Synthesis, X-ray structure and reactivity of a sterically protected azobisphenol ligand: On the quest for new multifunctional active ligands

Article abstract of DOI:10.1002/ejic.200701339

Different synthetic routes have been explored for the synthesis of the sterically protected 2,2′-dihydroxy-4,3,4,3′-tetra-tert- butylazobenzene ligand (2), which is an excellent candidate for the development of valence tautomeric complexes. As expected, s

Full text of DOI:10.1002/ejic.200701339

FULL PAPER
DOI: 10.1002/ejic.200701339
Synthesis, X-ray Structure and Reactivity of a Sterically Protected
Azobisphenol Ligand: On the Quest for New Multifunctional Active Ligands
Emi Evangelio,[a] Javier Saiz-Poseu,[b] Daniel Maspoch,[b] Klaus Wurst,[c] Felix Busque,*[d]
and Daniel Ruiz-Molina*[a,e]
Keywords: Azobisphenol ligand / Acid–base behaviour / Multifunctional ligands / Valence tautomerism / Chromophores
Different synthetic routes have been explored for the synthe-
sis of the sterically protected 2,2Ј-dihydroxy-4,3,4,3Ј-tetra-
tert-butylazobenzene ligand (2), which is an excellent candi-
date for the development of valence tautomeric complexes.
As expected, such a ligand exhibits good reactivity with tran-
sition-metal ions, as shown by the synthesis and characteri-
zation of the new cobalt complex [Co(22–)(H2O)3]Cl·
2.5EtOH·H2O (8). This fact together with the reversible de-
protonation/protonation of the phenol groups has been used
to create a chromophoric array of three states with signifi-
cantly different colours, which can interconvert reversibly
between them.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
must emphasize that even though the number of redox-
active ligands is considerable, those exhibiting valence taut-
omerism are rather limited since they must simultaneously
satisfy two conditions: (i) the degree of covalency in the
interaction between the metal ion and electroactive ligand
must be low, and (ii) the energy of their frontier orbitals
must be similar.[8]
Introduction
There is currently active interest in the development of
molecular electronic devices that can be used as optical and/
or magnetic data storage media.[1] Compounds of specific
interest are bistable molecular materials having two nearly
degenerated states with different optical and/or magnetic
properties.[2] These complexes have an appreciable sensitiv-
ity to the environment so an external perturbation, like
photons or temperature, may lead to an interconversion be-
tween the two degenerated electronic states. Examples of
electronic labile complexes are valence tautomeric com-
plexes.[3] Valence tautomeric metal complexes with at least
two redox-active centres, the metal ion and an electroactive
ligand, are characterized by the existence of two electronic
isomers (valence tautomers).[4] The interconversion between
the two electronic isomers is accomplished by a reversible
intramolecular electron transfer involving the metal ion and
the redox-active ligand. Interestingly, each isomer exhibits
different charge distributions, and consequently, different
optical, electric and magnetic properties.[5]
Most of the complexes that so far have shown to exhibit
valence tautomerism are transition-metal complexes of qui-
none or quinone-based ligands.[9] Nevertheless, the number
of electroactive ligands inducing valence tautomerism is ex-
panding with the inclusion of new electroactive ligands such
as tetraphenylporphyrin,[10] polychlorotriphenylmethyl[11]
and phenoxyl radicals.[12] Among them, phenoxyl radicals
are monovalent oxygen radical species that exhibit delocal-
ization of the unpaired electron over the aromatic ring with
ortho and para substituents that give steric protection.[13]
Several groups have prepared metal–phenoxylate complexes,
which give new insights into the chemical factors that gov-
ern the generation and stability of this type of radical.[14] It
is worth mentioning the work developed by Wieghardt et
al; they established that bidentate O,N-coordinated o-ami-
nophenolato ligands can be found in different protonation
and oxidation levels: o-imidophenolate(2–) anions, o-imino-
benzosemiquinonate(1–) radical monoanions or even o-imi-
nobenzoquinone. All these forms can exist in coordination
compounds, as confirmed by low-temperature X-ray crys-
tallography.[15] A further step has been the study and char-
acterization of O,N,O-coordinated ligands containing two
phenolate donor groups, such as 2-[(3,5-bis(1,1-dimethyl-
ethyl)-6-hydroxyl-2,4-cyclohexadien-1-ylidene)amino]-4,6-
