Tuning Metal–Metal Interactions through Reversible Ligand Folding in

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Tuning Metal−Metal Interactions through Reversible Ligand Folding in a Series of Dinuclear Iron Complexes Shaoguang Zhang,† Qiuran Wang, Laura M. Thierer, Alexander B. Weberg, Michael R. Gau, Patrick J. Carroll, and Neil C. Tomson* P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States

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S Supporting Information *

ABSTRACT: A dinucleating macrocyclic ligand with two redox-active, pyridyldiimine components was shown to undergo reversible ligand folding to accommodate various substitution patterns, metal ion spin states, and degrees of Fe− Fe bonding within the cluster. An unfolded-ligand geometry with a rectangular Fe2(μ-Cl)2 core and an Fe−Fe distance of 3.3262(5) Å served as a direct precursor to two different folded-ligand complexes. Chemical reduction in the presence of PPh3 resulted in a diamagnetic, folded ligand complex with an Fe−Fe bonding interaction (dFe−Fe = 2.7096(17) Å) between two intermediate spin (SFe = 1) Fe(II) centers. Ligand folding was also induced through anion exchange on the unfolded-ligand species, producing a complex with three PhS− ligands and a temperature-dependent Fe−Fe distance. In this latter example, the weak ligand field of the thiolate ligands led to a product with weakly coupled, high-spin Fe(II) ions (SFe = 2; J = −50.1 cm−1) that form a bonding interaction in the ground state and a nonbonding interaction in the excited state(s), as determined by SQUID magnetometry and variable temperature crystallography. Finally, both folded-ligand complexes were shown to reform an unfolded-ligand geometry through convergent syntheses of a complex with an Fe−Fe bonded Fe2(μ-SPh)2 core (dFe−Fe = 2.7320(11) Å). Experimentally validated DFT calculations were used to investigate the electronic structures of all species as a way to understand the origin of Fe−Fe bonding interactions, the extent of ligand reduction, and the nature of the spin systems that result from multiple, weakly interacting spin centers.

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INTRODUCTION The ability to vary the distances and spin interactions between multiple metal sites when binding and functionalizing small molecules is important for the proper functioning of many metalloenzymes1−4 and heterogeneous metallic surfaces.5−9 Recent work in molecular cluster chemistry has led to the development of di- and trinucleating ligand frameworks that allow for control over the distances between metal ions (Figure 1). This work has provided a means of evaluating the relationship between M−M bonds and the reaction chemistry that occurs between them. However, efforts to create synthetic systems that support geometric changes about a M−M bonded cluster core are in their infancy. Doing so would lead to more accurate modeling of the rearrangements that occur at enzymatic and metallic surface active sites as well as the development of new homogeneous catalysts.9−18 Betley and co-workers reported a trinucleating ligand scaffold that favors the formation of M−M bonded triangular cores.19−21 These species are able to exhibit μ3-X binding similar to that observed on metallic surfaces. Agapie and coworkers developed homobimetallic complexes of nickel and palladium supported by a terphenyl diphosphine. These show diverse metal−arene interactions and allow for carbon−carbon bond formation and bimetallic binding to heterocycles.22,23 © XXXX American Chemical Society

Uyeda and co-workers crafted a series of naphthyridine− diimine supported dinuclear metal complexes that use the redox-activity of the ligand and an exposed Ni−Ni interaction for several catalytic processes, such as reductive cycloaddition and cyclopropanation chemistry.24−26 Finally, Murray’s tris(βdiketiminate) cyclophane ligand framework has been used to create a substrate binding pocket with fixed internuclear distances that are long enough to preclude significant M−M bonding.27,28 This constraint appears to favor the combined action of these metal centers for binding and activating small molecules, including the Cu3 binding of N2 and the Fe3 reduction of N2 to form bridging amides. These examples may be compared to the exquisite use of M−M bonds by Lu, Thomas, and others to modulate the reactivity at one metal center in particular.29−37 These latter systems typically enforce chemistry at a position trans to the second metal, but in some cases, the release of a bridging ligand has provided simultaneous access to both.38 In the current study, we have investigated the ability of a dinucleating macrocyclic ligand framework to undergo dramatic geometric changes in response to the varying Received: June 5, 2019

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DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Formation of [Fe2Cl2]2+ and [Fe2Cl]+

