Synthesis and structural features of rhodium complexes of expanded ring n-Heterocyclic carbenes

Article abstract of DOI:10.1002/ejic.200801179

The rhodium complexes [Rh(NHC)(COD)Cl] and cis- [Rh(NHC)(CO)2Cl] of seven-membered N-heterocyclic carb-enes (NHC) bearing aromatic N-substituents (mesityl, xylyl and o-tolyl) were synthesised, and a comparison of their ste-ric and electronic properties wi

Full text of DOI:10.1002/ejic.200801179

FULL PAPER
DOI: 10.1002/ejic.200801179
Synthesis and Structural Features of Rhodium Complexes of Expanded Ring
N-Heterocyclic Carbenes
Manuel Iglesias,[a] Dirk J. Beetstra,[a] Benson Kariuki,[a] Kingsley J. Cavell,*[a]
Athanasia Dervisi,*[a] and Ian A. Fallis*[a]
Keywords: Carbene ligands / Rhodium / Carbonyl ligands / Electronic structure
The rhodium complexes [Rh(NHC)(COD)Cl] and cis-
[Rh(NHC)(CO)2Cl] of seven-membered N-heterocyclic carb-
enes (NHC) bearing aromatic N-substituents (mesityl, xylyl
and o-tolyl) were synthesised, and a comparison of their ste-
ric and electronic properties with those of the analogous five-
and six-membered NHC complexes was made on the basis
of infrared and solid-state data. The X-ray structures for the
complexes Rh(6-Mes)(COD)Cl, Rh(7-Mes)(COD)Cl and
Rh(7-oTol)(COD)Cl were determined.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Introduction
The structure and electronic properties of expanded ring
carbenes and their effect on catalytic performance is vir-
tually unexplored relative to their saturated and unsaturated
five-membered analogues.[1] Other than our work,[2] the
only complexes of seven-membered carbene complexes that
have been reported have been the palladium(II) complexes
prepared by Stahl and co-workers.[3] The PdII complexes
were synthesised by in situ deprotonation of the BF4 salt
with potassium tert-butoxide in the presence of the metal
precursor (Scheme 1). The most distinctive feature of these
complexes relative to five-membered and even six-mem-
bered carbenes is the torsional twist, which Stahl defines by
two parameters: The dihedral angle between the two aryl
rings of the biphenyl backbone and the torsional angle be-
Scheme 1. Synthesis of Stahl’s PdII carbene complex.
The donor abilities of carbene ligands have been studied
by comparing the C–O stretching frequencies for different
carbonyl NHC complexes. The carbonyl stretching fre-
quency is a common method for estimating the electron
density at the metal centre, and, therefore, the donating
abilities of the ligand.[5] The IR carbonyl frequencies ob-
tained for corresponding metal complexes with different
carbene ligands provides a measure of the overall electron
density donated by the carbene.
Here we report the first examples of rhodium complexes
of seven-membered NHCs bearing aromatic N-substituents.
By assessing the structural features of a similar group of
complexes, the ligand properties and influence of five-, six-
and seven-membered NHCs were examined and compared.
tween the two N···N–Cring planes of the carbene ring (Cring
–
N···N–Cring). The torsion of the ring, defined by these two
parameters, dictates the spatial disposition of the substitu-
ents in the coordination sphere.
A number of six-membered carbene complexes have been
published and compared with their five-membered ana-
logues in terms of their electronic and steric properties, as
well as their coordination chemistry and catalytic perform-
ance.[4] Analysis of their crystal structures shows that an
increasing ring size leads to the opening of the N–C–N an-
gle and the consequential decrease in the CNHC–N–CR an-
gle as the main feature. This results in an increase in the
steric hindrance around the metal core.
Results and Discussion
Recently we reported the synthesis of seven-membered
amidinium salts with aromatic substituents, their free carb-
enes and corresponding silver complexes.[2a] We have now
prepared several new rhodium cyclooctadiene and bis(car-
bonyl) complexes, with previously described procedures.[2b]
In addition to the previously reported carbene precursors
7-Mes·HBF4, 7-Xyl·HBF4 and 7-iPr·HBF4, the less encum-
bered amidinium salts, 6-oTol·HBF4 and 7-oTol·HBF4 were
[a] School of Chemistry, Cardiff University,
Main Building, Park Place, Cardiff, CF10 3AT, United King-
dom
Fax: +44-29-20874030
E-mail: dervisia@cardiff.ac.uk
Supporting information for this article is available on the
WWW under http://www.eurjic.org/ or from the author.
