Mechanistic studies of platinum(II)-catalyzed ethylene dimerization: Determination of barriers to migratory insertion in diimine Pt(II) hydrido ethylene and ethyl ethylene intermediates

Article abstract of DOI:10.1021/ja068118o

The diimine platinum(II) ethylene hydride complex [(N∧N)Pt(H)(ethylene) ][BAr′₄] (1, N∧N = [(2,6-Me₂C₆H ₃)N=C(An)-C(An)=N(2,6-Me₂C₆H₃)], An = 1,8-naphthalenediyl, Ar′ = 3,5-(CF₃)₂C ₆H₃) was prepared by protonation of the diethyl complex (N∧N)PtEt₂ with [H(OEt₂)₂] [BAr′₄], The energy barrier to interchange of the platinum hydride with the olefinic hydrogens in 1 was determined to be 19.2 kcal/mol by spin saturation transfer experiments. Complex 1 initiates ethylene dimerization; the ethyl ethylene complex (N∧N)Pt(Et)-(ethylene)+ (2) has been identified as the catalyst resting state. Trapping of 1 by ethylene to yield 2 is a second-order process; kinetic studies suggest this occurs via trapping of a reversibly formed β-agostic ethyl complex. Complex 2 has been isolated and characterized by X-ray crystallography. The barrier to migratory insertion of 2, the turnover-limiting step in catalysis, was determined to be 29.8 kcal/mol. The 1-butene hydride complex, (N∧N)Pt(H)(1-butene)⁺ (3), is a key intermediate in the dimerization cycle and has also been isolated and characterized. Surprisingly rapid rates of degenerate associative exchange of free ethylene with bound ethylene in complexes 1 and 2 as well as the rate of degenerate exchange of free nitrile with bound nitrile in (N∧N)Pt(Et) (CH₃CN)⁺ are reported.

Full text of DOI:10.1021/ja068118o

Published on Web 03/09/2007
Mechanistic Studies of Platinum(II)-Catalyzed Ethylene
Dimerization: Determination of Barriers to Migratory Insertion
in Diimine Pt(II) Hydrido Ethylene and Ethyl Ethylene
Intermediates
Masashi Shiotsuki, Peter S. White, Maurice Brookhart,* and Joseph L. Templeton*
Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received November 22, 2006; E-mail: mbrookhart@unc.edu
Abstract: The diimine platinum(II) ethylene hydride complex [(N∧N)Pt(H)(ethylene)][BAr′4] (1, N∧N ) [(2,6-
Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)], An ) 1,8-naphthalenediyl, Ar′ ) 3,5-(CF3)2C6H3) was prepared
by protonation of the diethyl complex (N∧N)PtEt2 with [H(OEt2)2][BAr′4]. The energy barrier to interchange
of the platinum hydride with the olefinic hydrogens in 1 was determined to be 19.2 kcal/mol by spin saturation
transfer experiments. Complex 1 initiates ethylene dimerization; the ethyl ethylene complex (N∧N)Pt(Et)-
(ethylene)+ (2) has been identified as the catalyst resting state. Trapping of 1 by ethylene to yield 2 is a
second-order process; kinetic studies suggest this occurs via trapping of a reversibly formed â-agostic
ethyl complex. Complex 2 has been isolated and characterized by X-ray crystallography. The barrier to
migratory insertion of 2, the turnover-limiting step in catalysis, was determined to be 29.8 kcal/mol. The
1-butene hydride complex, (N∧N)Pt(H)(1-butene)+ (3), is a key intermediate in the dimerization cycle and
has also been isolated and characterized. Surprisingly rapid rates of degenerate associative exchange of
free ethylene with bound ethylene in complexes 1 and 2 as well as the rate of degenerate exchange of
free nitrile with bound nitrile in (N∧N)Pt(Et)(CH3CN)+ are reported.
Introduction
able temperature NMR spectroscopic studies established that
the cationic metal alkyl complexes, (diimine)M-R+ (M ) Ni,
Investigations of late transition metal-catalyzed olefin poly-
merizations, oligomerizations, and dimerizations have attracted
attention from a large number of academic and industrial
research groups.1-9 Our group first reported that cationic aryl-
substituted R-diimine complexes of Ni(II) and Pd(II) bearing
ortho substituents on the aryl rings were active catalysts for
homopolymerization and oligomerization of ethylene and R-ole-
fins. Incorporating ortho-aryl substituents proved to be a critical
design feature that led to reduction of the rate of chain transfer
and thus production of high molecular weight polymers.
Working with the Versipol group at DuPont, these investigations
were expanded to include a broad range of monomers and
catalysts incorporating numerous R-diimine ligands.10-13 Vari-
Pd; R ) alkyl groups CnH2n+1, n > 1) adopt â-agostic structures
and provided information concerning the energetics of metal
migration along the alkyl chains in such complexes.12,14-18 The
barriers to migratory insertion in (diimine)M(ethylene)R+
complexes, frequently the catalyst resting state during ethylene
polymerization, were found to be in the range 13-14 kcal/mol
for Ni(II) complexes and ca. 5 kcal/mol higher for Pd(II)
complexes, 17-19 kcal/mol.
Little is known concerning the structure, dynamics, and
catalytic chemistry of the corresponding Pt(II) diimine com-
plexes. Ruffo19 has reported the synthesis of the cationic methyl
(10) Leatherman, M. D.; Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am.
Chem. Soc. 2003, 125, 3068-3081.
(11) Gates, D. P.; Svejda, S. A.; On˜ate, E.; Killian, C. M.; Johnson, L. K.; White,
P. S.; Brookhart, M. Macromolecules 2000, 33, 2320-2334.
(12) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M.
J. Am. Chem. Soc. 2000, 122, 6686-6700.
(13) Brookhart, M. S.; Johnson, L. K.; Killian, C. M.; Arthur, S. D.; Feldman,
J.; McCord, E. F.; McLain, S. J.; Kreutzer, K. A.; Bennett, M. A.; Coughlin,
E. B.; Ittel, S. D.; Parthasarathy, A.; Tempel, D. J. E. I. Du Pont de Nemours
& Co. and University of North Carolina at Chapel Hill. Patent PCT
WO9623010, April 3, 1995.
(1) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100, 1169-
1203.
(2) Mecking, S. Coord. Chem. ReV. 2000, 203, 325-351.
(3) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283-315.
(4) Fischer, K.; Jonas, K.; Misbach, P.; Stabba, R.; Wilke, G. Angew. Chem.,
Int. Ed. Eng. 1973, 12, 943-953.
(5) Cornils, B.; Herrmann, W. A. Applied Homogeneous Catalysis with
Organometallic Compounds; VCH Verlagsgesellshaft mbH: Weinheim,
1996.
(6) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; John Wily
& Sons: New York, 1992; p 72.
(14) Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics 1997, 16,
2005-2007.
(7) Pillai, S. M.; Ravindranathan, M.; Sivaram, S. Chem. ReV. 1986, 86, 353-
(15) Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 1999,
121, 10634-10635.
399.
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793.
(16) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137-1138.
(17) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414-6415.
(9) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox,
P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.;
Stro¨mberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999,
121, 8728-8740.
(18) Malinoski, J. M.; Brookhart, M. Organometallics 2003, 22, 5324-5335.
(19) Fusto, M.; Giordano, F.; Orabona, I.; Ruffo, F. Organometallics 1997, 16,
5981-5987.
