Chiral silver and gold rings: Synthesis and structural, spectroscopic, and photophysical properties of Ag and Au metallamacrocycles of bridging NHC ligands

Article abstract of DOI:10.1021/om200125w

The synthesis and silver(I) and gold(I) coordination chemistry of a new chiral, bidentate N-heterocyclic carbene (NHC) dehydrohexitol derivative (3) are reported. The imidazolium salt [H₂3][PF₆]₂ reacts with Ag₂

Full text of DOI:10.1021/om200125w

ARTICLE
pubs.acs.org/Organometallics
Chiral Silver and Gold Rings: Synthesis and Structural, Spectroscopic,
and Photophysical Properties of Ag and Au Metallamacrocycles of
Bridging NHC Ligands
Cristina Carcedo, James C. Knight, Simon J. A. Pope, Ian A. Fallis,* and Athanasia Dervisi*
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.
S Supporting Information
b
ABSTRACT: The synthesis and silver(I) and gold(I) coordination chemistry of a
new chiral, bidentate N-heterocyclic carbene (NHC) dehydrohexitol derivative (3)
are reported. The imidazolium salt [H23][PF6]2 reacts with Ag2O and Au(tht)Cl
(tht = tetrahydrothiophene) precursors to form the isostructural 18-membered
metallamacrocyclic dimers [Ag2(μ-3)2][PF6]2 and [Au2(μ-3)2][PF6]2 and the
monocarbene complex [(AuCl)2(μ-3)]. Single-crystal X-ray structures have been
determined for the bis-imidazolium precursor [H23][PF6]2 and corresponding
Ag(I) and Au(I) complexes of ligand 3. Comparison between the X-ray-derived
structures and solution-phase NMR data for [Ag2(μ-3)2][PF6]2 and [Au2(μ-3)2]-
[PF6]2 demonstrate that the complexes adopt a conformation in solution different
from that found in the solid state, implying a conformational flexibility of the
metallamacrocycle in solution. Both [(AuCl)2(μ-3)] and [Au2(μ-3)2][PF6]2 are
emissive in the solid state at ca. 380 nm (λex = 295 nm). Time-resolved
luminescence measurements indicate different excited-state lifetimes for the two
species, with [(AuCl)2(μ-3)] measured at 35 ns and [Au2(μ-3)2][PF6]2 at 379 ns.
The chiroptical properties of the silver and gold NHC complexes have been studied
by circular dichroism (CD).
’ INTRODUCTION
backbone.15 Isomannide is an inexpensive starch-industry by-
product obtained from the dehydration of mannitol. Our interest
in the use of this auxiliary in the development of multifunctional
ligands stems from the steric control exerted from its hinged
backbone. We have now extended the uses of isomannide in
ligand design by preparing the bis-imidazole carbene precursor
[H23][PF6]2 (Scheme 1) with 2,5-exo stereochemistry (the
endo and exo prefixes here refer to substitution with respect to
the bent V-shaped core of the fused dehydrohexitol ring). In this
paper we report the synthesis of the isomannide-derived bis-
methylimidazolium NHC precursor salt [H23][PF6]2 and its
deprotonation and in situ coordination with group 11 metals to
form chiral-NHC silver and gold metallamacrocyclic complexes.
Closed-shell silver and gold complexes have attracted much
interest due to their unusual bonding and photophysical pro-
perties.1 More recently, Ag(I) and Au(I) N-heterocyclic carbene
(NHC) ligand complexes2 have appeared in the literature with a
large array of structural motifs ranging from two-coordinate linear
molecules3 to complex helical structures,4 polymers,5 rings,6
cages,7 and clusters.8 Many of these have potential uses in high-
end materials applications such as luminescent chemosensors9
and optoelectronic onꢀoff switches.10 Ag(I)ꢀ and Au(I)ꢀNHC
complexes have also found many medicinal applications, the former
mainly due to their activity as antimicrobials11 and the latter as
chemotherapeutic agents.12 In addition, silver(I) carbenes are
extensively used as efficient ligand transfer agents for the majority
of imidazolium and other azolium salts, forming part of a
convenient and sometimes essential route for the synthesis of
metal carbene complexes.13
Of the many structurally characterized silver(I) and gold(I)
NHC complexes, there are relatively few examples with a
metallamacrocyclic structure and, to the best of our knowledge,
none with a chiral bridging framework.14 Previously, we have
exploited the chiral framework of the precursor isomannide (1;
Scheme 1) to prepare a chelating bis-diphenylphosphine with endo
stereochemistry which formed stable chelating complexes with
transition metals, despite the rigid sterically demanding fused-ring
’ RESULTS AND DISCUSSION
Synthesis of [H23][PF6]2. The bis-methylimidazolium iso-
idide salt [H23][PF6]2 was prepared from the commercially
available diol isomannide in three steps (Scheme 1). The ditosylate
2 was obtained from the reaction of isomannide with tosyl
chloride in cold pyridine in respectable yields. Heating the
ditosylate 2 with an excess of methylimidazole at 140 ꢀC afforded
Received: February 11, 2011
Published: April 06, 2011
r
2011 American Chemical Society
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Scheme 1a
a Reagents, conditions, and yields: (a) TsCl, pyridine, 4 ꢀC, 75%; (b) N-methylimidazole, 140 ꢀC, neat, 99%; (c) excess NH4PF6, H2O, 64%;
(d) Ag2O, NaBr, CH2Cl2, room temperature, 2 days, 85%; (e) for [Ag2(μ-3)2]Br2, excess Ag2O, [N(C3H7)4]Br; (f) [AuCl(tht)], NaOAc, 70ꢀ110
ꢀC, DMF, 30 min, 31% [Au(μ-3)]2[PF6]2 and 62% [(AuCl)2(μ-3)]; (g) [AuCl(tht)], NaOAc, 70ꢀ110 ꢀC, DMA, 2 h, 4% [Au(μ-3)]2[PF6]2 and 55%
[(AuCl)2(μ-3)].
