Verónica
Jornet-Mollá
a,
Carlos
Giménez-Saiz
a,
Laura
Cañadillas-Delgado
b,
Dmitry S.
Yufit
c,
Judith A. K.
Howard
*c and
Francisco M.
Romero
*a
aInstituto de Ciencia Molecular, Universitat de València, P. O. Box 22085, 46071 València, Spain. E-mail: fmrm@uv.es
bInstitut Laue-Langevin, 6 Rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France
cDepartment of Chemistry, Durham University, Durham DH1 3LE, UK
First published on 16th November 2020
The iron(II) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase. 1 undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W⋯O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at T1(LIESST) = 68 K and T2(LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice.
Spin crossover (SCO) metal complexes are suitable candidates for these synergic effects. These well-known bistable materials are able to interconvert between different spin states under an external perturbation, such as light irradiation, temperature change or application of pressure.5 Further, SCO is always accompanied by strong distortions of the metal coordination sphere that can lead to changes in the local electric dipoles. In many cases, SCO is associated with structural phase transitions that involve a breaking of symmetry and/or a concerted displacive motion of the molecules.6–8 This may result in enhanced dielectric responses.
Among the different compounds exhibiting SCO behaviour, we turned our attention to [Fe(bpp)2]2+ complexes (bpp = 2,6-bis(pyrazol-3-yl)pyridine, Chart 1).9,10 The reasons for that are manifold: first, these iron(II) complexes show light-induced excited-state spin trapping (LIESST) effects with relatively high relaxation temperatures;11 secondly, they have four pyrazolyl N–H moieties that can act as hydrogen-bond donors towards polytopic anionic hydrogen-bond acceptors.12 The high directionality and strength of hydrogen bonds allow for designing SCO salts with non-centrosymmetric structures.13 Further, the adventitious presence of solvent molecules acting as H-bond acceptors is responsible for the marked dependence of the magnetic properties of these materials on the extent of solvation.14,15 Following these design principles, we have recently obtained acentric packings based on low-spin [Fe(bpp)2]2+ isonicotinate salts. Reversible removal of solvent molecules yields a polar structure where half of the Fe2+ cations undergo crossover to the high-spin state. This allows for switching the electric polarisation and the magnetic moment of the material in the same temperature range.13
One of the main drawbacks of our approach is the high stabilisation of the polar phase, in such a way that the ferroelectric phase transition could not be detected even at high temperatures (T = 493 K). The high strength of hydrogen bonding hinders the relative displacement of positive and negative charges needed for reversal of electric polarisation.
In order to circumvent these problems, we focus now our interest in the dynamic processes at the basis of ferroelectric phase transitions. One of these processes is proton displacement between ditopic hydrogen-bond acceptors. Indeed, Rochelle salt and KH2PO4, the first ferroelectrics to be discovered, rely on hydrogen bonding.16 One of the first examples of a single-component organic ferroelectric crystal was dabcoHClO4 (dabco = 1,4-diazabicyclo[2.2.2]octane), in which the polarisation of the material depends on proton displacement along hydrogen-bonded chains.17 Also, ferroelectric cocrystals, with two or more components, that undergo a change of polarisation when protons shuttle between an acid and a base are being investigated.18 In these systems, it is important to match the pKa of the hydrogen bond acceptor and donor. Thus, binary systems composed of anilic acids and several hydrogen-bond acceptors like phenazine,19 5,5′-dimethyl-2,2′-bipyridine20 or the ‘proton sponge’ 2,3,5,6-tetra(2-pyridinyl)pyrazine21 show ferroelectric behaviour based on proton dynamics. Proton displacement usually takes place along short strong hydrogen bonds (SSHBs), in which the proton is trapped between the HB donor and the HB acceptor in a single or a low-barrier double potential well.22 As in the case of SCO, this effect can also be responsive to an external perturbation, including the application of an electric field or changes in temperature and pressure.23,24 Further, many enzymatic reactions involve proton transfer in SSHBs.25 The study of systems combining SCO and proton transfer can thus provide interesting synergies in terms of physical properties and also models of enzymatic action (considering SCO as mimicking the allosteric changes that activate the enzyme towards proton transfer). With these ideas in mind, we undertake now the study of a cocrystal comprising a [Fe(bpp)2]2+ SCO typical cation and a carboxyl(ate)-based SSHB. Herein, we describe the structural and (photo)magnetic properties of [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1), a SCO salt containing isonicotinic acid N-oxide (Chart 1) both in its free and deprotonated form. We will show that spin crossover in 1 is accompanied by proton migration and hydrogen bond formation in two steps, with breaking of symmetry and formation of a metastable high-spin/low-spin (HS/LS) pair. Details of the synthesis and characterisation of 1 are provided in the ESI.†
