BINUCLEAR PALLADIUM COMPLEXES BRIDGED BY BIS(DIMETHYLPHOSPHINO) METHANE
Abstract
The diphosphine, bis(dimethylphosphino)methane (dmpm) is prepared by the reaction of methylmagnesium iodide with bis(dichlorophosphino)methane (dcpm). Treatment of Pd(CO)Cl (,n) with dmpm leads to the formation of the bridged binuclear complex, Pd(,2)Cl(,2)(dmpm)(,2). The chlorides of this complex are labile and may be substituted by X('-)(X = Br, OH). The crystal and molecular structure of the Br derivative, Pd(,2)Br(,2)(dmpm)(,2), show it has a Pd-Pd separation of 2.603(1)(ANGSTROM), consistent with a net Pd-Pd bond. The complexes, Pd(,2)X(,2) (dmpm)(,2) (X = Cl, Br), are stable and soluble in H(,2)O. Substitution of hydroxide for the chloride ligands of Pd(,2)Cl(,2)-dmpm(,2) occurs in water to form Pd(,2)(OH)(,2) (dmpm)(,2). Reaction of Pd(,2)Cl(,2)(dmpm)(,2) with small molecules CO, CH(,3)NC, SO(,2) and CS(,2) leads to insertion into the Pd-Pd bond and formation of Pd(,2)((mu)-L)Cl(,2)-(dmpm)(,2) (L = CO, CH(,3)CN, SO(,2), CS(,2)). The crystal and molecular structure of the (mu)-CO complex, Pd(,2)((mu)-CO)Cl(,2)(dmpm)(,2) shows a Pd....Pd separation of 3.171(1)(ANGSTROM) consistent with no net Pd-Pd bond. Treatment of Pd(,2)Cl(,2)dmpm(,2) with excess CH(,3)NC leads to displacement of the terminal chlorides to form Pd(,2)((mu)-CH(,3)NC)(CH(,3)NC)(,2)(dmpm)(,2) Cl (,2). The complex, Pd(,2)Cl(,2)(dmpm)(,2), is an active catalyst precursor for water gas shift catalysis in aqueous solutions. Heating an aqueous solution of Pd(,2)Cl(,2)(dmpm)(,2) under CO leads to evolution of CO(,2) and H(,2) at an initial turnover rate of >1 per hour, decreasing over a period of a week to 0.2 per hour. The major species found in solution are Pd(,2)((mu)-CO)(OH)(,2)(dmpm)(,2), Pd(,2)((mu)-CO)(H)(,2)(dmpm)(,2) and Pd(,2)(OH)(CO)(dmpm)(,2) OH . The mechanism of CO(,2) evolution is proposed to involve a metallocarboxylic acid intermediate formed by migratory insertion of CO into a Pd-OH bond. Diphenoxide complexes of palladium, Pd(,2)(OAr)(,2)(dmpm)(,2) (Ar = Ph, p-C(,6)H(,4)F, 2,6(CH(,3))(,2)C(,6)H(,3)), are prepared by metathesis of the substituted phenoxide for the chlorides of Pd(,2)Cl(,2)(dmpm)(,2). The diphenoxide complexes insert CO into both Pd-Pd and Pd-O bonds of Pd(,2)(OAr)(dmpm)(,2), with eventual elimination of CO(,2). The palladium containing product formed is dependent on the reaction conditions and is either binuclear or trinuclear in nature. Organic products are formed during the reaction which suggest that these diphenoxide complexes can be used to deoxygenate phenols to give the corresponding arene.
Degree
Ph.D.
Subject Area
Chemistry
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