Outer-Sphere Reactivity Shift of Secondary Phosphine Oxide-Based Nickel Complexes: From Ethylene Hydrophosphinylation to Oligomerization

Article abstract of DOI:10.1002/chem.201701414

A new dimension for secondary phosphine oxide (SPOs) ligands is described in this article. Demonstrated on original π-allylic nickel structures, these self-assembled complexes trigger catalytic hydrophosphinylation reactions. Addition of a Lewis acid B(C₆F₅)₃ switches the reactivity towards migratory insertion and thus ethylene oligomerization through an unprecedented outer-sphere interaction with the coordinated SPO ligand. NMR experiments and X-ray analyses allowed for the observation of the formation of zwitterionic active species as well as their degradation pathway.

Full text of DOI:10.1002/chem.201701414

A Journal of
Accepted Article
Title: Outer-sphere reactivity shift from hydrophosphinylation to
oligomerisation through secondary phosphine oxide-based nickel
complexes
Authors: Rudy Lhermet, Emile Moser, Erwann Jeanneau, Hélène
Olivier-Bourbigou, and Pierre-Alain René Breuil
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To be cited as: Chem. Eur. J. 10.1002/chem.201701414
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Outer-sphere reactivity shift of secondary phosphine oxide-based
nickel complexes : From ethylene hydrophosphinylation to
oligomerization
Rudy Lhermet,[a] Emile Moser,[a] Erwann Jeanneau,[b] Hélène Olivier-Bourbigou,[a] and Pierre-Alain R.
Breuil*[a]
Abstract: A new dimension for secondary phosphine oxide (SPOs)
ligands is described in this article. Demonstrated on original π-allylic
nickel structures, these self-assembled complexes trigger catalytic
hydrophosphinylation reactions. Addition of a Lewis acid as B(C6F5)3
switches the reactivity towards migratory insertion and thus ethylene
oligomerisation through an unprecedented outer-sphere interaction
with the coordinated SPO ligand. NMR experiments and X ray
analyses allowed to observe the zwitterionic active species formed
as well as its degradation pathway.
While tertiary phosphines are among the most commonly used
ligands for transition metal homogeneous catalysis,[1] the
potential of the air- and moisture-stable secondary phosphine
oxides (SPOs) has been considered only recently although the
first example of SPO-based metal complex was disclosed in
1983 for the platinum-catalysed hydroformylation of terminal
alkenes.[2] The ligands generated from SPOs present a great
range of electronic properties thanks to their modularity
(monodentate or bidentate, neutral or anionic, see Figure 1) that
Figure 1. SPO modularity in coordination chemistry.
still needs to be assessed in catalysis.[3] They are considered for
gold nanoparticles stabilisation,[4] for effective asymmetric noble
metal-catalysed transformations as hydrogenation,[5] or for
Although prone to promote hydrophosphinylation reactions, we
coupling reactions.[6] By contrast, when considering catalyst
anticipated that structures presenting a cationic nickel(II) species
design and access to well-defined species, examples with first-
bearing metal-hydride or metal-carbon bonds could be of
row and low-cost transition metals such as nickel remain more
interest for migratory insertion of olefins to produce oligomers or
than sporadic despite their unique properties compared to noble
polymers.[9] However, due to their high reactivity and their
metals as palladium. To the best of our knowledge, only Kläui
instability, these organonickel compounds are usually generated
followed by Walther and Hartung reported such isolated
in situ from a nickel(II) halide precatalyst and an excess of
structures, without further reactivity studies.[7] More recently, Han
aluminum activator rather than isolated.[10] Herein, we describe a
and
hydrophosphinylation of terminal alkynes with a mixture of
[Ni(PPh2Me)4] and Ph2P(O)H,[8] generating in situ
five-
coworkers
disclosed
the
nickel-catalyzed
family of supramolecular organonickel complexes and their
reactivity shift from hydrophosphinylation towards ethylene
oligomerization using unprecedented outer-sphere interactions
of Lewis acids with SPOs.
a
coordinated hydrido phosphinito nickel complex (D, Figure 1),
characterized by 1H NMR spectroscopy but not isolated.