Fig. 1 Thermal ellipsoid plot of the crystal structure of 1 at 240 K showing the hydrogen-bonding connectivity. |
As expected, the two terdentate bpp ligands bind in meridional positions of the iron(II) coordination sphere and are arranged in almost perpendicular planes, with Fe–N bond lengths ranging from 2.115 Å to 2.190 Å (Table S4†). This is in agreement with 100% of the Fe2+ cations in the HS state being determined from magnetic measurements. A HS phase stable at 240 K is unusual for [Fe(bpp)2]2+ salts of simple anions in their hydrated forms.14,28 For comparison, the isonicotinate salt [Fe(bpp)2](isonic)2·2H2O exists only in the LS state even at temperatures much higher than room temperature.13 Noteworthy is the fact that the N-oxide moiety is not involved in hydrogen bonds with the [Fe(bpp)2]2+ cation, probably due to the weaker H-bond accepting character of pyridine N-oxides with respect to pyridines. This decreases the electron density of the non-coordinating nitrogen atoms and leads to weaker N–Fe donor-metal σ-interactions, destabilising the LS state.29
Another aspect of the crystal packing of [Fe(bpp)2]2+ salts with an impact on the SCO abruptness and hysteretic behaviour is the presence of π–π stacking interactions between adjacent bpp ligands. In 1, these contacts organise alternating chains of [Fe(bpp)2]2+ complexes along the c axis (Fig. S1a†). Isonicotinate N-oxide anions are located in the interchain space, preventing the formation of the terpyridine embrace motif commonly found in these salts. A description of these intermolecular contacts and their variation with temperature is given in Fig. S1.†
At 210 K, the crystal structure of 1 is essentially the same. The Fe–N lengths are slightly shorter, ranging from 2.106 Å to 2.182 Å, but still pointing to a HS state for this cation (Table S4†). All H-bond distances (Table 1), including those involving only isonicNO anions and/or H2O molecules, show a small decrease with respect to the values observed at 240 K. This is expected as a consequence of thermal contraction.
H-bond | 240 K (HS) | 210 K (HS) | 120 K (HS/LS)a | 120 K (HS/LS)b | 95 K (LS) | 50 K (PIHS)c | 50 K (PIHSrel)d | 95 K (LS)e |
---|---|---|---|---|---|---|---|---|
a Contacts within the A sublattice, except for O1WA⋯O9B, O3WA⋯O9B and O5WA⋯O2WB. b Contacts within the B sublattice, except for O1WB⋯O9A, O3WB⋯O9A and O5WB⋯O2WA. c Photoinduced high-spin phase. d Structure obtained from partial relaxation of the PIHS phase. e Structure obtained after full relaxation of the PIHS phase. | ||||||||
N1⋯O3W | 2.7031(19) | 2.6982(18) | 2.6845(19) | 2.6427(19) | 2.638(3) | 2.6880(18) | 2.662(2) | 2.645(3) |
N5⋯O4W | 2.832(2) | 2.825(2) | 2.808(2) | 2.7442(19) | 2.727(3) | 2.8188(18) | 2.777(2) | 2.741(3) |
N6⋯O4 | 2.8357(18) | 2.8320(16) | 2.8335(18) | 2.7507(18) | 2.758(3) | 2.8324(17) | 2.8017(19) | 2.757(3) |
N10⋯O8 | 2.7179(18) | 2.7155(16) | 2.7235(18) | 2.6883(17) | 2.697(3) | 2.7091(17) | 2.711(2) | 2.714(3) |
O1W⋯O6 | 2.8488(18) | 2.8402(16) | 2.7889(17) | 2.8158(17) | 2.778(2) | 2.8272(16) | 2.8037(17) | 2.780(3) |
O1W⋯O9 | 2.838(2) | 2.8311(17) | 2.8231(17) | 2.8153(17) | 2.816(3) | 2.8055(17) | 2.8054(18) | 2.823(3) |
O2W⋯O1W | 2.814(2) | 2.8030(18) | 2.7890(18) | 2.7694(18) | 2.765(3) | 2.7955(17) | 2.7780(18) | 2.771(3) |
O2W⋯O8 | 2.892(2) | 2.8883(17) | 2.8786(17) | 2.9321(18) | 2.928(3) | 2.8616(17) | 2.8857(18) | 2.938(3) |
O3W⋯O9 | 2.892(2) | 2.8832(18) | 2.8696(18) | 2.8453(17) | 2.853(3) | 2.8579(17) | 2.8503(19) | 2.857(3) |
O3W⋯O2 | 2.7630(18) | 2.7563(16) | 2.7603(17) | 2.6946(17) | 2.710(3) | 2.7511(17) | 2.7313(18) | 2.713(3) |
O4W⋯O7 | 2.723(2) | 2.7181(17) | 2.7135(18) | 2.7384(17) | 2.737(3) | 2.7136(17) | 2.7222(18) | 2.736(3) |
O4W⋯O5W | 3.062(2) | 3.044(2) | 3.0256(19) | 2.8562(19) | 2.858(3) | 3.0070(18) | 2.934(2) | 2.853(3) |
O5W⋯O3 | 2.743(2) | 2.7372(18) | 2.7437(18) | 2.7091(18) | 2.722(3) | 2.7397(17) | 2.7313(17) | 2.709(3) |
O5W⋯O2W | 2.854(2) | 2.8455(19) | 2.8387(19) | 2.8072(18) | 2.803(3) | 2.8410(18) | 2.8204(19) | 2.802(3) |
For Fe1B the φ angle increases (approaching to 180°) in comparison with Fe1A and the HS site at 240 K (Table S5†). The parameter Σ equals 100.54°, a value characteristic of a LS configuration and much lower than those observed for HS centers.9
A very interesting point arises after examining the second coordination sphere, defined by the hydrogen bonds between the four NH pyrazole functions and the four H-bond acceptors (two water molecules and two isonicotinate N-oxide anions). It is found (Table 1) that hydrogen bonds are stronger for the Fe1B site (mean distance N⋯O: 2.706 Å) as compared to the Fe1A site (mean distance N⋯O: 2.762 Å), the latter being similar to the H-bond distances observed at 240 K (mean value N⋯O: 2.772 Å). The increase in H-bond strength as the Fe(II) complex undergoes SCO to the LS state can be explained in terms of an electrostatic model: the LS state has a lower volume and, therefore, a higher charge density. This reinforces electrostatic attraction to the negative charges of the anions (and water dipoles), thus leading to shorter H-bond distances.
