Tuning the O2 Binding Affinity of Cobalt(II) Centers by Changing the Structural and Electronic Properties of the Distal Substituents on Azole-Based Chelating Ligands

Article abstract of DOI:10.1021/acs.inorgchem.8b02241

The effects of the substituents on the chelating ligands located in the secondary coordination sphere on the O₂ affinity of cobalt(II) centers have been explored. The combination of facially capping tridentate tris(pyrazolyl)borates (= Tp^Me2,4R) and bidentate bis(imidazolyl)borates (= [B(Im^N-Me)₂MeX]^- L^X) yields square-pyramidal cobalt(II) complexes. The structural properties of the substituent groups X attached to the boron center of L^X affect the arrangement of X in the resulting cobalt(II) complexes [Co^II(Tp^Me2,4R)(L^X)]. When the boron-attached moiety of X is a relatively bulky sp³-CH₂Y group (i.e., X:Y = Me:H and nBu:nPr), the alkyl group X faces the cobalt center, whereas for isopropoxy (OiPr) and phenyl (Ph) groups, of which the boron-attached atoms are a less hindered oxygen atom and a planer sp²-carbon, respectively, the X group is arranged away from the cobalt center. This flexible behavior of L^X is reflected in the O₂ affinity of the cobalt(II) center, which depends on the extent to which the complex sphere is shielded by the ligands. The dependence of the cobalt(II) oxidation potential on the X substituent of L^X is inconsistent with the O₂ affinity. On the other hand, the electronic properties of R, which is attached to the fourth position of the pyrazolyl rings in the rigid Tp^Me2,4R ligand, are reflected in the electrochemical properties and O₂ affinity of the cobalt center.

Full text of DOI:10.1021/acs.inorgchem.8b02241

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Tuning the O2 Binding Affinity of Cobalt(II) Centers by Changing the
Structural and Electronic Properties of the Distal Substituents on
Azole-Based Chelating Ligands
Toshiki Nishiura, Yosuke Chiba, Jun Nakazawa,* and Shiro Hikichi*
Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University 3-27-1 Rokkakubashi, Kanagawa-ku,
Yokohama 221-8686, Japan
S
* Supporting Information
ABSTRACT: The effects of the substituents on the chelating ligands
located in the secondary coordination sphere on the O2 affinity of
cobalt(II) centers have been explored. The combination of facially
capping tridentate tris(pyrazolyl)borates (= TpMe2,4R) and bidentate
bis(imidazolyl)borates (= [B(ImN‑Me)2MeX]− ; LX) yields square-
pyramidal cobalt(II) complexes. The structural properties of the
substituent groups X attached to the boron center of LX affect the
arrangement of X in the resulting cobalt(II) complexes [CoII(TpMe2,4R)-
(LX)]. When the boron-attached moiety of X is a relatively bulky sp3-
CH2Y group (i.e., X:Y = Me:H and nBu:nPr), the alkyl group X faces the
cobalt center, whereas for isopropoxy (OiPr) and phenyl (Ph) groups, of which the boron-attached atoms are a less hindered
oxygen atom and a planer sp2-carbon, respectively, the X group is arranged away from the cobalt center. This flexible behavior of
LX is reflected in the O2 affinity of the cobalt(II) center, which depends on the extent to which the complex sphere is shielded
by the ligands. The dependence of the cobalt(II) oxidation potential on the X substituent of LX is inconsistent with the O2
affinity. On the other hand, the electronic properties of R, which is attached to the fourth position of the pyrazolyl rings in the
rigid TpMe2,4R ligand, are reflected in the electrochemical properties and O2 affinity of the cobalt center.
We have designed O2-binding mononuclear nonheme
metallocomplexes involving “five azole-supported” transition-
metal centers10 by combining facially capping tridentate N3
donors, hydrotris(3,4,5-substituted-1-pyrazolyl)borates (=
[HB(pzR)3]−; TpR),11,12 with bidentate N2 donors, bis(1-
methyl-2-imidazolyl)borates (= [B(ImN‑Me)2MeX]−; LX).13−20
These chelating reagents act as monoanionic ligands composed
of tetra-coordinated boron centers and azoles. Both ligands
have great advantages for controlling the steric and electronic
properties of the resulting metal complexes. In the metal
complexes of TpR, the properties of the substituent groups (=
R3, R4, and R5) on the pyrazolyl rings of TpR affect the nature
of the metal centers. Interestingly, the substituent at the R4 site
of the pyrazolyl ring, which is positioned distal to the metal
center, affects the electronic nature without changing the steric
environment of the metal center.11,12,21 In the case of LX, the
boron-attached group X affects the dihedral angle of the two
imidazolyl rings as well as the arrangement of another boron-
attached methyl group.17 In this study, we have explored the
effects of the ligand substituent groups located in the
secondary coordination sphere (i.e., R of hydrotris(3,5-
dimethyl-4-R-pyrazolyl)borates (TpMe2,4R) and X of LX) on
the O2 affinity of the supported transition-metal centers. We
chose cobalt as the central metal because the dioxygen
1. INTRODUCTION
In enzymes that catalyze oxidation reactions and respiratory
pigments that function as O2 carriers in living systems,
dioxygen complexes of transition metals are formed via
oxidative addition of the dioxygen molecule to a coordinatively
unsaturated metal center.1−3 Therefore, investigations of
transition-metal complexes that exhibit an O2 binding ability
that mimics the functions of living systems have been of
continuous interest because these investigations provide useful
information for understanding O2 metabolic processes and for
constructing functional materials, such as catalysts, gas
absorbents, and permeation agents.4−6 Controlling the O2
affinity of the metal center by tuning the coordination
environment is a promising approach for obtaining highly
efficient materials. The dioxygen-binding metal centers in
biological O2 transport systems are supported by azole-based
ligands such as a porphyrin (polypyrrole) and a histidine
residue (imidazole). In hemoglobin/myoglobin, which are
mammalian O2 transport and storage heme proteins,
mononuclear iron centers are supported by five azoles
composed of an equatorial tetracyclic porphyrin and an axial
imidazole.7 In contrast, nonheme O2 transport proteins,
namely, hemocyanin and hemerythrin, involve dinuclear
metal centers supported by terminal imidazole ligands. O2 is
trapped between two copper(I) centers in hemocyanin,8
whereas in hemerythrin, one of two iron(II) centers, which
is supported by an N2O3 donor set, captures O2.9
Received: August 8, 2018
© XXXX American Chemical Society
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Scheme 1. Synthesis of Co(II) Complexes
complexation chemistry of cobalt(III) species is well
established,22−26 and interest in cobalt-O2 compounds from
the viewpoints of bioinorganic27−44 to material45−59 chemistry
has been growing. In addition, cobalt complexes are generally
robust against irreversible oxidative transformations, unlike the
related iron analog.60−62 Therefore, the study of cobalt
complexes would facilitate examination of the effects of the
ligands.