We reported recently that bis(cyclooctadiene)nickel(0)
[Ni(COD)2] reacts with sulfonamido-phosphines in the presence
of a second phosphine ligand, leading to a self-assembled
supramolecular organometallic species.[11] We thus wondered if
similar structure could be reached starting from simple SPO
preligands. Walther and Hartung described the reaction between
[Ni(COD)2] and two equivalents of diphenylphosphine oxide in
THF and identified [Ni(Ph2PO)2(COD)] as resulting complex,
after concomitant release of hydrogen and COD.[7] Interestingly,
performing this reaction in toluene, we isolated the diamagnetic
π-allylic nickel 1a bearing a phosphinito-phosphinous acid
system as a yellow solid in 96% yield (Scheme 1). The structure
of complex 1a was confirmed by X ray diffraction on monocrystal
as well as 1H, 13C, 31P NMR spectroscopies and elemental
analysis.
[a]
[b]
Dr. R. Lhermet, E. Moser, Dr. H. Olivier-Bourbigou, Dr. P.-A. R.
Breuil
IFP Energies nouvelles, Rond-point de l’échangeur de Solaize, BP
3, 69360 Solaize, France
E-mail: pierre-alain.breuil@ifpen.fr
Webpage : http://www.ifpenergiesnouvelles.com
Dr. E. Jeanneau
Centre de Diffractométrie Henri Longchambon
Site CLEA - Bât. ISA, 3ème étage, 5 rue de La Doua; 69100
Villeurbanne, France
Supporting information for this article is given via a link at the end of
the document.
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Under ethylene pressure and at 40°C, complexes 1a-1f conduct
to the formation of traces of tertiary ethyl phosphine oxides and
vinylcylooctene, confirmed by GC/MS, 1H and 31P NMR
spectroscopies, along with complex degradation. We postulated
an activation of complexes 1 under ethylene pressure, leading to
the zwitterionic alkylnickel intermediates 2 after departure of the
vinylcylooctene (Figure 4). These species probably coexist with
their neutral form 2’ through a P(σ4λ5)-P(σ3λ3) equilibrium. In
agreement with phosphorylation reactions, the formation of
these tertiary ethyl phosphine oxide species can result from
reductive elimination from Ni-ethyl intermediate 2’.[8,12]
a
Scheme 1. Synthesis of supramolecular SPO complexes. Reaction carried
without COD.
Complex 1a adopts a square-planar coordination geometry with
P-O bond lengths (1.553 and 1.559 Å) half way between single
and double bonds (1.500-1.630 Å), traducing the negative
charge delocalization (Figure 2). Equivalence of the two
phosphorus atoms is supported by 31P NMR, with a sole singlet
at 101 ppm. This proton exchange equilibrium is also pointed out
by the great O-H-O angle (175.24°), the short O-O distance
(2.431 Å) and the deshielded phosphinous acid proton (16.1
ppm). Using
a related protocol, complexes 1b-1f were
synthesized from the corresponding SPOs (Scheme 1).
Nevertheless, an additional amount of COD was necessary to
prevent metallic nickel formation. All these compounds are
diamagnetic and were characterized by NMR analyses. They
also present two equivalent phosphorus atoms with a single
signal between 98 and 110 ppm as well as a deshielded
phosphinous acid protons (see SI). Crystals of complex 1b,
suitable for X-ray analysis, were grown in toluene. 1b structure
displays similar square planar features to 1a (Figure 3), with an
almost flat O-H-O angle (167.58°) and a short O-O distance
(2.425 Å).
Figure 4. Reactivity path of SPO-based nickel complexes.
The catalytic feature of complex 1a for hydrophosphinylation
transformations was demonstrated with ethylene or 1-octyne
and Ph2P(O)H. Under catalytic conditions (5 mol % of Ni, 30 bar
of C2H4 and 70°C), complex 1a efficiently coupled Ph2P(O)H and
C2H4 in toluene to form Ph2P(O)Et with up to 73% yield. When
focusing on 1-octyne, well-defined species 1a led at 70°C in
toluene to a mixture of branched and linear regio-isomers with
33% and 28% yield, respectively.
