Influence of Pyridine on the Multielectron Redox Cycle of Nickel Diethyldithiocarbamate

Article abstract of DOI:10.1021/acs.inorgchem.9b02430

Two-electron (2e^-)-transfer reactions for monometallic complexes of first-row transition metals are uncommon because of the tendency of these metals to proceed through sequential one-electron (1e^-)-transfer pathways. For this chemistry to be observed, structural changes upon electron transfer are often needed to shift the 1e^- redox potentials to a condition of potential inversion where 2e^- transfer becomes favorable. Nickel(II) dithiocarbamate complexes take advantage of these conditions to drive 2e^- oxidation from Ni^II to Ni^IV. Here, we have studied the electrochemistry of Ni^II(dtc)₂, where dtc^- is N,N-diethyldithiocarbamate in an acetonitrile solvent as a function of the scan rate and added pyridine to gain further insight into the mechanism for its 2e^- oxidation to [Ni^IV(dtc)₃]⁺. The scan rate dependence revealed evidence for an ECE mechanism in which the chemical step constituted ligand exchange between [Ni^III(dtc)₂]⁺ and Ni^II(dtc)₂. A pseudo-first-order rate constant for this reaction of 34 s^-1 was obtained at 1 mM Ni^II(dtc)₂. The addition of pyridine to the electrolyte solution showed pronounced changes to the cyclic voltammetry (CV) that were consistent with the formation of a pyridine-bound Ni^III complex, [Ni^III(dtc)₂(py)₂]⁺, which was stable at high scan rates but decomposed to [Ni^IV(dtc)₃]⁺ at low scan rates. The observed decomposition rate constant was well modeled with two parallel decay pathways, one through the dipyridine [Ni^III(dtc)₂(py)₂]⁺ and another through a monopyridine [Ni^III(dtc)₂py]⁺. Overall, these data point to a mechanism for oxidation from Ni^II(dtc)₂ to [Ni^IV(dtc)₃]⁺ that proceeds through an undercoordinated [Ni^III(dtc)₂]⁺ complex, which can be trapped on the time scale of CV experiments using pyridine ligands. These studies provide insight into how we may be able to control 1e^- versus 2e^- redox chemistry using the coordination environment and nickel oxidation state.

Full text of DOI:10.1021/acs.inorgchem.9b02430

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Influence of Pyridine on the Multielectron Redox Cycle of Nickel
Diethyldithiocarbamate
Chase S. Richburg and Byron H. Farnum*
Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, United States
S
* Supporting Information
ABSTRACT: Two-electron (2e−)-transfer reactions for monome-
tallic complexes of first-row transition metals are uncommon
because of the tendency of these metals to proceed through
sequential one-electron (1e−)-transfer pathways. For this chemistry
to be observed, structural changes upon electron transfer are often
needed to shift the 1e− redox potentials to a condition of potential
inversion where 2e− transfer becomes favorable. Nickel(II)
dithiocarbamate complexes take advantage of these conditions to
drive 2e− oxidation from NiII to NiIV. Here, we have studied the
electrochemistry of NiII(dtc)2, where dtc− is N,N-diethyldithiocar-
bamate in an acetonitrile solvent as a function of the scan rate and
added pyridine to gain further insight into the mechanism for its 2e−
oxidation to [NiIV(dtc)3]+. The scan rate dependence revealed evidence for an ECE mechanism in which the chemical step
constituted ligand exchange between [NiIII(dtc)2]+ and NiII(dtc)2. A pseudo-first-order rate constant for this reaction of 34 s−1
was obtained at 1 mM NiII(dtc)2. The addition of pyridine to the electrolyte solution showed pronounced changes to the cyclic
voltammetry (CV) that were consistent with the formation of a pyridine-bound NiIII complex, [NiIII(dtc)2(py)2]+, which was
stable at high scan rates but decomposed to [NiIV(dtc)3]+ at low scan rates. The observed decomposition rate constant was well
modeled with two parallel decay pathways, one through the dipyridine [NiIII(dtc)2(py)2]+ and another through a monopyridine
[NiIII(dtc)2py]+. Overall, these data point to a mechanism for oxidation from NiII(dtc)2 to [NiIV(dtc)3]+ that proceeds through
an undercoordinated [NiIII(dtc)2]+ complex, which can be trapped on the time scale of CV experiments using pyridine ligands.
These studies provide insight into how we may be able to control 1e− versus 2e− redox chemistry using the coordination
environment and nickel oxidation state.
changes in the oxidation state. The differences in environments
A and B could be due to changes in the coordination number,
isomerization, or both. The differences in energy E1A° − E1B°
and E2A° − E2B° represent the difference in free energy
between the oxidized and reduced states in each coordination
environment, ΔG1AB and ΔG2AB, respectively. A large differ-
ence indicates that environment B is greatly preferred by the
higher oxidation state over the lower oxidation state. Likewise,
environment A is highly favored by the lower oxidation state
compared with the higher oxidation state. Potential inversion
therefore arises in this system if the most oxidized state MIV
strongly prefers environment B and the most reduced state MII
strongly prefers environment A. Under these conditions, a two-
INTRODUCTION
■
Electron-transfer reactions with first-row transition metals
typically follow one-electron (1e−) pathways. Therefore, many
synthetic first-row metal complexes that have shown multi-
electron reactivity incorporate multiple metal centers, where
each is oxidized/reduced by 1e−.1−16 For complexes that
achieve multielectron redox chemistry at monometallic centers,
however, structural changes around the metal center and/or
noninnocent ligands have been utilized to force a multielectron
pathway over single electron transfer.17−28 In these examples,
the 1e− redox potentials associated with the metal center are
shifted from their normal ordering to a condition known as
potential inversion.29
Figure 1 illustrates the concept of structure-induced
potential inversion. Here, E1A° and E2A° are 1e− redox
potentials associated with changes in the oxidation state of a
metal center, where the coordination environment (A) is
unchanged between oxidation states. These could be imagined
as the MIV/III (E1A°) and MIII/II (E2A°) reduction potentials in
their normal ordering; i.e., a more cathodic potential is
required to reduce MIII → MII than MIV → MIII. A change in
the structure to a new coordination environment (B) produces
two new reduction potentials at E1B° and E2B° for the same
electron (2e−) redox potential then emerges at E2e ° = (E2A° −
−
E1B°)/2. Importantly, the mechanism of the 2e− redox event
occurs through a rapid sequence of 1e− MIII/II and MIV/III redox
reactions, where the MIII oxidation state is an unstable
intermediate.
Examples of such structural changes known to induce
potential inversion include ligand coordination, ligand
Received: August 10, 2019
© XXXX American Chemical Society
A
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Figure 2. Chemical structures of NiII(dtc)2 and [NiIV(dtc)3]+, where
dtc− is N,N-diethyldithiocarbamate.
nickel, the 1,1-dithio functionality of the dithiocarbamates is
more difficult to oxidize than the metal, and therefore metal-
based oxidation is observed.
