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Multiple Bistability in Quinonoid-Bridged Diiron(II) Complexes: Influence of Bridge Symmetry on Bistable Properties Margarethe van der Meer,† Yvonne Rechkemmer,‡ Frauke D. Breitgoff,‡ Raphael Marx,‡ Petr Neugebauer,‡ Uta Frank,† Joris van Slageren,*,‡ and Biprajit Sarkar*,† †

Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34-36, D-14195 Berlin, Germany Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany

‡

S Supporting Information *

ABSTRACT: Quinonoid bridges are well-suited for generating dinuclear assemblies that might display various bistable properties. In this contribution we present two diiron(II) complexes where the iron(II) centers are either bridged by the doubly deprotonated form of a symmetrically substituted quinonoid bridge, 2,5-bis[4-(isopropyl)anilino]-1,4-benzoquinone (H2L2′) with a [O,N,O,N] donor set, or with the doubly deprotonated form of an unsymmetrically substituted quinonoid bridge, 2-[4-(isopropyl)anilino]-5hydroxy-1,4-benzoquinone (H2L5′) with a [O,O,O,N] donor set. Both complexes display temperature-induced spin crossover (SCO). The nature of the SCO is strongly dependent on the bridging ligand, with only the complex with the [O,O,O,N] donor set displaying a prominent hysteresis loop of about 55 K. Importantly, only the latter complex also shows a pronounced lightinduced spin state change. Furthermore, both complexes can be oxidized to the mixed-valent iron(II)−iron(III) form, and the nature of the bridge determines the Robin and Day classification of these forms. Both complexes have been probed by a battery of electrochemical, spectroscopic, and magnetic methods, and this combined approach is used to shed light on the electronic structures of the complexes and on bistability. The results presented here thus show the potential of using the relatively new class of unsymmetrically substituted bridging quinonoid ligands for generating intriguing bistable properties and for performing sitespecific magnetic switching.

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INTRODUCTION Quinonoid bridges have often been successfully used for generating di- or polynuclear assemblies that often display magnetic/optical bistability1a−t and other intriguing properties such as luminescence and cytotoxicity.1u−y Spin crossover (SCO),1m valence tautomerism (VT),1h,i and mixed-valent systems1a,q are prominent examples of bistable behavior. On combination of the inherent redox and photoactive nature of the quinonoid bridges with metal centers that can display magnetic bistability, designing systems displaying multiple bistability as a response to a variety of external perturbations should be possible. Despite this possibility, the majority of quinonoid-bridged metal complexes containing first-row transition metals have usually been investigated for either SCO or VT, but not for both.1 Investigations of the mixed-valent properties in quinonoidbridged diiron complexes were recently reported.2 However, in that case, the metal centers do not display SCO. A survey of the bridging quinone ligands used for these purposes reveals that a majority of the bridges used to date contain a symmetrically substituted bridge that have [O,O,O,O],1 [O,N,O,N],3−5 or [N,N,N,N]3,4g,6 donor sets (Figure 1). While ligands such as the doubly deprotonated form of H2L1 have been popular for some decades,1 only recent years have seen the extensive use of the doubly deprotonated forms of the ligands H2L2, H2L3, and H2L4 for generating metal complexes that display a set of very intriguing properties.3 H2L2, H2L3, and H2L4 are related to H2L1 through the [NR] for [O] isoelectronic substitution. © XXXX American Chemical Society

Figure 1. Examples of symmetrically and asymmetrically substituted quinonoid ligands.

Obvious advantages of using the ligands containing [NR] donors are the easy tuning of the steric and electronic properties of the ligands and in turn of that of their metal complexes through the R substituents on the nitrogen donor.3 Iron is arguably the most interesting metal center for investigating magnetic bi- and polystability in polynuclear complexes.7 However, to the best of our knowledge, there are no examples of diiron complexes known with the doubly deprotonated form of ligands such as H2L5 that contain an unsymmetrically substituted [O,O,O,N]4k,s,8 donor set.3 This is surprising because the asymmetric nature of such a bridge should allow for the site-selective perturbation of the spin sites. The Received: August 30, 2016

A

DOI: 10.1021/acs.inorgchem.6b02097 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. ORTEP views of [1](OTf)2 (left) and [2](OTf)2 (right). Hydrogen atoms, counteranions, and solvent molecules are removed for clarity.

