Iron ( II ) Spin Crossover Polymers of Planar N 2 O 2 Schiff Base Templates and 4 , 4 ’-bis ( pyridyl ) urea Bridges

Single crystal X-ray structure elucidation of the trinuclear module 4 and of the coordination polymer 5 reveals the presence of HS-LS-HS chains and all-HS infinite 1-D strands, respectively. As anticipated the presence of the bridging urea supports the supramolecular concatenation within an extended hydrogen-bonding network. Magnetic measurements reveal spin crossover behavior for four of the five complexes (1 – 4) that is strongly solvent dependent.


INTRODUCTION
Molecules that can shuttle between two or more energetically close-lying geometrical and/or electronic states qualify as sensor and actor materials.Being responsive to changes in environmental conditions or external stimuli, such molecular switches can undergo discrete stereo-electronic transitions among these states which, importantly, may be controlled by some operator.For this reason, switchable molecules attract ongoing interest of scientists from very interdisciplinary fields due to their application potential as molecular sensor and actors or for data storage at the molecular level [1 -4].Along the latter line of research, Spin Crossover (SCO) complexes are prominent protagonists as they undergo stimulated transitions (thermal agitation or irradiation of light, etc.) between discrete magnetic states with similarly discrete * Address correspondence to this author at the Department of Chemistry, Inorganic Chemistry II, University of Bayreuth, Bayreuth, Germany; Tel: +49 921 552555; Fax: +49 921 552157; E-mail: weber@uni-bayreuth.deoptical properties.As inherently the largest divergence in the material's read-out properties accompanies the largest aspect of the spin states' magnetic properties, the majority of the spin crossover complexes investigated so far relies on the non-toxic and very abundant iron(II) as metal center [5 -7].The predominance of iron(II) can thus be traced to the switching between a paramagnetic S = 2 High Spin (HS) state and a diamagnetic S = 0 Low Spin (LS) state which massively alters the population of anti-bonding e g -type orbitals and, through this, strongly affects the metal-ligand bond lengths.This effect on the inner coordination sphere with all of its stereochemical and elec-tronic consequences can be triggered by a wide range of different physical or chemical stimuli, e.g.change in temperature [8], pressure [9], pH [10], or light irradiation [11].In some cases, the combination of different stimuli leads to unexpected behavior, as for example a pressure-induced spincrossover photomagnet [12].Whereas SCO inherently is a molecular phenomenon, its amplification via supramolecular concatenation may give rise to the emergence of higher-order Scheme 1.General procedure for the synthesis of the iron(II) coordination polymers [Fe(L1a)(bpua)] (1), [Fe(L1b)(bpua)](0.5bpua)(2), and [Fe(L2a)(bpua)] ( 3) and used abbreviations.effects.For instance, a given system made of SCO-active units may express either of two macroscopically different magnetic states at the same temperature dependent on the history of the system [5, 13 -21].The underlying chemical forces which translate cooperativity among neighboring molecular units into magnetic bistability of the material are not known in detail in many of the reported cases.
A highly interesting aspect of SCO materials is that the spin state change can be combined with additional functionalities yielding multifunctional materials where synergetic effects between the different functions are possible [13,17].The combination of the SCO phenomenon with additional properties such as liquid crystal/phase transition behavior [22 -32], magnetic exchange interactions [33], or photoluminescence [34 -38] can be realized through modification of the ligand.This is highly relevant for potential applications, as either additional read-out possibilities are realized (e.g.change in luminescence) or new ways to trigger the spin state switch (e.g.phase transition) are established.Multiple functionalities can also be implemented through the combination of a cationic SCO complex with a functional counter ion (e.g.electrical conductivity [39 -41] or ferromagnetism [42]).Nano-sized SCO complexes are highly relevant for future applications and offer further pathways towards multifunctional systems e.g. through surface modification or to fine-tune the SCO properties.[43 -47] For all those multifunctional or nano-sized examples, high cooperativity leading to the observation of wide hysteresis loops is highly desirable.This is important for the application in the field of sensors with memory effect or data storage.There is growing evidence for supramolecular networks of hydrogen bridges being a preferential requisite of cooperativity in the solid state [48].Accordingly, there is a continuous need for new spin crossover systems that harness hydrogen-bond networks to impose new or improved SCO properties or new functionalities.Inspired by the results with N-(pyrid-4yl)isonicotinamide as bridging axial ligand, that yielded very wide (up to 88 K) and stable thermal hysteresis loops around room temperature [49], in the present study 4,4'-bispyridylurea (bpua) is used.We introduced bis-pyridine substituted urea moieties as N-ligating and potentially bridging ligands to our Schiff-base based square-planar templating tetradentate N 2 O 2 ligand platform in iron(II) complexes [50 -54].These axial ligands combine a moderate to strong ligand field with an almost linear directionality about the binding sites, implying minor steric stress in coordination polymers.Additionally, the ligand field strength of bpua can be influenced by the addition of molecules that interact with the protons of the urea moiety [55].It was demonstrated already that the energy of the hydrogen atoms in bpua can be changed by the addition of counter ions [55].In addition, the urea moiety is well known to support multiple hydrogen-bonding interactions, both as a donor and an acceptor.Herein we have combined this symmetric and rigid linker with several N 2 O 2 derivatives L1a/b and L2a/b and have studied the solid state structures and magnetic response through X-ray crystallography (single crystals and powder diffraction) and SQUID magnetometry.
CHN analyses were measured with a Vario El III from Elementar Analysen-Systeme.Mass spectra were recorded with a Finnigan MAT 8500 with a data system MASPEC II.Magnetic susceptibility data were collected using a MPMSXL-5 SQUID magnetometer under an applied field of 0.5 T over the temperature range 10 to 400 K in the sweep mode (5K/min).The samples were placed in gelatin capsules held within a plastic straw.The data were corrected for the dia- The diffraction data of 4 and 5 were collected with a STOE StadiVari diffractometer using graphite-monochromated MoK α radiation.The data were corrected for Lorentz and polarization effects.The structure was solved by direct methods (SIR-97 (SIR-97) [62 -64] and refined by full-matrix least-square techniques against Fo 2 -Fc 2 (SHELXL-97) [65] using WinGX [66].All hydrogen atoms were calculated in idealized positions with fixed displacement parameters.ORTEP-III [67,68] was used for the structure representation, Mercury [69] to illustrate the crystal packing.Cif files were deposited at the CCDC database (4: CCDC 1827146; 5: CCDC 1827147).

