Parallels between Metal-Ligand Cooperativity and Frustrated Lewis Pairs

Article abstract of DOI:10.1002/ejic.201900169

Metal ligand cooperativity (MLC) and frustrated Lewis pair (FLP) chemistry both feature the cooperative action of a Lewis acidic and a Lewis basic site on a substrate. A lot of work has been carried out in the field of FLPs to prevent Lewis adduct formation, which often reduces the FLP reactivity. Parallels are drawn between the two systems by looking at their reactivity with CO₂, and we explore the role of steric bulk in preventing dimer formation in MLC systems.

Full text of DOI:10.1002/ejic.201900169

A Journal of
Accepted Article
Title: Parallels between Metal-Ligand Cooperativity and Frustrated
Lewis Pairs
Authors: Evi Habraken, Andrew Jupp, Marie Brands, Martin Nieger,
Andreas Ehlers, and Chris Slootweg
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900169
Link to VoR: http://dx.doi.org/10.1002/ejic.201900169

10.1002/ejic.201900169
European Journal of Inorganic Chemistry
FULL PAPER
Parallels between Metal-Ligand Cooperativity and Frustrated
Lewis Pairs
Evi R. M. Habraken,[a] Andrew R. Jupp,[a] Maria B. Brands,[a] Martin Nieger,[b] Andreas W. Ehlers[a,c] and
J. Chris Slootweg*[a]
Abstract: Metal ligand cooperativity (MLC) and frustrated Lewis pair
(FLP) chemistry both feature the cooperative action of a Lewis acidic
and a Lewis basic site. A lot of work has been carried out in the field
of FLPs to prevent Lewis adduct formation, which often reduces the
FLP reactivity. Parallels are drawn between the two systems by
looking at their reactivity with CO2, and we explore the role of steric
bulk in preventing dimer formation in MLC systems.
groups on the donor and/or acceptor sites can induce frustration
and prevent adduct formation. The unquenched reactivity of the
Lewis acid and base has been exploited for the activation of small
molecules, such as H2 and CO2, and for the subsequent catalytic
hydrogenation of unsaturated substrates.[4] The Lewis acid and
base can be tethered to afford an intramolecular FLP (Scheme
1.iii), which allows for preorganization of the reactive site and can
reduce the (entropic) energy barrier for such activation
reactions.[5 , 6 ] The interplay between the electronic and steric
properties of the Lewis acids and bases is of paramount
importance in determining the activity of the FLP system.
Introduction
Over the past decades catalysis has been dominated by transition
metal complexes. The partially filled d-orbitals grant the metal
centre both donor and acceptor orbitals on a single atom, and
allows prototypical transition metal reactivity such as oxidative
addition of dihydrogen, shown in Scheme 1.i, which involves an
increase on the formal oxidation state of the metal by +2. In these
cases, the surrounding ligands are crucial for tuning the electronic
and steric properties of the metal centre, but they are not directly
involved in the reactions. Separating the donor and acceptor site
has led to new reaction pathways for catalysis. This reactivity
occurs when the ligand actively participates in substrate activation
together with the metal centre, and has been termed
bifunctionality or metal-ligand cooperativity (MLC), shown in
Scheme 1.ii. Noyori first demonstrated this concept with a
ruthenium-phosphine complex bearing an ethylenediamine ligand,
where the amine functionality cooperates with the metal.[1] In
these reactions the formal oxidation state of the metal is
unchanged on activation of the substrate. This topic has grown
rapidly and been reviewed on many occasions,[ 2 ] and has
important ramifications for catalyst design.
Scheme 1. Differing modes of dihydrogen activation by transition metal
complexes and frustrated Lewis pairs (M = transition metal, A = Lewis acidic
site, B = Lewis basic site, L = ligand).
