Bifunctional Fe(II) spin crossover-complexes based on ω-(1H-tetrazol-1-yl) carboxylic acids

To increase the supramolecular cooperativity in Fe(ii) spin crossover materials based on N1-substituted tetrazoles, a series of ω-(1H-tetrazol-1-yl) carboxylic acids with chain-lengths of C2-C4 were synthesized. Structural characterization confirmed the formation of a strong hydrogen-bond network, responsible for enhanced cooperativity in the materials and thus largely complete spin-state transitions for the ligands with chain lenghts of C2 and C4. To complement the structural and magnetic investigation, electronic spectroscopy was used to investigate the spin-state transition. An initial attempt to utilize the bifunctional coordination ability of the ω-(1H-tetrazol-1-yl) carboxylic acids for preparation of mixed-metallic 3d-4f coordination polymers resulted in a novel one-dimensional gadolinium-oxo chain system with the ω-(1H-tetrazol-1-yl) carboxylic acid acting as μ2-η2:η1 chelating-bridging ligand.


Introduction
First row transition metals with 3d 4 -3d 7 electron configurations allow for population of their 3d-orbitals with either a maximum, or a minimum of paired electrons. Depending on the coordinative environment around the metal centre, the energetic difference between these electron configurations (high-spin (HS) and low-spin (LS) state) may be small enough to be governed by an external stimulus such as temperature, 1,2 pressure, 1-3 light 4 or external electric field, 5-8 among others. 1 This phenomenon was first observed in the 1930s by Italian researchers, working on iron(III) N,N-dialkyldithiocarbamates and has since been known as spin crossover (SCO), or spin state transition. [9][10][11] The intrinsic correlation between the bistability of SCO materials and several of their physical properties makes them appealing candidates for novel miniaturized sensors, 12,13 memory/storage devices, 14 and a key technology in spintronics. 15 Today, SCO materials are primarily investigated by an academic bottom-up approach, resulting in highly specialized materials for devices on a nanoscale. Molecular actuators, 16 their utilization in quenching and affecting luminescence and fluorescence, 17,18 their sensing capacity in form of porous switchable materials, 12,19 or their combination with chirality [20][21][22] have recently been reported.
Since the occurrence of SCO under ambient conditions is very sensitive to the ligand field, it is strongly affected by the nature of the ligand system, the presence or absence of solvates 23 and counter anions applied. 24 (Substituted) azole ligands, including N1-substituted tetrazoles, are a promising platform for systematic development of SCO-materials. 1 Homoleptic hexa-coordinated Fe(II)-complexes based on N1substituted tetrazoles are often spin switchable, as coordination of Fe(II) by tetrazoles results in an appropriate ligand field strength. In this case, Fe(II) switches between a paramagnetic HS state with S = 2 and a diamagnetic LS state with S = 0.
In previous work, we have emphasized fine-tuning of the ligands' geometry and flexibility, shaping cooperativity and transition temperature on a molecular scale, 25,26 as well as post functionalization of the ligand backbone. 27 Therefore, in this work, a carboxylic acid moiety has been introduced to the ligand system, allowing for the investigation of the SCO in [Fe (ω-(1H-tetrazol-1-yl) carboxylic acid) 6 ] 2+ -cations. Alkylcarboxylic acid substituted tetrazoles provide three useful features: On a molecular level the COOH-group should establish a network of hydrogen bonds, increasing cooperativity and governing the abruptness of the spin state transition, [28][29][30] the COOH-group may act as an additional ligand allowing for the formation of multi-metallic networks with a second transition metal, or rare-earth element and, finally, the COOH termination of the ligand may allow for deposition on oxide surfaces.
In this contribution, we report the ω-(1H-tetrazol-1-yl) carboxylic acids with C 2 -C 4 alkyl chains and the magnetic, structural and spectroscopic characterization of their Fe(II)-SCO complexes, as well as an initial attempt to prepare a multimetallic 3d-4f coordination polymer.

