Tetrahedral and octahedral metallomesogenic Zn(II) complexes supported by pyridine-functionalised pyrazole ligands

Abstract

Pyridylpyrazoles with long-chained alkyloxyphenyl substituents [Hpz^R(n)py] (R(n) = C₆H₄OC_nH_2n+1, n = 12, 14, 16, 18) are versatile ligands for designing new zinc metallomesogens [Zn(Hpz^R(n)py)_m][X]₂ (m = 2, X = NO₃⁻; m = 3, X = BF₄⁻) with tetrahedral and octahedral coordination geometry, respectively. Molar conductivity measurements reveal the non-bonding character of the nitrate and tetrafluoroborate anions to Zn(II) and confirm the ionic nature of the complexes in solution. Polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) studies show that all of them are enantiotropic liquid crystals, exhibiting SmA mesophases. A layered packing with interdigitation of the alkyl chains is proposed on the basis of XRD results to explain the organisation in the SmA mesophase. TG experiments indicate that the zinc complexes are stable up to ca. 175 °C, the temperature at which mass loss is observed. On the basis of computational models using hyperchem-7 program the molecular geometry has been examined. The semi-empirical calculations suggest that, in the solid state, the counteranions could be involved in hydrogen bonds, which would contribute to obtaining the stable electronic structure.

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Introduction

The interest in metallomesogens or metal-containing liquid crystals has been the subject of intense research for the last few decades because they offer a viable approach to producing multifunctional materials.¹ The possibility that a compound displays a liquid crystal phase is related to its shape, dipolar properties, conformational dynamics, and specific intermolecular associations.² In particular in coordination compounds, the environmental geometry at the central metal is important in determining the mesogenic properties because of its influence on the overall molecular shape,³ metal ions with tetrahedral or octahedral coordination being less used than those with linear or square-planar geometry to generate mesomorphism. Metal ions such as rhodium(I) and palladium(II) with d⁸ electronic configurations and a preferred square-planar coordination geometry have been the most used to generate smectic^4–8 and discotic metallomesogens.^9–12 Additionally, metallomesogens of other metals such as Pt, Ni, Au, Cu, Ag, Fe and Mn have also been investigated.^13–20

The Zn metallomesogens reported in the literature have shown variable coordination geometries such as square planar with porphyrin ligands²¹ or trigonal-bypiramidal with ligands such as dithiobenzoates and tridentate pyridines.^22,23 However, tetrahedral or octahedral zinc complexes with mesomorphic properties have remained a hedged objective for a long time.¹⁴ Although the tetrahedral geometry has been considered unfavourable for mesogenic species, some of them have been prepared and characterised as liquid crystals in the last few years,^24,25 but to the best of our knowledge octahedral zinc complexes exhibiting liquid crystal behaviour have not been reported to date.

On the other hand, the effects of polar substituents on the molecular periphery have been investigated in many organic systems, but they have been less studied for metallomesogenic materials. In this context, we have recently reported the suitability of long-chained 3-(4-n-alkyloxyphenyl)-(5-pyridin-2-yl)pyrazole ligands [Hpz^R(n)py] to achieve liquid crystal behaviour in ionic silver complexes of the type [Ag(Hpz^R(n)py)₂][X] (R(n) = C₆H₄OC_nH_2n+1, n = 12, 14, 16, 18; X = NO₃⁻, BF₄⁻).²⁶ The introduction of a polar pyridine group as a substituent on the pyrazole group modifies the dispersive forces and the intermolecular interactions, thus favouring the induction of mesomorphism.

Following our research interest in pyrazole-based metallomesogens, the present work is focused on the preparation, characterisation and mesomorphic behaviour study of two new series of ionic pyridylpyrazole Zn(II) complexes based on [Hpz^R(n)py] ligands.

