Ethylene dimerization by cationic palladium(II) alkyl complexes that contain bis(heterocycle)methane ligands

Article abstract of DOI:10.1021/om700767r

The catalytic ethylene dimerization reactions of (N∧N)PdMe(L) ⁺ species that contain bidentate nitrogen donor ligands were studied (N∧N = (1-Me-imidazol-2-yl)₂CH₂ (a); (1-Me-imidazol-2-yl)₂CH(C₆H₁₃) (b), 1,1′di(triphenylmethyl)-4,4′-biimidazole (c), (5-Me-pyridin-2-yl) ₂CH₂ (d), (pyrazol-1-yl)₂CH₂ (e), (3,5-Me₂-pyrazol1-yl)₂CH₂ (f), (4-Me-C ₆H₄)N=CMeCMe=N(4-Me-C₆H₄) (g), and (2,6-ⁱPr₂-C₆H₃)N=CMeCMe=N(2,6- ⁱPr₂-C₆H₃) (h)). (N∧N)PdMe ₂ (2a-e,g) and (N∧N)Pd(Me)Cl (3f-h) complexes were converted to [(N∧N)Pd{C(=O)Me}CO][B(C₆F₅)₄)] (7a,c-h), [(N∧N)Pd(Me)(H₂C=CH₂)][B(C₆F ₅)₄)] (8a-g), and [(N∧N)Pd(Me)(H₂C=CH ₂)] [SbF₆] (8f′). The v_co values for 7a,c-f show that there is weak back-bonding in these species, the donor ability of the NAN ligand varies in the order imidazole > pyridine > pyrazole, and variation of the chelate ring size does not strongly affect the electron density at Pd. 8a,c-g and 8f′ dimerize ethylene by an insertion/β-H elimination mechanism. The catalyst resting state is (N∧N)Pd(Et)(H ₂C=CH₂)⁺ (9a,c-g). First-order rate constants for ethylene insertion of 8a-g and 8f′ (k_insert,Me) and 9a,c-g (k_insert,Et) were determined by NMR. The k_insert,Et and k_insert,Et values for analogous (N∧N)Pd(R)(H₂C=CH ₂)⁺ species are similar. Increasing the electrophilic character and the steric bulk of the (N∧N)Pd unit leads to moderate increases in ethylene insertion rates.

Full text of DOI:10.1021/om700767r

6726
Organometallics 2007, 26, 6726–6736
Ethylene Dimerization by Cationic Palladium(II) Alkyl Complexes
that Contain Bis(heterocycle)methane Ligands
Christopher T. Burns and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
ReceiVed July 29, 2007
The catalytic ethylene dimerization reactions of (N∧N)PdMe(L)+ species that contain bidentate nitrogen
donor ligands were studied (N∧N ) (1-Me-imidazol-2-yl)2CH2 (a); (1-Me-imidazol-2-yl)2CH(C6H13) (b), 1,1′-
di(triphenylmethyl)-4,4′-biimidazole (c), (5-Me-pyridin-2-yl)2CH2 (d), (pyrazol-1-yl)2CH2 (e), (3,5-Me2-pyrazol-
1-yl)2CH2 (f), (4-Me-C6H4)NdCMeCMedN(4-Me-C6H4) (g), and (2,6-iPr2-C6H3)NdCMeCMedN(2,6-iPr2-
C6H3) (h)). (N∧N)PdMe2 (2a–e,g) and (N∧N)Pd(Me)Cl (3f–h) complexes were converted to
[(N∧N)Pd{C(dO)Me}CO][B(C6F5)4)] (7a,c–h), [(N∧N)Pd(Me)(H2CdCH2)][B(C6F5)4)] (8a-g), and
[(N∧N)Pd(Me)(H2CdCH2)][SbF6] (8f′). The νCO values for 7a,c-f show that there is weak back-bonding
in these species, the donor ability of the N∧N ligand varies in the order imidazole > pyridine > pyrazole,
and variation of the chelate ring size does not strongly affect the electron density at Pd. 8a,c-g and 8f′
dimerize ethylene by an insertion/ꢀ-H elimination mechanism. The catalyst resting state is (N∧N)-
Pd(Et)(H2CdCH2)+ (9a,c-g). First-order rate constants for ethylene insertion of 8a-g and 8f′ (kinsert,Me
)
and 9a,c-g (kinsert,Et) were determined by NMR. The kinsert,Me and kinsert,Et values for analogous
(N∧N)Pd(R)(H2CdCH2)+ species are similar. Increasing the electrophilic character and the steric bulk
of the (N∧N)Pd unit leads to moderate increases in ethylene insertion rates.
high molecular weight polymers from ethylene, R-olefins,
internal olefins, and cyclic olefins when the N-aryl rings contain
ortho substituents.6 A key development in this area was
understanding that chain transfer in these oligomerization/
polymerization reactions proceeds by associative olefin ex-
change. The orthogonal orientation of the N-aryl rings with
respect to the metal square plane in (R-diimine)Pd species
positions the ortho substituents above and below the axial
coordination sites, which inhibits associative ligand exchange
processes and thus disfavors chain transfer.
Introduction
A wide variety of (N∧N)PdR+ olefin oligomerization and
polymerization catalysts that contain neutral bidentate nitrogen
donor ligands (N∧N) have been developed.1–4 Brookhart and
co-workers showed that the dimerization of ethylene by the
(phen)PdR+ system (phen ) phenanthroline) proceeds by an
insertion/ꢀ-H elimination mechanism and that the ethyl ethylene
complex (phen)Pd(Et)(H2CdCH2)+ is the catalyst resting state.5
Brookhart and co-workers also developed the chemistry of
(ArNdCRCRdNAr)PdR+ R-diimine catalysts, which produce
In previous work, we studied {R2C(pz)2}Pd(R)(H2CdCH2)+
complexes that contain bis(pyrazolyl)methane ligands (pz )
pyrazolyl).7 These species catalytically oligomerize ethylene to
C8-C24 internal olefins. However, ethylene insertion of
{Me2(pz)2}PdMe(H2CdCH2)+ is much slower than for
(phen)PdMe(H2CdCH2)+ or (diimine)PdMe(H2CdCH2)+ com-
plexes. Canty, Trofimenko, and others prepared a variety of other
{R2C(pz)2}Pd complexes, including {R2C(pz)2}PdMe2 and
{R2C(pz)2Pd(allyl)+.8,9 The (N∧N)Pd chelate rings in these
compounds adopt boat conformations and may undergo inver-
* Corresponding author. E-mail: rfjordan@uchicago.edu.
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10.1021/om700767r CCC: $37.00
2007 American Chemical Society
Publication on Web 12/08/2007

Cationic Palladium(II) Alkyl Complexes
Organometallics, Vol. 26, No. 27, 2007 6727
Chart 1
Scheme 1
Scheme 2
Scheme 3
sion, which results in exchange of the axial and equatorial CR2
substituents.10,11
In the present work we have explored the chemistry of
(N∧N)PdR(L)+ complexes that contain a wider range of
bis(heterocyclic) N∧N ligands, in order to probe how the
electronic and steric properties of the heterocycle influence the
ethylene insertion reactivity.
Results and Discussion
) cyclooctadiene), or (Me2S)2PdMe2 as shown in Scheme 1.8,21
In the solid state, 2e,g decompose within minutes (2e) or hours
(2g) at 25 °C. In CD2Cl2 solution, 2d,e,g decompose to free
N∧N ligand, CH4, C2H6, and Pd0 within minutes (2e) or hours
(2d,g) at 25 °C.8 Complexes 2b,c are more thermally stable,
but decompose photochemically in CD2Cl2 to (N∧N)PdCl2.15
Synthesis of (N∧N)Pd(Me)Cl Complexes (3f–h). The
(N∧N)Pd(Me)Cl complexes 3f–h were prepared by the reaction
of (cod)Pd(Me)Cl with 1f–h (Scheme 2).6a,d,22 Compounds 3f–h
are more thermally stable than 2a–e,g. However, 3g decomposes
slowly (days) in the solid state at 25 °C.
NMR and Dynamic Properties. Complexes 2a,b,d,e and
3f likely have boat conformations, as shown for 2e in Scheme
3, similar to those observed for the analogous complexes {1,3-
dioxolane)C(py)2}PdCl2, {Me2C(pz)2}PdCl2, {Ph2C(pz)2}PdCl2,
and {(n-hexyl)HC(mim)2}PdCl2.7,15,23–25 Previously, Canty
observed a singlet for the methylene bridge hydrogens in the
1H NMR spectrum of 2a down to -70 °C, consistent with rapid
inversion of the (N∧N)Pd chelate ring.8 In contrast, slower ring
inversion is observed for 2d,e, and 3f. The variable-temperature
1H NMR spectra (δ 6.9–5.8 region) for 2e are shown in Figure
N∧N Ligands. The N∧N ligands 1a-h in Chart 1 were
prepared by literature routes.12–20 Ligands 1a-h were chosen
for synthetic convenience and to enable investigation of how
differences in donor properties, chelate ring size, and steric
effects influence the rate of ethylene insertion in (N∧N)Pd-
(R)(H2CdCH2)+ species.
Synthesis of (N∧N)PdMe2 Complexes (2a–e,g). The
(N∧N)PdMe2 compounds 2a-e,g were synthesized by ligand
substitution reactions of [(pyridazine)PdMe2]n, (cod)PdMe2 (cod
(10) Jalón, F. A.; Manzano, B. R.; Otero, A.; Rodriguez-Perez, M. C.
J. Organomet. Chem. 1995, 494, 179.
(11) (a) Brown, D. G.; Byers, P. K.; Canty, A. J. Organometallics 1990,
9, 1231. (b) Byers, P. K.; Canty, A. J.; Honeyman, R. T. J. Organomet.
Chem. 1990, 385, 417.
(12) (N∧N) ) (1-Me-imidazol-2-yl)2CH2 (1a, (mim)2CH2), (1-Me-
imidazol-2-yl)2CH(C6H13) (1b, (mim)2CH(n-hexyl)), 1,1′-di(triphenyl-
methyl)-4,4′-biimidazole (1c, biTim), (5-Me-pyridin-2-yl)2CH2 (1d, (5-
Mepy)2CH2), (pyrazol-1-yl)2CH2 (1e, (pz)2CH2), (3,5-Me2-pyrazol-1-yl)2CH2
(1f, (3,5-Me2-pz)2CH2), (4-Me-C6H4)NdCMeCMedN(4-Me-C6H4) (1g,
p-tolyldiimine), and (2,6-iPr2-C6H3)NdCMeCMedN(2,6-iPr2-C6H3) (1h,
2,6-iPr2-diimine).
(13) Gorun, S. M.; Papaefthymiou, G. C.; Frankel, R. B.; Lippard, S. J.
J. Am. Chem. Soc. 1987, 109, 4244.
(14) Braussaud, N.; Ruther, T.; Cavell, K. J.; Skelton, B. W.; White,
A. H. Synthesis 2001, 626.
(21) Foley, S. R.; Stockland, R. A., Jr.; Shen, H.; Jordan, R. F. J. Am.
Chem. Soc. 2003, 125, 4350.
(15) Burns, C. T,; Shen, H.; Jordan, R. F J. Organomet. Chem. 2003,
683, 240.
(22) Rülke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
(23) Newkome, G. R.; Gupta, V. K.; Theriot, K. J.; Ewing, J. C.;
Wicelinski, S. P.; Huie, W. R.; Fronczek, F. R.; Watkins, S. F. Acta
Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, C40, 1352.
(24) Minghetti, G.; Cinellu, M. A.; Bandini, A. L.; Banditelli, G.;
Demartin, F.; Manassero, M. J. Organomet. Chem. 1986, 315, 387.
(25) (a) Canty, A. J.; Minchin, N. J.; Skelton, B. W.; White, A. H. Aust.
J. Chem. 1992, 45, 423. (b) Newkome, G. R.; Gupta, V. K.; Taylor, H. C. R.;
Fronczek, F. R. Organometallics 1984, 3, 1549.
