Cationic tetra-and pentacoordinate complexes of nickel based on POCN-and POCOP-Type pincer ligands: Synthesis, characterization, and ligand exchange studies

Article abstract of DOI:10.1039/d1nj01355f

Stirring acetonitrile solutions of the charge-neutral pincer complexes (POCN)NiBr (1, POCN = κP,κC,κN-{2-(i-Pr)2PO,6-CH2{c-N(CH2)5}-C6H3) and (POCOP)NiBr (2, POCOP = κP,κC,κP′-2,6-(i-Pr2OP)2C6H3) with AgSbF6 facilitates Br- abstraction to give the corresponding cationic acetonitrile adducts [(POCN)Ni(NCMe)]+, 1a, and [(POCOP)Ni(NCMe)]+, 2a. Treating 1a and 2a with pyridine (py), 2,2′-bipyridine (bipy), phenanthroline (phen), or 4,4′-bipyridine (bipy?) gave the corresponding monocationic adducts [(POCN or POCOP)Ni(ligand)]+ (ligand = py: 1b and 2b; κN,κN′-bipy: 1c and 2c; κN,κN′-phen: 1d and 2d) and the dicationic dinuclear adducts [(POCN or POCOP)2Ni2(μ-bipy?)]2+ (1e and 2e). The new adducts 1a-1d and 2b-2e have been characterized by NMR spectroscopy; complex 1e proved to be insoluble and could not be analyzed by NMR. Single crystal X-ray diffraction studies were used to establish the solid-state structures of 1a-1e, and 2b-2d. UV-vis spectra have also been recorded for the pentacoordinated complexes 1c, 1d, 2c, and 2d. Studying the equilibria that govern the displacement of halides in (pincer)NiX (X = Cl, Br) by the in-coming nucleophiles py, bipy, and phen has allowed us to determine the following order for the relative nucleophilicities of the ligands involved in the halide substitution equilibria: Cl- > Br- > phen > bipy ? py > MeCN. Similar Keq measurements showed that cationic species are better stabilized with the POCN platform compared to POCOP. This implies that POCN is a better net donor of electron density compared to POCOP, such that the relative Lewis acidity (electrophilicity) of the cationic fragments should follow the order [(POCOP)Ni]+ > [(POCN)Ni]+. Cyclic voltammetry measurements on the bipy adducts showed reversible, one-electron oxidation events occurring at a lower value for [(POCN)Ni(bipy)]+, implying a more electron-rich Ni(ii) centre with POCN vs. POCOP. Consistent with these assertions, mixing (POCN)NiBr and [(POCOP)Ni(phen)]+ in acetonitrile gave [(POCN)Ni(phen)]+ and (POCOP)NiBr. Similarly, the Keq value of 0.19 for the equilibrium exchange (POCN)NiCl + (POCOP)NiBr ? (POCN)NiBr + (POCOP)NiCl indicates that the [(POCOP)Ni]+ fragment is better stabilized by Cl-vs. Br-. These exchange reactions do not occur in THF or CH2Cl2, implying that they are driven by the nucleophilic character of the solvent. This journal is

Full text of DOI:10.1039/d1nj01355f

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Cationic tetra- and pentacoordinate complexes of
nickel based on POCN- and POCOP-type pincer
ligands: synthesis, characterization, and ligand
exchange studies†
Cite this: New J. Chem., 2021,
45, 15063
Naser Rahimi and Davit Zargarian
*
Stirring acetonitrile solutions of the charge-neutral pincer complexes (POCN)NiBr (1, POCN = kP,kC,kN-
0
{2-(i-Pr)2PO,6-CH2{c-N(CH2)5}-C6H3) and (POCOP)NiBr (2, POCOP = kP,kC,kP -2,6-(i-Pr2OP)2C6H3) with
AgSbF6 facilitates Brꢀ abstraction to give the corresponding cationic acetonitrile adducts
[(POCN)Ni(NCMe)]+, 1a, and [(POCOP)Ni(NCMe)]+, 2a. Treating 1a and 2a with pyridine (py), 2,20-
bipyridine (bipy), phenanthroline (phen), or 4,40-bipyridine (bipy*) gave the corresponding mo0nocationic
0
adducts [(POCN or POCOP)Ni(ligand)]+ (ligand = py: 1b and 2b; kN,kN -bipy: 1c and 2c; kN,kN -phen: 1d
and 2d) and the dicationic dinuclear adducts [(POCN or POCOP)2Ni2(m-bipy*)]2+ (1e and 2e). The new
adducts 1a–1d and 2b–2e have been characterized by NMR spectroscopy; complex 1e proved to be
insoluble and could not be analyzed by NMR. Single crystal X-ray diffraction studies were used to
establish the solid-state structures of 1a–1e, and 2b–2d. UV-vis spectra have also been recorded for the
pentacoordinated complexes 1c, 1d, 2c, and 2d. Studying the equilibria that govern the displacement of
halides in (pincer)NiX (X = Cl, Br) by the in-coming nucleophiles py, bipy, and phen has allowed us to
determine the following order for the relative nucleophilicities of the ligands involved in the halide
substitution equilibria: Clꢀ 4 Brꢀ 4 phen 4 bipy c py 4 MeCN. Similar Keq measurements showed
that cationic species are better stabilized with the POCN platform compared to POCOP. This implies
that POCN is a better net donor of electron density compared to POCOP, such that the relative Lewis
acidity (electrophilicity) of the cationic fragments should follow the order [(POCOP)Ni]+ 4 [(POCN)Ni]+.
