Synthesis of rhodium(i) complexes bearing bidentate nh,nr-nhc/phosphine ligands

Article abstract of DOI:10.1021/om200689r

The N-alkyphosphine-substituted benzimidazoles 5 (R = methylenedicyclohexylphosphine), 7 (R = ethylene-di(tBu)phosphine), and 8 (R = ethylenedicyclohexylphosphine) have been prepared. The benzimidazoles react with [RhCl(COE)₂]₂ in th

Full text of DOI:10.1021/om200689r

ARTICLE
pubs.acs.org/Organometallics
Synthesis of Rhodium(I) Complexes Bearing Bidentate
NH,NR-NHC/Phosphine Ligands
Abbas Raja Naziruddin, Alexander Hepp, Tania Pape, and F. Ekkehardt Hahn*
Institut f€ur Anorganische und Analytische Chemie, Westf€alische Wilhelms-Universit€at M€unster, Corrensstrasse 30, D-48149 M€unster,
Germany
S Supporting Information
b
ABSTRACT: The N-alkyphosphine-substituted benzimidazoles 5 (R = methyl-
enedicyclohexylphosphine), 7 (R = ethylene-di(tBu)phosphine), and 8 (R =
ethylenedicyclohexylphosphine) have been prepared. The benzimidazoles react
with [RhCl(COE)2]2 in the presence of tertiary phosphines under formal tautomer-
ization of the benzimidazole and chelating coordination of the resulting bidentate
NH,NR-NHC/phosphine ligand (P∧C) to give complexes [RhCl(P∧C)PR3]
[6]ꢀ[9]. Depending on the steric demand of the PR3, the phosphines of the
P∧C ligands, and on the spacer linking the benzimidazole ring nitrogen atom to the alkylphosphine, complexes with cis-P,P and trans-P,
P geometry have been obtained and crystallographically characterized.
’ INTRODUCTION
More recent efforts have been directed toward the design of
ligands that can bind and orient selected substrates via non-
covalent interactions. This is often achieved via hydrogen bonds
between a ligand coordinated to the catalytically active metal
center and a functional group of the substrate.8 Such an arrange-
ment enforced a substrate orientation that enabled highly
selective rhodium-catalyzed hydrogenation9 or hydroformylation10
reactions.
Many enzymatic reactions are based on the combination of
molecular recognition and catalysis. Multiple noncovalent inter-
actions between the substrate and the active site of the enzyme
can lead to a high degree of substrate selectivity and to a regio-
and stereoselective catalytic reaction.1 The transformation of
these principles to homogeneous catalysis with transition-metal
complexes has led to the emerging field of supramolecular
catalysis where the substrate selection by the catalyst is based
on specific supramolecular interactions.2 Various research groups
have tried to combine transition-metal catalysis with noncovalent
substrate recognition and binding,3 but only a few successful
approaches combining high substrate selectivity and rate en-
hancement have been reported. Among those are the asymmetric
hydrogenation of trisubstituted acrylic acids in the presence
of a chiral (aminoalkyl)ferrocenylphosphine, a reaction that is
believed to result from an attractive interaction of the amino
group of the ferrocenylphosphine with the carboxyl group of the
substrate.4 Metalloporphyrins with attached cyclodextrin groups
have been shown to catalyze the regioselective hydroxylation of
steroid derivatives via interaction of the steroid substituents with
the cyclodextrin groups, thereby causing the proper orientation
of the substrate.5 Hydrogen bonding between the carboxylic acid
groups of a Mn(μ-O2)Mn coordinated ligand and the carboxyl
group of a substrate also led to a specific substrate orientation
and a modified regioselectivity for the sp3 CꢀH oxidation.6
Finally, Breit et al. introduced the concept of a temporary
substrate-bound catalyst-directing group for catalytic hydrofor-
mylations. Here, the substrate is covalently linked to a phosphine.
Simultaneous coordination of the phosphine and the functional
group of the substrate to rhodium(I) allowed for highly regio-
and stereoselective transformations. The major drawback of this
approach is the required covalent linkage of the substrate to the
phosphine.7
We became interested in supramolecular catalysis using com-
plexes bearing N-heterocyclic carbenes (NHCs).11 NHC ligands
have been employed as spectator ligands for the preparation of
various catalytically active complexes12 where the NHC ligand
imparts the desired steric and electronic properties to the metal
center. In complexes with the commonly used NR,NR-substi-
tuted NHC ligands (A, Figure 1), all catalytic transformations
take place at the NHC coordinated metal center. As an alter-
native, we have also developed the coordination chemistry of
complexes bearing protic NHC ligands, that is, NHC ligands
featuring an NH,NR (B) or NH,NH (C) substitution pattern
(Figure 1),13 which are valuable building blocks for the template
synthesis of macrocyclic ligands featuring NHC donors.14
Furthermore, the NꢀH group of NH,NR-substituted NHC
ligands in complexes of type B can act as a hydrogen bond donor
and thus function as a recognition unit via the formation of
hydrogen bonds. In contrast to many related systems,6,9,10 the
recognition unit is located in close proximity to the catalytically
active metal center. As expected, tungsten complex D forms
a hydrogen bond between the NꢀH group of the NHC ligand
and the hydrogen bond acceptor DMPU in solution (Figure 1),
which was confirmed by 1H NMR spectroscopy.15
Received: July 27, 2011
Published: October 03, 2011
r
2011 American Chemical Society
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Scheme 2. Synthesis of Donor-Functionalized
Benzimidazolesa
Figure 1. Complexes bearing NR,NR-, NH,NR-, and NH,NH-substi-
tuted NHCs and hydrogen bonding between complex D and DMPU.
