Mechanisms of C-C and C-H alkane reductive eliminations from octahedral Pt(IV): Reaction via five-coordinate intermediates or direct elimination?

Article abstract of DOI:10.1021/ja029140u

The Pt(IV) complexes P₂PtMe₃R [P₂ = dppe (PPh₂(CH₂)₂PPh₂), dppbz (o-PPh₂(C₆H₄)PPh₂); R = Me, H] undergo reductive elimination reactions to form carbon-carbon or carbon-hydrogen bonds. Mechanistic studies have been carried out for both C-C and C-H coupling reactions and the reductive elimination reactions to form ethane and methane are directly compared. For C-C reductive elimination, the evidence supports a mechanism of initial phosphine chelate opening followed by C-C coupling from the resulting five-coordinate intermediate. In contrast, mechanistic studies on C-H reductive elimination support an unusual pathway at Pt(IV) of direct coupling without preliminary ligand loss. The complexes fac-P₂PtMe₃R (P₂ = dppe, R = Me, H; P₂ = dppbz, R = Me) have been characterized crystallographically. The Pt(IV) hydrides, fac-P₂PtMe₃H (P₂ = dppe, dppbz), are rare examples of stable phosphine ligated Pt(IV) alkyl hydride complexes.

Full text of DOI:10.1021/ja029140u

Published on Web 07/11/2003
Mechanisms of C-C and C-H Alkane Reductive Eliminations
from Octahedral Pt(IV): Reaction via Five-Coordinate
Intermediates or Direct Elimination?
Dawn M. Crumpton-Bregel and Karen I. Goldberg*
Contribution from the Department of Chemistry, Box 351700, UniVersity of Washington,
Seattle, Washington 98195-1700
Received October 29, 2002; E-mail: goldberg@chem.washington.edu
Abstract: The Pt(IV) complexes P2PtMe3R [P2 ) dppe (PPh2(CH2)2PPh2), dppbz (o-PPh2(C6H4)PPh2); R
) Me, H] undergo reductive elimination reactions to form carbon-carbon or carbon-hydrogen bonds.
Mechanistic studies have been carried out for both C-C and C-H coupling reactions and the reductive
elimination reactions to form ethane and methane are directly compared. For C-C reductive elimination,
the evidence supports a mechanism of initial phosphine chelate opening followed by C-C coupling from
the resulting five-coordinate intermediate. In contrast, mechanistic studies on C-H reductive elimination
support an unusual pathway at Pt(IV) of direct coupling without preliminary ligand loss. The complexes
fac- P2PtMe3R (P2 ) dppe, R ) Me, H; P2 ) dppbz, R ) Me) have been characterized crystallographically.
The Pt(IV) hydrides, fac-P2PtMe3H (P2 ) dppe, dppbz), are rare examples of stable phosphine ligated
Pt(IV) alkyl hydride complexes.
Introduction
importance of these bond cleavage/formation processes in
homogeneous catalysis has prompted numerous studies of the
Reductive elimination to form a carbon-carbon or carbon-
hydrogen bond is often the critical product release step in
organic transformations mediated by transition-metal com-
plexes.1,2 The microscopic reverse, oxidative addition of a
carbon-carbon or carbon-hydrogen bond serves as the sub-
strate activation step in many stoichiometric and catalytic
processes, including hydrocarbon functionalization.1-5 The
mechanisms of C-C and C-H oxidative addition and reductive
elimination reactions.1,3-6 Despite this large research effort,
however, it remains difficult to predict the detailed mechanistic
pathways of these reactions. Specifically, it is often unknown
whether a particular reaction will proceed directly or will require
ancillary ligand dissociation or even association prior to bond
cleavage/formation.1 Such explicit information concerning the
reaction mechanism would be of enormous value to current
efforts to rationally develop novel catalytic processes.
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Many metal catalyzed reactions proceed via pathways involv-
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followed later in the cycle by reductive elimination from an
(6) For some specific examples, see refs 7-28.
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9
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J. AM. CHEM. SOC. 2003, 125, 9442-9456
10.1021/ja029140u CCC: $25.00 © 2003 American Chemical Society

C−C and C−H Alkane Reductive Eliminations
A R T I C L E S
octahedral d6 center.1 The mechanisms of oxidative addition
reactions to d8 metal centers and reductive elimination reactions
from d6 metal centers are thus of considerable interest. The
formation of alkanes by C-C and C-H reductive elimination
from d6 octahedral complexes, in particular, has been extensively
studied. Carbon-carbon coupling of two alkyl groups is
frequently observed from Pt(IV) and Pd(IV) complexes,7-9 but
there are very few examples of this reaction from other d6 metal
centers.10 Mechanistic investigations of sp3C-sp3C reductive
elimination reactions from octahedral Pt(IV) and Pd(IV) com-
plexes have consistently found evidence for a five-coordinate
intermediate from which the C-C bond formation takes
place.7,8,11,12 This five-coordinate intermediate is formed by
ligand loss from the starting octahedral complex. The reactivity
and stability of Pt(IV) and Pd(IV) alkyl complexes vary
significantly with respect to the ancillary ligands present. For
Pt(IV), C-C bond formation is common for complexes with
phosphine ligands.7 In contrast, complexes with Cp or nitrogen
ligands are generally resistant to reductive elimination.8d,13,14
A notable exception is the recent report of ethane reductive
elimination from a nitrogen ligated five-coordinate Pt(IV)
trimethyl complex.9 For Pd(IV) alkyls, most phosphine ligated
species appear to be too unstable to isolate or even observe.8c,g,k
Instead, C-C reductive elimination from Pd(IV) has been
commonly observed and studied from Pd(IV) alkyls complexes
bearing nitrogen ligands.8 Very little has been reported on alkyl
C-C reductive elimination from late metal centers other than
platinum or palladium. However, it should be noted that alkyl-
alkyl coupling from an octahedral Rh(III) center was observed
only when a five-coordinate intermediate was accessible.10
Support for the involvement of five-coordinate intermediates
in sp2C-sp3C couplings from Ru(II) and Ir(III) has also been
presented.15 Thus, based on the limited empirical evidence,
alkane C-C reductive elimination from d6 octahedral complexes
seems to require a five-coordinate intermediate.11
C-H reductive elimination reactions from d6 octahedral metal
centers do not, in general, follow as distinctive a pattern in terms
of mechanism as C-C reductive elimination reactions. For alkyl
and hydrocarbyl hydride complexes of the late metals Ir(III),
Rh(III), Os(II), and Ru(II), some reactions are proposed to go
by direct reductive elimination, whereas others require ligand
dissociation prior to C-H coupling.16-19 In contrast, all of the
examples of C-H reductive elimination from Pt(IV) alkyl
hydrides that have been studied to date were found to proceed
via a preliminary ligand dissociation pathway.3d,g,h,12,20-22 In fact,
the recent discoveries of thermally stable Pt(IV) alkyl hydrides
have been attributed to the use of chelating and other types of
ligands which do not easily undergo dissociation to allow
formation of five-coordinate species.22,23 Of note, nearly all of
the Pt(IV) hydrocarbyl hydrides reported contain nitrogen
ancillary ligands.24 Nitrogen ligands generally stabilize Pt(IV)
and Pd(IV) to a greater extent than phosphine ligands.8,14,25
However, even with nitrogen ligands, there are no examples of
stable or even observable Pd(IV) alkyl hydrides. This has
necessarily precluded any studies of C-H reductive elimination
from Pd(IV). Overall, it seems that the mechanisms as well as
the thermodynamics of C-H reductive elimination are quite
sensitive to the identities of the metal and the ligand set. Further
studies are clearly required to gain general predictive power
concerning these reactions.
Although C-C and C-H reductive elimination of alkanes
is known to occur from several different late metals, Pt(IV)
shows the most examples of both reactions, and is therefore
the most promising for use in a study to directly compare the
mechanisms of C-C and C-H coupling. As noted above, in
virtually every investigation of either C-C or C-H reductive
elimination of alkanes from octahedral Pt(IV) complexes,
(16) (a) Basato, M.; Longato, B.; Morandini, F.; Bresadola, S. Inorg. Chem.
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Inorg. Chimica. Acta 1998, 270, 537. (j) Wiley, J. S.; Oldham, W. J.;
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D. Organometallics 1994, 13, 385.
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118, 5961. (b) Jenkins, H. A.; Yap, G. P. A.; Puddephatt, R. J.
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Organometallics 1999, 18, 4156. (d) Reinartz, S.; White, P. S.; Brookhart,
M.; Templeton, J. L. Organometallics 2000, 19, 3854. (e) Reinartz, S.;
White, P. S.; Brookhart, M.; Templeton, J. L. Organometallics 2001, 20,
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(22) A recent review of Pt(IV) hydrides does suggest that direct C-H reductive
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219, 157.
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(11) A possible exception: a kinetic fit analysis indicated that in the absence
of added L, 1-2% of thermally promoted C-C reductive elimination from
L2PtMe4 (L ) MeNC, 2,6-Me2C6H3NC) proceeds without ligand dissocia-
tion. However, significant decomposition of L under the reaction conditions
hindered a complete analysis of the data.7b
(12) Puddephatt, R. J. Angew. Chem., Int. Ed. Engl. 2002, 41, 261.
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(24) The only previously reported example of a Pt(IV) alkyl hydride complex
with phosphine ligands is (PEt3)2Pt(CH3)2(H)Cl.20a This complex undergoes
reductive elimination of methane, but mechanistic details were not reported.
(25) Hill, G. S.; Puddephatt, R. J. Organometallics 1998, 17, 1478.
9
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A R T I C L E S
Crumpton-Bregel and Goldberg
mechanistic evidence supports a reaction pathway of ancillary
ligand dissociation to form a five-coordinate intermediate prior
to reductive elimination.7,12,20,21 There have been no unambigu-
ous examples of direct alkyl-alkyl or alkyl-hydride coupling
from an octahedral Pt(IV) complex.11 This has led to a general
belief that such eliminations from Pt(IV) require a five-
coordinate intermediate.22 This proposal has been further
supported by the observation that Pt(II) centers which undergo
facile intermolecular activation of C-H bonds all share in
common the presence of a labile ligand or an alternative means
of generating a site at Pt(II) to which the hydrocarbon can
coordinate.9b,20e,h,k,26,27 This apparent “requirement” for C-H
oxidative addition to Pt(II) would be consistent, by the principle
of microscopic reversibility, with the preliminary ligand dis-
sociation observed for C-H reductive elimination from Pt(IV).
The oxidative addition of C-H bonds to Pt(II) is currently of
great interest as such reactivity is proposed as the key substrate
activation step in platinum-catalyzed selective alkane oxidation
reactions, so-called Shilov chemistry.3 It should also be noted
that C-C activation at Pt(II) has only been reported to occur at
unsaturated Pt(II) centers.28
Is this large pool of empirical evidence concerning prelimi-
nary ligand dissociation meaningful as a predictor of the
mechanism of C-H and C-C reductive eliminations from
Pt(IV) and oxidative additions to Pt(II), or is it merely a
consequence of the particular Pt(IV) complexes chosen for
study? In this contribution, we address this question by reporting
the first examples of isolable Pt(IV) alkyl hydrides bearing
phosphine ligands and mechanistic investigations of C-H
reductive elimination of methane from these complexes. Evi-
dence is presented for the first alkane reductive elimination
reaction from Pt(IV) that proceeds directly without ligand
dissociation. In addition, detailed mechanistic investigations of
C-C reductive elimination from the complementary Pt(IV) alkyl
complexes to produce ethane are also reported.29 Thus, Me-
Me and Me-H reductive elimination reactions from the same
metal center can be directly compared for the first time. Because
both C-C and C-H reductive elimination reactions proceed
via similar concerted three-center transition states involving cis
σ-only donor ligands on the metal center,1 these couplings have
often been used to model one another. However, because the
propensities for these two types of reactions differ so dramati-
cally, direct comparison of the couplings in the same metal
ligand environment has been challenging. If alkyl C-H reduc-
tive elimination is observable, then alkyl C-C coupling appears
Figure 1. ORTEP diagram for (dppe)PtMe4 (1), with hydrogen atoms
omitted for clarity.
to require a sufficiently high temperature such that competing
decomposition reactions ensue. Similarly, if alkyl C-C coupling
is observable, the corresponding metal alkyl hydride is too
reactive to detect even at much lower temperatures. Thermolyses
of the phosphine ligated Pt(IV) complexes L2PtMe3R (L2 ) dppe
(PPh2(CH2)2PPh2), dppbz (o-PPh2(C6H4)PPh2); R ) Me, H) has
allowed for a direct comparison of the mechanisms of these
coupling reactions: C-C reductive elimination takes place from
a five-coordinate intermediate, whereas the more facile C-H
elimination proceeds directly from the octahedral geometry.
Results
Synthesis and Characterization of L2PtMe3R (L2 ) dppe,
dppbz; R ) Me, Et or H). The tetraalkyl complexes (dppe)-
PtMe4 (1),30 (dppbz)PtMe4 (2), and (dppe)PtMe3Et (3) were
prepared by addition of an alkyllithium or alkyl Grignard reagent
to the corresponding L2PtMe3I (L2 ) dppe, dppbz) complex.
The isotopically labeled analogues of 1, fac-dppePtMe(CD3)3
(fac-1-d9) and dppePt(CD3)4 (1-d12), were prepared by similar
methods using deuterated methylating reagents.
The trimethylhydride complexes fac-(dppe)PtMe3H (4) and
fac-(dppbz)PtMe3H (5) were prepared by addition of a sodium
borohydride reagent (NaBH4 or NaBH3CN) to the trimethyl
acetate derivative L2PtMe3(OAc) (L2 ) dppe, dppbz).7g The
deuterated analogues of 4 and 5, fac-(dppe)PtMe3D (4-d1) and
fac-(dppbz)PtMe3D (5-d1) were prepared using borodeuteride
reagents. Significant amounts of the Pt(II) complexes, L2PtMe2
formed during the preparation of the Pt(IV) trimethylhydride
complexes and even after multiple recrystallizations, small
amounts of L2PtMe2 (9-21%) remained in the isolated samples
of 4, 4-d1, 5, and 5-d1. This was the only Pt impurity detected,
and it was determined that the presence of L2PtMe2 did not
affect the studies of methane reductive elimination from these
complexes (see below).
The complexes 1, 2, and 4 were characterized by X-ray
crystallography. The ORTEP diagrams are shown in Figures 1,
2, and 3. Table 1 contains the crystallographic data, and selected
bond lengths and angles are listed in Tables 2 and 3. In each
complex, the geometry deviates only slightly from octahedral,
as indicated by the bond angles around Pt (84°-95°). The Pt-
Me bond lengths vary only slightly with respect to the trans
ligand. For Pt-Me trans to phosphorus, the average Pt-C bond
length is 2.11 ( 0.01 Å (complexes 1, 2, and 4). The Pt-C
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1999, 121, 1974. (f) Heiberg, H.; Johansson, L.; Gropen, O.; Ryan, O. B.;
Swang, O.; Tilset, M. J. Am. Chem. Soc. 2000, 122, 10831. (g) Johansson,
L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122,
10 846. (h) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100.
(i) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. J. Am. Chem.
Soc. 2001, 123, 6579. (j) Harkins, S. B.; Peters, J. C. Organometallics
2002, 21, 1753. (k) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2002, 124, 1378. (l) Konze, W. V.; Scott, B. L.; Kubas, G. J.
J. Am. Chem. Soc. 2002, 124, 12550.
(27) (a) Vedernikov, A. N.; Caulton, K. G. Angew Chem. Int Ed. Engl. 2002,
41, 4102. (b) Vedernikov, A. N.; Caulton, K. G. Chem. Commun. 2003,
358. (c) Vedernikov, A. N.; Huffman, J. C.; Caulton, K. G. New J. Chem.
2003, 27, 665.
(28) (a) van der Boom, M. E.; Kratz, H.-B.; Hassner, L.; Ben-David, Y.; Milstein,
D. Organometallics 1999, 18, 3873. (b) Edelbach, B. L.; Lachicotte, R. J.;
Jones, W. D. J. Am. Chem. Soc. 1998, 120, 2843.
(30) (dppe)PtMe4 was previously prepared by addition of dppe to Pt2(µ-SMe2)2-
Me8. Lashanizadehgan, M.; Rashidi, M.; Hux, J. E.; Puddephatt, R. J. J.
Organomet. Chem. 1984, 269, 317.
(29) For a preliminary report, see ref 7f.
9
9444 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

