Ferrocene-bis(phosphinimine) Nickel(II) and Palladium(II) Alkyl Complexes: Influence of the Fe-M (M = Ni and Pd) Interaction on Redox Activity and Olefin Coordination

Article abstract of DOI:10.1021/acs.organomet.7b00626

The synthesis of several novel nickel(II) and palladium(II) ferrocene-bis(phosphinimine) alkyl complexes containing iron-nickel and iron-palladium interactions is reported. The redox behavior of all complexes was evaluated electrochemically and chemically

Full text of DOI:10.1021/acs.organomet.7b00626

Article
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Cite This: Organometallics XXXX, XXX, XXX-XXX
Ferrocene-bis(phosphinimine) Nickel(II) and Palladium(II) Alkyl
Complexes: Influence of the Fe−M (M = Ni and Pd) Interaction on
Redox Activity and Olefin Coordination
Mark Abubekerov, Saeed I. Khan, and Paula L. Diaconescu*
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
S
* Supporting Information
ABSTRACT: The synthesis of several novel nickel(II) and
palladium(II) ferrocene-bis(phosphinimine) alkyl complexes
containing iron−nickel and iron−palladium interactions is
reported. The redox behavior of all complexes was evaluated
electrochemically and chemically; in addition, reactions with
weak nucleophiles, such as acetonitrile and olefins, were also
investigated. DFT calculations were performed to understand
the electronic structure of the alkyl metal complexes.
olefin polymerization25−27 and Buchwald−Hartwig cross-
INTRODUCTION
■
coupling reactions,28 utilization of cobalt complexes for olefin
Catalytic processes that can be switched by an external stimulus
hydroalkoxylation,29 and of gold mesoionic carbenes for
can offer an additional, “biolike” control of chemical trans-
heterocycles synthesis.30 However, in all cases of redox-
formations.1,2 Early examples showed that switchable catalysts
controlled catalysis, the ferrocene unit is distant from the
could be used to speed up or slow down the rate of a reaction
primary metal center, and no direct interaction exists between
according to the presence of a specific trigger,3−12 which could
iron in ferrocene and the metal involved in a substrate
be either a physical or a chemical stimulus,1,2,13 or to change
transformation.
the stereochemical14−16 outcome of a reaction (on/off
Various bidentate ferrocene derived ligands reported in the
switches). In recent years, however, more advanced tasks
literature containing phosphorus,31−37 sulfur,38−41 and nitro-
have been accomplished, such as turning “on” and “off”
gen-based42−53 substituents are known to promote direct iron−
different forms of the same precatalyst in order to promote
metal interactions. However, the use of these complexes in
complementary reactions that take place from a mixture of
redox-switchable catalysis has not been investigated. For
different building blocks.17−23 Redox switchable catalysis uses
example, ferrocene-bis(phosphinimines) have been used as
supporting ligands,43,54 but not in catalytic examples. On the
redox reagents (chemical triggers) to turn on and off reactions.
The earlier works of Wright on et el.,12 Long et al.,3 and Plenio
basis of our interest in redox-switchable catalysis utilizing
ferrocene derivatives, we decided to prepare ferrocene-bis-
(phosphinimine) nickel and palladium alkyl complexes
containing iron−nickel and iron−palladium interactions and
investigate their efficacy for redox-switchable olefin polymer-
ization.
et al.5 illustrated the influence of the redox states of ferrocenyl
units, in supporting ligands, on catalysis. Our initial work
focused extensively on utilizing ferrocene-based ligands for the
redox control of ring-opening polymerizations (ROP) of cyclic
esters. Starting with indium, yttrium,4 and cerium24 phosfen
(phosfen = 1,1′-di(2-tert-butyl-6-diphenylphosphinimino-
phenoxy)ferrocene) complexes, an on/off activity for the
polymerization of L-lactide was observed based on the oxidation
RESULTS AND DISCUSSION
■
Preparation and Characterization of the Ferrocene-
bis(phosphinimine) Metal Complexes. Initial attempts to
carry out a direct ligand substitution using fc(NPPh3)2 to
prepare the halide-alkyl complexes of nickel and palladium were
unsuccessful (Scheme 1). However, the addition of NaBPh4 to
(PPh3)2NiCl(Ph) or (COD)PdCl(Me) (COD = 1,5-cyclo-
octadiene) in the presence of fc(NPPh3)2 in methylene chloride
resulted in the isolation of [fc(NPPh3)2NiPh][BPh4] and
[fc(NPPh3)2PdMe][BPh4] as red crystals in an 82.4% yield and
state of the iron center. These early efforts culminated in the
discovery of a system that displayed redox-controlled polymer-
ization of several cyclic monomers: first, in the case of L-lactide/
ε-caprolactone with a titanium thiolfan complex, (thiolfan*)-
Ti(OiPr)2 (thiolfan* = 1,1′-di(2,4-di-tert-butyl-6-thiophenoxy)-
ferrocene),18 and then for L-lactide/cyclohexene oxide utilizing
a zirconium salfan complex, (salfan)Zr(OtBu)2 (salfan = 1,1′-
di(2-tert-butyl-6-N-methylmethylenephenoxy)ferrocene).17
Recently, there have been efforts by our group and others to
expand the application of redox-controlled systems beyond
ROP.10 Such examples include ferrocene-based ligands capable
of modulating monomer selectivity in palladium systems for
Received: August 16, 2017
© XXXX American Chemical Society
A
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Scheme 1. Preparation of Nickel and Palladium Ferrocene-
bis(triphenylphosphinimine) Complexes
Figure 2. Molecular structure of [fc(NPPh3)2PdMe][BPh4] with
thermal ellipsoids at 50% probability; hydrogen, solvent, and borate
atoms are omitted for clarity. Selected distances (Å) and angles (deg):
N(1)−P(1), 1.620(1); N(2)−P(2), 1.606(1); N(1)−Pd(1), 2.072(1);
N(2)−Pd(1), 2.031(1); C(11)−Pd(1), 2.051(2); Fe(1)−Pd(1),
2.7957(5); C(11)−Pd(1)−N(1), 102.29(6); C(11)−Pd(1)−N(2),
97.14(7); N(1)−Pd(1)−Fe(1), 81.07(4); N(2)−Pd(1)−Fe(1),
79.46(4).
