pH-Induced Linkage Isomerism of Pd(II) Complexes: A Pathway to Air- and Water-Stable Suzuki–Miyaura-Reaction Catalysts

Abstract

The Pd(II) complexes of the ambidentate pyridyldiketones 2,2-dimethyl-5-(3- or 4-pyridyl)pentane-3,5-dione are readily prepared in their linkage isomeric forms by the appropriate pH control during syntheses. The isolated diketonate- or pyridine-bound species can be interconverted with essentially 100% efficiency by treatment with an acid or base, respectively. Under the normal basic conditions for a Suzuki-Miyaura coupling, only the diketonate forms are present and act as very efficient catalysts for this reaction. The dynamic nature of the presented complexes allows the catalytic process to be quenched and reactivated at any stage without the risk of losing the catalyst's activity.

Intrinsic Effect of Pyridine-N-Position on Structural Properties of Cu-Based Low-Dimensional Coordination Frameworks

“…The crystal structure established protonation of the pyridine nitrogen atom and H-bonding interactions between methine, two aromatic protons and the nitrate counterion ( Figure 3 a). The protonated ligand in its crystal adopts its enol form similarly to the previously described neutral version of L2 ( Figure 3 b) [ 24 ]. This tautomer is stabilized by a strong resonance-assisted intramolecular hydrogen bond [ 36 ] involving O1-H⋯O2 (1.618 Å).…”

“…The ligands were prepared by Claisen condensation, following a modified literature procedure and involving the reaction between the methyl esters of isonicotinic (for L1), nicotinic (for L2) or picolinic (for L3) acids and pinacolone in the presence of a strong base [ 24 , 35 ]. Depending on the position of the nitrogen atom in the pyridine ring, structurally distinct assemblies were obtained: two coordination polymers N1 ( para - N ) and N2 ( meta - N ) and a monomeric complex C1 ( ortho - N ).…”

“…The syntheses of 4,4-dimethyl-1-(pyridin-4-yl)-pentane-1,3-dione (L1), 4,4-dimethyl-1-(pyridin-3-yl)-pentane-1,3-dione (L2) and 4,4-dimethyl-1-(pyridin-2-yl)-pentane-1,3-dione (L3) were performed following previously described procedures involving Claisen condensation between appropriate pyridinecarboxylic acid methyl esters and pinacolone in the presence of strong base (NaH) ( Figures S1–S3 ) [ 24 , 35 ].…”

“…Coordination complexes containing ambidentate ligands that are capable of binding metal-ion centres in more than one way through different donor-atom combinations provide an efficient strategy in the construction of functional coordination assemblies [ 12 , 13 ].Of the many structurally distinct ambidentate ligands, pyridyl-β-diketonates have been widely employed in the generation of different coordination systems from simple mononuclear complexes, [ 14 ] through cages [ 15 ] and macrocycles [ 16 ] to larger architectures (e.g., polymers (CPs) and metal-organic frameworks) [ 15 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ] Importantly, coordination structures based on ambidentate ligands have shown real potential in many applications including catalysis, [ 24 , 25 , 26 ] bioinorganic modelling [ 27 , 28 ] and molecule magnetism [ 29 , 30 , 31 ] and in the design of molecular machines [ 32 ]. More recently, ambidentate ligands have been successfully utilized with metal ions such as Pd(II), Pt(II) and Cu(I) to provide a series of metallosupramolecular complexes with different levels of dynamicity and high catalytic activity in important processes such as the Suzuki–Miyaura, alkene hydrosilylation and Ullmann reactions [ 24 , 25 , 26 ].…”

DFT analysis of the linkage isomerism in penta(ammine)ruthenium(II/III) complexes of benzotriazole: Natural bond orbital method approach and a comprehensive energy decomposition analysis

“…In coordination chemistry, linkage isomerism can, in principle, occur whenever a transition metal coordinates with an ambidentate ligand . That way, a vast number of metal complexes are known to have linkage isomers that can be obtained thermically, photochemically, electrochemically, by pH effects, and even by pressure . The interest in investigating this type of isomerism comes from the possible applications of linkage isomers in bioinorganic chemistry, molecule magnets, and in the design of molecular machines .…”