An Incomplete Spin Transition Associated with a Z′=1→Z′=24 Crystallographic Symmetry Breaking

Abstract Crystalline [FeL 2][BF4]2 ⋅Me2CO (L=N‐[2,6‐di{pyrazol‐1‐yl}pyrid‐4‐yl]acetamide) is high‐spin at room temperature, and undergoes an abrupt, hysteretic spin‐crossover at T 1/2 =137 K (ΔT 1/2 =14 K) that proceeds to about 50 % completeness. This is associated with a crystallographic phase transition, from phase 1 (P21/c, Z=4) to phase 2 (P21, Z=48). The cations associate into chains in the crystal through weak intermolecular π⋅⋅⋅π interactions. Phase 2 contains a mixture of high‐spin and low‐spin molecules, which are grouped into triads along these chains. The perchlorate salt [FeL 2][ClO4]2 ⋅Me2CO also adopts phase 1 at room temperature but undergoes a different phase transition near 135 K to phase 3 (P21/c, Z=8) without a change in spin state.

The structural chemistry of spin-crossover (SCO) compounds [1][2][3] continues to be heavily studied.T he structural relationships underlying SCO functionality [4] are fundamentalt ot he de novo design of new SCO materials ford evice applicationso ri n nanoscience. [3,5] Moreover,S CO crystalsh ave proven especially useful for studying the fundamental physics of crystallographic phase transitions. [6,7] Crystallographic symmetry breaking during SCO is observed in an umber of materials. [8] Re-entrant symmetry breaking can lead to an intermediate crystal phase duringt he SCO process, containing am ixture of high-spina nd low-spinm olecules in its asymmetricu nit. [9][10][11][12][13][14] The resultant mixed spin-state population is generally retained over at emperature range, before undergoing another phase change accompanied by full conversion to the low-spinf orm. Alternatively,i rreversible symmetry breakingc an occur during SCO to al ow-temperature phase which can be either fully low-spin, [15] or contain am ixture of high-and low-spin molecules as before. [16,17] Symmetry-breaking involving ad oubling of the crystallographic asymmetric unit is most common in either scenario, with the lower symmetry phase containing distinct high-spina nd low-spinm olecules arranged in a0 D, 1D, or 2D sublattice. [9,15,16] However, SCO-induced phase changes involving tripling, [10,11,17] quadrupling, [12] six-fold, [13] or 7.5-fold expansion [14] of the asymmetric unit have also been reported, leading to more complicated patterning of spin-states in these low-symmetry phases.
As parto fo ur continuing investigations of complexes derived from [Fe(bpp) 2 ] 2 + (bpp = 2,6-di{pyrazol-1-yl}pyridine), [18,19] we now describe am aterial exhibiting cooperative but incomplete SCO, whose low-temperature phase shows a2 4-folde xpansiono ft he crystallographic asymmetric unit. As wella s being the most dramatic example of SCO-induced symmetry breaking yet reported, the low-symmetry phase contains one of the largestn umberso fc rystallographicallyi ndependent molecules (Z')observed in amolecular compound. [20] The new ligand N-(2,6-di{pyrazol-1-yl}pyrid-4-yl)acetamide (L) was prepared by treatment of 4-amino-2,6-di{pyrazol-1-yl}pyridine [21] with acetyl chloride. The reaction is sluggish, reflecting the de-activated nature of the (pyrid-4-yl)amino group, but proceeds in 67 %y ield if a6 .5x excesso fa cetyl chloride is used. The identity of L was confirmed crystallographically, which showedacomplicated pattern of acetamido group disorder and intermolecular hydrogen bonding, associatedw ith the partial occupancy of al attice water molecule. [22]  and are phase-pure by X-ray powder diffraction. [22] Solid 1[BF 4 ] 2 ·Me 2 CO is high-spin at room temperature, but undergoes an incomplete spin transition on cooling according to magnetic susceptibility data ( Figure 1). The transition is abrupt and shows as mall thermalh ysteresis loop, with T1 = 2 fl = 130 and T1 = 2 › = 142 K( scan rate 5Kmin À1 )i mmediately below the transition temperature, c M T = 2.0 cm 3 mol À1 K, which correspondst oa bout a4 1% low-spin population at that temperature. This slowly decreases to 1.7 cm 3 mol À1 K( 50 %l ow-spin) upon further cooling to 95 K. Below 95 Kt he sample remains in a1 :1 high:low-spin form, with an additional decrease in c M T below 50 Kr eflecting zero-field splitting of the residual highspin content of the sample. [23] Crystals of 1[BF 4 ] 2 ·Me 2 CO at 240 Ka dopt the monoclinic space group P2 1 /c,w ith one formula unit in the asymmetric unit (i.e. Z = 4). The complex's metric parameters imply it is high-spina tt hat temperature, as expected from the magnetic data. semblies, through NÀH···F hydrogen bonds between the acetamido substituents and BF 4 À ions (whicha re all disordered at that temperature;F igure 2). The only significant contact between cationsi nt he lattice is aw eak intermolecular p···p overlap between pyrazolyl rings, of nearest neighbors related by translation along the crystallographic a direction.
