Hysteretic thermal spin-crossover and symmetry breaking in heteroleptic Fe(II) complexes using alkyl chain substituted 2,2'-dipyridylamine ligands
The alkyl chain carrying ligands N, N-di(pyridin-2-yl)butanamide (LC4) and N, N-di(pyridin-2yl)decanamide (LC10) were combined with NCSco-ligands to form the neutral heteroleptic Fe(II) complexes trans-[Fe II (LC4)2(NCS)2] (1C4) and trans-[Fe II (LC10)2(NCS)2] (1C10). Variable temperature crystallographic studies revealed that 1C4 is in the orthorhombic space group Pna21 between 85-200 K whereas 1C10 is in the monoclinic space group P21/c between 85-105 K before undergoing a crystallographic phase transition to the triclinic space group P1 � by 140 K. The average Fe-N bond lengths suggest that at 85 K 1C4 contains LS Fe(II) centres; However, the ca. 0.18 Å increase in the average Fe-N bond lengths between 85 and 120 K suggests a spin-transition occurs within this temperature interval and the HS state is predominant beyond this. 1C10 contains LS Fe(II) centres between 85 and 105 K. Upon warming from 105 to 140 K the average Fe-N bond lengths increase by ca. 0.19 Å, which suggests that a spin-transition to the HS accompanies the P21/c to P1 � crystallographic phase transition. Solid-state magnetic susceptibility measurements showed that 1C4 undergoes semi-abrupt spincrossover with T1/2 = 127.5 K and a thermal hysteresis of ca. 13 K whereas, 1C10 undergoes an abrupt spin-crossover with T1/2 = 119.0 K, and is also accompanied by thermal hysteresis of ca. 4 K. The crystallographic and magnetic data show that the length of the complex's alkyl chain substituents can have a large impact on the structure of the crystal lattice as well as a subtle effect on the T1/2 value for thermal spin-crossover.
Introduction<!>Experimental General<!>Elemental analysis<!>X-ray crystallography<!>Magnetic Susceptibility Measurements<!>Synthetic procedures<!>N,N-Di(pyridin-2-yl)butanamide (LC4):<!>Results and Discussion<!>Structural description of 1C4•85K, 1C4•120K and 1C4•200K<!>Magnetic studies<!>Infrared spectroscopy<!>UV-vis spectroscopy<!>Conclusions
<p>The spin-crossover 1 (SCO) phenomenon is most commonly encountered in octahedral Fe(II) species where a reversible transition between the quantum mechanical ground state (low-spin (LS) state), and the thermodynamically stable high-spin (HS) state can be achieved by application of external influences such as temperature, 2 pressure, 3 irradiation 4,5 and applied magnetic fields. 6 Cooperative SCO systems rely on strong intermolecular interactions such as hydrogen bonds and π•••π contacts and this cooperativity can result in an abrupt spin-transition often accompanied by thermal hysteresis. This enables bistability between the two spin states, making SCO materials ideal candidates for use as switches or memory devices. This transition between the LS and HS states, with a subsequent change in the Fe(II) to ligand donor atom distance, results in a significant change in the material's magnetic and electric dipole moments, conductivity and optical properties. 7,8 This makes SCO materials attractive targets for both molecular-based spintronic devices [9][10][11] and hybrid multifunctional materials incorporating a secondary function such as luminescence or chirality. 