Accessing Ni(III)-thiolate versus Ni(II)-thiyl bonding in a family of Ni-N2S2 synthetic models of NiSOD

Article abstract of DOI:10.1021/ic503124f

Superoxide dismutase (SOD) catalyzes the disproportionation of superoxide (O₂^?-) into H₂O₂ and O₂(g) by toggling through different oxidation states of a first-row transition metal ion at its active site. Ni-containing SODs (NiSODs) are a distinct class of this family of metalloenzymes due to the unusual coordination sphere that is comprised of mixed N/S-ligands from peptide-N and cysteine-S donor atoms. A central goal of our research is to understand the factors that govern reactive oxygen species (ROS) stability of the Ni-S(Cys) bond in NiSOD utilizing a synthetic model approach. In light of the reactivity of metal-coordinated thiolates to ROS, several hypotheses have been proffered and include the coordination of His1-Nδ to the Ni(II) and Ni(III) forms of NiSOD, as well as hydrogen bonding or full protonation of a coordinated S(Cys). In this work, we present NiSOD analogues of the general formula [Ni(N₂S)(SR′)]^-, providing a variable location (SR′ = aryl thiolate) in the N₂S₂ basal plane coordination sphere where we have introduced o-amino and/or electron-withdrawing groups to intercept an oxidized Ni species. The synthesis, structure, and properties of the NiSOD model complexes (Et₄N)[Ni(nmp)(SPh-o-NH₂)] (2), (Et₄N)[Ni(nmp)(SPh-o-NH₂-p-CF₃)] (3), (Et₄N)[Ni(nmp)(SPh-p-NH₂)] (4), and (Et₄N)[Ni(nmp)(SPh-p-CF₃)] (5) (nmp^2- = dianion of N-(2-mercaptoethyl)picolinamide) are reported. NiSOD model complexes with amino groups positioned ortho to the aryl-S in SR′ (2 and 3) afford oxidized species (2^ox and 3^ox) that are best described as a resonance hybrid between Ni(III)-SR and Ni(II)-^?SR based on ultraviolet-visible (UV-vis), magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectroscopies, as well as density functional theory (DFT) calculations. The results presented here, demonstrating the high percentage of S(3p) character in the highest occupied molecular orbital (HOMO) of the four-coordinate reduced form of NiSOD (NiSOD_red), suggest that the transition from NiSOD_red to the five-coordinate oxidized form of NiSOD (NiSOD_ox) may go through a four-coordinate Ni-^?S(Cys) (NiSOD_ox-His_off) that is stabilized by coordination to Ni(II).

Full text of DOI:10.1021/ic503124f

Article
pubs.acs.org/IC
Accessing Ni(III)-Thiolate Versus Ni(II)-Thiyl Bonding in a Family of
Ni−N2S2 Synthetic Models of NiSOD
Ellen P. Broering,† Stephanie Dillon,‡ Eric M. Gale,† Ramsey A. Steiner,† Joshua Telser,§
Thomas C. Brunold,‡ and Todd C. Harrop*,†
†Department of Chemistry and Center for Metalloenzyme Studies, The University of Georgia, 1001 Cedar Street, Athens, Georgia
30602, United States
‡Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States
§Department of Biological, Chemical and Physical Sciences, Roosevelt University, 430 South Michigan Avenue, Chicago, Illinois
60605, United States
S
* Supporting Information
ABSTRACT: Superoxide dismutase (SOD) catalyzes the disproportionation of
superoxide (O2• −) into H2O2 and O2(g) by toggling through different oxidation states
of a first-row transition metal ion at its active site. Ni-containing SODs (NiSODs) are a
distinct class of this family of metalloenzymes due to the unusual coordination sphere
that is comprised of mixed N/S-ligands from peptide-N and cysteine-S donor atoms. A
central goal of our research is to understand the factors that govern reactive oxygen
species (ROS) stability of the Ni−S(Cys) bond in NiSOD utilizing a synthetic model
approach. In light of the reactivity of metal-coordinated thiolates to ROS, several
hypotheses have been proffered and include the coordination of His1-Nδ to the Ni(II)
and Ni(III) forms of NiSOD, as well as hydrogen bonding or full protonation of a
coordinated S(Cys). In this work, we present NiSOD analogues of the general formula
[Ni(N2S)(SR′)]−, providing a variable location (SR′ = aryl thiolate) in the N2S2 basal
plane coordination sphere where we have introduced o-amino and/or electron-
withdrawing groups to intercept an oxidized Ni species. The synthesis, structure, and properties of the NiSOD model complexes
(Et4N)[Ni(nmp)(SPh-o-NH2)] (2), (Et4N)[Ni(nmp)(SPh-o-NH2-p-CF3)] (3), (Et4N)[Ni(nmp)(SPh-p-NH2)] (4), and
(Et4N)[Ni(nmp)(SPh-p-CF3)] (5) (nmp2− = dianion of N-(2-mercaptoethyl)picolinamide) are reported. NiSOD model
complexes with amino groups positioned ortho to the aryl-S in SR′ (2 and 3) afford oxidized species (2ox and 3ox) that are best
described as a resonance hybrid between Ni(III)-SR and Ni(II)-•SR based on ultraviolet−visible (UV-vis), magnetic circular
dichroism (MCD), and electron paramagnetic resonance (EPR) spectroscopies, as well as density functional theory (DFT)
calculations. The results presented here, demonstrating the high percentage of S(3p) character in the highest occupied molecular
orbital (HOMO) of the four-coordinate reduced form of NiSOD (NiSODred), suggest that the transition from NiSODred to the
five-coordinate oxidized form of NiSOD (NiSODox) may go through a four-coordinate Ni-•S(Cys) (NiSODox-Hisoff) that is
stabilized by coordination to Ni(II).
1
ligand set, but the S = /2 Ni(III) ion is five-coordinate and
additionally bound to the imidazole-Nδ of His1 resulting in a
Ni−N3S2 square-pyramidal geometry (Figure 1).10−12
INTRODUCTION
■
Aerobic organisms utilize superoxide dismutase (SOD) metal-
loenzymes in their defense against the free radical superoxide
(O2• −), which if unregulated plays a role in a variety of disease
states.1−8 The most recently discovered class of SODs are the
nickel-containing SODs (NiSODs), which utilize Ni in a mixed
nitrogen and thiolate coordination environment to catalyze the
disproportionation of O2• − to H2O2 and O2 by cycling through
Ni(II/III) redox states (see eqs 1 and 2 in Figure 1).9 X-ray
crystal structures of NiSOD from two separate Streptomyces
strains confirm two distinct metal coordination geome-
tries.10−12 In the reduced form (NiSODred), the low-spin (S
= 0) Ni(II) ion lies in a square-planar N2S2 environment
composed of the N-terminal amine of His1, the deprotonated
peptido-N of Cys2, and two thiolato-S from Cys2 and Cys6
(Figure 1). The oxidized form (NiSODox) contains the same
Questions concerning the enzyme’s unusual N/S coordina-
tion sphere, particularly its stability toward O2 and reactive
oxygen species (ROS), in light of the reactive Ni−S(Cys)
bonds,13 and the catalytic mechanism have prompted
researchers to develop maquette-based,14−20 tripeptide,21−24
and low molecular weight25−38 analogues that seek to replicate
NiSOD’s structure and/or function.39,40 Indeed, several
experimental and theoretical efforts have suggested that the
mixed peptide-N/amine-N ligation serves as one key factor to
promote greater Ni-character in the redox-active molecular
Received: December 31, 2014
© XXXX American Chemical Society
A
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Figure 1. Structures of the NiSOD active site and the corresponding SOD half-reactions.
Chart 1. General Structure of the [Ni(nmp)(SR)]−
Complexes Reported in This Study
orbital of NiSOD, in particular for the Ni(II) or NiSODred state,
thus preventing unwanted S-oxidation/oxygenation.10,30,41 For
example, a direct electronic structure comparison of two planar
Ni(II)-N2S2 complexes (NiSODred models) with diamine/
dithiolate versus amine/carboxamide/dithiolate donors re-
vealed a significant increase in Ni character of the Ni(dπ)−
S(pπ) antibonding highest occupied molecular orbital
(HOMO) upon introduction of the carboxamide, i.e., 18%
Ni, 74% S for diamine; 33% Ni, 50% S for amine/
carboxamide.30 Direct density functional theory (DFT)
computations on truncated versions of NiSODred clearly
demonstrated more S(pπ) character than Ni(dπ) character in
the π-antibonding HOMO (25% Ni, 67% S) and HOMO−1
(37% Ni, 44% S).10 Taken together, these results support a
highly covalent Ni−S bond in NiSODred that, nonetheless, still
has significant S-character in the HOMO even with inclusion of
the Ni−Npeptide bond. Thus, the enzyme has evolved in such a
way to promote Ni-based oxidation via the destabilization of
the Ni(dπ) orbitals. This proposal has been verified by
examining the electronic structure of the DFT-generated
hypothetical four-coordinate (4C) planar version of NiSODox,
namely NiSODox-Hisoff, that revealed a HOMO composition of
58% Ni(dπ) and 24% S(pπ).10 On the other hand, the actual
NiSODox HOMO is predominantly Ni in character with the
a
a
R = 2-aminothiophenolate (2), 2-amino-4-(trifluoromethyl)-
benzenethiolate (3), 4-aminothiophenolate (4), 4-(trifluoromethyl)-
thiophenolate (5).