The fields of FLP and MLC chemistry have both grown
rapidly and, in general, separately. However, it is clear that the
underlying cooperativity for the activation of substrates is similar
in both cases. The distinction was further blurred by the advent of
transition metal-based FLPs, where a transition metal centre is
used as one of the Lewis acidic or basic sites in an FLP.[7] Wass
Another form of cooperation can be found in transition
metal-free frustrated Lewis pairs (FLPs), where the acceptor and
donor site are also on separate sites.[3] Lewis acids and bases
typically form Lewis adducts, however incorporation of bulky
[a]
E. R. M. Habraken, Dr. A. R. Jupp, M. B. Brands, Dr. A. W. Ehlers,
Assoc. Prof. Dr. J. C. Slootweg
and
co-workers
introduced
a
cationic
zirconocene-
phosphinoaryloxide complex, with the zirconium centre acting as
a Lewis acid and a pendant phosphine acting as a Lewis base for
the activation of dihydrogen (Scheme 2.i).[8] Just as with traditional
main-group FLPs, the balance of sterics and electronics is
important, as simply switching the [C5Me5]– (Cp*) ligands for
[C5H5]– (Cp) resulted in a strong Zr–P interaction, and shut down
the FLP reactivity. This reactivity could be equally well described
as FLP or MLC chemistry, and Wass noted this insight in
subsequent articles.[ 9 a,b] The analogy has also been noted
elsewhere, especially with the transition metal-based FLPs,[9,10]
and recently Bullock and co-workers cited guiding principles from
main-group and transition metal-based FLPs in the design of
Van ’t Hoff Institute for Molecular Sciences
University of Amsterdam, Science Park 904
1090 GD Amsterdam (The Netherlands)
E-mail: j.c.slootweg@uva.nl
[b]
[c]
Dr. M. Nieger
Department of Chemistry, University of Helsinki
A. I. Virtasen aukio 1, PO Box 55, Helsinki (Finland)
Dr. A. W. Ehlers
Department of Chemistry, Science Faculty
University of Johannesburg
PO Box 254, Auckland Park, Johannesburg (South Africa)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201900169
European Journal of Inorganic Chemistry
FULL PAPER
bifunctional Mo complexes for the controlled heterolytic cleavage
of dihydrogen (Scheme 2.ii).[11] Herein we further explore the
relationships between archetypal MLC and FLP systems, in
particular investigate the dimerization of the active MLC-monomer
by Lewis adduct formation, and to consolidate the knowledge
garnered from the two topics.
Figure 1. Top: Milstein system, activation of precursor by deprotonation with a
base resulting in the dearomatized species. Bottom: molecular orbital diagram
of MLC 3 (isopropyl groups omitted for clarity) and FLP 5 (left: HOMO, right:
LUMO) calculated at the ωB97X-D/6-311G(d,p) level of theory. Blue: Lewis
acid; red: Lewis base.
Scheme 2. Transition metal-based FLP reactivity, and/or MLC reactivity:
activation of dihydrogen by i) Wass’s Zr complex[8] and ii) Bullock’s Mo
complex[11]. Blue: Lewis acid; red: Lewis base.
To compare this traditional MLC system with a main-group
FLP, we opted to study the intramolecular FLP, 5 (Figure 1). The
acidic and basic components are preorganised by the methylene
bridge in the ideal orientation to activate a range of small molecule
substrates, including dihydrogen, carbon dioxide and isocyanates,
despite the lack of strong electron-withdrawing groups on the
boron centre.[6] The HOMO is the lone pair on phosphorus, and
the LUMO is predominantly the formally vacant p orbital on boron.
The parallels between the orbitals of 3 and 5 should bear out in
their reactivity, so we resorted to DFT calculations to provide
detailed mechanistic insight into the mode of activation of carbon
dioxide of the two systems.
Milstein and co-workers already partially elaborated on the
activation of CO2 for 4,[16] which we extended for 3 to investigate
the influence of the steric bulk, and this was compared to the
geminal FLP system 5 (Figure 2). The latter was also investigated
in the original publication, but at a different level of theory, so all
calculations herein were carried out using ωB97X-D/6-311G(d,p)
for ease and relevance of comparison. Pertinent bond metric data
are included in Table 1, including the bond distances between the
CO2 and the Lewis acidic and basic sites, as well as the bond
lengths and angle within the CO2 moiety. For both MLC systems,
first a van der Waals complex is formed with long distances
between the MLC and CO2, and the CO2 moiety has barely
deviated from linearity. The complex is energetically favourable,
but the ΔG values are slightly uphill due to a decrease in entropy.
This initial complexation is followed by a nucleophilic attack by the
ligand-based carbon to the carbon of CO2 in an asynchronous
concerted transition state (3 ΔGǂ = 3.3; 4 ΔGǂ = 3.8 kcal∙mol–1).