Although 2e− oxidation is highly favored, reduction of NiIV
back to NiII occurs through a different pathway involving two
sequential 1e− reductions (i.e., [NiIV(dtc)3]+ → NiIII(dtc)3 →
NiII(dtc)2) because of the strongly chelated cis-octahedral
environment. Previous studies have reported the electro-
chemistry of NiII(dtc)2 molecules but have not investigated the
mechanistic details for conversion between NiII and NiIV
oxidation states. All 2e−-transfer reactions must proceed
through 1e− intermediates, even if the intermediates are
extremely short-lived and conversion to the 2e− product is
rapid.29 This means that details surrounding the NiIII oxidation
state are greatly important in understanding the kinetic and
thermodynamic factors that dictate 2e− versus 1e− redox
chemistry. Research efforts to better understand what factors
influence this complicated redox cycle could lead to the
development of reversible 2e− couples based on nickel
dithiocarbamates. It is also notable that the ability of nickel-
based catalysts to proceed through 2e− versus 1e− pathways
has become an area of great interest.43−52
Figure 1. Electrochemical energy diagram showing how structural
changes between coordination environments (A and B) can induce
potential inversion of the 1e− redox potentials. In the hypothetical
redox cycle, the MIV oxidation state prefers environment B and the
MII oxidation state prefers environment A, thus forcing the MIV/III
potential (E1B°) more cathodic than the MIII/II potential (E2A°). A 2e−
−
redox potential will thus be favored at E2e ° = (E2A° − E1B°)/2.
dissociation, and changes in the ligand hapticity.17,20−23,26−28
For example, 2e− reduction of [NiII(bpy)3]2+, where bpy is
2,2′-bipyridine, results in the formation of Ni0(bpy)2 and loss
of a bpy ligand.22 Likewise, the reduction of some manganese
and ruthenium arene complexes has shown a decrease in the
hapticity from η6 to η4 upon 2e− reduction.17,27,28 In general,
increases in the coordination number around the metal center
lead to more cathodic reduction potentials (i.e., environment
B), while decreases lead to more anodic potentials (i.e.,
environment A). The energy required for the structural change
to induce potential inversion is typically large, and therefore
simple ligand coordination/dissociation without reorganization
of the coordination environment often does not result in 2e−
behavior. For example, axial ligand coordination/dissociation
to metal centers in tetraazamacrocycles does not typically
result in 2e− redox chemistry; however, there are some noted
cases.30,31
One class of molecules that is interesting in this context is
nickel dithiocarbamates (NiII(dtc)2), which are known to
undergo 2e− oxidation from NiII to NiIV.20,21,32 This reaction
takes advantage of structural changes around the metal center
to influence the 1e− NiIV/III and NiIII/II reduction potentials to
achieve potential inversion. Specifically, the oxidation process
converts a square-planar NiII(dtc)2 molecule into octahedral
[NiIV(dtc)3]+. Reorganization of the dithiocarbamate ligand
framework from a square-planar environment to cis-octahedral
coupled with the coordination of a third dtc− ligand imparts a
significant amount of reorganization to the system that
influences the 1e− reduction potentials. The structures of
both oxidation states for a number of dithiocarbamate ligands
have been characterized, with their general structures shown in
Figure 2.33−38 The assignment of the NiIV oxidation state is
unique from the perspective of other highly oxidized nickel−
sulfur complexes such as nickel dithiolenes where the 1,2-
dithio functionality of the ligand results in ligand-based
oxidation instead of the metal.39−42 When coordinated to
Lachenal proposed that favorable 2e− oxidation from NiII to
NiIV is due to the formation of an unstable [NiIII(dtc)2]+
intermediate following 1e− oxidation of NiII(dtc)2; however,
no evidence for this species has been presented. The
unsaturated coordination environment of this intermediate
may therefore be susceptible to influence by the addition of
ancillary ligands such as pyridine, which would allow for
competitive mechanistic studies to be performed. Here we
present an electrochemical study on the influence of pyridine
on the multielectron redox cycle of a NiII(dtc)2 complex, where
dtc− is N,N-diethyldithiocarbamate. The scan rate dependence
of NiII(dtc)2 with and without added pyridine uncovers new
information about the mechanism of 2e− transfer and the
importance of reactions related to transferring dithiocarbamate
ligands to oxidized nickel centers. Specifically, the addition of
pyridine shows the ability to trap intermediate NiIII oxidation
states prior to ligand exchange and subsequent oxidation to
NiIV.
EXPERIMENTAL SECTION
■
Synthesis and Characterization of NiII(dtc)2. Nickel(II)
diethyldithiocarbamate [NiII(dtc)2] was prepared as previously
described.20 A total of 2 equiv of sodium diethyldithiocarbamate
trihydrate (Sigma, >99.0%) was added to 1 equiv of nickel(II)
chloride hexahydrate (Alfa Aesar, 98%) dissolved in distilled water. A
green solid precipitated immediately and was filtered under vacuum
and washed with cold distilled water, ethanol, and ether with 96%
1
yield. Characterization of the light-green solid was performed by H
B
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Figure 3. (a) CV data for 1 mM NiII(dtc)2 in MeCN (0.1 M TBAPF6) at room temperature. The current is shown normalized to the scan rate (v)
by dividing by v1/2. Shifts in the peak currents and peak potentials indicate the presence of redox reactions coupled to chemical steps in a
homogeneous solution. (b) Epa(IV/II) plotted versus log(v).
NMR (acetonitrile-d3): δ 3.57 (q, −CH2−), 1.17 (t, −CH3) and UV−
vis spectroscopy [λmax, nm (ε, M−1 cm−1)]: 388 (5600), 323 (26700).
Electrochemistry. Tetrabutylammonium hexafluorophosphate
(TBAPF6; Sigma-Aldrich, 98%) was recrystallized in absolute ethanol,
then dried under vacuum, and stored in a desiccator. All cyclic
voltammetry (CV) experiments were performed with 0.1 M TBAPF6
and acetonitrile (MeCN; VWR Chemicals, HPLC grade) electrolytes
in a nitrogen-purged environment. Experiments were performed at
room temperature using a WaveDriver 20 bipotentiostat (Pine
Research) with a glassy-carbon-disk working electrode (Pine
Research, 5 mm diameter), a Ag/Ag+ nonaqueous reference electrode
(BASi Instruments) with 0.001 M AgNO3 in MeCN, and a platinum-
wire counter electrode. The glassy carbon working electrode was
polished with 0.05 μm alumina powder (Allied High Tech Products
Inc., DeAgglomerated). The reduction potential of ferrocene (Fc; Alfa
Aesar, 99%) was recorded before and after all electrochemical
experiments to confirm the consistency in the reference electrode. All
potentials are reported versus the Fc+/0 couple. All experiments were
collected after compensating for the internal solution resistance and
consisted of three continuous cycles starting at Eapp = −1.02 V and
scanned with an initial positive direction over the potential range.
Data shown are those of the third cycle. Pyridine (Sigma-Aldrich,
anhydrous, 99.8%) titrations were performed directly from a sample
of pure pyridine (12 M) purged with nitrogen.
Spectroelectrochemistry. Spectroelectrochemical experiments
were performed with a WaveDriver 20 bipotentiostat using 0.1 M
lithium bis(trifluoromethane)sulfonimide (Sigma, 99.95%) in MeCN
as the electrolyte. In this experimental setup, both the working and
counter electrodes were present on a platinum honeycomb electrode
(Pine Research), and the reference electrode was a low-profile Ag/Ag+
nonaqueous electrode (Pine Research). [NiII(dtc)2] in all electro-
chemical experiments was 1.0 mM. UV−vis absorbance spectra were
collected simultaneously using an AvaAspec-2048 fiber-optic spec-
trometer (Avantes) and a thin-path-length (2 mm) cuvette. Spectra
were recorded for each applied potential after being held for a period
of 30 s to ensure the establishment of redox equilibrium.
collected at 77 K and performed by fitting the cavity with a liquid-
nitrogen finger dewar. Spectra were recorded with a field modulation
frequency of 100 kHz, a modulation amplitude of 6.00 G, a
microwave power of 1.995 mW, and a frequency of 9.370 GHz.
RESULTS
■
Electrochemistry. CV experiments performed with
NiII(dtc)2 in MeCN solvents were qualitatively consistent
with those previously reported by Martin in acetone and
Lachenal in MeCN.20,21 Figure 3a shows the CV data for 1
mM NiII(dtc)2 as a function of the scan rate (v) with the
current normalized to v1/2, with peak potentials and peak
currents reported in Tables S1−S7. At v = 100 mV s−1, an
irreversible oxidation is observed at Epa(IV/II) = 0.27 V
assigned to the oxidation of four-coordinate NiII(dtc)2 to six-
coordinate [NiIV(dtc)3]+ via eq 1. The stoichiometry of this
process is complex because of the ligand-exchange reaction
that must occur in order to yield [NiIV(dtc)3]+. The
consumption of a secondary NiII(dtc)2 molecule results in an
overall number of electrons transferred per NiII(dtc)2 of n =
4/3, despite NiII being oxidized by 2e− to NiIV. Controlled
potential electrolysis experiments have confirmed this
stoichiometry.21 Subsequent reduction of [NiIV(dtc)3]+ to
NiII(dtc)2 occurs through two irreversible reductions at
Epc(IV/III) = −0.27 and Epc(III/II) = −0.74 V via eqs 2 and
3, respectively. Although all three redox waves are electro-
chemically irreversible, the entire cycle is reproducible and
always regenerates NiII(dtc)2 upon returning to cathodic
potentials.
