Thermally persistent high spin ground states in octahedral iron clusters

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Thermally persistent high spin ground states in octahedral iron clusters Raúl Hernández Sánchez, and Theodore A. Betley J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10181 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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Journal of the American Chemical Society

Thermally persistent high spin ground states in octahedral iron clusters Raúl Hernández Sánchez† and Theodore A. Betley* Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA.

ABSTRACT: Chemical oxidation and reduction of the all-ferrous (HL)2Fe6 in THF affords isostructural, coordinatively unsaturated clusters of the type [(HL)2Fe6]n: [(HL)2Fe6][BArF24] (1, n = +1; where [BArF24]– = tetrakis[(3,5-trifluoromethyl)phenyl]borate), [Bu4N][(HL)2Fe6] (2a, n = –1), [P][(HL)2Fe6] (2b, n = –1; where [P]+ = tributyl(1,3-dioxolan-2-ylmethyl)phosphonium), and [Bu4N]2[(HL)2Fe6] (3, n = –2). Each member of the redox transfer series was characterized by zero-field 57Fe Mössbauer spectroscopy, near-infrared spectroscopy, single-crystal X-ray crystallography, and magnetometry. Redox directed trends are observed when comparing the structural metrics within the [Fe6] core. The metal octahedron [Fe6] decreases marginally in volume as the molecular reduction state increases as gauged by the Fe–Feavg distance varying from 2.608(11) Å (n = +1) to 2.573(3) (n = –2). In contrast, the mean Fe–N distances and Fe–N–Fe angles correlate linearly with [Fe6] oxidation level; or alternatively, the changes observed within the local Fe–N4 coordination planes vary linearly with the aggregate spin ground state. In general, as the spin ground state (S) increases, the Fe–N(H)avg distances also increase. The structural metric perturbations within the [Fe6] core and measured spin ground states were rationalized extending the previously proposed molecular orbital diagram derived for (HL)2Fe6. Chemical reduction of the (HL)2Fe6 cluster results in an abrupt increase in spin ground state from an S = 6 for the all-ferrous cluster, to an S = 19/ in the monoanionic 2b and S = 11 for the dianionic 3. The observation of asymmetric intervalence charge transfer bands in 3 2 provides further evidence of the fully delocalized ground state observed by 57Fe Mössbauer spectroscopy for all species examined (1 – 3). For each of the clusters examined within the electron transfer series, the observed spin ground states thermally persist to 300 K. In particular, the S = 11 in dianionic 3 and S = 19/2 in the monoanionic 2b represent the highest spin ground states isolated up to room temperature known to date. The increase in spin ground state results from population of the antibonding orbital band comprised of the Fe–N σ* interactions. As such, the thermally persistent ground states arise from population of the resultant single spin manifolds in accordance with Hund's rules. The large spin ground states, indicative of strong ferromagnetic electronic alignment of the valence electrons, result from strong direct exchange electronic coupling mediated by Fe–Fe orbital overlap within the [Fe6] cores, equivalent to a strong double exchange magnetic coupling B for 3 that was calculated to be 309 cm–1.

1. INTRODUCTION Mixed valence compounds were first recognized as a unique class of chemical compounds in the late 1960s given their distinctive spectral, electrochemical, and magnetic properties.1-4 A consequence of having two or more metal sites within a complex at different formal oxidation states opens the possibility for electronic delocalization between the sites. The extent of electronic delocalization has been heavily studied in mixed valent dinuclear coordination complexes,5-8 but less so in larger molecular assemblies (e.g., polynuclear clusters) or bulk solids (e.g. minerals9). In particular, polynuclear clusters have attracted much attention given their potential utility as models of biologically relevant cofactors with regards to understanding their electronic structure.10-11 The biological mixed valent metal clusters represent a minor component of very large protein superstructures, complicating the cluster spectroscopic analysis and making rigorous electronic structure determination challenging. Nonetheless, significant strides have been taken towards the analysis of cofactor spin ground

states.12 In particular, super-spin proteins have been discovered wherein a one-electron redox reaction triggers the appearance of high spin states.13-16 Given the importance for the high spin electronic structures arising in mixed valent protein cofactors that mediate electron transfer and catalyze small molecule activation, gaining a detailed understanding of the electronic structure makeup for these systems and electronic structure analogues is warranted. Previously, well-defined polynuclear clusters could be obtained from dimensional reduction of 3D-lattices.17-19 Unfortunately the examples of dimensional reduction is limited in number and, to date, most current cluster syntheses rely on self-assembly reactions that lack control of the resulting cluster constitution, geometry, and local metal coordination environments.20-32 Thus reliable methods to assemble polynuclear clusters are required to rigorously undertake electronic structure evaluation of mixed valent clusters. Our lab and others33-48 have reported the synthesis of clusters with varying nuclearity (three metals and higher) templated within a polynucleating ligand architecture to predispose the metal ions to bind in a specific manner to form

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the polynuclear aggregate. The resulting clusters have been utilized to explore the nature of the metal-metal interactions by varying auxiliary ligand coordination to the polynuclear system, analyzing the effect of distance between the metal ions to develop magneto-structural correlations, and by assessing electronic structure perturbations as a function of cluster redox level.49-61 Our own interests have been to illustrate the electronic coupling effect engendered by cluster designs featuring close M–M interactions. For tri- and hexanuclear iron based clusters, we have observed that the direct electronic exchange interaction resulting from the direct overlap of Fe–Fe valence orbitals results in thermally persistent high-spin ground states.61-62 Furthermore, the ferromagnetic alignment is significantly enhanced upon formation of mixed valent species through the introduction of double exchange (DE) interactions as a consequence of electron delocalization.63-64 Fundamentally the double exchange electronic coupling augments ferromagnetic coupling within the interacting sites.65-66 Stabilization of high spin states in strongly delocalized species has provided the evidence of this interaction.67-74 Thus, double exchange not only supports the high spin states (e.g. S ≥ 7/2) observed in biologically relevant polynuclear clusters,7582 but it provides a roadmap to achieving high spin ground states relevant for a number of applications. One area where high spin electronic architectures are desirable is for the pursuit of single-molecule magnets (SMMs). High spin states have been long targeted to increase the spin reversal barrier energy (U = |D|S2) exhibited by these materials.83-84 The majority of materials exhibiting SMM behavior rely on superexchange electronic coupling between paramagnetic ions, thus research has focused on building larger and larger polynuclear aggregates to increase S. High spin ground states (S = 7  83/2)85-124 have been obtained in this manner, yet many only exhibit the high spin ground states at low temperatures (< 30 K). In contrast we have reported mixed valent trinuclear (S = 11/2) and hexanuclear (S = 19/2) iron clusters which feature thermally persistent ground states (to 300 K) owing to both direct and double exchange mechanisms.125-126 We report herein the formation of a coordinatively unsaturated hexanuclear cluster [(HL)2Fe6]n for which four redox levels are accessible (n = +1  –2). We find that electronic delocalization of the mixed valent clusters is observable structurally and spectroscopically. The high spin ground state arising from strong direct exchange previously reported for (HL)2Fe6 is reinforced by the double exchange electronic coupling introduced by mixed valency. The high spin ground state electronic structures observed for two of the anionic redox isomers (S = 19/2 [(HL)2Fe6]– and S = 11 [(HL)2Fe6]2–) represent the highest, thermally persistent ground states ever recorded.

2. EXPERIMENTAL SECTION 2.1. General considerations. All manipulations involving metal complexes were performed under an atmosphere of dry, oxygen-free N2 by means of standard Schlenk or glovebox techniques (MBraun glovebox equipped with a –35 °C freezer).