(16) Cliff, M. D.; Pyne, S. G. Synthesis 1994, 681.
(17) Li, X.; Gibb, C. L. D.; Kuebel, M. E.; Gibb, B. C. Tetrahedron
2001, 57, 1175.
(18) Jameson, D. L.; Castellano, R. K.; Reger, D. L.; Collins, J. E.;
Tolman, W. B.; Tokar, C. J. Inorg. Synth. 1998, 32, 51.
(19) Joshi, V. S.; Sarkar, A.; Rajamohanan, P. R. J. Organomet. Chem.
1991, 409, 240.
(20) Alcazar, J.; Begrup, M.; de la Hoz, A. J. Chem. Soc., Perkin Trans.
1 1995, 2467.

6728 Organometallics, Vol. 26, No. 27, 2007
Burns and Jordan
Scheme 4
observed for the analogous dichloride complex, {(n-
hexyl)HC(mim)2}PdCl2.15 Complexes 2c and 3g likely have
planar conformations, similar to the analogous complexes {2,2′-
bipyridine}PdCl2, (2,6-iPr-BIAN)PdCl2 (2,6-iPr-BIAN )
bis{2,8-(2,6-di-isopropylphenylimino)}acenaphthene), and (p-
Figure 1. Variable-temperature 1H NMR spectra (δ 6.9–5.8 region)
of {H2C(pz)2}PdMe2 (2e) in CD2Cl2 solution. The chemical shift
scale is in δ units. NMR spectra were not obtained above -10 °C
due to the thermal instability of 2e. The broadening of the
{H2C(pz)2}Pd resonances is due to chelate ring inversion.
MeO-BIAN)PdCl2 (p-MeO-BIAN
) bis{2,8-(4-methoxy-
phenylimino)}acenaphthene).28
Generation of (N∧N)PdMe(NMe2Ph)+ Species (5a–e,g).
The (N∧N)PdMe2 complexes 2a–e,g can be converted to
(N∧N)PdMe(L)+ species in a variety of ways.3b The (N∧N)-
PdMe(L)+ species described here are insufficiently thermally
stable to isolate and therefore were characterized in situ by
NMR. Complexes 2a–e,g were reacted with [HNMe2Ph]-
[B(C6F5)4] in CD2Cl2 at -60 °C to produce [(N∧N)PdMe-
(NMe2Ph)][B(C6F5)4] (5a–e,g; Scheme 4) in near quantitative
yield. Complexes 5a–e,g are quite stable at low temperature
(-60 °C) but decompose slowly at 25 °C to yield black solutions
and NMe2Ph; the fate of the (N∧N)PdMe+ unit was not
determined.
1. At -60 °C, two doublets at δ 6.61 (Hax) and 6.11 (Heq) are
observed for the methylene bridge hydrogens, which indicates
that inversion of the chelate ring is slow on the NMR time scale
at this temperature.26 However, as the temperature is raised to
-10 °C, the Hax and Heq resonances broaden selectively,
1
indicating that inversion occurs. H NMR spectra of 2e could
not be obtained above -10 °C due to the thermal instability of
this species. The spectra of 2e contain one set of pyrazole H3,
H4, and H5 resonances and one PdMe resonance at all
temperatures, consistent with the expected Cs-symmetric structure.
The ring inversion in 2e most likely occurs by a nondisso-
ciative process via a planar transition state as shown in Scheme
3.10 The free energy barrier for ring inversion in 2e, estimated
from the line broadening of the Hax and Heq resonances, is ∆Gq
) 12.4(1) kcal/mol at -20 °C.27 Similar results were observed
for 2d and 3f. The inversion barrier for 2d (∆Gq ) 13.1(1)
kcal/mol at -20 °C) is similar to that for 2e, while that for
3f is ca. 3 kcal/mol higher (∆Gq ) 15.6(1) kcal/mol at 25
°C). Ring inversion in 3f may be inhibited by the 3,5-Me2
substituents on the pyrazole rings, which would crowd the
Pd-Me and Pd-Cl groups in the planar transition state (cf.
Scheme 3).
1
The -60 °C H NMR spectra of 5a–e,g show that in each
case the sides of the N∧N ligand are inequivalent and one
NMe2Ph ligand is present. The NMe2Ph resonances are strongly
deshielded compared to the resonances of free NMe2Ph by
coordination to the (N∧N)PdMe+ unit.29 In 5a–e, one hetero-
cycle ring resonance (H4 in (mim)2CH2 and (mim)2CH(n-hexyl);
H2 in biTim; H6 in (5-Me-py)2CH2; H3 in (pz)2CH2) appears
at a higher field (δ e 5.95) compared to the other ring
resonances due to anisotropic shielding by the NMe2Ph ring.
One conformer is observed for 2b at both -60 and 25 °C. It
(28) (a) Canty, A. J.; Skelton, B. W.; Traill, P. R.; White, A. H. Aust.
J. Chem. 1992, 45, 417. (b) Coventry, D. N.; Batsanov, A. S.; Goeta, A. E.;
Howard, J. A. K.; Marder, T. B. Polyhedron 2004, 23, 2789.
is likely that the n-hexyl group occupies the axial position as
(29) (a) NMR data for free NMe2Ph: 1H NMR (CD2Cl2): δ 7.20 (m,
2H, o-Ph), 6.72 (m, 2H, m-Ph), 6.67 (t, J ) 7, 1H, p-Ph), 3.03 (s, 6H, Me).
13C{1H} NMR (CD2Cl2): δ 151.1 (C1), 129.3 (C2), 116.6 (C4), 112.8 (C3),
40.7 (Me). 1H NMR (CD2Cl2, -60 °C): δ 7.18 (m, 2H, o-Ph), 6.67 (m,
2H, m-Ph), 6.63 (t, J ) 7, 1H, p-Ph), 2.88 (s, 6H, Me). 13C{1H} NMR
(CD2Cl2, -60 °C): δ 150.2 (C1), 128.7 (C2), 115.8 (C4), 111.9 (C3), 40.3
(Me). (b) If excess [HNMe2Ph][B(C6F5)4] is used in the generation of 5a-e
and 5g, and the NMe2Ph is then displaced from 5a-e and 5g by another
ligand, the excess HNMe2Ph+ undergoes fast H+ exchange with free
NMe2Ph and a single set of NMe2Ph/HNMe2Ph+ resonances at the weighted
average of the chemical shifts of these species is observed.
(26) (a) The assignment of the bridge methylene hydrogen resonances
is based on Canty’s results for {MeHC(py)2}PdMe2, which exists as a 1/1
mixture of conformers at -10 °C (see ref 8). In this case, two doublets
were observed for the bridge CH in the 1H NMR spectrum at -10 °C. The
downfield CH resonance was assigned the conformer in which the C-H is
in the axial position. The downfield shift was ascribed to the proximity of
H
ax to Pd. (b) Deeming, A. J.; Rothwell, I. P.; Hursthouse, M. B.; New, L.
J. Chem. Soc., Dalton Trans. 1978, 1490. (c) Polyakov, V. A.; Ryabov,
A. D. J. Chem. Soc., Dalton Trans. 1986, 589.
(27) gNMR, v. 4.1.2; Adept Scientific: Letchworth, 2000.

Cationic Palladium(II) Alkyl Complexes
Organometallics, Vol. 26, No. 27, 2007 6729
Table 1. Pd-CO 13C NMR Chemical Shifts and νCO Values for
(N∧N)Pd{C(dO)Me}CO+ Complexes in CD2Cl2 Solution
(COMe)(CO)+ (νCO ) 2128 cm-1), shows that chelate ring size
does not dramatically affect the electron density at the Pd(II)
center.5 The high νCO values of 7g and 7h indicate that the
R-diimines 1g and 1h are weak donor ligands.35
complex
N∧N ligand
(mim)2CH2
biTim
(5-Me-py)2CH2
(pz)2CH2
(3,5-Me2-pz)2CH2
p-tolyldiimine
2,6-iPr2-diimine
δ
13C Pd-CO
νCO (cm-1
)
7a
7c
7d
7e
7f
7g
7h
173.8a
2122a
2123a
2128a
2133a
2132a
2130a
2132a
173.5b
Generation of [(N∧N)PdMe(H2CdCH2)][B(C6F5)4] Species
(8a–g). The methyl ethylene complexes [(N∧N)PdMe-
(H2CdCH2)][B(C6F5)4] (8a–g) were prepared by two routes,
as shown in Scheme 4. Reaction of 5a–e,g with ethylene at
-60 °C yields 8a–e,g by associative displacement of NMe2Ph
173.1b
175.0 (br)c
171.7d
172.4d
by ethylene. The reaction of 3f with
1 equiv of
a 23 °C. b -40 °C. c -20 °C. d -60 °C.
[Li(Et2O)2.8][B(C6F5)4] in the presence of ethylene yields 8f.
Complexes 8a–g are stable in CD2Cl2 solution at -60 °C for
long periods of time, but undergo insertion at higher tempera-
tures (above ca. -40 °C for 8g and ca. -20 °C for 8a–f), as
described below.
Complexes 8a–g were characterized by NMR at -60 °C in
CD2Cl2. Consistent with the displacement of NMe2Ph by
ethylene, the heterocycle ring and aryl resonances that were
shifted upfield in 5a–e,g by anisotropic shielding by the NMe2Ph
ring appear in their normal regions for 8a–g.
Intermolecular exchange of free and coordinated ethylene is
fast on the NMR chemical shift time scale for 8a,c–e,g at -60
°C in the presence of excess ethylene (ca. 1–9 equiv). This
exchange is stereospecific in the sense that the incoming ethylene
occupies the same coordination site as the departing ethylene,
so that the sides of the N∧N ligand remain inequivalent under
fast exchange conditions. It is presumed that ethylene exchange
occurs by a standard associative mechanism.5,6a,36 In contrast,
the -60 °C 1H NMR spectra of 8b,f in the presence of excess
ethylene contain separate resonances for bound (AA′BB′ pattern)
and free ethylene (δ 5.37), indicating that intermolecular
ethylene exchange is slow on the NMR chemical shift time scale
in these cases.37 For 8b, the axial n-hexyl substituent may inhibit
access to the axial coordination sites of the metal. For 8f, the
steric crowding and enhanced boating of the chelate ring due
to methyl substituents of the 3,5-Me2-pz rings inhibit associative
ligand exchange processes.
Similarly, in 5g, one of the aryl resonances appears at high field
(δ 6.24) due to anisotropic shielding by the NMe2Ph ring.
The amine ligand of 5a–e,g is easily displaced by olefins or
other ligands.7,30 The displaced NMe2Ph has no effect on
subsequent chemistry, but can be used as an internal standard
for NMR integration.29b
(N∧N)Pd{C(dO)Me}CO+ Species (7a,c–h). The acyl car-
bonyl complexes [(N∧N)Pd{C(dO)Me}CO][B(C6F5)4] (7a,c–h)
were prepared by two routes, as shown in Scheme 4. Exposure
of frozen CD2Cl2 solutions of 5a,c–e to CO followed by brief
warming to 25 °C yields 7a,c–e quantitatively. The reaction of
3f–h with 1 equiv of [Li(Et2O)2.8][B(C6F5)4] in CD2Cl2 in the
presence of CO at 25 °C affords 7f–h quantitatively. Complexes
7a,c–h are stable at low temperatures (-78 to -40 °C) but
decompose at 25 °C over the course of several hours.
