Structural characterization and catalytic activity of the rhodium-carbene complex Rh(PPh3)2(IMes)Cl (IMes = bis(1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene)

Article abstract of DOI:10.1016/S0022-328X(02)01638-8

The rhodium-carbene complex Rh(PPh₃)₂(IMes)Cl (2) is an active catalyst for the hydroboration of simple olefins at room temperature. The reactivity of 2 was also tested in the methylenation of aldehydes. The crystal structure of 2 is

Full text of DOI:10.1016/S0022-328X(02)01638-8

Journal of Organometallic Chemistry 658 (2002) 126Á131
/
www.elsevier.com/locate/jorganchem
Structural characterization and catalytic activity of the rhodiumÁ
carbene complex Rh(PPh3)2(IMes)Cl (IMesꢀbis(1,3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene)
/
/
Gabriela A. Grasa a, Zakhia Moore a, Kenneth L. Martin a, Edwin D. Stevens a, Steven
P. Nolan a,*, Vale´rie Paquet b, He´le`ne Lebel b,*
a Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA
b De´partement de Chimie, Universite´ de Montre´al, P.O. Box 6128, Station Downtown, Montre´al, Que´bec, Canada H3C 3J7
Received 13 May 2002; received in revised form 27 May 2002; accepted 6 June 2002
Abstract
The rhodiumÁcarbene complex Rh(PPh3)2(IMes)Cl (2) is an active catalyst for the hydroboration of simple olefins at room
/
temperature. The reactivity of 2 was also tested in the methylenation of aldehydes. The crystal structure of 2 is also reported.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Structural characterization; Catalytic activity; RhodiumÁcarbene complexes
/
1. Introduction
as IMes (IMesꢀ
zol-2-ylidene), IPr (IPrꢀ
nyl)imidazol-2-ylidene), or SIMes (SIMesꢀ
trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) can re-
sult in enhanced catalytic performance over phosphine-
bearing analogues in various catalytic transformations
/
bis(1,3-(2,4,6-trimethylphenyl)imida-
bis(1,3-(2,6-diisopropylylphe-
1,3-(2,4,6-
/
The Rh(PPh3)3Cl (1) complex (Wilkinson’s catalyst) is
well known as an active homogeneous catalyst for olefin
hydrogenation [1], hydrosilylation [2] and hydrobora-
tion [3]. Other rhodium [3b,4a,4b] and cationic iridium
complexes [3b,4c,4d] bearing bulky phosphines as
ligands have also proven to be active catalysts for
directed alkene hydroboration and hydrogenation.
Very recently, complex 1 has been found active in the
methylenation of aldehydes using trimethylsilyldiazo-
methane in the presence of triphenylphosphine and 2-
propanol [5].
/
which include metal-mediated CÃ
reactions [8], amination of aryl chlorides [9], hydro-
formylation [10], and olefin metathesis [11]. RhodiumÁ
/C cross coupling
/
carbene complexes have been extensively studied by
Lappert et al. [12]. However, there are few reports on the
catalytic acitvity of rhodiumÁcarbene complexes in
/
rhodium-mediated processes [10,12c].
Imidazole-2-ylidenes (N-heterocyclic carbenes, NHC)
[6] represent an alternative to the widely utilized
phosphine ligands [7] in homogenous catalysis. Studies
performed by us and others have demonstrated that
replacement of bulky phosphines by NHC (Fig. 1) such
Based on these findings and our continuing interest in
developing more efficient and stable catalysts, we wished
to examine whether we could influence the catalytic
activity of Rh(PPh3)3Cl (1) [9] for olefin hydroboration
and hydrogenation as well as for aldehyde methylena-
tion when a PPh3 ligand is replaced with the more
donating IMes. We now report the catalytic activity of
the previously synthesized Rh(PPh3)2(IMes)Cl (2) [10]
for olefin activation and methylenation of aldehydes as
well as its crystal structure.
* Corresponding authors. Tel.:
2806860
E-mail
lebelhe@chimie.umontreal.ca (H. Lebel).
ꢁ
/
1-504-2806311; fax:
ꢁ
/
1-504-
addresses:
snolan@uno.edu (S.P.
