New paramagnetic ruthenium complexes via one-electron reduction of metallacumulenes

Article abstract of DOI:10.1039/b008893p

One-electron reductions of [Cl(dppe)₂Ru=C=C=CR₂]PF₆ (R = Ph, Me) and [Cl(dppe)₂Ru=C=C=C=C=CPh₂]PF₆ with Co(C₅H₅)₂ provide radical species with the unpaired

Full text of DOI:10.1039/b008893p

New paramagnetic ruthenium complexes via one-electron reduction of
metallacumulenes†
Stéphane Rigaut,* Olivier Maury, Daniel Touchard* and Pierre H. Dixneuf
Institut de Chimie de Rennes, UMR 6509 CNRS-Université de Rennes: Organométalliques et Catalyse, Campus de
Beaulieu 35042 Rennes, France. E-mail: daniel.touchard@univ-rennes1.fr
Received (in Cambridge, UK) 6th November 2000, Accepted 11th January 2001
First published as an Advance Article on the web 8th February 2001
One-electron reductions of [Cl(dppe)2RuNCNCNCR2]PF6 (R
= Ph, Me) and [Cl(dppe)2RuNCNCNCNCNCPh2]PF6 with
Co(C5H5)2 provide radical species with the unpaired elec-
tron localized on the trisubstituted carbon atom of the
cumulene moiety as shown by hydrogen atom capture and
EPR spectroscopy.
reduction processes is a clear indication of the key role played
by the cumulene chain. It is then obvious that increasing the
length of the delocalized path on the cumulene ligand has a
crucial stabilizing influence on the LUMO.11
We attempted to generate in-situ the first reduced species
allowing further spin trapping experiments or EPR spectros-
copy investigations. The most appropriate reducing agent is the
cobaltocene, Cp2Co, since (i) the reduction potential12 (E° =
21.33 V vs. ferrocene) will selectively induce the first
reduction process in 1+ and 3+, and (ii) it is EPR-silent down to
temperatures considerably below that of liquid nitrogen.12b
Indeed, reduction of the deep colored cationic compounds 1+
(red), 2+ (green) and 3+ (blue), carried out with one equivalent
of cobaltocene in THF, quickly lead to colorless solutions.
Simultaneous addition of Ph3SnH, which is known as a specific
radical quencher by H· transfer,13 leads to the complete
disappearance of 1+, 2+ or 3+, on the basis of NMR data, and to
the formation of the pale yellow, neutral acetylide compounds
[Cl(dppe)2Ru–C·C–CHPh2] 4, [Cl(dppe)2Ru–C·C–CHMe2] 5
and [Cl(dppe)2Ru–C·C–C·C–CHPh2] 6 (Scheme 1). These
Transition metal complexes are useful to stabilize the otherwise
highly reactive :(CN)nCR2 species which behave as strong
electron withdrawing ligands. Following the development of
vinylidene derivatives1 [M]NCNCR2, the Selegue methodology2
has allowed the synthesis and reactivity study of a large variety
of metal allenylidene [M]NCNCNCR2.3,4 This chemistry has
been extensively developed towards material science5 and
alkene metathesis catalyst precursors.6 Stimulated by the fact
that radical species have been recently proposed as inter-
mediates in metathesis7 and by the design of new magnetic
materials with carbon-rich chains, monoelectronic reduction of
metal allenylidene and higher metallacumulenes now appears as
an important field to explore. Apart from a recent specific
reduction of a 3-alkylthioallenylidene complex8 which leads to
a heteroatom-bonded carbon-stabilized radical, no information
has yet been obtained on the nature of the reduction site, e.g. on
the metal or on the cumulene ligand. We now wish to report here
the preliminary results of a general study on reduction of all
1
complexes have been characterized by H, 13C, 31P NMR and
high resolution mass spectrometry.† In addition, characteristic
IR vibration stretches are obtained: nC·C = 2085 (4), 2086 (5),
2179 and 2023 (6) cm21. All of these observations show that
radical trapping only occurs at the end of the carbon skeleton.
Furthermore, achieving the reduction in situ of complexes 1+,
2+ and 3+ by dropping a crystal of cobaltocene in a THF solution
into a capped EPR tube allows the direct observation of the
radical species 1, 2 and 3. Reduced complexes 1 and 3 generated
an intense and persistent signal. By contrast, the EPR spectrum
generated from 2 disappears within few seconds, indicating fast
evolution of the reduced species as observed on the electro-
chemistry scale.14a,b All the reduced forms 1, 2 and 3 exhibit
signals at 293 K in a characteristic region for organic radicals
with g = 2.0042, 2.0097 and 2.0089 respectively. The signals
carbon
LnRuNCNCNCR2
metallacumulenes
and
including
allenylidene9
pentatetraenylidene10
LnRuNCNCNCNCNCPh2 complexes. We show on the basis of
electrochemical, spin trapping and EPR spectroscopic evi-
dences that: (i) monoelectronic reduction leads to a radical
stabilized on the most remote carbon of the cumulene chain as
a general trend; (ii) a striking difference of reactivity of
LnRuNCNCNCNCNCPh2 with respect to reduction vs. nucleo-
philic addition.
Complexes [Cl(dppe)2RuNCNCNCR2]PF6 [R = Ph (1+), Me
(2+)9a] and [Cl(dppe)2RuNCNCNCNCNCPh2]PF6 (3+)10 [dppe =
1,2-bis(diphenylphosphino)ethane], were studied using cyclic
voltammetry (Fig. 1). Allenylidene 1+ undergoes two, well
defined, one-electron reduction waves: the first one is reversible
(E° = 21.03 V vs. ferrocene, DEp = 60 mV, Ipa/Ipc  1),
whereas the second one is irreversible (Epc
= 22.11 V). By
contrast, complex 2+ undergoes only one irreversible reduction
(Epc = 21.31 V) which is consistent with the first reduced
species undergoing following chemical reaction. The pentate-
traenylidene complex 3+ shows a similar behavior to 1+, i.e. a
reversible reduction (E° = 20.63 V, DEp = 65 mV, Ipa/Ipc

