Copper(II) complexes of a hexadentate mixed-donor N3S 3 macrobicyclic cage: Facile rearrangements and interconversions

The potentially hexadentate mixed-donor cage ligand l-methyl-8-amino-3, 13, 16-trithia-6, 10, 19-triazabi-cyclo[6.6.6]eicosane (AMME-N3-S 3sar; sar = sarcophagine) displays variable coordination modes in a complex with copper(II). In the absence of coordi-nating anions, the ligand adopts a con-ventional hexadentate N3S3 binding mode in the complex [Cu(AMME-N3S3sar](ClO4)2 that is typical of cage ligands. This structure was determined by X-ray crystallography and solution spectroscopy (EPR and NIR UV/Vis). However, in the presence of bromide ions in DMSO, clean conversion to a five-coordinate bromido complex [Cu(AMME-N3S3sar)Br]+ is observed that features a novel tetradentate (N2S2)-coordinated form of the cage ligand. This copper(II) complex has also been characterized by X-ray crys-tallography and solution spectroscopy. The mechanism of the reversible inter-conversion between the six- and five coordinated copper(II) complexes has been studied and the reaction has been resolved into two steps; the rate of the first is linearly dependent on bromide ion concentration and the second is bromide independent. Electrochemis-try of both [Cu(AMME-N3S 3sar)]2+ and [Cu(AMME-N3S3sar)Br] + in DMSO shows that upon reduction to the monovalent state, they share a common five-coordinated form in which the ligand is bound to copper in a tetradentate form exclusively, regard-less of whether a six- or five-coordinat-ed copper(II) complex is the precursor. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.