N-alkyl functionalised expanded ring N-heterocyclic carbene complexes of rhodium(i) and iridium(i): Structural investigations and preliminary catalytic evaluation

Article abstract of DOI:10.1039/c2dt32823b

A series of new N-alkyl functionalised 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) pro-ligands 3-6 and their corresponding complexes of rhodium(i) and iridium(i), [M(NHC)(COD)Cl] 7-14 and [M(NHC)(CO) ₂Cl] 15-22 are described. The complexes have been characterised by ¹H and ¹³C{¹H} NMR, mass spectrometry, IR and X-ray diffraction. It is noted from X-ray diffraction studies that the N-alkyl substituents are found to orientate themselves away from the metal centre due to unfavourable steric interactions resulting in low percent buried volume (%V_bur) values in the solid state. The heterocycle ring size is also found to dictate the spatial orientation of the N-alkyl substituents in the neopentyl functionalised derivatives 10 and 14. The 7-membered derivative 14 allows for a conformational 'twist' of the heterocycle ring with the N-alkyl substituents adopting a mutually trans configuration with respect to each other, while the more rigid 6-membered system 10 does not allow for this conformational 'twist' and consequently the N-alkyl substituents adopt a mutually cis configuration. The σ-donor function of this new class of expanded ring NHC ligand has also been probed by measured IR stretching frequencies of the [M(NHC)(CO)₂Cl] complexes 15-22. A preliminary catalytic survey of the hydrogenation of functionalised alkenes with molecular hydrogen under mild conditions has also been undertaken with complex 10, affording an insight into the application of large ring NHC ancillary ligands bearing N-alkyl substituents in hydrogenation transformations.

Full text of DOI:10.1039/c2dt32823b

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N-alkyl functionalised expanded ring N-heterocyclic
carbene complexes of rhodium(I) and iridium(I):
structural investigations and preliminary catalytic
evaluation†
Cite this: DOI: 10.1039/c2dt32823b
a
b
a
b
Jay J. Dunsford,‡ Dorette S. Tromp, Kingsley J. Cavell,* Cornelis J. Elsevier and
a
Benson M. Kariuki
A series of new N-alkyl functionalised 6- and 7-membered expanded ring N-heterocyclic carbene (NHC)
pro-ligands 3–6 and their corresponding complexes of rhodium(I) and iridium(I), [M(NHC)(COD)Cl] 7–14
1
13
1
2
and [M(NHC)(CO) Cl] 15–22 are described. The complexes have been characterised by H and C{ H}
NMR, mass spectrometry, IR and X-ray diffraction. It is noted from X-ray diffraction studies that the N-alkyl
substituents are found to orientate themselves away from the metal centre due to unfavourable steric
interactions resulting in low percent buried volume (%Vbur) values in the solid state. The heterocycle ring
size is also found to dictate the spatial orientation of the N-alkyl substituents in the neopentyl function-
alised derivatives 10 and 14. The 7-membered derivative 14 allows for a conformational ‘twist’ of the
heterocycle ring with the N-alkyl substituents adopting a mutually trans configuration with respect to
each other, while the more rigid 6-membered system 10 does not allow for this conformational ‘twist’
and consequently the N-alkyl substituents adopt a mutually cis configuration. The σ-donor function of
this new class of expanded ring NHC ligand has also been probed by measured IR stretching frequencies
Received 26th November 2012,
Accepted 18th December 2012
2
of the [M(NHC)(CO) Cl] complexes 15–22. A preliminary catalytic survey of the hydrogenation of func-
tionalised alkenes with molecular hydrogen under mild conditions has also been undertaken with
complex 10, affording an insight into the application of large ring NHC ancillary ligands bearing N-alkyl
substituents in hydrogenation transformations.
DOI: 10.1039/c2dt32823b
www.rsc.org/dalton
3
of important transition metal mediated transformations.
1. Introduction
More recently our group and others have detailed the synthesis
N-heterocyclic carbene (NHC) ligands have become increas- of a range of novel expanded ring NHC ligand architectures (to
ingly popular in both organometallic chemistry and homo- date there have been examples of 6-, 7- and 8-membered ring
1
4
geneous catalysis over the past 20 years, in part due to their systems) and their corresponding complexes with an array of
2
5
6
7
8
9
10
11
12
readily tunable steric and electronic parameters. To date there late transition metals (Ag, Rh, Ir, Pt, Pd, Ni, Cu, Au
has been a plethora of literature reports detailing an extremely and Ru ). The application of complexes bearing expanded
wide and diverse range of NHC ligand architectures, metal ring NHC ancillary ligands in catalytic transformations such as
1
3
6
f,7
6c
complexes and their catalytic application in the vast majority transfer hydrogenation
and hydrogenation, hydrosilyl-
8
a,b
9c,d
ation,
nes,
C–C cross coupling
and the hydration of alky-
1
2a,b
have thus far demonstrated a number of encouraging
results and in some cases novel reactivity by comparison to tra-
ditional imidazolium based analogues. With these previous
investigations in mind we are interested in further understand-
a
School of Chemistry, Main Building, Cardiff University, Cardiff, CF10 3AT Wales,
UK. E-mail: cavellKJ@cf.ac.uk; Fax: (+44) 029-20874030; Tel: (+44) 029-20879316
9c
b
Molecular Inorganic Chemistry, Van’t Hoff Institute for Molecular Sciences,
University of Amsterdam, Science Park 904, Postbus 94157, 1090 GD Amsterdam,
ing the novel reactivity and scope of such expanded ring NHC
Electronic supplementary information (ESI) available: Structure refinement complexes through modulation of both σ-donor function and
The Netherlands. E-mail: c.j.elsevier@uva.nl
†
details for compounds 4, 5, 6, 10 and 14. CCDC 905711–905715. For ESI
the steric impact of the N-substituents.
and crystallographic data in CIF or other electronic format see DOI:
Early studies in the area by Buchmeiser and co-workers
detailed the synthesis of a 6-membered expanded ring NHC
ligand bearing N-alkyl substituents (iPr) and its coordination
10.1039/c2dt32823b
‡
Present Address: School of Chemistry, University Park, University of Notting-
ham, Nottingham, NG7 2RD, UK.
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1
4
chemistry with rhodium and palladium. There has also been
a previous example of a fused 7-membered ligand scaffold
based around a biphenyl backbone bearing neopentyl (Neo)
9
a
N-alkyl substituents detailed by Stahl and co-workers. Follow-
ing these reports, and our earlier investigations on saturated
expanded ring NHCs bearing N-aryl substituents, we became
interested in the idea of tuning the steric constraint imparted
by large ring NHC systems through the incorporation of less
sterically imposing N-alkyl substituents. We were particularly
interested in the influence in which the increasingly flexible
nature of such substituents would impart on both the struc-
tural conformation of the heterocycle ring and the catalytic
implications this may lead to. Imidazolium based N-alkyl func-
tionalised NHC complexes are well known within the literature
with numerous accounts of both their synthesis and appli-
1
5
cation. Arguably the most notable example of the catalytic
application of such systems comes in the form of the iridium
Scheme 1 Modified synthesis of N-alkyl functionalised formamidines 1 and 2
and expanded ring NHC·HX pro-ligands 3–6. (i) Amine (2 eq.), triethylorthofor-
(
III) complexes of Crabtree and co-workers consisting of a
mate (1 eq.), acetic acid (1 eq.), 140 °C, 17 h. (ii) NaOH(aq) (1.1 eq.), Et
2
O, 2 h. (iii)
1
6
bis-chelating NHC bearing pendant neopentyl substituents.
This set of compounds gave exceptional TOF values in the
transfer hydrogenation of unsaturated substrates.
dihalide (1.4 eq.), K
2 3
CO (0.5 eq.), MeCN, 70 °C, 17 h.
−
1
Herein, we present the synthesis and characterisation of
new N-alkyl functionalised expanded ring NHC ligands, utilis-
ing a modified synthetic protocol, and their coordination to
both rhodium(I) and iridium(I) precursors. Steric and elec-
tronic parameters of the resulting complexes, and preliminary
catalytic behaviour in the hydrogenation of alkynes were also
investigated.
1
13
1
a
Table 1 H and C{ H} NMR data for N–CH–N shifts of NHC·HX salts 3–6
1
13
1
Compound
H NMR shift
C{ H} NMR shift
3
4
5
6
8.21
7.89
7.92
8.94
149.9
156.2
155.6
160.4
a
1
3
Values given in ppm, measured in CDCl at 400 MHz ( H) and
25 MHz ( C{ H}).
1
3
1
1
2
. Results and discussion
functionalised expanded ring NHC·HX pro-ligands 3–6 in
yields ranging from 54 to 73% (Scheme 1).