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All glassware was oven-dried for a minimum of 10 h and cooled in an evacuated antechamber prior to use in the drybox. Diethyl ether and tetrahydrofuran (THF) were dried and deoxygenated on a Glass Contour System (SG Water USA, Nashua, NH) and stored over 4 Å molecular sieves (Strem) prior to use. Propylene carbonate (PC, Sure Seal) was purchased from Sigma Aldrich and brought into the N2-filled glovebox and stored over 4 Å molecular sieves (Strem) prior to use. Tetrabutylammonium chloride, [Bu4N]Cl; tributyl(1,3dioxolan-2-ylmethyl)phosphonium bromide (see Scheme 1), [P]Br; and ferrocenium hexafluorophosphate, Fc[PF6], were purchased from Sigma Aldrich and used without further purification. Likewise, naphthalene obtained from Alfa Aesar was used without further purification. Sodium tetrakis[3,5bis(trifluoromethyl)phenyl]borate, Na[BArF24], was purchased from TCI America and used without further purification. Salt metathesis between Fc[PF6] and Na[BArF24] to afford large crystals of Fc[BArF24] was carried out inside a dinitrogen filled glovebox according to the procedure reported by Chávez et al.127 (HL)2Fe6 was prepared according to the method previously reported by our laboratory.61 2.1.1. [(HL)2Fe6][BArF24] (1a). (HL)2Fe6 (204 mg, 0.163 mmol) was suspended in 4 mL of THF. To this was added a solution of Fc[BArF24] (172 mg, 0.164 mmol) in 2 mL of THF. The reaction mixture was stirred for 3 h at room temperature. The solvent was removed under vacuum and the resulting black powder washed in a fritted funnel with 5 x 2 mL of a 1:1 mixture of THF:Et2O. Addition of small amounts of THF to the fritted funnel produces a black filtrate solution and leaves behind a black powder. 57Fe Mössbauer on both filtrate and precipitate reveal the same species and the fractions were combined (80%). Single crystals of 1 can be obtained from a concentrated solution of THF:Et2O at –35 °C. Anal. Calcd. for 1a·2THF: C86H76BF24Fe6N12O2: C, 48.92; H, 3.63; N, 7.96%. Found: C, 48.84; H, 3.58; N, 8.09%. 2.1.2. [(HL)2Fe6(py)2][PF6] (1b). (HL)2Fe6 (105 mg, 0.084 mmol) was suspended in 2 mL of pyridine. To this was added a solution of Fc[PF6] (88 mg, 0.085 mmol) in 2 mL of pyridine. The reaction mixture was stirred for 3 h at room temperature. The solvent was removed under vacuum and the resulting black powder washed in a fritted funnel with 5 x 2 mL of a 1:1 mixture of THF:Et2O. From the frit 87 mg of black powder (1b) was isolated (74% yield). Single crystals of 1b can be readily grown by dissolving 1b in pyridine and placing this in a diethyl ether vapor diffusion cell. Anal. Calcd. for 1: C56H58F6Fe6N14P: C, 47.80; H, 4.15; N, 13.94%. Found: C, 47.66; H, 4.09; N, 13.85%. 2.1.3. [Bu4N][(HL)2Fe6] (2a). (HL)2Fe6 (100 mg, 0.080 mmol) was suspended in 2 mL of THF and cooled down to –35 °C. To this was added a freshly prepared solution of sodium naphthalenide by smearing sodium metal to the walls of a vial and adding naphthalene (10.3 mg, 0.080 mmol) in 2 mL of THF and stirred for 30 min at room temperature. The sodium naphthalenide solution was filtered through Celite in a pipette filter and added to the THF suspension of (HL)2Fe6 at –35 °C. Immediately following the sodium naphthalenide addition, [Bu4N]Cl (22.5 mg, 0.081 mmol) was added as a solid and the reaction mixture stirred for 3 h at room temperature. A dark brown precipitate was formed and filtered through a medium


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size fritted funnel and washed with 5 x 2 mL of THF. The collected brown powder was dissolved on the frit with propylene carbonate (PC) and collected directly into a diethyl ether vapor diffusion cell. Micron-sized crystals were obtained at room temperature from which the single-crystal structure was obtained. When the PC filtrate is dried in vacuo to remove volatiles, 29 mg of 2a was obtained (27% yield). Anal. Calcd. for 2a: C62H84Fe6N13: C, 55.30; H, 6.29; N, 13.52%. Found: C, 55.21; H, 6.18; N, 13.39%. 2.1.4. [P][(HL)2Fe6] (2b). The synthesis of 2b is analogous to that of 2a. (HL)2Fe6 (100 mg, 0.080 mmol) was suspended in 2 mL of THF and cooled down to –35 °C. To this was added a freshly prepared solution of sodium naphthalenide by smearing sodium metal to the walls of a vial and adding naphthalene (10.3 mg, 0.080 mmol) in 2 mL of THF and stirred for 30 min at room temperature. The sodium naphthalenide solution was filtered through Celite in a pipette filter and added to the THF suspension of (HL)2Fe6 at –35 °C. Immediately following the sodium naphthalenide addition, tributyl(1,3-dioxolan-2ylmethyl)phosphonium bromide ([P]Br, 29.6 mg, 0.080 mmol) was added as a solid and the reaction mixture was stirred for 3 h at room temperature. In contrast to the synthesis of 2a, no precipitate formed; thus, THF and volatiles from the reaction mixture were removed in vacuo. The resulting solid was placed on a medium porosity fritted funnel and washed with 5 x 2 mL of a 1:1 THF:Et2O mixture. After washing the solid, THF was used to dissolve the powder which was filtered directly into a vapor diffusion cell with diethyl ether. High-quality, bulk crystallization results at room temperature. Yield: 56 mg (50%). 2.1.5. [Bu4N]2[(HL)2Fe6] (3). (HL)2Fe6 (100 mg, 0.080 mmol) was suspended in 2 mL of THF and cooled down to –35 °C. To this was added a freshly prepared solution of sodium naphthalenide by smearing sodium metal to the walls of a vial and adding naphthalene (20.6 mg, 0.164 mmol) in 2 mL of THF and stirred for 30 min at room temperature. The sodium naphthalenide solution was filtered through Celite in a pipette filter and added to the THF suspension of (HL)2Fe6 at –35 °C. Immediately following the sodium naphthalenide addition,[Bu4N]Cl (43 mg, 0.162 mmol) was added as a solid and the reaction mixture was stirred for 3 h at room temperature. The dark brown precipitate was filtered through a medium size fritted funnel and washed with 5 x 2 mL of THF. The remaining solids were dissolved in pyridine and collected into a diethyl ether vapor diffusion cell. High quality crystals were obtained (49% yield) in this manner at –35 °C from which the X-ray single crystal structure was obtained. Compound 3 decomposes over a period of 12 h at room temperature and under inert atmosphere. Anal. Calcd. for 3: C78H120Fe6N14: C, 58.96; H, 7.61; N, 12.34%. Found: C, 58.88; H, 7.46; N, 12.18%. 2.2. X-ray Structure Determinations. Single crystals suitable for X-ray structure analysis were coated with deoxygenated Paratone N-oil and mounted in MiTeGen Kapton loops (polyimide). Data for compounds 1 – 3 were collected at 100 K on an APEX II DUO single-crystal diffractometer. None of the crystals showed significant decay during data collection. Raw data were integrated and corrected for Lorentz and polarization effects using Bruker APEX2 v.2009.1.128