Figure 2. ORTEP plot (50% probability displacement ellipsoids) of complex 1a.
Hydrogen atoms have been omitted for clarity (except for the OHO moiety).
Scheme 1. Nickel-catalyzed hydrophosphinylation reactions.
We hypothesized that addition of Lewis acids may allow an
outer-sphere modulation of the complex electronics and hamper
Figure 3. ORTEP plot (50% probability displacement ellipsoids) of complex 1b.
Hydrogen atoms have been omitted for clarity (except for the OHO moiety).
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GC. [f] 1a solubilized in CH2Cl2 (2 mL). [g] B(C6F5)3 replaced by GaCl3
(2 eq.).
in this example contribution of structure 2', preventing P(III)-P(V)
rearrangement which favors phosphorylation reactions and
moreover, preserve the electro-deficient character of the nickel
center profitable to ethylene oligomerization reactions.[9,13] The
proof-of-concept
was
established
with
To gain mechanistic insights, 1a was mixed in CD2Cl2 with one
equivalent of B(C6F5)3 and monitored by phosphorus decoupled
proton NMR spectroscopy. Within ten minutes, disappearance of
the phosphinito-phosphinous acid system signal at 101 ppm was
tris(pentafluorophenyl)borane as a remote activator (Table 2).
Indeed, mixing precursor 1a with B(C6F5)3 (2 equivalents) under
ethylene pressure triggered the ethylene consumption, leading
to short-chain linear olefins (C4 to C8), with up to 98.9% for 1-
butene in the C4 fraction, although deactivation was noticed after
1 hour of reaction (see SI). In similar conditions, complex 1b
provided a more active and stable catalyst (6467 goligo/(gNi.h)),
while no activity was noticed with complex 1c at 40°C, 60°C
being required for a weak reaction initiation, highlighting the
induction of SPO electronic properties (para-CF3 in 1b and para-
OMe in 1c). Introducing steric hindrance on the SPOs as in
complex 1d allows the access to a wider product distribution
(KSCHULZ-FLORY~0.7). Similarly to 1c, solely a slight reactivity could
be detected with benzyl substituted SPO complex 1e at 60°C.
This detrimental effect of electron-rich SPOs was confirmed with
the alkyl analog 1f, inert even at 60°C. Interestingly,
replacement of B(C6F5)3 with GaCl3 afforded a much more active
system in the case of 1a, with a selectivity of 93.6% towards
butenes. With para-CF3 SPO-based complex 1b, ethylene
uptake was lower than expected, with a butene selectivity up to
94.8%. Surprisingly, a 1:1 ratio between complex 1a and borane
(50 µmol) also resulted in an absence of activity. In addition,
when adding few equivalents of Ph2P(O)H to 1a and B(C6F5)3
observed. The rise of two new doublets indicated
a
disymmetrization of the system. Cis configuration of the P atoms
was preserved, as evidenced by a weak 31P coupling constant
(JPP = 32 Hz). Taking a closer look at the signal at 110 ppm
revealed a doublet of septuplet, due to a close interaction of one
phosphorus with the six ortho fluorine atoms of B(C6F5)3 (JPF = 5
Hz, Figure 5). Moreover, a new broad singlet on the proton NMR
spectra rose in the allylic area, attributed to a P-OH signal. All
these observations support formation of the zwitterionic
dissymmetric borate adduct 3a (Figure 4).
under
ethylene,
Ph2P(O)Et
is
obtained
as
the
hydrophosphinylation product. The control of the olefin
distribution brought by inner-sphere and outer-sphere
modifications, through the nature of the SPO pre-ligand or the
Lewis acid, respectively, lies in the heart of the industrial
developments to meet the flexibility requirements of the LAO
markets.
Figure 5. 31P NMR spectroscopy of complex 3 highlighting the interaction
between one phosphorus and B(C6F5)3.
