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and DFT calculations explain this phenomenon through a change between having the doubly-occupied orbital as dz2 or dyz, as the former is more influenced by the nearby positive charge.


INTRODUCTION
The organometallic chemistry of iron has been dominated by strong-field supporting ligands such as CO, CN, and phosphines, and by macrocyclic N ligands like porphyrins. [1][2][3][4][5][6] The active sites of hydrogenase enzymes incorporate S-based ligands, and these are low-spin due to the influence of carbonyl and cyanide donors. 7 However, the interesting reactions of nitrogenase enzymes, and a new generation of low-valent iron catalysts, have predominantly weak-field ligands and have led to increasing interest in organometallic iron complexes with higher spin states. [8][9][10][11] We focus here on iron coordination environments that result from a mixture of C and S donors -choices that are particularly compelling since the six "belt" iron atoms in the ironmolybdenum cofactor (FeMoco) of nitrogenase have a mixed C/S coordination sphere. [12][13][14] This unusual combination of potentially strong-field C donors and weak-field S donors could lead to changes in spin states during catalysis, which has been linked to changes in barriers and selectivity. [15][16][17][18] Thus, the study of C-and S-ligated iron has relevance for both fundamental coordination chemistry and bioinorganic mechanisms. [19][20][21] However, CO-free iron complexes with supporting ligands that coordinate through only carbon and sulfur donors are rare. [22][23][24][25][26][27][28] Of these examples, only one multidentate C/S ligand is known to support N2 binding. 26 In addition, Qu has provided important studies on Cp*-supported iron dimers bridged by dithiolates, and this C/S ligand sphere can bind and facilitate the reduction of nitrogenase-relevant NxHy substrates. [23][24][25]29 Our group's recent work on SCS systems began with a dithiolate ligand having a central arene that can interact with iron through backbonding into its π-system in low oxidation states. 26,30 While this hemilabile interaction stabilizes the reduced complexes, complete dissociation of the SCS ligand occurred in the presence of Brønsted acids, and backbonding into N2 competed with backbonding into the arene. In order to address this instability, we then moved to tridentate pincer 3 scaffolds in which the carbon forms a σ-bond to the iron center. Namely, we synthesized a new SCS pincer comprised of an N-heterocyclic carbene (NHC) ligand with two bulky thiolate arms, and isolated a number of iron complexes with this scaffold. 28 The geometries at iron sites with this supporting ligand were sometimes tetrahedral and sometimes square planar, indicating significant flexibility in the core. However, this NHC ligand was also sensitive to acids, and the ligand synthesis involved time-consuming protection and deprotection steps.
Here, we study the first iron complexes of pincer ligands containing a central aryl C donor and two S-donor arms. SCS pincers of this type have been studied in complexes of group 10 metals (Chart 1, top), but not in group 8 metals to our knowledge. [31][32][33][34][35][36] Thioamide arms are compelling, because they are easily prepared from readily available precursors and offer the potential for proton responsiveness. [37][38][39] There is no precedent for iron complexes with thioamide-aryl ligands, although there is a related iron SNS pincer complex with a central pyridine (Chart 1, lower left). [40][41] 4 Chart 1. Typical palladium SCS pincer complexes and closely-related iron complexes with SNS and SCS pincers. In the bottom pictures, the square represents a coordination site with various ligands.
Thioamides have two tautomers, which form the same anion upon deprotonation ( Figure   1, right). Most often, the thioamide coordinates as an iminothiolate through the S donor rather than the potential N donor as a result of the relative weakness of the C=S π bond and the better overlap of the S atom with metal orbitals. 42 After coordination, protonation at the N atom gives a formally neutral thione donor that maintains the M-S interaction (Figure 1, brackets). Even as a formally neutral ligand, the thione C-S bond is significantly polarized toward a C δ+ -S δform that affords some anionic character at S. 43 The ability to tune the donor properties of iron-coordinated sulfur via protonation/deprotonation of the supporting ligand could be used to probe how protonation state can affect the redox potential, spectroscopic characteristics, and reactivity of the iron center. The potential for proton responsiveness is also relevant to nitrogenase mechanisms. The Thorneley-Lowe kinetic scheme for nitrogenases proposes that one proton is transferred to the FeMoco with each reduction step, 44 but the location of these protons has been controversial.