bis(1,1-dimethylethyl)phenol (1).[16] These ligands, in ad-
dition to producing phenoxyl radicals in the presence of air,
exhibit better chelating capabilities and good π-donor
Ligands that display different oxidation states when co-
ordinated to metallic centres have been broadly studied by
both theoretical[6] and experimental means.[7] However, we
[a] Institut de Ciència dels Materials de Barcelona (CSIC),
Esfera UAB, 08193 Bellaterra, Catalonia, Spain
[b] Institut Català de Nanotecnologia,
Campus UAB, 08193 Bellaterra, Catalonia, Spain
[c] Institut für Allgemeine Anorganische und Theoretische
Chemie, Universität Innsbruck,
Innrain 52a, Innsbruck, Austria
[d] Chemistry Department, Universitat Autònoma de Barcelona,
Campus UAB, 08193 Bellaterra, Catalonia, Spain
[e] Centre de Nanociència i Nanotecnologia,
Campus UAB, 08193 Bellaterra, Catalonia, Spain
Fax: +34-935814777
E-mail: dani@icmab.es
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Sterically Protected Azobisphenol Ligand
atoms that stabilize higher oxidation states, a fact that has material and aniline 4 were recovered. Similarly, oxidative
allowed the observation of valence tautomerism in some of coupling of aniline 4 with cetyltrimethylammonium dichro-
their complexes.[17]
mate[27] failed, and degradation products were obtained.
The advantages of the valence tautomeric complexes ob- The tert-butylation reaction[28] of 2,2Ј-dihydroxy-azoben-
tained from the Schiff base ligand 1 over transition-metal zene was also tested, though no traces of the desired prod-
complexes with o-quinone ligands are considerable. First, uct were obtained.
valence tautomeric Schiff base complexes display higher
stabilities in atmospheric oxygen in solution and in the solid
state.[18] Second, the differences between the optical proper-
ties of the isomers involved in the valence tautomerism of
the cobalt Schiff base complex are enhanced relative to
those observed for cobalt complexes with o-quinone li-
gands. Third, the Schiff base ligand exhibits a richer electro-
chemical behaviour since it can exist in different oxidation
forms, ranging from +1 to –3, which may lead to stable
Scheme 1. Molecular structures of 1 and 2.
coordination complexes with several metal ions in a variety
of oxidation states.[19]
Fortunately, an alternative synthetic route, which in fact
is the most extended synthetic strategy for the synthesis of
this type of compounds,[25] was most successful (see
Scheme 2). Initially, nitrophenol 3 was obtained in a 73%
yield by nitration of phenol 5 following a modified experi-
mental route previously described.[29] The reaction was per-
formed at about –20 °C to avoid the side dinitration reac-
tion. Compound 3 was then reduced by using Pd/C in meth-
anol[30] to afford aniline 4 in an 84% yield. Finally, the
coupling reaction[25] of the diazonium salt 6, derived from
aniline 4 with phenol 5, afforded the diazo derivative 2 in
21% yield. This last reaction was performed under slightly
basic or neutral conditions to avoid conversion of the dia-
zonium salt to the corresponding diazohydroxide com-
pound.
Because of the interest in this family of ligands, we fo-
cused our attention on obtaining new bisphenol ligands
similar to 1, particularly bearing additional functional
groups that can add a multifunctional character to the val-
ence tautomeric behaviour. For this, 2,2Ј-dihydroxy-
4,3,4,3Ј-tetra-tert-butylazobenzene (2) would be an excel-
lent candidate. While keeping the same coordination capa-
bility and steric protection of the radicals through the pres-
ence of tert-butyl groups, the incorporation of the azo
group ensures a multifunctional character thanks to their
wide spread application as dyes, acid–base indicators and
histological stain applications, among others.[20] Moreover,
the possibility to use azo compounds as electron–acceptor
or radical ligands in transition-metal species has already
been reported.[21] However, in spite of its interest, reports
on 2 have so far been scarce – there is a report on a CuII
complex obtained as a side product[22] and on a cumber-
some and harsh low-yield synthesis.[23]
Herein we report on the exploration of several new syn-
thetic routes for the synthesis of azobisphenol 2, and in-
clude the one-pot synthesis of a related CoIII complex. In
addition, both its acid–base character based on the pres-
ence of the phenol groups and its reactivity with transition-
metal ions has been used to develop a chromophoric sensor.