vacant site on the cobalt center in 1 and 4, whereas the space
around the sixth vacant site on 2 and 3 is more narrow because
the Me and nBu groups face the metal center. To quantitatively
assess the structural effects of X on 1−4, the percentage of the
complex sphere shielded by the ligands was calculated based
on solid angle analyses.64 Complexes 2 and 3 containing LMe
and LnBu showed sphere-shielding percentages of 94.0% and
94.6%, respectively. In contrast, the corresponding values for 1
(LOiPr) and 4 (LPh) were 89.9 and 91.3%, respectively (see
drawings of the sphere-shielding and space filling diagrams
provided as Figures S8 and S9 in Supporting Information).
Similar trends in the arrangement of X have been observed for
[Ni(LX)2] (where X = OMe, H, Me, nBu, and Ph) and
[NiII(TpiPr2)(LX)] complexes (TpiPr2 = hydrotris(3,5-diiso-
propyl-1-pyrazolyl)borate and X = Me and nBu).17 The
observed trend in the orientation of the boron-attached X
groups has been attributed to the steric interactions around
both the boron center of LX and the metal center of the
resulting complex. In both previous homoleptic nickel
complexes [Ni(LX)2] and the presented heteroleptic cobalt
complexes [CoII(TpMe2,4H)(LX)], the bulkiness of the boron-
attached part of X affects the dihedral angles between two
imidazolyl rings of LX. When X is Me or nBu, two 1-methyl
groups on the imidazolyl rings and the boron-attached methyl
group of LX are oriented to the opposite site of the metal
center, and the steric repulsion between these methyl groups
leads the smaller dihedral angles (in [Ni(LX)2]: 118.6° (X =
Me) and 123.6° (X = nBu),17 in [CoII(TpMe2,4H)(LX)]: 123.5°
(X = Me; 2) and 129.6° (X = nBu; 3)) as well as approaching
of X to the metal center due to tetrahedral borate geometry. In
contrast, the planar phenyl group and the less sterically
demanding oxygen atom of the alkoxide groups are located in
the cleft formed by two 1-methyl groups on LPh and LOR, and
those result in the larger dihedral angles on two imidazolyl
rings (in [Ni(LX)2]: 129.2° (X = Ph) and 125.6° (X =
OMe),17 in [CoII(TpMe2,4H)(LX)]: 144.6° (X = Ph; 4) and
2. RESULTS AND DISCUSSION
2.1. Structural Characterization of Mixed Ligand
Complexes. In this study, we investigated a series of mixed
ligand complexes of cobalt(II), [CoII(TpMe2,4R)(LX)] (1−6;
see Scheme 1) and a related complex [CoII(TpMe2,4H)(Bp)] (7;
Bp denotes dihydrobis(pyrazolyl)borate). The mixed ligand
complexes were synthesized by the reaction of acetato
complexes [CoII(TpMe2,4R)(κ2-OAc)]63 with alkaline metal
salts of the bidentate azole ligands LiLX and KBp. The
molecular structures of all the synthesized cobalt(II)
complexes were revealed by X-ray crystallography and are
summarized in Figure 1 and Table 1.
2.1.1. Coordination Properties of LX in the TpMe2,4H
Complexes. The coordination structures of the cobalt(II)
centers in 1−4 are very similar to each other: The square-
pyramidal cobalt(II) centers are supported by three pyrazolyl
nitrogens of TpMe2,4H and two imidazolyl nitrogens of LX (1−4
in Figure 1). Interestingly, the arrangement of X bound to the
boron center can be categorized into two groups depending on
the structural properties of X. When X = Me and nBu, within
which the boron-attached α-carbon is a relatively bulky sp3-
CH2Y group (Y = H and nPr), the alkyl groups approach the
cobalt centers in 2 and 3. In contrast, the isopropoxy (OiPr)
and phenyl (Ph) groups, within which the boron-attached
atoms are a less hindered oxygen atom and a planer sp2-carbon,
respectively, are located away from the cobalt centers in 1 and
4. As a result, a large amount of space exists around the sixth
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−
Figure 1. ORTEP diagrams of [CoII(TpMe2,4R)(LX)] (1−6) and [CoII(TpMe2,4H)(Bp)] (7) and a ball and stick drawing of [CoIII(O2 )-
(TpMe2,4H)(LOiPr)] (1S). All hydrogen atoms are omitted for clarity; ellipsoids are set at 30% probability.
156.4° (X = OiPr; 1), and the boron-attached methyl group in
these complexes is arranged away from the metal center.
Notably, difference in the amplitude of the dihedral angles
between Lalkyl (2 and 3) and others (LPh and LOiPr; 4 and 1) in
[CoII(TpMe2,4H)(LX)] is larger than that observed in [Ni(LX)2].
In the entitled cobalt complexes 1−4, the cobalt centers are
surrounded by three methyl groups attached on the R3 position
of the pyrazolyl rings of TpMe2,4H, and steric repulsion between
these methyl groups and the C−H groups of fourth position of
imidazolyl rings seems to affect the dihedral angles of LX. In
2.1.2. Effect of R on TpMe2,4R. Then, we examined the effects
of R on TpMe2,4R by introducing an electron-donating methyl
(Me) or electron-withdrawing bromine (Br) group as the
substituent (= R) at the fourth position of the pyrazoles. LPh
was employed as a bidentate partner because its cobalt(III)-
superoxo derivative was the most stable in a series of TpMe2,4H
complexes, as will be shown later. As we expected, the
molecular structures of the TpMe2,4Br (5) and TpMe2,4Me (6)
derivatives were very similar to that of the TpMe2,4H complex
(4). In particular, identical arrangements of the phenyl group
in LPh were observed for complexes 4, 5, and 6.