Synthesis of N-alkyl functionalised expanded ring NHC·HX
ligands 3–6
Analysis of the N-alkyl functionalised NHC·HX pro-ligands
1
Synthetic routes toward previously described N-alkyl functiona- 3–6 by H NMR demonstrates an interesting feature. In pre-
lised expanded ring NHC pro-ligands have been based around viously reported expanded ring NHC·HX systems bearing
the functionalisation of a diamine backbone prior to ring N-aryl substituents, a trend in upfield shift of the NCHN
closure with an appropriate C1 building block (HC(OEt)
formaldehyde ). Our preferred entry into such expanded ring size. While this remains the case regarding the isopropyl
5
c
or proton resonance is observed with increasing heterocycle ring
3
1
4
4b
NHC systems has been based around a formamidine ring (iPr) functionalised derivatives 3 and 5 (8.21 ppm and
5
a,17
closure protocol,
both heterocycle ring size and N-substituents.
ment of this synthetic methodology for the synthesis of 6- and ively) (Table 1). This implies the NCHN proton of 6 is more
-membered N-alkyl functionalised derivatives is hampered by acidic than that of it’s 6-membered analogue 4, indicative of a
which allows for the facile modulation of 7.92 ppm, respectively), the neopentyl (Neo) analogues 4 and 6
5
a,6b
Employ- demonstrate a reverse trend (7.89 ppm and 8.94 ppm, respect-
7
2
0
13
1
the limited number of N-alkyl functionalised formamidine weaker conjugate base. However, analysis by C{ H} NMR
species reported to date. Hence, we have also prepaired reveals that the NCHN carbon demonstrates a downfield shift
N-alkyl functionalised N,N′-dialkylformamidines via a modifi- with increasing heterocycle ring size in both cases, in agree-
cation of a previously reported literature procedure. In this ment with previous reports (Table 1). This novel observation
approach utilisation of 1 equivalent of acetic acid (as apposed may be derived from the increasingly flexible nature of the
to a catalytic amount) affords the corresponding N,N′-dialkyl- both the 7-membered heterocycle and the neopentyl N-alkyl
formamidine salt, which is followed by a neutralisation step substituents.
1
8
19
enabling the isolation of N,N′-dialkylformamidines 1 and 2 as
Crystals suitable for X-ray diffraction were grown by the
stable, white crystalline solids in good yields of 82 and 89%, slow diffusion of diethyl ether into a concentrated dichloro-
respectively (Scheme 1). Ring closure of the stable N,N′-dialkyl- methane solution at ambient temperature. Analysis of the neo-
formamidines 1 and 2 yields both 6- and 7-membered N-alkyl pentyl functionalised compounds 4 and 6, showed the N-alkyl
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2
1
21
Fig. 1 ORTEP ellipsoid plot at 50% probability of the molecular structure of
. Hydrogens and solvent molecules have been omitted for clarity. Selected
bond lengths (Å) and angles (°) for 6-Neo·HBr: C(1)–N(1), 1.327(8); C(1)–N(2),
.325(8); N(1)–C(2), 1.477(8); N(2)–C(4), 1.456(9); N(1)–C(5), 1.475(9); N(2)–
Fig. 3 ORTEP ellipsoid plot at 50% probability of the molecular structure of
5. Hydrogens and solvent molecules have been omitted for clarity. Selected
bond lengths (Å) and angles (°) for 7-iPr·HI: C(1)–N(1), 1.316(18); C(1)–N(2),
1.310(17), N(1)–C(2), 1.476(18); N(2)–C(5), 1.433(18); N(1)–C(6), 1.471(18);
N(2)–C(9), 1.483(16); N(1)–C(1)–N(2), 130.2; C(1)–N(1)–C(2), 121.8(16); C(1)–
N(2)–C(5), 125.3(16); C(1)–N(1)–C(6), 118.5(13); C(1)–N(2)–C(9), 116.9(13).
4
1
C(10), 1.470(9); N(1)–C(1)–N(2), 122.7(7); C(1)–N(1)–C(2), 121.6(6); C(1)–N(2)–
C(4), 121.6(6); C(1)–N(1)–C(5), 118.9(6); C(1)–N(2)–C(10), 117.8(6).
respectively (Table 2). This observation is consistent with pre-
viously reported data for related N-aryl functionalised
5
a
expanded ring NHC systems. Through modulation of the
N-alkyl substituents to isopropyl in 5 (Fig. 3) the N–CNHC–N
bond angle is observed to increase further still to 130.2°. To
the best of our knowledge this represents the largest N–CNHC
–
N bond angle reported to date for any NHC·HX salt and even
rivals those of previously reported acyclic diaminocarbene
2
2
systems. It is also noteworthy to mention that all measured
N–CNHC–N bond angles of the N-alkyl functionalised systems
are found to be greater than those of their related 6- and
7
7
-membered N-aryl counterparts (6-Mes·HBr 114.7(13)° and
-Mes·HI 116.6(4)°).
2
1
Fig. 2 ORTEP ellipsoid plot at 50% probability of the molecular structure of
. Hydrogens and solvent molecules have been omitted for clarity. Selected
bond lengths (Å) and angles (°) for 7-Neo·HI: C(1)–N(1), 1.326(6); C(1)–N(2),
.324(6); N(1)–C(2), 1.480(7); N(2)–C(5), 1.475(7); N(1)–C(6), 1.469(7); N(2)–
5a
6
1
Synthesis of rhodium(I) and iridium(I) complexes
C(11), 1.474(7); N(1)–C(1)–N(2), 127.4(5); C(1)–N(1)–C(2), 125.0(4); C(1)–N(2)–
C(5), 123.4(4); C(1)–N(1)–C(6), 116.3(4); C(1)–N(2)–C(11), 117.7(4).
The N-alkyl functionalised NHC·HX amidinium salts 3–6 were
coordinated to both rhodium(I) and iridium(I) precursors
(
[M(COD)Cl]
2
) providing a comparison of physical and spectro-
scopic data with that of related complexes bearing N-aryl
Table 2 Selected bond lengths (Å) and angles (°) for NHC·HX ligands 4, 5,
and 6
5
–13
substituents.
As noted previously the [Rh(6-iPr)(COD)Cl]
1
4
complex 7 has been reported by Buchmeiser et al., herein we
will expand upon this work through the analysis of alternate
N-alkyl substituents (6-Neo) and larger ring NHC species (7-iPr
4
5
6
C(1)–N(1)
C(1)–N(2)
N(1)–C(alkyl
N(2)–C(alkyl
N(1)–C(1)–N(2)
N(1)–C(1)–C(alkyl
N(2)–C(1)–C(alkyl
1.327(8)
1.325(8)
1.475(9)
1.470(9)
122.7(7)
118.9(6)
117.8(6)
1.316(18)
1.310(17)
1.471(18)
1.483(16)
130.2
118.5(13)
116.9(13)
1.326(6)
1.324(6) and 7-Neo). The synthesis of the rhodium(I) and iridium(I)
)
)
1.469(7)
1.474(7)
127.4(5)
complexes 7–14 of the general formula, [M(NHC)(COD)Cl] was
achieved through the addition of in situ generated free carbene
)
)
116.3(4) directly to the corresponding [M(COD)Cl]
117.7(4)
2
dimer in THF
under inert conditions, affording the desired complexes 7–14
as air stable yellow solids in yields ranging from 69 to 91%
(
Scheme 2).
Analysis of the H NMR data for the neopentyl function-
1
substituents to be mutually cis to each other with the bulky
terminal tert-butyl groups serving to effectively block one side alised complexes 9, 10, 13 and 14, reveals a splitting of the
of the ligand coordination sphere (Fig. 1 and 2). However, this NCH C(CH ) proton resonances with two distinct signals
2
3 3
observation may not be representative of the solution state appearing at ca. 5.5 and 3.4 ppm, respectively. This feature is
conformation with free rotation enabled about the NCH C- possibly not surprising considering the degree of free rotation
CH3)3 bond. In analysis of the N–CNHC–N bond angles, enabled about the NCH C(CH ) bond in question. The term-
2
(
2
3 3
ligands 4 and 6 increase with increasing ring size from 6- to inal tert-butyl groups appear as a sharp singlet corresponding
2
3
13
1
7
-membered derivatives, with values of 122.7(7)° and 127.4(5)°, to 18 protons at ca. 1 ppm. Analysis of the C{ H} NMR shift
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Scheme 2 The synthesis of rhodium(I) and iridium(I) complexes [M(NHC)(COD)Cl] (7–14) and [M(NHC)(CO)
2
Cl] (15–22). (i) KHDMS (1.4 eq.), THF (10 ml), rt,
3
2
0 min. (ii) [Rh/Ir(COD)Cl] (0.5 eq.), THF (10 ml), rt, 1 h. (iii) CO(g) (1 atm), DCM (10 ml), rt, 20 min.
of the N–CNHC–N carbon of complexes 7–14 demonstrates steric constraint imposed by the bulky N-aryl substituents is
an observable downfield shift in resonance with increasing often found to lead to a slight increase in bond length with
9
c
5–13
heterocycle ring size, in line with previous observations.
increasing heterocycle ring size.
The N–CNHC–N bond
angles of complexes 10 and 14 become more acute with
increasing heterocycle ring size from 118.5(6)° to 114.1(7)°.
Solid-state analysis of rhodium(I) and iridium(I) complexes
Single crystals suitable for X-ray diffraction were grown by the This feature is unexpected in that all previous reports concern-
slow vapour diffusion of diethyl ether into a concentrated ing related N-aryl functionalised complexes demonstrate more
dichloromethane solution of the complex in question. The obtuse N–CNHC–N bond angles in the transition from 6- to
5
–13
conformations of the neopentyl substituents, relative to each 7-membered derivatives.