9
4058
J. AM. CHEM. SOC. 2007, 129, 4058-4067
10.1021/ja068118o CCC: $37.00 © 2007 American Chemical Society

Platinum(II)-Catalyzed Ethylene Dimerization
A R T I C L E S
Scheme 2. Synthesis of Ethylene Hydride Complex 1
Scheme 1. Pt(II) Aryl-Substituted Diimine Complexes 1-3
olefin complexes [ArNdC(Me)-C(Me)CdNAr]Pt(CH3)(olefin)+
(Ar ) 2,6-i-Pr2C6H3, olefin ) ethylene, propylene, styrene;
Ar ) 2,6-Et2C6H3, olefin ) ethylene). Line-broadening of the
bound ethylene resonance was observed in the presence of free
ethylene but no quantitative exchange rates were reported. An
X-ray diffraction study of a single crystal of the methyl ethylene
complex for Ar ) 2,6-Et2C6H3 showed, as expected, that the
C-C axis of the ethylene ligand and the aryl rings lie nearly
perpendicular to the coordination square plane. No insertion
chemistry of these simple olefin complexes was noted although
a later report by Ruffo20 showed that migratory insertion occurs
at 60 °C in the electron deficient olefin complex [ArNdC(Me)-
C(Me)CdNAr]Pt(CH3)(methyl acrylate)+ (Ar ) 2,6-Et2C6H3)
to yield a chelate complex involving the ester carbonyl
functionality analogous to that observed in the Pd(II) congener.
DFT calculations suggest that the aryl-substituted (diimine)Pt-
(H)(ethylene)+ complex should be a classical hydride and that
the barrier to alkyl migration in the analogous alkyl ethylene
complex should be ca. 26 kcal/mol, much higher than the Ni-
(II) (10-11 kcal/mol) and Pd(II) (16-18 kcal/mol) barriers.21-23
There are several additional reports relevant to the work
described here. Numerous square planar platinum(II) hydrido
olefin complexes bearing bidentate ligands have been character-
ized. Complexes bearing bidentate nitrogen ligands include (κ2-
trispyrazolylborate)Pt(H)(olefin)+ 24 and (â-diiminate)Pt(H)-
(olefin)25,26 systems. In each of these classes of complexes, the
olefin hydrides adopt classical hydride structures rather than
â-agostic geometries. Spencer has reported cationic platinum-
(II) olefin hydrides supported by a series of bidentate
phosphines.27-30 The structures of these species can be classical
olefin hydrides or â-agostic alkyl complexes depending on the
P-Pt-P angle, the bulk of the substituents on phosphorus, and
the substituents on the olefin. The energy difference between
the agostic and terminal hydride structures in these complexes
is small. The sole report of catalytic ethylene dimerization by
a homogeneous Pt(II) complex is that by Roddick who observed
that [(C2F5)2PCH2CH2P(C2F5)2]Pt(CH3)(OTf) initiates dimer-
ization at 65 °C in methylene chloride. A coordination-insertion
mechanism was established but neither the intermediate hydride
nor the ethyl ethylene complex could be observed.31 Vitagliano
has reported Pt(II)-catalyzed codimerization of ethylene and
substituted olefins but a coordination-insertion mechanism does
not apply. Rather the C-C bond forms by direct attack of the
nucleophilic substituted olefin on the electrophilic Pt(II)-bound
ethylene.32,33
We report here the synthesis, characterization and chemistry
of the Pt(II) aryl-substituted diimine complexes 1, 2, and 3
(Scheme 1). Complex 1, the ethylene hydride complex, initiates
catalytic ethylene dimerization, whereas complex 2, the ethyl
ethylene complex, is the catalyst resting state, and complex 3,
the butene hydride, is a key intermediate in the catalytic cycle.
Migratory insertion barriers are reported for 1 and 2 along with
the kinetics and mechanism for conversion of 1 to 2, which
permits quantitative comparison to the Ni(II) and Pd(II)
analogues.
Results and Discussion
Synthesis of (Diimine)PtEt2 (4). R-Diimine diethyl platinum
complex 4 was synthesized by treatment of (cod)PtEt2 (cod )
1,5-cyclooctadiene) with excess R-diimine ligand, N∧N (N∧N
) [(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)], An )
1,8-naphthalenediyl), in acetonitrile solution (Scheme 2). After
heating at 55 °C for 16 h, the solvent was removed in vacuo
and (N∧N)PtEt2 (4) was purified by column chromatography
on alumina. Recrystallization from CH2Cl2/pentane provided 4
as green microcrystals. Complex 4 is air stable in the solid state
and is moderately stable in solution (ca. 1 day). The diethyl
palladium analogue34 has been prepared by the reaction of
(N∧N)PdCl2 with 2 equiv of EtMgCl, but the same method was
not successful for preparing the (diimine)Pt diethyl complex.35,36
(20) Ganis, P.; Orabona, I.; Ruffo, F.; Vitagliano, A. Organometallics 1998,
17, 2646-2650.
(21) Stro¨mberg, S.; Zetterberg, K.; Siegbahn, P. E. M. J. Chem. Soc., Dalton
Trans. 1997, 4147-4152.
(22) Musaev, D. G.; Froese, R. D. J.; Morokuma, K. New. J. Chem. 1997, 21,
1269-1282.
(23) Musaev, D. G.; Morokuma, K. Top. Catal. 1999, 7, 107-123.
(24) Reinartz, S.; Baik, M.-H.; White, P. S.; Brookhart, M.; Templeton, J. L.
Inorg. Chem. 2001, 40, 4726-4732.
(31) White, S.; Bennett, B. L.; Roddick, D. M. Organometallics 1999, 18, 2536-
2542.
(25) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804-6805.
(26) Fekl, U.; Kaminsky, W.; Goldberg, K.; I. J. Am. Chem. Soc. 2003, 125,
15286-15287.
(32) Hahn, C.; Cucciolito, M. E.; Vitagliano, A. J. Am. Chem. Soc. 2002, 124,
9038-9039.
(33) Cucciolito, M. E.; D’Amora, A.; Vitagliano, A. Organometallics 2005, 24,
3359-3361.
(27) Mole, L.; Spencer, J. L.; Carr, N.; Orpen, A. G. Organometallics 1991,
10, 49-52.
(34) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20, 3975-3982.
(35) (Diimine)PtCl2 was prepared by modified procedure according to the
following paper: van Asselt, R.; Elsevier, C. J.; Amatore, C.; Jutand, A.
Organometallics 1997, 16, 317-328.
(28) Carr, N.; Dunne, B. J.; Mole, L.; Orpen, A. G.; Spencer, J. L. J. Chem.
Soc., Dalton Trans. 1991, 863-871.
(29) Carr, N.; Mole, L.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Dalton
Trans. 1992, 2653-2662.
(36) Here, the following analogous R-diimine ligand was used: [(2,4,6-
Me3C6H2)NdC(An)-C(An)dN(2,4,6-Me3C6H2)]. The main product was
(30) Spencer, J. L.; Mhinzi, G. S. J. Chem. Soc., Dalton Trans. 1995, 3819-
3824.
1
confirmed as mono alkylated (diimine)Pt complex by H NMR spectrum.
9
J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 4059

A R T I C L E S
Shiotsuki et al.
Complex 4 was fully characterized by H and 13C NMR
spectroscopy. The pattern of the aromatic signals of the diimine
ligand and the appearance of one singlet for the four xylyl
methyl substituents reveals the presence of two mirror planes,
consistent with C2V symmetry for 4. The methyl and methylene
hydrogens of the ethyl ligands appear as a triplet at 0.73 ppm
(3JPt-C ) 85 Hz) and a quartet at 2.05 ppm (2JPt-H ) 95 Hz),
respectively. As in the 1H NMR spectrum, the aromatic region
of the 13C NMR spectrum confirms the symmetry of 4 (see
Experimental Section). The xylyl methyl groups of the diimine
ligand appear at 17.5 ppm whereas the methyl and methylene
carbons of the ethyl ligands are found at 17.8 and -0.3 ppm
(1JPt-H ) 886 Hz), respectively.
