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Spin-State Versatility in a Series of Fe4 [2 × 2] Grid Complexes: Effects of Counteranions, Lattice Solvent, and Intramolecular Cooperativity Markus Steinert, Benjamin Schneider, Sebastian Dechert, Serhiy Demeshko, and Franc Meyer* Georg-August-University, Institute of Inorganic Chemistry, Tammannstrasse 4, D-37077 Göttingen, Germany S Supporting Information *

ABSTRACT: The new compartmental proligand 4-bromo3,5-bis{6-(2,2′-bipyridyl)}pyrazole (HLBr) was synthesized and shown to form robust [2 × 2] grid complexes [FeII4LBr4]X4 with various counteranions (X− = PF6−, ClO4−, BF4−, Br−). The grid [FeII4LBr4]4+ is stable in solution and features two high-spin (HS) and two low-spin (LS) ferrous ions in frozen MeCN, and its redox properties have been studied. Six all-ferrous compounds [Fe4LBr4]X4 with different counteranions and different lattice solvent (1a−f) were structurally characterized by X-ray diffraction, and their magnetic properties were investigated by Mössbauer spectroscopy and SQUID magnetometry. Variations in spin-state for the crystalline material range from the [4HS] via the [3HS1LS] to the [2HS-2LS] forms, with some grids showing thermal spin crossover (SCO). The series of [FeII4LBr4]4+ compounds allowed us to establish experimentally well-grounded correlations between structural distortion of the {FeN6} coordination polyhedra, quantified by using continuous shape measures, and the grid’s spin-state pattern. These correlations evidenced pronounced cooperativity for the multistep SCO transitions within the grid, imparted by the strain effects of the rigid bridging ligands, and a high stability of the dimixed-spin configuration trans-[2HS-2LS] that has identical sites at opposite corners of the grid. The results are in good agreement with recent quantum chemical calculations for such molecular [2 × 2] grids featuring strongly elastically coupled vertices.

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INTRODUCTION In the search of suitable molecules for miniaturized data storage and molecular electronics applications, [2 × 2] grid complexes composed of four metal ions and four compartmental ligand strands are discussed as promising candidates because such complexes can exhibit bi- or multistability based on a series of accessible redox or spin states.1 Spin-state switching exploits the so-called spin crossover (SCO) phenomenon, i.e., metal ions such as FeII (d6) in octahedral ligand environment can undergo a transition from a high-spin (HS) to a low-spin (LS) state and vice versa, induced by an external stimulus like pressure, light, or change of temperature.2 In this context, [2 × 2] grids have been proposed as attractive building blocks for potential application in molecular logic devices such as Quantum Cellular Automata (QCA), which encodes binary information (“0” and “1”) in elementary cells that can interconvert between two stable and energetically degenerate yet distinguishable states.3 If based on SCO transitions, QCA implementation with tetrametallic grid-type molecular entities requires a persistent dimixed-spin [2HS-2LS] configuration with identical spin states at opposite corners of the grid (trans-[2HS-2LS]; Figure 1) and the ability of switching between cell states. While numerous examples of [2 × 2] grid systems have been synthesized and reported over the years,1,4 grid-type homometallic all-ferrous Fe4 complexes have remained relatively © XXXX American Chemical Society

Figure 1. Schematic representations of trans and cis configurations of dimixed-spin Fe4 grid complexes and the two states of a four-dot QCA cell based on the trans configured grid.

scarce5−11 despite the prominent role of FeII in SCO research. In particular, FeII4 grids in the dimixed-spin [2HS-2LS] state are few in number, and their thermal SCO mostly shows a gradual or stepwise pattern;5,8−10 this SCO behavior is unfavorable for use in QCA, which requires a persistent [2HS-2LS] configuration. From a ligand design point of view, compartmental ligands with a central pyrazolate or triazole bridge appear to be good candidates for creating di- and oligonuclear complexes with mechanically coupled metal centers showing cooperative SCO behavior.12 We recently Received: November 27, 2015

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DOI: 10.1021/acs.inorgchem.5b02762 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry developed some robust, air- and moisture-stable [2 × 2] metallogrids based on a set of bis(tridentate) 3,5-bis(bipyridyl)pyrazole ligands.13 The Fe4 grid of the parent proligand HLH (Figure 2) exhibited redox as well as spin multistability, where

HS-LS], Figure 1), as was previously suggested for [Fe4LH4]4+ 10 and experimentally confirmed for the derivative [Fe4LMe4]4+.11 It should be noted though that intermolecular and crystal-packing effects are ignored in the available quantum chemical treatment of isolated grid molecules.15,16 Therefore, experimental investigation of a series of identical FeII4 grids with different arrangements in the crystalline solid, induced by, e.g., different counteranions or crystallization conditions, is highly warranted to demonstrate the general validity of the CSM-based analysis of the evolution of coordination polyhedra in such array of four electronically and mechanically coupled SCO centers. In this article we report a new derivative of the 3,5bis(bipyridyl)pyrazole ligand scaffold, namely, HLBr with a bromine substituent in the 4-position of the central pyrazole bridge (Scheme 1), and the structural and magnetic character-

Figure 2. Pyrazolate-bridged binucleating proligands HLR.

