Pop-the-Cork strategy in synthetic utilization of imines: Stabilization by complexation and activation via liberation of the ligated species

Article abstract of DOI:10.1021/ic034086j

Treatment of trans-[PtCl₄(RCN)₂] (R = Me, Et) with ethanol allowed the isolation of trans-[PtCl₄{E-NH=C(R)OEt}₂]. The latter were reduced selectively, by the ylide Ph₃P=CHCO₂Me, to trans-[PtCl₂{E-NH=C(R)OEt}₂]. The complexed imino esters NH=C(R)OEt were liberated from the platinum(II) complexes by reaction with 2 equiv of 1,2-bis (diphenylphosphino)ethane (dppe) in chloroform; the cationic complex [Pt(dppe)₂]Cl₂ precipitates almost quantitatively from the reaction mixture and can be easily separated by filtration to give a solution of NH=C(R)OEt with a known concentration of the imino ester. The imino esters efficiently couple with the coordinated nitriles in trans-[PtCl₄(EtCN)₂] to give, as the dominant product, [PtCl₄{NH=C(Et)N=C(R)OEt}₂] containing a previously unknown linkage, i.e., ligated N-(1-imino-propyl)-alkylimidic acid ethyl esters. In addition to [PtCl₄{NH=C(Et)N= C(Et)OEt}₂], another compound was generated as the minor product, i.e., [PtCl₄(EtCN){NH=(Et)N=C(Et)OEt}], which was reduced to [PtCl₂(EtCN){NH=C(Et)N=C(Et)OEt}], and this complex was characterized by X-ray singlecrystal diffraction. The platinum(IV) complexes [PtCl₄{NH=C(Et)N=C(R)OEt}₂] are unstable toward hydrolysis and give EtOH and the acylamidine complexes trans-[PtCl₄{Z-NH=C(Et)NHC(R)O}₂], where the coordination to the Pt center results in the predominant stabilization of the imino tautomer NH=C(Et)NHC(R)=O over the other form, i.e., NH₂C(Et)=NC(R)=O, which is the major one for free acylamidines. The structures of trans-[PtCl₄{Z-NH=C(Et)NHC(R)=O}₂] (R = Me, Et) were determined by X-ray studies. The complexes [PtCl₄{NH=C(Et)N=C(R)OEt}₂] were reduced to the appropriate platinum(II) compounds [PtCl₂{NH=C(Et)N=C(R)OEt}₂], which, similarly to the appropriate Pt(IV) compounds, rapidly hydrolyze to yield the acylamidine complexes [PtCl₂{NH=C(Et)NHC(R)=O}₂] and EtOH. The latter acylamidine compounds were also prepared by an alternative route upon reduction of the corresponding platinum(IV) complexes. Besides the first observation of the platinum(IV)-mediated nitrile-imine ester integration, this work demonstrates that the application of metal complexes gives new opportunities for the generation of a great variety of imines (sometimes unreachable in pure organic chemistry) in metal-mediated conversions of organonitriles, the storage of imino species in the complexed form, and their synthetic utilization after liberation.

Full text of DOI:10.1021/ic034086j

Inorg. Chem. 2003, 42, 3602−3608
Pop-the-Cork Strategy in Synthetic Utilization of Imines: Stabilization by
Complexation and Activation via Liberation of the Ligated Species
,†
Nadezhda A. Bokach,†,‡ Vadim Yu. Kukushkin,* Matti Haukka,§ Joa˜o J. R. Frau´sto da Silva,‡ and
,‡
Armando J. L. Pombeiro*
St. Petersburg State UniVersity, 198504 Stary Petergof, Russian Federation, Centro de Qu´ımica
Estrutural, Complexo I, Instituto Superior Te´cnico, AV. RoVisco Pais, 1049-001 Lisbon, Portugal,
and Department of Chemistry, UniVersity of Joensuu, P.O. Box 111, FIN-80101, Joensuu, Finland
Received January 27, 2003
Treatment of trans-[PtCl4(RCN)2] (R ) Me, Et) with ethanol allowed the isolation of trans-[PtCl4{E-NHdC(R)-
OEt}2]. The latter were reduced selectively, by the ylide Ph3PdCHCO Me, to trans-[PtCl2{E-NHdC(R)OEt}2]. The
2
complexed imino esters NHdC(R)OEt were liberated from the platinum(II) complexes by reaction with 2 equiv of
1,2-bis(diphenylphosphino)ethane (dppe) in chloroform; the cationic complex [Pt(dppe)2]Cl2 precipitates almost
quantitatively from the reaction mixture and can be easily separated by filtration to give a solution of NHdC(R)OEt
with a known concentration of the imino ester. The imino esters efficiently couple with the coordinated nitriles in
trans-[PtCl4(EtCN)2] to give, as the dominant product, [PtCl4{NHdC(Et)NdC(R)OEt}2] containing a previously
unknown linkage, i.e., ligated N-(1-imino-propyl)-alkylimidic acid ethyl esters. In addition to [PtCl4{NHdC(Et)Nd
C(Et)OEt}2], another compound was generated as the minor product, i.e., [PtCl4(EtCN){NHdC(Et)NdC(Et)OEt}],
which was reduced to [PtCl2(EtCN){NHdC(Et)NdC(Et)OEt}], and this complex was characterized by X-ray single-
crystal diffraction. The platinum(IV) complexes [PtCl4{NHdC(Et)NdC(R)OEt}2] are unstable toward hydrolysis
and give EtOH and the acylamidine complexes trans-[PtCl4{Z-NHdC(Et)NHC(R)dO}2], where the coordination to
the Pt center results in the predominant stabilization of the imino tautomer NHdC(Et)NHC(R)dO over the other
form, i.e., NH C(Et)dNC(R)dO, which is the major one for free acylamidines. The structures of trans-[PtCl4-
2
{Z-NHdC(Et)NHC(R)dO}2] (R ) Me, Et) were determined by X-ray studies. The complexes [PtCl4{NHdC(Et)-
NdC(R)OEt}2] were reduced to the appropriate platinum(II) compounds [PtCl2{NHdC(Et)NdC(R)OEt}2], which,
similarly to the appropriate Pt(IV) compounds, rapidly hydrolyze to yield the acylamidine complexes [PtCl2-
{NHdC(Et)NHC(R)dO}2] and EtOH. The latter acylamidine compounds were also prepared by an alternative
route upon reduction of the corresponding platinum(IV) complexes. Besides the first observation of the platinum-
(IV)-mediated nitrile−imine ester integration, this work demonstrates that the application of metal complexes gives
new opportunities for the generation of a great variety of imines (sometimes unreachable in pure organic chemistry)
in metal-mediated conversions of organonitriles, the “storage” of imino species in the complexed form, and their
synthetic utilization after liberation.
