Intramolecular C-H activation involving a rhodium-imidazol-2-ylidene complex and its reaction with H2 and CO

Article abstract of DOI:10.1021/om990922e

An orthometalated rhodium-carbene complex Rh(IMes)(H)(IMes′)Cl (2) was obtained in high yield by reacting 2 equiv of a nucleophilic carbene, 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) with [Rh(COE)₂Cl]₂ (1). The X-ray structure of 2 shows an intramolecular C-H activation of one aryl ortho methyl group. Complex 2 reacts rapidly with H₂ and CO at room temperature, leading to the formation of Rh(IMeS)₂(H)₂Cl (3) and Rh(IMes)₂(CO)Cl (4), respectively.

Full text of DOI:10.1021/om990922e

1194
Organometallics 2000, 19, 1194-1197
In tr a m olecu la r C-H Activa tion In volvin g a
Rh od iu m -Im id a zol-2-ylid en e Com p lex a n d Its Rea ction
w ith H2 a n d CO
J inkun Huang, Edwin D. Stevens, and Steven P. Nolan*,†
Department of Chemistry, University of New Orleans, New Orleans, Louisiana, 70148
Received November 22, 1999
Summary: An orthometalated rhodium-carbene com-
plex Rh(IMes)(H)(IMes′)Cl (2) was obtained in high yield
by reacting 2 equiv of a nucleophilic carbene, 1,3-bis-
(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) with
[Rh(COE)2Cl]2 (1). The X-ray structure of 2 shows an
intramolecular C-H activation of one aryl ortho methyl
group. Complex 2 reacts rapidly with H2 and CO at room
temperature, leading to the formation of Rh(IMes)2(H)2Cl
(3) and Rh(IMes)2(CO)Cl (4), respectively.
sesses similar electron-donating and significantly more
demanding steric properties than the bulky tertiary
phosphine ligands PCy3 and PiPr3.6a,7
In tr od u ction
Rhodium phosphine complexes such as Wilkinson’s
Catalyst have played a major role in the development
of homogeneous catalysis and continue to be a very
active area of investigation. The “RhL2Cl” fragment,
where L is a tertiary phosphine, is a key intermediate
in olefin hydrogenation and in alkane dehydrogenation
catalyzed by Rh(PMe3)2(CO)Cl.8
The use of tertiary phosphines as ligands is ubiqui-
tous in organometallic chemistry and homogeneous
catalysis.1 Recent developments in the area of nucleo-
philic carbenes have indicated that these compounds
could act as supporting ligation-mimicking tertiary
phosphines. Herrmann2 and more recently Cavell3 have
utilized members of this ligand family to assist in
palladium-mediated Heck coupling. Based on the ste-
reoelectronic properties of these nucleophilic carbenes,
we have developed a methodology for C-C4 and C-N5
bond formation mediated by palladium. Furthermore,
we have also recently reported on the synthesis and use
of thermally tolerant ruthenium olefin metathesis-active
complexes modified with these nucleophilic carbene
ligands.6
RCH2CH3 Rh(PMe ) (CO)Cl8 RCHCH2 + H2
(1)
3
2
hν
Goldman and co-workers have recently investigated
alkane dehydrogenation and aldehyde decarbonylation
mediated by Rh(PiPr3)2Cl.8
1/2[Rh(PiPr3)2Cl]2 + RCHO f
Rh(PiPr3)2(CO)Cl + RH (2)
Solution calorimetric and structural studies on com-
plexes bearing this ligand family help explain the
improved catalytic behavior encountered in the pal-
ladium and ruthenium systems.7 In a recent contribu-
tion, we have shown that the carbene ligand 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) pos-
In the Rh(PiPr3)2Cl complex, the bulky tertiary phos-
phine ligands are used to assist in stabilizing the
coordinatively unsaturated Rh(I) species. It is also of
note that tertiary alkyl phosphines are used in these
complexes which render the metal center highly electron
rich. Having recently examined the enhanced activity
in systems where the nucleophilic carbenes can replace
bulky tertiary phosphines and in view of the important
catalytic behavior of RhL2Cl species, we became inter-
ested in examining rhodium complexes of this type
bearing nucleophilic carbenes. We now report our initial
synthetic and reactivity studies in this area.
† E-mail: snolan@uno.edu.
(1) (a) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987. (b) Parshall, G. W.;
Ittel, S. Homogeneous Catalysis; J . Wiley and Sons: New York, 1992.
