One-Electron Oxidation of Ruthenocene
osmocene analogues,4 in spite of there being some possible
advantages with the heavier metals, including greater thermal
stability and stronger cyclopentadienyl-to-metal bonding.5
Reasoning that this disparity may be traced to the incom-
pletely understood redox chemistry of ruthenocene and
osmocene, we investigated the anodic behavior of RuCp2
(Cp ) η5-C5H5), 1, under gentle medium conditions, and
this paper offers a detailed view of the electrochemical
production and chemical fate of the ruthenocenium ion when
generated in the presence of a low-donor solvent and a
weakly coordinating anion (WCA).
2[RuCp2]+ h RuCp2 + [RuCp2]2+
(2)
the results of Gale and Job, who found that the oxidation
was a partially reversible one-electron process in molten
salts,8a or the fact that the decamethylruthenocenium ion does
not disproportionate.11 The disclosure by Hill et al.12 that
the oxidation of ruthenocene was a simple one-electron
processes in electrolyte solutions containing the WCA
[B(C6H3(CF3)2)4]-, BArF24, suggested that the traditional
anions were, in fact, responsible for promoting the two-
electron ruthenocene oxidation process by anion attack at
the strongly electrophilic Ru(III) center. However, the
voltammetric data in ref 12 did not address the long-term
(i.e., synthetic or electrolytic) fate of the ruthenocenium ion
(1+). Ruthenocene initially became of interest to us when
we fortuitously noticed a diminution of the chemical revers-
ibility of its anodic oxidation at reduced temperatures in
CH2Cl2 containing [NBu4][TFAB] (TFAB ) [B(C6F5)4]-)
as supporting electrolyte, an effect which was amplified at
increased concentrations of 1. We hypothesized that the
voltammetry was indicative of the generation of the bis(ru-
thenocenium) dication [Ru2Cp4]2+, 22+, and showed that this
Ru-Ru bonded dimer was isolable as the TFAB salt by low-
temperature (243 K) electrolysis.13 It was also reported at
that time that other products were produced in the electroly-
sis, becoming, in fact, the major products at ambient
temperature. In the present paper we identify those elec-
trolysis products and offer a coherent explanation of the
complex oxidative behavior of ruthenocene.
The present study is also relevant to literature on the
oxidation of osmocene. Although Hill et al.12 showed that it
also followed one-electron stoichiometry with formation of
[OsCp2]+ in CH2Cl2/[NBu4][BArF24], dramatically different
osmocene oxidation chemistry has been reported by Droege
et al.14 under other conditions. When oxidized by ceric
ammonium nitrate in hot acetonitrile, osmocene gave the
crystallographically characterized Os-Os bonded dimer
[Os2Cp4]2+ (dOs-Os ) 3.038 Å), which served as the model
for the Ru-Ru bonded dimer 22+ reported earlier.13 Warming
the osmocenium dimer in nitromethane resulted in the formal
loss of dihydrogen to give the dinuclear complex [Os2Cp2(σ:
η5-C5H4)2]2+, 42+, in which each Os atom is both π-bonded
and σ-bonded to a C5H4 group. As will be shown below, an
analogous dinuclear Ru complex, 32+, was observed at
temperatures higher than 289 K which reverts to 22+ in a
thermally reversible process at substantially colder temper-
atures.
In the same historic paper6 by Page and Wilkinson that
described the electrochemistry of ferrocene, the anodic
oxidation of ruthenocene was reported to be a reversible one-
electron process at a mercury electrode. However, it was later
shown that the isolated product, rather than being the
ruthenocenium ion, was the Hg(I) adduct, Hg[(RuCp2)2]+,
formed by fast coupling of the initially produced Ru(III)
species with the mercury being used as the anode.7 On the
basis of a number of attempts over the next couple of decades
to find conditions under which the ruthenocenium ion is
stable, it was generally concluded that it is unstable with
respect to disproportionation, accounting for the Ru(IV)
products often isolated after either the electrochemical8 or
the chemical9 oxidation of 1. On the basis of reactions such
as shown in eq 1 it was generally concluded that the
oxidation of ruthenocene was inherently a two-electron
process. It should be noted that all of the electrochemical
work was carried out in media that contained traditional
RuCp2 + 2X2 f [RuCp2X][X3]
(1)
supporting electrolyte anions, either [PF6]-, [BF4]-, or
[ClO4]-. Left unanswered was the question of why such a
two-electron process should occur for a compound so much
like ferrocene in its structural and electronic makeup.10
Furthermore, a direct disproportionation of [RuCp2]+ into
RuCp2 and [RuCp2]2+ (eq 2) was not consistent with either
(4) See Wyman, I. W.; Robertson, K. N.; Cameron, T. S.; Swarts, J. C.;
Aquino, M. A. S. Organometallics 2005, 24, 6055. for leading
references.
(5) (a) Bennett, M. A.; Bruce, M. I.; Matheson, T. W. In ComprehensiVe
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel,
E. W., Eds.; Pergamon Press: Elmsford, NY, 1982; Vol. 4, pp
759-767. (b) Adams, R. D.; Selegue, J. P. In ComprehensiVe
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel,
E. W., Eds.; Pergamon Press: Elmsford, NY, 1982; Vol. 4, pp
1018-1020.
(6) Page, J.; Wilkinson, J. J. Am. Chem. Soc. 1952, 74, 6149.
(7) Hendrickson, D. N.; Sohn, Y. S.; Morrison, W. H., Jr.; Gray, H. B.
Inorg. Chem. 1972, 11, 808. See also Morrison, W. H., Jr.; Hendrick-
son, D. N. Inorg. Chem. 1972, 11, 2912.
(8) (a) Gale, R. J.; Job, R. Inorg. Chem. 1981, 20, 42. (b) Denisovich,
L. I.; Zakurin, N. V.; Bezrukova, A. A.; Gubin, S. P. J. Organomet.
Chem. 1974, 81, 207. (c) Gubin, S. P.; Smirnova, L. I.; Denisovich,
L. I.; Lubovich, A. A. J. Organomet. Chem. 1971, 30, 243. (d)
Kuwana, T.; Bublitz, D. E.; Hoh, G. J. Am. Chem. Soc. 1960, 82,
5811.
(9) (a) Watanabe, M.; Sano, H. Chem. Lett. 1991, 555. (b) Smith, T. P.;
Kwan, K. S.; Taube, H.; Bino, A.; Cohen, S. Inorg. Chem. 1984, 23,
1943. (c) Sohn, Y. S.; Schlueter, A. W.; Hendrickson, D. N.; Gray,
H. B. Inorg. Chem. 1974, 13, 301.
(10) Yamaguchi, Y.; Ding, W.; Sanderson, C. T.; Borden, M. L.; Morgan,
M. J.; Kutal, C. Coord. Chem. ReV. 2007, 251, 515.
Experimental Section
Chemicals and Spectroscopy. The compounds used or generated
in this study were handled under strictly anaerobic (dinitrogen)
conditions, with the exceptions of the air-stable compounds
(11) (a) Koelle, U.; Grub, J. J. Organomet. Chem. 1985, 289, 133. (b)
Koelle, U.; Salzer, A. J. Organomet. Chem. 1983, 243, C27.
(12) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg. Chem. 1991, 30,
4687.
(13) Trupia, S.; Nafady, A.; Geiger, W. E. Inorg. Chem. 2003, 42, 5480.
(14) Droege, M. W.; Harman, W. D.; Taube, H. Inorg. Chem. 1987, 26,
1309.
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