Two chiral CoIII and NiII complexes with identical ligands presenting different nonlinear optical responses

Abstract

Using NaN₃ and a chiral N-donor ligand (1R,2R)-1,2-diphenylethylenediamine (L) to react with Co(ClO₄)₂(H₂O)₆ and Ni(ClO₄)₂(H₂O)₆, respectively, two chiral Co^III and Ni^II complexes with formulae [Co(L)₂(N₃)₂](ClO₄)(0.5H₂O) (1) and [Ni(L)₂(μ_1,3-N₃)]_n(ClO₄)_n(0.5H₂O)_n (2) were isolated and structurally characterized. Although 1 and 2 contain the same L and N₃⁻ ligands, they have completely different molecular structures. 1 shows a chiral mononuclear structure with N₃⁻ as the terminal ligand, in which the Co^II ion has been oxidized to the Co^III ion, while 2 exhibits a chiral one-dimensional (1D) Ni^II chain structure with N₃⁻ as the bridging ligand. Notably, a study of their nonlinear optical (NLO) properties demonstrates that 1 presents a moderate second-harmonic generation (SHG) response (0.15 × KDP), while 2 only displays a strong third-harmonic generation (THG) response (160 × α-SiO₂). This interesting finding gives the first example of the complexes with the same ligands but different molecular structures showing completely different NLO responses regulated by metal ions.