2.1.3. Bp Complex. To clarify the effects of the steric and
electronic properties of the bidentate ligands, an analogous
mixed ligand complex [CoII(TpMe2,4H)(Bp)] (7; where Bp =
dihydrobis(pyrazolyl)borate) was also investigated. The whole
molecular structure of the cobalt(II) complex 7 was similar to
those of complexes 1−4 with LX derivatives and previously
the Lalkyl complexes of nickel with the bulky TpiPr2
,
[NiII(TpiPr2)(LX)], however, the steric repulsion between the
metal-surrounding iPr groups and the boron-bounded alkyl
groups is critical to the amplitude of the dihedral angles of
Lalkyl, as found in the difference of the dihedral angles 129.9°
(X = Me) vs 160.3° (X = nBu).17
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a
Table 1. Structural Parameters of 1−7 and 1S
b
1
1S
2
3
4
5
6
7
Bond Lengths (Å)
Co1-N1
Co1-N3
Co1-N5
Co1-N7
Co1-N9
Co1-O1
O1-O2
2.107(3)
2.057(3)
2.044(3)
1.996(3)
1.971(3)
−
−
2.052(4)
1.978(3)
2.005(4)
1.962(4)
1.952(3)
1.901(3)
1.301(5)
2.115(3)
1.974(2)
1.974(2)
1.937(2)
1.937(2)
−
−
2.117(7)
1.959(7)
1.975(7)
1.922(7)
1.941(7)
−
−
2.119(2)
1.965(2)
1.965(2)
1.923(2)
1.923(2)
−
−
2.102(2)
1.974(2)
1.966(2)
1.919(2)
1.922(2)
−
−
2.092(3)
1.967(2)
1.967(2)
1.928(2)
1.928(2)
−
−
2.096(3)
1.986(2)
2.015(2)
1.990(2)
1.975(2)
−
−
Selected Angles (°)
N1-Co1-N3
N1-Co1-N5
N1-Co1-N7
N1-Co1-N9
N3-Co1-N5
N3-Co1-N7
N3-Co1-N9
N5-Co1-N7
N5-Co1-N9
N7-Co1-N9
Co-O1-O2
dihedral angles of LX (°)
89.3(1)
92.2(1)
103.6(1)
97.7(1)
82.3(1)
166.7(1)
91.9(1)
93.8(1)
168.5(1)
89.6(1)
−
89.9(2)
90.8(2)
93.2(2)
90.6(2)
88.4(2)
176.8(2)
91.3(2)
91.4(2)
178.5(2)
88.8(2)
119.6(3)
146.82
90.8(1)
90.8(1)
94.4(1)
94.4(1)
87.6(1)
174.7(1)
91.1(1)
91.1(1)
174.7(1)
89.7(1)
−
90.7(3)
91.6(3)
94.1(3)
95.1(3)
87.1(3)
175.0(3)
92.9(3)
91.3(3)
173.2(3)
88.1(3)
−
91.4(1)
91.4(1)
95.9(1)
95.9(1)
86.3(1)
172.6(1)
92.2(1)
92.2(1)
172.6(1)
88.4(1)
−
90.5(1)
90.2(1)
98.1(1)
96.3(1)
88.4(1)
171.4(1)
90.8(1)
92.0(1)
173.5(1)
87.9(1)
−
91.2(1)
91.2(1)
98.4(1)
98.4(1)
87.3(1)
170.4(1)
91.6(1)
91.6(1)
170.4(1)
87.9(1)
−
90.8(1)
90.6(1)
99.0(1)
96.8(1)
86.5(1)
170.3(1)
90.4(1)
92.9(1)
172.0(1)
88.9(1)
−
d
156.41
0.03
123.54
0
129.63
0.03
144.64
0
137.08
0.04
144.49
0
117.75
0.03
c
τ
0.03
a
Due to the presence of a mirror plane (on Co1-N1-B1), the atom numberings N5, N7, and N9 of complexes 2, 4, and 6 in this table correspond to
b
c
d
the N3′, N5, and N5′ atoms, respectively, in the crystallographic data. From ref 10. (The largest angle − the second-largest angle)/60°. Dihedral
angle of Bp.
reported TpR-BpR mixed ligand complexes.65,66 A τ value of
0.03 indicated that the cobalt center of 7 had a slightly
distorted square-pyramidal geometry. The dihedral angle
between the two pyrazoles of Bp in 7 was 125.0°, and one
of two hydrogen atoms attached to the boron center of Bp was
pointed toward the cobalt center. However, the distance from
the hydrogen atom to the cobalt center was 3.02 Å, and a large
open space surrounded the sixth vacant site on the cobalt
center. In fact, solid angle analysis revealed that the percentage
of the sphere shielded by the ligands was 90.5%, which was
comparable to the values for the LOiPr and LPh complexes 1 and
4.
The superoxide ligand sits in a cleft between the two methyl
groups of TpMe2,4H, and the distal oxygen atom is pointed away
from the boron-attached methyl group of LOiPr. The Co−N
bond lengths are 1.952(4)−2.052(4) Å, which are somewhat
shorter than the corresponding lengths in the cobalt(II)
precursor 1. Although shrinkage of the bond lengths between
the cobalt center and the N-donor ligands occurs, the structure
of the [Co(TpMe2,4H)(LOiPr)]+ moiety of 1S is almost the same
as that of 1. The boron-attached OiPr group is arranged away
from the cobalt(III) center in 1S, and the dihedral angle
between the two imidazolyl rings of LOiPr in 1S is 146.8°,
whereas that in the precursor compound 1 is 156.4°. Solid
angle analysis indicated that the percentage of the sphere
shielded was 99.3% for the superoxide-bound state of 1S and
89.3% for the superoxide ligand-removed [Co(TpMe2,4H)-
(LOiPr)]+ moiety of 1S, respectively. The latter value was very
close to that observed for 1. Therefore, the coordination
environment of the cobalt center seems to be retained during
the oxygenation process.
2.1.4. Cobalt(III)-Superoxo Complex 1S. The molecular
structure of an O2 adduct of 1 was determined, as reported in
the earlier communication.10 An O−O bond length of
1.301(5) Å is typical for a superoxide ligand. The IR spectrum
of this O2 adduct in the solution state exhibited an O−O bond
stretching vibration at 1147 cm−1 for the 16O2 derivative and at
1088 cm−1 for the 18O2 derivative. Additionally, the EPR
spectrum (measured at −198 °C) showed a signal at
approximately g = 2 with eight-line hyperfine coupling.
These spectroscopic properties are consistent with the
assignment of the O2 adduct of 1 as an end-on superoxide
In order to check the retention of the structures of the solid
state superoxo complex in solution, DR-UV−vis spectra of the
solid-state superoxo species and its precursor were measured.
We could not measure the spectra of the solid-state superoxo
species 1S−5S and 7S because of their lower thermal stabilities.
