Probing the limits of ligand steric bulk: Backbone C-H activation in a saturated N-heterocyclic carbene

Article abstract of DOI:10.1002/chem.201304243

The consequences of extremely high steric loading have been probed for late transition metal complexes featuring the expanded ring N-heterocyclic carbene 6-Dipp. The reluctance of this ligand to form 2:1 complexes with d-block metals (rationalised on the

Full text of DOI:10.1002/chem.201304243

DOI: 10.1002/chem.201304243
Full Paper
&
CÀH Activation
Probing the Limits of Ligand Steric Bulk: Backbone CÀH
Activation in a Saturated N-Heterocyclic Carbene
Nicholas Phillips, Remi Tirfoin, and Simon Aldridge*[a]
Abstract: The consequences of extremely high steric loading
have been probed for late transition metal complexes featur-
ing the expanded ring N-heterocyclic carbene 6-Dipp. The
reluctance of this ligand to form 2:1 complexes with d-block
metals (rationalised on the basis of its percentage buried
volume, %Vbur, of 50.8%) leads to CÀH and CÀN bond activa-
tion processes driven by attack at the backbone b-CH2 unit.
In the presence of IrI (or indeed H+) the net result is the for-
mation of an allyl formamidine fragment, while AuI brings
about an additional ring (re-)closure step via nucleophilic
attack at the coordinated alkene. The net transformation of
6-Dipp in the presence of [(6-Dipp)Au]+ represents to our
knowledge the first example of backbone CÀH activation of
a saturated N-heterocyclic carbene, proceeding in this case
via a mechanism which involves free carbene in addition to
the AuI centre.
Introduction
The use of extremely sterically demanding ancillary ligands has
proved to be a highly successful strategy for the isolation of
metal complexes displaying coordinative and/or electronic un-
saturation.[1] This approach has yielded landmark compounds
from across the Periodic Table (both in terms of unusual struc-
ture and reactivity),[2,3] and exploits the kinetic and thermody-
namic disincentives to increasing the metal coordination
number in the presence of significant steric crowding.
Among late transition metal systems, N-heterocyclic carbene
(NHC) ligands have in recent years been shown to be viable al-
ternatives to more traditional phosphine donors in the stabili-
sation of complexes of wide-ranging catalytic relevance.[4] The
strong s-donor characteristics of NHCs based on an unsaturat-
ed imidazol-2-ylidene core,[5] allied to the possibility for the in-
corporation of sterically demanding N-substituents means that
ligands such as ItBu, IMes and IDipp, are key components in
a number of low-coordinate systems implicated, for example,
in CÀH bond activation.[6]
7-membered heterocycles appears to be mitigated somewhat
by folding of the non-planar backbone. Thus, the percentage
[8]
buried volumes (%Vbur
)
calculated for 5-Mes (also known as
sIMes), 6-Mes and 7-Mes based on their respective silver(I)
halide complexes are 36.1,[8f] 44.0,[9] and 44.2%, respective-
ly.[7a,10] Such trends are also reflected in reactivity patterns.
Thus, among formally 14-electron complexes of the type
[L2IrH2]+ those with L=6-Mes or 7-Mes are uniquely stable to
air and moisture,[6k] as a consequence of the effective shroud-
ing of the metal centre by the mesityl substituents. An addi-
tional (but perhaps less desirable) consequence of greater
bending of the N-substituents towards the metal centre—as
manifested by the reactions of 5-Mes, 6-Mes and 7-Mes with
the IrI complex [Ir(COE)(m-Cl)]2—appears to be a greater ten-
dency towards activation of peripheral CÀH bonds for the
larger ring sizes (Scheme 1). Thus, under otherwise analogous
conditions, doubly activated products are observed for 6-Mes
and 7-Mes whereas only one CÀH bond is cleaved in
5-Mes.[6k,11] In the current study attempts are reported to ex-
plore the chemistry of the related 6-Dipp ligand;[7a,b] in the
event, the further augmented steric profile of this ligand
More recently, the use of so-called “expanded ring” NHCs,
featuring saturated 6- and 7-membered heterocyclic cores has
been pioneered, with a number of studies espousing the
stronger s-donor and more sterically demanding nature of
these ligands over their imidazolylidene counterparts.[7] From
the viewpoint of steric effects, the wider NCN angle is respon-
sible for the greater demands exerted by the N-substituents in
6-membered NHCs compared to 5-membered counterparts;
the effect of the additional methylene spacer inherent in
[a] N. Phillips, R. Tirfoin, Prof. S. Aldridge
Inorganic Chemistry Laboratory, Department of Chemistry
University of Oxford, South Parks Road, Oxford, OX1 3QR (UK)
E-mail: simon.aldridge@chem.ox.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304243.
