Room temperature reversible C-H activation mediated by a Pt(0) center, and stoichiometric biphenyl formation via solvent activation

Article abstract of DOI:10.1039/c2cc33847e

Room temperature reversible C-H activation mediated by a designed diphosphine platinum complex is presented. These findings are demonstrated through mechanistic studies involving kinetics, isotopic effects, and corroborated by DFT calculations. The coupling between two unactivated aromatic derivatives is also demonstrated.

Full text of DOI:10.1039/c2cc33847e

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Room Temperature Reversible C–H Activation Mediated by a Pt(0)
center, and Stoichiometric Biphenyl Formation via Solvent Activation
Emmanuel Nicolasa, Xavier-Frederic Le Goffa, Stéphane Bouchonnetb and Nicolas Mézailles*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Room temperature reversible C–H activation mediated by a
designed diphosphine platinum complex is presented. These
findings are demonstrated through mechanistic studies
involving kinetics, isotopic effects, and corroborated by DFT
10 calculations. The coupling between two unactivated aromatic
derivatives is also demonstrated.
C–H bond activation/functionalization is amongst the major
challenges of nowadays chemistry, and in this field, the reaction
of Pt centers with hydrocarbons have been extensively studied,
15 following the discovery made by Shilov in the early 70s.1,2 Most
studies have been performed on Pt(II)/Pt(IV) systems, such as
Periana’s system for the functionalization of methane in hot,
concentrated sulphuric acid.3,4 In parallel, numerous mechanistic
studies have been performed on these oxidized systems to probe
20 the different elementary steps of the mechanisms of C–H bond
activation and functionalization.5–12 On the other hand, Pt(0)
fragments have been studied by very few groups, despite
significant early findings. In particular, Halpern described in
1978 the ability of the complex [(PPh3)2Pt(H)(CH3)] to readily
25 eliminate methane at low temperature (-25°C), generating the
transient 14 electron complex that could be trapped with
triphenylphosphine to form the stable 16 electron Pt(0)
complex.13 A few years later, Whitesides and Ibers described the
thermolysis of the [(dcpe)Pt(H)(neopentyl)] complex (dcpe: bis-
30 dicyclohexylphosphinoethane), leading to a high energy L2Pt(0)
fragment, capable of activating C–H bonds of alkanes (in low
yield) or aromatics (quantitatively).14,9,15 These early findings
showed that the energetic demand of the oxidative addition of C–
H bonds is linked to the geometry of the 14 electron L2Pt(0)
35 fragment. In correlation, MO computations have predicted that a
small bite angle, obtained with a bidentate ligand, can lead to an
increased reactivity of the L2Pt(0) fragment (Scheme 1). We
reasoned that careful choice of the bite angle of a diphosphine
ligand should result in the stabilization of the L2Pt(0), while
40 keeping its ability to activate C–H bonds. Tuning the electronics
and sterics of the ligand should then allow both complexes to be
in equilibrium, opening ways for further transformations.
Scheme 1 Strategy of CH activation
in the reversibility of the insertion at room temperature, proved
50 by kinetic experiments. Secondly, the Pt(II) hydride complex was
shown to further activate C–H bonds of the solvent leading to C–
C bond formation from unactivated aromatic derivative. Overall,
the coupling of two molecules of the solvent to form the
corresponding biphenyl derivative at mild temperatures is proved.
55 DFT calculations rationalizing these findings are presented
herein.
The low temperature (-35°C) reduction of 1I in toluene or room
temperature reduction of 1Cl with two equivalents of KC8 resulted
in the formation of graphite and KX salts, indicative of the
60 efficient reduction of the Pt(II) complexes. The crude mixture,
analyzed by 31P{1H} NMR spectroscopy after 2 hours, did not
present the signal ( = 11.7 ppm, JPt-P = 3320 Hz for 1I, = 7.9
ppm, JPt-P = 3460 Hz for 1Cl) for the starting complex 1 and
consisted instead in several platinum complexes featuring, for
65 each species, two electronically different thus coupled
phosphorus signals. It pointed to the formation of C–H inserted
species (solvent and/or ligand insertions). This first experiment
showed a fast reduction process followed by facile C–H
insertions. Most interestingly, following the reaction in time, at
70 room temperature, revealed that the kinetic mixture evolved to
yield a single C–H inserted complex, within 2 days. This reaction
allowed for a full NMR characterization of the thermodynamic
product, 3H, after elimination of the graphite and the salts by
filtration. Most importantly, this complex presents a signal for the
Firstly, we show here that the reduction of a stable Pt(II)
precursor in toluene results in the formation of a Pt(0) fragment
45 which inserts quantitatively C–H bonds of the solvent at room
temperature. One of the key points of the designed system lies
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Fig. 1 X-Ray structure of complex 3H. Selected Bonds and Angles: Pt(1)-
C(30) 2.063(3), Pt(1)-P(2) 2.2687(8), Pt(1)-P(1) 2.304(1), Pt(1)-H
1.56(3), C(30)-Pt(1)-P(1) 93.1(1), P(2)-Pt(1)-P(1) 98.16(3), C(30)-Pt(1)-
H 84(1), P(2)-Pt(1)-H 84(1). The hydride has been refined using Shelx97.