Computational studies have showed that there are many potential sites of protonation on the FeMoco, including the sulfides, [45][46][47][48] the Mo-coordinated homocitrate, 46,49 and even the carbide. 47-5 48, 50 Studies on multi-site proton-coupled electron transfer (PCET) have demonstrated that various protonation sites can influence the redox potentials and bond dissociation free energies (BDFE). [51][52][53][54] These inspirations motivated us to pursue thermochemical studies on well-characterized (SCS)Fe complexes with proton-responsive ligands to examine the effects of distant protonation events on potentially biologically-relevant iron sites.
In the present study, we describe a new ligand scaffold that incorporates thioamides and an anionic aryl group to mimic the sulfur and carbon coordination environment found in FeMoco.
The ligand is easy to prepare on a useful scale, and the stability of the iron complexes is highlighted by the ability of an iron(III) complex to be handled in air and water. Although no iron complex of the new SCS ligand was observed to bind N2, we use EPR and Mössbauer spectroscopy alongside magnetometry measurements to elucidate the unusual electronic structures of complexes with different exogenous donors, including biologically-relevant amide, thiolate, ammonia, and CO. In the amide complex, the electronic structure may be influenced to the presence of nearby cations.
We also demonstrate the ability of the thioamide arm to be proton responsive, and quantify the connection between protonation state and redox state of iron.

RESULTS
Ligand Synthesis and Metalation. Using a procedure modified from the literature, 55 isophthalic acid was treated with thionyl chloride followed by 2,6-diisopropylaniline, which provided the diamide (Scheme 1). Reaction of this compound with P2S5 in toluene at 100 °C yielded the bis(thioamide) 1, in an overall yield of 69% in two steps from commercial starting materials. Treating 1 with a slight excess of Fe(PMe3)4 in Et2O at room temperature led to 6 effervescence and a color change from yellow-brown to dark green, and iron complex 2 was isolated in 93% yield (Scheme 1). [56][57] Scheme 1. Synthesis and metalation of SCS pincer ligand 1. Dipp = 2,6-diisopropylphenyl.
Crystallization of 2 gave green blocks suitable for X-ray diffraction (Figure 2, top). The diffraction data showed an octahedral iron site with a meridional pincer ligand as expected. The Fe−C bond length is 1.9502(18) Å, and the average Fe−S length is 2.268(9) Å. The three Fe−P bonds are only slightly different, with a length of 2.2484(3) Å for the phosphine trans to C and lengths of 2.2553(4) Å and 2.2608(4) Å for the phosphines cis to C. There was disorder at the thioamide N−H site, which was satisfactorily modeled with each thioamide having 50% hydrogen atom occupancy, consistent with single protonation of the supporting ligand in 2. The Mössbauer spectrum of 2 has a doublet with δ = 0.21 mm/s and |ΔEQ| = 1.25 mm/s ( Figure S46). Proton NMR spectra in either C6D6 or THF-d8 show signals in the range expected for a diamagnetic complex of low-spin iron(II). The most downfield resonance integrates 1:1 with proton groups from each side of the molecule ( Figure S5). Adding D2O made this signal disappear, and therefore it is assigned as the thioamide proton. Based on the NMR spectra, 2 is unsymmetric in solution, and it is best described has having one neutral thioamide S donor and one anionic iminothiolate donor. The solid-state IR spectrum of 2 shows a broad peak centered at 3336 cm -1 ( Figure 3, red trace), which is distinct from the analogous signal in free ligand 1 at 3136 cm -1 8 ( Figure 3, gray trace), consistent with an N-H stretching vibration. We hypothesize that the thioamide arm is deprotonated during the synthesis of 2 by a transient iron hydride to form H2, which explains the effervescence during metalation. Electron and Proton Transfer in Phosphine Complexes. We serendipitously discovered that neutral, fully-deprotonated iron(III) compound 4 could also be prepared in one pot from 1 and Fe(PMe3)4 by simply exposing the crude product 2 to air after metalation was complete. Proton NMR and UV-visible spectroscopy of air-exposed 2 in situ demonstrates rapid and complete conversion to 4, with no monoprotonated iron(III) transient species observed. This one-pot procedure afforded 4 in 88% yield from 2, and this method was used to prepare 4 for subsequent experiments. Additional proton and electron transfer reactions were carried out on 2 and 4.