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Scheme 1. Coordination and CÀH bond activation at iridium in 5-, 6- and 7-
membered saturated N-heterocyclic carbenes featuring pendant mesityl sub-
stituents.
(%Vbur =50.8%, based on LAuCl),[7e] leads to unprecedented
backbone CÀH and CÀN activation processes resulting from its
inability to form “normal” 2:1 complexes with d-block
metals.[12]
Figure 1. Molecular structures of 2 (upper) and 3 (lower) with H atoms
(except alkene bound Hs) omitted and Dipp iPr groups shown in wireframe
format for clarity; thermal ellipsoids set at the 50% probability level. Key
bond lengths [ꢁ] and angles [o]: (for 2) Ir(1)-Cl(2) 2.344(1), Ir(1)-N(3) 2.103(4),
Ir(1)-{C(2)-C(37)}centroid 1.981, Ir(1)-{C(29)-C(30)}centroid 2.039; (for 3) Ir(1)-
Cl(2) 2.3451(8), Ir(1)-N(3) 2.098(3), Ir(1)-{C(7)-C(8)}centroid 1.977, Ir(1)-{C(33)-
C(34)}centroid 2.005.
Results and Discussion
The reaction of [Ir(COE)2(m-Cl)]2 (1) with four equivalents of
6-Dipp was probed with a view to offering direct comparison
with the chemistry of the related 6-Mes system. In the case of
6-Mes, two NHC ligands are incorporated into the product,
each of which is bound both via the carbenic carbon, and
through a benzyl function derived from CÀH activation of an
ortho-methyl group (Scheme 1).[6k] With 6-Dipp, however, an
entirely different mode of NHC activation is observed, involv-
ing cleavage of both CÀH and CÀN bonds within the heterocy-
cle backbone (Scheme 2). The metal-containing product of this
reaction (3) features a pair of allyl-functionalised formamidine
ligands, one of which is bound to the resulting IrI centre via an
N-alkene chelating motif, with the other ligated solely via the
alkene donor (and thus featuring a pendant formamidine
moiety). While the low symmetry implied by the large number
structure of 3 features a planar four-coordinate geometry at
Ir(1), with the two cis orientated alkene donors aligned approx-
imately perpendicular to the coordination plane. The remain-
ing metal-bound ligands comprise the original chloride, and
a k1-formamidine donor, for which the metalÀligand bond
lengths (2.345(1) and 2.098(3) ꢁ, respectively) conform to litera-
ture precedent.[13]
In order to gain insight into potential pathways leading to
the formation of the ring-opened ligands found in 3, we exam-
ined the reactivity of 1 towards a single equivalent of 6-Dipp.
1
Monitoring of the reaction mixture in situ by H NMR spectros-
copy revealed the formation at short reaction times of a metal
complex featuring both cyclooctene and intact 6-Dipp ligands.
However, this species could not be isolated in pure form even
at low temperatures, being rapidly transformed into a second
1
of ligand signals for 3 in both H and 13C NMR spectra (e.g.,
eight Dipp CH signals in the 13C NMR spectrum), and the pres-
ence of signals due to coordinated alkene units, are consistent
with the proposed structure, definitive evidence of connectivi-
ty is reliant on X-ray crystallography (Figure 1). The solid state
1
species, for which H NMR data reveal: 1) retention of the cy-
clooctene ligand, but 2) rearrangement of the 6-Dipp ligand to
give a fragment of lower symmetry featuring a coordinated
alkene moiety. The identity of this complex (2; Scheme 2) has
been established by NMR spectroscopy, microanalysis and
X-ray crystallography. The latter (Figure 1) reveals a structure
closely related to that of 3, in which the 6-Dipp ligand has
been cleaved to give a chelating alkene/amidine donor. Consis-
tent with the differing reaction stoichiometries, 2 differs from
3 in that the second alkene ligand is a cyclooctene donor re-
tained from the starting material, rather than a further equiva-
lent of ring-opened 6-Dipp.
