Versatile Coordination and C-C Coupling of Diphosphine-Tethered Imine Ligands with Ni(II) and Ni(0)

Article abstract of DOI:10.1021/acs.inorgchem.8b01478

Ligands that can adapt their coordination mode to the electronic properties of a metal center are of interest to support catalysis or small molecule activation processes. In this context, the ability of imine moieties to bind in either an n¹(N)-fashion via σ-donation of the lone pair or, less commonly, in an n ²(C,N)-fashion via π-coordination is potentially attractive for the design of new metal-ligand cooperative systems. Herein, the coordination chemistry of chelating ligands with a diphosphine imine framework (PCNP) to nickel is investigated. The imine moiety binds in an n ¹(N)-fashion in a Ni(II)Cl₂ complex. The uncommon n ²(C,N)-interaction is obtained in Ni(0) complexes in the presence of a PPh₃ coligand. Increasing the bulk on the phosphine side-arms in the Ni(0) complexes, by substituting phenyl for o-tolyl groups, leads to a distinct binding mode in which only one of the phosphorus atoms is coordinated. In the absence of a coligand, a mixture of two different dimeric Ni(0) complexes is formed. In one of them, the imine adopts an uncommon n ¹(N) n ²(C,N) bridging mode of the ligand to nickel, while the second one may involve reactivity on the ligand by the formation of a new C-C bond by oxidative coupling. The latter is supported by the isolation and structural characterization of a crystalline bis-CO derivative featuring a C-C bond formed by oxidative coupling of two imine moieties.

Full text of DOI:10.1021/acs.inorgchem.8b01478

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Versatile Coordination and C−C Coupling of Diphosphine-Tethered
Imine Ligands with Ni(II) and Ni(0)
Dide G. A. Verhoeven,† Hidde A. Negenman,† Alessio F. Orsino,† Martin Lutz,‡
and Marc-Etienne Moret*,†
†Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Faculty of Science, Utrecht University,
Universiteitsweg 99, 3584 CG, Utrecht, The Netherlands
‡Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Padualaan 8,
3584 CH, Utrecht, The Netherlands
S
* Supporting Information
ABSTRACT: Ligands that can adapt their coordination mode to
the electronic properties of a metal center are of interest to
support catalysis or small molecule activation processes. In this
context, the ability of imine moieties to bind in either an η1(N)-
fashion via σ-donation of the lone pair or, less commonly, in an
η2(C,N)-fashion via π-coordination is potentially attractive for the
design of new metal−ligand cooperative systems. Herein, the
coordination chemistry of chelating ligands with a diphosphine
imine framework (PCNP) to nickel is investigated. The imine
moiety binds in an η1(N)-fashion in a Ni(II)Cl2 complex. The
uncommon η2(C,N)-interaction is obtained in Ni(0) complexes in
the presence of a PPh3 coligand. Increasing the bulk on the
phosphine side-arms in the Ni(0) complexes, by substituting
phenyl for o-tolyl groups, leads to a distinct binding mode in which
only one of the phosphorus atoms is coordinated. In the absence of a coligand, a mixture of two different dimeric Ni(0)
complexes is formed. In one of them, the imine adopts an uncommon η1(N)η2(C,N) bridging mode of the ligand to nickel,
while the second one may involve reactivity on the ligand by the formation of a new C−C bond by oxidative coupling. The
latter is supported by the isolation and structural characterization of a crystalline bis-CO derivative featuring a C−C bond
formed by oxidative coupling of two imine moieties.
Imine functionalities are prevalent as ligands in organo-
metallic chemistry, which is reflected in their numerous
applications in homogeneous catalysis.8,9 Examples are the
NNN-pincer Fe and Co complexes independently reported by
Gibson10 and Brookhart11 for alkene polymerization and
investigated subsequently by Chirik for various reactions
capitalizing on the redox noninnocent character of the imine
moieties,12 dinuclear Ni complexes as published by Uyeda that
catalyze hydrosilylation reactions,9j or diphosphine imine
complexes, reported by several groups, for olefin polymer-
ization and oligomerization.13−16 Generally, the imine nitrogen
atom is reported to bind the metal center via its lone pair in an
η1(N)-fashion, forming a σ dative bond. A second, and less
common, possibility is formation of π-complexes via an
η2(C,N)-coordination of the imine. This latter binding mode
can be described by the Dewar-Chatt-Duncanson (DCD)
model for coordination of π-ligands, i.e. the side-on bound and
the metallo-aza-cycle extreme. Limited examples of an
η2(C,N)-bound benzophenone-imine to Ni(0) are reported,17
INTRODUCTION
■
In light of the ongoing interest in the substitution of precious
metals by earth abundant metals in catalysis, the development
of systems displaying metal−ligand cooperativity is flourish-
ing.1−4 Cooperative ligands can stabilize the metal center upon
structural and electronic changes during catalytic processes by
either adapting their binding mode or by reversibly accepting
electrons, protons, or substrate fragments. For this purpose,
ligands with versatile binding modes, facilitating hemilabile or
adaptive behavior, or with the possibility of stabilizing
multielectron redox processes, are of interest. In particular,
π-ligands such as CC double bonds have been shown to act
as hemilabile ligands.5 In this context, CE (E = O, N)
double bonds could constitute an attractive element of ligand
design because of their ability to bind either through the E-
centered lone pair or through the CE π-system. Building on
this idea, we recently reported that the diphosphine ketone
ligand of 2,2′-bis(diphenylphosphino)benzophenone (Phdpbp)
can act as a hemilabile acceptor ligand: the ketone moiety does
not bind to the M(II)Cl2 fragment (M = Fe, Co, Ni) but binds
in an η2-fashion to M(I)Cl.6,7
Received: May 30, 2018
© XXXX American Chemical Society
A
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as well as a recently published bimetallic Ni complex with a
chelating NNN-bis(imino)pyridine ligand.18 Monometallic
complexes with a chelating ligand with an η2(C,N)-
coordinating imine functionality based on precious metals
Pd, Rh, and Ir are described as well.19−24
Here, we investigate the versatile binding of an imine ligand
flanked by two ortho-diphenylphosphino-substituted phenyl
rings to nickel. The design of the ligand allows for a chelated
binding structure via the phosphine arms to the metal center.
Synthesis of the diphosphine-imine ligand PPhCNPPh (1,
Scheme 1) and its complexation to Co(II),13,14 Ni(II),13,15
peaks sharpen, and the peak at δ 35.1 shifts to δ 8.9, suggesting
a spin transition. The latter possibly involves a high spin
tetrahedral structure at room temperature−where one chloride
anion is dissociated−converting to a diamagnetic low spin
square planar structure at low temperature (Supporting
Information, Figures S4, S5). X-ray crystal structure determi-
nation of crystals grown by slow vapor diffusion of hexanes
into CH2Cl2 showed that 2 has a strongly distorted square
pyramidal geometry around nickel (Figure 1). The PPhCNPPh
ligand is bound via both phosphorus atoms and the imine
moiety in an η1(N)-fashion. The PPhCNPPh ligand is
disordered by a 180° rotation around the imine bond in a
ratio 0.550(6):0.450(6) (see the Supporting Information).
