Electron-deficient trifluoromethyl-substituted sub-components affect the properties of M 4 L 4 tetrahedral cages

Two supramolecular tetrahedral cages based on a new electron-deficient trifluoromethyl-substituted pyridylimine ligand are synthesised by sub-component self-assembly. Their structures are characterised by NMR und UV-Vis spectroscopy, high-resolution mass spectrometry and single crystal X-ray diffraction. The iron(ii) complex shows host-guest chemistry, complex-to-complex transformations and novel electronic properties.


Introduction
The implementation of functionality into supramolecular aggregates is one of the major challenges in modern metallosupramolecular chemistry. 1This may include novel functional ligands, 2 utilising cavities and voids 3 in the aggregates or utilising the properties of the metal centres themselves.
Complex-to-complex transformations with varying ligand systems are also a field of major interest, allowing the use of the properties of one ligand and subsequent utilization of the second ligand. 5Possible applications of this procedure have been shown for structurally similar but "inverted" ligand systems.‡,6 Here, the electronic effects of the ligands play a major role both in supramolecular chemistry in general as well as other fields of interest, for example the field of spincrossover research. 7ence, we focused on investigating the properties of new ligand systems in order to both adjust the binding properties of the new ligands to metal centres like iron(II) and generate new supramolecular structures.Furthermore, its (supra-)molecular composition was proven by high-resolution mass spectrometry (SI).Finally, the crystal structure of the mixed hexafluorophosphate (PF 6 −

Results and discussion
) and triflate (CF 3 SO 3 − ) salt of 1 was obtained.Crystals were grown from a solution of 1 with triflate as the counter-anion and an excess of KPF 6 in a mixture of acetonitrile and chloroform by gas diffusion of diisopropyl ether.The cage 1 was found to crystallise in the space group I2/a with one half of the cage in the asymmetric unit.Iron(II) metal centres exhibit the low-spin configuration, with Fe-N bond lengths between 1.966(3) and 1.986(3).The cage was found to possess an internal cavity with a volume of 42 Å 3 , just big enough to accommodate a nitrate (NO 3 − ) anion.Indeed, addition of 1.2 equivalents of KNO 3 to a solution of cage 1 in acetonitrile and heating to 60°C for 1 hour resulted in the host-guest complex NO 3 − @1 in nearly quantitative yield.Addition of one equivalent of nitrate per cage 1 and equilibrating the system for one hour yielded a mixture of empty and filled cages. 9From this experiment we could deduce a binding constant of K = 10.8 ± 1.910³ M -1 (log(K)=4.03± 0.08).This is of the same order of magnitude as found for other, structurally similar cages. 10While the parent structure reportedly does not bind any guests the slight change in electronic properties and the added equilibration step might be the key for enabling the binding of small anions.While the slightly longer Zn-N bond length, hailing from the d 10 configuration of Zn, allows for the free rotation of aryl spacers in 2 already at room temperature, the free rotation of aryl spacers in 1 is only possible at temperatures above 313 K (SI), due to the low-spin d 6 configuration of its Fe(II) centres .
As expected for an electron-deficient pyridylimine derivative, some properties differ from those of the parent pyridylimine structure. 9Most notably, this is a slightly different ligand field splitting at the metal centres which, while mostly irrelevant for the closed-shell d 10 -configured zinc(II) centres, has a notable impact on the properties of the iron(II) centres.Shifted 1 A 1g → 5 T 2g transitions in the UV-Vis spectra (from 566 nm in the parent structure 8 to 580 nm in 1) are observed.Complex-to-complex transformation of 1 to the parent structure. [9]e reduced electron density in the pyridyl system and, hence, the reduced σ-donating character of the nitrogen lone pair results in a slightly reduced ligand field splitting (see the SI). 7ccordingly, a slight stabilisation of the 5 T 2g high-spin state compared to the 1 A 1g low-spin ground state can be observed by temperature-dependent NMR experiments, resulting in a slight population of the high-spin state at room temperature and above leading to temperature-dependent increased shifts in NMR spectra. 11nother interesting feature of systems involving electrondeficient ligands is the altered stability of the formed complexes.While the aggregates are stable in solution and gas phase, complex-to-complex transformations of the system can still be observed.By addition of more electron-rich ligand fragments (like e.g.pyridyl-2-carbaldehyde in a slight excess) 1 can be fully converted to the more stable parent structure (Figure 3).

Conclusions
In summary, we have synthesised and characterised two novel supramolecular tetrahedral Fe(II) and Zn(II)-based cages possessing a trifluoromethyl-substituted pyridylcarbaldehyde fragment.The tetranuclear iron(II) cage can encapsulate small anionic guests such as nitrate in its inner cavity.It furthermore exhibits novel properties such as e.g. a slight stabilization of paramagnetic 5 T 2g excited states as well as offering the possibility of complex-to-complex transformation which has not been established for aniline-backbone structures before.The presented systems were characterised by means of different NMR spectroscopic techniques, UV-Vis spectroscopy, high-resolution mass spectrometry and IR spectroscopy.Furthermore, 1 was also characterised by single crystal X-ray diffraction.