3NiII(dtc)2 F 2[NiIV(dtc)3]+ + NiII + 4e−
(1)
2{[NiIV(dtc)3]+ + e− F NiIII(dtc)3}
(2)
Electrochemical Modeling. DigiElch, version 8, was used to
model the CV data and generate a working curve for analysis of the
rate constants in the presence of pyridine. The Supporting
Information provides detailed information regarding the methods
and parameters.
Electron Paramagnetic Resonance (EPR). EPR samples were
prepared by dissolving NiII(dtc)2, pyridine, and acetylferrocenium
(AcFc+) in the proper ratio in a MeCN solvent and quickly
transferring to an EPR tube, where the sample was frozen using liquid
nitrogen. AcFc+ was synthesized according to previous reports.53
Continuous-wave EPR spectra were measured at the X-band (9 GHz)
frequency on a Bruker EMX spectrometer fitted with an ER-4119-HS
(high-sensitivity) perpendicular-mode cavity. All measurements were
2NiIII(dtc)3 + NiII + 2e− F 3NiII(dtc)2
(3)
The scan rate dependence of NiII(dtc)2 oxidation demon-
strates that Epa(IV/II) shifts positive with increasing scan rates.
Great care was taken to correct for internal solution resistance
to ensure the accuracy of the peak potentials. Figure 3b
exhibits that, at low scan rates, Epa(IV/II) versus log(v) shows
a linear dependence with a slope of 29 mV dec−1, while at high
scan rates, the data are linear with a slope of 58 mV dec−1. This
behavior is consistent with an ECE/DISP mechanism for the
oxidation of NiII to NiIV described by eqs 4−7.21,54−56 Here, E
C
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stands for an electrochemical step at the electrode surface, C
stands for a chemical reaction step in a homogeneous solution,
and DISP refers to an inherent disproportionation reaction
that is competitive with the second electrochemical step.
Within this mechanistic framework, electrochemical oxidation
from NiII(dtc)2 to [NiIII(dtc)2]+ occurs first (eq 4), followed
by ligand exchange with a second NiII(dtc)2 to produce
NiIII(dtc)3 (eq 5). Finally, NiIII(dtc)3 may be either oxidized by
the electrode via the second E step (eq 6) or disproportionate
with [NiIII(dtc)2]+ (eq 7). Because of the fact that the chemical
step consumes a second NiII(dtc)2 molecule, this mechanism is
specifically referred to as a radical substrate dimerization
(RSD-ECE/DISP) and is often observed for the electro-
chemistry of halides, sulfides, organic radicals, and some
organometallic species.54,56−58
Extrapolation of the low-scan-rate data in Figure 3b to the
condition of log(v) = 0 allowed for kc = 34 s−1 to be estimated.
On the basis of the complex stoichiometry of the ligand-
exchange reaction in eq 5, this observed first-order rate
constant is likely the result of a rate-determining step such as a
reaction between [NiIII(dtc)2]+ and NiII(dtc)2, which could be
taken as a pseudo-first-order reaction with excess NiII(dtc)2.
Taking the general assumption that D = 10−5 cm2 s−1,
extrapolation of high-scan-rate data to log(v) = 0 yielded an
estimate of α = 0.51 based on the slope of 58 mV dec−1 and kE
= 0.018 cm s−1 from the intercept.54,56,59
Epa(IV/II) = E° + 1.15(RT/nF) − (RT/2nF) ln λC
(8)
Epa(IV/II) = E° + 1.15(RT/αnF) − (RT/2αnF) ln λE
E
NiII(dtc)2 F [NiIII(dtc)2]+ + e−
2[NiIII(dtc)2]+ + NiII(dtc)2 F 2NiIII(dtc)3 + NiII
(9)
(4)
The reduction of [NiIV(dtc)3]+ to NiIII(dtc)3 by eq 2 showed
very little dependence on the scan rate, indicating that the
reduction occurs with a large electron-transfer rate constant.
Figure S3 shows a plot of Epc(IV/III) versus log(v), where the
cathodic peak remains between −0.26 and −0.27 V from 20 to
1600 mV s−1. In contrast, the reduction of NiIII(dtc)3 to
NiII(dtc)2 via eq 3 exhibits an obvious dependence of Epc(III/
II) on the scan rate (Figure S4). The slope of Epc(III/II) versus
log(v) is 38 mV dec−1 and indicates an EC mechanism
described by eqs 10 and 11. Here, 1e− reduction of NiIII(dtc)3
to [NiII(dtc)3]− occurs as the E step and ligand exchange with
a NiII ion occurs as the C step. Epc(III/II) can be analyzed by
eq 12, where λC has the same definition as above to determine
an apparent rate constant of kc(III/II) = 1.0 s−1 for the
chemical step. Given the complexity of the reaction shown in
eq 11, this observed rate constant is best viewed as that for a
rate-determining step. We do not presently know the nature of
this step but could envision [NiII(dtc)3]− dissociation or
reaction with a NiII ion as a possible pathway.
C
(5)
E
NiIII(dtc)3 F [NiIV(dtc)3]+ + e−
(6)
DISP
+ [NiIV(dtc)3]+
[NiIII(dtc)2]+ + NiIII(dtc)3 F NiII(dtc)2
(7)
The slope of 58 mV dec−1 at high scan rates indicates that
Epa(IV/II) is controlled by the quasi-reversible electron-
transfer kinetics associated with the oxidation of NiII(dtc)2 to
[NiIII(dtc)2]+. The slope of 29 mV dec−1 at low scan rates
indicates that the kinetics describing the ligand-exchange
reaction are exhibiting chemical control over the magnitude of
Epa(IV/II). In this regime, the scan rate is slow enough to allow
for the chemical step to proceed appreciably and thus control
the concentration of [NiIII(dtc)2]+ at the electrode surface.
The second electron-transfer step needed to oxidize NiIII(dtc)3
to [NiIV(dtc)3]+ or the disproportionation reaction has no
bearing on Epa(IV/II); however, they do influence the peak
current, ipa(IV/II).54,56 As shown in Figure S1, the scan-rate-
normalized current, ipa(IV/II)v−1/2, increases for low scan rates
because of the ability for the chemical step to sufficiently
produce NiIII(dtc)3 and thus allow for further oxidation. At
appreciably fast scan rates, the reduction of [NiIII(dtc)2]+ at the
electrode surface occurs faster than ligand exchange and the
current stabilizes to a value consistent with 1e− transfer.
The rate constants for eqs 4 and 5 can be extracted from the
scan rate dependence of Epa(IV/II) using eqs 8 and 9,
respectively.54,56 Here, E° is the standard reduction potential
for the [NiIII(dtc)2]+/NiII(dtc)2 redox couple, R is the ideal gas
constant, T is the temperature (298 K), F is Faraday’s constant,
n is the number of electrons transferred in eq 4 (n = 1), α is the
electron-transfer exchange coefficient, and λ is the unitless
kinetic parameter associated with each step. For a chemically
controlled step, λc = kcRT/nFv, where kc is the apparent first-
order rate constant (s−1) for the chemical step. For an
E
NiIII(dtc)3 + e− F [NiII(dtc)3]−
(10)
(11)
C
2[NiII(dtc)3]− + NiII F 3NiII(dtc)2
Epc(III/II) = E° − 0.78(RT/nF) + (RT/2nF) ln λC
(12)
The normalized peak current for NiIII(dtc)3 reduction
[ipc(III/II)v−1/2] was also observed to increase prominently
with faster scan rates (Figure S1). We believe that this behavior
is consistent with a preceding chemical step that controls the
concentration of NiIII(dtc)3 prior to reduction by eq 3. We
believe this step to be the reverse ligand-exchange reaction of
eq 5, i.e., 2NiIII(dtc)3 + NiII → 2[NiIII(dtc)2]+ + NiII(dtc)2.
