Bis(1,2,3-triazol-5-ylidenes)(i-bitz) as stable 1,4-bidentate ligands based on mesoionic carbenes (MICs)

Article abstract of DOI:10.1021/om200844b

Direct metalation of bis(1,2,3-triazolium) salts affords mononuclear rhodium(I) complexes, which feature a 1,4-bidentate bis(1,2,3-triazol-5-ylidene) (i-bitz) ligand. The topology of the ligand is similar to that of 2,20-bipyridines (bpy) and their congen

Full text of DOI:10.1021/om200844b

ARTICLE
pubs.acs.org/Organometallics
Bis(1,2,3-triazol-5-ylidenes) (i-bitz) as Stable 1,4-Bidentate Ligands
Based on Mesoionic Carbenes (MICs)
Gregorio Guisado-Barrios, Jean Bouffard, Bruno Donnadieu, and Guy Bertrand*
UCR-CNRS Joint Research Chemical Laboratory, Department of Chemistry, University of California, Riverside, California 92521,
United States
S Supporting Information
b
ABSTRACT: Direct metalation of bis(1,2,3-triazolium) salts affords mono-
nuclear rhodium(I) complexes, which feature a 1,4-bidentate bis(1,2,3-
triazol-5-ylidene) (i-bitz) ligand. The topology of the ligand is similar to that
of 2,20-bipyridines (bpy) and their congeners, as well as bis(1,2,4-triazol-5-
ylidenes) (bitz). As the former, but in contrast to the latter, the free i-bitz can
be isolated, which paves the way for various applications.
’ INTRODUCTION
2,20-Bipyridines (bpy) and their congeners (Figure 1), such as
phenanthrolines, are privileged 1,4-bidentate ligands1 in struc-
tural inorganic chemistry, in catalysis, and more recently in light-
emitting devices and solar cells.2 Their electronic properties, and
Figure 1. Schematic representation of bpy, bitz, and i-bitz, showing the
the strength of the resulting metalꢀligand bonds, can be tuned,
topological analogy.
but only to a limited extent, by the introduction of substituents
on the heterocyclic ring. Therefore, readily accessible ligands,
featuring the same topology but with strikingly different electro-
Scheme 1. Synthesis of Bis(triazoles) 1aꢀc, Bis(triazolium)
nic properties, are highly desirable. In this regard, neutral ligands
Salts 2aꢀc, and i-bitzꢀRhodium Complexes 3b,c and 4b,c
bearing a lone pair at carbon are attractive candidates.3 Peris,
Crabtree, et al.4 introduced 4,40-bis(1,2,4-triazol-5-ylidenes)
(bitz) as N-heterocyclic carbene (NHC) equivalents of 2,20-
bipyridines. Here we report the preparation of rhodium(I)
complexes bearing a bis(1,2,3-triazol-5-ylidene) ligand (i-bitz)
and show that, in contrast with their bitz isomers, the metal-free
bidentate ligand can be isolated and structurally characterized.
’ RESULTS AND DISCUSSION
Recently, the coordination chemistry of 1,2,3-triazol-5-yli-
denes, which are mesoionic carbenes (MICs),5 has attracted
considerable interest.6,7 They feature electron-donor properties
stronger than those of their 1,2,4-triazolylidene isomers,8 as well
as of any other five-membered NHCs.9 Importantly, ligands can
only find numerous applications if they are readily available in
large quantities. This is the case for 1,2,3-triazol-5-ylidenes, as well as
bis(1,2,3-triazoles),10 which prompted us to tackle the synthesis of
metal complexes featuring bidentate 4,40-bis(1,2,3-triazol-5-ylidenes)
(i-bitz). The protonated precursors 2aꢀc are rapidly prepared
on a multigram scale following the route shown in Scheme 1.
Bis(triazoles) 1aꢀc are obtained in a single pot in 51ꢀ84% yield
from the corresponding aniline and 1,4-bis(trimethylsilyl)butadiyne.
Alkylation with methyl trifluoromethanesulfonate affords the bis-
(triazolium) salts 2aꢀc in 42ꢀ70% yield.
The direct metalation of bis(triazolium) salts 2aꢀc with
[Rh(cod)(OEt)]2, following the approach described for the
formation of bitzꢀRh complexes,4b was first explored. In the
case of bitz, attempts to chelate Rh(I) were unsuccessful:
only dinuclear Rh(I) and pseudo-octahedral mononuclear
Rh(III) species were obtained, depending on the conditions
employed. In contrast, when [Rh(cod)(OEt)]2 was reacted with
Received: September 7, 2011
Published: October 11, 2011
r
2011 American Chemical Society
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Scheme 2. Rearrangement of Transient bitz and Preparation
of Stable i-bitz 7aꢀc
Figure 2. Solid-state structure of 4c with 50% probability ellipsoids.
