Aryl-bromide reductive elimination from an isolated Pt(iv) complex

Article abstract of DOI:10.1039/c001487g

A Pt(iv) complex bearing two aryl and two bromo ligands, which undergoes selective elimination of a bromoarene molecule has been prepared and fully-characterized. The mechanistic studies of this reaction are presented.

Full text of DOI:10.1039/c001487g

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Aryl-bromide reductive elimination from an isolated Pt(IV) complexw
Anette Yahav-Levi,a Israel Goldberg,a Arkadi Vigalok*a and Andrei N. Vedernikov*b
Received 22nd January 2010, Accepted 9th April 2010
First published as an Advance Article on the web 16th April 2010
DOI: 10.1039/c001487g
A Pt(IV) complex bearing two aryl and two bromo ligands, which
undergoes selective elimination of a bromoarene molecule has
been prepared and fully-characterized. The mechanistic studies
of this reaction are presented.
dominating undesired formation of C–C bonds and to increase
the likelihood of the C–Br reductive elimination, we decided to
replace one of the aryl ligands with C6F5, that forms stronger
bonds to transition metal centers.11 We also decided to utilize
the commercially-available quinoxaline-based ligand—(R,R)-
(ꢀ)-2,3-bis(tert-butylmethylphosphino)quinoxaline—that
would decrease the electron density at the metal center and
increase the steric strain while maintaining the necessary
rigidity. Reacting this ligand with (COD)Pt(4-FC6H4)C6F5
(COD = 1,5-cyclooctadiene) cleanly produced the diaryl
complex 1 (Scheme 2). As expected, the 31P{1H} NMR
Late transition metal-mediated formation of carbon–halogen
bonds is becoming increasingly important for the design of
new synthetic methods.1 It is well established that a combi-
nation of a late transition metal center with a source of an
electrophilic halogen can result in the catalytic halogenation of
aromatic compounds that bear pendant chelating groups.2,3
Only recently the examples of carbon–halogen bond forma-
tion from isolated metal complexes started to appear in the
literature mostly involving palladium complexes.4–7 Although
Pt(IV) compounds have historically been crucial to the under-
standing of some mechanisms of transition metal-mediated
C–X bond forming reactions, all examples reported thus far
involved the formation of carbon–iodine bonds.4,8 Herein, we
report the first example of a far more challenging direct C–Br
reductive elimination of an aryl bromide from an isolated
diaryl dibromo Pt(IV) complex, a type of reaction that was
previously presumed to be involved in brominolysis of some
diaryl Pt(II) complexes.9
spectrum of 1 showed two signals at 28.72 (br m, JPtP
=
2441.9 Hz) and 31.19 (br d, JPtP = 1649.6 Hz) due to two
inequivalent phosphine ligands. Interestingly, the 19F{1H}
spectrum of 1 showed five different signals due to the fluoro
substituents in the pentafluorophenyl group, indicating restricted
rotation around the metal–carbon bond. The X-ray structure
of 1 showed the square planar arrangement around the Pt(II)
center with the Pt–C bonds being nearly identical in length
(2.082(9) A). Both Pt–P distances were also found to be
identical (2.281(2) A).
Complex 1 undergoes clean oxidative addition of Br2 in
CH2Cl2 solution to give the Pt(IV) complex 2. Notably, this
reaction is reversible in more polar solvents (vide infra). The
31P{1H} NMR spectrum of 2 exhibits two signals at ꢀ6.33 ppm
(JPtP = 929.1 Hz) and ꢀ4.34 ppm (JPtP = 1404.9 Hz), which is
more than thirty ppm upfield compared with 1. The X-ray
structure of 2z shows that the two bromo atoms occupy the
apical positions of the octahedron with the Pt–Br bonds being
ca. 2.48 A. Interestingly, although the Pt–C bond distances of
ca. 2.113 A remained fairly equal (albeit longer than in 1), the
Pt–P bonds are now inequivalent with the Pt–P4 (trans to
the 4-FC6H4 group) bond being significantly longer than the
Pt–P5 bond, 2.462(3) A vs. 2.396(3) A, respectively.
We recently reported octahedral Pt(IV) complexes bearing
two aryl and two iodo ligands, the latter being in the mutual
trans-position, that can undergo a selective C–I reductive
elimination of an aryl iodide (Scheme 1).10 The thermodynami-
cally more stable cis-isomeric complexes undergo exclusive
C–C reductive elimination reaction. Based on these results, we
were interested in applying this strategy for C–X bond forma-
tion to significantly less reactive lighter halogens X. However,
although the trans-dibromo analogues of the complexes shown
in Scheme 1 could be prepared readily, they failed to exhibit
the C–Br bond elimination reactivity.9 To suppress the
Importantly, heating 2 in CH3CN for 24 h at 80 1C resulted
in its complete conversion to the Pt(II) complex 3 with the
concomitant formation of p-FC6H4Br as the major organic
product (yield 80–90%) along with 10–20% of fluorobenzene
(Scheme 2). The rate of disappearance of 2 followed first-order
kinetics with t1/2 being ca. 12 h at 70 1C, which corresponds to
the Gibbs activation energy of 27.7 kcal molꢀ1. To the best of
our knowledge, this is the first example of a directly observed
reductive elimination of a bromoarene from a Pt(IV) complex.