EXPERIMENTAL
Synthesis of [Fe(L1a)(bpua)] (1): 0.2 g (0.35 mmol) [FeL1a(MeOH) 2 ] and 0.22 g (1.05 mmol) bpua were dissolved in 20 mL MeOH and refluxed for 1 hour.In the dark red solution, a fine crystalline black precipitate was obtained overnight, that was filtered off, washed two times with 2 mL MeOH and dried in vacuum.Yield: 0.20 g (80% -1 .Please note, the carbon value obtained in the CHN analysis is too low due to incomplete combustion.The same difficulties were encountered for similar complexes of this ligand type with extended aromatic units [35,51,57,70]. In order to receive single crystals of the coordination polymers 1 -3 a diffusion setup was used employing a Schlenk tube separated in two parts with a glass wall.The starting iron(II) complex was placed on one side and the bridging ligand bpua on the other side of the glass wall and both parts were filled up with the solvent (MeOH or EtOH) to enable diffusion between the two chambers.This setup was left standing at room temperature for several weeks.By this, single crystals were grown of a trinuclear complex with the composition [{Fe(L1a)(µ-bpua)(MeOH)} 2 {Fe(L1a)}](8Me OH) (4) and the coordination polymer [Fe(L1b)(bpua)] (2EtOH) (5).In both cases, only few single crystals were obtained that allowed the determination of the X-ray structure and magnetic measurements but no further characterization of the material.Thus there are no results from elemental analysis and powder X-ray diffraction.The difference in the reaction conditions (room temperature vs. boiling point of the solvent) is most likely the reason for the different composition of the products.

Synthesis and General Characterization
Given in Scheme. 1 is the general pathway for the synthesis of the coordination polymers [Fe(Lxy)(bpua)] n (1 -3) and the used abbreviations.The bulk synthesis approach (mixing of the starting materials in a suitable solvent and heating to reflux) yielded the desired products as fine crystalline powders in good yield.In the case of [Fe(L1a)(bpua)] (1) and [Fe (L2a)(bpua)] (3) no solvent is included according to elemental analysis, whereas in the case of [Fe(L1b)(bpua)](0.5 bpua) (2) satisfactory elemental analysis results were obtained by assuming that half an additional molecule of bpua is included.The obtained coordination polymers were characterized using elemental analysis, mass spectrometry, IR spectroscopy and powder X-ray diffraction.Diffusion setups were used to grow single crystals of the complexes, however, as illustrated below, a different composition is obtained for the single crystals compared to the bulk material.A trinuclear complex with the composition [{Fe(L1a)(µ-bpua)(MeOH)} 2 {Fe(L1a)}](8MeOH) ( 4) is obtained from the diffusion setup with [Fe(L1a)(MeOH) 2 ] as starting material, whereas the coordination polymer [Fe(L1b)(bpua)](2 EtOH) (5) crystallized without of additional bpua in the crystal packing.

