Some inorganic and organometallic chemistry of palladium (II) with pyridine based ligands

This thesis describes the results of a study of inorganic and organometallic chemistry of palladium(II) with 2- and 2,6- disubstituted pyridine containing N-donor ligands. Ligands of this type have been used to assess the ability of palladium(II) to adopt coordination geometries higher than the characteristic square-planar geometry, to determine whether palladium(II) substrates will undergo metallation reactions with polydentate pyridine donor ligands, and to synthesise complexes in which palladium(II) is bound to carbon atoms of the ligands at sites other than those expected via cytlometallation reactions. Inorganic complexes derived from the reaction of ligands, L, with palladium(II) acetate and chloride substrates have allowed the isolation of complexes of the form [L\\(_n\\)PdX\\(_2\\)] (where X= OAc, Cl; n = 1 (polydentate), n = 2 (unidentate)), [LPdX]X (L = planar tridentate; X=OAc, Cl), [L\\(_2\\)Pd]X\\(_2\\) (L = tris(pyridin-2-yl)methane; X = NO\\(_3\\)), and the related poly(pyrazol-1-y1) complexes [(pz\\(_3\\)CH)\\(_2\\)Pd]X\\(_2\\) (X = NO\\(_3\\), BF\\(_4,\\)), and [(pz\\(_3\\)BH)\\(_2\\)Pd]. X-Ray structural analyses show that, [(pz\\(_3\\)BH)\\(_2\\)Pd] and the cations L\\(_2\\)Pd]\\(^{2+}\\) L = tris(pyridin-2-yl)methane, tris(pyrazol-1-y1)methane) have square planar coordination with two uncoordinated donor groups. An X-ray structural determination of the complex [LPd(OAc)](OAc) . 3H\\(_2\\)O (where L = the 2,6-disubstituted pyridine ligand meso-(pyPhMeC) \\(_2\\)C\\(_5\\)H\\(_3\\)N) has been obtained confirming the planar tridentate-, nature Of the ligand and presence of a unidentate acetate-group. In conjunction with the investigation of the coordination preferences of palladium(II), the coordination properties of the new ligand. (pyPhMeC) \\(_2\\)C\\(_5\\)H\\(_3\\)N, have also been assessed by the formation of methylmercury(II) nitrate complexes. Fractional crystallisation has allowed the separation of mesa and racemic forms of the complex, and further reaction with sodium cyanide has afforded the free ligand separated into its meso and racemic diastereoisomers. An X-ray structural study has shown that, the meso complex is four-coordinate and contains the ligand as a tripodal tridentate. Thus, the meso form of the ligand has been shown to exhibit two binding modes, planar-tridentate to palladium(II), and tripodal-tridentate to methylmercury(II). Attempted cyclopalladation reactions of the ligands py\\(_2\\)CH\\(_2\\), py\\(_n\\)Ph\\(_{3-n}\\)CH (n = 1-3), and py\\(_3\\)PhC, with palladium(II) acetate in glacial acetic acid has resulted in the isolation of a dinuclear (C,N1-cyclopalladated product of pyPh\\(_2\\)CH; dinuclear chloro- and mononuclear acetylacetonato-complexes have also been prepared to assist with spectral characterisation. The remaining ligands studied form bidentate chelate complexes, [LPd(OAc) \\(_2\\)], with no evidence of metallation products. Similar reactions using meta-disubstituted benzenes, for example, (pyMeHC) \\(_2\\)C\\(_6\\)H\\(_4\\), have resulted in metallation of the central benzene ring to give mononuclear (N,C,N) complexes, for example, [{(pyMeHC) \\(_2\\)C\\(_6\\)H\\(_3\\)}Pd(OAc)]. In contrast to palladation of pyPhCH\\(_ 2\\)and pyPh\\(_2\\)CH, which occurs at the ortho-position of a phenyl ring, lithiation, of these and related pyridines such as py\\(_2\\)CH\\(_2\\), occurs at the