The quest for stable σ-methane complexes: Computational and experimental studies

Article abstract of DOI:10.1039/c1nj20602h

A series of cationic late transition metal pincer complexes with tridentate, neutral pincer ligands and their corresponding metal methyl complexes have been investigated by density functional theory (DFT). The key calculated quantities of interest for eac

Full text of DOI:10.1039/c1nj20602h

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PAPER
The quest for stable r-methane complexes: computational and
experimental studiesw
Marc D. Walter, Peter S. White, Cynthia K. Schauer and Maurice Brookhart*
Received (in Gainesville, FL, USA) 11th July 2011, Accepted 27th September 2011
DOI: 10.1039/c1nj20602h
A series of cationic late transition metal pincer complexes with tridentate, neutral pincer ligands
and their corresponding metal methyl complexes have been investigated by density functional
theory (DFT). The key calculated quantities of interest for each metal–ligand pair were the energy
of the metal methyl hydride relative to the metal s-methane complex and the methane
dissociation enthalpy and free energy. A few promising pincer ligand frameworks emerged as
candidates for the syntheses of s-methane complexes with enhanced thermal stability.
The calculational predictions have been tested experimentally, and new iridium and rhodium
complexes of a tridentate pincer ligand, 2,6-bis(di-tert-butylphosphinito)-3,5-diphenylpyrazine
(N-PONOP) have been prepared as well as a cationic palladium methyl complex with
2,6-bis(di-tert-butylphosphinito)pyridine (PONOP) and subjected to several protonation
experiments. Protonation of the (N-PONOP)Ir methyl complex yielded the corresponding
five-coordinate iridium(III) methyl hydride cation. Kinetic studies of the C–H bond coupling and
reductive elimination have been carried out. Line broadening NMR spectroscopic techniques have
been established a barrier of 7.9(1) kcal molꢀ1 for H–Calkyl bond coupling in the iridium(III)
methyl hydride (ꢀ100 1C). A protonation of the iridium pincer complexes at the uncoordinated
pyrazine-N atom was not achieved.
photodissociation of CO from metal carbonyl precursors such
Introduction
as M(CO)6 (M = Cr, Mo and W) in methane matrices at very
low temperature (12 K) in the 1970s,22,23 even before the more
stable dihydrogen complexes were discovered.24
Carbon–hydrogen bond activation processes are not only reactions
of fundamental importance but also have wide potential utility
for conversion of alkane feedstocks to value-added chemical
intermediates and in catalytic synthesis of fine chemicals and
pharmaceuticals.1–8 While many researchers concentrate their
efforts on the development of practical organometallic oxidation
catalysts,9–14 the fundamental aspects of the C–H bond activation
are still not fully understood. For instance, it is now generally
accepted that s-alkane complexes are key intermediates in
C–H activation reactions whether involving oxidative addition
(and the reverse reductive elimination) reaction,15,16 metathesis
processes especially via s-CAM,17 protonation of metal alkyls,18,19
simple ligand substitution processes15,20 or alkane adsorption
on metal surfaces.21 Such s-complexes have been detected and
investigated by low-temperature experiments. Organometallic
alkane complexes were observed and characterized following
Sophisticated techniques such as fast time-resolved infrared
(TRIR) spectroscopy have been developed and applied to
these highly reactive systems. A systematic study of related
metal carbonyl alkane complexes by TRIR spectroscopy
showed an increased lifetime of these s-alkane complexes on
going both across and down Groups 5–7.25 The increased
stability of this class of compounds subsequently allowed their
characterization with 1H NMR spectroscopy.25–32 Photolysis of
CpRe(CO)3 in cyclopentane at 180 K yielded the corresponding
alkane complex which was the first alkane complex to be
characterized by 1H NMR spectroscopy.26 However, while there
is strong evidence of intermolecular metal–alkane interactions in
solution, these systems have up to now defied isolation and solid
state structural characterization.33 Intermolecular metal–alkane
complexes are therefore one of the most attractive targets in the
investigation of fundamental aspects of C–H bond activation.
Furthermore, a more detailed understanding of the mechanisms
of C–H bond activation should allow better reaction control, and is
thus crucial for the advancement of this chemistry.
Department of Chemistry, University of North Carolina at Chapel
Hill, Chapel Hill, North Carolina 27599-3290, USA.
E-mail: mbrookhart@unc.edu
w Electronic supplementary information (ESI) available: Experimental
details, computational energies and coordinates, and crystallographic
data. CCDC reference numbers 834658–834662. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c1nj20602h
We have recently reported the isolation of an unusually stable
five-coordinate 16VE Ir(III) hydrido methyl cation, [(PONOP)-
Ir(H)(CH3)][B(ArF)4] {where PONOP is 2,6-(tBu2PO)2C5H3N;
c
2884 New J. Chem., 2011, 35, 2884–2893
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Results and discussion
Computational results
Modern DFT calculations are a valuable tool for synthetic
chemists in their efforts to optimize catalytic reactions and to
explain the nature of the metal–ligand interaction and reactivity
patterns.35–40 Computational models can also be used to
examine a large number of potential synthetic targets and
identify the most promising complexes for synthesis and further
study. In this vein, we set out to probe a series of tridentate,
neutral pincer ligands and their corresponding metal complexes
by DFT methods with the goals of identifying the metal and
ligand combination that is computed to yield the most stable
s-methane complex (eqn (1)).
ð1Þ
Additionally, the relative energy of the corresponding metal
methyl hydride complex relative to the s-methane complex was
calculated (eqn (2)), which is a prediction of the ground state for
a particular metal–ligand combination.