Next to this, two chloride ligands are bound, of which the
apical Ni−Cl bond is strongly elongated to 2.6545(6) Å, versus
2.1889(5) Å for the in-plane chloride ligand. Hence, the
geometry can be seen as derived from a cationic square-planar
Ni(II) complex weakly interacting with a Cl− counterion. This
geometry is similar to that of the dibromide analogue of 2, as
published by Sun and co-workers, with Ni−Br bond lengths of
2.3222(11) Å for the in-plane Br and 2.7754(11) Å for the
apical Ni−Br bond.13 Furthermore, Ni−P and Ni−N distances
are similar for the Cl and Br analogues (2: Ni1−N1A:
1.963(3), Ni1−P1A: 2.1892(6), Ni1−P2A: 2.1837(6) Å
(Table 1). 2Br:13 Ni−N: 1.956(6), Ni−P(N-side): 2.154(2),
Ni−P(C-side): 2.1972(19) Å). The torsion angle for C−N
C−C is 175.4(6)°, indicating a slightly distorted flat
configuration of the ligand backbone (close to 180°) and the
largely sp2 character of the imine-carbon atom.
Scheme 1. Synthesis of PRCNPR Ligand 1
and Pd(II)14,15 with the imine backbone bound in an η1(N)-
fashion has been reported previously, mainly for use in olefin
oligomerization and polymerization reactions. In this con-
tribution, the coordination chemistry of PCNP ligands to
nickel is investigated, showing that the imine moiety is able to
adopt a variety of coordination modes (end-on, side-on,
bridging), changing upon varying the oxidation state from
Ni(II) to Ni(0) and upon the addition of steric bulk on the
phosphine tethers. Of particular interest is the observation of
an oxidative C−C coupling reaction in dimeric complexes, in
which a Ni(0)Ni(0) core is oxidized to Ni(II)Ni(0).
An η2(C,N)-coordination mode of the imine moiety was
accessed by synthesis of a Ni(0) complex. Reaction of ligand 1
with Ni(cod)2 (cod = 1,5-cyclooctadiene) in the presence of
PPh3 as coligand in THF afforded the Ni(0) complex
Ni(PPhCNPPh)(PPh3) (3) as a dark red solid after precipitation
from THF/hexanes. Analysis by NMR spectroscopy displays a
diamagnetic species with three signals in the 31P NMR
spectrum: a sharp doublet at δ 29.5 and two broad signals at δ
6.3 and 44.1. The broadening is possibly caused by the
presence of a small amount of coprecipitated free PPh3,
causing the spectrum to broaden by exchange of the coligand.
The 31P NMR signals sharpen both at −50 and 50 °C (Figure
S7). The sharp low-temperature spectrum shows the splitting
pattern and coupling constants for complex 3, from which it is
evident that the three 31P nuclei couple with each other (δ
RESULTS AND DISCUSSION
■
The diphosphine-imine ligand PPhCNPPh (1) was readily
synthesized via an imine condensation of the diphenylphos-
phine substituted aldehyde and amine compounds (Scheme
1).15,25 A Ni(II) complex was synthesized from reaction of
NiCl2(DME) with PPhCNPPh in CH2Cl2, resulting in Ni-
(PPhCNPPh)Cl2 (2) after isolation via precipitation from THF/
hexanes and extraction of the product (Scheme 2).13 Analysis
1
by H NMR spectroscopy at room temperature showed the
formation of a paramagnetic species, with broad aromatic
peaks in the region of δ 6 to 9, and one largely shifted broad
peak at δ 35.1. Upon lowering the temperature to −80 °C, the
46.80 (d, JPP = 41 Hz), 30.00 (d, JPP = 28 Hz), 4.16 (dd, JPP
=
Scheme 2. Overview of the Ligands and Ni-Complexes
B
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Figure 1. Molecular structures of 2, 3, 6, and 7 in the crystal. Displacement ellipsoids are drawn at the 50% probability level. Cocrystallized solvent
molecules and hydrogen atoms are omitted for clarity: 2: 0.5 Et2O and 0.5 CH2Cl2; 7: THF. Selected distances (Å) and angles (deg): 2: only the
major disorder component of the structure is shown here. Ni1−N1A: 1.963(3), Ni1−C7A: 2.913(4), N1A−C7A: 1.294(5), Ni1−P1A: 2.1892(6),
Ni1−P2A: 2.1837(6), Ni1−Cl1: 2.1889(5), Ni1−Cl2: 2.6545(6), P1A−Ni1−P2A: 153.13(3), Cl1−Ni1−N1A: 169.69(10). 3: Ni1−N1: 1.943(3),
Ni1−C7: 2.075(4), N1−C7: 1.364(5), Ni1−P1: 2.1888(11), Ni1−P2: 2.3234(11), Ni1−P3: 2.1761(11), P1−Ni1−P2: 121.89(4). 6: Ni1−N1:
1.969(2), Ni1−C1: 2.031(3), N1−C1: 1.358(4), Ni1−P1: 2.2996(8), Ni1−P2: 2.2189(8), Ni1−P3: 2.1904(9). 7: Ni1−N1: 1.864(3), Ni1−C1:
2.044(4), N1−C1: 1.368(4), Ni1−P1: 4.8493(12), Ni1−P2: 2.2048(11), Ni1−P3: 2.1475(12).
CN band in IR analysis.26 1H−13C HMQC 2D NMR
Table 1. Selected Bond Distances (Å) and Angles (deg) in
the X-ray Crystal Structures
b
analysis shows a cross peak for a signal at δ 84 in 13C NMR
and δ 6 in 1H NMR spectrum, which is assigned to the imine−
CH moiety (Figure S8). The considerable shift of the 13C
NMR signal toward high field is indicative of a strong
rehybridization toward sp3, i.e. a substantial metallacycle
character of the M−C−N unit. It is, however, somewhat
smaller than that observed in tricoordinate aldimine complexes
o f t h e ( d i p p e ) N i f r a g m e n t ( d i p p e
= b i s -
(diisopropylphosphino)ethane),17d in which this signal is
found at ca. δ 60. This difference likely arises from more
efficient π-backdonation from the (Ni−P) σ-antibonding in-
plane d-orbital in tricoordinate complexes. Crystallographic
analysis of 3 showed a distorted tetrahedral geometry around
the nickel center (Figure 1).27 The Ni(0) center is bound to 1
in a κ4(P,P,C,N)-fashion with an η2(C,N)-coordination of the
imine backbone and to the PPh3 coligand. The C−N distance
of the imine moiety is clearly larger in 3 (1.364(5) Å) than in
the Ni(II) complex 2 (1.294(5) Å), as a result of π-
backdonation to the antibonding π* orbital of the imine
CN bond.
a
Constraints were used for the P atoms in the two disorder
b
components. 2a is the major and 2b is the minor disorder
component of complex 2. For clarity, the phenylene linkers in all
structures are represented with gray lines.
28, 41 Hz)). Free PPh3 is not observed in this spectrum, which
is probably due to its low concentration. The free imine is no
longer present in 3, as the distinctive imine−CH peak in the
1H NMR spectrum at δ 9.32 in 1 is not observed, nor is the
Ligand Variation. The design of the ligand allows for facile
incorporation of different substituents on phosphorus,
including mixed ligands, as the building blocks of the imine
C
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a
Scheme 3. Synthesis of Dimeric Complexes Derived from 1
a
P = PPh2, L is a solvent molecule.
condensation can be adjusted. The influence of additional bulk
on the PRCNPR ligand was explored by the synthesis of the o-
tolyl substituted ligands (Scheme 2). The ligands PPhCNPoTol
(4) and PoTolCNPoTol (5) were synthesized accordingly, and
subsequent complexation via a reaction of the ligand with
Ni(cod)2 and PPh3 in toluene afforded Ni(0) complexes
Ni(PPhCNPoTol)(PPh3) (6) and Ni(PoTolCNPoTol)(PPh3) (7),
respectively, after precipitation with hexanes and isolation of
the solids.
with an intermediate hybridization, with a slightly higher
degree of sp3 character for 7.
