Di- and tri-valent metal complexes with tris-amide-functionalised 1,4,7-triazacyclononane chelators

Abstract

The reactions of a series of divalent 3d metal ions (Co, Ni, Cu, Zn) with two tris-amide functionalised tacn ligands, {PhNHC(O)CH₂}₃-tacn (1) and {ⁱPrNHC(O)CH₂CH₂}₃-tacn (2), in alcohol solution are described. The resulting complexes, [M(1)](NO₃)₂ and [M(2)](NO₃)₂ are characterised by elemental analysis, mass spectrometry, IR, UV-vis, ¹H and ¹³C{¹H} NMR spectroscopy, as appropriate, and by single crystal X-ray analysis for four representative examples. In all cases the ligands behave as hexadentate chelators to the divalent metal ion, with N₃O₃ donor sets through the tacn N-donor atoms and the O-bound carboxamide pendant arms. However, the reaction of 1 with Co(NO₃)₂·6H₂O produces the Co(III) complex, [Co(1-H)](NO₃)₂, via air oxidation. The X-ray crystal structure of this complex confirms a distorted octahedral N₄O₂ coordination environment at cobalt(III) through the three facial tacn amine groups, the anionic N atom from one deprotonated amide group, and two O-bound carboxamides. In comparison, the coordination of 1 towards the trivalent group 13 nitrates, M(NO₃)·9H₂O (M = Ga and In) at room temperature in MeOH yields the distorted octahedral [M(1)](NO₃)₃ salts initially (from NMR and IR spectroscopy and elemental analysis data) as colourless solids. However, they are less stable than the divalent complexes, undergoing slow amide hydrolysis in MeOH at room temperature over several hours, or more rapidly with heating. This process occurs more readily with Ga(III) than with the less Lewis acidic In(III) analogue. The crystal structure of one hydrolysis product, [Ga(3)](NO₃), is also reported (3 = {PhNHC(O)CH₂-tacn-(CH₂CO₂)₂}²⁻), in which two amide arms from 1 are converted to carboxylates.