N-assisted carbon-hydrogen bond activation by cobalt(I) complexes

Article abstract of DOI:10.1021/om8007844

Summary: Reactions of 2-(4′-R-phenylazo)-4-methylphenols (R = Me (1), Br (2)) with Co(PMe₃)₃Cl afford two organocobalt(III) complexes, Co(PMe₃)₂Cl(Me(C₆H ₃O∩N=NC₆H₃R)·

Full text of DOI:10.1021/om8007844

5434
Organometallics 2008, 27, 5434–5437
N-Assisted Carbon-Hydrogen Bond Activation by Cobalt(I)
Complexes
Aichen Wang, Hongjian Sun, and Xiaoyan Li*
School of Chemistry and Chemical Engineering, Shandong UniVersity, Shanda Nanlu 27,
250100 Jinan, People’s Republic of China
ReceiVed August 14, 2008
Scheme 1
Summary: Reactions of 2-(4′-R-phenylazo)-4-methylphenols (R
) Me (1), Br (2)) with Co(PMe3)3Cl afford two organo-
cobalt(III) complexes, Co(PMe3)2Cl(Me(C6H3O∩NdNC6H3R) ·
(H2NC6H4R)) (R ) Me (3), Br (4)), whereas the reactions of 1
and 2 with Co(PMe3)4 and Co(PMe3)4Me afford the dinuclear
complex [Co2(PMe3)4(MeC6H3O∩NH)2] (7) with the cleaVage
of the NdN bond. Crystal structures of 3, 4, and 7 were
determined by X-ray crystallography.
There has been considerable current interest in the utilization
of transition metals in promoting interesting chemical transfor-
mations of organic substrates. Such reactions often proceed via
a C-H activation of the organic substrate, leading to the
formation of a reactive organometallic intermediate, which then
undergoes further reactions to yield the final product. Thus,
C-H activation of hydrocarbons,1 particularly that involving
aromatic compounds and cyclometalation,2 is an important area
of research with respect to homogeneous catalysis. Klein
reported C-H activation of ortho metalations of azobenzene3
and phenyl phosphites4 using cobalt complexes supported by
trimethylphosphine (eqs 1 and 2).
membered chelate ring.5 In some cases, C-H activation from
an ortho position of the pendant phenyl ring was observed. The
2-(arylazo)phenol derivative ligand serves as a [C,N,O]-donor
by Ru,6 Ir,7 and Os8 (Scheme 1).
Herein we report on progress in the direction of cyclometa-
lation reactions involving C-H bond activation at cobalt centers
with 2-(arylazo)phenol as prechelate ligand in the reaction of
2-(arylazo)phenol with Co(PMe3)3Cl. The new ortho-chelated
cobalt complexes were isolated and characterized. NdN cleav-
age reactions were also observed, and a dinuclear cobalt complex
was obtained in the reactions of 2-(arylazo)phenol with
Co(PMe3)4 and Co(PMe3)4Me. The related formation mecha-
nisms of the complexes are discussed.
Reaction of Co(PMe3)3Cl with 2-(Arylazo)phenols. Reac-
tions of Co(PMe3)3Cl with 2-(4′-R-phenylazo)-4-methylphenols
(R
)
Me (1), Br (2)) afford the complex type
Co(PMe3)2Cl(Me(C6H3O∩NdNC6H3R) · (H2NC6H4R) (R ) Me
(3), Br (4)) via C-H activation (eq 3).
Crystallization at 0 °C in diethyl ether afforded dark green
crystals in 25% (3) and 29% yields (4) based on Co(PMe3)3Cl.
The two compounds are stable at room temperature in the solid
state for more than 4 h but quickly decompose in solution when
exposed to air.
Transition-metal complexes that contain an azo ligand or its
derivatives are well-known. Several coordination modes have
been documented. However, 2-(arylazo)phenols usually bind to
metal ions as bidentate [N,O]-donors, forming a five- or six-
(5) (a) Basuli, F.; Peng, S. M.; Bhattacharya, S. Polyhedron 1998, 18,
391. (b) Rath, R. K.; Nethaji, M.; Chakravarty, A. R. J. Organomet. Chem.
2001, 633, 79. (c) Sinha, C. R.; Bandyopadhyay, D.; Chakravorty, A.
J. Chem. Soc., Chem. Commun. 1988, 468. (d) Price, R. J. J. Chem. Soc. A
1969, 1296. (e) Jarvis, J. A. J. Acta Crystallogr. 1961, 14, 961.
(6) (a) Gupta, P.; Dutta, S.; Basuli, F.; Peng, S. M.; Lee, G. H.;
Bhattacharya, S. Inorg. Chem. 2006, 45, 460. (b) Halder, S.; Acharyya, R.;
Peng, S. M.; Lee, G. H.; Michael, G. B. D.; Bhattacharya, S. Inorg. Chem.
2006, 45, 9654.
(7) (a) Acharyya, R.; Basuli, F.; Peng, S. M.; Lee, G. H.; Wang, R. Z.;
Bhattacharya, S. J. Organomet. Chem. 2005, 690, 3908. (b) Acharyya, R.;
Basuli, F.; Wang, R. Z.; Bhattacharya, S. Inorg. Chem. 2004, 43, 704.
(8) Gupta, P.; Butcher, R. J.; Bhattacharya, S. Inorg. Chem. 2003, 42,
5405.
* To whom correspondence should be addressed. Fax: +86-531-
88564464. E-mail: xli63@sdu.edu.cn.
(1) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879.
(2) Trofimenko, S. Inorg. Chem. 1973, 12, 1215.
(3) Klein, H.-F.; Helwig, M.; Koch, U.; Floerke, U.; Haupt, H.-J. Z.
Naturforsch. 1993, 48b, 778.
(4) Klein, H.-F.; Schneider, S.; He, M.; Floerke, U.; Haupt, H.-J. Eur.
J. Inorg. Chem. 2000, 2295.
10.1021/om8007844 CCC: $40.75
2008 American Chemical Society
Publication on Web 10/08/2008

