Stereodynamics of metal-ligand assembly: what lies beneath the "simple" spectral signatures of C2-symmetric chiral chelates.

ABSTRACT

A series of C2-symmetric chiral tetra-dentate ligands were prepared by using [4,5]- or [5,6]-pinene-fused 2,2'-bipyridyl units that are supported across a rigid arylene-ethynylene backbone. These conformationally pre-organised chelates support stable 11 metal complexes, which were fully characterised by UV/Vis, fluorescence, circular dichroism (CD), and (1)H NMR spectroscopy. A careful inspection of the exciton-coupled circular dichroism (ECCD) and (1)H NMR spectra of the reaction mixture in solution, however, revealed the evolution and decay of intermediate species en route to the final 11 metal-ligand adduct. Consistent with this model, mass spectrometric analysis revealed the presence of multiple metal complexes in solution at high ligand-to-metal ratios, which were essentially unobservable by UV/Vis or fluorescence spectroscopic techniques. Comparative studies with a bi-dentate model system have fully established the functional role of the π-conjugated ligand skeleton that dramatically enhances the thermodynamic stability of the 11 complex. In addition to serving as a useful spectroscopic handle to understand the otherwise "invisible" solution dynamics of this metal-ligand assembly process, temperature-dependent changes in the proton resonances associated with the chiral ligands allowed us to determine the activation barrier (ΔG(≠)) for the chirality switching between the thermodynamically stable but kinetically labile (P)- and (M)-stereoisomers.