1,1′-Bis(N-benzimidazolylidene)ferrocene: Synthesis and study of a novel ditopic ligand and its transition metal complexes

Article abstract of DOI:10.1039/b908696j

Diiridum complexes containing 1,1′-bis(N-benzimidazolylidene) ferrocene, a novel ditopic ligand comprised of two N-heterocyclic carbenes (NHCs) linked directly via their N-substituents to each cyclopentadienyl ring of a ferrocene moiety, were synthesized.

Full text of DOI:10.1039/b908696j

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This paper is published as part of a Dalton Transactions themed issue on:
N-Heterocyclic Carbenes
Guest Editor Ekkehardt Hahn
Westfälische Wilhelms-Universität, Münster, Germany
Published in issue 35, 2009 of Dalton Transactions
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1,1¢-Bis(N-benzimidazolylidene)ferrocene: synthesis and study of a novel
ditopic ligand and its transition metal complexes†
C. Daniel Varnado Jr, Vincent M. Lynch and Christopher W. Bielawski*
Received 1st May 2009, Accepted 10th July 2009
First published as an Advance Article on the web 31st July 2009
DOI: 10.1039/b908696j
Diiridum complexes containing 1,1¢-bis(N-benzimidazolylidene)ferrocene, a novel ditopic ligand
comprised of two N-heterocyclic carbenes (NHCs) linked directly via their N-substituents to each
cyclopentadienyl ring of a ferrocene moiety, were synthesized. Crystallographic analyses of these
C2-symmetric bimetallic complexes revealed the benzimidazolylidene moieties were intramolecularly
stacked in nearly opposing orientations, effectively forming Janus-type bis(NHC) structures in the solid
state. Using a variety of electrochemical techniques, the oxidation potentials of the ferrocenyl groups in
these complexes were found to depend on the auxillary ligands coordinated to the Ir centers (i.e.,
1,5-cyclooctadiene vs. carbonyl). Similarly, the nCO of carbonyls ligated to the Ir centers varied in
accord with the oxidation state of the ferrocene contained with the bis(NHC) ligand. These results
suggest that the Ir and Fe centers in these complexes are electronically coupled and that the electron
donating ability of the bis(NHC) ligand reported herein can be tuned electrochemically.
metals.7 Capitalizing on the unique features inherent to NHCs and
their transition metal complexes, we believe such materials hold
Introduction
N-Heterocyclic carbenes (NHCs)1 have emerged as a highly ver-
tremendous potential in displaying novel physical and electronic
satile class of ligands for a broad range of transition metals.2 They
properties. Our efforts have focused primarily on Janus-type
generally coordinate with higher affinities than phosphines3 and,
in many cases, greatly enhance the catalytic activities displayed
by the metals to which they are ligated.4 These effects are often
amplified through the use of multi-topic NHCs, where a ligand
containing multiple NHCs is coordinated to a single metal. A
majority of these efforts have been directed toward bidentate
derivatives (e.g., A; Fig. 1).5 As a result of the chelate effect, these
monometallic complexes are generally more stable than analogues
containing two monodentate ligands.6 In addition, such metal
complexes often feature remarkable structural characteristics,
such as unusual bite angles or asymmetry, which can result in
pronounced catalytic activities or otherwise interesting physical
properties.
bis(NHC)s,8 which feature two linearly-opposed NHCs annulated
to a common arene backbone. These ditopic ligands have proven
to be useful for the synthesis of new classes of bimetallic complexes
(B)9 as well as polymeric,10 self-assembled,11 and catalytically-
active materials.12 Recently, Peris and co-workers demonstrated
that triazolyldiylidenes13 are also capable of binding two transition
metals (C) and can be used to form homo- as well as hetero-
binuclear complexes that exhibit useful catalytic properties.14 One
attractive feature of ditopic ligands such as B and C is that
the two NHCs are connected via p-conjugated linkers, which
creates opportunities for enabling electronic interactions between
coordinated metal centers.
The key to growth in this field lies in the development of
new molecular scaffolds for bridging transition metals. NHCs
functionalized with ferrocenes hold considerable potential in such
regard. Pioneered by Bildstein,15 this burgeoning field16 encom-
passes a broad range of NHCs,17,18 including derivatives with
additional donor groups19 (i.e., phosphines, sulfides, etc.) that can
be used as multi-dentate ligands. These ligands generally contain
one NHC moiety and have been primarily used as sterically-
encumbered or chiral groups to enhance the catalytic activities
and/or selectivities displayed by various types of transition metals
(see Fig. 2 for representative examples).
Fig. 1 Representative examples of transition metal complexes containing
ditopic N-heterocyclic carbenes.
We are generally interested in the development of multi-
topic NHC scaffolds that are poised to bind multiple transition
Department of Chemistry and Biochemistry, University of Texas at
Austin, 1 University Station A5300, Austin, TX 78712, USA. E-mail:
bielawski@cm.utexas.edu
† Electronic supplementary information (ESI) available: NMR spectra
and cyclic voltammograms (PDF). CCDC reference numbers 729832 (3),
729834 (6) and 729833 (7). For ESI and crystallographic data in CIF
format see DOI: 10.1039/b908696j
Fig. 2 Representative examples of previously reported ferrocenyl-substi-
tuted NHCs and diaminocarbenes.
