Cationic and neutral rhodium(I) oxazolinylcarbene complexes

Article abstract of DOI:10.1002/ejic.200400182

Reaction of [Rh(μ-OtBu)(diene)]₂ [diene = 1,5-cyclooctadiene (COD) or bicyclo[2.2.1]hepta-2,5-diene (NBD)] with the oxazolinylcarbene ligand precursor 2-(4,4-dimethyl)oxazolinyl-imidazolium bromide ("Me ₂carboxH⁺Br^-", 2) yielded the five-coordinate complexes [RhBr(Me₂carbox)(diene)] [diene = COD (3), NBD (4)]. Single-crystal X-ray structure analyses established a distorted trigonal bipyramidal configuration for 3, whereas the molecular structure of 4 is distorted square pyramidal. The dynamic properties of these five-coordinate complexes have been investigated by variable temperature ¹H NMR spectroscopy, indicating polytopal rearrangements of the five-coordinate complexes. The two compounds were converted into the square-planar cationic complexes [Rh(Me₂carbox)(diene)]⁺ (5 and 6, respectively), which have been isolated and fully characterised. The rhodium complex [RhBr(Me₂carbox)(CO)] (7) was obtained in high yield in a one-step reaction of the ligand precursor 2 and [Rh(acac)-(CO)₂] (acac = acetylacetonate). Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400182

FULL PAPER
Cationic and Neutral Rhodium( ) Oxazolinylcarbene Complexes
I
[a]
[a]
[a,b]
´
´
Vincent Cesar, Stephane Bellemin-Laponnaz,* and Lutz H. Gade*
Keywords: Carbenes / N ligands / Oxazolines / Rhodium / X-ray diffraction
Reaction of [Rh(µ-OtBu)(diene)]2 [diene = 1,5-cyclooctadiene
1H NMR spectroscopy, indicating polytopal rearrangements
(COD) or bicyclo[2.2.1]hepta-2,5-diene (NBD)] with the oxa- of the five-coordinate complexes. The two compounds were
zolinylcarbene ligand precursor 2-(4,4-dimethyl)oxazolinyl- converted into the square-planar cationic complexes
imidazolium bromide (‘‘Me2carboxH+Br−’’, 2) yielded the five-
[Rh(Me2carbox)(diene)]+ (5 and 6, respectively), which have
coordinate complexes [RhBr(Me2carbox)(diene)] [diene = been isolated and fully characterised. The rhodium complex
COD (3), NBD (4)]. Single-crystal X-ray structure analyses
established a distorted trigonal bipyramidal configuration for
3, whereas the molecular structure of 4 is distorted square
pyramidal. The dynamic properties of these five-coordinate
complexes have been investigated by variable temperature
[RhBr(Me2carbox)(CO)] (7) was obtained in high yield in a
one-step reaction of the ligand precursor 2 and [Rh(acac)-
(CO)2] (acac = acetylacetonate).
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
donor ligands (A) has been successfully employed in various
catalytic reactions.[6] The ligand synthesis is modular, as-
sembling both units in a single coupling step, which was
thought to be an efficient strategy in the development of
novel catalysts.
The combination of ligand donor units with very differ-
ent coordination properties in a di- or polydentate spectator
ligand (‘‘hetero-donor ligands’’) may permit a stereoelec-
tronic control of the reactivity at the remaining coordi-
nation sites in a transition metal complex. This has been an
underlying principle in the development of many phos-
phane-heteroatom-based polydentate ligands employed in
molecular catalysts.[1]
In recent years, N-heterocyclic carbenes have emerged as
a new class of spectator ligands in transition metal com-
plexes with ever increasing potential in homogeneous ca-
talysis.[2] They are strong σ-donors and in many respects
resemble phosphorus-donor ligands rather than classical
Fischer- or Schrock-type carbenes.[3] In this regard, they
may replace phosphane units in heterodonor ligands in
combination with more ‘‘classical’’ ligand functionali-
ties.[4,5] Among these novel chelating ligand systems, N-
functionalised carbene complexes have recently received
much attention and have given rise to exciting results in
homogeneous catalysis.[5,6]
The rigidity of the bidentate ligand A allows for the
modeling of a well-defined active space in its molecular
catalysts. In this paper we report a number of new com-
plexes of rhodium() containing a type A ligand which is
derived from a 2-(4,4-dimethyl)oxazolinylimidazolium salt.