Communications
Organometallics, Vol. 27, No. 21, 2008 5435
In the 1H NMR spectra, the two trimethylphosphine ligands
show a signal at 0.96 ppm for 3. In the 31P NMR spectra, signals
of the PMe3 ligands are registered at 7.74 ppm for 3.
Complexes 3 and 4 have similar molecular structures (Figures
1 and 2).9 They are composed of two components: the
organocobalt complex and the organic part. The cobalt atom is
hexacoordinate in an octahedral geometry, and the two trim-
ethylphosphines are trans in axial positions with the angle
P1-Co-P2 (170.35(12)°) in complex 3 and 173.32(6)° in
complex 4. The NdN lengths of 1.285(11) Å in complex 3 and
1.286(5) Å in complex 4 are in the range of normal NdN double
bonds (1.22-1.30 Å).6-8 The Co1-C1 bond (1.936(9) Å) in
complex 3 is longer than the Co1-C25 bond (1.896(5) Å) in
complex 4 because of the difference in electron properties
between the methyl group (electron-donating group) in complex
3 and the bromo atom (electron-withdrawing group) in complex
4. The organic part is 4-methylaniline in complex 3 and
4-bromoaniline in complex 4, indicating the cleavage of the
NdN bond.
The proposed formation mechanism of complexes 3 and 4 is
suggested in Scheme 2. The first step is the coordination of the
nitrogen atom of the azo function to the cobalt atom to give
rise to the intermediate 5. The C-H bond activation is supported
and compensated through cyclometalation with the azo nitrogen
as an anchoring group to form the hydrido intermediate 6. This
hydridocobalt species reacts with compound 1 or 2 with the
participation of the phenoxy hydroxy group to afford complex
3 or complex 4 together with 4-methylaniline (or 4-bromoa-
niline), by hydrogen transfer to compound 1 (or 2), through
the cleavage of the azo function (reduction). Unfortunately,
another byproduct, 2-amino-4-methylphenol, could not be
experimentally verified. In earlier work the similar process with
the reduction of 3-tert-butyl-5-methylsalicylaldehyde to 2-tert-
butyl-6-(hydroxymethyl)-4-methylphenol by hydrogen transfer
from the hydrido and phenoxy hydroxyl groups was also
observed.10 Instead of the product with two six-membered rings
in that system, two five-membered chelate rings play an
important role in this reaction for the formation of complexes
3 and 4. The tridentate phenyl-azo-phenoxy [C,N,O]-ligand
acts as a pincer ligand. Bruce thinks in some cases it would
