Unexpected formal aryl insertion in a cyclometalated diphenylplatinum (IV) complex: The first seven-membered cyclometalated platinum compound structurally characterized

Article abstract of DOI:10.1021/om020260k

The structural characterization of a seven-membered cyclometalated platinum compound was investigated. In regard with it, the aryl insertion in a cyclometalated diphenylplatinum (IV) complex was studied. The liability of the selenium based ligand enabled the reductive elimination of a C₆H₆ molecule and the formal aryl insertion of other phenyl ligand in the cyclometalated bond. The crystal structure of the triphenylphosphine derivative was determined by using x-ray analysis.

Full text of DOI:10.1021/om020260k

Organometallics 2002, 21, 3305-3307
3305
Un exp ected F or m a l Ar yl In ser tion in a Cyclom eta la ted
Dip h en ylp la tin u m (IV) Com p lex: Th e F ir st
Seven -Mem ber ed Cyclom eta la ted P la tin u m Com p ou n d
Str u ctu r a lly Ch a r a cter ized
Merce` Font-Bard´ıa,† Carlos Gallego,‡ Manuel Martinez,*,‡ and Xavier Solans†
Departament de Cristal.lografia, Mineralogia i Dipo`sits Minerals, Universitat de Barcelona,
Martı´ i Franque`s s/ n, E-08028 Barcelona, Spain, and Departament de Qu´ımica Inorga`nica,
Facultat de Quı´mica, Universitat de Barcelona, Martı´ i Franque`s 1-11,
E-08028 Barcelona, Spain
Received April 2, 2002
Summary: Cyclometalation of the (2-BrC6H4)CHNBzl
imine via oxidative addition on [PtII(Ph)2(SMe2)2] to
produce the corresponding [PtIVBr(Ph)2(CC5H3CHNBzl)-
(SMe2)] compound has been achieved, and the X-ray
crystal structure of its triphenylphosphine derivative has
been determined. The lability of the SMe2 ligand in this
complex enables the reductive elimination of a C6H6
molecule and the formal insertion of the other phenyl
ligand in the cyclometalated Pt-C bond, producing the
first structurally characterized seven-membered cyclo-
metalated platinum complex.
was carried out according to the already well-estab-
lished procedures.5 After 4 h of reaction, the solution
was taken to dryness. The proton NMR spectrum (250
MHz, acetone-d6, room temperature) of the residue
showed a set of signals which indicated that the activa-
tion of the C-Br bond has taken place, producing the
expected cyclometalated compound cis-[PtIVBr(Ph)2-
(CC5H4CHNBzl)(SMe2)]. The CH imine proton signal at
8.51 ppm shows a Pt(IV)-indicative coupling constant,
J
Pt-H, of 45.4 Hz;5 furthermore, the product reacts with
PPh3 to produce the corresponding triphenylphosphine
derivative, characterized by the corresponding proton
Substitution reactions on inert Werner-type coordina-
tion complexes have been studied for a long time and,
although a few important challenges still exist,1 the
results seem quite clear as far as the intimate mecha-
nism is concerned. The same sorts of studies on orga-
nometallic complexes are much more scarce, even
though their inherent interest is evident both from the
catalytic point of view and from that of activation
processes of organic substrates.2 Furthermore, the dis-
sociation reactions on these octahedral complexes have
been found to be a key step for the reductive elimination
processes.3 According to our interest in the substitution
and phosphorus NMR signals: CH, 7.82 ppm (J Pt-H
)
46 Hz); CH2, 3.64 and 5.79 ppm (J H-H ) 17.6 Hz); PPh3,
-5.93 ppm (J Pt-P ) 1001.8 Hz). On standing, the
acetone solution of the triphenylphosphine complex
produced good X-ray-quality crystals that were struc-
turally analyzed.6 Figure 1 shows the determined
structure for the P(IV) cyclometalated complex. The
complex has the well-established5 facial arrangement
of the three Pt-C bonds observed in complexes of the
same family, and the PPh3 ligand is located in the less
hindered available position of the octahedral coordina-
tion polyhedron. As a whole, the structure has the same
characteristics as those determined for complexes with
two methyl ligands instead of the two phenyl groups.4,5
The kinetics of the substitution reaction of SMe2 by
PPh3 were studied at variable temperature and sulfide
and phosphine concentrations, to establish possible
differences due to the size and trans influences on the
non-cyclometalated organometallic ligands.4 The reac-
tion mechanism and rate law found are indicative of the
operation of the established limiting dissociative mech-
anism, which involves the existence, under steady-state
conditions, of a pentacoordinated intermediate with no
SMe2 attached. Further reactions of this intermediate
with the entering phosphine ligand produce the final
complex.4 The kinetic and thermal activation param-
6
reactions on theoretically inert t2g Pt(IV) organome-
tallic complexes,4 we decided to continue the study of
such reactions (eq 1) when much bulkier groups, having
a smaller trans influence, are present in the molecule.
