Intermolecular C?C Coupling between 1-Methyl-1,2,3-Triazole and 2,2′-Bipyridine or 1,10-Phenanthroline in MoII Complexes

Article abstract of DOI:10.1002/chem.201703524

Unsupported 1-methyl-1,2,3-triazole has been coordinated to {Mo(η³-methallyl)(CO)₂(N-N)} (N-N=2,2′-bipyridine, bipy; or 1,10-phenanthroline, phen) fragments, yielding cationic complexes that can be regarded as metalated triazolium salts. Their reactivity towards a strong base led to the deprotonation of the C5?H group of the triazole moiety, followed by an intermolecular nucleophilic attack to the ortho CH group of a bipy or phen ligand affording cyclic, bimetallic dearomatized C?C coupling products. The reaction of the neutral bipy derivative with an acid led to the formation of dihydropyridyl units by protonation of a CH group of the dearomatized rings, the dimeric nature of complexes being mantained upon protonation.

Full text of DOI:10.1002/chem.201703524

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Accepted Article
Title: Intermolecular C-C Coupling Between 1-Methyl-1,2,3-Triazole
and 2,2'-Bipyridine or 1,10-Phenanthroline at Mo(II) Complexes
Authors: Lucia Riera, Julio Pérez, Sergio Fombona, and Jesús Díaz
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Intermolecular C-C Coupling Between 1-Methyl-1,2,3-Triazole and
2,2’-Bipyridine or 1,10-Phenanthroline at Mo(II) Complexes
Sergio Fombona,[b] Julio Pérez,[a, b] Jesús Díaz[c] and Lucía Riera*[a]
Abstract: Unsupported 1-methyl-1,2,3-triazole has been coordinated
to {Mo(η3-methallyl)(CO)2(N-N)} (N-N= 2,2’-bipyridine, bipy; or 1,10-
phenanthroline,phen) fragments yielding cationic complexes which
can be regarded as metalated triazolium salts. Their reactivity towards
a strong base led to the deprotonation of the C5-H group of the triazole
moiety, followed by an intermolecular nucleophilic attack to the ortho
CH group of a bipy or phen ligand affording cyclic, bimetallic
dearomatized C-C coupling products. The reaction of the neutral bipy
derivative with an acid led to the formation of dihydropyridyl units by
protonation of a CH group of the dearomatized rings, the dimeric
nature of complexes being mantained upon protonation.
formally result from the deprotonation of the CH group of a
triazolium precursor,[14] and are defined as abnormal or mesoionic
N-heterocyclic carbenes due to their substitution pattern. The
different properties of triazolylidenes compared to those of
imidazolylidenes, and their implications in catalysis have made
this new type of NHCs to evolve very rapidly.
We have reported several examples of synthesis of
relatively elaborate, non-symmetric, bidentate or tridentate
ligands by C-C coupling between smaller ligands which were
already coordinated to the same metal center.[15] This metal-
templated methodology proved successful even for coupling
partners considered very inert such as non-functionalized 2,2’-
bipyridine or 1,10-phenanthroline.[16] In them an internal
nucleophile is generated by C-H deprotonation of one of the
ligands using an external base. If the position where the ligand is
deprotonated is proximal to the diimine, the intramolecular
addition occurs with little distortion of the geometry about the
metal, as for the coupling this ligands needs just to tilt towards the
plane of the electrophilic diimine. In contrast, the generation of the
nucleophile by ligand deprotonation at a position pointing away
from the metal center could preclude intramolecular coupling.
Under these conditions, intermolecular coupling could be
expected instead, resulting in the formation of cyclic dimers. The
formation of such species from metal monomers usually results
from self-assembly of ditopic ligands and metal complexes
containing appropriately positioned vacant coordination sites.[17]
Our results in the application of covalent self-assembly to the
synthesis of cyclic dimers are discussed below.
1,2,3-Triazoles are potential ligands in transition metal
coordination chemistry.[1] However, few studies have been
reported before the year 2002 when the non-selective azide-
alkyne Huisgen cycloaddition,[2] employed for the synthesis of this
type of azoles, was dramatically improved by its Cu(I)- catalyzed
version. The Cu-catalyzed alkyne-azide cycloaddition (CuAAC)
reaction allowed the regioselective formation of 1,4-disubstituted
1,2,3-triazoles,[3] and later, the Ru-catalyzed version was
disclosed to yield, also regioselectively, the 1,5-disubsituted
derivatives.[4] 1,2,3-triazole species are potentially versatile
ligands offering several donor sites for metal coordination,[5] so
they may act as polydentate and bridging ligands, allowing the
preparation of polymeric coordination compounds.[1c] On the other
hand, in the last years a great number of bi and polydentate
ligands containing the 1,2,3-triazole unit have been synthesized,[6]
and their metal complexes have shown potential applications in a
wide range of areas such as material science,[1d,7] ion sensing,[8]
medicinal chemistry[9] and catalysis.[10]
Cationic Mo(II) compounds bearing
triazole (Metz) ligand, with formulae
a
1-methyl-1,2,3-
[Mo(η3-
methallyl)(CO)2(Metz)(N-N)]OTf (N-N= bipy 1a, phen 1b; OTf=
trifluoromethanesulfonate), were prepared from known
[Mo(OTf)(η3-methallyl)(CO)2(N-N)] precursors by substitution of
the labile triflate ligands by the triazole.[18] The new compounds
were obtained in good yields, selectively as the only products of
the reactions. Coordination of the triazole ligand occurs through
the better donor N3 nitrogen (compared to N2) as supported by
NOE data, X-ray structure determination,[19,20] and the natural
population analysis (NBO) charges of -0.07 for N2 and -0.27 for
N3.[18] Therefore, the starting compounds 1a,b can be considered
as N-metallated triazolium salt analogs. Next, we studied the
deprotonation of these new species, a reaction that, as far as we
know, lacks precedents.
