Rhodium-mediated C-C bond activation of 2-(2′,6′- dialkylarylazo)-4-methylphenols. Elimination and migration of alkyl groups

Article abstract of DOI:10.1021/om700826t

Upon reaction of 2-(2′,6′-dimethylphenylazo)-4-methylphenol with [Rh(PPh₃)₃Cl], the azo ligand undergoes an interesting rhodium-assisted C-C bond activation at one ortho position of the 2′,6′-dimethylphenyl fragment, leading to the elimination of the methyl group from that ortho position. A similar elimination of an ethyl group takes place when 2-(2′,6′-diethylphenylazo)-4-methylphenol is allowed to react with [Rh(PPh₃)₃Cl]. However, when 2-(2′,6′-diisopopylphenylazo)-4-methylphenol reacts with [Rh(PPh₃)₃Cl], an interesting migration of an isopropyl group from its original location (say the 2′ position) to the corresponding third position (say the 4′ position) takes place. In all the three complexes the modified azo ligands bind to rhodium as dianionic C,N,O-donors, and two triphenylphosphines and a chloride are also coordinated to the metal center. Structures of all three complexes have been determined by X-ray crystallography. These complexes show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on these complexes shows a Rh(III)-Rh(IV) oxidation on the positive side of SCE and a reduction of the coordinated azo ligand on the negative side.

Full text of DOI:10.1021/om700826t

6596
Organometallics 2007, 26, 6596–6603
Rhodium-Mediated C–C Bond Activation of
2-(2′,6′-Dialkylarylazo)-4-methylphenols. Elimination and Migration
of Alkyl Groups
Suparna Baksi,† Rama Acharyya,† Falguni Basuli,† Shie-Ming Peng,‡ Gene-Hsiang Lee,‡
Munirathinam Nethaji,§ and Samaresh Bhattacharya*,†
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity, Kolkata 700 032, India,
Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan, Republic of China, and
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
ReceiVed August 13, 2007
Upon reaction of 2-(2′,6′-dimethylphenylazo)-4-methylphenol with [Rh(PPh3)3Cl], the azo ligand
undergoes an interesting rhodium-assisted C–C bond activation at one ortho position of the 2′,6′-
dimethylphenyl fragment, leading to the elimination of the methyl group from that ortho position. A
similar elimination of an ethyl group takes place when 2-(2′,6′-diethylphenylazo)-4-methylphenol is allowed
to react with [Rh(PPh3)3Cl]. However, when 2-(2′,6′-diisopopylphenylazo)-4-methylphenol reacts with
[Rh(PPh3)3Cl], an interesting migration of an isopropyl group from its original location (say the 2′ position)
to the corresponding third position (say the 4′ position) takes place. In all the three complexes the modified
azo ligands bind to rhodium as dianionic C,N,O-donors, and two triphenylphosphines and a chloride are
also coordinated to the metal center. Structures of all three complexes have been determined by X-ray
crystallography. These complexes show characteristic 1H NMR signals and intense MLCT transitions in
the visible region. Cyclic voltammetry on these complexes shows a Rh(III)–Rh(IV) oxidation on the
positive side of SCE and a reduction of the coordinated azo ligand on the negative side.
ring in the arylazo fragment and bind to the metal center as
tridentate C,N,O-donors (II), affording interesting cyclometa-
lated complexes.4e,h,i,k,m,o,r In the present study we have utilized
three modified 2-(arylazo)phenols (III), viz., 2-(2′,6′-dimeth-
ylphenylazo)-4-methylphenol (L1), 2-(2′,6′-diethylphenylazo)-
4-methylphenol (L2), and 2-(2′,6′-diisopropylphenylazo)-4-
methylphenol (L3), in which both the ortho positions of the
phenyl ring in the arylazo fragment are strategically blocked
by three different alkyl groups. The main objective of this ligand
modification was to see whether these modified ligands (III)
undergo any C–C bond activation at the 2′ position. In order to
investigate the consequences of this ligand modification,
rhodium has been selected as the platinum metal, and as a source
of rhodium, the Wilkinson’s catalyst, [Rh(PPh3)3Cl], has been
utilized. Reactions of the three selected ligands (L1, L2, and
L3) with [Rh(PPh3)3Cl] have indeed afforded three interesting
organorhodium complexes via C–C bond activation at the 2′
position of the azo ligands. The chemistry of these complexes
Introduction
There has been significant current interest in the utilization
of transition metal complexes for bringing about interesting
chemical transformations of organic substrates.1 Such reactions
often proceed via a C–H or C–C activation of the organic
substrates,2,3 leading to the formation of reactive organometallic
complexes, which finally yields the desired product, and hence
synthesis of such organometallic species is of significant
importance. The present work has emerged from our interest
in the synthesis of organometallic complexes via platinum metal
mediated C–H and C–C activation of some selected organic
ligands, in general,4 and C–H activation of 2-(arylazo)-4-
methylphenols (I) in particular.4e,h,i,k,m,o,r It has been observed
that upon reaction with different triphenylphosphine complexes
of the platinum metals, these ligands (I) usually undergo C–H
activation at one ortho position (say the 2′ position) of the phenyl
* Corresponding author. E-mail: samaresh_b@hotmail.com.
