Redox Properties of Ferrocenyl Ene-diynyl-Bridged Cp?(dppe)M-C≡C-1,4-(C6H4) Complexes

Article abstract of DOI:10.1021/acs.organomet.8b00740

The complexes FcCH=C{1,4-C≡C-C₆H₄-C≡CM(dppe)Cp?}₂ (Fc = ferrocenyl (FeCp(η-C₅H₄-); M = Fe (1), Ru (2)) were prepared from FcCH=C{1,4-C≡C-C₆H₄-C≡CSiMe₃}₂ (3) via a desilylation/metalation protocol in good (2; 65%) to excellent (1; 97%) yield. The iron compound 1 could also be prepared in a stepwise fashion by desilylation of 3 to give FcCH=C{1,4-C≡C-C₆H₄-C≡CH}₂ (4), reaction with FeCl(dppe)Cp? to give the vinylidene complex FcCH=C{1,4-C≡C-C₆H₄-CH=C=Fe(dppe)Cp?}₂](PF₆)₂ (5(PF₆)₂; 65%), and deprotonation. The cyclic voltammograms of 1 and 2 are characterized by an initial oxidation wave resulting from the overlap of two closely spaced oxidation processes, the potentials of which are sensitive to the identity of M, and a subsequent, one-electron-oxidation wave. Thus, while the dications 1²⁺ and 2²⁺ could be prepared by oxidation with 2 equiv of ferrocenium hexafluorophosphate and isolated as the PF₆^- salts 1(PF₆)₂ and 2(PF₆)₂ at low temperature, the monocations 1⁺ and 2⁺ could only be detected and studied as comproportionated mixtures of 1, 1(PF₆), 1(PF₆)₂ and 2, 2(PF₆), 2(PF₆)₂. A combination of EPR spectroscopy, IR and NIR spectroelectrochemistry, and DFT quantum chemical calculations reveal subtle distinctions in the electronic structures of 1(PF₆)_n and 2(PF₆)_n (n = 0-2). The HOMOs of 1 and 2 are more heavily distributed over the metal-diethynylbenzene arm trans to the ferrocenyl moiety. While one-electron oxidation of 1 gives 1(PF₆), in which the spin density is similarly distributed along the branch of the molecule trans to the ferrocenyl group, the spin density in 2(PF₆) is more extensively, but not fully, delocalized. Further analysis of the ESR, NIR, and IR spectra reveals that charges are essentially localized in 1(PF₆) and 1(PF₆)₂ on the IR time scale, but ground-state exchange between the Fe(dppe)Cp? moieties can take place via the ferrocenyl moiety on the slower ESR time scale. For 2(PF₆) and 2(PF₆)₂, optical charge transfer processes between the ferrocenyl moiety and the organometallic branches can also be observed, consistent with the increased coupling between the Ru(dppe)Cp? and Fc moieties that are linked by a linear conjugation pathway through the bridging-ligand backbone.

Full text of DOI:10.1021/acs.organomet.8b00740

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Redox Properties of Ferrocenyl Ene-diynyl-Bridged Cp*(dppe)M−
CC−1,4-(C H ) Complexes
6
4
Rim Makhoul, Josef B. G. Gluyas, Kevin B. Vincent, Hiba Sahnoune, Jean-Franco̧
is Halet,*,†
†
‡
‡
§
†,∥
,
†
,†
Paul J. Low,* Jean-Rene
́
Hamon, and Claude Lapinte*
†
Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes)-UMR 6226, F-35000 Rennes, France
School of Molecular Sciences, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia
Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
‡
§
∥
Dep
artement de Chimie, Faculte des Sciences, Universite M’Hamed Bougara, Boumerdes, Algeria
́ ́ ́ ̀
*
S Supporting Information
ABSTRACT: The complexes FcCHC{1,4-CC−C H −
6
4
CCM(dppe)Cp*} (Fc = ferrocenyl (FeCp(η-C H −); M
2
5
4
=
Fe (1), Ru (2)) were prepared from FcCHC{1,4-CC−
C H −CCSiMe } (3) via a desilylation/metalation proto-
6
4 3 2
col in good (2; 65%) to excellent (1; 97%) yield. The iron
compound 1 could also be prepared in a stepwise fashion by
desilylation of 3 to give FcCHC{1,4-CC−C H −C
6
4
CH} (4), reaction with FeCl(dppe)Cp* to give the
2
vinylidene complex FcCHC{1,4-CC−C H −CHC
6
4
Fe(dppe)Cp*} ](PF ) (5(PF ) ; 65%), and deprotonation.
2
6 2
6 2
The cyclic voltammograms of 1 and 2 are characterized by an
initial oxidation wave resulting from the overlap of two closely
spaced oxidation processes, the potentials of which are
sensitive to the identity of M, and a subsequent, one-electron-oxidation wave. Thus, while the dications 1 and 22+ could
2+
−
be prepared by oxidation with 2 equiv of ferrocenium hexafluorophosphate and isolated as the PF salts 1(PF ) and 2(PF )
6
6 2
6 2
+
+
at low temperature, the monocations 1 and 2 could only be detected and studied as comproportionated mixtures of 1, 1(PF ),
6
1
(PF ) and 2, 2(PF ), 2(PF ) . A combination of EPR spectroscopy, IR and NIR spectroelectrochemistry, and DFT quantum
6 2 6 6 2
chemical calculations reveal subtle distinctions in the electronic structures of 1(PF ) and 2(PF ) (n = 0−2). The HOMOs of
6
n
6 n
1
and 2 are more heavily distributed over the metal−diethynylbenzene arm trans to the ferrocenyl moiety. While one-electron
oxidation of 1 gives 1(PF ), in which the spin density is similarly distributed along the branch of the molecule trans to the
6
ferrocenyl group, the spin density in 2(PF ) is more extensively, but not fully, delocalized. Further analysis of the ESR, NIR, and
6
IR spectra reveals that charges are essentially localized in 1(PF ) and 1(PF ) on the IR time scale, but ground-state exchange
6
6 2
between the Fe(dppe)Cp* moieties can take place via the ferrocenyl moiety on the slower ESR time scale. For 2(PF ) and
6
2
(PF ) , optical charge transfer processes between the ferrocenyl moiety and the organometallic branches can also be observed,
6 2
consistent with the increased coupling between the Ru(dppe)Cp* and Fc moieties that are linked by a linear conjugation
pathway through the bridging-ligand backbone.
INTRODUCTION
interactions can be constructive, giving rise to resonances and
effective electron transport (e.g., through a para-substituted
benzene), or destructive, giving rise to anti-resonances and
heavily restricted electron transport (e.g., through a meta-
■
In recent years, the search for more highly functional systems
that access unusual electronic structures and permit greater
control and manipulation of intramolecular charge transfer
processes has seen attention turned to systems bearing cross-
16,17
substituted benzene).
It has been proposed that the energy
1
−4
of a QI anti-resonance can not only be influenced by the
conjugated bridging moieties,
including mixed-valence
5
−8
structure of the molecular backbone such as linear vs cross-
derivatives.
These studies form part of a larger range of
9
,18−20
conjugated scaffolds or introduction of pendant groups
but also be tuned through the introduction of electron-donating
investigations that address more general questions about the
ability to tune the electrical behavior of cross-conjugated
systems through the manipulation of quantum interference
21
or -withdrawing groups to the molecular backbone (Figure 1).
9
−13
As QI effects are persistent in even quite large molecular
Received: October 11, 2018
features.
Quantum interference (QI) arises from the interaction of the
de Broglie waves of electrons traversing a molecular system with
the accessible molecular energy levels. The sum of these
15
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00740
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Organometallics
Article
1
4
systems, the opportunity to explore chemical control over QI
of QI can be modulated through the introduction of electron-
donating or -withdrawing groups pendant to the electron
transfer channel. The Grozema system proposes the protonation
of a pendant amine or carboxylate group or alkali-metal ion
coordination to a pendant crown to change the electrostatic
potential of the pendant group to chemically “gate” the
interference patterns of the propagating electron waves between
the donor and acceptor sites.
2
2,23
effects is an exciting area of contemporary chemistry.
In seeking to extend these concepts, our attention has been
drawn to electrochemically gated systems, with a redox-active
pendant introduced to a general donor−bridge−acceptor
structure. By tuning of the formal redox state of the gate, a
change in the QI patterns between the donor and acceptor
should be introduced (Figure 3). We have chosen to base our
Figure 3. Electrochemically gated analogue of Grozema’s QI transistor.
design strategy around mixed-valence compounds in which the
Figure 1. Cartoon sketch of an anti-resonance arising from QI through
a molecular junction formed from a cross-conjugated gem-diethynyle-
thene, on the basis of the work of Andrews et al. The energy of the
anti-resonance is sensitive to the chemical structure of the pendant
group, E.
donor and acceptor differ only in their formal oxidation
25−28
14
state.
