THE FIRST ISOLABLE ORGANOPALLADIUM FORMATO COMPLEXES - SYNTHESIS, CHARACTERIZATION, AND X-RAY STRUCTURE - FACILE AND CONVENIENT THERMAL GENERATION OF COORDINATIVELY UNSATURATED PALLADIUM(0) SPECIES

[L(2)Pd(2)(R)(2)(mu-OH)(2)] complexes (L = PPh(3), R = Ph, Me) react with formic acid in benzene to give the first stable organopalladium formates, [L(2)Pd(2)(R)2(mu-HCOO)(2)] (1, R = Ph; 2, R = Me). Addition of L to 1 and 2 results in the formation of the less stable, but still isolable, [L(2)Pd(R)(HCOO)] (3, R = Ph; 4, R = Me). Complexes 1-4 have been characterized by elemental analysis and NMR and IR spectral data. The structure of 2 has been,confirmed by X-ray analysis of its 1:1.5 benzene solvate: monoclinic, space group P2(1)/n; a = 9.785(4) Angstrom, b = 24.909(8) Angstrom, c = 18.854(7) Angstrom, beta = 102.92(3)degrees; V = 4479(3) Angstrom, Z = 4; D-calcd = 1.441 g/cm(3); R = 0.044, R(w) = 0.066. The thermal decomposition of 4-4 in benzene or toluene at 30-55 degrees C gives CO2, RH, and highly reactive [L(n)Pd] (n = 1, 2). An exchange between the sigma-Me groups and phenyls of the PPh(3) ligand is observed in the course of the thermal decomposition of 2 in toluene at 55 degrees C. The thermolysis of 1 in the presence of PhI gives rise to [L(2)Pd(2)(Ph)(2)(mu-I)(2)] (5); likewise, the decomposition of 2 in the presence of Mel furnishes [L(2)Pd(2)(Me)(2)(mu-I)(2)] (6). However, when 1 is decomposed in the presence of Mel, both 5 and 6 are formed, with the ratio of 5 to 6 being higher at higher concentrations of MeI. A similar effect is observed when 2 is thermolyzed in the presence of PhI. This ''reverse'' selectivity leads to the recognition of two distinct mechanisms governing the Pd-catalyzed reductive dehalogenation of aryl halides with formate anion.