Preparation of Novel Banana-Shaped Triple Helical Liquid Crystals by Metal Coordination

The synthesis of a series of banana-shaped structures has been carried out, in which the bend unit is formed by a 4,4’-methylenedianiline or 3,3’-methylenedianiline core bearing two symmetric pyridylimine linkages to di- and tri- alkoxyphenylester moieties on the side arms. The molecules, in addition to providing an elongated aromatic central core associated with liquid crystal (LC) molecules, also provide binding sites for metals. The methylenedianiline spacer incorporates phenylene groups that sterically prevent the two binding sites from co-ordinating to a single metal centre and the central methylene unit introduces enhanced flexibility into the ligand backbone. Furthermore, complexes have been formed by the co-ordination between 3, 3’-methylenedianiline containing ligands and Cu (I) ions [Cu2(3a-c)2][PF6]2. Electrospray Mass Spectrometry (ESMS) and Fast Atom Bombardment Mass Spectrometry (FABMS) showed the formation of dimeric species; [Cu (L)2][PF6]2. Finally, thermal analysis of the ligands (1a-d, 2a-d, 3a-c and 4a-d) and Cu complexes [Cu2(3a-c)2][PF6]2 has been carried out in order to investigate the phase properties of these materials. None of the banana-shaped ligands and the metal complexes [Cu2(3a-c)2][PF6]2 showed any mesophases.


Introduction
Although classical thermotropic liquid crystals (LCs) are commonly composed of rod-like molecules, many liquid crystals exhibit unconventional molecular structures. An example of such a molecular architecture is a banana-shaped one, with a bend in the middle of the mesogenic part [1][2][3][4]. Niori et al. [5] first observed ferroelectricity in a smectic phase which was produced from achiral banana-shaped molecules. Since then, several other research groups have [6] reported banana-shaped molecules that exhibited ferroelectric properties in the mesophase. The ferroelectricity is attributed to the polar packing of molecules with C 2v symmetry where the molecules are packed in the same direction. A great many analogous banana-shaped structures have been prepared, some with lateral substituents in the central aromatic ring, which exhibit the wide range of 'banana phases' commonly designated B 1 through to B 7 [7]. The existence of such mesophases depends on the length of the rigid core, as well as on the magnitude of the bend angle and its position. The structure and flexibility of the angled segment are also important [8].
Dating back to the 1980s there has been immense interest in metal-containing mesogens, and subsequently a novel subfield in liquid crystals has emerged, known as metallomesogens. One of the factors contributing to this growth of interest has been due to the presence of the metal atom causing significant change in the magnetic, electric and optical properties of liquid crystals [9][10][11]. Many liquid crystals calamites based on rare earth element complexes [9] have exhibited enhance magnetic susceptibility among liquid crystals, for example Dy and Tb complexes with Schiff's bases and βaminovinyl ketones demonstrate high anisotropies of magnetic susceptibility [9]. At present, the molecular motifs of the known metallomesogens are based on the classical rod-like [12][13][14][15] and discshaped mesogens [16]. One of the modern pioneers in this field is Bruce et al., who has demonstrated [12][13][14][15] that by a suitable choice of anisotropic ligand, it is possible to form rod-like liquid crystals based on the metals with octahedral coordination geometry [12,13]. Strong chelating moieties such as 2,2-bipyridine [12] and imine [13] moieties have been utilised as the metal chelating units in these metallomesogens. Also, Ziessel et al., have prepared several ligands based on 2,2'-bipyridine and these ligands assembled into a dinuclear double-helicate complex with copper that exhibits columnar mesophase [17].
Hannon et al. [18,19] have shown that bispyridylimine ligands can form double or triple stranded helicate structures. The triple helicate arises through the entwining of three bispyridylimine ligand strands around two metal cations, while the double helicates arise with tetrahedral metal ions. Herein, two novel series of banana-shaped molecules have been prepared that incorporate metal binding units (Scheme 1). The binding units contain 4,4'-methylenedianiline and 3,3'-methylenedianiline moieties as the central units which are linked to pyridine-2-carbaldehyde, affording imine structures. It is envisaged from the chemistry of the unsubstituted analogues (X-ray structures of their complexes are known [18,19,20]) that the helicates formed from the 4,4'-methylenedianiline core will form gross linear structures [18,21] and hence chemical modification at the terminal of this helicate with alkyl chains will result in classical rod-like mesogenic structures. Whereas, the helicates formed from the 3,3'-methylenedianiline which is structurally constrained will develop a kink at the centre of the helicate [20] and thus, with alkyl chains at the terminal of the helicate will form novel banana mesogenic structures [Cu 2 (3a-c) 2 ][PF 6 ] 2 . The thermal properties of these ligands (1a-d, 2a-d, 3a-c and 4a-d) and complexes [Cu 2 (3a-c) 2 ][PF 6 ] 2 have been investigated. Scheme 1. Desired banana-shaped molecules and the preparation of Cu complexes.

