Reductive coupling of CO templated by iron bound to the tris(pyrrolide)ethane scaffold

Article abstract of DOI:10.1021/om2003859

The reactivity of the high-spin (S = 2) [(^Mestpe)Fe(THF)] [Li(THF)₄] (1) complex (^Mestpe = tris(mesitylpyrrolide) ethane) with isocyanide and CO substrates is explored. Reaction of 1 with excess ^tBuNC forms a lo

Full text of DOI:10.1021/om2003859

ARTICLE
pubs.acs.org/Organometallics
Reductive Coupling of CO Templated by Iron Bound to the
Tris(pyrrolide)ethane Scaffold
Graham T. Sazama and Theodore A. Betley*
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
S Supporting Information
b
ABSTRACT: The reactivity of the high-spin (S = 2)
[(Mestpe)Fe(THF)][Li(THF)4] (1) complex (Mestpe = tris-
(mesitylpyrrolide)ethane) with isocyanide and CO substrates is
t
explored. Reaction of 1 with excess BuNC forms a low-spin
t
(S = 0), six-coordinate iron(II) species with three BuNC
ligands bound to iron, producing a notable tautomerization of
one of the pyrrolide units from N- to C-ligation to iron. Reaction
of 1 with an atmosphere of CO also produces a new diamag-
netic complex, wherein two molecules of CO are consecutively
reductively coupled, driven by the two-electron oxidation and fragmentation of the tris(pyrrolide)ethane ligand. The product features a
six-coordinate Fe(II) species bound toa dipyrrometheneligand(resulting fromoxidativefragmentationoftheMestpe ligand), an oxalyl-
imino pyrrole fragment from pyrrole coupling to two molecules of CO. The reactions of 1 with tBuNC and CO provide insight into
how tautomerization of the tris(pyrrolide) ligand upon substrate binding initiates the contiguous reductive coupling of CO.
he reduction of CO to form CÀC bonds is one of the two
key steps involved in the FischerÀTropsch (FT) process, in
Formation of [(Mestpe)Fe(THF)][Li(THF)4] (1) is accom-
plished by deprotonation of the (Mestpe)H3 ligand using base
(LiN(SiMe3)2, 3.05 equiv) in THF followed by subsequent
addition to a thawing slurry of divalent iron chloride in THF
(Scheme 1a; THF = tetrahydrofuran). Recrystallization from
THF and hexane affords a pale brown-yellow powder in 72%
T
which a heterogeneous catalyst combines CO and H2 to produce
long-chain hydrocarbons.1 Few homogeneous examples of FT-
relevant transformations are known, but have provided insight
into key challenges associated with FT chemistry: formation of
CÀC bonds from reduced carbonyl species, CÀH bond forma-
tion from metal-coordinated CO, and the role of exogenous
Lewis acids in promoting CO reduction. The first reports of facile
CÀC coupling from metalÀcarbonyl species arose from carbene
coupling following hydride or alkyl insertion into early transition
metal or lanthanide carbonyls2,3 or by alkali reduction of metal
carbonyl species followed by electrophilic trapping.4 These
reactions require the preparation of highly reducing transition
metal hydride species or utilize alkali metal reductants (e.g., Na)
to achieve carbonyl reduction. Two recent examples of FT-
relevant chemistry employ the formation of strong early transi-
tion metalÀoxygen bonds as a driving force to cleave the CÀO
bond and form new CÀC bonds.5 Moving away from strongly
reducing early transition metal hydride or alkyl precursors,
reductive coupling of metal carbonyls on a Re-based coordina-
tion complex was recently reported using a Lewis-acidic, pendant
borane to facilitate carbonyl activation via hydride insertion.6
From these precedents, a desirable reaction sequence would
employ coordination complexes that template the construction
of CÀC bonds from carbonyl ligands without requiring strong
chemical reductants. In this vein, we have investigated the
reactivity of the electron-rich [(Mestpe)Fe]À anion toward
carbonyl and isocyanide substrates, leading to a metal-templated
reductive coupling of CO initiated by the ligand framework.7
1
yield. The H NMR spectrum reveals 1 to be paramagnetic
(solution magnetic moment = 5.56(5) μB, consistent with a
spin-only value of S = 2),8 with 10 proton resonances apparent,
ranging from δ 80 to À6 ppm. The zero-field 57Fe M€ossbauer
spectrum (δ = 0.85 mm/s, |ΔEQ| = 2.55 mm/s) gives parameters
consistent with a high-spin formulation for 1 that are very similar
