The thermal and light induced spin transition in [FeL2](BF4)2

(L � 2,6-dipyrazol-1-yl-4-hydroxymethylpyridine)

V. A. Money,a J. Elhaïk,b M. A. Halcrow b and J. A. K. Howard*a

a Chemistry Department, Durham University, South Road, Durham, UK DH1 3LE.E-mail: j.a.k.howard@durham.ac.uk

b School of Chemistry, University of Leeds, Leeds, UK LS2 9JT

Received 26th January 2004, Accepted 16th April 2004First published as an Advance Article on the web 22nd April 2004

This communication presents the crystal structures of thehigh spin state at 300 K, the low spin state at 30 K and themetastable high spin state after irradiation at 30 K and anestimate of the critical LIESST temperature of [FeL2]-(BF4)2 which is shown to undergo a spin transition at271 K.

Since the first report of a spin crossover transition in the 1930sby Cambi et al. there has been much interest in materials whichshow this behaviour due to their potential for use in appli-cations such as molecular switches, display devices and inform-ation storage.1,2 In 1985 Decurtins et al. reported the firstexample of what is now well known as the LIESST effect (LightInduced Excited Spin State Trapping).3 On irradiation with anintense light source, it is possible to promote the low spinground state (LS) to a metastable high spin state (HS-2) with anextremely long lifetime, of the order of many hours or evendays, at low temperatures. Despite intense interest in this phen-omenon, crystallographic information about the HS-2 state isstill extremely unusual.4 Previous work on compounds derivedfrom the bpp1 ligand (bpp1 = 2,6-di(pyrazol-1-yl)pyridine) hasyielded compounds which exhibit a range of spin transitionbehaviour and have been shown to undergo LIESST transi-tions.4,5 Examination of the structure of the HS-2 state enablesa direct comparison of the low and high spin states without thecomplication of having to take into account the effects of tem-perature, allowing direct isolation of the effect of the spin tran-sition on the structure. For this reason crystallographic studiesof the metastable state are of critical importance in increasingthe understanding of this fascinating phenomenon (Fig. 1).

Diffraction quality, yellow–brown crystals of [FeL2](BF4)2 †were obtained by diffusion of ether into nitromethane.

SQUID magnetometry showed the thermal spin transitionfrom the high spin state (HS-1) to the LS state to take placeabruptly at 271 K with a very small hysteresis of <2 K, Fig. 2a.A variable temperature crystallographic study‡ showed that[FeL2](BF4)2 crystallises in the monoclinic space group Cc

Fig. 1 (a) 2,6-di(pyrazol-1-yl)pyridine (bpp1), (b) L =2,6-dipyrazol-1-yl-4-hydroxymethylpyridine.

between 360 K and 30 K. The asymmetric unit consists of onecomplex cation and two counter anions and there are fourformula units in the unit cell. The iron is shown to be in adistorted octahedral environment with the two ligands boundequatorially through three of their five nitrogen atoms, Fig. 3.At 300 K there is some thermal disorder in the hydroxy groupsof the ligands which is resolved at 30 K. Monitoring the changein unit cell parameters on cooling, reveals that there is a sharpdecrease of 48 Å3 or 1.6% in unit cell volume between 280 Kand 260 K, this coincides with the temperature at which there isa change in magnetisation observed by SQUID magnetometry,Fig. 2b. The spin transition is accompanied by a change incolour from dark yellow to brown. Transitions from the HS-1to the LS state cause a decrease in metal ligand bond lengths,which for iron() nitrogen systems is generally in the region of0.2 Å. For this system see Table 1. As a result of the decrease inFe–N bond lengths there is a concomitant decrease in theligand bite angle. As expected for iron() systems of this typethe LS state is less distorted than the HS state and this can bequantified using the Σ and υ parameters as defined by Guion-neau et al.6 A decrease in both parameters indicates that thefinal coordination polyhedron is closer to an ideal octahedron.There are hydrogen bonds between the OH groups of the cationligands, L, and the fluorine atoms of the anions, however thishydrogen bonding does not form a three dimensional networkthrough the solid and this may reduce the communicationbetween iron centres and be responsible for the small width ofthe hysteresis loop in the spin transition curve of the titlecompound.