A detailed structural study was intended to provide insight
into the nature of precursors and active rhodium catalysts
based on these systems.[6b]
Results and Discussion
We previously reported the direct coupling of oxazolines
and N-heterocyclic carbenes, and this new class of C,N-
Synthesis and Structural Characterization of the Bromo{1-
(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3-mesitylimidazol-2-
ylidene}rhodium(I) Complexes 3 and 4
As described in a previous paper,[6a] the imidazolium salt
2 is readily obtained from 1-mesityl-1H-imidazole and com-
mercially available 4,5-dihydro-4,4-dimethyloxazole. Fol-
lowing the procedure reported by Meyers and Novachek,
the reaction of the lithiated oxazole (tBuLi, THF, Ϫ78 °C)
with 1,2-dibromotetrafluoroethane gave the corresponding
[a]
´
Laboratoire de Chimie Organometallique et de Catalyse (UMR
´
7513), Institut Le Bel, Universite Louis Pasteur,
4, rue Blaise Pascal, 67070 Strasbourg, France
Fax: ϩ 33-390-241531
E-mail: gade@chimie.u-strasbg.fr
bellemin@chimie.u-strasbg.fr
[b]
Anorganisch-Chemisches Institut, Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: ϩ 49-6221-545609
E-mail: lutz.gade@uni-hd.de
3436
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DOI: 10.1002/ejic.200400182
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Cationic and Neutral Rhodium(I) Oxazolinylcarbene Complexes
FULL PAPER
Scheme 1. Synthesis of the 2-bromooxazoline 1 and the oxazolinyl imidazolium salt 2 as ligand precursor
2-bromo-4,5-dihydro-4,4-dimethyloxazole
1
in 65Ϫ70% 1677 cmϪ1; 4: 1664 cmϪ1) compared to the free oxazoline
yield (Scheme 1).[7] Treatment of 1 with 1-mesityl-1H-imi- in 2 (1691 cmϪ1). The details of the molecular structure
dazole in THF at 50 °C gave the ligand precursor, the 2- of both complexes were determined by single-crystal X-ray
(4,4-dimethyloxazolinyl)imidazolium salt (Me2oxcarb-
HϩBrϪ) 2, in about 80% yield (Scheme 1).[6a]
All attempts to generate the free carbene Me2oxcarb by
reaction of 2 with an external base were unsuccessful and
led to the nonspecific degradation of the compound. We
therefore chose to generate the corresponding rhodium
complexes by using precursors that already contain an
anionic ligand with sufficient basicity and are capable of
in situ deprotonation of the imidazolium salt. Following a
general procedure first reported by Herrmann and co-work-
ers,[8] we prepared the rhodium() alkoxide precursor [Rh(µ-
OtBu)(diene)]2 in situ by reaction of [Rh(COD)Cl]2 or
˚
Figure 1. Molecular structure of 3; selected bond lengths (A) and
[Rh(NBD)Cl]2 with potassium tert-butoxide at room tem-
angles (°): RhϪBr 2.7545(3), RhϪN(1) 2.255(2), RhϪC(6)
perature in THF. The resulting solution was then slowly
added to a suspension of the imidazolium salt 2 at Ϫ78
°C and the mixture was allowed to warm slowly to room
temperature overnight. Using this procedure, we obtained
2.022(2), RhϪC(18) 2.090(3), RhϪC(25) 2.096(3), RhϪC(21)
2.223(2), RhϪC(22) 2.251(2), C(21)ϪC(22) 1.371(4), C(18)ϪC(25)
1.428(4); C(6)ϪRhϪN(1) 77.47(9), C(6)ϪRhϪBr 86.98(7),
N(1)ϪRhϪBr 81.89(6)(4)
the rhodium complexes [Rh(Me2oxcarb)(COD)Br] (3) and diffraction studies and are depicted in Figure 1 and 2, along
[Rh(Me2oxcarb)(NBD)Br] (4) as orange, air-stable solids in with the principal bond lengths and interbond angles.
86% and 84% yield, respectively (Scheme 2).
The X-ray structure analyses confirm the bidentate nat-
The formation of the carbene complexes was confirmed ure of the oxazolinylcarbene ligand in these rhodium com-
by 13C NMR spectroscopy. Signals characteristic of N-het- plexes. The coordination geometry of 3 around the metal
erocyclic carbene carbon nuclei[4,5] at δ ϭ 177.2 ppm center is best approximated by a distorted trigonal bipyr-
(JRh,C ϭ 53 Hz) and at δ ϭ 183.6 ppm (JRh,C ϭ 55 Hz) were amidal arrangement with C(6), C(21) and C(22) occupying
observed for complexes 3 and 4, respectively Furthermore, the apical positions whereas the coordination geometry of
a significant coordination shift of the C(ϭN)O 13C NMR 4 is essentially square pyramidal. A steric interaction be-
resonances in both cases (3: δ ϭ 158.8 ppm; 4: δ ϭ tween the COD ligand and the mesityl ring seems to be the
155.6 ppm) in comparison with the ligand precursor 2 (δ ϭ origin of the trigonal bipyramidal arrangement of 3. The
148.8 ppm) indicates the coordination of the oxazoline ring. mesityl rings are oriented almost orthogonally to the imid-
The latter is also supported by the oxazoline ν(CϭN) vi- azolyl rings [dihedral angle for 3: C(14)ϪC(9)ϪN(3)ϪC(6)
brational band which is shifted to lower wavenumbers (3: 80.0°; dihedral angle for 4: C(10)ϪC(9)ϪN(2)ϪC(1) 78.6°].
Scheme 2. Synthesis of the five-coordinate complexes [RhBr(ligand)(diene)] 3 and 4
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´
V. C e sar, S. Bellemin-Laponnaz, L. H. Gade
FULL PAPER
structures of both complexes. In line with the ubiquitous
dynamic processes observed for pentacoordinate molecules,
we propose a sequence of Berry pseudorotations as dis-
played in Scheme 3.