The iron−nickel distance of 2.8244(6) Å is similar to that of
a related complex [fc(S2)Ni(PMe2Ph)] (2.886(1) Å)41 and
longer than those observed for the dicationic species [fc-
(NIm)2Ni(NCMe)][BF4]2 (2.6268(4) Å) and [fc(NIm)2Ni-
(PMe3)][BF4]2 (2.7376(4) Å, fc(NIm)2 = N,N′-bis(1,3-di-iso-
propyl-4,5-dimethylimidazolin-2-ylidene)-1,1′-ferrocenedi-
amine).44 The palladium−iron distance of 2.7957(5) Å is
comparable with values reported for the similar complexes
[fc(NIm)2Pd(PMe3)][BF4]2 (2.7475(4) Å), [fc(NIm)2Pd-
(PMe3)][BF4]2 (2.7424(3) Å),44 [(dppf)Pd(PPh3)][BF4]2
(2.877(2) Å, dppf = 1,1′-bis(diphenylphosphino)ferrocene),56
and [fc(S2)Pd(PPh3)] (2.878(1) Å)39 and is longer than those
observed for [fc(NIm)2Pd(NCMe)][BF4]2 (2.6297(4) Å),
[fc(NIm)2Pd(PPh3)][BF4]2 (2.6447(3) Å),44 and [fc-
orange crystalline material in 67.0% yield, respectively (Scheme
1). The solid-state molecular structures of [fc(NPPh3)2NiPh]-
[BPh4] (Figure 1) and [fc(NPPh3)2PdMe][BPh4] (Figure 2)
were investigated using single-crystal X-ray diffraction. The
coordination environments around the nickel and palladium
centers have a distorted square planar geometry with a τ value55
of 0.18 and 0.16, respectively.
(NPPh3)2Pd(Cl)][Cl] (2.67 Å).43 The H NMR spectrum of
1
[fc(NPPh3)2NiPh][BPh4] is consistent with the presence in
solution of a diamagnetic species and both the nickel and
palladium complexes display a wide separation between the Cp
signals (0.97 and 1.46 ppm, respectively), consistent with the
presence of an Fe−M (M = Ni and Pd ) interaction.43 In the
31P NMR spectra, the chemical shifts of 37.5 ppm (nickel) and
30.2 ppm (palladium) are downfield compared to that of
fc(NPPh3)2 (27.2 ppm).43 The presence of the borate
counterion was confirmed by singlets at −5.7 ppm and −5.9
ppm for the nickel and palladium complexes, respectively, in
the corresponding 11B NMR spectra.
Electrochemical studies performed on [fc(NPPh3)2NiPh]-
[BPh4] displayed a complicated and uninformative redox
behavior (Figure S40). Attempts to perform a chemical
oxidation using acetyl ferrocenium tetrakis(3,5-bis-
(trifluoromethyl)phenyl)borate ([AcFc][BArF], Scheme 2 and
Figure S26) or AgBF4 (Figure S27) and a reduction using
cobaltocene (Figure S24) on an NMR reaction scale did not
result in the formation of any new species. Utilizing KC8 as a
reducing agent resulted only in the formation of nickel black
Figure 1. Molecular structure of [fc(NPPh3)2NiPh][BPh4] with
thermal ellipsoids at 50% probability; hydrogen, solvent, and borate
atoms are omitted for clarity. Selected distances (Å) and angles (deg):
N(1)−P(1), 1.613(2); N(2)−P(2), 1.613(2); N(1)−Ni(1), 1.898(2);
N(2)−Ni(1), 1.895(2); C(11)−Ni(1), 1.891(3); Fe(1)−Ni(1),
2.8244(6); C(11)−Ni(1)−N(1), 102.4(1); C(11)−Ni(1)−N(2),
97.2(1); N(1)−Ni(1)−Fe(1), 80.94(6); N(2)−Ni(1)−Fe(1),
79.52(6).
B
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Scheme 2. Chemical Oxidation and Reduction of Nickel and
Palladium Ferrocene-bis(triphenylphosphinimine)
Complexes
Figure 3. Molecular structure of [fc(NPPh3)2Pd(THF)][BF4]2 with
thermal ellipsoids at 50% probability; hydrogen, solvent, and borate
atoms are omitted for clarity. Selected distances (Å) and angles (deg):
N(1)−P(1), 1.618(4); N(2)−P(2), 1.613(4); N(1)−Pd(1), 2.040(4);
N(2)−Pd(1), 2.042(4); O(1)−Pd(1), 2.142(3); Fe(1)−Pd(1),
2.6493(8); O(1)−Pd(1)−N(1), 99.9(1); O(1)−Pd(1)−N(2),
97.7(1); N(1)−Pd(1)−Fe(1), 80.9(1); N(2)−Pd(1)−Fe(1), 81.5(1).
trum, the chemical shift of 37.4 ppm is downfield from that of
[fc(NPPh3)2PdMe][BPh4] (30.2 ppm). The presence of the
BF4 ions was confirmed by 11B NMR spectroscopy showing a
singlet at −0.4 ppm and 19F NMR spectroscopy showing two
singlets at −152.0 and −152.1 ppm. The presence of two
signals in the 19F NMR spectrum is due to two naturally
occurring boron isotopes, 11B and 10B, both of which are NMR
active.
and liberation of fc(NPPh3)2 (Scheme 2, Figure S25).
