Cooperative H2 Activation on Dicopper(I) Facilitated by Reversible Dearomatization of an “Expanded PNNP Pincer” Ligand

Article abstract of DOI:10.1002/chem.201903724

A naphthyridine-derived expanded pincer ligand is described that can host two copper(I) centers. The proton-responsive ligand can undergo reversible partial and full dearomatization of the naphthyridine core, which enables cooperative activation of H₂ giving an unusual butterfly-shaped Cu₄H₂ complex.

Full text of DOI:10.1002/chem.201903724

A Journal of
Accepted Article
Title: Cooperative H2 Activation on Dicopper(I) Facilitated by
Reversible Dearomatization of an 'Expanded PNNP Pincer'
Ligand
Authors: Daniel Broere, Errikos Kounalis, and Martin Lutz
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Cooperative H2 Activation on Dicopper(I) Facilitated by Reversible
Dearomatization of an ‘Expanded PNNP Pincer’ Ligand
Errikos Kounalisa, Martin Lutzb and Daniël L. J. Broerea*
a) Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Faculty of Science, Utrecht University, Universiteitsweg 99,
3584 CG, Utrecht (The Netherlands)
b) Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science,Utrecht University Padualaan 8, 3584 CH,
Utrecht (The Netherlands)
Abstract:
A
naphthyridine-derived expanded pincer ligand is
we describe the synthesis of a dinucleating naphthyridine
diphosphine ligand (t-BuPNNP, Scheme 1), which can bind two
copper(I) centers in close proximity to each other. The so-called
‘expanded pincer’ ligand in the corresponding dicopper(I)
complexes can undergo reversible single and double
deprotonation of the ligand concomitant with partial (t-BuPNNP*)
and full (t-BuPNNP**) dearomatization of the naphthyridine core,
respectively (Scheme 1). Moreover, we demonstrate how the
expanded pincer ligand enables cooperative activation of H2 on
dicopper(I) and induces formation of a new type of copper hydride.
The expanded pincer ligand t-BuPNNP was prepared as an
air-sensitive off-white solid through a three-step procedure from
commercially available reagents in 11% overall yield.14 The 1H,
13C and 31P NMR spectra of t-BuPNNP in CD2Cl2 at 298 K show
the expected number of resonances for a C2v symmetric species.
The 31P{1H} NMR spectrum shows a single resonance at δ = 35.8
ppm, similar to the lutidine-derived t-BuPNP pincer ligand. 15
Reacting t-BuPNNP with two equiv of CuCl in THF gives dinuclear
complex 1 (Scheme 2), which was isolated as a pink solid in 78%
described that can host two copper(I) centers. The proton-responsive
ligand can undergo reversible partial and full dearomatization of the
naphthyridine core, which enables cooperative activation of H2 giving
an unusual butterfly-shaped Cu4H2 complex.