C−C and C−H Alkane Reductive Eliminations
A R T I C L E S
Table 1. X-ray Diffraction Data for P2PtMe3R Complexes
(dppe)PtMe4 (1)
(dppbz)PtMe4 (2)
C34H36P2Pt
(dppe)PtMe3H(4)
C29H34P2Pt
empirical formula
FW
C30H36P2Pt
653.62
701.66
639.6
T (K)
161(2)
293 (2)
161
wavelength (Å)
crystal description
crystal system, space group
unit cell dimensions (Å, deg)
0.71070
0.71070
0.71070
colorless plate
colorless cube
orthorhombic, Pna21
a ) 14.6533 (2)
b ) 11.7535 (2)
c ) 15.4973 (1)
R ) 90
â ) 90
γ ) 90
2669.06 (6)
4, 1.627
colorless plate
triclinic, P1
a ) 10.0224 (4)
b ) 11.1025 (4)
c ) 14.4028 (6)
R ) 83.390 (2)
â ) 71.766 (2)
γ ) 80.506 (2)
1497.88 (10)
2, 1.556
monoclinic, P21/n (No. 14)
a ) 10.05560 (10)
b ) 17.0058 (3)
c ) 16.1268 (3)
R ) 90
â ) 107.0455 (10)
γ ) 90
2636.60 (7)
4, 1.611
volume (Å3)
Z, density (g/cm3)
absorption coefficient (mm-1
F (000)
)
5.393
1296
4.811
696
5.458
1264
crystal size (mm)
reflection for indexing
θ range (deg)
0.31 × 0.29 × 0.23
0.35 × 0.25 × 0.18
0.13 × 0.10 × 0.08
1726
46
1077
3.08 to 28.66
-17 e h g 17
-14 e k g 14
-17 e l g 16
36929, 5191
81.9%
SORTAV
full-matrix least
squares on F2
0.047
2.99 to 23.33
-11 e h g 11
-12 e k g 12
-15 e l g 16
15983, 4087
94.1%
2.40 to 30.52
-13 e h g 13
-24 e k g 24
-23 e l g 23
108784, 7550
90.7%
index ranges
reflections collected/ unique
completeness to 2θ
absorption correction
refinement method
SORTAV
scalepack
full-matrix least
squares on F2
0.0826
full-matrix least
squares on F2
0.0222
Rint
parameters refined
goodness of fit on F2
final R, Rw (I > 2σI)a
298
1.028
0.0302, 0.0949
334
1.041
0.0380, 0.0900
292
1.028
0.0352, 0.0824
a w ) 1/[σ2(Fo2) + (aP)2 + bP]; P ) (Fo2 + 2Fc2)/3; a ) 0.0418 for 1, 0.0577 for 2 and 0.0316 for 4; b ) 0.9579 for 1, 7.5138 for 2 and 6.9896
for 4.
Table 2. Selected Bond Lengths and Angles for L2PtMe4
Scheme 1
Complexes
(dppe)PtMe4 (1)
(dppbz)PtMe4 (2)
bond lengths (Å)
2.139(7)
2.111(6)
Pt-C1
Pt-C4
2.151(8)
2.131(8)
Pt-C2
Pt-C3
2.115(6)
2.105(6)
2.108(7)
2.122(8)
Thermolysis of L2PtMe4 (L2 ) dppe (1), dppbz (2)).
Pt-P1
Pt-P2
2.3359(17)
2.3250(16)
2.3275(15)
2.3246(15)
Thermolysis of (dppe)PtMe4 (1) in solution at temperatures in
31
excess of 150 °C yielded ethane and (dppe)PtMe2 (6) in
bond angles (deg)
85.94(5)
84.0(3)
174.6(3)
95.4(2)
quantitative yield (Scheme 1). The reaction exhibited clean first-
order kinetic behavior in benzene-d6 (k ) 4.2 ( 0.1 × 10-6
s-1), THF-d8 (k ) 3.2 ( 0.1 × 10-6 s-1) and acetone-d6 (k )
4.9 ( 1.0 × 10-6 s-1).32 Activation parameters of ∆Hq ) 43 (
2 kcal/mol and ∆Sq ) 15 ( 4 eu were calculated from rate
constants measured in benzene-d6 over a temperature range of
165-205 °C. The Eyring plot is shown in Figure 4.
P1-Pt-P2
84.17(5)
87.3(4)
174.1(3)
93.9(3)
C2-Pt-C3
C1-Pt-C4
C2-Pt-P1
C3-Pt-P2
94.67(18)
94.4(2)
Table 3. Selected Bond Lengths and Angles for
Fac-(dppe)PtMe3H (4)
Crossover Study. A crossover study was performed in which
an equimolar mixture of (dppe)PtMe4 (1) and (dppe)Pt(CD3)4
(1-d12) was heated in benzene-d6 at 165 °C for slightly more
than one half-life (50 h). Analysis by mass spectrometry showed
that the gaseous products produced consisted of approximately
equal amounts of CH3CH3 and CD3CD3, with less than 4% CH3-
CD3. The Pt starting complexes and products were analyzed
by 31P NMR spectroscopy.33 Prior to thermolysis, only signals
bond lengths (Å)
bond angles (deg)
Pt-H
Pt-C1
Pt-C2
Pt-C3
Pt-P1
Pt-P2
1.70(4)
P1-Pt-P2
C2-Pt-C3
C2-Pt-P2
C3-Pt-P1
H-Pt-C1
86.04(3)
87.44(17)
92.93(11)
93.54(13)
174.0(14)
2.145(4)
2.110(4)
2.114(4)
2.3141(10)
2.3185(9)
bond trans to methyl has an average length of 2.13 ( 0.02 Å
(complexes 1 and 2). With hydride as the trans ligand, the Pt-
Me bond is 2.145(4) Å (complex 4). The hydride in 4 was
located and refined, and the Pt-H bond length was determined
to be 1.70(4) Å.
(31) Appleton, T. G.; Bennett, M. A.; Tomkins, I. B. J. Chem. Soc., Dalton
Trans. 1976, 439.
(32) See Supporting Information for kinetic plots.
(33) Attempts to characterize the Pt species present (both before and after
thermolysis) by mass spectrometry were unsuccessful. Molecular ions were
not detected using either direct insertion or electrospray techniques.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9445