However, electrochemical studies performed on [fc-
(NPPh3)2PdMe][BPh4] (Figure S41) displayed several quasi-
reversible redox processes in the range of 0−1.25 V (vs Fc/Fc+)
suggesting that a more potent oxidant than a ferrocenium salt
may be used. Similarly to the nickel case, utilizing [AcFc][BArF]
as an oxidant led to the formation of several minor species, with
the starting material remaining the predominant species
(Scheme 2, Figure S28). However, in the presence of excess
AgBF4, the formation of a single species was observed in the 31P
NMR spectrum (Figure S37). A closer look at the in situ
Next, we looked at the strength of the Fe−M (M = Ni and
Pd ) interaction. Previous reports by Cabrera et al.31 and Jess et
al.44 showed that the Fe−M interactions in similar complexes
are weak enough that a small molecule, such as acetonitrile, can
disrupt them. The resulting acetonitrile adducts, in the case of
palladium complexes, are nonsymmetrical species that can be
readily detected by heteronuclear NMR spectroscopy. Con-
sequently, solutions of [fc(NPPh3)2NiPh][BPh4] and [fc-
(NPPh3)2PdMe][BPh4] in a mixture of acetonitrile and
methylene chloride (1:1 vol %) both display singlets with no
apparent shift, suggesting that the acetonitrile−palladium and
acetonitrile−nickel interactions are not sufficient enough to
overcome the metal−metal interactions in these complexes.
Due to the lack of redox activity in [fc(NPPh3)2NiPh]-
[BPh4], we did not investigate its reactivity with olefins.
Although [fc(NPPh3)2PdMe)[BPh4] did not show a reversible
redox process, we wanted to determine whether the comound
would display the same redox behavior during a polymerization
process. However, we did not observe any polymerization
activity at ambient temperature in the reactions with ca. 100
equiv of norbornene, styrene, 1-hexene, or ethylene (1.0 atm)
in CD2Cl2, suggesting that the palladium−olefin interactions
are also too weak to overcome the iron−palladium interaction.
Elevating the reaction temperature resulted in the rapid
formation of palladium black and no additional reactivity.
The lack of activity of [fc(NPPh3)2PdMe][BPh4] toward
olefin polymerization prompted an investigation into alternative
phosphinimine ligands. It is possible that a more electron rich
derivative, such as the analogous tricyclohexylphosphinimine,
1
generated complex via H, 11B, and 19F NMR spectroscopies
(Figures S36, S38, and S39) revealed the loss of the methyl
group and the presence of tetrafluoroborate as the predominant
counterion in solution. The existence of palladium−fluorine
interactions in the newly formed palladium complex, in the
absence of a coordinating solvent, can be postulated based on
the broad signals observed in the 19F NMR spectrum. Scaling
up the reaction and the addition of THF resulted in the
isolation of [fc(NPPh3)2Pd(THF)][BF4]2 (Scheme 2) as dark
golden plates in a 73.2% yield. The results of an X-ray
diffraction study are displayed in Figure 3 along with selected
distances and angles. The coordination environment around
the palladium center is a distorted square planar geometry (τ =
0.14). The palladium−iron distance of 2.6493(8) Å is
significantly shorter than in [fc(NPPh3)2PdMe][BPh4] and is
now comparable with those observed for [fc(NIm)2Pd-
(NCMe)][BF4]2 (2.6297(4) Å), [fc(NIm)2Pd(PPh3)][BF4]2
(2.6447(3) Å),44 and [fc(NPPh3)2Pd(Cl)][Cl] (2.67 Å).43 In
1
the H NMR spectrum, the separation between the Cp signals
(1.76 ppm) is slightly larger (1.46 ppm) than for the parent
complex, [fc(NPPh3)2PdMe][BPh4]. In the 31P NMR spec-
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would yield a palladium complex with a weaker Fe−Pd
interaction.43 However, we could not prepare [fc-
(NPCy3)2PdMe][BPh4] (Scheme S1) although we used various
methods, possibly due to the large steric bulk of the supporting
ligand disfavoring the formation of a square planar fc-
(NPCy3)2PdCl(Me) intermediate. Alternatively, we prepared
a new ferrocene-bis(phosphinimine) derivative, fc(PEt3)2
(Scheme 3), combining a more electron-rich phosphine and a
Scheme 3. Preparation of Nickel and Palladium Ferrocene-
bis(triethylphosphinimine) Complexes
Figure 4. Molecular structure of [fc(NPEt3)2PdMe][BPh4] with
thermal ellipsoids at 50% probability; hydrogen, solvent, and borate
atoms are omitted for clarity. Selected distances (Å) and angles (deg):
N(1)−P(1), 1.624(2); N(2)−P(2), 1.612(2); N(1)−Pd(1), 2.054(2);
N(2)−Pd(1), 2.046(2); C(1)M−Pd(1), 2.056(2); Fe(1)−Pd(1),
2.7806(5); C(1)M−Pd(1)−N(1), 99.4(1); C(1)M−Pd(1)−N(2),
98.35(9); N(1)−Pd(1)−Fe(1), 80.90(6); N(2)−Pd(1)−Fe(1),
81.02(6).
a chemical oxidation using excess AgBF4 only resulted in a
partial counterion exchange while a new palladium species was
not observed. The addition of acetonitrile to a methylene
chloride solution of [fc(NPEt3)2PdMe][BPh4] yielded the same
results as were observed for [fc(NPPh3)2NiPh][BPh4] and
[fc(NPPh3)2PdMe][BPh4]. No change in the chemical shift or
reduced steric environment compared with that of the
fc(NPPh3)2 analogue. The reduction of the steric environment
manifests itself in the isolation of a stable halide−alkyl complex,
fc(NPEt3)2PdCl(Me) (Scheme 3), that was not achieved with
either fc(NPPh3)2 or fc(NPCy3)2. The lack of symmetry in
fc(NPCy3)2PdCl(Me) is clearly observed by a multitude of
signals, specifically, the presence of eight separate signals
(3.70−5.56 ppm) for the Cp protons in the 1H NMR spectrum
and two singlets (43.4 and 45.1 ppm) in the 31P NMR
spectrum.
Chloride abstraction from fc(NPEt3)2PdCl(Me) was accom-
plished with NaBPh4 affording [fc(NPEt3)2PdMe][BPh4]
(Scheme 3) as an orange crystalline material in a 79.3% yield.
