Synthesis and study of 5,5'-bibenzimidazolylidenes and their bimetallic complexes

Article abstract of DOI:10.1002/ejic.200801188

A new, high-yielding synthesis of 3, 3-,4,4-tetrachlorobi-phenyl was developed, facilitating access to a new family offluorescent 5,5-bibenzimidazolium salts bearing phenyl or bulky tertiary alkyl N-substituents. Deprotonation of thesesalts afforded the r

Full text of DOI:10.1002/ejic.200801188

FULL PAPER
DOI: 10.1002/ejic.200801188
Synthesis and Study of 5,5Ј-Bibenzimidazolylidenes and Their Bimetallic
Complexes
Joyce A. V. Er,[a] Andrew G. Tennyson,[a] Justin W. Kamplain,[a] Vincent M. Lynch,[a] and
Christopher W. Bielawski*[a]
Keywords: Nitrogen heterocycles / Carbenes / Benzimidazolium salts / Bimetallics / Rhodium / Polychlorinated biphenyls
A new, high-yielding synthesis of 3,3Ј,4,4Ј-tetrachlorobi- RhI/II redox couples. A bimetallic Rh carbonyl complex bear-
phenyl was developed, facilitating access to a new family of
fluorescent 5,5Ј-bibenzimidazolium salts bearing phenyl or
bulky tertiary alkyl N-substituents. Deprotonation of these
salts afforded the respective 5,5Ј-bibenzimidazolylidenes in
excellent isolated yields (Ն86%), which were characterized
in the solid state and solution. Treatment of these ditopic li-
gands with [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) af-
forded bimetallic complexes composed of two Rh atoms con-
nected through a bis(carbene) linker. Electrochemical analy-
ing a 5,5Ј-bibenzimidazolylidene was also synthesized and
found to exhibit νCO signals similar to a monotopic analogue.
Collectively, these results suggested that the two metal cen-
ters in the aforementioned bimetallic complexes were elec-
tronically decoupled. These observations were supported by
the crystal structures of one of the 5,5Ј-bibenzimidazolium
salts and its bis(carbene) derivative, which revealed rela-
tively large dihedral angles of 51.2° and 47.8°, respectively,
between the two aryl systems.
ses of these complexes revealed single, quasi-reversible oxi- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
dations at E1/2 = +0.54 to +0.59 V (vs. SCE) arising from the
Germany, 2009)
We recently reported[8] a new class of benzobisimidazol-
ylidenes (1) that feature two linearly opposed NHCs annu-
lated to a common arene backbone.[9,10] These ditopic li-
gands proved to be instrumental in the preparation of new
classes of bimetallic complexes[11] as well as polymeric[12]
and self-assembled[13] materials. Bis(NHC)s 1 are generally
prepared by deprotonation of their respective benzobisimid-
azolium (BBI) salts,[8] with two strategies available for the
syntheses of these precursors. In the first method, for-
mylative cyclization of 1,2,4,5-tetraaminobenzene followed
by alkylation affords a tetraalkylated BBI salt. One limita-
tion of this methodology is that only primary N-alkyl sub-
stituents can be installed. Although these BBIs are suitable
monomers for the synthesis of poly(enetetraamine)s,[14] pri-
mary N-alkyl groups are generally too small to enable isola-
tion of the respective free bis(NHC)s. This limitation was
circumvented with the development of a second strategy.
Four-fold aryl amination of 1,2,4,5-tetrabromo- or chloro-
benzene afforded the respective tetraaminobenzenes bear-
ing large N-substituents (R = tertiary alkyl, aryl) in excel-
lent yields (typically Ͼ90%).[15] After subsequent for-
mylative cyclization and deprotonation, this methodology
enabled isolation of the respective stable bis(NHC)s 1.[8] Al-
though the physical and electronic properties of 1 can be
tuned by modification of the N-substituents using the afore-
mentioned synthetic approaches,[11] we desired methodol-
ogy that would enable the incorporation of other aromatic
linkers into bis(NHC) scaffolds, in particular biphenyl,
leading to derivatives such as 2 (see Figure 1).
Introduction
The development of new molecular scaffolds capable of
bridging multiple transition metals has enabled the develop-
ment of organometallic and inorganic materials that exhibit
a variety of novel properties.[1] Ideally, the multitopic li-
gands employed for such purposes should: 1) be modular,
to allow fine-tuning of physical and electronic properties, 2)
be easy to synthesize, and 3) have strong affinities for a
wide variety of transition metals. N-Heterocyclic carbenes
(NHCs),[2] such as imidazolylidenes[3] and benzimidazolyl-
idenes,[4] satisfy all three criteria. Multiple synthetic meth-
odologies are available that facilitate variation of the N-
substituents as well as the heteroaromatic cores.[5] As a re-
sult, an impressively diverse range of derivatives has been
prepared and shown to form thermally robust complexes[6]
with a large number of transition metals.[2] Attention has
focused primarily on the synthesis and characterization of
monometallic complexes,[2] with much less emphasis on the
development of multitopic NHCs that can coordinate mul-
tiple transition metals.[7]
[a] Department of Chemistry and Biochemistry, The University of
Texas at Austin
Austin, TX 78712, USA
E-mail: bielawski@cm.utexas.edu
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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With an efficient, high-yielding route to 3 developed, we
shifted our efforts toward exploring the potential of this
tetrahaloarene to undergo fourfold aryl amination with
bulky alkyl (tert-butyl, tert-amyl and 1-adamantyl) as well
as aryl (phenyl and mesityl) amines. Although tetraami-
nobiphenyls bearing sterically shielding N-substituents can
be prepared by reaction of 3,3Ј-diaminobenzidine with
RC(O)Cl followed by reduction, this route can only be used
with redox inert functional groups and can only install pri-
mary alkyl N-substituents.[18] With 3, however, we discov-
ered that tertiary alkyl as well as aryl N-substituents could
be installed using Pd-catalyzed aryl amination.[19] As sum-
marized in Scheme 2, these reactions were performed by
heating a mixture of 4.1–5.0 equiv. of an amine relative to
3 (typical [3]0 = 0.2 ) and a 1:2 molar ratio of Pd-
(OAc)2/1,3-bis(2,6-diisopropylphenyl)imidazolylidene (IPr)
as the pre-catalyst (2 mol-% Pd relative to 3) in toluene at
110 °C for 12 h. The reaction mixture was then concen-
trated and washed with ethanol to give the desired products
in modest to excellent yields (40–93%). Spectroscopic
analyses of these compounds confirmed their structural as-
signments as 3,3Ј,4,4Ј-tetraaminobiphenyls 4. Surprisingly,
Figure 1. Structures of benzobisimidazolylidenes 1 and 5,5Ј-bi-
benzimidazolylidenes 2.
In analogy to BBI-based compounds, formylative cycli-
zation of 3,3Ј-diaminobenzidine followed by alkylation was
previously shown to afford the respective 5,5Ј-bibenzimid-
azolium salts bearing primary N-alkyl groups in excellent
yields.[12] Although deprotonation of these salts did not af-
ford the desired free bis(NHC)s, they did yield to a new
class of poly(enetetraamine)s with 2 as the repeating
unit.[14] Recently, Gehrhus and Lappert reported the first
example of an isolable 5,5Ј-bibenzimidazolylidene (2, R =
neopentyl) by reducing the corresponding bis(thiourea)
with KC8.[10]
We envisioned that a gentler, more efficient synthesis of
stable derivatives of 2 could be realized in a manner analo-
gous to 1 through deprotonation of 5,5Ј-bibenzimidazolium
salts featuring bulky N-substituents. Guided by our experi-
ence with 1, we sought to develop methodology that en-
abled access to such salts via exhaustive aryl amination of
a 3,3Ј,4,4Ј-tetrahalobiphenyl followed by formylative cycli-
zation. Deprotonation of these salts was envisioned to af-
ford 2 as their stable bis(NHC)s as well as bimetallic deriva-
tives supported by this ditopic ligand. A corollary of these
efforts was to explore the relationship between the geomet-
ric properties of 2 (i.e. the dihedral angles between its aryl
rings) and the degree of electronic communication across
the biphenyl linkage using fluorescence, electrochemical,
and other spectroscopic measurements.[16]
1
despite being connected to electron-rich arylamines, the H
NMR signals for the biphenyl protons were not signifi-
cantly upfield (δ = 6.9–7.4 ppm, CDCl3). Likewise, tetra-
aminoarenes 4 were found to be relatively stable toward oxi-
1
dation. For example, H NMR analysis of aerated CDCl3
solutions of these tetraamines revealed no oxidation prod-
ucts, even after several days.
