Mechanism of the competition between phenyl insertion and ligand reductive elimination on a hindered platinum(IV) cyclometalated complex

Article abstract of DOI:10.1021/om060818e

Careful tuning of the reaction conditions has been proved to be essential for the operation of two disjunctive reaction mechanisms occurring on the ortho-metalated platinum compounds [Pt^IVBr(Ph)₂-(C ₅CH₄CHNZ)(SMe₂)] (Z = Me, Bzl, CH ₂(2,4,6-Me₃C₆H₂)). In dilute solutions, where the already established lability of the SMe₂ ligand favors the existence of the pentacoordinated {Pt^IVBr(Ph) ₂(C₄CH₄-CHNZ)} species, the complex evolves to produce the insertion of one of the phenyl ligands into the cyclometalated Pt^IV-C bond to yield the complexes [Pt^IIBr(C ₄CH₃C₆H₄CHNZ)(SMe₂)], which contain a seven-membered cyclometalated ring. The process involves the formation of an hydride intermediate, which has been detected via low-temperature proton NMR for Z = Bzl, prior to the reductive elimination of benzene. In more concentrated solutions, or in the presence of large amounts (200-500 fold) of SMe₂, the existence of pentacoordinated species is reduced to a minimum and a reductive C-C coupling takes place between the metalated imine carbon and one of the phenyl ligands, yielding the coordination complexes [Pt^IIBr(Ph)(C₆H₃C₆H ₄CHNZ)(SMe₂)], which evolve rapidly to trans-[Pt ^IIBr(Ph)(SMe₂)₂] and free C₅H ₃C₆H₄CHNZ. The validity of the mechanisms proposed has been proved via stoichiometric and reactivity studies carried out under carefully controlled conditions, both on initial Pt^IV complex and on the final inserted complex, [Pt^IIBr(C₄CH ₃C₆H₄CHNZ)(SMe₂)]. The overall reactivity is rather surprising, given the generally accepted dissociative processes involved in the preliminary steps of reductive elimination reactions on Pt^IV complexes. DFT calculations have been carried out in order to check the energetic validity of the proposed disjunctive reaction mechanisms. From the data obtained, it is clear that the formation of the pentacoordinated species {Pt^IVBr(Ph)₂(C₅CH₄CHNZ)} could effectively lead to the standard C-C reductive coupling, but in our case the existence of the parallel insertion process is highly favored. As a consequence the observed reductive elimination reaction can only occur via the otherwise less favored direct C-C coupling on the octahedral [Pt ^IVBr(Ph)₂(C₄CH₄CHNZ)(SMe ₂)] starting material.

Full text of DOI:10.1021/om060818e

Organometallics 2007, 26, 527-537
527
Mechanism of the Competition between Phenyl Insertion and
Ligand Reductive Elimination on a Hindered Platinum(IV)
Cyclometalated Complex
Carlos Gallego,† Manuel Mart´ınez,*,† and Vicent Sixte Safont‡
Departament de Qu´ımica Inorga`nica, UniVersitat de Barcelona, Mart´ı i Franque`s 1-11,
E-08028 Barcelona, Spain, and Departament de Cie`ncies Experimentals, UniVersitat Jaume I,
Campus del Riu Sec, E-12080 Castello´, Spain
ReceiVed September 7, 2006
Careful tuning of the reaction conditions has been proved to be essential for the operation of two
disjunctive reaction mechanisms occurring on the ortho-metalated platinum compounds [PtIVBr(Ph)2-
(C5CH4CHNZ)(SMe2)] (Z ) Me, Bzl, CH2(2,4,6-Me3C6H2)). In dilute solutions, where the already
established lability of the SMe2 ligand favors the existence of the pentacoordinated {PtIVBr(Ph)2(C4CH4-
CHNZ)} species, the complex evolves to produce the insertion of one of the phenyl ligands into the
cyclometalated PtIV-C bond to yield the complexes [PtIIBr(C4CH3C6H4CHNZ)(SMe2)], which contain a
seven-membered cyclometalated ring. The process involves the formation of an hydride intermediate,
which has been detected via low-temperature proton NMR for Z ) Bzl, prior to the reductive elimination
of benzene. In more concentrated solutions, or in the presence of large amounts (200-500 fold) of SMe2,
the existence of pentacoordinated species is reduced to a minimum and a reductive C-C coupling takes
place between the metalated imine carbon and one of the phenyl ligands, yielding the coordination
complexes [PtIIBr(Ph)(C6H3C6H4CHNZ)(SMe2)], which evolve rapidly to trans-[PtIIBr(Ph)(SMe2)2] and
free C5H3C6H4CHNZ. The validity of the mechanisms proposed has been proved via stoichiometric and
reactivity studies carried out under carefully controlled conditions, both on initial PtIV complex and on
the final inserted complex, [PtIIBr(C4CH3C6H4CHNZ)(SMe2)]. The overall reactivity is rather surprising,
given the generally accepted dissociative processes involved in the preliminary steps of reductive
elimination reactions on PtIV complexes. DFT calculations have been carried out in order to check the
energetic validity of the proposed disjunctive reaction mechanisms. From the data obtained, it is clear
that the formation of the pentacoordinated species {PtIVBr(Ph)2(C5CH4CHNZ)} could effectively lead to
the standard C-C reductive coupling, but in our case the existence of the parallel insertion process is
highly favored. As a consequence the observed reductive elimination reaction can only occur via the
otherwise less favored direct C-C coupling on the octahedral [PtIVBr(Ph)2(C4CH4CHNZ)(SMe2)] starting
material.
and reductive elimination reactions to and from Pt(II) square-
planar centers with more than one Pt-C bond.11-17 Neverthe-
less, it has been recently demonstrated that the existence of such
an intermediate of lower coordination number is not absolutely
necessary for these reactions and that the nature of the inert
skeleton on the platinum center (namely the nature and rigidity
of the donors) plays a crucial role in the process.18,19 In this
respect, we have been involved in the study of the associativity-
dissociativity tuning of the substitution reactions of organome-
Introduction
The chemistry on octahedral organometallic complexes of
Pt(IV) is very often related exclusively to reductive elimination-
oxidative addition processes.1-5 Although substitution reactions
are generally linked to such processes, very few studies have
been carried out in that field.6-10 The existence of an intermedi-
ate species with a lower coordination number has been
repetitively established as crucial for both the oxidative addition
† Universitat de Barcelona.
‡ Universitat Jaume I.
(11) Puddephatt, R. J. Angew. Chem., Int. Ed. 2002, 41, 261-263.
(12) Felk, U.; Zahl, A.; van Eldik, R. Organometallics 1999, 18, 4156-
4164.
(1) Canty, A. J.; Rodemann, T. Inorg. Chem. Commun. 2003, 6, 1382-
1384.
(2) Puddephatt, R. J. Coord. Chem. ReV. 2001, 219-221, 157-185.
(3) Rendina, L. M.; Puddephatt, R. J. Chem. ReV. 1997, 97, 1735-1754.
(4) Crespo, M.; Mart´ınez, M.; de Pablo, E. J. Chem. Soc., Dalton Trans.
1997, 1231-1235.
(13) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995,
14, 4966-4968.
