Synthesis of amidine complexes by metal-mediated addition of amino acid esters to coordinated nitriles

Article abstract of DOI:10.1007/s11172-005-0330-4

The reaction of the nitrile platinum(IV) complex trans-[PtCl ₄(EtCN)₂] with amino acid esters H₂NC(R ¹)(R²)CO₂Me (R¹ = R² = H, H-Me, Me-Me, H-Ph) and H₂NCH<

Full text of DOI:10.1007/s11172-005-0330-4

610
Russian Chemical Bulletin, International Edition, Vol. 54, No. 3, pp. 610—614, March, 2005
Synthesis of amidine complexes by metalꢀmediated addition of
amino acid esters to coordinated nitriles
ꢀ
A. V. MakarychevaꢀMikhailova, E. A. Tronova, and V. Yu. Kukushkin
Department of Chemistry, St. Petersburg State University,
26 Universitetsky prosp., 198504 Stary Petergof, Russian Federation.
Fax: +7 (812) 428 6939. Eꢀmail: kukushkin@VK2100.spb.edu
The reaction of the nitrile platinum(IV) complex transꢀ[PtCl4(EtCN)2] with amino
acid esters H2NC(R1)(R2)CO2Me (R1 = R2 = H, H—Me, Me—Me, H—Ph) and
H2NCH2CH2CO2Me in CH2Cl2 produces the amidine complexes transꢀ[PtCl4{Zꢀ
NH=C(Et)NHC(R1)(R2)CO2Me}2] and transꢀ[PtCl4{ZꢀNH=C(Et)NHCH2CH2CO2Me}2],
which were isolated in 70—80% yields and characterized by elemental analysis, mass spectromꢀ
1
etry, IR spectroscopy, and H and 13C{1H} NMR spectroscopy. The structures of the comꢀ
plexes with R1 = R2 = H (1), R1 = H, R2 = Me (2), and R1 = H, R2 = Ph (4) were established
by Xꢀray diffraction analysis.
Key words: nitriles, amino acid esters, amidines, platinum(IV) complexes, nucleophilic
addition, reactivity of ligands.
The nucleophilic addition to the CN group is a wideꢀ
R2 = Me; R1 = R2 = Me; R1 = H, R2 = Ph) and
H2NCH2CH2CO2Me to the nitrile ligands of the
transꢀ[PtCl4(EtCN)2] complex giving rise to compounds
with functionalized amidine ligands.
spread reactivity mode of nitriles. This reaction is used to
prepare compounds of industrial (for example, acrylamide)
and/or pharmaceutical (for example, nicotinamide or
Sꢀ(+)ꢀibuprofen) importance. The reactivity of RCN speꢀ
cies in nucleophilic additions can be enhanced in a numꢀ
ber of ways, among which coordination to metal centers
plays a great role. Metalꢀmediated transformations of niꢀ
triles were considered in the reviews.1—3
Examples were given of metalꢀmediated reactions
of nitrile ligands with Nꢀnucleophiles containing the
sp3ꢀhybridized nitrogen atom (ammonia,4 primary and
secondary amines;5 for recent studies, see Refs 6—9).
These reactions afford amidine complexes with the
[M]—N(H)=C(R)NHR´ structural moiety. In most cases,
only simple amines (R´ = H, Alk, Ar) were used as nuꢀ
cleophiles. The reactions with functionalized amines were
studied only for amino alcohols.10 However, the introꢀ
duction of various peripheral groups (for example, hydroꢀ
philic) into the substituent R´ could, first, lead to an
increase in solubility of amidine complexes in water, which
is of importance in tests for antitumor activity of these
biologically active compounds,11 and, second, provide
the possibility to use the peripheral groups in further synꢀ
thetic transformations (for example, for the synthesis of
metallaligands12).
Results and Discussion
The reactions of nitriles in the platinum(IV) complex
transꢀ[PtCl4(EtCN)2] with amino acid esters
H2NCR1R2CO2Me and H2NCH2CH2CO2Me proceed at
room temperature in a CH2Cl2 solution and are comꢀ
pleted in one hour (Scheme 1).