Results and discussion

Synthesis and characterisation

Pyridylpyrazole ligands [Hpz^R(n)py] were prepared from 1,3-diketones by a procedure previously reported.²⁶ The zinc compounds [Zn(Hpz^R(n)py)_m][X]₂ (m = 2, X = NO₃⁻; m = 3, X = BF₄⁻) were obtained by reaction of the corresponding ligand [Hpz^R(n)py] with the salts Zn(NO)₃·6H₂O or Zn(BF₄)₂·6H₂O in a 2 : 1 or 3 : 1 (ligand : metal) molar ratio, respectively (Scheme 1). It is interesting to note that the reaction mixture always evolves to give the compounds [Zn(Hpz^R(n)py)_m][X]₂ (m = 2, X = NO₃⁻; m = 3, X = BF₄⁻) independent of the molar ratio used, although the highest yields were obtained under the stoichiometric conditions mentioned above. All the new complexes were isolated as white solids, stable at room temperature and insoluble in apolar organic solvents. They were characterised by elemental analysis and IR and ¹H-NMR spectroscopic techniques (see the Experimental section). All compounds were obtained with molecules of water of solvatation.

Scheme 1 Synthesis of the zinc compounds [Zn(Hpz^R(n)py)_m][X]₂ (X = NO₃⁻, BF₄⁻). The numbering used in the NMR assignment is also indicated.

The IR spectra of the new compounds in the solid state show the characteristic bands of the pyrazole ligand and those corresponding to the counteranion. Among the most significant ones, the ν(NH) band from the [Hpz^R(n)py] group appears at ca. 3200–3300 cm⁻¹ and the overlapped ν(C=N) and ν(C=C) band associated with the pyrazole and pyridine heterocycles is located at ca. 1612 cm⁻¹. In addition, a band assigned to the γ(CH) deformation of the pyridine group could also be observed at ca. 800 cm⁻¹.

The nitrate and tetrafluoroborate counteranions were identified by a band around 1384 cm⁻¹ assigned to the ν(N–O) stretching from the NO₃⁻ group in the nitrate complexes, or by a broad band centred at 1058 cm⁻¹ characteristic of the ν(B–F) from the BF₄⁻ group in the tetrafluoroborate derivatives. These bands are slightly shifted related to those of the free anions, suggesting that in the compounds the counteranion could be involved in coordinative or hydrogen-bond interactions.

The complexes behave as 1 : 2 electrolytes, in CH₃CN solution, showing molar conductivity values of ca. 230 Ω⁻¹ mol⁻¹ cm². Although, these results agree with the ionic nature of the compounds in solution, they do not avoid the counteranions participation in coordinative or hydrogen-bond interactions in the solid state as is suggested by IR spectroscopy.

The ¹H-NMR spectra of [Zn(Hpz^R(n)py)₂][NO₃]₂ and [Zn(Hpz^R(n)py)₃][BF₄]₂ in CDCl₃ or (CD₃)₂CO at room temperature display, in general, broad signals corresponding to the protons of the pyrazole and pyridine groups and the alkyloxyphenyl substituent, which are shifted downfield with respect to those in the free ligands [Hpz^R(n)py].

The broad signals mentioned above observed at 298 K are characteristic of the presence of dynamic processes. In order to highlight that behaviour, a variable temperature ¹H-NMR study was carried out for the four-coordinated [Zn(Hpz^R(n)py)₂][NO₃]₂ (n = 14, 18) complexes. Then we were able to observe how the starting broad signals which appear at room temperature were well defined and split as the temperature was decreasing. So, from −15 °C to −45 °C three sets of signals for each type of protons could be observed in almost all cases, suggesting the presence of three kinds of species. In particular, these features were clearly detected for the C(6)H and NH protons (Fig. 1). The results could be explained by considering the presence of an equilibrium between species having the suggested tetrahedral geometry (A) and those more planar produced by distortion of the tetrahedral one towards the square planar geometry (B, C), the latter exhibiting head-to-head and head-to-tail conformers corresponding to the different orientations of the chains substituent of the ligands (Fig. 2). The lowering of the temperature gave rise to the freezing of the equilibrium, therefore allowing detection of the three mentioned isomers.

Fig. 1 Details of the ¹H-NMR spectrum of the [Zn(Hpz^R(14)py)₂][NO₃]₂ compound in CDCl₃ solution at 258 K.

Fig. 2 Possible equilibrium proposed for the nitrate complexes in solution.

The related variable temperature ¹H-NMR study of [Zn(Hpz^R(12)py)₃][BF₄]₂ was also carried out. However, in any case we were able to improve the resolution of the spectra obtained at room temperature. This fact can be attributed to the presence of a fast interchange between the potential conformers related to the octahedral symmetry of the cationic part of the complexes.

