From triplesalen to triplesalalen and triplesalan - Strengthening the aromatic character of the ligand backbone in extended phloroglucinol ligands by prevention of heteroradialene formation

Article abstract of DOI:10.1002/ejic.201200993

The methyl-protected triplesalalen ligand Me₃H ₃talalen tBu 2 was synthesized from 2,4,6-tris(bromomethyl)-1,3,5- trimethoxybenzene and three equivalents of the secondary amine 4,6-di-tert-butyl-2-{[2-(methylamino)ethylimino]methyl}phenol. A reduction with NaBH₄ afforded the methyl-protected triplesalan ligand Me ₃H₃talan tBu 2. Deprotection efforts of the methyl-protected Me₃H₃talalen tBu 2 ligand with Lewis acids were unsuccessful. Using Cu^II ions as Lewis acid resulted in the formation of the methyl-protected triplesalalen complex [(Me ₃H₃talalen tBu 2){Cu^II(H₂O)} ₃](ClO₄)₃, which could be characterized by single-crystal X-ray diffraction. The bond-length analysis reveals the aromatic character of the central backbone without heteroradialene contribution, and this is corroborated by NMR spectra of the ligands, which exhibit a singlet assigned to the benzylic protons at δ = 3.50 ppm in the ¹H NMR and a resonance typical for an aromatic C^Ar-O group at δ = 160.0 ppm in the ¹³C NMR spectroscopy. FTIR spectra of the ligands and of the complexes exhibit the typical features of a central aromatic unit and do not show the characteristic intense bands for vibrations of the exocyclic C=C and C=O double bonds of a heteroradialene. Additionally, the characteristic strong absorption bands for heteroradialenes in the 26000-35000 cm^-1 region are absent in the UV/Vis/NIR spectra. The electrochemistry exhibits the reversible oxidation of the terminal phenolates of [(Me₃H ₃talalen tBu 2){Cu^II(H₂O)}₃] (ClO₄)₃ at +0.72 V vs Fc⁺/Fc, and the magnetic measurements reveal an uncoupled behavior due to the apical coordination of the central methoxy groups to the Cu^II ions. To synthesize the triplesalalen ligand H₆talalen tBu 2, the methyl-protected ligand Me₃H₃talalen tBu 2 has been prepared from 1. However, methyl deprotection was not successful but resulted in the complex [(Me ₃talalen tBu 2){Cu(H₂O)}₃](ClO ₄)₃. Copyright

Full text of DOI:10.1002/ejic.201200993

FULL PAPER
DOI:10.1002/ejic.201200993
From Triplesalen to Triplesalalen and Triplesalan –
Strengthening the Aromatic Character of the Ligand
Backbone in Extended Phloroglucinol Ligands by
Prevention of Heteroradialene Formation
Bastian Feldscher,[a] Anja Stammler,[a] Hartmut Bögge,[a] and
Thorsten Glaser*[a]
Keywords: Copper / Schiff bases / N,O ligands / Ligand design / Heteroradialenes
The methyl-protected triplesalalen ligand Me3H3talalentBu
was synthesized from 2,4,6-tris(bromomethyl)-1,3,5-trimeth-
oxybenzene and three equivalents of the secondary amine
4,6-di-tert-butyl-2-{[2-(methylamino)ethylimino]methyl}-
phenol. A reduction with NaBH4 afforded the methyl-pro-
= 3.50 ppm in the 1H NMR and a resonance typical for an
aromatic CAr–O group at δ = 160.0 ppm in the 13C NMR spec-
troscopy. FTIR spectra of the ligands and of the complexes
exhibit the typical features of a central aromatic unit and do
not show the characteristic intense bands for vibrations of the
exocyclic C=C and C=O double bonds of a heteroradialene.
Additionally, the characteristic strong absorption bands for
2
tected triplesalan ligand Me3H3talantBu . Deprotection efforts
2
of the methyl-protected Me3H3talalentBu ligand with Lewis
2
acids were unsuccessful. Using CuII ions as Lewis acid re- heteroradialenes in the 26000–35000 cm–1 region are absent
sulted in the formation of the methyl-protected triplesalalen
in the UV/Vis/NIR spectra. The electrochemistry exhibits
the reversible oxidation of the terminal phenolates of
[(Me3H3talalentBu ){Cu (H2O)}3](ClO4)3,
which
II
2
complex
[(Me3H3talalentBu ){Cu (H2O)}3](ClO4)3 at +0.72 V vs Fc+/Fc,
and the magnetic measurements reveal an uncoupled behav-
ior due to the apical coordination of the central methoxy
groups to the CuII ions.
