Tuning the liquid crystalline properties of palladium(II) metallomesogens: A study of rod-like to disc-like transition in cyclopalladated complexes with N-benzoyl thiourea derivatives

doi:10.1016/j.molliq.2017.08.013

Journal of Molecular Liquids

Volume 243, October 2017, Pages 151-156

https://doi.org/10.1016/j.molliq.2017.08.013 Get rights and content

Highlights

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New Pd(II) metallomesogens with N-benzoyl thiourea were designed.
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A transition from lamellar to columnar organization was evidenced.
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Pd(II) complex with one alkoxy group onto BTU ligand shows a SmA phase.
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Complexes with higher number of alkoxy groups onto BTU ligands show Col_h phases.

Abstract

A new family of cyclometalated palladium(II) complexes with the non-symmetric dimeric liquid crystal Schiff base α-(4-cyanobiphenyl-4′-yloxy)-ω-(4-n-decyloxyanilinebenzylidene-4′-oxy) hexane and alkoxysubstituted N-benzoyl thiourea (BTU) derivatives as auxiliary ligands was designed and prepared with the aim to study the influence of increasing the number of aliphatic chains on the molecular organization in the liquid crystal phase. The liquid crystalline behavior of these palladium(II) complexes was investigated by a combination of polarized optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction. A transition from the lamellar (complex 3a with two aliphatic chains on BTU ligand) to columnar organization (complexes 3b and 3c having three and four chains on BTU ligand, respectively) was evidenced for these palladium(II) complexes, which was correlated with the increase of the number of terminal chains at molecular periphery.

Graphical abstract

Introduction

Metal-containing liquid crystals (metallomesogens) represent one of the most fascinating areas in the field of liquid crystals as the unique properties of metals (geometry of coordination, electronic, magnetic, optic or just the structural role) can be combined in a successful manner with the properties of anisotropic fluids (anisotropy of physical properties and fast orientational response to external fields) specific to liquid crystals (LC) [1]. Moreover, the possibility of metal ions to adopt different geometries can give rise to rich structures and topologies of metal complexes with a direct consequence on the mesomorphic properties, different from those shown by pure organic liquid crystals [2], [3], [4]. The palladium(II) complexes are one of the most appealing class of metallomesogens, in particular cyclopalladated complexes, due to the significant advantages they possess, such is their square planar geometry which favors the stabilization of LC phases as well as their relative facile synthesis. The LC and emission properties of palladium(II) complexes could be finely tuned as multiple choices are available to change the chemical structure around the metal center [5], [6]. Various mono- or dinuclear palladium(II) complexes with lower transition temperatures and rich mesomorphic behavior ranging from calamitic to discotic materials were reported. [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22] Transitions from the lamellar to columnar organization in the liquid crystalline phases have been reported for cyclometalated palladium complexes with acetylacetonates and different cyclometalated ligands: imine, [23] 2-phenylpyrimidine [24], [25] or 2-phenylpyridine [24] derivatives. In general, these palladium(II) complexes show a smectic-to-columnar phase cross-over observed on increasing the chain number and distribution of the aliphatic chains on the acetylacetonate auxiliary ligands. Such a mesomorphic behavior (characteristic of both rods and discs) is seen in polycatenar systems that possess an extended rigid calamitic core and a different number of terminal aliphatic chains (typically up to six divided between the two ends of mesogen) [26], [27], [28]. In order to display a possible rod-like to disc-like transition, previous attempts to prepare a family of palladium(II) complexes based on cyclometalated imine ligands and N-benzoyl thiourea (BTU) derivatives with increasing number of the aliphatic chains failed. Either various series of mononuclear palladium(II) complexes displaying calamitic behavior (nematic and smectic phases) [29], [30], [31] or double cyclopaladated complexes with discotic behavior (hexagonal columnar phases) [32] were obtained. For instance, when the number of aliphatic chains on the BTU ligand was progressively increased from two to four, the mesogenic behavior was lost, and a possible contributing factor could be the monotropic character of these LC phases [30]. The same results, e.g. loss of mesogenic character, were displayed for palladium(II) complexes with more than two aliphatic chains attached to the imine ligands [30]. Therefore, to prevent such a poor LC behavior, a more extended imine derivative, the non-symmetric dimeric liquid crystal Schiff base α-(4-cyanobiphenyl-4′-yloxy)-ω-(4-n-decyloxyanilinebenzylidene-4′-oxy)hexane (1), has been selected as cyclometalating ligand. Recently, we have reported a series of palladium(II) and platinum(II) complexes based on the above mentioned Schiff base on one side and simple BTU derivatives on the other side which can self-organize into nematic and smectic A phases [33]. The mesomorphic behavior of such complexes was strongly related to the type of simple N-benzoyl thiourea derivatives as well as the metal center used, palladium(II) or platinum(II).

