The gem-dialkyl effect as a test for preliminary diphosphine chelate opening in a reductive elimination reaction

Article abstract of DOI:10.1021/om0500467

A significant kinetic effect of the placement of gem-dialkyl substituents in the carbon backbone of a bidentate phosphine ligand on a reductive elimination reaction involving chelate opening as a preliminary step has been documented. This observation conf

Full text of DOI:10.1021/om0500467

4624
Organometallics 2005, 24, 4624-4628
The gem-Dialkyl Effect as a Test for Preliminary
Diphosphine Chelate Opening in a Reductive
Elimination Reaction†
Kathryn L. Arthur, Qi L. Wang, Dawn M. Bregel, Nicole A. Smythe,
Bridget A. O’Neill, and Karen I. Goldberg*
Department of Chemistry, Box 351700, University of Washington,
Seattle, Washington 98195-1700
Kenneth G. Moloy*
Central Research and Development Department, E.I. du Pont de Nemours & Co., Inc.,
Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880-0328
Received January 21, 2005
A significant kinetic effect of the placement of gem-dialkyl substituents in the carbon
backbone of a bidentate phosphine ligand on a reductive elimination reaction involving
chelate opening as a preliminary step has been documented. This observation confirms the
viability of the gem-dialkyl effect as a mechanistic probe for preliminary phosphine
dissociation in reactions of complexes containing chelating diphosphine ligands.
Introduction
the chelate) may then be required to generate the open
site. Yet, it is notoriously difficult to confirm the
involvement of such preliminary chelate opening.3 The
traditional method to detect preliminary ligand dis-
sociation, useful with monodentate ligands, involves the
demonstration of rate inhibition by added ligand. This
test is not valid for bidentate ligands, as the concentra-
tion of added ligand can never approach the intrinsic
high effective concentration of the partially coordinated
ligand.
Numerous examples of reductive elimination, often
the product-forming and rate-determining step in a
catalytic cycle, have been shown to involve ancillary
ligand dissociation as a preliminary reaction step.3
Primarily, such reactions to form C-H and C-C bonds
have involved monodentate ligands, and obtaining
evidence for a preequilibrium ligand dissociation mech-
anism in these model systems via kinetic studies (rate
inhibition by added ligand) has been straightforward.
Recently, the use of the gem-dialkyl effect4 was
proposed as a test for detecting preliminary ligand
dissociation involving chelating ligands.5 Marcone and
Moloy studied the rates of C-CN reductive elimination
from the Pd(II) complexes L2Pd(CN)(CH2SiMe3) (L2 )
dppp, Et2dppp, cbdppp, Scheme 1).5 The geminal sub-
stitution of alkyl groups for hydrogen atoms on the
aliphatic backbone of the phosphine chelate dppp was
expected to hinder opening of the chelate ring via the
gem-dialkyl effect. This proposal was based on the very
large rate differences (as much as 1010)6 that have been
observed in organic cyclization reactions with gem-
Ancillary ligands are critical components in homogen-
eous transition metal catalyzed reactions. The steric,
electronic, and chiral properties of these ligands are
often the key to the effectiveness of the metal catalyst.1
Bidentate phosphine ligands, whose properties can be
easily modified by changing the groups attached to the
phosphorus atoms and by changing the number or
identity of the atoms in the backbone chain connecting
the phosphorus groups, have proven particularly profit-
able in catalytic processes.2 However, it is often chal-
lenging to predict which ligand modifications (steric and
electronic) will lead to the desired increases in rate or
selectivity. In some cases, the dissociation of an ancillary
ligand is required during the reaction, and this adds a
new element to the challenge of ligand design, particu-
larly when chelating ligands are used.
Ligand dissociation to open a coordination site at the
metal can be necessary to allow access to a low-energy
pathway for reaction steps such as oxidative addition,
reductive elimination, â-hydride elimination, and olefin
insertion.1 In complexes with chelating ancillary ligands,
dissociation of one end of the chelate (i.e., opening of
† Contribution #8606.
* To whom correspondence should be addressed. E-mail:
goldberg@chem.washington.edu; Kenneth.G.Moloy@USA.dupont.com.
