Cis/trans isomerization of o -phosphino-arenesulfonate palladium methyl complexes

Article abstract of DOI:10.1021/om501007q

The cis/trans isomerization of (PO-OMe)PdMe(lut) ([PO-OMe]^- = 2-{P(2-OMe-Ph)₂}-4-Me-benzenesulfonate) was studied to model the proposed isomerization in chain propagation in ethylene polymerization by (o-phosphino-arenesulfonate)PdR(ethylene) species. Nonequilibrium mixtures of cis-P,C- and trans-P,C-(PO-OMe)PdMe(2,6-lutidine) were generated by the reaction of Na[PO-OMe] and {Pd(μ-Cl)Me(2,6-lutidine)}₂ in CD₂Cl₂ at -25 °C. Kinetic studies revealed lutidine-catalyzed and noncatalyzed isomerization pathways. The lutidine-catalyzed pathway involves five-coordinate (PO-OMe)PdMe(lut)₂ intermediates that undergo Berry pseudorotation. Kinetic studies, structure-activity relationships, solvent effects, and density functional theory calculations for the noncatalyzed pathway are most consistent with a mechanism, originally proposed by Nozaki, Morokuma, and co-workers, which proceeds through a five-coordinate transition state with κ³-P,O,O coordination of the [PO]^- ligand.

Full text of DOI:10.1021/om501007q

Article
pubs.acs.org/Organometallics
cis/trans Isomerization of o‑Phosphino-Arenesulfonate Palladium
Methyl Complexes
Xiaoyuan Zhou, Ka-Cheong Lau, Benjamin J. Petro, and Richard F. Jordan*
Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: The cis/trans isomerization of (PO-OMe)PdMe-
(lut) ([PO-OMe]− = 2-{P(2-OMe-Ph)2}-4-Me-benzenesulfo-
nate) was studied to model the proposed isomerization in chain
propagation in ethylene polymerization by (o-phosphino-
arenesulfonate)PdR(ethylene) species. Nonequilibrium mix-
tures of cis-P,C- and trans-P,C-(PO-OMe)PdMe(2,6-lutidine)
were generated by the reaction of Na[PO-OMe] and {Pd(μ-
Cl)Me(2,6-lutidine)}2 in CD2Cl2 at −25 °C. Kinetic studies
revealed lutidine-catalyzed and noncatalyzed isomerization
pathways. The lutidine-catalyzed pathway involves five-
coordinate (PO-OMe)PdMe(lut)2 intermediates that undergo Berry pseudorotation. Kinetic studies, structure−activity
relationships, solvent effects, and density functional theory calculations for the noncatalyzed pathway are most consistent with a
mechanism, originally proposed by Nozaki, Morokuma, and co-workers, which proceeds through a five-coordinate transition
state with κ3-P,O,O coordination of the [PO]− ligand.
ligands. Observable and isolable (PO)Pd(R)(L) complexes
invariably have cis-P,R geometries.1,2 The DFT studies show
that the barrier to ethylene insertion of cis-P,R-(PO)Pd(R)-
(ethylene) is higher than those for isomerization of cis-P,R-
(PO)Pd(R)(ethylene) to trans-P,R-(PO)Pd(R)(ethylene) and
ethylene insertion of the trans-P,R isomer. These results imply
that chain propagation occurs by isomerization of the cis-P,R-
(PO)Pd(R)(ethylene) resting state to the trans-P,R isomer
followed by insertion, as shown in Scheme 1. Analogous
mechanisms were proposed for chain propagation by Ni
catalysts that contain salicylaldiminato and anilinotropone
ancillary ligands.5
INTRODUCTION
■
Palladium alkyl catalysts that contain o-phosphino-arenesulfo-
nate ligands ([PO]−, Scheme 1) have been studied intensively
Scheme 1
Three general mechanisms have been implicated in the cis/
trans isomerization of square planar complexes of Pd(II) and
other d8 metals.6 These include (i) associative processes
catalyzed by exogeneous ligands or solvents involving
consecutive ligand displacement steps or rearrangement of
five-coordinate species via multiple Berry pseudorotations or
