[<i>closo</i>-B₁₀H₈-1-CN-10-Azinium]⁻ Anions: Photoactive Heteroditopic Ligands for Metal Complexes

A new class of rigid, photoactive heteroditopic anionic ligands based on the 1,10-disubstituted [<i>closo</i>-B₁₀H₁₀]^2- anion was designed and six of these compounds were obtained from [<i>closo</i>-B₁₀H₁₀]^2- in three steps with yields in the range of 25-30%. The design includes two apical substituents, a metal coordinating cyano group and an azinium (4-cyanopyridinium, 4,4'-bipyridinium, pyrazinium, pyrimidinium, and pyridazinium), which provides a secondary binding site. The azinium substituent is involved in an efficient intramolecular charge transfer process and compensates one of the two delocalized negative charges of the boron cluster. The compounds exhibit intramolecular CT bands with maxima in the range of 340-410 nm (MeCN) and two are weakly fluorescent with significant Stokes shifts. A highly colored bis-zwitterionic byproduct with two clusters, [<i>closo</i>-1-CN-B₁₀H₈-10-(pyrazinium-1,4-diyl)-10'-B₁₀H₈-1'-CN-<i>closo</i>]^2-, was also obtained in a low yield. The ligands were structurally characterized (XRD) and their geometrical and photophysical properties were compared to those of the analogues lacking the CN group and the parent pyridinium derivative. A comparative analysis of experimental data was augmented with DFT and TD-DFT results. Two of the ligands (with the 4,4'-bipyridinium and pyrazinium) were converted to (η⁵-Cp)(dppe)Fe complexes and one of them was used to obtain a heterodinuclear complex with Cu(pdc)(H₂O)₃ to demonstrate the ditopic function of the ligand. All three iron complexes were characterized by the XRD method.