Unprecedented migration of a methyl group in 2-(2′,6′- dimethylphenylazo)-4-methylphenol mediated by ruthenium

Article abstract of DOI:10.1021/ic049494h

An unprecedented chemical transformation of 2-(2′,6′- dimethylphenylazo)-4-methylphenol has been observed upon its reaction with [Ru(PPh₃)₂(CO)₂Cl₂] whereby the methyl group at the 2′ position migrates to the 4′ or 6′ position.

Full text of DOI:10.1021/ic049494h

Inorg. Chem. 2004, 43, 4814−4816
Unprecedented Migration of a Methyl Group in
2-(2′,6′-Dimethylphenylazo)-4-methylphenol Mediated by Ruthenium
,†
Sumon Nag,† Parna Gupta,† Ray J. Butcher,‡ and Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity,
Kolkata 700 032, India, and Department of Chemistry, Howard UniVersity,
Washington, D.C. 20059
Received April 19, 2004
An unprecedented chemical transformation of 2-(2′,6′-dimethylphe-
nylazo)-4-methylphenol has been observed upon its reaction with
[Ru(PPh3)2(CO)2Cl2] whereby the methyl group at the 2′ position
migrates to the 4′ or 6′ position.
The present work has emerged from our interest in the
C-H and C-C activation of 2-(arylazo)phenols (1) and
related ligands.1 While ligand 1 has been observed to undergo
C-H activation at one ortho position of the phenyl ring in
the arylazo fragment leading to the formation of cyclom-
etalated species (2),1a,c-k,2 we have recently observed that a
modified 2-(arylazo)phenol, viz. 2-(2′,6′-dimethylphenylazo)-
4-methylphenol (3), in which both the ortho positions of the
phenyl ring in the arylazo fragment are strategically blocked
by methyl groups, displays interestingly different reactivity.1b
[Ru(PPh3)2(CO)2Cl2] has been selected as the ruthenium
Upon its reaction with [Ru(PPh3)3Cl2], or its osmium
analogue, ligand 3 has been observed to undergo an unusual
chemical transformation, whereby one methyl group from
the phenyl ring of the arylazo fragment migrates to the metal
center via oxidation to CO (4). Susceptibility of the metal
center toward oxidation to the +3 state was speculated to
play the key role in the observed oxidative migration of the
methyl group.1b This has encouraged us to explore the
reactivity of ligand 3 with other platinum metal complexes
where the metal center is not so easily oxidizable, and thus,
complex in the present study. The presence of strong π-acid
CO, in addition to PPh3, in the coordination sphere makes
the metal center in [Ru(PPh3)2(CO)2Cl2] much less oxidizable
relative to that in [Ru(PPh3)3Cl2]. It may be mentioned here
that reactivity of [Ru(PPh3)3Cl2] and [Ru(PPh3)2(CO)2Cl2]
toward a particular organic ligand has been found to be
significantly different.3 Herein, we wish to report an interest-
ing migration of a methyl group in ligand 3 that took place
upon its reaction with [Ru(PPh3)2(CO)2Cl2].
Reaction of ligand 3 with [Ru(PPh3)2(CO)2Cl2] was carried
out in refluxing 2-methoxyethanol in the presence of tri-
ethylamine, which afforded two green complexes 5 and 6,
which were separated by chromatography.4,5 The green band
of complex 5 separated first followed by that of complex 6.
Though ligand 3 contains three methyl groups, the 1H NMR
spectrum of complex 5 showed a single aromatic methyl
signal at 2.31 ppm. Besides that, a triplet (3H) and a quartet
(2H) were observed at 1.15 and 1.44 ppm, respectively, the
origin of which was initially not clear. In complex 6, three
methyl signals were observed at 1.25, 1.83, and 2.30 ppm.
* To whom correspondence should be addressed. E-mail: samaresh_b@
hotmail.com.
† Jadavpur University.
‡ Howard University.
(1) (a) Acharyya, R.; Basuli, F.; Wang, R. Z.; Mak, T. C. W.; Bhatta-
charya, S. Inorg. Chem. 2004, 43, 704. (b) Acharyya, R.; Peng, S.