A R T I C L E S
Crumpton-Bregel and Goldberg
Figure 2. ORTEP diagram for (dppbz)PtMe4 (2), with hydrogen atoms
omitted for clarity.
Figure 5. Equilibrium plot of fac-1-d9 (() and mer-1-d9 (b) at 100 °C in
benzene-d6.
among d8 M(II) (M ) Pt, Pd) complexes at elevated tempera-
tures has been reported by others.35
Intramolecular Isomerization of fac-1-d9. Although the
crossover experiment described above clearly showed that
intermolecular exchange of methyl groups at Pt(IV) does not
occur, intramolecular exchange of the methyl groups at Pt(IV)
does take place. This was observed directly when the partially
deuterated complex fac-(dppe)PtMe(CD3)3 (fac-1-d9) was heated
at 100 °C in benzene-d6. Slow isomerization of fac-1-d9 to a
1:1 mixture of fac-1-d9 and mer-1-d9 (Keq ) 1) occurred (Figure
5). The progress of the reaction was conveniently monitored
Figure 3. ORTEP diagram for fac-(dppe)PtMe3H (4), with hydrogen atoms
omitted for clarity.
1
by H NMR. The rate constant for the isomerization of fac-1-
d9 to mer-1-d9 at 100 °C in benzene-d6 was kf ) 1.7 × 10-6
s-1 32,36
.
Thermolysis of (dppbz)PtMe4 (2). The thermal behavior of
(dppbz)PtMe4 (2) was also examined. Similar to the dppe
complex 1, the solution thermolysis of 2 in THF-d8 at 165 °C
resulted in ethane elimination and the formation of the corre-
sponding Pt(II) dimethyl complex, (dppbz)PtMe2 (7). Reductive
elimination from 2 was significantly slower than from 1 and in
addition, the rate was not first-order in 2. The rate of the reaction
slowed considerably with time. In the first 100 h, 9-17%
conversion was observed, yet between 100 and 500 h, only
5-13% additional conversion was detected. The amount of
conversion measured varied within these ranges for each
individual sample.
A trace amount of an impurity was suspected to be the cause
of the fast initial rate. Although 2 was repeatedly purified by
recrystallization, thermolysis reactions of 2 continued to deceler-
ate with time. However, when the thermolyses of 2 were carried
out in the presence of 2% cross-linked polyvinylpyridine (PVP),
a known acid scavenger,7d the reaction slowed considerably.
After 300 h at 165 °C in THF-d8, only 4% conversion was
observed. This is approximately the same as that observed at
late reactions times without PVP present. It should be noted
that the addition of PVP to the thermolysis of 1, also in THF-
d8, did not affect the rate of the reaction.
Figure 4. Eyring plot for ethane elimination from 1 (165 to 205 °C) in
benzene-d6.
corresponding to 1 (δ 7.3, JPt-P ) 1145 Hz) and 1-d12 (δ 7.4,
JPt-P ) 1145 Hz) were observed in the 31P NMR spectrum.
After 50 h of thermolysis, these same two Pt(IV) phosphine
signals were observed and there was no evidence of deuterium
scrambling in the unreacted Pt(IV) species. However, 31P NMR
signals for three Pt(II) products, (dppe)PtMe2 (6, δ 47.35, JPt-P
) 1779 Hz), (dppe)PtMe(CD3) (6-d3, δ 47.42, JPt-P ) 1774
Hz), and (dppe)Pt(CD3)2 (6-d6, δ 47.49, JPt-P ) 1769 Hz) were
observed.34 Unfortunately, these dppePt(II) signals were not
baseline separated and integration was not possible, even at high
field (202.5 MHz). To confirm that the deuterium scrambling
among these Pt(II) complexes occurred after reductive elimina-
tion, an equimolar mixture of 6 and 6-d6 in benzene-d6 was
heated at 165 °C for 12 h. 31P NMR revealed the generation of
a significant amount of 6-d3. The exchange of alkyl groups
Thermolysis of (dppe)PtMe3Et (3). Thermolysis of a non-
equilibrium mixture of fac and mer isomers of 3 at 165 °C in
benzene-d6 generated the organic products ethylene, methane,
ethane and propane, and the Pt(II) products (dppe)PtMe2 (6)
and (dppe)PtMeEt (8). The disappearance of 3 is a first-order
process, with kobs ) (1.2 ( 0.1) × 10-4 s-1 at 165 °C. The
(34) An additional 31P NMR signal for 6-d3 may be hidden under the signals
for 6 and 6-d6.
(35) Casado, A. L.; Casares, J. A.; Espinet, P. Organometallics 1997, 16, 5730
and references therein.
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9446 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

C−C and C−H Alkane Reductive Eliminations
A R T I C L E S
Figure 7. Eyring plot for 4 (b, 40-75 °C) and 5 ((, 40-80 °C) in C6D6.
Figure 6. Equilibrium plot of fac-3 (() and mer-3 (b) at 100 °C in C6D6.
(1.3 ( 0.1) × 10-4 s-1 for complex 4, and kobs ) (1.2 ( 0.1)
× 10-4 s-1 for complex 5.32 The rates of reaction were
unaffected by small amounts of (dppe)PtMe2 (6) and (dppbz)-
PtMe2 (7) present in the starting material. The rate constants
from different samples of 4 and 5 (containing variable amounts
(9%-25%) of 6 and 7, respectively) were identical.
Scheme 2
Activation parameters for the reductive elimination of meth-
ane from 4 of ∆Hq ) 25.7 ( 1.0 kcal/mol and ∆Sq ) 3 ( 3 eu
were calculated from kinetic data over a temperature range of
40 to 75 °C. For 5, very similar activation parameters of ∆Hq
) 26.6 ( 0.4 kcal/mol and ∆Sq ) 6 ( 2 eu were found over
the temperature range of 40 °C to 80 °C. The Eyring plots for
the methane reductive elimination reactions from complexes 4
and 5 are shown in Figure 7.
organic products were identified by 1H NMR (ethylene δ 5.25;
3
methane δ 0.15; ethane δ 0.80; propane δ 0.86 (t, JH-H ) 7
Hz), 1.26 (m, 3JH-H ) 7 Hz)) and also by GC/MS. Analysis of
the organic products by GC/FID revealed the ratio of ethylene:
ethane:propane to be 75:18:7. The Pt(II) products, 6 and 8, were
Thermolysis of the deuteride complex 4-d1 in benzene-d6 at
50 °C led to the clean formation of 6 and CH3D. Under the
same reaction conditions, 5-d1 yielded 7 and CH3D. No other
isotopomers of methane were observed (1H NMR), and no
scrambling of deuterium into Pt-Me groups of either the starting
1
identified by H and 31P NMR, and the spectra matched those
of authentic samples. As determined by integration of 1H NMR
spectra, the yields of 6 and 8 were 91% and 4%, respectively.
Intramolecular Isomerization of 3. During the thermolysis
of 3 in benzene-d6 at 165 °C, isomerization to an equilibrium
mixture of fac and mer isomers was observed. At the lower
temperature of 100 °C, there was no reactivity leading to organic
or Pt(II) products, and the isomerization was the only process
observed. The mer isomer is favored, with Keq ) 6.3 at 100 °C
(Figure 6). The isomerization reaction was complete within 4
materials or of the products was detected (1H, 1H{31P}, and 31
P
NMR). Both reactions exhibited clean first-order kinetics, with
similar rate constants (kobs ) 5.9 × 10-5 s-1 for 4-d1 and kobs
) 4.8 × 10-5 s-1 for 5-d1).32 The kinetic deuterium isotope
effects for C-H(D) reductive elimination from 4 and 5 are kH/
kD ) 2.2 and kH/kD ) 2.5, respectively.
Discussion
h, and kf ) 3.3 × 10-4 s-1 32,36
.
Unfortunately, purification of
Mechanism of Ethane Elimination from (dppe)PtMe4 (1).
Thermolysis of 1 in solution requires temperatures in excess of
150 °C before appreciable amounts of C-C reductive elimina-
tion to form ethane and (dppe)PtMe2 (6) occurs. Complex 1 is
significantly more thermally stable than other reported Pt(IV)
trimethyl and tetramethyl complexes which undergo reductive
elimination to form ethane. For example, the production of
ethane from the Pt(IV) complexes (PR3)2PtMe3X and (RNC)2-
PtMe4 in solution is conveniently monitored at approximately
60 °C.7a,b Thermolysis of (L2)PtMe3X (L2 ) dppe (PPh2(CH2)2-
PPh2) or dppbz (o-PPh2(C6H4)PPh2)) in solution to yield C-X
and C-C reductive elimination products was studied at 79 °C
for X ) I, 99 °C for X ) OAc, and 120 °C for X ) OAr.7c,d,g
Mechanistic investigations of C-C reductive elimination reac-
tions from these other Pt(IV) complexes have revealed that
ancillary ligand dissociation precedes the carbon-carbon bond
formation step.7 In complexes with monodentate ligands such
as PR3 or RNC, the neutral ligands dissociate to form a five-
coordinate intermediate from which C-C coupling occurs.7a,b
3 involved column chromatography which resulted in substantial
fac to mer isomerization and despite great effort, we were unable
to reproducibly generate pure samples of 3 with high enough
enrichment in the fac isomer to carry out a full kinetic/
mechanistic study on the isomerization reaction.
Thermolysis of L2PtMe3H (L2 ) dppe, dppbz). In solution,
fac-(dppe)PtMe3H (4) undergoes C-H reductive elimination at
ambient temperature to yield CH4 and (dppe)PtMe2 (6) (Scheme
2). The complex fac-(dppbz)PtMe3H (5) exhibits similar be-
havior, and produces CH4 and (dppbz)PtMe2 (7). The identities
of the products were confirmed by comparison to NMR spectra
of authentic samples.
The thermolyses of complexes 4 and 5 were monitored by
1H and 31P NMR spectroscopy in benzene-d6 at 50 °C. Clean,
reproducible first-order reaction kinetics were observed for both
complexes. The observed rate constants for reductive elimination
of methane from 4 and 5 were essentially identical, with kobs
)
(36) Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper Collins: New York, 1987.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9447

A R T I C L E S
Crumpton-Bregel and Goldberg
Scheme 3
When a bidentate phosphine ligand such as dppe or dppbz is
employed, a higher energy process involving charge separation
occurs as dissociation of the anionic X group generates a cationic
five-coordinate intermediate prior to C-C bond formation.7c,d,g
The extraordinary kinetic stability of (dppe)PtMe4 (1) toward
reductive elimination is thus likely to result from a lack of facile
access to a five-coordinate intermediate species.7f This is
consistent with the recent report that reductive elimination of
ethane from 1 can be induced to occur rapidly at room
temperature by the addition of a catalytic amount of an
electrophilic Pt complex.37 The catalyst was proposed to abstract
a methide group to generate the five-coordinate cationic
intermediate, dppePtMe3+, from which reductive elimination
takes place.
Despite the high temperatures required, the thermally pro-
moted formation of ethane from 1 is a well-behaved first-order
process. Kinetics experiments were performed between 165 and
205 °C. At 165 °C, the reaction proceeds slowly with a rate
constant on the order of 4 × 10-6 s-1 in a variety of solvents
(benzene-d6, THF-d8 and acetone-d6). The lack of a solvent
effect on the rate of the reaction is in contrast to the related
thermal reductive elimination of ethane from (dppe)PtMe3X in
which the rate of reaction was found to significantly increase
with the polarity of the solvent.7c,d,g The mechanism of that
reaction was shown to involve the formation of ionic intermedi-
ates.
(dppe)PtMe4 (1) lacks both neutral monodentate ligands and
a potential anionic leaving group X. Thus, both mechanisms
previously established for C-C reductive elimination from
Pt(IV) are disfavored. Three reasonable alternative mechanisms
for the reductive elimination of ethane from the Pt(IV) tetra-
methyl complex 1 can then be considered (Scheme 3).
The first mechanism is a radical or intermolecular pathway
(A). With estimates of PtIV-C bond strengths of 30-37 kcal/
mol7a-c,38 and temperatures in excess of 150 °C required for
reductive elimination, a Pt-Me bond cleavage pathway appears
reasonable. However, evidence against mechanism A is provided
by the results of the crossover study. When 1 and 1-d12 were
heated at 165 °C for just over one half-life, < 4% of the
crossover product, CH3CD3, was observed. Thus, the major
pathway for ethane elimination from 1 involves an intramo-
lecular reaction, rather than a radical or other intermolecular
process.
The two intramolecular mechanisms to be considered are
direct elimination (pathway B) and chelate opening to form a
five-coordinate intermediate prior to C-C coupling (pathway
C). The latter pathway is analogous to the neutral ligand pre-
dissociation mechanism observed with monodentate ligands.
However, in contrast to the situation with monodentate ligands,
the chelate remains coordinated (κ1). Thus, paths B and C are
kinetically indistinguishable and both mechanisms should yield
first-order rate laws. Nevertheless, the measured activation
entropy for the reaction potentially provides some insight to
differentiate between the two mechanisms. The fairly large ∆Sq
value (15 ( 4 eu) is more consistent with pathway C, a
reversible chelate opening (ligand dissociation) prior to reductive
coupling. A value closer to zero would be expected for pathway
B (see data for reductive elimination from 4 and 5), and
potentially for pathway C if chelate-opening was rate-determin-
ing. Yet, it should be noted that had a ∆Sq value close to zero
actually been observed, the interpretation would have been more
ambiguous. A survey of the literature shows that despite similar
proposed reaction mechanisms, i.e., ancillary ligand dissociation,
∆Sq values previously reported for C-C reductive elimination
reactions from Pt(IV) alkyl complexes vary over a considerable
range (-21 to +44 eu, Table 4). Even within the closely related
series of complexes (PMe2Ph)2PtMe3X (X ) Cl, Br, I) which
all undergo reaction by preliminary phosphine dissociation, the
values vary from 4 ( 2 eu for X ) Cl up to 16 ( 1 eu for
X ) I.
More compelling evidence against the direct elimination
pathway (B) was provided by thermolysis of the closely related
complex, (dppbz)PtMe4 (2). The two bidentate phosphine
ligands, dppe and dppbz, are very similar in the sterics and
electronics of their binding to Pt(IV). The P-Pt-P bond angles
for dppe and dppbz are nearly identical; the P-Pt-P angle is
86° in (dppe)PtMe4 (1), and 84° in (dppbz)PtMe4 (2). These
values are consistent with literature reports for other com-
plexes.39 The platinum-phosphorus coupling constants for the
two complexes are almost identical (1145 Hz for 1 and 1147
Hz for 2). Little difference is also noted in the PtMe4 fragments
of these two complexes. The Pt-C bond lengths are very similar
(37) Hill, G. S.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1999, 18,
1408.
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9448 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