(14) Procelewska, J.; Zhal, A.; Liehr, G.; van Eldik, R.; Smythe, N. A.;
Williams, B. S.; Goldberg, K. I. Inorg. Chem. 2005, 44, 7732-7742.
(15) Arthur, K. L.; Wang, Q. L.; Bregel, D. M.; Smythe, N. A.; O’Neil,
B. A.; Goldberg, K. I.; Moloy, K. G. Organometallics 2005, 24, 4624-
4628.
(5) Crespo, M.; Mart´ınez, M.; Sales, J. Organometallics 1992, 11, 1288-
1295.
(6) Frey, U.; Helm, L.; Merbach, A. E.; Romeo, R. J. Am. Chem. Soc.
1989, 111, 8161-8165.
(7) Tobe, M. L.; Burgess, J. Inorganic Reaction Mechanisms; Long-
man: Essex, U.K., 1999.
(16) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471-2526.
(17) Wik, B. J.; Ivanovic-Burmazovic, I.; Tilset, M.; van Eldik, R. Inorg.
Chem. 2006, 45, 3613-3621.
(8) Peloso, A. Coord. Chem. ReV. 1973, 10, 123-181.
(9) Dixon, N. E.; Lawrance, G. A.; Lay, P. A.; Sargeson, A. M. Inorg.
Chem. 1984, 23, 2940-2947.
(18) Crumpton, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125,
9442-9456.
(19) West, N. W.; Reinartz, S.; White, P. S.; Templenton, J. L. J. Am.
Chem. Soc. 2006, 128, 2059-2066.
(10) Drougge, L.; Elding, L. I. Inorg. Chim. Acta 1986, 121, 175-183.
10.1021/om060818e CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/23/2006

528 Organometallics, Vol. 26, No. 3, 2007
Gallego et al.
Scheme 1
process which involves the reductive elimination of a biphenyl
ligand and further cyclometalation reaction, as already proposed
for similar systems (Scheme 2).11,25-27 Despite the fact that
dimethyl sulfide dissociation was found to be a key step for
the stoichiometric process,24 no further description of the
intimate mechanism has been available. In this paper we present
a comprehensive study of the intimate reaction mechanism by
which the stoichiometric insertion reaction takes place in these
types of systems.
The process has been determined to involve a proper insertion
reaction, and the corresponding kinetic, thermal, and baric
activation parameters have been measured. Furthermore, the
existence of a parallel path, which effectively produces the
reductive elimination process of a biphenyl imine ligand,
mentioned in the previous communication, has been established.
In the latter case no further C-H bond activation has been
observed leading to the formation of seven-membered cyclo-
metalated compounds; this fact has also been confirmed by
stoichiometric studies. The complete reaction scheme is that
indicated in Scheme 3, where the two competitive reaction paths
are shown. The kinetic, thermal, and baric activation parameters
of the reductive elimination process have also been measured
for Z ) Bzl, Me. The ground-state energies corresponding to
the complexes with Z ) Me appearing in this scheme have
also been determined by DFT calculations, justifying the
processes observed. Although some calculations had been
carried out on nondissociative sp2-sp2 reductive elimination
reactions on Pt(IV) complexes, as distinct from the equivalent
sp3-sp3 processes,28 the possible presence of labile ligands
capable of generating a lower coordination number intermediate
had not been considered.
Scheme 2
The activation parameters corresponding to kins agree with a
substantial enthalpy requirement (∆Hq on the order of 100 kJ
mol-1). They also involve ordering demands very dependent
on both the iminic ligand and the solvent used (∆Sq ) 64 J
K-1 mol-1 for Z ) Bzl and 25 J K-1 mol-1 for Z ) Me in
acetone; ∆Sq ) 64 J K-1 mol-1 in acetone and -9 J K-1 mol-1
in chloroform for Z ) Bzl). Despite this fact, few changes in
the volume on going to the transition state (∆Vq being ∼0 for
all the systems studied) are observed. As for the reductive
elimination reactions, kred, they are characterized by values of
activation enthalpy and entropy very similar to those already
indicated, including an important expansion process on going
to the transition state (∆Vq in the 10-12 cm3 mol-1 range).
For Z ) Me, the final inserted or reductively eliminated
platinum complexes, 6Me and 5Me, indicated in Scheme 3,
represent an energy gain with respect to the starting compound
1Me, thus justifying the fine stoichiometric tuning of the
processes that favors the formation of the inserted 5Me complex.
The solution isomerization behavior (as referred to the relative
positions of the halide and metalated phenyl ring) of the
phosphine-substituted derivative of 5Bzl has also been fully
established, as for other systems of the same family.29,30 An
important tuning of steric origin, capable of overruling the
electronic trans effect of the cyclometalated phenyl ring, has
been established and a remarkable degree of stereodiscrimination
found to be operative. The sequence observed agrees with the
possible side products observed in the insertion/reductive
elimination full process indicated in Scheme 3.
tallic cyclometalated Pt(IV) complexes (Scheme 1).20 In general,
the systems behaved in a dissociatively (k1, k-1, k2) activated
quasi-labile way.21,22 The establishment of the first associatively
activated substitution reaction on an octahedral Pt(IV) complex
(k2′) has been achieved for the R ) Me, Y4 ) F4, L ) SMe2,
E ) PMePh2, PEtPh2, PPh3 systems.23
During the studies of the reaction depicted in Scheme 1 with
R ) Ph, Y4 ) H4, X ) Br, S ) SMe2, the surprising formation
of a seven-membered metallacycle via a formal insertion of a
phenyl ring in a cyclometalated Pt-Caryl bond has been
established. A preliminary communication has already ap-
peared.24 For the stoichiometric process, two possible pathways
could be operative, depending on the reaction either occurring
through a real one-step insertion reaction or via a two-step
Results and Discussion
Insertion Reaction. The insertion reaction (1Z f 5Z)
indicated in Scheme 3, already stoichiometrically established
for 1Bzl,24 has been checked and found operative for the

A Hindered Pt(IV) Cyclometalated Complex
Organometallics, Vol. 26, No. 3, 2007 529
Scheme 3
Chart 1
compounds 1Me and 1CH2Mes. The isolation of the final 5Z
compounds has not been pursued for these two new compounds;
NMR spectroscopy has been found to be a sufficient indicator
for the confirmation of processes taking place during the
reaction. In our previous studies about the formation of the
compound 5Bzl from 1Bzl,24 it was not clear if the reaction
proceeded through a simple kins path (Scheme 3) or through a
two-step process involving the reductive elimination reaction
(kred path, Scheme 3), followed by a further C-H oxidative
addition-reductive elimination of benzene process on the
unobserved compound 6Bzl.25 In this respect, the reaction of
the new diphenylimine 2-(C6H5)C6H4CHNBzl, PhBzl, with the
compounds [{Pt(Me)2(µ-SMe2)}2], cis-[Pt(Ph)2(SMe2)2], and
trans-[PtBrPh(SMe2)2] (7) has been tried in order to establish
its reactivity with complexes that could, potentially, produce
the desired seven-membered metallacycle observed in the
complex 5Bzl. In all cases the product obtained from these
reactions did not correspond to the formation of the seven-
membered metallacycle. When the complexes [{Pt(Me)2(µ-
SMe2)}2] and cis-[Pt(Ph)2(SMe2)2] were used as starting ma-
terials, elimination of methane or benzene occurred, respectively,
with a simultaneous activation of the C-H bond in the 2′
position of the iminic phenyl ring. That is, the formation of the
standard five-membered metallacycle indicated in Chart 1 took
place.5,26,27,31
The formation of such five-membered cyclometalated com-
plexes has been demonstrated by the 1H 200 MHz NMR
spectrum of the final reaction mixture of the reaction of the
diphenyl 2-(C6H5)C6H4CHNBzl imine, PhBzl, with the afore-
mentioned starting materials. The spectra showed a set of signals
corresponding to a standard five-membered metallacycle: the
NCH proton appears at 8.64 ppm (JPtH ) 57 Hz) or at 8.65
ppm (JPtH ) 44 Hz), and the NCH2 protons appear at 5.13 ppm
or at 4.38 and 4.92 ppm (JHH ) 12 Hz), for the reaction with
[{Pt(Me)2(µ-SMe2)}2] or cis-[Pt(Ph)2(SMe2)2], respectively. In
both cases the NCH signal at 8.79 ppm (JPtH ) 122.2 Hz) of
the seven-membered metallacycle in 5Bzl is absent; that is, no
C-H bond activation of the distant phenyl ring has taken place.