to the previously reported pyridine adduct, [(Mestpe)Fe(py)]À.7
X-ray diffraction quality crystals can be obtained from a
THFÀhexane solution stored for two weeks at À35 °C. The
crystal structure (Figure 1) shows the tpe ligand bound k3 to the
Fe center (FeÀN distances: 2.005(3), 2.028(3), and 2.033(3) Å)
with one THF molecule occupying a fourth coordination site
(FeÀO bond length = 2.023(2) Å).9 The Fe-bound THF
molecule is parallel to two mesityl fragments and canted away
from the third, and thus does not fall on the molecular-C3 axis
(defined by the vector containing the tpe ligand bridgehead and
the Fe center), giving an overall pseudotetrahedral geometry at
iron. A similar deviation from C3 symmetry was also observed in
the reported pyridine complexes; however in the pyridine com-
plexes this can be attributed to a significant π-stacking interaction
with a ligand mesityl unit not present in the THF complex.7
Received: May 9, 2011
Published: July 19, 2011
r
2011 American Chemical Society
4315
dx.doi.org/10.1021/om2003859 Organometallics 2011, 30, 4315–4319
|

Organometallics
ARTICLE
Scheme 1a
Figure 2. Solid-state molecular structure of [(N,N,C-Mestpe)Fe-
(CNtBu)3]Li(THF) (2, thermal ellipsoids shown at 50% probability
and hydrogens and mesityl groups omitted for clarity). Selected bond
lengths (Å) and angles (deg) for 2: Fe1ÀN1 2.0180(16); Fe1ÀN2
2.0538(17); Fe1ÀC30 2.2531(19); C1aÀN4a 1.166(3); C1bÀN4b
1.166(3); C1cÀN4c 1.162(3); N3ÀC30 1.421(3); C30ÀC31 1.433(3);
C31ÀC32 1.376(3); C32ÀC33 1.430(3); C33ÀN3 1.333(3); C2ÀC10
1.499(3); C2ÀC20 1.504(3); C2ÀC30 1.548(3); N1ÀFeÀN2 87.82(7),
N1ÀFeÀC30 79.31(7), N2ÀFeÀC30 78.62(7). Dihedral angle: C10À
C2ÀC1ÀC20 122.36°.
a Reaction conditions: (a) 1. LiN(SiMe3)2 (3.05 equiv), THF, 25 °C, 1.5
t
h; 2. FeCl2 (1.1 equiv), thawing THF, 3 h; (b) BuNC (3.3 equiv),
benzene, 25 °C; (c) CO gas (1 atm), benzene, 25 °C.
results from a spin-state change (from S = 2 for 1 to S = 0 for 2)
and back-bonding from Fe to three π-acidic isonitrile ligands.
The large decrease observed in the quadrupole splitting is
likewise attributable to the change in spin state, wherein the
asymmetric population of the e set in the approximately tetra-
hedral 1 ((e)3(t2)3) changes to the electronically symmetric,
octahedral coordination in 2 (t2g)6(eg)0.
Large, translucent, deep red crystals of X-ray diffraction quality
of 2 grew from saturated benzene solutions standing overnight at
room temperature under an inert atmosphere. The X-ray diffrac-
tion crystal structure of 2 shows three bound isonitrile molecules
(see Figure 2). The isonitrile CÀN distances are between
1.162(3) and 1.166(3) Å, and a strong, broad FTIR absorbance
(νCÀN) was observed at 2093 cmÀ1 (free isonitrile νCÀN
=
2137 cmÀ1), indicative of weak π-back-bonding from the low-
spin iron(II). Most striking about the structure of 2 is the
observed tautomerization of one pyrrolide unit, resulting in
pyrrole C-ligation to Fe (Fe1ÀC30, 2.253(2) Å) and the now
imino nitrogen (N3ÀC33: 1.333(3) Å, consistent with a CdN
formulation) bound to lithium (N3ÀLi: 2.253(2) Å). The
lithium cation is bound η5 to a neighboring pyrrolide unit (C/
NÀLi distances range from 2.250(4) to 2.578(4) Å) and one
molecule of THF. The pyrrole C-ligation to iron produces a bond
contraction of 0.035 Å of two backboneÀpyrrole bonds
(C2ÀC10 and C2ÀC20, from 1.536(5) and 1.537(5) Å in 1
to 1.499(3) and 1.504(3) Å in 2). This change suggests a
strengthening of the pyrroleÀbackbone connection with two
pyrroles, providing evidence that this mode of ligation is inter-
mediate between the N,N,N-ligation and a dipyrromethene. The
change in (Mestpe) ligand hapticity from N,N,N-ligation to N,N,
C-ligation likely arises from the inability of the (Mestpe) to
accommodate the three large isocyanide ligands. While the steric
Figure 1. Solid-state molecular structure for [(Mestpe)Fe(THF)]À (1)
(thermal ellipsoids shown at 50% probability and hydrogens and THF-
bound lithium counterion omitted for clarity). Selected bond lengths
(Å) and angles (deg): FeÀN1 2.033(3); FeÀN2 2.028(3); FeÀN3
2.005(3); FeÀO 2.023(2); N1ÀFeÀN2 93.93(11); N1ÀFeÀN3
95.39(11); N2ÀFeÀN3 95.37(11).
