Synthesis of η1-butadienyl iron complex upon coupling of two organometallic η1-vinyl units and selective decomplexation at one end

Article abstract of DOI:10.1016/S0022-328X(97)00723-7

The symmetric binuclear iron μ-bis(ethylidene) complex [Cp*(dppe)Fe(=CHCH₂CH₂CH=)Fe(dppe)Cp*][PF ₆]₂ (2) reacts with the equivalent of two KOBu^t in THF to provide the mononuclear η¹-butadie

Full text of DOI:10.1016/S0022-328X(97)00723-7

Journal of Organometallic Chemistry 554 (1998) 203–205
Preliminary communication
Synthesis of p1-butadienyl iron complex upon coupling of two
organometallic p1-vinyl units and selective decomplexation at one end
Vale´rie Guillaume, Virginie Mahias, Claude Lapinte *
UMR CNRS 6509 Organome´talliques et Catalyse: Chimie et Electrochimie Mole´culaires, Uni6ersite´ de Rennes 1, Campus de Beaulieu, 35042,
Rennes Cedex, France
Received 29 July 1997; received in revised form 10 October 1997
Abstract
The symmetric binuclear iron v-bis(ethylidene) complex [Cp*(dppe)Fe(ꢀCHCH2CH2CHꢀ)Fe(dppe)Cp*][PF6]2 (2) reacts with
the equivalent of two KOBut in THF to provide the mononuclear p1-butadienyl complex Cp*(dppe)Fe(CHꢀCH–CHꢀCH2) (4)
isolated in 76% yield. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Synthesis; Organoiron; Iron butadienyl; Ethylidene
The formation of new carbon–carbon bonds is one
of the most important objectives in organo-transition
metal chemistry. The dimerization by ligand–ligand
coupling via 17-electron transition metal radicals con-
stitutes one of the possible reactions to reach this goal
[1,2]. This behavior is fairly broad and the coupling
through the ligand can be achieved with complex pos-
sessing of unsaturated hydrocarbon ligands such as
inter alia cyclopentadienyl [3], cyclooctatetraene [4,5],
vinylidene [6], alkynyl [7] or dienyl [8]. However, the
products formed in this way are symmetric by nature,
and this constitutes the principal limitation of this type
of C–C bond formation.
unique among the non-heteroatom stabilized carbene
derivatives of the first row transition metals [10–12]
and even in comparison with isostructural analog of the
third row transition metals [13]. This chemical inertness
is obviously associated with a loss of the electrophilic
reactivity usually observed with these types of carbene
derivatives [11]. Thus, the binuclear carbene complex 2
and its mononuclear analog [Cp*(dppe)Fe(ꢀCHCH3)]
[PF6]2 do not react with alkene or phosphane to yield
cyclopropane or phosphonium derivatives respectively.
This unusual chemical behavior offered us a unique
opportunity to find a route towards a mononuclear
derivative with a four carbon ligand by selective re-
moval of one of the |-bound terminal Cp*Fe(dppe)
units. This chemistry constitutes an independant and
complementary work to our project on the formation
of new carbon–carbon bonds by ligand–ligand cou-
pling of 17-electron radicals. Indeed, despite the high
yield of the coupling reactions, the interest of this
synthetic approach may remain limited without chemi-
cal routes to selectively remove the capping groups in
the symmetric binuclear compounds. In this communi-
cation we report the first step toward this goal.
Recently, we found that the 17-electron iron(III)
’+
vinyl radical cation [Cp*Fe(dppe)(CHꢀCH2)] (1) du-
plicates in solid state to give the stable bis(carbene)
complex
[Cp*(dppe)Fe(ꢀCHCH2CH2CHꢀ)Fe(dppe)
Cp*][PF6]2 (2) isolated with quite a good yield [9]. The
high thermal stability of the v-bis(ethylidene) 2 is very
* Corresponding author. Tel.: +33 2 99286361; fax: +33 2
99281646; e-mail: lapinte@univ-rennes1.fr
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00723-7