In order to confirm the molecular formula of the Zn(II) derivatives containing two or three [Hpz^R(n)py] ligands, MALDI-TOF mass spectra of [Zn(Hpz^R(18)py)₂][NO₃]₂·H₂O and [Zn(Hpz^R(12)py)₃][BF₄]₂·2H₂O as representative examples of each family were registered. For the nitrate derivative the ion [M–H₂O]⁺ at m/z 1166 can be detected. By contrast it was not observed for the related tetrafluoroborate one, which exhibited a peak at 1343 corresponding to [Zn(L)₃(H₂O) + 2Na]⁺ (L = [Hpz^R(n)py]) in agreement with the cationic molecular formulation proposed for those complexes. In addition, the fragment [Zn(L)₂]⁺ at m/z 1042 and 874 and those at 489 and 405 from the respective ligands [L]⁺ were also observed in both complexes.

On the other hand, all efforts to obtain suitable single crystals of both types of complexes for X-ray crystalline determination were unsuccessful. Then, the molecular structure was simulated using the HyperChem-7 program for the molecular mechanics and semi-empirical calculations. The conformational models of minimum energy obtained by computer simulation for [Zn(Hpz^R(12)py)₂][NO₃]₂ and [Zn(Hpz^R(12)py)₃][BF₄]₂ are shown in Fig. 3. The zinc atom in the nitrate compound is bound to four nitrogen atoms of the two pyridylpyrazole ligands, giving rise to a tetrahedral geometry. However, in the tetrafluoroborate derivative the zinc centre is hexacoordinated to three bidentate pyridylpyrazole ligands and adopts an octahedral geometry. As was deduced from conductance measurements in solution, the counteranions should be outside of the coordination sphere in both types of compounds. However, this result does not preclude the presence of potential hydrogen-bond interactions between the counteranions and the ligands in the solid state, as has been proposed from the IR data. Then, we consider this suggestion in a new simulation study and compare it with that in which the nitrate and tetrafluoroborate groups act as free counteranions. As an interesting result for the first option, the energy values of −13 253 and −19 659 kcal mol⁻¹ for nitrate and tetrafluoroborate derivatives, respectively, were lower than those obtained in the latter (−13 951 and −20 720 kcal mol⁻¹, respectively), so suggesting that the counteranions should be involved in hydrogen-bond interactions and therefore stabilizing the structure in the solid state.

Fig. 3 Computer simulation of [Zn(Hpz^R(12)py)₂][NO₃]₂ (left) and [Zn(Hpz^R(12)py)₃][BF₄]₂ (right), using HyperChem-7 (Hipercube Inc.).

Thermal behaviour

TG studies. Thermogravimetric (TG) experiments were carried out for [Zn(Hpz^R(12)py)₂][NO₃]₂·H₂O and [Zn(Hpz^R(18)py)₃][BF₄]₂·2H₂O as representative examples of both families of complexes in order to check their thermal stability. Under N₂ atmosphere the complexes are stable below 173.1 and 183.1 °C, respectively. The thermal decomposition of both compounds occurs in several stages.

The nitrate complex shows, in the first steps (173.1–239.8 °C), an overall mass loss of 7.5% corresponding to a dehydration process and the loss of one nitrate group (calculated 7.8%). The step of the highest weight loss was that above 240 °C, as a result of the loss of the two pyrazole ligands and the second nitrate group according to the molecular structure (total weight loss of ca. 83%, calculated 83.9%) (Fig. 4). For the tetrafluoroborate complex, two water molecules, two pyrazole ligands and one tetrafluoroborate group were lost in the first two decomposition steps (total weight loss ca. 63.6%, calculated 63.2%). The remaining tetrafluoroborate and the third ligand decomposed at 470 °C, corresponding to an ca. 30.5% of the mass loss (calculated 30.6%).

Fig. 4 Thermal decomposition of (a) [Zn(Hpz^R(12)py)₂][NO₃]₂·H₂O and (b) [Zn(Hpz^R(18)py)₃][BF₄]₂·2H₂O.

Although the nature of the gases formed was not established, the total weight loss observed (91 and 94% for nitrate and tetrafluoroborate complexes, respectively) allowed determining that the ZnO is the final residue obtained after dehydration and decomposition processes.

