A solvent- and temperature-dependent intramolecular equilibrium of diamagnetic and paramagnetic states in Co complexes bearing triaryl amines

Article abstract of DOI:10.1039/c8dt02538j

Complexes [Co(L)₂](ClO₄)₂ (L = o-substituted 2-(pyridine-2-yl)-1,10-phenanthrolines 1a-c) containing three redox active centres (a Co²⁺ ion and two triaryl amine (Tara) units) have been synthesised. The order of

Full text of DOI:10.1039/c8dt02538j

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A solvent- and temperature-dependent
intramolecular equilibrium of diamagnetic and
paramagnetic states in Co complexes bearing
triaryl amines†
Cite this: DOI: 10.1039/c8dt02538j
Linda Schnaubelt,a Holm Petzold, a Evgenia Dmitrieva,b Marco Rosenkranzb and
a
Heinrich Lang
*
Complexes [Co(L)2](ClO4)2 (L = o-substituted 2-(pyridine-2-yl)-1,10-phenanthrolines 1a–c) containing
three redox active centres (a Co2+ ion and two triaryl amine (Tara) units) have been synthesised. The order
of oxidation steps in [Co(L)2](ClO4)2 (L = 1a–c) was determined using cyclic voltammetry and EPR/UV-vis-
NIR spectroelectrochemistry. In acetonitrile solutions, at room temperature, the first oxidation is Co-
centred followed by the Tara oxidation at more anodic potentials. The order of oxidation is inverted in
solutions of the less polar solvent dichloromethane. The Co3+/2+-centred redox event leads to a spin tran-
sition between the paramagnetic high-spin (HS) Co2+ and the diamagnetic low-spin (LS) Co3+ state,
1
which was proven using H NMR and EPR spectroscopy. After one-electron oxidation of [Co(L)2](ClO4)2,
an equilibrium between the diamagnetic [Co3+(L)]3+ and paramagnetic [Co2+(L)(L+)]3+ state in [Co(L)2]3+
(L = 1a–c) was found. Cyclic voltammetry showed enhanced intermolecular electron transfer between
the [Co2+(L)2]2+ and [Co3+(L)2]3+ redox states mediated by [Co2+(L)(L+)]3+. Variable temperature vis-NIR
spectroscopy of in situ generated [Co(L)2]3+ revealed a temperature-dependent redox equilibrium
between the [Co3+(L)2]3+ and the [Co2+(L+)(L)]3+ states (L = 1a–c). Magnetic coupling between the
HS-Co2+ ion and the Tara+ radical in [HS-Co2+(L+)(L)]3+ (L = 1a,c) was deduced from broad and undetect-
able lines observed in the corresponding EPR spectra. Complete oxidation to [LS-Co3+(L+)2]5+ (L = 1a,c)
leads to characteristic EPR spectra of Tara biradicals with non-interacting spins.
Received 21st June 2018,
Accepted 21st August 2018
DOI: 10.1039/c8dt02538j
rsc.li/dalton
Stimuli are most commonly a change in temperature and/or
light-irradiation. In this context, polypyridine complexes of
Introduction
Metal complexes with multiple redox centres have gained Ru,1–3,15–20 Os21–24 and Zn,25–29 respectively, bearing triaryl
interest, for example, in the development of sensors, electro- amines (Taras) have attracted great attention, but less common
chromic materials and memory storage media.1–4 are cobalt complexes.30,31 The combination of redox-active
Intramolecular charge transfer (CT) between redox centres has cobalt ions and Taras in one molecule leads to hybrid
been widely investigated on inorganic mixed-valence com- materials with interesting properties such as enhanced light
plexes with two metal redox centres.5–10 Similarly, organic absorption,2,17 electrochromism,18,32 metal-to-ligand or
mixed-valence compounds11,12 have attracted attention but ligand-to-metal charge transfer16,33 (MLCT, LMCT) and spin
hybrid organic/inorganic redox-active complexes have pro- transition. The LS-Co3+/HS-Co2+ (LS = low spin, HS = high
voked a dynamic issue in CT research. Modulation of redox spin) redox couple has been an intriguing building block for
states and even spin multiplicities leads to characteristic charge
transfer
induced
spin
transition
(CTIST)
changes in optical, magnetic and structural properties.13,14 complexes.34–37 Common examples are Prussian-blue ana-
logues38,39 and catecholate/semiquinonate40–42 systems. Their
CTIST behaviour in the solid state has been investigated in
detail but there is a lack of examination of the CTIST in
solution.43,44
Recently, we reported on the synthesis, molecular structure
and redox behaviour of [Co(1a)2]2+,45 possessing two phenan-
throline-pyridyl based ligands which bear a redox-active Tara
unit. Cyclic voltammetry and coupled EPR/UV-vis-NIR spectro-
aTechnische Universität Chemnitz, Faculty of Natural Sciences, Institute of
Chemistry, Inorganic Chemistry, 09107 Chemnitz, Germany.
E-mail: heinrich.lang@chemie.tu-chemnitz.de
bCenter of Spectroelectrochemistry, Leibniz Institute for Solid State and Materials
Research (IFW Dresden), Helmholtzstraße 20, 01069 Dresden, Germany
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8dt02538j
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reported.46–48 To decrease the redox separation between both
redox sites, the Taras were alternated by introducing electron-
rich methoxy groups. Substitution of a methyl group by a
methoxy group reduces the redox potential of the Tara by
approximately 70 mV.49 Similarly, a difference in the solvent
polarity can have a substantial effect on the redox potentials of
the Co3+/2+ and Tara+/0 couples.50 Therefore, the redox behav-
iour of the complexes [Co(L)2](ClO4)2 (L = 1a–c) was investi-
gated using cyclic voltammetry and coupled EPR/UV-vis-NIR
spectroelectrochemistry in acetonitrile and dichloromethane.
The results are supported by variable temperature (VT) NMR
and vis-NIR spectroscopy of in situ oxidised [Co(1c)2](ClO4)2.
