A new class of modular oxazoline-NHC ligands and their coordination chemistry with platinum metals

Article abstract of DOI:10.1002/ejic.200800908

A novel family of enantiomerically pure imidazolium salts [(NHC-Ox)H] ⁺X^- (2a-g) has been generated bearing an oxazoline unit and in which both heterocycles are connected by a (dimethyl)methylene bridge. Deprotonation of the imidazolium salt 2f and subsequent reaction of the resulting free carbene with [Rh(nbd)Cl]₂ (nbd = norbornadiene) afforded the neutral rhodium(I) complex [(NHC-Ox)Rh(nbd)Br] (3) in which the ligand was found to be monodentate. Bromide abstraction lead to the air-stable cationic complex [(NHC-Ox)-Rh(nbd)]PF₆ (4) with the expected bidentate coordination mode of the ligand. Reaction of the imidazolium salt 2d with Karstedt's catalyst and one equivalent of KOtBu gave the trigonal planar platinum(0) complex [(NHC-Ox)Pt(dvtms)] (5) (dvtms = divinyltetramethylsiloxane) , which was oxidized by CsBr₃ to give the square planar platinum(II) complex [(NH-COx) PtBr₂] (6). Complex [(NHC-Ox)PtCl₂] (7) was directly prepared by reaction of the imidazolium salt with Ag₂O followed by the addition of [PtCl₂(1,5-cod)]. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800908

FULL PAPER
DOI: 10.1002/ejic.200800908
A New Class of Modular Oxazoline-NHC Ligands and Their Coordination
Chemistry with Platinum Metals
Nathanaëlle Schneider,[a] Stéphane Bellemin-Laponnaz,*[b] Hubert Wadepohl,[a] and
Lutz H. Gade*[a]
Keywords: Carbenes / Oxazolines / Platinum / Rhodium
A novel family of enantiomerically pure imidazolium salts
[(NHC-Ox)H]+X– (2a–g) has been generated bearing an ox-
azoline unit and in which both heterocycles are connected
by a (dimethyl)methylene bridge. Deprotonation of the imid-
azolium salt 2f and subsequent reaction of the resulting free
carbene with [Rh(nbd)Cl]2 (nbd = norbornadiene) afforded
the neutral rhodium(I) complex [(NHC-Ox)Rh(nbd)Br] (3) in
which the ligand was found to be monodentate. Bromide ab-
straction lead to the air-stable cationic complex [(NHC-Ox)-
Rh(nbd)]PF6 (4) with the expected bidentate coordination
mode of the ligand. Reaction of the imidazolium salt 2d with
Karstedt’s catalyst and one equivalent of KOtBu gave the tri-
gonal planar platinum(0) complex [(NHC-Ox)Pt(dvtms)] (5)
(dvtms = divinyltetramethylsiloxane), which was oxidized by
CsBr3 to give the square planar platinum(II) complex [(NHC-
Ox)PtBr2] (6). Complex [(NHC-Ox)PtCl2] (7) was directly pre-
pared by reaction of the imidazolium salt with Ag2O followed
by the addition of [PtCl2(1,5-cod)].
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
The design of efficient chiral ligands for asymmetric ca-
talysis is usually based on simple concepts and principles
such as molecular symmetry, modularity in the assembly
of polydentate ligands or the use of “privileged” structural
motifs.[1,2] Among all the stereoinducing elements, the oxaz-
oline ring has become one the most commonly employed.[3]
Indeed, such chiral enantiopure units are not only easily
accessible, but being rigid, quasi-planar five-membered
rings they also provide a good control of the first coordina-
tion sphere of the transition metal upon N-coordination.[4]
N-Heterocyclic carbenes[5] are excellent “anchor” units
for late transition metals which form strong metal–carbon
bonds[6] and have thus been widely used in homogeneous
catalysis.[7] They may be combined with other ligating units
by the appropriate functionalization of the N-atoms in their
heterocyclic structures.[8] Especially the combination of an
N-heterocyclic carbene with an oxazoline unit has led to
very efficient catalytic systems.[9–11] The first example of
such a combination was provided by Herrmann et al. who
reported the bidentate ligand A, where the two heterocycles
are connected by a methylene bridge (Scheme 1).[12] The
corresponding RhI complexes were employed as catalysts in
the hydrosilylation of ketones but afforded low enantio-
selectivities (ee values up to 11%).[13]
Scheme 1.
More recently, Pfaltz et al. developed an alternative syn-
thesis of such ligands; their IrI complexes were tested in
alkene hydrogenations and gave moderate to high enantio-
selectivities (ee values up to 90%).[14] We described the di-
rect coupling of oxazolines and N-heterocyclic carbenes
generating highly rigid type B chelate ligands.[15] This strat-
egy has provided the key to a highly efficient class of cata-
lysts for the asymmetric hydrosilylation of prochiral
ketones[16] and confirmed the considerable potential of
combining a N-heterocyclic carbene with oxazoline. How-
ever, this class of ligands imposes a rather constrained ge-
ometry upon coordination which limits its applicability for
a broad range of transition metals.[17]
We were therefore interested in NHC-oxazoline ligands
C in which the two heterocycles are connected by a (di-
methyl)methylene bridge. In comparison with the type A
ligands, the methyl groups on the methylene bridge are ex-
pected to kinetically stabilize the bridge under the condi-
tions of molecular catalysis. We report herein the synthesis
of chiral carbene precursors of C and their first use in the
coordination chemistry of platinum(0 or II) as well as rho-
dium(I).
[a] Anorganisch-Chemisches Institut, Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: lutz.gade@uni-hd.de
[b] Institut de Chimie, CNRS – Université Louis Pasteur,
1, rue Blaise Pascal, 67000 Strasbourg, France
E-mail: bellemin@chimie.u-strasbg.fr
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Results and Discussion
Synthesis of the Chiral Imidazolium Salts 2a–g
The synthesis of the imidazolium salts 2a–g, which are
the precursors of ligand C is summarized in Scheme 2, the
key step being the final coupling of the chiral imidazole
derivatives 1a,b with a series of alkyl halides. The substi-
tuted imidazoles 1a,b were obtained in good yield via a
three-step synthesis from imidazole, ethyl 2-bromoisobutyr-
ate and the chiral amino alcohol [(S)-valinol or (S)-tert-
leucinol].[18]
The corresponding imidazolium salts 2a–g were isolated
as relatively air-stable white powders. The 1H and 13C
NMR spectra of 2a–g are consistent with the proposed
structure of the molecule. The signals of the C2-H proton
of the imidazolium ring were observed at δ = 11.16–9.80,
reflecting the positive electronic charge located in the ring.
Intense molecular ion peaks [M – Br]+ were observed in the
mass spectra of all compounds, whilst the IR vibrational
bands ν(C=N) of the oxazoline units were found between
1672 and 1661 cm–1. Suitable crystals for an X-ray diffrac-
tion analysis of 2e were obtained to establish its structural
details (Figure 1). The molecule crystallizes as a pair of
conformers, which are related to one another by a pseudo
centre of inversion (disregarding the chiral centre). Both
Figure 1. Molecular structure of imidazolium salt 2e (only one of
the two independent moieties is shown, hydrogen atoms, except for
C(10)–H, omitted for clarity). Selected bond lengths [Å] and angles
[°] (values in square brackets refer to the second cation): C(10)–
N(2) 1.326(5) [1.329(5)], C(10)–N(3) 1.325(5) [1.315(5)], C(1)–N(1)
1.248(6) [1.245(5)], N(2)–C(10)–N(3) 108.9(4) [109.0(4)], N(1)–
C(1)–C(7)–N(2) –10.3(6) [–0.9(6)], C(1)–C(7)–N(2)–C(10) –89.1(5)
[77.5(5)]. Thermal ellipsoids drawn at 25% probability.
The imidazolium salts were readily deprotonated with
have similar bond lengths and angles and correspond to KHMDS (= potassium hexamethyldisilazide) to afford the
rotations along the C(13)–C(14), C(13)–C(24) and N(2)– corresponding free carbenes, following a procedure re-
C(7) bonds. The imidazole ring and the oxazoline ring are ported by Coleman et al.[19] The two carbenes derived from
almost orthogonal to each other (angles between the best 2e and 2f have been characterized spectroscopically in solu-
planes through the rings are 84 and 82°, respectively).
tion. The formation of the free carbenes was confirmed by
Scheme 2. Synthesis of the imidazolium salts 2a–g (note: the counterion of 2a is iodide instead of bromide). (i) BrC(CH3)2COOEt,
CH3CN, reflux, 3 d; (ii) amino alcohol, NaH cat., 120 °C, 4 h; (iii) MsCl, NEt3, CH2Cl2 then NaOH, MeOH/H2O.
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A New Class of Modular Oxazoline-NHC Ligands
the disappearance of the C2-H proton in the 1H NMR other possibility is that the oxazolinyl ring remains coordi-
spectra, and the typical chemical shift of the carbene car- nated and the metal center undergoes polytopal Berry-type
bon at low field (δ = 217.3 in the 13C-NMR spectra for rearrangements similar to those observed in complexes with
both carbenes).
ligand B as well as bromide association/dissociation pro-
cesses.[21] The characteristic vibrational band of the C=N
bond is only slightly shifted from 1669 (2f) to 1664 cm–1
(3), which in turn is consistent with a non-coordinated ox-
azoline. In order to resolve this structural ambiguity and to
establish the molecular structure of compound 3, an X-ray
diffraction study has been carried out (Figure 2). The com-
plex crystallizes with two independent molecules in the unit
cell, which are related by a pseudo centre of symmetry (dis-
regarding the oxazoline rings). The two diastereomers
roughly correspond to a 180° rotation of the NHC ligand
around the Rh–Ccarbene bond. The coordination geometry
around the metal center is approximately square planar as
observed in other structures of neutral NHC-RhI com-
plexes, and the ligand is monodendate.[22] The distortion
from an ideal square-planar coordination is due to the ri-
gidity and the small bite angle of the nbd ligand. To mini-
mize the steric hindrance, the imidazolyl ring is twisted by
almost 90° out of the coordination plane. The Ccarbene–Rh
bond length (2.070(2) and 2.065(2) Å] is within the range
reported in the Cambridge Structural Database for NHC-
RhI complexes (Ccarbene–RhI = 2.035 Å for 163 examples).
As expected, the Rh–Cnbd distances trans to the carbene
carbon atom are longer than those trans to the bromide
(average values are 2.180 vs. 2.121 Å). This trans influence
is also reflected in the difference in the C=C double bond
lengths of the coordinated nbd ligand (average values 1.381
vs. 1.407 Å).
Synthesis and Structural Characterization of the
Rhodium(I) Complexes 3 and 4
In order to study the coordination behavior of the (di-
methyl)methylene bridged ligand C, neutral and cationic
RhI complexes were prepared. The procedure previously
used in our group for the synthesis of the B-type rhodium
complexes, namely the reaction of the alkoxy-rhodium pre-
cursor with the imidazolium salts, could not be applied to
C, probably due to a different reactivity of the potassium
tert-butoxide towards the imidazolium salts 2.[12] However,
the latter being cleanly deprotonated by KHMDS and the
corresponding free carbene being relatively stable, a pro-
cedure similar to the one developed by Lassaletta et al. was
chosen (Scheme 3).[20] In a first step, the imidazolium salt
2f was deprotonated with KHMDS at –78 °C in thf and a
solution of [Rh(nbd)Cl]2 in thf was subsequently added. Af-
ter warming to ambient temperature and workup, complex
3 was isolated as a slightly air-sensitive yellow solid.
