Synthesis and structure of silver and rhodium 1,2,3-triazol-5-ylidene mesoionic carbene complexes

Article abstract of DOI:10.1021/om201104f

New mono- and bidentate 1,2,3-triazol-5-ylidene mesoionic carbene (MIC) ligands have been synthesized and reacted with silver oxide, forming cationic 1:2 and 2:2 Ag:MIC ([AgL₂]Br and [Ag₂L₂] [Ag₂Br₄]) complexes, respectively. These complexes have been isolated and characterized. They were further found to be potent transmetalation agents toward rhodium, resulting in the preparation of mono- and bimetallic rhodium complexes. The reaction of the 2:2 Ag:L complex with [Rh(COD)₂]BF₄ results in the first reported example of a cationic bimetallic rhodium MIC complex. This complex was fully characterized, and its X-ray structure is reported. Interestingly, this complex exhibits various secondary noncovalent interactions in the solid state of the general type C-HX, where X = F, Cl.

Full text of DOI:10.1021/om201104f

Article
pubs.acs.org/Organometallics
Synthesis and Structure of Silver and Rhodium
1,2,3-Triazol-5-ylidene Mesoionic Carbene Complexes
Eric C. Keske, Olena V. Zenkina, Ruiyao Wang, and Cathleen M. Crudden*
Department of Chemistry, Queen’s University, Chernoff Hall, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6
S
* Supporting Information
ABSTRACT: New mono- and bidentate 1,2,3-triazol-5-ylidene mesoionic carbene (MIC) ligands have been synthesized and
reacted with silver oxide, forming cationic 1:2 and 2:2 Ag:MIC ([AgL2]Br and [Ag2L2][Ag2Br4]) complexes, respectively. These
complexes have been isolated and characterized. They were further found to be potent transmetalation agents toward rhodium,
resulting in the preparation of mono- and bimetallic rhodium complexes. The reaction of the 2:2 Ag:L complex with
[Rh(COD)2]BF4 results in the first reported example of a cationic bimetallic rhodium MIC complex. This complex was fully
characterized, and its X-ray structure is reported. Interestingly, this complex exhibits various secondary noncovalent interactions
in the solid state of the general type C−H···X, where X = F, Cl.
transmetalation from a Ag-MIC complex.13 Although a wide
INTRODUCTION
N-heterocyclic carbenes (NHCs) have been shown to be
■
variety of silver NHC complexes with remarkable structural
diversity have been reported,13b,c until very recently, there were
no reports of structurally characterized Ag triazole-MIC
complexes.11a
powerful ligands in catalytic transformations1 and to provide
stabilization of reactive metal centers.2 As a result, the deve-
lopment of novel NHC ligands that vary in either their electro-
nic or their steric properties and the synthesis of late transition-
metal complexes of these ligands has been a highly active field
of chemistry.3 In comparison, heterocyclic carbenes that are
stabilized by only one heteroatom have received significantly
less attention.4 Because no neutral canonical representation of
these species is possible, these ligands are often termed abnormal
N-heterocyclic carbenes (aNHCs), or mesoionic carbenes
(MICs). In general, the decreased heteroatom stabilization of
these ligands results in stronger donation toward a metal center
than even the very basic imidazole-derived NHCs. As a result, the
properties of the carbenes themselves and their complexes are
being extensively studied by the groups of Bertrand,5,6 Albrecht,7,8
and others.9 Yet still, MICs represent a class of largely unexploited
ligands in homogeneous catalysis.6b,c,7c,8b,d,e,10,11
During the preparation of this paper, Bielawski and Sessler
reported the structure of the first 1,2,3-triazolylidene silver
complex in a 1:2 ([L2Ag4]) ligand-to-metal ratio.11a The ligand
in question was functionalized with a pyrrole wingtip group that
was also coordinated to silver. This complex underwent trans-
metalation to ruthenium to form a bimetallic complex that was
found to be active in ring-opening metathesis polymerization
(ROMP) reactions. Similarly, Schubert and co-workers have
reported the synthesis of a mixture of 1,2,3-triazolylidene silver
complexes that were transmetalated to ruthenium to give a
remarkable CNC pincer framework.12c Interestingly, this complex
displayed vibrant photophysical properties, further illustrating
the potential of triazole MIC complexes outside of catalysis. These
encouraging results, along with the remarkable strides being
made in the synthesis of these ligands and their complexes by the
Bertrand group,6 suggest that triazole MIC complexes have signifi-
cant value in a variety of applications.