Indeed, in recent times there has been a resurgence
of interest in mixed-valence models of intramolecular charge-
transfer processes, from the development of theoretical
descriptions and spectroscopic analysis of the charge-transfer
29
With these concepts in mind, Grozema’s proposal of a
chemically gated QI-based molecular transistor provided a
fascinating structural model through which to explore and
modulate molecule-mediated charge transfer processes between
event to the development of novel optoelectronic materials
30
and the design of molecular electronic components.
Some of the present authors have explored aspects, including
mixed-valence characteristics, of cross-conjugated 1,1-bis-
24
31
a donor and an acceptor (Figure 2). As noted above, the effects
(alkynyl)-2-ferrocenylethene derivatives (A), 3 including 1,1-
2
bis(ferocenylalkynyl)-2-ferrocenylethene (B) and tetrakis-
33
(
ferrocenylethynyl)ethene (C) (Chart 1). However, efforts
to incorporate the half-sandwich building blocks M(PP)Cp′,
which have been so successful in exploring other aspects of
organometallic mixed valency when these building blocks are
linked through linearly conjugated all-carbon and carbon-rich
3
4−40
bridging ligands,
within the cross-conjugated 1,1-bis-
(
alkynyl)-2-ferrocenylethene framework through desilylation−
metalation reactions of 1,1-bis(trimethylsilylethynyl)-2-ferroce-
nylethene (Chart 1; A, R = SiMe ) or via vinylidenes formed
3
from 1,1-bis(ethynyl)-2-ferrocenylethene (Chart 1; A, R = H)
32
have so far proved fruitless.
Here we now report the development of an extended
derivative of the 1,1-bis(alkynyl)-2-ferrocenylethene building
block A and the successful preparation of bis-M(dppe)Cp* (M
=
Fe (1), Ru(2)) complexes (Chart 1). The electronic structures
and intramolecular charge transfer processes through the
branched carbon-rich ligand framework within redox families
generated from these compounds have been explored through a
combination of electrochemical, spectroscopic, and computa-
tional methods, revealing the subtle differences that arise from
the use of the iron and ruthenium end caps.
RESULTS AND DISCUSSION
Syntheses and Characterization. Mono- and binuclear
alkynylmetal complexes in the M(dppe)Cp* series (M = Fe, Ru)
are usually prepared by one of two general routes.
■
Figure 2. Chemically gated QI systems proposed by Grozema and
24
34,41−43
colleagues.
Most
B
DOI: 10.1021/acs.organomet.8b00740
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Article
Chart 1
Scheme 1. Synthesis of 3
4
4
commonly, the readily available MCl(dppe)Cp* precursor is
reacted with a terminal alkyne to give the corresponding
vinylidene derivative. In turn, deprotonation of the vinylidene
gives the alkynyl metal complex.
(
Potassium tert-butoxide was used as both a base and a reducing
agent during the workup of the more acidic and oxidatively
sensitive homometallic iron complex 1. Alternatively, 1 could
also be obtained via an intermediate bis(vinylidene) complex
(Scheme 2). Desilylation of 3 (K CO /MeOH) gave FcCH
43,45−47
Alternatively, MCl-
dppe)Cp* (M = Fe, Ru) can be reacted with a trimethylsilyl-
protected alkyne in the presence of a fluoride source (such as KF
or NBu F) to give the metal alkynyl complex via an in situ
2
3
C{1,4-CC−C H −CC−H} (4), which was briefly char-
6
4
2
acterized and reacted directly with FeCl(dppe)Cp* in the
4
48−54
desilylation−metalation sequence.
presence of NH PF to give the intermediate vinylidene
4
6
In previous investigations by a number of the current authors
complex [FcCHC{1,4-CC−C H −CHCFe(dppe)-
6
4
the reaction of FcCHC(CCH) with RuCl(PPh ) Cp gave
Cp*} ](PF ) (5(PF ) ). Subsequent deprotonation of 5(PF )
2
3 2
2 6 2 6 2 6 2
t
the monovinylidene complex [FcCHC(CCH){CH
with KOBu gave 1 in quantitative yield.
The new complexes 1 and 2 were obtained and characterized
+
CRu(PPh ) Cp}] . However, all efforts to prepare trimetallic
3
2
1
31
FcCHC{CCRu(PPh ) Cp} from either FcCHC(C
by mass spectrometry, IR, H and P NMR, and UV−vis
3
2
2
CH) or the trimethylsilyl-protected analogue FcCHC(C
spectroscopy, and cyclic voltammetry. As previously noted for
2
32
31
1
CSiMe ) proved unsuccessful, likely due to excessive steric
the organic derivatives FcCHC(CCR) , the H NMR
3
2
2
crowding.
spectra of 1 and 2 confirm the free rotation of the ferrocenyl
moiety around the ferrocene−vinyl C−C bond, with the protons
of the η-C H ring observed as only two signals (δ 4.33, 4.80
In the current study, Sonogashira cross-coupling of FcCH
3
1
C(CCH)
with 4-(trimethylsilylethynyl)bromobenzene
2
5
4
H
was employed to give the “extended” precursor FcCH
(1); δ 4.41, 4.88 (2)). In 1, the vinyl proton resonance is
overlapped by the protons of the C H ring (δ 6.80−6.89) but
C{1,4-CC−C H −CC−SiMe } (3) (Scheme 1). Reac-
6
4
3
2
6
4
H
tion of 3 with MCl(dppe)Cp* (M = Ru, Fe) and KF in a 1/1 (v/
v) mixture of tetrahydrofuran and methanol gave 1 (97%) and 2
is clearly observed in 2 at δ 6.86. The protons of the Cp*
H
ligands attached to the iron atoms in 1 are observed as a single,
(
65%) as red and orange powders, respectively (Scheme 2).
slightly broadened resonance (δ 1.33 ppm), while those in 2
H
C
DOI: 10.1021/acs.organomet.8b00740
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Organometallics
Article
Scheme 2. Synthesis of 1, 2, 4, and 5(PF6)2
are observed as two very closely spaced, but well-resolved,
singlets (δ 1.57 and 1.58 ppm), as the E and Z arms of the
H
double bonds are not strictly equivalent. However, the electronic
and stereochemical differences are not detected in 3 P NMR
spectra, with the phosphorus atoms on the two M(dppe)Cp*
1
moieties resonating as singlets at δ 99.3 and 80.6 for 1 and 2,
P
respectively.
The IR spectra of 1 and 2 exhibit strong ν
bands at
M−CC
−
1
2
046 and 2062 cm , respectively. As is generally observed for
iron and ruthenium complexes M(CCR)(dppe)Cp*, the
frequency of the ν stretch is somewhat higher for the
M−CC
55,56
ruthenium complex than for its iron homologue.
Two weak
−
1
ν
bands at 2201 (sh) and 2187 cm (1) and a broad stretch
CC
−
1
at 2189 cm (2) arise from the triple bonds which connect the
vinyl ferrocenyl moiety to the phenyl rings. The spectra of 1 and
2
also show an intense band at 1591/1592 cm−1 which can be
assigned to the ν
vibration of the vinyl group. The significant
CC
intensity of this band is likely due to the dipole across the
Cp*(dppe)M−donor ferrocene−acceptor fragment.
Cyclic Voltammetry of 1 and 2. The initial scans in the
Figure 4. Cyclic voltammograms for 1 (0.2 mM) and 2 (0.2 mM)
cyclic voltammetry of 1 and 2 were recorded between −1.2 and
−
1
recorded at ν = 0.100 V s in 0.1 M NBu PF /CH Cl .
+
4
6
2
2
0
.6 V (vs. FeCp /FeCp at 0.00 V) in 0.1 M NBu PF /CH Cl .
