Cross-π-conjugated enediyne with multitopic metal binding sites

Abstract

The synthesis of an enediyne molecule functionlized with different metal coordination sites in a cross-π-conjugated fashion is reported. Using Pd-mediated cross-coupling reactions, 2,2'-bipyridine units were attached at the periphery of diazafluorenemethylidene to obtain a multitopic ligand. UV-vis spectrosopic investigations along with electrochemical analyses reveal electronic communication along the conjugated path reflected in red-shifted absorption spectra and shifts of reduction potentials. The properties of the ligand could be manipulated by coordinating [Ru(bpy)₂]²⁺ fragments at all three coordination spheres of the molecule while the different complexing imine moieties serve as possible coordination sites for various metal centres.

Fig. 1. Illustration of a cross-π-conjugated ligand based on diazafluorene core, decorated with 2,2′-bipyridine complexing units at the periphery.

Scheme 1. Synthetic sequence of 5-(ethynyl)-2,2′-bipyridine 5.

Scheme 2. Synthesis of ligand 7via Sonogashira–Hagihara coupling reaction.

Scheme 3. Synthesis of triply complexed ligand 7.

Fig. 2. UV-vis absorption spectra of ligand 7 and the linear conjugated counterpart 9 in CH₂Cl₂ at r.t.

Fig. 3. UV-vis absorption spectra of ligand 7 (magnified) and the corresponding Ru complex 8, measured in CH₂Cl₂ at r.t.

Fig. 4. Cyclic voltammogram of 7, performed in CH₂Cl₂ solution at room temperature containing 0.1 M ⁿBu₄NPF₆ with Ag/AgCl as reference electrode, Pt wire as counter electrode and Pt disk as working electrode; scan rate: 150 mV s⁻¹; all data referenced against external Fc⁺/Fc.

Fig. 5. Cyclic and differential pulse voltammogram of 8, performed in CH₂Cl₂ solution at room temperature containing 0.1 M ⁿBu₄NPF₆ with Ag/AgCl as reference electrode, Pt wire as counter electrode and glassy carbon as working electrode; scan rate: 100 (CV)/50 (DPV) mV s⁻¹; all data referenced against external Fc⁺/Fc.