The solid-state molecular structure of [fc(NPEt3)2PdMe]-
[BPh4] (Figure 4) was determined using single-crystal X-ray
diffraction. The coordination environment around the palla-
dium center is in a slightly distorted square planar geometry
with a τ value of 0.20. The iron−palladium distance of
2.7806(5) Å is similar to that of [fc(NPPh3)2PdMe][BPh4]
(2.7957(5) Å). The 1H NMR spectrum of [fc(NPEt3)2PdMe]-
[BPh4] shows a wide separation of 1.66 ppm between the
signals of the Cp protons. A downfield shift in the resonance
signal of [fc(NPEt3)2PdMe][BPh4] (57.3 ppm) compared to
both fc(NPEt3)2 (28.9 ppm) and fc(NPEt3)2PdCl(Me) (43.4
and 45.1 ppm) was observed in the 31P NMR spectra. The
BPh4 ion appears at −5.7 ppm in the 11B NMR spectrum,
similar to the fc(NPPh3)2 nickel and palladium analogues.
Electrochemical studies performed with [fc(NPEt3)2PdMe]-
[BPh4] displayed irreversible redox events similar to those for
[fc(NPPh3)2PdMe][BPh4] (Figure S43). Attempts to perform
formation of a nonsymmetrical species was observed in the 31
P
NMR spectrum. Furthermore, the species is thermally sensitive
and gradually decomposes at ambient temperature to form a
new major species (Figures S33 and S34). A loss of the methyl
group and the widening in the Cp proton separation was
observed in the 1H NMR spectrum as well as a further
downfield shift of the signal in the 31P NMR spectrum (Figures
S34), reminiscent of the formation of [fc(NPPh3)2Pd(THF)]-
[BF4]2 from [fc(NPPh3)2PdMe][BPh4]. Decomposition was
also clearly observed in the presence of styrene while no
olefin−palladium interaction was observed at ambient temper-
ature (Figure S35). Increasing the temperature only accelerated
the rate of decomposition of the palladium−methyl species.
The lack of reactivity toward nucleophiles is not surprising
when comparing the structural parameters of [fc-
(NPPh3)2PdMe][BPh4] and [fc(NPEt3)2PdMe][BPh4].
Although PEt3 is a more electron-rich phosphine than PPh3,
there are no significant observable differences in various
distances or the geometries around palladium (Table 1).
Therefore, in these two examples, a change in the phosphine
substituent did not heavily impact the electronic properties of
the resulting ferrocene−phosphinimine palladium complex.
However, a decrease in the steric bulk of the phosphinimine
ligand, upon a transition from a phenyl to an ethyl group, may
explain the decrease in the thermal stability of [fc-
(NPEt3)2PdMe][BPh4] and the observed loss of the methyl
ligand.
DFT Calculations. The extent of the Fe−M interaction can
also be determined from DFT calculations. Geometry
D
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Table 1. Summary of Structural Parameters and NMR Spectroscopic Data for Nickel and Palladium Complexes
[fc(NPPh3)2NiPh]+
[fc(NPPh3)2PdMe]+
[fc(NPPh3)2Pd(THF)]2+
2.6493(8)
[fc(NPEt3)2PdMe]+
Fe−M (Å)
M−C (Å)
2.8244(6)
1.891(3)
2.7957(5)
2.051(2)
2.7806(5)
2.056(2)
M−N (Å)
N−P (Å)
1.895(2), 1.898(2)
1.613(2), 1.613(2)
0.18
2.031(1), 2.072(1)
1.606(1), 1.620(1)
0.16
2.040(4), 2.042(4)
1.613(4), 1.618(4)
0.14
2.046(2), 2.054(2)
1.612(2), 1.624(2)
0.20
τ
1H NMR Cp−H (ppm)
11B NMR (ppm)
31P NMR (ppm)
3.39, 4.36
3.12, 4.58
3.63, 5.39
2.97, 4.63
−5.7
37.5
−5.9
30.2
−0.4
37.4
−5.7
57.3
optimizations for [fc(NPPh3)2NiPh]+, [fc(NPPh3)2PdMe]+,
and [fc(NPEt3)2PdMe]+ were performed for the present
study employing ADF2013.01,57−59 using the PW91 func-
tional60 and no frozen electron cores. Although the counterion
was omitted, all the atoms of the cations were included. The
optimized Fe−M distances (Table 2) are close to each other
complexes have two NLMOs with a bond order (BO) larger
than 0.02: NLMO89 (BO = 0.04) and NLMO95 (BO = 0.03)
for [fc(NPPh3)2NiPh]+, NLMO93 (BO = 0.03) and
NLMO219 (BO = 0.06) for [fc(NPPh3)2PdMe)]+, and
NLMO69 (BO = 0.03) and NLMO147 (BO = 0.06) for
[fc(NPEt3)2PdMe]+.
Topological analysis of the electron density was performed
via Bader’s atoms in molecule (AIM) theory.64,65 AIM identifies
bonds by calculating (3, − 1) critical points, and it differentiates
between covalent bonds and weak interactions, such as
hydrogen bonds, van der Waals, and donor−acceptor
interactions, by the value of the Laplacian (∇2 ). If ∇2 < 0,
Table 2. Computational Parameters for [fc(NPPh3)2NiPh]+,
[fc(NPPh3)2PdMe]+, and [fc(NPEt3)2PdMe]+
Ni
Pd (PPh3)
Pd (PEt3)
Fe−M (Å)
Fe−M Mayer bond order
2.81
2.82
0.27
2.84
0.27
ρ
ρ
then the interaction is considered covalent. If ∇2ρ > 0, then the
interaction is a weak interaction. AIM has been used to
calculate bond critical points between two metal centers66 and
in ferrocene complexes.67 All three [fc(NPPh3)2NiPh]+,
[fc(NPPh3)2PdMe]+, and [fc(NPEt3)2PdMe]+ species show
Fe−M bond critical points (Table 1) consistent with a weak,
noncovalent interaction (see the Supporting Information for
details).
