Elementary steps of iron catalysis: Exploring the links between iron alkyl and iron olefin complexes for their relevance in C-H activation and C-C bond formation

Article abstract of DOI:10.1002/anie.201410069

The alkylation of complexes 2 and 7 with Grignard reagents containing β-hydrogen atoms is a process of considerable relevance for the understanding of C-H activation as well as C-C bond formation mediated by low-valent iron species. Specifically, reaction of 2 with EtMgBr under an ethylene atmosphere affords the bis-ethylene complex 1 which is an active precatalyst for prototype [2+2+2] cycloaddition reactions and a valuable probe for mechanistic studies. This aspect is illustrated by its conversion into the bis-alkyne complex 6 as an unprecedented representation of a cyclo-addition catalyst loaded with two substrates molecules. On the other hand, alkylation of 2 with 1 equivalent of cyclohexylmagnesium bromide furnished the unique iron alkyl species 11 with a 14-electron count, which has no less than four β-H atoms but is nevertheless stable at low temperature against β-hydride elimination. In contrast, the exhaustive alkylation of 1 with cyclohexylmagnesium bromide triggers two consecutive C-H activation reactions mediated by a single iron center. The resulting complex has a diene dihydride character in solution (15), whereas its structure in the solid state is more consistent with an η³-allyl iron hydride rendition featuring an additional agostic interaction (14). Finally, the preparation of the cyclopentadienyl iron complex 25 illustrates how an iron-mediated C-H activation cascade can be coaxed to induce a stereoselective C-C bond formation. The structures of all relevant new iron complexes in the solid state are presented.

Full text of DOI:10.1002/anie.201410069

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Angewandte
Chemie
DOI: 10.1002/anie.201410069
Organoiron Complexes
Elementary Steps of Iron Catalysis: Exploring the Links between Iron
ꢀ
Alkyl and Iron Olefin Complexes for their Relevance in C H
Activation and C C Bond Formation**
ꢀ
Alicia Casitas, Helga Krause, Richard Goddard, and Alois Fꢀrstner*
Abstract: The alkylation of complexes 2 and 7 with Grignard
reagents containing b-hydrogen atoms is a process of consid-
erable relevance for the understanding of C–H activation as
well as C–C bond formation mediated by low-valent iron
species. Specifically, reaction of 2 with EtMgBr under an
ethylene atmosphere affords the bis-ethylene complex 1 which
is an active precatalyst for prototype [2+2+2] cycloaddition
reactions and a valuable probe for mechanistic studies. This
aspect is illustrated by its conversion into the bis-alkyne
complex 6 as an unprecedented representation of a cyclo-
addition catalyst loaded with two substrates molecules. On the
other hand, alkylation of 2 with 1 equivalent of cyclohexyl-
magnesium bromide furnished the unique iron alkyl species 11
with a 14-electron count, which has no less than four b-H atoms
but is nevertheless stable at low temperature against b-hydride
elimination. In contrast, the exhaustive alkylation of 1 with
cyclohexylmagnesium bromide triggers two consecutive C–H
activation reactions mediated by a single iron center. The
resulting complex has a diene dihydride character in solution
(15), whereas its structure in the solid state is more consistent
with an h3-allyl iron hydride rendition featuring an additional
agostic interaction (14). Finally, the preparation of the cyclo-
pentadienyl iron complex 25 illustrates how an iron-mediated
C–H activation cascade can be coaxed to induce a stereoselec-
work seem to switch this isomerization process on and off.