Dimeric Complexes. Reaction of 1 and Ni(cod)2 in
toluene without the addition of a coligand resulted in a mixture
of two species, major species 8a and minor species 8b (Scheme
3). A workup was performed by precipitation of the products
upon addition of hexanes to a THF solution, removing the
majority of cod in the filtrate, followed by extraction of the
solids in THF and evaporation of the solvent in vacuum and
remaining cod by high vacuum.28 The mixture mainly consists
of 8a, allowing for its spectroscopic characterization, which
shows the absence of a free imine moiety, as the imine-
Complex 6 gives rise to three broad signals in the 31P NMR
spectrum at δ 36, 29, and 1, all in the region of nickel bound
phosphorus compounds. The broad signals suggest fluxionality
in the complex, which is assumed to be caused by the increased
bulk. 1H NMR indicates a shift of the imine-CH moiety as the
distinctive imine-proton (δ 9.33 in 4) is no longer present. The
X-ray crystal structure on single crystals grown from vapor
diffusion of hexane into THF showed a tetrahedral
configuration around the nickel center, bound to PPh3, and
4 in a κ4(P,P,C,N)-fashion with an η2(C,N)-coordination of
the imine backbone (Figure 1). The imine C−N distance is
1.358(4) Å, which is similar to that in complex 3 (Table 1).
Likewise, the Ni−P distances closely resemble the analogues
distances of complex 3.
The bulkier substituents on the phosphorus atoms in the
tetra-ortho-tolyl PCNP ligand 5 and its nickel complex 7 result
in decoordination of one of the phosphine arms: the 31P NMR
spectrum of 7 shows again three signals, but in this case one
peak appears as a broad singlet at δ −28, indicating a
noncoordinated phosphorus atom. The remaining two
signals−also broad singlets−are found at δ 11 and 40,
consistent with coordination to Ni. Single crystal X-ray
structure determination confirms this interpretation: nickel is
bound to the ligand in a κ3(P,C,N)-fashion with an η2(C,N)-
coordination of the imine moiety (Figure 1). Next to this, PPh3
is bound, and the carbon-side phosphine of the PCNP ligand is
dissociated. The imine C−N bond length is 1.368(4) Å for 7,
which is comparable to the elongation of the imine backbone
in 3 and 6, indicating a similar degree of π-backbonding
despite the lower coordination number. The Ni−N bond
distance of 1.864(3) Å is however shorter compared to 3 and
6, which is likely caused by the lesser amount of geometric
strain due to the detachment of the second phosphine arm.
The torsion angle C−NC−C for 3, 6, and 7 is similar, with
151.4(3)° for 3, 147.0(3)° for 6, and 145.8(3)° for 7. These
angles differ from an sp2 (180°) and an sp3 (120°), consistent
1
hydrogen peak is shifted in H NMR and the according band
in the IR spectrum at 1621 cm−1 is not observed.26 Four
signals are observed in the 31P NMR spectrum: two doublets at
δ 35.4 and −7.9 (JPP = 9 and JPP = 70 Hz) and two double−
doublets at δ 35.5 and 22.9 (JPP = 2, 70 and JPP = 2, 9 Hz).
This indicates that the four nonequivalent phosphorus atoms
belong to a single product, which is likely to be caused by a
dimeric nature of 8a. Heating a sample of 8a/8b up to 100 °C
did not reveal exchange of the four inequivalent 31P signals of
8a on the NMR time scale (Figure S39). Next to 8a, the minor
species 8b is present in approximately 10 to 20%, which shows
two doublet signals in the 31P NMR spectrum at δ 1.1 and 38.3
(JPP = 43 Hz). No conditions could be identified that would
allow for bulk isolation of either 8a or 8b, but the ratio
between the species appears to be somewhat sensitive to the
conditions of precipitation (see below), suggesting that 8a and
8b are kinetic products that have not reached equilibrium. To
establish the overall composition of the mixture, one
equivalent of PPh3 with respect to nickel was added.
Quantitative conversion of 8a/8b into 3 was observed,
confirming the overall composition of [Ni(PPhCNPPh)]2n
(Scheme 3) and demonstrating the synthetic use of the
mixture as a source of the [Ni(PPhCNPPh)] fragment.
Diffusion of hexanes into a THF solution afforded crystals
suitable for X-ray diffraction, although only a minor fraction of
the material crystallized. The crystalline material was covered
by precipitation of a second compound, preventing bulk
isolation. An NMR spectrum of the solid materials revealed a
mixture of 8a and 8b (about 1:1), showing the increasing
presence of the minor species after crystallization. A crystal
could be harvested from the mixture and subjected to X-ray
diffraction. The obtained crystal structure presents a dimeric
species with two ligands and two nickel centers, identified as
the minor component 8b (Figure 2): even though no
D
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Figure 2. Molecular structures of 8b and 9b in the crystal. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are
omitted for clarity. Selected distances (Å) and angles (deg): 8b: N1−C1: 1.347(6), N2−C2: 1.361(6), N1−Ni1: 2.033(4), N1−Ni2: 1.992(4),
N2−Ni1: 2.004(4), N2−Ni2: 2.031(4), Ni1−Ni2: 2.5595(9), Ni1−C2: 2.063(5), Ni2−C1: 2.046(5). 9b: Formed C1−C2 bond is shown in red.
C1−C2: 1.550(4), N1−C1: 1.448(4), N2−C2: 1.460(4), Ni2−P2: 2.1545(10), Ni2−P3: 2.1529(10), Ni2−N1: 1.884(3), Ni2−N2: 1.884(3),
Ni1−P1: 2.2228(9), Ni1−P4: 2.2269(9), Ni1−C3: 1.765(4), Ni1−C4: 1.774(4).
crystallographic symmetry is found, the overall structure of 8b
possesses an approximate 2-fold rotation axis perpendicular to
the Ni−Ni axis, which suggests that its 31P NMR spectrum
should display only two signals. Each imine moiety acts as a
bridge, binding side-on to one metal and end-on to the other,
with the two P-donor sites of one ligand binding each to a
different Ni center. The structure has a rather short Ni−Ni
distance of 2.5595(3) Å which is likely sterically enforced by
the geometrical arrangement of the ligands. An electronic Ni−
Ni interaction is unlikely because both centers possess a d10
configuration. The CN bond distances in 8b are 1.347(6) Å
and 1.361(6) Å, both comparable to the monomeric Ni(0)
complexes discussed above; thus, the additional σ(N)-
interaction does not seem to contribute significantly to a
more activated imine.
phenylene linker by a pyridine group (Figure 3).29 Notably,
the Co−Co distance of ca. 2.9 Å is considerably longer than
the Ni−Ni distance of 2.5595(9) Å in 8b, which is likely a
result of the different local coordination geometry: 5-
coordinate, trigonal bipyramidal (TBP) for CoI vs tetracoordi-
nate, tetrahedral for Ni0.