NMR spectra.27-30 Because of rapid interchange between a
hydride olefin structure and a â-agostic alkyl structure on the
NMR time scale, distinct hydride signals can be observed only
by low temperature 1H NMR. These phosphine complexes have
hydride resonances in the range of -1.0 to -5.0 ppm, far
downfield from the hydride resonance of 1, with a small to
medium coupling constant to platinum (70∼700 Hz) due to
decreased Pt-H interactions. A reported â-agostic ethyl Pd
analogue bearing an R-diimine ligand shows a broad triplet
(2JH-H ) 16 Hz) at -8.90 ppm at -130 °C in CDCl2F,
corresponding to a â-agostic hydrogen resonance.34
When the ethylene hydride complex 1 in methylene chloride
is sparged with ethylene at room temperature the ethyl ethylene
complex, [(N∧N)Pt(Et)(ethylene)][BAr′4] (2), is formed via
hydride migration and trapping by ethylene. Complex 2 was
isolated in high yields as an air stable orange powder. The
analogous acetonitrile-trapped ethyl Pt complex, [(N∧N)Pt(Et)-
(CH3CN)][BAr′4] (5), was also formed in situ and observed via
NMR spectroscopy.
1
Whereas there is ample precedent in the literature for dialkyl
platinum complexes bearing a bidentate nitrogen ligand, there
are few examples of diethyl analogues.37,38 One relevant example
is (2,2′-bipyridyl)PtEt2 reported by Chaudhury et al.;38 reactivity
studies of (2,2′-bipyridyl)PtEt2 were limited by low solubility
in organic solvents and a satisfactory NMR spectrum was not
reported.
Synthesis and Reactivity of the Platinum Ethylene Hy-
dride Complex, [(N∧N)Pt(H)(ethylene)][BAr′4] (1). Diethyl
platinum complex 4 was converted to the cationic ethylene
hydride complex, [(N∧N)Pt(H)(ethylene)][BAr′4] (1), by pro-
tonation with a slight excess of [H(OEt2)2][BAr′4] (Ar′ ) 3,5-
(CF3)2C6H3) in diethyl ether at -78 °C (Scheme 2). Complex
1 was isolated as an air stable, light orange powder by repeated
recrystallization from CH2Cl2/pentane. In the 1H NMR spectrum
of 1 in CD2Cl2, the coordinated ethylene exhibited a broad
singlet at 4.02 ppm with platinum satellites (2JPt-H ) 64 Hz),
and the hydride resonance appeared as a sharp singlet at -21.7
ppm with coupling to platinum of 1107 Hz. The four methyl
groups of the xylyl substituents display two singlets at 2.31 and
2.42 ppm, consistent with different ligands trans to the two
nitrogen donors.
To the best of our knowledge, there is only one earlier
example of a cationic platinum hydride ethylene complex
bearing a bidentate N-donor ligand, namely [κ2-(HPz*)BH-
(Pz*)2]Pt(H)(ethylene)+ (Pz* ) 3,5-dimethylpyrazolyl).24 Con-
sistent with data for 1, the hydride resonance in the protonated
pyrazolylborate case appears at -22.19 ppm with coupling to
platinum of 1048 Hz and the coordinated ethylene signals appear
at 3.86 and 3.51 ppm. Other reported olefin-coordinated
analogues bearing bidentate N donor ligands25,26,39-46 have
chemical shifts for the hydride between -17 and -30 ppm and
Pt-H coupling constants from 1045 to 1360 Hz.
In the 1H NMR spectrum of 2, the hydrogens of the
coordinated ethylene resonate slightly upfield of the analogous
signal in 1, appearing as a broad singlet at 3.96 ppm with
platinum satellites (2JPt-H ) 72 Hz). A triplet at 0.39 ppm and
a quartet at 0.99 ppm (2JPt-H ) 74 Hz) are observed, consistent
with formation of a Pt-ethyl group. All protons of the naphthyl
moiety exhibit different shifts, and two xylyl methyl signals
appear as singlets at 2.28 (6H) and 2.37 (6H) ppm consistent
1
with only mirror symmetry for 2. The H NMR spectrum of
the nitrile-trapped analogue, 5, looks quite similar to that for 2
but contains a broad singlet at 2.03 ppm attributable to
coordinated nitrile and lacks the four proton ethylene signal.
The structure of complex 2 was confirmed by X-ray analysis.
Crystallographic data and an ORTEP drawing are shown in
Table 1 and Figure 1, respectively. The expected square planar
geometry of 2 is clearly evident. The sum of the four cis L-Pt-
L′ angles around the Pt center is 360.07°, and the distance
between the platinum atom and the least-squares mean plane
based on C3, N5, N26, and the centroid of C1-C2 bond is
0.014 Å. The bond length of Pt-N26 (2.181(5) Å) is longer
than that of the Pt-N5 bond (2.065(5) Å) by more than 0.1 Å;
these distances are in the range of reported examples of
(diimine)Pt(R)(olefin)+ complexes.19,47 The strong σ donation
of the ethyl group results in the elongation of the trans Pt1-
N26 bond. The same tendency has been reported for a methyl
analogue, (diimine)Pt(CH3)(ethylene)+.19
Cationic platinum hydride olefin complexes bearing bidentate
1
phosphine ligands show different characteristics in their H
(37) Uchida, H.; Sai, A.; Sato, M.; Ogi, K. JP Patent JP95-18177.7 19950718,
1997.
(38) Chaudhury, N.; Puddephatt, R. J. J. Organomet. Chem. 1975, 84, 105-
115.
(39) Holtcamp, M. W.; Henling, L. M.; Day, M. W.; Labinger, J. A.; Bercaw,
J. E. Inorg. Chim. Acta. 1998, 270, 467-478.
Ethylene Dimerization. Under excess ethylene, complex 2
catalyzes ethylene dimerization to produce 1-butene together
with cis- and trans-2-butene (eq 2). The reaction was monitored
(40) Kostelansky, C. N.; MacDonald, M. G.; White, P. S.; Templeton, J. L.
Organometallics 2006, 25, 2993-2998.
(41) Vedernikov, A. N.; Huffman, J. C.; Caulton, K. G. New J. Chem. 2003,
27, 665-667.
1
by H NMR spectroscopy using a 1,1,2,2-tetrachloroethane-d2
(42) Vedernikov, A. N.; Caulton, K. G. Chem. Commun. 2003, 358-359.
(43) Albano, V. G.; Castellari, C.; Ferrara, M. L.; Panunzi, A.; Ruffo, F.
J. Organomet. Chem. 1994, 469, 237-244.
(CDCl2CDCl2) solution of catalyst 2 and 30-60 equiv of
ethylene. The dimerization reaction at 100 °C is very clean;
1-butene and cis- and trans-2-butenes are the only products
(44) De Felice, V.; Ferrara, M. L.; Panunzi, A.; Ruffo, F. J. Organomet. Chem.
1992, 439, C49-C51.
(45) Johnson, J. A.; Sames, D. J. Am. Chem. Soc. 2000, 122, 6321-6322.
(46) Johnson, J. A.; Li, N.; Sames, D. J. Am. Chem. Soc. 2002, 124, 6900-
6903.
(47) Wik, B. J.; Lersch, M.; Krivokapic, A.; Tilset, M. J. Am. Chem. Soc. 2006,
128, 2682-2696.
9
4060 J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007

Platinum(II)-Catalyzed Ethylene Dimerization
A R T I C L E S
Table 1. Crystallographic Data and Collection Parameters for 2
formula
C64H45BF24N2Pt‚C5H12
mol wt.
