N-heterocyclic carbene rhodium complexes and their reactions with H2 andwith co

Article abstract of DOI:10.1002/ejic.200801169

The NHC-RhIcomplexes [RhCl(COE)(NHC)] 2 1 and 2 [COE= cyclooctene, NHC in 1 = N,N-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) and, in 2, N,N-bis(2,4,6-trimeth-ylphenyl)imidazol-2-ylidene (IMes)] react with H2 in hexaneto give the dimeric, mono-carbene dihydrido species [Rh(H)2-Cl(NHC)] 2 (NHC = IPr (3), IMes (4)). In the presence offurther NHC, the bis-carbene dihydrido species Rh(H)2Cl-(NHC)2 are formed; a crystal structure of the IPr complex 5is analogous to that of the known IMes analogue. The di-hydride-mixed-carbene species Rh(H)2Cl(IPr)(IMes) (5a) wasalso observed but not isolated. A benzene solution of 5 underD2 slowly generates the corresponding dideuteride. Reactions of the mono-carbenes (1/3, or 2/4) with CO in hexaneafford the respective dicarbonyl complexes RhCl(CO)2(NHC)[NHC = IPr (6), IMes (7)] , while CO reactions with the bis-carbene dihydrides give, respectively, the mono-carbonylcomplex RhCl(CO)(IPr)2 (8) and the known IMes analogue. All the complexes are characterized by elemental analysis, 1H and 13C{1H} NMR and IR spectroscopies and, in the caseof 5, by X-ray crystallography. The catalytic activities of 5and the previously reported Rh(H)2Cl(IMes)2 for hydrogenation of COE and 1-octene (and isomerization of the latter) areshowntobepoor. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009.

Full text of DOI:10.1002/ejic.200801169

FULL PAPER
DOI: 10.1002/ejic.200801169
N-Heterocyclic Carbene Rhodium Complexes and Their Reactions with H2 and
with CO
Xiao-Yan Yu,[a] Hongsui Sun,[a] Brian O. Patrick,[a] and Brian R. James*[a]
Keywords: Carbene ligands / Carbonyl ligands / Rhodium / Hydrogenation / Hydrides
The NHC-RhI complexes [RhCl(COE)(NHC)]2 1 and 2 [COE
= cyclooctene, NHC in 1 = N,N-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene (IPr) and, in 2, N,N-bis(2,4,6-trimeth-
ylphenyl)imidazol-2-ylidene (IMes)] react with H2 in hexane
to give the dimeric, mono-carbene dihydrido species [Rh(H)2-
Cl(NHC)]2 (NHC = IPr (3), IMes (4)). In the presence of
further NHC, the bis-carbene dihydrido species Rh(H)2Cl-
(NHC)2 are formed; a crystal structure of the IPr complex 5
is analogous to that of the known IMes analogue. The di-
hydride-mixed-carbene species Rh(H)2Cl(IPr)(IMes) (5a) was
also observed but not isolated. A benzene solution of 5 under
D2 slowly generates the corresponding dideuteride. Reac-
tions of the mono-carbenes (1/3, or 2/4) with CO in hexane
afford the respective dicarbonyl complexes RhCl(CO)2(NHC)
[NHC = IPr (6), IMes (7)], while CO reactions with the bis-
carbene dihydrides give, respectively, the mono-carbonyl
complex RhCl(CO)(IPr)2 (8) and the known IMes analogue.
All the complexes are characterized by elemental analysis,
1H and 13C{1H} NMR and IR spectroscopies and, in the case
of 5, by X-ray crystallography. The catalytic activities of 5
and the previously reported Rh(H)2Cl(IMes)2 for hydrogena-
tion of COE and 1-octene (and isomerization of the latter) are
shown to be poor.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
tems with those of classic Rh-tertiary phosphane sys-
tems.[17] There is literature on reactions of H2 and CO, and
related catalysis, within NHC-RhI systems,[5,9,13,18–21] but
not specifically with the COE/IPr/IMes precursors, and our
studies have led to identification of new species, as well as
some findings on catalytic hydrogenation and isomerization
of COE and 1-octene. Our findings on the O2 and H2 reac-
tions were first presented at a Canadian Chemistry confer-
ence.[22] During the writing of this paper, we heard that
Crudden’s group had also been studying reactivity of H2,
CO, and N2, toward the same NHC-RhI species, and their
work (with necessarily some overlap) is reported in this
journal issue.[23]
Introduction
Interest in NHC-Rh complexes (NHC = N-heterocyclic
carbene) shows no signs of abatement, as can judged by the
contents of this special issue of the Eur. J. Inorg. Chem., and
other recent publications[1–12] and the references contained
therein. Our initial studies in metal-carbene chemistry[13]
developed from attempts to replace oxidizable tertiary
phosphane ligands in a Ru system that showed catalytic ac-
tivity for conversion of N2O to N2 and O2.[14] We later re-
ported that (carbene-phosphanyl)-RhI species undergo
“standard” oxidative addition of O2 to give RhIII-peroxide
species,[15] while Crudden and co-workers have recently sug-
gested that such species are best considered as RhI with
coordinated singlet oxygen.[16] Our studies[13–15] have thus
far mainly utilized the simple, unfunctionalized NHCs,
N,N-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and
N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene
(IMes)
(see Scheme 1), in syntheses of the RhCl(diene)(NHC) and
[RhCl(COE)(NHC)]2 complexes, and reactivities of these
complexes with O2 (diene = NBD or COD, and COE =
cyclooctene). Because of our interests in catalytic hydrogen-
ation/hydroformylation, we then studied reactions of the
[RhCl(COE)(NHC)]2 complexes with H2 and with CO, with
an ultimate goal of comparing catalytic activity of such sys-
[a] Department of Chemistry, The University of British Columbia,
Vancouver, BC, V6T 1Z1, Canada
Fax: +1-604-822-2847
E-mail: brj@chem.ubc.ca
Scheme 1. Reactions of 1 and 2 with H2.
