Metathesis cleavage of an NN bond in benzo[: C] cinnolines and azobenzenes by triply-bonded ditungsten complexes

Article abstract of DOI:10.1039/c7cc08570b

A metathesis reaction of a WW bond and an NN bond was observed by reacting a W-W triply-bonded W(iii)₂ complex, (^tBuO)₃WW(O^tBu)₃ (1), with benzo[c]cinnoline derivatives to form biphenyl-linked dinuclear (imido)tungsten complexes 2-4. When azobenzene was used as the substrate, a trans to cis isomerization induced by blue-LED light was essential prior to the metathesis cleavage of the NN bond by the WW bond of (Me₂N)₂(TfO)WW(OTf)(NMe₂)₂ (6), affording the corresponding imido-bridged dinuclear tungsten complexes 7-9.

Full text of DOI:10.1039/c7cc08570b

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DOI: 10.1039/C7CC08570B
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Metathesis Cleavage of an N=N Bond in
Benzo[c]cinnolines and Azobenzenes by Triply-bonded
Ditungsten Complexes†
Received 00th January
2
0xx,
Accepted 00th January
0xx
2
Hideaki Ikeda, Kohei Nishi, Hayato Tsurugi,* and Kazushi Mashima*
DOI:
1
0.1039/x0xx00000x
www.rsc.org/
A metathesis reaction of a W≡W bond and an N=N bond was
observed by reacting a W-W triply-bonded W(III) complex,
(1), with benzo[c]cinnoline derivatives to
form biphenyl-linked dinuclear (imido)tungsten complexes 2—
. When azobenzene was used as the substrate, a trans to cis
isomerization induced by blue-LED light was essential prior to
the metathesis cleavage of the N=N bond by the W≡W bond of Figure 1. A cis-conformation of the N=N moiety in azo
Me N) (TfO)W≡W(OTf)(NMe
(6), affording the corresponding compounds was found to be crucial for the azo compounds to
2
t
t
(
BuO)
3
W≡W(O Bu)
3
Fig. 1 Metathesis cleavage of N=N bond by W≡W bond.
4
(
2
2
2 2
)
2
2
imido-bridged dinuclear tungsten complexes 7—9.
approach the ditungsten core to generate side-on bound -( , )-
azo species as the plausible intermediate before cleaving the N=N
bond.
Transition metal-mediated metathesis reactions have been
extensively investigated and alkene and alkyne metathesis
reactions are used to construct natural and pharmaceutical organic
Metathesis cleavage of an N=N bond occurred upon treating
(tBuO)
t
3
W≡W(O Bu)
benzo[c]cinnoline, in benzene to afford biphenyl-bridged
phenylimido)tungsten complex 2 as red-colored microcrystals in
3
(1) with
a
cyclic azo compound,
1
componds.
Notably, dinuclear metal complexes containing
(
metal-metal triple bonds, especially W≡W complexes, are known
to facilitate metathesis reactions of alkynes and nitriles, by which
metal-alkylidyne and metal-nitride species are generated through
cleavage of the C≡C and C≡N bonds of the substrates. In contrast,
metathesis-type cleavage of azo compounds is little explored,
except for group 5 metal dinuclear complexes: Cotton et al.
reported that a Ta=Ta bond of [Cl
reacted with an N=N bond of azobenzene to give a dinuclear
imidotantalum complex with doubly chloride-bridged ligands (eq
1
quantitative yield (eq 2). In the H NMR spectrum, one set of
signals for the aryl moiety was observed at  7.32, 7.08, 7.02, and
.91, suggesting a symmetric structure in solution. All the butoxy
ligands were detected as one broad signal, and the integral ratio of
the aromatic and butoxy protons indicated that the complexation
reaction of 1 and benzo[c]cinnoline occurred in a 1:1 ratio. The
overall molecular structure was clarified by an X-ray diffraction
2
O, 1,1'-biphenyl-
,2'-diamine was obtained in the reaction mixture, suggesting the
formula of the biphenyl-linked bimetallic (phenylimido)tungsten
complex. Thus, to the best of our knowledge, this is the first
example of metathesis cleavage of an N=N bond using metal-
metal triple bonds.