Reaction of the same diphenylimine ligand with trans-[PtBrPh-
(SMe2)2] (7) did not produce better results. Under the same
reaction conditions, even the coordination of the iminic nitrogen
to the Pt(II) center was not observed. Probably the lack of a
double statistical reductive elimination process that favors
cyclometalation in [{Pt(Me)2(µ-SMe2)}2] or cis-[Pt(Ph)2(SMe2)2]
prevents the advance of the reaction with such a hindered imine.
Furthermore, the presence of only one Pt-C bond in compound
(20) Bernhardt, P. V.; Gallego, C.; Mart´ınez, M.; Parella, T. Inorg. Chem.
2002, 41, 1747-1754.
(21) Bernhardt, P. V.; Gallego, C.; Mart´ınez, M. Organometallics 2000,
19, 4862-4869.
(22) Esteban, J.; Font-Bardia, M.; Gallego, C.; Gonza´lez, G.; Mart´ınez,
M.; Solans, X. Inorg. Chim. Acta 2003, 351, 269-277.
(23) Gallego, C.; Gonza´lez, G.; Mart´ınez, M.; Merbach, A. E. Organo-
metallics 2004, 23, 2434-2438.
(24) Font-Bard´ıa, M.; Gallego, C.; Mart´ınez, M.; Solans, X. Organo-
metallics 2002, 21, 3305-3307.
(25) Crespo, M.; Font-Bard´ıa, M.; Solans, X. Organometallics 2004, 23,
1708-1713.
(26) Crespo, M.; Evangelio, E. J. Organomet. Chem. 2004, 689, 1956-
1964.
(27) Crespo, M.; Evangelio, E.; Font-Bard´ıa, M.; Pe´rez, S.; Solans, X.
Polyhedron 2003, 22, 3363-3369.
(28) Ananikov, V. P.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc.
2002, 124, 2839-2852.
(29) Font-Bard´ıa, M.; Gallego, C.; Gonza´lez, G.; Mart´ınez, M.; Merbach,
A. E.; Solans, X. Dalton Trans. 2003, 1106-1113.
(30) Craig, A.; Crespo, M.; Font-Bardia, M.; Klein, A.; Solans, X. J.
Organomet. Chem. 2000, 601, 22-33.
(31) Crespo, M.; Mart´ınez, M.; Sales, J. Organometallics 1993, 12,
4297-4304.

530 Organometallics, Vol. 26, No. 3, 2007
Gallego et al.
7 forces the actuation of an associatively activated substitution
mechanism6,32,33 of SMe2 by PhBzl, the full process being very
disfavored with the bulky imine ligand. Figure S1 (Supporting
Information) collects the H NMR spectral monitoring of the
reactivity involved.
should then produce compound 7. Figure S1 also collects the
1H NMR spectral monitoring of the reactivity involved.
It is interesting to note that the triphenylphosphine derivatives
of complexes 1Z do not undergo such processes on the same
time scale. As expected from Scheme 3, the 31P NMR of a
solution of 1Bzl with a 4-fold excess of PPh3 does not show
the presence of any insertion product, 5Bzl, as indicated by the
lack of a signal at 18.7 ppm, corresponding to the opening of
the insertion product (see Seven-Membered Ring Opening).
Furthermore, after 4 days at room temperature only 5-10% of
the initial 1Bzl complex evolved, via reductive elimination, to
trans-[PtBrPh(PPh3)2], as indicated by its 31P NMR signal at
22.8 ppm with JPtP ) 3129 Hz. Probably the dramatic differ-
ences between the thioether and phosphine ligands account for
this fact. Not only is the better donor character of the phosphine
ligand bound to disfavor any reductive elimination process from
the metal center but also the bulkiness of the ligand induces a
more rigid orientation of the phenyl groups, thus disfavoring
the highly sterically demanding reductive elimination reac-
tion.
Seven-Membered Ring Opening. Decyclometalation of
complex 5Bzl, via substitutive decoordination of the iminic
nitrogen by SMe2, does not occur on the isolated complex.24
This lack of reactivity is surprising, given the important trans
inductive effect on the iminic nitrogen in the final ligand
distribution on the crystallized 5Bzl complex. The lack of
planarity observed in these systems can account for this fact.
Nevertheless, during all the insertion processes depicted in
Scheme 3 and studied under moderate [SMe2]added conditions,
an extra Z-dependent platinum-coupled signal around 2.75-
2.85 ppm appears in all cases (2.77 ppm, JPtH ) 52 Hz for Z )
Bzl; 2.86 ppm, JPtH ) 52 Hz for Z ) Me; 2.81 ppm, JPtH ) 52
Hz for Z ) CH2Mes). The position and coupling of the signal
correspond to the existence of a trans-[PtBr(C-substituted Ph)-
(SMe2)2] type of species.36 In order to establish the stability of
the formed seven-membered metallacycle in 5Z, the sulfide by
triphenyl phosphine substitution has been studied in full detail
for the 5Bzl compound. The process resulted in a rather
complicated sequence of changes in the 31P NMR spectra. The
speculative series of isomerization reactions involved are
depicted in Scheme 4; these are in fact consistent with
the structure found for complexes with bulkier DMSO and
PPh3 ligands in seven-membered metallacycle Pt(IV) com-
plexes.25,37
1
It is interesting to note that the cyclometalated Pt(IV)
complexes 1Z are a mixture of the two possible isomers,
showing a mer (Ph/Ph/SMe2) and fac (Ph/Ph/SMe2) distribution,
the latter being that indicated in Scheme 3 (see the Experimental
Section). The mer/fac isomerization process is found to be slow
enough at room temperature to make the two complexes
distinguishable by NMR spectroscopy.24,29 The facile intercon-
version of the pentacoordinated species formed on sulfide
decoordination, 2Z, can be held responsible for the isomerization
process.34 Theoretical calculations (see Calculations) indicate
that the energy demand of the transition state of the mer (Ph/
Ph/0) a fac (Ph/Ph/0) interconversion process between pen-
tacoordinated species, 2Z, is low. The free energy value in the
gas phase is only 8.72 kJ mol-1 for the mer to fac and 6.09 kJ
mol-1 for the fac to mer reactions for the 2Me complex, thus
favoring by 2.63 kJ mol-1 the mer isomeric form. In view of
the existence of these isomeric mixtures and the different
substitution labilities already observed for the mer (Me/Me/
SMe2) and fac (Me/Me/SMe2) complexes,29 some experiments
on the insertion reaction were carried out in order to determine
a possible isomeric discrimination for any of the reactions
indicated in Scheme 3. In all cases monitoring of the 1H NMR
spectra during the process did not indicate such discrimination,
the mer (Ph/Ph/SMe2) to fac (Ph/Ph/SMe2) isomeric ratio being
constant within experimental error during all the processes
leading to the final 5Z complexes.
