Study of the Reactivity of the [(PE1CE2P)Ni(II)] (E1, E2 = O, S) Pincer System with Acetonitrile and Base: Formation of Cyanomethyl and Amidocrotononitrile Complexes versus Ligand Decomposition by P-S Bond Activation

Article abstract of DOI:10.1021/acs.organomet.9b00645

Nickel(II) chloride complexes with PE¹CE²P (E = O, S) pincer ligands were used as precursors for the generation of cyanomethyl complexes in order to investigate the influence of variations (O vs S) in the side arms of the ligands on reactivity and stability of such compounds. In this regard, five hitherto unknown Ni(II) compounds were synthesized and fully characterized. Reaction of the Ni(II) chloride complex [(^iPrPOCSP^iPr)NiCl] (2-Cl) with 1 equiv of base and nitrile furnishes the cyanomethyl complex [(^iPrPOCSP^iPr)NiCH₂CN] (2-CM). Increase of the amount of base and nitrile results in the formation of 3-amidocrotononitrile complexes [(^iPrPOCOP^iPr)NiNHC(CH₃)CHCN] (1-ACN) and [(^iPrPOCSP^iPr)NiNHC(CH₃)CHCN] (2-ACN). In contrast, similar reactions of the bis(thiophosphinite) complex 3-Cl resulted in formation of a tetranuclear Ni cluster (4) or a dinuclear 1,3-dithiolate-bridged PSCSP complex 5 by unexpected cleavage of P-S bonds of the pincer ligand.

Full text of DOI:10.1021/acs.organomet.9b00645

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Study of the Reactivity of the [(PE1CE2P)Ni(II)] (E1, E2 = O, S) Pincer
System with Acetonitrile and Base: Formation of Cyanomethyl and
Amidocrotononitrile Complexes versus Ligand Decomposition by
P−S Bond Activation
Patrick Hasche, Anke Spannenberg, and Torsten Beweries*
Leibniz-Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
S
* Supporting Information
ABSTRACT: Nickel(II) chloride complexes with PE1CE2P
(E = O, S) pincer ligands were used as precursors for the
generation of cyanomethyl complexes in order to investigate
the influence of variations (O vs S) in the side arms of the
ligands on reactivity and stability of such compounds. In this
regard, five hitherto unknown Ni(II) compounds were
synthesized and fully characterized. Reaction of the Ni(II)
chloride complex [(iPrPOCSPiPr)NiCl] (2-Cl) with 1 equiv of base and nitrile furnishes the cyanomethyl complex
[(iPrPOCSPiPr)NiCH2CN] (2-CM). Increase of the amount of base and nitrile results in the formation of 3-amidocrotononitrile
complexes [(iPrPOCOPiPr)NiNHC(CH3)CHCN] (1-ACN) and [(iPrPOCSPiPr)NiNHC(CH3)CHCN] (2-ACN). In contrast,
similar reactions of the bis(thiophosphinite) complex 3-Cl resulted in formation of a tetranuclear Ni cluster (4) or a dinuclear
1,3-dithiolate-bridged PSCSP complex 5 by unexpected cleavage of P−S bonds of the pincer ligand.
On the basis of Zargarian’s initial report, our group recently
published the first Ni thiophosphinite [(iPrPOCSPiPr)NiCl] (2-
Cl) and Ni bis(thiophosphinite) [(iPrPSCSPiPr)NiCl] (3-Cl)
pincer complexes (POCSP = C6H4-2-(OP(iPr)2)-6-(SP-
(iPr)2); PSCSP = C6H4-2,6-(SP(iPr)2)2).37 Both complexes
showed significantly better activity for Kumada cross-coupling
than the well-known isostructural complex 1-Cl with activation
of chlorobenzene even at room temperature.
