Cyclopalladated acac and cp liquid crystals: a comparative study Donocadh P. Lydon, Gareth W. V. Cave and Jonathan P. Rourke*† Department of Chemistry, Warwick University, Coventry, UK CV4 7AL The cyclopalladation of mesogenic Schiff bases yields two series of novel metallomesogens. The flat acetylacetonate derivatives show both nematic and smectic A phases with extended mesogenic ranges. In contrast, the non-planar cyclopentadienyl complexes show nematic phases at much lower temperatures than those of the acetylacetonate complexes.There is currently much interest in the synthesis of metal- and the clearing points are essentially the same. The fact that acac complexes do not exhibit the more ordered smectic phases containing liquid crystals owing to the perceived advantages of combining the properties of liquid crystal systems with those shown by the ligands is not surprising when one considers that the shape of the complexes will be distorted away from of transition metals.The area has been well reviewed recently,1–5 with excellent new work appearing constantly.6–11 the calamitic shape of the ligand by the presence of the Pd(acac) moiety on the side. Presumably this group disrupts Cyclopalladated compounds have proved to be a particularly fertile area of research, with many different examples from the packing of the molecule within the crystal, resulting in the lower melting point of the complexes compared with the many different groups.10,12–22 We have been studying a number of cyclopalladated Schiff ligands.The absence of a nematic phase for compound 3d is not unexpected given the presence of the two long chains base compounds with two very different co-ligands and present our results here.(seven carbons) at either end of the molecule, which presumably stabilise the smectic phase. It is interesting to note that the clearing points of complexes 3a and 3b, which both have four Synthesis carbons on the biphenyl group, are very similar, as are those of 3c and 3d, which both have seven carbons. It is also The synthesis of the new compounds described here, 3 and 5, interesting to note that the stability of the smectic phase seems is summarised in Scheme 1.The synthesis of the 4-alkyloxy- to be related to the length of the chain on the cyclopalladated N-(4¾-alkyloxybiphenyl)benzylidene ligands 1 via a simple con- ring.Thus, complexes 3b and 3d, which both have seven densation of the appropriate aldehyde and aniline proceeded carbons on the cyclopalladated ring, locating the bulky in high yield. The cyclopalladation step to give the intermediate Pd(acac) moiety more towards the centre of the molecule. have 2 was essentially quantitative, and 2 was used without further the most stable smectic A phases.purification. The synthesis of the acetylacetonate (acac) deriva- The thermal behaviour of the cp complexes 5 is listed in tives 3 from 2 proceeded in good yield, and these complexes Table 1 and summarised in Fig. 1. Thus, all the complexes were purified by column chromatography. The synthesis of the showed only a nematic phase, with the exception of 5d, which cyclopentadienyl (cp) complexes 5 from 2 via 4 gave a poor exhibits a smectic A phase too.The phases were identified on yield overall, after purification. All homologues of compounds the basis of their optical texture, the nematic and smectic A 1, 3 and 5 were analysed by 1H and 13C NMR and gave good phases exhibiting classic textures.The melting