Heterobimetallic Carbene Complexes Bearing Cyclometalated IrIII/RhIII and Mixed NHC∧ Py/PPh3 Coordinated PdII Centers: Structures and Tandem Catalysis

Article abstract of DOI:10.1002/ejic.202001080

Heterobimetallic complexes bearing NHC donor ligands are gaining immense popularity in organometallic chemistry and tandem catalysis. It is known that the NHC reacts with Pd^II in the presence of pyridine to yield PEPPSI type complexes and the NHC ligands having ortho-C?H proton easily orthometalate to Ir^III or Rh^III centers. Combining these two methodologies in a stepwise fashion, we present here a series of heterobimetallic Ir^III?Pd^II and Rh^III?Pd^II complexes from a dicarbene donor ligand featuring cyclometalated Ir^III or Rh^III and mixed (Formula presented.) /PPh₃ coordinated Pd^II centers. All the heterobimetallic complexes have been structurally characterized by X-ray crystallographic analysis. The heterobimetallic complexes featuring mixed (Formula presented.) coordinated Pd^II centers show better activity in tandem Suzuki-Miyaura/transfer hydrogenation reactions compared to both, the heterobimetallic complexes possessing PEPPSI type Pd^IIcenters, and the equimolar mixture of their mononuclear Pd^II and Rh^III or Ir^III counterparts. The heterobimetallic complex featuring cyclometalated Ir^III and mixed (Formula presented.) coordinated Pd^II center shows excellent selectivity for 4-biphenylmethanol (isolated yield: 92 %) in tandem catalysis.

Full text of DOI:10.1002/ejic.202001080

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Very Important Paper
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Heterobimetallic Carbene Complexes Bearing
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Cyclometalated IrIII/RhIII and Mixed NHC^Py/PPh3
Coordinated PdII Centers: Structures and Tandem Catalysis
Adhir Majumder,[a] Tarak Nath Saha,[a] Niladri Majumder,[a] Rajat Naskar,[a] Kuntal Pal,[a] and
Ramananda Maity*[a]
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Dedicated to Professor F. Ekkehardt Hahn on the occasion of his 65th birthday
Heterobimetallic complexes bearing NHC donor ligands are
gaining immense popularity in organometallic chemistry and
tandem catalysis. It is known that the NHC reacts with PdII in
the presence of pyridine to yield PEPPSI type complexes and
the NHC ligands having ortho-CÀ H proton easily orthometalate
to IrIII or RhIII centers. Combining these two methodologies in a
stepwise fashion, we present here a series of heterobimetallic
IrIIIÀ PdII and RhIIIÀ PdII complexes from a dicarbene donor ligand
featuring cyclometalated IrIII or RhIII and mixed NHC^Py/PPh3
coordinated PdII centers. All the heterobimetallic complexes
have been structurally characterized by X-ray crystallographic
analysis. The heterobimetallic complexes featuring mixed
NHC^PPh3 coordinated PdII centers show better activity in
tandem Suzuki-Miyaura/transfer hydrogenation reactions com-
pared to both, the heterobimetallic complexes possessing
PEPPSI type PdIIcenters, and the equimolar mixture of their
mononuclear PdII and RhIII or IrIII counterparts. The heterobime-
tallic complex featuring cyclometalated IrIII and mixed
NHC^PPh3 coordinated PdII center shows excellent selectivity
for 4-biphenylmethanol (isolated yield: 92%) in tandem catal-
ysis.
Introduction
ligands.[10] Heterobimetallic complexes bearing mesoionic car-
bene (MIC) donor ligands have also been synthesized in a
stepwise fashion by following the controlled deprotonation
technique.[11] An unique approach for the construction of
heterobimetallic PdIIÀ MIII (M=Rh, Ir) complexes has been
described by Hahn and co-workers by single-step site-selective
metalation of a tris-carbene donor ligand having 2,3,4-substitu-
tion pattern of the central phenyl ring.[12] The PdII center in the
aforementioned complexes binds two NHC units in a chelating
fashion and the MIII (M=Rh, Ir) binds the monodentate NHC site
followed by additional orthometalation to the central phenyl
ring (Figure 1B). The same working group has also reported the
heterobimetallic and heterotrimetallic complexes bearing
phenylene-bridged dicarbene and tricarbene donor ligands,
respectively (Figure 1C).[13] However, such complexes have not
been tested as catalysts for tandem reactions.
The cyclometalation reactions in these complexes bearing
poly-NHC ligands featuring symmetric substitution pattern of
the central phenyl ring are usually facile and ended up with
one unreacted imidazolium unit.[13] The unreacted imidazolium
unit ultimately allows the synthesis of heterobimetallic com-
plexes. We have recently demonstrated the synthesis and
catalytic tandem reactions of heterobimetallic complex bearing
cyclometalated IrIII and PEPPSI type PdII centers, using the
aforementioned synthetic strategy.[14]
N-heterocyclic carbenes (NHCs) have received a profound
impact in a wide varieties of research fields such as fascinating
ligands in organometallic chemistry,[1] biologically active metal
complexes,[2] catalysis[3] and material science.[4] The application
of NHC complexes could be further augmented by the
incorporation of more NHC donor units to the central backbone
leading to poly-NHC complexes.[5] These poly-NHCs have
recently been used as spacer units in metallosupramolecular
chemistry[6] and also as ligand platform for the synthesis of
heterobimetallic NHC complexes.
