The first NH aldimine organometallic compound. Isolation and crystal structure.

Article abstract of DOI:10.1039/b211808d

The first NH aldimine organometallic derivative is unexpectedly formed by the cleavage of the nitrogen-carbon bond of the amino acid fragment of the Schiff base 2,4,6-Me3C6H2CH=NCH(CH2Ph)COOEt when the imine is treated with palladium acetate.

Full text of DOI:10.1039/b211808d

The first NH aldimine organometallic compound. Isolation and crystal
structure†
Joan Albert,a J. Magali Cadena,a Asensio González,b Jaume Granell,*a Xavier Solansc and Mercè
Font-Bardiac
a Departament de Química Inorgànica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.
E-mail: jaume.granell@qi.ub.es; Fax: 34 93 4907725
b Laboratorio de Química Orgánica, Facultat de Farmàcia, Universitat de Barcelona, Pl. Pius XII, s/n,
08028 Barcelona, Spain
c
Departament de Cristal.lografia i Dipòsits Minerals, Universitat de Barcelona, Martí i Franquès, s/n,
08028 Barcelona, Spain
Received (in Cambridge, UK) 28th November 2002, Accepted 9th January 2003
First published as an Advance Article on the web 20th January 2003
The first NH aldimine organometallic derivative is un-
expectedly formed by the cleavage of the nitrogen–carbon
bond of the amino acid fragment of the Schiff base
2,4,6-Me3C6H2CHNNCH(CH2Ph)COOEt when the imine is
treated with palladium acetate.
chloro-bridged exo-cyclopalladated dimer 1 (that contains the
imine in the Z-form) was isolated, after purification by SiO2
column chromatography, in the first colored band (Scheme 1).7
A complex mixture of products were eluted in the second
colored band. Reaction of dimer 1 with PPh3 afforded the
mononuclear exo-complex [PdCl(C–N)(PPh3)] 2. The reaction
between PPh3 and the mixture of compounds obtained in the
second colored band afforded a new mixture of compounds that
were separated by SiO2 column chromatography, to afford the
exo-derivative 3 (that contains the imine in the E-form), the
endo metallacycle 4 and the NH aldimine derivative 5, in 20, 25
and 15% yield, respectively. Compound 5 is unexpectedly
formed by the cleavage of the nitrogen–carbon bond of the
amino acid fragment and, to the best of our knowledge, is the
first NH aldimine organometallic derivative described.8
NH aldimines have been proposed as unstable intermediates in
many reactions and their highly unstable nature is well
documented.1 The first evidence for the existence of the NH
aldimines, containing a saturated alkyl group, was reported in
19821a and PhCHNNH was identified in 1985 by NMR studies
in C6D5CD3 at 270 °C and by the reaction with methylamine to
yield N-benzylidenemethylamine via a transimination re-
action.1b Only two NH aldimines have ever been isolated, in
both cases at low temperatures.1c,d Recently, Chen and Brown
have reported the synthesis of stable adducts of these imines
with boranes, as new intermediates for using these ligands in
organic synthesis.2 Besides this, the formation of NH aldimines,
during monoamine oxidases (MAO) catalyzed transformation
of amines, has been established by biosynthetic investigations.3
Monoamine oxidases are primarily responsible for norepinephr-
ine, serotonin and dopamine metabolism and MAO inhibitors
are used as antidepressant agents (depression has been asso-
ciated with decreased functional amine-dependent synaptic
transmission).4
All the new organometallic compounds obtained were
characterised by elemental analysis, IR spectra and 1H and 31
P
NMR experiments. In some cases 2D-NMR experiments and
positive FAB-mass spectra were carried to complete the
characterisation.9 The high field shift of the aromatic protons of
the palladated ring in compounds 2, due to the aromatic rings of
the phosphine, indicates the cis disposition of the phosphorous
relative to the metallated carbon atom, and the chemical shift of
the phosphorous confirms this arrangement.5,10
The structure of 2 was determined by X-ray diffraction (Fig.
1).11,12 This complex is a six-membered exo-metallacycle that
contains the imine in the Z-form.
The structure of the NH aldimine derivative 5 was also
determined by X-ray diffraction (Fig. 2).12,13 It should be noted
that the molecules in unit cell are linked in pairs by two N–
H…Cl intermolecular bonds.
All the attempts carried out for the synthesis of palladium
coordinated NH aldimines, by reaction between NH3 and
mesitylaldehyde in different solvents and reaction conditions
were unsuccessful. Nevertheless we found that compound 5 can
also be obtained from the cyclometallation reaction of Schiff
As part of our continuing studies on the synthesis of
metallacycles containing N-donor ligands5 we have investi-
gated the action of palladium acetate on the Schiff base
2,4,6-Me3C6H2CHNNCH(CH2Ph)COOEt, obtained by con-
densation of mesitaldehyde with the
L
-phenylalanine ethyl
ester.6 The reaction was performed in toluene for 1 h at room
temperature and the reaction residues were treated with LiCl in
acetone for 30 min to afford a mixture of compounds. The
† Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b2/b211808d/
Scheme 1 Reagents and conditions: (i) Pd(AcO)2, toluene, 1 h.; (ii) LiCl, acetone, 30 min; (iii) PPh3, acetone, 30 min.
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CHEM. COMMUN., 2003, 528–529
This journal is © The Royal Society of Chemistry 2003

