Synthesis, properties, and reactivity of palladium and nickel NHC complexes supported by combinations of allyl, cyclopentadienyl, and indenyl ligands

Article abstract of DOI:10.1021/om4002632

The synthesis of a series of Pd and Ni complexes containing combinations of 2-methylallyl (C₄H₇), cyclopentadienyl (C ₅H₅, Cp), and indenyl (C₇H₉, Ind) ligands is reported. In all cases these complexes are supported by the electron-donating N-heterocyclic carbene ligand 1,3-bis(2,6-diisopropylphenyl)- 1,3-dihydro-2H-imidazol-2-ylidene (IPr). The mixed Cp/2-methylallyl complexes (η¹-Cp)(η³-2-methylallyl)Pd(IPr) ( ^CpAllPd) and (η⁵-Cp)(η¹-2-methylallyl) Ni(IPr) (^CpAllNi) were synthesized through the reaction of IPr with (Cp)(2-methylallyl)M (M = Ni, Pd). The binding mode of the ligands is different in the two complexes, and as a result the total valence electron count around the metal is 18 for the Ni complex and only 16 for the Pd species. In the case of Pd, an analogue of ^CpAllPd containing an indenyl ligand, (η¹-Ind)(η³-2-methylallyl)Pd(IPr) ( ^IndAllPd), was synthesized through the reaction of (η³-Ind)Pd(IPr)Cl (^IndClPd) with (2-methylallyl) magnesium chloride. The corresponding Ni complex (η⁵-Ind) (η¹-2-methylallyl)Ni(IPr) (^IndAllNi) could not be isolated. The binding modes of the ligands in the mixed indenyl/Cp complexes (η¹-Ind)(η⁵-Cp)M(IPr) (M = Ni ( ^IndCpNi), Pd (^IndCpPd)) were the same for both Ni and Pd. ^IndCpPd was prepared through the reaction of ^IndClPd with NaCp, while ^IndCpNi was synthesized through the reaction of (η⁵-Cp)Ni(IPr)Cl (^CpClNi) with lithium indenyl. Similarly, the structures of the bis(Cp) complexes (η⁵-Cp) (η¹-Cp)Ni(IPr) (^CpCpNi) and (η⁵-Cp) (η¹-Cp)Pd(IPr) (^CpCpPd) were identical for the two different metals. In contrast to ^CpCpPd, which is an 18-electron complex, the related bis(indenyl) Pd complex (η³-Ind) (η¹-Ind)Pd(IPr) (^IndIndPd) is a 16-electron species, while no Ni analogue of ^IndIndPd was characterized. Preliminary reactivity studies with electrophiles indicate that, in all systems with mixed ligands, the η¹-ligand is nucleophilic and reacts selectively. The complexes ^CpAllPd, ^CpAllNi, ^CpCpPd, ^CpCpNi, ^IndClPd, ^IndAllPd, and ^IndIndPd were characterized by X-ray crystallography.

Full text of DOI:10.1021/om4002632

Article
pubs.acs.org/Organometallics
Synthesis, Properties, and Reactivity of Palladium and Nickel NHC
Complexes Supported by Combinations of Allyl, Cyclopentadienyl,
and Indenyl Ligands
Elizabeth A. Bielinski, Wei Dai, Louise M. Guard, Nilay Hazari,* and Michael K. Takase
The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut, 06520, United States
S
* Supporting Information
ABSTRACT: The synthesis of a series of Pd and Ni complexes containing
combinations of 2-methylallyl (C4H7), cyclopentadienyl (C5H5, Cp), and indenyl
(C7H9, Ind) ligands is reported. In all cases these complexes are supported by the
electron-donating N-heterocyclic carbene ligand 1,3-bis(2,6-diisopropylphenyl)-1,3-
dihydro-2H-imidazol-2-ylidene (IPr). The mixed Cp/2-methylallyl complexes (η1-
Cp)(η3-2-methylallyl)Pd(IPr) (CpAllPd) and (η5-Cp)(η1-2-methylallyl)Ni(IPr)
(
CpAllNi) were synthesized through the reaction of IPr with (Cp)(2-methylallyl)M
(M = Ni, Pd). The binding mode of the ligands is different in the two complexes,
and as a result the total valence electron count around the metal is 18 for the Ni
complex and only 16 for the Pd species. In the case of Pd, an analogue of CpAllPd
containing an indenyl ligand, (η1-Ind)(η3-2-methylallyl)Pd(IPr) (IndAllPd), was
synthesized through the reaction of (η3-Ind)Pd(IPr)Cl (IndClPd) with (2-
methylallyl)magnesium chloride. The corresponding Ni complex (η5-Ind)(η1-2-
methylallyl)Ni(IPr) (IndAllNi) could not be isolated. The binding modes of the ligands in the mixed indenyl/Cp complexes (η1-
Ind)(η5-Cp)M(IPr) (M = Ni (IndCpNi), Pd (IndCpPd)) were the same for both Ni and Pd. IndCpPd was prepared through the
reaction of IndClPd with NaCp, while IndCpNi was synthesized through the reaction of (η5-Cp)Ni(IPr)Cl (CpClNi) with lithium
indenyl. Similarly, the structures of the bis(Cp) complexes (η5-Cp)(η1-Cp)Ni(IPr) (CpCpNi) and (η5-Cp)(η1-Cp)Pd(IPr)
(
CpCpPd) were identical for the two different metals. In contrast to CpCpPd, which is an 18-electron complex, the related
bis(indenyl) Pd complex (η3-Ind)(η1-Ind)Pd(IPr) (IndIndPd) is a 16-electron species, while no Ni analogue of IndIndPd was
characterized. Preliminary reactivity studies with electrophiles indicate that, in all systems with mixed ligands, the η1-ligand is
nucleophilic and reacts selectively. The complexes CpAllPd, CpAllNi, CpCpPd, CpCpNi, IndClPd, IndAllPd, and IndIndPd were
characterized by X-ray crystallography.
several other catalytic reactions involving attack at the η3-allyl
INTRODUCTION
■
ligand by a nucleophile.9
Group 10 metal complexes containing allyl ligands play an
important role in metal-mediated organic synthesis as catalysts1
and precursors2 for a variety of bond-forming reactions.
Additionally, they are interesting on a fundamental level, as
the bonding and subsequent reactivity of complexes with allyl
ligands is heavily dependent on the identity of the metal center
and the nature of the ancillary ligands.3 In general, allyl ligands
bind to transition metals in one of two different coordination
modes; η1-allyls (Figure 1a) are nucleophilic and have been
invoked as a key intermediates in many reactions with
electrophiles4 such as CO2,5 aldehydes,6 and imines.6b,7
Conversely, η3-allyls (Figure 1b) are electrophilic and are
crucial intermediates in the Tsuji−Trost reaction,8 as well as
We have previously synthesized highly unstable complexes of
the type (η3-allyl)(η1-allyl)M(L) (M = Ni, Pd; L = PR3, IPr; IPr
= 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-
ylidene).5a,b These complexes react readily with CO2 to give
carboxylated products in which one molecule of CO2 inserts
into the nucleophilic M−η1-allyl bond (eq 1). In the case of Pd
the bis(allyl) species are highly active catalysts for the
carboxylation of allylstannanes5a,10 and allylboranes;10a how-
ever, we believe their poor thermal stability limits the substrate
scope. Mechanistic studies suggest that the η3-allyl ligand is
Figure 1. Depictions of the two most common binding modes of allyl
ligands: (a) η1-allyl; (b) η3-allyl.
Received: March 28, 2013
© XXXX American Chemical Society
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Figure 2. Ni and Pd complexes with combinations of allyl, Cp, and indenyl ligands prepared as part of this work.
Scheme 1
simply a spectator and only the η1-allyl ligand is required to
facilitate catalysis.5a,b Related work by Wendt and co-workers
using Pd−η1-allyl complexes supported by tridentate pincer
ligands is consistent with this proposal.5c However, our
attempts to use thermally stable Ni−η1-allyl complexes
supported by pincer ligands11 or PdI dimers with bridging
allyl ligands12 for catalytic carboxylation did not improve
catalyst efficiency. Therefore, we wished to explore alternative
ligand frameworks to support thermally stable Pd and Ni
complexes containing η1-allyl ligands.
thermally stable η1-allyl ligands, we were interested in further
exploring the synthesis and reactivity of Ni and Pd complexes
with both Cp and allyl ligands. Ideally these complexes would
be supported by a strongly donating ligand such as IPr,19 as this
was the ancillary ligand for our most active bis(allyl)Pd systems
for catalytic carboxylation.5a In addition, given the close
relationship between Cp and indenyl ligands,15 we wanted to
prepare Pd and Ni complexes with indenyl and allyl ligands
supported by IPr. Here, we report the synthesis, character-
ization, and preliminary reactivity of the series of IPr-supported
Ni and Pd complexes shown in Figure 2, which have
combinations of allyl, Cp, and indenyl ligands. The structures
and reactivity of these systems have been compared to those of
complexes of the type (η3-allyl)(η1-allyl)M(IPr) (M = Ni, Pd)
that we recently studied.5a,b
It is widely accepted that the allyl ligand is closely related to
both the cyclopentadienyl (Cp) and indenyl ligands.13
However, despite the close similarities between Cp and indenyl
ligands, they do not always bind in the same mode to a
transition metal.14 This is primarily due to the “indenyl effect”,
which stabilizes η3-binding of indenyl ligands, and as a
consequence they are far more likely to bind in an η3-fashion
than Cp ligands, which generally bind in an η1- or η5-manner.
Currently, a limited number of Pd and Ni complexes containing
combinations of allyl and Cp ligands have been prepared.15 For
example, as part of their studies toward the synthesis of PdI
dimers with bridging Cp and allyl ligands,16 Werner and co-
workers prepared the monomeric complex (η5-Cp)(η1-2-
chloroallyl)Pd{P(O-o-tolyl)3}, as well as related species with
different phosphine and allyl ligands.15e In particular, they
noted that (η5-Cp)(η1-2-chloroallyl)Pd{P(O-o-tolyl)3} was
stable at temperatures up to 50 °C. Furthermore, Werner
found that the related complexes (η5-Cp)(η1-2-methylallyl)Pd-
(PR3) (R = PiPr3, PCy3, PtBuPh2)15a showed unusual fluxional
behavior in solution and at −60 °C the (η5-Cp)(η1-2-
methylallyl)Pd(PR3) isomer could also be observed. This
species is a relatively rare example of a Pd complex with an η1-
Cp ligand.15b,17 At this stage there is only one example of a Pd
complex containing both an indenyl and allyl ligand,18 while to
the best of our knowledge Ni species containing mixed Cp and
allyl or indenyl and allyl ligand sets are unknown.