Introduction
trimerizations that some of these compounds are commonly
treated as elusiVe.1
Even though it has since long been known that most of
the imine reactions are affected by metal ions, surprisingly,
a fundamental theory predicting metal control on the reactiv-
ity of imines has not yet been devised.2 Indeed, the reactions
of coordinated imines strongly depend on a delicate (and in
In general, imines RR′CdNR′′ are rather reactive species
and often unstable toward hydrolysis and also involved in
di-, tri-, and polymerizations and in various redox type
conversions. In particular, the imines RR′CdNH with two
donor substituents are usually so reactive in hydrolysis and
* To whom correspondence should be addressed. E-mail: kukushkin@
VK2100.spb.edu (V.Yu.K.) and pombeiro@ist.utl.pt (A.J.L.P.).
† St. Petersburg State University.
(1) Prenzler, P. D.; Hockless, D. C. R.; Heath, G. A. Inorg. Chem. 1997,
36, 5845 and references therein.
(2) Constable, E. C. Metals and Ligand ReactiVity. An Introduction to
the Organic Chemistry of Metal Complexes; VCH: Weinheim, 1995;
p 72.
‡ Instituto Superior Te´cnico.
§ University of Joensuu.
3602 Inorganic Chemistry, Vol. 42, No. 11, 2003
10.1021/ic034086j CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/06/2003

Pop-the-Cork Strategy in Synthetic Utilization of Imines
Scheme 1
many instances unpredictable) balance between such factors
as fine structure of the ligand and nature of the metal center.
Thus, imines with very close structural characteristics being
ligated to a metal center can exhibit even the opposite
reactivity modes, for example, stabilization vs activation
toward hydrolysis.2
Recently we (see reviews3) and others4 found explicit
evidence that platinum group metals,5-9 rhenium,10 gold,11a,b
or silver11c centers provide enormous stabilization of the
potentially unstable imines RR′CdNH, and these ligands can
be “stored” without changes in the coordinated form under
normal conditions for a prolonged time. Moreover, it appears
that the formation of imines is one of the major driving forces
for some reactions, e.g., condensation of complexed ammonia
with ketones,11 reductions of oximes,12 oxidative dehydration
of amines (e.g., at Fe,13 Ru,14,15 Os,16 Re,17 and Pt18 centers),
and nucleophilic additions to metal-bound nitriles.3,4 We
expected that the combination of the inertness of coordinated
imines RR′CdNH with their high reactivity in the free state
could have some intrinsic practical, albeit not yet explored,
implications. Indeed, if the imine complex forms and if the
further replacement of the ligated imines is carried out in
nonaqueous dried solvents and the new complex (without
imine) precipitates and is removed by filtration, the liberated
reactive imines present in the filtrate can be immediately
used in situ for further reactions, and we felt that this
methodology warranted investigation.
Taking into account our interest in the reactions of metal-
activated organonitriles in general and the interest in the
nitrile-imine coupling3 in particular, the suggested research
plan of the present work was the following (Scheme 1): (i)
To use a metal center as the promoter for the formation of
imines which are unstable in the free state. For this part of
the study we addressed [PtCl4(RCN)2] (R ) Me, Et)
compounds and EtOH insofar as it has been demonstrated8
that the Pt(IV) center provides the facile hydroxide-free
nitrile-alcohol coupling to achieve imino ester complexes.
The latter species contain the imines unreactive in the
complexed form, while the free imino esters are quite reactive
in acid-free media.19 (ii) To perform the liberation of the
coordinated imines. Despite the overall inertness of the imino
ester complexes toward substitution, some specific displace-
ment methods have been previously developed,8,9,20,21 and it
was anticipated to apply them for the liberation. (iii) To
employ the released imines in situ for further reactions by
studying the yet unknown Pt(IV)-mediated nitrile-imino
ester coupling giving 1,3-diaza-1,3-dienes. All these resultss
indicating new directions in the synthetic utilization of imines
and their complexes and showing novel routes to 1,3-diaza-
1,3-diene and acylamidine speciessare reported herein.
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Inorganic Chemistry, Vol. 42, No. 11, 2003 3603

Bokach et al.
Scheme 2
nitrile coupling, and the formation of amidines from conver-
sions of alkyl nitriles at a Pt(II) center24 or on Re4Te4(TeCl2)4-
Cl8 clusters25 or imidoylamidines at a Ni(II) center26), only
a few works deal with nitrile-imine (or heteroimine)
coupling, i.e., the addition of Ph2CdNH to Pt(IV)-bound
nitriles27 and of Ph2SdNH to organonitriles at Pt(II)28 and
Pt(IV)29 centers; both Ph2CdNH and Ph2SdNH exhibit
substantial stability toward hydrolysis and dimerization. The
coupling between complexed nitriles and unstable (or reac-
tive) imines in general and imino esters in particular is yet
an uncovered area of nitrile chemistry. In the present work,
it has been found that the imino esters efficiently couple in
step III with the nitriles in trans-[PtCl4(EtCN)2] to give
complexed 1,3-diaza-1,3-diene species, and this is the first
observation of nitrile-imine ester integration.
Thus, to achieve 1,3-diaza-1,3-diene complexes, a ca. 10-
fold excess of the imino ester (prepared in step II) is added
to trans-[PtCl4(EtCN)2] in CHCl3 at room temperature, and
the solvent is evaporated almost immediately to give [PtCl4-
{NHdC(Et)NdC(R)OEt}2] (R ) Me 5, R ) Et 6) as the
dominant product (Scheme 2, step III). Both 5 and 6 gave
satisfactory C, H, N elemental analyses and expected
fragmentation/isotopic patterns in FAB+ mass spectra. IR
spectra display two stretching vibrations due to the different
CdN bonds [1699 and 1613 cm-1 for 5 and 1698 and 1617
cm-1 for 6], and also two CdN carbons [179.6 and 159.9
ppm for 5 and 178.8 and 162.4 ppm for 6] from the NHd
C(Et)NdC(R)OEt species were observed in the 13C{1H}
NMR spectra. The imino hydrogen was identified by the
characteristic IR ν(N-H) stretching vibrations and for 6 a
broad 1H NMR signal from HN displayed at 7.11 ppm (this
signal was not observed for 5 probably due to the line
broadening).
