Unusual transformation of trialkylamines mediated by platinum

Article abstract of DOI:10.1021/om0607494

An interesting chemical transformation of trialkyl-amines has taken place during the reaction of 2-(2′,6′-dimethylphenylazo)-4-methylphenol (1) with K₂[PtCl₄] in refluxing methanol in the presence of trialkylamines, leading to the formation of organoplatinum complexes (2 and 3), where ligand 1 is coordinated as a bidentate N,O donor and the transformed trialkylamines are coordinated as bidentate C,N donors.

Full text of DOI:10.1021/om0607494

© Copyright 2006
Volume 25, Number 26, December 18, 2006
American Chemical Society
Communications
Unusual Transformation of Trialkylamines Mediated by Platinum
Sarmistha Halder,† Michael G. B. Drew,‡ and Samaresh Bhattacharya*,†
Departments of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity, Kolkata 700 032, India, and
UniVersity of Reading, Whiteknights, Reading RG6 6AD, U.K.
ReceiVed August 18, 2006
Summary: An interesting chemical transformation of trialkyl-
amines has taken place during the reaction of 2-(2′,6′-
dimethylphenylazo)-4-methylphenol (1) with K2[PtCl4] in re-
fluxing methanol in the presence of trialkylamines, leading to
the formation of organoplatinum complexes (2 and 3), where
ligand 1 is coordinated as a bidentate N,O donor and the
transformed trialkylamines are coordinated as bidentate C,N
donors.
Recently we have observed very interesting C-H as well as
C-C bond activation of 2-(2′,6′-dimethylphenylazo)-4-meth-
ylphenol (1) mediated by ruthenium,3c,e osmium,3c and iridium,3b
which has encouraged us to explore the reactivity of ligand 1
with other platinum metals. During this exploration we have
observed an unusual chemical transformation of simple trialkyl-
amines, viz. triethylamine and N-ethyldiisopropylamine, medi-
ated by platinum and herein we wish to report our preliminary
findings on this particular reaction.
Reaction of ligand 1 with K2[PtCl4] has been first carried
out in refluxing methanol in the presence of triethylamine, which
has afforded a pinkish red complex (2).4 The structure deter-
mination of complex 2 by X-ray crystallography5 has revealed
that, apart from coordination of ligand 1 to platinum, an
unexpected interesting reaction has also taken place during the
formation of this complex (Scheme 1). The structure (Figure
There has been considerable current interest in metal-mediated
chemical transformations of organic molecules.1 Such reactions
often proceed via a C-H bond activation step, leading to the
formation of an organometallic complex as the reactive inter-
mediate,2 which then undergoes further reactions to yield the
final product. Metal-mediated C-H bond activation of suitable
organic molecules is therefore of significant importance, and
the present work has originated from our interest in this area.3
† 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,
6775. (e) Fan, Y.; Hall, M. B. Organometallics 2005, 24, 3827. (f) Driver,
T. G.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Organometallics 2005,
24, 3644. (g) Esteruelas, M. A.; Lopez, A. M. Organometallics 2005, 24,
3584. (h) Rybtchinski, B.; Cohen, R.; Jan, M. L.; Milstein, D. J. Am. Chem.
Soc. 2003, 125, 11041. (i) Pamplin, C. B.; Legzdins, P. Acc. Chem. Res.
2003, 36, 223. (j) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002,
102, 1731. (k) Rybtchinski, B.; Oevers, S.; Montag, M.; Vigalok, A.;
Rozenberg, A.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2001,
123, 9064. (l) Vigalok, A.; Milstein, D. Acc. Chem. Res. 2001, 34, 798.
(m) Slugovc, C.; Padilla-Martnez, I.; Sirol, S.; Carmona, E. Coord. Chem.
ReV. 2001, 213, 129. (n) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem.
Res. 2001, 34, 633. (o) Sundermann, A.; Uzan, O.; Milstein, D.; Martin, J.
