C-H activation by a terminal imidoiron(III) complex to form a cyclopentadienyliron(II) product

Article abstract of DOI:10.1016/j.ica.2010.11.031

Imido complexes of the late transition metals have the ability to activate C-H bonds through the abstraction of hydrogen atoms from hydrocarbons. This paper describes the apparent hydrogen atom transfer from cyclopentadiene (CpH) to the isolable imidoiron

Full text of DOI:10.1016/j.ica.2010.11.031

Inorganica Chimica Acta 369 (2011) 40–44
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
C–H activation by a terminal imidoiron(III) complex to form
a cyclopentadienyliron(II) product
⇑
Ryan E. Cowley, Patrick L. Holland
Department of Chemistry, University of Rochester, Rochester, NY 14627, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 26 November 2010
Imido complexes of the late transition metals have the ability to activate C–H bonds through the abstrac-
tion of hydrogen atoms from hydrocarbons. This paper describes the apparent hydrogen atom transfer
from cyclopentadiene (CpH) to the isolable imidoiron(III) complex LMeFeNAd (LMe = 2,4-bis(2,6-di-
isopropylphenylimido)pent-3-yl; Ad = 1-adamantyl) in the presence of 4-tert-butylpyridine (tBupy).
The isolated product is not the amidoiron(II) complex, but instead it is LMeFe(Cp)(tBupy), the result of
amido protonation by additional CpH. The crystal structures of both LMeFeCp and LMeFe(Cp)(tBupy) are
presented, and in the latter compound the Fe–C bonds are longer than in any previously characterized
cyclopentadienyliron complex.
This article is dedicated to Robert G. Bergman,
a teacher, researcher, and mentor
of the highest order.
Keywords:
Imido complexes
Iron
Ó 2010 Elsevier B.V. All rights reserved.
Half-sandwich complexes
C–H activation
Crystallography
1. Introduction
complex and an organic radical. This C–H activation is the first step
of a hypothetical catalytic cycle for hydrocarbon amination.
Reactions that result in the removal of a hydrogen atom from a
hydrocarbon play an important role in organometallic chemistry
[1]. Though the term ‘‘C–H activation’’ is most often used to de-
scribe the single-step oxidative addition of a hydrocarbon to a me-
tal complex as elucidated by Bergman and others [2,3], there are
other ways to activate C–H bonds. For example, nature often uses
enzymatic iron-oxo (Fe@O) species to break a C–H bond in hydro-
carbons by abstracting a hydrogen atom; the resulting organic rad-
ical reacts with the iron hydroxide in a ‘‘rebound’’ step, with the
ultimate result being formal insertion of an oxygen atom into the
R–H bond to give ROH [4,5]. These natural hydroxylation reactions
have inspired chemists to create synthetic iron-oxo complexes that
hydroxylate hydrocarbons [6]. In an effort to design and under-
stand the analogous insertion of an NR fragment into hydrocarbon
C–H bonds, we have synthesized LMeFe@NAd (LMe = 2,4-bis(2,6-di-
isopropylphenylimido)pent-3-yl; Ad = 1-adamantyl), a rare exam-
ple of an imidoiron(III) complex [7]. This complex has been isolated
and characterized using crystallography, NMR, EPR, magnetism,
and X-ray absorption spectroscopy [8]. In the presence of 4-tert-
butylpyridine, it abstracts hydrogen atoms from hydrocarbons
such as 1,4-cyclohexadiene and indene, giving an iron(II) amido
In this paper, we describe the reaction of LMeFe@NAd with
cyclopentadiene, a reaction in which the iron product has gained
a cyclopentadienyl ligand. Cyclopentadienylmetal complexes are
an important class of organometallic complexes, and have found
many applications including olefin polymerization [9], asymmet-
ric catalysis [10], and polymer science [11]. Protonation of metal–
amido (M–NR2) complexes by cyclopentadiene (CpH) is a useful
route to metal–Cp complexes, especially for early transition met-
als [12–17]. The reaction of cyclopentadiene with metal–imido
(M@NR) complexes, however, has received little attention,
despite a renaissance in late metal–imido chemistry [18]. In
literature reactions, CpH reacts with complexes containing both
amido and imido ligands, but reacts only with the amido moiety
[17]. Here, we demonstrate that an imidoiron(III) complex reacts
with CpH to afford
a cyclopentadienyliron(II) product with
complete loss of the NR group. We give evidence that C–H
activation of CpH through hydrogen atom abstraction is a key
step in this reaction.