RESULTS AND DISCUSSION
■
Synthesis and Structure of Ni and Pd Complexes
Supported by both Cyclopentadienyl and 2-Methylallyl
Ligands. Previously, we have demonstrated that the reaction of
IPr with the homoleptic Ni and Pd complexes (allyl)2M (M =
Ni, Pd) generates (allyl)2M(IPr) in high yield.5a Through the
reaction of the previously prepared mixed-ligand species
(Cp)(2-methylallyl)Pd5b with IPr at −35 °C, we were able to
synthesize (η1-Cp)(η3-2-methylallyl)Pd(IPr) (CpAllPd) in mod-
erate yield (Scheme 1a). Synthetic attempts at temperatures
greater than −35 °C gave mixtures of products, and significant
decomposition was observed if a toluene solution of CpAllPd was
left at room temperature for longer than 48 h. In addition,
CpAllPd was unstable when stored in the solid state at room
temperature; however, it could be stored indefinitely at −35 °C
in a nitrogen-filled drybox. When the unsubstituted allyl
complex (Cp)(allyl)Pd was mixed with IPr, only the dimeric
product (μ-Cp)(μ-allyl)Pd2(IPr)2 was isolated.20 Similar dimers
supported by phosphines have been studied by Werner and are
proposed to form through the monomeric compound (η1-
Cp)(η3-allyl)Pd(PR3).16 However, all attempts to spectroscopi-
Given the stability of (η5-Cp)(η1-2-chloroallyl)Pd{P(O-o-
tolyl)3} and our desire to prepare complexes containing
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cally characterize the related IPr-supported monomeric
intermediate were unsuccessful.
The solution-state hapticities of the Cp and 2-methylallyl
ligands in CpAllPd were determined using NMR spectroscopy.
Sergeyev has demonstrated that the 1H and 13C NMR chemical
shifts of the Cp ligand can be used as a diagnostic tool to
determine the coordination mode of Cp ligands in transition-
metal complexes.21 Typically, the H NMR chemical shift for
1
the Cp resonance in complexes where the Cp ring forms a σ
bond (η1) to the metal ranges from 5.6 to 6.2 ppm, while in
complexes where the Cp ring forms both σ and π bonds (η5),
the shift of the Cp resonance ranges from 4.0 to 4.8 ppm. In the
case of 13C NMR spectroscopy, chemical shifts for the Cp
resonance ranging from 113 to 118 ppm are proposed to be
indicative of η1-binding, while shifts ranging from 70 to 93 ppm
21
are consistent with η5-binding. The H NMR spectrum of
CpAllPd at −35 °C showed a single peak for all five Cp protons
at 6.13 ppm, which is consistent with η1-binding. This is further
substantiated by the 13C NMR shift of the Cp ligand at 108.9
ppm, which is also in the range proposed by Sergeyev for η1-
binding. It should be noted that Sergeyev’s ranges for the
chemical shift of the Cp ligand are based only on data from η1-
and η5-coordinated Cp ligands and at this stage no general
trends have been determined for η3-bound Cp ligands, which
are significantly less common.22 As such, it is not possible to
completely rule out the unlikely possibility of η3-binding of the
Cp ligand in CpAllPd on the basis of NMR spectroscopy;
however, X-ray crystallography (vide infra) is also consistent
with an η1-bound Cp. The resonances for the 2-methylallyl
1
Figure 3. ORTEP drawing of CpAllPd at the 30% probability level
(hydrogen atoms, solvent in crystal lattice, and the isopropyl groups of
IPr have been omitted for clarity). Selected bond lengths (Å) and
angles (deg): Pd(1)−C(1) = 2.186(4), Pd(1)−C(6) = 2.174(5),
Pd(1)−C(7) = 2.171(5), Pd(1)−C(8) = 2.171(5), Pd(1)−C(11) =
2.043(4), C(1)−C(2) = 1.446(7), C(2)−C(3) = 1.363(6), C(3)−
C(4) = 1.416(9), C(4)−C(5) = 1.381(8), C(1)−C(5) = 1.439(7),
C(6)−C(7) = 1.382(8), C(7)−C(8) = 1.420(7), C(7)−C(9) =
1.506(9); C(1)−Pd(1)−C(6) = 197.9(2), C(1)−Pd(1)−C(11) =
198.0(2), C(8)−Pd(1)−C(11) = 197.5(2), C(6)−Pd(1)−C(11) =
1163.0(2), C(1)−Pd(1)−C(6) = 197.5(2).
C(1)−C(5), respectively. The 2-methylallyl ligand is clearly
bound in an η3-fashion, although somewhat surprisingly the
binding of the ligand is symmetric and the Pd(1)−C(6),
Pd(1)−C(7), and Pd(1)−C(8) bond distances of 2.171(4),
2.178(5), and 2.173(5) Å, respectively, are equivalent. This
suggests that both the IPr ligand and the η1-Cp ligand exert
comparable trans influences. Overall, the solid-state structure of
CpAllPd is similar to that observed in the closely related (η1-
Cp)(η3-2-tert-butylallyl)Pd(PiPr)3.15b
1
ligand in the H NMR spectrum of CpAllPd are typical of those
expected for η3-binding of an allyl ligand in a C1-symmetric
complex.23 Four distinct resonances are observed for the
terminal protons, with each below 3.5 ppm.
Even at very low temperature, −60 °C, only one sharp
1
resonance was observed for the Cp ligand in the H NMR
spectrum of CpAllPd, suggesting that there is a low barrier for
metallotropic rearrangement (a 1,2-shift) which makes the Cp
protons equivalent. Werner and co-workers have previously
reported analogous NMR properties for compounds of the type
(η1-Cp)(η3-2-tert-butylallyl)Pd(PR3) (R = PiPr3, PCy3,
The Ni analogue of CpAllPd, (η5-Cp)(η1-2-methylallyl)Ni-
(IPr) (CpAllNi), was prepared using a synthetic route similar to
that for the Pd complex (Scheme 1b). Initially, the precursor
(Cp)(2-methylallyl)Ni was synthesized through the reaction of
(Cp)2Ni with (2-methylallyl)magnesium chloride in THF.24
Subsequently, treatment of (Cp)(2-methylallyl)Ni with 1 equiv
of IPr in benzene at room temperature gave CpAllNi in good
15a
1
PtBuPh2). In contrast, low-temperature H NMR spectros-
copy on complexes of the type (η1-Cp)(η3-2-methylallyl)Pd-
(PR3) (R = PiPr3, PCy3, PtBuPh2) suggested that both the 16-
electron species (η1-Cp)(η3-2-methylallyl)Pd(PR3) and the 18-
electron species (η5-Cp)(η1-2-methylallyl)Pd(PR3) were
present at low temperature,15b although the 16-electron species
was the major isomer. It may be that the more electron
donating NHC ligand in CpAllPd exclusively favors the 16-
electron isomer. Alternatively, the 16- and 18-electron isomers
may be in rapid equilibrium even at low temperature and
therefore would be indistinguishable by NMR spectroscopy.
The solid-state structure of CpAllPd was elucidated using X-
ray crystallography and clearly contains an η1-Cp ligand and an
η3-2-methylallyl ligand (Figure 3). The Pd(1)−C(1) distance
for the η1-Cp ligand is 2.186(4) Å, which is consistent with the
bond lengths observed in the few other examples of PdII species
containing η1-Cp ligands.17b The Pd−C distances to all other
carbon atoms of the η1-Cp ligand are greater than 2.85 Å. The
C−C bond distances within the Cp ligand are also indicative of
η1-coordination. Two short bond distances of 1.363(6) and
1.381(8) Å are observed for C(2)−C(3) and C(4)−C(5),
while three longer bond distances of 1.446(7), 1.416(9), and
1.439(7) Å are observed for C(1)−C(2), C(3)−C(4), and
1
yield. In contrast to the case for CpAllPd, the H and 13C NMR
chemical shifts for the resonances associated with the Cp ligand
in CpAllNi, 4.64 and 91.6 ppm, respectively, are consistent with
1
an η5-bound Cp. Furthermore, in the H NMR spectrum the
resonances associated with the 2-methylallyl ligand are
consistent with an η1-binding mode for the allyl ligand. Two
signals, both integrating to one proton, at 4.55 and 4.59 ppm,
are assigned as the terminal protons of the noncoordinated
olefin, while a resonance at 1.24 ppm, integrating to two
protons, is assigned as the protons of the metal-bound CH2
group. Their NMR shifts are similar to those that we observed
for the η1-2-methylallyl ligand in Pd complexes of the type (η3-
2-methylallyl)(η1-2-methylallyl)Pd(L) (L = PR3, IPr)5a and
those that Werner noted for (η5-Cp)(η1-2-methylallyl)Pd-
(PiPr)3.15b At room temperature the resonances associated
with the 2-methylallyl ligand are not fluxional, which indicates
that no exchange processes involving an η3-2-methylallyl ligand
are occurring. This stands in contrast to the previously reported
compound (η3-2-methylallyl)(η1-2-methylallyl)Ni(IPr), where
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rapid interconversion between η1- and η3-2-methylallyl ligands
is observed at −80 °C by H NMR spectroscopy.
Cp ligand range from 2.075(6) to 2.217(6) Å, with the longest
Ni−C bond length being observed for the carbon atom
opposite the 2-methylallyl ligand. The C−C bond lengths
within the Cp ligand are quite similar, ranging from 1.382(5) to
1.418(6) Å. It does not appear that a diene distortion of the Cp
ligand, which was observed by Crabtree25 in complexes of the
type (η5-Cp)Ni(NHC)Cl, is present in CpAllNi. However, in
complexes of the type (η5-Cp)Ni(PR3)Cl, Bergman26 has
previously described an ene−allyl distortion of the Cp ligand, in
which there is slight ring slippage.14a In CpAllNi there is some
evidence for an ene−allyl distortion of the Cp ligand, as the
shortest C−C bond distance inside the Cp ring (C(1)−C(5) =
1.383(8) Å) involves the same carbon atoms, which form the
longest Ni−C bonds (Ni(1)−C(1) = 2.203(5) Å and Ni(1)−
C(5) = 2.217(6) Å). However, the magnitude of the distortion
is small, and it is reasonable to describe CpAllNi as an 18-
electron complex. The 2-methylallyl ligand is bound in an η1-
fashion, with the C(6)−C(7) bond length of 1.474(3) Å being
significantly longer than the C(7)−C(8) bond length of
1.336(3) Å. The Ni−C bond length for the η1-bound 2-
methylallyl ligand (Ni(1)−C(6) = 1.971(2) Å) is similar to that
observed in (η3-2-methylallyl)(η1-2-methylallyl)Ni(IPr),5b
while the Ni−NHC bond distance is also consistent with
those previously reported in the literature.27 Overall, CpAllNi
exhibits a classic piano-stool geometry with a pseudo-five-
coordinate arrangement around Ni.