Treating the neutral iron(II) compound 2 with stoichiometric sodium bis(trimethylsilyl)amide provided anionic complex 3-Na. Oxidation of 3-Na using AgPF6 afforded neutral iron(III) complex 4. We generated the potassium analogue, 3-K, by treatment of neutral iron(III) compound 4 with stoichiometric KC8. Both have sharp peaks in their 1 H NMR spectra suggesting low-spin iron(II), and their structures were inferred based on these spectra ( Figures S7 and S9). Their yields (88% for 3-Na and 91% for 3-K) were determined by integration relative to an internal standard.
Alternatively, 4 could be formed in one step by hydrogen atom abstraction from 2 using TEMPO.  Figure S60). The peak currents displayed a linear dependence on the square root of the scan rate, indicating that the process is diffusion controlled ( Figure S61). Next, we tested the ability of different organic bases to deprotonate 2, using changes in the 31 P NMR chemical shifts to determine the ratio of 2 and deprotonated 2 since the conjugate base and acid are in rapid equilibrium. We assumed that the chemical shifts from the 31 P NMR spectrum of 3-K represent the shifts of fully-deprotonated 2. Addition of neither Et3N (pKaH = 12.5 in THF) nor DBU (pKaH = 16.9 in THF) gave changes in the NMR spectra of 2. 58 An initial 1.10 equiv addition of triazabicyclodecene (TBD, pKaH = 21.0 in THF) 58  Other redox and protonation tests were used as well. Electrochemical oxidation of 2 is irreversible under the same CV conditions used for 4 ( Figure S59). To test whether 4 could be protonated, it was treated with one equiv of [H(OEt2)2][BF4] in THF. A dark brown solid precipitated from the reaction mixture, and the IR spectrum of the solid showed a single N-H stretch at 3250 cm −1 , which is much lower than the N-H stretching frequency of 2 at 3336 cm −1 .
( Figure S88). Though purification and subsequent electrochemical testing of the putative protonated 4 was prevented by its insolubility, the IR spectrum of acid-treated 4 may indicate the feasibility of thioamide protonation. This direction was not pursued further.
Spectroscopy of Phosphine Complexes. The Mössbauer signal for 2 (δ = 0.21 mm/s and |ΔEQ| = 1.25 mm/s, Figure S46) is similar to those for 3-Na (δ = 0.24 mm/s and |ΔEQ| = 0.94 mm/s, Figure S47) and 3-K (δ = 0.25 mm/s and |ΔEQ| = 1.05 mm/s, Figure S48), consistent with low-spin iron(II) complexes. Solid 4 has a lower isomer shift of 0.16 mm/s and much larger quadrupole splitting of 3.45 mm/s, and its doublets are highly asymmetric, with ΓL = 0.57 mm/s, ΓR = 0.32 mm/s. To determine the spin state of 4, solid state and C6D6 solution magnetic moments were obtained. Both measurements gave µeff = 1.6 µB at 298 K, which indicates a ground-state spin of S = 1/2.     Figure S12). The number and integration of the peaks is consistent with C2v symmetry; a seventh expected peak integrating to 4H was not resolved, likely due to broadening. This is also consistent with the dimer breaking up, although it is also possible that there is some other dynamic phenomenon. The 1 H NMR spectra of 5-Solv in C6D6 have at least 12 peaks with considerable broadness, which prevented further analysis but points to a lowered symmetry in the dimer.
Cyclic voltammetry of 5 showed three irreversible reduction events ( Figure S62). An attempt to reference the reduction potentials to a ferrocene internal standard was unsuccessful due to reactivity with Fc and Fc + . The lack of electrochemical reversibility may be due to large structural changes occurring different oxidation states. Undeterred, we proceeded to stoichiometrically reduce 5 with 1 equiv of KC8 per iron at −78 °C in THF, which resulted in a deep purple solution. Crystallization from diethyl ether at −40 °C give iron(II) complex 6 in 46% yield. Its X-ray crystal structure, shown in Figure 6, shows a tetrameric assembly in the solid state,   THF-d8 shows at least 19 resonances between -50 and +15 ppm, reflecting C1 symmetry in solution. We were unable to assign resonances to specific proton environments due to peak broadness and overlapping in the 0 to 8 ppm region ( Figure S13).
Despite the complexity of the 1 H NMR spectrum and crystal structure, the solid-state Mössbauer spectrum of 6 ( Figure S52) has a single quadrupole doublet with an isomer shift similar to that of the starting material (0.34 mm/s for 6 vs. 0.35 mm/s for 5-Et2O), but a smaller quadrupole splitting (1.87 mm/s for 6 vs. 3.91 mm/s for 5-Et2O). The Mössbauer spectrum suggests that all iron sites are equivalent.