In stark contrast, coordination compounds of unsupported
1,4- or 1,5-disubstituted 1,2,3-triazole ligands remain relatively
unexplored, and examples of complexes bearing terminal 1,2,3-
triazole ligands are scarce.[11] A relatively recent[12] but steadily
growing family of derivatives are 1,2,3-triazolylidenes,[5,13] which
[a]
Dr. J. Pérez, Dr. L. Riera
Centro de Investigación en Nanomateriales y Nanotecnología-CINN
CSIC-Universidad de Oviedo-Principado de Asturias
Avda. de la Vega, 4-6. 33940 El Entrego (Spain)
E-mail: l.riera@cinn.es
[b]
[c]
S. Fombona, Dr. J. Pérez
Departamento de Química Orgánica e Inorgánica
Universidad de Oviedo
C/Julián Clavería, 8. 33006 Oviedo (Spain)
Dr. J. Díaz
The reaction of [Mo(η3-methallyl)(CO)2(Metz)(bipy)]OTf (1a)
with the equimolar amount of the strong base KN(SiMe3)2 in THF
at -78 ºC afforded complex 2a as the only product (Scheme 1a).
The IR spectrum clearly showed the formation of a neutral cis
dicarbonyl Mo(II) species (νCO bands at 1933, 1847 cm-1) from the
cationic precursor (IR νCO bands at 1955, 1872 cm-1). The 1H NMR
spectrum of 2a in CD2Cl2 featured distinct signals for every
hydrogen of the bipy moiety, and for each H of the methallyl ligand,
Departamento de Química Orgánica e Inorgánica
Universidad de Extremadura
Avda. de la Universidad s/n. 10071 Cáceres (Spain)
Supporting information for this article is given via a link at the end of
the document.
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indicating the asymmetry of the molecule of 2a. Besides, the
signals between 5.85 and 4.54 ppm are clearly indicative of a
dearomatized 2-pyridyl ring. Slow diffusion of hexane into a
concentrated solution of complex 2a in toluene at -20 ºC afforded
red crystals, one of which was used for an X-ray structure
determination.[21] The results, depicted in Figure 1a, show that 2a
is a bimetallic complex, resulting from the deprotonation of the
triazole C5-H group and subsequent C-C coupling with the C6
Complex 2b, obtained from deprotonation of the phen
precursor 1b, is also a bimetallic species analogous to 1a,
resulting from the C-C coupling between the C5 of the triazole and
an ortho carbon atom of the phen ligand. In this case, the
distortion of the diimine ligand because of loss of the aromaticity
is less pronounced than in the bipy complex, due to the presence
of the third aromatic ring that provides some rigidity to the ligand.
The dimeric structures found in 2a and 2b could be additionally
position of a bipy ligand of other {Mo(η3-methallyl)(CO)2} fragment. stabilized by a face-to-face π- π stacking interaction (the centroid-
The binuclear complex has
a
centrosymmetric molecular
centroid distances are 3.36 Å and 3.43 Å for the bipy and phen
complexes, respectively) between the two 1,2,3-triazole rings
(see Figure 1b).[22]
structure, the two metallic fragments being symmetry related (as
depicted in Fig. 1a with atom * labels). Each molybdenum atom
displays a pseudooctaedral geometry, if the η3-methallyl group is
regarded as occupying one coordination position. The two CO
ligands are mutually cis, and each being trans to a bipy nitrogen
atom, thus, the triazole is trans to the allyl ligand. The bipy ligand
has become dearomatized as a consequence of the nucleophilic
attack from the deprotonated triazole C atom, the bipy C6 being
now sp3 hibrydized as judged by the C6-N4 and C6-C7 bond
distances, of 1.472(4) Å and 1.509(4) Å respectively, typically of
single bonds. The new C-C bond, created by the C-C coupling of
C5 of the Metz and C6 of the bipy, of 1.520(4) Å, is also a single
bond, and the involved 2-pyridyl ring is no longer a planar moiety.
