Five-coordinate platinum(IV) complex as a precursor to a novel Pt(II) olefin hydride complex for alkane activation

Article abstract of DOI:10.1021/ja0258677

The five-coordinate platinum(IV) complex (nacnac)PtMe₃ (nacnac- = [{(o-iPr₂C₆H₃)NC(CH₃)}₂CH]_-) thermally eliminates ethane and methane to produce a novel olefin(hydrido)platinum(II) complex, where the olefin is part of the nacnac-type ligand. This Pt(II) product activates hydrocarbons, including alkanes under mild conditions, as indicated by scrambling of hydrogen and deuterium between the hydrocarbon solvent and selected positions on the ligand of the platinum complex. A mechanism in which olefin insertion into the metal hydride bond opens a site to allow hydrocarbon coordination and C-H bond oxidative addition is proposed. Copyright

Full text of DOI:10.1021/ja0258677

Published on Web 05/22/2002
Five-Coordinate Platinum(IV) Complex as a Precursor to a Novel Pt(II) Olefin
Hydride Complex for Alkane Activation
Ulrich Fekl and Karen I. Goldberg*
Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington, 98195-1700
Received February 8, 2002
Some of the most promising approaches to the selective
functionalization of alkanes involve platinum catalysts.1 Significant
recent progress has been made in the development of catalytic
systems for alkane oxidation,2 as well as in the understanding of
the mechanisms of these platinum(II/IV) based systems.3,4 Mecha-
nistic studies have shown that virtually all C-C and C-H reductive
elimination reactions to form alkanes from six-coordinate Pt(IV)
complexes involve prior dissociation of a ligand to form five-
coordinate Pt(IV) intermediates.4,5 Conversely, the oxidative addi-
tion of C-H bonds to Pt(II) must (by the principle of microscopic
reversibility) involve either a three-coordinate Pt(II) intermediate,
formed by dissociation of a ligand from square-planar Pt(II), or an
Figure 1. Thermal ellipsoid plot for 2-d27: heavy atoms as 30% probability
associative substitution step3d in which hydrocarbon replaces a
weakly bound ligand. Coordination of hydrocarbon within the
Pt(II) square plane appears to be a key factor for C-H activation
at Pt(II).6 In Shilov’s classic system, aquation of PtCl42- occurs to
generate a reactive species that has a good leaving group.2c,d More
recently, Pt(II) complexes with labile solvento ligands or weakly
bound anionic ligands have been found to activate hydrocarbon
C-H bonds.3a-g A Lewis acid has also been used to remove a
strongly bound anionic ligand for this purpose.3h Thus, all Pt(II)-
based C-H bond activation reactions have required that a platinum-
bonded ligand leaves. Herein we describe a different approach:
olefin insertion can create the open site and allow for selective
alkane activation under mild conditions.
envelopes, 1H as white spheres, 2H as dark spheres of arbitrary radius.
Selected distances and angles (Å, deg; centroid X defined halfway between
C27 and C29): Pt1-N1, 2.089(4); Pt1-N2, 2.011(4); Pt1-D1, 1.66(5);
Pt1-C27, 2.152(6); Pt1-C29, 2.128(6); Pt1-X, 2.023(6); N1-Pt1-N2,
92.4(2); N2-Pt1-D1, 87.0(2); N1-Pt1-X, 90.3(2); torsion angle N1-
Pt1-X-C27, 55.5(1) (olefin roughly halfway between coplanar and
perpendicular to the ligand plane); interplanar angle {C29-D29A-D29B}/
{C27-C28-C19}, 50(4).
The nearby isopropyl groups are subject to C-H oxidative addition
at either the methyl or methine positions to generate (“NNC”)-
PtMe(H). Since (“NNC”)PtMe(H) are five-coordinate Pt(IV) alkyl
hydride species, rapid C-H reductive elimination of methane occurs
to produce (“NNC”)Pt. â-Hydride elimination from (“NNC”)Pt then
generates the olefin hydride species 2. This reaction should be facile
since an open site exists on the metal cis to the cyclometalated
ligand bearing â-hydrogens. This mechanism is in agreement with
the methane product detected; when the thermolysis is conducted
in C6D6, CH4 with only a minor amount of CH3D (<5%) is formed.