Results and Discussion
Steps I and II: Complexed Imino Esters and Their
Liberation. We have recently reported that, in contrast to
the well-known (nitrile)Pt(II) systems,3 the addition of
alcohols to organonitrile platinum(IV) complexes occurs
under mild conditions and does not require a base as a
catalyst.8 Thus, treatment of trans-[PtCl4(RCN)2] (R ) Me,
Et) with ethanol at 45 °C allowed the isolation of the trans-
[PtCl4{E-NHdC(R)OEt}2] imino ester complexes (Scheme
2, step IA). These complexes were reduced selectively, by
the ylide Ph3PdCHCO2Me in accord with a recently
described method,7b to the corresponding isomers of (imino
ester)Pt(II) species trans-[PtCl2{E-NHdC(R)OEt}2] (R )
Me 1, R ) Et 2) without change in configuration of the imino
ester ligands (step IB). The coordinated imino ester species
exhibit significant stability and the solid complexes remain
intact for at least 6 months. In step II, the imino esters NHd
C(R)OEt (R ) Me 3, R ) Et 4) were liberated from the
platinum(II) complexes 1 and 2 by reaction with 2 equiv of
1,2-bis(diphenylphosphino)ethane (dppe) in chloroform. The
cationic complex [Pt(dppe)2]Cl2 precipitates almost quanti-
tatively from the reaction mixture and can be easily separated
by filtration to give a solution of NHdC(R)OEt (3 or 4)
with a known concentration of the imino ester. The liberated
imino esters were used in step III for the coupling.
In addition to 6, another compound was generated as the
minor product, i.e., [PtCl4(EtCN){NHdC(Et)NdC(Et)OEt}]
(7). The latter has been separated after column chromatog-
raphy on silica gel. Despite the small isolated amount, we
1
succeeded to characterize 7 by FAB-MS and H NMR
spectroscopy (see Experimental Section) and also to reduce
this compound to the platinum(II) complex [PtCl2(EtCN)-
{NHdC(Et)NdC(Et)OEt}] (8) and to perform the X-ray
crystallographic study (Figure 1). The X-ray single-crystal
diffraction study of 8 revealed a planar environment around
the metal center and the overall trans geometry with the NHd
C(Et)NdC(Et)OEt ligand in the Z configuration having bond
(23) Belluco, U.; Benetollo, F.; Bertani, R.; Bombieri, G.; Michelin, R.
A.; Mozzon, M.; Tonon, O.; Pombeiro, A. J. L.; Guedes da Silva, F.
C. Inorg. Chim. Acta 2002, 334, 437.
(24) Longato, B.; Bandoli, G.; Mucci, A.; Schenetti, L. Eur. J. Inorg. Chem.
2001, 3021.
(25) Emirdag-Eanes, M.; Ibers, J. A. Inorg. Chem. 2002, 41, 6170.
(26) Kryatov, S. V.; Nazarenko, A. Y.; Smith, M. B.; Rybak-Akimova, E.
V. Chem. Commun. 2001, 1174.
(27) Garnovskii, D. A.; Kukushkin, V. Yu.; Haukka, M.; Wagner, G.;
Pombeiro, A. J. L. J. Chem. Soc., Dalton Trans. 2001, 560.
(28) Kelly, P. F.; Slawin, A. M. Z. Chem. Commun. 1999, 1081. Kelly, P.
F.; Slawin, A. M. Z. Eur. J. Inorg. Chem. 2001, 263.
(29) Makarycheva-Mikhailova, A. V.; Bokach, N. A.; Kukushkin, V. Yu.;
Kelly, P. F.; Gilby, L. M.; Kuznetsov, M. L.; Holmes, K. E.; Haukka,
M.; Parr, J.; Stonehouse, J. M.; Elsegood, M. R. J.; Pombeiro, A. J.
L. Inorg. Chem. 2003, 42, 301.
Step III: Nitrile-Imine Ester Coupling. In spite of the
wealth of reports on the metal-mediated RCN-amine
integration3 (recent examples of the C-N bond making
include molybdenum-22 and platinum-mediated23 amine-
(22) Du, S.; Kautz, J. A.; McGrath, T. D.; Stone, F. G. A. J. Chem. Soc.,
Dalton Trans. 2002, 1553.
3604 Inorganic Chemistry, Vol. 42, No. 11, 2003

Pop-the-Cork Strategy in Synthetic Utilization of Imines
Figure 1. Molecular structure of 8. Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths (Å) and angles (deg): Pt(1)-
Cl(1) 2.305(2), Pt(1)-Cl(2) 2.299(2), Pt(1)-N(1) 1.987(5), Pt(1)-N(3)
1.983(5), N(1)-C(1) 1.287(7), C(1)-N(2) 1.364(8), N(2)-C(4) 1.283(8),
C(4)-O(1) 1.337(8), C(4)-C(5) 1.488(9), O(1)-C(7) 1.452(8), N(3)-C(9)
1.125(8), C(9)-C(10) 1.466(9), Pt(1)-N(1)-C(1) 130.2(4), C(1)-N(2)-
C(4) 125.8(6), N(2)-C(4)-O(1) 118.8(6), N(2)-C(4)-C(5) 128.3(6),
C(4)-O(1)-C(7) 117.2(5).
