J. MATER. CHEM., 1994, 4(5), 657-661 Syntheses of Soluble Polymeric Lewis Bases and their Adducts With Metal Alkyls Xiaochang Li,t Charles M. Lindall, Douglas F. Foster and David J. Cole-Hamilton* School of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY76 9ST The pendant double bonds in polybutadiene have been hydrosilated by the reaction of polybutadiene with chloro- dimethylsilane in toluene using H2PtCI, as catalyst. The hydrosilated polybutadiene was further functionalised with 2-picolyl, 4-picolyl, 4-dimethylaminophenyl and diphenylphosphinomethyl Lewis base groups thereby obtaining a series of new polymeric Lewis bases. All the polymers were found to react with group 13 metal alkyls and the 2- or 4-substituted pyridine functionalised polymers were also found to react with group 12 metal alkyls to form soluble polymer adducts.The polymers have been characterised by 'H nuclear magnetic resonance (NMR) spectroscopy, 13C NMR and elemental analysis. Integration of 'H NMR resonances indicates that polymer adducts with M :N or M :P ratios close to 1.O are obtained when using group 13 metal alkyls, whereas polymer adducts with M :N ratios close to 0.5 are obtained when using group 12 metal alkyls. The metal alkyls are easily lost during vacuum drying. The 'H NMR resonances from the alkyl groups on the metal centre shift to a high field upon coordination with N-containing polymeric Lewis bases whilst small downfield shifts are observed upon coordination of Me,M (M=Ga or In) with the P-containing polymeric Lewis bases.Organic Lewis bases that contain electron-donating elements such as 0, N, or P can form adducts with electron-deficient compounds such as group 13 metal alkyls.' These adducts can not only greatly reduce fire and toxicity hazards of the parent metal alkyls, but can also be employed to purify metal alkyls used in metal-organic chemical vapour deposition (MOCVD) through the process of adduct purification.2 Thus, volatile impurities can be removed by evacuation of the isolated adduct at temperatures below its dissociation tem- perature, whilst involatile impurities remain when the adduct is dissociated. By using this approach, different metal alkyls have been purified with adducts containing nitrogen-donor Lewis base?'' and phosphorus-donor Lewis bases.''-13 Although all these systems work fairly well, organic Lewis bases generally have some volatility so that traces of them can contaminate the purified metal alkyls, or can reduce the yield of purified materia1.4,5,10,'4 It has recently been rep~rted'~ that adducts can be prepared between the polymeric Lewis base, poly(viny1pyridine) (PVP), and main-group metal alkyls such as Me,M (n=2, M=Zn, Cd; n=3, M=Al, Ga or In).These adducts dissociate on heating to liberate the metal alkyls and may, therefore, be useful for the purification of metal alkyls and/or for the transport of metal alkyls in a safe manner." The parent metal alkyls spontaneously ignite in air whilst the adducts with PVP are only mildly air sensitive.All of the adducts between Me,M and PVP are insoluble in common organic solvents, largely as a result of the high molecular weight (M,=ca. 330000) of the polymer. This is ideal for the purification or transport of metal alkyls, but there are other applications of this kind of polymeric adduct, such as the formation of nanoparticles by a solution-based route,16 in which it is necessary to have available adducts of metal alkyls with polymeric Lewis bases that can be dissolved in organic solvents. In this paper, we report the synthesis of new polymeric Lewis bases, starting from readily available polybutadienes, together with adducts of these polymeric Lewis bases with main-group metal alkyls. The adducts are all soluble in common organic solvents.t Permanent address: Institute of Material Science and Engineering, East China University of Chemical Technology, Shanghai 