C−C and C−H Alkane Reductive Eliminations
A R T I C L E S
Table 4. Literature Values of ∆Sq for Ethane Elimination from Pt(IV)
‡
∆S
complex
(cal/mol K)
solvent
mechanism proposed
ref
(MeNC)2PtMe4
-21 ( 3
4 ( 2
11 ( 1
16 ( 1
44 ( 4
benzene
isonitrile dissociation
phosphine dissociation
phosphine dissociation
phosphine dissociation
phosphine dissociation
7b
7a
7a
7a
7a
(PMe2Ph)2PtMe3Cl
(PMe2Ph)2PtMe3Br
(PMe2Ph)2PtMe3I
(PMePh2)2PtMe3I
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
Scheme 4
(Table 2) and the JP-H coupling constants for the Pt(IV)-CH3
groups are nearly identical (for Me trans to Me, JP-H ) 6.4 Hz
in 1 and 5.9 Hz in 2; for Me trans to P, JP-H ) 6.8 Hz in both
1 and 2) Thus, the electron donating ability of these two
phosphine ligands appears to be comparable.
The most significant difference between dppe and dppbz
ligands is the flexibility of the backbone. The benzene backbone
of dppbz is considerably more rigid than the ethane backbone
of dppe. The relative flexibility of the phosphine chelate should
become important in reactions in which one end of the
phosphine chelate dissociates from the metal center. Differences
in phosphine chelate flexibility have been previously used in
attempts to probe possible phosphine dissociation in C-X
(X ) CN, SR) reductive eliminations from Pd(II) complexes.40
The flexibility of the phosphine chelate had no effect in these
Pd(II) systems, a result which was used to support direct
eliminations not requiring chelate opening (phosphine predis-
sociation). Similarly in a Pt(IV) system closely related to that
under discussion here, C-O reductive elimination reactions from
(dppe)PtMe3OAc and (dppbz)PtMe3OAc were found to have
the same rate constant, despite the different phosphines.7g The
mechanism for these Pt(IV) reductive elimination reactions was
found to involve initial dissociation of acetate, rather than
phosphine. The reductive elimination of ethane from (dppe)-
PtMe4 (1) is unique in that a very large effect on reactivity was
observed as a result of varying the phosphine chelate flexibility.
Thermolysis of (dppbz)PtMe4 (2), which has the less flexible
phosphine, showed only 4% conversion after 300 h at 165 °C
in THF-d8. To achieve 4% conversion of (dppe)PtMe4 (1) (a
first-order process) would require only ca. 3.5 h under the same
conditions. This ca. 100-fold difference between the rates of
ethane reductive elimination from 1 and 2 correlates with
phosphine chelate flexibility and provides strong support for a
mechanism involving phosphine dissociation prior to C-C
coupling for complex 1 (pathway C).
18:7 indicating that while â-hydride elimination dominates,
C-C reductive elimination to form propane and ethane is
competitive.
The mechanism most consistent with the formation of these
products from 3 is shown in Scheme 4. Dissociation of
phosphine to yield a five-coordinate intermediate occurs as the
first step. The â-hydride elimination reaction proceeds from
the five-coordinate intermediate to generate ethylene and the
Pt(IV) methyl hydride complex, (dppe)PtMe3H (4). Complex 4
undergoes C-H reductive elimination at ambient temperature
(as described above), and therefore should undergo rapid
elimination of methane at 165 °C. The final Pt product from
this pathway is (dppe)PtMe2 (6). Competitive with â-hydride
elimination, presumably from the five-coordinate intermediate,
is C-C reductive elimination to form propane and (dppe)PtMe2
(6) or ethane and the Pt(II) methyl ethyl complex (dppe)PtMeEt
(8). The Pt(II) complex 8 is a 16 e- species and could itself
undergo â-hydride elimination to generate ethylene and Pt-
(0).41 However, any such reactivity would be a minor contribu-
tion, as 95% of the Pt was accounted for as tractable Pt(II)
products (91% of 6, 4% of 8). Thus, the majority of the ethylene
results from â-hydride elimination from the Pt(IV) species 3
rather than the Pt(II) complex 8.
Evidence has also been obtained to show that mechanism C
is kinetically viable. That is, phosphine chelate opening does
occur, and it does so on a time scale that would allow it to
precede the C-C coupling reaction. The observation of â-hy-
dride elimination products upon thermolysis of the closely
related tetraalkyl complex (dppe)PtMe3Et (3) at 165 °C estab-
lishes that the phosphine dissociation to generate an open site
takes place readily at this temperature. â-hydride elimination
requires an open coordination site cis to the ligand with
â-hydrogens.1 Upon thermolysis of 3 in benzene-d6 at 165 °C,
the organic products ethylene, methane, ethane and propane were
observed. The ratio of ethylene to propane to ethane was 75:
Additional support for the kinetic competence of the phos-
phine dissociation pathway is provided by the isomerization of
3 to an equilibrium mixture of fac and mer isomers during the
thermolysis at 165 °C. At 100 °C, isomerization of 3 occurs
cleanly without further reaction to generate Pt(II) complexes
(Figure 6). Isomerization of octahedral Pt(IV) complexes is most
commonly proposed to occur via fluxional five-coordinate
(38) Low, J. J.; Goddard, W. A. J. Am. Chem. Soc. 1986, 108, 6115.
(39) Dierkes, P.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans. 1999,
1519, and references therein.
(40) (a) Mann, G.; Baranano, D.; Hartwig, J. F.; Rheingold, A. L.; Guzei, I. A.
J. Am. Chem. Soc. 1998, 120, 9205. (b) Marcone, J. E.; Moloy, K. G. J.
Am. Chem. Soc. 1998, 120, 8527.
(41) Bryndza, H. E.; Calabrese, J. C.; Marsi, M.; Roe, C. D.; Tam, W.; Bercaw,
J. E. J. Am. Chem. Soc. 1986, 108, 4805, and references therein.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9449

A R T I C L E S
Crumpton-Bregel and Goldberg
intermediates.20b,42 The long assumed high fluxionality of five-
coordinate Pt(IV) was recently supported by the observation
that the isolable five-coordinate Pt(IV) complex (nacnac)PtMe3
remains fluxional down to at least -50 °C.9a Isomerization of
3 is thus proposed to proceed via a five-coordinate intermediate
formed by dissociation of one end of the phosphine chelate
(Scheme 4).
favored thermodynamically on the basis of its stronger trans
influence, then reductive coupling of the ethyl ligand with a
basal methyl ligand may occur with greater frequency than
methyl-methyl coupling.
Given the discussion above which indicates that the proclivity
for phosphine dissociation (chelate opening) appears to be
greater for (dppe)PtMe3Et (3) versus (dppe)PtMe4 (1), it is
natural to question whether 3 is truly a sufficient model for 1
in terms of establishing the kinetic viability of phosphine
dissociation occurring on the C-C reductive elimination
pathway. A more direct analogue of 1 is the partially deuterated
tetramethyl complex (dppe)PtMe(CD3)3 (fac-1-d9). Although
fac/mer isomerization was indeed observed when fac-1-d9 was
heated at 100 °C in benzene-d6, the reaction proceeded almost
200 times more slowly than the isomerization of fac-3. This
large disparity in the rates of isomerization of fac-1-d9 and fac-3
can be attributed to a combination of the differences in rates of
phosphine chelate opening and the rates of isomerization of the
respective five-coordinate intermediates. However, the important
point to be noted here is that both of these isomerization
reactions are significantly faster than C-C reductive elimination.
Thus, even though isomerization of fac-1-d9 at 100 °C is a
relatively slow process (kf ) 1.7 × 10-6 s-1), the rate constant
for ethane elimination from 1 at 100 °C, as estimated from the
Eyring plot (Figure 2), is 6.8 × 10-10 s-1, which is 2500 times
slower than isomerization.
Mechanism of Methane Elimination from L2PtMe3H
(L2 ) dppe, dppbz). Similar to C-C reductive elimination of
alkyl groups from Pt(IV), previous studies of C-H reductive
elimination of alkanes from Pt(IV) have provided evidence for
ancillary ligand dissociation and C-H coupling from a five-
coordinate intermediate.20-22,47 However, all of these experi-
mental studies have involved Pt(IV) alkyl hydride complexes
with nitrogen donor ligands. Recently, calculations in phosphine
ligated systems have been used to examine the possibility of
direct C-H reductive elimination.46,48 In calculations using the
model complex Cl2(PH3)2Pt(CH3)H, reductive elimination of
methane was found to occur preferentially via a five-coordinate
intermediate formed by preliminary phosphine dissociation.46
No transition state for the direct elimination pathway could even
be found. When phosphine dissociation was disfavored by using
the more basic phosphine PMe3, a significant difference was
noted. Direct reductive elimination of methane from Cl2(PMe3)2-
Pt(CH3)H was found to be enthalpically faVored by 2 kcal/mol
over a phosphine dissociation pathway.48 However, the entropy
associated with phosphine dissociation cannot be neglected, and
would likely overcome, in practice, the slight enthalpic prefer-
ence for direct elimination. In contrast, with a chelating
phosphine, the entropic advantage of phosphine dissociation
would be significantly reduced and it was suggested that a
Pt(IV) alkyl hydride complex with a chelating phosphine ligand
might undergo direct C-H reductive elimination.
An interesting observation is made when comparing the
thermolyses reactions of 1 and 3. The C-C reductive elimina-
tion reaction of two methyl groups to form ethane is faster from
3 than from 1. The rate constant for ethane formation from 1 is
4.2 × 10-6 s-1 (165 °C, C6D6), whereas the estimated rate
constant for ethane elimination from 3 is 8.4 × 10-6 s-1 (165
°C, C6D6).43 Given that Me-Me coupling is statistically twice
as likely from 1 than from 3, the reductive elimination of ethane
is actually four times faster from 3 than from 1. This acceleration
of Me-Me coupling in the presence of an ethyl group can easily
be accommodated by a mechanism involving phosphine dis-
sociation occurring prior to rate-determining C-C reductive
elimination. Under these conditions (Path C, Scheme 3), the
equilibrium constant for phosphine dissociation (Kphos) will
factor into the observed rate constant for the Me-Me coupling.
The presence of an ethyl group should lead to a greater value
of Kphos on both steric and electronic grounds. The sterically
bulkier ethyl group should favor phosphine dissociation. In
addition, the Pt-P coupling constant observed for the phos-
phorus trans to ethyl in mer-3 (1JPt-P ) 992 Hz), as compared
to that trans to methyl (1JPt-P ) 1141 Hz), clearly illustrates
that ethyl is a stronger trans influence ligand than methyl. The
1
tetramethyl complex 1 has JPt-P ) 1145 Hz for phosphorus
trans to methyl. Thus, the phosphine trans to the ethyl group in
mer-3 appears to be more weakly bound to the Pt metal center
than that trans to a methyl group, and a greater Kphos is expected
for 3 than for 1.44 This greater Kphos should yield a larger kobs
for Me-Me coupling.
It was also noted in the thermolysis of 3 at 165 °C, that Me-
Et elimination to form propane was preferred over Me-Me
elimination to form ethane. Statistically, equal amounts of
propane and ethane are expected. Instead, the observed ratio of
propane to ethane was 2.5:1. A similar preference for ethyl
versus methyl coupling reactions was reported for the ther-
molysis of (PMe2Ph)2PtMeEt2I in which Et-Et coupling to form
butane was observed, but no Me-Et coupling to form propane
was detected.45 The preference for C-C reductive elimination
of propane from 3 could be a simple steric effect or it may be
due to the stronger trans influence of the ethyl ligand versus
the methyl ligand. Dissociation of the phosphine trans to ethyl
places the ethyl ligand in the apical position of the resulting
five-coordinate square-pyramidal complex. Concerted C-C (and
C-H) reductive elimination occurs with the apical ligand and
one of the basal ligands.25,46 If the placement of the ethyl ligand
in the apical position of the five-coordinate intermediate is
The effect of using a chelating phosphine on the stability of
L2PtMe3H is indeed dramatic. fac-(dppe)PtMe3H (4) and fac-
(dppbz)PtMe3H (5) are the first alkyl hydride complexes of
(42) (a) Crespo, M.; Puddephatt, R. J. Organometallics 1987, 6, 2548. (b)
Wehman-Ooyevaar, I. C. M.; Drenth, W.; Grove, D. M.; van Koten, G.
Inorg. Chem. 1993, 32, 3347, and references therein.
(43) This number was calculated by multiplying the observed rate constant for
thermolysis of (dppe)PtMe3Et at 165 °C (kobs ) 1.2 × 10-4 s-1) by the
relative percent ethane formation (7%) from the GC/FID analysis.
(44) Note that the fac-3/mer-3 equilibrium favors the mer isomer, and this
equilibrium is established rapidly.
(45) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E.; Lavington, S. W. J.
Chem. Soc., Dalton Trans. 1974, 1613.
(46) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. J. Am. Chem. Soc. 2000,
122, 1456, and references therein.
Pt(IV) bearing phosphine ligands that have been isolated.24
4
(47) With chelating ligands, although ligand dissociation (chelate-opening) has
been proposed, whether ligand dissociation occurs prior to or concurrent
with the C-H coupling is difficult to determine.
(48) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. Organometallics 2001, 20,
2669.
9
9450 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