Nolan),
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 6 3 8 - 8

G.A. Grasa et al. / Journal of Organometallic Chemistry 658 (2002) 126Á
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131
127
Fig. 1. Selected N-heterocyclic carbenes.
2. Results and discussion
2.1. Thermal stability of Rh(PPh3)2(IMes)Cl (2)
Complex 2 was synthesized according to the literature
procedure [10] by a simple ligand exchange reaction of
Rh(PPh3)3Cl (1) with IMes in toluene. Since we had
observed improved thermal stability of a ruthenium
olefin metathesis system when IMes was affixed to the
metal center [11a], the hope was that improved thermal
stability would also result for 2. To monitor the effect
the ligand displacement might have on the catalyst
activity was one important goal of this study. Upon
heating a THF-d8 solution of 2 at 65 8C for 24 h, no
phosphine dissociation nor decomposition occurs, in-
dicating the high thermal stability of complex 2. In
contrast, the 31P-NMR spectrum of a THF-d8 solution
of 1 indicated that heating resulted in the labilization of
one phosphine ligand and the stabilization of the
dimeric complex [(Rh(PPh3)2Cl]2 (3) (Eq. (1)). While
Fig. 2. ORTEP of Rh(PPh3)2(IMes)Cl (2) with ellipsoids drawn at 50%
probability.
Table 1
Crystallographic data for Rh(PPh3)2(IMes)Cl (2)
2
Empirical formula
Formula weight
Crystal system
Space group
C57H54ClN2P2Rh
967.32
Monoclinic
P21/n
˚
a (A)
19.9449(11)
11.2340(6)
˚
b (A)
kinetic studies [13] indicate the presence of
a
˚
c (A)
22.0168(12)
90.00
Rh(PPh3)2Cl species in a pre-equilibrium leading to
dimer formation, heating Rh(PPh3)2(IMes)Cl (2) in
THF-d8 does not lead to the corresponding [Rh(PPh3)-
(IMes)Cl]2 dimer formation, indicating a higher stability
of the monomeric form. To unequivocally establish the
structure of 2, single-crystal X-ray diffraction studies
were performed on crystals grown from slow diffusion
of hexanes into saturated dichloromethane solution of 2.
a (8)
b,(8)
g (8)
V (A )
Z
105.5890(10)
90.00
4751.6(4)
3
˚
4
1.352
Dcalc (Mg mꢂ3
)
Reflections collected
Independent reflections
Refined parameters
94 242
18 134 [Rintꢀ0.0585]
784
Final R indices [I ꢀ2s(I)]
R indices (all data)
R1ꢀ0.0272, wR2ꢀ0.0480
R1ꢀ0.0629, wR2ꢀ0.0499
therefore more donating IMes, results in small changes
P bond distances (Table 2) [14]. This
ð1Þ
in RhÃ
/
Cl and RhÃ
/
suggests that the trans-influence of PPh3 and IMes are
similar. This observation is further supported by similar
2.2. X-ray analysis of 2
31P coupling constants of Rh(PPh3)3Cl (1) (JPÃP
ꢀ38
/
Hz) and Rh(PPh3)2(IMes)Cl (2) (JPÃP
ꢀ39 Hz).
/
The ORTEP diagram of 2 is presented in Fig. 2 and
selected crystallographic data and bond lengths and
angles are given in Tables 1 and 2. As shown in the
ORTEP (Fig. 2), 2 adopts a distorted square-planar
coordination geometry around the rhodium center
(sum of bond angles around Rh is 360.5658, with two
To ascertain whether this simple substitution of IMes
for one PPh3 in 1 could lead to significant catalytic
behavior differences, the role of 2 in a number of
catalytic transformations was investigated.
