N-PtIV-H/N-H...PtII intramolecular redox equilibrium in a product of H-C(sp2) cleavage and unusual alkane/arene C-H bond selectivity of ([2.1.1]pyridinophane)PtII(CH3)

Article abstract of DOI:10.1039/b207797n

T-shaped 14 valence electron (η²-L)PtMe⁺ (based on DFT geometry optimization, L = [2.1.1]-2,6-pyridinophane) reacts with benzene to give (η³-L) Pt^IV(Ph)₂H⁺ and methane; the latter cation is in thermal equilibrium with the N-protonated Pt^II tautomer (η²-L-H)Pt(Ph)₂⁺, and these complexes react with ethane or cyclopentane to produce benzene and (L)PtH(olefin)⁺.

Full text of DOI:10.1039/b207797n

N–PtIV–H/N–H…PtII intramolecular redox equilibrium in a product of
H–C(sp2) cleavage and unusual alkane/arene C–H bond selectivity of
([2.1.1]pyridinophane)PtII(CH3)+†
Andrei N. Vedernikov* and Kenneth G. Caulton*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA
Received (in Purdue, IN, USA) 8th August 2002, Accepted 15th October 2002
First published as an Advance Article on the web 6th January 2003
T-shaped 14 valence electron (h2-L)PtMe+ (based on DFT
geometry optimization, L = [2.1.1]-2,6-pyridinophane) re-
acts with benzene to give (h3-L) PtIV(Ph)2H+ and methane;
the latter cation is in thermal equilibrium with the N-
protonated PtII tautomer (h2-L-H)Pt(Ph)2 , and these com-
+
plexes react with ethane or cyclopentane to produce benzene
and (L)PtH(olefin)+.
We have shown1 a synthetic route to [2.1.1]-2,6-pyridinophanes
(‘L’), a class of nearly threefold symmetric ligands (Scheme 1).
When attached to PdCl2 or Pt(CH3)2, this ligand is bound 2h, but
the pendant pyridine N is constrained to be nearby, and in fact
this N binds to Pt when the metal in LPtMe2 is protonated (i.e.
3
oxidized to PtIV). The constrained h binding imposed by a
slightly ‘oversized’ macrocycle leads to facile methane re-
ductive elimination (Scheme 1), and then to a transient
Fig. 1 DFT-optimized structure of (L)PtMe+ cation. Hydrogen atoms of L
(L)PtIIMe+ species2 which shows the ability, at and below 25
are omitted. Selected bond lengths and angles: Pt–N(5), 1.993 Å; Pt–N(14),
3.026 Å; Pt–N(25), 2.171 Å; Pt–C(41), 2.042 Å; N(5)–Pt–N(25), 85.3;
N(5)–Pt-C(41), 100.5; N(25)–Pt–C(41), 171.3.
°C, to effect oxidative addition of the C–H bonds of methane,
ethane, propane, n-butane, cyclopentane and cyclohexane.3
2
We report here the DFT-calculated structure of (h -L)PtMe+,
together with its reactivity towards benzene and the results of
Reaction of [PtMe2H(L)]BArF
(BArF
is B[3,5-
4
4
some alkane–arene competition experiments. Taken together,
these give an expanded view of the potential energy surface
accessible, at and below 25 °C, to the species (L)Pt(R)2H+ and
(L)PtR+ + RH, and thus the reversibility of such a hydrocarbon
attack system. Deuterium isotope labeling studies also indicate
(CF3)2C6H3]4) in the presence of benzene (10 equiv.) in
dichloromethane solution at RT leads (Scheme 2, a) to the
appearance of new 1H NMR signals in both high- and low-field
regions, 1.50 (JPtH = 68 Hz, 3H, Pt-Me), 219.42 (JPtH = 1370,
1H, Pt-H) which can be attributed to the formation of
[(L)PtMe(Ph)H]X, 1 (X = BArF ) (6% vs. BArF in 30 min).
2
the presence of methane and arene complexes of (h -
4
4
L)Pt(R)+.
Subsequently, a peak at 218.42 (JPtH = 1350 Hz) appears with
simultaneous disappearance of the signals of 1, due to the
formation of the diphenylhydrido complex, [(L) PtPh2H]X, 2a,
(Scheme 2, b). Such a spontaneous alkyl for aryl ligand
exchange at a Pt(IV) center has not been observed earlier, though
it can occur in the presence of tris(pentafluorophenyl)boron in
neat benzene.7,8 It is interesting to note that the hydride complex
2a exists in equilibrium (d)9 with its platinum(II)-hydrogen
bonded NH-tautomer, [PtPh2(H-L)]X, 2b, with a very charac-
teristic low-field signal at 17.32 ppm (br, JPtH = 86 Hz) for the
N–H proton. The observed NH/Pt coupling suggests hydrogen
bonding between H and Pt.10,11 Complex 2a, of mirror
symmetry according to 1H and 13C NMR spectroscopy, shows
one AX pattern for the pyridinophane ligand methylene bridges
and only two triplet signals for the para-protons of its pyridine
rings in a 1+2 ratio. In contrast, its platinum(II) tautomer, 2b, has
no symmetry as proven by NMR data, and platinum is thus
DFT calculations (program package Priroda,4 PBE func-
tional,5 SBK basis set6) reveal a T-shaped three-coordinate 14
electron structure (Fig. 1) for the [PtMe(L)]+ cation rather than
a 4-coordinated pseudotetrahedral geometry. Due to the trans-
effect of the methyl ligand the Pt–N(25) bond is 0.18 Å longer
than the Pt–N(5) bond. The location of the C2H4 linker allows
the third ligand donor, N(14), to be situated at a non-bonding
position, slightly more than 3 Å from platinum. As a result, the
platinum atom is intermolecularly accessible and forms a third
Pt–N bond in the course of hydrocarbon substrate addition,
thereby improving the reaction energetics.
Scheme 1
†
Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b2/b207797n/
Scheme 2
358
CHEM. COMMUN., 2003, 358–359
This journal is © The Royal Society of Chemistry 2003

