Cu(i) and Pb(ii) complexes containing new tris(7-naphthyridyl)methane derivatives: Synthesis, structures, spectroscopy and geometric conversion

Abstract

Two novel facial-capping tris-naphthyridyl compounds, 2-chloro-5-methyl-7-((2,4-dimethyl-1,8-naphthyridin-7(1H)-ylidene)(2,4-dimethyl-1,8-naphthyridin-7-yl))methyl-1,8-naphthyridine (L¹) and 2-chloro-7-((2-methyl-1,8-naphthyridin-7(1H)-ylidene)(2-methyl-1,8-naphthyridin-7-yl))methyl-1,8-naphthyridine (L²), as well as their Cu(I) and Pb(II) complexes, [CuL^a(PPh₃)]BF₄ (1) (PPh₃ = triphenylphosphine, L^a = bis(2,4-dimethyl-1,8-naphthyridin-7-yl)(2-chloro-5-methyl-1,8-naphthyridin-7-yl)methane), [CuL^b(PPh₃)]BF₄ (2) (L^b = bis(2-methyl-1,8-naphthyridin-7-yl)(2-chloro-1,8-naphthyridin-7-yl)methane), [Pb(OL^a)(NO₃)₂] (3) (OL^a = bis(2,4-dimethyl-1,8-naphthyridin-7-yl)(2-chloro-5-methyl-1,8-naphthyridin-7-yl)methanol) and [Pb(L^b)₂][Pb(CH₃OH)(NO₃)₄] (4), have been synthesized and characterized by X-ray diffraction analysis, MS, NMR and elemental analysis. The structural investigations revealed that the transfer of the H-atom at the central carbon to an adjacent naphthyridine-N atom affords L¹ and L² possessing large conjugated architectures, and the central carbon atoms adopt the sp² hybridized bonding mode. The reversible hydrogen transfer and a geometric configuration conversion from sp² to sp³ of the central carbon atom were observed when Pb(II) and Cu(I) were coordinated to L¹ or L². The molecular energy changes accompanying the hydrogen migration and titration of H⁺ to different receptor-N at L¹ were calculated by density functional theory (DFT) at the SCRF-B3LYP/6-311++G(d,p) level in a CH₂Cl₂ solution, and the observed lowest-energy absorption and emission for L¹ and L² can be tentatively assigned to an intramolecular charge transfer (ICT) transition in nature.