The chemical nature of major dimeric species 8a could not
be fully elucidated. Further analysis of the mixture shows the
presence of two distinctive signals in the 1H NMR spectrum at
δ 4.58 and 5.49 that belong to the major species 8a, besides
numerous aromatic signals. Even though the low solubility of
the compound leads to a low quality 13C NMR spectrum, the
1H−13C HMQC NMR spectrum shows clear cross peaks with
13C NMR signals at δ 78.7 and 86.90, respectively, which are
both shown to be CH signals, according to APT 13C NMR
analysis (Figures S35, S38). A possible explanation is the
incorporation of a new C−C bond in the species. The absence
of a large 3JHH between these two signals would then be due to
a constrained H−C−C−H dihedral angle in the low-coupling
Karplus region.30 Upon arrangement of two imine bonds in
close proximity to each other, possibly in the form of a dimer,
the imine-carbon atoms can undergo a coupling reaction in
which two electrons from a metal center are transferred to the
ligand, forming a new carbon−carbon bond (Scheme 3). A
similar reaction was reported by Rauchfuss and co-workers,31
where the coupling of two imine moieties from diphenylphos-
phino-2-benzimine ligands bound to iron(0) undergoes a
coupling reaction upon addition of ferrocenium, analogous to a
pinacol coupling observed on iron(0) in their earlier
research.32−35 However, it should be noted that a single
dimeric molecule with two inequivalent η2(C,N)-coordinating
imine moieties cannot be excluded on the basis of the obtained
The somewhat unusual μ−η1(N)η2(C,N) binding mode
observed in 8b has been previously reported by de Bruin and
co-workers in dinuclear rhodium(I) complexes formed by
deprotonation of the α-CH2 group of a bridging dipicolylamine
(dpa) ligand (Figure 3).19 Subsequent work by the same group
Figure 3. Complexes by de Bruin and Rauchfuss. The bold line in the
right drawing represents a pyridine ring of which only the bound C−
N is shown.
1
data, since a shift around δ 5 in the H NMR spectrum and δ
80 in the 13C NMR spectrum could also correspond to such a
structure.17d
has afforded a number of related (hetero)bimetallic com-
pounds featuring the same binding mode, which could also be
accessed directly from the imine analogue of the ligand
(dpi).21−23 Very recently, a similar CoI2(imine)2 core was
observed by Rauchfuss and co-workers in the dimer [CoMe-
Evidence supporting the structure containing a new C−C
bond arises from the reactivity of the 8a/8b mixture with CO.
A solution of 8a/8b in C6D6 was exposed to CO (1 atm), after
which the mixture was monitored in situ by NMR spectros-
copy. 31P NMR data shows the formation of two species: major
species 9a with two doublet signals at δ 33.0 and 16.2 (JPP = 11
Hz) and minor species 9b with two singlet signals at δ 39.8 and
(
Ph2PC2NHpy)]2, where Ph2PC2NHpy is a tridentate pyridine-
imine-phosphine (pyCNP) ligand, differing from the PCNP
framework by substitution of the second o-diphosphine-
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22.1 (approximate ratio of 3.3:1) (Figure S42). Crystallization
by slow vapor diffusion of hexanes into a benzene mixture
resulted in single crystals of the minor product 9b (Figure 2,
Scheme 3). The X-ray crystal structure shows a dimeric, mixed
valence Ni(0)Ni(II) complex with two added equivalents of
CO both bound to the Ni(0) center. The Ni(II) center is
surrounded by two neutral phosphine ligands and two anionic
amido (R2N−) ligands, an uncommon coordination environ-
ment that was also observed in the Ni(II) complex of the P2N2
ligand N,N′-bis[2-(diphenylphosphino)phenyl]propane-1,3-di-
amine published by Duckworth, McPartlin, and co-workers.34
Compared to Ni(II) complex 2, the Ni−N bonds are rather
short, 1.963(3) Å in 2 (major disorder component) and
1.884(3) Å and 1.884(3) Å in 9b, originating from the strong
π-donating nature of the nitrogen ligands. Noteworthy, the C−
H hydrogen bound to C2 in the formed C−C bond has a
rather short distance below 3 Å to the Ni(0) center. This is,
however, attributed to the rigid geometry of the structure
rather than a chemical interaction.
of the Ni(0) center in 9b. The formation of the interligand C−
C bond by a two-electron transfer from the metal to the ligand
resulting in 8a and 9b shows the possibility of the PCNP
ligand to engage into ligand-centered redox processes, possibly
opening new venues for cooperative processes besides the
versatile binding as observed in complexes 2, 3, 6, and 7.
Interestingly, the fact that the mixture 8a/8b can be
quantitatively converted to the imine complex 3 by addition
of PPh3 suggests that C−C bond formation may be reversible.
These properties make Ni(PCNP)-complexes potentially
interesting candidates for cooperative activation of substrates
and subsequent catalysis. Further reactivity of these complexes
is currently under investigation in our laboratories.
CONCLUSIONS
■
The coordination chemistry of chelating diphosphine-imine
PRCNPR ligands to nickel was studied. The potential adaptive
behavior of the phenyl-substituted ligand is exemplified in its
coordination to Ni(II) and Ni(0): the imine moiety binds in
an η1(N) fashion to Ni(II) and in an η2(C,N) fashion to the
more electron-rich Ni(0). The addition of steric bulk to the
PRCNPR framework in the form of o-tolyl substituents on the
phosphorus atoms affords mixed-ligand complex 6, where both
phosphine tethers of the PoTolCNPPh ligand are bound to
nickel, and tetra-o-tolyl ligand complex 7, where one
phosphine arm is dissociated from the metal center. When
no coligand is used in the synthesis of the Ni(0) complex with
the PPhCNPPh ligand, a mixture of dimeric structures is
obtained, with a) an η1(N)η2(C,N)-coordination of the two
imine ligands to both Ni centers as shown by its X-ray crystal
structure and b) a complex in which the imine functionalities
seem to undergo a coupling reaction forming a new C−C
bond. Addition of CO to the mixture leads to the isolation of a
derivative of the C−C bound complex. The current study
provides detailed insight into the coordination of η2(C,N)-
bonding imine ligands to nickel, which are becoming prolific
tools in the field of metal−ligand cooperativity. The observed
adaptive behavior of the ligand upon different oxidation states
and the redox-activity of the dimeric species make these
complexes highly interesting for investigation toward their
cooperative reactivity and catalytic activity. Further reactivity
of these complexes is currently under investigation in our
laboratories.
Crystals of 9b could be isolated from the 9a/9b mixture and
were analyzed by NMR. 31P NMR analysis indeed shows the
signals earlier attributed to 9b, without the presence of other
species, and the 1H NMR spectrum shows a number of
aromatic signals located in the range of δ 5.94 to 10.09. The
1
large shift of the aromatic signals was confirmed by H−13C
HMQC 2D NMR analysis, as coupling signals are observed for
these peaks with the aromatic region of the 13C NMR
spectrum. The exception is a broad singlet signal at δ 6.18,
which has a coupling signal in the 13C NMR spectrum at δ
70.35 and is identified as the amido−CH functionality of the
1
formed C−C bond. Furthermore, 2D H−31P HMBC NMR
analysis shows a coupling with both phosphorus substituents of
this proton (Figure S45). The IR spectrum of the crystals
contains two signals for the CO bands at 1938 and 1999 cm−1.
Complex 9b contains two Ni-centers, in two oxidation states,
i.e. the Ni(0) and Ni(II) center. The formal oxidation state of
the Ni(0) center is unchanged, starting from the Ni(0)
precursor Ni(cod)2. The Ni(II) center, on the other hand, was
formed via an intramolecular redox process, by transfer of its
electrons to the formed C−C bond originating from the imine
moieties (Scheme 3).
