Ligand assembly and metal ion complexation: synthesis and X-ray structure of Ni(II) and Cu(II) benzoate and 4-tert-butylbenzoate complexes of cyclam

Abstract

The syntheses and X-ray structures of trans-[Ni(O-benzoato)2(cyclam)], trans-[Cu(H2O)2(cyclam)]-(benzoate)2·2H2O and trans-[Cu(H2O)2(cyclam)](4-t-butyl-benzoate)2 (where cyclam is 1,4,8,11-tetraazacyclotetradecane) are described. The nickel complex has a tetragonally distorted octahedral coordination geometry with cyclam occupying the equatorial plane in a stable trans-III arrangement, with benzoate ligands filling the axial positions. The structure closely resembles the arrangement in the previously described hydrogen-bonded (metal-free) ligand assembly [(cyclamH2)·(4-t-butyl benzoate)2]·2(benzoic acid) if the latter benzoic acid moieties, which do not interact directly with the cyclam moiety, are ignored. The 2 : 1 fragment thus may be viewed as a ligand assembly “preorganized” for nickel ion complexation. In contrast to the nickel structure, the two trans-axial sites in both copper structures are occupied by aqua rather than O-carboxylate ligands. Although they do not form part of the inner coordination sphere in either complex, the two carboxylate anions remain hydrogen bound to the (coordinated) cyclam ligand, with the mode of packing being generally similar in each complex.