For this purpose the reaction of the Pt(II) compound
cis-[Pt(Ph)2(SMe2)2] with the (2-BrC6H4)CHNBzl imine
† Departament de Cristal.lografia, Mineralogia i Dipo`sits Minerals.
‡ Departament de Qu´ımica Inorga`nica.
(1) (a) Tobe, M. L.; Burgess, J . Inorganic Reaction Mechanisms;
Longmans Green: London, 1999. (b) de Vito, D.; Sidorenkova, H.;
Rotzinger, F. P.; Weber, J .; Merbach, A. E. Inorg. Chem. 2000, 39, 5547.
(2) Parshall, G. W.; Ittle, S. D. Homogeneous Catalysis, 2nd ed.;
Wiley: New York, 1992.
(3) Puddephatt, R. J . Angew. Chem., Int. Ed. 2002, 41, 261.
(4) (a) Bernhardt, P. V.; Gallego, C.; Martinez, M. Organometallics
2000, 19, 4862. (b) Bernhardt, P. V.; Gallego, C.; Martinez, M.; Parella,
T. Inorg. Chem. 2002, 41, 1471.
(5) (a) Crespo, M.; Martinez, M.; Sales, J . Organometallics 1993,
12, 4267. (b) Crespo, M.; Martinez, M.; Sales, J .; Solans, X.; Font-
Bard´ıa, M. Organometallics 1992, 11, 1288.
(6) Anal. Calcd (found) for PtC44H37BrNP: C, 59.7 (59.7); H, 4.2 (4.3);
N, 1.6 (1.3). X-ray data: Mr ) 885.72, space group P1h, triclinic, a )
10.1980(10) Å, b ) 11.5200(10) Å, c ) 17.6330(10) Å, R ) 71.85°, â )
86.43°, γ ) 67.70°, V ) 1817.3(6) Å3, T ) 293(2) K, Z ) 2, µ(Mo KR) )
5.035 mm-1, 10 761 reflections measured, 6719 unique reflections (Rint
) 0.0317) which were used in all calculations, final Rw(F2) ) 0.1447
(all data).
10.1021/om020260k CCC: $22.00 © 2002 American Chemical Society
Publication on Web 07/12/2002

3306 Organometallics, Vol. 21, No. 16, 2002
Communications
F igu r e 2. View of the X-ray structure of the seven-
membered cyclometalated platinum(II) compound pre-
pared, [PtIIBr(CC5H4C6H4CHNBzl)(SMe2)] (ellipsoids in-
dicate 20% probability). Relevant distances (Å): Pt-C(1),
2.021(5); Pt-N, 2.044(4); Pt-S, 2.2528(14); Pt-Br, 2.5254-
(5). Relevant angles (deg): C(1)-Pt-N, 87.40(14); C(1)-
Pt-S, 89.04(11); M-Pt-S, 176.39(10); C(1-6)-C(7-12),
577(3).
F igu r e 1. View of the X-ray structure of the cyclometa-
lated platinum(IV) compound prepared, [PtIVBr(Ph)2(CC5H4-
CHNBzl)(PPh3)] (ellipsoids indicate 20% probability). Rel-
evant distances (Å): Pt-C(1), 2.067(8); Pt-C(15), 2.045(7);
Pt-C(21), 2.095(8); Pt-N, 2.181(6); Pt-P, 2.4888(18); Pt-
Br, 2.5547(10). Relevant angles (deg): C(1)-Pt-N, 80.5-
(3); C(1)-Pt-C(15), 93.4(3); C(1)-Pt-C(21), 86.1(3); C(1)-
Pt-P, 97.69(174); C(1)-Pt-Br, 174.21(18).