the all-ferrous complex [FeII4LH4](BF4)4 featured gradual but incomplete two-step thermal SCO from the [4HS] to the [2HS-2LS] state.10 In contrast, the derivative [FeII4LMe4](BF4)4 that was obtained from the proligand HLMe bearing a methyl substituent in the backbone C-4 position of the central pyrazolate (Figure 2) was shown to stabilize the dimixed-spin [2HS-2LS] state over a wide temperature range.11 Because of its sought-after [HS-LS-HS-LS] configuration, with identical spin states at diagonally opposed vertices of the grid (trans[2HS-2LS]), the latter system appeared particularly suited for potential use in QCA devices. With respect to further elaboration of such Fe4 grids it remains a key question whether strong elastic coupling between the transitioning FeII vertices, mediated by the rigid pyrazolate bridges, can give rise to cooperativity effects that intrinsically favor either the trans-type [HS-LS-HS-LS] or the cis-type [HS-HS-LS-LS] configuration in the dimixed-spin state, perhaps even predominating any crystal-packing effects. Recent quantum-chemical (DFT) calculations on the spinstate multistability patterns and SCO pathways in selected ferrous [2 × 2] Fe4 squares14 and grid complexes,15,16 including the parent pyrazolate-based complex [Fe4LH4]4+,16 indeed revealed that their SCO behavior is not merely defined by the complex topology but is governed by the elastic communication between the transitioning FeII corners. Hence, the rigidity or flexibility of the bridging ligands that form the grid’s edges and interlock the metal ion vertices play a key role for the magnetic properties. In particular, it was emphasized that the deviation of the coordination polyhedron of the FeII ions from ideal octahedral symmetry, quantified by the method of continuous shape measure (CSM)17,18 and called octahedricity S(Oh), correlates with the spin state and the physics of SCO. Since the LS-FeII state is generally associated with a more regular {FeN6} octahedron,19 hence with lower octahedricity, smaller S(Oh) values for HS-FeII ions translate into a more facile HS → LS transition (provided that entropic changes are similar). Most importantly, it was concluded from the DFT-based CSM analysis16 that the rigid pyrazolatebridging ligands induce significant elastic coupling between the four vertices and cause severe structural responses for the entire [Fe4LH4]4+ grid upon SCO of a single FeII ion, in contrast to the cyanide-bridged Fe4 squares where elastic coupling through cyanide bridges is weak.14 Specifically, the first HS → LS transition in [Fe4LH4]4+, associated with a decrease of octahedricity at the respective metal ion, leads to lowering of S(Oh) also at the opposite corner of the grid and to an increase of S(Oh) at the neighboring sites. Consequently, the preferred configuration for the mixed-spin [2HS-2LS] form resulting after a second HS → LS transition should be the one with identical spin states at diagonally opposed positions ([HS-LS-

Scheme 1. Functionalization of the Known Ligand HLH (top), and Schematic [2 × 2] Fe4 Grid Formation (bottom)

ization of six solid compounds composed of the new [2 × 2] grid [Fe4LBr4]4+ and different counteranions and/or solvent molecules included in the crystal lattice. The large bromine substituent at the pyrazolate backbone was chosen in order to impart a more spherical overall shape to the [2 × 2] grid and thus minimize crystal-packing effects and structural distortion resulting from the entanglement of neighboring grid molecules or from interlocking of grids and neighboring anions or solvent molecules.

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RESULTS AND DISCUSSION Synthesis and Characterization. The parent ligand HLH 13 can be selectively brominated in the 4-position of the central pyrazole by treatment with elemental bromine in dichloromethane at 0 °C, giving the functionalized ligand HLBr in 93% yield (Scheme 1). The formation of [2 × 2] grid complexes [Fe4LBr4]4+ proceeds readily in a one-pot selfassembly reaction using stoichiometric amounts of HLBr, an iron salt, and a base such as NaOtBu. Counteranions were varied either by using different iron salts directly in the synthesis (for X = ClO4−, BF4−) or by subsequent anion exchange starting from [Fe4LBr4](ClO4)4 (for X = PF6−, Br−). Crystallization was generally achieved by slow diffusion of an ether solvent (DME, THF, or MTBE) into a solution of the grid complex in DMF. Single crystals suitable for X-ray diffraction could be obtained for six compounds 1a−f that differ B


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Inorganic Chemistry Table 1. Compounds 1a−f All Contain the [Fe4LBr4]4+ Cationa counteranion crystallization conditions lattice solvent

1a

1b

1c

1d

1e

1f

PF6− diffusion of THF DMF THF

PF6− diffusion of MTBE 4 DMF

ClO4− diffusion of THF b

ClO4− layered with DME DME,b 0.25 DMF

BF4− diffusion of THF b

Br− diffusion of THF 4 DMF, 2 H2O

a

This table shows their corresponding counteranions, lattice solvent, as well as their crystallization conditions, starting from a solution of the bulk material in DMF. bDue to the use of PLATON SQUEEZE in the crystallographic analysis the amount and identity of the lattice solvent molecules could not be readily determined.

Figure 3. Molecular structure of the grid-type cation of [Fe4LBr4](BF4)4 (1e) determined by X-ray diffraction; ball and stick model (left) and two different views of a space-filling model (right) illustrating how the bromine substituents lead to a more spherical shape; counterions, hydrogen atoms, and solvent molecules are omitted for clarity.

by the counteranion and/or by the solvent molecules included in the crystal lattice (Table 1). While their structures in the solid state show subtle but important differences, the overall [2 × 2] grid topology of the [Fe4LBr4]4+ complex cations is found in all those compounds, as expected. The molecular structure of the cation of [Fe4LBr4](BF4)4 (1e) is shown in Figure 3 as an example. All FeII ions in the various [Fe4LBr4]4+ complex cations are found six coordinated in a {N6} environment composed of two tridendate (terpyridine-like) binding pockets of two orthogonally arranged ligand strands. The grid’s overall charge of 4+ is balanced by four counterions. Fe−N distances in the six structures are comparable: LS-FeII ions are characterized by four bonds with d(Fe−N) = 2.00 ± 0.02 Å and two significantly shorter bonds to the central pyridines next to the pyrazolate with d(Fe−N) = 1.90 ± 0.01 Å, resulting in an average Fe−N distance of 1.97 ± 0.01 Å. HS-FeII ions feature four bonds with d(Fe−N) = 2.15 ± 0.02 Å and two even longer bonds to the peripheral pyridines with d(Fe−N) = 2.27 ± 0.07 Å, resulting in an average Fe−N bond length of 2.17 ± 0.03 Å. Structural trends among the series of complexes as well as magnetic properties determined by Mössbauer spectroscopy and SQUID magnetometry will be discussed below. Solution Studies. All complexes [Fe4LBr4]X4 (with X = PF6−, ClO4−, BF4−, and Br−) are soluble only in polar solvents such as MeCN, MeOH, or DMF, and hence, the presence of solvent-separated ion pairs is likely. This is supported by positive-ion ESI mass spectrometry (ESI-MS+) data that show the “naked” [Fe4LBr4]4+ ion as the dominant species in all cases; the ESI mass spectrum of a MeCN solution of [Fe4LBr4](ClO4)4 is shown in Figure 4 as an example. The ESI-MS results also confirm that the [Fe4LBr4]4+ core is robust and stable in dilute solutions under ambient conditions in the presence of air and moisture. Only at very low concentrations