sphere. In this way, the electronic properties of the complex can
be evaluated and tuned by changing Sexo. Prior work on Ni(II)-
nmp systems focused on synthetic methodology32 and the
electronic influence of second-sphere N−H···S hydrogen bonds
(H-bonds).33 None of these complexes afforded a stable
oxidized species. Instead, oxidation took place at Sexo to afford
the disulfide of Sexo (through a transient thiyl radical) and 1 in
quantitative yields. We hypothesized that enforcing a potential
N-donor near the Ni center and/or decreasing the basicity of
2
unpaired electron occupying the Ni dz orbital, as verified by
electron paramagnetic resonance (EPR) spectroscopy. Thus,
the electronic structural changes from NiSODred → NiSODox-
Hisoff → NiSODox gradually move toward more Ni-character in
the HOMO. This analysis begs the question as to what
prevents NiSODred from undergoing deleterious S-based redox
chemistry upon conversion to NiSODox. Could a Ni-stabilized
•S(Cys) be traversed in such a conversion? To date, few low
molecular weight analogues of NiSOD with mixed peptide-N/
amine-N/thiolate-S coordination have been shown to achieve
S
exo would allow us to intercept a Ni(III) or Ni(II)-thiyl species.
Indeed, the presence of Naxial ligands has proven beneficial in
spectroscopically observing other Ni(III)-N2S2 complexes
related to NiSOD.36,44 Thus, the primary objective of this
work is to gauge the impact of electronically deficient Sexo and
N
axial-tethered Sexo donors on the redox properties of the
• −
catalytic O2 disproportionation. Many such complexes are
corresponding NiSOD model complexes. The synthesis,
structure, properties, and reactivity of the following Ni−N2S2
complexes will be described: (Et4N)[Ni(nmp)(SPh-o-NH2)]
(2), (Et4N)[Ni(nmp)(SPh-o-NH2-p-CF3)] (3), (Et4N)[Ni-
(nmp)(SPh-p-NH2)] (4), and (Et4N)[Ni(nmp)(SPh-p-CF3)]
(5) (Chart 1). The NiSOD model complexes with amino
groups positioned ortho to the aryl-S (2 and 3) afforded
oxidized species that are best described as a resonance hybrid
between Ni(III)-(−SR) and Ni(II)-(•SR) and are the first
examples of the room temperature (RT) observation of Ni-
stabilized thiyl radicals with relatively long (hours) lifetimes.
prone to undergo irreversible S-based rather than Ni-based
oxidation although in some cases, a transient Ni(III) species has
been trapped and observed by EPR spectroscopy.35,42,43
The work presented here adopts the methodology developed
by our group for synthesizing asymmetric, 4C planar Ni−N2S2
models of NiSOD employing the nmp2− ligand (see Chart 1;
nmp2− = dianion of N-(2-mercaptoethyl)picolinamide).32
Synthesis of such models was achieved through splitting of a
proposed S,S-bridged dimer [Ni2(nmp)2] with exogenously
added thiolate ions or Sexo (where exo refers to the exogenously
added monodentate S-ligand) that afforded a variety of Ni(II)-
N2S2 complexes. We now report this S,S-bridged species to be
the tetramer [Ni4(nmp)4] (1). The use of Sexo to complete the
N2S2 coordination sphere is synthetically novel in that it
provides a specific point of control for modeling Cys6 (see
Figure 1) at a site-differentiated location in the coordination
EXPERIMENTAL SECTION
■
General Information. All reagents were purchased from
commercial sources and used as received, unless otherwise noted.
Acetonitrile (MeCN), tetrahydrofuran (THF), diethyl ether (Et2O),
and pentane were purified by passage through activated alumina
B
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Article
columns of an MBraun MB-SPS solvent purification system and stored
under an N2 atmosphere until use. N,N-dimethylformamide (DMF)
was purified with a VAC solvent purifier containing 4 Å molecular
sieves and stored under N2. The ligand N-2-(mercaptoethyl)-
picolinamide (nmpH2; H represents dissociable protons) and its
corresponding Ni(II) complex, [Ni4(nmp)4] (1), were prepared
according to the published procedure.32 All reactions were performed
under an inert atmosphere of N2 using Schlenk line techniques or
under an atmosphere of purified N2 in an MBraun Unilab glovebox.
The syntheses of Et4N+ thiolate salts from Et4NCl and the appropriate
sodium thiolate were carried out according to previously published
methods.45,46
3.45 (t, 2H, J = 6.5 Hz, −NCH2CH2S−), 3.22 (br, 9H integrates high
due to the presence of excess Et4N+), 2.32 (t, 2H, J = 6.5 Hz,
−NCH2CH2S−), 1.25 (br, 20H integrates high due to the presence of
excess Et4N+). FTIR (KBr pellet) νmax (cm−1): 3407 (m, νNH), 3309
(m, νNH), 2974 (w), 2915 (w), 2846 (w), 1670 (m, νCO of residual
DMF), 1617 (vs, νCO), 1593 (vs, νCO), 1568 (m), 1559 (m), 1475
(m), 1437 (w), 1391 (s), 1293 (w), 1172 (w), 1093 (s), 999 (m), 786
(w), 755 (m), 687 (w), 623 (w), 558 (w), 485 (w). UV-vis (MeCN,
298 K) λmax, nm (ε, M−1 cm−1): 453 (7200). LR-ESI-MS (m/z): [M−
Et4N]− calcd for C14H14N3OS2Ni, 362.0 (100.0), 364.0 (49.3), 363.0
(18.1), 365.0 (10.3), 366.0 (10.2); found: 361.9 (100.0), 363.9 (47.4),
362.9 (14.4), 364.9 (8.5), 365.8 (9.4). HR-ESI-MS (m/z): [M−
Et4N]− calcd for C14H14N3OS2Ni, 361.994 (100.0), 363.989 (47.6),
362.996 (15.2), 364.992 (7.2), 365.986 (5.4); found: 361.994 (100.0),
363.990 (48.9), 362.997 (15.0), 364.992 (6.9), 365.986 (8.1). Anal.
Calcd for C22H34N4OS2Ni·H2O·MeCN·0.5DMF: C, 52.00; H, 7.27;
N, 13.08. Found: C, 51.39; H, 6.56; N, 13.21.
(Et4N)[Ni(nmp)(SPh-o-NH2-p-CF3)] (3). To a 6 mL DMF slurry of
1 (0.2380 g, 0.2490 mmol) was added a bright yellow DMF (3 mL)
solution of (Et4N)(SPh-o-NH2-p-CF3) (0.3012 g, 0.9342 mmol). The
solvent gradually took up a reddish-brown color over time. The
mixture was stirred under N2 for 20 h in a 40 °C water bath. The
resulting dark red, mostly homogeneous solution was filtered to
remove unreacted 1, and the filtrate was evaporated to dryness via
short-path vacuum distillation. The resulting red-burgundy residue was
dissolved in 5 mL of THF, cooled to −20 °C, and filtered through a
plug of Celite. The dark red THF filtrate was removed under vacuum
to result in a black oily material. This residue was triturated with 5 mL
of Et2O or pentane overnight to afford a burgundy-black powder
(0.4150 g, 0.6935 mmol, 74%). 1H NMR (500 MHz, CD3CN, δ from
protio solvent): 8.48 (d, 1H, J = 5.0 Hz), 8.11 (d, 1H, J = 10.0 Hz),
7.83 (t, 1H, J = 8.5 Hz), 7.51 (d, 1H, J = 10.0 Hz), 7.25 (t, 1H, J = 8.5
Hz), 6.69 (s, 1H), 6.60 (d, 1H, J = 10.0 Hz), 5.13 (s, 2H, NH2), 3.22
(t, 2H, J = 6.5 Hz, −NCH2CH2S−), 3.15 (q, 23H, integrates high due
to the presence of excess Et4N+, J = 10.0 Hz), 2.12 (t, 2H, J = 6.5 Hz,
−NCH2CH2S−), 1.19 (t, 34H, integrates high due to the presence of
excess Et4N+, J = 7.5 Hz). FTIR (KBr pellet) νmax (cm−1): 3407
(w,νNH), 3286 (w,νNH), 2980 (w), 2917 (w), 2849 (w), 1765 (w),
1620 (vs, νCO), 1592 (vs, νCO), 1483 (m), 1432 (m), 1329 (vs), 1102
(m), 1072 (m), 999 (w), 787 (w). UV-vis (MeCN, 298 K) λmax, nm
(ε, M−1 cm−1): 448 (3800). LR-ESI-MS (m/z): [M−Et4N]− calcd for
C15H13F3N3OS2Ni, 430.0 (100), 432.0 (49.5), 431.0 (19.1), 433.0
(10.8), 434.0 (10.3); found: 429.8 (100.0), 431.8 (48.0), 430.8 (16.1),
432.8 (9.1), 433.8 (8.8). HR-ESI-MS (m/z): [M−Et4N]− calcd for
C15H13F3N3OS2Ni, 429.981 (100.0), 431.977 (47.6), 430.984 (16.2),
432.979 (6.2), 433.974 (5.3); found: 429.982 (100.0), 431.977 (47.2),
430.985 (16.9), 432.980 (8.1), 433.974 (8.1). Anal. Calcd for
C23H33F3N4OS2Ni·0.5Et2O: C, 50.18; H, 6.40; N, 9.36. Found: C,
50.10; H, 6.07; N, 9.09.
