X-ray structures of the first platinum complexes with Z configuration iminoether ligands: Trans-dichlorobis(1-imino-1-methoxy-2,2′dimethylpropane)platinum(II) and trans-Tetrachlorobis(1-imino-1methoxy-2,2′-dimethylpropane)platinum(IV)

Article abstract of DOI:10.1021/ic010850v

Platinum complexes with Z configuration iminoether ligands (trans-[PtCl₂{HN=C(OMe)Bu¹}₂], 1, and trans-[PtCl₄-{HN=C(OMe)Bu₁}₂], 2) have been structurally characterized for the first time. The nearly planar Pt-N-C-O-C chain, all atoms being in gauche conformation, brings the terminal Pt and C atoms very close to one another. The steric clash is released by considerably increasing the Pt-N-C, N-C-O, and C-O-C bond angles (133, 124, and 121° for 1, respectively; 147, 129, and 127° for 2, respectively), which are well above the expected values (120° for Pt-N-C and N-C-O; less than 120° for C-O-C owing to the repulsive effect exerted by the lone pair of electrons on the oxygen atom). In the platinum(II) case the smaller increase of bond angles is accompanied by a greater value of the Pt-N-C-O torsion angle (27.3 and 15.6° for 1 and 2, respectively). The stabilization of the Z configuration, notwithstanding the steric clashes described above, has been achieved by a careful choice of the R substituent in the iminoether moiety (a bulky tert-butyl group). The reactions of the platinum(IV) species (2) in basic and acidic conditions and with triphenylphosphine have been investigated. Bases and acids both interact with the coordinated ligand in such a way to weaken the coordinative bond and promote the release of the iminoether ligands. The phosphine promotes a ready and complete reduction of the platinum(IV) complex to the corresponding platinum(II) species (1). Compound 1 reacts with a stoichiometric amount of phosphine (1:1 molar ratio) to form cis-[PtCl₂(PPh₃){Z-HN=C(OMe)Bu¹}] and with excess phosphine to form [PtCl₂(PPh₃)₂] and free iminoether. The latter two reactions leading to formation of a mixed phosphine/iminoether platinum species and to free iminoether, which can be used as a synthon for further organic transformations, can be of synthetic utility.

Full text of DOI:10.1021/ic010850v

Inorg. Chem. 2002, 41, 470−478
X-ray Structures of the First Platinum Complexes with Z Configuration
Iminoether Ligands: trans-Dichlorobis(1-imino-1-methoxy-2,2′-
dimethylpropane)platinum(II) and trans-Tetrachlorobis(1-imino-1-
methoxy-2,2′-dimethylpropane)platinum(IV)
,†
Ana M. Gonzalez,† Renzo Cini,‡ Francesco P. Intini,† Concetta Pacifico,† and Giovanni Natile*
Dipartimento Farmaco-Chimico, UniVersity of Bari, Via E. Orabona 4, 70125 Bari, Italy, and
Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi, UniVersity of Siena,
Via E. Bastianini 12, I-53100 Siena, Italy
Received August 6, 2001
t
Platinum complexes with Z configuration iminoether ligands (trans-[PtCl2{HNdC(OMe)Bu}2], 1, and trans-[PtCl4-
t
{HNdC(OMe)Bu}2], 2) have been structurally characterized for the first time. The nearly planar Pt−N−C−O−C
chain, all atoms being in gauche conformation, brings the terminal Pt and C atoms very close to one another. The
steric clash is released by considerably increasing the Pt−N−C, N−C−O, and C−O−C bond angles (133, 124, and
121° for 1, respectively; 147, 129, and 127° for 2, respectively), which are well above the expected values (120°
for Pt−N−C and N−C−O; less than 120° for C−O−C owing to the repulsive effect exerted by the lone pair of
electrons on the oxygen atom). In the platinum(II) case the smaller increase of bond angles is accompanied by a
greater value of the Pt−N−C−O torsion angle (27.3 and 15.6° for 1 and 2, respectively). The stabilization of the
Z configuration, notwithstanding the steric clashes described above, has been achieved by a careful choice of the
R substituent in the iminoether moiety (a bulky tert-butyl group). The reactions of the platinum(IV) species (2) in
basic and acidic conditions and with triphenylphosphine have been investigated. Bases and acids both interact
with the coordinated ligand in such a way to weaken the coordinative bond and promote the release of the iminoether
ligands. The phosphine promotes a ready and complete reduction of the platinum(IV) complex to the corresponding
platinum(II) species (1). Compound 1 reacts with a stoichiometric amount of phosphine (1:1 molar ratio) to form
t
cis-[PtCl2(PPh3){Z-HNdC(OMe)Bu}] and with excess phosphine to form [PtCl2(PPh3)2] and free iminoether. The
latter two reactions leading to formation of a mixed phosphine/iminoether platinum species and to free iminoether,
which can be used as a synthon for further organic transformations, can be of synthetic utility.
Introduction
similar to transplatin in having two chloride and two
nitrogen-donor ligands; moreover, the nitrogen atoms carry
a proton suitable for hydrogen-bond formation.
Iminoether complexes are generally prepared from related
platinum nitrile complexes by reaction with alcohol under
basic conditions. They can have either E or Z configuration
It has recently been reported that some analogues of
clinically ineffective trans-diamminedichloroplatinum(II)
(transplatin) exhibit antitumor activity.1-3 One of these new
classes of platinum compounds contains iminoethers as
carrier ligands.4-9 The active iminoether complexes are
(4) Coluccia, M.; Nassi, A.; Loseto, F.; Boccarelli, A.; Mariggio`, M. A.;
Giordano, D.; Intini, F. P.; Caputo, P.; Natile, G. J. Med. Chem. 1993,
36, 510.
(5) Coluccia, M.; Boccarelli, A.; Mariggio`, M. A.; Cardellicchio, N.;
Caputo, P.; Intini, F. P.; Natile, G. Chem. Biol. Interact. 1995, 98,
251.
(6) Brabec, V.; Vrana, O.; Novakova, O.; Kleinwachter, V.; Intini, F. P.;
Coluccia, M.; Natile, G. Nucleic Acid Res. 1996, 24, 336.
(7) Zaludova, R.; Zakovska, A.; Kasparkova, J.; Balkarova, Z.; Vrana,
O.; Coluccia, M.; Natile, G.; Brabec, V. Mol. Pharmacol. 1997, 52,
354.
* Corresponding author. E-mail: natile@farmchim.uniba.it.
† University of Bari.
‡ University of Siena. E-mail: cini@unisi.it.
(1) Natile, G.; Coluccia, M. In Metallopharmaceuticals I; Clarke, M. J.,
Sadler, P. J., Eds.; Springer-Verlag: Berlin, 1999; p 74.