Scheme 2. CÀN and CÀH activation of the 6-Dipp ligand in the coordination
sphere of iridium(I). Key reagents and conditions: a) 6-Dipp (2 equiv per mol
of iridium dimer), THF, RT, 1 h, 72%; b) (from [Ir(COE)2(m-Cl)]2 6-Dipp (4 equiv
per mol of iridium dimer), THF, RT, 30 min, 66%.
We hypothesised that a more logical synthesis of 2 would
exploit the reaction of the preformed allylformamidine 4 with
1 (Scheme 3). In the event, 4 is readily synthesised from the
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C-protonated species [(6-Dipp)H]+ and the strongly basic free
carbene 6-Dipp allows for deprotonation at the b-CH2 unit of
the heterocyclic backbone and net elimination, via loss of
a stable imine leaving group. This process results in overall re-
arrangement of 6-Dipp to the acyclic allyl derivative 4, while
regenerating a further molecule of [(6-Dipp)H]+ to perpetuate
the catalytic cycle. A related (albeit 1,4) Hofmann-type elimina-
tion mechanism has been proposed by Nechaev and co-work-
ers to account for stoichiometric ring opening in unsaturated
7-membered heterocycles by an oxide base.[7b]
Scheme 3. Synthesis of N,N’-(diisopropylphenyl)-N-allylformamidine, 4 via
either acid-catalysed ring opening of 6-Dipp or from N,N’-(2,6-diisopropyl-
phenyl)formamidine and allyl bromide; subsequent use of 4 in the synthesis
of 2. Conditions: a) HCl in Et2O (0.1 equiv), Et2O, RT, 10 min, 92%; b) allyl bro-
mide (1.0 equiv), potassium carbonate (0.5 equiv), acetonitrile, 508C, 2 h,
41%; c) [Ir(COE)2(m-Cl)]2 (0.5 equiv), [D6]benzene, RT, 10 min, quantitative by
NMR spectroscopy.
A similar series of steps can be envisaged leading to the for-
mation of the same allylformamidine fragment in both 2 and 3
(Scheme 4b). In contrast to the less bulky NHC 6-Mes, which is
capable of forming iridium bis(NHC) complexes,[14] the greater
bulk of 6-Dipp leads to an increased likelihood of the existence
in solution of both iridium-bound and free 6-Dipp species (to
our knowledge there exist no examples in the literature of
metal complexes featuring two 6-Dipp ligands bound to the
same metal centre).[15] This scenario opens up a similar reaction
pathway to that proposed in the presence of H+, in that coor-
dination of one molecule of 6-Dipp to an iridium Lewis acid fa-
cilitates backbone deprotonation/elimination by a second
6-Dipp molecule. In the case of an electron-rich iridum(I)
centre, the resulting alkene represents a good potential ligand,
and rearrangement via protonation at the carbene carbon
allows for the formation of the iridium–alkene moiety common
to both 2 and 3. In the case of 2, fragmentation of the chlo-
ride-bridged diiridium unit can then be envisaged by subse-
quent coordination of the imine function of the pendant ami-
dine. The crucial role of ligand steric bulk in bringing about
CÀH and CÀN bond rupture in this way is clearly evident, since
in our hands at least, no hint of similar processes was seen in
the analogous reactions of 6-Mes and 7-Mes with 1.[6k]
parent N,N’-diarylformamidine and allyl bromide (see the Sup-
porting Information), and subsequent reaction with (dimeric)
1 in a 2:1 stoichiometry does indeed provide an alternative
route to 2.
