Coordination chemistry of Group 13, Technetium and Rhenium metal complexes with Hexadentate Amine Phenol Ligands

A series of neutral and monocationic group 13 metal complexes with hexadentate tripodal, linear and pyridine amine phenolates were prepared and characterized by elemental analyses and various spectroscopic techniques (IR, UV, LSIMS, NMR). X-ray crystallographic structural analyses of selected Al³⁺, Ga³⁺ and In³⁺ complexes were also performed. It was observed that the coordination behavior of the amine phenolates and the stability of the resulting metal complexes were closely related to the flexibility of the ligand, the type and spatial organization of the donor atoms, as well as to the electronic and steric demands of the coordinated metal ion. Coordination of tripodal N₃O₃ tris(aminomethyl)ethane based amine phenolates (XTAM³⁻) to Al³⁺, Ga³⁺ and In³⁺ resulted in neutral mono-ligand metal complexes. The N₃O₃ donor set was suitable for coordination to all three group 13 metal ions. As the size of the metal ion increased from Al³⁺ to In³⁺, XTAM³⁻ adjusted to the steric demands of the coordinated metal in a subtle fashion, such that no significant identifiable bond angle change was observed. This flexibility allowed it to coordinate to various metals of differing sizes. Paramagnetic Tc(III), Tc(IV) and Re(IV) complexes with XTAM³⁻ were prepared via the substitution and reduction/complexation methods. The coordination of XTAM³⁻ to Tc and Re demonstrated the potential use of the hexadentate amine phenolates as ligands for coordination to Tc and Re in intermediate oxidation states. Monocationic Ga³⁺ and In³⁺ complexes with linear amine phenolates (Xbad²⁻ and Xbadd²⁻) based on triethylenetetraamine and N,N’-(³⁻aminopropyl)ethylenediamine were prepared; no analytically pure, Al³⁺ complex could be isolated. The N₄O₂ donor set of the linear amine phenolates was softer than the N₃O₃ set of XTAM³⁻ and hence was less suitable for coordination to the harder Al³⁺ ion. Structural analyses of [Ga(Brbad)]ClO₄ and [Ga(Brbadd)] ClO₄, and NMR spectral data revealed the mode of coordination of the Xbad²⁻ ligand to be different from that of the longer backbone Xbadd²⁻. Increasing the length of the amine phenolate altered the mode of coordination. The introduction of pyridine nitrogen atoms into the amine phenol framework reduced the overall hardness of the donor atom set. Monocationic Ga and In complexes with N₂N₂O₂ pyridine amine phenolates (Xbbpen²⁻) were prepared with ease, while no Al complex could be isolated. Only minor differences between the structures of the Ga and In complexes were observed, suggesting that the cavity of Xbbpen²⁻ was large enough to accommodate ions as large as In³⁺ without causing significant steric strain to be imposed onto its framework. Stability constant study of Ga and In complexes with sulfonated linear amine phenolates (Sbad⁴⁻ and Sbadd⁴⁻) showed the Ga complexes to be ~3 orders of magnitude more stable than the corresponding In complexes. The selectivity of the linear amine phenolates for Ga³⁺ over In³⁺ was consistent with the fact that, while the affinity of amine for Ga³⁺ and In³⁺ was similar, 2-oxybenzyl groups definitely has an affinity for Ga³⁺ over In³⁺. In contrast, the stability constants of Ga and In complexes with sulfonated pyridine amine phenolate (Sbbpen⁴⁻ ) were very similar to each other. The introduction of the pyridine nitrogen atoms into the amine phenolate donor set muted the selectivity of the amine phenol for Ga³⁺ over In³⁺. The stability constants of the Ga and In complexes with Sbbpen⁴⁻ were 7-10 orders of magnitude higher than those of the linear amine phenolate complexes. This was due to the donor atoms in Xbbpen²⁻ being more pre-arranged for coordination than those of linear amine phenolates.Science, Faculty ofChemistry, Department ofGraduat