Hydrogen-Atom Noninnocence of a Tridentate [SNS] Pincer Ligand

Article abstract of DOI:10.1021/acs.inorgchem.8b00618

Double deprotonation of bis(2-mercapto-4-methylphenyl)amine ([SNS]H₃) followed by addition to NiCl₂(PR₃)₂ in air-free conditions afforded [SN(H)S]Ni(PR₃) (1a, R = Cy; 1b, R = Ph) complexes, characterized as diamagnetic, square-planar nickel(II) complexes. When the same reaction was conducted with 3 equiv of KH, the diamagnetic anions K{[SNS]Ni(PR₃)} were obtained (K[2a], R = Cy; K[2b], R = Ph). In the presence of air, the reaction proceeds with a concomitant one-electron oxidation. When R = Cy, a square-planar, S = ¹/₂ complex, [SNS]Ni(PCy₃) (3a), was isolated. When R = Ph, the bimetallic complex {[SNS]Ni(PPh₃)}₂ ({3b}₂) was obtained. This bimetallic species is diamagnetic; however, in solution it dissociates to give S = ¹/₂ monomers analogous to 3a. Complexes 1-3 represent a hydrogen-atom-transfer series. The bond dissociation free energies (BDFEs) for 1a and 1b were calculated to be 63.9 ± 0.1 and 62.4 ± 0.2 kcal mol^-1, respectively, using the corresponding pK_a and E°′ values. Consistent with these BDFE values, TEMPO^· reacted with 1a and 1b, resulting in the abstraction of a hydrogen atom to afford 3a and 3b, respectively.

Full text of DOI:10.1021/acs.inorgchem.8b00618

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Hydrogen-Atom Noninnocence of a Tridentate [SNS] Pincer Ligand
Kyle E. Rosenkoetter, Michael K. Wojnar, Bronte J. Charette, Joseph W. Ziller, and Alan F. Heyduk*
Department of Chemistry, University of California, Irvine, California 92697-2025, United States
*
S Supporting Information
ABSTRACT: Double deprotonation of bis(2-mercapto-4-
methylphenyl)amine ([SNS]H ) followed by addition to
3
NiCl (PR ) in air-free conditions afforded [SN(H)S]Ni(PR )
2
3 2
3
(
1a, R = Cy; 1b, R = Ph) complexes, characterized as diamagnetic,
square-planar nickel(II) complexes. When the same reaction was
conducted with 3 equiv of KH, the diamagnetic anions
K{[SNS]Ni(PR )} were obtained (K[2a], R = Cy; K[2b], R = Ph). In the presence of air, the reaction proceeds with a
3
1
concomitant one-electron oxidation. When R = Cy, a square-planar, S = / complex, [SNS]Ni(PCy ) (3a), was isolated. When
2
3
R = Ph, the bimetallic complex {[SNS]Ni(PPh )} ({3b} ) was obtained. This bimetallic species is diamagnetic; however, in
3
2
2
1
solution it dissociates to give S = / monomers analogous to 3a. Complexes 1−3 represent a hydrogen-atom-transfer series.
2
−
1
The bond dissociation free energies (BDFEs) for 1a and 1b were calculated to be 63.9 ± 0.1 and 62.4 ± 0.2 kcal mol ,
•
respectively, using the corresponding pK and E°′ values. Consistent with these BDFE values, TEMPO reacted with 1a and 1b,
a
resulting in the abstraction of a hydrogen atom to afford 3a and 3b, respectively.
INTRODUCTION
diphosphine ligands with an amine bridgehead that are used in
■
1
7−19
the design of electrocatalysts.
The tertiary amine
It has long been recognized that the management of proton
and electron inventories is a critical factor in the reactions of
small-molecule substrates including N , O , CO, and CO .
Such proton-coupled electron-transfer (PCET) reactions, or
hydrogen-atom-transfer (HAT) reactions, constitute an
important and active area of current research. Detailed
thermodynamic and kinetic analyses of HAT reactions have
elucidated the key factors that control HAT reactions in both
bridgehead does not coordinate to the metal but instead
shuttles protons to and from the metal center during catalysis.
A ligand platform that can serve as a reservoir for both
protons and electrons can also act as a HAT noninnocent
ligand. Herein, we report on the ability of a tridentate [SNS]
1
−4
2
2
2
pincer ligand ([SNS]H = bis(2-mercapto-4-methylphenyl)-
3
20
amine) to participate in HAT reactions when coordinated to
3
organic and inorganic systems. Two types of inorganic HAT
nickel(II). Tridentate pincer ligands have a well-established
coordination chemistry with transition-metal ions, and in many
coordination complexes have been studied in detail. In the first
class, metal hydride complexes, both the proton and electron
are transferred in and out of the primary coordination sphere
of the metal. The second class comprises coordination
complexes in which the metal ion serves as the electron
source and a coordinated ligand serves as the proton source.
Herein we investigate a third type of coordination complex
capable of participating in HAT reactions, where both proton
and electron transfer occurs from a noninnocent ligand.
Redox-active ligands are the most commonly discussed type
of noninnocent ligand platform. Catecholate, which can exist
in three different oxidation states when coordinated to a metal
21−28
cases, ligand-centered HAT chemistry has been observed.
Similarly, there have been several recent reports of redox-
noninnocent ligand platforms that can participate in HAT
5
−9
reactions.
The [SNS] ligand examined here is the sulfur
congener of the well-known ligand derived from bis(3,5-di-tert-
butyl-2-phenol)amine, which is stable in three oxidation states
5
−9
11,29,30
when coordinated to a metal ion.
Structural, spectro-
scopic, and computational data for the square-planar nickel
complexes reported here indicate that the [SNS] ligand
platform is redox-noninnocent and capable of electron, proton,
1
0
1
1,12
ion, is the prototypical redox-active ligand.
In redox-active
and hydrogen-atom transfer reactivity. Redox and pKa
ligand coordination complexes, ligand-based oxidation state
changes often operate in concert with or even in place of more
classical metal-based oxidation state changes during redox
measurements are used to benchmark the bond dissociation
free energy (BDFE) for the active ligand-bound hydrogen
atom, and preliminary HAT reactivity with TEMPO• is
reported.
1
3
reactions. A second type of noninnocent ligand is one that
exerts a secondary steric-type interaction on the metal
1
4
coordination sphere. Second-sphere interaction ligands
typically include functionalities that provide hydrogen-
bonding, dipolar, or van der Waals interactions within the
Special Issue: Applications of Metal Complexes with Ligand-
Centered Radicals
15,16
second coordination sphere of the metal ion.
These
second-sphere interactions can stabilize reactive species or
lower activation barriers. An example of the latter ligand type is
Received: March 13, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00618
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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RESULTS
characterized by a parent-ion peak at 1194 amu, indicative of a
■
dimer with the formula {[SNS]Ni(PPh )} ({3b} ).