† Jadavpur University.
(2) (a) Jaeger-Fiedle, U.; Arndt, P.; Baumann, W.; Spannenberg, A.;
Burlakov, V. V.; Rosenthal, U. Eur. J. Inorg. Chem. 2005, 14, 2842. (b)
Keuseman, K. J.; Smolinkova, I. P.; Dunina, V. V. Organometallics 2005,
24, 4159. (c) Shi, L.; Tu, Y. Q.; Wang, M.; Zhao, F. M.; Fan, C. A.; Zhao,
Y. M.; Xia, W. J. J. Am. Chem. Soc. 2005, 127, 10836. (d) Thalji, R. K.;
Ahrendt, K. A.; Bergman, R. G.; Ellman, J. A. J. Org Chem. 2005, 70,
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‡ National Taiwan University.
§ Indian Institute of Science.
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Weinheim, 2000. (b) Hegedus, L. S. Coord. Chem. ReV. 1998, 168, 49. (c)
Bellar, M., Bolm, C., Eds. Transition Metals for Organic Synthesis 1–2;
Wiley-VCH: Weinheim, 1998. (d) Cornils, B., Hermann, W. A., Eds.
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Liebeskind, L. S., Ed. AdVances in Metal-Organic Chemistry; Jai Press:
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10.1021/om700826t CCC: $37.00
2007 American Chemical Society
Publication on Web 10/30/2007

Rhodium-Mediated C–C Bond ActiVation
Organometallics, Vol. 26, No. 26, 2007 6597
is reported herein, with special reference to their formation,
structure, and spectroscopic and electrochemical properties.
Results and Discussion
Figure 1. Structure of complex 1.
As delineated in the Introduction, three 2-(2′,6′-dialkylphe-
nylazo)-4-methylphenols (III) have been utilized in the present
study with the definite objective to see whether one of the two
C–R bonds, at the 2′ and 6′ positions, undergoes C–C activation
upon reaction with [Rh(PPh3)3Cl]. Reaction of the 2-(2′,6′-
dimethylphenylazo)-4-methylphenol (L1) was first carried out
with [Rh(PPh3)3Cl] in refluxing toluene in the presence of
triethylamine, which afforded the blue complex 1. Preliminary
characterizations (microanalysis, mass, and 1H NMR) on
complex 1 hinted at the presence of two triphenylphosphines,
a chloride, and a ligand L1 and also indicated that ligand L1
probably underwent some chemical transformation in the course
of its binding to rhodium. For example, although ligand L1
contains three methyl groups, the 1H NMR spectrum of complex
1 showed two methyl signals of equal intensity at 1.82 and 2.33
ppm, indicating loss of a methyl group from the L1 ligand during
formation of complex 1. The mass spectrum of this complex
(Figure S1, Supporting Information) showed the [M + H]+ peak
at 887, which is 15 units less than that expected for a rhodium
complex containing two triphenylphosphines, a chloride, and a
deprotonated L1, and thus the mass spectral data also supported
loss of a methyl group from ligand L1. Microanalytical data
were also consistent with this speculated loss of a methyl group
from ligand L1. To verify this speculation, the identity of
complex 1 was unveiled by its structure determination by X-ray
crystallography. The structure is shown in Figure 1, and selected
bond parameters are listed in Table 1. The structure shows that
ligand L1 has indeed undergone an interesting chemical
transformation during the course of the synthetic reaction, in
which one methyl group has been eliminated from one ortho
position (say the 2′ position) of the phenyl ring in the arylazo
fragment and a direct metal–carbon bond has been established
between the rhodium center and that particular phenyl ring
carbon from which the methyl group has been lost. The modified
L1 ligand is thus coordinated to rhodium in a dianionic tridentate
C,N,O-fashion (IV, R ) CH3). Two triphenylphosphines and a
chloride are also bound to the metal center. Rhodium is therefore
sitting in a CNOP2Cl coordination environment, which is
distorted octahedral in nature, as reflected in all the bond
parameters around rhodium. The C,N,O-coordinated ligand and
the chloride constitute one equatorial plane with the metal at
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6598 Organometallics, Vol. 26, No. 26, 2007
Baksi et al.