In both cases the Ru-O and C-C bonds are still relatively long,
Results and Discussion
We chose to investigate the quintessential Ru-based PNP pincer
systems developed by Milstein, as it is well established that these
species
can
undergo
an
MLC
pathway
via
dearomatization/rearomatization of the pyridine ring.[2a] Treatment
of the precursors 1 and 2, which differ by the R group on the
phosphine, with base leads to deprotonation of one of the
methylene arms and loss of chloride to afford 3 and 4, respectively
(Figure 1).[12,13] These compounds feature a Lewis basic site on
the carbon and a Lewis acidic site on the Ru centre. This notion
was confirmed by our DFT calculations of the frontier molecular
orbitals of 3 at the ωB97X-D/6-311G(d,p) level of theory, which
showed that the highest occupied molecular orbital (HOMO) is
principally located on the deprotonated carbon, and the lowest
unoccupied molecular orbital (LUMO) is centred on the ruthenium.
In this case the “frustration” of the Lewis acidic and basic sites is
enforced by the rigid ligand framework. Otten and co-workers
have previously demonstrated the FLP-like reactivity of a related
Ru-based PNN system with nitriles,[ 14 ] in which the Ru/C
combination added in a cooperative fashion across the CN triple
bond, Milstein has also noted that the cooperative addition of CO2
across these pincer systems bears a resemblance to FLPs.[10,15]
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10.1002/ejic.201900169
European Journal of Inorganic Chemistry
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indicating an early transition state. Ring closure affords the
product with an overall energy difference of ΔG = 12.1 and 10.4
kcal∙mol–1 for 3 and 4, respectively. There is little energetic
difference between the isopropyl or tert-butyl groups during the
reaction profile, and the bond distances (largest difference 0.03
Å) and angles (largest difference 0.6 °) are similar in all cases.
Intramolecular FLPs can quench either via an
intramolecular interaction or dimerization, although “quench” in
this case is perhaps a misnomer, as it is now well established that
complete frustration is not necessary for typical FLP reactivity to
occur. Stephan and co-workers recently showed that even the
classical adduct of B(C6F5)3 and the strongly basic
proazaphosphatrane P(N(Me)CH2CH2)3N is capable of addition to
a range of heteroallenes.[17] In any case, the Lewis adduct is a
resting state; dissociation into the corresponding Lewis acid/base
components with the accompanying energy penalty is required to
induce FLP reactivity. Therefore, the control of reactivity via fine-
tuning of the steric environment is still a widely employed strategy
in FLP chemistry. In a similar manner, MLC systems are able to
dimerize via an intermolecular interaction of the Lewis acidic and
Lewis basic sites in the complex. For example, the dearomatized
Ru-PNS system dimerizes as shown in Figure 4, and
subsequently undergoes a decomposition pathway involving C-S
cleavage and loss of isobutene.[18]
Figure 2. Relative ωB97X-D/6-311G(d,p) Gibbs free energies (energy in
brackets) in kcal∙mol–1 for the reaction of CO2 and 3/4.
Insert Table 1 here (see below)
The reaction profile for 5 is similar (Figure 3). First a van der
Waals complex is formed with long distances between the FLP
and CO2 with an almost linear CO2. The reaction proceeds via an
asynchronous concerted transition state (ΔGǂ = 12.5 kcal∙mol–1),
where the Lewis basic phosphorus centre attacks the electrophilic
carbon of CO2, and the O1 is stabilised by interaction with the
boron centre. This is evidenced by the slightly longer C–O1 and
C–O2 bond lengths and the smaller bond O–C–O bond angle in
TS-5 than the analogous metrics in TS-3 and TS-4, and is in good
agreement with the previously reported bond order data for TS-
5.[6] The final product is formed by ring closure, which is exergonic
by 5.4 kcal∙mol–1. In both the MLC and FLP systems the
mechanism is the same, and differences in energies and bond
metrics are readily explained by the varying electronic nature of
the Lewis acidic and basic sites in the molecules.
Figure 4. Two examples of dimeric MLC complexes. Left: molecular X-ray
structure of [3]2 (displacement ellipsoids are set at 50% probability, isopropyl
groups on the phosphorus are omitted for clarity).[12] Selected bond lengths [Å]:
Ru1–C7’ 2.409(7), Ru1’–C7 2.403(7), P2–C7 1.797(6), C6–C7 1.456(8), C1–
C2 1.489(9), C1–P1 1.842(6), P2’–C7’ 1.803(6), C6’–C7’ 1.449(8), C1’–C2’
1.55(1), C1’–P1’ 1.843(6). Right: Milstein’s Ru(PNS) dimer A.[18] Blue: Lewis
acid; red: Lewis base.