a)
CO
OC
CO
OC
Mo
N
Mo
N
N
N
N
N
H
H
N
N
N
H
H
N
N
N
HOTf
N
N
N
N
N
N
N
N
a) E= N, X= H
Mo
Mo
CO
CO
OC
OC
bipy, 2a 55 %
phen, 2b 51 %
bipy, 3a 45 %
OTf
N
X
OC
OC
Mo
+
KN(SiMe3)2
b) E= CH
N
N
OC
OC
Mo
X= H
X
N
N
N
E
N
N
E= N; X= H; bipy, 1a
phen, 1b
CO
OC
Mo
c) E= CH, X= Cl
N
N
Cl
Cl
N
N
Scheme 1. Different outcomes from the deprotonation reactions of {Mo(η3-
methallyl)CO)2} complexes bearing: a) Metz and bipy (or phen); b) MeIm and
bipy, and c) MeIm and 4,4’-dichloro-2,2’-bipyridine ligands (MeIm= 1-
methylimidazole).
b)
The reaction described above is intermolecular yielding a
bimetallic complex (Scheme 1a), in contrast with the formation of
imidazol-2-yl complexes (Scheme 1b) or monometallic
dearomatized C-C coupling products (Scheme 1c) obtained from
the deprotonation of analogous bipy or 4,4’-dihalo-2,2’-bipy
derivatives, respectively, bearing a N-methylimidazole instead of
the triazole ligand.[23] Besides, note that once the C5-H group has
been deprotonated the intermolecular nucleophilic attack onto a
bipy ligand is preferred over the isomerization from N- to C-
coordination of the so formed triazolide ligand (in a similar way to
the one depicted in Scheme 1b), despite the fact that 1,2,3-
triazole ligands have been regarded as much weaker donors than
N-methylimidazole.[5,11a,11d] Nucleophilic additions to Mo-
coordinated η3-allyl groups are well known reactions.[24] However,
we have not detected any product of such reactions, nor of
addition to the CO ligands, which are also typical sites of
nucleophilic attack.[25]
Figure 1. a) Molecular structure of complex 2a determined by X-ray diffraction.
Thermal ellipsoids are shown at the 30% probability level. b) Views of the
molecular structure of the dearomatized phen derivative 2b, showing the
geometry of the metallocyclic structure and the π-stacking interaction between
the triazole rings.
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The addition of the equimolar amount of HOTf to a solution
of complex 2a in toluene at room temperature caused immediately
the precipitation of a red solid, that was washed with hexane.
Characterization of the new species, 3a, in solution indicated, by
means of the IR νCO bands reaction (at 1958 and 1869 cm-1 in
CH2Cl2), the formation of a cationic complex as consequence of
the protonation, and by means of the NMR data the persistence
of a dearomatized bipy ligand. Furthermore, the NMR spectra in
CD2Cl2 solution clearly indicated that the protonation has
occurred on a CH group of the dearomatized pyridyl affording a
CH2 moiety. The 2D 1H-1H correlation spectroscopic (COSY)
study clearly established that the two hydrogen multiplet at 3.98
ppm corresponds to the methylene unit. Accordingly, a 2D HSQC
experiment showed that this signal corresponds to a peak at 29.9
ppm in the 13C NMR spectrum, its CH2 nature being confirmed by
a DEPT-135 experiment. In Figure 2 the solid-state structure of
the cation of compound 3a, determined by X-ray diffraction, is
displayed showing that the bimetallic structure is maintained upon
protonation. The connectivity and disposition of the metal
fragments is similar to that found in the precursor 2a, except by
the new CH2 moiety formed at C9 (and C29) atoms of the
dearomatized pyridyl rings. Protonation at this position of a
coordinated bipy ligand over protonation of amide type nitrogen
(N4 and 24) had been previously found in the protonation of the
dearomatized C-C coupling products. The protonation of the bipy
neutral species do not produce the breaking of the dimeric product,
resulting in the protonation of a CH group over the amido-type N
of the dearomatized pyridyl ring.
Acknowledgements
Financial
support
from
Ministerio
de
Economía
y
Competitividad/FEDER
(grant
CTQ2015-70231-P)
and
Principado de Asturias (grant GRUPIN14-103) is gratefully
acknowledged.
Keywords: carbonyl complexes • C-C coupling •
dearomatization • molybdenum • triazole
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In summary we have demonstrated that N3-coordinated 1,2,3-
triazoles can be selectively deprotonated at the C5 position,
affording N-triazolide complexes. The evolution of these highly
reactive species occurs through an intermolecular nucleophilic
attack onto the 6 position of a bipy ligand affording bimetallic
[14] There are two exceptions: a) the synthesis from an ammonia adduct,
see: L. Schaper, K. Öfele, R. Kadyrov, B. Bechlars, M. Drees, M: Cokoja,
W. A. Herrman, F. K. Kühn, Chem. Commun. 2012, 48, 3857- 3859 ;
alkylation of neutral cyclometallated triazolyl complexes, see: b) E. M.
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