(Table S1 in the Supporting Information). In [1](OTf)2 each of the iron centers is coordinated to four N donors from the tmpa ligand and an O and an N donor from the bridging (L2′)2− ligand (Figure 2). For [2](OTf)2 containing the unsymmetrically substituted bridging ligand (L5′)2−, the Fe1 is coordinated to the four N donors of the tmpa ligand and through the N and O donors of the bridging ligand. Fe2, on the other hand, is coordinated through four N donors of the tmpa ligand and through the two O donors of the (L5′)2− bridge. A look at the bond lengths within the (L2′)2− bridge in [1](OTf)2 shows bond localization in the “upper” and “lower” parts of the bridge. This is apparent from the C1−C3 and C2− C3 distances of 1.417(6) and 1.360(6) Å, respectively (Figure 2 and Table S2 in the Supporting Information). Accordingly, the C1−N1 bond length is relatively short and the C2−O1 bond is relatively long. The “upper” and “lower” parts of the bridging ligand are connected through the C1−C2 bonds, the distances of which fall in the range of a single bond (Table S2). The coordinating heteroatoms of the bridge are thus best described as “alkoxide-type O−” and neutral “imine-type N” donors.3 A similar localized bonding situation is also observed for the unsymmetrically substituted bridging ligand (L5′)2− in [2](OTf)2 (Table S2). The coordinating heteroatoms of that bridge are thus best described as two “alkoxide-type O−” donors (O1 and O3), one “imine-type” neutral N donor (N1) and one “ketotype” neutral O donor (O3).3 For [1](OTf)2, at the measured temperature of 140 K, all the Fe−N and Fe−O bond lengths fall between typical values known for high-spin (HS) and low-spin (LS) iron(II) centers (Table S2).7 The average iron−ligand bond distance is 2.1 Å (Table S2), and the value of ∑7c that describes the sum of absolute deviations of all angles around an octahedral coordinated metal center from the ideal 90° is 122° (Table S3 in the Supporting Information). For [2](OTf)2, the measurements for which were carried out at 100 K, some interesting differences are observed in the iron−ligand bond lengths between the two iron(II) centers. For the Fe1 center which is bound through the O1 and N1 donors of the (L5′)2− bridge, all the iron−ligand bond lengths are significantly shorter in comparison to the bond lengths at the Fe2 center, which is bound through the O2 and O3 donors of the (L5′)2− bridging ligand (Figure 2 and Table S2). This trend is apparent, for instance, by looking at the Fe1−N2 bond length of 2.008(6) Å and the Fe2−N3 bond length of 2.298(6) Å to the amine donors of the two tmpa ligands. The average distance of all the bonds around the Fe1 center is 1.963(6) Å and that of the bonds around

asymmetric substitution pattern might also help to induce hysteresis during SCO by preferential locking of a particular spin state. Hysteresis is a property that is highly sought after for the generation of future data storage devices.9 Additionally, the redox-active nature of these bridges and their metal complexes should allow for the generation of mixed-valent states and the investigation of electronic bistability in the mixed-valent forms.3 Bridges such as (L5) 2− can also be used to compare electrochemical and electronic coupling in mixed-valent states with symmetrically substituted bridges such as (L2)2−. In the following, we present the syntheses of the complexes [1](OTf)2 and [2](OTf)2 (Figure 1) that contain either a symmetrically substituted or an unsymmetrically substituted quinonoid bridging ligand and tris(2-methylpyridyl)amine (tmpa) as stopper ligands. We use a combination of singlecrystal X-ray diffraction, (spectro)electrochemical measurements, magnetometry, and EPR spectroscopy to probe multiple bistability in these systems. Furthermore, we show the effects that the symmetry of a quinone bridge can have on the electrochemical, electronic, and magnetic coupling as well as on different bistable properties in these systems.

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RESULTS AND DISCUSSION

Synthesis and Crystal Structures. The ligands H2L2′ and H2L5′ were synthesized by the reaction of 2,5-dihydroxy-1,4benzoquinone with 4-isopropylaniline in acetic acid under reflux (see Experimental Section).4s Chromatographic purification allowed isolation of both ligands in pure form from the same reaction mixture. The complexes [1](OTf)2 and [2](OTf)2 were synthesized by the reaction of Fe(OTf)2·6H2O, tmpa and the respective ligands in acetonitrile in the presence of NEt3 as a base. Purification of the crude reaction mixtures led to the isolation of the metal complexes in the pure form in reasonable yield (see Experimental Section). The respective molecular peaks were clearly identified in the mass spectra, and elemental analysis data further showed the purity of these complexes. Final proof of the structures came from single-crystal X-ray diffraction studies, and all further physical studies of these complexes were performed on microcrystalline material. Single crystals of [1](OTf)2 as acetonitrile solvates were obtained by slow diffusion of diethyl ether into an acetonitrile/ toluene solution, and those of [2](OTf)2 were obtained as dichloromethane solvates by layering a dichloromethane solution with n-hexane. [1](OTf)2 crystallizes in the monoclinic P21/n space group and [2](OTf)2 in the triclinic P1̅ space group B