2. Experimental
2.1. General considerations
⇑
All manipulations were carried out under an N2 atmosphere in
an MBraun glovebox maintained at or below 1 ppm of O2 and H2O.
Corresponding author.
E-mail address: holland@chem.rochester.edu (P.L. Holland).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.11.031

R.E. Cowley, P.L. Holland / Inorganica Chimica Acta 369 (2011) 40–44
41
Proton NMR spectra were recorded using a Bruker Avance 500
instrument at 25 °C, and chemical shifts were referenced to resid-
ual C6D5H at d 7.16 ppm. IR spectra were recorded on a Shimadzu
8400S FTIR spectrometer as KBr pellets. UV–Vis spectra were re-
corded on a Cary 50 spectrometer using screw-cap cuvettes.
Solution magnetic susceptibilities were determined using Evans’
method [19]. Microanalysis samples were sealed into airtight cups
in a VAC Atmospheres glovebox under argon and analyzed with a
PerkinElmer 2400 Series II Analyzer. LMeFeNAd [8], LMeFe(NHAd)
(tBupy) [7] and NaCp [20] were synthesized using literature meth-
ods. [LMeFeBr]2 was synthesized analogously to [LMeFeCl]2 [27].
4-tert-Butylpyridine (Aldrich) was distilled from CaH2 and stored
over activated 3 Å molecular sieves. Diethyl ether and pentane
were purified by passage through activated alumina and Q5 col-
umns from Glass Contour Co. (Laguna Beach, CA). Benzene-d6
was dried over flame-activated alumina. Before use, an aliquot of
each solvent was tested with a drop of sodium benzophenone ketyl
in THF. All glassware was dried overnight at 150 °C, and Celite was
dried at 250 °C overnight under vacuum.
regions of reciprocal space were surveyed: four major sections of
frames were collected with 0.50° steps in at four different set-
tings and a detector position of À33° in 2h. The intensity data were
corrected for absorption [22]. Final cell constants were calculated
from the xyz centroids of ca. 4000 strong reflections from the actual
data collection. The structures were solved using SIR97 [23] and re-
fined using SHELXL-97 [24]. The space groups were determined by
x
u
t
ꢀ
systematic absences (P21/c, 1Á Bupy) and intensity statistics (P1,
1). All non-hydrogen atoms were refined with anisotropic displace-
ment parameters. All hydrogen atoms were placed in ideal posi-
tions and refined as riding atoms with relative isotropic
t
displacement parameters. In the structure of 1Á Bupy, a single
non-negligible density peak near the Cp ligand (1.27 e ÅÀ3
,
ꢀ2.25 Å from Fe1) was observed, consistent with a minor occupa-
tional disorder of Cp with a chloride ligand. When Cl was included
in the model, the occupancy refined to 0.979:0.021 (Cp:Cl), i.e.,
ꢀ2% occupancy by Cl. Since the overall statistics were not signifi-
cantly improved, Cl was omitted from the final model. The struc-
ture of 1 was refined as a pseudomerohedral twin: application of
twin law [1 0 0/0 À1 0/ 0 0 À1], a rotation around direct lattice
[1 0 0], improved the R1 residual from 0.1747 to 0.0565. The final
t
2.2. Synthesis of LMeFe(Cp)(tBupy)(1Á Bupy)
full matrix least squares refinement for
1 converged to
A resealable NMR tube was loaded with LMeFeNAd (23.4 mg,
R1 = 0.0565 (F2, I > 2
r
(I)) and wR2 = 0.1335 (F2, all data), and for
t
37.6
lmol), 4-tert-butylpyridine (22
lL, 150
lmol), and 0.2 mL
1Á Bupy converged to R1 = 0.0449 (F2, I > 2
r(I)) and wR2 = 0.1240
C6D6. A solution of freshly cracked CpH in C6D6 (0.68 mL of a
(F2, all data).