Scheme 1 Computational ligand set.
tBu is C(CH3)3; and B(ArF)4 is B[3,5-(F3C)2C6H3]4} which is
isoelectronic with the Pt(IV) methyl hydride species proposed in
the Shilov cycle.34 Additionally, rapid interchange between
the Ir–H and Ir–CH3 protons was observed and established
LM(CH4)n+ " LM(H)(Me)n+
(2)
The ligand set examined, a trimmed version of the PONOP
ligand in which t-Bu groups are replaced by Me groups, is
shown in Scheme 1. Computations as a function of metal
(M = Rh, Ir, Pd, Pt) were only carried out for the parent
ligand, I-H, and Table 1 shows the calculated enthalpies (DH1)
and free energies (DG1) of methane binding for this set. The
charge on the metal complex has a pronounced effect on the
methane binding enthalpy and free energy, with the dicationic
group 10 metal complex binding methane more strongly than
the group 9 metal complex (4.4 kcal molꢀ1 for Pd/Rh and
5.9 kcal molꢀ1 for Pt/Ir). For both group 9 and group 10 pairs,
the third row transition metal is the stronger binder of
methane (by 6 kcal molꢀ1 for Pt/Pd and 4.6 kcal molꢀ1 for
Ir/Rh pair).
reversible formation of
a s-methane complex. Density
functional theory (DFT) calculations suggested that the
[(PONOP)Ir(H)(CH3)]+ ground state lies only B5 kcal molꢀ1
below the [(PONOP)Ir(s-CH4)]+ complex.18 However, the
small ground state free energy difference between [(PONOP)-
Ir(H)(CH3)]+ and [(PONOP)Ir(s-CH4)]+ and the substantial
binding energy of the s-methane led us to investigate the Rh
analogue in which the Rh(I) oxidation state was stabilized
relative to the Rh(III) state. Indeed, protonation of [(PONOP)-
Rh(CH3)] at low temperature permitted the observation and
full characterization by NMR spectroscopy of a relatively
long-lived s-methane complex, [(PONOP)Rh(CH4)][B(ArF)4],
which is isoelectronic with the Pt(II)-s-methane complexes
postulated as intermediates in the Shilov-type oxidation of
methane.18
For Ir and Rh pincer complexes, a series structural modifi-
cations of the parent pincer were examined computationally.
In set I, a variety of substituents were introduced in the para
position on the pyridine ring ranging from the electron-
donating NMe2 group to the highly electron-withdrawing
In our synthetic approach, the s-bound alkane is generated
in the coordination sphere by protonation of the neutral
metal–alkyl complex. The cationic charge of the complex gives
rise to a significantly increased lifetime of the corresponding
s-alkane complex compared to the neutral complexes. How-
ever, even weakly coordinating solvents such as CHCl2F
bind more strongly than the corresponding alkane to the
cationic metal center, and therefore loss of bound alkane is
an irreversible process.18
+
cationic NMe3 group. In ligand II, the substituents on
phosphorus in the parent ligand were changed from CH3 to
CF3. In ligand III, the pyridine ring is replaced by a pyrazine
ring. This ligand offers the opportunity to examine the impact
Table 1 Calculated methane binding enthalpies and free energiesa,b
Intrigued by these initial results we report herein on our
computational efforts to screen a series of methane complexes
of cationic late transition metal pincer complexes supported by
various tridentate, neutral pincer ligands. Density functional
theory (DFT) methods were used to estimate the binding
energy of methane and the relative stabilities of the metal
methyl hydride complexes vs. the methane s-complexes. These
computational predictions were probed experimentally with
several selected examples.
Complex
DH1
DG1
[Rh(I-H)]+
[Pd(I-H)]2+
[Ir(I-H)]+
ꢀ17.2
ꢀ21.6
ꢀ21.8
ꢀ27.7
ꢀ7.8 (ꢀ11.8)
ꢀ12.7
ꢀ11.9 (ꢀ16.1)
[Pt(I-H)]2+
ꢀ18.1
a
b
Energies in kcal molꢀ1
(in parenthesis).
.
Free energies at 298.15 K and 173 K
c
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Table 2 Calculated methane binding enthalpies and free energiesa,b
for the oxidative addition reaction since for both Pd and Pt, the
methyl hydride complex is not a minimum on the potential
energy surface, and the methane complex is the ground state.
The structural parameters of the methane complexes as a
function of metal and ligand are given in Table 4. The
numbering scheme for the coordinated methane ligand is given
in eqn (1). In all of the complexes except for those of ligand
III-H+, a single C–H bond of the methane ligand is coordi-
nated to the metal as signified by relative values of M–H1 and
M–H2 as well as the corresponding C–H distances. For the
group 9 set, the Ir complexes, which have more favorable
methane binding free energies, have longer C–H1 bonds
(1.163–1.179 A) than the corresponding Rh complexes
(1.129–1.138 A). The electron-withdrawing para substituents
on the pyridine ring for a given metal shorten the C–H1
distance, while electron-withdrawing substituents on phosphorus
have the opposite effect, although both substituent positions
have a favorable influence on the methane binding free energy.
A similar trend is observed for the group 10 pair, with
Pt having a substantially longer C–H bond (1.164 A) than
Pd (1.133 A). Comparison of the group 9/10 pairs shows the
cationic group 10 methane complex, with a more favorable
methane binding free energy, has the shorter C–H1 bond. The
C–H1 bond length for the pyrazine-based ligand, III, is
Ir
Rh
DH1
DG1
DH1
DG1
Ligand
I-NMe2
I-H
ꢀ21.5
ꢀ21.8
ꢀ22.4
ꢀ24.8
ꢀ27.0
ꢀ22.4
ꢀ27.9
ꢀ11.5
ꢀ11.9
ꢀ12.4
ꢀ14.8
ꢀ16.4
ꢀ12.5
ꢀ18.4
ꢀ16.9
ꢀ17.2
ꢀ17.7
ꢀ19.6
ꢀ21.6
ꢀ17.8
ꢀ21.6
ꢀ7.4
ꢀ7.8
ꢀ8.4
I-CF3
I-NMe3
II
+
ꢀ12.2
ꢀ11.4
ꢀ8.8
III
III-H+
ꢀ12.2
a
b
Energies in kcal molꢀ1
.
Free Energies at 298.15 K.
on methane binding of protonating the uncoordinated nitrogen
of the pyrazine ring.
The calculated methane binding enthalpies and free energies
for the Rh and Ir complexes as a function of ligand are
reported in Table 2. For ligand set I, as expected, ligands with
the more electron-withdrawing substituent in the para position
gives rise to stronger methane binding. The methane binding
energy is only modestly impacted (B1 kcal molꢀ1) by the
range of neutral substituents examined (I-NMe2 to I-CF3),
while the introduction of a cationic substituent has the largest
impact (from 3.3 kcal molꢀ1 (Ir) to 4.8 kcal molꢀ1 (Rh) for
I-NMe2 compared to I-NMe3+). Placing more electron with-
drawing substituents on phosphorus, cis to the methane binding
site, has a favorable influence on the methane binding energy of
4.5 kcal molꢀ1 and 3.6 kcal molꢀ1 for Ir and Rh, respectively.
The pyrazine ligand, III, has methane binding energies similar
to I-CF3 for both Rh and Ir. As expected, protonation of the
pyrazine ligand increases the methane binding by 5.9 kcal molꢀ1
for Ir and 3.4 kcal molꢀ1 for Rh.