turnstile processes,7 (ii) dissociative processes involving
configurationally labile three-coordinate intermediates,8 and
(iii) unimolecular rearrangement via a distorted tetrahedral
transition state without ligand loss or gain.9
Ziegler’s DFT analysis of the cis/trans isomerization of (2-
P{2-OMe-Ph}2-benzenesulfonate)Pd(Pr)(CH2CH2) and re-
lated species implicated a unimolecular mechanism involving a
distorted tetrahedral transition state (TS-1) in which the Pr
group and the ethylene ligand are rotated by ∼90° from their
position in the ground state structure (Scheme 2).3 Morokuma
because of their ability to polymerize ethylene to linear
polyethylene and copolymerize ethylene with polar vinyl
monomers.1,2 Because of the unsymmetrical structure of the
PO− ligand, two modes of coordination and insertion of olefins
are possible for (PO)PdR species, with the alkyl group either cis
or trans to the phosphine (Scheme 1). Density functional
theory (DFT) studies by Ziegler and co-workers3 and by
Nozaki, Morokuma, and co-workers4 show that cis-P,R-
(PO)Pd(R)(ethylene) species are more stable than the
corresponding trans-P,R isomers, which are destabilized by
the trans arrangement of the strong donor phosphine and alkyl
Received: October 3, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om501007q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
generating nonequilibrium mixtures of cis-P,C- and trans-P,C-
(PO-OMe)PdMe(2,6-lutidine) (1a) and related complexes
with substituted 2,6-lutidine ligands, studies of their isomer-
ization to the thermodynamically favored cis-P,C-(PO-OMe)-
PdMe(2,6-lutidine) isomers, and observations related to the
mechanism of this process.
Scheme 2
RESULTS
■
Generation of Mixtures of cis-P,C- and trans-P,C-(PO-
OMe)PdMe(2,6-lutidine) Complexes. The reaction of
Na[PO-OMe] and {Pd(μ-Cl)Me(2,6-lutidine)}2 in CD2Cl2 at
−25 °C for 30 min generates a 4/1 mixture of trans-P,C-1a and
cis-P,C-1a in quantitative NMR yield (Scheme 3). The mixture
is not stable at −25 °C, and the cis-P,C-1a/trans-P,C-1a ratio
increases over time until trans-P,C-1a is completely converted
to cis-P,C-1a. The reaction temperature and time are critical for
the successful generation of mixtures with high trans-P,C-1a/
cis-P,C-1a ratios. No reaction is observed below −40 °C, and
mixtures with a large fraction of cis-P,C-1a are obtained above
−25 °C. The use of shorter reaction times at −25 °C results in
mixtures of trans-P,C-1a, cis-P,C-1a, and unreacted starting
materials.
and Nozaki’s analysis of the isomerization of the model species
trans-P,C-(2-PMe2-benzenesulfonate)Pd(Pr)(CH2CH2) to
cis-P,C-(2-PMe2-benzenesulfonate)Pd(Pr)(CH2CH2) re-
vealed an alternative mechanism involving coordination of a
second sulfonate oxygen to Pd, leading to a square-pyramidal
five-coordinate transition state (TS-2) with the phosphorus
atom located at the apical position with a very long Pd−P
contact (2.771 Å vs 2.352 Å in the ground state), and
subsequent Berry pseudorotations.4 TS-2 was found to be 4.2
kcal/mol more stable than TS-1 for this model system. The TS-
2 process proposed by Morokuma and Nozaki can be
considered to be an intramolecular version of the classical
ligand-catalyzed cis/trans isomerization.10 Morokuma and
Nozaki also found that isomerization of trans-P,C-(2-PMe2-
benzenesulfonate)Pd(Pr)(pyridine) to cis-P,C-(2-PMe2-
benzenesulfonate)Pd(Pr)(pyridine) via TS-2 is favored over a
sulfonate dissociation pathway (TS-3) by 9.1 kcal/mol.4a
However, site-specific solvation effects that were not considered
in the DFT analyses might stabilize TS-3.