the corresponding ditosylate salt [H23][Ts]2 as a thick brown oil
in quantitative yields. This reaction proceeds cleanly to furnish
the expected SN2 inversion product with no isomerization
byproduct observed in this case. Counterion metathesis with
ammonium hexafluorophosphate in water furnished [H23][PF6]2
as a crystalline white solid in 64% yield. The imidazolium salts
[H23][Ts]2 and [H23][PF6]2 have the expected exo stereochem-
istry at the 2- and 5-positions of the bicyclic ring, the same as in the
parent hexitol, iditol. The dicationic salts are readily soluble in polar
organic solvents such as acetonitrile and DMSO. The ditosylate salt
is also freely soluble in water, with the PF6 analogue dissolving only
sparingly in water at room temperature.
for compound 2 in Scheme 1. Assignments for the endo and exo
methylene protons (H1/6) were made on the basis of their vicinal
coupling values with the methine protons H2/5. The signal for the
bridgehead protons (H3/4) appears as a singlet as a result of the
close to 90ꢀ angle formed with the vicinal H2/5 protons, a common
feature for the exo-substituted iditol derivatives. The azolium
proton has a relatively high chemical shift at δ 8.47 ppm associated
with H bonding to the O atoms of the dioxabicyclo unit (vide
infra). Worth noting from the 13C NMR data (Table 1) is the
large downfield shift for the C3/4 atoms of [H23][PF6]2 in com-
parison with those of the ditosylate derivative 2. Such large
downfield changes from endo to exo dehydrohexitol derivatives
have been attributed to steric compression effects (field effect) of
the more crowded endo derivatives.16
1H NMR data for 3 (HPF6)2 are shown in Table 1, and the
3
numbering scheme adopted for the core bicyclic protons is shown
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Table 1. Selected 1H and 13C NMR Data for the Compounds of 3a
compd
endo-H1/6 (JHH
)
exo-H1/6 (JHH
)
H2/5 (JHH
)
H3/4 (JHH
)
C1/6
C2/5
C3/4
2b
3.65 (9.6, 7.7)
3.83 (9.6, 6.8)
4.78 (m)
4.40 (m)
5.02 (∼0)
4.74 (∼0)
5.07 (br, ∼0)
4.93 (br, ∼0)
71.6
79.9
81.6
[H23][PF6]2
4.34 (11.4, 4.9)
4.23 (11.0, 5.1)
4.44 (10.8, 6.0)
4.35 (10.8, 5.4)
4.27 (11.4, 2.4)
4.43 (11.0, 0.7)
4.55 (10.8, 3.3)
4.23 (10.8, 2.7)
5.13 (4.9, 2.4)
5.20 (5.1, 0.7)
5.18 (6.0, 3.3)
5.36 (5.4, 2.7)
72.86
73.16
71.20
71.46
66.21
66.51
65.98
66.14
88.15
88.59
87.82
87.48
[Ag2(μ-3)2][PF6]2
[Au2(μ-3)2][PF6]2
[(AuCl)2(μ-3)]
a In CD3CN unless otherwise noted. Chemical shifts are given in ppm and J values in Hz. b In CDCl3.
Figure 2. View of the molecular packing for [H23][PF6]2 along the
a axis.
the silver-bridged macrocycle [Ag2(μ-3)2][PF6]2. This was the
only silver complex observed even when an excess of Ag2O was
used in order to gain access to Agꢀmonocarbene complexes, e.g. 4.
Further attempts to produce a monomeric complex using excess
Ag2O and tetrapropylammonium bromide failed, giving only thꢀe
dimeric complex [Ag2(μ-3)2]X2, with a mixture of Brꢀ and PF6
counterions. 1H NMR spectra (CD3CN) taken during the course
of the reaction revealed the formation of the intermediate biscar-
bene-imidazolium complex [Ag(H3PF6)2][PF6] (see Scheme 1).
Figure 1. Displacement ellipsoid plots at 30% probability of the
imidazolium cation [H23][PF6]2. Two cations from the unit cell are
shown, with the PF6 anions and selected protons omitted for clarity.
Selected bond lengths (Å) and angles (deg): NꢀCNHC(av) = 1.339(10),
O(1) H(18) = 2.391, O(3) H(4) = 2.396; O(1) H(18)ꢀ
3 3 3
3 3 3
3 3 3
C(18) = 145.8, O(3) H(4)ꢀC(4) = 146.7.
3 3 3
Single crystals of [H23][PF6]2 suitable for X-ray studies were
grown from a water solution of the salt. Several hydrogen-
bonding interactions between the dioxolane and imidazolium
protons with the ether oxygen and PF6ꢀ anion are observed in
the solid state. Figure 1 shows two of the four [H23][PF6]2 mol-
ecules in the unit cell linked via hydrogen-bonding interactions
between the imidazolium proton and the ether function of the
neighboring cation. The molecular packing of the imidazolium
salt, supported by intermolecular hydrogen bonding, is shown in
Figure 2. This alternating packing arrangement produces wave-
like channels running parallel to the a axis.
Complexes of 3. The bis-NHC ligand 3 was anticipated to
form bridging, linear coordination polymers17 or cyclic arrays
with the metals Ag(I) and Au(I), bearing either one or two NHC
donors per metal. The bis-imidazolium salt [H23][PF6]2 was
reacted directly with Ag(I) and Au(I) precursors to form the
corresponding NHC complexes without the need to preform or
isolate the carbene ligand 3.
The isostructural gold complex [Au2(μ-3)2][PF6]2 was isolated
from the 1:1 reaction of Au(tht)Cl (tht = tetrahydrothiophene) and
[H23][PF6]2 in hot DMF in the presence of sodium acetate, as a
weak base. In this case, however, the linear gold complex
[(AuCl)2(μ-3)] was also formed in an approximate 2:1 molar
ratio (as determined by 1H NMR) with the digold macrocycle. It
is logical to suggest that the Au(tht)Cl precursor favors formation of
The NHC precursor [H23][PF6]2 was reacted with freshly
prepared silver oxide in dichloromethane or DMSO to produce
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Figure 3. ORTEP ellipsoid plots at the 30% probability level of the cations [Ag2(μ-3)2]2þ (left; side view of the Ag coordination axes) and [Au2(μ-3)2]2þ
(right; front view of the Au coordination axes). Selected bond lengths (Å) and angles (deg): [Ag2(μ-3)2][PF6]2, Ag(1)ꢀC(2) = 2.107(6),
Ag(1)ꢀC(16) = 2.103(6), Ag(2)ꢀC(14) = 2.096(6), Ag(2)ꢀC(28) = 2.123(6), C(16)ꢀAg(1)ꢀC(2) = 171.7(3), C(14)ꢀAg(2)ꢀC(28) =
171.7(3); [Au2(μ-3)2][PF6]2, C(1)ꢀAu(1) = 2.046(10), C(11)ꢀAu(1)i = 1.983(10), C(11)iꢀAu(1)ꢀC(1) = 177.6(4). Hydrogen-bonding
interactions are indicated by dashed lines.