On examination of the crystal structure of 3, as reported in
Milstein’s original publication,[12] we noted that this species is also
a dimer in the solid state. The ruthenium-carbon interatomic
distance between the two monomers in the X-ray structure (Ru1–
C7’ 2.409(7) and Ru1’–C7 2.403(7) Å) lies within the sum of the
van der Waals radii (3.75 Å)[19] suggesting a bonding interaction.
The C6–C7 bond distances (according to the atom labelling in
Figure 1) in [3]2 are 1.456(8) and 1.449(8) Å. These bond lengths
are much longer than that found in the gas-phase DFT optimized
monomer 3 (1.379 Å), but shorter than that found in the X-ray
structure of unactivated complex 1 (1.501(2) Å), which suggests
that the deprotonated arm features a C–C bond with partial double
bond character. In fact, the C6–C7 bond length is similar to that
found in A (1.458(4) Å; Figure 4).[18]
To probe the structural changes that occur during
dimerization, we examined the aromaticity of the pyridine ring of
the compounds using NICS(0) calculations.[20,21] As expected, the
unactivated precursors feature an aromatic ring (1: –6.4 ppm, 2:
Figure 3. Relative ωB97X-D/6-311G(d,p) Gibbs free energies (energy in
brackets) in kcal∙mol–1 for the reaction of CO2 and 5.
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10.1002/ejic.201900169
European Journal of Inorganic Chemistry
FULL PAPER
–6.5 ppm), whereas in the activated species dearomatization has
occurred (3: 2.0 ppm, 4: 1.3 ppm). These values are consistent
with previous studies by Gonçalves and Huang on similar
organometallic pincer complexes.[22,23] Interestingly, the dimer [3]2
has a value (–4.7 ppm) between that of 1 and 3, indicating partial
rearomatization of the pyridine ring and a contributing factor to the
stability of the dimer.
slightly (ΔG = 12.8 kcal∙mol–1), as benzene only weakly
coordinates to the Ru centre in an η2 fashion. Once again, the
more coordinating THF stabilises the monomer to a greater extent,
making it easier to cleave the dimer (ΔG = 5.5 kcal∙mol–1); we
anticipate that coordinating substrates will have a similar effect.
Interestingly, and reminiscent of the tenets of FLP chemistry,
increasing the steric bulk of the alkyl substituents on phosphorus
from isopropyl to tert-butyl (i.e. going from 3 to 4) destabilises
these Lewis acid/Lewis base interactions and precludes dimer
formation. It was not possible to locate a minimum on the potential
energy surface corresponding to the structure of [4]2, and all
attempts led to regeneration of the two monomers during the
optimization process.
Figure 5. Computed AIM bond paths of [3]2, a simplified framework is depicted
(isopropyl groups on P and all H atoms omitted for clarity); bond critical points
(BCP) in red, ring critical points (RCP) in green.
The bonding situation in [3]2 was further analysed using AIM
analyses,[24,25] which revealed a bond critical point (BCP) between
the Lewis acidic Ru site of one monomer and the Lewis basic C7
site of the other monomer (Figure 5, ρ = 0.047 a.u. (ξ = 0.21), Ru–
C7 2.499 Å), indicative of a weak interaction. Furthermore, a ring
critical point (RCP) is found in the dimer between the two
monomers. The examination of the Laplacian of the electron
densities (∇2ρ) in the C6–C7 bond reveals a weaker interaction
for the dimer compared to the monomer, yet still stronger than for
1 ([3]2: –0.66 a.u., 3: –0.83 a.u., 1: –0.58 a.u.). ETS-NOCV[26]
analyses of the dimer, which we have used to assess donor-
acceptor interactions, concur with these observations and
revealed an interaction between ruthenium and the carbon in the
backbone of both monomers, showing orbital interactions and
specifically σ donations of 24.4 and 20.7 kcal∙mol–1 from C7 to Ru.
The monomeric pincer complexes are the active species in
catalysis, therefore the dimer must first be broken before it can
react. DFT calculations at the ωB97X-D/6-311G(d,p) level of
theory reveal it costs ΔG = 15.4 kcal∙mol–1 (ΔE = 29.7 kcal∙mol–1)
to break up dimer [3]2 (Table 2; all values given per monomer). To
give a better reflection of the thermodynamics of this equilibrium
in solution, we augmented our computational method by including
implicit solvation effects (benzene and THF). As expected, this
lowers the amount of energy required to break the dimer, the more
polar solvent THF does this to a greater extent (benzene: ΔG =
13.6 kcal∙mol–1, THF: ΔG = 12.4 kcal∙mol–1). Explicit solvent
interactions are also important, especially for monomeric species
such as 3 where the Ru centre has a vacant coordination site that
can be stabilised by solvent. Including one benzene molecule per
monomer in the calculations lowered the energy required very
Figure 6. Top: cleavage of dimer [3]2 into monomer 3. Bottom: optimized
structures of 3ꞏTHF (left, oxygen coordinates to ruthenium) and 3ꞏC6H6 (right,
coordination in an η2 fashion to ruthenium).