INTRODUCTION
■
Transition metal pincer complexes were first reported in the
1970s by the groups of Shaw1,2 and van Koten.3−5 Since then,
pincer compounds have attracted considerable attention as
such complexes have been at the forefront of advances in
catalytic coupling reactions, hydrogenation and dehydrogen-
ation and various other transformations.6−17 Among these
complexes, metal centers ligated by tridentate, meridional
pincer ligands are most well-known due to their high thermal
stability and stabilization of unusual oxidation states of the
metal center. Combined with the possibility to influence the
electronic and steric properties of the pincer ligands and thus
the resulting complex, these compounds have greatly expanded
the field of pincer chemistry.18−26
Consequently, extensive attention has been focused on the
combination of pincer-type ligands with earth-abundant metals
due to their significantly lower cost compared to precious
metals.27−30 In this regard, Zargarian et al. reported on a series
of bis(phosphinite) nickel pincer complexes derived from
[(iPrPOCOPiPr)NiCl] (1-Cl) (POCOP = C6H4-2,6-(OP-
(iPr)2)2).31,32 Since its first publication in 2007, Zargarian’s
Ni pincer complex was used by many other groups due to its
great usability for many transformations, allowing for the
synthesis of hydride,33,34 thiolate,35 triflate,31 and many other
complexes. Another prominent example includes the trans-
formation to the Ni cyanomethyl pincer complex
[(iPrPOCOPiPr)NiCH2CN] (1-CM), which is generated by
treating the Ni chlorido pincer complex with in situ generated
LiCH2CN, as reported by Guan in 2013.36
Motivated by these findings, we were further interested in
exploring the chemistry of Ni complexes coordinated by
iPrPOCSPiPr and iPrPSCSPiPr pincer ligands. For this study, we
chose Guan’s cyanomethyl complex as a starting point, asking
ourselves the question whether introduction of sulfur into the
side arms of the pincer ligand would have a noticeable effect on
the complex preparation or its properties.38−40 That variation
of the pincer ligand (i.e., introduction of sulfur) can have a
significant effect on the coordination chemistry was shown
before for Rh complexes, where PSCSP pincer complexes
could be accessed by C−H bond activation even at room
temperature.41 In this regard, five new Ni complexes were
synthesized and fully characterized. In addition, we point out
the importance of the stoichiometry for the generation of Ni
cyanomethyl complexes as higher amounts of base and nitrile
lead to different complexes.
Received: September 30, 2019
© XXXX American Chemical Society
A
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cyanomethyl complex. The presence of similar complexes was
not reported before by Guan et al.36
The molecular structure shows the Ni center in distorted
square-planar coordination geometry. The Ni1−N1 bond
distance was found to be 1.9022(12) Å. PCP Ni amide
complexes are comparably rare.42−46
RESULTS AND DISCUSSION
■
The reaction of 2-Cl with LiCH2CN, formed by addition of n-
BuLi to MeCN in THF at −78 °C, affords complex 2-CM as
an orange-brownish solid with yields greater than 75%
(Scheme 1). Notably, this reaction is very fast as similar yields
Characteristic resonances in 1H NMR include a sharp singlet
at δ = 1.90 ppm for the CH3 group in the 3-amidocrotononi-
trile ligand, a broad signal at δ = 2.07 ppm for the NH proton,
as well as a singlet at δ = 3.60 ppm for the methine proton
adjacent to the CN group. The sharp singlet in 31P{1H} NMR
at δ = 181.9 ppm isas already mentionedshifted to higher
field compared to Guan’s complex [δ = 189.6 ppm (in THF-
d8)]36 or the starting material [(iPrPOCOPiPr)NiCl] [δ = 185.5
ppm (in CDCl3)].31
With the new POCOP complex 1-ACN in hand, we decided
to investigate if our analogous thiophosphinite and bis-
(thiophosphinite) complexes would show a similar behavior
in the presence of more than 1 equiv of base and nitrile. For
the [(iPrPOCSPiPr)NiCl] complex 2-Cl, we observed the same
reactivity: addition of 1 equiv of base and acetonitrile affords
the already mentioned cyanomethyl complex 2-CM, whereby
an excess of base and nitrile leads to a complex possessing a 3-
amidocrotononitrile ligand (2-ACN, Scheme 2).
Scheme 1. Synthesis of Complex 2-CM
were observed also after reaction times of less than 5 min
followed by immediate workup. Comparison with the NMR
and IR data of Guan’s complex [(iPrPOCOPiPr)NiCH2CN] (1-
CM)36 (cf. H NMR δ = 0.78 ppm in THF-d8; 13C NMR δ =
1
−23.9 ppm in CD2Cl2; IR ν = 2187 cm−1 (in CH2Cl2)) shows
a very good agreement with our data as 1H NMR spectroscopic
analysis shows a characteristic resonance for the two methylene
protons at δ = 0.75 ppm (3JP−H = 9.8 Hz, THF-d8) as a set of
two doublets due to the nonsymmetric character of this
complex. The methylene carbon resonance for complex 2-CM
was observed at δ = −24.4 ppm (2JP−C = 17 Hz) as a triplet.
The attenuated total reflectance/infrared spectroscopy (ATR-
IR) spectrum reveals a strong band at ν = 2187 cm−1 for
ν(CN), thus confirming the assignment as a cyanomethyl
complex. It should be noted that the resonances in the 31P{1H}
NMR spectra for 2-CM as well as for 1-CM are slightly shifted
to lower field compared to the corresponding chlorido
complexes.