Results and Discussion
The synthesis of 3,3Ј,4,4Ј-tetrachlorobiphenyl (3) has
been previously achieved via Suzuki–Miyaura coupling of
(3,4-dichlorophenyl)boronic acid to 1,2-dichloro-4-iodo-
benzene.[17] Although the yield of this reaction is good
(69%), the requisite boronic acid is prohibitively expensive
and requires multiple steps to synthesize. We reasoned that
3 might be prepared more directly through metal-mediated,
oxidative homocoupling of the respective aryllithium com-
pound. Indeed, as shown in Scheme 1, treatment of com-
mercially available 1,2-dichloro-4-iodobenzene with nBuLi
at –78 °C, followed by stoichiometric CuBr2, afforded 3 in
92% isolated yield.
Scheme 2. Syntheses of 3,3Ј,4,4Ј-tetraaminobiphenyls (4).
Collectively, these results were similar to those we ob-
tained with various 1,2,4,5-tetraaminobenzenes prepared
using an analogous fourfold aryl amination methodology
from tetrachloro- or tetrabromobenzene.[15] Comprehensive
structural analyses of 1,2,4,5-tetraaminobenzenes analo-
gous to 4 revealed that the nitrogen lone pairs were rotated
into the planes of their respective arene rings.[15] To deter-
mine if the bisarenes 4 exhibited similar structural charac-
teristics, single crystals of 4a were grown by slowly cooling
a saturated solution in toluene from reflux to ambient tem-
perature and then analyzed by X-ray diffraction.[20] As
shown in Figure 2, the tert-butyl groups on two of the nitro-
gen atoms were positioned above and below the planes of
the arene rings with torsion angles of approximately 130°.
This conformation effectively places the nitrogen lone pairs
on these amino groups orthogonal to the π-systems of the
Scheme 1. Synthesis of 3,3Ј,4,4Ј-tetrachlorobiphenyl (3).
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5,5Ј-Bibenzimidazolylidenes and Their Bimetallic Complexes
arene rings to which they are bonded. This structural ar-
Previous studies of BBI-based salts revealed that these
rangement attenuates the electron-donating properties typi- compounds exhibit rich photoluminescent properties.[22]
cally associated with amines and explains both the unexpec- Thus, we sought to investigate the spectroscopic character-
ted NMR spectroscopic properties and the high oxidative istics of their 5,5Ј-bibenzimidazolium analogues. The ab-
stabilities exhibited by 4.
sorption spectra for 5a–c were nearly indistinguishable from
each other, with almost identical absorption wavelengths
and extinction coefficients (see Table 1). Furthermore, the
emission profiles were superimposable, with fluorescence
maximizing at nearly identical wavelengths. These results
are consistent with the similar electronic characteristics of
the N-substituents in compounds 5a–c: each benzimidazol-
ium nitrogen is connected to one quaternary carbon, which
itself is connected to three CH2X moieties (where X can be
a proton or hydrocarbon chain or cycle).
Table 1. Spectroscopic properties of 5.[a]
λmax [nm]
logε
λem [nm]
Φ
Figure 2. ORTEP diagram showing 50% probability thermal ellip-
soids and selected atom labels for 4a. Solvent molecules and H
atoms have been omitted for clarity. Selected bond lengths [Å] and
angles [°]: N1–C1, 1.376(2); N1–C7, 1.463(2); N2–C2, 1.436(2);
N2–C11, 1.491(2); N3–C18, 1.379(2); N3–C21, 1.459(2); N4–C19,
1.437(2); N4–C25, 1.500(2); C4–C15, 1.480(2); C1–N1–C7,
129.90(16); C2–N2–C11, 117.00(13); C18–N3–C21, 130.05(17);
C19–N4–C25, 117.44(13); C1–C2–N2–C11, 103.14(17); C2–C1–
N1–C7, 10.63(16); C18–C19–N4–C25, 102.98(17); C19–C18–N3–
C21, 173.47(17). The dihedral angle (φ) between the two benzimid-
azole moieties is 26.0°.
5a
5b
5c
5d
292
292
294
285
4.24
4.27
4.24
4.40
341
340
342
415
0.67
0.47
0.88
0.61
[a] Measurements were performed in DMF under ambient condi-
tions. Molar extinction coefficients (ε, in –1 cm–1) were determined
from Beer’s law plots. Fluorescence quantum efficiencies (Φ) were
determined relative to quinine bisulfate in 0.1  H2SO4 (aq.).
Interestingly, the λmax for 5a is slightly longer than its
monomeric “half” [1,3-bis(tert-butyl)benzimidazolium]-
(BF4) (292 nm vs. 276 nm, respectively). This is consistent
with 5a being slightly more electron deficient compared to
its monomeric analogue, thus lowering the energy of the
πǞπ* transition. Considering that their emission wave-
lengths are also comparable (341 nm vs. 351 nm), we believe
that similar absorption and emission processes are occur-
ring in these two molecules, albeit with radically different
quantum yields (67% vs. 0.1%). Overall, these results sug-
gest that the exciton that forms in 5a is localized largely on
the benzimidazolium fragments, with minimal delocaliza-
tion across the biphenyl linkage.
Despite the similarities among 5a–c, the quantum yields
for these compounds varied significantly (5c Ͼ 5a Ͼ 5b)
and in accord with the identities of the N-substituents in
these compounds. Because the excitation and emission ener-
gies of 5a–c were nearly identical, the variation in quantum
yields enabled us to gain greater insight into their photo-
physical properties than would otherwise be possible. A
quantum yield is equal to the radiative decay rate divided
by the sum of the radiative and non-radiative decay rates.
Although a molecule can emit a photon when it relaxes
from an excited state, relaxation can also be induced by
intermolecular collisions or intramolecular vibrations with-
out concomitant emission. Because 1-adamantyl is more ri-
gid than tert-butyl, the former has fewer vibrational and
rotational modes and thus provides fewer non-radiative de-
cay pathways; this explains why the quantum yield of 5c Ͼ
5a. Similarly, because tert-amyl contains one more methyl
group than tert-butyl, it has more vibrational and rotational
modes available and thus the quantum yield of 5a Ͼ 5b.
With tetraamines 4 in hand, we then pursued the synthe-
sis of their corresponding N-aryl- and N-alkyl-5,5Ј-bibenz-
imidazolium salts (5). This was accomplished by treating
solutions of 4 in HC(OMe)3 or HC(OEt)3 with excess tetra-
fluoroboric acid at 65–140 °C (see Scheme 3). After 24 h,
the desired 5,5Ј-bibenzimidazolium salts (5) precipitated
from their respective reaction mixtures, facilitating isolation
in good to excellent yields (79–96%) with minimal workup.
Unfortunately, 5e failed to form under these conditions,
consistent with the failure of 1,2,4,5-tetra(mesitylamino)-
benzene to undergo formylative cyclization.[15] The 1H
NMR signals for the benzimidazolium protons in 5 were
found between 8.9 and 10.5 ppm ([D6]DMSO), which was
within the range expected for 1,3-disubstituted benzimida-
zolium salts.[4,15,21] X-ray analysis of a single crystal of 5a
(obtained by slowly cooling a hot solution of DMSO satu-
rated with this salt) revealed that the dihedral angle between
the two benzimidazolium moieties was φ = 51.2° (structure
not shown).[20]
Scheme 3. Synthesis of 5,5Ј-bibenzimidazolium salts (5).
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Replacing the alkyl N-substituents with phenyl groups
results in a 10 nm hypsochromic shift in absorption maxi-
mum, which can be understood by the diminished electron-
releasing ability of phenyl relative to tertiary alkyl groups.