Albrecht and co-workers have recently described a facile syn-
thesis of MICs based on 1,2,3-triazolium salts and investigated
their coordination properties with several transition metals.8a
Since this report, several examples have appeared of transition-
metal complexes utilizing these ligands.11,12 However, the free
1,2,3-triazol-5-ylidene carbenes tend to be unstable in com-
parison with traditional NHCs, which has limited the study of
their coordination chemistry. Because of this, the coordination
of these ligands to a metal is generally achieved through
We have been actively studying the preparation of rhodium
N-heterocyclic carbene complexes,14 including their catalytic
activity,15 their ability to activate small molecules,16,17 and their
Received: November 9, 2011
Published: December 16, 2011
© 2011 American Chemical Society
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crystal-to-crystal transformations.17 Although monomeric Rh
NHC complexes have shown significant promise as catalysts for
organic transformations,14a,18 homobimetallic NHC complexes
of rhodium are also interesting, as they have been demonstrated
to be active and potentially important species in both the
hydrosilylation of alkynes19 and the hydroformylation of
alkenes.20 Furthermore, they have been utilized in the fabri-
cation of perforated organometallic nanotubes.21 Interestingly,
all examples that we are aware of involve coordination through
traditional imidazole-based NHCs, and no work has appeared
utilizing more strongly donating triazole MIC ligands.19−22
To address this deficiency, we have prepared bimetallic
rhodium carbene complexes coordinated to triazole MICs in
the hopes that their stronger electronic donation, and facile
steric tuning, may lead to rhodium complexes that are highly
active in a variety of organic transformations. Herein, we report
the synthesis and structural characterization of cationic 1:2
and 2:2 Ag:L silver triazole MIC complexes and their
transmetalation to form monometallic ([Rh(L)(COD)Br])
and cationic homobimetallic ([L1Rh2][BF4]2) rhodium triazole
MIC complexes, which have been fully characterized.
bulk around the metal center. The synthesis of bistriazole 5
was easily achieved by the cycloaddition of 2,6-diisopropylphenyl
(Dipp) azide 3 with a protected version of 1,3-diethynyl
benzene.26b In a procedure similar to that described by
Moses,27 azide 3 could be prepared in situ and reacted directly
with TMS-protected acetylene 4 in the presence of TBAF under
standard conditions (CuSO4 and sodium ascorbate) (Scheme 1).