(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Organometallic Chemistry of Transition Metals: Principles and Use;
University Science Books: Mill Valley, CA, 1987. (b) Crabtree, R. H.
The Organometallic Chemistry of the Transition Metals, 3rd ed.; John
Wiley & Sons: New York, 2001.
(2) For some leading references, see: (a) Freixa, Z.; Van Leeuwen,
P. W. N. M. Dalton Trans. 2003, 1890. (b) van Leeuwen, P. W. N. M.;
Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Chem. Rev. 2000, 100,
2741. (c) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich, L.
M.; Gavney, J. A.; Powell, D. R. J. Am. Chem. Soc. 1992, 114, 5535.
(d) Casey, C. P.; Whiteker, G. T. Isr. J. Chem. 1990, 30, 299. (e) Noyori,
R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345. (f) Vineyard, B. D.;
Knowles, W. S.; Sabacky, M. J.; Bachman, G. L.; Wemkauff, D. J. J.
Am. Chem. Soc. 1977, 99, 5946.
(3) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 9442, and references therein.
(4) For explanations and discussion of the gem-dialkyl effect, see:
(a) Parrill, A. L.; Dolata, D. P. J. Mol. Struct. (THEOCHEM) 1996,
370, 187. (b) Jung, M.; Gervay, J. J. Am. Chem. Soc. 1991, 113, 224.
(c) Jung, M. E.; Piizzi, G. Chem. Rev. 2005, 105, 1735.
(5) Marcone, J. E.; Moloy, K. G. J. Am. Chem. Soc. 1998, 120, 8527.
(6) Milstein, S.; Cohen, L. A. J. Am. Chem. Soc. 1972, 94, 9158.
10.1021/om0500467 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 08/09/2005

The gem-Dialkyl Effect
Organometallics, Vol. 24, No. 19, 2005 4625
Scheme 1
Scheme 2
dialkyl substitutions.4 However, instead of the rate
variation expected if the reaction proceeded via phos-
phine dissociation (Et2dppp, cbdppp < dppp), the au-
thors found that the rates varied only slightly (less than
4×) following the order Et2dppp < dppp < cbdppp.5
Thus, the reaction was proposed to proceed without
chelate opening. When a larger group of bidentate
ligands were compared in this reductive elimination
reaction, an overall rate variation of 104 was observed
with dppe (1,2-bis(diphenylphosphino)ethane) < cbdppp
< dppp < Et2dppp < DIOP (2,3-O-isopropylidene-2,3-
dihydroxy-1,4-bis((diphenylphosphino)butane). This trend
was attributed to the effect of increasing the bite angle
of the bisphosphine ligands on direct C-C coupling from
the four-coordinate Pd(II) center.2a,5,7 What remained
unclear from the study, however, is whether gem-dialkyl
substitution at a carbon within a chelating ligand would
indeed register a significant effect on the rate of a
reaction involving chelate opening as a preliminary
reaction step. That is, can gem-dialkyl substitution in
the backbone of a phosphine chelate be used as a test
for chelate opening during a reaction?
While the gem-dialkyl effect has been frequently em-
ployed in organic chemistry, there are considerably
fewer examples of its application in inorganic and or-
ganometallic systems. Shaw has argued that to effec-
tively employ the gem-dialkyl effect in systems having
tetrahedral atoms that are larger than carbon, e.g.,
phosphorus or metals, larger groups such as tert-butyl,
rather than methyl or ethyl, would be required.8 Indeed,
Shaw made excellent use of bulky organic substituents
on phosphorus to promote cyclometalation reactions and
to favor the formation of large chelate rings.8 Another
illustration of the effective use of the gem-dialkyl effect
on the thermodynamics of inorganic systems dates back
almost fifty years. Through the measurement of forma-
tion constants, it was shown that Cu and Ni complexes
of 2,2-dimethyl-1,3-propanediamine were more stable
than those of 1,3-propanediamine.9 However, the kinetic
effects of gem-dialkyl substitution on inorganic and
organometallic reactions have not been adequately
explored.
