Palladium Phosphinothiolato Complexes. Syntheses and Crystal Structures of Mononuclear [PdCl(SC₂H₄PPh₂)PPh₃] and Binuclear [Pd₂Cl₂(μ-SC₃H₆PPh₂)₂] and Their Performance in Catalytic Carbonylation

Abstract

The phosphinothiolato complexes [PdCl(dppet)PPh(3)] (3), [Pd(dppet)(2)] (4), and [Pd(2)Cl(2)(dpppt-P,&mgr;-S)(2)] (6) (Hdppet = HSC(2)H(4)PPh(2); Hdpppt = HSC(3)H(6)PPh(2)) have been synthesized in good yield by various base-free ligand exchange reactions on Pd(II), but [Pd(dpppt)(2)] (5) could only be obtained as a pure product employing an oxidative route from Pd(0). Both complexes 3 ((31)P NMR, delta(P) 66.8 (dppet), 24.1 (PPh(3)); (2)J(PP) = 459 Hz) and 5 ((13)C NMR) are trans-P,P. Crystals of 3.CH(2)Cl(2) are orthorhombic (P2(1)2(1)2(1)), with a = 9.247(3) Å, b = 17.956(9) Å, c = 19.869(9) Å. In 3 the Pd-S distance of 2.270(2) Å is short compared to the very different Pd-P distances of 2.280(2) (Pd-PPh(2)R) and 2.343(2) Å (Pd-PPh(3)). Crystals of 6.CH(2)Cl(2) are monoclinic (P2(1)/n) with a = 12.701(3) Å, b = 12.040(4) Å, c = 22.495(2) Å, and beta = 97.36(1) degrees. The structure of 6 consists of two Pd(dpppt)Cl moieties meeting at an angle of 105.81(3) degrees, linked by asymmetric thiolato bridges in a syn-endo configuration. The difference in the chelate angles of dppet (85.98(9) degrees ) and dpppt (96.60(3) degrees and 97.42(3) degrees ) seems to be crucial for palladium to form an unusual mononuclear complex (3) or a binuclear complex (6). Bischelate complexes 4 and 5 are inactive, but 3 and 6 catalyze the hydroesterification of styrene with CO (30 bar) and MeOH at moderate temperatures (60 and 80 degrees C) with no additives. The velocities are slow, but with 6 and with 3 at the lower temperature no decomposition to Pd metal is observed. Only esters are produced and regioselectivities of ca. 84% toward 2-phenylpropanoic acid methyl ester are achieved.