and clearing elemental analyses (see Table 2, later). points of complexes 5 are much lower than those of both the ligands 1 and the acac complexes 3. The fact that cp complexes Thermal properties do not exhibit the more ordered smectic phases is not surprising when one considers the shape of the complexes. Whilst both The thermal behaviour of the ligands 1 is listed in Table 1 and the ligands and the acac complexes can be thought of as summarised in Fig. 1. Thus, all the ligands showed smectic F, essentially planar, the cp ring is perpendicular to the plane of smectic C and nematic phases before clearing. The phases were the ligand. Clearly this group disrupts the packing of the identified on the basis of their optical texture, the nematic and molecule within the crystal, resulting in the reduced melting smectic C phases exhibiting schlieren textures, the nematic and clearing points of the complexes and the destabilisation showing both four- and two-point brushes, the smectic C only of the smectic phases.The presence of a smectic phase for four-point brushes. The smectic F phase showed a schlieren- compound 5d is not surprising given the presence of the two mosaic type texture on cooling from the smectic C phase.The long chains (seven carbons) at either end of the molecule, and absence of any point disclinations allowed us to distinguish it this mirrors the behaviour of the acac complex 3d. It is from the smectic I phase. interesting to note that the melting points of complexes 5a The thermal behaviour of the acac complexes 3 is listed in and 5c, which both have four carbons on the cyclopalladated Table 1 and summarised in Fig. 1. Thus, all the complexes ring, are very similar, as are those of 5b and 5d, which both showed a smectic A phase and, with the exception of 3d, a have seven, and thus the bulky Pd(cp) moiety located more nematic phase before clearing. The phases were identified on towards the centre of the molecule.the basis of their optical texture. The smectic A phase appeared Compounds 5 decompose at ca. 180 °C on transition to the as a focal-conic fan texture which separated on cooling from isotropic liquid, whereas compounds 3 do not decompose at the isotropic as batonnets which consisted of growing focal- temperatures almost 100 K higher. Chemically, the major conic domains.The melting and clearing points of complexes difference between the two complexes is the electron count on 3 neatly mirror those of the ligands from which they are the palladium: the cp complex has a formal count of 18e-, derived: the melting points of complexes 3 are ca. 10 K lower, whereas the acac complex has one of 16e-. The geometry of the cp complex is formally trigonal bipyramidal, whilst that of the acac complex is square planar. The chemistry of pal- † Email: j.rourke@warwick.ac.uk J.Mater. Chem., 1997, 7(3), 403–406 403N CnH2 n+1O OCmH2m+1 N OCnH2n+1 CmH2m+1O N CnH2 n+1O OCmH2m+1 Pd Pd OAc AcO N CnH2 n+1O OCmH2m+1 Pd O O N OCnH2n+1 CmH2m+1O N CnH2 n+1O OCmH2m+1 Pd Pd Cl Cl N CnH2 n+1O OCmH2m+1 Pd 1 NaAcac Pd(OAc)2 3 Tl(C5H5) 4 5 HCl 2 a n = 4, m = 4 b n = 4, m = 7 c n = 7, m = 4 d n = 7, m = 7 Scheme 1 ladium(II) is dominated by square-planar 16e- species, with exhibit mesogenic behaviour, with both smectic A and nematic phases being observed.very few examples of 18e- complexes. Thus the observed Thus it can be seen that our results are consistent with thermal stabilities of our compounds are