Tuning of the o-Caromn bond-formation reactivity of aniline using ruthenium(II) templates

Article abstract of DOI:10.1021/ic101868w

Chemical reactions of aniline with two facecapped ruthenium(II) templates viz. CpRu^IICl(PPh₃)₂ and (Bnz) ₂Ru^II ₂Cl₄ have been studied to develop an insight into the role of the m

Full text of DOI:10.1021/ic101868w

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pubs.acs.org/IC
Tuning of the o-CaromꢀN Bond-Formation Reactivity of Aniline Using
Ruthenium(II) Templates
Subhas Samanta and Sreebrata Goswami*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
S Supporting Information
b
having labile coordination sites are crucial for bringing about9aꢀc
ABSTRACT: Chemical reactions of aniline with two face-
the above reactions. Similar arguments in favor of using high-
valent metal-mediator complexes for CaromꢀN bond fusion in
electron-rich anilido substrates were put forward by others.10 It
has now been shown that the lability of the mediator complexes is
a prerequisite for providing cis-geometrical coordination of
aniline for o-CaromꢀN bond fusion.9 However, the lability in a
high-valent metal-mediator complex is usually poor, which
causes difficulty in finding a suitable metal-mediator complex for
o-CaromꢀN bond-forming reactions.
capped ruthenium(II) templates viz. CpRuIICl(PPh3)2 and
(Bnz)2RuII Cl4 have been studied to develop an insight into
2
the role of the metal template for the rare type of o-CaromꢀN
bond-forming reaction in aniline.
arbonꢀnitrogen bond formation in aniline1ꢀ3 is one of the
Herein we disclose some novel results of a few planned
chemical reactions to demonstrate that the use of oxidizable
low-valent metal-mediator complexes can be a viable solution to
the above problem. We began our studies with the reactions of
anilines using the two ruthenium(II)-mediator complexes, viz.,
C
most important reactions for the syntheses of nitrogen-
containing aromatic organic compounds that are ubiquitous in
modern society as medicines and functional materials.4 Polyani-
line is one of such prominent examples that are of paramount
importance5 as conducting polymers. These are commonly
synthesized6 by the electrochemical or chemical oxidation reac-
tion (Scheme 1), which occurs via successive p-CaromꢀN cou-
pling reactions. In contrast, the corresponding o-CaromꢀN bond-
fusion reaction has scarcely been observed. Acid-catalyzed re-
arrangement of hydrazobenzene to semidine is the only textbook
example7 for the synthesis of the ortho dimer of aniline. Recently,
we have discovered8 that strongly oxidizing OsO4 can be used as
a template for bringing about multiple o-CaromꢀN bond-forming
reactions in aniline (Scheme 1). In this report, we describe an
investigation that has led to the development of an insight into
the controlling factors that are responsible for the o-CaromꢀN
bond-fusion reactions.
(Bnz)2RuII Cl4 (Bnz = benzene) and CpRuIICl(PPh3)2 (Cpꢀ =
2
cyclopentadienyl anion). Our primary purpose for selecting these
complexes originated from the consideration that these face-
capped complexes can offer only cis geometry to the incoming
aniline ligands. The redox behaviors of the two chosen mediators
are, however, different. While the benzene complex is redox-
inert, its analogous RuIICp complex is susceptible to oxidation.11
It is worth noting here that Bnz as a coligand in a complex
strongly resists12 oxidation of the metal center because of its
superior π-accepting ability.
In line with our strategy, a suspension of (Bnz)2RuII Cl4 in
2
aniline was stirred at 40ꢀ50 °C in air. A cis-bisaniline complex
(1-Cl) was isolated from the reaction mixture in high yield, 85%.
In contrast, a similar chemical reaction of aniline and CpRuIICl-
(PPh3)2 produced a new complex of the N-phenyl-1,2-diamino-
benzene ligand (2-Cl), formed because of the dimerization of
aniline via o-CaromꢀN bond fusion. The two reference reactions
are depicted in Scheme 2 for comparison. These results thus
indicate that the oxidizable RuIICp complex allows oxidative o-
CaromꢀN bond fusion, while its redox-inert congener fails to
bring about such a bond-fusion reaction in spite of cis coordina-
tion of two aniline ligands. However, the complex 1-Cl, when
heated at 373 K in the presence of aniline or 2-chloroaniline
produced a new diruthenium complex, 3, in moderate yield, 35%
(Scheme 3). The most notable observations in this conversion, 1-
Cl f 3, is that o-CaromꢀN bond-forming reactions occurred only
when the Bnz ligand is substituted by an aromatic amine. This is
as anticipated because the intermediate complex lost its redox-
inert character because of the loss of the Bnz ligand.12 We note
here that the complex 1-Cl remained unreactive in boiling
Scheme 1. CaromꢀN Bond-Fusion Reactions in Aniline
We have been working8,9 on metal-promoted regioselective
ortho dimerization of aniline via CaromꢀN bond-fusion reaction
for quite some time. The first example of such a reaction was
reported by us nearly a decade ago.9a Our results so far have
established that the high-valent metal-mediator complexes
Received: September 11, 2010
Published: March 16, 2011
r
2011 American Chemical Society
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Scheme 2. Reactions of Aniline with Two Mediator
Complexes
Scheme 3. o-CaormꢀN Bond Fusion in an Oxidizable
Condition
Figure 1. Molecular views of the complexes 1-Cl (top) and 2-Cl
(bottom). Counteranions and hydrogen atoms are omitted for clarity.
Selected bond lengths for 1-Cl: Ru1ꢀN1, 2.153(2); Ru1ꢀN2,
2.162(2). Selected bond lengths for 2-Cl: Ru1ꢀN1, 1.992(4);