the Au(NHC)Cl complex. A change in reaction solvent to
dimethylacetamide (DMA) shifted the product distribution of this
reaction to favor [(AuCl)2(μ-3)] as the major product (55% yield),
with only a minor [Au2(μ-3)2][PF6]2 component(4%yield) being
observed. The 2:1 reaction of Au(tht)Cl and [H23][PF6]2 in hot
DMA afforded the monocarbene complex [(AuCl)2(μ-3)], but
the product was contaminated with unreacted [H23][PF6]2
and ill-defined intermediate Au-NHC species. Alternatively,
the gold macrocycle can be obtained via transmetalation of the
silver complex [Ag2(μ-3)2][PF6]2. However, this approach
afforded the product in reduced yield. All complexes were
soluble in polar solvents such as acetonitrile, DMF, and DMSO.
[(AuCl)2(μ-3)] was also soluble in chlorinated solvents such as
CH3Cl and CH2Cl2.
The silver(I)ꢀcarbene complexes have been extensively used
as carbene transfer agents to other transition metals, most com-
monly palladium and to a lesser extent rhodium and iridium.13
Several attempts were made, employing different synthetic routes
for the transmetalation of [Ag2(μ-3)2][PF6]2 with various Pd(II)
salts. Reactions, however, led to partial protonolysis to the parent
imidazolium salt [H23][PF6]2, and any well-defined palladiumꢀ
NHC complexes formed proved difficult to isolate and charac-
terize further.
and 74.6(4)ꢀ for the silver cation. Due to this large opening no
argentophilic or aurophilic interactions are observed between the
two metal centers for either [Ag2(μ-3)2]2þ or [Au2(μ-3)2]2þ
cations, with intermetallic distances of 5.558 and 6.293 Å, re-
spectively.18 In the silver dimer the two metal atoms bend toward
each other with a 171.7(3)ꢀ CꢀAgꢀC angle, whereas in the gold
analogue the CꢀAuꢀC angle is closer to linear at 177.6(5)ꢀ.
The two carbenoid rings of the M(NHC)2 fragments adopt
nearly coplanar orientations. The interplanar “tilt” angle between
the adjacent heterocycle planes in the [Au2(μ-3)2]2þ cation is
8.7(9)ꢀ, with the gold atom deviating from the associated planes
by relatively small amounts, 0.0412(4) and 0.0917(5) Å, indicat-
ing essential coplanarity. In the case of the silver dimer, though,
bending of the N(1,2) imidazole ring relative to its AgꢀCNHC
bond is observed by ca. 12ꢀ, associated with the Ag(1) atom
divergence by 0.4091(4) Å from the N(1,2) ring plane. However,
small deviations are observed for the silver atoms from the remain-
ing carbenoid planes by 0.0710(4), 0.1711(4), and 0.1351(4) Å,
respectively, with the interplanar tilt angle for the N(3,4) and
N(7,8) containing rings at 4.0(5)ꢀ.
As shown in Figure 3, small differences are observed in
the AgꢀC(2,14,16,28) distances (rav = 2.107(6) Å) for [Ag2
(μ-3)2][PF6)]2; this average is somewhat longer when compared
to those for other bis-imidazolyl Ag complexes but probably
reflects the steric restrictions of a macrometallacyclic structure.2d,19
The AuꢀC(1,11) bond distances at 2.042(11) and 1.985(11) Å, as
expected, are significantly shorter than those of the Ag isomer, due
to the smaller covalent radii of Au,20 but are comparable to the
values for other bridging Au(NHC)2þ complexes.14,21
Solid-State Structures. The solid-state structures for both the
silver and gold metallacycle complexes were determined, con-
firming their ring structure. Both compounds are cationic
comprised of a bimetallic core, [M2(μ-NHC)2]2þ, linked by a
pair of bridging carbene ligands. The two isostructural dimers
adopt an “open” C2-symmetric conformation placing the CNHC
ꢀ
MꢀCNHC axes parallel to each other and the carbene rings on the
same metal atom in a coplanar orientation (Figure 3) with the gold
complex bearing a true C2 crystallographic axis, rendering the two
Au fragments equivalent. The dioxolane linker imposes a cleft
between the constituent planes of the cations with a fold angle
between the two M(NHC)2 planes at 82.4(7)ꢀ for [Au2(μ-3)2]2þ
Both structures in the solid state form H bonds with the PF6
anions and, more interestingly, H bonds between the “endo” oxygen
atoms (facing into the metallacycle ring) with the methine pro-
tons (H2/5) opposite them (Figure 3, Table 2). Values for the H
bonds are 2.57/2.72 and 2.31 Å for the silver and gold complexes,
respectively.
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a
Table 2. H Bonding between Endo Oxygen Atoms in [Au(μ-3)(PF6)]2 and [Ag(μ-3)(PF6)]2
compd
DꢀH
A
d(DꢀH), Å
d(H A), Å
d(D A), Å
—(DHA), deg
3 3 3
3 3 3
3 3 3
[Au2(μ-3)2][PF6)]2
[Ag2(μ-3)2][PF6)]2
C9ꢀH9 O1_#1
1.00
1.00
1.00
2.31
2.57
2.72
3.170(14)
3.439(8)
3.560(8)
144.1
145.1
142.3
3 3 3
C19ꢀH19 O1
3 3 3
C10ꢀH10 O4
3 3 3
a Symmetry transformation used to generate equivalent atoms: (#1) 1/2 ꢀ x, y, 1 ꢀ z.