The structure of adduct 3a was finally confirmed by elemental
analysis and X ray analysis on monocrystal, obtained by slow
diffusion of pentane in a toluene solution (Figure 6). Similarly,
addition of two equivalents of B(C6F5)3 to 1a led only to adduct 3
on the 31P NMR spectra, in solution with free borane detected by
19F NMR (see SI). Keeping the solution five days at room
temperature resulted in the complete conversion of 3a into a
new symmetrical compound 4a, fully characterized by NMR
spectroscopies and elemental analysis. This preferred reaction
path is in line with preliminary DFT calculations (wB97XD) from
the structures determined by XRD. The formation of the boron
adduct 3 from complex 1a is thermodynamically favored by 34.3
kcal.mol-1. In addition, the degradation path of adduct 3, leading
to the stable complex 4a and the departure of C6F5H is also
confirmed to be exergonic (ΔG = -15.0 kcal.mol-1). Compound 4a
displayed two equivalent phosphorous atoms but no deshielded
proton. Monocrystals could be obtained by slow diffusion of
pentane in a toluene solution (Figure 7), confirming the bridging
borane moiety through XRD.[14] Complex 4a is not active
towards ethylene at 40°C, even in the presence of additional
B(C6F5)3. Thus, the requirement of two equivalents of borane
during catalysis remains unclear. In situ 31P NMR study of 1a
Table 2. Catalytic evaluation of complexes 1a-f associated with
B(C6F5)3 and GaCl3.[a]
Complex
T
(°C)
moligo
(g) [b]
Prod.
Product distribution (%) [d]
[c]
[e]
+
C4 (1-C4
)
C6
25.1
22.8
20.3
6.2
C8
1a[f]
1b
40
40
60
40
60
60
40
40
8.6
14.2
<1
3919
6467
<500
8778
<500
-
56.1 (98.9)
67.4 (91.9)
59.0 (96.3)
14.5 (83.7)
31.8 (86.9)
-
18.8
9.8
1c
20.7
79.3
54.1
-
1d
19.3
<1
1e
14.1
-
1f
-
1a[f,g]
1b[g]
15.9
5.2
7224
2340
93.6 (59.7)
94.8 (78.5)
4.8
1.6
4.8
1.0
[a] Ni complex (25 μmol), B(C6F5)3 (50 μmol), toluene (30 mL), 30
bars of C2H4, 90 min. [b] Mass of oligomers formed during the
reaction in grams. [c] Productivity in goligo/(gNi.h). [d] In wt%,
determined by GC. [e] 1-Butene wt% in C4 fraction, determined by
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supporting information file. This material is available free of charge via
the Internet.
under ethylene with 1 or 2 equivalents of B(C6F5)3, suggests that
with one equivalent of borane, the decoordination of the SPO
ligands occurs while with 2 equivalents, the ligands coordination
is longer preserved, sustaining an enhanced catalyst lifetime
(see SI).
Acknowledgements
We thank IFP Energies nouvelles for financial support, Dr Pierre
Boulens for preliminary results.
Keywords: Secondary phosphine oxide ligand • nickel • Lewis
acid interaction • outer-sphere coordination • homogeneous
catalysis
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Figure 6. ORTEP plot (50% probability displacement ellipsoids) of complex 3a.
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Figure 7. ORTEP plot (50% probability displacement ellipsoids) of complex 4a.
Hydrogen atoms have been omitted for clarity.
In conclusion, we synthesized and fully characterized a new
family of supramolecular phosphinito-phosphinous acid π-allylic
nickel complexes. We highlight a unique remote control of the
metallic center and its electro-density through SPO ligands to
switch the complex reactivity towards elementary migratory
insertion and ethylene oligomerization and selectivity. Ultimately,
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Experimental Section
Ligand and complex syntheses and characterizations, crystal structure
determinations, DFT calculations and reactivity study are detailed in the
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Rudy Lhermet, Erwann Jeanneau,
Hélène Olivier-Bourbigou, Pierre-Alain
R. Breuil*
Page No. – Page No.
Title
A new dimension for secondary phosphine oxide (SPOs) ligands is described in this
article. Demonstrated on original π-allylic nickel structures, these self-assembled
complexes trigger catalytic hydrophosphinylation reactions. Addition of a Lewis acid
as B(C6F5)3 switches the reactivity towards migratory insertion and thus ethylene
oligomerisation through an unprecedented outer-sphere interaction with the
coordinated SPO ligand.
This article is protected by copyright. All rights reserved.