1576.07
triclinic
P1h
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
11.9369(5)
14.9994(5)
18.2760(8)
89.172(3)
86.881(3)
87.947(3)
3265.0(2)
2
Z
D
calc, mg m-3
1.603
F(000)
1568
cryst dimens, mm
temp, °C
radiation
theta range, deg
µ, mm-1
0.20 × 0.20 × 0.15
-173
Mo KR (0.71073)
1.12-25.00
2.261
20328
0.0528
reflections collected
Rf
Rw
0.0844
GOF
0.972
formed in the presence of excess ethylene. No other oligomeric
products such as hexenes or higher olefins were observed. Figure
2 shows a series of H NMR spectra which depict the growth
Figure 1. ORTEP drawing of the structure of 2. Ellipsoids are given with
50% probability. Selected bond distances in angstroms (Å): Pt1-C1 )
2.145(6), Pt1-C2 ) 2.151(6), Pt1-C3 ) 2.033(7), Pt1-N5 ) 2.065(5),
Pt1-N26 ) 2.181(5), C1-C2 ) 1.369(8), C3-C4 ) 1.522(9). Selected
bond angles in degrees (°): C1-Pt1-C2 ) 37.2(2), C1-Pt1-C3 ) 90.8-
(3), C2-Pt1-C3 ) 87.9(3), C1-Pt1-N5 ) 158.1(2), C1-Pt1-N26 )
96.2(2), C2-Pt1-N5 ) 164.3(2), C2-Pt1-N26 ) 98.3(2), C3-Pt1-
N5 ) 93.9(2), C3-Pt1-N26 ) 173.0(2), N5-Pt-N26 ) 79.26(19).
1
of the products over time. 1-Butene can be detected by the
appearance of olefinic signals at 5.92, 5.03, and 4.96 ppm and
resonances at 2.11 and 1.03 ppm for an ethyl moiety. Reso-
nances for cis- and trans-2-butene are observed at 1.64 and 1.67
ppm but the olefinic resonances of both cis- and trans-2-butene
are masked by excess ethylene at 5.4 ppm. Isomerization of
1-butene to 2-butene is favored at low ethylene concentrations.
Over long times under the reaction conditions, secondary
products begin to appear, which likely arise from butenes
reacting competitively with ethylene.
Kinetics of Ethylene Exchange in 1. We have examined
the rate of ethylene self-exchange in complex 1 using 1H NMR
line broadening techniques. Exposure of a CD2Cl2 solution of
1 at -81 °C to free ethylene results in line broadening of the
bound ethylene signal at 4.02 ppm. The change in width at half-
height is proportional to ethylene concentration implying that,
as expected for a Pt(II) complex, the exchange is associative.
The second-order rate constants can be computed using the slow
exchange approximation: k ) π(∆ν)/[ethylene] (∆ν ) change
in line width at half-height). Data is summarized in Table 2.
The calculated ethylene exchange second-order rate constant
using 0.5, 1.0, and 1.5 equiv of ethylene was ca. 1.0 × 105
M-1‚s-1. This exchange likely proceeds through a five-
coordinate species such as 8.
Kinetic Studies. Migratory Insertion in the Ethylene
Hydride Complex 1. Spin-saturation transfer (SST) experiments
were carried out to measure the rate of exchange of the hydride
with the olefinic hydrogens in 1. Irradiation and saturation of
the ethylene signal in 1 at 64 °C resulted in a substantial decrease
in the intensity of the hydride resonance. Combining this data
with T1 data yields an exchange rate of 2.4 s-1, corresponding
to a free energy of activation of 19.2 kcal/mol (see Experimental
Section for details). Three routes can be envisioned to account
for this exchange process (Scheme 3). Route A involves direct
formation of the 14-electron intermediate 6 in which all the three
hydrogens of the methyl group become equivalent. Routes B
and C involve the formation of agostic intermediate 7. Route B
achieves scrambling through an “in-place” rotation of the agostic
methyl group (never accessing a true 14-electron species), which
has previously been suggested for similar â-agostic ethyl
species.27-30,48-50 Route C accomplishes scrambling from
agostic 7 via formation of 14-electron 6.
Kinetics of Ligand Exchange Reactions in 2 and 5 and
Relative Binding Affinities of Ethylene and Acetonitrile.
Similar to 1, bound ethylene in ethyl ethylene complex 2 exhibits
associative exchange with free ethylene, and the second-order
rate constant can be determined through 1H NMR line-
broadening experiments. Experiments were carried out in CD2-
(50) (a) Derome, A. E.; Green, M. L. H.; Wong, L.-L. New J. Chem. 1989, 13,
747-753. (b) Bercaw, J. E.; Bugrer, B. J.; Green, M. L. H.; Santarsiero,
B. D.; Sella, A.; Trimmer, M.; Wong, L.-L. J. Chem. Soc., Chem. Commun.
1989, 10, 734-736. (c) McNally, J. P.; Cooper, N. J. Organometallics
1988, 7, 1704-1715. (d) Casey, C. P.; Yi, C. S. Organometallics 1991,
10, 33-35.
(48) Tempel, D. J; Brookhart, M. Organometallics 1998, 17, 2290-2296.
(49) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc. 2001, 123,
11539-11555.
9
J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 4061

A R T I C L E S
Shiotsuki et al.
Figure 2. 1H NMR profile depicting growth of 1- and 2-butenes (s: solvent, *: H2O, ×: impurities, O: the decomposed Pt complex). The spectra were
measured at r.t. Reaction conditions: catalysts 2 (4.5 mM), 60 equiv of ethylene, in CDCl2CDCl2, heated at 100 °C.
Scheme 3. Possible Pathways for Scrambling Hydride with
Table 2. Kinetic Data from Ethylene Exchanging on 1a
Hydrogens of Ethylene
[ethylene]
(mM)
half-height
line width (Hz)b
∆ν
(Hz)
kobs
k
∆
G‡
(kcal/mol)
1
1
1 c
(s-
)
(M-
‚
s-
)
0.00
0.63
1.54
2.45
9
30
62
89
-
21
53
80
-
-
-
6.5 × 101
1.7 × 102
2.5 × 102
1.0 × 105
1.1 × 105
1.0 × 105
6.7
6.7
6.7
a Condition: [1] ) 3.3 mM, in CD2Cl2 at -80.5 °C. b Coordinated
ethylene peak at 4.02 ppm. c k (M-1‚s-1) ) kobs/[ethylene].
Table 3. Kinetic Data from Ethylene Exchanging on 2a
[ethylene]
(mM)
half-height
width (Hz)b
∆ν
(Hz)
kobs
k
∆
G‡
(kcal/mol)
1
1
1 c
(s-
)
(M-
‚
s-
)
Cl2 at 27 °C. The change in the line width at half-height of the
resonance for the bound ethylene was measured as a function
of the concentration of added ethylene, and the second-order
rate constants were calculated as above for complex 1. Kinetic
data are summarized in Table 3. The second-order rate of
ethylene exchange in 2 is much slower than in 1. Whereas the
second-order exchange rate constant for 2 is ca. 4 × 103 M-1‚s-1
at 27 °C, even faster exchange rates, ca. 1 × 105 M-1‚s-1, are
observed for 1 at a temperature almost 100 °C lower! The free
energies of activation for the two exchange reaction differ by 6
kcal/mol (12.6 vs 6.7 kcal/mol).
In the case of acetonitrile complex 5, degenerate exchange
with free nitrile is sufficiently slow that in the presence of free
nitrile no line broadening of the signal for bound nitrile at 2.03
ppm is observed up to temperatures of 24 °C. The rate of ligand
exchange was thus monitored by treatment of 5 with excess
CD3CN in CD2Cl2 at 24 °C and measuring the decrease in the
integral of bound nitrile in 5 and the increase in the signal for
free CH3CN at 1.96 ppm. Second-order kinetics were observed
0.0
3.2
5.6
8.3
2.7
6.5
9.4
-
-
-
3.7
6.7
12.0
1.2 × 101
2.1 × 101
3.8 × 101
3.7 × 103
3.7 × 103
4.5 × 103
12.6
12.6
12.5
14.7
a Condition: [2] ) 4.7 mM, in CD2Cl2 at 27 °C. b Coordinated ethylene
peak at 3.96 ppm. c k (M-1‚s-1) ) kobs/[ethylene].
with k ) 6.4 × 10-4 M-1‚s-1, corresponding to ∆G‡ ) 21.7
kcal/mol for a 1.0 M concentration of free ligand (for details of
kinetic measurements see Supporting Information).