Cyclic voltammetry measurements on the bipy adducts showed reversible, one-electron oxidation
events occurring at a lower E1ꢁ=2 value for [(POCN)Ni(bipy)]+, implying a more electron-rich Ni(II) centre
with POCN vs. POCOP. Consistent with these assertions, mixing (POCN)NiBr and [(POCOP)Ni(phen)]+ in
acetonitrile gave [(POCN)Ni(phen)]+ and (POCOP)NiBr. Similarly, the Keq value of 0.19 for the equilibrium
exchange (POCN)NiCl + (POCOP)NiBr # (POCN)NiBr + (POCOP)NiCl indicates that the [(POCOP)Ni]+
fragment is better stabilized by Clꢀ vs. Brꢀ. These exchange reactions do not occur in THF or CH2Cl2,
implying that they are driven by the nucleophilic character of the solvent.
Received 19th March 2021,
Accepted 10th May 2021
DOI: 10.1039/d1nj01355f
rsc.li/njc
molecule activation.4 For the most part, the growth of pincer
chemistry has been fueled by the efforts of various research
Introduction
Since its inception nearly 5 decades ago,1–3 pincer chemistry groups to prepare new pincer complexes by combining old and
has evolved into a prominent branch of coordination chemistry new ligand types with various d- and p-block elements, and to
thanks to its major contributions to catalysis, materials development, study the reactivities, structures, and physical properties of
and fundamental explorations of structure, bonding, and small these new complexes. These developments have been captured
in a number of authoritative reviews.5
Although nickel complexes were among the first pincer
derivatives introduced by Shaw and van Koten,1,2 the chemistry
of pincer-based organonickel complexes did not develop in
earnest until about 15 years ago.6 The introduction of many
different families of pincer nickel complexes firmly established
their importance in catalysis.7–9 A particularly popular family of
pincer–Ni complexes feature the POCOP pincer platform
´
´
´
´
´
Departement de chimie, Universite de Montreal, Montreal (Quebec), H3C 3J7,
Canada. E-mail: zargarian.davit@umontreal.ca
† Electronic supplementary information (ESI) available: All details pertaining to
the implementation of the experimental procedures (compound synthesis, pur-
ification, and characterization; procedures for ligand exchange studies) and the
collected data (NMR spectra; details on X-ray diffraction studies; one additional
molecular structure diagram). CCDC 2071697–2071704. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/d1nj01355f
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Scheme 1 Unassisted displacement of bromide from (pincer)NiBr.
Chart 1 POCOP– and POCN–Ni complexes.
18-electron cations formed upon displacement of Brꢀ by a
bidentate ligand should be more resistant to the reverse reaction
(re-entry of Brꢀ). This reasoning was borne out by the results of
substitution reactions conducted with the charge-neutral
complexes 1 and 2. As illustrated in Scheme 1, treating
acetonitrile solutions of these complexes with py as in-coming
nucleophile did not show any net substitution, whereas the
bidentate ligands bipy and phen18 led to smooth but incomplete
formation of the anticipated cationic adducts.
(Chart 1). The popularity of POCOP–Ni complexes stems from
two main factors: (a) the facile preparation of the ligand from
resorcinol and its substituted derivatives, as well as the
convenient synthesis of the Ni complexes via C–H nickelation;
(b) the interesting catalytic transformations these complexes
promote.10,11 A related but less widely studied family of pincer–
Ni complexes is based on the POCN pincer platform (Chart 1)
that is also accessible via simple synthetic routes.12
Our group’s previous reports have described the impact of
electron-donating and electron-withdrawing substituents on the
structures and ligand exchange equilibria of the charge-neutral
complexes (R-POCOP)NiBr13 and their cationic derivatives
[(R-POCOP)Ni(NCMe)]+.14 Analogous studies have been reported
by other groups on POCOP complexes of Ir and Pt.15 In
continuation of our investigations, we have undertaken a new
study designed to (a) evaluate and compare the net electron
donating properties of the POCOP and POCN pincer platforms,
and (b) assess the impact of these platforms on ligand exchange
equilibria involving the displacement of halogens and the
solvolysis of various cationic adducts. Reported herein are the
syntheses, spectroscopic and structural characterization, redox
potential measurements, and ligand exchange reactivities of
the cationic adducts [(pincer)Ni(ligand)]+ featuring the pincer
platforms POCOP and POCN and the mono- and bidentate
Schiff-base ligands pyridine (py), bipyridine (bipy), phenanthro-
line (phen) and 4,40-bipyridine (bipy*).
Having confirmed that unassisted Brꢀ displacement is not
an effective route to our target cationic adducts, we resorted to
Ag+-assisted halide abstraction to prepare these compounds
and use them in subsequent ligand exchange studies. Surprisingly,
however, treating acetonitrile solutions of (pincer)NiBr with the
appropriate ligand and AgSbF6 gave pure samples for only one of
the 6 target adducts, namely: [(POCN)Ni(phen)]+. In the remaining
cases, we obtained mixtures of the target adduct and the corres-
ponding acetonitrile adduct. This finding suggested that the target
4- and 5-coordinate adducts are labile in the presence of a large
excess of acetonitrile, as shown in Scheme 2.
To ensure access to pure samples of the target complexes, we
adopted a two-stage synthetic methodology involving the initial
preparation of the acetonitrile adducts, followed by their
complete conversion to the target adducts. The acetonitrile
adducts [(POCN)Ni(NCMe)]+, 1a, and [(POCOP)Ni(NCMe)]+, 2a,
were thus obtained by treating acetonitrile solutions of the
charge-neutral bromo precursors with AgSbF6 in the absence of
other added ligands (Scheme 3). The conversion of the bromo
precursors 1 and 2 was signaled by the prompt disappearance
of the 31P NMR signal for the starting materials (200.9 ppm for
1, 188.5 ppm for 2), and by the concomitant emergence of new
singlet resonances for the corresponding acetonitrile adducts
(202.4 ppm for 1a, 193.2 ppm for 2a).
