Unexpected reactivity of a PONNOP 'expanded pincer' ligand

Article abstract of DOI:10.1039/d0cc02166k

We report the synthesis, characterization and coordination chemistry of a new naphthyridine-derived phosphinite PONNOP expanded pincer ligand. As envisioned, the dinucleating ligand readily binds two copper(i) centers in close proximity, but undergoes an unexpected rearrangement in the presence of nickel(ii) salts to form an interesting PONNP pincer platform. This journal is

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Unexpected Reactivity of a PONNOP ‘Expanded Pincer’ Ligand
Arthur R. Scheerder,[a] Martin Lutz[b] and Daniël L. J. Broere*[a]
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report the synthesis, characterization and coordination Milstein8 and Goldberg & Brookhart9 groups. The PONOP pincer
chemistry of a new naphthyridine‐derived phosphinite PONNOP ligand has been utilized to stabilize various metal complexes
expanded pincer ligand. As envisioned, the dinucleating ligand and, for example, has enabled the detailed study of dihydrogen
readily binds two copper(I) centers in close proximity, but complexes of Rh and Ir,10 σ‐alkane complexes,11 and more
undergoes an unexpected rearrangement in the presence of recently photolytic N2 reduction to NH3 using Re.12 Given the
nickel(II) salts to form an interesting PONNP pincer platform.
distinct differences between the PNP and PONOP pincer ligands
we were interested to study a PNNP analogue wherein the
The use of well‐defined bimetallic complexes wherein two reactive methylene linkers are replaced by oxygen atoms.
metal atoms are bound in close proximity is an emerging area Herein, we describe the synthesis, characterization and
i
‐
in homogeneous catalysis.1 The cooperative activation of coordination chemistry of a new expanded pincer ligand,
substrate molecules by two metal centers in such bimetallic PrPONNOP. Similar to the PNNP analogue, i‐PrPONNOP can bind
systems can enable distinct reactivity or improved catalytic two copper atoms in close proximity. However, we found that
performance when compared to monometallic analogues.2 The this ligand undergoes an unusual rearrangement in the
design and exploration of new dinucleating ligands that enable presence of nickel salts. Although this rearrangement provides
such metal‐metal cooperativity is key to further explore the access to a unique ‘regular’ pincer ligand, it can prohibit binding
potential of bimetallic catalysis. A motif well‐suited to bind two two metal centers in close proximity, and should therefore be
metals is the 1,8‐naphthyridine scaffold.3 Yet, relatively few taken into consideration in the design of naphthyridine‐derived
ligands derived thereof have been reported that are capable of dinucleating ligands featuring phosphinite donors.13
both binding two metal centers in close proximity while also
providing accessible adjacent coordination sites on both metals
to enable metal‐metal cooperativity.4 In the pursuit of such new
platforms we recently reported a dinucleating ‘expanded PNNP
pincer’ ligand,5 which was inspired by the mononucleating PNP
pincer ligands6 (Scheme 1, top). Similar to the PNP ligands the
CH2 linkers in the PNNP ligand can be deprotonated
concomitant with dearomatization of the naphthyridine core,
and this enabled the cooperative activation of H2 to give a
tetranuclear copper dihydride cluster.7
Exchanging the acidic CH2 linkers in the PNP pincer ligands by O
atoms affords the diphosphinite PONOP pincer ligand (Scheme
1, bottom), which was developed independently by the
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 Centre for Biomolecular Research Faculty
of Science, Utrecht University Padualaan 8, 3584 CH Utrecht (The Netherlands)
Electronic Supplementary Information (ESI) available: NMR, IR and X‐ray
Scheme 1. Comparison of the PNP and PONOP pincer ligands with the
crystallographic data and synthetic procedures for all compounds. See
DOI: 10.1039/x0xx00000x
PNNP and PONNOP expanded pincer ligands.