The design and synthetic approaches to heterobimetallic
NHC complexes on poly-NHC ligand platform have received
immense popularity owing to their applications as catalysts in
cooperative tandem catalysis.[7] Heterobimetallic complex fea-
turing cyclopentadienyl-annulated imidazol-2-ylidene ligand
has been demonstrated by an oxidative addition reaction of Pd0
to the C2À Cl bond of the ruthenocene derived imidazolium
salt.[8] Promising approaches to access the heterobimetallic
architectures have been made by Peris and co-workers,
describing the stepwise synthesis of heterobimetallic complexes
bearing 1,2,4-triazolyl-diylidene[9] and imidazolin-2-ylidene
[a] A. Majumder, T. Nath Saha, N. Majumder, R. Naskar, K. Pal, R. Maity
Department of Chemistry
Here, we present a series of heterobimetallic MIIIÀ PdII (M=Ir,
Rh) complexes from a dicarbene donor ligand featuring cyclo-
metalated MIII (M=Ir, Rh) and mixed NHC^Py/PPh3 coordinated
PdII centers along with their tandem catalytic applications and
structural diversities. The design of the heterobimetallic com-
plexes was based on the observations that the cyclometalated
University of Calcutta
92-APC Road, Kolkata 700009, India
E-mail: rmchem@caluniv.ac.in
http://www.caluniv.ac.in/academic/department/Chemistry.html
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202001080
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Figure 1. Reported dinuclear and trinuclear heterobimetallic complexes.
Scheme 1. Synthesis of heterobimetallic complexes [3]–[5].
complexes bearing MIII (M=Rh, Ir) are potent catalysts for the
transfer hydrogenation reaction for a wide range of organic
substrates[15] and PdII complexes featuring mixed NHC^Py/PPh3
donor ligands have received wide applicability as catalysts for a
wide range of organic transformations.[16] Therefore, we envi-
sioned that the heterobimetallic complexes could show a
combine action of both PdII and IrIII centers to a single substrate
leading to the product formation. All the heterobimetallic
complexes appeared as potent pre-catalysts for tandem Suzuki-
Miyaura coupling/transfer hydrogenation reaction.
atom (δ=157.3 ppm, 1JRhÀ C =34.5 Hz). The resonance for the
imidazolium C2 carbon atom of the unreacted imidazolium unit
appeared at δ=136.9 ppm. The formation of the complex [2]
was further confirmed by the ESI-HRMS (positive ions) spec-
trometry.
The free imidazolium unit in complex [2] can also be
metalated under suitable reaction conditions. Therefore, the
°
treatment of [2] with PdCl2 and K2CO3 at 84 C for 24 h in
pyridine or with the corresponding substituted pyridines,
resulted in the formation of PEPPSI-type palladium(II) com-
plexes [2]–[5] (Scheme 1). All the complexes were obtained in
reasonable yields (50–57%) and they were sparingly soluble in
solvents such as acetonitrile, dichloromethane and chloroform.
All the complexes are stable towards both air and moisture. The
formation of the heterobimetallic complexes was easily moni-
Results and Discussion
The reaction of the bis-imidazolium salt 1[14] with one or two
equiv. of [Rh(Cl)2(Cp*)]2 in the presence of Cs2CO3 and NaOAc in
°
acetonitrile at 60 C for 20 h yielded the mononuclear cyclo-
metalated RhIII complex [2]. The complex [2] is also featuring
one unreacted imidazolium unit (Scheme 1). The complex [2] is
well soluble in chlorinated solvents (dichloromethane and
chloroform) and is stable towards air and moisture for long
time (months) both in the solid state and in solution.
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1
tored by H NMR spectroscopy. The H NMR spectrum of each
complex features the disappearance of the imidazolium C2À H
proton resonance, which was observed at δ=9.86 ppm for the
parent complex [2]. The resonances of the pyridine protons
The formation of the cyclometalated RhIII complex [2] was
appeared at δ=9.02–8.97, 7.54, and 7.20–7.19 ppm in the H
1
1
easily monitored by H NMR spectroscopy. The resonance for
NMR spectrum of heterobimetallic complex [3]. However, two
signals attributed to the pyridine protons were observed for
each of the heterobimetallic complexes [4] and [5] ([4]: δ=
8.79–8.76 and 7.04–7.01 ppm; [5]: δ=8.95–8.93 and 7.18–
7.16 ppm). The resonance for the pyridine α-protons in all the
heterobimetallic complexes appeared downfield shifted (δ=
9.02–8.97 ppm) upon coordination with PdII (free Py: δ=
8.62 ppm).[17] The resonance for the aryl CÀ H proton in each
heterobimetallic complex ([3]: δ=8.42 ppm; [4]: δ=8.40 ppm;
[5]: δ=8.38 ppm) appeared downfield shifted compared to the
equivalent resonance in mononuclear cyclometalated complex
[2] ([2]: δ=7.41 ppm). Formation of the heterobimetallic
complexes [3]-[5] was also confirmed by ESI mass spectrometry.
the unreacted imidazolium C2À H proton appears at δ=
1
1
9.86 ppm in the H NMR spectrum of the complex. The H NMR
spectrum also depicts a broad signal at δ=7.41 ppm attributes
to the central aryl CÀ H proton. Two separate signals were also
observed for two chemically non-equivalent N-methyl protons
1
at δ=4.30 and 3.92 ppm in the H NMR spectrum of complex
[2]. The resonances for chemically non-equivalent methyl
protons connected to the central phenyl ring appeared at δ=
2.55 and 2.15 ppm. The resonance for the characteristic carbene
carbon atom appeared at δ=185.0 ppm (1JRhÀ C =53.6 Hz) as
doublet, which was more downfield shifted compared to the
doublet resonance observed for the metalated aryl carbon
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The reaction of PEPPSI type complexes [3] or [6][14] with 1.1
The corresponding mononuclear cyclometalated RhIII com-
plex [10] and the mononuclear palladium(II) complexes cis-/
trans-[12] have also been synthesized using the similar
procedure described in the stepwise synthesis of heterobimetal-
lic complex [7], starting from imidazolium salt 9[19] (Scheme 3)
and the earlier reported mononuclear PEPPSI type complex
[11],[14] respectively.