On the other hand, the IR spectra of the solid residues obtained by heating at 200 °C showed the disappearance of some of the characteristic absorption bands as well as the appearance of other new ones from the decomposition products. Related behaviour has also been observed in other zinc species containing nitrate as a counteranion.²⁷

POM and DSC studies. The thermal behaviour of the new series of compounds was studied by polarised optical microscopy (POM) and differential scanning calorimetry (DSC). The phase transition temperatures and their associated enthalpy data are gathered in Table 1.

Table 1 Phase behaviour of Zn(II) metallomesogens

X	n	Transition^a	T/°C (ΔH/kJ mol^−1)
a Cr = crystalline phase, SmA = smectic A mesophase, I = isotropic liquid, d = decomposition. b Enthalpies were not determined due to decomposition. c The transition temperatures were determined by POM and confirmed by XRD.
NO3^−	12	Cr → SmA	156 (50.8)
		SmA → I	172^b
	14	Cr → SmA	143 (35.0)
		SmA → I	d
	16	Cr → SmA	90 (18.9)
		SmA → I	d
	18	Cr → SmA	90
		SmA → I	d
BF4^−	12	Cr → SmA	150^c
		SmA → I	d
	14	Cr → SmA	135^c
		SmA → I	d
	16	Cr → SmA	120^c
		SmA → I	d
	18	Cr → SmA	100^c
		SmA → I	d

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All zinc complexes displayed liquid crystalline behaviour exhibiting SmA mesophases. However, in the nitrate derivatives, despite the enantiotropic behaviour, the typical fan-shaped and focal-conic textures of the SmA mesophases could be only observed upon heating (Fig. 5), probably due to the decomposition that they undergo at high temperatures. The tetrafluoroborate complexes exhibited a minor decomposition at temperatures close to the clearing, thus allowing the observation of the mesophases upon heating and also upon cooling. Contact preparation study of [Zn(Hpz^R(14)py)₃][BF₄]₂ with the previously characterised 3,5-di(4-n-butoxyphenyl)pyrazole mesogen⁴ as a reference phase allowed us to establish the smectic-A nature of the mesophases.

Fig. 5 Microphotographs observed by POM of (a) [Zn(Hpz^R(18)py)₂][NO₃]₂ at 196 °C upon heating and (b) [Zn(Hpz^R(16)py)₃][BF₄]₂ at 161 °C upon cooling.

The DSC thermograms of the new derivatives show, in the first heating cycle, an endothermic peak related to solid–mesophase transition (Table 1). However the degradation of the samples at temperatures even lower than that of the clearing, ca. 175 °C, avoids the observation of a second peak associated with the mesophase–isotrope transition (except in the case of the nitrate derivative with n = 12) which prevents a reliable assignment of the clearing temperatures. In spite of this fact, the SmA characteristic textures simultaneously to partial decomposition could be observed by POM at temperatures above 200 °C.

XRD studies. All of the mesomorphic compounds were also studied by X-ray diffraction at variable temperature (XRD) to confirm the type of mesophase. The observed reflections, proposed indexing, and lattice constants are included in Table 2. The XRD diffractograms show, in general, two diffraction peaks in the low angle region in a ratio of 1 : 1/2, indexed to the (001) and (002) reflections of a smectic mesophase (Fig. 6). In particular, in the tetrafluoroborate complexes with n = 12, 14, a sole sharp reflection from (001) reflection was observed. In addition, in all cases, a broad halo in the wide-angle region at ca. 4.43 Å corresponds to the liquid-like order of the molten alkyl chains. These facts, in addition to the texture observed by POM, confirm the smectic nature of the fluid phases. Likewise, when the temperature is higher than 175 °C, peaks corresponding to a decomposition process were also observed in agreement with the TG experiments and DSC thermograms.