2
2
4
6
2
2
In each case, the voltammograms are characterized by two quasi-
reversible and well-separated waves, the separation between the
simultaneous oxidation of the two M(CCR)(dppe)Cp* units,
anodic and cathodic peaks for each redox wave (ΔE = 0.08 ±
p
while the second waves can safely be assigned to the ferrocenyl
a
c
0
.01 V) being somewhat larger than the value expected for
moiety. The (i /i ) current ratios indicate some chemical
p p
electrochemically reversible systems (ΔE = 0.06 V) (Figure 4
irreversibility of the second wave, reflecting chemical instability
of the electrochemically generated trications. In line with these
assignments, the first oxidation process(es) of the Fe(dppe)Cp*
p
and Table 1). The first wave, which consumes approximately
double the current of the second, corresponds to the near-
D
DOI: 10.1021/acs.organomet.8b00740
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Table 1. Electrochemical Data for 1 and 2
Article
a
very similar spectrum was reported by Sato and co-workers for
the highly delocalized mixed-valence complex [Cp(dppe)Fe−
a
c
compd
redox site
E1/2 (V)
ΔE (V)
i /i
p
60
p
p
CC−(η-C H )FeCp](PF ). Moreover, the smaller g value
5
4
6
b
1
Fe(dppe)Cp*
Fc
Ru(dppe)Cp*
Fc
−0.55
0.09
0.08
0.08
0.09
0.99
0.77
∼1
0.76
and the narrower ESR signal obtained for the ruthenium
derivatives might result from a larger carbon character of the
SOMOs which contains the unpaired electrons.
+0.12
−0.17
b
c
2
+0.15
Given the failure to observe 1(PF ) in spectroelectrochemical
experiments (vide infra), in an additional experiment, complex 1
was reacted with 1 equiv of [Cp Fe](PF ) under the same
conditions to give a solution containing 1(PF ) comproportio-
nated with 1 and 1(PF ) . The ESR spectrum collected from this
solution is less intense but otherwise identical with that obtained
from 1(PF ) . In other words, the mono- and dications give rise
6
a
Potentials in 0.1 M CH Cl , NBu (PF ), platinum electrode, sweep
2
2
4
6
−
1
+
rate 0.100 V s , potentials are relative to external FeCp /FeCp at
0
half-wave potential from two unresolved redox processes. The small
separation of the redox processes E(1) and E(2) makes precise
measurement of the current ratio i /i difficult.
2
2
2
6
+
57 b
.00 V (internal reference Cp Co/[Cp Co] = −1.30 V). Apparent
2
2
c
6
6
2
a
p
c
p
6
2
to sufficiently similar ESR spectra that a distinction cannot be
drawn. This is consistent with DFT calculations in which the
spin density in 1+ and 1 is localized on one or both
Fe(dppe)Cp* fragments.
complex 1 occur(s) some 0.37 V more negative than that of the
ruthenium analogue 2, while oxidation potentials assigned to the
ferrocenyl groups in 1 and 2 differ by only 0.02 V (Figure 4).
In Situ ESR Spectra of 1(PF ) and 2(PF ) (n = 1, 2). The
2+
6
n
6 n
IR Spectroelectrochemical Studies of 1(PF ) (n = 0−3)
6
n
facile oxidation of 1 and 2 revealed by the electrochemical
measurements prompted consideration of the redox-related
products. The complexes 1 and 2 were each reacted with 2 equiv
of [Cp Fe](PF ) in THF at −60 °C for 1 h, and the resulting
and 2(PF ) (n = 0−2). UV−vis−NIR and IR spectroelec-
6
n
trochemical investigations of 1(PF ) (n = 0−3) and 2(PF ) (n
6
n
6 n
=
0−2) were undertaken to complement the electrochemical
2
6
and ESR studies of the chemically oxidized compounds. In the
IR spectra, the stepwise oxidation of 1 and 2 from the neutral
species to the dication induces a shift to lower wavenumbers of
solutions of 1(PF ) or 2(PF ) transferred to an ESR quartz
6
2
6 2
tube and immediately cooled to liquid nitrogen temperature.
ESR monitoring established that while the iron complex
diradical is stable in a glass at 66 K (−207 °C) or even as a
fluid THF solution below −60 °C, the concentration of ESR-
active iron species decreases above −40 °C and completely
decomposes within 5 min at 20 °C.
−
1
the ν
bands from 2046 to 1991 cm and from 2062 to
M−CC
−
1
1927 cm in the iron and ruthenium series, respectively. As the
oxidation of complexes 1 and 2 proceeds, the ν band
Fc−CC
−
1
stretch at 1591/1592 cm corresponding to the vinyl group
decreases while new less intense band envelopes appear at lower
energy, indicating a degree of delocalization along at least one of
the M−CCCHCHFc branches.
The ESR spectra of 1(PF ) and 2(PF ) recorded at 66 K
6
2
6 2
(
Figure 5) are each characterized by one broad signal (1(PF ) ,
6 2
In the iron series, the intensity of the band corresponding to
the neutral complex 1 continuously decreases while the band
corresponding to the dication 1(PF ) gradually appears. An
6
2
intermediate band associated with the transient formation of the
monocation 1(PF ) could not be detected. In contrast, for the
6
ruthenium analogue, transient features in the IR spectroelec-
trochemical experiment were observed at 2131 (w), 2040 (sh),
−
1
1
994 (m), 1891 (s), 1572 (m), and 1547 (m) cm during the
oxidation of 2 to 2(PF ) , corresponding to the presence of
6
2
2
(PF ) in the comproportionated mixture (Figure 6 and Table
6
2
). Taken together, the IR spectra suggest that the E and Z arms
Figure 5. X-band ESR spectra of 1(PF ) (top) and 2(PF ) (bottom)
at 66 K.
6
2
6
2
of the molecule are completely independent in the iron
complexes, while a degree of delocalization leading to enhanced
stability of the mixed-valence form 2(PF ) can be observed in
6
g = 2.1265; 2(PF ) , g = 2.1034) with a width from peak to peak
the case of the ruthenium complexes.
6
2
of ca. 330 G for 1(PF ) and 120 G for 2(PF ) (Figure 5). The
On further oxidation of 2(PF ) the sample evinced
6 2
6
2
6 2
ESR spectra of 1(PF ) and 2(PF ) strongly contrast with the
significant and rapid decomposition on the time scale of the
spectroelectrochemical experiment, which prevented confident
assignment of the resulting spectra. However, in the case of the
iron complex oxidation to a trication with sufficient chemical
stability to be observed could be achieved, with the assignment
being made with confidence following the recovery of the
spectrum of the dication on back-reduction. Oxidation of
1(PF ) to 1(PF ) , which could be followed by observing the
6
2
6 2
spectra of the iron and ruthenium radical cations [M(C
CR)(dppe)Cp*]PF (M = Fe, Ru), which show one signal with
6
5
three features (g , g , and g ) characteristic of low-spin d
complexes with a pseudo-octahedral geometry,
1
2
3
45,56,58
while
substituted ferrocenyl cations exhibit one signal with two tensor
59
components (g and g ).
||
⊥
In the case of the iron complex 1(PF ) , the presence of
6
2
6 2
6 3
partially resolved features in the rather broad ESR spectrum
suggests that the signal is close to decoalescence, consistent with
a degree of electron exchange between the Fe(dppe)Cp* and Fc
moieties on a rate comparable with the ESR time scale. In the
case of 2(PF ) , the ESR spectra shown in Figure 5 can be
low-energy edge of the electronic transitions that fall in the
−
1
window of the spectrometer (1000−10000 cm ), had little
effect on the ν and ν IR bands, consistent with a
M−CC
Fc−CC
ferrocenyl-localized oxidation.
NIR Spectroelectrochemical Studies of 1(PF ) (n = 0,
6
2
6 n
regarded as averaged signals resulting from a fast intramolecular
electron transfer at the ESR time scale (faster than 10 s)
between the M(dppe)Cp* moieties and the ferrocenyl group. A
2, 3) and 2(PF ) (n = 0−2). Upon oxidation of 1 to 1(PF ) in
6
n
6 2
−
9
the spectroelectrochemical cell (Figures 7 and 8), the character-
−
1
61
istic MLCT absorption band near 20000 cm
collapses,
E
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giving way to a lower-intensity LMCT feature near 12000
−
1
58
−1
cm . To even lower energy (ca. 5000 cm ), the characteristic
d−d (mixed with CC π character) transition of an Fe(III)
III
+
pseudo-octahedral [Fe (CCR)(dppe)Cp*] complex can be
5
8
observed growing in concurrently. Further oxidation to
1
(PF ) does not affect the Fe(III) d−d transitions associated
6
3
III +
with the [Fe (CCR)(dppe)Cp*] moieties but does cause a
collapse of the LMCT band and the appearance of a weak
−
1
absorption band envelope around 8000−10000 cm . Absorp-
tion bands at similar energy have been observed on oxidation of
other ferrocenyl ene-diynes and assigned to the ferrocenium-like
d−d transitions and LMCT transitions within the vinyl-
31
ferrocenium moiety. These data are consistent with the
suggestions made from other spectroscopic observations, with
largely independent oxidation of the Fe(dppe)Cp* moieties
being followed by oxidation of the ferrocene fragment. There is
no detectable absorption band that can be confidently assigned
to an intramolecular intervalence charge transfer (or IVCT)
transition.
