Synthesis, Structure, and Reactivity of Arylchlorobis(dialkyl sulfide)platinum(II) Complexes

Complexes
trans-[PtRCl(SR‘2)2],
where R = Ph, mesityl, and p-anisyl and R‘ = Me or Et,
have been synthesized
and their crystal and molecular structures determined. Crystals of
trans-[PtPhCl(SEt2)2]
(2) are triclinic (P1̄)
with a = 10.112(6) Å, b =
13.158(2) Å, c = 14.714(5) Å, α =
102.48(2)°, β = 94.394(4)°, γ = 90.22(3)°,
and
Z = 4. Crystals of
trans-[Pt(mesityl)Cl(SMe2)2]
(4) are monoclinic (P21/c)
with a = 13.158(2) Å, b =
9.170(1)
Å, c = 16.013(3) Å, β = 120.93(2)°, and
Z = 4, and crystals of
[Pt(p-anisyl)Cl(SMe2)2]
(5) are monoclinic
(P21/n) with a =
9.879(4) Å, b = 8.128(2) Å, c =
19.460(5) Å, β = 96.56(3)°, and Z = 4.
All complexes are
square-planar, featuring Pt−Cl distances between 2.40 and 2.42 Å,
indicating a large ground-state trans influence
of the aryl group. The coordination geometry is maintained in
methanol and chloroform solution as shown by
1H-NMR spectra. The kinetics of substitution of the
labile chloride trans to aryl by various nucleophiles
has
been studied in methanol by variable-temperature and -pressure
stopped-flow spectrophotometry. A two-term
rate law with a well-developed solvolytic pathway is followed.
Negative entropies and volumes of activation
indicate an associative mode of activation in all cases, independent of
steric blocking of the axial sites and a large
Pt−Cl ground-state bond-weakening. Comparison of the reaction
rates of the present series of complexes with
their bis(phosphine) analogues and with related cyclometalated
compounds shows that the triethylphosphine
complexes are 2−3 orders of magnitude less reactive than the
thioether complexes, which in turn are a factor
10−20 less reactive than the cyclometalated ones. This
reactivity increase can be rationalized mainly in terms
of a decrease in steric hindrance in the series. There seems to be
no inherent differences with regard to trans
labilizing ability of the aryl ligands in the various types of
complexes, including the cyclometalated ones.