Eur. J. Inorg. Chem. 2009, 1913–1919
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M. Iglesias, D. J. Beetstra, B. Kariuki, K. J. Cavell, A. Dervisi, I. A. Fallis
FULL PAPER
also prepared, Scheme 2. The free carbenes were generated
The carbonyl complexes Rh(7-Mes)(CO)2Cl and Rh(7-
from the corresponding BF4 salts by using KN(SiMe3)2 as Xyl)(CO)2Cl were also prepared (Scheme 3). Treatment of
the base and were isolated in good yields as stable crystal- dichloromethane solutions of Rh(7-Mes)(COD)Cl and
line solids. In contrast to the reactivity of the majority of Rh(7-Xyl)(COD)Cl with 1 atm carbon monoxide for
the HBF4 salts, deprotonation of the ortho-tolyl salt, 7- 20 min afforded the corresponding carbonyl complexes as
oTol·HBF4, with KN(SiMe3)2 formed the corresponding bright yellow solids in good yields. Their properties were
carbene 7-oTol together with the base adduct 7- compared to the previously reported analogues, 5-Mes[7]
oTol·HN(SiMe3)2 in a 55:45 ratio, whereas the six-mem- and 6-Mes[4c] carbene complexes.[8]
bered equivalent, 6-oTol·HBF4, formed cleanly the free car-
1H and 13C NMR spectra of the Rh(7-Mes)(COD)Cl
bene 6-oTol. The above reactivity of the tolyl carbenes may and Rh(7-Xyl)(COD)Cl complexes, recorded in CDCl3,
be due to their reduced steric demands, which allows more show broad aromatic resonances arising from the slow rota-
ready access to the carbene core. In addition, the apparent tion of the aryl rings on the NMR timescale. However,
higher basicity of the seven-membered carbenes may ac- spectra recorded in [D6]benzene show sharp signals, and
count for the difference in reactivity between the 7-oTol and two distinct resonances are observed for the ortho-methyl
6-oTol carbenes (vide infra). A similar behaviour was pre- groups. Hindered N–Ar rotation in [D6]benzene is also evi-
viously observed by Alder et al. for tetrahydropyrimidinium dent from the 13C NMR spectra, in which two resonances
salts.[6] The authors reported a competitive nucleophilic at- are observed for the methyl carbon atoms and six for the
tack of the amide anion instead of CNHC–H deprotonation aromatic ones (as opposed to four signals if the aromatic
of the azolium salt, to generate the NHC·HN(SiMe3)2 ad- rings were freely rotating). For comparison, NMR spectro-
duct. Interestingly, the base adduct in this case is in equilib- scopic data for the Rh(5-Mes)(COD)Cl complex (in CDCl3)
rium with the free carbene and the corresponding proton- indicate free rotation of the mesityl rings, and for Rh(6-
ated base HN(SiMe3)2.
Mes)(COD)Cl, broadened (although resolved) signals are
observed for the ortho-methyl groups.
1
The H NMR spectra recorded in [D6]benzene show a
large downfield shift for only one of the ortho-methyl sig-
nals of the 7-Mes and 7-Xyl complexes, Table 1. In the
more polar CDCl3 solvent, however, the two ortho-methyl
resonances are comparable. The above suggests that the
methyl group resonating at higher frequency in [D6]benzene
resides on the same side as the chlorido ligand, the signal
is shifted because of hydrogen bonding with the chlorido
ligand, whereas in CDCl3, intermolecular H-bonding with
the solvent is more likely, hence similar shifts are observed
for the ortho-methyl protons in this solvent. Indeed, close
Cl–H(Me) contacts are observed in the solid-state structure
of Rh(7-Xyl)(COD)Cl, vide infra.
Scheme 2. Synthesis of six- and seven-membered NHCs with N-
aryl substituents. Reagents and conditions: KN(SiMe3)2, thf.
Treatment of [Rh(COD)Cl]2 with 2 equiv. of the corre-
sponding free carbene in thf gave the complexes Rh(7-Mes)-
(COD)Cl and Rh(7-Xyl)(COD)Cl as yellow air-stable solids
in good yields (Scheme 3). The corresponding 7-oTol com-
plex, because of the instability of the free carbene, was gen-
erated by in situ deprotonation of 7-oTol·HBF4 with
KN(SiMe3)2 followed by subsequent reaction with
[Rh(COD)Cl]2. Attempts to produce a stable rhodium 7-iPr
complex failed, presumably because of the steric demands
Table 1. 1H NMR methyl shifts for the Rh(NHC)(cod)Cl com-
plexes.
Complex
Solvent
δ(Me) [ppm]
Rh(7-Mes)(COD)Cl
Rh(6-Mes)(COD)Cl
Rh(5-Mes)(COD)Cl
Rh(7-Xyl)(COD)Cl
C6D6
3.10
2.52
2.53
3.10
2.21
2.48
2.43
2.35
2.35
2.32
2.28
2.27
2.16
2.19
2.20
2.25
CDCl3
CDCl3
C6D6
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
imposed by the bulky isopropyl substituents. Only the unre- Rh(7-oTol)(COD)Cl
Rh(7-Xyl)(CO)2Cl
2.41
2.36
2.29
2.23
acted metal precursor, [Rh(COD)Cl]2, and the hydrolysis
Rh(7-Mes)(CO)2Cl
product of the free carbene, 7-iPr·H2O, were recovered from
Rh(6-Mes)(CO)2Cl
2.34
2.25
the reaction mixture.
Rh(5-Mes)(CO)2Cl
Similar observations were made for the carbonyl com-
plexes. For Rh(6-Mes)(CO)2Cl and Rh(7-Mes)(CO)2Cl, six
aromatic and two ortho-methyl resonances appear in the
13C NMR spectra, as well as two signals for the ortho-
1
1
methyl groups in the H NMR spectra. Both the H and
13C NMR spectra for complex [Rh(5-Mes)(CO)2Cl] show
only one resonance for the ortho-methyl groups and four
Scheme 3. Synthesis of Rh(NHC)(COD)Cl and Rh(NHC)(CO)2Cl
complexes. (a) [Rh(cod)Cl]2, thf; (b) CO (1 atm), CH2Cl2.