There are only a couple of examples in the literature reporting HS and LS phases of the same hydrogen-bonded [Fe(bpp)2]2+ complex in the same environment. For instance, the structure of [Fe(bpp)2](ClO4)2·1.75CH3COCH3·1.5Et2O displays a gradual and incomplete spin transition near 205 K.31 The HS and LS phases are isostructural and both contain two inequivalent Fe2+ cations in the asymmetric unit. Hydrogen bonds (mean distance N⋯O: 2.834 Å) are stronger for the LS phase as compared to the HS phase (mean distance N⋯O: 2.871 Å). Another example is the anhydrous isonicotinate salt [Fe(bpp)2](isonic)2, that presents a partial spin crossover centered at 324 K. In this case, the HS and LS phases are isostructural and possess only one independent Fe center at both temperatures. Here again, an increase in H-bond strength (mean distance N⋯acceptor: 2.669 Å) is observed in the LS phase in comparison with the HS phase (mean distance N⋯acceptor: 2.708 Å).13
In our case, the main consequence of the increase of the hydrogen bond strength in the second coordination sphere of the LS Fe1B center is a shortening of most other hydrogen bonds with respect to the values obtained for equivalent positions near the HS Fe1A cation, the latter being closer to those measured at 240 K (Table 1). An elongation of the hydrogen bond distance is only observed in two cases. For instance, in the bifurcated H bond mentioned above, as the distance between the carboxylate anion and the LS Fe1B complex becomes shorter (N10B⋯O8B: 2.6883(17) Å versus N10A⋯O8A: 2.7235(18) Å), the distance with respect to the water molecule increases (O2WB⋯O8B: 2.9321(18) Å versus O2WA⋯O8A: 2.8786(17) Å). Besides this, the most significant difference between A and B sublattices is the formation of a hydrogen bond between O4WB and O5WB (O4WB⋯O5WB: 2.8562(19) Å) that it is not present between their A counterparts (O4WA⋯O5WA: 3.0256(19) Å). This in turn causes a slight elongation of a second hydrogen bond (O4WB⋯O7B: 2.7384(17) Å versus O4WA⋯O7A: 2.7135(18) Å).
The short strong H-bond involving the two carboxylates is now split in two different interactions with similar strengths (O1A⋯O5B: 2.4257(16) Å; O1B⋯O5A: 2.4216(16) Å). Their geometrical parameters after structure refinement are very similar to those observed at 240 K but now the electron density distributions around H1OA and H1OB obtained by differential Fourier analysis are elongated along the bonds (indicating their bonding character) and less symmetric (Fig. 2). Indeed, the positions of the electron density maxima shift from O1A and O1B to, respectively, O5B and O5A (Table S3†). Smearing out of the electron density upon cooling is very rare. Often the opposite behavior is detected due to decreased thermal motion.32 This may be an indication of dynamic disorder (if a double well potential appears below the phase transition) and/or a shift of the potential energy minimum towards one of the carboxylate anions as a consequence of the spin crossover. In any case, this shift takes place without breaking the centrosymmetric character of the crystal structure.
The hydrogen-bonding distances are very similar to those observed for the B sublattice at 120 K (Fig. 4). The mean distance between the pyrazole NH units and the H-bond acceptors in the second coordination sphere of the [Fe(bpp)2]2+ cation has almost the same value (2.705 Å). The remaining H-bonding distances are very similar to those observed in the crystal structure at 120 K between equivalent positions near the LS Fe1B center. This means a shrinkage of these contacts with respect to high temperature, except for the few cases already mentioned. Now, 100% of O4W molecules are bonded to O5W with a distance of 2.858(3) Å.