+
similar to I-NMe3 for both Rh and Ir. Interestingly, the
methane ligand adopts a symmetrical binding mode with
two C–H bonds equally coordinated in the Rh and Ir com-
plexes of the cationic ligand, III-H+. Based on the computa-
tional screening, systems modeled after the promising
candidates [Pd(I-H)]2+ and [M(III-H)]2+ (M = Rh, Ir) were
selected for synthetic studies. For the case of Ir, [Ir(III-H)]2+
was the only iridium complex computed that was predicted to
have a s-methane ground state. The experimental work on
derivatives related to these complexes is described below.
The computed enthalpies and free energies for the oxidative
addition reaction as shown in eqn (2) are reported in
Table 3. Of the group 9 pair, the Rh-methane complex is
substantially stabilized relative to the methyl hydride complex
(by 8.7–14.9 kcal molꢀ1), while for Ir, the energy difference is
smaller (ꢀ5.1 to +1.8 kcal molꢀ1). The trends in the enthalpy
and free energy for the oxidative addition reaction as a
function of ligand remarkably parallel those for methane
binding, with the oxidative addition being less favored when
electron withdrawing substituents are introduced. The Ir
complex carrying the ligand III-H+ is the only complex that is
computationally predicted to have a methane complex ground
state. The group 10 metals were not included in this comparison
New ligand system
The new neutral, tridentate pyrazine-based N-PONOP ligand
1, a derivative of III, was prepared from 2,6-dihydroxo-3,
Table 4 Structural parameters for methane complexesa,b
Metal Ligand
C–H1 C–H2 M–C M–H1 M–H2 M–N M–P
Rh
Pd
Ir
Pt
Rh
I-H
I-H
I-H
I-H
I-NMe2
I-H
I-CF3
I-NMe3
II
1.135 1.109 2.367 1.897 2.135 2.012 2.263
1.133 1.103 2.388 1.910 2.222 2.009 2.327
1.171 1.100 2.359 1.797 2.281 2.009 2.266
1.164 1.097 2.391 1.811 2.371 2.004 2.326
1.138 1.106 2.374 1.875 2.169 2.009 2.261
1.135 1.109 2.367 1.897 2.135 2.012 2.263
1.134 1.110 2.364 1.904 2.122 2.010 2.265
1.130 1.114 2.357 1.943 2.079 2.002 2.270
1.140 1.105 2.362 1.872 2.188 2.022 2.230
1.129 1.113 2.362 1.935 2.083 1.998 2.270
1.123 1.123 2.343 2.004 2.005 1.971 2.285
1.179 1.098 2.362 1.777 2.319 2.009 2.264
1.171 1.100 2.359 1.797 2.281 2.009 2.266
1.169 1.101 2.358 1.803 2.270 2.006 2.268
1.163 1.104 2.354 1.826 2.240 1.995 2.273
1.171 1.100 2.353 1.800 2.296 2.021 2.233
1.164 1.103 2.358 1.820 2.244 1.993 2.273
1.133 1.133 2.318 1.986 1.987 1.955 2.291
Table 3 Calculated oxidative addition enthalpies and free energiesa,b
+
Ir
Rh
III
DH1
DG1
DH1
DG1
Ligand
III-H+
I-NMe2
I-H
Ir
I-NMe2
I-H
ꢀ5.0
ꢀ4.9
ꢀ4.7
ꢀ2.5
ꢀ0.9
ꢀ4.1
1.4
ꢀ5.3
ꢀ5.1
ꢀ4.9
ꢀ2.6
ꢀ0.3
ꢀ4.2
1.8
8.5
8.9
9.2
11.4
12.9
10.0
14.3
8.7
9.4
9.7
13.9
13.7
10.9
14.9
I-CF3
I-NMe3
II
III
III-H+
I-CF3
I-NMe3
II
+
+
III
III-H+
a
b
Bond distances given in A. Numbering scheme given in eqn (1).
a
b
Energies in kcal molꢀ1
.
Free Energies at 298.15 K.
c
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5-diphenylpyrazine41 with di-tert-butylchlorophosphine in the
presence of excess of NEt3 at 65 1C in THF (eqn (3)) and
characterized by multinuclear NMR spectroscopy and elemental
analysis.
The solid state structure of 1-IrMe is shown in Fig. 1 with
relevant metrical parameters listed in the figure caption.z The
isostructural complex 1-RhMe is shown in ESI (Fig. S2), and
selected bond distances and angles are given in the figure
caption. The X-ray diffraction data are consistent with the
expected square planar geometry about the metal center.
Preparation of the cationic 2-PdMe+ complex
The other promising candidates for protonation identified
were the dicationic PONOP-based Pd/Pt(II) methyl complexes.
The DFT calculations (vide supra) suggested a s-methane
ground state and methane binding energies sufficient to allow
observation of the s-methane complexes at ambient temperature.
These complexes are of particular significance since they are very
closely related to Shilov Pt(II) catalytic systems for methane
conversion.42–46 Within the scope of this paper we will only
discuss investigations on 2-PdMe+. The synthesis of 2-PdMe+
has been accomplished in a straightforward manner from the
reaction of the precursor (COD)Pd(Cl)(Me) with Na[B(ArF)4]
and ligand 2 in CH2Cl2 at ꢀ40 1C (Scheme 4). Crystallization
from pentane/CH2Cl2 afforded 2-PdMe+ as pale yellow crystals,
which were characterized by multidimensional NMR spectro-
scopy, elemental analysis and a single crystal X-ray diffraction
study (Fig. 2).z
ð3Þ
Metallation of 1 with Ir and Rh was readily accomplished
by treatment with either [(C2H4)2IrCl]2 or [(COE)2RhCl]2,
affording (N-PONOP)IrCl (1-IrCl) and (N-PONOP)RhCl
(1-RhCl), respectively, in good yields (Scheme 2). Complex
1-IrCl was isolated as dark purple powder and characterized
by 1H, 13C{1H}, and 31P{1H} NMR spectroscopy and
elemental analysis. The 1H NMR spectrum of 1-IrCl is consistent
with a molecular C2v symmetry. The 31P{1H} NMR spectrum
displays a singlet at d 185.9 which is downfield shifted relative to
the free ligand 1 (d 162.8). Complex 1-RhCl was obtained as dark
red powder and characterized by multinuclear NMR spectro-
scopy. A benzene-d6 solution of 1-RhCl shows the expected
number of resonances, and the 31P{1H} NMR spectrum
displays a doublet at d 206.3 (1JRh–P = 149 Hz).
