Synthesis, Crystal Structures, and Electrochemical Behavior of Fe–Ru Heterobimetallic Complexes with Bridged Metallocene Units

A series of Fe–Ru complexes was prepared by reactions of (2-phenylethyl)­ferrocene (<b>1</b>), (<i>E</i>)-(2-phenylethenyl)­ferrocene (<b>2</b>), and (phenylethynyl)­ferrocene (<b>3</b>) with [Ru­(η⁵-C₅R₅)­(MeCN)₃]­[PF₆] (R = H, Me) salts. These heterobimetallic complexes of the general formula [Fc-spacer-(η⁶-C₆H₅)­Ru­(η⁵-C₅R₅)]­[PF₆] (Fc = ferrocenyl, spacer = CH₂CH₂ (<b>4</b>), CHCH (<b>5</b>), CC (<b>6</b>)) were isolated as hexafluorophosphate salts and characterized by elemental analysis, multinuclear NMR spectroscopy, and electrospray ionization mass spectrometry. The solid-state structures of the complete series of [Fc-spacer-(η⁶-C₆H₅)­Ru­(η⁵-C₅Me₅)]Cl (resulting via anion exchange upon recrystallization from a halogenated solvent) and of [FcCCRu­(η⁶-C₆H₅)­(η⁵-C₅H₅)]­[PF₆] were determined by single-crystal X-ray diffraction analysis. In addition, a η⁴-butadiene complex [Ru­(η⁵-C₅H₅)­(η⁴-1,2-Fc₂-3,4-Ph₂C₄)]­[PF₆] (<b>7</b>[PF₆]), obtained along with some unidentified alkyne oligomers and <b>6a</b>[PF₆] upon the treatment of <b>3</b> with [Ru­(η⁵-C₅H₅)­(MeCN)₃]­[PF₆], was characterized similarly, including structure determination. Cyclic voltammetry measurements performed on <b>1</b>–<b>3</b> revealed that these compounds undergo a single reversible one-electron oxidation, which can be attributed to the ferrocene/ferrocenium redox couple. Their redox potential increases with increasing electron-withdrawing nature of the ferrocenyl substituent (<i>E</i>°′: <b>1</b> < <b>2</b> < <b>3</b>). The cationic Fe–Ru complexes show similar redox waves that are shifted to more positive potential due to coordination of the positively charged Ru­(η⁵-C₅R₅) fragment and are only marginally influenced by the substitution at the Ru-bonded cyclopentadienyl ring (C₅H₅ vs C₅Me₅). Furthermore, the metal–organic Fe–Ru dyads exert an irreversible reduction event below 2 V presumably due to reduction of the Ru center. Spectroelectrochemical measurements in the UV–vis–NIR region and DFT computations confirmed the anticipated nature of the observed oxidative redox processes and further suggested electronic communication between the metal centers in compounds possessing the conjugated linking groups