Rigid bis-zinc(ii) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis

Zinc-centred salen complexes: versatile and accessible supramolecular building motifs

Zinc(II) complexes of salen ligands, and particularly those of the salphen family of ligands, are extremely versatile synthons. This Frontier reviews the recent applications of these molecular building blocks in supramolecular chemistry, and their potential in various fields of material science.

New templating strategies with salen scaffolds (Salen=N,N'-bis(salicylidene)ethylenediamine dianion)

Templated approaches towards selective organic synthesis is a common feature in nature in which nucleic acid templated synthesis plays a crucial role in various fundamental biological processes. The key feature that allows control over the amazing selectivity found in natural processes is evidently the effective molarity of the reaction partners that is mediated by the macromolecular templation event. An ongoing challenge within many chemical sciences is to exploit similar templating principles and make use of synthetic systems that are designed for specific chemical conversions. Here, we describe the recent developments that involve (metallo)salen scaffolds that are used for diverse templating events (salen=N,N'-bis(salicylidene)ethylenediamine dianion).

Expedient Method for the Transmetalation of Zn(II)-Centered Salphen Complexes

Various mono- and bis-ZnII complexes derived from salphen ligands have been conveniently transmetalated with a number of transition metal (TM) acetates (M=Ni, Pd, Mn) in THF to afford their respective TM-salphen counterparts in excellent isolated yields (80-100%). This new transmetalation procedure allows in situ switching between supramolecular and catalytic functions of the metallosalphen complex.

Supramolecular bidentate phosphorus ligands based on bis-zinc(ii) and bis-tin(iv) porphyrin building blocks

Selective metal-ligand interactions have been used to prepare supramolecular bidentate ligands by mixing monodentate ligands with a suitable template. For these assemblies pyridine phosphorus ligands and a zinc(II) porphyrin dimer were used. In the rhodium-catalysed hydroformylation of 1-octene and styrene improved selectivities have been obtained for some of the assembled bidentate ligand systems. In the palladium catalysed asymmetric allylic alkylation similar effects were observed; the enantioselectivity increased by using a bisporphyrin template. The preparation of supramolecular catalyst systems was also explored using tin-oxygen interactions. Dihydroxotin(IV) porphyrin and carboxylic phosphorus ligands assemble into supramolecular ligands and the phosphorus donor atom coordinates to transition metals. The stronger oxygen-tin bond, compared to pyridine-zinc does not result in a better performance of the catalyst.