metal-organic compounds
Figure 2
A view of the hydrogen bonding (dashed lines) present in (I).
Figure 1
A view of (I), showing the atom-labelling scheme. Displacement
ellipsoids are drawn at the 30% probability level.
of decreasing the dielectric constant around the metal core, so
favouring electrostatic interactions. Among these is the
interaction between the lone pair of electrons on the O atom
and the positively charged metal core. This obviously requires
the imino ether to assume a Z–anti configuration. This
hypothesis appears to be supported by the observation that
the Clꢂ counter-ion is the acceptor of three hydrogen bonds
involving the iminic N atom of two adjacent complex mol-
ecules and the solvent water molecule, which bridges the Clꢂ
counter-ion and the chloride ligand. Such a hydrogen-bond
network would be favoured by a local decrease in the dieletric
constant.
The relative orientations of the imino ether ligands can be
head-to-head (HH) or head-to-tail (HT) (Scheme 2). In (I),
the relative orientation of the imino ether ligands is HT. The
two imino ether ligands are almost coplanar [dihedral angle
8.9 (2)ꢁ] and the dihedral angles between their planes and the
coordination plane are 74.9 (2) and 69.0 (2)ꢁ.
et al., 2000). The two imino ether ligands have similar
geometric parameters. The N—C, Csp2—O and Csp3—O bond
˚
lengths [average values = 1.27 (1), 1.33 (1) and 1.45 (1) A,
respectively] and the Pt—N—C and N—C—C angles [average
values = 128.5 (3) and 123.0 (4)ꢁ, respectively] are similar to
those previously observed in cis- and trans-bis[(E)-1-imino-1-
methoxyethane]dichloridoplatinum(II) (Cini et al., 1995), in
tetrakis[(E)-1-ethoxy-1-iminopropane]platinum bis(trifluoro-
methanesulfonate) (Prenzler et al., 1997) and in trans-
dichloridobis[(Z)-1-imino-1-methoxy-2,20-dimethylpropane]-
platinum(II) (Gonzalez et al., 2002), and are in accord with a
ꢀ-bond delocalized over the N—C—O group.
The imino ether ligands of (I) have the unprecedented
Z–anti configuration (Scheme 2). A Z configuration was
observed in our previous paper reporting the X-ray structure
of the complex trans-[PtCl2{Z-HN C(tBu)OMe}2] (Gonzalez
et al., 2002), but the conformation was syn and not anti as in
the present case. In the previously reported Z-imino ether
Fig. 2 shows the chloride anion interacting with the N—H
groups of two imino ether ligands from two adjacent complex
molecules and a water molecule as a trifurcated acceptor,
Clꢀ ꢀ ꢀH—X, with Clꢀ ꢀ ꢀX distances of 3.293 (3), 3.262 (4) and
complex there was a t-Bu substituent on the C atom of the
. . . . . .
N
C O group and the steric bulkiness of t-Bu was consid-
ered to be responsible for stabilization of the Z configuration
(t-Bu trans to the metal core with respect to the C N double
bond) over the E configuration (t-Bu cis to the metal core). In
that case, the bulkiness of t-Bu could also be responsible for
˚
3.337 (5) A for X = N, N and O, respectively, and Clꢀ ꢀ ꢀH—X
angles in the range 166 (4)–168 (4)ꢁ. Weaker C—Hꢀ ꢀ ꢀCl
hydrogen bonds (Table 2) are apparent between aryl or
methyl C—H groups and the chloride anion.
the stabilization of the syn conformation (the Me group on the
. . . . . .
O atom cis to Pt with respect to the N C O group and trans
. . .
Experimental
to t-Bu with respect to the C O quasi-double bond).
However, in the present case of a small Me group on the C
. . . . . .
For the preparation of trans-[PtCl{E-HN C(Me)OMe}2(PPh3)]Cl,
an Et2O (5 ml) suspension of trans-[PtCl2{E-HN C(Me)OMe}2]
(100 mg, 0.24 mmol) was mixed with 1.2 equivalents of PPh3 (73.4 mg,
0.29 mmol). After stirring for 12 h at room temperature, the
suspension turned from yellow to white. The white solid, trans-
[PtCl{E-HN C(Me)OMe}2(PPh3)]Cl, was filtered off, washed with
Et2O (3 ꢃ 5 ml) and dried under vacuum (yield 159.9 mg, >98%).
Single crystals of trans-[PtCl{Z-HN C(Me)OMe}2(PPh3)]ClꢀH2O
suitable for X-ray diffraction analysis were obtained by evaporation
atom of the N
C O group, there is no steric interaction
which can account for the stabilization of the Z configuration,
as also witnessed by the anti conformation of the O—Me
. . .
group (O—Me cis to C—Me with respect to the C O quasi-
double bond and trans to Pt with respect to the planar
. . . . . .
N
C O group). Why, then, does the presence of a PPh3
group in the coordination environment of Pt stabilize the Z
configuration of the imino ether, accompanied by the anti
conformation of the O—Me group? Our hypothesis is that
replacement of a chloride anion by a phosphane ligand has the
effect of increasing the net positive charge on Pt and possibly
from
a
solution of trans-[PtCl{E-HN=C(Me)OMe}2(PPh3)]Cl
(10.0 mg, 0.024 mmol) in acetone (2 ml), where it isomerises to the
E,Z and Z,Z species. The latter form, (I), is the less soluble and
crystallizes preferentially.
ꢄ
Acta Cryst. (2012). C68, m300–m302
Papa et al.
[PtCl(C3H7NO)2(C18H15P)]ClꢀH2O m301