Absorption corrections were applied using SADABS.129 Space group assignments were determined by examination of systematic absences, E-statistics, and successive refinement of the structures. The program PLATON130 was employed to confirm the absence of higher symmetry for any of the crystals. The positions of the heavy atoms were determined using direct methods using the program SHELXTL.131 Successive cycles of least-square refinement followed by difference Fourier syntheses revealed the positions of the remaining nonhydrogen atoms. Non-hydrogen atoms were refined with anisotropic displacement parameters, and hydrogen atoms were added in idealized positions. Crystallographic data for 1 – 3 are given in Table S1. 2.3. Magnetic Data Measurements. Magnetic data for 1a, 1b, 2b, and 3 were collected using a Quantum Design MPMS-XL Evercool SQUID Magnetometer. The following is a general procedure for sample preparation: bulk crystals were collected and washed thoroughly with Et2O. These crystals were crushed in the presence of Et2O and the resulting fine suspension was then dried under high vacuum. The sample powder was then immobilized within a size #4 gelatin capsule by adding melted eicosane at 50 – 60 °C. The gelatin capsule was inserted into a plastic straw. Samples were prepared under a dinitrogen atmosphere. Magnetization data at 100 K from 0 to 7 T were used to detect ferromagnetic impurities (Figure S10 – S13). Variable-temperature direct current (dc) magnetic susceptibility measurements were collected in the temperature range 1.8 – 300 K under applied fields of 0.1, 0.5, and 1 T. The magnetic susceptibility (MT) were collected repeatedly until they were reproduced at least three times where samples feature linear 100 K magnetization scans. Variable-temperature, variable-field magnetization data were acquired on heating from 1.8 to 10 K at increasing fields of 1, 2, 3, 4, 5, 6, and 7 T. Magnetic susceptibility data were corrected for diamagnetism of the sample, estimated using Pascal’s constants, in addition to contributions from the sample holder and eicosane. AC magnetic susceptibility data were collected at zero applied dc field and with a 4 Oe oscillating ac field. Additionally, magnetic hysteresis data were collected at 1.8 K; in addition, zero-field- (ZFC) and field-cooled (FC) data were obtained between 1.8 and 10 K by measuring under a dc applied field of 0.1 T. 2.4. Electrochemical Measurements. Cyclic voltammetry measurements were acquired on a CHI660d potentiostat. A three-electrode cell setup was used with a 3mm in diameter glassy carbon working electrode, Pt wire as counter electrode, and Ag/AgNO3 as reference electrode. Saturated AgNO3 solutions in MeCN for the reference electrode were prepared fresh before each experiment. All measurements were done under a dinitrogen atmosphere and at room temperature. A 0.1 M solution of [Bu4N][PF6] in THF was used as the supporting electrolyte. 2.5. Zero-Field 57Fe Mössbauer Spectroscopy. Zero-field 57Fe Mössbauer Spectroscopy was collected at 4.2 and 90 K for all compounds (1a, 1b, 2a, and 3). Solid samples (ca. 20 mg) were restrained with Paratone-N oil. The data were measured with a constant acceleration spectrometer (SEE Co., Minneapolis, MN). Isomer shifts are given relative to -Fe metal at 298 K.



The data of 1a and 3 were fit by employing WMOSS4.132 Additionally the 90 K data of 1a, 1b, and 3 were fit using an inhouse software written in IGOR Pro WaveMetrics by Evan King.

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[(HL)2Fe6]+ readily binds two trans-disposed equivalents of pyridine to generate [(HL)2Fe6(py)2]+ (cation of 1b, see Scheme S1).

2.6. Other Physical Measurements. Elemental analysis was performed by Complete Analysis Laboratories, Inc., Parsippany, New Jersey. UV-Vis-NIR spectra were collected in 1 cm path length cuvettes on a Varian 5000 spectrophotometer at room temperature. All solutions were prepared under N2 atmosphere in a glovebox and the cuvettes sealed with a J-Young screw Teflon cap. Absorbance values were kept under 1 for all concentrations measured.

3. RESULTS 3.1 Synthesis and Electrochemical Characterization. We have previously reported on the redox isomers of (HL)2Fe6 and the nature of redox delocalization of the mixed valent clusters.61-62 Upon oxidation, the [Fe6] cluster binds two equivalents of solvent (MeCN or DMF) to trans-disposed iron sites within the cluster for every one-electron oxidation incurred until the cluster becomes coordinatively saturated at the tricationic state [(HL)2Fe6(L’)6]3+ (L’ = MeCN or DMF). We were thus interested to examine the nature of redox delocalization within the core in the absence of solvent or anion binding to the [Fe6] core. Scheme 1. Synthesis of unsolvated [(HL)2Fe6]n clusters

To prevent solvent ligation, oxidation of (HL)2Fe6 in THF was effected using Fc[BArF24] ([BArF24]– = tetrakis[(3,5trifluoromethyl)phenyl]borate) to render the oxidized product THF-soluble. Thus, one equivalent of Fc[BArF24] in THF was added to a suspension of (HL)2Fe6 in THF at room temperature and stirred for 3 h. Removal of the volatiles in vacuo followed by washing with 1:1 THF:Et2O affords [(HL)2Fe6][BArF24] (1a) in 80% yield (Scheme 1). Micron size single crystals can be grown from a standing THF:Et2O concentrated solution of 1a at –35 °C. Akin to the solvent ligation pattern observed during oxidation of (HL)2Fe6 in MeCN,61 the unsolvated cluster core

Figure 1. (a) Molecular crystal structure of the dianion [(HL)2Fe6]2– in 3 obtained at 100 K (the ammonium ion [Bu4N]+ and the hydrogen atoms are omitted for clarity). Thermal ellipsoids are set at 50% probability level. The Fe, C and N atoms are colored orange, grey and blue, respectively. (b) Cyclic voltammogram of [P][(HL)2Fe6] (2b) in THF at room temperature. Scan rate: 10 mV/s. A 0.1 M [Bu4N][PF6] solution was used as supporting electrolyte. The labels on italics correspond to the oxidation level n in [(HL)2Fe6]n.


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Table 1. Structural metrics and spin ground state for unsolvated complexes 1a, 2a, 2b, and 3 n

+1 1a

(HL)

0

–1

–1

–2

a 2Fe6

2a

2b

3

Fe–Fecis (Å)

2.608(11)

2.597(1)

2.599(11)

2.597(9)

2.573(6)

Fe–N(H)avg (Å)

1.988(5)

2.01(2)

2.081(13)

2.078(10)

2.127(9)

Fe–N(H)–Feavg (deg)

82.0(6)

80.1(9)

77.3(8)

77.4(6)

74.4(1)

3.6044(4)

3.610(27) 3.651(13)

3.630(1)

3.7111(4) 3.7135(5)

3.757(9)

3.646(1)

---

19/

11

3.63(2) Fe–Fetrans (Å)b

3.66(1)

3.673(2)

3.78(5) S aPreviously

11/

2

6

2

3.643(1)

reported.61 bIndividual values reported to highlight the [Fe6] core asymmetry.