Table 2 X-Ray diffraction data

Compound	n	T (°C)	2θ (°)	d exp ^a (Å)	[hkl]^b	Parameters^c
a d exp is the measured diffraction spacing. b [hkl] are the Miller indices of the reflections. c Molecular volume: Vmol = Mw/(NAρ); where Mw is the molecular weight, NA is Avogadro's number and ρ is the density. Smectic periodicity: dcal = (Σld00l)/N00l; where N00l is the number of 00l reflections. Molecular cross-sectional area: Amol = 2Vmol/dcal.
[Zn(Hpz^R(n)py)2][NO3]2	12	165	2.2	40.6	001	d cal = 40.7 Å V mol = 1691 Å^3 A mol = 83.1 Å^2
			4.3	20.4	002
	14	160	2.1	42.6	001	d cal = 43.0 Å V mol = 1784 Å^3 A mol = 83.0 Å^2
			4.1	21.7	002
	16	160	2.0	44.6	001	d cal = 45.3 Å V mol = 1892 Å^3 A mol = 83.5 Å^2
			3.8	23.0	002
	18	155	1.9	46.8	001	d cal = 47.8 Å V mol = 1971 Å^3 A mol = 82.5 Å^2
			3.6	24.4	002
[Zn(Hpz^R(n)py)3][BF4]2	12	160	2.4	36.6	001	d cal = 36.6 Å V mol = 2477 Å^3 A mol = 135.4 Å^2
	14	135	2.3	39.0	001	d cal = 39.0 Å V mol = 2616 Å^3 A mol = 134.2 Å^2
	16	120	2.8	42.7	001	d cal = 42.7 Å V mol = 2786 Å^3 A mol = 130.5 Å^2
	18	120	2.0	43.8	001	d cal = 44.4 Å V mol = 2896 Å^3 A mol = 130.4 Å^2
			3.9	22.5	002

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Fig. 6 Powder 2D XRD diffraction pattern of (a) [Zn(Hpz^R(16)py)₂][NO₃]₂ and (b) [Zn(Hpz^R(18)py)₃][BF₄]₂.

The supramolecular organisation in the SmA mesophases can be estimated by analysing the low angle diffraction peaks, which are attributed to the layer molecular ordering in the mesophase. The lamellar periodicity measured for the SmA phase of nitrate and tetrafluoroborate derivatives was ca. 44 and 40 Å, respectively. On the other hand, the area occupied by the complexes in the smectic layers (A_mol) is found to be about 83 and 144 Ų, respectively. These results were obtained assuming that the density of these complexes in the mesophase is ∼1 g cm⁻³, a value acceptable and commonly used for the calculations.^28,29

On this basis, a lamellar structure as that represented in Fig. 7 is proposed for both kinds of complexes. The nitrate derivatives should be arranged in a layered structure, showing certain interdigitation of the alkyl chains, which is determined by alternating antiparallel disposition of the tetrahedral molecular structure. However, the lower lamellar periodicity of the tetrafluoroborate compounds allows suggesting that the similar alternating antiparallel disposition of the octahedral molecules yields a higher interpenetration in order to compensate for the uneven number of alkyl chains. Both proposals are consistent with the packing of the SmA phase identified by XRD.

Fig. 7 Proposed supramolecular organisation in the SmA mesophase of tetrahedral (left) and octahedral (right) Zn(II) complexes showing the inter-layer distances.

Conclusions

Novel metallomesogenic Zn(II) ionic complexes of the type [Zn(Hpz^R(n)py)_m][X]₂ (R(n) = C₆H₄OC_nH_2n+1, n = 12, 14, 16, 18; m = 2, X = NO₃⁻; m = 3, X = BF₄⁻) containing monocatenar N,N-donor pyrazolylpyridine ligands have been designed and synthesised. The compounds behave as liquid crystal materials in which the tetrahedral or octahedral environments do not constitute an impediment to achieve the supramolecular ordering of the mesophases. The hemispherical or planar nature of the corresponding counteranions (NO₃⁻ and BF₄⁻, respectively) appears to be responsible for the tetrahedral or octahedral stereochemistry of the Zn(II) cationic species.

The mesomorphic properties of the new zinc derivatives depend on the chain length, the molecular geometry and the nature of the counteranions. So, by increasing the chain length the melting point of the complexes is decreased regardless of the counteranion present, which suggests that the increase of the van der Waals interactions is a determinant of the mesophase ordering.

Related to the coordination geometry, it is interesting to note that the mesophase is reached at lower temperatures in the short-chain octahedral compounds, in agreement with the lamellar structure of the alternated octahedron in which the increase of interdigitation presents a limit from which the van der Waals interactions are less. The inverse behaviour is found by comparing the melting point of tetrahedral and octahedral complexes with the longest chains. As a consequence, the best liquid crystal properties are found for the tetrahedral Zn derivatives with chains of 16 and 18 carbon atoms.