Table 1. Selected Bond Distances and Bond Angles for Complexes
1, 2, and 3
such hydrogen-bonding interactions are of significant current
interest in crystal engineering as well as in biology.5
Elimination of a methyl group from ligand L1 upon its
reaction with [Rh(PPh3)3Cl] via rhodium-assisted C–C bond
activation has prompted us to explore similar reactions with
other 2-(2′,6′-dalkylphenylazo)-4-methylphenols. Hence the
other two related ligands, viz., 2-(2′,6′-diethylphenylazo)-4-
methylphenol (L2) and 2-(2′,6′-diisopropylphenylazo)-4-meth-
ylphenol (L3), were tried. Reactions of these two ligands with
[Rh(PPh3)3Cl] were carried out under experimental conditions
similar to those used before, and two new rhodium complexes,
2 and 3, were obtained. As observed in the case of complex 1,
preliminary characterizations on complex 2 again indicated that
two triphenylphosphines, a chloride, and a monodeethylated L2
ligand are coordinated to rhodium. The loss of an ethyl fragment
from L2 was indicated by both the 1H NMR spectrum of
complex 2, showing a signal for only one ethyl group, and its
mass spectrum (Figure S2, Supporting Information), showing
the [M + H]+ peak at 901, which is 29 units less than that
expected for a rhodium complex containing two triphenylphos-
phines, a chloride, and a deprotonated L2. The observed
microanalytical data also supported this loss of an ethyl group
from L2. For an unambiguous characterization of complex 2 as
well as to authenticate the status of ligand L2 in it, its structure
was determined by X-ray crystallography. The structure (Figure
3) shows that one ethyl fragment has indeed been lost from
ligand L2 during formation of complex 2, and the modified
ligand is bound to rhodium as a C,N,O-donor (IV, R ) C2H5).
Two triphenylphosphines, which are mutually trans, and a
chloride are also coordinated to the metal center. The structural
parameters of complex 2 are similar to those of complex 1
(Table 1). An examination of the packing pattern in the lattice
of complex 2 shows that C–H---π interactions, involving
different phenyl C–H’s of the PPh3 ligands and π-cloud over
the phenyl rings of both PPh3 and the azo-ligand, are active in
the lattice (Figure S3, Supporting Information).
Complex 1
Bond Distances (Å)
Rh–Cl1
Rh–P1
Rh–P2
Rh–O1
2.3786(12)
2.3565(9)
2.3727(9)
2.194(3)
Rh–N1
Rh–C8
C1–O1
N1–N2
1.953(3)
1.998(4)
1.303(5)
1.279(4)
Bond Angles (deg)
P1–Rh–P2
Cl1–Rh–N1
O1–Rh–C8
174.19(4)
178.22(9)
O1–Rh–N1
N1–Rh–C8
80.28(11)
79.11(14)
159.38(14)
Complex 2
Bond Distances (Å)
Rh–Cl1
Rh–P1
Rh–P2
Rh–O1
2.367(3)
Rh–N1
Rh–C15
C1–O1
N1–N2
1.941(9)
2.357(3)
2.355(3)
2.179(7)
1.998(11)
1.294(15)
1.253(13)
Bond Angles (deg)
P1–Rh–P2
Cl1–Rh–N1
O1–Rh–C15
174.07(10)
178.0(2)
159.2(4)
O1–Rh–N1
N1–Rh–C15
80.8(3)
78.4(4)
Complex 3
Bond Distances (Å)
Rh–Cl1
Rh–P1
Rh–P2
Rh–O1
2.379(2)
Rh–N1
Rh–C19
C1–O1
N1–N2
1.957(7)
2.000(8)
1.292(12)
1.280(10)
2.363(2)
2.357(2)
2.180(5)
Bond Angles (deg)
P1–Rh–P2
Cl1–Rh–N1
O1–Rh–C19
172.51(7)
177.3(2)
159.0(3)
O1–Rh–N1
N1–Rh–C19
80.1(3)
78.9(3)
the center, while the two PPh3 ligands occupy the remaining
two axial positions, and hence they are mutually trans. The
Rh–C, Rh–N, Rh–O, Rh–P, and Rh–Cl distances are all quite
normal, as usually observed in complexes of rhodium(III)
containing these bonds.4r
Preliminary characterization of complex 3 indicated the
presence of two triphenylphosphines, a chloride, and a ligand
L3. However, unlike complexes 1 and 2, no indication of loss
of any isopropyl group from ligand L3 during formation of
1
complex 3 was obtained. For example, the H NMR spectrum
of complex 3 shows clear signals for two isopropyl groups. At
the same time, the NMR spectrum also shows three singlets
(1H each) in the aromatic region at 5.85, 6.44, and 6.93 ppm,
and as only one such singlet could be expected from the para-
cresol fragment of the azo-ligand, the appearance of these three
singlets was rather surprising. The mass spectrum of complex
3 (Figure S4, Supporting Information) shows the [M + H]+
peak at 957, as expected for a rhodium complex containing two
The absence of any solvent of crystallization in the crystal
lattice of complex 1 indicates the possible existence of nonco-
valent interactions between the individual complex molecules.