In an initial study of cis/trans isomerization mechanisms of
(PO)Pd complexes, we investigated the degenerate pyridine
exchange in (PO-OMe)Pd(py)2+ ([PO-OMe]− = 2-{P(2-OMe-
Ph)2}-4-Me-benzenesulfonate) and the cis/trans isomerization
of {2-P(iPr)2-4-Me-benzenesulfonate}Pd(Cl){P(O-o-tolyl)3}.11
Kinetic studies of these model systems show that both
processes occur via five-coordinate intermediates formed by
the coordination of pyridine, P(O-o-tolyl)3, or chloride.
Noncatalyzed pathways were not observed in these cases. To
date, it has not been possible to observe cis/trans isomerization
of (PO)Pd(R)(L) alkyl species. Here we describe a method of
The structures of trans-P,C-1a and cis-P,C-1a are readily
apparent from the NMR spectral data of these species. The 1H
(−25 °C) and 13C (−60 °C) NMR spectra of trans-P,C-1a
each contain a doublet for the Pd-CH3 group with a large 31P
2
coupling constant (3JHP = 8.5 Hz; JCP = 102 Hz), consistent
with a trans arrangement of the methyl and phosphine
ligands.12 In contrast, the H NMR spectrum of cis-P,C-1a
1
(−25 °C) contains a doublet for the Pd-CH3 group with a small
3JHP value (3.0 Hz) and the 13C NMR spectrum (−60 °C)
contains a singlet for the Pd-CH3 group, as expected for a cis-
P,C isomer. Two broad singlets are observed for the 2-OMe-Ph
1
units in the H NMR spectrum of trans-P,C-1a at −60 °C,
which is indicative of restricted inversion of the puckered
chelate ring and/or rotation around the P−Cipso bonds.13
Similar results were observed for cis-P,C-1a at −60 °C. Several
analogues of trans-P,C-1a with different para-substituted 2,6-
lutidine ligands (lut-X) were also prepared, as shown in Scheme
3 (initial trans-P,C-1/cis-P,C-1 ratios of 2.3 for 1b, 1.9 for 1c,
and 1.0 for 1d).
Kinetics of Isomerization of trans-P,C-1a to cis-P,C-1a.
The kinetics of isomerization of trans-P,C-1a to cis-P,C-1a
1
were probed by H NMR in CD2Cl2 solution at −25 °C. The
isomerization displays first-order behavior in trans-P,C-1a with
a first-order rate constant k0 of 1.60(4) × 10−4 s−1 at −25 °C.
The isomerization is accelerated in the presence of free 2,6-
lutidine. For example, the observed first-order rate constant
(kobs) in the presence of 1.67 M 2,6-lutidine is 43(3) × 10−4
s−1. A plot of kobs versus 2,6-lutidine concentration is linear with
a non-zero intercept, indicating that the isomerization is also
Scheme 3
B
dx.doi.org/10.1021/om501007q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
first-order in 2,6-lutidine (Figure 1). These results are
consistent with the rate law in eq 1 and the operation of two
Influence of the Electronic Properties of the 2,6-
Lutidine Ligand on k0. The first-order rate constants for the
X
isomerization of trans-P,C-1b−d to cis-P,C-1b−d (k0 ) were
determined to probe the influence of electronic effects on the
rate of the non-lutidine-catalyzed isomerization. The results of
this study are summarized in Table 1. Using 1a as a benchmark,
X
Table 1. First-Order Rate Constants (k0 ) for the
Unimolecular Isomerization of trans-P,C-1a−d to cis-P,C-
1a−d Containing 4-X-2,6-Lutidine Ligands in CD2Cl2
Solution at −25 °C
k0 (×10−4 s−1
X
)
σp
+
X
NO2
H
1.13(5)
1.60(4)
3.38(3)
7.4 (3)
0.79
0
OMe
NMe2
−0.78
−1.7
incorporating a 4-NO2 group into the 2,6-lutidine decreases the
isomerization rate, while the presence of a 4-OMe or 4-NMe2
group increases the rate. A Hammett analysis of these data,
using 1a (k0; X = H) as the reference, is shown in Figure 3. A
Figure 1. Plot of the observed first-order rate constant for the
isomerization of trans-P,C-1a to cis-P,C-1a (kobs) vs the concentration
of free 2,6-lutidine. The inset is an expansion of the y-intercept region
of the kinetic plot. Conditions: −25 °C, CD2Cl2 solution.