Prior to measuring the kinetics for interchange of acetonitrile
and ethylene in 2 and 5, the relative binding affinities of the
two ligands were determined. Complex 2 (4.5 mM in CD2Cl2)
was treated with 10 equiv each of CH3CN and C2H4 at 24 °C.
1
The concentrations of 2 and 5 were monitored by H NMR
spectroscopy. The ratio of 2 : 5 reached a constant value after
ca. 1 h and remained constant over the next 3 h. From the ratios
of 2 : 5 : ethylene : acetonitrile the equilibrium constant for
eq 3 could be estimated as 0.50 corresponding to ∆G ) +0.4
9
4062 J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007

Platinum(II)-Catalyzed Ethylene Dimerization
A R T I C L E S
Scheme 4. Free Energy Diagrams
Table 4. Kinetic Data for the Formation of 2 and 5a
in Scheme 3. Assuming 6 is responsible for scrambling,
extrapolation of the rate of formation of 6 from 65 °C to -54
°C yields a rate constant of 3.0 × 10-7 s-1 (assuming a ∆S‡ of
0 for this intramolecular process), 3 orders of magnitude slower
than the observed rate. Although this is a large temperature range
for extrapolation, for the extrapolated rate constant to match
those in Table 4, a large and positive ∆S‡ (ca. +30 eu) would
need to apply for formation of 6, and this is clearly unreasonable
for an intramolecular migration reaction. Furthermore, because
the rate of trapping is dependent on ligand concentration, an
additional barrier for trapping must be taken into account, so
the rate of formation of 6 predicted from k1 establishes an upper
limit on the rate of trapping (eq 4).
1
b
1
1 b
L
equiv of L
kobs (s-
)
k (M-
‚
s-
)
∆
G‡ (kcal/mol)
ethylene
ethylene
CH3CN
CH3CN
10
30
10
30
1.0 × 10-4
2.4 × 10-4
1.4 × 10-4
4.2 × 10-4
2.2 × 10-3
1.8 × 10-3
3.1 × 10-3
3.1 × 10-3
15.3
15.4
15.2
15.2
b
a Condition: [1] ) 4.5 mM, in CD2Cl2 at -54 °C. kobs ) pseudo first-
order rate constant ) k[L].
kcal/mol. Thus the binding affinities of acetonitrile and ethylene
in this system are quite similar, with ethylene slightly favored.
The rate of displacement of ethylene from 2 by CH3CN was
measured by treatment of 2 with 10 and 30 equiv of CH3CN in
1
CD2Cl2 at 24 °C and monitoring conversion to 5 by H NMR
spectroscopy. Second-order kinetics were observed and ana-
lyzed, taking into account the final equilibrium ratios of 2:5.
The second-order rate constants in eq 3 above were determined
to be k1 ) 5.6 × 10-3 M-1‚s-1, ∆G‡ ) 20.4 kcal/mol and k-1
) 1.1 × 10-2 M-1‚s-1, ∆G‡ ) 20.0 kcal/mol.
After ruling out trapping of 6 as a mechanism of formation
of 2 and 5, two remaining possibilities are summarized in eqs
5 and 6. Equation 5 represents trapping of a reversibly formed
agostic intermediate 7. Because the rate of formation of 2 and
5 depends on [L], k-1 must be greater than k2[L]. Equation 6
represents a distinctly different route to 2 and 5 in which
migratory insertion occurs from a five-coordinate intermediate
8. The kinetic experiments above cannot distinguish between
rate-determining formation of 7 or rapid, reversible formation
of 8 followed by slow migratory insertion. From ethylene
exchange experiments described below, if 8 is involved it is
likely formed rapidly and reversibly.
A summary of the exchange reactions is captured in the free
energy diagram in Scheme 4.
Kinetics of Formation of 2 and 5 from 1. Rate constants
1
for the formation of 2 and 5 from 1 were determined by H
NMR spectroscopy at -54 °C by adding excess ligand (10 and
30 equiv) to 1 and monitoring formation of 2 and 5. Kinetics
followed pseudo-first order behavior with the formation of both
2 and 5 dependent on the concentration of L. Data are
summarized in Table 4. When L is ethylene, the ratio of the
two rate constants as a function of ethylene concentration (30:
10 equiv) was determined to be 2.4, whereas for acetonitrile,
the ratio was 3.0, indicating a first-order dependence on added
L in each case.
Are these low-temperature trapping rates compatible with
high-temperature scrambling results for a common 14-electron
intermediate, 6? In fact this data is inconsistent with formation
of 2 and 5 through trapping of the 14-electron intermediate 6
9
J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 4063

A R T I C L E S
Shiotsuki et al.
Table 5. Kinetic Data for Ethylene Dimerization Catalyzed by 2a
1
equiv of ethylene
k (s-
)
∆
G‡ (kcal/mol)
30
60
2.60 × 10-5
29.8
29.8
2.62 × 10-5
a Condition: [2] ) 4.5 mM, in CD2ClCD2Cl at 100 °C.
Scheme 5. Isomerization of the 1-Butene Complex 3
migratory insertion of ethylene into the Pt-Et bond to form a
Pt-n-butyl intermediate. The turnover frequency (TOF) of
ethylene dimerization catalyzed by 2 was determined by H
Figure 3. Kinetic Plot for Ethylene Dimerization Catalyzed by 2.
1
The structures and dynamics of related complexes bearing
bidentate phosphine ligands reported by Spencer, et al. are
relevant to the transformation in eq 5.27-30 Spencer showed that
there is a subtle energy balance between the agostic ethyl isomer
and the ethylene hydride isomer; the agostic species is favored
by both large P-Pt-P bond angles and sterically bulky
substituents as shown below for complexes 9-11. Furthermore,
judging from the reported temperature-dependent spectra of the
â-agostic ethyl species and the related t-butyl complex 11 the
barrier to “in-place” rotation around the CR-Câ bond in these
complexes is substantial, ca. 8-10 kcal/mol.50
NMR spectroscopy. The reaction was carried out in CDCl2-
CDCl2 at 100 °C using two different concentrations of ethylene,
30 and 60 equiv relative to the platinum catalyst. Figure 3 shows
a plot of reaction time versus turnover number (TON) calculated
from the total amount of 1- and cis- and trans-2-butenes formed
(one turnover consumes two equivalents of ethylene). The results
clearly show that the dimerization rate is independent of ethylene
concentration. From qualitative observations it appears that the
rate of isomerization of 1-butene to 2-butenes decreases as
ethylene concentration increases. After 48 h at 100 °C, the
product ratios were 1-butene:cis-2-butene:trans-2-butene )
28:21:51 for the experiment with 30 equiv ethylene and 55:13:
32 with 60 equiv ethylene. The rate constant for migratory
insertion of 2 calculated from the TOF is 2.61 × 10-5 s-1
corresponding to ∆G‡ ) 29.8 kcal/mol. Kinetic data are
summarized in Table 5.
Synthesis and Reactivity of [(N∧N)Pt(H)(1-butene)][BAr′4]
(3). The (N∧N)Pt(H)(η2-1-butene)+ complex (3) is presumed
to be an intermediate in the catalytic ethylene dimerization by
2. This complex could be isolated via displacement of ethylene
from 1 with 1-butene. Addition of a large excess of 1-butene
to a solution of 1 in CH2Cl2 at -78 °C followed by vacuum
removal of volatiles produced pure complex 3 as an orange
powder (eq 7). (If the reaction solution is warmed during the
ligand exchange a complex mixture of products is obtained.)
Complex 3 is reasonably stable in the solid state, remaining
unchanged at room temperature for at least one week. In solution
above 0 °C, 3 readily isomerizes to yield mixtures of 3 and the
cis- and trans-2-butene complexes, cis- and trans-12. The
equilibrium ratios are specified in Scheme 5. Spectroscopic
characterization of 3 and the isomeric 2-butene complexes 12
are described in the Experimental Section.