0.21
Mulliken Charges
Fe
M
−0.15
0.29
−0.13
0.66
−0.13
0.75
Hirshfeld Charges
Fe
0.02
0.02
0.40
0.10
0.09
0.02
0.40
0.10
0.09
M
0.25
Wiberg bond index
NLMO bond order
0.09
0.09
Natural Charge
0.14
CONCLUSIONS
■
Fe
M
0.17
0.58
0.17
0.58
The preparation of ferrocene-bis(phosphinimine) nickel and
palladium alkyl complexes, via halide abstraction, was described.
The ferrocene ligands are bound in a κ3 fashion featuring iron−
nickel and iron−palladium interactions. Electrochemically, only
irreversible redox processes were observed for the palladium
species. Chemically, [fc(NPPh3)2NiPh][BPh4] and [fc-
(NPEt3)2PdMe][BPh4] were redox inactive, while [fc-
(NPPh3)2PdMe][BPh4] underwent an irreversible oxidation
resulting in the loss of the alkyl group. Attempts at disrupting
the iron−palladium interactions using weak nucleophiles, such
as acetonitrile and olefins, were unsuccessful. Although DFT
calculations indicate that these interactions are relatively weak,
they are important to the overall electronic stabilization of each
metal complex, especially in the case of palladium. On the basis
of the results of this study, the ferrocene-bis(phosphinimine)
nickel and palladium alkyl complexes discussed herein are not
viable for applications in redox-switchable olefin polymer-
ization.
0.70
Bader Charge
0.66
Fe
M
0.71
0.39
0.03
0.05
0.71
0.39
0.03
0.05
0.59
a
ρ at Fe−M BCP
0.03
Laplacian (∇2 ) at Fe−M BCP
0.04
ρ
a
BCP is an Fe−M bond critical point (3, −1) identified by Bader
analysis.
and to the values obtained experimentally (Table S1). The
calculated Mayer bond orders61,62 are around 0.2 and are
similar to the values obtained for other palladium complexes
that show Fe−Pd interactions.31,32,35,45
Each of the analyzed compounds showed several molecular
orbitals that indicate bonding interactions between iron and
nickel or palladium (Figure 5 for selected orbitals; see Figures
S48, S50, and S52 for more MOs for all three complexes, ).
Some of these orbitals are relatively deep, especially for the
palladium complexes, for example, HOMO−16 and HOMO−
25 for [fc(NPPh3)2PdMe]+ and HOMO−13 for [fc-
(NPEt3)2PdMe]+. Interestingly, the palladium complexes also
contain π type iron−metal interactions; however, both the
bonding and antibonding components are occupied (Figures
S50 and S52).
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed using
standard Schlenk techniques or in an MBraun drybox (<1 ppm of O2/
H2O) unless noted otherwise. All glassware, cannulas, and Celite were
stored in an oven at >425 K before being brought into the drybox.
Solvents were purified using a two-column solid-state purification
system by the method of Grubbs68 and transferred to the glovebox
without exposure to air. NMR solvents were obtained from Cambridge
Isotope Laboratories, degassed, and stored over activated molecular
sieves prior to use. NMR spectra were recorded at ambient
temperature on Bruker AV-300, AV-500, and DRX-500 spectrometers
In order to understand further the Fe → Pd interaction,
natural bond orbital analysis was carried out using NBO 6.063
and the natural localized molecular orbitals (NLMOs) were
generated (Figures S49, S51, and S53). All three metal
E
DOI: 10.1021/acs.organomet.7b00626
Organometallics XXXX, XXX, XXX−XXX

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Article
Figure 5. Frontier molecular orbitals: HOMO−6 for [fc(NPPh3)2NiPh]+ (left) and HOMO−4 for [fc(NPPh3)2PdMe]+ (right); isosurface value =
0.03.
unless otherwise noted. Proton and carbon chemical shifts are given
relative to residual solvent peaks. Phosphorus, boron, and fluorine
chemical shifts are given relative to external standards, H3PO4, Et2O·
BF3, and 1% Freon-113 in C6D6, respectively. Compounds
(PPh3)2NiClPh,69 (COD)PdCl(Me) (COD = 1,5-cyclooctadiene),25
fc(NPPh3)2,43 fc(NPCy3)2,43 fc(N3)2,70 fc(NPCy3)2PdCl2,43 and
[AcFc][BArF]71 were prepared using literature procedures, and unless
otherwise noted, all reagents were acquired from commercial sources
and used as received. Elemental analysis of compounds [fc-
(NPPh3)2NiPh][BPh4], [fc(NPPh3)2PdMe][BPh4], [fc(NPPh3)2Pd-
(THF)][BF4]2, fc(NPEt3)2, and [fc(NPEt3)2PdMe][BPh4] was
performed on an Exeter Analytical, Inc. CE-440 Elemental Analyzer.
Elemental analysis of [fc(NPPh3)2Pd(THF)][BF4]2, [fc-
(NPEt3)2PdMe][BPh4], and fc(NPEt3)2PdCl(Me) was carried out
by Midwest Microlab, LLC, Indianapolis, IN.
[fc(NPPh3)2NiPh][BPh4]. A solution of fc(NPPh3)2 (316.9 mg,
0.413 mmol) and (PPh3)2NiClPh (261.5 mg, 0.375 mmol) in 10 mL
of methylene chloride was added to a suspension of NaBPh4 (128.6
mg, 0.375 mmol) in 5 mL of methylene chloride at 0 °C. The reaction
mixture was stirred for 1 h at 0 °C followed by a filtration through a
Celite plug. Volatile substances were removed under a reduced
pressure, and the remaining solids were washed with diethyl ether (5 ×
10 mL). The remaining solids were dissolved in 3 mL of methylene
chloride, layered with 9 mL of diethyl ether, and stored for several
hours at −35 °C. Decanting of the solution and washing with cold
diethyl ether yielded the product as a red crystalline material.
