Synthesis of nitrogen-, oxygen- and phosphine- donor palladium(II) complexes for the catalytic hydrogenation and methoxycarbonylation of alkenes and alkynes.
Tshabalala, Thandeka Adelinah.
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Reactions of N-(1H-benzoimidazol-2-ylmethyl-2-methoxy)aniline (L1) and N-(1H-benzoimidazol-2-ylmethyl-2-bromo)aniline (L2) with p-TsOH, Pd(AOc)2 and two equivalents of PPh3 or PCy3 produced the corresponding palladium complexes, [Pd(L1)(OTs)(PPh3)] (1), [Pd(L2)(OTs)(PPh3)] (2) and [Pd(L1)(OTs)(PCy3)] (3) respectively in good yields. The new palladium complexes 1-3, and the previously reported complexes [Pd(L1)ClMe] (4) and [Pd(L2)ClMe] (5) gave active catalysts in the methoxycarbonylation of terminal and internal olefins to produce branched and linear esters. The effects of complex structure, nature of phosphine derivative, acid promoter and alkene substrate on the catalytic activities and selectivity have been studied and are herein reported. The ligands N-(1H-benzoimidazol-2-ylmethyl-2-methoxy)aniline (L1), N-(1H-benzoimidazol-2-ylmethyl-2-bromo)aniline L2), N-(1H-benzoimidazol-2-ylmethyl)benzenamine (L3) and N-(1H-benzoimidazol-2-ylmethylamino)benzenethiol (L4), were synthesized following our published literature method. The palladium complexes [Pd(L1)Cl2] (6), [Pd(L2)Cl2] (7) [Pd(L3)Cl2] (8), [Pd(L4)Cl2] (9), [Pd(L2)ClMe] (10) and [Pd(L2)ClPPh3]BAr4 (11), were prepared following our recently published procedure. The (benzoimidazol-2-ylmethyl)amine palladium(II) complexes 6-11, have been employed as catalysts in the homogeneous hydrogenation of alkenes and alkynes under mild conditions. A correlation between the catalytic activity and the nature of the ligand was reaffirmed. Kinetic studies of the hydrogenation reactions of styrene established pseudo-first-order dependence on the styrene substrate. On the other hand, partial orders with respect to H2 and catalyst concentrations were obtained. The nature of the solvent used, influenced the hydrogenation reactions, in that coordinating solvents resulted in lower catalytic activities. Kinetics and mechanistic studies performed are consistent with the formation of a palladium monohydride active species. Reactions of 2-(2-methoxyethylimino)ethyl)phenol (L5), 2-(2-hydroxyethylimino)ethyl)phenol (L6), 2-(2-aminoethylimino)ethyl)phenol (L7) and 2-(2-hydroxyethylimino)methyl)phenol (L8) with [PdCl2(COD)] afforded the neutral palladium complexes [PdCl2(L5)] (12), [PdCl2(L6)] (13), [PdCl2(L7)] (14), [PdCl2(L8)] (15) respectively. Treatment of complex 12 with PPh3 gave the cationic complex [Pd(L5)ClPPh3]+ (16), while reactions of 15 with Pd(OAc)2, in the presence of PPh3 and p-TsOH produced the corresponding palladium complex, [Pd(L5)(OTs)(PPh3)] (17). he molecular structure of 15a (derivative of 15) contained two bidentate anionic ligand (L8). Complexes 12-17 formed active catalysts in hydrogenation of alkenes and alkynes, in which the catalytic activities were largely dependent on the pendant donor atom of the ligand motif. Isomerization reactions were dominant in terminal alkenes hydrogenation reactions, while hydrogenation of alkynes to alkanes occurred in two steps via alkene intermediates. Kinetics data were consistent with homogeneous active species. Density functional theory studies supported the hemi-labile nature of the ligands, and offered insights into the catalytic activity trends observed. Compounds [2-(2-(diphenylphosphino)benzylidene)methoxyethanamine] (L9), [2-(2-(diphenylphosphino)benzylideneamino)ethanol] L10), 2-(diphenylphosphino)benzylidene)ethane-1,2-diamine) and 2- (diphenylphosphino)benzylidene)diethylethane-1,2-diamine) (L12) were prepared by a condensation reaction between 2-(diphenylphosphino)benzaldehyde and the corresponding amines. The imino-diphenylphosphino palladium(II) complexes, [PdCl2(L9)] (18), [PdCl2(L10)] (19), [PdCl2(L11)] (20), [PdCl2(L12)] (21) were prepared from the reactions of L9-L12 with Pd(COD)Cl2. On the other hand, reactions of L9 with Pd(AOc)2 in the presence of equivalenct amounts of p-TsOH and PPh3 afforded [Pd(L9)(OTs)(PPh3)]+TsO- (22) while subsequent treatment of complex 18 with one equivalent of NaBAr4 (Ar4 = 3,5-(CF3)2C6H3) in the presence of PPh3 afforded the cationic complex, [Pd(L5)(Cl)(PPh3)] BAr4- (23). Complexes 18-23 were characterized by mass spectrometry, elemental analysis, 1H, 13C and 31P NMR spectroscopy. Complexes 18-23 were found to be active catalysts in the hydrogenation of alkenes and alkynes in wish isomerization of terminal alkenes also occurred. Kinetic experiments, stoichiometry poisoning, mercury poisoning and kinetic reproducible data indicated the homogeneous nature of the active species. The water-soluble palladium(II) complexes, [Pd(L9)(TPPMS)(PPh3)]+TsO- (24), [Pd(L10)(TPPMS)(OTs)]+TsO- (25), [Pd(L11)(TPPMS)(PPh3)]+TsO- (26) and [Pd(L12)(TPPMS)(PPh3)]+TsO- (27) were synthesized from the reactions of L9-L12 with Pd(AOc)2 and p-TsOH in the presence of PPh3 followed by adding TPPMS in butanone. The complexes, 24-27, were highly soluble in methanol, ethanol and water but were insoluble in chlorinated solvents. Complexes 24-27 were found to form active catalysts for the high pressure hydrogenation of alkenes and alkynes in biphasic media. The complexes were recyclable and retained significant catalytic activities after six cycles. Reaction parameters such as temperature, and aqueous/organic ratio affected the catalyst recycling efficiencies.