Results and discussion
Synthesis of the cationic adducts
It is well-known that 16-electron d8 complexes of divalent group
10 metals undergo associative ligand displacements that proceed
relatively readily when the in-coming ligands are sufficiently
nucleophilic.16 Nevertheless, our past investigations of halide
displacement reactions with the complexes (pincer)Ni(II)X (X =
Cl, Br) have shown that substitution of Xꢀ by neutral donors L is
sluggish and easily reversible. As a result, conversion of these
charge-neutral compounds to the adducts [(pincer)Ni(II)L]+ is
often slow and incomplete in most cases. For this reason,
suitable Ag+ salts are commonly used for driving the halide
substitution equilibria to completion, thanks to the formation of
sparingly soluble AgX salts.17
Next, THF (or CH2Cl2) solutions of these acetonitrile adducts
were treated with bipy or phen to generate the corresponding
adducts in good yields and purity; an analogous reaction was
conducted in pyridine to give the two pyridine adducts
(Scheme 3). The new complexes were identified readily based
on their 31P NMR signals (acetone-d6), as follows: 195.8 ppm
for 1b; 189.8 ppm for 1c; 188.5 ppm for 1d; 185.5 ppm for 2b;
We revisited these halide substitution reactions by examining
the unassisted displacement of Brꢀ in (POCN)NiBr, 1, and
(POCOP)NiBr, 2, using mono- and bidentate aromatic imines
(Schiff bases) as in-coming nucleophiles. Our reasoning was that
bidentate ligands would be more likely to effect Brꢀ displacement
in the absence of Ag+ salts, because the putative 5-coordinate,
Scheme 2 Cationic adducts via Ag+-assisted displacement of Brꢀ from
(pincer)NiBr.
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Scheme 5 Solvolysis equilibria for the cationic complexes 1b–1d and
2b–2d.
Scheme 3 Synthesis of cationic adducts from [(pincer)Ni(NCMe)]+.
176.2 ppm for 2c; 176.8 ppm for 2d. The characterization of
The ligand substitution (solvolysis) reactions shown in
Scheme 5 were studied by monitoring the 31P NMR spectra of
room temperature (298.3 K) acetonitrile solutions of the py,
bipy, and phen adducts (ca. 0.02–0.03 mM). Where possible, the
relative intensities (integration values) of the P-bearing species
were used to determine the extent of solvolysis and calculate
Keq values.19 Overall, the data shown in Scheme 5 reveals that
the substitution equilibria are very sensitive to the binding
power of the N-based ligands. For instance, [(POCN)Ni(phen)]+
resists substitution of the phen ligand in spite of the thousands
of equivalents of MeCN present in the reaction medium. On the
other hand, solvolysis was observed for the other POCN-based
adducts either partially (ca. 22% for substitution of bipy in 1c) or
fully (for substitution of py in 1b).20 Indeed, the displacement of
py proved so facile that monitoring the exchange equilibrium
[(POCN)Ni(py)]+ + MeCN # [(POCN)Ni(NCMe)]+ + py had to be
conducted in a THF solution containing only 1% (by volume)
MeCN in order to avoid complete displacement (see Fig. S43,
ESI†). A similar trend was observed for the POCOP-based
adducts: partial displacements were noted for bipy (ca. 29%)
and phen (13%), whereas py is completely displaced by MeCN.21
Thus, we conclude that the substitution lability of the ligands
follows the order py 4 bipy 4 phen.
The equilibrium data shown in Scheme 5 also reveal that the
extent of solvolysis depends to some extent on the pincer
platform. Thus, the POCN systems appear to be somewhat
more resistant to substitution by MeCN relative to their POCOP
counterparts, even though the Keq constants measured for the
solvolysis of bipy and phen adducts are not very different for the
complexes featuring POCN vs. POCOP platforms. We conducted an
additional test to provide a more direct and reliable comparison
of the relative solvolysis reactions as a function of pincer
ligand, as follows: a 1 : 1 mixture of [(POCN)Ni(NCMe)]+ and
1
these 16 or 18-electron adducts was completed by H and 13C
NMR spectroscopy, as well as single crystal X-ray diffraction
studies (for 1a–1e, and 2b–2d). The spectroscopic and solid-
state data will be presented later in this report.
Before proceeding to the ligand substitution studies, we
briefly explored the synthesis of cationic adducts of 4,40-
bipyridine (bipy*), a potentially bridging analogue of bipy.
The main objective of these synthetic attempts was to prepare
coordination polymers or various multinuclear macrocycles.
Treating the acetonitrile adducts [(pincer)Ni(NCMe)]+ with various
quantities of bipy* gave only the 2 : 1 dinuclear dications
N0
[(pincer)Ni(m-kN,k -4,40-bipyridine)Ni(pincer)]2+. Conducting these
reactions in THF using half an equivalent of bipy* gave 63–67% of
the dications 1e and 2e, as shown in Scheme 4. All attempts at
making heteroleptic dications featuring both pincer platforms were
unsuccessful. The limited solubility of 1e precluded spectroscopic
characterization for this dinuclear adduct, but we were able to
confirm its identity by single crystal diffraction studies. On the
other hand, complex 2e was soluble enough to allow spectroscopic
characterization, but did not yield single crystals suitable for X-ray
diffraction studies. The characterization data for these species will
be presented later in this report.
Ligand substitution studies
The successful preparation and characterization of the target
cationic adducts (Scheme 3) meant that these species could
be reliably detected and identified in various mixtures,
thus facilitating the ligand exchange reactions we hoped to
investigate. We began by probing the stability of these cationic
adducts in acetonitrile (Scheme 5).
Scheme 4 Synthesis of dicationic dinuclear adducts.
