Metal-Promoted Aromatic Ring Amination Reactions following Carbon-Nitrogen Bond Formation. Proof of Amine Coordination for ortho Amination

Article abstract of DOI:10.1021/om9903341

The present report exemplifies two novel aromatic ring amination reactions via metal mediation. A rationale with respect to site selection for the above amination reactions, based on relative labilities of the coordinated coligands, has been established.

Full text of DOI:10.1021/om9903341

3772
Organometallics 1999, 18, 3772-3774
Meta l-P r om oted Ar om a tic Rin g Am in a tion Rea ction s
follow in g Ca r bon -Nitr ogen Bon d F or m a tion . P r oof of
Am in e Coor d in a tion for or th o Am in a tion
Amrita Saha,† Amit K. Ghosh,† Partha Majumdar,† Kedar N. Mitra,†
Sujit Mondal,† Kajal K. Rajak,† Larry R. Falvello,‡ and Sreebrata Goswami*,†
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India, and Departamento de Quimica Inorganica, Universidad de Zaragoza,
50009 Zaragoza, Spain
Received May 5, 1999
Summary: The present report exemplifies two novel
aromatic ring amination reactions via metal mediation.
A rationale with respect to site selection for the above
amination reactions, based on relative labilities of the
coordinated coligands, has been established.
tallization gives5 the crystalline green compound 1 as
the major product (yield >80%).
ArNH2
2+
+
Co(pap)3
8
(i)
Co(pap-o-am)2
1a -c
Metal-mediated reactions which result in addition of
[NR]2- fragments belong to a reaction class of funda-
mental importance in chemistry in connection with
carbon-nitrogen bond formation processes.1 In this
respect, transition-metal imido complexes are implicated
as reactive intermediates2 in bringing about the above
transformations. Herein we wish to report our results
on aromatic ring amination of a pendant aryl ring of
coordinated 2-(phenylazo)pyridine (pap). These reac-
tions do not occur with free pap ligand. Upon coordina-
tion, both o- and p-C-H of the pendant phenyl group of
the diaza ligand are activated. As a result, amination
processes occurring at both ortho and para positions
were observed (Scheme 1). By the use of two suitable
metal complexes of completely different nature, it has
been possible for us to show that labilities of coligands
in the metal complexes play an important role for the
ortho-fusion process. Two sets of reactions of ArNH2
occurring at a labile complex, [Co(pap)3](ClO4)2, and an
inert complex, [RhCl2(pap)2]ClO4, have been used for
this purpose. It may be relevant to mention here that
amination of an aromatic ring3 by the activation of a
C-H bond is still relatively rare but is synthetically a
useful reaction.
ArNH2 ) aniline (1a ),
p-toluidine (1b), p-anisidine (1c)
These diamagnetic cobalt(III) compounds are 1:1 elec-
trolytes in acetonitrile and show highly resolved 1H
NMR spectra. The 1H NMR spectra of the cobalt
complexes are in complete agreement with the crystal
structure of a representative example (vide infra).
Notably, each kind of proton of the coordinated anionic
ligand pap-o-am- gave rise to one signal. Thus, the two
ligands in 1 are magnetically equivalent, indicating the
presence of a 2-fold symmetry axis. The product from
this unusual reaction was finally confirmed on the basis
of the X-ray structure determination6 of a representative
complex, 1a . The structural analysis of 1a indeed
authenticated the fusion of ArNH2 at a position ortho
to the aza group of pap (Figure 1). We refer to this as
pap-o-am. There are two such extended ligands in 1a ,
each of which acts as a N,N,N-tridentate donor with the
deprotonation of the amino nitrogens (N(4) and N(8)).
The geometry of the cationic complex is meridional, and
the bond distances indicate the presence of backbone
conjugation in the anionic pap-o-am- ligand. Thus, the
aza nitrogen to phenyl carbon distances as well as the
The brown complex4 Co(pap)32+ reacts smoothly with
neat ArNH2 on a steam bath to produce an intense
green mixture which on workup and subsequent crys-
(5) A mixture of [Co(pap)3](ClO4)2 (0.2 g) and ArNH2 (2.0 g) was
heated on a steam bath for 1 h. The cooled green mixture was throughly
washed with diethyl ether, and crystallization of the crude product
from a CH2Cl2/C6H14 mixture yielded 1 (80-85%) as green crystals.
