K⊂{[FeII(Tp)(CN)3]4[CoIII(pzTp)]3[CoII(pzTp)]}: a neutral soluble model complex of photomagnetic Prussian blue analogues

We report a new K⊂[Fe4Co4] cyanide box: a true soluble model of the photomagnetic FeCo Prussian blue analogues, which also shows photo-switchable SMM properties and remarkable redox versatility.


Introduction
Switchable molecular systems featuring electronic, magnetic, or optical bistability are attracting strong research interest because of their potential use as molecular memories, switches, actuators, or sensors, and are therefore showing promise in emerging elds such as molecular electronics. 1-3 Cyanide coordination chemistry has proven successful in providing access to a variety of responsive systems, whose optical and magnetic properties can be reversibly switched. 4,5 A representative example is the photomagnetic FeCo Prussian Blue Analogues (PBA) of the composition K 0.2 Co 1. 4 [Fe(CN) 6 ]$6.9H 2 O, which was rst described by O. Sato et al. 6 In this compound, light irradiation can induce an Electron Transfer Coupled to a Spin Transition (ETCST), which thus converts the diamagnetic {Fe II LS -CN-Co III LS } pairs into paramagnetic (Fe III LS -CN-Co II HS ) ones (LS: low spin, HS: high spin), leading to important changes in both optical and magnetic properties. The physical properties of these FeCo PBAs are however highly dependent on their chemical composition. In particular, the amount and the nature of the inserted alkali ions appears to play a crucial role in the occurrence of magnetic and optical bistability. 7,8 In recent years, intense research efforts have been devoted to the synthesis of lower dimensional models of the FeCo PBAs. Polynuclear FeCo complexes or one-dimensional photomagnetic systems have been studied. [9][10][11][12][13][14][15][16] The cyanide-bridged, cubic-shaped {Fe 4 Co 4 } compound reported by Holmes, Clérac, and Mathonière et al. in 2008 is a remarkable example of a photomagnetic entity exhibiting a notably long metastable life-time. 9 Since then, no other FeCo photomagnetic cubes have been reported, but several similar cubic systems showing interesting magnetic properties or redox exibility have been reported. [17][18][19][20][21][22] The interest in these systems also originates from their possible use as molecular sensors for the selective binding of alkali ions. 23,24 In this work, we present a novel mixed-valence {Fe 4 Co 4 } molecular box encapsulating a potassium ion. The overall neutral complex shows a photomagnetic effect and single molecule magnet behaviour in the solid state. Besides, it exhibits remarkable redox exibility in dichloromethane solution with six accessible redox states. Single crystals suitable for X-ray diffraction were obtained by the slow diffusion of pentane in a dichloromethane solution of 1. Under these conditions, a triclinic crystalline phase is obtained (P 1, Z ¼ 2) whose structure is composed of K3{Fe 4 Co 4 } cubic motifs (Fig. 1) and dichloromethane lattice molecules (12 per cubic unit). Selected bond lengths are listed in the ESI. ‡ Within the {Fe 4 Co 4 } core structure, the iron and cobalt ions occupy alternate corners of the heterocubane, which possesses cyanide ligands in bridging positions along the cube edges. The Fe-CN-Co edge lengths are almost identical, averaging 4.99Å. The Co-N-C and Fe-C-N angles are only slightly bent (from 171.1(3) to 178.8(3) , and from 174.1(3) to 178.4(3) , respectively). The four iron ions exhibit very similar octahedral C 3 N 3 environments, composed of three N donor atoms of the faccoordinated Tp ligands and three cyanide carbon atoms. The Fe-C bond lengths fall in the range of 1.866(4)-1.900(4)Å and are similar to those previously reported for other {Fe II (Tp)(CN) 3 } structural motifs. 9,14 The cyanide stretching vibration at 2103 cm À1 also supports the occurrence of Fe II ions (Fig. S1 ‡). 