II
2
could be characterized by single-crystal X-ray diffraction.
The bond-length analysis reveals the aromatic character of
the central backbone without heteroradialene contribution,
and this is corroborated by NMR spectra of the ligands,
which exhibit a singlet assigned to the benzylic protons at δ
plexes exhibit ferromagnetic interactions,[9–11] and by a sup-
ramolecular approach, we could assemble two trinuclear
Introduction
complexes [(talentBu ){M (solv)n}3]
serving as building
t
m+
2
The discovery of the fascinating properties of single-mo-
lecule magnets (SMMs)[1] and their potential applications
in memory devices, quantum computing, and molecular
spintronics[2] has attracted much interest for new types of
SMMs. To obtain a rational access to new types of SMMs,
we developed the ligand system triplesalen.[3,4] The first
blocks with a central hexacyanochromate [Mc(CN)6]3– to
heptanuclear complexes [{(talentBu )M 3}2{M (CN)6}]
subsequently abbreviated [Mt6Mc]n+ [MnIII6CrIII 3+ [12]
[MnIII6FeIII 3+ [13] [MnIII6CoIII 3+ [14]
MnIII 3+ [15] The complexes [MnIII6CrIII 3+
MnIII 3+
t
c
3+
2
:
] ,
]
,
]
,
and
[MnIII
-
-
6
]
.
]
and [MnIII
6
generation of triplesalen ligands (e.g., H6talentBu , Fig-
2
]
were shown to be single-molecule magnets. How-
ure 1a, R1 = R2 = tBu) is composed of a central 1,3,5-tri-
hydroxybenzene (phloroglucinol) unit, which was chosen to
enforce ferromagnetic interactions between the metal ions
by the spin-polarization mechanism.[5,6] The salen-like co-
ordination environment was chosen to induce a strong mag-
netic anisotropy through its strong axial ligand field.[7] Ad-
ditionally, the C3 symmetry of the ligand and hence of its
complexes should reduce the rhombicity of the trinuclear
complexes to minimize quantum-mechanical magnetization
tunneling.[8] Indeed, the trinuclear CuII and VIV=O com-
ever, the analysis of the magnetic properties in these hepta-
nuclear complexes[12–14] and in trinuclear MnIII [16] and
3
FeIII [17,18] complexes revealed that the interaction between
3
the MnIII (FeIII) ions in the triplesalen subunits is not as
expected ferromagnetic but weakly antiferromagnetic.
We identified a reason for this unexpected exchange cou-
pling by extensive NMR spectroscopy on our ligands on
the basis of the work of MacLachlan et al.[19] and later
others.[20] The central phloroglucinol backbone is not in the
usually assumed O-protonated tautomeric form (Figure 1d,
I and II) but in the N-protonated form (Figure 1d, III and
IV).[17,21–23] Furthermore, this N-protonated tautomer has
the keto–enamine resonance structure IV as a main contri-
bution to the resonance hybrid and not the enolate–imin-
ium resonance structure III. This is in agreement with stud-
[a] Lehrstuhl für Anorganische Chemie I, Fakultät für Chemie,
Universität Bielefeld,
Universitätsstr. 25, 33615 Bielefeld, Germany
E-mail: tglaser@uni-bielefeld.de
Homepage: www.uni-bielefeld.de/chemie/ac1chair
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Figure 1. Abbreviations of ligands (a–c). Tautomerism and mesomery in extended phloroglucinol ligands (d).