3
2
2
Synthesis and Characterization. Square-planar nickel
complexes of the bis(2-mercapto-4-methylphenyl)amine
X-ray diffraction experiments were performed on single-
crystal samples of complexes 1−3, revealing all six complexes
to contain square-planar nickel centers. Figure 1 shows the
(
[SNS]H ) proligand were prepared by taking advantage of
3
the different pK values of the amine and thiol functional
a
3
1
groups. Under a N atmosphere, the reaction of [SNS]H
2
3
with NiCl (PR ) in the presence of 2 equiv of triethylamine
2
3 2
led to metalation of the ligand via selective deprotonation of
the more acidic thiol groups to afford [SN(H)S]Ni(PR3)
complexes (1a, R = Cy; 1b, R = Ph), as shown along the top of
Scheme 1. When 3 equiv of the strong base potassium hydride
Scheme 1
was used in place of triethylamine, potassium salts of a
nickellate anion were obtained with the formula K[SNS]Ni-
(
PR ) (K[2a], R = Cy; K[2b], R = Ph). Electrospray
3
ionization mass spectrometry (ESI-MS) showed the presence
of the appropriate [SN(H)S]Ni(PR ) ions for [SN(H)S]Ni-
+
3
(
PCy ) (1a) and 1b; in the cases of K[2a] and K[2b], the MS
3
+
spectra showed the parent ion for the salt as K[SNS]Ni(PR ) .
3
All four of the above complexes proved to be diamagnetic in
31
1
solution, showing a single resonance in the P{ H} NMR
1
spectrum in the 25−35 ppm range. The H NMR spectra of
these complexes all showed resonances in the normal
diamagnetic range; notably, the spectra for 1a and 1b showed
singlet resonances at 7.84 and 8.31 ppm, respectively,
attributable to an intact N−H proton. This resonance was
1
not observed in the H NMR spectra of K[2a] and K[2b].
Figure 1. ORTEP diagrams of 1a (top), [K(2.2.2-crypt)][2a]
Metalation of bis(2-mercapto-4-methylphenyl)amine with
nickel(II) salts under aerobic conditions led to the formation
of one-electron-oxidized complexes that maintain square-
planar geometry around the nickel center. Solutions of
NiCl (PCy ) with bis(2-mercapto-4-methylphenyl)amine
(
middle), and 3a (bottom). Ellipsoids are shown at 50% probability.
A THF solvent molecule has been omitted from the structure of
K(2.2.2-crypt)[2a] for clarity.
2
3 2
and 3 equiv of triethylamine in air changed color from blue
to deep green and afforded a blue-green solid upon workup.
The ESI-MS spectrum of the product showed a parent-ion
peak at 596 amu, indicative of a monomer with the formula
structures of the family of nickel complexes containing the
⊖
PCy ligand, 1a−3a. The PPh complexes 1b and [2b] are
3
3
⊖
isostructural with the PCy complexes 1a and [2a] ,
3
respectively (see the Supporting Information), except for an
intermolecular hydrogen-bonding interaction between adjacent
molecules of 1b that is not present in the structure of 1a.
Complex 3b is dimeric, and its structure will be discussed
below. Crystals of 1a, 1b, and 3a were obtained from
tetrahydrofuran (THF) or acetonitrile (MeCN) solutions of
1
31
[
SNS]Ni(PCy ) (3a). Neither the H NMR nor the P{1H}
3
NMR spectrum of 3a showed prominent resonances,
1
consistent with its characterization as a paramagnetic, S = /
2
complex. When the same reaction was carried out using
NiCl (PPh ) as the nickel source, the isolated product was
2
3 2
B
DOI: 10.1021/acs.inorgchem.8b00618
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Table 1. Selected Distances (Å) and Angles (deg) for [SN(H)S]Ni(PR ) (1), [K(2.2.2-crypt)]{[SNS]Ni(PR )} (K(2.2.2-
3
3
crypt)[2]), and [SNS]Ni(PR ) (3) for R = Cy (a) and Ph (b)
3
[
SN(H)S]Ni(PR ) (1a, R = Cy; K(2.2.2-crypt){[SNS]Ni(PR )} (K(2.2.2-crypt)[2a], R = Cy; K(2.2.2-
{[SNS]Ni(PR )} (3a, R = Cy;
3
3
3
n
1
b, R = Ph)
crypt)[2b], R = Ph)
{3b} , R = Ph)
2
Ni−P
Ni−N
2.1983(6); 2.1683(5)
1.9843(16); 1.9652(14)
2.1750(4); 2.1630(5)
2.1865(6); 2.1872(5)
1.471(2); 1.461(2)
1.475(2); 1.469(2)
1.755(4); 1.7559(18)
1.751(2); 1.7714(18)
88.54(11); 89.72(4)
86.94(5); 87.38(4)
169.55(5); 179.53(5)
0.17; 0.08
2.2025(7); 2.1711(6)
1.9085(19); 1.8973(15)
2.1694(7); 2.1688(6)
2.1749(7); 2.1742(6)
1.398(3); 1.395(2)
1.395(3); 1.402(2)
1.754(2); 1.7526(19)
1.750(2); 1.7610(18)
87.61(6); 88.22(5)
88.07(6); 88.33(5)
174.15(6); 175.97(5)
0.08; 0.07
2.2192(6); 2.2090(4)
1.9224(18); 1.9067(12)
2.1665(7); 2.1856(4)
2.1814(7); 2.2117(4)
1.390(3); 1.3936(18)
1.392(3); 1.3797(18)
1.742(2); 1.7466(15)
1.743(2); 1.7436(15)
88.61(6); 87.48(4)
88.18(6); 87.84(4)
179.00(6); 142.81(4)
0.03; 0.32
Ni−S(1)
Ni−S(2)
N−C(1)
N−C(7)
S(1)−C(2)
S(2)−C(8)
N−Ni−S(1)
N−Ni−S(2)
N−Ni−P
τ4
the respective complex at 25 °C. For K[2a] and K[2b], single
crystals were obtained by vapor diffusion of diethyl ether into
THF and MeCN solutions, respectively, which also contained
2
.2.2-cryptand to sequester the potassium cation. Selected
metrical parameters for 1−3 are presented in Table 1. As is
evident from Figure 1, complexes 1a−3a all have four-
coordinate, square-planar nickel centers; accordingly, the
32
calculated τ value for each complex is close to zero. The
4
adjacent five-membered chelate rings, formed upon coordina-
tion of the [SNS] pincer ligand to the nickel center, generate
acute N−Ni−S angles that readily allow for accommodation of
33
the large PCy ligand (cone angle = 170°). Bond distances
3
across the PCy family of complexes are remarkably similar.