9,14 The four cobalt ions exhibit an octahedral N 6 environment, surrounded by three N atoms of the fac-pz Tp ligand and three N atoms of the cyanide ligands. Three cobalt ions (Co1, Co2, Co3, Fig. 1) show shorter Co-N bond lengths, ranging from 1.915(3) to 1.943(4)Å. These values are only slightly longer than those previously observed for low-spin Co III ions in related molecular squares and cubes (ca. 1.89-1.91Å). 9-14 However, they are far shorter than typical Co-N bond lengths, ca. 2.1Å, observed in high-spin cobalt(II) complexes. 9,10,12-14 The Co1, Co2, and Co3 coordination spheres also exhibit a moderate octahedral distortion (the sum of the deviation to 90 being S ¼ 14.7-18.3 ), which matches better with low-spin Co III ions. In contrast, the unique cobalt ion Co4 exhibits much longer Co-N distances (average: 2.01Å), though shorter than typical Co II HS -N ones, and a signicantly more distorted octahedral environment (S ¼ 28.6 ). This is rather indicative of a high-spin Co II ion. Although the Co-N-C angles are slightly bent, they do not show a clear trend or difference between the Co II and Co III ions. Overall, some structural disorder in the position of the Co II HS and Co III LS ions likely account for the slight deviations observed in the coordination sphere geometries compared to "usual" Co III LS and Co II HS geometries. This seems coherent with the localization of the potassium ion, 25 which appears to be disordered over different positions inside the cubic cage. As previously observed by Rauchfuss et al. in a related K3{Rh 4 Mo 4 } cyanide box, the K + Lewis acid is not located in the center of the cube, but establishes interactions with three cyanide p systems, with short K-C and K-N distances of $3.2-3.4Å. 26 Here, the potassium ion is displaced toward the {Co(NC) 3 } corners but exhibits a marked preference for the Co4 corner (occupancy 50%). It appears reasonable to assume that this preference for the {Co4(NC) 3 } corner correlates with the formal negative local charge of the latter. Despite our efforts to synthesise better-ordered crystal phases, the structural disorder seems to be a marked trend in these cubic systems.

Magnetic measurements and EPR spectroscopic studies
In order to conrm the electronic states of the metal ions in 1, we have performed solid state magnetic studies. The continuous-wave X-band EPR spectrum recorded at low temperature ( Fig. 2) is typical for octahedral Co II HS complexes with axial symmetry and a large positive zero-eld splitting (D ⪢ 9.34 GHz). In such a case, EPR transitions are only observed within the lowest Kramer doublet (with an effective electron spin of 1/2). 27,28 The simulation of the spectrum 29 leads to the spin-Hamiltonian parameters: g efft ¼ 1.85 and g effk ¼ 7.71, with hyperne coupling to the Co II HS ion (I ¼ 7/2) of A t ¼ 80 MHz and A k ¼ 960 MHz. These values are comparable to those observed for octahedral Co II HS complexes exhibiting, as in the present Scheme 1 Simplified synthesis of 1 (further details, see ESI ‡). case, a C 3 axis and containing related tris(pyrazolyl)borate or tris(pyrazolyl)methanide ligands. 30,31 Magnetic susceptibility measurements were performed in the 2-400 K temperature range on freshly ltered crystalline powders (Fig. 2). Up to room temperature, 1 exhibits a c M T vs. T curve (c M is the molar magnetic susceptibility per cubic unit) which can be analysed considering the presence of one isolated octahedral high-spin Co II ion per cubic unit. 32 Indeed, the magnetic data between 10 and 300 K can be simulated using the T-P isomorphism approach 33-35 and the following Hamiltonian, which is appropriate to describe isolated Co II ions: with the following contributions: l is the spin-orbit coupling constant, a is the orbital reduction factor and D is the axial distortion parameter. L and S are the orbital and spin operators, respectively, with L ¼ 1 and S ¼ 3/2. The least square t of the magnetic c M T data leads to the set of values: l ¼ À147 cm À1 , a ¼ 0.76, and D ¼ À3638 cm À1 , with a good agreement factor. 