An N-Heterocyclic Carbene with a Saturated Backbone and Spatially-Defined Steric Impact

Article abstract of DOI:10.1002/zaac.201800423

The synthesis and coordination chemistry of a saturated analogue of a “bulky-yet-flexible” N-heterocyclic carbene (NHC) ligand are described. “SIPaul” is a 4,5-dihydroimidazol-2-ylidene ligand with unsymmetrical aryl N-substituents, and is one of the grow

Full text of DOI:10.1002/zaac.201800423

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800423
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
An N-Heterocyclic Carbene with a Saturated Backbone and
Spatially-Defined Steric Impact
Gillian Laidlaw,[a] Susanna H. Wood,[a] Alan R. Kennedy,[a] and David J. Nelson*[a]
Dedicated to Dr. Mark Spicer in recognition of his 27 years of service to the Department of Pure and Applied Chemistry
SIPaul.HCl and its complexes with copper, silver, iridium, palladium,
and nickel, and its selenourea, are reported. The steric impact of the
ligand is quantified using percent buried volume (%Vbur), whereas the
electronic properties are probed and quantified using the Tolman Elec-
tronic Parameter (TEP) and δSe of the corresponding selenourea. This
work shows that despite the often very different performance of satu-
rated versus unsaturated carbenes in catalysis, the effect of backbone
saturation on measurable properties is very small.
Abstract. The synthesis and coordination chemistry of a saturated ana-
logue of a “bulky-yet-flexible” N-heterocyclic carbene (NHC) ligand
are described. “SIPaul” is a 4,5-dihydroimidazol-2-ylidene ligand with
unsymmetrical aryl N-substituents, and is one of the growing class of
“bulky-yet-flexible” NHCs that are sufficiently bulky to stabilize cata-
lytic intermediates, but sufficiently flexible that they do not inhibit
productive chemistry at the central metal atom. Here, the synthesis of
Introduction
The synthesis, characterization, and application of new li-
gands is a crucial area of research that drives progress in appli-
cations that rely upon transition metal complexes. NHCs[1–3]
have a number of often very favorable electronic[4,5] and steric
properties;[6,7] they tend to be very electron-donating, and
present their steric bulk in an “umbrella” arrangement rather
than, for example, in the form of a cone with a vertex at the
metal.[8] Importantly, most common imidazol-2-ylidenes and
4,5-dihydroimidazol-2-ylidenes are readily-prepared from the
corresponding amine in a short series of high-yielding and
scalable synthetic steps.[9] This typically convenient and mod-
ular approach to NHC synthesis provides the opportunity to
finely tune ligand structure, and therefore ligand properties,
by designing and preparing the appropriate amine (Scheme 1a
shows a general route).
Scheme 1. (a) Modular synthesis of imidazolium and 4,5-hydroimid-
azolium precursors to NHC ligands. (b) Synthesis of IPaul·HCl (2). (c)
Synthesis of SIPaul·HCl (4, this work).
We recently disclosed the synthesis of the ligand precursor
IPaul·HCl (1) and the coordination chemistry of the correspond-
ing NHC ligand IPaul (2), which bears 2-(diphenylmethyl)-4,6-
dimethylphenyl N-substituents (Scheme 1b).[10] This ligand ligand – was very similar for IPaul and analogous carbenes
was coordinated to copper, silver, nickel, and iridium. Two dif- such as IPr* [IPr* = 1,3-bis(2,6-diphenylmethyl-4-meth-
ferent rotamers of the ligand were obtained: one in which the ylphenyl)imidazol-2-ylidene]. Steric maps and buried volume
diphenylmethyl groups were on the same side of the imidazol- (%Vbur) calculations showed that while the overall steric im-
2-ylidene core (syn), and one, in which they were on opposite pact of 2 was similar to IPr in most coordination environments,
sides (anti). Unsurprisingly, the Tolman Electronic Parameter the distribution of this steric bulk was focused on one side of
(TEP)[8] – a measure of the net electron-donating effect of a the carbene [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene]. The copper complex [CuCl(2)] was shown to be more
active than analogous complexes with smaller (e.g. IMes) or
larger (e.g. IPr*) NHCs for the hydrosilylation of a prototypi-
cal substrate [IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-
2-ylidene].
Despite their structural similarity, 4,5-dihydroimidazol-2-
ylidenes can have quite different properties to the correspond-
ing imidazol-2-ylidenes. The TEPs for the latter tend to be
* Dr. D. J. Nelson
E-Mail: david.nelson@strath.ac.uk
[a] WestCHEM Department of Pure and Applied Chemistry
University of Strathclyde
295 Cathedral Street
Glasgow, G1 1XL, UK
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201800423 or from the au-
thor.