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equiv. of PPh3 in acetonitrile resulted in the formation of
heterobimetallic complexes [7] or [8] (Scheme 2). Both com-
plexes were obtained in good yields ([7]: 61% and [8]: 50%).
The solubility of the complexes [7] or [8] are similar to the
above mentioned heterobimetallic complexes ([3]–[5]). The
1
formation of complexes [7] and [8] was easily monitored by H
1
NMR spectroscopy. The H NMR spectra of both the complexes
The formation of the mononuclear cyclometalated complex
[10] was easily monitored by 1H NMR spectroscopy, which
shows the disappearance of the parent imidazolium CÀ H proton
resonance (δ=9.91 ppm)[19] upon complex formation. The 1H
NMR spectrum of complex [10] also revealed three distinct
resonances for the aryl CÀ H protons (δ=7.60, 6.88, and
6.73 ppm), which appeared as two multiplets (δ=7.48 and
7.36 ppm)[19] in the parent imidazolium salt 9. The resonance for
the characteristic carbene carbon atom was observed at δ=
no longer feature the resonances for the pyridine protons. Two
1
sets of signals were detected in the H NMR spectrum of each
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complex [7] and [8], which might be due to the restricted
rotations of the PdII coordinated NHC unit around the
NimidazoleÀ CAr bond in solution. The 1H NMR spectrum of complex
[7] in deuterated dmso shows several sets of signals at ambient
°
temperature. Upon heating the sample to 90 C, the signals of
the rotamers are merging, thereby leading to a comparatively
simpler spectrum (Figure S27, ESI). Similar observation has also
been demonstrated in literature with cyclometalated iridium(III)
1
184.0 (d, JRhÀ C =54.75 Hz) ppm, and this signal is significantly
downfield shifted compared to the corresponding resonance
1
1
complex possessing an unreacted imidazolium unit.[13b] The H
for the metalated aryl carbon atom (δ=157.6, d, JRhÀ C =33 Hz
NMR spectrum of each complex revealed multiplets for the
phosphine protons. Four singlets ([7]: δ=4.0, 3.94, 3.93 and
3.89 ppm; [8]: δ=3.93, 3.86, 3.84 and 3.83 ppm) were detected
in the 1H NMR spectrum of each complex for the N-CH3 protons.
The resonances for Cp* protons appeared at δ=1.84 and
1.83 ppm for complex [7] and at δ=1.83 and 1.81 ppm for
complex [8]. The 31P{1H} NMR spectrum of each complex [7] and
[8] also shows two sets of signals ([7]: δ=15.22 and 14.88 ppm;
[8]: δ=15.28 and 14.85 ppm). These resonances also fall in the
range reported for the mononuclear trans-palladium(II) com-
plexes bearing mixed NHC^PPh3 donor ligands, andtherefore,
indicate the exclusive formation of heterobimetallic trans-
complexes under the mentioned reaction conditions.[18] The two
sets of signals for each complex is most likely due to the
restricted rotations around NimidazoleÀ CAr bonds as mentioned
earlier. Although, literature reports demonstrated the trans-
formation of the trans- to cis-isomer for the mononuclear
palladium(II) complexes bearing mixed donor NHC^PPh3 li-
gands, however, the isolated trans-complexes [7] and [8] are
stable towards air and moisture, and no transformation to the
cis-product was detected in solution, which most likely due to
the steric factor associated with the Cp* rings in these
complexes.[18] The formation of both complexes was supported
by the ESI mass spectrometric analysis (positive ions).
ppm). The formation of the cyclometalated complex [10] was
further supported by ESI mass spectrometry (positive ions).
The cis-and trans-[12] complexes were separated by column
chromatography and both complexes were characterized by 1H,
13C{1H}, 31P{1H}, 2D correlation NMR spectroscopy and mass
1
spectrometry. The H NMR spectrum of each complex shows
the missing of resonances for pyridine protons. The resonances
1
for the coordinated PPh3 protons appear as multiplets in the H
NMR spectra of both cis-and trans-[12]. The resonances for the
characteristic carbene carbon atoms were observed at δ=159.8
and 165.5 ppm (2D NMR) in the 13C{1H} NMR spectra of trans-
[12] and cis-[12], respectively. A singlet was observed at δ=
15.80 ppm in the 31P{1H} NMR spectrum of trans-[12], which was
upfield shifted compared to the equivalent resonance (δ=
21.82 ppm) observed for cis-[12]. This is in agreement with the
resonances reported for the PdII complexes featuring mixed
NHC^PPh3 donor ligands.[18] Both cis-and trans-[12] complexes
show isomerization reaction in CDCl3.
Single crystals suitable for X-ray diffraction studies were
obtained for complexes [3], [4], [5], [7] and [8] by slow
evaporation of the solvents from a saturated dichloromethane:
Scheme 2. Synthesis of heterobimetallic complexes [7] and [8].