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Inorganic Chemistry the Fe2 center is 2.151(6) Å (Table S2). The angles around the Fe1 center are close to the ideal 90° expected for an ideal octahedron, and those around the Fe2 center are more distorted (Table S3). The ∑ values around the Fe1 and Fe2 centers are 74.1 and 145.9°, respectively. The bond lengths and bond angles around the iron centers in [2](OTf)2 thus point to the existence of a LS iron(II) center at Fe1 and a HS iron(II) center at Fe2 (filling of antibonding orbitals for HS iron(II) centers but not for LS iron(II) centers in a simplified octahedral model; Figure 2).7 Thus, the unsymmetrically substituted bridging ligand (L5′)2− that has two different kinds of coordination sites is capable of stabilizing two different spin states (see Magnetic Properties) of otherwise identical metal centers at the same temperature. Interestingly, the bond lengths around both the equivalent iron centers in [1](OTf)2 (Figure 2) fall between the bond lengths observed for the Fe1 and Fe2 centers in [2](OTf)2, thus further displaying that the bonding parameters observed for the iron(II) centers in [1](OTf)2 are between those typically known for a HS and a LS iron(II) center. The intramolecular iron−iron distance is 7.922(6) Å in [1](OTf)2 and 7.843(5) Å in [2](OTf)2. The smaller metal−metal distance in the latter is probably a consequence of the difference in the bridging ligand, as well as the existence of a LS iron(II) center in [2](OTf)2. Electrochemistry and Spectroelectrochemistry. In view of the presence of two redox-active iron(II) centers as well as redox-active bridging ligands in the two complexes, we carried out electrochemical and spectroelectrochemical measurements on them. Both [1]2+ and [2]2+ display two one-electron oxidation steps and one one-electron reduction step in CH2Cl2/0.1 M Bu4NPF6 (Figure 3 and Table 1).

Table 1. Standard Potentials from Cyclic Voltammetry 2+

[1] [2]2+

E1/2ox1/Va

E1/2ox2/Va

ΔE/Vd

K/107 c

E1/2red/V

−0.18 −0.11

0.24 0.42

0.42 0.53

1.7 129.2

−1.71b −1.43

a

Half-wave potentials from cyclic voltammetric measurements in MeCN or CH2Cl2, 0.1 M Bu4NPF6 for reversible processes at 298 K, with scan rate 100 mV s−1. Ferrocene/ferrocenium was used as an internal standard. bEpa for an irreversible process. cRT ln K = nF(ΔE).4 d ΔE = Eox1 − Eox2.

109 for the one-electron-oxidized forms of [1]2+ and [2]2+, respectively (Table 1).10 Hence, it is seen that the thermodynamic stability of the one-electron-oxidized form as exemplified by the Kc values is 2 orders of magnitude higher for [2]3+, which contains an unsymmetrically substituted bridging ligand, in comparison to [1]3+, which has a symmetrically substituted bridging ligand. This phenomenon is likely related to a better stabilization of the iron(III) form in the O,O coordination pocket of the unsymmetrically substituted (L5′)2− bridge in [2]3+. Thus, the coordination asymmetry in the (L5′)2− bridge seems to provide a larger electrochemical coupling in the mixed-valent form [2]3+ in comparison to [1]3+, where the bridge is a symmetrical one.10l All things being equal, the unsymmetrical bridge appears to be better at the thermodynamic stabilization of the one-electron-oxidized form in comparison to the symmetrical bridge. However, as has been discussed elsewhere, Kc values are not always good measures of electronic coupling in mixed-valent systems (see below).10 Even though the symmetry of the bridge certainly plays a role in influencing the aforementioned properties, it should also be mentioned that the conversion of “O” to “N-[4-isopropyl(phenyl)]” while moving from (L2′)2− to (L5′)2− is also likely to influence the solvation properties of their respective metal complexes. The complexes [1]2+ and [2]2+ display a one-electronreduction step at −1.71 and −1.43 V, respectively. For dinuclear complexes containing such bridging ligands, there is precedence in the literature for predominantly bridging ligand centered reductions.3 For the present case, the reduction steps are likely bridge-centered as well. The easier reduction for [2]2+ in comparison to [1]2+ can be explained by the presence of a higher number of oxygen atoms (in comparison to nitrogen atoms) in the (L5′)2− bridge in [2]2+ in comparison to the (L2′)2− bridge in [1]2+. UV−vis−NIR spectroelectrochemical measurements were carried out on both complexes to shed further light on the electronic structure of these metal complexes in their various redox states. [1]2+ displays a strong absorption band at 360 nm with an extinction coefficient of 9400 M−1 cm−1 (Figure 4 and Table 2). We tentatively assign this band to a metal to ligand charge transfer (MLCT) transition. Additionally, a weak low-energy band is observed at around 800 nm. Such weak low-energy bands are also observed for the free quinone ligands, and hence we assign this absorption to an intraligand transition within the (L2′)2− ligand.3 The absorption features observed for [2]2+ are similar to those observed for [1]2+ (Figure 4 and Table 2). On the spectroelectrochemical time scale the one-electron reduction of [1]2+ turned out to be reversible, but not that of [2]2+. On reduction to [1]+, the aforementioned MLCT band loses intensity. Additionally, a new low-wavelength band appears at around 1200 nm (Figure 4). Such low-wavelength bands are found typically for metal-bound organic radicals of the type (L2′)3•−.3 The UV−vis−NIR data and their comparison with

Figure 3. Cyclic voltammograms of [1]2+ (top) and [2]2+ (bottom). Conditions: CH2Cl2, 0.1 M Bu4NPF6, room temperature, scan rate 100 mV s−1.