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The first example of a ferrocene-based ligand contain-
ing two NHCs was reported by Coleman and co-workers.20
They synthesized a PdII chloride complex coordinated to 1,1¢-
bis(methyleneimidazolylidene) (D; Fig. 3), which contains one
imidazolylidene linked to each cyclopentadienyl (Cp) ring of a
ferrocene via an N-methylene spacer. Due to its unique geometry,
this ditopic ligand was found to coordinate to a metal center in an
unusual trans geometry. Notably, derivatives of these complexes
were subsequently shown to be useful catalysts for facilitating the
Heck and other coupling reactions.21
Results and discussion
To prepare a ditopic ligand analogous to the one shown in
complex F (Fig. 3), we initially envisioned synthesizing 1,1¢-bis-
(N-imidazole)ferrocene via Ullmann coupling of 1,1¢-dibromo-
ferrocene14 to imidazole, in accordance with a literature report
for an analogous reaction involving bromoferrocene,27 followed
by alkylation and metallation. Unfortunately, all attempts at
the aforementioned coupling reaction resulted in a low yield
of the desired product, which was contaminated with 1-bromo-
1¢-(N-imidazole)ferrocene. As a result, a different synthetic
pathway involving the amination of 2-fluoronitrobenzene with
1,1¢-diaminoferrocene was developed.
As summarized in Fig. 4, treatment of 1,1¢-diaminoferrocene45
with 2-fluoronitrobenzene in DMSO under basic conditions af-
forded the desired SNAr product, 1,1¢-bis(2-nitroanilino)ferrocene
(1), in 88% isolated yield. Guided by literature precedent,10
Pd-catalyzed reductive cyclization of 1 using sodium formate
in formic acid at 110 ◦C afforded bis(benzimidazole) 2 in 66%
yield. Unfortunately, attempts to alkylate 2 with various alkyl
iodides, including methyl iodide, did not yield the expected
bis(benzimidazolium) iodide salt but rather afforded benzimida-
zolium iodide and an intractable material. However, treatment
of 2 with triethyloxonium tetrafluoroborate successfully afforded
the desired bis(benzimidazolium) derivative, diethyl 1,1¢-bis(N-
benzimidazolium)ferrocene (BF4)2 (3), in 70% yield. The 1H NMR
signals for the benzimidazolium protons in 3 were found at d =
9.83 ppm (DMSO-d6), which was within the range expected for
1,3-disubstituted benzimidazolium salts.28
Fig. 3 Selected examples of ditopic N-heterocyclic carbenes and their
transition metal complexes.
Building upon these studies, we envisioned a new class of transi-
tion metal complexes containing a ditopic ligand that featured two
NHCs connected directly to each Cp ring of a ferrocene unit via
their N-substituents. As a result of the unique rotational processes
exhibited by metallocenes, this ligand should be capable of binding
to metals in two distinct ways. For example, coordination of
the ligand to a single metal center should afford E, potentially
as a C2-symmetric complex. In this binding arrangement, the
ligand, like the one shown in D,20 is an the NHC analogue of
1,1¢-bis(diphenylphosphino)ferrocene (dppf), a bisphosphine that
has found tremendous utility in the synthesis of a variety of
catalytically-active complexes.22 Alternatively, coordination of the
ditopic ligand to two metal centers should afford complex F. In this
structure, the metal centers are proximally connected to a ferrocene
moiety which may open new modes of electronic communication
between the various groups.
Herein, we describe the synthesis and study of derivatives of
complex F.‡ These efforts include the synthesis of a ditopic ligand
analogous to the one shown as well as its binuclear IrCl2(cod)
(cod = 1,5-cyclooctadiene) and IrCl2(CO)2 complexes. These
particular derivatives were selected because Ir-NHC complexes
are typically stable and have been extensively used for evaluating
the electron donating properties of a wide range of ligands,23
facilitating comparison to others reported in the literature.24
Finally, as part of our general interest in redox-active and
functionalized NHCs,18,25,26 we also probed whether the ferrocene
moiety is electronically coupled to the NHC-metal centers using a
range of electrochemical techniques.
Fig. 4 Synthesis of bis(benzimidazolium) salt 3.
X-Ray analysis of a single crystal of 3 (obtained by slow
diffusion of diethyl ether into a saturated solution of acetone)
confirmed the structure of this salt. As shown in Fig. 5, the two
benzimidazolium moieties were rotated out the plane of the Cp
rings of the ferrocene moiety by approximately 31.4◦. In addition,
the Cp rings were slightly tilted toward each other by 1.6◦, which
may be due to crystal packing effects. The packing diagram of this
structure (not shown) revealed that the benzimidazolium moieties
were intermolecularly stacked in a co-planar arrangement and
˚
separated by a distance of 3.58 A with a centroid-to-centroid
˚
offset distance of 1.18 A, a value that was consistent with a
‡ Complexes analogous to E will be described separately.