Scheme 2. Formation of Cyanomethyl versus
Amidocrotononitrile Complexes
When working with complex 1-Cl in the presence of excess
base, a high field shift of the resonances in 31P{1H} NMR
compared to the corresponding signals of the chlorido complex
occurred. Additionally, the 1H NMR spectra no longer showed
the characteristic signal for methylene protons but instead new
defined signals indicating the formation of a new hitherto
unknown complex. The molecular structure of this new
complex 1-ACN is depicted in Figure 1. To our surprise,
instead of the cyanomethyl ligand, which is formed by
deprotonation and coordination of only one molecule of
acetonitrile, a 3-amidocrotononitrile ligand was obtained by
formal insertion of acetonitrile into the Ni−C bond of a Ni
1H NMR data of complex 2-ACN resemble those of the
POCOP analogue 1-ACN (δ(CH3) = 1.90, δ(CH) = 3.68
ppm); however, the NH resonance is found at much higher
field (δ = 1.65 ppm). 31P{1H} NMR spectra show two
doublets at δ = 101.9 and 175.3 ppm, and both signals are
again shifted to higher field compared to the cyanomethyl
complex (δ = 121.1, 183.6 ppm) or the starting material, the
chlorido complex 2-Cl (δ = 105.0, 179.7 ppm).37
The molecular structure is depicted in Figure 2. With a bond
distance of 1.9167(12) Å, the Ni1−N1 bond in 2-ACN is
slightly longer than that in the POCOP analogue. In the
crystal, molecules of 1-ACN and 2-ACN, respectively, are
linked by weak intermolecular N−H···N hydrogen bonds
between the NH group and the N of the CN group, forming
chains along the c-axis (Figure 3 and Table 1).
Investigation of these complexes by ATR-IR spectroscopy
revealed intense bands at ν = 2145 cm−1 (1-ACN) and ν =
2146 cm−1 (2-ACN) for the ν(C≡N) vibrations, which are
shifted to lower wavenumber compared to cyanomethyl
complexes (Table 2). Similarly, bands for N−H vibration of
the amide group in complexes 1-ACN (3289 cm−1) and 2-
Figure 1. Molecular structure of complex 1-ACN. Displacement
ellipsoids correspond to 30% probability. Hydrogen atoms, except for
the amido and methine hydrogen, are omitted for clarity.
B
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Table 1. Hydrogen Bond Geometry for Complexes 1-ACN
and 2-ACN
a
a
a
N1−H1
H1···N2
(Å)
N1···N2
(Å)
N1−H1···N2
complex
(Å)
(deg)
1-ACN
2-ACN
0.84(2)
0.81(2)
2.42(2)
2.50(2)
3.229(2)
3.307(2)
162(2)
173(2)
a
Symmetry code for 1-ACN: x, 1/2 − y, −1/2 + z; for 2-ACN: x,
−1/2 − y, −1/2 + z.
Table 2. IR Data of Cyanomethyl and Amidocrotononitrile
Complexes
complex
ν(CN) (cm−1
)
ν(N−H) (cm−1
)
a
1-CM
2185
na
Figure 2. Molecular structure of complex 2-ACN. Displacement
ellipsoids correspond to 30% probability. Hydrogen atoms, except for
the amido and methine hydrogen, as well as the second position of the
disordered pincer ligand, are omitted for clarity.
2-CM
1-ACN
2-ACN
2187
2145
2146
na
3289
3279
a
Data taken from ref 36.
ACN (3279 cm−1) showed only minor differences due to
variation of the pincer backbone.
Although 3-aminocrotononitrile can be seen as a dimer
consisting of two molecules of acetonitrile, only one
acetonitrile unit requires deprotonation to form the dimer.
As a consequence, a plausible mechanism for generation of 3-
amidocrotononitrile complexes involves formal migratory
insertion of a second acetonitrile molecule into the Ni−C
bond of complexes 1-CM and 2-CM, followed by proton shift
from the CH2 group to the nitrogen in α-position (Scheme 3).
Also, dimerization of aliphatic nitriles is known to occur via
base-catalyzed self-condensation (Thorpe reaction).57
3-Amidocrotononitrile is a rather unusual ligand for
transition metal complexes; however, examples are known in
which two of these ligands bind to two metal centers via the
amide and nitrile nitrogen atom.47−55 Apart from that, only
one Zn complex exists, where 3-amidocrotononitrile does not
act as a bridging ligand, only coordinating via the amido
nitrogen to the metal center.56 The complexes presented
herein are, to the best of our knowledge, the first examples of
fully characterized metal pincer complexes, where 3-amidoc-
rotononitrile coordinates as a monodentate ligand to a single
metal center. Ni amidocrotononitrile complexes 1-ACN and 2-
ACN are sensitive toward protonation. Test reactions (cf.