Scheme 1. Concept of Interaction/Recognition of Substrates
with Complexes Bearing NH,NR-Substituted NHCs and
Complexes of Type F
a The numbering refers to the assignment of the NMR resonances.
of 3-butenoic acid ester, thereby preventing the deactivation of
the catalyst. Unfortunately, complex E is not stable under the
reaction conditions but tautomerizes to give complex E0 with the
N-coordinated benzimidazole ligand.15 The reverse reaction of
this tautomerization has been used for the synthesis of complexes
bearing NH,NR-substituted NHCs from azole complexes.13oꢀq
In this contribution, we describe the preparation of rhodium-
(I) complexes bearing donor-functionalized NH,NR-substituted
NHCs of type F with different linkers between the NHC
nitrogen atom and the attached phosphine. Because of the
simultaneous coordination of the NHC and the phosphine donor
in these P∧C-chelate ligands (Scheme 1, bottom), tautomeriza-
tion is not possible, which leads to stable complexes with NH,
NR-NHCs (Scheme 1).
’ RESULTS AND DISCUSSION
The N-alkylphosphine-substituted benzimidazoles 3, 4, and 5,
featuring either a methylene or an ethylene spacer between the
heterocycle and the phosphine donor, have been prepared
starting from 5,6-dimethylbenzimidazole using a methodology
previously described.16 The preparation of the methylene bridged
ligand precursor proceeded via the alcohol and the hydrochloride
The RhI complex E (Scheme 1) provided proof of the concept
of supramolecular recognition with protic NHC ligands. In
competitive hydrogenation experiments with 1-dodecene and
3-butenoic acid ester using E as a catalyst, the substrate with the
carbonyl function was clearly preferred, most likely by interaction
of the carbonyl group with the NꢀH group of the NH,NR-
NHC ligand under formation of a two-point interaction prior
to the catalytic transformation (Scheme 1). Hydrogenation
of the CdC double bond leads to a one-point interaction and
substitution of the hydrogenated substrate for another molecule
derivative 1 HCl,16 which was finally reacted with KOtBu and
3
LiPPh2 to give 3 (Scheme 2). Ligand precursors 4 and 5 were
obtained by alkylation of 5,6-dimethylbenzimidazole17 to give 2,16
followed by reaction with LiPtBu2 or LiPCy2 to give compounds 4
and 5, respectively. The N-alkylphosphine-substituted benzimi-
dazoles were obtained as hygroscopic, off-white solids. Related
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Scheme 3. Reactions of 3, 4, and 5 with [RhCl(COE)2] in the
Presence of Different Phosphinesa
Figure 2. Molecular structure of 4 (50% displacement ellipsoids,
hydrogen atoms have been omitted for clarity, only one of the two
essentially identical molecules in the asymmetric unit is shown).
Selected bond lengths (Å) and angles (deg) for molecule 1 [molecule
2]: N1ꢀCl 1.363(3) [1.356(3)], N2ꢀC1 1.313(3) [1.316(3)];
N1ꢀC1ꢀN2 114.7(2) [114.7(2)].
N-alkylpyridyl-functionalized benzimidazoles18 and diphenylpho-
sphine-substituted imidazoles19 have also been described.
In addition to NMR spectroscopic and microanalytical data
for 3, 4, and 5, compound 4 was also characterized by an X-ray
diffraction study. This study confirmed the connectivity in the
molecule (Figure 2). The N2ꢀC1 bond length (1.313(3) Å) is
significantly shorter than the N1ꢀC1 (1.363(3) Å, molecule 1)
distance, in accord with a localized NdC double bond. The
N1ꢀC1ꢀN2 angle (114.7(2)°) is larger than the equivalent
angle in N,N0-dialkylated benzimidazolium cations where
NꢀCꢀN angles of about 110° are normally observed.20
Next, we have reacted the N-alkylphosphine-substituted ben-
zimidazoles 3, 4, and 5 with [RhCl(COE)2]2 in the presence of
different tertiary phosphines (Scheme 3). Compounds 3 and 4
react with formation of square-planar RhI P∧C chelate com-
plexes. In the presence of the sterically demanding tricyclohexyl
phosphine, compound 3 yields a mixture of the cis-P,P ([6],
major 77%) and the trans-P,P complexes ([7], minor 23%). With
the sterically less demanding triphenyl phosphine, the same
reaction of 3 with [RhCl(COE)2] yields only the cis-P,P-complex
[8]. The P∧C ligand precursor 4 with an ethyl spacer between
the ring nitrogen atom and the di(tBu)phosphine donor yields,
in the presence of PCy3, exclusively the trans-P,P complex [9]
(Scheme 3). No defined reaction products apart from phosphine
complexes could be isolated from the reaction of [RhCl-
(COE)2]2 with 5 in the presence of PCy3. In all complex forma-
tions, the P∧C ligand precursor formally reacts under tautomer-
ization to a NH,NR-substituted NHC, which then coordinates in
a chelating fashion to the metal center. Most likely, the formation
of complexes [6]ꢀ[9] involves an initial oxidative addition of the
C2ꢀH bond to RhI, and selected features of this reaction will be
discussed below.
a The numbering refers to the assignment of the 31P{1H} NMR
resonances.
