A bipyridine-ligated zinc(II) complex with bridging flavonolate ligation: synthesis, characterization, and visible-light-induced CO release reactivity

摘要

Metal-flavonolate compounds are of significant current interest as synthetic models for quercetinase enzymes and as bioactive compounds of importance to human health. Zinc-3-hydroxyflavonolate compounds, including those of quercetin, kampferol, and morin, generally exhibit bidentate coordination to a single Zn~II center. The bipyridine-ligated zinc-flavonolate compound reported herein, namely bis(mu-4-oxo-2-phenyl-4H-chromen-3-olato)-κ~3O~3:O~3,O~4;κ~3O~3,O~4:O~3-bis[(2,2'-bipyridine-κ~2N,N')zinc(II)]bis(perchlorate),{[Zn_2(C_15H_9O_3)_2(C_10H_8N_2)_2]-(ClO_4)_2}_n, (1), provides an unusual example of bridging 3-hydroxyflavonolate ligation in a dinuclear metal complex. The symmetry-related Zn~II centers of (1) exhibit a distorted octahedral geometry, with weak coordination of a perchlorate anion trans to the bridging deprotonated O atom of the flavonolate ligand. Variable-concentration conductivity measurements provide evidence that, when (1) is dissolved in CH_3CN, the complex dissociates into monomers. ~1H NMR resonances for (1) dissolved in d_6-DMSO were assigned via HMQC to the H atoms of the flavonolate and bipyridine ligands. In CH_3CN, (1) undergoes quantitative visible-light-induced CO release with a quantum yield [0.004 (1)] similar to that exhibited by other mononuclear zinc-3-hydroxyflavonolate complexes. Mass spectroscopic identification of the [(bpy)_2Zn(O-benzoyl-salicylate)]+ ion provides evidence of CO release from the flavonol and of ligand exchange at the Zn~II center.