The observed reduction in bond
other differ with heterocycle ring size, in contrast to their angle from the 6- to 7-membered derivatives may be attributed
NHC·HX precursors (Fig. 1 and 2). In the 6-membered to the ‘twist’ conformation adopted by the 7-membered hetero-
complex, 10 the N-alkyl substituents are observed to adopt a cycle 14 as a consequence of the trans configuration of the neo-
mutually cis conformation with regard to each other and a pentyl N-alkyl substituents (Fig. 5). Other important bond
trans conformation in relation to the cyclooctadiene co-ligand angles to consider in such complexes are the CNHC–N–CNalkyl
(
Fig. 4). This is in contrast to the conformation adopted in angles. These angles become more acute with increasing ring
the 7-membered complex 14, where a trans conformation of size, 120.4(6) and 120.9(6) for 10 and 118.0(7) and 118.1(7) for
5
–13
the N-alkyl substituents is favoured, facilitated by the inherent 14, which is consistent with their N-aryl counterparts,
twist’ of the NHC heterocycle backbone (Fig. 5). This feature is although the influence these angles have upon the steric
‘
interesting in that it may have been predicted that a mutually environment around the metal coordination sphere may differ
cis conformation would be preferred with regard to alleviation from the corresponding N-aryl counterparts as a consequence
of steric impact with the bulky cyclooctadiene co-ligand. of the increased degree of flexibility associated with the N-alkyl
Selected bond lengths and angles for the neopentyl functiona- substituents.
lised iridium(I) complexes 10 and 14 are provided in Table 4.
The influence of the N-substituents upon the steric environ-
An analysis of the CNHC–Ir bond lengths reveals a slight ment of the metal centre may be quantified by the percent
2
5
decrease with increasing heterocycle ring size from 2.085(6) to buried volume (%Vbur) analysis. Subjection of complexes 10
.050(8) Å. This observation would be expected considering and 14 to such an analysis affords comparably low %Vbur
2
2
6
the previously described relationship of σ-donor function to values of 33.4 and 31.0, respectively (Table 4). These low cal-
NHC heterocycle ring size and from the downfield shift of the culated values reflect the fact that the neopentyl N-alkyl substi-
1
3
1
N–CNHC–N resonace in the
Table 3). This decrease in bond length is not always evident, as a consequence of the steric repulsion imparted by the bulky
for example in the related N-aryl functionalised systems the cyclooctadiene co-ligand (Fig. 4 and 5). The observed decrease
C{ H} NMR measurements tuents are orientated away from the metal coordination sphere
(
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2
1
21
Fig. 4 ORTEP ellipsoid plot at 50% probability of the molecular structure of
0. Hydrogens and solvent molecules have been omitted for clarity. Selected
Fig. 5 ORTEP ellipsoid plot at 50% probability of the molecular structure of
14. Solvent molecules have been omitted for clarity. Selected bond lengths (Å)
and angles (°) for [Ir(7-Neo)(COD)Cl]: C(1)–N(1), 1.358(11); C(1)–N(2), 1.358(12);
C(2)–N(1), 1.476.(12); C(5)–N(2), 1.477(12); C(1)–Ir(1), 2.050(6); C(16)–Ir(1),
2.131(9); C(17)–Ir(1), 2.119(9); C(20)–Ir(1), 2.170(9); C(21)–Ir(1), 2.157(9); Ir(1)–
Cl(1), 2.380(2); C(16)–C(17), 1.418(14); C(20–C(21), 1.366(17); N(1)–C(1)–N(2),
114.1(7); C(1)––N(1)–C(2), 120.1(8); C(1)–N(2)–C(5), 121.7(8); C(1)–Ir(1)–Cl(1),
88.4(3); C(1)–N(1)–C(6), 118.0(7); C(1)–N(2)–C(11), 118.8(7).
1
bond lengths (Å) and angles (°) for [Ir(6-Neo)(COD)Cl]: C(1)–N(1), 1.336(9); C(1)–
N(2), 1.347(9); N(1)–C(5), 1.470(9); N(2)–C(10), 1.456(9); C(1)–Ir(1), 2.085(6);
C(15)–Ir(1), 2.133(6); C(16)–Ir(1), 2.103(6); C(19)–Ir(1), 2.197(7); C(20)–Ir(1),
2
.161(7); Ir(1)–Cl(1), 2.3783(17); C(15)–C(16), 1.423(10); C(19)–C(20), 1.388(11);
N(1)–C(1)–N(2), 118.5(6); C(1)–N(1)–C(5), 115.2(6); C(1)–N(2)–C(10), 120.9(6);
C(1)–Ir(1)–Cl(1), 94.14(18); C(1)–N(1)–C(2), 123.6(6); C(1)–N(2)–C(4), 123.0(6).
1
3
1
in %Vbur value from the 6- to 7-membered derivative is again Table 3
C{ H} NMR N–CNHC–N shifts of N-alkyl functionalised rhodium(I) and
inverse to the expected trend, although possibly not surprising iridium(I) complexes 7–14
given the measured decrease in N–CNHC–N bond angle
1
3
a
Complex
C NMR shift (ppm)
(Table 4) and the spatial orientation adopted by the neopentyl
1
4
14
substituents in the 7-membered derivative 14. Again, it is [Rh(6-iPr)(COD)Cl]
important to stress that the values for the corresponding struc-
tures are calculated in the solid state and may not be represen-
tative of the solution state behaviour given the element of free [Rh(7-iPr)(COD)Cl] 11
2 3 3
rotation about the NCH C(CH ) bond.
7
204.1
—
208.0
202.2
224.8
209.1
b
[
[
[
Ir(6-iPr)(COD)Cl] 8
Rh(6-Neo)(COD)Cl] 9
Ir(6-Neo)(COD)Cl] 10
[Ir(7-iPr)(COD)Cl] 12
[Rh(7-Neo)(COD)Cl] 13
[Ir(7-Neo)(COD)Cl] 14
b
—
221.1
Infra-red studies
a
b
3
Measured in CDCl . Not observed.
In order to probe the σ-donor function of the N-alkyl functio-
nalised expanded ring NHC ancillary ligands in greater detail,
complexes of the general formula, [M(NHC)(CO) Cl] 15–22
2
were prepared. The complexes were accessed via the displace- dichloromethane solution of the corresponding complex for
4
b
ment of the cyclooctadiene co-ligand of the [M(NHC)(COD)Cl] 20 minutes at ambient temperature (Scheme 2). This method
complexes 7–14 via bubbling CO gas (1 atm) through a for the quantification of σ-donor function of ancillary ligands
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Table 4 Selected bond lengths (Å), angles (°) and percent buried volume σ-donor ligand. This observation may be due to changes in
(
%Vbur) values for complexes 10 and 14
heterocycle conformation in complex 14 in the solution state.
Although the solid-state structure demonstrates a trans con-
figuration of the neopentyl substituents relative to each other
and a ‘twist’ conformation of the heterocycle (Fig. 5), this may
not be representative of the solution state behaviour. In the
10
14
C
Ir–Cl
NHC–Ir
2.085(6)
2.3783(17)
2.133(6)
2.103(6)
2.197(7)
2.161(7)
1.423(10)
1.388(11)
118.5(6)
94.14(18)
120.4(6)
120.9(6)
33.4
2.050(8)
2.380(2)
2.123(9)
2.119(9)
2.170(9)
2.157(9)
1.418(14)
1.366(17)
114.1(7)
88.4(3)
Ir–Ctrans Cl
Ir–Ctrans Cl
Ir–Ctrans NHC
Ir–Ctrans NHC
CvCtrans Cl
CvCtrans NHC
N–CNHC–N
C
C
C
solution state with free rotation enabled about the NCH
2
C-
(CH3)3 bond, the conformation of the heterocycle may fall
4a,b
more in line with the related N-aryl complexes
increase in N–CNHC–N bond angle.
allowing an
NHC–Ir–Cl
NHC–N–CNalkyl
NHC–aN–CNalkyl
118.0(7)
118.1(7)
31.0
Hydrogenation catalysis
A preliminary evaluation of this class of ancillary NHC ligand
in the hydrogenation of alkenes was undertaken employing
the [Ir(6-Neo)(COD)Cl] complex 10. Initially, styrene was inves-
tigated as a model substrate as the non-hindered nature of the
terminal alkene functionality would be expected to allow the
effective reduction of the double bond under relatively mild
Cl] conditions (Table 6). Analysis of Table 6 shows that at relatively
low catalytic loadings (0.01 mol%) the influence of the solvent
employed is pronounced (Table 6, entries 1–6). The employ-
%
V
bur
a
Percent buried volume (%Vbur) calculated utilising the SambVca
2
4
software.
Table 5 Measured carbonyl IR stretching frequencies for [M(NHC)(CO)
2
a
complexes 15–22
ν (CO) cm−
1
ν
av (CO) cm−1
Complex
ment of primary alcohols, ethanol and methanol (entries 1
and 2) affords the quantitative conversion to ethylbenzene at
ambient temperature within 1 h at 5 bar hydrogen pressure.
Changing the solvent to the secondary alcohol, iPrOH (entry 3)
leads to a pronounced reduction in overall percentage yield
1
4
b
b
[
[
[
[
[
[
[
[
Rh(6-iPr)(CO)
Ir(6-iPr)(CO) Cl] 16
Rh(6-Neo)(CO) Cl] 17
Ir(6-Neo)(CO) Cl] 18
Rh(7-iPr)(CO)
Ir(7-iPr)(CO)
2
Cl] 15
1982, 2063
1982, 2062
1989, 2068
1988, 2066
1988, 2049
1981, 2056
1972, 2056
1972, 2054
2023
2022
2029
2027
2019
2019
2014
2013
2
2
2
2
Cl] 19
Cl] 20
2
(
40%). The employment of other common solvents also gives
Rh(7-Neo)(CO)
2
Cl] 21
Ir(7-Neo)(CO) Cl] 22
considerably lower overall percentage yields after 1 h at
ambient temperature (entries 4–6). The parameters of time
and pressure were also examined and it may be observed that
in the employment of conditions identical to entry 1, complete
conversion to ethylbenzene is observed in less than
2
a
b
3
Measured in CDCl . Measured in DCM.