12 In 2005 Réal and co-workers investigated Fe(II) complexes of di(pyridin-2-yl)amine (dpa), and showed that cis-[Fe II (dpa)2(NCS)2] underwent gradual and incomplete thermal SCO with a T1/2 value of 88 K. 13 This T1/2 value and nature of the spin transition can be changed by modification of the amine group of the dpa ligand with notable examples including derivatised dpa/triazine ligands including: trans-[Fe II (DPT)2(NCS)2]•CH3OH which undergoes a gradual incomplete spin-transition with T1/2 of 150 K; 14 α-trans-[Fe II (DBB)2(NCS)2] exhibiting a complete abrupt spin-transition (T1/2 = 170 K); 15 β-trans-[Fe II (DBB)2(NCS)2] showing a gradual but complete spin transition (T1/2 = 300 K); 15 and trans-[Fe II (Cladpat)2(NCS)2] undergoing a complete spin transition with a T1/2 value of 178 K 16 (DPT = 6-phenoxy-N 2 ,N 2 ,N 4 ,N 4 -tetra-2-pyridinyl-1,3,5-triazine-2,4-diamine; DBB= (N 2 ,N 2 ,N 4 ,N 4 -tetrabenzyl-N 6 ,N 6 -di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine; and Cladpat = 6-chloro-N ' -phenyl-N,N-di(pyridin-2-yl)-1,3,5triazine-2,4-diamine). The work outlined above set a precedent for investigation of Fe(II) systems that incorporate derivatised dpa ligands and NCX co-ligands (where X = is typically S, Se, BH3) in an attempt to tailor the magnetic properties of the resultant materials. This approach allows for the possibility of multifunctionality through selection of a desirable Nsubstituent, such as alkyl chains, on to the parent dpa scaffold.</p><p>A number of studies have focussed on utilising the conformational changes of alkyl chain containing materials to: probe the relationship between SCO and thermotropic liquid crystal behaviour; [17][18][19][20] develop dual functioning SCO-active materials such as those found in Langmuir-Blodgett films; 21 and study biomimetic models of phospholipid membranes. 22, with phase dependent magnetic properties. 24 It is encouraging that the alkyl chain component of such materials does not necessarily inhibit any desirable SCO properties such as abrupt/hysteretic spin-transitions and cooperative lattice effects. [25][26][27][28][29] Here we report a family</p><!><p>All reagents were purchased from Sigma-Aldrich (Merck) and were used as received without further purification. All solvents were purchased from Fisher Scientific.</p><p>Methanol was dried over 4 Å molecular sieves (20% w/w) for a minimum period of 48 h before use in synthetic procedures. All synthetic procedures were done under aerobic conditions unless otherwise stated. 1</p><!><p>Elemental analysis was obtained by MEDAC LTD analytical and chemical consultancy service.</p><!><p>X-ray crystallography was conducted on a sample of fresh single crystals of 1C4 and 1C10. The single crystal was selected from a vial containing the methanolic mother liquor under a nitrogen atmosphere and placed straight into oil to avoid drying. Variable temperature X-ray diffraction data for 1C4 were collected using an AFC12 goniometer and a HyPix-600HE detector, FR-E+ SuperBright Molybdenum X-Ray generator and a highly focused beam (70µm) achieved with VariMax VHF optics. The diffractometers were also equipped with the Oxford Cryosystems Cobra. Variable temperature X-ray diffraction data for 1C10 were collected using an AFC12 goniometer and a HyPix-600HE detector, FR-E+ SuperBright Molybdenum X-Ray generator and a highly focused beam (70µm) achieved with VariMax VHF optics. 1C4•85K was solved using intrinsic phasing and refined by full-matrix least-squares on F 2 with SHELXL.</p><!><p>Variable-temperature magnetic susceptibility measurements were performed on a Quantum Design MPMS 7T SQUID magnetometer over the temperature range 50-340 K at a 5 K min -1 rate for 1C4 and 1C10 under an applied field of 0.1 T. The crystals of 1C4 and 1C10 were in methanol under a nitrogen atmosphere but were filtered, air dried quickly and then encased in Vaseline to avoid any torqueing effects. These samples were held in a capsule at the centre of a drinking straw fixed to the sample rod. The magnetometer was calibrated by either a standard palladium sample or CuSO4•5H2O and diamagnetic corrections were applied using Pascal's constants. The molecular weight used in the magnetic measurements were based on the solvated forms of 1C4 and 1C10 as defined by the microanalysis (1C4•2H2O and 1C10•H2O).</p><!