coordinated to two nitrogen atoms of non-equivalent pyridine
rings of the pyridinophane. Its spectrum exhibits two AX
patterns due to two non-equivalent methylene bridges, three
triplets corresponding to the para-protons of three non-
equivalent pyridine rings in a 1+1+1 ratio and two sets of signals
of nonequivalent phenyls.
presence of two electron-withdrawing aryl (vs. methyl, since 1
does not show an N–H isomer) ligands decreases metal basicity
to a degree sufficient to match that of the nitrogen in the ligand
L and as a result to make observable for the first time the redox–
prototropic equilibrium PtR2H(L)+ with PtR2(H-L)+. On the
other hand, transformation of PtAlk(L)+ into PtH(olefin)(L)+
helps to stabilize an electrophilic platinum by the electron
donating olefin to such an extent that it overcomes the energy
losses when going from PtAr2H(L)+ to PtAlk(Ar)H(L)+ and
then to PtAlk(L)+. These features of the described C–H bond
cleavage system open new perspectives in arene functionaliza-
tion and clarify the energy balance and mechanism of
competition between electrophilic and oxidative addition path-
ways in hydrocarbon activation with platinum(II) complexes.
This work was supported by the Department of Energy. We
thank Dr D. N. Laikov for the use of his software. ANV is on
leave from the Chemical Faculty, Kazan State University,
Kazan, Russia. This work has been made possible in part due to
support from Russian Foundation for Basic Research (grant
#01.03.32692).
The methyl-to-phenyl exchange is almost quantitative
(498% vs. BArF ) and the redox-isomeric diphenyl products
4
2a+2b are present in molar ratio 1+9 and are stable in
dichloromethane solution at 25 °C for at least several days. This
mixture of the tautomeric diphenyl platinum complexes 2 of the
same composition has been obtained independently from HOTF
and (L)PtPh2, then NaBArF .
4
A
similar picture is observed for the reaction of
[(L)PtMe2H]X in C6D6 solution but isotopomers of methane are
formed: CH4, CDH3 and CD2H2 and CD3H in 1+0.6+0.4+0.4
mol ratio (NMR integration). The formation of CD2H2 and
CD3H is consistent with reversibility of the R/H reductive
2
coupling,12,13 and the intermediacy of methane h - complexes
2
(Scheme 2 c), e.g. [LPtPh(h -CDH3)]+ and the benzene
2
2
complex [LPt(CH2D)(h -HC6D5)]+. The structure of [k
(HTpA)Pt(H)(h -C6H6)]+, and its ability to be in equilibrium
2
with [k -(HTpA)PtPh(H)2]+, has been described recently.14
2
Notes and references
1 A. N. Vedernikov, J. C. Huffman and K. G. Caulton, Inorg. Chem.,
accepted.
2 DFT calculated (see ESI†) Gibbs free energy of methane binding to
(L)PtMe+ at 298 K is only 2 kcal mol21, so, formation of the transient
(L)PtMe+ is feasible.
3 A. N. Vedernikov, J. C. Huffman and K. G. Caulton, J. Am. Chem. Soc.,
submitted.
4 (a) Y. A. Ustynyuk, L. Y. Ustynyuk, D. N. Laikov and V. V. Lunin, J.
Organomet. Chem., 2000, 597, 182; (b) D. N. Laikov, Chem. Phys. Lett.,
1997, 281, 151.
5 J. A. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77,
3865.
6 (a) W. J. Stevens, H. Basch and M. Krauss, J. Chem. Phys., 1984, 81,