In contrast, the solid-state superoxo species 6S, which was
−
complex of mononuclear low-spin cobalt(III), [CoIII(O2 )-
(TpMe2,4H)(LOiPr)] (1S).
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obtained as dark brown precipitates by cooling of the solution
of 6S at −80 C°, was suitable to measure the spectrum at
ambient temperature and exhibited absorption bands around
400 and 530 nm. These absorption bands are similar to those
observed in the spectrum of the solution of 6S (Figure S10).
2.2. Elucidation of the Factors That Control the O2
Affinity. As described above, the LOiPr complex 1 reacted with
O2 to give the corresponding end-on cobalt(III)-superoxo
complex 1S. The other LX complexes 2−6 also reacted with O2
at low temperature to give the corresponding cobalt(III)-
superoxo complexes 2S−6S, as evidenced by the similarity in
the UV−vis spectra (Figure 2). All complexes exhibited two
dissociation of O2 to the cobalt centers. IR spectra of the
CH2Cl2 solutions of in situ generated 2S−6S (measured at −80
°C; Figure S-17) exhibited vibrational bands around 1150
cm−1 as found for 1S (1147 cm−1; described above). The Bp
complex 7 also exhibited a similar reversible O2-binding ability
as found for 1−6 (Figure S16). Therefore, the thermody-
namics of cobalt(II) oxygenation in complexes 1−7 were
analyzed. As described below, the O2 affinity of the cobalt(II)
centers, which were supported by the common 3N + 2N
scaffold, depended on the combination of LX and TpMe2,4R
.
2.2.1. Effects of the Electronic and Structural Properties of
LX. The effect of X on LX was examined by using TpMe2,4H
complexes 1−4 having different LX ligands. A comparison of
the temperature-dependent relative absorption intensities of
the O2-saturated solutions at 390 nm demonstrated that the
order of thermal stability of the superoxo complexes was 4S (X
= Ph) > 1S (OiPr) > 2S (Me) > 3S (nBu) (Figure 3a). This
order was quantitatively confirmed by the oxygen-binding
equilibrium constants (= KO ) determined by Drago’s
2
method,67 in which the change in absorbance at 390 nm
depended upon the partial pressure of oxygen supplied (Figure
4a and Table 2). The order of the KO values determined at
2
−50 °C was not consistent with the order of the cobalt(II)/
cobalt(III) oxidation potentials of 1−4. The order of the
oxidation potentials was 2 (0.07 V; X = Me) < 3 (0.12 V; nBu)
< 4 (0.18 V; Ph) < 1 (0.21 V; OiPr), which indicated that the
distal substituent group X affected the electronic nature of the
cobalt center (Figure 3b). On the other hand, the order of the
Figure 2. UV−vis spectral changes that occur during the reversible
binding and release of O2 to 2 (0.16 mM in THF) at −80 °C. Solid
lines: O2 atmosphere, dashed lines: Ar atmosphere (removal of O2 by
bubbling Ar through the solution of 2S). To confirm the reversibility,
bubbling of Ar and the following introduction of O2 were repeated
twice to the solution after the recording of the first set (blue dashed
spectrum; cobalt(II) complex under Ar, green spectrum: generated
cobalt(III)-superoxo species by the reaction with O2). Colors of
spectra mean as follows. Gray (dashed): first recovered cobalt(II)
compound, purple: second generated superoxo species, pale blue
(dashed): second recovered cobalt(II) compound, red: third
generated superoxo species.
K
values seemed to be correlated with the percentage of the
O2
complex sphere shielded in the corresponding cobalt(II)
precursors (see above). When X = Me or nBu, the substituent
faced the cobalt center, which led to a reduced O2 affinity
(Figure 5).
The thermodynamic parameters of the oxygenation process
obtained from van’t Hoff plots (Figure 4b) indicated that the
ΔH values for 3 (−28.0 kJ mol−1) and 2 (−31.6) were less
favorable than those for 1 (−43.7) and 4 (−59.8), as
summarized in Table 2. The entropy changes (= ΔS) for 2
(−134.0 J mol−1 K−1) and 3 (−127.6) were more positive
(thermodynamically favorable) than those for 1 (−179.0) and
4 (−243.5), as shown in Table 2. As estimated from the
crystallographic analysis of 1S, no significant structural changes
occurred during the oxygenation process. Therefore, the
intense bands at approximately 390 and 530 nm, which are
−
attributed to O2 to cobalt(III) charge transfer (CT) (Figures
S11−S15). Repeated O2 and Ar bubbling of the solutions of
1−6 at an appropriate temperature led to reversible changes in
the UV−vis spectra that corresponded to the addition and
Figure 3. (a) Thermal stability of 1S−4S in THF with a complex concentration of 0.16 mM and (b) cyclic voltammograms of 1−4 in CH2Cl2 with
n
0.1 M Bu4NPF6.
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Figure 4. (a) Spectral changes of 4 under various O2 partial pressures (= p(O2)) in THF at −50 °C. p(O2) = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 atm
starting from the bottom. The initial spectrum of 4 (p(O2) = 0 atm) is shown in the blue line and the final saturated spectrum when p(O2) = 1.0
atm is the red line. The inset shows a plot of p(O2) vs p(O2)/(A − A0) at 390 nm. (b) van’t Hoff plots for the oxygenation of cobalt(II) complexes
1−4.
Table 2. Equilibrium Constants (KO ; measured at −50°C) and Thermodynamic Parameters of 1−6
2
complex
KO at −50 °C (atm−1
)
ΔS (J·K−1·mol−1
)
ΔH (kJ·mol−1
)
ΔG at −50 °C (kJ·mol−1
)
2
1
2
3
4
5
6
7.40
2.62
0.77
18.88
2.99
61.41
−179.0 (21)
−134.0 (66)
−127.6 (18)
−243.5 (103)
−159.5 (25)
−292.5 (106)
−43.71 (5)
−31.58 (14)
−28.02 (4)
−59.79 (24)
−37.60 (5)
−72.95 (25)
−3.71
−1.78
+0.48
−5.45
−2.04
−7.64
low temperature to give the corresponding O2 adduct 7S,
which showed two intense bands at approximately 390 nm (ε
= 1100 M−1cm−1) and 530 nm (ε = 300 M−1cm−1) at −80 °C
(Figure S16), as observed for 1S−4S. A few related penta-
coordinated cobalt(II) complexes with a combination of TpR
and bis(pyrazolyl)borate ligands exist, but the O2 binding
ability of these complexes has not yet been reported.65,66 As
described above, the shielding extent of the cobalt(II) center of
7 is close to those of 1 (X = OiPr) and 4 (Ph). Such structural
property of 7 derived from the less-hindered Bp ligand would
be an advantage to access O2 to the cobalt(II) center.