M.; Lee, G. H.; Bhattacharya, S. Inorg. Chem. 2003, 42, 7378. (c)
Gupta, P.; Butcher, R. J.; Bhattacharya, S. Inorg. Chem. 2003, 42,
5405. (d) Majumder, K.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc.,
Dalton Trans. 2001, 284. (e) Dutta, S.; Peng, S. M.; Bhattacharya, S.
J. Chem. Soc., Dalton Trans. 2000, 4623. (f) Sui, K.; Peng, S. M.;
Bhattacharya, S. Polyhedron 1999, 19, 631. (g) Basuli, F.; Peng, S.
M.; Bhattacharya, S. Polyhedron 1998, 18, 391. (h) Pramanik, N. C.;
Bhattacharya, S. Polyhedron 1997, 16, 3047. (i) Sinha, P. K.;
Chakravarty, J.; Bhattacharya, S. Polyhedron 1997, 16, 81. (j) Sinha,
P. K.; Chakravarty, J.; Bhattacharya, S. Polyhedron 1996, 15, 2931.
(k) Chakravarty, J.; Bhattacharya, S. Polyhedron 1996, 15, 391.
(2) Lahiri, G, K.; Bhattacharya, S.; Mukherjee, M.; Mukherjee, A.;
Chakravorty, A. Inorg. Chem. 1987, 26, 3359.
(3) (a) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2001, 40,
1126. (b) Basuli, F.; Ruf, M.; Pierpont, C. G.; Bhattacharya, S. Inorg.
Chem. 1998, 37, 6113. (c) Dutta, S.; Basuli, F.; Castineiras, A.; Peng,
S. M.; Lee, G. H.; Bhattacharya, S. Unpublished results.
4814 Inorganic Chemistry, Vol. 43, No. 16, 2004
10.1021/ic049494h CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/14/2004

COMMUNICATION
However, three singlets (1H each) were observed in the
aromatic region at 6.05, 6.11, and 6.52 ppm, the origin of
1
which was again not clear. The H NMR spectral data of
complexes 5 and 6 thus indicate that ligand 3 has probably
undergone some unexpected chemical transformations during
its reaction with [Ru(PPh3)2(CO)2Cl2]. To verify this specula-
tion, identity of both the complexes 5 and 6 has been unveiled
by their structure determination by X-ray crystallography.
The structures reveal that the speculation has indeed been
correct. The structure of complex 5 (Figure 1)6 shows that
one methyl group of ligand 3 has migrated from its original
location (say, at the 2′ position) and is inserted (via loss of
a proton) into the C-CH3 bond at the 6′ position, thus
converting the methyl group (at 6′ position) into an ethyl
group. The modified ligand is coordinated to ruthenium as
a dianionic tridentate C,N,O-donor. Two PPh3 ligands and
a CO are also coordinated to the metal center. Structure of
complex 6 (Figure 2)7 shows that one methyl group of ligand
3 has again migrated from its original location (say at 2′
position); however, it is inserted (via loss of a proton) into
the aromatic C-H bond at the 4′ position. Thus, a new
methyl group is generated at the 4′ position. The transformed
Figure 1. Molecular structure of 5.
(4) Complexes 5 and 6: ligand 3 (34 mg, 0.14 mmol) was dissolved in
2-methoxyethanol (40 mL), and to it were added triethylamine (22
mg, 0.24 mmol) and [Ru(PPh3)2(CO)2Cl2] (100 mg, 0.14 mmol). The
mixture was then heated at reflux for 24 h to afford a deep brown
solution. The solution was then evaporated to dryness to give a solid
residue, which was subjected to purification by thin-layer chroma-
tography on a silica plate. With benzene as eluant, two deep green
bands (complexes 5 and 6, respectively) separated, which were
extracted with acetonitrile. Upon evaporation of the acetonitrile
extracts, complexes 5 and 6 were obtained as crystalline green solids.
Yield: 18% (for complex 5) and 24% (for complex 6). Calcd: C,
70.03%; H, 4.93%; N, 3.14% (for both the complexes 5 and 6).
