The complexes
Fe(η5-C5R5)(CO)2(C⋮CC⋮CSiMe3)
(R = Ph, 2a; R = Me, 2b) were
obtained
from the reaction of
(η5-C5Ph5)Fe(CO)2Br
(1a) or (Cp*)Fe(CO)2I (Cp* =
η5-C5Me5, 1b)
with
Me3SiC⋮CC⋮CLi (1.05 equiv) in THF at −80 °C
(70−75%). The butadiynyl complex (η5-C5Me5)Fe(dppe)(C⋮CC⋮CSiMe3)
(2c) was prepared upon photolysis of 2b in a
toluene/acetonitrile mixture (95/5 v/v) in the presence of dppe
(1,2-bis(diphenylphosphino)ethane)
(85%). Treatment of complexes 2a,2b with
1.1 equiv of potassium fluoride in a 50/50
CH3OH/THF mixture at 20 °C afforded the terminal butadiyne complex
(η5-C5R5)Fe(CO)2(C⋮CC⋮CH) (R = Ph, 3a; R = Me, 3b),
isolated in 85 and 95% yields respectively. Complex
3c was obtained by reaction of 2c with 0.2 equiv
of Bu
n
4NF in THF (95%).
The mononuclear
complexes 2a
−
c and
3a
−
c were characterized by cyclic
voltammetry and IR, 1H, 13C, and
31P NMR, and Mössbauer spectroscopy. The
binuclear complexes 5
a,b were obtained
(59−69%) in a one-step procedure by treatment of
(η5-C5R5)Fe(CO)2(C⋮CC⋮CH)
(3
a,b) with 1
equiv of Cp*Fe(dppe)Cl (4) in the presence of
KPF6 and KOBu
t
in methanol. The
lower IR
frequencies of the carbonyl groups in 5
a,b
relative to the isolated acceptor group in
2
a,b
and 3
a,b are indicative of the electronic
communication between the metal centers through
the −C4− spacer. In the complexes
5
a,b, the polarization of the spacer was shown by
the
13C NMR chemical shifts of the carbon of the
−C4− chain. The Mössbauer spectrum of
5b
establishes that the electron density at the two iron atoms is quite
different. The structure
of 5b has been determined by X-ray diffraction. Cyclic
voltammograms of complexes 5
a,b
from −0.8 to −1.4 V/SCE displayed one fully reversible wave and a
second one almost
reversible, showing that the binuclear systems undergo two successive
one-electron oxidations
at the electrode. Comparison of both redox potentials and current
ratio with those of the
corresponding monomers 2a
−
c
demonstrates that a strong electronic communication
between
the metal centers takes place across the all-carbon bridge.
Treatment of the neutral
complexes with 1 equiv of ferrocenium allowed isolation of the salts
[5
a,b
][PF
6
]
in 85−90%
yield. Mössbauer spectroscopy showed that both salts were
trapped Fe(II)-Fe(III) mixed-valence compounds. The g tensors and couplings with the
31P nuclei are very close for both
Fe(III) low-spin radicals. Intense absorption bands were
observed at 810 and 829 nm for
[5a][PF
6
]
and
[5b][PF
6
],
respectively. A second absorption band was observed in the
near-infrared for
[5b][PF
6
]
(1600 nm) and ascribed to an ICT band. It allowed determination
of
the electronic coupling between the electron-poor and the electron-rich
iron centers through
the all-carbon spacer (V
ab = 0.021 eV).
The reaction of [Ir(Cl)(cod)]2,
P(CH2CH2(CF2)5CF3)3,
and CO (1 atm) gives the title
compound (2, 97%).
Tri(n-octyl)phosphine (3) and
rhodium (4) analogues are similarly
prepared. Crystal structures of 2 and 4 show
anti CCCC conformations in the six
perfluoroalkyl groups (average torsion angle 169°), with four chains
(two per phosphorus)
in parallel coplanar arrays that define ca. 21 × 6 Å “rafts”.
The other two chains extend on
a common side of the raft, which pack back-to-back in stacks,
maximizing parallel chains in
every dimension. DSC shows one phase transition (melting,
2/4 75/79 °C) prior to thermal
decomposition (>200 °C). The IR νCO values
(2/3/4 1975/1942/1979
cm-1) show that the
CH2CH2 spacers do not completely insulate the metals from the
perfluoroalkyl groups. Both 2
and 4 are soluble in
CF3C6F11,
CF3C6H5, ether, THF, and acetone
but insoluble in hexane,
toluene, CHCl3, and CH2Cl2
(CF3C6F11/THF partition coefficient
>99.7:<0.3). Reactions of
2 with RI, H2, and 3O2
or 1O2 in
CF3C6F11 give
Ir(CO)(Cl)(R)(I)[P(CH2CH2(CF2)5CF3)3]2
(R =
CH2CH2(CF2)7CF3
(5), CH3 (6),
CH(CH3)CH2CH3 (7),
70−84%),
Ir(CO)(Cl)(H)2[P(CH2CH2(CF2)5CF3)3]2
(82%), and
Ir(CO)(Cl)(O2)[P(CH2CH2(CF2)5CF3)3]2
(9, 67%), respectively. Additions of RI occur by free-radical chain mechanisms (inhibition by
duroquinone; slower dark
reactions; rates 7 > 5 > 6) with no
evidence for polar pathways. Complex 9 forms
more
rapidly in THF, indicating that (relative to THF) 2 and
3O2 are stabilized by
CF3C6F11 more
than the transition state.
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