Nature has evolved a variety of metalloenzymes that catalyze
challenging chemical transformations using earth-abundant
metals under mild conditions. In several enzymes the active site
features complex architectures that enable multiple metals and
ligands to work together to facilitate bond activation processes
that are essential to enzyme function.1 Drawing inspiration from
nature, various research groups have developed synthetic
systems where metals and ligands cooperatively activate
chemical bonds.2 A prominent platform that enables such metal-
ligand cooperativity (MLC) is the lutidine-derived PNP pincer3
ligand, and derivatives thereof, which enable cooperative
substrate activation through reversible dearomatization of the
heteroaromatic core. This MLC has been thoroughly studied and
employed in catalysis by the Milstein group and others.4
Another avenue in cooperative bond activation involves
complexes wherein multiple metal centers are in close proximity
and work together to make or break chemical bonds.5 Ligands
derived from 1,8-naphthyridine have been shown to be well-suited
to place two metals in an orientation that enables metal-metal
cooperativity (MMC).6 The Tilley group has reported aryl group
transfer reactions7 and remarkable stabilization of alkyl ligands8
1
yield. The H, 13C and 31P NMR spectra of 1 in CD2Cl2 at 298 K
show that the C2v symmetry is retained upon binding two Cu
centers. The 31P{1H} NMR spectrum shows a single broad
resonance at δ = 26.1 ppm, consistent with binding of the
phosphines to Cu.16 The broadness of the resonance is common
for Cu(I) phosphine complexes, due to quadrupolar relaxation
arising from 63Cu and 65Cu (both I = 3/2) nuclei.17
on dinuclear copper complexes bearing
a
napthyridine
H
H
H
t-Bu2
P
t-Bu2
P
t-Bu2
P
bis(dipyridyl) ligand.9 The Uyeda group has demonstrated that
dinuclear metal complexes bearing redox-active naphthyridine
diimine ligands can catalyze a variety of chemical transformations
for which the MMC is essential for activity or selectivity.10 Very
recently, the same group reported that these dinuclear systems
can catalyze the reductive [4+1]-cycloaddition of vinylidenes and
dienes. 11 Both the MMC and the redox-active nature of the
ligand12 were found to be crucial for the observed reactivity. This
demonstrates the potential for combining other types of
cooperative elements in naphthyridine-derived ligands. However,
to the best of our knowledge there are no examples where these
dinucleating ligands display MLC beyond being redox-active.13
Our group is currently exploring ligand systems that can
both host multiple low-valent 1st-row transition metals and also
contain fragments that enable metal-ligand cooperativity. Herein,
H
N
N
N
N
N
N
- H+
+ H+
- H+
+ H+
P
t-Bu2
P
t-Bu2
P
t-Bu2
H
H
H
H
H
t-BuPNNP
t-BuPNNP*
t-BuPNNP**
Scheme 1. The dinucleating ‘expanded pincer’ ligand t-BuPNNP and the partial
and full dearomatization upon single and double deprotonation, respectively.
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shows that the expanded pincer ligand allows for flexibility
towards hosting two metals at varying distances. Partial
dearomatization of the naphthyridine is evident by the double
bond character of the C1-C2 bonds (see Table 1), which are
shorter by ~0.14 Å than the C9-C10 bonds, and the C1-C2
bond in 1. In addition, the observed metric parameters suggest
localized double C3-C4 bonds and single C2-C3 and C4-C5
bonds, which contrasts with the delocalized C5-C7, C7-C8 and
C8-C9 bonds (all ~1.38 Å). Interestingly, the partial
dearomatization makes both PN binding pockets distinct within
the t-BuPNNP* ligand. On the dearomatized sides, the Cu1-N1
distances are slightly shorter (Table 1) than observed for Cu2-
N2, in agreement with the expected increase of negative
charge on N1. Moreover, nearly flat 5-membered
metallacycles are observed wherein the Cu1 atoms are located
within the naphthyridine planes. In the other PN pockets, the
Cu2 and P2 atoms are found sticking out of the naphthyridine
planes, likely facilitated by the more flexible methylene linker.
Treatment of complex 2 with two equiv of Et3NHCl
results in quantitative reformation of 1 and HOt-Bu, showing
that the observed partial dearomatization is reversible.
Reacting 2 with one equiv benzyl potassium (KBn) at room
temperature gives 3 (Scheme 4), which was isolated as an
orange film in 95% yield. The 31P{1H} NMR spectrum of 3 in
THF-d8 at 298 K shows a single resonance at δ = 14.4 ppm
indicating two magnetically equivalent phosphorus atoms. This
gain in symmetry for 3 compared to 2 is also evident in the 1H
and 13C NMR spectra, which show the expected number of
H
t-Bu2
P
H
Cu
Cu
N
N
2 CuCl
THF, RT
Cl
Cl
t-BuPNNP
P
t-Bu2
H
H
Scheme 2. Synthesis of dinuclear complex 1.
