Spin-Crossover 2-D Hofmann Frameworks Incorporating an Amide-Functionalized Ligand: N -(pyridin-4-yl)benzamide

: Two analogous 2-D Hofmann-type frameworks, which incorporate the novel ligand N -(pyridin-4-yl)benzamide (benpy) [Fe II (benpy) 2 M(CN) 4 ] · 2H 2 O (M = Pd ( Pd(benpy) ) and Pt ( Pt(benpy) )) are reported. The benpy ligand was explored to facilitate spin-crossover (SCO) cooperativity via amide group hydrogen bonding. Structural analyses of the 2-D Hofmann frameworks revealed benpy-guest hydrogen bonding and benpy-benpy aromatic contacts. Both analogues exhibited single-step hysteretic spin-crossover (SCO) transitions, with the metal-cyanide linker (M = Pd or Pt) impacting the SCO spin-state transition temperature and hysteresis loop width ( Pd(benpy) : T 12 ↓↑ : 201, 218 K, ∆ T : 17 K and Pt(benpy) : T 12 ↓↑ : 206, 226 K, ∆ T : 20 K). The parallel structural and SCO changes over the high-spin to low-spin transition were investigated using variable-temperature, single-crystal, and powder X-ray diffraction, Raman spectroscopy, and differential scanning calorimetry. These studies indicated that the ligand–guest interactions facilitated by the amide group acted to support the cooperative spin-state transitions displayed by these two Hofmann-type frameworks, providing further insight into cooperativity and structure–property relationships.

Effective SCO communication in these materials comes from the metallocyanide groups and can be modulated by the organic ligand and encapsulated guest molecules. Hence, the choice of organic ligand incorporated into Hofmann frameworks has a significant effect on the SCO properties observed, and, thus, substantial focus has been directed at structurefunction studies with various ligand functional groups and lengths. For example, in the 2-D Hofmann framework [Fe II (pyridine) 2 M(CN) 4 ] (M = Pd or Pt) a thermally induced, singlestep hysteretic SCO transition was observed (T 1 2 ↓↑: 208, 213 K; ∆T = 5 and T 1 2 ↓↑: 208, 216 K; ∆T = 8, for the Pd and Pt analogues, respectively) [15]. When pyrazine was used in place of pyridine to form the 3-D Hofmann analogue [Fe II (pyrazine)Pd(CN) 4 ], a higher transition temperature and a wider hysteresis loop (T 1 2 ↓↑: 220, 240 K; ∆T = 20 K) was observed [15]. However, framework dimensionality alone is not the only possible influencing factor on SCO cooperativity. For example, pyridine functionalization in the 2-D framework [Fe II (4phenylpyridine) 2 Pd(CN) 4 ] resulted in an increased transition temperature compared to the pyridine analogue and an impressive hysteresis loop width of 40 K [23]. Structural studies on this analogue highlight that, alongside the framework connectivity, aromatic contacts play a large role in enhancing the SCO communication.  [24]. Hence, the concept of targeting increased SCO cooperativity by both incorporating SCO sites into CPs and also including functionalized ligands with hydrogen bonding and/or aromatic interaction capacity serves as the basis for the materials prepared in this report. We utilized the ligand N-(pyridin-4-yl)benzamide (benpy; Figure 1) [25] to facilitate SCO cooperativity via both hydrogen bonding from the amide functional group and aromatic contacts via the aromatic rings ( Figure 1). Amide-functionalized ligands were successfully applied to a range of SCO framework previously, resulting in a range of interesting structures and SCO features [26][27][28][29][30] based on interaction with the amide group. Here, we report the structure and SCO properties of the 2-D Hofmann frameworks' analogues [Fe II (benpy) 2 (M(CN) 4 )] (M = Pd and Pt) with an interest in the impact of amide group contacts on the structure and cooperativity of the spin-state transition.
Chemistry 2021, 3, FOR PEER REVIEW 2 or Ni) and connected (3-D) or spaced (2-D) into frameworks by N-donor aromatic organic ligands [9,[14][15][16][17][18][19][20][21][22]. Effective SCO communication in these materials comes from the metallocyanide groups and can be modulated by the organic ligand and encapsulated guest molecules. Hence, the choice of organic ligand incorporated into Hofmann frameworks has a significant effect on the SCO properties observed, and, thus, substantial focus has been directed at structure-function studies with various ligand functional groups and lengths. For example, in the 2-D Hofmann framework [Fe II (pyridine)2M(CN)4] (M = Pd or Pt) a thermally induced, single-step hysteretic SCO transition was observed (T½↓↑: 208, 213 K; ΔT = 5 and T½↓↑: 208, 216 K; ΔT = 8, for the Pd and Pt analogues, respectively) [15]. When pyrazine was used in place of pyridine to form the 3-D Hofmann analogue [Fe II (pyrazine)Pd(CN)4], a higher transition temperature and a wider hysteresis loop (T½↓↑: 220, 240 K; ΔT = 20 K) was observed [15]. However, framework dimensionality alone is not the only possible influencing factor on SCO cooperativity.  [24]. Hence, the concept of targeting increased SCO cooperativity by both incorporating SCO sites into CPs and also including functionalized ligands with hydrogen bonding and/or aromatic interaction capacity serves as the basis for the materials prepared in this report. We utilized the ligand N-(pyridin-4-yl)benzamide (benpy; Figure 1) [25] to facilitate SCO cooperativity via both hydrogen bonding from the amide functional group and aromatic contacts via the aromatic rings ( Figure 1). Amide-functionalized ligands were successfully applied to a range of SCO framework previously, resulting in a range of interesting structures and SCO features [26][27][28][29][30] based on interaction with the amide group. Here, we report the structure and SCO properties of the 2-D Hofmann frameworks' analogues [Fe II (benpy)2(M(CN)4)] (M = Pd and Pt) with an interest in the impact of amide group contacts on the structure and cooperativity of the spin-state transition.

