Thermochemistry of a Cobalt Complex with Ionisable Pyrazole Protons

Article abstract of DOI:10.1002/ejic.201800347

Herein, we present the thermodynamic analysis of a cobalt complex with a new pentadentate N-donor ligand bearing four ionisable pyrazole protons in aqueous solution. A detailed analysis of the Co^II complex [Co(L)(X)]^+/2+ in the solid state revealed that the 6^th ligand X at the metal centre depends on the cobalt source employed. Small anions such as Cl^– and NO₃^– coordinate to the metal ion, while larger anions that are weaker hydrogen-bond acceptors are found in the second coordination sphere of the complex and instead a solvent molecule coordinates. However, in aqueous KCl solution, the sixth ligand is always chloride forming [Co(L)Cl]Cl, 1^Cl. pH dependent species distribution studies revealed a pK_a of 7.3(3) for the first ionisable pyrazole proton in the cobalt(II) complex and 6.0(3) in the cobalt(III) complex (methanol/H₂O mixture). That is the oxidation state has a fairly minor influence on the pK_a of the pyrazole proton. The Co^III/Co^II redox pair of the complex with the fully protonated ligand exhibits a potential of 0.78 V vs. NHE. The BDFE of the hypothetical H-atom abstraction step of [Co^II(L)Cl]⁺ forming [Co^III(LH_–1)Cl]⁺ was determined to equal 336 kJ mol^–1.

Full text of DOI:10.1002/ejic.201800347

A Journal of
Accepted Article
Title: Thermochemistry of a Cobalt Complex with Ionisable Pyrazole
Protons
Authors: Mona Wilken, Merle Kügler, Christian Würtele, Frank
Chrobak, and Inke Siewert
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800347
Link to VoR: http://dx.doi.org/10.1002/ejic.201800347

European Journal of Inorganic Chemistry
10.1002/ejic.201800347
Thermochemistry of a Cobalt Complex with Ionisable Pyrazole Protons
[
a]
[a]
[a]
[a]
[a]
Mona Wilken, Christian Würtele, Merle Kügler, Frank Chrobak, Inke Siewert *
Abstract: Herein, we present the thermodynamic analysis of a cobalt complex with a new pentadentate N-donor
ligand bearing four ionisable pyrazole protons in aqueous solution. A detailed analysis of the Co(II) complex
+
/2+
th
[
Co(L)(X)]
in the solid state revealed that the 6 ligand X at the metal centre depends on the cobalt source
−
−
employed. Small anions such as Cl and NO
3
coordinate to the metal ion, while larger anions that are weaker
hydrogen bond acceptors are found in the second coordination sphere of the complex and instead a solvent
molecule coordinates. However, in aqueous KCl solution, the sixth ligand is always chloride forming [Co(L)Cl]Cl,
Cl
1
. pH dependent species distribution studies revealed a pK
a
of 7.3(3) for the first ionisable pyrazole proton in the
O mixture). That is the oxidation state has
cobalt (II) complex and 6.0(3) in the cobalt (III) complex (methanol/H
a fairly minor influence on the pK
2
III
II
a
of the pyrazole proton. The Co /Co redox pair of the complex with the fully
protonated ligand exhibits a potential of 0.78 V vs. NHE. The BDFE of the hypothetical H-atom abstraction step
II
+
III
+
−1
of [Co (L)Cl] forming [Co (LH−1)Cl] was determined to equal 336 kJmol .
3
3
3
3
4 Metal ion binding to imidazole or imidazole
5 containing ligands leads to a decrease of the pK of
1
2
3
4
5
6
7
8
9
Introduction
a
Electron transfer reactions are one of the
6 the imidazole protons in comparison to free
7 imidazole and the extend of the effect depends on the
fundamental reaction types in chemistry. Many, if
not most, electron transfer reactions in chemistry and
biology are coupled to proton transfer events.[1] For
example, multiple site proton-coupled electron
transfer (PCET) steps play a crucial role in three out
of four oxidation steps in the Kok cycle, that is in the
[
3a,8]
38 d-electron configuration of the metal ion.
We
3
4
4
4
9 expected an even more pronounced effect in an
0 isoelectronic pyrazole complex as the distance
1 between the acidic proton and the cobalt ion is
2 shorter than in the isoelectronic complex
water oxidation reaction forming O
2
in the
im
BF4
[9]
[
2]
43 [Co(L )(H O)](BF ) , A
4
4
46 imidazole is a stronger σ- and π-donor in comparison
4
(Figure 1). On the
2
4 2
1
1
1
1
1
1
1
1
1
1
2
2
2
0 photosystem II. Recently, it has been showed, that
1 metal complexes with ionisable imidazole units have
2 an impact on the electrochemical-driven water
3 oxidation catalysis. Such metal imidazole units
4 undergo PCET reactivity and show a thermodynamic
5 coupling between ligand-based proton transfer and
6 metal-based electron transfer.[ Very little is known
7 for isoelectronic pyrazole complexes, although
8 pyrazoles are also widely used a N-donor moieties in
III
II
4 other hand, the redox potential of the Co /Co redox
5 pair was expected to be higher than in A , because
BF4
[
3]
[
10]
7 to pyrazole.