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[(POCOP)Ni(NCMe)]+ was dissolved in acetonitrile, followed by
addition of ca. 1 equiv. of dry phen, stirring for an hour at r.t. to
allow equilibration, and analysis by 31P NMR spectroscopy. The
resulting spectrum (Fig. S51 in ESI†) showed the following ratios of
the phen and MeCN adducts: ca. 70 : 30 (for POCN) and ca. 20 : 80
(POCOP). Evidently, [(POCOP)Ni(NCMe)]+ is ca. 4 times more stable
than [(POCOP)Ni(phen)]+, whereas [(POCN)Ni(phen)]+ is more than
twice as stable as its acetonitrile analogue. In other words,
[(POCOP)Ni(phen)]+ is more prone to solvolysis than [(POCN)-
Ni(phen)]+. This discrepancy can be rationalized by the s-donating
and p-accepting properties of the ligands involved: greater synergy
can be anticipated between the POCN pincer ligand (a better
s-donor and poorer p-acceptor than POCOP) and the good
p-acceptor phen ligand; similarly, the acetonitrile adduct should be
more stable with POCOP (a better net p-acceptor compared to POCN)
because acetonitrile is a poor p-acceptor compared to phen.22
Next, we studied the conversion of (pincer)NiX (X = Cl, Br)
into the corresponding cationic species, with the aim of gaining
some insight on the factors that influence these reactions.
The samples studied consisted of 0.015–0.05 M acetonitrile
solutions containing equimolar quantities of the charge-neutral
halide precursors and the bidentate ligands phen or bipy. These
mixtures were allowed to equilibrate over several hours at r.t.
before analysis by 31P NMR spectroscopy to assess the extent to
which the bromide substitution proceeds.23 The Keq values were
determined by using the integration values for the various 31P
NMR signals as indications of the concentrations of the species
at equilibrium. The equations and the equilibria data are shown
in the eqn (1)–(6).24
Fig. 1 Cyclic voltammograms of [(POCN)Ni(bipy)]+ (1c, blue) and
[(POCOP)Ni(bipy)]+ (2c, orange). The measurements were carried out at
298 K on samples obtained from 1 mM THF solutions of the complexes
and containing [Bu4N][PF6] as electrolyte (0.1 M). A scan rate of 100 mV sꢀ1
was used, and the potentials were referenced to the Cp2Fe/Cp2Fe+ redox
couple.
magnitude less facile than the analogous substitution of acet-
onitrile in the corresponding cationic adducts. Inspection of
these Keq values also indicates that the factors affecting the
substitution reactions include the nucleophilic character of the
in-coming bidentate ligands (phen vs. bipy), the leaving ability
(or nucleophilic character) of the halide anion (Brꢀ vs. Clꢀ), and
the net electron-donating properties of the pincer platforms
(POCN vs. POCOP). We can thus draw the following conclusions:
(a) Of the two bidentate ligands, phen is much more
effective than bipy for the unassisted displacement of halides
from (POCN)NiX (eqn (1) vs. eqn (2); eqn (3) vs. eqn (4)) and
(POCOP)NiX (eqn (5) vs. eqn (6); eqn (7) vs. eqn (8)).
(b) Displacement of Brꢀ takes place more readily than that of
Clꢀ (eqn (1) vs. eqn (3); eqn (2) vs. eqn (4); eqn (5) vs. eqn (7);
eqn (6) vs. eqn (8)).
Keqꢂ13
Ð
ðPOCNÞNiBr þ phen
½ðPOCNÞNiðphenÞꢃþþBrꢀ (1)
Keqꢂ0:07ꢄ0:2
Ð
(c) Charge-neutral Ni-halide species based on the POCOP
pincer are more resistant to ionization (formation of cationic
adducts) than their analogous POCN derivatives (eqn (1) vs.
eqn (5); eqn (2) vs. eqn (6); eqn (3) vs. eqn (7); eqn (4) vs.
eqn (8)).
ðPOCNÞNiBrþbipy
½ðPOCNÞNiðbipyÞꢃþþBrꢀ
(2)
Keqꢂ0:11
Ð
ðPOCNÞNiClþphen
½ðPOCNÞNiðphenÞꢃþþClꢀ (3)
Keqꢂ7ꢅ10ꢀ5
Combining the above points (a) and (b) allows us to establish
the following order for the nucleophilic character of the ligands
involved in the substitution reactions: Clꢀ 4 Brꢀ 4 phen 4
bipy c py. Moreover, point (c) above implies that POCN is a
better net donor than POCOP. To corroborate the validity of this
finding, we turned to cyclic voltammetry measurements in order
to probe the oxidation potentials ofꢁPOCN- and POCOP-based
species. Thus, we found a smaller E1=2 value for the reversible,
one-electron redox events involving [(POCN)Ni(bipy)]+ relative to
[(POCOP)Ni(bipy)]+ (Fig. 1), confirming that the POCN platform
promotes a more facile oxidation.
The above conclusion regarding the greater net electron-
donating character of POCN vs. POCOP was also corroborated
by IR spectroscopy, as follows. We have reported previously14
that the n(CRN) values derived from the IR spectra of cationic
acetonitrile adducts [(R-POCOP)Ni(NCMe)]+ show a fairly clear
correlation with the electronic nature of the POCOP ring
substituents R. We observed, for example, that the n(CRN)
value of 2297 cmꢀ1 for the parent system (R = H) increased to
Ð
½ðPOCNÞNiðbipyÞꢃþþClꢀ
(4)
ðPOCNÞNiClþbipy
ðPOCOPÞNiBrþphen KÐeqꢂ0:042 ½ðPOCOPÞNiðphenÞꢃþþBrꢀ
(5)
Keqꢂ7ꢅ10ꢀ5
Ð
ðPOCOPÞNiBrþbipy
½ðPOCOPÞNiðbiPyÞꢃþþBrꢀ
(6)
ðPOCOPÞNiClþphen KÐeqꢂ0:012 ½ðPOCOPÞNiðphenÞꢃþþClꢀ
(7)
Keqꢂ4ꢅ10ꢀ5
Ð
ðPOCOPÞNiClþbipy
½ðPOCOPÞNiðbipyÞꢃþþClꢀ
(8)
The mostly small Keq values for the equilibria shown in
eqn (1)–(8) indicate, unsurprisingly, that the substitution of the
halides in these charge-neutral precursors is orders of
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Scheme 6 Proposed pathway for the exchange reaction shown in eqn (7).