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Rhodium Complexes of Expanded Ring N-Heterocyclic Carbenes
aromatic signals (13C NMR), which implies free N–Ar rota- Table 2. 13C NMR shifts [ppm] and coupling constants [Hz] for the
CNHC carbon atom of the rhodium complexes.
tion due to the reduced steric demands of 5-Mes relative to
Complex
δ (1JCRh
)
those of the 6- and 7-Mes ligands. Interestingly, in the
Rh(7-Cy)(CO)2Cl complex,[2b] the seven-membered ring
carbene 7-Cy (bearing cyclohexyl N-substituents) rotates
freely at ambient temperature on the NMR timescale, which Rh(7-Mes)(COD)Cl
suggests that aromatic N-substitution generates a more en-
Rh(7-oTol)(COD)Cl
Rh(7-Xyl)(COD)Cl
222.8 (45.0)
223.1 (46.3)
224.0 (46.7)
211.5 (46.9)
211.6 (48.2)
212.2
Rh(6-Mes)(COD)Cl
Rh(5-Mes)(COD)Cl
Rh(7-Xyl)(CO)2Cl
Rh(7-Mes)(CO)2Cl
cumbered ligand.
As a result of the hindered rotation of the Rh–NHC and
213.4
N–Ar bonds of the ortho-tolyl complex Rh(7-oTol)(COD)-
Cl, there are four possible rotamers, two with the ortho-
methyl groups in a syn orientation and two anti isomers
(enantiomers), Scheme 4. In a manner consistent with this,
the 1H NMR (CDCl3) spectrum shows three sharp doublets
for the ortho-H atoms, which corresponds to the three iso-
mers in a 3:4.5:10 ratio, with one of the syn rotamers as the
major isomer.
Rh(6-Mes)(CO)2Cl
Rh(5-Mes)(CO)2Cl
202.9
205.0
higher νav values obtained for the 5-Mes complex are con-
sistent with a reduced electron density on the metal centre
and thus a less basic carbene ligand in this case.
Table 3. Infrared ν(CO) for the Rh(CO)2(NHC)Cl complexes.[a]
Complex
νs,as [cm–1]
νav [cm–1] (literature)
Rh(5-Mes)(CO)2Cl
Rh(6-Mes)(CO)2Cl
Rh(7-Mes)(CO)2Cl
Rh(7-Xyl)(CO)2Cl
Rh(7-Cy)(CO)2Cl
1995, 2080
1987, 2071
1987, 2069
1986, 2071
2071, 1990
2037.5 (2038)[7]
2029 (2029)[4]
2028
2028.5
2030.5
[a] Measured in CH2Cl2.
Solid-State Analysis
Crystals of Rh(6-/7-Mes)(COD)Cl and Rh(7-oTol)-
(COD)Cl suitable for X-ray diffraction were obtained by
layering a dichloromethane solution of the corresponding
complex with hexane. The crystal structures are shown in
Figure 1, and selected bond lengths and angles can be
found in Table 4. The solid-state structure of the Rh(7-
oTol)(COD)Cl complex corresponds to the syn-exo con-
former (Scheme 4, Figure 1). Data for the Rh(5-Mes)-
(COD)Cl complex have also been included in Table 4 for
comparison.[9]
It is evident from the crystallographic data that expan-
sion of the NHC ring leads to larger N–CNHC–N angles.
As expected, there is a significant increase in the N–C–N
angle from 107.29(12)° in the 5-Mes rhodium complex to
117.0–118.0(4)° in the six- and seven-membered ring car-
bene complexes. This leads to large changes in the CAr–N–
Scheme 4. Rotamers of Rh(7-oTol)(COD)Cl.
Table 2 shows the 13C NMR chemical shifts of the CNHC CNHC angles, which are accordingly compressed from an
carbon atom of the rhodium complexes. A significant average of 127.3(13)° in Rh(5-Mes)(COD)Cl to 118.8(4)°
downfield shift is observed for the carbene carbon atom in in the six- and seven-membered NHC complexes (Table 4).
Rh(7-Mes)(COD)Cl and Rh(7-Mes)(CO)2Cl when com- Consequently, the aryl substituents on the nitrogen atoms
pared to those for Rh(5-/6-Mes)(COD)Cl and Rh(5-/6- are forced closer to the metal centre in the expanded carb-
Mes)(CO)2Cl. The same trend was observed for the free enes, and virtually block two faces of the metal coordina-
carbenes and the corresponding silver complexes.[2a]
tion sphere. This steric congestion may also be responsible
In Table 3, the carbonyl stretching frequencies of [Rh(5- for the longer Rh–CNHC bonds observed for the 6-Mes and
Mes)(CO)2Cl], [Rh(6-Mes)(CO)2Cl], [Rh(7-Mes)(CO)2Cl] 7-Mes complexes than that for Rh(5-Mes)(COD)Cl
and [Rh(7-Xyl)(CO)2Cl] are listed and compared to litera- (Table 4). In contrast, the Rh(7-oTol)(COD)Cl complex has
ture values. For the six- and the seven-membered carbene a much shorter Rh–CNHC bond [2.030(3) Å] than that for
complexes, electron density on the metal centre, as indicated Rh(7-Mes)(COD)Cl [2.085(3) Å], which can be attributed
by the carbonyl stretching frequencies, is essentially iden- to differences in the electronic effects of the mesityl and
tical, which implies similar donor abilities. However, the tolyl N-substituents. Close contacts between the chlorido
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M. Iglesias, D. J. Beetstra, B. Kariuki, K. J. Cavell, A. Dervisi, I. A. Fallis
FULL PAPER
Table 4. Bond lengths [Å] and angles [°] for the Rh(NHC)(COD)-
Cl complexes.