The optimized procedure involves three reactions performed
in one pot in good overall yield (63%) and avoids the isolation
of the potentially sensitive azide. The methylation of 5 was
easily performed with Me3OBF4 in dichloromethane, resulting
in triazolium salt 6 in 82% yield (Scheme 1). Clean bis-
methylation was confirmed by the presence of a sharp singlet at
4.55 ppm, integrating for 6Hs in the highly symmetrical 1H NMR
spectrum, in accordance with previous reports.28
Rather than employing the deprotonation strategy described
by Bertrand6a that requires isolation of the resulting free
carbenes, we opted for the preparation of silver complexes of
both 2 and 6 for eventual transmetalation to rhodium. The
conditions we employed were similar to those described by
Cavell,29 in which the triazolium salts were treated with excess
Ag2O and KBr in MeCN at room temperature. Thus exposure
of 2 and 6 to these conditions resulted in the formation of air
stable complexes 7 and 8, respectively, in which each silver
atom is bound to two MICs (see Scheme 2). The bis(MIC)
structure of 7 ([AgL][Br])30 was confirmed by high-resolution
mass spectrometry, which gave an m/z of 745.3148,
corresponding to [AgMIC2]+. In addition, the forma-
tion of complex 7 was accompanied by clean loss of the
RESULTS AND DISCUSSION
■
Our studies began with monodentate triazole MIC precursor 2
reported previously by Bertrand and co-workers.6a Methylation
of triazole 1 was performed with Me3OBF4 in CF3Ph at room
temperature, resulting in the formation of 2 in 88% isolated
yield (eq 1). The bidentate version of this species, MIC ligand
1
triazole C−H peak at 8.98 ppm, present in the H NMR
spectrum of ligand precursor 2. The 13C{1H} NMR spectrum
of 7 displays a characteristic resonance at 172 ppm
corresponding to the Ag−C moiety.31 Similarly, the formation
of complex 8 was accompanied by clean loss of the triazole
1
C−H peak at 8.77 ppm, present in the H NMR spectrum of
6, along with the formation of a characteristic resonance at
172 ppm in the 13C{1H} NMR spectrum, corresponding to the
Ag−C moiety. As full conversion of 6 could be observed by 1H
NMR of 8, the lower isolated yield of 8 relative to 7 may
indicate the presence of insoluble, oligomeric Ag-MIC species
that are removed during filtration.
The structure of complex 8 was unambiguously determined
by single-crystal X-ray diffraction after crystallization by slow
diffusion of hexanes into a concentrated solution of 8 in
CH2Cl2. The solid-state molecular structure of complex 8 is
depicted in Figure 1. Complex 8 crystallizes with a 2:2 (Ag:L)
molecular ratio with a molecular formula of [Ag2L2][Ag2Br4],
exhibiting a nearly planar geometry around each silver atom.
The two ligands are nearly parallel to each other and are
interconnected by two silver atoms. The two bridging phenyl
rings have no π−π interactions with each other, at a separation
distance of 5.891 Å. Each of the triazole rings is tilted with
6, was synthesized as shown in Scheme 1. Its structure was
modeled on the 1,3-bis(alkyl-imidazole) benzene framework
initially reported by Hollis.23 This framework has been reported
to be flexibile with regard to its coordination to metals, giving
both bridging and pincer-type complexes of rhodium19,24 and
iridium,21,24b,25 and similar MIC frameworks have been
reported for both bimetallic and pincer complexes of
ruthenium,11a,12c gold,11b and palladium.11g
A significant benefit of the 1,2,3-triazol-5-ylidene MIC ligand
framework is that these species can be constructed using copper-
catalyzed [2 + 3] Huigsen or “click” cycloaddition chemistry,26
which provides significantly greater flexibility for the introduction
of carbene wingtip groups compared to typical synthetic strategies
for the preparation of imidazole NHCs. For our first-generation
ligand, 2,6-diisopropylphenyl (Dipp) wingtip groups were
selected for the triazole in order to provide significant steric
Scheme 1. Synthesis of Ligand Precursor 6
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Scheme 2. Synthesis of Ag-MIC Complexes 7 and 8
highly symmetrical species, consistent with the formation of
only one isomer. This differs from a related neutral bimetallic
rhodium NHC complex reported by Hollis, which exists as two
interconverting diastereomeric complexes.19 In the 13C{1H}
NMR spectrum, the Ag−C resonance at 172 ppm disappeared,
and in its place, a new downfield resonance at 166 ppm was
observed, corresponding to the Rh−CMIC. The relatively large
1
103Rh−13C coupling constant of JRh−C = 47 Hz suggests a
strong Rh−CMIC bond.8a X-ray quality single crystals were grown
by slow diffusion of hexanes into a concentrated CH2Cl2−THF
solution of 9.
X-ray crystallographic analysis unambiguously confirms the
bimetallic nature of complex 9 with a metal-to-ligand ratio of
2:1 (Figure 2A). The remaining coordination sites on the
rhodium are occupied by one COD ligand, and one acetonitrile.