2329 and 2303 cmꢀ1 with p-COOMe and m-COOMe substituents,
The conclusion that the POCN platform is a better net donor
respectively, while decreasing to 2293 and 2284 cmꢀ1 for p-OMe of electron density implies that the cationic fragment
and m-OMe, respectively. Evidently, n(CRN) values increase [(POCOP)Ni]+ must have a greater Lewis-acidity than its POCN
fairly significantly with electron-withdrawing substituents, but analogue. Consistent with this expectation, mixing (POCN)NiBr
decrease only somewhat with electron-donating substituents. with the cationic adducts [(POCOP)Ni(NCMe)]+ or [(POCOP)Ni
This correlation was rationalized in terms of the MO description (phen)]+ led to complete (or nearly complete) ligand exchange
of bonding in acetonitrile. Recall that acetonitrile’s HOMO favoring the formation of (POCOP)NiBr, as shown in eqn (9)
possesses a certain degree of antibonding character with respect and (10) (see Fig. S47 and S48 (ESI†) for the spectra).
to the CRN bond; thus, MeCN - Ni s-donation reinforces the
MeCN
ðPOCNÞNiBr þ ½ðPOCOPÞNiðphenÞꢃþ ꢀ!
C–N bond and increases the value of n(CRN).25,26 Consistent
with this phenomenon, the n(CRN) values in the cationic
adducts [(R-POCOP)Ni(NCMe)]+ are much higher than that of
free acetonitrile (2252 cmꢀ1), which implies a net transfer of
charge from acetonitrile to the cationic Ni(II) centre.
r:t:
(9)
½ðPOCNÞNiðphenÞꢃþþðPOCOPÞNiBr
MeCN
Ð
ðPOCNÞNiBr þ ½ðPOCOPÞNiðNCMeÞꢃþ
r:t:
(10)
The above can be taken to mean that n(CRN) values of the
acetonitrile adducts [(pincer)Ni(NCMe)]+ should reflect the
approximate net electron donating character of the pincer
platform, better net donors reducing MeCN - Ni donation
and resulting in lower n(CRN) values, and vice versa. Based on
this, and on the IR spectrum of [(POCN)Ni(NCMe)]+, which
showed a n(CRN) band at 2285 cmꢀ1, we conclude that the
POCN platform is a better net donor of electron density to
cationic Ni(II) compared to POCOP.
½ðPOCNÞNiðNCMeÞꢃþþðPOCOPÞNiBr
The exchange reaction shown in eqn (7) can be envisioned to
proceed as shown in Scheme 6. The Keq values shown in
Scheme 5 support the notion that the interaction of [(POCOP)-
Ni(phen)]+ with MeCN, which is present in a very large excess
(41000 equiv.), would lead to a partial displacement of phen;
this would generate small quantities of the acetonitrile adduct
Fig. 2 UV-visible spectra for [(POCN)Ni(bipy)]+ (1c, orange), [(POCN)Ni(phen)]+ (1d, red-orange), [(POCOP)Ni(bipy)]+ (2c, green), and [(POCOP)Ni(-
phen)]+ (2d, blue) recorded in air on 1 ꢅ 10ꢀ4 M THF solutions.
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[(POCOP)Ni(NCMe)]+ and free phen. The latter would then dis- (ESI†) for the spectrum).
place Brꢀ in (POCN)NiBr to give the new cationic adduct
[(POCN)Ni(phen)]+ (see eqn (1) above). Finally, the Brꢀ generated
MeCN
Ð
ðPOCNÞNiBr þ ðPOCOPÞNiCl
in the latter step would react with either [(POCOP)Ni(NCMe)]+ or
[(POCOP)Ni(phen)]+ to form (POCOP)NiBr.
The exchange reaction shown in eqn (8) would likely proceed
via a mechanism similar to the one shown in Scheme 6:
sluggish bromide substitution in (POCN)NiBr by MeCN would
r:t:
(11)
ðPOCNÞNiCl þ ðPOCOPÞNiBr
Characterization of the new complexes. NMR
generate small concentrations of [(POCN)Ni(NCMe)]+ and Brꢀ; The dinuclear dicationic complex 1e proved to be insoluble and
the latter would then readily displace MeCN in [(POCOP)- could not be analyzed by NMR, but the remainder of the new
Ni(NCMe)]+ to form (POCOP)NiBr. The pathways proposed for cationic adducts presented in this report have been characterized
the exchange equilibria shown in eqn (7) and (8) involve a by their 1H, 13C{1H}, and 31P{1H} NMR spectra. As mentioned
partial solvolysis step to initiate the exchange reactions, which earlier, the 31P NMR spectra were particularly versatile for a
implies in turn that these reactions should not proceed in non- straight forward, rapid, and reliable identification of these
nucleophilic media. Consistent with this reasoning, these compounds owing to the highly characteristic singlet 31P
reactions did not proceed in CH2Cl2 or THF. Moreover, that resonances appearing over a rather narrow range (176–203 ppm).
these exchanges proceed to completion (or near completion) is This method was also of great utility for the ligand substitution
presumably because (POCOP)NiBr is significantly more resistant studies as it allowed easy identification of various species in complex
to ionization in MeCN, and hence the back reaction is disfavored. mixtures. The 1H NMR spectra also allowed simple identification of
This is consistent with the finding that allowing the charge- the complexes on the basis of the characteristic protons of the
neutral chloro and bromo complexes to equilibrate results in a central aromatic ring (a doublet and a triplet for POCOP complexes;
mixture favoring (POCN)NiCl over (POCN)NiBr and (POCOP)NiBr two doublets and a pseudo-triplet for POCN complexes). The NMR
over (POCOP)NiCl, as shown in eqn (11) (Keq B 5; see Fig. S49 spectra are presented in the ESI,† (Fig. S1–S25).
