A readily accessible PNP pincer ligand with a pyrrole backbone and its NiI/II chemistry

Article abstract of DOI:10.1039/c2dt32199h

The novel chelating PNP pincer ligand 2,5-bis((diphenylphosphino)methyl)- 1H-pyrrole (1) was prepared and its nickel coordination chemistry explored. The reaction with Ni(COD)₂ led to a diamagnetic dinuclear nickel(i) complex (4) which was also

Full text of DOI:10.1039/c2dt32199h

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COMMUNICATION
A readily accessible PNP pincer ligand with a pyrrole backbone
and its NiI/II chemistry†‡
Nora Grüger, Hubert Wadepohl and Lutz H. Gade*
Received 21st September 2012, Accepted 11th October 2012
DOI: 10.1039/c2dt32199h
The novel chelating PNP pincer ligand 2,5-bis((diphenyl-
phosphino)methyl)-1H-pyrrole (1) was prepared and its
nickel coordination chemistry explored. The reaction with
Ni(COD)2 led to a diamagnetic dinuclear nickel(I) complex
(4) which was also obtained by the reaction of the square
planar NiII complexes [(PNP-Ph2)NiX] (X = Cl (2), X = I (3))
Scheme 1 Ligand synthesis.
with Li(Et3BH).
Hybrid ligands containing both soft and hard donors have been
extensively studied in recent years and their metal complexes are
used in a wide range of stoichiometric and catalytic reactions.1–7
These include the amidophosphine ligands containing
SiMe2CH2 backbones studied by Fryzuk and co-workers which
have given rise to remarkable patterns of reactivity with metals
across the periodic table.8–11
The first monoanionic PNP pincer ligand was prepared in the
late sixties by Sacconi and co-workers.12,13 In recent years this
ligand and its complexes with late transition metals have
received increasing attention.14–17 A more robust and rigid
system was first reported by Liang and co-workers and also
studied by Ozerov et al.18–22 Cui, Hou and co-workers have syn-
thesized a monoanionic PNP ligand with a large bite angle based
on a carbazole framework.23
Here we report the synthesis of a novel monoanionic PNP
pincer ligand HPNP-Ph2 (1) containing a pyrrole backbone and
two diphenylphosphinomethyl “arms”. The protio-ligand is
obtained by the reaction of 2,5-bis[(trimethylammonio)-methyl]-
pyrrole diiodide24,25 with potassium diphenylphosphide which
was generated in situ and 1 has been characterized by NMR,
mass spectrometry and elemental analysis (Scheme 1).
Given the suitability for coordination of this ligand in square
planar complexes, we investigated its coordination chemistry
Scheme 2 Complex synthesis.
with nickel. The square-planar complex [(PNP-Ph2)NiCl] (2)
was prepared by salt metathesis of [(PNP-Ph2)Li] with
NiCl2DME at −78 °C and subsequent warming to ambient temp-
erature, giving the reaction product in 77% yield (Scheme 2),
and halide exchange with sodium iodide readily afforded
[(PNP-Ph2)NiI] (3). The 31P{1H} NMR spectra of 2 and 3
display one singlet at 30.6 ppm (2) and at 42.5 ppm (3) for the
two equivalent P atoms [δ(31P) = −16.5 ppm for the protio-
ligand]. A single crystal X-ray structure analysis of 3 established
the structural details associated with the coordination of the new
PNP pincer ligand.§ The structure of 3 is depicted in Fig. 1.
Ligand 1 was designed to chelate metals in a tridentate motif
with P(1), N and P(2) spanning a common plane with the aro-
matic backbone, and the structure confirms that the ligand is
bound to nickel in a tridentate geometry. The coordination geo-
metry of 3 is approximately square planar. The two Ni–P bond
Anorganisch-Chemisches Institut, Universität Heidelberg, Im
Neuenheimer Feld 270, 69120 Heidelberg, Germany.
E-mail: lutz.gade@uni-hd.de
†Dedicated to Professor Mary McPartlin on the occasion of her 80th
birthday.
‡Electronic supplementary information (ESI) available: Characterisation
data for 1–4 and crystallographic data for 3 and 4. CCDC 901720 and
901721. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt32199h
14028 | Dalton Trans., 2012, 41, 14028–14030
This journal is © The Royal Society of Chemistry 2012

