(Allenylidene)ruthenium Complexes with Redox‐Active Substituents and Ligands

Abstract

AbstractWe describe the allenylidene complexes [TpL2Ru=C=C=CPhR]+ SbF6− [Tp = HB(pz)3−, L2 = 2 PPh3 or 1,1′‐bis(diphenylphosphanyl)ferrocene (dppf), R = Ph or ferrocenyl] and their spectroscopic and electrochemical characteristics. Three of these compounds possess redox‐active, ferrocene‐based substituents or ligands − that are oxidized at lower potentials than the ruthenium center itself − attached either to the terminal carbon atom of the allenylidene ligand or to the ruthenium atom. The Fc/Ph‐substituted complexes 3a and 3b provide unique examples of hindered rotation of the allenylidene substituent around the M=C bond. For 3a (L2 = 2 PPh3), two isomers differing in the orientation of the vertically aligned, unsymmetrically substituted allenylidene ligand are discernible even at 388 K. The dppf‐substituted congener 3b has the allenylidene ligand in a horizontal orientation and exhibits a rotational barrier, as determined by dynamic 31P NMR spectroscopy, of ΔG≠ = 47 kJ/mol at TC = 238 K. The changes in the spectroscopic and electrochemical properties upon replacement of the PPh3 by a dppf ligand and the Ph by an Fc moiety can be explained in terms of the bonding within these systems. These effects are attenuated when the ferrocene‐based redox tags are oxidized, as shown by IR and UV/Vis spectroelectrochemistry. Thus, infrared spectroelectrochemistry reveals a blue shift of the allenylidene stretch upon oxidation of the dppf ligand, while oxidation of the ferrocene covalently linked to the unsaturated C3 ligand has the opposite effect. Oxidation of the ruthenium atom influences the bonding within the unsaturated ligand more profoundly. Results from IR spectroelectrochemistry point to a vinylidene structure in the RuIII state. Reduction enhances the contribution of alkynyl‐type resonance forms to the overall bonding description, as also shown by IR spectroelectrochemistry. For the ferrocenyl‐substituted allenylidene complexes, oxidation and reduction result in bleaching of the intense optical low‐energy absorption band attributed to a ferrocenyl‐to‐allenylidene charge‐transfer process. The EPR spectra of the paramagnetic reduced forms of these complexes also indicate spin delocalization into the aryl substituents attached to the allenylidene ligand. The complexes Tp(dppf)RuCl and [Tp(dppf)Ru=C=C=CPh2]+ SbF6− were also characterized by X‐ray crystallography. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)