(Et4N)[Ni(nmp)(SPh-p-NH2)] (4). To a 6 mL DMF slurry of 1
(0.1783 g, 0.1866 mmol) was added a pale yellow DMF solution (2
mL) of (Et4N)(SPh-p-NH2) (0.1709 g, 0.6717 mmol) dropwise at RT.
The DMF solvent gradually took up a burgundy-red color over time.
The mixture was stirred under N2 for 20 h in a 40 °C water bath. The
resulting dark burgundy, mostly homogeneous solution was filtered
through Celite to remove unreacted 1, and the filtrate was evaporated
to dryness via short-path vacuum distillation. The resulting brown-
black residue was dissolved in 5 mL of MeCN, cooled to −20 °C, and
filtered through Celite. The burgundy MeCN filtrate was removed
under vacuum and the resulting black residue was stirred in 5 mL of
Et2O overnight to afford 0.1741 g (0.3138 mmol, 47%) of an orange-
red powder. 1H NMR (500 MHz, CDCl3 containing 0.05% v/v TMS,
δ from TMS): 8.60 (d, 1H, J = 10.0 Hz), 7.69 (m, 4H), 7.12 (t, 1H, J =
10.0 Hz), 6.41 (d, 2H, J = 10.0 Hz), 3.44 (t, 2H, J = 6.5 Hz,
−NCH2CH2S−), 3.37 (s, 2H, NH2), 3.31 (q, 10H, integrates high due
to the presence of excess Et4N+, J = 10.0 Hz), 2.31 (t, 2H, J = 6.5,
−NCH2CH2S−), 1.28 (t, 15H, integrates high due to the presence of
excess Et4N+, J = 7.5 Hz). FTIR (KBr pellet) νmax (cm−1): 3315
(m,νNH), 3197 (m,νNH), 2983 (w), 2912 (w), 2841 (w), 1671 (m, νCO
of residual DMF), 1616 (vs, νCO), 1592 (vs, νCO), 1486 (s), 1455 (m),
Physical Methods. Fourier transform infrared (FTIR) spectra
were collected on a ThermoNicolet Model 6700 spectrophotometer
running the OMNIC software. All samples were run as solid samples
prepared as pressed KBr pellets. X-band (9.60 GHz) EPR spectra were
obtained using a Bruker ESP 300E EPR spectrometer controlled with a
Bruker microwave bridge at 10 K. The EPR was equipped with a
continuous-flow liquid He cryostat and a temperature controller (ESR
9) made by Oxford Instruments, Inc. EPR spectra were simulated
using the program QPOWA, as modified by J. Telser.47 Electronic
absorption spectra were collected at 298 K using a Varian Model Cary-
50 spectrophotometer containing a Quantum Northwest TC 125
temperature control unit. The UV-vis samples were prepared in gas-
tight Teflon-lined screw cap quartz cells with an optical path length of
1 cm. Cyclic voltammetry (CV) measurements were performed with a
PAR Model 273A potentiostat using a Ag/Ag+ (0.01 M AgNO3/0.1 M
nBu4NPF6 electrolyte in MeCN) reference electrode, a Pt counter
electrode, and a glassy carbon working milli-electrode (diameter = 2
mm). CV measurements were performed at ambient temperature
using 3.5 or 8.0 mM analyte in DMF under Ar containing 0.1 M
nBu4NPF6 as the supporting electrolyte. The “maximize stability” mode
and a low-pass 5.3 Hz filter were used in the PAR PowerCV software.
Analyte potentials were referenced against a 0.05−0.1 mM ferrocene
standard. 1H NMR spectra were recorded in the listed deuterated
solvent on a 400 MHz Bruker Model BZH 400/52 NMR spectrometer
or a Varian Unity Inova 500 MHz NMR spectrometer at RT with
chemical shifts internally referenced to tetramethylsilane (TMS =
Si(CH3)4), or the residual protio signal of the deuterated solvent.48
Low-resolution electrospray ionization mass spectrometry (LR-ESI-
MS) data were collected using a Bruker Model Esquire 3000 plus ion-
trap mass spectrometer. High-resolution electrospray ionization mass
spectrometry (HR-ESI-MS) data were collected using an Orbitrap
Elite system with precision to the third decimal place. Magnetic
circular dichroism (MCD) and low temperature absorption data were
collected using a Jasco Model J-715 spectropolarimeter, in conjunction
with an Oxford Instrument SM-4000 8T magnetocryostat. The sample
for these studies was prepared in a 1:4 (v/v) solvent mixture of MeCN
and butyronitrile and sparged with dry O2(g) for 5 min. Glass strain
contributions to the MCD signal were removed by taking the
difference between spectra collected with the magnetic field aligned
parallel and antiparallel to the light-propagation axis. Elemental
analysis for C, H, and N was performed at QTI-Intertek in
Whitehouse, NJ and ALS Environmental (formerly Columbia
Analytical Services) in Tucson, AZ.
(Et4N)[Ni(nmp)(SPh-o-NH2)] (2). To a 6 mL DMF slurry of 1
(0.1550 g, 0.1622 mmol) was added a yellow DMF solution (2 mL) of
(Et4N)(SPh-o-NH2) (0.1517 g, 0.5962 mmol) dropwise at RT. The
DMF gradually took up a burgundy-red color over time. The mixture
was stirred under N2 for 20 h in a 40 °C water bath. The resulting dark
red, mostly homogeneous solution was filtered through Celite to
remove unreacted 1, and the filtrate was evaporated to dryness via
short-path vacuum distillation. The resulting brown-black residue was
dissolved in 5 mL of MeCN, cooled to −20 °C, and filtered through a
plug of Celite. The dark red MeCN filtrate was removed under
vacuum, and the residue stirred in 5 mL of Et2O overnight to afford
1
0.2093 g (0.3554 mmol, 60%) of an orange-red powder. H NMR
(500 MHz, CDCl3 containing 0.05% v/v TMS, δ from TMS): 8.49 (d,
1H, J = 10.0 Hz), 8.13 (d, 1H, J = 10.0 Hz), 7.69 (m, 2H), 7.10 (t, 1H,
J = 8.0 Hz), 6.75 (t, 1H, J = 9.3 Hz), 6.48 (m, 2H), 4.81 (s, 2H, NH2),
C
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1396 (m), 1268 (w), 1174 (w), 1096 (s), 1002 (w), 817 (w), 794 (w),
766 (w), 718 (w), 704 (w), 688 (w), 631 (w). UV-vis (MeCN, 298 K),
λmax, nm (ε, M−1 cm−1): 458 (6100). LR-ESI-MS (m/z): [M−Et4N]−
calcd for C14H14N3OS2Ni, 362.0 (100.0), 364.0 (49.3), 363.0 (18.1),
365.0 (10.3), 366.0 (10.2); found: 361.9 (100.0), 363.9 (43.8), 362.9
(19.1), 364.9 (10.0), 365.9 (7.0). HR-ESI-MS (m/z): [M−Et4N]−
calcd for C14H14N3OS2Ni, 361.994 (100.0), 363.989 (47.6), 362.996
(15.2), 364.992 (7.2), 365.986 (5.4); found: 361.994 (100.0), 363.989
(47.3), 362.997 (15.5), 364.992 (6.5), 365.986 (8.0). Anal. Calcd for
C22H34N4OS2Ni·H2O·0.5THF·0.1DMF: C, 52.61; H, 7.40; N, 10.35.