Fig. 3 Molecular structures of (a) [(POCN)Ni(NCMe)][SbF6], 1a, (b) [(POCN)Ni(py)][SbF6], 1b, (c) [(POCN)Ni(phen)][SbF6], 1d, and (d) [(POCN)Ni(4,40-
bipy)Ni(POCN)]2[SbF6], 1e. The anions, all the hydrogen atoms, and the isopropyl CH3 groups have been omitted for clarity.
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Fig. 4 Molecular structure of (a) [(POCOP)Ni(py)][SbF6], 2b, (b) [(POCOP)Ni(bipy)][SbF6], 2c, and (c) [(POCOP)Ni(phen)][SbF6], 2d. The anions, all the
hydrogen atoms, the isopropyl groups of their CH3 groups, and all disordered P and C atoms have been omitted for clarity.
Absorption spectroscopy
analysis, but one adduct (2e) resisted crystallization despite
repeated attempts. The X-ray analyses undertaken on 8 crystals
gave high quality structure data for 7, whereas the data
obtained for adduct 1c allowed us merely to confirm the
connectivity. Front and side views of the molecular diagrams
for the 7 adducts are shown in Fig. 3 (1a, 1b, 1d, and 1e) and
Fig. 4 (2b, 2c, and 2d), the most pertinent structural parameters
for the high-quality structures are listed in Table 1, and some of
the main findings are discussed below. ESI† provide details for
the diffraction studies (Tables S1–S3, ESI†) and the molecular
diagram for 1c (Fig. S52, ESI†).
UV-vis spectra of the adducts 1b, 1c, 2b, and 2c were also
recorded as an additional characterization method. Fig. 2
shows the spectra that were run in dry THF (ca. 1 ꢅ 10ꢀ4
M
solutions). The low-energy bands (470–600 nm) display very low
intensities and appear fairly insensitive to the different pincer
platforms and auxiliary ligands; it is reasonable to attribute the
absorptions in this region to spin-allowed but Laporte-forbidden
d–d transitions. The next set of bands in the ca. 370–430 nm
range have greater intensities, but show little variations in terms
of lmax; these are likely due to MLCT absorptions. Finally, the
bands below 350 nm show fairly different lmax values and
intensities as a function of ligands, suggesting that they
represent p - p* transitions. Similar bands have been observed
in the UV-vis spectra of previously studied acetonitrile adducts
[(R-POCOP)Ni(NCMe)]+,14 implying that these bands might be
centered on both the pincer platforms as well as the bipy and
phen ligands. Normally, the observed differences in lmax values
and also intensities of these bands can be related to the charge
buildup in the excited states, thus informing us on the relative
electron-withdrawing or donating properties of the pincer
platform (POCN 4 POCOP). In this case, however, the ambiguity
in the precise origins of the absorptions that give rise to these
bands preclude reliable interpretation.
The nickel centre in the tetracoordinated adducts adopts a
lightly distorted square planar geometry, with the smaller-than-
ideal bite angle of the pincer platforms being the main
distortion: P–Ni–N E 159–1631 in the POCN complexes 1a,
Table
1
Selected bond distances (Å) and angles (deg.) for
[(pincer)Ni(ligand)][SbF6]
Complex Ni–C
Ni–P1
Ni–P2 or Ni–N Ni–Neq Ni–Nax
2.019(2) 1.905(2)
1a
2aa
1b
2b
2c
1.852(2)
2.1267(7)
1.882(2) avg 2.1674(8) avg 2.1681(7) avg 1.878(2)
1.858(3)
1.891(3)
1.896(4)
1.862(5)
1.904(6)
1.858(3)
1.863(3)
2.1291(8)
2.181(1)
2.144(7)
2.118(2)
2.178(1)
2.1347(9)
2.129(1)
2.017(2)
2.178(1)
2.203(1)
2.114(4)
2.227(1)
2.044(3)
2.066(3)
1.948(2)
1.937(3)
1.944(3) 2.144(4)
1.952(5) 2.181(5)
1.961(5) 2.178(5)
1.944(3)
1d
2d
1e
X-Ray diffraction studies
1.942(3)
Of the 9 new cationic adducts prepared during the course of the
present study, 8 yielded single crystals suitable for X-ray
a
This complex has been reported previously: ref. 14.
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1b, and 1e; P–Ni–P is ca. 1631 in 2b. The remaining cis and trans arising from the competitive binding of this nucleophilic sol-
angles are closer to their ideal values: the cis angles P–Ni–C and vent. Thus, such syntheses are best carried out in less nucleo-
N–Ni–C are ca. 81–851, cis angles the N–Ni–N and N–Ni–P are philic solvents such as THF and CH2Cl2. The impact of solvent
ca. 97–991, and the trans angles N–Ni–C are ca. 174–1771. The py nucleophilicity is also observed in the ionization of halide
fragments in the mononuclear Ni–py adducts 1b and 2b, and in precursors, i.e., (pincer)Ni(halide) - [(pincer)Ni(L)]+, which
the dinuclear bipy* adduct 1e make ca. 72–771 angles with their can be rationalized in view of the associative substitution
mean coordination planes; the two py fragments in 1e are tilted mechanism expected for these 16-electron, d8 species.
by ca. 311 with respect to each other.