7254 | Dalton Trans., 2009, 7253–7261
p–p* interaction.29 The average distance between the Fe center
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to form bis(urea)s, whereas free NHCs do not.8,32 Compared to 4,
a relatively downfield signal was found at d = 152.4 ppm (C6D6) in
the 13C NMR spectrum of 5 and assigned to the carbon atoms of
its carbonyl groups. Likewise, the FT-IR spectrum of 5 displayed
a carbonyl stretching frequency characteristic of an urea at nCO
=
1709 cm-1 (KBr).
Enetetramines often exist in equilibrium with their free
diaminocarbenes33 and have been used to prepare NHC-metal
complexes.34 Hence, Ir complex 6 was synthesized by treating 4
with a molar equivalent of {Ir(cod)Cl}2 in benzene (see Fig. 7).
After stirring the resulting mixture at ambient temperature for
12 h, excess hexanes was added which resulted in the formation
of a yellow precipitate. Collection of this precipitate followed
by further purification via column chromatography (eluent =
20 : 1 v/v CH2Cl2–CH3OH) afforded 6 in 61% yield. Complex
6 was found to be stable toward both oxygen and water, and
could be stored on the bench top for indefinite periods of time.
The diagnostic 13C signal for the metal ligated carbons of the
NHC ligands was observed at d = 189.9 ppm (CDCl3), in accord
with other Ir-NHC cod complexes reported in the literature,23,24
particularly those containing benzimidazolylidenes.35
Fig. 5 ORTEP diagram showing 50% probability thermal ellipsoids and
selected atom labels for bis(benzimidazolium) salt 3. Solvent molecules,
counter anions, and hydrogen atoms have been removed for clarity. Selected
◦
˚
bond lengths (A) and angles ( ): C1–N1, 1.351(5); C1–N2, 1.327(6);
C2–N1, 1.401(6); C7–N2, 1.392(7); N1–C1–N2, 109.6(4). The distance
˚
between the Fe center and a Cp centroid is 1.647 A. The angle between
the planes of the two benzimidazolium moieties is 64.01◦. The angle
between the planes of the two Cp rings is 1.340◦. The dihedral angle (j)
between the benzimidazolium moiety and the Cp ring (defined by torsion
C1–N1–C10–C14) is 32.8(4)◦.
˚
in this salt and each carbon atom of its Cp rings was 2.040 A. This
distance was nearly identical to the analogous average distance
30
˚
found in ferrocene (2.045 A). Otherwise, the key structural
parameters of the benzimidazolium fragments were comparable
to other known benzimidazolium salts.28 Hence, despite direct
attachment of two positively charged benzimidazolium groups to
ferrocene, these components had only marginal effects on each
other’s molecular structures.
As shown in Fig. 6, treatment of a THF solution of 3
with sodium hydride (and a catalytic amount of potassium
tert-butoxide to facilitate deprotonation) afforded a dark red
solid in 94% yield. This compound was tentatively assigned as
enetetramine 4 and presumed to form via dimerization of the
respective NHCs (generated in situ). In addition to a disappearance
of the benzimidazolium proton in 3 (d = 9.83 ppm; DMSO-d6),
a signal characteristic of an enetetramine31 was observed at d =
142.4 ppm (C6D6) in the13C NMR spectrum of 4. Unfortunately,
all attempts at obtaining a crystal of this compound suitable for
X-ray diffraction analysis were unsuccessful. To verify its structure,
a benzene solution of 4 was exposed to atmospheric oxygen which
rapidly afforded bis(urea) 5 in nearly quantitative yield. It has
been previously established that enetetramines react with oxygen
Fig. 7 Synthesis of binuclear Ir(cod) complex 6.
The solid-state structure of complex 6 was determined by single-
crystal X-ray diffraction analysis. X-Ray quality crystals were
grown by slow diffusion of pentane into a saturated solution of
CHCl3. As shown in Fig. 8, the structure of 6 was found to adopt
a C2-symmetric structure with each Ir center featuring a distorted
square planar geometry, as expected for Ir-NHC complexes of
this type.23,24,35 Likewise, the Ccarbene–Ir and other bond lengths
and angles of the Ir-NHC center were similar to those observed
in other known NHC-Ir complexes. The benzene rings of the
nearly co-planar (j = 8.0◦) benzimidazolylidene moieties were
intramolecularly stacked upon each other and separated by a
˚
centroid-to-centroid distance of 3.46 A with a centroid-to-centroid
˚
offset distance of 0.86 A, a value consistent with a favourable
p–p* interaction.29,36 These unique structural characteristics may
explain why the Cp rings were tilted toward each other by 6.08◦.
Presumably to minimize negative steric interactions, the angle
between the two NHC-Ir segments was found to be approximately
117◦. Surprisingly, these features resulted in a solid-state structure
that resembled a Janus-type bis(NHC)8 where, as illustrated
in Fig. 9, the two benzimidazolylidenes were held into nearly
opposing positions via a supramolecular interaction. As a result,
the planes of the benzimidazolylidenes and Cp rings were twisted
by approximately 42.5◦.