independent isomerization pathways: a 2,6-lutidine-catalyzed
process (k1[2,6-lutidine]) and a non-2,6-lutidine-catalyzed
pathway (k0).
rate = (k0 + k1[2,6‐lutidine])[trans‐P,C‐1a]
(1)
The isomerization of trans-P,C-1a to cis-P,C-1a was studied
over the temperature range of −25 to −5 °C, and the resulting
Eyring plot is shown in Figure 2. The activation parameters for
the isomerization are as follows: ΔG⧧(−25 °C) = 19(1) kcal/
mol; ΔH⧧ = 19(1) kcal/mol; ΔS⧧ = −0.97(8) eu.
Figure 3. Hammett plot for the unimolecular isomerization of trans-
P,C-1a−d to cis-P,C-1a−d. The X substituents are shown in the plot.
X
+
linear correlation between log(k0 /k0) and σp , the Hammett
constant for substituents that are conjugated with a reaction
center and a delocalized positive charge,14 is observed. The
negative ρ value [−0.34(3)] implies a buildup of positive
charge on the lutidine ligand and hence the Pd center in the
transition state, though the effect is small.
Solvent Effects on k0. The k0 values for the isomerization
of trans-P,C-1a in several solvent mixtures are listed in Table 2.
Table 2. First-Order Rate Constants (k0) for the
Unimolecular Isomerization of trans-P,C-1a to cis-P,C-1a in
Different Solvent Mixtures at −25 °C
Figure 2. Eyring plot for the isomerization of trans-P,C-1a to cis-P,C-
1a in CD2Cl2.
entry
solvent
k0 (×10−4 s−1
)
1
CD2Cl2
1.60(4)
2.15(2)
4.7(1)
6.0(1)
It is possible that, even in the absence of added 2,6-lutidine, a
low concentration of free 2,6-lutidine could be present due to
decomposition of 1a and could catalyze the isomerization.
However, addition of 6 mol % {(PO-3,5-tBu)PdMe}2 ([PO-
3,5-tBu]− = 2-{P(3,5-tBu2-Ph)2}-4-Me-benzenesulfonate) as a
lutidine trap has no effect on the isomerization rate, which rules
out this possibility.2h
a
b
2
2.3 M acetone-d6 in CD2Cl2
a
a
b
3
4
2.2 M nPrOH in CD2Cl2
b
4.1 M CD3OD in CD2Cl2
a
trans-P,C-1a is insoluble in CD3OD, nPrOH, and acetone-d6.
Addition of >17 vol % cosolvent resulted in precipitation of trans-
P,C-1a and cis-P,C-1a.
b
C
dx.doi.org/10.1021/om501007q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
The addition of 17 vol % (2.3 M) acetone-d6 (ε = 20.7;
Gutmann donor number DN = 21) to the benchmark solvent
CD2Cl2 (ε = 8.9; DN = 1) has a minimal effect on k0 (entry
2).15 The addition of 17 vol % (2.2 M) nPrOH (ε = 20.1; DN =
21) or 17 vol % (4.1 M) CD3OD (ε = 33; DN = 19) increases
the isomerization rate by a factor of 3−4 (entries 3 and 4).
These results show that the polarity, donor ability, and
hydrogen bonding ability of the solvent have only a minor
effect on the isomerization rate.
