Chiral 1,8-naphthyridine based ligands: Syntheses and characterization of Di- and tetranuclear copper (I) and silver (I) complexes

Oxazoline and camphor-pyrazole units are introduced on the 1,8-naphthyridine scaffold to access chiral ligands L-1, L-2 and L-3. Metalation of these chiral ligands with Cu(I) and Ag(I) precursors afforded di- and tetranuclear complexes [Cu4I4(L-1)(2)] (1), [Cu4I4(L-2)(2)] (2), [Cu2I2(L-3)] (3), [Cu2I(L-2)(2)](OTf) (4), [Ag-2(L-1)(2)](OTf)(2) (5) and [Ag-4(L-2)(4)Br](OTf)(3) (6), containing [M4Xn] (n= 1,4 and X = Br, I) or [M2Xn] (n = 0, 1, 2 and X = I) core. All complexes are structurally characterized. Naphthyridine-derived ligands reveal bridge-chelate coordination motif and hold two metal centers in close proximity. The tetranuclear complexes are dimer of dinuclear complexes bridged by the halides. Electronic absorption and emission spectra of copper complexes are reported. Catalytic utility of all complexes are examined for asymmetric transformations but they showed poor activity probably due to limited solubility and coordinative saturation at the metal centers. The best results are obtained with [L-3/Cu salt] combination for cyclopropanation of styrene, N-H bond insertion and nitroaldol (Henry) reactions with very low enantioselectivity.