3
The Ni−N bond distance is sensitive to the presence of a
proton on the ligand nitrogen with a long Ni−N distance of
1
.98 Å in 1a and shorter Ni−N distances of 1.91 and 1.92 Å in
⊖
[
2a] and 3a, respectively. Similarly, the N−C bond distances
Figure 2. ORTEP diagram of {3b} . Ellipsoids are shown at 50%.
2
within the pincer ligand are sensitive to the presence of the
Hydrogen atoms and MeCN solvent molecules are omitted for clarity.
protonated nitrogen with a long distance of 1.47 Å in 1a and
shorter N−C distances of 1.40 and 1.39 Å in [2a] and 3a,
⊖
respectively. The Ni−P bond distances vary over a narrow 0.02
Å range in 1a−3a and the Ni−S and S−C bond distances show
little if any variation across the series of three complexes. In
general, the similarity in the structures across the series 1a−3a,
including metal−ligand and intraligand bond distances, makes
it impossible to make clear-cut metal and ligand oxidation state
Spectroscopic Characterization of Oxidized [SNS]
Nickel Complexes. Complexes 3a and {3b} are one-electron
2
⊖
⊖
oxidation products of the anions [2a] and [2b] . Given that
2a] and [2a] are both closed-shell, diamagnetic, nickel(II)
⊖
⊖
[
complexes with a potentially redox-active [SNS] ligand, the
nature of the oxidation in 3a and {3b} is of interest. As
2
34
expected for a one-electron oxidation product, 3a is a
paramagnetic complex that is NMR-silent. Figure 3 shows
the X-band electron paramagnetic resonance (EPR) spectrum
of 3a in a THF solution at both 10 and 298 K (inset). The
room-temperature spectrum is an isotropic signal centered at g
assignments from the structural data.
The dimeric formulation of {3b} , indicated by the mass
2
spectral data, was confirmed in the solid state by single-crystal
X-ray diffraction experiments. Figure 2 shows the structure of
{
3b} , consisting of two four-coordinate [SNS]Ni(PPh )
2
3
=
2, consistent with the type of spectra observed for ligand-
fragments stacked on top of one another. The dimer is held
1
35
localized, S = / spin systems. Upon cooling to 10 K, the
2
together by long (∼2.4 Å) Ni−S interactions between adjacent
spectrum becomes rhombic, indicative of a nickel contribution
to the singly occupied molecular orbital (SOMO). The low-
temperature spectrum was fit with g values of 1.99, 2.01, and
[
SNS]Ni(PPh ) fragments, which stacks the [SNS] ligands on
3
top of one another with centroid-to-centroid distances of 3.45
and 3.63 Å. The dimerization causes a distortion from planarity
around each nickel center manifested in an P−Ni−N angle of
2
.03 (see the Supporting Information).
The magnetic behavior of dimeric {3b} is more complex
2
only 140°, which, in turn, affords a τ value of 0.32 (excluding
4
than that of monomeric 3a. When dissolved in acetonitrile-d3
the extra Ni−S interaction). Nickel−ligand bond distances
(CD CN), {3b} is NMR-active, affording slightly broadened
3
2
1
31
within a monomer unit vary slightly from the PCy derivative
H and P NMR signatures in the normal diamagnetic region
3
3
a, with slightly shorter Ni−P and Ni−N distances and slightly
longer Ni−S distances. Other key bond distances within the
SNS] ligand are identical between monomeric 3a and dimeric
3b}2.
of the spectrum. Consistent with the NMR results, MeCN
solutions of {3b} are EPR-silent. Both the NMR and EPR
2
[
{
results in MeCN agree with the dimeric formulation of {3b}2
observed by ESI-MS and single-crystal X-ray diffraction, where
C
DOI: 10.1021/acs.inorgchem.8b00618
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Figure 3. X-band EPR spectrum of 3a dissolved in THF at 10 and
2
98 K (inset).
antiferromagnetic coupling between an unpaired electron on
each monomer would lead to diamagnetic behavior. In a
benzene solution, however, the behavior of {3b}2 is the
opposite. Solutions of {3b} in C D are NMR-silent. The EPR
2
6
6
spectrum of {3b} dissolved in benzene shows an isotopic, S =
2
1
/2
signal at 298 K, which upon cooling became rhombic with g
values at 1.95, 2.00, and 2.02 (see the Supporting
Information). These results are analogous to those for
monomeric 3a, indicating that the dimeric formulation for
Figure 4. (a) Qualitative molecular orbital diagram and Kohn−Sham
molecular orbitals and (b) total spin-density plot for monomeric 3b.
{
3b} is dynamic in solution. Polar solvents favor the dimeric
2
[SNS]Ni(PPh ), and nonpolar solvents favor dissociation to
qualitative molecular orbital diagram with Kohn−Sham
molecular orbitals for 3b, correlated to the frontier molecular
3
monomeric [SNS]Ni(PPh3).
⊖
Density Functional Theory (DFT) Computations.
Nickel complexes 1b−3b were examined using DFT
computations to help elucidate their electronic structures.
Gas-phase DFT computations were carried out at the TPSS/
orbitals of [2b] . Given that 3b is the one-electron oxidation
⊖
product of anion [2b] , it was expected that an electron would
be removed from the HOMO of this anion. Accordingly, the
lowest-energy, unoccupied β-orbital component correlates well
36−38
⊖
def2-TZVP level of theory.
In particular, the locus of the
with the HOMO of [2b] and is best described as an [SNS]
unpaired electron in monomeric 3b (and by analogy 3a) was
of interest. For 1b and [2b] , the solid-state structures were
π* orbital with small contributions from the nickel center
(13%) and phosphine ligand (2%). The filled α component of
this [SNS] π* orbital is lower in energy, falling below both the
α and β components of the M−L π* orbital that is the
⊖
used as the starting points for the geometry optimizations.
⊖
Complexes 1b and [2b] were well-described by a singlet
⊖
closed-shell Kohn−Sham DFT solution. Metal−heteroatom
bond distances fell within 0.02 Å of the solid-state values,
whereas intraligand C−S, C−S, and C−C bond distances all
HOMO−1 in [2b] , as shown in Figure 4a. The total spin-
density plot, shown in Figure 4b, is consistent with these
orbital assignments, with 91% of the total spin density residing
on the [SNS] ligand.