Scheme 3
other mechanistic experiments, the observation of a
difference of 2 orders of magnitude in reactions rates
between the complex containing dppe and that contain-
ing the more rigid dppbz ligand provided strong support
that for reductive elimination of ethane from dppePtMe4
one end of the phosphine dissociates to form a five-
coordinate intermediate prior to carbon-carbon coup-
ling. Evidence was also presented that phosphine che-
late opening in this system was kinetically viable as a
preliminary step, or in other words, the rate of phos-
phine dissociation exceeds that of C-C reductive elimi-
nation.3 Thus, this L2PtMe4 system is ideally suited for
analyzing whether the gem-dialkyl effect is useful as a
means to detect such chelate opening. In addition,
studies of this system should provide the first estimate
of the magnitude of rate inhibition to be expected upon
such gem-dialkyl substitution in a reaction involving
preequilibrium chelate opening. Herein, we report the
results of such a study in which the six-coordinate d6
Pt(IV) complexes, L2PtMe4, where L2 ) dppp (1), Et2-
dppp (2), and cbdppp (3), have been synthesized and
characterized, and the kinetics of their thermolysis
reactions to produce ethane and L2PtMe2 measured and
compared.
Recently, strong evidence was presented that carbon-
carbon reductive elimination from a Pt(IV) tetramethyl
complex with a chelating bidentate ligand proceeds via
preequilibrium opening of the phosphine chelate.3 In
this study, the rates of reductive elimination of ethane
from L2PtMe4 (L2 ) dppe and dppbz (1,2-bis(diphen-
ylphosphino)benzene)) were measured and compared
(Scheme 2). Combined with the results of a variety of
Results and Discussion
Synthesis and Characterization of L2PtMe4. The
complexes L2PtMe4 (L2 ) dppp (1), Et2dppp (2), cbdppp
(3), Scheme 3) were prepared either by the reaction of
bisphosphine ligand L2 with Pt2Me8(µ-SMe2)2 or by the
reaction of L2PtMe3I, generated in situ from L2 and
[PtMe3I]4, with MeMgCl. Complexes 1-3 were recrys-
tallized from methylene chloride/pentane solutions and
(7) Brown, J. M.; Guiry, P. J. Inorg. Chim. Acta 1994, 220, 249.
(8) Shaw, B. L. Adv. Chem. Ser. 1982, 196, 101.
(9) Hares, G. B.; Fernelius, C.; Douglas, B. D. J. Am. Chem. Soc.
1956, 78, 1816.
characterized by H, 13C{1H}, and 31P{1H} NMR spec-
troscopy and elemental analysis.
1

4626 Organometallics, Vol. 24, No. 19, 2005
Arthur et al.
The NMR data for complexes 1, 2, and 3 are very
similar. In the 1H NMR spectra, values of Pt-H
coupling constants for Pt-CH3 groups are nearly identi-
2
cal. For Pt-CH3 trans to methyl, JPt-H is 43 Hz for 1
and 44 Hz for both 2 and 3. For Pt-CH3 trans to
2
phosphorus, JPt-H is 59 Hz for 1 and 2 and 60 Hz for
3. In addition, the value of the P-H coupling constant
for the Pt-bound methyl groups varies only slightly with
the phosphine ligand. For Pt-CH3 trans to methyl,
3JP-H is 5.7 Hz for 1 and 2 and 6.0 Hz for 3. For Pt-
3
CH3 trans to phosphorus, JP-H is 6.0 for 1, 6.7 for 2,
and 6.8 for 3. In the 31P{1H} NMR spectra, it is seen
that the Pt-P coupling constants are also similar: 1084
Hz for 1, 1082 Hz for 2, and 1068 Hz for 3.
The similarity in the spectroscopic data of complexes
1, 2, and 3 indicates that the three complexes have very
similar structures. This is analogous to the virtually
identical νCN stretches found for the Pd complexes, L2-
Pd(CN)CH2SiMe3 (L2 ) dppp, Et2dppp, cbdppp),5 fur-
ther suggesting that there are negligible electronic
differences between the members of this ligand set.
