Facile arene C-H bond activation and alkane dehydrogenation with anionic LPtIIMe2- in hydrocarbon-water systems (L = dimethyldi(2-pyridyl)borate)

Article abstract of DOI:10.1021/ja0643570

Anionic dimethyldi(2-pyridyl)borato dimethylplatinum(II) complexes react vigorously in 3:1 RH/water mixtures to produce Pt^II aryl (RH = C₆H₆, para-F₂C₆H₄) or hydrido Pt^II olefin complexes (RH = cyclo-C_nH_2n; n = 5,6); the Na+ cation accelerates the reaction rates dramatically. Copyright

Full text of DOI:10.1021/ja0643570

Published on Web 09/16/2006
Facile Arene C-H Bond Activation and Alkane Dehydrogenation with Anionic
-
LPtIIMe2 in Hydrocarbon-Water Systems (L ) Dimethyldi(2-pyridyl)borate)
Eugene Khaskin, Peter Y. Zavalij, and Andrei N. Vedernikov*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
Received June 20, 2006; E-mail: avederni@umd.edu
Alkane CH bond activation with PtII complexes (Shilov chem-
Scheme 1
istry1) is a rapidly developing, practically important, and challenging
area of current research.2-5 Despite the fact that the original Shilov
systems included water as a solvent,1 much less coordinating organic
solvents3,5 ranging from neat hydrocarbons6 to trifluoroethanol
(TFE)7 were used later. Important reasons for this was that formation
of PtII alkane complexes (eq 1-a), suggested key intermediates in
C-H bond cleavage,8 was inhibited dramatically by water7 and by
products of alkane functionalization, such as alcohols or ethers (eq
1-b):9
Scheme 2
At the same time, subsequent functionalization of organoplatinum
intermediates resulting from hydrocarbon activation may require
more polar media, including water (solv ) H2O).1-2,4,10
A plausible solution to the problem could be the use of relatively
lipophilic L′PtII in biphasic hydrocarbon-water systems which
might favor the RH over H2O competition by extracting more
hydrophobic RH-Pt-derived species into the organic phase11 and/
or to use L′PtII(H2O) species prone to more facile dissociation of
the aqua ligand. In the course of our studies of anionic dipyridine
ligands,10,11 in order to develop water-tolerant systems for alkane
analytically pure diphenyl complex NaLPtPh2, 11 (12 h, 88%
functionalization, we chose to explore Pt complexes of the anionic
isolated yield). In contrast, no phenyl complexes formed after 12 h
with the same batch of benzene when the Bu4N analogue 8 was
- 12
n
dimethyl dipyridylborate ligand, dp-BMe2
,
1 (Scheme 1). This
choice was based, in part, on a number of recent reports3,5,11,13-15
documenting CH bond activation with PtIIMe complexes supported
by other anionic NN donors,3,5 including 2,13 3,14 4,15 and 5.11 Some
of these systems showed tolerance of few equivalents of water.8a
An important argument to study lipophilic Pt complexes derived
from 1 came from our DFT modeling studies. The calculated
standard Gibbs energy of dissociation of the aqua ligand from the
plausible LPtIIMe(OH2) intermediate, 13.4 kcal/mol, was lower than
15.3 kcal/mol for cis-PtCl2(OH2)2, the species responsible for
methane activation in neat water in the Shilov system.1
In this paper, we report facile stoichiometric arene and alkane
CH bond activation with ionic dipyridylborato PtII complexes
MLPtIIMe2 (M ) Na, nBu4N; Scheme 2), which occurs in the mono-
and biphasic hydrocarbon-water systems. Interestingly, the sodium
cation dramatically accelerated the rate of these reactions.
used instead. The important role of water in this unusually facile
cation-dependent “double” CH activation of benzene14,15 with the
anticipated reaction product, complex 9, was revealed in separate
experiments.
We found that complex 10 does not react with benzene dried
over the Na benzophenone adduct even after 3 weeks at room
temperature (Scheme 2-1). In contrast, when dry benzene was
combined with 10 and 3 equiv of water to produce a biphasic
system, a slow reaction occurred leading in 1 day to the diphenyl
complex 12 in >90% isolated yield (Scheme 2-2 and 2-3). The
identity of 12 was confirmed by X-ray diffraction (Figure 1).
Formation of nBu4NLPtPh(Me), 13, in high yield, was evident after
the first 7 h of the reaction (Scheme 2-2). Most remarkably, reaction
between the sodium analogue, NaLPtMe2, benzene, and 2 equiv of
H2O was complete in less than 5 min to produce 11 (90% isolated
yield, Scheme 2-4). Four isotopologues CHnD4-n (n ) 1-4) were
observed in the 10:8:7:7 ratio when C6D6/D2O was used instead.
Anionic ligand 1 in a form of sodium salt NaL, 7, was prepared
by reacting hydrogen dimethyldi(2-pyridyl)borate12 (6) with an
excess of sodium hydride in dry THF. Subsequent metathesis of 7
with nBuN4Br afforded the tetra-n-butylammonium analogue nBuN4L,
8. Corresponding dimethylplatinum(II) derivatives MLPtMe2, 9 (M
) Na) and 10 (M ) nBu4N), were synthesized using a ligand
exchange reaction between 7 or 8 and Pt2Me4(µ-SMe2)2 in dry THF
-
The effect of the Na+ cation on the reactivity of LPtMe2 was
n
confirmed in experiments with Bu4NLPtMe2, wet benzene, and
an additive of NaBArF , sodium tetrakis(3,5-bis(trifluoromethyl)-
4
phenyl)borate, a good source of “naked” electrophilic Na+.17 An
initially slow reaction could be brought to completion virtually
1
and were fully characterized by H and 13C NMR spectroscopy
immediately upon addition of 0.5 equiv of NaBArF . Further