M. L. J. Am. Chem. Soc. 2000, 122, 7095.
‡ University of Reading.
(1) (a) Tsuji, J. Transition Metal Reagents and Catalysts; Wiley-VCH:
Weinheim, Germany, 2000. (b) Hegedus, L. S. Coord. Chem. ReV. 1998,
168, 49. (c) Bellar, M., Bolm, C., Eds. Transition Metals for Organic
Synthesis; Wiley-VCH: Weinheim, Germany, 1998; Vols. 1 and 2. (d)
Cornils, B., Hermann, W. A., Eds. Applied Homogenous Catalysis with
Organometallic Compounds: A ComprehensiVe Handbook in Two Volumes;
VCH: Weinheim, Germany, 1996. (e) Liebeskind, L. S., Ed. AdVances in
Metal-Organic Chemistry; JAI Press: Greenwich, CT, 1996. (f) Abel, E.,
Stone, F. G. A., Wilkinson, G., Eds. ComprehensiVe Organometallic
Chemistry; Pergamon Press: Oxford, U.K., 1995; Vol. 12. (g) Hegedus, L.
S. Transition Metals in the Synthesis of Complex Organic Molecules;
University Science Books: Mill Valley, CA, 1994; (h) Collman, J. P.;
Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of
OrganoTransition Metal Chemistry; University Science Books: Mill Valley,
CA, 1987; (i) Trost, B. M.; Verhoeven, T. R. In ComprehensiVe Organo-
metallic Chemistry; Abel, E., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon
Press: Oxford, U.K., 1982; Vol. 8.
10.1021/om0607494 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/17/2006

5970 Organometallics, Vol. 25, No. 26, 2006
Communications
Scheme 1. Formation of Complex 2
1) shows that 2-(2′,6′-dimethylphenylazo)-4-methylphenol (1)
is coordinated to platinum, via dissociation of the phenolic
proton, as a bidentate N,O donor, forming a six-membered
chelate ring. The other two coordination sites of platinum are
occupied by another bidentate C,N donor ligand, which has
originated from triethylamine. The structure determination thus
demonstrates that triethylamine, which was used to serve as a
base, has undergone an unusual chemical transformation during
the synthetic reaction and the transformed triethylamine remains
coordinated to the metal center, forming a five-membered
chelate ring. A closer look at the triethylamine-derived ligand
shows that while two ethyl groups of the parent triethylamine
have remained intact, one ethyl group has undergone several
chemical changes, whereby it has been converted into a C(H)d
C(H)CH2 fragment. The observed bond distances within this
fragment are consistent with its bond description. In complex
2 platinum is sitting in a CN2O coordination sphere, which is
Figure 1. Molecular structure of complex 2 (50% probability
ellipsoids). Hydrogen atoms bound to carbon atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg): Pt1-C1
) 2.028(9), Pt1-N3 ) 2.096(6), Pt1-N1 ) 1.977(6), Pt1-O1 )
2.071(6), C4-O1 ) 1.314(9), N1-N2 ) 1.300(7), C1-C2 )
1.501(11), C2-C3 ) 1.284(13), C3-N3 ) 1.487(11); C1-Pt1-
N3 ) 84.4(3), N1-Pt1-O1 ) 92.1(2), C1-Pt1-O1 ) 172.8(3),
N1-Pt1-N3 ) 179.3(2).
distorted from ideal square-planar geometry, as reflected in the
bond parameters around the metal center. However, the pendant
2′,6′-dimethylphenyl fragment of the coordinated ligand 1 as
well as the two ethyl groups of the triethylamine-derived ligand
are almost orthogonal to this square plane.
The observed transformation of triethylamine, involving
incorporation of a CH2 fragment via C-C and Pt-C bond
formation, and conversion of a C(H)C(H) fragment to a CdC
fragment, has been very interesting. To test the generality of
such a transformation, reaction of ligand 1 with K2[PtCl4] has
also been carried out in the presence of another base, viz.