Additional evidence that the three ligands, dppp, Et2-
dppp, and cbdppp, coordinate to the metal to form
similar six-membered rings is that only minimal dif-
ferences in P-M-P angles are observed. The P-Pt-P
angles of the Pt(II) complexes L2PtMe2 (L2 ) Et2dppp
and cbdppp) have been reported as 95.82(4)° and 95.39-
(5)°, respectively.10 For direct comparison to these
complexes, crystals of dpppPtMe2 were grown and
analyzed by X-ray diffraction; a P-Pt-P of 93.56(4)°
was found.11 Thus, despite predictions on the basis of
the Thorpe-Ingold hypothesis (P-Pt-P angle predicted
to vary as L2 ) cbdppp > dppp > Et2dppp),5 the six-
membered chelate ring containing the large metal and
two phosphorus atoms is apparently able to effectively
minimize the strain caused by changes in the C-C-C
angle resulting from the gem-dialkyl substitutions.
Although there is a variation of almost 5° in the C-C-C
internal chelate angles of the L2PtMe2 complexes (dppp
(114.22°), cbdppp (111.3°), Et2dppp (109.3°)), the P-Pt-P
angles for the three Pt(II) complexes are within 2° of
one another. As implied by the virtually identical NMR
coupling constants (JPt-H, JP-H, and JPt-P) for the
methyl and phosphine ligands attached to Pt in com-
plexes 1-3, comparable minimal differences in the
P-Pt-P bond angles are expected for the analogous Pt-
(IV) complexes, 1-3.
Figure 1. First-order kinetic plots of the disappearance
of L2PtMe4 (b ) 1, ( ) 2, 9 ) 3) in C6D6 at 165 °C.
Table 1. First-Order Rate Constants for the
Reductive Elimination of Ethane from 1-3 in C6D6
at 165 °C
complex
kobs (s-1
)
(dppp)PtMe4 (1)
(3.3 ( 0.2) × 10-4
(7.3 ( 0.2) × 10-6
(1.6 ( 0.1) × 10-5
(4.2 ( 0.1) × 10-6
(Et2dppp)PtMe4 (2)
(cbdppp)PtMe4 (3)
3
(dppe)PtMe4
reactions exhibited clean and reproducible first-order
kinetic behavior, and representative kinetic data are
shown in Figure 1. The rate constants for the reductive
elimination reactions of 1-3 are presented in Table 1.
It is of interest to first compare the rate constant for
reductive elimination from 1 (kobs ) (3.3 ( 0.2) × 10-4
s-1) to that measured for reductive elimination from
dppePtMe4 (kobs ) (4.2 ( 0.1) × 10-6 s-1)3 under the
same conditions; an increase of almost 80 times is
observed upon substituting dppp for dppe. Since a
detailed mechanistic study of carbon-carbon reductive
elimination from dppePtMe4 provided strong evidence
that the reaction proceeds via preliminary chelate
opening,3 it is reasonable to attribute the faster reaction
of dpppPtMe4 to the increased flexibility provided by the
three-carbon chain of dppp versus the two-carbon chain
of dppe. However, it has been reported that the same
replacement in L2Pd(CN)(CH2SiMe3) led to an increase
of 24 times in the rate of C-CN reductive elimination,
and acceleration in this system was attributed to the
increase in bite angle on going from dppe (natural bite
angle of 85°) to dppp (natural bite angle of 91°), with
the reaction proposed to proceed via a direct pathway
not involving ligand chelate opening.5 Similarly, an
increase of 46 times was observed for the same dppe to
dppp substitution in the rates of ethane elimination
from L2Ni(CH3)2.12 In this Ni system, the authors left
open the question of whether the rate difference was
due to a distortion of the starting complex structure
(ligand bite angle) or a result of chelate opening on the
reaction path. Note that in the Pd and Ni systems, the
P-M-P angle of the tetrahedral M(0) product will be
significantly greater than in the starting square planar
M(II) complex. In contrast, for the Pt example, the
octahedral Pt(IV) starting complex and the square
In summary, relevant spectroscopic and crystal-
lographic data support that the structures of complexes
1, 2, and 3 are similar. Thus, any differences in observed
reaction rates for reductive elimination from these
complexes cannot be readily attributed to ground state
structural or electronic differences between them.