Found: C, 52.74; H, 7.91; N, 10.37.
atom as O1 (one set) and O1′ (other set), respectively. The two sets
for the atoms (C1 and C2) were divided using the PART commands
and proper restraints. The set of C1 and C2 has 46% occupancy while
the other (C1′ and C2′) has 54% occupancy. The occupancies for O1
and O1′ are set at 50% each. The structure of 4 has two distinct, but
chemically indistinguishable Ni complexes. Selected data and metric
parameters for complexes 1, 2, and 4 are summarized in Table S1 in
the Supporting Information and Table 1, respectively. Perspective
views of the complexes were obtained using ORTEP.55
Table 1. Selected Bond Distances (Å) and Bond Angles
(deg) for [Ni4(nmp)4] (1), (Et4N)[Ni(nmp)(SPh-o-NH2)]
(2), and (Et4N)[Ni(nmp)(SPh-p-NH2)] (4)
(Et4N)[Ni(nmp)(SPh-p-CF3)] (5). To a 6 mL DMF slurry of 1
(0.0849 g, 0.0888 mmol) was added a pink DMF solution of
(Et4N)(SPh-p-CF3) (0.1023 g, 0.3328 mmol) dropwise at RT. The
solvent gradually took up a burgundy-red color over time. The mixture
was stirred under N2 for 20 h in a 40 °C water bath. The resulting dark
red, mostly homogeneous solution was filtered through Celite to
remove unreacted 1, and the filtrate was evaporated to dryness via
short-path vacuum distillation. The resulting brown-black residue was
dissolved in 5 mL of THF, cooled to −20 °C, and filtered through a
plug of Celite. The dark red THF filtrate was removed under vacuum
and the resulting dark black residue was stirred in 5 mL of Et2O
overnight to afford 0.0858 g (0.157 mmol, 47%) of a burgundy-black
sticky material. 1H NMR (400 MHz, CD3CN, δ from protio solvent):
8.53 (d, 1H, J = 6.0 Hz), 8.09 (d, 2H, J = 8.4 Hz), 7.83 (t, 1H, J = 8.0
Hz), 7.52 (d, 1H, J = 7.6 Hz), 7.24 (t, 1H, J = 7.0 Hz), 7.17 (d, 2H, J =
8.0 Hz), 3.23 (t, 2H, J = 6.5 Hz, −NCH2CH2S−), 3.15 (q, 14H,
integrates high due to the presence of excess Et4N+, J = 7.6 Hz), 2.16
(br, 4H, integrates high due to overlap with H2O, −NCH2CH2S−),
1.19 (t, 20H, integrates high due to the presence of excess Et4N+, J =
8.0 Hz). FTIR (KBr pellet) νmax (cm−1): 2981 (w), 2914 (w), 2839
(w), 1670 (s, νCO of residual DMF), 1623 (vs, νCO), 1593 (vs, νCO),
1560 (m), 1486 (m), 1458 (w), 1385 (s), 1323 (s), 1274 (w), 1256
(w), 1183 (w), 1155 (w), 1086 (s), 1058 (m), 1009 (w), 826 (w), 761
(w), 686 (w). UV-vis (MeCN, 298 K), λmax, nm (ε, M−1 cm−1): 432
(4600). LR-ESI-MS (m/z): [M−Et4N]− calcd for C15H12F3N2OS2Ni,
415.0 (100.0), 417.0 (49.4), 416.0 (18.7), 418.0 (10.6), 419.0 (10.3);
found: 414.8 (100.0), 416.8 (48.0), 415.8 (16.8), 417.8 (8.5), 418.7
(8.9). HR-ESI-MS measured with Na+ salt (m/z): calcd for
C15H12F3N2OS2Ni, 414.970 (100.0), 416.966 (47.6), 415.973 (16.2),
417.968 (6.2), 418.963 (5.3); found: 414.970 (100.0), 416.965 (47.3),
415.973 (15.8), 417.968 (7.1), 418.962 (8.1). Anal. Calcd for
C23H32F3N3OS2Ni: C, 50.56; H, 5.90; N, 7.69. Found: C, 49.41; H,
5.99; N, 6.29. A reasonable fit to C, H, N could not be obtained due to
the oily nature of the resulting product.
X-ray Crystallographic Data Collection and Structure
Solution and Refinement. Red needlelike crystals of 1 were
grown by Et2O layering over a CD3OD solution of Na3[{Ni-
(nmp)}3(BTAS)] (BTAS3− is the trianion of N1,N3,N5-tris(2-
mercaptoethyl)benzene-1,3,5-tricarboxamide) at RT under aerobic
atmosphere. Dark red bladelike crystals of 2 were grown by slow vapor
diffusion of Et2O into a solution of 2 in DMF:C6H5Cl (1:1) at RT. In
addition, dark red blade crystals of 4 were grown by slow vapor
diffusion of Et2O into a solution of 4 in THF:DMF (∼4:1) at RT.
Suitable crystals were mounted on a glass fiber. The X-ray intensity
data were measured at 100 K on a Bruker SMART APEX II X-ray
diffractometer system with graphite-monochromatic Mo Kα radiation
(λ = 0.71073 Å), using ω-scan technique controlled by the SMART
software package.49 The data were corrected for Lorentz and
polarization effects50 and integrated with the manufacturer’s SAINT
software. Absorption corrections were applied with the program
SADABS.51 Subsequent structure refinement was performed using the
SHELXTL 6.1 solution package operating on a Pentium computer.
The structure was solved by direct methods using the SHELXTL 6.1
software package.52,53 All non-hydrogen atoms were refined
anisotropically.54 Non-hydrogen atoms were located from successive
difference Fourier map calculations. In the structure of 1, two carbon
atoms (C1 and C2) and one oxygen atom (O1) were found
disordered in two sets of each: labeled for the carbon atoms as C1 and
C2 (one set) and C1′ and C2′ (other set); and labeled for the oxygen
a
1
2
4
Bond Lengths (Å)
Ni(1)−N(1)
Ni(1)−N(2)
Ni(1)−S(1)
Ni(1)−S(2)
1.866(4)
1.8750(16)
1.9418(16)
2.1386(6)
2.2173(6)
1.8698(14)
1.9450(14)
2.1474(5)
2.2160(5)
1.951(4)
2.1500(12)
2.2170(12)
Bond Angles (deg)
N(1)−Ni(1)−N(2)
N(1)−Ni(1)−S(1)
N(1)−Ni(1)−S(2)
N(2)−Ni(1)−S(1)
N(2)−Ni(1)−S(2)
S(1)−Ni(1)−S(2)
C(9)−S(2)−Ni(1)
83.47(16)
87.33(12)
178.39(14)
168.73(12)
95.78(11)
93.25(5)
83.36(7)
83.12(6)
88.62(5)
178.74(5)
171.97(5)
97.60(5)
90.41(2)
104.66(6)
87.88(4)
177.51(4)
168.97(4)
99.36(4)
89.621(18)
107.39(6)
108.44(17)
a
Parameters for one of the two unique, but chemically similar, Ni
complexes observed in the asymmetric unit of 4.
Computations. All computations were performed using the
ORCA 2.9.0 program developed by Dr. Frank Neese.56 The
crystallographically determined coordinates for 2 provided the initial
structures for the computational model (II) and its one-electron
oxidized derivative (IIox). Both models were then subjected to
unconstrained geometry optimizations within the framework of DFT
using Becke’s three-parameter hybrid functional and the Lee−Yang−
Parr correlation functional (B3LYP).57,58 Ahlrichs’ valence triple-ζ
basis set, TZV(P) basis set,59,60 was chosen for the Ni, N, and S atoms,
respectively, while Ahlrichs’ polarized split valence and auxiliary basis
sets,59 SV(P) and SV/C, were selected for all of the remaining atoms.
The spin-restricted formalism was chosen for II, while the spin-
unrestricted formalism was used for IIox, which was modeled as S =
1/2. Single-point DFT calculations on the geometry-optimized models
were performed using the same functional and basis sets as those used
in the geometry optimizations. The Pymol program61 was utilized to
generate isosurface plots of MOs and spin densities with isodensity
values of 0.05 and 0.005 a.u., respectively.
The ORCA 2.9.0 program was also employed to compute EPR
parameters for the geometry-optimized model IIox by using the DFT-
based coupled-perturbed self-consistent field approach in conjunction
with the B3LYP functional. This calculation was performed using the
CP(PPP) basis set62,63 for Ni, Kutzelnigg’s NMR/EPR (IGLO-III)
basis set64 for all N and S atoms, and the TZV(P) basis set for the
remaining atoms.
Oxidation of 3. To a solution of 3 containing 10.9 mg of 3 (0.0194
mmol) stirring in 4 mL of DMF was added ceric ammonium nitrate
(CAN) dropwise (1.748 mL of a 10.0 mM DMF solution, 0.0175
mmol) at RT. Upon the addition of CAN, the red-brown color
associated with the Ni(II)-N2S2 complex was replaced by a dark-green
color. The mixture was stirred for another 15 min at RT and filtered
through a 0.2 μm nylon filter to yield a green transparent DMF
solution. A red solid of a color consistent with a minor amount of 1
was evident in the filter. The oxidation was monitored by UV-vis and
EPR spectroscopy.
D
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Oxidation of 2. Chemical oxidation of 2 was performed in the
same manner as with 3, except for using 11.2 mg (0.0227 mmol) of 2
and 2.045 mL of CAN (10.0 mM DMF solution, 0.0205 mmol). The
red solution of 2 darkened to green upon the addition of CAN. Within
minutes, the green color paled with the appearance of a red precipitate.
Oxidation of 4. Chemical oxidation of 4 was performed in the
same manner as with 3, except for using 9.6 mg (0.020 mmol) of 4 and
1.752 mL of CAN (10.0 mM DMF solution, 0.0175 mmol). The
orange-red solution of 4 paled to light orange upon addition of CAN
with the appearance of a red precipitate.
complexes 2−5 were synthesized by reacting a 40 °C DMF
slurry of 1 with 4 equiv of the Et4N+ salts of various thiolates to
yield the monomeric Ni(II) complexes in modest-to-good
yields (50%−75%) (see Scheme 1). All complexes were isolated
a
Scheme 1. Synthesis of Ni(II) Complexes
Oxidation of 5. Chemical oxidation of 5 was performed in the
same manner as with 3, except for using 8.9 mg (0.016 mmol) of 5 and
1.459 mL of CAN (10.0 mM DMF solution, 0.0146 mmol). The
orange-red solution of 5 paled to light orange upon addition of CAN
with the appearance of a red precipitate.