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higher, which is suggestive of lower net electron density at the Paul (3) were carried out [B3LYP/6-31G(d) in the gas phase].
central metal atom;[11] however, this effect is most likely due These indicated that the anti conformer of 3 is favored by
to the increased back bonding from the electron-rich nickel(0) 2.6 kcalmol–1, and that the barrier for conversion between the
or iridium(I) atom to the non-aromatic dihydroimidazol-2-ylid- two isomers is 15.7 kcal·mol–1 (Figure 1a). These data can be
ene core, compared to the imidazol-2-ylidene core.[12–16] As compared to the analogous calculations for 2, which show that
well as changes in the electronic properties of the ligand, the the anti conformer is preferred by 2.8 kcal·mol–1, and that
saturated and flexible nature of the backbone can allow dif- there is a 16.8 kcal·mol–1 barrier to rotation (Figure 1b). These
ferent conformational and steric properties. These can be diffi- data are consistent with the observation of two distinct isomers
cult to gauge using static measures such as %Vbur but might of 1 and 4 in solution by NMR spectroscopy.
have considerable effects in catalysis.
Herein, we report complexes of the saturated analogue of
IPaul [referred to here as SIPaul (3)], and studies to probe its
electronic and steric properties. Scheme 1c outlines our overall
approach to SIPaul·HCl (4), which serves as the ligand precur-
sor for this work, and was coordinated to a range of metals
without the need to liberate and isolate the free carbene.[17]
Results and Discussion
The synthesis of the target ligand was achieved straightfor-
wardly from diimine 5 (Scheme 2).[10] The reduction of 5 with
aluminium hydride reagent 6 led to the diamine 7 in good
yield. Cyclization of this diamine was initially somewhat prob-
lematic, and attempts to achieve this thermally using methods
that are used to prepare SIPr were not successful, or provided
very low yields.[18] However, a microwave-mediated reaction
Figure 1. Computational study of rotamers of (a) SIPaul (3) and (b)
provided the target compound SIPaul·HCl (4) in 86% yield.
Notably, the synthesis of such bulky 4,5-dihydroimidazolium
salts is often very challenging, and alternative multi-step routes
IPaul (2).
A series of organometallic complexes were prepared using
can be required.[19] Compound 3 is one of the bulkier 4,5- established methodologies that preclude the need to first pre-
dihydroimidazol-2-ylidenes that has been prepared.
pare, isolate, and purify the corresponding free carbene. In
each case, 4 was exposed to a weak base in the presence of an
appropriate metal precursor (Scheme 3).[17] [AgCl(SIPaul)] (8)
was prepared using AgNO3,[20] [CuCl(SIPaul)] (9) using
CuCl,[21] and [IrCl(COD)(SIPaul)] (10) using [Ir(μ-Cl)
(COD)]2.[22] [NiCl(Cp)(SIPaul)] (11) was prepared by heating
4 in THF solution in the presence of NiCp2.[23] All new com-
plexes were characterized by methods including 1H and
13C{1H} NMR spectroscopy and X-ray crystallography.[24]
Most complexes were obtained as a mixture of rotamers in
solution, as judged by NMR analysis, with a syn or anti ar-
rangement of the diphenylmethyl groups, in varying ratios de-
pending on the ligand sphere of the complex.
The solid state molecular structures for these species are
presented in Figure 2. The syn arrangement was found to pre-
dominate in the solid state, as determined by X-ray diffraction
analyses of single crystals, despite this being the higher energy
conformation. Notably, similar trends were observed for IPaul
complexes.[10] This is likely to be as a result of favorable pack-
Scheme 2. Synthesis of SIPaul·HCl (4). The compound is obtained as
a 2:1 mixture of rotamers.
The resulting imidazolium salt (4) was fully characterized ing arrangements in the solid state. The crystal structures of 8
1
using H and 13C{1H} NMR spectroscopy, elemental analysis, and 9 were found to be mutually isomorphous and iso-
and high-resolution mass spectrometry. Analogously to structural. The structure of 11 contains a large amount of disor-
IPaul·HCl (1), 4 exists as a mixture of two rotamers: a syn dered CHCl3 solvent; this could not be modeled in a satisfac-
rotamer, where the two diphenylmethyl groups are on the same tory manner and so its contribution to the structure was re-
side of the NHC core, and an anti isomer, where the two di- moved using the SQUEEZE routine within PLATON.[25,26]
phenylmethyl groups are on opposite sides (Figure 1). DFT
The molecular structures were used to calculate the percent
calculations of the structure and behavior of IPaul (2) and SI- buried volume (%Vbur).[6,7] This metric quantifies the percen-
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Scheme 3. Synthesis of metal complexes of SIPaul (3). All complexes
are obtained as a mixture of rotamers (see text).
tage of a sphere of 3.5 Å around the central metal atom that
is occupied by the ligand. Complexes 8–11 feature different
coordination numbers and arrangements (linear two-coordi-
nate; square planar four-coordinate; piano-stool) and so pro-
vide a means to assess the behavior of 3 in different environ-
ments. The free web-based SambVca software was used to
[27,28]
calculate %Vbur
.
This prepares “steric maps”, which al-
low the steric impact of the ligand around the central metal
atoms to be visualized using a contour map.[7,27] Figure 3 dis-
plays the steric maps for complexes 8–11 and notes their
%Vbur (per-quadrant and overall). The buried volumes of these
complexes are similar to those for IPaul, especially given the
limits of accuracy in comparing %Vbur values[7] (Ag: 44.1 ver-
sus 43.3%; Cu: 46.9% versus 45.7%; Ir: 36.1% versus
36.5%; Ni: 38.4 versus 40.0%). The increased flexibility of
the backbone does not lead to a significant change in the steric
profile of the ligand, as far as can be determined by X-ray
crystallographic measurements in the solid state.