Reductive Elimination Reaction. Some experiments run at
high [SMe2]added were designed in order to prevent the formation
of the expected pentacoordinated species occurring in solution
(Scheme 3).20,21 The results indicated that the insertion reaction
from 1Z to 5Z complexes is not only considerably retarded but
also even totally avoided when high dimethyl sulfide concentra-
tions are used (ca. (200-500) × [Pt]). The same effect was
observed whenever the experiments were run at high platinum
complex concentration, which has the effect of increasing the
presence of non-sulfide dissociated 1Z complex in the reaction
medium. The 1H NMR monitoring of the aforementioned
experiments shows a dominant signal at 2.35 ppm (JPtH ) 59
Hz), corresponding to the SMe2 ligand in trans-[PtBr(Ph)-
(SMe2)2] (7), while the NCH and NCH2 proton signals showed
also the presence of important amounts of free diphenylimine
(8.36 and 4.72 ppm for PhBzl; see the Experimental Section).
The formation of 7 and free diphenylimines, PhZ, has to occur
via a reductive elimination process of the cyclometalated ligand
and of one of the coordinated phenyl groups on hexacoordinated
1Z, to produce the tetracoordinated 6Z complexes (Scheme 3).
The process is, in fact, fairly similar to that described for similar
sp2-sp2 reductive coupling in intermediate Pt(IV) complexes
detected for some tetraphenylene-generating reactions.35 The
already established preferential coordination of SMe2 versus the
bulky PhZ biphenylimines on the Pt(II) complex (see above),
plus the stoichiometric excess of SMe2 in the reaction medium,
When an stoichiometric amount of PPh3 is added to the
complex 5Bzl, a set of two 31P NMR signals at 16.5 ppm (JPtP
) 1832 Hz) and at 15.5 ppm (JPtP ) 1888 Hz) appear in the
first spectrum recorded. The signals should thus correspond to
a coupled isomerization/substitution of the dimethyl sulfide
substitution product, as evaluated from the position and platinum
coupling of the signal, which indicates a cis (N/P) geometrical
distribution.38 During a period of 30-60 min at room temper-
ature the intensity of the signal at 16.5 ppm diminishes in favor
of that at 15.5 ppm, already reported in the synthetic procedure
published.24 The two complexes must have the same basic
geometrical arrangement, given the extremely similar magnetic
environments of the phosphine. Probably the rigidity of the
cyclometalated ring and the bulkiness of the phosphine ligand
force the separation of the two possible ring-based isomers, not
(32) Romeo, R.; Plutino, M. R.; Scolaro, L. M.; Stoccoro, S.; Minghetti,
G. Inorg. Chem. 2000, 39, 4749-4755.
(33) Alibrandi, G.; Bruno, G.; Lanza, S.; Minniti, D.; Romeo, R.; Tobe,
M. L. Inorg. Chem. 1987, 26, 185-190.
(36) Hadj-Bagheri, M.; Puddephatt, R. J. Polyhedron 1988, 7, 2695-
2702.
(34) Wilkins, R. G. Kinetics and Mechanisms of Reactions of Transition
Metal Complexes; VCH: Weinheim, Germany, 1991.
(35) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc.
1998, 120, 2843-2853.
(37) Capape´, A.; Crespo, M.; Granell, J.; Vizcarro, A.; Zafrilla, J.; Font-
Bard, M.; Solans, X. Chem. Commun. 2006, 4128-4130.
(38) Crespo, M.; Solans, X.; Font-Bard´ıa, M. J. Organomet. Chem. 1996,
105-113.

A Hindered Pt(IV) Cyclometalated Complex
Organometallics, Vol. 26, No. 3, 2007 531
Scheme 4
observed in the SMe2 starting material due to a fluxional
conformation movement. In this respect, the hindered rotation
or sulfur inversion of the SMe2 plane in 5Bzl results in a very
wide 1H NMR signal at room temperature (Figure S1),29 which
prevents a more complete study on other possible fluxional
processes taking place simultaneously. During a period of days
at room temperature the signal at 15.5 ppm disappears and a
new signal at 12.7 ppm (JPtP ) 4319 Hz) becomes dominant;
the high value of the coupling constant agrees with a trans (N/
P) disposition in the complex and the transphobia concept.25,39,40
If an excess of phosphine is added to the same reaction mixture,
another intense signal at 18.7 ppm (JPtP ) 3156 Hz) appears.
This signal is associated with a decyclometalated trans-bis-
(phosphine) organometallic complex, from comparison with the
spectrum of trans-[PtBr(Ph)(PPh3)2] (22.7 ppm (JPtP ) 3129
Hz)), obtained from the reaction of PPh3 with trans-[PtBr(Ph)-
(SMe2)2] (7).
To summarize, the rigidity of the cyclometalated ring and
the bulkiness of the PPh3 ligand enable the NMR observation
of the movement between two cycle-based isomeric forms of
the diphenylphosphine derivative of 5Bzl. These evolve finally
to the stable cyclometalated complex with the same trans (Br/
C) configuration as 5Bzl. The good Lewis base characteristics
of the PPh3 ligand enable a PPh3 decyclometalation process,
not observed by SMe2 in 5Bzl. Although this substitution on
the final isolated 5Bzl is not observed in the presence of large
amounts of SMe2, the intermediate derivatives mentioned above
could be tentatively held responsible for the opening of the
chelate ring via a Pt-N bond cleavage by SMe2 in any of them.
The signal appearing at ca. 2.75-2.85 ppm can thus be
associated with the presence of the putative species trans-[PtBr-
(2-(CC5H4)-2-C6H4CHNZ)(SMe)2], formed from some transient
isomer of 5Bzl produced on SMe2 association to the tricoordi-
nated intermediate, 4Z, in Scheme 3.
Kinetics and Mechanism. (a) Insertion Process. When the
monitoring of the insertion reaction from compounds 1Z to 5Z
(Z ) Bzl, Me) is carried out at different increasing platinum or
added dimethyl sulfide concentrations, an important retardation
effect is observed, as already reported.24 Figure 1a is a clear
indication of this behavior; from the inverse plots in Figure 1b
it is obvious that the limiting values of kobs at low dimethyl
sulfide or platinum complex concentrations are the same within
error, which indicates that the rate acceleration due to the
decrease of both concentrations has the same basis. Parallel 1H
NMR monitoring of the processes observed indicated that the
UV-vis quantified reaction corresponds to the stoichiometric
processes indicated in the previous sections.
