Reversible guest-induced gate-opening with multiplex spin crossover responses in two-dimensional Hofmann clathrates†

Spin crossover (SCO) compounds are very attractive types of switchable materials due to their potential applications in memory devices, actuators or chemical sensors. Rational chemical tailoring of these switchable compounds is key for achieving new functionalities in synergy with the spin state change. However, the lack of precise structural information required to understand the chemical principles that control the SCO response with external stimuli may eventually hinder further development of spin switching-based applications. In this work, the functionalization with an amine group in the two-dimensional (2D) SCO compound {Fe(5-NH2Pym)2[MII(CN)4]} (1M, 5-NH2Pym = 5-aminopyrimidine, MII = Pt (1Pt), Pd (1Pd)) confers versatile host–guest chemistry and structural flexibility to the framework primarily driven by the generation of extensive H-bond interactions. Solvent free 1M species reversibly adsorb small protic molecules such as water, methanol or ethanol yielding the 1M·H2O, 1M·0.5MeOH or 1M·xEtOH (x = 0.25–0.40) solvated derivatives. Our results demonstrate that the reversible structural rearrangements accompanying these adsorption/desorption processes (1M ↔ 1M·guest) follow a gate-opening mechanism whose kinetics depend not only on the nature of the guest molecule and that of the host framework (1Pt or 1Pd) but also on their reciprocal interactions. In addition, a predictable and reversible guest-induced SCO modulation has been observed and accurately correlated with the associated crystallographic transformations monitored in detail by single crystal X-ray diffraction.


Physical characterization
Elemental analyses (C, H, and N) were performed with a CE Instruments EA 1110 CHNS Elemental analyzer.
Magnetic measurements were performed with a Quantum Design MPMS-XL-5 SQUID magnetometer working in the 2 to 400 K temperature range with an applied magnetic field of 0.1 T.
Experimental susceptibilities were corrected for diamagnetism of the constituent atoms by the use of Pascal's constants.
Calorimetric measurements were performed using a differential scanning calorimeter Mettler Toledo DSC 821e. Low temperatures were obtained with an aluminium block attached to the sample holder, refrigerated with a flow of liquid nitrogen and stabilized at a temperature of 110 K. The sample holder was kept in a dry box under a flow of dry nitrogen gas to avoid water condensation.
The measurements were carried out using around 15 mg of polycrystalline samples sealed in aluminium pans with a mechanical crimp. Temperature and heat flow calibrations were made with standard samples of indium by using its melting transition (429.6 K, 28.45 J g -1 ). An overall accuracy of ±0.2 K in temperature and ±2% in the heat capacity is estimated. The uncertainty increases for the determination of the anomalous enthalpy and entropy due to the subtraction of an unknown baseline.
Powder X-ray diffraction measurements where performed on a PANalytical Empyrean X-ray powder diffractometer (monochromatic CuKα radiation) in capillary measurement mode. Due to the spontaneous rehydration of 1 Pt and 1 Pd , these samples were prepared by heating the hydrated forms into open capillaries inside an oven at 120ºC during 1 hour and rapidly sealing them to avoid the entering of air.
Single crystal X-ray measurements. Single crystals were mounted on a glass fiber using a viscous hydrocarbon oil to coat the crystal and then transferred directly to the cold nitrogen stream for data collection. X-ray data were collected on a Supernova diffractometer equipped with a graphitemonochromated Enhance (Mo) X-ray Source (λ = 0.71073 Å). The program CrysAlisPro, Oxford Diffraction Ltd., was used for unit cell determinations and data reduction. Empirical absorption correction was performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The structures were solved by direct methods using SHELXS-2014 and refined by full matrix least-squares on F 2 using SHELXL-2014 (Sheldrick, G. M. Crystal Structure Refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8). Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions refined using idealized geometries (riding model) and assigned fixed isotropic displacement parameters.
Adsorption/desorption isotherms. Vapor adsorption measurements were recorded on a Micromeritics 3Flex apparatus at relative pressures up to 1 bar and performed ex situ on 1 M . Samples were degassed overnight at 150 ºC and 10 -6 Torr prior to analysis assuring the presence of the totally desolvated 1 M compounds. A Micromeritics' ISO Controller was used to keep the temperature constant at 293 K for the H 2 O, MeOH or EtOH adsorption measurements.
TGA experiments were carried out with a TA instruments TGA550 device equipped with a Pt/Rh oven (Tmax = 1000°C). The time dependent TGA experiments were performed by connecting the TGA apparatus to a flow mass controller. Thus, a controlled dry nitrogen flow (60 l/min) was passed through the desired solvent (water, methanol or ethanol) at room pressure and a temperature of 30°C and then the mixture (N 2 +solvent vapor) was driven until the TGA chamber where a previously desolvated sample 1 M was mounted in a Pt pan. Figure S1. Thermogravimetric analyses of a) 1 Pt ·H 2 O, b) 1 Pd ·H 2 O, c) 1 Pt ·0.5MeOH, d) 1 Pd ·0.5MeOH, e) 1 Pt ·0.4EtOH and f) 1 Pt ·25EtOH.   (1) Figure S3. Powder X-ray diffraction patterns of a) 1 M ·H 2 O, b) 1 M ·MeOH and 1 M ·EtOH and c) 1 M series. Simulated patterns are also displayed for comparison.