Hydrogens and the triflate anion are omitted for clarity. Selected
bond lengths (Å) and angles (deg): C1ꢀRh1 = 2.071(4), C4ꢀRh1 =
2.068(4), C1ꢀC2 = 1.399(5), C2ꢀC3 = 1.446(6), C3ꢀC4 =
1.391(5), C2ꢀN3 = 1.3501(5), C3ꢀN6 = 1.349(5), N3ꢀN2 =
1.324(5), N6ꢀN5 = 1.315(6), N2ꢀN1 = 1.350(4), N5ꢀN4 =
1.343(5), N1ꢀC1 = 1.353(5), N4ꢀC4 = 1.353(5); C1ꢀRh1ꢀC4 =
77.01(15), N1ꢀC1ꢀC2 = 101.9(3), N4ꢀC4ꢀC3 = 101.8(3),
C1ꢀC2ꢀN3 = 108.3(3), C4ꢀC3ꢀN6 = 108.2 (4), C2ꢀN3ꢀN2 =
111.4(3), C3ꢀN6ꢀN5
=
111.6(3), N3ꢀN2ꢀN1
= 103.7(3),
N6ꢀN5ꢀN4 = 103.6(3), N2ꢀN1ꢀC1 = 114.7(3), N5ꢀN4ꢀC4 =
114.7(4).
bis(triazolium) salts 2b,c, i-bitz-chelated Rh(I) complexes 3b,c
were obtained in 77ꢀ86% yield (Scheme 1).11 Addition of
carbon monoxide to 3b,c displaces the cycloctadiene ligand to
give complexes 4b,c.
Figure 3. Crystal structure of bis(1,2,3-triazol-5-ylidene) 7a with
ellipsoids shown at the 50% probability level. Selected bond lengths
(Å) and angles (deg): C1ꢀC2 = 1.395(2), C2ꢀC3 = 1.467(2), C3ꢀC4 =
1.400(2), N1ꢀC1 = 1.3791(19), N4ꢀC4 = 1.376(2), N1ꢀN2 =
1.3443(18), N4ꢀN5 = 1.3463(18), N2ꢀN3 = 1.3185(18), N5ꢀN6 =
1.3205(18), N3ꢀC2 = 1.378(2), N6ꢀC4 = 1.379(2); N1ꢀC1ꢀC2 =
99.59(14), N4ꢀC4ꢀC3 = 99.35(14), N3ꢀC2ꢀC1 = 109.16(13),
N6ꢀC3ꢀC4 = 109(13), N2ꢀN3ꢀC2 = 111.62(13), N5ꢀN6ꢀC3 =
111.39(13), N3ꢀN2ꢀN1 = 102.56(12), N4ꢀN5ꢀN6 = 102.58(12),
N2ꢀN1ꢀC1 = 117.06(13), N5ꢀN4ꢀC4 = 117.33(13).
According to a single-crystal X-ray diffraction study, complex
4c adopts a square-planar geometry (Figure 2). The RhꢀC
bonds (2.07 Å) are rather long, and the bite angle of 77.0° is very
similar to those found in bitzꢀ and bpyꢀRh complexes (78.6ꢀ79.9°
and 75.6ꢀ77.1°, respectively).4a,b,12
The donor properties of i-bitz were evaluated by measuring
the IR CO stretching frequencies of rhodium complexes 4b,c in
dichloromethane (νav(CO) = 2047 and 2048 cmꢀ1, respectively).
These values indicate that they are stronger electron donors than
bis(dicyclohexylphosphinoethane) (νav(CO) = 2059 cmꢀ1) and
phenantrolines (νav(CO) = 2069ꢀ2073 cmꢀ1).13 Note that, in
contrast, bitz has been calculated to be a weaker donor than bpy
and diphosphines.4b
139 °C). Single crystals of 7a suitable for X-ray analysis were
obtained from a toluene solution at ꢀ30 °C (Figure 3). The two
carbene centers adopt an antiperiplanar geometry with a torsion
angle (C1ꢀC2ꢀC3ꢀC4) of 166.1o. Both heterocycles are planar,
presenting bond lengths between those of single and double bonds,
and the carbene bond angles N(1)ꢀC(1)ꢀC(2) and N(4)ꢀC-
(4)ꢀC(3) (99.59 and 99.35o, respectively) are comparable to those
previously observed for 1,2,3-triazol-5-ylidenes.14
Direct metalation of protic carbene precursors offers a prac-
tical and expeditious synthesis of metal complexes but lacks
generality beyond some late transition metals, such as Rh. Ligand
substitution with the free carbenes offers a much broader scope but
requires carbenes that are sufficiently robust to show prolonged
lifetimes. Despite the excellent stability of 1,2,4-triazolylidenes,8 the
bitz ligand spontaneously rearranges into bitriazole 6 upon at-
tempted deprotonation of the bis(triazolium) salt 5 (Scheme 2),
limiting further applicability.4b Given the excellent stability of
MICs,14,15 we hypothesized that i-bitz 7aꢀc, linked by a strong
CꢀC bond, would not suffer from the fragility of the NꢀN linker
found in bitz. Gratifyingly, deprotonation of triazolium salts 2aꢀc
with KHMDS in diethyl ether or tetrahydrofuran at ꢀ78 °C allows
for the isolation of the free species 7aꢀc in 63ꢀ90% yield. The 1H
NMR spectra show the absence of the triazolium acidic proton,
located in the 9.3ꢀ10.2 ppm region, and in the 13C NMR spectra
there is a low-field signal (7a, 196.3 ppm; 7b, 200.9 ppm; 7c, 201.4
ppm) within the expected range for 1,2,3-triazol-5-ylidenes.14
Metal-free i-bitz 7aꢀc are stable in the solid state in the
absence of oxygen and moisture (mp: 7a, 134 °C; 7b, 129 °C; 7c,
’ CONCLUSIONS
In summary, bis(1,2,3-triazol-5-ylidenes) (i-bitz) are readily
available and can be isolated even with small flanking aryl groups
such as phenyl. They behave as 1,4-bidentate ligands, and the IR
CO stretching frequencies of the cationic (i-bitz)(CO)2RhI
complexes indicate that these novel chelating ligands are strong
electron donors. Research in our laboratory is currently under-
way to extend the scope of bis(mesoionic) carbenes and to
explore the properties of the corresponding complexes.
