Redox-Responsive Rhodocenium [O,O], [N,N] and [N,C,N]Metalloligands

Abstract

As we have recently shown, the doubly substituted [1,2-O,O]H, [1,2-O,N]H, [1,2-N,N]H, and [1,3-N,N]H pentafulvenes are convenient N/O-functionalized cyclopentadienide (Cp) precursors. Deprotonation of the pentafulvenes by potassium hydride followed by reaction with [CpRhCl2]2 or [CpIrCl2]2 gave access to the first functionalized rhodocenium and iridocenium salts that contain two acyl and/or imidoyl substituents. These air-stable compounds represent interesting bis(acyl/imidoyl) or mixed acyl/imidoyl metalloligand systems that combine an axially shielding, electrochemically active metallocene moiety with directly attached, conjugating oxygen and/or nitrogen donor sites. The structural properties of these novel metallocene metalloligands in solution and in the solid state were studied by NMR spectroscopy (1H, 13C, 103Rh) and by single-crystal structure analysis. The electrochemical investigations on complexes 5a, 6b, and 7a showed the effect of the appended functional groups on the potential and on the reversibility of the rhodocenium/rhodocene reduction and provided evidence for the dimerization that followed the reduction. Further redox processes that were due to the heteroatom functions included the stepwise reduction of the 1,2-diketo entity of 5a and the oxidation of the 1,2-diimino moiety of 7a.