5b
1
The η5-binding of the Cp ligand in CpAllNi means that the
total valence electron count around the Ni is 18 electrons,
whereas in CpAllPd, there are only 16 electrons around Pd. This
is in agreement with our results for (allyl)2M(L) complexes (M
= Ni, Pd, L = PR3, NHC), where Ni systems are more likely to
adopt an 18-electron structure with two η3-allyl ligands,
whereas Pd systems prefer 16-electron structures with one η1-
allyl and one η3-allyl ligand.5a,b In the bis(allyl) systems the
preference for Pd complexes to have a lower total valence
electron count is proposed to be related to the fact that Pd
forms stronger metal−ligand bonds and therefore an extra π-
type bond from the ligand is not required.13 This explanation is
also plausible for explaining the difference between CpAllNi and
CpAllPd, although in mixed-ligand systems alternative explan-
ations are also possible. Presumably, due to the difficulty of full
ring slippage from η5-Cp to η3-Cp,14b CpAllNi forms the 18-
electron structure through η5-binding of the Cp ligand and η1-
binding of the 2-methylallyl ligand, rather than η3-binding of
both ligands. Most likely due to its stable 18-electron
configuration, CpAllNi is stable both as a solid and in solution
at room temperature, although it does decompose to Ni0 when
heated at 40 °C for longer than 24 h.
The solid-state structure of CpAllNi clearly contains an η5-
bound Cp ligand (Figure 4). The Ni−C bond lengths for the
Synthesis and Structure of Ni and Pd Complexes
Supported by Two Cyclopentadienyl Ligands. For the
purpose of comparison with CpAllNi, the 18-electron complex
(η5-Cp)(η1-Cp)Ni(IPr) (CpCpNi), which was stable at room
temperature both as a solid and in solution, was prepared
through the reaction of (Cp)2Ni and IPr in toluene (Scheme
2a). Two different resonances, at 5.65 and 4.11 ppm, were
1
observed for the Cp ligands in the H NMR spectrum of
CpCpNi. This suggests the presence of both η1- and η5-bound Cp
ligands and also indicates that there is a relatively high barrier
for interconversion between the Cp ligands. Upon warming to
60 °C, the resonances associated with the two Cp ligands began
to broaden and at 80 °C no Cp resonances were observed,
presumably because they had broadened into the baseline. At
temperatures above 80 °C, the compound decomposed. The
1
room-temperature H NMR spectrum did not change upon
Figure 4. ORTEP drawing of CpAllNi at the 30% probability level
(hydrogen atoms and the isopropyl groups of IPr have been omitted
for clarity). Selected bond lengths (Å) and angles (deg): Ni(1)−C(1)
= 2.203(5), Ni(1)−C(2) = 2.075(6), Ni(1)−C(3) = 2.111(7), Ni(1)−
C(4) = 2.158(9), Ni(1)−C(5) = 2.217(6), Ni(1)−C(6) = 1.971(2),
Ni(1)−C(11) = 1.870(2), C(1)−C(2) = 1.42(1), C(2)−C(3) =
1.40(1), C(3)−C(4) = 1.405(9), C(4)−C(5) = 1.42(1), C(1)−C(5)
= 1.383(8), C(6)−C(7) = 1.474(3), C(7)−C(9) = 1.496(4), C(7)−
C(8) = 1.336(3); C(2)−Ni(1)−C(6) = 97.7(2), C(4)−Ni(1)−C(11)
= 108.2(2), C(6)−Ni(1)−C(11) = 110.2(2), C(2)−Ni(1)−C(11) =
172.6(2), C(4)−Ni(1)−C(6) = 156.1(2).
cooling to −60 °C, indicating that the barrier for metallotropic
rearrangement in the η1-Cp ligand is low. At temperatures
below −60 °C the compound lacked sufficient solubility to
obtain NMR spectra. In the room-temperature 13C{1H} NMR
spectrum, two sharp peaks at 108.8 and 92.8 ppm were
observed for the Cp ligands, consistent with both η1- and η5-
binding of the two Cp ligands. X-ray crystallography confirmed
this assignment (Figure 5a). To the best of our knowledge,
there is only one other example of a Ni complex containing
both η1- and η5-bound Cp ligands and it displays NMR
properties similar to those of CpCpNi.27
Scheme 2
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Figure 5. (a) ORTEP drawing of CpCpNi at the 30% probability level (hydrogen atoms, solvent in the crystal lattice, and the isopropyl groups of IPr
have been omitted for clarity). Selected bond lengths (Å) and angles (deg): Ni(1)−C(1) = 2.152(2), Ni(1)−C(2) = 2.077(2), Ni(1)−C(3) =
2.175(2), Ni(1)−C(4) = 2.179(2), Ni(1)−C(5) = 2.142(2), Ni(1)−C(6) = 2.026(2), Ni(1)−C(11) = 1.887(1), C(1)−C(2) = 1.406(3), C(2)−
C(3) = 1.426(3), C(3)−C(4) = 1.389(2), C(4)−C(5) = 1.426(3). C(1)−C(5) = 1.397(2), C(6)−C(7) = 1.457(2), C(7)−C(8) = 1.336(2), C(8)−
C(9) = 1.434(3), C(9)−C(10) = 1.335(2), C(6)−C(10) = 1.457(2); C(2)−Ni(1)−C(11) = 172.04(6), C(5)−Ni(1)−C(6) = 152.70(6), C(6)−
Ni(1)−C(2) = 93.96(6), C(6)−Ni(1)−C(11) = 93.91(5), C(11)−Ni(1)−C(4) = 109.26(6). (b) ORTEP drawing of CpCpPd at the 30% probability
level (hydrogen atoms, solvent in the crystal lattice, and isopropyl groups of IPr have been omitted for clarity). Selected bond lengths (Å) and angles
(deg): Pd(1)−C(1) = 2.233(2), Pd(1)−C(2) = 2.348(3), Pd(1)−C(3) = 2.364(3), Pd(1)−C(4) = 2.380(3), Pd(1)−C(5) = 2.373(3), Pd(1)−C(6)
= 2.115(2), C(1)−C(2) = 1.402(4), C(2)−C(3) = 1.381(4), C(3)−C(4) = 1.423(5), C(4)−C(5) = 1.391(4), C(1)−C(5) = 1.425(4), C(6)−C(7)
= 1.458(4), C(7)−C(8) = 1.363(4), C(8)−C(9) = 1.426(6), C(9)−C(10) = 1.359(5), C(6)−C(10) = 1.468(4); C(1)−Pd(1)−C(11) = 170.38(9),
C(3)−Pd(1)−C(6) = 148.4(1), C(6)−Pd(1)−C(2) = 114.45(9), C(6)−Pd(1)−C(11) = 92.18(8), C(11)−Pd(1)−C(4) = 112.18(9).
The Pd complex (η5-Cp)(η1-Cp)Pd(IPr) (CpCpPd) was
synthesized from the reaction of (Cp)Pd(IPr)Cl28 and sodium
cyclopentadienyl in THF (Scheme 2b). It is unstable in
solution at room temperature and decomposes to give the
dimeric product (μ-Cp)2Pd2(IPr)2.20 Furthermore, unlike
and η1-Cp ligand is confirmed by comparison of the M−C and
C−C bond lengths associated with the Cp ligands. In the η5-
bound Cp ligand, all the C−C bond lengths are similar
(between 1.381(4) and 1.426(3) Å), whereas in the η1-bound
Cp ligand there are three long C−C bonds (between 1.426(4)
and 1.448(4) Å) and two short C−C bonds (between 1.335(2)
1
CpCpNi, the room-temperature H NMR spectrum of CpCpPd
5
́
and 1.363(4) Å). The M−C bond lengths for the η -bound Cp
ligand range from 2.077(2) to 2.179(2) Å for Ni and from
2.233(2) to 2.380(2) Å for Pd, with the shorter bond lengths
for Ni reflecting its smaller size. The η5-Cp ligands in both
complexes display a small ene−allyl distortion, with the shortest
C−C bond in the ring corresponding to the atoms which form
the longest Pd−C or Ni−C bonds. In CpCpPd, the bond lengths
between the Pd and the η5-Cp ligand are considerably longer
than those observed between the Pd and the η3-2-methylallyl
ligand in CpAllPd. This implies that the Pd−C interactions in the
η5-Cp ligand are weaker than those for Pd−C in the η3-2-
methylallyl ligand. The M−C bond length for the η1-bound Cp
ligands is 2.115(2) Å for Pd and 2.026(1) Å for Ni. Although
examples of crystallographically characterized Ni and Pd
complexes containing η1-Cp ligands are rare, these bond
distances are comparable to those previously reported.17b,27,29
The overall “piano stool” geometry for both molecules is
analogous to that observed for CpAllNi.
Synthesis and Structure of Ni and Pd Complexes
Supported by both Indenyl and 2-Methylallyl Ligands.