Owing to this enigmatic behavior, we conjectured that this
system provides an opportunity to study and eventually
control iron-mediated C–H activation processes which may
be of more general interest.[8] Furthermore, complexes of the
general type [L2Fe(alkene)2] are also expected to be catalysts
for other transformations upon replacement of the olefinic
ligands by more reactive substrates. In any case, they should
serve as valuable mechanistic probes for reactions in which
low-valent iron species of unknown constitution are gener-
ated in situ by empirically optimized recipes.[9]
In this context, we planned to use the bis-ethylene
complex 1 for further mechanistic investigations into iron
catalysis (Scheme 1).[10–13] The bis(diisopropylphosphino)-
propane ligand (dippp) was chosen in accordance with
Scheme 1. a) EtMgBr, THF, ethylene, ꢀ58C, 77–97%.
literature to ensure the necessary solubility of 1 and its
precursor complex 2;[2–7,14,15] at the same time, dippp is
thought to balance the steric shielding of and the accessibility
to the unsaturated iron center in 1. However, initial attempts
to reduce complex 2 with activated magnesium or lithium
sand in the presence of ethylene in analogy to the liter-
ature[2–7] gave erratic results. Gratifyingly, we found EtMgBr
to be a much more reliable reducing agent. The structure of
1 in the solid state suggests that backbonding from the metal
center into the p* orbitals of the ligated olefins must be
substantial.[14,16]
ꢀ
tive C C bond formation. The structures of all relevant new
iron complexes in the solid state are presented.
T
he reduction of complexes of type [L2FeX2] (L = phos-
phine, X = Cl, Br) with magnesium in the presence of alkenes
(dienes) is a gateway to coordinatively unsaturated 16-
electron iron(0) bis-olefin (diene) complexes of the general
types [L2Fe(alkene)2] and [L2Fe(diene)].[1–5] Although several
members of these series have been well characterized, their
reactivity patterns remain somewhat opaque.[6] For example,
1,5-diene substrates may or may not get isomerized to the
corresponding 1,3-dienes upon complexation;[7] small changes
in the structure of the bidentate phosphorus ligand frame-
Despite their apparent stabilizing function,[17] it seemed
plausible that the olefins in 1 can be replaced by either 1,3-
dienes or by acetylenes which tend to bind low-valent iron
centers more tightly.[10,18] As such substrates are potentially
more reactive too,[19] it met with our expectation that
1 catalyzes the [2+2+2] cycloaddition of prototype alkynes
or alkyne/nitrile combinations (Scheme 2).[20] Although we do
not pretend that the bulky and electron-rich dippp is the
optimal ligand, we note the close analogy of complex 1 to
a recent report in which a structurally undefined cyclo-
trimerization catalyst was generated in situ by reduction of
FeI2 with zinc dust in the presence of diphenylphosphino-
propane.[21] This congruence corroborates the notion that
complex 1 is a valuable source of [L2Fe0] fragments for use in
catalysis and (mechanistic) organometallic chemistry.[22]
[*] Dr. A. Casitas, H. Krause, Dr. R. Goddard, Prof. A. Fꢀrstner
Max-Planck-Institut fꢀr Kohlenforschung
45470 Mꢀlheim/Ruhr (Germany)
E-mail: fuerstner@kofo.mpg.de
[**] Generous financial support by the MPG, the Fundaciꢁn Ramꢁn
Areces (fellowship for A.C.) and SusChemSys (Ziel 2 Programm
NRW 2007–2013) is gratefully acknowledged. We thank Dr. C. Farꢂs
for recording the low-temperature NMR spectra and for helpful
discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410069.
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Figure 1. Structure of complex 6 in the solid state; for clarity, the
isopropyl groups at both P atoms were removed (the full structure is
contained in the Supporting Information).
ment. The very fast reduction of 2 to 1 via complex 9 as the
presumed intermediate must therefore proceed by a different
mechanism, most likely by means of b-hydride elimination
(9!10!1).[31] This pathway, however, is by no means obvious
because the tetrahedral FeII phosphine complexes 8 were
previously shown to be paramagnetic high-spin species with
four unpaired electrons (meff = 4.9 mB); as a result, all metal
orbitals are at least singly occupied.[30,32] If an analogous
diethyl complex 9 is passed through en route to 1,[33] which
likely shares this geometrical and electronic configuration,
spin crossover (most probably associated with a rearrange-
ment from tetrahedral to square-planar coordination geom-
etry) must first vacate an empty orbital before b-hydride
elimination can actually take place.[34–36]
Scheme 2. a) 1 (10 mol%), THF, ꢀ508C!RT; b) 1, ꢀ58C, then
ꢁ
ꢁ
PhC CPh, 70%; c) FeBr2, dippp, PhC CPh, Mg, THF, 08C, 53%.