200237, China. Experimental Microanalyses were carried out by the University of St. Andrews Microanalytical Service. Nuclear magnetic resonance (NMR) spectra were recorded on Bruker Associates WP80 and AM300 spectrometers operating in the Fourier transform mode with (for 13C)noise proton decoupling. Chemical shifts are in ppm to high frequency of external tetramethylsilane. Differential scanning calorimetry (DSC)was carried out using a Perkin-Elmer DSC 7 instrument and thermal gravimetric analysis (TG) on a Stanton thermal recording balance. All the experiments were carried out under dry, oxygen- free argon, which had been purified by passing it through a column consisting of Cr2+ on silica.Greaseless joints and taps were employed and manipulations were carried out using standard Schlenk-line and catheter-tubing techniques. All the solvents were carefully dried by distillation from so-dium diphenylketyl. 2-Picoline (2-methylpyridine), 4-picoline (4-methylpyridine) and methyldiphenylphosphine were pur- chased from Aldrich and were distilled prior to use. 4-Bromo- N,N-dimethylaniline (Aldrich) was purified by recrystallisation from ethanol. Butyllithium (Aldrich, 1.6 mol dm-3 in hexane) was used as received. Polybutadiene (PB; 83% pendant, 17% trans- 1,4, M, =3000) was a commercial product (Nippon Soda Company) and was used after pumping for 2 h.Me3M, M=Al, Ga or In and Me2M, M=Zn, Cd, were prepared by standard literature method^.^*^*'^ Hy dr osila tion of Polybutadiene The hydrosilation of PB was carried out as described pre- viously'8 but on a larger scale (0.11 mol of double bonds) by the reaction of PB with HSiMe2C1 in toluene (100cm3) in the presence of H2[PtCI6] (0.55 cm3, 0.22 mol dm-3 in propanol) at 80 "C for 22 h. Alkylation of Hydrosilated PB With 4-Lithiomethylpyridine (and 2-Lithiomethylpyridine) 4-Lithiomethylpyridine (and 2-lithiomethylpyridine) was pre- pared by the reaction of butyllithium (70 cm3, 1.6 mol dm-3 in hexane) with 4-picoline (or 2-picoline) (0.11 mol) in tetra- hydrofuran (THF) (150 cm3) at -25 "C according to the 1iterat~re.l~The freshly prepared 4-lithiomethylpyridine (or J.MATER. CHEM., 1994, VOL. 4 2-lithiomethylpyridine) solution (at -25 "C) was added drop- wise, over 1 h, to the solution of hydrosilated PB (0.11 mol Me2SiCl units) in THF (200 cm3) at between -15 and 25 "C with stirring. The resulting red-brown mixture was allowed to warm to room temperature and stirred for a further 12 h before addition of methanol (ca. 15cm3) to hydrolyse any remaining alkylating agent and Si-C1 bonds. The light-yellow solution was poured into water (2.5 dm-3) in order to remove lithium chloride and other impurities. The upper sticky layer was collected and dissolved in CH,C12 (300 cm3). The polymer solution was then washed with distilled water (3 x 100 cmW3), filtered through celite, and evaporated to dryness in uacuo to give a yellow sticky oil [4-picolyl functionalised polybutadiene (4PySiPB) and 2-picolyl functionalised polybutadiene (2PySiPB)I. Yield >95%.With4-Lithiodimethylaminobenzene 4-Bromodimethylaminobenzene (6cm3 of a solution contain- ing 0.14mol in 90cm3 of diethyl ether) was added to the suspension of freshly cut lithium chips (0.3 mol) in diethyl ether (18 cm3) and the resulting mixture heated under reflux conditions for 5-10 min to initiate the reaction.20 The remain- ing 4-bromodimethylaminobenzene was added dropwise over 4 h. After filtration, to remove unreacted Li, the yellow solution was cooled to 0 "C and treated with dropwise addition of hydrosilated PB (0.1 1 mol, Me2SiC1 units) in THF (250 cm3) over 0.5 h.The resulting mixture was allowed to warm to room temperature, stirred for 12 h and treated with methanol (15 cm3) to terminate the reaction. Purification was as described above and the product was obtained as a sticky oil (DMASiPB). Yield >90%. With(Dipheny1phosphino)methyllithium-N,N,N',N'-Tetra-methyl-l,2-diaminoethaneComplex (Dipheny1phosphino)methyllithium-N,N, N', "-tetramethyl-1,2-diaminoethane (TMEDA) complex was prepared by the reaction of butyllithium-TMEDA (0.2 mol) with methyldi- phenylphosphine (0.2 mol) in light petroleum (100 cm3) at room temperature for 72 h according to the literature2' (with 63-67% yield). The freshly prepared yellow powder (0.13 mol, 43 g) was dissolved in THF (150 cm3) and cooled to 0 "C.After adding hydrosilated PB (0.1 1 mol, Me,SiCl units) in THF (200 cm3) dropwise over 0.5 h, the mixture was allowed to warm to room temperature and stirred for 12 h before adding methanol (ca. 10cm3) to terminate the reaction. A similar purification procedure to that described above was carried out to provide the product as a colourless sticky oil (DPPSiPb). Yield >95%. Synthesis of Adducts Between Polymeric Lewis Bases and Metal Alkyls The pure metal alkyl (or a solution of InMe, in THF) was added to a solution of polymer in toluene (ca. 10% w/v) during stirring and at room temperature. The mole ratio of metal alkyl to coordinating atom (N or P) of the polymer was usually 1.5.After stirring for 4 h, the mixture was cooled to -30 "C and the light petroleum added, to precipitate the adduct, before decanting the solvent containing any excess metal alkyl. The adduct was then dried in LUCUO at room temperature for 4-24 h. Results and Discussion Syntheses and Properties of Polymeric Lewis Bases There are two strategies for the synthesis of polymeric bases. The first is by polymerisation of monomers that contain Lewis base groups, the second is by chemical modification of reactive polymers by chemically attaching Lewis base groups to the polymer chain. Interest has been shown in the preparation of new polymers using catalytic functionalisation of polybutadi- enes since these polymers are available with a variety of different microstructures (1,2 or 1,4 addition of butadiene to the growing chain, which leads to double bonds pendant from or contained within the growing chain, respectively), and since the functional groups of interest can often be introduced in a highly selective manner.Examples of functional groups that have been introduced include epo~ides,~~,~~ketone^'^.'^ carboxylic a~ids~~*~~,~~ and Me,SiC13 -n.18*25 Polymers func- tionalised with -SiMe,Cl can then be reacted with 2-lithiomethylpyridine to give polymers with pendant pyridyl gro~ps.'~,~~These polymers have a substituent in the 2-position relative to the pyridyl nitrogen atom so they may suffer from adverse steric crowding when binding to, for example, metal complexes, although it has been shown that it is possible to bind, for example [Fe(CN),]3-.25 Therefore, a similar strategy has been used to synthesize polymers in which the point of attachment to the polymer is in the 4-position relative to the pyridyl nitrogen.In addition, it is known that Me,NC6H4CH,C6H4NMe,6~7 and Ph2PCH2CH2PPh2"-13 are both excellent Lewis bases for the purification of group 13 metal alkyls. We have, therefore, synthesized polymeric analogues of these compounds by reacting polymer bound -CH,SiMe,Cl groups with 4-LiC6H4NMe2 and LiCH,PPh,, respectively. For all of these polymeric Lewis bases, a PB with a high (83%) pendant double-bond content has been used because the hydrosilation reaction occurs preferentially on the pendant double bonds and with a relatively low molecular weight (M,=3000) so as to retain solubility for the polymeric Lewis bases in common organic solvents.In order to avoid crosslinking and gel formation, it is necessary to use excess SiMe,HCl during the hydrosilation reaction and excess lithium reagent during further functionalisation. The synthetic reactions are outlined in Scheme 1. The polymeric Lewis bases have all been characterised by their 'H and 13C NMR spectra (Table 1) and by microanalysis (Table2). In all cases, small amounts of unreacted pendant