Physical data for a representative sample of 1a are as follows. Anal.
Calcd for C34H26N8ClO4Co: C, 57.92; H, 3.69; N, 15.30. Found: C, 57.35;
H, 3.86; N, 15.45. UV/vis (CH3CN): λmax (ꢀ) 787 sh (12 930), 712 sh
(12 500), 405 (22 450), 308 sh (30 000), 246 nm (59 600). IR (KBr):
* To whom correspondence should be addressed. E-mail: icsg@
mahendra.iacs.res.in. Fax: 91-33-473-2805.
† Indian Association for the Cultivation of Science.
‡ Universidad de Zaragoza.
(1) (a) Gray, S. D.; Thorman, J . L.; Adamian, V. A.; Kadish, K. M.;
Woo, K. L. Inorg. Chem. 1998, 37, 1. (b) Zen, L.; Conser, K. R.;
J acobsen, E. N. J . Am. Chem. Soc. 1993, 115, 5326. (c) Evans, D. A.;
Faul, M. N.; Bilodeau, M. T. J . Am. Chem. Soc. 1994, 116, 2742. (d)
Li, Z.; Quan, R. W.; J acobsen, E. N. J . Am. Chem. Soc. 1995, 117, 5889.
(e) Bruncko, M.; Khuong, T.-A. V.; Sharpless, K. B. Angew. Chem., Int.
Ed. Engl. 1996, 35, 454.
(2) (a) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239. (b) Nugent,
W. A.; Mayer, J . H. Metal-Ligand Multiple Bonds; Wiley-Interscience:
New York, 1988. (c) Mohs, T. R.; Du, Y.; Plashko, B.; Mata, E. A. J .
Chem. Soc., Chem. Commun. 1997, 1707. (d) Au, S.-M.; Fung, W.-H.;
Huang, J .-S.; Cheung, K.-K.; Che, C.-M. Inorg. Chem. 1998, 37, 6564.
(3) Hartwig, J . F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2046 and
references therein.
ν(NdN) 1590, ν(C-N) 1250, ν(ClO4-) 1100, 625 cm-1
.
(6) X-ray structure of 1a : X-ray-quality crystals (0.4 × 0.4 × 0.4 mm)
of 1a were obtained by slow diffusion of a chloroform solution of the
complex into hexane. Cell parameters were obtained by least-squares
fits of 30 machine-centered reflections. Data were collected (3 e 2θ e
43°) on a Nicolet R3m/V diffractometer (295 K) with graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å). The structure was
solved by the Patterson heavy-atom method using the SHELXTL-plus
program package on a Micro VAXII computer. Refinement was done
by the full matrix method. Crystal and refinement data: C34H26
-
ClCoN8O4, triclinic, P1h, a ) 9.106(4) Å, b ) 10.360(3) Å, c ) 17.356(6)
Å, R ) 104.04(3)°, â ) 91.34(3)°, γ ) 93.74(3)°, V ) 1583.7(10) Å3, Z )
2, with 433 parameters refined on 4158 data with I > 2σ(I), R1 )
0.0397 and wR2 ) 0.0903, GOF ) 1.026.
(4) Mahapatra, A. K. Ph.D. Thesis, J adavpur University (India),
1989.
10.1021/om9903341 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 08/14/1999

Communications
Organometallics, Vol. 18, No. 19, 1999 3773
Sch em e 1
come in close proximity to the o-C-H of the pendant
phenyl group of pap for ortho fusion. In
this context, it is worthwhile to mention here that our
recent work9,10on metal-promoted ortho dimerization of
aromatic amines clearly demonstrated that these fusion
reactions proceed via amine coordination to the metal
center.