(2) Kelland, L. R.; Barnard, C. F. J.; Evans, I. G.; Murrer, B. A.; Theobald,
B. R. C.; Wyer, S. B.; Goddard, P. M.; Jones, M.; Valenti, M.; Bryant,
A.; Rogers, P. M.; Harrap, K. R. J. Med. Chem. 1995, 38, 3016.
(3) Farrell, N. In Metal Ions in Biological Systems; Marcel Dekker: New
York, 1996; Vol 32, p 603.
470 Inorganic Chemistry, Vol. 41, No. 3, 2002
10.1021/ic010850v CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/08/2002

Pt Complexes of Z Configuration Iminoether Ligands
depending upon the positions of the substituents with respect
to the CdN double bond (E and Z stand for the platinum
and the alkoxy groups trans and cis with respect to the double
bond, respectively).10,11,12 The X-ray structure of the first
platinum iminoether complex, [PtCl2{HNdC(OPri)Me}2],
was reported 10 years ago.12 The E configuration found in
the solid state was assumed to be the preferred one and to
stem from cis addition of the alcohol to the nitrile triple bond.
Figure 1. Possible configurations for a coordinated iminoether ligand. A,
B, and C represent the E, Z-syn, and Z-anti configurations, respectively.
The crystal structures of other two platinum iminoether
complexes, cis- and trans-[PtCl2{HNdC(OMe)Me}2], re-
ported somewhat later, had again E configuration of the
iminoether ligands.13 However it was demonstrated that the
E isomer was not the first isomer to be formed. Monitoring
of the reaction course by NMR indicated that, in basic (KOH)
alcoholic solution (MeOH), the platinum nitrile undergoes
trans addition of the alcohol to the CtN triple bond and
formation of the Z isomer. However, under the basic
conditions required for the addition of the alcohol to the
nitrile, fast isomerization of the Z into the E isomer takes
place, and this latter is generally obtained as final product.
interaction is expected to be even greater in the Z isomer
since the longer Pt-N-C-O-R′ skeleton will bring the
terminal R′ group (the R′ radical comes from the added
alcohol) even closer to the platinum center. (ii) In principle,
the Z iminoether ligand could avoid the steric clash between
the terminal R′ group and the Pt atom by switching from a
syn to an anti conformation (see Figure 1). Such a config-
uration (which has never been observed so far for iminoether
complexes) would release the Pt...R′ steric interaction but
place the terminal R and R′ groups, within the planar R-C-
O-R′ moiety, in a gauche conformation.
In a previous paper we searched the conditions for the
preparation of the Z isomer and found that it can be
conveniently prepared under conditions in which it precipi-
tates from solution as soon as it is formed. Therefore, high
concentration of the reactants, low temperature (0 °C), and
removal of the precipitate from the mother liquor soon after
formation ensure good yield of the Z isomer. In most cases
purification by column chromatography was also required
as a final step.13 However, different attempts to characterize
the Z isomer by X-ray crystallography were unsuccessful
either because undesired Z f E ligand isomerization took
place during the crystallization process or because the
crystals, obtained under fast crystallization conditions, were
not suitable for X-ray investigation.
New interest in the Z configuration of the iminoether
ligands arose from the discovery that the configuration of
the iminoether ligands can modulate the antitumor potential
of the platinum complexes and species with the Z configu-
ration can be more active than those with the E configur-
ation.14-16
In this paper we sought the conditions for the stabilization
of the Z configuration of the iminoether ligands and
succeeded in performing the X-ray structural characterization
of two of such complexes, one having a 4-coordinate square-
planar arrangement and the second a 6-coordinate octahedral
structure. The steric clash between the alkoxy group and the
platinum center, cis to one another with respect to the CdN
double bond, is particularly high, especially in the 6-coor-
dinate PtIV species. How the steric strain could influence the
reactivity of these species has also been investigated.
Despite the experimental difficulties, a full characterization
of the Z isomer of platinum iminoether complexes was highly
desirable for at least two reasons: (i) The X-ray structure
of the E isomer revealed a “quasi planarity” of the iminoether
moiety with a lone pair of electrons of the oxygen atom
participating in the imine π system and consequent electron
charge delocalization over the N-C-O moiety. In the E
isomer, the Pt atom and the R group, within the Pt-N-
C-R moiety (the R group comes from the former nitrile
ligand), are forced to be very close to one another with
consequent steric repulsion between the two. Such an
Results and Discussion
The reactivity of coordinated nitriles depends very much
upon the oxidation state of the platinum center, and weak
nucleophiles react only with platinum(IV) species but do not
react with the platinum(II) counterparts.17,18 However, even
in the case of platinum(II), the addition of a catalytic amount
of base (KOH) is sufficient for promoting the rapid and
complete addition of a weak nucleophile such as an alcohol
(R′OH) to the coordinated nitrile (RCtN). There is no
impediment to the latter reaction stemming from either steric
(8) Zaludova, R.; Natile, G.; Brabec, V. Anti-Cancer Drug Des. 1997,
12, 295.
(9) Coluccia, M.; Mariggio`, M. A.; Boccarelli, A.; Loseto, F.; Cardellic-
chio, N.; Caputo, P. A.; Intini, F. P.; Pacifico, C.; Natile, G. In Platinum
and Other Metal Coordination Compounds in Cancer Chemotherapy;
Pinedo, H. M., Schornagel, J. H., Eds.; Plenum Press: New York,
1996; Vol 2, p 27.
(10) Fanizzi, F. P.; Intini, F. P.; Natile, G. J. Chem. Soc., Dalton Trans.
1989, 947.
(11) Lopez, G.; Ruiz, J.; Garcia, G.; Vicente, C.; Mart`ı, J. M.; Hermoso,
J. A.; Vegas, A.; Martinez-Ripoll, M. J. Chem. Soc., Dalton Trans.
1992, 53.
(12) Casas, J. M.; Chisholm, M. H.; Sicilia, M. V.; Streib, W. E. Polyhedron
1991, 10, 1573.
(14) Boccarelli, A.; Coluccia, M.; Intini, F. P.; Natile, G.; Locker, D.; Leng,
M. Anti-Cancer Drug Des. 1999, 14, 253
(15) Coluccia, M.; Nassi, A.; Boccarelli, A.; Giordano, D.; Cardellicchio,
N.; Locker, D.; Leng, M.; Sivo, M.; Intini, F. P.; Natile, G. J. Inorg.
Biochem. 1999, 77, 31.
(16) Leng, M.; Locker, D.; Giraud-Panis, M. J.; Schwartz, A.; Intini, F.
P.; Natile, G.; Pisano, C.; Boccarelli, A.; Giordano, D.; Coluccia, M.
Mol. Pharmacol. 2000, 58, 1525.
(17) Kukushkin, V. Yu.; Packhomova, T. B.; Kukushkin, Yu. N.; Herrmann,
R.; Wagner, G.; Pombeiro, A. J. L. Inorg. Chem. 1998, 37, 6511.