4]
[
4i,5]
9 coordination chemistry.
This prompted us to
0 investigate the solution thermochemistry of
1 isoelectronic cobalt complex with ionisable pyrazole
2 protons.
4
8
4
5
9
0
Figure 1. Pentadentate N-donor ligands with ionisable pyrazole
[3a]
or imidazole protons.
2
2
2
2
2
2
2
3
3
3
3
3 PCET reactions are usually described by the
4 thermodynamic data of the individual steps, that are
5 the pK of the proton transfer step, the redox
a
5
1 Results and Discussion
2 Ligand and complex synthesis.
3 L can be synthesised in a 5 step synthesis starting
4 from pyridine-2,6-dicarboxylic acid and pyrazole as
5 shown in Scheme S 1. The synthesis has been
5
5
5
5
5
5
5
5
6
6
6
6
6 potential for electron transfer step, and the bond
7 dissociation free energy of the concerted hydrogen
8 atom transfer (HAT) step.[6] According to the
9 thermodynamic square scheme, the bond
0 dissociation free energy (BDFE) of the concerted
1 HAT step can be calculated from the thermodynamic
[
3a,11]
6 adapted from syntheses of similar ligands.
The
7 ligand was purified by recrystallisation from ethanol
8 and fully characterised by the usual methods. A
9 single crystal X-ray diffraction experiment affirmed
0 the structure. The result of the refinement is shown
1 in Figure S 1. L crystallises in the orthorhombic
2 space group Fdd2 (Table S 2). The solid state
3 structure shows a hydrogen bond network between
+
•
2 data of the individual steps and the H /H standard
[
7]
3 reduction potential.
[
a]
Universität Göttingen, Institut für Anorganische Chemie
Tammannstraße 4, 37077 Göttingen, Germany
E-mail: inke.siewert@chemie.uni-goettingen.de
www.siewertlab.de
Supporting information for this article is given via a link at the end of
the document.
64 two adjacent pyrazole moieties and one ethanol
6
6
5 molecule. The distance between the N3…O4 and
6 N7…O4 atoms as well as between the N3… O3 and
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201800347
BF4
1
2
3
4
N9…O3 atoms are in the typical range of hydrogen
bonds, i.e. ~2.8 Å (Table S 3). The ethanol molecule
is bound tightly and cannot be removed under
vacuum.
45 distortion is slightly more prominent in A than in
BF4
46 1 (Figure S 2 and Figure S 3). The angle between
47 the planes defined by the pyridine ring and the four
48 equatorial nitrogen atoms is 85° in A and 90° in
4
9 1 . The BF anions are bridged between two
BF4
BF4
4
5
5
6
7
The reaction of L and [Co(H
2
O)
6
](BF
4
)
2
in methanol
BF4
0 adjacent molecules of A through hydrogen bonds.
led to the complex with the desired structural motif
BF4
[CoL(MeOH)](BF
4
)
2
,
1
(Scheme 1). Mass
8
9
spectrometry confirmed the composition and
elemental analysis the purity of the complex. IR
−1
1
1
1
0 spectroscopy showed
a
band at 3368 cm ,
1 indicating that the pyrazole units were still
2 protonated.[
12]
5
1
BF4
5
5
2
3
Figure 2. Molecular structure of 1 in the solid state. Most of
4
the hydrogen atoms, two BF anions and one further methanol
54 molecule were omitted for clarity. Thermal ellipsoids are set at
1
1
3
5
5
5
6
the 50% level.
4
Scheme 1. Synthesis of the cobalt(II) and cobalt(III) complexes.
Table 1. Selected data for 1 /[Å].
1
1
1
1
1
2
2
5 In order to obtain the cobalt(III) complex, L and
6 Co(acac) were heated in HCl-acidified methanol
Atoms
1Cl
2.344(4)
2.092(3)
2.130(3)
1NO3
1BF4
3
Co1‒N1
Co1‒N2
Co1‒N4
Co1‒N6
Co1‒N8
2.2740(16) 2.2255(18)
2.1320(16) 2.1188(19)
2.1009(17) 2.1083(19)
2.1015(17) 2.1202(19)
2.1100(16) 2.1009(19)
7 solutions for 5 h. The material, which was obtained,
1
8 showed a diamagnetic H NMR spectrum indicating
2
9 the successful formation of [Co(L)Cl]Cl , 2. Indeed,
0 mass spectrometry supported this and elemental
1 analysis confirmed the purity of the product.