⊖
fell within 0.004 Å of the solid-state values. For anion [2b] ,
the highest occupied molecular orbital (HOMO) is mainly
Electrochemistry. To better elucidate the redox behavior
of the [SNS]Ni platform, voltammetric experiments were
carried out. The electrochemical response of complexes 2 and
3 were well-behaved in a MeCN solution containing 0.1 M
[Bu N][PF ] as the supporting electrolyte (see the Supporting
[
2
SNS] π* in character with a smaller nickel contributions of
0% (see the Supporting Information). The lowest unoccupied
molecular orbital (LUMO), as expected for a square-planar
nickel(II) complex, is nominally M−L σ* with the overall
4
6
+
/0
2
2
symmetry of the metal d
orbital (see the Supporting
Information). All potentials were referenced to the [Cp Fe]
x −y
2
2−
Information). Neutral 1b, with the protonated [SN(H)S]
redox couple using an internal standard. The cyclic voltammo-
⊖
⊖
ligand, has the analogous M−L σ* orbital as the LUMO, but
thanks to protonation of the pincer ligand, the HOMO is an
gram of [SNS]Ni(PCy ) ([2a] ) showed a reversible (i /i
3
pa pc
≅ 1; ΔE ≅ 70 mV) one-electron redox process at −0.64 V
+
/0
M−L π* orbital involving the nickel d orbital (see the
versus [Cp Fe] (Figure 5, top), corresponding to oxidation
xz
2
⊖
Supporting Information). In 1b, the metal contribution to this
M−L π* HOMO is 44%.
of the complex to 3a. An irreversible reduction of [2a] also
+/0
was observed at −2.74 V versus [Cp Fe] . When 3a was
analyzed by cyclic voltammetry, the same two redox processes
were observed with the expected shift in the open-circuit
2
1
Open-shell [SNS]Ni(PPh ) (3b) was modeled as an S = /
3
2
monomer using a spin-unrestricted DFT computational
scheme. To build a model for monomeric 3b, the geometry
optimization calculation was carried out by starting with half of
potential (see the Supporting Information). For the PPh
3
derivatives [2b]⊖ and {3b} , analogous one-electron redox
2
the solid-state dimer, {3b} . After geometry optimization,
processes were observed, shifted only slightly to −0.61 and
2
+/0
metal−heteroatom and intraligand C−S, C−S, and C−C bond
distances for this 3b model were consistent with those
obtained by X-ray diffraction for 3a. Figure 4a shows a
−2.54 V versus [Cp Fe] (see the Supporting Information).
2
⊖
In the case of [2b] and {3b} , the process at −0.61 V is
2
reversible (i /i ≅ 1; ΔE ≅ 78 mV), suggesting that the event
pa pc
D
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Inorganic Chemistry
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phenolate anions [SNS]Ni(PR )⊖ ([2a]⊖ and [2b] ) are
⊖
3
2
−
related by the removal of a proton from the [SN(H)S]
ligand, it was of interest to examine the acid−base properties of
these complexes. The addition of the strong base 1,8-
diazabicyclo[5.4.0]undec-7-ene [DBU; pK (MeCN) =
a
39
2
4.34] to an MeCN solution of 1a resulted in an immediate
color change from orange to blue, signaling formation of the
⊖
[
2a] anion. Similarly, the addition of DBU to the PPh
3
⊖
derivative 1b resulted in the quantitative formation of [2b] .
The protonation reactions also proceeded smoothly, with the
addition of 4-cyanoanilinium tetrafluoroborate [pK (MeCN) =
a
40
⊖
⊖
7
.0] to solutions containing either [2a] or [2b] resulting
in the quantitative formation of 1a or 1b, respectively.
⊖
⊖
Complexes 1a, 1b, and [2a] and [2b] show very different
spectroscopic features in the UV−vis region of the electro-
magnetic spectrum, allowing for the ready determination of
⊖
Figure 5. Cyclic voltammograms for [2a] (top) and 1a (bottom) in
2
−
[
SN(H)S] pKa values by spectrophotometric titration.
MeCN containing 0.1 M [Bu N][PF ] using a glassy carbon working
4
6
+/0
Accordingly, the portionwise addition of 2,4,6-trimethylpyr-
electrode. Potentials are referenced to the [Cp Fe] couple.
2
39
idine [pK (MeCN) = 14.98] to an MeCN solution of 1a
a
resulted in the growth of an intense charge-transfer band at
involves the monomeric form of 3b rather than the dimeric
form observed in the solid-state structure. The reversibility of
this reduction, along with its similarity to the analogous
reduction of 3a, further suggests that the monomer−dimer
⊖
3
68 nm, characteristic of the nickelate anion of [2a] . From
the titration data, a pK of 17.4 ± 0.5 was determined for
a
complex 1a, which contains the PCy ancillary ligand.
3
Analogous titration of 1b formed [2b]⊖ and afforded a pKa
equilibrium for 3b and {3b} is fast on the electrochemical
2
of 15.9 ± 0.5 for the derivative with the PPh ancillary ligand.
time scale.
3
HAT Reactivity. From a thermodynamic perspective,
aminophenolate complexes 1a and 1b are related to oxidized
complexes 3a and 3b by HAT. To test for a viable HAT
pathway relating these complexes, reactions with both
TEMPO• and TEMPO-H (BDFE = 66.5 kcal mol for
In the cases of the protonated derivatives 1a and 1b, an
irreversible reduction leads to the observation of redox features
consistent with the formation of complexes [2a] and [2b] ,
⊖
⊖
respectively. Upon an initial scanning in the cathodic direction
−1
+
/0
between −0.6 and −2.0 V versus [Cp Fe] , samples of 1a
2
3
TEMPO-H) were investigated. Aminophenolate complexes
showed an irreversible reduction at −1.76 V (Figure 5,
bottom). When the scan window was widened to include
potentials from −0.3 to −2.8 V, additional, reversible, one-
electron processes were observed at −2.74 and −0.64 V versus
•
1
a and 1b rapidly reacted with 1 equiv of TEMPO , resulting
in the formation of 3a and 3b, respectively, along with
TEMPO-H, in quantitative spectroscopic yields. Conversely,
when 3a or 3b were treated with 1 equiv of TEMPO-H, no
forward reaction was observed with complete recovery of the
starting materials after several hours at room temperature.
+
/0
[
Cp Fe] after the initial irreversible reduction. These redox
2
processes correspond to the reduction and oxidation processes,
respectively, of the [2a]⊖ anion, suggesting that the initial,
1
irreversible reduction of 1a is followed by the loss of / equiv
2
DISCUSSION
of H in an EC-type mechanism (Scheme 2). Analogous
■
2
Redox Behavior. The [SNS] ligand platform derived from
bis(2-mercapto-4-methylphenyl)amine is a direct structural
analogue of the well-known [ONO] pincer ligand derived from
Scheme 2
29
bis(3,5-di-tert-butyl-2-phenol)amine. When coordinated to a
metal ion, the [ONO] ligand is stable in three different
oxidation states: the trianionic catecholate form, the dianionic
semiquinonate radical form, and the monoanionic quino-
11,30
nate.