Scheme 3. Synthesis of mononuclear complexes.
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°
°
°
°
hexane solution of the corresponding complexes at ambient
temperature. The molecular structure analysis with these
crystals confirmed the formation of heterobimetallic complexes
featuring rhodacycles ([3], [4], [5] and [7]) and iridacycle ([8])
with the expected bonding connectivity (Figure 2 and Figure 3).
In each heterobimetallic complex the rhodium or iridium
center has a three legged piano-stool type of coordination
geometry,[14] where the three legs are defined as one NHC
donor, one iodide ion and an anionic aryl carbon donor from
the central phenyl ring. In all the rhodium(III) complexes MÀ CNHC
bond distances ([3]: 1.998(3), [4]: 2.003(14), [5]: 1.991(14) and
[7]: 1.956(18) Å) are shorter compare to the MÀ Caryl bond
distances ([3]: 2.022(3), [4]: 2.047(13), [5]: 2.014(14) and [7]:
2.04(2) Å). These observations are in consistence with the
equivalent distances observed for similar cyclometalated com-
plexes bearing imidazolylidene and triazolylidene ligands.[12,13]
The distance between the metal center (MIII) and the center of
Cp* ring in all the complexes are in the range of 1.865À 1.877 Å.
The bite angles CNHCÀ MÀ Caryl (M=Rh, Ir) observed for the 5-
membered chelate rings fall in the expected range ([3]:
77.23(14) , [4]: 77.1(6) , [5]: 76.9(5) , [7]: 77.7(8) and [8]:
1
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[20]
°
76.2(10) ).
In each heterobimetallic complex the planes of the NHC
donor bearing the PdII center is rotated out of the central aryl
ring plane to incorporate the palladium coordinated pyridine
moieties. The torsion angles between the NHC plane containing
PdII and the central aryl ring plane fall in the range �63.59 to
°
�76.73 .
The coordination geometry around Pd atom is distorted
square planar. The CNHCÀ PdÀ NPy and CNHCÀ PdÀ PPPh3 bond angles
in the heterobimetallic complexes are in the range 173.9(6)–
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°
°
176.4(4) and 168.7(5)–168.7(6) , respectively. These values, like
the PdÀ CNHC bond lengths ([3]: 1.960(3), [4]: 1.955(15), [5]:
1.943(15), [7]: 2.006(15) and [8]: 2.007(18) Å), fall in the range
previously described for palladium(II) NHC complexes.[14,21] The
PdÀ PPPh3 bond lengths in complexes [7] and [8] measure
2.328(4) Å and 2.332(5) Å, respectively. These distances are also
in the range reported for the palladium(II) complexes bearing
mixed NHC^PPh3 donors.[18,22]
All the heterobimetallic complexes ([3], [4], [5], [7] and [8])
have been employed for the one-pot tandem Suzuki-Miyaura
coupling/transfer hydrogenation of 4-bromobenzaldehyde in
iPrOH/THF at elevated temperature (Table 1) and the complexes
appeared as active precatalysts in the aforementioned tandem
catalysis. The heterobimetallic precatalysts were used in
0.5 mol% in the presence of KOtBu as a base for tandem CÀ C
coupling/transfer hydrogenation reactions. The heterobimetallic
MIIIÀ PdII (M=Rh, Ir) complexes bearing mixed NHC^PPh3 donor
ligands at PdII centers ([7] and [8]) delivered higher yield of 4-
biphenylmethanol compared to the heterobimetallic MIIIÀ PdII
(M=Rh, Ir) complexes featuring PEPPSI type PdII centers ([3]-
[5]). The yields obtained by using catalysts [7] and [8] are much
higher when compared with the yields from an equimolar
mixture of their mononuclear PdII and RhIII/IrIII counterparts
(Table 1). These results probably due to the dual action of PdII
and RhIII/IrIII centers during catalysis.[7b,10,14]
The catalytic activity of the heterobimetallic IrIIIÀ PdII complex
[8] featuring mixed NHC^PPh3 donor ligands at PdII center was
highest among all the heterobimetallic complexes.However, in
Figure 2. ORTEP plots of [3], [4] and [5]. Ellipsoids are drawn at 50%
probability. Hydrogen atoms have been omitted for clarity.
Table 1. Suzuki-Miyaura coupling/transfer hydrogenation reaction.[a]
Entry
Catalyst
Isolated yield [%]
1
2
3
4
5
6
7
[3]
[4]
[5]
[7]
[10]+[12]
[8]
6
9
14
44
07
67
11
[IrÀ CAr^NHC]14 +trans-[12]
8
[8]
92[b]
[a] General reaction conditions: 0.5 mol% cat. (0.5 mol% of each complex
[10] and [12]), 3.5 eq. KOtBu, iPrOH/THF, 2 h; [b] 1.0 mol% cat. loading and
for 9 h.
Figure 3. ORTEP plots of [7] and [8]. Ellipsoids are drawn at 50% probability.
Hydrogen atoms have been omitted for clarity.
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temperature. The resulting crude reaction mixture was filtered
all cases 4-bromobenzyl alcohol was obtained as major side
product. An excellent isolated yield of the 4-biphenylmethanol
(92%) was obtained using the heterobimetallic IrIIIÀ PdII complex
[8], when the tandem reaction was carried out for 9 h with
1.0 mol% catalyst loading. Tandem Suzuki-Miyaura coupling/
transfer hydrogenation reactions have also been demonstrated
in literature using heterobimetallic IrIIIÀ PdII complexes with
higher catalyst loading.[7b,d] However, the heterobimetallic
complex [8] shows excellent selectivity towards the formation
of 4-biphenylmethanol (isolated yield: 92%) with 1.0 mol%
loading. The mercury poisoning test reveals the homogeneous
and heterogeneous nature of the transfer hydrogenation and
CÀ C coupling reactions, respectively.