Fig. 1 Parabolic representation of the CTIST in complexes [Co(L)2]3+
(L = 1a–c). LS = low-spin, HS = high-spin.
Results and discussion
Synthesis and characterisation
Compounds 1a–c were synthesised from 2-(6-bromopyridine-2-
yl)-1,10-phenanthroline45 by a solvent-free melt reaction with
an appropriate potassium salt of the respective para-hydroxyl-
Tara (Tara = triaryl amine). The potassium salts of the
hydroxyl-Taras were prepared in THF solution using KOtBu.
After removing all volatiles, 2-(6-bromopyridine-2-yl)-1,10-phe-
nanthroline was added and the reaction mixture was heated
until both components melted (>150 °C). Appropriate work-up
and column chromatography afforded compounds 1a–c in
moderate to good yields. The Co2+ complexes were obtained
after mixing 1a–c with [Co(H2O)6](ClO4)2 in ethanol solutions
as air and moisture stable yellow solids. The synthesis method-
ologies are outlined in Scheme 1. All compounds were charac-
terised using elemental analysis, mass spectrometry, and 13C
NMR (diamagnetic compounds only) and 1H NMR spec-
troscopy. A detailed description of the synthesis and experi-
mental data can be found in the ESI.†
electrochemistry in acetonitrile evinced that the first oxidation
process of [Co(1a)2]2+ is Co-centred followed by the Tara-
centred oxidation. The redox separation of the Co- and Tara-
centred redox process in [Co(1a)2]2+–5+ amounts to ΔE°′ ≈
250 mV. However, an equilibrium (Fig. 1) between the redox
isomers [Co3+(1a)2]3+ and [Co2+(1a)(1a+)]3+ was not directly
observed. Co-centred redox events (reaction on the electrodes
and self-exchange reaction) involving the LS-Co3+/HS-Co2+
redox couple are mediated by the presence of the Tara unit,
indicating the presence of paramagnetic [Co2+(1a)(1a+)]3+ as a
small fraction of the [Co(1a)2]3+ redox state (parabolic repre-
sentation in Fig. 1).
In search of complexes that are in a genuine equilibrium
between the paramagnetic and diamagnetic state of [Co(L)2]3+,
we modified the para-substituents R1 and R2 of the Tara units.
The variation of the Tara para-substituents is an efficient way
of fine tuning its redox properties. Furthermore, enhanced
intramolecular electron transfer in relation to small redox
splitting (ΔE°′) of the donor and acceptor unit was
1H NMR spectra of complexes [Co(L)2](ClO4)2 (L = 1a–c) in
d3-acetonitrile reveal a paramagnetic behaviour caused by the
d7 high-spin configuration of the metal. At 298 K, proton reso-
Scheme 1 Synthesis of 1a–c and their corresponding Co2+ complexes [Co(1a–c)2](ClO4)2. The small italic numbers indicate the proton labelling.
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nances spread from about δ = 170 to −10 ppm. Only the aro- ible electronic coupling between the redox-active units. Thus,
matic proton resonances of the tolyl and anisyl groups retain the redox behaviour of [Co(L)2](ClO4)2 (L = 1a–c) was studied
their doublet character (3JH,H coupling) due to the spatial dis- using cyclic and square wave voltammetry. Cyclic voltammo-
tance to the paramagnetic Co2+ ion (1/r6 dependence of the grams (CVs) and square wave voltammograms (SWVs) were
relaxation enhancement). Short T1 relaxation times of the recorded from anhydrous acetonitrile or dichloromethane
proton resonances are the result of dipolar coupling of the solution of the respective complex (1 mM) with weakly
proton spins with the unpaired d-electrons of the cobalt ion coordinating [nBu4N][B(C6F5)4] (100 mM) as the supporting
(T1 = 1–4 ms for H1). With regard to the intermediate ligand electrolyte55,56 at 25 °C. All potentials were referenced to the
strength of o-substituted 2-(pyridine-2-yl)-1,10-phenanthro- ferrocene/ferrocenium (FcH/FcH+) redox couple.57 Pertinent
lines51,52 and previously reported details on [Co(1a)2]2+,45 com- data are summarised in Table 1. Fig. 2 presents the corres-
plexes [Co(L)2](ClO4)2 (L = 1a–c) are exclusively regarded as ponding CVs.
1
high-spin. The H NMR spectrum of [Co(1c)2](ClO4)2 at 198 K
The CVs of complexes [Co(L)2](ClO4)2 (L = 1a–c) in aceto-
exhibits a strong paramagnetic shift of the 2-(pyridine-2-yl)- nitrile solution exhibit two redox events. The first redox
1,10-phenanthroline proton resonances to
lower field process can be assigned to the Co3+/2+ redox process followed
a
accompanied by substantial line broadening in accordance by the Tara-centred redox process at higher potentials. The
with the Curie behaviour and the increased solvent viscosity. Tara groups are oxidised in a two-electron event consisting of
The rotation of the phenyl rings with H11 and H12 is slowed two reversible superimposed redox waves. This indicates an
down leading to additional line broadening for the resonances insignificant electrostatic interaction between the two Taras,
at δ = −5.36 and −16.56 ppm (line widths = 396 and 600 Hz). which is characteristic of distinct Tara nitrogen atoms.58,59 The
Sub-stoichiometric oxidation of [Co(1c)2]2+ to [Co(1c)2]3+ with Co3+/2+ redox potentials remain constant with E°′1(Co3+/2+) =
[N(p-C6H5Br)3][SbCl6] leads to spreading out of all resonances 284, 290, 289 mV in the series [Co(L)2](ClO4)2 (L = 1a–c).
at 298 K as a result of fast electron transfer between these two Usually, the Co3+/2+ redox process is characterised by large
species (Fig. 15-SI†). Cooling d6-acetone solutions down to peak-to-peak separation due to large inner reorganis-
193 K decelerates the electron transfer and consequently the
resonances sharpen for both [Co(1c)2]2+ and [Co(1c)2]3+,
respectively. Signals for the aromatic protons of [Co(1c)2]3+ are
observed between δ = 10 and 5.5 ppm revealing the diamag-
netic LS-Co3+ state. The assignment of the [Co3+(1c)2]3+ reso-
1
nances was achieved by measuring the H,1H COSY spectrum
and previously recorded EXSY spectra of related complexes.45
In Co3+ polypyridyl complexes, the Co–N bond lengths are
shorter than those in Co2+ polypyridyl species.53,54 Hence, the
rotation of the phenyl rings bearing H11 and H12 is hindered
and two sets of resonances (four resonances in total) can be
observed in the 1H NMR spectrum (Fig. 16-SI†).