Whereas 5a–c show relatively small Stokes shifts (50 nm),
the absorption and emission maxima for 5d are separated
by 130 nm. Given the minimal conjugation observed be-
tween the two benzimidazole fragments in 5a–c, we believe
that the large Stokes shift for 5d arises from exciton delocal-
ization onto the N-phenyl groups, rather than onto a neigh-
bouring benzimidazolium group.
Upon the synthesis and characterization of 5,5Ј-benzimid-
azolium salts 5, we then pursued the synthesis of their
respective free bis(NHC)s 2. As shown in Scheme 4, ad-
dition of two equivalents of either NaOtBu or NaN-
(SiMe3)2 (NaHMDS) to 5a–c afforded 2a–c in excellent
yields (Ն86%) after filtration and removal of solvent. Diag-
nostic 13C NMR signals for the carbene carbons were ob-
served between δ = 220 and 230 ppm, consistent with other
known annulated imidazolylidenes.[4,5,8] Deprotonation of
5d with NaHMDS afforded a complex mixture that appears
to be the expected poly(enetetraamine)[14] (formed in situ
by polymerization of 2d).[23]
Figure 3. ORTEP diagram showing 50% probability thermal ellip-
soids and selected atom labels for 2a. Solvent molecules and H
atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (°): C1–N1 1.372 (3), C1–N2 1.372 (3), N1–C2 1.404 (3),
N2–C7 1.398(3), C2–C3 1.392 (3), C3–C4 1.396 (3), C4–C4A 1.488
(4). The dihedral angle (φ) between the two benzimidazole moieties
is 47.8°.
48.6–49.6 Hz.[4,8] Unfortunately, all attempts at obtaining
crystals suitable for X-ray diffraction analysis of these com-
pounds were unsuccessful.
Scheme 5. Synthesis of bimetallic Rh complexes containing 5,5Ј-
bibenzimidazolylidenes (2).
Scheme 4. Synthesis of 5,5Ј-bibenzimidazolylidenes (2).
Slow cooling of a saturated solution of 2a in refluxing
toluene afforded crystals suitable for X-ray diffraction (see
Cyclic voltammetry of 6 in CH2Cl2 (for 6a, see Figure 4,
Figure 3).[20] The N1–C1–N2 bond angle of 104.1(2)° and A) revealed quasi-reversible RhI/II redox couples within a
the average N–C bond length of 1.372(3) Å in this structure narrow range (E1/2 = +0.54 to +0.59 V relative to SCE). In
are similar to those found in other benzimidazolylid- contrast, the RhI/II couple for an N-ethyl analogue (7; see
enes.[4,5,8] Notably, the two benzimidazole moieties of this Figure 5) occurred at nearly double the potential (Epa
=
compound are more twisted (φ = 47.8°) relative to each +0.95 V), consistent with the diminished electron-donating
other in the solid state than in the tetraamine 4a (φ = 26.0°) ability of ethyl vs. tertiary alkyl groups. Furthermore, this
but less than its respective bibenzimidazolium salt 5a (φ = oxidation was irreversible, presumably due to the decreased
51.2°).
steric shielding around the metal atom. The voltammog-
To explore in greater detail the degree of communication rams displayed only single metal-based redox events, indi-
across the two benzimidazole groups in 2, a series of bime- cating that the two metal centers in these complexes were
tallic Rh complexes containing these bis(NHC)s were syn- electronically decoupled.[25] To test this observation, we pre-
thesized and studied electrochemically as well as spectro- pared [Rh{1,3-di(tert-butyl)benzimidazolylidene}(COD)Cl]
scopically.[24] Rhodium complexes 6 were prepared by add- (8a) and its N-ethyl analogue 8b to model the monomeric
ing one equivalent of 2 (generated by in situ deprotonation halves of 6a and 7, respectively. Cyclic voltammetry of 8a
of 5 with NaHMDS) to one equivalent of [Rh(COD)Cl]2 revealed a single, quasi-reversible oxidation wave at E1/2
=
(COD = 1,5-cyclooctadiene) in toluene. After stirring for +0.58 V for the RhI/II couple, which is nearly identical to
12 h at ambient temperature, the solvent was removed un- that observed for 6a (+0.59 V). Similarly, the RhI/II oxi-
der reduced pressure and the resulting yellow solids were dation potential in 8b occurred close to that in 7 (+0.86 V
triturated with Et2O to afford 6 in yields of up to 85% (see vs. +0.95 V, respectively). The higher oxidation potentials
Scheme 5). The diagnostic 13C signals for the ligated car- observed for 6a and 7 are consistent with the greater elec-
bene carbon atoms were observed at 195.8–197.8 ppm, and tron-deficient character inherent to bibenzimidazole-based
exhibited the characteristic Rh–C splitting pattern with J = systems relative to benzimidazole analogues (see above).
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5,5Ј-Bibenzimidazolylidenes and Their Bimetallic Complexes
ucts 9 and 10 (observable by NMR spectroscopy) were
found to decompose upon concentration (presumably
through loss of CO)[28] and could not be cleanly isolated by
this method. However, by allowing the solvent to evaporate
under an atmosphere of carbon monoxide then triturating
the crude material with pentane at –78 °C (also under car-
bon monoxide), complexes 9 and 10 (Figure 5) were suc-
cessfully isolated in Ն 95% yield. The cis and trans (relative
to Rh) νCO bands for 9 were observed at 2003 and
2084 cm–1, respectively, in solution which were similar to
the analogous signals observed for its related monomeric
half 10 (νCO = 2002 and 2083 cm–1). Because 9 displays only
two νCO bands at nearly identical energies to 10, we con-
clude that the two metal centers in the former are electroni-
cally decoupled.[16] Hence, rather than viewing these com-
plexes as ligand-bridged bimetallic species, they may be
more properly viewed as dimers of monometallic benzimid-
azolylidene species.
Figure 4. (A) CV of 6a with [Fc*] as an internal standard (vs. SCE).
(B) DPV of the oxidation for 6a (✧) and single-peak deconvolution
(black line). (C) Plot of i(t)/iss vs. t–1/2 data (✧) for the oxidation of
6a. Linear fit (black line, slope = –0.25, y-intercept = –0.78, R2 =
0.99) allowed determination of D0 and n.
Conclusions
We have developed a new, highly efficient synthesis of
3,3Ј,4,4Ј-tetrachlorobiphenyl (3) from commercially avail-
able starting materials. Subsequent Pd-catalyzed coupling
of bulky alkyl or arylamines, followed by formylative cycli-
zation, afforded a new class of highly fluorescent bibenzimid-
azolium salts (5). The N-alkyl variants of these salts dis-
played identical absorption and emission energies with in-
creasing quantum yields correlating to increasing rigidity of
the N-substituent (1-adamantyl Ͼ tert-butyl Ͼ tert-amyl).
Although the emission from these salts appears localized
primarily on each benzimidazolium moiety, that from the
N-phenyl analogue suggests delocalization onto its N-sub-
stituents. Deprotonation of these salts afforded the respec-
tive bis(NHC)s 2 in high isolated yields. Treatment of these
ditopic ligands with [Rh(COD)Cl]2 afforded the expected
bimetallic complexes, which exhibited only single, metal-
based redox processes. Furthermore, comparison of the cy-
clic voltammograms of these bimetallic complexes to mod-
els of their monometallic “halves” revealed RhI/II redox
processes at similar potentials. Likewise, the νCO values ob-
served for a related bimetallic carbonyl complex were found
at frequencies similar to a monometallic model. Collec-
tively, the results from our fluorescence, electrochemical,
and spectroscopic experiments have demonstrated that Rh
metal centers ligated to 5,5Ј-bibenzimidazolylidenes are
largely electronically decoupled. As such, bimetallic com-
plexes supported by the ditopic bis(NHC)s described herein
hold potential for use as bifunctional catalysts[9d,29] as well
as connectable components in the emerging fields of nano-
and molecular electronics.[30]
Figure 5. Structures of various Rh–NHC complexes that were syn-
thesized and studied.