The single crystal of [FeL2](BF4)2 was irradiated at 30 K, onthe diffractometer, with red laser light (λ = 632.8 nm, 25 mW)for ten minutes. The laser was switched off and observation ofthe crystal showed that it had changed colour from brown todark yellow. Determination of the unit cell parameters showedan increase in unit cell volume demonstrating that conversion tothe metastable high spin state had probably taken place. A fulldata set was then collected at 30 K, no further irradiation wasrequired due to the long lifetime of the metastable state at thistemperature. Complete conversion to the metastable state hadbeen achieved as was demonstrated by the expected increase inthe Fe–N bond lengths and a ∆VSC of 44 Å3 or 1.4% which isvery close to that observed over the thermal spin transition.Further evidence that there has been complete conversion to theHS-2 state is provided by the fact that the Σ parameters of theHS-1 and HS-2 states are the same. Determination of the unitcell parameters after data collection yielded the same unit cellindicating that the decay of the HS-2 state over the course ofthe experiment is negligible and that the crystal is still 100% in

Table 1 Selected structural parameters at various temperatures for [FeL2](BF4)2

Spin state T /K Volume/Å3 Mean Fe–N/Å Bite angle/� Σ/� υ/%

HS-1 300 2956(1) 2.143(5) 73.8(2) 147.6 7.699LS 30 2801(1) 1.962(3) 79.5(1) 91.2 3.369HS-2 30 2845(1) 2.161(3) 73.8(1) 147.0 8.001

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Fig. 2 (a) Variation of χT with temperature for [FeL2](BF4)2, (b) variation of unit cell volume on cooling for [FeL2](BF4)2.

the HS-2 state. The simularity between ∆VSC for the thermaland light induced transitions is somewhat unusual. Two of thethree reported examples where attempts were made to decouplethe effect of the thermal spin transition from that of tem-perature, show a much smaller increase for the light inducedtransition with only the related compound, [Fe(bpp1)2](BF4)2

reported to have similar values for both transitions.4 Com-parison of the HS-1 and HS-2 states for [FeL2](BF4)2 shows thatthe arrangement of the coordination cores is the same, asdemonstrated by the Σ parameters, and that very minor differ-ences in the ligand configurations cause one of the ligands totwist slightly out of the plane in the HS-2 state. Monitoring theunit cell parameters during warming after irradiation showsthat relaxation back to the low spin state takes place between70 K and 80 K.

Photomagnetic studies are underway to confirm the exacttemperature of the relaxation, T c(LIESST) and to examine theeffect of continuous radiation on the transition, the LITH(Light Induced Thermal Hysteresis) effect. The effect ofquenching on the spin state of the crystal will also be investi-gated. We are also studying the related compound [FeL2]-(ClO4)2.

The authors would like to thank the EPSRC for two student-ships (VAM and JE) and for a Senior Research Fellowship(JAKH). Dr H. J. Blythe (University of Sheffield) is acknow-ledged for the susceptibility data.

Notes and References

† Analytical data for [FeL2](BF4)2: Found C, 40.5; H, 3.3; N, 19.7%.Calc for C24H22B2F8FeN10O2: C, 40.5; H, 3.1; N, 19.7%. Full details ofthe synthesis of L and its complexes will be published elsewhere.‡ Crystal Data: C24H22B2F8FeN10O2, Mr = 711.99, monoclinic, Cc,Z = 4.

Crystallographic data were collected on a Bruker SMART CCD 7

(ω-scan, 0.3�/frame) using graphite monochromated Mo Kα radiation

Fig. 3 Crystal structure of the cation at 30 K in the low spin state. Thehydrogen atoms have been omitted for clarity and ellipsoids are at 90%probability.

(λ = 0.71073 Å). Between 360 K and 110 K the crystals were cooled in aflow of chilled nitrogen gas using an Oxford Cryosystems Cryostream.8

Below 110 K cooling was carried out in a flow of chilled helium usingan Oxford Cryosystems HELIX.9 All data processing was carried outusing the SAINT 10 and XPREP 11 software packages. Absorption cor-rections were applied using SADABS. The structures were solved bydirect methods and refined on F 2 using full matrix least-squaresmethods within the SHELXTL suite. All non-hydrogen atoms wererefined anisotropically. The hydrogen atoms were placed geometricallyand treated with a riding model. For the LIESST experiment the samplewas irradiated for ten minutes whilst on the diffractometer using aHe–Ne laser (λ = 632.8 nm, 25 mW). 300(2) K: a = 12.115(2) Å, b =12.118(2) Å, c = 20.414(4) Å, β = 99.53(3)�, V = 2955(1) Å3, Dc = 1.600Mg m�3, µ = 0.604 mm�1. Selected square pyramidal crystal 0.22 × 0.12× 0.08 mm3 mounted in epoxy resin. 10525 reflections (2.394 < θ <24.0875�) yielding 6100 unique data (R(int) = 0.0429). Final wR(F 2) =0.1635 and R(F ) = 0.0959 for all data (446 refined parameters), GOF =1.026, residuals ∆ρmin,max = �0.318, 0.447 e Å�3. 30(2) K: a = 11.904(2)Å, b = 11.935(2) Å, c = 20.007(4) Å, β = 99.71(3)�, V = 2801(1) Å3, Dc =1.688 Mg m�3, µ = 0.638 mm�1. Selected square pyramidal crystal 0.26× 0.18 × 0.12 mm3 mounted in epoxy resin.. 6996 reflections (2.43 < θ <27.49�) yielding 5050 unique data (R(int) = 0.0364). Final wR(F 2) =0.1220 and R(F ) = 0.0461 for all data (431 refined parameters), GOF =1.064, residuals ∆ρmin,max = �0.439, 0.1220 e Å�3 The crystal was foundto be racemically twinned and this was refined using the TWIN routinein the SHELXL suite. 30(2) K after irradiation: a = 11.974(2) Å, b =12.001(2) Å, c = 20.014(4) Å, β = 98.44(3)�, V = 2845(1) Å3, Dc = 1.662Mg m�3, µ = 0.628 mm�1. Same crystal as 30 K. 8019 reflections (2.42 <θ < 28.21�) yielding 5414 unique data (R(int) = 0.0373). Final wR(F 2) =0.1071 and R(F ) = 0.0416 for all data (431 refined parameters), GOF =1.029, residuals ∆ρmin,max = �0.359, 0.717 e Å�3. CCDC reference num-bers 229749–229751. See http://www.rsc.org/suppdata/dt/b4/b401155d/for crystallographic data in CIF or other electronic format.