entirely reasonable. established precedent: compared with the acac group, the cp Compounds 5 represent the only isomerically pure halfgroup brings down both the melting and clearing points of the sandwich liquid crystals ever reported.The only other halfcomplexes and shows a strong preference for the nematic sandwich liquid crystals known are those reported by Ghedini phase.It is clear that the cp group will become a popular et al.,10 where an azobenzene cyclometallates to give two motif in metallomesogen chemistry. isomeric products. Ghedini’s compounds, which like ours contain three aromatic rings, only exhibited nematic phases and at very similar temperatures to ours. Ghedini et al. also Experimental synthesised a couple of compounds with only two aromatic General rings, but observed no mesogenic behaviour for these cp derivatives.In contrast, some earlier work by Espinet et al.14 All chemicals were used as supplied, unless noted otherwise. All NMR spectra were obtained on either a Bruker AC250 or had shown that the acac derivatives of a two-ring system did 404 J. Mater. Chem., 1997, 7(3), 403–406Table 1 Mesogenic behaviour of compounds 1, 3 and 5 compd. trans.T/°C trans. T/°C trans. T/°C trans. T/°C 1a C–SF 191 SF–SC 197 SC–N 201 N–I 272 1b C–SF 160 SF–SC 171 SC–N 216 N–I 252 1c C–SF 171 SF–SC 178 SC–N 215 N–I 255 1d C–SF 157 SF–SC 161 SC–N 217 N–I 254 3a C–SA 184 SA–N 209 N–I 260 3b C–SA 162 SA–N 241 N–I 261 3c C–SA 155 SA–N 225 N–I 249 3d C–SA 141 SA–I 245 5a C–N 125 N–I 182a 5b C–N 101 N–I 145 5c C–N 125 N–I 179a 5d C–SA 92 SA–N 165 N–I 178a *Some decomposition.on an AC400 in CDCl3 and are referenced to external SiMe4, Hn ] , 1.81 (4 H, m, Ho,o¾), 1.40 (16 H, m, Hp,p¾), 0.89 [6 H, t, 3J(HH) 7.0 Hz, Hq,q¾]; dC: 159.2 (Ca), 158.5 (Ce,m ), 138.1 (Ci), assignments being made with the use of decoupling, NOE and the DEPT and COSY pulse sequences. Thermal analyses were 132.9 (Cb/j), 130.5 (Cc), 127.7 (Ch), 127.2 (Ck), 121.2 (Cg), 114.7 (Cd/l), 114.6 (Cd/l), 68.0 (Cn), 67.8 (Cn¾), 31.7 (Cp,p¾), 29.1 (Co,o¾), performed on an Olympus BH2 microscope equipped with a Linkam HFS 91 heating stage and a TMS90 controller, at a 28.9 (Cp,p¾), 25.9 (Cp,p¾), 19.2 (Cp,p¾), 14.0 (Cq,q¾).The mesogenic behaviour of all homologues is summarised heating rate of 10 K min-1.All elemental analyses were performed by Warwick Microanalytical Service. in Fig. 1 and detailed in Table 1. Elemental analyses are detailed in Table 2. Preparation of 4-heptyloxy-N-(4¾-heptyloxybiphenyl) benzylidene, 1d Preparation of orthometallated palladium acetate complex, 2d Compound 2d is described in detail, all other homologues Compound 1d is described in detail, all other homologues were prepared similarly. 4-Heptyloxybenzaldehyde (1.61 g, were prepared similarly. Ligand 1d, (0.41 g, 8.5×10-4 mol) and palladium acetate (0.191 g, 8.5×10-4 mol) were dissolved 7.10×10-3 mol) was added to a solution of 4¾-heptyloxy-4- aminobiphenyl (2.00 g, 7.10×10-3 mol) in toluene (200 ml). in acetic acid (250 ml) at 60°C, and stirred (20 h). The solvent was removed, the crude product dissolved in chloroform, The mixture was heated at reflux for 2 h using a Dean Stark trap and in the presence of molecular sieves.The solvent was filtered to remove traces of palladium black and the yellow solution evaporated to dryness. Yield 0.55 g (98%, removed and the product recrystallised from chloroform. Yield 2.47 g (72%, 5.1×10-3 mol). 