To investigate the electronic communication across the
bibenzimidazolylidene linker in the aforementioned com-
plexes in detail, we analyzed 8a by differential pulse voltam-
metry (DPV) and chronoamperometry (CA). Deconvol-
ution of the DPV data for 6a revealed only one oxidation
peak (Figure 4, B). Performing CA on 6a using an ultramic-
roelectrode enabled us to measure the time-dependent and
steady-state currents, i(t) and iss, respectively. Dividing i(t)
by iss and plotting vs. t–1/2 (Figure 4, C) allowed us to deter-
mine D0 = 6.9ϫ10–6 cm2 s–1 and n = 1.8 for 6a.[26] Similar
analyses of 8a also revealed only one oxidation peak, but
with D0 = 2.7ϫ10–5 cm2 s–1 and n = 1.1. These n values
support the notion that the oxidation of 6a is a two-electron
process (comprising two independent, one-electron oxi-
dations at each metal center) whereas that of 8a is a one-
electron process. Furthermore, the diffusion coefficients are
consistent with the fact that 8a is significantly smaller than
6a.
We sought to complement our electrochemical evaluation
of the electronic communication between the metals in 6 by
incorporating ligands with diagnostic IR frequencies (i.e.
carbonyls). Thus, the [Rh(COD)Cl] complexes 7 and 8b
were converted into their analogous [Rh(CO)2Cl] com-
plexes by independently pressurizing CD2Cl2 solutions of
these complexes with carbon monoxide, followed by stirring
Experimental Section
General Procedures: Unless otherwise noted, all manipulations
were performed using standard Schlenk techniques under nitrogen
at ambient temperature.[27] Unfortunately, the desired prod- or in a nitrogen-filled glove box. THF was distilled from Na/benzo-
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C. W. Bielawski et al.
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phenone under nitrogen. Toluene was distilled from CaH2 and de-
gassed by three consecutive freeze-pump-thaw cycles. All other
49 mg (92% yield) of the desired compound as white powder. Spec-
troscopic data was in accord with literature values.[17] M.p. 172–
1
1
chemicals were used as received. H NMR and 13C NMR spectra
175 °C. H NMR (CDCl3): δ = 7.60 (d, J = 2.2 Hz, 2 H), 7.50 (d,
were recorded on a Varian 400 MHz DirectDrive spectrometer and J = 8.4 Hz, 2 H), 7.35 (dd, J = 8.4, 2.2 Hz, 2 H) ppm. 13C NMR
were referenced to residual protio solvent. 13C NMR spectra were
routinely run with broadband decoupling. Chemical shifts are re-
ported in delta (δ) units, expressed in parts per million (ppm) down-
field from tetramethylsilane using the residual solvent as an in-
ternal standard [1H: CDCl3, 7.24 ppm; C6D6, 7.15 ppm; [D6]-
DMSO, 2.49 ppm; C6D5CD3, 2.09 ppm; CD2Cl2, 5.32 ppm; 13C:
CDCl3, 77.0 ppm; C6D6, 128.0 ppm; [D6]DMSO, 39.5 ppm;
C6D5CD3, 20.4 ppm; CD2Cl2, 53.8 ppm]. 19F NMR spectra were
also run on a Varian 400 MHz DirectDrive spectrometer and refer-
enced to an external standard (19F: CFCl3, 0 ppm). To quantify the
extent of the anion metathesis reactions, a known quantity of 1,2-
dibromo-4,5-difluorobenzene (δ = –136.3 ppm) was added as an
internal standard. In all cases, the reactions were found to proceed
in Ͼ99% conversion. Infrared spectra were recorded using a Per-
kin–Elmer Spectrum BX FT-IR spectrometer in a solution cell
equipped with CaF2 windows. Melting points were determined un-
der 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 (%).
(CDCl3): δ = 138.7, 133.2, 132.5, 131.0, 128.8, 126.1 ppm. HRMS:
[M + H]+ calcd. for C12H7Cl4: 290.9302; found 290.9303.
1,1Ј,3,3Ј-Tetra(tert-butyl)-3,3Ј-diaminobenzidine (4a): In a 30-mL
vial, a mixture of Pd(OAc)2 (4.9 mg, 0.022 mmol), 1,3-bis(2,6-di-
isopropylphenyl)imidazolium chloride (19 mg, 0.045 mmol), Na-
OtBu (5.0 mg, 0.052 mmol), and toluene (2 mL) was stirred for
15 min. Afterward, 3,3Ј,4,4Ј-tetrachlorobiphenyl (3) (500 mg,
1.71 mmol), tert-butylamine (626 mg, 8.56 mmol), NaOtBu
(675 mg, 7.02 mmol), and toluene (6.6 mL) were added. The re-
sulting mixture was then sealed and stirred at 110 °C for 12 h. The
cooled reaction mixture was filtered and then concentrated under
reduced pressure. After washing the residual solids with excess
ethanol, they were dried under high vacuum to afford 700 mg (93%
yield) of the desired product as white powder; m.p. 148–151 °C. 1H
NMR (CDCl3): δ = 7.09 (d, J = 2.0 Hz, 2 H), 6.94 (dd, J = 8.2,
2.2 Hz, 2 H), 6.89 (d, J = 8.2 Hz, 2 H), 3.65 (s, 4 H), 1.31 (s, 18
H), 1.29 (s, 18 H) ppm. 13C NMR (CDCl3): δ = 138.8, 136.8, 134.0,
120.9, 118.1, 52.0, 51.9, 30.0, 29.9 ppm. HRMS: [M + H]+ calcd.
for C28H47N4: 439.3798; found 439.3795.
UV/Vis absorption and fluorescence emission spectra were re-
corded with a Perkin–Elmer Lambda 35 spectrophotometer and a
PTI QuantaMaster 4 L fluorimeter, respectively. All measurements
were made with matched 6Q Spectrosil quartz cuvettes (Starna)
with 1-cm path lengths and 3.0 mL of sample solution volumes.
Beer’s law measurements were performed using 10, 20, 30 and
40 µ sample concentrations. Emission spectra were acquired using
1.0 µ solutions of fluorophore. Quantum yields were determined
relative to 1.0 µ quinine sulfate in 0.1 N H2SO4.[31] All measure-
ments were performed in DMF under ambient conditions.
1,1Ј,3,3Ј-Tetra(tert-amyl)-3,3Ј-diaminobenzidine (4b): Using a pro-
cedure analogous to one used to prepare 4a,
3 (200 mg,
0.685 mmol) and tert-amylamine (298 mg, 3.42 mmol) afforded
193 mg (57% yield) of the desired compound as white powder; m.p.
1
94–98 °C. H NMR (CDCl3): δ = 7.05 (d, J = 2.2 Hz, 2 H), 6.90
(dd, J = 8.2, 2.2 Hz, 2 H), 6.85 (d, J = 8.2 Hz, 2 H), 3.64 (s, 4 H),
1.65 (q, J = 7.6 Hz, 4 H), 1.62 (q, J = 7.4 Hz, 4 H), 1.22 (s, 12 H),
1.19 (s, 12 H), 0.92 (t, J = 7.4 Hz, 12 H) ppm. 13C NMR (CDCl3):
δ = 138.9, 136.6, 133.9, 120.6, 118.1, 117.8, 54.5, 54.3, 35.0, 34.9,
27.2, 27.1, 8.7 ppm. HRMS: [M + H]+ calcd. for C32H55N4:
495.4415; found 495.4421.
Electrochemical experiments were conducted using CH Instru-
ments Electrochemical Workstations (series 630C and 700B) using
a gas-tight three-electrode cell under nitrogen. The electrochemical
cell contained platinum working and counter electrodes and a silver
wire as a quasi-reference electrode. Chronoamperometric experi-
ments were performed using a 15 µm Pt ultramicroelectrode as the
working electrode. Quasi-reversibility for 6a was confirmed by
scan-rate independence from 50 mV/s to 1 V/s. All measurements
were performed in dry CH2Cl2 using 1 m analyte, 0.1  [tetra-n-
butylammonium](PF6) as the electrolyte, and decamethylferrocene
[Fc*] as the internal standard ([Fc*]0/+ = –0.13 V vs. SCE).[26,32]
The potentials listed in the manuscript and experimental section
were determined at 100 mV/s scan-rates and adjusted to SCE.