X-ray Structure Analysis
In Fig.
(1) an ORTEP drawing of the asymmetric unit of the two complexes 4 and 5 is given.4 crystallizes in the triclinic space group P-1 with 2 molecules in the unit cell.5 crystallizes in the monoclinic space group P2 1 /c with 4 molecules in the unit cell.Selected bond lengths and angles are summarized in Table 1.An overview of crystallographic data is given in Table S1.
The trinuclear complex 4 consists of three [Fe(L1a)]-units that are connected through two bpua ligands.Consequently the iron(II) centre in the middle (Fe2) has an N 4 O 2 coordination sphere with two bpua as axial ligands, whereas the two outer iron centres (Fe1 and Fe3) have an N 3 O 3 coordination sphere with one methanol molecule in each case as the sixth ligand.As expected, the O1-Fe1-O2 and the O12-Fe3-O13 angles of 106° and 107°, respectively, are in the region typical for high spin iron(II).This is in agreement with previous results on similar complexes of this ligand type with an N 3 O 3 coordination sphere [71].The O7-Fe2-O8 angle of 88° is in the  In the case of 5, one iron(II) center per asymmetric unit is observed.It has an octahedral N 4 O 2 coordination sphere consisting of the equatorial tetradentate Schiff base-like ligand L1b and the bridging axial ligand bpua.The O1-Fe1-O2 angle is 107.65° and therefore typical for iron(II) HS complexes of this ligand type [72 -76].The Fe-L bond lengths within the first coordination sphere confirm this spin state.As illustrated in Fig. (2), the polymer chains are almost perfectly linear due to the rigid nature of the bridging bpua ligand.Only a slight bending between neighbouring planes of the equatorial ligand is observed with an angle of 31° (see Figure S1).Adjacent polymer chains are connected to each other along the b-axis via hydrogen bonds O5 … H5A-N5 with a distance of 2.111 Å and O5 … H6A-N6 with a distance of 1.979 Å.A further contact to solvent molecules is observed between O3 … H51-O51 with a distance of 2.156 Å.
The packing of the molecules of 4 in the crystal is displayed at the top of Fig. (2).The trinuclear molecules of 4 are stacked with an offset of the half length of the axial ligand to each other via hydrogen bonds from H4(N4) and H5(N5) of bpua via Methanol O201-H201 to O10 of the equatorial ligand.Thus, the offset of the trinuclear molecules can be explained by the formation of an infinite hydrogen bond network that interconnects the individual trinuclear molecules among each other.This network is spread in three directions.Along the a-axis, the trinuclear molecules are connected via the bpua N10-H10A to the equatorial ligand O15 with a distance of 2.458 Å and from N5-H5 of bpua to O3 of the equatorial ligand with a distance of 2.503 Å.The second connection along the bc-plane is made up by bpua N4-H4 via methanol O201-H201 (2.007 Å) to O10 (1.907 Å) of the equatorial ligand.Along the c-axis the endgroups are connected with each other via O5H5 to O12 (2.097 Å) and O16H16 to O2 (2.015 Å).As anticipated, the bpua ligand shows a high tendency for the formation of hydrogen bond networks.
In the Supporting Information, Figure S2, the calculated powder X-ray diffraction pattern of 4 and 5 is compared with that of the corresponding bulk complexes 1 and 2, respectively.In both cases, no similarities are observed.This is not surprising due to the differences in the composition, but also due to the strong impact of the hydrogen bond network on the crystal packing.

Magnetic Properties
Temperature dependent measurements of the magnetic susceptibility were performed for all five samples using a SQUID magnetometer.The results are presented as a plot of the molar magnetic susceptibility (χ M ) temperature (T) product versus T for samples 1 -4 in Fig. (3) in the 50 -250 K temperature region.The results of 5 and not completely dried powder samples of 1 (1*) are given in the Supporting Information, Figure S3.