Layering a CH2Cl2 solution of 1 with hexane yielded crystals
suitable for single-crystal X-ray diffraction. The solid-state
structure (Figure 1, left) confirmed the presence of two Cu centers
separated by 2.581(4) Å within the expanded pincer ligand. The
Cu centers are equivalent by symmetry (Z’= 0.5) and show a
distorted tetrahedral geometry.
Treatment of complex 1 with two equiv18 KOt-Bu at room
temperature gives complex 2 (Scheme 3), which was isolated as
a vivid red solid in 90% yield. The 31P{1H} NMR spectrum of 2 in
C6D6 at 298 K features two broad resonances at δ = 26.9 and 7.7
ppm, showing two magnetically inequivalent phosphorus atoms.
This loss of symmetry in 2 is also evident in the 1H spectrum,
which shows the expected increase in aromatic signals for an
unsymmetrically substituted naphthyridine, and an additional
singlet at δ = 1.88 ppm, which we assign to the µ2-Ot-Bu ligand.
In addition, a two-proton doublet at δ = 2.57 ppm (JPH = 7.4 Hz)
and a one-proton doublet at δ = 4.29 ppm (JPH = 1.8 Hz) agree
resonances for
a C2v symmetric species. Notably, the
resonances corresponding to the naphthyridine protons in the
1H NMR spectrum are significantly shifted upfield to δ = 6.12
and 5.56 ppm, and integrate equally to a doublet (JPH = 1.3 Hz)
at δ = 3.27 ppm, which we assign to the deprotonated
methylene linkers in the t-BuPNNP** ligand. Together, these
observations support full dearomatization of the naphthyridine
core of the ligand.
with
a
partially dearomatized t-BuPNNP* ligand. These
observations are comparable to those made upon
dearomatization of related mononuclear complexes bearing
lutidine-derived PNP pincer ligands.19
H
t-Bu2
P
H
2 KOt-Bu
K(THF)n
t-Bu2
P
KBn
- HOt-Bu
- 2 KCl
N
N
Cu
Cu
- Bn-H
Cu
Cu
N
N
Ot-Bu
1
Ot-Bu
2
2 Et3NHCl
Et3NHCl
2
- HOt-Bu
- 2 Et3N
P
t-Bu2
3
H
- Et3N
- KCl
P
t-Bu2
H
H
Scheme 3. Reversible deprotonation of 1 to give 2.
Scheme 4. Reversible deprotonation of 2 to give 3.
Crystals suitable for single-crystal X-ray diffraction were
obtained from a concentrated pentane solution of 2 at −40 °C.
Two independent molecules of 2 were found in the asymmetric
unit (Z’= 2). Each molecule features a t-BuPNNP* ligand bound
to two copper centers that display distorted trigonal planar
geometries and a µ2-Ot-Bu ligand (Figure 1, middle). A notable
feature is the Cu-Cu distance in the two molecules (3.0467(2)
and 3.0218(2) Å), which are ~0.4 Å longer than that observed
in 1. This is likely a result of the bulky µ2-Ot-Bu ligand, and it
Crude 3 is soluble in various nonpolar solvents (e.g.
pentane and Et2O) and tends to precipitate, oil out or remain in
solution. Fortunately, addition of 1 equiv of 18-crown-6 to a
solution of 3 gives a species (3-18c6) that displays nearly
identical NMR spectra in THF-d8 solution, yet is far less soluble
in nonpolar solvents. Crystals of 3-18c6 suitable for single-
crystal X-ray diffraction were obtained from a solution of 3 with
1 equiv 18-crown-6 in a THF/pentane mixture at -40 °C.