ØCo1−N 2.158
Co1‒X
2.144
2.135
ax
2.3357(14) 2.0722(14) 2.0161(17)
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
2 Solid state structures.
BF4
Cl
3 The structure of 1
was confirmed by a single
57 Table 2. Selected data for A, B, and 2 /[Å].
4 crystal X-ray diffraction experiment. The result is
ABF4
2Cl
Atoms
B
5 shown in Figure 2 and selected bond lengths in Table
Co1‒N1
Co1‒N2
Co1‒N4
ØCo1−N 2.151
Co1‒X
2.3389(17) 1.979(5)
2.1052(14) 1.956(3)
2.1028(14) 1.920(3)
1.9814(19)
1.932(2)
1.908(2)
1.932
BF4
6 1 and angles in Table S 4. 1
7 orthorhombic space group P2
8 cobalt(II) ion is bound to the ligand in a κ -N
crystallises in the
1 1 1
2 2 (Table S 1). The
5
5
-mode.
1.946
1.892(5)
ax
9 The sixth position of the distorted octahedron is
0 occupied by a solvent molecule, i.e. methanol. The
1 distances between the equatorial N atoms and the Co
2 ion is shorter than the distances between the axial N
3 atoms and the Co ion. The average N−Co distance of
4 2.135 Å points to a high spin cobalt(II) ion. The two
2.087(2)
2.1841(6)
58
5
6
6
6
6
6
9 We employed further cobalt salts such as
0 [Co(H O) ](Cl) or [Co(H O) ](NO in order to
2
6
2
2
6
3 2
)
1 synthesise 1. The single crystal experiments of the
2 respective products showed that the 6 ligand
th
4
5 BF anions are captured between two adjacent
6 pyrazole units via hydrogen bonds (Figure S 7).
3 depends on cobalt salt employed. It was methanol in
4 1 , a Cl− anion in the experiment employing
BF4
7 The structure of 1BF4 is similar to the one of ABF4
.
Cl
3
3
3
4
4
4
4
4
65 [Co(H
2
O)
6
](Cl)
2
as cobalt source, forming 1 , and a
8 The refinement result of single X-Ray diffraction
66 nitrate anion in the experiment employing
67 [Co(H O) ](NO ) , yielding 1 . All derivatives
2 6 3 2
68 have been fully characterised by usual spectroscopic
69 methods and by elemental analysis. The refinement
70 results of the X-Ray experiments are shown in
BF4
BF4
NO3
9 measurements of A is shown in Figure S 3. A
0 crystallises in the orthorhombic space group Pnma
1 (Table S 2). The cobalt ion has a distorted octahedral
2 coordination geometry and the average N‒Co
3 distance of 2.151 Å corroborates the previously
Cl
NO3
71 Figure S 4 and Figure S 5. 1 and 1
72 in the orthorhombic space groups Pnna and Pbca,
crystallised
[
3a]
4 reported high spin state (Table 2). The octahedral
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201800347
1
2
3
4
5
6
7
8
9
respectively (Table S 1). The bond lengths and the
overlay of the molecular structures in the solid state
show that the bond distances and angles are very
similar in the three complexes (Table 1, Figure 3).
43 atom is shifted 0.20 Å out of the plane defined by the
44 N pyrazole atoms.
4
5 Structure in aqueous solution.
eq
NO3
46 Since the sixth ligand of the cobalt(II) complex
47 depends on the cobalt source in the solid state, we
48 investigated the solution structures by paramagnetic
The average N ‒Co distances of 2.112 in 1 , of
Cl
BF4
2.111 Å in 1 , and of 2.112 in 1 are very similar
BF4
to that of A , but slightly longer than in
1
O)]2 and [Co(PY5)(H
+
2+
49 H NMR spectroscopy. The proton resonances of the
[Co(pz
2.067 Å and 2.059 Å, respectively, Py5 = 2,6-
4 2 2
depy)(H O)] (cf.
5
5
5
0 pyridine and pyrazole protons depend on the anion
1 of the cobalt salt. The shifts of the three complexes
2 differ largely in MeOD-d , indicating that the sixth
4
1
1
1
1
1
1
1
1
1
1
2
2
2
2
0 (bis(bis-2-pyridyl)-methoxymethane)-pyridine,
1 pz4depy 2,6-bis(1,1-di(1H-pyrazole-1-
2 yl)ethyl)pyridine) . All metal ions show an axial
3 distortion, which is more pronounced than in
=
[
13]
53 ligand is not substituted by a solvent molecule in
4 solution (Figure S 12). Upon adding 0.1 M potassium
5
2+
ax
[13]
55 chloride to the methanol solutions of 1, the proton
4 [Co(pz
4
depy)(H
2
O)] (cf. Co‒N = 2.133 Å)
.