1 L. Cambi and A. Cagnasso, Atti Accad. Naz. Lincei, 1931, 13, 809.2 P. Gütlich, A. Hauser and H. Spiering, Angew. Chem., Int. Ed. Engl.,

1994, 33, 2024; P. Gütlich, Y. Garcia and H. A. Goodwin,Chem. Soc. Rev., 2000, 29, 419.

3 S. Decurtins, P. Gütlich, K. M. Hasselbach, H. Spiering andA. Hauser, Inorg. Chem., 1985, 24, 2174.

4 J. Kusz, H. Spiering and P. Gütlich, J. Appl. Crystallogr., 2001, 34,229; M. Marchivie, P. Guionneau, J. A. K. Howard, A. E. Goeta,G. Chastanet, J-F. Létard and D. Chasseau, J. Am. Chem. Soc.,2002, 124, 194; V. A. Money, I. R. Evans, M. A. Halcrow, A. E.Goeta and J. A. K. Howard, Chem. Commun., 2003, 158; E. J.MacLean, C. M. McGrath, C. J. O’Conner, C. Sangregorio, J. M. W.Seddon, E. Sinn, F. E. Sowrey, S. J. Teat, A. E. Terry, G. B. M.Vaughan and N. A. Young, Chem. Eur. J., 2003, 921, 5314.

5 J. M. Holland, J. A. McAllister, Z. Lu, C. A. Kilner, M. Thorton-Pett and M. A. Halcrow, Chem. Commun., 2001, 577; J. M. Holland,J. A. McAllister, C. A. Kilner, M. Thorton-Pett, A. J. Bridgeman andM. A. Halcrow, J. Chem. Soc., Dalton Trans., 2002, 548; V. A.Money, I. Radosavljevic Evans, M. A. Halcrow, A. E. Goeta andJ. A. K. Howard, Chem. Commun., 2003, 158; J. Elhaïk, V. A.Money, S. A. Barrett, C. A. Kilner, I. Radosavljevic Evans andM. A. Halcrow, Dalton Trans., 2003, 2053; V. A. Money, J. Elhaïk,I. Radosavljevic Evans, M. A. Halcrow and J. A. K. Howard, DaltonTrans., 2004, 65; V. A. Money, J. Elhaïk, I. Radosavljevic Evans,M. A. Halcrow and J. A. K. Howard, Acta Crystallogr., Sect. B,2004, 60, 41.

6 P. Guionneau, C. Brigonteix, Y. Barrans, A. E. Goeta, J-F Létard,J. A. K. Howard, J. Gaultier and D. Chasseau, C. R. Acad. Sci., Ser.IIi: Chim., 2001, 4, 161; P. Guionneau, M. Marchivie, G. Bravic,J-F Létard and D. Chasseau, J. Mater. Chem., 2002, 12, 2546.

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7 SMART-1000 CCD, Bruker AXS, Madison, Wisconsin, USA.8 Cryostream Cooler, Oxford Cryosystems Ltd, Oxford, UK.9 Oxford Helix, Oxford Cryosystems Ltd, Oxford, UK; A. E. Goeta,

L. K. Thompson, C. L. Sheppard, S. S. Tandon, C. W. Lehmann,J. Cosier, C. Webster and J. A. K. Howard, Acta. Crystallogr.,Sect. C, 1999, 55, 1243.

10 G. M. Sheldrick, SHELXS-97, Program for solution of crystalstructures, University of Göttingen, Germany, 1997; G. M. Shel-drick, SHELXL-97, Program for refinement of crystal structures,University of Göttingen, Germany, 1997.

11 G. M. Sheldrick, SHELXTL Version 5.1, Bruker AXS, Madison,Wisconsin, USA, 1998.

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