Modulation of Fe–Fe distance and spin in diiron complexes using tetradentate ligands with different flanking donors

Abstract

Here we report the synthesis and characterization of diiron complexes containing triaryl N₄ and N₂S₂ ligands derived from o-phenylenediamine. The complexes display significant differences in Fe–Fe distances and magnetic properties that depend on the identity of the flanking NMe₂ and SMe donor groups.

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Dinuclear iron complexes are renowned for their functional roles in metalloenzymes and biomimetic complexes.¹ They are also highly relevant to efforts aimed at understanding chemical factors that govern the strength of metal–metal interactions with first-row transition metals, especially when combined with weak field ligands.^2,3 These 3d complexes are often paramagnetic and adopt different spin configurations that depend sensitively on the degree of coupling between the metals. This has led to interest in understanding how ligand and structural modifications can be used to control spin states and associated magnetic properties for spin-based applications.^3,4

Amido ligands have been featured prominently in efforts aimed at preparing dinuclear complexes with iron. Examples include Fe₂[N(SiMe₃)₂]₄ and related complexes that form a diamond Fe₂N₂ core in the solid state.^5–7 Other examples include amido ligands containing appended metal-donor groups.⁸ These have been used most extensively to prepare diiron complexes that are C₃ symmetric with respect to the Fe–Fe axis,⁹ but amido ligands derived from o-phenylenediamine have also been used to assemble iron complexes of various nuclearity. Betley and coworkers reported tripodal ligands containing three o-phenylenediamine units that can yield metal clusters containing up to six Fe atoms.^10,11 Similarly, a dinuclear Fe₂N₂ complex containing N,N′-bis(pentafluorophenyl)-o-phenylenediamide was recently described.¹²

We previously reported the triaryl N₄ and N₂S₂ ligands H₂(L1) and H₂(L2) (Fig. 1).^13,14 Like the examples described above, these tetradentate ligands are derived from o-phenylenediamine, but they contain flanking aryl groups with NMe₂ and SMe donor substituents. Here we report the synthesis and properties of homoleptic Fe(II) complexes with L1 and L2. Unlike previous examples with Ni and Ru,^13,14 and prior studies of square-planar Fe(II) complexes with stronger field PNNP ligands,¹⁵ the aryl groups and donor atoms in L1 and L2 do not remain conjugated and coordinated in the same plane. Instead, they give rise to dinuclear Fe complexes with structures and spin configurations that are highly dependent on the identity of the flanking NMe₂ and SMe donor groups.

Fig. 1 Top: Synthesis of 1 (X = NMe₂) and 2 (X = SMe) and structural isomer observed with X = NMe₂ (1a). Bottom: Molecular structures of 1 (left), 2 (center), and 1a (right). Thermal ellipsoids are drawn at the 50% level (data collected at 150 K). Hydrogen atoms and co-crystallized solvent were removed from the figure. Only one of the two crystallographically unique complexes in the unit cell of 1a is shown.

Mixing H₂(L1) or H₂(L2) with in situ generated Fe₂[N(SiMe₃)₂]₄ in thf resulted in dark red solutions, and subsequent workup and crystallization by vapor diffusion of pentane into concentrated benzene solutions yielded large dark red crystals of 1 and 2. Both complexes were isolated as single crystals in good yields (67% and 85%, respectively).

The dinuclear structures of 1 and 2 were revealed by single-crystal X-ray diffraction studies. Each complex has a butterfly Fe₂N₂ core supported by bridging amido groups on separate ligands (Fig. 1). The most apparent difference between the two structures is the Fe–Fe distances of 2.5072(5) and 2.7666(6) Å for 1 and 2, respectively (Fig. 2). The Fe–Fe distance in 1 is significantly shorter than those reported for amido-bridged complexes Fe₂[N(SiMe₃)₂]₄, Fe₂[N(SiMe₃)₂]₂[OC(CF₃)₂Ph]₂, and Fe₂(NPh₂)₄ at 2.663(2), 2.674(6), and 2.715(2) Å, respectively,⁶ and is effectively identical to the 2.5128(4) Å distance very recently reported for Fe₂(TMP)₂(C₆F₅)₂ (TMP = tetramethylpiperidide).⁷ The distance corresponds to a formal shortness ratio (FSR) of 1.08, which is just beyond the range expected for a formal Fe–Fe single bond.³

Fig. 2 Summary of distances (Å) in the Fe₂N₂ core in 1, 2, and 1a and comparison to the RT μ_eff values in μ_B (susceptibility balance). Distances for 1a are shown for one of two crystallographically unique complexes in the asymmetric unit cell, but they are representative of both complexes.

The close Fe–Fe distances are supported by bridging, but inequivalent, Fe–N bonds in the Fe₂N₂ core. Each bridging amido group forms a covalent N–Fe bond (X-type donor) with one Fe and a dative N → Fe bond (L-type donor) with the other, as indicated by the differing Fe–N distances (Fig. 2). The Fe–N bonds in 2 are 0.04 Å shorter on average compared to those in 1.