Figure 2. Molecular structure of 9. Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths (Å) and angles (deg): Pt(1)-
Cl(1) 2.3201(11), Pt(1)-Cl(2) 2.3239(11), Pt(1)-N(1) 2.022(4), N(1)-
C(1) 1.293(6), C(1)-C(2) 1.508(6), C(1)-N(2) 1.360(6), N(2)-C(4)
1.388(6), C(4)-O(1) 1.215(6), C(4)-C(5) 1.497(7), Cl(1)-Pt(1)-Cl(2)
90.43(5), N(1)-Pt(1)-Cl(1) 94.15(11), N(1)-Pt(1)-Cl(2) 92.11(12), Pt(1)-
N(1)-C(1) 134.2(3), N(1)-C(1)-N(2) 119.7(4), N(1)-C(1)-C(2) 119.2-
(4), C(1)-N(2)-C(4) 129.6(4), N(2)-C(4)-O(1) 122.5(5), N(2)-C(4)-
C(5) 114.2(4), N(2)‚‚‚Cl(1) 3.150(4), N(2)-H(2)‚‚‚Cl(2) 149.8.
lengths and angles of normal values.27-30 In 8, the newly
formed ligand [N-(1-imino-propyl)-alkylimidic acid ethyl
esters] represents the previously unknown linkage.
It was proved that both 3 and 4 do not react with EtCN
under the reaction conditions. Furthermore, no side reactions
were reported for the first step of the Pinner synthesis3 when
imino esters are formed (in the absence of any activating
metal center) from nitriles and both species are jointly present
in the reaction mixture. All this implies that the nitrile-
imine ester coupling is metal mediated. We believe that the
(1,3-diaza-1,3-diene)Pt(IV) complexes are presumably formed
by nucleophilic attack of the imine N atom on the electro-
philically activated carbon atom of the organonitrile.
One further point deserves discussion. In many instances,
the synthetic utilization of imines involves their hydrochlo-
rides insofar as the protonation stabilizes RR′CdNH from
other reactions. Step III illustrates some advantages of the
application of metal-bound imines over their HCl salts.
Indeed, careful neutralization of 1 equiv of HCl in nonaque-
ous media is often a technically complicated task especially
when the synthesis is performed in microscales. Moreover,
products of neutralization, e.g., H2O, can affect the condition
of either the imines or a product of its transformation.
Similarly to protonation, coordination to a metal center
stabilizes imines and they can be stored in the complexed
form for a prolonged time, but their liberation in many
instances is easier to perform than that of dehydrochlorina-
tion. If the substitution is conducted in nonaqueous dried
solvents and the complex formed is separated (e.g., by
filtration), the liberated imines can be used in situ for further
reactions.
Figure 3. Molecular structure of 10. Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths (Å) and angles (deg): Pt(1)-
Cl(1) 2.3192(9), Pt(1)-Cl(2) 2.3207(10), Pt(1)-Cl(3) 2.3300(9), Cl(4)-
Pt(1) 2.3145(10), Pt(1)-N(1) 2.025(3), Pt(1)-N(3) 2.031(3), C(1)-N(1)
1.291(5), C(1)-N(2) 1.369(5), N(2)-C(4) 1.382(5), C(4)-O(1) 1.212(5),
N(3)-C(7) 1.291(5), C(7)-N(4) 1.370(5), N(4)-C(10) 1.400(5), C(10)-
O(2) 1.205(5), C(10)-C(11) 1.492(7), C(11)-C(12A) 1.371(10), C(11)-
C(12B) 1.439(12), Cl(1)-Pt(1)-Cl(3) 179.08(4), Cl(2)-Pt(1)-Cl(4)
179.18(4), N(1)-Pt(1)-N(3) 179.52(13), N(3)-C(7)-N(4) 119.0(3), N(4)-
C(10)-O(2) 121.9(4), N(1)-C(1)-C(2) 120.6(3), N(2)-C(4)-O(1) 122.3(4),
N(2)‚‚‚Cl(3) 3.126(3), N(2)-H(2)‚‚‚Cl(3) 143.3, N(4)‚‚‚Cl(1) 3.238(3),
N(4)-H(4)‚‚‚Cl(1) 150.9.
6), prepared in step III, are unstable toward hydrolysis. When
5 or 6 was dissolved in CDCl3 containing traces of water,
the formation of both EtOH and a new platinum complex
was observed. The latter was isolated upon column chro-
matography with nondried eluents. Physicochemical data
along with X-ray crystallography (Figures 2 and 3) for the
complexes trans-[PtCl4{Z-NHdC(Et)NHC(R)dO}2] (R )
Me 9, R ) Et 10) allowed the establishment of the
acylamidine structure of the ligated imino species. Indeed,
Step IV: Hydrolysis of the Coordinated 1,3-Diaza-1,3-
dienes. The complexes [PtCl4{NHdC(Et)NdC(R)OEt}2] (5,
(30) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
Inorganic Chemistry, Vol. 42, No. 11, 2003 3605

Bokach et al.
Scheme 3
the NdC bonds in the Pt-N(H)dC moieties and the CdO
bonds have typical double-bond character, while the two
bonds in the C-N(H)-C moiety are single ones. Coordina-
tion to the Pt center results in the predominant stabilization
of the imino tautomer NHdC(Et)NHC(R)dO over the other
form, i.e., NH2C(Et)dNC(R)dO. The imino tautomer is the
minor one when the acylamidines are free.31 Presumably 9
and 10 are formed via the [PtCl4{NHdC(Et)NdC(R)OH}2]
intermediate (Scheme 2) followed by its conversion to trans-
[PtCl4{NHdC(Et)NHC(R)dO}2].
It is worthwhile to mention that although acylamidines
are well-known species,32,33 their coordination chemistry is
still poorly investigated and reported examples are restricted
only to copper(II)31 and nickel(II)34 complexes where acyl-
amidines form N,O-chelates. Structures 9 and 10 are the first
ones where these ligands are involved in the monodentate
coordination.
Step V: Synthesis of 1,3-Diaza-1,3-diene Pt(II) Com-
plexes and Hydrolysis of the 1,3-Diaza-1,3-diene Ligands.
We endeavored to verify an oxidation state dependence of
the hydrolysis, and for this reason complexes [PtCl4{NHd
C(Et)NdC(R)OEt}2] (5, 6) were reduced, by the ylide
Ph3PdCHCO2Me (Scheme 3, step VA), to the appropriate
platinum(II) compounds [PtCl2{NHdC(Et)NdC(R)OEt}2]
(R ) Me 11, R ) Et 12). The latter have physicochemical
characteristics similar to those of the parent platinum(IV)
compounds; the most significant difference was observed in
the 195Pt NMR spectra, which display a ca. 2000 ppm shift.