Next, we sought to test 6 for the ability to bind N2 at low temperatures using variabletemperature UV-visible spectroscopy. Negligible spectral changes were observed between 25 and -100 °C in THF, Et2O, or toluene ( Figure S90a-S90c). Addition of stoichiometric amounts of 18crown-6 slightly shifted the absorption maxima, but the resulting species also did not exhibit notable temperature-dependent spectral changes ( Figure S90d-S90f). Thus, it does not appear that 6 undergoes speciation changes or N2 binding at these concentrations and temperatures. Further reduction of the iron(II) tetrameter 6 led to a silent NMR spectrum and a broad, featureless UV-Vis spectrum. The products formed in this reaction are unknown, and attempts to isolate or identify the species present were unsuccessful.

Synthesis of Monomeric Iron Complexes With S and N Donors.
To study monomeric complexes of our SCS iron framework in the absence of strong phosphine donors, we next treated 5-Et2O with ligands containing S and N donors, which gave the products in Scheme 4 within 1 hour at room temperature. Their syntheses, NMR spectra, and IR spectra are described here, and their magnetism, EPR spectra and electronic structures are described in following sections.  Figure S82). The geometry is distorted from square planar, with τ4 = 0.20. The Fe−SAr bond distance is 2.2448(8) Å, which is shorter than the Fe−SAr bond in 7-crown (Table 1). Unlike the structure of the THF adduct 7-crown, one mesityl group of THF-free 8-crown is twisted to cover an axial iron site; however, the long 3.06 Å distance between iron and the arene centroid is 19 inconsistent with a direct electronic interaction. We previously observed a similar orientation of this thiolate ligand in another SCS iron complex and ascribed this phenomenon to crystal packing effects. 28 We were unable to crystallize four-coordinate thiolate 8 without crown ether. The 1 H NMR spectrum of 8 in THF-d8 has ten broad peaks between 15 and −63 ppm ( Figure S14). Upon dissolution in THF-d8, the color of 8 changes from dark brown to red, indicating that the THFadduct 7 is likely formed. The number and integration of the signals suggests C2v symmetry, which implies that the crystallographically observed lack of symmetry caused by the bulky aryl thiolate is not preserved in THF-d8 solution.
Amide complex 9 was synthesized in 87% yield by treating 5 with KN(TMS)2, and it was crystallized with and without 18-crown-6 to give structures of 9-crown ( Figure S84) and 9 (Scheme 4, middle, and Figure S83), respectively. Crown-free 9 is unstable as a solid at room temperature and in solution even at −40 °C, as evidenced by the appearance of multiple new peaks between 11 and −43 ppm in its NMR spectrum after a few days ( Figure S17). Once this unknown impurity formed, we have been unable to remove it by recrystallization or washing. 9-Crown, however, is more stable. The crystallographic structures of 9 and 9-crown lack axial solvent coordination (despite 9 being crystallized from THF), and their color is the same in coordinating and non-coordinating solvents. These observations suggest that solvent coordination to the N(TMS)2 adduct is unfavorable, possibly because the TMS groups block the axial sites.  (Table 1). This difference may arise because the thiolate and amido ligands exert a larger trans influence than the ammine. 61,62 The solid-state IR spectrum of 10 shows four weak bands at 3352, 3299, 3236, and 3159 cm −1 that are assigned as N-H stretches ( Figure S44). Their frequencies are not suggestive of any significant coordination-induced N-H bond weakening. The 1 H NMR spectrum of 10 in THF-d8 has nine resonances between 175 ppm and −78 ppm, consistent with C2v symmetry ( Figure   S19). Since the complex is Cs symmetric with THF coordination on one face, the spectrum in THF-

22
d8 could indicate fast THF exchange on both sides of the complex. Its spectrum in C6D6 has severe broadening compared to THF-d8, and there are at least 15 resonances ( Figure S20). Assuming that coordinated THF has two different proton environments and that NH3 rotation on the NMR timescale renders its protons equivalent, a Cs symmetric geometry (with the mirror plane perpendicular to the pincer) would predict 14 peaks. Thus, the number of peaks in the C6D6solvated NMR spectrum suggests 10 has no symmetry in C6D6, though the reason for the low symmetry is not obvious.   Figure S58). Low isomer shifts such as that of 11 are frequently observed in iron carbonyl complexes, reflecting the withdrawal of d-electron density from the iron nucleus by backbonding. 63 The 1 H and 13 C NMR spectra of 11 show 7 and 14 narrow peaks, respectively ( Figures S21 and S22). These spectra are consistent with a C2v-symmetric diamagnetic species.