The iron-catalyzed cyclotrimerization of diarylalkynes is
known to be considerably more difficult than that of aliphatic
alkynes.[23] Therefore it was hoped that the reaction of 1 with
tolane would allow reactive intermediates to be studied.[24]
Despite the electron-rich nature of 1, however, the formation
of the expected metallacycle of type 3 (or a descendent
thereof) was not observed;[25,26] rather, treatment of 1 with
tolane furnished the bis(alkyne) Fe0 complex 6, which seems
to be without precedent.[27,28] Massive electron back-donation
from the metal center into the p* orbitals of the acetylenic
Under the premise of such a “two-state reactivity”,[37] it is
tempting to speculate that iron(II) phosphine complexes
might exist with alkyl substituents amenable to b-hydride
elimination that are sufficiently long lived in the tetrahedral
high-spin state to be isolated and characterized. Although we
have not yet managed to obtain a dialkyl variant of such an
elusive species, reaction of 2 with cyclohexylmagnesium
chloride at ꢀ458C delivered the unorthodox iron(II) mono-
alkyl complex 11. Its structure in the solid state shows a 14-
electron diphosphine iron(II) compound bearing an alkyl
residue with no less than four hydrogen atoms amenable to b-
hydride elimination, which is nonetheless stable at low
temperature for extended periods of time (Figure 2). This
pattern seems to be unprecedented;[35,36] complex 11 can
therefore be regarded as the missing link between the known
dialkyliron complexes 8[30] and the transient diethyliron
intermediate 9 invoked in the formation of the iron(0) bis-
ethylene complex 1 outlined above.
ꢁ
ligands can be inferred from the very elongated C C bonds
(1.286(2)/1.290(2) ꢀ in 6 versus 1.198(3) ꢀ in free tolane)[29]
and the high degree of rehybridization expressed in the
bending of the phenyl rings away from linearity with the
ꢁ
former triple bond (the bond angles C C-C(ipso) are
138.9(1)/140.9(1)8 and 141.6(2)/144.6(2)8) (Figure 1). More-
over, the two ligated alkynes are twisted against each other,
a geometry that is likely enforced by the sterically demanding
propellers of the forward-pointing P-iPr groups. This partic-
ular geometry, however, obstructs significant orbital overlap
between the ligand p bonds and hence prevents metallacycle
formation. Complex 6 is therefore thought to represent the
loaded catalyst ready to enter into the catalytic cycle of
a [2+2+2] cycloaddition reaction, but which is “locked” in
a pre-reactive state on steric grounds.
As mentioned above, the use of EtMgBr opens a conven-
ient and reliable entry into the iron ethylene complex 1. This
result in itself is nontrivial, since alkylation of the closely
related precursor complex 7 (X = Br) with dialkylmagnesium
reagents devoid of b-hydrogen atoms had been previously
shown to give isolable 14-electron FeII alkyl complexes 8
(Scheme 3).[30] Their meta stability implies that reductive
ligand coupling is slow in this particular coordination environ-
Attempts at isolating the analogous bis-cyclohexyl com-
plex 12 have so far met with failure. Rather than decomposing
to the corresponding bis(cyclohexene) complex 13 (L =
cyclohexene) as a sibling of the ethylene complex 1, this
fleeting intermediate evolves into another remarkable spe-
cies. Single-crystal X-ray diffraction showed that the resulting
2
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1408).[39] The Fe H bond length is
ꢀ
ꢀ
1.39(3) ꢀ, and the H···Fe H bond
angle is close to linearity (171(2)8).