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N-Heterocyclic Carbene Rhodium Complexes
We were unable to obtain any useful data for 3 or 4 from
either MALDI-TOF- or ESI-mass spectrometry investi-
gations, and thus we have no direct evidence for their di-
Results and Discussion
Reactions Involving H2
Reactions of the yellow complexes [RhCl(COE)(NHC)]2 meric nature. The empirical formulation Rh(H)2Cl(NHC),
(1, NHC = IPr; 2, IMes) with H2 at ambient conditions were implying a four-coordinate RhIII complex, certainly re-
1
initially studied in C6D6, where in-situ H NMR spectro- minded us of Nolan’s work on related Rh-NHC species
scopic data showed the formation of cyclooctane (COA, δ formed by steps involving double C–H activation of the
= 1.50 ppm, br. s) and new Rh-hydride species indicated by ItBu ligand [ItBu = N,N-bis(tert-butyl)imidazol-2-ylidene],
a high-field doublet resonance at δ = –22.78 ppm, J = 27 Hz loss of H2, and chloride removal,[19] but the chemistry in
(from 1) or a broad singlet at δ –21.61 ppm (from 2), conver- the syntheses of 3 and 4 leads us tentatively to the dimeric
sion being complete in ca. 1 h. Carrying out the reactions in formulation (as assumed also for the RhI-precursors
hexane, where 1 and 2 are present as suspensions, allowed [RhCl(COE)(NHC)]2[15,19]).
for isolation of the respective hydride products [Rh(H)2-
The monomeric, dihydrido-bis(NHC) complex Rh(H)2-
Cl(IPr)]2 (3) and [Rh(H)2Cl(IMes)]2 (4) in ca. 75% yield Cl(IMes)2 (5Ј) is a known, crystallographically charac-
(Scheme 1), which were both obtained as yellow solids con- terized compound, and was synthesized previously from the
taining 0.5 molecule COA as solvate. The complexes 3 and room temperature reaction (in THF) of H2 with Rh(H)Cl-
4 were characterized by elemental analysis, and NMR and (IMesЈ)(IMes),[18] the cyclo-metallated complex noted
IR spectroscopies. For 3, as well as the high-field doublet res- above. In this work, we synthesized the analogous Rh(H)2-
1
onance for the equivalent hydrides, the H NMR spectrum Cl(IPr)2 complex (5) in high yield by the room temperature
(in C6D6) shows a δ = 6.43 ppm singlet for the NCH proton, reaction of H2 with a suspension of 1 in hexane in the pres-
a septet signal at δ = 2.92 ppm for the IPr-methine protons, ence of 1 equiv. of IPr per Rh (Scheme 2). In situ 1H NMR
and doublets at δ = 1.34 and 1.02 ppm for two types of spectroscopic data on the initially formed clear yellow solu-
Me-groups; the signal integrations and J values support the tion revealed complete conversion to 5 (and the COE of 1
formulation, as well the 13C{1H} data, which include a to COA), and cooling to 0 °C yielded the light yellow crys-