Protonation attempts
The double protonation of 1-IrMe and protonation of
2-PdMe+ was attempted at various temperatures and using
different acids and acid concentrations.
Complexes 1-IrCl and 1-RhCl were methylated using
MgMe2 in a toluene/THF solution to afford (N-PONOP)-
IrCH3 (1-IrMe) and (N-PONOP)RhCH3 (1-RhMe), respectively
(Scheme 3). Complexes 1-IrMe and 1-RhMe were characterized
Protonation of 1-IrMe. Protonation at the Ir-center with
[H(OEt2)2][B(ArF)4], was readily accomplished as previously
Scheme 2 Preparation of 1-RhCl and 1-IrCl.
by NMR spectroscopy, elemental analysis and X-ray diffraction.
The presence of a M–CH3 moiety was confirmed by the
z Crystal data for 1-IrMe: C33H49IrN2O2P2, M = 759.88, monoclinic,
a = 11.1882(3) A, b = 12.8593(3) A, c = 23.2460(6) A, a = 90.001,
b = 94.8910(10)1, g = 90.001, V = 3332.28(15) A3, T = 100(2)K,
space group P21/c, Z = 4, m(Cu-Ka) = 8.896 mmꢀ1, 22 893 reflections
measured, 6154 independent reflections (Rint = 0.0303). The final R1
values were 0.0257 (I 4 2s(I)). The final wR(F2) values were 0.0616
(I 4 2s(I)). The final R1 values were 0.0313 (all data). The final wR(F2)
values were 0.0641 (all data). The goodness of fit on F2 was 1.028.
Crystal data for 2-PdMe+: C55H56BCl2F24NO2P2Pd, M = 1469.06,
monoclinic, a = 13.7687(5) A, b = 23.5697(9) A, c = 19.7027(7) A,
a = 90.001, b = 98.591(2)1, g = 90.001, V = 6322.3(4) A3, T =
100(2)K, space group P21/c, Z = 4, m(Cu-Ka) = 4.631 mmꢀ1, 59 378
reflections measured, 11956 independent reflections (Rint = 0.0310).
The final R1 values were 0.0463 (I 4 2s(I)). The final wR(F2) values
were 0.1241 (I 4 2s(I)). The final R1 values were 0.0502 (all data). The
final wR(F2) values were 0.1273 (all data). The goodness of fit on F2
was 1.034.
observation of a 3H triplet at d 2.36 (3JP–H = 4.9 Hz) and a
=
2
3H doublet of triplets at d 0.73 (3JP–H = 4.9 Hz, JRh–H
3.1 Hz) for 1-IrMe and 1-RhMe, respectively. Additionally,
the 31P{1H} NMR spectra display a single resonance at d 192.3
for 1-IrMe and a doublet at d 208.5 (1JRh–P = 171 Hz)
for 1-RhMe. In both cases the resonances are slightly
shifted downfield relative to the starting materials 1-IrCl and
1-RhCl. It should be noted that trace impurities of MgI2 in
MgMe2 cause problems in the synthesis of 1-RhMe. In this
case, 1-RhI is formed which does not metathesis with MgMe2
to 1-RhMe (see ESI, Fig. S1, and Experimental Section for
details).
c
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Scheme 3 Preparation of 1-IrMe and 1-RhMe.
Fig. 2 ORTEP diagram of 2-PdMe+ (50% probability ellipsoids).
Selected bond distances (A): Pd1–N1 2.064(2), Pd1–C22 2.060(3),
Pd1–P1 2.2932(8), Pd1–P2 2.2896(8).
Fig. 1 ORTEP diagram of 1-IrMe (50% probability ellipsoids).
Selected bond distances (A): Ir1–N1 2.030(3), Ir1–C33 2.107(3),
Ir1–P1 2.2391(8), Ir1–P2 2.2302(8).
to the formation of [(N-PONOP)Ir(CDCl2F)]+. The rate of
methane loss was monitored by 31P{1H} NMR spectroscopy,
and yields a first-order rate constant of k = 4.3(1) ꢁ 10ꢀ5 sꢀ1 at
19 1C, corresponding to a barrier of DGz = 22.9(2) kcal molꢀ1
(see ESI for details, Fig. S3). The barrier for methane loss from
1-Ir(H)(Me)+ is very similar to the barrier for 2-Ir(H)(Me)+ of
DGz = 22.4(2) kcal molꢀ1, therefore this modification of the
pincer backbone has no pronounced effect on the complex
stability.
described for 2-IrMe. However, protonation at the p-nitrogen
of the pyrazol-backbone did not occur even in the presence of
excess acid, and above ꢀ40 1C, the excess acid induced
phosphinite (O–P) bond cleavage. An excess of HN(SO2CF3)2
gives rise to the formation of a 6-coordinate species in CD2Cl2
or CDCl2F solvent as suggested by a downfield shift of the Ir–H
resonance from d ꢀ40.9 in 1-Ir(H)Me+ to d ꢀ16.6. This
downfield shift is consistent with a sixth ligand trans to the
Ir–H axis.
Cation 1-Ir(H)(Me)+ was characterized by multinuclear,
low-temperature NMR spectroscopy. At ꢀ130 1C in CDCl2F
1
solution the H NMR spectrum of 1-Ir(H)(Me)+ displays a
Since no double protonation was achieved, the focus of
these investigations was shifted to study of the stability and
reversible C–H activation process in 1-Ir(H)(Me)+ and the
comparison to 2-Ir(H)(Me)+ and our computational predictions.
The product of methane loss was tentatively assigned as the
iridium(I) solvate cation, [(N-PONOP)Ir(CDCl2F)]+. Free
methane was detected concomitant with and roughly proportional
triplet resonance at d ꢀ40.90 (2JP–H = 13.3 Hz) assigned to the
Ir–H and a broad resonance at d 2.15 assigned to the Ir–CH3
moiety. When the temperature is increased to ꢀ90 1C, the
Ir–H and Ir–CH3 resonances exhibit significant line broad-
ening. The rate of site exchange between the Ir–H and Ir–CH3
positions was examined by line shape analysis47 of selectively
Scheme 4 Preparation of 2-PdMe+
.
c
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decoupled 1H{31P}-NMR spectra. At ꢀ100 1C in CDCl2F
solvent, a rate constant of 630(10) sꢀ1 was observed for the
Ir–H and Ir–CH3 exchange, which corresponds to a barrier of
7.9(1) kcal molꢀ1 for the interchange of the iridium-hydride
and iridium-methyl protons by reductive coupling and oxidative
addition. It is interesting to note that the new pincer backbone
slightly stabilizes 1-Ir(H)(Me)+ with respect to methane elimination
while the barrier to Ir–H and Ir–CH3 exchange is lower (DGz =
7.9(1) kcal molꢀ1 vs. 9.3(4) kcal molꢀ1).
of a second proton to the uncoordinated nitrogen was not
observed before ligand degradation via P–O bond cleavage.