The reduction of [NiIII(dtc)2]+ by the electrode or
disproportionation by eq 7 could then result in NiII(dtc)2.
To test this hypothesis, excess dithiocarbamate ligands
Na(dtc) were added to the electrolyte to coordinate free NiII
ions produced during oxidation. Figure 4 shows voltammo-
grams measured with 1 mM Na(dtc) and 1 mM NiII(dtc)2 as a
function of the scan rate. The reduction of NiIII(dtc)3 to
NiII(dtc)2 near −0.75 V exhibits a much larger scan-rate
dependence in the presence of excess dtc− than in its absence.
Much like the data shown in Figure 3, ipc(III/II) increases with
larger scan rates. However, in the presence of dtc−, ipc(III/II)
increases to values equal to that of ipc(IV/III). This indicates
that NiIII(dtc)3 is further preserved in the presence of excess
2
electrochemically controlled step, λE = kE RT/αnFDv, where kE
is the rate constant for electron transfer at the electrode surface
(cm s−1) and D is the diffusion coefficient (cm2 s−1) of the
redox-active molecule. The standard reduction potential for
the [NiIII(dtc)2]+/NiII(dtc)2 redox couple was estimated to be
E° = 0.25 V based on data collected between 4000 and 40000
mV s−1 (Figure S2), where the reduction of [NiIII(dtc)2]+
could be observed by outcompeting the chemical step.
D
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MeCN with controlled additions of pyridine (py) to
investigate the effect of a more strongly coordinating solvent
ligand. Figure 5a shows CV data collected at 1000 mV s−1 for 1
mM NiII(dtc)2 in MeCN over a range of [py] = 0−383 mM.
The NiII(dtc)2 oxidation peak was found to shift in a cathodic
direction from +0.30 to −0.02 V over this concentration range,
while the reversibility of the redox wave increased. The
increase in the reversibility of this shifted redox feature was
concomitant with a noticeable loss in the peak current
associated with the reduction of [NiIV(dtc)3]+ , i.e., ipc(IV/
III). This could also be observed in the scan-rate-dependent
normalized voltammograms shown in Figures 5b and S5,
where the rise of the new cathodic peak near −0.1 V at higher
scan rates occurred with a decrease in the peak current for
ipc(IV/III).
Figure 4. CV data for 1 mM NiII(dtc)2 with 1 mM Na(dtc) in MeCN
(0.1 M TBAPF6) at room temperature.
The peak splitting of the shifted redox wave was found to be
ΔEp = Epa − Epc = 62 mV at v = 100 mV s−1 and 91 mV at v =
1000 mV s−1, consistent with a quasi-reversible 1e−-transfer
reaction. This new redox wave is therefore proposed to be the
result of NiII(dtc)2 oxidation to a pyridine-coordinated
[NiIII(dtc)2(py)x]+ complex (eq 13). EPR experiments
confirmed the presence of NiIII using AcFc+ as a chemical
oxidant (Figure S6). The observed signal was in the range of g
dithiocarbamate ligands. We believe that this behavior is due to
the coordination of excess dtc− ligands with NiII ions, which
lowers the concentration of NiII available for ligand exchange.
Other notable features observed in the presence of excess dtc−
include the irreversible oxidation of dtc− to dtc• at Epa = −0.45
V (100 mV s−1) and an increase in the current associated with
1
∼ 2.13, consistent with other reports of S = /2 NiIII species;
reversible oxidation from NiIII(dtc)3 to [NiIV(dtc)3]+ at Epa
=
however, the signal changed significantly with increased
pyridine concentration.60,61 Further experiments are currently
underway to decipher this behavior and characterize the
pyridine-coordinated NiIII structure. Notably, the EPR signal
was only observed in the presence of oxidant and pyridine,
with oxidant only conditions yielding and EPR-silent species
believed to be low-spin [NiIV(dtc)3]+.34
−0.21 V (100 mV s−1). This behavior has been described
previously as the chemical oxidation of NiII(dtc)2 by dtc• to
produce NiIII(dtc)3, whereby 1e− oxidation may proceed to
[NiIV(dtc)3]+.20 Interestingly, the peak potential and current
for the oxidation of NiII(dtc)2 to [NiIV(dtc)3]+ were not greatly
affected by excess dtc−. This is believed to be the result of dtc−
depletion at the electrode surface due to oxidation and
consumption at more negative potentials, thus leaving no
significant concentration available to coordinate to the
oxidized nickel center.
NiII(dtc)2 + xpy → [NiIII(dtc)2(py)x]+ + e−
(13)
[NiIII(dtc)2]+ + xpy → [NiIII(dtc)2(py)x]+
(14)
E1/2 = Eo − (RT/F) ln(K[py]x)
Pyridine Addition. The ligand-exchange reaction shown in
eq 5 is clearly an important step in both the oxidation of
NiII(dtc)2 and the reduction of NiIII(dtc)3. The details of this
reaction are not currently well understood and likely involve
multiple individual steps to achieve the overall reaction. We
surmise that the solvent must play a role in stabilizing species
such as [NiIII(dtc)2]+, NiII ions, or possible intermediates such
as bridged dimers capable of facilitating ligand exchange. Thus,
CV experiments were conducted for NiII(dtc)2 dissolved in
(15)
Equation 13 likely proceeds by an EC mechanism where
NiII(dtc)2 is first oxidized to [NiIII(dtc)2]+, followed by the
addition of pyridine. This is reasoned based on the equilibrium
constant for NiII(dtc)2(py)2 formation reported to be 0.087
1
M−2 and supported by H NMR and UV−vis data collected
here, which show little change in the NiII(dtc)2 properties over
the concentration range of pyridine studied (Figures S7 and
Figure 5. (a) CV data of 1 mM NiII(dtc)2 in MeCN with added pyridine at v = 1000 mV s−1. (b) CV data of 1 mM NiII(dtc)2 and 383 mM
pyridine in MeCN collected as a function of the scan rate. All data were collected with a 0.1 M TBAPF6 electrolyte at room temperature.
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S8).62 Using this value, we calculate [NiII(dtc)2(py)2] = 0.013
mM at the highest concentration of pyridine (383 mM), or
1.3% of the total NiII(dtc)2 in solution. The equilibrium
constant for pyridine coordination to [NiIII(dtc)2]+ (eq 14)
was determined using the shift in E1/2 as a function of the
pyridine concentration (Table 1) according to eq 15, where E°
called pure kinetic region that is achieved when the redox wave
is completely irreversible.54−56 Furthermore, the quasi-
reversibility of the pyridine-coupled wave is actually related
to the decomposition of [NiIII(dtc)2(py)2]+, not its formation.
Evidence for this assignment is found in the inverse
dependence of the cathodic peak currents between the
[NiIII(dtc)2(py)2]+/NiII(dtc)2 and [NiIV(dtc)3]+/NiIII(dtc)3
redox couples (Figures 5b and S5), which suggests that
irreversible decomposition of [NiIII(dtc)2(py)2]+ into
[NiIV(dtc)3]+ must be occurring on the time scale of the CV
experiment. We can therefore view the decomposition reaction
as the rate-determining chemical step of an ECE/DISP
mechanism, where the first E step is the pyridine-coupled
oxidation to [NiIII(dtc)2(py)2]+ and the second E step is
oxidation of the decomposition product to [NiIV(dtc)3]+.