4.2×10-4 mol).NMR data: dH: 8.44 (1 H, s, Ha), 7.87 (2 H, AA¾XX¾, Hc), 7.57 (2 H, AA¾XX¾, Hh ) , 7.55 (2 H, AA¾XX¾, Hk ) , 7.28 (2 H, NMR data: dH: 7.59 (1 H, s, Ha), 7.51 (2 H, AA¾XX¾, Hm), AA¾XX¾, Hg ) , 6.96 (2 H, AA¾XX¾, Hd), 6.95 (2 H, AA¾XX¾, Hl), 7.35 (2 H, AA¾XX¾, Hj), 7.18 [1 H, d, 3J(HH) 8.4 Hz, Hg], 6.95 4.03 [2 H, t, 3J(HH) 7.0 Hz, Hn¾], 4.01 [2 H, t, 3J(HH) 7.0 Hz, (2 H, AA¾XX¾, Hn ) , 6.78 (2 H, AA¾XX¾, Hi ) , 6.59 [1 H, dd, 3J(HH) 8.4 Hz, 4J(HH) 2.3 Hz, Hf], 6.03 [1 H, d, 4J(HH) 2.3 Hz, Hd], 4.05 [2 H, t, 3J(HH) 6.4 Hz, Hp¾], 4.00 [2 H, t, 3J(HH) 6.7 Hz, Hp], 1.90 (3H, s, Ht), 1.81 (4 H, m, Hq,q¾), 1.40 (16 H, m, Hr,r¾), 0.88 [6 H, t, 3J(HH) 7.1 Hz, Hs,s¾].Table 2 Elemental analysis data for compounds 1, 3 and 5 found (expected) compd. n m C (%) H (%) N (%) 1a 4 4 80.9 (80.8) 7.8 (7.8) 3.4 (3.5) 1b 4 7 81.2 (81.2) 8.4 (8.4) 3.1 (3.2) 1c 7 4 81.6 (81.2) 8.4 (8.4) 3.2 (3.2) 1d 7 7 81.3 (81.6) 8.7 (8.9) 3.0 (2.9) 3a 4 4 63.4 (63.4) 6.2 (6.2) 2.3 (2.3) 3b 4 7 64.5 (64.9) 6.6 (6.7) 2.0 (2.2) 3c 7 4 64.7 (64.9) 6.7 (6.7) 2.3 (2.2) 3d 7 7 66.3 (66.0) 7.2 (7.3) 1.9 (2.0) 5a 4 4 69.9 (67.2) 6.2 (6.2) 2.4 (2.5) 5b 4 7 68.1 (68.5) 6.6 (6.7) 2.1 (2.3) 5c 7 4 68.2 (68.5) 6.9 (6.7) 2.2 (2.3) 5d 7 7 69.5 (69.6) 7.6 (7.2) 2.2 (2.1) Fig. 1 Phase behaviour of compounds 1, 3 and 5 J. Mater. Chem., 1997, 7(3), 403–406 405Preparation of palladium acetylacetonate complex, 3d NMR data: dH: 7.86 (1 H, s, Ha), 7.54 (4 H, m, Hi,m ), 7.52 [1 H, d, 3J(HH) 8.4 Hz, Hg], 7.35 (2 H, AA¾XX¾, Hj ) , 7.24 Compound 3d is described in detail, all other homologues [1 H, d, 4J(HH) 2.3 Hz, Hd], 6.97 (2 H, AA¾XX¾, Hn ) , 6.59 were prepared similarly.Sodium acetylacetonate (0.038 g, [1 H, dd, 3J(HH) 8.4 Hz, 4J(HH) 2.3 Hz, Hf], 5.80 (5 H, s, 3.08×10-4 mol) was added to a solution of the acetate bridged Ht), 4.02 [2 H, t, 3J(HH) 6.9 Hz, Hp¾], 4.01 [2 H, t, 3J(HH) palladium complex 2d (0.200 g, 1.54×10-4 mol) in acetone 6.9 Hz, Hp], 1.84 (4 H, m, Hq,q¾), 1.40 (16 H, m, Hr,r¾), 0.90 (150 ml) at room temperature and stirred (2 h).The solvent [6 H, t, 3J(HH) 8.2 Hz, Hs,s¾]; dC: 164.3 (Ca), 158.9 (Ce), 157.7 was removed and the product was purified by column chroma- (Co), 139.6 (Cb/c/h), 138.9 (Cb/c/h), 138.4 (Cb/c/h), 138.1 (Cb/c/h), tography on silica, eluting with a 50550 mixture of dichloro- 132.4 (Ck), 131.8 (Cg), 130.5 (Cl), 128.0 (Cj/m), 127.9 (Cj/m ), methane and hexane.Yield 0.135 g (64%, 1.96×10-4 mol). 125.6 (Cd), 123.0 (Ci), 114.8 (Cn), 110.5 (Cf), 95.8 (Ct), 68.0 (Cp), 67.8 (Cp¾), 31.8 (Cq), 31.3 (Cq¾), 29.2 (Cr,r¾), 26.0 (Cr), 19.2 (Cr,r¾), 14.1 (Cs/s¾), 13.9 (Cs/s¾). The mesogenic behaviour of all homologues is summarised in Fig. 1 and detailed in Table 1. Elemental analyses are detailed in Table 2.We thank the University of Warwick for financial support NMR data: dH: 8.03 (1 H, s, Ha), 7.58 (2 H, AA¾XX¾, Hj), (D.P.L.), and Johnson-Matthey for loan of chemicals. 7.51 (2 H, AA¾XX¾, Hm), 7.46 (2 H, AA¾XX¾, Hi), 7.31 [1 H, m, 3J(HH) 8.1 Hz, Hg], 7.14 [1 H, d, 4J(HH) 2.3 Hz, Hd], 6.97 [2 H, AA¾XX¾, 3J(HH) 8.4 Hz, Hn], 6.60 [1 H, dd, 3J(HH) 8.3 Hz, 4J(HH) 2.3 Hz, Hf], 5.36 (1 H, s, Hv), 4.08 [2 H, t, References 3J(HH) 6.5 Hz, Hp¾], 4.02 [2 H, t, 3J(HH) 6.5 Hz, Hp], 2.10 (3H, s, Hx), 1.91 (3H, s, Ht), 1.84 (4 H, m, Hq,q¾), 1.40 (16 H, 1 S.A. Hudson and P. M. Maitlis, Chem. Rev., 1993, 93, 861. m, Hr,r¾), 0.90 [6 H, t, 3J(HH) 8.2 Hz, Hs,s¾]; dC: 188.3 (Cu/w), 2 A-M. Giroud-Godquin and P. M. Maitlis, Angew. Chem., Int.Ed. Engl., 1991, 30, 375. 