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through celite to get a clear solution. The acetonitrile was removed
in vacuo and the crude mixture was loaded onto a silica gel
column. Elution with dichloromethane: methanol (97:3, v:v) solvent
mixture afforded the compound as a brown solid. Yield: 0.024 g
(0.032 mmol, 66%). 1H NMR (300 MHz, CDCl3): δ=9.86 (s, 1H,
NÀ CHÀ N), 7.81 (s, br, 1H, Himidazole), 7.67 (s, 1H,Himidazole), 7.41 (s, 1H,
H
aryl), 7.32 (s, br, 1H, Himidazole), 7.14 (s, br, 1H, Himidazole), 4.30 (s, 3H,
NÀ CH3), 3.92 (s, 3H, NÀ CH3), 2.55 (s, 3H, ArÀ CH3), 2.15 (s, 3H,
ArÀ CH3), 1.83 (s, 15H, Cp*À CH3) ppm. 13C{1H} NMR (75 MHz, CDCl3):
1
1
δ=185.0 (d, JRhÀ C =53.6 Hz, CNHC-Rh), 157.3 (d, JRhÀ C =34.5 Hz, CAr-
Rh), 147.9 (CAr), 136.9 (Cimidazole), 133.3 (CArÀ H), 129.5 (Cimidazole), 127.0
(Cimidazole), 123.9 (Cimidazole), 122.4 (Cimidazole), 121.9 (CAr), 119.2 (Cimidazole),
98.7 (d, 1JRhÀ C =4.5 Hz, CCp*), 38.5 (NÀ CH3), 37.7 (NÀ CH3), 15.1
(ArÀ CH3), 14.1 (ArÀ CH3), 10.6 (CpÀ CH3) ppm. HRMS (ESI, positive
ions): m/z=631.0884 (calcd for [[2]-I]+ 631.0805).
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Compound [3]: To a mixture of mononuclear RhIII complex [2]
(0.025 g, 0.033 mmol), K2CO3 (0.0092 g, 0.067 mmol), PdCl2
(0.0064 g, 0.036 mmol) and KI (excess) was added pyridine (5 mL).
Conclusions
°
The resulting suspension was heated to 84 C for 24 h. The reaction
We have presented a series of heterobimetallic PdIIÀ MIII (M=Rh,
Ir) NHC complexes starting from a symmetrical bis-imidazolium
salt featuring 2,3-dimethyl-1,4-di(NHC) substitution pattern of
the central phenyl ring. All the heterobimetallic complexes are
prepared by using selective and sequential addition of RhIII or
IrIII followed by PdII. All the heterobimetallic complexes
appeared as active precatalysts for tandem Suzuki-Miyaura
coupling/transfer hydrogenation reactions. The heterobimetallic
complexes bearing mixed NHC^PPh3 donor ligands at PdII
center delivered much higher yields in tandem Suzuki-Miyaura/
transfer hydrogenation reaction when compared with the
combined action of their mononuclear counterparts. The
heterobimetallic complex [8], featuring cyclometalated IrIII and
mixed NHC^PPh3 donor ligands at PdII center, shows the best
activity along with excellent selectivity for 4-biphenylmethanol
in the tandem reaction, among all the heterobimetallic
complexes. Further research work in our lab is directed towards
the synthesis of heterobimetallic NHC complexes with short
metal-metal distances and their applications in cooperative
tandem catalysis.
mixture was allowed to stand at ambient temperature. The reaction
mixture was filtered, and the residue was washed several times
with dichloromethane. The solvent was removed, and the crude
solid was added onto a silica gel column. Elution with dichloro-
methane resulted a yellow solid. Yield: 0.020 g (0.019 mmol, 57%).
1H NMR (300 MHz, CDCl3): δ=9.02–8.97 (m, 2H, HPy), 8.42 (s, 1H,
HAr), 7.83 (s, br, 1H, Himidazole), 7.54 (m, 1H, HPy), 7.20–7.19 (m, 2H, HPy),
7.11 (s, br, 1H, Himidazole), 7.06–7.04 (m, 2H, Himidazole), 4.17 (s, 3H,
NÀ CH3), 3.96 (s, 3H, N-CH3), 2.56 (s, 3H, CH3À Ar), 2.19 (s, 3H, Ar-CH3),
1.88 (s, 15H, Cp*À CH3)ppm. HRMS (ESI, positive ions): m/z=
862.9090 (calcd for [[3]-IÀ Py]+ 862.8817). The 13C{1H} data could not
be obtained for complex [3]due to sparingly soluble nature of the
complex in most of the deuterated solvents.
Compound [4]: To a mixture of mononuclear RhIII salt [2] (0.025 g,
0.033 mmol), PdCl2 (0.0064 g, 0.036 mmol), K2CO3 (0.0136 g,
0.098 mmol), 4-methoxy pyridine (0.018 g, 0.165 mmol) and KI
(excess) was added 6 mL of acetonitrile and the reaction mixture
°
was heated to 84 C for 24 h. The reaction mixture was allowed to
stand at atambient temperature. The reaction mixture was filtered
and the residue was washed several times with dichloromethane.