Found: C, 70.10%; H, 4.93%; N, 3.19% for complex 5 and C, 70.06%;
H, 4.95%; N, 3.18% for complex 6. Complex 5 1H NMR: 1.15 (CH3,
t, J ) 7.5); 1.44 (CH2, quartet, J ) 7.3); 2.31 (CH3), 5.92 (d, 1H, J
) 8.3); 6.06 (s, 1H); 6.33 (d, 1H, J ) 8.3); 6.81 (d, 1H, J ) 7.2);
6.85 (d, 1H, J ) 7.2); 7.14 (t, 1H, J ) 7.5); 7.17-7.69 (2PPh3). 13C
NMR: 127.90, 129.84, 132.37, and 134.45 (2PPh3); 20.11(CH3); 24.37-
(CH2); 30.10(CH3); 116.41; 121.02; 124.71; 128.87; 128.97; 132.19;
132.24; 132.47; 132.55; 138.09; 141.10; 164.35 (Ru-C(Ar)); 175.67-
(CO). 31P NMR: 50.69(s). Electronic spectral data in dichloromethane
solution: (λ, nm (ꢀ, M-1 cm-1)): 686 (5500), 452 (5500), 436 (6100),
396 (6000), 354 (12 000), 266 (33 500). Cyclic voltammetric data:
oxidation1, E1/2 ) 0.42 V, ∆Ep ) 70 mV; oxidation2, Epa ) 1.20 V.
Complex 6 1H NMR: 1.25(CH3); 1.83(CH3); 2.30 (CH3); 5.93 (d,
1H, J ) 8.2); 6.05 (s, 1H); 6.11 (s, 1H); 6.33 (d, 1H, J ) 7.7); 6.52
(s, 1H); 7.43-7.71 (2PPh3). 13C NMR: 127.84, 129.76, 132.26, and
134.49 (2PPh3); 19.44 (CH3); 21.70 (CH2); 30.10 (CH3); 116.45;
121.20; 124.23; 128.86; 128.96; 132.08; 132.33; 132.44; 132.56;
138.10; 140.77; 163.36 ((Ru-C(Ar)); 175.89 (CO). 31P NMR: 50.75-
(s). Electronic spectral data in dichloromethane solution: (λ, nm (ꢀ,
M-1 cm-1)): 686 (5400), 450 (5100), 432 (5800), 396 (6500), 356
Figure 2. Molecular structure of 6.
ligand is coordinated to ruthenium as a dianionic tridentate
C,N,O-donor. As before, two PPh3 ligands and a CO are
also coordinated to the metal center. Complexes 5 and 6
therefore represent examples of two structural isomers.
To check whether the solvent (2-methoxyethanol) or the
base (triethylamine) has served as the source of a methyl or
ethyl group, reaction of ligand 3 has been carried out with
[Ru(PPh3)2(CO)2Cl2] in toluene and also in benzonitrile using
triethylamine, as well as Na2CO3, as base. From each of these
reactions, both complex 5 and 6 have been obtained. This
clearly indicates that neither the solvent (2-methoxyethanol)
nor the base (NEt3) has served as the source of a methyl or
ethyl group in the formation of complexes 5 and 6.
(11 100), 268 (27 500). Cyclic voltammetric data: oxidation1 E1/2
)
0.40 V, ∆Ep ) 70 mV; oxidation2, Epa ) 1.00 V.
The chemical transformations of ligand 3 observed upon
its reaction with [Ru(PPh3)2(CO)2Cl2] are significantly dif-
ferent than that observed upon its reaction with [Ru(PPh3)3-
Cl2]. However, the present chemical transformations of ligand
1
(5) A H NMR spectrum of the reaction mixture, prior to separation of
complexes 5 and 6 by chromatography on silica, has been recorded.
The spectrum looks complicated, because of the presence of other
species; however, it clearly indicates the presence of both complex 5
and 6 in the reaction mixture.
(6) Crystallographic data for 5. C52H44N2O2P2Ru, monoclinic, space group
P21/n, a ) 18.566(8) Å, b ) 11.982(4) Å, c ) 20.130(10) Å, â )
102.239(16)°, V ) 4376(3) Å3, Z ) 4, λ ) 0.710 73 Å, R1 ) 0.0752,
wR2 ) 0.1319, GOF ) 1.083. Selected distances [Å] and angles [deg]:
Ru-C(1), 2.037(5); Ru-C, 1.869(5); Ru-N(1), 2.042(4); Ru-O(1),
2.197(4); Ru-P(1), 2.3633(15); Ru-P(2), 2.3657(15); C(9)-C(91),
1.511(8); C-O, 1.151(6); N(1)-N(2), 1.278(5); C(12)-O(1), 1.308-
(6); C(5)-C(51), 1.457(10); C(51)-C(52), 1.4994(11); P(1)-Ru-
P(2), 174.70(5); C(1)-Ru-O(1), 154.90(18); C-Ru-N(1), 177.3(2);
N(1)-Ru-C(1), 77.14(19); N(1)-Ru-O(1), 77.77(15).