This fact adds value to the synthesis of 2 since the colour
change of chromogenic sensors provides an informative and
important signal for its use in analyte-sensing, environmen-
tal and biomedical applications.[24]
Results and Discussion
Scheme 2. Route for the synthesis of 2.
Synthesis of 2
In the present work, four different synthetic approaches
for the synthesis of azobisphenol 2 were initially envis-
aged.[25–28] Our first synthetic trials toward the synthesis
Crystal Structure of 2
The molecular and crystal structure of 2 was investigated
of 2 (Scheme 1) were done following three different by X-ray single crystal analysis. The crystallographic data
routes,[26–28] without any success. When nitro derivative 3 and experimental parameters are summarized in Table 4.
was exposed to reductive coupling conditions, either with Selected bond lengths and angles are also given in Table 1.
[26a]
LiAlH4
or with the Pb0/HCOONH4 system,[26b] no de- Molecule 2 crystallizes in the C2/c monoclinic space group,
sired diazo product 2 was obtained, and only the starting and the cell parameters are reported in Table 4. An ORTEP
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FULL PAPER
view of 2 is shown in Figure 1. The high molecular sym- molecule (Figure 2a), which most likely takes place through
metry is reflected by the presence of an inversion centre, the formation of the monoanionic form. Finally, it is impor-
which leads to a trans geometry of the hydrazone group tant to emphasize that this process is fully reversible upon
with respect to both quinone moieties. This configuration addition of acid (HCl), and therefore the absorption spec-
seems to be favoured by two intramolecular hydrogen trum of the neutral form 2 and the yellow colour of the
bonds between O1–H···N1 with distances of 1.695 Å and solution can be recovered. In any case, protonation of the
angles of 148.6°.
azo group was not detected after addition of controlled
amounts of acid.
Table 1. Selected bond lengths [Å] and angles [°] for 2.
Bond lengths
N1–N1Ј
N1–C2
C1–C2
C2–C3
C3–C4
1.280(2)
1.406(2)
1.406(2)
1.401(2)
1.373(2)
O1–C1
1.357(1)
C4–C5
C5–C6
C6–C1
1.406(2)
1.390(2)
1.498(2)
Angles
C2–N1–N1Ј
O1–C1–C2
C1–C2–N1
117.1(1)
120.7(1)
125.1(1)
Figure 1. ORTEP drawing of the azobisphenol 2 at the 30% prob-
ability level. Hydrogen atoms have been omitted for clarity.
Acid–Base Equilibria of 2
The acid–base activity of azobisphenol 2 was investigated
by means of UV/Vis absorption spectroscopy. An ethanol
solution of 2 displays two intense broad absorption bands
at 347 nm, most likely assigned to the n–π* transition of
the azo (N=N) group, and at 453 nm associated with the
presence of the hydroxyl groups. Both bands are expected
to be sensitive and to change upon acid/base addition, espe-
cially in the case of the second band.[31]
Figure 2 displays the changes measured in the absorption
spectrum of an ethanol solution of 2 (C0 = 5.7ϫ10–5 )
upon addition of base [80 µL of tetrabutylammonium hy-
droxide (tba) C0 = 4.3ϫ10–3 ]. As the pH increases, the
intensity of the band at 347 nm associated with the n–π*
transition decreases . However, a new band at 520 nm ap-
pears, and the intensity of the initial peak at 453 nm, associ-
ated with the hydroxyl groups, decreases until the band al-
most disappears. Deprotonation of the phenol groups under
basic conditions leads to a charge density shift and conse-
quently a bathochromic shift of the new band, which is ac-
companied by a colour change from yellow to violet. The
clean isosbestic point at 493 nm also indicates an equilib-
Figure 2. (a) Schematic representation of the acid–base equilibrium
of 2. (b) UV/Vis spectra of 2 (0.057 m) in ethanol as the pH in-
creases. (c) UV/Vis spectra of 2 (0.057 m) in thf as the pH in-
creases. In (b) and (c) each line corresponds to the addition of
40 µL of tba (0.4 m) until total deprotonation of 2 occurs. The
rium between the acid (2) and the basic (22–) form of this process is fully reversible upon addition of HCl.
