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 (1), (E)-(2-phenylethenyl)­ferrocene
(2), and (phenylethynyl)­ferrocene (3) 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 (4), CHCH (5), CC (6)) 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] (7[PF6]), obtained along with some unidentified
alkyne oligomers and 6a[PF6] upon the treatment
of 3 with [Ru­(η5-C5H5)­(MeCN)3]­[PF6], was characterized similarly,
including structure determination. Cyclic voltammetry measurements
performed on 1–3 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 (E°′: 1 < 2 < 3). 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.