Notes and references
1 (a) J. C. Guillemin and J. M. Denis, Angew. Chem., Int. Ed. Engl., 1982,
21, 690; (b) D. R. Boyd, P. B. Coulder, R. Hamilton, N. T. Thompson,
N. D. Sharma and M. E. Stubbs, J. Chem. Soc., Perkin Trans 1, 1985,
2123; (c) B. Bodganovic and M. Velic, Angew. Chem., Int. Ed. Engl.,
1967, 6, 803; (d) D. R. Boyd, R. Hamilton, N. T. Thompson and M. E.
Stubbs, Tetrahedron Lett., 1979, 20, 3201.
2 G. Chen and H. C. Brown, J. Am. Chem. Soc., 2000, 122, 4217.
3 A. R. Battersby, D. G. Buckley, J. Staunton and P. J. Williams, J. Chem.
Soc., Perkin Trans 1, 1979, 2550.
4 W. Z. Potter and L. E. Hollister, in Basic and Clinical Pharmacology,
ed. B. G. Katzung, Appleton & Lange, Connecticut, 1998, pp.
483–491.
5 (a) J. Albert, M. Gómez, J. Granell, J. Sales and X. Solans,
Organometallics, 1990, 9, 1405; (b) J. Albert, J. Granell, J. Sales, X.
Solans and M. Font-Bardia, Organometallics, 1995, 14, 1393.
6 (a) For the synthesis of organometallic complexes with biologically
important ligands, see: K. Severin, R. Bergs and W. Beck, Angew.
Chem., Int. Ed., 1998, 37, 1634; (b) G. Jaouen, A. Vessiers and I. S.
Butler, Acc. Chem. Res., 1993, 26, 361; (c) H. Chen, S. Ogo and R. H.
Fish, J. Am. Chem. Soc., 1996, 118, 4993; (d) A. D. Ryabov, Angew.
Chem., Int. Ed., 1991, 30, 931.
Fig. 1 ORTEP plot of 2. Selected bond lenghts (Å) and angles (°): Pd–C(1)
2.002(2), Pd–N 2.109(2), Pd–P 2.2543(7), Pd–Cl 2.4010(7), O(1)–C(9)
1.164(4), O(2)–C(9) 1.294(4), N–C(12) 1.264(4), N–C(8) 1.483(3); C(1)–
Pd–N 81.78(9), C(1)–Pd–P 90.58(7), N–Pd–Cl 89.79(6), P–Pd–Cl
98.88(3).
7 Imines can undergo metallation on different carbon atoms, giving
organometallic complexes of different structures: endo-metallacycles, if
the CNN bond is included in the metallacycle, or exo-derivatives: A.
Crispini and M. Ghedini, J. Chem. Soc., Dalton Trans., 1997, 75.
8 Characterization data for 5: elemental analysis: calc. for
C28H27ClNPPd: C, 61.11, H, 4.94, N, 2.55; found: C 61.1, H 4.9, N
2.6%. 1H NMR (400 MHz, CDCl3): d 2.15 (s, 3H, Me); 2.36 (s, 3H,
Me); 2.81 (d, 2H, 3JHP 5.6 Hz, CH2Pd); 6.08 (s, 1H, aromatic); 6.74 (s,
1H, aromatic); 7.39–7.42 (m, 6H, aromatic); 7.63–7.70 (m, 9H,
aromatic); 8.49 (t, 1H, 4JHP 13.2, 3JHH 13.2 Hz, HCNN); 9.22 (br d, 1H,
3JHH 13.2 Hz, HN), 31P{1H} NMR (101.26 MHz, CDCl3), d 34.19 s.
9 Complete racemization of the Schiff base takes place during the
cyclopalladation reaction.
10 (a) The destabilizing effect of two soft ligands in mutual trans positions
has been called antisymbiosis, see: J. A. Davies and F. R. Hartley, Chem.
Rev., 1981, 81, 79; (b) R. G. Pearson, Inorg. Chem., 1973, 12, 712; (c)
R. Navarro and E. P. Urriolabeitia, J. Chem. Soc., Dalton Trans, 1999,
4111. Recently the term transphobia has been proposed to describe the
