title: 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 1:1 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 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 π-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 (ΔG(≠)) for the chirality switching between the thermodynamically stable but kinetically labile (P)- and (M)-stereoisomers.
authors: Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, USA.
authors: Jiyoung
authors: J
authors: Jung
authors: Junyong
authors: J
authors: Jo
authors: Moitree
authors: M
authors: Laskar
authors: Dongwhan
authors: D
authors: Lee
mesh_terms: D002942:Circular Dichroism; D056831:Coordination Complexes / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008968:Molecular Conformation; D009281:Naphthalenes / Q000737:chemistry*; D009284:Naphthols / Q000737:chemistry*; D012996:Solutions; D013237:Stereoisomerism; D013816:Thermodynamics; D015032:Zinc / Q000737:chemistry*
chemical_list: C406943:BINAP, 2-naphthol; C406944:BINOL, naphthol; D056831:Coordination Complexes; D008024:Ligands; D009281:Naphthalenes; D009284:Naphthols; D012996:Solutions; D015032:Zinc