5-Mes
6-Mes
7-Mes
7-oTol
Rh–CNHC
Rh–Cl
2.0513(14)
2.3665(3)
127.62(13),
127.06(12)
107.29(12)
91.86(4)
59.0
2.078(4)
2.085(3) 2.030(3)
2.3876(13) 2.3817(8) 2.4470(7)
CAr–N–CNHC
119.5(4),
119.1(4)
117.0(4)
86.75(13) 85.27(9) 86.59(8)
83.5
8.2
119.1(2), 117.1(2),
117.2(2) 120.5(3)
118.0(3) 117.2(3)
N–CNHC–N
NHC–Rh–Cl
tilt angle θ
α (CAr–N···N–CAr
C
87.8
28.0
81.1
46.0
)
28.1
The torsional angle α between the planes defined by the
Ary–N···N–CAr atoms is a useful gauge for the spatial ori-
C
entation of the aromatic N-substituents, which point di-
rectly into the coordination sphere of the metal. The 6-Mes
complex has the smallest torsional angle with value of 8.2°.
The 5-Mes and 7-Mes complexes display similar angles
(28.1° and 28.0°, respectively), and the 7-oTol complex has
the largest torsional angle with a value of 46.0°.
All complexes in Figure 1 show small pyramidal distor-
tions for the carbenic carbon atom, as measured by the de-
viation of the CNHC carbon atom from the N–Rh–N plane.
The orthogonal distance between CNHC and the N–Rh–N
plane is 0.11 Å for the 6- and 7-Mes carbene complexes and
0.06 Å for the 7-oTol complex.
In the six- and seven-membered carbene complexes, the
NHC ligand adopts an almost perpendicular arrangement
with respect to the coordination plane (defined by the
C
NHC–Rh–Cl atoms). The tilt angle θ (defined by the coor-
dination and N–C–N planes) is 83.5° for Rh(6-Mes)(COD)-
Cl, 87.8° for Rh(7-Mes)(COD)Cl and 81.1° for Rh(7-oTol)-
(COD)Cl. However, the tilt angle θ is much smaller for
Rh(5-Mes)(COD)Cl, with a value of 59.0°.
Conclusions
The sterically crowded seven-membered N-heterocyclic
carbenes with aromatic N-substituents, 7-Mes, 7-Xyl and 7-
oTol, form stable rhodium complexes. Although by increas-
ing further the bulk of the ortho substituents, as in the case
of 7-iPr, coordination to the rhodium centre is hindered.
The donor ability of the seven-membered NHCs, on the
basis of infrared data, appears to be comparable to that of
their six-membered ring equivalents but greater than that
of the five-membered ring NHCs.
Figure 1. Solid-state molecular structures of Rh(6-Mes)(COD)Cl,
Rh(7-Mes)(COD)Cl and Rh(7-oTol)(COD)Cl.
ligand and the ortho-methyl protons were observed for the
6- and 7-Mes complexes [2.630 Å (av.) and 2.542 Å (av.),
respectively]. Similarly, Rh(7-oTol)(COD)Cl displays close
contacts between the chlorido ligand and the ortho-tolyl
protons, with an average Cl···H distance of 2.742 Å.
Experimental Section
General Remarks: The solid reactants were combined together in a
Schlenk tube in a glove box under a nitrogen atmosphere, and then
removed from the glove box and transferred to a Schlenk line where
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Rhodium Complexes of Expanded Ring N-Heterocyclic Carbenes
the solvents were added by syringe. Diethyl ether, tetrahydrofuran
and hexane were distilled from sodium and benzophenone under a
N2 atmosphere. Dichloromethane was distilled from calcium hy-
dride under a N2 atmosphere. [Rh(COD)Cl]2, [Rh(COD)Cl]2 and
[Pt(nbe)3] were synthesised according to literature methods. The
salts 7-Mes·HBF4, 7-Xyl·HBF4 and 7-iPr·HBF4 and the corre-
sponding free NHCs were prepared according to the syntheses re-
ported previously by us.[2a] 1H- and 13C spectra were recorded by
using a Bruker Advance DPX400 spectrometer. Chemical shifts (δ)
were expressed in ppm downfield from tetramethylsilane by using
CHCOD), 3.75–3.82 (m, 4 H, NCH2), 3.30 (m, 2 H, CHCOD), 2.53
(s, 6 H, CH3), 2.27 (s, 6 H, CH3), 2.25 (s, 6 H, CH3), 1.70–1.72
(m, 4 H, CH2COD), 1.43–1.47 (m, 4 H, CH2COD) ppm. 13C NMR
1
(100 MHz, CDCl3, room temperature): δ = 211.6 (d, JCRh
=
48.2 Hz, CCarbene), 137.4 (s, CMes), 136.8 (s, CMes), 135.2 (s, CMes),
134.2 (s, CMes), 128.9 (s, CHMes), 127.3 (s, CHMes), 96.1 (s, d, 1JRhC
1
= 7.1 Hz, CHCOD), 66.5 (s, d, JRhC = 7.5 Hz, CHCOD), 50.3 (s,
NCH2), 31.6 (s, CH2COD), 27.1 (s, CH2COD), 20.1 (s, CH3), 18.9 (s,
CH3), 17.3 (s, CH3) ppm. MS (ES, CH3CN): m/z = 558.2354 [M –
Cl + CH3CN+]. C31H41N3Rh requires 558.2356.