204
V. Guillaume et al. / Journal of Organometallic Chemistry 554 (1998) 203–205
Scheme 1.
The bis(ethylidene) complex 2 prepared as previously
bulky Cp*Fe(dppe) unit has never been observed to be
coordinated in the p2-mode with any unsaturated C2
ligands. In particular, it was observed that the stable
16-electron species [Cp*Fe(dppe)][PF6] does not react in
THF with ethylene [20]. Moreover, under the experi-
mental conditions, the in situ generated [Cp*Fe-
(dppe)]+ fragment reacts with the second equivalent of
KOtBu to decompose. The first step of the proposed
mechanism is supported by the easy deprotonation of
the Fisher-type bis(carbene) compound [Cp*(dp-
pe)Fe{ꢀC(OCH3)CH2CH2C(OCH3)ꢀ}Fe(dppe)Cp*][P-
F6]2 (5) with two equivalents of KOtBu in similar
conditions. In this case, the vinyl-heterocarbene inter-
mediate resulting from the first deprotonation is stable
enough to undergo a second proton abstraction and the
expected symmetric dimer [Cp*(dppe)Fe{ꢀC(OCH3)–
CHꢀCH–C(OCH3)ꢀ}Fe(dppe)Cp*] with the dimeth-
described was reacted with two equivalent potassium
tert-butoxides in THF at −80°C. Upon warming to
20°C, the initialy brown solution turned progressively
yellow over 1 h. After removal of the solvent and
pentane extraction, a red–orange powder correspond-
ing
to
the
iron
p1-butadienyl
complex
Cp*(dppe)Fe(CHꢀCH–CHꢀCH2) (4) was obtained in
76% yield (Scheme 1). Further recrystallization from
pentane at −20°C provided an analytically pure com-
pound. Under the above-mentioned conditions, the use
of one equivalent of potassium tert-butoxide resulted in
uncomplete conversion of 2. Complex 4 was character-
ized by mass spectrometry, multinuclear magnetic reso-
nance spectroscopy and cyclic voltammetry.
The formation of 4 should proceed through a one-
proton abstraction reaction with KOtBu providing the
vinyl-carbene 3 as a putative intermediate. A subse-
quent 1,2-hydride migration at the carbene side yields
the butadienyl ligand. Similar migrations in monomeric
alkylidene complexes are well known [14,15]. It has also
been established that this process is accelerated by the
presence of an electron releasing substituant at the Ci
position [16–19] and it is anticipated that the first
deprotonation should increase the electron density on
the Ci atom of the remaining alkylidene fragment. As a
consequence, the intramolecular hydride shift occurs
faster than the intermolecular second deprotonation,
even if the reaction is carried out with 5-fold excess of
the base. After the hydride migration step, the decoor-
dination of the iron building block from the CꢀC
double bond should be a favored process. Indeed, the
oxybutadiendiyl
bridge
is
isolated
almost
quantitativelty [21].
The new air-sensitive complex 4 shows the character-
istic 1H NMR resonances for the C4 p1-coordinated
ligand: a broad doublet at l 7.96 (C6D6, TMS) with
3J(HH) of 16.4 Hz, a doublet of doublets at l 5.63 with
3J(HH) of 16.2 and 16.4 Hz, a double triplet at l 6.71
3
with J(HH) of 16.2 and 10 Hz, and doublet at l 4.49
with 3J(HH) of 10 Hz corresponding to the proton
bound to the h–l carbon atoms respectively. In the 13C
NMR spectrum, the h–l carbon resonances of the
butadienyl ligand are observed at l 180.6, 146.5, 144.2
and 99.8 respectively. The high resolution FAB mass
spectral analysis of a sample of 4 dissolved in metani-
trobenzylic alcohol (m-NBA) shows the expected

V. Guillaume et al. / Journal of Organometallic Chemistry 554 (1998) 203–205
205
molecular pic for this mononuclear complex (Mcalc
=
References
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cyclic voltammogram of 4 from −1 to +0.7 V (versus
SCE) at a platinum electrode (CH2Cl2, 0.1 M tetrabuty-
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tetrenediyl-bridged binuclear complex [Cp*(dppe)
Fe(ꢀCH–CHꢀCH–CH2–CH2–CHꢀCH–CHꢀFe(dppe)
Cp*][PF6]2 could be an accessible synthetic target upon
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Acknowledgements
We are grateful to Drs. P. Gue´not and S. Sinbandhit
(CRMPO, Rennes, France) for mass spectrometry and
NMR measurements. Dr. F. Paul (Rennes) is gratefully
acknowledged for stimulating discussions.
.