Cooling the crystal below the SCO transition temperature causedt he appearance of new,c losely spaced diffraction spots, [22] implying at ransition to al ower symmetry phase (phase 2) with al arge unit cell. Allowing for the change in spin states, the unit cell transformation to form phase 2i sa' = 2 c, b' = b, c' = 6 a and b' = b,g iving V % 42 800 3 which is 12 larger than for phase1.V ariable temperature unit cell data show the phase 1$phase 2t ransition occurs at 135 AE 5K in cooling mode and 145 AE 5Kin warming mode,w hichr eproduces the thermal hysteresis in the magnetochemical transition. [22] After severala ttempts from different crystals and diffractometers, as atisfactory refinement of phase 2w as achieveda t 130 K, in the space group P2 1 (Z = 48). The loss of the crystallo-graphic c glide and inversion centeri np hase 2, together with its unit cell volumee xpansion, generates 24 unique molecules in its asymmetric unit which are labelled 'A'-'X' (Figure 3). The refinement of phase 2i so fm oderate precision, reflecting the size of the model and the lower data resolution from the very large unit cell. However the main features of the structure are clear.
Molecules A-J in the refinement are fully or predominantly high-spina ccording to their metricp arameters;m olecules O-X are fully or predominantly low-spin;a nd molecules K-N have a mixed high:low-spin population at the temperature of measurement ( Figure 3). That is consistentw ith the approximate 1:1h igh:low-spin ratio expected from the magnetic data ( Figure 1). The same pattern of NÀH···F hydrogen bonding occurs in phase 2a si np hase 1a lthough the acetamido substituents, and around half of the anionsa nd solventm olecules, have become crystallographically ordered at the lower temperature.
As before,c ations in the lattice associate by weak intermolecular p···p interactions into chains, whichr un parallel to the unit cell c axis in phase 2. The asymmetric unit contains four uniquec hains, whose molecules have aH S-HS-HS-LS-LS-LS or HS-HS-MS-LS-LS-MS( HS = high-spin; LS = low-spin; MS = mixed spin state population)s pin-state patterning. The four mixedspin molecules are well-separatedf rom each other in the lattice (Figure 3), and some or all of these might gradually increase their low-spin population upon further cooling. That could explain the small additional decrease in c M T observed between 125 and 95 K (Figure 1).