N-ethyldiisopropylamine. This particular base has been picked
up as it has two different alkyl groups, viz. ethyl and isopropyl,
and thus provides an opportunity to see which of these two alkyl
groups actually undergoes the transformation. This new reaction
has also afforded a pinkish red complex (3),6 and its structure
determination by X-ray crystallography7 shows that, as before,
(3) (a) Gupta, P.; Dutta, S.; Basuli, F.; Peng, S. M.; Lee, G. H.;
Bhattacharya, S. Inorg. Chem. 2006, 45, 460. (b) Acharyya, R.; Basuli, F.;
Peng, S. M.; Lee, G. H.; Wang, R. Z.; Mak, T. C. W.; Bhattacharya, S. J.
Organomet. Chem. 2005, 690, 3908. (c) Nag, S.; Gupta, P.; Butcher, R. J.;
Bhattacharya, S. Inorg. Chem. 2004, 43, 4814. (d) Acharyya, R.; Basuli,
F.; Wang, R. Z.; Mak, T. C. W.; Bhattacharya, S. Inorg. Chem. 2004, 43,
704. (e) Acharyya, R.; Peng, S. M.; Lee, G. H.; Bhattacharya, S. Inorg.
Chem. 2003, 42, 7378. (f) Gupta, P.; Butcher, R. J.; Bhattacharya, S. Inorg.
Chem. 2003, 42, 5405. (g) Pal, I.; Dutta, S.; Basuli, F.; Goverdhan, S.;
Peng, S. M.; Lee, G. H.; Bhattacharya, S. Inorg. Chem. 2003, 42, 4338. (h)
Majumder, K.; Peng, S. M.; Bhattacharya, S. Dalton Trans. 2001, 284. (i)
Das, A.; Basuli, F.; Falvello, L. R.; Bhattacharya, S. Inorg. Chem. 2001,
40, 4085. (j) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2001,
40, 1126. (k) Dutta, S.; Peng, S. M.; Bhattacharya, S. Dalton Trans. 2000,
4623.
(4) Complex 2: to a solution of 2-(2′,6′-dimethylphenylazo)-4-meth-
ylphenol (58 mg, 0.24 mmol) in hot methanol (30 mL) was added
triethylamine (120 mg, 1.20 mmol) followed by K2[PtCl4] (50 mg, 0.12
mmol). The mixture was then heated at reflux for 24 h, whereby a pinkish
red solution was generated along with a black precipitate. The solution was
filtered, and evaporation of the filtrate gave a dark solid, which was subjected
to purification by thin-layer chromatography on a silica plate. Using 1:1
hexane-benzene as the eluant, a pinkish red band separated, which was
extracted with acetonitrile. Evaporation of the acetonitrile extract gave
pinkish red crystals of 2. Yield: 53 mg (40%). Anal. Calcd for C22H29N3-
OPt: C, 48.3; H, 5.3; N, 7.7. Found: C, 47.8; H, 5.5; N, 7.6. Mass spectral
data (positive ion ES): m/z 570 (M + Na), 547 (M + H). 1H NMR (CDCl3,
25 °C): δ 7.58 (s, C(8)H), 7.42 (d, J ) 8.8 Hz, C(5)H), 7.07 (d, J ) 7.5
Hz, C(13)H and C(15)H), 7.02 (t, J ) 6.5 Hz, C(14)H), 6.79 (d, J ) 8.7
Hz, C(6)H), 5.19 (m, C(3)H), 4.78 (m, C(2)H), 3.46 (m, 2H, C31), 2.42
(m, 2H, C33), 2.30 (s, 3H, C71), 2.22 (s, 6H, C121 and C161), 1.51 (m,
2H, C1), 1.42 (t, J ) 7.12 Hz, 6H, C32 and C34). 13C NMR (CDCl3, 25
°C): δ 154.0 (C4), 151.8 (C9), 141.7 (C11), 138.5 (C5), 135.0 (C8), 132.4
(C6), 129.3 (C14), 127.5 (C13 and C15), 126.3 (C7), 124.4 (C12), 124.3
(C16), 57.8 (C33), 29.3 (C31), 19.6 (C71), 19.4 (C3), 16.7 (C121 and C161),
13.9 (C2), 12.6 (C32 and C34), 3.11 (C1). UV-vis spectral data (aceto-
nitrile): λmax/nm (ꢀ/dm3 mol-1 cm-1) 538 (2100), 506 (2400), 434 (1500),
372 (4200), 310 (5300), 254 (14 800).