><p>Scheme 1 Synthetic route to ligands LCx ligands from di(pyridin-2-yl)amine (dpa),</p><p>where Cx is the number of carbon atoms in the alkyl chain substituent including that of the carbonyl group.</p><!><p>Di(pyridin-2-yl)amine (dpa) (1.712 g, 10 mmol) and triethylamine (Et3N) (2.65 cm 3 , 19 mmol) were dissolved in 10 cm 3 of CH2Cl2, cooled over an ice bath and placed under an atmosphere of N2. Butyryl chloride (1.03 cm 3 , 10 mmol) was dissolved in 10 cm 3 of CH2Cl2 and added to the dpa/Et3N solution via syringe. The resulting solution was stirred for 12 h under N2. 50 cm 3 of a saturated aqueous ammonium chloride solution was added to the reaction solution and the organic phase was separated. The aqueous phase was then extracted with 2 x 20 cm 3 of CH2Cl2 and the organic extracts combined. The combined organic phases were then extracted with 3 x 30 cm 3 of a 10% (w/v) aq. NaCl solution. The organic phase was then washed firstly with 50 cm 3 of 0.1 M aq. NaOH and finally 50 cm 3 of 0.1 M aq. HCl solution before the organic phase was dried over anhydrous MgSO4. This solution was then gravity filtered and the solvent evaporated under reduced pressure to afford LC4 as a pale orange solid. Yield = 1.86 g, 7.7 mmol (77%). 1</p><p>, 802s, 783m, 765s, 690m, 650m.</p><!><p>Variable temperature crystallographic studies</p><!><p>.   1). The neutral, heteroleptic complex is formed by two bidentate chelating LC4 ligands in equatorial positions and two NCSanions in the trans axial sites to form a slightly distorted octahedron with an FeN6 core (Fig. 1). The unit cell of 1C4•85K is depicted in Fig. S1. The two LC4 ligands coordinate via the pyridyl nitrogen atoms (N1, N2, N4 and N5) and the NCSligands coordinate through the N atom of the nitrile groups (N3 and N6). A selection of bond lengths and angles for the three crystal structures can be found in typical values for an Fe(II) ion undergoing a LS to HS spin-transition. 4,16,27,34,35 In 1C4•200K the Fe-N bond lengths are similar to that of 1C4•120K, suggesting the presence of a central HS Fe(II) ion (Table 2).</p><p>, (θ i = N-Fe-N) where perfect octahedrons produce a value Σ = 0° and deviation from this value coincides with structural deformation of the coordination polyhedron.</p><p>1C4•85K has a distorted octahedral geometry, with an associated octahedral distortion parameter Σ = 29.4°. The spin transition in the Fe(II) ions between 85-120 K is accompanied by further deformation of the FeN6 polyhedron. This is demonstrated by the increase in the parameter Σ to 39.0° in 1C4•120K and 39.7° in 1C4•200K. 5,35,36 The lack of structural transformation between the 120 and 200 K structures of 1C4 suggests that the spin transition is mostly complete by 120 K and the vast majority of the Fe(II) centres will be in the thermodynamically stable HS state. The LC4 and NCSligands also distort to accommodate the spin transition (Fig. 2). The pyridyl moieties of the ligands rotate about their free axis with the bridging N7 and N8 atoms to accommodate the increasing Fe-N distances for the HS Fe(II) ions. This is manifested by a decrease in the N1-N7-N2 and N4-N8-N5 angles for each ligand unit (Fig. 2). For 1C4•85K the N1-N7-N2 angle is 71. to accommodate the change in spin state, which is a common feature noted by Brooker et al. 37 In 1C4•200K the Fe-N-C angles remain similar to that in 1C4•120K with values of 145.9 (6)   and 148.8(6)° (Table 2).   3). This results in a diagonal expansion of these 1D chains of complexes along the bc plane with a subsequent increase in the unit cell parameters b and c and a concomitant decrease in a. In all of the structures there are short C-H•••O contacts between the oxygen atoms of the carbonyl moieties of the LC4 ligands and the pyridyl C-H groups of a nearby complex (Fig. 3b). 38,39 These C-H•••O contacts range from 3.138 (11)</p><p>Variable temperature crystallography was performed at 85K (1C10•85K), 105 K (1C10•105K) and 140 K (1C10•140K) on the same single crystal of 1C10 in a similar vein to that reported above for 1C4. 