Figure 1. (a, left) Plot of the dependence of the observed rate constant for the insertion reaction for compounds 1Bzl to produce 5Bzl on
the concentration of (b) platinum complex ([SMe2]added ) 0) or (O) added SMe2 ([Pt] ) 2.0 × 10-4 M). (b) Inverse plot of the same
dependence. Conditions: acetone solution, 313 K.

532 Organometallics, Vol. 26, No. 3, 2007
Gallego et al.
Figure 3. Solvent dependence of the thermal activation parameters
for the insertion reaction depicted in Scheme 3 occurring on
compound 1Bzl: (9) acetone solution; (b) chloroform solution.
Figure 2. Plot of the dependence of 1/kobs on [SMe2] according to
eq 1 for the insertion reaction of compound 1Bzl in acetone solution
at different temperatures and platinum complex concentrations: (2)
[Pt] ) 1.25 × 10-4 M; (b) [Pt] ) 2.50 × 10-4 M).
Table 1. Kinetic and Thermal and Baric Activation
Parameters for the Rate Constants Indicated in Scheme 3
calculated values for K1 for the 1Bzl complex derived from the
slope/intercept ratio are in the (0.6-1.4) × 10-3 M range for
the systems studied, which, together with the values of k1,
represent a good indication of the fast preequilibrium nature of
the SMe2 dissociation. No further discussion can be carried out,
given the high errors involved in the determination.
The platinum concentration dependence of kobs (Figure 1a)
also indicates that, as expected, only at relatively high concen-
trations of platinum complex does the amount of free dimethyl
sulfide not increase significantly on decreasing the complex
concentration. The good empirical agreement of the values of
kobs at platinum concentrations of about (1-3) × 10-4 M,
without SMe2 added, with the values determined for the
reciprocal of the intercept in the 1/kobs versus [SMe2] plots (see
Figure S2 and Table S1, Supporting Information) enabled us
to reduce the amount of kinetic experiment runs. From the
temperature and pressure dependence of the value of kins, the
thermal and baric activation parameters associated with the
insertion rate constant were determined and are also collected
in Table 1. The values agree with a highly enthalpic process
taking place, with little compression or expansion to go to the
transition state, despite the definite positive (deorganization)
value found for ∆Sq for some of the systems. The differences
indicated in Table 1 related to the solvent used, although not
large, are evident. Definite differences are observed when Z
changes from Bzl to Me (Figure 3 collects graphically some of
these observations). The trend is the same as that observed for
previously studied substitution reactions on these complexes and
is related to the higher tendency of the benzylic protons to
promote hydrogen bonding with acetone.20 In this way an
increase in dissociation is produced on going to the transition
state, where the platinum center has a less acidic character.
a From ref 24.
In this respect, Figure 2 clearly shows that the choice of high
platinum complex concentrations is not needed to evaluate the
limiting value for kobs, provided [SMe2]added is kept high enough.
Our previous knowledge about substitution reactions on these
types of complexes allowed us to relate this behavior with the
establishment of a relatively fast dimethyl sulfide dissociation
prequilibrium process.22,29 The dissociation rate constants (k1),
as well as their thermal activation parameters, have already been
established for the complex 1Bzl and are collected in Table 1.
According to the reaction mechanism proposed in Scheme 3,
eq 1 gives the rate law expected. For the insertion process
The mechanism involved in the process could go through a
fully concerted transition state, such as that indicated in Chart
2a, or via the intermediate formation of an hydride complex
3Z, indicated in Chart 2b, resembling that proposed in the
literature as oxidative hydrogen (phenyl, in our case) migration.41
This hydride complex should rapidly (in a non-rate-determining
step) undergo a reductive elimination of benzene to produce
the final 5Z complex, after association with SMe2. The entropy
requirements involving the fully concerted transition state
occurring at low [SMe2], the equation is further simplified, as
indicated. The values of kins derived from the plots of 1/kobs
versus [SMe2] (Figure 2) are also collected in Table 1. The
(39) Cuevas, J. V.; Garc´ıa-Herbosa, G.; Miguel, D.; Mun˜oz, A. Inorg.
Chem. Commun. 2002, 5, 340-343.
(40) Vicente, J.; Abad, J. A.; Frankland, A.; Ram´ırez de Arellano, M.
C. Chem. Eur. J. 1999, 5, 3066-3074.
(41) Oxgaard, J.; Muller, R. P.; Goddard, W. A.; Periana, R. A. J. Am.
Chem. Soc. 2004, 126, 352-363.

A Hindered Pt(IV) Cyclometalated Complex
Organometallics, Vol. 26, No. 3, 2007 533
Chart 2
An alternative mechanism could involve the direct reductive
elimination of benzene from one of the phenyl ligands and a
proton from the other, producing a benzyne ligand PtII inter-
mediate. Evolution of this complex by insertion of the alkyne
into the cyclometalated Pt-C bond is plausible. Nevertheless,
the observation of the hydride complex stated before leads us
to believe that the existence of the putative benzyne derivative
is not probable.
(b) Reductive Elimination Process. The reproducibility of
the absorbance versus time traces for the insertion process is
very good, but when the added dimethyl sulfide concentration
increases considerably, the kinetic profiles show a more complex
behavior, with an important induction period and much smaller
absorbance changes. When [SMe2] increases further, the ab-
sorbance changes completely invert from those previously
observed (Figure 4). Even in some cases, as expected from eq
1, composite diphasic absorbance versus time traces are obtained
in experiments run at intermediate [SMe2], which were conse-
quently avoided for rate measuring. Room-temperature 1H NMR
monitoring of the changes occurring at these high SMe2
concentrations indicated that no insertion process is taking place.
Instead, the appearance of compound 7 is taking place via the
reductive elimination plus substitution process indicated in
Scheme 3 (Figure S1b). The values of kobs determined for these
processes, collected in Table S1, clearly indicated that the rate
of the process is independent of the concentration of added
dimethyl sulfide at the studied platinum complex concentrations.
This agrees with the reaction mechanism proposed in Scheme
3 and the very simple rate law kred ) kobs, obtained from eq 1
at high [SMe2]. The reductive elimination rate constants and
the corresponding thermal and baric activation parameters
derived from their temperature and pressure dependence are also
collected in Table 1. The process involves both an important
expansion (positive ∆Vq) and enthalpic demand to go to the
transition state, while ∆Sq values are small and positive; similar
results have been obtained for other reductive elimination
processes.12,17,18,47 The solvent dependence for this process has
only been checked for the Z ) Bzl system. The effect, if existent
(Figure S4, Supporting Information), is much smaller than that
observed for the insertion processes and only affects activation
entropies, which indicates a certain involvement of the acetone
solvent in the process. The large errors involved make any deep
discussion meaningless. Although these observations are in good
agreement with a transition state such as that indicated in Chart
2c, the reductive elimination of the diphenylimine from the
hydride complex 3Z, indicated in Chart 2b, is a plausible
alternative for the process. The second possibility can, neverthe-
less, be disregarded in view of the phenomenology of the SMe2
dependence of the processes; in the absence of added dimethyl
sulfide the reductive elimination process should also be ob-
served, and this is not the case.