Indenyl and Cp ligands are often compared because of their
ability to bind to metal centers in η1-, η3-, and η5-fashions,
although their reactivity can differ significantly due to the
“indenyl effect”.14b We were interested in exploring the
consequences of replacing the Cp ligand of CpAllNi and CpAllPd
with an indenyl ligand. Unfortunately, the Pd and Ni complexes
(Ind)(2-methylallyl)M (M = Ni, Pd) have not been reported,
and attempts to prepare them as part of this work resulted in
extremely unstable products, which could not be used as
precursors for direct reaction with IPr. We postulated that
showed one broad resonance at approximately 5.24 ppm for the
Cp ligands. When the spectrum was recorded at −40 °C, the
single broad resonance split into one sharp resonance at 4.69
ppm and one broad resonance at 5.90 ppm, which suggests the
presence of both η1- and η5-bound Cp ligands. This was
confirmed by X-ray crystallography (Figure 5b). Cooling the
sample further resulted in significant loss of solubility, and as a
result it was not possible to record spectra at lower
1
temperatures. In the H NMR spectrum at 60 °C both Cp
signals had broadened and decreased in intensity, similar to
what was seen with CpCpNi. However, formation of the dimeric
product (μ-Cp)2Pd2(IPr)2 was accelerated at higher temper-
atures and precluded us from obtaining spectra above 60 °C,
where two Cp peaks would coalesce. Overall, the 1H NMR data
for CpCpPd suggests that two fluxional processes are occurring:
(i) an η5/η1-exchange of the two Cp ligands, most likely
through an intermediate structure which contains either two η1-
Cp ligands or one η3- and one η1-Cp ligand, and (ii) a lower
energy metallotropic rearrangement, which makes the protons
of the η1-Cp ligand equivalent. Process (i) appears to be more
facile for CpCpPd than for CpCpNi, probably because it is more
favorable for Pd to form a structure with a lower total valence
electron count around the metal. Similar fluxional behavior has
been observed by Werner in compounds of the type (η5-
Cp)(η1-Cp)Pd(PR3).17b The 13C{1H} NMR spectrum at room
temperature of CpCpPd showed two broad resonances at 98.7
and 107.6 ppm for the Cp ligands, which become sharper at
lower temperature.
The solid-state structures of CpCpNi and CpCpPd are
isomorphous. In both structures, the presence of both an η5-
E
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Scheme 3
treatment of the unknown Pd complex (η3-Ind)Pd(IPr)Cl
(
(
IndClPd) and the known Ni species (η3-Ind)Ni(IPr)Cl
IndClNi)30 with (2-methylallyl)magnesium chloride could
generate the desired CpAllNi and CpAllPd complexes. Previously,
Zargarian31 and co-workers reported the syntheses of (η3-
Ind)Pd(PR3)Cl (R = Ph, Cy, Me) through the reaction of {(η3-
Ind)Pd(μ-Cl)}2 with the appropriate free phosphine. The
reaction of 2 equiv of IPr with {(η3-Ind)Pd(μ-Cl)}2 in diethyl
ether generated IndClPd as a bench-stable orange solid, which
served as the starting material for our further studies with
indenyl-supported Pd complexes (Scheme 3).
Analysis of the 13C{1H} NMR spectrum of IndClPd allowed
for the coordination mode of the indenyl ligand to be
determined. While η1-indenyl ligands have their own character-
1
istic H NMR pattern, more sophisticated analysis is required
to differentiate between η3- and η5-indenyl ligands. Baker32 and
Marder33 have shown that whether an indenyl ligand binds in
an η3- or η5-mode can be determined by comparing the average
13C NMR chemical shifts of the hinge carbons (labeled 3a and
7a in Figure 6) of the indenyl ligand with those of sodium
Figure 7. ORTEP drawing of IndClPd at the 30% probability level
(hydrogen atoms and the isopropyl groups of IPr have been omitted
for clarity). Selected bond lengths (Å) and angles (deg): Pd(1)−Cl(1)
= 2.3265(7), Pd(1)−C(1) = 2.270(2), Pd(1)−C(2) = 2.167(2),
Pd(1)−C(3) = 2.173(2), Pd(1)−C(4) = 2.608(2), Pd(1)−C(9) =
2.643(2), Pd(1)−C(10) = 2.007(2), C(1)−C(2) = 1.468(8), C(2)−
C(3) = 1.34(1), C(3)−C(4) = 1.415(9), C(4)−C(5) = 1.406(1),
C(5)−C(6) = 1.36(1), C(6)−C(7) = 1.38(1), C(7)−C(8) = 1.41(1),
C(8)−C(9) = 1.391(9), C(1)−C(9) = 1.451(8), C(4)−C(9) =
1.411(8); Cl(1)−Pd(1)−C(1) = 100.64(7), Cl(1)−Pd(1)−C(3) =
160.12(6), C(1)−Pd(1)−C(10) = 166.95(8), Cl(1)−Pd(1)−C(10) =
92.05(5), C(3)−Pd(1)−C(10) = 106.66(8).
Figure 6. Numbering scheme of the indenyl ligand.
indenyl. The magnitude of this difference, defined as Δδav (Δδav
= δav(C(3a),C(7a) of M-Ind) − δav(C(3a),C(7a) of Na+Ind−))
is related to the solution hapticity of the indenyl ligand. A value
of Δδav ≪ 0 is indicative of η5-coordination and a value of Δδav
≫ 0 is indicative of η3-coordination. Values of Δδav close to
zero indicate intermediate hapticity between η3- and η5-
coordination. For IndClPd, Δδav was equal to 8.7, a value
consistent with η3-coordination.14b
C(2), and C(3) and C(1), C(3), C(3a), and C(7a), while the
fold angle defines the angle between the planes formed by
C(1), C(2), and C(3) and C(3a), C(4), C(5), C(6), C(7), and
C(7a), with numbering consistent with Figure 6.31 IndClPd has a
hinge angle of 14.83° and a fold angle of 12.53°. These values
along with the ΔM−C value are consistent with other examples
of η3-indenyl compounds.14b,31 A similar solid-state structure
has been described for the analogous compound IndClNi,
although in this case the hapticity of the indenyl ligand is more
ambiguous and is proposed to be between η3- and η5-binding.36
The reaction of 1 equiv of 2-methylallyl magnesium chloride
with IndClPd in THF generated IndAllPd in good yield. IndAllPd is
stable at room temperature in the solid state for up to 3 weeks
and is stable in solution at room temperature overnight.
Significant formation of Pd0 was observed when a benzene
In the solid state IndClPd also contains an η3-bound indenyl
ligand (Figure 7). The bond distances between Pd and C(1),
C(2), and C(3) are 2.270(2), 2.167(2), and 2.173(2) Å,
respectively. All of these values are comparable with those
observed in similar complexes.14b,34 The Pd−C distances to the
hinge carbons of the indenyl ligand are significantly longer:
2.608(2) and 2.643(2) Å. As a result, the ΔM−C value, which
is defined as ΔM−C = (average M−C(3a),C(7a) distance) −
(average M−C(1),C(3) distance), is 0.40(2) Å.32,33 The
disparity in the Pd(1)−C(1) and Pd(1)−C(3) bond distances
is likely due to the difference in trans influence between the Cl
and IPr ligands, with the shorter Pd−C bond distance observed
for the carbon opposite the Cl ligand. This asymmetry has also
been observed in related phosphine-supported indenyl Ni
complexes.31,35 The coordination of indenyl ligands to
transition metals can be further described by the hinge and
fold angles, which quantify the degree to which the indenyl ring
has slipped from the idealized η5-coordination. The hinge angle
is defined as the angle between the planes formed by C(1),
1
solution of IndAllPd was heated above 50 °C overnight. The H
NMR spectrum of IndAllPd shows 2-methylallyl resonances
similar to those of CpAllPd, with four signals below 3.5 ppm
integrating to one proton each. This is consistent with an η3-2-
methylallyl ligand.23 The H NMR spectrum indicates that the
1
indenyl ligand is coordinated in an η1-fashion. The aromatic
region shows four resonances above 7.0 ppm, which
correspond to the protons of the benzene ring of the indenyl
ligand. An apparent triplet at 6.57 ppm and two broad
resonances at 5.97 and 5.90 ppm are associated with the five-
F
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membered ring of the indenyl ligand. These resonances are
consistent with literature examples of η1-indenyl complex-
es.14b,34a When the sample is cooled to −40 °C in toluene, the
broad resonances separate into two unique signals at 6.00 and
5.64 ppm, each integrating to one proton. The apparent triplet
at 6.57 ppm does not change upon cooling. Loss of solubility
below −40 °C prevented analysis at lower temperatures. X-ray
crystallography confirmed the presence of both an η1-indenyl
and an η3-2-methylallyl ligand (Figure 8). The Pd−C bond
five-membered ring of the indenyl ligand are also suggestive of
η1-coordination. The C(2)−C(3) bond distance of 1.34(1) Å is
consistent with a double bond, while the C(1)−C(9) distance
of 1.451(8) Å is significantly longer.
Unfortunately, the Ni compound IndAllNi was not successfully
isolated. Two different routes were attempted, as summarized
in Scheme 4. The reaction of (2-methylallyl)magnesium
chloride with IndClNi (synthesized using a literature
route)36,37 at low temperature (−78 °C) gave a product
which displayed broad signals in the 1H NMR spectrum
(Scheme 4a). We were unable to characterize this product, as
when the solution was raised above −78 °C rapid
decomposition to Ni0 occurred. An alternative route using
AllClNi as the starting material was attempted. Initially, the
dimeric compound {(η3-2-methylallyl)Ni(μ-Cl)}2 was synthe-
sized in situ from the reaction of Ni(COD)2 and 2-methylallyl
chloride (Scheme 4b).38 Subsequent addition of IPr generated
AllClNi. The addition of lithium indenyl to AllClNi at both room
temperature and −35 °C gave a mixture of products.
Performing the reaction at −78 °C gave a deep red solution
1
that showed broad H NMR signals. These signals were the
same as those observed in the reaction between (2-methylallyl)
magnesium chloride and IndClNi. The sample quickly
decomposed to Ni0 when the solution was raised even slightly
above −78 °C. Although both routes resulted in the same
species, we cannot say with certainty that this is our desired
IndAllNi product, due to the extreme thermal instability of the
complex, which prevented further characterization.
Figure 8. ORTEP drawing of IndAllPd at the 30% probability level
(hydrogen atoms and the isopropyl groups of IPr have been omitted
for clarity). Selected bond lengths (Å) and angles (deg): Pd(1)−C(1)
= 2.162(6), Pd(1)−C(10) = 2.073(5), Pd(1)−C(11) = 2.188(5),
Pd(1)−C(12) = 2.171(5), Pd(1)−C(13) = 2.132(5), C(1)−C(2) =
1.468(8), C(2)−C(3) = 1.34(1), C(3)−C(4) = 1.415(9), C(4)−C(5)
= 1.40(1), C(5)−C(6) = 1.36(1), C(6)−C(7) = 1.38(1), C(7)−C(8)
= 1.41(1), C(8)−C(9) = 1.391(9), C(1)−C(9) = 1.451(8), C(4)−
C(9) = 1.411(8), C(11)−C(12) = 1.400(8), C(12)−C(13) =
1.400(8), C(12)−C(14) = 1.495(9); C(1)−Pd(1)−C(13) =
93.0(2), C(1)−Pd(1)−C(10) = 95.7(2), C(10)−Pd(1)−C(11) =
104.6(2), C(1)−Pd(1)−C(11) = 159.7(2), C(10)−Pd(1)−C(13) =
167.3(2).