0.11 M solution, 75 lmol) was added, causing an immediate color
change from red to orange. The 1H NMR spectrum showed that
3. Results and discussion
t
LMeFe(Cp)(tBupy) (1Á Bupy) was formed in 50% yield versus an
t
internal integration standard. At room temperature, 1Á Bupy exists
3.1. Reaction of imidoiron complex with cyclopentadiene
in equilibrium with pyridine-free 1, precluding full solution
t
characterization. 1H NMR (C6D6, 4 equiv Bupy): d 44 (1H,
a-H),
We recently showed that in the presence of 4-tert-butylpyridine
(tBupy) the iron(III) imido complex LMeFeNAd reacts with indene to
cleanly form the iron(II) amido complex LMeFe(NHAd)(tBupy)
through H-atom (HÅ) transfer [7,25]. Here we investigate the reac-
tion of LMeFeNAd with cyclopentadiene (CpH), a similar but smaller
substrate. Two equivalents of freshly cracked CpH were added to a
pentane solution of LMeFeNAd and 4 equiv of tBupy. The color chan-
ged from bright red to orange-brown within 10 s, and the 1H NMR
spectrum showed a new paramagnetic species as the major prod-
uct. Interestingly, this 1H NMR spectrum showed no amidoiron(II)
complex LMeFe(NHAd)(tBupy), though this was the sole product of
the reaction of LMeFeNAd with indene [25]. The product of the CpH
reaction was identified by X-ray crystallography as the half-
12 (6H, CH3), 9 (tBupy), 8 (tBupy), 1 (tBupy), À4 (12H, CH(CH3)2),
À7 (4H, m-Ar or CH(CH3)2), À23 (5H, Cp-H), À27 (2H, p-Ar), À47
(12H, CH(CH3)2), À96 (4H, m-Ar or CH(CH3)2).
2.3. Synthesis of LMeFeCp (1)
To a slurry of [LMeFeBr]2 (423 mg, 0.382 mmol) in Et2O (10 mL)
was added a slurry of NaCp (67 mg, 0.76 mmol) in Et2O (2 mL). The
mixture was stirred for 16 h, and the volatile materials were re-
moved under vacuum. The residue was extracted into pentane
(10 mL), and a white solid was removed by filtration over a pad
of Celite. The orange supernatant solution was concentrated to
5 mL, and orange-brown crystals were deposited upon storing
the solution at À45 °C. The mother liquor was decanted from the
crystals and concentrated to 2 mL, affording a second crop of crys-
tals at À45 °C. The crystalline products were combined and dried
under vacuum (163 mg + 135 mg, total yield 73%). 1H NMR
sandwich iron(II) complex LMeFe(Cp)(tBupy) (1Á Bupy) (Fig. 1b).
t
The Fe–Ndiketiminate bond lengths (2.076(1) and 2.090(1) Å) are
the longest of any (diketiminate)–iron complex in the Cambridge
Structural Database (CSD) [26], perhaps as a result of steric pres-
t
sure from the Cp and Bupy ligands. The iron–Cp distance is also
(C6D6): d 58 (1H,
a
-H), 19 (6H, CH3), À5 (12H, CH(CH3)2), À10
unusually long (Fe–Cpcentroid = 2.11 Å; Fe–Cpplane = 2.09 Å). This
feature is discussed in further detail below.
(4H, m-Ar or CH(CH3)2), À13 (5H, Cp-H), À27 (2H, p-Ar), À51
(12H, CH(CH3)2), À105 (4H, m-Ar or CH(CH3)2). leff (C6D6, 25 °C):
4.9 0.1
lB. IR (KBr): 3057 (w), 3018 (w), 2962 (s), 2927 (m),
2868 (m), 1528 (s), 1433 (s), 1388 (s), 1317 (s), 1267 (m), 1252
(w), 1231 (w), 1175 (m), 1100 (w), 1055 (w), 1030 (w), 1009 (w),
933 (w) cmÀ1. UV–Vis (pentane): 330 (10 000 MÀ1 cmÀ1), 360
(sh, ꢀ5500 MÀ1 cmÀ1), 390 (sh, ꢀ3300 MÀ1 cmÀ1), 500 (sh,
3.2. Proposed mechanism
Based on the precedent of H-atom abstraction reactions of
LMeFe(NAd)(tBupy) (e.g., with indene) [25], homolytic cleavage of
the Cp–H bond by the imide is reasonable, thus affording the
iron(II) amide LMeFe(NHAd)(tBupy) and CpÅ. We have shown
that the anilido complex LtBuFe(NHdipp) (dipp = 2,6-iPr2C6H3;
LtBu = HC[C(tBu)N(2,6-iPr2C6H3)]2) is protonated by the weak acids
HOtBu (pKa = 32 in DMSO) and HCCPh (pKa = 29 in DMSO) to give
H2Ndipp and the corresponding alkoxo and acetylide complexes
[27]. Thus, if formed in the reaction, the amido complex
LMeFe(NHAd)(tBupy) is expected to be protonated by CpH (pKa =
18 in DMSO). To test this hypothesis, we added 1 equiv CpH to
an independently prepared sample of LMeFe(NHAd)(tBupy) [7].