bridging (carbon atom to give the salts pyR\\(_1\\)R\\(_2\\)C\\(^-\\)Li\\(^+\\) (where R\\(_1\\) and R\\(_2\\) are hydrogen, phenyl, or pyridin-2-yl substituents). Organolithium derivatives of these ligands have: been used in attempts to synthesise organopaIladium(II) complexes in which palladium is bound to the bridging carbon atom, and to synthesise organomercury(II) reagents for possible use synthesis of organopalladium(II) complexes. Thus, investigation of the reaction of the anions-pyMe\\(_2\\)C\\(^- \\), py\\(_n\\)Ph\\(_{2-n}\\)RC\\(^-\\) (R = H, Me; n = 1,2), and py\\(_n\\)Ph\\(_{3-n}\\)C\\(^-\\) (n = 1-3), with mercuric iodide have been performed in an attempt to obtain diorganomercurial reagents, [R\\(_2\\)Hg]. The reactions have resulted in the isolation of oxidative coupling products (pyMe\\(_2\\)C) \\(_2\\), (py\\(_n\\)Ph\\(_{2-n}\\)RC) \\(_2\\) (n = 1,2; R = H,Me), and polymeric products for the triarylmethanes, respectively. Similar reactions with Ph\\(_2\\)CH\\(^-\\) and Ph\\(_3\\)CH\\(^-\\) have resulted in isolation of (Ph\\(_2\\)HC) \\(_2\\) and triphenylmethylperoxide, respectively. Isolation of oxidative coupling products and triphenylmethyl-peroxide has been explained in terms of decomposition of transient [R\\(_2\\)Hg] species. The coordination properties of two dimers, (pyPhHC) \\(_2\\) and (py\\(_2\\)HC) \\(_2\\), have been investigated by preparation of their palladium(II) acetate complexes. In the former case selective crystallisation of the mononuclear racemic complex was observed, while in the latter case a bridged dinuclear complex [(py\\(_2\\)HC) \\(_2\\) {Pd (OAc) \\(_2\\)}\\(_2\\)] was obtained. The ligand anions py\\(_n\\)Ph\\(_{2-n}\\)HC\\(^-\\) (n = 1,2) have been reacted with trans-(4-methylpyridine) \\(_2\\)PdCl\\(_2\\), [(˜í¬•-pic) \\(_2\\)PdCl\\(_2\\)]. For n = 1 a number of products have been obtained and two organometallic dinuclear complexes have been isolated and structurally characterised. One complex has the formulation trans-(N,N') - [{˜í¬¿-py-N) (PhHC-C) } ( ˜í¬• -pic-N) PdCl] \\(_2\\). CH\\(_2\\)Cl\\(_2\\), with square-planar coordination geometry and two bridging [pyPhHC-C, N] groups. For n = 2, reaction under similar conditions resulted in the isolation of decomposition products including traces of the ligand oxidative coupling product (py\\(_2\\)HC) \\(_2\\). As an alternative route to synthesis of organopalladium(II) complexes with palladium bonded to bridging carbon atoms, oxidative addition reactions of py\\(_n\\)Ph\\(_{3-n}\\)CCl (n = 1-3) with bis(dibenzylideneacetone)palladium (0), [(dba) \\(_2\\)Pd], have been studied resulting in isolation of palladium(II) complexes for n = 1 and 2, but not for n = 3. For n = 1 a chloro-bridged dinuclear allylic complex, similar to the oxidative addition product formed, between triphehylmethylchloride and [(dba) \\(_2\\)Pd], has been obtained. For n = 2 a dinuclear complex has been obtained and structurally characterised. The complex has a formulation trans-(N-N') - [{(˜í¬¿-py-N) (Py-N') (PhC-C) } PdCl] \\(_2\\). [(CH\\(_3\\)) \\(_2\\)CO.CH\\(_2\\)Cl\\(_2\\)]\\(_{0.5}\\) with the ligand [py\\(_2\\)PhC-C,N,N']\\(^--\\) ˜ìvâ-bound to palladium via the bridging carbon atom. In each ligand one pyridine N-donor bridges two palladium atoms while the second forms a four-membered (C,N-chelate ring to the supporting palladium atom. Methods for the synthesis of pyridine-based ligands, including the preparation of a number of new ligands, are presented.