A key question to answer about the [SNS] platform
is the degree of redox activity present in the ligand that
contains sulfur donors in place of the oxygen atoms. While
dithiolate ligands were some of the first noninnocent ligand
platforms to be studied, the substitution of sulfur for oxygen in
catecholate-type ligands typically results in a decrease in the
redox activity because sulfur does not form strong π bonds
with carbon as readily as oxygen does.
electrochemical behavior is observed for 1b, with the
irreversible reduction occurring at −1.60 V versus
+
/0
The synthesis of anionic [SNS]Ni(PR ) ([2a]⊖ and
⊖
[
Cp Fe] . To test for the generation of hydrogen gas upon
2
3
⊖
reduction of 1a and 1b, an MeCN solution of 1b was treated
[2b] ) and the observation of a reversible, one-electron
with 1 equiv of decamethylcobaltocene (Cp* Co). Analysis of
the reaction headspace by thermal conductivity gas chroma-
tography revealed the formation of 0.45 equiv of hydrogen gas
oxidation in the cyclic voltammogram of each anion provided
an opportunity to examine the redox activity of the [SNS]
ligand platform. For both PCy and PPh derivatives, these
2
3
3
(
90% yield), whereas UV−vis spectroscopy of the resulting
anionic nickel complexes contained diamagnetic, square-
⊖
⊖
solution confirmed the quantitative formation of [2b] .
pKa Determinations. Given that the aminophenolate
planar, nickel(II) centers. The [SNS] ligand in [2a] and
⊖
[2b] is consistent with a trianionic pincer-type ligand, as has
complexes [SN(H)S]Ni(PR ) (1a and 1b) and the amido-
been previously observed when the ligand is coordinated to
3
E
DOI: 10.1021/acs.inorgchem.8b00618
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
2
0,41−43
3,49
tungsten or molybdenum.
[
When metalation of the
Scheme 4.
For the PCy derivative, 1a, a pK of 17.4 ± 0.5
3
a
+
/0
SNS] ligand platform was carried out in air, oxidized versions
of the nickel complexes were obtained. In the case of the PCy
derivative, 3a, the product was a square-planar, S = / nickel
complex. In the case of the PPh derivative, the complex
and an E°′(0/−) of −0.64 V versus [Cp Fe] afforded an
2
3
1
2
Scheme 4
3
dimerizes in the solid state as {3b} but appears to establish a
2
rapid monomer−dimer equilibrium based on solution EPR
spectroscopic and electrochemical data. This dimerization
behavior is reminiscent of the behavior of some copper(II)
thiolate complexes with ligands that favor planar geo-
4
4−46
metries.
b show the type of single-derivative signal at g = 2.00 that is
diagnostic for ligand-centered radicals in oxidized, semi-
At 298 K, the X-band EPR spectra of 3a and
3
1
1,35
quinonate ligands.
Upon cooling of the sample to 10 K,
the signal broadens to a rhombic pattern that is still centered at
gave = 2.01. This rhombic character and the slight deviation of
the g values from 2.00 are suggestive of nickel d orbital
35
participation in the SOMO. Consistent with this suggestion,
the low-temperature EPR spectra of 3a and 3b both include
31
hyperfine coupling to the P of the phosphine ligand,
indicating that the SOMO is at least partially delocalized
onto the nickel ion. Authentic nickel(III) complexes typically
−1
N−H BDFE of 63.9 ± 0.8 kcal mol . In the case of the PPh
3
derivative 1b, a slightly more acidic pK of 15.9 ± 0.5 and an
a
show rhombic spectra with more pronounced deviations of g
ave
E°′(0/−) of −0.61 V afforded an N−H BDFE of 62.4 ± 0.8
47
from the free electron. For example, the nickel(III) complex
derived from the N,N′-(2,6-dimethylphenyl)-2,6-pyridinedi-
carboxamide pincer ligand displays a rhombic EPR spectrum at
−1
kcal mol . Similar N−H BDFE values were recently reported
for bidentate aminophenolate ligands coordinated to palladium
50
and platinum. The calculated N−H BDFE values for 1a and
b are similar, owing to the minor electronic impact that
results from the difference between ancillary PCy and PPh
ligands, respectively. Assuming that the difference in the steric
profiles does not impact the N−H bond strength, the
difference between these two ancillary ligands lies in their
2
7
7
7 K with g values of 2.00, 2.23, and 2.32. By way of
1
−
comparison, oxidation of [PNP]NiCl ([PNP] = N[2-
P(CHMe ) -4-methylphenyl] ) by one electron generates an
S = / complex with a slightly rhombic EPR at 292 K (g =
3
3
2
2
2
1
2
2
.01, 2.02, and 2.04), which was assigned as a nickel(II)
48
species with the hole residing mostly on the [PNP] ligand.
donor ability. While PCy is a marginally stronger donor than
3
−
1
−1 33
To further elucidate the electronic structures of 3a and 3b,
DFT calculations were carried out on monomeric 3b. These
computations support the contention that the unpaired
electron and the hole in 3b are localized on the [SNS] ligand
with only a small contribution from the nickel ion. The
presence of an unpaired electron on the [SNS] ligand of 3a
and monomeric 3b constitutes evidence for one-electron
PPh (2056 cm vs 2069 cm ), this difference in donor
3
ability has contradictory effects on the calculated BDFE. While
the stronger donor PCy ligand pushes more electron density
toward the [SN(H)S]Ni fragment, this donation manifests a
3
higher pK but a more negative E°′(0/−). As a result, the N−
a
H BDFE values for 1a and 1b are within experimental error of
one another. It is worth noting that more dramatic changes to
the ancillary ligand may result in a larger spread in the BDFE
values.
⊖
ligand-based redox activity. As such, the conversion of [2] to
3
can be written as a primarily ligand-based oxidation, with the
[
SNS] ligand adopting a trianionic, catecholate oxidation state
The calculated N−H BDFE values for 1a and 1b are
consistent with the preliminary HAT reactivity studies. The
addition of a stoichiometric quantity of TEMPO to a solution
⊖
cat 3−
in [2] (denoted as [SNS ] ) and a dianionic, open-shell,
sq• 2−
•
semiquinonate oxidation state in 3 (denoted as [SNS ] in
Scheme 3).
of 1a or 1b resulted in a rapid reaction to produce TEMPO-H
and 3a or 3b, respectively. Conversely, the reverse reactions
using 3a and 3b with stoichiometric TEMPO-H did not
produce any observable 1a and 3b. These results are consistent
3
Scheme 3
−
1
with a BDFE for TEMPO-H of 66.5 kcal mol in MeCN.
The BDFE values measured for 1a and 1b fall at the low end of
the range typically observed for transition-metal HAT or
PCET systems, especially for those where the hydrogen atom
or proton binds to a nitrogen or oxygen atom of a coordinated
3
ligand. Given that the trends in the ligand-based redox
potentials are fairly well established, it stands to reason that a
great deal of control over the BDFE, and hence the
thermodynamics of HAT, can be realized through judicious
choice of the redox-active ligand platform in a given metal
complex.