C−C and C−H Alkane Reductive Eliminations
A R T I C L E S
Scheme 5
The deuterium isotope effects for C-H(D) reductive elimina-
tion from 4 and 5 are both normal (>1), and are similar for
both complexes. The kinetic isotope effect for C-H(D) coupling
from 4 was kH/kD ) 2.2, and from 5 was kH/kD ) 2.5. The
isotope effects measured for 4 and 5 do not help to distinguish
between the possible mechanisms of direct elimination and
phosphine dissociation, as both normal and inverse isotope
effects have been observed for C-H eliminations from transition
metal alkyl hydrides occurring by both paths.20a The normal
isotope effects for C-H reductive elimination from 4 and 5
do, however, confirm that C-H coupling is the rate-determining
step.
has even been crystallographically characterized. Both 4 and 5
undergo C-H reductive elimination to form methane at ambient
temperature. Similar to C-C reductive elimination from the
tetramethyl analogue 1, both a direct elimination pathway (D)
and a chelate opening (ligand dissociation) pathway (E) are
considered as possible mechanisms for the C-H reductive
elimination reaction (Scheme 5).
The phosphine dissociation pathway (E) would be analogous
to the mechanism supported for C-C reductive elimination from
(dppe)PtMe4 (Pathway C, Scheme 3), as described above. The
most convincing evidence for the involvement of phosphine
dissociation in the C-C reductive elimination reaction from 1
was the dramatic rate inhibition that occurred when dppe was
replaced by the more rigid chelating ligand dppbz. In contrast,
C-H reductive elimination reactions from fac-(dppe)PtMe3H
(4) and fac-(dppbz)PtMe3H (5) both proceed with virtually
identical first-order rate constants (ca. 1 × 10-4 s-1 at 50 °C in
benzene-d6) and have very similar activation parameters
(∆Hq ≈ 26 kcal/mol, ∆Sq ≈ 3-6 eu). The similarity in the rates
and activation parameters for the complexes of dppe and the
more rigid chelate dppbz provide compelling support for a
mechanism of direct elimination, without prior ligand dissocia-
tion (Pathway D).
A ∆Sq value close to zero would be expected for a direct
elimination and so the relatively low values of ∆Sq observed
for methane elimination from 4 and 5 would be consistent with
a mechanism that does not involve initial chelate opening. Yet,
it is important to note that the small ∆Sq values do not preclude
a chelate-opening mechanism. Particularly, if the chelate-
opening were the rate-determining step, the resulting increase
in entropy might not be realized in the transition state. Similar
to C-C reductive elimination reactions, examination of ∆Sq
for a variety of C-H reductive elimination reactions from d6
late transition metal complexes shows that the values span a
considerable range, particularly for those reactions proposed to
proceed via ligand dissociation pathways (Table 5). It is,
however, difficult to reconcile a chelate-opening step (rate-
determining or otherwise) with the observation of virtually
identical activation parameters (both ∆Hq and ∆Sq) for dppe
and dppbz, in view of the fact that the chelates are of such
differing rigidity. In addition, the fact that C-H reductive
elimination is completely independent of the rigidity of the
chelate, whereas C-C reductive elimination is extremely
sensitive to this parameter, strongly argues (even more so than
does each observation individually) that the C-C elimination,
but not the C-H eliminations, proceeds via chelate-opening.
In light of this difference in mechanism, the significantly
different values of ∆Sq for C-C reductive elimination (15 ( 4
eu) and for C-H reductive elimination (3-6 eu) appear quite
reasonable.
C-C vs C-H Reductive Elimination from L2PtMe3R
(L2 ) dppe, dppbz; R ) Me, H). The series of complexes
(L2PtMe3R; L2 ) dppe, dppbz; R ) Me, H) provide a unique
opportunity to directly compare Me-Me and Me-H reductive
elimination reactions.49 For the first time, both reactions are
observed from the same metal ligand environment. Typically,
if C-H reductive elimination is observed from a metal alkyl
hydride complex, the analogous metal dialkyl complex is stable
to C-C reductive elimination.50 Often, other decomposition
pathways are found to dominate upon thermolysis of the metal
dialkyl species.50a,c Conversely, for those systems in which C-C
reductive elimination is observed, the corresponding metal alkyl
hydrides are too unstable, even at low temperatures, to allow
for adequate mechanistic study.
Indeed, in the Pt(IV) bisphosphine system described here, a
dramatic temperature difference for reductive elimination from
the trialkyl hydride complexes versus the tetramethyl complexes
is observed. The L2PtMe3H complexes undergo reductive
elimination of methane at 50 °C, with kobs ) 1 × 10-4 s-1. To
achieve a comparable rate constant for C-C reductive elimina-
tion from (dppe)PtMe4 (1), a temperature of nearly 200 °C
(extrapolation from the Eyring data) would be required.
However, perhaps even more significant than the difference in
rates of reaction is that C-C and C-H reductive elimination
proceed via different mechanisms. C-H reductive elimination
from fac-(dppe)PtMe3H (4) and fac-(dppbz)PtMe3H (5) proceeds
directly from the six-coordinate geometry. C-C reductive
elimination from (dppe)PtMe4 (1) has a much higher activation
barrier and does not proceed via a direct coupling pathway.
Rather, dissociation of one end of the phosphine chelate occurs
and C-C coupling takes place from a five-coordinate species.
Calculations have confirmed that C-H and C-C coupling
reactions are more facile from five-coordinate Pt(IV) centers
than from the six-coordinate complexes.46,48 However, the
enthalpic cost of ligand dissociation must also be considered.
When the entropic advantage of ligand loss is thwarted by
employment of a chelating ligand, then direct elimination may
become feasible. C-H reductive elimination has, in general, a
lower activation barrier than C-C reductive elimination, and
with a chelating phosphine C-H reductive elimination from
Pt(IV) occurs directly from the six-coordinate species. The
enthalpic barrier to C-C reductive elimination is significantly
higher and C-C coupling apparently does not take place from
(49) Norton, J. R.; Acc. Chem. Res. 1979, 12, 139. (b) Glueck, D. S.; Bergman,
R. G. Organometallics 1991, 10, 1479 (c) Rees, W. M.; Churchill, M. R.;
Li, Y.-J.; Atwood, J. D. Organometallics 1985, 4, 1162.
(50) For example: (a) Cp*(PMe3)Ir(Me)(H) versus Cp*(PMe3)Ir(Me)2, ref 49b
(b) MeIr(H)2(CO)(PPh3)2 versus Me2Ir(CO)(PPh3)2I, ref 49c (c) (N2)PtMe2-
(H)X versus (N2)PtMe3X where N2 ) bpy or tmeda, refs 13b, 20a, 21, 25.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9451

A R T I C L E S
Crumpton-Bregel and Goldberg
Table 5. Literature Values of ∆Sq for C-H Elimination from Pt(IV), Ir(III) and Rh(III)
q
compound
∆S (eu)
T (K)/solvent
mechanism proposed
ref
(tmeda)Pt(CH2Ph)HCl2
(tmeda)PtMe2HCl
cis-[(BPMA)PtMe2H]BF4
-18.5
4.1
6.7
24.1
13
230-249, CD2Cl2
238-258, CD3OH
293-313, CD2Cl2
298-311, CD2Cl2
363-403, C6D6
291-317, C6D6
398-413, C6D6
300-317 C6D6
Cl- dissoc.
20a
20a
20c
20c
20h
16c
16b
17a
Cl- dissoc.
a
pyridyl dissoc.
pyridyl dissoc.
pyrazolyl dissoc.
direct elim.
direct elim.
PMe3 dissoc.
b
trans-[(BPMA)PtMe2H]BF4
TpMe2PtMe2H
(dppe)(CO)IrEtH2
-10.9
10
Cp*(PMe3)Ir(C6H11)H
(PMe3)3Rh(CH2OMe)HCl
5.3
a The hydride is cis to the amine ligand of BPMA. b The hydride is trans to the amine ligand of BPMA.
Microanalytical Services Ltd., Delta, B. C., Canada or Atlantic
Microlab, Inc., Norcross, GA.
the six-coordinate complex. Rather a lower energy route
involving ligand loss is followed. Despite this lower energy
pathway, the C-C coupling still requires a temperature some
150 °C higher than the C-H reductive elimination reaction.
Thus, although theoretically possible, direct sp3C-sp3C reductive
elimination from six-coordinate Pt(IV) without preliminary
ligand dissociation appears to be an unfavorable reaction
pathway.
1H NMR spectra were collected using Bruker AC 200, DPX 200,
AF 301, DRX 499, and AM 500 spectrometers. The spectra were
referenced to residual protiated solvent and chemical shifts are reported
in parts per million downfield of tetramethylsilane. 13C{1H} NMR
spectra were collected using Bruker DRX 499 (125 MHz) and AM
500 (125 MHz) spectrometers. The spectra were referenced to residual
protiated solvent peaks or a known chemical shift standard. Chemical
shifts are reported in parts per million downfield of tetramethylsilane.
31P{1H} NMR spectra were collected using Bruker AC 200 (81 MHz),
DPX 200 (81 MHz), AF 301 (121.5 MHz) and AM 500 (202.5 MHz)
spectrometers. The spectra were referenced to an external standard of
85% phosphoric acid, and chemical shifts are reported in parts per
million downfield from this reference. All coupling constants are
reported in Hertz.
Summary
The Pt(IV) complex (dppe)PtMe4 (1) undergoes reductive
elimination of ethane at very high temperatures in solution.
Despite the chelate effect of dppe, considerable evidence
supports that reductive elimination from 1 proceeds via a
mechanism that involves dissociation of one end of the
phosphine chelate to form a five-coordinate intermediate prior
to C-C bond formation (pathway C, Scheme 3). Replacing the
chelating bisphosphine dppe with the more rigid bisphosphine
chelate dppbz results in a dramatic reduction in the rate of ethane
elimination; reductive elimination of ethane from (dppbz)PtMe4
(2) is 2 orders of magnitude slower than from 1. The observation
of isomerization of fac-(dppe)PtMe3Et (3) and fac-(dppe)PtMe-
(CD3)3 (fac-1-d9) at a significantly lower temperature, in
conjunction with â-hydride elimination from (dppe)PtMe3Et (3),
demonstrates the kinetic viability of dppe chelate opening prior
to C-C coupling.
NMR tube reactions were carried out in flame-sealed Wilmad 504-
PP medium-walled tubes. Sealed tubes were prepared either using a
tube previously attached to a ground glass joint or by attaching the
tube to a Kontes vacuum stopcock via a Cajon adapter.51 The volume
of the solution in the tube was measured by comparison to a previously
calibrated 504-PP tube. As a safety precaution, the tubes were
surrounded by stainless steel jacket tubes during heating and rapid
cooling. The oil bath used for heating was a Neslab Excal EX-250 HT
elevated temperature bath.
The crossover study was carried out in a Wilmad pressure/vacuum
valve tube (524-PV-7). The tube was attached to a Kontes vacuum
stopcock via Swagelok and Cajon fittings. The gases produced were
analyzed by a Kratos direct insertion mass spectrometer.
The novel Pt(IV) alkyl hydride complexes fac-(dppe)PtMe3H
(4) and fac-(dppbz)PtMe3H (5) were found to undergo C-H
reductive elimination of methane at ambient temperature in
solution. 4 and 5 are the first examples of Pt(IV) alkyl hydrides
ligated by phosphorus which are stable enough to be isolated
and 4 has been crystallographically characterized. In contrast
to the C-C reductive elimination reactions from the analogous
complexes 1 and 2, C-H reductive elimination reactions from
4 and 5 occur at virtually identical rates with similar activation
parameters. The similarity of the facility of the reaction
irrespective of the rigidity of the phosphine chelate strongly
supports the proposal that these reactions occur directly from
the six-coordinate geometry. This is the first time that mecha-
nistic evidence for the direct reductive elimination of alkanes
from octahedral Pt(IV) alkyl complexes has been reported.
The gases produced in the thermolysis of (dppe)PtMe3Et were
analyzed by injection into a Kratos mass spectrometer fitted with a
Chrompack plot fused silica capillary column (50 in × 0.53 mm ID,
30 ft) with an alumina/KCl coating. The injector temperature was 50
°C, the oven was cooled to 0 °C, and the source temperature was 200
°C. The relative amount of each gas was determined by injection into
a Hewlett-Packard GC/FID fitted with the same Chrompack column,
with the oven cooled to 10 °C and a flow rate of 4 psi. The syringe
used for these experiments was a 50 µL Gastight syringe with a locking
valve.
Unless otherwise specified, all reagents were purchased from
commercial suppliers and used without further purification. The
phosphines [dppe (PPh2(CH2)2PPh2) and dppbz (o-PPh2(C6H4)PPh2)]
were recrystallized before use. Toluene, ether, and THF were distilled
under nitrogen from sodium/benzophenone. Pentane was distilled under
nitrogen from CaH2. Dioxane, benzene-d6, and THF-d8 were vacuum
transferred from sodium/benzophenone. Acetone-d6 and hexamethyl-
Experimental Section
52
disiloxane were vacuum transferred from activated 4Å sieves. [PtMe3I]4
General Information. Unless otherwise noted, all reactions under
nitrogen were carried out in a Vacuum Atmospheres VAC-MO-40-M
drybox. Samples were weighed using a Mettler-Toledo B154 College
analytical balance. Outside the box, samples were weighed using a
Mettler-Toledo AG204 DeltaRange analytical balance. Samples requir-
ing microgram accuracy were weighed on a Mettler MT5 analytical
balance. Elemental analyses were carried out by either Canadian
and (dppe)PtMe3I7c were prepared as previously described. The
deuterated analogues of these compounds were prepared in the same
manner, with deuterated alkylating reagents substituted for the protiated
(51) Wayda, A. L.; Darensbourg, M. Y. Experimental Organometallic Chem-
istry: A Practicum in Synthesis and Characterization; ACS Symposium
Series 357; American Chemical Society: Washington, DC, 1987.
(52) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411.
9
9452 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