Both 11 and 12 are unstable toward hydrolysis in the
presence of traces of water and give, in step VC, the
acylamidines [PtCl2{NHdC(Et)NHC(R)dO}2] (13, 14) and
EtOH. The complexes 13 and 14 were also prepared by the
alternative route VB upon reduction of the corresponding
platinum(IV) complexes. In the NMR experiments, we do
not feel the difference in the hydrolysis rates of the Pt(IV)
and Pt(II) complexes.
Final Remarks. The results from this work may be
considered from two perspectives. From a narrower, specific
point of view it demonstrates that the addition of imino esters
to coordinated organonitriles is Pt(IV)-mediated and leads
to ligated previously unknown 1,3-diaza-1,3-dienes and the
reaction represents the first example of nitrile-imino ester
coupling. We are continuing to explore the metal-mediated
nitrile-imine integration and to apply for that purpose more
labile systems, e.g., involving high oxidation state hard metal
centers, on one hand, to achieve a high metal-activation effect
for such reactions with an easier liberation of the 1,3-diaza-
1,3-diene ligands and, on the other hand, to discover novel
routes for generation of aza compounds and/or nitrogen
heterocycles under catalytic conditions, and this work will
be reported in due course. The possibility of stepwise
assembly of imines and nitriles, namely, by a sequence of
double steps as those achieved in this work consisting of
the liberation of the coupled ligand followed by its new
coupling to a nitrile ligand, appears particularly promising
to the metal-mediated synthesis of polyimines or aza
heterocycles formed upon secondary conversions of the
polyimines.
In a broader sense, the work shows that the application
of metal complexes gives new opportunities for (i) the
generation of a great variety of imines (sometimes unreach-
able in pure organic chemistry) in the metal-mediated
conversions of organonitriles,3 (ii) the “storage” of imino
species in the complexed form, and (iii) their synthetic
utilization after liberation. It is anticipated that all these issues
combined together might have a further impact on both
coordination and organic chemistries of imines.
(31) Eberhardt, J. K.; Fro¨hlich, R.; Venne-Dunker, S.; Wu¨rthwein, E.-U.
Eur. J. Inorg. Chem. 2000, 1739 and references therein.
(32) For review on acylamidines see: Mikhailov, B. M. Pure Appl. Chem.
1977, 49, 749.
(33) For recent works on acylamidines see: Cunha, S.; Kascheres, A. J.
Braz. Chem. Soc. 2001, 12, 481; Chem. Abstr. 2001, 135, 303742.
Thiagarajan, K.; Puranik, V. G.; Deshmukh, A. R. A. S.; Bhawal, B.
M. Tetrahedron 2000, 56, 7811.
(34) Bart, J. C. J.; Bassi, I. W.; Calcaterra, M.; Peroni, M. Inorg. Chim.
Acta 1978, 28, 201.
Experimental Section
Materials and Instrumentation. Solvents were obtained from
commercial sources and used as received. [PtCl4(RCN)2] (R ) Me,
Et) were prepared accordingly to the published method.35 C, H,
3606 Inorganic Chemistry, Vol. 42, No. 11, 2003

Pop-the-Cork Strategy in Synthetic Utilization of Imines
Table 1. Crystal Data for Compounds 8-10
{NHdC(R)OEt}2]. trans-[PtCl2{NHdC(Me)OEt}2] (1) has been
previously characterized.8
8
9
10
trans-[PtCl2{NHdC(Et)OEt}2] (2). Anal. Calcd for C10H20N4-
Cl2O2Pt: C, 25.65; H, 4.74; N, 5.98. Found: C, 25.54; H, 4.86; N,
5.98. FAB+-MS, m/z: 466 [M]+, 432 [M - Cl]+, 395 [M - 2Cl
- 2H]+. IR spectrum, selected bands, cm-1: 3246 m ν(N-H), 2985
empirical formula
fw
temp (K)
λ (Å)
cryst syst
space group
a (Å)
C11H21N3Cl2OPt C10H20N4Cl4O2Pt C12H24N4Cl4O2Pt
477.30
150(2)
0.71073
monoclinic
P21/c
569.19
150(2)
0.71073
tetragonal
P42/n
593.24
150(2)
0.71073
tetragonal
I41/a
1
and 2943 m-w ν(C-H), 1641 s ν(CdN). H NMR in CDCl3, δ:
7.7388(3)
12.3831(5)
17.0456(8)
97.886(3)
1618.04(12)
4
14.9692(2)
14.9692(2)
8.6265(2)
90
1933.00(6)
4
21.2748(2)
21.2748(2)
17.5715(2)
90
7953.16(14)
16
7.72 (s, br, 1H, CdNH), 4.02 (q, 6.9 Hz, 2H) and 1.35 (t, 6.9 Hz,
3H) (OCEt), 3.11 (q, 7.7 Hz, 2H) and 1.26 (t, 7.7 Hz, 3H)
(NdCEt). 13C{1H} NMR in CDCl3, δ: 173.32 (CdN), 63.82 and
13.56 (OEt), 28.82 (JPt-C 25.4 Hz) and 10.22 (NdCEt). 195Pt NMR
in CDCl3, δ: -1894.7 (590 Hz).
Step II. Dppe (40 mg, 0.10 mmol) is added to trans-[PtCl2-
{NHdC(R)OEt}2] (25 mg, 0.05 mmol) in CHCl3 (2 mL) and the
reaction mixture left to stand at 0 °C for 20 min, whereupon the
colorless precipitate of [Pt(dppe)2]Cl2 is removed by filtration and
the filtrate is used in step III. For characterization of NHdC(Me)-
OEt (3) see ref 8.
b (Å)
c (Å)
â (deg)
V (Å3)
Z
F
calc (Mg/m3)
1.959
1.942
1.982
µ(Mo KR) (mm-1
R1a (I g 2σ)
)
8.995
7.818
7.606
0.0321
0.0696
0.0300
0.0519
0.0216
0.0512
wR2b (I g 2σ)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc )2]/∑[w(Fo )2]]1/2
.