2JP1P2 = 28.0 Hz, P1) ppm. Two additional resonances at slightly
different chemical shifts of δ 68.3 (dd, 1JP2Rh = 156.0 Hz, 2JP2P1
339 Hz, P2) ppm and δ 31.7 (dd, JP1Rh = 137.0 Hz, JP1P2
=
=
1
2
339.0 Hz, P1) ppm, but featuring the large 2JPP coupling constant
of 339 Hz,21 indicate the presence of complex [7] with the trans-
P,P geometry. The 13C {1H}NMR spectrum shows the reso-
nance for the carbene carbon atom of the cis-P,P complex [6] at δ
193.0 (ddd, 1JCRh = 46.5 Hz, 2JCP2 = 103.4 Hz, 2JCP1 = 14.0 Hz)
ppm, whereas the resonance for the carbene carbon atom of [7]
was not observed due to its low concentration in the mixture of
complexes. Both the chemical shift and the observed coupling
constants of the 13C NMR carbene resonance in [6] match
previously reported values obtained for RhI complexs bearing
classical NR,NR-functionalized NHCs and two additional phos-
phine ligands in cis and trans positions to the NHC ligand.22
Given the high steric demand of the PCy3 ligand, which, under
standard conditions, yields with RhI the 14-electron complex
[RhCl(PCy3)2],23 the formation of the two isomeric complexes
[6] and [7] due to steric interactions of the phosphine ligands
is not surprising. To avoid the formation of geometric isomers,
we reacted the ligand precursor 3 with [RhCl(COE)2]2 in the
presence of the sterically less demanding triphenyl phosphine
(Scheme 3). As expected, this reaction gives exclusively the cis-P,
P complex [8] in an excellent yield of 92% (Scheme 3). Complex
[8] (cis-P,P) was also identified by NMR spectroscopy. The
In addition to X-ray diffraction structure analyses for [6] and
[9], all new complexes have been characterized by NMR spectro-
scopic methods. Complex [6] could be isolated in pure form by
multiple recrystallization steps from the mixture [6]/[7]. This
was not possible for complex [7] due to its low concentration in
the mixture. In the mixture of [6] and [7], complex [6] (cis-P,P)
can be distinguished from the trans-P,P derivative [7] by the
2
magnitude of the JP1P2 coupling constant. The 31P NMR
spectrum for [6] showed the resonances for the two chemically
1
different phosphorus atoms at δ 78.0 (dd, JP2Rh = 209.0 Hz,
1
2JP2P1 = 28.0 Hz, P2) ppm and δ 31.8 (dd, JP1Rh = 111.4 Hz,
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Figure 4. Molecular structure of the complex [6] in [6] 0.5C6H6 (50%
3
displacement ellipsoids, hydrogen atoms have been omitted with the
exception of the hydrogen atom bound to N2). Selected bond lengths
(Å) and angles (deg): RhꢀCl1 2.4286(12), RhꢀP1 2.1951(12),
RhꢀP2 2.3693(11), RhꢀC1 1.980(4), P1ꢀC10 1.878(5), N1ꢀC10
1.438(6), N1ꢀC1 1.364(5), N2ꢀC1 1.352(5); ClꢀRhꢀP1 166.84(4),
ClꢀRhꢀP2 89.10(4), ClꢀRhꢀC1 85.63(13), P1ꢀRhꢀP2 103.58(4),
P1ꢀRhꢀC1 81.63(13), P2ꢀRhꢀC1 174.63(13), N1ꢀC1ꢀN2
104.7(4).
Figure 3. 31P{1H} NMR spectrum of [8] in THF.
13C{1H} NMR spectrum features the resonance for the carbene
carbon atom at δ 194.0 (ddd, 1JCRh = 46.4 Hz, 2JCP2 = 111.2 Hz,
2JCP1 = 12.3 Hz) ppm with coupling to the rhodium atom and the
two phosphorus atoms. The 31P{1H} NMR spectrum (Figure 3)
features two resonances for the phosphorus atoms at δ 82.3
phosphorus atom P1, leading to a larger P1ꢀRhꢀCcarbene angle,
and the bulkiness of the tricylohexyl phosphine,23 contribute to
the formation of the trans-P,P complex [9] compared to the cis-P,
P complexes [6] and [8]. However, even with the P∧C chelate
ligand featuring a methylene spacer obtained from 3, formation
of trans-P,P complexes like [7] as a minor reaction was observed
in the presence of a bulky monodentate phosphine like PCy3.
The conclusions drawn from NMR spectroscopy were con-
firmed by X-ray molecular structure analyses. Complex [6]
(1JP2Rh = 196.0 Hz, 2JP2P1 = 30.0 Hz, P2) ppm and δ 33.7 (1JP1Rh
=
2
118.8 Hz, JP1P2 = 30.0 Hz, P1) ppm. The NMR data and,
in particular, the small 2JPP coupling constants for [6] and [8] are
quite similar, indicating the cis-P,P configuration for both com-
plexes. This geometrical arrangement was confirmed by X-ray
crystallography for [6] (vide infra).
crystallized from benzene as [6] 0.5C6H6. The structural analy-
3
Finally, we synthesized the rhodium(I) complex from the
ligand precursor 4 in the presence of tricyclohexyl phosphine.
In 4, the phosphine donor is linked by an ethylene spacer to the
benzimidazole ring nitrogen atom (Scheme 3). Upon coordina-
tion of the P∧C chelate ligand, a six-membered chelate ring
forms. It is expected that the P1ꢀRhꢀCcarbene angle in this six-
membered chelate ring is larger than the equivalent angle in
complexes [6]ꢀ[8], featuring five-membered chelate rings. This
angle expansion should have consequences for the complex
geometry and, in particular, for the position of the monodentate
phosphine. In fact, the reaction of [RhCl(COE)2]2 with 4 in the
presence of PCy3 leads exclusively to the formation of complex
[9] (70%) with a trans-P,P arrangement of the phosphine
ligands. The larger P1ꢀRhꢀCcarbene angle in [9] apparently
prevents the coordination of a second bulky phosphine ligand in
a cis position.
sis (Figure 4) confirmed the cis arrangement of the phosphine
donors. The RhꢀC1 distance (1.980(4) Å) is remarkably
short for the RhI complex with an NHC in a trans position to
a phosphine where normally RhꢀC separations of about 2.05 Å
are observed.22b This may be a consequence of the incorporation
of the NHC into a chelate ligand and the small hydrogen
substituent at atom N2. We take the short RhꢀCcarbene separa-
tion as another indication for the high stability of complexes of
type [6], which cannot undergo tautomerization easily. Contrary
to the free benzimidazole 4, the metric parameters within the
diamino heterocycle indicate the presence of a typical NHC with
equally long NꢀCcarbene bond distances and an NꢀCcarbeneꢀN
angle of 104.7(4)°.11 The RhꢀP distances are significantly
different with the RhꢀP1 separation (2.1951(12) Å, trans to
the σ,π-donor Cl) significantly shorter than the RhꢀP2 distance
(2.3693(11) Å, trans to the σ-donor/π-acceptor tricyclohexyl
phosphine). The square-planar coordination geometry around
the metal center is strongly distorted with P1ꢀRhꢀC1 and
ClꢀRuꢀC1 angles of only 81.63(13)° and 85.63(13)°, respec-
tively. The coordination geometry around the metal center is
clearly determined by the bulky phosphine donors.