Complexes 7a,c–h were characterized by NMR at 23 °C (7a)
or low temperature (7c–h) in the presence of excess CO. The
Pd-CO 13C NMR chemical shifts of 7a,c–g are listed in Table
1. The Pd-CO signals fall within a small range (δ 171.4 to 175.0)
and are shifted upfield from the free CO resonance (δ 184.0).31
The acyl carbon signals appear in the range δ 209 to 217. These
values are similar to those for (phen)Pd{C(dO)Me}CO+ (δ
173.0 (Pd-CO), 216.5 (acyl)).5 Separate Pd-CO and free CO
resonances are observed for 7a (25 °C) and 7c–h (low
temperature), indicating that CO exchange is slow on the NMR
time scale.32
Generation of [{H2C(3,5-Me2-pz)2}PdMe(H2CdCH2)][SbF6]
(8f′). The reaction of 3f with Ag[SbF6] in Et2O at 23 °C affords
[{H2C(3,5-Me2-pz)2}PdMe(OEt2)][SbF6] (6f′, Scheme 5) in near
quantitative yield.38,39 Complex 6f′ is quite stable at low
temperature (-60 °C), and the labile Et2O is easily displaced
by olefins or other ligands. The reaction of 6f′ with ethylene at
The νCO and νacyl values for 7a,c–h were determined by
solution IR spectroscopy and are listed in Table 1. Complexes
7a,c–h exhibit high νCO values (2122–2133 cm-1; cf. 2139 cm-1
for free CO in CD2Cl2) characteristic of minimal d-π* back-
bonding and electrophilic metal centers.5,31,33 Comparison of
the νCO values for 7a,c–f shows that the donor ability of the
heterocycles varies in the order imidazole > pyridine >
pyrazole.8,34 Comparison of the νCO values for 7e and 7f, and
of 7g and 7 h, shows that the addition of alkyl groups does not
strongly affect the donor property of the ligand. Comparison
of the νCO values of 7a and 7c, and of 7d and (phen)Pd-
(35) (a) Gasperini, M.; Ragaini, F.; Cenini, S. Organometallics 2002,
21, 2950. (b) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2002, 124, 1378.
(36) (a) The -CPh3 groups of 8c are too remote from the Pd to strongly
influence the associative ethylene exchange. For comparison, in [Cu(µ-η1-
NO3)(NO3)(biTim)]2, the distance between the -CPh3 carbon and the Cu
atom is 5.59 Å and the shortest H-Cu distance between the -CPh3 groups
and the Cu atom is 5.28 Å. The Cu-N distances are similar to Pd-N
distances in related Pd compounds. See: Aromi, G.; Gamez, P.; Kooijman,
H.; Spek, A. L.; Driessen, W. L.; Reedijk, J. Eur. J. Inorg. Chem. 2003,
1394. (b) Jiang, A.; Krüger, C.; Pfeil, B. Watkins, S. F. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1987, C43, 2334.
(30) Wu, F.; Foley, S. R.; Burns, C. T.; Jordan, R. F. J. Am. Chem.
Soc. 2005, 127, 1841.
(31) (a) Guo, Z.; Swenson, D. C.; Guram, A. S.; Jordan, R. F.
Organometallics 1994, 13, 766. (b) Lupinetti, A. J.; Strauss, S. H.; Frenking,
G. Prog. Inorg. Chem. 2001, 49, 1. (c) Willner, H.; Aubke, F. Organome-
tallics 2003, 22, 3612.
(37) At 0 °C, the bound ethylene resonance of 8b is broadened by
exchange. The linewidth is greater at higher free ethylene concentrations,
consistent with an associative exchange mechanism.
(32) For 7e at -10 °C, the Pd-CO and free CO 13C resonances are
broad, indicating that CO exchange is faster in this case. CO exchange by
a normal associative mechanism should occur stereospecifically without
permutation of the sides of the (pz)2CH2 ligand. However, the 1H NMR
(-20 °C) and 13C NMR (-10 °C) spectra of 7e in the presence of free CO
contain one set of pz resonances, indicating that the sides of the (pz)2CH2
ligand are equivalent on the NMR time scale. Also, the 1H NMR spectrum
of 7f (25 °C) contains one sharp set of 3,5-Me2-pz signals, indicating that
permutation of the sides of the (3,5-Me2-pz)2CH2 ligand occurs. These results
suggest that reversible decomplexation of the pz rings occurs.
(33) Foley, S. R.; Shen, H.; Qadeer, U. A.; Jordan, R. F. Organometallics
2004, 23, 600.
(38) 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.; Brookhart, M. (E. I. Du Pont
de Nemours & Co.). WO Patent 9623010, 1996 Chem. Abstr. 1996, 125,
222773. 1H NMR (CD2Cl2): δ 7.29 (d, J ) 8, 2H, Ar H3), 7.26 (d, J ) 8,
2H, Ar H3′), 6.89 (d, J ) 8, 2H, Ar H2), 6.82 (d, J ) 8, 2H, Ar H2′), 2.39
(s, 6H, ArMe), 2.15 (s, 3H, NdCMe), 2.04 (s, 3H, NdCMe), 0.44 (s, 3H,
PdMe).
(39) McCord, E. F.; McLain, S. J.; Nelson, L. T. J.; Arthur, S. D.;
Coughlin, E. B.; Ittel, S. D.; Johnson, L. K.; Tempel, D.; Killian, C. M.;
Brookhart, M. Macromolecules 2001, 34, 362.
(34) Canty, A. J.; Lee, C. V. Organometallics 1982, 1, 1063.

6730 Organometallics, Vol. 26, No. 27, 2007
Burns and Jordan
Table 2. First-Order Rate Constants (kinsert,Me) for Ethylene
Insertion of (N∧N)PdMe(H2CdCH2)+ Complexes in CD2Cl2
at -10 °C
Scheme 5
compound
N∧N ligand
kinsert,Me (10-4 s-1
)
8a
8b
8c
8d
8e
8f
8f′
8g
(mim)2CH2
(mim)2CH(n-hexyl)
biTim
(5-Me-py)2CH2
(pz)2CH2
(3,5-Me2-pz)2CH2
(3,5-Me2-pz)2CH2
p-tolyldiimine
p-tolyldiimine
2,6-iPr2-diimine
1.2(1)
1.6(1)
1.0(1)
9.0(9)
3.6(3)
13(1)
12(1)
39(4)
3.9(4)
17a
8g (-30 °C)
8 h (-30 °C)
a Data from ref 6a.
Consistent with Scheme 6, NMR monitoring of the reaction
of 8a–g with ethylene shows that as 8 is consumed, propylene
and 9 form, and that 9 subsequently produces a mixture of
1-butene and cis- and trans-2-butene.40 Complex 9 is the only
major Pd species present in solution until all the ethylene is
consumed, at which point Pd0 formation is observed. Once the
ethylene is consumed, the butenes are oligomerized by an
unknown mechanism.
Scheme 6
Characterization of [(N∧N)PdEt(H2CdCH2)][B(C6F5)4]
(9a,c–g). Complexes 9a,c–g were characterized by 1H NMR at
low temperature. The 1H NMR spectra of 9a,c,g contain quartets
for the PdCH2CH3 methylene hydrogens, which are coupled to
triplets for the PdCH2CH3 hydrogens. The 1H NMR spectra of
9d,e contain two signals (pentets for 9d, multiplets for 9e) for
the PdCH2CH3 hydrogens and a triplet for the PdCH2CH3
methyl group; in these cases the PdCH2 hydrogens are inequiva-
lent due to slow inversion of the (N∧N)Pd chelate ring. The 1H
NMR spectrum of 9f at -60 °C contains a triplet at δ 0.67 for
the PdCH2CH3 group, which is slightly upfield of the corre-
sponding resonance for 9e. The PdCH2CH3 signals of 9f were
not observed at -60 °C due to broadening and overlap with
other resonances. However, the spectrum contains two doublets
for the (3,5-Me2-pz)2CH2 hydrogens, which implies that chelate
ring inversion is slow, as observed for 3f, 6f′, 7f, and 8f.
1
The H NMR spectra of 9a,c–e and 9g, at -60 °C in the
presence of excess ethylene, contain one resonance for free and
coordinated ethylene, indicating ethylene exchange is fast on
the NMR time scale under these conditions. In all cases the
sides of the N∧N donor ligands are inequivalent, consistent with
the expected stereospecific exchange mechanism. In contrast,
for 9f separate resonances are observed for free and coordinated
ethylene, indicating ethylene exchange is slow on the NMR time
scale under these conditions for this case.
-60 °C yields the ethylene complex 8f′. The 1H NMR spectrum
-
of 8f′ is identical to that of the corresponding B(C6F5)4 salt
8f. As for 8f, associative ethylene exchange is slow on the NMR
time scale for 8f′. This route to (N∧N)PdMe+ species avoids
the use of thermally sensitive (N∧N)PdMe2 precursors.
Catalytic Ethylene Dimerization by (N∧N)PdMe(H2Cd
CH2)+ (8a–g,8f′). Complexes 8a–g and 8f′ catalytically dimerize
ethylene to a mixture of butenes. The mechanism for this
reaction is shown in Scheme 6 and is directly analogous to that
established by Brookhart for ethylene dimerization by
(phen)PdMe(H2CdCH2)+.5 Ethylene insertion into the Pd-Me
bond of 8 followed by ꢀ-H elimination forms intermediate Pd
hydride propylene complex B. Subsequent exchange of the
coordinated propylene by ethylene yields intermediate C.
Ethylene insertion of C followed by ethylene coordination forms
ethyl ethylene complex 9, which is the catalyst resting state.
Complex 9 inserts ethylene to form a Pd butyl cation (D), which
undergoes ꢀ-H elimination to form Pd hydride 1-butene complex
E. Intermediate E can undergo olefin exchange with ethylene
to liberate 1-butene and re-form C, or undergo 2,1 insertion of
1-butene to form Pd sec-butyl complex F. ꢀ-H elimination of
F followed by olefin exchange yields C and cis- or trans-2-
butene.
Kinetics of Ethylene Insertion into the Pd-Me Bond of
8a–g and 8f′ The rate of ethylene insertion of 8a–g and 8f′ at
1
-10 °C was measured by H NMR by monitoring the disap-
pearance of the Pd-Me resonance. These studies show that the
rate of ethylene insertion is zero-order in ethylene and first-
order in palladium. The first-order rate constants for insertion
of the Pd-Me cations (kinsert,Me) are listed in Table 2. The
kinsert,Me values for 8f and 8f′ are identical, which shows that
-
-
the difference in noncoordinating anion (B(C6F5)4 vs SbF6
)
has no effect on the insertion rate.
Kinetics of Ethylene Insertion into the Pd-Et Bond of
9a,c–g. Since complex 9 is the resting state for the ethylene
dimerization process in Scheme 6, the rate of ethylene insertion
(40) The 1-butene/2-butene ratios were as follows: 8a: 1/7; 8c: 1/9; 8d:
1/4; 8e,f: 1/7; 8g: 1/6. The cis/trans 2-butene ratios were as follows: 8a-e:
ca. 1/1; 8f: 1/1.5; 8g 1/2.

Cationic Palladium(II) Alkyl Complexes
Organometallics, Vol. 26, No. 27, 2007 6731
Table 3. First-Order Rate Constants (kinsert,Et) for Ethylene
Insertion of (N∧N)PdEt(H2CdCH2)+ Complexes in CD2Cl2
at -10 °C
columns containing activated molecular sieves and Q-5 oxygen
scavenger. Pentane, hexanes, toluene, and benzene were purified
by passage through columns of activated alumina and BASF R3-
11 oxygen scavenger. Diethyl ether and tetrahydrofuran were
distilled from Na/benzophenone ketyl. Dichloromethane was re-
fluxed for 24 h over CaH2 and distilled. CDCl3 and CD2Cl2 were
dried over CaH2 for 24 h, degassed by freeze–pump–thaw cycles,
and vacuum transferred to a storage vessel. Acetone-d6 was dried
over 4 Å molecular sieves and then distilled onto 4 Å molecular
sieves. CO and 13CO were purchased from Aldrich and used as
received. Ethylene (research grade) was obtained from Matheson
and used as received. [HNMe2Ph][B(C6F5)4] and [Li(Et2O)2.8]-
[B(C6F5)4] were obtained from Boulder Scientific and used as
received. The Et2O content of the [Li(Et2O)2.8][B(C6F5)4] salt was
determined by 1H NMR with C6Me6 as internal standard. (cod)PdCl2
and Ag[SbF6] were obtained from Strem and used as received. The
compounds (pz)2CH2 (1e),18 (mim)2CO,14 (mim)2CH2 (1a),14
(mim)2CH(n-hexyl) (1b),15 (5-Me-py)2CO,17 1,1′-di(triphenyl-
methyl)-4,4′-biimidazole (1c),16 (5-Me-py)2CH2 (1d),14 (3,5-Me2-
pz)2CH2 (1f),19 (4-Me-C6H4)NdCMeCMedN(4-Me-C6H4) (1g),20
(cod)Pd(Me)Cl,22 (cod)PdMe2,21 (pyridazine)PdMe2,8 {H2C(mim)2}-
PdMe2 (2a),8 {H2C(pz)2}PdMe2 (2e),8 {(2,6-iPr2-C6H3)Nd
CMeCMedN2,6-iPr2-C6H3)}PdMe2 (2h),6 {(4-Me-C6H4)Nd
CMeCMedN(4-Me-C6H4)}Pd(Me)Cl (3g),38 {(2,6-iPr2-C6H3)Nd
CMeCMedN(2,6-iPr2-C6H3)}Pd(Me)Cl (3h),6d [{H2C(mim)2}-
PdMe(NMe2Ph)][B(C6F5)4] (5a),30 and [{H2C(5-Me-py)2}PdMe-
(NMe2Ph)][B(C6F5)4] (5d)30 were prepared by literature procedures.