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Figure 1: Displacement ellipsoid plots (50ꢀ% probability) of complex 1 (left,
symmetry code i: 1-x, -y, z), 2 (middle), and the anion of 3 (right). Most
hydrogen atoms, and cations (for 3) are omitted, and t-Bu groups on P are
depicted as wireframe for clarity. For 2 only one molecule in the asymmetric
unit is depicted. For 1 and 3, only the major disorder component is shown.
ligand have localized double bonds between C1-C2, C3-C4,
C7-C8 and C9-C10 (see Table 1). Deprotonation of both arms
of related lutidine-based pincer ligands is possible but typically
results in complexes that are too sensitive for isolation.20
With the series of complexes 1-3 in hand some clear
trends in metric parameters (Table 1 and Figure S28) can be
observed that reflect the extent of dearomatization. The
increased ionic character on N1 and N2 upon partial and full
dearomatization results in shortening of N-Cu distances, as
expected. However, between 1-3 the P-Cu distances do not
differ significantly, which is also the case for the Cu-µ2-O
distances between 2 and 3. The most notable changes upon
dearomatization are the shorter C1-C2/C9-C10 (~0.14 Å) and
C1-P1/C10-P2 (~0.07 Å) distances. Additionally, the P1-C1-C2
and P2-C10-C9 angles increase upon dearomatization, in
agreement with the increased double bond character of the C-
C bond. Although the N-C bonds within the naphtyridine core
do elongate upon dearomatization as expected, the trend is
less clear. Together, our findings fit well with the resonance
structures for each protonation state of the PNNP ligand
depicted in Scheme 1.
Table 1: Selected distances and angles of compounds 1-3 in Å and °
Atoms
Cu-Cu
P1-Cu1
P2-Cu2
N1-Cu1
N2-Cu2
Cu1-O1
Cu2-O1
C1-P1
1
2a
3-18c6
2.9259(6)
2.1818(10)
2.1815(10)
2.081(3)
2.104(3)
1.907(5)c
1.897(5)c
1.754(4)
1.751(4)
1.380(5)
1.440(5)
1.351(5)
1.411(5)
1.434(5)
1.400(5)
1.352(5)
1.432(5)
1.399(5)
2.581(4)
2.186(3)
2.186(3)b
2.204(7)
2.204(7)b
-
3.0467(2)
2.1807(4)
2.1650(4)
2.1277(11)
2.1870(11)
1.8929(10)
1.8761(10)
1.7770(14)
1.8338(15)
1.3743(19)
1.4542(19)
1.339(2)
3.0218(2)
2.1740(4)
2.1737(4)
2.1259(11)
2.1638(11)
1.8915(10)
1.8880(9)
1.7753(14)
1.8444(14)
1.3782(19)
1.4539(18)
1.340(2)
-
1.837(10)
1.837(10)b
1.515(12)
1.403(9)
1.362(10)
1.442(9)
1.366(12)
1.442(9)b
1.362(10)b
1.403(9)b
1.515(12)b
C10-P2
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C5-C7
C7-C8
C8-C9
C9-C10
1.431(2)
1.435(2)
H
1.4345(18)
1.3840(19)
1.382(2)
1.4345(18)
1.386(2)
H
N
P
t-Bu2
1.385(2)
N
H
Cu
H2
1.388(2)
1.387(2)
H
Cu
H
1/2
3
P
t-Bu2
t-Bu2
1.515(2)
1.513(2)
- KOt-Bu
THF, 40 oC
P
Cu
Cu
C2-N1
N1-C6
C6-N2
1.327(10)
1.361(8)
1.3974(17) 1.3887(17)
1.3507(16) 1.3503(17)
1.3731(17) 1.3701(17)
1.3558(17) 1.3578(17)
1.391(4)
1.391(4)
1.366(4)
H
N
t-Bu2
P
N
1.327(10)b
4
H
1.361(8)b
1.386(4)
H
N2-C9
P1-C1-C2
115.6(6)
120.57(11)
119.35(10)
120.4(3)
Scheme 5. Cooperative H2 activation by 3 to give complex 4.
P2-C10-C9
115.6(6)b
112.66(9)
112.10(9)
121.0(3)
a Two independent molecules in the asymmetric unit; b The molecule has
exact C2 symmetry. Only half of the parameters are independent. c Major
disorder component.