Figure 4. Positive-ion ESI mass spectrum of [Fe4LBr4](ClO4)4 in MeCN; (inset) experimental (upper) and simulated (lower) isotopic distribution pattern for the peak pattern around m/z = 510 characteristic for the [Fe4LBr4]4+ ion.

could an additional minor peak assigned to the “corner complex” [Fe(HLBr)LBr]+ be observed in the ESI mass spectra. To obtain information about the intrinsic properties of the grid-type [Fe4LBr4]4+ core in solution and for comparison with the related [Fe4LH4]4+ and [Fe4LMe4]4+ grids that have a H atom or a Me group instead of the bromine substituent at the pyrazolate-C4 position, [Fe4LBr4](ClO4)4 in MeCN was investigated as a representative example of the present series of complexes. Cyclic voltammetry showed two separated and quasi-reversible oxidations at 816 and 983 mV vs SCE (Figure 5, Table 2). These are assigned to the sequential oxidation of two FeII ions in the grid, likely at opposite (diagonally disposed) corners, to give the [Fe III Fe II 3 L Br 4 ] 5+ and [FeIII2FeII2LBr4]6+ species; the one-electron nature of the two redox processes was confirmed by potentiostatic coulometry C


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grid the subsequent oxidations of the remaining ferrous ions to give the [FeIII3FeIILBr4]7+ and [FeIII4LBr4]8+ species cannot be observed; these oxidations coincide with further anodic processes that occur with larger currents at around 1.5 V vs SCE, likely corresponding to irreversible ligand oxidation (Figure S2). In the cathodic regime, several electrochemical processes take place between −1.0 and −1.7 V, likely representing the stepwise reduction of the four ligands (Figure S2). Oxidation of the [Fe4LBr4]4+ grid was monitored by UV− vis−NIR spectroelectrochemistry (Figures 6 and S3). The allferrous grid shows three intense (ε > 5 × 104 L mol−1 cm−1) absorptions at 242, 272, and 331 nm originating from ligand π−π* transitions, similar to the previously reported pyrazolatebased Fe4 grids. In the vis range a peak at 587 nm (ε ≈ 5700 L mol−1 cm−1) is assigned to MLCT transitions. Upon oxidation at an applied potential of 1.3 V vs SCE (covering the first two oxidation steps) the band at 587 nm decreases in intensity and shifts to slightly higher energy (as expected for an L ← FeII MLCT transition at the remaining FeII corners), while a new band arises at 800 nm. During this process the solution color changes from red to green. The latter is assigned to LMCT transitions, FeIII ← L, at the oxidized grid corners. Although an applied potential of 1.3 V vs SCE promotes a sequential twoelectron oxidation (the rather closely spaced individual steps could not be clearly separated by spectroelectrochemistry), a pseudo-isosbestic point is observed (Figure S3), suggesting a rather localized nature of the optical transitions and largely independent oxidation of two diagonally placed FeII ions in the grid. Subsequent reduction at an applied potential of 0 V vs SCE indicates chemical reversibility, but the oxidized species [FeIII2FeII2LBr4]6+ is not stable at room temperature over longer periods of time. When an electrochemically oxidized sample was stored under inert atmosphere in a Schlenk cuvette and UV−vis spectra were recorded over several days, gradual decomposition was evident (Figure S3), going along with a color change from intense green to brown. 57 Fe Mössbauer measurements of a frozen solution of [Fe4LBr4](PF6)4 in MeCN reveal the presence of two high-spin FeII ions and two low-spin FeII ions ([2HS-2LS] state) at 80 K with two characteristic quadrupole doublets (isomeric shift δ = 0.35 mm s−1 and quadrupole splitting ΔEQ = 1.07 mm s−1 for LS-FeII; δ = 1.09 mm s−1 and ΔEQ = 2.44 mm s−1 for HS-FeII; Figure 7). A [2HS-2LS] state was also found for the parent grid complex [Fe4LH4](BF4)4 in frozen solution at 80 K (in DMF or nitromethane, due to poor solubility in MeCN), whereas [Fe4LMe4](BF4)4 was shown to have a [1HS-3LS] configuration

Figure 5. Cyclic voltammogramm of [Fe4LBr4](ClO4)4 in MeCN solution (0.1 M [NBu4]PF6) at room temperature and a scan rate of 100 mV/s in the range 0.4−1.3 V; potentials vs SCE.

Table 2. Comparison of the Electrochemical Properties (anodic regime) of [Fe4LBr4]4+, [Fe4LH4]4+,10 and [Fe4LMe4]4+ 11 in MeCN Solutiona ox. step

[Fe4LBr4]4+ E1/2 (mV) (ΔEp (mV))

[Fe4LH4]4+ E1/2 (mV) (ΔEp (mV))

[Fe4LMe4]4+ E1/2 (mV) (ΔEp (mV))

1 2 3 4

816 (122) 983 (122)

642 (67) 783 (73) 1257 (85) 1438 (80)

518 (102) 674 (108) 1177 (135) 1401 (138)

ΔEp denotes the separation of the anodic and cathodic peak potentials for the individual redox events. a

during complete electrochemical oxidation at 1.3 V vs SCE (Figure S1). The previously reported grids [Fe4LH4]4+ and [Fe4LMe4]4+ both showed four sequential oxidations of the iron ions, finally giving fully oxidized [FeIII4LR4]8+. These four steps were grouped in two closely spaced pairs (ΔE1/2 roughly 150 mV) with a much larger gap between the second and the third oxidation (ΔE1/2 roughly 450 mV; Table 2), which was explained by the diagonal placement of the pairwise oxidations where electrostatic interaction is minimized. The third (as well as fourth) oxidation then occurs at a corner that already has two neighboring ferric ions, thus requiring much higher potential. In the case of [Fe4LBr4]4+ the first two oxidations are shifted by roughly 200 mV to higher potentials compared to [Fe4LH4]4+ and by 300 mV compared to [Fe4LMe4]4+, which comes as no surprise as the bromo substituent has a −I effect and therefore lowers the electron density at the ligand donor atoms, whereas the methyl group has a +I effect. It shows, however, that the backbone substituent at the pyrazolate-C4 position has a decisive effect on the redox properties of the [Fe4LR4]4+ grids. Due to the anodically shifted potentials of the Br-substituted

