Ligand-Based Reactivity of Oxygenation and Alkylation in Cobalt Complexes Binding with (Thiolato)phosphine Derivatives

Article abstract of DOI:10.1021/acs.inorgchem.9b03740

In our efforts to understand the nature of metal thiolates, we have explored the chemistry of cobalt ion supported by (thiolato)phosphine ligand derivatives. Herein, we synthesized and characterized a square-planar CoII complex binding with a bidentate (thiolato)phosphine ligand, Co(PS1″)2 (1) ([PS1″]- = [P(Ph)2(C6H3-3-SiMe3-2-S)]-). The complex activates O2 to form a ligand-based oxygenation product, Co(OPS1″)2 (2) ([OPS1″]- = [PO(Ph)2(C6H3-3-SiMe3-2-S)]-). In addition, an octahedral CoIII complex with a tridentate bis(thiolato)phosphine ligand, [NEt4][Co(PS2*)2] (3) ([PS2*]2- = [P(Ph)(C6H3-3-Ph-2-S)2]2-), was obtained. Compound 3 cleaves the C-Cl bond in dichloromethane via an S-based nucleophilic attack to generate a chloromethyl thioether group. Two isomeric products, [Co(PS2*)(PSSCH2Cl*)] (4 and 4′) ([PSSCH2Cl*]- = [P(Ph)(C6H3-3-Ph-2-S)(C6H3-3-Ph-2-SCH2Cl)]-), were isolated and fully characterized. Both transformations, oxygenation of a CoII-bound phosphine donor in 1 and alkylation of a CoIII-bound thiolate in 3, were monitored by spectroscopic methods. These reaction products were isolated and fully characterized. Density functional theory (DFT, the B3LYP functional) calculations were performed to understand the electronic structure of 1 as well as the pathway of its transformation to 2.

Full text of DOI:10.1021/acs.inorgchem.9b03740

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Article
Ligand-Based Reactivity of Oxygenation and Alkylation in Cobalt
Complexes Binding with (Thiolato)phosphine Derivatives
Yi-Ying Wu, Jia-Cheng Hong, Ruei-Fong Tsai, Hung-Ruei Pan, Bo-Hua Huang, Yun-Wei Chiang,
Gene-Hsiang Lee, Mu-Jeng Cheng,* and Hua-Fen Hsu*
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ABSTRACT: In our efforts to understand the nature of metal
thiolates, we have explored the chemistry of cobalt ion supported by
(thiolato)phosphine ligand derivatives. Herein, we synthesized and
characterized a square-planar CoII complex binding with a bidentate
(thiolato)phosphine ligand, Co(PS1″)2 (1) ([PS1″]− = [P-
(Ph)2(C6H3-3-SiMe3-2-S)]−). The complex activates O2 to form a
ligand-based oxygenation product, Co(OPS1″)2 (2) ([OPS1″]−
=
[PO(Ph)2(C6H3-3-SiMe3-2-S)]−). In addition, an octahedral CoIII
complex with a tridentate bis(thiolato)phosphine ligand, [NEt4][Co-
(PS2*)2] (3) ([PS2*]2− = [P(Ph)(C6H3-3-Ph-2-S)2]2−), was
obtained. Compound 3 cleaves the C−Cl bond in dichloromethane
via an S-based nucleophilic attack to generate a chloromethyl thioether
group. Two isomeric products, [Co(PS2*)(PSSCH Cl*)] (4 and 4′)
2
([PSSCH Cl*]− = [P(Ph)(C6H3-3-Ph-2-S)(C6H3-3-Ph-2-SCH2Cl)]−),
2
were isolated and fully characterized. Both transformations, oxygen-
ation of a CoII-bound phosphine donor in 1 and alkylation of a CoIII-
bound thiolate in 3, were monitored by spectroscopic methods. These
reaction products were isolated and fully characterized. Density functional theory (DFT, the B3LYP functional) calculations were
performed to understand the electronic structure of 1 as well as the pathway of its transformation to 2.
Metal complexes with noninnocent ligands have offered a
diverse chemistry where ligands are not just considered as
supporting roles for tuning the steric and electronic feature of
metal centers.7 Instead, the reactivity of metal complexes might
be based on ligands that are involved in redox change or
chemical tranformation.8 For instance, metal thiolates might
display various sulfur-based reactivity such as the formation of
disulfide,9 the generation of thiyl radical,10 oxygenation,11 and
alkylation.12
The nucleophilic nature of metal bound thiolate has been
emphasized, partially due to the finding of thiolate methylation
in ZnII-dependent proteins.13 The reactivity is based on a
hypothesis of a four-electron destabilizing interaction between
sulfur lone pairs and fulfilled metal d orbitals.14 This chemistry
has been well established in many synthetic metal thiolates.12,15
However, examples are relatively rare showing strong
INTRODUCTION
■
Cobalt ion plays essential roles in several metalloproteins.1
One such example is cobalt nitrile hydratase, which contains a
low-spin CoIII ion in its active site and catalyzes the conversion
of nitriles to amides. The metal center binds to two
deprotonated amide nitrogen atoms and three cysteine
thiolates, in which the two sulfur atoms are oxidized to
sulfenate (SO) and sulfinate (SO2).2 Synthetic analogues of
this enzyme have been investigated extensively in the effort to
understand the nature of the catalytic site.3 There is no cobalt
containing protein found in nature that binds or activates
dioxygen. However, cobalt-substituted hemoglobin has been
shown to bind dioxygen reversibly.4 In addition, cobalt-
substituted dioxygenases display activities similar to native
enzymes. Recently, it was found that the FeII center in
homoprotocatechuate 2,3-dioxygenase (HPCD) can be
replaced by a CoII ion with comparable activity. The O2
adduct of the CoII substituted enzyme was characterized as a
low-spin CoIII−superoxide species.5 Thus, O2-dependent
chemistry of cobalt complexes has been developed to
understand the whole paradigm of dioxygen activation by
metalloproteins.6
Received: December 26, 2019
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nucleophilicity of metal bound thiolate that activates a weak
electrophile such as dichloromethane. They include a
bis(sulfide)dimolybdenum and a bis(thiolate)diiron ligated by
cyclopentadienyl ligands.16 PdII, RuII, and VIII complexes
binding with (thiolato)phosphine ligand derivatives also show
the reactivity of dichloromethane activation.12,15c,17 In
addition, a CoIII center with bis-axial cyanide coordination
was able to induce in-plane thiolato donors toward dichloro-
methane.18
In cobalt thiolate complexes, several examples of ligand-
based reactivity have been reported. They include the
interconversion occurring between CoII disulfide and CoIII
thiolate.19 Remarkably, a square-planar CoII thiolate displays
reactivity toward S-alkylation with CH3I.20 Sequential
oxidations of cobalt bound thiolate to sulfenate and sulfinate
have been demonstrated in several model complexes.3c,21
These studies provide implications for post-translational
modification of cysteinyl thiolate in cobalt nitrile hydratases.