C−C and C−H Alkane Reductive Eliminations
A R T I C L E S
versions. (dppe)PtMe3(OAc)7d and (dppbz)PtMe3(OAc)7e were prepared
as previously described.
stir overnight. The reaction was exposed to air and the excess CH3Li
was quenched by slow addition of water (0.5 mL) to the mixture
(0 °C). The volatiles were removed under vacuum. CH2Cl2 was added
to the crude product to dissolve the Pt compounds, leaving behind the
inorganic salts. The solution was filtered, and the volatiles were removed
from the filtrate under vacuum. This material contained 70% dppbz-
PtMe4 (2) and 30% dppbzPtMe2 (7) (31P NMR). Chromatography on
neutral alumina (Brockmann II-III) was used to separate 2 from the
Pt(II) impurity. 2 was eluted with 3:1 pentane/CH2Cl2 and the Pt(II)
eluted with pure CH2Cl2. 2 was recrystallized from CH2Cl2/pentane,
Synthesis, Purification and Characterization of (dppe)PtMe4 (1).
An alternative preparation to that previously reported for 1 was used.30
Under nitrogen, an ether solution of CH3Li (11 mL, 15 mmol) was
added dropwise with stirring to a cooled suspension (-35 °C) of (dppe)-
PtMe3I (394 mg, 0.515 mmol) in toluene (15 mL). The reaction was
allowed to come to ambient temperature, and after stirring overnight,
the solution was pale yellow and slightly cloudy. In the air, water (2
mL) was added dropwise to the reaction mixture (0 °C). The volatiles
were removed under vacuum and toluene (20 mL) was added to the
flask to separate the organometallic product from the inorganic salts.
The mixture was filtered through pipets packed with glass wool, and
the volatiles were removed from the filtrate, which left behind a pale
yellow oil. 1 was separated from the (dppe)PtMe2 (6) byproduct of the
reaction via chromatography on neutral alumina. 1 was eluted with
3:1 pentane/CH2Cl2 and pure CH2Cl2 was used to elute 6. The fractions
containing only 1 (determined by TLC and/or NMR) were combined
and evaporated to dryness. Recrystallization from CH2Cl2/pentane
afforded colorless needles in 60% yield (0.202 mg). 1H NMR: (C6D6)
1
and white crystals were isolated in 33% yield (68.7 mg). H NMR:
2
3
(THF-d8) δ -0.51 (t, w/Pt satellites, 6H, JPt-H ) 44, JP-H ) 6.0,
Pt-CH3 trans to methyl), 0.67 (t, w/Pt satellites, 6H, 2JPt-H ) 60, 3JP-H
) 7.0, Pt-CH3 trans to phosphorus), 7.2-7.7 (complex aromatic
region); (C6D6) δ 0.18 (t, w/Pt satellites, 6H, 2JPt-H ) 43, 3JP-H ) 5.9,
Pt-CH3 trans to methyl), 1.44 (t, w/Pt satellites, 6H, 2JPt-H ) 60, 3JP-H
) 6.8, Pt-CH3 trans to phosphorus), 6.8-7.7 (complex aromatic
1
region). 13C NMR (THF-d8): δ -2.1 (s, w/Pt satellites, JPt-C ) 400,
1
Pt-CH3 trans to methyl), 4.0 (dd, w/Pt satellites, JPt-C ) 548, trans-
2JP-C ) 125, cis-2JP-C ) 4.8, Pt-CH3 trans to phosphorus), 128.2, 130.3,
131.8, 133.9, 136.1 (aromatic -CH), 131.1, 144.6 (aromatic quater-
2
3
δ 0.14 (t, w/Pt satellites, 6H, JPt-H ) 44, JP-H ) 6.4, Pt-CH3 trans
1
nary). 31P NMR (THF-d8): δ 15.6 (s, w/Pt satellites, JPt-P ) 1148).
2
3
to methyl), 1.6 (t, w/Pt satellites, 6H, JPt-H ) 60, JP-H ) 6.9, Pt-
CH3 trans to phosphorus), 2.2 (m, 4H, P-CH2-CH2-P), 7.0-7.4
(complex pattern, aromatic signals); (THF-d8) δ -0.54 (t, w/Pt satellites,
Elemental analysis calculated for C34H36P2Pt: C, 58.20; H, 5.17.
Found: C, 58.40; H, 5.23.
2
3
6H, JPt-H ) 44, JP-H ) 6.4, Pt-CH3 trans to methyl), 0.82 (t, w/Pt
satellites, 6H, 2JPt-H ) 60, 3JP-H ) 6.8, Pt-CH3 trans to phosphorus),
2.8 (m, 4H, P-CH2-CH2-P), 7.2-7.7 (complex pattern, aromatic
Synthesis and Purification of (dppe)PtMe3Et (3). Under nitrogen,
EtMgBr (3.0M solution in Et2O, 6.6 mmol) was added to a cooled
suspension (-35 °C) of (dppe)PtMe3I (0.965 g, 1.26 mmol) in toluene
(180 mL). The reaction was allowed to stir overnight. The reaction
was exposed to air and the excess EtMgBr was quenched by slow
addition of water (0.5 mL) to the reaction mixture (0 °C). The volatiles
were removed under vacuum. The remaining solid consisted primarily
of inorganic salts, (dppe)PtMe2 (6) and 3. The solubility of 6 in Et2O
is significantly less than that of 3. To achieve crude separation of 3
from the rest of the mixture, the solid residue was treated with ca. 80
mL of Et2O, and the ether solution was transferred to another flask.
Evaporation of ether left a residue, which was treated with ca. 50 mL
of Et2O in a similar fashion. Evaporation of the ether provided 0.402
g of solid, of which 77% was 3. Approximately 51% was fac-3 and
26% was mer-3 (31P NMR). Chromatography on alumina was used to
separate 3 from Pt(II) impurities. Optimal separation was achieved with
neutral alumina (Brockman II-III) and a benzene/pentane solvent
system. The Pt(IV) species eluted with 1:1 benzene/pentane and the
Pt(II) complexes with pure benzene. While on the column, nearly all
of the fac-3 isomerized to mer-3 and formation of (dppe)PtMe4 (1)
was observed. Later fractions contained progressively more 1. The
fractions with more than 5% 1 were combined and purified with a
second column (mer-3 elutes before 1). The final yield of 3 after
recrystallization from CH2Cl2/pentane was 7% (58.8 mg). These samples
typically contained 3-5% 1. fac-(dppe)PtMe3Et (fac-3); 1H NMR
signals). 13C NMR (C6D6): δ -4.6 (s, w/Pt satellites, JPt-C ) 398,
1
1
Pt-CH3 trans to methyl), 3.4 (dd, w/Pt satellites, JPt-C ) 547, trans-
2JP-C ) 127, cis-2JP-C ) 4.9, Pt-CH3 trans to phosphorus), 28.6
(complex pattern, P-CH2-CH2-P), 128-134 (aromatic signals, unre-
solved). 31P NMR data: (C6D6) δ 7.3 (s, w/Pt satellites, 1JPt-P ) 1145);
(acetone-d6) δ 7.7 (s, w/Pt satellites, 1JPt-P ) 1176); (CDCl3) δ 6.8 (s,
1
w/Pt satellites, JPt-P ) 1151). Elemental analysis calculated for
C30H36P2Pt: C, 54.90; H, 5.33. Found: C, 55.13; H, 5.55.
Synthesis, Purification and Characterization of (dppe)Pt(CD3)4
(1-d12). Under nitrogen, an ether solution of CD3Li (9 mL, 4.5 mmol)
was added dropwise with stirring to a cooled suspension (-35 °C) of
(dppe)Pt(CD3)3I (151 mg, 0.195 mmol) in toluene (10 mL). The rest
of the procedure used to isolate and purify 1-d12 is identical to that
reported above for 1. Colorless needles of 1-d12 were isolated in 51%
yield (66.2 mg). 1H NMR (C6D6): δ 2.2 (m, 4H, P-CH2-CH2-P), 6.9-
7.5 (complex aromatic region). 31P NMR (C6D6): δ 7.4 (s, w/Pt
1
satellites, JPt-P ) 1145).
Synthesis, Purification and Characterization of fac-(dppe)PtMe-
(CD3)3 (fac-1-d9). Under nitrogen, a THF solution of MeMgCl (2 mL,
5.6 mmol) was added to a suspension of (dppe)Pt(CD3)3I (125 mg,
0.161 mmol) in toluene (15 mL). The reaction was brought out of the
box under nitrogen in a glass vessel sealed with a vacuum stopcock
and placed in an oil bath (40 °C). The reaction was heated, with stirring,
for 24 h. In the air, water (3 mL) was added slowly to the reaction
mixture (0 °C). The organic layer (containing the Pt products) was
pipetted away from the aqueous layer, and filtered through glass wool.
The aqueous layer was washed three times with 5 mL portions of
toluene, which were pipetted away and filtered. The filtrate was a clear,
colorless solution. A colorless oil remained after the volatiles were
removed under vacuum. Recrystallization from CH2Cl2/pentane afforded
colorless needles of fac-1-d9 in 88% yield (94.1 mg). 1H NMR
2
3
(C6D6): δ 0.14 (t, w/Pt satellites, 3H, JPt-H ) 42, JP-H ) 6.4, Pt-
CH3 trans to ethyl), 0.73 (tq, w/Pt satellites, 2H, 2JPt-H ) 53, 3JH-H
)
7.7, 3JP-H ) 7, Pt-CH2CH3), 1.16 (t, w/Pt satellites, 3H, 3JPt-H ) 33,
3JH-H ) 7.7, Pt-CH2CH3), 1.51 (t, w/Pt satellites, 6H, JPt-H ) 61,
2
3JP-H ) 7.0, Pt-CH3 cis to ethyl), 2.3 (m, 4H, P-CH2-CH2-P), 7.0-
7.4 (complex aromatic region). 13C NMR (C6D6): δ -4.5 (s, w/Pt
satellites, 1JPt-C ) 380, Pt-CH3 trans to ethyl), -2.8 (s, w/Pt satellites,
1JPt-C ) 396, Pt-CH2CH3), 3.4 (dd, w/Pt satellites, 1JPt-C ) 567, trans-
2JP-C ) 122, cis-2JP-C ) 6.4, Pt-CH3 cis to ethyl), 13.1 (s (br), Pt-
CH2CH3), 27.4 (complex pattern, P-CH2-CH2-P), 128-133 (aromatic
2
3
(C6D6): δ 0.13 (t, w/Pt satellites, 3H, JPt-H ) 43, JP-H ) 6.4, Pt-
CH3), 2.2 (m, 4H, P-CH2-CH2-P), 6.8-7.6 (complex aromatic region).
1
signals, unresolved). 31P NMR (C6D6): δ 5.2 (s, w/Pt satellites, JPt-P
1
31P NMR (C6D6): δ 7.4 (s, w/Pt satellites, JPt-P ) 1138).
) 1147). mer-(dppe)PtMe3Et (mer-3); 1H NMR (C6D6): δ 0.12 (t, w/Pt
Synthesis, Purification and Characterization of (dppbz)PtMe4
(2). Under nitrogen, dppbz (0.133 g, 0.297 mmol, dppbz ) 1,2-bis-
(diphenylphosphino)benzene) and [PtMe3I]4 (0.133 g, 0.0742 mmol)
in THF (10 mL) were allowed to stir overnight. CH3Li (1.4 M in Et2O,
2.1 mmol) was then added, and the reaction mixture was allowed to
satellites, 6H, 2JPt-H ) 44, 3JP-H ) 6.4, Pt-CH3 trans to methyl), 1.52
2
3
(t, w/Pt satellites, 3H, JPt-H ) 61, JP-H ) 7.0, Pt-CH3 trans to
3
4
phosphorus), 1.58 (dt, w/Pt satellites, 3H, JPt-H ) 54, JP-H ) 11.6,
3JH-H ) 7.7, Pt-CH2CH3), 2.2 (m, 4H, P-CH2-CH2-P), 2.32 (tq, w/Pt
satellites, 2H, 2JPt-H ) 69, 3JP-H ) 8, 3JH-H ) 7.7, Pt-CH2CH3), 7.0-
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9453