This data suggests that an agostic intermediate, 7, should be
readily accessible from 1. If the barrier to in-place rotation in
7 is 8-10 kcal/mol, then the energy barrier between 1 and 7
must be ca. 9-11 kcal/mol to account for the observed exchange
barrier of 19.2 kcal/mol. The accessibility of 7 is consistent with
a significant barrier to trapping of 7 by ethylene and acetonitrile.
Insertion from a five-coordinate species 8 (eq 6) is appealing
based on our previous observations for CO insertion in four-
and five-coordinate Ni(II) systems. The insertion barrier in
(dppp)Ni(CO)2(CH3)+ is 2 kcal/mol lower than insertion in the
four-coordinate (dppe)Ni(CO)(CH3)+.51 However, the vast dif-
ference in rates between degenerate ethylene exchange in 2 and
acetonitrile exchange in 5, both passing through five-coordinate
intermediates, makes the five-coordinate insertion pathway less
attractive, because ethylene and acetonitrile react with 1 to form
2 and 5, respectively, at very similar rates. Although kinetically
the two pathways cannot be distinguished, we favor the agostic
intermediate route shown in eq 5.
Kinetics of Ethylene Dimerization. The ethyl ethylene
complex 2 is the catalyst resting state for ethylene dimerization.
Analogous complexes are consistent with observations concern-
ing ethylene dimerization and polymerization using Pd and Ni
diimine analogues.12,14-18 The turnover-limiting step is the
The reaction of complex 3 with ethylene was examined at
low temperatures. Upon addition of 2 equiv of ethylene to a
CD2Cl2 solution of 3 at -78 °C, the ethylene hydride complex
3 and free 1-butene were observed by H NMR spectroscopy
at -78 °C (eq 8). The reaction proceeds extremely rapidly at
1
(51) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. J. Am. Chem. Soc. 2001,
123, 9172-9173.
9
4064 J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007

Platinum(II)-Catalyzed Ethylene Dimerization
A R T I C L E S
Scheme 6. Catalytic Cycle of Ethylene Dimerization Catalyzed
by 2
species is not assisted by ethylene and appears to be a simple
first-order migratory insertion reaction. It is informative to
compare this barrier, 29.6 kcal/mol, to barriers measured for
analogous (diimine)Ni(R)(ethylene)+ species (13-14 kcal/mol)
and (diimine)Pd(R)(ethylene)+ species (17-19 kcal/mol).12,14-18
DFT computations on related species are in general agreement
with these trends and the observation that there is a much greater
difference in the migration barriers between the second and third
row systems than between the first and second row systems.21-23
DFT calculations also are consistent with the observation that
the Pt ethylene hydride is a classical hydride whereas the Ni
and Pd systems exist as â-agostic alkyl complexes.
Experimental Section
General Methods. All manipulations were performed using standard
Schlenk techniques. Nitrogen was purified by passage through columns
1
of BASF R3-11 catalyst (Chemalog) and 4 Å molecular sieves. H
and 13C NMR spectra were recorded on a Bruker Avance 400 MHz.
Chemical shifts are reported relative to solvent peaks (CD2Cl2: δ 5.32
for 1H, δ 53.8 for 13C, CDCl2CDCl2: δ 6.02 for 1H). Elemental analyses
were performed by Atlantic Microlab of Norcross, GA. Probe temper-
atures in NMR experiments were measured using the MeOH calibration
method.
Materials. Methylene chloride, toluene, diethyl ether, acetonitrile,
and pentane were purified using procedures reported by Pangborn et
al.52 Methylene chloride-d2 and 1,1,2,2-tetrachloroethane-d2 were
purchased from Cambridge Isotope Laboratories. Methylene chloride-
d2 was dried over CaH2 and then distilled by vacuum transfer, whereas
1,1,2,2-tetrachloroethane-d2 was used without further purification.
Polymer-grade ethylene and 1-butene were purchased from Matheson
Tri-Gas and Aldrich, respectively, and used without further purification.
The R-diimine ligand (N∧N),53-55 (cod)PtEt2,56 and [H(OEt2)2][BAr′4]57
were prepared based on standard literature procedures.
-78 °C as expected from the observation of very rapid
degenerate ethylene exchange in 1. Kinetics cannot be measured,
but it seems safe to assume this is an associative displacement.
Catalytic Cycle for Ethylene Dimerization. A complete
-
Representative BAr′4- NMR Data. 1H and 13C data for the BAr′4
1
counterion are reported separately for simplicity. H NMR (CD2Cl2):
δ 7.77 (br, 8H, o-Ar′), 7.60 (br, 4H, p-Ar′). 13C NMR (CD2Cl2): δ
162.1 (1:1:1:1 pattern,1JB-C ) 51 Hz, Cipso), 135.3 (Cortho), 129.4
(qq,2JC-F ) 30 Hz,4JC-F ) 5 Hz, Cmeta), 125.0 (q,1JC-F ) 295 Hz, CF3),
117.8 (Cpara).
catalytic cycle for ethylene dimerization can now be proposed
(Scheme 6). Three intermediates (1, 2, and 3) have been isolated
and fully characterized. Complex 2 is the catalyst resting state
and undergoes first-order migratory insertion (∆G‡ ) 29.8 kcal/
mol) to form the butyl complex 13 which, following â-hydrogen
elimination, produces the 1-butene hydride, 3. Ethylene very
rapidly displaces 1-butene from 3 to yield ethylene hydride 1.
Alternatively 3 can isomerize to cis-12 and trans-12 which, upon
reaction with ethylene, yields cis- and trans-2-butenes and 1.
Higher ethylene concentrations result in more rapid interception
of 3 and higher fractions of 1-butene product. To close the
catalytic cycle, ethylene hydride 1 converts to 2 via a process
which is first-order in ethylene (∆G‡ ) 15.4 kcal/mol).
[(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)]PtEt2 (4). A
solution of R-diimine ligand (N∧N) (772 mg, 1.99 mmol) in acetonitrile
(50 mL) was added to a solution of (cod)PtEt2 (598 mg, 1.65 mmol) in
acetonitrile (10 mL) at room temperature. The dark orange solution
was stirred at 55 °C for 16 h. After cooling, all solvents were removed
on a vacuum line leaving a green solid. The residue was dissolved in
toluene and purified by column chromatography (alumina, toluene).
From the collected green solution, toluene was removed under vacuum.
Recrystallization from CH2Cl2/pentane, filtration, and drying in vacuo
gave 668 mg (63% yield) of 4 as green microcrystals. 1H NMR (CD2-
Cl2, 400 MHz): δ 8.28 (d, 2H, J ) 8.4 Hz, An-pH), 7.33 (m, 8H,
An-mH, ArH), 6.81 (d, 2H, J ) 7.2 Hz, An-oH), 2.30 (s, 12H, ArCH3),
2.05 (q, 2H,3JH-H ) 7.6 Hz,2JPt-H ) 94.8 Hz, PtCH2CH3), 0.73 (t,
3H,3JH-H ) 7.6 Hz,3JPt-H ) 85.2 Hz, PtCH2CH3). 13C NMR (CD2Cl2,
400 MHz): δ 170.0, 146.0, 143.4, 133.2, 130.9, 130.4, 129.4, 128.6
(overlapped), 126.7, 121.1 (JPt-C ) 59 Hz), 17.8 (PtCH2CH3), 17.5
(ArCH3), -0.3 (PtCH2CH3,1JPt-C ) 886 Hz). Anal. Calcd for C32H34N2-
Pt: C, 59.89; H, 5.34; N, 4.37. Found: C, 59.87; H, 5.47; N, 4.44.
Concluding Remarks
Three key intermediates in this Pt-based catalytic system for
dimerization of ethylene have been isolated and characterized:
ethylene hydride complex 1, ethyl ethylene complex 2, and
butene hydride 3. Kinetic studies have provided detailed
information concerning the nature and the energetics of each
step. Perhaps most instructive is the observation that the
conversion of ethylene hydride 1 to ethyl ethylene complex 2
is first-order in ethylene and does not involve trapping of a true
fourteen-electron (diimine)PtCH2CH3+ intermediate. We cannot
distinguish between pathways involving either ethylene trapping
of a reversibly formed â-agostic ethyl intermediate or migratory
insertion of a reversibly formed five-coordinate bis-ethylene
hydride. Conversion of ethyl ethylene complex 2 to the Pt-butyl
(52) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518-1520.