Table 2. Energy (ΔE) and Gibbs free energy (ΔG) required to break dimer
[3]2 in kcal∙mol–1 (all values given per monomer).
ΔG [kcal mol–1
]
ΔE [kcal mol–1
]
No solvent added
Implicit THF
Implicit benzene
Explicit THF added, implicit THF
15.4
12.4
13.6
5.5
29.7
26.4
28.1
14.1
Explicit benzene added, implicit
Benzene
12.8
22.6
To corroborate these insights on the dimerization of 3 and 4
in different solvents, we analysed the diagnostic 1H NMR
chemical shift of the Ru-bound hydride, both computationally and
experimentally (Table 3). The computed shift for monomer 3 is
approximately –20 ppm, while the corresponding shift for [3]2 is
relatively deshielded and is computed to be approximately –10
ppm, with little dependence on the identity of the solvent.
Experimentally, the hydride in benzene solutions of 3 was found
to resonate at δ –13.04 ppm,[12] while in THF it is at δ –20.05 ppm.
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10.1002/ejic.201900169
European Journal of Inorganic Chemistry
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The latter is a very good match with the predicted monomeric Conclusions
structure, while the former is closer to the dimeric species, and
suggests the existence of a monomer/dimer equilibrium. These
data follow the trends predicted by the computations above, in
that the quenching of the MLC is more likely to occur in less
coordinating solvents such as benzene. These data are further
supported by the fact that the analogous hydride in A (Figure 4),
which is known to rapidly dimerise, resonates relatively downfield
at δ –11.83 ppm in the non-coordinating solvent CD2Cl2.[18]
The notion that the difference in the experimental chemical
shift of the hydride of 3 in THF and benzene is related to
dimerization is reinforced by the fact that the analogous
experimental values for 4, a system where dimerization is not
possible, are very similar in the two solvents (benzene: δ –25.78
ppm; THF: δ –26.17 ppm, implicit solvent added). It should be
noted that the computed values in this case are not very accurate,
as they predict the resonance at approximately –18.5 ppm,
depending on the solvent, and thus caution is advised in analysing
the close correlation between the computed and observed values
for 3 in THF above. However, the fact that the experimental values
of 3 are significantly different in the two solvents, while the
corresponding values for 4 are almost identical, is evidence for
the presence of monomer/dimer equilibrium effects.
We have shown that FLP and MLC chemistry both involve the
cooperative action of a Lewis acid and a Lewis base, and that
steric control to prevent quenching of the reactive sites is just as
important in both paradigms. There are many reactions in the
literature that have been given one label or the other on a fairly
arbitrary basis, and we believe both schools of thought should be
united so that lessons from one field can be used to benefit the
other – whether that is using principles and reactions from main-
group FLPs to broaden the scope of MLC reactivity, or using the
wealth of knowledge on ligand design and properties to rationally
construct new backbones for preorganised intramolecular main-
group FLPs.
Experimental Section
All manipulations regarding the preparation of air-sensitive compounds
were carried out under an atmosphere of dry nitrogen using standard
Schlenk and drybox techniques. Solvents were purified, dried and
degassed according to standard procedures. 1H NMR spectra were
recorded on a Bruker AV 400 or on a Bruker AV300-ll and internally
referenced to the residual solvent resonances (THF-d8: 1H δ 3.58, 1.72
ppm; C6D6: 1H δ 7.16 ppm; Tol-d8: 1H δ 2.08, 6.97, 7.01, 7.09 ppm). 31P{1H}
NMR spectra were recorded on a Bruker AV 400 or on a Bruker AV300-ll
and externally referenced (85% H3PO4). Chemical shifts are reported in
ppm. High resolution mass spectra were recorded on a Bruker MicroTOF
with ESI nebulizer (ESI) at –45 °C.
Insert Table 3 here (see below)
Finally, we wanted to show that consideration of steric bulk
is important for regulating the quenching of the Lewis acid/Lewis
base components in other organometallic pincer systems.