To sort this out, the packing pattern in the lattice has been
examined, which shows that a hydrogen-bonding interaction of
a single type, viz., C–H---π interaction, is active in the lattice
(Figure 2). One methyl C–H in the arylazo fragment of each
complex molecule is hydrogen-bonded to the π-cloud over a
phenyl ring of a PPh3 ligand belonging to an adjacent complex
molecule. In addition, phenyl C–H’s of different PPh3 ligands
are found to be involved in intermolecular hydrogen-bonding
interactions with the π-cloud over phenyl rings of PPh3 ligands
as well as of the azo ligand. Through such C–H---π interactions
each complex molecule is linked to six neighboring molecules,
and these hydrogen-bonding interactions are extended through-
out the entire lattice; they appear to be responsible for holding
the crystal together. It may be mentioned in this context that
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1229.

Rhodium-Mediated C–C Bond ActiVation
Organometallics, Vol. 26, No. 26, 2007 6599
Figure 2. C–H---π interactions in complex 1.
3 as well as to ascertain the status of ligand L3 in it, its structure
has also been determined by X-ray crystallography. The structure
(Figure 4) reveals that an interesting transformation of ligand
L3 has taken place during formation of complex 3, whereby
one isopropyl group has migrated from its original location (say
the 2′ position) and is inserted (via loss of a proton) into the
C–H bond at the corresponding 4′ position, thus generating an
isopropyl group at the 4′ position, and the modified ligand is
bound to rhodium as a C,N,O-donor (V). The other structural
features of complex 3 are similar to those of complexes 1 and
2 (Table 2). An examination of the packing pattern in the lattice
of complex 3 shows that intermolecular C–H---π interactions,
similar to those observed in complex 2, have linked the different
molecules in the lattice and thus have given it the observed
stability (Figure S5, Supporting Information).
The exact mechanisms behind the observed transformations
of ligand L1, L2, and L3 during formation of complexes 1, 2,
and 3, respectively, are not completely clear to us. However,
some speculated sequences shown in Scheme 1 and Scheme 2
seem probable.
During reaction of ligand L1 or L2 with the rhodium(I) center
of [Rh(PPh3)3Cl], the phenolic O–H bond of the azo-ligand
seems to undergo rhodium-assisted activation in the initial step,
whereby it oxidatively binds to the metal center, generating a
hydride complex of rhodium(III) as the reactive intermediate
Figure 3. Structure of complex 2.
triphenylphosphines, a chloride, and a deprotonated L3. The
observed microanalytical data were also in accordance with this
formulation. For an unambiguous characterization of complex

6600 Organometallics, Vol. 26, No. 26, 2007
Baksi et al.
Table 2. Electronic Spectral and Cyclic Voltammetric Data
electronic spectral dataa max, nm (ε, M-1 cm-1
compound
λ
)
cyclic voltammetric dataa,b
1
2
3
630(11200), 592(9800), 384c(10500), 339c (18700), 289(62300)
626(11700), 585c(10600), 386c(10900), 333c (19100), 290(43000)
624(10200), 588c(10300), 382c(12101), 342c(15923), 289(32484)
0.79d(90) e, -1.05f
0.97d(270)e, -0.99 f
1.03d (327) e, -1.25 f
d
a In acetonitrile. b Supporting electrolyte, TBAP; scan rate 50 mV s-1; reference electrode, SCE. c Shoulder. E1/2 ) 0.5(Epa+Epc),where Epa and Epc
f
are anodic and cathodic peak potentials, respectively. e ∆Ep ) (Epa – Epc). Epc value.