Reactivity. UV-vis Monitoring of 3 and 3ox with NaN3 and N-
methylimidazole. To a 5 mM DMF stock (2.5 mL) of 3 was added 10
equiv of N-methylimidazole (N-MeIm) or NaN3, and the UV-vis
spectrum was recorded. Solutions were oxidized in the same manner as
above in the presence of 10 equiv of N-MeIm or NaN3 to observe any
changes in the electronic absorbance profile to indicate anion binding
to the metal center.
a
R = SPh-o-NH2 (2), SPh-o-NH2-p-CF3 (3), SPh-p-NH2 (4), and
SPh-p-CF3 (5).
as analytically pure (except 5) dark-red to orange-red solids of
moderate stability to air. However, decomposition to the free
thiol and 1 occurred when dissolved in protic solvents such as
MeOH or H2O, even under anaerobic conditions.
Reactivity with KO2. To 5 mM DMF solutions of 3 or 3ox
(generated by adding 1 equiv of CAN to 3) was added 1 equiv of
KO2 in DMF containing 18-crown-6 ether (4.1 equiv).65 Workup of
the reaction is as stated in Chart S2 in the SI.
Spectroscopic Properties. Complexes 2−5 are soluble in
polar solvents such as DMF and MeCN to form deeply colored
red-brown solutions arising from charge-transfer (CT) bands in
the 430−460 nm range (ε = 3800−7200 M−1 cm−1 in MeCN at
298 K). NiSODred displays an absorption profile with a λmax at
450 nm (ε = 480 M−1 cm−1) and a shoulder at 543 nm (ε =
150 M−1 cm−1) arising from d−d transitions,10 a spectroscopic
benchmark of 4C planar Ni(II) centers in an N2S2 coordination
environment.40 These ligand-field bands are masked in 2−5 by
the intense CT features that involve electronic transitions
between py-N and Ni(II).33 Further solution measurements
RESULTS AND DISCUSSION
■
Synthesis. In previous work,32,33 an S,S-bridged Ni-nmp
unit was used as a metallosynthon to generate 4C planar
Ni(II)-N2S2 complexes of the general formula [Ni(nmp)(SR)]−
as active site models of NiSODred. The ligand nmp2− is a
tridentate chelate that contains pyridine-N, carboxamide-N, and
alkyl thiolate-S donors that replicate the asymmetric ligand
environment derived from His1 and Cys2 in the enzyme. In
these reports, various ways of changing R on the monodentate
thiolate (analogous to Cys6) were explored, and most
syntheses were based on splitting a putative [Ni2(nmp)2] S,S-
bridged dimeric complex with various exogenous thiolate
ligands. Elemental analysis and FTIR data of the solid were
consistent with the proposed formula, but the poor solubility of
this complex precluded more detailed spectroscopic and
structural analyses. Based on a structure reported herein (vide
infra), we describe this complex to be a tetrameric S,S-bridged
species [Ni4(nmp)4] (1) (Figure 2). While this result alters our
prior synthetic schemes, it does not change the ratio of
exogenous thiolate-to-Ni(nmp) used, i.e., 1:1 when considering
single Ni(nmp) units in the S,S-bridged complex. As such,
1
such as H NMR experiments revealed that all complexes are
diamagnetic, as expected for a 4C planar Ni(II) ion (see Figures
S5, S7, S9, and S11 in the Supporting Information (SI)). These
results do not exclude higher coordination numbers, as low-
spin tbp or square-pyramidal Ni(II) would also be S = 0, but
NMR results of known o-aminothiophenolate N,S-coordinated
planar Ni(II) complexes exhibit shifts in the NH2 resonance
(δNH ) by +1.4 ppm upon coordination.66 In addition, the
2
symmetric and asymmetric N−H stretches (νNH) of the NH2
group red-shift by ∼200 cm−1 upon coordination.66 The NH2
proton resonance is close (+0.15−0.28 ppm) to the value
obtained for the free thiolate and there are no significant
changes in νNH in 2−4. These minor changes in δNH and νNH
2
confirm that no Ni(II)−NH2Ph bond exists in 2−4 under the
conditions used.
Structural Characterization. The tetrameric complex 1
and monomeric complexes 2 and 4 have been characterized by
X-ray diffraction (XRD), the details of which can be found in
the Experimental Section, Table 1, and Table S1 in the SI.
Complex 1 crystallizes with four separate Ni(nmp) fragments
bridged by the alkyl-S of another Ni(nmp) complex (see Figure
2). A summation of the significant bond lengths and angles is
provided in Table 1, and a comparison with other monomeric
[Ni(nmp)(SR)]− complexes is provided in Table 2. The
tetramer is formed through specific Ni−S−Ni linkages (e.g.,
Ni1−S2−Ni2−S1′), such that each nmp-S bridges a different
4C planar Ni(II) ion. Compared to other reported [Ni(nmp)-
(SR)]− structures with NiN2S2 coordination,32,33 the average
Ni−Ncarboxamide distance (1.862 Å) is significantly shorter than
Figure 2. ORTEP of [Ni4(nmp)4] (1) with the atom labeling scheme
at 50% thermal probability. H atoms are omitted for clarity.
E
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a
Table 2. Selected Bond Distances for [Ni(nmp)(SR)]− NiSOD Model Complexes
Bond Distances (Å)
R
Ni−Ncarboxamide
Ni−Npy
Ni−Snmp
Ni−Sexo
SPh-p-Cl
1.8638(14)
1.882(2)
1.9470(14)
1.9635(19)
1.947(3)
2.1492(5)
2.1629(7)
2.1518(12)
2.156(3)
2.2139(4)
2.1938(7)
2.1939(14)
2.172(3)
StBu
b
S-o-babt
1.877(3)
c
S-meb
1.863(7)
1.944(7)
SPh-o-NH2
SPh-p-NH2
1.8750(16)
1.8698(14)
1.9418(16)
1.9450(14)
2.1386(6)
2.1474(5)
2.2173(6)
2.2160(5)
average
1.872 0.008
1.948 0.008
2.151 0.008
2.201 0.018
a
b
c
Bond distances reported for Ni(1), N(1), N(2), S(1), and S(2) of the unit cell. S-o-babt = anion of o-benzoylaminobenzenethiol. S-meb = anion
of N-(2-mercaptoethyl)benzamide.
Figure 3. Different views of the ORTEP of the anion of (Et4N)[Ni(nmp)(SPh-o-NH2)] (2) with the atom labeling scheme at 50% thermal
probability. H atoms are omitted for clarity.
Figure 4. Different views of the ORTEP of the anion of (Et4N)[Ni(nmp)(SPh-p-NH2)] (4) with the atom labeling scheme at 50% thermal
probability. H atoms are omitted for clarity.
that for Ni−Npy (1.943 Å), because of strong σ-donation by the
carboxamide ligand. This property, along with the monodentate
nature of SR, generally results in a longer Ni−Sexo versus Ni−
graphic details and selected bond distances and angles are
presented in Tables 1 and 2. Complex 4 crystallizes with two
unique, but chemically equivalent, Ni(II)-N2S2 complexes.
Bond distances are comparable to 1 and other [Ni(nmp)-
(SR)]− structures (Ni−Ncarboxamide: 1.8750 Å (2), 1.8698 Å (4);
Ni−Npy: 1.9418 Å (2), 1.9450 Å (4); Ni−Snmp: 2.1386 Å (2),
2.1474 Å (4); Ni−Sexo: 2.2173 Å (2), 2.2160 Å (4); see Table 2
for a comparison with all structurally characterized Ni-nmp
complexes) and reflect a trend in the electronic nature or Lewis
basicity of the differing donor atoms. To emphasize the strong-
field nature of the carboxamide donor, other 4C planar
NiSODred models (N = 4)29,31,35,43 with multidentate chelates
exhibit short Ni−Ncarboxamide bonds (average: 1.861 Å) and long
Ni−Strans‑carboxamide bonds (average: 2.175 Å) in their crystal
structures. Thus, the lengthening of the Ni−Sexo bonds in 2 and
S
nmp bond distance. Indeed, this bond is considerably longer in
1 (2.208 Å) than the Ni−S bond originating from the
coordinated nmp2− ligand (2.155 Å). In this case, Sexo
represents a metallothiolate from another Ni(nmp) unit instead
of an exogenously added thiolate ligand in prior reports. The
average Ni···Ni separation is 3.4 Å, which precludes any direct
Ni−Ni interaction.
Analogous to previously published [Ni(nmp)(SR)]− struc-
tures,32,33 2 and 4 (Figures 3 and 4, respectively and Figure S1
in the SI) demonstrated distorted 4C planar Ni(II) centers with
two N- and two S-ligands in a cis disposition, resulting in a
Ni(II)-N2S2 coordination sphere as in NiSODred. Crystallo-
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4 is not solely due to the monodentate Sexo coordination.