1
the residual proton as an internal standard (CDCl3, H 7.26 ppm
Rh(6-Mes)(COD)Cl: A solution of 6-Mes (74 mg, 0.22 mmol) in
dry thf (10 mL) was added to a solution of [Rh(COD)Cl]2 (50 mg,
0.10 mmol) in dry thf (10 mL) and stirred at ambient temperature
for 1 h under an argon atmosphere. The solvent was then removed
in vacuo to give a dark yellow solid, which was subsequently tritu-
rated with hexanes to give 71 mg (0.12 mmol) of Rh(6-Mes)-
(COD)Cl as a bright yellow powder (62% yield). Crystals suitable
for X-ray analysis were obtained by layering diethyl ether on a
dichloromethane solution of the complex. 1H NMR (400 MHz,
CDCl3, room temperature): δ = 6.98 (br., 2 H, CHMes), 6.88 (br., 2
H, CHMes), 4.16 (m, 2 H, CHCOD), 3.27 (m, 4 H, NCH2), 3.13 (m,
2 H, CHCOD), 2.52 (br. s, 6 H, o-CH3), 2.28 (s, 6 H, CH3), 2.20 (br.
s, 6 H, o-CH3), 2.08 (br., 2 H, CH2), 1.39 (m, 4 H, CH2COD), 1.19
(m, 4 H, CH2COD) ppm. 13C NMR (100 MHz, CDCl3, room tem-
1
and 13C 77.0 ppm; [D6]benzene H 7.15 ppm and 13C 128.0 ppm).
Coupling constants are expressed in Hertz. HRMS were obtained
on a Waters LCT Premier XE instrument and are reported as m/z
(%). Infrared spectra were recorded by using a JASCO FT/IR-660
Plus spectrometer and analysed in solution (dichloromethane).
1,3-Bis(2-methylphenyl)-4,5,6,7-tetrahydro-3H-[1,3]diazepin-1-ium
Tetrafluoroborate (7-oTol·HBF4): A suspension of N,NЈ-bis(o-tolyl)-
formamidine (2.24 g, 10.0 mmol), K2CO3 (0.69 g, 5.0 mmol) and
1,4-diiodobutane (3.26 g, 10.5 mmol) in acetonitrile (30 mL) was
heated to reflux overnight. Subsequently, the reaction mixture was
allowed to reach ambient temperature, at which point a solution of
NaBF4 (1.40 g, 10.0 mmol) in water (20 mL) was added. The reac-
tion mixture was stirred for 10 min, the volatiles were subsequently
partially evaporated, and the product precipitated in the remaining
water. The product was isolated by filtration, washed thoroughly
with water, and dissolved in dichloromethane. The residual water
was separated, and the dichloromethane solution dried with
MgSO4. The solution was filtered and concentrated, and ether was
slowly added to precipitate 7-oTol·HBF4 as a white, crystalline ma-
terial. Yield: 2.95 g (8.1 mmol, 81%). 1H NMR (CDCl3, 400 MHz,
room temperature): δ = 7.71 (br., 2 H, o-CH), 7.36 (s, 1 H, NCHN),
7.28–7.22 (m, 6 H, m,p-CH), 4.34 (br., 4 H, N–CH2), 2.40 (br., 4
H, N–CH2–CH2), 2.36 (s, 6 H, CH3) ppm. 13C NMR (CDCl3,
100 MHz, room temperature): δ = 156.5 (s, NCHN), 141.8 (s, ipso-
1
perature): δ = 211.5 (d, JCRh = 46.9 Hz, CCarbene), 142.3 (s, CMes),
1
137.8 (s, CMes), 130.6 (s, CHMes), 128.5 (s, CHMes), 94.3 (d, JCRh
1
= 7.4 Hz, CHCOD), 67.3 (d, JCRh = 14.7 Hz, CHCOD), 48.0 (s,
NCH2), 32.7 (s, CH2COD), 32.0 (s, CH2COD), 21.6 (s, NCH2CH2),
21.4 (s, CH3), 20.6 (s, CH3), 19.4 (s, CH3) ppm. MS (ES): m/z =
531.2221 [M – Cl+]. C30H40N2Rh requires 531.2247.
Rh(7-Mes)(COD)Cl: A solution of 7-Mes (71 mg, 0.22 mmol) in
dry thf (10 mL) was added to a solution of [Rh(COD)Cl]2 (50 mg,
0.10 mmol) in dry thf (10 mL) and stirred at ambient temperature
for 1 h under an argon atmosphere. The solvent was then removed
in vacuo to give a dark yellow solid, which was subsequently tritu-
rated with hexanes to give 73 mg (0.12 mmol) of 5 as a bright yel-
low powder (63% yield). Crystals suitable for X-ray analysis were
obtained by layering diethyl ether on a tetrahydrofuran solution of
CoTol), 131.7 (s, CoTolCH3), 130.7 (s, CHoTol), 129.1 (s, CHoTol),
127.4 (s, CHoTol), 126.5 (s, CoTol CH3), 54.3 (s, N-CH2), 24.4 (s,
NCH2CH2), 16.8 (s, CH3) ppm. MS (ES): m/z = 279.1853 [M –
BF4]+. C19H23N2 requires 279.1861.