(methane in the case of 1 and ethane in the case of 2). Upon
reaction of ligand L3 with [Rh(PPh3)3Cl], a similar hydride
intermediate is formed in the first step (Scheme 2), which is
followed, as before, by formation of a metal–carbon bond with
simultaneous loss of the hydride. In the next step, unlike loss
of the alkyl group as seen before, loss of the central proton
from the isopropyl group at the 2′ position seems to be
happening. Subsequently a C–C bond is formed between the
central carbon of the deprotonated isopropyl group and the ring
carbon at the 3′ position having the positive charge. This is
followed by migration of the C(CH3)2 fragment, which was
bound in an η2-fashion to the 2′,3′ ring carbons, to the
corresponding 3′,4′ position. In the next step usual rearrange-
ments in the η2-bound C(CH3)2 fragment take place, affording
complex 3. Formation of complex 3, from the hydride inter-
mediate, thus occurs via loss of molecular hydrogen.
Magnetic susceptibility measurements showed that all the
complexes are diamagnetic, which corresponds to the +3
oxidation state of rhodium (low-spin d6, S ) 0) in these
complexes. 1H NMR spectra of the complexes have been
recorded in CDCl3 solution. Each complex shows broad signals
within 7.0–7.7 ppm for the coordinated PPh3 ligands. The methyl
signal from the phenol fragment is observed as a distinct peak
near 1.8 ppm. The other aliphatic proton signals originating from
Figure 4. Structure of complex 3.
the methyl, ethyl, and isopropyl groups of the coordinated
ligands appear at their usual positions. Among the expected
aromatic proton signals for the coordinated azo-ligands, most
have been clearly observed, while a few could not be detected
due to their overlap with the PPh3 signals. Infrared spectra of
Scheme 1. Probable Steps in the Formation of Complexes 1
and 2
the complexes show sharp bands near 746, 696, and 518 cm-1
,
due to the coordinated PPh3 ligands. Strong bands are also
observed near 1483, 1434, 1380, 1305, 1244, 1190, and 1093
cm-1 in all these complexes, which are absent in the infrared
spectrum of [Rh(PPh3)3Cl], and hence these bands are attributed
1
to the coordinated azo-ligands. The H NMR and IR spectral
data of the complexes are therefore consistent with their
compositions.
The complexes 1, 2, and 3 are found to be soluble in
dichloremethane, chloroform, acetonitrile, acetone, etc., produc-
ing intense blue solutions. Electronic spectra of these complexes
have been recorded in acetonitrile solution. Each complex shows
several intense absorptions in the ultraviolet and visible region
(Table 2). The absorptions in the ultraviolet region are assignable
to transitions within the ligand orbitals. To understand the origin
of the absorptions in the visible region, EHMO calculations have
been performed on computer-generated models of all the three
complexes,6 where phenyl rings of the PPh3 ligands have been
replaced by hydrogens. Compositions of selected molecular
orbitals are given in Table 3, and the partial MO diagram of
complex 1 is shown in Figure 5. Partial MO diagrams of
complexes 2 and 3 are deposited as Figure S6 and Figure S7
(Supporting Information). The calculations show that the highest
occupied molecular orbital (HOMO) has major (>74%) con-
tribution from the metal d-orbitals, whereas the lowest unoc-
(Scheme 1). This is followed by formation of a metal–carbon
bond from one ortho position (say the 2′ position) of the phenyl
ring in the phenylazo fragment, causing loss of aromaticity of
that ring and generating a positive charge at the 3′ position.
Formation of the rhodium–carbon bond is associated with
simultaneous dissociation of the coordinated hydride, which
probably triggers loss of the R group from the 2′ position as a
carbocation, and thus the lost aromaticity of the phenyl ring is
regained, affording the organorhodium complexes 1 and 2.
Considering the last two steps together, formation of complexes
1 and 2 from the hydride intermediate occurs via loss of RH
(6) (a) Mealli, C.; Proserpio, D. M. CACAO Version 4.0; Italy, 1994.
(b) Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.