One possible pathway interconverting the two square-
pyramidal complexes A and B (each with a ‘‘marked’’ Cϭ
C double bond) involves the intermediates IϪIII, which are
connected by Berry-type polytopal rearrangements. How-
ever, in the low-temperature-limit spectra of both 3 and 4
the (four inequivalent) olefin hydrogen atoms give rise to
two signals and not four, as expected. This may be due to
fast BrϪ exchange even at these low temperatures. Dis-
sociation and the subsequent reassociation of the anion to
the ionic planar intermediate, combined with the Berry re-
arrangements of the pentacoordinate complexes, would in-
deed lead to the equilibration of the four olefin protons in
both complexes. This rapid bromide dissociation and as-
˚
Figure 2. Molecular structure of 4; selected bond lengths (A) and
angles (°): RhϪBr 2.7615(3), RhϪN(3) 2.178(2), RhϪC(1)
2.016(2), RhϪC(18) 2.070(2), RhϪC(19) 2.105(3), RhϪC(21)
2.192(2), RhϪC(22) 2.177(2), C(21)ϪC(22) 1.357(5), C(18)ϪC(19)
1.401(4); C(1)ϪRhϪBr 95.13(7), C(1)ϪRhϪN(3) 77.49(8),
N(3)ϪRhϪBr 89.12(5)
As expected, the RhϪCCOD distances in 3, which are trans sociation mechanism is supported by the observed fast in-
to the carbene carbon atom, are longer than the RhϪCCOD termolecular bromide exchange between 3 and 4 and the
˚
bond lengths trans to the oxazoline [2.223(2)Ϫ2.251(2) A respective square-planar complex cations, as discussed in
˚
versus 2.090(3)Ϫ2.096(3) A]. This pronounced trans influ- the following section
ence[5j,9] is also reflected in the difference in the CϭC
double bond distances of the coordinated COD ligand. For
Synthesis and Structural Characterization of the Cationic
{1-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-mesitylimidazol-
the double bond trans to the carbene a CϭC bond length
2-ylidene}(diolefin)rhodium(
I) Complexes 5 and 6
˚
of 1.371(4) A is found, in comparison with the value of
˚
1.428(4) A for the double bond trans to the oxazoline N-
The bromo ligand in the pentacoordinate complex 3 can
atom. In contrast, due to the more rigid conformation and be removed by the addition of an excess of KPF6 in di-
smaller bite angle of the NBD ligand in complex 4, com- chloromethane (Scheme 4). The cationic complex [Rh(Me2-
pared to COD, the trans influence of the N-heterocyclic car- carbox)(COD)](PF6) (5) was isolated in nearly quantitative
bene in 4 on the RhϪC bond lengths of the diolefin is less yield as a red solid. In the same fashion, the analogous
˚
pronounced [2.192(2)Ϫ2.178(2) A versus 2.105(3)Ϫ2.070(2) cationic complex [Rh(Me2carbox)(NBD)](BF4) (6) derived
˚
A trans to the oxazoline unit]. The distances between the from 4 was obtained by addition of an excess of NaBF4 in
˚
rhodium and the carbene carbon atoms are 2.022(2) A and a dichloromethane/water solvent system.
˚
2.016(2) A for 3 and 4, respectively, and are similar to those
The formulation of compounds 5 and 6 was substan-
tiated by their analytical data, and the molecular structures
found in related structures.[4b,10]
Signals attributable to the COD ligand are not observed displayed in Scheme 4 are consistent with the resonance
in the 1H NMR spectrum (recorded at 300 MHz) of 3 at 25 patterns in the NMR spectra. The 1H NMR spectra of
°C, indicating a coalescence phenomenon and thus a dy- complexes 5 and 6, recorded at room temperature, display
namic process. Upon lowering the temperature to Ϫ60 °C, two signals for the olefin protons since both square-planar
four broad signals between δ ϭ 1.5 and 2.5 ppm appear, complexes possess Cs symmetry. In the 13C NMR spectra
which correspond to the -CH2- groups, while two reson- of 5 and 6 the carbene signals are observed at δ ϭ
ances for the olefin protons are observed at δ ϭ 3.6 and 174.2 ppm (JC,Rh ϭ 54 Hz) and δ ϭ 175.3 ppm (JC,Rh
ϭ
5.3 ppm. This exchange process has a free activation en- 59 Hz), respectively.
thalpy ∆G‡ of about 54 kJ/mol. The fact that only two sig-
In order to establish the details of their molecular struc-
nals of the four olefin protons are observed at low tempera- tures, both compounds were characterized by single-crystal
ture, in spite of their chemical inequivalence, suggests that X-ray diffraction which confirmed the expected square-
a second fast-exchange process takes place which has a low planar arrangement of the ligands around the metal centre
activation barrier (vide infra).
(Figure 4 and 5). As for complexes 3 and 4, the mesityl
1
In the H NMR spectrum of complex 4, recorded at 25 ring is oriented almost orthogonally to the imidazolyl ring
°C, only one signal for the olefin protons is observed (δ ϭ [dihedral angle for 5: C(10)ϪC(9)ϪN(3)ϪC(8) 99.3°; di-
3.59 ppm) indicating fast exchange at that temperature.[11] hedral angle for 6: C(14)ϪC(9)ϪN(3)ϪC(8) 87.4°]. Both
˚
At Ϫ22 °C, this resonance coalesces, and two signals at δ ϭ the RhϪC distances [RhϪC(8) ϭ 2.037(7) A for 5 and
˚
4.67 ppm and at δ ϭ 2.67 ppm are detected at Ϫ70 °C, as RhϪC(8) ϭ 2.039(6) A for 6] and the nitrogen to rhodium
shown in Figure 3. The estimated ∆G‡ of the exchange pro- distances [RhϪN(1) ϭ 2.136(3) A and RhϪN(1) ϭ 2.120(5)
˚
[5a][5l][5m]
˚
cess associated with this coalescence is ca. 46 kJ/mol. The A] are within the expected range.
A trans influence
mechanism operating in the dynamic processes observed for of the carbene ligand is also found, but appears to be less
3 and 4 has to explain the rapid exchange of the two in- pronounced than for the pentacoordinate neutral com-
equivalent CϭC double-bond positions in the molecular plexes. This is also reflected in the CϭC double-bond
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Cationic and Neutral Rhodium(I) Oxazolinylcarbene Complexes
FULL PAPER
1
Figure 3. Variable temperature H NMR spectra of 4 in the region δ ϭ 2.5Ϫ5.0 ppm (recorded in CD2Cl2)
lengths. In complex 5, for instance, the bond length of the
˚
double bond trans to the carbene is 1.357(8) A, compared
˚
with the value of 1.371(7) A for the double bond trans to
˚
˚
the oxazoline N-atom [1.371(4) A and 1.428(4) A in the
neutral compound 3].