The rate constant for decomposition (kdec) was determined
by evaluating the ratio of the cathodic-to-anodic peak currents
Table 1. Summary of E1/2 and kdec Measured for the
NiIII(dtc)2(py)2/NiII(dtc)2 Redox Couple as a Function of
the Pyridine Concentration
[py]/mM
E
1/2/V vs Fc+/0
k
dec/s−1
0
3
6
12
25
49
98
195
383
0.14
0.12
0.09
0.06
0.03
0.00
−0.03
−0.06
9.9
6.3
4.2
2.8
2.1
1.6
1.0
0.8
(ipc/ipa) for the [NiIII(dtc)2(py)2]+/NiII(dtc)2 wave.54,63,64
A
plot of ipc/ipa versus log(v) is shown in Figure 7a, where kdec
was determined using a working curve for an ECE/DISP
mechanism generated using DigiElch software (Table S9).
Values of kdec over a range of pyridine concentrations are given
in Table 1. For smaller values of kdec, irreversibility (ipc/ipa = 0)
of the redox wave is obtained at lower scan rates. As kdec
increases, the ipc/ipa curve shifts uniformly toward higher scan
rates; thus, the potential must be scanned faster to obtain a
quasi-reversible wave. Note that the reversible limit in Figure
7a occurs at ipc/ipa = 0.72. This is due to that fact that
uncorrected peak currents were used for analysis in order to
avoid errors in correcting for background, nonfaradic currents.
Working curves were generated based on the same principle.
Table 1 shows that the decomposition rate constant
decreases with higher [py], consistent with stabilization of
[NiIII(dtc)2(py)2]+. Furthermore, the inverse dependence on
pyridine indicates that decomposition of [NiIII(dtc)2(py)2]+ to
yield [NiIV(dtc)3]+ must involve an equilibrium reaction that
dissociates pyridine from the nickel metal center. Therefore,
we propose that decomposition of [NiIII(dtc)2(py)2]+ occurs
through two parallel reaction pathways: one in which
dipyridine [NiIII(dtc)2(py)2]+ decays to [NiIV(dtc)3]+ and
another in which monopyridine [NiIII(dtc)2py]+ decays to
[NiIV(dtc)3]+. The second pathway is controlled by the
equilibrium constant between the di- and monopyridine-
coordinated species shown in eq 16. If a steady-state
approximation is made for the decay of [NiIII(dtc)2py]+, then
the parallel reaction scheme can be described by an overall kdec
given by eq 17. The rate constants k2 and k1 describe the
decomposition of [NiIII(dtc)2(py)2]+ and [NiIII(dtc)2py]+,
respectively, while k+py and k−py describe the rate constants
for pyridine coordination and dissociation, respectively, of the
second pyridine ligand with NiIII. A simplified expression can
be obtained with k1′ = k1/K3,py2, where K3,py2 = k+py/k−py, which
is suitable for fitting the data. A complete derivation of this
expression is shown in the Supporting Information.
= 0.25 V is the standard potential for the [NiIII(dtc)2]+/
NiII(dtc)2 couple. A plot of K[py]x versus [py] is shown in
Figure 6, where a clear second-order dependence on pyridine
(x = 2) can be observed, indicating that [NiIII(dtc)2(py)2]+ is
the ultimate product. The equilibrium constant for the
coordination of two pyridine ligands was extracted with a
quadratic fit to yield an average estimate of K = 1.4 ( 0.2) ×
106 M−2, obtained using data collected over a range in the scan
rates of 100−1600 mV s−1 (Table S8). On the basis of this
equilibrium constant, E1/2 = −0.24 V would be expected in a
pure pyridine solvent ([py] = 12.4 M). Indeed, Figure S9
shows the CV data collected in pure pyridine, where E1/2
=
−0.28 V was experimentally measured, consistent with our
calculated estimate.
Figure 6. Determination of the equilibrium constant [K = 1.4 ( 0.2)
× 106 M−2] for pyridine coordination to [NiIII(dtc)2]+ based on eqs
14 and 15
[NiIII(dtc)2(py)2]+ F [NiIII(dtc)2py]+ + py
(16)
k1k−py
k1′k−py
k1′ + k−py[py]
The rate constant for pyridine coordination could not be
measured using scan-rate-dependent techniques like those used
to analyze the chemical step in the absence of pyridine, given
the fact that the addition of pyridine results in a quasi-
reversible redox feature. These methods are only valid when
the chemically coupled electron-transfer reaction is in the so-
kdec = k2 +
= k2 +
k1 + k+py[py]
(17)
Analysis of kdec as a function of [py] is shown in Figure 7b,
where the solid line represents a fit to eq 17. The extracted rate
constants are k2 = 0.54 s−1, k1′ = k1/K3,py2 = 0.10 M s−1, and
F
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Figure 7. (a) Ratio of the cathodic-to-anodic peak currents associated with the [NiIII(dtc)2(py)2]+/NiII(dtc)2 redox couple plotted versus log(v).
The decomposition rate constant (kdec) was determined for each [py] using the working curves (dashed lines) generated for an ECE/DISP
mechanism. (b) Plot of kdec versus [py] showing a decrease in kdec for higher [py]. The solid overlaid line represents a fit to eq 17 with the rate
constants given in the figure. The dashed overlaid line represents the limiting form of eq 17 at high [py]. The inset shows a plot of kdec[py] versus
[py] to highlight the linear dependence at high [py]. Overlaid lines are the same as those of the main figure.
k−py = 7.2 s−1. At a high pyridine concentration, eq 17 takes the
limiting form kdec = k2 + k1′/[py]. Multiplication by [py]
allows for a linear expression to be obtained, kdec[py] = k2[py]
+ k1′. Fits to these limiting expressions (kdec and kdec[py]) are
shown as dashed lines in Figure 7b, with the inset plot
highlighting the linear behavior at high [py]. As the pyridine
concentration increases further, kdec is expected to asymptoti-
cally approach k2, representative of decomposition only
through [NiIII(dtc)2(py)2]+.
pyridine. We believe that this is likely due to the
decomposition of [NiIV(dtc)3]+ into NiII(dtc)2 and (dtc)2
(thiuram disulfide) that has been reported to occur in the
presence of pyridine and light.33,34
DISCUSSION
■
Oxidation/Reduction of NiII(dtc)2 in the Absence of
Pyridine. The multielectron redox cycle of NiII(dtc)2 in
MeCN can be described using the thermochemical cycle
shown in Scheme 1. Horizontal arrows indicate 1e−-transfer
The reductions of [NiIV(dtc)3]+ and NiIII(dtc)3 were not
significantly impacted by the presence of pyridine, as shown by
the similar cathodic peak positions compared to blank MeCN.
Figure S4 shows a comparison of Epc(III/II) versus log(v) for 0
and 383 mM pyridine concentrations. The slope for both plots
is 38 mV dec−1, indicating an EC mechanism, as described by
eqs 10 and 11. Analysis of the rate constant for the chemical
step in the presence of pyridine using eq 12 yields kc(III/II) =
3.3 s−1, an estimate similar to that obtained in blank MeCN.
Spectroelectrochemistry. The spectroelectrochemistry of
NiII(dtc)2 in MeCN with and without pyridine showed no
evidence for the [NiIII(dtc)2(py)2]+ intermediate and instead
showed only oxidation from NiII(dtc)2 to [NiIV(dtc)3]+ in both
cases. Figure S10a shows UV−vis absorption spectra of
NiII(dtc)2 collected as a function of the applied potential in
the absence of pyridine. At increasingly positive applied
potentials, decay of the NiII(dtc)2 absorbance spectrum with
λmax = 388 nm gives rise to a broad increase in absorbance
across the visible region with λmax = 445 nm, consistent with
the formation of [NiIV(dtc)3]+.20 Figure S10b shows an
identical experiment performed with [py] = 98 mM in which
clean conversion from NiII(dtc)2 to [NiIV(dtc)3]+ can be
observed with no intermediate absorbance features. Con-
Scheme 1. Thermochemical Cycle Describing the Multiple
Electron-Transfer and Ligand-Transfer Reactions Involved
a
in the [NiIV(dtc)3]+/NiII(dtc)2 Redox Couple
a
The redox potentials are reported versus Fc+/0
.
reactions and vertical arrows describe equilibria between bis-
and tris-chelated nickel complexes, as stated in eq 18.