The present paper describes the design, preparation and LC properties of a new family of palladium(II) complexes with the dimeric liquid crystal Schiff base 1 and BTU derivatives containing different numbers of long alkoxy mesogenic group in the terminal positions. Depending on the number of terminal alkoxy chains on the BTU ligands, different mesophase organization including smectic A and hexagonal columnar were found for these palladium (II) complexes. In particular, the columnar phases are of high interest, as stacking of molecules into columns could lead to an improved anisotropic charge transportation [34], [35].

Section snippets

Materials and characterization methods

The chemicals were used as supplied. C, H, N analyses were carried out on EuroEA 3300 instrument. IR spectra were recorded on a Jasco FTIR 4200 spectrometer coupled to ATR PIKE GladiATR device. ¹H and ¹³C NMR spectra were recorded on a Bruker Avance spectrometer operating at 300 MHz, using CDCl₃ solvent. ¹H chemical shifts were referenced to the residual solvent peak position, δ 7.26 ppm. The phase assignments and corresponding transition temperatures for all the palladium complexes were

Results and discussion

The preparation of cyclometalated mononuclear palladium(II) complexes with Schiff base 1 and BTU derivatives with a different number of alkoxy terminal chains, along with their numbering scheme, is presented in Scheme 1.

The dinuclear palladium(II) complex 2 was prepared by reacting the Schiff base 1 with palladium(II) acetate in dichloromethane, following the method previously reported [33]. Further, the mononuclear palladium(II) complexes 3a–c were prepared by reacting the dinuclear

Liquid crystal properties

All palladium complexes were investigated for their liquid crystal properties by hot stage polarizing optical microscopy, differential scanning calorimetry and powder X-rays diffraction. The thermal data are summarized in Table 1. The mesophases were assigned based on their optical texture [45], [46] and the representative pictures are presented in Fig. 1.

Complexes 3a shows three transitions on the first heating run: a first peak assigned to a crystal to crystal phase transition, a second

Conclusions

A transition from lamellar to columnar organization was evidenced for a novel family of cyclometalated palladium(II) complexes with the non-symmetric dimeric liquid crystal Schiff base α-(4-cyanobiphenyl-4′-yloxy)-ω-(4-n-decyloxyanilinebenzylidene-4′-oxy) hexane and alkoxysubstituted N-benzoyl thiourea derivatives as auxiliary ligands.

The complex 3a, possessing a BTU ligand with only two aliphatic chains, displays an enantiotropic SmA phase, stable on a large temperature interval, ranging from

Acknowledgements

This work was supported by a grant of the Romanian Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE-2011-3-0384 and Core Programme PN 16 48-3/2016.

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Tuning the liquid crystalline properties of palladium(II) metallomesogens: A study of rod-like to disc-like transition in cyclopalladated complexes with N-benzoyl thiourea derivatives

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and characterization methods

Results and discussion

Liquid crystal properties

Conclusions

Acknowledgements

J. Organomet. Chem.

Polyhedron

Inorg. Chem. Commun.

Tetrahedron

Inorg. Chem. Commun.

Inorg. Chem. Commun.

J. Mol. Liq.

J. Organomet. Chem.

Polyhedron

Inorg. Chim. Acta

J. Mol. Struct.

Coord. Chem. Rev.

Polyhedron

Polyhedron

Metallomesogens by ligand design

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Mesomorphism of ortho-metallated imine complexes of the β-diketonatoplatinum(II) fragment. Crystal and molecular structure of three platinum imine complexes

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Possible rod-disc transition in ortho-metallated imine complexes of palladium(II). Crystal and molecular structure of three palladium complexes

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New heteropolynuclear metallomesogens: copper(II), palladium(II), nickel(II) and oxovanadium(IV) chelates with [3]ferrocenophane-containing Schiff's base and – aminovinylketone

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Eur. J. Inorg. Chem.