A stable Janus bis(maloNHC) and its zwitterionic coinage metal complexes

Article abstract of DOI:10.1039/c4cc01200c

The first stable dianionic Janus-type bis(maloNHC) was isolated and characterized. Details on the chemistry of this biscarbene with respect to its ability to support catalytically relevant metal complexes are provided. The molecular structures of two dinu

Full text of DOI:10.1039/c4cc01200c

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A stable Janus bis(maloNHC) and its zwitterionic
coinage metal complexes†
Daniela Tapu,*a Zachary McCartya and Colin McMillenb
Cite this: Chem. Commun., 2014,
50, 4725
Received 14th February 2014,
Accepted 22nd March 2014
DOI: 10.1039/c4cc01200c
www.rsc.org/chemcomm
The first stable dianionic Janus-type bis(maloNHC) was isolated and
characterized. Details on the chemistry of this biscarbene with
respect to its ability to support catalytically relevant metal com-
plexes are provided. The molecular structures of two dinuclear gold
and silver complexes were determined by X-ray crystallography.
Since the discovery of the first stable nucleophilic carbene by
Arduengo over two decades ago,1 the area of carbene chemistry
has emerged as one of the most rapidly investigated and
promising fields of study in modern chemistry.2–5 Due to their
functional and synthetic diversity, as well as a high affinity
toward a wide range of main group and transition metals,
archetypal nucleophilic carbenes, namely N-heterocyclic carbenes
(NHCs), have shown remarkable utility as ligands for organo-
metallic catalysts,6 and as organocatalysts.7 Moreover, NHCs have
found utility in a broad range of medicinal, luminescent, and
other functional materials.8–13 Among NHCs, polyNHCs have
attracted great interest because they allow for the preparation
of metal complexes with a variety of geometries and reactivities.
Scheme 1
However, the vast majority of the reported bis-, tris-, and tetra- complexes,18–20,26,27,29,37,41–44 organic and organometallic poly-
NHCs were designed to bind in a chelating, pincer or tripodal mers,18–20,38,45–47 and supramolecular structures.10,24,31,34,48
fashion leading to monometallic complexes.14–17 Vital to the
Having observed the well documented success of these
advancement of NHC-based materials has been the design and Janus-type NHCs as a new class of bridging ligands, we sought
synthesis of new rigid multitopic NHCs featuring geometrically to extend the existing frameworks to a dianionic system of type
isolated carbene moieties.10,18–40 Among this very restrictive class 5 composed of two linearly opposed NHC moieties connected
of NHCs, compounds 1–4 in Scheme 1 are the only known systems by a common phenylene fragment. In contrast to carbenes 1–4,
containing two linearly opposed NHC moieties (Janus-type coordi- all neutral ‘‘L2’’-type ligands according to the Green formalism,
nation). While still in their infancy, these systems have found carbene 5 contains two remote anionic functionalities (malonate
tremendous utility as building blocks for homo- and heterometallic moieties) within the heterocyclic backbone. This translates into a
(ꢀ2) overall charge of the ligand. As a consequence, metalation can
occur concomitant to the displacement of other anionic ligands
(e.g. halides) from various transition metal precursors, thus resulting
in neutral bis-zwitterionic species with potentially valuable advan-
tages relative to the classical cationic metal complexes produced by
neutral NHCs.49 The idea to incorporate anionic moieties into 5
emerged by noticing the recent success that maloNHCs 650 have had
as new topologically distinct ensembles of six-membered NHCs.
Investigations into the transition-metal coordination chemistry
a Department of Chemistry and Biochemistry, Kennesaw State University,
1000 Chastain Road, Kennesaw, GA, 30144, USA. E-mail: dtapu@kennesaw.edu;
Fax: +1-770-423-6744; Tel: +1-678-797-2259
b X-ray Crystallography Laboratory, Department of Chemistry, Clemson University,
Clemson, SC, 29634, USA
† Electronic supplementary information (ESI) available: Experimental details of
all reported compounds. X-ray crystallographic data of 7, 10 and 11. CCDC
985394–985396. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc01200c
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Scheme 3 Synthetic procedures for 8 and 9. (a) N,N0-bis(2,6-Di-isopropyl-
phenyl)formamidine, dichloromethane, 0 1C, 25 min; (b) Et3N, 0 1C, 3 h;
(c) KHMDS, d8-THF, 30 min, r.t.
Scheme 2 Synthetic procedures for 5 and 7 reaction conditions: (a) N,N0-
bis(2,6-di-isopropylphenyl)formamidine, dichloromethane, 0 1C, 25 min;
(b) Et3N, 0 1C, 3 h; (c) KHMDS, THF, 30 min, r.t.
common organic solvents. The 13C NMR spectrum of 5 exhibited a
single diagnostic signal at d 249.88 ppm (THF-d8) indicative of its
formulation as a NHC and also of its highly symmetric structure.
For comparative purposes, the anionic monocarbene 9 was also
generated (Scheme 3). The 13C resonance of the carbene nucleus
was observed at d 249.37 ppm (THF-d8). Based on the similarity
observed in the spectroscopic data of 5 and 9, we postulated
that the reactivity and affinity toward transition metals of each
NHC moiety of 5 should be similar to that exhibited by its
monotopic analogue 9.
Due to the emergence of numerous catalytic, photophysical and
biomedical applications of NHC–coinage metal complexes,58 homo-
bimetallic complexes of gold and silver were targeted to investigate
the nucleophilic behavior of 5. Treatment of the in situ generated
carbene 5 with the phosphine-stabilized precursors Ph3PAuCl or
Ph3PAgOTf yields the corresponding complexes 10 and 11 in 76.5%