Therefore, the O2 affinity of 7 was compared with those of 1−
Figure 5. Schematic drawings of the structural difference of the cobalt
centers depending on LX.
−
different properties of X affected the strength of the Co−O2
bonds (i.e., the enthalpy change).
To clarify the effects of the steric and electronic properties of
the bidentate ligands, the O2 affinity of Bp complex 7 was also
investigated as a control. A solution of 7 was reacted with O2 at
4. The KO value between 7 and 7S was 0.79 atm−1 at −80 °C
2
and was smaller than the KO value between 3 and 3S (8.07),
2
Figure 6. (a) Spectral changes of 7 under various O2 partial pressures (= p(O2)) in THF at −80 °C. p(O2) = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 atm. The
initial spectrum of 7 (p(O2) = 0 atm) is shown in the blue line, and the final saturated spectrum when p(O2) = 1.0 atm is the red line. The inset
shows a plot of p(O2) vs p(O2)/(A − A0) at 390 nm. (b) van’t Hoff plots for the oxygenation of cobalt(II) complexes 3 and 7.
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Table 3. Equilibrium Constants and Thermodynamic Parameters of Co(II) Complexes 3 and 7 at −80°C
complex
KO at −80 °C (atm−1
)
ΔS (J·K−1·mol−1
)
ΔH (kJ·mol−1
)
ΔG at −80 °C (kJ·mol−1
)
2
3
7
8.07
0.79
−127.6 (18)
−106.1 (22)
−28.02 (4)
−20.06 (4)
−3.35
+0.39
which was lowest in the series of LX complexes 1−4 with the
same TpMe2,4H due to the shielding effect of nBu, at the same
temperature (Figure 6, Table 3). Moreover, ΔH for the
oxygenation of 7 was −20.1 kJ mol−1, which was more positive
(less favorable toward oxygenation) than that for 3 (see
above). In Bp complex 7, the oxidation potential of cobalt(II)
to cobalt(III) was 0.52 V (Figure 7), which was higher than the
bromine substituent on TpMe2,4Br weakened the ligand field,
2 2 2
which lowered the energy levels of the empty dx −y and dz
orbitals of the low-spin cobalt(III) center. In contrast, the λmax
values of the bands of 6S were similar to those of 4S. However,
the molar absorption coefficients of the absorption bands
followed the order 5S > 4S > 6S. Therefore, the overlap
−
between the orbitals of cobalt and O2 increased with
decreasing electron donation from TpMe2,R. The observed
trends can be explained by the trans influence between the
axial pyrazole donor and the superoxide ligand.
The order of thermal stability was 6S > 4S > 5S, which was
estimated in a similar manner as that for 1S−4S (Figure 9).
Figure 7. Cyclic voltammograms of 5 and 7 in CH2Cl2 with 0.1 M
nBu4NPF6.
potentials observed for 1−4. These electrochemical trends are
consistent with the lower Lewis basicity of pyrazole compared
to that of 1-methylimidazole.68 Therefore, these observations
suggest that the electronic property of Bp rather than its
structural property is responsible for the low O2 affinity of 7,
whereas the O2 affinity of LX complexes 1−4 is mainly
controlled by the steric properties.
Figure 9. Thermal stability of 4S−6S in 0.16 mM THF solution.
Additionally, the KO values determined at −50 °C were 61.4
2.2.2. Electronic Effects of TpMe2,4R. Then, we examined the
effects of R on TpMe2,4R. In the series of LPh complexes 4−6,
the cobalt(II)/cobalt(III) oxidation potentials followed the
order 6 (R = Me; 0.11 V) < 4 (H; 0.18 V) < 5 (Br; 0.36 V), as
expected from the electronic properties of R (Figure 8). Both 5
and 6 reacted with O2 to give the corresponding cobalt(III)-
superoxo species, as evidenced by the observation of two
intense absorption bands in their UV−vis spectra (Figures S12
and S13). Notably, in complex 5S with the brominated ligand,
these two bands were red-shifted compared to those in 4S and
6S. In complex 5S, the electron-withdrawing nature of the
2
atm−1 for 6, 18.9 atm−1 for 4, and 3.0 atm−1 for 5. These trends
were consistent with the order of the oxidation potentials of
the cobalt(II) precursors. In addition, the ΔH values (in kJ
mol−1) for the oxygenation of each complex were −73.0 (for
6), −59.8 (4), and −37.6 (5) (Figure 10). Therefore, the
electronic properties of R directly affected the enthalpic
stability of the cobalt(III)-superoxo species. Notably, the
oxidation potentials of the cobalt(II) centers of TpMe2,4H-Lalkyl
complexes 2 (0.07 V) and 3 (0.12 V) were close to that of the
complex with the highest O2 affinity (6; 0.11 V), which was
composed of LPh and TpMe2,4Me. The KO value for 6 at −50 °C
2
was 20−80 times larger than those for 2 and 3. These facts
indicate that the steric effect from LX is dominant to the O2
affinity of the cobalt(II) centers of which the oxidation
potentials show close values.
3. CONCLUSION
We have revealed the effects of the distal substituent groups in
the ligands on the O2 affinity of the supported metal center.
The structural properties of the boron-attached group X on LX
affect the shielding percentage of the cobalt(II) centers and are
critical for changing the enthalpic stabilization of the cobalt-O2
adducts, although dependence of the electrochemical proper-
ties of the metal centers on X is inconsistent with the O2
affinities. Such unprecedented behavior arises from the
structural flexibility of LX. On the other hand, the electronic
Figure 8. Cyclic voltammograms of 4−6 in CH2Cl2 with 0.1 M
nBu4NPF6.
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Figure 10. (a) Spectral changes of 6 at various O2 partial pressures (= p(O2)) in THF at −50 °C. p(O2) = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 atm starting
from the bottom. The inset shows a plot of p(O2) vs p(O2)/(A − A0) at 390 nm. (b) van’t Hoff plots for the oxygenation of the cobalt(III)-
superoxo complexes (4S−6S).