Design
The exact relationship between angled molecular structures and liquid crystalline properties is still not very clear to date, but empirical rules about molecular design [22][23][24][25][26][27] have shown that this new class of banana mesogen is extremely sensitive to structural modification regarding the attainment of mesophases. For example, flexibility [22,28], bend angle [7,28], nature of the central core [22,[29][30][31], length [26,31] and nature [32] of the terminal chains, substituents both on the central [7,26], and on the side arms [33], dipoles [26], and direction of linking groups [34] are among the main variants contributing to the capability of a material to exhibit liquid crystalline properties. Here a synthetic scheme has been devised that allows the incorporation of 4,4'-methylenedianiline and 3,3'-methylenedianiline units as the central core of the molecular architecture, which as well as providing the bent core for the banana-shaped molecules, provide pyridylimine and methylene dianiline units for binding to metal ions.

Synthesis
The syntheses of the novel banana-shaped compounds (1a-d, 2a-d, 3a-c and 4a-d) were achieved via a series of reactions as illustrated in Scheme 2. The commercially available di-and trihydroxy benzoic acids 6 and 7 were converted to their respective methyl esters in the presence of a catalytic amount of concentrated H 2 SO 4 in MeOH to afford compounds 8 and 9. Alkylation of compounds 8 and 9 was achieved under basic conditions (K 2 CO 3 ) with n-bromopentane, n-bromoheptane, n-bromononane and n-bromododecane, respectively affording compounds 10a-d and 11a-d [13]. The esters 10a-d and 11a-d were hydrolysed to the corresponding acids 12a-d and 13a-d under basic conditions (NaOH) [13]. The commercially available 5-hydroxy-2-methylpyridine (14) was directly oxidised with H 2 O 2 to afford the N-oxide 15. The N-oxide was subsequently heated in acetic anhydride to afford the acetate 16. Under acidic conditions (concentrated HCl) compound 16 was deprotected to afford the alcohol 17. Selective oxidation of the alcohol moiety was achieved in the presence of a mild oxidising agent, MnO 2 [35], to afford compound 18. Compound 18 was then coupled with the acids 12a-d and 13a-d through dicyclohexylcarbodiimide (DCC) [13] coupling to afford the respective aledehydes 19a-d and 20a-d. The target imine compounds 1a-d, 2a-d, 3a-c and 4a-d were obtained via stirring under N 2 atmosphere at room temperature between the respective aldehydes 19a-d and 20a-d and methylene dianilines [36]. Finally, the complexes [Cu 2 (3a-c) 2 ][PF 6 ] 2 were formed by coordination of molecules 3a-c to Cu (I) ions at room temperature in iso-PrOH [18,21].

Thermal Properties of Bent-shaped Ligands
Thermal analysis was carried out on compounds 1a-d, 2a-d, 3a-c and 4a-d using variable temperature optical polarised microscope (OPM) between crossed polarisers, and differential scanning calorimetry (DSC). OPM revealed that upon heating from solid to the liquid phase, no birefringent liquids were formed, and the materials passed straight to the isotropic liquid phase. On cooling the IK transition was observed at slightly lower temperatures, confirming the lack of any mesophase formation. However, for a few of the compounds (2a, 4c-d) the IK transition was not observed. This result suggests that these compounds decomposed through the KI transition. As expected, for all the 1a-d, 2a, 2d, 4c and 4d derivatives, the KI transition decreased with increasing the length of the carbon chain. The DSC traces are illustrated in Figure 1 and the melting points are listed in Table 1 6 ] 2 , all show broad peaks in the aromatic region suggesting that more than one species is present. As previously described [18,20,21] these dinuclear double stranded arrays can exist as helicate (rac-isomer) and box (mesoisomer) and these two are commonly interchanging in the NMR timescale leading to broad NMR peaks. The meso-isomer is usually the enthalpic product (the dominant solid state species) while the helicate is favoured entropically and is the major product in solution at room temperature [18,20,21]. In particular, [Cu 2 (3c) 2 ][PF 6 ] 2 , the NMR peaks were extremely broad making peak assignments difficult.
Liquid crystal properties of these free ligands 1a-d, 2a-d, 3a-c, and 4a-d and the three copper Cu (I) complexes [Cu 2 (3a-c) 2 ][PF 6 ] 2 were investigated using an OPM. Unfortunately, none of the compounds exhibited liquid crystalline phases. Possible reasons for this could lie in the ligands having too much flexibility about the central methylene unit. Secondly the two isomeric forms of the copper (I) complexes may prevent the molecules from becoming sufficiently ordered in the mesophase.