Reaction of 1 with tert-butylisocyanide (tBuNC, 3.3 equiv) at
room temperature in benzene immediately resulted in the
precipitation of a bright orange powder (92% yield, Scheme 1b).
The product, [(Mestpe)Fe(CNtBu)3]Li(THF) (2), was sparingly
soluble in benzene, but saturated solutions of 2 in C6D6 were
1
1
sufficient for obtaining assignable H NMR spectra. The H
NMR spectrum of 2 revealed a diamagnetic species, which
was reflected in the marked shift of the spectral parameters
obtained for 2 in the 57Fe M€ossbauer spectrum (δ = 0.12 mm/s,
|ΔEQ| = 0.47 mm/s). The significant decrease in isomer shift
4316
dx.doi.org/10.1021/om2003859 |Organometallics 2011, 30, 4315–4319

Organometallics
ARTICLE
Figure 3. Space-filling model of 2 where the three tBuNC carbon atoms
are colored gray and the (Mestpe) ligand carbon atoms are colored black.
Figure 4. Solid-state molecular structure of one-half of the dimeric
structure for {[(Mesdpme)Fe(CO)2(MesNC4H2-C(O)C(O))]Li}2 (3)
(thermal ellipsoids shown at 50% probability; hydrogens, mesityl
groups, one bridging lithium, and solvent molecules omitted for
clarity).12 Selected bond lengths (Å) for 3: Fe1ÀN1 2.0267(17);
Fe1ÀN2 2.0823(17). N1ÀFe1ÀN2 bond angle: 88.31(7)°. Dihedral
angle C10ÀC2ÀC1ÀC20: 173.36°.
pressure caused by binding of the three tert-butylisocyanide ligands
is apparent from viewing the space-filling model of 2 (see
Figure 3), another, more subtle driving-force is reorganization of
the (Mestpe) trianion to enable octahedral coordination of the
boundmetalion. The three NÀFeÀN bond angles in high-spin
1 range from 93.93(11)° to 95.39(11)°, but expand to an
average of 96.05(7)° in the diamagnetic [(Mestpe)Zn(py)]À
anion. Upon tautomerizing in 2 to the N,N,C-ligation, the
lithium atoms. Two molecules of THF complete the coordination
sphere for the lithium bridging via only one oxalato oxygen (see
full structure, available in SI). Bond lengths suggest an imino
formulation of the pyrrole nitrogen (N3ÀC34 = 1.339(3) Å,
consistent with a CÀN double bond). This fragment contains
π-bonds conjugated through the pyrrole-oxalyl fragment (see
Scheme 1, 3), terminating in an acyl carbon bound to iron
(Fe1ÀC29 = 1.948(3) Å). The two unfunctionalized bound
CO molecules display CÀO bond lengths of 1.144(3) and
1.150(3) Å, similar to lengths seen for other low-spin Fe2+
dicarbonyl complexes reported in the literature.11
The information obtained from the crystal structure data for
complexes 2 and 3 allow us to propose a reaction mechanism for
the formation of 3 from 1 (Scheme 2). We propose that upon
coordination of 3 equiv of CO, 1 initially forms a tautomer
intermediate analogous to the structure of 2 (Scheme 2, 2a). The
nucleophilic carbon-bound pyrrole subunit migrates to a bound
carbonyl to form a four-membered aza-metallacyclobutene (2b),
oxidizing the (Mestpe) trianion to the monoanionic dipyrro-
methene.13 The enolate-like structure of proposed interme-
diate 2b migrates to a second bound carbonyl, forming an R,
β-diketo-azametallacyclopentane (2c), presumably to relieve steric
strain from the aza-metallacyclobutene intermediate 2b, which
subsequently undergoes tautomerization to form the thermo-
dynamically stable imino-acyl dimer product 3. The steric bulk of
the tert-butylisocyanide ligands in 2 likely prevents the formation
of CÀC coupled product akin to 3,14 arresting the above reaction
sequence at the ligand tautomer 2.