The trans-P,P complex [9] was identified by 13C{1H} NMR
spectroscopy, showing the resonance for the carbene carbon
atom at δ 197.0 (1JCRh = 53.3 Hz, 2JCP = 15.1 Hz, 2JCP = 13.7 Hz.)
ppm. More information about the geometry of the complex can
be drawn from the 31P{1H} NMR spectrum. Here, two reso-
2
nances at δ 47.6 (1JP1Rh = 154.0 Hz, JP1P2 = 325.2 Hz, P1)
Complex [9] has been crystallized as [9] CH2Cl2 H2O
3
3
2
ppm and δ 27.4 (1JP2Rh = 149.0 Hz, JP2P1 = 325.2 Hz, P2)
from a dichloromethane solution. The X-ray diffraction analysis
(Figure 5) confirmed the trans arrangement of the phosphine
ligands in a slightly distorted square-planar rhodium(I) complex.
This arrangement only leads to a small change of the RhꢀCcarbene
separation (1.980(4) Å in [6], 1.922(3) Å in [9]). The same is
true for the RuꢀCl distances, which are identical within experi-
mental error for [6] and [9]. Because the phosphine ligands
are arranged in trans positions to each other in [9], the RhꢀP
separations are not as different as in [6] but approach almost
ppm have been observed. The very large 2JPP coupling constants
of 325.2 Hz clearly indicate the presence of the trans-P,P
complex,21 whereas the cis-P,P complexes [6] and [8] show
much smaller 2JPP coupling constants of about 30 Hz. The 2JPP
2
coupling constant in [9] is of similar magnitude as the JPP
coupling constant in [7] (2JP1P2 = 339 Hz), confirming the
assignment of [7] as a trans-P,P complex. Both, the length of
the spacer between the benzimidazole nitrogen atom and the
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Scheme 4. Oxidative Addition of 2-Chloro-N-methylbenzi-
midazoles to Pd0 and Pt0 (Top) and Proposed Mechanism for
the Redox Tautomerization of C2ꢀH Azoles (Bottom)
Figure 5. Molecular structure of the complex [9] in [9] CH2Cl H2O
3
3
(50% displacement ellipsoids, hydrogen atoms have been omitted with
the exception of the hydrogen atom bound to N2). Selected bond
lengths (Å) and angles (deg): RhꢀCl 2.4253(8), RhꢀP1 2.3402(9),
RhꢀP2 2.3219(9), RhꢀC1 1.922(3), P1ꢀC11 1.861(3), N1ꢀC10
1.458(4), N1ꢀC1 1.368(4), N2ꢀC1 1.379(4); ClꢀRhꢀP1 95.83(3),
ClꢀRh1ꢀP2 88.04(3), ClꢀRh1ꢀC1 171.34(9), P1ꢀRhꢀP2
165.53(3), P1ꢀRhꢀC1 86.56(9), P2ꢀRhꢀC1 91.69(9), N1ꢀC1ꢀN2
104.0(2).
equidistant values (RhꢀP1 2.3402(9) Å, RhꢀP2 2.3219(9) Å).
The angle P1ꢀRuꢀC1 within the chelate ring is significantly
expanded to 86.56(9)° in comparison to [6] (81.63(13)°).
Overall, the larger chelate ring formed by the P∧C chelate in
[9] and the trans arrangement of the phosphine donors lead to a
much less distorted square-planar coordination geometry in [9]
than was observed for [6]. The trans arrangement of the NHC
and the chloride is optimal for a catalytic hydrogenation reaction
with previous substrate recognition/orientation. The remaining
labile ligand (PCy3) for substrate coordination in a Wilkinson-
type hydrogenation is located in a cis position to the NH,NR-
NHC ligand containing the NH group for hydrogen bonding to
selected substrates.
The mechanism for the formation of complexes [6]ꢀ[9] has
not unambiguously been established. Related complexes bearing
bidentate imidazolin-2-ylidene/donor18,19 or benzimidazolin-2-
ylidene/donor16 ligands have been described as formed by
tautomerization of the azole to the NH,NR-substituted NHC,
which then coordinates to the metal center. This rather simple
type of reaction is not very likely to be operative. We have
recently shown that neutral 2-chloro-N-methylbenzimidazole,
similar to the well-known reaction of C2ꢀX substituted azolium
cations,24 can oxidatively add to Pd0 and Pt0, leading to the
intermediate G, which contains a strongly basic, partly anionic
nitrogen atom within the heterocycle (Scheme 4). Because of
this basic ring nitrogen atom, complex G subsequently either
dimerizes to the neutral dinuclear complexes H or reacts with a
proton source to give the cationic complexes I with a neutral NH,
NR-substituted NHC ligand.25 We propose that the ligands 3
and 4 initially also react with rhodium(I) under C2ꢀH oxidative
addition to give a five- or six-coordinated intermediate J. This
type of oxidative addition has been observed multiple times with
the C2ꢀH bond of azolium salts.24 In contrast to intermediate G,
the hydrido complex J is not stable but reacts under reductive
elimination of a proton, which subsequently protonates the basic
nitrogen atom of the heterocycle under transformation of the
anionic carbene ligand into a neutral NH,NR-substituted NHC
ligand. The related reductive elimination of a proton from NHC/
hydrido complexes is a well-documented reaction.26 We propose
to name the reaction of C2ꢀH-substituted azoles with transition
metals under formation of complexes with NH,NR-substituted
NHC ligands a redox tautomerization.