1) followed by an irreversible one (Epc = 21.67 V).
It is worth noting that by replacing the phenyl groups in 1+
with methyl groups in 2+ induces a significant cathodic shift of
first the reduction wave by more than 250 mV. Lengthening the
carbon chain from Ru–(C)3 1+ to Ru–(C)5 3+ surprisingly
lowers the reduction potential by 400 mV and shows that the
Ru–(C)5 3+ species is dramatically easier to reduce. The strong
impact of these modifications of the metallacumulene on the
Fig. 1 Cyclic voltammetry responses (CH2Cl2, Bu4NPF6 0.1 M, scan rate
100 mV s21) vs. internal ferrocene for (a) 1+, (b) 2+ and (c) 3+. Insets show
first reduction waves.
† Electronic supplementary information (ESI) available: selected spectro-
scopic data. See http://www.rsc.org/suppdata/cc/b0/b008893p/
DOI: 10.1039/b008893p
Chem. Commun., 2001, 373–374
This journal is © The Royal Society of Chemistry 2001
373

[Cl(dppe)2Ru–C·C–C·C–CHPh2] 6 (58%) shows the unknown
electrophilic character of the Ce atom. The fact that 7 is not
observed in the free radical trapping experiment with H· is very
promising, suggesting the location of the radical on the Ce atom
in 3 and the higher selectivity of radical addition vs. nucleo-
philic attack.
This last observation associated with cyclic voltammetry,
trapping experiments and EPR spectroscopy re-enforces the
conclusion that the reduction of Ru(II) allenylidene or pentate-
traenylidene generate radical species located on the termini of
the cumulene chain that can be written as shown in Scheme 1.
More detailed investigations are in progress, including the study
of the striking difference in selectivity between reduction/
radical trapping and nucleophilic attack on the pentate-
traenylidene complex and the potential of cumulenylidene in
radical chemistry.
We thank the CNRS and the Université de Rennes for
support, P. Guénot from the Centre de Mesures Physiques de
l’Ouest and F. Monnier for assistance.
Scheme 1
are resolved only in the case of 1 and 2. The EPR spectrum of
2 gave a poorly resolved quintet [Fig. 2(b)] showing the
Notes and references
1 M. I. Bruce, Chem. Rev., 1991, 91, 197.
hyperfine coupling with the four phosphorus nuclei with aP
=
2 J. P. Selegue, Organometallics, 1982, 1, 217.
3.0 G. For compound 1, the EPR spectrum is complex [Fig.
2(a)]. This could be best rationalized by the coupling of the
unpaired electron with the four phosphorus nuclei on one hand
and further coupling with the ortho, meta and para hydrogens of
the phenyl group of the carbon-rich bridge. These results
suggest that the radical is centered on the organic bridges in 1,
2, 3 and is stabilized by delocalization along the two
neighbouring phenyl rings in 1. In the case of 2, identification of
such a radical neighbouring methyl groups is quite unexpected.
These EPR data added to the trapping experiments indicate the
organic nature of radicals 1, 2 and 3 and show that the radical
stabilization on the cumulene chain takes place at the trisub-
stituted carbon atom and thus is not controlled by the presence
of a heteroatom bonded to the unsaturated chain.8
3 H. Werner, Chem. Commun., 1997, 903; M. I. Bruce, Chem. Rev., 1998,
98, 2797.
4 D. Touchard and P. H. Dixneuf, Coord. Chem. Rev., 1998, 178–180,
409.
5 M. Tamm, T. Jentzsh and W. Werncke, Organometallics, 1997, 16,
1418; G. Roth, H. Fischer, T. Meyer-Friedrichsen, J. Heck, S.
Houbrechts and A. Persoons, Organometallics, 1998, 17, 1511.
6 A. Furstner, M. Picquet, C. Bruneau and P. H. Dixneuf, Chem.