1,1Ј,3,3Ј-Tetra(1-adamantyl)-3,3Ј-diaminobenzidine (4c): In a man-
ner analogous to the procedure used to prepare 4a, 3 (300 mg,
1.03 mmol) and 1-adamantylamine (873 mg, 4.16 mmol) afforded
309 mg (40% yield) of the desired compound as a white powder.
Note that one minor modification to the workup procedure was
employed: after cooling and filtering the reaction mixture, the re-
sidual solids were rinsed with toluene and extracted with CHCl3.
The combined extracts were then concentrated under reduced pres-
sure to obtain the desired product; m.p. 316–319 °C. 1H NMR
(CDCl3): δ = 7.14 (s, 2 H), 6.94 (d, J = 8.1 Hz, 2 H), 6.91 (d, J =
8.1 Hz, 2 H), 3.64 (s, 4 H), 2.07 (br., 12 H), 1.91 (br., 12 H), 1.86
(br., 12 H), 1.64 (br., 24 H) ppm. 13C NMR (CDCl3): δ = 138.6,
135.6, 134.3, 122.8, 119.3, 117.9, 52.7, 43.3, 36.6, 29.8 ppm.
HRMS: [M + H]+ calcd. for C52H71N4: 751.5663; found 751.5973.
3,3Ј,4,4Ј-Tetrachlorobiphenyl (3): A 25 mL round-bottomed flask
was
charged
with
1,2-dichloro-4-iodobenzene
(100 mg,
1,1Ј,3,3Ј-Tetraphenyl-3,3Ј-diaminobenzidine (4d): In a manner anal-
ogous to the procedure used to prepare 4a, 3 (500 mg, 1.71 mmol)
and aniline (646 mg, 6.94 mmol) afforded 763 mg (86% yield) of
the desired compound as a brown powder; m.p. 160–163 °C. 1H
NMR (CDCl3): δ = 7.38 (d, J = 2.2 Hz, 2 H), 7.32 (s, 2 H), 7.30
(s, 2 H), 7.25 (d, J = 8.4 Hz, 2 H), 7.14 (m, 8 H), 7.08 (dd, J = 8.4,
2.2 Hz, 2 H), 6.96 (m, 8 H), 6.74 (m, 4 H) ppm. 13C NMR (CDCl3):
δ = 143.8, 143.7, 135.6, 135.0, 133.9, 129.4, 129.3, 121.2, 120.6,
120.3, 118.4, 117.3, 117.2 ppm. HRMS: [M + H]+ calcd. for
C36H31N4: 519.2533; found 519.2543.
0.366 mmol) and a stir bar, and then placed under nitrogen. After
dry THF (1.5 mL) was introduced into the flask via syringe, the
resulting solution was cooled to –78 °C using a dry ice/acetone
bath. nBuLi (2.2  in hexanes, 0.18 mL, 0.403 mmol) was then
added dropwise over 30 min. The resulting reaction mixture was
then stirred at –78 °C for an additional 30 min. CuBr2 (81.7 mg,
0.366 mmol) was then added in a single portion and the resulting
mixture was stirred for 3 h at –78 °C. Afterward, the mixture was
quenched with an aqueous solution saturated with NH4Cl and the
organic material was extracted with hexanes (3ϫ50 mL). The re-
sultant organic extracts were then combined and dried with
MgSO4. Residual solvent was removed using a rotary evaporator
under reduced pressure. The residual solids were collected by fil-
tration, washed with ethanol, and dried under vacuum to afford
[1,1Ј,3,3Ј-Tetra(tert-butyl)-5,5Ј-bibenzimidazolium](BF4)2 (5a):
A
mixture of 4a (700 mg, 1.59 mmol), HBF4 (1.08 mL, 50% in Et2O),
and trimethyl orthoformate (20 mL) was heated at 65 °C for 24 h.
Afterward, precipitated solids were collected by filtration, washed
1734
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1729–1738

5,5Ј-Bibenzimidazolylidenes and Their Bimetallic Complexes
with THF, and then dried under vacuum to afford 786 mg (90%
yield) of the desired compound as a white powder; m.p. 224–248 °C
(dec.). 1H NMR ([D6]DMSO): δ = 8.87 (s, 2 H), 8.42 (d, J = 8.9 Hz,
2 H), 8.39 (s, 2 H), 7.97 (d, J = 8.9 Hz, 2 H), 1.86 (s, 18 H), 1.86
(s, 18 H) ppm. 13C NMR ([D6]DMSO): δ = 139.0, 137.1, 131.0,
130.1, 125.7, 117.5, 116.2, 61.9, 61.7, 40.1, 40.0, 35.1, 35.0, 29.2,
m.p. 219 °C. 1H NMR (C6D6): δ = 7.97 (s, 2 H), 7.50 (d, J = 8.2 Hz,
2 H), 7.41 (dd, J = 8.5, 1.7 Hz, 2 H), 2.09 (q, J = 7.5 Hz, 4 H),
2.04 (q, J = 7.5 Hz, 4 H), 1.82 (s, 12 H), 1.81 (s, 12 H), 0.75 (t, J
= 7.5 Hz, 6 H), 0.74 (t, J = 7.5 Hz, 6 H) ppm. 13C NMR (C6D6):
δ = 228.1, 136.7, 135.4, 135.1, 120.7, 113.8, 113.1, 60.3, 60.2, 33.8,
33.7, 29.0, 28.9, 8.5 ppm. HRMS: [M + H]+ calcd. for C34H51N4:
29.1 ppm. 19F NMR ([D6]DMSO): δ = –148.2, –148.3 ppm. 515.4114; found 515.4103.
HRMS: [M]2+ calcd. for C30H44N4: 230.1782; found 230.1778. UV/
1,1Ј,3,3Ј-Tetra(1-adamantyl)-5,5Ј-bibenzimidazolylidene (2c): In a
manner analogous to the procedure used to prepare 2a, 5c
(30.0 mg, 0.032 mmol), NaHMDS (11.9 mg, 0.0649 mmol) and tol-
uene (1 mL) afforded 23.2 mg (86% yield) of the desired compound
Vis: λmax = 292 nm, ε = 1.74ϫ104 –1 cm–1. Fluorescence: λem
341 nm, Φ = 0.67.
=
[1,1Ј,3,3Ј-Tetra(tert-amyl)-5,5Ј-bibenzimidazolium](BF4)2
(5b):
1
Using a procedure analogous to one used to prepare 5a, 4b
(375 mg, 0.76 mmol) afforded 360 mg (79% yield) of the desired
compound as a beige solid; m.p. 219–222 °C (dec.). 1H NMR ([D6]-
DMSO): δ = 8.90 (s, 2 H), 8.46 (d, J = 8.8 Hz, 2 H), 8.44 (d, J =
1.4 Hz, 2 H), 7.99 (dd, J = 8.8, 1.4 Hz, 2 H), 2.24 (q, J = 7.0 Hz,
4 H), 2.23 (q, J = 6.8 Hz, 4 H), 1.87 (s, 12 H), 1.85 (s, 12 H), 0.69
(t, J = 7.4 Hz, 6 H), 0.68 (t, J = 7.2 Hz, 6 H) ppm. 13C NMR ([D6]-
DMSO): δ = 140.5, 137.7, 131.4, 130.6, 127.0, 166.9, 115.4, 64.9,
64.7, 31.8, 26.1, 26.0, 8.1, 7.9 ppm. 19F NMR ([D6]DMSO): δ =
–148.2, –148.3 ppm. HRMS: [M]2+ calcd. for C34H52N4: 258.2088;
found 258.2091. UV/Vis: λmax = 292 nm, ε = 1.86ϫ104 –1 cm–1.