36 Above approximately 300 K, the c M T vs. T experimental data deviate from the theoretical curve and show a steady increase that is likely due to a thermally-induced ETCST. In contrast to the previously reported photomagnetic {Fe 4 Co 4 } cube, 9 which shows a steep thermally-induced transition from the diamagnetic {Fe II LS Co III LS } 4 state to the paramagnetic {Fe III LS Co II HS } 4 state near 250 K, 1 shows a gradual transition. The c M T values obtained at 400 K and the absence of a plateau indicate a partial ETCST. The measured c M T value of 4.5 cm 3 mol À1 K at 400 K roughly corresponds to ca. 33% of the value expected for the fully paramagnetic state. The shi in the transition temperature is likely associated with the anionic charge and stronger donor character of the pz Tp ligand that stabilises the Co III redox state compared to the related neutral ligand (2,2,2-tris(pyrazolyl) ethanol) used by Holmes et al. 9 It should also be noticed that the thermally-induced ETCST is not reversible aer heating the sample up to 400 K (see ESI ‡). This is likely associated with the loss of crystallization solvent molecules, as previously observed in other FeCo molecular switches. 11b More interestingly, 1 shows signicant photomagnetic effects upon irradiation in the visible and near-infrared range. Signicant increases in magnetisation are observed upon irradiation with laser light at 405, 532, 635, 808 and 900 nm (Fig. S2, ESI ‡). As observed in related {Fe 2 Co 2 } squares, 14 the 808 nm wavelength is the most efficient, and shows the highest photoconversion rate. The thermal stability of the photo-induced metastable state was probed by measuring the c M T vs. T curve aer irradiation at 808 nm (empty circles plot in Fig. 2). The metastable state undergoes thermal relaxation at ca. T relax ¼ 80 K. This temperature is notably lower than that observed for the only other {Fe 4 Co 4 } photomagnetic cube (T relax z 175 K). 9 Finally, the dynamic magnetic properties of 1 were probed by measuring the alternating current magnetic susceptibility as a function of temperature, frequency and dc magnetic eld in both the ground state and the photo-induced one (see ESI ‡). In the ground-state, out-of-phase susceptibility signals (c 00 M ) were observed under a magnetic eld (in the range of 0-3 kOe) with an optimal value of 1.8 kOe (Fig. 3). Only very weak c 00 M signals are observed at zero-eld (Fig. S4 ‡), likely because of a fast relaxation through quantum tunnelling of the magnetisation (QTM). The Cole-Cole plots 37 at 1.8 kOe show a semi-circle shape (Fig. 3) and can be tted using a generalized Debye model in order to obtain the temperature dependence of the relaxation time s. The linear regime observed in the ln(s) versus (1/T) curve at high temperature is coherent with an Orbach (thermallyinduced) relaxation process (Fig. S5). The t of the linear region using an Arrhenius law leads to an effective energy barrier for the reversal of the magnetization of 34 cm À1 and a relaxation time of s 0 ¼ 1.1 Â 10 À7 s (see ESI ‡). The discrepancies from the linear regime at low temperature are due to other possible relaxation processes such as QTM and Raman relaxation processes. Overall, the complex 1 shows Single-Molecule Magnet (SMM) behaviour, which is reminiscent of that recently observed for other anisotropic Co II complexes. 38-40 It is worth noticing that the slowrelaxation disappears in the photomagnetic state so that 1 could be considered as a light-switchable SMM.

NMR spectroscopic studies in solution and mass spectrometry
A noticeable feature of 1 is its solubility in organic solvents such as CH 2 Cl 2 . All the solution-based analytical characterisation data point to the remarkable stability of the K3{Fe 4 Co 4 } box in CH 2 Cl 2 solution. For example, the EPR spectrum of 1 in frozen CH 2 Cl 2 solution is very similar to that obtained for the powder sample, and suggests a similar electronic state (Fig. S6 ‡).