double bonds are present in the polymer but all -SiMe2C1 groups are converted to -SiMe2L (L=2- or 4-CH,C5H4N, -C6H,NMe2 or CH,PPh2). In some cases, there are small resonances close to the main resonance from the Me,% groups.These may arise from a small amount of hydrosilation of the backbone double bonds or, more likely, from the presence of traces of degraded polymer (see below). Table 1 'H (and 13C)NMR results for polymeric Lewis bases" polymer SiCH, SiCHz CH=CH polymer CH, CH2 base group 2PySiPB -0.05 (-4.0) 0.40(10) 5.30 (128, 132) 1.0-2.0(27, 34, 38) 2.3, 6.9, 7.45, 8.35(119, 122, 135, 149, 161) DMASiPB 0.2 (-2.8) 0.6 (11) 5.3(129, 132) 4PySiPB -0.1 (-4.0) 0.40(10) 5.30(128, 132) 1.0-1.9(27, 34, 38) 1.0-2.0(27, 34, 38) 2.5, 6.7, 7.4(40, 112, 135, 151) 2.0(26), 6.8, 8.3(123, 149, 150) DPPSiPB -0.1 (-2.0) 0.4 (13) 5.4 1.0-2.0(27, 34, 38) 1.4, 7.2, 7.4(128, 132, 141) "In CdC13 at 298 K; chemical shifts in ppm.assignment -CH2-2Py -CH2-4Py -C,H,-NMe, -CH,P(C& J. MATER. CHEM., 1994, VOL. 4 + HSiMe2CI HdPtCIG] in toluene-80“C, 22 h + LCHZ + LiCHzG N + LiCHz-P(CGH5)z*Th4EDA CH3-Si-CH3 I CI 1c 2PySi PB CH3-Si-CH,I 4pYSiPB CH3-Si-CH,I DMASiPB CH3-Si-CH,IQ N CH3’ ‘CH3 DPPSiPB CH3-Si-CH3 I Scheme 1 Synthesis of polymeric Lewis bases Table 2 Elemental analysis results of polymeric Lewis bases polymeric Lewis bases measured value (%) C H N calculated value (%y C H N 2PySiPB 4PySiPR DMASiPB 69.36 69.65 71.02 10.89 11.17 9.40 5.36 4.83 5.00 70.19 70.19 72.83 9.02 9.02 9.92 6.93 6.93 5.73 DPPSiPB 73.67 8.48 0.00 73.60 8.12 0.00 “Assuming all pendant double bonds are converted to Lewis bases, as in Scheme 1.All the polymers are sticky oils that are soluble in common organic solvents of medium polarity such as diethyl ether, THF, CH,Cl, and toluene. They are stable if stored in THF or diethyl ether in the dark but we find that in the light, especially in the absence of solvent, all of the polymers decompose to give insoluble resins presumably vra a cross- linking reaction. The order of stability of the polymeric Lewis bases is -C,H,NMe, >2-CH,C5H4N >CH ,PPh, >4-CH,C,H,N. Because of the low stability of the 4-pyridyl functionalised polymer, we have studied this in more detail. Studies by ‘H NMR of this polymer in CDC1, show that over a period of days the signals from the -CH2C5H4N attached to the silicon atoms of the polymer decrease in intensity whilst J.MATER. CHEM., 1994, VOL. 4 B7 7 BI MA B = (CH312Na I (c6H5)2p4H2-B = -CH2a, 4 H 2 G MA = InMe3, GaMe3, AIMe3, CdMe2, ZnMe2 M = Cd, Zn Fig. 1 Structures of adducts between polymeric Lewis bases and metal alkyls new resonances from free 4-methylpyridine increase. At the same time, the resonance from the methyl groups on silicon decreases in intensity whilst a new resonance increases at 6 0.0. We tentatively conclude that a photochemical crosslink- ing reaction, which releases free 4-methyl pyridine, occurs. Similarly, samples of -CH,PPh, functionalised polymers also lose Ph,MeP when they are allowed to stand in the light, particularly in the dry state.Nevertheless, all the polymers can be stored for months in THF solution in the dark or for up to 3 weeks in the dry form in the dark. DSC and TG of the polymers under nitrogen show that they are generally stable up to 200 "C (300 "C for -CGH,NMe2) but that above this temperature they start to decompose, darkening in colour and losing weight. Adducts with Main-group Metal Alkyls As expected, all of the polymeric Lewis bases react with Me,M to give adducts. For L=-C,H,NMe, or -CH,PPh, only reactions with Me,M (M=Al, Ga or In) were successful? whilst for L =2-or 4-CH2C5H,N, adducts were prepared with Me,M' (M'=Zn or Cd) and Me3A1. In all cases, the adducts formed from toluene, THF or diethyl ether were soluble in common organic solvents so their stoichiometries could be determined from 'H NMR studies.A check was also made by measuring the change in the weight