Syntheses of mixed donor homogeneous and immobilized palladium(II) complexes catalysts for methoxycarbonylation and hydrogenation reactions.
Authors:   , Akiri Saphan Owino 
Published: January 2024

Reactions of ligands (E)-N'-(2,6-diisopropylphenyl)-N-(4-methylpyridin-2-yl)benzimidamide (L1), (E)-N'-(2,6-diisopropylphenyl)-N-(6-methylpyridin-2-yl)benzimidamide (L2), (E)-N'-(2,6-dimethylphenyl)-N-(6-methylpyridin-2-yl)benzimidamide (L3), (E)-N'-(2,6-dimethylphenyl)-N-(4-methylpyridin-2-yl)benzimidamide (L4) and (E)-N-(6-methylpyridin-2-yl)-N'-phenylbenzimidamide (L5) with [Pd(NCMe)2Cl2] furnished the corresponding palladium(II) pre-catalysts (Pd1-Pd5), in good yields. Molecular structures of Pd2 and Pd3 revealed an N^N bidentate coordination mode to afford square planar compounds. Activation of the palladium(II) complexes with para tolyl sulfonic acid (PTSA) afforded active catalysts in the alkenes methoxycarbonylation. The resultant catalytic activities were controlled by both the complex structure and alkene substrate. While aliphatic substrates favoured the formation of linear esters (>70%), styrene substrate resulted in predominantly branched esters of up to 91%. The water-soluble ligands; sodium 4-hydroxy-3-((phenylimino)methyl)benzenesulfonate (L6), sodium 3-(((2,6-dimethylphenyl)imino)methyl)-4-hydroxybenzenesulfonate (L7) and sodium 3-(2,6-diisopropylphenyl)imino)methyl)-4-hydroxybenzenesulfonate (L8) reacted with with Pd(OAc)2 afford their respective palladium(II) complexes [Pd(6)2] (Pd6), [Pd(L7)2] (Pd7) and [Pd(L8)2] (PdL8). In addition, treatment of the non-water-soluble ligands 2-((phenylimino)methyl)phenol (L9), 2-(((2,6-dimethylphenyl)imino)methyl)phenol (L10) and 2-((2,6-diisopropylphenyl)imino)methyl)phenol (L11) with Pd(OAc)2 yielded complexes [Pd(L9)2] (Pd9), [Pd(10)2] (Pd10) and [Pd(L11)2] (Pd11), respectively in good yields. Solid-state structures of compounds Pd6 and Pd9 revealed bis(chelated) square planar neutral compounds. All the complexes formed active catalysts in the methoxycarbonylation of 1- hexene, affording yields of up to 92% within 20 h and regioselectivity of 73% in favour of linear esters. The activities and selectivities of the compounds depended on the steric encumbrance around the coordination centre. The water-soluble complexes displayed comparable catalytic behaviour to the non-water-soluble systems. The complexes could be recycled five times with minimal changes in both the catalytic activities and regio-selectivity. Reactions of (amino)phenyl ligands, (E)-N-((Z)-4-(phenylamino)pent-3-en-2-ylidene)aniline (L12) and N,N'E,N,N'E)-N,N'-(3-(3-(triethoxysilyl)propyl)pentane-2,4-diylidene)dianiline (L13) with [Pd(NCMe)2Cl2] led to the formation of homogeneous complexes Pd13 and Pd14. Besides, supporting of complex Pd14 with either MCM-41, SBA-15, or Fe3O4 magnetic nanoparticles gave immobilized complexes P15-Pd17, respectively. Using varying metal loading in the MCM-41 immobilization of complex Pd14 produced complexes Pd18 and Pd 19. In addition, calcination of complex Pd16 at 150oC and 200oC led to the formation of complexes Pd20 and Pd21, respectively. All the complexes were received in good yields. The catalytic activities and selectivities of the homogeneous complexes were influenced by the coordination sphere, with the complexes predominantly forming linear esters. On the other hand, the catalytic behaviours of the immobilized catalysts depended on the nature of support and calcination temperatures. In addition, the catalytic activities were observed to depend on the reaction temperature, catalyst loading, amounts of PPh3 and acid promoters. The immobilized complexes Pd15, Pd16 and Pd17, were recycled up to five times. The homogeneous and silica immobilized palladium(II) complexes of ligands (2-phenyl-2-((3(triethoxysilyl)propyl)imino)ethanol) (L14), (4-methyl-2-((3(triethoxysilyl)propyl)imino)methyl)phenol ) (L15 ), [L14-MCM-41 (L16), and [L15- MCM-41 (L17)]. The homogeneous complexes [Pd(L14)2] (Pd22), [Pd(L14)2] (Pd23), [Pd(L14)(Cl2)] (Pd24), [Pd(L15)(Cl2)] (Pd25) were obtained from homogenous ligands L14, L15, L16 and L17 respectively. In addition, the silica immobilized compounds [Pd(L14)2]-MCM-41] (Pd26) and [Pd(L15)2)-MCM-4] (Pd27) were obtained through convergence immobilization of complexes Pd22 and Pd23, respectively. Comparatively, immobilized complexes [Pd (L14)(Cl2)-MCM-41] (Pd28) and [Pd(L15)(Cl2)]-MCM-41] (Pd29) were obtained from the complexation of immobilized ligands L16 and L17. Both sets of complexes gave active catalysts in molecular hydrogenation of alkenes, alkynes and functionalized benzenes. The catalytic activities and product distribution in these reactions were largely dictated by the nature of the substrate. The kinetic studies revealed reaction orders dependence on styrene for both the homogeneous and supported catalysts. Significantly, the selectivity of both sets of catalysts was comparable in the hydrogenation of alkynes and multi-functionalized benzenes. The supported catalysts could be recycled up to four times with minimum reduction in catalytic activities and showed the absence of any leaching from hot filtration experiments. Kinetics and poisoning studies established the presence of active homogeneous species for complexes Pd22-Pd5 and Pd(0) nanoparticles for the immobilized complexes Pd26-Pd29, respectively.