Figure 6. (Left) UV−vis bands of [FeII4LBr4]4+. (Right) Vis bands of [FeII4LBr4]4+ (black) and [FeII2FeIII2LBr4]6+ (red). D


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Inorganic Chemistry

ions present in every compound, but this low-temperature feature will not be specifically mentioned in each case.22 Mössbauer data were recorded at 7 K to exclude any SCO below 30 K. Furthermore, the FeII ions within a grid structure are considered as independent spin centers due to the very weak magnetic coupling known from previously studied pyrazolate-based Fe4 grids.10,23 Mö ssbauer parameters of crystalline samples of all 6 compounds, recorded at 80 K, are compiled in Table 3. Table 3. Mössbauer Parameter of the Crystalline Samples of 1a−f at 80 K compound

Figure 7. Mössbauer spectrum of solution at 80 K.

[FeII4LBr4](PF6)4

in frozen MeCN

1a 1b 1c

under those conditions. While a preference of the LS state in solutions of donor solvents such as MeCN is documented20 and has been attributed to H bonding between the ligands and the solvent molecules or to some dissociation equilibria,21 we assume that the differences in the ligand field strength of [LMe]−, [LH]−, and [LBr]− is more relevant in the present case. The solution studies confirmed that the new [2 × 2] grid [Fe4LBr4]4+ is robust, and they allowed for elucidating the effect of the backbone bromine substituent on intrinsic electronic and magnetic properties of the grid in comparison with the related [Fe4LH4]4+ and [Fe4LMe4]4+ complexes. The following main part of this contribution will focus on solid state effects for the new series of [Fe4LBr4]4+ complexes, namely, the effects of lattice solvent, counterions, and structural details. Influence of Lattice Solvent on the Magnetic Properties. Initial investigations of the magnetic properties of solid [Fe4LBr4](ClO4)4 indicated differences for freshly prepared crystals and air-dried material. Further studies showed that the SQUID and Mössbauer responses also depend on the types of counterions and solvent molecules included in the crystal lattice. A thorough study of these effects was thus initiated for the whole series of six compounds of [Fe4LBr4]X4 with four different counteranions X and crystallized under different conditions, as summarized in Table 1 (crystallization of [Fe4LBr4](PF6)4 from DMF/THF gave two different modifications, but since the structural parameters of the cation and the magnetic properties are almost identical just one will be discussed; see Figure S8 for the second modification). In order to correlate magnetic and spectral data with the crystallographic results, it is essential to avoid loss of the lattice solvents during SQUID and Mössbauer measurements. This was achieved by carefully separating crystals from the mother liquor, drying and weighing them quickly, and protecting the crystalline material in a hydrophobic oil. For Mössbauer measurements a mineral oil and for SQUID measurements a fluorinated polyether were used (see SI for details). For powder measurements, singlecrystalline material from the same batch was collected, air dried, and carefully grinded. Table S2 in the Supporting Information summarizes the sample composition (with corresponding molecular weights) that was used for analyzing the SQUID data of all crystalline or dried material and the analytical methods to derive these compositions. Table S2 also shows the errors in χMT that might be caused by the potential inaccuracy of the solvent content and molecular weight. All SQUID data discussed below show a decrease of χMT below 30 K due to zero-field splitting of the HS-FeII (S = 2)

1d 1e 1f

LS-Fe(II) HS-Fe(II) HS-Fe(II) LS-Fe(II) HS-Fe(II) LS-Fe(II) HS-Fe(II) LS-Fe(II) HS-Fe(II) LS-Fe(II) HS-Fe(II)

δ (mm/s)

ΔEQ (mm/s)

A (%)

0.37 1.04 1.05 0.39 1.05 0.38 1.05 0.39 1.05 0.38 1.06

1.07 1.90 2.37 0.94 2.70 0.95 2.68 0.91 2.73 0.94 2.70

49 51 100 29 71 25 75 25 75 26 74

To investigate effects of the lattice solvent on the magnetic properties of the Fe 4 grid, both [Fe4 L Br4 ](PF6 ) 4 and [Fe4LBr4](ClO4)4 were crystallized from DMF with two different ether solvents. In the former case crystallization with THF yielded compound 1a, and crystallization with MTBE yielded 1b. The latter shows an almost constant χMT value of 14.6 cm3 K mol−1 in the temperature region between 30 and 300 K, corresponding to a [4HS] state (Figure 8; Table 4). This configuration is confirmed by a Mössbauer measurement at 80 K that exhibits just one doublet with an isomeric shift δ = 1.05 mm s−1 and a quadrupole splitting ΔEQ = 2.37 mm s−1 typical for octahedral FeII in the high-spin state. Also, the X-ray structure is in good agreement, showing four high-spin FeII ions at 133 K. By changing the ether solvent for crystallization from MTBE to THF the magnetic properties are altered drastically. SQUID data for 1a reveal a [2HS-2LS] state between 30 and 250 K with χMT = 7.44 cm3 K mol−1 and only a small rise above 250 K. This is in accordance with Mössbauer data showing two doublets in a 1:1 ratio at 80 K, one with δ = 0.37 mm s−1 and ΔEQ = 1.07 mm s−1 (LS-FeII) and the other with δ = 1.04 mm s−1 and ΔEQ = 1.90 mm s−1 (HS-FeII). Furthermore, crystallographic data reveal two symmetry-related LS-FeII and two HS-FeII sites at 133 K. The second example for illustrating the influence of lattice solvent is [Fe4LBr4](ClO4)4, which was crystallized with THF to yield 1c and with DME to obtain 1d. The latter is found in the [3HS-1LS] state with only a minor rise of χMT from 9.50 cm3 K mol−1 at 50 K up to 10.46 cm3 K mol−1 at 300 K. Again, the [3HS-1LS] state is substantiated by X-ray data at 133 K and Mössbauer data at 80 K (LS-FeII δ = 0.38 mm s−1, ΔEQ = 0.95 mm s−1; HS-FeII δ = 1.05 mm s−1, ΔEQ = 2.68 mm s−1; in 1:3 area ratio). Compound 1c also shows a [3HS-1LS] state at low temperatures (χMT = 9.24 cm3 K mol−1 at 80 K), but an almost complete SCO to the [4HS] state takes place between 240 and 300 K, resulting in a χMT value of 12.86 cm3 K mol−1 at 320 K. While the [3HS-1LS] state at low temperatures has been confirmed by Mössbauer data at 7, 80, and 180 K (LS-FeII δ = E