(
Mestpe) bond angles decrease (N1ÀFeÀN2 87.82(7)°,
N1ÀFeÀC30 79.31(7)°, N2ÀFeÀC30 78.62(7)°). This change
in binding mode provides a greater volume to accommodate the
three linear tert-butylisocyanide ligands, facilitating the octahedral
coordination at iron in 2.
Exposure of 1 to one atmosphere of CO gas affords a markedly
different product from 2 that is soluble in most organic solvents
(Scheme 1c). 1H and 13C NMR (C6D6 solution) and M€ossbauer
spectra of this product represent a clean diamagnetic material,
with desymmetrization of the ligand environment apparent by
1H NMR. FTIR spectroscopy reveals four absorption bands at
1960, 1816, 1528, and 1480 cmÀ1, where the first two are likely
assignable to the CÀO stretches of terminally bound carbonyl
ligands (free CO νCO = 2143 cmÀ1). The latter two stretching
frequencies are consistent with ketone-like functionalities. The
M€ossbauer parameters obtained for3 (δ = 0.06 mm/s and|ΔEQ| =
1.20 mm/s) are similar to those obtained for 2. The low isomer
shift (relative to that obtained for 1, 0.85 mm/s) reflects a likely
spin-state change analogous to that observed in 2, with the
addition of π-back-bonding to CO ligands. Changes in the
quadrupole splitting, while less pronounced than in 2, again reflect
a change from high-spin to low-spin Fe2+.
Crystals suitable for X-ray crystallography can be grown from a
C6D6 solution after standing for two weeks at room temperature
under one atmosphere of CO. The crystal structure reveals that
the product, {[(Mesdpme)Fe(CO)2(MesNC4H2-C(O)C(O))]Li}2
(3, Figure 4, full structure available in SI), is a dimer of mono-
nuclear iron complexes in which the tpe ligand has undergone
oxidative fragmentation to form a dipyrromethene fragment10
and a separate, dicarbonylated pyrrole unit. The pyrrole unit has
nucleophilically coupled two reactant CO molecules, forming a
bidentate, oxalyl-imino moiety, giving rise to the two lower
carbonyl stretching frequencies observed in the IR spectrum of 3.
The two mononuclear Fe complexes are bridged via lithium
cations bound to the two oxalato-oxygens (O3 and O4 in
Figure 1c), with a crystallographic C2 symmetry axis through the
The proposed mechanism necessitates a closer inspection of
the role of lithium in the reductive coupling process. Given the
precedent for Lewis-acid activation of CO,6,15,16 the lithium
cation could bind and activate a bound CO toward nucleophilic
attack. In opposition to this, however, the CO coupling reaction
proceeds even in THF solutions, which should sequester the Li+
and preclude Lewis-acid activation of a CO ligand. Alternatively,
the presence of Li+ may facilitate the formation of tautomer 2a,
priming the nucleophile for the observed reductive coupling.
Substitution of the lithium for bulky, noncoordinating cations
4317
dx.doi.org/10.1021/om2003859 |Organometallics 2011, 30, 4315–4319

Organometallics
ARTICLE
Scheme 2. Proposed Mechanism of CO Reductive Coupling Reaction
+
(e.g., PPh4 , [Ph3PdNdPPh3]+) results in highly insoluble
species, but further examination may yet still elucidate the role
of lithium in the coupling process.
simulated with Igor Pro 6 software (WaveMetrics, Portland, OR) using
Lorentzian fitting functions.