A R T I C L E S
Crumpton-Bregel and Goldberg
1
7.4 (complex aromatic region). 13C NMR (C6D6): δ -3.8 (s, w/Pt
satellites, 1JPt-C ) 416, Pt-CH3 trans to methyl), 3.2 (dd, w/Pt satellites,
1JPt-C ) 573, trans-2JP-C ) 124, cis-2JP-C ) 5.5, Pt-CH3 trans to
phosphorus), 9.7 (dd, w/Pt satellites, 1JPt-C ) 547, trans-2JP-C ) 124,
cis-2JP-C ) 4.7, Pt-CH2CH3), 15.1 (s (br), Pt-CH2CH3), 28.5 (complex
pattern, P-CH2-CH2-P), 128-133 (aromatic signals, unresolved). 31P
NMR (C6D6): δ 6.4 (d, w/Pt satellites, JPt-P ) 1141, JP-P ) 8.8,
phosphorus trans to methyl), 7.4 (d, w/Pt satellites, 1JPt-P ) 992, 2JP-P
) 8.8, phosphorus trans to ethyl).
(50.3 mg) of the theoretical yield. H NMR (C6D6): δ -8.3 (t (br),
w/Pt satellites, 1H, 1JPt-H ) 689, 2JP-H ) 13.2, Pt-H), -0.27 (dt, w/Pt
satellites, 3H, 2JPt-H ) 45, 3JP-H ) 6.4, 3JH-H ) 2.9, Pt-CH3 trans to
hydride), 1.50 (dt, w/Pt satellites, 6H, 2JPt-H ) 60, 3JP-H ) 7.1, 3JH-H
) 1.1, Pt-CH3 trans to phosphorus), 6.7-7.8 (complex aromatic
region). 13C NMR (THF-d8): δ -9.5 (d, w/Pt satellites, 1JPt-C ) 517,
2JH-C ) 116, Pt-CH3 trans to hydride), 3.7 (d, w/Pt satellites, 1JPt-C
618, JP-C ) 99, Pt-CH3 trans to phosphorus), 129-137 (aromatic
signals, unresolved. 31P NMR (C6D6): δ 24.7 (s, w/Pt satellites,
1JPt-P ) 1218).
)
1
2
2
Synthesis, Purification and Characterization of fac-(dppe)-
PtMe3H (4). Under nitrogen, NaCNBH3 (99.4 mg, 1.58 mmol) was
added to a solution of fac-(dppe)PtMe3(OAc) (356 mg, 0.511 mmol)
in THF (10 mL). The reaction was allowed to stir for 15 min, and then
the volatiles were removed under vacuum. A colorless oil remained.
Et2O (10 mL) was added to the oil and then removed under vacuum
(repeated three times) to leave a slightly off-white solid remained. Under
nitrogen, some of the crude product was removed for analysis by 31P
NMR, which showed 75% 4 and 25% (dppe)PtMe2 (6). The crude
product was rinsed with Et2O (25 mL total), and the solution was filtered
through a pipet packed with glass wool, followed by removal of the
volatiles under vacuum. Recrystallization from Et2O/pentane gave
colorless crystals. The first crop contained few crystals, but the material
was pure 4 by NMR. A second recrystallization afforded significantly
more material, but the crystals contained 79% 4 and 21% 6 (31P NMR).
Of the solid isolated in the second crop (178 mg), only 79% was 4
Synthesis, Purification and Characterization of fac-(dppbz)-
PtMe3D (5-d1). Under nitrogen, NaBD4 (17.3 mg, 0.413 mmol) was
added to a solution of (dppbz)PtMe3(OAc) (54.1 mg, 0.0725 mmol) in
THF (10 mL). The reaction was allowed to stir for 1 h, and then the
volatiles were removed under vacuum. The resulting white solid was
left under vacuum overnight. Under nitrogen, the solid was rinsed with
toluene (20 mL), and the solution was filtered through a pipet packed
with glass wool and a plug of diatomaceous earth. The volatiles were
removed under vacuum and the resulting solid was recrystallized from
THF/pentane. White crystalline solid was isolated (28.9 mg), which
contained 85% 5-d1 and 15% (dppbz)PtMe2 (7). The amount of 5-d1
1
in the crystals was calculated to be 49% (24.6 mg). H NMR (C6D6):
δ -0.27 (t, w/Pt satellites, 3H, 2JPt-H ) 45, 3JP-H ) 6.4, Pt-CH3 trans
2
3
to deuteride), 1.50 (t, w/Pt satellites, 6H, JPt-H ) 60, JP-H ) 7.1,
Pt-CH3 trans to phosphorus), 6.7-7.8 (complex aromatic region). 31
P
1
1
NMR (C6D6): δ 24.7 (s, w/Pt satellites, JPt-P ) 1218).
(141 mg), which is 43% of the theoretical yield. H NMR (C6D6): δ
-8.6 (br t w/Pt satellites, 1H, 1JPt-H ) 706, 2JP-H ) 12.9, Pt-H), -0.14
Synthesis and Purification of (dppe)PtMe2 (6). An alternative
preparation to that previously reported for 6 was used.31 Under nitrogen,
an ether solution of CH3Li (32 mL, 45 mmol) was slowly added with
stirring to a cooled suspension (-35 °C) of (dppe)PtCl2 (5.840 g, 8.789
mmol) in toluene (80 mL). The reaction was allowed to come to ambient
temperature, and after stirring overnight, the solution was yellow and
cloudy. In the air, water (2 mL) was added dropwise to the reaction
mixture (0 °C). The volatiles were removed under vacuum and hot
toluene (180 mL) was added to the flask to separate the organometallic
product from the inorganic salts. The mixture was filtered through pipets
packed with glass wool, and the volatiles were removed from the filtrate,
which left behind a yellow solid. The solid was dissolved in CH2Cl2
(150 mL), and the solution was filtered through a small plug of alumina.
The filtrate was concentrated with gentle heat (45 mL), and the solution
was place in the freezer (-20 °C). Colorless crystals were isolated
and washed with cold CH2Cl2 and pentane. After drying under vacuum,
the yield was 89% (4.869 g).
(dt, w/Pt satellites, 3H, 2JPt-H ) 43, 3JP-H ) 6.4, 3JH-H ) 2.8, Pt-CH3
2
3
trans to hydride), 1.45 (dt, w/Pt satellites, 6H, JPt-H ) 60, JP-H
)
3
7.1, JH-H ) 1.3, Pt-CH3 trans to phosphorus), 1.8, 2.3 (m. 4H,
P-CH2-CH2-P), 7.0-7.4 (complex aromatic region). 13C NMR (THF-
d8): δ -8.5(d, w/Pt satellites, 1JPt-C ) 516. 1JH-C ) 117, Pt-CH3 trans
to hydride), -1.1 (d, w/Pt satellites, JPt-C ) 610, trans-2JP-C ) 98,
1
cis-2JP-C ) 5.5, Pt-CH3 trans to phosphorus), 27 (complex pattern,
P-CH2-CH2-P), 127-133 (aromatic signals, unresolved). 31P NMR:
1
(C6D6) δ 24.3 (s, w/Pt satellites, JPt-P ) 1218); (THF-d8) δ 24.4 (s,
1
w/Pt satellites, JPt-P ) 1232).
Synthesis, Purification and Characterization of fac-(dppe)-
PtMe3D (4-d1). Under nitrogen, NaBD4 (21.6 mg, 0.516 mmol) was
added to a solution of fac-(dppe)PtMe3(OAc) (71.9 mg, 0.103 mmol)
in THF (15 mL). The reaction was allowed to stir for 15 min, then
filtered through a pipet packed with glass wool. The volatiles were
removed under vacuum. The resulting yellow oil was dissolved in Et2O
(5 mL), which was removed under vacuum. This was repeated three
times, and yellow solid remained. Recrystallization from Et2O/pentane
yielded a pale yellow powder (45.4 mg), which contained 80% 4-d1
and 20% (dppe)PtMe2 (6) (31P NMR). The amount of 4-d1 in the powder
Synthesis and Characterization of (dppe)PtMeEt (8). Under
nitrogen, an ether solution of EtMgBr (0.2 mL, 0.6 mmol) was slowly
added with stirring to (dppe)PtMeCl (70.6 mg, 0.110 mmol) in toluene
(10 mL). The reaction was allowed to stir overnight, after which time
the solution was cloudy. In the air, water (0.2 mL) was added dropwise
to the reaction mixture (0 °C). The volatiles were removed under
vacuum and toluene (20 mL) was added to the flask to separate the
organometallic product from the inorganic salts. The mixture was
filtered through pipets packed with glass wool, and the volatiles were
removed from the filtrate, which left behind an off-white solid.
Recrystallization from CH2Cl2/pentane at -24 °C produced colorless
1
was calculated to 55% (36.3 mg) of the theoretical yield. H NMR
(C6D6): δ -0.14 (t, w/Pt satellites, 3H, 2JPt-H ) 43, 3JP-H ) 6.3, Pt-
CH3 trans to deuteride), 1.46 (t, w/Pt satellites, 6H, 2JPt-H ) 60, 3JP-H
) 7.0, Pt-CH3 trans to phosphorus), 1.7, 2.3 (m. 4H, P-CH2-CH2-P),
6.9-7.4 (complex aromatic region). 31P NMR (C6D6): δ 24.4 (s, w/Pt
1
satellites, JPt-P ) 1219).
Synthesis, Purification, and Characterization of fac-(dppbz)-
PtMe3H (5). Under nitrogen, a THF solution of BH3‚THF (0.120 mL,
0.120 mmol) was added to a solution of fac-(dppbz)PtMe3(OAc) (87.7
mg, 0.118 mmol) in THF (10 mL). The reaction was allowed to stir
for 30 min, and then the volatiles were removed under vacuum and
the resulting pale yellow solid was left under vacuum overnight. Under
nitrogen, the solid was rinsed with toluene (30 mL), which yielded a
colorless solution and insoluble yellow solid. The solution was filtered
through a pipet packed with glass wool and a plug of diatomaceous
earth. The volatiles were removed under vacuum, and the resulting
white solid was recrystallized from THF/pentane. White crystals were
isolated (55.3 mg), which contained 91% 5 and 9% (dppbz)PtMe2 (7)
(31P NMR). The amount of 5 in this mixture was calculated to be 62%
1
crystalline needles, 30.7 mg (44% yield). H NMR (C6D6): δ 1.57 (t,
w/Pt satellites, 3H, JPt-H ) 72, JP-H ) 7.4, Pt-CH3), 1.74 (dt, w/Pt
satellites, 3H, JPt-H ) 70, JH-H ) 8.7, JP-H ) 7.7, Pt-CH2CH3),
2
3
3
3
4
2
1.85 (m, 4H, P-CH2-CH2-P), 2.31 (tq, w/Pt satellites, 2H, JPt-H
)
75, 3JH-H ) 8.7, 3JP-H ) 7.7, Pt-CH2CH3), 7.0-7.4 (complex aromatic
region). 31P NMR (C6D6): δ 48.6 (d, w/Pt satellites, JPt-P ) 1850,
1
2JP-P ) 7.3, phosphorus trans to methyl), 45.0 (d, w/Pt satellites,1JPt-P
2
) 1536, JP-P ) 7.3, phosphorus trans to ethyl). Elemental analysis
calculated for C29H32P2Pt: C, 54.62; H, 5.05. Found: C, 54.51; H, 4.97.
Crystal Structure Determination. Colorless cubes of (dppe)PtMe4
(1) were obtained from a solution of 1 in CH2Cl2 that was layered
with pentane, then cooled to -20 °C. The crystals were rinsed with
9
9454 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