BF4
NO3
5
5
5
5
6 resonances of 1 and 1
shift to the resonances
5 The nitrate structure also shows a slight equatorial
6 distortion due to hydrogen bonds between two NH
7 pyrazole units and the nitrate ion. The angle between
8 the plane defined by the pyridine ring and the
9 equatorial N
0 1
1 and Figure S 5). 1 and 1
Cl
−
7 of 1 , which confirms initial methanol, NO3 , and
−
BF4
NO3
Cl
8 Cl binding in 1 , 1 , and 1 , respectively, and
9 chloride binding of the metal ion in the presence of
BF4
Cl
60 ex. KCl (Figure S 13). In methanol/water 80/20
4
-plane is 90° in 1
and 1 , whereas
BF4
NO3
61 mixtures, the resonance of the protons in 1
62 1
and
exhibits an angle of 80° (Figure S 2, Figure S 4,
NO3
Cl
NO3
are the same, indicating that the nitrate anion in
form an extended
6
6
6
6
6
6
6
7
7
7
7
7
3 1NO3 is substituted by a solvent molecule (Figure S
4 14). Since no solvent molecule was bound in
5 methanol, we assume that water is bound to the metal
6 ion, because it is smaller. The proton resonances in
2 network in the solid state through hydrogen bonds
3 (Figure S 9, Figure S 10).
Cl
BF4
7 1 differ from those of 1 , which indicates that the
8 chloride anion is still bound. Upon adding 0.1 M
9 potassium chloride to the methanol/water solutions,
BF4
NO3
Cl
0 the resonances of 1 and 1
shift to those of 1
1 (Figure 4, Figure S 15). That is to say, that the
II
+
2 [Co L(Cl)] cation is present in methanolic and
3 aqueous potassium chloride solution independently
4 of the cobalt salt employed in the synthesis.
2
4
BF4
NO3
2
2
5
6
Figure 3. Overlay of the molecular structures of 1 (red), 1
(green), and 1 (blue) in the solid state.
Cl
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
7 A single crystal X-ray diffraction experiment
8 confirmed the structure of 2 . The result of the
Cl
9 refinement process is shown in Figure S 6 and
0 selected bond lengths in Table 2 and angles in Table
7
5
Cl
1 S 5. 2 crystallises in the orthorhombic space group
1
NO3
BF4
Cl
7
7
6
7
4 2
Figure 4. H-NMR spectra of 1 , 1 , and 1 in MeOD-d /D O
1
2 Pna2 (Table S 2). The cobalt ion has a distorted
80/20 and 0.1 M KCl.
3 octahedral coordination geometry composed of the 5
4 N atoms of the ligand and one chlorido ligand. All
5 four pyrazole units are protonated. The distances
6 between the nitrogen atoms and the cobalt ion are
78 Subsequently, we investigated the solution
79 speciation in MeOH/H O (80/20) by titration
80 experiments. The ligand was soluble in MeOH/water
81 (80/20) mixtures over the whole pH range of 2-11.
82 Fitting of the UV/Vis data with SPECTFIT™
a
83 revealed one protonation equilibrium at pK of 3.20
84 (Figure S 16, Table 3). The equilibrium likely
85 belongs to the pyridine unit, because pyridinium has
86 a higher pK
2
7 considerably shorter than in 1 and similar to the
im
[14]
8 distances in [Co(L )(OH)](Cl)
2
, B, which is in line
9 with a low-spin cobalt(III) state. The structure shows
0 a slight equatorial distortion. The angle between the
1 plane defined by the pyridine ring and the equatorial
2 N
4
-plane exhibits 82° (Figure S 6), that is, the cobalt
a
than pyrazolium due to the repulsion of
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201800347
[
15]
53 other, 6.0 and 6.7.[19] The two pK
54 deprotonation of two pyrazole units. The pK
1
2
3
4
5
6
7
8
9
two adjacent NH units (cf. pyrazole pK
a
= 2.49,
= 5.23) . The protonation of the
pyrazole units is out of the experimental pH range (<
2). In order to corroborate ligand’s pK , NMR
a
are ascribed to the
[
16]
pyridine pK
a
a
of the
55 pyrazole protons is similar to the pK determined for
a
III
3+
[9a]
a
56 pyrazole in [(NH
57 The difference in the pK
58 cobalt complex is fairly minor.
3
)Co (pzH)] (cf. pK
a
= 6.07)
.