2
2
and N elemental analyses were carried out by the Microanalytical
Service of the Instituto Superior Te´cnico. Positive-ion FAB mass
spectra were obtained on a Trio 2000 instrument by bombarding
3-nitrobenzyl alcohol (NBA) matrices of the samples with 8 keV
(ca. 1.28 × 1015 J) Xe atoms. Mass calibration for data system
acquisition was achieved using CsI. Infrared spectra (4000-400
cm-1) were recorded on a BIO-RAD FTS 3000MX instrument in
1
NHdC(Et)OEt (4). H NMR in CDCl3, δ: 6.64 (s, br, 1H,
CdNH), 4.08 (q, 7.2 Hz, 2H) and 1.25 (t, 7.2 Hz, 3H) (OCEt),
2.21 (q, 7.5 Hz, 2H) and 1.08 (t, 7.5 Hz, 3H) (NdCEt). 13C{1H}
NMR in CDCl3, δ: 174.25 (CdN), 61.19 and 14.13 (OEt), 26.16
and 10.08 (NdCEt).
Step III. A significant excess of the imino ester prepared in the
step II (0.50 mmol or more) is added to [PtCl4(EtCN)2] (20 mg,
0.045 mmol) in CHCl3 at room temperature, and almost immediately
the solvent is evaporated and the mixture obtained is separated by
column chromatography (first fraction; eluent is CH2Cl2:Et2O )
4:1, v/v, silica gel 70-230 mesh; 60 Å, Aldrich).
1
KBr pellets. H, 13C{1H}, and 195Pt NMR spectra were measured
on Varian UNITY 300 spectrometer at ambient temperature. 195Pt
chemical shifts are given relative to Na2[PtCl6] (by using K2[PtCl4],
δ ) -1630 ppm, as a standard), and the half-height line width is
given in parentheses.
X-ray Structure Determinations. The X-ray diffraction data
were collected with a Nonius KappaCCD diffractometer using Mo
KR radiation (λ ) 0.71073 Å). The Denzo-Scalepack36 program
package was used for cell refinements and data reduction. All
structures were solved by direct methods using the SHELXS97
program and the WinGX graphical user interface.37,38 A multiscan
absorption correction based on equivalent reflections (XPREP in
SHELXTL v5.1)39 was applied to all data (Tmin/Tmax values were
0.11842/0.15193, 0.11160/ 0.17024, and 0.25637/0.33691 for 9,
10, and 11, respectively). Structural refinements were carried out
with SHELXL97.40 In structure 10, one of the ethyl groups was
disordered in two positions with occupation parameters 0.51/0.49.
All hydrogens were placed in idealized positions and were
constrained to ride on their parent atom. The crystallographic data
are summarized in Table 1. Selected bond lengths and angles are
shown in Figures 1-3.
[PtCl4{NHdC(Et)NdC(Me)OEt}2] (5). Anal. Calcd for
C14H28N4Cl4O2Pt: C, 27.07; H, 4.54; N, 9.02. Found: C, 26.75;
H, 4.02; N, 9.15. FAB+-MS, m/z: 665 [M + 2Na - H]+, 629 [M
- Cl + 2Na - 2H]+, 593 [M - 2Cl + 2Na - 3H]+. IR spectrum,
selected bands, cm-1: 3291 and 3215 m-w ν(N-H), 2979 m-w,
2934 w ν(C-H), 1699 m and 1613 s ν(CdN), 1292 s ν(C-O). 1H
NMR in CDCl3, δ: 4.26 (q, 7.1 Hz, 2H) and 1.27 (t, 7.1 Hz, 3H)
(OEt), 2.61 (q, 7.5 Hz, 2H) and 1.18 (t, 7.4 Hz, 3H) (dCEt), 2.02
(s, 3H, dCMe). 13C{1H} NMR in CDCl3, δ: 179.6 (CdN-Pt),
159.9 (CdN-C), 64.4 and 13.9 (OEt), 33.4 and 10.1 (dCEt), 20.6
(dCMe). 195Pt{1H} NMR in CDCl3, δ: -99.8 (880 Hz).
[PtCl4{NHdC(Et)NdC(Et)OEt}2] (6). Anal. Calcd for C16H32N4-
Cl4O2Pt: C, 29.30; H, 4.97; N, 8.63. Found: C, 29.32; H, 5.02; N,
8.54. FAB+-MS, m/z: 687 [M + K]+, 617 [M - 2Cl + K]+, 578
[M - 2Cl]+. IR spectrum, selected bands, cm-1
: 3356 m
ν(N-H), 2979 w, 29334 m ν(C-H), 1698 s and 1617 m-s
ν(CdN). 1H NMR in CDCl3, δ: 7.11 (s, br, 1H, NH), 4.30 (q, 7.2
Hz, 2H) and 1.31 (t, 7.2 Hz, 3H) (OEt), 2.64 (q, 7.5 Hz, 2H) and
1.20 (t, 7.5 Hz, 3H) (PtNdCEt), 2.39 (q, 7.5 Hz, 2H), and 1.20 (t,
7.5 Hz, 3H) (NdCEt). 13C{1H} NMR in CDCl3, δ: 178.80
(CdN-Pt), 162.38 (CdN), 64.30 and 13.90 (OEt), 33.57 and 10.24
(Pt-NdCEt), 28.21 and 9.73 (NdCEt). 195Pt{1H} NMR in CDCl3,
δ: -100.6 (719 Hz).
Synthetic Work. Step I. The platinum(IV) complexes trans-
[PtCl4{NHdC(Me)OEt}2] and trans-[PtCl4{NHdC(Et)OEt}2] were
prepared by addition of ethanol to the nitrile complexes [PtCl4-
(RCN)2] (R ) Me, Et) in accord with the published method.8
The reduction of trans-[PtCl4{NHdC(R)OEt}2] with the ylide
Ph3PdCHCO2Me7b gave the platinum(II) complexes trans-[PtCl2-
(35) Luzyanin, K. V.; Haukka, M.; Bokach, N. A.; Kuznetsov, M. L.;
Kukushkin, V. Yu.; Pombeiro, A. J. L. J. Chem. Soc., Dalton Trans.
2002, 1882.
(36) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology, Volume
276, Macromolecular Crystallography, Part A; Carter, C. W., Jr.,
Sweet, R. M., Eds.; Academic Press: New York, 1997; pp 307-326.