The isolated compound 9b is however not the major species
in the reaction mixture. The majority of the material (9a)
shows two intense CO signals in the IR spectrum at 1929 and
1991 cm−1, slightly shifted from the CO signals in 9b, and
NMR analysis shows the presence of aromatic signals in the 1H
EXPERIMENTAL SECTION
■
1
and 13C NMR spectra. In addition, the H−13C HMQC 2D
General Considerations. All reagents were purchased from
commercial sources and used as received unless stated otherwise. All
reactions were performed under N2 using standard Schlenk and
glovebox techniques unless stated otherwise. Deuterated benzene
(C6D6) and deuterated dichloromethane (CD2Cl2) were degassed
using the freeze−thaw−pump method (4×) and subsequently stored
over molecular sieves. Dichloromethane (CH2Cl2) was distilled over
calcium hydride, and tetrahydrofuran (THF) was distilled over
sodium/benzophenone before use; both were degassed by bubbling
N2 through it for 30 min and stored over molecular sieves. Dry diethyl
ether (Et2O), hexanes, acetonitrile (MeCN), and toluene were
acquired from a MBRAUN MB SPS-80 solvent purification system
and further dried over molecular sieves before use. MeCN was filtered
over alumina prior to use. 2-Diphenylphosphanyl-benzaldehyde37−39
and 2-diphenylphosphinoaniline40 were synthesized according to
literature procedures.
NMR spectrum shows a coupling signal for a peak at δ 7.8 in
1H NMR and δ 155 in 13C NMR consistent with a
noncoordinating imine-CH, which suggests the monomeric
structure depicted in Scheme 3 for compound 9a. More
complex, possibly oligomeric structures can however not be
fully excluded.
Combining the obtained data of compounds 8a, 8b and 9a,
9b suggests that the CO ligands act here as a trapping agent for
the structure with the C−C bond, 8a, making its isolation
possible. In the case of 8a, solvent molecules such as THF are
likely coordinating to the Ni(0) center, which upon dissolution
in benzene could be replaced by a benzene molecule.36
Isolation is facilitated by replacing loosely bound solvents for
stronger binding CO ligands, resulting in 9b. The higher
apparent symmetry of 9b (two 31P NMR signals) with respect
to 8a (four 31P NMR signals) may be due to higher fluxionality
1H, 13C, and 31P NMR spectra (respectively 400, 100, and 161
MHz) were recorded on an Agilent MRF400 or a Varian AS400
spectrometer at 25 °C. 1H and 13C NMR chemical shifts are reported
F
DOI: 10.1021/acs.inorgchem.8b01478
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
in ppm relative to TMS using the residual solvent resonance as
internal standard. 31P NMR chemical shifts are reported in ppm and
externally referenced to 85% aqueous H3PO4. Infrared spectra were
recorded using a PerkinElmer Spectrum One FT-IR spectrometer
equipped with a general liquid cell accessory under a N2 flow. ESI-MS
spectra were recorded on a Walters LCT Premier XE KE317
Micromass Technologies spectrometer. Compounds of which
elemental analysis is reported were either recrystallized or precipitated
and dried under high vacuum overnight before submission, and
analysis was performed by the Mikroanalytisches Laboratorium Kolbe,
NMR (d8-toluene, 25 °C): δP 44.1, 29.5, 6.1 ppm. 31P NMR (d8-
toluene, −50 °C): δP 46.80 (d, JPP = 41 Hz), 30.00 (d, JPP = 28 Hz),
4.16 (d, JPP = 28, 41 Hz) ppm. IR: 3050, 1574, 1554, 1477, 1455,
1432, 1398, 1313, 1179, 1152, 1113, 1089, 1027, 815, 739, 693, 502,
436, 417 cm−1. Crystals for X-ray analysis were grown from THF/
hexanes. The compound is too sensitive for transportation for
elemental analysis.
2-Di-o-tolyl-phosphinoaniline. Triethylamine (0.25 mL, 1.79
mmol) was added to a solution of 2-iodoaniline (375 mg, 1.71
mmol) and di-o-tolyl-phosphine (365 mg, 1.70 mmol) in MeCN (9
mL) in a round-bottom flask under an N2 atmosphere, to which
subsequently a suspension of Pd(PPh3)4 (20.4 mg, 0.018 mmol) in
H2O/MeCN (12 mL, 1:2 ratio) was added. The pale orange mixture
was refluxed for 64 h, after which all solvents were evaporated.
Degassed CH2Cl2 (10 mL) was added, the organic layer was washed
with degassed H2O (3 × 10 mL) and collected, and the solvents were
evaporated. The pale brown solid was washed with cold, degassed
MeOH (3 × 4 mL) and dried in vacuum, resulting in the product as
an off-white to pink solid with a yield of 87% (452.1 mg, 1.48 mmol).
1H NMR (C6D6): δH 7.14 (m, 2H, Ar−H), 7.08−6.89 (m, 8H, Ar−
H), 6.57, (t, J = 7 Hz, Ar−H), 6.32 (t, J = 7 Hz, Ar−H), 3.73 (s, 2H,
−NH2), 2.39 (s, 6H, −CH3) ppm. 13C NMR (C6D6): δC 151.10 (d,
Mulheim an der Ruhr, Germany.
̈
2-Ph2PC6H4NC(H)C6H4PPh2 (PPhCNPPh, 1). This was adapted
from a literature procedure.15 2-Diphenylphosphinobenzaldehyde
(6.0 g, 0.021 mol), 2-diphenylphosphinoaniline (5.73 g, 0.021 mol),
and p-toluene sulfonic acid (0.11 g, 0.58 mmol) were dissolved in
toluene (500 mL) in a 3-necked round-bottom flask under an N2
atmosphere. A Dean−Stark trap was attached, and the collecting end
was filled with molecular sieves (4 Å). The brown solution was heated
to reflux for 17 h, after which all solvents were evaporated. The
product was precipitated from a DCM/MeOH mixture, after which
the solids were collected and washed with MeOH until the
supernatant was colorless. Drying of the yellow solid resulted in the
1
product with a yield of 60% (6.94 g, 0.013 mol). H NMR: δH 9.32
J
CP = 21 Hz), 142.80 (d, JCP = 27 Hz), 135.11 (d, JCP = 2 Hz), 134.19
(d, J = 6 Hz, 1H, −NCH−), 8.39 (dd, J = 4, 4 Hz, 1H, Ar−H), 7.45
(t, J = 7 Hz, 4H, Ar−H), 7.28 (m, 4H, Ar−H), 6.93−7.02 (m, 16H,
Ar−H), 6.82−6.89 (m, 2H, Ar−H), 6.58 (dd, J = 3, 5 Hz, 1H, Ar−H)
(d, JCP = 8 Hz), 133.26, 130.74, 130.58 (d, JCP = 4 Hz), 129.13,
126.65, 117.10 (d, JCP = 6 Hz), 115.32 (d, JCP = 3 Hz), 21.32 (d, JCP
=
20 Hz) ppm. 31P NMR (C6D6): δP −36.56 ppm. IR cm−1: 3457, 3344,
3055, 3005, 1613, 1599, 1475, 1439, 1304, 1245, 1161, 1081, 1033,
799, 746, 717, 556, 517, 456. Elemental analysis: calcd: C 78.67, H
6.60, N 4.59; found: C 78.86, H 6.77, N 4.57. ESI-MS (MeCN/
formic acid) m/z: calcd: 306.1412, found: 306.1478.
ppm. 13C NMR: δC 158.15 (dd, JCP = 2.0, 24.0 Hz), 155.04 (d, JCP
=
18.1 Hz), 140.09 (d, JCP = 17.55 Hz), 138.67 (d, JCP = 19.3 Hz),
138.16 (d, JCP = 13.7 Hz), 136.89 (d, JCP = 11.1 Hz), 134.5 (dd, JCP
=
2.6, 18.2 Hz), 133.63 (d, JCP = 13.8 Hz), 133.29 (d, JCP = 33.4 Hz),
131.15, 130.09, 129.29, 129.12, 129.01 (d, JCP = 7.0 Hz), 128.70,
128.63, 126.29, 117.50 ppm. 31P NMR: δP −13.5, −14.9 ppm. IR:
3054, 1621, 1561, 1465, 1432, 1358, 1309, 1263, 1189, 1157, 1090,
1069, 1026, 999, 751, 738, 695, 499, 409 cm−1. ESI-MS (MeCN/
formic acid) m/z: [M + H]+ calcd: 550.1854, Found: 550.1780.