The stepwise oxidation of the ruthenium-based complex 2
was also carried out in a spectroelectrochemical cell (Figure 9).
As the potential in the cell is increased to more positive values,
−
1
the MLCT (or dπ−π*) band near 25000 cm , which typifies
62
Ru(CCR)(PP)Cp′ complexes, begins to collapse, giving
way to a complex series of band envelopes in the NIR region.
The band shape evolves with increasing applied potential, with
the presence of an isosbestic point near 20000 cm− and the
reversibility of the spectral changes on the reverse potential
sweep consistent with the establishment of the comproportio-
nated equilibrium of 2, 2(PF ), and 2(PF ) . The definitive
1
6
6 2
assignment of the transitions responsible for the NIR absorption
features is an extraordinarily difficult task, given the range of
conformations that can be adopted by the rotation of the
sandwich, half-sandwich, and phenylene moieties relative to
38−40
−1
each other.
However, the distinct feature near 5500 cm is
similar to the IVCT band observed in weakly coupled mixed-
5
2,63
valence Ru(dppe)Cp* complexes.
This band grows during
the early stages of oxidation at lower (less positive) potentials,
where the equilibrated solution contains 2(PF ) comproportio-
6
nated with 2 and 2(PF ) , and collapses at higher (more
6
2
positive) potentials, where the solution is expected to be
essentially wholly 2(PF ) . It is therefore very likely that this
6
2
−
1
band envelope near 5500 cm can be assigned to an IVCT
transition between the Ru(dppe)Cp* moieties. The NIR bands
−
1
nearer 10000−15000 cm likely arise from a combination of Fc
+
49,64,65
→
{CCRu(dppe)Cp*} charge transfer
and LMCT
62
transitions in 2(PF ) , which is the principal species in the
6
2
solution at higher potentials. While these spectroscopic changes
were reversible within the cell, as noted for the IR
spectroelectrochemical experiments, attempts to generate
2
(PF6)3 by further electrolysis resulted only in extensive
decomposition of the sample.
Computational Studies. Density functional theory (DFT)
calculations were carried out on compounds 1 and 2 and their
cationic species to gain further insight into the structural
arrangements, electronic structures, and bonding properties at
the PBE0/LANL2DZ level of theory (see the Experimental
Section for computational details), which complement earlier
studies of cross-conjugated carbon scaffolds and complexes cited
Figure 6. IR spectra of 1 and 2 collected spectroelectrochemically in 0.1
66−68
above and elsewhere.
The optimized molecular structure of
M CH Cl /NBu PF . The spectra of the transiently observed
2
2
4
6
the energetically most stable conformer found in each case and
atom labels are shown in Figure 10, and important bond lengths
and angles are summarized in Table 3.
comproportionated mixture between the neutral and dicationic states
is shown as a solid black trace in the lowest panel, the unique features
+
being attributed to 2 .
F
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Article
Table 2. Characteristic IR Data for 1n+ and 2n+ (n = 0−3) (CH Cl /0.1 M NBu PF )
2
2
4
6
1 (cm−1)
calcd (cm−1
2182 (129)
2047 (1779)
)
a
2 (cm−1
)
calcd (cm−1
2184 (138)
2057 (2028)
)
a
n
0
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
2202 (w,sh), 2187 (w)
2046 (vs)
2189 (vw)
2062 (s)
1592 (s)
2135 (w)
1994 (m), 1891 (s)
1572 (m), 1547 (m)
2191 (w)
CC
M−CC
Fc−CC
CC
M−CC
Fc−CC CC(Ar)
CC
M−CC
Fc−CC CC(Ar)
CC
M−CC
1591 (s)
1590 (278), 1472 (291)
2191 (1148), 2000 (2580)
2040 (1999)
1590 (314), 1472 (304)
2156 (1036)
1
2
3
1953 (62580)
/ν
1548 (2130)
2181 (436)
2025 (458)
1551 (574)
2206 (w, sh), 2191 (w)
2150 (3974)
1914 (1729)
1512 (1975)
2045 (vw, sh), 1991 (w)
1590 (w), 1574 (w), 1563 (w)
2205 (w, sh), 2190 (w)
2040 (vw, sh), 1993 (w)
1591 (w), 1575 (w), 1562 (w)
1927 (m)
1629 (w), 1581 (w, br)
/ν
/ν
Fc−CC CC(Ar)
a
Values in parentheses are given in km/mol.
n+
Figure 8. Expanded view of the UV−vis−NIR spectra of 1 collected
spectroelectrochemically (CH Cl /0.1 M NBu PF ). At this scale the
2
2
4
6
2+
3+
difference between the spectra of 1 and 1 cannot be discerned and
hence 13 is omitted.
+
Figure 7. NIR spectra of 1n+ collected spectroelectrochemically
CH Cl /0.1 M NBu PF ) (top). The expansion of the lower energy
(
2
2
4
6
region (bottom) highlights the extremely weak, approximately
Gaussian shaped bands observed for 12 and 1 . In this lower energy
+
3+
n+
region, neutral 1 is featureless.
Figure 9. UV−vis−NIR spectra of 2 collected spectroelectrochemi-
cally (CH Cl /0.1 M NBu PF ), including the unique spectroscopic
2
2
4
6
The molecular geometry around the −CC-M(dppe)Cp*
end-cap units in 1 (M = Fe) and 2 (M = Ru) is similar to that of
features of the comproportionated mixture.
G
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Article
rings here or Cp in the case of the ferrocenyl moiety described
below) determined as τ = 67 and 46° for 1 and 2, respectively.
To favor π delocalization, the two phenyl rings are nearly
coplanar in both compounds. With regard to the ferrocene
#
moiety, an average Fe−Cp distance of 1.65 Å is computed for 1
and 2. This value compares very well with that experimentally
70
measured in FeCp2.
The HOMOs of complexes 1 and 2 are substantially
energetically separated from the LUMO by 3.19 and 3.20 eV,
respectively (Figure 11). The nodal properties of the HOMOs
Figure 10. Plots illustrating the DFT-optimized molecular structures of
complexes 1 (top) and 2 (bottom). Blue, orange, and gray spheres are
metal, phosphorus, and carbon atoms, respectively. Hydrogen atoms
are omitted for clarity.
model component monometallic complexes Fe(CCPh)-
5
3,55,56
(
dppe)Cp* and Ru(CCPh)(dppe)Cp*,
with bond
34,69
lengths and angles in the previously established ranges.
The
Figure 11. DFT molecular orbital diagram of 1 (left) and 2 (right).
M2/Fe1(Fc)/carbon-backbone/M3 percentage contributions are
given in italics (M = Fe, Ru).
two end caps are twisted relative to each other with the torsion
#
angle (τ) between planes defined by Cp −M2−M3 and M2−
#
#
M3−Cp (Cp represents the centroid of the respective Cp*
Table 3. Important DFT-Optimized Bond Lengths (Å) and Angles (deg) for Complexes 1n+ and 2n+ (n = 0−2)
1
1+
12+
a
2
2+
22+
1.931
1.931
a
#
M2−Cp* (centroid)
1.741
1.741
1.742
1.770
1.909
1.908
1.918
#
M3−Cp* (centroid)
1.768
1.647/1.646
1.846
1.855
1.373
1.424
1.425
1.222
1.222
1.423
1.418
1.421
1.240
1.238
2.212
2.204
2.295
2.274
176.1
175.7
179.6
178.7
178.4
179.5
179.3
125.0
118.7
84.6
1.770
1.649/1.644
1.868
1.867
1.373
1.425
1.427
1.221
1.221
1.423
1.422
1.426
1.237
1.237
2.306
2.280
2.308
2.280
175.2
175.7
179.4
177.9
176.6
178.2
178.0
123.5
118.8
114.3
1.920
1.654/1.649
1.952
1.944
1.384
1.417
1.418
1.225
1.225
1.415
1.412
1.410
1.246
1.248
2.309
2.300
2.313
2.304
174.5
174.6
179.4
179.1
179.0
179.7
179.9
124.1
118.6
99.0
#
Fe1-Cp (centroid)
1.646/1.645
1.869
1.870
1.396
1.427
1.428
1.222
1.221
1.424
1.424
1.424
1.239
1.239
2.212
2.203
2.211
2.202
176.4
176.3
179.0
179.3
179.5
179.8
179.1
124.5
119.7
67.6
1.646/1.645
1.986
1.987
1.369
1.427
1.428
1.222
1.221
1.424
1.424
1.424
1.237
1.237
2.293
2.285
2.293
2.285
175.2
175.3
179.3
179.2
179.0
179.9
179.4
124.3
119.6
46.0
1.652/1.646
1.923
1.924
1.379
1.421
1.422
1.223
1.224
1.417
1.416
1.410
1.252
1.253
2.343
2.329
2.343
2.329
174.5
174.5
179.0
179.0
177.4
178.7
178.2
123.6
118.7
156.9
M2−C3
M3−C9
C1−C2
C2−C8
C2−C14
C7−C8
C13−C14
C6−C7
C12−C13
C4−C5
C3−C4
C9−C10
M2−P1
M2−P2
M3−P3
M3−P4
M2−C3−C4
M3−C9−C10
C3−C4−C5
C9−C10−C11
C6−C7−C8
C12−C13−C14
C2−C14−C13
C1−C2−C8
C1−C2−C14
b
τ
a
b
Triplet state configuration. See text.