As part of our evaluation of the electronic properties of com-
plexes containing 4 (see below), derivatives containing ligands with
diagnostic IR frequencies (i.e., carbonyls) were also synthesized.
Pressurizing a CH2Cl2 solution of 6 with carbon monoxide
Fig. 6 Synthesis of enetetramine 4 and bis(urea) 5.
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d = 180.6 ppm (CDCl3) and in accordance with other NHC-Ir
carbonyl complexes.23,24 The nCO for 7 were found at 1989 and
2068 cm-1 in solution (CH2Cl2) (see below for further discussion).
The structure of 7 was confirmed by single-crystal X-ray
diffraction. X-Ray quality crystals were grown by slow diffusion
of pentane into a saturated solution of CHCl3. As shown in
Fig. 11, complex 7 exhibited a similar solid-state structure as 6.
The benzene rings of the benzimidazolylidene fragments in 7 were
stacked upon each other and separated by a centroid-to-centroid
˚
distance of 3.54 A with a centroid-to-centroid offset distance
˚
of 1.07 A. As noted above, these structural features suggests a
favourable p–p* interaction,29,36 although the distance between the
benzene rings in 7 was slightly longer compared to the analogous
˚
distance observed in 6 (3.46 A), which may be due to the electron
deficient character inherent to the former complex. As observed
in the solid-state structure of 6, the two NHC-Ir units in 7 were
positioned in a nearly directionally-opposed (~128◦) orientation
(see Fig. 12). However, compared to 6, the tilt of the planes of two
Cp rings in 7 slightly increased to 8.03◦ while the angle between
the planes of the benzimidazolylidenes and the Cp rings decreased
to 37.2◦.
Fig. 8 ORTEP diagram showing 50% probability thermal ellipsoids and
selected atom labels for 6. Solvent molecules and hydrogen atoms have
◦
˚
been removed for clarity. Selected bond lengths (A) and angles ( ): Ir1–C1,
2.026(4); Ir1–C1a, 2.182(4); Ir1–C2a, 2.181(4); Ir1–C5a, 2.119(4); Ir1–C6a,
2.106(5); Ir1–Cl, 2.3772(10); C1–N1, 1.371(5); C1–N2, 1.355(5); N2–C2,
1.400(5); N1–C7, 1.395(5); C2–C7, 1.386(5); N1–C1–N2, 105.8(3). The
distance between the centroids of the benzimidazolylidene benzene rings
˚
˚
is 3.46 A. The distance between Fe1 and a Cp centroid is 1.662 A. The
angle between the planes of the two benzimidazolylidenes is 8.00◦. The
angle between the planes of the two Cp rings is 6.08◦. The dihedral angle
(j) between the benzimidazolylidene and the Cp ring (defined by torsion
angle = C10–C14–N1–C7) is 42.5(6)◦. Complex 6 sits on a crystallographic
two-fold rotation axis along 21 , y, 14 . The two-fold rotation axis passes
through the iron atom.
Fig. 11 ORTEP diagram showing 50% probability thermal ellipsoids and
selected atom labels for 7. Solvent molecules and hydrogen at◦oms have
˚
been removed for clarity. Selected bond lengths (A) and angles ( ): Ir–C1,
2.084(4); Ir–C1a, 1.885(4); Ir–C2a, 1.883(5); Ir-Cl, 2.3466(12); C1–N1,
1.353(5); C1–N2, 1.359(5); N1–C2, 1.391(5); N2–C7, 1.407(5); C2–C7,
1.387(5); N1–C1–N2, 106.0(3)◦. The distance between the centroids of the
˚
benzimidazolylidene benzene rings is 3.54 A. The distance between Fe1
˚
and a Cp centroid is 1.658 A. The angle between the planes of the two
Fig. 9 Alternate view of the ORTEP of 6 showing the co-planar
benzimidazolylidenes and the nearly opposed NHC-Ir moieties.
benzimidazolylidenes is 12.04◦. The angle between the planes of the two
Cp rings is 8.03◦. The dihedral angle (j) between the benzimidazolylidene
and Cp ring (defined by torsion C7–N2–C10–C11) is 37.2(5)◦.
followed by stirring at ambient temperature for 1 h afforded 7 as a
yellow solid in 95% yield, after removal of solvent and trituration
with pentane (see Fig. 10). Compared to 6, the 13C signal for the
carbene atoms of the NHC ligands in 7 was observed upfield at
Fig. 12 Alternate view of the ORTEP of 7 showing the wide angle (~128◦)
Fig. 10 Synthesis of binuclear Ir carbonyl complex 7.
7256 | Dalton Trans., 2009, 7253–7261
between the NHC-Ir moieties.
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relative oxidation potentials of 6 and 7 were consistent with the
greater p-acidity of carbonyl ligands compared to cod. These
results were surprising in light of the solid-state structure of 3
which revealed that the benzimidazolium components of this salt
had a near negligible effect on the key structural characteristics of
its ferrocene moiety.