We previously reported that chemical reduction of a suspension of (HL)2Fe6 in THF with sodium naphthalenide will afford the monoanionic, coordinatively unsaturated cluster [Na(OEt2)2(NCMe)2][(HL)2Fe6].61 Alternatively, the addition of [Bu4N]Cl to the reaction mixture immediately following chemical reduction (3 h, room temperature) yields the cation metathesis product [Bu4N][(HL)2Fe6] (2a, Scheme 1). Initial electrochemical examination of 2a was challenging owing to the poor-to-negligible solubility of 2a in THF (or any other suitable solvent for electrochemical studies). Thus, other cations were examined to solubilize [(HL)2Fe6]–. We found that by performing the salt metathesis with tributyl(1,3dioxolan-2-ylmethyl)phosphonium bromide ([P]Br) after one-electron reduction of (HL)2Fe6 in the same manner 2a was prepared, results in the THF-soluble compound [P][(HL)2Fe6] (2b) in 50% yield. Bulk crystallization of 2b can be carried out by diffusing Et2O into a concentrated THF solution at room temperature. The electrochemical properties of the unsolvated [Fe6] core were investigated by employing the THF-soluble species 2b. Cyclic voltammetry in THF reveals one reduction event, as expected to result from reduction to the dianionic cluster core [(HL)2Fe6]2– (E1/2 = –2.41 V vs Fc0/+, EP = 196 mV); and two oxidation events corresponding to the formation of the neutral all-ferrous and monocationic [(HL)2Fe6]+ core (E1/2 = – 1.68 and –1.18 V vs Fc0/+; EP = 158 and 192 mV, respectively). The calculated comproportionation constant of the mixed valent unsolvated [Fe6] core at the oxidation level of 2b is thus 2.9 x 1012; which is slightly larger than that found for the pyridine species [(HL)2Fe6(py)2]– (1.3 x 1010).125 The large comproportionation constants obtained for both species indicates strong electron delocalization within the hexanuclear iron core.62 The cyclic voltammogram of 2b showed a reversible reduction indicating a doubly reduced species was accessible. The doubly reduced cluster [Bu4N]2[(HL)2Fe6] (3) could be obtained in 49% yield following the same synthetic procedure described above for 2a using two equivalents of

sodium naphthalenide. Compound 3 is completely insoluble in THF, but readily soluble in pyridine. Crystals of 3 suitable for X-ray analysis can be grown by diffusing Et2O into a concentrated pyridine solution of 3 at –35 °C. The doubly reduced species 3 could be maintained as a crystalline powder at –35 °C, though it readily decomposes upon standing at room temperature in contrast to the thermally stable 2a and 2b. The X-ray molecular crystal structure of the clusters in compounds 1a, 2a, 2b, and 3 resemble that of the parent allferrous (HL)2Fe6, where all the iron sites remain locally 4coordinate square planar (neglecting Fe–Fe interactions, see Figure 1a). Given their isostructural nature a direct comparison of the bond metrics of the all-ferrous parent cluster and 1a, 2a, 2b, and 3 reveals significant perturbations on the bond lengths involving Fe–N(H)avg bonds and Fe– N(H)–Feavg angles; conversely, the Fe–Feavg distances change minimally. The volume of the M6 octahedron decreases marginally with cis-disposed Fe–Feavg distances of 2.597(1) (n = 0 (HL)2Fe6), 2.608(11) (n = +1, 1a), 2.599(11) (n = –1, 2a), and 2.573(6) Å (n = –2, 3) (data summarized in Table 1). Given the almost identical parameters within the [(HL)2Fe6]– core fragment in 2a and 2b, only the metrics of 2a will be discussed. Moreover, the Fe–N and Fe–NH distances increase linearly with the increasing overall cluster reduction level: 2.006(4) and 1.969(4) Å (n = +1, 1); 2.037(8) and 1.998(17) Å (n = 0, (HL)2Fe6); 2.099(17) and 2.063(21) Å (n = –1, 2a); and 2.144(8) and 2.110(15) Å (n = –2, 3). In a similar fashion, the Fe–N–Fe and Fe–NH–Fe angles become more acute with increasing core reduction level [Fe–N–Fe, Fe–NH–Fe (°)]: 80.2(5), 83.9(4) (n = +1, 1); 79.2(2), 81.1(2) (n = 0, (HL)2Fe6); 76.8(11), 77.9(12) (n = –1, 2a); and 74.2(1), 74.6(1) (n = –2, 3). 3.2. Zero-Field 57Fe Mössbauer and Near-IR Spectroscopy. The Mössbauer spectrum of the hexanuclear all-ferrous cluster (HL)2Fe6, in addition to the electron transfer families



of compounds: [(HL)2Fe6(NCMe)m]n+ (n = 1  4), [(HL)2Fe6(DMF)m]n+ (n = 2  4), and [(HL)2Fe6(CN)6]n–6 (n = 3  6) displayed a single quadrupole doublet for geometrically equivalent coordination sites,61-62 indicative of a delocalized ground state. One exception to this general observation was the singly reduced [Na(Et2O)2(NCMe)2][(HL)2Fe6], whose spectrum featured an asymmetric doublet at 90 K.61, 125 The spectral asymmetry was attributed to a proposed mixture of neutral (HL)2Fe6 and reduced [Na(Et2O)2(NCMe)2][(HL)2Fe6] clusters, owing to the presumed instability of the reduced [Fe6]–. We later ascertained the spectral asymmetry was due to slow magnetic relaxation in related monoanionic clusters [(HL)2Fe6(L’)2]– that is not fully resolved at 90 K,125 which has also been observed on other iron-based SMMs.133-134 Analysis of the Mössbauer spectrum of 1a and 2a gave insight into concluding that its asymmetry is caused by the slow relaxation of the magnetization imparted by the ground state, |𝑆,𝑚𝑠⟩.51

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electronically delocalized species.137 The intensity (max), position (max), full-width at half-maximum (1/2), and shape of this band is related to the extent of delocalization.1-4 The corresponding data for [Bu4N][(HL)2Fe6(py)2] display no absorption bands in the near-IR region of the spectrum where it is customary to find the IVCT bands.125 In the same way, electronic absorption data were collected for 1a, 2b, and 3. In the visible

Zero-field, 57Fe Mössbauer spectra were collected for 1a, 1b, 2a, and 3 at 4.2 and 90 K; an additional spectrum at 240 K was also collected for 2b. The spectrum of the cationic cluster [(HL)2Fe6][BArF24] (1a) at 4.2 K displays a hyperfine split spectrum (Figure 2a). Based on the almost identical heights of all absorption lines we presumed that this spectrum is composed by several individual sextets. Similar to 1a, compound 2a displays a hyperfine split spectrum at 4.2 K; although the spectral features are significantly broader than that of 1a (Figure 2b). The spectrum of 3 at 4.2 K (and 90 K) displays a single broad quadrupole doublet (Figure 2c). This is remarkable since the analogous unsolvated salts at n = +1 and –1 (clusters in 1a and 2a) display well-defined-toincipient hyperfine split spectra, respectively. The fit parameters that best model the spectrum of 3 at 4.2 K are [, |EQ| (mm/s)]: 0.62, 1.44. The zero–field spectrum of 1a was modeled by employing a nuclear Hamiltonian including, apart from the electric quadrupole interaction, the nuclear Zeeman interaction, 𝐻 = I ∙ 𝑄 ∙ I + 𝑔𝑛𝛽𝑛H ∙ I;135 where 𝑄 is proportional to the electric field gradient. Note that the only contribution to H originates from the internal magnetic field (Hint) since Hext = 0. The data were fit by employing the software WMOSS4 model 5.132 Three distinct crystallographic sites are observed per [Fe6] thus three sextets were considered when fitting the spectrum of 1a. The fit components are shown above the experimental data of 1a in Figure 2a and have the following parameters [, EQ (mm/s), Hint (T)]: 0.83, –2.45, 9.6 (33%, blue trace); 0.83, –2.45, 2.8 (33%, green trace); and 0.37, – 2.45, 6.8 (34%, brown trace). The composite fit is depicted by the red trace. On warming the relaxation rate of Hint increases and at 90 K averages to zero (Figure S6).136 At this temperature a single quadrupole doublet is observed that is best fit by [, |EQ| (mm/s)]: 0.40, 2.45 (Figure S6). The breadth of the spectrum of 2a precluded further analysis (Figure 2b). However, as temperature is increased to 90 and 240 K the spectrum coalesces into a single quadrupole doublet (Figure S8). A common feature for mixed valence compounds is the appearance of intervalence charge transfer bands (IVCTs) for

Figure 2. Zero-field 57Fe Mössbauer spectra for: (a) [(HL)2Fe6][BArF24] (1a); (b) [Bu4N][(HL)2Fe6] (2a); and (c) [Bu4N]2[(HL)2Fe6] (3) at 4.2 K. The blue, green and brown tarces correspond to the individual components of the fit for 1. The red traces indicate the overall fit to the data as described in the text. (d) Near-IR spectrum of 3 in pyridine at room temperature. Inset: UV–Vis part of the spectrum. The red trace corresponds to the fit of the spectrum considering three gaussian absorption bands (dashed and dotted grey, and green trace) between 7400 and 17500 cm–1.