Thermolysis of Complexes 1-3. When the Pt(IV)
complexes (dppp)PtMe4 (1), (Et2dppp)PtMe4 (2), and
(cbdppp)PtMe4 (3) were heated in C6D6 at 165 °C,
ethane and the corresponding Pt(II) product, L2PtMe2,
formed as the exclusive products (Scheme 3). The
(10) Smith, D. C., Jr.; Haar, C. M.; Stevens, E. D.; Nolan, S. P.;
Marshall, W. J.; Moloy, K. G. Organometallics 2000, 19, 1427.
(11) Complete crystallographic data for dpppPtMe2‚1/2(C6H5CH3)
supplied as CIF in the Supporting Information (C65H27P4Pt2, MW )
1367.27, colorless prism, monoclinic, space group ) P21/c, T ) 130(2)
K, a ) 12.1540(2) Å, b ) 14.9520(3) Å, c ) 18.4300(3) Å, R ) 90°, â )
120.5791(10)°, γ ) 90°, Z ) 2, R1 (I > 2σ(I)) ) 0.0354, wR2 (all data)
) 0.0928, GOF (F2) ) 1.044).
(12) Kohara, T.; Yamamoto, T.; Yamamoto, A. J. Organomet. Chem.
1980, 192, 265.

The gem-Dialkyl Effect
Organometallics, Vol. 24, No. 19, 2005 4627
planar Pt(II) product have similar P-Pt-P angles.13
Overall, it then appears difficult to distinguish the
effects of increased bite angle and increased flexibility
on substitution of dppp for dppe, compromising this
substitution as an unambiguous test for chelate opening.
In contrast, spectral and crystallographic data imply
that the P-M-P angles for coordinated dppp, Et2dppp,
and cbdppp are similar and the ground state structures
of 1, 2, and 3 highly comparable. Thus, rate differences
in reductive elimination from 1, 2, and 3 should be
directly attributable to differences in the flexibility of
the chelate caused by the gem-dialkyl substitution.
These differences in the rates of reductive elimination
reactions to form ethane and L2PtMe2 from 1, 2, and 3
are significant (Table 1). The gem-dialkyl-substituted
dppp complexes, Et2dpppPtMe4 (2) and cbdpppPtMe4
(3), eliminate ethane 45 times and 21 times slower,
respectively, than the unsubstituted dppp complex 1.
This substantial difference in the rates of elimination
from 2 and 3 compared to that of 1 is consistent with
the proposal that these reductive eliminations follow the
mechanism previously supported for ethane elimination
from the analogous bisphosphine Pt(IV) tetramethyl
complex (dppe)PtMe4.3 In this case, chelate opening
occurs as a preequilibrium step and is followed by rate-
determining carbon-carbon coupling from the resultant
five-coordinate intermediate.14 The results reported here
demonstrate that gem-dialkyl substituents can signifi-
cantly affect the rates of reactions wherein partial
chelate dissociation is involved as an initial step. Thus,
the gem-dialkyl effect shows considerable potential as
a viable method for detection of chelate opening on the
reaction pathway.
As noted earlier, gem-dialkyl substitution has been
widely used in organic chemistry to accelerate cycliza-
tion reactions, to stabilize cyclic structures, and as a
mechanistic tool.4 Its application in inorganic and
organometallic chemistry, however, has been primarily
limited to thermodynamic observations, i.e., favoring
ring-closed structures with gem-dialkyl substitution.8,9
As such, its potential as a diagnostic tool in kinetic/
mechanistic analyses of metal-based systems has not
yet been recognized. However, it is interesting that the
kinetic results for the organometallic system reported
here can in fact be explained on the basis of thermody-
namics. The reductive elimination of ethane consists of
an equilibrium step of chelate opening (K1) followed by
rate-determining carbon-carbon bond formation (k2)
(kobs ) K1k2; see Scheme 2).14 Assuming that the gem-
dialkyl substitution destabilizes the chelate-opened
form, the overall rate constant is decreased by dimin-
ishing the equilibrium constant (K1) for the preliminary
ligand dissociation step. Thus, the reductive elimina-
tions from gem-dialkyl-substituted 2 and 3 are much
slower than from unsubstituted 1. Note that changing
the identity of the gem-alkyl substituents does not
appear to have a significant effect, as there is only a
small difference (ca. 2×) in the rates of the reductive
elimination from 2 (Et2dppp) and 3 (cbdppp). This is
similar to thermodynamic observations by Busch in
which although the formation constants for Ni(II) and
Cu(II) complexes of gem-substituted 1,1-R2-1,3-pro-
panediamine were in general greater than that of 1,3-
propanediamine, there was little variation found in
Ni(II) formation constants with the size of R for 1,1-R2-
1,3-propanediamine (R ) Me, Et) or with ring size for
1,1-di(aminomethyl)cycloR′ (R′ ) propane, butane, pen-
tane, hexane).15
In summary, gem-dialkyl substitution on the dppp
backbone led to a significant decrease in the rate of
reductive elimination from 2 and 3 as compared to 1.