For 2-PdMe+, the same acids used to protonate the group
9 analogues were not sufficiently strong to protonate the group
10 complex, Protonation of 2-PdMe+ was achieved under
superacid conditions, but the methane complex was not stable
in this solvent system. These experimental results illustrate the
limitations of a protonation strategy to yield complexes of a
weak ligand like methane. Metals sufficiently electrophilic to
bind methane strongly will make the methyl group less subject
to protonation, thereby requiring superacids. A more electro-
philic metal complex can also be generated by protonation of a
coordinated ligand, but the site of protonation must be the
most basic site in the molecule after protonation at metal to
avoid undesired site reactions. Our quest continues for a
methane complex stable at room temperature.
The experimental result is consistent with a lower lying
1-Ir(s-CH4)+ state relative to 2-Ir(s-CH4)+ as initially
predicted by DFT calculations (DDG1 = 0.9 kcal molꢀ1).
The computed barriers to exchange are 7.2 kcal molꢀ1 for
[(III)Ir(H)(Me)]+ and 7.4 kcal molꢀ1 for [(I-H)Ir(H)(Me)]+,
and therefore of similar magnitude to the experimental result
for the untrimmed ligand. For the iridium complexes, since the
ground state is the methyl hydride, a computed barrier to
methane loss can be estimated by summing the energy difference
between the [(L)Ir(H)(Me)]+ and [(L)Ir(s-CH4)]+ with an
appropriately weighted entropic contribution to the free energy
of methane loss from [(L)Ir(s-CH4)]+ (see computational
methods section). Based on the assumption that 40% of the
reaction entropy is realized at the transition state for methane
loss, the computed barriers for methane loss are 20.8 for
[Ir(I-H)(H)(Me)]+ and 20.6 kcal molꢀ1 for [Ir(III)(H)(Me)]+,
which are in reasonable agreement with the experimental results
for the untrimmed ligands.
Experimental details
General considerations
All reactions, unless otherwise stated, were conducted under
an atmosphere of dry, oxygen free argon using standard high-
vacuum, Schlenk, or drybox techniques. Argon was purified
by passage through BASF R3-11 catalyst (Chemalog) and 4 A
molecular sieves. 1H, 13C{1H}, 1H–1H COSY, 1H–13C HSQC,
1H–1H NOESY, 1H–1H TOCSY and 13C DEPT135 NMR
spectra were recorded on a Bruker DRX 500 MHz, a Bruker
DRX 400 MHz, or a Bruker 400 MHz AVANCE spectro-
Protonation of 2-PdMe+. No protonation of 2-PdMe+
occurred with an excess of [H(OEt2)2][B(ArF)4] or
HN(SO2CF3)2 in CD2Cl2 or CDCl2F solvent at ꢀ80 1C. More
importantly, no significant protonation of 2-PdMe+ was
observed on warming of the sample to ambient temperature
or after storing the reaction mixture for several days at room
temperature. We then resorted to the superacidic conditions,
i.e. magic acid (HSO3F/SbF5, 1 : 1 ratio) with H0 = ꢀ2048 as
the proton source. Cation 2-PdMe+ was protonated under
these conditions at ꢀ80 1C in triflic anhydride, (CF3SO2)2O,
solvent. Unfortunately, a solvent coordinated species was
formed, and no dicationic 2-Pd(HMe)2+ complex was observed.
This result suggests that the stability of the methane complex
may have been overestimated by DFT calculations. Further, it
demonstrates that significantly stronger acids are required to
protonate a monocationic methyl species in comparison to a
neutral species.
1
meter. H and 13C Chemical shifts are referenced relative to
residual CHCl3 (d 7.24 for 1H), CH(D)Cl2 (d 5.32 for 1H),
CHCl2F (d 7.47 for 1H), C6HD5 (d 7.15 for 1H), 13CD2Cl2
(d 53.8 for 13C), 13CDCl3(d 77.0 for 13C), 13CDCl2F (d 104.2
for 13C) and 13C6D6 (d 128.0 for 13C); 31P chemical shifts are
referenced to an external standard of H3PO4. Probe tempera-
tures were calibrated using ethylene glycol and methanol as
previously described.49 Due to strong 31P–31P coupling in the
1
pincer ligand, many H and 13C{1H} NMR signals appear as
virtual triplets (vt) and are reported as such with the apparent
coupling noted. Elemental analyses were carried out by
Robertson Microlit Laboratories of Madison, NJ.
Materials
All solvents were deoxygenated and dried by passage over
columns of activated alumina.50,51 CD2Cl2, purchased from
Cambridge Laboratories, Inc., was dried over CaH2, vacuum
transferred to a Teflon sealable Schlenk flask containing 4 A
molecular sieves, and degassed via three freeze-pump-thaw
cycles. Freon (CDCl2F) was prepared according to a literature
procedure and stored over activated 4 A molecular sieves at
ꢀ25 1C.52 IrCl3 and RhCl3 were purchased from J&J Materials
or obtained from W. C. Heraeus GmbH and used as received.
[H(OEt2)2][B(ArF)4],53 Na[B(ArF)4],54 [(COE)2MCl]2 (M =
Ir,55 Rh56), [(COD)Pd(Me)(Cl)],57 MgMe2,58 2,6-Dihydroxy-
3,5-diphenylpyrazine,41 2,6-(tBu2PO)2C5H3N (PONOP),34
were synthesized according to literature methods. All other
reagents were purchased from Aldrich, Acros, Alphar Aesar
or Strem Chemicals and used as received.
Conclusions
Computational studies were used to identify pincer ligands
and metal complexes expected to yield more stable s-methane
complexes. Among the ligands explored computationally, a
pyrazine-based pincer ligand III was predicted to give rise to a
more stable s-methane complex upon protonation of the
uncoordinated nitrogen. Further, in the parent pincer ligand
system, I, dicationic group 10 analogues were predicted to form
more stable s-methane complexes than the group 9 analogues.