crystallization of [(3PDI2)Fe2(μ-Cl)2(thf)2][BArF4]2 from THF/hexane in 48% yield and high purities. The two [Fe2Cl2]2+ salts displayed qualitatively identical electronic absorption profiles (Figure S16), indicating that the two dications are of a similar composition. Crystallographic analysis of the borate salt revealed an unfolded, “stair-step”-like ligand geometry (Figure 3a). Each metal center was bound in a roughly octahedral coordination environment that includes a PDI ligand fragment, a thf ligand, and two bridging chlorides one axial and one equatorial with respect to placing the PDI unit in the equatorial plane. The imino Cim−Nim and Cim−Cpy distances are known to be diagnostic of the amount of electron density being held within the PDI π* manifold, reflecting the physical oxidation state of the ligand fragment.47−50 These values were recently combined into a single parameter, Δ, which provides a convenient numeric expression of the extent of reduction of the ligand, with values in the range of 0.17 reflecting PDI0 and those near 0.10 corresponding to PDI−.50 The values for [Fe2Cl2]2+ reflected neutral PDI fragments [(3PDI2)0, Δ = 0.192, Table 1], indicating that the metal centers are in the +2 oxidation state. The long Fe−Fe distance (3.3262(5) Å) points to the lack of a meaningful bonding interaction between the metals. This interpretation is consistent with the electron count for the cluster (36 e−),51 which represents an 18 e− count for each metal center at the exclusion of an Fe−Fe bond. Further analysis found that the electronic structure of [Fe2Cl2]2+ is that of a ferromagnetically coupled ground state involving two magnetic Fe(II) ions. The 1H NMR spectrum for [Fe2Cl2]2+ exhibited broad, paramagnetically shifted signals (Figure S1), and the long Fe−NPDI distances suggest partial occupation of the Fe−NPDI σ* manifold. The system may thus consist of either intermediate-spin (SFe = 1; Stot = 2) or highspin (SFe = 2; Stot = 4) metal centers. Spin-only magnetic moments of 4.9 and 8.9 μB would be expected for Stot = 2 or Stot = 4 states, respectfully. Solution phase magnetometry at 293 K revealed a magnetic moment of 7.7 ± 0.1 μB (CD2Cl2), suggesting the presence of ferromagnetically coupled HS Fe(II) ions. Variable temperature solid-state magnetometry similarly indicated a weakly ferromagnetically coupled Stot = 4 system. Modeling the SQUID data with a spin Hamiltonian that includes two, spin-only S = 2 ions and the presence of axial zero-field splitting (Dzfs) resulted in a satisfactory fit when g = 2.173, J = 2.5 cm−1, and |Dzfs| = 5.2 cm−1 (see Supporting Information).

Figure 1. Representative examples of multimetallic complexes.

electronic requirements of the metal centers.39 This work makes use of 3PDI2, a tert-butylated version of a macrocyclic, pyridyldiimine-based scaffold (Figure 2)39−43 that we recently

Figure 2. Illustration of the dinucleating, 3PDI2 ligand architecture.

employed for mediating the putative formation of transient dicobalt bridging nitrides.44 The Co2N complexes parallel proposed surface nitrides that develop during bulk Rh/Ircatalyzed NH3 decomposition/oxidation chemistry.45,46 Variations in the reaction conditions resulted in a wide array of products, suggesting that an improved understanding of the factors that govern changes in the ligand geometry will lead to enhanced reaction selectivity. The present report describes a series of diiron complexes in which we are able to observe geometric changes as a function of charge, spin state, and the identity of ancillary ligands. The information gained from this work will provide a platform for developing new catalysts that take advantage of the cooperative action of multiple metal centers within a geometrically and electronically flexible ligand framework.

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RESULTS AND DISCUSSION The addition of 2.0 equiv of FeCl2 to a solution of (3PDI2)Sr(OTf)2 in THF generated a purple slurry containing the dinuclear complex [( 3 PDI2 )Fe2 (μ-Cl) 2(thf) 2][OTf] 2 ([Fe2Cl2]2+, Scheme 1).40−43 This material exhibited limited solubility in low polarity solvents, but exchange of the triflate counterions for [BArF4]− (ArF = 3,5-(CF3)2C6H3) allowed for B

DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Depictions of the crystallographically determined cationic and neutral portions of [Fe2Cl2]2+ (a, top left), [Fe2Cl]+ (b, top right), [Fe2(SPh)3]+ (c, bottom left) and [Fe2(SPh)2]0 (d, bottom right); all hydrogen atoms and counterions were removed for clarity.

Table 1. Important Bond Lengths (Å) and Angle (deg) Data of Diiron Complexes cluster EAN Fe−Fe avg Fe−(μ-X) avg Fe−Npy avg Fe−Nim avg Nim−Cim avg Cim−Cpy ∠Fe−X−Fe Δ FSR

[Fe2Cl2][BArF4]2

[Fe2Cl][OTf]

[Fe2(SPh)3][OTf]

[Fe2(SPh)2]0

36 3.3262(5) 2.4348 2.0781(19) 2.232 1.278 1.498 86.12(2) 0.192 1.42

34 2.7096(17) 2.295 1.820 1.947 1.309 1.456 72.34(8) 0.119 1.16

32 3.1733(5) 2.3547 2.0638 2.1783 1.287 1.490 84.727(17) 0.176 1.36

34 2.7320(11) 2.2535 1.826(3) 1.970 1.331 1.433 74.63(4) 0.075 1.17

methylene groups of the bridging propylene linkers, and the P{1H} NMR spectrum of [Fe2Cl]+ in THF-d8 showed a single resonance at 39.8 ppm. These data are comparable to those reported previously for a folded macrocycle geometry that is static on the NMR time scale.44 The proposed geometry was confirmed by X-ray crystallography, which revealed an Fe2(μ-Cl) core, with ∠Fe−Cl−Fe = 72.34(8)° and d(Fe−Fe) = 2.710(2) Å (Table 1). The crystal structure further reveals significant reduction of the 3PDI2 ligand compared to [Fe2Cl2]2+. The imino group Cim−Nim and Cim−Cpy distances indicate that each PDI fragment houses one electron’s worth of electron density (Δ = 0.119), rendering a cluster core that contains two Fe(II) metal centers. The 2.710(2) Å Fe−Fe distance and the diamagnetism of the complex are consistent with a weak single bond between the two metals. The formal shortness ratio for this complex, defined as the ratio of the experimental M−M distance to the sum of the radii of the elements,52,53 is consistent with this interpretation (FSR = 1.16).54−56