Further compounds were made in order to evaluate the steric
and electronic properties of SIPaul. Iridium complex 10 was
exposed to carbon monoxide in chloroform solution, forming
[IrCl(CO)2(SIPaul)] (12) (Scheme 4). IR analysis of a DCM
solution of 12, as a thin film on sodium chloride plates, gave
two signals for the carbonyl stretching vibrations (at 2065 and
1981 cm–1). Like the corresponding IPaul complex, only two
Figure 2. Molecular structures for 8, 9, 10, and 11, as determined by
X-ray crystallographic analysis. Most hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at 50% probability.
The selenourea derivative of 3 (compound 13) was prepared
signals were observed; the signals for each rotomeric form are using previously published methodology, by stirring 4, excess
therefore very close, and the limited resolution of IR spectrom- selenium powder, and potassium tert-butoxide in anhydrous
eters (typically 1–2 cm–1) precludes resolution of two signals THF overnight (Scheme 5).[15,29,30] We recently reported the
for each rotamer. The average signal for 12 is 2023 cm–1, corresponding derivative of IPaul (14).[30] As first reported by
which compares to 2026 cm–1 for the IPaul congener. This sug- Ganter,[14] δSe for the selenourea provides a measurement of
gests that SIPaul is slightly more electron donating than IPaul, the degree of back-bonding into the NHC.[15,16] For more elec-
but the difference is relatively small.
tron-rich NHC scaffolds, a zwitterionic resonance form can be
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Finally, initial catalytic tests were undertaken by applying 9
in the hydrosilylation of benzophenone (Scheme 6, Figure 4).
Our previous work showed that the corresponding IPaul deriv-
ative was highly effective,[10] outperforming complexes that
bear smaller NHCs (such as IPr and IMes), whereas complexes
of bulkier NHCs (such as IPr*) were very poor for this reac-
tion. A kinetic study of this reaction, using 9 as a catalyst,
showed that SIPaul is a substantially poorer ligand for this
reaction than IPaul. The link between hydrosilylation perform-
ance and backbone saturation remains poorly understood;
[CuX(SIMes)] complexes outperform the [CuX(IMes)] ana-
logues, yet [CuX(SIPr)] complexes are far less effective cata-
lysts for hydrosilylation than [CuX(IPr)] species.[32] While this
preliminary assessment of the potential for SIPaul as a ligand
for homogeneous catalysts is not particularly positive, the in-
terplay of ligand structure and catalytic reactivity is complex
and not well understood, and so we hope that it might find
applications in alternative reactions.
Figure 3. Steric maps and %Vbur for complexes 8, 9, 10, and 12. The
crystal structure of 10 contains two independent molecules, of which
only one is presented here; the other has %Vbur = 36.2 and a very
similar steric map.
Scheme 6. Hydrosilylation of benzophenone catalysed by complex 9.
Scheme 4. Synthesis of [IrCl(CO)2(IPaul)] (12). The complex is ob-
tained as a mixture of two rotamers.
envisaged.[31] This therefore gives valuable extra information
about the electronic properties of NHCs. 77Se NMR analysis
of 15 reveals two values for δSe of 151 and 140 ppm (versus
64 and 51 ppm for the IPaul derivative). This fits with other
trends observed in this chemical shift value: δSe for SIPaul-Se
lies between the values for SIPr-Se (δ = 190 ppm) and SIMes-
Se (δ = 110 ppm), in the same way that the values for IPaul-
Se are between those for IPr-Se (δ = 90 ppm) and IMes-Se (δ
= 27 ppm).
Figure 4. Kinetic study of the hydrosilylation of benzophenone cata-
lysed by complex 9; data for the reaction catalysed by the analogous
IPaul complex are taken from our previous study.[10]
Conclusions
This manuscript presents the synthesis of an NHC ligand
(“SIPaul”) in which a saturated backbone is combined with
unsymmetrical N-aryl substituents. Silver, copper, iridium,
nickel, and selenourea compounds of this ligand are prepared
and characterized, and used as models to examine the steric
and electronic properties of the new carbene. SIPaul is one of
the bulkier saturated carbenes that has been reported so far.
SIPaul presents a broadly similar steric profile to IPaul, as
judged from buried volume calculations. While a saturated
Scheme 5. Synthesis of 13, which is obtained as a mixture of two
rotamers.