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i
The phosphinite expanded pincer ligand ‐PrPONNOP was
prepared in a three‐step procedure from commercially available
starting materials in an overall yield of 37%. The first two steps
comprise the synthesis of 2,7‐dihydroxy‐1,8‐naphthyridine
according to a modified literature procedure.14 Subsequent
phosphorylation was achieved by a reaction with a slight excess
of chlorodiisopropylphosphine in the presence of excess Et3N in
THF at 65 oC (Scheme 2) affording i‐PrPONNOP as a brown oil in
82% yield and 90% purity after workup. Several attempts to
further enhance the purity solely resulted further decrease in
purity due to the facile hydrolysis of the P‐O bonds in the Figure 1. Displacement ellipsoid plot (50% probability) of complex
1 in
expanded pincer ligand (see ESI for more detail). The related the crystal. Hydrogen atoms and THF solvent molecule are omitted, and
2,6‐dihydroxypyridine‐derived i‐PrPONOP pincer ligand was also i‐Pr groups on P are depicted as wireframe for clarity.
reported in 90% purity.8 Fortunately, the unidentified impurities
in the expanded pincer ligand are readily removed in As the P‐O bonds in the phosphinite expanded pincer ligand are
1
subsequent reactions (see below). The H, 13C and 31P NMR highly susceptible to hydrolysis, the use of solvents containing
i
spectra of ‐PrPONNOP in C6D6 at 298 K show the expected trace water typically results in precipitation of 2,7‐dihydroxy‐
number of resonances for a C2v symmetric species. The 31P{1H} 1,8‐naphthyridine and the formation of a partial hydrolysis
NMR spectrum shows a singlet at δ = 148.2 ppm, similar to the product i‐PrPONNOH. The latter monophosphinite displays four
observed resonance for the i‐PrPONOP pincer (δ = 149.1 ppm).8 characteristic aromatic resonances in 1H NMR spectra of
reaction mixtures and is also observed as an intermediate in the
i
synthesis of ‐PrPONNOP (See ESI for more detail). Both these
undesired naphthyridine containing products were also
observed in several reactions as a result of alcoholysis or
aminolysis. For example, a reaction in THF of i‐PrPONNOP with 2
equiv NiCl2 containing residual EtOH resulted in the formation
of an insoluble 2,7‐dihydroxy‐1,8‐naphthyridine‐derived
Scheme 2. The synthesis of the i‐PrPONNOP “expanded pincer” ligand.
species and trans‐NiCl2(P(OEt)(i‐Pr)2)2, which was isolated and
crystallographically characterized (see ESI for more detail).
i
Reacting ‐PrPONNOP with 2 equiv of CuCl in THF or CH2Cl2 Nonetheless, when using rigorously dried solvents these
(Scheme 3) gives the dicopper(I) complex
as yellow‐orange powder in 77% yield. The 1H, 13C and 31P NMR
spectra of in CD2Cl2 show that the C2v symmetry is retained formation of an insoluble precipitate and a single species in
upon binding the Cu centers. The 31P NMR spectrum of in solution that features four aromatic resonances in the 1H NMR
1
, which was isolated undesired side reactions can be prevented.
Reacting i‐PrPONNOP with 2 equiv NiBr2 in THF results in the
1
1
CD2Cl2 features a single broad resonance at δ = 123.7 ppm. VT spectrum, indicative of a loss of C2v symmetry. Unlike the partial
NMR analysis (Figure S14‐19) showed that the broadness is not hydrolysis product i‐PrPONNOH, this unsymmetrical species
due to a fluxional process on the NMR time scale. This suggests contains two diisopropylphospinite moieties, which appear as
that it originates from quadrupolar relaxation arising from 63Cu two doublets (2JPP = 326 Hz) at δ = 192.0 and 131.5 ppm in the
and 65Cu (both I = 3/2) nuclei, which is not uncommon for Cu(I) 31P NMR spectrum in CD2Cl2. To our surprise single‐crystal X‐ray
phosphinite complexes.15 Crystals suitable for single‐crystal X‐ structure determination (Figure 2) did not reveal a dinuclear
ray diffraction were grown by vapor diffusion of hexane into a complex but showed mononuclear complex
THF solution of
at ambient temperature. The solid‐state consistent with the 1H, 13C and 31P NMR spectra as well as the
structure of
2, which is
1
1
(Figure 1) revealed a nearly flat dinucleating CHN elemental analysis of the obtained solid. Furthermore,
‐
phosphinite ligand bound to a diamond‐shaped Cu2Cl2 core. The complex
structure is similar to the previously reported t‐BuPNNPCu2Cl2
2
can also be prepared in 89% yield by a reaction of i
PrPONNOP ligand with 1 equiv NiBr2 in CH2Cl2 (Scheme 4). The
7
but features a Cu‐Cu distance (2.7224(9) Å) that is ~0.14 Å solid state structure (Figure 2) shows that the expanded pincer
longer, and PN bite angles (∠ ∠ P2‐ ligand underwent a rearrangement to form an interesting
P1‐Cu1‐N1 = 83.33(14)o and
Cu2‐N2 = 83.00(14)o) that are smaller by ~3o (see table S3‐5 for naphthyridone‐like i‐PrPONNP ‘regular’ pincer ligand bound to
more detail).