indicated in Chart 2a needing to be highly negative,42 the known
stability of Pt(IV)-H bonds,2 and the coordinatively unsaturated
nature of the 2Z species prompted us to believe that the process
involves the two-step process mentioned above.41 If, as found
for other systems,17,43 the enthalpy requirement for the formation
of the Pt-H bond is much lower than that needed for the R-H
reductive elimination, the platinum hydride should exist for a
longer period of time at very low temperatures, despite any
thermodynamic requirement. Effectively, the cooling of a 1 ×
10-4 M sample of 1Bzl in acetone solution at -70 °C gave a
signal at -29.6 ppm in a 300 MHz NMR spectrometer that
was tentatively assigned to complex 3Z, in good agreement with
the position of the signals observed for Pt(IV)-H compounds.44
195Pt satellites could not be observed, given the extremely low
concentration of the hydride (it represents only a small fraction
of the complex, the concentration of which is at the 3 × 10-4
M level) and the CSA relaxation expected for compounds
containing Pt-H bonds.45,46 The thermodynamic requirements
of the system are made clear by the fact that the signal is lost
after longer periods of time, even at low temperature.
The volumes and entropies of activation are expected to have
both positive and negative contributions, indicating that the
slightly positive (∆Sq) or zero values (∆Vq) observed are
reasonable. In this respect, the transition state leading to the
formation of the putative hydride intermediate, indicated in Chart
2b, should also produce an important differential charge
generation responsible for the differences observed when
chloroform and acetone are used as solvents. Those are mainly
related to activation entropies and are more important for the
complexes with the bulkier benzyl-substituted iminic ligand.
Nevertheless, the uniformity of the mechanism involved is clear
from the ∆Hq versus ∆Sq compensation plot, which produces
an isokinetic temperature of ca. 60 °C (Figure S3, Supporting
Information).
Calculations. The complexes involved in the reactions
indicated in Scheme 3, shown in Figure 5, have been calculated
at the B3LYP/LANL2DZ theoretical level for Z ) Me. Table
S2 collects the energetic parameters obtained in the gas phase,
and Cartesian coordinates of the optimized structures are
reported in Table S3 (Supporting Information). Given the solvent
effect observed in some of the processes studied, we have also
conducted single-point calculations on the gas-phase optimized
structures obtained, by means of the polarizable continuum
model methodology, using chloroform and acetone as solvents.
We have not considered direct bonding of discrete solvent
(42) Mart´ınez, M.; Muller, G.; Panyella, D.; Rocamora, M. Organome-
tallics 1995, 14, 5552-5560.
(43) Wik, B. J.; Lersch, M.; Krivokapic, A.; Tilset, M. J. Am. Chem.
Soc. 2006, 128, 2682-2896.
(44) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc. 2002, 124,
12116-12117.
(45) Claridge, T. D. W. High-resolution NMR Techniques in Organic
Chemistry; Pergamon: Oxford, U.K., 1999.
(46) Ismail, I. M.; Kerrison, S. J. S.; Sadler, P. J. Polyhedron 1982, 1,
57-59.
(47) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961-5976.

534 Organometallics, Vol. 26, No. 3, 2007
Gallego et al.
Figure 4. UV-vis spectral changes observed at 370 nm of an acetone solution of compound 1Bzl with [SMe2]added ) 0 M (left, 303 K)
or [SMe2]added ) 2.25 × 10-2 M (right, 293 K).
Figure 5. Theoretically calculated structures for the complexes involved in Scheme 3 and Figure 6 (see Table 2).
Table 2. Total Free Energies (G) and Relative Free
Energies (∆G) in the Gas Phase in Chloroform and in
Acetone for the Indicated Structuresa
extra stabilization of the species and changes between sol-
vents.48,49
From these results, it is concluded that the theoretically
calculated energetic parameters favor the insertion process (1Me
f 2Me f 3Me f 4Me f 5Me; Scheme 3, Figure 6), given
the fact that the seven-membered metallacyclic compound 5Me
is the most stable. Hence, under thermodynamically controlled
conditions, its formation its predicted to predominate. The
reductive elimination process yielding the mixture of products
PhMe and 7 (1Me f 6Me f PhMe + 7; Scheme 3, Figure 6)
will be less favored, as 6Me is predicted to have a higher
stability than the final imine dissociated product. Experimentally,
though, PhBzl does not react with compound 7, as indicated
before, and complex 7 is the species detected in the final reaction
mixture. The need for an associatively activated substitution
process to take place in such complexes with only one metal-
carbon bond22 and the bulkiness of the iminic ligands can be
easily held responsible for the 7 + PhMe lack of reactivity via
a prohibitively high energetic transition state for the formation
of such a complex. Nevertheless, if the calculations are correct,
∆Gb/
Gc/hartree ∆Gc/kJ Gd/hartree ∆Gd/kJ
kJ mol-1 (particle)-1 mol-1 (particle)-1 mol-1
1Me + SMe2
2Me + 2 SMe2
3Me + 2 SMe2
0
-1139.78
0
-1139.79
0
-12.17 -1139.76
-0.33 -1139.77
52.94 -1139.77
14.11 -1139.76
47.48
16.05
4Me + 2 SMe2 + C6H6 -124.24 -1139.79 -33.94 -1139.81 -45.78
5Me + SMe2 + C6H6 -134.60 -1139.82 -92.52 -1139.83 -99.57
6Me + SMe2
-88.85 -1139.80 -63.05 -1139.82 -64.35
-42.52 -1139.79 -25.71 -1139.80 -28.43
-75.78 -1139.79 -13.85 -1139.80 -24.07
54.55 -1139.74 114.55 -1139.75 108.83
PhMe + 7
8Mee + 2SMe2
TS2-8 + 2 SMe2
2Mefac + 2 SMe2
TS2fac-2mer + 2 SMe2
2Memer + 2 SMe2
0
-1139.76
0
-1139.77
0
6.09 -1139.76
2.04 -1139.77
2.67
-2.63 -1139.76 -4.99 -1139.78 -5.26
a Extra molecules of SMe2 an/or C6H6 have been added to all structures,
except for the seventh entry, in order to properly compare the energies of
all entries. b Gas phase. c Chloroform. d Acetone. e 8Me is the species
resulting from SMe2 elimination from 6Me (see Figure 6).
molecules to a Pt orbital; the theoretical level used and the
system size prevents such calculations to be made in a
reasonable time. The results are reported in Table 2, where no
meaningful differences due to the solvent are detected. Only
the consideration of possible hydrogen bonding including
discrete solvent molecules in the system is bound to produce
(48) Aullo´n, G.; Bernhardt, P. V.; Bozoglia´n, F.; Font-Bard´ıa, M.;
Macpherson, B. P.; Mart´ınez, M.; Rodriguez, C.; Solans, X. Inorg. Chem.
2006, 45, 8551-8562.
(49) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry;
Wiley-VCH: Weinheim, Germany, 2003.