(6) Complex 3: complex 3 was prepared similarly to complex 2, using
N-ethyldiisopropylamine instead of triethylamine. Yield: 42 mg (30%).
Anal. Calcd for C24H33N3OPt: C, 50.1; H, 5.7; N, 7.3;. Found: C, 50.5;
H, 5.6; N, 7.5. Mass spectral data (positive ion ES): m/z 598 (M + Na),
1
575 (M). H NMR (CDCl3, 25 °C): δ 7.55 (s, C(8)H), 7.42 (d, J ) 8.95
Hz, C(5)H), 7.06 (d, J ) 7.4 Hz, 2H, C13 and C15), 7.01 (t, J ) 7.4 Hz,
C(13)H), 6.76 (d, J ) 8.8, C(6)H), 4.98 (s, C(2)H), 3.79 (m, 1H, C31),
3.01 (m, 2H, C342), 2.29 (s, 3H, C121), 2.27 (s, 3H, C161), 2.19 (s, 3H,
C71), 1.50 (d, J ) 6.2, 3H, C32), 1.48 (m, 2H, C1), 1.30 (d, J ) 7.93 Hz,
3H, C352), 1.29 (d, J ) 7.93, 2H, C33), 1.26 (s, 3H, C37), 0.88 (m, 3H,
C352). 13C NMR (CDCl3, 25 °C): δ 154.0 (C4), 151.8 (C9), 142.8 (C11),
141.0 (C5), 135.5 (C8), 132.8 (C6), 129.9 (C14), 128.2 (C13), 128.0 (C15),
126.8 (C7), 125.0 (C12), 124.8 (C16), 61.6 (C31), 49.4 (C342), 30.1 (C37),
21.6 (C71), 20.2 (C121 and C161), 19.8 (C2), 17.4 (C32), 17.3 (C33), 13.5
(C3), 10.5 (C352), 2.7 (C1). UV-vis spectral data (acetonitrile): λmax/nm
(ꢀ/dm3 mol-1 cm-1) 544 (2000), 510 (2400), 434 (2100), 374 (5700), 312
(7600), 252 (18 500).
(5) (a) Crystallographic data for 2: C22H29N3OPt, Mr ) 546.57, triclinic,
space group P1, a ) 7.941(9) Å, b ) 9.115(12) Å, c ) 16.865(17) Å, R )
81.037(10)°, â ) 88.203(10)°, γ ) 69.624(10)°, V ) 1130(2) Å3, Z ) 2,
µ ) 6.223 mm-1, T ) 293 K, λ ) 0.710 73 Å, R1 ) 0.0367 wR2 ) 0.0784,
GOF ) 1.07.
(7) (a) Crystallographic data for 3: C24H33N3OPt, Mr ) 574.62,
orthorhombic, space group P212121, a ) 7.965(9) Å, b ) 16.941(17) Å, c
) 17.97(2) Å, V ) 2425 (5) Å3, Z ) 4, µ ) 5.804 mm-1, T ) 293 K, λ
) 0.710 73 Å, R1 ) 0.1037, wR2 ) 0.1529, GOF ) 1.17.