Changes to the N-aryl substituent clearly influence the π- backbone might be expected to impart additional flexibility to
accepting properties of the resulting NHC, and so differences the ligand, the static picture from X-ray crystallography is very
in reactivity between the corresponding NHC complexes may similar to that of the more constrained IPaul ligand. However,
not be purely steric in nature.
it must be noted that the situation for coordinatively-unsatu-
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2.22 (s, 6 H, CH3), 2.19 (s, 6 H, CH3). 13C{1H} NMR (CDCl3,
rated intermediates in catalysis, in solution, might be rather
101 MHz): δC = 143.4 (Ar C), 143.2 (Ar C), 137.0 (Ar C), 131.8 (Ar
C), 131.7 (Ar C), 130.4 (Ar CH), 129.7 (Ar CH), 129.1 (Ar CH), 128.4
(Ar CH), 126.4 (Ar CH), 51.9 (CHPh2), 49.5 [N(CH2)2N], 21.1 (CH3),
18.7 (CH3). M. p. 143–145 °C. HR-MS. m/z calcd. for C44H45N2 [M
+ H]+ 601.3577; found 601.3566.
different. There is still not a practical and informative way to
reliably assess the dynamic steric influence of ligands and thus
understand the degree and role of flexibility. SIPaul appears to
be slightly more electron-donating, according to TEP measure-
ments; δSe indicates that SIPaul is a better π-acceptor than
IPaul, consistent with trends observed previously for carbene
ligands with saturated versus unsaturated backbones.
1,3-Bis(2-diphenylmethyl-4,6-dimethylphenyl)-4,5-dihydroimida-
zolium Chloride (4): A microwave vial equipped with a stir bar was
charged with 7 (2.04 g, 3.4 mmol, 1.0 equiv.), triethyl orthoformate
Current research in our lab is focussed on deploying IPaul
and SIPaul complexes in catalysis, and more fully understand- (700 μL, 624 mg, 4.2 mmol, 1.2 equiv.), HCl in 1,4-dioxane (1 mL of
a 4 mol·L–1 solution, 4 mmol, 1.2 equiv.), and 1,4-dioxane (15 mL).
The reaction was heated in the microwave at 145 °C for 1 h. Hexane
ing the effect of structural features on catalytic activity, and on
steric and electronic properties.
(6 mL) was added to the resulting thick suspension, and the product
was collected by filtration as a white powder. Yield 1.90 g, 2.9 mmol,
86%. The product was obtained as a 2:1 mixture of rotamers (see text).
Experimental Section
1
Rotamer A: H NMR (CDCl3, 400 MHz): δH = 9.27 (s, 1 H, N(CH)
N), 7.37–7.05 (m, 20 H, Ar CH), 7.00 (s, 2 H, Ar CH), 6.55 (s, 2 H,
Ar CH), 5.85 (s, 2 H, CHPh2), 4.03–3.88 [m, 2 H, N(CH2)2N], 3.61–
3.45 [m, 2 H, N(CH2)2N], 2.35 (s, 3 H, CH3), 2.20 (s, 3 H, CH3).
Computational Details: Calculations were carried out using the Ar-
chie-WeSt High Performance Computer and Gaussian 09 Rev.
D.01.[33] The B3LYP functional and 6-31G(d) basis set were used
throughout. All calculations were carried out in the gas phase without
a solvent model. Geometry optimizations were carried out without
symmetry constraints, and the nature of each stationary point was veri-
fied using frequency calculations.
1
Rotamer B: H NMR (CDCl3, 400 MHz): δH = 9.67 [s, 1 H, N(CH)
N], 7.37–7.05 (m, 20 H, Ar CH), 7.02 (s, 2 H, Ar CH), 6.58 (s, 2 H,
Ar CH), 5.63 (s, 2 H, CHPh2), 4.42–4.31 [m, 2 H, N(CH2)2N], 3.23–
3.13 [m, 2 H, N(CH2)2N], 2.47 (s, 3 H, CH3), 2.20 (s, 3 H, CH3). Both
rotamers: 13C{1H} NMR (CDCl3, 101 MHz): δC = 160.9 [N(CH)N,
rot. A], 160.8 [N(CH)N, rot. B], 142.8 (Ar C), 142.6 (Ar C), 142.3
(Ar C), 142.2 (Ar C), 141.4 (Ar C), 141.2 (Ar C), 140.7 (Ar C), 140.6
(Ar C), 135.9 (Ar C), 135.6 (Ar C), 131.3 (Ar CH), 131.0 (Ar CH),
130.4 (Ar C), 130.3 (Ar C), 130.1 (Ar CH), 130.0 (Ar CH), 129.8 (Ar
CH), 129.7 (Ar CH), 129.5 (Ar CH), 129.1 (Ar CH), 128.9 (Ar CH),
128.7 (Ar CH), 127.5 (Ar CH), 127.3 (Ar CH), 127.1 (Ar CH), 126.9
(Ar CH), 52.3 [N(CH2)2N, rot. B], 52.0 [N(CH2)2N, rot. A], 51.8
(CHPh2, rot. A), 51.7 (CHPh2, rot. B), 21.5 (CH3), 18.8 (CH3), 18.5
(CH3). C45H43ClN2: calcd. C 83.5, H 6.7, N 4.3%; found: C 82.9, H
6.6, N 4.3%. LR-MS (LCMS, MeCN/H2O, ESI + ACPI) m/z: 611.4,
612.4. HR-MS (LTQ, Orbitrap XL, DCM/MeOH) m/z: 611.3403.
C45H43N2 requires 611.3421.