(c) Pignolet, L. H., Ed. Homogeneous Catalysis with Metal Phosphine
Complexes; Plenum: New York, 1983.
(2) (a) Herrmann, W A.; Goosen, L. J .; Spiegler, M. J . Organomet.
Chem. 1997, 547, 357-366. (b) For
a comprehensive review see:
Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. Engl. 1997, 36,
2163-2187.
Resu lts a n d Discu ssion
(3) McGuiness, D. S.; Green, M. J .; Cavell, K. J .; Skelton, B. W.;
White, A. H. J . Organomet. Chem. 1998, 565, 165-178.
(4) (a) For Suzuki coupling see: Zhang, C.; Huang, J .; Trudell, M.
L.; Nolan, S. P. J . Org. Chem. 1999, 64, 3804-3805. (b) For Kumada
coupling see: Huang, J .; Nolan, S. P. J . Am. Chem. Soc. 1999, 121,
9889-9890.
When 4 equiv of IMes are reacted with 1 equiv of [Rh-
(COE)2Cl]2 (1) in THF at room temperature, an orange
(8) (a) Hun, K.; Rosini, G. P.; Goldman, A. S.; Nolan, S. P. J . Am.
Chem. Soc. 1995, 117, 5082-5088, and references cited. (b) Hun, K.;
Goldman, A. S.; Li, C.; Nolan, S. P. Organometallics 1995, 14, 4010-
4013. (c) Hun, K.; Emge, T. J .; Goldman, A. S.; Li, C.; Nolan, S. P.
Organometallics 1995, 14, 4929-4936. (d) Liu, F.; Pak, E. B.; Singh,
B.; J ensen, C. M.; Goldman, A. S. J . Am. Chem. Soc. 1999, 121, 4086-
4087, and references therein.
(5) Huang, J .; Grasa, G.; Nolan, S. P. Org. Lett. 1999, 1, 1307-1309.
(6) (a) Huang, J .; Stevens, E. D.; Nolan, S. P.; Petersen, J . L. J . Am.
Chem. Soc. 1999, 121, 2674-2678. (b) Scholl, M.; Trnka, T. M.; Morgan,
J . T.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247-2250.
(7) Huang, J .; Schanz, H.-J .; Stevens, E. D.; Nolan, S. P. Organo-
metallics 1999, 18, 2370-2375.
10.1021/om990922e CCC: $19.00 © 2000 American Chemical Society
Publication on Web 02/16/2000

Notes
Organometallics, Vol. 19, No. 6, 2000 1195
Sch em e 1
solution rapidly develops. The microcrystalline product
1
is isolated in high yield after a simple workup. The H
NMR spectrum of the complex was suprisingly complex
1
and showed no coordinated COE moiety present. A H
signal at -27 ppm indicated the presence of a Rh-H
moiety in the new complex. The spectroscopy also
indicated nonequivalent IMes ligands bound to the
rhodium center. To unequivocally establish the struc-
ture of the complex, a single-crystal diffraction was
performed on a crystal grown by slow diffusion of hexane
over a saturated THF solution of 2. The X-ray structure
shows that 2 is actually an orthometalated complex
resulting from C-H bond activation of an internal aryl
methyl C-H bond of one of the IMes ligands, producing
a five-coordinate Rh(III) alkyl hydride complex. (Scheme
1 and Figure 1). This was somewhat unexpected since
Herrmann and co-workers9 had recently reported on a
related ligand substitution occurring from the [Rh-
(COD)Cl]2 complex [COD ) cyclooctadiene] resulting in
[CODRhLCl] products[L ) nucleophilic carbene]. The
COE ligand is a more weakly coordinating ligand than
COD, and the realtive ease of COE dissociation may
prove the key in accessing RhL2Cl complexes. The
F igu r e 1. ORTEP of Rh(IMes)(H)(IMes′)Cl (2) with el-
lipsoids drawn at 50% probability. Selected bond distances
and angles: Rh(1)-C(33), 2.0242(17); Rh(1)-C(10), 2.0480-
(17); Rh(1)-C(40), 2.0793(18); Rh(1)-Cl(1), 2.4363(5); Rh-
(1)-H(1), 1.455(17); C(33)-Rh(1)-C(10), 172.33(7); C(33)-
Rh(1)-C(40), 81.21(7); C(10)-Rh(1)-C(40), 95.66(7); C(33)-
Rh(1)-Cl(1), 93.05(5); C(10)-Rh(1)-Cl(1), 91.13(5); C(40)-
Rh(1)-Cl(1), 169.07(5); C(33)-Rh(1)-H(1), 87.4(7); C(10)-
Rh(1)-H(1), 85.3(7); C(40)-Rh(1)-H(1), 83.5(7); Cl(1)-
Rh(1)-H(1), 105.6(7).