(37) Sheldrick, G. M. SHELXS97, Program for Crystal Structure Deter-
mination; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(38) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(39) Sheldrick, G. M. SHELXTL, version 5.1; Bruker Analytical X-ray
Systems, Bruker AXS, Inc.: Madison, WI, 1998.
(40) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
[PtCl4(EtCN){NHdC(Et)NdC(Et)OEt}] (7). This complex
formed as a minor byproduct in step III and can be isolated in pure
form after column chromatography on silica gel. Despite instability
and the small isolated amount, we succeeded in characterizing the
1
complex by FAB-MS and H NMR spectroscopy, reducing it to
the platinum(II) complex [PtCl2(EtCN){NHdC(Et)NdC(Et)OEt}]
(8; see later), and determining the X-ray structure of the latter
(Figure 1). FAB+-MS, m/z: 512 [M - Cl]+, 477 [M - 2Cl]+. 1H
NMR in CDCl3, δ: 6.85 (s, br, 1H, NH), 4.33 (q, 2H, CH2 from
OEt), 3.20 (q, 2H, CH2 from NtCEt), 2.72 (q, 2H, CH2 from
Inorganic Chemistry, Vol. 42, No. 11, 2003 3607

Bokach et al.
Pt-NdCEt), and 2.42 (q, 2H, CH2 from NdCEt), 1.47, 1.32, 1.21,
and 1.22 (four t, 12H, CH3 from Et).
H, 5.60; N, 9.38. IR spectrum, selected bands, cm-1: 3247 m
ν(N-H), 2979 and 2938 m ν(C-H), 1686 s and 16189 m ν(Cd
N). FAB+-MS, m/z: 506 [M - 2Cl]+. 1H NMR in CDCl3, δ: 7.22
(s, br, 1H, CdNH), 4.28 (q, 7.1 Hz, 2H) and 1.36 (t, 7.1 Hz, 3H)
(OEt), 2.66 (q, 7.4 Hz, 2H, CH2) and 1.23 (t, 7.4 Hz, 3H, CH3)
(NdCEt), 2.44 (q, 7.4 Hz, 2H, CH2) and 1.13 (t, 7.4 Hz, 3H, CH3)
(Pt-NdCEt). 13C{1H} NMR in CDCl3, δ: 178.68 (CdN-Pt),
163.69 (CdN), 63.37 and 13.96 (OEt), 32.29 (JPtH 37.03 Hz) and
10.02 (Pt-NdCEt), 26.65 and 10.27 (CNEt). 195Pt NMR in CDCl3,
δ: -2076.7 (583 Hz). The complex is unstable toward hydrolysis
and gives [PtCl2{NHdC(Et)NHC(Et)dO}2] and EtOH. In nondried
CDCl3 (Aldrich), ca. 50% hydrolytic conversion was observed after
12 h at room temperature.
Step IV. The imino ester prepared in step II (0.10 mmol) is added
to [PtCl4(EtCN)2] (20 mg, 0.045 mmol) in CHCl3, and the reaction
mixture is left to stand for 2 h, whereupon the product (9 or 10) is
separated by column chromatography (first fraction; eluent is CH2-
Cl2:Et2O ) 4:1, v/v, silica gel 70-230 mesh; 60 Å, Aldrich). In
the case of using well-dried solvents and a high rate of the elution,
a mixture of [PtCl4{NHdC(Et)NdC(R)OEt}2] and 9 or 10 is
obtained. If the reaction mixture is eluted at a lower rate, pure 9 or
10 is isolated. The latter can also be obtained from the mixture of
the two products after its recrystallization at 45 °C from commercial
nondried solvents (CH2Cl2 and Et2O).
[PtCl4{NHdC(Et)NHC(Me)dO}2] (9). Anal. Calcd for C10H20N4-
Cl4O2Pt: C, 21.25; H, 3.57; N, 9.91. Found: C, 21.46; H, 3.77; N,
9.57. FAB+-MS, m/z: 565 [M + H]+, 529 [M - Cl]+, 496 [M -
2Cl + 2H]+, 459 [M - 3Cl]+, 423 [M - 4Cl]+. IR spectrum,
selected bands, cm-1: 3284 and 3220 m-w ν(N-H), 2920 w, 2851
m ν(C-H), 1738 s ν(CdO), 1629 vs ν(CdN). 1H NMR in CDCl3,
δ: 10.13 (s, br, 1H, dCNH), 6.83 (s, br, 1H, CdNH), 3.17 (q, 7.5
Hz, 2H) and 1.30 (t, 7.4 Hz, 3H) (dCEt), 2.21 (s, 3H, dCMe).
13C{1H} NMR in CDCl3, δ: 173.21 (CdN), 168.23 (CdO), 29.80
(s + d, JPtC 25.0 Hz) and 9.84 (Et), 25.17 (Me). 195Pt{1H} NMR in
CDCl3, δ: -236.3 (410 Hz).
[PtCl2(EtCN){NHdC(Et)NdC(Et)OEt}] (8). For preparation
of this complex see above, i.e., step III. IR spectrum, selected bands,
cm-1: 3251 m ν(N-H), 2946 m-w ν(C-H), 1688 and 1678 s ν(Cd
1
N). H NMR in CDCl3, δ: 6.75 (s, br, 1H, NH), 4.30 (q, CH2
from OEt), 2.76 (q, CH2 from Pt-NtCEt), 2.65 (q, CH2 from Pt-
NdCEt), 2.47 (q, CH2 from NdCEt), 1.32 and 1.20 (four t, CH3
from Et). Crystals suitable for X-ray crystallography were obtained
by evaporation of a CH2Cl2-Et2O solution.
[PtCl2{NHdC(Et)NHC(Me)dO}2] (13). Anal. Calcd for
C10H20N4Cl2O2Pt: C, 24.30; H, 4.08; N, 11.34. Found: C, 24.78;
H, 4.40; N, 10.92. FAB+-MS, m/z: 457 [M - Cl]+, 422 [M - 2Cl
- H]+. IR spectrum, selected bands, cm-1: 3336 m-w, 3267 w
and 3198 m-w ν(N-H), 2945 w ν(C-H), 1732 m ν(CdO), 1637
s ν(CdN). 1H NMR in CDCl3, δ: 10.25 (s, br, 1H, dCNH), 6.97
(s, br, 1H, CdNH), 2.92 (q, 7.5 Hz, 2H) and 1.15 (t, 7.5 Hz, 3H)
(NdCEt), 2.16 (s, 3H, Me). 13C{1H} NMR in CDCl3, δ: 172.47
(CdN), 168.53 (CdO), 29.29 (JPt-C 25.4 Hz) and 9.64 (NdCEt),
24.97 (Me). 195Pt NMR in CDCl3, δ: -2083.5 (824 Hz).