Ni(PCNP)Cl2 (2). PPhCNPPh (1) (499 mg, 0.908 mmol) was
dissolved in CH2Cl2 (2 mL). A solution of NiCl2(DME) (199.9 mg,
0.910 mmol) in CH2Cl2 (8 mL) was added, after which the color of
the mixture turned from a bright yellow suspension to a yellow/brown
solution. The mixture was stirred for 3 h 15 min, after which all
solvents were removed in vacuum. The remaining solids were
dissolved in a minimum amount of THF, and addition of hexanes
caused precipitation. The solids were filtered off and collected via
filtration with CH2Cl2. The product was obtained as a brown solid
after evaporation of all solvents with a yield of 95% (0.584 g, 0.860
2-Di-o-tolyl-phosphinobenzaldehyde. The compound is reported
in the literature,39 but an adapted synthesis method was used. 2-
Bromobenzaldehyde (3.00 mL, 0.476 g, 25.7 mmol), di-o-tolyl-
phosphine (5.51 g, 25.7 mmol), and Pd(PPh3)4 (16.2 mg, 0.014
mmol) were dissolved in toluene (90 mL) in a round-bottom flask
under an N2 atmosphere, to which triethylamine (3.60 mL, 2.60 g,
25.8 mmol) was added. The orange suspension was refluxed for 8 h,
after which the mixture was filtered, and washed with subsequently
NH4Cl (3 × 50 mL) and brine (1 × 50 mL). The solvents were
removed in vacuum, and the remaining solids were washed with cold
degassed MeOH (3 × 40 mL) and dried in vacuum, resulting in the
product as an off-white solid to brown with a yield of 93% (7.65 g,
24.0 mmol). 1H NMR (C6D6): δH 10.61 (d, J = 6 Hz, 1H, CHO),
7.77 (ddd, J = 1, 3, 4 Hz, 1H, Ar−H), 7.06−6.84 (m, 11 H, Ar−H),
2.34 (s, 6H, −CH3) ppm. 13C NMR (C6D6): δC 190.62 (d, JCP = 22
Hz), 143.02 (d, JCP = 27 Hz), 140.02 (d, JCP = 26 Hz), 139.61 (d, JCP
= 16 Hz), 135.03 (d, JCP = 11 Hz), 134.17, 133.77, 133.54, 130.67 (d,
JCP = 5 Hz), 130.66, 129.42, 129.02, 126.75, 21.35 (d, JCP = 23 Hz)
ppm. 31P NMR (C6D6): δP −28.54 ppm. IR: 3054, 2963, 2908, 2824,
1693, 1584, 1449, 1391, 1290, 1261, 1199, 1116, 1034, 843, 823, 800,
746, 716, 556, 528, 507, 481 cm−1.
1
mmol). H NMR (CD2Cl2): δH 35.09, 8.63, 8.22, 7.94, 7.88, 7.76,
7.10, 7.01, 6.91, 6.75, 6.28 ppm. 31P and 13C NMR: no signal
detected. ESI-MS (MeCN) [M − Cl]+ m/z: calcd: 642.0817, found:
642.0851. IR: 3051, 1585, 1572, 1223, 1481, 1434, 1309, 1280, 1182,
1155, 1096, 1067, 998, 766, 747, 729, 691, 575, 521, 501 cm−1
.
Crystals for X-ray analysis were grown from CH2Cl2/hexanes.
Elemental analysis, calcd: C 65.43, H 4.30, N 2.06, found: C 65.29,
H 4.52, N 2.04.
PPhCNPoTol (4). The whole procedure was performed under inert
conditions and with dry and degassed solvents. 2-Di-o-tolyl-
phosphanyl-benzaldehyde (4.00 g, 0.013 mol), 2-diphenylphosphi-
noaniline (3.48 g, 0.013 mol), and p-toluene sulfonic acid (60 mg,
0.32 mmol) were dissolved in toluene (110 mL) in a 3-necked round-
bottom flask under an N2 atmosphere. A Dean−Stark trap was
attached, and the tap was filled with molecular sieves (3 Å). The clear
yellow-brown solution was heated to reflux for 17 h, and the color
changed to red/yellow-brown, after which all solvents were
evaporated. CH2Cl2 was added (20 mL), subsequently MeOH was
added (20 mL), and the mixture was cooled in an ice bath for 20 min
to precipitate the crude product. The solids were washed with MeOH
until the supernatant was colorless, and the solids were dried in
vacuum resulting in 4 as a yellow powder with a yield of 47% (3.43 g,
Ni(PPhCNPPh)(PPh3) (3). PPhCNPPh (1) (200 mg, 0.364 mmol),
PPh3 (95.4 mg, 0.364 mmol), and Ni(COD)2 (100.1 mg, 0.364
mmol) were combined in a vial, and Et2O (9 mL) was added,
resulting in a yellow-brown turbid mixture. The mixture was stirred
for 5 h, in which the color changed to red-brown, after which the
solvent was evaporated. THF (2 mL) was added to the solids,
followed by addition of hexanes (6 mL) to precipitate the product as a
red-brown solid, which was isolated via filtration and collected using
THF. After evaporation of all solvents, 5 was obtained with a yield of
1
94% (299.3 mg, 0.344 mmol). H NMR (C6D6) due to extended
overlap between the signals and with the solvent, integrals could not
be assigned: δH 7.79 (m, Ar−H), 7.49 (t, J = 8 Hz, Ar−H), 7.39 (s,
broad, Ar−H), 7.35−7.19 (m, Ar−H), 7.08−6.61 (m, Ar−H), 6.00 (t,
J = 5 Hz, Ar−H) ppm. 13C NMR (C6D6): (note: precise assignment
of signals hampered due to quality of the spectrum) δC 137.9, 137.5,
133.6, 133.5, 133.4, 133.2, 133.1, 132.9, 132.4, 132.2, 132.2, 132.0,
129.4, 128.2, 127.0, 125.1, 124.7, 120.4, 120.0, 99.6, 83.8 ppm. 31P
1
5.94 mmol). H NMR (C6D6): δH 9.33 (d, J = 6 Hz, 1H, −N
CH−), 8.43 (ddd, J = 8, 4, 1 Hz, 1H, Ar−H), 7.46−7.42 (m, 4H, Ar−
H), 7.06−6.93 (m, 16H, Ar−H), 6.87−6.80 (m, 4H, Ar−H), 6.59
(ddd, J = 8, 4, 1 Hz, 1H, Ar−H), 2.34 (s, 6H, −CH3) ppm. 13C NMR
(C6D6): δC 158.08 (dd, JCP = 2, 28 Hz), 155.13 (d, JCP = 18 Hz),
G
DOI: 10.1021/acs.inorgchem.8b01478
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
142.76 (d, JCP = 25 Hz), 140.49 (d, JCP = 17 Hz), 138.14 (d, JCP = 12
Hz), 137.21 (d, JCP = 19 Hz), 134.79, 134.61 (d, JCP = 21 Hz),
133.95, 133.74, 133.59 (d, JCP = 13 Hz), 133.10, 131.34, 130.59 (d,
= 3, 8 Hz, 1H, Ar−H (or imine−CH)), 2.48 (broad singlet, 6H,
oTol−Me) 2.43 (broad singlet, 3H, oTol−Me), 1.76 (broad singlet,
3H, oTol−Me) ppm. 13C NMR (C6D6): δC 165.23 (broad), 149.68
(broad), 143.10, 142,71, 141.82, 135.86, 135.49, 134.19 (d, JCP = 14
Hz), 133.87, 133.40, 132.98, 131.91, 131.28, 130.79, 130.34, 130.05,
129.11, 128.69, 126.68, 126.35, 126.04, 125.93, 123.47, 120.16,
117.71, 23.07, 21.70, 21.48, 21.12 ppm. 31P NMR (C6D6): δP 39.90,
10.80, − 27.72 ppm. IR: 3052, 2969, 2924, 2854, 1952, 1919, 1811,
1669, 1573, 1454, 1435, 1392, 1309, 1260, 1202, 1159, 1118, 1093,
1026, 803, 748, 716, 695, 572, 521, 487, 460 cm−1. Crystals for X-ray
analysis were grown from THF/hexanes. The compound is too
sensitive for transportation for elemental analysis.
