Synthesis and Some Properties of Bis(ruthenocenyl)thiophene Derivatives – Possible Spin‐Coupling in the Two‐Electron Oxidized Species of Dinuclear Ruthenocenes Bridged by Thiophene Derivatives

Abstract

The binuclear ruthenocene derivatives bridged by thiophene, 3,4-ethylenedioxythiophene, 2,2Ј-bithiophene, thieno[3,2-b]thiophene, or 3,6-dimethylthieno[3,2-b]thiophene were prepared by the Suzuki coupling of 2-ruthenocenyl-4,5-tetramethyl-1,3-dioxaborolane with the corresponding diiodo compounds. In addition, thiophene-and bithiophenebridged binuclear pentamethylruthenocenes were prepared by the reaction of bis(pentamethylruthenocenyl)diyne and -tetrayne with NaSH, respectively. The cyclic voltammograms of these complexes exhibit one-step two-electron redox waves in the lower potential region (0-0.3 V versus FcH/ FcH + ). The two-electron oxidized species of the thiophene-

Synthesis and some properties of binuclear ruthenocene derivatives bridged by both ethene and thiophene derivatives

“…A similar trend is also observed in the binuclear ruthenocene bridged by thiophene derivatives [10]. However, this tendency is different from that observed in the Rc * (CC) n Rc * series [8], in which the oxidation potentials shift to higher potential region with the increase of conjugation in the linker.…”

“…As shown in Table 1, the n value in the thin-layer coulometry was approximately 2 and the peak current parameter I pa was about 2000, suggesting that the first oxidation wave near 0.15 V in 5, 8, and 10 and near 0.0 V in 13-15 can be assigned to the one-step two-electron process that corresponds to the Ru(II)/Ru(III) couple at the two ruthenocenyl sites. Along with these waves, their cyclic voltammograms all are attended by the irreversible oxidation wave near 0.5 V. Such behavior also resembles with those of dinuclear ruthenocene derivatives bridged by only thiophene derivatives [10]. The magnitude of the wave near 0.5 V was nearly equal to that of the wave near 0.15 V in 5, 8, and 10 and near 0.0 V in 13-15.…”

“…The Wittig reaction of 7 with 4 in the presence of LDA gave 5-(2 0 -ruthenocenyl-E-ethenyl)-2-ruthenocenylthieno[3,2-b]thiophene (8) in 47% yield. Similarly, the reaction of 5 0 -formyl-5-iodo-2,2 0 -bithiophene with the dioxaborolane 1 gave 5 0 -formyl-5-ruthenocenyl-2,2 0 -bithiophene (9), the Wittig reaction of which led to 5 0 -(2 00 -ruthenocenyl-E-ethenyl)-5-ruthenocenyl-2,2 0 -bithiophene (10) in 62% yield, along with a trace amount of its Z-isomer. The Wittig reaction of 4-ruthenocenylbenzaldehyde, which was prepared from the Suzuki reaction between 4-bromobenzaldehyde and the dioxaborolane 1, with the phosphonium salt 4 and LDA led to 4-(2 0 -ruthenocenyl-E-ethenyl)-1-ruthenocenylbenzene (11) in moderate yield.…”

“…These observations suggest that the second oxidation wave of these complexes may be assigned to the two-electron oxidation process corresponding to the Ru(III)/Ru(IV) couple in the two ruthenocenyl sites, because mononuclear ruthenocene shows irreversible oxidation wave of a Ru(II)/Ru(IV) couple at 0.55 V. The observation of the first oxidation wave at low-potential region suggests that the two-electron oxidized species of complexes 5, 8, and 10, and 13-15 may be stable. It is noteworthy that the oxidation wave for 5 (+ 0.12 V) is observed in a lower potential region than that of 2, 5-bis(ruthenocenyl)thiophene (0.24 V) [10]. This may be due to the incorporation of the ethene part in the bridge of 5, because the oxidation wave in bis(ruthenocenyl)ethene appeared at +0.03 V [7].…”

“…It has been found that the binuclear ruthenocne derivatives bridged by ethenes [7] or ethynes [8] are subject to reversible one-step two-electron redox process and the spin-coupling of the singlet di-radical cations in the two-electron oxidized species leads to facile structural isomerization. Among aromatic linkers, benzene or naphthalene derivatives were ineffective for the spin-coupling interaction between the two ruthenocenyl sites in the two-electron oxidized species of binuclear ruthenocenes [9], but thiophene derivatives were much effective candidates [10]. Recently, a good reversibility of the ruthenocene oxidation in the analogous complex was reported [11].…”