1C10•85K and 1C10•105K are in the monoclinic space group P21/c but 1C10•140K is in the triclinic space group P1 � indicating that a crystallographic phase transition has taken place between 105 and 140 K (Table 4). The trans-[Fe II (LC10)2(NCS)2] molecular structure of the complex is similar to that of 1C4 (Figs. S2-4) with an identical coordination sphere and steric arrangement of the ligands, differing only in the length of the alkyl chain tail. In 1C10•85K and 1C10•105K the asymmetric unit contains one half of the molecular unit, with an inversion centre located on the central Fe(II) ion whereas the asymmetric unit of 1C10•140K contains three total molecular units, made up of two whole and two half complete molecular units.</p><p>The unit cells for 1C10•85K, 1C10 Fe(II) centres. See Table S1) suggests that the system is completely HS at 140 K and that a relatively abrupt spin-transition between 105 K and 140 K must occur. This must be accompanied by a crystallographic phase transition from P21/c to P1 � (see below) and this type of transition is often observed in systems that undergo abrupt thermal SCO. 24,40,41 The molecular units of 1C10•85K, 1C10•105K and 1C10•140K have a distorted octahedral geometry with Σ = 28.4°, 28.5° and 35.9°(av.) respectively, supporting the conclusion of a spin-transition between 105 K and 140 K.</p><p>The packing in 1C10•85K and 1C10•105K is directed by the same short contacts as 1C4 (Table 6).  6). The similarity in packing between the LS structures of 1C10•85K and 1C10•105K is expected and the 2D networks of complexes formed through the intermolecular contacts are separated by their trans alkyl chain tails, which extend mostly down the a axis to form layers in the bc plane, where the 2D sheets of complexes are between two regions of hydrophobic alkyl chains (Fig. 5).</p><p>35.9 a Bond lengths and angles for 1C10•140K are calculated using the average values obtained from the equivalent atoms across the four independent Fe(II) centres in this structure. b Octahedral distortion parameters are obtained using the equation:</p><p>, (θ i = N-Fe-N) where perfect octahedrons produce a value Σ = 0° and deviation from this value coincides with structural deformation of the coordination polyhedron. Legend: iron, turquoise; nitrogen, blue; carbon, grey; oxygen, red; sulfur, yellow.  as expected (Table 6). In 1C10•85K and 1C10•105K the 2D networks of LS Fe(II) centres are separated by alkyl chains along the a axis (Figs. S10 and S11), however in 1C10•140K, these networks propagate perpendicular to the a axis and are separated diagonally across the bc plane (Figs. S12 and S13). Apart from the orientation of the 2D sheets of complexes and the direction of their alkyl chain tails with respect to the crystallographic axes, the LS and HS structures of 1C10 pack identically. (right). Hydrogen atoms omitted for clarity. Structures were overlaid using the atomic coordinates of the FeN6 cores in Olex2. 31,32 For LS 1C10•105K the Fe-N-C angles of the coordinated NCSanion are 158.62(12)°, whereas for HS 1C10•140K these angles decrease to an average of 150.2° across all the Fe(II) centres in the asymmetric unit. These changes in the Fe-NCS angles of trans isomers are in-line with the observations in 1C4 and are in agreement with the analysis of trans-[Fe II (Rdpt)2(NCX)2] complexes (Rdpt = 4-(R)-3,5-di(2-pyridinyl)-4H-1,2,4-triazole, X = S, Se or BH3) performed by Rodríguez-Jiménez and Brooker. 