Structural insight into the cooperativity of spin crossover compounds

Macroscopic physical properties are correlated with microscopic structural changes in the orthorhombic and monoclinic polymorphs of the spin crossover compound [Fe(PM-BiA)2(NCS)2] by employing single crystal X-ray diffraction, magnetization and DSC measurements. Focus is given on the kinetic behavior and a theoretical model is proposed for the calculation of the non-equilibrium spin-phase fraction.


Supporting information
Table S1 : Selected experimental crystal data for both polymorphs Fe(PM-BiA) 2 (NCS) 2 in the two spin states (HS & LS).

Figure S2 :
Figure S2 : Rietveld refinement of the powder diffraction pattern for (a) Bia-P mono measured at PSI at 270 K (b) Bia-P ortho measured at ESRF at 250 K.The blue and green ticks correspond to the Bragg reflections originating from Bia-P ortho and Bia-P mono , respectively.Small impurity peaks in (a) correspond to the orthorhombic polymorph with a phase fraction of nearly 5%.The open red circles are observed data points and the solid black line corresponds to the Rietveld fit.The pink solid line represents the difference between observed and calculated patterns.

Figure S3 :
Figure S3 : Temperature evolution of the unit cell volume for the two polymorphs.The filled black square symbols black for Bia-P mono and the filled red circles for Bia-P ortho are from this study.Open stars are from ref [a]: (Buron-Le Cointe et al., 2012).Lines are guides to the eyes.Error bars are the same size or smaller than the symbols.

Figure S4 :
Figure S4 : Fe−N distances, Fe(U 11 ), and Fe(x) as a function of temperature for Bia-P ortho for the second cooling (a) and heating (b) cycles across the SCO transition (169-190 K).

Figure S5 :
Figure S5 : (Top) The Fe−N−C(S) angle as a function of temperature.(Bottom).The NCS -angle as a function of temperature.The red circles (Bia-P ortho ) and the full blue and open dark blue squares (Bia-P mono ) are from this study (the two colors are be used to differentiate the two NCS -branches).Open blue and dark blue stars are from ref [a]:(Marchivie et al., 2003).Red open star symbol are from ref [b]: (Létard et al., 1998).The standard deviations are indicated by the shaded area.Lines are guides to the eyes.

Figure S6 :
Figure S6 : Comparison of the evolution of the a-lattice parameter of Bia-P mono with various parameters.Red dashed lines represented two temperature points (219 K and 183 K) and highlight the changes in trend.The gray shaded regions show the temperature range (243 K -219 K), where part of the π-π interactions is changing their strength from moderate to strong.(a) Comparison to the high spin fraction estimated from the magnetization measurement (b) Comparison to the torsion angle between two biphenyl rings.(c) Comparison to the van der Waals interaction between two carbon atoms.(d) Comparison to the angle between the Fe−N−C in the thiocyanate arm.

Figure S7 :
Figure S7 : (Right) The N Bia −Fe−N NCS angle as function of temperature; full red circles (Bia-P ortho : N1−Fe−N3) and full blue and open dark blue squares (Bia-P mono : N1−Fe−N3, N2−Fe−N4, respectively) are from this study.(Left) the N Pm −Fe−N Pm angle a function of temperature; full red circles (Bia-P ortho: N5−Fe−N5) and full black squares (Bia-P mono N5−Fe−N6) are from this study.Blue, dark blue, and black open stars are from ref [a]: (Marchivie et al., 2003).Lines are guides to the eyes.Error bars are the same size or smaller than the symbols.

Figure S8 :
Figure S8 : (Right) The N Bia −Fe−N NCS angle as function of temperature; red circle (Bia-P ortho : N1−Fe−N3) and full blue and open dark blue squares (Bia-P mono : N1−Fe−N4, N2−Fe−N3, respectively) are from this study.(Left) the N Bia −Fe−N Bia angle a function of temperature; red circles (Bia-P ortho: N3−Fe−N3) and full black squares (Bia-P mono N3−Fe−N4) are from this study.Blue, dark blue, and black open stars are from ref [a]: (Marchivieet al., 2003).Lines are guides to the eyes.

Figure S9 :
Figure S9 : The N PM −Fe−N NCS angle as function of temperature; red circles (Bia-P ortho : N1−Fe−N5) and full blue and open dark blue squares (Bia-P mono : N1−Fe−N3, N2−Fe−N6, respectively) are from this study.Blue and dark blue open stars are from ref [a]:(Marchivie et al., 2003).Lines are guides to the eyes.

Figure S10 :
Figure S10 : (Top) Σ: Angular distortion parameter of the 12 N−Fe−N bond angles.(Middle) ζ: Bond length distortion parameter.(Bottom) Θ: the deviation from ideal octahedral towards a trigonal prismatic coordination.Red circles (Bia-P ortho ) and black squares (Bia-P mono ) are from this study.Red and black open stars are from ref [a]:(Buron-Le Cointe et al., 2012); open red circles and open black squares are from ref [b]:(Marchivie et al., 2003); open red and black diamonds are from ref [c]:(Marchivie et al., 2005).Lines are guides to the eyes.Error bars are the same size or smaller than the symbols.

Figure S11 :
Figure S11 : The evolution of the C−C bond length in the pyridine ring as a function of temperature.(Left) Bia-P mono, (right) Bia-P ortho .Lines are guides to the eyes.

Figure S12 :
Figure S12 : The evolution of the C−C bond length in the phenylene rings as a function of temperature.(left)Bia-P mono, (right) Bia-P ortho .Lines are guides to the eyes.

Figure S13 :
Figure S13: 3D packing of Bia-P mono showing the shortest contacts between phenyl rings.The strength depends on the distance between two centroids and the angle between the normal vectors of the two centroids.The interaction between the inner phenyl rings (centroid 1 and centroid 5) shows a strong interaction at all temperature points; however, centroid 7 & centroid 1 and, centroid 3 & centroid 5 exhibit moderate strengths in the HS state.At certain temperature points, they become strong, and in the LS state, they restore their moderate strength.

Figure S14 :
Figure S14 : The N−Fe−N angle (N corresponds to NSC − group) as a function of temperature.Red circles correspond to Bia-P ortho and black squares correspond to Bia-P mono .Filled symbols are obtained from this study.Open stars symbols are obtained from ref [a]:(Marchivie et al., 2003).Lines are guides to the eyes.Error bars are the same size or smaller than the symbols.

Figure S15 :
Figure S15 : (Right) The C•••C intermolecular shortest distance as a function of temperature in Bia-P ortho exist only in the LS state.Contacts are considered short if C•••C < 3.5 Å. C 11 •••C 1 and C 14 •••C 14 are the shortest contact in the intersheet layers.C 19 •••C 10 is the shortest contact in the intrasheet layers.(Left) The shortest C•••C intermolecular distances as a function of temperature in Bia-P mono .Contacts are considered short if C•••C < 3.5 A˚.C 27 •••C 6A and C 15 •••C 11 are the shortest contact in the intersheet layers.C 31 •••C 6A, C 2A •••C 14, C 3A •••C 29, C 19 •••C 10 are shortest contact in the intrasheet layers.C 2A … C 14 shortest contact in the intrasheet layers shows a similar change in its trend as the a lattice parameter with decreasing the temperature.