6
2
2
(Me
2
S)Ta]
2
(-Cl)
2
2
(-SMe )
study (vide infra). After quenching 2 with H
2
3
a
1
). Herein we report that the W≡W scaffold in X
2
YW≡WX
, Y = OTf) could cleave an
2
Y
t
complexes (X = Y = O Bu; X = NMe
N=N bond of azo compounds via a metathesis pathway to afford
dinuclear (imido)tungsten complexes as schematically shown in
2
4
The ORTEP drawing of 2 is shown in Figure 2. The N=N bond
of benzo[c]cinnoline is cleaved to form biphenyl-linked bimetallic
(phenylimido)tungsten complex 2. Thus, the distance of N1 and
Department of Chemistry, Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531, Japan
E-mail: mashima@chem.es.osaka-u.ac.jp; tsurugi@chem.es.osaka-u.ac.jp
†
Electronic Supplementary Information (ESI) available: Experimental data for all
new compounds. CCDC 1584131 ( ), 1584132 ( ). For ESI and crystallographic data
2
7
in CIF or other electronic format See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C7CC08570B
2
2
Fig. 3 Reported dinuclear -( , )-(benzo[c]cinnoline) complexes.
A linear azo compound, azobenzene, did not react with
Fig. 2 ORTEP drawing of complex 2 with 50% thermal ellipsoid. All the
ditungsten complexes 1 and (Me
due to the difficult approach of the trans isomer to the ditungsten
core. We accordingly used ditungsten complex,
Me (TfO)W≡W(OTf)(NMe (6), bearing a labile triflate
ligand and blue-LED irradiation conditions. When 5 was mixed
with azobenzene in the presence of 2 equiv of Me SiOTf in
2 3 2 3
N) W≡W(NMe ) (5), probably
hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond
o
angles [ ]; W1—W1* : 2.6782(5), W1—N1 : 1.7642(12), N1—N1* :
a
3
2
.0926(17), W1—O1 : 1.8822(15), W1—O2 : 1.9276(11), W1—O3
.0301(10), W1-N1-C1 : 162.87(11).
:
(
2
N)
2
2 2
)
3
benzene, complex 6 was formed, but no reaction was observed.
Irradiation with 450-470 nm blue-LED light, however, led to the
formation of phenylimido-bridged tungsten complex 7 (Scheme 1).
N1* (3.0926(17) Å) is reasonably elongated similarly to that of a
normal N—N single bond. The W—N1 bond length (1.7642(12)
5
Without the addition of Me
product from the reaction mixture, suggesting that the vacant site
was indispensable for the NMe -bound ditungsten complex 5. In
3
SiOTf, we detected no reaction
Å) and the W-N-Cipso angle (162.87(11)°) are consistent with the
terminal 6-electron donor imido ligands to early-transition metal
centers with a high oxidation state.6 The bond length of two
tungsten atoms is 2.6782(5) Å, which is in the typical range for a
2
addition, coordination of in situ-generated cis-azobenzene to the
ditungsten core of 6 might proceed to generate B, similarly to the
reaction of 1 with benzo[c]cinnoline, forming A. Because
complex 7 was sparingly soluble and no signal was detectable in
7
W—W single bond. Complex 2 has two types of tert-butoxido
ligands, bridged and terminal ligands, and the bond length of W—
O3 is slightly longer than that in terminal tert-butoxido ligands
1
the H NMR spectrum, structural determination was performed by
(
2
(
W1—O1 = 1.8822(15) Å; W1—O2 = 1.9276(11) Å; W1—O3 =
.0301(10) Å). The torsion angle of W1—W1* and C6—C6*
[W1]—[centroid of W1—W1*]—[centroid of C6—C6*]—[C6])
the X-ray diffraction study (Figure 4). Complex 7 contains two
W—W units linked by two bridging triflate ligands. Similar to
complex 2, the distance of N1 and N2 (2.819(7) Å) indicates
is 79.7°, a value that suggests a perpendicular approach of the
benzo[c]cinnoline to the ditungsten core (vide infra).