Rhodium-Mediated C–C Bond ActiVation
Organometallics, Vol. 26, No. 26, 2007 6601
Scheme 2. Probable Steps in the Formation of Complex 3
Table 3. Composition of Selected Molecular Orbitals
% contribution of fragments to
compound
contributing fragments
HOMO
HOMO–1
41
HOMO–2
LUMO
13
LUMO+1
LUMO +2
1
Rh
Cl
78
5
57
5
2
ONCligand
Rh
Cl
ONCligand
Rh
Cl
9
55
49
17
24
16
32
44
84 (NdN,50)
91
2
91
2
3
75
13
6
74
4
21
2
75 (NdN,46)
12
52
34
89
97
92
2
ONCligand
10
68
38
81 (NdN,50)
91
cupied molecular orbital (LUMO) is primarily (>75%) delo-
calized over the coordinated C,N,O-donor ligand and is
concentrated heavily (>46%) on the azo fragment. Hence the
lowest energy absorption in the visible region is assignable to
a charge-transfer transition from the filled rhodium orbital
(HOMO) to the π* (azo)-orbital (LUMO) of the tridentate
ligand. The other absorptions in the visible region are attributable
to transitions occurring from the other filled orbitals to the higher
energy vacant orbitals.
Electrochemical properties of the complexes have been
studied by cyclic voltammetry in acetonitrile solution (0.1 M
TBAP). Each complex shows one oxidative response on the
positive side of the SCE and one reductive response on the
negative side (Table 2). A selected voltammogram is shown in
Figure 6. Keeping the composition of the HOMO in view, the
oxidation is assigned to Rh(III)–Rh(IV) oxidation. Similarly in
view of composition of the LUMO, the reduction is attributed
to reduction of the azo group of the C,N,O-coordinated ligand.
The oxidative responses are quasi-reversible in nature, while
the reductive responses are completely irreversible, which
indicates that the oxidized species has little stability but the
reduced species is totally unstable in nature and undergoes rapid
decomposition. One-electron stoichiometry of both the redox
responses has been established by comparing their current
heights (ipa for oxidative response and ipc for reductive response)
with those of the standard ferrocene–ferrocenium couple under
identical experimental conditions.

6602 Organometallics, Vol. 26, No. 26, 2007
Baksi et al.
formed as reported in the literature.8 All other chemicals and
solvents were reagent-grade commercial materials and were used
as received.
Preparation of Compounds. L1. 2′,6′-Dimethylaniline (5 g, 0.04
mol) was dissolved in 1:1 HCl (30 mL). The solution was diazotized
with NaNO2 (2.8 g, 0.04 mol) in water (20 mL) at 0 °C. Separately
p-cresol (4.5 g, 0.04 mol) was dissolved in 10% NaOH solution
(30 mL) and the solution was cooled to 0 °C. The diazotized
solution was then added slowly to the alkaline solution of p-cresol
with continuous stirring in an ice-bath (0 °C). Then it was acidified
with 1:1 HCl. Stirring was continued for 30 min. The red precipitate
was collected by filtration, thoroughly washed with water, and dried
in vacuo over P4O10. Purification was achieved by thin-layer
chromatography on a silica plate with hexane as the eluant. A red
band separated, which was extracted with hexane. Upon slow
evaporation of the extract, 2-(2′,6′-dimethylphenylazo)-4-meth-
ylphenol (L1) was obtained as a red solid. Yield: 70%. Anal. Calcd
for C15H16N2O: C, 75.00; H, 6.66; N, 11.66. Found: C, 75.15; H,
6.54; N, 11.67. 1H NMR:9 2.43 (s, 6H); 2.47 (s, 3H); 6.95 (d, 1H,
J ) 8.5); 7.13–7.26* (4H); 7.75 (s, 1H); 12.71 (s, OH).
Figure 5. Partial molecular orbital diagram of complex 1.
L2. This was prepared by following the same above procedure
using 2′,6′-diethylaniline instead of 2′,6′-dimethylaniline. Instead
of a red precipitate (as observed for L1), an orange oily layer
separated, which was extracted with hexane. The hexane extract
was evaporated to yield an orange oily liquid, which was purified
by thin-layer chromatography on a silica plate with hexane as the
eluant. An orange band separated, which was extracted with hexane.
Upon slow evaporation of the extract, 2-(2′,6′-diethylphenylazo)-
4-methylphenol (L2) was obtained as a thick orange liquid. Yield:
60%. Anal. Calcd for C17H20N2O: C, 76.12; H, 7.46; N, 10.45.