All other chemicals were purchased from Aldrich and used without
further purification. Elemental analyses were performed by Midwest
Microlabs (Indianapolis, IN). Infrared spectra were obtained at 25
°C under an N2 atmosphere using a Nicolet NEXUS 470 FT-IR
spectrometer. GC-MS analyses were performed on a HP-6890
instrument with a HP-5973 mass selective detector.
compound
N∧N ligand
kinsert,Et (10-4 s-1
)
9a
9c
9d
9e
9f
(mim)2CH2
biTim
(5-Me-py)2CH2
(pz)2CH2
(3,5-Me2-pz)2CH2
p-tolyldiimine
1.0(1)
0.48(5)
2.0(2)
2.0(2)
7.0(7)
12(1)
9g
into the Pd-Et bond is equal to the rate of butene formation.
The rate of butene formation by 9a,c–g at -10 °C was measured
1
by H NMR. These experiments show that the rate of butene
formation and hence ethylene insertion into the Pd-Et bond is
zero-order in ethylene and first-order in palladium. First-order
rate constants for ethylene insertion, kinsert,Et, are listed in
Table 3.
Reactivity Trends in (N∧N)Pd(R)(H2CdCH2)+ species
(R ) Me, Et). The kinsert values in Tables 2 and 3 provide an
initial picture of how N∧N ligand properties influence the
insertion reactivity of (N∧N)Pd(R)(H2CdCH2)+ species. The
kinsert,Me values for 8a and 8c are essentially identical, which
shows that the chelate ring size does not dramatically affect
the rate of ethylene insertion. Ethylene insertion of 8e is 3 times
faster than for 8a. Since 8e and 8a have very similar structures,
this difference can be attributed to greater electrophilic character
at Pd in 8e versus 8a, which results from the difference in donor
ability of the pyrazole and imidazole ligands (cf. Table 1).
Ethylene insertion of 8f is ca. 3 times faster than for 8e, which
may reflect the steric crowding generated by the methyl
substituents of the 3,5-Me2-pz rings of 8f. Ethylene insertion
of 8d is 8 times faster than for 8a, due to the combination of
the poorer donor ability and the larger ring size and concomitant
greater steric crowding of the pyridine unit compared to the
imidazole unit in the N∧N ligands. Thus, increasing the
electrophilic character and the steric bulk of the {bis(heterocycle)-
methane}Pd unit leads to moderate (up to ca. 10-fold) increases
in ethylene insertion rates. The kinsert,Et values for 9a,c-g are
somewhat lower than kinsert,Me values for the corresponding
Pd-Me complexes 8a,c-g.
NMR spectra were recorded in flame-sealed or Teflon valve tubes
on Bruker AMX-360, AMX-400, or AMX-500 spectrometers at
1
ambient probe temperature unless otherwise indicated. H and 13C
chemical shifts are reported versus SiMe4 and were determined by
reference to the residual 1H and 13C solvent peaks. 19F and 11B
chemical shifts were referenced to external neat CFCl3 and BF3 · Et2O
respectively. Coupling constants are reported in Hz. NMR probe
temperatures were calibrated by a MeOH thermometer.41
The NMR spectra of cationic Pd compounds contained signals
-
of the free B(C6F5)4 anion. 13C{1H} NMR (CD2Cl2, -60 °C): δ
Interestingly, the insertion rate of 8f, which is the most
reactive of the {bis(heterocycle)methane}Pd(R)(ethylene)+ spe-
cies studied here, is 3 to ca. 12 times slower than those of the
(R-diimine)Pd(R)(ethylene)+ species 8g and 8h. As the elec-
tronic properties of 8f-h are quite similar (based on the νCO
data in Table 1), this difference must result from differences in
the steric properties and perhaps the degree of rigidity of the
(N∧N)Pd units in these systems.
147.5 (dm, J ) 241, C2), 137.8 (dm, J ) 238, C4), 135.8 (dm, J
) 249, C3), 123.6 (br, C1). 11B NMR (CD2Cl2, -60 °C): δ -
16.9 (s). 19F NMR (CD2Cl2, -60 °C): δ -133.7 (br s, 2F, o-F),
-163.0 (t, J ) 23, 1F, p-F), -167.0 (t, J ) 19, 2F, m-F). NMR
spectra of 7a,c–e, 8a–e,g, and 9a,c–e and species derived from these
species contain resonances for free NMe2Ph.29 Samples of CD2Cl2
solutions of cationic species generated in situ from the reaction of
3f–h and [Li(Et2O)2.8][B(C6F5)4] contain LiCl. Samples of CD2Cl2
solutions of cationic species generated in situ from the reaction of
3f and Ag[SbF6] contain AgCl.
Conclusions
Cationic {bis(heterocycle)methane}PdR+ species catalyze the
dimerization of ethylene by an insertion/ꢀ-H elimination mech-
anism. The catalyst resting state is the {bis(heterocycle)-
methane}Pd(Et)(ethylene)+ complex. Increasing the electrophilic
character (heterocycle ) pyrazole > pyridine > imidazole) and
the steric bulk of the {bis(heterocycle)methane}Pd unit leads
to moderate (up to ca. 10-fold) increases in ethylene insertion
rates of {bis(heterocycle)methane}Pd(R)(ethylene)+ species.
Atom-labeling schemes for the ligands (mim)2CH2, biTim, (5-
Me-py)2CH2, and (pz)2CH2 and complexes derived from these
ligands are given in Chart 2.
{(n-Hexyl)HC(mim)2}PdMe2 (2b). A slurry of (cod)PdMe2
(0.218 g, 0.893 mmol) and (mim)2CH(n-hexyl) (0.197 g, 0.759
mmol) in pentane (30 mL) was stirred for 30 min at -78 °C. The
mixture was warmed to 0 °C and stirred for 1 h. The white solid
was isolated by cannula filtration at 0 °C, washed with pentane
(30 mL) at 0 °C, and dried under vacuum at 0 °C (0.100 g, 33%).
2b was stored at -35 °C. 1H NMR (CD2Cl2): δ 7.05 (s, 2H, mim
H4/H5), 6.83 (s, 2H, mim H4/H5), 4.08 (t, J ) 8, 1H, CH), 3.66
(s, 6H, mim NMe), 2.31 (q, J ) 8, 2H, CH2), 1.24 (m, 8H,
Experimental Section
General Procedures. All manipulations were performed under
N2 or vacuum using standard Schlenk or high-vacuum techniques
or in a N2-filled drybox. Nitrogen was purified by passage through
(41) Van Geet, A. L. Anal. Chem. 1970, 42, 679.

6732 Organometallics, Vol. 26, No. 27, 2007
Burns and Jordan
Chart 2
Me-C6H4) (0.66 g, 2.5 mmol) in Et2O (80 mL) was added via
cannula, and a red solid immediately precipitated. The red suspen-
sion was warmed to -10 °C, and H2O (1.5 mL) was added by
syringe to quench any remaining MeLi. The mixture was filtered
immediately to afford a red solid. The solid was rinsed with Et2O
(3 × 10 mL) and dried under vacuum. The solid was transferred
to a 100 mL Schlenk flask, cooled to -20 °C, and dissolved in
CH2Cl2 (50 mL). The red solution was filtered through a plug of
Celite (LiCl removal) into a Schlenk flask kept at -78 °C. The
clear red filtrate was warmed to 0 °C, and the solvent was removed
under vacuum. The resulting red solid was suspended in Et2O (20
mL), stirred for 5 min, filtered, rinsed with Et2O (2 × 5 mL), and
dried under vacuum (0.64 g, 64%). 1H NMR (CD2Cl2, -60 °C): δ
0.7.22 (d, J ) 8, 4H, Ar), 6.74 (d, J ) 8, 4H, Ar), 2.33 (s, 6H,
4-Me), 2.01 (s, 6H, NdCMe), -0.40 (s, 6H, PdMe2). 13C{1H}
NMR (CD2Cl2, -60 °C): δ 170.3 (NdMe), 144.3, 135.1, 129.0,
120.4, 20.7, 19.7, -5.9 (PdMe2). 2g is insufficiently stable for
elemental analysis.
CHCH2(CH2)4CH3), 0.84 (t, J ) 4, 3H, CH2CH3), -0.03 (s, 6H,
PdMe). 1H NMR (CD3COCD3): δ 6.99 (s, 2H, mim H4/H5), 6.94
(s, 2H, mim H4/H5), 4.50 (t, J ) 8, 1H, CH), 3.83 (s, 6H, mim
NMe), 2.33 (m, 2H, CH2), 1.24 (m, 8H, CHCH2(CH2)4CH3), 0.83
(t, J ) 7, 3H, CH2CH3), -0.40 (s. 6H, PdMe). 13C{1H} NMR
(CD3COCD3): δ 145.6 (mim C2), 125.8 (mim C4/C5), 120.3 (mim
C4/C5), 37.0, 32.9, 32.6, 31.3, 26.6, 22.1, 13.3, -10.3 (PdMe).
One signal unobserved due to overlap with acetone-d6. Anal. Calcd
for C17H30N4Pd: C, 51.44; H, 7.61; N, 14.11. Found: C, 51.15; H,
7.43; N, 13.90.
{1,1′-Di(triphenylmethyl)-4,4′-biimidazole}PdMe2 (2c). In the
dark, a flask was charged with (pyridazine)PdMe2 (0.108 g, 0.498
mmol) and 1,1′-di(triphenylmethyl)-4,4′-biimidazole (0.309 g, 0.499
mmol), and CH2Cl2 (20 mL) was added by syringe. The solution
was stirred for 5 min in the dark at 23 °C, and the color changed
from orange to yellow. The solution was poured into pentane (180
mL), and a white solid precipitated. The solid was collected by
filtration, rinsed with acetone (5 mL), Et2O (10 mL), and pentane
(30 mL), and dried under vacuum to yield {1,1′-di(triphenylm-
ethyl)-4,4′-biimidazole}PdMe2 as a white solid (0.258 g, 68%).
1H NMR (CD2Cl2, -60 °C): δ 7.50 (s, 2H, imidazole H2), 7.34
(m, 18H, trityl H3 and H4), 7.16 (m, 12H, trityl H2), 6.83 (s, 2H,
imidazole H5), -0.18 (s, 6H, PdMe). 13C NMR (CD2Cl2, -60 °C):
δ 141.0 (imidazole C2 and trityl C4), 136.2 (imidazole C4), 135.0
(trityl ipso C), 129.4 (trityl C2/C3), 128.2 (trityl C2/C3), 115.7
(imidazole C5), 75.7 (C(C6H5)3), -13.5 (PdMe). 2c was stored at
-35 °C due to its thermal instability and is insufficiently stable
for elemental analysis.