The solvent was removed from the collected solutionand the crude
solid was purification by column chromatography (silica gel) using
dichloromethane. Yield: 0.019 g (0.017 mmol, 52%). 1H NMR
(300 MHz, CDCl3): δ=8.79–8.76 (m, 2H, HPy), 8.40 (s, 1H, HAr), 7.81 (d,
3
3
1H, JHÀ H =2.1 Hz, Himidazole), 7.07(d, 1H, JHÀ H =1.8 Hz, Himidazole), 7.04–
7.01 (m, 2H, HPy), 6.67–6.64 (m, 2H, Himidazole), 4.10 (s, 3H, NÀ CH3),
3.94 (s, 3H, NÀ CH3), 3.72 (s, 3H, OÀ CH3), 2.54 (s, 3H, ArÀ CH3), 2.12 (s,
3H, Ar-CH3), 1.86 (s, 15H, Cp*À CH3) ppm. HRMS (ESI, positive ions):
m/z=862.8851 (calcd for [[4]-I-(4-OMePy)]+ 862.8817). The 13C{1H}
data could not be obtained for complex [4]due to sparingly soluble
nature of the complex in most of the deuterated solvents.
Experimental Section
All reactions were carried out under argon atmosphere using
°
standard Schlenk techniques. Glassware was oven dried at 100 C.
Solvents were distilled by standard procedures prior to use. H, 31P
1
{1H} and 13C{1H} NMR spectra were recorded on Bruker Avance 300
spectrometer. Chemical shifts (δ) are expressed in ppm downfield
from tetramethylsilane using the residual protonated solvent as an
internal standard. All coupling constants are expressed in Hertz and
Compound [5]: Complex [5] was prepared as described for [4] from
mononuclear RhIII salt [2] (0.025 g, 0.033 mmol), PdCl2 (0.0064 g,
0.036 mmol), K2CO3 (0.031 g, 0.224 mmol), 4-chloro pyridine
(0.020 g, 0.133 mmol) and KI (excess) in acetonitrile (4 mL). Yield:
1
only given for H,1H couplings unless mentioned otherwise. Mass
spectra were obtained with Water XevoG2-SQ-TOF. Compound
[Ir(Cl)2Cp*]2,[23][Rh(Cl)2Cp*]2 and the bis-imidazolium salt 1[14] were
[24]
1
0.018 g (0.016 mmol, 50%). H NMR (300 MHz, CDCl3): δ=8.95–8.93
prepared as described in the literature. PdCl2 was purchased from
commercial sources and were used as received without further
purification.
(m, 2H, HPy), 8.38 (s, 1H, HAr), 7.80 (s, br, 1H, Himidazole), 7.18–7.16 (m,
3
3
2H, HPy), 7.08 (d, JHÀ H =2.1 Hz, 1H, Himidazole), 7.03 (d, JHÀ H =1.8 Hz,
2H, Himidazole), 4.11 (s, 3H, NÀ CH3), 3.93 (s, 3H, NÀ CH3), 2.53 (s, 3H,
ArÀ CH3), 2.16 (s, 3H, ArÀ CH3), 1.85 (s, 15H, Cp*À CH3) ppm. HRMS
(ESI, positive ions): m/z=862.9031 (calcd for [[5]-I-ClPy]+ 862.8817).
The 13C{1H} data could not be obtained for complex [5]due to
sparingly soluble nature of the complex in most of the deuterated
solvents.
Compound [2]: To a mixture of bis-imidazolium salt 1[14] (0.025 g,
0.048 mmol), Cs2CO3 (0.031 g, 0.095 mmol), NaOAc (0.008 g,
0.098 mmol), [Rh(Cl)2Cp*]2 (0.015 g, 0.024 mmol) and KI (excess) was
added acetonitrile (6 mL). The resulting suspension was heated to
°
60 C for 20 h. The reaction mixture was allowed to cool at ambient
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Compound[7]: To a mixture of compound [3] (0.020 g, 0.019 mmol)
Cis-isomer: Yield: 0.015 g (0.0191 mmol, 13%).1H NMR (300 MHz,
CDCl3): δ=7.33–7.30 (m, 4H, HAr), 7.21–7.12 (m, 15H, HAr), 6.88–6.85
(m, 2H, Himidazole), 3.76 (s, 3H, NÀ CH3), 1.65(s, 3H, CH3À Ar) ppm. 31P
{1H} NMR (121.5 MHz, CDCl3): δ=21.82 ppm. 13C{1H} NMR (75 MHz,
CDCl3): δ=165.5 (CNHCÀ Pd, from 2D NMR), 137.4 (CAr), 134.4 (d,
2JPÀ C =11.3 Hz, CAr), 132.7 (CAr), 132.0 (CAr), 131.8 (CAr), 130.66 (d,
1
2
3
4
5
6
7
8
9
and PPh3 (0.0055 g, 0.021 mmol) was added acetonitrile (6 mL). The
reaction mixture was stirred at room temparature for 24 h. The
solvent was removed in vacuoand the crude solid was loaded onto
a silica gel column. Elution with dichloromethane yielded a yellow
solid. Yield: 0.014 g (0.011 mmol, 61%).1H NMR (300 MHz, CDCl3):
δ=8.2 (s, HAr), 7.88–7.86 (m, HAr), 7.70–7.53 (m,HAr), 7.07–6.96 (m,
HAr), 4.00 (s, NÀ CH3), 3.94 (s, NÀ CH3), 3.93 (s, NÀ CH3), 3.89 (s, NÀ CH3),
2.58 (s, CH3À Ar), 2.56 (s, CH3À Ar),2.19 (s, CH3À Ar), 2.17 (s, CH3À Ar),
2.14 (s, CH3À Ar), 1.84 (s, Cp*À CH3), 1.83 (s, Cp*À CH3) ppm.31P{1H}
NMR (121.5 MHz, CDCl3): δ=15.22 (s, PPh3) and 14.88 (s, PPh3) ppm.