37 Although there may be a relationship between the Fe-NCS angle and the spin state or even the SCO activity of the heteroleptic complexes of this sort, it remains unclear whether this is applicable to all systems with the general formula cis/trans-[Fe II (Rdpa)2(NCX)2]. The following coordination compounds: trans-[Fe(bdpp)(NCS)2] (bdpp = bis(2',2''-dipyridylamine)pyrimidine), 42 cis-[Fe II (DDE)2(NCSe)2] (DDE = N 2 ,N 2 ,N 4 ,N 4 -tetraethyl-N 6 ,N 6 -di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), 43 trans-[Fe II (DPPyT)(NCS)2] and trans-[Fe II (DPPyT)(NCSe)2] (DPPyT = 1-[4,6-bis(dipyridin-2-ylamino)-1,3,5-triazin-2-yl]pyridin-4(1H)one) 14 do not show a consistent decrease in the Fe-NCX angles with the transition from LS to HS in these SCO-active complexes. This suggests that the distortion of the Fe-NCS angles in 1C4 and 1C10 between the LS and HS states may be relevant only to the systems reported here.</p><p>The changes in the molecular structure of 1C10 between the LS P21/c and HS P1 � crystal structures are subtle and are observed mainly in some slight ligand distortion as well as a slight "bending" of the alkyl chain tails towards the equatorial sites (Fig. 6).</p><p>Overall it is clear from the variable temperature crystallographic studies that both complexes undergo a spin-transition between 85 K and 120 K for 1C4 and between 105 K and 140 K for 1C10. In the case of 1C10 this is accompanied by a crystallographic phase transition as evidenced by the change of space group from P21/c to P1 � .</p><!><p>From the crystallographic data is it clear that we would expect a spin-transition between 85 K and 120 K for 1C4 and between 105 K and 140 K for 1C10. To confirm this variable temperature magnetic susceptibility measurements were obtained on single crystals of 1C4 and 1C10 down to 50 K at a heating/cooling rate of 5 K min -1 in an applied DC field of 0.1 T (Fig. 7). The single crystals were removed from the mother liquor and dried in air momentarily before being analysed so that oxidation or decomposition was minimised. Upon cooling from 300 K, 1C4 has a χMT value of 3.50 cm 3 mol -1 K down to 175 K, corresponding to a fully HS Fe(II) system. Below 175 K the χMT value decreases slightly to ca. 3.30 cm 3 mol -1 K at 150 K before a small plateau region is established down to 140 K. An abrupt spin transition then occurs between 140 and 100 K as the value of χMT decreases from 3.25 to 0.13 cm 3 mol -1 K, which signifies an almost quantitative LS population of Fe(II) centres. The value of χMT then subtly decreases to 0.09 cm 3 mol -1 K at 50 K. To check for thermal hysteresis, the samples were then reheated from 50 K. Upon heating, the χMT value rises gradually, reaching 0.45 cm 3 mol -1 K at 122 K. An abrupt spin transition then occurs between 122 and 146 K and the value of χMT reaches 3.22 cm 3 mol -1 K. This value increases to 3.51 cm 3 mol -1 K at 200 K where it plateaus and the HS state is restored (Fig. 7 top). The SCO in 1C4 is consistent with the crystallographic studies (see above) and shows thermal hysteresis with ↓T1/2 = 121 K and ↑T1/2 = 134 K, giving a thermal hysteresis width (ΔT) of 13 K. The observation of thermal hysteresis in 1C4 is surprising as the single crystal structure does not show a temperature-dependent crystallographic phase change from Pna21, which often accompanies thermal hysteresis in the SCO transition temperature (T1/2). 40 However, it is possible that the hysteresis in this instance is a collective response due to long-range elastic interactions. 1C10 has a χMT value of 3.37 cm 3 mol -1 K at 200 K which is expected of a HS Fe(II) system. The value of χMT then decreases slightly to 3.24 cm 3 mol -1 K at 123 K before abruptly decreasing to 0.05 cm 3 mol -1 K at 111 K. A plateau region is then observed down to 50 K with χMT stabilised at 0.01 cm 3 mol -1 K. The sample was then heated from 50 K, and the value of χMT gradually increases to 0.26 cm 3 mol -1 K by 116 K before abruptly increasing to 3.42 cm 3 mol -1 K at 125 K. The fully HS system is then stable beyond this temperature (Fig. 7 bottom). The SCO in 1C10 is thermally hysteretic as was the case with 1C4, although the width of the hysteresis is smaller in the former. For 1C10 ↓T1/2 = 117 K and ↑T1/2 = 121 K, giving ΔT = 4 K, which is 9 K smaller than the 13 K observed in 1C4. The hysteretic behaviour in 1C10 aligns well with the observation of symmetry breaking in the single crystal structures between 105 and 140 K, which correspond to completely LS (P21/c) and HS (P1 � ) systems, respectively, on the χMT vs. T plot.