(7) Crystallographic data for 6. C52H44N2O2P2Ru, monoclinic, space group
P21/c, a ) 18.343(8) Å, b ) 12.071(4) Å, c ) 20.628(13) Å, â )
109.90(2)°, V ) 4295(4) Å3, Z ) 4, λ ) 0.710 73 Å, R1 ) 0.0325,
wR2 ) 0.0814, GOF ) 1.031. Selected distances [Å] and angles [deg]:
Ru-C(1), 2.075(3); Ru-C, 1.875(3); Ru-N(1), 2.056(2); Ru-O(1),
2.229(2); Ru-P(1), 2.3723(10); Ru-P(2), 2.3829(10); C-O, 1.161(3);
O(1)-C(15), 1.319(3); N(1)-N(2), 1.301(3); C(3)-C(4), 1.492(5);
P(1)-Ru-P(2), 174.91(3); C(1)-Ru-O(1), 154.66(9); C(1)-Ru-
N(1), 77.21(10); N(1)-Ru-O(1), 77.45(8).
Inorganic Chemistry, Vol. 43, No. 16, 2004 4815

COMMUNICATION
Scheme 1. Probable Steps in the Formation of Complexes 5 and 68
or 3′ and 4′ positions in an η2-fashion. Dissociation of a
+
proton from the η2-bound CH3 ion, followed by usual
rearrangements, affords the final complex 5 or 6.
Electronic spectra of complexes 5 and 6 are found to be
similar. Each complex shows several intense absorptions in
the visible and ultraviolet regions. The absorptions in the
ultraviolet region are believed to be due to transitions within
the ligand orbitals, and those in the visible region are
attributable to metal-to-ligand charge-transfer transitions.
Redox activity of complexes 5 and 6 has been tested by
cyclic voltammetry.10 Both the complexes show two oxida-
tive responses on the positive side of SCE. The first oxidation
near 0.4 V is reversible and is assigned to Ru(II)-Ru(III)
oxidation. The second oxidation near 1.0 V is irreversible
and is tentatively assigned to oxidation of the CNO-
coordinated 2-(arylazo)phenolate ligand.
The present study shows that C-C activation of selective
alkyl groups, introduced at the ortho position of the phenyl
ring in the arylazo fragment of 1, may be easily achievable
by reaction of such ligands with suitable platinum metal
complexes. Such possibilities are currently under exploration.
Acknowledgment. Financial assistance received from the
University Grants Commission, New Delhi [Grant F.12-56/
2001(SR-I)], is gratefully acknowledged. S.N. and P.G. thank
CSIR for their fellowship.
Supporting Information Available: Crystallographic details
for complexes 5 and 6 in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC049494H
(9) (a) Cohen, R.; Martin, J. M. L.; Milstein, D. Organometallics 2004,
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3 are again very unusual and, to our knowledge, unprec-
edented. The mechanism of the observed chemical transfor-
mations is not completely clear to us. However, the
speculated sequences shown in Scheme 1 seem probable.8
Upon reaction with [Ru(PPh3)2(CO)2Cl2], ligand 3 binds to
the metal center as a tridentate C,N,O-donor via loss of the
phenolic proton and a CH3+ ion from one ortho position (say,
in the 2′ position) in the 2′,6′-dimethylphenylazo fragment.
Such metal assisted CsC bond activation is well documented
+
in the literature.9 The dissociated CH3 ion links itself
(10) Cyclic voltammetry was carried out in dichloromethane solution (0.1
selectively to the electron rich CdC bond within 1′ and 6′
M TBAP) at a scan rate of 50 mV s-1. E1/2 ) 0.5(Epa + Epc), where
E
pa and Epc are anodic and cathodic peak potentials respectively; ∆Ep
(8) The two axially coordinated PPh3 ligands are not shown for clarity.
) Epa - Epc.
4816 Inorganic Chemistry, Vol. 43, No. 16, 2004