It has been previously shown that the electron-donating nature
of various ligands can be probed by analyzing the nCO of metal
carbonyl complexes which contain them.37 Hence, the ability of 4’s
redox-active ferrocene unit to modulate the donating properties
of the NHCs and ultimately the ligated Ir centers to which
it is connected was evaluated by examining the nCO of 7 as
a function of the iron’s oxidation state.38 To accomplish this
task, a spectroelectrochemical experiment¶ that combined bulk
electrolysis with time-resolved FT-IR spectroscopy was devised.39
A thin-layer cell was assembled and a solution of complex 7 was
recorded as the background. To selectively oxidize the Fe center, a
potential of +1.0 V was applied while FT-IR spectra were recorded
over time. As shown in Fig. 13c, signals attributable to 7 (nCO
=
2067 and 1988 cm-1) disappeared as a new material (assigned
to 7+) exhibiting nCO = 2080 and 2002 cm-1 formed over the
same time period.40 These changes are consistent with diminished
electron density at the Ir center induced by the development of
positive charge at the redox-active Fe center (i.e., FeII → FeIII). To
place these results into context, the Tolman electronic parameters
(TEP)41 of 7 and 7+ were calculated to be 2053.7 and 2064.7 cm-1,
respectively, using Nolan’s method.24 The former value is similar
to TEPs displayed by weakly donating NHCs whereas the latter is
similar to weakly donating phosphines.ꢀ Collectively, these results
further support the notion that the NHC ligated Ir centers and the
ferrocenyl units in these complexes are electronically coupled and
suggest that the electron donating properties of the NHC ligands
can be tuned electrochemically.
Fig. 13 Cyclic voltammograms of 6 (A) and 7 (B) with Fc* added as an
internal standard (referenced to -0.057 V vs. SCE). Conditions: CH2Cl2
as solvent, 0.1 M [(Bu)4N](PF6) as electrolyte, 100 mV s-1 scan-rate. (C)
Superimposed difference FT-IR spectra showing the disappearance of 7
(nCO = 2067, 1988 cm-1) with concomitant formation of 7+ (nCO = 2080,
2002 cm-1) upon oxidation (E = +1.0 V) over the duration of 30 s.
Conditions: CH2Cl2 as solvent, [7]0 = 10 mM, 0.1 M [(Bu)4N](PF6) as
electrolyte.
Conclusions
We report the synthesis and study of two new diiridium complexes
linked together via 1,1¢-bis(N-benzimidazol-ylidene)ferrocene, a
novel ditopic ligand comprised of two N-heterocyclic car-
benes linked directly to each Cp ring of a ferrocene via their
N-substituents. These complexes were found to adopt Janus-like
bis(NHC)s structures in the solid-state, which may be attributed
to a favourable p–p* interactions formed between adjoining benz-
imidazolylidenes. It was determined that the oxidation potentials
of the ferrocene moieties in complexes 6 and 7 were dependent
on the electronic nature of the ligated Ir centers. Similarly, the
electron donating properties of the NHC ligands in 7 were tuned by
changing the oxidation state of its ferrocene moiety. Based on these
results, the ditopic ligand reported herein is poised for use in the
formation of novel bimetallic complexes, including redox-active
variants and those that may be used as bifunctional catalysts,14,42
as well as connectable components in the growing fields of nano-
and molecular electronics.43
Upon synthesis and characterization, the electrochemical prop-
erties of Ir complexes 6 and 7 were evaluated using a variety of
techniques. As shown in Fig. 13a and b, the cyclic voltammograms
of both complexes exhibited quasi-reversible redox processes,
but at different potentials: E1/2 = +0.75 V and +0.89 V for 6
and 7, respectively, (relative to SCE).§ Considering Ir carbonyl
complexes containing NHC ligands typically show irreversible
oxidations and bis(urea) 5 exhibited a redox couple at a similar
potential (E1/2 = +0.56 V), the aforementioned processes were
attributed to the FeII/III redox couple. The different redox potentials
observed in these complexes suggested to us that the Ir centers were
electronically coupled to the ferrocene moieties. Furthermore, the
¶ The use of chemical oxidants to oxidize ferrocene containing NHC-metal
complexes has been previously determined to be problematic; see ref. 18.
§ Irreversible oxidations were observed at Epa = +1.05 V and approximately
+1.50 V for 6 and 7, respectively, and attributed to IrI/II redox couples; see
the ESI for the full CVs of these complexes.†
ꢀ Similar results were obtained using methods reported by Crabtree and
Plenio; see ref. 24.
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solution containing 10% (w/v) sodium carbonate (400 mL). The
resulting mixture was then extracted with ethyl acetate (3 ¥
150 mL). The combined organic extracts were washed with brine
and then dried over anhydrous sodium sulfate. Removal of the
residual solvent under vacuum afforded a hygroscopic yellow solid.
The crude product was then purified via column chromatography
(silica gel) using 20 : 1 CH2Cl2–CH3OH as the eluent (r.f. = 0.32)
to afford 0.60 g (66% ◦yield) of the desired product as a yellow
solid. M.p. = 169–172 C. 1H NMR (CDCl3): d 8.03 (s, 2H), 7.81
(d, J = 7.2 Hz, 2H), 7.50 (d, J = 7.2 Hz, 2H), 7.27 (t, J = 7.2 Hz,
2H), 7.23 (t, J = 7.2 Hz, 2H), 4.75 (t, J = 1.8 Hz, 4H), 4.40
(t, J = 1.8 Hz, 4H). 13C NMR (CDCl3): d 144.2, 142.8, 133.7,
123.7, 122.9, 120.8, 110.9, 94.8, 67.8, 63.3. HRMS: [M]+ calcd for
C24H19N4Fe, 419.0956; found, 419.09536.