Materials and Methods
Reagents were purchased commercially from Sigma-Aldrich or Merck and not further purified before use. Care was taken, handling Fe(ClO4)2·6H2O only in small amounts to avoid any potential explosions.

Synthesis of N-(Pyridin-4-Yl)Benzamide (Benpy)
The 4-aminopyridine (1.69 g, 18.0 mmol) and triethylamine (2.28 g, 22.5 mmol) were dissolved in CH2Cl2 (30 mL). This solution was placed in an ice bath, wherein benzoyl chloride (2.11 g, 15.0 mmol) was added with continuous stirring once the temperature reached 0 °C. The mixture was then allowed to return to room temperature while stirring

Materials and Methods
Reagents were purchased commercially from Sigma-Aldrich or Merck and not further purified before use. Care was taken, handling Fe(ClO 4 ) 2 ·6H 2 O only in small amounts to avoid any potential explosions.

Synthesis of N-(pyridin-4-yl)benzamide (benpy)
The 4-aminopyridine (1.69 g, 18.0 mmol) and triethylamine (2.28 g, 22.5 mmol) were dissolved in CH 2 Cl 2 (30 mL). This solution was placed in an ice bath, wherein benzoyl chloride (2.11 g, 15.0 mmol) was added with continuous stirring once the temperature reached 0 • C. The mixture was then allowed to return to room temperature while stirring for 1 h. The translucent, needle-like precipitate was collected via filtration and washed with excess CH 2 Cl 2 . The crude product was recrystallized using 1:

Thermogravimetric Analysis (TGA)
Crystalline samples were loaded onto a platinum pan and measured on a TA Instruments Discovery TGA Analyzer at a ramp rate of 1 • C min −1 (RT-500 • C) under a dry N 2 gas environment (20 mL min −1 ).

Variable-Temperature, Single-Crystal X-ray Diffraction Analysis (VT-SCXRD)
Data were collected at the Australian Synchrotron macromolecular crystallography (MX1) beamline radiation source [31]. Data were collected using a Dectris Eiger 9M detector and Si<111> monochromated synchrotron radiation (λ = 0.71073 Å). The data were collected using the BlueIce software [32] and were processed using the XDS software [33]. All structures were solved using SHELXT [34] and refined using SHELXL [35] within the OLEX2 graphical user interface [36]. Hydrogen atoms were placed at idealized positions and refined using the riding model. Details of the crystallographic data collection and refinement parameters are summarized in Table 1 and selected parameters are presented in Table S2. The data have been deposited in the Cambridge Crystallographic Data Centre (CCDC) and are freely available at CCDC: 2049372-2049375.