ies by Plass et al. on a comparable ligand system.[24] The the imine functions to amine functions. A problem that one
resonance structure IV closely resembles [6]radialenes and faces with extended phloroglucinol ligands bearing central
has thus been named heteroradialene.[25] [6]Radialenes are ketimine units as a starting material (like in H6talenR2) is
cross-conjugated alicycles without a delocalized π system that the three reductions lead to the formation of three
and thus have no aromatic character.[26,27] Moreover, we stereo centers and thus to diastereomeric mixtures. As we
could show that also in our trinuclear complexes the main expect this to have severe consequences for the complex for-
resonance structure is the heteroradialene IV but to a lower mation, we developed extended phloroglucinol ligands
extend compared to the ligands.[17,21,22,28] As the spin- bearing central aldimine units for the reduction experi-
polarization mechanism is anticipated to be a π mecha- ments.[21,22] We applied conventional reductive routines,
nism,[6,29] we identified the heteroradialene formation ac- using NaBH4 and LiAlH4, which easily convert salen li-
companied by the reduction of the aromaticity of the cen- gands to salan ligands. However, despite extensive efforts,
tral ring as one major reason for the weak ferromagnetic we could not obtain an experimental indication for the
interactions in the CuII and (VIV=O)3 complexes and the presence of three amine functions on a phloroglucinol
3
weak antiferromagnetic interactions in the MnIII3 and FeIII
backbone. The heteroradialene nature of the starting mate-
rials – the anticipated imine functions simply do not exist –
3
complexes.[29]
In this contribution, we present an approach to pre- provides an obvious explanation for the failure of this re-
venting heteroradialene formation. As the heteroradialene ductive route. Therefore, for the synthesis of triplesalen li-
formation is based on the conjugation of the sp2-hybridized gands that exhibit amine functions at the central phloro-
C=N groups, we intend to formally substitute the imine glucinol ring instead of imine units, that is, triplesalalen or
groups in the 2,4, and 6 position by amine groups (H6tala- triplesalan ligands (Figure 1b, c), we had to develop a new
R2
lenR and H6talan , Figure 1b, c). We report our results for synthetic approach.
2
the reduction of the imine groups to amine functions and
for the introduction of amine groups by a substitution ap-
proach.
Another way to prepare asymmetrical salan (i.e., tetra-
hydrosalen) or salalen (i.e., dihydrosalen) ligands is to syn-
thesize a half-unit that bears a secondary amine, which can
be used in a nucleophilic substitution reaction, for example,
with σ-(bromomethyl)phenols.[30,34] To follow this synthetic
route for the preparation of triplesalalen ligands, the re-
quired synthon would be 2,4,6-tris(bromomethyl)-1,3,5-
trihydroxybenzene. To the best of our knowledge, this has
Results and Discussion
Synthesis
The most obvious route for the conversion of a tri- not yet been described in the literature. We used the methyl-
plesalen ligand to a triplesalan ligand is the reduction of protected derivative 2,4,6-tris(bromomethyl)-1,3,5-trimeth-
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oxybenzene (1) for the preparation of the methyl-protected talantBu . Unfortunately, this compound was also unstable
2
triplesalalen ligand Me3H3talalentBu (Scheme 1), which under the harsh reaction conditions used in attempts to de-
2
should afford the free triplesalalen ligand H6talalentBu
upon demethylation by Lewis acids.
protect Me3H3talalentBu , and we were not able to isolate
2
2
H6talantBu
.
2
We therefore envisioned deprotection by an internal
Lewis acid like CuII or FeIII, that is, a metal ion already
coordinated in the NNOPh coordination pocket. This pre-
coordination should also suppress the cleavage of the ligand
backbone. Following this line of thought, we studied the
reaction of Me3H3talantBu with CuII(ClO4)2·6H2O,
2
CuII(BF4)2·3H2O, CuIICl2·2H2O, FeII(ClO4)2·6H2O, and
FeIIICl3·6H2O, varying the solvent (MeOH, EtOH, dmf,
ethylglycol), the temperature, and the reaction time, but we
could not detect a deprotected product. However, the reac-
tion of Me3H3talalentBu with CuII(ClO4)2·6H2O in meth-
2
anol yielded green crystals, which were characterized by sin-
gle-crystal X-ray diffraction analysis, elemental analysis,
FTIR spectroscopy, and mass spectrometry and were iden-
tified as [(Me3talalentBu ){Cu (H2O)}3](ClO4)3.