Experimental section

Materials and physical measurements

All commercial reagents were used as supplied. Elemental analyses for carbon, hydrogen and nitrogen were carried out by the Microanalytical Service of Complutense University. IR spectra were recorded on a FTIR Thermo Nicolet 200 spectrophotometer with samples as KBr pellets in the 4000–400 cm⁻¹ region: w (weak), m (medium) and s (strong). ¹H-NMR spectra were collected at room temperature on a Bruker DPX-300 spectrophotometer (NMR Service of Complutense University) from solutions in (CD₃)₂CO or CDCl₃. The variable temperature studies were carried out on a Bruker Avance-500 spectrophotometer, requiring CDCl₃ to maintain dissolved compounds at low temperatures. Chemical shifts δ are listed relative to Me₄Si using the signal of the deuterated solvent as reference (2.05 and 7.26 ppm, respectively), and coupling constants J are in hertz. Multiplicities are indicated as s (singlet), d (doublet), t (triplet), m (multiplet), br (broad signal). The ¹H chemical shifts and coupling constants are accurate to ±0.01 ppm and ±0.3 Hz, respectively. MALDI-TOF-MS analyses were performed in a MALDI-TOF/TOF Bruker model ULTRAFLEX spectrometer at the Mass Spectrometry Service of Complutense University, with dithranol as a matrix. TG experiments were obtained on a Perkin-Elmer Pyris 1 TGA, with a heating rate of 10 K min⁻¹. The conformational models were obtained using HYPERCHEM-7 program for molecular simulation. Geometry optimisations of the structures were accomplished using MM+ method with a steepest descent calculation algorithm. The obtained configurations were optimised further by Fletcher–Reeves algorithm and semi-empirical calculations using PM3 method with a conjugate directions algorithm.

Phase studies were carried out by optical microscopy using an Olympus BX50 microscope equipped with a Linkam THMS 600 heating stage. The temperatures were assigned on the basis of optic observations with polarised light. Measurements of the transition temperatures were made using a Perkin Elmer Pyris 1 differential scanning calorimeter with the sample (1–4 mg) sealed hermetically in aluminium pans and with a heating or cooling rate of 10 K min⁻¹. The X-ray diffractograms at variable temperature were recorded on a Panalytical X'Pert PRO MPD diffractometer with Cu Kα (1.54 Å) radiation in a θ–θ configuration equipped with an Anton Paar HTK1200 heating stage (X-Ray Diffraction Service of Complutense University).

Synthesis of the compounds

Pyrazoles [Hpz^R(n)py]. The pyrazoles [Hpz^R(n)py] used as ligands were synthesised by the procedure described by us in a previous work.²³

Complexes [Zn(Hpz^R(n)py)₂][NO₃]₂·xH₂O. To a solution of Zn(NO₃)₂·6H₂O (0.1 mmol) in 5 mL of ethanol (96%) was added a solution of the corresponding pyrazole [Hpz^R(n)py] (0.2 mmol) in 15 mL of EtOH. After 24 h of stirring at room temperature, the white solid obtained was filtered off in vacuo and purified by recrystallisation in CH₂Cl₂–hexane. All the compounds prepared have been characterised by spectroscopic and analytical techniques.

[Zn(Hpz^R(12)py)₂][NO₃]₂·H₂O: colorless solid (75%). Found: C, 61.0; H, 6.7; N, 11.1%. ZnC₅₂N₈H₇₂O₉ requires C, 61.3; H, 7.1; N, 11.0%. ν_max/cm⁻¹ 3214w ν(N–H), 1612s ν(C=C + C=N), 1385s ν(N=O), 785m γ(C–H)_py. δ_H (300 MHz; (CD₃)₂CO; Me₄Si) 0.88 (6H, t, ³J 6.9, CH₃), 1.28 (36H, m, CH₂), 1.81 (4H, m, CH₂), 4.09 (4H, t, ³J 6.5, OCH₂), 7.12 (4H, d, ³J 8.7, H_m), 7.60 (2H, s, 4′-H), 7.79 (2H, m, 5-H), 7.87 (4H, d, ³J 8.7, H_o), 8.30 (2H, m, 3-H), 8.30 (2H, m, 4-H), 8.66 (2H, d, ³J 4.4, 6-H), 13.9 (2H, s, NH).