1
the complex. H NMR (400 MHz, C6D6, room temperature): δ =
1,3-Bis(2-methylphenyl)-3,4,5,6-tetrahydro-pyrimidin-1-ium Tetra-
fluoroborate (6-oTol·HBF4): The reaction was performed in an
analogous manner to that described for 7-oTol·HBF4, on a 2.9-
mmol scale. Yield: 0.99 g (2.8 mmol, 97%), white, crystalline mate-
7.06 (s, 2 H, CHMes), 6.96 (s, 2 H, CHMes), 5.00 (m, 2 H, CHCOD),
3.70 (m, 2 H, NCH2), 3.18 (m, 2 H, CHCOD), 3.10 (s, 6 H, CH3),
2.93 (m, 2 H, NCH2), 2.39 (m, 2 H, NCH2CH2), 2.32 (s, 6 H, CH3),
2.19 (s, 6 H, CH3), 1.70 (m, 2 H, CH2COD), 1.42–1.58 (m, 4 H,
1
rial. H NMR (CDCl3, 400 MHz, room temperature): δ = 7.61 (d,
CH2COD), 1.32 (m, 2 H, NCH2CH2) ppm. 13C NMR (100 MHz,
3JHH = 8.0 Hz, 2 H, o-CH), 7.55 (s, 1 H, NCHN), 7.30–7.16 (m, 6 C6D6, room temperature): δ = 224.0 (d, JCRh = 46.7 Hz, CNHC),
1
H, m,p-CH), 3.71 (br., 4 H, NCH2), 2.45 (br., 2 H, NCH2CH2), 143.2 (s, CMes), 136.5 (s, CMes), 135.4 (s, CMes), 133.5 (s, CMes),
1
2.30 (s, 6 H, CH3) ppm. 13C NMR (CDCl3, 100 MHz, room tem-
perature): δ = 153.7 (s, NCHN), 140.4 (s, ipso-C), 133.7 (s,
129.6 (s, CHMes), 127.0 (s, CHMes), 93.2 (d, JCRh = 6.8 Hz,
1
CHCOD), 65.9 (d, JCRh = 14.6 Hz, CHCOD), 53.6 (s, NCH2), 31.5
C
oTolCH3), 132.0 (s, CHoTol), 130.7 (s, CHoTol), 128.7 (s, CHoTol),
(s, CH2COD), 26.5 (s, CH2COD), 23.5 (s, NCH2CH2), 20.4 (s, CH3),
19.5 (s, CH3), 18.5 (s, CH3) ppm. MS (ES, CH3CN): m/z =
545.2379 [M – Cl+]. C31H42N2Rh requires 545.2403.
127.9 (s, CHoTol), 47.4 (s, NCH2) 19.8 (s, NCH2CH2), 17.8 (s, CH3)
ppm. MS (ES): m/z = 265.1699 [M – BF4]+. C18H21N2 requires
265.1705.
Rh(7-Xyl)(COD)Cl: A solution of 7-Xyl (67 mg, 0.22 mmol) in dry
thf (10 mL) was added to a solution of [Rh(COD)Cl]2 (50 mg,
0.10 mmol) in dry thf (10 mL) and stirred at ambient temperature
for 1 h under an argon atmosphere. The solvent was then removed
in vacuo to give a dark yellow solid, which was subsequently tritu-
rated with hexanes to give 78 mg (0.14 mmol) of 3 as a bright yel-
low powder (70% yield). Crystals suitable for X-ray analysis were
obtained by layering hexanes on a dichloromethane solution of the
Rh(5-Mes)(COD)Cl: A solution of 5-Mes (67 mg, 0.22 mmol) in
dry thf (10 mL) was added to a solution of [Rh(COD)Cl]2 (50 mg,
0.10 mmol) in dry thf (10 mL) and stirred at ambient temperature
for 1 h under an argon atmosphere. The solvent was then removed
in vacuo to give a dark yellow solid, which was subsequently tritu-
rated with hexanes to give 63 mg (0.11 mmol) of 1 as a bright yel-
low powder (57% yield). Crystals suitable for X-ray analysis were
obtained by layering diethyl ether on a dichloromethane solution
1
complex. H NMR (400 MHz, C6D6, room temperature): δ = 7.06
1
of the complex. H NMR (400 MHz, CDCl3, room temperature):
(t, 3JHH = 7.4 Hz, 2 H, CHXy), 7.05 (d, 3JHH = 7.1 Hz, 4 H, CHXy),
δ = 6.96 (s, 2 H, CHMes), 6.91 (s, 2 H, CHMes), 4.40 (m, 2 H, 5.00 (m, 2 H, CHCOD), 3.66 (m, 2 H, NCH2), 3.10 (m, 2 H,
Eur. J. Inorg. Chem. 2009, 1913–1919
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1917

M. Iglesias, D. J. Beetstra, B. Kariuki, K. J. Cavell, A. Dervisi, I. A. Fallis
FULL PAPER
CHCOD), 3.10 (s, 6 H, CH3), 2.85 (m, 2 H, NCH2), 2.33 (m, 2 H, temperature): δ = 6.87 (s, 4 H, CHMes), 3.37 (m, 4 H, NCH2), 2.35
NCH2CH2), 2.16 (s, 6 H, CH3), 1.70 (m, 2 H, CH2COD), 1.42– (s, 6 H, CH3), 2.31 (s, 6 H, CH3), 2.29 (m, 2 H, NCH2CH2), 2.25
1.53 (m, 4 H, CH2COD), 1.23 (m, 2 H, NCH2CH2) ppm. 13C NMR (s, 6 H, CH3) ppm. 13C NMR (100 MHz, CDCl3, room tempera-
1
1
(100 MHz, C6D6, room temperature): δ = 145.5 (s, CXyl), 137.0 (s,
ture): δ = 202.9 (d, JRhC = 40.8 Hz, CNHC), 186.1 (d, JRhC =
1
CXyl), 133.8 (s, CXyl), 129.1 (s, CHXyl), 127.0 (s, CHXyl), 93.5 (d, 52.5 Hz, CO), 183.7 (d, JRhC = 67.6 Hz, CO), 141.7 (s, CMes),
1JRhC = 14.8 Hz, CHCOD), 66.3 (d, JRhC = 6.8 Hz, CHCOD), 53.5 138.4 (s, CMes), 136.7 (s, CMes), 134.5 (s, CMes), 130.4 (s, CHMes),
1
(s, NCH2), 31.4 (s, CH2COD), 26.6 (s, CH2COD), 23.5 (s, NCH2CH2), 129.3 (s, CHMes), 47.2 (s, NCH2), 21.5 (s, CH3), 21.1 (s,
20.5 (s, CH3), 18.6 (s, CH3) ppm. 13C NMR (100 MHz, CDCl3, NCH CH ), 19.9 (s, CH ), 18.6 (s, CH ) ppm. IR: ν = 1987, 2071
˜
2
2
3
3
room temperature): δ = 223.1 (d, 1JCRh = 46.3 Hz, CNHC), 145.2 (s,
CXyl), 136.6 (br., CXyl), 134.2 (br., CXyl), 128.8 (br., CHXyl), 126.6
(br., CHXyl), 126.5 (s, CHXyl), 93.8 (d, JRhC = 7.1 Hz, CHCOD),
(CH2Cl2).