Variable-Temperature, Powder X-ray Diffraction Analysis (VT-PXRD)
Data were collected at the Powder Diffraction beamline BL-10 (20.0 keV, 0.589062 Å) at the Australian Synchrotron [37]. A polycrystalline sample was loaded into 0.7-mmdiameter borosilicate glass capillary and mounted onto the powder diffraction beamline. The capillaries were rotated at~1 Hz during data collection to assist in powder averaging. The beamline was set up with a nominal wavelength of 1.0 Å; the wavelength was determined accurately using NIST SRM660b LaB 6 standard. Temperature-dependent data were then collected continuously over the range 175-250 K. Data were collected using a Mythen microstrip detector [38] from 1.5 • to 75 • in 2θ. To cover the gaps between detector modules, two data sets, each of 60 s in duration, were collected with the detector set 5 • apart and they were then merged to give a single data set using PDViper [39]. A slit size of 2 mm was used to ensure that the fraction of the capillary illuminated by the X-ray beam was the same as the isothermal zone on the cryostream. Le Bail analysis and peak fitting were performed using the TOPAS software package [40].

Variable-Temperature Magnetic Susceptibility
Variable-temperature magnetic susceptibility data were obtained using a Quantum Design VersaLab magnetometer with a Vibrating Sample Magnetometer (VSM) accessory attached. Measurements were taken continuously over the range 150-300 K under a 0.3 T magnetic field with sweep rates of 0.5, 1, 2, and 4 K min −1 . The crystalline samples were loaded into polypropylene holders (Formolene 4100 N) and clipped into a brass half-tube for analysis.

Differential Scanning Calorimetry (DSC)
Measurements were taken at 10 K min −1 (294-190 K) using a Netzsch DSC 204 F1 Phoenix with a liquid nitrogen cooling cryostat. The crystals were placed into a 40-µL aluminum crucible and sealed. Instrument calibrations were made using sapphire standards with the sample holder kept dry and under a constant, dry N 2 flow (20 mL min −1 ). The Netzsch Proteus Analysis 2019 software was used to process the data.

Calculation of the Magnetic Susceptibility
The magnetic susceptibility was calculated from the Slichter-Drickamer model [41,42], which predicts that the HS fraction, γ HS , is given by where ∆H and ∆S are, respectively, the enthalpy and entropy differences between the HS and LS states, R is the gas constant, and Γ parameterizes the interactions between metal centers. We neglected magnetic interactions between metals in the HS state, whence one finds that the magnetic susceptibility is given by where χ LS (χ HS ) is the susceptibility in the LS (HS) state. For both Pd(benpy) and Pd(benpy) we obtained the best fits on constraining χ LS = 0 and χ HS T = 3.6 cm 3 K mol −1 .
No closed form solution was known for the transcendental Equation (1), so a numerical solution was obtained self-consistently for a range of parameters (∆H, ∆S, and Γ), with the best fit taken as the parameters with the lowest root-mean-square error relative to the measured susceptibility. We swept through parameters three times, each time narrowing the range but moving through finer steps, to obtain accurate solutions in a reasonable time.
The Slichter-Drickamer model is equivalent to a mean field treatment of the Ising-Wajnflasz-Pick model [42,43]. Like all mean field theories, it is more accurate in higher dimensions. So long as the interlayer elastic interactions are not too small (compared to k B T), Pd(benpy) and Pd(benpy) are three-dimensional materials and mean field theory should be reasonable. The anisotropy between the intra-and interlayer elastic interactions is only captured on average but does not affect the applicability of the theory. If the interlayer elastic interactions are small compared to k B T, then Pd(benpy) and Pd(benpy) are two-dimensional materials. As we did not discuss critical behavior below a mean field, approximation is reasonable in either case.

Variable-Temperature Raman Spectroscopy
Raman spectra were collected on a Renishaw inVia-Qontor upright microscope. The excitation wavelength of 785 nm (20 mW at 10% laser power, L ×50 objective) was used to acquire the Raman spectra in the range 150-2300 cm −1 with a 10-s exposure time. The samples were loaded into 0.7-mm-diameter glass capillaries and mounted on a Linkam-FTIR 600 variable temperature stage. Liquid nitrogen was continuously flowed onto the Linkam stage with a temperature programmer and cooling pump connected. Data were collected over the range 170-250 K at 10-K steps.