(18) Kukushkin, V. Yu.; Packhomova, T. B.; Bokach, N. A.; Wagner, G.;
Kuznetsov, M.; Galanski, M. L.; Pombeiro, A. J. L.; Inorg. Chem.
2000, 39, 216.
(13) Cini, R.; Caputo, P. A.; Intini, F. P.; Natile, G. Inorg. Chem. 1995,
34, 1130.
Inorganic Chemistry, Vol. 41, No. 3, 2002 471

Gonzalez et al.
or electronic properties of the coordinated nitrile and of the
incoming alcohol. The new ligand resulting from the addition
reaction has been named “iminoether” since the pioneer work
of Chisholm et al.12 Imidoester was initially used by
ourselves;10 however, the former name has been widely
accepted and will be adopted also in this paper.
Coordinated iminoethers can have either E or Z configu-
ration depending upon the relative position of the substituents
about the CdN double bond (the platinum and the alkoxy
groups cis or trans to one another for the Z and E isomers,
respectively). A cis addition of the alcohol to the nitrile triple
bond would lead to direct formation of a E isomer, while a
trans addition would lead to formation of a Z isomer. In a
previous paper it was demonstrated that always the first
formed isomer has a Z configuration indicating that the
addition of the alcohol is of trans type; i.e., the alkoxy group
and the proton add to the triple bond from opposite sides.13
The Z configuration is generally unstable under basic
conditions (the same conditions used for promoting the
addition of the alcohol to the coordinated nitrile) and readily
undergoes isomerization to the E form, which appears to be
thermodynamically favored. The greater stability of the E
isomer appears to be of steric origin. The involvement of
the oxygen atom in electron conjugation with the adjacent
CdN π-system renders the whole iminoether skeleton nearly
planar. Already in the E isomer the terminal groups in the
sequence Pt-N-C-R come close to one another, and some
steric clash between the Pt and R groups was evident from
the X-ray structures. Such a steric clash would be even
greater in the Z isomer for which the Pt-N-C-O-R′ chain
(all members being coplanar and in gauche positions) is one
unit longer (Figure 1). These considerations suggested the
way how to increase the thermodynamic stability of the Z
isomer with respect to the E isomer.
Stability of the Z Isomer for trans-[PtCl2{HNdC-
(OMe)But}2], 1. Being that the steric interaction between
the platinum center and either the R′ (Z isomer) or the R
substituent (E isomer) is the dominant factor in determining
the relative stabilities of the Z and E isomers, an increase of
the bulk of the R group with respect to R′ should destabilize
the E isomer with respect to the Z isomer. Hence the platinum
complex with tert-butyl nitrile trans-[PtCl2(NCBut)2] was
reacted with methanol in the presence of a catalytic amount
of base. The formed iminoether complex kept the initial
configuration (expected to be Z) and did not undergo
isomerization after prolonged standing in basic solution. The
Z configuration of the iminoether was deduced not only from
the reaction sterochemistry, which contemplates trans addi-
tion of the alcohol to the nitrile triple bond, but also from
the chemical shift values of the ligand protons, particularly
those of the alkoxy group, which in a Z configuration (the
methoxy group close to an axial site of the platinum center)
experience a strong deshielding effect exerted by the platinum
center (the resonance of the methoxy group falls at ca. 5
ppm in the present isomer while it is expected to be close to
3.8 ppm in a E isomer).13
Figure 2. Common reaction intermediate in the base-catalyzed alkoxide
exchange and configuration change for a coordinated iminoether ligand.
and not to a kinetic impediment. Substitution of CD3O for
CH3O in the coordinated iminoether ligand took place readily
in deuteriomethanol under basic conditions, indicating that
the reaction intermediate (or transition state), which is
common to the methoxide exchange and to the isomerization
reaction, was readily accessible (Figure 2).10
The platinum(II) compound with HNdC(OMe)But imino-
ether ligands is the only one, among the several species
investigated (R ) Me, Et, Pri, But; R′ ) Me, Et, Pri), for
which the Z configuration is the only one observed in solution
as well as in the solid state. This is because the size of the
R group (But) is such that no release of steric strains can be
attained by placing the But group, instead of the OMe group,
in a position cis to platinum with respect to the CdN double
bond.
Platinum(IV) Iminoether Complex [PtCl4{Z-HNdC-
(OMe)But}2], 2. Given the stability of the Z isomer for the
iminoether with a bulky C-But substituent, we wanted to
investigate if the shielding of the metal center by the O-Me
group could have any effect on the oxidation of platinum-
(II) to platinum(IV) by chlorine and on the stability of the
6-coordinate platinum species obtained therefrom.
The oxidation reaction by chlorine took place readily in
chlorinated solvents and led to the quantitative formation of
the related platinum(IV) species. This latter could be
crystallized from chloroform/hexane to give yellow crystals
suitable for X-ray diffraction analysis.
The chemical shift value of the O-Me protons was 0.5
ppm at higher field with respect to the chemical shift of the
corresponding platinum(II) species but still ca. 0.7-0.8 ppm
at lower field with respect to the chemical shift value
observed for O-Me in a platinum(II) iminoether complex
with E configuration. To demonstrate that a reduced down-
field shift of the iminoether alkyl substituent gauche to
platinum (O-R′ and C-R for Z and E isomers, respectively)
is a common feature of 6-coordinate platinum(IV) species
with respect to the 4-coordinate platinum(II) counterparts,
platinum(II) iminoether complexes with established E or Z
configuration of the iminoether ligands were oxidized to the
corresponding platinum(IV) species by chlorine and analyzed
by NMR. It was observed that the chemical shifts of the
OMe groups for the Z and E ligands, respectively, which
fall at 4.96 and 3.77 ppm in trans-[PtCl2{Z-HNdC(OMe)-
Me}{E-HNdC(OMe)Me}], fall at 4.07 and 3.92 ppm in
trans-[PtCl4{Z-HNdC(OMe)Me}{E-HNdC(OMe)Me}]. Sim-
ilarly, the chemical shift of the C-Me groups for the Z and
E ligands, respectively, which fall at 2.20 and 2.60 ppm in
It was possible to demonstrate that the lack of isomeriza-
tion was due to a thermodynamic preference for the Z isomer
472 Inorganic Chemistry, Vol. 41, No. 3, 2002

Pt Complexes of Z Configuration Iminoether Ligands
Figure 4. Acid-catalyzed release of an iminoether ligand from a platinum-
(IV) substrate.
Figure 3. Base-catalyzed release of an iminoether ligand from a platinum-
(IV) substrate.
Reaction of the PtIV Complex with PPh3. Addition of a
stoichiometric amount of a weak reducing agent such as PPh3
to a chloroform solution of the platinum(IV) species (2) leads
to the quantitative formation of the platinum(II) complex (1).