</p><!><p>Infrared spectra for 1C4, 1C10, LC4 and LC10 can be found in Figs. S14-S16. Infrared spectroscopy was conducted at room temperature on single crystals of 1C4 and 1C10.Both complexes 1C4 and 1C10 contain axial thiocyanate ligands which produce characteristic nitrile absorption bands that occur within two distinct regions depending on the spin state of the central Fe(II) ion due to the vibrational differences between electronic states. For Fe(II) the LS 1 A1g and HS 5 T2g ground electronic states are vibrationally discrete, where the longer coordination bonds of the HS species produce weaker bond force constants, thus oscillating at lower frequencies. [44][45][46] For Fe(II) species, the nitrile stretch usually appears at ca. 2107 cm -1 for the LS 1 A1g state and between 2040-2070 cm -1 for the HS 5 T2g state. 47 Both 1C4 and 1C10 exhibit strong IR absorption bands at 2043 and 2046 cm -1 , respectively, confirming the HS state of both 1C4 and 1C10 at room temperature consistent with both the crystallographic and magnetic studies.</p><!><p>Acetone solutions of 1C4 and 1C10 rapidly oxidise in atmospheric conditions which is observed as a colour change in solution from yellow to red. This occurs visibly over a matter of minutes and so the Fe(II) species was not stable for a long enough period of time for it to be detected in the UV-visible spectra. 1C4 and 1C10 exhibit identical UV-vis properties in solution (Fig. S17), with slight deviations in molar extinction coefficient (ε) values, which may be due to solid particulates remaining in solution as a consequence of the need to quickly measure the samples after forming the solutions. In acetone, 1C4 and 1C10 exhibit two main absorption bands that are not observed in the spectrum for the ligands LC4 and LC10. These bands have λmax values of 405 and 515 nm (Fig. S17). The band at 515 nm has a molar extinction coefficient value (ε) of 131 dm 3 mol -1 cm -1 for 1C4 and 107 dm 3 mol -1 cm -1 for 1C10. The relatively low value of ε at 515 nm is suggestive of weak d-d transitions originating from a HS Fe(III) species forming in solution. For the band at 405 nm ε = 979 dm 3 mol -1 cm -1 for 1C4 and 1130 dm 3 mol -1 cm -1 for 1C10 which is likely to be a weak CT process, due to the magnitude of ε far exceeding that which is typical for d-d transitions. 37</p><!><p>Complexes 1C4 and 1C10 vary only by the length of their alkyl chain substituents, which has a significant effect on both their structural and magnetic properties. 1C4 undergoes a quasistepped, semi-abrupt spin-crossover centred at 127.5 K, accompanied by a non-uniform thermal hysteresis where ΔT = 13 K. 1C10 undergoes an abrupt spin-crossover centred at 119.0 K and is also accompanied by a thermal hysteresis (ΔT = 4 K). The crystallographic phase change in 1C10 and subsequent symmetry breaking between the LS and HS states is likely to be the cause of the more abrupt spin-crossover in 1C10 compared to 1C4. The inclusion of hydrophobic alkyl chain substituents on Fe(II) complexes did not inhibit spin-crossover behaviour, but rather demonstrated high cooperativity which resulted in hysteretic and abrupt thermal spin-crossover in the solid state. This is an encouraging sign for future development of multifunctional materials that might rely on alkyl chains for the incorporation of secondary functionalities such as liquid crystal properties or low-melting point spincrossover liquids.</p>