On standing, the solutions of the aforementioned
Pt(IV) dimethyl sulfide complexes produced, to our
knowledge, the first structurally characterized seven-
membered cyclometalated platinacycle, [PtIIBr-
(CC5H4C6H4CHNBzl)(SMe2)]. From these solutions X-
ray-quality crystals could be collected and analyzed.8
The structure (Figure 2) shows a highly hindered Pt-
(II) center, due to the extreme steric demands of the
aryl groups of the cyclometalated ligand. In this respect
all attempts to carry out an oxidative addition on the
complex with MeI failed; only the starting material was
recovered. Reaction with excess triphenylphosphine of
this compound leads to the substitution of the SMe2 on
the Pt(II) complex (31P NMR signal at 17.8 ppm, J Pt-P
) 1889 Hz); no further opening of the cyclometalated
ring is observed, as opposed to other cases.9
The reductive process from the Pt(IV) compound to
produce the new seven-membered cyclometalated Pt-
(II) complex is not straightforward. It is clear from the
stoichiometry that a reductive elimination of benzene
has taken place, but a simultaneous formal phenyl
insertion into the Pt-C(cyclometalated) bond has also
had to occur. Preliminary kinetic observations indicate
that the overall reaction is quite clean, with no buildup
of intermediates, even at low temperature. The reaction
rate observed is inversely dependent both on the con-
centration of the platinum compound and on that of
added SMe2, which suggests that the well-established
easy dissociation of dimethyl sulfide on these types of
complexes, as measured, plays a crucial role in the
process.3,4,10 Both sequences of reactions indicated in-
Scheme 1 agree with the stoichiometric process ob-
served.
eters found for the process, that is, the sulfide ligand
293
dissociation (k1
) 0.12 s-1, ∆Hq ) 60 ( 3 kJ mol-1
,
∆Sq ) -58 ( 11 J K-1mol-1), agree with the substitution
mechanisms proposed and are of the same order as
those found for systems with methyl ligands.4 The
negative activation entropy can be related to the exist-
ence of a highly ordered dissociative transition state,
produced by the steric relief occurring on partial deco-
ordination of the leaving sulfide ligand. Nevertheless,
the existence of any other sort of ordered interaction in
the molecule following this partial decoordination would
produce the same effect (see below).
When the substitution process was studied with
platinum complex solutions that were not freshly pre-
pared, the absorbance-time traces showed a consistent
complex behavior. Furthermore, the solutions of the
dimethyl sulfide complex showed, on standing, sets of
1
new signals in the H NMR spectrum that correspond
to a new Pt(II) cyclometalated compound, as indicated
by the much larger platinum coupling constant of the
CH proton (now at 8.79 ppm), 122.2 Hz; after long
periods all Pt(IV) complex signals disappear. These
results would be expected if a reductive elimination has
taken place on the quasi-labile Pt(IV) organometallic
complex;7 in any case the simple process would require
the elimination of a biphenyl molecule that could not
be easily detected by the proton NMR spectrum.
(8) Anal. Calcd (found) for PtC22H22BrNS: C, 42.5 (43.5); H, 4.4 (3.7);
N, 2.4 (2.3). X-ray data: Mr ) 607.47, space group P21/c, monoclinic,
a ) 10.2770(10) Å, b ) 11.0820(10) Å, c ) 18.6390(10) Å, â ) 96.45°,
V ) 2109(4) Å3; T ) 293(2) K, Z ) 4, µ(Mo KR) ) 8.651 mm-1, 10 918
reflections measured, 4430 unique reflections (Rint ) 0.0432) which
were used in all calculations, final Rw(F2) ) 0.0786 (all data).
(9) Crespo, M.; Font-Bard´ıa, M.; Solans, X. Polyhedron 2002, 21,
105.
(7) (a) Crumpton, D. M.; Goldberg, K. I. J . Am. Chem. Soc. 2000,
122, 962. (b) Barlett, K. L.; Goldberg, K. I.; Borden, W. T. Organome-
tallics 2000, 20, 2669. (c) Puddephatt, R. J . Coord. Chem. Rev. 2001,
219-221, 157.

Communications
Organometallics, Vol. 21, No. 16, 2002 3307
Sch em e 1
On one hand, sequence A involves a real C6H4
insertion reaction into the Pt-C bond with a concerted
shift of a hydrogen to the reductive eliminating benzene.