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Fig. 1 Molecular structure of 3 (thermal ellipsoids at 50% probability
level). Hydrogen atoms are omitted for clarity. Selected distances (Å)
and angles (°): Ni–P(1) 2.195(1), Ni–P(2) 2.208(1), Ni–N 1.860(2),
Ni–I 2.481(1), P(1)–Ni–P(2) 166.52(3), N–Ni–I 174.75(6).
Fig. 2 Molecular structure of 4 (thermal ellipsoids at 50% probability
level). Hydrogen atoms are omitted for clarity. Selected distances (Å):
Ni(1)–Ni(2) 2.3259(2), Ni(1)–P(2) 2.1240(3), Ni(1)–P(3) 2.7805(3),
Ni(1)–P(4) 2.2333(3), Ni(1)–N(3) 1.9194(8), Ni(2)–P(1) 2.2386(3),
Ni(2)–P(2) 2.6856(3), Ni(2)–P(3) 2.1337(3), Ni(2)–N(2) 1.9273(8).
lengths of 2.1952(1) Å and 2.2084(1) Å and the Ni–N distance
of 1.860(2) Å are in the range of literature values.26 The P(1)–
Ni–P(2) angle is 166.52(3)°, the deviation from the ideal value
for square planar coordination being due to the steric constraints
of the chelating ligand. The N–Ni–I angle is 174.75(6)° and thus
close to 180°. The tridentate ligand adopts a slightly twisted con-
formation which makes the two methylene groups stick slightly
out of the coordination plane. The distances between the plane
spanned by the four donor atoms and the two methylene carbon
atoms are 0.624 Å and 0.388 Å.
Previously, Liang, Ozerov and Kemp have shown with other
monoanionic PNP ligands that [(PNP)NiH] complexes can be
formed via oxidative addition of the N–H bond to Ni(0).19,22,27
Remarkably, treatment of ligand 1 with Ni(COD)2 gave the
dinuclear nickel(I) complex 4 instead of the expected nickel(II)
hydride complex without adding reducing agents (Scheme 2).
Despite the fact that NiI complexes play an important role in
several chemically important processes, well defined NiI com-
plexes, generally prepared by reduction with magnesium or KC8,
are relatively rare due to the reactivity of these mostly radical
species.28–31 The 1H NMR spectrum of the isolated compound 4
displayed only the signals corresponding to the ligand and no
resonance for a hydride. The 31P{1H} NMR spectrum exhibited
two doublets at 14.4 ppm and 25.2 ppm (2J(P,P) = 21.9 Hz).
Both NMR spectra are consistent with an overall C2-symmetry
of the dinuclear species in solution.
The dinuclear core structure of the molecule and the details of
the ligand coordination are confirmed by the single crystal X-ray
structure analysis of a toluene solvate (Fig. 2).§ The two d9
metals are interacting strongly, forming a diamagnetic complex.
The Ni–Ni distance of 2.3259(2) Å is in the range of metal–
metal bond lengths found in other Ni–Ni complexes.26 In
complex 4 one phosphine of each PNP ligand is coordinated to
a nickel atom with Ni–P bond lengths of 2.2333(3) Å and
2.2386(3) Å, respectively. The two other phosphine atoms are
located at bridging positions between the two nickel atoms but
the distances are asymmetric with two short bonds of 2.1240(3) Å
and 2.1337(3) Å to the respective other Ni atom and two weaker
interactions of 2.7805(3) Å and 2.6856(3) Å to the Ni atom
bearing the ligand.32 The angle between the two planes
Ni(1)–P(2)–Ni(2) and Ni(1)–P(3)–Ni(2) is 117.05(1)°. There is
precedent for a dinuclear NiI complex with a monoanionic PNP
pincer ligand synthesized by Mindiola.33,34 However, that
species contained the central amido nitrogen of the pincer ligand
in the bridging position and the two phosphine donors coordinat-
ing to the two metal centres. The interatomic distance between
the two nickel atoms in the dinuclear NiI complex synthesized
by Mindiola and co-workers is in the same range (2.3288(7) Å)
as in 4. However, the former shows paramagnetic behaviour and
thus diradical character.
A structurally related complex with a cyclobutane-like Ni2N2
core was described by Zargarian and co-workers.35,36 They co-
ordinated a POCN pincer type ligand in a dinuclear nickel(II)
complex characterized by a NiII–NiII distance of about 2.51 Å