Changes in Calculated Atomic Charges. As noted
above, Morokuma and Nozaki found by DFT calculations
that isomerization of trans-P,C-(2-PMe2-benzenesulfonate)Pd-
(Pr)(pyridine) to cis-P,C-(2-PMe2-benzenesulfonate)Pd(Pr)-
(pyridine) via TS-2 is favored over a sulfonate dissociation
pathway (TS-3) by 9.1 kcal/mol. Our own DFT calculations
confirm this result and also show that the TS-2 pathway is
favored over the TS-1 pathway by 11 kcal/mol for this system
(B3LYP/6-31G* and LANL2DZ).11 We also calculated NBO
atomic charges of the Pd and P atoms along the TS-1 and TS-2
pathways. In TS-1, the Pd gains positive charge (+0.03 unit)
and the P gains negative charge (−0.08 unit) relative to trans-
P,C-(2-PMe2-benzenesulfonate)Pd(Pr)(pyridine). In TS-2,
even though the Pd is formally five-coordinate, Pd gains
substantial positive charge (+0.129 unit) while P gains negative
charge (−0.118 unit) compared to trans-P,C-(2-PMe2-
benzenesulfonate)Pd(Pr)(pyridine). This result is consistent
with the long Pd−P distance and nearly complete displacement
of the stronger donor P atom by the weaker donor sulfonate O
atom in TS-2.
cis-P,C-1a. This process results in exchange of free and
coordinated 2,6-lutidine.16
Five possible mechanisms may be considered for the non-
lutidine-catalyzed pathway, as summarized in Schemes 5 and 6.
Mechanism A proceeds via a distorted tetrahedral transition
state analogous to TS-1. Mechanism B proceeds via a five-
coordinate transition state with κ3-P,O,O coordination of the
[PO]− ligand analogous to TS-2. Mechanism C proceeds by
dissociation of the sulfonate to form a configurationally labile
three-coordinate, zwitterionic species analogous to TS-3.
Mechanism D is a classical solvent-catalyzed pathway involving
initial coordination of solvent followed by three consecutive
Berry pseudorotations to effect isomerization and solvent loss,
as detailed in Scheme 6. Mechanism E proceeds by dissociation
of the lutidine ligand to form a configurationally labile three-
coordinate intermediate.
Mechanism D, which involves a bimolecular solvent
coordination step, is inconsistent with the near-zero ΔS⧧, the
observed (though small) accelerating effect of electron-
donating substituents on the lutidine ligand, and the small
influence of solvent donor ability on the isomerization rate.
Mechanism E, which involves dissociation of a lutidine ligand, is
inconsistent with the near-zero ΔS⧧ and the observed rate law
for the isomerization (see the Supporting Information). While
the experimental results do not rule out mechanism A, this
mechanism may be discounted by the DFT results for the
model complex (2-PMe2-benzenesulfonate)Pd(Pr)(pyridine),
which show that TS-1 is disfavored by 11 kcal/mol relative to
TS-2.
Distinguishing between mechanisms B and C is more
challenging. Both of these unimolecular mechanisms are
consistent with the observed near-zero ΔS⧧ and the positive
charge buildup on Pd in the transition state implied by the
Hammett analysis. However, solvent effects on mechanism B
should be minimal, while mechanism C should be strongly
favored by polar, coordinating, and especially hydrogen-
bonding solvents, which would stabilize the zwitterionic species
TS-3. For comparison, the anchimerically assisted ionization of
4-methoxyneophyl tosylate, shown in Scheme 7, is 10 times
faster in acetone but ∼2000 times faster in MeOH than in THF
(ε = 7.5). Methanol stabilizes the departing tosylate anion
through hydrogen bonding.17 The relatively small solvent
effects on the isomerization rate of trans-P,C-1a and the
similarity of the effects of non-hydrogen-bonding (acetone) and
hydrogen-bonding additives (nPrOH and CD3OD) argue
against mechanism C. Furthermore, sulfonate dissociation is
rarely observed for (o-phosphino-arenesulfonate)Pd species
and is disfavored by the rigid phenylene linker. An unusual
example of this process is the reaction of [{PO-OMe}Pd-