(Thiolato)phosphine ligand derivatives have shown a
noninnocent nature in several well-defined metal complex-
es.12,15c,22 From this perspective, the reactivity of cobalt
complexes with several (thiolato)phosphine derivatives were
explored (Chart 1). In this particular study, a four-coordinate
RESULTS AND DISCUSSION
■
Syntheses and Characterization of [Co(PS1″)2] (1). A
square-planar CoII thiolate complex, [Co(PS1″)2] (1), was
obtained from a reaction of CoCl2 and [PS1″]− ([PS1″]− =
[P(Ph)2(C6H3-3-SiMe3-2-S)]−) in a 1:2 ratio. The obtained
orange precipitate was recrystallized from dichloromethane
and methanol to give a crystalline solid of 1 (79% yield). The
elemental analysis is consistent with the formula of Co(PS1″)2
(1). The ESI-MS data gives a set of peaks at 789.13 m/z, and
its isotope pattern is in agreement with the calculated one of 1
(Figure S1). The structure resolved from X-ray crystallographic
data contains a four-coordinate CoII ion superimposed with a
crystallographic inversion center (Figure 1). The metal ion
Chart 1
Figure 1. Thermal ellipsoid plots (35% probability) of 1 (A) and 3·
2CH3OH·0.5THF (B). The H atoms, cation, and solvated molecules
are omitted for clarity.
binds to two bidentate PS1″ ligands and has a square-planar
geometry with a τ4 value of 0.00, where the τ4 value is an index
of distortion between Td and D4h, ranging from 1.00 to 0.00
from a tetrahedron to a square planar, respectively.23 Two
phosphine or two thiolato donors are trans to each other. The
CoII−S distance is 2.1518(7) Å, similar to those in a reported
square-planar CoII thiolate (Co−Save = 2.168 Å).20
square-planar CoII complex, Co(PS1″)2 (1) ([PS1″]−
=
[P(Ph)2(C6H3-3-SiMe3-2-S)]−), and an octahedral CoIII
thiolate complex, [NEt4][Co(PS2*)2] (3) ([PS2*]2−
=
[P(Ph)(C6H3-3-Ph-2-S)2]2−), were isolated and characterized.
Complex 1 activates O2, leading to a ligand-based oxygenation
where metal-bound phosphine donor converts to phosphine
oxide, as shown in Scheme 1. Complex 3 showed a strong
nucleophilicity of bound thiolate that activates the C−Cl bond
in dichloromethane, as shown in Scheme 2.
Complex 1 dissolved in CH2Cl2 showed an orange color.
The electronic spectrum of complex 1 displays three intense
bands at 391 nm (ε = 6.48 × 103 M−1cm−1), 427 nm (ε = 5.08
× 103 M−1cm−1), and 520 nm (ε = 4.81 × 103 M−1cm−1),
likely associated with charge-transfer transition (Figure S2). A
weaker band appears at 822 nm (7.28 × 101 M−1cm−1) might
be attribited to d−d transition. The noninnocent nature of
(benzenethiolato)phosphine derivatives was found in vana-
dium and ruthium complexes, where the electronic structures
Scheme 1
of metal-thiyl radical species were also considered.10a,24
A
characteristic intense band appearing at the NIR region for the
reported metal thiyl radical species is absent in complex 1,
indicating the electronic structure of 1 should be viewed as a
Scheme 2
B
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Co(II)-thiolate species rather than a Co(I)-thiyl radical
species.10a,24 The calculation data is in agreement with the
findings (vide infra). The magnetic data of 1 was measured
with a SQUID magnetometer in a 1 T applied magnetic field
and at a temperature range of 1.8−300 K (Figure S3). The
obtained μ value of 1.81 μB at 50 K and 2.07 μB at 300 K is
close to the spin-only value for a ground state S = 1/2 (1.73
μB), indicating a low-spin d7 configuration present in a square-
planar geometry. The 1H NMR spectrum gave resonance peaks
at the paramagnetic region (23.73 and −18.88 ppm) associated
with three protons in benzenethiolate of the PS1″ ligand
(Figure S4). The X-band EPR spectra of 1 was measured at 77
K in 2-MeTHF (Figure S5). It displays rhombic signals with a
large anisotropy in g values, which is a characteristic feature for
square-planar low-spin Co(II) species.25 The fits gave the
parameters with g values of 2.9, 1.98, and 1.975 and A values of
220, 30, and 10 MHz, respectively. The pattern of g1 > g2 ∼ g3
with a g3 value less than 2 is an indication of having an
unpaired electron in a dyz orbital rather than a dz2 orbital.26
The finding is further confirmed by density functional theory
(DFT) calculations (vide infra).