Coordination Polymers
At room temperature, for all coordination polymers a χ M T product in the region of 3.1 -3.4 cm 3 K mol −1 per iron center is obtained that is typical for iron(II) in the HS state.Upon cooling, the χ M T product does not change with temperature for 5, in agreement with the results from single crystal XRD (HS at 133 K).Thus 5 is a pure HS complex.All other samples show different types of SCO in the temperature range between 200 K and 100 K.
In the case of 1 an incomplete Spin Transition (ST) with T 1/2 ↓ = 196 K and T 1/2 ↑ = 199 K is observed, revealing a 3 K wide thermal hysteresis loop.At 100 K the χ M T product of 1.76 cm 3 K mol −1 is in the region expected for a mixture of LS and HS states.Such a behavior is very frequently observed for coordination polymers of this ligand type [75,77].If the measurements are performed on a not completely dried sample (see SI, Figure S3), initially an incomplete gradual SCO is observed and the hysteresis appears after heating to 400 K.For the completely dried sample the SCO behavior does not change upon heating to 400 K in line with the results of elemental analysis that indicates the absence of solvent molecules.
In the first cooling/heating cycle 2 shows an almost complete ST with T 1/2 ↓ = 145 K and T 1/2 ↑ = 149 K (4 K hysteresis) and a χ M T product of 0.46 cm 3 K mol −1 at 100 K.
After annealing at 400 K, the ST is shifted to slightly higher temperatures (T 1/2 ↓ = 152 K and T 1/2 ↑ = 174 K) with a significantly broadened hysteresis loop (22 K) and a less complete ST with a χ M T product of 2.34 cm 3 K mol −1 at 100 K.
In the case of 3 an incomplete two-step SCO is observed with T 1/2_ 1 = 149 K and T 1/2_ 2 = 75 K.At 125 K the χ M T product is 2.07 cm 3 K mol −1 and at 50 K the χ M T product is 0.99 cm 3 K mol −1 .As for 1, elemental analysis indicates the absence of solvent molecules and the SCO behavior does not change upon heating to 400 K.
Powder XRD patterns were recorded for the powder samples 1 -3 at room temperature and 125 K to analyze whether the spin state change is accompanied by a phase transition.The results are displayed in Fig. (4).In all three cases, changes in the PXRD pattern are observed upon the spin transition.However, most of the patterns are only shifted thus the changes are most likely due to the change in the bond lengths and not due to a supplementary phase transition.

Trinuclear Complex
In the case of the trinuclear molecule 4 the χ M T product of 10.1 cm 3 K mol −1 is in the region expected for a complex with three iron(II) centers in the HS state.Upon cooling, a very gradual SCO is observed in the 200 -125 K temperature region.At 100 K the χ M T product of 7.3 cm 3 K mol −1 is slightly larger than expected for a trinuclear complex with two HS and one LS iron(II) and in good agreement with the results from single crystal XRD.Upon heating of the sample up to 400 K to remove the methanol molecules included in the crystal packing the SCO behavior is lost and a pure HS species is obtained.

CONCLUSION
In the manuscript, the synthesis and characterization of five new iron(II) complexes derived from a square-planar templating N 2 O 2 ligand platform (L1a/b and L2a/b) in the presence of the urea derivative bpua are presented.The bridging ligand bpua was introduced in order to foster supramolecular communication via the formation of interchain hydrogen bond neworks; through this cooperative SCO behavior with thermal hysteresis loop should be favored.Four of the complexes are obtained as coordination polymers; this conclusion is substantiated by single-crystal X-ray diffraction of 5 revealing infinite 1-D strands of urea bridged octahedral complex units with HS configuration.Although the crystal structure of 1-3 remains to be determined, we conclude similar topology based upon the results from elemental analysis and the magnetic properties.Compound 5 which derives from ligand L1b is a trinuclear complex consisting of two terminal MeOH] units and one central LSconfigured [Fe(L1b)] moiety; the overall octahedral coordination spheres are completed by two equivalents of urea bpua, acting as a N 2 -bidentate bridging ligand.Three of the coordination polymers and the trinuclear complex show spin crossover behavior whereas the coordination polymer 4 is a pure HS complex.The results from single crystal XRD indeed show that in both cases hydrogen bond networks are formed.For the complexes 1 and 2 a cooperative ST with hysteresis is observed.

Fig. ( 1
Fig. (1).ORTEP drawing of the asymmetric units of the trinuclear molecule 4 (left) and the polymeric complex 5 (right) and used atom numbering scheme.
Fig. (2).Top: Packing of the trinuclear complex 4 illustrating the hydrogen bond network discussed in the manuscript.Middle: ORTEP drawing of a 1D chain of 5 and Bottom: excerpt of the packing of the polymeric complex 5 to illustrate the hydrogen bond network discussed in the manuscript.

Fig. ( 3 ).
Fig. (3).Plot of the χ M T product vs.T in the 250 -50 K range for the complexes 1 -4 discussed in this manuscript.Prior to the second cycle the sample was heated to 400 K.