From a mechanistic standpoint, we were intrigued to ob-
serve that the allyl functionalised system 4 can also be formed
in high yield by the simple addition of catalytic strong acid to
a solution of 6-Dipp itself (Scheme 3). Thus, the addition of
10 mol% of HCl in Et2O to a solution of 6-Dipp in diethyl ether
leads to quantitative ring opening to give 4. We propose that
this acid-catalysed rearrangement proceeds as outlined in
Scheme 4a, in which the presence in solution of both the
Given the role of the proton in the proposed mechanism for
the acid-catalysed conversion of 6-Dipp to its ring-opened allyl
formamidine isomer, we wondered whether the isolobal
[LAu]+ fragment might offer a demonstration of the wider
scope of this reactivity.[16] Thus, we investigated the reactivity
of 6-Dipp towards the extremely strong Lewis acid
[(6-Dipp)Au]+, generated in situ by halide abstraction from
[(6-Dipp)AuCl] in fluorobenzene.[7e] However (at least superfi-
cially), the addition of Na[BArf4] to an equimolar mixture of
[(6-Dipp)AuCl] and 6-Dipp appears to lead to an alternative
type of activation process. This reaction yields complex 5, fea-
turing the [(6-Dipp)Au] fragment ligated by an alkyl donor de-
rived from backbone CÀH activation of the second 6-Dipp unit,
as shown in Scheme 5. The overall cationic charge of this spe-
cies is derived from the presence of a pendant amidinium unit,
and is balanced by a single [BArf4]À counter-ion.
1
The identity of 5 is suggested by a H NMR spectrum which
features two sets of Dipp signals (e.g., five CH3 resonances in
the ratio 6:6:6:6:24) and a multiplet at dH =1.56 ppm, in the
region typically associated with gold-bound CHR2 fragments
(c.f. dH =1.50 ppm for that in [(IDipp)Au{CH(CN)CH2NiPr2}]).[17]
In addition, the 13C NMR spectrum features signals at dC =
153.4 and 209.2 ppm associated with the protonated and
gold-bound carbene quaternary carbons (c.f. 152.8 and
Scheme 4. Proposed mechanisms for: a) the ring opening of 6-Dipp by cata-
lytic acid to give 4; and b) formation of the related acyclic ligand fragment
in 2. [Ir]=[Ir(COE)(m-Cl)2Ir(COE)(6-Dipp)].
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[(6-Dipp)AuCl]/Na[BArf4] system in the presence of allylform-
amidine 4. In the event, this reaction also leads to the forma-
tion of 5 in good yield, implying that a possible mechanistic
pathway for the formation of 5 from [(6-Dipp)Au]+ and 6-Dipp
involves the intermediacy of the ring-opened species 4. The in-
itial conversion of 6-Dipp to 4 mediated by [(6-Dipp)Au]+
might reasonably proceed along similar lines to those pro-
posed for IrI and H+ (Scheme 4). However, the known ability of
[LAu]+ systems to facilitate nucleophilic attack at CÀC multiple
bonds means that ring closure via attack of the imine function
would likely be facile if the pendant alkene is subsequently co-
ordinated at gold(I). Such 6-endo-trig processes are known to
be facile, and in this case cyclisation is also highly selective,
with no evidence being found for the regioisomeric product
arising from the related 5-exo-trig process (Scheme 6).[19,20]
Scheme 5. Formation of NHC-supported gold(I) alkyl complex 5 from cyclic
and acyclic precursors. Key reagents and conditions: a) [(6-Dipp)AuCl]
(1 equiv), fluorobenzene, then Na[BArf4] (1.0 equiv), RT, 10 min, 46%;
b) [(6-Dipp)AuCl] (1 equiv), fluorobenzene, then Na[BArf4] (1.0 equiv), RT,
5 min, quantitative by NMR spectroscopy.
193.1 ppm for [(6-Dipp)H]+ and [(6-Dipp)AuCl], respective-
ly).[7a,b,e] While this backbone activation of 6-Dipp offers superfi-
cial analogies with the generation of abnormal carbene species
from unsaturated heterocycles in the presence of extremely
sterically encumbered metal centres,[17] structural data for 5 are
clearly consistent with a description as a simple secondary
alkyl ligand (Figure 2). Thus, the Au(1)-C(35) distance
(2.081(5) ꢁ, c.f. 2.107(11) for [(IDipp)Au{CH(CN)CH2NiPr2}]) and
the sum of the Au-C-C and C-C-C angles at C(35) [330.2(5)8] are
consistent with the presence of an a-hydrogen atom, that is,
H(351).[17,18]
Scheme 6. Proposed mechanism for the formation of 5 from 4 at
[(6-Dipp)Au]+.