Prior to assessing the magnetism of the complexes, unit cell checks were performed in addition to elemental analysis to confirm the purity of crystals obtained for different batches. It was during these checks that we discovered a second structural isomer of 1 in one of the batches prepared in the latter part of our investigation. To distinguish between the two structures, we will refer to the second isomer as 1a (Fig. 1). Crystals of both isomers were grown by vapor diffusion of pentane into benzene solutions of 1. Crystals of 1 with the intact Fe₂N₂ diamond core crystallize as relatively large blocks in the monoclinic space group P2₁/c with 1.5 equivalents of co-crystallized benzene, whereas 1a crystallizes as small irregular prisms in the triclinic space group P1̄ and has an equivalent of co-crystallized benzene and disordered solvent presumed to be pentane. The biggest change in the structure of 1a with respect to 1 is the opening of the Fe₂N₂ core. The L-type bonds associated with the amido bridges in 1 are no longer present in 1a. As a result, the Fe–Fe distance in 1 at 2.5072(5) Å elongates to 2.627(2) Å in 1a, and the X-type Fe–N bonds shorten by 0.05–0.08 Å. A second polymorph of 2 containing co-crystallized benzene was also discovered (2a), but unlike 1 and 1a, there were only subtle differences in their structures (Fig. S1, ESI†).

Magnetometry studies were performed to investigate the magnetic properties of 1 and 2. Given the possibility of two isomers for 1, we investigated the room temperature magnetic moments first in solution and then in the solid state on crystallographically authenticated samples of both complexes. Evans method magnetic measurements performed on benzene solutions of 1 and 2 revealed effective magnetic moments of 7.6 and 6.3 μ_B, respectively. Solid-state measurements made using a magnetic susceptibility balance at room temperature (294 K) yielded effectively identical magnetic moments of 7.6(1) μ_B for 1 and 6.2(1) μ_B for 2. This suggests that the differences in μ_eff for 1 and 2 cannot be attributed to phase-dependent differences in structure or crystal packing. It may also suggest that the structure of 1 persists in solution or that the structure of 1a has little influence on the magnetic moment.

Dc magnetic susceptibility data were collected on polycrystalline samples of 1 and 2 using a SQUID magnetometer field of 1.0 T to measure the temperature dependence of the molar magnetic susceptibility times temperature (χ_MT vs. T) for each sample (Fig. 3). The room temperature effective magnetic moments of 7.43 μ_B (1) and 6.02 μ_B (2) are in excellent agreement with the moments determined via Evans method and susceptibility balance (Fig. S2–S5, ESI†). For 1, the room temperature χ_MT value of 6.900 cm³ K mol⁻¹ remains largely unchanged when lowering the temperature to ∼110 K, below which a gradual decrease in χ_MT value of 5.002 cm³ K mol⁻¹ is monitored (Fig. 3). Below ∼10 K a steep drop in χ_MT is observed, which is largely attributed to zero-field splitting and/or antiferromagnetic coupling. The magnetic data was modeled under consideration of a weak antiferromagnetic coupling interaction between the two Fe(II) centers with a J of −0.22(3) cm⁻¹ (Table S2, ESI†). For 2, exhibiting the longer Fe–Fe distance, the trend in progression of the dc magnetic susceptibility data from 2 to 300 K varies significantly relative to 1. The room temperature χ_MT value is 4.528 cm³ K mol⁻¹, which declines almost linearly with decreasing temperatures, resulting in a χ_MT value of 0.030 cm³ K mol⁻¹ at 2 K, which is close to zero. Such static magnetic susceptibility behavior is indicative of strong antiferromagnetic coupling and hence, was modeled accordingly, yielding a J value of −37(1) cm⁻¹. This magnitude of J represents a 168-fold increase in magnetic coupling for 2 compared to 1 despite the longer Fe–Fe distance in 2 (Table S3, ESI†).

Fig. 3 Left: ⁵⁷Fe Mössbauer spectra of 1 and 2 at 4 K. Right: Variable-temperature (zero-field-cooled (zfc)) dc magnetic susceptibility data for restrained polycrystalline samples of 1 and 2 collected under a 1.0 T applied dc field. The black lines represent fits to the data from 30 to 300 K for 1 and 2, respectively.

⁵⁷Fe Mössbauer spectra were collected on 1 and 2 to evaluate Fe oxidation states and electronic environment. The spectrum of 1 at 20 K revealed two quadrupole doublets (Fig. S6, ESI†). The major component (83%) had an isomer shift of δ 0.80 (ΔE_Q = 1.25 mm s⁻¹), consistent with high-spin Fe(II). A minor component was observed at a nearly identical isomer shift of δ 0.84, but it displayed a larger quadrupole splitting (2.43 mm s⁻¹). Cooling the sample further to 4 K yielded no effective change in the major component (δ 0.81; 1.25 mm s⁻¹; 85%), but the minor component appears to undergo additional hyperfine splitting (Fig. 3). As with our magnetic measurements, random unit cell checks of the crystal batch used for Mössbauer analysis were uniformly consistent with 1, but it is possible that the minor component could be attributed to small amounts of co-crystallized 1a. We think it is more likely that the minor component is attributed to a non-integer spin impurity, which would be consistent with the additional hyperfine splitting observed upon sample cooling.