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region 1a displays two broad absorption bands at max = 24907 and 19608 cm–1 (Figure S14), while no other discernible peak is observed down to 6000 cm–1. The absorption spectrum of 2b displays one peak maximum in the UV region at 33900 cm–1 and a second one in the visible

at 23900 cm–1 (Figure S16). Similar to 1a, no absorption band is observed in the near-infrared for 2b. For 3 only one broad absorption is seen in the visible region at 23200 cm–1 (Figure 2d inset). In contrast, in the near-IR two

Figure 3. Magnetic data collected for unsolvated complexes [(HL)2Fe6][BArF24] (1a), [P][(HL)2Fe6] (2b), and [Bu4N]2[(HL)2Fe6] (3). (a) VT magnetic susceptibility of 1a (), 2b (), and 3 () collected under a 0.1 T field. (b) Reduced magnetization data of 1a (), 2b (), and 3 () at temperatures from 1.8 to 10 K and fields 1 to 7 T. The continuous black traces in (a) and (b) correspond to the fit of the data as described in the text. (c) Plot of 𝜒′MT versus T of 1a (), 2b (), and 3 () at an oscillating field frequency of 1 Hz.

bands are observed between 13000 and 6000 cm–1. The low energy absorption band is asymmetric and skewed as predicted for moderately-to-strongly delocalized complexes.138 Thus the absorption profile fit is limited to max of the lowest absorption band; while the high energy side of the IVCT retains a Gaussian-shape, the low energy side does not.139 To gain further insight, the data between 7400 and 17500 cm–1 were fit to three Gaussian-shaped curves, where two (high and intermediate energy, dotted and dashed traces) are used only to provide an adequate absorption profile for the lowest energy band (Figure 2d). The fit parameters of the lowest absorption band (Figure 2d green trace) are /max = 0.1253 mol–1L ( = 890 mol–1cm–1L), max = 7107 cm–1, and 1/2 = 3911 cm–1. As can be observed the Gaussian fit overestimates the real width of the absorption band. To estimate the experimental 1/2, the difference between the high and low energy values of the fit and experimental data, respectively, was taken at half the maximum intensity, giving an estimate of 2696 cm–1. The extent of electron delocalization can be quantified by calculating the electronic coupling (Hab) between the donor and acceptor ground state energy surfaces using the spectral deconvolution results. Two methods were employed to assess the extent of electron delocalization: 1) electronic coupling calculated from the Hush formula (Hab, considering the donor-acceptor distance rab as the average Fe–Fe distance),1-4 and 2) a treatment introducing the parameter  that classifies the mixed valent species into: weakly coupled class II (0 <  < 0.1), moderately coupled class II (0.1 <  < 0.5), borderline class II-III (  0.5), and class III ( > 0.5).138 Hab and  can be calculated from Eq. S1 and S2 (See Supplementary Information). Thus, we obtain from the Hush formula Hab = 1045.4 cm–1; and alternatively, from the second method  = 0.33.

3.3. Spin Ground State Analysis via Magnetometry. To investigate the nature of the spin ground state in compounds 1a, 1b, 2b, and 3, magnetic susceptibility data (𝜒MT, 1.8 – 300 K) and variable-temperature, variable-field (VTVH) magnetization data (1.8 – 10 K) were collected. The ground state of the all-ferrous species (HL)2Fe6 has been determined to be a well-isolated S = 6 that persists to room temperature.61, 125 Its magnetic anisotropy is dictated by an axial zero-field splitting parameter (D) of +1 cm–1 and intermediate rhombicity of |E/D| = 0.16 (giso = 2.004) extracted from VTVH magnetization data. To date, the clusters prepared in this laboratory featured the highest, thermally persistent ground state electronic structures yet reported (S = 19/2, [Bu4N][(HL)2Fe6(L’)2] (L’ = pyridine or DMF;125 S = 6, (HL)2Fe661, (PhL)Fe3(thf)353 and (tbsL)Fe3(thf)60, 126). Additionally, ac magnetic susceptibility data for[Bu4N][(HL)2Fe6(L’)2] (L’ = py or DMF) reveal singlemolecule magnet (SMM) behavior at low temperatures for the anions with magnetic blocking temperatures (TB) of 2.6 (L’ = py) and 2.1 K (L’ = DMF) and effective spin reversal barriers (Ueff) of 42.5(8) and 33.5(1) cm–1, respectively.125 Magnetic data collected on [(HL)2Fe6][BArF24] (1a) display a plateau above 20 K (𝜒MT = 16.0 cm3K/mol) that persists to room temperature (𝜒MT300 K = 18.1 cm3K/mol). The room temperature 𝜒MT is slightly above the predicted spin-only value for an S = 11/2 (17.87 cm3K/mol, g = 2.0). In contrast, on decreasing temperature to 1.8 K a sudden decrease to 𝜒MT = 5.2 cm3K/mol is observed and is likely the result of zero-field splitting (Figure 3a). While oxidation of the all-ferrous cluster (HL)2Fe6 (S = 6) decreases the spin ground state of the [Fe6] core, reduction of (HL)2Fe6 increases the spin ground state significantly as observed in 2b (Figure 3a). VT 𝜒MT data of 2b display a plateau from 40 (𝜒MT = 49.5 cm3K/mol) to 300 K (𝜒MT = 48.9



cm3K/mol); in addition to a precipitous drop to 2.2 cm3K/mol at 1.8 K. This magnetic behavior is rather similar to the previously reported S = 19/2 clusters [Bu4N][(HL)2Fe6(L’)2] (L’ = py, dmf).125 The doubly reduced cluster [(HL)2Fe6]2– displayed

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even higher 𝜒MT values. VT 𝜒MT data of [Bu4N]2[(HL)2Fe6] (3) display a room temperature susceptibility of 𝜒MT300 K = 65.2 cm3K/mol (Figure 3a), remarkably close to the spin only value

Figure 4. Alternating-current magnetic susceptibility data for [(HL)2Fe6][BArF24] (1a, a-c) and [P][(HL)2Fe6] (2b, d-f) under zero-applied external dc field. Out-of-phase (𝜒′′M, a and d) and in-phase (𝜒′M, c and f) components of the ac magnetic susceptibility versus frequency (𝜈). (b and e) Cole-Cole plots. The solid continuous lines (a-f) represent a fit to the data as described in the text. (g) Relaxation times (ln ) versus inverse temperature (1/T) obtained by fitting the data in (a-f) to an Arrhenius temperature law as described in the text. Temperature labels in (a) are applicable to (a-c); similarly, those in (d) are applicable to (d-f). (h) Magnetic hysteresis of 2b at 1.8 K cycling from +7  –7 T. Zoomed-in region (+1  –1 T) rate is 0.18 mT/s. Inset: zero-field-cooled (ZFC) and field-cooled (FC) data for 2b at a temperature sweeping rate of 0.057 K/min.