185.7 (Cu/w), 172.4 (Ca), 160.4 (Ce), 158.7 (Co), 146.3 (Ch), 139.7 3 P. Espinet, M. A. Esteruelas, L. A. Oro, J. L. Serrano and E. Sola, (Cb/c), 138.8 (Cb/c), 132.6 (Ck), 129.5 (Cg), 127.9 (Cm), 127.6 Coord. Chem. Rev., 1992, 117, 215. (Cl), 126.5 (Cj), 123.6 (Ci ), 115.7 (Cd), 114.7 (Cn), 111.4 (Cf), 4 D. W. Bruce, in Inorganic Materials, ed. D. W. Bruce and D. 100.1 (Cv), 67.7 (Cp), 67.5 (Cp¾), 31.2 (Cq,q¾), 31.1 (Cq,q¾), 29.2 O’Hare, Wiley, Chichester, 1992. (Cr,r¾), 27.8 (Ct), 27.4 (Cx), 25.9 (Cr,r¾), 19.1 (Cr,r¾), 13.8 (Cs,s¾). 5 A. P. Polishchuk and T. V. Timofeeva, Russ. Chem. Rev., 1993, The mesogenic behaviour of all homologues is summarised 62, 291. in Fig. 1 and detailed in Table 1. Elemental analyses are 6 A. Omenat and M.Ghedini, J. Chem. Soc., Chem. Commun., 1994, 1309. detailed in Table 2. 7 R. Ishii, T. Kaharu, N. Pirio, S-W. Zhang and S. Takahashi, J. Chem. Soc., Chem. Commun., 1995, 1215. Preparation of chloro-bridged palladium complex, 4d 8 J. P. Rourke, D. W. Bruce and T. B. Marder, J. Chem. Soc., Dalton T rans., 1995, 317. Compound 4d is described in detail, all other homologues 9 R. Deschenaux, I.Kosztics and B. Nicolet, J. Mater. Chem., 1995, were prepared similarly. One equivalent of 0.4 mol dm-3 5, 2291. methanolic hydrogen chloridewas added to the acetato bridged 10 M. Ghedini, D. Pucci and F. Neve, Chem. Commun., 1996, 137. palladium complex 2d, (0.356 g, 3.1×10-4 mol) dissolved 11 R. Deschenaux, M. Schweissguth and A-M. Levelut, Chem. in chloroform (250 ml) at room temperature causing the Commun., 1996, 1275.clear yellow solution to become cloudy. The solvent was 12 J. Barbera�, P. Espinet, E. Lalinde, M. Marcos and J. L. Serrano, L iq. Cryst., 1987, 2833. removed and the crude product was washed with acetone 13 P. Espinet, J. Pe�rez, M. Marcos, M. B. Ros, J. L. Serrano, (15 ml) and filtered to collect the product. Yield 0.21 g (54%, J.Barbera� and A. M. Levelut, Organometallics, 1990, 9, 2028. 1.72×10-4 mol). 14 M. J. Baena, P. Espinet, M. B. Ros and J. L. Serrano, Angew. Chem., Int. Ed. Engl., 1991, 30, 711. Preparation of palladium cyclopentadienyl complex, 5d 15 M. J. Baena, J. Barbera�, P. Espinet, A. Ezcurra, M. B. Ros and J. L. Serrano, J. Am. Chem. Soc., 1994, 116, 1899. Compound 5d is described in detail, all other homologues 16 M.Ghedini, S. Morrone, O. Francescangeli and R. Bartolino, were prepared similarly. Thallium cyclopentadienide (0.200 g, Chem. Mater., 1992, 4, 1119. 7.64×10-4 mol, 4 equiv.) was added to a solution of 17 M. Ghedini, S. Morrone, O. Francescangeli and R. Bartolino, the chloro-bridged palladium complex 4d (0.239 g, Chem. Mater., 1994, 6, 1971. 18 M. Ghedini, D. Pucci, N. Scaramuzza, L. Komitov and S. T. 1.91×10-4 mol) in THF (250 ml) and heated at reflux (1 h). Lagerwall, Adv. Mater., 1995, 7, 659. The mixture was filtered and the solvent removed. The crude 19 L. Zhang, D. Huang, N. Xiong, J. Yang, G. Li and N. Shu, Mol. product was purified by column chromatography on silica, Cryst., L iq. Cryst., 1993, 237, 285. eluting with chloroform to yield a red solid. Yield 0.040 g 20 N. Hoshino, H. Hasegawa and Y. Matsunaga, L iq. Cryst., 1991, (28%, 5.16×10-5 mol). 9, 267. 21 M. Marcos, J. L. Serrano, T. Sierra and M. J. Gime�nez, Chem. Mater., 1993, 5, 1332. 22 K. Praefcke, S. Diele, J. Pickardt, B. Gu�ndogan, U. Nu�tz and D. Singer, L iq. Cryst., 1995, 18, 857. Paper 6/05791H; Received 20th August, 1996 406 J. Mater. Chem., 1997, 7(3), 4