The first oxidation step for both complexes appears at comparable potentials: −0.18 V for [1]2+ and −0.11 V for [2]2+. These comparable values are probably a first indication of a predominantly iron centered first oxidation for both complexes. The potential for the second oxidation step for [1]2+ is 0.24 V, and that for [2]2+ is 0.42 V. The difference in the potentials between the first and the second oxidation steps are thus 420 and 530 mV for [1]2+ and [2]2+, respectively. These values translate into comproportionation constants (Kc) of 1.7 × 107 and 1.3 × C


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Inorganic Chemistry

Figure 5. Changes in the UV−vis−NIR spectrum of [2]2+ during in situ spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6 at room temperature: (top) first oxidation; (bottom) second oxidation.

with a cutoff observed in the high-energy region (Figure 4 and Figure S1 in the Supporting Information). The Γ value that is used to classify mixed-valent systems is 0.51 for [1]3+.13 All the above data point to a borderline class II/class III classification and a tendency for delocalization of the mixed-valent [1]3+ form with the symmetrically substituted bridging quinone ligand. For [2]3+, a NIR band of IVCT nature is observed at 1615 nm with an extinction coefficient of only 600 M−1 cm−1. The experimental bandwidth at half-height of this band is 2537 cm−1, a value that is close to the theoretically calculated value of 3793 cm−1 according to the Hush formulation. This band is symmetrical, as can be seen from Figure 5 and Figure S1. The Γ value for [2]3+ is 0.33, and accordingly the mixed-valent form [2]3+ can be classified as class II in the Robin and Day classification scheme. As would be expected for mixed-valent compounds, on further oxidation of the complexes either to [1]4+ or to [2]4+, the aforementioned IVCT bands disappear.10 Thus, it is seen that the symmetry of the bridge has a decisive influence on the positions, shapes, and intensities of the IVCT bands. Furthermore, the bridging ligand also dictates the extent of delocalization in these mixed-valent forms. The symmetric bridge in [1]3+ leads to a more delocalized situation, whereas the asymmetric bridge in [2]3+ leads to a localized class II case. It should be noted here that although electrochemical data deliver a Kc value for [2]3+ that is about 2 orders of magnitude higher than that for [1]3+, analysis of the IVCT bands clearly shows that electronic coupling is stronger in [1]3+ in comparison to [2]3+. Magnetic Properties. The magnetic properties and potential bistability of compounds [1](OTf)2 and [2](OTf)2 were probed by temperature-dependent magnetic susceptibility measurements. Figure 6 shows the temperature dependence of the product of the molar paramagnetic susceptibility χ and the temperature T for [1](OTf)2, in both the heating and cooling modes (1 K min−1). At 300 K, χT adopts a value of 6.91 cm3 mol−1 K, consistent with the presence of two S = 2 iron(II) centers with g = 2.15. With decreasing temperature χT remains essentially constant until ca. 250 K, below which a rapid decrease is observed, leading to a minimum χT value of 4.55 cm3 mol−1 K at 130 K. We attribute this decrease to spin crossover. However, for complete conversion to the LS-LS species χT is expected to reach 0, showing that in [1](OTf)2 some of the iron(II) centers

Figure 4. Changes in the UV−vis−NIR spectrum of [1]2+ during in situ spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6 at room temperature: (top) first reduction; (middle) first oxidation; (bottom) second oxidation.

Table 2. UV−Vis−NIR Data of the Complexes in Various Redox Formsa λ/nm (ε/103 M−1 cm−1) +

[1] [1]2+ [1]3+ [1]4+ [2]2+ [2]3+ [2]4+ a

255 (15.4); 360 (7.3); 480 (sh); 1220 (0.4) 255 (13.5); 360 (9.4); 800 (sh) 260 (17.7); 365 (7.5); 430 (sh); 690 (sh); 865 (5.8) LMCT; 1775 (2.8) IVCT 240 (17.5); 325 (10.3); 500 (6.6); 825 (sh) 225 (19.9); 255 (22.1); 340 (15.4); 760 (2.4) 225 (21.8); 255 (23.7); 350 (12.2); 825 (5.0) LMCT; 1615 (0.6) IVCT 225 (15.4); 310 (9.1); 530 (sh); 860 (0.6)

From OTTLE11 spectroelectrochemistry in CH2Cl2, 0.1 M Bu4NPF6.