0.0270), and 11440 reflections (7, Rint = 0.0714) were collected. The
equivalent reflections were merged. The linear absorption coefficient,
μ, for Mo-Kα radiation was 6.01 cm−1 for 2, 5.93 cm−1 for 3, 40.31
cm−1 for 5, 5.31 cm−1 for 6, and 7.10 cm−1 for 7. A numerical
absorption correction was applied and resulted in transmission factors
ranging from 0.8893 to 0.8893 for 2, from 0.8813 to 0.9103 for 3,
from 0.5193 to 0.5843 for 5, from 0.8646 to 0.9366 for 6, and from
0.8528 to 0.9586 for 7. The data were corrected for Lorentz and
polarization effects. Structural solution by a direct method (SIR-92)70
and refinement by full-matrix least-squares (SHELXL-2014/7)71
against F2 with all reflections were performed on WinGX software.72
All nonhydrogen atoms were refined anisotropically. Hydrogen atoms
adjacent to carbon atoms were placed in calculated positions with C−
H = 0.96 Å (for methyl groups) or 0.93 Å (for aromatic rings) with
Uiso(H) = 1.2 Uiso(attached atom). Hydrogen atoms on boron
atoms were located by difference Fourier synthesis and refined
isotropically with B−H = 1.10 Å. The molecular structure was drawn
in the ORTEP-3 for Windows program.73 Crystal information files
(CIF) of the complexes reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publications CCDC-1843685 (2) CCDC-1843686 (3), CCDC-
1843687 (5), CCDC-1843688 (6), and CCDC-1893689 (7). These
data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre at www.ccdc.cam.ac.uk/data_request/cif. The
crystallographic data and structure refinement parameters of the three
catalysts are given in Table S1.
properties of the R substituent attached to the fourth position
of the pyrazolyl rings of the rigid TpMe2,4R ligands are reflected
in the electrochemical nature and the O2 affinity of the metal
center. In summary, the O2 affinity of the five azole-supported
metal centers is controlled by the structural and electronic
properties of the ligand substituent groups located in the
secondary coordination sphere.
4. EXPERIMENTAL
4.1. General. Elemental analysis was performed on a Perkin-Elmer
CHNS/O analyzer 2400II. IR spectra were recorded on a JASCO
FT/IR 4200 spectrometer with solid samples (as KBr pellets). NMR
spectra were recorded on a JEOL ECA-600 spectrometer. UV−vis
spectra were measured on an Agilent 8453 UV−vis spectrometer with
a UNISOK CoolSpeK cell holder. Cyclic voltammetry was performed
on an ALS Model 600C electrochemical analyzer. All commercial
reagents and solvents were used without further purification unless
otherwise noted. The preparation of oxygen-sensitive compounds was
performed by Schlenk techniques under an Ar atmosphere. LX (X =
OiPr, Me, nBu, Ph)14,17 and [CoII(TpMe2,4R)(OAc)] (R = H, Me,
Br)63 were prepared according to the literature.
4.2. X-ray Diffraction Study. Diffraction data were collected for
a single crystal of 2 using Rigaku “VariMax Mo” optics and multilayer
mirror monochromated Mo-Kα radiation with a Saturn70 CCD
detector at the Institute for Materials Chemistry and Engineering,
Kyushu University. Diffraction data for single crystals of 3, 5, 6, and 7
were collected using a Rigaku Saturn 70 CDD area detector system
with graphite monochromated Mo-Kα radiation at Kanagawa
University. The block- (2, 3, 5, 6) or plate (7)-shaped crystals were
mounted on a cryoloop with liquid paraffin and flash cooled to 113 K
(for 3, 5, 6), 123 K (for 2), or 193 K (for 7) by a flow of cold N2 gas
on the goniometer. For complex 2, two sets of data sweeps (at ϕ = 90
and 270°) were conducted using ω oscillations (sweep 1: ω = −110.0
to 40.0° at ϕ = 90° and sweep 2: ω = −35.0 to 40.0° at ϕ = 270°)
with 0.5° steps at χ = 45° with an exposure rate of 4.0 [s/°]. The
detector swing angle was −20°, and the crystal-to-detector distance
was 45 mm (total of 450 oscillation images). For complex 3, two sets
of data sweeps (at ϕ = 0 and 90°) were conducted using ω
oscillations from −110.0 to 70.0° in 0.5° steps at χ = 45° with an
exposure rate of 36.0 [s/°]. The detector swing angle was −20°, and
the crystal-to-detector distance was 45 mm (total of 720 oscillation
images). For complexes 5, 6, and 7, three sets of data sweeps (at ϕ =
0, 90, and 180°) were conducted using ω oscillations from −115.0 to
65.0° in 0.3° steps at χ = 45° with exposure rates of 100.0 [s/°] (for
5), 50.0 [s/°] (for 6), and 133.3 [s/°] (for 7). The detector swing
angle was −25°, and the crystal-to-detector distance was 55 mm (total
of 1800 oscillation images). Data collection and processing were
performed using Rigaku CrystalClear software.69 In total, 14717
reflections (2, Rint = 0.1030), 11107 reflections (3, Rint = 0.0618),
4.3. Preparation and Characterization Cobalt(II) Com-
plexes. 4.3.1. [CoII(TpMe2,4R)(LX)] (1−6). As a typical example, the
synthetic procedure is described for cobalt(II) complex 1. A THF
suspension (30 mL) of LiLOiPr (241.2 mg, 0.949 mmol) was slowly
added into a THF solution (30 mL) of [CoII(TpMe2,4H)(OAc)] (440
mg, 1.06 mmol). After stirring for 4 h, the volatile solvent was
evaporated, and the residue was redissolved in CH2Cl2. The solution
was then passed through a filter with Celite to remove any inorganic
salts. After evaporation of CH2Cl2, the residue was redissolved in
MeCN, and the solution was passed through a filter with Celite. After
reduction of the solvent volume by evaporation, the solution was
subjected to recrystallization at −30 °C to give the title complex as a
red-purple powder (178 mg, 0.295 mmol, 31.1%). Recrystallization
from a CH2Cl2/MeCN solution at −30 °C gave a red needle crystal
suitable for X-ray crystallography.
[CoII(TpMe2,4H)(LOiPr)] (1). FT/IR (KBr): ν = 3124 (w), 2980 (m),
2961 (vs), 2927 (vs), 2865 (vs), 2512 (m, νBH), 1283 cm−1 (vs, νBC).