The incorporation of deuterium into the isopropyl groups shows
that a species capable of C-H activation is generated. Activation
of C6D6 is expected to occur via one of the three-coordinate Pt(II)
intermediates, (nacnac)PtMe or (“NNC”)Pt. The lack of significant
amounts of CH3D in C6D6 argues against the involvement of the
former species. Shown on the right side of Scheme 1 is the proposed
mechanism. Oxidative addition of a C-D bond of C6D6 to
(“NNC”)Pt generates the five-coordinate Pt(IV) deuteride species
(“NNC”)PtR(D) (R ) C6D5).12 Then, while C-D reductive
elimination of C6D6 to regenerate the three-coordinate species of
the type (“NNC”)Pt is nonproductive, C-D elimination of the
isopropyl group produces the three-coordinate Pt(II) species (nac-
nac-d1)PtR. Oxidative addition of a C-H bond of one of the
isopropyl groups then produces (“NNC”-d1)PtR(H). C-H reductive
elimination of the C6D5-H solvent molecule occurs and is followed
by oxidative addition of another R-D solvent molecule. Repetition
of this sequence (at least 26 more times) leads to the product 2-d27.
Since only 2-d27 (and no 2) is observed upon thermolysis of 1
in C6D6, the oxidative addition of C6D6 to (“NNC”)Pt must either
be much faster than â-hydride elimination or â-hydride elimination
to form 2 must be reversible. There is strong literature precedent
Recently, we reported the isolation and structural characterization
of the five-coordinate platinum(IV) alkyl complex (nacnac)PtMe3
(1), nacnac- ) [{(o-iPr2C6H3)NC(CH3)}2CH]-.7 This study pro-
vided the first physical model for the five-coordinate alkyl inter-
mediates proposed in alkane reductive elimination reactions from
Pt(IV) complexes.7,8 We now report that thermolysis of 1 in C6D6
at 150 °C in the dark produces within 10 min ethane (the organic
product of C-C reductive elimination), methane, and a novel Pt(II)
1
complex.9 The H NMR spectrum of this new complex exhibits
the CH3 and the CH signals of the ligand backbone as well as the
aryl protons of the ligand. However, isopropyl hydrogens of the
1
ligand are not observed in the H NMR spectrum. Crystallization
of the product allowed the identification of this species as the olefin-
(deuterido)platinum(II) complex 2-d27 (Figure 1, relevant parameters
in the legend).10 The isopropyl groups, the isopropenyl group, and
the metal hydride position in this compound are completely
deuterated. If the reaction is instead carried out in C6H6, the
analogous olefin(hydrido)platinum(II) complex 2, unambiguously
characterized by NMR spectroscopy, is formed.11
A straightforward mechanism is proposed for the formation of
2 (left side of Scheme 1). The five-coordinate Pt(IV) complex 1
undergoes reductive elimination of ethane to generate the Pt(II)
intermediate (nacnac)PtMe. This three-coordinate Pt(II) species
should be highly reactive toward oxidative addition of C-H bonds.
* Corresponding author. E-mail: goldberg@chem.washington.edu.
9
6804
J. AM. CHEM. SOC. 2002, 124, 6804-6805
10.1021/ja0258677 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
in support of the latter; several cationic alkene hydride complexes
of Pt(II) with phosphine ligands have been observed to undergo
reversible olefin insertion/â-hydride elimination.13 There appears
to be only a small energy difference with a low activation barrier
between the Pt(II) alkene hydride complex and the Pt(II) alkyl with
a C-H agostic interaction.13,14
References
(1) (a) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of
Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer:
Boston, 2000. (b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 2181.
(2) (a) Lin, M.; Shen, C.; Garcia-Zayas, E. A.; Sen, A. J. Am. Chem. Soc.
2001, 123, 1000. (b) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.;
Satoh, T.; Fuji, H. Science 1998, 280, 560. (c) Shilov, A. E.; Shul’pin, G.
B. Chem. ReV. 1997, 97, 2879. (d) Goldshleger, N. F.; Eskova, V. V.;
Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1353.
(3) (a) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002,
124, 1378. (b) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. J.
Am. Chem. Soc. 2001, 123, 6579. (c) Thomas J. C.; Peters, J. C. J. Am.
Chem. Soc. 2001, 123, 5100. (d) Johansson, L.; Tilset, M. J. Am. Chem.
Soc. 2001, 123, 739. (e) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 2000, 122, 10846. (f) Holtcamp, M. W.; Henling,
L. M.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta
1998, 270, 467. (g) Brainard, R. L.; Nutt, W. R.; Lee, T. R.; Whitesides,
G. M. Organometallics 1988, 7, 2379. (h) Wick, D. D.; Goldberg, K. I.
J. Am. Chem. Soc. 1997, 119, 10235.
(4) (a) Puddephatt, R. J. Coord. Chem. ReV. 2001, 219, 157. (b) Fekl, U.;
Zahl, A.; van Eldik, R. Organometallics 1999, 18, 4156. (c) Jenkins, H.