The rapid dissociative BrϪ exchange postulated for com-
pounds 3 and 4 is supported by the observation of rapid
intermolecular bromide exchange between these pentacoor-
dinate complexes and their corresponding square-planar
cations 5 and 6. The1H NMR spectrum of a 1:1 mixture of
the neutral complex 4 and the ionic complex 6, dissolved in
CD2Cl2, shows an average resonance pattern of those of 4
and 6, instead of the spectra of separate species. The same
average resonance is observed at Ϫ70 °C and is compared
in Figure 6 with those of 4 and 6 recorded at the same tem-
perature. This implies fast intermolecular exchange at low
temperature (Scheme 5) and indicates that such a dissoci-
ative mechanism is probably at the origin of the low energy
fluxional process in the neutral pentacoordinate complexes.
Synthesis and Structural Characterization of the Square-
Planar Complex Bromo{1-(4,4-dimethyl-4,5-dihydrooxazol-
2-yl)-3-mesitylimidazol-2-ylidene}(carbonyl)rhodium(I) (7)
Scheme 3. A sequence of Berry pseudo-rotations leading to the
exchange of the positions of the CϭC double bonds in the chelat-
ing diolefin ligands relative to the C,N bidentate ligand. This pro-
cess, together with the dissociation and re-association of BrϪ, ex-
plains the exchange pattern observed in the variable temperature
1H NMR spectra of 3 and 4
Both the square-planar complexes 5 and 6 are cations.
As a neutral analogue we synthesized a rhodium carbonyl
derivative by direct reaction of the imidazolium precursor
2
with [Rh(acac)(CO)2] (Scheme 6).[12] The complex
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V. C e sar, S. Bellemin-Laponnaz, L. H. Gade
FULL PAPER
Scheme 4. Synthesis of the cationic four-coordinate complexes [Rh(ligand)(diene)]ϩ 5 and 6
sulting carbene by coordination to the rhodium() centre.
The ν(CϭO) band of the carbonyl ligand is observed in the
IR spectrum at 1974 cmϪ1 and the detection of a signal at
δ ϭ 184.0 ppm in the 13C NMR spectrum, with a character-
istic JC,Rh coupling constant of 60 Hz, established the for-
mation of the carbene complex.
˚
Figure 4. Molecular structure of 5; selected bond lengths (A) and
angles (°): RhϪN(1) 2.136(3), RhϪC(8) 2.037(7), RhϪC(18)
2.122(4), RhϪC(19) 2.116(4), RhϪC(22) 2.226(5), RhϪC(23)
2.206(5),
C(18)ϪC(19)
1.371(7),
C(22)ϪC(23)
1.357(8);
C(8)ϪRhϪN(1) 78.7(2), C(18)ϪRhϪC(23) 81.5(2)
˚
Figure 5. Molecular structure of 6; selected bond lengths (A) and
[RhBr(Me2carbox)(CO)] (7) was obtained in high yield by
reacting the two components in THF at room temperature.
The synthesis is based on the protonation of the acetylace-
angles (°): Rh(1)ϪC(8) 2.039(6), Rh(1)ϪN(1) 2.120(5),
Rh(1)ϪC(18) 2.098(7), Rh(1)ϪC(19) 2.106(7), Rh(1)ϪC(23)
2.196(7),
Rh(1)ϪC(24)
2.189(7),
C(18)ϪC(19)
1.38(1),
C(23)ϪC(24) 1.35(1); C(8)ϪRhϪN(1) 78.2(2), C(19)ϪRhϪC(24)
tonate by the imidazolium precursor and trapping the re- 67.1(3)
1
Figure 6. H NMR spectra of complex 4 (top), a 1:1 mixture of 4 and 6 (middle) and 6 (bottom) recorded in CD2Cl2 at 203 K
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Cationic and Neutral Rhodium(I) Oxazolinylcarbene Complexes
FULL PAPER
Scheme 5. Rapid intermolecular BrϪ exchange between complexes 4 and 6
Scheme 6. Synthesis of the neutral four-coordinate complexes [RhBr(ligand)(CO)] (7)
The single-crystal X-ray diffraction study of compound 7 precursor which readily coordinates to rhodium() complex
confirmed the square-planar coordination geometry (Fig- fragments. The rigidity of the ligand allows for only small
ure 7). The carbonyl ligand is disposed trans to the oxazo- variations of the metalϪligand bond lengths and angles, re-
line unit whereas the bromide is coordinated trans to the gardless of the coordination number or charge of the com-
carbene. The rhodiumϪcarbene distance [RhϪC(2) ϭ plex. The structural details concerning the binding of this
˚
1.950(4) A] and the rhodiumϪnitrogen distance class of oxazolinylcarbene ligands have been established in
[13]
˚
[RhϪN(3) ϭ 2.143(3) A]) are within the expected range.
this work. They provide the basis for a systematic use of
these functionalised N-heterocyclic carbene ligands in ca-
talysis. The first results in catalysis, which we obtained re-
cently, indicate their considerable potential.[6]
Experimental Section
All manipulations were performed under an inert atmosphere of
dry nitrogen using standard Schlenk techniques or by working in
a glove box. THF and diethyl ether were distilled from sodium/
benzophenone. Pentane was distilled from a sodium/potassium al-
loy, dichloromethane was dried over CaH2 and subsequently dis-
tilled. 1H and 13C NMR spectra were recorded on a Bruker Avance
300 NMR spectrometer at 300 MHz and 75 MHz and were refer-
enced to the residual proton solvent peak. Infrared spectra were
obtained on a FT-IR PerkinϪElmer 1600 spectrometer and the
˚
Figure 7. Molecular structure of 7; selected bond lengths (A) and
angles (deg): RhϪBr 2.5072(5), RhϪC(1) 1.807(4), RhϪC(2)
1.950(4), RhϪN(3) 2.143(3), C(1)ϪO(1) 1.152(4); BrϪRhϪC(1)
90.1(1), BrϪRhϪC(2) 173.0(1), C(2)ϪRhϪN(3) 78.4(1).