A dramatic change in both the geometry of the ligand and the nature of the interaction between the iron centers was observed on chemical reduction of [Fe2Cl2]2+. In previous work on related 34 e− dicobalt complexes, the 3PDI2 ligand was repeatedly observed in a folded conformation.44 These clusters exhibited weak Co−Co bonding, as judged by the diamagnetism of the complexes and the intermediate range internuclear distances. However, the dicobalt complexes were formed from Sr(II)-bound macrocycles through a one-pot synthetic sequence, which failed to give information on the structural flexibility of the 3PDI2 ligand prior to forming the dinuclear, folded-ligand products. In the present example, we were pleased to find that the stair-step geometry of [Fe2Cl2]2+ could be used as a starting point for the formation of diiron, folded-ligand complexes. Reduction of [Fe2Cl2][OTf]2 with 2.0 equiv of KC8 in the presence of 2.0 equiv of PPh3 led to dark brown crystals of [(3PDI2)Fe2(μ-Cl)(PPh3)2][OTf] ([Fe2Cl]+, Scheme 1) following crystallization from fluorobenzene/pentane. The diamagnetic product displayed diastereotopic signals for the

31

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DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX

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(thiolate) product is paramagnetic at room temperature. SQUID magnetometry revealed a temperature-dependent magnetic moment, as illustrated by a plot of μeff vs T. The trace exhibited a moment of 0.56 μB at 4 K and steadily increased with temperature to 5.09 μB at 300 K (Figure 4).

Taken together, these factors preclude the presence of lowspin Fe(II) and suggest that the electronic structure of [Fe2Cl]+ is best described as resulting from two intermediatespin Fe(II) centers (S = 1), each of which undergoes antiferromagnetic coupling to both a ligand-based spin and the other metal center. This view of the electronic structure is supported by DFT calculations, which allowed us to identify a low-lying, dz2-based Fe−Fe σ-antibonding orbital as the LUMO+1 (see Supporting Information, Figure S32). The PDIπ* manifold was heavily mixed into the occupied valence manifold and found to contain approximately one electron’s worth of electron density, consistent with the crystallographic data. A more complete accounting of the bonding within an isomorphic, electron transfer series of phosphine-bound, folded-ligand complexes will be presented elsewhere. The ligand folding that occurred to form [Fe 2 Cl] + accompanies the formation of an Fe−Fe bonding interaction as a way to satisfy the valence within the 34 e− cluster, but it was also found that ligand folding could be induced by salt metathesis chemistry. Treatment of [Fe2Cl2]2+ with three equivalents of NaSPh in THF resulted in a color change from purple to brown. Subsequent crystallization provided the tris(thiophenolate) product, [( 3 PDI 2 )Fe 2 (μ-SPh)(SPh)2] [OTf] ([Fe2(SPh)3]+) (Scheme 2).57 The product

Figure 4. Experimental (black squares) and simulated (red line) variable temperature solid-state magnetometry data of [Fe2(SPh)3] [OTf].

Scheme 2

This latter value correlated well with the room temperature solution phase magnetic susceptibility of 4.7 ± 0.1 μB (CD3CN, 297.2 K), indicating that the observed magnetic response in the solid state is in fact due to molecular magnetism, not extended magnetic coupling within the crystal lattice. Fitting the SQUID data with a two-site spin Hamiltonian that includes axial ZFS at each S = 2 spin center provided a satisfactory fit when J = −50.1 cm−1, g = 2.655, and |Dzfs| = 27.8 cm−1; a 0.9% S = 5/2 impurity was required to fit the residual magnetic susceptibility at low T. DFT calculations indicated a preference for the Stot = 0 state, in accord with the magnetometry data. This state is best described as a broken symmetry singlet that results from the antiferromagnetic coupling of two high-spin (SFe = 2) Fe(II) centers (BS(4,4); Figure 5). The calculated coupling constant relating the energy of the singlet and nonet states was found to be in good agreement with the experimental data (JDFT = −83.5 cm−1; Jexp = −50.1 cm−1). A small magnitude coupling constant in a broken symmetry solution implies weak-overlap covalent bonding between the fragments that compose the magnetic orbitals, resulting in closely spaced bonding and antibonding combinations on either side of the HOMO− LUMO gap. If one sets the Fe−Fe bond along the z-axis, the magnetic orbitals of [Fe2(SPh)3]+ may be considered to result from inphase, bonding combinations of dz2 (Sαβ = 0.58), dx2‑y2 (0.10), and dxz (0.02) as well as an out-of-phase, antibonding combination of dxy (0.16) (Figure 5). The presence of three in-phase, Fe−Fe bonding combinations implies that thermal population of higher spin states will lead to the transfer of bonding electron density into the low-lying antibonding manifold. This view was supported by geometry optimization calculations along Stot > 0 surfaces, which led to significant increases in the Fe−Fe distance, from 2.84 Å for the Stot = 0 state to 2.95, 2.98, 3.14, and 3.01 Å for the Stot = 1, 2, 3, and 4 states, respectively. Connecting this trend with the results from

gave paramagnetically broadened and shifted signals between −15 and +85 ppm by 1H NMR spectroscopy. Crystallographic analysis at 100 K revealed the three thiolate ligands in bridging and terminal positions of the folded-ligand complex, with similar Fe−SPh bond lengths (ca. 2.32−2.35 Å). The metric parameters within the PDI units indicate that the ligand resides in its neutral form, (3PDI2)0 (Δ = 0.176), meaning that each metal center remains as Fe(II). The long Fe−Fe distance (3.1733(5) Å; FSR = 1.36) indicates the lack of a significant bonding interaction between the metal centers, and the long Fe−NPDI distances are consistent with metals that have adopted a high spin (SFe = 2) configuration as a result of the weak-field PhS − ligands. The average Fe−N im /Fe−N py distances increase from 1.947/1.820 Å in [Fe2Cl]+, a species that was assigned as an intermediate-spin Fe(II), to 2.1783/ 2.0638 Å in [Fe2(SPh)3]+, which appears to contain two highspin Fe(II) ions. Magnetometry measurements on [Fe2(SPh)3]+ were used to probe the interaction between these two S = 2 spin centers. In contrast to the folded ligand complex [Fe2Cl]+, the trisD

DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Plot of the crystallographically determined Fe−Fe distance within [Fe2(SPh)3][OTf] as a function of temperature. Estimated standard deviations are shown with vertical brackets.