HRMS (ESI, positive ions): m/z=1125.0343 (calcd for [[7]-I]+
1124.9734), 862.9097 (calcd for [[7]-I-PPh3]+ 862.8817). The 13C{1H}
data could not be obtained for complex [7]due to sparingly soluble
nature of the complex in most of the deuterated solvents.
3
4JPÀ C =2.3 Hz, CAr), 129.3 (CAr), 128.28 (d, JPÀ C =10.5 Hz, CAr), 128.1,
(CAr), 126.8 (CAr), 124.7 (Cimidazole), 122.7 (d, Cimidazole), 39.4 (NÀ CH3),
18.9 (ArÀ CH3) ppm. HRMS (ESI, positive ions): m/z=667.0174 (calcd
for [[12]-I]+ 667.0003).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
General procedure CÀ C coupling/transfer hydrogenation
Reaction
The 4-bromobenzaldehyde (0.092 g, 0.50 mmol), phenylboronic
acid (1.2 mmol) and the corresponding complexes (0.5 mol%) were
mixed with KOtBu (3.5 equiv.) in a Schlenk tube under an inert gas
atmosphere, to this mixture was added dry iPrOH and THF (1:1, v:
Compound [8]: Complex [8] was prepared as described for [7] from
heterobimetallic PdIIÀ IrIII complex [6] (0.120 g, 0.104 mmol) and
PPh3 (0.030 g, 0.114 mmol) in acetonitrile (10 mL). The crude solid
was loaded onto a silica gel column and eluted with dichloro-
methane to give a yellow solid. Yield: 0.069 g (0.051 mmol, 50%).
1H NMR (300 MHz, CDCl3): δ=8.04 (s, HAr), 7.84–7.74 (m, HAr), 7.62–
7.45 (m,HAr), 7.01–6.90 (m, HAr), 3.93 (s, NÀ CH3), 3.86 (s, NÀ CH3), 3.84
(s, NÀ CH3), 3.83 (s, NÀ CH3), 2.52 (s, CH3À Ar), 2.50 (s, CH3À Ar), 2.16 (s,
CH3À Ar), 2.11 (s, CH3À Ar), 1.83 (s, Cp*À CH3), 1.81 (s, Cp*À CH3) ppm.
31P{1H} NMR (121.5 MHz, CDCl3): δ=15.28 (s, PPh3) and 14.85 (s,
PPh3) ppm. HRMS (ESI, positive ions): m/z=952.9959 (calcd for [[8]-
I-PPh3]+ 952.9282). The 13C{1H} data could not be obtained for
complex [8]due to sparingly soluble nature of the complex in most
of the deuterated solvents.
°
v). The mixtures were heated at 90 C for 2 h and cooled to ambient
temperature. The reaction was quenched with H2O (15 mL) and
extracted with dichloromethane. The organic part was dried over
MgSO4 and the solvent was removed. The crude mixture was
loaded onto a silica gel column and eluted with hexane:ethyl
acetate (100:5, v:v) to get the pure product.
X-ray Crystallography
Single crystals suitable for X-ray diffraction studies were obtained
for the complexes [3], [4], [5], [7] and [8] by slow evaporation of the
solvents from a saturated dichloromethane:hexane solution of the
corresponding complexes at ambient temperature. X-ray diffraction
data were collected at T=100 K with a Bruker Kappa Apex 2 duo or
a Bruker SMART APEX-II diffractometer equipped with a rotation
anode using graphite-monochromated Mo-Kα radiation (λ=
0.71073 Å). The strategy for the data collection was evaluated by
using the CrysAlisPro CCD or Smart software. The data were
collected by the standard ‘phi-omega scan techniques’. The data
integration and reduction were processed with SAINT software.
Empirical or multi scan absorption corrections were applied to the
collected reflections with SADABS using XPREP. Cell constantswer-
eobtainedfromtheleast-squaresrefinementofthree-dimensionalcen-
troidsbyrecording narrow ω rotation frames until completion of
almost all reciprocal space in the stated θ range. The space-
groupofthesecompoundswasdetermined basedonthe lackofsyste-
maticabsencesandintensitystatistics. Thestructures were solvedu-
singSHELXL-97 or olex2 and refined using SHELXL[25] (as
implementer in Olex2-V 1.3). Full-matrix least-squares/difference
Fourier cycles were performed to locate the remaining non-
hydrogen atoms. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms bonded to
carbon were refined isotropically in calculated positions. Crystal
qualities for [4], [5], and [7] were moderate, however, the same for
[8] was very poor after several attempts. The best data (using best
strategy) what we collected was used for structure determination. A
special type of residual difference density was found near heavy
atoms, usually within ~1 Å for [3], [4], [5], [7] and [8]. These are the
artefacts and might be related to systematic effects or absorption
correction, however a best choice of methods have been used to
get minimized the error. There were several atoms having multi
position disorder with less than 5% occupancy which might be
reflected as little high ellipsoid in thermal parameters. Such
disorders were not stable under disorder refinement using part
instruction in SHELXL. For some of structures, there were
unidentified, multi-positional disordered solvent molecules which
were removed from reflection using squeeze instruction as
Compound [10]: To a mixture of imidazolium salt 9[19] (0.04 g,
0.133 mmol) and Ag2O (0.034 g, 0.147 mmol) was added dichloro-
methane (10 mL). The resulting mixture was stirred at ambient
temperature for 18 h under the exclusion of light. To the reaction
mixture was added NaOAc (0.026 g, 0.317 mmol), [Rh(Cl)2Cp*]2
(0.041 g, 0.066 mmol) and KI (excess). The reaction mixture was
stirred at ambient temperature for an addtional 18 h. The reaction
mixture was filtered through a pad of Celite to get a clear solution.