1
The H and 13C-NMR spectroscopic data are consistent
with the formation of a carbene complex. At room tempera-
ture, most of the H NMR signals are very broad as a con-
1
sequence of fluxional processes, and a fully assignable
NMR spectrum could be obtained at 263 K. The coordina-
tion of the imidazolyl ring to rhodium is confirmed by the
coupling of the carbene 13C nuclei with rhodium, 1J(Rh-C)
=
56.1 Hz. The four inequivalent CH2/3/5/6-nbd protons and the
The bromide ligand in 3 could be abstracted by stirring
diastereotopic CH2 nbd protons appear as separate reso- of 3 with an excess of KPF6 in a CH2Cl2/water solvent sys-
nances. The 15N-NMR chemical shift of the Noxa resonance tem (Scheme 3). The cationic complex 4 was isolated from
1
at δ = 180.3 is inconclusive as to the role of the oxazoline the organic phase as an air-stable red compound. The H-,
donor and indicates a probable coordination/decoordina- 13C- and 15N-NMR spectra of 4 recorded at 263 K are very
tion equilibrium of the oxazolinyl ring to the metal center similar to those of 3, and do not show clear evidence for
in solution at low temperature. The observed fluxional be- the generation of the cationic complex. The typical signal
haviour at room temperature may be due to rapid coordina- of the carbene carbon as a doublet is observed at δ = 177.6
1
tion/decoordination of the oxazoline ring (vide infra). An- with a coupling constant of J(Rh–C) = 56.5 Hz. Finally, the
Scheme 3. Synthesis of the neutral rhodium(I) complex 3 and its cationic derivative 4.
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Figure 2. Molecular structure of complex 3 (only one of the two
independent molecules is shown, hydrogen atoms omitted for clar-
ity). Selected bond lengths [Å] and angles [°] (values in square
brackets refer to the second molecule): Rh(1)–C(11) 2.070(2)
[2.065(2)], Rh(1)–Br(1) 2.5029(3) [2.5031(4)], Rh(1)–C(35)/C(36)
2.143(2)/2.107(2) [2.115(2)/2.119(2)], Rh(1)–C(38)/C(39) 2.178(2)/
2.183(2) [2.169(2)/2.188(2)], C(35)–C(36) 1.363(2) [1.451(3)], C(38)–
C(39) 1.377(3) [1.384(3)], C(11)–Rh(1)–Br(1) 91.38(5) [91.03(5)],
C(11)–Rh(1)–C(35)/C(36) 101.85(7)/99.10(6) [102.61(7)/101.94(6)],
Br(1)–Rh(1)–C(38)/C(39) 94.44(5)/96.01(5) [93.40(6)/96.87(5)],
C(35)–Rh(1)–C(39) 67.47(7) [65.92(7)], C(36)–Rh(1)–C(38)
67.69(6) [65.77(7)], N(2)–C(8)–C(1)–N(1) 156.68(2) [137.5(2)].
Thermal ellipsoids drawn at 35% probability.
Figure 3. Molecular structure of complex 4 (hydrogen atoms and
counterion omitted for clarity). Selected bond lengths [Å] and
angles [°]: Rh–C(1) 2.025(1), Rh–N(3) 2.095(1), Rh–C(37)/C(38)
2.100(1)/2.143(1), Rh–C(40)/C(41) 2.240(1)/2.189(2), C(37)–C(38)
1.404(2), C(40)–C(41) 1.383(2), C(1)–Rh–N(3) 84.11(5), C(1)–Rh–
C(37)/C(38) 99.77(5)/104.49(5), N(3)–Rh–C(40)/C(41) 105.57(5)/
94.89(5), C(37)–Rh–C(41) 67.43(6), C(38)–Rh–C(40) 65.30(6),
N(1)–C(4)–C(7)–N(3) –37.3(2). Thermal ellipsoids drawn at 40%
probability.
infrared spectrum of 4 displays a vibrational band at
1637 cm–1, which is the signature of the coordination of the
oxazoline unit.
Diffraction quality single crystals of 4·CDCl3 were grown
by slow diffusion from pentane/CDCl3. The complex
adopts a distorted square-planar geometry where the ox-
azolinyl/carbene ligand is chelating through both donor
functions (Figure 3). As observed in the structure of com-
plex 3, the distortion is due to the rigidity and the small
bite angle of the nbd ligand [C(37)–Rh–C(41) 67.43(6),
C(38)–Rh–C(40) 65.30(6)°], but also due to the chelate an-
gle of the oxazolinyl/carbene ligand (C(1)–Rh–N(3)
84.11(5)°]. The imidazolyl and oxazolyl rings are non-co-
back-donation from the electron rich metal center.[23] The
C
carbene–Rh bond length [C(1)–Rh 2.025(1) Å] is within the
range reported for NHC–RhI complexes and similar to 3.
Synthesis and Structural Characterization of Platinum(0)
and (II) Complexes
The platinum(0) complex [(NHC-Ox)Pt(dvtms)] (5)
planar and oriented so that the tert-butyl group of the oxaz- (dvtms = divinyltetramethylsiloxane) was synthesized by re-
oline points in one direction and the bridging (dimethyl)- action of the imidazolium salt 2d with potassium tert-bu-
methylene in the other and out of the coordination plane toxide in presence of Karstedt’s catalyst, according to a pro-
[N(1)–C(4)–C(7)–N(3) –37.3°], to form a boat-shaped (Rh– cedure reported by Markó et al.[24] (Scheme 4). The product
C–N–C–C–N) chelate ring. The nbd ligand is bent away was purified by column chromatography and isolated as an
from the ideal coordination plane to avoid unfavorable re- air-stable white solid.
pulsion with the tert-butyl substituent [C(1)–Rh–C(40)
The 1H-, 13C- and 15N-NMR spectra in CD2Cl2 solution
165.51(5)°, N(3)–Rh–C(38) 170.67(5)°]. This arrangement at 273 K display two sets of signals having the same inten-
also allows an overlap between the π*-orbital of the C=Cnbd sity, which indicates that the complex exists as two con-
bonds and the high energy dz2 metal orbital, enhancing formers in an equimolar ratio. The coordination of the
Scheme 4. Synthesis of the platinum(0) complex 5.
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A New Class of Modular Oxazoline-NHC Ligands
imidazolyl ring to platinum is confirmed by the large coup-
ling constants of the carbene carbons with 195Pt of
1J(Pt-C) = 1375.7 Hz and 1384.3 Hz. The isopropyl protons
have similar environments as indicated by their chemical
shifts (δ = 0.97, 0.95, 0.89 and 0.88) and the 15N NMR
resonance of Noxa at δ = 259.5, 253.9 is typical for a non-
coordinated oxazoline. The characteristic vibrational band
of the C=N bond is only slightly shifted from ν = 1670 (2d)
˜
to 1663 cm–1 (5), which is also evidence for non-binding of
the oxazoline. The coordination of the dvtms ligand to the
metal center is confirmed by the upfield shift of the vinyl-
silane protons, from δ = 6.1–5.7 for free dvtms to δ = 2.2–
1.6 in 5. Moreover, the 195Pt nuclei couple with several vinyl
2
protons and 13C nuclei: J(Pt-H) = 53 Hz for the CH=CH2
1
protons, J(Pt-C) = 165 Hz for the CH=CH2 carbons and
1J(Pt-C) = 120 Hz for the CH=CH2 carbons. Another inter-
esting observation is the splitting of the SiMe2 proton reso-
nance into two distinct signals for the pseudo-equatorial
and pseudo-axial positions at δ = 0.3 and –0.5 respec-
tively.[25] This behavior has already been reported for other
Pt(0) complexes bearing a chelating dvtms ligand. The two
conformers observed in solution may correspond to the two
pseudo-chair conformations of the dvtms, or to hindered
rotations of the (dimethyl)methylene bridge.
The molecule crystallizes as a pair of conformers in the
unit cell. Both have similar bond lengths and angles and
are related by rotation around the N(1)–C(13) and N(2)–
C(4) bonds. The metal center adopts a trigonal planar ar-
rangement which is the characteristic coordination mode of
[Pt0(olefin)n] complexes (Figure 4).[26] This trigonal planar
conformation around platinum provides a better overlap be-
tween the Pt d-orbitals and the olefinic π* acceptor orbitals,
thus enhancing the backbonding.[27] The oxazoline ring is
not coordinated to the metal center, whereas the bidentate
dvtms ligand adopts a pseudo-chair conformation. To mini-
mize interligand repulsion, the NHC ligand is tilted by al-
most 90° out of the coordination plane spanned by the
Figure 4. Molecular structure of complex 5 (only one of the two
independent molecules is shown, hydrogen atoms omitted for clar-
ity). Selected bond lengths [Å] and angles [°] (values in square
brackets refer to the second molecule): Pt(1)–C(1) 2.06(1) [2.06(1)],
Pt(1)–C(26)/C(27) 2.11(1)/2.15(1) [2.12(1)/2.15(1)], Pt(1)–C(32)/
C(33) 2.16(1)/2.14(1) [2.13(1)/2.1181)], C(26)–C(27) 1.43(2)
[1.42(2)], C(32)–C(33) 1.44(2) [1.42(2)], C(1)–Pt(1)–C(27) 132.0(5)
[132.3(5)], C(1)–Pt(1)–C(32) 132.5(5) [132.4(5)], N(1)–C(1)–Pt(1)–
C(26) –83(1) [85(1)], N(1)–C(1)–Pt(1)–C(33) 94(1) [–91(1)], N(2)–
C(4)–C(7)–N(3) 143(1) [61(2)]. Thermal ellipsoids drawn at 25%
probability.
Scheme 5. Synthesis of the platinum(II) complex 6 by oxidation of
5 with CsBr3.
platinum center and the vinyl carbon atoms. The Ccarbene
–
Pt bond length [2.06(1) Å] is within the range reported in
the Cambridge Crystallographic Structural Database for
mono NHC–Pt0 complexes (average of 2.047 Å for 21 ex- 15N-NMR resonance of Noxa is shifted from δ = 259.5,
amples). The C=C bond lengths of the dvtms ligand 253.9 (5) to 189.0 (6), and the characteristic vibrational
(average value 1.43 Å) are halfway between a single and a band of the C=N bond from 1663 (5) to 1641 cm–1 (6). All
double bond as a result of the backbonding.
these data indicate that the carbene/oxazolinyl ligand acts
The transformation of the platinum(0) complex 5 into a as a bidentate ligand.
platinum(II) species leads to the coordination of the oxazol-
Suitable crystals for an X-ray structure analysis were
ine unit. A clean oxidation of 5 was achieved by stoichio- grown by vapor diffusion of hexane into a solution of 6 in
metric reaction with CsBr3, which acts as mild source of CH2Cl2 (Figure 5). The coordination geometry at the plati-
Br2 (Scheme 5).[28,29] The platinum(II) complex 6 was iso- num center is distorted square planar. The imidazolyl and
lated after purification by column chromatography as a oxazolinyl rings are not coplanar and disposed in such a
white solid in 56% yield.
way that the isopropyl group is bent in one direction and
The oxidation of the platinum is confirmed in the 13C- the bridging dimethylmethylene group in the other and out
NMR spectrum by the upfield shift of the carbene carbon of the coordination plane [N(2)–C(7)–C(1)–N(1) 39.4(5)°]
resonance from δ = 184.5 (5) to 144.2 (6).[30] In the 1H to form a boat-shaped (Pt–C–N–C–C–N) chelate ring, sim-
NMR spectrum, the oxazolyl ring proton signals are ob- ilar to the one observed in 4. The Ccarbene–Pt bond length
served downfield compared to the platinum(0) complex (δ [C(10)–Pt 1.957(3) Å] is within the range for NHC–PtII
= 4.36–3.89 (5), 4.52–4.35 (6)) and the isopropyl protons complexes. The strong trans influence of the NHC ligand is
show very different chemical shifts (δ = 0.57, 0.07). The reflected in the lengthening of the Pt–Br distance in trans
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disposition to the carbene ligand compared to the Pt–Br ture (Scheme 6).[31] Complex 7 derived from imidazolium
distance trans to the oxazoline-N donor atom [2.4807(4) vs. salt 2e has spectroscopic data which are very similar to
2.4334(4) Å].
those of its bromo analogue and was fully characterized by
X-ray diffraction. Its molecular structure is represented in
Figure 6 along with the main bond lengths and angles.