[
17]
spectra at different pH were recorded. In line with
the UV/Vis experiment, the pyridine proton
resonances held constant between a pH of 5 and 12,
and shift to lower field with lower pH of the solution
(Figure S 17, Figure S 18).
a
of the oxidised and reduced
100
2+
[Co(LH )Cl]
-2
[CoLCl]
8
0
0
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
0 Complex formation studies of Co2+ ions and L
1 revealed one equilibrium over the pH range of 2-9
2 (methanol/water 80/20, I = 0.1 M KCl Table 3).
3 However, the deprotonated complex is not fully
+
[Co(LH )Cl]
6
-1
40
a
4 soluble and therefore only an approximate pK of the
20
5 pyrazole protons can be given. Upon increasing pH,
6 we observe one deprotonation step around a pH of
7 7.5 and the analysis of the first derivative of four
3
+
Co
0
5.0
5.5
6.0
6.5
7.0
7.5
8 titration curves led to an average pK
a
of 7.3(3)
pH
5
9
9 (average of 7.25, 7.25 and 7.50, 7.35). We
0 tentatively assigned this to two deprotonation steps,
Cl
6
6
0
1
Figure 5. Species distribution of 2 as determined by UV/Vis
titration, solvent: methanol/water (80/20); I = 0.1 M KCl.
im
1 because complexes of the similar ligand L and of
2 L (see below) showed usually two pK
a
at a similar or
62 Redox properties.
63 In order to determine the BDFE of the formal H-
64 atom transfer step, we determined the redox potential
3 same values. The third and fourth deprotonation step
4 are outside the range of the experiment. The binding
5 of the pyrazole to the metal ion leads to a decrease
III/II
+/2+
65 of the [Co LCl]
redox pair. We collected CV
Cl
6 of the pK
7 orders of magnitude (cf. pK
8 The pK of 1 is slightly lower than the one of A (cf.
9 pK
= 9.79[3a]), which can be rationalised by the
0 shorter distance between the acidic H-atom and the
1 metal centre in 1. The difference in the pK of 1 and
a
of the acidic pyrazole proton of about 7
66 data of 2 under acidic conditions in order to
67 investigate the electron transfer (ET) step at different
68 concentrations and scan rates (Figure 6, Figure S 21).
[
18]
a
of pyrazole = 14)
.
a
a
6
7
7
7
7
7
9 The broad oxidation feature, the separation of the
0 reduction and oxidation waves as well as the peak
1 potential shift with increasing scan rates indicates a
2 rather slow electron transfer rate. This is typical for
a
2 A is very similar to the difference previously
3 observed in isoelectronic copper imidazole and
II
III
3 hs-Co /ls-Co redox pairs as the spin state change
[
4i]
4 pyrazole complexes.
[
13,20]
4 results in large structural changes.
75 were obtained using reasonable values to describe
6 the redox process. The potential was determined to
7 be E° = 0.78 V vs. NHE at a rather slow electron
8 transfer rate k
s
of 1×10−3 cms . Representative
9 overlays are depicted in Figure 6 and Figure S 21.
0 The potential of the Co
1 higher than the one of [Co(pz
2 considerably higher than of A due to the weaker σ-
3 and π-donor strength of pyrazole in comparison to
Good fits
2
+
3+
3
3
3
3
5
6
7
8
Table 3. The protonation constants of L with Co and Co ions
°
−3
at 25.0(2) C, I = 0.1 moldm (KCl), solvent: methanol/water
80/20. Errors of calculated values are given in parentheses.
Charges were omitted for clarity.
7
7
7
7
8
8
8
8
−1
L
CoII
CoIII
pK
pK
pK
a
a
a
(LH)
(CoLCl)
(CoLH−1Cl)
3.20(8)
III/II
redox pair is slightly
depy)(H
O)]2+ and
7.3(3) 6.0(3)
7.3(3) 6.7(4)
4
2
3
9
[
13]
4
4
4
4
4
4
4
4
4
4
5
5
5
0 Subsequently, we conducted pH-dependent UV-Vis
84 imidazole
(cf.
E
=
0.63
V
in
Cl
2+
[3a]
1 titration
2 methanol/water at 0.1 M KCl in order to estimate the
3 ligand’s pK in the cobalt(III) complex. The low
experiments
employing
2
in
85 [Co(pz
4
depy)(H
2
O)] ; E = 0.51 V in A vs. NHE).
a
Cl
4 solubility of 2 in methanol/water mixtures did not
5 allow for NMR or conductivity experiments. Upon
6 increasing the pH, the transition at 270 nm decreases,
7 albeit, only minor changes have been observed
8 (Figure S 20). This is similar to previously
9 investigated copper pyrazole complexes, which also
0 showed only minor changes in the UV/Vis data upon
[
4i]
1 deprotonation. Fitting of the titration data with
2 SPECTFIT™[ led to two pK
14]
values close to each
a
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201800347
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
9 The reduction potential of the [CoIII/II(LH−1)Cl]+/0
0 redox pair has been calculated to be 0.70 V
1 according to the square scheme formalism, and the
III/II
0/−
[21]
2 one of the [Co (LH−2)Cl] pair to be 0.67 V.