which is larger than the NiI–NiI distance in complex 4.
The formation of the NiI complex 4 from the reaction of the
protio-ligand 1 with Ni(COD)2 raised the question of the reac-
tion pathway involved. Liang et al. reported one case in which
the targeted nickel hydride complex was not isolated but the
product of COD insertion into a Ni–H bond instead.19 A similar
reaction could be involved in this case, and further studies to
back up this hypothesis were carried out. Upon treating 1 with
Ni(COD)2 in an NMR tube no gas formation was observed and
no signal attributable to molecular hydrogen was detected.
However, the partially hydrogenated diolefin cyclooctene (COE)
could be identified by NMR and GC-MS. Additionally, different
COD isomers were identified by GC-MS. This indicates the
possible formation of a nickel hydride complex followed by an
insertion of COD into the Ni–H bond. The resulting complex
may then react with a second nickel hydride complex leading to
COE and complex 4. Alternatively β-hydrogen elimination
would generate the different COD isomers observed. The detec-
tion of 1,3-cyclooctadiene is consistent with the reversibility of
the insertion into the Ni–H bond. Addition of iodine to complex
4 led to [(PNP-Ph2)NiI] (3) by oxidative addition across the two
metal sites.
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Scheme 3 Following an alternative route starting from a nickel(II) complex gave complex 4 with 5 as an intermediate.
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In an alternative approach for the synthesis of complex 4
(Scheme 3) Li(Et3BH) was added to a solution of complexes 2
or 3 in benzene-d6. In these cases evolution of a gas could be
observed and the 1H NMR spectrum showed a singlet at
4.47 ppm corresponding to the hydrogen molecule. Carrying out
this reaction at low temperature (−40 °C) a hydrido species
[(PNP-Ph2)NiH] (5) was detected by NMR spectroscopy
(Scheme 3). The 1H NMR spectrum revealed a triplet at
−16.72 ppm, 2J(P,H) = 56 Hz, the 31P NMR spectrum a
singlet at 52.0 ppm. Under these reaction conditions, a hydrido
species is an intermediate in the formation of the dinuclear NiI
complex 4.
In conclusion, a novel monoanionic PNP pincer type ligand
based on a pyrrole backbone was synthesized and its nickel(I)
and nickel(II) coordination chemistry was investigated. Interest-
ingly, a diamagnetic dinuclear nickel(I) complex was directly
formed without addition of reducing agents. Further studies into
the coordination chemistry of this ligand are ongoing in our
laboratory.
This work has been supported by the Deutsche Forschungs-
gemeinschaft (SFB 623, TP B6).
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Notes and references
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§Crystal data: complex 3: C30H26INNiP2, monoclinic, space group P21/
c, a = 14.291(9), b = 16.499(11), c = 11.466(6) Å, β = 107.243(9)°, V =
2582(3) Å3, Z = 4, μ = 2.092 mm−1, F000 = 1296, T = 100(2) K, θ range
1.5 to 32.2°. Index ranges h, k, l (indep. set): −21⋯20, 0⋯24, 0⋯17.
Reflections measd: 53 737, indep.: 8602 [Rint
= 0.0493], obsvd
[I > 2σ(I)]: 6783. Final R indices [Fo > 4σ(Fo)]: R(F) = 0.0329,
wR(F2) = 0.0691, GooF = 1.059.
Complex 4·1.5 toluene: C70.5H63.5N2Ni2P4, triclinic, space group P1,
28 S. Miyazaki, Y. Koga, T. Matsumoto and K. Matsubara, Chem. Commun.,
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ˉ
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a = 13.4842(2), b = 14.8729(2), c = 16.2602(3) Å, α = 65.464(2), β =
81.949(1), γ = 72.130(1)°, V = 2823.09(7) Å3, Z = 2, μ = 0.827 mm−1
,
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[Fo > 4σ(Fo)]: R(F) = 0.0260, wR(F2) = 0.0652, GooF = 1.032.
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14030 | Dalton Trans., 2012, 41, 14028–14030
This journal is © The Royal Society of Chemistry 2012