DOI: 10.1039/b409887k
This journal is © The Royal Society of Chemistry 2004 3662 Dalton Trans ., 2004, 3662–3668
Dalton
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F U L L P A P E R
Pseudo-rotation mechanism for fast olefin exchange and substitution
processes at orthometalated C,N-complexes of platinum(II)†
Stefanus Otto‡,
a
Pavel V. Samuleev,
a
Vladimir A. Polyakov,
b
Alexander D. Ryabov
c
and
Lars I. Elding*
a
a
Inorganic Chemistry, Department of Chemistry, Chemical Center, Lund University,
P. O. Box 124, SE-221 00 Lund, Sweden. E-mail: LarsI.Elding@inorg.lu.se
b
D. I. Mendeleev Moscow University of Chemical Technology, Miusskaya sq. 9,
125820 Moscow, Russia
c
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh,
PA 15213, USA
Received 30th June 2004, Accepted 22nd September 2004
First published as an Advance Article on the web 7th October 2004
Bridge splitting in chloroform of the orthometalated chloro-bridged complex [Pt(l-Cl)(2-Me
2
NCH
2
C
6
H
4
)]
2
(1),
with ethene, cyclooctene, allyl alcohol and phosphine according to 1 + 2 L → 2 [PtCl(2-Me
2
NCH
2
C
6
H
4
)(L)],
where L = C
2
H
4
(3a), C
8
H
14
, (3b), CH
2
CHCH
2
OH (3c), and PPh
3
(4a and 4b) gives monomeric species with L
coordinated trans or cis to aryl. With olefins the thermodynamically stable isomer with L coordinated cis to aryl is
formed directly without an observable intermediate. With phosphine and pyridine, the kinetically controlled trans-
product isomerizes slowly to the more stable cis-isomer. Bridge splitting by olefins is slow and first-order in 1 and
L, with largely negative DS
‡
. Substitution of ethene cis to aryl by cyclooctene and allyl alcohol to form 3b and 3c,
and substitution of cot from 3b by allyl alcohol to form 3c are first order in olefin and complex, ca. six orders of
magnitude faster than bridge cleavage due to a large decrease in DH
‡
, and with largely negative DS
‡
. Cyclooctene
exchange at 3b is first-order with respect to free cyclooctene and platinum complex. All experimental data for
olefin substitution and exchange are compatible with a concerted substitution/isomerization process via a turnstile
twist pseudo-rotation in a short-lived labile five-coordinated intermediate, involving initial attack on the labile
coordination position trans to the r-bonded aryl. Bridge-cleavage reactions of the analogous bridged complexes
occur similarly, but are much slower because of their ground-state stabilization and steric hindrance.
Introduction
The chemistry of cycloplatinated compounds has been widely
studied.
1–12
The bridge splitting reaction (eqn. (1)) offers a direct
and effective route to a large assortment of monomeric cyclo-
metalated derivatives.
(1)
Reactions similar to (1) are also mechanistically interest-
ing, since the bridging unit [Pt(l-Cl
2
)Pt] is a coordinatively
unsaturated moiety of a kind which plays a key role in metal
complex homogeneous catalysis. A few quantitative studies
of the kinetics and mechanism for cleavage of halide-bridged
dinuclear square-planar complexes have appeared. For in-
stance, cleavage of [Pt
2
Br
6
]
2−
with amines
13
and alkenes,
14,15
of
[Pd
2
I
6
]
2−
with iodide,
16
of [PtCl
2
(cyclooctene)]
2
with methanol,
acetonitrile and cyclooctene,
17
and of dimethylaminomethyl-
phenyl-C,N pallada- and platinacycles with pyridines
5
have
been studied in detail. In the case of cyclometalated dinuclear
complexes, there are also some qualitative studies, for instance
the splitting of [Pt{C
6
H
3
(CH
2
)
3
CN(cyclohexyl)}Cl]
2
with
carbon monoxide which results in the monomeric species
[Pt(C
6
H
3
(CH
2
)
3
CN(cyclohexyl)}Cl(CO)] with the CO finally
coordinated cis to the aryl carbon as confirmed by X-ray
crystallography.
18
Cleavage of dinuclear C,N-cycloplatinated complexes with
alkenes and substitution and exchange processes of alkenes at
the corresponding mononuclear cycloplatinated complexes is
the focus of the present study. These reactions offer particular
mechanistic challenges, since the resulting metal centers coor-
dinate an alkene with strong p-back bonding and p-trans effect
together with a strongly r-bonded aryl with a strong r-trans
effect and ground-state trans influence. Both these ligands
create very labile coordination positions trans to themselves.
This study is related to some previous work in our respec-
tive laboratories, viz. studies of the mechanistically simpler
cleavage of C,N-cyclometallated complexes with pyridines,
5
and the mechanism for exchange of ethene at Zeise’s anion,
[PtCl
3
(C
2
H
4
)]
−
,
19
and splitting of trans-[PtCl
2
(cyclooctene)]
2
with cyclooctene,
17
which both involve platinum centers coordi-
nating simultaneously two alkene molecules.
Our attention here was directed at: (i) bridge-splitting kinetics
for the reaction between the dinuclear platinum(II) complex
[Pt(l-Cl)(2-Me
2
NCH
2
C
6
H
4
)]
2
(1), derived from N,N-dimethyl-
benzylamine, and olefins, resulting in formation of complexes
3a, 3b, and 3c, (ii) for comparison, bridge splitting of 1 with
triphenylphosphine, resulting in formation of complexes 4a
and 4b, (iii) the reaction mechanism for the exchange and sub-
stitution of olefins at the metal centers 3a, 3b and 3c with the
strongly trans-labilizing aryl donor ligand, and (iv) structural
characterization of the reaction products by NMR and X-ray
crystallography. The cyclometalated complexes used are shown
in Chart 1.
Experimental
Chemicals
Ethene (99.95%, Cambridge Isotope Laboratories) was used as
received, CDCl
3
(Cambridge Isotope Laboratories Inc), cyclo-
octene (ACROS) and allyl alcohol (ACROS) were dried over
† Electronic supplementary information (ESI) available: Observed rate
constants for all reactions studied and carbon-13 NMR data for the
complexes studied. See http://www.rsc.org/suppdata/dt/b4/b409887k/
‡ Present address: SASOL Technology R&D, P.O. Box 1, Sasolburg,
South Africa
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