Table 2. Crystallographically Determined Fe−Fe Distance (Å) within [Fe2(SPh)3][OTf] at Different Data Collection Temperatures T/K

d(Fe−Fe)/Å

100 150 200 250

3.1726(5) 3.1930(5) 3.2154(6) 3.2344(8)

and one bridging 3p element donors. However, chemical reduction causes another rearrangement in the ligand geometry to allow for the system to recoup Fe−Fe bonding character. The addition of 2.0 equiv of KC8 to [Fe2(SPh)3]+ generated a soluble, brown material that showed paramagnetically shifted resonances in its 1H NMR spectrum. Identical, paramagnetically shifted 1H NMR spectroscopic data were obtained following treatment of the reduced complex [Fe2Cl]+ with NaSPh in THF, suggesting that these reaction sequences converge on a single product. Crystallization from Et2O at room temperature yielded brown crystals suitable for XRD. This analysis revealed an unfolded 3PDI2 ligand framework with a symmetric Fe2(μSPh)2 core: (3PDI2)Fe2(μ-SPh)2 ([Fe2(SPh)2]0, Scheme 2). This charge-neutral species displays crystallographic inversion symmetry with pseudosquare pyramidal geometries about the iron centers. The PDI fragments each contain one electron’s worth of electron density (Δ = 0.075) (Table 1), suggesting that the physical oxidation state of each metal center is +2. The acute Fe−S−Fe angle of 74.63(4)° accompanies a shortened Fe−Fe distance of 2.7321(7) Å (FSR = 1.17), similar to that observed for the folded-ligand complex [Fe2Cl]+. The short Fe−Fe distance in [Fe2(SPh)2]0 (cf. d(Fe−Fe) in [Fe2Cl2]2+ = 3.3262(5) Å) and the displacement of the Fe atoms by 0.22 Å toward one another from the planes generated by the PDI fragments highlights the structural adaptability afforded by the 3 PDI2 ligand system. While multiple routes were available to generate [Fe2(SPh)2]0, recalcitrant impurities prevented firm conclusions about its solid-state magnetic behavior (see Supporting Information). We thus resorted to the aforementioned computational methods that had provided excellent agreement between experiment and theory to help interpret the electron

Figure 5. Plots of the highest-lying α-spin (left) and β-spin (right) unrestricted corresponding orbitals (UCOs) for the geometry optimized BS(4,4) singlet state of [Fe2(SPh)3]+.

SQUID magnetometry suggested that the Fe−Fe distance should vary with temperature. Indeed, XRD data collected on a single crystal of [Fe2(SPh)3]+ at 100, 150, 200, and 250 K revealed that the Fe−Fe distance increased from 3.1726(5) Å at 100 K to 3.2344(8) Å at 250 K (Figure 6, Table 2), indicating (i) that the change in spin state distribution observed by SQUID magnetometry in fact corresponds to a change in the bonding within the dinuclear core and (ii) that the computational method used in this study provides a suitable representation of the complex electronic structure in this ion. We note that the discrepancy between the calculated Fe−Fe distances and the experimental XRD data reflect, in part, the dependence of the calculated Fe−Fe distance on the specific spin state being probed, coupled with the high spinstate density of this system (AF-coupled HS Fe(II) with Jexp = −50.1 cm−1). Even at 100 K, one would expect significant population of magnetic states, all of which exhibit long Fe−Fe distances. We next investigated the chemical reduction of [Fe2(SPh)3]+. A 2 e− reduced product would be a weak-field analog to the Fe−Fe bonded [Fe2Cl]+, in that both would be 34 e− species in folded ligand geometries with two terminal E

DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry distribution within [Fe2(SPh)2]0. Geometry optimizations were performed along the singlet and triplet spin surfaces. The triplet state was favored by 2.8 kcal/mol at 298 K and 0.2 kcal/mol at 0 K. These energy differences are smaller than the resolving power of common DFT functionals, precluding support for one spin state over the other, but suggesting that one may be thermally accessible from the other. Bond distances within the optimized structures were similar to one another, except for the Fe−Fe distance, which was better modeled with the singlet state (2.683 Å) over the triplet (2.958 Å) when compared to the experimental value of 2.7321(7) Å. The gross experimental structure was also better described by the singlet state geometry; the RMS deviations from overlays of the Fe2S2 cores were lower by a factor of 6 for the singlet, and the singlet model better captured the pitch of the macrocycle with respect to the Fe2S2 core: ∠Fe−Fe−Cpara = 146.0° (experimental), 147.0° (singlet), and 134.6° (triplet). In line with the crystallographically determined physical oxidation states of the metal and PDI fragments, the valence region may be interpreted in a similar fashion as [Fe2Cl]+, in which two intermediate spin (SFe = 1) Fe(II) centers each undergo antiferromagnetic coupling to a PDI•− fragment. On the singlet surface, the two metal centers are also coupled, resulting in an alternating pattern between the spin centers: L↑−Fe↓↓−Fe↑↑−L↓. However, scission of the Fe−Fe bond is responsible for generating the triplet state in [Fe2(SPh)2]0, not decoupling of the Fe−L antiferromagnetic interaction. Inspection of the spin density within the triplet reveals a L↓−Fe ↑↑ −Fe↑↑−L↓ configuration (Figure 7). The now

(Figure S13). This suggests that the product has a singlet ground state with a low-lying magnetic state that is thermally populated to an appreciable extent at room temperature.58−60 Analysis of the valence electronic structure reveals that the close proximity of the calculated singlet and triplet states is made possible by the weak-overlap covalent bonding within the diiron core. This results from the ability of the 3PDI2 ligand to hold the metal centers in proximity to one another while limiting the strength of the interaction between them.