Conclusion
The consequences in terms of CÀE bond activation brought
about by extremely high ligand steric loading have been
probed for the expanded ring NHC 6-Dipp. The reluctance of
this ligand to form 2:1 complexes with d-block metals, ration-
alised on the basis of a buried volume (%Vbur) of 50.8%,[7e]
leads to CÀH and CÀN bond activation processes driven by
attack at the backbone b-CH2 unit by free carbene. In the pres-
ence of H+ or IrI the net result is the formation of an allyl for-
mamidine fragment, while AuI allows for an additional ring
(re-)closure step via nucleophilic attack at the coordinated
alkene.[19] Although metallation at carbons other than C(2) to
give “abnormal” NHC coordination modes is a topic of much
interest for imidazolylidenes,[18] the net transformation of
6-Dipp to 5 in the presence of [(6-Dipp)Au]+ represents, to our
knowledge, the first example of backbone CÀH activation of
a saturated N-heterocyclic carbene.
Figure 2. Molecular structure of 5 with H atoms [except H(351)] omitted and
Dipp iPr groups shown in wireframe format for clarity; thermal ellipsoids set
at the 50% probability level. Key bond lengths [ꢁ] and angles [o]: Au(1)-C(2)
2.054(5), Au(1)-C(35) 2.081(5), C(2)-Au(1)-C(35) 172.4(2).
The cationic component of 5 is an isomer of the unknown
bis(NHC) complex [(6-Dipp)2Au]+; by contrast the correspond-
ing 6-Mes system [(6-Mes)2Au]+ is readily accessible from
[(6-Mes)AuCl], and can be shown crystallographically to feature
a pair of conventionally bound NHC ligands (see the Support-
ing Information). These observations serve to further highlight
the overwhelming steric bulk of the 6-Dipp ligand, and the
consequences—in terms of accessing alternative avenues of
reactivity—of attempts to coordinate two 6-Dipp ligands at
a single metal centre.
Experimental Section
Included here are synthetic, spectroscopic and crystallographic
data for compounds 2 and 5. General procedures and the corre-
sponding data for all other novel compounds, together with CIFs
for all X-ray crystal structures are included in the Supporting Infor-
mation. CCDC-966244 (2) and -966246 (5) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
In order to probe possible pathways leading to the forma-
tion of 5, we also examined the behaviour of the
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tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
2s(I), and R1 =0.0399, wR2 =0.0823 for all unique reflections. Max./
min. residual electron densities 1.56, À1.50 eꢁÀ3
.
All manipulations were carried out using standard Schlenk line or
dry-box techniques under an atmosphere of argon. With the ex-
ception of fluorobenzene, solvents were degassed by sparging
with argon and dried by passing through a column of the appro-
priate drying agent using a commercially available Braun SPS; fluo-
robenzene was dried by refluxing over calcium hydride, distilled,
sparged and stored over activated molecular sieves. NMR spectra
were measured in [D6]benzene or [D2]dichloromethane, which
were dried over potassium or molecular sieves, respectively, and
stored under argon in Teflon valve ampoules. NMR samples were
prepared under argon in 5 mm Wilmad 507-PP tubes fitted with
Synthesis of 5: A suspension of Na[BArf]4 (32 mg, 0.04 mmol) in
fluorobenzene (10 mL) was added to a stirred mixture of [(6-Dip-
p)AuCl] (23 mg, 0.04 mmol) and 6-Dipp (15 mg, 0.04 mmol) also in
fluorobenzene (20 mL). After 10 min, the solution was filtered and
the solvent removed in vacuo. The white product was washed
with hexanes (2ꢂ20 mL, and crystals suitable for X-ray diffraction
were obtained by layering a fluorobenzene solution with hexane
and storage at 208C. Yield: 31 mg, 46%. 1H NMR (300 MHz,
[D2]dichloromethane, 298 K) [6-Dipp’ refers to the backbone acti-
3
vated ligand]: dH =1.08, 1.12, 1.18, 1.28 (d, 6H, J(H,H)=6.6 Hz; CH3
of 6-Dipp’ iPr), 1.28 (d, 24H, 3J(H,H)=6.6 Hz; CH3 of 6-Dipp iPr),
1
3
J. Young Teflon valves. H and 13C NMR spectra were recorded on
1.56 (m, 1H; AuCH), 2.35 (quin, 2H, J(H,H)=5.8 Hz; CH2 of 6-Dipp),
3
Varian Mercury-VX-300 or Bruker AVII-500 spectrometers and refer-
enced internally to residual protio-solvent (1H) or solvent (13C) reso-
nances and are reported relative to tetramethylsilane (d=0 ppm).