Communications
Scheme 2. Formation of Complex 3
Organometallics, Vol. 25, No. 26, 2006 5971
Scheme 3. Probable Paths for the Formation of Complexes
2 and 3
a similar interesting transformation of the N-ethyldiisopropy-
lamine has taken place during the formation of this complex
(Scheme 2). The structure (Figure 2) shows that 2-(2′,6′-
dimethylphenylazo)-4-methylphenol (1) is coordinated to plati-
num as in complex 2 and another ligand, originating from
N-ethyldiisopropylamine, is also coordinated to the metal center
as a bidentate C,N donor. An inspection of the N-ethyldiiso-
propylamine-derived ligand shows that one isopropyl group has
undergone a similar chemical transformation as before, whereby
it has been converted into a C(CH3)dC(H)CH2 fragment, while
the other isopropyl and ethyl groups have remained intact. The
observed bond distances in this fragment are in good agreement
with its bond description.
The observed chemical transformation of the trialkylamines,
viz. triethylamine and N-ethyldiisopropylamine, has been very
unusual and, to our knowledge, unprecedented. Incorporation
of the CH2 fragment in the trialkylamines has been the most
interesting aspect of the observed transformations, and it may
be relevant to mention in this context that such incorporation
of a CH2 fragment in organic molecules is rare.8 Two sources
of this CH2 fragment are in principle probable: viz. the solvent
methanol or ligand 1. To sort this out, reaction of ligand 1 with
K2[PtCl4] has also been carried out in the presence of triethy-
lamine similarly as before, using CD3OD as the solvent instead
of methanol. The pinkish red complex 4, similar to complex 2,
is obtained from this reaction. Characterizations (1H NMR, 2H
NMR, 13C NMR and mass spectra) of complex 4 show it to be
almost identical with complex 2, except the metal-bound CH2
fragment in 2 is replaced by a CD2 fragment in 4.9 This clearly
indicates that methanol has served as the source of the CH2
fragment. It may be recalled at this point that complexes 2 and
3 are obtained in relatively low yields along with a notable
quantity of a black precipitate.4,6 It may be also be mentioned
in this context that merely heating a solution of K2[PtCl4] in
(8) (a) Wu, H. J.; Chiang, C. M. J. Phys. Chem. B 1998, 102, 7075. (b)
Tadayoshi, M. Huaxue 1995, 53, 115. (c) Tsuneo, I. Shokubai 1986, 28, 208.
(9) Complex 4: complex 4 was prepared similarly to complex 2, using
CD3OD instead of methanol. Yield: 53 mg (40%). Mass spectral data
(positive ion ES): m/z 549 (M + H). 1H NMR (CDCl3, 25 °C): δ 7.58 (s,
1H), 7.41 (d, J ) 8.6 Hz, 1H), 7.09-6.99 (m, 3H), 6.79 (d, J ) 8.9 Hz,
1H), 5.19 (d, J ) 8.4 Hz, 1H), 4.78 (d, J ) 8.5 Hz, 1H), 3.46 (m, 2H),
2.45 (m, 2H), 2.30 (s, 3H), 2.22 (s, 6H), 1.42 (t, J ) 7.13 Hz, 6H). 2H
NMR (CHCl3, 25 °C): δ 1.47 (s). 13C NMR (CDCl3, 25 °C):12 δ 154.2
(C4), 151.9 (C9), 141.8 (C11), 138.6 (C5), 135.1 (C8), 132.5 (C6), 129.4
(C14), 127.6 (C13 and C15), 126.4 (C7), 124.5 (C12), 124.4 (C16), 57.8
(C33), 29.6 (C31), 19.7 (C71), 19.5 (C3), 16.8 (C121 and C161), 14.0 (C2)
12.7 (C32 and C34). UV-vis spectral data (acetonitrile): λmax/nm (ꢀ/dm3
mol-1 cm-1), 531 (2200), 504 (2400), 433 (1700), 372 (4500), 310 (6500),
252 (18 300).