structure of 2 shows the complex to be a slightly
distorted trigonal bipyramid with IMes moieties trans
to one another (172.33(7)°). One of the IMes aryl methyl
(9) (a) Herrmann, W A.; Goosen, L. J .; Spiegler, M. Organometallics
1998, 17, 2162-2168. (b) Herrmann, W. A.; Elison, M.; Fischer, J .;
Kocher, C.; Ofele, K., US Patent 5,728,839, Mar 17, 1998.

1196 Organometallics, Vol. 19, No. 6, 2000
Notes
trigonal bipyramidal dihydride Rh(IMes)2(H2)Cl (3). The
orthometalated complex also reacts rapidly with CO to
effectively reverse the orthometalation and produce a
four-coordinate, square planar Rh(IMes)2(CO)Cl (4). The
carbonyl stretching frequency of 4 shows the IMes
ligand to be more electron donating than the phosphine
in Rh(PiPr3)2(CO)Cl. The mechanism by which the
orthometalation is reversed as well as the catalytic
activity of these complexes is presently being investi-
gated in our laboratories.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations involving
organorhodium complexes were performed under inert atmo-
spheres of argon or nitrogen using standard high-vacuum or
Schlenk tube techniques or in an argon-filled MBraun glovebox
containing less than 1 ppm oxygen and water. Solvents
including deuterated solvents for NMR analysis were dried
and distilled under nitrogen before use employing standard
drying agents.12 For example, tetrahydrofuran was stored over
sodium wire, distilled from sodium benzophenone ketyl, stored
over Na/K alloy, and vacuum transferred into flame-dried
glassware prior to use. 1,3-Bis(2,4,6-trimethylphenyl)imidazol-
F igu r e 2. ORTEP of Rh(IMes)2(H)2Cl (3) with ellipsoids
drawn at 50% probability. Selected bond distances and
angles: Rh(1)-C(10), 2.0183(18); Rh(1)-C(31), 2.0216(18);
Rh(1)-Cl(1), 2.4185(6); Rh(1)-H(1), 1.44(2); Rh(1)-H(2),
1.647(17); C(10)-Rh(1)-C(31), 177.24(8); C(10)-Rh(1)-Cl-
(1), 89.93(5); C(31)-Rh(1)-Cl(1), 92.68(5); C(10)-Rh(1)-
H(1), 90.7(8); C(31)-Rh(1)-H(1), 86.7(8); Cl(1)-Rh(1)-
H(1), 143.5(8); C(10)-Rh(1)-H(2), 90.8(5); C(31)-Rh(1)-
H(2), 86.8(5); Cl(1)-Rh(1)-H(2), 169.4(6); H(1)-Rh(1)-
H(2), 47.1(8).
2-ylidene (IMes)13 and [Rh(COE)2Cl]2 were synthesized ac-
14
cording to literature procedures. H2 and CO were purchased
from Matheson and purified prior to use (passed through
columns of molecular sieves/manganese oxide). NMR spectra
were recorded using a Oxford-400 MHz spectrometer. Infrared
spectra were recorded on a Perkin-Elmer 2000 FT-IR spec-
trometer. Elemental analyses were performed by Desert
Analysis, Tucson, AZ.
C-H bonds has been activated with a Rh-C bond
length of 2.0793(18) Å and a Rh-H bond distance10 of
1.455(17) Å. To minimize the steric congestion about the
metal center, the IMes ligands are staggered with the
imidazole rings displaying a dihedral angle of 49.19(7)°.
Reacting a THF solution of 2 with 1 atm of H2 at room
temperature leads to the rapid formation of the dihy-
dride complex 3. The complex was successfully crystal-
lized from THF/hexane, and an ORTEP of 3 is presented
in Figure 2.