of the polymer on adduct formation, although this method is unreliable because: (i) not all of the polymer may be collected after precipitation; (ii) it is difficult to remove the last traces of solvent from the polymers; and (iii) dynamic vacuum can remove some or all of the coordinated metal alkyl even at room temperature. The NMR studies suggest that the adducts of -CH,C,H,NMe, with Me,M, M=Al, Ga or In have a j. For L=-C,H,NMe,, reactions with Me,M' (M'=Zn or Ca) give products that contain only traces of the metal alkyl (<5%). limiting 1 :1 M :N ratio, although for Ga and In the observed ratio is generally less than this because prolonged pumping on the isolated adducts to remove residual traces of solvents also removes some of the metal alkyl.For Me,AI, the binding to the polymer is stronger as evidenced by (i) no loss of Me,Al on prolonged pumping at room temperature and (ii) the observation that the solution warms noticeably during the formation of this adduct whereas this is not the case for the other metal alkyls. For the adducts of polymers containing 2-CH2Py or 4-CH2Py with Me,M' (M'=Zn or Cd), NMR studies on isolated and vacuum-dried compounds show that only traces of the metal alkyl are present, although if an adduct is prepared in solution, precipitated and its 'H NMR spectrum run without having dried the sample, the Me,Zn :pyridine ratio is 0.5: 1, as has been observed for adducts of Me2Zn with small N donor Lewis ba~es,4*~,'.~ which include pyri- dine.30 The nature of the adducts is shown in Table 3 and their proposed structures in Fig.1. Table 3 Preparation and properties of polymeric adducts M/Nratio Solubility"adducts appearance 2PySiPB/Me2Cd yellow resin 2PySiPB/Me,Zn yellow solid 2PySiPB/Me,Al yellow solid 4PySiPB/Me2Cd light-yellow solid 4PySiPB/Me2Zn light-yellow solid 4PySiPB/Me,Al yellow solid DM ASiPB/Me,Ga yellow resin DM ASiPB/Me,In yellow resin DM ASiPB/Me,Al white solid DPPSiPB/Me,Ga transparent resin DPPSiPB/Me,In transparent resin DPPSiPB/Me,Al transparent resin a~:soluble; x: insoluble. Table 4 'H NMR measurement of polymeric adducts (ppm)" adducts 2PySiPB/Me2Cd 2PySiPB/Me2Zn 2PySiPB/Me,AI 4PySiPB/Me,Cd 4PySiPB/Me2Zn 4PySiPB/Me,Al DMASiPB/Me,In DM ASiPB/Me,Ga DMASiPB/Me,Al DPPSiPB/Me,In DPPSiPBjMe,Ga DPPSiPB/Me, A1 "In C2H,]benzene Si-CH, CH =CH 0.0 5.40 0.0 5.42 0.0 5.42 0.0 5.42 0.0 5.38 0.0 5.40 0.2 5.38 0.2 5.42 0.2 5.48 -0.1 5.5 -0.1 5.5 -0.1 5.5 base group M-CH, 0.5 toluene(v), THF(v) 0.5 toluene(v), THF(v) 1.o toluene(v), THF(v) 0.5 toluene(x), THF(v) 0.5 toluene (x), THF( v) 1.o toluene(x), THF(v) 1.o toluene(v), THF(v) 1.o toluene(v), THF(v) 1.o toluene(v), THF(v) 1.o toluene(v), THF(v) 1.o toluene(v), THF(v) 1.o toluene(v), THF(v) M-CH,( free) 2.34, 6.95, 7.48, 8.30 -0.62 -0.53 2.32, 6.95, 7.48, 8.40 -0.82 -0.63 2.35, 6.96, 7.50, 8.40 -0.93 -0.34 2.15, 6.95, 8.40 -0.67 -0.53 2.10, 6.90, 8.30 -0.72 -0.63 2.25, 7.14, 8.38 -0.85 -0.34 2.35, 6.5, 7.35 -0.25 -0.23 2.30, 6.70, 7.32 -0.40 -0.12 2.30, 6.95, 7.35 -0.70 -0.34 1.20, 6.90, 7.28 -0.10 -0.23 1.10, 6.90, 7.20 -0.10 -0.12 1.28, 7.15, 7.35 -0.40 -0.34 at 298 K except the 4PyPB adducts, which were in ['H,]THF.J. MATER. CHEM., 1994, VOL. 4 66 1 All of the polymeric adducts are soluble in THF and all except those containing 4-CH2Py are soluble in aromatic solvents. 'H NMR studies (Table 4) show that the resonances from the polymer backbone are unaffected by adduct forma- tion whilst those from the donor groups shift to a higher field on account of the removal of electron density by the Lewis 6 7 8 9 D.F. Foster, S. A. Rushworth, D. J. Cole-Hamilton, A. C. Jones and J. P. Stagg, Chemtronics, 1988,3,38. D. F. Foster, S. A. Rushworth and D. J. Cole-Hamilton, UK Pat., 8 704 657,1987. H. M. Yates, J. 0. Williams, I. L. J. Patterson and D J. Cole- Hamilton, J. Cryst. Growth, 1993,129,215. D. F. Foster, I. L. J. Patterson, L. D. James, D. J. Cole-Hamilton, acid. The