Time dependent TGA measurements
Kinetic water, methanol and ethanol adsorption experiments were performed through TGA measurements for activated 1 Pt and 1 Pd compounds ( Figure S2a and S2b). As observed in Figure 2a, the tetracyanoplatinate derivative adsorbs 0.97/0.41/0.14 molecules of water/methanol/ethanol per Fe II ion. However, in the same conditions, the tetracyanopalladate network adsorbs a smaller fraction of guest molecules (0.91/0.25/0.01 molecules of water/methanol/ethanol per Fe II ion) during the same range of time. In order to estimate the adsorption rates exhibited by each derivative for the different guests, the experimental isotherm curves were fitted with the kinetic Avrami equation: where α is the adsorbed fraction at time t, A is the total amount of adsorbed guest at time ∞, K av is the rate constant, and n is the Avrami exponent which defines the cooperativity of the adsorption process. The kinetic parameters resulting from the fittings are gathered in Table S1. Whereas methanol adsorption curves show higher uptake rates, those of water exhibit higher n values reflecting a certain degree of cooperativity throughout the adsorption process. Besides, for a given guest, 1 Pt displays higher kinetic constants and lower n values than 1 Pd . Figure S4. Time dependent TGA measurements registered during the adsorption of water, methanol and ethanol at room temperature and pressure. Solid lines correspond to the corresponding fittings following the Avrami's equation. Table S3. Avrami parameters extracted from fitting of the experimental TGA curves depicted in Figure 4. 1 Pd ·EtOH has been excluded from fitting because it does not present significant adsorption at the studied conditions.      Figure S9. SCO properties recorded at 2 K min -1 of 1 M ·H 2 O (M = (a) Pt or (b) Pd) before heating (left), after heating at 400 K for 30 minutes (middle) and after exposure of 1 Pt to air for 3 hours (right). Figure S10. a)  M T vs T curves measured at 2 K min -1 of 1 Pt ·0.4EtOH, 1 Pd ·0.5MeOH and 1 Pt ·0.5MeOH in the 50-250 K range.  Figure S13. T vs T plots for 1 Pt ·0.5MeOH at increasing pressures (scan rate: 2 K min -1 ).  Figure S14. Powder X-ray diffraction pattern of 1 Pd ·EtOH soaked in ethanol (blue line). Diffraction patterns of 1 Pd (red line) and 1 Pt ·EtOH are also showed for comparison. Three insets corresponding to the 2 ranges delimited by the red dashed areas are also depicted to see the spectra in more detail. As explained in the main text, the pattern of 1 Pd ·EtOH is very similar to that of 1 Pd indicating that a low quantity of EOH is adsorbed by 1 Pd and consequently the spectrum is barely modified. However, the inset plots reveal some weak peaks (indicated by black arrows) which are reminiscent of the 1 Pd ·EtOH. Figure S15.  M T vs T curves measured at 2 K min -1 for freshly dehydrated (a) 1 Pt or (