The two triazole rings are twisted 53.51° in opposite directions
with respect to the bridging phenyl ring. This conformation is
likely caused by the steric hindrance provided by the COD
groups. This hindrance is further observed in the Dipp wingtip
groups, which are tilted with respect to their corresponding
triazole rings with dihedral angles of 77.71°.
Figure 1. Crystallographically determined structure of [Ag2L2]-
(Ag2Br4)·3CH2Cl2 (8) displaying thermal ellipsoids drawn at the
50% confidence level. Hydrogen atoms, counteranion (Ag2Br4), and
solvent (CH2Cl2) are omitted for clarity. There is 1/4 of a molecule of
complex 8 per asymmetric unit. Selected interatomic distances [Å] and
angles [deg]: Ag(1)−C(6), 2.075(7); N(2)−N(3), 1.340(8); N(1)−
N(2), 1.321(9); N(3)−C(6), 1.350(8); N(1)−C(7), 1.477(9); C(6)−
Ag(1)−C(6B), 173.9(3).
Interestingly, multiple weak, noncovalent C−H···F and C−
respect to the plane of the central phenyl ring with an angle of
46.15°. Each Dipp wingtip group is nearly perpendicular with
respect to the triazole rings, both displaying dihedral angles of
89.60°. The Ag−CMIC bond is 2.075(7)Å, which is nearly
identical to that in the Ag-MIC complex reported by Bielaweski
(2.074(4)Å),11a and similar to that in related Ag-NHC
complexes.32 To the best of our knowledge, this is the first
structurally characterized example of a cationic 2:2 (Ag:L) silver
triazole MIC complex.
−
H···Cl interactions are observed between the ligand, the BF4
counterion, and the CH2Cl2 that was incorporated into the
crystal lattice (Figure 2B). These contacts may reasonably be
considered to be nonclassical C−H···X hydrogen bonds since
they are less than the sum of the van der Waals radii (2.67 and
2.95 Å for H···F and H···Cl, respectively.33 These interactions
appear to play a crucial role in the solid-state organization of 9.
This complex appears to be the first reported example of a
homobimetallic rhodium triazole MIC complex. In an attempt
to prepare bidentate versions of 9, we examined the effect of
decreasing the ratio of the Rh precursor to carbene complex 8,
but in all cases, fewer equivalents of [Rh(COD)2]BF4 resulted
in the incomplete transmetalation of 8.
The transmetalation of Ag-MIC complex 7 was performed
with [Rh(COD)Cl]2 in dichloromethane in order to produce
the neutral monomeric complex [Rh(MIC)(COD)X] (X = Cl,
Br) 10 (Scheme 4). To avoid any halide scrambling, excess KBr
(10 equiv) was added. The related Ir complex [Ir(MIC)-
(COD)Cl] has been prepared in situ by the Bertrand group en
Transmetalation to Rhodium. Both complexes 7 and 8
were investigated for their ability to transfer their MIC ligands
to rhodium. The transmetalation of complex 8 to rhodium was
investigated at room temperature by the addition of 8 to
stoichiometric quantities (per potential MIC ligand) of [Rh-
(COD)2]BF4 in MeCN. This resulted in the near immediate
1
formation of a white precipitate. After 3 h, the H NMR spec-
trum of the resulting mixture showed the complete
disappearance of 8, along with the formation of a new complex
9 (see Scheme 3). The 1H NMR spectrum of 9 also indicated a
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Scheme 3. Formation of Rhodium Complex 9 by Transmetalation of Cationic Complex 8 with [Rh(COD)2]BF4
Figure 3. Crystallographically determined structure of [RhL(COD)-
Br] (10), displaying thermal ellipsoids drawn at the 50% confidence
level. Hydrogen atoms are omitted for clarity. Selected interatomic
distances [Å] and angles [deg]: Rh(1)−C(1), 2.044(6); Rh(1)−Br(1),
2.4937(10); C(2)−C(16), 1.465(9); N(1)−C(3), 1.436(7); N(1)−
C(1), 1.372(8); N(2)−N(3), 1.308(7); N(3)−C(15), 1.483(8);
Br(1)−Rh(1)−C(1), 87.54(17).
more tilted with respect to the triazole ring with a dihedral
angle of 71.77°. The triazole ring of the carbene unit is com-
pletely planar. The Rh−CMIC bond length in 10 (2.044(6) Å) is
the same as that of 9 (2.048(4) Å) within error and is nearly
identical to the Rh triazole MIC complex reported by Albrecht
(2.027(6)Å).8a No supramolecular contacts, similar to those of
9, are observed in 10, likely due to the overall neutrality of the
complex.