doublet for the carbene carbon at δ = 182.7 ppm (JRhC
=
talline 5, which was fully characterized by elemental analy-
58 Hz), the normal range for carbene-Rh complexes.[13,24] sis, IR (νRhH) and NMR spectroscopy, and X-ray analysis.
1
The hydride resonances are also in the range reported for The H NMR spectrum in C6D6 shows resonances for two
the hydride ligands of Rh(H)Cl(IMesЈ)(IMes), where IMesЈ equivalent IPr ligands and a doublet at δ = –22.90 ppm
is the cyclometallated carbene formed from a methyl group (JRhH = 33 Hz) for two equivalent hydrides; the 13C{1H}
via intramolecular C–H activation,[15,18] and of Rh(H)2Cl- NMR doublet for the carbene-carbon (δ = 194.5 ppm, JRhC
(IMes)2 (see below).[18] For 4, the 1H NMR spectrum shows = 36 Hz) is ca. 12 ppm downfield from that of the di-
also singlets (of correct integrations) at δ = 6.11 ppm for the hydrido-monocarbene complex 3. The high-field hydride 1H
NCH proton, and at δ = 2.18 and 2.13 ppm, respectively, for NMR signal slowly disappeared when the C6D6 solution of
the p-Me and o-Me groups of the IMes ligands. The expected 5 was frozen, the NMR-tube evacuated, then subjected to
carbene–carbon resonance of 4 is surprisingly absent in the a D2 atmosphere, and warmed to room temperature; over
13C{1H} NMR, although the same absence has been noted 24 h with intermittent shaking of the NMR-tube, the corre-
for 2, the precursor complex.[15] This is not understood but sponding dideuteride 5-D2 was formed (δD = –22.96 ppm,
1
the signal is presumably broadened into the base-line by br. s), with H NMR spectroscopic data of the IPr ligands
some exchange process, and reflects in some way the rela- being the same as those of 5. The reverse exchange was
tively labile and weak metal–carbene bond in this system also demonstrated. The observations establish the reversible
(formation of 4 from 2 is not reversed on subjecting a solu- nature of H2/D2 binding possibly to a “RhCl(IPr)2” species,
tion to vacuum). Of note, rapid exchange of N-heterocyclic although reactivity via η2-H2/D2 intermediates cannot be
ligands in various Ag systems, including between dinuclear ruled out. In this regard, it should be noted that in the
Ag2 and tetranuclear Ag4 species, is well known.[25]
conversion of 5 to 5-D2, the disappearing δ = –22.90 ppm
Complexes 3 and 4 have νRhH IR bands at 2124 and hydride doublet of 5 was accompanied by formation of a
2103 cm–1, respectively; these are in the range seen for new doublet at δ = –22.96 ppm with the same J value of
(hydride-tertiary phosphane)-RhIII species,[26] and so useful 33 Hz, and this presumably pertains to some isomeric form
comparisons of electronic properties of NHC vs. PR3 li- of 5; both doublets slowly disappeared as the dideuteride
gands cannot be made from these particular IR data. Data formed. More detailed studies of this conversion are on-
on νRhH IR bands do not appear to have been noted pre-
viously for NHC-containing species. More careful IR stud-
ies with measurements in the same media (solid state/solu-
tion) for NHC vs. corresponding PR3 complexes could con-
tribute to a better understanding of the electronic properties
of the two ligands, which remain debatable, particularly the
location of electron density in their complexes with transi-
tion metals.[27] Corresponding data on someνCO values are
discussed below.
Scheme 2. Formation of Rh(H)2Cl(IPr)2 (5).
Eur. J. Inorg. Chem. 2009, 1752–1758
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X.-Y. Yu, H. Sun, B. O. Patrick, B. R. James
FULL PAPER
going. There is no evidence for C–H activation of an isopro-
pyl methyl group to give a hydrido-alkyl species, similar to
that seen for the analogous “RhCl(IMes)2” species.[15,18]
The IMes analogue of 5 (the known complex 5Ј[18]) was also
synthesized in high yield by the procedure used for synthe-
sis of 5.
X-ray quality crystals of 5 were obtained by slow evapo-
ration of a saturated hexane solution of the complex, the
collected crystals being washed with Et2O and dried in
vacuo. The structure reveals a hexane/Et2O-solvated, “sup-
posed” trigonal bipyramidal, five-coordinate dihydride
complex (Figure 1), analogous to that of the unsolvated
Rh(H)2Cl(IMes)2 complex 5Ј, where the hydride ligands
were located.[18] In our case, the Rh–H hydrides were not
located even though residual electron density around the
Rh was evident. The two IPr ligands are mutually trans
[177.1(2)°] and the chloride is almost perpendicular to the
IPr ligands [91.43(12)°]; the diisopropylphenyl substituents
are twisted by an average of 81.97° with respect to the cen-
tral imidazole ring. The Rh–C and Rh–Cl bond lengths of
2.029(4) and 2.4083(14) Å are close to those reported for
other (C2-carbene)-RhIII complexes (e.g. ref.[2,4,5,9,18,28]).