3
0 minutes (entry 7). Reduction of the hydrogen pressure to 3
has been utilised extensively within the literature since the
2
7
introduction of the Tolman electronic parameter (TEP).
The average carbonyl stretching frequencies of complexes
−
1
1
5–22 fall between 2029 and 2013 cm (Table 5). The 6-mem-
Table 6 Catalyst optimisation for the hydrogenation of styrene with [Ir(6-Neo)-
bered neopentyl functionalised complexes 17 and 18 demon-
a
−
1
(COD)Cl], 10
strate average values of 2029 and 2027 cm respectively, while
their 7-membered analogues 21 and 22 demonstrate a shift to
lower wavenumbers of 2014 and 2013, respectively. This
decrease in average wave number demonstrates the increased
basicity of the 7-membered ancillary ligands, in line with
4
b
observations noted for their N-aryl counterparts. Similar
b
Entry
Solvent
t (min)
P (bar)
% Yield
observations are noted for the isopropyl functionalised com-
9
c
plexes 15, 16, 19 and 20 (Table 5). NHC ligands. Attempting
to correlate the σ-donor strength of the ancillary ligands, as
measured by average carbonyl bond stretching frequencies to
1
2
3
4
5
6
7
8
9
EtOH
MeOH
iPrOH
THF
Acetone
DCE
EtOH
EtOH
EtOH
60
60
60
60
60
60
30
30
30
5
5
5
5
5
5
5
3
1
100
100
40
12
34
4
b
measured N–CNHC–N bond angles
is hampered by the
unusual decrease in bond angle demonstrated by complex 14
in comparison with 10. Although there is a marked decrease in
N–CNHC–N bond angle, (ca. 4°) due to the alteration in hetero-
cycle conformation due to the orientation adopted by the
neopentyl substituents, the measured decrease in average
carbonyl bond stretching frequency from 10 to 14 is still
4
100
100
58
a
Reaction conditions: Styrene (1 mmol), [Ir(6-Neo)(COD)Cl] 10
(
0.01 mol%, THF stock solution), solvent (20 ml), decane (1 mmol,
b
internal standard). Percentage yields based upon consumption of
indicative of the 7-membered complex 14 being a stronger substrate by GCMS.
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Table 7 Substrate scope for the catalytic hydrogenation of terminal alkenes conversion within 1 h at 5 bar hydrogen pressure. For the steri-
a
utilising [Ir(6-Neo)(COD)Cl] complex 10
cally demanding 2,3-dimethyl-1,3-butadiene substrate, a sig-
nificant decrease in activity was observed (entry 6). The overall
b
Entry
1
Substrate
Product
t (h)
% Yield
percentage conversion after 1 h at 5 bar is found to be 52%,
which is consistent with the value obtained for α-methylsty-
rene substrate (entry 3). No partially hydrogenated 2,3-
dimethyl-1,3-butadiene could be detected. Hydrogenation of
the cis-internal alkene cyclooctene (COE) substrate also proved
challenging (46%) even after extended reaction periods of
16 h. These observations are consistent with the steric demand
of the substrate. Similarly, for increasingly hindered cis- and
trans-internal alkenes (entries 8 and 9) conversions after pro-
longed reaction times are significantly reduced with only very
small yields of reduced products observed.
A preliminary study of functional group tolerance of
complex 10 was also been undertaken (Table 7, entries 10–14).
Terminal alkene species bearing ketone functionalities were
reduced routinely in quantitative yield, without further
reduction of the ketone to its corresponding alcohol (entries
0.5
100
2
3
0.5
1
100
43
4
5
0.5
0.5
100
100
6
7
1
16
16
52
46
>1
10 and 13). Complex 10 was also found to tolerate halide-
containing substrates (entries 11 and 14). It is interesting to
note that the overall percentage yield afforded in entry 11 is
8
9
69% in comparison to the quantitative conversion achieved for
the related styrene substrate (entry 1). It appears that the
bromide in the ortho-position has a detrimental effect on the
catalytic activity, possibly as a consequence of unfavourable
steric interactions.
16
1
>1
10
100
1
1
1
1
69
3
. Conclusions
3
4
1
1
100
100
In summary, a range of new 6- and 7-membered N-alkyl func-
tionalised expanded ring NHC·HX ancillary ligands have been
synthesised via modified synthetic procedures. The ligands
have been employed in the synthesis of both rhodium(I) and
iridium(I) complexes of the general formula, [M(NHC)(COD)-
1
a
Reaction conditions: Substrate (1 mmol), catalyst (0.01 mol%, THF
stock solution), EtOH (20 ml), decane (1 mmol, internal standard), H
2
b
(
5 bar). Percentage yield based upon consumption of substrate by Cl]. The novel structural parameters of the complexes have
GCMS.
been probed by solid-state analysis, affording an insight into
the influence in which modulation of N-substituents has upon
heterocycle conformation, and N–CNHC–N bond angles in par-
ticular. To gain an insight into the σ-donor function of the
bar was found to have no effect on the overall catalytic activity ancillary N-alkyl functionalised ligands, [M(NHC)(CO) Cl]
2
displayed by the system although a further reduction to atmos- derivatives were prepared from their corresponding [M(NHC)-
pheric pressure leads to a significant decrease to 58% conver- (COD)Cl] complexes allowing an analysis of measured carbonyl
sion after 1 h (entries 8 and 9). With optimal conditions bond stretching frequencies. A preliminary evaluation of
established, complex 10, [Ir(6-Neo)(COD)Cl] was tested with a complex 10 in the hydrogenation of unsaturated C–C contain-
range of substituted alkene substrates (Table 7). It may be ing substrates was also undertaken. This study demonstrated
noted that similar activities to styrene are observed in the 10 to be effective in the reduction of non-hindered substrates
hydrogenation of allyl benzene (entry 2) although in the utili- under mild conditions, although employment of increasingly
sation of the increasingly hindered α-methylstyrene, a pro- hindered substrates facilitated less satisfactory outcomes.
nounced decrease in activity is observed with a percentage Current work in our laboratory is focused upon the employ-
conversion of 43% observed after 1 h at 5 bar hydrogen ment of the N-alkyl functionalised expanded ring NHC
pressure. The hydrogenation of non-hindered terminal ancillary ligands in alternate transition metal mediated trans-
alkenes, such as 1-octene and 3,3-dimethyl-1-butene (entries 4 formations, the results of which will be reported in due
and 5) were also found to proceed effectively, reaching full course.
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(
3 and 4) or 1,4-diiodobutane (5 and 6) (24 mmol) was
4
. Experimental
added and the solution heated to reflux for 17 h. Upon
cooling the solvent was removed in vacuo before the
General remarks
All manipulations were performed using standard Schlenk residue was taken up in DCM (20 ml), filtered to remove all
techniques under an argon atmosphere, except where other- potassium salts and recrystallised via the slow addition of
wise noted. All complexes after their formation were treated diethyl ether yielding a white crystalline suspension isolated
under aerobic conditions. Solvents of analytical grade were by filtration.
freshly distilled using an MBraun SPS-800 solvent purification
system. Deuterated solvents for NMR measurements were dis-
6-iPr·HBr (3). White crystalline solid, yield 73% (3.64 g).
1
3
H NMR (CDCl , 400 MHz, 298 K): δ 8.12 (1H, s, NHCN); 3.94
3
tilled from the appropriate drying agents under N
ately prior to use, following standard literature methods. All (2H, q, J
2
immedi- (2H, s, NCH(CH
3
)
2
); 3.34 (4H, t, ); 1.99
= 5.0 Hz, NCH CH ); 1.29 (12H, d, J = 6.7 Hz,
HH
J
HH = 5.8 Hz, NCH
2
2
8
3
3
HH
2
2
1
3
1
commercially available reagents were used as received. NMR NCH(CH
spectra were obtained on a Bruker Advance AMX 400 or 500. (s, NCHN); 56.1 (s, NCH(CH
The chemical shifts are given as dimensionless δ values and NCH(CH ) ); 17.9 (s, NCH CH ). MS (ESI) m/z 169.1707
3
)
2
). C{ H} NMR (CDCl
3
, 125 MHz, 298 K): δ 149.9
3
)
2
); 38.8 (s, NCH
2
); 18.9 (s,
3
2
2
2
+
are frequency referenced relative to the peak for TMS for (M − I) – C11
23 2
H N requires 169.1703.
1
13
H and C. Coupling constants J are given in Hertz as positive
6-Neo·HBr (4). White crystalline solid, yield 54% (3.29 g).
1
values regardless of their real individual signs. The multi-
plicity of the signals is indicated as “s”, “d”, “t” or “m” for (2H, t, JHH = 5.7 Hz, NCH
singlet, doublet, triplet or multiplet, respectively. High- (2H, q, JHH = 5.2 Hz, NCH
resolution mass spectra were obtained in electrospray (ES)
mode unless otherwise reported on a Waters Q-Tof micromass (s, NCH
spectrometer. GCMS data was obtained on an Agilent Techno- (s, NCH
logies 6890N GC system with an Aligent Technologies 5973 requires 225.2334.
H NMR (CDCl , 400 MHz, 298 K): δ 7.89 (1H, s, NCHN); 3.46
3
3
2
2
); 3.32 (4H, s, NCH
2
C(CH
3
)
3
); 2.12
).