1
Found: C, 76.35; H, 7.43; N, 10.44. H NMR: 1.23 (t, 6H, J )
7.8); 2.40 (s, 3H); 2.71 (q, 4H, J ) 7.4); 6.96 (d, 1H, J ) 8.3);
7.17–7.28*(4H); 7.74 (s, 1H); 12.60 (s, OH).
L3. This was obtained as an orange liquid by following the same
above procedure (as in L2) using 2′,6′-diisopropylaniline instead
of 2′,6′-diethylaniline. Yield: 65%. Anal. Calcd for C19H24N2O: C,
77.03; H, 8.11; N, 9.46. Found: C, 77.15; H, 8.16; N, 9.51. 1H
NMR: 1.27 (d, 12H, J ) 6.7); 2.44 (s, 3H); 3.11 (m*, 2H)
7.02–7.40*(5H); 7.82 (s, 1H); 12.54 (s, OH).
1. To a solution of L1 (26 mg, 0.11 mmol) in toluene (40 mL)
was added [Rh(PPh3)3Cl] (100 mg, 0.11 mmol) followed by
triethylamine (11 mg, 0.11 mmol). The resulting mixture was then
heated at reflux for 24 h to yield a dark red solution. The solvent
was then evaporated to give a solid mass, which was subjected to
purification by thin-layer chromatography on a silica plate. With
benzene as the eluant, a blue band separated, which was extracted
with acetonitrile. Evaporation of this extract gave complex 1 as a
dark crystalline solid. Yield: 35%. Anal. Calcd for
C50H42N2OP2ClRh: C, 67.76; H, 4.74; N, 3.16. Found: C, 67.35;
H, 4.76; N, 3.14. 1H NMR: 1.82 (s, 3H); 2.33 (s, 3H); 5.88 (s,
1H); 6.25 (d, 1H, J ) 8.7); 6.34 (t, 1H, J ) 7.6); 6.50 (d, 2H, J )
6.9); 7.08–7.55* (2PPh3).
Figure 6. Cyclic voltammogram of complex 2 in acetonitrile
solution (0.1 M TBAP) at a scan rate of 50 mV s-1
.
Conclusion
The present study shows that blocking both the 2′,6′ positions
of the 2-(arylazo)phenol (I) by alkyl groups can successfully
induce metal-mediated C–C bond activation at one of these two
positions, which has been manifested by reaction of the 2-(2′,6′-
dialkylphenylazo)-4-methylphenols (III) with [Rh(PPh3)3Cl].
Each of the three selected azo ligands (L1, L2, and L3) is found
to undergo interesting chemical transformation, viz., loss or
migration of an alkyl group from the 2′ position, and this study
also demonstrated that the nature of the transformation of the
azo ligand is dependent on the nature of the alkyl group at the
2′,6′ positions.
The other two complexes (2 and 3) were prepared by following
the same above procedure using ligand L2 and L3, respectively,
instead of L1.
2. Yield: 28%. Anal. Calcd for C51H44N2OP2ClRh: C, 68.04; H,
1
4.89; N, 3.11. Found: C, 68.35; H, 4.86; N, 3.13. H NMR: 1.83
Experimental Procedures
(s, 3H); 2.75 (q, 2H, J ) 7.5); 5.90 (s, 1H); 6.28 (d, 1H, J ) 7.9);
6.39 (t, 1H, J ) 7.5); 6.56 (d, 2H, J ) 7.9); 7.08 (d, 1H, J ) 7.8);
7.14–7.72* (2PPh3). 3. Yield: 25%. Anal. Calcd for
C55H52N2OP2ClRh: C, 69.07; H, 5.55; N, 2.93. Found: C, 68.89;
Materials. Rhodium trichloride was purchased from Arora
Matthey, Kolkata, India. 2,6-Dimethylaniline, 2,6-diethylaniline, and
2,6-diisopropylaniline were obtained from Loba Chemie, Mumbai,
India. Triphenylphosphine and p-cresol were purchased from S. D.
Fine-Chem Limited, Mumbai, India. [Rh(PPh3)3Cl] was prepared
by following a reported procedure.7 Preparation of tetrabutylam-
monium perchlorate (TBAP) for electrochemical work was per-
(8) (a) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry
for Chemists; Wiley: New York, 1974; pp 167–215. (b) Walter, M.;
Ramaley, L. Anal. Chem. 1973, 45, 165.
(9) Chemical shifts are given in ppm, and multiplicity of the signals
along with the associated coupling constants (J in Hz) are given in
parentheses. Overlapping signals are marked with an asterisk.
(7) Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1967, 10, 67.