X-ray Crystallography Procedures. All structures were col-
lected on a Bruker three-circle platform goniometer equipped with
either a Bruker Apex I or Apex II CCD and an Oxford Cryostream
cooling device. Radiation was from a graphite fine-focus sealed tube Mo
KR (0.71073 Å) source. Crystals were mounted on a cryoloop using
Paratone-N oil. All structures were collected at 100 K. Data were
collected as a series of j and/or ω scans. Data were integrated using
SAINT (Bruker AXS) and scaled with a multiscan absorption correction
using SADABS (Bruker AXS).17 The structures were solved by direct
methods or Patterson maps using SHELXS-9718 and refined against F2
on all data by full matrix least-squares with SHELXL-97. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed at idealized positions and refined using a riding model. The
isotropic displacement parameters of all hydrogen atoms were fixed to
1.2 times the U value of the atoms they are linked to (1.5 times for
methyl groups). Further details on several structures are noted below.
Note on 2: The structure of 2 contained a Li-bound THF that exhibited
positional disorder. No restraints were necessary to refine the model.
The major component of the disordered THF refined to 75%. Note on
3: A free benzene solvent molecule was disordered over two positions
and refined with the aid of similarity restraints, with the major
contributor refining to 68%. Note on 4: The structure of 4 contained
a Li-bound THF that exhibited positional disorder. EADP, SIMU, and
DELU restraints were used to refine the model, and the two parts refined
to 50% each.
We have observed the reductive coupling of two CO ligands
under mild reaction conditions (1 atm of CO), without the use of
strong exogenous reducing agents.16 The hapticity change of the
(
Mestpe) trianion facilitates coordination environment changes at
the metal ion, where N,N,N-ligation is preferred for tetrahedral
complexes and N,N,C-ligation preferentially stabilizes an octahe-
dral field. The facile interchange of the ligand coordination mode
generates a metal-bound nucleophile to trigger migration to a
metal-bound carbonyl, giving rise to CO reduction with con-
comitant (Mestpe) oxidative fragmentation in the absence
of an alkali metal reductant. A second carbonyl coupling is
likely facilitated by the strained four-membered azametallacycle
formed via the initial carbonyl insertion, where incorporation of a
second equivalent of CO results in a more stabilized five-mem-
bered oxalyl-imino pyrrole product. The results presented herein
allude to more general reactions based on a ligand platform similar
to that found in the final product and employing extant nucleo-
philes; this possibility is especially appealing given our group’s
concurrent research into the chemistry of dipyrromethene com-
plexes. We are currently pursuing further investigation of general-
ization of this reactivity.
’ EXPERIMENTAL SECTION
[(tpe)Fe(THF)][Li(THF)4] (1). In separate 20 mL scintillation vials,
tris(pyrrolyl)ethane (MestpeH3, 0.635 g, 1.10 mmol) and Li(N(SiMe3)2)
(0.554 g, 3.31 mmol) were eachdissolvedin4 mL ofTHF. The solution of
Li(N(SiMe3)2) was added to the solution of tpeH3 and stirred at room
temperature for1 h. In another 20 mLvialFeCl2 (0.166 g, 1.31 mmol) was
slurried in 2 mL of THF. Both vials were placed in a liquid nitrogen cooled
cold well until frozen. The ligand solution was thawed and added to the
stirring slurry of FeCl2. The reaction mixture became a translucent dark
brown solution as it warmed. The reaction was stirred at room tempera-
ture for 2 h, after which removal of the solvent in vacuo gave a brown
residue. The residue was extracted into 5 mL of diethyl ether and filtered
through diatomaceous earth to remove lithium chloride. THF (5 drops)
was added to the dark brown ether filtrate, and a yellow-brown precipitate
(1) formed. The solution was cooled to À35 °C overnight, and the
precipitate was collected on a medium fritted funnel. The solid was
purified by precipitation from THF with hexane, and 0.729 g (72% yield)
of yellow powder was collected and stored at À35 °C. Crystals suitable for
X-ray diffraction were grown from a mixture of THF and hexane at
À35 °C. 1H NMR (400 MHz, C6D6): δ/ppm: 78.84 (br s), 69.79 (br s),
48.22 (br s), 28.81 (br s), 26.69 (sh), 12.91 (br s), 4.10 (br s), 1.61 (br s),
1.02 (br s), À5.39 ppm (br s). Comb. Anal. Calcd for [C45H50-
FeN3O]À[LiC16H32O4]+: C, 73.25; H, 8.26; N, 4.20. Found: C, 73.19;
H, 8.08; N, 4.31.