appear that formation of H2 in the metalation is a more relatively
difficult process than the small-molecule HCl elimination.11 The
metalation of the ligand by loss of hydrogen was reported with
a hydridoiridium complex.12 We think in our case the reduction
with hydrogen occurs because of the presence of an active azo
compound.
Figure 1. Molecular structure of 3 (all hydrogen atoms are omitted
for clarity). Selected bond distances (Å) and angles (deg): Co1-N7
) 1.843(7), Co1-C1 ) 1.936(9), Co1-O1 ) 1.988(7), Co1-P1
) 2.210(3), Co1-P2 ) 2.225(3), Co1-Cl1 ) 2.279(3), N7-N8
) 1.285(11); N7-Co1-C1 ) 81.9(4), N7-Co1-O1 ) 83.7(3),
C1-Co1-O1 ) 165.5(3), N7-Co1-P1 ) 94.2(2), C1-Co1-P1
) 88.8(2), O1-Co1-P1 ) 92.2(2), N7-Co1-P2 ) 94.2(2),
C1-Co1-P2 ) 87.7(3), O1-Co1-P2 ) 93.47(19), P1-Co1-P2
) 170.35(12), N7-Co1-Cl1 ) 177.2(3), C1-Co1-C11 )
101.0(2), O1-Co1-C11 ) 93.47(19), P1-Co1-C11 ) 85.97(12),
P2-Co1-C11 ) 85.89(9), N7-N8-C6 ) 108.7(8).
Figure 2. Molecular structure of 4 (all hydrogen atoms are omitted
for clarity). Selected bond distances (Å) and angles (deg): Co-O
) 2.014(3), Co-Cl ) 2.2602(15), Co-N1 ) 1.847(4), Co-C25
) 1.896(5), Co-P1 ) 2.2359(17), Co-P2 ) 2.2388(17), N1-N2
) 1.286(5); N1-Co-C25 ) 82.2(2), N1-Co-O ) 83.72(16),
C25-Co-O ) 165.86(19), N1-Co-P1 ) 92.10(13), C25-Co-P1
) 88.43(15), P1-Co-O ) 91.52(12), N1-Co-P2 ) 93.86(13),
C25-Co-P2 ) 89.39(15), P2-Co-O ) 92.14(12), P1-Co-P2
) 173.32(6), N1-Co-Cl ) 178.28(13), C25-Co-Cl ) 99.44(16),
O-Co-Cl ) 94.68(11), P1-Co-Cl ) 87.30(6), P2-Co-Cl )
86.84(6), N1-N2-C21 ) 108.7(4).
Reaction of Co(PMe3)4 and Co(PMe3)4Me with
2-(Arylazo)phenols. Reactions of Co(PMe3)4 and Co(PMe3)4Me
with 2-(4′-R-phenylazo)-4-methylphenols (R ) Me (1), Br (2))
(9) CCDC-686118 (3), CCDC-686120 (4), and CCDC-686117 (7)
contain the supplementary crystallographic data for this paper. These data
can be obtained free of charge on application to the CCDC, 12 Union Road,
Cambridge CB21EZ, U.K. (fax (+44)1223-336-033; email
deposit@ccdc.cam.ac.uk).
(10) Sun, H.; Li, X.; Klein, H.-F.; Floerke, U.; Haupt, H.-J. Organo-
metallics 2005, 24, 2612.
(11) Bruce, M. I. Angew. Chem., Int. Ed. Engl. 1977, 16, 73.
(12) Duff, J. M.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1972, 2219.
afforded
a
kind of dinuclear cobalt(II) complex,
[Co(PMe3)2(MeC6H3O∩NH)]2 (7). Complex 7 was isolated and
characterized by X-ray diffraction. The o-phenoxy-imido-phenyl