{H2C(3,5-Me2-pz)2}Pd(Me)Cl (3f). A flask was charged with
(cod)Pd(Me)Cl (0.530 g, 2.00 mmol) and (3,5-Me2-pz)2CH2 (0.413
g, 2.02 mmol), and Et2O (30 mL) was added by syringe. A white
precipitate formed rapidly. The mixture was stirred at 23 °C for
4 h. The white solid was collected by filtration, rinsed with Et2O
(4 × 10 mL) and pentane (3 × 10 mL), and dried under vacuum
for 1 h to yield {H2C(3,5-Me2-pz)2}Pd(Me)Cl as a white solid
(0.706 g, 97%). 1H NMR (CD2Cl2): δ 7.14 (br d, J ) 14, 1H, CHeq),
6.00 (br d, J ) 14, 1H, CHax), 5.95 (s, 1H, pz H4/H4′), 5.82 (s,
1H, pz H4/H4′), 2.37 (s, 6H, pz Me), 2.33 (s, 3H, pz Me), 2.32 (s,
1
3H, pz Me), 0.82 (s, 3H, PdMe). H NMR (CD2Cl2, -20 °C): δ
7.08 (d, J ) 15, 1H, CHeq), 5.97 (d, J ) 15, 1H, CHax), 5.95 (s,
1H, pz H4/H4′), 5.82 (s, 1H, pz H4/H4′), 2.36 (s, 3H, pz Me),
1
2.35 (s, 3H, pz Me), 2.30 (s, 6H, pz Me), 0.77 (s, 3H, PdMe). H
NMR (CD2Cl2, -60 °C): δ 7.03 (d, J ) 15, 1H, CHeq), 5.95 (d, J
) 15, 1H, CHax), 5.94 (s, 1H, pz H4/H4′), 5.81 (s, 1H, pz H4/
H4′), 2.33 (s, 3H, pz Me), 2.30 (s, 3H, pz Me), 2.27 (s, 3H, pz
1
Me), 2.25 (s, 3H, pz Me), 0.71 (s, 3H, PdMe). H NMR (CD2Cl2,
-80 °C): δ 7.01 (d, J ) 15, 1H, CHeq), 5.94 (d, J ) 15, 1H, CHax),
5.95 (s, 1H, pz H4/H4′), 5.81 (s, 1H, pz H4/H4′), 2.34 (s, 3H, pz
Me), 2.32 (s, 3H, pz Me), 2.28 (s, 3H, pz Me), 2.27 (s, 3H, pz Me)
0.67 (s, 3H, PdMe). 13C{1H} NMR (CD2Cl2): δ 153.0 (3/3′-pz),
152.2 (3/3′-pz), 141.0, (5/5′-pz), 140.0 (5/5′-pz), 108.4 (4/4′-pz),
107.5 (4/4′-pz), 58.0 (CH), 14.6 (pz Me), 13.6, (pz Me), 11.6 (pz
Me), 11.0 (pz Me), -7.5 (PdMe). 13C{1H} NMR (CD2Cl2, -80
°C): δ 151.4 (3/3′-pz), 150.6 (3/3′-pz), 140.5, (5/5′-pz), 139.3 (5/
5′-pz), 107.2 (4/4′-pz), 106.4 (4/4′-pz), 57.0 (CH), 14.1 (pz Me),
12.9, (pz Me), 11.2 (pz Me), 10.6 (pz Me), -7.7 (PdMe). 3f was
stored at -35 °C due to its thermal instability and is insufficiently
stable for elemental analysis.
{H2C(5-Me-py)2}PdMe2 (2d). A flask was charged with (py-
ridazine)PdMe2 (0.217 g, 1.00 mmol) and (5-Mepy)2CH2 (0.218
g, 1.10 mmol), and Et2O (30 mL) was added by syringe. An off-
white precipitate formed within 15 min. The mixture was stirred
at 23 °C for a total of 30 min The solid was collected by filtration,
rinsed with Et2O (2 × 20 mL), and dried under vacuum to yield
{H2C(5-Me-py)2}PdMe2 as a white solid (0.185 g, 55%). 1H NMR
(CD2Cl2, -60 °C): δ 8.33 (s, 2H, py H6), 7.50 (d, J ) 8, 2H, py
H4), 7.25 (d, J ) 8, 2H, py H3), 4.65 (d, J ) 14, 1H, CH2 Hax),
4.03 (d, J ) 14, 1H, CH2 Heq), 2.24 (s, 6H, py 5-Me), 0.00 (s, 6H,
Generation of [{(n-Hexyl)HC(mim)2}PdMe(NMe2Ph)]-
[B(C6F5)4] (5b). A valved NMR tube was charged with {(n-
hexyl)HC(mim)2}PdMe2 (0.0123 g, 0.0310 mmol) and
[HNMe2Ph][B(C6F5)4] (0.0247 g, 0.0310 mmol), and CD2Cl2 (0.7
mL) was added by vacuum transfer at -78 °C. The tube was shaken
at -78 °C until both solids had dissolved to produce a clear yellow
solution. The tube was kept at -78 °C and transferred to an NMR
probe that had been precooled to -60 °C, and NMR spectra were
recorded. Complete conversion to 5b was observed. 1H NMR
(CD2Cl2, -60 °C): δ 7.81 (d, J ) 8.0, 2H, o-Ph), 7.42 (t, J ) 8,
2H, m-Ph), 7.28 (t, J ) 8, 1H, p-Ph), 6.87 (s, 1H, mim H4/H5),
6.86 (s, 1H, mim H4/H5), 6.53 (s, 1H, mim H4/H5), 4.85 (s, 1H,
mim H4), 4.08 (t, J ) 8, 1H, CH), 3.63 (s, 3H, mim NMe), 3.54
(s, 3H, mim NMe), 3.03 (s, 3H, NMe), 2.82 (s, 3H, NMe), 2.67
(m, 1H, CH2), 1.29 (m, 1H, CH2), 1.18 (br s, 8H,
CHCH2(CH2)4CH3), 0.79 (br s, 6H, CH2CH3 and PdMe). 13C{1H}
NMR (CD2Cl2, -60 °C): δ 153.1 (ipso-Ph), 145.6 (mim C2/C2′),
144.9 (mim C2/C2′), 129.4 (o-Ph), 127.6 (p-Ph), 127.2, 125.6, 122.2
1
PdMe). H NMR (CD2Cl2, -40 °C): δ 8.36 (s, 2H, py H6), 7.50
(d, J ) 8, 2H, py H4), 7.25 (d, J ) 8, 2H, py H3), 4.69 (d, J ) 13,
1H, CH2 Hax), 4.03 (d, J ) 13, 1H, CH2 Heq), 2.26 (s, 6H, py 5-Me),
0.02 (s, 6H, PdMe). 1H NMR (CD2Cl2, -20 °C): δ 8.38 (s, 2H, py
H6), 7.50 (d, J ) 8, 2H, py H4), 7.25 (d, J ) 8, 2H, py H3), 4.61
(br d, 1H, CH2 Hax), 4.03 (br d, 1H, CH2 Heq), 2.27 (s, 6H, py
5-Me), 0.05 (s, 6H, PdMe). 13C{1H} NMR (CD2Cl2, -60 °C): δ
152.4 (py C2), 150.1 (py C6), 137.8 (py C4), 132.8 (py C5), 123.3
(py C3), 45.4 (CH2), 17.8 (py 5-Me), -7.4 (PdMe). 2d was stored
at -35 °C due to its thermal instability and is insufficiently stable
for elemental analysis.
{(4-Me-C6H4)NdCMeCMedN(4-Me-C6H4)}PdMe2 (2g). A
suspension of trans-PdCl2(SMe2)2 (0.75 g, 2.5 mmol) in Et2O (140
mL) was cooled to -78 °C, and MeLi (1.5 M in Et2O, 3.6 mL, 5.5
mmol) was added via syringe. The mixture was stirred for 1 h,
during which time most of the orange solid disappeared. The
solution was warmed to -50 °C and stirred for 3 h, to afford a
clear,colorlesssolution.Asolutionof(4-Me-C6H4)NdCMeCMedN(4-

Cationic Palladium(II) Alkyl Complexes
Organometallics, Vol. 26, No. 27, 2007 6733
(m-Ph), 121.5, 121.4, 56.2 (NMe), 50.2 (NMe), 38.2, 34.3, 33.9,
33.6, 31.7, 29.3, 27.5, 22.8, 14.2, 2.2 (PdMe).
7a had formed quantitatively. 1H NMR (CD2Cl2): δ 7.03 (br s,
2H, mim H4/H5), 6.86 (s, 1H, mim H4/H5), 6.83 (s, 1H, mim H4/
H5), 4.14 (s, 2H, CH2), 3.74 (s, 3H, mim NMe), 3.72 (s, 3H, mim
NMe), 2.66 (s, 3H, COMe). 13C{1H} NMR (CD2Cl2): δ 217.4
(C(O)Me), 173.7 (PdCO), 141.8, 140.4, 128.5, 127.5, 123.5, 123.2,
40.8 (COMe), 34.8 (mim NMe), 34.2, (mim NMe), 23.1 (CH2).
The 13C NMR assignments were confirmed by 13CO experiments.
IR (CD2Cl2, cm-1): 2121 (VCO), 1734 (Vacyl).
Generation of [{1,1′-Di(triphenylmethyl)-4,4′-biimidazole}-
PdMe(NMe2Ph)][B(C6F5)4] (5c). This compound was generated
quantitatively from 2c and [HNMe2Ph][B(C6F5)4] using the pro-
cedure for 5b. 1H NMR (CD2Cl2, -60 °C): δ 7.61 (s, 1H, imidazole
H5/H5′), 7.52 (d, J ) 8, o-Ph), 7.33 (m, 18H, trityl H3 and H4),
7.08 (d, J ) 8, 6H, trityl H2), 6.90 (d, J ) 8, 6H, trityl H2), 6.80
(s, 1H, imidazole H2), 6.73 (t, J ) 8, 1H, p-Ph), 5.30 (s, 1H,
imidazole H2′), 3.01 (s, 3H, NMe), 2.98 (s, 3H, NMe), 0.82 (s,
3H, PdMe). 13C NMR (CD2Cl2, -60 °C): 153.4, 140.4, 140.2, 136.8
(imidazole C4/C4′), 136.6 (imidazole C4/C4′), 135.0 (imidazole
C2/C2′), 132.4 (imidazole C2/C2′), 129.3, 129.0, 128.5, 128.4,
128.2, 128.1, 126.5, 116.4 (imidazole C5/C5′), 116.1 (imidazole
C5/C5′), 76.8 (C(C6H5)3), 75.8 (C(C6H5)3), 52.1 (NMe2Ph), 0.04
(PdMe).
Generation of [{1,1′-Di(triphenylmethyl)-4,4′-biimidazole}-
Pd{C(dO)Me}(CO)][B(C6F5)4] (7c). This compound was generated
quantitatively from 2c, [HNMe2Ph][B(C6F5)4], and CO (5 atm) using
1
the procedure for 7a. H NMR (CD2Cl2, -40 °C): δ 7.57 (s, 1H,
imidazole H), 7.38 (m, 19 H, trityl and imidazole H), 7.25 (d, J ) 8,
12 H, trityl H2), 7.11 (s, 1H, imidazole H), 7.09 (s, 1H, imidazole H),
2.61 (s, 3H, COMe). 13C{1H} NMR (CD2Cl2, -40 °C): δ 213.0
(C(O)Me), 173.5 (PdCO), 140.5, 140.1, 139.2 (imidazole C4/C4′),
138.5 (imidazole C4/C4′), 134.3 (imidazole C2/C2′), 132.4 (imidazole
C2/C2′), 129.4 (2 C), 128.8, 128.7, 128.5, 128.4, 77.5, 77.1, 41.8
(COMe). IR (CD2Cl2, cm-1): 2123 (VCO). The Vacyl stretch was not
observed due to overlap with a solvent stretch.
Generation of [{H2C(pz)2}PdMe(NMe2Ph)][B(C6F5)4] (5e).