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Sterically Protected Azobisphenol Ligand
The acid–base activity of 2 was also investigated in thf
solution (Figure 2c). In this solvent, the spectrum of 2 also
displays the two intense broad absorption bands at 347 nm
and 453 nm, with no significant solvatochromic shift. Upon
addition of base, deprotonation of the phenol groups takes
place, as described previously for the ethanol solution,
which leads to the appearance of a new band at 565 nm,
and the intensity of the initial peak at 453 nm decreases.
However, two main differences can be noted: (i) a solva-
tochromic effect is detected in the absorbance spectra for
the 22– species (the spectral parameters for the species 2 and
22– are summarized in Table 2), which could be tentatively
ascribed to changes in solvent polarity as well as in the
solvent hydrogen-bond-accepting capacity,[32] and (ii) the
intensity of the band at 347 nm decreases more consider-
ably.
Table 2. UV/Vis spectroscopic data for the species 2 and 22– in etha-
nol and thf solutions.
State
Solvent
EtOH
λ [nm]
ε [–1 cm–1]
2
347, 453
347, 520
347,453
347, 565
13899, 9322
7834, 11624
15789, 10488
5623, 14348
22–
2
thf
Figure 3. (a) Schematic representation of the complexation equilib-
rium of 2. (b) UV/Vis spectra of 2 (0.057 m) in the presence of
Co(CH3COO)2·4H2O (1 m), added at regular intervals of 24 µL.
22–
To shed more light into the reactivity of ligand 2 and the
nature of the complex formed upon reaction with a cobalt
Complexation Ability of 2
The complexation ability of 2 with transition-metal ions salt, we considered the straight chemical synthesis of a tran-
such as CoII was initially studied in solution (Figure 3a). sition-metal complex bearing ligand 2. For this, we were
First, the deprotonated form 22– was obtained after an eth- inspired by the methodology described by Pierpont[17a] and
anol solution of 2 (3 mL, C0 = 1ϫ10–5 ) was made basic Dei et al.[17b] for the synthesis of transition-metal complexes
with tba (5 µL, C0 = 5ϫ10–2 ), as confirmed by UV/Vis bearing ligand 1. These authors showed that 3d metal ions
spectroscopy. An aqueous solution of CoCl2·6H2O (25 µL, react with 3,5-di-tert-butylcatechol (3,5-DTBCatOH) and
C0 = 9ϫ10–4 ) was then added. Immediately, the intensity aqueous ammonia in the presence of air to yield complexes
of the band at 520 nm, characteristic of the 22– form, of the Schiff base bisquinone ligand. Moreover, for the case
started to decrease and that of the new band at 560 nm of the copper metal ion, excess ammonia induces a redox
with two shoulders at 520 and 484 nm, associated with the cascade reaction coupled with air oxidation, which is re-
formation of the corresponding cobalt complex 2·Co, in- sponsible for the formation of a related azophenolate com-
creases. The appearance of the new band is accompanied plex.[22] By taking these precedents into account, and only
by a colour change from violet to red–purple. The complex- after several different reaction trials did the reaction of 3,5-
ation ability of ligand 2 was also tested by direct reaction DTBCatOH (6 equiv.), ammonia (excess) under aerobic
of 2 with an ethanol solution of cobalt acetate, whose basic conditions and a mixture EtOH/H2O (50:50) and CoCl2·
character is expected to deprotonate the azobisphenol li- 6H2O (1 equiv.) yield complex [Co(22–)(H2O)3]Cl·2.5EtOH·
gand. The UV/Vis spectra of an ethanol solution of 2 upon H2O (8), as characterized by X-ray diffraction and chemical
addition of cobalt acetate are shown in Figure 3b. Addition analysis (vide infra). Most likely, the reaction mechanism
of a solution of Co(CH3COO)2·4H2O (240 µL, C0
=
takes place through the formation of complex [Co(Cat-N-
1ϫ10–3 ) to the ethanol solution of 2 (C0 = 5.5ϫ10–5 ) SQ)(Cat-N-BQ)], where (Cat-N-BQ)– and (Cat-N-SQ)2– re-
also induces a shift in the band at 453 nm, which is charac- fer to the monoanionic and dianionic forms of ligand 1,
teristic of 2, to 560 nm. The shoulders at 520 and 484 nm respectively. Indeed, upon addition of excess concentrated
arise as a result of the formation of the complex 2·Co. As aqueous ammonia to a solution of 3,5-DTBCatOH and
expected, the acetate counterion of the metal salt is suffi- CoCl2·6H2O, the solution turned dark violet, characteristic
ciently acidic to deprotonate the azo ligand, which leads to of the formation of [Co(Cat-N-SQ)(Cat-N-BQ)]. However,
the formation of complex 2·Co. Moreover, the process is after 10 min whilst stirring, the solution gradually became
fully reversible on addition of acid (HCl) and the absorp- a lighter violet colour and finally the characteristic red–
tion spectrum of the neutral form 2 is thus recovered, and purple colour of complex 8. After 4 h of stirring, the solu-
consequently, the yellow colour of the solution.