The reaction rate was only slightly affected by light or presence
of water additives. For example, a 50% conversion of 2 was
observed after 5 h of reaction at 80 1C under bright light of a
100 W incandescent lamp vs. 40% conversion observed in the
dark. Addition of 10% of water to an acetonitrile solution of 2
did not result in any significant increase in the reaction rate.
Addition of 10 equiv. n-Bu4NBr to the reaction mixture did
Scheme 1
a School of Chemistry, The Raymond and Beverly Sackler Faculty of
Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel.
E-mail: avigal@post.tau.ac.il; Fax: +972-3-6409293;
Tel: +972-3-6408617
b Department of Chemistry and Biochemistry, University of Maryland,
College Park, MD 20742, USA. E-mail: avederni@umd.edu;
Tel: +1-301-405-2784
w Electronic supplementary information (ESI) available: Synthesis
and characterization of compounds 1–3. CCDC 763113–763115. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c001487g
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Scheme 2
not slow down the overall reaction rate significantly, although
the fraction of p-FC6H4Br noticeably decreased whereas the
amount of PhF, the C–H elimination byproduct, increased.
Interestingly, performing the reaction in acetone, DMSO or
NMP resulted in the unexpected reductive elimination of Br2
and formation of 1. Continued heating of those solutions led
to the formation of complex 3, p-FC6H4Br and substantial
amounts of fluorobenzene, the C–H elimination product. The
formation of some fluorobenzene byproduct in MeCN can be
explained by partial elimination of Br2 (vide infra), bromi-
nation of the reactive solvent (acetone, DMSO, NMP but not
much so MeCN) to produce HBr and a subsequent reaction of
the latter with complex 1 leading to PhF and complex 3.
The possible mechanisms of the reactions described here
were studied using the high-level DFT calculations. Two
possible pathways were analyzed for the Ar–Br elimination
in acetonitrile solutions (Scheme 3): the path a involving a
concerted p-fluorophenyl and bromide ligands C–Br coupling
from neutral species 2 via the transition state TSa and the path
b involving dissociation of one of the bromide ligands from 2
to form the cationic five-coordinate intermediate 4 along
with a free bromide anion. The intermediate 4 undergoes a
subsequent C–Br coupling via the transition state TSb leading
ultimately to complex 3 and p-FC6H4Br. The solvation of 2, 4
and both transition states in acetonitrile was modeled using a
Poisson–Boltzmann continuum solvation model (PBF) imple-
mented in the Jaguar program12 package. For bromide
anion an experimental value of the Gibbs energy of solva-
tion of ꢀ69.2 kcal molꢀ1 13 was used (the calculated value is
ꢀ75.0 kcal molꢀ1). The results are presented in Scheme 3. The
path a includes an ion-pair like transition state TSa with both
Pt–Br bonds being significantly stretched with the energy of
36.8 kcal molꢀ1, which is noticeably higher than the experi-
mental value of 27.7 kcal molꢀ1. Note that a mechanism similar
to the one in the path a was also studied computationally and
was shown to be consistent with experimental observations in
the case of an aryl C–I reductive elimination from trans-di-
(p-fluorophenyl) diiodo Pt(IV) 1,2-bis(dimethylphosphino)benzene
complex 5 (Fig. 1).10 A concerted mechanism for C–Br elimi-
nation of vinyl bromides was also suggested previously for
some octahedral dibromo Pd(IV) complexes 6 (Fig. 1).14
The path b includes a pair of free ions, 4 and Brꢀ, with the
Gibbs energy of 20.4 kcal molꢀ1 and the low energy transition
state TSb, 24.8 kcal molꢀ1. Assuming that the solvation energy
of cationic complex 4 is slightly overestimated by the PBF
model as it is seen, for instance, in the case of bromide anion,
the TSb energy should be higher than 24.8 kcal molꢀ1 and,
hence, more consistent with the experimental Gibbs activation
energy value of 27.7 kcal molꢀ1. A decrease of fraction of
Fig. 1 d6 Metal complexes undergoing concerted Ar–X reductive
Scheme 3
elimination.
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p-FC6H4Br caused by n-Bu4NBr additives is consistent with
the path b,15 whereas it is difficult to account for this effect in
terms of the concerted mechanism in the path a. Interestingly,
reductive elimination of an Ar–I from the iodo-analogue 5 in
DMF solution was not sensitive to the presence of n-Bu4NI
suggesting that 5 reacts via a concerted path analogous to the
path a.10 Since formation of 1 from 2 was observed in some of
our experiments, a computational analysis of the Ar–Br
elimination from 1 as a result of a direct electrophilic attack
of free Br2 at an aryl carbon atom in 1 was attempted but no
such reaction path could be found.