Experimental difficulties in realizing the computational predictions
were encountered. In the metal complexes of the pyrazine-based
ligand, 1-IrMe, monoprotation occurs at the metal, but addition
c
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New J. Chem., 2011, 35, 2884–2893 2889

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Computational details
A table of calculated electronic energies, enthalpies, and free
energies in the gas phase for all ground states and transition
state calculated, and tables of Cartesian coordinates (A) for
the optimized structures and transition states in the gas phase
are included as ESI.
All density functional theory (DFT) calculations were
performed by using the Gaussian 03 package.59 The basis-set/
functional selection was based on a prior study of methane
binding,18 and consists of the built-in 6-31G** basis set for
all non-transition metal atoms, the Stuttgart-Dresden basis
set-pseudo relativistic effective core potential combination for
the transition metals60,61 with a single f-type polarization function
for Rh and Ir (exponent = 1.062 (Rh); 0.685 (Ir)), which are the
geometric average of the two f exponents given in the appendix of
reference 61, and the functional PBE0, the hybrid variant of PBE
that contains 25% Hartree–Fock exchange62 for geometry
optimizations. The PBE0 functional was found to yield results
in better agreement with experimental data than the B3LYP
functional in an Ir pincer system,63 and has been endorsed as
one of the best performing functionals for late transition metal
systems.64 A similar basis set combined with the PBE0 functional
was used to calculate weak RhꢂꢂꢂH–C interactions in another
system,65 and in our recent study of methane binding energies.18
For each metal–ligand combination, geometries were optimized
in the gas phase for [(L)M]n+, [(L)M(Me)(H)]n+ and the
agostic complexes [(L)M(s-CH4)]n+. Frequency calculations
were carried out on all minimum structures, and the resulting
frequencies all had positive values. The non-scaled vibrational
frequencies formed the basis for the calculation of vibrational zero
point corrections and the standard thermodynamic corrections for
the conversion of electronic energies to enthalpies and free energies
at 298.15 K and 1 atm.
Preparation of N-PONOP (1)
2,6-Dihydroxo-3,5-diphenylpyrazine (1.2 g, 4.54 mmol),
di-tert-butylchlorophosphine (1.64 g, 9.1 mmol), triethylamine
(5 mL), and THF (10 mL) were combined in a Schlenk flask. A
colorless precipitate immediately formed and the reaction
mixture was heated to 65 1C for 2 h. The volatiles were
removed under dynamic vacuum, leaving a yellow solid. The
solid was extracted into toluene and filtered. The solvent was
removed under dynamic vacuum, leaving a bright yellow
crystalline solid. Yield: 1.52 g (3.32 mmol, 73%). 1H NMR
3
(C6D6, RT): d 8.33 (d, 2H, JHH = 8.4 Hz, o-C6H5), 7.33
For [(I-H)Ir(s-CH4)]+ and [(III)Ir(s-CH4)]+, the transition
state for the oxidative addition reactions were optimized in the gas
phase using the Synchronous Transit-Guided Quasi-Newton
(STQN) method implemented in Gaussian. Frequency calculations
yielded one imaginary frequency for all transition states, and IRC
calculations were carried out to confirm that the transition state
identified connected the correct minima.
(d, 4H, 3JHH = 7.8 Hz, m-C6H5), 7.20 (t, 2H, 3JHH = 7.6 Hz,
3
p-C6H5), 1.15 (d, 36 H, JP–H = 12.0 Hz, C(CH3)3). 31P{1H}
NMR (C6D6, RT): d 162.8 (s). 13C{1H} NMR (C6D6, RT):
2
d 155.3 (d, JCP = 9.0 Hz, C3), 135.7 (s, C4), 129.7 (s, C6),
128.34 (s, C8), 128.3 (s, C7), 35.8 (d, 1JCP = 30.4 Hz, C2), 27.9
2
(d, JCP = 17.6 Hz, C2). Anal. Calcd for C24H46P2O2N2: C,
63.15; H, 10.15; N, 6.14. Found: C, 63.05; H, 10.02; N, 5.99.
Calculation of the barrier for methane loss at room temperature
is not directly obtained from the calculations. For dissociation of a
neutral ligand from a transition metal complex that does not
rearrange following dissociation like the complexes calculated in
this manuscript, assuming the recoordination of methane is
enthalpically barrierless, the methane dissociation enthalpy
(DH0) is an upper limit to the DGz for methane dissociation
(Fig. 3). The actual DGz for methane release will be determined
by the degree to which the favorable entropy of methane
dissociation is reflected in the transition state.66 For comparison
to experimental data for [(I-H)Ir(s-CH4)]+ and [(III)Ir(s-CH4)]+,
it is assumed that 40% of the reaction entropy is realized in the
transition state as calculated for k3-TpPt(Me)(CH4).66
Preparation of (N-PONOP)IrCl (1-IrCl)
A Schlenk tube was charged with [(COE)2IrCl]2 (0.4 g,
0.45 mmol) and diethyl ether (10 mL). On a Schlenk line,
the orange solution was stirred under 1 atm of C2H4 at ꢀ40 1C
until the solution became very pale yellow. Then a solution of
N-PONOP (0.25 g, 0.95 mmol) dissolved in Et2O was added
via cannula to the reaction mixture, and immediate color
change to dark purple was observed. The reaction vessel was
slowly warmed to ambient temperature, and the volatiles were
removed under dynamic vacuum. The resulting deep purple
powder was extracted with toluene, fitered, and the solvent
removed. The resulting red-purple powder was washed with
cold pentane (10 mL) yielding 0.45 g (0.65 mmol, 73%) of
1
3
1-IrCl. H NMR (C6D6, RT): d 8.27 (d, 2H, JHH = 8.3 Hz,
3
o-C6H5), 7.31 (t, 4H, JHH = 7.7 Hz, m-C6H5), 7.16 (t, 2H,
3JHH = 7.3 Hz, p-C6H5), 1.42 (vt, 36 H, JP–H = 7.3 Hz,
C(CH3)3). 31P{1H} NMR (C6D6, RT): d 185.9 (s). 13C{1H}
NMR (C6D6, RT): d 159.1 (vt, JCP = 4.2 Hz, C3), 135.7
(s, C5), 135.7 (vt, JCP = 1.7 Hz, C4), 129.2 (s, C6), 128.4 (s, C8),
2
128.3 (s, C7), 41.2 (vt, JCP = 8.4 Hz, C2), 27.9 (vt, JCP
=
3.7 Hz, C2). Anal. Calcd for C24H46P2O2N2IrCl: C, 42.13; H,
Fig. 3 Reaction coordinate diagram for methane dissociation. The
marked quantities are for the forward methane dissociation reaction.