Figure 2. Electron spin-density plot of 1 in a doublet state with a
square-planar geometry. Top view (left); side view (right).
from orange to bluish green. This solution was layered with
methanol, and a navy blue crystalline solid of [CoII(OPS1″)]2
(2) ([OPS1″]− = [OP(Ph)2(C6H3-3-SiMe3-2-S)]−) was
precipitated (93% yield) (Scheme 1). Alternatively, a synthetic
method for generating complex 2 can be completed through
the reaction of CoCl2 and Li[OPS1″] in methanol. The solvent
was removed from the reaction mixture, and the residue was
dissolved in CH2Cl2 followed by layering CH3OH. The
crystalline solid of complex 2 was produced at 90% yield
(based on CoCl2). The elemental analysis is consistent with
the molecular formula of 2. The ESI-MS data displays a set of
peaks at 821.12 m/z, and its isotope pattern is in agreement
with the calculated one of 2 (Figure S8). The X-ray
crystallographic structure of complex 2 was reported previously
in the literature,32 where crystals were obtained by electro-
chemical oxidation of a sacrificial cobalt anode in the presence
of the [OPS1″]− ligand solution. The analogue of 2,
CoII(OPS1)2 ([OPS1]− = [OP(Ph)2(C6H4-2-S)]−, obtained
Four-coordinate CoII complexes are mostly stabilized in a
tetrahedral geometry; complexes with a square-planar structure
are relatively few. Where a square-planar geometry is found, its
underlying structure is determined by different ligand
substituents or through a second coordination sphere
interaction.25c,27 In CoII thiolate complexes, most geometries
are also tetrahedral.28 Precisely three examples with a square-
planar configuration exist in the literature: CoIIL where L is a
bipyridine-bis(thiolato) ligand;20 bis(maleonitrile-dithiolato)-
CoII;29 bis(dithioacetylacetonato)CoII complexes.30 The struc-
ture of a zinc(II) complex with two PS1″ ligands, [Zn(PS1″)2],
was reported in the literature.31 It has a tetrahedral geometry,
further implying that an unusual square-planar geometry
adopted in 1 is not attributed to the ligand constrain or steric
effect from a bulky trimethylsilyl substituent in the PS1″
ligand. Density functional theory (DFT, the B3LYP functional)
calculations were performed to provide more insights for
interpreting the unusual geometry adopted in 1 as well as have
an understanding of its electronic structure. Our calculations
show that 1 has a doublet ground state with a square-planar
geometry around Co. The only unpaired electron is localized at
the metal center (spin density = 1.15 |e|, Mulliken analysis),
indicating that the electronic structure of 1 is Co2+[(PS1″)2]2−
rather than Co1+[(PS1″)2]•1−. In contrast, the quartet state of
1 adopts a tetrahedral conformation and is 8.6 kcal/mol higher
in energy with most of its three unpaired electrons localized at
Co (spin density = 2.63 |e|). The electron spin-density plots for
both configurations are shown in Figures 2 and S6,
respectively. Furthermore, a single point calculation based on
the optimized structure of the pure doublet state was
performed to obtain a broken-symmetry doublet state with
two α unpaired electrons localized at Co and one β unpaired
electron localized on the ligand (Figure S7). However, we find
that this broken-symmetry doublet state is 30.0 kcal/mol
higher in energy than the pure doublet state. Importantly, the
optimization of this broken-symmetry doublet state led to the
pure doublet state. These calculations indicate that the
supporting ligand prefers to be innocent and Co prefers to
have a +2 oxidation state.
from a reaction of CoCl2 with PS1 ligand ([PS1]−
=
[P(Ph)2(C6H4-2-S)]−) was also reported.33 In both reported
works, only X-ray structures were described and other physical
properties were not investigated. The structural parameters of
complex 2 that we have obtained are similar to those
previously reported.32 According to the data, one asymmetric
unit contains two molecules and each molecule has a distorted
tetrahedral CoII center formed from bonds with two bidentate
OPS1″ ligands with τ4 values of 0.77 and 0.81, respectively.
The average Co−S and Co−O bond distances are 2.276 and
1.976 Å, respectively. The Co−S distances are approximately
0.1 Å longer than those in complex 1. The magnetic data of 2
measured by a SQUID magnetometer gave μ values of 4.11 μB
that is slightly higher than the spin-only value (3.87 μB) for a
configuration with a ground state S = 3/2, likely due to orbital
contribution (Figure S9). Complex 2 gives no EPR signal at
room temperature and 77 K, likely due to the fast spin−lattice
relaxation.34 However, it gives signals at 4 K with g = 4.80,
2.75, and 2.01, implying a S = 3/2 configuration of the Co(II)
center (Figure S10). The 1H NMR spectrum taken in CD2Cl2
gave paramagnetic shifts at 43.06, 33.63, and −13.50 ppm,
associated with three protons of the benzenthiolato group in 2
(Figure S11).
The isolation of complex 2 by the reaction of 1 with
dioxygen unequivocally indicates that the square-planar CoII
center in 1 activates dioxygen to form the ligand oxygenation
product, OPS1″. The reaction can be monitored by UV−vis−
NIR and NMR spectroscopies. The spectrum of 1 was taken
every hour for a 18 h period in a dioxygen atmosphere. As
shown in Figure 3, the spectrum gradually changed to the one
similar to that of complex 2 (Figure 3, inset). Two isosbestic
Oxygenation of Bound Phosphine Donor in
[CoII(PS1″)2] (1). The addition of dioxygen to a CH2Cl2
solution of 1 at room temperature causes a slow color change
C
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and 18O2 in a 1:1 ratio was monitored by ESI-MS. As shown in
Figure S13, [Co(PS1″)(OPS1″)] formed and then disap-
peared, followed by the formation of the product, [Co-
(OPS1″)2]. The analysis of isotope distribution of the
intermediate species is consistent with the theoretic pattern
of [1 + 16O] and [1 + 18O] in a 50:50 ratio. The composition
for the final product agrees with the theoretic pattern of [1 +
16O + 18O], [1 + 216O], and [1 + 218O] in a 50:25:25 ratio.