tion was filtered and a purple compound was obtained,
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FULL PAPER
which was characterized by chemical analysis as complex
The ν(O–H) mode for the three coordinated water mole-
[Co(Cat-NSQ)(Cat-N-BQ)]. This result partially confirms cules appears as a broad band from 3300–3100 cm–1. The
the formation of this species as an intermediate along the electronic spectra in ethanol consists of an intense band
reaction. The solution was then kept for 2 h, and crystals with a maximum at 553 nm (1.2ϫ104 –1 cm–1) and two
of complex 8 suitable for X-ray analysis were collected.
shoulders at 512 and 476 nm. Interestingly, these bands are
Crystallographic data and experimental parameters used similar to those observed for 2·Co in solution, which con-
for the resolution of the X-ray structure of 8 are summa- firms the series of experiments as described in Figure 3.
rized in Table 4. Selected bond lengths and angles are given For comparison purposes, the electronic spectrum of
¯
in Table 3. Complex 8 crystallizes in the P1 triclinic space complex 8 was also monitored in two additional solvents.
group with the cell parameters as reported in Table 4. An The most intense band shifts its maxima to 560 nm
ORTEP drawing is shown in Figure 4. The asymmetric unit (7.4ϫ103 –1 cm–1)
in
thf
and
to
565 nm
contains one molecule of 8, one chloride counterion, two (6.2ϫ103 –1 cm–1) in toluene. The shoulders shift to 523
and a half ethanol molecules and one water molecule of and 489 nm in thf and 529 and 492 nm in toluene, respec-
crystallization. The cobalt ion displays an octahedral coor- tively, which confirms the existence of a solvatochromic ef-
dination geometry by binding to two oxygen and one nitro- fect as described previously for compound 22–. In addition,
1
gen atoms of 22– and three water molecules. The Co1–O1, the H NMR spectra of deuterated methanol solutions of
Co1–O2, Co1–N1 distances are 1.868(4), 1.856(4) and 2·Co and 8 also suggest that both complexes are almost
1.849(6) Å, respectively, so 22– is a tridentately coordinated identical. Indeed, as described in the Experimental Section,
to the cobalt ion. In this coordination mode, the azobis- the signals attributed to the four tert-butyls groups and the
phenolate ligand forms both typical five- and six-membered four aromatic protons are very similar, with small shift dif-
chelate rings with the cobalt atom. All bond lengths and ferences that are expected according to the different coordi-
angles are characteristic of and similar to those previously nation environments of the CoIII ion in 2·Co and 8.
reported for other azobisphenolate-based complexes.[22]
The magnetization temperature dependence data in the
temperature range 5–300 K under an external applied mag-
netic field of 0.5 T exhibits diamagnetic behaviour along the
whole temperature range. These results are consistent with
the formulation of the ligand in the form of diamagnetic
azophenolate (22–) and CoIII ions. The extra positive charge
is compensated for by the presence of the chloride anion.
This charge assignment was also confirmed by cyclic vol-
tammetry. Cyclic voltammetry of complex 8 shows two re-
versible one-electron waves at +1.093 V, which is assigned
as an oxidation ligand-centred process, and at –1.083 V,
which is assigned to the reduction of CoIII to CoII or, to a
lesser extent, of a ligand-centred process.
Table 3. Selected bond lengths and angles for 8.