According to Scheme 1, oxidation of NiII(dtc)2 to
[NiIV(dtc)3]+ must proceed through two 1e−-transfer steps
and one equilibrium. This may be achieved by three pathways:
(1) sequential oxidation of NiII(dtc)2 to [NiIV(dtc)2]2+,
followed by ligand exchange to [NiIV(dtc)3]+ (i.e., E1° → E2°
→ K4); (2) ligand exchange to [NiII(dtc)3]− followed by
sequential oxidation (i.e., K2 → E3° → E4°); or (3) oxidation
to [NiIII(dtc)2]+, followed by ligand exchange to NiIII(dtc)3
and oxidation to [NiIV(dtc)3]+ (i.e., E1° → K3 → E4°). The
ECE/DISP mechanism described by eqs 4−7 constitutes
pathway 3 and was originally proposed by Lachenal.21
version from NiII(dtc)2 to [NiIV(dtc)3]+ occurred with E1/2
=
0.10 V in the absence of pyridine and E1/2 = −0.06 V in the
presence of pyridine. This shift in E1/2 was consistent with the
CV results discussed above. The ultimate formation of
[NiIV(dtc)3]+ in the spectroelectrochemical experiments is
also consistent with the mechanism proposed from CV studies
in which [NiIII(dtc)2(py)2]+ decays to [NiIV(dtc)3]+ at slow
scan rates or, equivalently, long time scales. Notably, the
concentration of [NiIV(dtc)3]+ produced in the absence of
pyridine was larger than that produced in the presence of
2[Nix(dtc)2]x−2 + NiII(dtc)2 F 2[Nix(dtc)3]x−1 + NiII
Kx
(18)
The standard redox potential E1° = 0.25 V versus Fc+/0 was
estimated here using CV data collected at high scan rates. The
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redox potentials of E3° = −0.71 V and E4° = −0.24 V were
reported previously for electrochemical studies on
[NiIV(dtc)3]+ and are consistent with the data presented
here.21 The potential E2° for [NiIV(dtc)2]2+/[NiIII(dtc)2]+ has
not been previously reported and was not determined in the
present study. We anticipate that this potential lies at an
extremely positive voltage because of the unlikely formation of
a four-coordinate NiIV complex. Almost all NiIV complexes
reported in the literature are six-coordinate,43−45,47−49,51,65−71
where only a few examples of lower coordination have been
observed using bulky ligands that prevent higher coordination
numbers.72,73 Estimates of K2, K3, and K4 are not available in
the literature; however, the difference in E3° and E1° can be
used to estimate the ratio of equilibrium constants K3/K2 = 1.7
× 1016 based on the relationship (E1° − E3°) = (RT/F) ln(K3/
K2).
The scan-rate-dependent CV data presented here strongly
support the ECE/DISP pathway. This is evidenced by the
slopes of Epa(IV/II) versus log(v) extracted from Figure 3b,
which indicate regions of kinetic control by both chemical
(∼30 mV dec−1) and electrochemical (∼60 mV dec−1) steps as
well as the increase in the normalized peak current at low scan
rates. The overall ligand-exchange reaction expressed in eq 5
indicates complexity of the reaction beyond a simple transfer of
the dithiocarbamate ligand. Given that the only source for dtc−
ligands in the electrolyte is NiII(dtc)2, we propose that the first
step in the overall ligand-exchange process is a reaction
between [NiIII(dtc)2]+ and NiII(dtc)2, according to eq 19, to
form a sulfur-bridged dimer, [Ni2(dtc)4]+. Dithiocarbamate-
bridged dimers have been observed previously, with the
structures of the [Co2(dtc)5]+ and [Ru2(dtc)5]+ dimers
characterized to show bridging sulfur atoms, which also
chelate to the metal centers.32,74−76 Ligand-exchange reactions
between FeIII, NiII, and HgII complexes containing different
dithiocarbamate derivatives have also been well documented
and proposed to occur through dimer intermediates.32 The
ultimate fate of the proposed [Ni2(dtc)4]+ dimer must be the
generation of NiIII(dtc)3 and free NiII ions according to eq 5.
We speculate that the dimer may dissociate via eq 20 to yield
NiIII(dtc)3 and [NiII(dtc)]+. The resulting [NiII(dtc)]+, likely
coordinated to MeCN, may then undergo disproportionation
to yield free NiII ions and NiII(dtc)2 (eq 21).
8500 M−1 s−1 if an ECE pathway is operative and kLE = 17000
M−1 s−1 if a DISP1 pathway is operative. Notably, a third
possibility may occur where ligand exchange occurs as a
preequilibrium to disproportionation. This mechanism is
referred to as a DISP2 pathway and exhibits a characteristic
Epa versus log(v) slope of 19.7 mV dec−1. Given the observed
29 mV dec−1 slope, the DISP2 pathway was not considered
further.
Distinguishing between an ECE and DISP1 pathway can be
very difficult without knowledge of kD. The disproportionation
reaction described by eq 7 arises from the difference in the
redox potentials for the two intermediate NiIII species,
[NiIII(dtc)2]+ and NiIII(dtc)3. Using the reduction potentials
in Scheme 1, ΔGDISP° = −nF(E1° − E4°) = −0.49 eV can be
calculated. We note that, given the bridging nature of
dithiocarbamate ligands, this reaction may proceed through
an inner- or outer-sphere pathway. The large driving force of
the disproportionation step highlights the impact of ligand
coordination in determining the redox potential of the metal
center. For example, coordination of a third dtc− ligand to NiIII
results in a shift of 0.96 V in the NiIII/II reduction potential
from the bis- to tris-coordinated species. This large difference
in the potentials allows for a potential inversion between E1°
and E4°, resulting in thermodynamically favorable dispropor-
tionation. The key to the observed 2e− oxidation from
NiII(dtc)2 to [NiIV(dtc)3]+ is therefore the large difference in
the equilibrium constants for ligand exchange, K2 and K3. An
estimate of K2 has never been measured in the literature, likely
because of its extremely small magnitude, and [NiII(dtc)3]− has
never been isolated and characterized. The instability of this
species is due to the strong preference of NiII toward a square-
planar geometry when coordinated to dithiocarbamate ligands
as a result of its d8 electronic configuration and strong π
donation from the conjugated ligand.38 Notably, [Fe(dtc)3]−
(d6) and [Co(dtc)3]− (d7) are stable complexes, and CV
studies exhibit sequential 1e− waves for their MIV/III and MIII/II
redox couples.77,78
On the other end of the redox cycle, NiIV is present in a six-
coordinate environment with a low-spin electronic config-
uration.34 Other known examples of NiIV complexes also
exhibit low-spin, six-coordinate geometries.43−45,49,52 There-
fore, the equilibrium constant K4 in Scheme 1 is expected to be
extremely large. Considering Scheme 1 as a whole, we can see
that K4 ≫ K3 ≫ K2 and the regions of stability are tilted
toward the corners of NiII(dtc)2 and [NiIV(dtc)3]+. The
equilibrium K3 is then critical to the ability to facilitate 1e−
versus 2e− reactions. If K3 ≪ 1, only the 1e− [NiIII(dtc)2]+/
NiII(dtc)2 process would be observed; however, if K3 ≫ 1, then
the ECE mechanism described above would proceed.
