Flexible linkers and dinuclear metallic nodes build up an original metal–organic framework

Abstract

A Metal–Organic Framework (MOF) of formula [Cu₂(μ₃-tmds)₂(imH)₄]·H₂tmds (imH = imidazole; H₂tmds = 1,3-bis(3-carboxypropyl)tetramethyldisiloxane) 1 has been synthesized by successive additions of H₂tmds and imidazole to a solution of copper(II) acetate. The crystal structure of compound 1 has been investigated by single-crystal X-ray diffraction. It crystallizes in the C2/c monoclinic space group. The crystals are composed of an extended coordination network welcoming H₂tmds molecules. The network contains paddle-wheel dicopper(II) moieties where the two copper atoms are bridged by one of the carboxylate groups of two tmds²⁻ anion. The basal plane of their coordination sphere is complemented by two imidazole molecules whereas the apical position is occupied by one oxygen atom of the second carboxylate group of a neighboring tmds²⁻ anion leading to an overall square-pyramidal environment of the metal ions. These short and long bridging modes of the flexible tmds²⁻ dianion build up a complex bidimensional coordination network where the nodes are dinuclear clusters rather than simple metal centres. H-bonds are found within the network with imidazole as a H-bond donor and tmds²⁻ as a H-bond acceptor leading to a cis-oide conformation of the ligand. The coordination network is further H-bonded to H₂tmds molecules leading to an overall three-dimensional supramolecular network mixing coordination and hydrogen bonds. Variable-temperature magnetic susceptibility measurements reveal antiferromagnetic interactions between the copper(II) ions within the paddle-wheel dinuclear units (J = −34.4(2) cm⁻¹; H = −2JS₁S₂).