The observation implies that the addition of the O atom is
stepwise and likely involves an intermolecular path. The DFT
calculation was performed to interpret this finding (vide infra).
The addition of O2 to the solution of 1 does not cause an
instant color change between room temperature and −40 °C.
However, the solution changed to a deep greenish-brown
immediately when the temperature progressed down to −80
°C. While the temperature was raised, complex 1 was
recovered. The process was reversible and recorded by UV−
vis−NIR spectra (Figure S14). It has been reported in some
Co(II) complexes that O2 coordination is thermodynamically
unfavorable and often only observed at a very low
temperature.6d,35 This transit intermediate observed at low
temperature is likely associated with the O2 adduct of 1,
[Co(PS1″)2(O2)], as found in many other cobalt sys-
tems.6d,35,36 The X-band EPR measurements were performed
at 77 K to characterize this transit intermediate (Figure S15).
The spectrum of complex 1 with the addition of dioxygen in
frozen 2-MeTHF display a S = 1/2 signal with resolved
hyperfine coupling splitting to 59Co and 31P. The best fit gave
parameters of g1 = 2.036, g2 = 2.018, and g3 = 2.010 (|A1| = 38
MHz, |A2| = 13 MHz, |A3| = 56 MHz for 59Co; |A1| = 10 MHz, |
A2| = 21 MHz, |A3| = 26 MHz for 31P). A small magnetic
anisotropy observed in the spectrum compared to the data of 1
clearly indicates an O2 interaction to the Co(II) center, which
produces an effective axial ligand field.37 Furthermore, the
magnitude of 59Co hyperfine coupling constants are relatively
Figure 3. Variation of the UV−vis−NIR spectrum of [Co(PS1″)2]
(1) in CH2Cl2 (1.3 × 10−4 M) in a O2 atmosphere. The spectrum was
taken over an 18 h period: initial (red line) and final (blue line)
spectra. Inset: The UV−vis−NIR spectrum of [Co(OPS1″)2] (2) in
CH2Cl2.
points at 378 and 560 nm were observed, implying no
detectable intermediates. According to the absorbance of the
final spectrum, the formation yield of 2 is nearly quantitative
(∼95%). In addition, the transformation is also clearly
1
evidenced by following the reaction with H NMR measure-
ments (Figure S12). The spectrum taken under an O2
atmosphere showed the characteristic peaks of 1 at 23.73
and −18.88 ppm that gradually disappeared as well as the
appearance of peaks at 43.06, 33.63, and −13.50 ppm (Figure
S12A). In addition, the peaks at 0 to −10 ppm associated with
trimethylsilane groups of ligands display the same growth and
decline during the reaction (Figure S12B).
Isotopic labeling studies were used to understand the path of
O2 addition to complex 1. The reaction with a mixture of 16O2
Figure 4. DFT calculations for pathways of 1 reacting with O2 to form 2. The ground state multiplicity for each stationary point along the pathway
is marked as D for doublet states, Q for quartet states, and Qui for quintet states.
D
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small, implying that an unpaired electron density mainly
localizes in the bound O2 moiety. Thus, the electronic
× 103 M−1cm−1) in the UV−vis−NIR spectrum (Figure S21).
−
1
The diamagnetic nature of complex 3 is revealed by the H
structure of the O2 adduct of 1, [Co(PS1″)2(O2)], should be
NMR spectrum through resonance peaks appearing at the
range of 0−10 ppm (Figure S22), further indicating a low-spin
d6 configuration. The 31P NMR spectrum gives two peaks at
82.33 and 88.71 ppm attributed to two nonequivalent bound
phosphine donors (Figure S23). The structure of 3 was well
resolved by X-ray crystallography. It contains a distorted
octahedral CoIII center bound by two tridentate [PS2*]2−
ligands (Figure 1). Two phosphine atoms are in the cis
position. The average distance of the Co−S bonds is 2.286 Å.
Alkylation of Bound Thiolate in [NEt4][Co(PS2*)2] (3).
Dissolving complex 3 in CH2Cl2 yielded a reddish-brown
solution initially, but the color slowly changed to violet in
several days. This violet solution was layered with methanol to
give crystalline solids of two isomeric products, [Co(PS2*)-
considered as a Co(III)-superoxide species.5b,6d,35,38
To understand whether the oxidation process of phosphine
donor to phosphine oxide in 1 is mediated by the CoII center,
the reactivity of ZnII analogue with dioxygen was explored.