NiSODred exhibits similar distances (average from the two
reported Streptomyces structures): Ni-NH2(His1): 1.97 0.14
Å; Ni−Npeptide(Cys2): 1.93 0.02 Å; Ni−S(Cys2): 2.20 0.06
Å; Ni−S(Cys6- trans to peptide-N): 2.19 0.01 Å.11,12 The
trends expected for the different Ni−N and Ni−S bonds are
presumably muted in the enzyme structures due to poor
resolution and the redox heterogeneity in the crystals (equal
mix of Ni(II) and Ni(III) ions). In 2 and 4, the closest Ni−
NH2 distance is 4.4 Å, establishing the absence of any five-
coordinate species in the solid state. The Ni···NHis distance is
also long in NiSODred, 4.26 Å (PDB: 1T6U) and 3.81 Å (PDB:
1Q0D), indicative of the nonbonding nature of this potential
N-donor in the enzyme. Because of the ortho-positioning of the
aniline-NH2 in 2, the N and coordinated Sexo are separated by
3.024 Å (sum of the van der Waals radii for N and S is 3.55 Å),
which would suggest an intramolecular hydrogen bond with the
ligated S. However, the N−H−S angle is estimated to be 110°
(if H is placed in an ideal position), suggesting relatively poor
orbital overlap between the donor−acceptor pair. Strong
hydrogen bonds are usually linear in nature, i.e., ∠N−H···S ≈
180°.67 To emphasize the importance of this angle, hydrogen
bonding to coordinated S-ligands in other NiSOD model
compounds usually results in a decrease by 0.02−0.03 Å in the
Ni−S distance due to relief of the Ni(dπ)−S(pπ) antibonding
interaction, as the S lone pair is engaged in the hydrogen
bond.33,41 No such contraction of the Ni−S bonds was
observed, when compared to the non-hydrogen-bonded
complex 4. Collectively analyzing the six structurally charac-
terized complexes in this family (Table 2) revealed that
distances to the nmp2− ligating atoms are largely invariable. The
largest variations appear in Sexo (Ni−S(avg.): 2.201 0.018 Å),
which range from 2.172 to 2.217 Å and generally correlate with
the electronic nature of this particular S-donor.
Figure 5. CV of an 8 mM solution of 5 in DMF at RT (glassy carbon
working electrode, 0.1 M nBu4NPF6 as the supporting electrolyte, 100
mV/s scan speed). The arrow shows the direction of the scan.
PhSSPh (Eox = −0.49, Ered = −2.12 V vs Fc/Fc+ in DMF)68 and
even more structurally complex thiolates that display
qualitatively similar CVs.69 The CV of a related 4C planar
Ni(II)-NS3 complex, [Ni(pdmt)(SPh)]− (pdmt = pyridine-2,6-
dimethanethiolate), which also owes its electrochemical activity
to the monodentate thiolate PhS−, resembles the CV of 5 with
values (Eox = −0.47, Ered = −1.73 V in DMF) shifted
approximately +0.20 V from that of free PhSSPh (above) due
to coordination to Ni(II).70 The comparative positive shift of
∼0.40 V in the Ered/ox associated with 5 from PhSSPh is
consistent with the electron-withdrawing p-CF3 substituted aryl
thiolate ligand. Complex 4 with the p-NH2 substitution on SR
behaved similarly (Eox = −0.51 V in DMF) with additional low
current intensity waves that we have yet to identify (see Figure
S2 in the SI). Again, comparison with p-NH2-PhSSPh-p-NH2
(Eox = −0.88, Ered = −2.33 V vs Fc/Fc+ in DMF)68 revealed a
slight shift upon coordination of the thiolate to Ni(II).
Regardless, the CVs of 4 and 5 only reflect ligand-based
electrochemistry with no indication of a Ni(III/II) process.
Additional verification of the reactions displayed in Scheme 2
(left side) comes from bulk oxidation experiments with CAN or
Fc+, which yielded 1 and the corresponding disulfide in near
stoichiometric yield. Collectively, the [NiII(nmp)(SR)]−
complexes with simple alkyl/aryl R-groups do not go through
any observable Ni(III) state, as all oxidation chemistry is
associated with Sexo (even the electron-deficient Sexo in 5) and is
suggestive of the high S-character in the HOMO of these
complexes.
Electrochemistry. To assess the role of the o-NH2 group in
the electrochemical activity of the Ni-nmp complexes, we
performed cyclic voltammetry (CV) on 2−5. Previously
reported [NiII(nmp)(SR)]− complexes display irreversible
waves in their CVs with cathodic and anodic peaks assigned
to redox activity associated with the coordinated RS− ligand,
i.e., [NiII(nmp)(SR)]− → 0.25[NiII4(nmp)4] (1) +
0.5RSSR.32,33 Similar CVs and redox assignments were also
observed with the peptide-based [NiII(GC-OMe)(SR)]− (GC-
OMe is the dianion of H2N-Gly-L-Cys-OMe) complexes.34 In
both cases, R represented simple aryl/alkyl thiolates or more
complex thiolates with appended hydrogen-bonding moieties.
In this study, we anticipated that providing a potential N-donor
ligand from the o-aminothiophenolate would provide a viable
mechanism to capture an oxidized species related to NiSODox.
Representative CVs are displayed in Figures 5 and 6, as well
as Figures S2 and S3 in the SI. All CVs were recorded in DMF
at RT and E values are reported versus an externally referenced
ferrocene/ferrocenium (Fc/Fc+) couple using a glassy carbon
The CVs of 2 and 3 displayed similar irreversible waves with
the exception of a quasi-reversible wave observed at E1/2
=
−0.69 V and −0.43 V for 2 and 3, respectively (DMF, RT), that
was slightly superimposed on Eox. As expected, the oxidation of
2 is easier than 3 due to the p-CF3 substitution on Sexo. Since
the free o-aminothiophenolate is irreversibly oxidized at more
positive potentials, this E1/2 may be assigned as a metal-
centered Ni(III/II) electron-transfer process. For simplicity, we
will limit our discussion to complex 3, but similar CVs were
observed for 2 (see Figure S3 in the SI). The possibility of
isolating and characterizing an oxidized NiSOD model, as
suggested by the electrochemistry, led us to generate this
species (termed 3ox) by chemical oxidation with CAN and to
study its redox properties. Interestingly, the CV of in situ-
generated 3ox displayed a redox-couple at E1/2 = −0.43 V (ΔEp
n
working electrode and 0.1 M Bu4NPF6 as the supporting
electrolyte. Electrochemical measurements on 4 and 5,
complexes without an o-NH2 group, revealed irreversible
waves (ΔEp ≈ 1.5 V) in the CV that are due to Sexo redox.
The CV of 5 (containing the p-CF3 group), for example,
displayed two distinct electrochemical events at potentials of
−1.64 V (Ered) and −0.19 V (Eox) (Figure 5) that we assign as
the RS• (or 0.5 RSSR) → RS− and RS− → RS• (or 0.5 RSSR)
electrochemical reactions, respectively. These values are on par
with reported potentials for simple organic disulfides such as
G
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Figure 6. CV of a 3.5 mM solution of 3ox in DMF at RT from different scan directions (see arrow, glassy carbon working electrode, 0.1 M nBu4NPF6
as the supporting electrolyte).
Scheme 2. General Redox Activity of [Ni(nmp)(SR)]− Complexes
Figure 7. (Left) UV-vis spectrum of 3 (red trace) and in situ-generated 3ox (green trace) by addition of CAN to 3 in DMF at 298 K. (Right) X-band
EPR spectrum (black trace) and simulation (red trace) of in situ generated 3ox in DMF at 10 K. Simulation parameters: g = [2.132, 2.028, 2.004], W
= 35, 30, 45 MHz.
≈ 0.060 V; and ipa/ipac ≈ 1) that was remarkably similar to the
CV of 3 (Figure 6 and Figure S3 in the SI). Additional metrics
such as the linear (r2 = 0.991, of 3ox) ip vs v1/2 plot support the
fact that this couple is a reversible, diffusion-controlled process.
Scanning from either side of the isolated E1/2 does not result in
any changes (Figure 6). It is important to reiterate that
[NiII(nmp)(SR)]− complexes do not exhibit any reversibility in
their CVs (see Figure 5). The appearance of such a wave in the
CV of 3 is thus a new feature in these otherwise isostructural
NiSOD model complexes. A unique feature of 3 is the presence
of the o-NH2 group that must be the stabilizing feature to allow
observation of such an oxidized species on the CV time scale.
We propose that this redox event is due to one of the following
couples: (i) [NiII(nmp)(SR)]−/[NiIII(nmp)(SR)] or (ii)
[NiII(nmp)(SR)]−/[NiII(nmp)(•SR)]. In case (ii), the o-NH2
group supports a coordinated S-radical to prevent the
formation of RSSR and 1 that would result in the typical
irreversible CVs (Scheme 2). Of course, a resonance
description of [NiIII(nmp)(SR)] ↔ [NiII(nmp)(•SR)] is the
more-realistic possibility (vide infra) (Scheme 2, right side).