Conclusions
In conclusion, a novel family of mixed carbene/oxazoli-
nyl ligands has been developed. The modular synthetic
strategy gives facile access to several new imidazolium salts.
The coordination behavior of the corresponding carbenes
has been investigated by preparing rhodium and platinum
complexes. In general, the ligand readily adapts to the stereo-
electronic requirements of the metal center and the methyl
groups on the methylene bridge are expected to kinetically
stabilize the bridge under the conditions of molecular catal-
ysis and to allow a better control of the steric environment
around the metal. To which extent the system at hand will
prove useful as a stereodirecting ligand in catalytic transfor-
Figure 5. Molecular structure of complex 6 (hydrogen atoms omit-
ted for clarity). Selected bond lengths [Å] and angles [°]: Pt–C(10)
1.957(3), Pt–N(1) 2.013(3), Pt–Br(1) 2.4807(4), Pt–Br(2) 2.4334(4),
C(1)–N(1) 1.278(4), C(10)–Pt–N(1) 85.21(13), N(1)–Pt–Br(1)
90.49(8), Br(1)–Pt–Br(2) 88.76(1), C(10)–Pt–Br(2) 95.68(9), N(2)– mations is currently being investigated.
C(7)–C(1)–N(1) –39.4(5). Thermal ellipsoids drawn at 50% prob-
ability.
Alternatively, the [(NHC)PtX2] complexes could be ob-
tained by reaction of the imidazolium salt with Ag2O and
subsequent transmetallation with [PtX2(1,5-cod)] (X = Cl),
according to a procedure previously reported in the litera-
Experimental Section
General: All manipulations of air and moisture-sensitive materials
were performed under an inert atmosphere of dry argon using stan-
dard Schlenk techniques. Solvents were purified and dried by stan-
dard methods. (S)-valinol,[32] (S)-tert-leucinol,[33] 1-(1-ethoxycar-
bonyl-1-methylethyl)imidazole and 1-{1-methyl-1-[(4S)-isopropyl-
4,5-dihydrooxazol-2-yl]ethyl}imidazole (1a),[34] bromo(dinaphth-1-
yl)methane,[35] [Rh(nbd)Cl]2[36] and [PtCl2(1,5-cod)][37] were synthe-
sized according to reported procedures. KOtBu was purified by
sublimation prior to use. Karstedt’s catalyst and all other reagents
were obtained commercially and used as received. 1H, 13C and 15
N
spectra were recorded on Bruker Avance 400 and 600 NMR spec-
trometers and were referenced using the residual protio solvent
(1H) or solvent (13C) resonances or externally to 15NH3. Infrared
spectra were recorded on a Varian 3100 FT-IR spectrometer. Mass
spectra and elemental analyses were recorded by the analytical ser-
vice of the Heidelberg Chemistry Department.
Scheme 6. Synthesis of the platinum(II) complex 7.
N-[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]-2-[(imidazol-1-yl)-
methyl]propanamide: (S)-tert-leucinol (2.7 g, 22 mmol), 1-(1-
ethoxycarbonyl-1-methylethyl)imidazole (4.1 g, 22 mmol) and a
catalytic amount of NaH (60% in mineral oil) were placed in a
Schlenk tube and heated at 120 °C for 4 h. After removal of the
generated ethanol in vacuo, the resulting viscous oil was purified
by column chromatography (SiO2, AcOEt/hexane, 3:1) to yield the
reaction product as a white solid (3.16 g, 55 %). 1H NMR
(400 MHz, CDCl3): δ = 7.57 (m, 1 H, CHimid), 7.06 (m, 2 H,
3
CHimid), 5.74 (d, J = 8.7 Hz, 1 H, NH), 4.07 (m, 1 H, OH), 3.77–
3.66 (m, 2 H, NHCH + CH2), 3.40 (m, 1 H, CH2), 1.78 (s, 3 H,
C(CH3)2), 1.76 (s, 3 H, C(CH3)2), 0.79 (s, 9 H, C(CH3)3) ppm.
13C{1H} NMR (100 MHz, CDCl3): δ = 173.3 (NCO), 135.4, 129.7,
117.6 (CHimid), 62.3 (C(CH3)2), 61.5 (CH2), 59.1 (NHCH), 33.8
(C(CH3)3), 26.7 (C(CH3)3), 26.5, 26.4 (C(CH3)2) ppm. MS (EI):
m/z (%) = 254.3 (100) [M + H]+, 222.3 (20) [M + H – CH2OH]+,
154.2 (8) [M + 2H – C6H13O]+, 109.2 (51) [M + H – C7H14NO2]+.
Figure 6. Molecular structure of complex 7 (hydrogen atoms omit-
ted for clarity). Selected bond lengths [Å] and angles [°]: Pt(1)–
C(10) 1.983(8), Pt(1)–N(1) 1.977(7), Pt(1)–Cl(1) 2.373(2), Pt(1)–
Cl(2) 2.297(2), C(1)–N(1) 1.34(1), C(10)–Pt(1)–N(1) 87.1(3), N(1)–
Pt(1)–Cl(1) 89.2(2), C(10)–Pt(1)–Cl(2) 94.2(2), Cl(1)–Pt(1)–Cl(2)
89.16(8), N(2)–C(7)–C(1)–N(1) –38(1). Thermal ellipsoids drawn at
25% probability.
FT-IR (KBr): ν = 1668 (s, νC=O) cm–1. C13H23N3O2 (253.34): calcd.
˜
C 61.63, H 9.15, N 16.59; found C 61.11, H 8.85, N 16.00.
5592
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 5587–5598

A New Class of Modular Oxazoline-NHC Ligands
1-{1-[(4S)-4-tert-Butyl-4,5-dihydrooxazol-2-yl]-1-methylethyl}-
CH(CH3)2), 0.89 (d, 3J = 6.8 Hz, 3 H, CH(CH3)2), 0.83 (d, 3J =
imidazole (1b): N-[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]-2- 6.8 Hz, 3 H, CH(CH3)2) ppm. 13C{1H} NMR (100 MHz, CDCl3):
[(imidazol-1-yl)methyl]propanamide (1.3 g, 5.1 mmol) was dis-
solved in CH2Cl2 (50 mL), and Et3N (1.8 mL, 12.8 mmol) and me-
syl chloride (0.5 mL, 6.4 mmol) were added dropwise at 0 °C. The
resulting orange solution was warmed to ambient temperature and
was, after two hours, washed with an aqueous solution of NH4Cl
(5%, 20 mL). The organic layer was decanted and the aqueous
phase was extracted with additional CH2Cl2 (2ϫ20 mL). The or-
ganic phase was dried with Na2SO4 and evaporated to give an
orange oil which was directly used in the following reaction. A
solution of NaOH (0.45 g, 11.3 mmol) in MeOH/H2O (1:1, 40 mL)
was added to the orange oil and the mixture was refluxed for three
hours. After evaporation of methanol, the aqueous phase was ex-
tracted with CH2Cl2 (4ϫ50 mL). The organic phase was dried with
Na2SO4, concentrated in vacuo and the oily residue was purified
by column chromatography (SiO2, CH2Cl2/MeOH, 97:3) to give
the reaction product 1b as a colorless oil (778 mg, 65%). 1H NMR
(400 MHz, CDCl3): δ = 7.62 (br. s, 1 H, CHimid), 7.03–7.01 (m, 2
δ = 165.4 (NCO), 137.3 (N2C), 133.0 (CPh), 129.4, 129.3 (CHPh),
121.2, 120.3 (CHimid), 72.2 (CHoxa), 71.7 (CH2 oxa), 61.1
(C(CH3)2), 53.4 (CH2Ph), 32.4 (CH(CH3)2), 26.8, 26.6 (C(CH3)2),
18.5, 18.0 (CH(CH3)2 ppm. HR-MS (ESI) m/z (%): calcd. for
C19H26N3O ([M – Br]+) 312.207, found 312.207 (100); calcd. for
C19H28N3O2 ([M + H2O – Br]+) 330.217, found 330.217 (10). FT-
IR (KBr): ν = 1665 (s, νC=N) cm–1. C19H26BrN3O (392.33): calcd.
˜
C 58.17, H 6.68, N 10.71; found C 58.07, H 6.80, N 10.53.
1-{1-[(4S)-Isopropyl-4,5-dihydrooxazol-2-yl]-1-methylethyl}-3-[(R)-
naphth-1-ylmethyl]imidazolium Bromide (2c): Imidazole derivative
1a (485 mg, 2.2 mmol) and 2-(bromomethyl)naphthalene (533 mg,
2.4 mmol) were placed in a Schlenk tube and dissolved in CH3CN
(15 mL). After reacting for 12 h at 60 °C, the solvent was evapo-
rated and the resulting solid was washed with Et2O (2ϫ5 mL) and
pentane (5 mL) and dried in vacuo to yield the imidazolium salt 2c
1
as a white powder (860 mg, 89%). H NMR (400 MHz, CDCl3): δ
= 11.02 (br. s, 1 H, NCHN), 8.04 (br. s, 1 H, CHNp), 7.85–7.75 (m,
3 H, CHNp), 7.61 (dd, 3J = 8.5, 4J = 1.8 Hz, 1 H, CHNp), 7.50–
7.45 (m, 2 H, CHNp), 7.38 (dd, 4J = 1.8, 4J = 1.8 Hz, 1 H, CHimid),
7.29 (dd, 4J = 1.9, 4J = 1.9 Hz, 1 H, CHimid), 5.92 (s, 2 H, CH2Np),
3
H, CHimid), 4.14 (dd, 2J = 8.8, J = 10.1 Hz, 1 H, CH2 oxa), 4.05
2
3
3
(dd, J = 8.8, J = 7.6 Hz, 1 H, 1 H, CH2 oxa), 3.85 (dd, J = 10.1,
3J = 7.6 Hz, 1 H, 1 H, CHoxa), 1.81 (s, 6 H, C(CH3)2), 0.81 (s, 9
H, C(CH3)3) ppm. 13C{1H} NMR (150 MHz, CDCl3): δ = 167.3
(NCO), 135.0, 129.0, 117.0 (CHimid), 75.5 (CHoxa), 69.4 (CH2 oxa),
56.1 (C(CH3)2), 33.7 (C(CH3)3), 26.9, 26.8 (C(CH3)2), 25.6
(CH(CH3)3) ppm. MS (EI): m/z (%) = 234.2 (100) [M – H]+. FT-
2
3
2
4.30 (dd, J = 8.5, J = 9.7 Hz, 1 H, CH2 oxa), 4.00 (dd, J = 8.3,
3J = 8.3 Hz, 1 H, CH2 oxa), 3.90 (ddd, J = 9.7 Hz, J = 7.9 Hz, J
3
3
3
= 6.2 Hz, 1 H, CHoxa), 1.98 (s, 3 H, C(CH3)2), 1.96 (s, 3 H,
3
3
3
C(CH3)2), 1.68 (qqd, J = 6.7 Hz, J = 6.7 Hz, J = 6.7 Hz, 1 H,
CH(CH3)2), 0.84 (d, 3J = 6.8 Hz, 3 H, CH(CH3)2), 0.79 (d, 3J =
6.8 Hz, 3 H, CH(CH3)2) ppm. 13C{1H} NMR (100 MHz, CDCl3):
δ = 165.2 (NCO), 136.9 (N2C), 133.3, 133.0, 130.4 (CNp), 129.3,
129.0, 128.1, 127.7, 126.9, 126.7, 126.0, (CHNp), 121.6, 120.3
(CHimid), 72.1 (CHoxa), 71.6 (CH2 oxa), 60.9 (C(CH3)2), 53.4
(CH2Np), 32.3 (CH(CH3)2), 26.7, 26.6 (C(CH3)2), 18.4, 17.9
(CH(CH3)2) ppm. HR-MS (FAB+) m/z (%): calcd. for C23H28N3O
IR (KBr): ν = 1669 (s, νC=N) cm–1. C13H21N3O (235.33): calcd. C
˜
66.35, H 8.99, N 17.86; found C 66.22, H 8.97, N 17.24.