The strong 3c–4e− H-bond interaction is maintained (O1⋯O5: 2.429(2) Å). The shape of the electron density changes with respect to 120 K and is more similar to the 240 K data, becoming more spherical but slightly unsymmetric (Fig. 2). Also, the position of the maximum density (Table S3†) is shifted back towards O1, as expected for a reentrant phase transition.
The crystal structures of 1 determined by X-ray diffraction have clearly shown the presence of a strong H-bond interaction of quasi covalent nature with a proton being trapped between the two isonicotinate anions isonicNO(3) and isonicNO(6). Although this hydrogen atom H1O was found in Fourier differential density maps, its position could not be determined precisely. Further, the value of the atomic displacement parameter for H1O for the different temperatures (Table S2†) is much larger than those observed for the other refined H atoms (those of water molecules). These facts point to the presence of H1O in a single potential energy well with an energy minimum located approximately halfway between the H-bond acceptors (Fig. 5a). This is an interesting situation as any subtle variation in the crystal structure (such as spin crossover) can lead to proton displacement (Fig. 5b) and/or the appearance of static disorder due to the change of the potential energy curve or to the difficulty to overcome the activation energy barrier (Fig. 5c) at low temperatures. Since these phenomena are at the basis of H-bonded ferroelectrics,18–21 a neutron crystallography study is justified.
Other important hydrogen-bonding parameters to discuss are the N–H⋯O and O–H⋯O bond angles (θ), which can be accurately determined from neutron diffraction and can differ randomly from the estimated XRD values by as much as 6°. It was found that the N–H⋯O bond angles in the second coordination sphere are lower for the LS species (Table S8†). They decrease from an average value of 170° at 240 K to a value of 164° at 95 K. This is striking, as in most cases shorter H bonds tend to be more linear. The depart from linearity on going from the HS to the LS state can be ascribed to the change of the bpp bite angle below the phase transition. Concerning the O–H⋯O angles, their variation does not seem to correlate with the change in H bond distances. The formation of the H bond between O4W and O5W in the LS structure entails an increase of the O4W–H⋯O5W θ angle (172.4(6)°) with respect to the value at 240 K (167.7(7)°), in this case approaching linearity.
The most remarkable observation concerns the SSHB interaction O1⋯O5. In the HS phase at 240 K, the H1O proton is located halfway between the two oxygen atoms (Table 2), in qualitative agreement with the XRD study. For this very strong H bond, with the H atom lying in an almost symmetric environment, the position of the nucleus and the center of the electron density distribution are almost coincident. In the mixed-spin phase at 120 K, this contact is split into two. While the O1A⋯H1OB⋯O5B interaction keeps the proton practically in the middle of the H bond, in the O1B⋯H1OA⋯O5A interaction the proton separates slightly from O1B. In the reentrant LS phase at 95 K, the symmetry is restored but the H1O proton still deviates from the center of the H bond. SSHBs with O⋯O distances smaller than 2.45 Å are expected to show broad single-well potentials suitable for proton migration. Normally, the proton is located halfway between the heteroatoms at high temperature and becomes off-centered as temperature decreases.32,34 Additional neutron diffraction data of 1 were thus collected at 50 K for both the mixed-spin and LS phases (the latter being obtained after thermal relaxation at 95 K for 4 h). For the mixed-spin phase, the geometries of the SSHBs are similar to those described at 120 K, with a clearly symmetric O1A⋯H1OB⋯O5B interaction (Table 2) and a slightly more asymmetric O1B⋯H1OA⋯O5A interaction. For the LS phase, the asymmetry of the SSHB increases and is larger than that observed at 95 K.
T (K) | d(O1⋯H1O) (Å) | d(H1O⋯O5) (Å) | D(O1⋯O5) (Å) |
---|---|---|---|
a The O1⋯H1O⋯O5 interaction is split into two: O1A⋯H1OB⋯O5B (top line) and O1B⋯H1OA⋯O5A (bottom line) due to symmetry breaking. b Temperature was held at 95 K for 4 h prior to data collection. | |||
240 | 1.203(7) | 1.206(7) | 2.407(4) |
120a | 1.216(7) | 1.213(7) | 2.427(4) |
1.221(7) | 1.198(7) | 2.416(4) | |
95b | 1.222(6) | 1.200(6) | 2.420(4) |
50a | 1.213(8) | 1.209(8) | 2.420(5) |
1.227(8) | 1.191(8) | 2.414(5) | |
50b | 1.237(7) | 1.184(7) | 2.419(4) |
At all working temperatures (Fig. 2), the nuclear density of the H atom is well localized, with thermal parameters that are similar to those corresponding to other protons present in the structure (Table S9†). This clearly discards a temperature-dependent proton disorder resulting from the presence of a low-activation energy barrier and confirms a genuine proton migration. This can be due either to an asymmetric potential energy well; or to a change of the potential energy surface with temperature.32 The latter seems to apply in our case, given the rather symmetric environment of the H atom. The similar behaviour of the LS phase and the LS sublattice of the mixed-spin phase confirms that the modulation of the potential energy surface and the subsequent proton migration is a consequence of the SCO process. The shift in the proton position is small (Table 2) but significant (8 times the experimental error) and compares well with similar shifts observed in H-bonded ferroelectrics.18–21
Variable temperature neutron diffraction data down to 10 K were also collected using the single-crystal Laue technique. No further transformations were observed upon cooling down to 10 K (ESI†).