45
Scheme 2 Summary of reactions
hydride at -1.53 ppm in the 1H NMR spectrum (dd, JPtrans-H
=
179.3 Hz, JPcis-H= 21.2 Hz, JPt-H= 579 Hz), and two coupled
doublets at 18.8 (JP-P= 20 Hz, JPt-P= 1800 Hz) and 31.4 ppm (JP-
P= 20 Hz, JPt-P= 1785 Hz) in the 31P{1H} NMR spectrum. The
similar reaction carried out in deuterated toluene lead to the
analogous complex, by 31P{1H} NMR, but for which no hydride
signal could be observed, proving the C–H insertion into the
5
Fig. 2 ORTEP representation of complex 6 - Selected Bond and Angles:
50 Pt(1)-H(1PT) 1.48(4), Pt(2)-H(2PT) 1.59(4), P(1)-Pt(1)-P(2) 98.74(4),
P(3)-Pt(2)-P(4) 99.17(4), Angle between planes (P1,Pt1,P2) and
P3,Pt2,P4): 85.02(4) Hydrides have been refined using Shelx97.
10 solvent to form complex 3D. Final proof of the insertion in the
solvent was given by X-ray crystal analysis, which showed the
insertion at the meta position (Figure 1).
3 was kept at room temperature for several days, it evolved partly
and colorless crystals deposited from the crude mixture. It was
55 later found that this reaction could be performed within few hours
at 80°C (scheme 3). This complex, 6, was characterized by usual
NMR spectroscopy. The 31P{1H} spectrum was particularly
informative, indeed, it presented one signal in solution for all P
Further evidence of the room temperature reversibility of the C–
H insertion was obtained when complex 3H was dissolved in D8-
15 toluene. The signal for the hydride decreased slowly to eventually
disappear. The kinetics of the C–H insertion/elimination was then
studied using complexes 4H and 4D obtained by the reduction of 1
in benzene and C6D6 respectively. The disappearance of the
hydride signal form 4H dissolved in C6D6, and the disappearance
20 of the corresponding deuteride signal form 4D dissolved in C6H6
were followed (Scheme 2).
Since complexes 4H and 4D are not soluble in non aromatic
solvents, we could not measure a direct kinetic isotope effect.
However, the study of the kinetics of the disappearance of 4H in
25 C6D6 and 4D in C6H6 allowed us to measure the kinetic constants
k-H and k-D of disappearance of complexes, thus confirming the
first order kinetic, and the equilibrium of all Pt(II) species with
the Pt(0) species at room temperature. Moreover, the study of the
proportions of complexes at equilibrium in a C6H6/C6D6 mixture
30 allowed us to compute the full kinetic isotope effect (See
supplementary information for details of calculations), we thus
observed an inverse kinetic isotope effect of 0.78, typical of
exchange studies.16
1
2
atoms, which were coupled with two Pt centers with J and J
60 coupling constants pointing to the formation of a dimer (1JPt-P
=
2510 Hz 2JPt-P = 415 Hz). Moreover, it is consistent with previous
reports by L. Mole who describes the cationic analogues of
similar dimers.17 Final proof of the structure of complex 6 was
given by an X-ray analysis, an ORTEP of which is presented in
65 Figure 2. This unprecedented reactivity presented above
prompted us to carry a theoretical study within the frame of DFT,
for the case of benzene. These calculations were performed with
the Gaussian09 suite of programs, using the hybrid density
functional ω-B97XD in order to describe more precisely long
70 range interactions, as well as thermodynamics, and kinetics
values. The Def2-TZVP basis set and its associated core potential
was used for platinum, the 6-31G* basis set being used for all
other atoms but for mobile hydrogen atoms which were described
with the 6-311+G** basis set.18 The different energies of the
75 intermediates as well as transition states were computed with the
real system, the results are shown in scheme 3.