difficulty of coordinating mutually trans phosphine and aryl ligands in
palladium complexes, see (d) J. Vicente, J. A. Abad, A. D. Frankland
and M. C. Ramírez de Arellano, Chem. Eur. J., 1999, 5, 3066; (e) J.
Vicente, A. Arcas, D. Bautista and P. G. Jones, Organometallics, 1997,
16, 2127; (f) J. Vicente, J. A. Abad, F. S. Hernández-Mata and P. G.
Jones, J. Am. Chem. Soc., 2002, 124, 3848.
Fig. 2 ORTEP plot of 5. Selected bond lenghts (Å) and angles (°): Pd(1)–
C(10) 2.041(5), Pd(1)–N 2.067(3), Pd(1)–P(2) 2.2597(10), Pd(1)–Cl
2.4354(12), N–C(1) 1.275(5), C(1)–C(2) 1.454(6); C(10)–Pd(1)–N
82.39(17), C(10)–Pd(1)–P(2) 94.15(12), N–Pd(1)–Cl 88.67(12), P(2)–
Pd(1)–Cl 95.17(4).
base 2,4,6-Me3C6H2CHNNCH(Ph)COOMe, which contains the
2-phenylglycine fragment. These results show that the reactivity
of the amino acid fragment plays a crucial role in the synthesis
of 5.
In conclusion we have shown that the highly unstable NH
aldimines can be obtained by reaction between palladium
acetate and phenylalanine derivatives and that these species,
when coordinated to palladium, are so stable that can easily be
characterized and their crystal structure determined by X-ray
diffraction. The synthesis of new NH aldimines from other
biologically important ligands, in the presence of transition
metals, and experiments for clarifying the mechanism of the
formation of the NH aldimine derivatives are currently in
progress.
11 Crystallographic data: for 2: C39H39ClNO2PPd, Mw = 726.53, triclinic,
¯
P1, a = 10.9750(10), b = 11.0460(10), c = 15.1750(10) Å, a =
75.4050(10), b = 80.8580(10), g = 82.3680(10)°, V = 1749.4(3) Å3, Z
= 2. For 5884 observed reflections the final R (on F) factor was 0.034,
wR (on F2)
= 0.078 for I > 2s(I). CCDC 184188. For 5:
C28H27ClNPPd, Mw = 550.33, monoclinic, P21/c, a = 10.1670(10), b
= 24.9410(10), c = 10.6240(10) Å, b = 114.2640(10)°, V = 2456.0(3)
Å3, Z = 4. For 3524 observed reflections the final R (on F) factor was
0.045, wR (on F2) = 0.136 for I > 2s(I). CCDC 1841887. See http://
www.rsc.org/suppdata/cc/b2/b211808d/ for crystallographic data in
CIF or other electronic format.
12 Data collected in an MAR345 diffractometer with a image plate
detector, graphite monochromatized Mo-Ka radiation. The structures
were solved by Direct methods, using SHELXS computer program (G.
M. Sheldrick, 1977, A computer program for determination of crystal
structure, University of Göttingen, Germany) and refined by a full-
matrix least-squares method, with the SHELX97 computer program.
This work was supported by the Ministerio de Ciencia y
Tecnología (project: BQU2000-0652) and by the Comissionat
per a Universitats i Recerca (project: 2001SGR-00054).
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