An Unprecedented Oxidative Migration of a Methyl Group from 2-(2′,6′-Dimethyl phenylazo)-4-methylphenol Mediated by Ruthenium and Osmium

Article abstract of DOI:10.1021/ic034968i

An unprecedented chemical transformation of 2-(2′,6′ -dimethylphenylazo)-4-methylphenol has been observed upon its reaction with [M(PPh₃)₃X₂] (M = Ru, Os; X = Cl, Br) whereby one methyl group from the phenyl ring of the arylazo fragment migrates to the metal center via oxidation to CO.

Full text of DOI:10.1021/ic034968i

Inorg. Chem. 2003, 42, 7378−7380
An Unprecedented Oxidative Migration of a Methyl Group from
2-(2′,6′-Dimethylphenylazo)-4-methylphenol Mediated by Ruthenium and
Osmium
,1a
Rama Acharyya,1a Shie-Ming Peng,1b Gene-Hsiang Lee,1b and Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity, Kolkata 700 032,
India, and Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan, R.O.C.
Received August 15, 2003
An unprecedented chemical transformation of 2-(2′,6′-dimethylphen-
and [Ir(PPh3)3Cl]) ligand 1 always binds to the metal center
as a tridentate C,N,O-donor (2) affording cyclometalated
complexes.2g-i,3 With the intention of inducing a different
mode of coordination in such ligands, we selected 2-(2′,6′-
dimethylphenylazo)-4-methylphenol (3), in which both the
ortho positions of the phenyl ring in the arylazo fragment
are blocked by methyl groups. In order to investigate the
consequences of this ligand modification, reaction of 3 was
carried out with the same triphenylphosphine complexes of
the platinum metals listed above. In this Communication we
disclose an interesting chemical transformation of 3 that took
place during its reaction with the [M(PPh3)3X2] (M ) Ru,
Os; X ) Cl, Br) complexes.
ylazo)-4-methylphenol has been observed upon its reaction with
[M(PPh3)3X ] (M) Ru, Os; X ) Cl, Br) whereby one methyl group
2
from the phenyl ring of the arylazo fragment migrates to the metal
center via oxidation to CO.
The present report has originated from our interest in the
chemistry of platinum metal complexes of 2-(arylazo)-4-
methylphenol (1).2 It has been observed that upon reaction
Reaction of 3 was first carried out with [Ru(PPh3)3Cl2] in
refluxing ethanol in the presence of triethylamine (containing
0.5% water), which afforded a deep green complex (4).4
Though 3 contains three methyl groups, the 1H NMR
spectrum of 4 shows only two methyl signals at 1.82 and
2.31 ppm. The infrared spectrum of 4 shows a strong
vibration at 1928 cm-1 indicating the possible presence of
1
coordinated CO. The H NMR and IR spectral data thus
indicate that ligand 3 probably has undergone some unex-
pected chemical transformation during the course of the
synthetic reaction. Identity of the green complex (4) has been
revealed by its structure determination by X-ray crystal-
(3) Lahiri, G, K.; Bhattacharya, S.; Mukherjee, M.; Mukherjee, A.;
Chakravorty, A. Inorg. Chem. 1987, 26, 3359.
with different triphenylphosphine complexes of the platinum
metals (e.g., [Ru(PPh3)3Cl2], [Os(PPh3)3Br2], [Rh(PPh3)3Cl],
(4) Complex 4: Ligand 3 (30 mg, 0.13 mmol) was dissolved in ethanol
(40 mL), and to it were added triethylamine (containing 0.5% water)
(22 mg, 0.22 mmol) and [Ru(PPh3)3Cl2] (100 mg, 0.10 mmol). The
mixture was then heated at reflux for 3 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, a deep green band
separated, which was extracted with acetonitrile. Upon evaporation
of the acetonitrile extract complex 4 was obtained as a crystalline green
solid. Yield: 40%. Calcd: C, 69.78%; H, 4.79%; N, 3.19%. Found:
C, 69.37%; H, 4.82%; N, 3.14%. 1H NMR: 1.82(CH3); 2.31(CH3);
5.91(d, 1H, J ) 8.6); 6.05(s, 1H); 6.22(t, 1H, J ) 7.4); 6.30(d, 1H, J
) 7.3); 6.34(d, 1H, J ) 8.6); 6.86(d, 1H, J ) 7.4); 7.15-7.39(2PPh3).