1-{1-[(4S)-Isopropyl-4,5-dihydrooxazol-2-yl]-1-methylethyl}-3-meth-
ylimidazolium Iodide (2a): Iodomethane (0.52 g, 3.6 mmol) and 1a
(0.41 g, 1.8 mmol) were placed in a Schlenk tube and dissolved in
thf (15 mL). The reaction mixture was stirred at room temperature
for 4 h. After evaporation of the solvents in vacuo, the resulting
solid was washed with a thf/Et2O mixture (1:5, 2ϫ8 mL), pentane
(5 mL) and dried in vacuo to yield the imidazolium salt 2a as a
white powder (0.56 g, 84%). 1H NMR (400 MHz, CDCl3): δ =
([M – Br]+) 362.223, found 362.222 (100). FT-IR (KBr): ν = 1672
˜
(s, νC=N) cm–1. C23H28BrN3O (442.39): calcd. C 62.44, H 6.38, N
9.50; found C 62.58, H 6.45, N 9.41.
4
4
3
10.31 (dd, J = 1.7, J = 1.7 Hz, 1 H, NCHN), 7.43 (dd, J = 1.7,
4J = 1.7 Hz, 1 H, CHimid), 7.35 (dd, 3J = 1.7, 4J = 1.7 Hz, 1 H,
CHimid), 4.36 (dd, J = 8.5, J = 9.6 Hz, 1 H, CH2 oxa), 4.22 (s, 3
3-(Diphenylmethyl)-1-{1-[(4S)-isopropyl-4,5-dihydrooxazol-2-yl]-1-
methylethyl}imidazolium Bromide (2d): The same procedure as the
one described for 2c was followed. Reacting imidazole derivative 1a
(0.90 g, 4.1 mmol) and bromodiphenylmethane (1.31 g, 4.7 mmol)
yielded the imidazolium salt 2d as a white powder (1.65 g, 87%).
2
3
2
3
H, CH3), 4.06 (dd, J = 8.3, J = 8.0 Hz, 1 H, CH2 oxa), 3.96 (ddd,
3J = 9.6, 3J = 7.9, 3J = 6.1 Hz, 1 H, CHoxa), 2.00 (s, 3 H,
C(CH3)2), 2.00 (s, 3 H, C(CH3)2), 1.74 (m, 1 H, CH(CH3)2), 0.90
(d, 3J = 6.8 Hz, 3 H, CH(CH3)2), 0.84 (d, 3J = 6.8 Hz, 3 H,
CH(CH3)2) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 165.1
(NCO), 136.4 (N2C), 123.7, 120.4 (CHimid), 72.1 (CHoxa), 71.6
(CH2 oxa), 60.9 (C(CH3)2), 37.3 (CH3), 32.2 (CH(CH3)2), 26.8, 26.7
(C(CH3)2), 18.5, 17.9 (CH(CH3)2) ppm. HR-MS (ESI) m/z (%):
calcd. for C13H22N3O ([M – I]+) 236.156, found 236.176 (100);
calcd. for C26H44N6O2Cl ([2M + Cl – 2I]+) 507.321, found 507.321
4
1H NMR (400 MHz, CDCl3): δ = 10.70 (pseudo-t, J = 1.6 Hz, 1
H, NCHN), 8.05 (s, 1 H, CHPh2), 7.43 (pseudo-t, J = 1.6 Hz, 1 H,
CHimid), 7.40–7.38 (m, 6 H, CHPh), 7.34–7.30 (m, 4 H, CHPh), 7.14
(pseudo-t, J = 1.6 Hz, 1 H, CHimid), 4.38 (dd, 2J = 8.4, 3J = 9.6 Hz,
2
3
1 H, CH2 oxa), 4.07 (dd, J = 8.4, J = 8.4 Hz, 1 H, CH2 oxa), 3.94
(ddd, 3J = 9.8, 3J = 8.1, 3J = 6.4 Hz, 1 H, CHoxa), 2.03 (s, 3 H,
C(CH3)2), 2.02 (s, 3 H, C(CH3)2), 1.71 (qqd, J = 6.8, J = 6.8, J
= 6.8 Hz, 1 H, CH(CH3)2), 0.89 (d, J = 6.8 Hz, 3 H, CH(CH3)2),
3
3
3
3
(10). FT-IR (KBr): ν = 1668 (s, νC=N) cm–1. C13H22IN3O (363.24):
˜
0.85 (d, 3J = 6.8 Hz, 3 H, CH(CH3)2) ppm. 13C{1H} NMR
(100 MHz, CDCl3): δ = 165.5 (NCO), 137.5 (N2C), 136.7 (CPh),
129.1, 129.0, 128.4 (CHPh), 120.7, 120.3 (CHimid), 72.1 (CHoxa),
72.0 (CH2 oxa), 66.1 (CHPh2), 61.1 (C(CH3)2), 32.4 (CH(CH3)2),
26.5, 26.4 (C(CH3)2), 18.4, 18.1 (CH(CH3)2) ppm. 15N NMR
(60 MHz, CD2Cl2): δ = 235.5 (Noxa), 194.4, 192.1 (Nimid) ppm. HR-
MS (FAB+) m/z (%): calcd. for C25H30N3O ([M – Br]+) 388.239,
calcd. C 42.99, H 6.10, N 11.57; found C 43.39, H 6.30, N 11.48.
3-Benzyl-1-{1-[(4S)-isopropyl-4,5-dihydrooxazol-2-yl]-1-methyl-
ethyl}imidazolium Bromide (2b): The same procedure as the one
described for 2a was used. Reacting benzyl bromide (237 mg,
1.3 mmol) and imidazole 1a (150 mg, 0.7 mmol) yielded the imid-
azolium salt 2b as a white powder (202 mg, 76 %). 1H NMR
(400 MHz, CDCl3): δ = 11.09 (dd, 4J = 1.6, 4J = 1.6 Hz, 1 H,
NCHN), 7.55–7.50 (m, 2 H, CHPh), 7.42–7.35 (m, 3 H, CHPh), 7.28
found 388.239 (100). FT-IR (KBr): ν = 1670 (s, νC = N ).
˜
C28H30BrN3O (504.46): calcd. C 64.10, H 6.46, N 8.97; found C
62.77, H 6.45, N 8.59.
3
4
3
4
(dd, J = 1.9, J = 1.9 Hz, 1 H, CHimid), 7.20 (dd, J = 1.8, J =
2
3
1.8 Hz, 1 H, CHimid), 5.78 (s, 2 H, CH2Ph), 4.34 (dd, J = 8.5, J
= 9.7 Hz, 1 H, CH2 oxa), 4.04 (dd, 2J = 8.4, 3J = 8.1 Hz, 1 H,
3-(Dinaphth-1-ylmethyl)-1-{1-[(4S)-isopropyl-4,5-dihydrooxazol-2-
3
3
3
CH2 oxa), 3.94 (ddd, J = 9.7, J = 7.9, J = 6.2 Hz, 1 H, CHoxa), yl]-1-methylethyl}imidazolium Bromide (2e): Same procedure as the
2.02 (s, 3 H, C(CH3)2), 2.00 (s, 3 H, C(CH3)2), 1.72 (m, 1 H, one described for 2c. Reacting imidazole derivative 1a (1.14 g,
Eur. J. Inorg. Chem. 2008, 5587–5598
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5593

N. Schneider, S. Bellemin-Laponnaz, H. Wadepohl, L. H. Gade
FULL PAPER
5.1 mmol) and bromodinaphth-1-ylmethane (1.98 g, 5.6 mmol) for C26H32N3O ([M – Br]+) 402.254, found 402.249 (100). FT-IR
yields the imidazolium salt 2e as a white powder (2.51 g, 86%).
Suitable crystals for an X-ray diffraction study were obtained by
vapor diffusion of hexane in a solution of 2e in CH2Cl2 at room
temperature. 1H NMR (400 MHz, CDCl3): δ = 11.04 (dd, 4J = 1.6,
4J = 1.6 Hz, 1 H, NCHN), 9.33 (s, 1 H, CHNp2), 8.41 (m, 2 H,
CHNp), 7.90 (m, 2 H, CHNp), 7.88 (m, 2 H, CHNp), 7.57 (m, 2 H,
CHNp), 7.52 (m, 2 H, CHNp), 7.32 (m, 2 H, CHNp), 7.32 (m, 1 H,
(KBr): ν = 1670 (s, νC=N) cm–1.
˜
3-(Dinaphth-1-ylmethyl)-1-{1-[(4S)-isopropyl-4,5-dihydrooxazol-2-
yl]-1-methylethyl}imidazole-2-ylidene: The imidazolium salt 2e
(100 mg, 0.17 mmol) was suspended in thf (10 mL) and cooled to
–78 °C. A suspension of KN[Si(CH3)3]2 (36 mg, 0.18 mmol) in thf
(5 mL) was added dropwise to 2e at –78 °C and the reaction mix-
ture subsequently stirred for 30 min. The resulting yellow mixture
was gradually warmed to room temperature and then evaporated
under reduced pressure to give a yellow solid. The product was
extracted with warm pentane (4 ϫ 15 mL). The solvent was re-
moved at reduced pressure to give the reaction product as a yellow
4
4
3
CHimid), 6.91 (dd, J = 1.6, J = 1.6 Hz, 1 H, CHimid), 6.85 (d, J
= 7.2 Hz, 2 H, CHNp), 4.34 (dd, 2J = 8.6, 9.7 Hz, 1 H, CH2 oxa),
2
3
3
4.04 (dd, J = 8.2, J = 8.2 Hz, 1 H, CH2 oxa), 3.93 (ddd, J = 9.7,
3J = 7.9, 3J = 6.3 Hz, 1 H, CHoxa), 2.04 (s, 6 H, C(CH3)2), 1.69
3
3
3
3
(qqd, J = 6.7, J = 6.7, J = 6.7 Hz, 1 H, CH(CH3)2), 0.88 (d, J
= 6.7 Hz, 3 H, CH(CH3)2), 0.84 (d, J = 6.7 Hz, 3 H, CH(CH3)2)
1
solid (45 mg, 52%). H NMR (600 MHz, C6D6): δ = 8.83 (s, 1 H,
3
3
3
CHNp2), 8.36 (d, J = 8.8 Hz, 1 H, CHNp), 8.33 (d, J = 8.8 Hz, 1
H, CHNp), 7.69 (m, 2 H, CHNp), 7.65 (m, 2 H, CHNp), 7.29–7.22
ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 165.8 (NCO), 138.4
(N2C), 134.0, 133.2, 130.7 (CNp), 130.3, 128.6, 128.2, 126.8, 125.6,
124.7, 124.2 (CHNp), 121.6, 119.5 (CHimid), 72.1 (CHoxa), 71.9
(CH2 oxa), 61.4 (CHNp2), 61.3 (C(CH3)2), 32.5 (CH(CH3)2), 26.4
(C(CH3)2), 18.4, 18.1 (CH(CH3)2) ppm. 15N NMR (60 MHz,
CD2Cl2): δ = 235.2 (Noxa), 195.7, 189.1 (Nimid) ppm. HR-MS
(FAB+) m/z (%): calcd. for C33H34N3O ([M – Br]+) 488.270, found
488.273 (100). FT-IR (KBr): 1661 cm–1 (s, νC=N). C33H34BrN3O
(568.55): calcd. C 69.71, H 6.03, N 7.39; found C 69.92, H 5.95, N
7.43.
