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or with suitable palladium(II) precursors such as [Pd(h3-
C3H5)(h5-C5H5)],[19] [Pd(h3-1-PhC3H4)(h5-C5H5)],[17e]
process, in which the trinegative Cht ligand delivers three
electrons to afford the neutral C7H7 radical and subsequently
its dimer (C7H7)2 (9). Hence, assignment of a formal + 4
oxidation state to the titanium atom in 1 gives a tricationic
(CpTi)3+ fragment, which furnishes 8 by coordination of three
acetate ions. This oxidation can also be accomplished with
silver(I) acetate, which reacts cleanly with 1 in a 1:3 ratio as
shown in Scheme 3. It should be noted that comparison of the
formal redox potential of the tropylium ion C7H7+ (e. g. E0’ =
À0.65 V in CH3CN)[27] with those of the Ag+/Ag and Pd2+/Pd
couples suggests that further oxidation of ditropyl (9) should
occur in the presence of Ag(OAc) or Pd(OAc)2. However,
[(tmeda)PdMe2],[20] [Pd(h3-2-methylallyl)Cl]2,[21] and [(h2:h2-
1,5-octadiene)PdBr2],[22] which provide palladium(0) by
reductive elimination steps. For palladium(II) salts such as
Pd(OAc)2, however, the phosphane itself has to act as the
reducing agent with formation of the corresponding phos-
phane oxide.[23] The system Pd(OAc)2/PPh3 was particularly
well studied, and it was suggested that the intermediate
palladium(II) complex [Pd(OAc)2(PPh3)2] slowly rearranges
through an intramolecular mechanism to form initially the
palladate(0) [Pd(PPh3)(OAc)]À and the phosphonium ion
[Ph3P(OAc)]+, which are subsequently converted into the
catalytically active species [Pd(PPh3)2(OAc)]À and Ph3PO by
reaction with excess phosphane or residual water, respec-
tively. In contrast, a very recent study involving [Pd(OAc)2-
(PCy3)2] showed that this complex cannot be reduced to
[Pd(PCy3)2] in the presence of an excess of PCy3, but follows
a base-promoted pathway to form Cy3PO and a monophos-
phane-palladium(0) species, which is trapped in the presence
of PhBr and water to form the dinuclear acetato- and
À
this process, which involves C C bond cleavage, seems to be
unfavorable under these reaction conditions, since no indica-
tion of 2,4,6-cycloheptatrienyl acetate[28] formation was found
by treatment of 9 (or 1) with a large excess of metal acetate.
In contrast, four-electron oxidation of 1 can indeed be
accomplished by use of silver(I) trifluoromethanesulfonate
(triflate) [AgOTf], in CH2Cl2, and tropylium tetrakis(trifluor-
omethanesulfonato)titanate(IV) (10) can be isolated in high
yield (81%, Scheme 3). This salt is composed of (C7H7)+
cations and [CpTi(OTf)4]À anions, and the structure of the
latter is best described as a pseudosquare pyramid with the h5-
C5H5 ligand in the apical position and with the four CF3SO3-
kO ligands forming the basal plane (Figure 3). We believe
hydroxo-bridged
complex
[{(Cy3P)Pd(Ph)}2(m-OAc)(m-
OH)].[24]
To elucidate a possible mechanism for the palladium
reduction by 4 and 5, the reaction of Pd(OAc)2 with two
equivalents of the phosphane ligand was monitored by
NMR spectroscopy in [D8]toluene solution. The 31P NMR
spectra showed rapid formation of the corresponding palla-
dium(0) complexes with no indication of phosphane oxide
formation, while the 1H NMR spectrum showed characteristic
multiplets in a 2:2:2:1 ratio at d = 6.54, 6.11, 5.22 and
2.03 ppm, which point to the formation of a cycloheptatriene
species. It was therefore supposed that the redox process for
palladium(0) generation might involve the troticene rather
than the phosphane moiety. Accordingly, 1 was treated with
excess Pd(OAc)2 in CH2Cl2, thus instantaneously furnishing
a brownish precipitate of elemental palladium. Filtration and
evaporation of the reaction mixture gave a yellow solid, from
which 7,7’-bi-1,3,5-cycloheptatriene (9; ditropyl) was isolated
by extraction with n-pentane,[25] and the remaining solid was
identified as practically pure [(h5-C5H5)Ti(OAc)3] (8) by
NMR spectroscopy and X-ray structure determination
(Scheme 3).[26]
Figure 3. ORTEP drawing of 10 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths [ꢂ] and angles [8]:
Ti–O1 2.017(2), Ti–O4 2.005(2), Ti–O7 2.001(2), Ti–O10 1.963(2);
O1-Ti-O7 138.68(8), O4-Ti-O10 133.34(8).[31]
that the different reactivity of 1 towards AgOAc and AgOTf
can be ascribed to the different nature of the two counter-
ions.[27] Whereas the acetate can be regarded as a relatively
strongly coordinating anion, which binds to the Ti atom in
a bidentate fashion and would afford a covalent ester with the
tropylium cation, the triflate anion is a more weakly
coordinating anion and therefore capable of forming a ther-
modynamically favorable tropylium salt. Thereby, the high
electrophilicity of the cyclopentadienyl–titanium fragment
will require coordination of four triflate ions and conse-
quently result in the formation of the ate-complex 10.
Scheme 3. Oxidation of troticene (1) by palladium(II) or silver(I)
acetate and by silver trifluoromethanesulfonate (AgOTf).
Considering the stoichiometry of the above redox process
(Scheme 3), the reactions between the phosphanes 4 and 5
and Pd(OAc)2 were repeated with an L:Pd ratio of 8:3 (2.67-
fold phosphane excess according to a three-electron redox
process with 0.67 equiv of L providing two electrons). In fact,
the palladium(0) complexes were isolated in significantly
Since it has been demonstrated several times that the
substantially covalently bound Cht ligand in 1 is better
conceived as a À3 ligand rather than a + 1 ligand,[9] the
formation of 8 and 9 can be formally described as an oxidation
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 8638 –8642