Experimental
General considerations
Unless otherwise noted, all manipulations were performed using
standard Schlenk techniques under an atmosphere of nitro-
gen or in a nitrogen-filled glove box. THF was distilled from
Na/benzophenone under nitrogen. Toluene was distilled from
CaH2 and degassed by three consecutive freeze-pump-thaw cycles.
DMSO was distilled under nitrogen from calcium hydride. All
other chemicals were used as received. H NMR and 13C NMR
1
spectra were recorded on a Varian Mercury 400 MHz spectrometer
and were referenced to residual protio solvent. 13C NMR spectra
were routinely run with broadband decoupling. Chemical shifts
are reported in (d), expressed in parts per million (ppm) downfield
from tetramethylsilane using the residual solvent as an internal
standard (1H: CDCl3, 7.24 ppm; C6D6, 7.15 ppm; DMSO-d6,
2.49 ppm; 13C: CDCl3, 77.0 ppm; C6D6, 128.0 ppm; DMSO-d6,
39.5 ppm). Infrared spectra were recorded using a Perkin-Elmer
Spectrum BX FT-IR spectrometer in a solution cell equipped
with CaF2 windows. Unless otherwise noted, melting points were
performed on a Mel-Temp apparatus under ambient atmosphere
and are uncorrected. High-resolution mass spectra (HRMS) were
obtained with a VG analytical ZAB2-E or a Karatos MS9
instrument and are reported as m/z (relative intensity). Elemental
analyses were performed by Atlantic Microlabs (Norcross, GA).
Electrochemical analyses were performed on CH Instruments
Electrochemical Workstations (series 660B and 700B) using an
air-free three electrode cell under an atmosphere of nitrogen.
The electrochemical cell contained platinum working and counter
electrodes and a silver wire as a quasi-reference electrode. All
measurements were performed in dry CH2Cl2 using 1 mM analyte,
0.1 M [(Bu)4N](PF6) as the electrolyte, and decamethylferrocene
(Fc*) as the internal standard (Fc*0/+ = -0.057 V vs. SCE).44 The
potentials listed were determined at 100 mV s-1 scan-rates and
adjusted to saturated calomel electrode (SCE).
Diethyl 1,1¢-bis(N-benzimidazolium)ferrocene (BF4)2 (3).
A
20 mL vial was charged with 2 (400 mg, 0.89 mmol), triethyloxo-
nium tetrafluoroborate (600 mg, 3.0 mmol), CH2Cl2 (10 mL) and
a stirring bar, and then stirred for 4 h at ambient temperature. The
reaction mixture was quenched with excess methanol (5 mL) and
stirred at ambient temperature for an additional 12 h. The resulting
mixture was poured into excess diethyl ether (50 mL), which caused
yellow solids to precipitate. The solids were collected via filtration,
washed with diethyl ether and then dried under vacuum to afford
480 mg (70% yield) of the desired product as a yellow powder. 1H
NMR (DMSO-d6): d 9.83 (s, 2H), 8.07 (d, J = 8.1 Hz, 2H), 7.83
(d, J = 8.4 Hz, 2H), 7.62 (dd appearing as t, J = 7.2 Hz, 2H),
7.53 (dd appearing as t, J = 7.2 Hz, 2H), 5.47 (t, J = 1.8, 4H),
4.77 (t, J = 4.8, 4H), 4.28 (q, J = 7.2 Hz, 4H), 1.48 (t, J = 7.2,
6H). 13C NMR (DMSO-d6): d141.5, 130.3, 129.6, 127.0, 126.6,
113.6, 92.9, 69.2, 64.5, 42.3, 13.7. HRMS: [M - 2BF4]2+/2 calcd
for C28H28FeN4, 238.0832; found, 238.0826.