3
(m, 4 H, CHNp), 7.19–7.13 (m, 4 H, CHNp), 7.00 (d, J = 1.7 Hz,
1 H, CHimid), 6.71 (d, 3J = 1.7 Hz, 1 H, CHimid), 3.90 (dd, 2J =
3
3
3
3
7.8, J = 9.4 Hz, 1 H, CH2 oxa), 3.78 (dd, J = 9.4, J = 7.8, J =
6.1 Hz, 1 H, CHoxa), 3.69 (dd, 2J = 7.8, 3J = 7.8 Hz, 1 H, CH2 oxa),
2.15 (s, 6 H, C(CH3)2), 1.61 (m, 1 H, CH(CH3)2), 0.95 (d, 3J =
6.7 Hz, 3 H, CH(CH3)2), 0.83 (d, 3J = 6.7 Hz, 3 H, CH(CH3)2)
ppm. 13C{1H} NMR (150 MHz, C6D6): δ = 217.3 (N2C), 169.2
(NCO), 138.0, 137.9, 134.4, 131.9, 131.9 (CNp), 129.0, 128.8, 127.0,
126.8, 126.7, 126.1, 125.5, 124.9 (CHNp), 119.1, 117.3 (CHimid),
72.4 (CHoxa), 70.6 (CH2 oxa), 63.4 (CHNp2), 58.8 (C(CH3)2), 33.7
(CH(CH3)3), 28.3, 28.2 (C(CH3)2), 18.7, 18.5 (CH(CH3)2) ppm. MS
(ESI): m/z (%) = 488.5 (100) [M + H]+.
1-{1-[(4S)-tert-Butyl-4,5-dihydrooxazol-2-yl]-1-methylethyl}-3-(di-
naphth-1-ylmethyl)imidazolium Bromide (2f): The same procedure
as the one described for 2c was followed. Reacting imidazole deriv-
ative 1b (1.40 g, 6.0 mmol) and bromo(dinaphth-1-yl)methane
(2.08 g, 6.0 mmol) yielded the imidazolium salt 2f as a white pow-
der (2.79 g, 80%). 1H NMR (400 MHz, CDCl3): δ = 10.85 (m, 1
H, NCHN), 9.27 (s, 1 H, CHNp2), 8.49 (m, 2 H, CHNp), 7.87 (m,
2 H, CHNp), 7.85 (m, 2 H, CHNp), 7.47 (m, 4 H, CHNp), 7.41 (m,
1 H, CHimid), 7.32 (m, 2 H, CHNp), 6.91 (dd, 3J = 1.7, 4J = 1.7 Hz,
1 H, CHimid), 6.83 (m, 1 H, CHNp), 6.81 (m, 1 H, CHNp), 4.26 (dd,
2J = 8.8, 3J = 10.1 Hz, 1 H, CH2 oxa), 4.10 (dd, 2J = 8.8, 3J =
8.0 Hz, 1 H, CH2 oxa), 3.86 (dd, 3J = 10.1, 3J = 8.0 Hz, 1 H, CHoxa),
1.98 (s, 3 H, C(CH3)2), 1.96 (s, 3 H, C(CH3)2), 0.79 (s, 9 H,
C(CH3)3) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 165.6
(NCO), 138.0 (N2C), 133.8, 133.0, 130.6, 130.5 (CNp), 130.1, 128.5,
127.9, 126.6, 126.5, 125.5, 124.7, 124.0, 121.5, 119.8 (CHimid), 75.4
(CHoxa), 70.3 (CH2 oxa), 61.2 (CHNp2), 61.2 (C(CH3)2), 33.6
(C(CH3)3), 26.3, 26.2 (C(CH3)2), 25.5 (C(CH3)3) ppm. 15N NMR
(60 MHz, CD2Cl2): δ = 234.0 (Noxa), 195.8, 189.0 (Nimid) ppm. HR-
MS (FAB+) m/z (%): calcd. for C34H36N3O ([M – Br]+) 502.286,
1-{1-[(4S)-tert-Butyl-4,5-dihydrooxazol-2-yl]-1-methylethyl}-3-(di-
naphth-1-ylmethyl)imidazole-2-ylidene: Same procedure as the one
described above. Reacting imidazolium salt 2f (100 mg, 0.17 mmol)
with KN[Si(CH3)3]2 (36 mg, 0.18 mmol) in thf at –78 °C gives the
reaction product as a yellow solid (65 mg, 75 %). 1H NMR
3
(600 MHz, C6D6): δ = 8.71 (s, 1 H, CHNp2), 8.25 (d, J = 8.5 Hz,
3
1 H, CHNp), 8.21 (d, J = 8.5 Hz, 1 H, CHNp), 7.61–7.53 (m, 4 H,
CHNp), 7.18–7.11 (m, 4 H, CHNp), 7.08–7.02 (m, 4 H, CHNp), 6.89
3
3
(d, J = 1.5 Hz, 1 H, CHimid), 6.59 (d, J = 1.5 Hz, 1 H, CHimid),
3.75–3.73 (m, 2 H, CH2 oxa + CHoxa), 3.64 (dd, 2J = 7.8, 3J =
10.5 Hz, 1 H, CH2 oxa), 2.03 (s, 6 H, C(CH3)2), 0.78 (s, 9 H,
C(CH3)3) ppm. 13C{1H} NMR (150 MHz, C6D6): δ = 217.3 (N2C),
169.1 (NCO), 137.9, 137.8, 134.3, 131.7 (CNp), 128.8, 128.7, 126.9,
126.8, 126.7, 125.9, 125.4, 124.4 (CHNp), 119.0, 117.3 (CHimid),
75.8 (CHoxa), 68.9 (CH2 oxa), 63.3 (CHNp2), 58.6 (C(CH3)2), 33.6
(C(CH3)3), 28.1 (C(CH3)2), 25.8 (C(CH3)3) ppm. MS (FAB): m/z
(%) = 502.5 (100) [M + H]+.
found 502.286 (100). FT-IR (KBr): ν = 1669 (s, νC=N) cm–1
.
˜
Bromo{1-{1-[(4S)-tert-butyl-4,5-dihydrooxazol-2-yl]-1-methylethyl}-
3-(dinaphth-1-ylmethyl)imidazol-2-ylidene}(η4-2,5-norbornadiene)-
rhodium(I) (3): A solution of the imidazolium salt 2f (100 mg,
0.17 mmol) in thf (5 mL) was added to a suspension of
KN[Si(CH3)3]2 (36 mg, 0.17 mmol) in thf (5 mL) at –78 °C. The
resulting yellow solution was stirred for 30 min and a solution of
[Rh(nbd)Cl]2 (40 mg, 0.08 mmol) in thf (3 mL) was added. The
C34H36BrN3O (582.57): calcd. C 70.10, H 6.23, N 7.21; found C
70.07, H 6.42, N 6.84.
1-{1-[(4S)-tert-Butyl-4,5-dihydrooxazol-2-yl]-1-methylethyl}-3-(di-
phenylmethyl)imidazolium Bromide (2g): Same procedure as the one
described for 2c. Reacting imidazole derivative 1b (400 mg,
1.7 mmol) and bromodiphenylmethane (460 mg, 1.9 mmol) yielded
the imidazolium salt 2g as a white powder (506 mg, 61 %). 1H mixture was stirred and warmed to ambient temperature and
NMR (400 MHz, CDCl3): δ = 10.74 (m, 1 H, NCHN), 8.07 (s, 1 stirred for 12 h. The orange-red solution was then filtered and the
H, CHPh2), 7.39–7.28 (m, 11 H, 10 CHPh + CHimid), 7.09 (m, 1 H, volatiles were removed in vacuo. The resulting orange solid was
2
3
CHimid), 4.30 (dd, J = 8.9, J = 10.2 Hz, 1 H, CH2 oxa), 4.15 (dd,
2J = 8.9, 3J = 7.9 Hz, 1 H, CH2 oxa), 3.88 (dd, 3J = 10.2, 3J =
7.9 Hz, 1 H, CHoxa), 2.03 (s, 3 H, C(CH3)2), 2.01 (s, 3 H,
C(CH3)2), 0.82 (s, 9 H, C(CH3)3) ppm. 13C{1H} NMR (100 MHz,
CDCl3): δ = 165.6 (NCO), 137.9 (N2C), 137.0, 136.9 (CPh), 129.3,
129.2, 129.1, 128.6 (CHPh), 120.6, 120.0 (CHimid), 75.6 (CHoxa),
washed twice with Et2O/CH2Cl2 (5:1, 2ϫ5 mL) and hexane (5 mL)
to yield 3 as a yellow solid (114 mg, 86%). Suitable crystals for an
X-ray diffraction study were grown from vapor diffusion of hexane
into a solution of 3 in CH2Cl2. 1H NMR (600 MHz, CD2Cl2,
263 K): δ = 8.36 (d, 3J = 6.3 Hz, 1 H, CHNp), 8.11 (s, 1 H, CHNp2),
3
8.01 (d, J = 6.1 Hz, 1 H, CHNp), 7.96–7.95 (m, 2 H, CHNp), 7.91
3
70.5 (CH2 oxa), 66.1 (CHNp2), 61.2 (C(CH3)2), 33.7 (C(CH3)3), 26.4 (d, J = 8.0 Hz, 1 H, CHNp), 7.65–7.64 (m, 2 H, CHNp), 7.53 (dd,
3
3
3
(C(CH3)2), 25.6 (C(CH3)3) ppm. HR-MS (FAB+) m/z (%): calcd. 3J = 7.6, J = 7.6 Hz, 1 H, CHNp), 7.49 (dd, J = 7.4, J = 7.4 Hz,
5594 Eur. J. Inorg. Chem. 2008, 5587–5598
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A New Class of Modular Oxazoline-NHC Ligands
3
3
1 H, CHNp), 7.48 (dd, J = 6.6, J = 6.6 Hz, 1 H, CHNp), 7.42 (m,
1 H, CHimid), 7.34 (dd, 3J = 7.3, 3J = 7.3 Hz, 1 H, CHNp), 7.25
(60 MHz, CD2Cl2, 263 K): δ = 198.9, 196.7 (Nimid), 189.8 (Noxa)
ppm. HR-MS (FAB+) m/z (%): calcd. for C41H43N3ORh ([M –
3
3
3
(dd, J = 7.5, J = 7.5 Hz, 1 H, CHNp), 6.85 (d, J = 7.0 Hz, 1 H,
PF6]+) 696.246, found 696.241 (80); calcd. for C34H35N3ORh ([M –
CHNp), 6.74 (d, J = 6.9 Hz, 1 H, CHNp), 6.62 (m, 1 H, CHimid), C7H8 – PF ]+) 604.183, found 604.129 (100). FT-IR (KBr): ν =
3
˜
6
5.20 (m, 1 H, CH 2/3/5/6-nbd), 4.84 (dd, J = 9.6, J = 9.6 Hz, 1 H, 1637 (s, νC=N) cm–1. C41H43F6N3OPRh (841.67): calcd. C 58.51, H
2
3
CH2 oxa), 4.62 (m, 2 H, CH 2/3/5/6-nbd + CH2 oxa), 3.69–3.67 (m, 2 5.15, N 4.99; found C 58.96, H 5.25, N 4.79.