The solvent was removed in vacuo and the crude mixture was
loaded onto a silica gel column. Elution with dichloromethane
1
resulted a brown solid. Yield: 0.015 g (0.028 mmol, 21%). H NMR
(300 MHz, CDCl3): δ=7.70 (d, 3JHÀ H =2.1 Hz, 1H, Himidazole), 7.60 (d,
3JHÀ H =7.5 Hz, 1H, HAr), 6.99 (d, 3JHÀ H =2.1 Hz, 1H, Himidazole), 6.88 (t, br,
3JHÀ H =7.5 Hz, 1H, HAr), 6.73 (d, 3JHÀ H =7.2 Hz, 1H, HAr), 3.90 (s, 3H,
NÀ CH3), 2.54 (s, 3H, ArÀ CH3), 1.82 (s, 15H, Cp*À CH3), ppm. 13C{1H}
1
NMR (75 MHz, CDCl3): δ=184.0 (d, JRhÀ C =54.75 Hz, CNHC-Rh), 157.6
(d, 1JRhÀ C =33 Hz, CAr-Rh), 144.6 (CAr), 137.2 (CAr), 126.3 (CAr), 124.3
(CAr), 121.4 (Cimidazole), 118.5 (CAr), 98.1 (d, 1JRhÀ C =4.5 Hz, CCp*À Rh),
38.2 (NÀ CH3), 21.0 (ArÀ CH3),10.5 (CpÀ CH3) ppm. HRMS (ESI, positive
ions): m/z=537.0283 (calcd for [[10]+H]+ 537.0274), 409.1036
(calcd for [[10]-I]+ 409.1151).
Compound [12]: To
a
mixture of compound [11][14] (0.09 g,
0.147 mmol) and PPh3 (0.0424 g, 0.1617 mmol) was added dichloro-
methane (5 mL). The reaction mixture was stirred at room temper-
ature for 24 h. The crude solid was loaded onto a silica gel column
and eluted with dichloromethane to give the isomeric compounds.
1
Trans-isomer: Yield: 0.033 g (0.042 mmol, 28%). H NMR (300 MHz,
CDCl3): δ=8.16 (d, 3JHÀ H =7.5 Hz, 1H, HAr), 7.52–7.42 (m, 7H, HAr),
7.40–7.26 (m, 11H, HAr), 7.10–7.08 (m, 2H, Himidazole), 3.97 (s, 3H,
NÀ CH3), 2.31 (s, 3H, CH3À Ar) ppm. 31P{1H} NMR (121.5 MHz, CDCl3):
δ=15.81 ppm.13C{1H} NMR (75 MHz, CDCl3): δ=159.8 (CNHCÀ Pd,
2
from 2D NMR), 138.3 (CAr), 135.3 (d, JPÀ C =10.5 Hz, CAr), 135.0 (CAr),
132.9 (CAr), 132.3 (CAr), 131.4 (CAr), 130.2 (d, 4JPÀ C =2.3 Hz, CAr), 129.05
3
(CAr), 127.6 (d, JPÀ C =10.5 Hz, CAr), 126.0, 124.0 (Cimidazole), 123.0 (d,
Cimidazole), 39.05 (NÀ CH3), 19.8 (ArÀ CH3) ppm.
Eur. J. Inorg. Chem. 2021, 1104–1110
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doi.org/10.1002/ejic.202001080
Trans. 2009, 9392; c) Y. Han, L. J. Lee, H. V. Huynh, Chem. Eur. J. 2010,
implemented in Olex2. The details of refinement parameters and
instructions were appended into cif files for each case. The
structure solution was well agreements with all other spectroscopic
characterization and provides meaningful information to the
related chemistry. Detail crystallographic data parameters were
placed in Table S1 (the Supporting Information).
1
2
3
4
5
16, 771; d) A. Rit, T. Pape, F. E. Hahn, J. Am. Chem. Soc. 2010, 132, 4572;
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6
7
8
9
Deposition Numbers 2039360 (for 3), 2039361 (for 4), 2039363 (for
5), 2039377 (for 7), and 2039379 (for 8) contain the supplementary
crystallographic data for this paper. These data are provided free of
charge by the joint Cambridge Crystallographic Data Centre and
Fachinformationszentrum Karlsruhe Access Structures service
www.ccdc.cam.ac.uk/structures.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
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53
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57
Acknowledgements
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Conflict of Interest
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The authors declare no conflict of interest.
Keywords: Carbene ligands · Heterobimetallic complexes ·
Structure elucidation
hydrogenation
·
Tandem catalysis
·
Transfer
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Manuscript received: November 27, 2020
Revised manuscript received: December 27, 2020
Accepted manuscript online: December 30, 2020
[6] For selected examples see:a) C. Radloff, F. E. Hahn, T. Pape, R. Fröhlich,
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