6026; (b) W. J. Stevens, H. Basch, M. Krauss and P. Jasien, Can. J.
Chem., 1993, 98, 5555.
7 D. D. Wick and K. I. Goldberg, J. Am. Chem. Soc., 1997, 119,
10235.
8 S. Reinartz, P. S. White, M. Brookhart and J. L. Templeton,
Organometallics, 2001, 20, 1709.
9 I. C. M. Wehman-Ooyevarr, D. M. Grove, P. De Vaal, A. Dedieu and G.
Van Koten, Inorg. Chem., 1992, 31, 5484.
10 W. Yao, O. Eisenstein and R. H. Crabtree, Inorg. Chim. Acta, 1997, 254,
105.
11 D. Braga, F. Grepioni, E. Tedesco, K. Biradha and G. R. Desiraju,
Organometallics, 1997, 16, 1846.
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Acta, 1997, 265, 117; (b) M. W. Holtcamp, J. A. Labinger and J. E.
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15 B. B. Coussens, F. Buda, H. Oevering and R. J. Meier, Organometallics,
1998, 17, 795.
The equilibrating system d in Scheme 2 shows alkane C–H
cleavage chemistry. Recently we reported3 facile alkane
dehydrogenation using [PtMe2H(L)]+ and have shown that
formation of the corresponding hydridoplatinum(II) olefin
complex is thermodynamically favorable. Now we demonstrate
that this driving force is enough to overcome the expected
higher affinity of [PtR(L)]+ species for arene C–H bonds as
compared with alkane C–H bonds in the course of reversible
oxidative addition. An equilibrium mixture of tautomers 2a+2b,
reacts in CD2Cl2 with cyclopentane (3 M) at 25 °C (RT) over 3
days to produce [PtH(cyclo-C5H8)(L)]BArF4 and two moles of
benzene in 80% yield vs. BArF4 (eqn. 1).
[(L)PtPh2H]X + cyclo-C5H10
?
2
(1)
{(L)PtH(h -cyclo-C5H8)]X + 2 PhH
This observation proves the reversibility of arene C–H bond
oxidative addition to the [(L)PtR]+ species and shows its
unusual alkane–arene C–H bond selectivity in favor of an
alkane C–H bond caused by the high thermodynamic prefer-
ence15–18 for a cationic hydridoplatinum(II) olefin complex over
its 14 electron alkylplatinum(II) isomer.
An alkane–arene competition experiment has been conducted
when both substrates were present in comparable concentra-
tions. A solution of benzene and ethane taken at 1.6+1 mol ratio
reacts with [PtMe2H(L)]+ at RT in the course of 8 h to produce,
with 100% yield vs. BArF , the tautomeric mixture (1+9) of
4
[PtPh2H(L)]+ and [PtPh2(H-L)]+. Then, in the course of several
days, the slow accumulation of the hydridoplatinum(II) ethene
complex (analogous to eqn. 1) has been observed so that in 4
days the yield of the ethene complex reached 10%. The slow
rate of alkane consumption indicates that benzene at high
concentration significantly inhibits alkane oxidative addition,
presumably due to formation of a p-complex with transient
[(L)PtR]+ species.
16 R. Romeo, G. Alibrandi and L. M. Scolaro, Inorg. Chem., 1993, 32,
4688.
17 N. Carr, L. Mole, A. G. Orpen and J. L. Spencer, J. Chem. Soc., Dalton
Trans., 1992, 2653.
18 L. Mole, J. L. Spencer, N. Carr and A. G. Orpen, Organometallics,
1991, 10, 49.
The observed chemistry can be related both to the ability of
L to not ‘overstabilize’ PtR2H(L)+,13 and also to the electro-
philicity of platinum in the described complexes. Thus, the
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