´
mass spectra were recorded by the ‘‘service de spectrometrie de
´
masse de lЈUniversite Louis Pasteur’’ on an Autospec HF mass
It is interesting to compare the CϪRhϪN bite angles of
the chelating oxazolinylcarbene ligand in the molecular
structures of compounds 3Ϫ7. The values found for the
five-coordinate complexes 3 and 4 are 77.47(9)° and
77.49(8)°, respectively, while the corresponding data of the
square-planar cations 5 and 6 are 78.7(2)° and 78.2(2)°,
respectively. In the neutral complex 7 this value is 78.4(1)°.
The rigidity of the ligand upon coordination to a metal cen-
tre thus imposes very well defined metric data, with little
variation, regardless of the coordination number or charge
of the complex.
spectrometer. The elemental analyses were performed by the ana-
lytical services of the Strasbourg and Heidelberg Chemistry De-
partments. [RhCl(COD)]2 and [RhCl(nbd)]2 were prepared accord-
ing to literature procedures.[14] [Rh(acac)(CO)2] was synthesized by
reacting [RhCl(CO)2]2 with Na(acac) in THF. Potassium tert-but-
oxide was sublimed prior to use. RhCl3·3H2O was provided by
BASF AG (Ludwigshafen). All other reagents were commercially
available and used as received.
2-Bromo-4,5-dihydro-4,4-dimethyloxazole (1): tBuLi (17.7 mL, 1.7
 in pentane, 30.0 mmol) was added dropwise to a solution of 4,5-
dihydro-4,4-dimethyloxazole (3.0 mL, 27.3 mmol) in anhydrous
THF (80 mL) at Ϫ78 °C over a period of 5 min. The resulting
yellow solution was stirred for an additional 15 min prior to the
addition of 1,2-dibromo-1,1,2,2-tetrafluoroethane (4.84 mL,
36.8 mmol). The solution was allowed to warm to ambient tem-
perature overnight and was subsequently concentrated to about
Conclusion
The direct condensation of a 2-bromooxazoline deriva-
tive with an imidazole provides a modular bidentate ligand 15 mL. The brown crude product was purified by a short bulb-to-
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´
V. C e sar, S. Bellemin-Laponnaz, L. H. Gade
FULL PAPER
bulb distillation to yield a colorless solution of the expected bromo-
oxazoline in THF (concentration of about 18% w/w) (2.58 g of pure
(175 mg, 84%). Crystallization from CH2Cl2/Et2O gave small or-
ange crystals suitable for an X-ray diffraction study. 1H NMR
1
3
compound, 53%). H NMR (CDCl3): δ ϭ 4.12 (s, 2 H, oxa CH2),
(CDCl3): δ ϭ 7.30 (d, J ϭ 2.1 Hz, 1 H, 4/5-im CH), 6.95 (s, 2 H,
1.34 (s, 6 H, CH3) ppm. 13C{1H} NMR (CDCl3): δ ϭ 141.4 (NCO), CHmes), 6.62 (d, J ϭ 2.1 Hz, 1 H, 4/5-im CH), 4.52 (s, 2 H, oxa
3
76.3 (CH), 71.6 (CH2), 25.5 (CH3) ppm.
CH2), 3.58 (br. m, 4 H, 2/3/5/6-nbd CH), 3.45 (m, 2 H, 1/4-nbd
CH), 2.30 (s, 3 H, para CH3), 2.20 (s, 6 H, ortho CH3), 1.45 (s, 6
1-(4,5-Dihydro-4,4-dimethyloxazol-2-yl)-3-mesitylimidazolium Bro-
mide (2): 1-Mesityl-1H-imidazole (1.11 g, 5.96 mmol) was added to
a solution of 1 (1.11 g, 6.23 mmol) in THF (ca. 1.0 ) and the
mixture was heated at 50 °C for 2 hours. A white solid precipitated
during this period of time. After cooling, diethyl ether (10 mL) was
added to complete the precipitation. The product was filtered,
washed with diethyl ether (2 ϫ 10 mL) and dried in vacuo to yield
1.80 g (82%) of 2 as a white powder. 1H NMR (CDCl3): δ ϭ 10.57
H, oxa CH3), 0.98 (s, 2 H, CH2nbd) ppm. 13C{1H} NMR (CDCl3):
13
δ ϭ 183.6 (d, 1J103
ϭ 55 Hz, N2C), 155.6 (NCO), 139.5
Rh,
C
(Cmes), 135.6 (Cmes), 133.6 (Cmes), 128.8 (CHmes), 123.5 (CHim),
115.6 (CHim), 84.6 (oxa CH2), 67.3 (oxa C4), 60.1 (nbd CH2 ), 49.2
(1/4-nbd CH), 28.5 (oxa CH3), 21.1 (para CH3), 18.6 (ortho CH3)
ppm. MS (ESI): m/z (%) ϭ 478.13 (100) [M Ϫ Br]ϩ. FT-IR (KBr):
ν˜ ϭ 1664 cmϪ1 (s, νCϭN). C24H29BrN3ORh (558.32): calcd. C
51.63, H 5.23, N 7.53; found C 51.11, H 5.19, N 7.34.