The two most-downfield 13   In an effort to produce complexes with fewer CO ligands, we treated 6 with substoichiometric CO. This treatment gave solutions with 1 H NMR spectra showing a large number of peaks with chemical shifts indicative of multiple paramagnetic species. Though we were unable to isolate any of these species, the mixture converted to diamagnetic 11 upon addition of greater than three equiv of CO ( Figure S91). nearly-degenerate singly-occupied orbitals, while the second feature at 7114 eV is assigned to an Fe 1s to LUMO excitation. As expected for iron in the +3 oxidation state, the large contributions of ligand S orbitals to the frontier QROs indicate a large degree of Fe-S bond covalency.  The magnetization and EPR fit parameters are summarized in Table 2. Our magnetization data suggests that D < 0 for 8 and 9, and the intense absorption at g = 6.15 in the spectrum of 10

X-Ray Absorption
suggests the sign of D is negative in this compound as well. Fits are plotted as black traces beneath their respective experimental spectrum. Signals marked by asterisks are assigned to impurities, which were modeled as S = 1/2 species for 8 and 10 in 1% relative concentration to the S = 3/2 species.    (Table 3). Since the crystal structure of 9 shows a potassium cation only 3.13 Å from one S atom in the SCS pincer, we hypothesized that including K + may be necessary to properly describe its electronic structure. Thus, we started from the crystallographic structure of 9 and performed Mössbauer calculations on structures that included K + : one with geometry optimization of all atoms (model A) and one with only H atoms optimized (model B) (Figure 12). Model C in Figure   12 is the optimized structure of potassium-free 9, whose predicted Mössbauer parameters are listed in Table 3. In the crystal structure, the potassium is also coordinated to a Dipp aryl group of a neighboring molecule of 9, and this second arene was modeled as benzene in the calculations.
Mössbauer calculations on models A and B predicted parameters in agreement with experiment for 9 despite a significant distortion of the pincer plane in model A ( Figure 12). Model

32
A is predicted to have δ = 0.19 mm/s and ΔEQ = 0.97 mm/s while B is predicted to have δ = 0.22 mm/s and ΔEQ = 0.84 mm/s. The Mössbauer calculations on model C give a very different quadrupole splitting, and these are listed in Table 3 as the calculated parameters of 9. These results are summarized in Figure 12, and the drastic difference will be discussed below. components. To test the isolated effect of a distant and free potassium ion, we removed the two THF molecules and the 18-crown-6 that were coordinated to potassium in optimized 9-crown and repeated the Mössbauer calculations ( Figure S95). This model gave δ = 0.33 mm/s and ΔEQ = −4.08 mm/s, which is not close to the experimental spectrum of 9-crown and instead is much closer to that calculated for model C (anionic 9). In contrast to the structure containing THF and 18-crown-6, the Löwdin charge on potassium was found to be +0.99 in this structure. These results indicate that the THF and 18-crown-6 may be accepting most of the positive charge of the potassium cation, which then places enough positive charge close to the iron to alter its electronic structure. Charge reorganization, albeit in varying degrees, has been implicated as a stabilizing factor in the formation of alkali-crown complexes in previous theoretical studies. [70][71] Additionally, although the anionic model of thiolate complex 8 was in agreement with experiment, we calculated Mössbauer parameters for optimized 8-crown with potassium included to evaluate its consistency with experiment. This calculation gave δ = 0.28 mm/s and ΔEQ = 3.96 mm/s, also in agreement with the spectrum of 8-crown and essentially unchanged from the potassium-free predictions for 8 that are shown in Table 3. Taken together, the experimental and computed Mössbauer spectra for the models described above suggest that the amide donor in 9 leads to an unusual situation in which its electronic structure is dependent on the presence of a nearby cation.