JCP = 4 Hz), 130.09, 129.39, 129.33, 128.68, 128.60, 128.41 (d, JCP =
4 Hz), 126.84, 126.25, 117.47 (d, JCP = 2 Hz), 21.39 (d, JCP = 22 Hz)
ppm. 31P NMR (C6D6): δP −13.49, −31.62 ppm. IR: 3048, 3004,
2965, 2941, 2912, 2875, 1698, 1627, 1618, 1561, 1466, 1431, 1357,
1265, 1191, 1118, 1093, 1068, 1024, 861, 766, 747, 740, 695, 555,
503, 457 cm−1. ESI-MS (MeCN/formic acid) m/z: calcd: 578.2167,
found: 578.2456. Elemental analysis: calcd: C 81.09, H 5.76, N 2.42;
found: C 80.68, H 5.89, N 2.41.
P
oTolCNPoTol (5). The same method was used as for 4 with adjusted
amounts: 2-di-o-tolyl-phosphanyl-benzaldehyde (0.258 g, 0.810
mmol), 2-di-o-tolyl-phosphinoaniline (0.252 g, 0.825 mmol), p-
toluene sulfonic acid (8 mg, 0.042 mmol), and toluene (20 mL). After
the procedure 5 was obtained as an off-white/yellow powder with a
yield of 82% (0.400 g, 0.66 mmol). 1H NMR (C6D6): δH 9.32 (d, J =
6 Hz, 1H, −NCH−), 8.38 (m, 1H, Ar−H), 6.79−7.10 (m, 22H,
Ar−H), 6.59 (dd, J = 3, 5 Hz, 1H, Ar−H), 2.51 (s, 6H, −CH3), 2.33
(s, 6H, −CH3) ppm. 13C NMR (C6D6): δC 158.12 (dd, JCP = 2, 27
Hz), 155.72 (d, JCP = 19 Hz), 142.92 (d, JCP = 26 Hz), 142.79 (d, JCP
= 27 Hz), 140.59 (d, JCP = 18 Hz), 137.35 (d, JCP = 18 Hz), 135.96
(d, JCP = 13 Hz), 134.79 (d, JCP = 10 Hz), 134.15, 133.96, 133.52 (d,
JCP = 46 Hz), 131.63 (d, JCP = 12 Hz), 131.28, 130.58 (d, JCP = 4.6
Hz), 130.29 (d, JCP = 4.4 Hz), 130.16, 129.30 (d, JCP = 6 Hz), 129.31,
128.81, 128.35, 126.82, 126.47, 126.31, 125.70, 21.61−21.28 (21.61,
21.49, 21.38, 21.28:4 signals, 2d, assignment unknown) ppm. 31P
NMR (C6D6): δP −30.28, −31.56 ppm. IR: 3057, 2967, 2934, 2864,
1625, 1586, 1564, 1464, 1380, 1356, 1264, 1129, 1033, 800, 750, 733,
717, 555, 485, 453 cm−1. ESI-MS (MeCN/formic acid) m/z: [M]+
calcd: 606.2479, found: 606.2273. Elemental analysis: calcd: C 81.30,
H 6.16, N 2.31; found: C 80.91, H 6.36, N 2.17.
[Ni(PPhCNPPh)]2 (8). PPhCNPPh (1) (150.2 mg, 0.273 mmol) was
suspended in toluene (2 mL). In a second vial, Ni(COD)2 (74.9 mg,
0.272 mmol) was suspended in toluene (6 mL) and transferred to the
ligand suspension. The yellow suspension directly changed color to a
turbid yellow-brown mixture and was left to stir for 5 h. All solvent
was evaporated, and the remaining solids were dissolved in THF (1
mL). The product was precipitated by addition of 8 mL of hexanes,
followed by cooling the mixture to −35 °C before the solids were
collected by filtration. The solids were collected by dissolution in
THF, after which the solvents were evaporated. The mixture was
dried under vacuum for 2 nights, after which the product mixture was
obtained with a yield of 96% (159.6 mg, 0.131 mol). 1H NMR (C6D6)
major species 8a: δH 7.83−7.78 (m, Ar−H), 7.66−7.61 (m, Ar−H),
7.58−7.53 (m, Ar−H), 7.47 (ddd, J = 1, 3, 5 Hz, Ar−H), 7.35−7.30
(m, Ar−H), 7.25 (dt, J = 2, 8 Hz, Ar−H), 7.13−6.44 (m, Ar−H),
6.39−6.35 (m, 1H, Ar−H), 5.49 (d, J = 2 Hz, 1H, CH−CH or
imine−CH), 4.58, (d, J = 6 Hz, 1H, CH−CH or imine−CH) ppm.
Minor species 8b: δH 7.91 (t, J = 8 Hz, Ar−H), 7.21 (s, Ar−H), 7.19
(s, Ar−H), 6.29 (t, J = 8 Hz, Ar−H), 6.17 (t, J = 7 Hz, Ar−H), 5.31
(m), 3.88 (s) ppm. 13C NMR (C6D6) 8a + 8b: δC 166.28 (d, JCP = 38
Hz), 152.39 (d, JCP = 38 Hz), 151.91 (dd, JCP = 5, 15 Hz), 143.64 (d,
JCP = 12 Hz), 143.16 (d, JCP = 11 Hz), 140.95, 140.66 (d, JCP = 10
Ni(PPhCNPoTol)(PPh3) (6). PPhCNPoTol (4) (252.4 mg, 0.437 mmol),
PPh3 (115.8 mg, 0.441 mmol), and Ni(cod)2 (120.5 mg, 0.438 mmol)
were combined in a vial, and THF (5 mL) was added, resulting in a
red/brown solution. The mixture was stirred overnight, after which
the mixture was concentrated to ∼2 mL. Hexanes (6 mL) were added,
and the vial was placed at −35 °C for 1 h. The solids were isolated by
filtration and redissolved in toluene. After evaporation of all solvents,
6 was obtained with a yield of 90% (352.1 mg, 0.392 mmol). Single
crystals for XRD analysis were grown by slow vapor diffusion of
Hz), 140.43, 140.16, 139.92, 139.12, 139.04, 138.78, 138.601 (d, JCP
=
7 Hz), 140.65 (d, JCP = 10 Hz), 139.13, 139.04, 138.78, 137.61 (d, JCP
= 7 Hz), 136.78 (d, JCP = 5 Hz), 136.42, 136.16, 135.72, 135.56,
135.08, 134.93, 133.70, 133.61, 133.52, 133.46, 133.39, 133.26,
133.11, 133.01, 132.88, 132.73, 132.63, 132.21, 132.09, 131.56,
130.72, 130.06, 129.39, 129.34, 129.27, 129.13, 128.91, 128.82,
128.69, 128.57, 128.50, 127.53, 127.33, 125.70, 123.54, 121.24,
120.39, 117.32, 87.24 (d, JCP = 26 Hz), 79.13 (d, JCP = 6 Hz). 31P
NMR (C6D6) 8a: δP 35.5 (dd, JPP = 2, 70 Hz), 35.4 (d, JPP = 9 Hz),
22.9 (dd, JPP = 2, 9 Hz), − 7.9 (d, JPP = 70 Hz); 8b: 38.32 (d, JPP = 43
Hz), 1.05 (d, JPP = 43 Hz) ppm. IR: 3048, 2856, 1580, 1568, 1477,
1449, 1431, 1325, 1251, 1202, 1090, 1065, 1026, 919, 850, 735, 693,
497 cm−1.