’ CONCLUSIONS
We have prepared the phosphine-functionalized benzimida-
zoles 3, 4, and 5. The precursors for bidentate NH,NR-NHC/
phosphine ligands 3 and 4 react with [RhCl(COE)2]2 under
formal tautomerization to give the phosphine-substituted NH,
NR-NHCs, which coordinate to RhI in a chelating fashion. The
resulting complexes are stable and cannot tautomerize under
formation of the N-coordinated azole complexes. The length of
the spacer between the benzimidazole nitrogen atom and the
attached phosphine phosphorus atom and the steric demand of
the additional monodentate phosphine determine the geometry
of the resulting rhodium(I) complexes. On the basis of published
data, it appears that the formal tautomerization observed during
complex formation with the ligand precursors 3 and 4 actually
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involves an oxidative addition of the C2ꢀH bond of the
benzimidazole moiety to RhI, followed by reductive elimination
of the metal bound hydrogen atom as a proton, which subse-
quently protonates the free nitrogen atom of the heterocycle.
This type of “redox tautomerization” may be operative for the
various formal tautomerization reactions described in the litera-
ture leading to complexes with NH,NR-NHCs. Further studies in
this laboratory are directed toward the elucidation of the mechanism
for the reaction of C2ꢀH-substituted benzimidazoles with transi-
tion metals and toward the use of complexes of type [9] in
catalytic hydrogenation reactions.
(0.61 mmol, 40%). 1H NMR (400 MHz, C6D6): δ 7.90 (s, 1H, H1), 7.16
(s, 1H, H4), 7.11 (s, 1H, H7), 3.84 (m, 2H, H10), 2.26 (s, 3H, H9), 2.20
(s, 3H, H8), 1.56 (m, 2H, H11), 0.93 (s, 9H, tBu-H), 0.90 (s, 9H, tBu-
H). 13C{1H} NMR (100 MHz, C6D6): δ 144.0 (s, C3), 142.4 (s, C1),
132.8 (s, C2), 131.4 (s, C6), 130.7 (s, C5), 121.7 (s, C4), 110.0 (s, C7),
45.8 (d, 2JCP = 39.5 Hz, C10), 31.2 (d, 1JCP = 21.7 Hz, C11), 29.5 (d, 2JCP
=
14.1 Hz, CꢀCH3), 22.8 (d, 1JCP = 25.1 Hz, CꢀCH3), 20.7 (s, C9), 20.3
(s, C8). 31P{1H} NMR (162 MHz, C6D6): δ 23.5. MS (EI, 50 eV)
m/z (%): 318 (100) [4]+. Anal. Calcd: C, 71.66; H, 9.81; N, 8.80.
Found: C, 71.54; H, 9.27; N, 9.59.
Compound 5. A sample of LiPCy2 was prepared by the addition
of nBuLi (0.77 mL of a 1.6 M solution, 1.23 mmol) to a solution of
dicyclohexyl phosphine (242 mg, 1.22 mmol) in THF (10 mL) at
ꢀ78 °C. This solution was added to a solution of 2-chloroethyl-5,6-
dimethyl benzimidazole 2 (220 mg, 1.05 mmol) in THF (8 mL), and the
reaction mixture was stirred at ambient temperature for 2 days. The
reaction mixture was then quenched with methanol (2 mL), and all
solvents were subsequently removed in vacuo. The residue was sus-
pended in water (20 mL), and compound 5 was extracted with benzene
(3 ꢁ 10 mL). The combined organic phases were dried over Na2SO4,
and the volume of the solution was reduced to 2 mL. Addition of pentane
(20 mL) led to precipitation of 5 as a colorless solid. Yield: 240 mg
(0.65 mmol, 62%). 1H NMR (400 MHz, C6D6): δ 7.92 (s, 1H, H4), 7.58
(s, 1H, H1), 7.10 (s, 1H, H7), 3.83 (d, 3JHP = 7.2 Hz, H10), 2.27 (s, 3H,
H9), 2.20 (s, 3H, H8), 1.56 (d, 2JHP = 8.6 Hz, 2H, H11), 1.69ꢀ1.04 (m,
22H, Cy-H). 13C{1H} NMR (100 MHz, C6D6): δ 144.3 (s, C3), 142.3
(s, C1), 132.9 (s, C2), 131.5 (s, C6), 130.6 (s, C5), 121.0 (s, C4), 110.0
(s, C7), 44.6 (d, 2JCP = 33.8 Hz, C10), 33.5 (d, 1JCP = 13.9 Hz, Cy-C),
30.5 (d, 2JCP = 15.0 Hz, Cy-C), 29.2 (d, 2JCP = 8.6 Hz, Cy-C), 27.5 (d,
3JCP = 11.3 Hz, Cy-C), 27.4 (d, 3JCP = 7.0 Hz, Cy-C), 26.4 (s, Cy-C), 22.9
(d, 1JCP = 21.6 Hz, C11), 20.7 (s, C9), 20.3 (s, C8). 31P{1H} NMR (162
MHz, C6D6): δ ꢀ8.9. MS (EI, 50 eV) m/z (%): 370 (100) [5]+. Anal.
Calcd: C, 74.56; H, 9.52; N, 7.56. Found: C, 73.41; H, 9.42; N, 7.42.
Complex [6]. A thick-walled screw-capped vial was charged with
compound 3 (25.0 mg, 0.07 mmol), tricyclohexyl phosphine (20.0 mg,
0.07 mmol), [RhCl(COE)2]2 (25 mg, 0.035 mmol), and THF (8 mL).