Fig. 6 DSC plot of 1 (empty circles) and 1d (solid circles). Blue and red colours refer to cooling and heating modes, respectively. Scan rate: 10 K min−1. |
When hydrogen bonding influences spin crossover, thermodynamic properties can be very sensitive to isotopic composition.36 We thus undertook DSC measurements on samples crystallised from deuterated solvents (1d). A similar calorimetric plot was obtained with T↓ = 155 K and T↑ = 162 K (Fig. 6), with similar values of ΔH = 2.24 KJ mol−1 and ΔS = 13.8 J K−1 mol−1. The thermal hysteresis (ΔT = 7 K) is higher than that observed for 1 but the most striking result is the 6 K decrease of the critical temperature of the phase transition for 1d, indicating a destabilisation of the mixed HS/LS phase.
Since the FeN6 environment in the iron(II) coordination sphere should be the same, this destabilisation has to be ascribed to the free energy gain associated to the reorganisation of hydrogen bonds, particularly to the formation of the H-bond between O4WB and O5WB. As a result of the lower zero-point vibrational energy of O–D bonds relative to O–H bonds,33 it is known that hydrogen bond lengths increase upon deuteration (Ubbelohde effect). Hence, H-bond enthalpies should decrease and this could be the reason for the destabilisation effect observed in the thermal properties. However, the enthalpy variation ΔH is very similar or even higher for 1d. It seems then that the shift in T1/2 is due to differences in vibrational entropy (note that ΔS increases in our case for 1d). This is in contrast with the seminal report on the isotope effects of the tris(picolylamine)iron(II) complex, in which the SCO shifts to higher temperatures upon deuteration.37 In this and other cases,38 the isotope effect has been ascribed to differences between the zero-point vibrational levels of the HS and LS sites in their deuterated and non-deuterated forms, while here we are focusing on the intermolecular hydrogen-bond interactions. In any case, the existence of isotope effects demonstrates the crucial role of hydrogen bonding in defining the magnetic behaviour of the salts.
Below 25 K, the magnetic moment of the sample decreases fast and this is a signature of the zero-field splitting (ZFS) expected for the HS fraction present in the material. An essentially identical curve is obtained after heating the sample from 2 K to 300 K and even after subsequent temperature cycles (not depicted), showing the reversibility of the process. The lack of hysteresis contrasts with the small value (ΔT = 5 K) observed in the DSC measurements, the latter being performed at a much higher sweeping rate (10 K min−1).
The magnetic properties of 1d were studied exactly in the same conditions and a very similar plot was obtained (Fig. 7). The limiting value of χT at 300 K equals 3.68 emu K mol−1, matching that observed for 1. On lowering the temperature, χT decreases and reaches an inflection point at T1 = 161 K. Note the 6 K decrease in the SCO temperature, in perfect agreement with the DSC measurements. Further cooling yields a constant value of χT = 1.90 emu K mol−1, corresponding to a HS fraction γHS ≈ 0.5. Finally, at very low temperatures, an additional decrease due to ZFS is observed. Again, the full process is reversible as confirmed by the equivalent heating and cooling curves (not shown). Clearly, magnetic properties show the higher stability of the HS phase for the deuterated material, thus confirming the crucial role of hydrogen-bonding in the SCO process.
The photomagnetic experiments clearly suggest three intervals of stability: (i) at T > 167 K, the HS phase is the stable one; (ii) 116 K < T < 167 K, where the stable phase has a 50:50 HS:LS distribution of the Fe2+ sites; (iii) at T < 116 K, where this mixed spin phase becomes metastable and the most stable phase is LS. This behavior is very similar to that previously observed for an iron(II) bischelated complex of 2,6-bis(3-methylpyrazol-1-yl)pyridine.40 Of interest for our discussion is the fact that the magnetic properties of this compound were strongly dependent of the presence of H2O molecules in the crystal. However, in that case, the crucial role of hydrogen bonding could not be studied due to crystallographic disorder.