General: Unless otherwise stated, all materials were obtained from
commercial sources and used as supplied. Compound 5 and IPaul·HCl
(1) were prepared according to literature procedures.[10] Anhydrous
solvents were obtained from an Innovative Technologies PureSolv sol-
vent drying system. NMR analyses were conducted with a Bruker
1
AV3–400 spectrometer equipped with a liquid nitrogen cryoprobe. H
NMR spectra are referenced to residual solvent signals, and 13C{1H}
NMR spectra – which are typically from J-modulated spin echo
(JMOD) experiments to differentiate quaternary, CH, CH2, and CH3
signals – were referenced to solvent signals.[34] 2D NMR experiments
1
such as [1H, H] COSY, [1H, 13C] HSQC, and [1H, 13C] HMBC were
used to assign signals, as appropriate. GC-MS analyses were carried
out with an Agilent 7890A instrument coupled to an Agilent 5975C
mass spectrometer with electron impact ionization. IR spectra were
acquired with a Shimadzu IRAffinity-1 spectrometer. LC-MS analyses
were carried out with an Agilent 1200 HPLC coupled to an Agilent
6130 mass spectrometer operating in ACPI/ESI mode. High resolution
mass spectrometry data were acquired with an LTQ Orbitrap XL in-
strument at the University of Swansea. Elemental analyses were con-
ducted either at Strathclyde, with a Perkin–Elmer 2400 Series II instru-
ment, or at London Metropolitan University.
1,3-Bis(2-diphenylmethyl-4,6-dimethylphenyl)-4,5-dihydroimida-
zol-2-yl)silver(I) Chloride (8): A microwave vial was equipped with
a stirrer bar and charged with 4 (80.7 mg, 0.125 mmol, 1.0 equiv.),
AgNO3 (28.6 mg, 0.168 mmol, 1.3 equiv.), K2CO3 (106.2 mg,
0.768 mmol, 6.1 equiv.), and DCM (2 mL). The reaction was placed
in a nitrogen atmosphere, wrapped in aluminum foil and stirred for 36
h. The crude reaction mixture was filtered through silica, the pad was
washed with DCM, and the resulting solution was concentrated under
reduced pressure. The addition of pentane led to the precipitation of
the product as a white solid. Yield 69.0 mg, 0.092 mmol, 73%. The
N,NЈ-bis(2-Diphenylmethyl-4,6-dimethylphenyl)-ethane-1,2-di-
amine (7): A round-bottom flask was charged with N,NЈ-bis(2-di- product was obtained as a 5:4 mixture of rotamers (see text). 1H NMR
phenylmethyl-4,6-dimethylphenyl)-ethane-1,2-diimine (5) (250 mg,
0.45 mmmol, 1.0 equiv.) and anhydrous toluene (25 mL). The flask
was purged with nitrogen for 20 min, and sodium bis(methoxyethoxy)
aluminium dihydride (Red-Al ) (6) (5 mL, 1.35 mmol, 3.0 equiv.) was
added slowly, resulting in the formation of gas. The reaction was
(CDCl3, 400 MHz): δH = 7.40–6.97 (m, 22 H, Ar CH), 6.68 (d, J =
1.4 Hz, 2 H, Ar CH, rot. B), 6.61 (d, J = 1.3 Hz, 2 H, Ar CH, rot. A),
5.88 (s, 2 H, CHPh2, rot. A), 5.76 (s, 2 H, CHPh2, rot. B), 3.70–3.61
[m, 2 H, N(CH2)2N, rot. B], 3.49–3.32 [m, 2 H, N(CH2)2N, rot. A],
3.23–3.13 [m, 2 H, N(CH2)2N, rot. B], 2.99–2.83 [m, 2 H, N(CH2)2N,
stirred in a nitrogen atmosphere for 2 h. NaOH (5% w/w) aqueous rot. A], 2.31 (s, 6 H, CH3), 2.29 (s, 6 H, CH3), 2.26 (s, 6 H, CH3).