Enetetramine 4. A 20 mL vial with a Teflon lined cap was
charged with 3 (65 mg, 0.01 mmol), sodium hydride (10 mg,
043 mmol), potassium tert-butoxide (1 mg, 0.009 mmol), THF
(2 mL), and a stirring bar. The resulting red mixture was removed
from the glovebox and heated to 60 ◦C for 6 h. After cooling
to ambient temperature, the mixture was returned to an inert
atmosphere glovebox, diluted with hexanes (5 mL) and then
filtered through a PTFE filter. Concentration of the resulting
solution under reduced pressure afforded 45 mg (94% yield) of the
1,1¢-Bis(2-nitroanilino)ferrocene (1). A 40 mL pressure tube
was charged with 1,1¢-diaminoferrocene45 (1.55 g, 7.10 mmol),
2-fluoronitrobenzene (2.10 g, 15.0 mmol), sodium bicarbonate
(1.80 g, 21.5 mmol), DMSO (6 mL) and a stirring bar. The resulting
mixture was then heated to 120 ◦C for 24 h. Upon cooling to
ambient temperature, a black solid precipitated from solution. The
filtrate was then poured into 500 mL of H2O, causing additional
precipitate to form. The precipitates were collected and combined,
triturated with isopropanol in a sonicator, filtered, and then dried
under vacuum to afford 3.0 g (88%◦yield) of the desired product
as a black powder. M.p. = 177–182 C. 1H NMR (CDCl3): d 9.08
(s, 2H), 8.10 (d, J = 8.1 Hz, 2H), 7.28 (dd appearing as t, J =
7.5 Hz, 2H), 7.16 (d, J = 8.4 Hz, 2H), 6.67 (dd appearing as t,
J = 7.5 Hz, 2H), 4.41 (br, 4H), 4.23 (br, 4H). 13C NMR (CDCl3):
d 144.4, 135.7, 132.6, 126.5, 116.9, 115.9, 95.6, 67.7, 66.5. HRMS:
[M]+ calcd for C22H18O4FeN4, 458.0675, found, 458.0672.
◦
desired compound as a red solid. M.p. = 155–160 C (capillary
tube sealed with vacuum grease). 1H NMR (C6D6): d 6.78 (m, 4H),
6.68 (d, J = 7.2 Hz, 2H), 6.59 (d, J = 6.8 Hz, 2H), 4.32 (t, J =
2.1, 4H), 3.93 (t, J = 1.8, 4H), 3.47 (q, J = 6.8, 4H), 0.90 (t, J =
6.8 Hz, 6H). 13C NMR (C6D6): d 142.4, 139.4, 121.1, 120.3, 118.3,
110.1, 107.2, 94.1, 68.6, 67.8, 41.3, 10.9. HRMS: [M]+ calcd for
C28H27FeN4, 475.1581; found, 475.1580.
Bis(urea) 5. In a nitrogen-filled dry box, a 20 mL flask was
charged with 4 (45 mg, 0.095 mmol), benzene (5 mL), and a stirring
bar. The resulting mixture was then removed from the dry box
and stirred under an atmosphere of oxygen (balloon) for 10 min.
The solution rapidly changed from cherry red to orange brown.
Removal of the residual solvent under vacuum afforded 50 mg
(99% yield) of the desired product as an orange solid. M.p. = 162–
164 ◦C. 1H NMR (C6D6): d 7.41 (d, J = 7.6 Hz, 2H), 6.88 (t, J =
8.0 Hz, 2H), 6.83 (t, J = 7.6 Hz, 2H), 6.30 (d, J = 8.0 Hz), 5.01 (t,
J = 1.6, 4H), 4.01 (t, J = 1.6, Hz, 4H), 3.33 (q, J = 7.2 Hz, 4H),
1,1¢-Bis(N-benzimidazole)ferrocene (2). A 50 mL flask was
charged with 1 (1.00 g, 2.20 mmol), sodium formate (3.2 g,
47 mmol), Pd/C (5% Pd, 500 mg, 0.23 mmol), formic acid (88%
aqueous, 10 mL), and a stirring bar, and was then heated to
110 ◦C for 12 h. After cooling to ambient temperature, the reaction
mixture was filtered through a PTFE filter into an aqueous
7258 | Dalton Trans., 2009, 7253–7261
This journal is
The Royal Society of Chemistry 2009
©

0.87 (t, J = 7.2 Hz, 6H). 13C NMR (C6D6): d 152.4, 129.1, 128.9,
121.1, 120.8, 109.8, 106.8, 93.8, 66.3, 63.9, 30.2, 13.3. HRMS:
[M + H]+ calcd for C28H27N4O2Fe, 507.1482; found, 507.1478. IR
(CH2Cl2): 1708 cm-1. IR (KBr): 1709 cm-1. E1/2 (FeII/III) = 0.56 V
(quasi-reversible). Epa = +0.60 V.