Complexes 1—4 and transꢀ[PtCl4{NH=C(Et)ꢀ
NHCH2CH2CO2Me}2] (5) were isolated in 70—80%
yields and characterized by elemental analyses (C, H,
and N), mass spectrometry, IR spectroscopy, and 1H and
13C{1H} NMR spectroscopy. In the reaction mixture, we
also found small amounts of the iminol platinum(IV) comꢀ
plex transꢀ[PtCl4{NH=C(OH)R}2], which has been charꢀ
acterized earlier.17 Apparently, the latter is formed by
hydrolysis of the propionitrile ligands in [PtCl4(EtCN)2],
which occurs in undried dichloromethane.
The results of elemental analyses of complexes 1—5
showed that all these compounds contain two amidine
ligands formed by the nucleophilic addition of the amino
acid esters to the coordinated nitriles. The observed fragꢀ
mentation and the character of isotopic patterns in the
electrospray mass spectra of the complexes, which were
measured in acetone solutions at concentrations of
∼2•10–6 mol L–1, are similar to those expected for amidine
As part of continuing studies (see the reviews1,2
and recent publications9,10,12—16) of the reactions of
Nꢀnucleophiles with nitrile platinum(IV) complexes, we
investigated the nucleophilic addition of amino acid esꢀ
ters H2NC(R1)(R2)CO2Me (R1 = R2 = H; R1 = H,
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 601—605, March, 2005.
1066ꢀ5285/05/5403ꢀ0610 © 2005 Springer Science+Business Media, Inc.

Synthesis of amidine complexes
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 3, March, 2005
611
Scheme 1
R1 = R2 = H (1); R1 = H, R2 = Me (2); R1 = R2 = Me (3); R1 = H, R2 = Ph (4).
derivatives; [M – H]– and [M + Na]+ are characteristic
fragments.
(see below) demonstrated that all complexes in the solid
state have an intramolecular hydrogen bond between the
H atom of the amido group and the chloride ligand.
The NOE spectra were recorded for all complexes.
These data revealed a correlation between the signal for
the proton of the imino group [Pt]—NH=C(CH2CH3)
and the signal for the methylene protons in
[Pt]—NH=C(CH2CH3). This is evidence that22 the
amidine ligands in solutions of complexes 1—5, like those
in the solid state (see below), have a Z configuration.
Lightꢀorange crystals of complexes 1, 2, and 4 were
prepared by evaporation of solutions of these compounds
in a mixture of toluene and methanol. The coordination
polyhedra of these compounds are slightly distorted octaꢀ
hedra. The amidine ligands adopt a Z configuration. The
C=N bond lengths vary in the range of 1.275—1.319 Å
and are consistent with the average lengths in other imino
platinum complexes.10,13 Xꢀray diffraction analysis conꢀ
firmed the presence of a hydrogen bond between the amide
hydrogen atom and the chloride ligand. The Pt—Cl bond
lengths (2.303—2.327 Å) are typical of chloride derivaꢀ
tives of platinum(IV).17,19
The IR and NMR spectroscopic data also confirm the
hypothesis of the addition of amino acid esters to the
RCN ligands. A comparison of the IR spectra of the reꢀ
sulting compounds with the spectra of the starting comꢀ
plexes shows the absence of the absorption band ν(C≡N)
at 2350—2300 cm–1 (in the transꢀ[PtCl4(EtCN)2] comꢀ
plex, ν(C≡N) 2340 cm–1 17
and reveals a very intense
)
stretching band ν(C=N) at 1642—1625 cm–1 and weaker
bands ν(N—H) in the range of 3387—3317 cm–1, which
are absent in the IR spectrum of the transꢀ[PtCl4(EtCN)2]
complex. An analogous situation is observed for the
addition of other Nꢀdonors (ammonia,4 amino
alcohols H2NCH(R´)CH(R″)OH,10 imino esters
18
NH=C(R)OR´,15 or benzophenone imine HN=CPh2
)
or OHꢀnucleophilic agents (for example, oximes
HON=CRR 19—21 or alcohols HOR´ 22) to nitriles in the
[PtCl4(RCN)2] complexes.