predicted for an S = 11 (66 cm3K/mol, g = 2.0). The susceptibility increases to 68.2 cm3K/mol on decreasing the temperature to 80 K. Below 3.5 K the effect of zero-field splitting is manifest and a sudden decrease in 𝜒MT is observed to 55.6 cm3K/mol at 1.8 K. The 𝜒MT data of 1a, 2b and 3 were fit using PHI140 according 2 to the spin Hamiltonian 𝐻 = 𝐷𝑆𝑧 + 𝑔𝑖𝑠𝑜𝜇𝐵S ∙ H, for an S = 11/2 (1a), 19/2 (2b), and 11 (3). The fit parameters that best reproduced the data of 1a are: g = 1.91, D = –0.01 cm–1, and TIP = 5 x 10–3 cm3/mol. Similarly, for 2b and 3 the fits are: g = 2.04 and 2.03; and D = +10.9 and ~0 cm–1, respectively. Further insight into the ground states of these clusters was obtained by collecting VTVH magnetization data. In all cases the reduced magnetization data display non-superimposable isofield curves indicative of zero-field splitting (ZFS); although for 3 this appears to be negligible (Figure 3b). At 1.8 K and under a 7 T magnetic field saturation of magnetization in 1a, 2b, and 3 occurs at 8.1, 14.9, and 22.0 B, respectively (Figure 3b). Saturation in 3 occurs at the expected value for an S = 11

when ZFS is absent; in contrast the magnetic saturation for 1a and 2b fall short of the expected values of 11 and 19 B, respectively. To quantify this effect more reliably the data 2 2 2 were fit to the spin Hamiltonian 𝐻 = 𝐷𝑆𝑧 +𝐸(𝑆𝑥 ― 𝑆𝑦) + 𝑔𝑖𝑠𝑜𝜇𝐵S ∙ H, considering the ground states established. The data of 1a is best modeled by an S = 11/2 with g = 1.87, D = – 1.13 cm–1, and |E/D| ≈ 0. Comparably high-quality fits were obtained for 2b and 3 resulting in the following parameters: g = 2.04 and 2.003; D = –0.76 and +0.07 cm–1; and |E/D| = 0.04 and 0.07, respectively. As a final note the ground state assignment of 1a, 2b, and 3 was complemented by analyzing the 𝜒′MT vs. T dependence. When dealing with large spin ground states, low lying excited states have the potential to be populated under the traditional dc magnetic susceptibility measurement. To circumvent this complication, the susceptibility is recorded using a small ac field in place of the dc measurement.109, 121 The ac measurement resembles the measurement acquired under a static dc field as long as the oscillation frequency of


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the ac field is slow enough to allow the magnetization vector to reorient. At 1.8 K 1a displays a 𝜒′MT value of 14.3 cm3K/mol (Figure 3c). On slight heating it reaches a plateau at T > 2.4 K (16.8 cm3K/mol). Further heating increases 𝜒′MT subtly to approach the spin-only value of an S = 11/2 (17.87 cm3K/mol; 𝜒′MT10 K = 17.77 cm3K/mol). For 2b a marked transition is observed where at low temperatures 𝜒′MT increase mildly from 1.68 to 3.59 cm3K/mol, from 1.8 to 3 K, respectively; followed by a sudden increase in 𝜒′MT which occurs between 3 and 4.4 K. For T > 4.4 K a monotonic increase brings the moment up and by T = 10 K the recorded 𝜒′MT is 37.6 cm3K/mol. The closely related S = 19/2 solvated species [Bu4N][(HL)2Fe6(L’)2] displays similar behavior.125 Of greater significance, the downturn in the susceptibility product corresponds to a decrease in the in-phase susceptibility (𝜒′M), which marks the occurrence of slow relaxation of the magnetization as has been described elsewhere.141 In contrast, species 3 displays at the lowest temperature a moment that corresponds already with population of an S = 11 (𝜒′MT1.8 K = 62.7 cm3K/mol, Figure 3c). 3.4. Slow Relaxation of the Magnetization. The relaxation dynamics of 1a, 2b, and 3 were probed by ac magnetic susceptibility at zero dc-field. The frequencies of the 4 Oe oscillating field were in the range of 1 to 1488 Hz. To our surprise 3 does not display an out-of-phase (𝜒′′M) component indicative of SMM behavior. Therefore only [(HL)2Fe6][BArF24] (1a) and [P][(HL)2Fe6] (2b) were investigated in detail. At 1.9 K a peak in the 𝜒′′M vs.  plot is evident for 1a which shifts to higher frequencies as the temperature is increased (Figure 4a). An equivalent behavior for 2b is observed at T ≥ 3.8 K (Figure 4d). Accordingly, a decrease in 𝜒′M is observed for both species (Figure 4c and f).142 The SMM behavior can also be visualized by the semicircular profiles displayed in the Cole-Cole plots (𝜒′′M vs. 𝜒′M) illustrated in Figure 4b and e; where a single relaxation pathway is invoked judging from the appearance of a single arc at any given temperature.143 The relaxation dynamics were fit according to a generalized Debye model.144 IGOR Pro Wavemetrics was used to fit 𝜒′′M and 𝜒′M vs. 𝜈 to a distribution of single relaxation processes.145-146 CC-FIT was employed to fit the Cole-Cole plots.147 Panels (a) through (f) in Figure 4 were fit independently and the extracted temperature-dependent relaxation times () fit to an Arrhenius temperature law 𝜏 = 𝜏0exp(𝑈eff/kBT). The average values from all the three methods fit to an effective spin relaxation barrier of Ueff = 21.7(1) and 43.8(5) cm–1 and an attempt time of 𝜏0 = 8.7(6) x 10–9 and 6(1) x 10–9 s for 1a and 2b, respectively (Figure 4g). Based on the definition of Gatteschi et al. of the magnetic blocking temperature (TB) as that at which the relaxation rate (𝜏) becomes 100 s,142 we calculate a TB of 1.3 and 2.7 K for 1a and 2b, respectively. Further evidence of magnetization blocking was manifest from magnetic hysteresis experiments. Saturation of the magnetization at 1.8 K was observed at ±8.16 and ±14.9 µB when cycling between ±7 T for 1a and 2b, respectively. As expected from the calculated blocking temperature,

compound 1a did not exhibit any magnetic hysteresis (Figure S18); in contrast 2b displayed a remnant magnetization of 4.9 µB and a coercive field of ~0.7 T (Figure 4h). In addition to the magnetic hysteresis experiments, zero-field-cooled (ZFC) and field-cooled (FC) data were collected at 0.1 T to explore the magnetization dynamics. Divergence between the ZFC and FC data were observed only for 2b, while the corresponding data for 1a superimposes at all temperatures (Figure S19). In 2b overlapping magnetization is observed at T > 2.9. The FC data display a plateau between 1.8 and 3 K at ~1.25 µB; in comparison the ZFC data diverge dramatically displaying a maximum of 1.22 µB at 2.9 K to subsequently drop to 0.21 µB at 1.8 K (Figure 5h inset).