Fluorescence: λem = 340 nm, Φ = 0.47.
as a white solid; m.p. 344 °C. H NMR (C6D5CD3): δ = 8.17 (d, J
= 1.5 Hz, 2 H), 7.73 (d, J = 8.4 Hz, 2 H), 7.53 (dd, J = 8.4, 1.6 Hz,
2 H), 2.65 (br., 12 H), 2.60 (br., 12 H), 2.14 (br., 12 H), 1.76–1.67
(m, 24 H) ppm. 13C NMR (C6D5CD3): δ = 225.9, 136.5, 134.9,
134.7, 119.8, 114.4, 113.7, 58.4, 58.3, 43.6, 43.5, 37.1, 37.0, 30.6,
30.5 ppm. HRMS: [M + H]+ calcd. for C54H67N4: 771.5367; found
771.5366.
[{Rh(COD)Cl}2(2a)] (6a): A 30-mL vial was charged with 5a
(107 mg, 0.168 mmol), NaHMDS (62.9 mg, 0.343 mmol) and a
Teflon®-coated stirbar. THF (10 mL) was then added and the
slurry was stirred at ambient temperature for 20 min. The resulting
cloudy light brown solution was filtered through a 0.4 µm PTFE
filter to give a solution of 2a. [Rh(COD)Cl]2 (82.7 mg, 0.168 mmol)
was then added to the solution of freshly generated 2a and the
mixture was stirred at ambient temperature for 12 h. THF was then
removed and the resulting yellow solid was triturated with diethyl
ether (3ϫ20 mL) and dried under vacuum to afford 127 mg (80%
yield) of the desired product as a light yellow powder. 1H NMR
(CDCl3): δ = 7.79 (d, J = 1.3 Hz, 2 H), 7.75 (d, J = 8.7 Hz, 2 H),
7.34 (dd, J = 1.3, 8.7 Hz, 2 H), 5.01 (br., 4 H), 2.99 (br., 4 H),
approx. 2.41 (m, 8 H), 2.40 (d, 36 H), 1.76 (m, 8 H) ppm. 13C
NMR (CDCl3): δ = 196.1 (d, J = 48.6 Hz), 136.1, 134.9, 134.4,
120.7, 114.8, 113.7, 93.4, 67.6, 67.4, 60.5, 32.3, 32.2, 28.6 ppm.
HRMS: [M – Cl]+ calcd. for C46H66ClN4Rh2: 915.3090; found
915.3081. E1/2 (RhI/II) = +0.59 V (quasi-reversible).
[1,1Ј,3,3Ј-Tetra(1-adamantyl)-5,5Ј-bibenzimidazolium](BF4)2
(5c):
Using a procedure analogous to one used to prepare 5a, 4c
(100 mg, 0.133 mmol) afforded 113 mg (88% yield) of the desired
compound as a beige solid; m.p. 347–352 °C (dec.). 1H NMR ([D6]-
DMSO): δ = 8.89 (s, 2 H), 8.55 (d, J = 8.9 Hz, 2 H), 8.47 (d, J =
1.2 Hz, 2 H), 7.97 (d, J = 8.9, 1.4 Hz, 2 H), 2.51 (br., 12 H), 2.46
(br., 12 H), 2.32 (br., 6 H), 2.28 (br., 6 H), 1.88–1.78 (br., 24 H)
ppm. 13C NMR ([D6]DMSO): δ = 139.1, 137.2, 131.0, 130.1, 125.8,
117.6, 116.3, 61.9, 61.7, 35.1, 29.2, 29.1 ppm. 19F NMR ([D6]-
DMSO): δ = –148.2, –148.3 ppm. HRMS: [M]2+ calcd. for
C54H68N4: 386.2721; found 386.2717. UV/Vis: λmax = 294 nm, ε =
1.74ϫ104 –1 cm–1. Fluorescence: λem = 342 nm, Φ = 0.88.
[1,1Ј,3,3Ј-Tetraphenyl-5,5Ј-bibenzimidazolium](BF4)2 (5d): Using a
procedure analogous to one used to prepare 5a, combination of 4d
(500 mg, 0.96 mmol) and triethyl orthoformate (20 mL) at 140 °C
for 24 h afforded 648 mg (96% yield) of the desired compound as
[{Rh(COD)Cl}2(2b)] (6b): A 5 mL vial was charged with 5b (22 mg,
0.043 mmol), [Rh(COD)Cl]2 (21.1 mg, 0.0428 mmol), THF (2 mL),
and a Teflon®-coated stirbar and then stirred at ambient tempera-
ture for 12 h. After removal of the solvent, the resulting yellow
solid was triturated with diethyl ether (3ϫ10 mL) and then dried
under vacuum to give 32.7 mg (76% yield) of the desired product
as a light yellow powder. 1H NMR (CDCl3): δ = 7.82 (s, 2 H), 7.77
(d, J = 8.5 Hz, 2 H), 7.32 (d, J = 8.5 Hz, 2 H), 5.02 (br., 4 H), 3.10
(br., 12 H), 2.96 (br., 4 H), 2.71 (m, 4 H), 2.43 (m, 8 H), 2.19 (s, 6
H), 2.15 (s, 6 H), 1.77 (m, 12 H), 0.59 (t, J = 6.8 Hz, 12 H) ppm.
13C NMR (CDCl3): δ = 197.8 (d, J = 49.4 Hz), 136.1, 134.9, 134.3,
120.8, 114.4, 113.2, 93.2, 67.5, 63.7, 34.6, 32.2, 31.8, 29.1, 29.0,
28.6, 8.5 ppm. HRMS: [M – 2Cl]2+ calcd. for C46H66N4Rh2:
468.2010; found 468.2006. E1/2 (RhI/II) = +0.59 V (quasi-revers-
ible).
1
a beige solid; m.p. 296–300 °C (dec.). H NMR ([D6]DMSO): δ =
10.48 (s, 2 H), 8.20 (s, 2 H), 8.18 (d, J = 8.6 Hz, 2 H), 8.02 (d, J =
8.8 Hz, 2 H), 7.95 (m, 8 H), 7.77 (m, 12 H) ppm. 13C NMR ([D6]-
DMSO): δ = 143.9, 138.7, 133.0, 132.9, 131.7, 131.1, 130.9, 130.8,
130.5, 130.4, 127.9, 125.4, 125.3, 114.5, 112.7 ppm. 19F NMR ([D6]-
DMSO): δ = –148.2, –148.3 ppm. HRMS: [M]2+ calcd. for
C38H28N4: 270.1155; found 270.1152. UV/Vis: λmax = 285 nm, ε =
2.51ϫ104 –1 cm–1. Fluorescence: λem = 415 nm, Φ = 0.61.
1,1Ј,3,3Ј-Tetra(tert-butyl)-5,5Ј-bibenzimidazolylidene (2a): In a 5-
mL vial, a suspension of bibenzimidazolium salt 5a (20.9 mg,
0.0328 mmol), NaOtBu (6.5 mg, 0.067 mmol) and benzene (1 mL)
was stirred at ambient temperature for 10 h. The reaction mixture
was then filtered through a 0.4 µm PTFE filter and concentrated
under reduced pressure to afford 14.9 mg (99% yield) of the desired
[{Rh(COD)Cl}2(2c)] (6c): A 30-mL vial was charged with 5c
(100 mg, 0.106 mmol), NaHMDS (40 mg, 0.22 mmol) and a Tef-
lon®-coated stirbar. Toluene (25 mL) was then added and the re-
sulting mixture was stirred at ambient temperature for 45 min. The
resulting cloudy, light brown mixture was then filtered through a
0.4 µm PTFE filter to afford a solution of 2c. [Rh(COD)Cl]2
(98 mg, 0.21 mmol) was added to the solution of freshly generated
2c and the resulting mixture was stirred at ambient temperature for
12 h. Toluene was then removed and the resulting brown solid was
1
compound as a beige powder; m.p. 222 °C. H NMR (C6D6): δ =
7.96 (s, 2 H), 7.49 (d, J = 8.7 Hz, 2 H), 7.42 (d, J = 8.7 Hz, 2 H),
1.79 (s, 12 H), 1.78 (s, 12 H) ppm. 13C NMR (C6D6): δ = 224.0,
136.3, 135.2, 134.8, 120.8, 114.4, 113.7, 59.1, 59.0, 32.9 ppm.