It is known that platinum(IV) species can be readily reduced
to platinum(II) complexes by several reducing agents such
as N2H4‚H2SO4, NH2OH‚HCl, oxalate, and ascorbate (in
water solution) and SO2, formaldehyde, and olefins (in
nonaqueous media). More recently carbonyl-stabilized phos-
phorus ylides (Ph3PdCHCO2R) have been proposed as
versatile reagents for the reduction of different classes of
PtIV compounds to the corresponding PtII species in non-
aqueous media.19 Triphenylphosphine can be used as well
although, if used in excess, it can also lead to ligand
substitution in the newly formed platinum(II) complex (see
following discussion).
trans-[PtCl2{Z-HNdC(OMe)Me}{E-HNdC(OMe)Me}], fall
at 2.52 and 2.69 ppm in trans-[PtCl4{Z-HNdC(OMe)Me}-
{E-HNdC(OMe)Me}]. It can be seen how in the PtIV species
the chemical shifts of a given substituent does not change
much upon its position cis or trans to platinum with respect
to the CdN double bond; therefore, the chemical shifts of
the O-R′ and C-R alkyl groups, which were very diagnostic
for the iminoether configuration in the case of 4-coordinate
platinum(II) species, are no longer so in the case of
6-coordinate platinum(IV). In the present case, however, that
the iminoether ligands keep the original Z configuration upon
oxidation of PtII to PtIV was clearly demonstrated by the
X-ray structures of both compounds.
Reaction of the PtIV Complex with Acids and Bases.
The platinum(IV) species is stable in chloroform solution
and only after several days at 25 °C gives a small amount
of decomposition products (free ligand and some PtII species).
The PtIV species reacts readily with acids (HCl) and bases
(KOH). Both reagents lead to displacement of the iminoether
ligands which are released in the free (KOH solution) or
protonated forms (HCl solution). The metallic species could
be recovered as hexachloroplatinate anion in both cases (in
the reaction with base it was sufficient to neutralize with
HCl before recovering the complex anion).
The fast displacement of the iminoether ligand from the
platinum(IV) species was rather unexpected owing to the
well-known inertness of 6-coordinate PtIV substrates. More-
over it was possible to demonstrate that the platinum(IV)
complex remains stable in the presence of excess chloride
under neutral conditions, indicating that are the acidic or
basic conditions responsible for the weakening of the
platinum-iminoether bond. We propose that the mechanism
by which the metal-iminoether bond is weakened is similar
to that by which the acidic and basic conditions catalyze the
iminoether isomerization (E T Z) in the platinum(II) species.
The catalysis under basic conditions has been already
described (Figure 2). It takes place through nucleophilic
attack of an alkoxide anion (alcohol + base) upon the carbon
atom of the imino residue leading to the formation of an
amido ketal which, afterward, reeliminates the alkoxide
restoring the iminoether ligand.13 It is conceivable that an
alkoxide anion from the amido ketal intermediate could be
transferred to the platinum center while the reconstituted
iminoether ligand is released (Figure 3).
Reaction of the Pt(II) Complex with PPh3. It was already
anticipated that the platinum(II) complex formed by reaction
of the platinum(IV) counterpart with PPh3 reacts with PPh3
present in excess to give ligand substitution. The reaction
of the platinum(II) species with a stoichiometric amount of
PPh3 was investigated in detail in chloroform solution. The
first step of the reaction is substitution of one chloro ligand
by the phosphine and formation of trans-[PtCl(PPh3){Z-HNd
C(OMe)But}2]+. The formation of this reaction product is
in accord with the greater trans effect of chloro ligands (as
compared with N-donor iminoether ligands) in the starting
substrate and accounts for the chloride being the leaving
group in the reaction of the same substrate with biologically
relevant molecules (glutathione, nucleotides, etc.). However,
on standing of the species in methanol solution, displacement
of one iminoether ligand from the previously released
chloride and formation of cis-[PtCl2(PPh3){Z-HNdC(OMe)-
But}] and free iminoether is observed. The latter reaction is
probably favored by the low dielectric constant of the solvent
(4.8 for chloroform as compared to 78.5 for water) which
could favor the formation of an ionic couple between the
cationic trans-[PtCl(PPh3){Z-HNdC(OMe)But}2]+ species
and the chloride anion. The latter reaction can represent an
alternative route to the synthesis of cis-platinum complexes
with mixed ligands (in the present case phosphine and
iminoether).
In the presence of excess PPh3 both iminoether ligands
are displaced from platinum leading to formation of [PtCl2-
(PPh3)2] and free iminoether. This latter reaction can have a
practical application in case the iminoether molecules are
used as synthons for further organic transformations.
Under acidic conditions an electrophilic attack of the
proton on the CdN π system was proposed to lead to a
N-bisprotonated species, which could undergo Z f E
isomerization before releasing of the added proton.10 It is
conceivable that N-protonation also weakens the Pt-N bond
favoring the dissociation of the iminoether ligand in its
protonated form (Figure 4).
(19) Wagner, G.; Packhomova, T. B.; Bokach, N. A.; Frausto da Silva, J.
J. R.; Vicente, J.; Pombeiro, A. J. L.; Kukushkin, V. Yu. Inorg. Chem.
2001, 40, 1683.
Inorganic Chemistry, Vol. 41, No. 3, 2002 473

Gonzalez et al.
