Rigid, strained, and flexible: a DFT study of a backbone-affected monohydride formation of salen and salan complexes

The monohydride formation of some palladium(II)-sulfosalen and sulfosalan catalysts was studied by DFT methods. The coordination of the hydrogen molecule to the metal center and the following heterolytic dissociation of the coordinated hydrogen could occur in a two-step or a concerted process resulting in a monohydride complex and having a protonated dissociated phenolate arm. The effect of the backbone frame of the ligands (the molecular unit between two nitrogen atoms) strongly determines the energetics and the type of the hydride formation. Rigid, strained, and flexible molecular structures were studied covering a wide range of planar and spherical types of backbones. Besides the previously studied Direct 1 and Direct 2 mechanisms, three other mechanisms of direct monohydride formation were found. Known and fictive structures were studied to predict kinetically and thermodynamically preferred pathways as well as complexes for this type of reaction.

Graphical Abstract

Palladium (II)-Salan Complexes as Catalysts for Suzuki-Miyaura C-C Cross-Coupling in Water and Air. Effect of the Various Bridging Units within the Diamine Moieties on the Catalytic Performance

Water-soluble salan ligands were synthesized by hydrogenation and subsequent sulfonation of salens (N,N'-bis(slicylidene)ethylenediamine and analogues) with various bridging units (linkers) connecting the nitrogen atoms. Pd (II) complexes were obtained in reactions of sulfosalans and [PdCl₄]^2-. Characterization of the ligands and complexes included extensive X-ray diffraction studies, too. The Pd (II) complexes proved highly active catalysts of the Suzuki-Miyaura reaction of aryl halides and arylboronic acid derivatives at 80 °C in water and air. A comparative study of the Pd (II)-sulfosalan catalysts showed that the catalytic activity largely increased with increasing linker length and with increasing steric congestion around the N donor atoms of the ligands; the highest specific activity was 40,000 (mol substrate) (mol catalyst × h)^-1. The substrate scope was explored with the use of the two most active catalysts, containing 1,4-butylene and 1,2-diphenylethylene linkers, respectively.