All manipulations were carried out in the absence of water and
dioxygen using standard Schlenk techniques or in an MBraun inert
atmosphere glove box under a dinitrogen atmosphere. All glassware was
oven-dried for a minimum of 1 h and cooled in an evacuated ante-
chamber prior to use in the glove box. Benzene, diethyl ether, n-hexane,
tetrahydrofuran, and toluene were dried and deoxygenated on a Glass
Contour System (SG Water USA, Nashua, NH) and stored over 4 Å
molecular sieves (Strem) prior to use. Benzene-d6 was purchased from
Cambridge Isotope Laboratories and degassed and stored over 4 Å
molecular sieves prior to use. Pyridinium p-toluenesulfonate, ferroce-
nium hexafluorophosphate, dihydroanthracene, and anhydrous pyridine
were purchased from Aldrich and used as received. Anhydrous iron(II)
chloride was purchased from Strem and used as received. Celite 545
(J. T. Baker) and 4 Å molecular sieves were dried in either a Schlenk
flask or a vacuum oven for 24 h under dynamic vacuum while heating to
at least 150 °C.
1
Characterization and Physical Measurements. H and 13C
NMR spectra were recorded on Varian Mercury 400 MHz or Varian
Unity/Inova 500 MHz spectrometers. 1H and 13C NMR chemical shifts
are reported relative to residual solvent peaks as reference. Elemental
Analyses were carried out at Complete Analysis Laboratories, Inc.
(Parsippany, NJ).
[(tpe)Fe(CNtBu)3]Li(THF) (2). A benzene solution of tert-butyl iso-
cyanide (28.3 mg, 0.340 mmol) was added to a benzene solution of 1
(103.7 mg, 0.1037 mmol) and stirred two hours at room temperature.
57Fe M€ossbauer spectra were measured with a constant acceleration
spectrometer (SEE Co, Minneapolis, MN). Isomer shifts are quoted
relative to Fe metal at room temperature. Data were analyzed and
4318
dx.doi.org/10.1021/om2003859 |Organometallics 2011, 30, 4315–4319

Organometallics
ARTICLE
A bright orange precipitate formed out of the dark red-orange solution
and was collected and dried on a medium fritted funnel (91.4 mg, 92%).
Crystals suitable for X-ray diffraction were grown from a solution of 2 in
122, 1559. (k) Miller, R. L.; Toreki, R.; LaPointe, R. E.; Wolczanski,
P. T.; Van Duyne, G. D.; Roe, D. C. J. Am. Chem. Soc. 1993, 115, 5570.
(l) Wolczanski, P. T. Polyhedron 1995, 14, 3335.
1
(3) (a) Manriquez, J. M.; Fagan, P. J.; Marks, T. J.; Day, C. S.; Day,
V. W. J. Am. Chem. Soc. 1978, 100, 7112. (b) Marks, T. J. Prog. Inorg.
Chem. 1979, 25, 224. (c) Fagan, P. J.; Manriquez, J. M.; Marks, T. J. In
Organometallics of the f-Elements; Marks, T. J.; Fischer, R. D., Eds.; Reidel
Publishing Co.: Dordrecht, Holland, 1979; Chapter 4. (d) Fagan, P. J.;
Manriquez, J. M.; Marks, T. J.; Day, V. W.; Vollmer, S. H.; Day, C. S.
J. Am. Chem. Soc. 1980, 102, 5393. (e) Katahira, D. A.; Moloy, K. G.;
Marks, T. J. Organometallics 1982, 1, 1723.
(4) (a) Lam, C. T.; Corfield, P. W. R.; Lippard, S. J. J. Am. Chem. Soc.
1976, 21, 91. (b) Bianconi, P. A.; Williams, I. D.; Engeler, M. P.; Lippard,
S. J. J. Am. Chem. Soc. 1986, 108, 311. (c) Bianconi, P. A.; Vrtis, R. N.;
Rao, Ch. P.; Williams, I. D.; Engeler, M. P.; Lippard, S. J. Organometallics
1987, 6, 1968. (d) Carnahan, E. M.; Protasiewicz, J. D.; Lippard, S. J. Acc.
Chem. Res. 1993, 26, 90.