Experimental General remarks
All substances were purchased from Alfa Aesar, J.T. Baker, Sigma-Aldrich or VWR Chemicals and used without further purification.5-(Trifluoromethyl)-pyridyl-2-carbaldehyde was purchased from Fluorochem and used without further purification.NMR spectra were recorded on a Bruker Advance DPX 700, DPX 500 or DPX 300.Chemical shifts δ are given in Please do not adjust margins Please do not adjust margins ppm relative to residual solvent signals (1 H). 12 For labelling schemes of the individual nuclei of metallosupramolecular cages 1 and 2 see SI. ESI mass spectra were recorded on a Fischer Scientific LTQ Orbitrap XL.FT-IR spectra were recorded on a Thermo Nicolet 380 FT-IR.UV-Vis spectra were recorded on a Jena Analytic Specord 200 spectrometer using a 10 mm cuvette.

Synthesis and characterisation of 1
A solution of 15 mg pararosaniline base (0.05 mmol, 4 eq), 25 mg 5-(trifluoromethyl)-pyridyl-2-carbaldehyde (0.15 mmol, 12 eq) and 37 mg iron(II) triflimide hexahydrate (0.5 mmol, 4 eq) in 2 mL acetonitrile (HPLC grade) were degassed by one freeze-pump-thaw cycle and flushed with argon and heated to 65 °C for 18 hours.After cooling the solution to room temperature the solution was poured in 20 mL diethyl ether, the precipitate was filtered off, washed with dichloromethane and generous amounts of diethyl ether and dried in a stream of air to yield 59 mg (0.01 mmol, 85 %) purple-pink powder.

Synthesis and characterisation of 2
A solution of 15 mg pararosaniline base (0.05 mmol, 4 eq), 25 mg 5-(trifluoromethyl)-pyridyl-2-carbaldehyde (0.15 mmol, 12 eq) and 37 mg zinc(II) triflimide hexahydrate (0.5 mmol, 4 eq) in 2 mL acetonitrile (HPLC grade) was degassed by one freeze-pump-thaw cycle and flushed with argon and heated to 65 °C for 18 hours.After cooling the solution to room temperature the solution was poured in 20 mL diethyl ether, the precipitate was filtered off, washed with dichloromethane and generous amounts of diethyl ether and dried in a stream of air to yield 61 mg (0.01 mmol, 86%) bright pink powder.

Transformation from 1 to the parent structure
A solution of 10.0 mg (0.0017 mmol, 1 eq) 1 and 2.5 mg (0.023 mmol, 13.2 eq) pyridyl-2-carbaldehyde in 0.6 mL acetonitrile-d3 was heated to 50 °C for 18 hours.The resulting mixture was analysed by NMR-spectroscopy.Afterwards the solution was poured in diethyl ether, filtered, the remaining precipitate was washed with dichloromethane and diethyl ether and dried in a stream of air to yield the pure parent compound in 6 mg (0.0013 mmol, 72%) isolated yield.The resulting 1H-NMR spectrum is in accordance with previously published NMR data. 10

Synthesis and characterisation of NO 3 -@1
A solution of 10.0 mg (0.0017 mmol, 1 eq) 1 and 0.17 mg (0.0017 mmol, 1.0 eq) KNO 3 in 0.6 mL acetonitrile-d3 was heated to 60 °C for 2 hours and the resulting solution was analysed by NMR spectroscopy.Please do not adjust margins Please do not adjust margins

Crystallographic analysis of 1_PF6_TfO
Single crystal X-ray diffraction data were collected on an Agilent SuperNova Dual diffractometer with an Atlas detector using mirror-monochromatized Cu-Kα radiation (λ = 1.54184Å) from a microfocus source, equipped with an Oxford Cryostream cooling system.CrysAlisPro13 software was used for data collection, integration and reduction as well as applying the analytical and semi-empirical absorption corrections.
The geometries of PF 6 − anions were restrained to be regular and identical, while the CF 3 SO 3 − anion was modelled as a rigid body using the coordinates extracted from a published structure, CSD refcode MEFHOW. 21Their atomic displacement parameters were also appropriately restrained.Moreover, three acetonitrile molecules could be located and modelled, one of which with half-occupancy.Their geometries were restrained to be the same, and the rigid-body restraints were applied to their atomic displacement parameters.Finally, the rest of the solvent molecules were found to be too badly disordered and could not be modelled explicitly.The SQUEEZE 22 routine of PLATON

Figure 2 .
Figure 2. The X-ray structure of the cationic unit of 1. Thermal ellipsoids are shown at 30% probability level.Colour code: Grey -carbon, blue -nitrogen, red -oxygen, yellow -fluorine.Hydrogen atoms are omitted for clarity.