Bond lengths
Co1–O1
Co1–N1
N1–C2
O1–C1
C1–C2
1.868(4)
1.849(6)
1.422(8)
1.334(8)
1.391(10)
Co1–O2
N1–N2
N2–C8
O2–C7
C7–C8
1.856(4)
1.266(8)
1.386(9)
1.326(8)
1.417(10)
Angles
O1–Co1–O2
O2–Co1–N1
C2–N1–N2
N1–Co1–O5
177.0(2)
96.1(2)
118.4(6)
177.6(2)
O1–Co1–N1
N1–Co1–O3
C8–N2–N1
86.6(2)
88.9(3)
119.9(6)
Finally, the redox activity of ligand 2 described pre-
viously inspired us to explore the possible existence of val-
ence tautomerism for complex 8 by means of variable-tem-
perature UV/Vis spectroscopy in the 280–370 K tempera-
ture range. An external stimuli such a temperature may in-
duce a reversible intramolecular electron transfer between
the redox-active ligand and the metal ion. However, in spite
of the different solvents and solvent mixtures used, no val-
ence tautomerism could be detected for 8. In all cases, a
large reduction in the intensity of the bands, followed by
the appearance of new bands at very short wavelengths, is
observed. This behaviour has been attributed to the decom-
position of the complex under the experimental conditions,
as confirmed by the irreversibility of the process.
Figure 4. ORTEP drawing of the azobisphenol–Co complex 8 at
the 30% probability level. Hydrogen atoms, the chloride counterion
and the solvent molecules are omitted for clarity.
Three-State Switching Array
Elemental analysis and electrospray–(+) measurements
The studies described previously confirmed the suitabil-
in methanol of a polycrystalline sample of 8 are in agree- ity of 2 to create a chromophoric array of three states with
ment with the structure previously described (see Experi- significantly different colours, thanks to the presence of the
mental Section). The FTIR spectrum (KBr pellet) of 8 exhi- azo group, which can interconvert reversibly between them
bits peaks at 1268 and 1450 cm–1, which are characteristic by means of acid/base and complexation reactions. The
of the ν(C–O) and ν(N=N) modes.[31]
UV/Vis spectra of the three species 2, 22– and 2·Co and a
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Sterically Protected Azobisphenol Ligand
schematic representation of the different mechanisms that shown to take place under basic and acidic conditions,
can be followed to interconvert between them are shown in respectively. Moreover, ligand 2 exhibits good reactivity
Figure 5. The yellow colour of 2 changes to violet and red– with transition-metal ions such as cobalt, as shown UV/Vis
purple upon addition of a base such as tba and Co(Cl)2· spectroscopy in solution. Both the acid–base character and
6H2O, respectively. In the first case, the dianionic com- the complexation ability of 2 have been used to create a
pound 22– is formed, whereas addition of the cobalt salt chromophoric array of three states (2, 22– and 2·Co) with
induces the formation of the 2·Co complex. In both cases, significantly different colours (enhanced by the presence of
subsequent addition of HCl leads to 2, which shows that the azo group), which can interconvert reversibly between
the process is fully reversible. The colour changes can nicely them. The reactivity of ligand 2 has also been used to pre-
be detected by the naked eye, as shown in Figure 5a. This pare complex 8, although no evidence of valence tautomer-
fact was also observed for complex 8. Upon addition of ism was detected. Further work is currently underway to
HCl to an ethanol solution of complex 8, crystals of pure prepare a new series of complexes bearing this ligand with
2 (40% yield) were obtained by recrystallization from the different N,NЈ counter ligands, which will allow us to study
solution, which opens an alternative high-yield methodol- valence tautomerism.
ogy for obtaining 2 to those described in the previous sec-
tion. Finally, the reverse reaction, i.e. addition of further
base to the acidic solution of 2 and the free cobalt ion did
not process. For this process to work, further addition of
Co(CH3COO)2·4H2O is necessary.
Experimental Section
All reagents were purchased from Aldrich Chemical Co. and used
as received. The solvents were dried by distillation over the appro-
priate drying agents. All reactions were performed avoiding moist-
ure by standard procedures and under an atmosphere of nitrogen
and monitored by analytical thin-layer chromatography (TLC) by
using silica gel 60 F254 pre-coated aluminium plates (0.25 mm
thickness). Flash column chromatography was performed using sil-
ica gel 60 Å, with particle sizes 35–70 µm.