5436 Organometallics, Vol. 27, No. 21, 2008
Scheme 2. Formation of Compounds 3 and 4
Communications
is a bridged ligand, formed through the cleavage of the NdN bond
(eq 4). The reaction mechanism according to eq 4 is not clear, and
more investigation on it is ongoing.
whereas the Co-N bonds of the two five-membered chelate
rings (Co1-N2 ) 1.918(2) and Co2-N1 ) 1.922(2) Å) are
shorter than the other two Co-N bonds (Co1-N1 ) 1.966(2)
and Co2-N2 ) 1.961(2) Å).
In conclusion, reactions of 2-(4′-R-phenylazo)-4-meth-
ylphenols (R ) Me (1), Br (2)) with Co(PMe3)3Cl afford
Crystallization at 0 °C afforded dark crystals of 7 in pentane
and diethyl ether solution in 34-37% yield. The compound is
very stable both in the solid state and in solution in the presence
of moisture. The N-H stretching bands were recorded at 3283
and 3181 cm-1, which are comparable with those of a bridging
amide group.13
In the crystal structure of complex 79 two imidophenola-
tobis(trimethylphosphine)cobalt(II) units are combined through
an ordinary metal-metal bond (Co-Co ) 2.3997(5) Å)
which is almost symmetrically bridged by two NH groups
(Co1-N-Co2 ) 76.23(8) and 76.42(8)°) to give a butterfly-
shaped [N2Co2] core (N-Co-N ) 79.94(9)°) without a
center of symmetry. The four trimethylphosphine ligands are
pairwise attached to the cobalt atoms. Those ligands align
with a [Co, N] vector. The axial Co-P bonds (Co1-P1 )
2.2469(8) and Co2-P2 ) 2.2405(8) Å) are longer than those
(Co1-P3 ) 2.2110(8) and Co2-P4 ) 2.214(7) Å) positioned
trans to N atoms because of the strong [Co,Co] interaction,
Figure 3. Molecular structure of 7 (all hydrogen atoms are omitted
for clarity). Selected bond distances (Å) and angles (deg): Co1-N1
) 1.966(2), Co1-N2 ) 1.918(2), Co1-O2 ) 1.9706(18), Co1-P1
) 2.2469(8), Co1-P3 ) 2.2110(8), Co1-Co2 ) 2.3997(5),
Co2-N1 ) 1.922(2), Co2-N2 ) 1.961(2), Co2-O1 ) 1.9675(17),
Co2-P2 ) 2.2405(8), Co2-P4 ) 2.214(7), O2-C2 ) 1.325(3),
O1-C10 ) 1.319(3), N2-C1 ) 1.412(3), N1-C9 ) 1.419(3);
N2-Co1-N1 ) 79.94(9), N1-Co2-N2 ) 79.95(9), N2-Co1-O2
) 84.41(8), P3-Co1-P1 ) 99.02(3), N1-Co1-Co2 ) 51.05(6),
P3-Co1-Co2)112.77(2),P1-Co1-Co2)142.04(3),P4-Co2-P2
) 100.18(3), P4-Co2-Co1 ) 112.98(2), P2-Co2-Co1 )
140.74(3), Co1-N1-Co2 ) 76.23(8), Co1-N2-Co2 ) 76.42(8),
Co1-N2-C1)111.67(17),Co1-O2-C2)110.67(16),O2-C2-C1
) 118.9(2), C2-C1-N2 ) 113.4(2).
(13) (a) Klein, H.-F.; Schmidt, A.; Flo¨rke, U.; Haupt, H.-J. Inorg. Chim.
Acta 2003, 342, 171–178. (b) Mu¨ller, J.; Oliveira, G. M.; Pickardt, J. J.
Organomet. Chem. 1987, 327, 241.

Communications
Organometallics, Vol. 27, No. 21, 2008 5437
two organocobalt(III) complexes, Co(PMe3)2Cl(Me
Acknowledgment. We gratefully acknowledge support by
(C6H3O∩NdNC6H3R) · (H2NC6H4R) (R ) Me (3), Br (4)),
the NSF of China, Nos. 20772072 and 20872080.
whereas the reactions of compounds
1 and 2 with
Supporting Information Available: Text and tables giving
experimental details and X-ray data for 3, 4, and 7 and CIF files
giving X-ray data for 3, 4, and 7. This material is available free of
charge via the Internet at http://pubs.acs.org.
Co(PMe3)4 and Co(PMe3)4Me afford the dinuclear cobalt
complex [Co2(PMe3)4(MeC6H3O∩NH)2] (7) with the
complete cleavage of the NdN bond. Crystal structures
of the complexes 3, 4, and 7 were determined by X-ray
crystallography.
OM8007844