Commun., 1998, 1315; M. Picquet, C. Bruneau and P. H. Dixneuf,
Chem. Commun., 1998, 2249; A. Furstner, A. F. Hill, M. Liebl and
J. D. E. T. Wilton-Ely, Chem. Commun., 1999, 615; M. Picquet, D.
Touchard, C. Bruneau and P. H. Dixneuf, New J. Chem., 2000, 24, 141;
D. Sémeril, J. Le Nôtre, C. Bruneau, P. H. Dixneuf, A. F. Kolomiets and
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7 V. Amir-Ebrahimi, J. G. Hamilton, J. Nelson, J. J. Rooney, J. M.
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Commun., 1999, 1621.
8 R. F. Winter, Eur. J. Inorg. Chem., 1999, 2121.
9 (a) D. Touchard, P. Haquette, A. Daridor, A. Romero and P. H. Dixneuf,
Organometallics, 1998, 17, 3844; (b) D. Touchard, N. Pirio and P. H.
Dixneuf, Organometallics, 1995, 14, 4920.
10 D. Touchard, P. Haquette, A. Daridor, L. Toupet and P. H. Dixneuf,
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11 These results are in agreement with previous theoretical studies on
analogous allenylidene [(C5H5)(CO)(PPh3)RuNCNCNCH2]+ and
[(C9H7)(PPh3)2RuNCNCNCH2]+ suggesting that the LUMO is strongly
located on the cumulenic ligand. See: (a) M. A. Esteruelas, A. V.
Gomez, A. Lopez, J. Modrego and E. Oñate, Organometallics, 1997, 16,
5826; (b) V. Cadierno, M. P. Gamasa, J. Gimeno, M. González-Cueva,
E. Lastra, J. Borge, S. García-Granda and E. Pérez-Carreño, Organome-
tallics, 1996, 15, 2137.
12 (a) N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 877;
(b) J. H. Ameter and J. D. Swallen, J. Phys. Chem., 1972, 57, 678.
13 (a) D. D. Tanner, G. E. Diaz and A. Potter, J. Org. Chem., 1985, 50,
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Inorg. Chem., 1992, 31, 66.
14 (a) The irreversibility of the reduction wave of 2+ was also observed in
THF at room temperature and at low temperature (230 °C). (b) Many
organometallic species are known to undergo reductive coupling
reactions via radical intermediates. For examples see: ref. 13(b); Z. Hou,
A. Fujita, H. Yamazaki and Y. Wakatsuki, J. Am. Chem. Soc., 1996,
118, 7843; E. J. Roskamp and S. F. Pedersen, J. Am. Chem. Soc., 1987,
109, 3152. However, in the present reaction we have observed that
evolution of 2 led to a mixture of products. The two main compounds
have been identified as 5 and [Cl(dppe)2Ru–C·C–C(NCH2)CH3],
certainly result from an intermolecular hydrogen abstraction [such an
intermolecular abstraction reaction has already been reported for ketyl
radicals, see ref. 13(b).
Fig. 2 EPR spectra resulting from reduction of (a) 1+, g = 2.0042; (b) 2+,
g = 2.0097.
The synthesis of complexes 4–6 were achieved on successive
addition to the cumulene chain of one electron and one
hydrogen atom. This led us to study the reactivity difference
with the simultaneous addition, i.e. the nucleophilic addition, of
H2. Nucleophiles are susceptible to attack either the Ca or the
Cg atom of an allenylidene ligand,3,11a but using the bulky
phosphines additions only occur on Cg.9b Reductions were
performed using NaBH4 in THF (Scheme 1). As expected for 1+
and 2+, additions of H2 take place at the Cg carbons yielding the
acetylide compounds [Cl(dppe)2Ru–C·C–CHPh2]
4 and
[Cl(dppe)2Ru–C·C–CHMe2] 5. When complex 3+ was re-
duced, a mixture of two compounds was obtained. The presence
1
of [Cl(dppe)2Ru–C·C–CHNCNCPh2] 7 (42% estimated by H
NMR), which displays a 13C NMR signal at d 216.7 character-
istic for the cumulenic carbon (CNCNC),† is consistent with the
electrophilicity of carbon Cg in 3+.10 The main product
374
Chem. Commun., 2001, 373–374