Table 1. Selected Crystal Data and Structure Refinement for trans-[PtIICl2{Z-HNdC(OMe)But}2], 1, and trans-[PtIVCl4{Z-HNdC(OMe)But}2], 2
param
1
2
empirical formula
fw
C12H26Cl2N2O2Pt
496.34
C12H26Cl4N2O2Pt
567.24
temp/K
293(2)
293(2)
wavelength/Å
cryst system, space group
unit cell dimens
a/Å
0.710 73
monoclinic, C2/m
0.710 73
monoclinic, P21/c
15.796(1)
8.647
b/Å
c/Å
11.420(1)
5.075(1)
102.14(1)
895.0(2)
11.116(1)
10.880(1)
109.52(2)
985.7(1)
â/deg
V/Å3
Z, calcd density/Mg m-3
abs coeff/mm-1
cryst size/mm
θ range for data collcn/deg
reflcns collcd/unique
refinement method
data/restraints/params
final R indices [I > 2σ(I)]
final R indices (all data)
2, 1.842
8.137
2, 1.911
7.664
0.10 × 0.15 × 0.20
0.20 × 0.15 × 0.10
2.22-30.00
2.50-25.00
1636/1369 [R(int) ) 0.0130]
full-matrix least-squares on F2
1369/0/115
2334/1733 [R(int) ) 0.0345]
full-matrix least-squares on F2
1733/0/98
R1 ) 0.0160, wR2 ) 0.0400
R1 ) 0.0160, wR2 ) 0.0400
R1 ) 0.0274, wR2 ) 0.0714
R1 ) 0.0351, wR2 ) 0.0772
Table 3. Selected Bond Angles (deg) for
trans-[PtIICl2{Z-HNdC(OMe)But}2], 1, and
trans-[PtIVCl4{Z-HNdC(OMe)But}2], 2a
vectors
1
2
Cl(1)-Pt(1)-Cl(2)
Cl(1)-Pt(1)-Cl(2)#1
N(1)-Pt(1)-Cl(1)
N(1)-Pt(1)-Cl(2)
Cl(1)-Pt(1)-Cl(1)#1
N(1)-Pt(1)-Cl(1)#1
N(1)-Pt(1)-Cl(2)#1
N(1)-Pt(1)-N(1)#1
C(1)-N(1)-Pt(1)
C(1)-O(1)-C(6)
N(1)-C(1)-O(1)
O(1)-C(1)-C(2)
N(1)-C(1)-C(2)
C(3)-C(2)-C(1)
C(4)-C(2)-C(1)
C(5)-C(2)-C(1)
C(4)-C(2)-C(3)
C(3)-C(2)-C(5)
C(4)-C(2)-C(5)
C(3A)-C(2)-C(1)
C(4A)-C(2)-C(1)
C(5A)-C(2)-C(1)
C(3A)-C(2)-C(4A)
C(3A)-C(2)-C(5A)
C(4A)-C(2)-C(5A)
88.38(6)
91.62(6)
95.66(13)
83.50(13)
180.0
84.34(13)
96.50(13)
180.0
147.3(4)
127.1(5)
129.0(5)
110.2(4)
120.8(5)
109.3(4)
112.5(4)
106.3(5)
109.1(5)
110.3(5)
109.4(5)
94.61(12)
180.0
85.39(12)
180.0
133.3(4)
120.7(5)
124.1(5)
111.4(5)
124.5(5)
112.3(7)
113.7(9)
103.8(8)
113.9(14)
105.4(13)
109.0(15)
106.8(10)
108.9(10)
113.8(10)
106(2)
Figure 5. Drawing of the complex molecule for trans-[PtIICl2{Z-HNd
C(OMe)But}2], 1. Ellipsoids enclose 30% probability.
Table 2. Selected Bond Lengths (Å) for
trans-[PtIICl2{Z-HNdC(OMe)But}2], 1, and
trans-[PtIVCl4{Z-HNdC(OMe)But}2], 2a
vector
Pt(1)-N(1) 2.020(4) 2.044(4) 2.016(4) 2.017(4) 1.999(7) 2.010(8)
Pt(1)-Cl(1) 2.309(1) 2.316(1) 2.313(1) 2.300(2) 2.303(2)
Pt(1)-Cl(2) 2.320(1)
1
2
3
4
5
6
O(1)-C(1) 1.323(7) 1.290(7) 1.334(7) 1.325(7) 1.31(1) 1.33(1)
O(1)-C(6) 1.438(8) 1.420(8) 1.475(8) 1.444(8) 1.43(1) 1.45(1)
N(1)-C(1) 1.280(7) 1.269(6) 1.276(7) 1.280(7) 1.30(1) 1.27(1)
C(1)-C(2) 1.525(7) 1.536(7) 1.487(7) 1.500(7) 1.51(1) 1.52(1)
C(2)-C(3) 1.510(16) 1.519(8)
105(2)
116(2)
a Symmetry transformations used to generate equivalent atoms: for 1,
#1 ) -x + 1, -y, -z + 1; for 2, #1 ) -x + 1, -y, -z.
C(2)-C(4) 1.483(15) 1.522(8)
C(2)-C(5) 1.570(18) 1.529(8)
C(2)-C(3A) 1.54(2)
C(2)-C(4A) 1.51(3)
binary axis. Since the iminoether ligand is located in general
position, two distinct orientations of the coordination square
around the x axis are present in the cells at 50% probability
each (see Experimental Section for the occupancies of the
atoms).
C(2)-C(5A) 1.46(2)
a Previously reported bond distances for Pt(II) iminoether complexes are
also listed for comparison: cis-[PtIICl2{E-HNdC(OPri)Me}2], 3 (ref 12),
[PtII{E-HNdC(OEt)Et}4]2+, 4 (Prenzler, P. D.; Hockless, D. C. R.; Heath,
G. A. Inorg. Chem. 1997, 36, 5845), trans-[PtIICl2{E-HNdC(OMe)Me}2],
5 (ref 13), and cis-[PtIICl2{E-HNdC(OMe)Me}2], 6 (ref 13).
The bond distances relevant to the Pt-Cl (2.309(1) Å)
and Pt-N (2.020(4) Å) linkages and all bond lengths within
the iminoether ligands are in excellent agreement with those
previously found for trans- and cis-[PtCl2{E-HNdC(OMe)-
Me}2].13
X-ray Crystallography of trans-[PtIICl2{Z-HNdC-
(OMe)But}2], 1. The Pt atom of 1 has the usual square planar
coordination arrangement (Figure 5 and Tables 2 and 3). The
chlorine atoms occupy positions trans to each other and are
located on the mirror plane (perpendicular to the b axis).
The metal atom is also located on the mirror plane and sits
on the 2-fold axis. The two iminoether ligands (trans to each
other), as well as the chlorine ligands, are related by the
The configuration of the iminoether ligand can be de-
scribed as Z, the methoxy group and the platinum atom being
cis to one another with respect to the CdN double bond. As
a consequence, the C-OMe branch of the ligand molecule
474 Inorganic Chemistry, Vol. 41, No. 3, 2002

Pt Complexes of Z Configuration Iminoether Ligands
Table 4. Contact Distances between the Pt Atom and the Closest H(C) Atoms for the Selected Structures of Pt(II) Iminoether Complexesa
C‚‚‚Pt/Å, σ ) 0.006
[H‚‚‚Pt/Å, σ ) 0.08]
C‚‚‚norm/Å, σ ) 0.005
[H‚‚‚norm/Å, σ ) 0.07]
compd
trans-[PtCl2{Z-HNdC(OMe)But}2]
trans-[PtCl2{E-HNdC(OMe)Me}2]b
3.164 [2.672]
1.558 [0.964]
2.284 [1.343]
2.434 [1.550]
2.278 [1.223]
3.290 (C2) [2.655]
3.357 (C5) [2.741]
3.322 [2.715]
cis-[PtCl2{E-HNdC(OMe)Me}2]b
a The distances between the Pt atom and the closest H(C) atom, as well as those between the normal to the plane which passes through the metal center
(norm) and the closest H(C) atoms have also been reported. b From ref 13.
protrudes over the coordination plane. It has to be recalled
that both the trans- and cis-[PtCl2{E-HNdC(OMe)Me}2]
complexes described in ref 13 had the methoxy group trans
to the platinum atom with respect to the CdN double bond
(E configuration of the ligand) while was the C-Me group
to be cis to platinum and protruding toward the metal center.