Complex 3 is thermally stable but readily reacts with
traces of proton sources to give 2. In agreement with this
observation, treatment of 3 with one equiv of Et3NHCl results
The solid-state structure (Figure 1, right) revealed a flat
fully dearomatized t-BuPNNP** ligand that holds the Cu2(µ2-Ot-
Bu) core in the naphthyridine plane. The anionic complex is
separated from the potassium cation, which is sequestered by
18-crown-6 and two THF molecules. In agreement with the
t-
in quantitative formation of 2. To explore whether the
BuPNNP** ligand enables cooperative bond activation we
placed a THF-d8 solution of 3 under an atmosphere of H2.
Excitingly, we observed full conversion of 3 over the course of
30 h at 40 °C concomitant with formation of hydride complex 4
(Scheme 5) in 79% spectroscopic yield. Complex 4 was also
NMR spectra, the metric parameters reveal
a
fully
dearomatized ligand where both halves of the t-BuPNNP**
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independently synthesized in 67% yield by reacting 2 with
Ph2SiH2, which is converted to Ph2SiHOt-Bu according to H
location of the hydrides were substantiated by DFT optimized
geometries of 4.14 Copper hydrides with fewer than six metal
centers are uncommon21 and a CCSD search22 did not reveal
any Cu4H2 butterfly-shaped clusters as featured in 4. Given the
prevalence of copper hydrides in catalysis, 23 this unique
tetranuclear dihydride architecture might enable distinct
reactivity from other multinuclear hydrides, and we are
currently exploring this.
1
NMR spectroscopy and GC-MS analysis. The single crystal X-
ray structure revealed a tetranuclear ‘butterfly-shaped’ copper
cluster featuring two t-BuPNNP* ligands (Figure 2). The two Cu
atoms within each t-BuPNNP* ligand are separated by
2.5106(6) and 2.5122(6) Å, which is similar to the distance
between the Cu atoms in each non-dearomatized PN pocket
(Cu21-Cu22 2.4778(5) Å). In contrast, the distance between
the Cu atoms in the dearomatized PN pockets is ~ 0.9 Å longer
(Cu11-Cu12 3.4144(6) Å), which results in the overall butterfly
shaped tetracopper(I) core. Two hydrides atoms that display a
µ3-binding mode on each side of the cluster were located in the
difference-Fourier maps (Figure 2, bottom). The Cu-H
distances were slightly shorter for the Cu atoms that are
situated in the dearomatized pocket of the t-BuPNNP* ligands
(Cu12-H2 1.61(4) Å and Cu11-H2 1.67(4) Å) than for the other
Cu sites (1.76(4) – 1.81(4) Å) in 4. Although the experimental
distances were not significantly (>3σ) different, the shorter Cu-
H distances in the dearomatized PN pocket as well as the
The 1H, 13C and 31P{1H} NMR spectra of 4 in THF-d8 or
C6D6 at 298 K show the characteristic features of a partially
dearomatized t-BuPNNP* ligand. Notably, in both solvents the
1H NMR spectra show distinct resonances for each of the three
protons on the methylene and methine linkers and four
resonances for the t-Bu groups are observed, indicating that
the CH2 protons and t-Bu groups are diastereotopic. This
shows that the mirror plane containing the expanded pincer
plane is lost, implying that the tetranuclear complex assembly
stays intact in solution. Although no hydride resonances were
1
2
observed in the H NMR spectra, the H NMR spectrum of a
sample of 4 that was synthesized by reacting 2 with Ph2SiD2
showed a resonance at δ = 1.2 ppm (Figure S23). This shows
1
that the hydride resonance in the H NMR spectrum is likely
obscured by the resonances of the protons on the t-Bu
2
groups.24 This same resonance was observed in the H NMR
spectrum of a THF-d8 solution of 3 that was exposed to an
atmosphere of D2 at 40 °C for 48 h (Figure S26). Moreover, in
the 1H NMR spectrum both H-D and an equal decrease in the
intensity of the CH and CH2 resonances in the ligand arms was
observed. This suggests a reversible cooperative activation of
D2 that results in partial D incorporation into the ligand arms,
which was confirmed by 2H NMR analysis (Figure S27).