Table 1 Summary of X-ray experimental details for 3, 6 and 7
7a
3
6
Formula
C28H28B2F8FeN4 C44H50Cl2FeIr2N4 C33H27Cl5FeIr2
O4N4
FW/g mol-1
Morphology
650.01
Yellow blocks
1146.03
1161.09
Orange prisms Yellow plates
Dimensions/mm3 0.30 ¥ 0.1 ¥ 0.08 0.12 ¥ 0.08 ¥ 0.06 0.23 ¥ 0.21 ¥ 0.12
{Ir2(COD)2Cl2}(4) (6). A 20 mL vial was charged with 4
(48 mg, 0.10 mmol), {Ir(cod)Cl}2 (75 mg, 0.11 mmol), benzene
(2 mL), and a stirring bar. The resulting mixture was stirred for
12 h at ambient temperature during which time its colour changed
from red to brown, and the mixture became cloudy. Addition of
excess hexanes (10 mL) caused yellow solids to precipitate. The
solids were then collected via filtration, dissolved in a mixture of
CH2Cl2–CH3OH (20 : 1 v/v), and then filtered through a plug
of silica gel. Removal of the residual solvent under vacuum to
afforded 70 mg (61% yield) of the desired compound as a yellow
Crystal system
Space group
Monoclinic
C2/c
Monoclinic
C2/c
Monoclinic
P21/c
˚
a/A
28.853(2)
9.4328(12)
20.816(2)
90
98.506(2)
90
22.6278(6)
10.6387(4)
18.5406(6)
90
120.241(2)
90
15.8100(2)
14.5490(2)
17.1930(2)
90
110.5520(10)
90
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
5603.1(10)
8
1.541
3855.9(2)
4
1.974
3703.02(8)
4
2.083
Z
r
calcd/g cm-3
m/mm-1
0.619
2656
7.434
2224
7.959
2200
1
powder. H NMR (CDCl3): d 7.71 (d, J = 7.8 Hz, 2H), 7.34 (t,
F(000)
J = 7.8 Hz, 2H), 6.77 (t, J = 7.8 Hz, 2H), 6.74 (m, 2H), 6.78
(d, J = 7.8 Hz, 2H), 4.75–4.66 (br m, 6H), 4.53–4.19 (br m, 8H),
2.49 (br m, 2H), 2.10 (br m, 10H), 1.63 (br m, 8H), 1.49 (t, J =
7.2 Hz, 6H). 13C NMR (CDCl3): d 189.9, 133.3, 132.9, 124.1,
121.6, 112.6, 111.5, 108.4, 98.1, 85.1, 84.7, 84.7, 70.2, 66.7, 65.0,
61.7, 53.4, 52.6, 43.3, 33.4, 32.7, 29.6, 28.9, 14.4. HRMS: [M–Cl]+
calcd for C44H50N4FeIr2Cl, 1111.2316; found, 1111.2326. Anal.
calcd. for C44H50Cl2FeIr2N4: C 46.11, H 4.40, N 4.89; found: C
46.39, H 4.62, N 4.94%. E1/2 (FeII/III) = +0.75 V (quasi-reversible).
Epa (FeII/III) = +0.81 V. Epa (IrI/II) = +1.05 V (irreversible).
q range/◦
Total/unique
reflections
1.98–25.00
9421/4933
2.08–27.49
25 726/4435
1.89–30.00
53 217/10 804
Completeness to 99.9
99.9
99.9
2q (%)
Data/restraints/ 4933/309/428 4435/0/241
parameters
10 804/0/411
GOF
1.452
1.089
1.075
0.0349, 0.0869
c
R1b, wR2 [I >
0.0605, 0.1530 0.0274, 0.0604
2s(I)]
R1b, wR2 (all data) 0.0744, 0.1592 0.0365, 0.0639
0.0508, 0.0913
2.819/-1.384
c
Largest diffraction 0.820/-0.411
peak/hole/e A )
1.733/-1.285
-3
˚
a A molecule of what appeared to be chloroform was disordered. At-
{Ir2(CO)4Cl2}(4) (7). A 20 mL vial was charged with 6 (40 mg,
0.035 mmol), CH2Cl2 (2 mL), and a stirring bar. The resulting
solution was then stirred for 1 h under an atmosphere of carbon
monoxide (balloon). After removal of the residual solvent under
reduced pressure, the resulting yellow solid was triturated with
pentane (2 ¥ 10 mL), filtered, and then dried under vacuum to
afford 34 mg (95% yield) of the desired product as a yellow powder.
1H NMR (CDCl3): d 7.75 (d, J = 8.4 Hz, 2H), 7.46 (t, J = 8.4 Hz,
2H), 6.95 (t, J = 8.1 Hz, 2H), 6.84 (d, J = 8.1 Hz, 2H), 6.27
(br m, 2H), 4.65–4.35 (br m, 10H), 1.46 (t, J = 7.2 Hz, 6H). 13C
NMR (CDCl3): d 180.6, 180.6, 166.8, 132.7, 132.4, 125.1, 123.2,
112.2, 109.8, 97.2, 70.1, 67.8, 65.6, 62.6, 43.8, 14.2. HRMS: [M -
Cl]+ calcd for C32H26N4O4FeClIr2, 1007.0245; found, 1007.0245.
IR (CaF2, CH2Cl2): 2068, 1989 cm-1. IR (KBr): 2062, 1979 cm-1.
E1/2 (FeII/III) = +0.89 V (quasi-reversible). Epa (FeII/III) = +0.93 V.
ꢀ
tempts to model the disorder were unsatisfactory. b R1
=
||Fo| -
ꢀ
ꢀ
ꢀ
|Fc||/ |Fo|. c wR2 = { [w(Fo - Fc2)2]/ [w(Fo2)2]}1/2. The contri-
butions to the scattering factors due to the solvent molecule were removed
by use of the utility SQUEEZE50 in PLATON.51 PLATON was used as
incorporated in WinGX.52
2
Acknowledgements
We thank the National Science Foundation (CHE-0645563), the
Office of Naval Research (N00014-08-1-0729), the Robert A.
Welch Foundation (F-1621), and the Arnold and Mabel Beckman
Foundation for their generous financial support. C. W. B. is a
Cottrell Scholar of Research Corporation.
Notes and references
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