Synthesis
The spin crossover behaviour of Fe(II)-N1-alkyl-substituted tetrazole complexes has been shown to be governed by the length of the N1-alkyl substituent and the choice of weakly coordinating anion. 24,31 Therefore, in the current study, the length of the alkyl-skeleton was varied from C 2 -C 4 (see Scheme 1) and BF 4 − and ClO 4 − were used as weakly coordinating tetrahedral The ligands 2-4COOHTz (1-3) were prepared from the corresponding amino-acids based on the Franke-tetrazole synthesis, 32 in analogy to the reported synthesis of (1H-tetrazol-1yl)-2-acetic acid (2). [33][34][35]

Magnetic properties
The dependence of molar magnetic susceptibility on temperature (10 to 300 K) was investigated for all six Fe(II)-complexes (Fig. 1a). The resulting χ mol T curves show three distinct tendencies: partial, incomplete spin crossover (5), complete spin crossover (4, 6, 7 and 9) and no spin crossover (8). All changes in χ mol T below 50 K may be attributed to zero-field splitting, expected for residual HS Fe(II). χ mol T decreases for 5 gradually below 170 K from 3.51 cm 3 K mol −1 at 300 K to 2.86 cm 3 K mol −1 at 50 K. This seems to correspond to ∼25% of Fe(II) sites in a LS state. [Fe (2COOHTz) 6 ](BF 4 ) 2 ·2 MeCN (4) shows the highest T 1/2 of the series with 227 K and decreases to a full LS-state below 180 K.

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with T 1/2 at 132 K and a final χ mol T of 2.75 cm 3 K mol −1 , corresponding to a ∼25% LS-state. The completely desolvated compound 4b is spin crossover inactive, remaining in the HS-state for all temperatures. Similarly, the desolvated ClO 4 − -complex 7b shows no spin crossover anymore (Fig. 1b). This demonstrates, how crucial the incorporated solvate molecules are in this case additional to the H-bonding network. All in all, the differing picture obtained from this comparison of the SCO-behaviour is not only caused by the weak-coordinating anion (as obvious by comparing 4 vs. 7, or 6 vs. 9), but rather attributed to a combination of different effects. Additional to the impact of the anion, as discussed in the case of 2COOHTz the 2 MeCN solvate makes the game. Furthermore, there is the variation in the length of the alkylspacer, which for 4COOHTz seems to be a good fit between establishing cooperativity and preventing too much motional freedom in form of a shock-absorber effect, 24,31 which was evidenced in the past for increasing chain-lengths. Missing the structural characterization of the 3COOHTz-compounds, the differing SCO-behaviour is probably related to the odd number of C-atoms, which already for unsubstituted alkyl-tetrazoles were found challenging in the past. 24,31 Crystallographic analysis Single crystals suitable for determination of the molecular structure could be grown for 4, 6, 7 and 9 by vapour-diffusion of diethyl ether (Et 2 O) into a concentrated MeCN-solution of the corresponding compounds. No suitable crystals of 5 and 8 could be obtained by similar means. Single crystal diffraction data were collected at 100 and 200 K for 4, 6, 7 and 9 in order to determine both low-and high-spin molecular structures. In addition variable temperature P-XRD patterns were acquired for 4 and 7 from 100-300 K.
[Fe(2COOHTz) 6 ](BF 4 ) 2 ·2 MeCN (4). 4 crystallizes as MeCN solvate in the trigonal R3 space group and this symmetry is retained both at 100 K (low-spin, see Fig. 2) and 200 K (highspin). The iron-centre is octahedrally surrounded by six 2COOHTz-ligands coordinating via the exo N4-nitrogen. The Fe-N bond lengths for all six ligands are equal (1.987 Å), typical for a Fe(II) LS-state. At 200 K the distance increases ∼9% to 2.176 Å, characteristic for the Fe(II) HS-state (see Table S1 †). 1 The packing in the crystal is governedas predicted for a COOH-groupby an H-bond network composed of infinite C(5) type chains between the COOH-groups. Viewed along the b axis, the [Fe(2COOHTz) 6 ] 2+ -cations stack in layers, with three of the ligands pointing upwards and three downwards (see Fig. 2) to the adjacent layer. This arrangement is further stabilised by the above-mentioned H-bonds (see Fig. 3). The Fe-atoms are located on the Wyckoff-position 3a, alternating with an acetonitrile, an apex-up and an apex-down BF 4 − group. The BF 4 − groups interact strongly with the [Fe(2COOHTz) 6 ] 2+ -cations through C-H⋯F-bonds with both the tetrazolic CH and CH 2 -group (see Fig. 4) in both the highand low-spin states.