Synthesis and Characterization
The two materials Pd(benpy) and Pt(benpy) were successfully synthesized as bulk crystalline solids via slow diffusion methods. Their purity was confirmed via elemental analysis (CHN), IR spectroscopy, Le Bail analysis of the PXRD data ( Figures S4 and S5) and TGA measurements. The TGA data ( Figure S1) showed an initial mass loss below~50 • C (Pd(benpy): 4.90% and Pt(benpy): 4.33%) corresponding to a theoretical loss of two solvent water molecules per Fe II site (calculated for Pd(benpy): 4.58% and Pt(benpy): 4.07%). There was no further mass loss until~300 • C, when framework decomposition occurred.

Variable-Temperature Magnetic Susceptibility
A color change from yellow to red was observed when the crystals of Pd(benpy) and Pt(benpy) were cooled with liquid N 2, indicative of a HS to LS transition ( Figure  S2). Variable-temperature magnetic susceptibility measurements (300-150 K) revealed hysteretic, single-step SCO transitions (Figure 2a,b). The χ M T values remained constant at~3.50 cm 3 Kmol −1 from 300 to 196 K for Pd(benpy) and 300-203 K for Pt(benpy), corresponding to HS Fe II sites (S = 2) [10]. Below this, a rapid decrease in χ M T values occurred, to~0.1 cm 3 Kmol −1 , indicative of a complete HS to LS (S = 0) transition of the Fe II sites. The scan rate dependence of the SCO transition was explored (rates: 0.5, 1, 2, and 4 K min −1 ; Figure 2a temperature observed in the Pt analogue compared to the Pd analogue was consistent with other Hofmann-type frameworks [9,[14][15][16][17][18][19][20][21][22]. The abrupt transition character and approximate transition temperatures and hysteresis loop width were consistent with other 2-D Hofmann frameworks [44].

Pt(benpy)).
It can be shown [43] that the Slichter-Drickamer model is equivalent to the meanfield solution of the long-range Ising-Wajnflasz-Pick model and, thence, that where , is the coupling between metal centers unit cells apart in the crystallographic a-direction and unit cells apart in the crystallographic b-direction. Thus, Γ measures the sum of the microscopic interactions, , , which, in turn, can be related to
It can be shown [43] that the Slichter-Drickamer model is equivalent to the mean-field solution of the long-range Ising-Wajnflasz-Pick model and, thence, that where J n,m is the coupling between metal centers n unit cells apart in the crystallographic a-direction and m unit cells apart in the crystallographic b-direction. Thus, Γ measures the sum of the microscopic interactions, J n,m , which, in turn, can be related to the underlying elastic interactions in the material [43]. Thus, the large positive values of Γ (37% of ∆H in Pd(benpy) and 28% of ∆H in Pt(benpy)) showed that the sum of the elastic interactions in both materials was significant and ferroelastic. Overall, the modeling showed that the spin-state transitions in both Pd(benpy) and Pt(benpy) were strongly cooperative.

Variable-Temperature Powder X-ray Diffraction
Data were collected to monitor the structure variation over the SCO transition (250-175 K). Figure 3a and Figure S8a show the evolution of a selected Bragg peak (2θ = 6.5-6.8 • , hkl: 011) for Pd(benpy) and Pt(benpy), respectively, over the HS to LS to HS transition. These data are also presented as peak position versus temperature ( Figures S8b and S9), showing SCO character and transition temperatures consistent with magnetic susceptibility data. All of the data over the temperature range 250-175-250 K were fitted by Le Bail refinement. These data: (1) confirmed the bulk purity of the samples (Figures S6 and S7), (2) confirmed the symmetry and space group, and (3) allowed the temperature dependence of the unit cell parameters to be assessed. Figure 3b and Figure S11d show the unit cell volume evolution for Pd(benpy) and Pt(benpy), respectively, confirming the abrupt single-step volumetric change over the reversible spin-state transition and an overall volume change of~5.5%, consistent with a complete HS to LS transition. The evolution of the unit cell lengths for Pd(benpy) and Pt(benpy) are shown in Figures S10 and S11, respectively. The transition temperatures from VT-PXRD data were ca. 5 K lower than that observed by magnetic susceptibility. This is not uncommon due to the differences in experimental conditions. Here, the ramp rate for data collection was much faster for VT-PXRD than magnetic susceptibility. Chemistry 2021, 3, FOR PEER REVIEW 7 the underlying elastic interactions in the material [43]. Thus, the large positive values of Γ (37% of ΔH in Pd(benpy) and 28% of ΔH in Pt(benpy)) showed that the sum of the elastic interactions in both materials was significant and ferroelastic. Overall, the modeling showed that the spin-state transitions in both Pd(benpy) and Pt(benpy) were strongly cooperative.