Figure 4. ORTEP ellipsoid plot at the 30% probability level of
[(AuCl)2(μ-3)]. Selected bond lengths (Å) and angles (deg): C-
(2)ꢀAu(1) = 1.969(10), C(12)ꢀAu(2) = 1.974(13), Au(1)ꢀCl(1) =
Figure 5. Line drawing representation of the side and top views of the
metallamacrocyclic ring of [Ag2(μ-3)2][PF6)]2 in a D2-symmetric con-
formation.
2.278(3), Au(2)ꢀCl(2) = 2.285(3), Au(1) Au(2) = 3.2948(7);
3 3 3
Cl(1)ꢀAu(1)ꢀC(2) = 170.8(4), Cl(2)ꢀAu(2)ꢀC(12) = 173.4(4), C-
(2)ꢀAu(1)ꢀAu(2) = 91.7(3), Cl(1)ꢀAu(1)ꢀAu(2) = 95.85(10), C-
(12)ꢀAu(2)ꢀCl(2) = 173.4(4), Cl(2)ꢀAu(2)ꢀAu(1) = 93.9(4),
Cl(2)ꢀAu(2)ꢀAu(1) = 92.54(9), Cl(1)ꢀAu(1)ꢀAu(2)ꢀC(12) =
55.4(4), Cl(2)ꢀAu(2)ꢀAu(1)ꢀC(2) = 49.0(4).
Data from the 1H and 13C NMR spectra of the silver and gold
1
complexes of 3 are given in Table 1. The H NMR spectra of
[Ag2(μ-3)2][PF6]2 and [Au2(μ-3)2][PF6]2 show four distinct
resonances for the dioxolane core protons, suggesting a con-
formation of higher symmetry in solution than the C2 symmetry
observed in the solid state. The “open”chain conformation (eq 1) of
the crystal structures shown in Figure 3 renders all eight protons
of the dioxolane unit nonequivalent, whereas in the higher D2
symmetry of the “folded” chain conformations (Figure 5) the
respective protons H1/6(endo), H1/6(exo), H2/5, and H3/4 are
equivalent (numbering shown in Scheme 1, structure 2).
An ORTEP representation of the solid-state structure of the
gold chloride complex [(AuCl)2(μ-3)] is shown in Figure 4. In
the solid state, [(AuCl)2(μ-3)] molecules are linked by closed
shell d10ꢀd10 aurophilic interactions forming infinite chains,
with the two interacting gold units arranged in a staggered
fashion. The Au(1) Au(2) separation between adjacent gold
3 3 3
centers is 3.2948(7) Å, close to the sum of their van der Waals
radii (3.32 Å).22 Although shorter Au Au distances have been
3 3 3
previously reported for AuCl(NHC) complexes (ca. 3.16 Å),3d,9
the observed Au(1) Au(2) separation for [(AuCl)2(μ-3)] is
3 3 3
well within the limits for aurophilic interactions.1a
Solution Studies. The silver and gold metallacycle complexes
of 3 preserve their dimeric structure in solution, as confirmed by
NMR, conductivity, and MS measurements. Electrospray mass
spectrometry shows maximum mass peaks at m/z 907 and 1087
for the silver and gold complexes, respectively, corresponding
to loss of one of the PF6 anions from the parent molecule,
[M(μ-3)]2[PF6]2.
The D2-symmetric conformation shown in Figure 5 is steri-
cally less demanding than the C2 conformation adopted in the
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Figure 6. Left: excitation and emission spectra for [(AuCl)2(μ-3)] (gray) and [Au2(μ-3)2][PF6]2 (black), in the solid state. Right: decay profiles for
[(AuCl)2(μ-3)] (gray) and [Au2(μ-3)2][PF6]2 (black).
solid state (Figure 3), with the former conformation placing the
two carbenoid rings of the M(NHC)2 unit orthogonal to each
other, thus minimizing steric repulsions. In contrast, in the
C2-symmetric conformation the heterocyclic rings are coplanar.
In the 13C NMR spectrum of the silver complex [Ag2(μ-3)2]-
[PF6]2 in d3-acetonitrile a carbene resonance was not observed.23
However, in d6-dmso [Ag2(μ-3)2][PF6]2 displays a sharp pair of
doublets at δ 179.7 ppm for the carbene carbon with coupling
constants of 183.5 (1J107AgC) and 212.3 Hz (1J109AgC), respectively.
The coupling values are in agreement with a silver bis-carbene
species formed rather than a monocarbene complex. As we have
previously reported, AgꢀC coupling constants in monocarbene
107AgC
complexes are ca. 40ꢀ50 Hz greater, in the region of 220 (J
)
and 260 Hz (J109AgC), respectively.24
Molar Conductance. Molar conductivity measurements (ΛM)
were carried out for the gold and silver dimers at room tem-
perature within the range of 10ꢀ3 to 10ꢀ5 M concentrations, in
acetonitrile. The molar conductivity values at 1.0 mM concen-
trations are 252 and 416 Ωꢀ1 cm2 molꢀ1 for the gold and silver
complexes, respectively. The value for [Au2(μ-3)2][PF6]2 is
within the 220ꢀ300 Ωꢀ1 cm2 molꢀ1 range for 2:1 electrolytes;25
however, that for [Ag2(μ-3)2][PF6]2 is well above the upper limit.
Comparison between plots of ΛM vs c1/2 (c = molar con-
centration) allows the determination of the nature of electrolytes,
where the same types of electrolytes are expected to have similar
slopes. Plots of this nature for the silver and gold complexes
[M2(μ-3)2][PF6]2 show the molar conductivity decreasing with
increasing concentration, as expected due to changes in the
interionic forces (Figure S5, Supporting Information). The sig-
nificant feature of this plot is that the two sets of data have very
similar slopes, indicating that both complexes are 2:1 electrolytes.