Dalton Transactions Paper
Variable temperature P-XRD of bulk (4) and (7). Magnetic measurements of bulk 4 and 7 show an incomplete spin transition, whereas the Fe-N bond distances obtained from the single crystal data suggest a largely complete transition. Therefore, variable temperature P-XRD was used to follow any structural changes of the bulk material during the SCO (see Fig. 6). For both compounds, the calculated diffractograms obtained from the HS and LS molecular structures are in reasonable agreement with the powder-patterns of the bulk material. For both 4 (Fig. 6a) and 7 (Fig. 6b) the temperature dependent PXRD measurements show more pronounced changes in the powder pattern (dotted lines as a guide) than to those just expectable from thermal contraction. On this basis it seems clear that both bulk and single crystalline samples are the same compound.
[Fe(4COOHTz) 6 ](BF 4 ) 2 ·Et 2 O (6). 6 crystallizes as an Et 2 O solvate, as a result of vapour-diffusion crystallization, in the triclinic P1 space group. The structure was determined at 100 K (LS-state) and 200 K (HS-state). The asymmetric unit (see Fig. 7 The relevant crystallographic parameters for all Fe-containing structures are given in Table 1.

Spectroscopic characterization
MIR and FIR spectroscopy. For homoleptic Fe(II)-tetrazolecomplexes IR-spectroscopy allows for confirmation of successful SCO-complex formation, as well as gaining insight into the spin state present, since the tetrazolic CH-vibration is sensitive to the N4 coordination environment. 36 In Fig. 10, the IR spectra of 2COOHTz (1) and its ClO 4 − -complex (7) in the HSstate (red) and LS-state (blue) are compared. At 3156 cm −1 the free ligand shows its tetrazolic CH-vibration, which upon successful coordination is split and shifted to 3159 cm −1 and 3152 cm −1 . On cooling to the LS-state, at 100 K the CHvibration is shifted to 3164 cm −1 and 3159 cm −1 , respectively. Around 1600 cm −1 the ν N2vN3 stretching vibration of the tetrazole after coordination is similarly affected by the spin-state transition, shifting from 1612 cm

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Those vibrations are nearly entirely decoupled from other vibrational modes and therefore indicative of spin state. For the HS-state, this vibration is found at 226 cm −1 , for the LSstate the characteristic vibrations are located at 470 and 443 cm −1 (see Fig. 11).
UV-VIS/NIR spectroscopy. Electronic transitions are also affected by the spin state transition. For homoleptic Fe-tetrazole SCO complexes the HS-state absorbs around 850 nm in the near-infrared, causing a broad and often weak absorption in the ligand field spectrum. This 5 T 2 → 5 E transition weakens with cooling and spin-state transition, giving rise to two absorptions in the visible region of the spectrum at 545 nm and 386 nm, responsible for the pink colour of the LS-state. These absorption bands correspond to the 1 A 1 → 1 T 1 transition, as well as the 1 A 1 → 1 T 2 transition. From the maxima observed in the HS and the LS state, a ligand-field strength 10 Dq can be estimated. 37 In the case of 7, measured in the solid state, a value of 1.65 Dq is obtained (Fig. 12).

Towards a mixed-metallic 3d-4f coordination polymer
The functionalization of alkyl-tetrazoles with a COOH-group introduces an H-bond donor-acceptor system thereby enhan-

Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2020 cing supramolecular cooperativity of SCO. To take advantage of the coordination ability of the COOH-group and the bifunctional ligand, we attempted to prepare a multi-metallic 3d-4f coordination polymer. Coronado et al. have success-fully applied such an approach to the formation of a mixed Fe(III)-Fe(II) SCO compound using a COOH-functionalized ligand. 38 By reacting 3COOHTz (2) with Fe(ClO 4 ) 2 ·6H 2 O and GdCl 3 ·6H 2 O in aqueous MeOH and subsequent evaporation a slightly yellow oil was obtained, which did not be solidify. After a few weeks at 4°C a few colourless crystals had formed, allowing for determination of the structure by X-ray diffraction. Instead of a mixed-metallic coordination polymer, [Gd (3COOTz) 2 (H 2 O) 3 ]Cl (10) had crystallized in the monoclinic C2/ c space group. During the reaction/crystallization process the COOH-groups were deprotonated and the resulting carboxylate groups act as chelating, bridging ligands to the gadoliniumatoms in a µ 2 -η 2 :η 1 fashion (see Fig. 13). Each gadolinium atom is coordinated nine-fold by oxygen-donors as the centre of a monocapped square antiprismatic coordination polyhedron. Due to the µ 2 -η 2 :η 1 coordination of the carboxylateligands one-dimensional chains are formed, in which the Gd-atom is coordinated by four 3COOTz-ligands: two of them     Fig. 15). The crystallographic parameters for 10 are given in Table 2.

