Coordination-driven self-assembly of chiral palladium(ii)-based supramolecular triangle structures

Abstract

In the present work, four chiral supramolecular coordination complexes (SCCs) are described. The self-assembly of ditopic bis(4R-1H-imidazolyl)benzene ligands (L^R, with R = H or Me) and achiral palladium(II) complexes [((en)Pd(NO₃)₂)] (1a) afford supramolecular coordination complexes 2 and 4 (where en = ethylenediamine). Similarly, self-assembly of L^H and achiral [((tmeda)Pd(NO₃)₂)] (1b) or chiral enantiopure [(((R,R)-Me₄-dach)Pd(NO₃)₂)] (1c) yield compounds 3 and 5 respectively (where tmeda = (N,N,N′,N′)-tetramethylethylenediamine and (R,R)-Me₄-dach = (R,R)-(N,N,N′,N′)-tetramethylcyclohexane-1,2-diamine). SCCs 2–5 are fully characterized in solution by ¹H (1-D and 2-D experiments), ¹³C, DOSY NMR spectroscopy, coldspray ionization mass spectrometry. These analyses clearly demonstrate an equilibrium in solution between the triangular (2a–5a) and square (2b–5b) structures, albeit as minor components. The palladium(II) corners of 2a–5a exhibit planar chirality and are homochiral. The solution structures are rare examples of homochiral square planar palladium(II) supramolecular coordination complexes.