General
The starting materials which were commercially available were purchased from Aldrich and used as received. The solvents were purchased from Fisher Scientific (CH 2 Cl 2 , EtOAc, Et 2 O and MeOH) or Aldrich (anhydrous EtOH). Solvents were either used as received or dried; DCM was distilled from CaCl 2 under a N 2 atmosphere. Yields refer to chromatographically pure products. Thin-layer chromatography (TLC) was carried out on aluminum sheets coated with silica gel 60 (Merck 5554 mesh). Column chromatography was performed on silica gel 60 (Merck 230-400). Microanalyses were performed by the University of London microanalytical laboratories or by the University of Birmingham microanalytical services. Electron impact (EI) mass spectra were recorded at 70eV on a VG ProSpec mass spectrometer. Liquid secondary ion mass spectra (LSIMS) were recorded on a VG ZaBSpec mass spectrometer equipped with a cesium ion source and utilizing m-nitrobenzyl alcohol containing a trace of sodium acetate as the liquid matrix. 1 H-NMR spectra were recorded on a Bruker AC 300 (300 MHz) spectrometer. 13 C-NMR spectra were recorded on a Bruker AC 300 (75.5 MHz) spectrometer. The chemical shift values are expressed as  values and the coupling constant values (J) are in Hertz (Hz). The following abbreviations are used for the signal multiplicities or characteristics: s, singlet; d, doublet; dd, doublet of doublets; t, triplet; m, muliplet; q, quartet; quint; quintet; br, broad. Transition temperatures were measured using a Mettler FP82 HT hot stage and central processor in conjunction with Leitz DMFRT Polarizing microscope as well as differential scanning calorimetry (DSC7 Perkin-Elmer). (8) Methyl 3,4-dipentyloxybenzoate (10a): A suspension of K 2 CO 3 (9.90 g, 71.63 mmol) in a solution of methyl 3,4-dihydroxybenzoate (8, 3.00 g, 17.84 mmol) and n-bromopentane (6.75 g, 44.69 mmol) in MeCN (100 mL) was stirred and heated under reflux overnight. The resultant brown mixture was allowed to cool to room temperature and concentrated in vacuo (30 mL). Water (250 mL) was added and the aqueous layer was extracted by washing with Et 2 O (3 x 100 mL). The combined organic layers were washed with brine solution (50 mL), dried (MgSO 4 ), filtered and solvent removed in vacuo, yielding an orange oil as the crude product. The crude product was purified via silica gel column chromatography (gradient elution: 0 to 10 % EtOAc in hexane, increase polarity in increments of 5% per 150 mL of eluent used). The solvent was removed in vacuo to yield a colourless oil (4.84 g, 88%). Using the same general procedure as described for 10a, the following compounds were prepared:  150 mL), an aqueous solution of sodium hydroxide (15 mL, 2.10 mol L -1 ) was added and the mixture was stirred and heated under reflux overnight. The resultant colourless solution was allowed to cool to room temperature and HCl (2 M) was added dropwise to acidify the solution, whereupon a white precipitate was formed, which was filtered off. The filtrate was concentrated in vacuo (20 mL) and water (100 mL) was added. The aqueous layer was extracted with Et 2 OAc (3 x 100 mL). The combined organic layers were washed with brine solution (20 mL), dried (MgSO 4 ) and filtered. The solvent was removed in vacuo, yielding a white solid (4.00 g, 87%). mp: 126-128 C; Following the same general procedure described for 12a, the following compounds were prepared:   (16): 5-Hydroxy-2-methylpyridine N-oxide (15, 17.5 g, 0.14 mol) was slowly added to (CH 3 CO) 2 O (170 mL) at 110 C. The resultant dark brown reaction mixture was stirred for 2 hrs at 140 C. EtOH (400 mL) was added and the solution was concentrated in vacuo to yield brown oil. CHCl 3 (100 mL) was added and the solution was neutralised with saturated NaHCO 3 . The organic layers were collected through washings with saturated NaHCO 3 (2 x 20 mL), dried (MgSO 4 ) and filtered. The solvent was removed in vacuo to yield a brown oil, which was further dried under high vacuum for 2 hrs (26.1 g, 89%). 1

Conclusions
In summary, we have successfully synthesised a series of novel banana-shaped compounds, in which the bend unit is formed by a 4,4'-methylenedianiline or 3,3'-methylenedianiline core bearing two symmetric pyridylimine linkages. Also, a number of complexes [Cu 2 (3a-c) 2 ][PF 6 ] 2 were formed by coordination with Cu(I) cations. Mass spectrometry techniques have shown that dinuclear double stranded arrays were formed. Thermal analyses of the ligands 1a-d, 2a-d, 3a-c and 4a- 6 ] 2 revealed that these do not display any mesophase behavior. However, the generic structure of the target series 1a-d, 2a-d, 3a-c and 4a-d appears to fulfill, in principle, the same criteria of the banana-shaped mesogens known to date [37]. Further work is being carried out to prepare and investigate the liquid crystalline properties of the Cu (I) complexes formed from ligands 1a-d, 2a-d and 4a-d, because such metal containing structures conjure up potential application of magnetic LC phases.