previous reports of related compounds thus deliver the best formulation of the one-electron-reduced complex to be [(tmpa)FeII(L2′)3•−FeII(tmpa)]•+. On one-electron oxidation both complexes display similar trends in the vis−NIR region. The MLCT band loses in intensity in both cases (Figures 4 and 5 and Table 2). Additionally, a new band appears at 865 nm for [1]3+ and at 825 nm for [2]3+. Both of these bands have similar intensities. We tentatively assign these bands to a ligand to metal charge transfer (LMCT) transition for the mixed-valent forms. The most interesting aspect for the mixed-valent forms is observed at further lower energies in the NIR region. As for [1]3+, which contains a symmetrically substituted bridging ligand, a band is observed at 1775 nm with an extinction coefficient of 2800 M−1 cm−1, which we assign to an intervalence charge transfer (IVCT) transition for the mixedvalent form. The experimental bandwidth at half-height for this band is 1758 cm−1. This value is much lower than the theoretical value of 3609 cm−1 calculated according to the Hush formulation.12 Additionally, the shape of the band is asymmetric D


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Inorganic Chemistry

coupling between the two S = 2 iron centers, Di and Ei are the individual axial and rhombic zero-field splitting parameters, respectively, and gi are the respective g values. To avoid overparametrization, the same single-ion parameters were assumed for both the iron(II) centers and the exchange coupling as well as the g values were taken as isotropic. The maximum of χT as well as the magnetization data were best reproduced by assuming a molar fraction of the trapped HS-HS species of x(HSHS,trapped) = 0.60 ± 0.01 and the remaining sample being in the LS-LS state at low temperatures. With these parameters kept fixed, the spin-crossover behavior was modeled by applying an Ising-like model14−17 and assuming two subsequent SCO processes, the first being the conversion from the LS-LS to the mixed LS-HS form and the second corresponding to the final LSHS to HS-HS conversion. For the first process, the best simulation was obtained with a transition temperature of T1/2 = 195 ± 5 K, an effective energy gap of Δeff = 1500 ± 100 K, and a cooperativity parameter of Jcoop = 70 ± 30 K. The second process shows slight thermal hysteresis, which was accounted for by different transition temperatures for the cooling and heating modes. The best agreement between experiment and simulation was achieved with T1/2(cooling) = 230 ± 2 K, T1/2(heating) = 235 ± 1 K, Δeff = 1900 ± 300 K, and Jcoop = 200 ± 10 K. These parameters lie in the typical range observed for iron-based SCO compounds.4p,16,17 The χT vs T curves were finally simulated by taking into account the sum of the calculated contributions of the LS-LS, LS-HS, and HS-HS fractions. Clearly, the assumption of two SCO processes easily leads to overparametrization; however, applying a one-step model did not allow for satisfactory reproduction of the shape of the χT vs T curves in the temperature range between 150 and 250 K (Figure S5 in the Supporting Information). The final simulations together with the calculated temperature dependence of the LS-HS and the HS-HS fractions are shown in Figure 6 and Figure S5. Figure 8 shows the χT vs T curves for [2](OTf)2 in the heating and cooling modes, respectively (1 K min−1). In comparison to

Figure 6. Temperature dependence of the product of the molar paramagnetic susceptibility χ and the temperature T for compound [1](OTf)2: (black circles) experimental data points; (black solid lines) simulations based on a two-step Ising model (see main text). Dashed gray lines show the temperature dependence of the HS-HS fraction, while dotted gray lines correspond to the LS-HS fraction.

remain trapped in the HS state. Incomplete SCO is further corroborated by the fact that toward lower temperatures χT increases again, exhibiting a pronounced maximum with χT = 6.01 cm3 mol−1 K at 8.0 K. At 1.8 K a value of 5.23 cm3 mol−1 K is reached. The observation of such a maximum is typical for ferromagnetic exchange coupling and thus points to the presence of the HS-HS form of [1](OTf)2. This suggests that the SCO involves a (partial) transition from HS-HS to LS-LS rather than HS-HS to HS-LS. In the latter case, no significant exchange interactions are expected. In combination with field-dependent magnetization data (Figure S2 in the Supporting Information) as well as EPR spectroscopic results (Figure 7 and Figures S3 and

Figure 7. High-field EPR spectra recorded on a pellet of [1](OTf)2 at 370 GHz and various temperatures: (solid lines) experimental spectra; (dotted lines) simulations with J = 3.2 cm−1, D1 = D2 = 4.10 cm−1, E1 = E2 = 0.18D, and g1 = g2 = 2.15.

S4 in the Supporting Information) the position and shape of this maximum were used for the determination of the corresponding exchange-coupling constant. On application of the spin Hamiltonian given in eq 1 2

/ = −S1̂ ·J·S2̂ +

⎡

∑ Di⎢Sẑ2,i − 2 + i=1

⎣

Figure 8. Temperature dependence of the product of the molar paramagnetic susceptibility χ and the temperature T for compound [2](OTf)2: (black circles) experimental data points; (black solid lines) simulations based on a two-step Ising model (see main text). Dashed gray lines show the temperature dependence of the HS-HS fraction, while dotted gray lines correspond to the LS-HS fraction. Red triangles show the temperature dependence of χT after irradiation.

