Synthesis and characterisation of Pd(II) complexes with a derivative of aminoazobenzene -: Dynamic 1H-NMR study of cyclopalladation reactions in DMF

Three new Pd(II) complexes, i.e. [PdCl2L](2) (A), PdCl2L2 (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, by IR, UV-vis and solid-state C-13-NMR spectra. Complexes B and C are additionally characterized by H-1-, C-13- and P-31-NMR and electrospray mass spectrometry (ESMS) studies using dimethylformamide (DMF) as a solvent. In DMF solution adducts A and B undergo spontaneous rearrangement into the cyclopalladated complex C. Dynamic H-1-NMR study of this rearrangement as well as of the reactions of L with PdCl2 and Na2PdCl4 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[PdCl2(L)(DMF)]. Complex M was also the first intermediate in the reaction of L with PdCl2. 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 [PdCl3(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. (C) 2003 Elsevier B.V. All rights reserved.