solution was added, dropwise at first, to quench the reaction, and the 13C{1H} NMR (CDCl3, 101 MHz): δC = 210.1 (d, JC–Ag = 17.5 Hz,
reaction was stirred for 20 min. The organic layer was washed with
water (3ϫ70 mL), with the final wash having a neutral pH. The or-
ganic layer was washed with brine (2ϫ40 mL), dried on MgSO4, and
concentrated to yield a yellow powder. Yield 160 mg, 0.26 mmol,
68%. 1H NMR (CDCl3, 400 MHz): δH = 7.32–7.17 (m, 12 H, Ar CH),
C-Ag), 207.7 (d, JC–Ag = 16.1 Hz, C-Ag), 143.5 (Ar C), 143.3 (Ar C),
142.68 (Ar C), 142.65 (Ar C), 142.0 (Ar C), 141.8 (Ar C), 139.1 (Ar
C), 139.0 (Ar C), 135.8 (Ar C), 135.2 (Ar C), 130.9 (Ar CH), 130.0
(Ar CH), 129.7 (Ar CH), 129.6 (Ar CH), 128.92 (Ar CH), 128.86
(Ar CH), 128.79 (Ar CH), 128.76 (Ar CH), 127.01 (Ar CH), 126.96
7.07 (app. d, 8 H, Ar CH), 6.91 (s, 2 H, Ar CH), 6.51 (s, 2 H, Ar CH), (Ar CH), 126.8 (Ar CH), 52.0 (CHPh2), 51.65 [N(CH2)2N], 51.57
5.76 (s, 2 H, CHPh2), 3.03 (br. s, 2 H, NH), 2.75 (s, 4 H, N(CH2)2N), [N(CH2)2N], 51.5 (CHPh2), 51.2 [N(CH2)2N], 51.1 [N(CH2)2N], 21.5
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(CH3), 18.45 (CH3), 18.37 (CH3). C45H43ClAgN2: calcd. C 71.6, H 126.3 (Ar CH), 126.1 (Ar CH), 125.9 (Ar CH), 125.7 (Ar CH), 85.3
5.7, N 3.7%; found: C 71.1, H 5.6, N 3.7%. (COD CH), 85.1 (COD CH), 83.9 (COD CH), 54.3 (COD CH), 51.8
[N(CH2)2N], 51.43 [N(CH2)2N], 51.42 [N(CH2)2N], 51.6 (CHPh2),
(1,3-Bis(2-diphenylmethyl-4,6-dimethylphenyl)-4,5-dihydroimida- 51.5 (COD CH), 51.3 (COD CH), 50.6 (CHPh2), 50.1 (CHPh2), 34.4
zol-2-yl)copper(I) Chloride (9): A microwave vial was equipped with (COD CH2), 33.5 (COD CH2), 32.8 (COD CH2), 29.6 (COD CH2),
a stirrer bar and charged with 4 (73.1 mg, 0.113 mmol, 1.0 equiv.), 28.7 (COD CH2), 27.5 (COD CH2), 21.49 (CH3), 21.47 (CH3), 21.45
CuCl (50.6 mg, 0.511 mmol, 4.5 equiv.), K2CO3 (114.3 mg, (CH3), 21.0 (CH3), 19.4 (CH3), 19.0 (CH3). C53H54ClIrN2: calcd. C
0.827 mmol, 7.3 equiv.) and acetone (3 mL). The reaction was placed 67.2, H 5.8, N 3.0%; found: C 65.3, H 6.2, N 2.7%. Despite repeated
in a nitrogen atmosphere and heated to 60 °C for 20 h. The crude attempts, satisfactory results could not be obtained for this compound,
reaction mixture was filtered through a pad of silica, which washed
with DCM. The solvent was concentrated under reduced pressure and
perhaps as a result of incomplete combustion.
the product was precipitated by addition of pentane to yield a white (1,3-Bis(2-diphenylmethyl-4,6-dimethylphenyl)-4,5-dihydroimida-
solid. Yield 73.9 mg, 0.104 mmol, 92%. The product was obtained as zol-2-yl)(η5-cyclopentadienyl)nickel(II) Chloride (11): A vial was
1
a 4:3 mixture of rotamers (see text). H NMR (CDCl3, 400 MHz): δH equipped with a stirrer bar and charged with 4 (131.6 mg, 0.204 mmol,
= 7.38–7.09 (m, 20 H, Ar CH), 7.03 (s, 2 H, Ar CH), 6.69 (d, J =
1.0 equiv.) and [NiCp2] (71.3 mg, 0.377 mmol, 1.9 equiv.). The vial
1.1 Hz, 2 H, Ar CH, rot. A), 6.61 (d, J = 1.3 Hz, 2 H, Ar CH, rot. B), was evacuated, backfilled with argon, and charged with anhydrous
5.95 (s, 2 H, CHPh2, rot. B), 5.87 (s, 2 H, CHPh2, rot. A), 3.61–3.52 THF (2 mL). The reaction was stirred at 80 °C for 24 h, during which
[m, 2 H, N(CH2)2N, rot. A], 3.38–3.24 [m, 2 H, N(CH2)2N, rot. B], time the reaction turned from green to purple/red. The solvents were
3.08–2.99 [m, 2 H, N(CH2)2N, rot. A], 2.83–2.69 [m, 2 H, N(CH2)2N, removed in vacuo and the residue was taken up in DCM, and purified
rot. B], 2.34 (s, 3 H, CH3), 2.33 (s, 3 H, CH3), 2.26 (s, 3 H, CH3). by chromatography on silica gel using DCM; the initial green and
13C{1H} NMR (CDCl3, 101 MHz): δC = 203.9 (C-Cu, rot. A), 203.8 yellow fractions were discarded, and the red/pink fraction was col-
(C-Cu, rot. B), 143.5 (Ar C), 143.3 (Ar C), 143.0 (Ar C), 142.9 (Ar
C), 142.1 (Ar C), 141.9 (Ar C), 138.9 (Ar C), 138.8 (Ar C), 135.8 (Ar
lected. The DCM solution was concentrated under reduced pressure,
and pentane was added to precipitate the product as a pink solid. Yield
C), 135.11 (Ar C), 135.07 (Ar C), 130.78 (Ar CH), 130.75 (Ar CH), 90.7 mg, 0.118 mmol, 58%. The product is obtained as a ca. 5:4 mix-
129.9 (Ar CH), 129.81 (Ar CH), 129.77 (Ar CH), 129.68 (Ar CH),
129.67 (Ar CH), 128.75 (Ar CH), 128.73 (Ar CH), 127.0 (Ar CH),
126.9 (Ar CH), 126.7 (Ar CH), 52.0 (CHPh2, rot. B), 51.6 (CHPh2,
ture of rotamers, which are difficult to distinguish by NMR spec-
troscopy (see text); data here are reported for both rotamers. 1H NMR
(CDCl3, 400 MHz): δH = 7.54–7.01 (m, 20 H, Ar CH), 7.14 (s, 2 H,
rot. A), 51.4 [N(CH2)2N], 50.9 [N(CH2)2N], 21.5 (CH3), 18.51 (CH3), CHPh2), 6.77 (s, 4 H, Ar CH), 4.77 (s, 5 H, Cp CH, rot. A), 4.64 (s,
18.47 (CH3). C45H43ClCuN2: calcd. C 76.0, H 6.1, N 3.9%; found: C
75.4, H 6.0, N 3.9%.