This rate inhibition illustrates that the gem-dialkyl
effect has potential as a mechanistic probe for prelimi-
nary chelate opening on a reaction pathway. The results
reported here also offer the first insight into the
magnitude of a kinetic effect that might be expected by
such substitution. Our kinetic results parallel earlier
observations of the gem-dialkyl effect on the thermody-
namics of chelation in inorganic and organometallic
systems.
Experimental Section
General Considerations. Unless otherwise noted, all
reactions were carried out under a N2 atmosphere in a Vacuum
Atmospheres VAC-MO-40-M drybox. Pentane and CH2Cl2 were
dried over CaH2. Benzene, THF, diethyl ether, and toluene
were dried over sodium and benzophenone. All solvents were
distilled before use. Benzene-d6 and dioxane were dried over
sodium benzophenone and vacuum transferred prior to use.
1H NMR, 31P NMR, 13C NMR, DEPT-135, HMQC, and
HMBC spectra were collected on Bruker DPX, DRX, and
AVANCE spectrometers. The 1H NMR spectra were referenced
to residual protiated solvent, and chemical shifts are reported
in parts per million downfield of tetramethylsilane. The 13C
NMR, DEPT-135, HMQC, and HMBC spectra were referenced
to residual solvent peaks or a known chemical shift standard.
31P NMR spectra were referenced to an external standard of
85% phosphoric acid, and all chemical shifts are reported in
parts per million (ppm) downfield of this reference. Coupling
constants are reported in hertz (Hz). Elemental analyses were
performed by Atlantic Microlab, Inc.
Pt2Me8(µ-SMe2)2,16 [PtMe3I]4,17 Et2dppp,18 and cbdppp19 were
prepared according to literature procedures. Unless otherwise
noted, all other reagents were used as supplied from com-
mercial sources.
Synthesis and Characterization of (dppp)PtMe4 (1).
Under nitrogen, dppp (169 mg, 0.409 mmol) and [PtMe3I]4 (146
mg, 0.0994 mmol) in THF (10 mL) were allowed to stir
overnight. The solvent was removed under vacuum and
toluene (10 mL) was added to the residue. MeMgCl (2.8 M in
THF, 0.8 mmol) was added, and the reaction was allowed to
stir overnight. The reaction was exposed to air, and water (0.5
mL) was slowly added to the reaction mixture (0 °C) to quench
the excess MeMgCl. The volatiles were removed under vacuum.
CH2Cl2 (10 mL) was added to dissolve the Pt compounds. The
suspension was filtered through tightly packed glass wool, and
the volatiles were removed from the filtrate under vacuum.
(15) Newman, M. S.; Busch, D. H.; Cheney, G. E.; Gustafson, C. R.
Inorg. Chem. 1972, 11, 2890.
(13) The PPtP angle is 86.04(3)° in (dppe)PtMe4 (ref 3) and 84.73(5)°
in dppePtMe2 (ref 10).
(16) Lashanizadehgan, M.; Rashidi, M.; Huz, J. E.; Puddephatt, R.
J.; Ling, S. S. M. J. Organomet. Chem. 1984, 269, 317.
(17) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411.
(18) Bianchini, C.; Lee, H. M.; Meli, A.; Moneti, S.; Vizza, F.; Fontani,
M.; Zanello, P. Macromolecules 1999, 32, 4183.