This compound was generated quantitatively from 2e and
[HNMe2Ph][B(C6F5)4] using the procedure for 5b. 1H NMR
(CD2Cl2, -60 °C): δ 7.79 (d, J ) 8, 2H, o-Ph), 7.71 (d, J ) 3, 1H,
5-pz), 7.59 (d, J ) 2, 1H, 5′-pz), 7.56 (d, J ) 2, 1H, 3-pz), 7.47 (t,
J ) 8, 2H, m-Ph), 7.34 (t, J ) 8, 1H, p-Ph), 6.87 (d, J ) 14, 1H,
CHax), 6.41 (t, J ) 2, 1H, 4-pz), 6.15 (d, J ) 14, 1H, CHeq), 6.05
(t, J ) 2, 1H, 4′-pz), 5.27 (d, J ) 2, 1H, 3′-pz), 3.19 (s, 3H, NMe),
2.83 (s, 3H, NMe), 1.09 (s, 3H, PdMe). 13C NMR (CD2Cl2, -60
°C): δ 152.0, 143.5, 141.9, 132.4, 132.0, 129.3, 127.3, 121.7, 108.2,
107.5, 63.3 (CH2), 55.5 (NMe), 49.6 (NMe), 4.9 (PdMe).
Generation of [{H2C(5-Me-py)2}Pd{C(dO)Me}(CO)][B-
(C6F5)4] (7d). This compound was generated quantitatively from
2d, [HNMe2Ph][B(C6F5)4], and CO (5 atm) using the procedure
1
for 7a. H NMR (CD2Cl2, -40 °C): δ 8.23 (s, 1H, py H6/H6′),
8.12 (s, 1H, py H6/H6′), 7.74 (d, J ) 8, 1H, py H4/H4′), 7.68 (d,
J ) 8, 1H, py H4/H4′), 7.47 (d, J ) 8, 1H, py H3/H3′), 7.44 (d, J
) 8, 1H, py H3/H3′), 4.71 (d, J ) 15, 1H, CH2 Hax), 4.30 (d, J )
15, 1H, CH2 Heq), 2.64 (s, 3H, COMe) 2.33 (s, 3H, py 5-Me), 2.30
(s, 3H, py 5-Me). 13C{1H} NMR (CD2Cl2, -40 °C): δ 218.0
(C(O)Me), 173.0 (PdCO), 151.9 (py C2/C2′), 151.1 (py C6/C6′),
150.8 (py C2/C2′), 148.7 (py C6/C6′), 142.3 (py C4/C4′), 141.5
(py C4/C4′), 135.3 (py C5/C5′), 135.1 (py C5/C5′), 125.7 (py C3/
C3′), 125.4 (py C3/C3′), 45.7 (COMe), 40.9 (CH2) 18.1 (py 5-Me),
17.8 (py 5-Me). IR (CD2Cl2, cm-1): 2128 (VCO), 1741 (Vacyl).
Generation of [(p-Tolyldiimine)PdMe(NMe2Ph)][B(C6F5)4]
(p-tolyldiimine ) (4-Me-C6H4)NdCMeCMedN(4-Me-C6H4))
(5g). This compound was generated quantitatively from 2g and
[HNMe2Ph][B(C6F5)4] using the procedure for 5b. 1H NMR
(CD2Cl2, -60 °C): δ 7.27 (d, J ) 8, 2H, Ar), 7.21 (m, 2H,
NMe2Ph), 7.16 (m, 3H, NMe2Ph), 6.97 (d, J ) 8, 2H, Ar), 6.79
(d, J ) 8, 2H, Ar), 6.24 (d, J ) 8, 2H, Ar), 2.61 (s, 6H, NMe2Ph),
2.35 (s, 3H, 4-Me-C6H4), 2.29 (s, 3H, 4-Me-C6H4), 2.05 (s, 3H,
NdCMe), 1.93 (s, 3H, NdCMe), 0.43 (s, 3H, PdMe). 13C NMR
(CD2Cl2, -60 °C): δ 178.5 (NdCMe), 173.5 (NdCMe), 151.9,
143.9, 143.8, 137.3, 136.1, 130.2, 129.8, 129.0, 126.5, 120.7, 120.6,
118.5, 52.2 (NMe2), 21.7, 20.7, 20.6, 20.1, 13.2 (PdMe).
Generation of [{H2C(pz)2}Pd{13C(dO)Me}(13CO)][B(C6F5)4]
(7e). This compound was generated quantitatively from 2e,
[HNMe2Ph][B(C6F5)4], and 13CO (1 atm) using the procedure for
7a. 1H NMR (CD2Cl2, -20 °C): δ 7.81 (br s, 2H, pz H5/H5′),
7.64 (br s, 2H, pz H3/H3′), 6.39 (br s, 2H, pz H4/H4′), 6.39 (br s,
2H, CH2), 2.76 (d, JC-H ) 6, 3H, COMe). 13C{1H} NMR (CD2Cl2,
-10 °C): δ 209.8 (C(O)Me), 175.0 (br s, PdCO),144.5 (br s, pz
C5/C5′), 129.2 (pz C3/C3′), 109.1 (pz C4/C4′), 63.7 (CH2), 40.0
(d, JC-C ) 32, COMe). IR (CD2Cl2, cm-1): 2133 (VCO), 1756 (Vacyl).
Generation of [{H2C(3,5-Me2-pz)2}PdMe(OEt2)][SbF6] (6f′). A
valved NMR tube was charged with 3f (0.0072 g, 0.020 mmol)
and Ag[SbF6] (0.0069 g, 0.020 mmol), and Et2O (1.0 mL) was
added by vacuum transfer at -78 °C. The tube was sealed, briefly
warmed to 23 °C, and vigorously shaken for 10 min. A slurry of a
fine white solid in a colorless supernatant was obtained. The
volatiles were removed under vacuum. The tube was cooled to -78
°C, and CD2Cl2 (0.7 mL) was added by vacuum transfer. The tube
was kept at -78 °C and transferred to a precooled (-60 °C) NMR
Generation of [{H2C(3,5-Me2-pz)2}Pd{C(dO)Me}(CO)]-
[B(C6F5)4] (7f). A valved NMR tube containing a CD2Cl2 (0.7 mL)
solution of 3f (0.0056 g, 0.016 mmol) and [Li(Et2O)2.8][B(C6F5)4]
(0.014 g, 0.016 mmol) was cooled to -196 °C and exposed to CO
(5 atm). The tube was sealed and warmed to -78 °C. The tube
was briefly warmed to 23 °C and vigorously shaken. A slurry of a
fine white solid in a colorless supernatant was obtained. The tube
was kept at -78 °C and transferred to a precooled (-60 °C) NMR
1
probe, and NMR spectra were recorded. The H NMR spectrum
1
established that 6f′ had formed quantitatively. H NMR (CD2Cl2,
-60 °C): δ 6.78 (d, J ) 15, 1H, CH2 Hax), 6.21 (d, J ) 15, 1H,
CH2 Heq), 6.07 (s, 1H, pz H4/H4′), 5.86 (s, 1H, pz H4/H4′), 3.70
(m, 4H, coord. Et2O CH2), 2.37 (s, 3H, pz Me), 2.33 (s, 3H. pz
Me), 2.27 (s, 3H, pz Me), 2.14 (s, 3H, pz Me), 1.63 (t, J ) 7, 6H,
coord. Et2O CH3), 0.79 (s, 3H, PdMe). 13C{1H} NMR (CD2Cl2,
-60 °C): δ 152.7 (3/3′-pz), 150.6 (3/3′-pz), 142.3 (5/5′-pz), 140.7
(5/5′-pz), 108.1 (4/4′-pz), 106.9 (4/4′-pz), 71.8 (Et2O CH2), 57.2
(CH2), 15.6 (Et2O CH3), 14.5, 12.9, 11.2, 10.6, -4.5 (PdMe).
Resonances for free Et2O were also present.
1
probe, and NMR spectra were recorded. The H NMR spectrum
1
established that 7f had formed quantitatively. H NMR (CD2Cl2,
-60 °C): δ 6.43 (d, J ) 15, 1H, CH2 Hax), 6.05 (d, J ) 15, 1H,
CH2 Heq), 6.03 (s, 1H, pz H4/H4′), 5.97 (s, 1H, pz H4/H4′), 2.62
(s, 3H, (COMe)), 2.36 (s, 3H, pz Me), 2.34 (s, 3H. pz Me), 2.22
(s, 3H, pz Me), 2.11 (s, 3H, pz Me). 1H NMR (CD2Cl2, 25 °C): δ
6.45 (br s, 1H, Hax), 6.22 (br s, 1H, Heq), 6.04 (s, 2H, pz H), 2.65
(s, 3H, COMe), 2.39 (s, 6H, pz Me), 2.22 (s, 6H, pz Me). 13C{1H}
NMR (CD2Cl2, -60 °C): δ 211.3 (C(O)Me), 171.7 (PdCO), 153.1
(3/3′-pz), 152.2 (3/3′-pz), 142.8 (5/5′-pz), 141.1 (5/5′-pz), 108.7
(4/4′-pz), 107.6 (4/4′-pz), 56.9 (CH2), 40.4 (COMe), 13.9, 13.6,
11.0, 10.6. IR (CD2Cl2, cm-1): 2132 (VCO). The Vacyl band was not
observed due to overlap with a solvent band.
Generation of [{H2C(mim)2}Pd{C(dO)Me}(CO)][B(C6F5)4] (7a).
A solution of 5a in CD2Cl2 (0.7 mL) was generated in a valved
NMR tube from 2a (0.0096 g, 0.031 mmol) and
[HNMe2Ph][B(C6F5)4] (0.025 g, 0.031 mmol) and cooled to -196
°C. The tube was exposed to CO (5 atm), sealed, and warmed to
-78 °C. The tube was briefly warmed to 23 °C and vigorously
shaken. The tube was kept at -78 °C prior to NMR and IR analysis
Generation of [(p-Tolyldiimine)Pd{C(dO)Me}(CO)][B(C6F5)4]
(7g). This compound was generated quantitatively from 3g,
[Li(Et2O)2.8][B(C6F5)4], and CO (5 atm) using the procedure for
1
at ambient temperature. The H NMR spectrum established that

6734 Organometallics, Vol. 26, No. 27, 2007
Burns and Jordan
7f. 1H NMR (CD2Cl2, -40 °C): δ 7.29 (br d, 2H, Ar), 7.25 (br d,
2H, Ar), 6.89 (br d, 2H, Ar), 6.78 (br d, 2H, Ar), 2.35 (s, 6H,
4-Me-C6H4), 2.32 (s, 3H, N d CMe), 2.31 (s, 3H, NdCMe), 2.09
(s, 3H, COMe). 13C{1H} NMR (CD2Cl2, -40 °C): δ 212.3
(C(O)Me), 180.4 (Nd CMe), 172.4 (PdCO and NdCMe), 143.7,
142.6, 131.3, 130.6, 130.5, 122.1, 120.2, 119.9, 36.4 (COMe), 21.2,
20.9, 20.0. IR (CD2Cl2, cm-1): 2130 (VCO), 1752 (Vacyl).
procedure for 8a. Under these conditions (-60 °C), exchange of
coordinated and free ethylene is fast on the NMR chemical shift
time scale. 1H NMR (CD2Cl2, -60 °C): δ 8.16 (s, 1H, py H6/
H6′), 7.95 (s, 1H, py H6/H6′), 7.72 (d, J ) 8, 1H, py H4/H4′),
7.63 (d, J ) 8, 1H, py H4/H4′), 7.44 (d, J ) 8, 1H, py H3/H3′),
7.39 (d, J ) 8, 1H, py H3/H3′), 5.35 (br s, free and coordinated
H2CdCH2), 4.71 (d, J ) 14, 1H, CH2 Hax), 4.25 (d, J ) 14, 1H,
CH2 Heq), 2.33 (s, 3H, py 5-Me), 2.29 (s, 3H, py 5-Me), 0.71 (s,
3H, PdMe). 13C{1H} NMR (CD2Cl2, -60 °C): δ 151.8 (py C2/
C2′), 150.6 (py C2/C2′), 149.8 (py C6/C6′), 147.4 (py C6/C6′),
141.1 (py C4/C4′), 140.5 (py C4/C4′), 134.9 (py C5/C5′), 134.8
(py C5/C5′), 122.9 (br s, exchanging H2CdCH2), 45.3 (CH2), 17.8
(py 5-Me), 17.7 (py 5-Me), 9.0 (PdMe).