’ EXPERIMENTAL SECTION
Materials. Unless otherwise noted, all reagents including solvents
were obtained from commercial suppliers and used directly without
further purification. Anhydrous Et2O and THF were obtained after
distillation over sodiumꢀbenzophenone ketyl under an argon atmo-
sphere. Anhydrous toluene was obtained after distillation over sodium
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under an argon atmosphere. Anhydrous benzene and hexane were obtained
after distillation over potassium. Anhydrous CH2Cl2 and CH3CN were
obtained after distillation over calcium hydride under an argon atmosphere.
Column chromatography was performed with silica gel (32ꢀ63 μM).
General Methods, Instrumentation, and Measurements.
Synthetic manipulations that required an inert atmosphere (unless
otherwise noted) were carried out in flame-dried glassware equipped
with magnetic agitators under argon using standard Schlenk techniques
or in an inert-atmosphere glovebox. NMR (1H, 13C, and 19F) spectra
were recorded on 300, 400, and 500 MHz spectrometers. The chemical
shift data for each signal are given in units of δ (ppm) relative to
tetramethylsilane (TMS), where δ(TMS) = 0, and are referenced to the
residual solvent resonances. Splitting patterns are denoted as s (singlet),
d (doublet), t (triplet), q (quartet), sept (septet), m (multiplet), and br
(broad). IR spectra were recorded on a Bruker Equinox 55 FTIR
spectrometer. High-resolution mass spectra (HR-MS) were acquired on
an LC-TOF instrument using electrospray ionization mode (ESI). Melting
points (open or sealed capillaries) are reported without correction.
Synthesis of 1,10-Diphenyl-1H,10H-4,40-bis(1,2,3-triazole) (1a). The
title compound was prepared by combining two reported methods.16,10a
To a stirred solution of aniline (6.34 g, 68.2 mmol) in CH3CN (40 mL) at
0 °C, were added first tert-butyl nitrite (10.2 g, 99.0 mmol), then
trimethylsilyl azide (9.28 g, 80.6 mmol). The reaction mixture was
warmed to room temperature, while stirring was maintained for 2 h.
Afterward, 1,4-bis(trimethylsilyl)buta-1,3-diyne (6.27 g, 31.0 mmol),
pyridine (24.5 g, 310 mmol), potassium carbonate (8.5 g, 62.0 mmol),
(3.1 g, 12.4 mmol) and sodium ascorbate (4.9 g, 24.8 mmol) in H2O
(40 mL) was added. The mixture was stirred overnight at room
temperature. The solution was concentrated; a 9/1 DCM/NH4OH
solution mixture was added, and this mixture was stirred overnight.
The organic phase was extracted with dichloromethane, dried over
magnesium sulfate, and the solvent was evaporated to yield 1c as a
1
yellowish solid (11.6 g, 82%). Mp: 289 °C. H NMR (CDCl3, 25 °C,
300 MHz): δ 8.26 (s, 2 H, Htr), 7.55 (t, 2 H, Hdipp, J = 7.8 Hz), 7.34 (t, 4 H,
Hdipp, J = 7.8 Hz), 2.38 (s, 4 H, CH, J = 6.7 Hz), 1.20 (d, 24 H, CH3,
J = 6.81 Hz). 13C NMR (CDCl3, 25 °C, 75 MHz): δ 146.3, 140.0, 133.1,
131.2, 124.1, 123.8, 28.6, 24.4, 24.2. HR-MS (ESI, CH2Cl2): m/z calcd
for C28H36N6 [M + H] 457.3074, found 457.3096.
Synthesis of 3,30-Dimethyl-1,10-diphenyl-1H,10H-4,40-bis(1,2,3-triazolium)
Trifluoromethanesulfonate (2a). 3,30-Dimethyl-1,10-diphenyl-1H,10H-
4,40-bis(1,2,3-triazolium) trifluoromethanesulfonate 2a was prepared by
an adaptation of a reported method.14 To a stirred solution of 1,10-
diphenyl-1H,10H-4,40-bis(1,2,3-triazole) 1a (0.45 g, 1.57 mmol) in 1,2-
dichloroethane (20 mL), previously cooled to ꢀ78 °C, was added
methyl trifluoromethanesulfonate (0.57 g, 3.54 mmol). The reaction
mixture was warmed to room temperature, and then heated to 100 °C in
a sealed Schlenk flask for 48 h. The solvent was evaporated; addition of
diethyl ether prompted the precipitation of a white solid after stirring the
crude solution for 30 min. Afterward, the precipitate was washed with
hexane and diethyl ether. Crystals suitable for X-ray study were obtained
by slow solvent evaporation of a solution of 2a in acetone. Yield: 0.75 g
(97.6%). Mp: 251 °C. 1H NMR (d6-DMSO, 25 °C, 400 MHz): δ 10.18
(s, 2 H, Htr), 8.08 (s, 4 H, Har), 7.81 (s, 6 H, Har), 4.57 (s, 6 H, CH3). 13
C
and an aqueous solution containing CuSO4 5H2O (3.10 g, 1.24 mmol)
3
NMR (d6-DMSO, 25 °C, 100 MHz): δ 134.5, 132.4, 131.25, 130.6, 127.4,
121.9, 39.8. HR-MS (ESI, acetone): m/z calcd for C19H18F3N6O3S+
467.1108 [M ꢀ OTf], found 467.1102.
and sodium ascorbate (4.90 g, 2.48 mmol) in H2O (40 mL) were added.