ꢀ400 MÀ1 cmÀ1), 880 (25 MÀ1 cmÀ1
)
nm. Anal. Calc. for
C34H46FeN2: C, 75.82; H, 8.61; N, 5.20. Found: C, 75.66; H, 8.73;
N, 5.14%.
t
2.4. Crystal structures of 1 and 1Á Bupy
Crystals were placed onto the tip of a ꢀ0.1 mm diameter glass
fiber and mounted on a Bruker SMART APEX II CCD Platform
diffractometer [21] for a data collection at 100.0(1) K using Mo
Ka radiation and a graphite monochromator. Randomly oriented

42
R.E. Cowley, P.L. Holland / Inorganica Chimica Acta 369 (2011) 40–44
t
Fig. 1. Structures of (a) LMeFeCp (1) and (b) LMeFe(Cp)(tBupy) (1Á Bupy) using 50% probability ellipsoids. Hydrogen atoms are removed for clarity, and only one unique
molecule in the asymmetric unit of 1 is shown. Selected bond distances (Å) and angles (°) for 1: Fe1–N11 1.902(2); Fe1–N21 1.904(2); Fe1–Cpcent 1.68, average CCp–CCp
t
1.402(6); N11–Fe1–N21 93.19(9); N11–Fe1–Cpcent 133.3; N21–Fe1–Cpcent 133.5. For 1Á Bupy: Fe1–N11 2.076(1); Fe1–N21 2.090(1); Fe–N14 2.221(1); Fe1–Cpcent 2.11,
average CCp–CCp 1.405(2); N11–Fe1–N21 89.61(4); N11–Fe1–N14 94.27(4); N21–Fe1–N14 99.33(4); N11–Fe1–Cpcent 131.3; N21–Fe1–Cpcent 123.3; N14–Fe1–Cpcent 111.8.
Scheme 1.
t
The 1H NMR spectrum immediately showed complete conversion
We also investigated the conversion of 1 to 1Á Bupy. Interest-
ingly, the 1H NMR chemical shifts of 1 are dependent on the con-
centration of tBupy between 70 mM and 3.1 M, indicating that
tBupy binds 1 rapidly, reversibly, and weakly. An equilibrium
t
to 1Á Bupy (91% yield), showing that LMeFe(NHAd)(tBupy) is a
kinetically competent intermediate in the conversion of LMeFeNAd
t
to 1Á Bupy. Together, the data are consistent with the proposed
mechanism in Scheme 1.
An alternative description of the second step in Scheme 1 could
be a second HÅ transfer from CpH, giving iron(I) and CpÅ, which
would then form the observed iron(II)/CpÀ product via electron
transfer. However, less acidic substrates such as 1,4-cyclohexadi-
ene do not react with LMeFe(NHAd)(tBupy), thus arguing against
this alternative mechanism.
3.3. Synthesis of pyridine-free LMeFeCp (1)
For comparison, we also synthesized the pyridine-free complex
LMeFeCp (1). Complex 1 was synthesized as a bright orange solid in
73% yield from [LMeFeBr]2 by anion metathesis with NaCp. Eight
paramagnetically shifted peaks are observed in the 1H NMR spec-
trum of 1, consistent with an average C2v symmetry on the NMR
timescale. A broad resonance in the 1H NMR spectrum of 1 can
be assigned to the Cp hydrogens based on its integration of five
protons. The ability to observe these protons by 1H NMR spectros-
copy is surprising because
(diketiminate)Fe(alkyl) are too broad to be observed [28,29]. The
solution effective magnetic moment of 1 (4.9 B) is consistent with
a and b protons in the alkyl group of
l
a high-spin iron(II) ground state (S = 2). Complex 1 was also
characterized by X-ray crystallography (Fig. 1a). The complex
crystallizes with two independent molecules in the asymmetric
unit, and both show very similar structural features. In contrast
t
to 1Á Bupy, compound 1 has a more typical Fe–Cp distance
Fig. 2. Histogram of all iron–Cp centroid distances for iron–C5H5 in the CSD
(n = 8187) [26]. The locations of compounds 1 and 1Á Bupy are indicated.
t
(Fe–Cpcentroid = 1.68 Å; Fe–Cpplane = 1.68 Å).