It is known that, for these bischelated iron(II) complexes, T(LIESST) can be correlated to the SCO temperature T1/2 by the expression T(LIESST) = T0 − 0.3T1/2, where T0 = 150 K. For T1/2 = T2 = 116 K, a value of T(LIESST) = 115 K should be expected. It has been pointed out that when the expected T(LIESST) and T1/2 are similar, the thermal SCO depends strongly on kinetic factors and this often results in partial spin conversion,41 as observed in the present case, and reduction of the T(LIESST) parameter with respect to the expected values. Alternatively, an increase of T(LIESST) resulting from reorientation of solvent molecules during the HS to LS relaxation has been recently observed in [FeL2][BF4]2·CH3CN (L = 4-{isopropylsulfanyl}-2,6-di{pyrazol-1-yl}-pyridine).42 Interestingly, in our case, the determination of T(LIESST) from the first derivative plot (Fig. 8, inset) shows clearly a relaxation in two steps: a first process with T1(LIESST) = 68 K, in which the photoexcited HS phase relaxes to a mixed HS/LS material; and a second process with T2(LIESST) = 76 K, where the remaining HS Fe2+ cations relax to their LS states. Clearly stepped LIESST plots, generally ascribed to the successive relaxation of inequivalent Fe2+ ions, are not very common.43 An example showing two clearly separated relaxation events differing in more than 40 K has been reported in iron(II) complexes derived from dipyrazolylpyrazine.44 Most often, however, the LIESST curve exhibits an inflexion point indicating the presence of two relaxation processes.45 Normally, the presence of this characteristic point in the LIESST plot reflects the stepped character of the thermal SCO. The parameters defining the thermal SCO (T1/2 and cooperativity) are generally mirrored in the LIESST properties.46 We can thus ascribe the first relaxation process at T1(LIESST) = 68 K to the partial SCO observed at T1 = 167 K. Here also, the data fall outside the T(LIESST) vs. T line, as expected for kinetically trapped spin states.
In another experiment, compound 1 was cooled down to 95 K and kept at this temperature for 4.5 h (Fig. 9). In these conditions, 1 relaxes completely to the LS phase. Then, the compound was cooled further to 10 K and irradiated with red laser light (λ = 630 nm). A 30-fold increase of the magnetic susceptibility was observed after 10–15 min, as fast as that measured by irradiating the HS/LS phase. Further, there was no evidence of a plateau that would indicate a two-step process. It seems then that the photoswitching process proceeds directly from the LS to the PIHS phase.
Then, the T(LIESST) curve was recorded in the dark. After an initial increase owing to ZFS effects, χT reached a plateau in the 40–55 K region, with a value of 3.97 emu K mol−1, corresponding to 100% population of the HS state. Heating further yields a drastic decrease of the magnetic signal, vanishing again at 80 K. The values of T(LIESST) obtained from the first derivative plot (T1(LIESST) = 68 K, T2(LIESST) = 76 K) are the same, irrespective of the initial phase present before irradiation. This is not always the case: in a similar compound showing also two-step relaxation, the sample can undergo photoexcitation from the low-spin phase or from a metastable mixed-spin phase with different results (the latter being more effective).47 Thus, the fact that T(LIESST) curves of 1 obtained from photoexcitation of, respectively, the LS phase and the mixed HS/LS phase are identical in the whole temperature range, strongly suggests that the photoexcited HS phase is the same in both cases.
(1) |
Fig. 10 Time dependence of the high-spin fraction of Fe2+ cations at different temperatures. Red lines correspond to the best-fit data according to eqn (1). The inset shows Arrhenius plots corresponding to the fast (red) and slow (blue) processes, together with the best-fit to Arrhenius equation. |
However, due to cooperative effects, both rate constants depend also on the remaining fraction of HS centers (γHS) in such a way that the two exponential curves become sigmoidal and dependent on a self-acceleration factor, that is also a function of temperature:48
(2) |
(3) |
The observed behaviour is very similar to that reported for [Fe(bppI)2](BF4)2, a complex showing two T(LIESST) values of 65 and 75 K. In that example, simulation of the relaxation curves was accomplished with a model consisting in the sum of a fast exponential decay and a slow self-accelerated process.49
From the best-fit data shown in Fig. 10, values of ki and αi can be extracted at different temperatures below Ti(LIESST). Results are gathered in Table S10.† Note that the two rate constants k1 and k2 differ in one order of magnitude. This justifies the fit to a double exponential law. The calculated values of the rate constants follow an Arrhenius law (inset Fig. 10) indicating a thermally activated process with apparent activation energies, Ea1 = 935.7 cm−1 and Ea2 = 997.9 cm−1, that compare well with values reported for similar systems.49,50 The apparent preexponential factors k1(∞) and k2(∞) are 2.563 × 105 s−1 and 1.059 × 105 s−1, respectively. The ratio k1(∞)/k2(∞) is close to 2, the expected statistical value for these similar processes. The energy associated to cooperativity, lies in the expected range observed for these complexes. Instead, the α2 parameter does not follow a simple thermal variation.