H
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Article
Figure 12. Plots (from left to right) of the HOMO-1 (−4.78 eV), HOMO (−4.63 eV), LUMO (−1.44 eV), and LUMO+1 (−0.85 eV) of complex 1.
3
1/2
Contour values are ±0.03 (e/bohr )
.
Figure 13. Plots (from left to right) of the HOMO-1 (−4.74 eV), HOMO (−4.59 eV), LUMO (−1.39 eV), and LUMO+1 (−0.86 eV) of complex 2.
3
1/2
Contour values are ±0.03 (e/bohr )
.
5
8,61
of 1 (Figure 12) and 2 (Figure 13) are comparable, forming part
of the “t ” set expected for pseudo-octahedral metal centers, and
are π in character, heavily weighted on the metal centers and on
to date.
From the data in Table 3 it can be seen that the
Fe2−C3 and Fe3−C9 distances contract by only some 0.02 Å on
oxidation of 1 to 1 , before elongating again in the triplet
2
g
+
the carbon backbone with little to no (HOMO-1) or only a weak
dication. The changes in formal metal oxidation state are more
readily observed through the Fe−Cp*(centroid) and Fe−P
distances. Therefore, if we consider the optimized geometries of
(
1
HOMO) contribution of the Fc and ethenyl moieties (Figures
1−13). It is noteworthy that the closely lying HOMO and
+
2+
+
HOMO-1 are not equally distributed over the two branches of
the molecules; rather, each largely forms the π system associated
with one “branch” of the cross-conjugated carbon backbone.
These orbitals have significant M2−C3 and M3−C9 antibond-
ing and C3−C4 and C9−C10 bonding character. The LUMO of
1, 1 , and 1 we see that on oxidation from 1 to 1 the Fe(3)−
Cp*centroid distance increases from 1.741 to 1.768 Å and the
Fe(3)−P(3)/Fe(3)−P(4) distances elongate from 2.211/2.202
to 2.295/2.274 Å, consistent with oxidation of this metal site,
whereas Fe(2) parameters are largely unchanged (i.e., localized
2+
1
and 2 is mostly centered on the ethenyl group and shows a
oxidation at Fe(3)). On oxidation to 1 , the parameters at
+
strong π* character. A noticeable participation of the ferrocenyl
Fe(3) remain similar to those of 1 , while Fe(2) exhibits the
moiety in the LUMO of 1 and 2 is observed (Figure 11).
expected changes that accompany oxidation. In contrast, the
+
+
n+
The geometries of the redox-related monocationic 1 and 2
ruthenium analogues [Ru(CCR)(dppe)Cp*] feature more
2
+
2+
and dicationic 1 and 2 species were also calculated, the
results of which are summarized in Table 3 (metrical parameters
of only the triplet states are given for the dicationic species, being
largely energetically preferred over the singlet states by more
than 1.8 eV). In the absence of a complete set of spectroscopic
Ru−CC character in the HOMO/SOMO and hence
structural changes are more evident in the Ru−C and CC
distances on oxidation (Table 3).
Despite the fact that the energies of the HOMO and LUMO
are rather similar in both 1 and 2 (Figure 11), the computed
adiabatic ionization potentials (IPs) differ; values of 10.53 and
data permitting comparison of 13 and 2 , these tricationic
species were not calculated. Unsurprisingly given the M−C
antibonding and CC bonding character of the HOMOs,
oxidation of 1 and 2 leads to some shortening of the M2−C3 and
M3-C9 distances and a slight lengthening of the adjacent CC
bonds in the case of 2 (Table 3). The ethenyl C1−C2 bond
length remains almost constant across each series. The Fe−
C(alkynyl) bond length is rather insensitive to changes in the
metal oxidation state in Fe(CCR)(dppe)Cp* complexes,
differences falling within the experimental statistical differences
in the examples that have been crystallographically characterized
+
3+
2
+
2+
11.39 eV for the dications 1 and 2 were found, respectively.
2+
2+
The higher IP of 2 in comparison with that of 1 is consistent
with the higher oxidation potential of 2 in comparison to 1
(Table 1). We note that a direct correlation of the gas-phase IPs
and electrochemical potentials should not be expected, given the
sensitivity of the electrochemical data to solvation, ion pairing,
and inner-sphere reorganization energies.
The energies of the key molecular vibrations were computed
n+
n+
for 1 and 2 (n = 0−2) to provide a point of reference between
the experimental observations and the computational results.
I
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Article
+
+
3
Figure 14. Spatial distribution of the computed spin density of 1 (left) and 2 (right). Isocontour value: ±0.002 e/bohr .
Figure 15. Spatial distribution of the computed spin density of the triplet states of 12+ (left) and 22+ (right). Isocontour value: ±0.002 e/bohr3.
1
For the neutral complexes 1 and 2, the ν
and ν
cm− not being clearly resolved in the experimental spectra. For
M−CC
CC
−
1
+
vibrational frequencies are computed at 2047 cm (1779 km/
the monocationic system 1 for which no experimental values
are observed, strong and moderate ν
2000 cm (2580 km/mol) and 2191 cm (1148 km/mol),
whereas a ν
mol). Overall, these values reflect localized oxidation of one
Fe(dppe)Cp* moiety. The vinyl ν
1548 cm (2130 km/mol) for 1 .
In the case of 2 , a very intense ν
−
1
−1
mol) and 2183 cm (129 km/mol) (1) and at 2057 cm (2028
km/mol) and 2184 cm− (138 km/mol) (2) in excellent
agreement with the experimentally determined values (Table 2).
bands are calculated at
CC
1
−1
−1
−
1
band is calculated at 2040 cm (1999 km/
M−CC
−
1
The vinyl ν
bands are calculated at 1472 cm (291 km/
CC
−
1
−1
mol) and 1590 cm (278 km/mol) for 1, and 1472 cm (304
km/mol) and 1590 cm− (314 km/mol) for 2. Again, these
values are in excellent agreement with the observed bands at
band shifts somewhat to
CC
1
−1
+
+
band is calculated at
M−CC
−
1
−1
1
591 and 1592 cm , the bands calculated to fall below 1500
1953 cm (62580 km/mol) and involved a coupled oscillation
J
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Article
n+
n+
of both Ru−CC fragments. Vibration of the CC bonds
ESR properties of the complexes 1 (n = 1, 2) and 2 (n = 1,
2) were also computed. The resulting g tensor components are
given in Table 4 for a comparison with experiment. The
−
1
associated with the vinyl moiety are calculated at 2156 cm
(
1036 km/mol). The very high oscillator strength of the ν
M−CC
+
band arising from vibration along the electron-transfer axis in 2
accounts for the observation of a band from this species in the
experimental spectroelectrochemical experiment, despite the
low equilibrium concentration of this species.
n+
Table 4. Computed (Experimental) ESR Parameters for 1
n+
and 2 (n = 1, 2)
compd
g1
g2
g3
giso
Δg
2+
For the dicationic (triplet) system 1 , two moderately intense
+
−
1
1
2.003
1.980
2.006
1.880
1.165
1.994
2.083
2.114
2.044
1.947
1.181
2.105
2.114
2.222
2 0.075
2.019
4.473
2.140
2.067 (2.1265)
2.106 (2.1265)
2.042
1.949 (2.1034)
2.273
0.111
0.242
0.069
0.139
3.308
0.146
ν
and ν
vibrations are calculated at 2025 cm (458
M−CC
CC
2+
−
1
1
2
2
6
7
km/mol) and 2181 cm (436 km/mol), respectively, while the
vinyl ν
experimentally indistiguishable from that in 1 . These values
+
−
1
band at 1551 cm (574 km/mol) would be
CC
2+
+
+
compare very well with the spectroelectrochemically observed
+
−
1
2.079
bands at 2191 (ν
), 1991 (ν
), and 1563 (ν
) cm .