The reaction mixture was stirred at ambient temperature for 10 min,
followed by heating to 90 °C for 6 h. A brown solution was obtained,
which was cooled to ambient temperature, and the solvent was removed
under vacuo to give a brown solid. The solid was washed with pentane
(3 ꢁ 10 mL) and then dried in vacuo to give a mixture of complexes [6]
(77%) and [7] (23%) as a brownish solid. Yield: 30 mg (0.04 mmol, 57%
of the complex mixture). Both complexes in the mixture have been
identified by 31P{1H} NMR spectroscopy. Pure [6] was obtained by
recrystallization from benzene, whereas [7] could not be isolated in
analytically pure form due to its low concentration in the mixture and
’ EXPERIMENTAL SECTION
General Comments. All syntheses were carried out under an
argon atmosphere using conventional Schlenk techniques. Solvents
were dried and degassed by standard methods prior to use. 1-Chloro-
methyl-5,6-dimethylbenzimidazolium hydrochloride,16 1 HCl; 2-chloro-
3
27
ethyl-5,6-dimethylbenzimidazole,16 2; and [RhCl(coe)2]2 have been
prepared by published methods. NMR spectra were recorded with a
Bruker Avance I 400 NMR spectrometer. MALDI, EI, and ESI-HRMS
mass spectra were obtained with Bruker Reflex IV, Finnigan MAT 95,
and Bruker Daltronics MicroTof spectrometers, respectively. For NMR
signal assignments and numbering, see Scheme 1.
Compound 3. A sample of 1-chloromethyl-5,6-dimethylbenzimi-
dazolium hydrochloride, 1 HCl (400 mg, 1.73 mmol), in THF (10 mL)
3
cooled to 0 °C was transformed into the free amine by addition of
194 mg (1.73 mmol) of KO-tBu. The resulting solution was stirred at
0 °C for 30 min. Removal of the solvent gave a white precipitate. To this
precipitate was added at 0 °C a solution of LiPCy2 obtained by treatment
of PHCy2 (343 mg, 1.73 mmol) with 1.1 mL of 1.6 M nBuLi
(1.76 mmol) in THF (10 mL) at ꢀ78 °C. The reaction mixture was
stirred at ambient temperature for 12 h and was then quenched by
addition of 2.0 mL of methanol while vigorously stirring for another
10 min. All solvents were then removed in vacuo. The solid residue was
extracted with ethyl acetate (20 mL), and the extract was washed with
water. Drying of the organic phase over Na2SO4 and removal of the
solvent under reduced pressure gave compound 3 as a highly hygro-
1
scopic white solid. Yield: 230 mg (0.65 mmol, 38%). H NMR (400
MHz, THF-d8): δ 7.90 (s, 1H, H1), 7.37 (s, 1H, H7), 7.28 (s, 1H, H4),
4.35 (d, 2JHP = 3.2 Hz, 2H, H10), 2.35 (s, 3H, H9), 2.31 (s, 3H, H8), 1.74
(m, 11H, Cy-H), 1.26 (m, 11H, Cy-H). 13C{1H} NMR (100 MHz,
THF-d8): δ 144.3 (s, C3), 143.1 (d, 3JCP = 7.0 Hz, C1), 134.2 (s, C2),
131.4 (s, C6), 130.4 (s, C5), 121.1 (s, C4), 111.3 (s, C7), 40.3 (d, 1JCP
=
21.7 Hz, C10), 33.9 (d, 1JCP = 15.0 Hz, Cy-C), 30.6 (d, 2JCP = 13.5 Hz,
Cy-C), 30.0 (d, 2JCP = 15.0 Hz, Cy-C), 28.6 (d, 3JCP = 8.3 Hz, Cy-C),
28.1 (d, 3JCP = 10.8 Hz, Cy-C), 27.3 (s, Cy-C), 20.7 (s, C9), 20.3 (s, C8).
31P{1H} NMR (162 MHz, THF-d8): δ ꢀ9.7 (s). MS (EI, 50 eV) m/z
(%): 356 (100) [3]+. Anal. Calcd: C, 74.11; H, 9.34; N, 7.86. Found: C,
73.23; H, 9.30; N, 7.78.
1
contamination with [6]. Analytical data for complex [6]: H NMR
(400 MHz, THF-d8): δ 11.40 (s, 1H, NH), 7.18 (s, 1H, H4), 7.05 (s, 1H,
H7), 3.74 (d, 2JHP = 4.8 Hz, 2H, H10), 2.29 (s, 3H, H9), 2.28 (s, 3H,
H8), 1.81ꢀ1.69 (m, 33H, PCy3-H), 1.34ꢀ1.23 (m, 22H, PCy2-H).
1
13C{1H} NMR (100 MHz, THF-d8): δ 193.0 (ddd, JCRh = 46.5 Hz,
Compound 4. Compound 4 was obtained by addition of 235 mg
2JCP2 = 103.4 Hz, 2JCP1 = 14.0 Hz, C1), 132.4 (d, 3JCRh = 2.3 Hz, C3),
131.7 (d, 3JCRh = 9.8 Hz, C2), 130.3 (s, C5), 129.6 (s, C6), 111.3 (s, C7),
110.5 (s, C4), 42.6 (dd, 1JCP1 = 28.6 Hz, 2JCRh = 4.5 Hz, C10), 36.7 (d,
1JCP = 20.9 Hz, Cy-C), 33.3 (d, 1JCP = 28.4 Hz, Cy-C), 28.2 (d, 1JCP = 9.6
Hz, Cy-C), 27.6 (d, 1JCP = 9.1 Hz, Cy-C), 27.0 (d, 1JCP = 9.3 Hz, Cy-C),
24.8ꢀ24.0 (m, Cy-C), 19.2 (s, C9), 19.3 (s, C8). 31P{1H} NMR (162
MHz, THF-d8): δ 78.0 (dd, 1JP2Rh = 209.0 Hz, 2JP2P1 = 28.0 Hz, P2),
31.8 (dd, 1JP1Rh = 111.4 Hz, 2JP1P2 = 28.0 Hz, P1). MS (ESI HRMS) m/
z: 739.3753 (calcd for [6 ꢀ Cl]+ 739.3756). Anal. Calcd (for [6]/[7]):
C, 61.97; H, 8.58; N, 3.61. Found: C, 61.39; H, 8.72; N, 2.96. NMR data
for [7] in the complex mixture [6]/[7]: 31P{1H} NMR (162 MHz,
THF-d8): δ 68.3 (dd, 1JP2Rh = 156.0 Hz, 2JP2P1 = 339 Hz, P2), 31.7 (dd,
1JP1Rh = 137.0 Hz, 2JP1P2 = 339.0 Hz, P1).