The steric bulk of the tert-butyl group renders the E
configuration highly unfavorable for the present case.
The N(1), C(1), C(2), and O(1) ligand atoms are coplanar
within (0.003(3) Å (plane 1) as expected owing to the sp2
Figure 6. Drawing of the complex molecule for trans-[PtIVCl4{Z-HNd
hybridization of the N(1) and C(1) atoms linked by a double
bond. Moreover, as already pointed out in a previous paper,
C(OMe)But}2], 2. Ellipsoids enclose 30% probability.
whenever an oxygen atom is bound to a carbon atom engaged
in a double bond, a lone pair of the oxygen atom gets
involved in the π-system.13 The π-bond delocalization over
the N-C-O moiety extends the planarity of the iminoether
ligand also to the O-Me carbon atom (C(6)). The C(6) and
Pt atoms deviate significantly from plane 1 (0.288(7) and
methoxy methyl and the platinum coordination center is
present in 1.
As expected, the two iminoether molecules are nearly
perpendicular to the coordination plane (dihedral angle
between the plane defined by the coordination square atoms
(plane 2) and the plane of the iminoether ligands (plane 1)
-0.660(1) Å, respectively). Such a deviation suggests the
of 95.3(5)°).
presence of a steric interaction between the O-Me group
Weak intermolecular interactions could be found between
N(1) and Cl(1) (-x + 1, -y, -z) (contact distance 3.469(6)
Å), between O(1) and C(3) (-x + 1, -y - 1, -z + 1)
(contact distance 3.505(7) Å), and between Cl(1) and several
and the platinum center which can be relieved by displace-
ment of the Pt and C(6) atoms from the ligand plane (Pt-
N(1)-C(1)-O(1) and N(1)-C(1)-O(1)-C(6) torsion angles
of 27.3(5) and 12.9(5)°, respectively). In the corresponding
methyl groups from the tert-buthyl group (contact distances
platinum(II) species with E configuration of the iminoether
in the range 3.75(1)-4.00(1) Å).
ligands the maximum deviations from the ligand plane (plane
X-ray Crystallography of trans-[PtIVCl4{Z-HNdC-
1) were 0.019(3) and -0.195(1) Å for the carbon atom of
the methoxy group and the Pt atom, respectively.
(OMe)But}2], 2. The complex molecule is represented in
Figure 6, and selected geometrical parameters are listed in
Tables 2 and 3. The coordination sphere arrangement can
be described as pseudooctahedral with the two nitrogen atoms
from the iminoether ligands occupying the axial positions
and the four chloride ligands occupying the equatorial sites.
The Pt-Cl (average, 2.316(1) Å) and Pt-N (2.044(4) Å)
bond lengths are not significantly longer than those found
for 1 and for other Pt(II) iminoether complexes.
The presence of a relevant steric interaction between the
O-Me and the platinum center is also evident from the
analysis of bond angles. The Pt-N(1)-C(1), N(1)-C(1)-
O(1), and C(1)-O(1)-C(6) bond angles (133.3(4), 124.1-
(5), and 120.7(5)°, respectively) are all above the expected
values (120° for Pt-N(1)-C(1) and N(1)-C(1)-O(1) and
less than 120° for C(1)-O(1)-C(6) owing to the repulsive
effect exerted by the lone pair of electrons on the oxygen
atom) and larger than corresponding values for the iminoether
complexes with E configuration (average values 126.2, 123.0,
and 118.5°, respectively).13 The difference is particularly
relevant for the Pt-N(1)-C(1) bond angle.
Notwithstanding the torsions along the Pt-N(1)-C(1)-
O(1)-C(6) chain and the enlargements of the Pt-N(1)-
C(1), N(1)-C(1)-O(1), and C(1)-O(1)-C(6) bond angles,
the O-Me is much closer to Pt in 1 (C(6)‚‚‚Pt contact
distance of 3.162(6) Å) than was the C-Me in trans- and
cis-[PtCl2{E-HNdC(OMe)Me}2] (average C‚‚‚Pt contact
distance of 3.32 Å;13 see Table 4). It is to be noted that the
sum of the van der Waals radii for C and Pt atoms is 3.3
Å.20 All these facts suggest that a severe clash between the
Some bond angles, within the coordination sphere, deviate
significantly from the idealized value of 90°. Two angles
are particularly large (95.7(1) and 96.5(1)° for N(1)-Pt-
Cl(1) and N(1)-Pt-Cl(2) (-x + 1, -y, -z), respectively),
and two are particularly small (83.5(1) and 84.3(1)° for
N(1)-Pt-Cl(2) and N(1)-Pt-Cl(1) (-x + 1, -y, -z),
respectively). The largest angles involve the nitrogen atom
and the two chloride ligands which are syn with respect to
the O-Me group. This indicates a relevant steric interaction
between the methoxy group and the chloride ligands close
to it (intramolecular C(6)...Cl contact distances of 3.335(6)
and 3.376(6) Å for Cl(1) and Cl(2), respectively).
(20) Bondi, A. J. Phys. Chem. 1964, 68, 441.
Inorganic Chemistry, Vol. 41, No. 3, 2002 475

Gonzalez et al.
The N(1), C(1), C(2), and O(1) atoms of the iminoether
ligands are coplanar within (0.008(3) Å (plane 1), and as
already observed for the platinum(II) complex, the Pt and
C(6) atoms deviate significantly (although less than in
complex 1) from the least-squares iminoether plane (dis-
placements of Pt and C(6) from plane 1 of -0.337(1) and
0.243(3) Å, respectively). The dramatic increase of inter-
ligand steric interaction on passing from 4-coordinate plati-
num(II) to 6-coordinate platinum(IV) is revealed by the Pt-
N(1)-C(1) (147.3(4)°), N(1)-C(1)-O(1) (129.0(5)°), and
C(1)-O(1)-C(6) (127.1(5)°) bond angles which are greater
by 14, 5, and 6°, respectively, than corresponding values
for complex 1 (see Table 3).
been investigated. Both the acid and the base favor the
displacement of the strained iminoether ligand. Such a
displacement is most probably promoted by an interaction
of either the base or the acid with the ligand moiety, resulting
in a weakening of the coordinative bond with platinum. The
reaction with phosphine leads to fast reduction of the
platinum(IV) species (2) to the corresponding platinum(II)
complex (1). The reaction of the platinum(II) species with
excess phosphine leads to the displacement of the iminoether
ligands. This latter reaction can be of synthetic utility in case
the iminoethers are used as synthons for further organic
transformations. It is interesting to note that the reaction with
a stoichiometric amount of PPh3 can lead to the formation
of mixed phosphine-iminoether platinum(II) species; this
latter reaction can also be of synthetic utility.
The iminoether plane (plane 1) is almost staggered with
respect to the Pt-Cl bonds (Cl(1)-Pt-N(1)-C(1) torsion
angle of 28.0(5)°) so that the interligand steric repulsions
are minimized.