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Inorganic Chemistry

Figure 8. Effect of lattice solvents on the magnetic properties. (Left) Magnetic data for 1a (filled triangles) and 1b (open circles); solid lines represent expectation values for the [4HS] and [2HS-2LS] configurations with g = 2.24. (Middle) Mössbauer data of 1a at 80 K. (Right) Mössbauer data of 1b at 80 K.

Table 4. Overview of the Spin States of Crystalline 1a−f at 80 K and SCO Temperature 1a at 80 K SCO

2HS2LS

1b 4HS

1c 3HS1LS 270 K

1d 3HS1LS

1e 3HS1LS 250 K

1f 2HS-2LS/3HS1LS 50−300 K

0.39 mm s−1, ΔEQ = 0.94 mm s−1; HS-FeII δ = 1.05 mm s−1, ΔEQ = 2.70 mm s−1; in 1:3 ratio), the [4HS] state at high temperatures is corroborated by a single-crystal X-ray diffraction analysis at room temperature showing four crystallographically equivalent HS-FeII ions (additional Mössbauer data Figures S4 and S5; additional magnetic data Figure S6). Influence of Counteranions on the Magnetic Properties. To single out the effect of counterions on the magnetic properties of the Fe4 grid, [Fe4LBr4]4+ was crystallized with different counteranions while all other parameters such as the solvent combination (DMF/THF) were kept the same. Four different compounds 1a, 1c, 1e, and 1f (with PF6−, ClO4−, BF4−, and Br−, respectively) could be isolated, and their structures were determined by X-ray diffraction. It is interesting to note that 1c and 1e, which have counterions of the same molecular symmetry (Td) and similar size, show similar behavior. Crystals of the isomorphous 1c and 1e undergo a phase transition upon cooling from RT, which results in destruction of the respective single crystal. The compounds have therefore been measured at RT in a sealed glass capillary containing some mother liquor. The molecular structure of 1c could be determined as well, but the crystals were of rather low quality, and parameters derived from the crystal structure will not be discussed. However, the spin cross-over temperature T1/2 shifts by 20 K from 270 K for 1c to 250 K for 1e. The compound 1f with the spherical bromide anion shows a very gradual SCO over a wide temperature range, with χMT rising from values larger than expected for the [2HS-2LS] state at low temperatures (χMT = 8.64 cm3 K mol−1 at 80 K) to beyond the [3HS-1LS] state at 300 K (χMT = 11.46 cm3 K mol−1); it should be noted that expectation values shown in Figure 9 assume g = 2.2, but g values for the HS-FeII ions may differ significantly for the series of complexes. A predominant [3HS1LS] state for 1f at 80 K is suggested by Mössbauer data (LSFeII δ = 0.38 mm s−1, ΔEQ = 0.94 mm s−1; HS-FeII δ = 1.06 mm s−1, ΔEQ = 2.70 mm s−1; 1:3 ratio) and by the X-ray crystallographic analysis performed at 133 K. Taken together there are three different magnetic signatures for the [Fe4LBr4]4+ grid depending on the counterions: a constant [2HS-2LS] state over a wide temperature range for 1a; a very gradual transition from a situation close to the [2HS-2LS] state to beyond the

Figure 9. Effects of counteranions on the magnetic properties of the Fe4 grid (obtained under the same crystallization conditions; crystals in oil): 1a (filled triangles), 1e (filled circles), 1f (open circles); solid lines represent expectation values for the [4HS], [3HS-1LS], and [2HS-2LS] configurations with g = 2.2.

[3HS-1LS] state for 1f; and a more abrupt SCO from the [3HS-1LS] to the [4HS] state for 1c and 1e (Figure 9; additional Mössbauer data in Figures S4 and S5; additional magnetic data in Figure S6). To further demonstrate the importance of the lattice solvent for well-defined spin states, SQUID and Mössbauer measurements were also performed for all compounds after air drying and careful grinding of the samples (Figures S4−S6). The general observation is that spin-state patterns of the [Fe4LBr4]4+ grid are less clear for the powder samples (i.e., the Mössbauer and SQUID data do not reveal a clear correlation with 2:2, 3:1, or 4:0 ratios of HS- and LS-FeII ions) and not as easily reproducible as they are for the pristine crystals (i.e., they depend on details of the sample treatment). However, in most cases the amount of LS-FeII is higher for the powder samples, most prominently for 1f, which shows 63% instead of 26% LSFeII. It is also interesting that air drying and grinding do not change the spin states of the PF6−-containing complexes, neither for 1b which remains in the [4HS] state nor for 1a which remains in the [2HS-2LS] state. These examples demonstrate that, as is well known for many other systems,24 crystal-packing effects related to counteranions and lattice solvent play a decisive role in determining the spin state also for the present [Fe4LBr4]4+ grid. Correlation of Structures and Magnetic Properties. The distortion of the individual {FeN6} octahedra can be described via continuous symmetry measures (CSM).17,18 For LS-FeII the values for S(Oh) are lower than 2.50, indicating a F


