Metal Complexes of Acyclic and Macrocyclic Multifunctional Ligands

Abstract

This thesis describes the design, synthesis and study of metal derivatives of new acyclic and macrocyclic ligands containing pyridine and amide groups. Chapter 1 provides an overview of metal-carboxamide and pyridinamide chemistry including a number of important pincer compounds, macrocyclic involvement in formation of metal-templated rotaxanes and catenanes, oxidation catalysts, anion receptors and bimetallic complexes. Chapter 2 discusses new palladium(II) complexes of an acyclic ligand bearing pendant 2-pyridyl-6-methyl arms, N,N_-bis(6-methyl-2-pyridinyl)-2,6-pyridinedicarboxamide (H2LMe). H2LMe formed a dimer [Pd(LMe)]2 when treated with palladium(II) salts and non-ligating bases, but in the presence of DBU the palladium-DBU adduct, Pd(LMe)(DBU), was formed. Reaction of Pd(LMe)(DBU) with methyl iodide resulted in the displacement of the DBU ligand and the concomitant formation of cationic monomeric complex, [PdI(LMe{Me}2)]I and dimeric N-methylpyridinium complex, [Pd(LMe{Me})]2I2. A series of ligands, N,N_-bis(x-tolyl)-2,6-pyridinedicarboxamide (x = 2, 3, 4) (H2Lxtol), bearing ortho-, meta- and para-tolyl groups, was prepared and these were coordinated to palladium(II) in their deprotonated form so that the effect of the pendant pyridine rings and steric environment around the metal on the reactivity of metal derivatives could be investigated. Stable palladium(II) derivatives of the deprotonated H2Lxtol ligands, Pd(Lxtol)(E) (E = DBU, n-butylamine, p-tolylisocyanide) were prepared. The p-tolylisocyanide adducts reacted with pyrrolidine or p-toluidine to afford the stable bis(amino)carbene complexes, Pd(Lxtol)(=C(NH-p-tolyl)(pyrl)) and Pd(Lxtol)(=C(NH-p-tolyl)2), respectively. The coordinated DBU ligands in Pd(Lxtol)(DBU) and Pd(LMe)(DBU) were displaced by n-butylamine to afford the corresponding n-butylamine adducts, and their relative rates of exchange were determined by 1H NMR spectroscopy. Chapter 3 discusses palladium(II) complexes of dicationic N-methylpyridinium ligands prepared by treating H2LMe or the series of ligands, N,N_-bis(x-pyridinyl)-2,6- pyridinedicarboxamide (x = 2, 3, 4) (H2Lxpy) that contain pendant 2-, 3-, or 4-pyridyl groups, with methyl triflate to form [H2LMe{Me}2][OTf]2 or [H2Lxpy{Me}2][OTf]2, respectively. These ligands were coordinated in their deprotonated forms to palladium(II) to give [PdCl(Lxpy{Me}2)]OTf. The chloro ligands in these metallated complexes were displaced on treatment with silver triflate in acetonitrile or water to afford the corresponding solvent adducts. The coordinated solvent molecules in [Pd(LMe{Me}2)(NCCH3)][OTf]2 and [Pd(Lopy{Me}2)(OH2)][OTf]2 could in turn be displaced by p-tolylisocyanide to form isocyanide adducts, [Pd(LMe{Me}2)(CN-ptolyl)][ OTf]2 and [Pd(Lopy{Me}2)(CN-p-tolyl)][OTf]2. Dicationic bis(amino)carbene complexes [Pd(LMe{Me}2)(=C(NH-p-tolyl)2)][OTf]2 and [Pd(Lopy{Me}2)(=C(NH-ptolyl) 2)][OTf]2 were prepared by treating the corresponding isocyanide precursors with p-toluidine. A 1H NMR spectroscopic study was performed to compare the relative rates of reaction of p-toluidine with the neutral tolyl isocyanide complexes Pd(Lxtol)(CN-p-tolyl) and the dicationic isocyanide complexes [Pd(LMe{Me}2)(CN-ptolyl)][ OTf]2 and [Pd(Lopy{Me}2)(CN-p-tolyl)][OTf]2 to determine the influence of the steric and electronic environments on the reactivity of the isocyanide ligand. On deprotonation of the amide groups in [H2LMe{Me}2][OTf]2 and [H2Lopy{Me}2][OTf]2 the neutral free bis(imine) compounds LMe{Me}2 and Lopy{Me}2 could be isolated. Chapter 4 discusses extended acyclic ligands H4LpdnA and H4LSpyA (H4LxA) that were derived from the precursor N,N_-bis(6-acrylamido-2-pyridinyl)pyridine-2,6- dicarboxamide (H4LacrA) through Michael addition of pyrrolidine or 2- mercaptopyridine, respectively, to the acrylyl groups. The double-helical dimers [M(H2LxA)]2 were formed when these ligands were treated with palladium(II) or mercury(II) acetate, and in the presence of DBU the adducts Pd(H2LxA)(DBU) were formed. In the absence of added base, palladium(II) acetate coordinated between the tail amine groups of the ligand H4LpdnA which bears terminal pyrrolidyl groups. Chapter 5 discusses complexes of macrocycles formed from double Michael-type addition of the amines n-butylamine, 2-(aminomethyl)pyridine, 2-(aminoethyl)pyridine, N,N-dimethylethylenediamine and N,N_-bis(2-pyridylmethyl)ethylenediamine to the pendant acrylyl groups of H4LacrA. The macrocycle synthesised from addition of nbutylamine, H4LnBu, reacted with palladium(II) acetate and DBU to form a complex in which palladium was coordinated in the macrocycle headgroup and an aminolactam resulting from hydrolysis of DBU was coordinated on the fourth site of the metal, Pd(H2LnBu)(NH2Lac[7]). A palladium derivative of H4LnBu with a labile water ligand, Pd(H2LnBu)(OH2), was prepared and used for subsequent syntheses of n-butylamine, DBU and p-tolylisocyanide adducts. When treated with p-toluidine, the isocyanide ligand of the macrocyclic p-tolylisocyanide adduct was displaced to form a p-toluidine adduct. Modified macrocycles with other amine donors incorporated into the tail were prepared in order to provide an additional site for metal complexation. The macrocycle with an additional N,N-dimethylamino group, H4Ldmen, reacted with metal salts to form complexes where metallation had taken place at the tail amide groups and the tail amine group interacted with the metal.