H, CHoxa + CH 1/4-nbd), 3.57 (m, 1 H, CH 1/4-nbd), 3.49 (m, 1 H,
{3-(Diphenylmethyl)-1-{1-[(4S)-isopropyl-4,5-dihydrooxazol-2-yl]-1-
CH 2/3/5/6-nbd), 3.45 (m, 1 H, CH 2/3/5/6-nbd), 2.76 (s, 3 H, C(CH3)2),
methylethyl}imidazol-2-ylidene}(1,1,3,3-tetramethyl-1,3-divinylsilox-
ane)platinum(0) (5): To a solution of the imidazolium salt 2d
(468 mg, 1 mmol) and Karstedt’s catalyst (2.1–2.4% Pt in xylene,
2.00 (s, 3 H, C(CH3)2), 1.87 (d, 2J = 8.3 Hz, 1 H, CH2 nbd), 1.39 (s,
2
9 H, C(CH3)3), 0.93 (d, J = 8.3 Hz, 1 H, CH2 nbd) ppm. 13C{1H}
NMR (150 MHz, CD2Cl2, 263 K): δ = 177.4 (d, 1J(103Rh13C) =
9.3 g, 1 mmol) in thf (5 mL) was added a solution of KOtBu
56.1 Hz, N2C), 170.2 (NCO), 135.4, 133.9, 133.4, 133.3, 130.5
(178 mg, 1.4 mmol) in thf (5 mL) at –78 °C. The mixture was stirred
and warmed to ambient temperature and stirred for 12 h. The
orange-red solution was then filtered and the solvents were re-
moved in vacuo. The resulting orange oil was purified by column
chromatography (SiO2, AcOEt/hexane, 35:65) to yield 5 as a white
solid (615 mg, 80 %). Suitable crystals for an X-ray diffraction
(CNp), 130.0 (CHNp), 129.2 (CNp), 129.2, 128.9, 128.3, 127.3, 126.8,
126.2, 126.0, 125.1, 125.0, 123.7, 123.3, 122.0 (CHNp), 120.3, 118.7
(CHimid), 82.1 (d, 1J(103Rh13C) = 2.4 Hz, CH 2/3/5/6-nbd), 73.8 (d,
1J(103Rh13C) = 3.0 Hz, CH 2/3/5/6-nbd), 73.1 (CHoxa), 71.8 (CH2 oxa),
65.0 (CH2 n b d ), 62.1 (CHNp2), 60.5 (C(CH3)2), 60.2 (d,
1J(103Rh13C) = 9.5 Hz, CH 2/3/5/6-nbd), 57.0 (d, 1J(103Rh13C) =
study were grown from a saturated solution of 5 in pentane. 1H
9.8 Hz, CH 2/3/5/6-nbd), 52.7, 50.9 (CH 1/4-nbd), 35.5 (C(CH3)2), 33.2
NMR (600 MHz, CD2Cl2, 273 K): δ = 7.37–7.33 (m, 12 H, CHPh),
(C(CH3)3), 24.8 (C(CH3)3), 23.5 (C(CH3)2) ppm. 15N NMR
3
7.29 (s, 1 H, CHPh2), 7.28 (d, J = 2.2 Hz, 1 H, CHimid), 7.20 (s, 1
(60 MHz, CD2Cl2, 263 K): δ = 198.4, 197.5 (Nimid), 180.3 (Noxa
)
H, CHPh2), 7.19 (m, 1 H, CHimid), 7.08–7.03 (m, 8 H, CHPh), 6.97
ppm. HR-MS (FAB+) m/z (%): calcd. for C41H43N3ORh ([M –
3
3
(d, J = 1.1 Hz, 1 H, CHimid), 6.93 (d, J = 1.1 Hz, 1 H, CHimid),
Br]+) 696.246, found 696.246 (100); calcd. for C34H35N3ORh ([M –
2
3
2
4.36 (dd, J = 8.4, J = 9.5 Hz, 1 H, CH2 oxa), 4.26 (dd, J = 8.5,
C H – Br]+) 604.183, found 604.183 (75). FT-IR (KBr): ν = 1664
˜
3J = 9.7 Hz, 1 H, CH2 oxa), 4.04 (dd, J = 8.2, J = 8.2 Hz, 1 H,
2
3
7
8
(s, νC=N) cm–1.
2
3
CH2 oxa), 4.00 (dd, J = 8.3, J = 8.2 Hz, 1 H, CH2 oxa), 3.98 (m, 1
H, CHoxa), 3.89 (ddd, 3J = 9.7, J = 8.3, J = 6.3 Hz, 1 H, CHoxa),
3
3
{1-{1-[(4S)-tert-Butyl-4,5-dihydrooxazol-2-yl]-1-methylethyl}-3-(di-
naphth-1-ylmethyl)imidazol-2-ylidene}(η4-2,5-norbornadiene)rhodi-
um(I) Hexafluorophosphate (4): To a solution of 3 (80 mg,
0.10 mmol) in CH2Cl2 (5 mL) were added KPF6 (29 mg,
0.15 mmol) and water (5 mL). The orange-red solution was stirred
for 30 min at room temperature. The organic layer was decanted
and the aqueous phase was extracted with additional CH2Cl2
(2 ϫ 10 mL). The organic layers were combined, dried under
Na2SO4 and the volatiles were removed in vacuo. The resulting
solid was washed with Et2O (5 mL) and hexane (5 mL) to yield 4
as a yellow solid (78 mg, 90%). Suitable crystals for an X-ray dif-
fraction study were obtained by slow diffusion of pentane into a
2.25 (d, J(195Pt1H) = 53.2, J = 11.3 Hz, 1 H, CH2=CHSi), 2.20
2
3
2
3
(d, J(195Pt1H) = 50.6, J = 11.4 Hz, 1 H, CH2=CHSi), 1.95 (m, 1
H, CH2=CHSi), 1.95 (s, 3 H, C(CH3)2), 1.93 (s, 3 H, C(CH3)2),
1.89 (m, 1 H, CH2=CHSi), 1.89 (s, 3 H, C(CH3)2), 1.86 (s, 3 H,
C(CH3)2), 1.83–1.68 (m, 9 H, 4 CH2=CHSi + 3 CH2=CHSi + 2
CH(CH3)2), 1.59 (d, 2J(195Pt1H) = 52.3, 3J = 13.5 Hz, 1 H,
3
3
CH2=CHSi), 0.97 (d, J = 6.8 Hz, 3 H, CH(CH3)2), 0.95 (d, J =
6.8 Hz, 3 H, CH(CH3)2), 0.89 (d, 3J = 6.8 Hz, 3 H, CH(CH3)2),
3
0.88 (d, J = 6.8 Hz, 3 H, CH(CH3)2), 0.28 (s, 3 H, SiCH3 eq), 0.27
(s, 3 H, SiCH3 eq), 0.25 (s, 3 H, SiCH3 eq), 0.23 (s, 3 H, SiCH3 eq),
–0.28 (s, 3 H, SiCH3 ax), –0.30 (s, 3 H, SiCH3 ax), –0.47 (s, 3 H,
SiCH3 ax), –0.51 (s, 3 H, SiCH3 ax) ppm. 13C{1H} NMR (150 MHz,
CD2Cl2, 273 K): δ = 184.9 (s, 1J(195Pt13C) = 1375.7 Hz, N2C), 184.1
1
solution of 4 in CDCl3. H NMR (600 MHz, CD2Cl2, 263 K): δ =
3
8.31 (d, J = 7.2 Hz, 1 H, CHNp), 8.06 (s, 1 H, CHNp2), 8.00 (d,
1
(s, J(195Pt13C) = 1384.3 Hz, N2C), 167.9, 167.5 (NCO), 140.0,
3
3J = 7.2 Hz, 1 H, CHNp), 7.94–7.92 (m, 2 H, CHNp), 7.89 (d, J =
139.8, 139.7 (CPh), 128.6, 128.5, 128.4, 128.3, 127.8, 127.7 (CHPh),
121.2 (s, 3J(195Pt13C) = 43.3 Hz, CHimid), 121.0 (s, 3J(195Pt13C) =
44.7 Hz, CHimid), 119.0 (s, 3J(195Pt13C) = 31.4 Hz, CHimid), 118.8
(s, 3J(195Pt13C) = 33.3 Hz, CHimid), 72.2, 72.2 (CHoxa), 70.8, 70.7
(CH2 oxa), 67.5 (s, 3J(195Pt13C) = 48.4 Hz, CHPh2), 67.4 (s,
3J(195Pt13C) = 47.2 Hz, CHPh2), 60.3, 59.9 (C(CH3)2), 43.7 (s,
1J(195Pt13C) = 163.4 Hz, CH2=CHSi), 43.0 (s, 1J(195Pt13C) =
166.4 Hz, CH2=CHSi), 42.3 (s, 1J(195Pt13C) = 162.8 Hz,
CH2=CHSi), 42.1 (s, 1J(195Pt13C) = 164.5 Hz, CH2=CHSi), 33.8 (s,
1J(195Pt13C) = 122.2 Hz, CH2=CHSi), 33.6 (s, 1J(195Pt13C) =
119.8 Hz, CH2=CHSi), 33.5 (s, 1J(195Pt13C) = 121.4 Hz,
CH2=CHSi), 33.4 (s, 1J(195Pt13C) = 121.0 Hz, CH2=CHSi), 32.6
(CH(CH3)2), 27.6, 27.4, 27.2 (C(CH3)2), 18.6, 17.8 (CH(CH3)2), 1.2
(SiCH3 eq), –2.1, –2.2, –2.9 (SiCH3 ax) ppm. 15N NMR (60 MHz,
CD2Cl2, 273 K): δ = 259.5, 253.9 (Noxa), 253.8, 252.8, 252.8, 251.8
(Nimid) ppm. MS (EI+): m/z (%) = 769.2 (50) [M + H]+, 582.2 (85)
3
8.1 Hz, 1 H, CHNp), 7.63–7.60 (m, 2 H, CHNp), 7.50 (dd, J = 7.5,
3J = 7.5 Hz, 1 H, CHNp), 7.46 (dd, 3J = 7.5, 3J = 7.5 Hz, 1 H,
3
3
3
CHNp), 7.43 (d, J = 8.6 Hz, 1 H, CHNp), 7.31 (dd, J = 7.5, J =
3
3
7.5 Hz, 1 H, CHNp), 7.22 (dd, J = 7.6, J = 7.6 Hz, 1 H, CHNp),
3
7.14 (m, 1 H, CHimid), 6.82 (d, J = 7.2 Hz, 1 H, CHNp), 6.86 (d,
3J = 7.0 Hz, 1 H, CHNp), 6.58 (m, 1 H, CHimid), 4.86 (m, 1 H,
2
3
CH 2/3/5/6-nbd), 4.59 (dd, J = 9.8, J = 4.3 Hz, 1 H, CH2 oxa), 4.57
(m, 1 H, CH 2/3/5/6-nbd), 4.53 (dd, 2J = 9.8, 3J = 9.5 Hz, 1 H,
CH2 oxa), 3.64 (m, 1 H, CH 1/4-nbd), 3.57 (m, 1 H, CH 2/3/5/6-nbd),
3.54 (dd, 3 J = 9. 5, 3 J = 4. 3 Hz, CHo x a ), 3.50 (m, 1 H,
CH 2/3/5/6-nbd), 3.43 (m, 1 H, CH 1/4-nbd), 2.71 (s, 3 H, C(CH3)2),
1.92 (s, 3 H, C(CH3)2), 1.17 (d, 2J = 8.5 Hz, 1 H, CH2 nbd), 1.10 (s,
9 H, C(CH3)3), 0.92 (d, J = 8.5 Hz, 1 H, CH2 nbd) ppm. 13C{1H}
2
NMR (150 MHz, CD2Cl2, 263 K): δ = 177.6 (d, 1J(103Rh13C) =
56.5 Hz, N2C), 170.7 (NCO), 135.6, 134.3, 133.8, 133.5, 130.8
(CNp), 130.4 (CHNp), 129.6 (CNp), 129.6, 129.3, 128.7, 127.7, 127.2,
126.6, 126.3, 125.5, 125.4, 124.1, 123.6, 122.2 (CHNp), 120.7, 118.4
(CHimid), 81.8 (d, 1J(103Rh13C) = 5.5 Hz, CH 2/3/5/6-nbd), 74.2 (d,
1J(103Rh13C) = 3.1 Hz, CH 2/3/5/6-nbd), 73.4 (CHoxa), 71.9 (CH2 oxa),
[M – C8H18OSi2 + H]+. FT-IR (KBr): ν = 1663 (s, νC=N) cm–1.