Electronic Absorption, Luminescence, and Circular Di-
chroism Spectroscopic Measurements. Luminescence stud-
ies undertaken on crystalline samples of the gold complexes
[Au2(μ-3)2][PF6]2 and [(AuCl)2(μ-3)] showed that they are
luminescent in the solid state. In accordance with previous
studies into the optical and photophysical behavior of car-
bene-derived Au(I) complexes, excitation in the UV region was
required for observable emission.14,21 The electronic absorption
spectrum of [Au2(μ-3)2][PF6]2 in MeCN displays a complex
absorption band with a λmax at 245 nm. Although it is red-shifted,
Figure 7. Short-wavelength electronic (top) and circular dichroism
(bottom) spectra of [Ag2(μ-3)2][PF6]2 (solid curve), [Au2(μ-3)2]-
[PF6]2 (long dashes), [H23][PF6]2 (short dashes), and [(AuCl)2-
(μ-3)] (dots). Spectra were obtained as approximately equimolar
(0.1 mM) acetonitrile solutions at 21 ꢀC.
the main band has an intensity comparable to that of a similar
band found in the UV spectrum of the precursor imidazolium salt
[H23][PF6]2. Following excitation at 270ꢀ300 nm (Figure 6)
[Au2(μ-3)2][PF6]2 was found to be emissive at ca. 380 nm. The
related complex [(AuCl)2(μ-3)] was similarly emissive, although
with a slightly broader emission profile. Complementary time-
resolved luminescence studies showed that the complexes exhibited
different emission lifetimes. In the solid state, [(AuCl)2(μ-3)]
possessed a lifetime (λex 295 nm; λem 380 nm) of 35 ns, whereas
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Table 3. X-ray Crystallographic Data
[H23][PF6]2
[Au2(μ-3)2][PF6]2 H2O
[Ag2(μ-3)2][PF6]2
[(AuCl)2(μ-3)]
3
empirical formula
C
14H20F12N4O2P2
C28H38Au2F12N8O5P2
C28H36Ag2F12N8O4 P2
1054.33
C14H18Au2Cl2N4O2
739.16
formula wt
566.28
1250.54
cryst syst, space group
monoclinic, P21
12.762(2)
11.951(2)
14.504(2)
90
orthorhombic, I212121
13.2240(5)
17.0310(6)
20.7230(8)
90
monoclinic, P21
10.2132(1)
20.4615(3)
10.8911(2)
90
monoclinic, P21
8.3312(3)
12.6030(4)
9.4992(4)
90
a/Å
b/Å
c/Å
R/deg
β/deg
99.815(6)
90
90
106.504(1)
90
114.071(2)
90
γ/deg
90
Z
4
4
2
2
no. of data/restraints/params
final R indices (F2 > 2σ(F2)): R1, wR2
R indices (all data): R1, wR2
6163/1/612
0.0689, 0.1483
0.1044, 0.1723
4980/0/260
0.0561, 0.1257
0.0759, 0.1336
9389/1/509
0.0545, 0.1373
0.0583, 0.1403
3742/7/220
0.0371, 0.0818
0.0444, 0.0853
the decay profile of [Au2(μ-3)2][PF6]2 fitted well to two con-
tributions: a dominant (86%), long component of 379 ns, and a
minor (14%), shorter component (35 ns). The latter contribu-
tion is actually attributed to a small amount of [(AuCl)2(μ-3)] in
the sample, which was quantitatively identified in solution by 1H
NMR spectroscopy.
hence implying a small or negligible contribution to the CD
derived from the cyclic nature of [Au2(μ-3)2]2þ. Given the short
wavelengths of these bands, it is not unreasonable to assume that
these are largely ligand-centered processes, and we tentatively
suggest that the complexity of the Au spectra arises from a larger
degree of metalꢀligand covalency, resulting in perturbed ligand
π levels.27 This notion is verified to some extent by comparing
the bisignate nature of the CD spectrum of [Au2(μ-3)2]2þ at short
wavelengths, implying exciton coupling between ligand chromo-
phores within the constraints of the cyclic chiral environment,28 to
the single signed (uncoupled) spectrum of [(AuCl)2(μ-3)].
Gold complexes with short aurophilic contacts can display
interesting luminescence properties in the solid state, where the
nature of the goldꢀgold interaction can influence the spectral
properties.3a,b,14,26 However, our observations for the AuꢀCl
and AuꢀPF6 complexes of 3 with the presence or absence of
aurophilic interactions as determined by X-ray diffraction (AuꢀAu
distances 3.2948(7) and 6.293(6) Å, respectively) suggest that
such interactions do not dominate the emissive character of these
complexes. The emission wavelengths of the two goldꢀ3 com-
plexes are actually comparable to those of the ligand precursor
[H23][PF6]2, although the peak appearance of the proligand is
much broader (Figure S6, Supporting Information) and the
emission lifetime of the imidazolium salt is extremely short
(0.9 and 4.1 ns with ca. 50/50 weighting) and indicative of
fluorescence. Therefore, the wavelength positioning of the ob-
served emission from the complexes probably suggests an excited
state which is dominated by ligand-centered character, albeit one
significantly perturbed (as indicated by the lifetime decay
profiles) by the gold.
In order to measure the effect of the macrocyclic struc-
ture on the chiroptical properties of [Ag2(μ-3)2][PF6]2 and
[Au2(μ-3)2][PF6]2, circular dichroism (CD) measurements in
the UV region were undertaken. For purposes of comparison, the
CD and absorption spectra in the 200ꢀ300 nm region of [Ag2-
(μ-3)2][PF6]2, [Au2(μ-3)2][PF6]2, [H23][PF6]2, and [(AuCl)2-
(μ-3)] in acetonitrile solution are presented in Figure 7. While
no attempt is made here to rigorously assign these spectra, several
general observations may be made. The metallamacrocycles
[Ag2(μ-3)2]2þ and [Au2(μ-3)2]2þ both display moderately large
negative CD values in comparison to the acyclic imidazolium salt
[H23]2þ. In addition, we observe that while both the CD and
absorption spectra of [Ag2(μ-3)2]2þ are relatively simple, the
absorption spectra of [Au2(μ-3)2]2þ and [(AuCl)2(μ-3)] are
more complex, which is reflected in the complex envelopes of
their CD spectra. It is worth noting that, if ligand concentrations
are taken into account, the magnitudes of the CD observed for
[Au2(μ-3)2]2þ and [(AuCl)2(μ-3)] are roughly comparable,
’ CONCLUSION
A short synthetic route to the chiral, ether-functionalized
NHC precursor [H23][PF6]2 has been developed. The bridging
ligand 3 forms either cationic dimeric cyclic structures with Ag(I)
and Au(I) metals or, in the case of Au(I), neutral bimetallic
arrays, depending on the choice of counterion. The neutral and
cationic gold complexes are luminescent in the solid state, and
lifetime measurements show distinctly different profiles for these
two complexes, with the metallamacrocycle [Au2(μ-3)2][PF6]2
having an order of magnitude longer emission lifetime (379 ns)
in comparison to the acyclic complex [(AuCl)2(μ-3)] (35 ns).