HAT Reactivity. Complexes 1−3 are related by electron-
and proton-transfer and HAT reactions. Given the conclusion
that the redox change associated with the oxidation of 2 to 3 is
localized primarily on the [SNS] ligand platform, then the
HAT reaction that relates complexes 1 and 3 also must be
localized primarily on the ligand platform. Hess’ law allows for
EXPERIMENTAL SECTION
■
determination of the N−H BDFE associated with the HAT
General Considerations. All manipulations were carried out
sq•
reaction relating 1 and [SNS ]Ni(PR ) (3), as outlined in
using standard vacuum-line, Schlenk-line, and glovebox techniques
3
F
DOI: 10.1021/acs.inorgchem.8b00618
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
5
1
unless otherwise noted. Hydrocarbon and ethereal solvents were
Crystallographic Methods. X-ray diffraction data were collected
at low temperature on a single crystal covered in Paratone and
mounted on a glass fiber. Data were acquired using a Bruker SMART
APEX II diffractometer equipped with a CCD detector using Mo Kα
radiation (λ = 0.71073 Å), which was wavelength-selected with a
single-crystal graphite monochromator. The SMART program pack-
age was used for determination of the unit-cell parameters and for
sparged with argon before being deoxygenated and dried by serial
passage through Q5 and activated alumina columns. To test for
effective oxygen and water removal, aliquots of each solvent were
treated with a few drops of a purple solution of sodium benzophenone
ketyl radical in tetrahydrofuran (THF). Halogenated solvents were
sparged with argon and dried by passage through two activated
alumina columns. Acetonitrile-d (CD CN) was dried over calcium
hydride and distilled under reduced pressure prior to use.
Triphenylphosphine (Sigma) and tricyclohexylphosphine (20 wt %
in toluene, Sigma) were all used as received. The ligand precursor
57
data collection. The raw frame data were processed using SAINT
3
3
5
8
and SADABS to yield the reflection data file. Subsequent
59
calculations were carried out with SHELXTL. The structures were
2
solved by direct methods and refined on F by full-matrix least-squares
2
0
bis(2-mercapto-4-methylphenyl)amine and the nickel starting
materials NiCl (PR ) (R = Cy, Ph) were prepared as previously
techniques. Analytical scattering factors for neutral atoms were used
52
60
throughout the analyses. Hydrogen atoms were generated in
2
3 2
described in the literature. Ferrocene and decamethylferrocene
Acros) were purified by sublimation, and tetrabutylammonium
hexafluorophosphate (Acros) was recrystallized from ethanol three
times and dried under vacuum.
Spectroscopic Methods. Elemental analyses were conducted on
calculated positions and refined using a riding model. ORTEP
61
(
diagrams were generated using ORTEP-3 for Windows.
Theoretical Calculations. All calculations were performed with
53
62,63
TURBOMOLE 7.2
using the TPSS meta-generalized-gradient-
36
approximation (meta-GGA) functional. For computational effi-
a PerkinElmer 2400 Series II CHNS elemental analyzer. NMR spectra
ciency, initial geometry optimizations were performed using a
moderate split-valence plus polarization basis set (def2-SVP) and
37
were collected on a Bruker Avance 600 MHz spectrometer in dry,
1
degassed CD CN. H NMR spectra were referenced to tetrame-
then refined using a triple-ζ valence plus polarization basis set (def2-
3
38
thylsilane using the residual proteo impurities of the solvent (1.94
TZVP). Atomic coordinates from the solid-state structure of 3a
54
31
1
ppm for CD CN). All P{ H} NMR spectra were referenced to
were used as the starting point for the geometry optimization of
3
phosphoric acid using an external standard (85% H PO ). Chemical
[SNS]Ni(PCy ); no molecular symmetry was imposed. For
3
4
3
shifts are reported using the standard δ notation in parts per million.
Electronic absorption spectra were recorded with a Jasco V-670
absorption spectrometer or a Cary 60 UV−vis spectrometer equipped
with fiber-optic cables. UV−vis spectra were recorded for samples
dissolved in dry, degassed acetonitrile (MeCN) and contained in 10
mm quartz cells. Electrospray ionization mass spectrometry (ESI-MS)
data were collected on samples dissolved in dry, degassed MeCN or
THF using a Waters LCT Premier mass spectrometer. Electron
paramagnetic resonance (EPR) spectra were collected on a Bruker
EMX X-band spectrometer equipped with an ER041XG microwave
bridge.
monomeric 3b, the solid-state structure of dimeric {3b} was split
2
in half and used as the starting point for the geometry optimization of
[SNS]Ni(PPh ). Molecular geometries and orbital energies were
3
evaluated self-consistently to tight convergence criteria (energy
converged to 0.1 μHartree; maximum norm of the Cartesian gradient
−
4
≤10 au). Molecular orbital and spin-density surface images were
64
rendered using Visual Molecular Dynamics (VMD 1.9.3) software.
[SN(H)S]Ni(PCy ) (1a). Under a N atmosphere, a 20 mL
3
2
scintillation vial was charged with NiCl (PCy ) (673 mg, 0.977
2
3 2
mmol, 1.00 equiv), toluene (7 mL), and a magnetic stir bar. To the
stirring suspension was added [SNS]H (256 mg, 0.980 mmol, 1.00
3
Electrochemical Methods. Electrochemical data were collected
on a Gamry Series G 300 potentiostat/galvanostat/ZRA (Gamry
Instruments, Warminster, PA) using a standard three-electrode
configuration comprising a 3.0 mm glassy carbon working electrode,
a platinum wire auxiliary electrode, and a silver wire pseudoreference
electrode. Electrochemical experiments were performed at 25 °C in a
equiv) dissolved in 2 mL of toluene. The addition of triethylamine
(275 μL, 1.97 mmol, 2.02 equiv) caused an immediate color change
to dark orange. The mixture was stirred for 30 min at ambient
temperature, filtered through a glass frit, and concentrated to
approximately 5 mL under reduced pressure. The addition of 100
mL of pentane did not induce precipitation; however, when the total
solution volume was reduced to approximately 20 mL, an orange
microcrystalline solid formed. This solid was collected by filtration,
washed with cold pentane, and dried under reduced pressure to afford
the product in 51% yield (256 mg). X-ray-quality crystals of the
product were obtained from an MeCN solution of the complex at 25
°C. Anal. Calcd for C H NNiPS : C, 64.28; H, 7.96; N, 2.13.