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temperature the [2 × 2] grid molecules contain four crystallographically equivalent HS-FeII ions arranged in an almost perfect square. As reflected by the CSM parameters the coordination polyhedron is significantly distorted from octahedral. The ligand is just slightly bent from planarity at one side, whereas the bending is more pronounced on the other side. 1f has a [3HS-1LS] configuration at 133 K with two HS-FeII related by symmetry and two crystallographically distinct FeII ions, one in the HS and the other in the LS state. The four metal centers are arranged as a rhomb (Fe−Fe−Fe angles 75° and ∼105°). While one HS-FeII has a moderately distorted octahedral environment (S(Oh) 5.91) the other two are severely distorted toward an ideal trigonal prism (itp) situation (S(Oh) 8.97). All ligands are highly twisted away from planarity. 1d contains eight crystallographically distinct FeII ions in two independent cations, in both cases three of them in the HS state and one in the LS state. Unlike 1f, which has the same spin configuration, the metal centers are arranged almost in a square. The HS FeII ions have S(Oh) values between 4.57 and 7.96 and therefore coordination polyhedra in between octahedral and itp. Two of the ligands are staggered with a Br−C−C−Br torsion angle of 16.5°/17.9°, the other two are more eclipsed (5.5°/3.2°). The unit cell of 1b contains two grid molecules, each with four crystallographically distinct FeII ions; structural parameters of the two grids are nearly identical; values are listed in Table 5. The HS-FeII have S(Oh) values around 7 and are therefore somewhere in between octahedral and itp. The ligands are twisted at angles ranging from 12.8° to 14.6°. While all other grids of the present series contain at least three HS-FeII, 1a (Figure 12) is an exception. The complex features two LS-FeII ions and two HS-FeII ions at opposite corners of the grid. The HS-FeII ions are highly distorted (S(Oh) around 8) resulting in coordination polyhedra closer to itp than an octahedron. The ligands are not planar but severely curved; in each cation two of them are staggered (Br−C−C−Br torsion angle of 12.3), while the other two are almost eclipsed (6.2°). As already discussed above, the recently reported complex [Fe4LMe4](BF4)4 shows a similar magnetic behavior as 1a.12 Also, the solid state structure shows some analogy as the Fe− Fe−Fe angles and the Fe−Fe distances are roughly the same (Figure 13). Figure 14 shows a graphical representation of the S(Oh) values for the series of [Fe4LBr4]4+ grid complexes, including [Fe4LMe4]4+. There are each two compounds with [4HS], [3HS-1LS], and [2HS-2LS] configuration, representing a solid basis for correlating structures and spin states. Two features are evident: (i) S(Oh) values for all HS-FeII (red and green; mostly in the range 6.5−7.2 for the [4HS] cases) are much higher than for the LS-FeII (blue; between 2.1 and 2.3), as expected; (ii) upon SCO at a single grid corner, S(Oh) for the opposite HSFeII (trans position) decreases significantly (green; 4.5/5.7), while S(Oh) for the neighboring HS-FeII (cis position with respect to the LS-FeII) increases (up to 9.0 in some cases); (iii) in the [2HS-2LS] cases the LS-FeII are trans-disposed and S(Oh) values for the HS-FeII sites are all very high (>8). These correlations provide clear experimental evidence that mechanical coupling and elastic communication via the bridging ligands is strong for the class of pyrazolate-based [Fe4LR4]4+ grids, as has been seen before for dinuclear SCO systems.25 Structural reorganization after the first SCO directs the second SCO to

close to ideal octahedral symmetry, as expected for a low-spin d6 ion. S(Oh) in the case of HS-FeII ranges from 4.57 to 8.96 and reflects a much higher deviation from octahedral symmetry (Table 5).19 The four ferrous ions of complexes 1a and 1f are Table 5. Structural Parameters of All Grid Complexes 1a−fa complex

Fe atom

1a

1, 1′ 2, 2′ 1 2 3 4 11d 12d 13d 14d 1 2 3 4 5d 6d 7d 8d 1, 1′, 1″, 1‴ 1 2, 2′ 3

1b

1d

1ee 1f

dmean (Å)

spin state

S(Oh)b

S(itp)b

1.97 2.20 2.19 2.19 2.19 2.19 2.18 2.19 2.18 2.19 2.20 2.15 2.18 1.97 1.96 2.20 2.09 2.20 2.19

LS HS HS HS HS HS HS HS HS HS HS HS HS LS LS HS HS HS HS

2.23 8.09 7.09 6.73 6.80 6.99 7.11 6.97 6.37 7.19 6.56 5.70 6.31 2.19 2.12 7.96 4.57 7.08 6.70

10.51 4.89 6.10 6.60 6.28 6.01 5.62 6.66 6.87 5.92 6.94 7.23 6.93 9.95 10.39 5.80 7.98 5.95 6.34

1.97 2.20 2.18

LS HS HS

2.26 8.97 5.91

11.10 4.56 7.73

torsion (deg) Br1− C1−C2−Br2c 6.2/12.3 14.0/14.6

14.4/12.8

5.5/16.5

3.2/17.9

10.8 5.2/5.2

a

Mean Fe−N distances, CSM values, and torsion angle of the ligands (see Figure 11). bReference 17 and 18, itp = ideal trigonal prism. cSee Figure 10. dSecond crystallographically independent cation if present. e Measured at ambient temperature.

almost perfectly located within a plane. For the two complexes 1d and 1e the average distance of the FeII ions from the mean Fe4 plane is 0.23 Å, while 1b exhibits a larger average distance of 0.34 Å, and its four FeII ions adopt butterfly-like arrangement (Figure 10). To describe the overall distortion of the individual

Figure 10. (Left) Butterfly-shaped Fe4 arrangement of 1b. (Right) Views of the pyrazolate rings of parallel ligand strands from two different viewpoints to determine the different degree of staggering that is reflected by the torsion angles Br1−C1−C2−Br2.

ligands strands, the angle between the planes defined by the pyrazolate ring and by the terminal pyridine rings has been determined (gray values in Figure 11). Furthermore, to describe the rotation of parallel ligand strands in a grid with respect to each other, the torsion angles Br1−C1−C2−Br2 have been considered which reflect the mutual rotation of parallel pyrazolate rings away from an eclipsed configuration (Figure 10). Due to the physical properties of the single crystals of 1e the X-ray data were collected at ambient temperature. At this G


Article

Inorganic Chemistry

Figure 11. Fe···Fe distances, Fe−Fe−Fe angles (black), and angle of the plane of the terminal pyridine ring of each ligand with the plane of the pyrazolate ring (gray): HS-FeII in red, LS-FeII in blue.

