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Synthesis and characterisation of Pd(II) complexes with diethyl [alpha-(4-benzenazoanilino)-2-hydroxybenzyl]phosphonate. Dynamic 1H NMR study of cyclopalladation reactions in N, N-dimethylformamide.

Three new Pd(II) complexes, i.e. [PdCl_2L]_2 (A), PdCl_2L_2 (B) and [Pd(mu-Cl)(L-H)]2 (C), each with two diethyl [alpha-(4-benzenazoanilino)-2-hydroxybenzyl]phosphonates (L) bound to either one or two palladium atoms, are synthesized and characterized by elemental analysis, X-ray powder diffraction, magnetic measurements and by IR, UV/Vis and 1H NMR studies. B and C are additionally characterized by 13C and 31P NMR and ES-MS studies. In DMF solution adducts A and B undergo spontaneous rearrangement into the cyclopalladated complex C. Dynamic 1H NMR study of this rearrangement as well as of the reactions of L with PdCl_2 and Na_2PdCl_4 revealed a complex equilibrium in DMF solutions and enabled the formation mechanism of all involved species to be resolved. The complex A is immediately solvolyzed producing two molecules of intermediate M [PdCl_2(L)(DMF)]. M was also the first intermediate in the reaction of L with PdCl_2. Once present in concentration above 10^-5 mol dm^-3 M dimerizes very fast into chloro-bridged dimer [PdCl(mu-Cl)(L)]2 (D) which undergoes cyclopalladation and converts into the complex C. The formation of C from the intermediate D is clearly demonstrated by the concentration dependence of the cyclopalladation reaction which has order greater than one. Chloride ions, released by cyclopalladation, react with D by splitting chloro-bridge and binding to metal atoms to produce byproduct [PdCl_3(L)]- (T). The same species (T) are formed in the reaction of L with Na2PdCl4 whereby a chloride ion is replaced by the ligand L. The complex B undergoes similar, but slower, solvolytic reaction producing M and L while further reaction steps are identical as in the solvolysis of A.

palladium; cyclopalladation; aminoazobenzene; dynamics

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