[PtCl4{NHdC(Et)NHC(Et)dO}2] (10). Anal. Calcd for
C12H24N4Cl4O2Pt: C, 24.30; H, 4.08; N, 9.44. Found: C, 24.58;
H, 4.31; N, 9.19. FAB+-MS, m/z: 617 [M + Na]+, 594 [M]+, 559
[M - Cl + H]+, 523 [M - 2Cl + H]+. IR spectrum, selected bands,
cm-1: 3358 m-w and 3214 m-w ν(N-H), 2980 w, 2936 m ν(C-
H), 1698 s ν(CdO), 1621 vs ν(CdN). 1H NMR in CDCl3, δ: 10.05
(s, br, 1H, dCNH), 6.81 (s, br, 1H, CdNH), 3.17 (q, 7.4 Hz, 2H)
and 1.29 (t, 7.5 Hz, 3H) (NdCEt), 2.46 (q, 7.5 Hz, 2H) and 1.20
(t, 7.5 Hz, 3H) (OdCEt). 13C{1H} NMR in CDCl3, δ: 173.19 (Cd
N), 172.07 (CdO), 31.42 and 9.90 (OdCEt), 29.87 (JPt-C 25.4 Hz)
and 8.38 (NdCEt). 195Pt{1H} NMR in CDCl3, δ: -244.5 (707 Hz).
Step V. The ylide Ph3PdCHCO2Me (4 mg, 0.01 mmol) is added
to a solution of 9 or 10 (0.01 mmol) in CH2Cl2 (2 mL) and left to
stand at room temperature for 12 h, whereupon the solvent is
evaporated and the yellow residue of the corresponding Pt(II)
complex formed is washed with two 2-mL portions of Et2O,
dissolved in a minimum amount of CH2Cl2 at room temperature,
and purified on a column filled with silica gel (70-230 mesh; 60
Å, Aldrich). Complexes 8, 11, and 12 were reduced analogously
but at 45 °C for 6 h and with monitoring of the reaction by TLC.
[PtCl2{NHdC(Et)NdC(Me)OEt}2] (11). Anal. Calcd for
C14H28N4Cl2O2Pt: C, 30.55; H, 5.13; N, 10.18. Found: C, 30.09;
H, 5.15; N, 9.97. FAB+-MS, m/z: 665 [M + 2Na - H]+, 629 [M
- Cl + 2Na - 2H]+, 593 [M - 2Cl + 2Na - 3H]+. IR spectrum,
selected bands, cm-1: 3224 m ν(N-H), 2978 m-w ν(C-H), 1692
[PtCl2{NHdC(Et)NHC(Et)dO}2] (14). Anal. Calcd for
C12H24N4Cl2O2Pt: C, 27.59; H, 4.63; N, 10.73. Found: C, 27.88;
H, 4.55; N, 10.52. IR spectrum, selected bands, cm-1: 3314 m-w,
3268 m-w ν(N-H), 2981 and 29.39 w ν(C-H), 1732 m ν(CdO),
1
1633 s ν(CdN). FAB+-MS, m/z: 522 [M]+. H NMR in CDCl3,
δ: 10.23 (s, br, 1H, dCNH), 6.90 (s, br, 1H, CdNH), 2.95 (q, 7.4
Hz, 2H, CH2 from NdCEt), 2.40 (two q, 7.4 Hz, 2H, CH2 from
C(dO)Et), 1.18 (t, 7.4 Hz, 6H, both CH3). 13C{1H} NMR in CDCl3,
δ: 172.64 (CdN), 172.40 (CdO), 31.09 and 9.70 (NdCEt), 29.48
(JPt-C 25.4 Hz) and 8.60 (C(dO)Et). 195Pt NMR in CDCl3, δ:
-2092.0 (803 Hz).
Acknowledgment. N.A.B. expresses gratitude to the
INTAS for Grant YSF 2002-93. V.Yu.K. thanks the Russian
Fund for Basic Research for Grant 03-03-32362, the Inter-
national Science Foundation (Soros Foundation) for a Soros
Professorship (2001 and 2002), and the Academy of Finland
for a grant. A.J.L.P., J.J.R.F.S., and V.Yu.K. are grateful to
the FCT (Foundation for Science and Technology) (Portugal)
and the POCTI program (POCTI/QUI/43415/2001) for
financial support of these studies. The authors also thank
Dr. M. Caˆndida Vaz for the elemental analysis service and
Mr. Indale´cio Marques for running the FAB+-MS spectra.
1
m and 1625 m-s ν(CdN). H NMR in CDCl3, δ: 7.06 (s, br, 1H,
CdNH), 4.30 (q, 7.20 Hz, 2H) and 1.36 (t, 7.20 Hz, 3H) (OEt),
2.45 (q, 7.46 Hz, 2H) and 1.11 (t, 7.46 Hz, 3H) (NdCEt), 2.28 (s,
3H, Me). 13C{1H} NMR in CDCl3, δ: 178.80 (CdN at Pt), 160.57
(CdN), 63.41 and 13.99 (OEt), 32.07 and 9.98 (NdCEt), 19.15
(Me). 195Pt NMR in CDCl3, δ: -2038.4 (420 Hz). The complex is
unstable toward hydrolysis and gives [PtCl2{NHdC(Et)NHC(Me)d
O}2] and EtOH. In nondried CDCl3 (Aldrich), ca. 50% hydrolytic
conversion was observed after 12 h at room temperature.
Supporting Information Available: Tables of crystallographic
data for 8-10. Crystallographic data in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
[PtCl2{NHdC(Et)NdC(Et)OEt}2] (12). Anal. Calcd for
C16H32N4Cl2O2Pt: C, 33.22; H, 5.58; N, 9.69. Found: C, 33.36;
IC034086J
3608 Inorganic Chemistry, Vol. 42, No. 11, 2003