alkyl resonances shift to a high field on coordination with N-containing Lewis bases but to a low field when Me,Ga or Me& are bound to the -CH2PPh2 functionalised polymer. In contrast to the free metal alkyls, the polymeric Lewis base adducts are non-pyrophoric and only slightly air sensi- tive. DSC and TG studies suggest that they all dissociate on 10 11 12 D.N. Armstrong, H. M. Yates, A. C. Wright and J. 0. Williams, Adv. Muter. Opt. Electron., 1994,3, 163. D. F. Foster, S. A. Rushworth, D. J. Cole-Hamilton, R. Cafferty, J. Harrison and P. Parkes, J. Chem. Soc., Dalton Trans., 1988,7. D. C. Bradley and H. Chudzynska, Polyhedron, 1988,7, 1289. A. H. Moore, M. D. Scott, J. I. Davies, D. C. Bradley, M. M. Faktor and H. J. Chudzynska, J. Cryst. GroKth, 1986, heating to liberate the metal alkyl so that they may be suitable for purification of the metal alkyls. However, since the adducts liberate substantial amounts of metal alkyl when they are pumped in uacuo for removal of solvent residues, and since the polymeric Lewis bases themselves decompose on heating above ca. 200 "C, their application for purification is limited.13 14 15 16 77, 19. D. C. Bradley, H. Chudzynska and M. M. Faktor, Uorld Put., 04405,1985. K. H. Thiele, Z. Anorg. Allg. Chem., 1964,330, 8. X. Li, D. F. Foster and D. J. Cole-Hamilton, Polym. Adv. Technol., in the press. V. Saukarau, J. Yue, R. E. Cohen, R. R. Schrock and R J. Sibley, Indeed, attempts to liberate pure metal alkyls from them have led to only low recoveries (30%). The fact that the adducts are soluble in organic solvents (in contrast to the PVP adducts) means that they are much more suitable for studies of their reaction chemistry. We are, therefore, studying the use of these polymeric adducts for the production of a wide range of nanoscale particles of a variety of semiconducting materials.,l 17 18 19 20 21 22 Chem.Muter., 1993,5, 1133. D. F. Foster and D. J. Cole-Hamilton, Inorg. Synth., in the press. A. Iraqi, S. Seth, C. A. Vincent, D. J. Cole-Hamilton, M. D. Watkinson, I. M. Graham and D. Jeffrey, J. Matm. Chem., 1992, 2, 1057. 0.F. Beumel, W. N. Smith and B. Rybalka, Synthesis, 1374,1,43. W. Kaminski and D. L. Esmay, J. Org. Chem., 1960,25, 1870. N. E. Schore, L. S. Benner and B. E. Labelle, Inorg. Chem., 1981, 20,3200. M. Gahagan, A. Iraqi, D. C. Cupertino, R. K. Mxkie and We thank the Japanese Soda Company for the gifts of polybutadiene, the Royal Society for a K. C.Wong Fellowship 23 24 D. J. Cole-Hamilton, J. Chem. SOC.,Chem. Commun., 1989, 1688. A. Iraqi and D. J. Cole-Hamilton, Polyhedron, 1991, 10. 993. P. Narayanan, B. G. Clubley and D.J. Cole-Hamilton, J. Chem. (X.L.) and SERC for Fellowships (C.M.L. and D.F.F.). 25 Soc., Chem. Commun., 1991, 1628. A. Iraqi, M. Watkinson, J. A. Crayston and D. J. Cole-Hamilton, J. Chem. SOC., Chem. Commun., 1991, 1767. References 26 27 A. Iraqi and D. J. Cole-Hamilton, J. Muter. Chem., 1992,2, 183. P. Narayanan, A. Iraqi and D. J. Cole-Hamilton, J. Ma!er. Chem., 1 D. G. Tuck, in Comprehensive Organometallic Chemistry, ed. 1992,2, 1149. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon, Oxford, 28 P. Narayanan, B. Kaye and D. J. Cole-Hamilton, J. Maaer. Chem., 1982, vol. 1, p. 683. 2 D. J. Cole-Hamilton, Chem. Br., 1990, 852. 29 1993,3, 19. X,Guo, R. Farwaka and G. L. Rempel, Mucromolecriles, 1990, 3 P. R. Jacobs, D. V. Shenai-Khatkhate, E. D. Orrell, J. B. Mullin 23, 5047. and D. J. Cole-Hamilton, UK Put. 8 509 055, 1985. 30 G. Levy, P. de Loth and F. Gallais, C.R. Acad. Sci., Ser. C., 1974, 4 P. R. Jacobs, E. D. Orrell, D. V. Shenai-Khatkhate, J. B. Mullin 278,1405. and D. J. Cole-Hamilton, Chemtronics, 1986, 1, 13. 31 X. Li, D. F. Foster and D. J. Cole-Hamilton, in preparation. 5 D. V. Shenai-Khatkhate, E. D. Orrell, J. B. Mullin, D. C. Cupertino and D. J. Cole-Hamilton, J. Cryst. Growth, 1986,77,38. Paper 3/07521D; Received 22nd December, 1993