Experimental Section
Weak intermolecular interactions involve the Cl(1) atom
and the methyl groups C(5) (-x + 1, y + 0.5, -z + 0.5)
and C(6) (x, -y - 0.5, z + 0.5) (contact distances 3.771(7)
and 3.891(7) Å, respectively) and the O(1) atom and the
methyl group C(6) (-x + 1, -y + 1, -z) (contact distance
3.831(7) Å).
Starting Materials. Commercial reagent grade chemicals were
used without further purification. The complex trans-[PtCl2-
(NCBut)2] was prepared by the reported procedure.21
Preparations. trans-[PtCl2{Z-HNdC(OMe)But}2], 1. The pro-
cedure was similar to that described for the preparation of similar
compounds in ref 13. trans-[PtCl2(NCBut)2] (0.25 g, 0.58 mmol)
dissolved in 10 mL of methanol was treated with KOH (0.1 g, 1.8
mmol) and kept under stirring at 20 °C for 10 min; meanwhile a
yellow solid separated out. This was collected by filtration of the
mother liquor, washed with methanol, and dried. Yield: 65%. Anal.
Calcd for C12H26Cl2N2O2Pt (1): C, 29.0; H, 5.3; N, 5.6. Found:
C, 29.3; H, 5.3; N, 5.5. 1H NMR in CDCl3 (δ): 1.17 (s, 9H, CBut),
5.08 (s, 3H, OMe), 6.70 (s, br, 1H, NH).
Conclusions
The conditions for the stabilization of the Z configuration
in complexed iminoether ligands have been found. Since in
the planar Pt(H)NdC(OR′)R moiety more sterically demand-
ing substituents will tend to go trans to one another with
respect to the CdN double bond, a R radical (But) larger
than R′ (Me) will go preferentially trans to platinum as found
in the Z configuration. In the iminoether ligands the oxygen
atoms gets involved in electron delocalization with the Cd
N π system; as a consequence, not only the oxygen atom
but also the R′ radical is forced to be coplanar with the
azomethene moiety. In principle the R′ group could be either
syn or anti to R; however only the latter conformation has
been found both in 4-coordinate platinum(II) and 6-coordi-
nate platinum(IV) complexes.
The X-ray structures (the first for an iminoether complex
with Z configuration) has shown that the terminal atoms in
the sequence Pt-N-C-O-C (all five atoms being nearly
coplanar and gauche to one-another) come very close to one
another. The steric clash is released by an increase of the
Pt-N-C, N-C-O, and C-O-C angles (respectively 133,
124, and 121° in the case of platinum(II) and 147, 129, and
127° in the case of platinum(IV)) which are well above the
expected values (120° for Pt-N-C and N-C-O and less
than 120° for C-O-C). In the case of the C-O-C bond
angle a value smaller than 120° is expected and, indeed, has
been observed in iminoether complexes with E configuration,
because of the repulsion exerted by the lone pair of electrons
on the oxygen atom. In the case of platinum(II) the smaller
increase of bond angles (as compared to the platinum(IV)
case) is compensated by greater values of Pt-N-C-O (27°)
and N-C-O-C (13°) torsion angles.
trans-[PtCl4{Z-HNdC(OMe)But}2], 2. trans-[PtCl2{Z-HNd
C(OMe)But}2] (0.063 g, 0.127 mmol) dissolved in 2 mL of CHCl3
was treated with a saturated solution of chlorine in carbon
tetrachloride (3 mL). A layer of hexane was placed on the reaction
solution and the reaction vessel left standing at room temperature
overnight. Yellow crystals of the desired product were formed.
Evaporation of the mother liquor under reduced pressure allowed
the recovery of a further crop of the desired product. Yield: 90%.
Anal. Calcd for C12H26Cl4N2O2Pt (2): C, 25.4; H, 4.6; N, 4.9.
1
Found: C, 25.5; H, 4.7; N, 4.9. H NMR in CDCl3 (δ): 1.30 (s,
9H, CBut), 4.54 (s, 3H, OMe), 7.64 (s, br, 1H, NH).
trans-[PtCl{Z-HNdC(OMe)But}2(PPh3)]Cl. A chloroform so-
lution (30 mL) of 1 (0.52 g, 1.01 mmol) was treated with PPh3
(0.32 g, 1.2 mmol) and stirred at room temperature for 15 min.
The colorless solution was then covered with a supernatant layer
of hexane (300 mL) and left on standing 4 h at 4 °C. The formed
white precipitate was recovered by filtration of the mother liquor.
Yield: 80%. Anal. Calcd for C61H83N4O4P2Cl7Pt2 ([PtCl{Z-HNd
C(OMe)But}2(PPh3)]Cl‚1/2CHCl3): C, 44.7; H, 5.1; N, 3.4.
1
Found: C, 44.3; H, 5.0; N, 3.3. H NMR in CDCl3 (δ): 0.87 (s,
18H, CBut), 4.86 (s, 6H, OMe), 7.98 (s, br, 2H, NH), 7.50 (m, 9H,
meta + para PPh), 7.86 (d, 6H, ortho PPh).
cis-[PtCl2{Z-HNdC(OMe)But}(PPh3)]. trans-[PtCl{Z-HNd
C(OMe)But}2(PPh3)]Cl (0.35 g, 0.46 mmol) was dissolved in CHCl3
(50 mL) and the solution kept for 4 h at 55 °C. After cooling, the
solution was covered with a supernatant layer of hexane (500 mL)
and left on standing at 4 °C overnight. The resulting white
The reactivity of the Pt(IV) complex (2) toward a base
(KOH), an acid (HCl), and a phosphine ligand (PPh3) has
(21) Cini, R.; Fanizzi, F. P.; Intini, F. P.; Maresca, L.; Natile, G. J. Am.
Chem. Soc. 1993, 115, 5123.
476 Inorganic Chemistry, Vol. 41, No. 3, 2002

Pt Complexes of Z Configuration Iminoether Ligands
crystalline solid was collected by filtration of the mother liquor,
washed with hexane, and dried in a stream of dry air. Yield: 80%.