Synthesis and Structure of Ni and Pd Complexes
Supported by both Indenyl and Cyclopentadienyl
Ligands. The mixed Cp/indenyl complex IndCpPd was
synthesized through the reaction of IndClPd with NaCp in
THF at 0 °C (Scheme 5a). While low temperature was required
during the synthesis to avoid the formation of multiple
products, IndCpPd is stable for up to 1 week in solution at
room temperature under a nitrogen atmosphere and indef-
initely stable as a solid. 1H NMR spectroscopy indicates IndCpPd
has an η1-indenyl ligand and an η5-Cp ligand. In a fashion
similar to that for IndAllPd, IndCpPd shows four aromatic
resonances above 6.8 ppm associated with the benzene ring of
the indenyl ligand. Of these four resonances, two are coincident
with the aryl protons of the IPr ligand. The remaining
resonances appear as a doublet at 7.54 ppm and a doublet at
6.88 ppm. The proton bound to the η1-carbon of the five-
membered indenyl ring appears as a singlet at 4.54 ppm. The
two olefinic protons of this ring appear as an apparent triplet at
6.40 ppm and a broad resonance at 6.74 ppm. These shifts are
consistent with what has been reported for η1-indenyl
complexes of Pt and Pd.14b,39 Furthermore, the 1H NMR
spectrum shows a single resonance at 4.63 ppm for the five Cp
distances for the 2-methylallyl ligand are 2.188(5), 2.171(5),
and 2.132(5) Å, respectively. The longest Pd−C bond is trans
to the η1-indenyl ligand. This bond is slightly longer than the
Pd−C bond to the 2-methylallyl ligand for the carbon atom
trans to the η1-Cp ligand in CpAllPd, suggesting that the η1-
indenyl ligand is a stronger donor than the η1-Cp ligand. The
Pd only interacts with one carbon of the indenyl ligand, and the
Pd(1)−C(1) bond length is 2.162(6) Å. This is longer than the
Pd−C bond length in other reported η1-indenyl complexes,
presumably due to the increased trans influence of the allyl
ligand as compared to Cl, the most common trans ligand in
reported structures.34a The C−C bond distances within the
Scheme 4
G
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Scheme 5
protons, well within the range cited by Sergeyev for η5-bound
Cp rings.21
For unknown reasons we were unable to synthesize a pure
sample of the analogous Ni complex IndCpNi through the
reaction of IndClNi with sodium cyclopentadienyl. However,
IndCpNi was synthesized through the reaction of (η5-Cp)Ni-
(IPr)Cl40 CpClNi) with lithium indenyl in THF at −40 °C
(
(Scheme 5b). IndCpNi is stable as a solid at room temperature
under a nitrogen atmosphere and slowly decomposes to Ni0 in
solution over the course of 3 days at room temperature. The 1H
and 13C{1H} NMR shifts for the Cp ring, 4.17 and 92.5 ppm,
respectively, confirm that the Cp is η5-bound. 1H NMR
spectroscopy suggests that as in IndAllPd and IndCpPd, the
indenyl ligand is η1-bound. A singlet at 3.82 ppm corresponds
to the proton on the η1-bound carbon of the indenyl ring. The
two olefinic protons for the five-membered ring appear at 6.56
ppm as an apparent triplet and as a complex multiplet
coincident with an indenyl aryl peak at 6.79 ppm.
Synthesis and Structure of Ni and Pd Complexes
Supported by Two Indenyl Ligands. The Pd species with
two indenyl ligands IndIndPd was synthesized through the
reaction of IndClPd with lithium indenyl at −35 °C in diethyl
ether (Scheme 6). The dark pink solid is not stable at room
Figure 9. ORTEP drawing of IndIndPd at the 30% probability level
(hydrogen atoms and the isopropyl groups of IPr have been omitted
for clarity). Selected bond lengths (Å) and angles (deg): Pd(1)−C(1)
= 2.115(4), Pd(1)−C(10) = 2.017(4), Pd(1)−C(11) = 2.217(5),
Pd(1)−C(12) = 2.220(5), Pd(1)−C(13) = 2.610(4), Pd(1)−C(19) =
2.345(4), C(1)−C(2) = 1.468(8), C(2)−C(3) = 1.348(7), C(3)−
C(4) = 1.453(6), C(4)−C(5) = 1.396(6), C(5)−C(6) = 1.391(7),
C(6)−C(7) = 1.397(8), C(7)−C(8) = 1.381(6), C(8)−C(9) =
1.383(7), C(1)−C(9) = 1.480(6), C(4)−C(9) = 1.417(8), C(11)−
C(12) = 1.406(5), C(12)−C(13) = 1.450(7), C(13)−C(14) =
1.403(6), C(14)−C(15) = 1.391(8), C(15)−C(16) = 1.399(7),
C(16)−C(17) = 1.390(6), C(17)−C(18) = 1.394(8), C(13)−C(18)
= 1.435(6), C(18)−C(19) = 1.443(6), C(11)−C(19) = 1.420(8);
C(10)−Pd(1)−C(12) = 169.7(2), C(1)−Pd(1)−C(12) = 97.3(2),
C(1)−Pd(1)−C(10) = 92.8(2), C(10)−Pd(1)−C(19) = 109.6(2),
C(1)−Pd(1)−C(19) = 157.3(2), C(12)−Pd(1)−C(19) = 60.2(2).
Scheme 6
2.115(4) Å. All other Pd−C distances to this indenyl ligand are
greater than 2.9 Å. The other indenyl ring is coordinated in an
η3-fashion, similar to that seen in IndClPd. The Pd(1)−C(11)
and Pd(1)−C(12) bond lengths are similar at 2.217(5) and
2.220(5) Å, respectively, while the Pd(1)−C(19) distance is
considerably longer at 2.345(4) Å, indicating the increased
trans influence of the η1-indenyl ligand compared to IPr. The
Pd−C distances to the hinge carbons are 2.610(4) and
2.674(5) Å, resulting in a ΔM−C value of 0.36(4) Å.32,33 As
with IndClPd, the hinge and fold angles of 12.53 and 13.53° for
the η3-indenyl ligand of IndIndPd are consistent with previous
examples for η3-indenyl complexes.34a,41 The Pd(1)−C(1)
bond distance to the η1-indenyl ligand is shorter than that in
IndAllPd, presumably because the η3-allyl ligand exerts a stronger
trans influence than the η3-indenyl ligand. It is noteworthy that
temperature and decomposes to give (μ-Ind)2Pd2(IPr)2 as the
major product with a minor amount of Pd(IPr)2.20 The H
1
NMR spectrum at −40 °C displays 14 unique indenyl
resonances. The eight aromatic resonances appear above 6.31
ppm, and the remaining six protons on the five-membered rings
appear below 6.25 ppm. Attempts to prepare the Ni analogue of
IndIndPd either through the reaction of (Ind)2Ni with IPr or the
treatment of IndClNi with lithium indenyl were unsuccessful and
resulted in complex mixtures of products.
X-ray-quality crystals of IndIndPd were grown from a
concentrated solution of diethyl ether at −35 °C (Figure 9).
The solid-state structure clearly shows two different coordina-
tion modes for the two indenyl ligands. One indenyl ligand is
bound in a η1-fashion, with a Pd(1)−C(1) bond distance of
H
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IndIndPd is a 16-electron species, with one η1-indenyl ligand and
one η3-indenyl ligand, whereas the related species CpCpPd is an
18-electron species with one η1-Cp ligand and one η5-Cp
ligand. This highlights the “indenyl effect”14b with the indenyl
ligand adopting an η3-binding mode, while the Cp ligand
prefers an η5-binding mode.
Reactivity of Ni and Pd Compounds with Electro-
philes. Previously, it has been proposed that in complexes of
the type (η3-allyl)(η1-allyl)M(L) (M = Ni, Pd; L = PR3, IPr),
electrophiles such as Brønsted acids, aldehydes, and CO2 react
with the nucleophilic η1-allyl ligand.1a,2c,4,6a,42 In comparison,
relatively little is known about the reactivity of η1-Cp and η1-
indenyl ligands bound to Pd and Ni with these substrates.43 As
a result, we were interested in probing the reactivity of the
mixed compounds prepared as part of this work with simple
electrophiles. Table 1 summarizes our results for the reactions
Legzdins and co-workers reported the reaction of tungsten
compounds supported by both η5-pentamethycyclopentadienyl
and η3-allyl ligands with triphenylcarbenium tetafluoroborate to
give allylated triphenylmethyl products.44 The first step in these
reactions was proposed to be the isomerization of the allyl
ligand from an η3- to an η1-binding mode, followed by
nucleophilic attack of the terminal carbon of the η1-allyl ligand
(as opposed to the metal-bound carbon) on triphenylcarbe-
nium tetafluoroborate to form a new C−C bond. In our
systems, the reaction of CpAllNi with 1 equiv of triphenylcarbe-
nium chloride gave quantitative conversion to the new organic
compound (2-methylallyl)triphenylmethane and CpClNi. The
organic compound was isolated using column chromatography.
The reaction with CpAllPd showed orthogonal reactivity, giving
AllClPd and several isomers of the organic product, cyclo-
pentadienyltriphenylmethane, which have previously been
characterized by Werner.45 Similar reactions were performed
on the remaining compounds of the series, as shown in Table 2.