In summary, we have investigated the coordination chemistry
of triazolium MICs by the formation of silver and rhodium
complexes. A novel bridging MIC ligand based on alkylated
bis(1,2,3-triazolium salts) was synthesized, and its reactivity
toward Ag2O was investigated, resulting in the isolation of
the first structurally characterized example of a cationic 2:2
(Ag:L) silver triazole MIC complex. This complex was further
transmetalated to Rh to give a cationic bimetallic rhodium com-
plex. To the best of our knowledge, this is the first example of a
cationic bimetallic mesoionic carbene complex of rhodium
reported in the literature.19−22 In addition, the previously
reported monodentate MIC 2 was prepared and reacted with
Ag2O and then transmetalated to rhodium to give complex 9,
which was fully characterized. Complexes 9 and 10 represent
some of the first reported and structurally characterized
examples of Rh-MIC complexes. We are currently investigating
the catalytic activity of complexes 9 and 10, as well as the trans-
metalation of Ag-MIC complex 8 to other metal sources as
Figure 2. (A) Crystallographically determined structures of
[Rh2L2(COD)2(MeCN)2][BF4]2CH2Cl2 (9), displaying thermal
ellipsoids drawn at the 50% confidence level. There is /2 of a mole-
1
cule of complex 9 per asymmetric unit. Hydrogen atoms, counteranion
(BF4), and solvent (CH2Cl2) are omitted for clarity. Selected inter-
atomic distances [Å] and angles [deg]: Rh(1)−C(6), 2.048(4);
Rh(1)−N(4), 2.060(3); C(28)−C(29), 1.459(6); N(2)−N(3),
1.314(4); N(1)−N(2), 1.346(4); N(1)−C(6), 1.361(5); N(3)−
C(7), 1.468(4); N(4)−Rh(1)−C(6), 86.70(13). (B) 2D structure of
[Rh2L2(COD)2(MeCN)2][BF4]2CH2Cl2 showing multiple hydrogen
C−H···F and C−H···Cl interactions. Non-hydrogen bonding H atoms
are omitted for clarity.
route to a biscarbonyl compound employed to assess the donor
properties of the MIC derived from 2,6a but this compound was
not isolated. Thus, the isolation and characterization of 10 by
X-ray crystallography also serves as support for the structure of
its Ir congener.
Complex 10 exhibits nearly planar geometry around the
rhodium atom (Figure 3). A common plane between the
aromatic wingtip groups (Ph and Dipp) is not observed, and
they lie tilted at different angles from the central triazole ring.
The phenyl wingtip group is only slightly twisted with respect
to the plane of the triazole ring, at an angle of 33.29°. Likely
due to steric considerations, the Dipp wingtip group is much
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Scheme 4. Formation of Complex 10 by Transmetalation of Ag-MIC Complex 7 with [Rh(COD)Cl]2
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synthetic routes to pincer complexes, and to prepare other
bimetallic complexes.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic procedures and characterization data for all
compounds and selected crystallographic data for compounds
7−10. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: cathleen.crudden@chem.queensu.ca.
■
ACKNOWLEDGMENTS
■
The Natural Sciences and Engineering Research Council of
Canada (NSERC) is thanked for support of this work in terms
of Discovery, Accelerator and Equipment Grants to CMC. The
Ontario Government and Queen’s University are thanked for
Ontario Graduate Scholarships and Queen’s Graduate Awards
to E.C.K. The Canada Foundation for Innovation (CFI) is
thanked for instrumentation in support of this research.
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