Unexpected migration of the indenyl ligand in the reaction of alkynyl(η5-indenyl)iron complexes with phosphanes and phosphites from iron to the alkynyl group

Article abstract of DOI:10.1039/a902785h

Novel indenylvinylidene iron complexes are formed in the reaction of alkynyl(η⁵-indenyl)iron complexes with phosphites and phosphanes in large excess by an intermolecular transfer of the indenyl ligand from iron to the C(β) atom of the alkynyl ligand; mechanistic studies indicate that the reaction proceeds by a radical pathway.

Full text of DOI:10.1039/a902785h

Unexpected migration of the indenyl ligand in the reaction of
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alkynyl(h -indenyl)iron complexes with phosphanes and phosphites from iron
to the alkynyl group
Helmut Fischer* and Peter A. Scheck
Fakulta¨t fu¨r Chemie, Universita¨t Konstanz, Fach M727, D-78457 Konstanz, Germany.
E-mail: hfischer@dg6.chemie.uni-konstanz.de
Received (in Cambridge, UK) 8th April 1999, Accepted 29th April 1999
Novel indenylvinylidene iron complexes are formed in the
are derived from 1a–e by substitution of P(OMe)3 for one CO
ligand, addition of two more molecules of P(OMe)3 and an
unusual transfer of the indenyl group from iron to the Cb atom
of the alkynyl ligand. Such a migration is without precedence.
The migration is highly selective. In the reaction of 1e with
P(OMe)3, in addition to 2e, only the vinylidene complex 3e is
observed. The alternative butatrienylidene complex that would
have been formed by migration of the indenyl ligand to the Cd
atom of the butadiynyl ligand could not be detected.
The product ratio 2a–e:3a–e is strongly temperature depend-
ent. With increasing temperature it is shifted towards the
substitution product 2. On prolonged heating at 70 °C in neat
P(OMe)3 the alkynyl complexes 2a–d are also transformed into
the vinylidene complexes 3a–d.
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reaction of alkynyl(h -indenyl)iron complexes with phos-
phites and phosphanes in large excess by an intermolecular
transfer of the indenyl ligand from iron to the Cb atom of the
alkynyl ligand; mechanistic studies indicate that the reac-
tion proceeds by a radical pathway.
Many associative substitution reactions of coordinatively
saturated cyclopentadienyl and indenyl complexes have been
documented.1 These reactions have been interpreted as proceed-
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3
5
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ing via an h ?h ?h ring slippage.2 The conversion of h - to
3
1
isolable h - or even h -cyclopentadienyl or indenyl complexes
has also been observed.3 The cleavage of the cyclopentadienyl
ligand as [C5H5]2 from [(h -Cp)(CO)(NO)Re–Me] or mer-
5
1
[(h -Cp)(NO)(PMe3)3Re–Me] in the reaction with PMe3 in
When the nucleophilicity of the alkynyl Cb atom is changed
by variation of its substituent (R = Me, C6H4OMe-p, C6H4Me-
p, Ph, C6H4Br-p, SiMe3) the product distribution and the
reaction rate are only slightly influenced. Therefore, a bimo-