UV−vis (CH2Cl2, r.t.): λ = 532 nm (ε = 94.2 M−1 cm−1), 505 nm (ε
1
= 94.2 M−1 cm−1), 427 nm (ε = 72.1 M−1 cm−1). H NMR (600
MHz, CDCl3, 25 °C, TMS): δ = 54.97 (br, 1H; iPr), 42.41 (br, 3H;
Pz-H), 38.41 (br, 9H; Pz-CH3), 37.35 (br, 6H; Im-CH3), 30.83 (br,
2H; Im), 25.31 (br, 3H; B-CH3), 23.53 (br, 6H; iPr), −55.45 (br, 2H;
Pz-CH3), −71.31 (br, 6H; Pz-CH3). ESI-MS+ (MeCN): m/z = 603
(M+). Magnetic moment (Evans method, r.t., CDCl3): μeff = 4.1 μB.
Elemental analysis calcd (%) for [CoII(TpMe2,4H)(LOiPr))]
15476 reflections (5, Rint = 0.0260), 14782 reflections (6, Rint
=
H
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(C27H42N10B2OCo): C 53.76, H 7.02, N 23.22; found: C 53.39, H
6.75, N 22.98.
(C30H37N10B2Br3Co): C 42.00, H 4.35, N 16.33; found: C 41.70,
H 4.53, N 15.94.
[CoII(TpMe2,4Me)(LPh)] (6). The crude complex was recrystallized
from MeCN at −30 °C to give the title complex as a reddish-brown
powder (173 mg, 0.261 mmol, 11.7%). Recrystallization from a
MeCN solution at −30 °C gave a red block crystal suitable for X-ray
crystallography. FT-IR (KBr): ν = 3119 (w), 3041 (w), 2995 (w),
2913 (vs), 2860 (m), 2505 (vs, νBH), 1284 cm−1 (s, νBC). UV−vis
(CH2Cl2, r.t.): λ = 681 nm (ε = 27.7 M−1 cm−1), 498 nm (ε = 116.7
The other complexes were prepared by following the same
procedure. In the synthesis of the complexes 2−6, LiLX, which was
generated by the reaction of HLX with n-butyllithium at −80 °C,
reacted with the corresponding acetate complexes [CoII(TpMe2,4R)-
(OAc)] (where R = H for 2−4, Br for 5, and Me for 6, respectively).
[CoII(TpMe2,4H)(LMe)] (2). The crude complex was recrystallized from
MeCN at −30 °C to give the title complex as a light pink powder
(50.7 mg, 0.091 mmol, 46.4%). Recrystallization from a CH2Cl2/
MeCN solution at −30 °C gave a red block crystal suitable for X-ray
crystallography. FT/IR (KBr): ν = 3117 (w), 2911 (vs), 2517 (m,
νBH), 1287 cm−1 (vs, νBC). UV−vis (CH2Cl2, r.t.): λ = 537 nm (ε =
60.0 M−1 cm−1), 500 nm (ε = 71.8 M−1 cm−1), 420 nm (ε = 71.0 M−1
cm−1). 1H NMR (600 MHz, CDCl3, 25 °C, TMS): δ = 59.01 (s, 1H;
B-H), 37.98 (s, 3H; Pz-H), 31.14 (s, 2H; Im), 29.60 (s, 6H; Im-CH3),
25.96 (br, 9H; Pz-CH3), 1.54 (s, 6H; B-CH3), −17.77 (br, 2H; Im),
−43.88 (br, 9H; Pz-CH3). ESI-MS+ (MeCN): m/z = 388 ([M − LMe
+ MeCN]+), 544 ([M − CH3]+). Magnetic moment (Evans method,
r.t., CDCl3): μeff = 3.5 μB. Elemental analysis calcd (%) for
[CoII(TpMe2,4H)(LMe)] (C25H38N10B2Co): C 53.70, H 6.85, N
25.05; found: C 53.65, H 7.06, N 24.79.
1
M−1 cm−1), 420 nm (ε = 103.3 M−1 cm−1). H NMR (600 MHz,
CDCl3, 25 °C, TMS): δ = 56.81 (br, 1H; B-H), 33.61 (br, 2H; Im),
23.67 (s, 9H; Pz-CH3), 22.43 (s, 6H; Im-CH3), 16.36 (s, 2H; Ph),
15.81 (s, 2H; Ph), 14.34 (s, 1H; Ph), 7.28 (br, 9H; Pz-CH3), 6.10 (s,
3H; B-CH3), −24.90 (br, 2H; Im), −43.24 (br, 9H; Pz-CH3). ESI-
MS+ (MeCN): m/z = 664 ([M + H]+). Magnetic moment (Evans
method, r.t., CDCl3): μeff = 3.4 μB. Elemental analysis calcd (%) for
[CoII (TpMe2,4Me)(LPh)]·0.5H2O (C33H47N10B2CoO0.5): C 58.95, H
7.05, N 20.83; found: C 58.89, H 7.25, N 20.94.
4.3.2. [CoII(TpMe2,4H)(Bp)] (7). This Bp complex was also synthesized
by the same procedure for 1, where KBp was used instead of LiLOiPr
.
This complex was recrystallized from MeCN at −30 °C to give the
title complex as a red-purple powder (193.3 mg, 0.348 mmol, 38.9%).
Recrystallization from a CH2Cl2/pentane solution at room temper-
ature gave a red block crystal suitable for X-ray crystallography. FT/IR
(KBr): ν = 3133 (w), 2959 (vs), 2927 (vs), 2861 (vs), 2509 (vs, νBH),
2432 cm−1 (vs, νBH). UV−vis (CH2Cl2, r.t.): λ = 537 nm (ε = 77.4
M−1 cm−1), 506 nm (ε = 81.5 M−1 cm−1), 422 nm (ε = 55.0 M−1
[CoII(TpMe2,4H)(LnBu)] (3). The crude complex was recrystallized from
MeCN at −30 °C to give the title complex as a brown powder (89.5
mg, 0.149 mmol, 30.3%). Further recrystallization from a CH2Cl2/
MeCN solution at −30 °C gave a brown block crystal suitable for X-
ray crystallography. FT-IR (KBr): ν = 3120 (w), 2948 (vs), 2925 (vs),
2866 (vs), 2511 (m, νBH), 1285 cm−1 (m, νBC). UV−vis (CH2Cl2,
r.t.): λ = 539 nm (ε = 77.8 M−1 cm−1), 501 nm (ε = 89.2 M−1 cm−1),
1
cm−1). H NMR (600 MHz, CDCl3, 25 °C, TMS): δ = 137.04 (br,
2H; B(Bp)-H), 74.54 (br, 1H; Pz(Tp)-H), 73.15 (s, 2H; Pz(Tp)-H),
55.76 (s, 2H; Pz(Bp)-H), 45.95 (s, 3H; Pz(Tp)-CH3), 44.88 (s, 6H;
Pz(Tp)-CH3), 16.16 (s, 2H; Pz(Bp)-H), −49.48 (s, 6H; Pz(Tp)-
CH3), −50.68 (s, 3H; Pz(Tp)-CH3), −67.03 (s, 2H; Pz(Bp)-H). ESI-
MS+ (MeOH): m/z = 503 (M+). Magnetic moment (Evans method,
r.t., CDCl3): μeff = 4.2 μB. Elemental analysis calcd (%) for
[CoII(TpMe2,4H)(Bp)] (C21H30N10B2Co): C 50.14, H 6.01, N 27.84;
found: C 49.70, H 5.53, N 27.39.