Generation of [{(2,6-iPr2-C6H3)NdCMeCMedN(2,6-iPr2-
42
C6H3)}Pd{C(dO)Me}(CO)][B(C6F5)4] (7h).
This compound
was generated quantitatively from 3h, [Li(Et2O)2.8][B(C6F5)4], and
1
CO (5 atm) using the procedure for 7f. H NMR (CD2Cl2, -40
°C): δ 7.41–7.27 (m, 6H, 2,6-iPr2-C6H3 Ar), 2.74 (m, J ) 7, 4H,
CHMe2), 2.39 (s, 3H, NdCMe), 2.27 (s, 3H, NdCMe), 2.01 (s,
3H, COMe) 1.37 (d, J ) 7, 12H, CHMe2), 1.19 (d, J ) 7, 6H,
CHMe2), 1.10 (d, J ) 7, 6H, CHMe2). IR (CD2Cl2, cm-1): 2132
(VCO), 1757 (Vacyl).
Generation of [{H2C(pz)2}PdMe(H2CdCH2)][B(C6F5)4] (8e).
This compound was generated quantitatively from 2e,
[HNMe2Ph][B(C6F5)4], and ethylene (ca. 5 equiv) using the
procedure for 8a. Under these conditions (-60 °C), exchange of
coordinated and free ethylene is fast on the NMR chemical shift
Generation of [{H2C(mim)2}PdMe(H2CdCH2)][B(C6F5)4] (8a).
A solution of 5a (0.016 mmol) in CD2Cl2 (0.7 mL) was generated
in a valved NMR tube from 2a (0.0048 g, 0.016 mmol) and
[HNMe2Ph][B(C6F5)4] (0.012 g, 0.016 mmol), as described above,
and cooled to -196 °C. The tube was exposed to ethylene (ca. 5
equiv) and sealed. The tube was warmed to -78 °C. The tube was
kept at -78 °C and transferred to a precooled (-60 °C) NMR probe,
1
time scale. H NMR (CD2Cl2, -60 °C): δ 7.81 (br s, 1H, pz H),
7.71 (br s, 2H, pz H), 7.47 (br s, 1H, pz H), 6.54 (m, 1H, pz H4/
H4′), 6.50 (d, J ) 15, CH2 Hax), 6.43 (m, 1H, pz H4/H4′), 6.21 (d,
J ) 15, CH2 Heq), 5.28 (s, free and coordinated H2CdCH2), 0.88
(s, 3H, PdMe). 13C{1H} NMR (CD2Cl2, -60 °C): δ 142.1 (pz C5/
C5′), 140.8 (pz C5/C5′), 133.5 (pz C3/C3′), 132.5 (pz C3/C3′),
115.2 (br s, exchanging H2CdCH2), 108.0 (pz C4/C4′), 107.9 (pz
C4/C4′), 40.3 (CH2), 8.9 (PdMe).
1
1
and a H NMR spectrum was recorded. The H NMR spectrum
established that 8a had formed (100% versus NMe2Ph). Under these
conditions (-60 °C), exchange of coordinated and free ethylene is
fast on the NMR chemical shift time scale. Fast exchange was also
observed when 2.5 equiv of ethylene was used. 1H NMR (CD2Cl2,
-60 °C): δ 7.04 (s, 1H, mim H4/H5), 7.00 (s, 2H, mim H4/H5),
6.65 (s, 1H, mim H4/H5), 5.22 (br s, free and coordinated
H2CdCH2), 4.09 (s, 2H, CH2), 3.73 (s, 3H, mim NMe), 3.67 (s,
3H, mim NMe), 0.55 (s, 3H, PdMe). 13C{1H} NMR (CD2Cl2, -60
°C): δ 140.3 (mim C2/C2′), 138.9 (mim C2/C2′), 124.8, 122.8,
122.7, 121.9, 114.0 (br s, free and coord H2CdCH2), 34.3 (mim
NMe), 33.8 (mim NMe), 22.5 (CH2), 7.0 (PdMe).
Generation of [{H2C(3,5-Me2-pz)2}PdMe(H2CdCH2)]-
[B(C6F5)4] (8f). A valved NMR tube containing a CD2Cl2 (0.7 mL)
solution of 3f (7.2 mg, 20.0 µmol) and [Li(Et2O)2.8][B(C6F5)4] (17.8
mg, 20.0 µmol) was cooled to -196 °C and exposed to ethylene
(ca. 3 equiv). The tube was sealed and warmed to -78 °C. The
solution was kept at -78 °C and transferred to a precooled (-60
1
°C) NMR probe, and a H NMR spectrum was recorded at -60
°C. The 1H NMR spectrum established that 8f had formed (100%
versus Et2O). Under these conditions (-60 °C), exchange of
coordinated and free ethylene (δ 5.37) is slow on the NMR chemical
Generation of [{(n-Hexyl)HC(mim)2}PdMe(H2CdCH2)]-
[B(C6F5)4] (8b). This compound was generated quantitatively from
2b, [HNMe2Ph][B(C6F5)4], and ethylene (ca. 4 equiv) using the
procedure for 8a. Under these conditions (-60 °C), exchange of
coordinated and free ethylene (δ 5.37) is slow on the NMR chemical
1
shift time scale. H NMR (CD2Cl2, -60 °C): δ 6.60 (d, J ) 15,
1H, CH2 Hax), 6.05 (s, 1H, pz H4/H4′), 6.02 (d, J ) 15, 1H, CH2
Heq), 5.88 (s, 1H, pz H4/H4′), 5.15 (br s, 2H, H2CdCH2), 4.89 (br
s, 2H, H2CdCH2), 2.37 (s, 3H, pz Me), 2.30 (s, 3H, pz Me), 2.28
(s, 3H, pz Me), 2.17 (s, 3H, pz Me), 0.80 (s, 3H, PdMe). 13C{1H}
NMR (CD2Cl2, -60 °C): δ 151.8 (3/3′-pz), 150.9 (3/3′-pz), 141.6
(5/5′-pz), 140.9 (5/5′-pz), 108.2 (4/4′-pz), 107.9 (4/4′-pz), 89.4
(H2CdCH2), 57.2 (CH2), 13.9, 13.1, 10.9, 10.6, 5.9 (PdMe).
1
shift time scale. H NMR (CD2Cl2, -60 °C): δ 6.99 (s, 1H, mim
H4/H5), 6.96 (s, 1H, mim H4/H5), 6.92 (s, 1H, mim H4/H5), 6.68
(s, 1H, mim H4/H5), 4.86 (br s, 2H, H2CdCH2), 4.72 (br s, 2H,
H2CdCH2), 4.21 (t, J ) 7, 1H, CH), 3.73 (s, 3H, mim NMe), 3.67
(s, 3H, mim NMe), 2.09 (m, 2H, CH2), 1.15 (br s, 8H,
CHCH2(CH2)4CH3), 0.77 (br s, 3H, CH2CH3), 0.57 (s, 3H, PdMe).
Generation of [{1,1′-Di(triphenylmethyl)-4,4′-biimidazole}-
PdMe(H2CdCH2)][B(C6F5)4] (8c). This compound was generated
quantitatively from 2c, [HNMe2Ph][B(C6F5)4], and ethylene (ca.
10 equiv) using the procedure for 8a. Under these conditions (-60
°C), exchange of coordinated and free ethylene is fast on the NMR
Generation of [{H2C(3,5-Me2-pz)2}PdMe(H2CdCH2)][SbF6]
(8f′). A solution of 6f′ (0.020 mmol) in CD2Cl2 in a valved NMR
tube was generated as described above and cooled to -196 °C,
and ethylene (ca. 8 equiv) was added by vacuum transfer. The tube
was sealed and warmed to -78 °C. The tube was maintained at
-78 °C and transferred to a precooled (-60 °C) NMR probe and
1
1
NMR spectra were recorded. A H NMR spectrum was recorded
chemical shift time scale. H NMR (CD2Cl2, -60 °C): δ 7.62 (s,
at -60 °C and showed that 8f′ had formed. Under these conditions
(-60 °C), exchange of coordinated and free ethylene (δ 5.37) is
1H, imidazole H2/H2′), 7.32 (m, 18H, trityl H3 and H4), 7.29 (s,
1H, imidazole H2/H2′), 7.11 (d, J ) 8, 6H, trityl H2/ H2′), 7.07
(d, J ) 8, 6H, trityl H2/ H2′), 7.00 (s, 1H, imidazole H5/H5′),
6.99 (s, 1H, imidazole H5/H5′), 5.31 (s, free and coordinated
H2CdCH2), 0.57 (s, 3H, PdMe). 13C NMR (CD2Cl2, -60 °C):
140.4, 140.1, 136.1 (imidazole C4/C4′), 135.9 (imidazole C4/C4′),
135.0 (imidazole C2/C2′), 132.8 (imidazole C2/C2′), 129.3, 129.2,
128.6, 128.4, 128.3, 128.2, 119.9 (br s, free and coordinated
H2CdCH2), 117.2 (imidazole C5/C5′), 116.8 (imidazole C5/C5′),
76.9 (C(C6H5)3), 76.6 (C(C6H5)3), 3.6 (PdMe).
1
slow on the NMR chemical shift time scale. H NMR (CD2Cl2,
-60 °C): δ 6.66 (d, J ) 15, 1H, CH2 Hax), 6.14 (d, J ) 15, 1H,
CH2 Heq), 6.03 (s, 1H, pz H4/H4′), 5.88 (s, 1H, pz H4/H4′), 5.15
(br s, 2H, H2CdCH2), 4.90 (br s, 2H, H2CdCH2), 2.39 (s, 3H, pz
Me), 2.34 (s, 3H, pz Me), 2.29 (s, 3H, pz Me), 2.18 (s, 3H, pz
Me), 0.79 (s, 3H, PdMe). 13C{1H} NMR (CD2Cl2, -60 °C): δ 151.5
(3/3′-pz), 150.6 (3/3′-pz), 141.8 (5/5′-pz), 141.3 (5/5′-pz), 108.0
(4/4′-pz), 107.7 (4/4′-pz), 89.3 (H2CdCH2), 57.3 (CH2), 13.9, 13.2,
10.9, 10.6, 5.8 (PdMe).
Generation of [{H2C(5-Me-py)2}PdMe(H2CdCH2)][B(C6F5)4]
(8d). This compound was generated quantitatively from 2d,
[HNMe2Ph][B(C6F5)4], and ethylene (ca. 8 equiv) using the
Generation of [(p-Tolyldiimine)PdMe(H2CdCH2)][B(C6F5)4]
(8g).Thiscompoundwasgeneratedquantitativelyfrom2g,[HNMe2Ph]-
[B(C6F5)4], and ethylene (ca. 6 equiv) and handled using the
procedure for 8a. Under these conditions (-60 °C), exchange of
coordinated and free ethylene is fast on the NMR chemical shift
(42) van Asselt, R.; Gielens, E. E. C. G.; Rulke, R. E.; Vrieze, K.;
Elsevier, C. J. J. Am. Chem. Soc. 1994, 116, 977.

Cationic Palladium(II) Alkyl Complexes
Organometallics, Vol. 26, No. 27, 2007 6735
time scale. 1H NMR (CD2Cl2, -60 °C): δ 7.30 (d, J ) 8, 4H, Ar),
6.79 (d, J ) 8, 2H, Ar), 6.70 (d, J ) 8, 2H, Ar), 5.20 (s, free and
coordinated H2CdCH2), 2.35 (s, 6H, 4-Me-C6H4), 2.28 (s, 3H,
NdCMe), 2.15 (s, 3H, NdCMe), 0.16 (s, PdMe). 13C{1H} NMR
(CD2Cl2, -60 °C): δ 180.4 (NdCMe), 174.5 (NdCMe), 141.6,
140.6, 137.8, 137.7, 130.4, 129.9, 120.2, 118.8, 117.1 (br s,
exchanging H2CdCH2), 21.2, 20.7, 20.6, 20.2, 13.7 (PdMe).