13C NMR: 127.89, 129.84, 132.22 and 134.48 (2PPh3); 19.59 (CH3);
20.06 (CH3); 116.53; 121.42; 121.51; 123.34; 128.19; 132.05; 132.40;
134.77; 135.99; 139.99; 141.96; 165.34 (Ru-C(Ar)); 173.67 (CO).
31P NMR: 50.75 (s).
* Author to whom correspondence should be addressed. E-mail:
samaresh_b@hotmail.com.
(1) (a) Jadavpur University. (b) National Taiwan University.
(2) (a) Chakravarty, J.; Bhattacharya, S. Polyhedron 1996, 15, 391. (b)
Sinha, P. K.; Chakravarty, J.; Bhattacharya, S. Polyhedron 1996, 15,
2931. (c) Sinha, P. K.; Chakravarty, J.; Bhattacharya, S. Polyhedron
1997, 16, 81. (d) Pramanik, N. C.; Bhattacharya, S. Polyhedron 1997,
16, 3047. (e) Basuli, F.; Peng, S. M.; Bhattacharya, S. Polyhedron
1998, 18, 391. (f) Sui, K.; Peng, S. M.; Bhattacharya, S. Polyhedron
1999, 19, 631. (g) Dutta, S.; Peng, S. M.; Bhattacharya, S. J. Chem.
Soc., Dalton Trans. 2000, 4623. (h) Majumder, K.; Peng, S. M.;
Bhattacharya, S. J. Chem. Soc., Dalton Trans. 2001, 284. (i) Acharyya,
R.; Basuli, F.; Wang, R. Z.; Mak, T. C. W.; Bhattacharya, S.
Unpublished results.
7378 Inorganic Chemistry, Vol. 42, No. 23, 2003
10.1021/ic034968i CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/18/2003

COMMUNICATION
Figure 1. The molecular structure of 4.
Figure 2. The molecular structure of 6.
lography. The structure (Figure 1)5 shows that upon reaction
with [Ru(PPh3)3Cl2] ligand 3 has indeed undergone an
interesting chemical transformation, whereby one methyl
group of 3 has migrated to the metal center via oxidation to
CO and the modified ligand is coordinated to ruthenium as
a dianionic C,N,O-donor ligand (5, M ) Ru). Two PPh3
ligands are also coordinated to ruthenium. The observed
chemical transformation of 3 is not only unusual but also,
to our knowledge, unprecedented.
7 has taken place, and no carbonyl has been formed in this
reaction. Results of the above experiments clearly show that
the origin of the CO in complex 4 was not the alcoholic
solvent but the displaced methyl group of ligand 3.
Encouraged by the observed transformation of ligand 3
upon its reaction with [Ru(PPh3)3Cl2], a similar reaction of
it was also carried out with an analogous osmium complex,
viz., [Os(PPh3)3Br2], in order to check whether it again
undergoes a similar transformation. The reaction afforded a
light green complex (6),9 which shows 1H NMR (two methyl
signals at 1.89 and 2.26 ppm) and IR (νCO at 1922 cm-1)
spectral properties similar to those of the ruthenium complex
4. The structure of 6 has also been determined by X-ray
crystallography, and the structure (Figure 2)10 shows that
ligand 3 indeed underwent the same chemical transformation
as before (5, M ) Os) and the osmium complex has
composition and stereochemistry similar to those of the
ruthenium complex. Complex 6 can also be synthesized using
To verify that the origin of the coordinated CO in complex
4 is the methyl group lost from ligand 3 and not the solvent
(ethanol), reaction of ligand 3 with [Ru(PPh3)3Cl2] has also
been carried out in another alcoholic solvent that cannot
undergo decarbonylation, viz., tert-butyl alcohol, as well as
in a nonalcoholic solvent, viz., benzene, under refluxing
conditions in the presence of triethylamine. The same green
complex 4 has been obtained from these reactions,6,7 which
shows that the origin of CO is not the solvent. Furthermore,
reaction of a similar ligand, viz., 2-(2′-methylphenylazo)-4-
methylphenol (7), has been carried out with [Ru(PPh3)3Cl2]
in refluxing ethanol in the presence of triethylamine, which
has afforded a cyclometalated complex of ruthenium(III) (8;
M ) Ru, X ) Cl).8 No loss of methyl group from this ligand
(6) Complex 4 was also synthesized following the same above procedure
given in ref 4, using tert-butyl alcohol instead of ethanol, and the
reflux time was 5 h instead of 3 h. Yield: 45%.