(53) van Asselt, R.; Elsevier, C. J.; Smeets, W. J. J.; Spek, A. L.; Benedix, R.
Recl. TraV. Chim. Pays-Bas 1994, 113, 88-98.
(54) tom Dieck, H.; Svoboda, M.; Grieser, T. Naturforsch 1981, 36b, 823-
832.
(55) Svoboda, M.; tom Dieck, H. J. Organomet. Chem. 1980, 191, 321-328.
(56) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411-428.
(57) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920-
3922.
9
J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 4065

A R T I C L E S
Shiotsuki et al.
[[(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)]Pt(H)-
(ethylene)][BAr′4] (1). A solution of [H(OEt2)2][BAr′4] (127 mg, 0.126
mmol) in diethyl ether (10 mL) cooled to -78 °C was added dropwise
to a solution of diethyl complex 4 (80.2 mg, 0.125 mmol) in diethyl
ether (40 mL) at -78 °C via a cannula. The combined solutions were
placed under vacuum at -78 °C for 4.5 h then warmed to r.t. where
solvents were removed under vacuum. Recrystallization of the orange
residue from CH2Cl2/pentane and filtration was repeated until the
supernatant had no purple color. Drying under vacuum gave 149.4 mg
of 1 as light orange powder (81% yield). 1H NMR (CD2Cl2, 400
MHz): δ 8.28 (d, 1H, J ) 8.4 Hz, An-pH), 8.25 (d, 1H, J ) 8.4 Hz,
An-pH), 7.61-7.57 (m, 2H, An-oH), 7.40-7.37 (m, 6H, ArH), 6.84
(d, 1H, J ) 8.8 Hz, An-oH), 6.82 (d, 1H, J ) 8.8 Hz, An-oH), 4.02
(brs, 4H,2JPt-H ) 64.4 Hz, CH2dCH2), 2.42 (s, 6H, ArCH3), 2.31 (s,
6H, ArCH3), -21.7 (s, 1H,1JPt-H ) 1107 Hz, Pt-H). 13C NMR (CD2-
Cl2, 400 MHz): δ 178.0, 175.1, 149.1, 146.4, 142.0, 134.6, 133.8, 132.1,
130.6, 130.5, 130.1, 130.0, 129.5, 129.4, 128.8, 128.2, 126.5, 125.9,
124.5, 124.4, 69.9 (CH2dCH2), 18.2 (ArCH3), 17.8 (ArCH3), Anal.
Calcd for C62H41BF24N2Pt: C, 50.46; H, 2.80; N, 1.90. Found: C, 50.78;
H, 2.78; N, 1.88.
vinyl H), 2.42 (s, 3H, ArMe), 2.41 (s, 3H, ArMe), 2.34 (s, 3H, ArMe),
2.27 (s, 3H, ArMe), 2.12-1.98 (m, 2H, CH2 of 1-butene), 1.14 (t, 3H,
J ) 7.2 Hz, CH3 of 1-butene), -23.6 (s, 1H,1JPt-H ) 1267.7 Hz, Pt-
H). 13C NMR (CD2Cl2, 100 MHz, at 0 °C): δ 178.1, 174.2, 148.7,
145.7, 141.0, 134.4, 133.4, 131.7, 131.2, 130.6, 130.5, 130.1, 130.1,
130.0, 130.0, 129.3, 128.7, 128.5, 128.0 (overlapped), 126.4, 125.6,
124.2, 123.9, 101.9 (internal sp2-C of 1-butene), 68.0 (terminal sp2-C
of 1-butene), 30.9 (CH2 of 1-butene), 18.1 (ArCH3), 18.1 (ArCH3), 17.7
(ArCH3), 17.5 (ArCH3), 14.1 (Me of 1-butene). Anal. Calcd for C64H45-
BF24N2Pt: C, 51.11; H, 3.02; N, 1.86. Found: C, 51.37; H, 3.15; N,
1.88.
[[(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)]Pt(H)(cis- and
trans-2-butene)][BAr′4] (cis- and trans-12). These isomers were
generated by warming 3 in CD2Cl2 solution at r.t. to establish an
equilibrium between 3 and cis- and trans-12. 1H NMR(CD2Cl2,
400Mhz): trans-12: The aromatic signals of the R-diimine ligand (N∧N)
overlap the signals of the other isomers. δ 4.49 (q, 2H, J ) 4.0 Hz,2JPt-H
) ca. 77 Hz vinyl H), 2.49 (s, 3H, ArMe), 2.42 (s, 3H, ArMe), 2.40
(s, 3H, ArMe), 2.20 (s, 3H, ArMe), 1.76 (d, 6H, J ) 4.8 Hz,3JPt-H
)
45.2 Hz, CH3 of trans-2-butene), -24.1 (s, 1H,1JPt-H ) 1317.7 Hz,
Pt-H). cis-12: The aromatic signals of the R-diimine ligand (N∧N)
overlap the signals of the other isomers. δ 4.47 (m, 2H,2JPt-H ) ca. 81
Hz, vinyl H), 2.43 (s, 6H, ArMe), 2.33 (s, 6H, ArMe), 2.01 (d, 6H,
J ) 4.4 Hz,3JPt-H ) 63.2 Hz, CH3 of cis-2-butene), -24.5 (s, 1H,1JPt-H
) 1419.3 Hz, Pt-H).
[[(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)]Pt(Et)-
(ethylene)][BAr′4] (2). A solution of complex 1 (100 mg, 67.8 mmol)
in CH2Cl2 (3.0 mL) at 0 °C was purged with ethylene for 5 min. Pentane
(20 mL) was then added to form an orange precipitate which was
collected by filtration and dried under vacuum to give 89.6 mg (88%
yield) of the isolated product 2. 1H NMR (CD2Cl2, 400 MHz): δ 8.31
(d, 1H, J ) 8.3 Hz, An-pH), 8.25 (d, 1H, J ) 8.5 Hz, An-pH), 7.63-
7.39 (m, 8H, An-mH, ArH), 6.81 (d, 1H, J ) 7.2 Hz, An-oH), 6.74 (d,
1H, J ) 7.5 Hz, An-oH), 3.96 (brs, 4H,2JPt-H ) 71.5 Hz, CH2dCH2),
2.37 (s, 6H, ArCH3), 2.28 (s, 6H, ArCH3), 0.99 (q, 2H,3JH-H ) 7.6
Ethylene Dimerization. For the kinetic study of ethylene dimer-
ization catalyzed by 2, the sample was prepared as follows: The catalyst
2 (4.0 mg, 2.7 µmol) was added to a tared J-Young tube(Wilmad).
After adding CDCl2CDCl2 (0.60 mL), the tube was capped with a
J-Young valve equipped with a rubber capped terminated glass tube.
The solution was subjected to three freeze-pump-thaw cycles. Then
ethylene gas (2.0-4.0 mL gas at 25 °C, 30-60 equiv based on 2) was
injected via gastight syringe through the rubber cap into the frozen
solution (liquid N2). The J-Young valve was closed and the tube was
warmed to room temperature. The ethylene dimerization was carried
out by immersing the tube in a 100 °C oil bath. The reaction was
quenched at various time intervals in a stream of air (25 °C). Turnover
numbers at these intervals were determined by 1H NMR spectroscopy
at room-temperature based on the integrals of all three butene
isomers: one of the vinyl protons of 1-butene (4.96 ppm) and methyl
groups of cis- and trans-2-butenes (1.64 and 1.66 ppm, respectively).
Plots of TON versus time are shown in Figure 3.