2.3. Catalytic hydroboration
inequivalent phosphine ligands cis and trans to IMes
˚
(bond length RhÃ
(trans-IMes)ꢀ
sphine ligand in 1 with the more nucleophilic, and
/
P (cis-IMes)ꢀ
/
2.0527 A and RhÃ
/
P
The catalytic performance of 2 in the hydroboration
of 1-hexene by catecholborane was investigated and
compared to the activity of related active complexes for
˚
/
2.3053 A). Replacing a triphenylpho-

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G.A. Grasa et al. / Journal of Organometallic Chemistry 658 (2002) 126Á131
/
Table 2
[Ir(COD)(PCy3)(Py)]PF6 [11a,11b,11c,11d], usually ac-
tive for olefin hydrogenation, also led to high conver-
sions of this reaction in 24 h. The mechanism for
rhodium-mediated hydroboration is said to involve
two separate manifolds, one involving initial ligand
(PPh3) loss and the second one involving initial oxida-
tive-addition of the borane without prior ligand dis-
sociation [3c,16]. The similar activities displayed by 1
and 2 and the less facile triphenylphosphine dissociation
observed for 2 would suggest the initial oxidative
addition manifold might be the preferred mode of
activation in the hydroboration mediated by 2.
˚
Selected bond lengths (A) and angles (8) for Rh(PPh3)2(IMes)Cl (2),
and Rh(PPh3)3Cl (1) [14]
2
1 [14]
Bond lengths
RhÃC(1)(carbene)
RhÃP(1)(c-carbene) 2.2158(4)
RhÃP(2)(t-carbene)
RhÃCl
2.0527(14) RhÃP(1) (c to Cl) 2.322, 2.335
RhÃP(1)(t to Cl) 2.212
2.3053(4)
2.3941(4)
RhÃCl
2.377
Bond angles
P(1)(c-carbene)Ã
RhÃC(carbene)
P(2)(t-carbene)Ã
RhÃC(carbene)
P(1)ÃRhÃP(2)
C(1)ÃRhÃCl
98.43(4)
164.10(4)
96.238(15)
85.71(4)
2.4. Catalytic hydrogenation
P(1)ÃRhÃCl
P(2)ÃRhÃCl
173.835(15)
80.187(13)
Using a similar protocol, we wished to compare the
catalytic activity of compound 2 to the Wilkinson and
Crabtree catalysts under hydrogenation conditions (am-
bient temperature and pressure of hydrogen) (Table 4).
As expected, complex 2 yields lower conversions of the
hydrogenated product compared to the very active
Crabtree [11e] or Crabtree-type iridium catalysts [11e].
The lower catalytic activity may be explained in terms of
the higher stability with respect to phosphine dissocia-
tion, since phosphine dissociation (followed by olefin
insertion) is required in order to obtain a coordinatively
unsaturated species during the catalytic cycle.
both hydroboration (3) and hydrogenation 4a4d[15].
These results are presented in Table 3. Although the
hydroboration of olefins can be performed with hydro-
borating reagents such as 9-BBN or catecholborane,
these reaction require elevated temperatures and long
reaction times [3b] (Table 3, entry 1). However, the
hydroboration reaction of simple olefins in the presence
of rhodium-based catalysts takes place at room tem-
perature [3b]4a4b with relatively short reaction time.
The hydroboration mediated by 2 was performed using
1 mol% of catalyst in 2 ml of CH2Cl2. The catalytic
activity of complex 2 is comparable to that displayed by
Wilkinson’s catalyst (Table 3, entries 2 and 3), with
predominant formation of the anti-Markovnikov linear
product. For comparison purposes, cationic iridium
2.5. Catalytic olefination
The catalytic activity of 2 was also tested in the
olefination reaction using trimethylsilyldiazomethane, 2-
propanol and triphenylphosphine. In contrast to reac-
tions mediated by 1, the methylenation of hydrocinna-
maldehyde proceeded very slowly at room temperature
with 2 (entries 1 and 7). Only 20% of the desired
terminal olefin was observed after 24 h, with some 10%
of various by-products and 70% of starting material.