3 The protonation state of pyrazole has a fairly minor
III/II
4 influence on the redox potential of the Co
5 pair in comparison to the potential shift of the Ru
6 redox pair in the series of cis-[(bpy) Ru(pzH−1
and cis-
(cf. 0.54 V, 0.93 V, and 1.42 V vs.
redox
III/II
2
2
) ],
+
7 cis-[(bpy)
8 [(bpy) Ru(pz)
9 NHE, respectively). In other words, the pK
2
Ru(pz)(pzH−1)] ,
2+
2
2
]
[
5]
a
of the
0 pyrazole unit decreases only slightly by oxidising the
1 metal centre. This could be rationalised by a
2 stabilisation of the protonated cobalt(III) complex
3 via hydrogen bonds to the chloride anions as
4 observed in the solid state structure. This effect is
1
45 likely less pronounced in the cobalt(II) complex, as
4
4
6 the complex exhibits a charge of +1 instead of +2 as
7 in 2 .
2
3
4
5
Figure 6. CV data of 0.27 mM 2 in MeOH/H
rt (0.1 M KCl); black lines: experimental data; red dotted lines:
1
simulation with the parameter values E = 0.78 V vs. NHE, α =
2
O 80/20 at pH 2 at
Cl
1
−3
−1
0.5, k
s
= 1×10 cms .
48 In order to substantiate the BDFE, 1 was reacted with
4
5
5
5
5
5
5
5
5
9 a HAT transfer reagent in methanol/water.
6
7
8
9
Determination of the BDFE.
0 4-Dimethylaminophenol (DMAP) has a BDFE of
The BDFE of the hypothetical HAT can be
determined according to eq. 1, where R is the gas
constant, T is temperature, and F is the Faraday
−1
a
1 329 kJmol and a pK of 10.1 in water, and thus
III
+
−
2 should react with [Co (LH−2)Cl] under H /e -
[6]
3 transfer.
Indeed, UV/Vis-monitoring of the
4 reaction of 10 eq. DMAP and 2 H−2 at pH of 9
5 indicates a slow reaction (Figure S 22). The rather
[
7a]
1
1
1
1
1
1
1
0 constant. The first two terms describe the redox
1 potential of the complex and its acid/base
2 equilibrium. c is equivalent to the H /H standard
G
3 reduction potential in the respective solvent and thus,
4 is independent of the nature of the species
5 involved. It has been calculated to be 226.3
Cl
[22]
+
•
6 slow reaction can be rationalised by the ls/hs
[23]
7 transition upon reduction of the cobalt(III) ion.
[
7]
5
5
6
6
6
6
6
6
6
8 Conclusion
9 We synthesised a new ligand with a N -donor set and
5
−1
[4i]
6 kJmol in MeOH/H
2
O 80/20.
0 its Co(II) and Co(III) complexes. Characterisation in
1 the solid state and solution showed the expected
2 distorted octahedral complexes with the Co(II) ion in
3 high-spin state (1) and the Co(III) ion in low-spin
4 state (2). The sixth ligand in 1 depends on the
5 employed Co(II)-precursor in the solid state and was
6 shown to be chloride in solutions with 0.1 M KCl.
ꢀ
1
7 BDFE = 2.301푅푇p퐾 + 퐹퐸 + 푐 (1)
a
G
1
1
2
2
2
2
2
8 Based on the thermodynamic data of the various
9 species, i.e. E°’ of [Co (L)Cl] , pK of
0 [Co (L)Cl] , [Co (LH−1)Cl], [Co (L)Cl] , and
1 [Co (LH−1)Cl] , we determined a BDFE for the two
2 hypothetical H-atom-abstraction steps of 336
3 kJmol (80.2 kcal mol ) and 332 kJmol (79.4 kcal
4 mol ), respectively (Scheme 2).
III/II
2+/+
a
II
+
II
III
2+
III
+
−1
−1
−1
6
6
6
7
7
7
7
7
7
7
7
7
7
8
a
7 The pK values of the acidic protons in 2 were
8 determined to be 6.0 and 6.7 and were be assigned to
9 the deprotonation of two of the pyrazole units of the
−1
0 ligand, yielding a neutral complex at higher pH. The
Cl
1 pK
a
of 1 was found to be 7.3. This is more than two
2 orders of magnitude lower than in A, likely because
3 of the shorter cobalt−NH distance in pyrazole
4 complex compared to imidazole complex. Cyclic
Cl
5 voltammetry of 2 under acidic conditions revealed
6 a potential of 0.78 V vs. NHE for the Co(II)/Co(III)
7 redox pair. The influence of the oxidation state on
8 the pK of ligand in the complex are small compared
a
9 to previously reported ruthenium complex with
0 pyrazole units, likely due to hydrogen bonding of the
2
5
Cl Cl
2
2
2
6
7
8
Scheme 2. Thermodynamic square scheme for 1 /2 .