To compare the electronic structures of the computational models whose Mössbauer parameters match experiment to those that do not, we plotted the frontier quasi-restricted orbitals (QROs) for models A (reproduces experimental Mössbauer parameters; all atoms optimized with K + ) and C (does not reproduce experimental Mössbauer parameters; all atoms optimized without K + ) in Figure 13. The QROs show that the shape of the doubly-occupied d orbital (DOMO) for model A resembles dz 2 while the doubly-occupied d orbital for model C resembles dyz. The dz 2 DOMO has 3.7% Fe s character, while the dyz DOMO has 0.0% Fe s character, explaining the significant change in the isomer shift. The ordering of the dxz and dxy orbitals is also switched, but the dx 2 −y 2 orbital is always higher in energy than the remaining four d orbitals due to its s antibonding interactions with the SCS pincer. QRO analysis of optimized 9-crown shows the same d orbital ordering as model A (all atoms optimized with K + ), though dz 2 is 0.90 eV more stabilized than dxy in 9-crown. ( Figure S96) Additional experiments were performed to probe the influence of the cation on the electronic structure of 9. First, we computationally sampled a wide range of potential locations for a positive charge to learn about the orientational dependence, but the results were not conclusive ( Figure S97). In addition, we experimentally sought to "remove" the potassium completely by orders of magnitude less acidic than free thioamides, which lie in the range 11-15 ( Figure 14). [72][73][74] It is seen from the Bordwell equation that the high pKa of 2 raises the BDFE relative to organic thioamides, but the very negative reduction potential (E1/2 = -1.42 V vs. Fc + /Fc) of the iron(II) center lowers the BDFE to a much larger extent. This potential is 1.0 V more negative than E1/2 for the Fe 3+/2+ couple in a four-coordinate SNS pincer complex with an NHC as the fourth ligand, and 1.4 V more negative than the analogous five-coordinate SNS scaffold with two THF molecules. 41 The more negative reduction potential in our system may result from the negativelycharged aryl pincer in place of the pyridine pincer, and also from the coordination of three strong phosphine donors.  doubly-occupied dz 2 orbital, and the EPR spectrum was rationalized on the basis of strong spinorbit coupling of the ground state with the dz 2 (β) → dyz and dxy(β) → dxz excited states. In our case, mixing of the ground state with an excited state created by promotion of dz 2 (β) into the half-filled dyz orbital could also account for the large deviation in g from ge in 9. Interestingly, computational analysis of the cited systems with anomalously small ΔEQ also revealed a doubly-occupied dz 2 orbital, as we found for 9 ( Figure 13). Thus, the small ΔEQ values imply that the large positive contribution to the z-component of the electric field gradient by the planar ligand is counteracted by a large negative contribution from the dz 2 ground state. Double occupation of the dz 2 orbital also accounts for the decreased isomer shift (which decreases with increasing iron s-electron density) of 9 by greater mixing of the doubly-occupied 3dz 2 with the 4s orbital, 94 which is corroborated by our DFT calculations.

39
The question that arises is then: why do amide complexes 9 and 9-crown show evidence for a dz 2 ground state electronic structure while thiolate complexes 8 and 8-crown as well as ammonia complex 10 do not? First, we propose that the π-donating orbitals of the N(TMS)2 ligand in 9 exert a stronger destabilizing influence on the π-symmetry iron dxy and dyz orbitals than the thiolate donor in 8 and certainly more than the ammine donor in 10. Our proposition of a stronger interaction in 9 than 8 is tied to its structure, which shows a short Fe−N bond and a nearly 90° angle between the N(TMS)2 plane and the pincer plane, permitting excellent overlap between dxy and the N px orbital and the dyz and sp 2 -like orbitals aligned roughly along the N−Si bonds. This highly π-donating ligand field closes the energy gap between dz 2 and dxz/dyz in the typical square planar splitting pattern. This explanation alone does not account, however, for the extreme variability in the calculated Mössbauer parameters for 9 depending on whether there is a positive charge near iron. On the basis of our DFT results, we therefore also suggest that the dz 2 orbital can be further stabilized by the presence of a nearby positive charge. Though the complete description of the cation interaction with each of the d orbitals is not elucidated by our work, selective stabilization of the dz 2 orbital may be a result of its shape. A positive charge in the plane of the pincer could interact with the torus-shaped portion of the dz 2 orbital, and one along the z axis can interact with the lobes pointing above and below the pincer plane. The stabilizing interaction of charge with the lobes of dz 2 is illustrated by the comparison between the QRO energies of model A in Figure 13 with optimized 9-crown ( Figure S96). Model A has positive charge with non-zero projections both onto the z axis and xy-plane while 9-crown has positive charge that is along the z axis. Accordingly, the dz 2 orbital is 0.35 eV lower in energy than the DOMO+1 in 9-crown versus model A.