28
(1.56 mmol) of solid LiPtBu2 to a THF (10 mL) solution of
2-chloroethyl-5,6-dimethyl benzimidazole 2 (322 mg, 1.54 mmol) at
ꢀ60 °C. The reaction mixture was stirred at ꢀ60 °C for 1 h and was then
slowly warmed up to ambient temperature. During this warm up, the
color of the initially green solution changed to pale orange. The reaction
mixture was stirred at ambient temperature for 1 h and then heated
under reflux for another 3 h. After cooling to ambient temperature, the
reaction mixture was quenched with methanol (3 mL) and all solvents
were removed in vacuo. The solid obtained was dissolved in dichloro-
methane (20 mL), and the solution was washed with water (20 mL).
The organic phase was dried over MgSO4, and the volume of the
solution was reduced to 2 mL. Addition of pentane (20 mL) led to
precipitation of 4 as a highly hygroscopic off-white solid. Yield: 194 mg
5864
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Organometallics
ARTICLE
Complex [8]. A thick-walled screw-capped vial was charged with
compound 3 (21.0 mg, 0.06 mmol), triphenyl phosphine (16.0 mg,
0.06 mmol), [Rh(COE)2Cl]2 (22.0 mg, 0.03 mmol), and THF (6 mL).
The mixture was stirred at ambient temperature for 10 min, followed by
heating to 90 °C for 6 h. The solvent was then removed in vacuo, and the
solid brown residue was washed with pentane (4 mL). Drying of the
resulting solid in vacuo gave [8] as a dark orange solid. Yield: 42 mg
(0.055 mmol, 92%). 1H NMR (THF, 400 MHz): δ 11.40 (s, 1H, NH),
7.88ꢀ7.84 (m, 6H, PPh3-H), 7.28 (s, 9H, PPh3-H), 7.26 (s, 2H, H4 and
H7), 3.76 (d, 2JHP = 4.9 Hz, 2H, H10), 2.32 (s, 3H, H8), 2.30 (s, 3H,
H9), 1.72ꢀ1.31 (m, 11H, Cy-H), 1.06ꢀ0.88 (m, 11H, Cy-H). 13C{1H}
NMR (100 MHz, THF-d8): δ 194.0 (ddd, 1JCRh = 46.4 Hz, 2JCP2 = 111.2
Hz, 2JCP1 = 12.3 Hz, C1), 139.3 (d, 1JCP = 31.8 Hz, PPh3-C), 136.7 (d,
γ = 90.239(3)°, V = 1893.03(15) Å3, Fcalc = 1.117 g cmꢀ3, MoꢀKα
3
radiation (λ = 0.71073 Å), μ = 1.258 mmꢀ1, ω- and j-scans, 11 114
measured intensities (6.5° e 2θ e 143.0°), semiempirical absorption
correction (0.674 e T e 0.940), 6406 independent (Rint = 0.0534) and
4974 observed intensities (I g 2σ(I)), refinement of 413 parameters
against |F2| of all measured intensities with hydrogen atoms on cal-
culated positions. R = 0.0528, wR = 0.1439, Rall = 0.0659, wRall = 0.1514.
The asymmetric unit contains two formula units featuring identical
metric parameters within experimental error limits.
[6] 0.5C6H6. Crystals suitable for an X-ray diffraction study were
3
obtained by recrystallization from benzene. C43H69N2ClP2Rh: M =
814.30 g molꢀ1, pale yellow crystal, 0.06 ꢁ 0.04 ꢁ 0.02 mm3, triclinic,
3
space group P1, Z = 2, a = 9.5232(5) Å, b = 10.4892(6) Å, c =
2JCP = 12.3 Hz, PPh3-C), 133.4 (d, 3JCRh = 3.0 Hz, C3), 132.1 (d, 3JCRh
3.0 Hz, C2), 131.8 (s, C5), 131.3 (s, C6), 129.4 (s, PPh3-C), 128.0 (d,
3JCP = 8.9 Hz, PPh3-C), 112.7 (s, C7), 111.6 (s, C4), 42.8 (dd, 1JCP1
=
21.0399(12) Å, α = 86.6200(10)°, β = 86.9030(10)°, γ =
79.1000(10)°, V = 2058(2) Å3, Fcalc = 1.314 g cmꢀ3, MoꢀKα radiation
3
(λ = 0.71073 Å), μ = 0.589 mmꢀ1, ω- and j-scans, 20 701 measured
intensities (1.9° e 2θ e 45.0°), semiempirical absorption correction
(0.966 e T e 0.988), 9453 independent (Rint = 0.0568) and 6560
observed intensities (I g 2σ(I)), refinement of 433 parameters against
|F2| of all measured intensities with hydrogen atoms on calculated
positions. R = 0.0545, wR = 0.1247, Rall = 0.0915, wRall = 0.1419. The
=
26.6 Hz, 2JCRh = 4.5 Hz, C10), 35.3 (d, 1JCP = 23.2 Hz, Cy-C), 30.1 (d,
3JCP = 5.18 Hz, Cy-C), 27.9 (d, 2JCP = 9.1 Hz, Cy-C), 27.3 (s, Cy-C),
20.3 (s, C9), 20.2 (s, C8). 31P{1H} NMR (162 MHz, THF-d8): δ 82.3
(dd, 1JP2Rh = 196.0 Hz, 2JP2P1 = 30.0 Hz, P2), 33.7 (dd, 1JP1Rh = 118.8
Hz, 2JP1P2 = 30.0 Hz, P1). MS (ESI HRMS) m/z: 721.2346 (calcd for
[8 ꢀ Cl]+ 721.2348). Anal. Calcd (for [8]): C, 63.45; H, 6.39; N, 3.70.