Table 1. Summary of Reactivity of Species Prepared in This
a
Work with Acid (2,6-Lutidinium Chloride)
Table 2. Summary of Reactivity of Species Prepared in This
a
Work with Triphenylcarbenium Chloride
metal
product
b
b
compd
conditions
organic product
CpAllPd
CpAllNi
toluene, −35 °C, 4 h
toluene, room temp, 4 h
AllClPd
CpClNi
AllClPd
cyclopentadiene
2-methylpropene
indene
IndAllPd toluene, 0 °C to room temp,
metal
product
b
b
2 h
compd
conditions
organic product
IndCpPd toluene, 0 °C to room temp,
CpClPd
CpClNi
indene
indene
c
CpAllPd
toluene, −35
°C to room
temp, 24 h
AllClPd
cyclopentadienyltriphenylmethane
2 h
IndCpNi toluene, 0 °C to room temp,
2 h
CpAllNi
benzene, room
temp, 12 h
CpClNi
AllClPd
2-methylallyltriphenylmethane
CpCpPd benzene, room temp, 1 h
CpCpNi
CpClPd
CpClNi
cyclopentadiene
cyclopentadiene
c
IndAllPd toluene, 0 °C
to room
indenyltriphenylmethane
toluene, room temp, 3 h
a
A representative reaction is shown above the body of the table.
Products determined using 1H NMR spectroscopy. See the
temp, 24 h
b
c
IndCpPd toluene, 0 °C
to room
CpClPd
CpClNi
indenyltriphenylmethane
Experimental Section for more information.
temp, 24 h
c
IndCpNi toluene, 0 °C
to room
indenyltriphenylmethane
of CpAllPd, CpAllNi, IndAllPd, IndCpPd, and IndCpNi with the solid
source of HCl, 2,6-lutidinium chloride. Although different
reaction conditions needed to be employed depending on the
specific reaction to obtain clean conversion, in all cases with
mixed ligands selective protonation at the η1-ligand was
observed and high yields of the organic product were detected
temp, 24 h
c
c
CpCpPd benzene, room
CpClPd
CpClNi
cyclopentadienyltriphenylmethane
cyclopentadienyltriphenylmethane
temp, 12 h
CpCpNi
benzene, room
temp, 12 h
a
A representative reaction is shown above the body of the table.
b
1
by H NMR spectroscopy. For example, in the case of CpAllPd,
Products determined using 1H NMR spectroscopy. See the
c
which contains an η1-Cp ligand and an η3-2-methylallyl ligand,
the sole product of protonation was (η3-2-methylallyl)Pd(IPr)-
Cl (AllClPd), whereas for CpAllNi, which contains an η5-Cp
ligand and an η1-2-methylallyl ligand, the only product was (η5-
Cp)Ni(IPr)Cl (CpClNi). From the results of these reactions we
believe that two conclusions can be drawn: (i) in the case of
acid, where the reaction is proposed to proceed directly with
the carbon bound to the metal center, η1-allyl, η1-Cp, and η1-
indenyl ligands all display similar reactivity and (ii) protonation
with acid can almost certainly be used as a general technique to
confirm the identity of the ligand bound in an η1-fashion in
mixed-ligand complexes of the type described here.
Experimental Section for more information. Several different isomers
were formed.
In contrast to the reactions with CpAllPd and CpAllNi, the
reactions with indenyl ligands did not give quantitative
conversion. Multiple organic and metal-containing products
1
were seen by H NMR spectroscopy; however, the metal-
containing product reported in Table 2 was the major product.
Similarly, the major organic product was indenyltriphenyl-
methane.46 It is noteworthy that, although these reactions were
not clean, there was no evidence to support a reaction taking
place at either the η3-2-methylallyl ligand in the case of IndAllPd
or the η5-Cp ligand in the case of IndCpPd or IndCpNi. Overall,
the reactions described in Table 2 suggest that in an analogous
The above reactions with a Brønsted acid led us to
investigate reactivity with nonacidic electrophiles. In 2009,
I
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Article
fashion to η1-allyl ligands, η1-Cp and η1-indenyl ligands can also
act as nucleophiles with substrates where the initial point of
attack of the electrophile is not proposed to be the metal-bound
carbon. However, the exact pathway for the reaction of
triphenylcarbenium chloride with the η1-Cp and η1-indenyl
species studied in this work still needs to be determined.
diethyl ether, benzene, toluene and THF; thus, when any of these
solvents were used, traces of all these solvents were in the atmosphere
and could be found intermixed in the solvent bottles.) Moisture- and
air-sensitive liquids were transferred by stainless steel cannula on a
Schlenk line or in a drybox. Solvents were dried by passage through a
column of activated alumina followed by storage under dinitrogen. All
commercial chemicals were used as received, except where noted.
(Cp)2Ni was purchased from Alfa Aesar and triphenylcarbenium
chloride from Fisher Scientific Co. Deuterated solvents were obtained
from Cambridge Isotope Laboratories. C6D6 and d8-toluene were dried
over sodium metal and vacuum-transferred prior to use. NMR spectra
were recorded on Bruker AMX-400 and -500 spectrometers at ambient
probe temperatures, unless otherwise noted. Chemical shifts are
reported in ppm with respect to residual internal protio solvent for 1H
and 13C{1H} NMR spectra; J values are given in Hz. Robertson
Microlit Laboratories, Inc., performed the elemental analyses.
Literature procedures were used to prepare the following compounds:
(Cp)(2-methylallyl)Pd,48 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-
2H-imidazol-2-ylidene (IPr),49 (2-methylallyl)magnesium chloride,50
(η5-Cp)Pd(IPr)Cl,28 sodium cyclopentadienyl,51 {(η3-Ind)Pd(μ-
Cl)}2,52 (η5-Cp)Ni(IPr)Cl,40 AllClNi,38 lithium indenyl,53 and 2,6-
lutidinium chloride.54
X-ray Crystallography. Low-temperature diffraction data (ω
scans) were collected on either a Rigaku SCXmini diffractometer
coupled to a Mercury275R CCD detector with Mo Kα radiation (λ =
0.71073 Å), a Rigaku R-AXIS RAPID diffractometer coupled to a R-
AXIS RAPID imaging plate detector with Mo Kα radiation (λ =
0.71073 Å) or a Rigaku MicroMax-007HF diffractometer coupled to a
Saturn994+ CCD detector with Cu Kα (λ = 1.54178 Å). All structures
were solved by direct methods using SHELXS55 and refined against F2
on all data by full-matrix least squares with SHELXL-9756 using
established refinement techniques.57 All non-hydrogen atoms were
refined anisotropically. All hydrogen atoms were included into the
model at geometrically calculated positions and refined using a riding
model. The isotropic displacement parameters of all hydrogen atoms
were fixed to 1.2 times the U value of the atoms to which they are
linked (1.5 times for methyl groups). All disorders were refined with
the help of similarity restraints on the 1,2- and 1,3-distances and
displacement parameters as well as rigid bond restraints for anisotropic
displacement parameters. Further details of the crystal and refinement
data are given in the Supporting Information.
CONCLUSIONS
■
A family of Pd and Ni complexes containing combinations of 2-
methylallyl, Cp, and indenyl ligands, as well as an ancillary IPr
ligand, has been synthesized. Unfortunately, a single general
synthetic route cannot be used to access the family of
complexes, due to differences in the stability of the precursors.
For example, CpAllPd and CpAllNi can be prepared through the
reaction of IPr with (Cp)(2-methylallyl)Pd and (Cp)(2-
methylallyl)Ni, respectively. However, because (Cp)2Pd does
not exist, an alternative strategy was used to synthesize CpCpPd.
In general, the hapticities of the 2-methylallyl, Cp, and
indenyl ligands in the complexes synthesized in this work can
be explained by the following two observations: (i) Cp ligands
prefer to adopt either an η5- or an η1-binding mode as opposed
to an η3-binding mode and (ii) Ni complexes prefer to be 18-
electron species, whereas Pd complexes prefer to be 16-electron
complexes.47 As a result, in CpAllPd, the Cp ligand binds in an
η1-fashion and the 2-methylallyl ligand in an η3-fashion to give a
complex with 16 electrons around Pd. In the corresponding Ni
complex CpAllNi, the Cp ligand binds in an η5-fashion and the 2-
methylallyl ligand in an η1-fashion so that Ni has 18 electrons.
Normally, observation (i) takes priority over (ii). Therefore,
CpCpPd is an 18-electron species with one η1- and one η5-bound
Cp ligand. An exception to our observations is IndCpPd, an 18-
electron species with one η1-bound indenyl ligand and one η5-
bound Cp ligand, whereas we would predict a 16-electron
species with one η1-bound Cp ligand and one η3-bound indenyl
ligand. In contrast to the Cp ligand, due to the indenyl effect,
the indenyl ligand can bind in an η1-, η3-, or η5-fashion to satisfy
observation (ii), although the structure of IndCpPd suggests that
even for the indenyl ligand η3-binding is not favored. In this
work, the only compound isolated with both indenyl and allyl
ligands was IndAllPd; therefore, at this stage there are not
enough data to evaluate the preferred hapticity of the allyl
ligand in comparison to the indenyl ligand. Overall, we believe
that the trends observed here will be general for related Pd and
Ni complexes with combinations of allyl, Cp, and indenyl
ligands.
Synthesis and Characterization of Compounds. (η1-Cp)(η3-2-
methylallyl)Pd(IPr) (CpAllPd). A solution of (Cp)(2-methylallyl)Pd
(0.025 g, 0.110 mmol) in 5 mL of pentane was added to a solution of
IPr (0.042 g, 0.110 mmol) in 5 mL of pentane at −35 °C. The solution
was stirred at −35 °C for 14 h. The volatiles were removed under
reduced pressure at −35 °C to give a pale yellow solid. The solid was
washed three times with 5 mL portions of cold pentane and dried
under vacuum at −35 °C to give CpAllPd as a pale yellow solid. Yield:
0.040 g, 53%. Due to the thermal instability of this compound
elemental analysis was not performed. X-ray-quality crystals were
grown by slow diffusion of pentane into a saturated toluene solution at
−35 °C.
Preliminary reactivity studies of our new complexes with
electrophiles indicate that the η1-ligand reacts selectively,
regardless of whether it is an allyl, Cp, or indenyl ligand.
Therefore, whereas acid protonates the Cp ligand in CpAllPd, it
protonates the 2-methylallyl ligand in CpAllNi. Similar reactivity
is observed using the nonacidic electrophile triphenylcarbenium
chloride. Although it is well-known that Pd and Ni complexes
with η1-allyl ligands will act as nucleophiles, comparable
reactions with η1-Cp and η1-indenyl ligands are significantly
less common. Further work will look to explore the reactivity of
these species with less activated electrophiles such as aldehydes
and CO2.