Crudden and co-workers have synthesized complex 5 from
the nitrogen complex trans-RhCl(N2)(IPr)2, and have also
obtained a crystal structure for 5 which co-crystallized with
the RhII complex RhCl2(IPr)2; their determined crystallo-
graphic geometrical parameters for a non-solvated crystal
of 5 are close to our values, and they were able also to
locate the hydrides (Rh–H = 1.45 Å).[23]
Figure 1. ORTEP diagram of Rh(H)2Cl(IPr)2 (5) with 50% prob-
ability thermal ellipsoids. Selected bond lengths [Å] and angles [°]
with estimated standard deviations in parentheses: Rh(1)–C(1)
2.029(4), Rh(1)–Cl(1) 2.4083(14); C(1)–Rh(1)–C(1*) 177.1(2),
C(1)–Rh(1)–Cl(1) 91.43(12).
far thwarted attempts to separate the materials. Not sur-
prisingly, the same chemistry results by reaction of H2 with
[RhCl(COE)(IMes)]2 (2) in the presence of one equivalent
of IPr.
Of note, 5 also reacts rapidly in benzene with O2 to form
the dioxygen species RhCl(O2)(IPr)2, which has been iso-
lated by another route and purported by X-ray (O–O =
1.315 Å), IR, Raman, and XAS data, to be a (singlet oxy-
gen)-RhI complex.[16] We have also structurally charac-
terized the oxygen complex and find an O–O bond length of
1.393 Å,[29] approaching the length for a standard peroxide
complex.[15] The nature of this important “discrepancy” is
currently under investigation via collaboration with
Crudden and co-workers.
In attempts to make hydrido, mixed-NHC complexes, re-
actions of [RhCl(COE)(IPr)]2 (1) with 1 equiv. of IMes per
Rh were examined. In C6D6 at ambient conditions under
H2, an in situ mixture of a new mixed-carbene complex
Rh(H)2Cl(IPr)(IMes) (5a), Rh(H)2Cl(IPr)2 (5), and the
known Rh(H)2Cl(IMes)2 (5Ј),[18] was obtained (Scheme 3),
as judged by the three, respective, high-field doublets at δ
–23.21 (JRhH = 36 Hz), –22.90 (JRhH = 33 Hz), and
–22.78 ppm (JRhH = 35 Hz). On the basis of data for the
1
isolated 5 and 5Ј, the other H resonances of the in-situ
mixture fit well data for the three Rh species. The integra-
tions of the three hydride signals changed slowly with time,
indicating equilibria among the species, and after ca. 6 h,
Reactions Involving CO
Reactions of 1 or 3 in hexane solution with 1 atm of CO
the final ratio of the three products was close to 1:1:1. A at room temperature generated cis-RhCl(CO)2(IPr) (6), and
synthetic scale reaction in hexane led to isolation of the the IMes analogue 7 was formed from the corresponding
hexane-soluble 5, and a mixture of 5a and 5Ј in 25% yield, CO reactions with 2 or 4; use of the hydrido species 3 and
1
from which the H NMR signals for 5a were assigned (see 4 involved loss of H2, which was detectable in the solution
Experimental); similar solubilities of 5a and 5Ј have thus at δH = 4.18 ppm. The CO reaction with 5 gave the biscar-
Scheme 3. Formation of the mixed NHC complex Rh(H)2Cl(IPr)(IMes) (5a); the correct stoichiometry requires a factor of 2/3 for each
product.
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N-Heterocyclic Carbene Rhodium Complexes
bene complex trans-RhCl(CO)(IPr)2 (8) (Scheme 4). The tertiary phosphanes) result in higher electron density at a
CO reactions were instantaneous, and the products were RhI centre.[5] Contrary to this generally accepted behaviour
isolated in almost quantitative yields from the hexane solu- of the two types of ligands, recent K-edge XAS spectra sug-
tions as white solids. Complexes 6–8 are well characterized gest the opposite for some RuII systems.[27]
by elemental analyses, and NMR and IR spectroscopies. The
A crystal structure of complex 8 (synthesized from trans-
1H NMR spectroscopic data are straightforward, and the RhCl(N2)(IPr)2) is reported elsewhere in an article in this
13C{1H} data generally so. For 6, three 13C{1H} doublets journal issue.[23]
are seen at δ = 186.0 (JRh–C = 54 Hz), 184.2 (JRh–C = 72 Hz)
and 182.1 ppm (JRh–C = 45 Hz) for the one carbene-carbon
Catalytic Hydrogenations
and the two inequivalent carbonyls; signal assignment is not
obvious, because JRh–C values for the carbene carbon atom
are typically in the 35–65 Hz range (e.g.ref.[2,4–7,9,13,15]). Two
νCO bands at 2073 and 1990 cm–1 indicate efficient back-
bonding from RhI to the carbonyl ligands. Other cis-
RhCl(CO)2(NHC) complexes, made by reactions of CO
with RhCl(COD)(NHC), also have symmetric and asym-
metric νCO bands (KBr) in the 2088–2069 and 2002–
1994 cm–1 ranges, respectively.[5,30] For 7, the 13C{1H} spec-
Hydrogenation of olefinic substrates using Rh-NHC cat-
alysts, either with[9,33] or without[9,30] ancilliary phosphane
ligands, has been reported, but we were unaware of any
attempts at such catalysis using characterized Rh-hydrido-
NHC complexes as catalyst precursors. Thus, we tested via
a few preliminary experiments the Rh(H)2Cl(NHC)2 com-
plexes 5 (NHC = IPr) and 5Ј (NHC = IMes) for catalytic
hydrogenation of cyclooctene and 1-octene at ambient con-
ditions, and compared data with those we obtained also
using the well-known RhCl(PPh3)3 system.[34] However, the
Rh-NHC compounds are not useful catalysts. For example,
in benzene under conditions given in the Experimental sec-
tion, conversions of cyclooctene over 24 h even at 45 °C
reached only about 15%, and at this temperature the solu-
tions darkened likely due to formation of colloidal metal
that eventually precipitated; RhCl(PPh3)3 gave about three
times the conversion with no visible signs of decomposition.