A Hindered Pt(IV) Cyclometalated Complex
Organometallics, Vol. 26, No. 3, 2007 535
Figure 6. Energetic pathways (green) for the insertion/reductive elimination processes from complex 1Me in acetone solution. Full points
indicate species either kinetically or spectrometrically detected, and empty points refer to calculated-only species. The red line represents
the standard path expected for the reductive elimination processes on organometallic Pt(IV) complexes.
complex 6Me should be the end point of the reductive
elimination sequence.
8Me, and final reductive elimination of benzene to give 4Me,
after final SMe2 addition). Complexes 6Z are not available
synthetically from the reaction of 7 and PhZ, and seven-
membered-ring cyclometalation from the transient 6Z cannot
be ruled out if substitution of imine by sulfide competes in 6Z
complexes with C-H oxidative addition. However, these types
of processes have been shown not to take place experimentally;
instead, the preferential obtention of five-membered metalla-
cycles occurs (Chart 1).
This disagreement between the theoretical results and the
experimental observations can be attributed to the weakness of
the Pt-S bond compared with the Pt-N bond, as described by
the calculations using the double-ς pseudo-orbital basis set
LanL2DZ. On going from (1Me + SMe2) to (2Me + 2 SMe2),
only a small increase of 55.44 kJ mol-1 in the potential energy
in the gas phase is obtained, corresponding to the Pt-S bond
breaking in 1Me (Table S2). This demand is largely compen-
sated by the entropic factor that favors the process (∆G )
-12.17 kJ mol-1); in solution this term is less important and
the enthalpy dominates the ∆G values in chloroform (+52.94
kJ mol-1) and acetone (+47.48 kJ mol-1) solutions. Further-
more, although the calculated Pt-S bond distance in 5Me is
2.44 Å, much larger than the experimental value of 2.25 Å
determined24 and close to the values found for similar Pt(IV)
systems,20,21,29 the calculated Pt-N distance is 2.03 Å, very close
to the value found in its crystal structure (2.04 Å),24 confirming
the aforementioned fact. As a whole, these discrepancies could
be easily related to the π-acid character of the S atom and seem
to indicate that the Pt(II) complexes 5Me and 6Me could be
considerably more stable than the calculations carried out. The
effect ought to be even more important for complex 7, given
the fact that two Pt(II)-S bonds are present in this compound.
The consequence is that the energy of complexes 5Me, 6Me,
and 7 indicated in Figure 6 represent a high limit and that the
relative values of 6Me and 7 could be reversed, as observed in
the experimental results.
A simpler alternative path corresponds to a one-step process
involving benzene elimination coupled with seven-membered-
ring metalation on the hexacoordinate 1Me complex, with a
transition state similar to that indicated in Chart 2a, giving 5Me.
The transition structure (TS1-5) for this path should account
for two coupled Pt-Caryl bonds being broken, with a concomi-
tant H migration from a phenyl ligand, that will form part of
the final PhMe, to the other phenyl ligand that leaves the
complex as benzene. A reduction from six- to four-coordination
of the platinum center occurs simultaneously with this Pt(IV)
to Pt(II) reduction. The complex fine tuning demanded for this
TS to take place can preclude its formation, and all attempts
made to locate such a delicate TS have been unsuccessful.
Furthermore, the stoichiometric [SMe2]added dependence ob-
served for the overall process and the NMR detection of 3Bzl
agree with the nonviability of this process (see above).
As for the reductive elimination process, the formation of
6Me from 1Me occurs through TS1-6 that should describe the
cleavage of two Pt-Caryl bonds, the formation of a new Caryl
-
Caryl bond, plus the reduction from 6 to 4 in the coordination
sphere of platinum. However, this transition state is much
simpler than the one described in the preceding paragraph and
we have not been able to locate it. The equivalent TS involving
the SMe2-dissociated derivatives, 2Me to 8Me (TS2-8, Figure
S5 and Table 2), has been localized instead, as found for the
majority of reductive elimination processes studied on Pt(IV)
complexes.14 From this perspective, it is conceivable that the
process 1Me f 6Me corresponds, in fact, to a 1Me f 2Me f
With reference to the insertion process, from 1Me the
formation of 2Me is not the thermodynamically preferred path,
which could make the insertion process difficult in an important
way. From 1Me, the formation of 6Me is thermodynamically
more favorable. From this complex, compound 5Me can be
achieved directly via C-H bond activation to produce a seven-
membered-ring Pt(IV) hydride octahedral intermediate followed
by benzene elimination (or via its SMe2-decoordinated analogue,

536 Organometallics, Vol. 26, No. 3, 2007
Gallego et al.
formation from two freely rotating metalated phenyl rings is
detected in the reaction medium; the rigidity of the cyclometa-
lated ring is clearly responsible for this entropic fact. Although
DFT calculations pose a reasonable free energy barrier of ca.
110 kJ mol-1 for the standard reductive elimination from
pentacoordinated 2Z (middle step in the 1Z f 2Z f 8Z f 6Z
sequence in Figure 6) only that occurring from 1Z has been
observed (1Z f 6Z sequence). There must be a much more
favorable energetic balance promoting the 1Z f 2Z f 3Z f
4Z f 5Z insertion sequence (Figure 6) from complex 2Z that
is only available in the complexes studied.
In summary, results indicate that the 1Z f 6Z reaction path
is available with the complexes used, probably due to entropic
factors, while the insertion 2Z f 5Z reaction path directly
competes in these complexes with the 2Z f 6Z reductive
elimination path, observed in most of the literature reports.
Experimental Section
Instruments. NMR spectra were recorded with a Varian XL-
200, a Bruker 250 DRX, and a Varian Unity 300 Plus instrument.
1
Chemical shifts (in ppm) were measured relative to SiMe4 for H
and to TMP for 31P; the solvents used were acetone (d6) or
chloroform (d1). All spectra were obtained in the Unitat de RMN
d’Alt Camp de la Universitat de Barcelona at room temperature,
unless stated.
Compounds. The noncyclometalated complexes [{Pt(Me)2(µ-
SMe2)}2] and cis-[Pt(Ph)2(SMe2)2] have been prepared according
Figure 7. Structures for the mer (Ph/Ph/0) a fac (Ph/Ph/0)
interconversion process between the 2Me pentacoordinated species.
1
to the published procedures;50,51 their H NMR characterization
agreed with the values previously reported. The complex trans-
[PtBr(Ph)(SMe2)2] has been prepared from trans-[Pt(Br)2(SMe2)2]
according to the standard procedure involving reaction with LiPh.
8Me f 6Me path (Figure 6), followed by 6Me f PhMe + 7
to the reductive elimination, competing with 1Me f 2Me f
3Me f 4Me f 5Me leading to the insertion process. The
stoichiometric and kinetic evidence indicated above do not agree
with this sequence; both processes are expected to be slowed
down, or even stopped, in with the presence of large amounts
of added SMe2, and this is not the case.
The values found for its proton NMR spectrum (2.35 ppm, JPtH
)
59 Hz) are very similar to those indicated36 for the analogous trans-
[PtCl(Ph)(SMe2)2] and agree with the expected for its formulation;
therefore, further characterization was not pursued.