The chiral nature of these isomannide-derived compounds has
enabled their study by circular dichroism spectroscopy, which, to
the best of our knowledge, is the first example of an examination
of the chiroptical properties of a carbene complex. Further work
is currently in progress on the development of other dehydro-
hexitol-based NHC derivatives and their applications.
’ EXPERIMENTAL SECTION
General Remarks. All manipulations were performed using stan-
dard glassware under aerobic conditions, except where otherwise noted.
Solvents of analytical grade were freshly distilled from sodium/benzo-
phenone (thf, toluene, hexane) or from calcium hydride (CH2Cl2, MeCN).
DMF (dimethylformamide) was purified by a Braun-SPS solvent puri-
fication system. Anhydrous DMA (dimethylacetamide) was purchased
from Sigma-Aldrich and used as received. Deuterated solvents for NMR
measurements were distilled from the appropriate drying agents under
N2 immediately prior to use, following standard literature methods.29 All
other reagents were used as received. Au(tht)Cl was prepared following
a literature procedure.30 NMR spectra were obtained on Bruker Avance
2559
dx.doi.org/10.1021/om200125w |Organometallics 2011, 30, 2553–2562

Organometallics
ARTICLE
AMX 400 and 500 and JEOL Eclipse 300 spectrometers. The chemical
shifts are given as dimensionless δ values and are frequency referenced
relative to TMS for 1H and 13C. Coupling constants J are given in hertz
(Hz) as positive values, regardless of their real individual signs. The
multiplicities of the signals are indicated as s, d, and m for singlets,
doublets, and multiplets, respectively. The abbreviation br is given for
broadened signals. Emission, excitation, and lifetime solid-state spectra
were acquired on a Jobin Yvon-Horiba Fluorolog spectrometer fitted
with a JY TBX picosecond photodetection module. CD spectra were
obtained using an Applied Photophysics Chirascan spectrometer equipped
with a Xe arc source and a thermostated (21 ꢀC) sample holder. In all
cases five spectra were accumulated and averaged. Spectra were mea-
sured in the region 200ꢀ600 nm at a concentration of approximately
5 ꢁ 10ꢀ4 M in 0.1 mm quartz cuvettes (Helma). Mass spectra and high-
resolution mass spectra were obtained on a Waters LCT Premier XE
instrument and are reported as m/z (relative intensity).
[Ag2(μ-3)2][PF6]2. The imidazolium salt [H23][PF6]2 (0.220 g, 0.39
mmol), Ag2O (0.092 g, 0.40 mmol), and NaBr (0.410 g, 4.0 mmol) were
added to 40 mL of dichloromethane. The mixture was stirred in the dark
for 2 days, until the black color of the silver oxide had dissipated. The
reaction was monitored by NMR and deemed complete with the dis-
appearance of 3 (HPF6)2. The reaction mixture was filtered over Celite,
3
which was then washed with dichloromethane (3 ꢁ 20 mL). The filtrate
was collected and dried under vacuum to afford the silver complex as a
white powder. Recrystallization by diffusion of hexane into a dichlor-
omethane solution afforded the product as needle-shaped crystals. Yield:
0.179 g, 85%. 1H NMR (250 MHz, CD3CN): δ 3.78 (s, 3H; CH3), 4.23
(dd, 2J = 11.0, 3J = 5.1, 1H; endo-H1/6), 4.43 (dd, 2J = 11.0, 3J = 0.7, 1H;
exo-H1/6), 4.74 (d, 1H, 3J ≈ 0, H3/4), 5.20 (dd, 3J = 5.1, 3J = 0.7, 1H;H2/5),
7.12 (d, 3J = 1.8, 1H; imid-H), 7.16 (d, 3J = 1.8, 1H; imid-H). 13C NMR
(125 MHz, CD3CN): δ 38.57 (s, CH3), 66.40 (s, C2/5), 72.60 (s, C1/6),
88.16 (s, C3/4), 120.73 (s, imid-C), 122.93 (s, imid-C), CNHC signal not
1
observed. H NMR (400 MHz, d6-DMSO): δ 3.90 (s, 3H; CH3), 4.4
Syntheses. 1,4:3,6-2.5-di-O-(p-tolylsulfonyl)-D-mannitol (2). A
modified literature procedure was used for the synthesis of 2.31 To a
solution of 8.5 g (58.2 mmol) of isomannide in 40 mL of dry pyridine
cooled in an ice bath was added p-tolylsulfonyl chloride (25.6 g, 135 mmol)
portionwise. The slurry that formed was stirred overnight. Iceꢀwater
(50 mL) was added to the reaction mixture, and the slurry that formed
was extracted with chloroform (120 mL). The collected organic layer
was washed with 1.25 N hydrochloric acid (3 ꢁ 75 mL) until the water
layer remained acidic and then with water and finally dried over MgSO4.
The chloroform solution was subsequently filtered and the solvent
removed under reduced pressure to leave a thick brown oil (22.1 g).
Recrystallization from ethanol afforded 2 as a white powder. Yield: 19.9 g,
75%. 1H NMR (400 MHz, CDCl3): δ 2.41 (s, 6H), 3.65 (dd, 1H, 2J =
9.6, 3J = 7.7, endo-H1/6), 3.83 (dd, 1H, 2J = 9.6, 3J = 6.8, exo-H1/6), 4.40
(m, 2H, H3/4), 4.78 (m, 2H, H2/5), 7.31 (m, 4H, Ar), 7.75 (m, 4H, Ar).