40
The influence of the alkali metal cation is a distinctive aspect of the work reported here, and it pushes forward the growing understanding of the potential tuning role of cations near a transition-metal center. In a relevant example, Yang has shown that appended cations cause substantial electric fields, 95 which can have beneficial influences on catalysis. [96][97] Electronic structure calculations in these systems did not show that the orbital energies were differentially affected by the cation, leading to the conclusion that the cation mainly exerts an electrostatic effect.
Tolman has shown the influences of nearby cations on O-H bond dissociation energies. 98 In more recent work, Tomson observed shifts in redox potentials that were attributed to stabilization of a dz 2 orbital, and supported this idea with DFT calculations. 99 Redox potentials were primarily used to probe cation effects in these examples, but incorporation of iron in our complexes enabled us to use Mössbauer spectra that can be directly related to the electronic structure of iron. Through the combination of spectroscopy and DFT, we identified cation-dependent changes in orbital energetics. While the overall spin state is conserved throughout, a b electron shifts to a cationstabilized dz 2 orbital, manifested in Mössbauer spectra by low δ and |ΔEQ|. In the future, this effect may be utilized to control reactivity that depends on the ordering of orbital energies.
The conclusions from our electronic structure studies of complexes 8 (thiolate), 9 (amide), and 10 (ammine) utilizing EPR spectroscopy, SQUID magnetometry, Mössbauer spectroscopy, and DFT calculations are summarized as follows. EPR spectroscopy in combination with variabletemperature measurements demonstrated large negative D with E ≠ 0 in each case. Mössbauer spectra of 8 and 10 showed isomer shifts and quadrupole splitting values consistent with other known intermediate spin iron(III) complexes, and DFT could accurately predict these parameters without including a counterion in 8. The quadrupole splitting of 9, however, was anomalously low, and DFT predicted a large |ΔEQ| when no counterion was included. Inclusion of potassium in the 41 model of 9 gave calculated Mössbauer parameters in agreement with experiment, and the energetic ordering of dz 2 relative to the other d orbitals is likely to determine whether a typical large |ΔEQ| versus an atypical small |ΔEQ| is observed. Independently, the relatively low distortion from planarity, strong π donation from the amide ligand, and short Fe−N bond in 9 are hypothesized to enact relative orbital energies that are amenable to changing order in the presence of a cation.
Clearly, the SCS scaffold gives access to a range of electronic structures depending on the identity of the fourth donor ligand and on the cation location.
Comparisons to the Nitrogenase FeMoco. The C and S-ligated iron complexes have the same donor atoms as the iron sites in the FeMoco, and we briefly explore the comparison of molecular and electronic structures. The most thoroughly-characterized state of FeMoco is the enzyme resting state (E0), and in this structure, the high-spin belt irons are bridged by sulfide ligands and share a central carbide, leading to a 3S/1C environment. 12 Our complex 8 likewise has an iron center ligated exclusively by three S donors and one C donor. Its Fe−C bond length of 2.00 Å is very close to the 2.01 Å Fe−C distance in FeMoco. [13][14]100 The Fe-S lengths, which range from 2.24 to 2.25 Å, are also highly similar to Fe−S bond lengths in FeMoco (2.25 to 2.27 Å).
Despite being in an intermediate spin state, these bond distances are closer to those in resting state FeMoco than the two previously-reported (SCS)iron complexes, which were in lower oxidation states and thus had longer Fe−C/S bonds. 26,28 The final state of the enzyme during catalysis is an iron-ammonia adduct (E8). 101 Because turnover from E8 to E0 is redox neutral, ammonia would be expected to bind to either the iron(II) or iron(III) oxidation states. 101 We were able to isolate the stable ammonia adduct in high yield in an intermediate-spin iron(III) oxidation state, thus demonstrating the feasibility of an iron(III) ammonia adduct in a C and S-ligand field.

CONCLUSIONS
A new pincer ligand bearing only sulfur and carbon donor atoms can support new iron complexes. The ligand scaffold has a proton-responsive site, gives access to unusual electron structures, and mimics the sulfide and carbide donors found in the cofactor of nitrogenase enzymes. Adducts of thiolate, amide, ammonia, and CO demonstrate the versatility of this SCS pincer system to support a variety of nitrogenase-relevant donors that induce an electronic structure that is highly sensitive to the iron surroundings.