Fig. 2 Cyclic (solid) and square wave (dotted) voltammograms of (a)
[Co(1a)](ClO4)2, (b) [Co(1b)2](ClO4)2 and (c) [Co(1c)2](ClO4)2 in CH3CN (left)
and in CH2Cl2 (right). Measurement conditions: 1.0 mmol L−1 analyte,
[nBu4N][B(C6F5)4] (0.1 mol L−1) as the supporting electrolyte, scan rates
100 mV s−1 (CV) and 2 mV s−1 (SWV), glassy carbon electrode, 25 °C.
Electrochemistry
Complexes [Co(L)2](ClO4)2 (L = 1a–c) contain three redox-active
centres, a Co2+ ion and two Tara units. Electrochemical studies
are frequently employed to examine redox potentials and poss-
Table 1 Cyclic voltammetry data of [Co(1a–c)2](ClO4)2 in CH3CN and CH2Cl2. E°’ is the formal potential, ΔEp is the difference between the oxidation
and reduction peak potentials, and ΔE°’ is the difference between the Co3+/2+ and Tara+/0 formal potentials. Measurement conditions: 1.0 mmol L−1
analyte, [nBu4N][B(C6F5)4] (0.1 mol L−1) as the supporting electrolyte, scan rate 100 mV s−1, glassy carbon electrode, 25 °C
Solvent CH3CN
E°′1(Co3+/2+) [mV]
ΔEp(Co3+/2+) [mV]
E°′2(Tara+/0) [mV]
ΔEp(Tara+/0) [mV]
ΔE°′ [mV]
[Co(1a)2](ClO4)2
[Co(1b)2](ClO4)2
[Co(1c)2](ClO4)2
284
290
289a
106
70
531
440
374a
68
247
150
85
80
60a
90a
Solvent CH2Cl2
E°′1(Tara+/0) [mV]
ΔEp(Tara+/0) [mV]
E°′2(Co3+/2+) [mV]
ΔEp(Co3+/2+) [mV]
ΔE°′ [mV]
[Co(1a)2](ClO4)2
[Co(1b)2](ClO4)2
[Co(1c)2](ClO4)2
468a
364
296
96a
74
68
598a
578
581
84a
90
98
130
214
285
a Determined using square wave voltammetry.
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ation.45,54,60 So, noteworthy is the decrease in the peak-to-peak dichloromethane solutions with ΔE°′ = 130 mV is found
separation of the Co-centred redox process within the series of for [Co(1a)2](ClO4)2. The difference between the Tara+/0 and
[Co(L)2](ClO4)2 (ΔEp(Co3+/2+) = 106 (L = 1a), 70 (L = 1b), and 60 Co3+/2+ redox potentials increases in [Co(L)2](ClO4)2 (L = 1b,c)
(L = 1c) mV) concomitant with a smaller gap between the with the increasing electron donating character of the para-
redox potentials of Co3+/2+ and Tara+/0. This can be explained substituents at the Tara units (ΔE°′ = 214, 285 mV).
by the mediator effect of the Tara unit, which catalyses the
UV-vis-NIR spectroelectrochemistry
Co3+/2+ redox process.45,61 In this case, the Tara-centred oxi-
dation to [Co2+(L)(L+)]3+ is kinetically favoured due to the small UV-vis-NIR spectra of [Co(L)2](ClO4)2 (L = 1a–c) exhibit one
inner reorganisation energy of the Tara+/0 redox couple. An intense absorption around λ ≈ 300 nm both in CH3CN and
intramolecular electron transfer leads to the thermo- CH2Cl2. This absorption can be attributed to a π–π* transition
dynamically more stable [Co3+(L)2]3+ species.45 The Co3+/2+ within the phenanthroline-pyridyl ligand and/or
a Tara
reduction peak in the CV of [Co(1a)2](ClO4)2 is sluggish. In absorption.49,61–64 The complexes are transparent at λ >
contrast, this process is clearly visible in the CV of [Co(1b)2] 400 nm. In contrast, the UV-vis-NIR spectra of Co3+- and par-
(ClO4)2. The Tara-centred redox process is shifted cathodically ticularly Tara+-centred transitions show characteristic absorp-
from E°′2(Tara+/0) = 531 mV in [Co(1a)2](ClO4)2 to E°′2(Tara+/0) = tions. An absorption band related to the Co3+/2+ redox process
440 mV in [Co(1b)2](ClO4)2 and E°′2(Tara+/0) = 374 mV in [Co of [Co3+/2+(1a)2]3+/2+ in CH3CN was observed at λ = 382 nm.45
(1c)2](ClO4)2, due to the electron donor effect of the methoxy The Tara+-centred transitions of [Co3+(1a+)2]5+ appeared at λ =
groups in the Tara units. The Co- and Tara-centred redox pro- 682 nm with a shoulder at λ = 558 nm and a higher energy
cesses in [Co(1c)2](ClO4)2 are merged with a redox separation transition at λ = 365 nm.45 These are characteristic absorptions
of ΔE°′ = 85 mV. Therefore, SWV was used for the determi- for Tara radicals.49,61 To verify these statements UV-vis-NIR
nation of the redox potentials. Cooling to −5 °C leads to a spectroelectrochemistry measurements of [Co(L)2](ClO4)2 (L =
cathodic shift of the redox potentials in [Co(1c)2](ClO4)2 1a–c) (2 mM) were carried out in anhydrous CH3CN and
(E°′1(Co3+/2+) = 210 mV and E°′2(Tara+/0) = 320 mV). The redox CH2Cl2, respectively, using an OTTLE65 cell (optically transpar-
separation with ΔE°′ = 110 mV has increased compared to the ent thin-layer electrochemistry cell, quartz windows) with
situation at 25 °C as a result of more facile formation of the [nBu4N][B(C6F5)4] (0.1 M) as the supporting electrolyte at 25 °C.