The addition products of amino acid esters
H2NCR1R2CO2Me and H2NCH2CH2CO2Me to nitriles
in the transꢀ[PtCl4(EtCN)2] complex were studied by
1H and 13C{1H} NMR spectroscopy. The spectra of
compounds 1—5 show fragments corresponding to the
NH=C(Et) group and the amide moiety of the ligand. In
the 13C{1H} NMR spectra of the complexes, the signal for
the C atom of the imino group NH=C emerges at δ ∼170,
i.e., at substantially lower field compared to the signal for
the nitrile carbon atom (δ 119) in the spectrum of the
The nucleophilic addition of amino acid esters to niꢀ
triles observed in the present study is promoted by
C(6)
O(1)
O(2)
C(5)
Cl(2)
C(4)
1
Pt
starting transꢀ[PtCl4(EtCN)2] complex. The H NMR
spectra show signals for the hydrogen atom of the NH=C
group at δ 5.4—5.7 and the signals for the proton of the
—NH— amide group at δ 7.5—8.1 (assignment of the
signals for NH was made based on the NOESYTP NMR
experiment). The downfield shift of the signal of the amide
group indicates that the proton of the —NH— group is
involved in hydrogen bonding. Xꢀray diffraction study
N(2)
N(1)
C(1)
C(2)
Cl(1)
C(3)
Fig. 1. Molecular structure of complex 1.

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MakarychevaꢀMikhailova et al.
C(3)
C(2)
Cl(1)
Cl(2)
N(1)
C(1)
C(12)
O(3)
N(4)
Pt(1)
C(13)
N(2)
C(14)
C(4)
C(6)
C(11)
C(8)
O(4)
C(5)
O(1)
Cl(4)
N(3)
O(2)
C(9)
Cl(3)
C(7)
C(10)
Fig. 2. Molecular structure of complex 2.
C(20)
C(21)
C(19)
C(3)
C(8)
C(9)
Cl(2)
Cl(3)
C(2)
C(1)
C(18)
C(22)
C(7)
C(6)
N(1)
Pt(1)
C(17)
C(16)
C(23)
C(10)
C(5)
C(4)
N(4)
N(2)
C(11)
O(1)
O(4)
Cl(1)
Cl(4)
C(13)
C(14)
C(15)
N(3)
O(2)
O(3)
C(24)
C(12)
Fig. 3. Molecular structure of complex 4.
solution of amino acid ester hydrochloride (0.190 mmol) in
CH2Cl2 (4 mL). The reaction mixture was vigorously shaken in
a separatory funnel and the organic layer was separated. Then a
solution of the amino acid ester in CH2Cl2 (2 mL) was added to
a suspension of transꢀ[PtCl4(EtCN)2] (40.0 mg, 0.089 mmol) in
CH2Cl2 (1 mL), and the reaction mixture was stirred at room
temperature for 30 min. After removal of the solvent, the orange
oily residue was washed with diethyl ether (3×3 mL), and the
orange crystalline powder was obtained. The powder was dried
at room temperature in air. The yields were 70—80%.
Tetrachloro[bis(methylꢀNꢀpropanimidoylꢀNꢀglycinato)]plaꢀ
tinum(IV), [PtCl4{NH=C(Et)NHCH2CO2Me}2] (1). Found (%):
C, 22.81; H, 3.65; N, 8.67. C12H24N4Cl4O4Pt. Calculated (%):
C, 23.05; H, 3.87; N, 8.96. MS, m/z: 648 [M + Na]+. M.p.
159 °C, Rf = 0.54 (Et2O—CH2Cl2, 1 : 25, as the eluent). IR,
ν/cm–1: 3387 (N—H); 1748 (C=O); 1625 (C=N). 1H NMR
(CDCl3), δ: 8.00 (t, 1H, NH); 5.75 (t, 1 H, NH); 3.83 (s, 1 H,
OCH3); 2.86 (q, 2 H, CH2NH); 2.53 (q, 2 H, CH2); 1.30 (t, 3 H,
CH3). 13C{1H} NMR (CDCl3), δ: 171.87 (COO); 170.01 (C=N);
53.28 (OCH3); 44.75 (CH2NH); 27.09 (CH2); 10.11 (CH3).
platinum(IV) because free reactants do not react with
each other even under much more drastic conditions
(2 days, 40 °C).
