The number and shape of lattice solvent molecules controls spin-crossover in an isomorphous series of crystalline solvate salts

Crystals of [FeL2][BF4]2 nMeCN (L = N-(2,6-di{pyrazol-1-yl}pyrid-4yl)acetamide; n = 1 or 2) and [FeL2][ClO4]2 MeCN are isomorphous. When n = 1 the compounds exhibit an abrupt, hysteretic spintransition below 200 K, but when n = 2 the material remains high-spin on cooling. [FeL2]X2 EtCN (X = BF4 or ClO4 ) are isomorphous with the MeCN solvates and undergo their spin-transition at almost the same temperature. However this now occurs in two-steps via a re-entrant mixed-spin intermediate phase, which correlates with crystallographic ordering of the bent propionitrile molecule.

Crystal engineering of spin-crossover (SCO) molecular materials is of continuing interest, for their development as molecular switches for macro-, micro-and nanoscale device applications. 1 Such compounds also have wider relevance, as models for mechanistic studies of phase transitions in molecular crystals. 2 Isomorphous families of compounds are particularly useful in that regard, in showing how small structure changes can influence the temperature and cooperativity of a thermal spin transition within the same crystal lattice structure. 3 While some groups of isomorphous compounds exhibit very consistent SCO behaviour, [4][5][6] in other cases a surprising variation in the cooperativity or occurrence of SCO is observed. [7][8][9][10][11] We recently described the solvate crystals [FeL 2 ]X 2 ÁMe 2 CO (1X 2 Á Me 2 CO, Scheme 1; L = N-(2,6-di{pyrazol-1-yl}pyrid-4-yl)acetamide; X À = BF 4 À or ClO 4 À ). The BF 4 À salt undergoes an abrupt spin-transition on cooling which proceeds to ca. 50% completeness, and is accompanied by an unusual crystallographic symmetry breaking involving a twelve-fold expansion of the unit cell. 12 2 ] 2+ (bpp = 2,6-di{pyrazol-1-yl}pyridine) derivatives show a range of f values between 150 r f r 1801, and are less likely to exhibit SCO as f decreases. 13,14 That reflects the unfavourable activation barrier required to transform a highly distorted high-spin molecule in a solid lattice, to the more regular coordination geometry (f 4 1701) preferred by the low-spin state. 13,15 The f = 167 AE 11 distortion in high-spin 1X 2 ÁnMeCN lies on the cusp, where SCO is possible but is rarely observed in practise. 16 However, when high-spin [Fe(bpp) 2 ] 2+ salts with that degree of distortion do undergo SCO, the large structural rearrangement involved imparts cooperativity to the transition. 16,17 Thus, the change in f between the spin states is Df = 5.9(2)1 in 1[BF 4 ] 2 ÁMeCN and 6.6(2)1 in 1[ClO 4 ] 2 ÁMeCN, which leads to atomic displacements of up to 1.0 Å at the periphery of the molecules during SCO (Fig. S4, ESI ‡). This structural rearrangement is the most likely source of the abrupt, hysteretic cooperativity in their spin-transitions.
The cations and anions in 1X 2 ÁnMeCN form discrete [{FeL 2 }X 2 ] assemblies by N-HÁ Á ÁF or N-HÁ Á ÁO hydrogen bonding. The cations associate into chains by translation along the crystallographic a direction, through weak intermolecular pÁ Á Áp overlap of their pyrazolyl rings. The only other close inter-cation contact is a C-HÁ Á ÁH-C steric clash involving another pyrazolyl group, in molecules related by the crystallographic inversion centre. That contact only occurs in the high-spin crystals with n = 1, and leads to disorder of that pyrazolyl ring. The basic disposition of the cations, anions and solvent in 1 (Fig. 1).
The extra solvent molecules in 1[BF 4 ] 2 Á2MeCN form centrosymmetric pairs within a new cavity centred at lattice coordinates 1/2, 1/2, 0 ( Fig. 1, top). This cavity has a volume of 145 Å 3 , and is created by the following changes in 1[BF 4 ] 2 Á2MeCN compared to 1[BF 4 ] 2 ÁMeCN: a ca. 1.5 Å displacement of nearest neighbour cations within the (100) plane, which also changes the handedness of the triclinic unit cell from a o 901 to a 4 901; 18 a ca. 51 rotation of the cation within its lattice site, which is coupled to a small decrease in f; and 0.5-0.8 Å displacements of each anion. Notably the extensive anion and solvent disorder in high-spin 1[BF 4 ] 2 ÁMeCN is mostly quenched in 1[BF 4 ] 2 Á2MeCN. That implies the packing in the latter crystal is generally more compact.
Freshly prepared 1[BF 4 ] 2 ÁnMeCN is predominantly the n = 2 phase by microanalysis and TGA ( Fig. S11 and S12, ESI ‡). The material loses 1 equiv. MeCN abruptly at 368 K in the TGA, with the second equivalent being lost more gradually on further heating; the solvent-free material is only achieved above ca. 500 K. Samples of 1[BF 4 ] 2 ÁnMeCN lose one equivalent of MeCN on drying in vacuo at room temperature, affording analytically pure 1[BF 4 ] 2 ÁMeCN (Fig. 2) (Fig. 2). Magnetic data from 1[ClO 4 ] 2 ÁMeCN also contain a small fraction of a  high-spin phase, although this appears to be solvent-free material generated by in situ solvent loss (Fig. S13, ESI ‡).
The propionitrile solvates 1X 2 ÁEtCN (X À = BF 4 À or ClO 4 À ) are isomorphous with the acetonitrile crystals (triclinic, P% 1, Z = 2). They are phase-pure by powder diffraction, and are also highly stable to solvent loss by TGA ( Fig. S26 and S27, ESI ‡). These also exhibit abrupt and hysteretic thermal spin-transitions, but now with a clear plateau near 50% conversion (Fig. 3). Interestingly, the average T 1/2 for the two SCO steps in each 1X 2 ÁEtCN material corresponds well with T 1/2 for the one-step SCO in the corresponding 1X 2 ÁMeCN sample ( Table 1). The discontinuous SCO in 1[ClO 4 ] 2 ÁEtCN reflects re-entrant crystallographic symmetry-breaking to an intermediate phase (phase 2) near the midpoint temperature of the spin-transition (Fig. S22, ESI ‡). 19,20 The intermediate phase (triclinic, P% 1, Z = 4) retains the same space group as the initial phase, but with a doubled unit cell volume containing 1 : 1 mixture of high-and low-spin molecules. These are segregated into alternating layers within the crystallographic (010) plane (Fig. 4).
The cation molecular structures in phases 1 and 2 resemble those in 1X 2 ÁMeCN (Table S6,  ions. Most striking is the EtCN molecule, which is equally disordered over ''up'' and ''down'' orientations in high-spin (HS) phase 1 (Fig. S17, ESI ‡). This disorder is mostly quenched in phase 2 where both unique solvent sites adopt the up configuration, but one molecule retains ca. 15% of the down orientation. In low-spin (LS) phase 1, the single EtCN molecule is now fully ordered in the ''up'' orientation. Hence, the phase 1(HS) -2 -1(LS) transformations are accompanied by stepwise crystallographic ordering of the solvent molecules in the lattice.
No crystallographic evidence for phase 2 was found in 1[BF 4 ] 2 ÁEtCN, whose SCO plateau spans a narrower temperature range (Fig. 3). However, that does not rule out a symmetry breaking near 190 K in that crystal, which could occur locally but without long-range order in the lattice. 20 For comparison, the mixed-anion salt 1[BF 4 ] z [ClO 4 ] 2Àz ÁEtCN (z E 1) was also crystallised. The spin-transition properties of this material lie midway between the pure BF 4 À and ClO 4 À salts, and phase 2 was again observed crystallographically in its plateau temperature region. The ordering of the EtCN molecule between the phases of the mixed-anion crystal follows the same sequence as in 1[ClO 4 ] 2 ÁEtCN. Crystal structures of 1X 2 Á2MeNO 2 (X À = BF 4 À or ClO 4 À ),