[Co3+(L)2]3+ species rather than [Co2+(L)(L+)]3+ (L = 1c) at lower
temperatures (Fig. 1-SI†).
An absorption band related to the Co3+/2+ redox process
appears at λmax = 382, 379, 380 nm for complexes [Co3+(L)2]3+
Redox processes are widely influenced by the solvents used. (L = 1a–c) in CH3CN solutions. This transition was verified
Polar solvents (εr > 15) are more capable of stabilising highly using EPR/UV-vis-NIR spectroelectrochemistry measurements
charged cations such as metal redox couples, Mn+/m+ (m ≠ 0, (vide infra) and appears at lower potentials than the Tara+-
n > m > +1).50 Neutral and singly charged cations such as related absorption bands in [Co(L)2](ClO4)2 (L = 1a,b) (Fig. 2
organic radicals are less susceptible to such effects. In conse- and 3-SI†). For [Co(1c)2](ClO4)2 a concomitant growth of the
quence we can expect a different response of potentials for the Co3+/2+ transition and Tara+ absorptions is observed
Co3+/2+ and Tara+/0 redox processes. A decrease in polarity of (Fig. 4-SI†). The Tara+ absorption of [Co3+(L+)2]5+ (L = 1a–c)
the solvent destabilises [Co3+(L)2]3+ relative to the isomer struc- appears at λmax = 682, 720, 753 nm in CH3CN as intense bands
ture [Co2+(L)(L+)]3+ owing to the higher charge of the cobalt (εmax = 25 000–50 000 L mol−1 cm−1) accompanied by a weaker
ion. Possibly an inversion of the order of redox processes can shoulder between λ = 450 and 600 nm and a higher energy
be achieved by variation of the solvent from CH3CN (εr = 35.9) transition at λ = 365, 362 and 363 nm. A bathochromic shift of
to CH2Cl2 (εr = 8.93). The CVs and SWVs of [Co(L)2](ClO4)2 (L = the absorption in the vis-region is observed with an increasing
1a–c) were recorded in CH2Cl2 under stationary measurement M-donor effect of the Tara+ substituents R1/2 within the series
conditions.
of [Co3+(L+)2]5+ (L = 1a–c). This tendency is also observed for
The CVs of [Co(L)2](ClO4)2 (L = 1a–c) in CH2Cl2 again the Tara+ transition in CH2Cl2 solutions (λmax = 698, 736,
exhibit two redox events, as discussed earlier (Fig. 2). However, 778 nm for [Co3+/2+(L+)2]5+/4+ (L = 1a–c)). The higher energy
compared to the situation in CH3CN, the order of oxidation transition of Tara+ in CH2Cl2 is shifted around 10–16 nm to
steps in CH2Cl2 is inversed. The Tara+/0 redox events occur as λmax = 349, 352, 353 nm for [Co3+/2+(L+)2]5+/4+ (L = 1a–c) com-
two overlapping independent redox waves at E°′1(Tara+/0) = pared to the situation in CH3CN. A transition which is con-
468, 364, 296 mV for [Co(L)2](ClO4)2 (L = 1a–c), which are cath- nected to the Co3+/2+ redox process was found at λmax = 369,
odically shifted up to 80 mV relative to the CH3CN solutions. 368, 366 nm for [Co3+/2+(L+)2]5+/4+ (L = 1a–c) in CH2Cl2. Its
The Tara redox process is followed by a Co3+/2+ redox event at intensity is smaller with respect to the measurements in
higher potentials. The E°′ values for the Co3+/2+ redox poten- CH3CN solutions, possibly due to the spin interaction
tials in the series of [Co(L)2](ClO4)2 (L = 1a–c) are similar (vide infra). Charge transfer bands, assignable to a direct electron
(E°′2(Co3+/2+) = 598, 578, 581 mV). The Co-centred redox process transfer between the Co and Tara redox centres in their oxi-
is shifted anodically by ≈300 mV in comparison with CH3CN dised or reduced state, cannot be observed in both solvents.
solutions. The Tara+/0 and Co3+/2+ potential shifts verify the This is mainly caused by the large distance (d ≈ 8.4 Å)45
above mentioned influence of the solvent permittivities on between the two redox centres and the consequent weak elec-
differently charged species. The smallest redox separation in tronic coupling. Similarly, indication for an ICVT charge trans-
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Fig. 3 UV-vis-NIR spectra of [Co(1c)2](ClO4)2 at rising potentials vs. Ag/
AgCl in CH2Cl2. The black dashed line is recorded after a series of oxi-
dation measurements at −0.3 V to prove the reversibility. Measurement
conditions: 2.0 mmol L−1 analyte, [nBu4N][B(C6F5)4] (0.1 M) as the sup-
porting electrolyte, 25 °C.
Fig. 4 [left] VT vis-NIR spectra of [Co(1c)2](ClO4)2 which is partly oxi-
dised to [Co(1c)2]3+ using 1 eq. of [N(p-C6H5Br)3][SbCl6] in CH3CN.
[right] Examined solution of [Co(1c)2]3+ in CH3CN at −40 °C and +50 °C.