4
3
4
(t, J ϭ 1.5 Hz, 1 H, NCHN), 8.14 (dd, J ϭ 2.1, J ϭ 1.5 Hz, 1
H, 4,5-imidazolium CH), 7.60 (dd, 3J ϭ 2.1, 4J ϭ 1.5 Hz, 1 H, 4,5-
imidazolium CH), 7.00 (s, 2 H, mesityl CH), 4.53 (s, 2 H, CH2),
2.32 (s, 3 H, para CH3), 2.16 (s, 6 H, ortho CH3), 1.45 (s, 6 H,
oxazoline CH3) ppm. 13C{1H} NMR (CDCl3): δ ϭ 148.8 (CNO),
142.2 (mesityl C), 138.3 (N2C), 134.4, 130.5 (mesityl C), 130.4
(mesityl CH), 125.8, 121.3 (imidazolium CH), 83.2 (CH2), 68.7 (ox-
azoline C4), 28.4 (ortho CH3 ), 21.5 (para CH3), 18.4 (oxazoline
CH3) ppm. MS (FAB): m/z (%) ϭ 284.0 [M]ϩ (100), 187.0 [M Ϫ
(η4-1,5-Cyclooctadiene){1-(4,5-dihydro-4,4-dimethyloxazol-2-yl)-3-
mesitylimidazol-2-ylidene}rhodium(
I)
Hexafluorophosphate (5):
Solid KPF6 (34 mg, 0.185 mmol, 1.5 equiv.) was added to an or-
ange solution of 3 (70 mg, 0.122 mmol) in CH2Cl2 (5 mL). De-
gassed water (5 mL) was subsequently added and the mixture was
stirred vigorously for 30 minutes. The organic layer was decanted
and the aqueous layer was washed with an additional 5 mL of
CH2Cl2. The organic layers were combined, dried over Na2SO4 and
the volatiles were removed in vacuo. After washing with Et2O
(5 mL) and drying, an orange powder was isolated (76 mg, 98%).
Suitable crystals for an X-ray diffraction study were obtained by
slow diffusion of Et2O into a saturated solution of 5 in CH2Cl2/
Et2O. 1H NMR (CDCl3): δ ϭ 7.50 (d, 3J ϭ 2.1 Hz, 1 H, 4/5-im
C5H8NO]ϩ (22). FT-IR (KBr): ν˜
ϭ
1691 cmϪ1 (s, νCϭN).
C17H22BrN3O (364.29): calcd. C 56.05, H 6.09, N 11.54; found C
55.66, H 6.08, N 11.49.
Bromo(η4-1,5-cyclooctadiene){1-(4,5-dihydro-4,4-dimethyloxazol-2-
yl)-3-mesitylimidazol-2-ylidene}rhodium(I) (3): [RhCl(1,5-COD)]2
3
CH), 6.96 (s, 2 H, CHmes), 6.75 (d, J ϭ 2.1 Hz, 1 H, 4/5-im CH),
(48 mg, 0.098 mmol) and KOtBu (24 mg, 2.2 equiv.) were stirred in
THF (5 mL) for 30 min at room temperature. The resulting dark
yellow solution was then slowly added to a suspension of the imida-
zolium salt 2 (72 mg, 2.0 equiv.) in THF (10 mL) at Ϫ78 °C. The
mixture was allowed to warm to room temperature overnight. The
bright yellow solution was centrifuged, the supernatant was sepa-
rated and the solvents evaporated to give a yellow powder which
was washed with pentane (2Ϫ3 mL) and dried in vacuo to yield
compound 3 (97 mg, 86%). Suitable crystals for an X-ray diffrac-
tion studies were obtained by slow diffusion of Et2O into a solution
of 3 in CH2Cl2. 1H NMR (CDCl3, 298 K): δ ϭ 7.69 (d, 3J ϭ 2.1 Hz,
1 H, 4/5-im CH), 6.96 (s, 2 H, mes CH), 6.82 (d, 3J ϭ 2.1 Hz, 1 H,
5/4-im CH), 4.66 (s, 2 H, oxa CH3), 2.29 (s, 3 H, para CH3), 2.12
(br, 4 H, COD CH2), 2.08 (s, 6 H, ortho CH3), 1.74 (br, 4 H, COD
5.32Ϫ5.29 (m, 2 H, COD CH), 4.70 (s, 2 H, oxa CH2), 3.66Ϫ3.64
(m, 2 H, COD CH), 2.33 (s, 3 H, para CH3), 2.33Ϫ2.10 (m, 4 H,
COD CH2), 2.10 (s, 6 H, ortho CH3), 1.98Ϫ1.92 (m, 2 H, COD
CH2), 1.82Ϫ1.76 (m, 2 H, COD CH2), 1.49 (s, 6 H, oxa CH3) ppm.
1
13
13C{1H} NMR (CDCl3, 298 K): δ ϭ 174.2 (d, J103
C ϭ 54 Hz,
Rh,
N2C), 160.2 (NCO), 140.8, 134.2, 133.0 (Cmes), 129.7 (CHmes),
13
Rh,
125.3 (CHim), 118.2 (CHim), 97.6 (d, J103
ϭ 7.5 Hz, COD
C
13
CH), 85.1 (oxa CH2), 74.4 (d, J103
ϭ 13 Hz, COD CH), 67.9
Rh,
C
(oxa C4), 31.8, 29.1 (COD CH2), 27.4 (oxa CH3 ), 21.2 (para
CH3 ), 17.6 (ortho CH3 ) ppm. MS (ESI): m/z (%) ϭ 486.09 (100)
[RhL(CH3CN)2(H2O)]ϩ, 527.12 (46) [RhL(CH3CN)3(H2O)]ϩ. FT-
IR (KBr): ν˜ ϭ 1664 cmϪ1 (s, νCϭN). C25H33F6N3OPRh (639.42):
calcd. C 46.96, H 5.20, N 6.57; found C 46.9, H 5.35, N 6.65.