ZnII(PS1″)2 and ZnII(OPS1″)2 were prepared by following
literature procedures.31 The reactivity of ZnII(PS1″)2 with
dioxygen was monitored by NMR measurements. As shown in
Figures S16 and S17, the addition of dioxygen to a solution of
1
ZnII(PS1″)2 does not cause any change in the H and 31P
NMR spectra, implying no formation of ZnII(OPS1″)2. In
addition, H[PS1″] in solution at a dioxygen atmosphere is
stable at least for 1 day, as recorded at NMR spectra (Figures
S18 and S19). Thus, the CoII center was involved in the
activation of dioxygen, leading to the oxygenation of a bound
phosphine donor in 1. Similar chemistry has been observed in
a low-spin tetrahedral Co(II) system, [PhBP3]CoI ([PhBP3] =
[PhB(CH2PPh2)3]−).39
(PSSCH Cl*)] (4 and 4′) ([PSSCH Cl*]− = [P(Ph)(C6H3-3-Ph-
2-S)(C6H3-3-Ph-2-SCH2Cl)]−), in a 68% total yield (Scheme
2). The ESI-MS data of these products show a set of peaks at
1060.09 m/z, consistent with the theoretical isotope pattern of
4 and 4′. The formula agrees with the measurement of
elemental analysis. Attempts to separate 4 and 4′ by
recrystallization with various solvent systems were not
successful. Two types of crystals were identified in the
precipitated solid, and both were measured by X-ray
crystallography; the major product has a needle shape and
gave the structure of 4, and the minor one has a block-like
appearance and yielded the structure of 4′. Both structures
contain CoIII centers ligated by one [PS2*]2− ligand and one
2
2
DFT calculations were conducted to understand how 1 is
oxidized by O2 to form 2. We find that O2 first coordinates
with 1 to form 1-O2 with ΔG = 3.7 kcal/mol (Figure 4). This
is followed by insertion of the entire O2 into Co−P to form 1-
OO with a Co-OO-P moiety (ΔG⧧ = 17.4 kcal/mol and ΔG =
−15.7 kcal/mol). In the next step, a structural rearrangement
takes place in 1-OO to form 1-O(O) with ΔG⧧ = 4.7 kcal/mol
and ΔG = −33.0 kcal/mol. The oxo group of 1-O(O) has a
strong radical character (spin density = 0.98 |e|), suggesting
that this intermediate is very reactive. Indeed, all possible
subsequent reactions for 1-O(O) are very facile. The insertion
of oxo of 1-O(O) into the other Co−P forms 2 with ΔG⧧ =
2.7 kcal/mol and ΔG = −71.4 kcal/mol.
[PSSCH Cl*]− ligand, where one of the thiolato groups has been
2
alkylated by the chloromethyl group to form a thioether
moiety (Figure 5). The metal centers both adopt an octahedral
Besides the monomolecular reaction, 1-O(O) could react
with another Co complex. The coordination of 1-O(O) with
another 1 forms [1-O]-O-[1] with ΔG = −26.8 kcal/mol.
Then, the bridged O inserts into Co−P of the [1] moiety to
form two isolated 1-Os with ΔG⧧ = 9.1 kcal/mol and ΔG =
−37.8 kcal/mol. Alternatively, 1-(O)O could coordinate with a
1-O to form [1-O]-O-[1-O] with ΔG = −37.6 kcal/mol. This
is followed by a fast insertion of the bridged O into one of the
1-O moieties to form 1-O plus 2 with ΔG⧧ = −0.4 kcal/mol
and ΔG = −33.8 kcal/mol, where ΔG⧧ is negative due to the
inclusion of the ZPVE correction and is an indication that
there is no barrier for this elementary step. Overall, we find
that the 1-O(O) prefers to proceed through the bimolecular
reactions. This explains why two oxygens in 2 could come from
two different O2 molecules, as evidenced by isotope labeling
studies. The rate-determining step for the oxidation of 1 by O2
to 2 is the insertion of O2 into Co−P of 1.
Syntheses and Characterization of [NEt4][Co(PS2*)2]
(3). A six-coordinate CoIII complex, [NEt4][Co(PS2*)2] (3)
([PS2*]2− = [P(Ph)(C6H3-3-Ph-2-S)2]2−), was obtained from
a reaction of CoCl2 and [PS2*]2− in methanol with a 1:2 ratio.
The addition of the cation, tetraethylammonium, to the
reaction mixture gave a brown precipitate. The solid was
recrystallized from THF and methanol to give crystals of 3·
2CH3OH·0.5THF in a 75% yield. The formula is confirmed by
elemental analysis. The ESI-MS data gives a set of peaks at
1011.09 m/z, and its isotope pattern is consistent with the
calculated one of [Co(PS2*)2]− (the anion of 3) (Figure S20).
Dissolving complex 3 in CH3CN yielded a reddish brown
solution and showed an absorption band at 483 nm (ε = 2.35
Figure 5. Thermal ellipsoid plots (35% probability) of 4·5CH3OH
(A) and 4′·CH2Cl2 (B). The H atoms and solvated molecules are
omitted for clarity.
geometry by binding with two phosphine donors, three
thiolato groups, and one chloromethyl thioether. The two
structures are isomeric; the thioether group is trans to the
thiolate in 4 while in 4′ it is trans to the phosphine donor. The
average distances of Co−S bonds in both structures are 2.267
and 2.275 Å, respectively, similar to that in 3. Notably, the
Co−S bond distances decrease slightly upon alkylation; for
example, Co−S4 = 2.2754 Å in 3 changes to 2.266(1) Å in 4;
Co−S3 = 2.3103(5) Å in 3 changes to 2.295(1) Å in 4′. The
comparable distances of Co−Sthiolate and Co−Sthioether bonds
have been found in the alkylation of other metal thiolates and
E
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well discussed by the Grapperhaus group.15c The variation of
metal−sulfur bond distance is due to two types of orbital
interaction upon alkylation of metal-bound thiolate to
thioether; (1) the π*-bonding interaction between metal and
sulfur decreases, leading to a shortened M−S bond, and (2)
the σ-bonding interaction is less for the thioether donor
compared to thiolate, resulting in the elongation of the M−S
bond. Consequently, these two competing factors compensate
each other, giving rise to similar Co−S bond distances upon
alkylation.
