Bis(morpholine) hydrogen bond pincer – a novel series of heteroleptic Cu(ii) coordination compounds as receptors for electron rich guests

Abstract

Crystallisation from morpholine (morph) solutions of copper(II) salts with monovalent anions (A) and 1,3-diketones (Hdkt) yielded nine heteroleptic coordination compounds [Cu(dkt)(morph)₂A]. The coordination polyhedron of the copper ion in these compounds is a square pyramid with a monovalent anion (A) at the apex and a diketonate as well as two N-bonded morpholine ligands forming the base of the pyramid. Through such coordination, the morpholine amine N–H groups are brought into a relative position that enables them to simultaneously form N–H⋯B hydrogen bonds with an electron rich guest B. The combination of the ligands, in conjunction with the coordination preference of the central ion, has thus led to the formation of a bis(morpholine) hydrogen bond pincer, capable of binding electron rich guests into the second coordination sphere of the copper ion. The bonding of guests into this receptor site in the solid state was studied by single crystal X-ray diffraction as well as FT-IR spectroscopy. In six compounds the pincer binds apical ligands of neighbouring coordination molecules leading to aggregation of molecules into supramolecular chains. In two cases the guest bonded in the pincer was found to be an uncoordinated chloride ion, and in one a water molecule. The bonding into the pincer was also found to have a significant effect on the IR spectra of the compounds, and may lead to shifting, broadening, and, in case of asymmetrical bonding of the guest, splitting of the bands corresponding to the stretching of morpholine N–H bonds.