(7) Complex 4 was also synthesized following the same above procedure
given in ref 4, using benzene instead of ethanol. Yield: 57%.
(8) Acharyya, R.; Peng, S. M.; Lee, G. H.; Bhattacharya, S. Unpublished
results.
(9) Complex 6: This complex was synthesized by following the same
above procedure as for complex 4, using 2-methoxyethanol instead
of ethanol and [Os(PPh3)3Br2] instead of [Ru(PPh3)3Cl2]. The reflux
time was 7 h instead of 3 h. Yield: 32%. Calcd: C, 63.35%; H, 4.35%;
1
N, 2.90%. Found: C, 63.07%; H, 4.33%; N, 2.98%. H NMR: 1.89
(CH3); 2.26 (CH3); 6.01 (d, 1H, J ) 8.5); 6.06 (s, 1H); 6.24 (t, 1H, J
) 7.6); 6.31 (d, 1H, J ) 7.6); 6.38 (d, 1H, J ) 8.4); 6.96 (d, 1H, J )
8.5); 7.13-7.40 (2PPh3). 13C NMR: 127.90, 129.98, 133.91 and
134.57 (2PPh3); 19.29 (CH3); 19.95 (CH3); 117.80; 120.04; 121.73;
121.84; 129.98; 130.52; 133.70; 134.13; 135.35; 138.50; 143.37;
166.13 (Ru-C (Ar)); 175.06 (CO). 31P NMR: 46.73(s).
(5) Crystallographic data for 4: C51H42N2O2P2Ru, monoclinic, space group
P21/c, a ) 18.1758(7) Å, b ) 12.0548(4) Å, c ) 20.6448(8) Å, â )
112.663(1)°, V ) 4174.1(3) Å3, Z ) 4, λ ) 0.71073 Å, R1 ) 0.0515,
wR2 ) 0.1090, GOF ) 1.146. Selected distances [Å] and angles [deg]:
Ru-C(1), 1.858(3); Ru-C(13), 2.045(3); Ru-N(1), 2.040(2); Ru-
O(2), 2.195(2); Ru-P(1), 2.360(8); Ru-P(2), 2.379(8); C(1)-O(1),
1.144(4); N(1)-N(2), 1.280(3); C(2)-O(2), 1.308(4); P(1)-Ru-P(2),
176.88(3); C(13)-Ru-O(2), 154.59(11); C(1)-Ru-N(1), 177.21(13);
N(1)-Ru-C(13), 76.75(11); N(1)-Ru-O(2), 77.83(9); O(1)-C(1)-
Ru, 178.8(3).
(10) Crystallographic data for 6: C51H42N2O2P2Os, monoclinic, space group
P21/c, a ) 18.2014(1) Å, b ) 12.0662(1) Å, c ) 20.6995(2) Å, â )
112.7269(4)°, V ) 4193.09(6) Å3, Z ) 4, λ ) 0.71073 Å, R1 )
0.0377, wR2 ) 0.0785, GOF ) 1.083. Selected distances [Å] and
angles [deg]: Os-C(1), 1.874(6); Os-C(13), 2.059(5); Os-N(1),
2.062(4); Os-O(2), 2.206(3); Os-P(1), 2.365(12); Os-P(2), 2.376-
(12); C(1)-O(1), 1.117(6); N(1)-N(2), 1.288(5); C(2)-O(2), 1.304-
(5); P(1)-Os-P(2), 177.15(4); C(13)-Os-O(2), 153.16(17); C(1)-
Os-N(1), 177.55(18); N(1)-Os-C(13), 76.17(18); N(1)-Os-O(2),
77.00(13); O(1)-C(1)-Os, 179.2(6).
Inorganic Chemistry, Vol. 42, No. 23, 2003 7379

COMMUNICATION
Scheme 1. Probable Steps in the Formation of Complexes 4 and 613
this CNOP2X coordination environment the bivalent metal
gets oxidized to the trivalent state,2h,3 and the metalated CH2
fragment undergoes further C-H activation, via loss of HX,
affording a reactive carbene species. Upon reaction with
water (present in the commercial solvent as well as in
triethylamine) the MdC-H fragment gets converted into a
M(H)-CdO fragment via elimination of hydrogen. Reduc-
tive elimination of hydride, followed by migration of the
CO fragment, yields the carbonyl complex of the bivalent
metals. Similar metal-assisted C-C bond activation is well
documented in the literature.15
The present study shows that C-H activation of selective
alkyl groups, linked to the ortho carbon of the phenyl ring
in the arylazo fragment of 1, may be easily achievable by
reaction of such ligands with the [M(PPh3)3X2] (M ) Ru,
Os; X ) Cl, Br) complexes, leading to the formation of
interesting cyclometalated complexes. Such possibilities are
currently under exploration.