The colour changes from yellow ([Co3+(1c)2]3+) to green ([Co2+(1c)
(1c+)]3+).
fer band assignable to charge transfer between Tara and Tara+
in [Co3+(L)(L+)]4+ redox states is not found.59 All oxidations
from [Co2+(L)2]2+ to [Co3+(L+)2]5+ (L = 1a–c) were reversible,
which was proven by recording an additional spectrum after
reduction at −0.3 V (Fig. 3).
transition reactions, having the maximum possible number of
unpaired electrons with increasing temperature.68–71 A similar
temperature dependent equilibrium between the Co2+/Tara+
and Co3+/Tara states was observed for [Co(1a)2]3+ in CH2Cl2
(Fig. 8-SI†). At ambient temperature, the Tara oxidation occurs
130 mV prior to the Co-centred oxidation; thus, addition of 1
eq. of “magic blue” to [Co2+(1a)2]2+ leads to the formation of
the blue coloured [Co2+(1a+)(1a)]3+ complex in CH2Cl2 at T =
23 °C. The absorption spectrum shows a characteristic band at
λmax = 685 nm with a weaker shoulder. Cooling to T = −90 °C
leads to the formation of the yellow coloured [Co3+(1a)2]3+
species and consequently, to a significant decrease in the
absorption band at λmax = 685 nm.
VT vis-NIR spectroscopy
The CVs of [Co(1c)2](ClO4)2 in CH3CN indicated a difference
between the Co- and Tara-centred redox potentials of only
ΔE°′ = 85 mV. However, electronic communication between Co
and Tara is too weak to be detected in the UV-vis-NIR spectro-
electrochemical measurements for the in situ generated
[Co(1c)2]3+ species. Previous VT 1H NMR (VT = variable tem-
perature) spectroscopic measurements on partly oxidised
[Co(1a)2](ClO4)2 revealed an accelerated self-exchange of the
Co3+/2+ redox couple in comparison with Co3+/2+ complexes
without a redox mediator.45 This self-exchange can be frozen
EPR/UV-vis-NIR spectroelectrochemistry
out at low temperatures. Such behaviour was also found for For verification of the above statements in the oxidation order
partly oxidised [Co(1c)2](ClO4)2 (vide supra). Apart from the of complexes [Co(L)2](ClO4)2 (L = 1a,c), coupled EPR/UV-vis-
kinetic effect, redox equilibria involving the Co3+/2+ redox NIR (electron paramagnetic resonance/ultraviolet-visible-near
couple are often strongly temperature dependent. This thermo- infrared) spectroelectrochemistry experiments were conducted.
dynamic effect is a consequence of the large entropy difference Tara radical and Tara biradical cations with non-interacting
between HS-Co2+ and LS-Co3+ (≈100 J mol−1
K
−1).41,66,67 spins exhibit characteristic temperature-dependent EPR line-
A temperature dependent shift of the equilibrium between the shapes.59 The HS-Co2+ ion is EPR-active but as a result of its fast
[Co2+(L+)(L)]3+ and [Co3+(L)2]3+ states must exhibit character- electron relaxation (S = 3/2), only broad lines are observed at
istic changes of the absorption in the vis-NIR region. T > 20 K.72,73 In contrast, the LS-Co3+ ion is EPR silent due to its
Therefore, [Co(1c)2](ClO4)2 was partly oxidised to [Co(1c)2]3+ diamagnetic behaviour. Thus it is expected that valid EPR
using 1 eq. of [N(p-C6H5Br)3][SbCl6] (“magic blue”) in CH3CN. spectra are limited to in situ generated radicals [Co2+(L)(L+)]3+
From this solution vis-NIR spectra were recorded in the temp- and [Co3+(L)(L+)]4+, and biradical complexes [Co3+(L+)2]5+ and
erature range of −40 °C and +50 °C. The reduced oxidant, tris [Co2+(L+)2]4+ (L = 1a,c). Increased electron relaxation caused by
(4-bromophenyl)amine shows no vis-NIR absorption.59 The dipolar magnetic coupling between the Co2+ magnetic moment
spectra are depicted in Fig. 4 and show the obvious thermo- and the Tara+ radical further restricts the EPR observable
chromic behaviour of the solutions. The yellow colour and the species to the Co3+ state, namely the [Co3+(L)(L+)]4+ and
absence of any absorptions in the range of 500–900 nm indi- [Co3+(L+)2]5+ species. The measurements were carried out with
cate the preference of the [Co3+(1c)2]3+ species at −40 °C. With an X-band CW spectrometer (microwave power of 5 mW,
increasing temperature the colour changes from yellow 100 kHz modulation) at 25 °C. The analyte dissolved in an-
(−40 °C) to green (+50 °C). An absorption band at λmax
745 nm arises, which is characteristic of Tara+ radicals (vide supporting electrolyte was placed in an optical EPR cavity.