5 H, Cp CH, rot. B), 3.02–2.93 [m, 2 H, N(CH2)2N, rot. B], 2.88–2.78
[m, 2 H, N(CH2)2N, rot. A], 2.32 (s, 3 H, CH3), 2.31 (s, 3 H, CH3),
2.30 (s, 3 H, CH3), 2.20–2.12 [m, 2 H, N(CH2)2N, rot. A], 2.08 (s, 3 H,
(1,3-Bis(2-diphenylmethyl-4,6-dimethylphenyl)-4,5-dihydroimida- CH3), 2.06–1.99 [m, 2 H, N(CH2)2N, rot. B]. 13C{1H} NMR (CDCl3,
zol-2-yl)(η2,η2-1,5-cyclooctadienyl)iridium(I) Chloride (10): A vial
equipped with a stirrer bar and charged with 4 (98.2 mg, 0.152 mmol,
1.0 equiv.), [Ir(μ-Cl)(cod)]2 (52.5 mg, 0.078 mmol, 0.5 equiv.), K2CO3
101 MHz): δC = 203.2 (C-Ni), 199.7 (C-Ni), 145.3 (Ar C), 145.0 (Ar
C), 144.4 (Ar C), 144.3 (Ar C), 143.6 (Ar C), 143.5 (Ar C), 138.0 (Ar
C), 137.9 (Ar C), 137.6 (Ar C), 137.0 (Ar C), 135.6 (Ar C), 130.7 (Ar
(237.7 mg, 1.72 mmol, 11 equiv.) and acetone (3 mL) was placed in a CH), 130.6 (Ar CH), 130.3 (Ar CH), 130.1 (Ar CH), 129.9 (Ar CH),
nitrogen atmosphere. The reaction was stirred at 60 °C for 24 h. The
solvent was removed under reduced pressure, and the residue was
taken up in the minimum amount of DCM and filtered through silica.
The pad was washed with DCM until the filtrate was clear. The DCM
129.5 (Ar CH), 128.52 (Ar CH), 128.46 (Ar CH), 128.3 (Ar CH),
128.0 (Ar CH), 126.7 (Ar CH), 126.4 (Ar CH), 126.2 (Ar CH),
126.0 (Ar CH), 93.3 (Cp CH), 92.7 (Cp CH), 51.6 (CHPh2), 51.2
[N(CH2)2N], 50.6 (CHPh2), 50.5 [N(CH2)2N], 21.6 (CH3), 19.5 (CH3),
solution was concentrated under reduced pressure, and pentane was 18.5 (CH3). C50H48ClNiN2: calcd. C 78.0, H 6.2, N 3.6%; found: C
added to precipitate the product as a yellow solid. Yield 96.3 mg,
0.102 mmol, 67%. The product is obtained as a 5:3 mixture of rota-
mers (see text); the presence of these rotamers, and the overlap be-
77.7, H 6.3, N 3.5%.