3
CH ); 0.94 (18H, s, NCH
2
2
C(CH )
3 3
1
3
1
C{ H} NMR (CDCl , 125 MHz, 298 K): δ 156.2 (s, NCHN); 66.7
3
2
C(CH
3 3 2 2 3 3
) ); 46.6 (s, NCH ); 27.3 (s, NCH C(CH ) ); 19.3
+
2
CH ). MS (ESI): m/z 225.2331 (M − Br) – C14H N
2
29 2
inert MS detector with MSD.
7-iPr·HI (5). White crystalline solid, yield 65% (4.03 g).
1
H NMR (CDCl
3
, 400 MHz, 298 K): δ 7.92 (1H, s, NCHN); 3.96
General procedure for synthesis of N-Alkyl formamidines
3
(
2H, m, NCH(CH ) ); 3.65 (4H, t, J
= 5.8 Hz, NCH ); 2.04
2
); 1.29 (12H, d, JHH = 6.7 Hz,
3
2
HH
3
3
A round bottom flask was charged with the appropriate amine (4H, q, JHH = 5.1 Hz, NCH
2
CH
). C{ H} NMR (CDCl
30 mmol). The reaction mixture was heated to 140 °C for 17 h (s, NCHN); 58.0 (s, NCH(CH ) ); 42.9 (s, NCH2); 24.4
2
1
3
1
(
(
60 mmol), triethylorthoformate (30 mmol) and acetic acid NCH(CH
3
)
2
3
, 125 MHz, 298 K): δ 155.6
3
2
with stirring prior to the removal of all volatiles in vacuo fur- (s, NCH
nishing the acetic acid salt. The acetic salt was dissolved in a (M − I) – C11
2
CH
2
); 19.2 (s, NCH(CH
3 2
) ). MS (ESI): m/z 183.1864
+
23 2
H N
requires 183.1861.
minimum amount of diethyl ether and aqueous NaOH
1.1 eq.) added slowly and the mixture stirred at ambient temp-
erature for 2 h. The organic layer was collected, the aqueous (4H, t, JHH = 5.8 Hz, NCH
7-Neo·HI (6). White crystalline solid, yield 58% (4.23 g).
1
(
H NMR (CDCl
3
, 400 MHz, 298 K): δ 8.49 (1H, s, NCHN); 3.80
); 3.51 (4H, s, NCH C(CH ); 2.15
layer washed with diethyl ether (3 × 30 ml), dried over MgSO4 (4H, q, J = 4.9 Hz, NCH CH ); 0.99 (18H, s, NCH C(CH ) ).
3
2
2
3 3
)
3
HH
2
2
2
3 3
1
3
1
and evaporated to dryness to furnish the desired formamidine
species.
C{ H} NMR (CDCl
3
, 125 MHz, 298 K): δ 160.4 (s, NCHN); 69.3
); 27.2 (s, NCH C(CH ); 25.1
(s, NCH C(CH ); 53.2 (s, NCH
2
3
)
3
2
2
3 3
)
+
N,N′-Bis(isopropyl)formamidine (1). White crystalline solid, (s, NCH CH ). MS (ESI): m/z 239.2487 (M − I) – C H N
2
2
15 31 2
1
yield 82% (3.15 g). H NMR (CDCl
3
, 400 MHz, 298 K): δ 7.26 requires 239.2484.
(
1H, s, NCHN), 2.29 (2H, s, NCH
2
C(CH ), 0.87 (18 H, s,
3 3
)
1
3
1
NCH C(CH ) ). C{ H} NMR (CDCl , 125 MHz, 298 K): δ 165.0
2
3 3
3
General procedure for synthesis of [Rh(NHC)(COD)Cl]
complexes
(
(
s, NCHN), 49.0 (s, NCH
ESI): m/z 129.1395 (M + H) – C
2
C(CH
3
)
3
2 3 3
), 27.1 (s, NCH C(CH ) ). MS
+
7
17 2
H N requires 129.1392.
N,N′-Bis(2,2-dimethylpropane)formamidine (2). White crys- A flame dried Schlenk was loaded with the desired NHC·HX
1
talline solid, yield 89% (4.91 g). H NMR (CDCl
3
, 400 MHz, salt (0.15 mmol) and dried under vacuum for ca. 30 min
2
98 K): δ 7.38 (1H, s, NHCN), 4.07 (1H, s (broad), NH), 3.45 prior to the addition of dry, degassed THF (10 ml) and
3
13
(
{
(
(
2H, m, NCH(CH ) ), 1.11 (12H, d, J = 7.2, NCH(CH3)2).
C
KHDMS (0.21 mmol). The solution was stirred for 30 min
, 125 MHz, 298 K): δ 149.1 (s, NHCN), 49.5 at ambient temperature and transferred via an oven dried
), 24.2 (s, NCH (CH ). MS (ESI): m/z 184.1936 filter cannula to a stirred THF (5 ml) solution of [Rh(COD)Cl]
M + H) – C H N requires 184.1939.
3
2
HH
1
H} NMR (CDCl
3
s, NCH(CH
3
)
2
2
3
)
2
2
+
(0.075 mmol). The orange solution turned bright
yellow immediately upon addition of the free carbene solution.
The reaction mixture was allowed to stir for 1 h at ambient
temperature before the THF was removed under vacuum
1
1
24 2
General procedure for synthesis of N-alkyl functionalised
NHC·HX salts
A round bottom flask was charged with N,N′-bis(alkyl)forma- and the resultant yellow residue triturated with n-pentane
midine (20 mmol), K CO (10 mmol) and suspended in aceto- to furnish the title compounds as microcrystalline yellow
2
3
nitrile (80 ml). To this stirred solution 1,3-dibromopropane solids.
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General procedure for synthesis of [Ir(NHC)(COD)Cl]
complexes
MeCN) – C24
H
43
N
3
Ir requires 564.3038. Anal. Calcd for
C H N IrCl: C, 47.17; H, 7.20; N, 5.00. Found: C, 47.19;
2
3
40 2
H, 6.94; N, 4.61.
A flame dried schlenk was loaded with the desired NHC·HX
salt (0.15 mmol); and dried under vacuum for 30 min prior to 69% (44.3 mg). H NMR (CDCl , 400 MHz, 298 K): δ 6.31 (2H,
[Rh(7-iPr)(COD)Cl] (11). Yellow microcrystalline solid, yield
1
3
3 2
the addition of dry, degassed THF (10 ml) and KHDMS m, NCH(CH ) ), 4.93 (2H, m, CHCOD), 3.42 (2H, m, CHCOD);
3
3
(
0.21 mmol). The solution was stirred at ambient temperature 3.31 (2H, t, JHH = 5.8 Hz, NCH
2
), 3.26 (2H, t, JHH = 5.9 Hz,
for 30 min and transferred via oven dried filter cannula to a NCH ), 2.19 (4H, m, CH ), 1.80 (4H, m, CH2 COD), 1.58
2
2 COD
3
3
stirred THF (5 ml) solution of [Ir(COD)Cl]
bright yellow solution darkened immediately upon addition of NCH(CH
the free carbene solution. The reaction mixture was allowed to not observed, 92.3 (d, J
2
(0.075 mmol). The (4H, q, JHH = 5.0 Hz, NCH
2
CH
2
), 1.26 (12H, d, JHH = 5.8 Hz,
, 125 MHz, 298 K): δ (NCN)
1
3
1
3
)
2
). C{ H} NMR (CDCl
3
1
1
= 6.3 Hz, CHCOD), 70.7 (d, J
=
CRh
CRh
stir for 1 h at ambient temperature before the THF was 13.7 Hz, CHCOD), 56.7 (s, NCH2), 41.4 (s, NCH(CH
removed under vacuum and the resultant yellow/brown oil CH2 COD), 28.5 (s, CH2 COD), 24.9 (s, NCH CH ), 20.1 (s, NCH-
triturated with n-pentane furnishing a brown solid. The iso- (CH ) ). MS (ESI): m/z 393.1778 (M − Cl) – C H N Rh
3 2
) ), 30.9 (s,
2
2
+
3
2
19 34 3
lated solid was further purified via flash column chromato- requires 393.1777.
graphy (silica/DCM) and eluted as a yellow band in the first [Ir(7-iPr)(COD)Cl] (12). Yellow microcrystalline solid, yield
fraction, the eluent was reduced in vacuo to afford the title 75% (58.3 mg). H NMR (CDCl , 400 MHz, 298 K): δ 6.09 (2H,
1
3
3
compounds as bright yellow microcrystalline solids.
[
8
m, NCH(CH
Rh(6-iPr)(COD)Cl] (7). Yellow microcrystalline solid, yield 5.9 Hz, NCH
8% (54.7 mg). The physical data obtained was found to be in CHCOD), 2.08 (4H, m, CH
3 2 HH
) ), 4.28 (2H, m, CHCOD), 3.40 (2H, t, J =
1
4
3
2
), 3.21 (2H, t, JHH = 5.9 Hz, NCH
2
), 2.88 (2H, m,
), 1.79 (4H, m, CH2 COD), 1.56
), 1.25 (12H, d, JHH = 5.8 Hz,
, 125 MHz, 298 K): δ 209.1
2 COD
3
3
line with the previously reported complex of Buchmeiser.