CH2), 1.49 (s, 6 H, oxa CH3) ppm. 13C{1H} NMR (CDCl3, 298 K):
{1-(4,5-Dihydro-4,4-dimethyloxazol-2-yl)-3-mesitylimidazol-2-
13
δ ϭ 177.2 (d, 1J103
ϭ 53 Hz, N2C), 158.8 (NCO), 140.4
Rh,
C
ylidene}(η4-2,5-norbornadiene)rhodium(
I)
Tetrafluoroborate (6):
(Cmes), 134.5 (Cmes), 133.6 (Cmes), 129.5 (CHmes), 125.2 (CHim),
118.8 (CHim), 84.3 (oxa CH2 ), 67.8 (oxa C4 ), 30.5 (br, COD CH2 ),
27.8 (oxa CH3), 21.2 (para CH3 ), 18.1 (ortho CH3) ppm. MS
(ESI): m/z (%) ϭ 486.09 (22) [M Ϫ Br Ϫ COD ϩ 2CH3CN ϩ
H2O]ϩ, 527.12 (100) [M Ϫ Br Ϫ COD ϩ 3CH3CN ϩ H2O]ϩ. FT-
IR (KBr): ν˜ ϭ 1677 cmϪ1 (s, νCϭN). C25H33BrN3ORh (574.36):
calcd. C 52.28, H 5.79, N 7.32; found C 51.82, H 5.66, N 7.38.
Solid NaBF4 (37 mg, 0.340 mmol, 2 equiv.) was added to an orange
solution of 4 (95 mg, 0.170 mmol) in CH2Cl2 (5 mL). Degassed
water (5 mL) was then added and the resulting red mixture was
stirred vigorously for 30 min. The organic layer was removed and
the aqueous layer was washed with an additional 5 mL of CH2Cl2.
The organic layers were combined, dried over Na2SO4 and the vol-
atiles were removed in vacuo. Crystallization of the crude material
from CH2Cl2/Et2O gave reaction product 6 in the form of long, red
needles (77 mg, 80%). Suitable crystals for an X-ray diffraction
study were obtained by slow diffusion of Et2O into a saturated
Bromo{1-(4,5-dihydro-4,4-dimethyloxazol-2-yl)-3-mesitylimidazol-2-
ylidene}(η4-2,5-norbornadiene)rhodium(
I) (4): Solid [RhCl(nbd)]2
(86 mg, 0.186 mmol) and KOtBu (46 mg, 0.41 mmol, 2.2 equiv.)
were weighed and placed in a Schlenk tube. THF (6 mL) was then solution of 4 in CH2Cl2/Et2O. 1H NMR (CDCl3): δ ϭ 7.52 (d, 3J ϭ
added and the reaction mixture was stirred for 30 minutes at ambi-
ent temperature. The solution thus obtained was slowly added to a
2.1 Hz, 1 H, 4/5-im CH), 6.81 (s, 2 H, CHmes), 6.67 (d, 3J ϭ 2.1 Hz,
1 H, 4/5-im CH), 5.13 (br, 2 H, nbd CH), 4.76 (s, 2 H, oxa CH2),
suspension of the imidazolium salt 2 (136 mg, 0.373 mmol, 2.0 3.87 (br, 2 H, nbd CH), 3.52 (br, 2 H, 1/4nbd CH), 2.26 (s, 3 H,
equiv.) in THF (12 mL) at Ϫ78 °C. The mixture was allowed to
warm to ambient temperature overnight and was centrifuged. The
resulting orange-red supernatant was separated and the solvents
evaporated in vacuo. The crude solid was washed two or three times
with pentane (5 mL). Complex 4 was obtained as an orange powder
para CH3), 2.07 (s, 6 H, ortho CH3), 1.33 (s, 6 H, oxa CH3), 1.31
(s, 2 H, nbd CH2) ppm. 13C{1H} NMR (CDCl3): δ ϭ 175.3 (d,
13
1J103
C ϭ 59 Hz, N2C), 161.8 (NCO), 140.5, 134.4, 132.5 (Cmes),
Rh,
129.2 (CHmes), 124.7 (CHim), 117.4 (CHim), 85.8 (oxa CH2), 82.2
(oxa C4), 66.3, 65.8 (nbd CH), 59.0 (nbd CH2), 53.9, 53.5 (1/4-nbd
3442
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2004, 3436Ϫ3444

Cationic and Neutral Rhodium(I) Oxazolinylcarbene Complexes
FULL PAPER
Table 1. X-ray experimental data of compounds 3Ϫ7
3
4
5
6
7
Formula
C25H33BrN3ORh
C24H29BrN3ORh
C25H33BrN3ORhPF6
C72H87N9O3Rh3·3BF4·
2CH2Cl2·H2O
1883.57
C18H21BrN3O2Rh
Mol. mass
574.38
558.33
639.43
494.20
Crystal system
Space group
orthorhombic
Pbca
13.3030(2)
18.9492(2)
18.6447(3)
monoclinic
P121/n (no. 1)
10.8649(1)
9.2177(1)
22.4762(3)
91.557(5)
2250.15(4)
4
1.65
1128
2.555
173
monoclinic
C1c (no. 1)
11.6730(2)
23.2279(5)
10.1204(3)
97.450(5)
2720.9(1)
4
1.56
1304
0.751
293
monoclinic
P121/c (no. 1)
26.2844(2)
11.5110(1)
28.0873(2)
97.268(5)
8429.8(1)
4
1.48
3832
0.785
294
monoclinic
P121/c (no. 1)
10.6735(3)
11.6164(3)
16.3699(4)
103.738(5)
1971.60(9)
4
1.66
984
2.908
173
˚
a (A)
˚
b (A)
˚
c (A)
β (°)
3
˚
V (A )
4700.0(1)
8
1.62
2336
2.449
173
Z
ρcalcd. (g·cmϪ3
F000
)
µ (mmϪ1
)
Temperature (K)
˚
Wavelength (A)
0.71073
13757
0.71073
11954
0.71073
7090
0.71073
25736
0.71073
9055
Number of data meas.