Synthesis of 2,2Ј-Dihydroxy-4,3,4Ј,3Ј-tetra-tert-butylazobenzene: A
solution of sodium nitrite (0.027 g, 0.40 mmol) in a mixture of
water (0.2 mL) and 35% HCl acid (0.1 mL, 0.90 mmol) was first
cooled to 5 °C. A thf solution (0.6 mL) of aniline 4 (0.080 g,
0.36 mmol) was then added, and the resulting mixture was stirred
at 5 °C for 20 min. This solution was then added dropwise to an-
other solution of compound 5 (0.075 g, 0.36 mmol) and NaOH
(0.036 g, 0.90 mmol) in a mixture of thf (0.5 mL) and water
(0.5 mL). The whole mixture was stirred at 10 °C for 30 min, ad-
justed to a pH of 6–7 and extracted with ethyl acetate (3ϫ10 mL).
The entire organic extracts were dried with magnesium sulfate, fil-
tered and concentrated under reduced pressure to yield an oil that
was chromatographed through silica gel by using a mixture of ethyl
acetate/ethyl ether (15:1) to afford 0.033 g of azobisphenol 2
(0.08 mmol, 21% yield). FTIR (KBr): ν = 3428 [ν(O–H)], 2963,
˜
2905, 2862 [ν(C–H)], 1613, 1479, 1461, 1428 [ν(N=N)] ), 1393,
1360, 1266 [ν(C–O)], 1249, 1220, 1197, 1169 cm–1. C28H42N2O2
(438.31): calcd. C 76.0, H 9.5, N 6.4; found C 75.9, H 9.5, N 6.3.
ES–(+) (MeOH): m/z
=
439 [M+, C28H43O2N2]. 1H NMR
(400 MHz, [D8]toluene): δ = 1.46 (s, 18 H, CH3), 1.81 (s, 18 H,
CH3), 7.67–7.72 (m, 4 H, aromatic H), 13.70 (s, 2 H, OH) ppm.
Figure 5. (a) Schematic representation of the three-state switching
array. (b) UV/Vis spectra of an ethanol solution of 2 (0.055 m)
with the initial addition of 20 µL tba (0.8 ) to generate 22– and
subsequent addition of 240 µL of Co(Cl)2·6H2O (1 m) to generate
2·Co.
Synthesis of Complex 2·Co: An ethanol solution (20 µL) of
Co(CH3COO)2·4H2O (28 mg, 0.11 mmol) was added to an ethanol
solution (0.75 mL) of 2,2Ј-dihydroxy-4,3,4Ј,3Ј-tetra-tert-butylazo-
benzene (2) (1.3 mg, 0.003 mmol) in the NMR tube, whilst stirring.
The colour of the mixture turned to a lighter violet colour, and the
1
obtention of the product was followed by H NMR spectroscopy.
1H NMR (400 MHz, [D4]MeOH): δ = 1.41 (s, 18 H, CH3), 1.62 (s,
9 H, CH3), 1.66 (s, 9 H, CH3), 7.35 (s, 1 H, aromatic H), 7.49 (s, 1
H, aromatic H), 7.77 (s, 1 H, aromatic H), 8.10 (s, 1 H, aromatic
H) ppm.
Conclusions
A new synthetic route for the synthesis of the sterically
protected azobisphenol 2 has been described. The reversible
Synthesis of Complex 8: An ethanol solution (25 mL) of 3,5-di-
deprotonation/protonation of the phenol groups has been tert-butylcatechol (5 g, 23 mmol) was added to an aqueous solution
Eur. J. Inorg. Chem. 2008, 2278–2285
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2283

E. Evangelio, J. Saiz-Poseu, D. Maspoch, K. Wurst, F. Busque, D. Ruiz-Molina
FULL PAPER
(25 mL) of CoCl2·6H2O (6.2 g, 26 mmol) whilst stirring. Immedi-
ately, an aqueous solution (25 mL) of ammonia was added. The
colour of the mixture turned to dark violet, which changed to a
lighter violet colour after 10 min whilst stirring. The mixture was
then stirred for a further 4 h, and after filtration, crystals of 8 were
X-ray Data and Structure Determination of 2 and 8: Data collection
was performed on a Nonius Kappa CCD equipped with graphite-
monochromatized Mo–Kα radiation (λ = 0.71073 Å) and a nominal
crystal-to-area-detector distance of 36 mm. Intensities were inte-
grated by using DENZO and scaled with SCALEPACK. Several
scans in φ and ω direction were made to increase the number of
collected from the solution. FTIR (KBr): ν = 3109 [ν(H O)], 2955,
˜
2
2905, 2866 [ν(C–H)], 1612, 1480, 1438, 1360, 1278, 1254, redundant reflections, which were averaged in the refinement cycles.