(4H, q, JHH = 4.9 Hz, NCH
2
CH
2
1
3
1
[
Ir(6-iPr)(COD)Cl] (8). Yellow microcrystalline solid, yield NCH(CH
3
)
2
). C{ H} NMR (CDCl
3
1
7
9% (59.8 mg). H NMR (CDCl , 400 MHz, 298 K): δ 6.24 (2H, (s, NCN), 78.9 (s, CHCOD), 56.8 (s, CHCOD), 52.4 (s, NCH ), 42.1
3
2
m, NCH(CH
3
)
2
), 4.84 (2H, m, CHCOD), 3.93 (2H, m, CHCOD), (s, NCH(CH
3
)
2
), 32.3 (s, CH
), 20.2 (s, NCH(CH
= 5.5 Hz, NCH CH ), 1.25 (M − Cl + MeCN) – C H N Ir requires 522.2594.
2
COD), 28.4 (s, CH
) ). MS (ESI): m/z 522.2589
3 2
2
COD), 24.9
3
3
.22 (4H, t, JHH = 5.8 Hz, NCH
2
3
), 2.91 (4H, m, CH2 COD), 1.98 (s, NCH
2
CH
2
+
(
(
4H, m, CH
), 1.41 (2H, q, J
COD HH
2
2
2
21 37 3
3
13
1
12H, d,
JHH = 6.8 Hz, NCH(CH
3
)
2
). C{ H} NMR (CDCl
3
,
[Rh(7-Neo)(COD)Cl] (13). Yellow microcrystalline solid, yield
1
1
25 MHz, 298 K): δ (NCN) not observed, 96.1 (s, CHCOD), 74.9 83% (60.3 mg). H NMR (CDCl , 400 MHz, 298 K): δ 4.82 (2H,
3
2
(s, CHCOD), 58.2 (s, NCH ), 40.3 (s, NCH(CH ) ), 32.6 (s, d, J
= 15.3 Hz, NCH C(CH ) ), 4.56 (2H, m, CHCOD), 3.37
HH 2 3 3
2
3 2
2
CH
2
COD), 29.5 (s, CH
2
COD), 22.5 (s, NCH
2
CH
2
), 19.4 (s, (2H, d, JHH = 15.4 Hz, NCH
2
3 3
C(CH ) ), 3.19 (2H, m, CHCOD),
+
3
NCH(CH
3
)
2
). MS (ES) m/z: 503.1932 (M + H) – C18
H
33
N
2
IrCl 2.25 (4H, t, JHH = 5.8 Hz, NCH
2
), 1.79 (4H, m, CH2 COD), 1.53
requires 503.1932.
(4H, m, CH ), 1.04 (18H, s, NCH C(CH ) ), 0.37 (4H, q,
2
COD
2
3 3
3
13
1
[
Rh(6-Neo)(COD)Cl] (9). Yellow microcrystalline solid, yield
J
HH = 5.6 Hz, NCH
2 2 3
CH ). C{ H} NMR (CDCl , 125 MHz,
1
1
7
1% (50.1 mg). H NMR (CDCl
3
, 400 MHz, 298 K): δ 5.59 (2H, 298 K): δ (NCN) not observed, 96.3 (d, JCRh = 6.3 Hz, CHCOD),
2
1
d, J
= 15.4 Hz, NCH C(CH ) ), 4.78 (2H, m, CH
), 3.39 72.1 (s, NCH C(CH ) ), 66.6 (d, J
= 15 Hz, CHCOD), 55.7 (s,
HH
2
3 3
COD
2
3 3
CRh
2
3
(
2H, d,
J
HH = 15.5 Hz, NCH
2
C(CH
3
)
3
), 3.15 (2H, t,
J
HH
=
NCH
2
), 31.9 (s, CH
2
COD), 28.1 (s, NCH
2 3 3
C(CH ) ), 27.0 (s,
3
5
.8 Hz, NCH
2
), 3.05 (2H, t, JHH = 5.8 Hz, NCH
2
3
), 2.89 (2H, m, CH
2
COD), 23.5 (s, NCH
2
CH ). MS (ESI): m/z 449.2403
2
+
CHCOD), 2.27 (4H, m, CH2 COD), 1.87 (2H, q, JHH = 5.1 Hz, (M − Cl) – C H N Rh requires 449.2396.
23
42 2
NCH
2
CH
2
), 1.79 (4H, m, CH
2
COD), 1.06 (18H, s, NCH
2
C-
[Ir(7-Neo)(COD)Cl] (14). Yellow microcrystalline solid, yield
1
3
1
1
(
CH
3
)
3
). C{ H} NMR (CDCl
3
, 125 MHz, 298 K): δ 208.0 (d, 76% (65.5 mg). H NMR (CDCl
3
, 400 MHz, 298 K): δ 4.41 (2H,
= 15.4 Hz, NCH C(CH ) ), 4.23 (2H, m, CHCOD), 3.47
1
1
2
JCRh = 47.5 Hz, NCN), 94.2 (d, J
= 6.3 Hz, CHCOD), 69.7 (s, d, J
HH
CRh
2
3 3
1
3
NCH
2
C(CH
3
)
3
), 68.9 (d, JCRh = 15 Hz, CHCOD), 45.9 (s, NCH
2
), (2H, m, CHCOD), 3.26 (4H, t, JHH = 5.9 Hz, NCH
2
), 2.92 (2H, d,
2
3
2
C
1.6 (s, CH2 COD), 28.8 (s, NCH
0.6 (s, NCH CH ). MS (ESI): m/z 435.2289 (M − Cl) – (4H, m, CH2 COD), 1.47 (4H, q, J
2
C(CH ), 28.0 (s, CH2 COD),
3
)
3
JHH = 15.6 Hz, NCH C(CH ) ), 2.10 (4H, m, CH2 COD), 1.58
2 3 3
+
3
= 5.0 Hz, NCH CH ), 1.01
2 2
2
2
HH
1
3
1
22
H
40
N
2
Rh requires 435.2257.
(18H, s, NCH
2
C(CH
3
)
3
).
C{ H} NMR (CDCl
3
, 125 MHz,
C-
[Ir(6-Neo)(COD)Cl] (10). Yellow microcrystalline solid, yield 298 K): δ 221.2 (s, NCN), 81.2 (s, CHCOD), 71.7 (s, NCH
2
1
9
1% (76.5 mg). H NMR (CDCl , 400 MHz, 298 K): δ 5.38 (2H, (CH ) ), 56.2 (s, CH
), 50.4 (s, NCH ), 32.1 (s, CH2 COD), 28.3
3
3 3
COD
2
2
d, JHH = 15.5 Hz, NCH
2
C(CH
3
)
3
), 4.33 (2H, m, CHCOD), 3.21 (s, CH2 COD), 28.1 (s, NCH
2
C(CH
3
)
3
), 25.4 (s, NCH
2
CH
Ir requires
NCH C(CH ) ), 3.05 (2H, t, J = 5.8 Hz, NCH ), 2.51 (2H, m, 580.3223. Anal. Calcd for C H N IrCl: C, 48.10; H, 7.37; N,
2
). MS
3
2
+
(2H, t,
J
HH = 5.8 Hz, NCH ), 3.14 (2H, d, JHH = 15.3 Hz, (ESI): m/z 580.3243 (M − Cl + MeCN) – C25H N
2
45 3
3
2
3 3
HH
2
24 42 2
CHCOD), 2.07 (4H, m, CH2 COD), 1.55 (4H, m, CH2 COD), 1.43 4.88. Found: C, 48.39; H, 7.46; N, 4.48.
3
(
2H, q, JHH = 5.1 Hz, NCH
2
CH
2 2 3 3
), 1.02 (18H, s, NCH C(CH ) ).
1
3
1
General procedure for synthesis of [Rh/Ir(NHC)(CO)
2
Cl]
C{ H} NMR (CDCl , 125 MHz, 298 K): δ 202.2 (s, NCN), 78.9
3
complexes
(
s, CHCOD), 68.9 (s, NCH
NCH ), 32.3 (s, CH2 COD), 31.4 (s, CH2 COD), 28.6 (s, NCH
CH ) ), 20.9 (s, NCH CH ). MS (ESI): m/z 564.3036 (M − Cl + ing [Rh/Ir(NHC)(CO) Cl] complex and dissolved in DCM
2 3 3
C(CH ) ), 52.5 (s, CHCOD), 46.8 (s,
2
2
C- A 50 ml round bottom flask was loaded with the correspond-
+
(
3
3
2
2
2
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+
(
10 ml) followed by CO(g) (1 atm) bubbled through the solution (ESI): m/z 460.1568 (M − Cl + MeCN) – C14
for 20 min where upon the yellow solution lightened. The 460.1576. FTIR: 1981 (ν(CO I)), 2056 (ν(CO II)).
solution was reduced in vacuo to afford a yellow residue [Rh(7-Neo)(CO) Cl] (21). Pale yellow microcrystalline solid,
which was triturated with n-pentane to yield the corresponding yield 93% (16.6 mg). H NMR (CDCl
25 3 2
H N O Ir requires
2
1
3
, 400 MHz, 298 K): δ 4.22
2
2
[
Rh/Ir(NHC)(CO) Cl] complex as a pale yellow microcrystalline (2H, d, JHH = 15.4 Hz, NCH C(CH ) ), 3.46 (2H, d, JHH
=
2
2
3 3
3
solid.
15.4 Hz, NCH
2
C(CH
3
)
3
), 2.29 (4H, t, JHH = 5.9 Hz, NCH
CH ), 0.99 (18H, s, NCH
C{ H} NMR (CDCl , 125 MHz, 298 K): δ (NCN) not observed,
2
), 1.23
C(CH ) ).