The Rh(IMes)2(H)2Cl (3) complex also adopts a trigo-
nal bipyramidal structure where the two IMes ligands
are now 177.24(8)° apart. A more nearly trans arrange-
ment of the IMes ligands results from the formal release
of the ortho position by Rh in 2. A dihedral angle
between the two imidazole rings of 47.43(6)° is found
in 3. Two Rh-H bonds are present in the structure:
(Rh(1)-H(1), 1.44(2) Å, and Rh(1)-H(2), 1.647(17) Å).11
Reaction involving 2 and CO affords Rh(IMes)2(CO)-
Cl. The carbonyl stretching frequency of 2 (νCO ) 1935
cm-1) clearly indicates the increased electron density
provided by IMes compared to the PiPr3 (νCO ) 1943.2
cm-1).8a This trend is similar to the one observed in our
initial thermochemical studies on Cp*Ru(L)Cl com-
plexes where the IMes ligand displays enthalpies of
reaction on the order of 6 kcal/mol more exothermic than
PiPr3.6a
Syn th esis. Experimental procedures, leading to isolation
of previously unreported complexes, are described below.
Rh (IMes)(H)(IMes′)Cl (2). A 100 mL flask was charged
with 0.995 g (1.387 mmol) of [Rh(COE)2Cl]2, 1.700 g (5.547
mmol) of IMes, and 50 mL of dry THF in the glovebox. The
flask was taken out of the box and connected to a Schlenk line.
The clear orange solution was stirred at room temperature for
60 min, after which the solvent was removed under vacuum.
The residue was washed with pentane (2 × 10 mL), filtered,
and dried under vacuum, which afforded the yellow powder.
Yield: 1.702 g, 82%. 1H NMR (400 MHz, THF-d8) δ: -27.30
(br, Rh-H), 2.10 (s, 18 H, Mes-2, 6-CH3), 2.20 (s, 3 H, Mes-
2,6-CH3), 2.24 (s, 3 H, Mes-4-CH3), 2.36 (s, 9 H, Mes-4-CH3),
2.41 (s, 2 H, Rh-CH2), 6.42 (s, 1 H, NCHCHN of IMes’), 6.66
(s, 1 H, NCHCHN of IMes′), 6.76 (s, 2 H, NCHCHN of IMes),
6.84 (s, 4 H, Mes-3, 5-H), 6.95 (s, 2 H, Mes-3, 5-H), 6.87 (d, 1
H, J ) 2 Hz, Mes′-3(5)-H), 7.42 (d, J ) 2 Hz, Mes′-3(5)-H) ppm.
Anal. Calcd for C42H48ClN4Rh: C, 67.51; H, 6.47; N, 7.50.
Found: C, 67.69; H, 6.34; N, 7.14.
Rh (IMes)2(H)2Cl (3). A 50 mL flask was charged with 0.202
g (0.270 mmol) of Rh(IMes)(H)(IMes′)Cl and 20 mL of dry THF
in the glovebox. The flask was taken out of the box and
connected to a Schlenk line. The clear yellow solution was
purged with H2 at room temperature for 60 min, after which
the solvent was removed under vacuum. The residue was
washed with pentane (2 × 10 mL), filtered, and dried under
vacuum, which afforded the light yellow powder. Yield: 0.188
Su m m a r y
We have shown that the reaction of the IMes nucleo-
philic carbene with [Rh(COE)2Cl]2 forms an orthometa-
lated complex 2. This complex is very reactive and in
the presence of H2 rapidly forms the five-coordinate
(12) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: New York, 1988.
(13) (a) Arduengo, A. J ., III; Harlow, R. L.; Kline, M. J . J . Am. Chem.
Soc. 1991, 113, 361-363. (b) Arduengo, A. J ., III; Gamper, S. F.;
Calabrese, J . C.; Davidson, F. J . J . Am. Chem. Soc. 1994, 116, 4391-
4393. (c) Adducts of a related carbene ligand with group II and XII
metallocenes have been reported: Arduengo, A. J ., III; Davidson, F.;
Kraftczyk, R.; Marshall, W. J .; Tamm, M. Organometallics 1998, 17,
3375-3382. (d) Arduengo, A. J ., III. Acc. Chem. Res. 1999, 32, 913-
921.
(10) All hydrogens contained in complex
2 were located on a
difference Fourier map and refined in full-matrix least-square refine-
ments.
(11) All hydrogens contained in complex
3 were located on a
difference Fourier map and refined in full-matrix least- square
refinements.
(14) Werner, H.; Wolf, J .; Hohn, A. J . Organomet. Chem. 1985, 287,
395-407.