[H23][Ts]2. A sealed pressure tube containing a solution of 4.5 g
(9.9 mmol) of 1,4:3,6-2.5-di-O-(p-tolylsulfonyl)-D-mannitol (2) in
4.7 mL (59.4 mmol) of 1-methylimidazole was heated at 140 ꢀC for 3
days. The reaction was followed by TLC (100% ethyl acetate; Rf for 2 =
0.8) until all the ditosylate 2 had reacted. The thick brown oil obtained
was washed several times with diethyl ether, to remove the excess
methylimidazole. Upon drying [H23][Ts]2 was obtained as a clear
foamy material and was used in subsequent reactions without further
purification. Yield: 6.07 g, 99%. 1H NMR (400 MHz, d6-DMSO): δ 2.31
(s, 3H, PhMe), 3.83 (s, 3H, NMe), 4.23 (m, 2H, H1/6), 5.12 (d, 1H, 3J ≈
0, H3/4), 5.25 (m, 1H, H2/5), 7.11 (m, 2H, Ar), 7.49 (m, 2H, Ar), 7.82 (m,
2H, imid-H), 9.19 (s, 1H, CNHCH). MS (ES, CH3CN): m/z 447.19 (M ꢀ
(C7H7SO3)þ; C21H27N4O5S requires 447.17).
(dd, 2J = 11.0, 3J = 5.1, 1H; endo-H1/6), 4.5 (d, 2J = 11.0, 1H; exo-H1/6),
4.9 (d, 1H, 3J ≈ 0, H3/4), 5.40 (d, 3J = 5.1, 1H; H2/5), 7.5
(d, 1H; imid-H), 7.5 (d, 1H; imid-H). 13C NMR (75 MHz, d6-DMSO):
δ (CH3 signal obscured by acetone peak) 66.51 (s, C2/5), 73.16 (s, C1/6),
88.59 (s, C3/4), 121.92 (s, imid-C), 124.10 (s, imid-C), 179.74 (dd,
1J107AgC = 183.5, 1J109AgC = 212.3, CNHC). UVꢀvis (CH3CN, 0.47 mM):
λmax/nm (ε/103 Mꢀ1 cmꢀ1) 232 (42.8). MS (accurate mass, ESþ,
CH3CN): m/z (%) calcd mass for [C28H36107Ag2F6N8O4P]þ 907.0603,
measured 907.0643.
[Au2(μ-3)2][PF6]2 and [(AuCl)2(μ-3)]: Reaction in DMF. A round-
bottom flask containing DMF (3.8 mL) was heated to 70 ꢀC, and sub-
sequently Au(tht)Cl (0.203 g, 0.63 mmol) was added. The clear solution
was stirred until the entire gold precursor was dissolved. [H23][PF6]2
(0.340 g, 0.63 mmol) and NaOAc (0.125 g, 1.5 mmol) were added to the
solution and the mixture heated to 110 ꢀC for 30 min. While hot, the
solution was filtered over Celite into a flask containing 10 mL of water. The
precipitated white solid was filtered and the filtrate washed with cold
methanol (2 mL) and ether (3 ꢁ 5 mL) and dried under vacuum. This
afforded 0.258 g of a 1:2 molar mixture (by 1H NMR) of [Au(μ-3)]2[PF6]2
(yield ∼31%) and [(AuCl)2(μ-3)] (yield ∼62%). X-ray-quality, colorless
crystals of [Au(μ-3)]2[PF6]2 were obtained after storing a saturated DMF
solution of the complex for 3 days in the dark.
[Au2(μ-3)2][PF6]2 and [(AuCl)2(μ-3)]: Reaction in DMA. A similar
procedure was followed in DMA. Au(tht)Cl (0.189 g, 0.59 mmol) was
suspended in DMA (3 mL) and heated to 60 ꢀC until the gold complex
was dissolved. [H23][PF6]2 (0.332 g, 0.59 mmol) and NaOAc (0.107 g,
1.31 mmol) were added to the solution and the mixture heated to 70 ꢀC
for a few minutes and then to 50 ꢀC for 2 h. Some decomposition to
metallic gold was observed during the reaction. The solution was filtered
while hot over Celite, and 30 mL of methanol was added to the filtrate to
precipitate the product as a white solid. This afforded 0.125 g of a 1:13.5
[H23][PF6]2. To a water solution of [H23][Ts]2 (3.16 g, 5.1 mmol,
0.5 M) heated to 45 ꢀC was added a 30 mL water solution of ammonium
hexafluorophosphate (8.0 g, 49.1 mmol). During the reaction an off-
white solid precipitated, and the mixture was stirred at 45 ꢀC for 1 h and
then left at room temperature to complete precipitation of the product.
The solid obtained was filtered and dried under vacuum. Yield: 1.85 g,
64%. 1H NMR (400 MHz, d6-DMSO): δ 3.85 (s, 3H, CH3), 4.26 (dd,
1
molar mixture (by H NMR) of [Au(μ-3)]2[PF6]2 (yield ∼4%) and
[(AuCl)2(μ-3)] (yield ∼55%). Needle-shaped colorless crystals of
[(AuCl)2(μ-3)] were isolated after slow evaporation of an acetonitrile
solution.
2
3
2
3
1H, J = 11.0, J = 2.4, exo-H1/6), 4.31 (dd, 1H, J = 11.0, J = 4.7,
endo-H1/6), 5.04 (s, 1H, H3/4), 5.25 (m, 1H, H2/5), 7.76 (m, 2H, imid-
H), 9.13 (s, 1H, CNHCH). 1H NMR (400 MHz, CD3CN): δ 3.86 (s,
3H, NCH3), 4.27 (dd, 1H, 2J = 11.4, 3J = 2.4, exo-H1/6), 4.34 (dd, 1H,
2J = 11.4, 3J = 4.9, endo-H1/6), 5.02 (d, 3J ≈ 0, 1H, H3/4), 5.13 (ddd,
2J = 4.9, 3J = 2.4, 3J ≈ 0, 1H, H2/5), 7.42 (m, 2H, imid-H), 8.48 (s, 1H,
1
[Au2(μ-3)2][PF6]2. H NMR (400 MHz, CD3CN): δ 3.90 (s, 3H;
CH3), 4.44 (dd, 2J = 10.8, 3J = 6.0, 1H; endo-H1/6), 4.55 (dd, 2J = 10.8, 3
J = 3.3, 1H; exo-H1/6), 5.07 (d, 1H, 3J ≈ 0, H3/4), 5.18 (ddd, 3J = 6.0, 3J =
3.3, 3J ≈ 0, 1H; H2/5), 7.24 (d, 3J = 1.9, 1H; imid-H), 7.28 (d, 3J = 1.9,
1H; imid-H). 13C NMR (125 MHz, CD3CN): δ 38.19 (s, CH3), 65.98
(s, C2/5), 71.20 (s, C1/6), 87.816 (s, C3/4), 118.44 (s, CH), 123.52 (s,
CH), 183.72 (s, CNHC). 13C NMR (100 MHz, CD3CN): δ 37.48 (s,
CH3), 66.14 (s, C2/5), 71.46 (s, C1/6), 87.48 (s, C3/4), 118.20 (s, CH),
123.10 (s, CH), CNHC not observed. UVꢀvis (CH3CN, 0.42 mM):
λmax/nm (ε/103 Mꢀ1 cmꢀ1) 235 (25.9), 245 (28.7), 261 (26.9).