Ei ̂2 2 ⎤ (Sx , i − Sŷ , i)⎥ Di ⎦

2

+

∑ μBSî ·gi·B i=1

[1](OTf)2, a significantly lower room-temperature value of χT = 4.58 cm3 mol−1 K is obtained. With decreasing temperature, a drop of χT due to SCO is observed below 240 K, leading to a value of 3.41 cm3 mol−1 K at 210 K. However, in the heating mode the corresponding step in the χT curve is observed at a much higher temperature of ca. 285 K, indicating pronounced

(1)

the best simulation of all experimental data was obtained with J = 3.2 ± 0.2 cm−1, D1 = D2 = 4.1 ± 0.2 cm−1, E1 = E2 = (0.18 ± 0.02) D, and g1 = g2 = 2.15 ± 0.01. Here, J describes the exchangeE


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Inorganic Chemistry

metrically substituted bridging quinonoid ligand ([2](OTf)2). Structural and magnetic data clearly show that the unsymmetrically substituted bridge is capable of site-selective stabilization of the spin states of the iron(II) centers. Both complexes display two reversible one-electron-oxidation steps. The Kc values obtained from these data point to a thermodynamic stability 2 orders of magnitude higher for [2]3+ with the unsymmetrical bridge in comparison to [1]3+ with the symmetrical bridge. Both complexes also display IVCT transitions for the in situ electrochemically generated mixed-valent forms. Analysis of these bands clearly shows that the electronic coupling and hence the extent of delocalization is higher in [1]3+ in comparison to [2]3+. The contrasting trends observed for the Kc values and the results from the analysis of the IVCT bands point to the different factors that contribute to those two phenomena.10 Temperaturedependent magnetic susceptibility measurements revealed that both [1](OTf)2 and [2](OTf)2 show thermally induced twostep spin crossover. However, only [2](OTf)2 shows substantial thermal hysteresis as well as light-induced spin state switching, which can be explained by the site-selective spin state stabilization mediated by the asymmetric bridge. We have conclusively shown here that the symmetry of the bridge plays a decisive role in determining the electrochemical, electronic, and magnetic coupling in such quinonoid-bridged metal complexes. Furthermore, the symmetry of the bridge also has a strong influence on optical and magnetic bistability. In particular, the unsymmetrically substituted bridge seems to be particularly suitable for generating large hysteresis loops in SCO processes and for performing site-specific magnetic switching. The symmetry of the bridge certainly plays an important role in modulating all the aforementioned properties. However, it should also be mentioned that the presence or the absence of an additional 4-(isopropyl)phenyl group on the ligand backbone might also have an influence on the properties of these metal complexes through either different solvation effects in solution or through different packing effects in the solid state. It will be intriguing to see if the properties observed here can also be transferred to extended systems in the future.

thermal hysteresis with a hysteresis width of ca. 55 K. The curves obtained in both the heating and cooling modes show a continuous slight decrease of χT toward lower temperatures, followed by a steep decrease below 10 K, which is attributed to zero-field splitting. At 1.8 K, χT reaches 2.20 cm3 mol−1 K. The absence of strong deviation from Curie behavior below 50 K suggests the absence of strong exchange interactions. This in turn indicates that the main SCO transition is from HS-HS to HS-LS. For the simulation of the low-temperature χT data the spinHamiltonian parameters for Fe1 were fixed to those obtained for [1](OTf)2, while those of Fe2 were estimated to D2 = −3.0 cm−1, E2 = 0, and g2 = 2.15. Unfortunately, no EPR data of sufficient quality were obtained for [2](OTf)2, precluding the more accurate determination of these parameters. The low-temperature molar fractions of the different high-spin/low-spin combinations were determined to x(LS-LS) = 0.19 ± 0.02 and x(LS-HS) = 0.81 ± 0.02. In agreement with the X-ray crystallographically determined bond lengths, the main part of the sample thus consists of the mixed HS-LS species with Fe1 preferring the LS configuration and Fe2 the HS configuration. For [1](OTf)2, simulating the susceptibility data for [2](OTf)2 in the entire temperature range required the assumption of two subsequent spin-crossover processes. The best agreement between experiment and simulation (Figure 8) was obtained with T1/2 = 150 ± 10 K, Δeff = 500 ± 100 K, and Jcoop = 0 for process 1 and T1/2(cooling) = 230 ± 2 K, T1/2(heating) = 285 ± 1 K, Δeff = 1900 ± 500 K, Jcoop(cooling) = 200 ± 20 K, and Jcoop(heating) = 300 ± 20 K for process 2. Part of the LS-HS species does not participate in the SCO, leading to a composition of x(LS-HS) ≈ 0.65 and x(HS-HS) ≈ 0.35 at 300 K. Although the meaning of the absolute values should not be overestimated, the results strongly indicate a more site selective stabilization of spin states due to the asymmetry of the bridging ligand in [2](OTf)2. Furthermore, the thermal hysteresis is much more pronounced in comparison to that of [1](OTf)2. While the symmetry of the bridging ligand certainly plays a strong role in influencing the magnetic properties of these metal complexes, the presence or absence of an additional 4-(isopropyl)phenyl substituent on the ligand backbone might also influence packing effects in the solid state and thus influence the magnetic properties of the respective metal complexes. Since spin crossover is not only a thermally driven process but can also be induced by irradiation (light-induced excited spin state trapping, LIESST)18 we performed photomagnetic experiments on [1](OTf)2 and [2](OTf)2. The samples were irradiated with green light (520 ± 20 nm) at a constant temperature of 15 K while the change in magnetization was recorded. When saturation was reached, the lamp was switched off and the temperature dependence of χT was measured while the sample was heated. For [1](OTf)2, no significant increase in χT during irradiation was observed. In contrast, [2](OTf)2 displayed a clear increase of χT, showing one more promising property related to the asymmetric bridging ligand. The temperature dependence of χT after irradiation is shown in Figure 8. The initial value after stopping irradiation is χT = 3.34 cm3 mol−1 K, suggesting nearly complete conversion to the LSHS form of [2](OTf)2 (χTexpected = 3.40 cm3 mol−1 K). While the sample was heated, thermal relaxation back to the mixed LS-LS and LS-HS composition occurred, being completed around 50 K.