Supporting Information, p S5) with water and methanol
resulted in formation of free aminocrotononitrile along with
decomposition of the pincer ligand. Reactions with HCl result
in clean formation of the chloride complexes 1-Cl and 2-Cl,
respectively.
Scheme 3. Formal Insertion of MeCN into the Ni−C Bond
Figure 3. Packing diagram of complex 1-ACN along the a-axis. Hydrogen bonds are depicted as dashed lines and are along the c-axis. Hydrogen
atoms, except for the amido hydrogen, are omitted for clarity.
C
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Scheme 4. Control Experiments for Generation of Complex 1-ACN
Attempts to verify this proposal failed as cyanomethyl
complexes 1-CM and 2-CM are stable in MeCN, even after
prolonged heating. The addition of stoichiometric and
substoichiometric amounts of base to evaluate base-mediated
dimerization also has no effect. Treatment of both, 1-CM and
2-CM with n-BuLi under reaction conditions did not result in
formation of 1-ACN or 2-ACN; instead, only the starting
cyanomethyl complexes as well as ill-defined decomposition
products could be isolated (Scheme 4a). Increasing the
amount of nitrile by performing the reaction in pure
acetonitrile instead of THF had no influence as we still
could not find any evidence for the formation of 1-ACN or 2-
ACN (Scheme 4b). Attempts to synthesize the 3-amidocro-
tononitrile complexes with only 1 equiv of base failed so far;
that is the reaction of 1-Cl with 1 equiv of n-BuLi and 4 equiv
of MeCN only led to formation of 1-CM (Scheme 4c).
Dimerization in the coordination sphere of Ni is thus unlikely.
As free 3-aminocrotononitrile is reported to be synthesized
at comparably high temperatures (e.g., in refluxing toluene)
using other bases (e.g., KOtBu),58 we checked whether it can
also be generated under our reaction conditions, that is, at −78
°C with slow warming to room temperature. Indeed, when
performing the reaction at −78 °C without adding the Ni
chlorido complex, we observed rapid formation of 3-amino-
crotononitrile as a mixture of E- and Z-isomers (δ(CH3) =
1.90, 2.08; δ(CH) = 3.80, 4.10; δ(NH2) = 4.38, 4.70 ppm))
after quenching the reaction mixture with water (Scheme 4d).
However, when adding 3-aminocrotononitrile to a THF
solution of [(iPrPOCOPiPr)NiCl] in the presence of n-BuLi,
no deprotonation and coordination to the Ni center was
observed by NMR (Scheme 4e). Instead, slow decomposition
of the pincer ligand occurred. Also, with weaker bases such as
NEt3, no conversion was observed. Adding n-BuLi first to a
THF solution of 3-aminocrotononitrile, which was then stirred
for 2 h before 1-Cl was added (i.e., reversed order of addition
of 1-Cl and MeCN), had no effect as we still observed slow
decomposition of the pincer ligand and no formation of
complex 1-ACN (Scheme 4f).
The presence of excess base that is needed for the formation
of amidocrotononitrile complexes could, in principle, be
explained by simultaneous chloride abstraction and MeCN
deprotonation. Outer-sphere nucleophilic attack of thus
obtained NCCH2− on an acetonitrile adduct [(iPrPECEPiPr)-
Ni(NCMe)]+ could formally result in formation of an
amidocrotononitrile complex. To evaluate this, we have
reacted LiCH2CN with [(iPrPOCOPiPr)Ni(NCMe)]OTf but
did not observe formation of 1-ACN (Figure S13). From these
experiments, it remains unclear why the second equivalent of
base is needed to generate the Ni amidocrotononitrile
complexes as Himmel’s Zn complex is formed with only 1
equiv of base, supporting the insertion mechanism depicted in
Scheme 3.56
When we attempted to access the cyanomethyl complex 3-
CM by reaction of 3-Cl with equimolar amounts of n-BuLi and
an excess of MeCN, yellow to orange oily residues were
obtained after filtration and evaporation of the solvent, which
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were immediately analyzed by NMR analysis. Although NMR
spectra clearly showed the formation of the cyanomethyl
complex 3-CM by a sharp triplet in the H NMR at δ = 0.55
liquor, from which the crystals of 4 grew, no longer showed
signals that point to the presence of 3-CM as it completely
decomposed to 4 and some other ill-defined decomposition
products.