Because of the facile reversible C–H insertion, the stable Pt
35 hydride complex 3 was envisaged as a competent source of a 14
electron fragment. Accordingly, trapping of this fragment with
several two electron donors were carried out providing the
expected 16 electron complexes. The example of
diphenylacetylene is given here, for which complete formation of
40 complex 5 was observed within 5 minutes, and isolated in
excellent 91% yield.
We chose to focus the discussion on the ΔH rather than on the
ΔG for two main reasons: first, it is well known that the entropic
contribution is typically overestimated, and since the reactions
80 involve multiple variations in the number of molecules, the
overestimation is greatly increased. Secondly, the reaction is
carried out in the solvent, which concentration is therefore very
high and decreases its entropic contribution to the different steps.
Quite unexpectedly, it was found that when a solution of complex
2
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Scheme 3 Energy profile of the reaction
This was confirmed thanks to the computation of the transition
relative energies of complexes with formal oxidation states
between 0 and 4. Pt(0), Pt(II) and even Pt(IV) species may be in
equilibrium under readily accessible thermal conditions. Work is
currently underway to further develop this coupling process
50 between unactivated aromatic derivatives.
state energy of the dissociation of 4H through Eyring’s method19:
we obtained, using the data of the previous kinetic study, a
transition state energy of 22.2 kcal/mol, which fits well to the
computed TSBC (See ESI for calculation details).
5
The calculations show that the hypothetical free 14 electron
fragment A is higher in energy than both the solvent adduct B and
Notes and references
10 the C–H inserted complex C. The reversibility of the insertion is
readily explained by the energy of TSBC which lies only 20.9
kcal/mol higher than C, indicating a facile process at room
temperature, and confirming our working hypothesis. Several
possibilities for the formation of the coupling products were
15 searched, involving either the intermediacy of Pt(IV) centers or
via metathesis reactions for which only Pt(II) centers are
involved. The process involving Pt(IV) was the lowest in energy.
Thus Pt(IV) dihydride complex D was calculated only at 12.5
kcal/mol (25.5 kcal/mol in G) higher than C. The TSCD was
20 found at 28.0 kcal/mol above C. The formation of the “Pt(H)2Pt”
complex was not searched because of the size of the system.
Rather, a path leading to a monomeric “Pt(H)2” complex, E,
which could then react with the Pt(0) complex B to form the
observed dimer was searched from complex D. The reductive
25 elimination from complex D does in fact lead to the formation of
complex E, but the transition state connecting these two
complexes, TSDE, was calculated at 41.0 kcal/mol. On the other
hand, the TSDF leading to the elimination of H2 and the formation
of complex F was found at 30.0 kcal/mol, indicating a favored
30 path. Reductive elimination leading to fragment A was found to
require 33.1 kcal/mol from F (TSFA at 37.2 kcal/mol). Finally, the
oxidative addition of H2 to lead to complex E was found
barrierless. Overall, the calculations are in accord with the
experimental facts, namely showing that inserted complex C is
35 the preferred kinetic species of the reduction, and that an overall
high activation energy of 37.2 kcal/mol is required to eventually
form the thermodynamic complex, the Pt(I) dimer via complex E.
In conclusion, we show here that using a strongly donating and
flexible diphosphine ligand, featuring a propyl bridge between
40 the P atoms, a reactive Pt(0) fragment can be generated, which
does insert C–H bonds reversibly at room temperature. It was
also showed, and corroborated by DFT calculations, that the
Pt(II) complex can then further react with the solvent to yield
biaryl derivatives at relatively low temperatures. The nature of
45 the bidentate ligand therefore influences to a great extent the
This journal is © The Royal Society of Chemistry [year]
a Laboratoire « Hétéroéléments et Coordination », UMR CNRS 7653,
Ecole Polytechnique, 91128 Palaiseau CEDEX, France,Tel: +33 1 69 33
44 02; Mail: nicolas.mezailles@polytechnique.edu
55 b Laboratoire « Mécanismes Réactionnels », UMR CNRS 7651, Ecole
Polytechnique, 91128 Palaiseau CEDEX, France
† Electronic Supplementary Information (ESI) available: Full
experimental procedures, characterizations of products, CIF files for
crystal structures, details of kinetic studies and geometry of optimized
60 structures are contained in the electronic supplementary information. See
DOI: 10.1039/b000000x/
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A « (diphosphine)Pt(0) » fragment allows the room temperature reversible C–H activation as
well as the coupling of two solvent molecules.