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EXPERIMENTAL SECTION

General Considerations. Tris(2-methylpyridyl)amine (tmpa)19 and Fe(OTf) 2 ·6H 2 O20 were prepared according to literature procedures. All other reagents were commercially available and were used as received. All solvents were dried and distilled using common techniques unless otherwise mentioned. All reactions were carried out under a N2 atmosphere. Instrumentation. Cyclic voltammetry was carried out in 0.1 M Bu4NPF6 solution using a three-electrode configuration (Pt or glassycarbon working electrode, Pt counter electrode, Ag wire as pseudoreference) and PAR VersaSTAT 4 potentiostat. The ferrocene/ferrocenium (Fc/Fc+) couple served as internal reference. UV− vis−NIR absorption spectra were recorded on an Avantes spectrometer system: Ava Light-DH-BAL (light source), AvaSpec-ULS2048 (UV−vi -detector) and AvaSpec-NIR256-2.5TEC (NIR detector). Spectroelectrochemical measurements were carried out using an optically transparent thin-layer electrochemical (OTTLE) cell.11 Elemental analysis was performed on a PerkinElmer Analyzer 240 instrument. Xray diffraction measurements were carried out on a BRUKER Smart AXS diffractometer (graphite-monochromated Mo Kα radiation, λ = 0.71073 Å). SHELXS-97 and SHELXL-97 were used to solve and refine the structure.21 CCDC 971161 and 971160 contain crystallographic data for [1](OTf)2·2CH3CN and [2](OTf)2·3CH2Cl2, respectively. These data can be obtained free of charge from www.ccdc.cam.ac.uk/ data_request/cif. 1H NMR spectra were recorded on a JEOL ECS 400 spectrometer. Chemical shifts (δ) are expressed in ppm downfield from

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CONCLUSION Summarizing, we have presented here diiron complexes with either a symmetrically substituted ([1](OTf)2) or an unsymF


Article

Inorganic Chemistry

Calcd for C53H49N9O9Fe2F6S2·3CH2Cl2·0.6CH3CN: C, 45.02; H, 3.79; N, 8.82. Found: C, 45.02; H, 3.60; N, 8.84.