Experimental
The transꢀ[PtCl4(EtCN)2] complex was prepared according
to a procedure described earlier.17 Amino acid ester hydrochloꢀ
rides were synthesized from the corresponding amino acids acꢀ
cording to a known procedure.23 The melting points were deterꢀ
mined on a Büchi Bꢀ540 instrument. The TLC analysis was
carried out on Merk 60 F254 plates. The electrospray mass specꢀ
TM
tra were measured on a Bruker Esquire3000
instrument. The
IR spectra were recorded on a Perkin Elmer FTIR instrument
in the 4000—400 cm–1 region in KBr pellets. The 1H and
13C{1H} NMR spectra were measured on a Bruker DPX 400
spectrometer at ambient temperature.
Addition of amino acid esters to coordinated nitriles. An aqueꢀ
ous NaHCO3 solution (16.8 mg, 0.20 mmol) was added to a

Synthesis of amidine complexes
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 3, March, 2005
613
Table 1. Crystallographic data and parameters of structure refinement of complexes 1, 2, and 4
Parameter
1
2
4
Molecular formula
М
C
12H24N4Cl4O4Pt
625.23
C14H28N4Cl4O4Pt
653.28
C24H32N4Cl4O4Pt
777.42
Crystal system
Space group
a/Å
b/Å
Monoclinic
Р2(1)/n
10.721(2)
8.835(2)
11.373
Monoclinic
Р2(1)
7.788(2)
18.839(4)
8.485
Monoclinic
Р2(1)
8.561(2)
13.881(3)
12.381
c/Å
β/deg
104.13(2)
1044.7(5)
2
1.990
7.25
107.19(3)
1189.3(5)
2
1.824
6.37
95.66(3)
1464.1(5)
2
1.763
5.19
V/Å3
Z
dcalc/g cm–3
µ/mm–1
Radiation
Scan range
Number of measured reflections
MoꢀKα (λ)
2.1—25.0
2145
2.3—25.0
1972
2.4—25.0
2502
Number of reflections with I > 2σ
1195
1995
2339
R1
wR2
0.039
0.094
0.019
0.049
0.015
0.015
Tetrachloro[bis(methylꢀNꢀpropanimidoylꢀNꢀalaninato)]plaꢀ
tinum(IV), [PtCl4{NH=C(Et)NHCH(Me)CO2Me}2] (2).
Found (%): C, 25.56; H, 4.31; N, 8.63. C14H28N4Cl4O4Pt. Calꢀ
culated (%): C, 25.74; H, 4.32; N, 8.58. MS, m/z: 676
[M + Na]+. M.p. 149 °C, Rf = 0.68 (Et2O—CH2Cl2, 1 : 25, as
the eluent). IR, ν/cm–1: 3317 (N—H); 1746 (C=O); 1642
Tetrachloro[bis(methylꢀNꢀpropanimidoylꢀNꢀβꢀalaꢀ
ninato)]platinum(IV), [PtCl4{NH=C(Et)NHCH2CH2CO2Me}2]
(5). Found (%): C, 25.56; H, 4.25; N, 8.36. C14H28N4Cl4O4Pt.
Calculated (%): C, 25.74; H, 4.32; N, 8.57. MS, m/z:
651 [M – H]–, 689 [M + Cl]–. M.p. 146 °C, Rf = 0.44
(MeCOMe—CH2Cl2, 1 : 20, as the eluent). IR, ν/cm–1: 3332
(N—H); 1720 (C=O); 1630 (C=N). 1H NMR (CDCl3), δ: 7.54
(t, 1 H, NH); 5.45 (t, 1 H, NH); 3.72 (s, 1 H, OCH3); 3.58 (q,
2 H, CH2NH); 2.62 (t, 2 H, CH2); 2.55 (q, 2 H, CH2); 1.30 (t,
3 H, CH3). 13C{1H} NMR (CDCl3), δ: 171.33 (COO); 169.68
(C=N); 52.66 (OCH3); 38.98 (CH2NH); 34.57 (CH2); 26.82
(CH2 from Et); 10.53 (CH3).