2 3 3
14
3
[
Rh(6-iPr)(CO)
yield 88% (15.4 mg). The physical data obtained was found to
be in line with the previously reported complex of Buchmeiser. 188.5 (d, JRhC = 55.1 Hz, CO), 181.5 (d, JRhC = 74.2 Hz, CO),
Ir(6-iPr)(CO) Cl] (16). Pale yellow microcrystalline solid, 71.6 (s, NCH C(CH ), 54.2 (s, NCH ), 29.7 (s, NCH C(CH ),
yield 86% (15.5 mg). H NMR (CDCl , 400 MHz, 298 K): δ 5.84 25.8 (s, NCH CH ). MS (ESI): m/z 426.1622 (M − Cl + MeCN) –
2
Cl]. (15). Pale yellow microcrystalline solid, (4H, q, JHH = 5.5 Hz, NCH
2
2
1
3
1
3
1
1
[
2
2
3
)
2
2
2
3 3
)
1
+
3
2
2
3
(2H, m, NCH(CH
3
)
2
), 3.21 (4H, t, JHH = 5.8 Hz, NCH
2
), 2.34
), 1.54 (12 H, d, JHH = 5.9 Hz, (ν(CO II)).
[Ir(7-Neo)(CO) Cl] (22). Pale yellow microcrystalline solid,
18 33 3 2
C H N O Rh requires 426.1628. FTIR: 1972 (ν(CO I)), 2056
3
3
2 2
(2H, q, JHH = 5.4 Hz, NCH CH
1
3
1
NCH(CH ) ). C{ H} NMR (CD CN, 125 MHz, 298 K): δ 203.6
3
2
3
2
1
(
s, NCN), 189.4 (s, CO), 181.4 (s, CO), 56.2 (s, NCH(CH
s, NCH ), 23.6 (s, NCH(CH ), 19.9 (s, NCH(CH
982 (ν(CO I)), 2062 (ν(CO II)).
Rh(6-Neo)(CO) Cl] (17). Pale yellow microcrystalline solid,
yield 97% (17.2 mg). H NMR (CDCl
3 2 3
) ), 41.8 yield 78% (14.2 mg). H NMR (CDCl , 400 MHz, 298 K): δ 4.27
2
3
(
1
2
3
)
2
3
)
2
). FTIR: (2H, d,
J
HH = 15.2 Hz, NCH
5.8 Hz, NCH ), 3.42 (2H, t, J
2
C(CH
3 3 HH
) ), 3.51 (2H, t, J =
3
= 5.8 Hz, NCH ), 2.76 (2H, d,
2
HH
2
2
3
[
2
J
HH = 15.2 Hz, NCH
2
C(CH
3
)
3
), 1.88 (2H, q,
), 1.73 (2H, q, JHH = 5.6 Hz, NCH CH
NCH C(CH ) ). C{ H} NMR (CDCl , 125 MHz, 298 K): δ 179.7
J
HH = 5.6 Hz,
), 0.98 (18H, s,
1
3
3
, 400 MHz, 298 K): δ 4.72 NCH
2
CH
2
2
2
2
3
13
1
(2H, d, JHH = 15.3 Hz, NCH C(CH ) ), 3.16 (2H, t, JHH =
2 3 3
2
3 3
3
3
5
.9 Hz, NCH
HH = 15.4 Hz, NCH
2
), 3.12 (2H, t, JHH = 5.9 Hz, NCH
2
), 3.07 (2H, d, (s, NCN), 167.8 (s, CO), 155.7 (s, CO), 72.9 (s, NCH
2
C(CH
3 3
) ),
2
3
J
2
C(CH
3
)
3
), 1.85 (2H, q,
J
HH = 5.0 Hz, 56.9 (s, NCH
2
), 27.5 (s, NCH
2
C(CH
3
)
3
), 24.5 (s, NCH
2
CH ). MS
2
1
3
1
+
NCH CH ), 0.99 (18H, s, NCH C(CH ) ). C{ H} NMR (CDCl , (ESI): m/z 526.2170 (M − Cl + MeCN) – C H N O Ir requires
2
2
2
1
3 3
3
19 33 3 2
1
25 MHz, 298 K): δ 199.4 (d, JRhC = 39.1 Hz, NCN), 185.0 (d, 526.2179. FTIR: 1972 (ν(CO I)), 2054 (ν(CO II)).
1
1
J
RhC = 53.7 Hz, CO), 183.7 (d, JRhC = 77.5 Hz, CO), 70.7 (s,
NCH C(CH ) ), 47.0 (s, NCH ), 32.2 (s, NCH C(CH ) ), 19.4 (s,
2
3 3
2
2
3 3
General hydrogenation procedure
+
NCH
2
CH
2
). MS (ESI): m/z 424.1489 (M − Cl + MeCN) –
Rh requires 424.1471. FTIR: 1989 (ν(CO I)), 2068
A 250 ml Parr autoclave equipped with a stirrer bar was
loaded with ethanol (20 ml), decane (1 mmol, internal
standard), the desired substrate (1 mmol) and the pre-catalyst
stock solution (THF). This solution was degassed with
nitrogen for ca. 5 minutes prior to the sealing to the vessel and
purging with molecular hydrogen. The reaction was stirred
at ambient temperature for 1 h prior to the venting of the
hydrogen and subsequent analysis of the reaction mixture by
GCMS. Samples were prepared by the dilution of a 0.3 ml
sample with 2 ml HPLC DCM and the elution through a small
silica column. Program for analysis: initial temperature at
C
18
H
31
N
3
O
2
(
ν(CO II)).
Ir(6-Neo)(CO)
yield 89% (16.1 mg). H NMR (CDCl
[
2
Cl] (18). Pale yellow microcrystalline solid,
1
3
, 400 MHz, 298 K): δ 4.84
2
3
(2H, d, JHH = 15.4 Hz, NCH C(CH ) ), 3.16 (2H, t, JHH
=
2
3 3
3
5
.8 Hz, NCH
2
), 3.12 (2H, t, JHH = 5.8 Hz, NCH
2
3
), 3.04 (2H, d,
2
2 3 3
JHH = 15.4 Hz, NCH C(CH ) ), 1.91 (2H, q, JHH = 4.9 Hz,
1
3
1
NCH CH ), 0.99 (18H, s, NCH C(CH ) ). C{ H} NMR (CDCl ,
2
2
2
3 3
3
1
7
1
C
25 MHz, 298 K): δ 195.1 (s, NCN), 179.9 (s, CO), 168.6 (s, CO),
2 3 3 2 2 3 3
0.2 (s, NCH C(CH ) ), 47.6 (s, NCH ), 28.1 (s, NCH C(CH ) ),
+
9.3 (s, NCH CH ). MS (ESI): m/z 514.2031 (M − Cl + MeCN) –
2
2
40 °C, held for 2.5 minutes, ramp 5 °C per minute next 150 °C,
18
31 3 2
H N O Ir requires 514.2046. FTIR: 1989 (ν(CO I)), 2068
ramp 10 °C per minute next 220 °C, held for 10 minutes. The
temperature of the injector and detector were maintained at
240 °C.
(
ν(CO II)).
Rh(7-iPr)(CO) Cl] (19). Pale yellow microcrystalline solid,
[
2
1
3
yield, 94% (16.5 mg). H NMR (CDCl , 400 MHz, 298 K): δ 5.54
3
(2H, m, NCH(CH
3
)
2
), 3.39 (4H, t, JHH = 5.8 Hz, NCH
2
), 2.12
3
3
(
4H, q, J
= 5.5 Hz, NCH CH ), 1.51 (12H, d, J
2 2 HH
) ). C{ H} NMR (CDCl , 125 MHz, 298 K): δ (NCN)
3 2 3
= 5.8 Hz,
HH
Identification hydrogenation products
1
3
1
NCH(CH
1
1
All products were identified through GCMS analysis or
not observed, 186.3 (d, JRhC = 54.2 Hz, CO), 183.4 (d, JRhC
7
NCH(CH
=
1
H NMR and were found to be in line with previously reported
7.6 Hz, CO), 57.8 (s, NCH ), 41.8 (s, NCH(CH ) ), 28.1 (s,
2
3 2
data.
3
)
2
), 18.2 (s, NCH
2
2
CH ). FTIR: 1988 (ν(CO I)), 2060
(
ν(CO II)).
Ir(7-iPr)(CO) Cl] (20). Pale yellow microcrystalline solid,
[
2
1
3
yield 97% (17.5 mg). H NMR (CDCl , 400 MHz, 298 K): δ 5.53
(
(
Acknowledgements
3
3 2 2
2H, m, NCH(CH ) ), 3.36 (4H, t, JHH = 5.8 Hz, NCH ), 1.55
3
3
4H, q, J
= 5.5 Hz, NCH CH ), 1.18 (12H, d, J
= 5.9 Hz, We would like to acknowledge the EPSRC and the Cardiff
HH
HH
2
2
1
3
1
NCH(CH
3
)
2
). C{ H} NMR (CDCl
3
, 125 MHz, 298 K): δ (NCN) Catalysis Institute (CCI) for JJD’s PhD studentship and
), 41.5 Johnson Matthey for the generous loan of rhodium and
s, NCH(CH ) ), 27.6 (s, NCH(CH ) ), 17.6 (s, NCH CH ). MS iridium salts.
not observed, 179.9 (s, CO), 165.3 (s, CO), 56.7 (s, NCH
(
2
3
2
3 2
2
2
Dalton Trans.