Anal. Calcd for C24H28NOCl2PPt: C, 44.8; H, 4.4; N, 2.2. Found:
C, 45.1; H, 4.4; N, 2.0. 1H NMR in CDCl3 (δ): 0.80 (s, 9H, CBut),
4.85 (s, 3H, OMe), 6.09 (s, br, 1H, NH), 7.44 (m, 9H, meta +
para PPh), 7.76 (m, 6H, ortho PPh).
cis-[PtCl2(PPh3)2]. trans-[PtCl2{Z-HNdC(OMe)But}2] (0.06 g,
0.12 mmol) and an excess of PPh3 (0.15 g, 0.6 mmol) were
dissolved in CHCl3 (20 mL), and the solution was kept under stirring
for 20 h at room temperature. After evaporation of the solvent under
reduced pressure, the white residue was transferred on a filter,
washed carefully with diethyl ether (which removes excess PPh3),
and dried in a stream of dry air. Yield: 80%. Anal. Calcd for C36H30-
Cl2P2Pt: C, 54.7; H, 3.8. Found: C, 54.7; H, 3.9. 1H NMR in CD2-
Cl2 (δ): 7.48 (m, 2H, ortho PPh), 7.21 (m, 2H, meta PPh) 7.37
(m, 1H, para PPh). The cis configuration of the compound was
deduced on the basis of spectral features already reported for this
compound22,23
least-squares cycles. The hydrogen atoms for the methyl groups
were located in computed positions and allowed to ride on the
carbon atoms to which they are linked. All the hydrogen atoms
were refined isotropically, and their thermal parameters were
restrained to 1.2 times those (U(eq)) of the atoms to which they
are bound. The agreement factors converged to R1 ) 0.0160 and
wR2 ) 0.0400 over the independent observed reflections with
I > 2σ(I).
All the calculations were carried out using XSCANS,24 XEMP,25
SHELXS 97,26 SHELXL 97,27 PARST 97,28 PLATON 98,29
XPMA-ZORTEP,30 and ORTEP 331 computer programs imple-
mented on Pentium machines.
trans-[PtIVCl4{Z-HNdC(OMe)But}2], 2. (a) Data Collection.
A yellow prism of 2 (dimensions 0.10 × 0.15 × 0.20 mm) was
used for the X-ray analysis through the techniques and the devices
already reported above for 1. The cell constants (see Table 1) were
determined from the values of 42 carefully centered randomly
selected reflections in the range 11° e 2θ e 44°. The data were
collected at 293 ( 2 K and then corrected for Lorentz-polarization
and absorption effects (ψ-scan technique). The independent ob-
served reflections (I > 2σ(I)) were 1445 out of a total of 2334
collected reflections (R(int) ) 0.0345). The space group (P21/c,
No. 14) is in agreement with the systematic extinctions and with
the structure solution and refinement (see below). Three standard
reflections were monitored periodically (97 reflections) during the
data collection; no appreciable decay was observed.
(b) Structure Solution and Refinement. The structure was
solved through the Patterson and Fourier techniques. The Pt atom
is located on an inversion center. Two chloride ligands and a
Z-HNdC(OMe)But ligand, which complete the asymmetric unit,
are in general positions. As a consequence the structure of the
complex molecule is pseudooctahedral. All the non-hydrogen atoms
were treated as anisotropic during the subsequent 12 cycles of least-
squares refinement. All the hydrogen atoms for the Z-HNdC(OMe)-
But group were located in computed positions, allowed to ride on
the atoms to which they are linked, and refined isotropically. The
agreement factors converged to R1 ) 0.0274 and wR2 ) 0.0714
over the independent observed reflections with I > 2σ(I).
All the calculations were carried out using the packages and
machines described above for 1.
Physical Measurements. IR spectra in the range 4000-400
cm-1 were recorded as KBr pellets; spectra in the range 400-200
cm-1 were recorded as polythene pellets on a Perkin-Elmer FT 1600
1
spectrophotometer. H NMR spectra were obtained with a Bruker
Avance DPX 300WB spectrometer.
X-ray Structure Determinations. trans-[PtIICl2{Z-HNdC-
(OMe)But}2], 1. (a) Data Collection. A pale yellow prism of 1
(dimensions 0.10 × 0.15 × 0.20 mm) was selected at the polarizing
microscope and mounted on a glass fiber. The diffraction data were
collected through a four-circle automatic Siemens P4 diffractometer,
by using graphite-monochromatized Mo KR radiation (λ, 0.710 73
Å). The cell constants (see Table 1) were determined via full-matrix
least-squares refinement of the values of 26 carefully centered
randomly selected reflections in the range 11° e 2θ e 42°. The
data were collected at 293 ( 2 K and then corrected for Lorentz-
polarization and absorption effects (ψ-scan technique). The inde-
pendent observed reflections (I > 2σ(I)) were 1369 out of a total
of 1636 collected reflections ([R(int) ) 0.0130]). The space group
(C2/m, No. 12) is in agreement with the systematic extinctions and
with the structure solution and refinement (see below). Three
standard reflections were monitored periodically (97 reflections)
during the data collection; no appreciable decay was observed.
Acknowledgment. This work was supported by the
Ministero della Universita` e della Ricerca Scientifica e
Tecnologica (Italy), the Italian National Research Council
(Italy), the Ministerio de Education y Cultura (Spain), and
the European Community (COST D8 97/007 and COST D20
01/001). The authors gratefully acknowledge Mr. F. Berrettini
(b) Structure Solution and Refinement. The structure was
solved through the Patterson and Fourier techniques. The Pt atom
is located on a mirror plane and on a 2-fold crystallographic axis,
whereas the chlorine atom is located on the mirror plane. All the
other atoms are located in general position. As a consequence, the
structure is disordered as regard to the iminoether ligands. The
occupancy of atoms N(1), C(1), C(2), O(1), and C(6) was fixed to
0.5. Two sets of three peaks each was generated by the Fourier-
difference synthesis around C(2). This was interpreted as a further
statistical disorder of the tert-butyl group. The two sets of peaks
were assigned as carbon atoms (namely, C(3), C(4), and C(5) and
C(3A), C(4A), and C(5A)) at the occupancy of 0.25 each. All the
non-hydrogen atoms were treated as anisotropic during the subse-
quent 12 cycles of least-squares refinement. The Fourier-difference
synthesis performed at this stage showed a low intensity new peak,
close to the N(1) atom. This peak was assigned to a hydrogen atom
linked to N(1), and its position was refined freely in the subsequent
(24) XSCANS Users Manual; Siemens Analytical X-ray Instruments Inc.:
Madison, WI, 1994.
(25) Xemp Empirical Absorption Correction Program; Siemens Analytical
X-ray Instruments Inc.: Madison, WI, 1994
(26) Sheldrick, G. M. SHELXS 97, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(27) Sheldrick, G. M. SHELXL 97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(28) Nardelli, M. PARST 97, A System of Computer Routines for Calculating
Molecular Parameters from Results of Crystal Structure Analyses;
University of Parma: Parma, Italy, 1997.
(29) Spek, A. L. PLATON 98; Utrecht University: Utrecht, The Nether-
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Inorganic Chemistry, Vol. 41, No. 3, 2002 477

Gonzalez et al.
and some metrical parameters which do not appear in the text. This
material is available free of charge via Internet at http:/pubs.acs.org.
(CIADS, Siena University) for technical assistance in the
X-ray data collection.
Supporting Information Available: X-ray crystallographic
files, in CIF format and listings of positional and thermal parameters
IC010850V
478 Inorganic Chemistry, Vol. 41, No. 3, 2002