1
(C=N). H NMR (CDCl3), δ: 8.10 (d, 1 H, NH); 5.41 (t, 1 H,
NH); 4.25—4.19 (m, 1 H, CH); 3.81 (s, 1 H, OCH3); 2.53 (q,
2 H, CH2); 1.58 (d, 3 H, CH3CH); 1.29 (t, 3 H, CH3).
13C{1H} NMR (CDCl3), δ: 171.16 (COO); 169.36 (C=N);
53.42 (CH); 52.44 (OCH3); 26.69 (CH2); 18.96 (CH3CH);
10.54 (CH3).
Tetrachloro[bis(methylꢀ2ꢀmethylꢀNꢀpropanimidoylꢀNꢀalaꢀ
ninato)]platinum(IV), [PtCl4{NH=C(Et)NHC(Me)2CO2Me}2]
(3). Found (%): C, 27.98; H, 4.86; N, 8.34. C16H32N4Cl4O4Pt.
Calculated (%): C, 28.21; H, 4.73; N, 8.22. MS, m/z: 680
[M – H]–. M.p. 186 °C (decomp.), Rf = 0.34 (CH2Cl2, as the
eluent). IR, ν/cm–1: 3341 (N—H); 1733 (C=O); 1628 (C=N).
1H NMR (CDCl3), δ: 7.95 (t, 1 H, NH); 5.54 (t, 1 H, NH); 3.79
(s, 1 H, OCH3); 2.36 (q, 2 H, CH2); 1.63 (s, 6 H, CH3); 1.28 (t,
3 H, CH3 from Et). 13C{1H} NMR (CDCl3), δ: 174.55 (COO);
170.37 (C=N); 58.50 (C(Me)2); 53.46 (OCH3); 27.75 (2 CH3);
26.14 (CH2); 11.03 (CH3).
Xꢀray diffraction study. Xꢀray diffraction data sets for comꢀ
pounds 1, 2, and 4 were collected on an automated Enrafꢀ
Nonius CADꢀ4 diffractometer (monochromator, MoꢀKα raꢀ
diation, 293(2) К, θ/2θ scanning technique). Principal crystalꢀ
lographic parameters and details of structure refinement are
given in Table 1.
All calculations were carried out with the use of the
SHELXꢀ97 program package.24 The atomic coordinates and
complete tables of the bond lengths and bond angles were deꢀ
posited with the Cambridge Structural Database.
Tetrachloro{bis[methylphenyl(propanimidoylꢀNꢀamino)aceꢀ
tato]}platinum(IV), [PtCl4{NH=C(Et)NHCH(Ph)CO2Me}2] (4).
Found (%): C, 37.45; H, 3.95; N, 7.02. C24H32N4Cl4O4Pt. Calꢀ
culated (%): C, 37.08; H, 4.15; N, 7.21. MS, m/z: 799
[M + Na]+. M.p. 176 °C (decomp.), Rf = 0.33 (Et2O—CH2Cl2,
1 : 20, as the eluent). IR, ν/cm–1: 3385 (N—H); 1735 (C=O);
1637 (C=N). 1H NMR (CDCl3), δ: 8.80 (d, 1 H, NH); 7.55 (d,
2 H, Ph); 7.42—7.37 (m, 3 H, Ph); 5.79 (t, 1 H, NH); 5.21 (d,
1 H, CH); 3.78 (s, 3 H, OCH3); 2.47 and 2.26 (both m, 1 H each,
CH2); 1.15 (t, 3 H, CH3). 13C{1H} NMR (CDCl3), δ: 169.22
(C=N); 136.39 (Ph—Cipso); 129.74, 129.44, and 127.52
(Ph—Cmeta,ortho,para); 60.39 (CH); 53.76 (OCH3); 27.00 (CH2);
10.02 (CH3).
We thank Prof. V. K. Belsky (L. Ya. Karpov Institute
of Physical Chemistry) for performing Xꢀray diffraction
analysis, Prof. B. K. Keppler (University of Vienna)
for putting ESMS facilities at the our disposal, and
M. Galanski (University of Vienna) for performing the
NOESYTP NMR experiment.
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 03ꢀ03ꢀ32362)
and the Competitive Center of Fundamental Natural
Sciences (Grants A03 2.11ꢀ61, M03ꢀ2.5Kꢀ91, and
PD 03ꢀ1.3ꢀ28).

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Received June 15, 2004;
in revised form October 6, 2004