More convincing structural evidence for the stability of 1 is provided by the NMR study (Fig. 4). The 1 H NMR spectrum recorded in the 183-293 K temperature range is indeed fully consistent with the ascribed {Fe II 4 Co III 3 Co II } redox states. The occurrence of one paramagnetic Co II HS ion in one of the vertices of the cubic unit has several impacts both on the NMR spectrum itself and the interpretation and assignment of the signals. (i) Strongly shied 1 H signals are observed in both the positive and negative frequency regions of the spectrum. This effect can help to assign the signals as the chemical shis are expected to be more important for those protons closely connected to the paramagnetic center. 41 (ii) Because of the nuclear-electron dipolar interaction, line broadening effects are observed. This interaction depends on the proton-metal distance and also helps to assign the proton signals. 41 (iii) The chemical shis show a signicant temperature dependence, with an overall increase in the absolute value upon cooling (Fig. S7-S9 ‡). This behaviour is typical of paramagnetic molecules, as their chemical shis exhibit a signicant contribution from the hyperne interaction. Here again, the changes in the chemical shis mainly affect the proton signals located in the vicinity of the paramagnetic ion, bearing a signicant amount of spin density. 42 The occurrence of one non-equivalent Co(II) ion per cubic unit also induces a lowering of the symmetry from T d to C 3v (assuming that the potassium ion resides on the C 3 axis; see scheme in Fig. 4). Consequently, the pyrazolyl entities of the Tp and pz Tp ligands, whose boron atoms are not located on the C 3 axis, are not equivalent any more. In order to visualise this effect, the non-equivalent pyrazolyl groups are depicted as bars with different colours in the schematic drawing in Fig. 4 (note that the non-coordinated pz groups of the pz Tp ligand are omitted for clarity). Remarkably, all of the 24 expected 1 H NMR signals of the pyrazolyl entities can be observed in the 1 H NMR spectrum, provided that the temperature is low enough (in our case, 233 K). 43 To some extent, the assignment of the pyrazolyl proton signals was either possible due to the above mentioned remarks and using the relative intensity of the signals (see details in the ESI ‡), or, for those signals possessing favourable nuclear relaxation, by applying the 1 H gCOSY method. The 11 B NMR spectrum recorded at room temperature also supports this structural analysis (Fig. S10 ‡). Three 11 B signals are observable, i.e. one for the paramagnetic {Co II ( pz Tp)} unit, which strongly shis with temperature, and two signals at 1.6 ppm and at À13.5 ppm, which can be assigned to the boron atoms of the {Co III ( pz Tp)} units and the {BH} moiety of the {Fe II (Tp)} units. Finally, it is worth noticing that the K3{Fe 4 Co 4 } box is quite stable in solution over three months, as the NMR spectra recorded during this period only show very small amounts of degradation products (Fig. S11 ‡).
Diffusion NMR studies were performed on a dichloromethane solution of 1. In spite of the reduced spin-lattice relaxation times of the proton due to the paramagnetic nature of the complex, the mean diffusion coefficient could be estimated as D ¼ 6.88 Â 10 À10 m 2 s À1 . This corresponds to a spherical hydrodynamic radius of 7.6Å, which is in line with the expected value for a fully stable species in solution (see ESI ‡).
Mass spectrometric studies provided further proof for the stability of the cubic moieties in CH 2 Cl 2 . In the ESI-MS spectrum (cation mode), a molecular peak corresponding to the mono-oxidized [K3{Fe II 4 Co III 4 }] + species was observed, with the expected isotopic pattern for an exact molecular mass of M ¼ 2779.4 g mol À1 (Fig. S13 ‡). The only other detected peak, at m/z ¼ 617, corresponds to residual traces of the [Co III ( pz Tp) 2 ] + by-product from the last purication step of the synthesis. In the anion mode ESI-MS spectrum, the only detectable signal (m/z ¼ 2814) corresponds to an adduct between the neutral 1 and a chloride anion from the solvent, furnishing [1$Cl] À .