Figure 2. Molecular structure of complex 3 (50% probability
ellipsoids). Hydrogen atoms bound to carbon atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg): Pt-C1 )
1.95(2), Pt-N3 ) 2.11(2), Pt-N1 ) 1.976(15), Pt-O1 ) 2.067-
(15), C4-O1 ) 1.33(3), N1-N2 ) 1.28(2), C1-C2 ) 1.47(3),
C2-C3 ) 1.29(4), C3-N3 ) 1.51(4); C1-Pt-N3 ) 80.9(9), N1-
Pt-O1 ) 88.9(6), C1-Pt-O1 ) 173.4(8), N1-Pt-N3 ) 175.9-
(6).

5972 Organometallics, Vol. 25, No. 26, 2006
Communications
methanol has been observed to produce black particles of
elemental platinum along with formaldehyde.10 The mechanism
of the observed chemical transformation of the trialkylamines
during the formation of complexes 2 and 3 is not completely
clear to us. However, the speculative sequences illustrated in
Scheme 3 seem probable. In the initial step an intermediate (A)
is believed to be formed, which contains ligand 1, coordinated
as a bidentate N,O donor via loss of the phenolic proton,
chloride, and trialkylamine coordinated to platinum through the
nitrogen. In the next step formadehyde, generated in situ, binds
to the metal center in a η2 fashion (intermediate B), which is
followed by a condensation reaction between the formaldehyde
and a terminal CH3 fragment of the trialkylamine (intermediate
C). The new CdC fragment thus formed remains π-bonded to
the metal center. In the final step elimination of a hydrogen
ion and new Pt-C bond formation takes place associated with
simultaneous cleavage of the Pt-Cl bond, yielding the orga-
noplatinum complex (2 or 3). It may be mentioned here that
though such transformation of trialkylamines has no precedence,
metal-mediated transformation of triisopropylphosphine of
similar nature is documented in the literature.11
The present study shows that, in combination with ligand 1,
platinum can mediate interesting transformations of trialkyl-
amines in methanol medium involving incorporation of a CH2
fragment from the solvent. This study also demonstrates that,
given an option between ethyl and isopropyl groups, the latter
undergoes the transformation.
Acknowledgment. We thank the Department of Science and
Technology, New Delhi, India (Grant No. SR/S1/IC-15/2004),
for financial assistance and the RSIC at Central Drug Research
Institute, Lucknow, India, for the C,H,N analysis data. We thank
Prof. B. C. Ranu (Department of Organic Chemistry, Indian
Association for the Cultivation of Science) for useful discus-
sions. We also thank Mr. Ashutosh Banik (Service Engineer,
Bruker) and Mr. E. Padmanaban (Indian Institute of Chemical
Biology, Kolkata) for recording 2H NMR spectrum of complex
4. M.G.B.D. thanks the EPSRC (U.K.) and the University of
Reading for funds for the Image Plate System. S.H. thanks the
University Grants Commission, New Delhi, India, for her
fellowship (Grant No. 10-2(5)2004(1)-E.U.II).
Supporting Information Available: CIF files giving crystal-
lographic data for 2 and 3. This material is available free of charge
via the Internet at http://pubs.acs.org. These data have also been
deposited at the Cambridge Crystallographic Data Center (CCDC
No. 256356 for 2 and 288927 for 3).
(10) Generation of formaldehyde was verified by preparing its 2,4-
dinitrophenyl hydrazone derivative.
(11) (a) Baya, M.; Buil, M. L.; Esteruelas, M. A.; Onate, E. Organo-
metallics 2005, 24, 5180. (b) Baya, M.; Buil, M. L.; Esteruelas, M. A.;
Onate, E. Organometallics 2005, 24, 2030. (c) Baya, M.; Esteruelas, M.
A.; Gonzalez, A. I.; Lopez, A. M.; Onate, E. Organometallics 2005, 24,
1225. (d) Esteruelas, M. A.; Gonzalez, A. I.; Lopez, A. M.; Onate, E.
Organometallics 2004, 23, 4858. (e) Baya, M.; Buil, M. L.; Esteruelas, M.
A.; Onate, E. Organometallics 2004, 23, 1416.
OM0607494
(12) The same atom-labeling scheme, used for complex 2 (Figure 1),
has been followed for complex 4.