lecular mechanism via a nucleophilic attack of the alkynyl Cb
atom at an indenyl ligand is unlikely. In contrast, the steric
requirements of the P-donors considerably influence the
reaction rate and the product distribution. For instance, 1a reacts
significantly more slowly with P(OEt)3 than with P(OMe)3.
When the even bulkier P(OPri)3 or PPh3 are employed in the
reaction with 1a no monocarbonyl-vinylidene complex analo-
gous to 3a–e is formed but rather a dicarbonyl-vinylidene
complex [(CO)2(PR3)3FeNCNC(inden-1-yl)Ph] (R = OPri 4 or
large excess to give ionic trans-[(NO)(PMe3)4Re–
Me]+[C5H5]2 has also been described.4 Finally, the loss of the
indenyl ligand formally as a cation and the formation of
[R3P(Ind)]+ in the reaction of [Cp(h -Ind)(CO)2Mo]2+ with PR3
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was recently proposed by Roma˜o and coworkers.5 We now
report on (i) an unusual temperature-controlled product se-
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lectivity in the reaction of (alkynyl)(h -indenyl)iron complexes
with phosphites and phosphanes, (ii) the novel transformation
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of (alkynyl)(h -indenyl)iron complexes into vinylidene com-
plexes by reaction with PR3, and (iii) the first evidence for the
release of the indenyl ligand as a radical.
5
5
The alkynyl(dicarbonyl)(h -indenyl)iron complexes [(h -
Ind)(CO)2Fe–C·CR] (R = Ph 1a, C6H4Me-p 1b, Me 1c, SiMe3
1d or C·CBun 1e) react in refluxing Bu2O with a slight excess
of P(OMe)3 within a few minutes by substitution of P(OMe)3
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Ph 5 in addition to the substitution product [(h -Ind)-
(CO)(PR3)Fe–C·CPh] (Scheme 2). The latter cannot be
transformed into a vinylidene complex.
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for one CO ligand to form a racemic mixture of [(h -
Labelling studies show that the Fe/Cb migration of the
indenyl ligand proceeds by an intermolecular pathway. The
Ind)(CO){P(OMe)3}Fe–C·CR] 2a–e. However, when 1a–e is
heated in neat P(OMe)3 for ca. 4 h at 70 °C, the complexes 2a–e
are formed in only minor amounts. The major products are the
novel monocarbonyl-vinylidene complexes [CO{P(OMe3}3-
FeNCNC(inden-1-yl)R] 3a–e (Scheme 1).† The complexes 3a–e
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reaction of a 1:1 mixture of undeuterated 1a and [(h -
1,3-C9H5D2)(CO)2Fe–C·C–C6D5] ([D7]-1a) with P(OMe)3 at
70 °C affords [D0]-, [D2]-, [D5]- and [D7]-3a in a ≈ 1:1:1:1
ratio as determined by mass spectrometry.
In the reaction of 1a with PPh3, complex 5 is formed
independent of whether the reaction is carried out in toluene,
EtOH or EtOH–H2O (20:1). Therefore, the formation of
[C7H9]+ and [(CO)2(PPh3)2Fe–C·CPh]2 or of [C7H9]2 and
[(CO)2(PPh3)2Fe–C·CPh]+ as intermediates is unlikely. Since
[(CO)2(PPh3)2Fe–C·CPh]2 and [C7H9]2 are rapidly trapped in
a large excess of EtOH–H2O as [(CO)2(PPh3)2FeNCNC(H)Ph]
and indene, respectively, complex 5 should not be formed.
Scheme 1
Scheme 2
Chem. Commun., 1999, 1031–1032
1031