To establish the above proposition on the ortho-
amination process, we performed another reaction using
RhCl2(pap)2 as the starting compound.11 Unlike Co-
+
(pap)32+, which is very labile and loses one pap readily
+
in solution, RhCl2(pap)2 is substitutionally inert. It is
known that a substitution reaction12 at this cationic
rhodium complex, even under the most forceful condi-
tions, results only in the monosubstituted complex
n+
RhCl(N)(pap)2 (N ) nucleophile; n ) 1, 2). Thus, if
coordination to the metal center is a prerequisite for
ortho amination, one would expect that ortho amination
F igu r e 1. ORTEP plot and atom-numbering scheme for
[Co(pap-o-am)2]+ (1a ) in [Co(pap-o-am)2]ClO4. Hydrogen
atoms are omitted for clarity.
at best can occur at only one of the two coordinated pap
+
ligands of RhCl2(pap)2
.
amine nitrogen of pap-o-am- bonds to the phenyl group
of pap are shorter than a C-N single bond. The N-N
aza distances in these anionic ligands are considerably
longer than that observed7 in [papH]ClO4, in agreement
with the notion of electron delocalization along the
ligand backbone in the anionic pap-o-am-. In this
reaction two out of the three coordinated pap ligands
in the starting compound, Co(pap)32+, get aminated
ortho to the aza nitrogen to result in the bis-chelated
[Co(pap-o-am)2]+. During this process one bidentate
ligand pap is eliminated from Co(pap)32+, leaving behind
two vacant coordination sites. Coordination of two Ar-
NH2 residues at the vacant sites followed by oxidative
fusion8 at the ortho carbon of the pendant phenyl ring
of coordinated pap ligands are the two most plausible
steps for this reaction. On coordination, ArNH2 residues
+
The reaction of neat ArNH2 with RhCl2(pap)2 pro-
duced13 a deep blue mixture which on chromatographic
purification yielded the ink blue crystalline compound
2 in ca. 65% yield.
ArNH2
+
RhCl2(pap)2
8
[RhCl(pap-o-am)(pap-p-amH)]+
2a -c
(ii)
ArNH2 ) aniline (2a ),
p-toluidine (2b), p-anisidine (2c)
This compound is diamagnetic (Rh(III), 4d6) and showed
1
a very complex H NMR spectrum due to the presence
(9) Mitra, K. N.; Majumdar, P.; Peng, S.-M.; Castin˜eiras, A.;
Goswami, S. Chem. Commun. 1997, 1267.
(10) Mitra, K. N.; Peng, S.-M.; Goswami, S. Chem. Commun. 1998,
1685.
(7) Majumdar, P.; Peng, S.-M.; Goswami, S. J . Chem. Soc., Dalton
Trans. 1998, 1569.
(8) The reactions under consideration do not occur in the absence
of aerial oxygen. The overall transformation pap f papamH is a 2e-
+ 2H+ transfer reaction.
(11) Deb, A. K.; Goswami, S. J . Chem. Soc., Dalton Trans. 1989,
1635.
(12) Majumdar, P.; Ghosh, A. K.; Goswami, S. Unpublished results.

3774 Organometallics, Vol. 18, No. 19, 1999
Communications
group of pap; we shall refer to this as pap-p-amH. This
ligand is bidentate and chelates to rhodium through the
pyridine nitrogen N(1) and the aza nitrogen N(8). The
amine nitrogen atom N(15) of this bears a hydrogen
atom, which is in marked opposition to the correspond-
ing pap-o-am-, which binds as an anionic ligand. The
second ligand, pap-o-am-, acts as a monoanionic tri-
dentate ligand which is similar to that observed in the
2+
Co(pap)3 and ArNH2 reaction. The bidentate pap-p-
amH and tridentate pap-o-am- along with one chloride
complete a distorted-octahedral coordination environ-
ment about rhodium. Furthermore, comparison of bond
distances in this molecule indicates the presence of more
delocalization in the anionic pap-o-am- than that in the
neutral pap-p-amH.
Thus, the above results have clearly demonstrated
that an ortho-fusion process is favored in the case of a
labile metal complex. It seems reasonable, since the
coordination of an amine residue to the metal ion at the
vacant site of the leaving ligand can bring it in close
proximity to the o-C-H of the pendant phenyl group to
promote ortho amination. In the absence of any such
vacant site at the metal center, the ortho carbon is not
approachable by ArNH2 and fusion occurs at the para
carbon as a second choice. Therefore, the site selectivity
for this amination process can be controlled by the
proper selection of metal complex.