and 91.6% yield, respectively (Scheme 4). As expected, the negative
charges of the two zwitterionic metal complexes are delocalized
in the remote malonate backbone of the ligand in the outer-
coordination sphere of the complex. The related monometallic
complexes 12 and 13 (Scheme 5) were synthesised in a similar
fashion. Complexes 12 and 13 are very soluble in halogenated
solvents, whereas complexes 10 and 11 have poor solubilities in
almost all common organic solvents. Gold complexes 10 and 12
display their representative carbene carbon resonances as doublets
at d = 199.8 ( JP–C = 120.1 Hz) and 201.11 ( JP–C = 119.4 Hz) ppm,
respectively. Owing to the limited solubility of complex 11 and
of 6 have led to the synthesis of a series of zwitterionic metal
complexes with interesting catalytic properties.50–54 In addition, it has
been shown that by direct reaction of the malonate functionality with
an electrophile, the electronic properties of 6 can be finely tuned post-
complexation with various transition metals without disrupting the
steric environment created around the metal center.53
As shown in Scheme 2, the zwitterionic precursor 7 was
obtained by coupling 1,4-phenylenedimalonyl tetrachloride55
with N,N0-bis(2,6-di-isopropylphenyl)formamidine56 in dichloro-
methane in the presence of excess triethylamine. Compound 7
was found to be neither air nor moisture sensitive. This resistance
to moisture advantageously facilitated the removal of the Et3NꢁHCl
from the reaction mixture.
The crystallographic analysis for 7 revealed the expected
framework (Fig. 1) and confirmed the bis-zwitterionic structure
outlined in Scheme 1. The average N–C1 distance (1.311 Å) and the
N–C–N angle (122.0(6)1) measured were in accord with those pre-
viously reported for the NHCꢁH+ precursors of mono maloNHC 6.50
The central phenylene moiety is twisted by 31.791 relative to
each pyrimidine ring. As expected, the N1–C4 and N2–C2 bonds
(1.473(8) and 1.465(8) Å, respectively) are very long for amide
functionalities, a result of competitive delocalization of the
negative charge over the C4–C3–C2 fragment which reduces
the multiple bond character of the amidic C–N bonds.50,57
Having isolated 7, subsequent efforts were focused on
exploring the synthesis and reactivity of the dianionic carbene 5.
The free carbene was generated cleanly and quantitatively by treat-
ment of 7 with 2 equivalents of potassium bis(trimethylsilyl)amide,
(KHMDS) at room temperature. Carbene 5 is stable for at least
several days and it is only slightly soluble in THF. Despite our
best efforts, all attempts to grow X-ray quality crystals of 5 were
not successful, at least partly a result of its limited solubility in
Scheme 4 Syntheses of bimetallic complexes 10 and 11: reaction conditions:
(a) for 10: PPh3AuCl, THF, r.t., 12 h; for 11: PPh3AgOTf, THF, r.t, 12 h.
Fig. 1 Solid state molecular structure of 7 with 50% probability ellipsoids.
Hydrogen atoms and solvent molecules are omitted for clarity.
Scheme 5 Monometallic complexes 12 and 13.
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since it gives access to neutral complexes of monocationic metals
bearing otherwise neutral ligands, a feature not available with the
current arsenal of Janus-type carbenes. The utility of this ligand in
the synthesis of new organometallic materials has been demon-
strated by its incorporation into two new dinuclear zwitterionic
gold and silver complexes. These complexes have been fully
characterized by spectroscopic techniques and X-ray crystallo-
graphy. The similar chemical shifts of the carbenic carbons of 5
and 9, in both free and coordinated forms, suggest that the
reactivity and the affinity of each carbene ‘‘face’’ of the Janus ligand
5 toward transition metals do not deviate abnormally from that of
their monotopic analogue 9. The facile synthesis of 5 and its
unique topological properties renders this new system a promising
dianionic scaffold well suited to function as a building block in the
synthesis of other functional organic and metal–organic frame-
works. Further studies in order to extend the coordination of this
ligand to other metals and also afford neutral main-chain zwitter-
ionic organometallic polymers are underway.
Fig. 2 Solid state molecular structure of 10 with 50% probability ellipsoids.
Hydrogen atoms and solvent molecules are omitted for clarity.
the expected multiplicity of the resonance arising from coupling
with the two silver isotopes and the phosphorus atom, the reso-
nance signal for the carbene carbons in the 13C{1H} NMR spectrum
was not detected. The carbene carbon of 13 appears as two doublets
of doublets centered at 200.5 ppm, consistent with splitting by the
three 1/2 nuclei: 107Ag, 109Ag and 31P. The 13C–107,109Ag coupling
constants are large (199.5 and 233.2 Hz, respectively). These
couplings suggest that 13 does not experience rapid ligand
exchange on the NMR time scale.59
To elucidate the solid structures of the two dinuclear com-
plexes 10 and 11, single crystals were obtained by slow evapora-
tion of saturated solutions of 10 or 11 in halogenated solvents
(CHCl3 and 1 : 1 mixture of CH2Cl2 : CHCl3, respectively) and
subjected to X-ray diffraction analysis. The determinations
revealed similar molecular structures for the two complexes
as depicted in Fig. 2 and 3. The complexes crystallize in the
monoclinic P21/c group and they are heavily solvated. The metal
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for support of this
research (Grant # 50126-UNI1). This work has also been partially
supported by internal grants from Kennesaw State University.
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