C−C and C−H Alkane Reductive Eliminations
A R T I C L E S
pentane and excess solvent was removed under vacuum. The same
procedure was used to obtain colorless plates of (dppbz)PtMe4 (2).
Colorless plates of fac-(dppe)PtMe3H (4) were obtained under a nitrogen
atmosphere from a solution of 4 in Et2O that was layered with pentane
and cooled to -35 °C. The crystals were rinsed with pentane and excess
solvent was removed under vacuum.
of the base peak for each isotopomer of ethane was calculated. These
values were used to represent the amount of each isotopomer present
in the mixture. This analysis showed that less than 4% of gas mixture
was CH3CD3, with approximately 43% CH3CH3 and 53% CD3CD3.
Thermolysis of 6 and 6-d6 in C6D6. An NMR tube was loaded with
6 (6.002 mg, 9.625 × 10-3 mmol) and 6-d6 (6.098 mg, 9.685 × 10-3
mmol). C6D6 (0.35 mL) was vacuum transferred into the tube and the
tube was flame-sealed. The initial 31P NMR spectrum showed only 6
and 6-d6. The tube was then heated in an oil bath at 165 °C for 12 h.
After heating, the 31P NMR spectrum showed 6 (δ 47.35, 1JPt-P ) 1779),
6-d6 (δ 47.49, 1JPt-P ) 1769) and a significant amount of 6-d3 (δ 47.42,
1JPt-P ) 1774).34
GC/MS Analysis of (dppe)PtMe3Et (3) Thermolysis Products.
A bomb was charged with 3 (52.8 mg, 0.0791 mmol) and benzene (20
mL). After three freeze-pump-thaw cycles, the solution was heated
for 12 h at 165 °C. The solution was cooled to -78 °C and the volatiles
were transferred under static vacuum to a bomb with a septum sideport,
cooled to -196 °C. The volatiles were sampled with a Gastight syringe
and injected into the GC/MS. The methane appeared immediately, and
was overlapping with a large air peak. This was followed by ethane,
then ethylene and finally propane. The gases were identified by their
unique mass spectral peaks.
Calibration for GC/FID Analysis. Using a known volume bulb
(16.95 mL), equal pressures of ethane, ethylene and propane (ca. 170
Torr, 0.15 mmol) were transferred to a bomb with a septum sideport,
cooled to -196 °C. This mixture was sampled with a Gastight syringe
equipped with a locking valve, and then injected into the GC/FID. The
three peaks were integrated and the data from two separate runs was
compared. The signals for ethane and ethylene were found to be lower
than expected, and correction factors were calculated. Ethane intensities
needed to be multiplied by 1.014, and ethylene intensities needed to
be multiplied by 1.032.
Crystallographic data for 1 and 2 were collected at the University
of Washington X-ray crystallography lab, using a Nonius Kappa CCD
with Mo KR radiation. The crystals of 1 and 2 were mounted on a
glass capillary with epoxy. Non-hydrogen atoms were refined aniso-
tropically by full-matrix least-squares. Hydrogen atoms were placed
with idealized geometry and refined using a riding model. All intensities
were integrated and subsequently scaled with the Denzo-SMN pack-
age.53
Crystallographic data for 4 were also collected at the University of
Washington X-ray crystallography lab, using a Nonius Kappa CCD
with Mo KR radiation. The crystals of 4 were mounted on a glass
capillary under oil. Non-hydrogen atoms were refined anisotropically
by full-matrix least-squares. All hydrogen atoms were located from
difference maps except for H(1C) on C1 (methyl hydrogen), which was
placed. Hydrogen atoms were refined with a riding model except for
the hydride, H(1Pt), which was refined isotropically. All intensities
were integrated and subsequently scaled with the Denzo-SMN pack-
age.53 The crystallographic data for complexes 1, 2, and 4 can be found
in Table 1. Selected bond lengths and angles appear in Tables 2 and 3.
Kinetics Experiments for 1, 2, and 3. In a typical kinetics
experiment, the desired compound was weighed to microgram accuracy
(typically 3-10 mg), and transferred to a sealable medium-walled NMR
tube. The tube was evacuated and deuterated solvent was vacuum
transferred into the tube. An internal standard, either dioxane or
hexamethyldisiloxane (ca. 4 Torr), was added via a known volume bulb
(16.95 mL, 0.004 mmol). The tube was flame sealed under vacuum.
After warming to room temperature, the volume of the solution was
GC/FID Analysis of (dppe)PtMe3Et (3) Thermolysis Products.
Under nitrogen, a bomb was charged with 3 (20.854 mg, 0.031 234
mmol), ferrocene (4.459 mg, 0.02397 mmol) and C6D6 (2 mL). An
aliquot (0.3 mL) was removed, and transferred to an NMR tube. Initial
1H and 31P NMR spectra were collected, and the only signals
corresponded to 3 and a small amount of (dppe)PtMe4 (1, ca. 5%).
The bomb was heated 12 h at 165 °C, then frozen at -78 °C. Under
static vacuum, the volatiles were transferred to bomb with a septum
sideport. The volatiles were condensed in the septum bomb at -196
°C (liquid nitrogen). The reaction mixture was warmed to room
temperature, and the transfer process was repeated two more times.
The gaseous products of the thermolysis were then sampled with a
gastight syringe equipped with a locking valve and injected into the
GC/FID. Peaks for methane (3.7 min), ethane (4.0 min), ethylene (4.5
min) and propane (6.4 min) were detected and integrated. The injections
were repeated until the signal/noise was too poor to continue. The
relative amounts of ethylene, ethane and propane were then calculated
using the correction factors from the calibration (see above). The ratio
of ethylene to propane to ethane was 75:18:7. The numbers for methane
were not an accurate representation of the methane produced in the
reaction, because methane does not completely condense at -196 °C,
so not all of it was collected in the septum bomb. Finally, a small
aliquot (0.3 mL) of the post-thermolysis reaction mixture was transferred
1
measured (ca. 0.30 mL). An initial H NMR spectrum was collected.
The tube was heated in an oil bath at the desired temperature for a
specified amount of time, then quickly cooled in an ice bath. Three 1H
NMR spectra were acquired after each heating period, and the integrals
were averaged. The reaction was followed by monitoring the disap-
pearance of the axial methyl signal relative to the dioxane (T1 ) 17 s,
pulses space 90 s apart) or hexamethyldisiloxane (T1 ) 8 s, pulses
spaced 40 s apart) standard. The tube was frozen at -20 °C when not
in the oil bath or spectrometer. Kinetics samples were followed for at
least three half-lives (ca. 10 points) and a rate constant was obtained
by a least squares linear regression method.
Crossover Study. 1 (6.849 mg, 0.01048 mmol) and 1-d12 (6.981
mg, 0.01049 mmol) were dissolved in acetone (1.00 mL). An aliquot
(0.67 mL) was transferred to a pressure/vacuum valve NMR tube, and
the solvent was removed under vacuum. C6D6 (0.3 mL) was vacuum
transferred into the tube, which was then sealed by closing the valve.
An initial 31P NMR spectrum was collected, which showed only 1 and
1-d12. The tube was heated in an oil bath at 165 °C for 50 h. The 31P
1
NMR spectrum showed 1, 1-d12, 6 (δ 47.35, JPt-P ) 1779), 6-d6 (δ
1
47.49, JPt-P ) 1769), and what has been assigned as 6-d3 (δ 47.42,
1JPt-P ) 1774). Unfortunately, even at high field (202.5 MHz), baseline
1
resolution of the Pt(II) products was not observed. In the H NMR
1
to an NMR tube and analyzed by H and 31P NMR, which showed
(500 MHz) spectrum, a sharp signal for CH3CH3 (δ 0.80) was observed.
No other signals which could be assigned to CH3CD3 were detected.
The gases produced in the thermolysis were analyzed directly by high-
resolution mass spectrometry. The spectrum was compared to the high-
resolution mass spectra of CH3CH3, CH3CD3, and CD3CD3. The mass
spectra of the authentic samples provided information about the relative
intensities of the fragments. The base peak (highest intensity) was C2H4
for ethane, C2H2D2 for ethane-d3 and C2D4 for ethane-d6. In the analysis
of the gaseous products from the thermolysis of 1 and 1-d12, the intensity
mostly (dppe)PtMe2 (6, 91% from integration of 1H NMR) and a small
1
amount of (dppe)PtMeEt (8, 4% from integration of H NMR).
Kinetics Experiments for 4 and 5. In a typical kinetics experiment,
the desired compound was weighed (3-5 mg), under nitrogen, to the
nearest tenth of a milligram. The compound was then transferred into
a sealable NMR tube. The tube was evacuated and deuterated solvent
was vacuum transferred into the tube. Dioxane (ca. 3 Torr) was added
via a known volume bulb (16.95 mL, 3 × 10-3 mmol) as an internal
standard. The tube was flame sealed under vacuum or under a partial
pressure of nitrogen (ca. 500 Torr). After warming to room temperature,
(53) Otinowski, Z.; Minor, W. Methods Enymol. 1996, 276, 307.
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A R T I C L E S
Crumpton-Bregel and Goldberg
the volume of the solution (ca. 0.30 mL) was measured. An initial 1H
NMR spectrum was collected. Two methods were employed for
monitoring the thermolysis reaction. NMR tubes sealed under vacuum
were heated in an oil bath at the desired temperature for a specified
cases, kinetics samples were followed for three half-lives (ca. 10 points)
and a rate constant was obtained by a least squares linear regression
method. The identity of the products was confirmed by comparison to
NMR spectra of authentic samples.
1
amount of time, then quickly cooled in an ice bath. Three H NMR
Acknowledgment. This work was supported by the National
Science Foundation. The X-ray structure determinations were
performed by Dr S. Lovell and Dr. W. Kaminsky (UW).
spectra were acquired after each heating period, and the integrals were
averaged. The reaction was followed by disappearance of the axial
methyl signal of fac-L2PtMe3H relative to the dioxane standard (T1 )
17 s, pulses spaced 90 s apart). The tube was frozen at -20 °C when
not in the oil bath or spectrometer. Several kinetics samples were heated
in the NMR probe (AM 500 or DRX 499). Due to solvent reflux when
sealed under vacuum, these NMR tubes were sealed under a partial
pressure of nitrogen. The spectrometer was programmed to collect a
specified number of spectra, each spectrum separated by a specified
amount of time, which was at least 90 s to allow for full relaxation of
the dioxane standard. The progress of the reaction was monitored by
integrating the axial methyl signal against the dioxane standard. In both
Supporting Information Available: Kinetics plots for the
thermolyses of 1, 4, 4-d1, 5, and 5-d1 and kinetics plots for the
isomerizations of complexes fac-1-d9 and fac-3 (PDF); X-ray
crystallographic data files for complexes 1, 2, and 4 (CIF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA029140U
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