1
The H NMR spectra show two sets of absorption peaks
associated with methylene groups in chloromethyl thioether
derived from 4 and 4′ (Figures 6A and S24). Two protons are
Figure 7. Variation in UV−vis−NIR spectrum of 3 in CH2Cl2. The
spectrum was monitored over a 70 h period: the initial (red line) and
the final (blue line) spectra. Inset: the UV−vis−NIR spectrum of 4
and 4′ in CH2Cl2.
bound thiolate via a nucleophilic reaction. To confirm the
nucleophilic nature of the bound thiolate in 3, the reactions
with other strong electrophiles such as benzyl chloride and
methyl iodide were carried out and monitored by UV−vis−
NIR spectra (Figures S26 and S27). With 100 equiv of methyl
iodide, the reaction proceeded immediately, and with benzyl
chloride, the reaction completes at around 7 h. The final
spectra of the two reactions were alike. The ESI-MS analysis of
the reaction products indicates dialkylation species of 3, [3 +
2CH3]+1 and [3 + 2CH2Ph]+1, respectively (Figures S28 and
S29).
Figure 6. (A) 1H NMR spectrum of 4 and 4′ in CD2Cl2 at a range of
4.50 to 4.00 ppm. (B) The 31P NMR spectrum of 4 and 4′ in CD2Cl2
The CoII-bound thiolate in complex 1 does not show
nucleophilicity toward dichloromethane or stronger electro-
philes such as benzyl chloride. Even with the addition of 100
equiv of reactive alkylating reagent, CH3I, the reaction does
not proceed for several hours (Figure S30), in contrast to a
literature example of a square-planar CoII thiolate that
undergoes S-alkylation with CH3I.20
▲
at a range of 120 to 60 ppm. Resonance peaks associated with 4 (
and 4′ ( ).
)
●
diastereotopic and give two sets of doublets for each species:
4.35 and 4.14 ppm for 4 with 2JH−H = 10.36 Hz; 4.39 and 4.08
2
ppm for 4′ with JH−H = 10.15 Hz. According to the ratio of
intensity, the composition of 4 and 4′ is around 5:1. The 31P
NMR data gives two resonance peaks at 91.58 and 85.02 ppm,
attributed to two nonequivalent phosphines in 4 (Figure 6B).
The set of peaks at 86.10 and 81.34 ppm are associated with
those in 4′.
CONCLUSIONS
■
In summary, we have isolated and characterized a four-
coordinate CoII complex with a P2S2 coordination environ-
ment. The structure of the CoII center adopts a square-planar
geometry, which is unusual relative to a pseudotetrahedron.
The finding is supported by the DFT calculations that 1 has a
doublet ground state with a square-planar geometry around
Co. Importantly, the complex activates dioxygen and leads to a
ligand-based oxygenation product: a distorted tetrahedral CoII
complex binding with two (thiolato)phosphine oxides. Such a
metal-mediated oxo transformation derived from dioxygen to
the substrate is unprecedented in a well-defined square-planar
CoII complex. In addition, a low-spin octahedral CoIII complex
binding to two tridentate bis(thiolato)phosphine ligands was
obtained. The bound thiolato group of this complex shows a
strong nucleophilic reactivity to activate CH2Cl2, which is a
weak electrophile. The reaction leads to the formation of CoIII
complexes containing a chloromethyl thioether moiety. It has
been well addressed by Ashby and Enemark that the
nucleophilicity of bound sulfur in metal thiolates is attributed
to a four-electron destabilizing interaction of lone pairs on
thiolate sulfur and fulfilled metal d orbitals.14 This hypothesis
has been further elaborated by Grapperhaus and co-workers
The conversion of 3 reacting with CH2Cl2 to give 4 and 4′
was monitored by UV−vis−NIR spectra. As shown in Figure 7,
the electronic spectrum of 3 in CH2Cl2 shows an absorption at
483 nm initially, but slowly changes to the one with a broad
band between 500 and 600 nm. The final spectrum resembles
the one obtained from the mixtures of 4 and 4′ in CH2Cl2. The
conversion of 3 to 4 and 4′ is marked by an isosbestic point at
575 nm. Isotope studies combined with ESI-MS spectrometric
investigations concur with our results. The molecular ion peaks
of the reaction products have isotopic distributions at 1060.09
m/z, consistent with the theoretic pattern of [Co(PS2*)-
(PSSCH Cl*)] (4 and 4′) (Figure S25A). When complex 3
2
reacts with CD2Cl2 instead of CH2Cl2, the isolated products
show a molecular ion peak at 1062.09 m/z with the isotope
distribution in agreement with the theoretic one of [Co-
(PS2*)(PSSCD2Cl*)] (Figure S25B). It is clear that the
chloromethyl moiety of thioether group in 4 and 4′ is from
the activation of the C−Cl bond in CH2Cl2.
The formation of 4 and 4′ from the reaction of 3 with
CH2Cl2 indicates that the C−Cl bond is activated by CoIII-
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SADABS absorption correction was made.46 The SHELXTL
structural refinement program was employed.47 All non-hydrogen
atoms were refined with anisotropic displacement factors. All
hydrogen atoms were calculated using the riding model. CCDC
1574445, CCDC 1574262, CCDC 1574279, and CCDC 1574278
contain the supplementary crystallographic data of 1, 3, 4, and 4′,
respectively, for this paper.
Synthesis of [Co(PS1″)2] (1). H[PS1″] (0.1 g, 0.273 mmol) and
lithium (0.0028 g, 0.403 mmol) were dissolved in CH3OH to
generate a solution. The addition of a methanol solution of CoCl2
(0.0177 g, 0.136 mmol) to the reaction mixture produced an orange
precipitate. The obtained solid was recrystallized from CH2Cl2 and
CH3OH to give reddish-orange crystals with a yield of 79% (based on
CoCl2). Anal. Calcd for C42H44P2S2Si2Co: C, 63.86; H, 5.61; S, 8.12.