Changing the solvent to dichloromethane did not
improve the conversion, as the reaction was completely
inhibited in this solvent (entry 8). These results clearly
complexes
[Ir(COD)(SIMes)(Py)]PF6
[11e]
and
Table 3
Hydroboration of 1-hexene with catecholborane using various cata-
lysts
illustrate that the rhodiumÁ/carbene complex 2 is less
Table 4
Hydrogenation of cyclohexene using various catalysts
a
Entry Catalyst
Time Linear Branched Yield
(hrs)
a,b
a,b
(%)
Catalyst
Time (h) T (8C) Yield (%)
c
3b
1
2
3
4
5
9-BBN/ 688 C
Rh(PPh3)3Cl (1)
Rh(IMes)(PPh3)2Cl (2)
8
1
1
B99
99
99
B99
100
95
[Ir(COD)(PCy3)(Py)]PF6
[Ir(COD)(SIMes)(Py)]PF6
Rh(PPh3)3Cl (1)
B0.5
2
24
RT
RT
RT
RT
45
100 [11e]
100 [11e]
1
1
8
9
[Ir(COD)(SIMes)(Py)]PF6 24
[Ir(COD)(Pcy3)(Py)]PF6 24
B99
B99
92
92
Rh(IMes)(PPh3)2Cl (2)
Rh(PPh3)3Cl (1)
Rh(IMes)(PPh3)2Cl (2)
24
24
36
18
24
45
a
Reaction conditions: 1 mol% catalyst, 1 mmol 1-hexene, 2 ml
CH2Cl2, 1.1 mmol catecholborane.
a
Reaction conditions: 1 mol% catalyst, 1 mmol cyclohexene, 5 ml
CH2Cl2, 1 atm H2.
b
GC yield, average of two runs.
The product is 1-(9-BBN-)-hexane.
c
b
GC yield, average of two runs.

G.A. Grasa et al. / Journal of Organometallic Chemistry 658 (2002) 126Á
/
131
129
potent at mediating the formation of methylenetriphe-
nylphosphorane from trimethylsilyldiazomethane, tri-
phenylphosphine and 2-propanol at room temperature
than its Rh(PPh3)3Cl (1) parent.
Table 5
Olefination of aldehydes using various catalysts
a
The higher thermal stability displayed by 2, compared
to 1, suggested that improved catalytic conversions
might be possible for 2 at higher temperatures. Indeed,
when the reaction was carried out at 50 8C using
complex 2, the terminal olefin derived from hydrocin-
namaldehyde was obtained in an 88% yield after 6 h
(entry 9). Similar results were obtained with cyclohex-
anecarboxaldehyde, which produced the terminal olefin
quantitatively at 50 8C with 2 (entry 10). We have also
tested the catalytic activity of 1 and 2 with substrates
that exhibit lower rate of conversion, such as 4 and 5.
The terminal alkenes of aldehydes 4 and 5 were
produced, respectively in 91% after 8 h and in 79%
after 7 h using Wilkinson’s catalyst at room tempera-
ture. Both catalysts exhibit comparable activity at
50 8C with aldehyde 4 and 5 leading to 80Á
/
ꢀ98% of
/
the corresponding alkene after 2 h. Complex 2 was
slightly more efficient with aldehyde 4 whereas 1
provided slightly superior yields with substrate 5. Early
mechanistic work shows the rhodium-catalyzed olefina-
tion catalytic cycle to involve the association of the
trimethylsilyldiazomethane with the rhodium catalyst
through nitrogen coordination [17]. This suggests that
PPh3 ligand dissociation may have a major effect on the
relative catalytic activity of both rhodium catalysts.
a
The methylenation reaction was conducted using 1.4 equiv. Of
TMSCHN2, 1.1 equiv. Of 2-propanol, 1.1 equiv. Of triphenylpho-
sphine and 2.5 mol% of Rh-catalyst in THF.b GC yield, average of
two runs.c Isolated yield5.d DCM was the solvent.
Indeed, we observed an induction period (ca. 30Á45
/
min) when the olefination reaction was carried out with
the carbene complex 2 at 50 8C that could be explained
by phosphine dissociation and formation of an active
species. As a control experiment, complex 2 was
premixed with trimethylsilyldiazomethane in THF and
stirred for 30 min at room temperature, thus favoring
ligand exchange; triphenylphosphine, 2-propanol and
the aldehyde were then added. Under those conditions,
we observed the formation of the terminal olefin at
25 8C, with a maximal conversion of 57% [18]. This
experiment clearly suggests that there is an initial
activation of the catalyst with trimethylsilyldiazo-
methane and that might be responsible for the observed
induction period once the trimethylsilyldiazomethane is
added at the end. As for the hydrogenation reaction, the
PPh3 ligand dissociation seems to have a major effect
contributing to the increased activity of 1 over that
displayed by 2 in rhodium-catalyzed olefination (Table
5).