(1,3-Bis(2-diphenylmethyl-4,6-dimethylphenyl)-4,5-imidazol-2-yl)
tween COD, methyl, and backbone protons makes the proton NMR dicarbonyliridium(I) Chloride (12): Compound 10 (39.1 mg,
spectrum rather complex and it is difficult to assign signals to specific
rotamers. 1H NMR (CDCl3, 400 MHz): δH = 4.47 (s, 1 H, CHPh2),
0.041 mmol) was dissolved in chloroform (2 mL) and carbon mon-
oxide was bubbled through the solution for 5 min. The color of the
7.46–7.37 (m, Ar CH), 7.23 (s, 2 H, CHPh2), 7.10–6.99 (m, Ar CH), solution changed from dark yellow to pale yellow. The vial was then
6.67 (s, 2 H, Ar CH), 6.62 (d, J = 1.8 Hz, Ar CH), 6.42 (s, 1 H,
CHPh2), 4.74–4.64 (m, 2 H, COD CH, rot. B), 4.55–4.45 (m, 2 H,
COD CH, rot. A), 3.48–3.39 (m, 2 H, COD CH, rot B.), 3.18–3.10
(m, 2 H, COD CH, rot A.), 3.07–3.01 [m, 2 H, N(CH2)2N], 2.97–2.79
[m, 4 H, N(CH2)2N], 2.52 (CH3), 2.28 (CH3), 2.27 (CH3), 2.25–2.08
[m, 2 H, N(CH2)2N], 2.21–1.14 (m, COD CH2). 13C{1H} NMR
(CDCl3, 101 MHz): δC = 210.4 (C-Ir, rot. B), 204.8 (C-Ir, rot. A),
145.31 (Ar C), 145.26 (Ar C), 144.7 (Ar C), 144.6 (Ar C), 144.38 (Ar
C), 144.34 (Ar C), 144.2 (Ar C), 143.6 (Ar C), 142.3 (Ar C), 138.4
(Ar C), 137.59 (Ar C), 137.56 (Ar C), 137.44 (Ar C), 137.35 (Ar C),
137.2 (Ar C), 137.1 (Ar C), 135.6 (Ar C), 135.5 (Ar C), 131.0 (Ar
CH), 130.6 (Ar CH), 130.5 (Ar CH), 130.3 (Ar CH), 130.14 (Ar CH),
sealed and kept under a carbon monoxide atmosphere using a balloon
filled with CO. The reaction was stirred for 3 h at room temperature.
Diethyl ether was added to precipitate the product, which was washed
with pentane to yield a pale yellow solid. Yield 20.1 mg, 0.022 mmol,
55%. The product is obtained as a ca. 1:1 mixture of rotamers (see
text). Rotamer A: 1H NMR (CDCl3, 400 MHz): δH = 7.65 (d, 4 H,
ArH), 7.25–6.95 (m, 20 H, ArH), 6.81 (s, 2 H, CHPh2) 2.63–2.57 (m,
2 H, m, CH2), 2.25 (s, 6 H, CH3) 2.11 (s, 2 H, CH2), 1.86 (s, 6 H,
CH3). Rotamer B: 1H NMR (CDCl3, 400 MHz): δH = 7.71 (d, 4 H,
ArH), 7.25–6.95 (m, 20 H, ArH), 6.81 (s, 2 H, CHPh2). 2.70 (s, 2 H,
CH2) 2.41 (s, 6 H, CH3), 2.28 (s, 2 H, CH2), 1.89 (s, 6 H, CH3) ppm.
IR (DCM solution): ν˜ = 2064.7, 1981.2 cm–1. Poor solubility in
130.08 (Ar CH), 130.0 (Ar CH), 129.9 (Ar CH), 129.7 (Ar CH), 128.8 CDCl3, C6D6, and [D8]THF precluded 13C{1H} NMR analysis.
(Ar CH), 128.7 (Ar CH), 128.4 (Ar CH), 128.2 (Ar CH), 127.9 (Ar
CH), 127.8 (Ar CH), 127.7 (Ar CH), 126.7 (Ar CH), 126.4 (Ar CH),
C47H42ClIrN2O2: calcd. C 63.1, H 4.7, N 3.1%; found: C 63.0, H 4.6,
N 2.7%.
Z. Anorg. Allg. Chem. 0000, 0–0
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6
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Journal of Inorganic and General Chemistry
ARTICLE
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1,3-Bis(2-diphenylmethyl-4,6-dimethylphenyl)-4,5-dihydroimida- funded Archie-WeSt High Performance Computer (www.archie-
zole-2-selenone (15): Compound 4 (129.8 mg, 0.201 mmol), KOtBu west.ac.uk; EPSRC grant no. EP/K000586/1). We thank Mr Stephen
(78.5 mg, 0.700 mmol, 3.5 equiv.), and selenium (102.4 mg, Boyer at the London Metropolitan University for conducting some of
1.297 mmol, 6.5 equiv.) were added to a vial fitted with a septum cap. the elemental analyses reported in this manuscript.
The vial was evacuated, backfilled with argon, and charged with anhy-
drous THF (2 mL). The reaction was stirred for 24 h at room tempera-
ture. The solvent was removed in vacuo, and the residue was taken up
Keywords: N-heterocyclic carbenes; Coordination chemis-
try; Organometallic chemistry ((р=CE / QC: no from list -
in DCM and filtered through celite. The DCM solvent was removed
insertOchangeOreplace KWЈs?))
in vacuo and the residue was washed with hexane to yield the product
as a white powder. Yield 63.5 mg, 0.092 mmol, 46%. The product was
obtained as a ca. 3:2 mixture of rotamers (see text); some key signals
for each rotamer could be identified. 1H NMR (CDCl3, 400 MHz): δH
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We thank the University of Strathclyde for funding (consumables and
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for a studentship via the GSK/Strathclyde Centre for Doctoral Training
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Journal of Inorganic and General Chemistry
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G. Laidlaw, S. H. Wood, A. R. Kennedy,
D. J. Nelson* ..................................................................... 1–9
An N-Heterocyclic Carbene with a Saturated Backbone and
Spatially-Defined Steric Impact
Z. Anorg. Allg. Chem. 0000, 0–0
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