Much faster consumption of 1-octene was seen (90% in 5 h
at room temperature using 5 as catalyst, and in 3 h using
5Ј) but the major reaction was isomerization to cis- and
mainly trans-2-octene, followed by slower hydrogenation of
the cis- and trans-isomers, a well known phenomenon;[35]
with RhCl(PPh3)3 hydrogenation was complete in ca. 3 h,
with only trace isomerization observed. Under Ar at room
temperature, complexes 5 and 5Ј do catalyze the isomeriza-
tion but more slowly (ca. 30–40% after 5 h). Our findings
generally parallel closely more detailed studies reported by
Herrmann et al.,[9] who tested RhCl(COD)(NHC) and
[Rh(COD)(NHC)2]+ in toluene for catalytic hydrogenation
of cyclohexene and 1-octene; in these systems, addition of
PPh3 was needed to generate effective catalysts. Nolan’s
group has reported low activity for RhCl(IMes)(PPh3)2 for
hydrogenation of cyclohexene,[20] but this study was carried
out in CH2Cl2 not generally a favoured solvent because hy-
drogen transfer to the solvent can occur with liberation of
HCl.[36] That 5 and 5Ј were synthesized under H2, and
showed no signs of decomposition under H2 up to 45 °C,
implies that under the catalytic conditions an olefin might
be replacing at least one of the NHC ligands, this leading
to destabilization of the RhI state and decomposition to
metal.
trum shows only one doublet at δ = 186.0 ppm (JRh-C
=
54 Hz) for the two carbonyl ligands, and one for the car-
bene-carbon at δ = 179.2 ppm (JRh-C = 45 Hz), although
two νCO IR bands at 2070 and 1990 cm–1 are consistent
with inequivalent carbonyls, at least in the solid-state struc-
ture. This phenomenon has been found in other carbene-
carbonyl-Rh complexes.[19] For the mono(carbonyl) com-
plex with two trans NHC ligands (8), two 13C{1H} doublets
at δ = 189.8 (JRh-C = 42 Hz) and 188.1 ppm (JRh-C = 79 Hz)
show the presence of equivalent IPr ligands and one car-
bonyl, and the single νCO band is seen at 1938 cm–1. A νCO
value of 1935 cm–1 in CH2Cl2 solution has been recorded
for the analogous RhCl(CO)(IMes)2 complex made from
the reaction of CO with the cyclometallated complex Rh(H-
)Cl(IMesЈ)(IMes),[18] but the paper does not report any
13C{1H} data. Reported νCO values (cm–1) for trans-
RhCl(CO)(PPh3)2 are 1983 (in KBr)[31] and 1980 (in
CHCl3),[32] while values for cis-RhCl(CO)2(PPh3), a sug-
gested reaction intermediate, are 2088 and 2002 cm–1 (in
hexadecane);[32] these phosphane species are the analogues
of the carbene complexes 8 (and the IMes analogue[18]), and
6/7, respectively. The IR data, even allowing for the dif-
ferent media, imply that PPh3 is a better π-acceptor than
IPr and IMes in these RhI complexes, and are consistent
with similar data from Herrmann’s group that carbenes (vs.
Conclusions
The carbene-RhI complexes [RhCl(COE)(NHC)]2, NHC
= IPr and IMes, are used to synthesize via reactions with
H2: the mono-carbene dihydride complexes [Rh(H)2–
Scheme 4. Reactions of complexes 1, 2 and 5 with CO; complexes
3 and 4 could be used, respectively, instead of 1 and 2.