Although insertion reactions in Pt-C bonds are rare and
limited,11,12 the transition state proposed involves a
hydrogen shift that could facilitate the reaction via a
concerted reductive elimination of benzene.13 This one-
step mechanism should be facilitated by a certain degree
of Pt-H interaction in the transition state, given the
important degree of positive charge density of the Pt-
(IV) center.14 On the other hand, sequence B seems, in
principle, much more feasible. After the reductive
elimination reaction of an imine-attached diphenyl
group, further oxidative addition of a C-H bond to the
Pt(II) center, followed by a fast reductive elimination
of benzene, produced the final compound.15 Neverthe-
less, such a C-H activation process does not seem very
likely, given the lack of planarity/aromaticity of the final
metallacycle formed. Only oxidative addition of C-H
bonds having the above-mentioned planarity/aromatic-
ity characteristics have been observed on these com-
plexes, even when the C-H bond has been further
activated by the presence of a fluorine in an ortho
position.5 Furthermore, the absence of any intermediate
Pt(II) species in solution that could be detected by low-
temperature NMR measurements indicates that the
C-H bond activation should be relatively fast.
From the results, and the images in Figure 1 and
Scheme 1, it is clear that, if SMe2 does not previously
dissociate, the structure is too rigid for such a sterically
demanding transition state. The fact that the corre-
sponding phosphine derivative [PtIVBr(Ph)2(CC5H3-
CHNBzl)(PPh3)] does not evolve in the same direction
is a clear indication that the feasibility of the dissocia-
tion of the sixth ligand is fundamental for the process.3
Further studies are under way in order to ascertain the
mechanism of the reaction observed; some degree of
interaction with the leaving hydrogen atom on the
complex, in a way that could be considered as having a
certain degree of hydride character, has been observed.16
The existence of this sort of interaction can also account
for the negative values found for the activation entropy
for the sulfide by the phosphine substitution reactions
studied (see above).
Ack n ow led gm en t. We acknowledge financial sup-
port for the BQU2001-3205 project from the Ministerio
de Ciencia y Tecnolog´ıa.
Su p p or tin g In for m a tion Ava ila ble: Listings of observed
rate constants for the substitution process studied, the mech-
anism and rate law operating for the reaction, and the crystal
structure data for the compounds [PtIVBr(Ph)2(CC5H3CHNBzl)-
(PPh3)] and [PtIIBr(CC5H4C6H4CHNBzl)(SMe2)]. This material
is available free of charge via the Internet at http://pubs.acs.org.
(10) Values of the observed rate constants are 2.7 × 10-4, 5.9 × 10-4
,
and 9.8 × 10-4 s-1 for [Pt(IV)] ) 2.5 × 10-4 M and [SMe2] ) 2.0 ×
10-3, 1.0 × 10-3, and 5.0 × 10-4 M, respectively; and 2.0 × 10-3, 1.5 ×
10-3, and 1.2 × 10-3 s-1 for [Pt(IV)] ) 2.5 × 10-4, 7.5 × 10-4, and 12.5
× 10-4 M, respectively, with no SMe2 added.
(11) (a) Stang, P. J .; Zhong, Z.; Arif, A. M. Organometallics 1992, 1,
1017. (b) Kayaki, Y.; Tsukamoto, H.; Kaneko, M.; Shimizu, I.; Yama-
moto, A.; Tachikawa, M.; Nakajima, T. J . Organomet. Chem. 2001,
622, 199.
OM020260K
(16) The preparation of [PtIIPh(CC5H4CHNBzl)(SMe2)] was carried
out in order to establish the possible insertion of the phenyl ring into
the PtII-C(cyclometalated) bond on addition of LiBr to the solution
medium. Neither in chloroform nor in acetone solution was evolution
of the compound to the possible [PtIIPh(CC5H4C6H4CHNBzl)(SMe2)]
observed. Nevertheless, the proton NMR spectrum of the complex [PtII-
(12) (a) de Felice, V.; de Renzi, A.; Tesauro, D.; Vitagliano, A.;
Organometallics 1992, 11, 3669. (b) Bergamini, P.; Costa, E.; Orpen,
A. G.; Pringle, P. G.; Smith, M. B. Organometallics 1995, 14, 3178. (c)
Yagyu, T.; Suzaki, Y.; Osakada, K. Organometallics 2002, 21, 2088.
(13) Reinarzt, S.; White, P. S.; Brookhart, M.; Templeton, J . L. J .
Am. Chem. Soc. 2001, 123, 12724.
Ph(CC5H4CHNBzl)(SMe2)] showed a signal at 9.41 ppm (dd, J H-H
1.5, 7.4 Hz) indicative of a certain through-space interaction between
the two platinum-bound aromatic rings.
)
(14) Fan, Y.; Hall, M. B. J . Chem. Soc., Dalton Trans. 2002, 713.
(15) Puddephatt, R. J . Angew. Chem., Int. Ed. 2002, 41, 261.