As seen previously, the photomagnetic studies reveal a two-step relaxation process with T1(LIESST) = 68 K and T2(LIESST) = 76 K. After the first step, the value of γHS = 0.5 suggests the presence of a mixed 1:1 HS/LS material similar to the metastable phase measured at 120 K. In order to characterise this mixed-spin phase by photocrystallography, a single crystal of 1 mounted on a X-ray diffractometer was irradiated (λ = 630 nm) at 50 K for 1 h. Then, it was heated to 69 K (scan rate of 0.3 K min−1), allowing for partial relaxation. Then, the crystal was quenched rapidly to 50 K and measured. The structure is still centrosymmetric (P) but the unit cell volume (V = 2090.17 Å3) decreases with respect to the value of the PIHS phase. There is a single independent Fe(II) center, exhibiting Fe–N bond lengths in the 2.034–2.106 Å range (Table S4†). The mean length dFe–N = 2.075 Å and the octahedral distortion parameters are intermediate between those corresponding to the HS and LS structures (Table S5†), confirming the efficient trapping of a mixed-spin phase with 50% of the iron sites in each spin state. Similar considerations apply to the second sphere H-bonding N⋯O distances, with a mean value (2.738 Å) that lies exactly between those observed for the HS and LS phases (Table 1). An intermediate distance between O4W and O5W water molecules (2.934(2) Å) is also observed. Isotropic displacement parameters are much higher than those measured for the other structures (Table S2†). This translates into larger ellipsoids (Fig. S7†), specially for those pyrazolyl C and N atoms that undergo a higher displacement with the change of the bpp bite angle, and for water molecules (O3W and O4W) located in the second coordination sphere of the complex.
These results point to a random distribution of HS and LS Fe(II) complexes in the crystal lattice. We did not found any evidence of superstructure reflections that might indicate an ordering of the two spin species in the crystal, even in a modulated manner. A possible explanation for the two-step relaxation in the magnetic properties and the lack of a symmetry breaking event in the structural analysis can be the existence of a short-range interaction defining LS dimers that distributes randomly throughout the lattice. This explanation is consequent with the cooperativity of H-bonds. Indeed, the analysis of thermal parameters for the different isonicNO anions shows higher values for atoms (O7 and O8) involved in the H-bonding cooperative pathway described above. This illustrates how the analysis of a metastable structure obtained after partial relaxation can be useful in the definition of the cooperative interactions present in the solid.
Finally, the crystal structure of 1 was measured after full relaxation at 95 K. As expected, an isostructural phase to that measured at the same temperature without irradiation was observed (Fig. S8 and Table S1†).
Fig. 11 TIESST experiment: thermal variation of χT after quenching a sample of 1 at 10 K, then heating it at 0.3 K min−1. The inset shows the first derivative of the TIESST plot. |
Marked differences between the relaxation kinetics of the metastable HS phases obtained by photoinduced (LIESST) or thermal (TIESST) trapping have been reported.52 The HS Fe2+ sites obtained by irradiation of the LS material at low temperatures have a less distorted octahedral environment in comparison to the Fe2+ sites obtained by supercooling, the latter being frozen in a coordination environment similar to that observed in the HS phase stable at high temperature. These subtle differences in the coordination spheres propagate throughout the lattice, making the whole structure of the photoinduced HS phase more similar to that of the LS phase, thus decreasing the apparent activation energy of the SCO process.
The divergence between T(TIESST) and T(LIESST) is indicative of different relaxation mechanisms for the metastable phases obtained by thermal quenching and light irradiation. This can be confirmed by measuring the relaxation of a precooled sample at different temperatures within the metastability region (T < 114 K). For instance, the relaxation curve γHS = f(t) at 72 K of the photoinduced material decays completely after 72 min, whereas that corresponding to the thermally quenched sample at 75 K contains less than 10% LS centers even after 10.5 h (Fig. S9†). At higher temperatures, 85 K and 95 K, complete decay is reached in 8 h and 4 h, respectively. Instead, at 102.5 K, 20% of Fe2+ cations still remain in the HS state after 12 h, whereas a change of only 1% in the magnetic signal is observed at 105 K after 3.2 h. For the last two points, relaxation rates decrease when the temperature is increased, indicating that the compound enters into the thermal range corresponding to thermodynamic equilibrium, its behaviour being governed by the master equation.53
At T < 114 K, the shape of the χT = f(T) plot strongly depends on the cooling procedure and, at a sufficiently low cooling rate, thermal hysteresis should be measured. Fig. 12 shows the thermal variation of χT of 1 at a low cooling rate. First, the material was cooled down to 105 K at 3 K min−1. Then, the rate was changed to 0.04 K min−1 and the sample cooled further to 85 K. Below 101 K, χT decreases steadily, reaches an inflection point at T↓ = 97 K, and diminishes further to reach a constant value of 0.17 emu K mol−1 at 85 K. Heating now the sample results in a fast increase of the magnetic signal around T↑ = 111 K to yield an apparent thermal hysteresis ΔT = 14 K. The observation of a low-temperature HS residue indicates that kinetic effects are noticeable even in such a slow cooling conditions. This can be best appreciated by the superimposition of isothermal relaxation experiments, starting from the mixed-spin phase, in the same plot. It is possible to notice that, at 85 and 95 K, relaxation proceeds to values below those observed in the hysteresis plot. At 97.5 K, the quasistatic χT value matches the heating branch of the loop, indicating that the hysteresis has a purely kinetic origin at this temperature. At 102.5 K, complete relaxation is not observed even after 12 h, confirming that the quasistatic hysteresis loop is much thinner than the apparent one. Above 114 K, the χT plot merges with the one obtained at higher sweeping rates, suggesting that, at least in the range 105 K < T < 114 K, true bistability is observed.