[Zn(Hpz^R(14)py)₂][NO₃]₂·H₂O: colorless solid (64%). Found: C, 62.9; H, 7.2; N, 10.4%. ZnC₅₆N₈H₈₀O₉ requires C, 62.6; H, 7.5; N, 10.4%. ν_max/cm⁻¹ 3219w ν(N–H), 1612s ν(C=C + C=N), 1385s ν(N=O), 784m γ(C–H)_py. δ_H (300 MHz; (CD₃)₂CO; Me₄Si) 0.87 (6H, t, ³J 6.9, CH₃), 1.29 (44H, m, CH₂), 1.81 (4H, m, CH₂), 4.07 (4H, t, ³J 6.5, OCH₂), 7.08 (4H, d, ³J 8.6, H_m), 7.58 (2H, s, 4′-H), 7.77 (2H, m, 5–H), 7.85 (4H, d, ³J 8.6, H_o), 8.25 (2H, m, 3-H), 8.25 (2H, m, 4-H), 8.55 (2H, m, 6-H).

[Zn(Hpz^R(16)py)₂][NO₃]₂·2H₂O: colorless solid (56%). Found: C, 63.0; H, 7.3; N, 9.8%. ZnC₆₀N₈H₉₀O₁₀ requires C, 62.7; H, 7.9; N, 9.7%. ν_max/cm⁻¹ 3220w ν(N–H), 1612s ν(C=C + C=N), 1384s ν(N=O), 782m γ(C–H)_py. δ_H (300 MHz; (CD₃)₂CO; Me₄Si) 0.87 (6H, t, ³J 6.9, CH₃), 1.28 (52H, m, CH₂), 1.81 (4H, m, CH₂), 4.08 (4H, t, ³J 6.4, OCH₂), 7.10 (4H, d, ³J 8.1, H_m), 7.60 (2H, s, 4′-H), 7.77 (2H, m, 5-H), 7.87 (4H, d, ³J 8.1, H_o), 8.30 (2H, m, 3-H), 8.30 (2H, m, 4-H), 8.60 (2H, m, 6-H), 13.9 (2H, s, NH).

[Zn(Hpz^R(18)py)₂][NO₃]₂·H₂O: colorless solid (62%). Found: C, 65.1; H, 7.8; N, 9.3%. ZnC₆₄N₈H₉₆O₉ requires C, 64.8; H, 8.1; N, 9.4%. ν_max/cm⁻¹ 3221w ν(N–H), 1612s ν(C=C + C=N), 1384s ν(N=O), 781m γ(C–H)_py. δ_H (300 MHz; (CD₃)₂CO; Me₄Si) 0.86 (6H, m, CH₃), 1.28 (60H, m, CH₂), 1.81 (4H, m, CH₂), 4.06 (4H, t, ³J 6.6, OCH₂), 7.07 (4H, d, ³J 8.8, H_m), 7.58 (2H, s, 4′-H), 7.78 (2H, m, 5-H), 7.85 (4H, d, ³J 8.8, H_o), 8.19 (2H, m, 3-H), 8.19 (2H, m, 4-H), 8.60 (2H, m, 6-H).

Complexes [Zn(Hpz^R(n)py)₃][BF₄]₂·xH₂O. A solution of the corresponding pyrazole [Hpz^R(n)py] (0.3 mmol) in 15 mL of EtOH was added to a solution of Zn(BF₄)₂·6H₂O (0.1 mmol) in a minimum volume of distilled water. The reaction mixture was stirred at room temperature for 24 h. The white precipitate formed was filtered off in vacuo and recrystallised in CH₂Cl₂–hexane. Elemental analysis and spectroscopic data are given as follows.

