G. Bandoli et al. / Inorganica Chimica Acta 346 (2003) 143ꢂ
/
150
147
luble in chloroform, having only one lipophilic PPh3
ligand, and precipitates as a white solid (Eq. (1)).
signals at 5.84, 6.88, and 4.77 ppm, respectively, while
t
the NH signal at 5.86 ppm correlates with the Bu signal
2
ꢃ
ꢁ
at 0.96 ppm. Therefore, two isomers are present in
[
Pt (PPh ) fN(H)C(R)Og ] ꢃ2NO3
2
3 2
4
2
2
solution: one symmetrical with N O donor atoms
around the two platinum atoms (tert-butyl signal at
0.96 ppm and NH signal at 5.86 ppm) and one non-
?
[Pt (PPh )(NO )fN(H)C(R)Og ](NO )¡ꢃPPh (1)
In order to prove the above hypothesis we synthesized
2
3
3
4
3
3
3
the same bisphosphine compound with a non coordinat-
symmetrical with N O donor atoms around one plati-
3
num atom and NO donor atoms around the second
ꢁ
ꢁ
ing counter ion (ClO4 or BF4 ). As expected the
bisphosphine compound was stable even in the absence
of excess triphenylphosphine. We could also notice that
the chemical shifts of the NH protons are strongly
affected by the nature of the counter ion. For instance
substitution of BF4 for NO3 causes the chemical
shifts of the NH protons at 7.08, 5.66, and 5.31 ppm to
be shifted at 6.26, 4.82 and 4.84 ppm, respectively. The
shift is greater for the NH’s of the N Oꢀ
subunit (0.82ꢂ0.84 ppm) than for the NH of the NO
subunit (0.47 ppm). The shift of the NH signals was even
greater when the nitrate anion was replaced by perchlo-
rate (new signals at 5.95, 4.78, and 4.71 ppm, respec-
tively). The deshielding effect of the nitrate anion, with
respect to perchlorate and tetrafluoroborate anions, is
platinum atom (tert-butyl signals at 1.08, 0.91, and 0.89
ppm and NH signals at 5.84, 6.88, and 4.77 ppm). As
already observed for the acetamidate complex 1 also the
3
3
N O/NO isomers of 2 is unstable in the absence of
excess triphenylphosphine and, while its proton NMR
signals decrease in intensity, three new signals rise at
ꢁ
ꢁ
1.21, 1.04, and 0.90 ppm (intensity ratios 1:2:1). The
new signals belong to a new compound which is formed
3
/
platinum-
3
/
from 2 by substitution of a phosphine ligand by a water
3
3
t
molecule,
O} ](NO ) (3). The new compound was obtained in
N O/NO ꢂ[Pt (H O)(PPh ){N(H)C(Bu )-
/
2 2 3
4
3 2
crystalline form and its structure was determined by X-
ray diffraction. Complex 3 is the first non-symmetric
3
3
ꢁ
N O/NO lantern-type platinum dimer with four brid-
ging amidate ligands to be characterized by X-ray
diffraction.
indicative of a greater ability of the NO3 anion to give
H-bond interactions with the NH’s. Such an interaction
could be favoured by the planar shape of the nitrate
anion which can interact with the NH hydrogen atoms
Similarly to the acetamide system, also for the
pivaloamide system the dissociation of an apical phos-
phine is a reversible process. Therefore, complex 3 in the
presence of an excess of triphenylphosphine reverts to
complex 2.
without severe steric clash with the PPh ligand. This
3
would not be possible for perchlorate and tetrafluor-
oborate having a tetrahedral geometry. The greater
3
downfield shift of the NH’s of the N Oꢀ
/
Pt subunit
Pt subunit could be a
3
Similarly to complex 1, also in the case of 2,
with respect to the NO ꢀ
/
ꢁ
ꢁ
substitution of ClO4 for NO3 deeply influences the
NMR spectrum in CDCl solution. Thus substitution of
ClO4 for NO3 causes a shift of the NH signals from
consequence of the nitrate anions concentrating around
the former subunit.
3
ꢁ
ꢁ
6
4
.88, 5.86, 5.84, and 4.77 ppm to 5.86, 5.21, 5.09, and
t
.1.1. [Pt (PPh ) {N(H)C(Bu )O} ](NO ) (2) and
3
3
.63 ppm, respectively. For the N O/NO isomer the
3
2
3 2
4
3 2
t
Pt (H O)(PPh ){N(H)C(Bu )O} ](NO ) (3)
[
3
2
2
3
4
3 2
shift is greater for NH’s of the N O subunit (1.02ꢂ
/
0.63
3
ppm) than for the NH of the NO subunit (0.14 ppm),
1
Complex 2 was investigated by H NMR spectro-
scopy in CDCl . Four signals (1.08, 0.96, 0.91, and 0.89
3
2
while for the cis-N O isomer the shift of the NH signal
2
ppm) were observed in the region of tert-butyl groups.
The intensity ratios of signals at 1.08, 0.91, and 0.89
ppm were 1:2:1, respectively, and remained constant in
different samples while the relative intensity of the
fourth signal changed from sample to sample. Three
signals were observed in the region of NH protons (6.88,
is 0.77 ppm.
3
3
Differently from the N O/NO isomer which is stable
in the presence of excess PPh3 (and unstable in the
2
2
absence of free phosphine), the cis-N O isomer is
unstable in CHCl3 solution, and decomposes in an
irreversible way, if free triphenylphosphine is present
in excess. The rate of decomposition is greater if the
concentration of free phosphine is higher.
5
ppm (intensity ratio 2:1) showed a small coupling with
.86, and 4.77 ppm). The NH signals at 6.88 and 4.77
3
1
31
P, consequently a P decoupled spectrum was regis-
1
31
The { H} P spectra showed a signal at ꢁ
/
5.5 ppm for
1.0 and ꢁ9.6
ppm for the N O/NO isomer of 2, and a signal at 30.6
tered in order to better evaluate the chemical shifts and
intensities of the NH signals. The signal at 5.86 ppm did
2
2
the cis-N O isomer of 2, two signals at ꢁ
/
/
3
3
3
1
not change in the P decoupled spectrum with respect
to the coupled one. The NOESY spectrum revealed the
existence at a fourth NH’s signal at 5.84 ppm and
overlapping with the signal at 5.86. Moreover, it
3
1
ppm for complex 3. The presence of only one P signal
(ꢁ5.5 ppm) for the symmetrical isomer of 2 is in accord
with a N O configuration and rules out the N /O
/
2
2
4
4
t
1
indicates that the Bu signals at 1.08, 0.91, and 0.89
ppm (intensity ratios 1:2:1) are correlated with the NH
configuration which could be compatible with the H
NMR data.