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SUMMARY AND CONCLUSIONS The complexes described above highlight the adaptability of the 3PDI2 ligand set for supporting diiron cluster chemistry in a range of structural and electronic configurations. The 36 e− cluster [Fe2Cl2]2+ displays ferromagnetic coupling between two high-spin Fe(II) centers arranged in a rectangular cluster core supported by an unfolded ligand geometry. The 3PDI2 framework underwent dramatic changes in its structure during both chemical reduction and salt metathesis chemistry. Chemical reduction caused the displacement of a bridging chloride, which allowed for the formation of an Fe−Fe bond in the resulting 34 e− species [Fe2Cl]+. Metathesis of chloride and triflate anions at [Fe2Cl2]2+ with PhS− formed the tris(thiolate) folded ligand complex [Fe2(SPh)3]+. The Fe−Fe interaction in [Fe2(SPh)3]+ was found to be temperature dependent as a result of thermal population of Fe−Fe nonbonding, higher multiplicity spin states at elevated temperatures. Either salt metathesis at [Fe2Cl]+ with NaSPh or reduction of [Fe2(SPh)3]+ with KC8 formed the unfolded ligand complex [Fe2(SPh)2]0. The ligand-based redox activity, coupled with the weak-overlap covalent bonding between the metal centers in this system, appears to generate a high density of states at the valence region. Further investigations into the ability of this ligand system to undergo controlled rearrangements are currently being pursued in an effort to understand how multinuclear systems use the combined action of multiple metal centers for performing bond activation and catalysis.

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EXPERIMENTAL SECTION

General Information. All reactions containing transition metals were performed under an inert atmosphere of N2 using glovebox techniques. Glassware, stir bars, and filter aid (Celite) were dried in an oven at 150 °C for at least 12 h before use. All solvents (THF, fluorobenzene, n-pentane, n-hexane, and diethyl ether) were dried by passage through a column of activated alumina and stored over 4 Å molecular sieves under an inert atmosphere. Deuterated solvents were purchased from Cambridge Isotope Laboratories, dried over either Na0/benzophenone (C6D6, THF-d8) or calcium hydride (CD3CN, CD2Cl2), and degassed by freeze−pump−thaw cycles; the solvents were then isolated via vacuum transfer and stored under an inert atmosphere over 4 Å molecular sieves. Sr(OTf)2,61 NaBArF4,62 and (3PDI2)Sr(OTf)244 were prepared according to literature procedures. KC8 was also prepared according to the literature procedure63 and stored at −35 °C under dry nitrogen in a glovebox prior to use. PPh3 was purchased from MilliporeSigma, purified by recrystallization from hot ethanol64 and dried at 30 mbar and 40 °C for 6 h before use. NaSPh, and anhydrous FeCl2 were purchased from Sigma-Aldrich and Strem Chemicals, respectively, and used without further purification. 1 H, 13C{1H}, 31P{1H}, 19F{1H}, 11B{1H}, 1H−13C HSQC, and 1 H−1H COSY NMR spectra were recorded on either Bruker UNI 400 or UNI 500 NMR spectrometers. All chemical shifts (δ) are reported in units of ppm, with references to the residual protio-solvent resonance for proton and carbon chemical shifts. External H3PO4, CFCl3, and BF3·OEt2 were used for referencing 31P, 19F, and 11B NMR chemical shifts, respectively. Elemental analyses were either

Figure 7. Spin density plot for the triplet state of [Fe2(SPh)2]0. The numbers represent the amount of unpaired α (positive, purple) and β (negative, cyan) spin density at each spin center.

ferromagnetically coupled dinuclear core exhibits a spin of +3.6 e−, evenly distributed between the two metal centers, with a small amount of spin polarization on the bridging sulfur atoms (+0.15 e− total). The π* manifolds of the PDI fragments each contain −0.9 e−, as expected for a radical anion. Together these data indicate that the triplet state results from high spin character within the metal-centered dz2 system and not from decoupling of the M−L interaction. Based on these results, we surmise that the system may exhibit thermal population of multiple spin states at room temperature, as seen for [Fe2(SPh)3]+. The low temperature crystallographic data match better with the calculated singlet geometry, but the observed paramagnetism in solution at room temperature is consistent with the presence of an occupied triplet state. Indeed, lowering the temperature of the sample while performing 1H NMR spectroscopy caused the paramagnetically shifted signals to converge on the portion of the spectrum typically associated with diamagnetic compounds F

DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Summary of Crystallographic Data for Compounds Reported in This Work formula Mr T/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dc/g·cm−3 μ/mm−1 F(000) crystal size, mm collection range (deg) index ranges

reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] largest diff. peak/hole (e Å−3) Flack parameter

[Fe2Cl2][BArF4]2

[Fe2Cl][OTf]

[Fe2(SPh)3][OTf]