Notes
Organometallics, Vol. 19, No. 6, 2000 1197
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t for 2 a n d 3
2
3
empirical formula
fw
C
42H48ClN4Rh
C42H50ClN4Rh
749.22
747.23
wavelength (Å)
cryst syst
space group
unit cell dimens
0.71073
monoclinic
P21/n
a ) 16.1160(6) Å
b ) 12.2405(5) Å
c ) 20.8719(8) Å
R ) 90°, â ) 103.1°, γ ) 90°
4010.4(3)
0.71073
Orthorhombic
Pbca
a ) 16.935(2) Å
b ) 19.226(3) Å
c ) 23.173(3) Å
R ) 90°, â ) 90°, γ ) 90°
7545.1(18)
volume (Å3)
Z
4
8
density (g/cm3)
abs coeff
1.302
1.319
0.528 mm-1
0.558 mm-1
F(000)
1644
3136
crysta size (mm3)
θ range for data collection
index ranges
0.10 × 0.20 × 0.30
0.15 × 0.45 × 0.50
1.81-30.00°
2.12-27.50°
-22 e h e 22, -17 e k e 17,
-29 e l e 29
-22 e h e 17, -24 e k e 24,
-29 e l e 30
59 394
no. of reflns collected
no. of ind reflns
no. of data/restraints/params
goodness-of-fit on F2
final R indices
66 596
11 697 [R(int) ) 0.0665]
11697/36/674
8661 [R(int) ) 0.0542]
8661/434/633
0.690
R1 ) 0.0239, wR2 ) 0.0364
R1 ) 0.0640, wR2 ) 0.0378
0.829
R1 ) 0.0291, wR2 ) 0.0544
R1 ) 0.0647, wR2 ) 0.0585
R indices (all data)
g, 93%. 1H NMR (400 MHz, THF-d8) δ: -23.04 (d, J ) 34.8
Hz, 2 H, Rh-H), 1.86 (s, 24 H, Mes-2,6-CH3), 2.42 (s, 12 H,
Mes-4-CH3), 6.765 (d, 8 H, Mes-3, 5-H), 6.86 (s, 4 H, NCHCHN)
ppm. Anal. Calcd for C42H50ClN4Rh: C, 67.33; H, 6.73; N, 7.48.
Found: C, 67.05; H, 6.57; N, 7.10.
Rh (IMes)2(CO)Cl (4). A 50 mL flask was charged with
0.1935 g (0.259 mmol) of Rh(IMes)(H)(IMes′)Cl and 10 mL of
dry THF in the glovebox. The flask was taken out of the box
and connected to a Schlenk line. The clear yellow solution was
purged with CO at room temperature for 60 min, after which
the solvent was removed under vacuum. The residue was
washed with pentane (2 × 10 mL), filtered, and dried under
vacuum, which afforded the off-white powder. Yield: 0.173 g,
86%. 1H NMR (400 MHz, THF-d8) δ: 1.81, 1.93 (both br, 24
H, Mes-2,6-CH3), 2.43 (s, 12 H, Mes-4-CH3), 6.84 (d, 8 H, Mes-
3, 5-H), 7.00 (s, 4 H, NCHCHN) ppm. IR (CH2Cl2, ν): 1935
cm-1. Anal. Calcd for C43H48ClN4ORh: C, 66.62; H, 6.24; N,
7.23. Found: C, 66.45; H, 6.22; N, 7.48.
temperature using graphite-monochromated Mo KR radiation
on a Bruker diffractometer fitted with a CCD detector. The
structure was solved using direct methods (SHELXS-86) and
refined by full-matrix least-squares techniques. Initial frac-
tional coordinates for the Rh atom were determined by heavy-
atom methods, and the remaining non-hydrogen atoms were
located by successive difference Fourier calculations, which
were performed with algorithms provided by SHELXTL IRIS
operating on a Silicon Graphics IRIS Indigo workstation.
Crystallographic data can be found in Table 1.
Ack n ow led gm en t. S.P.N. acknowledges the Na-
tional Science Foundation (Grant No. CHE-9985213),
and DuPont (Grant in Aid) for partial support of this
work.
Su p p or tin g In for m a tion Ava ila ble: Details of crystal
structure determinations for 2 and 3 (PDF) are available. This
material is available free of charge via the Internet at
http://pubs.acs.org.
X-r a y Diffr a ction Mea su r em en ts. A single crystal of 2
or 3 [grown by slow diffusion of hexane through a saturated
THF solution of 2 or by cooling a saturated THF/hexane
solution of 3] was coated with paratone oil and then sealed in
a glass capillary tube. The X-ray data were collected at low
OM990922E