Rhodium-Mediated C–C Bond ActiVation
Organometallics, Vol. 26, No. 26, 2007 6603
Table 4. Crystallographic Data for Complexes 1, 2, and 3
1
2
3
empirical formula
fw
C50H42ClN2OP2Rh
887.16
C51H44N2ClOP2Rh
901.18
C55H52ClN2OP2Rh
957.29
space group
a (Å)
b (Å)
monoclinic, P21/c
18.1980 (7)
11.9925 (5)
20.5405 (8)
112.5470 (10)
4140.1 (3)
4
monoclinic, P21/n
18.463 (10)
12.007 (7)
20.898 (12)
92.519 (10)
4628 (5)
monoclinic, P21/c
15.887 (5)
12.016 (4)
25.410 (9)
100.139 (7)
4775 (3)
4
c (Å)
ꢀ (deg)
V (Å3)
Z
4
λ (Å)
0.71073
0.71073
0.71073
cryst size (mm3)
T (K)
0.12 × 0.15 × 0.20
0.03 × 0.11 × 0.34
0.16 × 0.22 × 0.65
295
293
293
µ (mm-1
R1a
)
0.595
0.0565
0.1282
1.18
0.534
0.1199
0.3744
0.98
0.521
0.0750
0.2113
0.88
wR2b
GOFc
2
2 2
2
2 2
a R1 ) ∑||Fo| – |Fc||/∑|Fo. b wR2 ) [∑{w(Fo – Fc ) }/∑{w(Fo2)}]1/2
.
c GOF ) [∑(w(Fo – Fc ) )/(M – N)]1/2, where M is the number of reflections
and N is the number of parameters refined.
1
H, 5.46; N, 3.03. H NMR: 0.918 (d, 6H, J ) 6.9); 1.81 (s, 3H);
5.85 (s, 3H); 6.26 (d, 1H, J ) 8.8); 6.93 (s, 1H); 7.11–7.68 (2PPh3).
Physical Measurements. Microanalyses (C, H, N) were done
using a Heraeus Carlo Erba 1108 elemental analyzer. Magnetic
susceptibilities were measured using a PAR 155 vibrating sample
magnetometer fitted with a Walker Scientific L75FBAL magnet.
Mass spectra were recorded with a Micromass LCT electrospray
(Qtof Micro YA263) mass spectrometer by electrospray ionization
method. IR spectra were obtained on a Shimadzu FTIR-8300
spectrometer with samples prepared as KBr pellets. 1H NMR spectra
in CDCl3 solutions were obtained on a Bruker Avance DPX 300
NMR spectrometer using TMS as the internal standard. Electronic
spectra were recorded on a JASCO V-570 spectrophotometer.
Electrochemical measurements were made using a CH Instruments
model 600A electrochemical analyzer. A platinum disk working
electrode, a platinum wire auxiliary electrode, and an aqueous
saturated calomel reference electrode (SCE) were used in the cyclic
voltammetry experiments. All electrochemical experiments were
performed under a dinitrogen atmosphere. All electrochemical data
were collected at 298 K and are uncorrected for junction potentials.
Crystallography. Single crystals of complexes 1, 2, and 3 were
obtained by slow evaporation of the acetonitrile solutions of the
respective complexes. Selected crystal data and data collection
parameters are given in Table 4. Data were collected on a Bruker
CCD diffractometer using graphite-monochromated Mo KR radia-
tion. X-ray data reduction and structure solution and refinement
were done using the SHELXS-97 and SHELXL-97 programs.10
The structures were solved by the direct methods.
Acknowledgment. Financial assistance received from the
Department of Science and Technology (Grant No. SR/S1/
IC-15/2004) is gratefully acknowledged.
Note Added after ASAP Publication. The spelling of an
author’s name was corrected and ref 4a was updated in the version
reposted on November 2, 2007.
Supporting Information Available: Mass spectrum of complex
1 (Figure S1), mass spectrum of complex 2 (Figure S2), C–H---π
interactions in complex 2 (Figure S3), mass spectrum of complex
3 (Figure S4), C–H---π interactions in complex 3 (Figure S5), partial
MO diagrams of complex 2 (Figure S6) and complex 3 (Figure
S7), and X-ray crystallographic data of complexes 1, 2, and 3 in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM700826T
(10) Sheldrick, G. M. SHELXS-97 and SHELXL-97, Fortran programs
for crystal structure solution and refinement; University of Gottingen:
Gottingen, Germany, 1997.