2
Hz, JPt-H ) 74.0 Hz, PtCH2CH3), 0.39 (t, 3H,3JH-H ) 7.6 Hz,
PtCH2CH3). 13C NMR (CD2Cl2, 400 MHz): δ 179.8, 172.1, 148.4,
140.8, 140.2, 133.3, 132.1, 130.5, 130.4, 130.1, 130.1 (overlapped),
130.0, 129.8, 129.3, 128.2, 127.3, 125.6, 125.5, 124.7, 75.5 (1JPt-H
)
212.6Hz,CH2dCH2),18.1(ArCH3),17.5(ArCH3),13.5(PtCH2CH3,2JPt-H
) 146.9 Hz), 9.7 (PtCH2CH3,1JPt-H ) 637.4 Hz). Anal. Calcd for
C64H45BF24N2Pt: C, 51.11; H, 3.02; N, 1.86. Found: C, 51.12; H, 2.94;
N, 1.85.
[[(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)]Pt(Et)-
(CH3CN)][BAr′4] (5). To a solution of complex 1 (49 mg) in CH2Cl2
(10 mL) was added CH3CN (1 mL) at -78 °C, and then warmed up to
r.t. After 30 min, solvent and excess CH3CN were evaporated by
vacuum to give deep orange residue. The formation of 5 was confirmed
by 1H and 13C NMR spectra. 1H NMR (CD2Cl2, 400 MHz): δ 8.27 (d,
1H, J ) 11.2 Hz, An-pH), 8.22 (d, 1H, J ) 11.2 Hz, An-pH), 7.56-
7.29 (m, 8H, An-mH, ArH), 6.81 (d, 1H, J ) 7.2 Hz, An-oH), 6.74 (d,
1H, J ) 7.3 Hz, An-oH), 2.37 (s, 6H, ArCH3), 2.27 (s, 6H, ArCH3),
2.03 (brs, 3H, NCCH3), 1.54 (q, 2H,3JH-H ) 7.6 Hz,2JPt-H ) 85.9 Hz,
Rates of Formation of 2 and 5 from 1. The rates were determined
by 1H NMR measurements on samples prepared by the following
method: Complex 1 (4.0 mg, 2.7 µmol) was weighed into a tared NMR
tube and then dissolved in CD2Cl2 (0.60 mL). After cooling the solution
to -78 °C, the ligand (either ethylene for 2 or acetonitrile for 5) was
added via gastight syringe or a microsyringe. The tube was then
transferred into a -78 °C precooled NMR probe. The reaction was
started by warming to -54.2 °C for both 2 and 5. The hydride signal
of starting complex 1 at -21.7 ppm was used to calculate the amount
of the remaining 1. Kinetic plots are shown in Figure S1 (see Supporting
Information).
PtCH2), 0.58 (t, 3H,3JH-H ) 7.6 Hz,3JPt-H ) 38.2 Hz, PtCH2CH3). 13
C
NMR (CD2Cl2, 100 MHz): δ 178.5, 170.6, 147.3, 142.5, 141.8, 133.1,
132.3, 132.2, 130.5, 130.3 (overlapped), 129.8, 129.5, 129.3, 128.9,
128.5, 126.2, 125.8, 125.1, 124.4, 118.3 (CH3CN), 17.8 (ArCH3), 17.5
(ArCH3), 16.7 (CH3CN), 4.1 (1JPt-C ) 445.3 Hz, PtCH2), 3.0
(PtCH2CH3).
Spin Saturation Transfer Measurement of the Rate of Exchange
of the Pt Hydride with the Vinyl Hydrogens of Ethylene. Complex
1 (8.0 mg) in an NMR tube was dissolved in CDCl2CDCl2 and degassed
via three freeze-pump-thaw cycles. At 64.4 °C, the coordinated
ethylene signal (4.06 ppm) of 1 was saturated by irradiation, and the
decrease in intensity of the hydride (-21.7 ppm) was measured. The
relaxation time T1 for the hydride was determined to be 1.000 s using
the inversion-recovery method and analyzed with the spectrometer
T1 routine. The exchange rate constant was calculated using the equation
kobs ) (1/T1)(M(0)/M(irr)-1) ) 1.62 s-1 where M(0) and M(irr) are
the intensities of the hydride resonance before and after spin saturation,
respectively. The rate constant k for migration was calculated using
[[(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)]Pt(H)(1-
butene)][BAr′4] (3). An excess amount of 1-butene (∼1 mL as a liquid
at -78 °C) was added to a solution of complex 1 (50 mg, 36 µmol) in
CH2Cl2 (21 mL) at -78 °C. After stirring at -78 °C for 1 h, methylene
chloride and excess 1-butene were evaporated under vacuum while
keeping the solution temperature under 0 °C. Complete solvent removal
gave 3 in 80% isolated yield as an orange powder. 1H NMR (CD2Cl2,
400 MHz, 0 °C): δ 8.28 (d, 1H, J ) 8.3 Hz, An-pH), 8.24 (d, 1H,
J ) 8.4 Hz, An-pH), 7.61-7.56 (m, 2H, An-mH), 7.40-7.36 (m, 6H,
ArH), 6.81 (d, 1H, J ) 7.4 Hz, An-oH), 6.78 (d, 1H, J ) 7.3 Hz,
An-oH), 4.62 (m, 1H,2JPt-H ) 73.8 Hz, vinyl H), 3.98 (d, J ) 14.0
Hz,2JPt-H ) 57.8 Hz, vinyl H), 3.83 (d, J ) 7.8 Hz,2JPt-H ) 63.7 Hz,
9
4066 J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007

Platinum(II)-Catalyzed Ethylene Dimerization
A R T I C L E S
the equation k ) (3/2)kobs ) 2.43 s-1 assuming that for every three
migration events only two hydrogens undergo site exchange.
Degenerate Ethylene Exchange Rates in 1 and 2. The exchange
rate between free ethylene and the coordinated ethylene of 1 and 2
was determined by 1H NMR line broadening experiments at 192.5 and
298 K, respectively. Samples were prepared by following procedure:
In an argon-filled drybox, the corresponding complex was weighed (4
mg) into a tared NMR tube and then dissolved in dry, degassed CD2-
Cl2 (0.60 mL) at room temperature to give a homogeneous solution.
After capping with a septum and Parafilm, the NMR spectrum was
acquired at the appropriate temperature for each complex. Ethylene
gas was added to the solution via a gastight syringe and a second NMR
spectrum acquired at the corresponding temperature. The molarities of
free ethylene and the ethylene complex were calculated using the BAr′4
resonances as an internal standard. Linewidths (w) were measured at
half-height in units of Hertz for complexed ethylene. Linewidths (w0)
in the absence of ethylene were also measured. The second-order
exchange rates were determined from the standard equation for the slow
exchange approximation: k ) π(∆ν)/[ethylene], where ∆ν is equal to
w - w0 and [ethylene] is the molar concentration of free ethylene. Data
are summarized in Tables 2 and 3.
X-ray Structure of [[(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-
Me2C6H3)]Pt(Et)(ethylene)][BAr′4] (2). Crystals of 2 were grown from
slow diffusion of pentane into a CH2Cl2 solution of 2. The X-ray data
were collected on a Bruker SMART APEX-II using the ω scan mode.
Experimental details are given in Table 1. The structure was solved by
direct methods using SHELXS-97. Final agreement indices were Rf )
5.3% and Rw ) 8.4%, with hydrogen placed in computed positions
and included in the refinement using a riding model. All other atoms
were refined anisotropically.
Acknowledgment. We thank the National Science Foundation
(CHE-0414726(JLT) and CHE-0615704(MB)) for support of
this work.
Supporting Information Available: Kinetic plots for rates
of formation of 2 and 5 from 1, the rate of formation of 5 from
2, the rate of degenerate acetonitrile exchange in 5, and a plot
showing the determination of the equilibration of 2 and 5. CIF
file for complex 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA068118O
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J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 4067