F igu r e 2. ORTEP plot and atom-numbering scheme for
[RhCl(pap-o-am)(pap-p- amH)]+ (2c) in [RhCl(pap-o-am)-
(pap-p-amH)]ClO4.
of large numbers of unique protons. Fortunately, the
product, 2c, obtained from the reaction of p-anisidine
+
with RhCl2(pap)2 formed crystals suitable for the
determination14 of its X-ray structure. In this case,
unlike reaction (i), two types of fusion of ArNH2 with
the pap moiety have occurred to result in isomeric pap-
amH ligands (Figure 2). In the present rhodium complex
2, one of the extended ligands is neutral, namely that
which contains N(1), N(8), and N(15). This ligand has
the p-anisidine fragment fused to pap at the para
position (relative to the aza fragment) of the phenyl
(13) A mixture of [Rh(pap)2Cl2]ClO4 (0.2 g) and ArNH2 (2.0 g) was
heated on a steam bath for 1 h. The cooled green mixture was throughly
washed with diethyl ether and was purified on a silica gel column.
Finally, crystallization from a CH2Cl2/C6H14 mixture yielded 2 (60-
65%) as dark blue crystals. Physical data for a representative sample
of 2a are as follows. Anal. Calcd for C34H27N8Cl2O4Rh: C, 51.97; H,
3.43; N, 14.26. Found: C, 51.79; H, 3.50; N, 13.96. UV/vis (CH3CN):
λmax (ꢀ) 801 sh (6180), 600 (25 500), 375 sh (13 800), 266 (63 400), 220
nm (96 000). IR (KBr): ν(N-H) 3500, ν(NdN) 1580, ν(C-N) 1300,
Finally, we wish to note here that we have been
successful in isolating both isomers of the aminated
organic compounds, viz. pap-o-amH and pap-p-amH, as
free ligands by the removal of the respective metal ions.
The coordination chemistry of these pyridyl aza-amines
appears to be rich and is currently under scrutiny.
ν(ClO4-) 1100, 620, ν(Rh-Cl) 340 cm-1
.
(14) X-ray structure of 2c: geometric and intensity data on 2c were
gathered from a dark, block-shaped crystal (0.49 × 0.43 × 0.36 mm),
obtained by slow diffusion of a dichloromethane solution into hexane,
using a Nonius CAD-4 diffractometer. After the initial reflection search
and indexing, axial photographs were taken for a, b, c, and [111] to
verify the dimensions of the triclinic lattice. The scan parameters for
data collection were based on two-dimensional (ω-θ) plots of 18
Ack n ow led gm en t. We are grateful to the Depart-
ment of Science and Technology, Council of Scientific
and Industrial Research, New Delhi, and the Director-
ate for Higher Education of Spain (Grant PB95-0792)
for financial support.
reflections. Absorption corrections were based on
a full three-
Su p p or tin g In for m a tion Ava ila ble: Complete details on
the X-ray analyses of 1a and 2c, including tables of bond
distances and angles, atomic coordinates, anisotropic displace-
ment parameters, and hydrogen atom coordinates, and figures
giving representative 1H NMR spectra for complexes 1a and
2a . This material is available free of charge via the Internet
at http://pubs.acs.org.
dimensional absorption model derived from 15 ψ-scans with bisecting-
mode Eulerian angle ø between -31 and +44°. The structure was
solved by direct methods and developed and refined in the usual
alternating series of least-squares cycles and difference Fourier maps,
which showed residual extrema at 0.75 and -0.50 e Å-3. Crystal and
refinement data: C36H31Cl2N8O6Rh, triclinic, P1h, a ) 9.0138(5) Å, b )
12.6198(10) Å, c ) 16.695(3) Å, R ) 109.968(9)°, â ) 100.865(9)°, γ )
93.809(7)°, V ) 1735.2(4) Å3, Z ) 2, with 602 parameters refined on
2
6074 data with Fo g 2σ(Fo2), R1 ) 0.0370 and wR2 ) 0.1035, GOF )
1.041.
OM9903341