Found: C, 63.22; H, 5.54; S, 8.07. 1H NMR in CD2Cl2: δ 23.73 (2H,
Ph), 12.41 (3H, Ph), 11.26 (5H, Ph), 9.37 (4H, Ph), 6.46 (8H, Ph),
1.49 (1H, Ph), 0.09 (2H, Ph), −4.35 (18H, Si(CH3)3), −18.88
(1H,Ph) ppm. UV−vis−NIR in CH2Cl2 (λ, nm; ε, M−1 cm−1): 391
(6.48 × 103), 427 (5.08 × 103), 520 (4.81 × 103), 822 (7.28 × 101).
ESI-MS: calcd for C42H44CoP2S2Si2 789.12, found 789.13.
Synthesis of [Co(OPS1″)2] (2). For Method A, [Co(PS1″)2] (1)
(0.100 g, 0.127 mmol) was dissolved in CH2Cl2, and purging
dioxygen to the solution for 30 s caused the color to gradually change
from orange to bluish-green. The solution was concentrated by
vacuum and layered with CH3OH. A crystalline solid of
[CoII(OPS1″)2] (2) was produced after a few days (0.105 g, 0.118
mmol, ca. 93% based on 1). For Method B, H[OPS1″] (0.100 g,
0.261 mmol) and lithium (0.0022 g, 0.317 mmol) were dissolved in
CH3OH to generate a solution. The addition of a CH3OH solution of
CoCl2 (0.017 g, 0.131 mmol) to the reaction mixture gave a bluish-
green precipitate that was recrystallized from CH2Cl2 and CH3OH
(0.193 g, 0.108 mmol, ca. 90% based on CoCl2). Anal. Calcd for
C42H44P2O2S2Si2Co: C, 61.37; H, 5.40; S, 7.80. Found: C, 61.16; H,
5.52; S, 7.82. 1H NMR in CD2Cl2: δ 43.06 (2H, Ph), 33.63 (2H, Ph),
9.01 (10H, Ph), 7.72 (6H, Ph), 0.21 (4H, Ph), −5.84 (18H,
Si(CH3)3), −13.50 (2H,Ph) ppm. UV−vis−NIR in CH2Cl2 (λ, nm; ε,
M−1 cm−1): 363 (6.75 × 103), 413 (1.73 × 103), 427 (1.50 × 103),
556 (4.16 × 102), 616 (3.17 × 102), 676 (1.56 × 102). ESI-MS: calcd
for C42H44CoO2P2S2Si2 821.11, found 821.12.
that the reactivity is enhanced when the metal d and sulfur p
orbitals have comparable energy and good overlap.10c,40 Thus,
for low-spin octahedral CoIII in complex 3, the small ionic radii
and fulfilled t2g electrons might lead to a significant metal−
sulfur π-antibonding orbital interaction and large electron
repulsion, consequently resulting in a strong nucleophilicity.
The CoII center in 1 induces dioxygen activation and leads
to the oxygenation of phosphine donors. Such CoII-mediated
dioxygen cleavage has also been seen in a well-defined square-
planar complex and in a complex with NN3 tripodal ligand that
provides an intramolecular H-bonding interaction.41 Further-
more, CoII complexes, [Co(PEt3)2Cl2] and [Co(acac)2],
catalyzing phosphine oxidation with dioxygen have been
documented.42 ESI-MS data combined with isotope labeling
studies show that the addition of the O atom is stepwise and
likely involves an intermolecular path. The DFT calculation
provides a feasible reaction pathway that the first oxygenation
occurs in a single cobalt center, followed by the second
oxygenation with a dicobalt transition state. Although
[Co(O2)]2+ and [Co(O)]2+ metastable species have been
identified in Co complexes with other ligand systems,36,43
efforts for isolating such intermediates in this system are
continuing in our laboratory.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out
under an atmosphere of dinitrogen with a standard Schlenk technique
or glovebox. Ligands, [P(Ph)2(C6H3-3-SiMe3-2-SH)], [PO-
(Ph)2(C6H3-3-SiMe3-2-SH)], and [P(Ph)(C6H3-3-Ph-2-SH)2], were
synthesized according to the literature procedures.44 Solvents were
dried and distilled by standard procedures. Otherwise, all starting
materials were obtained commercially and used without further
purification.
Physical Methods. Electronic spectra were recorded in the range
of 300−1100 nm on a Hewlett-Packard 8453 spectrometer. NMR
spectra were obtained on a Bruker AMX400 and Bruker
AVIIIHD700X. The samples were prepared in a sealed NMR tube
Synthesis of [N(C2H5)4][CoIII(PS2*)2] (3). H2[PS2*] (0.1000 g,
0.209 mmol) and lithium (0.0032 g, 0.461 mmol) were reacted to
generate a clear solution. After the addition of CoCl2 (0.0136 g, 0.105
mmol) in methanol, the solution color changed to reddish-brown.
The addition of the cation, [N(C2H5)4]Br (0.0221 g, 0.105 mmol), in
CH3OH to the reaction mixture gave a brown precipitate. The solid
was recrystallized from THF and methanol to give crystals of 3·
2CH3OH·0.5 THF after 10−14 days (0.090 g, 0.079 mmol, ca. 75%
based on CoCl2). Anal. Calcd for C72H74CoNO2.5P2S4: C, 69.60; H,
6.00; N, 1.13; S, 10.32. Found: C, 70.69; H, 5.83; N, 1.12; S, 10.65.
1H NMR in CD2Cl2: δ 6.60−7.90 (42H, Ph), 2.56 (8H, N(C2H5)4),
0.68 (12H, N(C2H5)4) ppm. 31P NMR in CD2Cl2: δ 82.33, 88.71
ppm. UV−vis−NIR in CH2Cl2 (λ, nm; ε, M−1 cm−1): 483 (2.35 ×
103). ESI-MS: calcd for [C60H42CoP2S4]− 1011.10, found 1011.09.