1H NMR (d8-toluene, −35 °C, 400 MHz): 7.19 (t, J = 6.6 Hz, 2H,
p-H ArIPr), 7.12 (d, J = 6.6 Hz, 4H, m-H ArIPr), 6.55 (s, 2H, HCCH),
6.13 (s, 5H, Cp), 3.36 (sept, J = 6.3 Hz, 2H, (CH3)2CH), 3.03 (s, 1H,
CH2 allyl), 2.91 (sept, J = 6.3 Hz, 2H, (CH3)2CH), 2.30 (s, 1H, CH2
allyl), 1.51 (s, 1H, CH2 allyl), 1.28 (d, J = 6.3 Hz, 12H, (CH3)2CH),
1.22 (s, 3H, CH3 allyl), 1.04 (d, J = 6.3 Hz, 12H, (CH3)2CH), 0.77 (s,
1H, CH2 allyl). 13C{1H} NMR (d8-toluene, −35 °C, 126 MHz): 191.7,
145.8, 145.4, 109.1, 87.6, 47.2, 28.7, 26.2, 25.5, 25.0, 23.5, 23.4, 22.7,
22.4.
(Cp)(2-methylallyl)Ni. There is a literature procedure for the
synthesis of (Cp)(2-methylallyl)Ni,58 however we believe that the
procedure described below represents an improved protocol.
A solution of (Cp)2Ni (0.250 g, 1.32 mmol) in 10 mL THF was
cooled to 0 °C and 2-methylallyl magnesium chloride (2.09 mL of a
0.657 M solution in diethyl ether, 1.32 mmol) was added dropwise
with stirring. The resulting solution was allowed to warm to room
temperature and stirred for twelve hours to give a dark purple solution.
EXPERIMENTAL SECTION
■
General Methods. Experiments were performed under a
dinitrogen atmosphere in an M. Braun drybox or using standard
Schlenk techniques, unless otherwise noted. (Under standard glovebox
conditions, purging was not performed between uses of pentane,
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The solvent was removed at 30 Torr and the resulting residue
extracted three times with 10 mL portions of pentane. The filtrate was
concentrated at 30 Torr to give (Cp)(2-methylallyl)Ni as a dark
75%. X-ray-quality crystals were grown by slow diffusion of pentane
into a saturated toluene solution at room temperature. 1H and
13C{1H} NMR spectra of IndClPd are provided in the Supporting
Information.
1
purple oil. Yield: 0.200 g, 84.7%. The H NMR data was consistent
with that previously reported in the literature.58
1H NMR (C6D6, 400 MHz): 7.37 (t, J = 7.7 Hz, 2H, p-H ArIPr),
7.26 (d, J = 7.7 Hz, 2H, m-H ArIPr), 7.08 (d, J = 7.7 Hz, 2H, m-H
ArIPr), 6.97 (s, 2H, HCCH), 6.78 (d, J = 7.4 Hz, 1H, ArInd), 6.60 (t, J =
7.4 Hz, 1H, ArInd), 6.21 (m, 2H, ArInd), 5.66 (d, J = 7.4 Hz, 1H, CpInd),
5.48 (t, J = 2 Hz, 1H, CpInd), 5.22 (t, J = 2 Hz, 1H, CpInd), 3.03 (sept, J
= 6.8 Hz, 2H, (CH3)2CH), 2.57 (sept, J = 6.8 Hz, 2H, (CH3)2CH),
1.36 (d, J = 6.7 Hz, 6H, (CH3)2CH), 1.10 (d, J = 6.7 Hz, 6H,
(CH3)2CH), 0.97 (d, J = 6.7 Hz, 6H, (CH3)2CH), 0.85 (d, J = 6.7 Hz,
6H, (CH3)2CH). 13C{1H} NMR (C6D6, 126 MHz): 177.9, 146.3,
138.6, 137.7, 136.8, 130.2, 124.8, 118.4, 116.3, 110.2, 86.1, 69.5, 28.8,
28.7, 26.4, 25.7, 23.7, 22.4. Anal. Calcd (found) for C36H43N2PdCl: C,
66.87 (65.09); H, 6.86 (5.69); N, 4.33 (4.12).
(η5-Cp)(η1-2-methylallyl)Ni(IPr) (CpAllNi). A solution of (Cp)(2-
methylallyl)Ni (0.183 g, 1.02 mmol) was dissolved in 5 mL benzene
and added to a solution of IPr (0.397 g, 1.02 mmol) in 5 mL benzene.
The solution was stirred at room temperature for twelve hours and the
solvent removed under reduced pressure. The residue was washed
three times with 10 mL portions of cold pentane and dried under
vacuum to give CpAllNi as a pale green solid. Yield: 0.350 g, 60%. X-ray
quality crystals were grown from a concentrated pentane solution at
−35 °C.
1H NMR(d8-toluene, 400 MHz): 7.25 (t, J = 7.3 Hz, 2H, p-H ArIPr),
7.16 (d, J = 7.6 Hz, 4H, m-H ArIPr), 6.50 (s, 2H, HCCH), 4.64 (s, 5H,
Cp), 4.59 (d, J = 2.6 Hz, 1H, CH2 allyl), 4.55 (d, J = 1.9 Hz, 1H, CH2
allyl), 3.06 (sept, J = 6.8 Hz, 4H, (CH3)2CH), 1.45 (s, 3H, CH3 allyl),
1.40 (d, J = 6.8 Hz, 12H, (CH3)2CH), 1.24 (s, 2H, CH2 allyl), 0.98 (d,
J = 6.8 Hz, 12H, (CH3)2CH). 13C{1H} NMR (d8-toluene, 126 MHz):
188.0, 158.7, 146.1, 138.0, 129.8, 124.1, 101.6, 91.6, 28.9, 25.8, 24.0,
22.4, 1.0. Anal. Calcd (found) for C36H48N2Ni: C, 76.06 (76.23); H,
8.69 (8.41); N, 4.93 (4.83).
(η1-Ind)(η3-2-methylallyl)Pd(IPr) (IndAllPd). IndClPd (0.050 g, 0.077
mmol) was dissolved in 3 mL of THF, and the solution was cooled to
0 °C. (2-Methylallyl)magnesium chloride (0.117 mL of a 0.657 M
THF solution, 0.077 mmol) was added dropwise with stirring, and the
resulting solution was stirred at 0 °C for 90 min to give a bright yellow
solution. The solvent was removed under reduced pressure and the
resulting residue extracted twice with 5 mL portions of pentane. The
combined extracts were dried under vacuum to give IndAllPd as a bright
yellow solid. Yield: 0.037 g, 68%. X-ray-quality crystals were grown
(η5-Cp)(η1-Cp)Ni(IPr) (CpCpNi). (Cp)2Ni (0.010 g, 0.052 mmol) was
dissolved in 2 mL of benzene at room temperature. A 3 mL benzene
solution of IPr (0.020 g, 0.052 mmol) was added and the resulting
suspension stirred at room temperature for 10 h to give a dark red
solution. The solvent was removed under reduced pressure. The
residue was extracted in 5 mL of pentane and dried under vacuum to
give CpCpNi as a dark red solid. Yield: 0.025 g, 82%. X-ray-quality
crystals were grown by layering pentane on top of a saturated toluene
solution at −35 °C.
1
from a saturated pentane solution at −35 °C. H and 13C{1H} NMR
spectra of IndAllPd are provided in the Supporting Information.
1H NMR (C6D6, 500 MHz): 7.46 (d, J = 6.5 Hz, 1H, ArInd), 7.24 (t,
J = 7.7 Hz, 2H, p-H ArIPr), 7.21 (d, J = 1.3 Hz, 1H, ArInd), 7.19 (d, J =
1.6 Hz, 1H, ArInd), 7.14 (d, J = 7.7 Hz, 2H, m-H ArIPr), 7.10 (d, J = 7.7
Hz, 2H, m-H ArIPr), 6.64 (s, 2H, HCCH), 6.56 (t, J = 3.2 Hz, 1H,
CpInd), 5.97 (br, 1H, CpInd), 5.90 (br, 1H, CpInd), 3.38 (sept, J = 6.8
Hz, 2H, (CH3)2CH), 3.01 (sept, J = 6.8 Hz, 2H, (CH3)2CH), 2.31 (d,
J = 3.2 Hz, 1H, CH2 allyl), 2.13 (s, 1H, CH2 allyl), 1.50 (s, 1H, CH2
allyl), 1.30 (d, J = 6.8 Hz, 6H, (CH3)2CH), 1.21 (d, J = 6.7 Hz, 6H,
(CH3)2CH), 1.05 (d, J = 6.8 Hz, 6H, (CH3)2CH), 1.01 (d, J = 6.8 Hz,
6H, (CH3)2CH), 1.00 (s, 3H, CH3 allyl), 0.95 (d, J = 2.8 Hz, 1H, CH2
allyl), one ArInd peak is coincidental with solvent. 13C{1H} NMR
(C6D6, 75 MHz): 191.8, 146.4, 146.2, 138.6, 137.8, 130.5, 124.8, 124.6,
124.5, 121.2, 120.1, 119.9, 78.5, 51.3, 29.3, 29.2, 26.7, 25.9, 23.8, 23.2,
23.1, 1.77. Anal. Calcd (found) for C40H51N2Pd: C, 72.11 (70.89); H,
7.72 (7.66); N, 4.20 (4.20).
1H NMR (d8-toluene, 400 MHz): 7.26 (t, J = 7.6 Hz, 2H, p-H
ArIPr), 7.15 (d, J = 7.6 Hz, 4H, m-H ArIPr), 6.48 (s, 2H, HCCH), 5.65
(s, 5H, Cp), 4.11 (s, 5H, Cp), 3.20 (s, br, 4H, (CH3)2CH), 1.31 (d, J =
6.6 Hz, 12H, (CH3)2CH), 0.98 (d, J = 6.5 Hz, 12H, (CH3)2CH).
13C{1H} NMR (d8-toluene, 126 MHz): 182.6, 145.8, 137.4, 130.0,
124.1, 108.8, 92.3, 28.8, 28.4, 25.6, 24.5. Anal. Calcd (found) for
C37H46N2Ni: C, 76.82 (76.92); H, 8.19 (7.92); N, 4.84 (4.58).
(η5-Cp)(η1-Cp)Pd(IPr) (CpCpPd). (Cp)Pd(IPr)Cl28 (0.019 g, 0.032
mmol) was dissolved in 2 mL of THF at room temperature. A 2.0 M
THF solution of sodium cyclopentadienyl (0.016 mL, 0.032 mmol)
was added and the resulting solution stirred at room temperature for 1
h to give a dark green solution. The solvent was removed under
reduced pressure, and the residue was extracted twice in 5 mL of
pentane. The combined extracts were dried under vacuum to give
CpCpPd as a dark green solid. Yield: 0.014 g, 71%. X-ray-quality crystals
were grown by layering pentane on a saturated solution of diethyl
ether at −35 °C.