Thermodynamic values in square brackets have been calculated
according to Hess’s law.
81 anion to the complex. From the thermodynamic data
8
8
2 the BDFE for the formal HAT`s was calculated to be
3 336 kJmol−1 and kJmol , respectively, which is
−1
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201800347
1
2
3
4
5
6
higher than in a previously reported copper complex
53 1Cl: 1H-PARA-NMR (300 MHz, MeOD-d
54 (ppm) = 54.5 (2 H, Py-3,5-H), 42.1 (4 H, Pz-H),
55 41.7 (4 H, Pz-H), 17.4 (6 H, OCH ), 13.8 (1 H, Py-
56 4-H). ESI(+)-MS m/z (%) = 229.0 (100, [CoL‒
4
): δ
with pyrazole containing ligands (cf. 279
−
1 [4i]
kJmol ).
This shows, that the BDFE can be
3
readily tuned by the respective metal/ligand entity,
which could be exploited in in small molecule
2+
2+
57 MeOH] ), 245.1 (88, [CoL] ), 432.2 (91), 454.2
[
1e,6,24]
+
-1
activation and organic synthesis.
58 (80), 489.1 (54, [Co(L‒H)] . IR ν (cm ) = 608 (s),
5
6
6
6
6
9 760 (s), 781 (m), 795 (s), 1067 (s), 1126 (m), 1196
0 (s), 1225 (s), 1356 (s), 1443 (s), 1454 (s), 1524 (s),
1 1572 (s), 1591 (s), 2953 (s), 3437 (br). Micro
2 analysis (%) C21H N O CoCl : calc.: C 43.54, H
23 9 3 2
3 4.00, N 21.76; found: C 43.58, H 3.71, N 21.54.
7
8
9
Experimental Section
General. Manipulations of air-sensitive reagents
were carried out in an MBraun glove box, or by
1
1
1
1
1
1
1
1
1
1
2
2
0 means of Schlenk-type techniques involving the use
1 of a dry nitrogen atmosphere. Millipore water and
2 methanol were degassed separately by bubbling
3 argon through it before using it. All reagents were
4 purchased as reagent grade or with higher quality
5 and used without further purification. CCDC-
6
6
6
6
6
6
7
7
7
7
7
7
7
4 2: L (typically 50 mg, 0.10 mmol, 1.00 eq.) and
5 [Co(acac) ] (34 mg, 0.10 mmol, 1.00 eq.) were
3
6 dissolved in methanol (5 mL) and 6 drops of
7 hydrochloric acid (37%) were added. The reaction
8 mixture was heated under reflux for 5 h.
9 Diethylether was added to the solution at ambient
0 temperature, the precipitate was filtered off and
1 washed with diethylether and chloroform several
2 times. Single crystals also suitable for X-ray analysis
3 were obtained by slow diffusion of diethylether into
4 a solution of 2 in methanol. The crystals were filtered
5 off and dried in vacou. Isolated yield typically
6 around 50%.
6 1837824,
CCDC-1837825,
CCDC-1837826,
7 CCDC-1837827, CCDC-1837829, CCDC-1837829
8 contain the supplementary crystallographic data for
9 this paper. The data can be obtained free of charge
0 from The Cambridge Crystallographic Data Centre
1 via https://www.ccdc.cam.ac.uk/structures/.
2
2 Complex Synthesis.
2
2
2
2
2
2
2
3
3
3 1: L (typically 100 mg, 0.19 mmol, 1.00 eq.) and
4 [Co(H O) ]X (0.19 mmol, 1.00 eq.) were stirred in
2 6 2
4
7 ): δ (ppm) = 8.36 (dd,
1H-NMR (300 MHz, MeOD-d
8 J = 7.03 Hz, 1H), 8.27 (d, J = 2.8 Hz, 4H), 8.25 –
9 8.21 (m, 2H), 7.03 (d, J = 2.8 Hz, 4H), 4.28 (s, 6H).
7
7
7
8
8
8
8
8
8
8
8
8
8
9
9
5 methanol (3mL) for 2 h. The resulting orange
6 solution was reduced to 1.5 mL, filtered and the
7 target compound crystallised by slow diffusion of
8 diethylether. The resulting orange crystal were also
9 suitable for X-ray analysis. The crystals were filtered
0 off and dried in vacuo. Isolated yield typically
1 around 65%.