Variable-Temperature Powder X-ray Diffraction
Data were collected to monitor the structure variation over the SCO transition (250-175 K). Figure 3a and Figure S8a show the evolution of a selected Bragg peak (2θ = 6.5-6.8°, hkl: 011) for Pd(benpy) and Pt(benpy), respectively, over the HS to LS to HS transition. These data are also presented as peak position versus temperature (Figures S8b and  S9), showing SCO character and transition temperatures consistent with magnetic susceptibility data. All of the data over the temperature range 250-175-250 K were fitted by Le Bail refinement. These data: (1) confirmed the bulk purity of the samples (Figures S6 and  S7), (2) confirmed the symmetry and space group, and (3) allowed the temperature dependence of the unit cell parameters to be assessed. Figures 3b and S11d show the unit cell volume evolution for Pd(benpy) and Pt(benpy), respectively, confirming the abrupt single-step volumetric change over the reversible spin-state transition and an overall volume change of ~5.5%, consistent with a complete HS to LS transition. The evolution of the unit cell lengths for Pd(benpy) and Pt(benpy) are shown in Figures S10 and S11, respectively. The transition temperatures from VT-PXRD data were ca. 5 K lower than that observed by magnetic susceptibility. This is not uncommon due to the differences in experimental conditions. Here, the ramp rate for data collection was much faster for VT-PXRD than magnetic susceptibility.

Variable-Temperature Raman Spectroscopy
VT-Raman spectra were collected (150-2300 cm −1 ; 250-150 K) to monitor vibrational transitions that occur with spin-state change. Figure 4a and Figure S12 show representative HS and LS patterns for Pd(benpy) and Pt(benpy), respectively. The largest frequency shifts between spin states were the bands corresponding to the Fe-N and C≡N stretching, in agreement with the literature on Hofmann frameworks [44]. The characteristic Fe-N stretching peaks (<250 cm −1 ) and C≡N bands (~2200 cm −1 ) were upshifted from the HS to the LS state (Figure 4b and Figures S13 and S14). The C≡N bands also increased in intensity over the HS to LS transition (Figure 4b and Figure S14b, Pd(benpy) and Pt(benpy), respectively).
Chemistry 2021, 3, FOR PEER REVIEW 8 the LS state (Figure 4b and Figures S13 and S14). The C≡N bands also increased in intensity over the HS to LS transition (Figures 4b and S14b, Pd(benpy) and Pt(benpy), respectively).

Variable-Temperature, Single-Crystal X-ray Diffraction
Overall Structural Description Powder diffraction analyses at 250 K confirmed that Pd(benpy) crystallizes in the primitive monoclinic space group: P21/n and Pt(benpy) in the monoclinic C-centered space group C2/m ( Figures S6 and S7). SCXRD data collected on each agreed with this symmetry and space group assignment ( Table 1). The asymmetric unit (ASU; Figure S15) of Pd(benpy) contained one Fe II site, a half of a [Pd(CN)4)] 2-group, one benpy ligand, and one water molecule. The ASU ( Figure S16) of Pt(benpy) contained one Fe II site, a quarter of a [Pd(CN)4)] 2− group, one benpy ligand, and one water molecule. The main distinction between the two is that the benpy ligands showed a long-range, ordered, torsional twist around the amide bond in Pd(benpy) (Figure 5a). In Pt(benpy), the benzyl ring of the ligand showed two-fold disorder (Figure 5b). Hence, the same ligand torsional ligand was observed but with short-range order, thus accounting for the space group difference between the analogues. The water molecule in Pt(benpy) was also disordered over two positions across the mirror plane.
Aside from the difference in symmetry, the overall formula for both was  (Figure 6b,c). Neighboring benpy ligands along the a-direction were oriented in the same direction ( Figure S17a), i.e., within each ligand the oxygen and amide groups pointed in opposite directions and were oriented in the same direction compared to adjacent ligands. The trans-benpy ligands, i.e., those coordinated to the same Fe II site, were related by inversion symmetry and, thus, the amide func-