(5) (a) Matsuo, T.; Kawaguchi, H. J. Am. Chem. Soc. 2005,
127, 17198. (b) Shima, T.; Hou, Z. J. Am. Chem. Soc. 2006, 128, 8124.
(6) (a) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2008, 130, 11874. (b) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2010, 132, 3301. (c) Miller, A. J. M.; Labinger, J. A.; Bercaw,
J. E. Organometallics 2010, 29, 4499.
C6D6 at room temperature in an NMR tube. H NMR (500 MHz,
C6D6): δ/ppm 7.88 (d), 6.99 (s), 6.93 (s), 6.80 (m), 6.50 (s), 6.37 (s),
6.04 (s), 2.70 (s), 2.34 (m), 2.25 (s), 2.19 (s), 1.80 (s) 1.13 (s), 0.95 (s),
0.89 (s). Comb. Anal. Calcd for C60H77FeLiN6O: C, 74.98; H, 8.08; N,
8.74. Found: C, 74.86; H, 8.01; N, 8.59.
[(dpme)Fe(COCOC4H2NMes)]2[Li(THF)]2 (3). A 100 mL recovery
flask was charged with a 10 mL benzene solution of 1 (357.3 mg, 357.3
mmol) and sealed with a fresh rubber septum under nitrogen. Carbon
monoxide gas (35 mL at 1 atm, 1.43 mmol) was added via syringe to the
flask, and the reaction mixture was stirred for one hour at room
temperature. The solution was frozen and lyophilized, and 343.2 mg
of dark red-brown powder was isolated. The resultant powder was
purified by dissolving in hexane, filtering through diatomaceous earth to
remove unreacted 1, and collecting and evaporating the filtrate in vacuo,
providing dark red-brown 3 as a powder. X-ray diffraction quality crystals
1
were grown in saturated C6D6 solutions over a period of 2 weeks. H
NMR (500 MHz, C6D6): δ/ppm 7.38 (d, J = 4.4 Hz, 1 H), 7.24 (br s, 1
H), 7.03 (d, J = 4.4 Hz, 2 H), 6.90 (br s, 2 H), 6.75 (s, 1 H), 6.77 (s, 1 H),
6.51 (br s, 1 H), 6.39 (d, J = 3.9 Hz, 1 H), 6.27 (d, J = 3.9 Hz, 1 H), 6.22
(d, J = 4.4 Hz, 1 H), 3.53 (br s, 9 H), 2.71 (s, 3 H), 2.40 (s, 3 H),
2.25À2.34 (m, 6 H), 2.13À2.24 (m, 9 H), 1.97 (s, 6 H), 1.80 (s, 3 H),
1.36 (br s, 9 H). 13C NMR (126 MHz, C6D6): δ/ppm 212.4, 211.8,
184.8, 171.3, 164.9, 163.2, 161.9, 149.2, 142.8, 142.1, 140.8, 139.2, 138.9,
138.2, 138.0, 137.5, 137.2, 136.8, 136.7, 136.4, 135.2, 130.1, 129.8, 129.3,
129.0, 128.9, 128.1, 127.9, 127.4, 124.8, 123.0, 120.3, 119.6, 68.5, 26.0,
22.9, 22.8, 22.2, 22.2, 22.1, 21.8, 21.7, 21.6, 21.5, 20.7, 3.0. Comb. Anal.
Calcd for C49H50FeLiN3O5: C, 71.45; H, 6.12; N, 5.10. Found: C, 71.33;
H, 6.13; N, 5.19.
(7) Sazama, G. T.; Betley, T. A. Inorg. Chem. 2010, 49, 2512.
(8) Evans, D. F. J. Chem. Soc. 1959, 2003.
(9) Compound 1: C61H82FeLiN3O5, Mr = 1000.09, monoclinic,
P2(1)/c, a = 13.875(5) Å, b = 24.856(9) Å, c = 15.973(6) Å, R = 90°, β =
94.945(8)°, γ = 90°, V = 5488(3) Å3, Z = 4, Fcalcd = 1.210 Mg/m3, μ =
0.326 mmÀ1, R1(I > 2σ(I)) = 0.0630, wR2 = 0.1508, GOF = 1.107.