Scheme 3. Synthesis of the dicopper(I) complex
2 | J. Name., 2012, 00, 1‐3
1
.
Scheme 4. Synthesis of the mononuclear Ni complex 2.
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nickel. The nickel atom exhibits a slightly distorted square PONNP isomers are similar in energy and that there is a
pyramidal geometry with a Br atom in the apical position. relatively high barrier for their interconversion. Heating a
i
o
Nickel(II) pentacoordination is uncommon in pincer chemistry16 toluene solution of ‐PrPONNOP for multiple hours at 110 C
and all reported NiX2 (X = Cl or Br) complexes bearing related showed no conversion into a new species based on NMR
lutidine‐derived PNP,17,18,19 PONOP20 and PNNNP21 pincer analysis indicating that either the barrier for the
ligands display square planar geometries with
coordinated halide anion. The nickel bromide bond lengths in
a
non‐ interconversion is too high or that the PONNOP isomer is the
thermodynamic product.24 Notably, when the PONNOP ligand is
2
differ greatly, and the distance is significantly longer for the bound to either CuCl or NiBr2 the calculated free energies of the
bromide in the apical position (Ni1‐Br1 =2.7852(8) Å than what PONNP isomer are significantly more stable than the PONNOP
is observed for the bromide in the basal position (Ni1‐Br2 = isomer. Similarly, the calculated energy barrier for the
2.3200(8) Å).
dibromo{bis[2‐(diphenylphosphanyl)ethyl]amine}Ni(II)
A
similar observation was made for
a
conversion of the PONNOP isomer to the PONNP is also
significantly lower. Upon analysis of the corresponding
complex, which showed an apical Ni‐Br bond of 2.698(7) Å and transition state geometries (Figure 3, bottom) it can be seen
a basal Ni‐Br bond of 2.333(7) Å.22 A rare example of a related that the P atom, which migrates from O to N is bound to the
pentacoordinated Ni(PNNNP) pincer complex with an anionic transition metal atom. Although this also results in significant
phosphinite instead of a basal Br ligand, showed an significantly bending of the naphthyridine core (Figure S33), we reason that
shorter apical Ni‐Br bond of 2.535(4) Å.23 The intraligand bond this provides the additional stabilization of the transition state
metrics are distinctly different from those in complex
1 (Table to enable the isomerization towards the more stable PONNP
S6). The C7‐O2 bond length of 1.214(6) Å is significantly shorter pincer motif. We hypothesize that these effects are most
than the C1‐O1 bond length of 1.346(5) Å. This observation pronounced when the ligand is bound to NiBr2 because the TS
indicates a dearomatization of the six‐membered heterocycle and PONNP geometries are closer to the preferred coordination
containing N2 to form a localized naphthyridone, and this is geometry of Ni(II) than of that of Cu(I) (see ESI for more detail).
further underlined by the clear localization of single and double Although dinuclear complex
1 was obtained in high yield, we
bonds in this ring (Figure S29). We reason that the reacted i‐PrPONNOP with one equiv of CuCl since the calculated
i
rearrangement of the ‐PrPONNOP ligand to the pincer ligand free energy profile indicated facile formation of a PONNP pincer
present in complex
2 is also enabled by the ability to form a complex of CuCl. Unfortunately, no conclusions can be drawn
naphthyridone structure.
from this experiment as it resulted in the formation of an
inseparable mixture of unidentified unsymmetrical species. The
broadness of the resonances due to coordination to Cu also
obscured the observation of a potential characteristic trans
coupling like that observed in the 31P NMR spectrum of
reason that the facile formation of is likely fast and irreversible
2. We
2
under the experimental conditions, and this is in agreement
with the calculated free energy profile that features a low
barrier and is very exergonic. In contrast, it is conceivable that
with Cu the binding of a second CuCl is associated with a lower
energy barrier, and therefore kinetically favored over the
rearrangement. An alternative explanation is that the
Figure 2. Displacement ellipsoid plot (50% probability) of complex
2 in
the crystal. Hydrogen atoms and THF solvent molecules are omitted,
and i‐Pr groups on P are depicted as wireframe for clarity.