HRMS: [M + H]+ calcd. for C30H43N4: 459.3488; found 459.3486.
1,1Ј,3,3Ј-Tetra(tert-amyl)-5,5Ј-bibenzimidazolylidene (2b): In a man-
ner analogous to the procedure used to prepare 2a, 5b (20.5 mg,
0.030 mmol) and NaHMDS (11.1 mg, 0.0605 mmol) afforded triturated with diethyl ether (3ϫ20 mL). Drying the residual solids
14.9 mg (98% yield) of the desired compound as a yellow solid; under vacuum afforded 120 mg (85% yield) of the desired product
Eur. J. Inorg. Chem. 2009, 1729–1738
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1735

C. W. Bielawski et al.
FULL PAPER
as a light yellow powder. 1H NMR (CDCl3): δ = 7.81 (s, 2 H), 7.77
(d, J = 8.5 Hz, 2 H), 7.32 (d, J = 8.5 Hz, 2 H), 4.99 (br., 4 H), 3.50
EtOAc/30% hexanes as eluent) and then dried to afford 96.2 mg
(42% yield) of the desired product as pale yellow powder. 1H NMR
(m, 12 H), 3.02 (m, 16 H), 2.42 (br., 24 H), 1.90 (m, 35 H) ppm. (CDCl3): δ = 7.67 (dd, J = 6.26, 3.13 Hz 2 H), 7.13 (dd, J = 6.26,
13C NMR (CDCl3): δ = 195.8 (d, J = 49.6 Hz), 135.7, 134.6, 133.6,
120.0, 115.6, 114.2, 92.4, 67.5, 62.4, 62.3, 43.0, 36.5, 36.4, 32.4,
32.3, 30.4, 28.8, 28.7 ppm. HRMS: [M – 2Cl]2+ calcd. for
3.13 Hz 2 H), 4.98 (br., 2 H), 2.97 (br., 2 H), 2.42 (m, 4 H), 2.36
(s, 18 H), 1.74 (m, 4 H) ppm. 13C NMR (CDCl3): δ = 194.4 (d, J
= 49.4 Hz), 135.4, 120.8, 114.7, 93.1, 93.0, 67.5, 67.3, 60.3, 32.2,
C70H90N4Rh2: 596.2628; found 596.2632. E1/2 (RhI/II) = +0.54 V 32.1, 28.6 ppm. HRMS: [M – Cl]+ calcd. for C23H34N2Rh:
(quasi-reversible).
441.1775; found 441.1772. E1/2 (RhI/II) = +0.58 V (quasi-revers-
ible).
[1,1Ј,3,3Ј-Tetraethyl-5,5Ј-bibenzimidazolium](BF4)2: A 30-mL vial
was charged with [1,1Ј,3,3Ј-tetraethyl-5,5Ј-bibenzimidazolium]-
[1,3-Diethylbenzimidazolium](Br): A 30-mL vial was charged with
benzimidazole (416 mg, 3.52 mmol), NaHCO3 (607 mg,
7.22 mmol), acetonitrile (5 mL), and a stir bar. The vial was then
sealed and the resulting reaction mixture was stirred at 90 °C for
1 h. After cooling to ambient temperature, bromoethane (1.6 mL,
21 mmol) was added. The resulting mixture was stirred at 90 °C for
12 h. After cooling to ambient temperature, precipitated NaBr was
removed by vacuum filtration and the filtrate was concentrated to
afford 792.3 mg (Ͼ99% yield) of the desired product as a white
powder. 1H NMR ([D6]DMSO): δ = 10.0 (s, 1 H), 8.09 (dd, J =
6.31, 3.08 Hz, 2 H), 7.66 (dd, J = 6.31, 3.08 Hz, 2 H), 4.54 (q, J =
7.34 Hz, 4 H), 1.53 (t, J = 7.34 Hz, 6 H) ppm. 13C NMR ([D6]-
DMSO): δ = 141.7, 130.9, 126.4, 113.6, 42.0, 14.2 ppm. HRMS:
[M]+ calcd. for C11H15N2: 175.1227; found 175.1230.
[14]
(Br)2
(202 mg, 0.398 mmol), dry CH3CN (5 mL), triethylox-
onium tetrafluoroborate (182 mg, 0.956 mmol) and a stir bar.[33]
The reaction mixture was stirred at ambient temperature for 12 h.
Excess triethyloxonium tetrafluoroborate was quenched by adding
ethanol (2 mL) followed by stirring for an additional 2 h. The re-
sulting solution was poured into excess ether and white solids
(186.1 mg, 85%) were collected by vacuum filtration. 1H NMR
([D6]DMSO): δ = 9.82 (s, 2 H), 8.52 (s, 2 H), 8.23 (d, J = 8.7 Hz,
2 H), 8.18 (d, J = 8.7 Hz, 2 H), 4.57 (m, 8 H), 1.58 (m, 12 H) ppm.
13C NMR ([D6]DMSO): δ = 142.5, 137.6, 131.8, 130.8, 126.2,
114.3, 112.2, 42.2, 42.1, 14.3 ppm. 19F NMR ([D6]DMSO): δ =
–148.2, –148.3 ppm. HRMS: [M]2+ calcd. for C22H28N4: 174.1155;
found 174.1152. UV/Vis: λmax = 297 nm, ε = 1.39ϫ104 –1 cm–1.
Fluorescence: λem = 347 nm, Φ = 0.83.
[1,3-Diethylbenzimidazolium](BF4): A 30-mL vial was charged with
[1,3-diethylbenzimidazolium](Br) (291 mg, 1.29 mmol), dry
CH3CN (5 mL), triethyloxonium tetrafluoroborate (295 mg,
1.55 mmol) and a stir bar.[33] The reaction mixture was stirred at
ambient temperature for 12 h. Excess triethyloxonium tetrafluo-
roborate was quenched by adding ethanol (2 mL) followed by stir-
ring for an additional 2 h. The resulting solution was concentrated
and washed with methanol (3 mL) to afford 187 mg (92% yield) of
[{Rh(COD)Cl}2{1,1Ј,3,3Ј-tetraethyl-5,5Ј-bibenzimidazolylidene}]
(7): A 30-mL vial was charged with [1,1Ј,3,3Ј-tetraethyl-5,5Ј-bi-
benzimidazolium](BF4)2 (110 mg, 0.211 mmol), KOtBu (49.7 mg,
0.443 mmol), [Rh(COD)Cl]2 (104 mg, 0.211 mmol), THF (3 mL),
and a Teflon®-coated stirbar. The resulting slurry was then stirred
at ambient temperature for 12 h. This produced a yellow solution
which was filtered through a 0.4 µm PTFE filter. The filtrate was
1
concentrated to afford 178 mg (98% yield) of the desired product
the desired product as a white powder. H NMR ([D6]DMSO): δ
1
as pale yellow powder. H NMR (CDCl3): δ = 7.37 (m, 6 H), 5.14 = 9.74 (s, 1 H), 8.08 (dd, J = 6.31, 3.08 Hz, 2 H), 7.69 (dd, J =
(br., 4 H), 5.08 (m, 4 H), 4.84 (m, 4 H), 3.37 (br., 4 H), 2.45 (m, 8
H), 2.00 (m, 8 H), 1.64 (m, 12 H) ppm. 13C NMR (CDCl3): δ =
196.7 (d, J = 50.7 Hz), 135.9, 135.1, 133.9, 122.2, 122.1, 110.1,
108.8, 108.7, 100.2, 68.7, 68.6, 43.7, 43.6, 32.9, 28.8, 15.1, 15.0 ppm.
HRMS: [M – Cl]+ calcd. for C38H50ClN4Rh2: 803.1832; found
803.1829. Epa (RhI/II) = +0.95 V (irreversible).
6.31, 3.08 Hz, 2 H), 4.50 (q, J = 7.34 Hz, 4 H), 1.53 (t, J = 7.34 Hz,
6 H) ppm. 13C NMR ([D6]DMSO): δ = 141.6, 131.0, 126.5, 113.6,
42.0, 14.2 ppm. 19F NMR ([D6]DMSO): δ = –148.3 ppm. HRMS:
[M]+ calcd. for C11H15N2: 175.1232; found 175.1230. UV/Vis: λmax
= 277 nm, ε = 6.28ϫ103 –1 cm–1. Fluorescence: none detectable,
possibly due to extremely low Φ, insufficient Stokes shift, or both.