The most probable mechanism involves dissociation of the
indenyl ligand as a radical and subsequent addition of [C7H9]· to
the alkynyl Cb atom of either [(CO)(PRA3)3Fe–C·CR]· or
[(CO)2(PRA3)2Fe–C·CR]·. This mechanistic proposal is sup-
ported by two observations: (i) addition of the radical-scavenger
galvinoxyl to the reaction mixture of 1a and P(OMe)3
efficiently inhibits the formation of 3a but does not affect the
formation of the substitution product 2a; (ii) dimerization of the
indenyl radical to give bisindenyl, C14H18, is preferentially
observed when the addition of the indenyl radical to the alkynyl
Cb atom is hindered by the sterically demanding mesityl (Mes)
substituent. Then, only small amounts of the corresponding
several other related 17-electron iron complexes [(CO)2-
(PR3)2FeX] (X = Br or I)7 and [{P(CH2CH2PPh2)3}FeC·CR]8
have been isolated and structurally characterized.
Financial support of this work by the Fonds der Chemischen
Industrie is gratefully acknowledged.
Notes and references
† Selected spectroscopic data: IR (CH2Cl2): n(CO): 1906 (3a), 1917 (3b),
1917 (3c), 1902 (3d), 1918 (3e), 1907, 1974 (4), 1896, 1962 cm21 (5); 13
C
NMR (CDCl3, 0 °C): d(FeNC) 312.8 (q, J 42 Hz) (3a), 313.8 (q, J 42 Hz)
(3b), 307.9 (q, J 42 Hz) (3c), 310.6 (q, J 41 Hz) (3d), 315.5 (q, J 41 Hz) (3e),
325.5 (t, J 63 Hz) (4), 325.2 (t, J 48 Hz) (5); 31P NMR (CDCl3): d 177.54
(3a), 177.68 (3b), 178.93 (3c), 180.16 (3d), 175 (3e), 165.05 (4), 73.54
(5).
vinylidene
complex,
[CO{P(OMe)3}3FeNCNC(inden-
1-yl)Mes], are formed. In addition, small amounts of 1,4-bis-
(mesityl)butadiyne are obtained. The butadiyne is presumably
formed by decomposition of the 17-electron intermediate
[CO{P(OMe)3}3Fe–C·CMes].
1 See, for example: J. A. S. Howell and P. M. Burkinshaw, Chem. Rev.,
1983, 83, 557; F. Basolo, Inorg. Chim. Acta, 1985, 100, 33; J. M.
O’Connor and C. P. Casey, Chem. Rev., 1987, 87, 307; F. Basolo,
Polyhedron, 1990, 9, 1503.
The indenyl radical intermediates are very likely stabilized by
addition to the phosphites or phosphanes which are present in
the reaction mixture in a large excess. The reversible addition of
radicals to PR3 is well known.6 The resulting phosphoranyl
radical [R3P(Ind)]· would then act as a mediator. The assump-
tion is supported by the following observation: when the
2 A. J. Hart-Davis and R. J. Mawby, J. Chem. Soc. A, 1969, 2403.
5
3
3 See, for example: h ?h : T. C. Forschner, A. R. Cutler and R. K.
5
1
Kullnig, Organometallics, 1987, 6, 889; h ?h : H. Werner and A. Ku¨hn,
Angew. Chem., 1979, 91, 416; Angew. Chem., Int. Ed. Engl., 1979, 91,
447; C. P. Casey and W. D. Jones, J. Am. Chem. Soc., 1980, 102,
6154.
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methylindenyl complex [(h -1-Me-Ind)(CO)2Fe-C·CPh] is
treated with P(OMe)3 in the presence of a mixture of P(Ind)3
and P(Ind)2OMe, both [CO{P(OMe)3}3FeNCNC(1-Me-inden-
1-yl)Ph] and [CO{P(OMe)3}3FeNCNC(inden-1-yl)Ph] are
formed in nearly equal amounts.
4 C. P. Casey, J. M. O’Connor and K. J. Haller, J. Am. Chem. Soc., 1985,
107, 1241.
5 C. A. Gamelas, E. Herdtweck, J. P. Lopes and C. C. Roma˜o,
Organometallics, 1999, 18, 506.
6 See, for example: W. G. Bentrude, Phosphorus Sulfur, 1977, 3, 109; B. P.
Roberts, Adv. Free Radical Chem., 1980, 6, 225.
7 H. Kandler, C. Gauss, W. Bidell, S. Rosenberger, T. Bu¨rgi, I. L.
Eremenko, D. Veghini, O. Orama, P. Burger and H. Berke, Chem. Eur. J.,
1995, 1, 541.
8 C. Bianchini, F. Laschi, D. Masi, F. M. Ottaviani, A. Pastor, M.
Peruzzini, P. Zanello and F. Zanobini, J. Am. Chem. Soc., 1993, 115,
2723.
The results demonstrate a novel route to vinylidene com-
plexes. Presumably, the transformation mode can also be
extended to other complexes such as alkynyl(allyl) and
alkynyl(fluorenyl) complexes. Based on previous results,4,5 we
initially assumed that the transformation most likely proceeds
by an ionic pathway via either an indenyl anion or cation.
However, all observations indicate a radical mechanism.
Presumably, more organometallic reactions proceed by a radical
mechanism than anticipated. There are some experimental hints
that the proposed 17-electron intermediate is rather long-lived
and can be intercepted also with other substrates. Recently,
Communication 9/02785H
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Chem. Commun., 1999, 1031–1032