[NiIII(dtc)2]+ + NiII(dtc)2 → [Ni2(dtc)4]+
(19)
[Ni2(dtc)4]+ → NiIII(dtc)3 + [NiII(dtc)]+
(20)
2[NiII(dtc)]+ → NiII(dtc)2 + NiII
(21)
The consumption of a sacrificial NiII(dtc)2 molecule during
the ECE/DISP pathway further classifies this reaction as a
radical substrate dimerization mechanism, for which there are
numerous examples, especially related to organic radi-
cals.54,56−58 For the specific case of an RSD-ECE/DISP
pathway, Saveant has shown that the rate constant for the
chemical step can be defined further by the relationship kc =
4kLE[NiII(dtc)2]0 or kc = 2kLE[NiII(dtc)2]0, where kLE is the
second-order rate constant for the ligand-exchange reac-
tion.54,56 The former expression refers to the case where kLE
is larger than the disproportionation rate constant (kD) and
represents a true ECE mechanism, where NiIII(dtc)3 is oxidized
quickly at the electrode surface. The latter expression is
reserved for the case where kD > kLE and represents a DISP1
The reduction of [NiIV(dtc)3]+ back to NiII(dtc)2 during CV
experiments occurs predominately through sequential 1e−-
transfer reactions, followed by equilibration of [NiII(dtc)3]− to
NiII(dtc)2 (i.e., E4° → E3° → K2 from Scheme 1). This is due
to the relative stability of the tris-chelated NiIII(dtc)3 complex.
However, the notable difference in the peak currents for the
two redox waves suggests some degree of instability. Lachenal
proposed that the instability of NiIII(dtc)3 was due to an
internal decomposition via oxidation of a dtc− ligand to yield
NiII(dtc)2 and (dtc)2.21 Controlled potential electrolysis of
[NiIV(dtc)3]+ at a potential between E4° and E3° indeed
revealed the ultimate production of NiII(dtc)2 and (dtc)2.
However, this reaction involved the reduction of a
[NiIV(dtc)3]+ starting material and would necessarily produce
mechanism. Using kc = 34 s−1 and [NiII(dtc)2]0 = 1 mM, kLE
=
H
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uncoordinated dithiocarbamate species.21 Electrochemical
studies presented here with Na(dtc) added to NiII(dtc)2
solutions showed that the irreversible oxidation of dtc− to
dtc• gave rise to an increase in the current for the oxidation of
NiIII(dtc)3 to [NiIV(dtc)3]+. Such an increase indicates that
spontaneous and kinetically fast oxidation of NiII(dtc)2 by dtc•
occurs in a homogeneous solution to generate NiIII(dtc)3. This
reaction is formally the reverse of that proposed by Lachenal
and clearly indicates favorability in the formation of NiIII(dtc)3,
not decomposition.
for pyridine coordination to [NiIII(dtc)2]+ are described by
K3,py1 and K3,py2, of which the product K3,py1K3,py2 = K = 1.4
( 0.2) × 106 M−2. The equilibrium constants for pyridine
coordination to NiII(dtc)2 are smaller than we can detect using
UV−vis and 1H NMR methods; however, the overall
equilibrium K2,py1K2,py2 = 0.087 M−2 has been measured
using the magnetic susceptibility in a mixture of 95% pyridine,
2.5% CH2Cl2, and 2.5% tert-butyl alcohol.62 The small
magnitude for pyridine coordination is attributed to the strong
square-planar coordination environment induced by the
dithiocarbamate ligands and has been noted previously in the
literature.38 Coordination of other nitrogen bases such as 1,10-
phenanthroline and n-hexylamine to NiII(dtc)2 have also been
noted in the literature, with the chelating 1,10-phenanthroline
(phen) yielding a significantly higher equilibrium constant
(Kphen = 2240 M−1) because of the formation of a cis-
coordinated adduct.79,80 Using the literature value for the
formation of NiII(dtc)2(py)2 and the value measured here for
the formation of [NiIII(dtc)2(py)2]+, we can estimate the redox
potential of Epy2° = −0.18 V based on Scheme 2. The
magnitude of Epy1° is still unknown without knowledge of the
individual equilibrium constants for pyridine coordination
(K3,py1, K3,py2, K2,py1, and K2,py2).
In the present case, where the oxidation of NiII(dtc)2 is
followed by the reduction of [NiIV(dtc)3]+ on the return scan,
decomposition of NiIII(dtc)3 must occur on the time scale of
the CV experiment. We propose that this happens via a
reaction between NiIII(dtc)3 and NiII ions liberated during the
oxidation step to regenerate [NiIII(dtc)2]+. Formally, this
reaction is the reverse of the ligand-exchange step shown in eq
5. Production of [NiIII(dtc)2]+ then results either in reduction
at the electrode surface where the applied potential is now less
than E1° or through the disproportionation reaction described
in eq 7.
Oxidation/Reduction of NiII(dtc)2 in the Presence of
Pyridine. The addition of pyridine to the MeCN solution
resulted in a shift of the NiII(dtc)2 oxidation peak in the
negative direction and an increase in the reversibility. The
pyridine-coupled redox feature can best be described as a
[NiIII(dtc)2(py)2]+/NiII(dtc)2 redox couple where the equili-
brium constant for pyridine coordination to [NiIII(dtc)2]+ was
determined to be K = 1.4 ( 0.2) × 106 M−2. The negative shift
in Epa(IIIpy/II) is characteristic of ligand-coupled electron
transfer (LCET). The LCET process is analogous to proton-
coupled electron transfer (PCET) in which the reduction of a
molecule is coupled with proton transfer in order to stabilize
the more electron-rich reduced species. A characteristic result
of PCET is a positive shift in the cathodic peak potential for
increased [H+]. In the case of NiII(dtc)2 oxidation, the electron
density from the coordinated pyridine ligands is necessary to
stabilize the electron-deficient NiIII metal center. Because of
this stabilization effect with added [py], a negative shift in Epa
is observed, indicating that oxidation is easier in the presence
of pyridine.
We can estimate that the concentration of NiII(dtc)2(py)2
over the range of 0−383 mM pyridine never exceeds 0.013
mM or 1.3% of NiII species based on K2,py1 2,py2 = 0.087 M−2.
K
This indicates that the pyridine-coupled oxidation observed by
CV mostly occurs through an ECC process (E1° → K3,py1
→
K
3,py2), with pyridine coordination to [NiIII(dtc)2]+ occurring
after oxidation. The reduction of [NiIII(dtc)2(py)2]+ to
NiII(dtc)2 may occur through either the reduction of
NiII(dtc)2(py)2 followed by rapid dissociation of pyridine
ligands (ECC mechanism; Epy2° → K2,py2 → K2,py1) or through
pyridine dissociation followed by the reduction of
[NiIII(dtc)2py]+ to NiII(dtc)2py before loss of the second
pyridine (CEC mechanism; K3,py2 → Epy1° → K2,py1).
The structure of the [NiIII(dtc)2(py)2]+ complex is presently
unknown, but we can speculate that the species exists in a six-
coordinate environment with either a cis or trans conformation
(Figure 8). The reversibility of the pyridine-coupled wave at
Scheme 2 summarizes the effect of pyridine coordination on
the NiIII/II reduction potential. Here, the equilibrium constants
Scheme 2. Thermochemical Cycle Describing NiIII/II
Reduction Potentials as a Function of Pyridine
Coordination
Figure 8. Proposed trans- and cis-octahedral structures for
[NiIII(dtc)2(py)2]+.
high scan rates could indicate that a trans conformation is
favored, resulting in a lower energy barrier for pyridine
coordination, where the dithiocarbamate ligand framework
would not undergo significant reorganization. However, the cis
form is more similar to the ultimate structure of [NiIV(dtc)3]+,
which exists in a tris-chelated octahedral environment. We
note that theoretical calculations for the coordination of
ethylamine to NiII(dtc)2 have indicated that the trans isomer is
7.19 kcal mol−1 more favorable.80
I
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Article
The instability of the [NiIII(dtc)2(py)2]+ complex at low
scan rates resulted in voltammograms that strongly resembled
those without pyridine but with a shifted anodic wave. The
consistency in the appearance of 1e− reduction waves for
[NiIV(dtc)3]+ and NiIII(dtc)3 at low scan rates and the
spectroelectrochemical data that show the ultimate formation
of [NiIV(dtc)3]+ indicate that [NiIII(dtc)2(py)2]+ must
decompose to [NiIV(dtc)3]+. The rate constant for this
reaction was measured using the ratio of cathodic-to-anodic
peak currents versus scan rate, where the dependence of kdec on
the pyridine concentration was well described by a parallel
decay mechanism. The two reactions are proposed to be
decomposition via [NiIII(dtc)2(py)2]+ or [NiIII(dtc)2py]+.