glovebox under an atmosphere of N . Electrochemical samples were
2
1
.0 mM analyte solutions in MeCN containing 0.1 M [Bu N][PF ] as
4 6
the supporting electrolyte. All potentials were referenced to the
+/0
[
−
Cp Fe] couple using ferrocene or decamethylferrocene (E°′ =
2
55
0.59 V) as an internal standard.
pKa Determinations. The pKa values for 1a and 1b were
32
46
2
1
determined by spectrophotometric titration following published
Found: C, 64.22; H, 7.75; N, 2.34. H NMR (500 MHz, CD CN): δ
3
56
methods. In a typical experiment, an MeCN stock solution of the
SN(H)S]Ni(L) complex (L = PPh , 1.67 mM; L = PCy , 1.42 mM)
7.83 (br s, 1H, −NH), 6.99 (m, 2H, aryl-H), 6.85 (m, 2H, aryl-H),
[
6.57 (m, 2H, aryl-H), 2.19 (m, 6H, −CH ), 2.05 (m, 6H, −Cy), 1.74
3
3
3
3
1
1
was prepared inside a nitrogen-filled glovebox. Aliquots (200 μL)
were removed from the stock solution and diluted with 3.0 mL of
MeCN to give concentrations between 60 and 70 μM and a total
solution volume of 3.20 mL. After an initial spectrum was obtained,
the solution was titrated with 2,4,6-collidine (2.5−10 μL per addition)
using a volumetric syringe. Spectral changes after each addition were
recorded with a Cary 60 UV−vis spectrometer equipped with fiber-
optic cables.
(m, 15H, −Cy), 1.28 (m, 9H, −Cy). P{ H} NMR (162 MHz,
−1 −1
CD CN): δ 33.9 (s). UV−vis [MeCN; λ , nm (ε, M cm )]: 244
3
max
+
(27000), 270 (22500), 374 (2200). ESI -MS (THF): m/z 598
([M] ).
+
[SN(H)S]Ni(PPh ) (1b). Under a N atmosphere, a 20 mL
3
2
scintillation vial was charged with NiCl (PPh ) (655 mg, 1.00
2
3 2
mmol, 1.00 equiv), toluene (7 mL), and a magnetic stir bar. To the
stirring suspension was added [SNS]H (261 mg, 1.00 mmol, 1.00
3
Gas Chromatography. To quantify hydrogen production,
headspace analyses were performed using an Agilent 7890B gas
chromatograph system equipped with a thermal conductivity detector
equiv) dissolved in 2 mL of toluene. An immediate color change was
observed from dark blue to maroon. After 5 min of stirring,
triethylamine (280 μL, 2.00 mmol, 2.00 equiv) was added, which
resulted in the immediate precipitation of an orange solid. The solid
was collected on a glass filter frit and washed with toluene (2 × 20
mL) and pentane (3 × 20 mL). The orange solid was collected and
dried under vacuum (423 mg, 73%). X-ray-quality crystals were
obtained from an MeCN solution of the complex at 25 °C. Anal.
Calcd for C H NNiPS : C, 66.23; H, 4.86; N, 2.41. Found: C,
(
GC-TCD). The GC-TCD system was equipped with a 30 m HP-
Molesieve column (catalog no. 19095P-MS6) with a diameter of
.530 mm and a film thickness of 25 μm. The method for hydrogen
separation consisted of a 4 min run with an inlet temperature of 200
C (splitless), a detector temperature of 220 °C, a 40 °C isotherm,
0
°
−
1
and a flow rate of 4.7 mL min for the nitrogen carrier gas. The
retention time of molecular hydrogen under these conditions was 2.26
min. Hydrogen was quantified for the method using a calibration
curve obtained from six known hydrogen concentrations.
3
2
28
2
1
65.37; H, 4.78; N, 2.33. H NMR (500 MHz, CD CN): δ 8.31 (br s,
3
1H, −NH), 7.78 (m, 5H, aryl-H), 7.52 (m, 2H, aryl-H), 7.42 (m, 6H,
aryl-H), 7.20 (m, 6H, aryl-H), 6.89 (m, 4H, aryl-H), 6.62 (m, 2H,
G
DOI: 10.1021/acs.inorgchem.8b00618
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
aryl-H), 2.33 (s, 6H, CH3). 31P{ H} NMR (162 MHz, CD CN): δ
1
CD CN): δ 7.66 (m, 2H, aryl-H), 7.64 (m, 2H, aryl-H), 7.62 (m, 4H,
3
3
−
1
−1
2
9.0 (s). UV−vis [MeCN; λ , nm (ε, M cm )]: 260 (23700).
aryl-H), 7.60 (m, 4H, aryl-H), 7.51 (m, 15H, PPh ), 7.44 (m, 15H,
max
3
+
+
ESI -MS (THF): m/z 579 ([M] ).
PPh ) (note: −CH resonances for the [SNS] ligand were not
3
3
K(2.2.2-crypt){[SNS]Ni(PCy )} (2a). Under a N atmosphere, a 20
mL scintillation vial was charged with KH (43 mg, 1.1 mmol, 3.2
equiv), THF (3 mL), and a magnetic stir bar. A solution of [SNS]H3
distinguishable because they overlapped with proteo-MeCN in
3
2
3
1
1
CD CN). P{ H} NMR (162 MHz, CD CN): δ 26.0 (s). UV−vis
−1 −1
3
3
[MeCN; λ , nm (ε, M cm )]: 282 (46800), 378 (sh, 13700), 661
max
+
+
(
89 mg, 0.34 mmol, 1.0 equiv) in THF (3 mL) was added, resulting in
(8200). ESI -MS (THF): m/z 1194 ([M] ).
the liberation of gas. Once the gas evolution had ceased, solid
NiCl (PCy ) (236 mg, 0.343 mmol, 1.00 equiv) had an immediate
One-Electron Reduction of 1b. A 50 mL round-bottomed flask
was charged with a magnetic stir bar, complex 1b (3.4 mg, 5.8 μmol),
2
3 2
color change to dark greenish blue. The mixture was stirred for 30
min and filtered through a fritted glass filter. The filtrate was
concentrated to approximately 1 mL, and 100 mL of pentane was
added to force the precipitation of a baby-blue solid. The solid was
collected by filtration, washed with pentane (3 × 20 mL), and dried
under vacuum to afford 161 mg of product (74%). X-ray-quality
and decamethylcobaltocene (Cp* Co; 2.5 mg, 7.6 μmol). The round-
2
bottomed flask was sealed with a ribbed rubber septum inside a
nitrogen-filled glovebox. To the vessel was added by syringe 15 mL of
dry, degassed MeCN. The initial bright-orange solution rapidly turned
to emerald green. The mixture was stirred for 10 min before a sample
of the headspace was removed using a gastight syringe. Analysis of this
gas by GC-TCD confirmed the formation of 2.7 μmol of hydrogen
(0.45 equiv, 90% yield). Analysis of the reaction solution by UV−vis
crystals were obtained by diffusion of diethyl ether (Et O) into a THF
2
solution of the complex and 2.2.2-cryptand at 25 °C. Anal. Calcd for
C H N NiPO S K (with 2,2,2-cryptand): C, 59.28; H, 8.06; N,
spectrometry revealed the quantitative formation of [CoCp*
{[SNS]Ni(PPh )}.