4. DISCUSSION 4.1. Synthesis and structure. Oxidation of the all-ferrous cluster (HL)2Fe6 affords the monocationic, unsolvated cluster [(HL)2Fe6][BArF24] (1a), while isostructural species are also obtained under reduction conditions to yield the monoanionic [Bu4N][(HL)2Fe6] (2a), [P][(HL)2Fe6] (2b) and dianionic [Bu4N]2[(HL)2Fe6] (3) (Scheme 1). Direct comparison between the overall oxidation level (n) and the core ironanilide structural metrics yield a remarkable correlation. In general, while the average metal-metal distance within the hexanuclear octahedron decreases slightly throughout the unsolvated clusters (n = +1  –2, Figure 5a) the iron-anilide mean distance expands with increasing number of valence electrons (VE) at [Fe6] (Figure 5b). The opposite trend is seen for the Fe–N(H)–Feavg angle which becomes more acute upon increasing the number of VE (Figure 5c). The comparison of the Fe–Feavg in the pyridine-bound species [Bu4N][(HL)2Fe6(py)2] (n = –1; VE = 37)125 and [(HL)2Fe6(py)2][PF6] (1b, n = +1; VE = 35) is less straight forward due to the significant distortion enforced by solvent ligation, the Fe–N(H)avg distance and Fe–N(H)–Feavg angle preserves the overall trend observed in the unsolvated clusters (Figure S21). 4.2. Electronic structure. Remarkably the spin ground states of the reduced clusters undergo a significant electronic ground state perturbation that drastically increases from an S = 6 in the all-ferrous (HL)2Fe6 to an S = 19/2 for 2b and 11 for 3. In contrast, the removal of an electron from the [Fe6] core reduces the spin to an S = 11/2 for unsolvated 1a where upon ligation of pyridine decreases further to 9/2 in [(HL)2Fe6(py)2][PF6] (1b) (Figure S17 and S22). A qualitative molecular orbital bonding approach developed previously for hexanuclear edge-bridged Nb and Ta clusters148 has been adapted to the all-ferrous cluster (HL)2Fe6 to account for its S = 6 ground state.61 The axis system at each iron site was chosen so that the dz2 orbital is oriented normal to the local N4 plane comprised of the ligand anilides to which the dx2–y2 orbital is directed. The frontier iron 3d orbitals were combined according to symmetry considerations and filled with the available 36 iron valence electrons. To account for the observed S = 6, the symmetry adapted linear combinations (SALCs) of the iron-anilido (Fe– N) antibonding interactions (1a2g, 2eg and 2t2u) are highest in energy and must exceed the mean



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Figure 5. Structural comparison of (a) Fe–Feavg and (b) Fe–N(H)avg distances, and (c) Fe–N(H)-Feavg angle in the series 1a, 2a, 2b, and 3 (n = +1  –2) including the all-ferrous cluster (HL)2Fe6 (n = 0). The black open symbol at n = –1 correspond to the metrics of 2b. Qualitative molecular orbital diagram derived for neutral (HL)2Fe6 (e);61 oxidized [(HL)2Fe6]+ (1a, d); and reduced [(HL)2Fe6]– (2a or 2b, f), [(HL)2Fe6]2– (3, g) to explain the observed spin ground state.

spin-pairing energy149 to remain unpopulated (ΔE(1a2g  1eg) > spin pairing energy; Figure 5e). As described in our previous report for [Bu4N][(HL)2Fe6(L’)2] the addition of an electron to (HL)2Fe6 likely proceeds by population of the σ*(FeN) band (1a2g, 2eg, 2t2u), weakening the Fe–N bonds as a result.125 The latter effect is structurally manifest in longer Fe–N bonds (Table 1) and follows a correlation with the overall oxidation level n (Figure 5b), tracking with the spin ground state (i.e., longer Fe–N with increasing S). Following our proposed MO diagram, we predict that removal of an electron from the HOMO (2 t1u) in the absence of ligand perturbations would result in an S = 11/2 species (Figure 5d). This is confirmed experimentally by the S = 11/2 cluster 1a. Pyridine ligation to trans-disposed iron sites in 1b affords [(HL)2Fe6(py)2][PF6] that results in a spin ground state of an S = 9/2 (Figure S17 and S22). The decrease in spin ground state going from 1a to 1b, can be explained where two nominally dz2 based SALCs rise in energy due to pyridine coordination, thus effectively decreasing the number of energetically accessible molecular orbitals and forcing the spin to decrease to 9/2 (Figure S22ab). 4.2.1. Spin ground state of monoanionic 2. Previously125 we speculated that perturbation of two dz2-based SALCs arising from pyridine ligation raised two orbitals high enough in energy so as to remain unpopulated akin to the effects of

solvent ligation in the cationic clusters. Furthermore, we posited that removal of the pyridine might result in a maximally high-spin configuration of S = 23/2, but that is not observed. Indeed, the ground spin-state observed for [P][(HL)2Fe6] (2b) is identical (S = 19/2) to the solvento adducts [(HL)2Fe6(L)2] (L = py, dmf). Thus, the molecular orbital configuration proposed consistent with the true orbital ordering, the observed spin ground state (S = 19/2) must solely arise from partial population of the σ*(FeN) band (1a2g, 2eg, 2t2u), leaving two nominally antibonding orbitals of x2y2 parentage empty (Figure 5f and S22d-e). A closer inspection of the FeN bond metrics of 2b reveal a clear contraction in the xy plane for two, trans-disposed iron centers of the anion. Each of the anions in the asymmetric unit (constituting one half of each [(HL)2Fe6] of crystal structure of 2b feature one iron site with short FeN bonds (2.010(4), 2.033(4) Å) and two iron sites with longer FeN bonds (2.109(4), 2.104(4) Å). Thus, two iron sites per anionic cluster each feature an orbital of x2-y2 parentage that remains empty in the partially filled σ*(FeN) band (1a2g, 2eg, 2t2u) giving rise to the distortion observed. Furthermore, the two highest lying orbitals that remain unpopulated simply exceed the spin-pairing energy for iron in this configuration, rather than for the reasons previously proposed (Figure 5f).125


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4.2.2. Spin ground state of dianionic 3. Addition of an electron to [(HL)2Fe6]– affords the dianionic species [Bu4N]2[(HL)2Fe6] (3) that has been determined to have an S = 11 ground state. From our molecular orbital considerations, 2b has not populated all the available antibonding SALCs (1a2g, 2eg, or 2t2u) yielding an S = 19/2, not 23/2 spin ground state (Figure 5f). Upon reduction of [(HL)2Fe6]– to 3, however, the additional electron promotes a maximally high spin species via population of the entire FeN antibonding set, thus achieving an S = 11 configuration (Figure 5g). Consistent with that assessment, 3 features the longest Fe–Navg bonds (2.127(9) Å) measured in the electron-transfer series (Figure 5b), consistent with the bond metrics observed for trinuclear clusters that feature maximally high spin ground states with iron centers in similar geometries (i.e., (PhL)Fe3(thf)3, dFeN 2.167(6) Å).53 Each successive reduction ([Fe6]  [Fe6]  [Fe6]2) adds an electron to nominally antibonding orbitals of σ*FeFe or σ*Fe-N origin (Figure 5E). Population of the FeN antibonding orbital set (σ*Fe-N) lowers the overall band energy, promoting a higher population of the σ*FeN orbitals, resulting in a higher overall spin for the anion and dianion. Thus, assuming the high spin electronic configurations maximizes the electron exchange interaction stabilization effect, offsetting the destabilizing impact of populating a greater number of antibonding interactions. Conversely, oxidation of the [Fe6] core stabilizes the cluster by removing electrons from antibonding orbitals, thus the electronic configurations arrange into lower spin states. For 1  3 the spin ground states remain isolated up to room temperature. We attribute the available intracore direct orbital overlap to provide a direct exchange pathway that results in the observed S = 6 ground state for (HL)2Fe6. Both theoretical64 and experimental69, 78, 150 work has shown that double exchange enhances the ferromagnetic interaction in mixed valence complexes. For the clusters studied herein, the double exchange pathway is the same as the direct exchange pathways mediated by direct FeFe orbital overlap. This electron exchange pathway enhancement is manifest in the modest shortening of the Fe–Feavg distance upon reduction (Figure 5a), similar to other strongly coupled clusters, e.g. [(tbsL)Fe3]–.126 4.3. Mixed valency and redox delocalization. Fully delocalized compounds have spin-dependent IVCTs given by 𝐸op(𝑆) = 2 |𝐵|(𝑆 + 1 2),66 where B corresponds to the double exchange interaction energy. For 3 the calculated value of B is 309 cm–1. Fully delocalized complexes have no thermal barrier for electron transfer and thus the ground state adiabatic energy surface has a single minimum.139 This occurs when the following condition is met 2Hab/max ≥ 1. Two methods were presented earlier by which the extent of delocalization is quantified. The first (Hab) is well-known to work best for partially delocalized complexes (Class II) and significantly underestimates Hab as delocalization increases towards borderline Class II-III or fully delocalized Class III (the underestimation reaches 50% when 2Hab = max).151 Thus, via the Hush formulae Hab is calculated to be 1045.4 cm–1; while by the second method we obtain  = 0.33. Both methods point towards a moderately coupled Class II species; in