(py)2][SbF6] and pyridine to generate [κ1-P-{PO-OMe}Pd-
(py)3][SbF6]; however, this process is reversible (Keq = 0.28
M−1 at 20 °C), and the latter species could be isolated only
from neat pyridine.11
Mechanism of Isomerization of trans-P,C-1a to cis-P,C-
1a. The 2,6-lutidine-catalyzed pathway can be explained by the
classical ligand-catalyzed cis/trans isomerization mechanism, as
shown in Scheme 4. In this process, 2,6-lutidine binds ot trans-
P,C-1a to form the five-coordinate intermediate trans-P,C-2a,
which undergoes two consecutive Berry pseudorotations to
form cis-P,C-2a. Subsequent dissociation of 2,6-lutidine affords
Scheme 4
CONCLUSIONS
■
The isomerization of trans-P,C-(PO-OMe)PdMe(lut) species
to the thermodynamically favored cis-P,C isomers occurs by two
independent pathways, a lutidine-catalyzed pathway and a
unimolecular pathway. The lutidine-catalyzed pathway pro-
ceeds through five-coordinate (PO)PdMe(lut)2 intermediates
that undergo Berry pseudorotations. The non-lutidine-catalyzed
pathway exhibits a near-zero ΔS⧧, is slightly accelerated by
electron-donating substituents on the lutidine, and is only
D
dx.doi.org/10.1021/om501007q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 5
Scheme 6
Scheme 7
DFT results for related species show that TS-1 is significantly
disfavored relative to TS-2. The experimental results argue
against mechanisms involving configurationally labile three-
coordinate species generated by dissociation of the sulfonate or
lutidine ligands, or five-coordinate species formed by
coordination of a solvent molecule. These results also suggest
that under catalytic polymerization conditions, where ethylene
is present in large excess relative to (PO)PdR species, an
ethylene-catalyzed cis/trans isomerization pathway via five-
coordinate (PO)PdR(ethylene)2 intermediates may outcom-
pete the unimolecular pathway.
slightly accelerated by addition of polar, coordinating, and
hydrogen-bonding solvents. These results favor a mechanism
originally proposed by Nozaki, Morokuma, and co-workers that
proceeds through a five-coordinate transition state with κ3-
P,O,O coordination of the [PO]− ligand (TS-2).4a The
experimental results are also consistent with a mechanism
involving a distorted tetrahedral transition state (TS-1), but
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis and characterization of compounds, experimental and
computational details, figures, and tables. This material is
available free of charge via the Internet at http://pubs.acs.org.
E
dx.doi.org/10.1021/om501007q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Organometallics 1998, 17, 954. (m) Macchioni, A.; Bellachioma, G.;
Cardaci, G.; Travaglia, M.; Zuccaccia, C.; Milani, B.; Corso, G.;
Zangrando, E.; Mestroni, G.; Carfagna, C.; Formica, M. Organo-
metallics 1999, 18, 3061.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rfjordan@uchicago.edu.
■
(8) (a) Romeo, R.; Minniti, D.; Trozzi, M. Inorg. Chem. 1976, 15,
1134. (b) Alibrandi, G.; Minniti, D.; Scolaro, L. M.; Romeo, R. Inorg.
Chem. 1988, 27, 318. (c) Alibrandi, G.; Scolaro, L. M.; Romeo, R.
Inorg. Chem. 1991, 30, 4007. (d) Romeo, R.; Grassi, A.; Scolaro, L. M.
Inorg. Chem. 1992, 31, 4383. (e) Minniti, D. Inorg. Chem. 1994, 33,
2631. (f) Casado, A. L.; Casares, J. A.; Espinet, P. Inorg. Chem. 1998,
37, 4154. (g) Romeo, R.; D’Amico, G.; Sicilia, E.; Russo, N.; Rizzato,
S. J. Am. Chem. Soc. 2007, 129, 5744. (h) Gossage, R. A.; Jenkins, H.
A.; Jones, N. D.; Jones, R. C.; Yates, B. F. Dalton Trans. 2008, 3115.
(9) (a) Azizian, H.; Dixon, K. R.; Eaborn, C.; Piddock, A.; Shuaib, N.
M.; Vinzixa, J. Chem. Commun. 1982, 1020. (b) Wouters, J. M. A.;
Klein, R. A.; Elsevier, C. J.; Haming, L.; Stam, C. H. Organometallics
1994, 13, 4586. (c) Tsuji, Y.; Nishiyama, K.; Hori, S.; Ebihara, M.;
Kawamura, T. Organometallics 1998, 17, 507. (d) Tsuji, Y.; Obora, Y. J.