4.4. Thermodynamic Analysis of the Oxygenation of 1−7.
UV−vis spectra of 1−7 in THF were measured at low temperature
under various O2 partial pressures (p(O2)). Argon and dioxygen gas
were mixed using two mass flow controllers and passed through
approximately 0.17 mM solutions (gas flow 100 mL min−1) of the
complex in a quartz cell (1 cm path length). The gas mixture was
bubbled through the solution for 0.5 h before the measurement to
ensure that equilibrium had been reached. The growth of absorption
band shoulders, absent in the deoxy form, upon oxygenation were
monitored at 390 nm. The strong binding affinity resulted in
saturation at high pO2 values, and the measurements obtained at these
partial pressures could not be used in the determination of the
1
420 nm (ε = 79.4 M−1 cm−1). H NMR (600 MHz, CDCl3, 25 °C,
TMS): δ = 58.83 (br, 1H; B-H), 36.78 (s, 3H; Pz-CH3), 30.41 (br,
2H; Im), 29.49 (br, 2H; nBu-CH2), 28.33 (s, 6H; Im-CH3), 25.54 (br,
9H; Pz-CH3), 1.49 (br, 3H; B-CH3), −10.32 (s, 3H; nBu-CH3),
−16.14 (br, 2H; Im), −21.91 (s, 2H; nBu-CH2), −22.80 (br, 2H; nBu-
CH2), −44.15 (s, 9H; Pz-CH3). ESI-MS+ (MeCN): m/z = 544 ([M −
C4H9]+). Magnetic moment (Evans method, r.t., CDCl3): μeff = 3.6
μB. Elemental analysis calcd (%) for [CoII(TpMe2,4H)(LnBu)]
(C28H44N10B2Co): C 55.93, H 7.38, N 23.30; found: C 55.68, H
7.01, N 23.25.
[CoII(TpMe2,4H)(LPh)] (4). The crude complex was recrystallized from
DMF at −30 °C to give the title complex as a brown powder (45.1
mg, 0.073 mmol, 7.4%). Recrystallization from a CH2Cl2/MeCN
solution at −30 °C gave a red block crystal suitable for X-ray
crystallography. FT-IR (KBr): ν = 3120 (w), 2953 (vs), 2909 (vs),
2513 (m, νBH), 1285 cm−1 (m, νBC). UV−vis (CH2Cl2, r.t.): λ = 663
1
nm (ε = 21.1 M−1 cm−1), 496 nm (ε = 91.2 M−1 cm−1). H NMR
(600 MHz, CDCl3, 25 °C, TMS): δ = 48.87 (br, 1H; B-H), 32.51 (br,
3H; Pz-H), 30.37 (br, 2H; Im), 20.46 (br, 9H; Pz-CH3), 19.94 (br,
6H; Im-CH3), 15.25 (br, 2H; Ph), 13.56 (br, 2H; Ph), 13.49 (br, 1H;
Ph), 4.19 (br, 3H; B-CH3), −18.93 (br, 2H; Im), −37.67 (br, 9H; Pz-
CH3). ESI-MS+ (MeCN): m/z = 606 ([M − CH3]+), 397 ([M − LPh
equilibrium constant (KO ).
2
+ MeCN]+). Magnetic moment (Evans method, r.t., CDCl3): μeff
=
3.6 μB. Elemental analysis calcd (%) for [CoII(TpMe2,4H)(LPh)]
(C36H40N10B2Co): C 58.00, H 6.49, N 22.55; found: C 57.60, H 6.29,
N 22.38.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02241.
[CoII(TpMe2,4Br)(LPh)] (5). The crude complex was washed with a
small amount of pentane to give the title complex as a beige powder
(130 mg, 0.152 mmol, 22.7%). Recrystallization from a CH2Cl2/
iPrOH solution at room temperature gave a pale brown block crystal
suitable for X-ray crystallography. FT-IR (KBr): ν = 3125 (w), 3045
(w), 2989 (w), 2955 (w), 2902 (vs), 2521 (s, νBH), 1285 cm−1 (s,
νBC). UV−vis (CH2Cl2, r.t.): λ = 684 nm (ε = 21.6 M−1 cm−1), 493
nm (ε = 98.2 M−1 cm−1). 1H NMR (600 MHz, CDCl3, 25 °C, TMS):
δ = 46.68 (br, 1H; B-H), 29.72 (s, 2H; Im), 19.46 (br, 9H; Pz-CH3),
19.02 (s, 6H; Im-CH3), 15.04 (s, 2H; Ph), 13.33 (s, 1H; Ph), 12.74 (s,
2H; Ph), 2.37 (br, 3H; B-CH3), −16.22 (br, 2H; Im), −36.38 (br,
9H; Pz-CH3). ESI-MS+ (MeCN): m/z = 899 ([M + MeCN + H]+).
Magnetic moment (Evans method, r.t., CDCl3): μeff = 3.3 μB.
Elemental analysis calcd (%) for [CoII(TpMe2,4Br)(LPh)]
1H NMR spectra of the cobalt(II) complexes 1−7,
drawings of the sphere-shielding of 1−7 and 1S, space
filling diagrams of 1−7 and 1S, DR-UV−vis spectra of
the solid samples of 6 and 6S, UV−vis spectra in the
reversible O2 sorption/desorption experiments on 1 and
3−7, IR spectra of in situ generated 2S−7S and 2−7 in
CH2Cl2 solutions, crystallographic data and structure
refinement parameters for 1−7 and 1S, and KO values of
2
1−7 under various temperatures (PDF)
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Accession Codes
CCDC 1843685−1843689 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jnaka@kanagawa-u.ac.jp.
*E-mail: hikichi@kanagawa-u.ac.jp.
ORCID
Jun Nakazawa: 0000-0003-4665-6657
Shiro Hikichi: 0000-0001-6001-9780
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was funded by CREST, JST (JPMJCR16P1),
and Kanagawa University (ordinary budget for 411)
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