II
2
NMR Spectroscopy
1
The H NMR spectrum of the model compound Me32
exhibits singlets at δ = 3.81, 3.44, and 2.14 ppm, triplets at
δ = 2.37 and 0.85 ppm, and two multiplets at δ = 1.46 and
1.26 ppm. The multiplets between 2.37 and 0.85 ppm can
be assigned to the butyl group, and the singlet at δ =
2.14 ppm can be assigned to the NCH3 group, whereas the
singlet at δ = 3.81 ppm belongs to the OCH3 moiety, and
the singlet at δ = 3.44 ppm belongs to the methylene group
next to the central ring.
These spectral features are different from those of hetero-
radialene ligands,[17,21–23] which exhibit two multiplets at δ
≈ 11.3 and 10.9 ppm. These signals are due to the NH pro-
tons, which couple with the vinylic CH proton (δ = 8.3–
8.0 ppm) and the CH2 protons (δ = 3.43 ppm) of the ethyl-
ene bridge. One set of signals can be assigned to the C3h
Scheme 1.
To find suitable conditions for the substitution reaction, symmetric isomer, and three sets of signals can be assigned
we first synthesized the model compound Me32 by the reac- to the CS symmetric isomer.
tion of 1 with N-methylbutylamine in toluene (Scheme 1).
Also, the 13C NMR spectrum of Me32 is less complex
The presence of KOH leads to the precipitation of KBr, than the 13C NMR spectra of the heteroradialene ligands.
which is an additional driving force for the formation of Me32 exhibits signals at δ = 159.9 and 122.4 ppm, which
Me32. In analogy, the protected triplesalalen ligand can be assigned to the CArOMe and the CArCCN moiety of
Me3H3talalentBu was obtained by the reaction of 1 with the central ring, respectively. In contrast, the CArO moieties
2
half-unit 3[44] and KOH in toluene (Scheme 1). The identity of the heteroradialene ligands show signals between 188
and purity of both compounds was confirmed by NMR and 183 ppm,[17,21–23] which are more characteristic for
spectroscopy, FTIR spectroscopy, mass spectrometry, and ketones (Figure 1, IV).[34] Also, the resonance of the
elemental analysis.
CArCCN carbon at δ ≈ 105 ppm shows the strong distortion
To obtain the deprotected ligand H6talalentBu , we tried of the aromaticity of the central ring.
2
The ligands Me3H3talalentBu and Me3H3talantBu show
2
2
different approaches. Common routes for the demethylation
of phenols use Lewis acids like BBr3,[31] BF3·SMe2,[32] or almost identical NMR spectra. In the H NMR spectra,
1
AlCl3.[33] However, employing these reaction conditions for singlets at δ = 3.77 and 3.56 ppm for Me3H3talalentBu and
2
the deprotection of Me3H3talalentBu yielded no pure prod- at δ = 3.80 and 3.50 ppm for Me3H3talantBu can be as-
ucts, but the cleavage of the terminal imine was frequently signed to the OCH3 and the CArCH2 moieties, respectively.
observed. Therefore, to enhance the stability of the pro- The main difference between the 1H NMR spectra of
2
2
tected ligand, we reduced the imine group with NaBH4 to Me3H3talalentBu and Me3H3talantBu is a singlet at δ =
2
2
afford the protected triplesalan ligand precursor Me3H3- 8.31 ppm for the terminal imine of Me3H3talalentBu , which
2
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forms a new broad singlet at δ = 3.68 ppm for the benzylic
unit upon reduction. The 13C NMR spectra of Me3H3tala-
lentBu and Me3H3talantBu show identical signals at δ =
160.0 and 122.1 ppm for the CArOMe and CArCCN units of
2
2
the central ring, respectively. For Me3H3talantBu , an ad-
2
ditional singlet can be detected at δ = 166.4 ppm, which is
assigned to the C=N resonance of the terminal imine.
The comparison of the chemical shifts of the CArO reso-
nance of the central ring of Me32, Me3H3talalentBu , and
2
Me3H3talantBu with aromatic phloroglucinol (δ
=
2
158.8 ppm) or methoxybenzene (δ = 159.7 ppm) unambigu-
ously demonstrates that the central ring is, in contrast to
that of heteroradialene ligands, aromatic.