Ru1ꢀN2, 2.024(4); C1ꢀN1, 1.331(6); C2ꢀC1, 1.425(8); C3ꢀC2,
1.353(8); C4ꢀC3, 1.436(8); C5ꢀC4, 1.362(9); C6ꢀC5, 1.429(7);
C6ꢀN2, 1.336(6).
toluene (383 K). To have further insight into the transformation,
a similar reaction was carried out with a di-ortho-substituted
aniline like 2,6-dimethylaniline in place of unsubstituted aniline.
A complex 3c analogous to 3a and 3b was obtained. The result
clearly indicates that dimerization of coordinated cis-anilines in 1-
Cl via o-CaromꢀN bond fusion occurs only when benzene is
substituted by the aromatic amines.
Details of the chemical reactions are submitted as Supporting
Information. Microanalytical, positive-ion electrospray ioniza-
tion mass spectrometry (ESI-MS) spectra, together with NMR
spectral data of the compounds 1-Cl, 2-Cl, and 3, convincingly
support their formulations as shown in the above schemes.
Characterization data of the products are submitted as Support-
ing Information (Figures S1ꢀS17). Their identities were finally
confirmed by determination of the three-dimensional X-ray
structures of the representative complexes (Figures S18ꢀS20
and Tables S1 in the Supporting Information). For example, the
chloro-substituted complex 3b formed a suitable crystal for X-ray
structure determination. However, the physicochemical proper-
ties of the other two complexes 3a and 3c are similar to those of
the above chloro derivative.
Molecular views and selected structural parameters of the
complexes 1a-Cl and 2-Cl are shown in Figure 1 and that of
the compound 3b are displayed in Figure 2. The bond parameters
exhibit the following trend: for 2-Cl, the values for CdNH(1.331Å),
CdNPh (1.336 Å), and CdC(meta) bond lengths at 1.360 Å,
and for the compound 3, the values for CdNH (1.331 Å),
Figure 2. Molecular view of the complex 3b. Hydrogen atoms are
omitted for clarity. Selected bond lengths: Ru1ꢀN1, 1.953(2);
Ru1ꢀN2, 1.988(2); Ru1ꢀN3, 2.152(2); C1ꢀN1, 1.332(3); C2ꢀC1,
1.422(3); C3ꢀC2, 1.356(4); C4ꢀC3, 1.441(5); C5ꢀC4, 1.351(5);
C6ꢀC5, 1.422(3) C6ꢀN2, 1.331(3); N2ꢀC7, 1.445(3).
CdNPh (1.332 Å), and CdC(meta) bond lengths at 1.355A.
This trend in the bond parameter is very similar to our previously
reported results indicating an o-semiquinonediimine rather than
an o-quinonediimine formulation for the coordinated ligands.13
The compounds are diamagnetic at room temperature. This is
ascribed13,14 to strong antiferromagnetic interactions between the
unpaired spins on the ligand and the trivalent ruthenium centers.
To probe the oxidation state of the coordinated bidentated
semidine ligand in these two complexes, we examined their redox
behaviors. The voltammogram of 2-Cl displays one reversible
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Inorganic Chemistry
COMMUNICATION
oxidation at 1.05 V in addition to an irreversible reduction
feature at ꢀ0.85 V in the potential range from þ1.5 to ꢀ1.5 V
(Figure S21 in the Supporting Information), while the voltam-
mograms of 3 showed one irreversible reduction near about
ꢀ0.75 V and an irreversible oxidation near about 0.65 V. The
reversible oxidative response of 2-Cl was studied by electron
paramagnetic resonance (EPR) spectroscopy. Oxidative electro-
lysis of the complex in dichloromethane at 77 K yielded a ligand-
based signal at g = 1.998, signifying the generation of a
ruthenium(IV) semiquinonediimine radical15 complex upon
oxidation. The EPR spectrum of [2]2þ is submitted as Figure
S22 in the Supporting Information.
The detailed mechanism of the reaction remains unresolved as
yet. We have not been able to identify or isolate the intermediates
of the above reactions. It is worth noting that CꢀN bond-fusion
reactions do not occur under anaerobic conditions. Thus, we
conclude that an oxidizable metal-mediator complex with parti-
cipation of air brings about this transformation. The chemical
reaction of CpRuIICl(PPh3)2 with aniline under anaerobic con-
ditions resulted in a mixture of products, as evidenced from ESI-
MS analysis of the crude product.
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In summary, the present results have clearly demonstrated
that low-valent metal complexes with oxidizable character can act
as excellent mediators for o-CaromꢀN bond-making reactions in
anilido substrates. Such a mediator is more preferable because of
its labile nature, which is crucial for cis coordination of aniline to
the mediator complex. Thus, the redox-inert areneruthenium(II)
complex (Bnz)2RuII Cl4 failed to bring about CaromꢀN bond-
2
fusion reaction, while the oxidizable complex CpRuIICl(PPh3)2
mediated the reaction successfully. However, the Bnz ligand in
1-Cl upon substitution became susceptible to oxidation and
freely mediated oxidative CꢀN bond-formation reactions. Studies
on similar reactions using other suitable metal-mediator com-
plexes and substituted anilines are in progress.
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’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data in
b
CIF format, experimental details of the compounds 1-Cl, 2-Cl,
and 3 along with their spectra and ORTEP diagrams. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: icsg@iacs.res.in.
’ ACKNOWLEDGMENT
The research was supported by the Department of Science and
Technology (DST), India (Project SR/S1/IC/0031/2010).
Crystallography was performed at the DST-funded National
Single Crystal Diffractometer Facility at the Department of
Inorganic Chemistry, IACS. S.S. is thankful to CSIR, New Delhi,
India, for fellowship support.
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