The presence or absence of SCO in solid, high-spin [Fe(bpp) 2 ] 2 + derivativeso ften correlatesw ith their coordination geometry.T his is conveniently expressed by the parameters q (the dihedral angle between the least squares planes of the ligands)a nd f (the trans-N{pyridyl}-Fe-N{pyridyl} bond angle, which is N(2)-Fe(1)-N(22) in Figure 1). [18,22] An ideal D 2d symmet-   H igh-spin cationsa re coloredw hite,low-spin cations are purple andcations with am ixed high/low-spin population are pink;a nions and solvent( yellow)a re de-emphasized for clarity.The letter labels for each uniquem olecule in the modela re also shown. [24] ric complex gives q = 908 and f= 1808.M ostl ow-spin [Fe(bpp) 2 ] 2 + derivatives approach these values, but high-spin complexes show much more variation. In practice, high-spin complexes deviating more strongly from the ideal values of q and f are less likely to transform to their low-spins tate upon cooling. [18,25] Notably,n ine of the ten high-spinc ations in phase 2h ave a more distorted coordination geometry than the high-spin molecule in phase 1, which could explain why they remainh ighspin at low temperatures ( Figure 4). Interestingly,t hese follow an ear-linear q versus f relationship, which is not usual in plots of this type. [19] That implies the high-spinm olecules all distort along the same structural pathway,w hich should be af unction of the anisotropic plasticity of the crystal lattice.That is reasonable, since the molecules are all approximately co-aligned in the lattice (Figure 3). All the low-spin molecules, and three of the four mixed-spin iron sites, have lessd istortedg eometries than the phase 1molecule as expected.
Crystalline 1[ClO 4 ] 2 ·Me 2 CO also adopts high-spin phase 1a t room temperature, and af ull structure refinement at 170 K showedo nly minor differences to this phase with the BF 4 À salt. However,n oS CO waso bserved upon cooling 1[ClO 4 ] 2 ·Me 2 CO to 100 Ko nt he diffractometer.R ather,a t1 35 AE 5Kthe crystals transform to an ew phase (phase 3), which retains the P2 1 /c space group but with ad oubled unit cell a dimension( as well as small increases in c and b). [22] Both uniquem olecules in phase 3, labelled 'A' and 'B',a re fully high-spin from their metric parameters, with molecule Bs howings ignificantly reduced q and f values comparedt op hase 1. [22] The p···pstacked cation chains, which now run parallel to the unit cell a axis, contain alternating Aand Bc ations.
Magnetic susceptibility data confirmed that 1[ClO 4 ] 2 ·Me 2 CO indeed remains predominantly high-spin between 5-300 K. However,a na brupt reduction of c M T from 3.3 to 3.0 cm 3 mol À1 Ko ccurs reproducibly near 145 K, close to the crystallographic phase transition temperature ( Figure 1). For a phase change to have such an effect on c M T,without an associated spin transition, is unusual in ac ompound of this type. [26] Howeverh igh-spin [Fe(bpp) 2 ] 2 + derivativesw ith reduced values of q and f,a si nm olecule Bo fp hase 3, [22] can exhibit magnetic moments up to 10 %l ower than their undistorted analogues. [27] Hence, rather than indicatingac hange in spinstate population,t he magnetochemical feature at 145 Km ight simplyr eflect the changes in molecular coordination geometry duringt he high-spin phase 1!phase 3t ransition.
In conclusion, thermal SCO in 1[BF 4 ] 2 ·Me 2 CO yields al ow temperature phase with an approximate1 :1 high:low-spin population, that is distributed between 24 crystallographically unique molecules (i.e. Z' = 24 [20] ). This is the mosts evere example of symmetry breaking yet observed in an SCO crystal. [8] Moreover, notwithstandingo ne compound with Z' = 56, [28] crystalsw ith such high Z' values as phase 2a re very rare. [20,29] High Z' crystals have been proposed to be kinetic intermediates in the crystallization pathway;o r, to arise from frustrated, mutuallyo rthogonal packing interactions in the lattice. [20] Either descriptionc ould apply to phase 2. On one hand, phase 2m ay be an intermediate in the SCO of 1[BF 4 ] 2 ·Me 2 CO, with around half the molecules kinetically trapped in their high-spinf orm. [10,30] On the other,c ompeting ferroelastic and antiferroelastic interactions between molecules over different length scales in the lattice, are also known to stabilizem ixedspin phases in SCO materials. [31] Experimental Section Synthetic procedures, crystallographic data, and details of the instrumentation used for the spectroscopic and crystal structure measurements are given in the Supporting Information. [22]