4
and elemental analysis. Remarkably, when benzene, which was not
dried carefully enough, was used instead of THF in the attempted
synthesis of 9, a reaction occurred which produced along with free
Me2S a solid, sparingly soluble in PhH, which turned out to be an
experiments showed that the fast reaction between 9, PhH, and H2O
occurred in the organic phase.
Benzene saturated with water reacted with a solution of 9 in
PhH instantaneously to cleanly produce complex 11. Thus, the
9
13054
J. AM. CHEM. SOC. 2006, 128, 13054-13055
10.1021/ja0643570 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
Figure 1. ORTEP drawings (50% probability ellipsoids) of complex 12.
Hydrogen atoms are omitted for clarity.
-
homogeneous reaction between LPtMe2 and benzene could be
catalyzed with even trace amounts of water, whereas the Na+ ions
dramatically enhanced the catalytic effect. Similar to benzene, clean
CH bond activation with NaLPtMe2 in the presence of 3 equiv of
water to form NaLPtAr2 could be achieved with para-difluoroben-
zene (Ar ) 2,5-F2C6H3, 14, 92% isolated yield).
Importantly, NaLPtMe2 was also shown to carry out CH bond
activation in the presence of larger amounts of H2O, in biphasic
3/1 benzene/water systems. The reaction was complete in less than
2 min, but upon removal of all volatiles under vacuum, the solid
residue was found to be a mixture of diphenyl complex 11 (80%
yield) and a hydroxo phenyl complex NaLPtPh(OH), 15 (15%)
(Scheme 2-5). Longer reaction times, 10-12 min, led to the
disappearance of the kinetic product 11 and formation of 15
exclusively (>90%). The intermediacy of 11 was proven by reacting
a pure sample of this complex with a 3/1 PhH/H2O mixture which
produced 15 in >90% NMR yield after 10 min (Scheme 2-6).
Interestingly, 11 was stable in wet C6D6 or in pure D2O for at least
a few days, though showing complete H/D exchange between the
PtPh fragments and D2O in both cases after <15 h.
amounts of water could favor transformation of 11 to the thermo-
dynamic product 15.
To the best of our knowledge, a combination of high basicity of
the PtIIMe2 fragment,20 high reactivity of the related PtII transients
toward CH bonds, and tolerance of water is currently the unique
feature of the system presented here. In summary, PtII dipyridine
complexes can be electronically tuned for facile alkane and arene
CH bond activation in hydrocarbon-water systems.
Acknowledgment. We thank the University of Maryland, the
Donors of the American Chemical Society Petroleum Research
Fund, and NSF (CHE-0614798) for the financial support.
High reactivity of 9 toward arenes prompted us to test it in alkane
CH bond cleavage. When 3 equiv of water was added to a stirred
suspension of 9 in cyclohexane, a vigorous gas evolution occurred
at the water/cyclohexane interface. After removal of the solvent
under vacuum and extraction of the strongly alkaline residue with
cyclohexane, hydrido cyclohexene complex LPtH(cyclo-C6H10), 16,
was isolated in 42% yield (Scheme 2-7). Under the same conditions,
cyclopentane could be also dehydrogenated to produce LPtH(cyclo-
C5H8), 17, in 33% isolated yield. We suggest that poor solubility
of 9 in alkanes might be responsible for low yields. Finally, in 2:1
alkane:water mixtures, both substrates reacted with 9 to produce
LPtH(olefin) complexes in essentially the same yields as with 3
equiv of water. Dehydrogenation of various alkanes with PtIVR2H
and PtIVMe3 complexes is well established18 but, to the best of our
knowledge, was never observed in hydrocarbon-water systems.
The mechanism of the reaction between complex 9 and hydro-
carbons might involve protonation of anionic LPtIIMe2- with H2O
to form a very reactive lipophilic LPtIVMe2H which could be
efficiently extracted into the organic phase.11 In water-poor systems,
the Na+ ion could coordinate few equivalents of H2O, so suppress-
ing hydrolysis of Pt-C bonds, enhancing acidity of H2O, and thus
accelerating formation of LPtIVMe2H. According to our DFT
calculations,19 the CH reductive coupling of LPtIVMe2H is facile
(Scheme 3-1). Considering the MeH ligand in the LPtMe(MeH)
intermediate as a good leaving group, we suggest that MeH for
PhH (Scheme 3-2) and MeH for H2O substitution has similar
activation barriers, so that PhH can win a kinetic competition with
H2O for PtII in the organic phase, where the [PhH]/[H2O] ratio is
high. Subsequent benzene CH bond oxidative cleavage might lead
to LPtIVPh(Me)H, 18 (Scheme 3-3). 18 could lose H+ in a reaction
with OH-, producing LPtPh(Me)-, eliminate methane (Scheme 3-4),
or react with the second PhH and form LPtPh2- in a similar reaction
sequence.19 Similar energies of TSPhH and TS′MeH are consistent
with the observed multiple deuterium incorporation in the methane
liberated in reaction between 9 and C6D6 and involving 18. Finally,
the Na+ ion could enhance acidity of water in the organic phase
and accelerate formation of LPtR2H in water-rich biphasic systems,
too, since both 9 and 11 are benzene-soluble. The presence of large
Supporting Information Available: Experimental and computa-
tional details, additional discussion, and CIF file for 12. This material
is available free of charge via the Internet at http://pubs.acs.org.
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