6.78; N, 4.09. Found: C, 42.02; H, 6.65; N, 4.15.
c
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1
Preparation of (N-PONOP)RhCl (1-RhCl)
31P{1H} NMR (C6D6, RT): d 208.5 (d, JRh–P = 171 Hz).
13C{1H} NMR (C6D6, RT): d 155.1 (vt, JP–C = 4.3 Hz, C3),
136.8 (s, C5), 132.6 (s, C4), 128.8 (s, C6), 128.4 (s, C8), 128.2
(s, C7), 41.2 (dvt, JP–C = 3.3 Hz, JRh–C = 3.5 Hz, C2),
A 50 mL Schlenk tube was charged with N-PONOP (0.35 g,
1.32 mmol), [(COE)2RhCl]2 (0.47 g, 0.65 mmol) and 20 mL of
toluene. The dark red solution was stirred at ambient
temperature for 24 h, volatiles were removed in vacuo, and
the dark red powder was washed with cold pentane affording
0.52 g (0.874 mmol, 66%) of 1-RhCl. 1H NMR (CD2Cl2, RT):
2
27.9 (vt, JCP = 4.7 Hz, C2). Anal. Calcd for C25H49P2O2-
N2Rh: C, 52.26; H, 8.60; N, 4.88. Found: C, 52.45; H, 8.45;
N, 4.62.
3
3
d 8.11 (d, 2H, JHH = 7.5 Hz, o-C6H5), 7.50 (t, 4H, JHH
=
3
7.8 Hz, m-C6H5), 7.42 (t, 2H, JHH = 7.3 Hz, p-C6H5), 1.49
(vt, 36 H, JP–H = 7.3 Hz, C(CH3)3). 31P{1H} NMR (CD2Cl2,
Preparation of (N-PONOP)RhI (1-RhI)
Under an argon atmosphere a heavy walled glass reaction
vessel was charged with 1-RhCl (0.10 g, 0.17 mmol), MgI2
(0.047 g, 0.17 mmol) and THF solvent (ca. 2 mL). The reaction
mixture was heated to 65 1C for 2 d. Volatiles were removed
under dynamic vacuum, and the residue was extracted with
toluene. The deep red toluene extracts were concentrated, a
layer of pentane added and cooled to ꢀ35 1C. Dark red-brown
blocks of 1-RhI formed within a few days. Yield: 0.105 g
(0.158 mmol, 93%). 1H NMR (C6D6, RT): d 8.14 (d, 2H,
1
RT): d 206.3 (d, JRh–P = 149 Hz). 13C{1H} NMR (CD2Cl2,
RT): d 157.5 (vt, JP–C = 4.2 Hz, C3), 136.7 (s, C5), 135.7
(vt, JCP = 1.5 Hz, C4), 129.1 (s, C8), 128.9 (s, C7), 128.5 (s, C6),
41.2 (dvt, JP–C = 4.1 Hz, JRh–C = 2.1 Hz, C2), 27.9
(vt, 2JCP = 4.2 Hz, C2). Anal. Calcd for C24H46P2O2N2RhCl:
C, 48.45; H, 7.79; N, 4.71. Found: C, 48.53; H, 7.50; N, 4.65.
Preparation of (N-PONOP)IrCH3 (1-IrMe)
3JHH = 7.7 Hz, o-C6H5), 7.51 (t, 4H, JHH = 7.6 Hz,
3
Under an argon atmosphere a heavy walled glass reaction
vessel was charged with 1-IrMe (0.25 g, 0.37 mmol) and
Mg(CH3)2 (0.020 g, 0.37 mmol). A solvent mixture of THF
(ca. 0.1 mL) and toluene (ca. 4 mL) was added via cannula.
The reaction mixture was heated to 130 1C for 3 days and a
color change to purple was observed. Volatiles were removed
under dynamic vacuum, and the residue was extracted with
toluene. The deep purple toluene extracts were concentrated, a
layer of pentane added and cooled to ꢀ35 1C. Dark purple
blocks formed within a few days. Yield: 0.12 (0.18 mmol,
3
m-C6H5), 7.43 (t, 2H, JHH = 7.3 Hz, p-C6H5), 1.52 (vt, 36 H,
JP–H = 7.2 Hz, C(CH3)3). 31P{1H} NMR (C6D6, RT): d 215.4
(d, JRh–P = 140 Hz). 13C{1H} NMR (C6D6, RT): d 156.7
(vt, JP–C = 4.2 Hz, C3), 136.0 (s, C5), 133.5 (vt, JCP = 1.7 Hz,
1
C4), 129.2 (s, C8), 128.6 (s, C7), 128.3 (s, C6), 40.9 (dvt, JP–C
=
2
4.4 Hz, JRh–C = 2.3 Hz, C2), 27.9 (vt, JCP = 3.9 Hz, C2).
Anal. Calcd for C24H46P2O2N2RhI: C, 42.0; H, 6.75; N, 4.08.
Found: C, 42.12; H, 6.85; N, 3.95.
1
3
49%). H NMR (C6D6, RT): d 8.42 (d, 2H, JHH = 7.7 Hz,
o-C6H5), 7.36 (t, 4H, JHH = 7.8 Hz, m-C6H5), 7.18 (t, 2H,
Observation of [(N-PONOP)Ir(H)(CH3)][B(ArF)4]
(1-Ir(H)(Me)+)
3
3JHH = 7.3 Hz, p-C6H5), 2.36 (t, 3H, JP–H = 4.9 Hz,
2
Ir–CH3), 1.36 (vt, 36 H, JP–H = 7.0 Hz, C(CH3)3). 31P{1H}
NMR (C6D6, RT): d 192.3 (s). 13C{1H} NMR (CD2Cl2, RT):
A screw cap NMR tube was charged with [H(OEt2)2][B(ArF)4]
(0.027 g, 0.027 mmol) and 1-IrMe (0.017 g, 0.026 mmol). At
77 K approximately 500 mL CDCl2F was transferred onto the
tube via a cannula. The NMR tube was transferred into a
cooling bath which was precooled to ꢀ100 1C. After the
sample thawed, the screw cap was removed and the sample
was mixed under a flow of argon with glass rod until a
homogeneous solution formed. The NMR tube was capped
again and cooled to 77 K. The sample was maintained at 77 K
until inserted into the precooled NMR probe at ꢀ143 1C.