The notion of a rather “advanced”
agostic state in 14 is corroborated by
the NMR data of this diamagnetic
species. When the NMR spectrum is
recorded at ꢀ808C, the presence of
a single signal in the hydride region at
dH = ꢀ19.37 ppm with an integral of
2H suggests that the complex in THF
solution features a diene dihydride
character 15 on the NMR timescale;
all other 1H, 13C, and 31P NMR data are
in line with the proposed symmetric
structure, which is reached by fully
ꢀ
breaking the formerly agostic C H
bond. Complex 14 is reminiscent of
the classical species 16[BF4][40] and
17[SO3F],[41] which provided early
illustrations for the peculiar three-
center/two-electron bonding mode
that later became commonly known
as “agostic” interaction.[39] It is of note,
however, that the duality of the new
species 14/15 is a distinctive feature, as
is the preparation of this neutral com-
plex by C–H activation; we recall that
the cationic complexes 16 and 17 were
Scheme 3. a) EtMgBr, THF, ethylene, ꢀ58C, 77–97%; b) C6H11MgCl (1 equiv), THF, ꢀ358C, 51%
(11); c) C6H11MgCl (2 equiv), THF, cyclohexene (5 equiv), ꢀ308C, 79% (14/15).
Figure 2. Structure of complex 11 in the solid state; cocrystallized THF
was removed for clarity; for the entire structure, see the Supporting
Information.
Figure 3. Structure of complex 14 in the solid state; the two explicitly
shown H atoms interacting with the Fe center have been localized in
a difference Fourier map and their positions refined.
product 14 is an h3-allyliron hydride complex formed by
prepared by protonation of a diene iron precursor in strongly
acidic or even superacidic media.[40,41]
ꢀ
activation of the allylic C H bond of what originally must
have been a cyclohexene ligand in a transient complex of type
13 (L = neutral ligand).[38] For the iron center in 14 to gain an
18-electron count, it engages in a second C–H activation that
has not yet progressed much beyond the agostic state
(Figure 3). However, the recorded 1.66(3) ꢀ for the Fe···H
bond length occurs at the very low end of such agostic
interactions (usually between 1.8 and 2.3 ꢀ), whereas the H-
C-Fe bond angle (101(1)8) in 14 is in the typical range (90–
The ready formation of 14 at low temperature infers that
allylic C–H activation of an alkene ligand by the low-valent
iron center can be very facile.[42] Therefore, it was no surprise
that treatment of 18[15] with cyclopentadiene gives the cyclo-
pentadienyl iron ethyl complex 20,[14] which is apparently
formed by initial C–H activation and migratory insertion of
the remaining ethylene ligand into the transient iron hydride
bond (Scheme 4). Even more involved is the formation of
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.
Angewandte
Communications
compound 25, which was obtained from 1 and 1,3-cyclo-
hexadiene.[2] In this case, the ethyl and the cyclohexadienyl
groups of the presumed intermediate 23, generated by an
initial C–H activation, undergo reductive ligand coupling with
formation of a substituted iron(0) diene intermediate of type
24, which is set up for a second C–H activation that positions
the ethyl substituent endo relative to the iron center and
ꢀ
forms the observed Fe H bond (1.40(4) ꢀ). This transforma-
tion is a striking case of two consecutive C–H activations
ꢀ
intimately coupled with a stereoselective C C bond forma-
tion, all of which are mediated by a single iron center.
Although complex 25 and close analogues have previously
been described in the literature, they are either difficult to
retrieve and/or have not been fully delineated.[2,7b,c,43] There-
fore the structure of this noteworthy product in the solid state
is displayed in Figure 4.
Figure 4. Structure of complex 25 in the solid state; the hydrogen
ꢀ
atom of the Fe H bond has been localized in a difference Fourier map
and its position refined.
a true diene dihydride complex 15 in solution.