(14) The precedent of the dppePtMe4 reaction wherein rate-
determining C-C coupling occurs after preequilibrium phosphine
chelate opening3 strongly argues against the possibility of rate-
determining phosphine dissociation in the analogous dppp system. The
rate of phosphine dissociation from Pt(IV) for the more flexible dppp
should be even greater than that of the dppe ligand.
(19) Prepared analogously to Et2dppp (ref 18) via the reaction of
(CH2)3C(CH2OTs)2 (Houk, J.; Whitesides, G. M. J. Am. Chem. Soc.
1987, 109, 6825) with LiPPh2.

4628 Organometallics, Vol. 24, No. 19, 2005
Arthur et al.
3
This material contained 84% (1) and 16% (dppp)PtMe2 (de-
termined by 31P{1H} NMR). Recrystallization from THF/
pentane afforded white crystals of 1 in 15% yield (37.3 mg).
1H NMR (500 MHz, C6D6): δ 0.62 (t, w/Pt satellites, 6H, 2JPt-H
60, JP-H ) 6.8, Pt-CH3 trans to phosphorus), 1.27 (t, 4H,
3JH-H ) 7.2, CH2-CH2-CH2 ), 1.47 (p, 2H, 3JH-H ) 7.7, CH2-
CH2-CH2), 2.79 (q, 4H, 3JH-H ) 8.0, 2JP-H ) 5.7, P-CH2), 7.04
and 7.49 (br m, 12H, 8H, phenyl signals). 13C{1H} NMR (50
3
1
2
) 43, JP-H ) 5.7, Pt-CH3 trans to methyl), 1.08 (t, w/Pt
MHz, CDCl3): δ -5.02 (t, w/Pt satellites, JPt-C ) 411, JP-C
2
3
satellites, 6H, JPt-H ) 59, JP-H ) 6.0, Pt-CH3 trans to
phosphorus), 1.55 (br m, 2H, P-CH2-CH2-CH2-P), 2.16 (br
m, 4H, P-CH2-CH2-CH2-P), 7.04 and 7.33 (br m, 12H, 8H,
phenyl protons). 13C{1H} NMR (50 MHz, CD2Cl2): δ -5.52 (t,
) 4.3, Pt-CH3 trans to methyl), 7.96 (dd, w/Pt satellites, 1JPt-C
2
2
) 543, cis JP-C ) 6.6, trans JP-C ) 124, Pt-CH3 trans to
phosphorus), 16.4 (s, CH2-CH2-CH2), 35.6 (vt20 with apparent
J ) 15.4, P-CH2), 38.6 (vt20 with apparent J ) 6.7, CH2-
CH2-CH2), 40.5 (s, quaternary carbon), 127.8, 129.8, and 133.8
(phenyl carbons). 31P{1H} (202 MHz, C6D6): δ -35.2 (s, w/Pt
satellites JPt-P ) 1068 Hz). Anal. Calcd for C34H42P2Pt: C,
57.70; H, 5.98. Found: C, 56.95; H, 5.93.
1
2
w/Pt satellites, JPt-C ) 408, JP-C ) 4.7, Pt-CH3 trans to
methyl), 8.65 (dd, w/Pt satellites, 1JPt-C ) 546, cis 2JP-C ) 7.7,
2
1
trans JP-C ) 124, Pt-CH3 trans to phosphorus), 20.19 (s,
P-CH2-CH2-CH2-P), 24.36 (virtual coupling pattern,20
P-CH2-CH2-CH2-P), 128.3, 130.2, and 133.9 (phenyl car-
bons). 31P{1H} NMR (202 MHz, C6D6): δ -32.9 (s, w/Pt
satellites, 1JPt-P ) 1084). Anal. Calcd for C31H38P2Pt: C, 55.77;
H, 5.74. Found: C, 56.06; H, 5.85.