The above halide substitutions were also found to depend
Somewhat greater structural distortions are detected in the on the nature of the halide (Clꢀ is much harder to displace than
three pentacoordinated adducts wherein the nickel centre Brꢀ), the pincer platform (POCOP resists ionization more than
adopts a square pyramidal geometry. The pentagonal distortion POCN), and also the in-coming nucleophile L (phen 4 bipy 4
is reflected, primarily, in the position of the Ni centre, which is py). The solvolyses of the cationic py-, bipy-, and phen-adducts
lifted off the square plane by 0.27 Å in the bipy adduct 2c and by in acetonitrile depend on not only the binding force of these
ca. 0.20–0.21 Å in the phen adducts 1d and 2d. Interestingly, the ligands (the bidentate ligands being less easy to displace) but
bite angles in the pentacoordinated complexes are even smaller also to some extent on the pincer platform (POCOP adducts
than those found in tetracoordinated adducts: P–Ni–N E 146– undergoing solvolysis somewhat more readily). The solvolysis
1531 in the 4 independent molecules of 1d; P–Ni–P E 146–1541 of the pentacoordinated cationic bipy- and phen-adducts
in 2c and 2d. The bidentate ligands show bite angles (Nax–Ni– reactions also raise the intriguing question of whether they proceed
N
eq) of ca. 801, and the Nax and Neq atoms form the following cis associatively (via 20-electron intermediates) or dissociatively.
angles: Nax–Ni–P B 100–1131, Nax–Ni–N B 92–941, Nax–Ni–C B The equilibrium data obtained for the above-described
97–1001, Neq–Ni–P B 95–1041, Neq–Ni–N B 961. The angles ligand exchange reactions have been interpreted as indicating
between the mean coordination planes and the central chelate that the POCN platform is a better net donor of electron density
ring of the phen ligand in 1d is about 771, whereas the relative to its POCOP analogue. This conclusion is also sup-
corresponding angles for the POCOP analogue 2d and the bipy ported by cyclic voltammetry measurements carried out on
adduct 2c are nearly perpendicular (871 and 841, respectively). [(pincer)Ni(bipy)]+ (POCN-based adduct undergoes oxidation
Finally, there is little systematic variation in the transoid bond at a lower potential) and, to a lesser extent, from a comparison
angles C–Ni–N (172–1791) across all comparable tetra- and of the n(CRN) values in [(POCN)Ni(NCMe)]+ and [(POCOP)-
pentacoordinate complexes, regardless of the type of pincer Ni(NCMe)]+. It is also worth noting that these observations
ligand platform.
are consistent with our previous findings that POCN–Ni(II)
Looking at bond distances, much shorter Ni–C and Ni–P complexes are readily converted to the thermally stable trivalent
distances are observed in the POCN adducts compared to the species (POCN)NiBr2,12,27 whereas the analogous POCOP
corresponding distances in the POCOP complexes. For complexes do not yield trivalent species.28
instance, the longest Ni–C distance in the pentacoordinate
POCN adduct 1d is shorter than the shortest Ni–C distance in
the tetracoordinate POCOP complex 2a (1.862(5) vs. 1.882(2) Å). Conflicts of interest
Similarly, the longest Ni–P distance in the pentacoordinate
We declare that we have no financial or personal relationships
with other people or organizations that can inappropriately
influence our work, and that there is no professional or other
personal interest of any nature or kind in any product, service
and/or company that could be construed as influencing the
position(s) presented in this manuscript.
POCN adduct 1d is shorter than the shortest Ni–P distance in
the tetracoordinate POCOP complex 2a (2.118(2) vs. 2.144(7) Å).
On the other hand, the longest Ni–Nax distance is found in the
POCN adduct 1d. Not surprisingly, in all cases the Ni–Nax
distances are longer than the Ni–Neq distances. Finally, it is
worth noting that the Ni–C distances show only minor variations
within each group of adducts based on the same pincer plat-
form, whereas statistically significant differences are observed
for the Ni–P distances within both groups (see Table 1).
Acknowledgements
The authors are grateful to: (a) the Natural Sciences and
Engineering Research Council of Canada (NSERC) for financial
Conclusion
support of this study; (b) to Dr Loic P. Mangin for fruitful
discussions and assistance with this study; (c) to Dr Daniel
Chartrand for help with the resolution of the crystal data.
This study has informed us on the stabilities of cationic Ni(II)
complexes featuring the pincer platforms POCN and POCOP,
and has provided interesting observations about the ligand
exchange reactions that these complexes undergo. We learnt,
for instance, that conducting the syntheses of py, bipy, and phen
adducts of POCN–Ni and POCOP–Ni cations in acetonitrile can
engender complications due to the solvolytic side-reactions
Notes and references
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(a) V. Gomez-Benıtez, O. Baldovino-Pantaleon, C. Herrera-
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676; (k) J. Zhang, T. Liu, Q.-Q. Ma, S. Li and X. Chen, Dalton 18 It should be noted that we used 1,10-phenanthroline mono-
Trans., 2018, 47, 6018; (l) J. Zhang, Y. Ding, Q.-Q. Ma,
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hydrate (as opposed to its dry form) in most preparative
procedures and ligand substitution reactions. A reviewer of
our manuscript has pointed out that the residual moisture in
this starting material might have considerable impact on
bromide displacement equilibria. We have probed this issue
by conducting additional tests in the presence of added water
and by using dry phen. The results of these tests confirmed
that whereas relatively large quantities of residual water can
have major influence on the displacement of bromide, only
minor and negligible impact is observed from the small
amounts of water arising from the use of hydrated ligand
(ca. 1 equiv.). We estimate that this impact should be on the
same order as the residual water in the ‘‘dried’’ solvent. See
the ESI† for more details on the tests.