Finally, a prototype reaction cascade outlines how
ꢀ
to combine C–H activation with stereoselective C
C bond formation. Attempts to generalize these
unusual observations, to characterize the new
complexes in more electronic detail, and to
explore the obvious links of this chemistry to
iron catalysis[44] are subject to ongoing studies in
this laboratory.[45]
Received: October 14, 2014
Published online: && &&, &&&&
Keywords: agostic interactions · C–H activation ·
.
cycloadditions · iron · homogeneous catalysis
[1] a) H. Hoberg, K. Jenni, K. Angermund, C. Krꢁger,
Angew. Chem. Int. Ed. Engl. 1987, 26, 153 – 155;
Angew. Chem. 1987, 99, 141 – 142; b) H. Hoberg, K.
Jenni, C. Krꢁger, E. Raabe, Angew. Chem. Int. Ed.
Scheme 4. a) EtMgBr, THF, ethylene, ꢀ208C, then cyclopentadiene, ꢀ208!RT,
65%; b) EtMgBr, THF, ethylene, ꢀ58C, then 1,3-cyclohexadiene, ꢀ58C!RT, 40%.
Engl. 1986, 25, 810 – 811; Angew. Chem. 1986, 98,
819 – 820; c) H. Hoberg, K. Jenni, J. Organomet.
Chem. 1986, 301, C59 – C61; d) H. Hoberg, K. Jenni,
E. Raabe, C. Krꢁger, G. Schroth, J. Organomet. Chem. 1987, 320,
325 – 338; e) H. Hoberg, K. Jenni, J. Organomet. Chem. 1987,
322, 193 – 201.
To summarize, alkylation of diphosphine iron complexes 2
and 7 with alkyl Grignard reagents is an intriguing process.
On the one hand, it provides access to Fe0 diethylene
[2] A. J. Frings, Dissertation, Ruhr-Universitꢂt Bochum, 1988.
[3] K. Jonas, Pure Appl. Chem. 1990, 62, 1169 – 1174.
[4] See also: a) J. R. Bleeke, R. J. Wittenbrink, J. Organomet. Chem.
1991, 405, 121 – 132; b) H. Kubo, M. Hirano, S. Komiya, J.
Organomet. Chem. 1998, 556, 89 – 95.
[5] a) For a related (aminocarbene)iron(0) diene complexes, see: H.
Zhang, Z. Ouyang, Y. Liu, Q. Zhang, L. Wang, L. Deng, Angew.
Chem. Int. Ed. 2014, 53, 8432 – 8436; Angew. Chem. 2014, 126,
8572 – 8576; b) for a bis-carbene iron(0) arene complex, see: B.
Blom, G. Tan, S. Enthaler, S. Inoue, J. D. Epping, M. Driess, J.
Am. Chem. Soc. 2013, 135, 18108 – 18120.
complexes such as 1 and 18, which are valuable starting
points for investigations into iron catalysis and the organo-
metallic chemistry of this element. This aspect is illustrated by
the use of 1 as a catalyst for prototype [2+2+2] cycloaddition
reactions and by the preparation of an unprecedented Fe0
bis(alkyne) complex that mimics the loaded catalyst of such
transformations. On the other hand, alkylation of 1 with
cyclohexylmagnesium chloride furnished the unique FeII
cyclohexyl complex 11. Despite its 14-elecron count and
four H atoms at appropriate positions, 11 is surprisingly
resistant to b-hydride elimination. In contrast, treatment of
1 with an excess of cyclohexylmagnesium chloride gave
a remarkable complex with a chameleon nature, which
transmutes from a h3-allyliron hydride bonding mode in 14
with an additional agostic interaction in the solid state to
[6] Stoichiometric reactions showed such complexes to be nucleo-
philic at carbon, cf. Ref. [1].
[7] a) S. Geier, R. Goddard, S. Holle, P. W. Jolly, C. Krꢁger, F. Lutz,
Organometallics 1997, 16, 1612 – 1620; b) B. Gabor, R. Goddard,
S. Holle, P. W. Jolly, C. Krꢁger, R. Mynott, W. Wisniewski, Z.
Naturforsch. B 1995, 50, 503 – 513; c) B. Gabor, S. Holle, P. W.
Jolly, R. Mynott, J. Organomet. Chem. 1994, 466, 201 – 209;
4
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