tetramethylsilane using the residual protonated solvent as an internal standard. All coupling constants are expressed in hertz and are only given for 1H,1H couplings unless mentioned otherwise. Mass spectra were obtained with an Agilent 6210 ESI-TOF instrument. Magnetic measurements were performed on a Quantum Design MPMS 3 SQUID magnetometer. Temperature-dependent susceptibility measurements on bulk samples were performed at applied fields of 0.1 T (for T < 50 K) and 1.0 T (for T > 40 K). Samples were studied as slightly pressed, Teflon-wrapped pellets. The data were corrected for diamagnetic contributions by using Pascal’s constants.22 Photomagnetic measurements were carried out on thin layers of the sample in transparent silicone grease. For sample irradiation the radiation of a Xe lamp (LSE 140 by LOT-Quantum Design) was filtered with the help of a band-pass filter (520 ± 20 nm) and guided to the sample using an optical fiber. The measured data were scaled to match the data of the bulk sample. Highfrequency EPR (HFEPR) spectra at various temperatures and frequencies between 95 and 440 GHz were recorded on a home-built spectrometer featuring an Anritsu signal generator, a VDI amplifier− multiplier chain, a Thomas Keating quasi-optical bridge, an Oxford Instruments 15/17 T solenoid cryomagnet, and a QMC Instruments InSb hot electron bolometer. The samples were studied either as pure pellets ([1](OTf)2) or as pellets of a mixture with eicosane ([2](OTf)2). EPR spectra and magnetization data were simulated using the Easyspin program.23 Syntheses. H2L2′ and H2L5′. The synthesis was performed according to the procedure published in ref 4s. 2,5-Dihydroxy-1,4benzoquinone (0.5 g, 3.6 mmol, 1 equiv) was dissolved in acetic acid (20 mL) followed by the slow addition of 4-isopropylaniline (0.5 mL, 3.6 mmol, 1 equiv). The reaction mixture was heated to reflux for 4 h. After the mixture was cooled to room temperature, water was added, resulting in a dark precipitate which was filtered off, washed with water, and dried in vacuo. The crude product was purified by column chromatography (silica, first only CH2Cl2 and then CH2Cl2/CH3OH 2/1). H2L2′ was obtained as a purple solid (180 mg, 0.5 mmol, 14%) and H2L5′ as a dark purple solid (400 mg, 1.6 mmol, 44%). Data for H2L2′ are as follows. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.07 (s, 2 H, NH), 7.44−7.00 (m, 8 H, Ar-H), 6.01 (s, 2 H, Q-H), 2.92 (hept, J = 6.9 Hz, 2 H, CH(CH3)2), 1.24 (d, J = 6.9 Hz, 12 H, CH3). ESI-MS: calcd for C24H26N2O2 (M + Na+) m/z 397.19, found 397.19. Anal. Calcd for C24H26N2O2: C, 76.98; H, 7.00; N, 7.48. Found: C, 76.54; H, 7.19; N, 7.24. Data for H2L5′ are as follows. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.90 (s, 1 H, NH), 7.25 (d, J = 8.2 Hz, 2 H, Ar-H), 7.14 (d, J = 8.2 Hz, 2 H, Ar-H), 6.01 (s, 1 H,QH), 5.98 (s, 1 H, Q-H), 2.90 (hept, J = 6.9 Hz, 1H, CH-(CH3)2), 1.23 (d, J = 6.9 Hz, 6 H, CH3). ESI-MS: calcd for C15H15NO3 (M + Na+) m/z 280.09, found 280.09. Anal. Calcd for C15H15N1O3·0.3AcOH· 0.55CH2Cl2: C, 60.24; H, 5.42; N, 4.35. Found: C, 60.21; H, 5.48; N, 4.30. [1](OTf)2. Fe(OTf)2·6H2O (139 mg, 0.3 mmol, 2 equiv) and tmpa (87 mg, 0.3 mg, 2 equiv) were dissolved in acetonitrile (10 mL) and stirred for 30 min at room temperature. H2L2′ (56 mg, 0.15 mmol, 1 equiv) and NEt3 (0.5 mL) were added, and the reaction mixture was stirred for another 2 days at room temperature. The solvent was removed in vacuo, and the residue was dissolved in CH2Cl2 (10 mL) and filtered via cannula filtration. The solution was layered with hexane (5 mL) to yield a green solid of [1](OTf)2 (160 mg, 0.12 mmol, 80%). Single crystals suitable for X-ray diffraction were obtained by slow diffusion of diethyl ether into a concentrated solution of [1](OTf)2 in an acetonitrile/toluene (4/1) solution. ESI-MS: calcd for C60H60Fe2N10O2 (M2+) m/z 532.18, found 532.18. Anal. Calcd for C62H60N10O8Fe2F6S2· 1.5CH2Cl2: C, 51.17; H, 4.32; N, 9.40. Found: C, 51.33; H, 3.55; N, 9.15. [2](OTf)2. Fe(OTf)2·6H2O (139 mg, 0.3 mmol, 2 equiv) and tmpa (87 mg, 0.3 mg, 2 equiv) were dissolved in acetonitrile (10 mL) and stirred for 30 min at room temperature. H2L5′ (39 mg, 0.15 mmol, 1 equiv) and NEt3 (0.5 mL) were added, and the reaction mixture was stirred for a further 2 days at room temperature. The solvent was removed in vacuo, and the residue was dissolved in CH2Cl2 (10 mL) and filtered via cannula filtration. The solution was layered with hexane (5 mL) to yield green crystals of [2](OTf)2 (71 mg, 0.06 mmol, 40%). ESIMS: calcd for C51H49Fe2N9O3 (M2+) m/z 473.63, found 473.63. Anal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02097. Structural details, selected bond lengths and angles, HF EPR spectra, and magnetization curves (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.v.S.: [email protected]. *E-mail for B.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Deutsche Forschungsgemeinschaft (DFG, SL104/2-1, SA1580/5-1, SPP 1601) for the financial support of this work.

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REFERENCES

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