[1,3-Bis(tert-butyl)benzimidazolium](BF4): A 5-mL vial was charged
[Rh(1,3-Diethylbenzimidazolylidene)(COD)Cl] (8b): A 30-mL vial
with [1,3-bis(tert-butyl)benzimidazolium](Cl)[34] (41.8 mg, 0.157 was charged with [1,3-diethylbenzimidazolium](BF4) (99.7 mg,
mmol), dry CH3CN (3 mL), triethyloxonium tetrafluoroborate
(41.7 mg, 0.219 mmol) and a stir bar.[33] The reaction mixture was
stirred at ambient temperature for 12 h. Excess triethyloxonium tet-
rafluoroborate was quenched by adding ethanol (2 mL) followed
by stirring for an additional 2 h. The resulting solution was concen-
trated and washed with methanol (3 mL) to afford 47.6 mg (95%
yield) of the desired compound as a white powder. 1H NMR ([D6]-
DMSO): δ = 8.84 (s, 1 H), 8.31 (dd, J = 6.34, 3.17 Hz, 2 H), 7.64
(dd, J = 6.34, 3.17 Hz, 2 H), 1.81 (s, 18 H) ppm. 13C NMR ([D6]-
DMSO):δ=138.7,130.9,125.8,116.6,60.9,28.1ppm.19FNMR([D6]-
0.380 mmol), KOtBu (47.0 mg, 0.419 mmol), [Rh(COD)Cl]2
(93.8 mg, 0.190 mmol), THF (3 mL), and a Teflon®-coated stirbar.
The resulting slurry was then stirred at ambient temperature for
12 h. The resulting yellow solution was filtered through a 0.4 µm
PTFE filter and concentrated to give 158.8 mg (99% yield) of the
desired product as yellow powder. 1H NMR (CDCl3): δ = 7.29 (dd,
J = 5.81, 3.08 Hz, 2 H), 7.18 (dd, J = 5.81, 3.08 Hz, 2 H), 5.11 (br.,
2 H), 5.02 (m, 2 H), 4.77 (m, 2 H), 3.34 (br., 2 H), 2.43 (m, 4 H),
1.98 (m, 4 H), 1.60 (t, J = 7.5 Hz, 2 H) ppm. 13C NMR (CDCl3):
δ = 195.1 (d, J = 50.7 Hz), 134.4, 122.1, 109.8, 99.9, 99.8, 68.6,
DMSO): δ = –148.2, –148.3 ppm. HRMS: [M]+ calcd. for 68.5, 43.5, 32.8, 28.7, 14.9 ppm. HRMS: [M – Cl]+ calcd. for
C15H23N2: 231.1859; found 231.1856. UV/Vis: λmax = 276 nm, ε =
C19H26N2Rh: 385.1141; found 385.1146. Epa (RhI/II) = +0.86 V
(irreversible).
6.98ϫ103 –1 cm–1. Fluorescence: λem = 351 nm, Φ = 1.1ϫ10–3.
[Rh{1,3-Bis(tert-butyl)benzimidazolylidene}(COD)Cl] (8a): A 5-mL
vial was charged with [1,3-bis(tert-butyl)benzimidazolium](Cl)[34]
(110 mg, 0.412 mmol), NaHMDS (83.1 mg, 0.453 mmol),
[{Rh(CO)2Cl}2{1,1Ј,3,3Ј-tetraethyl-5,5Ј-bibenzimidazolylidene}] (9):
A solution of 7 (20 mg, 0.024 mmol) and CH2Cl2 (2 mL) was
stirred under CO (balloon) with slow purging until the solvent was
[Rh(COD)Cl]2 (102 mg, 0.206 mmol), THF (4 mL), and a Teflon®- evaporated. The resulting yellow solid was then washed with pen-
coated stirbar. The resulting slurry was then stirred at ambient tem-
perature for 12 h. This produced a yellow solution that was filtered
through a 0.4 µm PTFE filter. After removal of solvent, the crude
product was purified by flash chromatography (silica gel, 70%
tane (3ϫ3 mL) at –78 °C under an atmosphere of CO. Removal of
the residual pentane under vacuum afforded 16.9 mg (96% yield)
1
of the desired product as a yellow solid. H NMR (CD2Cl2): δ =
7.67 (d, J = 1.54 Hz, 2 H), 7.64 (dd, J = 8.44, 1.54 Hz, 2 H), 7.58
1736
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Eur. J. Inorg. Chem. 2009, 1729–1738

5,5Ј-Bibenzimidazolylidenes and Their Bimetallic Complexes
Reichel, M. Handke, F. Hampel, Angew. Chem. Int. Ed. 1998,
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For selected examples of bimetallic complexes formed between
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through saturated linkers, see: a) W. A. Herrmann, M. Elison,
J. Fischer, C. Köcher, G. R. J. Artus, Chem. Eur. J. 1996, 2,
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(d, J = 8.44 Hz, 2 H), 4.73 (m, 8 H), 1.60 (t, J = 7.26 Hz, 6 H),
1.59 (t, J = 7.26 Hz, 6 H) ppm. 13C NMR (CD2Cl2): δ = 186.2 (d,
J = 53.7 Hz), 186.0 (d, J = 42.9 Hz), 183.0 (d, J = 73.8 Hz), 137.1,
135.0, 133.9, 123.9, 111.5, 110.0, 44.4, 44.3, 15.1, 15.0 ppm. IR
(CaF , CD Cl ): ν = 2084 (trans νCO), 2003 (cis νCO) cm–1. HRMS:
˜
2
2
2
[M – Cl]+ calcd. for C26H26ClN4O4Rh2: 698.9753; found 698.9747.
Epa (RhI/II) = +1.16 V (irreversible).
[Rh(1,3-Diethylbenzimidazolylidene)(CO)2Cl] (10): A solution of 8b
(30 mg, 0.071 mmol) and CH2Cl2 (3 mL) was stirred under slight
pressure of CO (balloon) with slow purging until the solvent was
evaporated. The resulting yellow solid was then washed with pen-
tane (3ϫ3 mL) at –78 °C under CO. Removal of the residual pen-
tane under vacuum afforded 26.0 mg (Ͼ99% yield) of the desired
product as a yellow solid. 1H NMR (CD2Cl2): δ = 7.49 (dd, J =
5.81, 3.08 Hz, 2 H), 7.37 (dd, J = 5.81, 3.08 Hz, 2 H), 4.67 (m, 4
H), 1.55 (t, J = 7.2 Hz, 6 H) ppm. 13C NMR (CD2Cl2): δ = 186.2
(d, J = 53.8 Hz), 184.5 (d, J = 42.3 Hz), 183.1 (d, J = 73.8 Hz),
134.3, 123.7, 111.1, 44.2, 14.9 ppm. HRMS: [M – Cl]+ calcd. for
[6]
[7]
C H N O Rh: 333.0109; found 333.0105. IR (CaF , CD Cl ): ν
˜
13 14
2
2
2
2
2
= 2083 (trans νCO), 2002 (cis νCO) cm–1. Epa (RhI/II) = +0.88 V
(irreversible).
Supporting Information (see also the footnote on the first page of
this article): NMR spectra and cyclic voltammograms.
Acknowledgments
We are grateful to the National Science Foundation (NSF) (CHE-
0645563) and the Robert A. Welch Foundation (F-1621) for their
generous financial support. We would also like to thank Alec Ne-
pomnyashchii and Joaquin Lopez for valuable electrochemical as-
sistance and fruitful discussions.
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These data can be obtained free of charge from The Cambridge
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www.eurjic.org
1737

C. W. Bielawski et al.
FULL PAPER
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Received: December 7, 2008
Published Online: February 6, 2009
1738
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Eur. J. Inorg. Chem. 2009, 1729–1738