Materials and methods
All operations involving Fe(II) were carried out under inert gas atmosphere (argon 5.0). The glassware used was oven dried at 120°C before use for at least 2 hours. All solvents for the complexation reactions were dried before use and stored over molecular sieve 3 Å under argon. 39 Unless otherwise stated, all starting materials were commercially obtained and used without further purification. All NMR spectra were recorded in dry deuterated solvents on a Bruker Avance UltraShield 400 MHz. Chemical shifts are reported in ppm; 1 H and 13 C shifts are referenced against the residual solvent resonance. For the measurement of MIR and FIR spectra, a PerkinElmer Spectrum 400, fitted with a coolable/heatable PIKE Gladi ATR unit was used within the range of 4000-100 cm −1 . Solid state UV/Vis/NIR spectra were recorded with a PerkinElmer Lambda 900 spectrophotometer between 300 and 1600 nm in diffuse reflectance against BaSO 4 . A Harrick coolable/heatable powder sample holder in "Praying Mantis" configuration was used. Melting points were determined by differential scanning calorimetry, using a Netzsch STA 449 C Jupiter® with heating rates of 2 K min −1 , see Fig. S1-S3. † The magnetic moment of the Fe(II) complexes was measured using a Physical Property Measurement System (PPMS®) by Quantum Design. The

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experimental setup consisted of a vibrating sample magnetometer attachment (VSM), bearing a brass sample holder with a quartz-glass powder container. The magnetic moment was determined in an external field of 1 T in the range of 10 K to 300 K, measuring all 5 K with a previous thermal stabilization of 5 minutes. Variable temperature mid-range (4000-450 cm −1 ) infrared spectra were recorded by the ATR technique on a PerkinElmer Spectrum 400, fitted with a coolable/heatable PIKE Gladi ATR Unit. 25 Single crystals were attached to a glass fiber by using perfluorinated oil and were mounted on a Bruker KAPPA APEX II diffractometer equipped with a CCD detector with Mo K α radiation (Incoatec Microfocus Source IµS: 30 W, multilayer mirror, λ = 0.71073 Å). For all measurements data were reduced to intensity values by using SAINT Plus, 40 and an absorption correction was applied by using the multi scan method implemented by SADABS. 40 For the iron(II) complexes, protons were placed at calculated positions and refined as riding on the parent C atoms. All non-H atoms were refined with anisotropic displacement parameters. For 2024832 (10), a Bruker KAPPA APEX II diffractometer equipped with a CCD detector was used and data were collected at 100 K. The powder X-ray diffraction measurements were carried out on a PANalytical X'Pert Pro diffractometer in Bragg-Brentano geometry using Cu K α1,2 radiation filtered with a BBHD mirror and an X'Celerator linear detector. For in situ experiments below ambient temperature an Oxford PheniX Cryochamber from Oxford Cryosystems was used. The powder sample were mounted on a copper sample holder on top of a background-free silicon support. The sample chamber was evacuated, and all measurements were carried out under vacuum. The actual sample temperature is directly monitored by a thermocouple on the sample holder. The diffractograms were evaluated using the PANalytical program suite HighScorePlus, correcting for the background.

Conclusions
Fe(II)-SCO complexes with bifunctional ω-(1H-tetrazol-1-yl) carboxylic acids allow formation of supramolecular hydrogen bonding networks that enhance the cooperativity of the material. Furthermore, the COOH-group can act as anchorgroup for deposition on oxidic surfaces, or for formation of multi-metallic coordination polymers. Three ligands bearing a COOH-group with alkyl-chain lengths of C 2 -C 4 were synthesized and coordinated to Fe(II) with BF 4 − and ClO 4 − as weak-coordinating anions. Complete and sharp spin state transitions were observed in the cases of 4, 6, 7 and 9. An initial attempt to prepare a 3d-4f mixed-metallic coordination polymer with Gd 3+ resulted in the unexpected formation of one-dimensional Gd-oxo chains with the 3COOHTz-ligand having been deprotonated and coordinating to the Gd 3+ -atoms in a chelating-bridging µ 2 -η 2 :η 1 fashion. Future work will focus on a more suitable synthetic approach to realise a coordination on both ligand functional groups.

Conflicts of interest
There are no conflicts to declare.