DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1998, Pages 2109–2110 2109 Metal-template synthesis and co-ordination properties of a palladium complex containing a novel and stable imidazole-substituted phosphine C]P bidentate chelate Huifang Lang, Jagadese J. Vittal and Pak-Hing Leung * Department of Chemistry, National University of Singapore, Kent Ridge 119260, Singapore An organopalladium complex promoted Diels–Alder reaction between 1-phenyl-3,4-dimethylphosphole and 1-vinylimidazole gave a novel imidazole-substituted phosphanorbornene bidentate ligand which co-ordinated to the palladium template via the C2 carbon atom of the imidazole group and the bridgehead phosphorus donor atom. Transition-metal complexes containing imidazole and its derivatives play an important role in bioinorganic chemistry.1 These compounds are frequently considered as models in the development of metal-based enzymes and proteins.In terms of co-ordination chemistry, imidazole may be considered as an ambidentate ligand. For instance, it has been well established that the imidazole unit co-ordinates to transition-metal ions, such as palladium(II) and platinum(II), predominantly via one of its nitrogen atoms.In some rare cases, however, the cyclic unit may also co-ordinate as a carbene or an amidine ligand via its C2 carbon atom. This interesting mode of organometallic bonding has been observed in a small number of Cr0, Fe0, RuII and RuIII complexes.2 Apart from their biological interest, the availability of stable C-bound imidazole heavy-metal complexes is important for the study of the trans influence 3 and other related phenomena that are pertinent to the design and development of eYcient support ligands for homogenous catalysis.4 Indeed, it has been reported that palladium complexes containing C-bound imidazole ligands are eYcient catalysts for crosscoupling reactions.5 In general, however, palladium complexes containing monodentate ligands are kinetically labile.We believe that the development of a new class of stable C-bound imidazole complexes may significantly influence the design of catalysts in homogenous catalysis. Here we report the palladium-template synthesis of the first imidazole-substituted tertiary phosphine bidentate ligand in which the C2 carbon of the imidazole unit is involved in metal chelation. In the absence of the transition-metal ion, no Diels–Alder reaction was observed between 1-phenyl-3,4-dimethylphosphole (DMPP) and 1-vinylimidazole.Upon co-ordination to palladium, however, DMPP is activated towards the [4 1 2] cycloaddition reaction. Thus, when [PdCl2(DMPP)2] or the organopalladium complex 1 6 was treated with the dienophile in 1,2-dichloroethane at 84 8C, the exo-cycloadduct 2 was obtained as the sole product (Scheme 1).† When the template complex 1 was used, the cycloaddition reaction was completed in 30 d. Interestingly, during the course of heating, the N3 nitrogen of the imidazole unit underwent a parallel N-alkylation reaction with 1,2-dichloroethane and produced HCl as the side product.7 The HCl thus generated led to further chemoselective cleavage of the orthometallated benzylamine ligand from the palladium template and thus facilitate the formation of the dichloro complex 2 as the final product.6 The * E-Mail: chmlph@nus.edu.sg molecular structure and the co-ordination chemistry of 2 have been determined by X-ray structural analysis (Fig. 1).‡ The study reveals that the reaction of 1 with 1-vinylimidazole in 1,2- dichloroethane has resulted in the removal of the benzylamine chelate and the imidazole substituted exo-phosphanorbornene ligand created co-ordinates to palladium as a bidentate chelate via the bridgehead phosphorus atom and the C2 carbon atom of the imidazole group. The palladium atom is in a slightly distorted square-planar geometry with the bond angles in the ranges 84.8(1)–94.8(2) and 167.0(1)–172.2(2)8. Due to the aromaticity of the imidazole ring, all C]C and C]N bonds [1.350(8)–1.383(6) Å] within the five-membered ring are noticeably shorter than the two attached C]N bonds [1.473(7)– 1.453(7) Å].The Pd(1)]C(11) distance of 1.993(5) Å