We further checked the effect of the substituent at the 3,8-
positions of the benzo[c]cinnoline: both of 3,8-dimethyl- and 3,8-
5
complete scission of the N=N bond of azobenzene. Phenylimido
ligands asymmetrically bridge two tungsten atoms: one of four
W—N bonds in the bridging phenylimido ligands is significantly
elongated (W1—N1 = 2.106(5) Å) compared with other reported
dimethoxy-benzo[c]cinnolines could be also applicable to the
dinuclear tungsten(V) -imido complexes due to the trans
t
metathesis cleavage by W
2
(O Bu)
6
(1) to give the corresponding
6b,11
influence of the electron-donating NMe
2
ligand,
while the
bis(imido) tungsten complexes 3 and 4 in quantitative yields,
other W—N bonds of the -phenylimido moiety (W1—N2 =
.988(5) Å; W2—N1 = 1.897(5) Å; W2—N2 = 1.931(5) Å) are
1
13
whose structures were characterized by H and C NMR analyses.
1
In addition, protonolysis of 3 and 4 afforded the corresponding
shorter due to the presence of terminal and bridging triflate ligands
in their trans position. In addition to the two -phenylimido
ligands, one dimethylamido ligand bridges two tungsten centers.
The W1—W2 bond length of 2.5255(4) is shorter than that in
complex 2 as well as previously isolated and structurally
characterized dinuclear tungsten(V) complexes due to the
4
,4'-dimethyl- and 4,4’-dimethoxy-1,1'-biphenyl-2,2'-diamines.
For the mechanism of cleavage of the N=N bond,
(tBuO)
2
2
t
t
2
2 2
W{-( , )-(benzo[c]cinnoline)}(-O Bu) W(O Bu)
species A is proposed to be an intermediate prior to cleavage of
the N=N bond, in which the W-W bond and N-N bond were
perpendicular. A similar perpendicular coordination mode was
presence of three bridging ligands, two -phenylimido and one
2
2
observed for some dinuclear -( , )-(benzo[c]cinnoline)
6b,6c,7,11
dimethylamido.
The dihedral angle of W1-N1-W2 and
8
9
10
complexes of diniobium, diiron, and dizirconium (Figure 3), in
which the N=N bond was reduced to a single bond via two-
electron transfer from two metal centers.
W1-N2-W2 planes is 135.3°, which is typical for the butterfly
12,13
structure of doubly -arylimido-bridged bimetallic complexes.
2
| J. Name., 2012, 00, 1-3
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This work was supported by JSPS KAKENHI Grant NoVsi.ewJPA2rt6ic7le0O8n0li1n2e
DOI: 10.1039/C7CC08570B
(
(
Grant-in-Aid for Young Scientist (A)) to H.T. and JP15H05808
Precisely Designed Catalysis with Customized Scaffolding) to K.M.
Notes and references
1
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Synth. Catal., 2007, 349, 93. e) A. Fürstner, Angew. Chem., Int. Ed.,
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013, 52, 2794.
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R. R. Schrock, M. L. Listemann and L. G. Sturgeoff, J. Am. Chem.
Soc., 1982, 104, 4291.
a) F. A. Cotton, S. A. Duraj and W. J. Roth, J. Am. Chem. Soc., 1984,
Scheme 1. Metathesis cleavage of the N=N bond of azobenzenes.
1
06, 4749. b) J. A. M. Canich, F. A. Cotton, S. A. Duraj and W. J.
Roth, Polyhedron, 1986, , 895.
In addition to the amine formation, reaction of
4 equiv) in C at 65 °C for 24 h produced 2,2'-
5
4
5
2
with benzaldehyde
(
6 6
D
1
bis(benzylideneamino)biphenyl in 92% yield (by H NMR) via
metathesis reaction of W=NR and PhCH=O.
a) G. Fochi, C. Floriani, J. C. J. Bart and G. Giunchi, J. Chem. Soc.,
Dalton Trans., 1983, 1515. b) J. A. M. Canich, F. A. Cotton and L. R.