1162 cm–1. C28H52ClCoN6O5 (646.69): calcd. C 51.4, H 8.5, N 12.8;
This procedure replaces, in a good approximation, an empirical
absorption correction. The structures were solved with direct meth-
found C 50.3, H 7.9, N 12.6. ES–(+) (MeOH): m/z = 512 [M+,
1
C28H41CoN2O3]. H NMR (400 MHz, [D4]MeOH): δ = 1.347 (s, 9 ods SHELXS86 and refined against F2 SHELX97. The crystallo-
H, CH3), 1.353 (s, 9 H, CH3), 1.44 (s, 9 H, CH3), 1.48 (s, 9 H,
CH3), 7.29 (s, 1 H, aromatic H), 7.34 (s, 1 H, aromatic H), 7.70 (s,
1 H, aromatic H), 8.15 (s, 1 H, aromatic H) ppm.
graphic data for compounds 2 and 8 are listed in Table 4. CCDC-
682913 and -682914 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif.
Physical Measurements: Microanalyses were performed by the
Servei dЈAnalisi of the Universitat Autònoma de Barcelona. Elec-
tronic absorption spectra were recorded with a Varian Cary05e
spectrophotometer equipped with a thermostatted cell holder that
can operate between 280 K and 370 K. The stability of the tem-
perature was better than Ϯ5 K. Spectra were collected on samples
that were previously thermally equilibrated at each temperature for
10 min. Magnetic susceptibilities of both samples were measured
in the temperature range 1.8–350 K with a Quantum Design
MPMS®XL superconducting SQUID magnetometer operating at
a magnetic field strength of 0.1 T. The paramagnetic susceptibility
was corrected for the holder capsules and the tabulated Pascal con-
stants. Cyclic voltammograms were recorded with a Perkin–Elmer
instrument at 20 °C in CH3CN solution containing 0.10  commer-
cial tetrabutylammonium hexafluorophosphate as supporting elec-
trolyte, with a glassy carbon working electrode and an Ag/AgCl
reference electrode. Ferrocene was used as an internal standard,
and potentials are referenced versus the ferrocinium/ferrocene cou-
ple (Fc/Fc+). NMR experiments were performed at the Servei de
Ressonància Magnètica Nuclear of the Universitat Autònoma de
Barcelona. 1H NMR spectra were recorded on Bruker DPX250
(250 MHz) and Bruker ARX400 (400 MHz) spectrometers. Proton
chemical shifts are reported in ppm (CDCl3, δ = 7.26 or [D6]-
DMSO, δ = 2.50 ppm). Infrared spectra were recorded on a
Sapphire-ATR Spectrophotometer; peaks are reported in cm–1.
High resolution mass spectra (HRMS) were recorded at Micro-
mass-AutoSpec using (ESI+ or ESI–).
Acknowledgments
This work was supported by the European Network of Excellence
MAGMANet through the project MAT2006-13765-C02. F. B. and
D. M. thank the Ministerio de Ciencia y Tecnología and the Euro-
pean Regional Development Fund for a RyC contract. E. E. thanks
the Ministerio de Ciencia y Tecnología for a pre-doctoral grant.
The authors also thank J. M. Pérez for the magnetic measurements
and A. Bernabe for the spectroscopic measurements.
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[Co(22–)(H2O)3]Cl·2.5EtOH·H2O (8).
2
8
Formula
MW
Color
C28H42N2O2
438.64
yellow plates
C33H63N2O8.50CoCl
718.23
red plates
Crystal system monoclinic
triclinic
P1
¯
Space group
a [Å]
b [Å]
c [Å]
α [°]
C2/c
19.4693(2)
6.2499(3)
21.6187(7)
10.9158(4)
14.2039(5)
14.8742(6)
101.344(2)
103.320(2)
99.697(2)
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Published Online: April 4, 2008
Eur. J. Inorg. Chem. 2008, 2278–2285
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2285