R.E. Cowley, P.L. Holland / Inorganica Chimica Acta 369 (2011) 40–44
43
t
Fig. 3. Comparison of the cores of 1 (left) and 1Á Bupy (right), looking perpendicular to the C5H5 plane. Fe–C (in bold) and C–C (in italic) distances are given in Å. The atoms
N11 and N21 are the ligating atoms from the diketiminate ligands, and N14 is the ligating atom from tBupy.
constant of Keq = 1.3 0.1 MÀ1 was calculated from the [tBupy]
dependence of the chemical shifts using an iterative method [30].
This binding constant is ꢀ104 smaller than the binding affinity of
pyridine to the iron(II) complex LtBuFeCl (Keq = 41 000 6000)
[31]. This is most likely a steric effect, in which the bulkier Cp li-
gand creates a much smaller pyridine binding pocket. This steric
effect is also manifested in the Fe–Npy bond length of 2.221(1) Å
C–H bond is homolyzed. Cyclopentadiene is also acidic enough to
protonate the amido ligand in the amidoiron(II) species, affording
t
the observed final product, LMeFe(Cp)(tBupy) (1Á Bupy). The depro-
tonation of weak acids by a group 8 amido complex has been char-
acterized thoroughly by Bergman and coworkers [33]. The
t
structural changes between 1 and 1Á Bupy are also remarkable,
with the Cp ligand pushed 0.43 Å further away from Fe in the pres-
t
t
in 1Á Bupy, which is significantly longer than iron–pyridine dis-
ence of tBupy. The greatly decreased Fe–Cp bonding in 1Á Bupy
tances in other complexes of the type (diketiminate)FeII(X)(py)
causes the overall binding of the pyridine to 1 to be surprisingly
(average Fe–Npy = 2.12(2) Å, n = 9) [26].
weak.
3.4. Structural comparison of Fe–Cp complexes
Acknowledgements
t
The presence of Bupy significantly changes the Fe–Cp interac-
P.L.H. acknowledges funding from the National Science Founda-
tion (CHE-0911314), and R.E.C. acknowledges the NSF and the
University of Rochester for fellowships. Microanalytical data were
obtained at the CENTC Elemental Analysis Facility funded by the
NSF (CHE-0650456). We thank Dr. William Brennessel for assis-
tance with X-ray crystallography and for obtaining microanalytical
data.
tion, as evidenced by the lengthening of the Fe–Cpcentroid distance
t
from 1 (1.68 Å) to 1Á Bupy (2.11 Å). To compare these distances
to literature values, we surveyed the Cambridge Structural Data-
base [26] for all crystallographically characterized Fe–C5H5 dis-
tances (Fig. 2). The Fe–Cp centroid distance in compound 1 is
unremarkable, lying near the mean Fe–C5H5 distance of
t
1.68 0.04 Å. In contrast, however, 1Á Bupy lies significantly out-
side the range of previously observed distances.
Appendix A. Supplementary material
t
A closer inspection of the coordination sphere of 1Á Bupy shows
that although the Cp ring is planar (largest deviation from the
least-squares plane is 0.0018 Å), the Cp ring is slightly tilted
(6.9°) with respect to the Fe–Cpcentroid vector, and pushed away
from the crowding pyridine (Fig. 3). As a result, the two FeÁ Á ÁC
interactions abutting the pyridine ligand (2.54 and 2.50 Å) are
slightly longer than the other three (2.30, 2.35, and 2.42 Å). This
could be described as partial ring slippage, resulting in a resonance
CCDC 785971 and 785972 contain the supplementary crystallo-
t
graphic data for 1Á Bupy and 1, respectively. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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a useful synthon for assembling half-sandwich compounds. In par-
ticular, the reaction of LMeFe(NAd)(tBupy) with CpH likely proceeds
through a stepwise reaction in which the first step is hydrogen
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t
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