Variable-Temperature, Single-Crystal X-ray Diffraction Overall Structural Description
Powder diffraction analyses at 250 K confirmed that Pd(benpy) crystallizes in the primitive monoclinic space group: P2 1 /n and Pt(benpy) in the monoclinic C-centered space group C2/m ( Figures S6 and S7). SCXRD data collected on each agreed with this symmetry and space group assignment ( Table 1). The asymmetric unit (ASU; Figure S15) of Pd(benpy) contained one Fe II site, a half of a [Pd(CN) 4 )] 2group, one benpy ligand, and one water molecule. The ASU ( Figure S16) of Pt(benpy) contained one Fe II site, a quarter of a [Pd(CN) 4 )] 2− group, one benpy ligand, and one water molecule. The main distinction between the two is that the benpy ligands showed a long-range, ordered, torsional twist around the amide bond in Pd(benpy) (Figure 5a). In Pt(benpy), the benzyl ring of the ligand showed two-fold disorder (Figure 5b). Hence, the same ligand torsional ligand was observed but with short-range order, thus accounting for the space group difference between the analogues. The water molecule in Pt(benpy) was also disordered over two positions across the mirror plane.
Aside from the difference in symmetry, the overall formula for both was [Fe II (benpy) 2 M(CN) 4 (Figure 6b,c). Neighboring benpy ligands along the a-direction were oriented in the same direction ( Figure S17a), i.e., within each ligand the oxygen and amide groups pointed in opposite directions and were oriented in the same direction compared to adjacent ligands. The trans-benpy ligands, i.e., those coordinated to the same Fe II site, were related by inversion symmetry and, thus, the amide functional groups were facing in opposite directions ( Figure S17b). Additionally, the [Fe II (M(CN) 4 )] layers undulated along the ab-plane, as illustrated in Figure 6c.  (Figure 6b,c). The ligands of adjacent layers stacked in a head-to-tail fashion (Figure 6c) with near overlap. The nearly eclipsed overlap of the interdigitated ligands and close packing along the b-axis allowed for weak offset π-stacking, evidenced by the centroid-centroid distances between benpy ligands of ~3.5 Å (Figure 7). Two water molecules per Fe II site were located in the interlayer spacing. These water molecules were engaged in a range of host-guest hydrogen-bonding interactions with the amide group of the benpy ligands (Figure 7). Similar arrays of host-guest interactions involving amide functional groups have been observed in other SCO frameworks [26][27][28][29][30].  Neighboring 2-D [Fe II (M(CN) 4 )(benpy) 2 ] layers were stacked along the c-axis, with efficient packing provided by ligand interdigitation from neighboring layers (Figure 6b,c). The ligands of adjacent layers stacked in a head-to-tail fashion (Figure 6c) with near overlap. The nearly eclipsed overlap of the interdigitated ligands and close packing along the b-axis allowed for weak offset π-stacking, evidenced by the centroid-centroid distances between benpy ligands of~3.5 Å (Figure 7). Two water molecules per Fe II site were located in the interlayer spacing. These water molecules were engaged in a range of host-guest hydrogen-bonding interactions with the amide group of the benpy ligands (Figure 7). Similar arrays of host-guest interactions involving amide functional groups have been observed in other SCO frameworks [26][27][28][29][30].  Variable-Temperature Data VT-SCXRD data were collected at 10-K intervals over the range of 250-100 K (HS to LS, cooling) for Pd(benpy) and 100-250 K (LS to HS, warming) for Pt(benpy). Plots of the unit cell parameters (a-, b-, and c-axes) and unit cell volume versus temperature (Figures S18 and S19) matched the transition temperature anticipated from magnetic susceptibility. An overall 5.0% volume change was observed for both analogues over the HS to LS transition, consistent with a complete spin-state conversion. Notably, the evolution of the unit Variable-Temperature Data VT-SCXRD data were collected at 10-K intervals over the range of 250-100 K (HS to LS, cooling) for Pd(benpy) and 100-250 K (LS to HS, warming) for Pt(benpy). Plots of the unit cell parameters (a-, b-, and c-axes) and unit cell volume versus temperature (Figures S18 and S19) matched the transition temperature anticipated from magnetic susceptibility. An overall 5.0% volume change was observed for both analogues over the HS to LS transition, consistent with a complete spin-state conversion. Notably, the evolution of the unit cell parameters was different for Pd(benpy) for Pt(benpy). In particular, both the interlayer spacing (i.e., the c-axis) and the β angle increased over the HS to LS transition for Pd(benpy) but decreased for Pt(benpy). The reason for this difference was not entirely clear but may have arisen from the relative structural support provided by the long-versus short-range order of the ligand torsion and water molecules for Pd(benpy) for Pt(benpy), respectively.
Detailed structural analyses for Pd(benpy) for Pt(benpy) were conducted at 250 and 100 K to assess the structure changes over the HS to LS transition. Relevant parameters are summarized in Table S2. The overall variation to average Fe-N bond length for both was~0.2 Å, which was consistent with a complete spin-state transition [10]. The octahedral distortion parameter (Σ), defined as the sum of the deviation from 90 • of the 12 cis-N-Fe-N angles, is also a useful indicator of spin-state change. Typically, it is more regular in the LS state compared to the HS state [10]. However, for Pd(benpy) the degree of distortion was seen to increase over the HS to LS transition (HS = 8.16 • and LS = 13.20 • ) and for Pt(benpy) there was no substantial change over the SCO transition. This is often an indicator of elastic frustration [46][47][48][49][50], which arises from a competition between interactions and the volumetric change over the spin-state transition. In most cases, this results in multi-stepped SCO transitions, but here the cooperative one-step transition suggests that there was a degree of elastic frustration. However, it was not great enough to overcome the preferred energetics of the bulk material to attain spin-state homogeneity. Other minor changes to the structures included a contraction in host-guest hydrogen bonds and ligand-ligand aromatic contacts, as well as a more planar Hofmann layer in the LS state, as evidenced by change in Fe-N≡C angles toward 180 • . Furthermore, the benpy torsional twist for both analogues decreased substantially with HS to LS transition. A notable distinction between Pd(benpy) and Pt(benpy) was that the guest-accessible void space was larger in Pt(benpy), but for both analogues the void volume decreased with HS to LS transition (Pd(benpy): 5% (73 Å 3 ) to 3% (37 Å 3 ) and Pt(benpy): 13% (195 Å 3 ) to 6% (94 Å 3 )). This distinction may underlie the difference in long-range and short-range ordering of the ligand torsional twist in Pd(benpy) and Pt(benpy). Indeed, on closer inspection, the ligand-water contacts were slightly longer in Pt(benpy) compared to Pd(benpy) in the HS state (O1·water: 2.137(2) and 2.006(2) Å for Pt and Pd, respectively). These contacts may play a role in the ligand ordering.