After the sample thawed, 1-Ir(H)(Me)+ was monitored by
multinuclear NMR spectroscopy. A typical experiment yielded
100% conversion of 1-IrMe to 1-Ir(H)(Me)+. 1H NMR
d 156.5 (vt, JP–C = 4.0 Hz, C3), 137.2 (s, C5), 134.5 (vt, JCP
1.7 Hz, C4), 129.1 (s, C6), 128.6 (s, C8), 128.3 (s, C7), 40.6
=
2
(vt, JP–C = 8.4 Hz, C2), 27.8 (vt, JCP = 4.8 Hz, C2),
2
ꢀ22.9 (t, JPC
= 6.6 Hz, Ir–CH3). Anal. Calcd for
C25H49P2O2N2Ir: C, 45.23; H, 7.44; N, 4.22. Found: C,
45.16; H, 7.35; N, 4.15.
Preparation of (N-PONOP)RhCH3 (1-RhMe)
Under an argon atmosphere a heavy walled glass reaction
vessel was charged with 1-RhCl (0.25 g, 0.4 mmol), Mg(CH3)2
(0.022 g, 0.4 mmol) and THF solvent (ca. 2 mL). The reaction
mixture was heated to 65 1C for 2 d. The conversion was
monitored by 31P{1H} NMR spectroscopy, and it was found
that the reaction time was strongly dependent on the amount
of solvents. With more concentrated solutions faster conver-
sion was observed. Volatiles were removed under dynamic
vacuum, and the residue was extracted with toluene. The deep
red toluene extracts were concentrated, a layer of pentane
added and cooled to ꢀ35 1C. Dark red-brown blocks of
1-RhMe formed within a few days. Yield: 0.13 g (0.28 mmol,
3
(CDCl2F, ꢀ130 1C): d 8.16 (d, 2H, JHH = 7.5 Hz, o-C6H5),
3
7.80 (s, 8H, o-Ar, B(ArF)4), 7.61 (t, 4H, JHH = 7.2 Hz,
m-C6H5), 7.55 (p-C6H5, overlapped with p-Ar B(ArF)4
resonance), 2.15 (br.s, 3H, CH3), 1.37 (br.s., 36H, C(CH3)3),
2
ꢀ40.9 (t, 1H, JP–H = 13.3 Hz, Ir–H). 1H NMR (CDCl2F,
3
ꢀ30 1C): d 8.21 (d, 1H, JHH = 8.2 Hz, o-C6H5), 7.61 (t, 1H,
3JHH = 7.5 Hz, m-C6H5), 7.58 (p-C6H5, overlapped with
p-Ar B(ArF)4 resonance), 1.42 (vt, 36H, J = 8.1 Hz,
C(CH3)3), no Ir–H and Ir–CH3 resonances detected. 31P{1H}
NMR (CDCl2F, ꢀ110 1C): d 192.7. 13C{1H} NMR (CDCl2F,
ꢀ120 1C): d 162.2 (q, 37 Hz, ipso-Ar, B(ArF)4), 153.5 (br.s, C3),
136.3 (br. s, C4), 135.2 (o-Ar, B(ArF)4), 133.9 (s, C5), 130.9 (s,
C8), 129.4 (s, C6), 129.3 (q, 31 Hz, m-Ar, B(ArF)4), 128.8 (s, C7),
125.0 (q, 273 Hz, CF3, B(ArF)4), 117.9 (p-Ar, B(ArF)4),
1
3
57%). H NMR (C6D6, RT): d 8.46 (d, 2H, JHH = 7.8 Hz,
o-C6H5), 7.36 (t, 4H, JHH = 7.4 Hz, m-C6H5), 7.19 (t, 2H,
3
3JHH = 7.4 Hz, p-C6H5), 1.35 (vt, 36 H, JP–H = 6.7 Hz, C(CH3)3),
2
0.73 (dt, 3H, JRh–H = 3.1 Hz, JP–H = 4.9 Hz, Rh–CH3).
3
c
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Under an argon atmosphere a Schlenk tube was charged with
2 (0.2 g, 0.5 mmol), Na[B(ArF)4] (0.44 g, 0.5 mmol)
and [(COD)Pd(Me)(Cl)] (0.132 g, 0.5 mmol) and cooled to
ꢀ78 1C. To this precooled CH2Cl2 (ca. 15 mL) was added via
cannula to form a yellow suspension. The reaction mixture
was stirred at ꢀ78 1C for 10 min, and then slowly warmed to
room temperature. The yellow solution was filtered, concen-
trated and a layer of pentane added. Slow cooling to ꢀ30 1C
yielded pale yellow crystals of 2-PdMe+, which incorporate
one molecule of CH2Cl2. Yield: 0.44 g (0.30 mmol, 60%).
3
1H NMR (CD2Cl2, RT): d 7.82 (t, JHH = 8.0 Hz, 1H,
p-C6H3N), 7.80 (s, 8H, o-Ar, B(ArF)4), 7.56 (s, 4H, p-Ar,
3
B(ArF)4), 6.91 (d, JHH = 8.3 Hz, 2H, m-C6H3N), 1.36
3
(vt, 36H, JP–H = 8.1 Hz, C(CH3)3), 0.87 (t, JPH = 4.8 Hz,
3H, Pd-CH3). 31P{1H} NMR (CD2Cl2, RT): d 191.0 (s).
13C{1H} NMR (CD2Cl2, RT): d 162.2 (q, 37 Hz, ipso-Ar,
B(ArF)4), 161.0 (vt, JPC = 1.8 Hz, C3), 145.4 (s, C5), 135.2
(o-Ar, B(ArF)4), 129.3 (q, 31 Hz, m-Ar, B(ArF)4), 125.0
(q, 273 Hz, CF3, B(ArF)4), 117.9 (p-Ar, B(ArF)4), 103.7 (s, C4),
40.7 (vt, JPC = 5.7 Hz, C2), 27.3 (vt, JPC = 3.8 Hz, C1), ꢀ22.9
(t, 2JPC
=
6.6 Hz, Pd-CH3). Anal. Calcd for
C54H54BF24NO2P2Pd*CH2Cl2: C, 44.97; H, 3.84; N, 0.95.
Found: C, 44.65; H, 3.93; N, 0.88.
Acknowledgements
We thank the National Science Foundation (CHE 1010170)
for support, the Alexander-von-Humboldt Foundation for a
Feodor-Lynen-Fellowship (MDW), and Dr David L. Harris
for his help with NMR spectroscopy.
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