CC
M−CC
CC
The observation of additional bands in the experimental spectra
is not unexpected, given the conformational flexibility of these
complexes with regard to the dihedral angles among the key
agreement is moderately satisfactory, although the computed
values indicate some anisotropy of the rhombic g tensor (Δg = g
1
2+
−
g ) for both compounds, which is not observed
Fe(dppe)Cp*, C H , and Fc moieties. For the Ru analogue 2 ,
3
6
4
+
2+
−
1
experimentally. Moreover, the values calculated for 1 and 1
ν
and ν
vibrations are calculated at 1914 cm (1729
km/mol) and 2150 cm (3974 km/mol), respectively. The
vibration is computed at 1512 cm (1975 km/mol).
These values are comparable to those experimentally measured
M−CC
CC
+
−
1
differ substantially; this suggests that the concentration of 1 in
the comproportionated mixture studied experimentally is too
low to be observed. In addition, the small values of Δg computed
for the mono-oxidized forms 1 (0.111) and 2 (0.069) are
consistent with an important degree of delocalization of the odd
electron in these mixed-valence species, especially for the
ruthenium compound. However, these results must be taken
with caution, as it is known that the rotational orientation of the
M(dppe)Cp* fragments around the M−CC axis relative to
the conjugated ligand can strongly influence the g-tensor
values. Despite these ambiguities, the smaller values of the g-
tensor components for 2 and 2 with respect to 1 and 1
reflect more delocalization in the ruthenium compounds, as
inferred from the experimental spectroscopic data and electronic
structure calculations.
−
1
vinyl ν
CC
+
+
(
Table 2).
With the vibrational data giving confidence in the relevance of
the optimized molecular geometries to the experimental
samples, attention was turned to further exploration of the
electronic structures. Mulliken atomic spin densities of the
34
+
+
monocationic species 1 and 2 were computed and compared
to gain insight concerning the (de)localization of the unpaired
electron over the molecule, as well as some indication about the
electronic communication between the metal end groups via the
Fc-carbon backbone. The results reveal a quite asymmetric iron
71
+
2+
+
2+
+
cation 1 with the unpaired electron mostly localized on one
branch, especially on the −CC−Fe(dppe)Cp* end-cap unit
In order to explore the involvement of the Fc group in the ESR
properties of 1 and 2 (n = 1, 2), g-tensor computations were
(Figure 14, left). The spin density on Fc is very small (0.02 e),
n+
n+
indicating that the oxidation occurs mainly at one iron−ethynyl
+
+
performed on the iron models [FcCHCH ] (6 ) and
+
2
unit. For 2 , the situation differs, with the unpaired electron
+
+
[
CH C{1,4-CC−C H −CC−Fe(dppe)Cp*} ] (7 )
2
6
4
2
more extensively delocalized over the whole molecule with
comparable contribution on the Ru atoms and Fc (Ru2, 0.13 e;
Ru3, 0.17 e; Fc, 0.12 e; carbon backbone, 0.88 e) (Figure 14,
right). This indicates that electron density is removed from three
metal centers and the carbon backbone upon oxidation of 2.
Finally, it is worth mentioning that a more substantial spin
density on the β-carbon atom close to the ferrocenyl group is
for comparison. As expected, an axial g tensor is obtained for
+
6
with two different tensor components (g = 1.173 and g =
.473). Such values are comparable to those expected for Fc
Computations on model 7 predict a g value
of 2.079 and a rhombic splitting of the g tensor with g = 1.994, g
2.105, and g = 2.140 which differs very slightly from that
computed for 1 (Table 4). These results seem to indicate a
relatively minor role of the Fc group on the ESR properties of 1
|| ⊥
+
4
72−74
+
complexes.
iso
l
2
=
3
+
+
+
n+
computed for 2 (0.20 e) in comparison to 1 (0.04 e).
2
+
n+
Mulliken atomic spin densities of the dicationic species 1
and 2 .
2
+
and 2 with their triplet electronic configuration were also
computed (Figure 15). Note that the broken-symmetry singlets
CONCLUSION
■
(
BSs) featuring the antiferromagnetic states are computed to be
The complexes FcCHC{1,4-CC−C H −CCM(dppe)-
6
4
almost isoenergetic (less stable by 0.001 eV (ca. 0.2 kcal/mol)).
For the iron species, the spin density is mainly localized on the
iron atoms of the Fe(dppe)Cp* fragments (Fe2, 1.13 e; Fe3,
Cp*} (Fc = ferrocenyl (FeCp(η-C H -); M = Fe (1), Ru (2))
2
5
4
and their redox-related products have provided further
opportunities to explore the electronic differences and
characteristics of putative mixed-valence complexes derived
from the half-sandwich {M(dppe)Cp*} moieties, through an
1
.13 e) and to a lesser extent on the carbon bridge (0.67 e),
consistent with the formal assignment of Fe(III) oxidation states
for these fragments. The contribution on the Fe atom of the
ferrocenyl (Fc) moiety is very weak (0.02 e). In the case of the
ruthenium species, the spin density is more evenly distributed
over the whole molecule with 1.69 e on the carbon bridge and
“
extended” cross-conjugated bridging ligand. Despite the low
(unresolved) separation of the first two oxidation processes, in
the case of the ruthenium species 2 a combination of UV−vis−
NIR and IR spectroelectrochemistry can be used to detect the
presence of mixed-valence 2 in the comproportionated mixture.
Electron exchange between the metal complex “branches” of 2
appears to be fast on the EPR time scale, with significant
contributions from the carbon chain and organic-like singlet in
+
0
.47 and 0.46 e on the Ru2 and Ru3 ruthenium centers,
+
respectively. Note that, while the participation of the ferrocenyl
iron atom Fe1 is small (0.07 e), this is still significantly greater
than in the iron species (0.02 e).
K
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Article
the EPR spectrum. Quantum chemical analysis supports the
description of 2 in terms of a polarized, but still rather
(APCI), ESI, MALDI, or EI ionization techniques. Elemental analyses
were performed at the London Metropolitan University.
+
FcCHC{1,4-CC−C H −CCSiMe } (3). A mixture of
extensively delocalized, system with substantial spin density over
the bridging ligand framework. In contrast, the iron complex 1
gives rise to much more metal localized redox behavior.
Apparently the more limited delocalization into the carbon-
rich bridging ligand lowers the thermodynamic stability of
6
4
3 2
FcCHC{CCH} (762 mg, 2.93 mmol), 4-(trimethylsilylethynyl)-
2
bromobenzene (1.56 g, 6.15 mmol), Pd(PPh ) (160 mg, 147 μmol),
3
4
CuI (28.0 mg, 147 μmol), and triethylamine (50 mL) was heated at
reflux for 17 h and then cooled to ambient temperature. Following
removal of the solvent, the residue was purified by column
chromatography (eluent: hexanes/ethyl acetate (98/2 (v/v))) to
afford FcCHC{1,4-CC−C H −CCSiMe } as a red solid (661
+
mixed-valence 1 , which cannot be detected spectroscopically
using the methods available here. Thus, as has been observed
6
4
3 2
37
1
recently in linearly conjugated systems, the limited d−π
mixing associated with 3d metals such as Fe promotes more
localized behavior in mixed-valence complexes featuring carbon-
rich bridging ligands, while heavier 4d metals such as Ru mix
more extensively with the ligand, leading to more bridge-based
redox character.