Organomet. Chem. 2000, 611, 343. (e) Goossen, L. K.; Koley, D.;
Hermann, H. L.; Theil, W. Organometallics 2005, 24, 2398. (f) Zheng,
F.; Hutton, A. T.; van Sittert, C. G. C. E.; Moss, J. R.; Mapolie, S. F.
Dalton Trans. 2013, 42, 11163.
(10) For other cases in which cis/trans isomerization or equivalent
exchange processes are mediated by intramolecular coordination of
hemilabile ligands, see: (a) Rotondo, E.; Battaglia, G.; Arena, C. G.;
Faraone, F. J. Organomet. Chem. 1991, 419, 399. (b) Imhoff, P.; van
Asselt, R.; Ernsting, J. M.; Vrieze, K.; Elsevier, C. J.; Smeets, W. J. J.;
Spek, A. L.; Kentgens, A. P. M. Organometallics 1993, 12, 1523.
(c) Casares, J. A.; Espinet, P. Inorg. Chem. 1997, 36, 5428.
(11) Conley, M. P.; Jordan, R. F. Angew. Chem., Int. Ed. 2011, 50,
3744.
(12) (a) Fanjul, T.; Eastham, G.; Fey, N.; Hamilton, A.; Orpen, A. G.;
Pringle, P. G.; Waugh, M. Organometallics 2010, 29, 2292.
(b) Marquard, S.; Rosenfeld, D. C.; Hartwig, J. F. Angew. Chem., Int.
Ed. 2010, 49, 793.
(13) (a) Feng, G.; Conley, M. P.; Jordan, R. F. Organometallics 2014,
33, 4486. (b) Neuwald, B.; Caporaso, L.; Cavallo, L.; Mecking, S. J.
Am. Chem. Soc. 2013, 135, 1026.
(14) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(15) (a) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225. (b) Montalti,
M. Handbook of Photochemistry, 3rd ed.; CRC Press: Boca Raton, FL,
2006; p 555.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(Grants CHE-0911180 and CHE-1048528).
REFERENCES
■
(1) (a) Nakamura, A.; Anselment, T.; Claverie, J.; Goodall, B.;
Jordan, R. F.; Mecking, S.; Rieger, B.; Sen, A.; van Leeuwen, P.;
Nozaki, K. Acc. Chem. Res. 2013, 46, 1438. (b) Nakamura, A.; Ito, S.;
Nozaki, K. Chem. Rev. 2009, 109, 5215. (c) Berkefeld, A.; Mecking, S.
Angew. Chem., Int. Ed. 2008, 47, 2538.
(2) (a) Drent, E.; Dijk, R.; Ginkel, R.; Oort, B.; Pugh, R. I. Chem.
Commun. 2002, 744. (b) Drent, E.; van Dijk, R.; van Ginkel, R.; van
Oort, B.; Pugh, R. I. Chem. Commun. 2002, 964. (c) Vela, J.; Lief, G.
R.; Shen, Z.; Jordan, R. F. Organometallics 2007, 26, 6624. (d) Luo, S.;
Vela, J.; Lief, G. R.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 8946.
(e) Weng, W.; Sheng, Z.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129,
15450. (f) Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2010, 132, 52.
(g) Shen, Z.; Jordan, R. F. Macromolecules 2010, 43, 8706. (h) Cai, Z.;
Shen, Z.; Zhou, X.; Jordan, R. F. ACS Catal. 2012, 2, 1187. (i) Kochi,
T.; Yoshimura, K.; Nozaki, K. Dalton Trans. 2006, 25. (j) Ota, Y.; Ito,
S.; Kuroda, J.; Okumura, Y.; Nozaki, K. J. Am. Chem. Soc. 2014, 136,
11898. (k) Guironnet, D.; Runzi, T.; Gottker-Schnetmann, I.;
Mecking, S. Chem. Commun. 2008, 4965. (l) Runzi, T.; Frohlich, D.;
̈
̈
Mecking, S. J. Am. Chem. Soc. 2010, 132, 17690. (m) Wucher, P.;
Schwaderer, J. B.; Mecking, S. ACS Catal. 2014, 4, 2672. (n) Skupov,
K. M.; Marella, P. R.; Simard, M.; Yap, G. P. A.; Allen, N.; Conner, D.;
Goodall, B. L.; Claverie, J. P. Macromol. Rapid Commun. 2007, 28,
2033. (o) Skupov, K. M.; Hobbs, J.; Marella, P.; Conner, D.; Golisz, S.;
Goodall, B. L.; Claverie, J. P. Macromolecules 2009, 42, 6953.