Recrystallization from methylene chloride/diethyl ether (1:3 vol %)
layering at ambient temperature and washing with diethyl ether
yielded analytically pure product (406.1 mg, 82.4%). X-ray quality
crystals were obtained from methylene chloride/diethyl ether (1:3 vol
%) layering at ambient temperature. Crystals of [fc(NPPh3)2NiPh]-
[BPh4] always contain a mixture of methylene chloride and diethyl
ether per molecule of compound as supported by NMR data. 1H NMR
(CDCl3, 500 MHz, 298 K): δ (ppm) 3.39 (t, 4H, Cp-H), 4.36 (t, 4H,
Cp-H), 5.77 (t, 2H, Ni-(m-Ph)), 5.92 (t, 1H, Ni-(p-Ph)), 6.69 (d, 2H,
Ni-(o-Ph)), 6.89 (t, 4H, B-(p-Ph)), 7.05 (t, 8H, B-(m-Ph)), 7.31−7.57
(m, 38H, B-(o-Ph), P-(o-Ph), P-(m-Ph), P-(p-Ph)). 13C NMR
(CDCl3, 126 MHz, 298 K): δ (ppm) 69.0 (d, Cp-C), 72.4 (s, Cp-
C), 80.6 (d, Cp-C), 121.6 (s, NiPh), 121.7 (s, BPh), 124.8 (d, PPh),
125.6 (m, BPh), 125.7 (s, NiPh),128.9 (d, PPh), 133.2 (d, PPh), 133.5
(d, PPh), 136.6 (s, BPh), 138.9 (s, NiPh), 141.9 (s, NiPh), 164.5 (q,
1JCB = 49.9 Hz, B-C). 11B NMR (CDCl3, 161 MHz, 298 K): δ (ppm)
−5.7 (s). 31P{1H} NMR (CDCl3, 203 MHz, 298 K): δ (ppm) 37.5 (s).
Anal. Calcd: [fc(NPPh3)2NiPh][BPh4]·(Et2O)0.5(CH2Cl2)-
(C79H70BCl2FeN2NiO0.5P2): C, 72.23; H, 5.37; N, 2.13. Found: C,
72.60; H, 5.33; N, 1.96.
[fc(NPPh3)2PdMe][BPh4]. A solution of fc(NPPh3)2 (293.0 mg,
0.398 mmol) and (COD)PdCl(Me) (101.3 mg, 0.382 mmol) in 6 mL
of methylene chloride was added to a suspension of NaBPh4 (130.8
mg, 0.382 mmol) in 4 mL of methylene chloride at ambient
temperature. The reaction mixture was stirred for 30 min at ambient
temperature followed by a filtration through a Celite plug. Volatile
substances were removed under a reduced pressure and the remaining
solids were were dissolved in 3 mL of THF. Orange crystalline
material forms from THF after several hours at ambient temperature.
Decanting of the solution and washing with cold THF yielded the
product as an orange crystalline material (319.6 mg, 67.0%). X-ray
quality crystals were obtained from methylene chloride/diethyl ether
vapor diffusion. Crystals of [fc(NPPh3)2PdMe][BPh4] always contain
a molecule of THF per molecule of compound as supported by NMR
1
data. H NMR (CD2Cl2, 500 MHz, 298 K): δ (ppm) −0.07 (s, 3H,
Pd−CH3), 3.12 (s, 4H, Cp-H), 4.58 (s, 4H, Cp-H), 6.86 (t, 4H, B-(p-
Ph)), 7.01 (t, 8H, B-(m-Ph)), 7.31 (br s, 8H, B-(o-Ph)), 7.53 (m, 12H,
P-(o-Ph)), 7.67 (t, 6H, P-(p-Ph)), 7.73 (m, 12H, P-(m-Ph)). 13C
NMR (CD2Cl2, 126 MHz, 298 K): δ (ppm) −4.7 (t, Pd-CH3), 68.2
(d, Cp-C), 72.6 (s, Cp-C), 88.3 (d, Cp-C), 122.2 (s, BPh), 126.1 (q,
BPh), 126.9 (d, PPh), 129.5 (d, PPh), 133.8 (d, PPh), 134.0 (d, PPh),
1
136.5 (q, BPh), 164.6 (q, JCB = 49.4 Hz, B-C). 11B NMR (CD2Cl2,
161 MHz, 298 K): δ (ppm) −5.9 (s). 31P{1H} NMR (CD2Cl2, 203
MHz, 298 K): δ (ppm) 30.2 (s). Anal. Calcd: [fc(NPPh3)2PdMe]-
[BPh4]·(THF)1 (C75H69BFeN2OP2Pd): C, 72.10; H, 5.57; N, 2.24.
Found: C, 72.13; H, 5.61; N, 2.29.
[fc(NPPh3)2Pd(THF)][BF4]2. To a suspension of AgBF4 (54.4 mg,
0.279 mmol) in 2 mL of methylene chloride was added [fc-
(NPPh3)2PdMe][BPh4] (116.3 mg, 0.093 mmol) in 5 mL of
methylene chloride at ambient temperature. The reaction solution
was stirred for 10 min at ambient temperature before being filtered
through a Celite plug. The filtrate was concentrated to 2 mL followed
by the addition of a few drops of THF, layering with 2 mL of diethyl
ether, and storing overnight at −35 °C. Decanting of the solution and
washing with cold diethyl ether yielded the product as dark golden
plates (79.9 mg, 73.2%). X-ray quality crystals were obtained from a
methylene chloride/THF (1:2 vol %) layering at ambient temperature.
Crystals of [fc(NPPh3)2Pd(THF)][BF4]2 always contain 1.5 molecules
of methylene chloride per molecule of compound as supported by
1
NMR spectroscopic data. H NMR (CD2Cl2, 500 MHz, 298 K): δ
(ppm) 1.61 (m, 4H, CH2), 3.53 (m, 4H, OCH2), 3.63 (t, 4H, Cp-H),
5.39 (t, 4H, Cp-H), 7.66 (m, 12H, o-Ph), 7.75 (m, 18H, m-Ph, p-Ph).