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1
H 6.7, N 6.1. IR: ν = 2103 cm–1. H NMR (C D ): δ = 6.75 (s, 4
˜
Cl(NHC)]2, NHC = IPr (3) and IMes (4), the bis-carbene
dihydride Rh(H)2Cl(IPr)2 (5), which was characterized crys-
tallographically, and the known IMes analogue 5Ј. The
mixed dihydride, Rh(H)2Cl(IPr)(IMes) was also charac-
terized in situ. Complex 5 undergoes reversible exchange
with D2 to form the dideuteride. Complexes 5 and 5Ј are
not effective precursors for catalytic hydrogenation of cyclo-
octene and 1-octene. Reactions of CO with the [RhCl-
(COE)(NHC)]2 or [Rh(H)2Cl(NHC)]2 complexes yield the
dicarbonyl compounds RhCl(CO)2(NHC), while the mono-
carbonyl complex RhCl(CO)(IPr)2 is prepared by reaction
of CO with the dihydride 5. The νCO data for these RhI
complexes support the premise that PPh3 is a better π-ac-
ceptor than the NHC ligands.
6
6
H, m-ArH), 6.11 (s, 2 H, NCH), 2.18 (s, 6 H, p-CH3), 2.13 (s, 12
H, o-CH3), –21.61 (br. s, 2 H, Rh-H) ppm. 13C{1H} NMR (C6D6):
δ = 138.3 (s, NC), 137.7 (s, p-ArC), 136.6 (s, o-ArC), 129.5 (s, m-
ArCH), 121.9 (s, NCH), 21.8 (s, CH3), 19.3 (s, CH3) ppm.
Synthesis of Rh(H)2Cl(IPr)2 (5): A yellow suspension of 1 (64 mg,
0.050 mmol) and IPr (39 mg, 0.100 mmol) in hexane (10 mL) was
purged with H2, with stirring for 2 h at room temperature. The
clear yellow solution was then concentrated to one third of the
volume; cooling the solution at 0 °C afforded a light yellow crystal-
line solid that was collected and dried in vacuo. Yield 81 mg (88%).
C54H74ClN4Rh (916.46): calcd. C 70.71, H 8.14, N 6.11; found C
1
70.6, H 8.3, N 6.2. IR: ν = 2125 cm–1. H NMR (C D ): δ = 7.26
˜
6
6
(t, J = 7 Hz, 4 H, p-ArH), 7.05 (d, J = 7 Hz, 8 H, m-ArH), 6.44 (s,
4 H, NCH), 3.04 [sept, J = 7 Hz, 8 H, CH(CH3)2], 1.10 [d, J =
6 Hz, 24 H, CH(CH3)2], 1.00 [d, J = 6 Hz, 24 H, CH(CH3)2], –22.90
(d, J = 33 Hz, 2 H, Rh-H) ppm. 13C{1H} NMR (C6D6): δ = 194.5
(d, JRh-C = 36 Hz, NCN), 146.8 (s, NC), 138.2 (s, iPr-C), 129.6 (s,
p-CH), 124.4 (s, m-CH), 123.9 (s, NCH), 28.9 [s, CH(CH3)2], 26.3
(s, CH3), 23.8 (s, CH3) ppm.
Experimental Section
General: All manipulations were performed under Ar, using stan-
dard Schlenk or glove-box techniques. Reagent grade solvents
(Fisher Scientific) were dried using standard procedures, and prior
to use were purged with a stream of Ar. Deuterated solvents (Cam-
bridge Isotope Laboratories) were similarly dried, and then distilled
under N2 prior to use. Common chemicals were obtained from
Fisher Scientific, and were used as received. The [RhCl(COE)-
(NHC)]2 complexes 1 and 2 (NHC = IPr, IMes, respectively),[15]
and RhCl(PPh3)3,[37] were synthesized according to the reported
procedures.
Synthesis of Rh(H)2Cl(IPr)(IMes) (5a): A yellow suspension of 1
(64 mg, 0.050 mmol) and IMes (30 mg, 0.100 mmol) in hexane
(10 mL) was purged with H2, with stirring for 6 h at room tempera-
ture. The yellow suspension was then filtered through Celite and
the solid obtained was washed with hexane (5 mL) followed by cold
Et2O (3 mL). The hexane filtrate was vacuumed to dryness to give
5 in ca. 25% yield, and the collected yellow solid was the known
Rh(H)2Cl(IMes)2 (5Ј).[18] The cold ether filtrate was vacuumed to
dryness to give a light yellow mixture of 5a and 5Ј. Yield of mix-
ture: 81 mg (25%). Data for 5a: IR: ν = 2094 cm–1. 1H NMR
˜
NMR spectra were recorded at room temperature (ca. 20 °C) with
Bruker AV 300 (300.0 MHz for H and 75.0 MHz for 13C{1H}) or
1
(C6D6): δ = 7.27 (t, J = 7 Hz, 2 H, p-ArH-IPr), 7.06 (d, J = 7 Hz,
4 H, m-ArH-IPr), 6.62 (s, 4 H, m-ArH-IMes), 6.54 (s, 2 H, NCH-
IPr), 6.08 (s, 2 H, NCH-IMes), 3.04 [sept, J = 7 Hz, 4 H, CH-
(CH3)2], 2.29 (s, 6 H, p-CH3), 2.11 (s, 12 H, o-CH3), 1.28 [d, J =
7 Hz, 12 H, CH(CH3)2], 1.05 [d, J = 7 Hz, 12 H, CH(CH3)2], –23.21
(d, J = 36 Hz, 2 H, Rh-H) ppm.