Rh(7-Mes)(CO)2Cl: Complex [Rh(7-Mes)(COD)Cl] (50 mg,
0.082 mmol) was dissolved in dichloromethane (10 mL) and stirred
under an atmosphere of carbon monoxide for 30 min. The solvent
was then removed in vacuo, and the resulting solid was washed
with cold hexanes to remove residual 1,5-cyclooctadiene. The re-
sulting solid was dried in vacuo to afford 40 mg (0.077 mmol) of a
1
66.3 (d, 1JRhC = 14.9 Hz CHCOD), 54.6 (s, NCH2), 54.4 (s, NCH2),
31.2 (s, CH2COD), 26.3 (s, CH2COD), 24.0 (s, NCH2CH2), 19.1 (s,
CH3), 17.5 (s, CH3) ppm. MS (ES, CH3CN): m/z = 517.2110 [M –
Cl+]. C29H38N2Rh requires 517.2090.
1
pale yellow solid (94% yield). H NMR (400 MHz, CDCl3, room
Rh(7-oTol)(COD)Cl: KNSi(Me3)2 (0.200 g, 1.0 mmol) and 7-
oTol·HBF4 (0.361 g, 1.0 mmol) were placed into a Schlenk tube,
followed by the addition of diethyl ether (10 mL). The solution was
stirred for 30 min and subsequently filtered into a Schlenk tube
containing a thf solution (10 mL) of [Ir(COD)Cl]2 (0.336 g,
0.5 mmol); an immediate colour change was observed from light to
dark yellow. After the reaction mixture was stirred at room tem-
perature for 1 h, the solvent was removed in vacuo. The precipitate
was washed with hexane and dried under vacuum to afford a brown
solid. The product was dissolved in dcm (5 mL) and precipitated
with diethyl ether to yield 140 mg of a yellow microcrystalline solid
(0.25 mmol, 25%). 1H NMR (500 MHz, CDCl3, room tempera-
temperature): δ = 6.89 (s, 4 H, CHMes), 4.00 (m, 2 H, NCH2), 3.71
(m, 2 H, NCH2), 2.43 (s, 6 H, CH3), 2.36 (s, 6 H, CH3), 2.34 (m,
2 H, NCH2CH2), 2.23 (s, 6 H, CH3), 2.11 (m, 2 H, NCH2CH2)
ppm. 13C NMR (100 MHz, CDCl3, room temperature): δ = 213.4
(d, 1JRhC = 40.8 Hz, CNHC), 187.1 (d, JRhC = 52.5 Hz, CO), 184.5
1
1
(d, JRhC = 77.7 Hz, CO), 143.6 (s, CMes), 138.2 (s, CMes), 136.6 (s,
CMes), 134.3 (s, CMes), 130.5 (s, CHMes), 129.4 (s, CHMes), 55.2 (s,
NCH2), 25.4 (s, NCH2CH2), 21.4 (s, CH3), 20.5 (s, CH3), 19.3 (s,
CH ) ppm. IR: ν = 1987, 2069 (CH Cl ). MS (ES, CH CN): m/z
˜
3
2
2
3
= 506.1682 [M – Cl – CO + CH3CN]+. C26H33N3ORh requires
506.1679.