˜
C33H47N3O2PtSi2 (768.99): calcd. C 51.54, H 6.16, N 5.46; found
C 51.72, H 6.22, N 5.45.
1
65.4 (CH2 nbd), 62.5 (CHNp2), 60.9 (d, J(103Rh13C) = 10.4 Hz,
Dibromo{3-(diphenylmethyl)-1-{1-[(4S)-isopropyl-4,5-dihydrooxazol-
CH 2/3/5/6-nbd), 60.8 (C(CH3)2), 57.6 (d, 1J(103Rh13C) = 9.4 Hz, 2-yl]-1-methylethyl}imidazol-2-ylidene}platinum(II) (6): CsBr3
CH 2/3/5/6-nbd), 53.2, 51.4 (CH 1/4-nbd), 35.7 (C(CH3)2), 33.7 (93 mg, 0.25 mmol) was added to a solution of 5 (190 mg,
(C(CH3)3), 25.1 (C(CH3)3), 23.5 (C(CH3)2) ppm. 15N NMR 0.25 mmol) in toluene (15 mL) and the mixture was allowed to re-
Eur. J. Inorg. Chem. 2008, 5587–5598
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5595

N. Schneider, S. Bellemin-Laponnaz, H. Wadepohl, L. H. Gade
act for 20 min at 60 °C. The formation of a white precipitate was clusion of light. After stirring for five days at room temperature, a
FULL PAPER
observed and the solvent was evaporated. The resulting solid was
purified by column chromatography (SiO2, CH2Cl2/MeOH, 97:3)
to yield 6 as a white solid (104 mg, 56%). Suitable crystals for X-
ray diffraction were obtained by vapor diffusion of hexane ito a
solution of [PtCl2(1,5-cod)] (195 mg, 0.52 mmol) in CH2Cl2 (5 mL)
was added and the mixture was allowed to react for one day at
room temperature. The solution was then filtered through celite
and the solvent was evaporated. The resulting yellow solid was
solution of 6 in CH2Cl2 at 0 °C. 1H NMR (600 MHz, CDCl3): δ = purified by column chromatography (SiO2, CH2Cl2/MeOH, 97:3)
8.57 (s, 1 H, CHPh2), 7.39 (m, 1 H, CHPh), 7.38 (m, 1 H, CHPh), to yield 7 as a white solid (210 mg, 54%). Suitable crystals for X-
7.34–7.29 (m, 6 H, CHPh), 7.22 (m, 1 H, CHPh), 7.20 (m, 1 H, ray diffraction were obtained by vapor diffusion of hexane into a
3
3
CHPh), 7.04 (d, J = 2.2 Hz, 1 H, CHimid), 6.86 (d, J = 2.2 Hz, 1
solution of 7 in CH2Cl2 at 0 °C. 1H NMR (400 MHz, CDCl3): δ =
2
3
3
H, CHimid), 5.20 (m, 1 H, CHoxa), 4.52 (dd, J = 9.2, J = 10.2 Hz, 9.83 (s, 1 H, CHNp2), 8.86 (d, J = 8.1 Hz, 1 H, CHNp), 8.00 (d,
1 H, CH2 oxa), 4.35 (dd, J = 9.2, J = 5.9 Hz, 1 H, CH2 oxa), 2.61 3J = 8.0 Hz, 1 H, CHNp), 7.85 (m, 2 H, CHNp), 7.80 (m, 2 H,
2
3
3
3
(s, 3 H, C(CH3)2), 1.91 (m, 1 H, CH(CH3)2), 1.88 (s, 3 H,
CHNp), 7.59 (m, 1 H, CHNp), 7.52 (dd, J = 7.1, J = 7.1 Hz, 1 H,
C(CH3)2), 0.57 (d, 3J = 7.0 Hz, 3 H, CH(CH3)2), 0.07 (d, 3J = CHNp), 7.44 (dd, J = 7.2, J = 7.2 Hz, 1 H, CHNp), 7.32 (dd, J
3
3
3
6.7 Hz, 3 H, CH(CH3)2) ppm. 13C{1H} NMR (150 MHz, CDCl3): = 7.8, J = 7.5 Hz, 1 H, CHNp), 7.30 (dd, J = 8.1, J = 7.5 Hz, 1
3
3
3
3
3
δ = 168.7 (NCO), 144.2 (N2C), 139.2, 138.3 (CPh), 129.6, 128.9,
H, CHNp), 7.17 (dd, 3J = 7.7, J = 7.7 Hz, 1 H, CHNp), 6.94 (d, J
3
128.7, 128.5, 128.2, 128.0 (CHPh), 120.7, 116.5 (CHimid), 72.2 = 2.1 Hz, 1 H, CHimid), 6.76 (d, J = 7.2 Hz, 1 H, CHNp), 6.53 (d,
(CH2 oxa), 68.7 (CHoxa), 66.5 (CHPh2), 60.5 (C(CH3)2), 29.9 3J = 2.1 Hz, 1 H, CHimid), 6.48 (d, J = 6.1 Hz, 1 H, CHNp), 5.10
3
(C(CH3)2), 29.7 (CH(CH3)2), 23.6 (C(CH3)2), 17.4, 14.7 (CH- (m, 1 H, CHoxa), 4.53 (dd, 2J = 9.7, 3J = 9.7 Hz, 1 H, CH2 oxa),
(CH3)2) ppm. 15N NMR (60 MHz, CDCl3): δ = 199.3, 193.4
(Nimid), 189.0 (Noxa) ppm. HR-MS (FAB+) m/z (%): calcd. for
4.42 (dd, 2J = 9.2, 3J = 5.8 Hz, 1 H, CH2 oxa), 2.53 (s, 3 H,
C(CH3)2), 1.91 (m, 1 H, CH(CH3)2), 1.88 (s, 3 H, C(CH3)2), 0.68
C25H29BrN3OPt ([M – Br]+) 662.114, found 662.117 (40); calcd. for (d, 3J = 6.9 Hz, 3 H, CH(CH3)2), 0.42 (d, 3J = 6.5 Hz, 3 H,
C25H28N3OPt ([M – 2Br – H]+) 581.188, found 581.165 (100). FT-
CH(CH3)2) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 168.9
(NCO), 141.6 (N2C), 137.0, 134.5, 134.0, 133.8, 131.7, 130.9 (CNp),
129.6, 128.5, 128.4, 128.3, 128.1, 127.4, 127.0, 126.3, 126.2, 125.3,
124.8, 124.6, 124.5, 123.8 (CHNp), 121.6, 115.7 (CHimid), 71.9
(CH2 oxa), 67.8 (CHoxa), 61.4 (CHNp2), 60.1 (C(CH3)2), 30.5
(C(CH3)2), 29.5 (CH(CH3)2), 23.5 (C(CH3)2), 17.6, 15.4 (CH-
(CH3)2) ppm. MS (FAB+): m/z (%) = 717.1 (40) [M – Cl]+, 681.1
IR (KBr): ν = 1641 (s, νC=N) cm–1.
˜
Dichloro{3-(dinaphth-1-ylmethyl)-1-{1-[(4S)-isopropyl-4,5-dihydro-
oxazol-2-yl]-1-methylethyl}imidazol-2-ylidene}platinum(II) (7): The
imidazolium salt 2e (300 mg, 0.52 mmol) was dissolved in CH2Cl2
(10 mL) and silver oxide (86 mg, 0.37 mmol) was added under ex-
Table 1. Details of the crystal structure determinations of 2e, 3, 4, 5, 6 and 7.
2e
[C33H34N3O][Br] C41H43BrN3ORh [C41H43N3ORh][PF6] C33H47N3O2PtSi2 C25H29Br2N3OPt C33H33Cl2N3OPt
3
4
5
6
7
Formula
Crystal system
Space group
triclinic
P1
triclinic
P1
monoclinic
P21
monoclinic
P21
orthorhombic
P212121
orthorhombic
P212121
a [Å]
b [Å]
c [Å]
α [°]
9.5637(9)
9.9034(10)
15.1840(15)
82.572(2)
89.163(2)
88.962(2)
1425.7(2)
2
11.1212(13)
12.0126(14)
14.2028(16)
65.563(2)
83.671(2)
78.971(2)
1694.5(3)
2
10.0434(7)
19.8604(14)
10.8516(8)
12.8775(11)
15.6965(14)
17.4003(15)
7.6433(4)
17.0957(9)
18.8429(9)
8.0091(4)
14.6503(8)
29.5201(16)
β [°]
104.6900(10)
104.910(2)
γ [°]
V [Å3]
2093.8(3)
2
3398.7(5)
4
2462.2(2)
4
3463.8(3)
4
Z
Mr
568.54
1.324
592
776.60
1.522
796
1.721
961.03
1.524
980
769.01
1.503
1552
742.42
2.003
1424
753.61
1.445
1488
dc [Mgm–3]
F(000)
µ(Mo-Kα) [mm–1]
Transmission factors
max., min.
1.472
0.702
4.231
8.967
4.232
0.7457, 0.6261
150(2)
0.9189, 0.8467
100(2)
0.7445, 0.5995
100(2)
1.9 to 32.2
–15 ... 14,
–29 ... 29,
0 ... 15
0.7457, 0.6149
200(2)
1.2 to 28.3
–17 ... 16,
–20 ... 20,
0 ... 23
0.7464, 0.5075
100(2)
1.6 to 31.5
–11 ... 11,
0 ... 25,
0.8163, 0.3633
298(2)
1.34 to 25.0
–9 ... 9,
0 ... 17,
0 ... 35
Data collection temp. [K]
θ range [°]
Index ranges (indep. sets)
h,k,l
2.1 to 25.0
–11 ... 11,
–11 ... 11,
–18 ... 18
23903
1.6 to 32.2
–16 ... 16,
–17 ... 17,
–20 ... 21
42316
0 ... 27
60855
Refl. measured
52083
35584
20849
Refl. unique [Rint
]
9930 [0.0590]
8079
693
20702 [0.0723]
12708
858
13724 [0.0383]
13391
531
16717 [0.0446]
11660
756
8101 [0.0606]
7274
293
6120 [0.0547]
4547
365
Refl. obsd. [IՆ2σ(I)]
Parameters refined
R indices [FϾ4σ(F)]
R(F), wR(F2)
0.0431, 0.0874
0.0572, 0.1353
0.0232, 0.0580
0.0422, 0.0782
0.0261, 0.0467
0.0396, 0.0919
R indices (all data)
0.0618, 0.0950
0.1140, 0.1637
0.0242, 0.0587
0.0755, 0.0876
0.0337, 0.0489
0.0563, 0.0984
R(F), wR(F2)
GooF on F2
1.001
0.851
1.068
1.007
1.081
0.944
Absol. structure parameter
0.009(6)
0.035(5)
1.475, –1.535
–0.012(9)
0.889, –0.322
0.034(12)
1.260, –0.556
–0.017(5)
1.546, –0.807
–0.018(11)
1.305, –1.612
Largest residual peaks [e·Å–3] 0.473, –0.302
5596
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Eur. J. Inorg. Chem. 2008, 5587–5598

A New Class of Modular Oxazoline-NHC Ligands
(80) [M – 2Cl – H]+. FT-IR (KBr): ν = 1655 (s, νC=N) cm–1.
C33H33Cl2N3OPt (753.62): calcd. C 52.59, H 4.41, N 5.58; found
C 52.80, H 4.75, N 5.36.
Díez-González, S. P. Nolan, Coord. Chem. Rev. 2007, 251, 874–
883.