The mixture was stirred overnight at room temperature. The solution
was concentrated. A 9/1 DCM/NH4OH solution mixture was added to
the concentrate, and this mixture was stirred overnight. The organic
phase was extracted with dichloromethane, dried over magnesium
sulfate, and the solvent was evaporated to yield 1a as an off-white solid
Synthesis of 1,10-Dimesityl-3,30-dimethyl-1H,10H-4,40-bis(1,2,
3-triazolium) Trifluoromethanesulfonate (2b). 1,10-Dimesityl-3,30-di-
methyl-1H,10H-4,40-bis(1,2,3-triazolium) trifluoromethanesulfonate 2b was
prepared by an adaptation of a reported method.14 To a stirred solution
of 1,10-dimesityl-1H,10H-4,40-bis(1,2,3-triazole) 1b (4.7 g, 12.6 mmol)
in DCE (20 mL), previously cooled to ꢀ78 °C, was added methyl
trifluoromethanesulfonate (4.58 g, 27.7 mmol). The reaction mixture
was warmed to room temperature, then heated to 100 °C in a sealed
Schlenk flask for 48 h. The solvent was evaporated; addition of diethyl
ether prompted the precipitation of a white solid after stirring the crude
solution for 30 min. Afterward, the precipitate was washed with hexane
and diethyl ether. Yield: 6.11 g (70%). Mp: 274 °C. 1H NMR (CD3CN,
25 °C, 400 MHz): δ 9.33 (s, 2 H, Htr), 7.35 (s, 4 H, Har), 4.58 (s, 6 H,
CH3), 2.52 (s, 6 H, CH3), 2.04 (m, 6 H, CH3). 13C NMR (CD3CN,
25 °C, 75 MHz): δ 144.3, 135.6, 131.1, 130.8, 128.5, 117.0, 41.4, 21.2,
17.4. HR-MS (ESI, acetone): m/z calcd for C25H30F3N6O3S+ 551.2047
(M ꢀ OTf), found 551.2060.
1
(4.58 g, 51.3%). Mp: 251 °C, H NMR (CDCl3, 25 °C, 300 MHz):
δ 8.57 (s, 2 H, Htr), 7.83 (d, 4H, J = 7.95 MHz), 7.57 (m, 4H), 7.48 (m,
2H). 13C NMR (CDCl3, 25 °C, 75 MHz): δ 140.6, 137.1, 130.1, 129.2,
120.7, 119.0. HR-MS (ESI, CH2Cl2): m/z calcd for C16H12N6 289.1196,
found 289.1200.
Synthesis of 1,10-Dimesityl-1H,10H-4,40-bis(1,2,3-triazole) (1b). The
title compound was prepared by following the reported method.10a In a
250 mL flask containing 60 mL of a 1/1 tert-butyl alcohol/water mixture
were added 2-azido-1,3,5-trimethylbenzene (5.0 g, 31.0 mmol),17,18 1,4-
bis(trimethylsilyl)buta-1,3-diyne (3.01, 15.5 mmol), pyridine (12.26 g,
15.5 mmol), potassium carbonate (4 g, 28.9 mmol), CuSO4 5H2O (1.44 g,
3
6.21 mmol), and sodium ascorbate (2.45 g, 12.4 mmol) at 0 °C. The
mixture was stirred overnight at room temperature. A 9/1 DCM/
NH4OH solution mixture was added, and this mixture was stirred
overnight. The organic phase was extracted with dichloromethane,
dried over magnesium sulfate, and the solvent was evaporated to yield
1b as a yellowish solid (4.88 g, 84.5%). Mp: 314 °C. 1H NMR
(CDCl3, 25 °C, 300 MHz): δ 8.20 (s, 2 H, Htr), 7.04 (s, 4 H, Har), 2.38
(s, 6 H, CH3), 2.05 (s, 12 H, CH3). 13C NMR (CDCl3, 25 °C, 75
MHz): δ 140.4, 140.2, 135.3, 133.5, 129.4, 122.8, 21.3, 17.5. HR-MS
(ESI, CH2Cl2): m/z calcd for (C22H24N6 [M + Na] 395.1955, found
395.1974.
Synthesis of 1,10-Bis(2,6-diisopropylphenyl)-3,30-dimethyl-1H,10H-4,
40-bis(1,2,3-triazolium) Trifluoromethanesulfonate (2c). 1,10-Bis(2,6-
diisopropylphenyl)-3,30-dimethyl-1H,10H-4,40-bis(1,2,3-triazolium) tri-
fluoromethanesulfonate 2c was prepared by following the reported
method.14 To a stirred solution of 1,10-bis(2,6-diisopropylphenyl)-
1H,10H-4,40-bis(1,2,3-triazole) 1c (5.0 g, 10.9 mmol) in DCM (20 mL),
previously cooled to ꢀ78 °C, was added methyl trifluoromethanesulfo-
nate (3.97 g, 24.1 mmol). The reaction mixture was warmed to room
temperature, and stirring was maintained overnight. The solvent was
evaporated; addition of diethyl ether prompted the precipitation of a
white solid after stirring the crude mixture for 30 min, which was filtered
and washed with hexane. The title compound 2c was obtained as a white
solid after column chromatography (DCM/MeOH 95/5). Yield: 5.72 g
(66.7%). Mp: 265 °C. 1H NMR (CDCl3, 25 °C, 300 MHz): δ 9.61 (s,
2 H, Htr) 7.67 (t, 2 H, Hdipp, J = 7.8 Hz), 7.36 (t, 4 H, Hdipp, J = 7.9 Hz),
4.54 (s, 6 H, CH3), 2.29 (s, 4 H, CH, J = 6.7 Hz), 1.10 (m, 24 H, CH3, J =
6.63 Hz). 13C NMR (CDCl3, 25 °C, 75 MHz): δ 145.9, 135.0, 133.5,
Synthesis of 1,10-Bis(2,6-diisopropylphenyl)-1H,10H-4,40-bis(1,2,
3-triazole) (1c). The title compound was prepared by combining two
reported methods.16,10a To a stirred solution of diisopropylaniline
(12.08 g, 68.2 mmol) in CH3CN (40 mL) at 0 °C, were added first
tert-butyl nitrite (10.2 g, 99.0 mmol), then trimethylsilyl azide (9.28 g,
80.6 mmol). The reaction mixture was warmed to room temperature, while
stirring was maintained for 2 h. Afterward, 1,4-bis(trimethylsilyl)buta-
1,3-diyne (6.03 g, 31.0 mmol), pyridine (24.5 g, 310 mmol), potassium car-
bonate (8.5 g, 62 mmol), and an aqueous solution containing CuSO4 5H2O
3
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130.4, 128.0, 126.8, 124.9, 40.6, 23.9, 23.8. HR-MS (ESI, CH2Cl2): m/z
calcd for C31H42F3N6O3S+ [M ꢀ OTf] 635.2986, found 635.2678.