11B and 19F NMR spectra were referenced with respect to Et2O·BF3
and CFCl3, respectively. Chemical shifts are quoted in d [ppm] and
coupling constants in Hz. Elemental analyses were carried out at
London Metropolitan University. Starting materials 6-Dipp,[7b]
6-Mes,[7a] [Ir(COE)2(m-Cl)]2,[21] bis(N,N’-diisopropylphenyl)formami-
dine,[22] [(6-Dipp)AuCl],[7e] [(6-Mes)AuCl],[7e] Na[BArf4],[23] and
[H(OEt2)2][BArf4],[24] were prepared by literature procedures.
2.74, 2.77 (sept, 2H, J(H,H)=6.6 Hz; CH of 6-Dipp’ iPr), 2.99 (over-
lapping m, 6H; H of NCH2 of 6-Dipp’ and CH of 6-Dipp iPr), 3.25
(apptr, 2H; H of NCH2 of 6-Dipp’), 3.44 (tr, 4H, 3J(H,H)=5.8 Hz;
NCH2 of 6-Dipp), 7.12–7.41 (overlapping m, 13H; arom-CH and
NCHN), 7.56(s, 4H; p-CH of [BArf4]À), 7.72 ppm (s, 8H; o-CH of
[BArf4]À); 13C NMR (75 MHz, [D2]dichloromethane, 298 K): d=20.5
(CH2 of 6-Dipp), 24.1, 24.3, 24.4, 24.5 (CH3 of 6-Dipp’ iPr), 24.7 (CH3
of 6-Dipp iPr), 25.3 (AuCH), 28.9 (CH of 6-Dipp iPr), 29.2, 29.2 (CH
of 6-Dipp’ iPr), 47.8 (NCH2 of 6-Dipp), 59.3 (NCH2 of 6-Dipp’), 117.8
1
(m, p-CH of [BArf4]À), 124.6 (m-CH of 6-Dipp), 125.0 (quart, J(C,F)=
271 Hz; CF3 of [BArf4]À), 125.0, 125.5 (m-CH of 6-Dipp’), 129.1 (p-CH
of 6-Dipp), 129.2 (m, m-C of [BArf4]À), 131.0 (p-CH of 6-Dipp’), 135.2
(br, o-CH of [BArf4]À), 136.6 (o-C of 6-Dipp’), 141.6 (o-C of 6-Dipp),
145.4, 145.7 (NC of 6-Dipp’), 146.3 (NC of 6-Dipp), 153.4 (NCHN of
6-Dipp’), 162.1 (quart, 1J(C,B)=50 Hz CB of [BArf4]À), 209.2 ppm
(NCN of 6-Dipp); ESI-MS: m/z (%): 1005.6 (100) [M]+; elemental
analysis: calcd (%) for C88H92N4AuBF24: C 56.54, H 4.96, N 3.00;
found: C 56.20, H 4.59, N 3.10. Crystallographic data (for 5):
C88H92N4AuBF24, Mr =1869.45, monoclinic, P21/c, a=13.2907(1), b=
27.0645(2), c=24.7244(2) ꢁ, b=105.5210(2)8, V=8569.19(12) ꢁ3,
Z=4, 1c =1.449 MgmÀ3, T=150(2) K, l=0.71073 ꢁ; 19530 inde-
pendent reflections [R(int)=0.053], used in all calculations; R1 =
0.0489, wR2 =0.0826 for I>2s(I), and R1 =0.0868, wR2 =0.1150 for
all unique reflections. Max./min. residual electron densities 4.77,
Synthesis and characterisation
Synthesis of 2: 6-Dipp (100 mg, 0.24 mmol) was dissolved in THF
(30 mL) and was added to a stirred solution of [Ir(COE)2(m-Cl)]2
(111 mg, 0.12 mmol) also in THF (20 cm3). The reaction mixture was
allowed to stir for 1 h, then the volatiles were removed under
vacuum. Extraction of the residue into hexanes (3ꢂ20 mL), concen-
tration and storage at À308C yielded 2 as a yellow-orange crystal-
line product. X-ray quality crystals were obtained from a concen-
trated solution in Et2O at À308C. Yield: 132 mg, 72%. NMR and mi-
croanalytical measurements were carried out on crystalline samples
which had been subject to drying in vacuo. H and 13C NMR meas-
1
urements indicate negligible quantities of retained diethyl ether,
and microanalytical calculations are based on the expected values
À2.72 eꢁÀ3
.