Notably, no H/D scrambling was observed upon heating a
THF-d8 solution of 4 (with or without KOt-Bu) under an
atmosphere of D2 at 40 °C, suggesting that the reversible
cooperative bond activation step takes place prior to formation
of the dimer 4.
In conclusion, we prepared a new proton-responsive
expanded pincer ligand, t-BuPNNP. The ligand can host two
copper(I) centers and shuttle reversibly between three
protonation states concomitant with partial or full
t-
dearomatization of the naphthyridine core. Moreover, the
BuPNNP ligand enables cooperative H2 activation on
dicopper(I) and induces formation of a unique tetranuclear
copper dihydride cluster. To the best of our knowledge, this
work demonstrates the first example of metal-ligand
cooperativity in naphthyridine-based bimetallic complexes, and
of cooperative H2 activation on copper. These studies further
the development of new bimetallic systems that employ both
metal-metal and metal-ligand cooperativity to activate
chemical bonds. In addition to investigating the further
reactivity of the systems reported herein, we are exploring
various other avenues to discover if the t-BuPNNP ligand can
be as valuable in bimetallic bond activation and catalysis as
the lutidine-derived PNP analogues are in mononuclear
chemistry.
Figure 2: Displacement ellipsoid plots (50ꢀ% probability) of 4 (top) and a
depiction of the tetranuclear butterfly-shaped Cu core (bottom). Most
hydrogen atoms are omitted and t-Bu groups on P are depicted as wireframe
for clarity. Selected distances [Å] and angles [o]: Cu11-CuC21 2.5106(6),
Cu11-Cu22 2.7310(6), Cu21-Cu22 2.4778(5), Cu21-Cu12 2.7338(6), Cu22-
Cu12 2.5122(6), Cu11-Cu12 3.4144(6), Cu11-Cu21-Cu12 81.123(17),
Cu11-Cu22-Cu12 81.149(17), Cu11-Cu21-Cu22 66.384(16), Cu12-Cu21-
Cu22 57.385(15). Atoms Cu21, Cu22, H1, and H2 are approximately in one
plane.
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Acknowledgements
This work was supported by The Netherlands Organization for
Scientific Research (START-UP grant 740.018.019 to D.L.J.B.)
and the European Union's Horizon 2020 research and innovation
program (agreement 840836, MSCA-IF to D.L.J.B., BiMetaCat).
Access to supercomputer facilities was sponsored by NWO
Exacte en Natuurwetenschappen (Physical Sciences). The X-ray
diffractometer was financed by the NWO. Gerard van Koten, Bert
Klein-Gebbink and Marc-Etienne Moret are acknowledged for
valuable discussions and suggestions.
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reported ligand by the Tilley group could also display chemical
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[14] See the supporting information for additional details.
[15] Simler, T.; Karmazin, L.; Bailly, C.; Braunstein, P.; Danopoulos, A. A.
Organometallics 2016, 35, 903–912.
Keywords: bimetallics • copper hydride • metal-ligand
cooperativity • ligand design • metal-metal interactions
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10.1002/chem.201903724
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
E. Kounalis, M. Lutz, D.L.J. Broere*
Page No. – Page No.
Cooperative H2 Activation on
Dicopper(I) Facilitated by Reversible
Dearomatization of an ‘Expanded
PNNP Pincer’ Ligand
Double Trouble A new dinucleating ligand is described that can bind two copper(I)
centers. Reversible dearomatization of the ligand’s naphthyridine core enables
cooperative activation of H2 giving an unusual butterfly-shaped Cu4H2 complex.
This article is protected by copyright. All rights reserved.