Kinetics Experiments. In a typical kinetics experiment,
the platinum compound (1, 2, or 3, 3-6 mg) was loaded into a
Wilmad 504-PP medium-walled NMR tube that was sealed
onto a 14/20 joint. A stopcock adapter was attached to the tube,
then the tube was evacuated and deuterated solvent was added
via vacuum transfer. Dioxane, the internal standard, was
added via a known volume bulb, and the tube was flame-sealed
under vacuum. An initial 1H NMR spectrum was collected. The
tube was immersed in a Neslab Excal EX-250 HT elevated
temperature oil bath at 165 °C for a specified amount of time
and then quickly cooled in an ice bath. As a safety precaution,
the tubes were surrounded by stainless steel jackets during
heating and rapid cooling. Three 1H NMR spectra were
acquired after each heating period, and the integration values
were averaged. The reaction progress was quantified by
integration of the ligand backbone methylene signal relative
to the dioxane. The tube was frozen at -20 °C when not in
the oil bath or spectrometer. Reactions were monitored for a
minimum of three half-lives, and the rate constant was
calculated by a least squares linear regression method. Samples
were then heated to completion, and mass balance was
confirmed by the integration of products against the internal
standard.
Synthesis and Characterization of (Et2dppp)PtMe4 (2)
and (cbdppp)PtMe4 (3). Et2dpppPtMe4 (2) and cbdpppPtMe4
(3) were prepared by a method adapted from a procedure
reported for a similar compound.16 Pt2Me8(µ-SMe2)2 (21.8 mg,
0.0343 mmol) and diphosphine ligand (0.0700 mmol) were
dissolved in toluene (2 mL), and the solution was allowed to
stir for 3 h in a sealed reaction vessel. The solvent was removed
under vacuum, and the product was recrystallized from
methylene chloride layered with pentane at -35 °C. Et2dppp-
PtMe4 (2): Colorless blocks were produced (34.0 mg, 76%
1
3
yield). H NMR (500 MHz, C6D6): δ 0.35 (t, JH-H ) 7.2, 6H,
2
3
CH2-CH3), 0.66 (t, w/Pt satellites, 6H, JPt-H ) 44, JP-H
)
5.7, Pt-CH3 trans to methyl), 0.92 (q, 4H, 3JH-H ) 7.3, CH2-
CH3), 1.19 (t, w/Pt satellites, 6H, 2JPt-H ) 59, 3JP-H ) 6.7, Pt-
CH3 trans to phosphorus), 2.62 (m, 4H, P-CH2), 7.04 and 7.57
(br m, 12H, 8H, phenyl protons). 13C{1H} NMR (50 MHz,
1
2
C6D6): δ -4.33 (t, w/Pt satellites, JPt-C ) 411, JP-C ) 4.3,
Pt-CH3 trans to methyl), 7.51 (s, CH2-CH3), 9.42 (dd, w/Pt
satellites, 1JPt-C ) 548, cis 2JP-C ) 6.5, trans 2JP-C ) 124, Pt-
CH3 trans to phosphorus), 32.8 (vt20 with apparent J ) 15.5,
P-CH2), 34.0 (vt20 with apparent J ) 5.7, CH2-CH3), 39.9 (s,
quaternary carbon), 130.1, 134.5, and 135.2 (phenyl carbons).
31P{1H} NMR (202 MHz, C6D6): δ -35.0 (s, w/Pt satellites
1JPt-P ) 1082). Anal. Calcd for C35H46P2Pt: C, 58.08; H, 6.41.
Found: C, 58.14; H, 6.40. For cbdpppPtMe4 (3): A white
powder was produced (36.0 mg, 68% yield). 1H NMR (500 MHz,
Acknowledgment. The authors thank the National
Science Foundation for support of this work. Dr. W.
Kaminsky is acknowledged for the solution of the X-ray
structure of dppePtMe2 and Colonial Metals, Inc. for a
donation of (COD)PtMe2.
2
3
C6D6): δ 0.64 (t, w/Pt satellites, 6H, JPt-H ) 44, JP-H ) 6.0,
Supporting Information Available: X-ray crystallo-
graphic data for (dppp)PtMe2 (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Pt-CH3 trans to methyl), 1.15 (t, w/Pt satellites, 6H, 2JPt-H
)
(20) Redfield, A. A.; Nelson, J. H.; Cary, L. W. Inorg. Nucl. Chem.
Letters 1974, 10, 727.
OM0500467