The 2-BrC6H4CHNZ imines used in this study have been
prepared according to the standard aldehyde plus amine condensa-
tion reaction widely applied.52 For Z ) Bzl the proton NMR
spectrum agrees with that published, while for Z ) Me and Z )
CH2Mes the new imines have proton NMR spectra that agree with
the expected formulation: δ 3.59 [3H], 8.66 [1H] ppm for Z )
Me and δ 2.29 [3H], 2.44 [6H], 4.91 [2H], 8.75 [1H] ppm for Z )
CH2Mes. The new 2-(C6H5)C6H4CHNBzl imine PhBzl has been
prepared by the same procedure from 2-(C6H5)C6H4COH and H2-
NBzl: δ 4.72 [2H], 8.36 [1H] ppm.
Finally, as indicated before, the transition state of the mer
(Ph/Ph/0) a fac (Ph/Ph/0) interconversion process between
2Me pentacoordinated species, expected to be low in energy,
has been calculated. This transition structure (TS2fac-2mer; Figure
7, Table 2) and the results obtained agree, this time very well,
with the experimental trend showing the mer isomer to be
slightly more stable than that having a fac conformation.
Conclusions. The disjunctive insertion/reductive elimination
process indicated in Scheme 3 and in Figure 6 has been proved
from a kinetic, thermodynamic, and stoichiometric study. The
reactivity observed is surprising in view of the generally
accepted fact that pentacoordinate complexes are needed for
reductive elimination reactions on organometallic Pt(IV) com-
pounds. The only reductive elimination processes studied so
far occurring on fully octahedral Pt(IV) complexes correspond
to C-H and H-H couplings. Some sp2-sp2 reductive elimina-
tion systems have also been seen to react directly from
octahedral complexes; nevertheless, the lack of labile ligands
in the coordination sphere of the Pt(IV) center and the laterality
of the study do not allow for comparison with ours. Our systems
undergo C-C reductive coupling from the hexacoordinate
compounds 1Z, while the parallel insertion reaction takes place
from the pentacoordinate intermediates 2Z. It is clear that
important differences in reaction mechanisms can be induced
from changes in steric and electronic tuning of the metal center.
For the reactions studied, the C-C coupling observed always
takes place between a freely rotating phenyl group and a very
rigid phenyl ring of a cyclometalated imine. No diphenyl
The cyclometalated complexes 1Bzl, 1Me, and 1CH2Mes have
been prepared by oxidative addition of the C-Br bond of the
corresponding 2-BrC6H4CHNZ imines to the cis-[Pt(Ph)2(SMe2)2]
1
complex.21,24,25 Their H NMR spectra agree with the formulation
expected (Table S4); in all cases the proton NMR spectra of the
complexes indicated the coexistence of mer (Ph/Ph/SMe2) and fac
(Ph/Ph/SMe2) isomeric forms of the compounds in the reaction
mixture.20,29 No elemental analyses were carried out on these
complexes, given their high reactivity to produce immediate parallel
insertion and reductive elimination reactions, which prevented the
existence of a 100% pure sample of the dimethyl sulfide derivatives.
Nevertheless, the substituted triphenylphosphine derivative of 1Bzl
has already been described and its X-ray crystal structure deter-
mined.24
(50) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643-
1648.
(51) Rashidi, M.; Fakhroeian, Z.; Puddephatt, R. J. J. Organomet. Chem.
1991, 406, 261-267.
(52) Go´mez, M.; Granell, J.; Mart´ınez, M. Inorg. Chem. Commun. 2002,
5, 67-70.

A Hindered Pt(IV) Cyclometalated Complex
Organometallics, Vol. 26, No. 3, 2007 537
The seven-membered cyclometalated inserted complex 5Bzl has
already been fully characterized and described.24 The corresponding
5Me and 5CH2Mes complexes have been prepared by the same
method, and their 1H NMR spectra (Table S4) show the distinctive
resonance of the NCH proton around 8 ppm with a very large JPtH
value of about 120 Hz. No further characterization was pursued,
given the simultaneous presence in the reaction medium of the free
PhZ imine, trans-[PtBr(Ph)(SMe)2], and the putative trans-[PtBr-
(2-(CC5H4)2-C6H4CHNZ)(SMe)2] complexes (see Results and
Discussion).
basis set LanL2DZ, in which Pt, S, and P atoms are represented
by the relativistic core LanL2 potential of Los Alamos,57 was used.
Solvent effects were taken into account by means of polarized
continuum model calculations58,59 using standard options. The
energies of solvation were computed in chloroform (ꢀ ) 4.9) and
acetone (ꢀ ) 20.7) at the geometries optimized in the gas phase.
Acknowledgment. We acknowledge financial support of the
CTQ2006-14909-C02-02/BQU and BQU2003-04168-C03-03
projects from the Ministerio de Educacio´n y Ciencia and the
Fundacio´ Bancaixa-UJI (project P1-1B2005-15). We also thank
the SerVei d’Informa`tica and the Departament de Cie`ncies
Experimentals of the Universitat Jaume I for computer facilities.
Kinetics. The reactions were followed for the 1Bzl and 1Me
complexes by UV-vis spectroscopy in the 500-330 nm range,
where none of the solvents absorb. Atmospheric pressure runs were
recorded on an HP8452A or Cary50 instrument equipped with a
thermostated multicell transport. Observed rate constants were
derived from absorbance versus time traces at the wavelengths
where a maximum increase and/or decrease of absorbance was
observed. For runs at variable pressure, a previously described
pressurizing system and pillbox cell was used;53 the system was
connected to a J&M TIDAS spectrophotometer, which was used
for the absorbance measurements. No dependence of the values of
the observed rate constants on the selected wavelengths was
detected, as expected for reactions where a good retention of
isosbestic points is observed. The general kinetic technique is that
previously described.4,20,24 When SMe2 was added to the reaction
mixture, pseudo-first-order conditions were maintained; but in some
cases (see Results and Discussion) no dimethyl sulfide was added
to the reaction mixture. The platinum complex concentration was
maintained at a minimum of 2.5 × 10-4 M (unless stated) to avoid
undesired decomposition reactions and platinum complex rate
dependence, both caused by the preequilibrium dissociative reaction
described.21 Table S1 collects all the obtained kobs values for all
the systems studied as a function of the starting complex, process
studied, platinum and dimethyl sulfide concentrations, solvent,
pressure, and temperature. All post-run fittings were carried out
by the standard available commercial programs.
Supporting Information Available: Table S1, giving the
observed rate constants for the systems studied as a function of
starting material, SMe2 concentration, solvent, temperature, and
pressure, Table S2, giving energies calculated at the B3LYP/
LANL2DZ level in the gas phase for the compounds studied, Table
S3, giving Cartesian coordinates for all the optimized structures of
1
the species involved in this work, Table S4, giving H NMR data
for all of the new complexes prepared, Figure S1, showing changes
1
occurring in the SMe2 signal zone of the H NMR spectra for the
reaction of 1Bzl, Figure S2, giving a plot of the dependence of
1/kobs on [SMe2] for the insertion reaction of compound 1Bzl, Figure
S3, giving a ∆Hq versus ∆Sq compensation plot, Figure S4, showing
the solvent dependence of the thermal activation parameters for
the reductive elimination occurring on compound 1Bzl, and Figure
S5, showing the theoretically calculated structure for the complex
8Me and for TS2-8. This material is available free of charge via
the Internet at http://pubs.acs.org.
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