Metal-organic frameworks from chiral square-pyramidal copper(II) complexes: Enantiospecific inclusion and perfectly polar alignment of guest and host molecules

The physical properties of [CuL12(H2O)] (1) and [CuL22(H2O)] (2) and preparation and crystal structures of the inclusion compounds 1·(P)-C2H4Br2, 2·(M)-C2H4Br2, 1·CH3CN and 2·CH3CN are described. HL1 and HL2 (H represents the dissociable phenolic proton) are the N,O-donor chiral reduced Schiff bases N-(2-hydroxy-5-nitrobenzyl)-(R)-α-methyl-benzylamine and N-(2-hydroxy-5-nitrobenzyl)-(S)-α-methylbenzylamine, respectively. All the compounds crystallize in the non-centrosymmetric space group C2. In the crystal lattice, the host [CuLn2(H2O)] (1 and 2) molecules connected by O−H⋯O and C−H⋯O interactions form perfectly polar two-dimensional networks. In these chiral and polar host frameworks, enantiospecific inclusion with polar ordering of the right-handed (P) and the left-handed (M) gauche form of 1,2-dibromoethane as well as polar alignment of acetonitrile molecules are observed. The host and guest molecules are linked by C−H⋯O interactions. The O-atoms of the nitro substituent on the ligands of 1 and 2 act as the acceptors in all these intermolecular O−H⋯O and C−H⋯O interactions. The structures reported in this work provide rare examples of enantiospecific trapping of the chiral rotamers of 1,2-dibromoethane as well as perfectly polar alignment of both guest and host molecules.

The square-pyramidal Cu(II) complexes [CuLn2(H2O)] with the bidentate HLn (HL1=N-(2-hydroxy-5-nitrobenzyl)-(R)-α-methyl-benzylamine and HL2=N-(2-hydroxy-5-nitrobenzyl)-(S)-α-methylbenzylamine) form 1:1 host−guest compounds with Br(CH2)2Br and CH3CN. The X-ray structures of these species reveal the enantiospecific confinement of the chiral rotamers of Br(CH2)2Br and perfectly polar ordering of both host and guest molecules in the crystal lattice. The figure shows the polar alignments of (a) [CuL12(H2O)]·(P)-C2H4Br2 and (b) [CuL22(H2O)]·CH3CN.Figure optionsDownload as PowerPoint slide