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(a) V. Pandarus and D. Zargarian, Chem. Commun., 2007,
978; (b) V. Pandarus and D. Zargarian, Organometallics,
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12 For representative reports on POCN-Ni complexes see:
(a) D. M. Spasyuk and D. Zargarian, Inorg. Chem., 2010, 19 Although the exchange reactions proceeded readily upon
49, 6203; (b) B.-S. Zhang, W. Wang, D.-D. Shao, X.-Q. Hao,
J.-F. Gong and M.-P. Song, Organometallics, 2010, 29, 2579;
(c) J.-L. Niu, Q.-T. Chen, X.-Q. Hao, Q.-X. Zhao, J.-F. Gong
dissolution of the solids, a 1 h delay was allowed before
recording the NMR spectra to ensure that equilibration was
complete.
and M.-P. Song, Organometallics, 2010, 29, 2148; 20 According to their 31P NMR spectra, [(POCN)Ni(phen)]+ is
(d) D. M. Spasyuk, S. I. Gorelsky, D. E. A. Van and
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Y.-J. Liu, J.-F. Gong and M.-P. Song, Organometallics, 2011,
30, 3793; ( f ) J. Sanford, C. Dent, J. D. Masuda and A. Xia,
Polyhedron, 2011, 30, 1091; (g) D. M. Spasyuk, D. Zargarian
completely intact (Fig. S40, ESI†), whereas [(POCN)Ni(py)]+
is completely converted to [(POCN)Ni(NCMe)]+ (Fig. S42,
ESI†) and the MeCN solution of [(POCN)Ni(bipy)]+ consists
of an 78 : 22 mixture of this adduct and its solvolysis product
[(POCN)Ni(NCMe)]+ (Fig. S41, ESI†).
and D. E. A. Van, Organometallics, 2009, 28, 6531; 21 According to their 31P NMR spectra, the MeCN solution of
(h) J. B. Smith and A. J. M. Miller, Organometallics, 2015,
34, 4669.
13 B. Vabre, D. M. Spasyuk and D. Zargarian, Organometallics,
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14 S. Lapointe, B. Vabre and D. Zargarian, Organometallics,
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15 (a) J. Choi, A. H. R. MacArthur, M. Brookhart and
A. S. Goldman, Chem. Rev., 2011, 111, 1761; (b) K. Zhu,
P. D. Achord, X. Zhang, K. Krogh-Jespersen and
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(c) K. Krogh-Jespersen, M. Czerw, K. Zhu, B. Singh,
M. Kanzelberger, N. Darji, P. D. Achord, K. B. Renkema
and A. S. Goldman, J. Am. Chem. Soc., 2002, 124, 10797;
[(POCOP)Ni(phen)]+ consists of an 87 : 13 mixture of this
adduct and its solvolysis product [(POCN)Ni(NCMe)]+
(Fig. S44, ESI†) and the analogous bipy adduct leads to a
71 : 29 mixture (Fig. S45, ESI†), whereas the py adduct undergoes
complete solvolysis to [(POCOP)Ni(NCMe)]+ (Fig. S46, ESI†).
22 A reviewer of our manuscript has pointed to steric factors as
important contributors to the lower stability of the POCOP
adducts featuring more bulky donor moieties. In principle,
this proposal is reasonable, and we have searched for
evidence of steric crowding in the solid-state structures of
py, bipy, and phen adducts of POCN and POCOP platforms,
including the bite angles (P–Ni–N and P–Ni–P) and the tilt
angles (the angle between the coordination plane and the
mean plane of the Schiff bases). The bite angles are very
similar for both pincer ligands, but the Schiff-base ligands
adopt a more upright, less tilted configurations in the
POCOP vs. POCN adducts (84–871 vs. 771). On the other
hand, at least in some cases the observed tilts and bending
of the bipy and phen ligands appear to be caused by
(presumably favorable, not repulsive) interactions between
the C–H bonds of the i-Pr P-substituents and the aromatic
ring of the Schiff-base ligands. Thus, it is difficult to
conclude definitively whether the minor structural differ-
ences observed for the analogous adducts serve to relieve
greater steric crowding in the solid states of POCOP com-
plexes or to enhance electronic interactions.
¨
(d) I. Gottker-Schnetmann, P. White and M. Brookhart,
J. Am. Chem. Soc., 2004, 126, 1804; (e) D. M. Roddick, Top.
Organomet. Chem., 2013, 40, 49.
16 R. G. Wilkin, Kinetics and Mechanism of Reactions of Transi-
tion Metal Complexes, VCH, Weinheim, 2nd edn, 1991, ch
4.6, p. 232, ISBN: 3-527-28389-7.
17 For representative reports on Ag+-assisted preparation of
cationic adducts see: (a) S. Lapointe and D. Zargarian,
Dalton Trans., 2016, 45, 15800; (b) B. Vabre, Y. Canac,
C. Lepetit, C. Duhayon, R. Chauvin and D. Zargarian,
Chem. – Eur. J., 2015, 21, 17404; (c) A. Salah, M. Corpet,
N. U.-H. Khan, D. Zargarian and D. Spasyuk, New J. Chem.,
2015, 39, 6649; (d) B. Vabre, Y. Canac, C. Duhayon,
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23 Recall that the halide precursors do not undergo substitu-
tion with the monodentate ligand pyridine.
Int. J. Quantum Chem., 2016, 116, 1285, DOI: 10.1002/
qua.25174.
´
´
24 See ESI† for the methods used for calculating these Keq 27 B. Mougang-Soume, F. Belanger-Gariepy and D. Zargarian,
values. Organometallics, 2014, 33, 5990.
25 S. Liang, H. Wang, T. Deb, J. L. Petersen, G. T. Yee and 28 It should be emphasized that this statement applies only to
M. P. Jensen, Inorg. Chem., 2012, 51, 12707.
resorcinol-derived POCOP platforms featuring a central
aromatic moiety, sometimes represented as POCsp2OP,
whereas the analogous platforms derived from 1,3-
propandiol, POCsp3OP, do stabilize trivalent species. See
ref. 11b and c.
26 P. Chaquin, Y. Canac, C. Lepetit, D. Zargarian and
R. Chauvin., Estimating Local Bonding/Antibonding Char-
acter of Canonical Molecular Orbitals from their Energy
Derivatives. The Case of Coordinating Lone Pair Orbitals,
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2021
New J. Chem., 2021, 45, 15063–15073 | 15073