Properties of 3ox. Exposure of DMF solutions of 2 or 3 to
air or O2(g) resulted in an immediate color change of the
solution from red to green accompanied by the eventual
formation of a red precipitate. These solutions afforded a broad,
low-energy feature at ∼900 nm in the RT electronic absorption
spectrum. This transformation occurs on the minute (2, full
H
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formation of 1 within minutes) or hour (3, ∼70% 1 after 24 h)
time scale with the red precipitate confirmed to be 1, suggesting
the same ultimate fate as seen with other [Ni(nmp)(SR)]−
complexes (Scheme 2). However, the difference here is that
transient oxidized complexes are spectroscopically observable.
Furthermore, chemical reduction of in situ-generated 3ox with
[Co(Cp)2] resulted in regeneration of the reduced species 3 as
and a wide variety of Ni(III) tetraazamacrocyclic77,78 and amino
acid79,80 complexes, can be described by g⊥ ≈ 2.2−2.3 and g∥ =
2.00, which arise from a low-spin d7 configuration with the
single unpaired electron residing in the Ni dz orbital.80 EPR
2
parameters for relevant, formally Ni(III) complexes are
summarized in Table S2 in the SI. This classical Ni(III) EPR
pattern is the reverse of that seen here for 3ox, so the present
1
21
verified by UV-vis, H NMR, and ESI-MS experiments. Thus,
complex cannot be an exclusive Ni(III) species with a dz
bulk chemical oxidation and reduction mirrored the electro-
chemical observations and was also a reversible process. Due to
the longer lifetime of 3ox, we limit our discussion below to this
species, although similar spectroscopic observations were noted
for 2.
ground state.
EPR spectra previously reported for Ni(III)−N2S2 complexes
that more closely resemble 3 do not exhibit quite the classical
pattern described above, as the symmetry of these complexes is
often lower, leading to more rhombic EPR spectra, but most do
have g⊥ ≈ 2.2−2.3 and g∥ ≈ 2.00.10,36,43,44,81−83 Moreover, their
spectra are more anisotropic with g-spreads, gmax − gmin ≈ 0.3,
as opposed to 0.128 for 3ox, and with an overall greater
deviation from the free electron (radical) g value (ge = 2.00)
than for 3ox (2.055), namely with gavg ≈ 2.2.10,36,44,81,83 The
lower g-spread in 3ox, smaller deviation from ge, and ambiguous
electronic ground state (see computational section below)
would be more consistent with a coordinated S-radical with a
smaller degree of electron delocalization onto the Ni d-orbitals,
as observed in Ni-S4 (bis-dithiolene) complexes,84,85 than in
classical Ni(III) complexes and even in other oxidized Ni(II)
complexes with N2S2 donor sets. Thus, taken together, the UV-
vis and EPR spectra support a Ni(III)-(SR) ↔ Ni(II)-(•SR)
description for 3ox, rather than a genuine Ni(III)-SR species.
Low Temperature Absorption and MCD. The low
temperature (4.5 K) absorption spectrum of 2ox, generated by
sparging 2 with O2(g) for 5 min, exhibited a broad band
centered at ∼22 000 cm−1 (455 nm) and a lower energy feature
at ∼10 000 cm−1 (1000 nm) (Figure 8, top). The MCD
spectrum of this sample (Figure 8, bottom) exhibited negatively
signed features corresponding to the bands observed in the low
temperature absorption spectrum, as well as a prominent
feature at ∼16 000 cm−1 (625 nm). All MCD features increase
in intensity with decreasing temperature, characteristic of C-
To better identify the oxidized complex, we performed in situ
spectroscopic characterization of the reaction mixture after
chemically oxidizing 3 with oxidants such as CAN or Fc+ salts.
For example, addition of a stoichiometric amount of CAN to a
DMF solution of 3 at RT immediately generated a green
solution with a broad and intense absorption band at 910 nm,
concomitant with the disappearance of the CT band associated
with 3 at 450 nm (Figure 7). Low-energy bands in this region
have been observed in 4C planar Ni(III) complexes with N2S2
dicarboxamido-dithiolato coordination and were attributed to
S-to-Ni(III) CT transitions,44 a common spectroscopic feature
for Ni(III)-N/S coordination complexes. Interestingly, Ni(II)-
N2S2 complexes containing o-aminothiophenolate ligands with
radical character also exhibit a low-energy band in a similar
range in their UV-vis spectra. However, it must be noted that
these bands are observed for the Ni-coordinated o-iminosemi-
quinonato form that would require the loss of two protons and
one electron from the original o-aminothiophenol (see Chart
S1 in the SI).66 A close structural comparison of the C−C (avg:
1.397
0.011 Å), C−S (1.773 Å), and C−N (1.392 Å)
distances of Sexo in 2 and, by analogy, 3 did not reveal any
contraction in the C−N (1.348 Å), C−S (1.724 Å) distances
nor the quinoid structure (average = 1.405 0.0226 Å) typical
of a o-iminosemiquinonate and are more consistent with a
coordinated thiophenolate anion71 (see Chart S1 in the SI). It
is thus unlikely that an N,S-coordinated o-iminosemiquinonate
is present in 2ox or 3ox. However, spectroscopically charac-
terized metal-thiyl complexes with thiophenolate ligands of
Ni(II) (λ = 670−800 nm in CH2Cl2),72 Co(III) (λ = 780 nm in
CH2Cl2),73 Ru(III) (λ = 850 nm in acetone),74 and V(IV) (λ =
910 nm in MeCN)75 exhibit broad and distinct low-energy
bands similar to 3ox. These are significantly red-shifted from the
broad transition observed for the free phenylthiyl radical (λ =
460−500 nm) generated by pulse radiolysis in pH 11 aqueous
solution.71 Most of these metal-thiyl complexes have been
observed at low temperature, due to rapid formation of the
disulfide, which is not surprising considering the second-order
decay rate for disulfide formation from free PhS• (k = 9.6 × 109
M−1 s−1).71 These facts make the spectroscopic observation of
3ox even more noteworthy, in that this species is stable for
hours at 298 K. After 24 h in DMF, the green color of 3ox faded
and the red precipitate of 1 was isolated in ∼70% yield.
An X-band EPR spectrum was collected for in situ-generated
3ox at 10 K in a DMF glass and is shown in Figure 7, along with
a simulated fit. According to the simulation, the three g-values
observed are g = 2.132 (∼g∥), 2.028, and 2.004 (gavg = 2.055; g⊥
≈ 2.016) with line widths of 35, 30, and 45 MHz, respectively.
The typical EPR pattern for Ni(III), as seen in complexes that
unambiguously contain Ni(III), such as NiIIIF430M,76 the
oxidized cofactor from methyl coenzyme M reductase (MCR),
Figure 8. Low temperature absorption (4.5 K, top) and MCD (4.5, 8,
15, and 25 K, bottom) spectra of 2ox generated in situ by sparging 2 in
a 1:4 (v/v) solvent mixture of MeCN and butyronitrile with O2(g) for
5 min.
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Article
term behavior and indicative of a paramagnetic species. Note
that the three features observed in the MCD spectrum of 2ox
coincide with the three absorption bands observed at RT of in
situ-generated 3ox (Figure 7), providing further evidence that
2ox and 3ox have nearly identical electronic structures. Given the
relatively high intensities of the MCD features exhibited by 2ox,
it can be concluded that, in this species, and by analogy in 3ox,
significant unpaired spin density resides on the Ni atom. In
support of this conclusion, the large ratio of MCD-to-
absorption intensity of the feature at ∼16 000 cm−1 (625
nm) indicates that the corresponding transition contains
significant Ni 3d → 3d character.
Computations. To obtain further insight into the geo-
metric and electronic structures of 2ox, DFT geometry
optimization and property calculations were performed for a
model of this species (termed IIox). An analogous geometry
optimization for a model of the crystallographically charac-
terized precursor 2 (model II) yielded Ni−ligand bond
distances that agree reasonably well with the experimental
values, thus validating our approach used to generate a model
of the structurally ill-defined complex 2ox (Table 3). During the
is in line with the intense MCD features and sizable g-spread
displayed by 3ox relative to a free thiyl.
Reactivity of 3ox with Azide and Superoxide. The azide
−
anion (N3 ) is often employed as a redox-inert analogue of
• −
superoxide to obtain information on M−O2 interactions.
−
NiSOD shows no direct Ni−N3 interaction based on EPR,
UV-vis, and resonance Raman (rR) studies.10,12 To evaluate the
anion affinity of the Ni center in 3 and 3ox, we added excess (10
equiv) NaN3 to this NiSOD model and monitored its UV-vis
spectrum. As observed for NiSOD, the UV-vis spectrum of 3 or
3ox did not change upon the addition of N3− (Figure 10), even
after 1 h. Despite the presence of the potentially coordinating
amine-N from Sexo, we tested whether the presence of 10 equiv
−
of N-MeIm impacted any Ni−N3 bonding. Indeed, the
addition of N-MeIm resulted in a ∼30 nm blue-shift in λmax of 3
that may imply a Ni···N-MeIm interaction; however, no other
direct evidence for a Ni-NMeIm bond was obtained. Analogous
to the results in the absence of N-MeIm, no change was
−
observed in the UV-vis spectrum when 10 equiv of N3 are
added (Figure 10); thus, much like NiSOD, the Ni center in 3
and 3ox does not bind N3 .