Bruker AV 400 (400.0 MHz for 1H, 61 MHz for 2D, and 100.6 MHz
for 13C{1H}) spectrometers. Shifts are reported relative to external
TMS; a residual protonated species in C6D6 (δ = 7.15 ppm) was
used as the internal reference for 1H data, with J values given in
Hz (s = singlet, d = doublet, t = triplet, sept = septet, br. = broad).
IR spectra (KBr) were recorded with ATI Mattson Genesis or
Bomem–Michelson MB-100 FT-IR spectrometers. Elemental
analyses were performed by Mr. M. Lakha of this department on
a Carlo Erba EA 1108 analyzer. GC analyses for product yields of
the catalytic hydrogenations were performed on a Hewlett–Packard
HP 5890, equipped with an FID and an HP-17 capillary column
(with pre-column).
Synthesis of RhCl(CO)2(IPr) (6): A yellow suspension of 1 (64 mg,
0.050 mmol) in hexane (5 mL) was purged with CO, with stirring
for 1 h at room temperature, and then the mixture was concentrated
to half the volume. The precipitated white solid was collected and
dried in vacuo. Yield 56 mg (96%). C29H36ClN2O2Rh (582.15):
calcd. C 59.78, H 6.23, N 4.81; found C 60.1, H 6.4, N 4.6. IR: ν
˜
= 2073, 1990 cm–1. H NMR (C6D6): δ = 7.20 (t, J = 7 Hz, 2 H,
1
p-ArH), 7.15 (d, J = 7 Hz, 4 H, m-ArH), 6.63 (s, 2 H, NCH), 3.14
[sept, J = 7 Hz, 4 H, CH(CH3)2], 1.72 (d, J = 7 Hz, 12 H, CH3),
0.96 (d, J = 7 Hz, 12 H, CH3) ppm. 13C{1H} NMR (C6D6): δ =
186.0 (d, JRh-C = 54 Hz, CO ?), 184.2 (d, JRh-C = 72 Hz, CO), 182.1
(d, JRh-C = 45 Hz, NCN ?), 146.7 (s, NC), 135.9 (s, iPr-C), 131.1
(s, p-CH), 125.3 (s, m-CH), 124.7 (s, NCH), 29.5 [s, CH(CH3)2],
26.9 (s, CH3), 23.4 (s, CH3) ppm.
Synthesis of [Rh(H)2Cl(IPr)]2 (3): A yellow suspension of 1 (64 mg,
0.050 mmol) in hexane (5 mL) was purged with H2, with stirring
for 2 h at room temperature, and then the mixture reduced to half
the volume under vacuum. The resulting yellowish suspension was
refrigerated and then, while cold, filtered through Celite. The col-
lected solid was dried in vacuo at room temperature. Yield 41 mg
(74%). C54H76Cl2N4Rh2·0.5C8H16 (1112.42): calcd. C 62.57, H
1
7.60, N 5.03; found C 62.4, H 7.6, N 4.8. IR: ν = 2124 cm–1. H
˜
Synthesis of RhCl(CO)2(IMes) (7): This complex was prepared in
a manner analogous to that described for 6 but using 2 as precursor
(58 mg, 0.050 mmol). Yield 49 mg (98%). C23H24ClN2O2Rh
NMR (C6D6): δ = 7.23 (t, J = 7 Hz, 2 H, p-ArH), 7.07 (d, J =
6 Hz, 4 H, m-ArH), 6.43 (s, 2 H, NCH), 2.92 [sept, J = 7 Hz, 4 H,
CH(CH3)2], 1.34 [d, J = 5 Hz, 12 H, CH(CH3)2], 1.02 [d, J = 5 Hz,
12 H, CH(CH3)2], –22.78 (d, J = 27 Hz, 2 H, Rh-H) ppm. 13C{1H}
NMR (C6D6): δ = 182.7 (d, JRh-C = 58 Hz, NCN), 147.8 (s, NC),
137.5 (s, iPr-C), 131.2 (s, p-CH), 126.6 (s, m-CH), 124.4 (s, NCH),
33.1 [s, CH(CH3)2], 27.0 (s, CH3), 24.4 (s, CH3) ppm.