mg, ∼80% purity by H NMR). Analytically pure FcCHC{1,4-C
H } was obtained following preparative TLC of
6 4 3 2
this solid (eluent: hexanes/ethyl acetate (98/2 (v/v))). which was
carried out in small batches immediately prior to its use in further
C−C
−CCSiMe
reactions. For a typical preparative TLC purification, 109 mg of crude 3
1
gave 53 mg of pure product. H NMR (500 MHz, CDCl ): δ 0.26 (s,
3
9
H, Si(CH ) ), 0.27 (s, 9H, Si(CH ) ), 4.22 (5H, s, C H ), 4.55 (m,
3 3 3 3 5 5
2
H, C H ), 4.88 (m, 2H, C H ), 6.99 (s, 1H, CCH), 7.41−7.51 (m,
5
4
5
4
EXPERIMENTAL SECTION
13
8H, C H ). C NMR (125 MHz, CDCl ): δ 0.07 (Si(CH ) ), 69.9
■
6
4
3
3 3
All reactions were carried out using standard Schlenk techniques under
dry, inert atmospheres of argon (reactions involving FeCl(dppe)Cp*)
or nitrogen (all other reactions). Triethylamine was distilled over
potassium hydroxide; hexanes, diethyl ether, and tetrahydrofuran were
dried by passage over an alumina column or distilled from sodium
benzophenone ketyl. Methanol was dried and distilled from magnesium
methoxide. Dichloromethane was dried either by distillation under
argon from P O and then Na CO or on an Inert Technologies solvent
(C
5
5
), 70.0 (C−H, C
5
H
4
), 70.9 (C−H, C
5
H
4
), 79.6 (C−CHC,
C
H
5
96.4 (CC−Si), 96.7 (CC−Si), 98.9 (CC), 104.8 (CC), 104.9
(CC), 122.8 (C−CC, C
H
), 123.2 (C−CC, C
H
6
), 123.5 (C−
), 131.4
6
4
4
CC, C
(C−H, C
H
H
), 123.5 (C−CC, C
H
4
), 131.3 (C−H, C
H
6
4
6
6
4
), 132.0 (C−H, C
H
4
), 132.2 (C−H, C
H
), 145.6 (C
6
4
6
6
4
C−H). FT-IR (CH
Cl
2
): ν 2209 (CC), 2156 (CC−Si), 1576
2
+
cm (CC). ASAP-MS(+): m/z 605.2 [M + H] . Anal. Calcd for
2
5
2
3
purification system. Triethylamine and methanol were further
deoxygenated by sparging with N or Ar before use. Other solvents
were standard reagent grade and used as received. No special
precautions were taken to exclude air or moisture during workup,
except where otherwise indicated. The compounds FeCl(dppe)Cp*·
C
38
2
Fe: C, 75.48; H, 6.00. Found: C, 75.40; H, 6.12.
FcCHC{1,4-CC−C −CCH} (4). A Schlenk tube was
charged with FcCHC{1,4-CC−C −CCSiMe (218 mg,
0.36 mmol) and an excess of K CO (299 mg, 2.76 mmol, 6 equiv), and
H
2
6
4
2
H
6
}
3 2
4
2
3
the solids were dissolved in 2/1 MeOH/THF (18 mL). The reaction
was stirred at room temperature for 4 h, after which the solvents were
removed under reduced pressure and the solid residue was extracted
with diethyl ether. The extracts were combined, the solvent was
removed under reduced pressure, and the resulting red powder was
dried in vacuo for 1 h to yield 4, which was briefly characterized before
44
31
CH Cl2 and RuCl(dppe)Cp*, FcCHC{CCH} , and Pd-
2
2
75
(
PPh3)4 were prepared according to published procedures. The
compound 4-(trimethylsilylethynyl)bromobenzene was prepared by a
76
minor variation of the published route, as detailed in the Supporting
Information. Other chemicals were purchased from commercial
sources and used without further purification.
1
further use (166 mg, 0.36 mmol, nominally 100%) . H NMR (400
Instruments. Solid-state infrared spectra were obtained as KBr
pellets with a Bruker IFS28 FT-IR infrared spectrophotometer (4000−
MHz, CDCl ): δ 2.98 (s, 1H, CCH), 3.00 (s, 1H, CCH), 4.02 (s,
3
5H, C H ), 4.25 (s, 2H, C H ), 4.69 (s, 2H, C H ), 6.82 (s, 1H, C
5
5
5
4
5
4
−
1
4
00 cm ), and solution spectra on a Cary 660 instrument. Near-IR and
CH), 7.06 (s, 2H, C H ), 7.26 (s, 2H, C H ), 7.31 (m, 4H, C H ).
6 4 6 4 6 4
UV−visible spectra were recorded in solution using a 1 cm length
[FcCHC{1,4-CC−C H −CHCFe(dppe)Cp*} ](PF ) (5-
)
6 2
6
4
2
6 2
quartz cell on a Cary 5000 spectrophotometer. NMR spectra were
(PF
(149 mg, 0.32 mmol), FeCl(dppe)Cp*·CH
2.1 equiv), and NH PF (109 mg, 0.67 mmol), before addition of 2/1
). A Schlenk tube was charged with a freshly prepared sample of 4
1
13
recorded at 25 °C on a Bruker Avance III 600 ( H, 600.1 MHz; C,
2
Cl (477 mg, 0.67 mmol,
2
31
1
1
50.9 MHz; P, 242.9 MHz), a Bruker Avance III 500 ( H, 500.1 MHz;
4
6
13
31
1
C, 125.8 MHz; P, 202.4 MHz) or a Bruker Avance 400 ( H, 400.1
MeOH/THF (15 mL). The reaction medium was stirred at room
temperature for 48 h, the solvents were removed, and the product was
extracted with dichloromethane (2 × 10 mL). The combined extracts
were concentrated to 5 mL under vacuum, and addition of methanol
(15 mL) caused the precipitation of a red solid that was collected by
MHz; 13C, 100.6 MHz) spectrometer using CDCl or CD Cl as the
3
2
2
solvent. Chemical shifts (ppm) were determined relative to internal
1
13
77
31
residual solvent signals ( H, C) or external 85% H PO ( P, δ 0.0
3
4
ppm). Cyclic voltammetry was carried out in a nitrogen-filled glovebox
3
+
using a PalmSens Emstat potentiostat, with platinum working
electrode, a platinum-plated titanium-wire counter electrode, and a
platinum-plated titanium wire pseudoreference electrode, from
solutions of the complex (ca. 0.2 mM) in dichloromethane containing
filtration, washed with methanol (3 × 10 mL), and dried in vacuo to
1
afford 5(PF
(400 MHz, CDCl
2.98 (m, 4H, CH ), 4.15 (s, 5H, C
), 5.04 (m, 2H, FeCCH), 6.21 (m, 2H, C
)
(401 mg, 0.208 mmol, 65%), as a red powder. H NMR
6
2
): δ 1.51 (s, 30H, C
(CH ) ), 2.42 (m, 4H, CH
),
), 4.81 (s, 2H,
), 6.51 (m, 2 H,
4
3
5 3 5 2
H
), 4.40 (s, 2H, C
5
H
2
5
5
4
−
1
0
.1 M NBu PF as the electrolyte: ν = 100 mVs . The cobaltocene/
C
C
C
C
5
6
6
6
H
4
6
H
4
6
cobaltocenium couple was used as an internal reference for potential
H
H
H
4
4
5
), 6.90 (s, 1 H, CCH), 7.02 (m, 2 H, C
+m-C dppe), 7.26 (m, 8H, m-C dppe), 7.39 (m, 16H, o-
dppe), 7.52 (m, 8H, p-C dppe). P NMR (162 MHz, CDCl ):
).
6
H
4
), 7.10 (m, 10H,
+
measurements such that Cp Co/[Cp Co] falls at −1.30 V relative to
H
6
5
/
H /
6 5
2
2
+
40
31
external Cp Fe/[Cp Fe] at 0.00 V. Electron spin resonance (ESR)
/
H
6
5
/
3
2
2
1
spectra were recorded on a Bruker EMX-8/2.7 (X-band) spectrometer
at 77 K (liquid nitrogen). Spectroelectrochemistry was conducted in an
δ = 86.5 (s, dppe), 86.6 (s, dppe), −144.5 (septet, JP−F = 710 Hz, PF
FT-IR (KBr): ν 2184 s (CC), 1640 s (FeCC), 831 s (PF
6
)
6
78
−1
OTTLE cell, using solutions in dichloromethane containing 0.1 M
cm .
nBu NPF as the supporting electrolyte. Spectra were recorded on an
FcCHC{1,4-CC−C
6
H
4
−CC−Fe(dppe)Cp*}
2
(1). Route A.