1 (a) A. Vigalok, Chem.–Eur. J., 2008, 14, 5102; (b) T. D. Sheppard,
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14374.
Finally, we also studied computationally Br2 elimination
from 2. This reaction is only slightly uphill (Gibbs energy
change is 1.7 kcal molꢀ1). A homolytic loss of one of the
bromine atoms from 2 is prohibitively endergonic with the
reaction Gibbs energy of 39.8 kcal molꢀ1. At the same time, a
heterolytic mechanism with an intermediacy of 4 (and, possibly,
a derived solvento complex) and Brꢀ favored by polar solvents
and a subsequent virtually barrierless attack of Brꢀ at the
bromo ligand in 4 allows one to account for facile formation
of 1 and Br2 elimination from 2 in acetone, DMSO or NMP
and in the presence of additives of n-Bu4NBr.16
7 (a) S. R. Whitfield and M. S. Sanford, J. Am. Chem. Soc., 2007,
129, 15142; (b) T. Furuya and T. Ritter, J. Am. Chem. Soc., 2008,
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A. L. Rheingold, Polyhedron, 2002, 21, 2357.
9 A. Yahav-Levi, I. Goldberg and A. Vigalok, J. Am. Chem. Soc.,
2006, 128, 8710.
In conclusion, we have demonstrated the first example of an
aryl bromide C–Br reductive elimination from an isolated
Pt(IV) complex. The reaction probably proceeds via the forma-
tion of a cationic five-coordinate Pt(IV) intermediate 4.17 This
mechanism is different from the mechanism of concerted
reductive elimination of aryl iodides from Pt(IV) center suggested
by us previously for similar complex 5. Interestingly, the
pentacoordinate intermediate 4 also participates in the reduc-
tive elimination of Br2, making the product distribution highly
dependent on the reaction conditions.
10 A. Yahav-Levi, I. Goldberg, A. Vigalok and A. N. Vedernikov,
J. Am. Chem. Soc., 2008, 130, 724.
11 This bond can still participate in the reductive elimination
chemistry: (a) A. Kaspi, A. Yahav-Levi, I. Goldberg and
A. Vigalok, Inorg. Chem., 2008, 47, 5; (b) T. Koizumi,
A. Yamazaki and T. Yamamoto, Dalton Trans., 2008, 3949.
12 Schrodinger Inc., JAGUAR, Version 7.6, Schrodinger Inc., Portland,
OR, 2009.
13 (a) Y. Marcus, Ion Solvation, Wiley, Chichester, 1985;
(b) J. Richardi, P. H. Fries and H. Krienke, J. Chem. Phys.,
1998, 108, 4079.
We acknowledge the support from US–Israel Binational
Science Foundation. A. Y. thanks the Levi Eshkol Founda-
tion for the fellowship.
14 R. van Belzen, C. J. Elsevier, A. Dedieu, N. Veldman and
A. L. Spek, Organometallics, 2003, 22, 722.
Notes and references
15 An additive of n-Bu4N+Brꢀ would decrease
concentration of 4 and suppress formation of p-FC6H4Br
a
steady-state
z X-Ray structure data for 2: C30H32Br2F6N2P2Pt, M = 951.43, 0.3 ꢂ
0.15 ꢂ 0.15 mm, orthorhombic, space group P212121, a = 10.3414(2),
b = 12.3660(3), c = 30.6670(7) A, a = b = g = 901, V = 3921.75(15) A3,
Z = 4, rcalcd. = 1.611 g cmꢀ3, ymax = 27.701, Nonius KappaCCD,
and 3.
16 An additive of n-Bu4N+Brꢀ would decrease
a steady-state
concentration of 4 but accelerate the subsequent nucleophilic
attack of Brꢀ at the bromo ligand 4 in the equal extent leading
to a net zero order in [Brꢀ] for the rate for Br2 elimination
from 2.
MoKa radiation (l
= 0.71073 A), graphite monochromator,
T = 110(2) K, 22 362 collected reflections, 8896 unique reflections
(Rint = 0.0760). R1 = 0.0532, wR2 = 0.1384 for data with I 4 2s(I),
and R1 = 0.0884, wR2 = 0.1524 for all unique data. The lattice
contains also severely disordered pentane solvent, as indicated by
several residual electron-density peaks (within 1.0–1.7 e Aꢀ3) present
in areas between molecules of the Pt-compound. The pentane solvent
could not be modeled by discrete atoms, and was therefore excluded
from the structural model.
17 Such cationic intermediates have been shown to be highly reactive
in reductive elimination reactions from Pt(IV) complexes, see for
example: (a) J. Procelewska, A. Zahl, G. Liehr, R. van Eldik,
N. A. Smythe, B. S. Williams and K. I. Goldberg, Inorg. Chem.,
2005, 44, 7732; (b) G. S. Hill and R. J. Puddephatt, Organometallics,
1998, 17, 1478.
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3326 | Chem. Commun., 2010, 46, 3324–3326