F
DOI: 10.1021/acs.organomet.7b00626
Organometallics XXXX, XXX, XXX−XXX

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Article
13C NMR (CD2Cl2, 126 MHz, 298 K): δ (ppm) 25.9 (s, CH2), 69.1 (s,
OCH2), 73.2 (d, Cp-C), 83.8 (s, Cp-C), 106.9 (d, Cp-C), 123.9 (d,
PPh), 130.8 (d, PPh), 133.3 (d, PPh), 135.4 (d, PPh). 11B NMR
(CD2Cl2, 161 MHz, 298 K): δ (ppm) −0.4 (br s). 19F NMR spectrum
(CD2Cl2, 282 MHz, 298 K) of [fc(NPPh3)2Pd(THF)][BF4]2: δ
(ppm) −152.0 (br s), −152.1 (br s). 31P{1H} NMR (CD2Cl2, 203
MHz, 298 K): δ (ppm) 37.4 (s). Anal. Calcd: [fc(NPPh3)2Pd(THF)]-
[(BF4)2]·(CH2Cl2)1.5 (C51.5H49B2Cl3F8FeN2OP2Pd): C, 50.86; H,
4.06; N, 2.30. Found: C, 51.20; H, 3.97; N, 2.28.
NMR-Scale Polymerizations. To
a
small vial, [fc-
(NPPh3)2PdMe][BPh4] (5 μmol), monomer (0.5 mmol), and 0.5
mL of CD2Cl2 were added. The contents of the vial were stirred, and
the homogeneous solution was transferred to a J. Young NMR tube
equipped with a Teflon valve. The NMR tube was sealed, taken out of
the box, and placed in an oil bath. The polymerization activity was
1
monitored by H NMR spectroscopy. In the case of ethylene, the J.
Young NMR tube was degassed via three cycles of freeze−pump−thaw
and refilled with 1.0 atm of ethylene.
fc(NPEt3)2. To fc(N3)2 (194.0 mg, 0.724 mmol) in 8 mL of hexanes
was added dropwise PEt3 (0.21 mL, 1.45 mmol) in 8 mL of hexanes
under subdued light. The reaction solution was stirred for 1 h at
ambient temperature. The hexanes solution was decanted and volatile
substances were removed under a reduced pressure. The remaining
orange crystalline solids were dissolved in 2 mL of diethyl ether,
layered with 2 mL of hexanes, and stored for 18 h at −35 °C.
Decanting of the solution and washing with 2 mL of cold hexanes
yielded the product as red crystals (295.1 mg, 90.9%). 1H NMR
(CDCl3, 500 MHz, 298 K): δ (ppm) 1.13 (m, 18H, CH2CH3), 1.85
(m, 12H, CH2CH3), 3.66 (t, 4H, Cp-H), 3.74 (t, 4H, Cp-H). 13C
NMR (CDCl3, 126 MHz, 298 K): δ (ppm) 6.6 (d, CH2CH3), 18.0 (d,
CH2CH3), 61.3 (d, Cp-C), 64.8 (s, Cp-C), 111.6 (s, Cp-C). 31P{1H}
(CDCl3, 203 MHz, 298 K): δ (ppm) 28.9 (s). Anal. Calcd: fc(NPEt3)2
(C22H38FeN2P2): C, 58.94; H, 8.54; N, 6.25. Found: C, 59.15; H, 8.55;
N, 6.18.
fc(NPEt3)2PdCl(Me). To (COD)PdCl(Me) (80.2 mg, 0.303
mmol) in 3 mL of methylene chloride was added fc(NPEt3)2 (123.3
mg, 0.275 mmol) in 4 mL of methylene chloride, and the reaction
mixture was stirred for 1 h at ambient temperature. The reaction
mixture was filtered through Celite, volatile substances removed under
a reduced pressure, and the product extracted with 5 mL of toluene.
Removal of toluene under a reduced pressure yielded the product as
an orange oil (120.9 mg, 72.6%). 1H NMR (C6D6, 500 MHz, 298 K):
δ (ppm) 0.72 (m, 9H, CH2CH3), 0.81 (m, 9H, CH2CH3), 1.21 (s, 3H,
Pd−CH3), 1.45 (m, 3H, CH2CH3), 1.70 (m, 6H, CH2CH3), 2.39 (m,
3H, CH2CH3), 3.75 (br s, 1H, Cp-H), 3.77 (br s, 1H, Cp-H), 3.80 (br
s, 1H, Cp-H), 3.88 (br s, 1H, Cp-H), 4.05 (br s, 1H, Cp-H), 4.08 (br s,
1H, Cp-H), 5.19 (br s, 1H, Cp-H), 5.52 (br s, 1H, Cp-H). 13C NMR
(C6D6, 126 MHz, 298 K): δ (ppm) −9.8 (t, Pd-CH3), 6.8 (d,
CH2CH3), 7.2 (d, CH2CH3), 18.3 (d, CH2CH3), 18.8 (d, CH2CH3),
63.1 (s, Cp-C), 63.7 (s, Cp-C), 65.4 (s, Cp-C), 65.8 (s, Cp-C), 67.0 (s,
Cp-C), 67.1 (s, Cp-C), 68.1 (s, Cp-C), 70.1 (s, Cp-C), 109.1 (d, Cp-
C), 109.3 (m, Cp-C). 31P{1H} NMR (C6D6, 203 MHz, 298 K): δ
(ppm) 43.3 (s), 45.1 (s). Anal. Calcd: fc(NPEt3)2PdCl(Me)
(C22H40FeN2P2Pd): C, 45.64; H, 6.83; N, 4.63. Found: C, 45.74; H,
6.71; N, 4.72.