3
3
ture): δ = 8.50 (d, JHH = 7.9 Hz, 2 H, CHAr), 7.32 (d, JHH
=
cis-[Rh(7-Xyl)(CO)2Cl]: Complex [Rh(7-Xyl)(COD)Cl] (50 mg,
0.090 mmol) was dissolved in dichloromethane (10 mL) and stirred
under an atmosphere of carbon monoxide for 30 min. The solvent
was then removed in vacuo, and the resulting solid was washed
with cold hexanes to remove residual 1,5-cyclooctadiene. The re-
sulting solid was dried in vacuo to afford 35 mg (0.070 mmol) of a
7.8 Hz, 2 H, CHAr), 7.22 (m, 4 H, CHAr), 4.39 (m, 2 H, CHCOD),
3
3
4.19 (t, JHH = 12.6 Hz, 2 H, NCH2), 3.42 (d, JHH = 13.3 Hz, 2
H, NCH2), 2.46 (m, 2 H, NCH2CH2), 2.04 (m, 2 H, CHCOD), 2.21
3
(s, 6 H, CH3), 1.77 (d, JHH = 7.7 Hz, 2 H, NCH2CH2), 1.42 (m,
2 H, CH2COD), 1.23 (m, 2 H, CH2COD), 1.19 (m, 2 H, CH2COD),
1.04 (m, 2 H, CH2COD) ppm. 13C NMR (125 MHz, CDCl3, room
1
pale yellow solid (78% yield). H NMR (400 MHz, CDCl3, room
1
temperature): δ = 222.8 (d, JCRh = 45.0 Hz, CNHC), 146.8 (s, ipso-
3
temperature): δ = 7.12 (t, JHH = 7.0 Hz, 2 H, CHXyl), 7.05 (d,
CAr), 133.5 (s, CAr), 133.0 (s, CAr), 130.0 (s, CAr), 127.4 (s, CAr),
3JHH = 7.5 Hz, 4 H, CHXyl), 4.04 (m, 4 H, NCH2), 3.76 (m, 4 H,
NCH2CH2), 2.48 (s, 6 H, CH3), 2.41 (s, 6 H, CH3), 2.34 (m, 4 H,
CH2COD), 2.15 (m, 4 H, CH2COD) ppm. 13C NMR (100 MHz,
1
1
126.6 (s, CAr), 95.3 (d, JRhC = 19.8 Hz, CHCOD), 66.8 (d, JRhC
=
15.1 Hz, CHCOD), 53.3 (s, NCH2), 31.6 (s, CH2COD), 27.4 (s,
CH2COD), 24.0 (s, NCH2CH2), 18.4 (s, CH3) ppm. MS (ES,
CH3CN): m/z = 489.1776 [M – Cl+]. C29H38N2Rh requires
489.1790.
1
CDCl3, room temperature): δ = 212.2 (d, JRhC = 40.8 Hz, CNHC),
1
1
184.5 (d, JRhC = 52.5 Hz, CO), 182.6 (d, JRhC = 77.3 Hz, CO),
144.5 (s, CXyl), 135.6 (s, CXyl), 133.3 (s, CXyl), 128.4 (s, CHXyl),
127.4 (s, CHXyl), 127.1 (s, CHXyl), 53.7 (s, NCH2), 24.1 (s,
Rh(5-Mes)(CO)2Cl: Complex [Rh(5-Mes)(COD)Cl] (50 mg,
0.090 mmol) was dissolved in dichloromethane (10 mL) and stirred
under an atmosphere of carbon monoxide for 30 min. The solvent
was then removed in vacuo, and the resulting solid was washed
with cold hexanes to remove residual 1,5-cyclooctadiene. The re-
sulting solid was dried in vacuo to afford 38 mg (0.076 mmol) of a
NCH CH ), 18.0 (s, CH ), 13.1 (s, CH ) ppm. IR: ν = 1986, 2071
˜
2
2
3
3
(CH2Cl2). MS (ES, CH3CN): m/z = 478.1377 [M – Cl – CO +
CH3CN+]. C24H29N3ORh requires 478.1366.
X-ray Crystallography: Suitable crystals were selected, and a data
set for Rh(7-oTol)(COD)Cl was measured on a Bruker-Nonius
APEX II CCD camera on a κ-goniostat, while data sets for Rh(7-
1
pale yellow solid (83% yield). H NMR (400 MHz, CDCl3, room
temperature): δ = 6.88 (s, 4 H, CHMes), 3.92 (s, 4 H, NCH2), 2.35
Mes)(COD)Cl and Rh(6-Mes)(COD)Cl were measured on
a
(s, 12 H, CH3), 2.23 (s, 6 H, CH3) ppm. 13C NMR (100 MHz,
Bruker-Nonius KappaCCD area detector, all at the window of a
Bruker-Nonius FR591 rotating anode [λ(Mo-Kα) = 0.71073 Å]
driven by COLLECT[10] and processed by DENZO[11] software at
120 K. Structures were determined by SHELXS-97 and refined by
SHELXL-97.[12] CCDC-711927 [for Rh(7-Mes)(COD)Cl], -711954
[for Rh(7-oTol)(COD)Cl] and -711928 [for Rh(6-Mes)(COD)Cl]
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
1
CDCl3, room temperature): δ = 205.0 (d, JRhC = 40.8 Hz, CNHC),
1
1
184.2 (d, JRhC = 57.3 Hz, CO), 181.9 (d, JRhC = 74.4 Hz, CO),
135.1 (br., CMes), 137.6 (s, CMes), 133.9 (s, CMes), 128.5 (s, CHMes),
50.5 (s, NCH ), 20.1 (s, CH ), 17.6 (s, CH ) ppm. IR: ν = 1995,
˜
2
3
3
2080 (CH2Cl2).
Rh(6-Mes)(CO)2Cl: Complex [Rh(6-Mes)(COD)Cl] (50 mg,
0.088 mmol) was dissolved in dichloromethane (10 mL) and stirred
under an atmosphere of carbon monoxide for 30 min. The solvent
was then removed in vacuo, and the resulting solid was washed
with cold hexanes to remove residual 1,5-cyclooctadiene. The re-
sulting solid was dried in vacuo to afford 40 mg (0.078 mmol) of a
Supporting Information (see footnote on the first page of this arti-
cle): 1H NMR spectrum from the deprotonation of 7-oTol·HBF4
with K[HMDS].
1
pale yellow solid (88% yield). H NMR (400 MHz, CDCl3, room
1918
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1913–1919

Rhodium Complexes of Expanded Ring N-Heterocyclic Carbenes
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Acknowledgments
We would like to acknowledge the EPSRC UK National Crystal-
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[8] The complexes Rh(5-Mes)(cod)Cl, Rh(6-Mes)(cod)Cl and
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Received: December 5, 2008
Published Online: March 5, 2009
Eur. J. Inorg. Chem. 2009, 1913–1919
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1919