Figure 13. Comparison of the solid state structure of 1a with that of the recently reported grid complex [Fe4LMe4](BF4)4.11

compartmental proligand HLBr now allowed us to significantly expand the family of pyrazolate-based [2 × 2] FeII4 grid compounds. Solution studies confirmed that the new complex [Fe4LBr4]4+ is rugged and adopts an intrinsic dimixed-spin [2HS-2LS] configuration in the absence of crystal-packing effects and that the backbone bromine substituent has the predictable (based on the −I effect of the bromine) effect on oxidation potentials. Most importantly, six all-ferrous compounds [Fe4LBr4]X4 could be structurally and magnetically characterized, with the same [2 × 2] FeII4 grid but different counteranions and different solvent molecules in the crystal lattice. This allowed us to establish an experimentally wellgrounded correlation of structural parameters and spin-states pattern in such tetranuclear metal ion clusters with compact and well-defined topology. The results are in excellent agreement with recent quantum chemical calculations15,16 and confirm a high degree of cooperativity for the multistep SCO

Figure 12. Molecular structure of 1a with all pyrazolates perpendicular to the readers view (counterions, hydrogen atoms, and solvent molecules omitted for clarity). Ferrous ions are located at the corners of a rhomb, and all ligands are highly curved.

occur at the trans position, and it even facilitates a second SCO to occur. In the resulting [2HS-2LS] form the sites are then locked and further SCO is hampered. Distortion from octahedral shape, reflected by the S(Oh) values, is a crucial factor governing the spin-state configuration. On the other hand, correlations with the bending and loss of planarity of the individual [LR]− ligands strands are much less obvious.

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CONCLUSIONS Building upon the experience gained with the previous [Fe4LH4](BF4)4 and [Fe4LMe4](BF4)4 grids,10,11 the new H


Article

Inorganic Chemistry

Figure 14. Graphical representation of the S(Oh) values for the series of [Fe4LBr4]4+ grid complexes, including [Fe4LMe4]4+. The xy planes depict the positions of the iron atoms in the grids: HS-FeII in red and green, LS-FeII in blue. 57

Co source in a Rh matrix using an alternating constant acceleration Wissel Mössbauer spectrometer operated in the transmission mode and equipped with a Janis closed-cycle helium cryostat. Isomer shifts are given relative to iron metal at ambient temperature. Simulation of the experimental data was performed with the Mfit program.26 Temperature-dependent magnetic susceptibilities were measured by using a SQUID magnetometer (Quantum Design MPMS XL-5). Analysis of the experimental data was performed with the julX program.26 Around 10% error margin for the experimental χMT values is estimated to result from potential partial loss of lattice solvent during sample preparation for the SQUID measurements and because isolation of the material from the mother liquor and sample preparation had to be done rapidly in some cases to prevent as much as possible the loss of lattice solvent. While no problems were encountered during this work, perchlorate salts are potentially explosive and should be handled with proper precautions. Bromine generates highly toxic and corrosive vapors of high density. Working in a properly f unctioning fume hood is advised. HLBr. HLH (1.00 g, 2.66 mmol) was dissolved in DCM (25 mL), and aqueous Na2CO3 (25 mL, 10%) was added. The mixture was cooled to 0 °C and stirred vigorously, and then bromine (0.5 mL, 9.76 mmol) was added in one go. After 30 min a yellow precipitate had formed and was collected on a fine fritted glass filter, washed with water (5 × 50 mL), and dried in vacuo to give 1.12 g (93%) of the product. 1H NMR (300 MHz), CDCl3: δ (ppm) = 7.40 (ddd, J = 7.6, 4.8, 1.2 Hz, bpy-5′-H, 2 H), 7.97 (td, J = 7.9, 2.0 Hz, bpy-4′H, 2 H), 8.03 (t, J = 7.9 Hz, bpy-4H, 2H), 8.48 (dd, J = 7.8 Hz, 1.0 Hz, bpy-5H, 2H), 8.65 (dd, J = 7.8, 1.0 Hz, bpy-3H, 2H), 8.70−8.76 (m, bpy-6′H, 2H), 8.85 (d, J = 8.0 Hz, bpy-3′H, 2H). 13C NMR (75 MHz), CDCl3: δ (ppm) = 121.8 (bpy-3′C), 123.2 (bpy-3C), 124.2 (bpy-5′C), 124.5 (bpy-5C), 137.2 (bpy-4′C), 137.7 (bpy-4C), 145.9 (bpy-2C), 149.2 (bpy-6′C), 155.6 (bpy-2′C), 155.7 (bpy-6C), 170.0 (py-2C). IR (KBr, cm−1): 2954, 2924, 2854, 1580, 1560, 1460, 1431, 1096, 833, 767, 740, 638. ESI-MS (m/z): [M + H]+ 455.3 (100%); [M + Na]+ 477.3 (22%). UV−vis (MeCN): λmax (εM, L mol−1 cm−1): 236 (13 520), 266 (11 320), 304 (9180), 366 (1160) nm. [Fe4LBr4](ClO4)4. HLBr (200 mg, 0.440 mmol) and NaOtBu (84.0 mg, 0.875 mmol) were dissolved in MeOH (7 mL). After stirring for 10 min the solution was slowly added to a solution of Fe(ClO4)2· 6H2O (162 mg, 0.446 mmol, dried in vacuo for 1 h) in MeOH (3 mL). The mixture was stirred overnight and then filtered over Celite. The crude product was washed with MeOH (3 × 20 mL) and extracted with MeCN (150 mL). The solvent was removed under

transitions within the grid, imparted by the strain effects of the rigid bridging ligands. In particular, the first SCO transition at one corner facilitates the second SCO at the opposite corner, but further SCO steps are severely hampered. The distortion of the {FeN6} polyhedra from octahedral symmetry, reflected by the S(Oh) values, is a key parameter, and evolution of the S(Oh) values indicates strong elastic coupling mediated by this type of compartmental pyrazolate ligand. In essence this leads to a high stabilization of a dimixed-spin configuration [HS-LS-HS-LS]. It remains to be seen whether the strong structural determinants are detrimental for SCO switching between the two degenerate spin-state configurations of this trans-[2HS-2LS] form (Figure 1).

