Coordination chemistry of a mono-dibenzofuran derivative of 1,4,7,10-tetraazacyclododecane

Abstract

Copper(II) and cadmium(II) complex of a mono-dibenzofuran derivative of 1,4,7,10-tetraazacyclododecane (cyclen), L1, have been prepared by reaction of the mono-dibenzofuran-substituted cyclen with either copper(II) or cadmium(II) perchlorate in acetonitrile. This yielded the corresponding divalent metal complexes, [Cu(L1)](ClO 4 ) 2 ·H 2 O (C1) and [Cd(L1)](ClO 4 ) 2 ·H 2 O (C2), which were isolated as single crystals suitable for X-ray crystallography by diffusing diethyl ether into an acetonitrile solution of each complex. For the copper(II) complex, C1, the X-ray crystal structure revealed a distorted square pyramidal Cu(II) coordination sphere occupied by four amine nitrogens from the cyclen ring and one oxygen from the amide linkage present in L1. On the other hand, the metal center in the cadmium complex is seven coordinate, with two weakly bound acetonitrile molecules occupying two additional coordination sites about the Cd(II) center; these bind cis to the amide oxygen. The coordination geometry is best described as monocapped trigonal prismatic. In both complexes, the secondary amine nitrogens on the cyclen macrocycle form hydrogen bonds with perchlorate counterions present in the crystal lattice. Titration of L1 with various metal ions (Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , and Cd 2+ ) in acetonitrile revealed a gradual quenching of the benzofuran emission with increasing [M 2+ ] consistent with formation of metal complexes with a 1 : 1 M 2+ : L1 stoichiometry. From the emission data, the conditional stability constant was determined to be 10-20 times higher for the Cu(II) complex than the other divalent metal complexes investigated.

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“…However, the data also demonstrates the “vulnerability” of the Cu­(II)-cyclen system in vivo , since (i) the natural amino acids along with other substances containing primary amines and (ii) endogenous metal chelators such as ATP and glutamate would all have the potential to bind and inactivate the Cu­(II)-cyclen moiety. In fact, to some extent, these findings are potentially relevant to any metallodrug that has not lived up to its promise in vivo , since by definition the metal center must leave coordination sites available for its interaction with the biological target, , which also makes it vulnerable toward unwanted ligand exchange reactions . To address this issue for the Cu­(II)-cyclen system, it would be interesting to explore the potential of a dynamic, intramolecular cap in the so-called “second coordination sphere” .…”

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“…Pyrene, benzofuran, and benzothiazole chromophores can also be used to build other tacn- and cyclen-based ligands capable of the fluorescence-based sensing of selected analytes (representative examples from our own work are shown in Figure ). − Notably, Zn­(II) 2 – 67 , a complex incorporating one Fc unit and two pyrene units, is a good example of a receptor for biological phosphate anions. Dual-mode electrochemical and fluorescence-based detection of polyphosphate anions, such as PPi, ATP and ADP, was achieved with this receptor .…”