Cyclic voltammetry studies
In view of the structural integrity of 1 in organic solvents and in order to probe its redox properties, cyclic voltammetry studies were performed in CH 2 Cl 2 at room temperature. Interestingly, 1 presents six accessible redox states (Fig. 5). The rst redox process at E pa ¼ À0.23 V (vs. Fc/Fc + ) is ascribed to the one-electron oxidation of the high-spin Co(II) centre into a low-spin Co(III). The corresponding reduction wave is evidenced at E pc ¼ À0.95 V (see the ESI ‡ for further details). The large difference between the two half waves (ca. 0.93 V) is due to the structural reorganisation that accompanies the spin-state change. Such behaviour is typical for a redox process which is coupled to a spin transition and has already been observed for related cobalt complexes. 31,44 At more positive potential values, four well-separated, quasi-reversible redox processes are observable, which are tentatively assigned to the following redox couples:    20 Differential absorption spectra of 1 recorded under controlled potential (from À0.40 to 1.00 V and from 1.00 to À1.15 V) were measured to follow the changes in the optical properties which accompany the redox changes (Fig. 5, bottom). Before oxidation, two intense absorption bands are observed in the visible range. The absorption at l ¼ 410 nm (3 410 ¼ 3585 L mol À1 cm À1 ) compares well with that observed at l z 405 nm in the [Fe II (L)(CN) 3 ] 2À complexes (L ¼ Tp and pz Tp), and is ascribed to a metal-to-ligand charge transfer transition (MLCT). The very broad and intense band centred at l ¼ 618 nm (3 618 ¼ 3500 L mol À1 cm À1 ) is reminiscent of that observed around 550 nm in K x [Co y [Fe(CN) 6 ] z PBAs, 6,45 and is assigned to a Fe II -Co III charge transfer transition (MMCT). Upon oxidation of the Co II ion, from À0.4 to +0.2 V, a blue shi of the band ascribed to the Fe II -Co III CT transition is observed, together with an increase in its intensity (the absorption shis from 620 nm to ca. 590 nm). As the potential is further increased to oxidize the four Fe II ions, a signicant increase in the intensity of the Fe-centred MLCT band (near ca. 500 nm) is observed, as previously reported for the related {Fe 4 Fe 4 } complex. 20 The phenomenon appears to be fully reversible as the successive reduction of the four Fe(III) ions induces a decrease of this band. The initial state is then recovered near ca. À1.1 V in the present experimental conditions as the low-spin Co III ion requires a much lower potential to be reduced (see cyclic voltammetry studies above). However, if the strongly reductive potential is maintained for a few minutes at this stage, slow decomposition of the cube can be observed.

Conclusions
The rst cyanide-bridged K3{Fe 4 Co 4 } "molecular box" (1) containing an inserted potassium ion is reported. As with the original K 0.2 Co 1. 4 [Fe(CN) 6 ]$6.9H 2 O inorganic polymer, 1 exhibits a remarkable photomagnetic effect at low temperature, which is ascribed to a photo-induced electron transfer. Remarkably, the complex also behaves as a photo-switchable single molecule magnet. As evidenced by various methods such as NMR and EPR spectroscopy, ESI-MS spectrometry, and electrochemistry, compound 1 is soluble and stable in CH 2 Cl 2 solution. Indeed, the overall neutral complex remains fully undissociated in solution, still possessing the potassium-lled heterocubane-type structure. Moreover, cyclic voltammetry studies reveal that 1 exhibits remarkable redox exibility with six accessible redox states. The stability and the electronic exibility of 1 are appealing features, which open up perspectives for the insertion of this switchable molecule into hybrid materials. Our current efforts are devoted to more fundamental aspects, as we are currently exploring the role of the nature of the inserted alkali ions on the electronic and magnetic properties of similar "molecular boxes".