Catalytic Properties of [Pd(COOMe)nX2-n(PPh3)2] (n=0, 1, 2; X = Cl, NO2, ONO2, OAc and OTs) in the Oxidative Carbonylation of MeOH

cis-[Pd(ONO2)(2)(PPh3)(2)] (1) reacts under mild conditions with CO in methanol)MeOH) in the presence of pyridine (py), yielding trans-[Pd(COOMe)(ONO2)(PPh3)(2)] (1a). The use of NEt3 instead of py leads to a mixture of 1a, trans-[Pd(COOMe)(2)(PPh3)(2)] (2), and [Pd(CO)(PPh3)(3)]. Pure 2 was prepared by reacting cis-(Pd(OTs)(2)(PPh3)(2)] with CO in MeOH and subsequently adding NEt3. The nitro complex trans-{Pd(COOMe)(NO2)(PPh3)(2)] (3a) was prepared by reacting trans-[Pd(COOMe)Cl(PPh3)(2)] with AgNO2 or with AgOTs and NaNO2. New syntheses for 1 and trans-[Pd(NO2)2(PPh3)21 (3) are also reported. All complexes have been characterized by IR and H-1 and P-31{H-1} NMR spectroscopies. Complexes 1 and 2 exchange irreversibly and quantitatively one nitrato with one carbomethoxy ligand, yielding la. 2 in CD2Cl2 at 40 degrees C decomposes with the formation of dimethyl carbonate (DMC), whereas under 4 atm of CO, DMC and dimethyl oxalate (DMO) are formed, ca. 12% each; in the presence of PPh3 and in the absence of CO, decomposition occurs at 60 C with the formation of DMC only, suggesting that decarbonylation involves a five-coordinate intermediate or predissociation of a PPh3 ligand. The oxidative carbonylation of MeOH does not occur when using NaNO2 or NaNO3 as the oxidant and 1, 1a, 3, or 3a as the catalyst precursor. On the contrary, when using benzoquinone (BQ) as the oxidant, these complexes, 2, or [Pd(COOMe)(2-n)X-n(PPh3)(2)] (X = Cl, OAc, OTs; n = 1, 2) promote selective catalysis to DMO. After catalysis the precursors are transformed into [Pd(BQ)(PPh3)(2)](2)center dot H(2)BQ, Pd(CO)(PPh3)](3) and Pd(CO)(PPh3)(3)]. Also the last with BO gives selective catalysis to DMO. The solid-state structures of 1 center dot CH2Cl2 and la have been determined by means of single-crystal X-ray diffraction.