Fig. 12 Thermal variation of χT for 1 at a slow scan rate (0.04 K min−1) showing the thermal hysteresis. Superimposed are isothermal relaxation experiments at the given temperatures. |
A clear correlation between intermolecular interactions, particularly hydrogen bonding, and the presence of multistep SCO is present in several studies concerning Hofmann-type frameworks based on triazole ligands. In these systems, the reversible exchange of guest species allows for tuning the competing ferro- and antiferroelastic interactions in such a way that elastic frustration can be reversibly switched.54 It seems clear that hydrogen bonding is playing a crucial role also in our case.
A strong dependence of the SCO properties on the temperature scan rate is usually associated to large structural differences between the HS and LS phases. Of course, this dissimilarity has its origin in specific intermolecular interactions with a strong impact on the respective crystal packings.55 The wide temperature range of metastability of our mixed-spin phase is thus probably correlated to the formation of the hydrogen bond between O4W and O5W. Indeed, it has been shown that changes in the location of solvent molecules and/or anions can lead to very impressive thermomagnetic properties such as wide hysteresis loops or even double spin transitions.56
The possibility of populating different metastable states depending on the external perturbation (multimetastability) has been verified in some SCO Fe2+ complexes.52,57 In the present case, due to the high difference in T(LIESST) and T(TIESST) values, it seems clear that the mixed-spin metastable phases obtained by, respectively, light irradiation and thermal trapping are markedly different. This has been confirmed by photocrystallography measurements.
An important outcome of our work is the impact of SCO on proton migration along a SSHB. Although neutron diffraction is the method of choice for the study of these processes, a temperature-dependent X-ray diffraction analysis gives some complementary information. Electronic and nuclear densities do not necessarily follow the same pattern.32 While neutron diffraction reflects only vibrational (thermal) effects, X-ray yields information on electron sharing. For compound 1, SCO induces a small displacement of the proton position but has a strong impact on the electron density being more delocalised and shifted towards O5A and O5B in the mixed-spin phase. The advantage of neutron diffraction is that it can discard the presence of disorder of the H atoms over two positions in a double-well potential, a situation that is not commonly observed in SSHB but seems to appear here as a consequence of the lattice expansion induced by irradiation.
Very recently, Sato and coworkers have described the first examples of coupling between proton transfer and spin crossover.58 They have shown that the change in the spin state of an Fe(II) bischelated complex triggers an intramolecular proton transfer from a coordinated hydrazone unit to an appended pyridine moiety. The disymmetric character of this H-bond enables an unambiguous assignment of proton location, without the need of neutron crystallography experiments. While in this study the driving force for proton transfer is the change of the bite angle of the chelating ligand, in our case the effect seems to be purely electrostatic. An advantage of our intermolecular approach is that the proton can be located in a strictly symmetric environment, occupying a special position that becomes general after the SCO process, lowering the symmetry of the system, a situation that is needed in the design of ferroelectrics.
Further, the present work establishes that SCO can have an impact on proton displacement even on remote positions, mimicking the allosteric changes that activate enzymes towards proton transfer, and opening the way to the design of SCO systems exhibiting proton conductivity and/or H-bonded ferroelectricity.
Footnote |
† Electronic supplementary information (ESI) available: Experimental section; Tables S1–S9: crystallographic data; Table S10: kinetic parameters derived from relaxation after LIESST; Fig. S1 and S4: details of crystal packing and H-bond cooperativity; Fig. S2 and S3: comparative plot of H-bonding parameters obtained from neutron and X-ray data; Fig. S5: thermal variation of crystal cell parameters; Fig. S6–S8: crystal structures under different conditions; Fig. S9: thermal relaxation experiments at different temperatures. CCDC 1873604, 1832988, 1873602 and 1832962 contain X-ray crystal data for 1 at 95, 120, 210 and 240 K, respectively. CCDC 1984961, 1984960 and 1984957 contain the neutron crystal data for 1 at 240 K, 120 K and 50 K, respectively. CCDC 1984959 and 1984958 contain the neutron crystal data for 1 at 95 K and 50 K after full relaxation to the LS state, respectively. CCDC 1984962, 1984963 and 1984964 contain the X-ray data for PIHS 1 at 50 K, 1 at 50 K after partial relaxation and at 95 K upon full relaxation to the LS state, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc04918b |
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