˜
[7] Reviews: a) W. A. Herrmann, Angew. Chem. 2002, 114, 1342–
1363; Angew. Chem. Int. Ed. 2002, 41, 1290–1309; b) E. Colac-
ino, J. Martinez, F. Lamaty, Coord. Chem. Rev. 2007, 251, 726–
764; c) E. A. B. Kantchev, C. J. O’Brien, M. G. Organ, Angew.
Chem. 2007, 119, 2824–2870; Angew. Chem. Int. Ed. 2007, 46,
2768–2813.
[8] Review: S. T. Liddle, I. S. Edworthy, P. L. Arnold, Chem. Soc.
Rev. 2007, 36, 1732–1744.
[9] Review: V. César, S. Bellemin-Laponnaz, L. H. Gade, Chem.
Soc. Rev. 2004, 33, 619–636.
X-ray Crystal Structure Determinations: Crystal data and details of
the structure determinations are listed in Table 1. Intensity data
were collected with Bruker AXS Smart 1000 and Apex CCD dif-
fractometers (Mo-Kα, graphite monochromator, λ = 0.71073 Å).
Data were corrected for air and detector absorption, Lorentz and
polarization effects;[38] absorption by the crystal was treated with a
semiempirical multiscan method.[39,40] The structures were solved
by the heavy atom method combined with structure expansion by
direct methods applied to difference structure factors[41] (com-
pounds 2e, 3, 6 and 7), by conventional direct methods[42,43] (com-
plex 4) or by direct methods with dual-space recycling (“Shake-
and-Bake”)[44] (complex 5). Refinement was carried out with full-
matrix least-squares methods based on F2 against all unique reflec-
tions.[43,45] All non-hydrogen atoms were given anisotropic displace-
ment parameters. Hydrogen atoms were generally placed in calcu-
lated positions and refined with a riding model. When justified by
the quality of the data the positions of some hydrogen atoms (those
on the carbon atoms involved in coordination to the metal) were
taken from difference Fourier syntheses and refined.
[10] Reviews: a) L. H. Gade, S. Bellemin-Laponnaz, Coord. Chem.
Rev. 2007, 251, 718–725; b) L. H. Gade, S. Bellemin-Laponnaz,
Top. Organomet. Chem. 2007, 21, 117–157.
[11] a) M. T. Powell, D.-R. Hou, M. C. Perry, X. Cui, K. Burgess,
J. Am. Chem. Soc. 2001, 123, 8878–8879; b) M. C. Perry, X.
Cui, M. T. Powell, D.-R. Hou, J. H. Reibenspies, K. Burgess,
J. Am. Chem. Soc. 2003, 125, 113–123; c) X. Cui, J. W. Ogle,
K. Burgess, Chem. Commun. 2005, 672–674; d) C. Bolm, T.
Focken, G. Raabe, Tetrahedron: Asymmetry 2003, 14, 1733–
1746; e) Y. Yuan, G. Raabe, C. Bolm, J. Organomet. Chem.
2005, 690, 5747–5752; f) L. Ren, A. C. Chen, A. Decken, C. M.
Crudden, Can. J. Chem. 2004, 82, 1781–1787.
[12] W. A. Herrmann, L. J. Goossen, M. Spiegler, Organometallics
1998, 17, 2162–2168.
[13] L. J. Goossen, Ph. D. Thesis, Technische Universität München,
Germany, 1997.
[14] S. Nanchen, A. Pfaltz, Chem. Eur. J. 2006, 12, 4550–4558.
[15] V. C ésar, S. Bellemin-Laponnaz, L. H. Gade, Organometallics
2002, 21, 5204–5208.
[16] a) L. H. Gade, V. César, S. Bellemin-Laponnaz, Angew. Chem.
2004, 116, 1036–1039; Angew. Chem. Int. Ed. 2004, 43, 1014–
1017; b) V. César, S. Bellemin-Laponnaz, H. Wadepohl, L. H.
Gade, Chem. Eur. J. 2005, 11, 2862–2873.
Due to severe disorder and fractional occupancy, electron density
attributed to solvent of crystallization (hexane) was removed from
the structure (and the corresponding Fobs) of 7 with the BYPASS/
SQUEEZE procedure,[46] as implemented in PLATON.[47]
CCDC-701546 (for 2e), -701550 (for 3), -701551 (for 4), -701547
(for 5), -701549 (for 6) and -701548 (for 7) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[17] N. Schneider, V. César, S. Bellemin-Laponnaz, L. H. Gade, J.
Organomet. Chem. 2005, 690, 5556–5561.
[18] N. Schneider, V. César, S. Bellemin-Laponnaz, L. H. Gade, Or-
ganometallics 2005, 24, 4886–4888.
Acknowledgments
[19]
S. Dastgir, K. S. Coleman, A. J. Cowley, M. L. H. Green, Orga-
nometallics 2006, 25, 300–306.
We thank the Deutsche Forschungsgemeinschaft (DFG) (SFB
623), the Deutsch-Französische Hochschule (DFH-UFA) for finan-
cial support of this work.
[20]
M. Alcarazo, S. J. Roseblade, E. Alonso, R. Fernández, E. Al-
varez, F. J. Lahoz, J. M. Lassaletta, J. Am. Chem. Soc. 2004,
126, 13242–13243.
[21]
[22]
[23]
[24]
[25]
V. C ésar, S. Bellemin-Laponnaz, L. H. Gade, Eur. J. Inorg.
Chem. 2004, 3436–3444.
C.-Y. Wang, Y.-H. Liu, S.-M. Peng, S. T. Liu, J. Organomet.
Chem. 2006, 691, 4012–4020.
C. H. Leung, C. D. Incarvito, R. H. Crabtree, Organometallics
2006, 25, 6099–6107.
[1] a) J. K. Whitesell, Chem. Rev. 1989, 89, 1581–1590; b) C. Mob-
erg, Angew. Chem. 1998, 110, 260–281; Angew. Chem. Int. Ed.
1998, 37, 248–268; c) S. E. Gibson, M. P. Castaldi, Chem. Com-
mun. 2006, 3045–3062; d) L. H. Gade, S. Bellemin-Laponnaz,
Chem. Eur. J. 2008, 14, 4142–4152.
[2] J. Mulzer in Comprehensive Asymmetric Catalysis (Ed.:E. N.
Jacobsen, A. Pfaltz, H. Yamamoto) Springer 1999, Ch. 3, p.
33–100.
[3] a) G. Helmchen, A. Pfaltz, Acc. Chem. Res. 2000, 33, 336–345;
b) H. A. McManus, P. J. Guiry, Chem. Rev. 2004, 104, 4151–
4202; c) A. I. Meyers, J. Org. Chem. 2005, 70, 6137–6151; d) S.
Dagorne, S. Bellemin-Laponnaz, A. Maisse-François, Eur. J.
Inorg. Chem. 2007, 913–925; e) R. Rasappan, D. Laventine, O.
Reiser, Coord. Chem. Rev. 2008, 252, 702–714.
[4] C. Foltz, B. Stecker, G. Marconi, S. Bellemin-Laponnaz, H.
Wadepohl, L. H. Gade, Chem. Eur. J. 2007, 13, 9912–9923.
[5] Reviews: a) A. J. Arduengo III, Acc. Chem. Res. 1999, 32, 913–
921; b) D. Bourissou, O. Guerret, F. Gabbaï, G. Bertrand,
Chem. Rev. 2000, 100, 39–92; c) W. A. Herrmann, C. Köcher,
Angew. Chem. 1997, 109, 2256–2282; Angew. Chem. Int. Ed.
Engl. 1997, 36, 2162–2187; d) F. Glorius, Top. Organomet.
Chem. 2007, 21, 1–20.
I. E. Markó, S. Stérin, O. Buisine, G. Berthon, G. Michaud, B.
Tinant, J.-P. Declercq, Adv. Synth. Catal. 2004, 346, 1429–1434.
G. Berthon-Gelloz, O. Buisine, J.-F. Brière, G. Michaud, S.
Stérin, G. Mignani, B. Tinant, J.-P. Declercq, D. Chapon, I. E.
Markó, J. Organomet. Chem. 2005, 690, 6156–6168.
F. G. A. Stone, Acc. Chem. Res. 1981, 14, 318–325.
N. Rösch, R. Hoffmann, Inorg. Chem. 1974, 13, 2656–2666.
L. H. Gade, G. Marconi, C. Dro, B. D. Ward, M. Poyatos, S.
Bellemin-Laponnaz, H. Wadepohl, L. Sorace, G. Poneti, Chem.
Eur. J. 2007, 13, 3058–3075.
[26]
[27]
[28]
0
Oxidation of (NHC)Pt complexes to (NHC)PtII complexes
[29]
has also been investigated using I2, see: D. Brissy, M. Skander,
P. Retailleau, A. Marinetti, Organometallics 2007, 26, 5782–
5785.
S. Fantasia, J. L. Peterson, H. Jacobsen, L. Cavallo, S. P. No-
lan, Organometallics 2007, 26, 5880–5889.
M. Poyatos, A. Maisse-François, S. Bellemin-Laponnaz, L. H.
Gade, Organometallics 2006, 25, 2634–2641.
A. Abiko, S. Masamune, Tetrahedron Lett. 1992, 33, 5517–
5518.
[30]
[31]
[32]
[6] Reviews: a) L. Cavallo, A. Correa, C. Costabile, H. Jacobsen,
J. Organomet. Chem. 2005, 690, 5407–5413; b) C. M. Crudden,
D. P. Allen, Coord. Chem. Rev. 2004, 248, 2247–2273; c) S.
Eur. J. Inorg. Chem. 2008, 5587–5598
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5597

N. Schneider, S. Bellemin-Laponnaz, H. Wadepohl, L. H. Gade
FULL PAPER
[33] D. A. Evans, G. S. Peterson, J. S. Johnson, D. M. Barnes, K. R.
Campos, K. A. Woerpel, J. Org. Chem. 1998, 63, 4541–4544.
[34] N. Schneider, V. César, S. Bellemin-Laponnaz, L. H. Gade, Or-
ganometallics 2005, 24, 4886–4888.
J. M. M. Smits, S. Garcia-Granda, R. O. Gould, DIRDIF-2008,
Raboud University Nijmegen, The Netherlands, 2008.
[42] SHELXS-86, University of Göttingen, 1986; G. M. Sheldrick,
Acta Crystallogr., Sect. A 1990, 46, 467–473.
[43] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
[44] M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G. L.
Cascarano, L. De Caro, C. Giacovazzo, G. Polidori, R.
Spagna, SIR2004, CNR IC, Bari, Italy, 2004; J. Appl. Crys-
tallogr. 2005, 38, 381–388.
[45] G. M. Sheldrick, SHELXL-97, University of Göttingen, 1997.
[46] P. v. d. Sluis, A. L. Spek, Acta Crystallogr., Sect. A 1990, 46,
194–201.
[47] PLATON, Utrecht University, The Netherlands; A. L. Spek, J.
Appl. Crystallogr. 2003, 36, 7–13.
[35] E. Weber, W. Seichter, I. Goldberg, Chem. Ber. 1990, 123, 811–
820.
[36] E. W. Abel, M. A. Bennet, G. Wilkinson, J. Chem. Soc. 1959,
3178–3182.
[37] D. Drew, J. R. Doyle, A. G. Shaver, Inorg. Synth. 1990, 28, 346–
349.
[38] SAINT, Bruker AXS, 2007.
[39] R. H. Blessing, Acta Crystallogr., Sect. A 1995, 51, 33–38.
[40] G. M. Sheldrick, SADABS, Bruker AXS, 2004–2008.
[41] P. T. Beurskens in: G. M. Sheldrick, C. Krüger, R. Goddard
(Eds.), Crystallographic Computing 3, Clarendon Press, Oxford,
UK, 1985, p. 216; P. T. Beurskens, G. Beurskens, R. de Gelder,
Received: September 10, 2008
Published Online: November 14, 2008
5598
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Eur. J. Inorg. Chem. 2008, 5587–5598