To gain insight into why the ligand rearrangement is only
observed with NiBr2 and not with CuCl we studied the
interconversion of the PONNOP and PONNP isomers
computationally. Both isomers and the transition state
geometries for their conversion were optimized (BP86‐D3, def2‐
TZVP) without a metal, bound to CuCl or bound to NiBr2 (See ESI
for more details and discussion). Unfortunately, the
heterogeneous nature of the studied reactions and the high
i
sensitivity of the ‐PrPONNOP ligand prohibited experimental
validation of the computational methods beyond a good
agreement of the experimental and computational geometries
of complex 2. Hence, we refrain from putting much value on the
absolute energies and choose to only look at the general trends, Figure 3. DFT calculated (BP86‐D3, def2‐TZVP) free energy profiles
which provide some insight. The calculated free energy profile (ΔG0298K in kcal mol‐1) for the rearrangement of PONNOP to PONNP (top)
for the free ligand (Figure 3, top) shows that the PONNOP and and the transition state geometries (bottom).
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J. Name., 2013, 00, 1‐3 | 3
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6484–6491; b) T. ‐T. Zhou, D. R. Hartline, T. J. Steiman, P. E.
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rearrangement is reversible in the presence of excess CuCl, and
perhaps mediated by binding to
a second Cu center.
i
‐
Unfortunately, since various attempts at reactions of
PrPONNOP with 1 equiv CuCl or 2 equiv NiBr2 did not give a
monocopper or dinickel species, respectively, we have thus far
been unable to verify these hypotheses.
In conclusion, we have described the synthesis,
5
E. Kounalis, M. Lutz, D. L. J. Broere Organometallics 2020, 39,
characterization and coordination chemistry of
a new
585‐592.
i
diphosphinite expanded pincer ligand, ‐PrPONNOP. This new
dinucleating ligand does not feature the acidic methylene
linkers that are present in the related PNNP ligand, and binds
two copper centers in close proximity in a similar fashion. In
contrast, attempts to bind two nickel centers resulted in a
ligand rearrangement concomitant with the formation of a
unique naphthyridone PONNP pincer ligand. This work adds a
new, readily synthesized dinucleating ligand to the bimetallic
catalysis toolkit. Moreover, the results herein demonstrate that
naphthyridine‐derived ligands featuring phosphinite donors
should be used with caution as they are able to undergo a
rearrangement under certain conditions to form a ‘regular’
pincer ligand. Although the latter is undesired for bimetallic
chemistry, it does provide access to an otherwise inaccessible
ligand class for mononuclear chemistry.
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This work was supported by Utrecht University (tenure track
start‐up package to D.L.J.B), 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. Lada
Dabranskaya is acknowledged for her artistic input for the TOC
graphic. NMR and ORCA data files can be obtained free of
charge from: http://doi.org/10.4121/uuid:8fd78884-01b0-4c37-
a4ca-fb1cf4af8b61
12 Q. J. Bruch, G. P. Connor, C. ‐H. Chen, P. L. Holland, J. M. Mayer,
F. Hasanayn, A. J. M. Miller J. Am. Chem. Soc. 2019, 141, 20198‐
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13 While this manuscript was in preparation an interesting ligand
system for the selective synthesis of heterobimetallic complexes
that features such a naphthyridine‐derived phosphinite system
was reported: S. Deolka, O. Rivada Wheelaghan, S. Aristizábal, R.
Fayzullin, S. Pal, K. Nozaki, E. Khaskin, J. Khusnutdinova (2020):
Metal‐Metal Cooperative Bond Activation by Heterobimetallic
Alkyl, Aryl, and Acetylide PtII/CuI Complexes. ChemRxiv. Preprint.
https://doi.org/10.26434/chemrxiv.11742687.v1
14 P. S. Lemport, P. N. Ostapchuk, A. A. Bobrikova, P. V. Petrovskii,
N. D. Kagramanov, G. V. Bodrin, E. E. Nifant’ev Mendeleev
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Conflicts of interest
There are no conflicts to declare.
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the thermodynamic product (by only 2 kcal mol‐1), we consider
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4 | J. Name., 2012, 00, 1‐3
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