contrast the experimental shape of the IVCT band in 3 (Gaussian on the high energy and skewed on the low energy side) suggest strong delocalization. While the transitions observed could also be of d-to-d origin, the low energy band asymmetry displays a common characteristic of IVCTs. Other experimental evidence of delocalization in 3 is seen in the Mössbauer spectrum at 4.2 K where the six iron centers display a single iron environment indicative of complete shared oxidation, therefore the electron transfer rate within the [Fe6] core occurs faster than 10–7 s. 4.4. Single molecule magnetism behavior. The monoanionic 2b features a spin ground state (S = 19/2) equal to the solvated clusters [(HL)2Fe6(L)2] (L = py, dmf).125 Cluster 2b also displays slow relaxation of the magnetization. The effective spin reversal barrier has an almost identical magnitude, 42.5(8) and 43.8(5) cm–1 for [Bu4N][(HL)2Fe6(py)2] and 2b, respectively. In contrast they differ quite significantly below the blocking temperature, for instance the pyridine-bound cluster displays a well-resolved hyperfine split Mössbauer spectrum at 4.2 K and a step-like behavior in its magnetic hysteresis at 1.8 K, while 2b features a broad and unresolved spectrum with narrower magnetic hysteresis measured under identical conditions. Magnetic analysis of 1a confirm its SMM behavior with Ueff = 21.7(1) cm–1. The theoretical values calculated from 𝑈 = |𝐷|(𝑆2 ― 1 4) for 1a (33.9 cm–1) and 2b (68.4 cm–1) yield values higher than the observed Ueff which is typical of systems where the spin reversal does not follow strictly the thermal barrier, but instead tunnels through the barrier at some higher energy ms levels.83, 152 We previously observed that structural distortions of the [Fe6] octahedron accompany the emergence of SMM behavior. In fact, elongation on the solvated clusters [(HL)2Fe6(L’)2]– (L’ = pyridine or DMF) was observed along the Fe–Fetrans sites coordinated by L’.125 However, we now note that the solvation sites do not occur at the iron sites where an xy-contraction is observed. Similarly, the [Fe6] clusters in 1a, 2a and 2b distort, each featuring two short (almost identical) and one long Fe–Fetrans distance (Table 1). In comparison 3 has a highly symmetric [Fe6] octahedron with Fe–Fetrans distances of 3.630(1), 3.643(1) and 3.646(1) Å; thus, 3 would be expected to be absent of slow relaxation of the magnetization effects. In agreement, Mössbauer data at 4.2 K and ac magnetic susceptibility studies on 3 corroborated the absence of SMM behavior. To date the most successful methods to achieve high spin ground states in polynuclear systems rely on the use of superexchange electronic coupling interactions. Alternatively, the current account demonstrates that significantly strong magnetic interaction in mixed valence complexes can be achieved by the introduction of double exchange. This has been previously articulated150 and shown experimentally in a few dinuclear coordination complexes.6774 Nonetheless, the spin ground states of [P][(HL) Fe ] (2b) 2 6 and [Bu4N]2[(HL)2Fe6] (3) represent the highest spin ground states recorded to date that persist to room temperature. Manipulation of the auxiliary ligands around the iron centers may provide a pathway for enhanced SMM behavior as already shown in [(HL)2Fe6(L’)2]– (L’ = pyridine or DMF).



5. CONCLUSIONS

2.

A series of isostructural octahedral hexairon clusters are shown to display spin ground states varying from S = 9/2 to S = 11, marking thus the record of the highest spin states that remain isolated up to 300 K. Such behavior is attributed to double exchange manifested in the strongly delocalized clusters. Consequences of the latter are the hyperfine splitting of the Mössbauer spectrum at low temperature and the emergence of SMM behavior. As seen in the structural metrics, the existence of magnetic anisotropy can be predicted by the asymmetry of the [Fe6] octahedron and further manipulated by simple solvation of the cluster. The latter provides a pathway by which the magnetic behavior can be tuned and enhanced by simple exploration of other auxiliary ligands.

Hush, N. S., Intervalence-Transfer Absorption. Part 2. Theoretical Considerations and Spectroscopic Data. In Prog.

Inorg. Chem., John Wiley & Sons, Inc.: 1967; pp 391-444. 3.

Robin, M. B.; Day, P., Mixed Valence Chemistry-A Survey and Classification. In Advances in Inorganic Chemistry and

Radiochemistry, Emeléus, H. J.; Sharpe, A. G., Eds. Academic Press: 1968; Vol. Volume 10, pp 247-422. 4.

Hush, N. S., "Homogeneous and heterogeneous optical and thermal electron transfer." Electrochim. Acta. 1968, 13, 1005.

5.

Creutz, C., "Mixed-valence complexes of d5-d6 metal centers."

Prog. Inorg. Chem. 1983, 30, 1. 6.

Ward, M. D., "Metal-metal interactions in binuclear complexes exhibiting mixed-valency: molecular wires and switches."

Chem. Soc. Rev. 1995, 24, 121.

ASSOCIATED CONTENT

7.

Supporting Information

Kaim, W.; Klein, A.; Glockle, M., "Exploration of mixed valence chemistry: Inventing new analogues of the Creutz-

The Supporting Information is available free of charge on the ACS Publications website. X-ray crystal structure images, crystallographic data (cif), Mössbauer and near-infrared spectra, and magnetometry data.

8.

AUTHOR INFORMATION

9.

Taube ion." Acc. Chem. Res. 2000, 33, 755. Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J., "The localizedto-delocalized transition in mixed-valence chemistry." Chem.

Rev. 2001, 101, 2655. Burns, R. G., "Intervalence transitions in mixed-valence minerals of iron and titanium." Annu. Rev. Earth Pl. Sc. 1981,

Corresponding Author

9, 345.

*[email protected]

10. Beinert, H.; Holm, R. H.; Munck, E., "Iron-sulfur clusters:

Present Addresses †Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, PA 15260, United States

Nature's modular, multipurpose structures." Science 1997,

277, 653. 11. Rao, P. V.; Holm, R. H., "Synthetic analogues of the active sites of iron-sulfur proteins." Chem. Rev. 2004, 104, 527.

Notes The authors declare no competing financial interest.

12. Hagen, W. R., "EPR spectroscopy of iron-sulfur proteins."

Adv. Inorg. Chem. 1992, 38, 165.

ACKNOWLEDGMENT This work was supported by a grant from the NIH (GM 098395), DOE (DE-SC0008313), and Harvard University. R.H.S. gratefully acknowledges Consejo Nacional de Ciencia y Tecnología (CONACYT) and Fundación México for a doctoral fellowship. T.A.B. gratefully acknowledges support from the Dreyfus Foundation (Teacher-Scholar Award). ChemMatCARS Sector 15 is supported by the National Science Foundation under grant number NSF/CHE-1346572. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DEAC02-06CH11357.

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