olefin hydroboration. This catalyst displays lower activ-
ity with respect to iridium based Crabtree’s and
modified Crabtree’s catalysts in catalytic hydrogenation
of olefins at ambient pressure. The catalytic activity of 2
was virtually identical to that of Rh(PPh3)3Cl (1) in the
rhodium-catalyzed methylenation of aldehydes using
trimethylsilyldiazomethane at 50 8C. Structural and
thermal stability studies reveal that the replacement of
one phosphine with IMes lowers the ease of displace-
ment of the remaining coordinated triphenylphosphines.
Investigations into the synthesis and catalytic uses of
related rhodiumÁcarbene complexes are ongoing.
/
4. Experimental
4.1. General considerations
3. Conclusions
All reactions were carried out under an atmosphere of
dry Ar using standard Schlenk techniques or in a
MBraun glovebox containing less than 1 ppm of oxygen
and water. NMR spectra were recorded using Varian
400 or 300 MHz spectrometers. GC analyses were
The replacement of one triphenylphosphine by a N-
heterocyclic carbene in Wilkinson’s catalyst results in a
catalytic species displaying comparable activity towards

130
G.A. Grasa et al. / Journal of Organometallic Chemistry 658 (2002) 126Á131
/
performed on a Hewlett-Packard HP 5890 II equipped
with a FID and a HP-5 column. All reported yields for
catalytic reactions were determined by GC.
150 K. The structures were solved using direct methods
SHELXS-86) and refined by full-matrix least-squares
techniques. Crystallographic data can be found in Table
1 and selected bond distances and angles in Table 2.
(
4.2. Reagents
Substrates for catalysis were purchased from com-
mercial suppliers and either used as received (solid
compounds) or degassed prior to use by purging with
5. Supplementary material
Details of the crystal structure determination for 2 are
available. Crystallographic data for the structure analy-
sis have been deposited with the Cambridge Crystal-
lographic Data Center, CCDC no. 186952 for
compound 2. Copies of this information may be
obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (Fax (int.
Ar for 20Á30 min. Dichloromethane was dried by
/
passage through an alumina tower and stored in a
glovebox. IMesHCl, IMes [19], [Ir(COD)(SIMes)-
(Py)]PF6 [11e] and Rh(PPh3)2(IMes)Cl [10] were pre-
pared according to the literature procedures.
4.3. Catalytic hydroboration
code): ꢁ44-1223-336033; e-mail: deposit@ccdc.cam.
/
ac.uk or www: http://www.ccdc.cam.ac.uk).
Catalyst (0.01 mmol), CH2Cl2 (2 ml), 1-hexene (1
mmol), catecholborane (1.1 mmol) were loaded into
either a Schlenk tube or a scintillation vial fitted with a
TFE/silicone liner and a screw cap inside a glove box.
After removal from the glovebox the resulting yellow
solution was stirred at room temperature for the
indicated time. Reaction progress was monitored by
analyzing aliquots by GC.
Acknowledgements
The National Science Foundation and the Petroleum
Research Fund administrated by ACS are gratefully
acknowledged for support of the work carried out at the
University of New Orleans. S.P.N. thanks J.M. for
generous gifts of the Crabtree and Wilkinson catalysts.
H.L. gratefully acknowledges support of this work by
NSERC (Canada), F.C.A.R (Que´bec), the Canadian
Foundation for Innovation, the Research Corporation
and the Universite´ de Montre´al.
4.4. Catalytic hydrogenation
A solution of 0.01 mmol of metal catalyst and 1.0
mmol of cyclohexene in 5 ml of CH2Cl2 was loaded into
a 50 ml scintillation vial equipped with a screw cap and
TFE/silicone liner inside a glove box. The vial was then
purged with hydrogen from a Schlenk line for 1 min.
The reaction was allowed to stir under atmospheric
pressure of hydrogen and the product formation was
monitored by GC. In some cases, the catalytic solution
was loaded into a Schlenk tube, purged with hydrogen
three times, then allowed to stir in an oil bath at 45 8C
for the indicated time.
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/
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