supra) and is associated with the formation of the [Co2+(1c+) In situ oxidation of [Co2+(1a)2]2+ to [Co3+(1a)2]3+ in CH3CN
(1c)]3+ species. This is also in line with entropy-driven spin leads to an increase of an absorption band at λ = 378 nm
=
hydrous CH3CN or CH2Cl2 with 0.1 M [nBu4N][B(C6F5)4] as the
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(Fig. 9-SI†). Thus this band is a marker for the Co3+/2+-related
In CH2Cl2 solutions, the Co- and Tara-centred redox pro-
redox event; at the same time, EPR spectra remain featureless. cesses of [Co(1a)2](ClO4)2 are merged (vide supra), which is
Upon further increase of the oxidation potential an EPR signal reflected by the plot of the potential dependence of the absorp-
appears with a simultaneously rising absorption band at λ = tion of characteristic UV bands and the EPR detected spin
682 nm. This feature indicates the in situ formation of the density obtained from combined EPR/UV-vis-NIR spectroelec-
[Co3+(1a)(1a+)]4+ and/or [Co3+(1a+)2]5+ species. The g value trochemical experiments (Fig. 10-SI†). At E = 0.5 V complex
amounts to 2.0031, close to the value of a free electron, which [Co(1a)2]2+ is partly oxidised, and only a weak and broad EPR
is characteristic of triphenyl amine radicals.59,74
signal is detected. A quantitative model for the complicated
The CV in CH3CN for [Co(1c)2](ClO4)2 exhibits two overlap- equilibria between the different redox states involved in the
ping redox processes; this is also reflected by simultaneously successive oxidation cannot be derived. However, UV-vis
rising absorption bands in the UV-vis-NIR spectra for the Co- markers indicating the presence of Tara+ radicals increase
and Tara-centred oxidation (λmax(Co3+)
λmax(Tara+)
[Co(1c)2]3+ is found in
=
380 nm and slightly before the EPR detected spin density. This finding is
599/748 nm). Hence, in CH3CN solution consistent with a dominant first oxidation of [Co2+(1a)2]2+ to
genuine equilibrium between the Tara+ radical [Co2+(1a)(1a+)]3+. The latter shows no EPR
=
a
[Co3+(1c)2]3+ and [Co2+(1c)(1c+)]3+. With the in situ formation of response (under the experimental conditions used) due to
[Co3+(1c+)2]5+ an EPR signal with a five line pattern appears dipolar coupling to the HS-Co2+. A simultaneous formation of
with a g value of 2.0032. In contrast to [Co(1a)2](ClO4)2, for the the EPR silent [Co3+(1a)2]3+ with a weak marker band for Co3+
methoxy-substituted complex [Co(1c)2](ClO4)2 the potential in CH2Cl2 cannot be ruled out. The first oxidation step is
dependence of the EPR intensity does not match the potential immediately followed by the oxidation to the EPR active Tara+
dependence of the Tara+ absorption band intensity (Fig. 11-SI† radical [Co3+(1a)(1a+)]4+ and/or biradical [Co3+(1a+)2]5+. The
bottom left). The EPR signal appears at higher potentials than EPR signal with significant higher intensity appears at E = 0.85
the Tara+ absorptions. It increases by further oxidation, while V with g = 2.0037.
the absorptions reach a plateau. This can be explained by elec-
The case of [Co(1c)2]3+ in CH2Cl2 solution is the antipode to
tron spin exchange between HS-Co2+ and Tara+ due to overlap- [Co(1a)2]3+ in CH3CN. The CV measurements revealed a Tara-
ping redox processes (vide infra). To obtain a better resolved centred oxidation occurring 285 mV prior to the Co-centred
line shape the EPR measurement for [Co3+(1c+)2]5+ was oxidation. During the first oxidation step [Co2+(1c)(1c+)]3+ and
repeated at T = 100 K (Fig. 12-SI†). The g factor is anisotropic [Co2+(1c+)2]4+ complexes are formed. Both complexes contain
with g1 = 2.0155, g2 = 2.0029 and g3 = 1.9898. Additionally, an Tara+ units but only a broad and small 3-line EPR signal is
EPR signal could not be detected at half-field, indicating no detected in CH2Cl2 at E = 0.5 V (Fig. 5). Dipolar coupling to the
interaction between the two Tara•+ spin centres (d ≈ 14 Å).75,76
HS-Co2+ ion and collisions in solution between paramagnetic
Fig. 5 [top left] In situ absorption spectra of [Co(1c)2](ClO4)2 in CH2Cl2. [bottom left] Potential dependence of the absorption bands and EPR inten-
sity. [right] EPR spectra recorded at E = 0.5 V ([Co2+(1c+)2]4+) and E = 0.7 V ([Co3+(1c+)2]5+) at T = 298 K.
Dalton Trans.
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metal ions and organic radicals cause broadening of the solvent
purification
system
SPS-800
by
MBraun.
organic radical EPR signal.73,77,78 The interaction leads to an Tetrahydrofuran was purified by distillation from sodium/
electron–spin exchange and “transfer” of the short relaxation benzophenone ketyl. 2-(6-Bromopyridin-2-yl)-1,10-phenanthroline,
times of the HS-Co2+ ions to the Tara+ spins. This behaviour is 1a and [Co(1a)2][(ClO4)2] were synthesised as previously
immediately quenched by the oxidation of the paramagnetic reported in the literature.45 The synthesis of employed triaryl
Co2+ ion to diamagnetic Co3+ (E = 0.7 V) resulting in an intense amines is described in the ESI.† All other chemicals were
EPR signal for the [Co3+(1c+)2]5+ species (g = 2.0033) with a purchased from commercial suppliers and were used without
hyperfine splitting (triplet). The intensity of the EPR signal further purification.
drops immediately when [Co3+(1c+)2]5+ is reduced to
NMR spectra were recorded with a Bruker Avance III 500
[Co2+(1c+)2]4+ in the reverse scan. The dipolar coupling spectrometer; chemical shifts for 1H and 13C NMR are refer-
between Tara+ and HS-Co2+ was also observed for [Co(1a)2] enced internally to the residual protons and to the 13C NMR
(ClO4)2 in CH2Cl2 and [Co(1c)2](ClO4)2 in CH3CN, but in less signal of the deuterated solvent. Elemental analyses were per-
extent.
formed using a Thermo FlashAE 1112 analyser. Mass spectra
were recorded with a Bruker micrOTOF-QIIa mass spectro-
meter operating in the ESI mode (ESI
ionisation).