1
ppm (in toluene-d8) for the CH2 protons and a singlet in
31P{1H} at δ = 104.2 ppm, which is shifted to lower field
compared to 3-Cl [δ = 98.9 ppm (in toluene-d8)],37 all
attempts to isolate 3-CM failed as the compound quickly
decomposed into an ill-defined black mixture when we tried to
extract it with various solvents. However, dissolving a freshly
synthesized sample of 3-CM in a small amount of acetonitrile
at room temperature led to the formation of black crystals
suitable for X-ray analysis. The molecular structure, to our
surprise, shows a cluster-like compound consisting of four Ni
atoms and four P−S activated ligand fragments (i.e., cleavage
of the P(iPr)2 group), all of which coordinated via the P and
ipso-C atoms to a Ni center, and the remaining S atom bridges
When increasing the amount of base, isolation of a red solid
material was possible. 31P{1H} NMR analysis of this
compound revealed a singlet at δ = 95.9 ppm at slightly
higher field compared to that with the starting material 3-Cl (δ
= 98.9 ppm).37 However, characteristic signals indicating the
presence of methine or methylene groups as part of a
cyanomethyl or amidocrotononitrile ligand were absent in
1H NMR spectra. Unequivocal characterization of this complex
was possible by X-ray analysis from crystals that could be
obtained from a saturated THF solution layered with n-
pentane at −30 °C. The molecular structure shows a dinuclear
complex 5, possessing a 1,3-dithiolate moiety, formed by P−S
activation of a pincer ligand, that bridges two [(iPrPSCSPiPr)-
Ni] fragments (Figure 5 and Scheme 5).
Figure 5. Molecular structure of complex 5. Displacement ellipsoids
correspond to 30% probability. Hydrogen atoms as well as the second
position of the disordered isopropyl groups are omitted for clarity.
Figure 4. Molecular structure of complex 4. Displacement ellipsoids
correspond to 30% probability. Hydrogen atoms are omitted for
clarity.
The nickel−sulfur bond distances of Ni1−S1 = 2.2199(7) Å
and Ni2−S2 = 2.2392(7) Å are in the same range as the others
already reported for Ni pincer thiolate complexes.35,59−61
Unfortunately, neither the fate of the phosphine moiety nor
the origin of the proton in the 2-position of the bridging aryl
ring could be clarified.
Scheme 5. Stoichiometric versus Excess Route for
[(iPrPSCSPiPr)NiCl] Complexes
Ligand decomposition such as in the formation of 4 and 5
was not observed for POCOP or POCSP analogues before,
indicating a higher stability of P−O bonds compared to that of
P−S bonds in the presence of base. We observed a very similar
behavior before when investigating mechanistic aspects of
Kumada coupling reactions with Ni complexes 1-Cl, 2-Cl, and
3-Cl.37 Whereas 3-Cl immediately formed an unknown species
with evident ligand decomposition upon stoichiometric
reaction with the Grignard component, the corresponding
alkyl complexes were formed with 1-Cl and 2-Cl, respectively.
Taking into consideration that 3-Cl still was by far the best
precatalyst for Kumada cross-coupling reactions and PSCSP
ligands are therefore promising ligands to be used and tested in
other reactions in the future, it seems important to consider
the limited stability of those ligands toward stronger bases,
leading to unusual products as we showed with the formation
of 4 and 5.
CONCLUSION
■
two further Ni centers (4, Figure 4 and Scheme 5). The
31P{1H} NMR spectrum only shows one sharp singlet at δ =
In summary, we have presented the synthesis and character-
ization of a new Ni(II) cyanomethyl complex displaying the
phosphinito-thiophosphinito POCSP pincer ligand. Further-
more, we established a protocol which gives access to two new
Ni(II) pincer complexes, both coordinated by the in situ
1
113.7 ppm (in THF-d8) for complex 4, whereas the H NMR
spectrum displays the expected signals for both the iPr groups
and aromatic protons. NMR analysis of the acetonitrile mother
E
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Experimental details, NMR spectra, and crystallographic
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Accession Codes
CCDC 1910466−1910468 and 1922864 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
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AUTHOR INFORMATION
Corresponding Author
*Phone: (+49) 381 1281 104. E-mail: torsten.beweries@
catalysis.de.
■
́
(15) Selander, N.; Szabo, K. J. Catalysis by Palladium Pincer
ORCID
Complexes. Chem. Rev. 2011, 111, 2048−2076.
Torsten Beweries: 0000-0002-2416-8874
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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We thank our technical and analytical staff, in particular,
Benjamin Andres for assistance. Financial support by the
Leibniz association is gratefully acknowledged.
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