Synthesis of [RhI(cod)(7b)]+OTfꢀ (3b). The metal complex
[RhI(cod)(7b)]+OTfꢀ 3b was prepared by following slight modifica-
tions of the reported methods.19,20 NaH (24.32 mg, 1.02 mmol) was
dissolved in EtOH (5 mL), and the resulting solution was trans-
ferred via cannula to a supension of [RhCl(cod)]2 (100 mg, 0.20
mmol) in EtOH (5 mL). The reaction mixture was stirred for 30 min
at room temperature. 1,10-Dimesityl-3,30-dimethyl-1H,10H-4,40-bis-
(1,2,3-triazolium) trifluoromethanesulfonate 2b (294.2 mg, 0.42 mmol)
was added to the reaction mixture at ꢀ78 °C, warmed to room
temperature after 10 min, and then stirred for another 12 h. Solvent
was evaporated; dichloromethane was added to dissolve the product, and
the mixture was filtered. After removal of the solvent, 3b was obtained as
an orange solid(256.5 mg, 77.3%). Mp: 223 °C. 1H NMR (CDCl3, 25 °C,
500 MHz): δ 6.98 (s, 2H, Har), 4.68 (s, 6 H, CH3), 4.09 (s, 4H, CHcod),
2.37 (s, 6H, CH3), 2.11 (s, 12H, CH3), 2.00 (m, 4 H, CH2 cod), 1.73 (m, 4 H,
CH2 cod), 1.13 (d, 12 H, CH3, J = 6.5 Hz). 13C NMR (CDCl3, 25 °C, 125
MHz): δ 172.9 (d, 1JRh,C = 40 Hz, NCC), 143.4, 141.2, 134.7, 134.4, 129.6,
88.0, 40.5, 31.0, 21.5, 17.6. 19F NMR (CDCl3, 25 °C, 376 MHz) δ ꢀ78.8.
HR-MS (ESI, CH2Cl2): m/z calcd for C32H40N6Rh+ [M+] 611.2364, found
611.2383.
13C NMR (CDCl3, 25 °C, 100 MHz): δ 187.1 (d, 1JRh,C = 57.1 Hz, CO),
174.8 (d, 1JRh,C = 45.3 Hz, NCC), 145.8, 143.9, 132.1, 128.5, 124.4, 40.8,
28.6, 24.5, 23.8. 19F NMR(CDCl3, 25 °C, 376 MHz): δ ꢀ79.2; MS (ESI,
CH2Cl2): m/z calcd for C32H40N6O2Rh+ (M+) 643.2262, found 643.2333.
IR (CH2Cl2, cmꢀ1): ν 2076 (ν(CO)), 2020 (ν(CO)).
Synthesis of 4,40-Bis(1-phenyl-3-methyl-1H-1,2,3-triazol-5-ylidene)
(7a). In a flame-dried Schlenk flask were added 1,10-diphenyl-3,30-dimethyl-
1H,10H-4,40-bis(1,2,3-triazolium) trifluoromethanesulfonate 2b (0.400 g,
0.65 mmol) and potassium bis(trimethylsilyl)amide (0.280 g, 1.43
mmol). The Schlenk flask was cooled to ꢀ78 °C, and then tetrahydrofuran
(15 mL) was added. After 10 min, the cold bath was removed; the
reaction mixture was warmed to room temperature. After 1 h the solvent
was removed under vacuum. The metal-free 7a was extracted by
trituration in dry benzene (2 ꢁ 10 mL) and the extract filtered via
cannula. Solvent was evaporated to yield 7a as a purple-white solid
(0.184 g, 90%). Note: colorless crystals suitable for X-ray analysis were
obtained after dissolving the title compound in toluene at room
temperature and slowly cooling the solution to ꢀ30 °C under an argon
atmosphere. Mp: 134 °C. 1H NMR (C6D6, 25 °C, 300 MHz): δ 8.71
(d, 4 H, J = 7.86 Hz, Har), 7.34 (m, 4 H, Har), 7.21 (t, 2 H, J = 7.29 Hz,
Har), 4.6 (s, 6 H, CH3). 13C NMR (C6D6, 25 °C, 75 MHz): δ 196.0,
142.8, 129.5, 128.7, 121.7, 39.1.