(p) Piche, L.; Daigle, J.-C.; Rehse, G.; Claverie, J. P. Chem.Eur. J.
2012, 18, 3277. (q) Anselment, T. M. L.; Wichmann, C.; Anderson, C.
E.; Herdtweck, E.; Rieger, B. Organometallics 2011, 30, 6602.
(r) Anselment, T. M. J.; Anderson, C. E.; Rieger, B.; Bele
Boeddinghaus, M.; Fassler, T. F. Dalton Trans. 2011, 40, 8304.
(s) Borkar, S.; Newsham, D. K.; Sen, A. Organometallics 2008, 27,
3331.
(3) (a) Haras, A.; Anderson, G. D. W.; Michalak, A.; Rieger, B.;
Ziegler, T. Organometallics 2006, 25, 4491. (b) Haras, A.; Michalak, A.;
Rieger, B.; Ziegler, T. Organometallics 2006, 25, 946.
(4) (a) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.;
Morokuma, K.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14088.
(b) Nozaki, K.; Kusumoto, S.; Noda, S.; Kochi, T.; Chung, L.;
Morokuma, K. J. Am. Chem. Soc. 2010, 132, 16030.
̈
(16) Separate and sharp resonances for coordinated and free 2,6-
1
lutidine are observed in the H NMR spectrum of the trans-P,C-1a/
cis-P,C-1a mixture, indicating that intermolecular 2,6-lutidine ex-
change is slow on the NMR T2 time scale at −25 °C.
(17) Smith, S. G.; Fainberg, A. H.; Winstein, S. J. Am. Chem. Soc.
1961, 83, 618.
(5) (a) Chan, M. S. W.; Deng, L.; Ziegler, T. Organometallics 2000,
19, 2741. (b) Deubel, D. V.; Ziegler, T. Organometallics 2002, 21,
4432. (c) Michalak, A.; Ziegler, T. Organometallics 2003, 22, 2069.
(6) (a) Anderson, G. K.; Cross, R. J. Chem. Soc. Rev. 1980, 9, 185.
(b) Cross, R. J. Chem. Soc. Rev. 1985, 14, 197.
(7) (a) Cooper, D. G.; Powell, J. Can. J. Chem. 1973, 51, 1634.
(b) Redfield, D. A.; Nelson, J. H.; Henry, R. A.; Moore, D. W.;
Jonassen, H. B. J. Am. Chem. Soc. 1974, 96, 6298. (c) Verstuyft, A. W.;
Nelson, J. H. Inorg. Chem. 1975, 14, 1501. (d) Redfield, D. A.; Cary, L.
W.; Nelson, J. H. Inorg. Chem. 1975, 14, 50. (e) Price, J. H.; Birk, J. P.;
Wayland, B. B. Inorg. Chem. 1978, 17, 2245. (f) van Eldik, R.; Palmer,
D. A.; Kelm, H. Inorg. Chem. 1979, 18, 572. (g) MacDougall, J. J.;
Mathey, F.; Nelson, J. H. Inorg. Chem. 1980, 19, 1400. (h) Favez, R.;
Roulet, R. Inorg. Chem. 1981, 20, 1598. (i) Yamamoto, A.; Yamamoto,
T.; Komiya, S.; Ozawa, F. Pure Appl. Chem. 1984, 56, 1621.
(j) Frankcombe, K. A.; Cavell, K. J.; Yates, B. F.; Knott, R. B.
Organometallics 1997, 16, 3199. (k) Stockland, R. A., Jr.; Anderson, G.
K. Organometallics 1998, 17, 4694. (l) Casado, A.; Espinet, P.
F
dx.doi.org/10.1021/om501007q | Organometallics XXXX, XXX, XXX−XXX