= electrospray
Conclusions
Synthetic procedure
Herein, a series of [Co2+(L)2]2+ (L = 1a–c) complexes is pre-
sented. The complexes have three redox-active sites that can be
4-((6-(1,10-Phenanthrolin-2-yl)pyridin-2-yl)oxy)-N-(4-methoxy-
oxidised to form the tri-, tetra- and penta-cationic analogues. phenyl)-N-(p-tolyl)aniline (1b). 410 mg (1.34 mmol) of
Particularly in solution, the tri- and tetra-cationic complexes 4-((4-methoxyphenyl)(p-tolyl)amino)phenol and 126 mg
[Co(L)2]3+ and [Co(L)2]4+ (L = 1a–c) exist in an equilibrium state (1.12 mmol) of potassium tert-butoxide were dissolved in 3 mL
of different redox isomers. Depending on the solvent and the of anhydrous THF and heated to 60 °C for 2 h. After removing
temperature either the HS-Co2+ ion is oxidised to the LS-Co3+ all volatile materials under vacuum, 264 mg (0.78 mmol) of 2-
ion (HS = high-spin, LS = low-spin) or the Tara substituents are (6-bromopyridin-2-yl)-1,10-phenanthroline were added in a
oxidised to the Tara+ radical. This equilibrium, e.g. between single portion. The respective reaction mixture was heated to
[Co2+(L)(L+)]3+ and [Co3+(L)2]3+, is strongly temperature and 200 °C overnight. The molten material was cooled down to
solvent dependent. In CH3CN, the Co2+ oxidation occurs at ambient temperature and was then dissolved in 20 mL of
lower potentials than the Tara oxidation. The oxidation order CH2Cl2 and extracted twice with an aqueous NaHCO3 solution.
is reversed in CH2Cl2, due to different solvation energies. The The organic phase was dried over Na2SO4 and all volatiles were
equilibrium involves two species with different spin multiplici- removed under vacuum. The crude product was purified via
ties (e.g. S = 0 and S = 2) representing a CTIST equilibrium column chromatography over ALOX (column size: 2.5 × 20 cm)
(CTIST = charge transfer induced spin transition). Together with a mixture of n-hexane/CH2Cl2 (1 : 1; v/v) and 3% NEt3 as the
with the different absorptions of the Tara and Tara+ radicals eluent. Yield: 110 mg, yellow solid (25% based on 2-(6-bromo-
thermochromic and solvatochromic behaviours are observed pyridin-2-yl)-1,10-phenanthroline). EA calcd for C37H28N4O2
for [Co(L)2]3+ (L = 1a–c). Coupled EPR/spectroelectrochemical (%): C 79.27, H 5.03, N 9.99; found: C 79.21, H 5.07, N 10.32.
measurements proved the existence of this equilibrium. 1H NMR (500 MHz, CDCl3, 298 K): 9.24 (dd, J = 4.3, 1.7 Hz, 1H,
Moreover, characteristic absorption bands in the UV-vis-NIR as H1), 8.74 (dd, J = 7.5, 0.5 Hz, 1H, H8), 8.62 (d, J = 8.4 Hz, 1H,
well as the pattern in the EPR spectra served as indicators for H6), 8.31 (d, J = 8.4 Hz, 1H, H7), 8.27 (dd, J = 8.0, 1.7 Hz, 1H,
the equilibrium position. VT 1H NMR spectroscopy and VT vis- H3), 7.89 (t, J = 7.8 Hz, 1H, H9), 7.83 (d, J = 8.8 Hz, 1H, H5),
NIR spectroscopy further verified these findings. The solvato- 7.79 (d, J = 8.8 Hz, 1H, H4), 7.65 (dd, J = 8.0, 4.3 Hz, 1H, H2),
chromic behaviour was ascribed to the different solvation ener- 7.14–7.06 (m, 8H, H11, H12, anisyl-H13, tolyl-H13), 7.01 (d, J =
gies of the Co3+/2+ and Tara+/0 redox couples. The thermochro- 8.5 Hz, 2H, tolyl-H14), 6.93 (dd, J = 8.1, 0.6 Hz, 1H, H10), 6.86
mic behaviour is the consequence of the strongly different (d, J = 9.0 Hz, 2H, anisyl-H14), 3.81 (s, 3H, OMe-H15), 2.31 (s,
entropies of the redox isomers. In summary, the complexes 3H, Me-H15) ppm. 13C NMR (125.80 MHz, CDCl3, 298 K):
[Co(L)2]3+/4+ represent interesting building blocks for the 163.56, 155.98, 155.86, 154.38, 150.58, 148.78, 146.56, 145.97,
construction of molecular switches.
145.81, 145.29, 141.34, 140.50, 137.00, 136.32, 131.83, 129.95,
129.19, 128.98, 126.89, 126.71, 126.69, 123.84, 123.46, 123.06,
122.10, 121.13, 117.31, 114.89, 111.48, 55.65, 20.88 ppm.
MS-ESI: 560.2207 ([M], calcd 560.2212).
4-((6-(1,10-Phenanthrolin-2-yl)pyridin-2-yl)oxy)-N,N-bis(4-
methoxyphenyl)aniline (1c). 750 mg (2.3 mmol) of 4-(bis(4-
Experimental
General remarks
All reactions handling sensitive chemicals were carried out methoxyphenyl)amino)phenol79 and 215 mg (1.9 mmol) of
under an atmosphere of argon using standard Schlenk and potassium tert-butoxide were dissolved in 5 mL of anhydrous
cannula techniques. Anhydrous ethanol was purchased com- THF and heated to 60 °C for 2 h. After removing all volatiles
mercially from Acros Organics. Toluene was obtained from a under vacuum, 575 mg (1.7 mmol) of 2-(6-bromopyridin-2-yl)-
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1,10-phenanthroline were added in a single portion. The reac-
[Co(1c)2](ClO4)2. According to the general procedure, 200 mg
tion mixture was heated to 200 °C overnight. The molten (0.35 mmol) of 1c and 64 mg (0.175 mmol) of Co(ClO4)2·6H2O
material was cooled down to ambient temperature and dis- were treated. After appropriate work-up, 195 mg of the title
solved in 40 mL of CH2Cl2 and extracted twice with an complex could be isolated as a yellow solid (79% based on 1c).