[Fe2(SPh)2]0

C104H86B2Cl2F48Fe2N6O2 2568.01 100 triclinic P1̅ 12.5847(5) 13.0578(6) 18.1229(7) 85.123(2) 75.419(2) 71.197(2) 2728.4(2) 2 1.563 0.449 1296.0 0.25 × 0.1 × 0.08 3.294 ≤ 2θ ≤ 55.24 −16 ≤ h ≤ 16 −16 ≤ k ≤ 17 −23 ≤ l ≤ 23 70673 12407 [R(int) = 0.0361] 12407/305/861 1.033 R1 = 0.0550 wR2 = 0.1332 R1 = 0.0765 wR2 = 0.1461 1.20/−1.21 −

C69H76ClF3Fe2N6O3P2S 1335.50 100 tetragonal P4̅21c 25.4711(9) − 20.7911(7) − − − 13488.8(11) 8 1.315 0.606 5584.0 0.12 × 0.1 × 0.06 2.262 ≤ 2θ ≤ 55.024 −33 ≤ h ≤ 33 −33 ≤ k ≤ 33 −26 ≤ l ≤ 26 172876 15493 [R(int) = 0.2414] 15493/293/872 1.023 R1 = 0.0688 wR2 = 0.1770 R1 = 0.1343 wR2 = 0.1484 1.31/−0.45 −0.008(11)

C51H61F3Fe2N6O3S4 1102.99 100 monoclinic P21/c 14.4799(6) 16.4311(7) 22.4459(9) − 96.743(2) − 5303.4(4) 4 1.381 0.762 2304.0 0.15 × 0.15 × 0.1 2.832 ≤ 2θ ≤ 55.07 −18 ≤ h ≤ 18 −21 ≤ k ≤ 21 −29 ≤ l ≤ 29 134819 12146 [R(int) = 0.0579] 12146/258/663 1.015 R1 = 0.0323 wR2 = 0.0697 R1 = 0.0507 wR2 = 0.0767 0.48/−0.41 −

C44H56Fe2N6S2 844.76 100 monoclinic C2/c 30.1232(15) 13.4720(7) 10.1153(5) − 96.017(2) − 4082.4(4) 4 1.374 0.853 1784.0 0.3 × 0.22 × 0.06 6.224 ≤ 2θ ≤ 55.192 −39 ≤ h ≤ 39 −15 ≤ k ≤ 17 −13 ≤ l ≤ 13 31793 4702 [R(int) = 0.0852] 4702/0/249 1.221 R1 = 0.0702 wR2 = 0.1150 R1 = 0.1019 wR2 = 0.1272 0.63/−0.75 −

̂ + Ĥ CF + Ĥ ZEE Ĥ = ĤSO + HEX

performed by Midwest Microlab, LLC or on a Costech ECS 4010 analyzer. Solution phase effective magnetic moment data were determined by Evans’ method.65 Variable Temperature Solid-State Magnetometry. Variable temperature solid-state magnetic data were collected on a Quantum Design Magnetic Property Measurement System (MPMS-7) under applied 1 T DC fields from 2 to 300 K. Field dependent magnetization data were performed at 2 K with varying applied magnetic field strengths ranging from 0 to 7 T. Diamagnetic corrections for the samples were made using Pascal’s constants.66 Sample preparation was performed under an atmosphere of N2 in a glovebox, wherein each sample was secured in a heat-sealed compartment of a polypropylene tube. Prior to use, polypropylene tubes were stored under dynamic vacuum overnight to remove residual water from the surface. An airtight seal was established 9.5 cm from the top of the sample tube using preheated forceps, and the tube’s mass was measured three times. The analyte (10−30 mg) was loaded into the tube using a plastic spatula, and the mass was again measured three times; the final mass of the sample was determined by taking the difference in the mass averages. Glass wool (dried at 250 °C under vacuum) was pushed into the tube on top of the sample and packed tightly using a glass pipet tip. The tube was sealed directly above the glass wool with preheated forceps, thereby forming an airtight compartment 2σ(I). Because the crystal was known to be weak, a large redundancy of reflections was collected. Large redundancies can increase the Rint but improve the overall data. Temperature-dependent X-ray crystallographic data of [Fe2(SPh)3] [OTf] were collected on the Bruker APEX-II instrument at 100, 150, 200, and 250 K. Rotation frames were integrated using SAINT,68 producing a listing of unaveraged F2 and σ(F2) values. The intensity data were corrected for Lorentz and polarization effects and for absorption using SADABS. 69 The structures of [Fe2(SPh)2]0, [Fe2(SPh)3][OTf], [Fe2Cl][OTf] were solved by direct methods using SHELXT70 and refined based on F2 using all reflections with SHELXL-2017.71 The structure of [Fe2Cl2][BArF4]2 was solved by direct methods using SHELXT and refined based on F2 using G

DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX

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filtered through Celite, and the volatile materials were removed under reduced pressure. The solid residue was washed with diethyl ether, extracted with 7 mL of fluorobenzene, and filtered through Celite. The dark brown filtrate was layered with 10 mL of n-pentane at −35 °C for 9 days to afford [Fe2Cl][OTf] as dark brown blocks. Yield: 56 mg (36%). 1H NMR (500 MHz, THF-d8, 298 K): δ = 7.30 (m, 10H, pyr-H + ArH), 7.17 (t, 3JHH = 7.2 Hz, 12H, ArH), 6.90 (t, 3JHH = 7.0 Hz, 12H, ArH), 3.38 (m, 4H, CH2), 2.27 (d, 2JHH = 12.0 Hz, 4H, CH2), 1.95 (m, 2H, CH2), 1.75 (s, 12H, CH3), 1.43 (s, 18H, C(CH3)3), 1.31 (m, 2H, CH2) ppm; 13C{1H} NMR (126 MHz, THF-d8, 298 K): δ = 160.60 (s, Cimine), 160.23 (s, pyr-C), 146.07 (s, pyr-C), 133.84 (m, ArC), 129.92 (s, ArC), 128.90 (m, ArC), 114.9 (s, pyr-C), 54.93 (s, CH2), 36.04 (s, C(CH3)3), 32.06 (s, C(CH3)3), 31.03 (s, CH2), 14.70 (s, CH3) ppm; 19F{1H} NMR (376 MHz, THFd8, 298 K): δ = −78.47 (s, CF3) ppm; 31P{1H} NMR (162 MHz, THF-d8, 298 K): δ = 39.79 (s, P(C6H5)3) ppm. Anal. Calcd for C69H76ClF3Fe2N6O3P2S (1335.50 g/mol): C, 62.04; H, 5.74; N, 6.30. Found: C, 60.49; H, 5.65; N, 6.42. Synthesis of [(3PDI2)Fe2(μ-SPh)(SPh)2][OTf] ([Fe2(SPh)3] [OTf]). To a stirred solution of (3PDI2)Sr(OTf)2 (100 mg, 0.111 mmol) in THF (7 mL) was added solid, anhydrous FeCl2 (28.2 mg, 0.222 mmol) at room temperature. After being stirred for 2 h, the purple slurry was treated with NaSPh (44 mg, 0.333 mmol), which led to the immediate formation of a dark brown slurry. After being stirred for an additional 12 h at room temperature, the reaction mixture was filtered through Celite. n-Pentane was diffused into the dark brown filtrate at room temperature for 10 days to afford [Fe2(SPh)3][OTf] as dark brown needles. Yield: 75 mg (63%). 1H NMR (500 MHz, CD3CN, 298 K): δ = 81.03 (4H, pyr-H), 63.02, 44.64, 16.42, 14.56, 10.98 (5H, SPh, o-H), 10.66 (10H, SPh, m-H and p-H), 2.19 (18H, C(CH3)3), −9.00, −10.89, −11.89, −12.99 ppm; 19F{1H} NMR (376 MHz, CD3CN, 300 K): δ = −79.31 (s, CF3) ppm. Anal. Calcd for C51H61F3Fe2N6O3S4 (1102.99 g/mol): C, 55.53; H, 5.58; N, 7.62. Found: C, 55.89; H, 5.74; N, 7.62. μeff = 4.7 ± 0.1 μB (CD3CN, 293 K). Synthesis of (3PDI2)Fe2(μ-SPh)2 ([Fe2(SPh)2]0). To a stirred solution of (3PDI2)Sr(OTf)2 (50 mg, 0.056 mmol) in THF (7 mL) was added solid, anhydrous FeCl2 (14.1 mg, 0.111 mmol) at room temperature. After being stirred for 2 h, the purple slurry was treated with PPh3 (29.2 mg, 0. 111 mmol). The reaction mixture was stirred for an additional 10 min and then chilled to −35 °C. KC8 (15 mg, 0.111 mmol) in THF (2 mL) was chilled to −35 °C and then added into the reaction mixture, which resulted in a dark green slurry, which gradually turned dark brown on warming to room temperature and stirring for 1 h. The reaction mixture was then filtered through Celite and treated with NaSPh (14.7 mg, 0.111 mmol). After being stirred for 12 h at room temperature, the dark brown solution was filtered, and the volatile materials were removed under reduced pressure. The solid residue was extracted with 3 mL diethyl ether and filtered through Celite. Slow evaporation at room temperature for 3 days afforded [Fe2(SPh)2]0 as dark brown blocks. The product was isolated by decanting and washing with 4 mL of cold diethyl ether. Yield: 18 mg (38%). 1H NMR (400 MHz, THF-d8, 298 K): δ = 111.81 (4H, pyr-H), 94.53, 62.73, 17.70, 10.91, 1.17 (18H, C(CH3)3), −10.95, −14.20, −18.12, −86.71 ppm. Anal. Calcd for C44H56Fe2N6S2 (844.76 g/mol): C, 62.55; H, 6.69; N, 9.95. Found: C, 63.85; H, 7.30; N, 7.52. Repeated attempts to obtain analytically pure [Fe2(SPh)2]0 were unsuccessful; PPh3 was identified by NMR spectroscopy as a persistent impurity through multiple recrystallization cycles. Alternative Synthesis of (3PDI2)Fe2(μ-SPh)2 ([Fe2(SPh)2]0). A dark brown solution of [Fe2(SPh)3][OTf] (21 mg, 0.019 mmol) in THF (7 mL) was stirred and chilled to −35 °C. KC8 (5.2 mg, 0.038 mmol) in THF (2 mL) was chilled to −35 °C and added, creating a dark brown mixture that was allowed to warm to room temperature. After being stirred for 1 h, the volatile materials were removed under reduced pressure to leave a solid residue, which was dissolved in C6D6 and examined by NMR spectroscopy. The 1H NMR spectrum revealed [Fe2(SPh)2]0 as the primary constituent of the reaction mixture.