Found: C, 63.60; H, 6.65; N, 3.21.
asymmetric unit contains one formula unit [6] 0.5C6H6. The benzene
3
molecule resides on an inversion center between two asymmetric units;
no hydrogen positions for the benzene molecule were calculated.
[9] CH2Cl2 H2O. Crystals suitable for an X-ray diffraction study
Complex [9]. A thick screw-capped vial was charged with com-
pound 4 (23 mg, 0.072 mmol), [Rh(COE)2Cl]2 (25 mg, 0.035 mmol),
tricyclohexyl phosphine (21 mg, 0.075 mmol), and THF (5 mL). The
reaction mixture was stirred at ambient temperature for 10 min, followed
by heating to 90 °C for 6 h. During this period, the color of the mixture
changed from dark red to light orange. The reaction mixture was cooled
to ambient temperature, and the solvent was removed in vacuo, giving a
yellow solid. This solid was washed with pentane and dried in vacuo to
give [9] as a yellow solid. Yield: 36 mg (0.049 mmol, 70%). 1H NMR
(400 MHz, THF-d8): δ 9.98 (s, 1H, NH), 7.00 (s, 1H, H4), 6.94 (s, 1H,
H7), 5.61 (m, 2H, H10), 4.42 (d, 2JHP = 17.2 Hz, 2H, H11), 2.28 (s, 3H,
H8), 2.25 (s, 3H, H9), 1.77ꢀ1.58 (m, 30H, Cy-H), 1.29 (s, 9H, tBu-H),
1.26 (s, 9H, tBu-H), 1.11ꢀ1.04 (m, 3H, Cy-H). 13C{1H} NMR (100
3
3
were obtained by recrystallization from dichloromethane.
C38H68N2Cl3OP2Rh: M = 840.14 g molꢀ1, orange crystal, 0.21 ꢁ
3
0.16 ꢁ 0.07 mm3, monoclinic, space group C2/c, Z = 8, a = 30.542(5) Å,
b = 14.614(2) Å, c = 23.612(6) Å, β = 122.925°, V = 8846(3) Å3, Fcalc
=
1.262 g cmꢀ3, MoꢀKα radiation (λ = 0.71073 Å), μ = 669 mmꢀ1
,
3
ω- and j-scans, 50 917 measured intensities (3.2° e 2θ e 60.0°),
semiempirical absorption correction (0.872 e T e 0.955), 12 898
independent (Rint = 0.0236) and 10 631 observed intensities (I g
2σ(I)), refinement of 450 parameters against |F2| of all measured
intensities with hydrogen atoms on calculated positions. R = 0.0513,
wR = 0.1642, Rall = 0.0630, wRall = 0.1785. The asymmetric unit contains
one molecule of [9] and one water molecule in addition to 1/2 CH2Cl2
with SOF = 0.5. A second CH2Cl2 molecule (SOF = 1.0) resides on a
2-fold axis between two asymmetric units. No hydrogen positions have
been calculated for the CH2Cl2 molecules and the water molecule.
MHz, THF-d8): δ 197.0 (ddd, 1JCRh = 53.3 Hz, 2JCP = 15.1 Hz, 2JCP
=
13.6 Hz, C1), 134.7 (d, 4JCP = 1.9 Hz, C3), 133.1 (s, C2), 130.7 (s, C6),
2
130.1 (s, C5), 129.7 (s, C4), 109.2 (s, C7), 45.4 (dd, JCP1 = 7.9 Hz,
3JCRh = 4.2 Hz, C10), 36.9 (dd, 1JCP = 7.5 Hz, 2JCRh = 3.3 Hz, C11), 35.7
1
1
(d, JCP2 = 14.2 Hz, Cy-C), 32.8 (d, JCP2 = 18.0 Hz, Cy-C), 32.1 (d,
1JCP2 = 12.8 Hz, Cy-C), 31.5 (s, Cy-C), 30.9 (d, 2JCP2 = 5.9 Hz, Cy-C),
30.1 (s, CꢀCH3), 29.0 (d, 2JCP2 = 10.2 Hz, Cy-C), 28.5 (d, 2JCP2 = 9.0
Hz, Cy-C), 27.8 (s, Cy-C), 27.5 (s, Cy-C), 27.0 (s, Cy-C), 26.2 (s, Cy-
C), 20.2 (s, C8), 20.1 (s, C9), 19.8 (m, CꢀCH3). 31P{1H} NMR (162
MHz, THF-d8): δ 47.6 (dd, 1JP1Rh = 154.0 Hz, 2JP1P2 = 325.2 Hz, P1),
27.4 (dd, 1JP2Rh = 149.0 Hz, 2JP2P1 = 325.2 Hz, P2). MS (ESI HRMS)
m/z: 717.3524 [9 ꢀ Cl + O]+ (calcd for [9 ꢀ Cl + O]+ 717.3549).
X-ray Diffraction Studies. X-ray diffraction data were collected at
T = 153(2) K with a Bruker AXS APEX CCD diffractometer equipped
with a rotation anode using monochromated CuꢀKα radiation (λ =
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic files for
b
complexes 4, [6] 0.5C6H6, and [9] CH2Cl2 H2O in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
3
3
3
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: fehahn@uni-muenster.de.
1.54178 Å) for 4 or MoꢀKα radiation (λ = 0.71073 Å) for [6] 0.5C6H6
3
and [9] CH2Cl2 H2O. Diffraction data were collected over the full
3
3
sphere and were corrected for absorption. Structure solutions were
found with the SHELXS-9729 package using direct methods and were
refined with SHELXL-9729 against |F2| using, first, isotropic and, later,
anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen
atoms were added to the structure models on calculated positions
(for exceptions, see description of the individual molecular structures).
4. Crystals suitable for an X-ray diffraction study were obtained by
diffusion of pentane into a dichloromethane solution of the compound.
’ ACKNOWLEDGMENT
The authors thank the Deutsche Forschungsgemeinschaft
(IRTG 1444, SFB 858). A.R.N. thanks GSC-MS for a predoctoral
grant.
’ REFERENCES
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