Synthesis of 4,40-Bis(1-mesityl-3-methyl-1H-1,2,3-triazol-5-ylidene)
(7b). In a flame-dried Schlenk flask were added 1,10-dimesityl-3,30-dimethyl-
1H,10H-4,40-bis(1,2,3-triazolium) trifluoromethanesulfonate 2b (0.4 g,
0.57 mmol) and potassium bis(trimethylsilyl)amide (0.250 g, 1.25 mmol).
The Schlenk was cooled to ꢀ78 °C, and then diethyl ether (7 mL) was
added. After 10 min the cold bath was removed, and the reaction mixture
was warmed to room temperature. After 1 h the solvent was removed
under vacuum. Dry benzene (15 mL) was added to the crude mixture,
and the free carbene was extracted by filtration via cannula. Afterward,
solvent was removed under vacuum to yield 7b as an off-white solid
(0.186 g, 81%). Mp: 129 °C. 1H NMR (C6D6, 25 °C, 300 MHz): δ 6.88
(s, 4 H, Har), 4.71 (s, 6 H, CH3), 2.26 (s, 12 H, CH3), 2.22 (d, 12 H,
CH3). 13C NMR (C6D6, 25 °C, 75 MHz): δ 200.9, 141.8, 140.2, 139.0,
135.3, 129.7, 39.5, 21.6, 18.4.
Synthesis of [RhI((cod)(7c)]+OTfꢀ (3c). The metal complex
[RhI(cod)(7c)]+OTfꢀ was prepared by following slight modifications
of the reported methods.19,20 NaH (12.16 mg, 0.51 mmol) was dissolved
in EtOH (5 mL), and the resulting solution was transferred via cannula to
a suspension of [RhCl(cod)]2 (50 mg, 0.10 mmol) in EtOH (5 mL).
The reaction mixture was stirred for 30 min at room temperature.
1,10-Bis(2,6-diisopropylphenyl)-3,30-dimethyl-1H,10H-4,40-bis(1,2,3-
triazolium) trifluoromethanesulfonate 2c (164 mg, 0.21 mmol) was
added to the reaction mixture at ꢀ78 °C, warmed to room temperature
after 10 min, and then stirred overnight for 12 h. Solvent was evaporated,
dichloromethane was added to dissolve the product, and the mixture was
filtered. After removal of the solvent, 3c was obtained as an orange solid
(158 mg, 86%). Mp: 239 °C. 1H NMR(CDCl3, 25 °C, 500 MHz): δ 7.50
(t, 2H, Hdipp, J = 7.5 Hz), 7.28 (d, 4H, Hdipp, J = 8.0 Hz), 4.71 (s, 6 H,
CH3), 4.02 (s, 4 H, CHcod), 2.49 (4 H, CHdipp, J = 7.0 Hz), 1.87 (m, 4 H,
CH2 cod), 1.72 (m, 4 H, CH2 cod), 1.33 (d, 12 H, CH3, J = 6.5 Hz), 1.13
(d, 12 H, CH3, J = 6.5 Hz). 13C NMR(CDCl3, 25 °C, 125 MHz): δ 172.1
Synthesis of 4,40-Bis(1-(2,6-diisopropylphenyl)-3-methyl-1H-1,2,3-
triazol-5-ylidene) (7c). In a flame-dried Schlenk flask were added 1,10-
bis(2,6-diisopropylphenyl)-3,30-dimethyl-1H,10H-4,40-bis(1,2,3-triazolium)
trifluoromethanesulfonate 2c (0.3 g, 0.38 mmol) and potassium bis-
(trimethylsilyl)amide (0.161 g, 0.84 mmol). The Schlenk flask was cooled
to ꢀ78 °C, and then diethyl ether (7 mL) was added. After 10 min the
dry iceꢀacetone bath was removed. The reaction mixture was warmed
to room temperature and stirred for 1 h. Volatiles were removed under
vacuum. The free carbene was extracted by filtration via cannula.
Afterward, solvent was removed under vacuum to afford 7c as an off-
white solid (115 mg, 62%). Mp: 139 °C. 1H NMR (C6D6, 25 °C, 300
MHz): δ 7.39 (m, 2 H, Hdipp), 7.29 (t, 4 H, Hdipp, J = 8.5 Hz), 4.70 (s, 6
H, CH3), 2.85 (s, 4 H, CH, J = 6.8 Hz), 1.32 (d, 12 H, CH3, J =
6.6 Hz), 1.25 (d, 12 H, CH3, J = 6.7 Hz). 13C NMR (C6D6, 25 °C,
100 MHz): δ 201.4, 145.5, 140.4, 139.3, 129.7. 129.3, 124.7, 39.3, 29.2,
24.6, 24.5.
1
(d, JRh,C = 41.2 Hz, NCC), 145.4, 142.9, 134.3, 131.8, 124.2, 87.9,
55.6, 30.7, 28.8, 25.7, 22.4. HR-MS (ESI, CH2Cl2): m/z calcd for
C40H58N6Rh+ 695.3303 [M+], found 695.3354.
Synthesis of [Rh(CO)2(7b)]+OTfꢀ (4b). The preparation of
[Rh(CO)2(7b)]+OTfꢀ was carried out by following a procedure similar
to the reported method.14 In a solution of [Rh(cod)(7b)]+OTfꢀ (256
mg, 0.32 mmol) in chloroform (5 mL) was bubbled CO over 30 min.
Evaporation of the solvent afforded 4b as a yellow solid (230 mg, 99%).