MS (accurate mass, ESþ, CH3CN): m/z (%) calcd mass for
[C28H36197Au2F6N8O4P]þ 1087.1833, measured 1087.1835.
C
NHCH). 13C NMR (125 MHz, d6-acetone): δ 36.86 (s, CH3), 66.21
(s, C2/5), 72.86 (s, C1/6), 88.15 (s, C3/4), 122.39 (s, imid-C), 125.37
(s, imid-C), 137.01 (s, CNHCH). UVꢀvis (CH3CN, 0.28 mM): λmax
/
nm (ε/103 Mꢀ1 cmꢀ1): 212 (7.4). IR (KBr) 839 (s), 1101 (s), 1172
(s), 1585 (m), 2912 (w), 3170 (sh, m), 3425 (br, w). MS (ES,
CH3CN): m/z 421.1229 (M ꢀ PF6þ; C14H20N4O2PF6 requires
421.1228).
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Organometallics
ARTICLE
[(AuCl)2(μ-3)]. 1H NMR (250 MHz, CD3CN): δ 3.79 (s, 3H; CH3),
4.23 (dd, 2J = 10.8, 3J =2.7, 1H;exo-H1/6), 4.35 (dd, 2JHH = 10.8, 3JHH =5.4,
1H; endo-H1/6), 4.93(d, 1H, 3J≈0, H3/4), 5.36 (ddd, 3J=5.4, 3J=2.7,3J≈0,
1H; H2/5), 7.24 (d, 3J = 1.9, 1H; imid-H), 7.28 (d, 3J = 1.9, 1H; imid-H).
13C NMR (100 MHz, CD3CN): δ 37.48 (s, CH3), 66.14 (s, C2/5), 71.46
(s, C1/6), 87.48 (s, C3/4), 118.20 (s, CH), 123.10 (s, CH), CNHC not
43, 4412. (e) Yam, V. W. W.; Chan, C. L.; Li, C. K.; Wong, K. M. C.
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)
231 (14.7), 235 (28.6), 248 (19.5). MS (ESþ, CH3CN/d6-DMSO): m/z
(%) calculated mass for [M ꢀ Cl]þ 703.04, measured 703.05 (80).
X-ray Crystallography. All single-crystal X-ray data were collected
at 150 K on a Bruker/Nonius Kappa CCD diffractometer using graphite-
monochromated Mo KR radiation (λ = 0.710 73 Å), equipped with an
Oxford Cryostream cooling apparatus. Crystal parameters and details of
the data collection, solution, and refinement are presented in Table 3.
The data were corrected for Lorentz and polarization effects and for
absorption using SORTAV.32 Structure solution was achieved by direct
methods (Sir-92 program system)33 and refined by full-matrix least
squares on F2 (SHELXL-97)34 with all non-hydrogen atoms assigned
anisotropic displacement parameters. Hydrogen atoms were placed in
idealized positions and allowed to ride on their parent atoms. In the final
cycles of refinement, a weighting scheme that gave a relatively flat analysis of
variance was introduced and refinement continued until convergence was
reached. Molecular structures in the figures were drawn with ORTEP 3.0 for
Windows (version 1.08)35 and X-Seed.36 The [Ag2(μ-3)2][PF6]2 and
[Au2(μ-3)2][PF6]2 structures contained large regions of diffusely scat-
tered electron density indicative of severely disordered solvent, which
could not be modeled. The SQUEEZE routine in PLATON37 was used
to remove the contributions of the disordered solvent from diffraction
intensities in order to improve the refinement of ordered parts within the
structures. SQUEEZE details have been appended to the CIF files. The
final F(000), calculated density, and MW values reflect known species
only. Crystallographic data for all compounds have been deposited with
the Cambridge Crystallographic Data Centre as supplementary publica-
tions CCDC 750149 for [H23][PF6]2, 750151 for [Ag2(μ-3)2][PF6]2,
750150 for [Au2(μ-3)2][PF6]2 and 808468 for [(AuCl)2(μ-3)]. Copies
of the data can be obtained free of charge on application to the CCDC,
12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (þ44) 1223 336033;
e-mail, deposit@ccdc.cam.ac.uk).
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’ ASSOCIATED CONTENT
S
Supporting Information. Text and figures giving mo-
b
lecular packing diagrams, a molar conductivity graph, and NMR
spectra and CIF files giving X-ray crystallographic data for the
compounds [H23][PF6]2, [Ag2(μ-3)2][PF6]2, [Au2(μ-3)2][PF6]2,
and [(AuCl)2(μ-3)]. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*A.D.: e-mail, dervisia@cardiff.ac.uk; tel, þ 44 (0)29 20874081;
fax, þ 44 (0)29 20874030.
’ ACKNOWLEDGMENT
We thank Johnson Matthey plc for the loan of hydrated KAuCl4.
(14) (a) Radloff, C.; Wiegard, J. J.; Hahn, F. E. Dalton Trans.
2009, 9392. (b) dit Dominique, F. J. B.; Gornitzka, H.; Sournia-Saquet,
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Eur. J. Inorg. Chem. 2009, 2835. (e) Samantaray, M. K.; Pang, K.; Shaikh,
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