Kirchner[27] and Huang[28] have replaced the methylene bridges in
Milstein’s PNP pincer system with the more acidic NH moiety
(Figure 7).[29] Calculations at the ωB97X-D/6-311G(d,p) level of
theory (with no solvent modelled) reveal a dimer is feasible for
[(iPrPNNNP)RuH(CO)], and it costs ΔG = 11.3 kcal∙mol–1 to break
up the dimer (per monomer), which is slightly lower than for the
Milstein system. Once again, for the tert-butyl complex
[(tBuPNNNP)RuH(CO)], the added steric protection around the
acidic and basic sites prevents dimerization. However, the altered
electronics of the Lewis basic site in these complexes compared
to 3 and 4 have drastic consequences, as the activation of CO2 is
no longer feasible (ΔG = 15.3 and ΔG = 23.7 kcal∙mol–1 for
[(iPrPNNNP)RuH(CO)] and [(tBuPNNNP)RuH(CO)], respectively).
Synthesis of diisopropylphosphine. Diisopropylphosphine was
prepared according to a modified literature procedure of A. S. Glodman et
al.[30] A solution of ClPiPr2 (4.92 g, 0.032 mol, 1.0 equiv) diethyl ether (55
mL) was added dropwise to a slurry of LiAlH4 (0.37 g, 0.01 mol, 0.3 equiv)
in diethyl ether (30 mL) in an ice/water bath. The mixture was stirred
overnight and conversion was checked by 31P{1H} NMR. Degassed H2O
(20 mL) was added slowly and the organic layer was dried over MgSO4.
The water layer was extracted with diethyl ether (3 x 15 mL) and dried over
the same MgSO4. The MgSO4 was filtered off (with a cannula filter) and
rinsed with diethyl ether (3 x 15 mL). All volatiles were removed in vacuo
while the Schlenk vessel was cooled in an ice/water bath to afford
diisopropylphosphine as a colourless clear liquid in 81% (3.08 g, 0.026
mol). If some phosphine was oxidized a Schlenk to Schlenk distillation was
performed. Note, the presence of some diethyl ether does not influence
the next step. 1H NMR (400.1 MHz, C6D6, 291 K): δ 2.93 (dt, 1JH,P = 192.3
3
Hz, JH,H = 5.9 Hz, 1H; PH), 1.77 (m, 2H; CH(CH3)), 1.01 (m, 12H;
CH(CH3)). 31P{1H} NMR (162.0 MHz, C6D6, 295 K): δ –16.5 (s).
Lithiation of diisopropylphosphine. Lithium diisopropylphosphide was
prepared according to a modified literature procedure of A. Jansen and S.
Pitter.[31] n-Butyllithium (1.6M in hexanes, 5.2 mL, 8.347 mmol, 1.4 equiv)
was added dropwise to a solution of diisopropylphosphine (7.045 mg,
5.962 mmol, 1.0 equiv) in n-pentane (15 mL) at 0 C with a glass stirring
bean and stirred for an additional 30 min, after which the solution was
allowed to warm to room temperature. The resulting colourless/pale yellow
solution was stirred for 16 hours during which an off-white solid
precipitated. The solids were collected by filtration, subsequently washed
with n-pentane (2 x 15 mL) and evaporated to dryness to give lithium
diisopropylphosphide as an off-white solid in 76% (562.7 mg, 4.535 mmol).
Figure 7. Representation of the activated [(R-PNNNP)RuH(CO)] complex, R =
iPr or tBu. Blue: Lewis acid; red: Lewis base.
1H NMR (400.1 MHz, THF-d8, 297 K): δ 2.25 (dsept, 2JH,P = 6.8 Hz, 3JH,H
=
2
3
4.7 Hz, 2H; CH(CH3)2), 1.07 (dd, , JH,P = 11.3 Hz, JH,H = 6.8 Hz, 12H;
CH(CH3)2). 7Li NMR (155.5 MHz, THF-d8, 297 K): 1.3 (s). 13C NMR (100.6
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10.1002/ejic.201900169
European Journal of Inorganic Chemistry
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MHz, THF-d8, 297 K): δ 26.93 (d, 3JC,P = 14.2 Hz; CH(CH3)2), 23.4 (d, 2JC,P
= 25.3 Hz; CH(CH3)2). 31P{1H} NMR (162.0 MHz, THF-d8, 297 K): δ 1.5 (s).
= 55.2°, 50402 reflections, of which 5365 were independent (Rint = 0.034),
239 parameters, R1 = 0.019 (for 4830 I > 2σ(I)), wR2 = 0.044 (all data), S
= 1.06, largest diff. peak / hole = 0.35 / -0.49 e Å-3.