A.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1997, 16, 1946. (d)
Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118,
5961. (e) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics
1995, 14, 4966.
(5) (a) Williams, B. S.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 2576.
(b) Crumpton, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2000, 122, 962.
(c) Goldberg, K. I.; Yan, J.; Breitung, E. M. J. Am. Chem. Soc. 1995,
117, 6889. (d) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E. J. Chem.
Soc., Dalton Trans. 1974, 2457.
(6) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem.
Soc. 2001, 123, 12724.
(7) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123,
6423.
If â-hydride elimination from (“NNC”)Pt to form 2 is reversible,
the deuteration of 2 should not depend on an in situ preparation
from 1. Isolated samples of 2 should be capable of activating
hydrocarbon C-D bonds. Indeed, heating of 2 in C6D6 to 85 °C
for 16 h led to the formation of 2-d27. Even more remarkable is
that 2 is also able to activate alkane C-H bonds under mild
conditions. Deuterium incorporation (ca. 50%) into the positions
shown for 2-d27 in Scheme 1 was observed when 2 was heated in
pentane-d12 to 85 °C for 18 h. After 62 h, deuteration at these
positions was virtually complete.15 The regioselectivity of the alkane
H/D exchange was determined by heating 2-d27 in pentane-h12
wherein the formation of 2 and partially deuterated pentane was
observed. Deuteration of the primary positions of pentane was
favored over that of the secondary positions by a statistically
2
corrected factor of ca. 6 (determined by H NMR).16
In summary, thermolysis of a five-coordinate platinum(IV) com-
plex has led to the formation of an olefin(hydrido)platinum(II)
complex, where the olefin moiety is part of a nacnac-type ligand.
This complex activates alkanes by a novel mechanism; olefin inser-
tion into the Pt-H bond opens a coordination site at Pt(II) allowing
alkane coordination and C-H oxidative addition. Experiments to
apply this promising alkane activation concept to the functional-
ization of alkanes are underway in our laboratories. It is im-
portant in this respect that the desirable selectivity for terminal
C-H bonds, typical for Pt(II)-based systems,1 was also observed
in H/D exchange reactions with this olefin(hydrido)platinum(II)
complex.16
(8) (a) Simultaneous discovery of five-coordinate dihydrido(silyl)platinum-
(IV): Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am.
Chem. Soc. 2001, 123, 6425. (b) Recent review: Puddephatt, R. J. Angew.
Chem., Int. Ed. 2002, 41, 261.
(9) The yield of 2-d27 is 50-60%. As a soluble side product, free ligand
nacnac-H is formed in ca. 5% yield.
(10) 2-d27: C29H13D27N2Pt, MW ) 638.88, clear yellow-brown prism; mono-
clinic, space group P21/c, T ) 130(2) K, a ) 8.7950(4) Å, b ) 26.0160-
(14) Å, c ) 11.6170(6) Å, â ) 102.853(3)°, Z ) 4, R1 ) 0.0421, wR2 )
0.0781. GOF (F2) ) 0.716.
(11) NMR data are given in the Supporting Information.
(12) Both isomers must be accessible, since scrambling occurs into isopropyl
CH and CH3 positions.
(13) Spencer, J. L.; Mhinzi, G. S. J. Chem. Soc., Dalton Trans. 1995, 3819
and references therein.
Acknowledgment. The authors thank the National Science
Foundation for support of this work. U.F. is grateful to DAAD
(Deutscher Akademischer Austauschdienst) for a postdoctoral
fellowship award. The authors acknowledge W. Kaminsky and K.
Claborn (UW) for the solution of the X-ray structure of 2-d27.
(14) Creve, S.; Oevering, H.; Coussens, B. B. Organometallics 1999, 18, 1967.
(15) Additionally, significant deuteration at the CH3 positions of the ligand
backbone was observed, as indicated by appearance of CH2D (1:1:1)
triplets along with small peaks due to double-deuteration (CHD2, quintets).
This process is slow and apparently an intermolecular reaction; if free
ligand nacnac-H is present in the sample, partial deuteration of analogous
positions on the free ligand takes place.
(16) This selectivity may stem from the initial activation step or from possible
subsequent reactions (C-H reductive coupling/oxidative cleavage or
reversible â-H elimination) which could isomerize the alkyl to the
thermodynamically favored terminal position.
Supporting Information Available: Synthetic procedures and
NMR spectral data for 2 and 2-d27 (PDF); X-ray crystallographic data
for 2-d27 (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0258677
9
J. AM. CHEM. SOC. VOL. 124, NO. 24, 2002 6805