13
1
0
C{ H}- NMR
1 (ppm) = 160.1 (C
2 (Py-4), 139.3 (Pz-3), 122.9 (Py-3,5), 107.1 (Pz-4),
3 83.6 (C , C-OCH ), 58.1 (OCH ). ESI(+)-MS m/z
4 (%) = 229.0 (13, [CoL‒MeOH] ), 245.1 (12,
(75 MHz, MeOD-d
4
)
훿
q
, Py-2,6), 151.7 (C , Pz-5), 145.1
q
q
3
3
2+
2
+
2+
5 [CoL] ), 262.5 (44, [CoLCl] ), 524.1 (100, [Co(L‒
4
: ): δ
1H-PARA-NMR (300 MHz, MeOD-d
3
3
3
3
3
3
3
3
4
4
4
2 1NO3
3 (ppm) = 53.5 (2 H, Py-3,5-H), 43.9 (4 H, Pz-H),
4 41.1 (4 H, Pz-H), 17.4 (6 H, OCH ), 11.1 (1 H, Py-
5 4-H). ESI(+)-MS m/z (%) = 489.1 (20, [Co(L-H)] ),
6 552.1 (76, [CoL(NO )] ), 603.1, 1166.2 (14,
3
2 2 3 3
7 [Co L (NO ) ] ). IR ν (cm ) = 605 (s), 757 (s), 777
+
-1
6 H)Cl] . IR ν (cm ) = 608 (s), 716 (s), 748 (s), 779
7 (s), 934 (s), 1078 (s), 1138 (s), 1211 (s), 1242 (s),
8 1366 (br), 1458 (m), 1526 (m), 1599 (s), 2916 (br),
3
+
23 9 3 3
9 3430 (br). Micro analysis (%) C21H N O CoCl :
0 calc.: C 41.03, H 3.77, N 20.51; found: C 41.00, H
1 3.72, N 20.60.
+
+
−1
8 (m), 1028 (s), 1067 (s), 1123 (s), 1191 (s), 1312
9 (s), 1460 (s), 1521 (s), 1596 (s), 2971 (br), 3422
9
2 Acknowledgements
[
25]
0 (br) NO
3
-anion: 1384 (s) .
Co: calc.: C 41.05, 3.45, 25.08; found:
2 C 41.06, H 3.16, N 25.19,.
Micro analysis (%)
1 C21
H
21
N
11
O
8
93 This work was supported by funding from the DFG
94 (Emmy Noether Programm: SI 1577/2 (IS), INST
9
9
9
9
5 186/1086-1 FUGG (X-Ray)), the Fonds der
6 Chemischen Industrie and the Dr. Otto Röhm
7 Gedächtnisstiftung. We would like to thank Dr.
8 Julian Holstein for help in X-Ray crystallography.
4
4
4
4
4
4
4
5
5
5
3 1BF4 1H-PARA-NMR (300 MHz, MeOD-d
: ): δ
4
4 (ppm) = 55.5 (2 H, Py-3,5-H), 43.8 (4 H, Pz-H),
5 41.1 (4 H, Pz-H), 18.3 (6 H, OCH ), 10.8 (1 H, Py-
6 4-H). ESI(+)-MS m/z (%) = 489.1 (44, [Co(L-H)] ),
3
+
+
7 577.1 (9, [CoL(BF
4
)] ), 603.1 (92). IR
99 Keywords: Cobalt • Coordination Chemistry • N
8 ν (cm ) = 523 (s), 604 (s), 761 (s), 805 (s), 1127 100
−1
Ligands • Thermochemistry
1
01
9 (m), 1358 (s), 1454 (s), 1524 (s), 1595 (s), 2951 (br),
[
26]
102 [1] Selected reviews: a) S. J. Slattery, J. K. Blaho,
0 3368 (br), BF
1 (%) C23
2 17.31; found: C 37.72, H 4.09, N 17.84.
4
-anion: 1071 (s).
Micro analysis
1
1
1
1
03
04
05
06
J. Lehnes, K. A. Goldsby, Coord. Chem. Rev.
1998, 174, 391–416; b) S. J. Formosinho, M.
Barroso, Proton-Coupled Electron Transfer: A
Carrefour of Chemical Reactivity Traditions,
H
29
N
9
O
4
CoB : calc.: C 37.94, H 4.01, N
2 8
F
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201800347
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European Journal of Inorganic Chemistry
10.1002/ejic.201800347
FULL PAPER
Thermochemistry
A
cobalt complex with
a
pentadentate N-donor ligand with
four ionisable pyrazole units was
studied in solid state and solution.
The pKa and E°’ of the individual
steps as well as the bond
dissociation free energy of the
concerted hydrogen atom transfer
step were determined.
M. Wilken, C. Würtele, M. Kügler, F.
Chrobak, I. Siewert.*
Page No. – Page No.
Thermochemistry of
a
Cobalt
Complex with Ionisable Pyrazole
Protons
1
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