Chemistry of P,N-ligated methylpalladium(II) alkoxide complexes: Syntheses, structural features in the solid state and solution and hydrogen-bond formation

Abstract

The reaction of P,N-ligated dimethylpalladium(II) complexes [Pd(Me)2(P~N)] [P~N = o-(diphenylphosphino)-N,N-dimethylbenzylamine (PCN), (diphenylphosphino)-N,N-dimethylethyleneamine (PN)] with an equimolar amount of HOCH(CF3)2 or HOC6H4-4-X (X = H, Cl, OMe, Me, CN, NO2, OH) affords methylpalladium(II) alkoxide and aryl oxide complexes [Pd(Me)(OR)(P~N)] [R = CH(CF3)2, C6H4-4-X; P~N = PCN, PN], which have been isolated in high yields as white or pale orange solids. NMR spectroscopic data indicate that the complexes have the alkoxide or aryl oxide ligand (OR) positioned trans with respect to the P-atom. Reaction of [Pd(Me)2(PCN)] with 2 equiv of phenol affords the phenol adduct [Pd(Me)(OC6H5)(PCN)]·HOC6H5, which has been isolated as a white solid. The transesterification reaction of [Pd(Me)(OC6H5)(PCN)] with C6H5SC(O)Me affords the arenethiolate complex [Pd(Me)(SC6H5)(PCN)]. Furthermore, the alkoxide complex [Pd(Me)(OCH(CF3)2)(PN)] reacts with phenylacetylene to afford an isolable alkynylpalladium(II) complex [Pd(Me)(CCPh)(PN)]. Thermolysis of the latter complex results in reductive elimination of MeCCPh in 98% yield. When this reductive elimination is performed under a CO atmosphere, MeCCPh (87%) is formed together with small amounts of the insertion product, MeC(O)CCPh (3%). Crystals of [Pd(Me)(OC6H5)(PCN)]·CH2Cl2 and its adduct with HOC6H5 have been subjected to X-ray diffraction studies. The molecular structure of the latter adduct shows an O-H···O hydrogen bond between the phenol and the oxygen atom of the phenoxide unit. The 31P NMR chemical shift and 3JH,P for the Pd-CH3 group of the complexes [Pd(Me)(OC6H4-4-X)(PN)] have been correlated with various Hammet substituent constants; the best linearity was achieved with parameters that represent the sum of resonance and inductive/field effects of the substituent X.