Cl
4
6
Agilent Technologies Cary 660 FT-IR, an Agilent Technologies Cary
Potassium fluoride (8.0 mg, 130 μmol) and FeCl(dppe)Cp*·CH
2
2
5
000 UV−vis−NIR, or an Avantes diode array UV−vis−NIR system
(98 mg, 138 μmol) were added to a solution of 3 (42 mg, 69 μmol) in
tetrahydrofuran (4 mL) and methanol (4 mL), and the mixture was
heated at reflux for 20 h. Following cooling to ambient temperature, a
solution of potassium tert-butoxide in methanol (10 mL, 0.1 M) was
added, affording a deep red precipitate. The precipitate was filtered
under Schlenk conditions, washed with a solution of potassium tert-
butoxide in methanol (10 mL, 0.1 M) and hexanes (10 mL), and dried
comprising two light sources (UV−vis, AvaLight-DH-S-Bal; vis−NIR,
AvaLight-Hal-S) and two spectrometers (UV−vis, AvaSpec-ULS204−
8
built sample holder by bifurcated fiber optic cables. The Vis−NIR light
source was attenuated with a band-pass filter transparent between ∼900
and 4700 nm. Mass spectrometry was carried out employing ASAP
L-USB2; NIR, AvaSpec-NIR256-2.5TEC) connected to a custom-
L
DOI: 10.1021/acs.organomet.8b00740
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
8
6
under vacuum to afford 1 as a red powder (109 mg, 67 μmol, 97%). The
product was transferred directly to a nitrogen-filled glovebox, and all
further manipulations were carried out therein.
program.8 Orbital compositions were obtained using the AOMix
7,88
program.
89−91
The Amsterdam Density Functional (ADF) program
was
Route B. A Schlenk tube was charged with the bis-vinylidene
employed to compute the EPR properties of the cationic species using
geometries optimized via Gaussian09. Electron correlation was treated
within the local density approximation (LDA) in the Vosko−Wilk−
t
complex 5(PF ) (400 mg, 0.208 mmol), KOBu (58.0 mg, 0.52 mmol,
6
2
2
.5 equiv), and THF (20 mL). The reaction mixture was stirred at room
temperature for 2 h. The solvent was evaporated to dryness under
vacuum, and the crude residue was extracted with dichloromethane (2
10 mL). After removal of the solvent from the combined extracts
under reduced pressure, the solid material was washed with pentane (10
92
Nusair parametrization. Nonlocal corrections were added to the
80
exchange and correlation energies using the PBE0 functional.
Calculations were performed using the standard ADF triple-ζ quality
×
basis set. The ESR procedure developed by van Lenthe and co-workers
93−95
was used.
The g-tensor components were obtained using self-
mL) and dried in vacuo to give 1 as a red powder (340 mg, 0.208 mmol,
1
consistent spin-unrestricted DFT calculations after incorporating the
1
00% yield). H NMR (500 MHz, CD Cl ): δ 1.42 (br s, 30H,
2
2
relativistic spin−orbit coupling by first-order perturbation theory from
C (CH ) ), 2.00 (br s, 4H, CH ), 2.63 (br s, 4H, CH ), 4.21 (br s, 5H,
5
3
5
2
2
89,90
a ZORA Hamiltonian.
C H ), 4.41 (br s, 2H, C H ), 4.90 (br s, 2H, C H ), 6.80−6.89 (m, 5H,
5
5
5
4
5
4
C H and CCH), 7.17−7.45 (m, 36H, C H and C H ), 7.85 (br s,
6
4
6
4
6
5
31
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00740.
8
2
H, C H ). P NMR (202 MHz, CD Cl ): δ 99.3. FT-IR (CH Cl ): ν
6 5 2 2 2 2
■
*
−
1
201 w,sh, 2187 w (CC), 2046 s,br (CC−Fe), 1591 s cm (C
S
C). Anal. Calcd for C1 H P Fe : C, 76.29; H, 5.91. Found: C, 76.67;
04
96
4
3
+
56
H, 5.51. HRMS (ESI ): m/z calculated for C H P Fe3,
635.43877; calculated for C104H P Fe ([M + H] ), 1636.4466;
found, 1636.4508 (0 ppm) ([M + H] ).
1
04 96 4
5
6
+
1
97 4 3
+
Plots of the UV−vis spectra of 1 and 2, plots of the
intermediate NIR spectra collected during the spectroe-
lectrochemical oxidation of 1, revised synthetic procedure
for the synthesis of 4-(trimethylsilylethynyl)-
bromobenzene, and plots of NMR spectra (PDF)
FcCHC{1,4-CC−C H −CC−Ru(dppe)Cp*} (2). Potassi-
6
4
2
um fluoride (10 mg, 175 μmol) and RuCl(dppe)Cp* (117 mg, 175
μmol) were added to a solution of 3 (53.0 mg, 88.0 μmol) in
tetrahydrofuran (5 mL) and methanol (5 mL), and the mixture was
heated at reflux for 16 h. After the mixture was cooled to ambient
temperature, methanol (15 mL) was added, affording an orange
precipitate. The precipitate was collected by filtration, washed with
methanol (2 × 15 mL) and hexanes (2 × 15 mL), and dried under
Cartesian coordinates for all calculated geometries
(MOL)
1
vacuum to afford 2 as an orange powder (99 mg, 57 μmol, 65%). H
AUTHOR INFORMATION
Corresponding Authors
NMR (600 MHz, CD Cl ): δ 1.57 (s, 15H, C (CH ) ), 1.58 (s, 15H,
2
2
5
3
5
■
C (CH ) ), 2.11 (m, 4H, CH ), 2.69 (m, 4H, CH ), 4.20 (s, 5H, C H ),
5
3
5
2
2
5
5
4
2
1
.41 (m, 2H, C H ), 4.88 (m, 2H, C H ), 6.71 (m, 2H, C H ), 6.76 (m,
5 4 5 4 6 4
*E-mail for J.-F.H.: jean-francois.halet@univ-rennes1.fr.
H, C H ), 6.86 (s, 1H, CCH), 7.15 (m, 2H, C H ), 7.21−7.24 (m,
6
4
6
4
*E-mail for P.J.L.: paul.low@uwa.edu.au.
0H, C H and C H ), 7.37−7.39 (m, 24H, C H ), 7.77 (m, 8H,
6
4
6
5
6
5
*
E-mail for C.L.: claude.lapinte@univ-rennes1.fr.
31
−1
C H ). P NMR (242 MHz, CD Cl ): δ 80.6. FT-IR (CH Cl /cm ):
6
5
2
2
2
2
ORCID
ν(CC) 2189 w, ν(RuCC) 2062 s,br, ν(CC) 1592 s. MALDI-
+
MS(+): m/z 1729.3 [M + H] . Anal. Calcd for C H P FeRu : C,
Paul J. Low: 0000-0003-1136-2296
1
04 96
4
2
7
2.30; H, 5.60. Found: C, 72.15; H, 5.63.
Notes
In Situ Preparation of [FcCHC{1,4-CC−C H −CC−
6
4
The authors declare no competing financial interest.
Fe(dppe)Cp*} ](PF ) (1(PF ) , n = 0−2). A Schlenk tube was
2
6 n
6 n
charged with 1 (0.030 g, 0.018 mmol) and THF (7 mL). The solution
was cooled to −60 °C prior to adding ferrocenium hexafluorophos-
phate (0.0050 g, 0.018 mmol, 1 equiv) in a single portion. The reaction
mixture was warmed to room temperature overnight before adding 10
mL of pentane with vigorous stirring. The resulting precipitate was
collected by filtration, washed with pentane (2 × 5 mL), and dried in
ACKNOWLEDGMENTS
■
The authors thank Dr. F. Gendron and Prof. Abdou Boucekkine
(Rennes) for helpful discussions. Part of this work was
conducted within the scope of the CNRS International
Associated Laboratories “Molecular Materials and Catalysis
(MMC)” (University of Durham-University of Rennes) and
vaccuo to yield 0.014 g of a comproportionated mixture of 1, 1(PF6),
and 1(PF6) as a red powder (0.0078 mmol, 40%). For ESR
2
“Assemblages organomet
pour la photonique et l’el
́
alliques redox-actifs et multipolaires
ectronique moleculaire (REDO-
measurements, the low-temperature-generated red solution was
directly transferred via cannula into an ESR tube conserved at liquid
nitrogen temperature.
́
́
CHROM)” (University of Western Australia-University of
Rennes). P.J.L. and J.B.G.G. thank the Australian Research
Council for financial support (DP140100855).
In Situ Generation of [FcCHC{1,4-CC−C H −CC−Fe-
dppe)Cp*} ](PF ) (1(PF ) ). The dioxidized product 1(PF ) was
2 6 2 6 2 6 2
6
4
(
synthesized in a manner similar to that described above, from 1 (0.050
g, 0.030 mmol) and ferrocenium hexafluorophosphate (0.020 g, 0.060
mmol, 2.0 equiv). Yield: 0.03 g (0.015 mmol, 52%) of a red powder.
The same procedure as above was carried out to prepare the ESR
samples.
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