Preparation of 2,6-bis-(diisopropylphosphino-methyl)-pyridine. 2,6-
Bis-(diisopropylphosphino-methyl)-pyridine was prepared according to a
modified literature procedure of A. Jansen and S. Pitter.[31] A solution of
2,6-bis(chloromethyl)pyridine (0.26 g, 1.477 mmol, 1.0 equiv) in THF (2.0
mL) as added slowly to a solution of the lithiated phosphine (0.40 g, 3.223
mmol, 2.2 equiv), using a glass stirring bean, in THF (4.0 mL) at –78 °C
and was stirred for an additional 15 min at the same temperature. During
the addition, the solution changed colour from yellow to orange, and
subsequently was allowed to warm to room temperature in 16h. Addition
of degassed water (0.2 mL) resulted in a colour change to yellow and the
mixture was dried over Na2SO4. The solution was filtered and the Na2SO4
was washed with THF (3 x 4 mL). The combined solution was dried in
vacuo, extracted with pentane (3 x 5 mL) and evaporated to dryness to
give iPrPNP as a yellow oil in 88% (0.44 mg, 1.296 mmol, 95% pure). Note,
some remaining diisopropylphosphine can be observed. 1H NMR (400.1
MHz, C6D6, 298 K): δ 7.13–7.04 (m, 1H; p-PyH), 7.02–6.95 (m, 2H; m-
CCDC 1884053 (1) contains the supplementary crystallographic data for
this paper. This data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2
2
PyH), 2.97 (d, JH,P = 1.5 Hz, 4H; CH2), 1.71 (dsept, JH,P = 7.1, 1.7 Hz,
4H; CH(CH3)2), 1.05 (m, 24H; CH(CH3)2). 31P{1H} NMR (162.0 MHz, C6D6,
297 K): δ 11.4 (s; product, 95%), –12.1 (s; impurity, presumably iPr2PH,
5%).
Figure 8. Molecular structure of
1 [(iPrPNP)RuHCl(CO)] (displacement
ellipsoids are set at 50% probability, hydrogen atoms omitted for clarity).
Selected bond lengths [Å]: Ru–P1 2.3221(4), Ru–P2 2.3061(4), P1–C1
1.8415(14), P2–C7 1.8435(15), C1–C2 1.5051(19), C6–C7 1.501(2).
Synthesis of 1. [(iPrPNP)RuHCl(CO)] was prepared according to a
literature procedure.[32] X-ray quality crystals were grown at –20 °C from a
saturated solution of [(iPrPNP)RuHCl(CO)] in THF layered with n-pentane.
Computational Details: Density functional calculations were performed
at the ωB97X-D[35] level of theory using Gaussian09, revision D.01.[36]
Geometry optimizations were performed using the 6-311G(d,p)[37] basis
set for atoms C, H, N, O, P, Cl, in combination with the Def2TZVP[38] basis
set for Ru. ZPE and Gibbs free energies (G°) were obtained from
frequency analyses performed at the same level of theory. Calculations of
large dimer systems with solvation (SCRF, THF or benzene) were
obtained from frequency analyses after optimization without solvation. The
NICS[ 39 ] analyses was performed at the ωB97X-D/6-311G(d,p), Ru
Def2TZVP // B3LYP[40]/6-311G(d,p), Ru Def2TZVP level of theory. The
AIM[ 41 ] and ETS-NOCV[42] analyses were performed at the ZORA-
BP86/TZ2P[42] level of theory using ADF2016.102[42]. Structures were
optimized using the same functional and basis set prior to the analysis
using ZORA-BP86/TZ2P. NMR calculations were performed at ωB97X-
D/6-311G(d,p), Ru Def2TZVP level of theory, using solvation (THF or
benzene) and are corrected for the TMS value[39,43] and obtained with the
Synthesis of 2. [(tBuPNP)RuHCl(CO)] was prepared according to a
literature procedure.[13]
Synthesis of 3. [(iPrPNP)RuH(CO)] was prepared according to a slightly
modified literature procedure.[12] To
a
solution of complex
[(iPrPNP)RuHCl(CO)] 1 (50 mg, 0.099 mmol) in THF (5 mL) was added
KOtBu (11.1 mg, 0.099 mmol) at –30 to –35 C. Subsequently, the mixture
was stirred at room temperature for 4 hrs, then filtered. The orange filtrate
was dried under vacuum and washed with n-pentane (3 x 3 mL) and dried
under vacuum to afford a yellowish powder in 56% yield (26 mg, 0.055
mmol).