Mp: 199 °C. 1H NMR (CDCl3, 25 °C, 300 MHz): δ 7.02 (s, 4, Har), 4.73
(s, 6 H, CH3), 2.36 (6 H, CH3), 2.07 (s, 12 H, CH3). 13C NMR (CDCl3,
25 °C, 75 MHz): δ 187.3 (s, CO), 173.6 (d, 1JRh,C = 44.0 Hz, NCC),
144.3, 141.6, 135.0, 134.2, 128.8, 128.2, 40.5, 28.2, 21.4, 17.6, 19F NMR
(CDCl3, 25 °C, 282 MHz): δ ꢀ78.8. MS (ESI, CH2Cl2): m/z calcd for
C26H28N6O2Rh+ [M+] 559.1323, found 559.1392. IR (CH2Cl2, cmꢀ1):
ν 2076 (ν(CO)), 2019 (ν(CO)).
’ ASSOCIATED CONTENT
Synthesis of [Rh(CO)2(7c)]+OTfꢀ (4c). The preparation of
[Rh(CO)2(7c)]+OTfꢀ was carried out by following an adaptation of a
reported method.14 The red powder [Rh(cod)(7c)]+OTfꢀ 3c (159 mg,
0.18 mmol) was suspended in chloroform (5 mL). The solution was
bubbled with CO over 30 min. Evaporation of the solvent yielded 4c as a
yellow solid (55 mg, 60%). Crystals suitable for X-ray diffraction were
obtained in a glovebox by slow evaporation of the solvent of a solution of
4c in CDCl3. Mp: 233 °C. 1H NMR (CDCl3, 25 °C, 400 MHz): δ 7.47
(t, 2H, Hdipp, J = 7.6 MHz), 7.27 (d, 4H, Hdipp, J = 7.6 MHz), 4.74 (s, 6 H,
CH3), 2.32 (4 H, CH, J = 6.8 Hz), 1.14 (dd, 24 H, CH3, J = 6.4 Hz).
S
Supporting Information. NMR spectra for all new com-
b
pounds, and X-ray crystallographic data for 2a, 4c, and 7a.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: guy.bertrand@ucr.edu.
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dx.doi.org/10.1021/om200844b |Organometallics 2011, 30, 6017–6021

Organometallics
ARTICLE
’ ACKNOWLEDGMENT
Fratila, R. M.; Balentova, E.; Sagartzazu-Aizpurua, M.; Miranda, J. I. Org.
Lett. 2010, 12, 1584.
(11) Direct metalation of bis(triazolium) 2a gave rise to complex
mixtures.
(12) (a) Dadci, L.; Elian, H.; Frey, U.; H€ornig, A.; Koelle, U.; Merbach,
A. E.; Paulus, H.; Schneider, J. S. Inorg. Chem. 1995, 34, 306. (b) Ogo, S.;
Hayashi, H.; Uehara, K.; Fukuzumi, S. Appl. Organomet. Chem. 2005,
19, 639. (c) Riis-Johannessen, T.; Schenk, K.; Severin, K. Inorg. Chem. 2010,
49, 9546.
(13) (a) Del Zotto, A.; Costella, L.; Mezzetti, A.; Rigo, P. J. Organomet.
Chem. 1991, 414, 109. (b) Clauti, G.; Zassinovich, G.; Mestroni, G. Inorg.
Chim. Acta 1986, 112, 103.
(14) 1,2,3-Triazol-5-ylidenes have recently been isolated: Guisado-
Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int.
Ed. 2010, 49, 4759.
(15) Other types of MICs have been isolated: (a) Lavallo, V.; Dyker,
C. A; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5411.
(b) Fernꢀandez, I.; Dyker, C. A.; DeHope, A.; Donnadieu, B.; Frenking,
G.; Bertrand, G. J. Am. Chem. Soc. 2009, 131, 11875. (c) Aldeco-Perez,
E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.;
Bertrand, G. Science 2009, 326, 556. (d) Ung, G.; Bertrand, G. Chem. Eur.
J. 2011, 17, 8269.
(16) Barral, K.; Moorhouse, A. D.; Moses, J. E. Org. Lett. 2007,
9, 1809. Note: the reaction leading to the formation of 2,6-diisopropyl-
phenyl azide is highly exothermic following a variable induction period,
which can lead to a runaway reaction and pose a safety hazard (explosion).
Consequently, we advise caution in performing this reaction, especially
on a large scale. In particular, even when the reaction is performed at
room temperature, it should be conducted with a large heat sink (e.g.
flask immerged in a water bath at room temperature) and with efficient
heat transfer (e.g. rapid and efficient stirring, oversized glassware).
(17) Ugi, I.; Perlinger, H.; Behringer, L. Chem. Ber. 1958, 91, 2330.
(18) Bochmann, M.; Sarsfield, M. J. (BP Chemicals Limited, U.K.)
Application: WO 2000047592 A1.
Thanks are due to the NIH (R01 GM 68825) and DOE (DE-
FG02-09ER16069) for financial support and to FQRNT for a
postdoctoral fellowship to J.B.
’ REFERENCES
(1) (a) Smith, A. P.; Fraser, C. L. Bipyridine Ligands. In Compre-
hensive Coordination Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.;
Elsevier: Oxford, U.K., 2004; pp 1ꢀ23. (b) Kaes, C.; Katz, A.; Hosseini,
M. W. Chem. Rev. 2000, 100, 3553. (c) Accorsi, G.; Listorti, A.; Yoosaf.,
K.; Armaroli, N. Chem. Soc. Rev. 2009, 38, 1690.
(2) (a) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.