[Zn(Hpz^R(12)py)₃][BF₄]₂·2H₂O: colorless solid (58%). Found: C, 63.9; H, 7.1; N, 8.6%. ZnC₇₈N₉H₁₀₉O₅B₂F₈ requires C, 64.4; H, 7.3; N, 8.7%. ν_max/cm⁻¹ 3321w ν(N–H), 1612s ν(C=C + C=N), 1058s ν(B–F), 785m γ(C–H)_py. δ_H (300 MHz; (CD₃)₂CO; Me₄Si) 0.88 (9H, t, ³J 6.9, CH₃), 1.27 (54H, m, CH₂), 1.77 (6H, m, CH₂), 3.94 (6H, t, ³J 6.4, OCH₂), 6.91 (6H, d, ³J 8.6, H_m), 6.97 (3H, s, 4′-H), 7.51 (3H, m, 5-H), 7.61 (6H, d, ³J 8.6, H_o), 8.03 (3H, br, 3-H or 4-H), 7.89 (3H, br, 3-H or 4-H), 8.47 (3H, br, 6-H), 12.16 (3H, s, NH).

[Zn(Hpz^R(14)py)₃][BF₄]₂·2H₂O: colorless solid (64%). Found: C, 65.0; H, 7.4; N, 8.1%. ZnC₈₄N₉H₁₂₁O₅B₂F₈ requires C, 65.5; H, 7.7; N, 8.2%. ν_max/cm⁻¹ 3316w ν(N–H), 1612s ν(C=C + C=N), 1059s ν(B–F), 785m γ(C–H)_py. δ_H (300 MHz; (CD₃)₂CO; Me₄Si) 0.88 (9H, t, ³J 6.8, CH₃), 1.26 (66H, m, CH₂), 1.77 (6H, m, CH₂), 3.95 (4H, t, ³J 6.4, OCH₂), 6.93 (6H, d, ³J 8.6, H_m), 6.99 (3H, s, 4′-H), 7.53 (3H, m, 5-H), 7.64 (6H, d, ³J 8.6, H_o), 7.89 (3H, br, 3-H or 4-H), 8.04 (3H, br, 3-H or 4-H), 8.45 (2H, br, 6-H), 12.15 (3H, s, NH).

[Zn(Hpz^R(16)py)₃][BF₄]₂·3H₂O: colorless solid (64%). Found: C, 64.0; H, 7.7; N, 7.3%. ZnC₉₀N₉H₁₃₅O₆B₂F₈ requires C, 64.4; H, 8.1; N, 7.5%. ν_max/cm⁻¹ 3311w ν(N–H), 1613s ν(C=C + C=N), 1058s ν(B–F), 784m γ(C–H)_py. δ_H (300 MHz; (CD₃)₂CO; Me₄Si) 0.88 (9H, t, ³J 6.9, CH₃), 1.26 (78H, m, CH₂), 1.75 (6H, m, CH₂), 3.95 (6H, t, ³J 6.4, OCH₂), 6.93 (6H, d, ³J 8.6, H_m), 6.99 (3H, s, 4′-H), 7.55 (4H, m, 5-H), 7.64 (6H, d, ³J 8.1, H_o), 7.88 (3H, br, 3-H or 4-H), 8.04 (3H, br, 3-H or 4-H), 8.47 (3H, br, 6-H), 12.0 (3H, s, NH).

[Zn(Hpz^R(18)py)₃][BF₄]₂·2H₂O: colorless solid (53%). Found: C, 66.3; H, 8.0; N, 6.8%. ZnC₉₆N₉H₁₄₅O₅B₂F₈ requires C, 66.1; H, 8.4; N, 7.2%. ν_max/cm⁻¹ 1615s ν(C=C + C=N), 1056s ν(B–F), 784m γ(C–H)_py. δ_H (300 MHz; (CD₃)₂CO; Me₄Si) 0.87 (9H, t, ³J 6.9, CH₃), 1.25 (90H, m, CH₂), 1.76 (6H, m, CH₂), 3.94 (6H, t, ³J 6.5, OCH₂), 6.92 (6H, br, H_m), 7.00 (3H, s, 4′-H), 7.58 (6H, m, 5-H), 7.63 (6H, br, H_o), 7.87 (3H, br, 3-H or 4-H), 8.03 (3H, m, 3-H or 4-H), 8.45 (3H, br, 6-H), 12.0 (3H, s, NH).

Acknowledgements

The authors are grateful to the Ministerio de Economía y Competitividad (Spain), project CTQ2011-25172. We would like to thank Ignacio Sánchez Martínez for his contribution in the metallomesogens characterisation by POM and María José Torralvo Fernández for her help with the TG experiments.

Notes and references

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Footnote

† Dedicated to the memory of Prof. José Vicente Heras Castelló.

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