SYNTHESIS AND THERMOLYSIS BEHAVIOR OF MONOETHYLPALLADIUM COMPLEXES, ETPD(X)(PME(3))(2) (X= ELECTRONEGATIVE LIGANDS)

A series of monoethylpalladium complexes coordinated with various electronegative ligands, trans-[PdEt(X)(PMe(3))(2)], where X = OPh (2a), O2CCH3 (2b), O2CCH2Cl (2c), O2CCHCl2 (2d), O2CCl3 (2e), O2CCH2CH=CH2 (2f), SPh (2g), SCOCH3 (2h), Cl (2i), Br (2j), and I (2k) have been prepared by protonolysis of trans-PdEt(2)(PMe(3))(2) with HX or by metathesis of the known monoethylpalladium acetate complex. These complexes were characterized by means of H-1-, 13C{1H}-, and (31)p{H-1}-NMR, IR, and elemental analysis. Complexes 2a and 2b are thermolyzed in solution with evolution of ethylene whereas complexes 2c-2e and 2g-2k are decomposed with evolution of ethylene and ethane, Kinetic studies on the thermolysis of the ethylpalladium complexes revealed that they decompose according to the first-order rate law in concentration of the palladium complexes. The thermolysis is hindered by addition of X(-) to the solution. A detailed analysis of the thermolysis revealed that there are two thermolysis routes, one major route involving dissociation of the X ligand to generate an unstable cationic ethylpalladium species that is rapidly thermolyzed, evolving ethylene, and another minor route proceeding from the neutral complex without dissociation of the anionic ligand. Comparison of the rate constant k(1) for dissociation of the anionic ligand X from trans-PdEt(X)(PMe(3))(2) showed that k(1) is the smallest for the Cl- ligand dissociation. Activation parameters for the dissociation of the acetate ligand from 2b were found as follows: Delta H double dagger = 9.9(+/-1) kcal/mol, Delta S double dagger = -41(+/-2) cal/(K.mol). The large negative entropy is consistent with a mechanism where dissociation of X is assisted by solvent coordination to generate a solvent-coordinated cationic ethylpalladium complex susceptible to beta-H elimination. In fact removal of the chloride ligand on treatment of trans-PdEt(Cl)(PMe(3))(2) with AgBF4 gave a thermally unstable complex trans-[PdEt(solvent)(PMe(3))2](BF4-)-B-+ which is readily decomposed to liberate ethylene above -30 degrees C.