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EXPERIMENTAL SECTION

Methods and Materials. All syntheses of the metal complexes were performed under an inert argon atmosphere using standard Schlenk techniques, but salt metathesis reactions and crystallization experiments were carried out under ambient conditions with HPLCgrade solvents, and crystallization was done under ambient conditions without exclusion of air. The ligand 3,5-bis{6-(2,2′-bipyridyl)}pyrazol (HLH)13 was synthesized as previously described. Solvents were dried according to established procedures and deoxygenated by purging with argon for 1 h. Diatomaceous earth (Celite 545) was purchased from Acros Organics and dried by heating. All other reagents were obtained from commercial vendors and used without further purification. Mass spectrometry was performed with a Bruker HCT ultra (ESI). UV−vis spectra were recorded with a Varian Cary 5000. Elemental analyses were measured by the Analytical Laboratory of the Institute of Inorganic Chemistry at the Georg-August-University Göttingen using an Elementar Vario EL III instrument. IR spectra of solid samples were recorded using a Cary 630 FTIR spectrometer equipped with DialPath and Diamond ATR accessory (Agilent) placed in a glovebox (MBRAUN UNIlab, argon atmosphere). NMR spectra were recorded on Bruker Avance 200 and 300 MHz spectrometers and referenced to the residual solvent signal: δ(CDCl3) = 7.24 (1H) and 77.1 (13C) ppm. Cyclic voltammetry was performed at room temperature with a potentiostat/galvanostat PerkinElmer model 263A with a glassy carbon working electrode and platinum reference and counter electrodes in MeCN/0.1 M NBu4PF6. Decamethylferrocene was used as internal standard. Mössbauer spectra were recorded with a I


Article

Inorganic Chemistry Author Contributions

reduced pressure, giving 234 mg (87%) of the dark red complex. Slow diffusion of THF (for 1c) in a solution of this complex dissolved in DMF gave dark red crystals suitable for X-ray analysis. A solution of the crude product in DMF layered with DME (for 1d) gave dark red crystals with different properties in the solid state. IR (KBr, cm−1): 1596, 1574, 1564, 1514, 1456, 1437, 1384, 1308, 1122, 1109, 998, 779, 701, 636, 626. ESI-MS (m/z): [Fe4LBr4]4+ 510.1 (100%). UV−vis (MeCN): λmax (εM, L mol−1 cm−1): 242 (80 100), 272 (60 700), 331 ( 5 0 40 0) , 5 8 7 ( 5 7 0 0 ) nm . F o r 1 c , A n a l. C al c d f o r C92H56Br4Cl4Fe4N24O16·C4H8O·4H2O: C, 44.65; H, 2.81; N, 13.02. Found: C, 44.87; H, 2.53; N, 12.66. [Fe4LBr4](BF4)4. HLBr (200 mg, 0.440 mmol) and NaOtBu (84.0 mg, 0.875 mmol) were dissolved in MeOH (5 mL). After stirring for 10 min the solution was slowly added to a solution of Fe(BF44)2· 6H2O (163 mg, 0.482 mmol, dried in vacuo for 1 h) in MeOH (5 mL). The mixture was stirred for 4 days and then filtered over Celite. The crude product was washed with MeOH (3 × 20 mL) and extracted with MeCN (150 mL). The solvent was removed in vacuo, giving 226 mg (86%) of the dark red complex. Slow diffusion of THF in a solution of this complex dissolved in MeOH gave dark red crystals of 1e. Because of the high BF4− content of [Fe4LBr4](BF4)4, no satisfactory CHN elemental analysis could be obtained. Crystalline material was used for X-ray diffraction, SQUID, and Mössbauer measurements. [Fe4LBr4](PF6)4. KPF6 (3.00g, 16.3 mmol) in water (50 mL) was given to a solution of [Fe4LBr4](ClO4)4 (200 mg, 81.9 μmol) in MeCN (10 mL). The mixture was cooled to 0 °C for 1 h. The precipitate was collected on a fine fritted glass filter. Slow diffusion of THF (for 1a) or MTBE (for 1b) into a solution of the complex in DMF gave dark red crystals. Due to the partial loss of lattice solvent the crystals rapidly crumble after separation from the mother liquor. The composition of the air-dried compounds was determined by elemental analyses. For 1a: Anal. Calcd for C92H56Br4F24Fe4N24P4 (2620.4): C, 42.17; H, 2.15; N, 12.83. Found: C, 41.35; H, 2.38; N, 12.63. The thermogravimetric analysis (TGA) of air-dried 1a confirms the absence of any solvent molecules, as the mass loss over the temperature range 30−200 °C is negligible (ca. 0.1%; the calculated mass difference for loss of one water molecule is 0.7%). For 1b: anal. calcd for C92H56Br4F24Fe4N24P4· 2C3H7NO (2766.3): C, 42.54; H, 2.55; N, 13.16. Found: C, 42.54; H, 2.51; N, 13.23. [Fe4LBr4]Br4. A solution of KBr in MeOH (100 mL, satd) was given to a solution of [Fe4LBr4](ClO4)4 (200 mg, 81.9 μmol) in MeCN (10 mL). After stirring for 1 h the solvent was removed in vacuo. The bromide complex was extracted with small portions of MeOH (5 mL) until the residue was colorless. The solvent was removed in vacuo, and the complex was extracted with MeCN (3 × 5 mL). After removing the solvent in vacuo dark red crystals of 1f suitable for X-ray diffraction could be obtained by slow diffusion of THF in a solution of the compound in DMF. Because of the high bromine content (27.08%) of [Fe4LBr4]Br4, no satisfactory CHN elemental analysis could be obtained. Crystalline material was used for X-ray diffraction, SQUID, and Mössbauer measurements.

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This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft (SFB 1073, project B06) is gratefully acknowledged.

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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.5b02762. Further electrochemical, spectroscopic, magnetic, and crystallographic details (PDF) X-ray crystallographic data in CIF format (CIF)

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REFERENCES

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