(498.06): calcd. C 55.42, H 4.86, N 5.62; found C 55.3, H 5.0, N
1
5.7. IR: ν = 2070, 1990 cm–1. H NMR (C D ): δ = 6.76 (s, 4 H,
˜
6
6
ArH), 6.13 (s, 2 H, NCH), 2.25 (s, 12 H, o-CH3), 2.05 (s, 6 H, p-
CH3) ppm. 13C{1H} NMR (C6D6): δ = 186.0 (d, JRh-C = 54 Hz,
CO), 179.2 (d, JRh-C = 45 Hz, NCN), 139.8 (s, NC), 136.1 (s, p-
ArC), 135.9 (s, o-ArC), 129.4 (s, m-CH), 123.7 (s, NCH), 23.4 (s,
p-CH3), 19.0 (s, o-CH3) ppm.
[Rh(H)2Cl(IMes)]2 (4): This complex was prepared in a manner
analogous to that described for 3, but using 2 as the precursor
(58 mg, 0.050 mmol). Yield 36 mg (76%). C42H52Cl2N4Rh2·
0.5C8H16 (944.23): calcd. C 58.46, H 6.40, N 5.93; found C 58.5,
Synthesis of RhCl(CO)(IPr)2 (8): This complex was prepared in a
manner analogous to that described for 6 but using 5 as precursor
1756
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1752–1758

N-Heterocyclic Carbene Rhodium Complexes
(46 mg, 0.050 mmol). Yield 44 mg (93%). C55H72ClN4ORh
converged (largest parameter shift was 0.00 times its esd) with un-
(942.44): calcd. C 70.03, H 7.70, N 5.94; found C 70.1, H 7.4, N
weighted and weighted agreement factors of: R1 = Σ ||Fo| – |Fc||/Σ
1
2 2
6.1. IR (KBr): ν = 1938 cm–1. H NMR (C D ): δ = 7.29 (t, J = |Fo| = 0.084, and wR2 = [Σ (w(Fo2 – Fc ) )/Σw(Fo2)2]1/2 = 0.080. The
˜
6
6
8 Hz, 4 H, p-ArH), 7.09 (d, J = 8 Hz, 8 H, m-ArH), 6.51 (s, 4 H, standard deviation of an observation of unit weight was 0.84, the
NCH), 3.14 [sept, J = 7 Hz, 8 H, CH(CH3)2], 1.14 [d, J = 7 Hz, 24 weighting Scheme being based on counting statistics. The maxi-
H, CH(CH3)2], 1.02 [d, J = 7 Hz, 24 H, CH(CH3)2] ppm. 13C{1H} mum and minimum peaks on the final difference Fourier map cor-
NMR (C6D6): δ = 189.8 (d, JRh-C = 42 Hz, NCN), 188.1 (d, JRh-C responded to 0.69 and –0.63 e–/Å3, respectively.
= 79 Hz, CO), 146.9 (s, NC), 138.0 (s, iPr-C), 131.2 (s, p-CH), 124.8
(s, m-CH), 124.5 (s, NCH), 29.0 [s, CH(CH3)2], 26.8 (s, CH3), 23.5
(s, CH3) ppm.
Acknowledgments
Catalytic Hydrogenation: Inside a glove-box, a benzene solution
We thank the Natural Sciences and Engineering Research Council
of Canada (NSERC) for financial support.
(10 mL) containing 0.005 mmol of Rh complex and 0.10 mmol of
cyclohexene or 1-octene was loaded into a 50 mL Schlenk-tube
equipped with a septum and stirrer. The Schlenk was then vacu-
umed and refilled with 1 atm H2 from a Schlenk-line. The reaction
mixture was then stirred, and product formation was monitored by
GC; in some cases, the “catalytic solution” was stirred in an oil-
bath at 45 °C. With this set-up, even if hydrogenation were com-
plete, the H2 pressure remains essentially constant.
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X-ray Crystallographic Analysis: Measurements were made at
173.0(1) K on a Rigaku/ADSC CCD area detector with graphite
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graphic data for 5 are shown in Table 1. Data were collected using
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Table 1. Crystallographic data for complex 5.
Data
5·1/2C6H14·1/2(C2H5)2O
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Formula
Fw
Crystal color, habit
Crystal size [mm]
Crystal system
Space group
a [Å]
C59H86ClN4O0.5Rh
997.68
yellow, block
0.25ϫ0.20ϫ0.20
orthorhombic
P21212 (#18)
12.952(3)
20.569(4)
10.599(2)
2824(4)
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V [Å3]
Z
2
µ [mm–1]
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Rint
0.389
29862
6022
0.052
No. variables
R1 [IϾ2σ(I)]
wR2[a]
290
0.084 (all data)
0.080 (all data)
0.84 (all data)
Gof
[a] w = 1/[σ2(Fo ) + (0.0287 P)2 + 0.00 P], where P = (Fo + 2Fc2)/3
2
2
Eur. J. Inorg. Chem. 2009, 1752–1758
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1757

X.-Y. Yu, H. Sun, B. O. Patrick, B. R. James
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Published Online: February 18, 2009
1758
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Eur. J. Inorg. Chem. 2009, 1752–1758