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Dalton Trans., 2011, 40, 9131; (d) U. Siemeling, C. Färber,
C. Bruhn, S. Fürmeier, T. Schulz, M. Kurlemann and
S. Tripp, Eur. J. Inorg. Chem., 2012, 9, 1413; (e) P. Hauwert,
J. J. Dunsford, D. S. Tromp, J. J. Weigand, M. Lutz,
K. J. Cavell and C. J. Elsevier, Organometallics, DOI:
10.1021/om300930w.
Notes and references
1
For reviews see: (a) F. E. Hahn and M. C. Jahnke, Angew.
Chem., Int. Ed., 2008, 47, 3122; (b) W. A. Herrmann, Angew.
Chem., Int. Ed., 2002, 41, 1290; (c) T. Dröge and F. Glorius,
Angew. Chem., Int. Ed., 2010, 49, 6940; (d) E. A. B. Kantchev,
C. J. O’Brien and M. G. Organ, Angew. Chem., Int. Ed., 2007, 10 (a) C. J. E. Davies, M. J. Page, C. E. Ellul, M. F. Mahon and
4
6, 2768.
(a) D. G. Gusev, Organometallics, 2009, 28, 6458;
b) R. Tonner and G. Frenking, Organometallics, 2009, 28,
901.
M. K. Whittlesey, Chem. Commun., 2010, 46, 5151;
(b) P. D. Newman, K. J. Cavell and B. M. Kariuki, Organo-
metallics, 2010, 29, 2724.
2
(
3
11 P. D. Newman, K. J. Cavell and B. M. Kariuki, Chem.
Commun., 2012, 48, 6511.
12 (a) T. W. Hudnall, A. G. Tennyson and C. W. Bielawski,
Organometallics, 2010, 29, 4569; (b) J. J. Dunsford,
K. J. Cavell and B. M. Kariuki, Organometallics, 2012, 31,
4118.
3
4
For a review see: S. Diez-Gonzalez, N. Marion and
S. P. Nolan, Chem. Rev., 2009, 109, 3612.
(a) M. Iglesias, D. J. Beetstra, J. C. Knight, L. Ooi, A. Stasch,
S. Coles, L. Male, M. B. Hursthouse, K. J. Cavell, A. Dervisi
and I. A. Fallis, Organometallics, 2008, 27, 3279;
(
b) W. Y. Lu, K. J. Cavell, J. S. Wixey and B. Kariuki, Organo- 13 (a) R. Armstrong, C. Ecott, E. Mas-Marzá, M. J. Page,
metallics, 2011, 30, 5649.
M. F. Mahon and M. K. Whittlesey, Organometallics, 2010,
29, 991; (b) V. Friese, S. Nag, J. Wang, M.-P. Santoni,
A. Rodrigue-Witchel, G. S. Hanan and F. Schaper,
Eur. J. Inorg. Chem., 2011, 1, 39.
5
(a) M. Iglesias, D. J. Beetstra, J. C. Knight, L. L. Ooi,
A. Stasch, S. Coles, L. Male, M. B. Hursthouse, K. J. Cavell,
A. Dervisi and I. A. Fallis, Organometallics, 2008, 27, 3279;
(
b) E. L. Kolychev, I. A. Portnyagin, V. V. Shuntikov, 14 M. Mayr, K. Wurst, K.-H. Ongania and M. R. Buchmeiser,
V. N. Khrustalev and M. S. Nechaev, J. Organomet. Chem.,
Chem.–Eur. J., 2004, 10, 1256.
2
009, 694, 2454; (c) W. A. Herrmann, S. K. Schneider, 15 (a) F. E. Hahn, C. Holtgrewe, T. Pape, M. Martin, E. Sola
K. Ofele, M. Sakamoto and E. Herdtweck, J. Organomet.
Chem., 2004, 689, 2441.
and L. A. Oro, Organometallics, 2005, 24, 2203;
(b) J. R. Miecznikowski and R. H. Crabtree, Organometallics,
2004, 23, 629; (c) R. Corberan and E. Peris, Organometallics,
2008, 27, 1954; (d) R. Corberan, M. Sanau and E. Peris,
Organometallics, 2007, 26, 3492; (e) A. Zanardi, E. Peris and
J. A. Mata, New J. Chem., 2008, 32, 120; (f) F. Hanasaka,
K.-i. Fujita and R. Yamaguchi, Organometallics, 2004, 23,
1490; (g) J. A. Brown, S. Irvine, A. R. Kennedy, W. J. Kerr,
S. Andersson and G. N. Nilsson, Chem. Commun., 2008,
1115.
6
(a) M. Iglesias, D. J. Beetstra, A. Stasch, P. N. Horton,
M. B. Hursthouse, S. J. Coles, K. J. Cavell, A. Dervisi and
I. A. Fallis, Organometallics, 2007, 26, 4800; (b) A. Binobaid,
M. Iglesias, D. J. Beetstra, B. Kariuki, A. Dervisi, I. A. Fallis
and K. J. Cavell, Dalton Trans., 2009, 7099;
(
c) P. D. Newman, K. J. Cavell, A. J. Hallett and
B. M. Kariuki, Dalton Trans., 2011, 40, 8807; (d) P. Bazinet,
G. P. A. Yap and D. S. Richeson, J. Am. Chem. Soc., 2003,
1
25, 13314; (e) C. C. Scarborough, I. A. Guzei and 16 M. Albrecht, J. R. Miecznikowski, A. Samuel, J. W. Faller
S. S. Stahl, Dalton Trans., 2009, 2284; (f) M. Iglesias, and R. H. Crabtree, Organometallics, 2002, 21, 3596.
D. J. Beetstra, B. Kariuki, K. J. Cavell, A. Dervisi and 17 For the original ‘click’ ring closure protocol see: R. Jazzar,
I. A. Fallis, Eur. J. Inorg. Chem., 2009, 1913; (g) D. Mercan,
E. Çetinkaya and B. Çetinkaya, J. Organomet. Chem., 2011,
H. Liang, B. Donnadieu and G. Bertrand, J. Organomet.
Chem., 2006, 691, 3201.
6
96, 1359; (h) I. Özdemir, S. Demir, B. Çetinkaya and 18 K. Hirano, S. Urban, C. Wang and F. Glorius, Org. Lett.,
E. Çetinkaya, J. Organomet. Chem., 2005, 690, 5849;
2009, 11, 1019.
(i) N. Imlinger, K. Wurst and M. R. Buchmeiser, J. Organo- 19 K. M. Kuhn and R. H. Grubbs, Org. Lett., 2008, 10, 2075.
met. Chem., 2005, 690, 4433; ( j) C. Segarra, E. Mas-Marzá, 20 E. M. Higgins, J. A. Sherwood, A. G. Lindsay, J. Armstrong,
J. P. Lowe, M. F. Mahon, R. C. Poulten and
M. K. Whittlesey, Organometallics, DOI: 10.1021/
om300984v.
R. S. Massey, R. W. Alder and A. C. O’Donoghue, Chem.
Commun., 2011, 47, 1559.
21 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
7
8
A. Binobaid, M. Iglesias, D. Beetstra, A. Dervisi, I. Fallis 22 (a) R. W. Alder, P. R. Allen, M. Murray and A. G. Orpen,
and K. J. Cavell, Eur. J. Inorg. Chem., 2010, 2010, 5426.
(a) J. J. Dunsford, K. J. Cavell and B. Kariuki, J. Organomet.
Chem., 2011, 696, 188; (b) P. D. Newman, K. J. Cavell and
B. M. Kariuki, Dalton Trans., 2012, 41, 12395.
Angew. Chem., Int. Ed., 1996, 35, 1121; (b) R. W. Alder,
M. E. Blake, S. Bufali, C. P. Butts, A. G. Orpen, J. Schutz
and S. J. Williams, J. Chem. Soc., Perkin Trans. 1, 2001,
1586.
9
(a) C. C. Scarborough, A. Bergant, G. T. Sazama, I. A. Guzei, 23 This feature has also been observed in a related silver(I)
L. C. Spencer and S. S. Stahl, Tetrahedron, 2009, 65, 5084;
b) C. C. Scarborough, M. J. W. Grady, I. A. Guzei,
complex in an earlier report by Stahl and co-workers (see
ref. 9a).
(
B. A. Gandhi, E. E. Bunel and S. S. Stahl, Angew. Chem., Int. 24 A. Poater, B. Cozenza, A. Correa, S. Giudice, F. Ragone,
Ed., 2005, 44, 5269; (c) J. J. Dunsford and K. J. Cavell,
V. Scarano and L. Cavello, Eur. J. Inorg. Chem., 2009, 1759.
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Parameters applied for SambVca calculations: 3.50 Å was 26 For a comprehensive review of %Vbur values of NHC
selected as the value for the sphere radius, 2.00 Å was used
as distances for the metal–ligand bond, hydrogen atoms
complexes see: H. Clavier and S. P. Nolan, Chem. Commun.,
2010, 46, 841.
were omitted, and bondi radii scaled by 1.17 were used. 27 (a) C. A. Tolman, Chem. Rev., 1977, 77, 313;
The above parameters applied are identical to those of all
literature examples discussed allowing a direct comparison
of calculated values.
(b) R. A. I. Kelly, H. Clavier, S. Giudice, N. M. Scott,
E. D. Stevens, J. Bordern, I. Samardjiev, C. D. Hoff,
L. Cavallo and S. P. Nolan, Organometallics, 2008, 27, 208.
2
5 J. Huang, H. J. Schanz, E. D. Stevens and S. P. Nolan, 28 D. D. Perrin and W. F. A. Amarego, Purification of Laboratory
Organometallics, 1999, 18, 2370. Chemicals, Oxford, Pergamon, 1988.
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