−
To evaluate the potential of 3 and 3ox as SOD catalysts,
experiments were performed with 18-crown-6 ether solubilized
KO2 in DMF at 298 K. Addition of a stoichiometric amount of
KO2 to a DMF solution of 3 resulted in no reaction, as
monitored by UV-vis. However, the same reaction with in situ
generated 3ox produced an immediate solution color change
from green to red-brown. To obtain more information
regarding the products formed in this reaction, fractional
precipitation and isolation of the components present in the
reaction mixture were performed (see Chart S2 in the SI). After
vacuum distilling DMF from the reaction mixture, MeCN was
added that afforded a red-orange precipitate and a brown
MeCN-soluble portion. The MeCN-insoluble component was
identified as the tetrameric species 1, based on FTIR data. Two
separate reactions of 3ox and O2• − revealed that the bulk of the
Ni-nmp ends up as 1 (avg: 65% yield). Further attempts at
separating components in the MeCN-soluble portion by
selective precipitation with THF and Et2O revealed the
Table 3. Ni−Ligand Bond Distances in DFT-Optimized
Models of II and IIox, Compared with X-ray
Crystallographically Determined Values for 2
DFT
X-ray Structure of 2
II
IIox
(Et4N)[Ni(nmp)(SPh-o-NH2)]
Ni−Sexo
Ni−Snmp
Ni−Ncarboxamide
Ni−Npy
2.278
2.198
1.895
1.981
2.211
2.169
1.880
1.990
2.218
2.138
1.875
1.942
geometry optimization of IIox, the Ni atom retained a 4C planar
geometry with Ni−ligand bond distances somewhat shorter
than those in its one-electron reduced counterpart II (Table 3).
A DFT calculation of the EPR parameters for IIox yielded g-
values of 2.16, 2.06, and 2.04, which agree well with the g-values
determined experimentally for 3ox of 2.132, 2.028, 2.004
(Figure 7). Therefore, an analysis of the computed electronic
structure description for IIox is warranted.
−
presence of 3 and the free unbound sulfinate (RSO2 ) and
−
A comparison of the spin-up (α) and spin-down (β)
manifolds of MOs obtained from a spin-unrestricted DFT
calculation for IIox (Figure 9) revealed that the βLUMO
(βMO#93) corresponds to the formally singly occupied MO, as
its α-counterpart (αMO#83, not shown) is occupied. To higher
energy, the βLUMO+2 (βMO#95) can be identified as the Ni
sulfonate (RSO3 ) of Sexo based on ESI-MS (see Figure S4 in
the SI). Unfortunately, quantitative isolation of species other
than 1 was not possible due to similar solubility profiles and the
presence of CAN-related species. In addition, no O2(g)
generation was observed utilizing electrochemical detection
methods.86 One can envision two primary reaction paths of 3ox
with superoxide: (i) O2• − behaves as an outer-sphere reductant
to furnish the corresponding Ni(II) complex 3 and O2 (Scheme
3i), or (ii) O2• − oxygenates the Sexo atom of 3ox to generate the
2
2
3dx −y -based MO, while the βLUMO+1 (βMO#94) is mainly
localized on the aryl-carboxamide moiety of the supporting
nmp2− ligand. As expected for a 4C planar complex, the
2
occupied Ni 3dz -, 3dxy-, 3dxz-, and 3dyz-based orbitals
−
resulting sulfinate (RSO2 ) and tetramer 1 (Scheme 3ii).
(βMO#88, #87, #84, and #81, respectively) are significantly
Scheme 3i would be more in line with a Ni(III) assignment for
3ox, whereas Scheme 3ii is more consistent with a Ni(II)-thiyl.
However, one could envision the formation of similar products
in both pathways if the product Ni(II) species 3 in Scheme 3i
reacted with O2 in a manner other than as an outer-sphere
Ni(II) oxidant (i.e., “other products” in Scheme 3i). Either
scenario explains the lack of any detectable O2(g) via
electrochemistry, although the path in Scheme 3ii is more
consistent with the significant amount of isolated 1 in the
reaction mixture. Regardless of the path at work, these results
advocate for a Ni(II)-thiyl assignment for 3ox.
2
2
stabilized relative to the unoccupied Ni 3dx −y -based βMO#95.
The βLUMO has predominant S 3p (53% total: 26% from
Sexo and 27% from Snmp) and Ni 3d (21%; mainly 3dxz and
some 3dyz) orbital character, suggesting that the removal of an
electron from II results in partial oxidation of both the thiolates
and the Ni(II) ion. Consistent with this prediction, a plot of the
spin density for IIox shows that the unpaired electron is
delocalized over the two S and the Ni atoms (Figure 9, inset).
Consequently, our computational results obtained for IIox
indicate that 2ox is best described as a hybrid of [Ni(III)-
(−SR)] ↔ [Ni(II)-(SR)] resonance structures. This description
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Article
Figure 9. Relevant portion of the MO diagram and isosurface plots of select βMOs obtained from a spin-unrestricted DFT calculation for IIox. Inset:
Spin density plot for IIox (red = positive spin density; white = negative spin density).
oxidized Ni(III) complex that would be structurally and
electronically analogous to the Ni−N3S2 coordination sphere
of NiSODox. We hypothesized that the incorporation of a weak
Naxial ligand in SR′ from o-aminothiophenolate could mimic the
long (2.3−2.6 Å) Ni-NHis bond in NiSOD and would be
required to obtain such an oxidized species. Indeed, site-
directed variants and DFT calculations revealed that the axial
Ni-NHis bond in NiSODox is crucial for maintaining the rate (k
≈ 109 M−1 s−1)87,88 and appropriate electrochemical potential
(0.090 V vs Ag/AgCl, pH 7.4, phosphate buffer) for
turnover.10,89 In addition, the few low molecular weight
NiSOD models that access Ni(III) or that exhibit some SOD
activity contain an Naxial ligand.27,34,35 Chemical oxidation of
NiSOD models 4 and 5 that lack this potential N-donor
resulted in the quantitative formation of 1 and the disulfide
(R′SSR′) of the monodentate thiolate. This result was not too
surprising, given the expected high percentage of S-character in
the HOMO of such Ni(II)-N2S2 complexes, although we had
initially anticipated the electron-deficient thiolate in 5 would
prevent such chemistry. Chemical oxidation of models 2 and 3
that contain a potential N-donor ligand resulted in the transient
formation of an oxidized species (2ox and 3ox) that is best
described as a resonance hybrid of a Ni(III)-thiolate and
Ni(II)-thiyl species: [Ni(III)-(SR)] ↔ [Ni(II)-(•SR)]. This
assignment was confirmed through a variety of experimental
(X-ray, UV-vis, MCD, EPR, reactivity) and theoretical (DFT)
techniques. These results may at first appear incongruent with
Figure 10. UV-vis spectra of 3 (black) and 3ox (black dash); 3 + 10
equiv N-MeIm (blue) and 3ox + 10 equiv N-MeIm (blue dash); 3 + 10
−
−
equiv N3 (red) and 3ox + 10 equiv N3 (red dash). All experiments
were performed in DMF at 298 K.
CONCLUSIONS
■
In summary, we have prepared and characterized four Ni(II)-
N/S complexes of the general formula [Ni(N2S)(SR′)]−, where
SR′ represents a site-differentiated location of the N2S2
coordination sphere as models of NiSOD. The SR′ ligand
was functionalized with electron withdrawing substituents and/
or a tethered N-donor in efforts to promote the formation of an
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Article
Scheme 3. Possible Scenarios for the Reaction of 3ox with Superoxide
the actual mechanism of NiSOD. Most experimental work
supports an outer-sphere electron transfer mechanism for O2
ACKNOWLEDGMENTS
■
•−
T.C.H. acknowledges financial support from a National Science
Foundation CAREER Award (No. CHE-0953102) and the
Office of the Vice President for Research (OVPR) and the
Office of the Provost at the University of Georgia (UGA).
T.C.B. acknowledges financial support by the National
Institutes of Health (Grant No. GM 64631). S. D. acknowl-
edges the NIH (Grant No. T32-GM008505) for support and
the NSF (Grant No. CHE-0840494) for computational
resources.
disproportionation with the Ni(III/II) center possibly remain-
ing 5C throughout catalysis.8,39,40 In the first turnover,
conversion of 4C NiSODred to 5C NiSODox likely results in a
4C planar intermediate (NiSODox-Hisoff) before formation of
NiSODox due to the kinetics of electron transfer versus Ni−
N(His) bond formation. Although this transient species has
never been observed experimentally, DFT computations on
NiSODox-Hisoff indicate a predominant Ni-based HOMO and
support a Ni(III) oxidation state (vide supra) even before axial
His1 binding. This assignment is in contrast to the Ni(III)-
thiolate ↔ Ni(II)-thiyl resonance description for the NiSOD
models 2ox and 3ox. However, •S(Cys) radicals have been
observed in other enzymes,90 some of which function through
such a radical species.91−97 Based on the EPR spectrum, Ni
exists in the 3+ state in NiSODox with no indications of a
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