Conclusions
Two isostructural 2-D Hofmann-like framework materials, [Fe II (benpy) 2 M(CN) 4 ]·2H 2 O (M = Pd, Pd), were prepared with a novel amide-functionalized ligand (benpy). Variation of M (Pd or Pt) resulted in subtle changes to structure and SCO properties, which we comprehensively assessed by variable temperature magnetic susceptibility, DSC, PXRD, Raman spectroscopy, and SCXRD analyses, and validated by calculation of the magnetic susceptibility data. The main change to the structure between the Pd and Pt analogues was the long-or short-range ordering of a torsional twist in the benpy ligand around the amide group. Structural analyses suggested this difference arose due to the relative steric bulk of Pd versus Pt and, hence, the packing efficiency and contacts that can be achieved. Both the Pd and Pt analogues showed abrupt and hysteretic one-step SCO transitions, with the Pt transition occurring at a slightly higher temperature. This difference was ascribed to a subtle variation HS stability arising from a combination of the greater electron density and size of Pt compared to Pd. Comparison of the HS and LS structures for both analogues revealed a degree of elastic frustration due to the increase in distortion over the HS to LS transition, opposed to the normal decrease, in particular, for the Pd analogue. Although, rather than a multi-step SCO transition, which can arise in such cases, a cooperative one-step character prevailed, indicating that the frustration effects were not large enough to overcome the preferential energetics of spin-state homogeneity. Consistent with this theory, we found that the net interactions were ferroelastic and that the transition was strongly cooperative. Overall, the inclusion of an amide-functionalized ligand into these SCO Hofmann frameworks successfully led to a range of through-bond (i.e., coordination bridges) and through-space (i.e., supramolecular contacts) pathways. Rather than functioning antagonistically, these functioned collectively to produce cooperative spin-state switching properties. This study supports the role of various structural modes of elastic coupling propagation in SCO materials.