1
hexanes into THF. H NMR (C6D6): δH 7.77 (t, J = 9 Hz, 2H, Ar−
H), 7.40 (s, broad, Ar−H), 7.30 (t, J = 6 Hz, Ar−H), 7.24 (m, Ar−H),
7.13−6.86 (m, Ar−H), 6.79−6.68 (m, Ar−H), 5.88 (t, J = 7 Hz, 1H,
Ar−H), 2.54 (s, 3H, oTol−Me), 1.88 (s, 3H, oTol−Me) ppm. 13C
NMR (C6D6): δC 166.83 (dd, JCP = 4, 28 Hz), 153.86 (d, JCP = 33
Hz), 142.06, 141.95, 138.41, 138.15, 137.87, 137.52, 137.28, 136.24,
136.09, 135.50, 133.97 (d, JCP = 13 Hz), 133.50 (d, JCP = 14 Hz),
132.95, 132.95 (d, JCP = 11 Hz), 131.83 (m), 131.49 (d, JCP = 4 Hz),
130.16, 129.34, 126.24, 126.03 (dd, JCP = 4, 11 Hz), 125.74 (m),
125.15 (d, JCP = 4 Hz), 120.27 (d, JCP = 5 Hz), 119.11 (broad), 22.98,
22.91 ppm. 31P NMR (C6D6): δP 36.02, 28.79, 1.09 ppm. IR: 3047,
1585, 1570, 1453, 1433, 1387, 1297, 1089, 1026, 818, 740, 693, 514,
484, 460, 407 cm−1. Crystals for X-ray analysis were grown from
THF/hexanes. Elemental analysis: calcd: C 76.19, H 5.38, N 1.56;
found: C 75.99, H 5.57, N 1.46.
[Ni(PPhCNPPh)]2(CO)2 (9a and 9b). 8 (8.4 mg, 6.9 μmol, the
obtained mixture of 8a and 8b was used) was dissolved in C6D6 (0.6
mL) and transferred to a Young-type NMR tube. The mixture was
degassed to remove the N2 atmosphere (2 freeze−pump−thaw
cycles) and charged with CO (1 atm) upon thawing of the solution.
The tube was regularly shaken, and the color of the mixture changed
overnight from turbid red to clear orange. The progress of the
reaction was monitored by NMR analysis (see Supporting
Information, Figure S42). The NMR tube was placed back in the
glovebox, after which the mixture was filtered and transferred to a
small reaction tube. The reaction tube was placed in a vial, and
hexanes (1 mL) was placed in the vial (crystallization setup) for
crystallization to take place. After 2 days, crystals were formed, and
the liquid phase was removed by decantation. The crystals were
washed with hexanes (3 × 1.5 mL) and dried in vacuum for no longer
than 5 min, resulting in 9b. The liquid phase was dried in vacuum as
well, resulting in the bulk material 9a.
Ni(PoTolCNPoTol)(PPh3) (7). PoTolCNPoTol (5) (40.0 mg, 0.066
mmol), PPh3 (17.3 mg, 0.066 mmol), and Ni(cod)2 (18.2 mg,
0.066 mmol) were combined in a vial, and THF (5 mL) was added,
resulting in a red/brown solution. The mixture was stirred overnight,
after which the mixture was concentrated to ∼1.5 mL. Hexanes (8
mL) was added, and the vial was placed at −35 °C for 2 h. The solids
were isolated by filtration, washed with hexanes (3 × 2 mL), and
collected by dissolution in toluene. After evaporation of all solvents, 7
was obtained with a yield of 94% (57.5 mg, 0.062 mmol). Single
crystals for XRD analysis were grown by slow vapor diffusion of
hexanes into THF. 1H NMR (C6D6): δH 7.73 (broad singlet, 2H, Ar−
H), 7.41 (broad singlet, 6H, Ar−H), 7.20 (broad singlet, 1H, Ar−H),
7.12−6.72 (m, 30H, Ar−H), 6.45 (t, J = 7 Hz, 1H, imine−CH (or
Ar−H)), 6.17 (broad singlet, 1H, Ar−H (or imine−CH)), 5.81 (dd, J
9b: 9b was isolated as red-brown crystals with a yield of 21% (1.8
mg, 1.4 μmol). 1H NMR (C6D6): δH 10.10 (dd, J = 4, 8 Hz, 2H, Ar−
H), 7.91 (t, J = 2, 8 Hz, 4H, Ar−H), 7.75 (t, J = 8 Hz, 4H, Ar−H),
7.47 (m, 6H, Ar−H), 7.31 (m, 4H, Ar−H), 7.12−6.86 (m, 20H, Ar−
H), 6.79−6.66 (m, 10H, Ar−H), 6.58 (t, J = 8 Hz, 2H, Ar−H), 6.50
(s, broad, 2H, Ar−H), 6.18 (t, J = 7 Hz, 2H, CH−CH), 5.95 (d,
H
DOI: 10.1021/acs.inorgchem.8b01478
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
broad, J = 9 Hz, 2H, Ar−H) ppm. 13C NMR (C6D6): δC 198.24
(−CO), 150.97, 134.75, 134.69, 134.62, 133.65, 133.59, 133.21,
133.12, 133.07, 132.70, 132.14, 132.07, 131.61, 130.09, 129.96,
129.51, 128.69, 126.53, 113.81, 112.79, 70.35 ppm. 31P NMR (C6D6):
δP 39.81, 22.12 ppm. IR: 3051, 2963, 2922, 2853, 1999, 1938, 1680
(broad), 1578, 1479, 1452, 1435, 1328, 1260, 1095, 1026, 799, 740,
692, 526, 511, 499, 456 cm−1.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01478.
Spectroscopic data (PDF)
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emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: M.Moret@uu.nl.
■
ORCID
Marc-Etienne Moret: 0000-0002-3137-6073
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Sectorplan Natuur-en Scheikunde (Tenure-track
grant at Utrecht University) and The Netherlands Organ-
ization for Scientific Research (NWO, ECHO-STIP grant) for
financial support. Support with NMR spectroscopic analysis by
Dr. J. T. B. H. Jastrzebski is gratefully acknowledged. The X-
ray diffractometer was financed by NWO. This work was
sponsored by NWO Exacte en Natuurwetenschappen (Phys-
ical Sciences) for the use of supercomputer facilities, with
financial support from The Netherlands Organization for
Scientific Research (NWO). The DFT work was carried out on
the Dutch national e-infrastructure with the support of the
SURF Foundation.
I
DOI: 10.1021/acs.inorgchem.8b01478
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