Falvello, Inorg. Chim. Acta, 1988, 143, 185. c) P. J. Walsh, F. J.
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a) G. R. Clark, A. J. Nielson and C. E. F. Rickard, J. Chem. Soc., Chem.
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H. Tanahashi, H. Ikeda, H. Tsurugi and K. Mashima, Inorg. Chem.,
2
016, 55, 1446.
a) M. H. Chisholm, W. E. Streib, D. B. Tiedtke and D.-D. Wu, Chem.
Eur. J., 1998, , 1470. b) M. H. Chisholm, D. R. Click, J. C. Gallucci,
Fig. 4 ORTEP drawing of complex 7 with 50% thermal ellipsoid. All hydrogen
7
8
4
atoms and solvent molecules are omitted for clarity. Selected bond lengths
C. M. Hadad and P. J. Wilson, Organometallics, 2003, 22, 4725.
T. Saito, H. Nishiyama, K. Kawakita, M. Nechayev, B. Kriegel, H.
Tsurugi, J. Arnold and K. Mashima, Inorg. Chem., 2015, 54, 6004.
o
[
Å] and bond angles [ ]; W1—W2 : 2.5255(4), W1—N1 : 2.106(5), W1—N2 :
1
.988(5), W2—N1 : 1.897(5), W2—N2 : 1.931(5), N1—N2 : 2.819(7), W1-
N1-W2 : 78.04(17), W1-N2-W2 : 80.22(18), N1-W1-N4 : 170.67(19)
9
1
R. J. Doedens, Inorg. Chem., 1970, 9, 429.
0 M. Gonzàlez-Maupoey, G. M. Rodrìguez and T. Cuenca, Eur. J. Inorg.
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1 L. P. H. Lopez, R. R. Schrock and P. Muller, Organometallics, 2008,
27, 3857.
Under the same reaction conditions, N=N bond cleavage of
substituted azobenzenes proceeded as follows: 4,4’-
1
dimethylazobenzene and 4,4’-difluoroazobenzene reacted with 5 12 K. Kaleta, P. Arndt, T. Beweries, A. Spannenberg, O. Theilmann and
U. Rosenthal, Organometallics, 2010, 29, 2604.
under blue-LED irradiation in the presence of Me
3
SiOTf to afford
as dark-red
microcrystals in high yield (8: 71% yield; 9: 83% yield). These
1
3 a) A. K. Burrell and J. C. Bryan, Organometallics, 1993, 12, 2426. b)
N. A. McLeod, L. G. Kuzmina, A. V. Churakov, P. Mountford and G.
I. Nikonov, Dalton Trans., 2014, 43, 188.
corresponding tungsten complexes
8 and 9
complexes were characterized by elemental analysis. It is 14 Y. Einaga, O. Sato, T. Iyoda, A. Fujishima and K. Hashimoto, J. Am.
reasonable to assume that blue-LED light irradiation caused the
isomerization of trans-azobenzene to the cis form.14 In our
Chem. Soc., 1999, 121, 3745.
experiments, after exposing a C
to blue-LED light for 2 h, cis-azobenzene was partially formed
19%), and all of the cis-azobenzene was rapidly consumed by
adding the tungsten complex 5 and Me SiOTf.
In summary, we found that metathesis reactions of an N=N
bond and a W W bond in X YW WX Y complexes (1, X = Y =
O Bu; 6, X = NMe , Y = OTf) afforded the corresponding
6 6
D solution of trans-azobenzene
(
3
≡
2
≡
2
t
2
dinuclear bis(imido)ditungsten complexes 2—4, and 7—9, some
of which were characterized by X-ray diffraction studies. The
different reactivites of trans and cis-azobenzene indicated that -
2
2
(
 , )-(N,N)-coordination was an important step for cleaving the
N=N bond via a metathesis pathway.
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