9e were confirmed by a COSY experiment due to overlap of one
of the methylene resonances with the cis- and trans-2-butene methyl
resonance.
Generation
of
[{H2C(3,5-Me2-pz)2}PdEt(H2CdCH2)]-
[B(C6F5)4] (9f). A valved NMR tube containing a CD2Cl2 solution
of 8f and 13 equiv of ethylene was kept at -10 °C for 26 min. A
1H NMR spectrum was recorded at -60 °C and showed that the
following species were present (% relative to free NMe2Ph): 8f
(14%), propylene (66%), cis- and trans-2-butenes (66%), and
{H2C(3,5-Me2-pz)2}PdEt(H2CdCH2)+ (9f, 85%). Under these
conditions (-60 °C), exchange of coordinated and free ethylene is
slow on the NMR chemical shift time scale. 1H NMR of {H2C(3,5-
Me2-pz)2}PdEt(H2CdCH2)+ (CD2Cl2, -60 °C): δ 6.54 (d, J ) 15,
1H, CH2 Hax), 6.99 (d, J ) 15, 1H, CH2 Heq), 4.97 (br d, J ) 15,
2H, H2CdCH2), 4.87 (br d, J ) 10, 2H, H2CdCH2), 0.63 (t, J )
7, 3H, PdCH2Me). The pz H4 and pz Me resonances of 9f could
not be distinguished from those of 8f. The PdCH2CH3 resonances
of 9f were not observed at -60 °C due to broadening and overlap
with other resonances.
Generation of [{H2C(mim)2}PdEt(H2CdCH2)][B(C6F5)4] (9a).
A valved NMR tube containing a CD2Cl2 solution of 8a and 5 equiv
of ethylene was kept at -10 °C for 90 min. A 1H NMR spectrum
was recorded at -10 °C and showed the following species were
present in solution (% relative to free NMe2Ph): unreacted 8a (27%),
propylene (61%), cis- and trans-2-butenes (24%), and
{H2C(mim)2}PdEt(H2CdCH2)+ (9a, 73%). Under these conditions
(-10 °C), exchange of coordinated and free ethylene is fast on the
NMR time scale. 1H NMR of {H2C(mim)2}PdEt(H2CdCH2)+
(CD2Cl2, -10 °C): δ 5.20 (br s, free and coordinated H2CdCH2
of 8a and 9a), 1.50 (q, J ) 8, 2H, PdCH2Me), 0.85 (t, J ) 8, 3H,
PdCH2Me). The mim, bridge-CH2, and NMe methyl resonances of
9a could not be distinguished from those of 8a.
Generation of [(p-Tolyldiimine)PdEt(H2CdCH2)][B(C6F5)4]
(9g). A valved NMR tube containing a CD2Cl2 solution of 8g and
4.5 equiv of ethylene was kept at -10 °C for 13 min. A 1H NMR
spectrum was recorded at -60 °C and showed the following species
were present (% relative to free NMe2Ph): propylene (43%), cis-
and trans-2-butenes (44%), 1-butene (32%), and (p-
tolyldiimine)PdEt(H2CdCH2)+ (9g, 86%). Under these conditions
(-60 °C), exchange of coordinated and free ethylene is fast on the
NMR chemical shift time scale. 1H NMR of (p-
tolyldiimine)PdEt(H2CdCH2)+ (CD2Cl2, -60 °C): δ 7.30 (br m,
4H, Ar), 6.86 (br d, J ) 7, 2H, Ar), 6.69 (br d, J ) 7, 2H, Ar),
5.23 (br s, free and coordinated H2CdCH2 and 9g), 2.36 (br s, 3H,
4-Me-C6H4), 2.35 (br s, 3H, 4-Me-C6H4), 2.28 (br s, 3H, NdCMe),
2.14 (br s, 3H, NdCMe), 1.05 (q, 2H, J ) 7, PdCH2Me), 0.28 (t,
J ) 7, 3H, PdCH2Me).
Generation of [{1,1′-Di(triphenylmethyl)-4,4′-biimidazole}-
PdEt(H2CdCH2)][B(C6F5)4] (9c). A valved NMR tube containing
a CD2Cl2 solution of 8c and 14 equiv of ethylene was kept at -10
°C for 7 h. A 1H NMR spectrum was recorded at -10 °C and
showed that 7c had completely disappeared, and the following
species were present (% relative to free NMe2Ph): propylene
(100%), cis- and trans-2-butenes (100%), 1-butene (23%), and
{1,1′-di(triphenylmethyl)-4,4′-biimidazole}PdEt(H2CdCH2)+ (9c,
100%). Under these conditions (-10 °C), exchange of coordinated
and free ethylene is fast on the NMR chemical shift time scale. 1H
NMR of {1,1′-di(triphenylmethyl)-4,4′-biimidazole}PdEt(H2Cd
CH2)+ (CD2Cl2, -10 °C): δ 7.65 (s, 1H, imidazole H2/H2′), 7.38
(br s, 18H, trityl H3 and H4), 7.31 (s, 1H, imidazole H2/H2′), 7.12
(br s, 12H, trityl H2/ H2′), 7.03 (br s, 2H, imidazole H5/H5′), 5.34
(br s, free and coordinated H2CdCH2),1.50 (br q, J ) 8, 2H,
PdCH2Me), 0.85 (br t, J ) 8, 3H, PdCH2Me).
Kinetics of Insertion of (N∧N)PdMe(H2CdCH2)+ Species. The
first-order rate constants (kinsert,Me) for the insertion of ethylene into
the Pd-Me bond of (N∧N)PdMe(H2CdCH2)+ species 8a-g and
8f′ were determined by 1H NMR. The procedure for 8a is described
here. Analogous procedures were used for 8b-g and 8f′. Details
and kinetic plots are provided in the Supporting Information.
Procedure for 8a: A CD2Cl2 solution of 8a containing 4 equiv of
excess free ethylene was generated in a valved NMR tube. The
tube was placed in a -10 °C constant temperature bath for 20 min,
placed in a -78 °C bath for 3 min, and transferred to a precooled
(-60 °C) NMR probe where a 1H NMR spectrum was recorded at
-60 °C. This procedure was repeated at 20 min intervals. Values
of I0,PdMe, IPdMe, and INMe2Ph, where I0,PdMe ) the integral of the
Pd-Me resonance of 8a (δ ) 0.61) at the start of the experiment,
IPdMe ) the integral of the Pd-Me resonance of 8a at the end of
each 20 min interval, and INMe2Ph ) the integral of the NMe2Ph
resonance (δ ) 2.92), were determined by integration. A plot of
Generation of [{H2C(5-Me-py)2}PdEt(H2CdCH2)][B(C6F5)4]
(9d). A valved NMR tube containing a CD2Cl2 solution of 8d and
1
9 equiv of ethylene was kept at -10 °C for 125 min. A H NMR
spectrum was recorded at -60 °C and showed that 8d had
completely disappeared, and the following products were present
(% relative to free NMe2Ph): propylene (42%), cis- and trans-2-
butenes
(100%),
1-butene
(32%),
and
{H2C(5-Me-
py)2}PdEt(H2CdCH2)+ (9d, 80%). Under these conditions (-60
°C), exchange of coordinated and free ethylene is fast on the NMR
time scale. 1H NMR of {H2C(5-Me-py)2}PdEt(H2CdCH2)+
(CD2Cl2, -60 °C): δ 8.35 (s, 1H, py H6/H6′), 7.92 (s, 1H, py H6/
H6′), 7.73 (d, J ) 8, 1H, py H4/H4′), 7.61 (d, J ) 8.0, 1H, py
H4/H4′), 7.45 (d, J ) 8, 1H, py H3/H3′), 7.38 (d, J ) 8, 1H, py
H3/H3′), 4.65 (d, J ) 14, 1H, CH2 Hax), 4.25 (d, J ) 14, 1H, CH2
Heq), 5.32 (br s, free and coordinated H2CdCH2), 2.35 (s, 3H, py
5-Me), 2.26 (s, 3H, py 5-Me), 1.72 (pentet, J ) 8, 1H, PdCH2Me),
1.44 (pentet, J ) 8, 1H, PdCH2Me), 0.65 (t, J ) 8, 3H, PdCH2Me).
ln(APdMe/A0,PdMe) versus time (at -10 °C), where APdMe ) IPdMe
/
INMe2Ph and A0,PdMe ) I0,PdMe/INMe2Ph, was linear. The slope of this
plot equals -kinsert,Me. For 8a, kinsert,Me ) (1.2 ( 0.1) × 10-4 s-1 at
-10 °C (ca. 3 half-lives).
Generation of [{H2C(pz)2}PdEt(H2CdCH2)][B(C6F5)4] (9e).
A valved NMR tube containing a CD2Cl2 solution of 8e and 4 equiv
of ethylene was kept at -22 °C for 2 h. A 1H NMR spectrum was
recorded at -65 °C and showed the following species were present
(% relative to free NMe2Ph): 8e (15%), propylene (85%), cis- and
trans-2-butenes (36%), and {H2C(pz)2}PdEt(H2CdCH2)+ (9e,
85%). Under these conditions (-65 °C), exchange of coordinated
and free ethylene is fast on the NMR chemical shift time scale. 1H
NMR of {H2C(pz)2}PdEt(H2CdCH2)+ (CD2Cl2, -65 °C): δ 5.20
(br s, free and coordinated H2CdCH2 of 8e and 9e), 1.95 (m, 1H,
PdCH2Me), 1.57 (m, 1H, PdCH2Me), 0.85 (t, J ) 7, 3H, PdCH2Me).
The pz, and bridge CH2 resonances of 9e could not be distinguished
from those of 8e. The 1H NMR assignments for the PdEt group of
Kinetics of Insertion of (N∧N)PdEt(H2CdCH2)+ Species
9a,c–g. The first-order rate constants (kinsert,Et) for the insertion of
ethylene into the Pd-Et bond of (N∧N)PdEt(H2CdCH2)+ species
9a,c-g were determined by 1H NMR. The procedure for 9a is
described here. Analogous procedures were used for 9c-g, and details
and kinetic plots are provided in the Supporting Information. Procedure
for 9a: A CD2Cl2 solution (0.7 mL) of 8a was generated in a valved
NMR tube as described above. The tube was transferred to a precooled
(-40 °C) NMR probe. A 1H NMR spectrum was recorded at -40 °C
and showed that 8a had formed (100% versus NMe2Ph). Under these
conditions (-40 °C), exchange of coordinated and free ethylene is
fast on the NMR time scale. The NMR probe was warmed to -10

6736 Organometallics, Vol. 26, No. 27, 2007
Burns and Jordan
°C, thermally equilibrated, and maintained at this temperature, and
1H NMR spectra were recorded periodically. Values of Ibutenes and
INMe2Ph, where Ibutenes ) the integral of the methyl resonances of trans-
2-, cis-2- (δ ) 1.59) and 1-butene (δ ) 0.98) and INMe2Ph ) the integral
of the NMe2Ph resonance (δ ) 2.94), were determined by careful
integration of each spectrum and used to determine the moles butenes
produced/moles of catalyst. A plot of turnovers (moles butenes
produced/moles of catalyst) versus time was linear (r2 ) 0.991). The
slope of this plot equals kinsert,Et. For 9a, kinsert,Et ) (1.0 ( 0.1) × 10-4
s-1 at -10 °C.
Acknowledgment. This work was supported by the U.S.
Department of Energy (DE-FG-02-00ER15036).
Supporting Information Available: Variable-temperature NMR
spectra of 2d and 3f, details of kinetics studies, additional synthetic
procedures, and representative NMR spectra of 2d, 2g, 3f, 5d, 7d,
8d, and 9d. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM700767R