Electrochemical Studies. Cyclic voltammetry studies were
carried out in a 20 mL scintillation vial with electrodes fixed in
position by a rubber stopper, in a 0.10 M tetrabutylammonium
hexafluorophosphate or tetrapropylammonium tetrakis(3,5-bis-
(trifluoromethyl)phenyl)borate solution in methylene chloride. A
glassy carbon working electrode (planar circular area = 0.071 cm2), a
platinum reference electrode (planar circular area = 0.031 cm2), and a
silver-wire pseudoreference electrode were purchased from CH
Instruments. Before each cyclic voltammogram was recorded, the
working and auxiliary electrodes were polished with an aqueous
suspension of 0.05 μm alumina on a Microcloth polishing pad. Cyclic
voltammograms were acquired with a CH Instruments CHI630D
potentiostat and recorded with CH Instruments software (version
13.04) with data processing on Origin 9.2. All potentials are given with
respect to the ferrocene-ferrocenium couple.
X-ray Crystallography. X-ray quality crystals were obtained from
various concentrated solutions placed in a −35 °C freezer in the
glovebox unless otherwise specified. Inside the glovebox, the crystals
were coated with oil (STP Oil Treatment) on a microscope slide,
which was brought outside the glovebox. The X-ray data collections
were carried out on a Bruker SMART 1000 single crystal X-ray
diffractometer using Mo Kα radiation and a SMART APEX CCD
detector. The data was reduced by SAINTPLUS and an empirical
absorption correction was applied using the package SADABS. The
structure was solved and refined using SHELXTL (Bruker 1998,
SMART, SAINT, XPREP, AND SHELXTL, Bruker AXS Inc.,
Madison, WI). Tables with atomic coordinates and equivalent
isotropic displacement parameters, with all the distances and angles,
and with anisotropic displacement parameters are listed in the
Supporting Information.
DFT Calculations. All structures were optimized using the
ADF2013.01 software suite.57−59 Full molecules were used for
calculations. Optimizations were performed at the PW9160 theory
level, with full electron (no frozen cores) triple-ζ-potential (TZP)
basis sets and using the relativistic scalar ZORA approximation. Mayer
bond orders and atomic properties were calculated using the defaults
implemented in the ADF2013.01 program suite. The optimized
coordinates were used for further analysis with NBO 6.0 and Bader’s
Atoms In Molecules (AIM) methods, implemented in ADF.
[fc(NPEt3)2PdMe][BPh4]. To fc(NPEt3)2PdCl(Me) (156.5 mg,
0.258 mmol) in 8 mL of methylene chloride was added solid NaBPh4
(88.5 mg, 0.258 mmol), and the resulting suspension stirred for 30
min at ambient temperature. The reaction mixture was filtered through
Celite followed by removal of volatile substances under reduced
pressure. The remaining solids were dissolved in 2 mL of methylene
chloride and layered with 6 mL of diethyl ether. Orange crystalline
material forms after several hours at ambient temperature. Decanting
of the solution and washing with diethyl ether yielded the product as
an orange crystalline material (181.8 mg, 79.3%). X-ray quality crystals
were obtained from methylene chloride/diethyl ether vapor diffusion.
Crystals of [fc(NPEt3)2PdMe][BPh4] always contain a molecule of
methylene chloride per molecule of compound as supported by NMR
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00626.
NMR spectra, cyclic voltammograms, X-ray, and DFT
1
calculation data (PDF)
spectroscopic data. H NMR (CDCl3, 500 MHz, 298 K): δ (ppm)
0.91 (s, 3H, Pd−CH3), 1.09 (m, 18H, CH2CH3), 1.82 (m, 12H,
CH2CH3), 2.97 (t, 4H, Cp-H), 4.63 (t, 4H, Cp-H), 6.89 (t, 4H, B-(p-
Ph)), 7.04 (t, 8H, B-(m-Ph)), 7.41 (br s, 8H, B-(o-Ph)). 13C NMR
(CDCl3, 126 MHz, 298 K): δ (ppm) −11.1 (t, Pd-CH3), 6.1 (d,
CH2CH3), 18.1 (d, CH2CH3), 67.5 (d, Cp-C), 72.5 (s, Cp-C), 87.6 (d,
Cartesian coordinates (XYZ)
Accession Codes
CCDC 1569203−1569206 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Cp-C), 121.8 (s, BPh), 125.6 (q, BPh), 136.5 (q, BPh), 164.4 (q, 1JCB
=
49.3 Hz, B-C). 11B NMR (CDCl3, 161 MHz, 298 K): δ (ppm) −5.7
(s). 31P{1H} NMR (CDCl3, 203 MHz, 298 K): δ (ppm) 57.3 (s).
Anal. Calcd: [fc(NPEt3)2PdMe][BPh4]·(CH2Cl2) (C48H63BCl2FeN2-
P2Pd): C, 59.19; H, 6.52; N, 2.88. Found: C, 59.18; H, 6.49; N, 2.78.
G
DOI: 10.1021/acs.organomet.7b00626
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(26) Chen, M.; Yang, B.; Chen, C. Angew. Chem., Int. Ed. 2015, 54
(51), 15520−15524.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: pld@chem.ucla.edu.
ORCID
Paula L. Diaconescu: 0000-0003-2732-4155
Notes
The authors declare no competing financial interest.
(27) Zou, W.; Pang, W.; Chen, C. Inorg. Chem. Front. 2017, 4 (5),
795−800.
(28) Feyrer, A.; Breher, F. Inorg. Chem. Front. 2017, 4 (7), 1125−
1134.
(29) Shepard, S. M.; Diaconescu, P. L. Organometallics 2016, 35 (15),
2446−2453.
(30) Klenk, S.; Rupf, S.; Suntrup, L.; van der Meer, M.; Sarkar, B.
Organometallics 2017, 36 (10), 2026−2035.
(31) Cabrera, K. D.; Rowland, A. T.; Szarko, J. M.; Diaconescu, P. L.;
Bezpalko, M. W.; Kassel, W. S.; Nataro, C. Dalton Trans. 2017, 46,
5702−5710.
ACKNOWLEDGMENTS
■
We thank the NSF, Grant 1362999 and CHE-1048804 for
NMR spectroscopy.
́ ́
(32) Blass, B. L.; Hernandez Sanchez, R.; Decker, V. A.; Robinson,
M. J.; Piro, N. A.; Kassel, W. S.; Diaconescu, P. L.; Nataro, C.
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