Reactions of 1a and 1b with TEMPO . In a typical experiment, a
]-
2
5
0
81
3
6 2
1
4
.15. Found: C, 58.93; H, 8.19; N, 3.80. H NMR (600 MHz,
3
•
CD CN): δ 7.27 (s, 2H, aryl-H), 6.87 (d, J = 1.60 Hz, 2H, aryl-H),
3
6
.36 (m, 2H, aryl-H), 2.16 (s, 6H, −CH ), 2.06 (m, 6H, −Cy), 1.79
20 mL scintillation vial was charged with 1 equiv of the nickel
3
•
(
m, 15H, −Cy), 1.75 (m, 3H, −CH (PCy )), 1.28 (d, J = 7.78 Hz,
aminophenolate complex 1a or 1b and 1 equiv of TEMPO in 15 mL
3
31
1
3
9
H, −Cy). P{ H} NMR (162 MHz, CD CN): δ 33.0 (s). UV−vis
of MeCN. The initial orange color of the solution rapidly turned to
dark blue. After 1 h, the reaction volatiles were removed under
reduced pressure, and the product was washed with pentane. Analysis
of the solid residue by UV−vis and NMR or EPR spectroscopy
revealed the formation of the corresponding oxidized nickel complex
3a or 3b.
1
−1
[
(
MeCN; λ , nm (ε, M− cm )]: 236 (33600), 276 (31000), 368
max
+
+
−
10700). ESI -MS (THF): m/z 635 ([M + K] ). ESI -MS (THF):
−
m/z 596 ([M] ).
K(2.2.2-crypt){[SNS]Ni(PPh )} (2b). Complex 2b was prepared
3
using a procedure analogous to that used for the preparation of 2a.
Using 255 mg of [SNS]H (0.977 mmol, 1.00 equiv), 117 mg of KH
Reactions of 3a and 3b with TEMPO-H. In a typical experiment, a
20 mL scintillation vial was charged with 1 equiv of the nickel
complex 3a or 3b and 1 equiv of TEMPO-H. The resulting dark-blue
solution was stirred for 1 h with no observed color change. After
removal of the volatiles under reduced pressure and washing of the
solid residue with pentane, spectroscopic analysis (UV−vis and EPR
or NMR) showed only the presence of an unreacted nickel starting
material with no evidence for the formation of 1a or 1b, respectively.
3
(
2.93 mmol, 3.00 equiv), and 638 mg of NiCl (PPh ) afforded 400
2 3 2
mg of product as a microcrystalline purple solid (83% yield). X-ray-
quality crystals were obtained by the diffusion of Et O into an MeCN
2
solution of the complex and 2,2,2-cryptand at 25 °C. Anal. Calcd for
C H NNiPS K: C, 62.15; H, 4.40; N, 2.26. Found: C, 60.16; H,
3
2
27
2
1
4
7
6
6
.35; N, 2.17. H NMR (600 MHz, CD CN): δ 7.82 (m, 5H, aryl-H),
3
.43 (d, J = 7.10 Hz, 2H, aryl-H), 7.39 (t, J = 7.21 Hz, 10H, aryl-H),
.80 (br s, 2H, aryl-H), 6.43 (d, J = 8.24 Hz, 2H, aryl-H), 2.14 (br s,
31
1
H, −CH ). P{ H} NMR (162 MHz, CD CN): δ 36.0 (s). UV−vis
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00618.
3
3
■
1
−1
+
[
MeCN; λ , nm (ε, M− cm )]: 294 (29200), 364 (12300). ESI -
max
*
S
+
MS (MeCN): m/z 617 ([M + K] ).
[
SNS]Ni(PCy ) (3a). In air, a 250 mL round-bottomed flask was
3
charged with NiCl (PCy ) (553 mg, 0.802 mmol, 1.00 equiv) in
2
3 2
toluene (50 mL) to give a magenta suspension. The proligand
SNS]H (208 mg, 0.797 mmol, 0.984 equiv) dissolved in 3 mL of
[
X-ray diffraction data, NMR and EPR spectra, and
3
toluene was added to the suspension but did not result in an
immediate color change. Triethylamine (500 μL, 3.60 mmol, 3.70
equiv) was added by syringe, resulting in an immediate color change
to a dark-blue/purple solution with the concomitant formation of a
precipitate. The reaction mixture was stirred for 2 h, and the volatiles
were removed under reduced pressure. The solid residue was
suspended in THF (50 mL) and stirred for 30 min. The suspension
spectrophotometric pK titrations (PDF)
a
Accession Codes
CCDC 1831023−1831028 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
was filtered to remove Et NHCl, and the solvent was removed from
3
the filtrate. The dark-greenish-blue solid was dissolved in 5 mL of
THF, and excess pentane was added to induce the precipitation of a
dark-blue solid in 36% yield (215 mg). X-ray-quality crystals of the
product were obtained from a concentrated analyte solution in MeCN
at 25 °C. Anal. Calcd for C H NNiPS : C, 64.33; H, 7.49; N, 2.34.
AUTHOR INFORMATION
Corresponding Author
*E-mail: aheyduk@uci.edu.
ORCID
■
3
2
45
2
−
1
Found: C, 64.29; H, 7.59; N, 2.80. UV−vis [MeCN; λmax, nm (ε, M
−
1
+
cm )]: 271 (16700), 300 (14200), 376 (6100), 677 (1900). ESI -
+
MS (THF): m/z 596 ([M] ).
Alan F. Heyduk: 0000-0003-2183-7099
{
[SNS]Ni(PPh3)}2 ({3b} ). Following the same procedure as that
2
used for the preparation of 3a, 1.25 g of NiCl (PPh ) (1.91 mmol,
Notes
2
3 2
1
.00 equiv) was suspended in toluene (70 mL) in a 250 mL round-
bottomed flask equipped with a magnetic stir bar. To the suspension
The authors declare no competing financial interest.
was added 504 mg of [SNS]H 1.93 mmol, 1.01 equiv) and 800 μL of
3
ACKNOWLEDGMENTS
This work was funded by the National Science Foundation
Grant CHE-1464832). The authors thank Prof. Andy Borovik
for the use of his EPR spectrometer and Claudia Ramirez for
■
(
triethylamine (5.80 mmol, 3.00 equiv) in 75 mL of toluene, which
afforded 626 mg of product (56% yield) as a blue-green solid. X-ray-
quality crystals were obtained from a concentrated analyte solution in
MeCN at 25 °C. Anal. Calcd for C H N Ni P S : C, 66.34; H, 4.70;
6
4
54
2
2 2 4
1
N, 2.42. Found: C, 65.97; H, 5.36; N, 2.34. H NMR (600 MHz,
assistance with fitting EPR spectra.
H
DOI: 10.1021/acs.inorgchem.8b00618
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
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22) Gunanathan, C.; Milstein, D. Bond Activation and Catalysis by
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