Synthesis of 4. [(tBuPNP)RuH(CO)] was prepared according to a
literature procedure.[13]
Gauge-Independent Atomic Orbital (GIAO)[44]
.
X-ray crystal structure determination: The single-crystal X-ray
diffraction study (see Figure 8) was carried out on a Bruker D8 Venture
diffractometer with Photon100 detector at 123(2) K using Mo-Ka radiation
(l = 0.71073 Å). Dual space (intrinsic) methods (SHELXT)[33] were used for
structure solution and refinement was carried out using SHELXL-2014
(full-matrix least-squares on F2)[34]. Hydrogen atoms were localized by
difference electron density determination and refined using a riding model
(H(Ru) free). A semi-empirical absorption correction and an extinction
correction were applied.
Acknowledgments
This work was supported by the Council for Chemical Sciences of
The Netherlands Organization for Scientific Research (NWO/CW)
by a VIDI grant (J.C.S.) and a VENI grant (A.R.J.). We gratefully
acknowledge Ed Zuidinga for mass spectrometric analyses.
Keywords: Lewis acid • Lewis base • Frustrated Lewis pairs •
Metal ligand cooperativity • DFT calculations
1. Colourless blocks, C20H36ClNOP2Ru, Mr = 504.96, crystal size 0.24 ×
0.18 × 0.10 mm, monoclinic, space group P21/c (No. 14), a = 13.7461(6)
Å, b = 11.7625(5) Å, c = 14.4290(7) Å, β = 95.022(2)°, V = 2324.05(18)
Å3, Z = 4, ρ = 1.443 Mg/m-3, µ(Mo-Kα) = 0.94 mm-1, F(000) = 1048, 2max
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European Journal of Inorganic Chemistry
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European Journal of Inorganic Chemistry
FULL PAPER
Table 1. Computed bond metric data for the van der Waals complexes (vdW), transition states (TS) and products (P) during the reactions of CO2 with 3, 4 and 5.
LA-O1 [Å]
LB-CCO2 [Å]
C-O1 [Å]
C-O2 [Å]
O-C-O [°]
vdW
TS
P
3
4
5
2.55
2.56
3.63
3.14
3.16
3.46
1.16
1.16
1.16
1.15
1.15
1.16
175.9
176.0
177.6
3
4
5
3
4
5
2.43
2.45
3.00
2.26
2.29
1.57
2.43
2.42
2.26
1.59
1.59
1.88
1.18
1.18
1.20
1.27
1.27
1.28
1.16
1.16
1.19
1.22
1.22
1.21
158.9
158.3
147.8
129.3
129.1
128.8
LA = Lewis acid (Ru in 3, 4; B in 5); LB = Lewis base (C in 3, 4; P in 5).
Table 3. Experimental and computational data of the hydride shift of various compounds.
a
]
Compound
Experimental data [ppm]
Computational data [ppm][
THF[b]
Benzene[b]
3
–20.23
–19.90
3 in THF-d8
3 in C6D6
–20.05
–13.04
[c]
[3]2
–10.16
–18.58
–10.19
–18.36
4
4 in THF-d8
4 in C6D6
[4]2
–26.17
–25.78
n.a.
n.a.
[a]
A in CD2Cl2
–11.83
[a] Calculations were performed using ωB97X-D/6-311G(d,p), Ru Def2TZVP level of theory. [b] Calculated using implicit solvent interactions; n.a. = not applicable,
as the dimeric structure could not be obtained computationally. [c] Both hydrides have the same shift (–10.16 or –10.19).
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10.1002/ejic.201900169
European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
FULL PAPER
The burgeoning fields of frustrated
Lewis pair (FLP) chemistry and
metal-ligand cooperativity (MLC)
have developed largely
Lewis acids and bases
Evi R. M. Habraken, Andrew R. Jupp,
Maria B. Brands, Martin Nieger,
Andreas W. Ehlers and
independently. In this article, we
explore the similar mode of reactivity
of archetypal systems from each
field with carbon dioxide, and show
that both sets of compounds act via
cooperative action of a Lewis acid
and a Lewis base on the substrate.
We also highlight the key role of
sterics in preventing the dimerization
of the MLC systems, by analogy
with the fundamental principles of
FLPs.
J. Chris Slootweg*
Page No. – Page No.
Parallels between Metal-Ligand
Cooperativity and Frustrated Lewis
Pairs
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