1
under a nitrogen atmosphere in a glovebox. H NMR was referenced
to the residual proton peaks of the solvents. 31P NMR was referenced
to −6 ppm of PPh3 as external standard. Elemental analyses were
measured with an Elementar vario EL III. The electrospray ionization
(ESI) mass data were taken in CH2Cl2 or THF with the LTQ
Orbitrap XL, Thermo-Fisher spectrometer. The magnetization data
was recorded on a SQUID magnetometer (Quantum Design MPMS
SQUID VSM System) with an external 1 T magnetic field at a
temperature range of 1.8 to 300 K. The susceptibility data were
corrected for diamagnetic contributions by using Pascal constants.
EPR samples of 1 and 2 were prepared under a nitrogen atmosphere
in a glovebox. The sample of 1 reacting with dioxygen was obtained
by purging O2 into the 2-MeTHF solution of 1. The spectra of 1 in
both a N2 and O2 atmosphere were obtained at the X band by using a
Bruker ELEXSYS E580 spectrometer at 77 K. The measurement
parameters were as follows: microwave frequency, 9.65 GHz;
modulation amplitude, 3 G; modulation frequency, 100 kHz;
power, 15 mW. The spectrum of 2 was recorded at 4 K with a
microwave frequency of 9.62 GHz, modulation amplitude of 10 G,
and power of 2 mW. Spectral analysis and simulations were performed
using the EasySpin program.45
Synthesis of [CoIII(PS2*)(PSSCH Cl*)](4 and 4′). [N(C2H5)4]-
2
[CoIII(PS2*)2] (3) was dissolved in CH2Cl2 and layered with
CH3OH. A violet black crystalline solid of Co(PS2*)(PSSCH Cl*) (4
2
and 4′) was produced after 3 days with a 68% yield. Anal. Calcd for
C61H44ClCoP2S4: C, 69.01; H, 4.18; S, 12.08. Found: C, 68.13; H,
4.26; S, 11.90. 1H NMR in CD2Cl2: δ 6.70−7.80 (42H, Ph), 4.39 (d,
2
2
1H, JH−H = 10.15 Hz, CH2Cl, 4′), 4.35 (d, 1H, JH−H = 10.36 Hz,
2
CH2Cl, 4), 4.14 (d, 1H, JH−H = 10.36 Hz, CH2Cl, 4), 4.08 (d, 1H,
2JH−H = 10.15 Hz, CH2Cl, 4′) ppm. 31P NMR in CD2Cl2: δ 91.58 (4),
86.10 (4′), 85.02 (4), 81.34 (4′) ppm. ESI-MS: calcd for
C61H44ClCoP2S4 1060.08, found 1060.09.
X-ray Structure Determination. Crystallographic data and
refinement parameters of 1, 3·2CH3OH·0.5THF, 4·5CH3OH, and
4′·CH2Cl2 are summarized in Tables S1−S4. Selected bond lengths
and angles are listed in Tables S5−S8. Each crystal was mounted on a
glass fiber and quickly coated in epoxy resin. Diffraction measure-
ments were performed with a Nonius Kappa diffractometer equipped
with a CCD detector. Least-squares refinement of the positional and
anisotropic thermal parameters for the contribution of all non-
hydrogen atoms and fixed hydrogen atoms was based on F2. A
Computational Details. The B3LYP functional combined with
the LACVP** basis set (LanL2DZ for Co and 6-31G** for the
others) was used for the geometry optimizations and the zero-point
vibrational energy (ZPVE). To obtain more accurate electronic
energies, single point energy calculations using a larger basis set
(LACV3P++**, LanL2TZ for Co, and 6-311++G** for the others)
G
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with Grimme’s D3 empirical correction were performed on the
B3LYP/LACVP** optimized structures. All the calculations were
performed using the Gaussian 16 package. The energies discussed
here are Gibbs free energies obtained using the equation as follows for
nonlinear molecules:
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Science and
Technology in Taiwan (MOST 107-2113-M-006-003, 107-
2113-M-006-008-MY2, and 108-2113-M-007-029) and Na-
tional Cheng Kung University in Taiwan. We thank Mr. Ming-
Feng Chen and Mr. Tsung-Lun Kan in the Establishment of
the Instrument Development Center at National Cheng Kung
University for assisting with the measurements of ESI-MS data
and NMR data.
hv
G298K = Eelec + ZPVE + 4kT + ∑
ehv/kT − 1
v
− T × (Strans + Srot + Svib)
4kT is replaced by 3.5kT for O2(g)
.
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sı
* Supporting Information
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Additional characterization, spectra, and crystallographic
data (PDF)
Accession Codes
CCDC 1574262, 1574278, 1574279, and 1574445 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Hua-Fen Hsu − Department of Chemistry, National Cheng
Kung University, Tainan 701, Taiwan; orcid.org/0000-
0001-8021-193X; Email: konopka@mail.ncku.edu.tw
Mu-Jeng Cheng − Department of Chemistry, National Cheng
Kung University, Tainan 701, Taiwan; orcid.org/0000-
0002-8121-0485; Email: mjcheng@mail.ncku.edu.tw
Authors
Yi-Ying Wu − Department of Chemistry, National Cheng Kung
University, Tainan 701, Taiwan
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Jia-Cheng Hong − Department of Chemistry, National Cheng
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Ruei-Fong Tsai − Department of Chemistry and Frontier
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Taiwan
Hung-Ruei Pan − Department of Chemistry, National Cheng
Kung University, Tainan 701, Taiwan
Bo-Hua Huang − Department of Chemistry, National Cheng
Kung University, Tainan 701, Taiwan
Yun-Wei Chiang − Department of Chemistry and Frontier
Research Center on Fundamental and Applied Sciences of
Matters, National Tsing Hua University, Hsinchu 30013,
Taiwan; orcid.org/0000-0002-2101-8918
Gene-Hsiang Lee − Department of Chemistry, National
Taiwan University, Taipei 106, Taiwan
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
H
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