AMINE-THIOETHER AND AMINE-PYRIDINE COMPLEXES OF PALLADIUM(II) AND THEREACTIVITY OF THE METHYL COMPLEXES TOWARDS CO AND ALLENES

The synthesis and characterization of the complexes [PdX(2)(L)] and [P dX(Me)(L)] (X = Cl, Br, I; L = S-methyl-D/L-cysteine methyl ester (H[M e]cysMe-N,S (a)), D/L-methionine methyl eater (HmetMe-N,S (b)), 2-[ami nomethyl] pyridine (Py(1)-N,N' (c)), 2-[2-amino-ethyl]pyridine (Py(2)- N,N' (d))) have been reported. A single crystal X-ray determination of [PdCl(Me)(H [Me] cysMe-N,S)] (2a) showed chelate coordination of the NS ligand. The square planar surrounding is completed by the chloride and the methyl group, which is positioned cis to the sulfur atom. The crystal structure determination of [PdCl(Me)(HmetMe-N,S)] (2b) shows a n analogous geometry with the HmetMe-NS ligand forming a six-membered chelate ring with palladium. Again the methyl group is cis to the sulf ur atom. The structure of [PdCl(Me)(Py(2)-N,N')] (2d) shows the presen ce of an amine-pyridine ligand also forming a six-membered chelate rin g with Pd(II), with the methyl group positioned cis to the amine group . The unexpectedly stable methylpalladium complexes reacted with CO to give the corresponding acyl complexes. The structure of [PdCl(C(O)Me) (HmetMe-N,S)] (5b) in the solid state shows the presence of a six-mem bered chelate ring. The acyl group is cis to the sulfur atom. The NS a nd the NN' complexes (1a-4b) contain, also in solution, a chelating li gand L as demonstrated by NMR. The complexes [PdX(R)(L)] (R = Me, C(O) Me; L = HmetMe-N,S; H[Me] cysMe-N,S) exist in two diastereoisomeric fo rms which differ by the position of the methyl substituent on the S at om and can be distinguished at low temperatures. The free energy value s (Delta G double dagger) of the interconversion varies between 49.5 a nd 62.1 kJ mol(-1). Reaction of [PdX(R)(L)] (R =Me,C(O)Me); L = HmetMe -N,S; H[Me]cysMe-N,S; Py(1)-N,N') with allenes afforded [Pd(eta(3)-all yl)(L)]Cl. This insertion reaction is faster for the NS complexes cont aining the six-membered rings than for the five-membered NS containing complexes. The kinetics of the allene insertion show a two term rate law, k(obs) = k(1) + k(2)[allene], and depend on the nature of both th e ligand and the allene substrate; either an allene dependent (k(2); f or 2a + 3-methyl-1,2-butadiene, 2b + 3-methyl-1,2-butadiene and 2b + 1 ,2-heptadiene) or an allene independent (k(1); for 2b + 2,4-dimethyl-2 ,3-pentadiene) pathway is the dominant one.