Number of data with
I Ͼ 3σ(I)
Number of variables
R
Rw
GOF
4481
280
0.029
0.036
1.032
5853
271
0.030
0.054
1.028
5347
332
0.036
0.047
1.012
16749
1039
0.069
0.089
1.209
3680
226
0.029
0.049
1.026
Largest peak in final
Ϫ3
˚
difference (e·A
)
0.509
2.088
0.405
1.482
0.402
CH), 27.7 (oxa CH3), 21.1 (para CH3), 17.7 (ortho CH3) ppm. MS
(ESI): m/z (%) ϭ 478.12 (24) [M Ϫ BF4]ϩ, 510.11 (64) [M Ϫ BF4
Ϫ nbd ϩ 3CH3CN]ϩ, 537.12 (100). FT-IR (KBr): ν˜ ϭ 1662 cmϪ1
(s, νCϭN).
X-ray Crystallographic Study of Compounds 3؊7: Suitable crystals
of complexes 3, 4, 5, 6 and 7 were obtained by layering concen-
trated solutions of the compounds in dichloromethane with pen-
tane, hexane or diethyl ether (vide supra) and allowing slow dif-
fusion at room temperature. The crystal data were collected at
Ϫ100 °C on a Nonius KappaCCD diffractometer and transferred
to a DEC Alpha workstation; for all subsequent calculations the
Nonius OpenMoleN package was used.[15] The structures were
solved by direct methods with absorption corrections being part of
the scaling procedure of the data reductions. After refinement of
the heavy atoms, difference Fourier maps revealed the maxima of
residual electron density close to the positions expected for the hy-
drogen atoms; they were introduced as fixed contributors in the
Bromo(carbonyl){1-(4,5-dihydro-4,4-dimethyloxazol-2-yl)-3-mesityl-
imidazol-2-ylidene}rhodium(
I)
(7):
[Rh(acac)(CO)2]
(75 mg,
0.290 mmol) and the imidazolium salt 2 (106 mg, 1.0 equiv.) were
placed in a Schlenk tube and dissolved in THF (8 mL). A rapid
evolution of a gas (CO) was observed and the solution color im-
mediately turned bright yellow. After stirring for 50 minutes at am-
bient temperature, the reaction mixture was centrifuged, the super-
natant was separated and the solvents evaporated to dryness. The
crude product was washed with pentane (2 ϫ 4 mL) to yield the
analytically pure compound 7 as a yellow microcrystalline solid
(122 mg, 85%). Crystallization from CH2Cl2/pentane gave yellow
crystals suitable for X-ray diffraction. 1H NMR (CDCl3): δ ϭ 7.28
(d, 3J ϭ 2.3 Hz, 1 H, 4/5-im CH), 7.04 (s, 2 H, CHmes), 6.70 (d,
3J ϭ 2.3 Hz, 1 H, 4/5-im CH), 4.62 (s, 2 H, oxa CH2), 2.32 (s, 3
H, para CH3), 2.10 (s, 6 H, ortho CH3), 1.72 (s, 6 H, oxa CH3)
˚
structure factor calculations with fixed coordinates (CϪH: 0.95 A)
2
˚
and isotropic temperature factors [B(H) ϭ 1.3 Beqv(C) A ] but not
refined. Full least-square refinements on F2. A final difference map
revealed no significant maxima of electron density. The scattering
factor coefficients and the anomalous dispersion coefficients were
taken from the literature.[16] Crystal data and experimental details
for the crystals of compounds 3؊7 are given in Table 1.
1
13
Rh,
ppm. 13C{1H} NMR (CDCl3): δ ϭ 187.4 (d, J103
ϭ 78 Hz,
C
CCDC-229779 ... -229783 (for 3Ϫ7, respectively) contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html [or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: ϩ44-
1223-336033; E-mail: deposit@ccdc.cam.ac.uk].
13
CO), 184.0 (d, 1J103
ϭ 60 Hz, N2C), 158.5 (NCO), 140.4
Rh,
C
(Cmes), 135.0 (Cmes), 134.0 (Cmes), 129.4 (CHmes), 123.5 (CHim),
114.7 (CHim), 86.4 (oxa CH2), 67.4 (oxa C4), 28.2 (oxa CH3), 21.2
(para CH3), 18.0 (ortho CH3) ppm. FT-IR (KBr): ν˜ ϭ 1974 cmϪ1
(s, νCϭO), 1670 cmϪ1 (s, νCϭN). C17H21BrN3O2Rh (494.19): calcd.
C 43.75, H 4.28, N 8.50; found C 42.93, H 4.05, N 8.14.
Intermolecular Br؊ Exchange between Complexes 4 and 6: Equal
amounts of compounds 4 and 6 (20 µmol) were weighed, placed
into an NMR tube and dissolved in CD2Cl2 (0.5 mL). In an 1H
NMR spectrum, which was recorded at 203 K, the signals of com-
plexes 4 and 6 were not observed separately but a set of resonances
at the weighted mean chemical shift indicated a fast intermolecular
exchange of bromide between 4 and 6.
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