Within the context of the ligand-exchange mechanism outlined
above for the absence of pyridine, we speculate that the decay
of [NiIII(dtc)2(py)2]+ and [NiIII(dtc)2py]+ yields NiIII(dtc)3 as
the initial decomposed product. This is reasoned based on the
fact that ligand exchange must certainly be present in order to
form a tris-chelated nickel complex and NiIII(dtc)3 would
provide a direct precursor to [NiIV(dtc)3]+.
reaction, which occurs in blank MeCN with a rate constant of
17000 M−1 s−1. Instead, the formation of [NiIII(dtc)2py]+ and
[NiIII(dtc)2(py)2]+ result in partially stable coordination
complexes with much slower rate constants for conversion to
NiIII(dtc)3. The apparent first-order rate constants for
[NiIII(dtc)2py]+ and [NiIII(dtc)2(py)2]+ decomposition were
−1
measured to be k1 = 0.1K
s
and k2 = 0.54 s−1. Although a
3,py2
NiII(dtc)2 concentration dependence was not performed,
conversion of these values to second-order rate constants
based on [NiII(dtc)2]0 = 1 mM results in k1′ = 100K3,py2 M−1
s−1 and k2′ = 540 M−1 s−1. The parallel decay mechanism
points to a small equilibrium constant for coordination of the
second pyridine. A reasonable estimate of K3,py2 < 10 M−1 thus
results in k1′ < 1000 M−1 s−1. The rate constants k1′ and k2′ are
therefore much smaller than kLE and justify the quasi-reversible
electrochemistry observed for [NiIII(dtc)2(py)2]+. The product
of each ligand-exchange reaction is proposed to be NiIII(dtc)3,
which could undergo further oxidation to [NiIV(dtc)3]+ either
at the electrode surface or through a disproportionation
reaction in solution. Additional disproportionation reactions
emerge in the presence of pyridine because of the multiple
pyridine-coordinated NiIII species. These are described
generally by the DISP reaction in Scheme 3.
Scheme 3 offers a summary of these reaction pathways in
relation to the ECE/DISP mechanism outlined in blank
Scheme 3. Proposed ECE/DISP Mechanism Describing the
Oxidation of NiII(dtc)2 to [NiIV(dtc)3]+ in the Presence of
Pyridine
Digital Simulations of the CV Data. In order to fully
describe the complex redox cycle for the [NiIV(dtc)3]+/
NiII(dtc)2 couple in MeCN with and without pyrdine, we have
used digital simulation software (DigiElch) to model the
experimental CV data as a function of the scan rate based on
Schemes 1−3. In the case of MeCN only, known redox
potentials E1°, E3°, and E4° were held constant in addition to
the rate constants for electron transfer kE = 0.018 cm s−1,
determined here for the [NiIII(dtc)2]+/NiII(dtc)2 redox couple,
and kLE = 17000 M−1 s−1 associated with a DISP1 mechanism.
A detailed description of the fitting procedure is provided in
the Supporting Information. Figure 9a shows a comparison of
the experimental cyclic voltammograms for 1 mM NiII(dtc)2 at
100 and 1000 mV s−1 with simulations overlaid as dashed lines.
The agreement between the simulations and the experimental
data is excellent with only small differences in the region of the
NiIII(dtc)3/NiII(dtc)2 peak. Importantly, the simulations model
the changes in Epa(IV/II), ipa(IV/II), and ipc(III/II) as a
function of the scan rate quite well.
MeCN. Following oxidation from NiII(dtc)2 to [NiIII(dtc)2]+,
rapid coordination of pyridine prevents the ligand-exchange
Figure 9. Comparison of the experimental (solid lines) and simulated (black dashed lines) CV data collected for NiII(dtc)2 in (a) MeCN and (b)
MeCN with 383 mM pyridine at 100 and 1000 mV s−1.
J
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Article
On the basis of these simulations, an equilibrium constant
for eq 5 of K3 = 0.079 was calculated. From this value, an
estimate of K2 = 4.6 × 10−18 can be reasonably made. The
magnitude of K3 indicates that ligand exchange between
[NiIII(dtc)2]+ and NiII(dtc)2 is slightly disfavored. We would
have expected that K3 > 1 based on the observed 2e− oxidation
and 1e− reduction processes; however, 2e− oxidation may still
proceed if kd is sufficiently fast.54 Indeed, the disproportiona-
tion rate constant was best simulated near the diffusion limit
and was therefore held constant at 1 × 1010 M−1 s−1. The large
magnitude of this estimate is consistent with the large driving
pyridine coordination in the NiIII oxidation state, which
kinetically inhibits further oxidation to [NiIV(dtc)3]+. Organo-
metallic nickel complexes reported by Sanford and Mirica have
shown similar behavior, where scorpionate ligands have been
used to rapidly coordinate and stabilize NiIII upon 1e−
oxidation from NiII.45,46,49,50,52 Controlling this chemistry has
led to both 1e− and 2e− catalytic pathways. In the case of
nickel dithiocarbamates, both ligand coordination and
reorganization of the ligand framework appear to be important
in driving the overall 2e− oxidation from NiII to NiIV.
Coordination of pyridine alone does not lead to potential
inversion of the NiIV/III and NiIII/II redox potentials, instead
displaying 1e− redox chemistry through stabilization of NiIII.
The ultimate formation of [NiIV(dtc)3]+ even in the presence
of pyridine shows that nickel dithiocarbamate bonding is still
greatly favored and rearrangement of the ligand framework to a
cis-octahedral environment is a significant factor in driving 2e−
oxidation. Further studies are underway using derivatized
pyridine ligands to enhance the stability of [NiIII(dtc)2(py)2]+
in order to slow down conversion to [NiIV(dtc)3]+ and
produce isolable NiIII complexes for structural study.
force for disproportionation and supports our use of kLE
=
17000 M−1 s−1 (DISP1 pathway) in the simulations. The fact
that K3 is close to 1 is consistent with our proposal of
NiIII(dtc)3 decomposition through ligand exchange/dispropor-
tionation prior to being reduced at E3° = −0.71 V.
Additionally, CV data were collected with only NiII(dtc)2
present in the solution and no added NiII ions to balance
the equilibrium. Therefore, the only NiII ions in solution are
produced as a result of oxidation, the low concentration of
which may skew the equilibrium toward the favorable
formation of NiIII(dtc)3. This suggests that the addition of
NiII ions to solution may have a pronounced effect on CV.
Studies of this nature are currently under investigation in our
laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02430.
Figure 9b shows digital simulations overlaid with the
experimental CV data collected with [py] = 383 mM. All
experimentally obtained rate constants, reduction potentials,
and equilibrium constants were fixed during simulations, while
unknown values were allowed to float to obtain a best fit. The
proposed model agrees well with the experimental data but
misses some in the region of the NiIII(dtc)3/NiII(dtc)2 peak,
similar to what was observed in the absence of pyridine. A
further understanding of this reduction step is needed in order
to refine the model for better fitting in this region. Regardless,
the model does an excellent job of fitting the change in
reversibility of the [NiIII(dtc)2(py)2]+/NiII(dtc)2 couple as a
function of the scan rate.
1
CV, UV−vis absorbance, H NMR, and EPR, and CV
simulation data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: farnum@auburn.edu.
■
ORCID
Byron H. Farnum: 0000-0001-9152-1909
Notes
Given the complexity of the mechanism proposed in Scheme
3 and the limited amount of known thermodynamic and
kinetic information, the errors associated with the fitting
parameters such as the equilibrium and rate constants are
much greater than what was found in the absence of pyridine.
Nonetheless, the simulations provide support for a small
equilibrium constant between [NiIII(dtc)2(py)2]+ and
[NiIII(dtc)2py]+. The best fit to the experimental data resulted
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by startup funds provided by Auburn
University. The authors thank Eduardus Duin for help with
EPR measurements.
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