(η1-Ind)(η5-Cp)Pd(IPr) (IndCpPd). IndClPd (0.050 g, 0.077 mmol) was
dissolved in 3 mL of THF, and the solution was cooled to 0 °C. A
solution of sodium cyclopentadienyl (0.038 mL of a 2 M THF
solution, 0.077 mmol) was added dropwise with stirring and the
resulting solution stirred at 0 °C for 90 min. The solvent was removed
under reduced pressure and the residue extracted twice with 5 mL
portions of pentane. The solvent from the combined extracts was
removed under vacuum to give IndCpPd as an orange solid. Yield: 0.040
g, 71%.
1H NMR (C6D6, 400 MHz): 7.25 (m, J = 7.5 Hz, 2H, p-H ArIPr),
7.13 (d, J = 7.6 Hz, 4H, m-H ArIPr), 6.56 (s, 2H, HCCH), 5.51 (br s,
5H, Cp), 5.07 (br s, 5H, Cp), 3.08 (sept, J = 6.5 Hz, 4H, (CH3)2CH),
1.30 (d, J = 6.8 Hz, 12H, (CH3)2CH), 0.98 (d, J = 6.9 Hz, 12H,
1H NMR (C6D6, 400 MHz): 7.53 (d, J = 7.1 Hz, 1H, ArInd), 7.26 (t,
J = 7.7 Hz, 2H, p-H ArIPr), 7.20−7.11 (m, 6H, ArInd and ArIPr), 6.89 (d,
J = 7.2 Hz, 1H, ArInd), 6.74 (br, 1H, CpInd), 6.56 (s, 2H, HCCH), 6.40
(m, 1H, CpInd), 4.63 (s, 5H, Cp), 4.54 (br, 1H, CpInd), 3.19 (sept, J =
6.8 Hz, 2H, (CH3)2CH), 3.06 (sept, J = 6.8 Hz, 2H, (CH3)2CH), 1.32
(d, J = 7.0 Hz, 6H, (CH3)2CH), 1.25 (J = 6.8 Hz, 6H, (CH3)2CH),
1.01 (d, J = 7.0 Hz, 6H, (CH3)2CH), 0.99 (d, J = 7.0 Hz, 6H,
(CH3)2CH). 13C{1H} NMR (C6D6, 75 MHz): 181.5, 146.5, 146.3,
141.8, 137.8, 130.6, 124.9, 124.8, 124.6, 124.2, 122.9, 121.9, 121.1,
118.1, 99.2, 34.9, 29.4, 26.3, 22.8, 23.0, 14.6. Anal. Calcd (found) for
C41H49N2Pd: C, 72.82 (73.09); H, 7.30 (7.11); N, 4.14 (4.13).
(η1-Ind)(η5-Cp)Ni(IPr) (IndCpNi). (η5-Cp)Ni(IPr)Cl (0.050 g, 0.091
mmol) was dissolved in 2 mL of THF and cooled to −40 °C. Lithium
indenyl (0.0012 g, 0.100 mmol) in 1 mL of THF was added dropwise
with stirring. The solution was warmed to room temperature over 1 h
and stirred for an additional 2 h at room temperature. The solvent was
removed under vacuum and the residue extracted twice with 5 mL
portions of pentane to give a red solution. The solvent was removed
1
(CH3)2CH). H NMR (d8-toluene, 400 MHz, −30 °C): 7.23 (t, J =
7.5 Hz, 2H, p-H ArIPr), 6.47 (s, 2H, HCCH), 5.86 (s, 5H, Cp), 4.64 (s,
5H, Cp), 3.04 (m, 4H, (CH3)2CH), 1.28 (d, J = 6.4 Hz, 12H,
(CH3)2CH), 0.97 (d, J = 6.8 Hz, 12H, (CH3)2CH). The m-H ArIPr
protons are under the solvent resonances. 13C{1H} NMR (d8-toluene,
100 MHz): 181.1, 146.4, 137.6, 130.5, 124.6, 124.4, 109.7 (br), 100.2
(br), 29.5, 26.1, 22.9. Anal. Calcd (found) for C37H46N2Pd: C, 71.08
(71.15); H, 7.42 (7.66); N, 4.48 (4.26).
(η3-Ind)Pd(IPr)Cl (IndClPd). {(η3-Ind)Pd(μ-Cl)}2 (0.060 g, 0.116
mmol) and IPr (0.090 g, 0.232 mmol) were suspended in 15 mL of
diethyl ether and stirred at room temperature for 1 h. The resulting
orange suspension was filtered and the filtrate concentrated to
approximately 5 mL under reduced pressure. A 20 mL portion of cold
pentane was added to the concentrated filtrate, and an orange solid
precipitated out of solution. The solid was isolated via filtration,
washed three times with 5 mL portions of cold pentane, and dried
under vacuum to give IndClPd as a dark orange solid. Yield: 0.113 g,
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from the combined extracts to give IndCpNi as a red solid. Yield: 0.045
g, 75%.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR (C6D6, 500 MHz): 7.51 (d, J = 7.2 Hz, 1H, ArInd), 7.30 (t,
J = 7.7 Hz, 3H, ArInd and p-H ArIPr), 7.21−7.19 (m, 5H, ArInd and m-H
ArIPr), 6.79 (m, 2H, ArInd and CpInd), 6.56 (t, J = 3.3 Hz, 1H, CpInd),
6.55 (m, 1H, CpInd), 6.52 (s, 2H, HCCH), 4.17 (s, 5H, Cp), 3.82 (br,
1H, CpInd), 3.43 (br, 2H, (CH3)2CH), 3.08 (br, 2H, (CH3)2CH), 1.38
(d, J = 6.5 Hz, 6H, (CH3)2CH), 1.20 (d, J = 6.5 Hz, 6H, (CH3)2CH),
0.98 (d, J = 6.8 Hz, 12H, (CH3)2CH). 13C{1H} NMR (C6D6, 126
MHz): 184.0, 158.9, 146.5, 146.4, 146.2, 146.0, 142.3, 138.3, 138.1,
130.6, 130.5, 125.4, 124.7, 124.1, 122.2, 121.6, 120.9, 120.0, 117.1,
97.5, 34.7, 29.4, 26.3, 26.1, 22.8, 19.8. Anal. Calcd (found) for
C41H49N2Ni: C, 78.35 (78.07); H, 7.86 (8.01); N, 4.46 (4.47).
(η3-Ind)(η1-Ind)Pd(IPr) (IndIndPd). IndClPd (0.018 g, 0.028 mmol) was
dissolved in 5 mL of diethyl ether at −35 °C. The solution was added
to lithium indenyl (0.003 g, 0.028 mmol) in 5 mL of diethyl ether at
−35 °C and stirred at this temperature for 20 min to give a cloudy,
dark pink solution. The precipitate was removed by quickly filtering
the mixture through a Celite plug. The solvent was removed from the
filtrate under reduced pressure to give IndIndPd as dark pink solid. Due
to the thermal instability of this compound, elemental analysis was not
performed, nor was a 13C NMR spectrum recorded. X-ray-quality
crystals were grown in a concentrated solution of diethyl ether at −35
°C. Yield: 0.016 g, 80%.
CIF files and tables giving X-ray information for CpAllPd,
CpAllNi, CpCpPd, CpCpNi, IndClPd, IndAllPd and IndIndPd and
figures giving 1H and 13C{1H} NMR spectra of selected
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nilay.hazari@yale.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge support from the National Science
Foundation through Grant CHE-1150826. We are grateful to
Matthew J. Chalkley and Damian P. Hruszkewcyz for
informative discussions.
1H NMR (d8-toluene, −40 °C, 500 MHz): 7.78 (d, J = 6.8 Hz, 1H,
ArInd), 7.47 (t, J = 7.7 Hz, 1H, ArInd), 7.34 (t, J = 7.7 Hz, 1H, ArInd),
6.86 (d, J = 4.8 Hz, 1H, ArInd), 6.71 (t, J = 7.4 Hz, 1H, ArInd), 6.61 (m,
2H, ArInd), 6.35 (d, J = 6.61 Hz, 1 H, ArInd), 6.30 (s, 1H, HCCH), 6.22
(t, J = 3 Hz, 1H, CpInd), 6.13 (t, J = 3 Hz, 1H, CpInd), 6.06 (br, 1 H,
CpInd), 5.69 (d, J = 7.6 Hz, 1H, CpInd), 4.94 (br, 1H, CpInd), 4.54 (br,
1H, CpInd), 3.01 (br, 4H, (CH3)2CH), 0.94 (br, 24H, (CH3)2CH).
Reactivity Studies. General Procedure for the Reaction of Pd
and Ni Compounds with 2,6-Lutidinium Chloride. A solution of 2,6-
lutidinium hydrochloride (0.010g, 0.006 mmol) in 0.25 mL d8-toluene
was added to a solution of the metal complex (0.006 mmol) in 0.25
mL of d8-toluene at the desired temperature in an NMR tube. The
solution was allowed to stand at the desired temperature for the
specified time (see Table 1 for specific temperatures and reaction
times). Quantitative formation of the known complexes (η3-2-
methylallyl)Pd(IPr)Cl1b and (η5-Cp)Pd(IPr)Cl59 was observed by
1H NMR spectroscopy. (η5-Cp)Ni(IPr)Cl40 precipitated out of
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solution and was isolated by filtration and then identified using H
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Reaction between CpAllPd and Triphenylcarbenium Chloride.
Triphenylcarbenium chloride (0.002 g, 0.008 mmol) was added to a
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̈
Reaction between CpAllNi and Triphenylcarbenium Chloride.
Triphenylcarbenium chloride (0.002 g, 0.008 mmol) was added to a
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1
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1H NMR (CDCl3, 400 MHz): 7.29−7.15 (m, 15H, phenyl), 4.66 (s,
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́
Danowski, F.; Bellenbaum, M.; Benn, R.; Rufinska, A.; Schroth, G.;
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dx.doi.org/10.1021/om4002632 | Organometallics XXXX, XXX, XXX−XXX

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M
dx.doi.org/10.1021/om4002632 | Organometallics XXXX, XXX, XXX−XXX