Chem. Rev. 2010, 110, 6595. (b) Nazeeruddin, M. K.; Gr€atzel, M. Transition
Metal Complexes for Photovoltaic and Light Emitting Applications. In
Photofunctional Transition Metal Complexes; Yam, V. W. W., Ed.; Springer-
Verlag: Berlin, 2007; Structure and Bonding Series 123; pp 113ꢀ175. (c)
Dixon, I. M.; Lebon, E.; Sutra, P.; Igau, A. Chem. Soc. Rev. 2009, 38, 1621.
(3) For recent books and reviews, see for example: (a) N-Hetero-
cyclic Carbenes, From Laboratory Curiosities to Efficient Synthetic Tools;
Díez-Gonzꢀalez, S., Ed.; Royal Society of Chemistry: London, 2011. (b)
N-Heterocyclic Carbenes in Transition Metal Catalysis; Glorius, F. A., Ed.;
Springer-Verlag: Berlin, 2007. (c) N-Heterocyclic Carbenes in Synthesis;
Nolan, S. P., Ed.; Wiley-VCH: Weinheim, Germany, 2006. (d) Melaimi,
M.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed. 2010, 49, 8810.
(e) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122.
(4) (a) Poyatos, M.; McNamara, W.; Incarvito, C.; Peris, E.;
Crabtree, R. H. Chem. Commun. 2007, 2267. (b) Poyatos, M.; McNamara,
W.; Incarvito, C.; Clot, E.; Peris, E.; Crabtree, R. H. Organometallics 2008,
27, 2128. (c) Corberꢀan, R.; Mas-Marzꢀa, E.; Peris, E. Eur. J. Inorg. Chem.
2009, 1700. (d) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009,
109, 3677.
(5) For the origin of the acronym MIC, see: Araki, S.; Wanibe, Y.;
Uno, F; Morikawa, A.; Yamamoto, K.; Chiba, K.; Butsugan, Y. Chem. Ber
1993, 12, 1149.
(6) For reviews on MICs and related species, see: (a) Schuster, O.;
Yang, L.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009,
109, 3445. (b) Albrecht, M. Chem. Commun. 2008, 31, 3601. (c) Arnold,
P. L.; Pearson, S. Coord. Chem. Rev. 2007, 251, 596. (d) Albrecht, M.
Chimia 2009, 63, 105.
(19) Riederer, S. U.; Gigler, P.; Hoegerl, M.; Herdtweck, E. B.;
Bechlars, B.; Herrmann, W. A.; Kuehn, F. E. Organometallics 2010,
29, 5681.
(20) Koecher, C.; Herrmann, W. A. J. Organomet. Chem. 1997,
532, 261.
(7) For recent reports on the coordination chemistry of 1,2,3-triazol-
5-ylidenes, see: (a) Mathew, P.; Neels, A.; Albrecht, M. J. Am. Chem. Soc.
2008, 130, 13534. (b) Karthikeyan, T.; Sankararaman, S. Tetrahedron
Lett. 2009, 50, 5834. (c) Nakamura, T.; Ogata, K.; Fukuzawa, S.-i. Chem.
Lett. 2010, 39, 920. (d) Lalrempuia, R.; McDaniel., N. D.; M€uller-Bunz,
H.; Bernhard, S.; Albrecht, M. Angew. Chem., Int. Ed. 2010, 49, 9765.
(e) Kilpin, K. J.; Paul, U. S. D.; Lee, A.-L.; Crowley, J. D. Chem. Commun.
2011, 47, 328. (f) Nakamura, T.; Terashima, T.; Ogata, K.; Fukuzawa,
S.-i. Org. Lett. 2011, 13, 620. (g) Prades, A.; Peris, E.; Albrecht, M.
Organometallics 2011, 30, 1162. (h) Bouffard, J.; Keitz, B. K.; Tonner, R.;
Guisado-Barrios, G.; Frenking, G.; Grubbs, R. H.; Bertrand, G. Organo-
metallics 2011, 30, 2617. (i) Keitz, B. K; Bouffard, J.; Bertrand, G.;
Grubbs, R. H. J. Am. Chem. Soc. 2011, 133, 8498. (j) Saravanakumar, R.;
Ramkumar, V.; Sankararaman, S. Organometallics 2011, 30, 1689.
(k) Schulze, B.; Escudero, D.; Friebe, C.; Siebert, R.; G€orls, H.; K€ohn,
U.; Altunas, E.; Baumgaertel, A.; Hager, M. D.; Winter, A.; Dietzek, B.;
Popp, J.; Gonzꢀalez, L.; Schubert, U. S. Chem. Eur. J. 2011, 17, 5494.
(l) Bernet, L.; Lalrempuia, R.; Ghattas, W.; Mueller-Bunz, H.; Vigara, L.;
Llobet, A.; Albrecht, M. Chem. Commun. 2011, 47, 8058. (m) Poulain,
A.; Canseco-Gonzalez, D.; Hynes-Roche, R.; M€uller-Bunz, H.; Schuster,
O.; Stoeckli-Evans, H.; Neels, A.; Albrecht, M. Organometallics 2011,
30, 1021.
(8) Enders, D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles, J. H.;
Melder, J. P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed. 1995, 34, 1021.
(9) Dr€oge, T.; Glorius, F. Angew. Chem., Int. Ed. 2010, 49, 6940.
(10) (a) Fletcher, J. T.; Bumgarner, B. J.; Engels, N. D.; Skoglund,
D. A. Organometallics 2008, 27, 5430. (b) Aizpurua, J. M.; Azcune, I.;
6021
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