The Mössbauer spectrum of 2 at 4 K revealed a quadrupole doublet that was modeled as a single high-spin Fe(II) species. The isomer shift of δ 0.76 is similar to the δ 0.81 shift for 1, but the spectrum for 2 displays a larger quadrupole splitting of 2.07 mm s⁻¹. This increase is consistent with the reduction in spin due to the increased antiferromagnetic coupling. For example, Betley and coworkers showed that a reduction in S = 4 to S = 2 in o-phenylenediamido-supported triiron clusters capped with ancillary pyridine ligands at 105 K led to an increase in quadrupolar splitting from ΔE_Q = 1.48 mm s⁻¹ to ΔE_Q = 2.22 mm s⁻¹,¹¹ but with nearly identical isomer shifts of δ 0.85 and 0.82, respectively, as observed for 1 and 2.

DFT calculations (B3LYP-D3/def2-TZVP)¹⁶ were performed to evaluate the energies and structural preferences of 1 with respect to 1a. All attempts to optimize 1 resulted in geometries with bond distances consistent with 1a, indicating that this species lies lower in energy (Tables S4–S6, ESI†). The DFT calculated Fe–Fe distance for 1a was 2.687 Å, only a 0.060 Å (2.3%) deviation from experiment. To calculate the structure of 1, the Fe atoms were fixed at their experimental positions while the remaining ligand atoms were allowed to relax. This fixed species, which is denoted herein as 1′, is nearly 3 kcal mol⁻¹ higher in energy than 1a according to DFT (Tables S5 and S6, ESI†). Likewise, CASPT2¹⁷ calculations yielded a consistent energy difference of 5.6 kcal mol⁻¹ (Table S5, ESI†). These relatively small energy differences agree with our ability to isolate both structures.

Geometry optimizations of 2 also yielded a structure in excellent agreement with experiment (Fe–Fe distance of 2.807 Å; 0.040 Å (1.5%) deviation from XRD), as shown with overlays of the computed and experimental structures (Fig. S10, ESI†). Although DFT optimizations were performed only for the S = 4 DFT ground state (Tables S7 and S8, ESI†), CASPT2 calculations revealed that the lowest energy spin configuration for 1′, 1a, and 2 was S = 0 (Table S9, ESI†). However, the other spin states (S = 1, 2, 3, and 4) were all within 1.2 kcal mol⁻¹, which falls within the error of the CASPT2 method. This indicates that the assignment of the ground spin state would require careful treatment of both spin–orbit coupling and electron correlation. The observation of closely spaced spin multiplets is consistent with no interaction between the Fe centers, as can be seen in the CASSCF natural orbitals (Fig. S8, ESI†). On the other hand, some orbital overlap is present in DFT (Fig. S9, ESI†), resulting in a Mayer bond-order between the metal ions of 0.1 for 1′ and 1a and 0.05 for 2 (Table S10, ESI†). These differing observations are consistent with the more localized bonding in the CASSCF natural orbitals, which suggest no Fe–Fe bonding, compared to the DFT Kohn–Sham orbitals, which suggest the existence of very weak Fe–Fe bonding.

In summary, we have described how triaryl tetradentate ligands derived from o-phenylenediamine form diiron complexes that have Fe–Fe distances and magnetic properties that are highly sensitive to the identity of the flanking donor substituents. These results compliment studies showing how ancillary ligand field strength can significantly alter Fe–Fe distances and spin states in triiron complexes supported by hexadentate ligands derived from o-phenylenediamine.¹¹ Rather than using ancillary ligands, we have shown here that exchanging flanking donors can be used to achieve similar outcomes. In addition to our ongoing studies with iron, we are currently investigating these dinucleating ligand effects with other first-row transition metals. These efforts will be described in several upcoming reports.

KDS, DS, and SRD thank the NSF for support (CHE-1650894), and Dale Swenson for collecting the XRD data. The structure of 1a was collected using the instrument supported by NSF CHE-1828117. JJS, HRP, and EJC thank the University of Iowa for support. GPH and CMT acknowledge the NSF for support (CHE-2101002). SD and FB also thank the NSF for support through the CAREER award to SD (CHE-2339595). SRC and BV thank the U.S. DOE, Office of Science, BES, Computational Chemical Sciences program for support under award number DE-SC0019463. Computations supporting this project performed on high-performance computing systems were made possible by resources funded by NSF award OAC-1626516.

Data availability

Input and output files associated with all calculations are available both in a FigShare repository (https://doi.org/10.6084/m9.figshare.26121724) and in an ioChem-BD repository (https://doi.org/10.19061/iochem-bd-6-379). Crystallographic data has been deposited at the CCDC under 2357235–2357238. The remaining data have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and computational details. CCDC 2357235–2357238. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc02522a

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