Acknowledgment. Financial assistance received from the
Department of Science and Technology, New Delhi [Grant
No. SP/S1/F33/98], the Council of Scientific and Industrial
Research, New Delhi [Grant No. 01(1675)/00/EMR-II], and
the University Grants Commission, New Delhi [Grant No.
F.12-56/2001(SR-I)], is gratefully acknowledged. The authors
thank Dr. Falguni Basuli for useful discussions.
Supporting Information Available: Crystallographic details
for 4 and 6 in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
tert-butyl alcohol or benzene as solvent instead of 2-meth-
oxyethanol.11,12 Reaction of ligand 7 with [Os(PPh3)3Br2] in
refluxing 2-methoxyethanol in the presence of triethylamine
has also afforded a cyclometalated complex of osmium(III)
(8; M ) Os, X ) Br).8 These results again show that it is
the displaced methyl group of ligand 3, not 2-methoxyetha-
nol, which was the source of CO in complex 6.
Mechanism of the observed chemical transformation of 3
is not completely clear to us. However, the speculated
sequences illustrated in Scheme 1 seem probable.13 In the
initial step, ligand 3 coordinates to the metal center as a
tridentate C,N,O-donor via loss of the phenolic proton and
another proton from one methyl group in the arylazo
fragment. This particular coordination mode of ligand 3 has
been observed by us in some other complexes,2i and such
C-H activation has also been reported by many others.14 In
IC034968I
(14) (a) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Patertson, T. H.
Acc. Chem. Res. 1995, 28, 154. (b) Hall, C.; Perutz, R. N. Chem.
ReV. 1996, 96, 3125. (c) Shilov, A. E.; Shul’pin, G. B. Chem. ReV.
1997, 97, 2879. (d) Martin, R. L. J. Am. Chem. Soc. 1999, 121, 9459.
(e) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2001, 2437. (f)
Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (g) Iron, M. A.;
Lo, H. C.; Martin, J. M. L.; Keinan, E. J. Am. Chem. Soc. 2002, 124,
7041. (h) Hagen, H.; Boersma, J.; van Koten, G. Chem. Soc. ReV.
2002, 31, 357. (i) Bijkstra, H. P.; Albrecht, M.; van Koten, G. Chem.
Commun. 2002, 126. (j) Rybtchinski, B.; Cohen, R.; Ben-David, Y.;
Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2003, 125, 11041.
(15) (a) van der Boom, M. E.; Kraatz, H. B.; Hassner, L.; Ben-David, Y.;
Milstein, D. Organometallics 1999, 18, 3873. (b) Sundermann, A.;
Ujan, O.; Milstein, D.; Martin, J. M. L. J. Am. Chem. Soc. 2000, 122,
7095. (c) Cohen, R.; van der Boom, M. E.; Shimon, L. J. W.; Milstein,
D. J. Am. Chem. Soc. 2000, 122, 7723. (d) Cao, Z.; Hall, M. B.
Organometallics 2000, 19, 3338. (e) Coalter, J. N.; Huffman, J. C.;
Caulton, K. G. Organometallics 2000, 19, 3569. (f) Rybtchinski, B.;
Oevers, S.; Montag, M.; Vigalok, A.; Rozenberg, H.; Martin, J. M.
L.; Milstein, D. J. Am. Chem. Soc. 2001, 123, 9064. (g) Gauvin, R.
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(11) Complex 6 was also synthesized following the same above procedure
given in ref 9, using tert-butyl alcohol instead of 2-methoxyethanol,
and the reflux time was 11 h instead of 7 h. Yield: 39%.
(12) Complex 6 was also synthesized following the same above procedure
given in ref 9, using benzene instead of 2-methoxyethanol. Yield: 46%.
(13) The two axially coordinated PPh3 ligands are not shown for clarity.
7380 Inorganic Chemistry, Vol. 42, No. 23, 2003