Structural study of Ga(III), In(III), and Fe(III) complexes of triaza-macrocycle based ligands with N3S3 donor set

Two new ligands, 1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclodecane (L2) and 9-methyl-1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclodecane (L3), have been synthesized by reaction of the free triazacycloalkanes with ethylene sulfide. Complexes of L2 with Ga(III), In(III), and Fe(III), as well as Ga(III) and In(III) complexes of L3, have been characterized by single crystal X-ray diffraction. In all cases, the metal ions are coordinated in a trigonal-antiprismatic environment with varying degrees of distortion. L2 complexes are isostructural and exhibit the chair-Delta(lambda lambda)(delta delta delta) conformation, whereas GaL3 and InL3 adopt twist-boat-Delta(lambda lambda)(delta delta delta) and twist-boat-Delta(lambda lambda)(lambda delta delta) configurational isomers and their enantiomers, respectively. In addition, we report the crystal structure of the Fe(III) complex of the known ligand 1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclononane (TS-TACN, L1), which is isostructural to its Ga(III) and In(III) complexes. All aforementioned complexes are compared (including literature data of Ga(TS-TACN) and In(TS-TACN)), and the influence of ring size and backbone substitution on coordination geometries is discussed. Furthermore, the solution structures of Ga(III) and In(III) complexes of the new ligands have been investigated employing NMR spectroscopy and density functional theory (DFT) calculation. The latter revealed that for all compounds, the most stable structures are those where the six-membered chelate ring adopts chair conformation. NMR measurements confirmed this proposal for the L2 complexes. In contrast, the GaL3 complex is present as a mixture of chair and twist-boat isomers. This implies that for this complex, interconversion between both forms is occurring, which necessarily includes intermediate partial decoordination of the metal ion. InL3 occurs as a single species, but an unambiguous determination of its conformation was not possible.