1
for ether-free samples. H NMR (300 MHz, [D6]benzene 298 K) [see
Supporting Information for numbering scheme]: dH =0.96 (tr, 4H,
3JHH =6.8 Hz, H33,34 COE), 1.04 (d, 6H, 3JHH =6.9 Hz, H26, H51), Acknowledgements
1.09, 1.13 (d, 3H, 3J(H,H)=6.9 Hz; H38), 1.17 (d, 3H, 3J(H,H)=
6.9 Hz; H17), 1.31 (br, 2H; H31,36 COE), 1.33 (d, 3H, 3J(H,H)=
We acknowledge the EPSRC (studentship for N.P., and access
3
6.9 Hz; H14), 1.44–1.65 (m, 6H; H32,35 COE), 1.72 (d, 6H, J(H,H)=
to the National Mass Spectrometry Service Facility, Swansea
University).
3
6.9 Hz; H25, H52), 1.79 (d, 3H, J(H,H)=6.9 Hz; H15), 2.57 (br, 2H;
2
H37), 2.82 (d, 1H, J(H,H)=15.6 Hz; H6), 2.95 (br, 1H; H16), 3.11 (br,
2H; H29,30 COE), 3.40 (sept, 2H, 3J(H,H)=6.9 Hz; H24, H50), 4.37
(d, 1H, 2J(H,H)=15.6 Hz; H6’), 4.44 (br, 1H; H13), 5.05 (vbr, 1H;
H2), 6.97–7.23 (m, 6H; H9, H20–22, H75–76), 7.18 ppm (s, 1H; H4);
13C NMR (75 MHz, [D6]benzene, 298 K): d=14.3 (C33–34 COE), 22.9
(C51), 23.0 (C15), 24.2 (C38), 24.6 (C52), 24.8 (C14), 25.1 (C17), 26.3,
26.4, 26.8, 28.0 (C31–32, C35–36 COE), 28.6 (C16), 28.9 (C24, C50),
29.1 (C13), 31.9 (C29, C30 COE), 43 (br, C2), 54.1 (C6), 123.2, 123.3
(C9, C76), 124.6, 125.0 (C20, C22), 126.7, 129.6 (C21, C75), 140.0,
142.6, 145.1, 146.3, 146.6, 147.2 (C7, C8, C12, C18, C19, C23),
155.2 ppm (C4); EI-MS: m/z (%): 402.9 (15) [LÀ2H]+; elemental
analysis: calcd (%) for C36H54N2IrCl·C4H10O: C 58.83, H 7.90, N 3.43;
Keywords: CÀH activation · gold · iridium · N-heterocyclic
carbenes · steric loading
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.
¯
C36H54N2IrCl C4H10O, Mr =816.63, triclinic, P1, a=11.8594(1), b=
12.9290(1), c=13.4021(1) ꢁ, a=109.325(1), b=93.226(1), g=
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,
T=
150(2) K, l=0.71073 ꢁ; 8758 independent reflections [R(int)=
0.033], used in all calculations; R1 =0.0306, wR2 =0.0713 for I>
Chem. Eur. J. 2014, 20, 3825 – 3830
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Full Paper
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Received: October 30, 2013
Revised: December 11, 2013
Published online on February 24, 2014
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