Stereodynamics of metal-ligand assembly: What lies beneath the 'simple' spectral signatures of c2-symmetric chiral chelates

摘要

A series of C2-symmetric chiral tetra-dentate ligands were prepared by using [4,5]- or [5,6]-pinenefused 2,2'-bipyridyl units that are supported across a rigid arylene-ethynylene backbone. These conformationally pre-organised chelates support stable 1:1 metal complexes, which were fully characterised by UV/Vis, fluorescence, circular dichroism (CD), and 1H NMR spectroscopy. A careful inspection of the exciton-coupled circular dichroism (ECCD) and 1H NMR spectra of the reaction mixture in solution, however, revealed the evolution and decay of intermediate species en route to the final 1:1 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 p-conjugated ligand skeleton that dramatically enhances the thermodynamic stability of the 1:1 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 (DG) for the chirality switching between the thermodynamically stable but kinetically labile (P)- and (M)-stereoisomers.