SHELXL-2018. For [Fe2Cl][OTf], there was a region of disordered solvent for which a reliable disorder model could not be devised; the X-ray data were corrected for the presence of disordered solvent using SQUEEZE.72 Crystal parameters and refinement results are given in Table 3. CCDC 1916938 ([Fe2Cl2][BArF4]2), 1916939 ([Fe2(SPh)3] [OTf]), 1916940 ([Fe2Cl][OTf]), and 1916941 ([Fe2(SPh)2]0) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Computational Details. All density functional theory (DFT) calculations were performed with the ORCA program package, v3.0.3.73 Geometry optimizations were carried out at the B97-D3 level of DFT,74−76 using model compounds derived from crystallographic data. The def2-TZVP basis sets and the def2-TZVP/J auxiliary basis sets (used to expand the electron density in the resolution-of-identity (RI) approach) were used for Fe, Cl, S, P, and N.77−79 All other atoms were described using the def2-SV(P) basis sets and def2-SV/J auxiliary basis sets. The conductor-like screening model (COSMO; dipole moment corresponding to acetonitrile) was implemented throughout. The SCF calculations were tightly converged (1 × 10−8 Eh in energy, 1 × 10−7 Eh in the density change, and 5 × 10−7 in the maximum element of the DIIS error vector). In all cases the geometries were considered converged after the energy change was less than 1 × 10−6 Eh, the gradient norm and maximum gradient element were smaller than 3 × 10−4 Eh bohr−1 and 1 × 10−4 Eh bohr−1, respectively, and the root-mean square and maximum displacements of all atoms were smaller than 6 × 10−4 bohr and 1 × 10−3 bohr, respectively. Numerical frequency calculations were used to verify that the calculated structures represented local minima on the potential energy surface. The reported energies are Gibbs free energies, calculated for 298.15 K and 1.00 atm, as obtained from numerical frequency calculations on the optimized geometries. Orbital and spin density plots were generated using the program Chimera,80 with isosurface cutoffs of |0.03| and |0.003| au, respectively. The unrestricted corresponding orbitals (UCOs) used to visualize the magnetic coupling interactions in [Fe2(SPh)3]+ and [Fe2(SPh)2]0 result from orbital transformations that allow for a valence bond-type interpretation of the electronic structure. In this case, the spatial overlap between the α- and β-spin components of an orbital (Sαβ) varies from 0 (spatially unique) to 1 (spatially identical) and allows the orbital to be classified as doubly occupied (Sαβ ≳ 0.85), spinpaired (0.85 ≳ Sαβ ≳ 0.05), or uncoupled (Sαβ ≲ 0.05).81−83 Synthesis of [( 3 PDI 2 )Fe 2 (μ-Cl) 2 (thf) 2 ][BAr F 4 ] 2 ([Fe 2 Cl 2 ] [BArF4]2). To a stirred solution of (3PDI2)Sr(OTf)2 (100 mg, 0.111 mmol) in THF (7 mL) was added anhydrous FeCl2 (28.2 mg, 0.222 mmol) as a solid at room temperature. After being stirred for 2 h, the purple slurry was treated with NaBArF4 (196.8 mg, 0.222 mmol), resulting in the immediate formation of a purple solution. After being stirred for an additional 2 h, the solution was filtered through Celite and layered with 12 mL of n-hexane at room temperature. After 3 days, [Fe2Cl2][BArF4]2 was afforded as dark purple blocks. Yield: 137 mg (48%). 1H NMR (400 MHz, CD2Cl2, 296 K): δ = 81.39 (4H, pyrH), 36.82, 25.11, 7.86 (16H, BArF4 o-H), 7.83 (8H, BArF4 p-H), 5.33 (12H, CH3), −1.55 (18H, C(CH3)3), −13.86, −60.93 ppm; 11B{1H} NMR (128 MHz, CD2Cl2, 298 K): δ = −7.00 (s, BArF4) ppm; 19 1 F{ H} NMR (376 MHz, CD2Cl2, 298 K): δ = −61.08 (s, BArF4) ppm. Anal. Calcd for C104H86B2Cl2F48Fe2N6O2 (2568.01 g/mol): C, 48.61; H, 3.38; N, 3.27. Found: C, 48.92; H, 3.57; N, 3.33. μeff = 7.7 ± 0.1 μB (CD2Cl2, 293 K). Synthesis of [(3PDI2)Fe2(μ-Cl)(PPh3)2][OTf] ([Fe2Cl][OTf]). To a stirred solution of (3PDI2)Sr(OTf)2 (100 mg, 0.111 mmol) in THF (7 mL) was added solid, anhydrous FeCl2 (28.2 mg, 0.222 mmol) at room temperature. After being stirred for 2 h, the purple slurry was treated with PPh3 (58.2 mg, 0. 222 mmol). The reaction mixture was stirred for an additional 10 min and then chilled to −35 °C. KC8 (15 mg, 0.111 mmol) in THF (2 mL) was chilled to −35 °C and then added into the reaction mixture. The resulting dark green slurry was warmed to room temperature and stirred for 1 h, during which time the slurry gradually turned dark brown. The reaction mixture was H

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01673. NMR spectra, UV−vis spectra, variable temperature solid-state magnetometry data, and supplementary computational details (PDF) Accession Codes

CCDC 1916938−1916941, 1943751−1943753, and 1943844 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*(N.C.T.) E-mail: [email protected]. Telephone: +1 (215) 898-6208. ORCID

Shaoguang Zhang: 0000-0002-0931-321X Michael R. Gau: 0000-0002-4790-6980 Neil C. Tomson: 0000-0001-9131-1039 Present Address †

Department of Process Research and Development, Merck & Co., Inc., Rahway, NJ 07065. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Pennsylvania and the donors of the Petroleum Research Fund (57346-DNI3), administered by the American Chemical Society, for support of this research. We also thank Prof. Jay Kikkawa for assistance with SQUID magnetometry data collection.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01673 Inorg. Chem. XXXX, XXX, XXX−XXX