Compound 2: C69H86FeLiN6O, Mr = 1078.23, triclinic, P1, a =
10.7409(7) Å, b = 12.5697(8) Å, c = 23.8076(15) Å, R = 88.352(4)°,
β = 84.975(4)°, γ = 72.407(4)°, V = 3052.1(3) Å3, Z = 2, Fcalcd = 1.173
Mg/m3, μ = 0.294 mmÀ1, R1 (I > 2σ(I)) = 0.0536, wR2 = 0.1113, GOF =
1.016. Compound 3: C47H57Fe3N6P3, Mr = 966.45, monoclinic, C2/c,
a = 25.163(4) Å, b = 10.6991(16) Å, c = 34.829(5) Å, R = 90°, β =
90.582(3)°, γ = 90°, V = 9376(2) Å3, Z = 8, Fcalcd = 1.278 Mg/m3, μ =
0.374 mmÀ1, R1(I > 2σ(I)) = 0.0566, wR2 = 0.1159, GOF = 1.035.
CCDC-788652 (1), 788653 (2), 788654 (3) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
(10) King, E. R.; Betley, T. A. Inorg. Chem. 2009, 48, 2361.
(11) (a) Bart, S. C.; Chzopek, K.; Bill, E.; Bouwkamp, M. W.;
Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc.
2006, 128, 13901. (b) Vendemiati, B.; Prini, G.; Meetsma, A.; Hessen,
B.; Teuben, J. H.; Traverso, O. Eur. J. Inorg. Chem. 2001, 2001, 707.
(12) Additional structural details are available in SI.
(13) Although we cannot rule out the formation of a metallocyclo-
propane followed by tautomerization, this transformation seems un-
likely to occur due to the steric constraints of the Mestpe ligand.
(14) Berg, F. J.; Petersen, J. L. Organometallics 1991, 10, 1599.
(15) (a) Butts, S. B.; Holt, E. M.; Strauss, S. H.; Alcock, N. W.;
Stimson, R. E.; Shriver, D. F. J. Am. Chem. Soc. 1979, 101, 5864.
(b) Collman, J. P.; Finke, R. G.; Cawse, J. N.; Brauman, J. I. J. Am.
Chem. Soc. 1978, 100, 4766.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data for
b
1À3. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: betley@chemistry.harvard.edu.
’ ACKNOWLEDGMENT
The authors thank Harvard University and the NSF (CHE-
0955885) for financial support (T.A.B.), the NSF for a predoc-
toral fellowship (G.T.S.), and Dr. Shao-Liang Zheng for his
assistance with X-ray crystallography.
’ REFERENCES
(1) Khodakov, Y.; Chu, W.; Fongarland, P. Chem. Rev. 2007,
107, 1692.
(16) (a) Wong, A.; Atwood, J. D. J. Organomet. Chem. 1980, 199, C9.
(b) Wong, A.; Atwood, J. D. J. Organomet. Chem. 1981, 210, 395.
(c) Okazaki, M.; Ohtani, T.; Inomata, S.; Tagaki, N.; Ogino, H. J. Am.
Chem. Soc. 1998, 120, 9135.
(17) Bruker AXS. APEX II; Bruker AXS: Madison, WI, 2009.
(18) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(2) (a) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw,
J. E. J. Am. Chem. Soc. 1976, 98, 6733. (b) Calderazzo, F. Angew. Chem.,
Int. Ed. Engl. 1977, 16, 299. (c) Fachinetti, G.; Floriani, C.; Roselli, A.;
Pucci, S. J. Chem. Soc., Chem. Commun. 1978, 269. (d) Labinger, J. A.;
Wong, K. S.; Scheidt, R. J. Am. Chem. Soc. 1978, 100, 3254. (e) Wood,
C. D.; Schrock, R. R. J. Am. Chem. Soc. 1979, 101, 5421. (f) Wolczanski,
P. T.; Bercaw, J. E. Acc. Chem. Res. 1980, 13, 121. (g) Barger, P. T.;
Santarsiero, B. D.; Armantrout, R.; Bercaw, J. E. J. Am. Chem. Soc. 1984,
106, 5178. (h) Erker, G. Acc. Chem. Res. 1984, 17, 103. (i) Toreki, R.;
LaPointe, R. E.; Wolczanski, P. T. J. Am. Chem. Soc. 1987, 109, 7558.
(j) Hofmann, P.; Stauffert, P.; Frede, M.; Tatsumi, K. Chem. Ber. 1989,
4319
dx.doi.org/10.1021/om2003859 |Organometallics 2011, 30, 4315–4319