is similar to the Pd]C bonds observed in other reported complexes containing the orthometallated benzylamine [2.004(11) Å] and naphthylamine units (2.006 Å) which experience a similar trans electronic influence from a chloro ligand.8 The Pd(1)]P(1) distance of 2.207(1) Å is also within the normal range observed for this class of phosphanorbornene complex.The Pd ? ? ? Cl(3) distance is 3.622(2) Å indicating that there is no interaction between the two heavy atoms. Interestingly, when [PdCl2(DMPP)2] was used for the Diels– Alder reaction, a much lower reaction rate was observed. The 31P NMR studies indicated that 80% of [PdCl2(DMPP)2] Scheme 1 Pd N P Cl Me Me Me Ph P Cl Pd Cl Ph Me or [PdCl2(DMPP)2] N N N N 1 2 ClCH2CH2 ClCH2CH2Cl 2 3 1 † Preparation of complex 2.A solution of the organopalladium complex 1 (0.46 g, 0.97 mmol) in 1,2-dichloroethane (40 cm3) was treated with silver perchlorate (0.2 g, 0.97 mmol) in water (1 cm3) for 30 min. The resulting mixture was filtered through a layer of Celite to remove silver chloride and the organic layer was dried over anhydrous MgSO4. The dried solution was treated with 1-vinylimidazole (0.37 g, 3.87 mmol) and the reaction mixture was then stirred at 84 8C for 30 d.The solution was washed with water (50 cm3) and then dried over MgSO4. The solvent was removed under reduced pressure to give a yellow residue. Upon crystallization from acetonitrile–diethyl ether, the dichloro complex 2 was obtained as yellow prisms (0.21 g, 40%), m.p. 256– 258 8C (decomp.). 31P NMR (CDCl3): d 105.3 (s). ‡ Crystal data for 2: C19H22Cl3N2PPd, M = 522.11, orthorhombic, space group Pna21, a = 14.9398(1), b = 16.5294(1), c = 8.6207(1) Å, U = 2128.85(3) Å3, Z = 4, Dc = 1.629 g cm23, T = 293 K, m(Mo-Ka) = 13.29 cm21, F(000) = 1048, R1 = 0.0408, wR2 = 0.0885 for 4214 independent observed reflections [I > 2s(I), 1.84 < 2q < 29.318] and 236 parameters.The Flack parameter was refined to 20.02(4). CCDC reference number 186/1018.2110 J. Chem. Soc., Dalton Trans., 1998, Pages 2109–2110 remains unchanged after the complex was treated with 1-vinylimidazole for 30 d under similar reaction conditions.Nevertheless, a small quantity of 2 was observed in the 31P NMR spectra of the reaction mixture. Clearly, the co-ordinated cyclic diene in Fig. 1 Molecular structure and co-ordination chemistry of complex 2. Selected bond lengths (Å) and angles (8): Pd(1)]P(1) 2.207(1), Pd(1)]C(11) 1.993(5), Pd(1)]Cl(1) 2.391(2), Pd(1)]Cl(2) 2.358(2), P(1)]C(1) 1.859(6), P(1)]C(4) 1.842(5), N(1)]C(6) 1.473(7), N(1)]C(11) 1.375(7), N(1)]C(9) 1.383(6), C(9)]C(10) 1.350(8), C(10)]N(2) 1.381(7), N(2)]C(11) 1.357(7), N(2)]C(12) 1.453(7), C(13)]Cl(3) 1.781(9); P(1)]Pd]Cl(1) 167.0(1), P(1)]Pd]Cl(2) 84.8(1), P(1)]Pd] C(11) 87.8(2), C(11)]Pd]Cl(1) 94.8(2), C(11)]Pd]Cl(2) 172.2(2), Cl(1)] Pd]Cl(2) 92.0(1), C(1)]P(1)]C(4) 81.8(3), C(6)]N(1)]C(11) 124.0(4), N(1)]C(9)]C(10) 106.3, C(9)]C(10)]N(2) 107.7(4), C(10)]N(2)]C(11) 110.7(4), N(2)]C(11)]N(1) 104.4(4) the template complex 1 receives a higher degree of activation as compared with its counterparts in [PdCl2(DMPP)2]. We are currently investigating the optical resolution and the catalytical properties of transition-metal complexes containing the imidazole-substituted phosphine ligand.Acknowledgements We are grateful to the National University of Singapore for support of this research (Grant No. RP972667) and research scholarships (to H. F. L.). References 1 R. J. Sundberg and R. B. Martin, Chem. Rev., 1974, 74, 471; M. Grebl and B. Krebs, Inorg. Chem., 1994, 33, 3877. 2 F. A. Cotton and G. Wilkinson, in Advanced Inorganic Chemistry, John Wiley & Son, New York, 5th edn., 1988, ch. 10. 3 M. F. Tweedle and H. Taube, Inorg. Chem., 1982, 21, 3361. 4 K. H. Weiss, in Transition Metal Carbene Complexes, Verlag Chemie, Weinheim, 1983, p. 227. 5 W. A. Herrmann and C. Kocher, Angew. Chem., Int. Ed. Engl., 1997, 36, 2162. 6 S. Selvaratnam, P. H. Leung, A. J. P. White and D. J. Williams, J. Organomet. Chem., 1997, 542, 61. 7 C. A. Ghilardi, S. Midollini, S. Moneti, A. Orlandini and J. A. Ramirez, J. Chem. 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