1[BF 4 ] 2
ÁMeOH and 1[ClO 4 ] 2 Á1/2EtOH are presented in the ESI. ‡ These materials remain high-spin on cooling which, in some cases, can be related to a strong f distortion in their molecular ) materials from magnetic susceptibility data (Fig. 2 and 3 and ESI; scan rate 5 K min À1 )   geometries, as above. [13][14][15] Solutions of 1[BF 4 ] 2 in CD 3 CN and (CD 3 ) 2 CO exhibit SCO on cooling, with T 1/2 = 201 AE 2 K (Fig. S33, ESI ‡). That agrees reasonably with our published correlation for pyridyl-substituted [Fe(bpp) 2 ] 2+ derivatives, 21 which predicts T 1/2 = 190 K for [FeL 2 ] 2+ bearing NHC{O}R pyridyl substituents with a s P + Hammett parameter of À0. 6. 22 This study gives new insight into the importance of lattice solvent for SCO switching in molecular materials, which was recognised in the 1980s 23 but is rarely rationalised in detail. [9][10][11]24 SCO in 1X 2 ÁMeCN, containing linear MeCN molecules, occurs cooperatively in one step. However, the extra solvent equivalent in 1[BF 4 ] 2 Á2MeCN quenches the SCO, without changing its crystal symmetry or the molecular structure of the complex. That could simply reflect more compact crystal packing in the presence of the extra solvent molecule, which would inhibit the structural changes associated with SCO. 25 The same lattice in 1X 2 ÁEtCN exhibits two-step SCO via a reentrant mixed-spin intermediate phase. This correlates with a crystallographic order/disorder transition between two orientations of the bent propionitrile molecule. Discontinuous spintransitions involving re-entrant intermediate crystal phases are well-known. 19,20 However the creation of a re-entrant mixedspin phase during SCO in 1X 2 ÁEtCN, by simply changing the shape of the lattice solvent molecule, is a rarer observation. 9 The average T 1/2 for the two SCO steps in each EtCN solvate is the essentially same as T 1/2 for the corresponding MeCN solvate salt (Table 1). Hence, the temperature of SCO in 1X 2 Ánsolv (n = 1) is influenced by the X À anion, while the solvent controls the form of the transition.
The authors thank Dr Mark Howard for the solution magnetic susceptibility measurement, and Algy Kazlauciunas for the TGA data. This work was funded by the Leverhulme Trust (RPG-2015-095) and the EPSRC (EP/K012568/1 and EP/N509681/1).

Conflicts of interest
There are no conflicts to declare.