aqueous NaHCO3 solution. The organic phase was dried over EA calcd for C74H56Cl2CoN8O14·12CH3CN·H2O (%): C 61.38, H
1
Na2SO4 and all volatiles were removed under vacuum. The 4.22, N 8.11; found: C 61.55, 3.89, N 8.13. H NMR (500 MHz,
crude product was purified via column chromatography over CD3CN, 298 K): 171.41 (s, LW = 312 Hz, T1 = 4 ms, 2H, H1),
ALOX (column size: 2.5 × 20 cm) with a mixture of n-hexane/ 114.13 (s, LW = 54 Hz, T1 = 11 ms, 2H, H7), 88.37 (s, LW = 39
CH2Cl2 (1 : 1; v/v) and 3% NEt3 as the eluent. Yield: 549 mg, Hz, T1 = 13 ms, 2H, H8), 45.24 (s, LW = 16 Hz, T1 = 37 ms, 2H,
yellow solid (56% based on 2-(6-bromopyridin-2-yl)-1,10-phe- H10), 43.95 (s, LW = 18 Hz, T1 = 31 ms, 2H, H2), 36.12 (s, LW =
nanthroline). EA calcd for C37H28N4O3·12H2O (%): C 75.98, H 14 Hz, T1 = 53 ms, 2H, H5), 29.41 (s, LW = 16 Hz, T1 = 32 ms,
4.85, N 9.59; found: C 75.88, H 4.99, N 9.57. 1H NMR 2H, H6), 23.36 (s, LW = 11 Hz, T1 = 80 ms, 2H, H4), 10.59 (s, LW
(500 MHz, CDCl3, 298 K): 9.25 (dd, J = 4.3, 1.7 Hz, 1H, H1), = 12 Hz, T1 = 53 ms, 2H, H9), 6.71 (d, J = 8.6 Hz, T1 = 841 ms,
8.73 (dd, J = 7.5, 0.7 Hz, 1H, H8), 8.63 (d, J = 8.4 Hz, 1H, H6), 8H, H14), 6.37 (d, J = 8.7 Hz, T1 = 380 ms, 8H, H13), 3.63 (s, LW
8.31 (d, J = 8.4 Hz, 1H, H7), 8.27 (dd, J = 8.0, 1.7 Hz, 1H, H3), = 5 Hz, T1 = 1.1 s, 12H, H15), 3.01 (s, LW = 15 Hz, T1 = 132 ms,
7.88 (t, J = 7.8 Hz, 1H, H9), 7.83 (d, J = 8.8 Hz, 1H, H5), 7.80 (d, 4H, H12), −1.03 (s, LW = 11 Hz, T1 = 74 ms, 2H, H3), −10.06 (s,
J = 8.8 Hz, 1H, H4), 7.65 (dd, J = 8.0, 4.3 Hz, 1H, H2), 7.12–7.07 LW = 94 Hz, T1 = 17 ms, 4H, H11) ppm. 1H NMR (500 MHz,
(m, 6H, H12, H13), 7.04 (d, J = 9.0 Hz, 2H, H11), 6.92 (dd, J = (CD3)2CO, 193 K): 295.89 (s, LW = 4720 Hz, 2H), 203.12 (s, LW =
8.1, 0.7 Hz, 1H, H10), 6.85 (d, J = 9.0 Hz, 4H, H14), 3.80 (s, 6H, 449 Hz, 2H), 154.64 (s, LW = 360 Hz, 2H), 61.24 (s, LW =
H15) ppm. 13C NMR (125.80 MHz, CDCl3, 298 K): 163.50, 174 Hz, 2H), 58.93 (s, LW = 177 Hz, 2H), 56.93 (s, LW = 117 Hz,
155.74, 155.59, 154.25, 150.48, 148.16, 146.45, 145.69, 145.54, 2H), 56.33 (s, LW = 178 Hz, 2H), 28.82 (s, LW = 73 Hz, 2H),
141.45, 140.33, 136.86, 136.16, 129.05, 128.84, 126.76, 126.57, 14.21 (s, LW = 111 Hz, 2H), 6.22 (s, LW = 21 Hz, 8H), 5.50 (s,
125.94, 122.91, 122.62, 121.92, 120.99, 117.15, 114.72, 111.26, LW = 33 Hz, 8H), 3.18 (s, LW = 11 Hz, 12H), −5.36 (s, LW = 396
55.53 ppm. MS-ESI: 576.2156 ([M], calcd 576.2161); 577.2234 Hz, 4H), −14.75 (s, LW = 81 Hz, 2H), −16.56 (s, LW = 600 Hz,
([M + H]+, calcd 577.2240); 599.2054 ([M + Na]+, calcd 4H) ppm. MS-ESI: 1310.3134 ([M − ClO4]+, calcd 1310.3140).
599.2059); 615.1793 ([M + K]+, calcd 615.1798).
General syntheses of complexes [Co(1b,c)2](ClO4)2. 2 eq. of
the appropriate ligand and 1 eq. of cobalt perchlorate hexa-
hydrate were suspended in anhydrous ethanol and were stirred at
Conflicts of interest
ambient temperature overnight. The reaction mixture was centri- There are no conflicts to declare.
fuged and the precipitate was washed with 2 mL of ethanol and
5 mL of diethyl ether consecutively. The crude product was dis-
solved in 2 mL of acetonitrile and filtered over Celite. The filtrate
was mixed with diethyl ether until a precipitate was formed and
Acknowledgements
centrifuged. The remaining solid was washed with 5 mL of This work was financially supported by the Deutsche
diethyl ether twice and dried under vacuum.
Forschungsgemeinschaft DFG (PE1513/3-3).
[Co(1b)2](ClO4)2. According to the general procedure,
131 mg (0.23 mmol) of 1b and 43 mg (0.12 mmol) of
Co(ClO4)2·6H2O were treated. After appropriate work-up, 138 mg
of the title complex could be isolated as a yellow solid (83%
based on 1b). EA calcd for C74H56Cl2CoN8O12·12CH3CN (%): C
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