Structural Characterization
The crystal structure of the complex [(Me3talalentBu )-
2
{CuII(H2O)}3](ClO4)3·4H2O·CH3OH was determined by
single-crystal X-ray diffraction analysis, and the molecular
structure is displayed in Figure 2. Selected interatomic dis-
tances and angles are summarized in Table 1. The structure
contains three independent cations [(Me3talalentBu ){Cu -
II
2
(H2O)}3]3+, which all exhibit crystallographic C3 symmetry.
The deprotonated ligand (Me3talalentBu
)
acts as a triply
3–
2
tetradentate ligand for three CuII ions, which are in a dis-
torted square-pyramidal coordination environment (τ val-
[35]
ues:
τ
= 0.32, τCu2 = 0.28, τCu3 = 0.35) and are coordi-
Cu1
nated by phenolate, amine, and imine donors of the ligand
and by a water molecule in the basal plane. The ether func-
tion CAr–O–CH3 of the central phloroglucinol is coordi-
nated in the apical position. The Cu–OPh bond lengths are
dCu1 = 1.90 Å, dCu2 = 1.89 Å, and dCu3 = 1.89 Å, whereas
the Cu–OCH bond lengths are much longer (dCu1 = 2.38 Å,
3
dCu2 = 2.40 Å, and dCu3 = 2.36 Å). All Cu–Nimine bonds
have the same length (dCu = 1.91 Å) and are shorter than
the Cu–Namine bond lengths (dCu1 = 2.08 Å, dCu2 = 2.09 Å,
and dCu3 = 2.10 Å).
The mean C–C bond length of the central ring is 1.40 Å
for the molecule including Cu1 (molecule 1) and for the
molecule including Cu3 (molecule 3), and it is 1.38 Å for
the molecule including Cu2 (molecule 2). These values are
significantly shorter than the mean bond lengths in a typi-
cal heteroradialene complex like [(talentBu )Cu 3] with
II
2
1.43 Å[10] and are roughly the same as those of benzene
(1.39 Å).[36] However, for an evaluation of the aromaticity,
not only the mean C–C bond length has to be taken into
account, but a complete delocalization of the C–C single
bonds and the C=C double bonds is also required. These
two criterions (bond lengths and their alternation) are con-
sidered in the HOMA value (harmonic oscillator model of
aromaticity).[27,37] The HOMA value is 1 for benzene and 0
for the model nonaromatic benzene system with localized
double and single bonds. The HOMA values for the three
Figure 2. Molecular structure of [(Me3talalentBu ){Cu (H2O)}3]
II
3+
2
(molecule 1 and molecule 3) in crystals of [(Me3talalentBu ){Cu -
(H2O)}3](ClO4)3·4H2O·CH3OH. Molecule 1 (a) and molecule 3 (b)
drawn perpendicular to the central benzene ring of the phloro-
glucinol backbone. Molecule 1 (c) and molecule 3 (d) drawn
parallel to the central benzene ring of the phloroglucinol backbone.
Hydrogen atoms and counterions are omitted for clarity. The con-
figuration of the tertiary amine results in different orientations of
the terminal ring (e). tBu groups are omitted for clarity.
II
2
molecules in the crystal structure of [(Me3talalentBu )-
2
{CuII(H2O)}3](ClO4)3·4H2O·CH3OH are 0.95 (molecule 1),
1.00 (molecule 2), and 0.96 (molecule 3), which clearly con-
firms the aromatic character of the central phloroglucinol
backbone in the complexes.
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Table 1. Selected interatomic distances [Å] and angles [°] for
There are four possible combinations for the composi-
tion of a crystal with three such independent molecules
in a chiral space group: the “equal-enantiomeric” forms
3ϫ R and 3ϫ S and the “mixed-enantiomeric” forms (2ϫ
R + 1ϫ S) and (2ϫ S + 1ϫ R). The examined single crystal
exhibits molecule 2 and molecule 3 in the R configuration,
whereas molecule 1 is in the S configuration (Figure 2e),
therefore, the analyzed crystal is in the mixed-enantiomeric
form 2 ϫ R + 1ϫ S.
[(Me3talalentBu ){Cu (H2O)}3](ClO4)3·4H2O·CH3OH.
II
2
Bond
[Å]
Angle
[°]
Cu1–O11
Cu1–O12
Cu1–O13
Cu1–N11
Cu1–N12
O11–C1
N11-C11
C1–C2
2.379(3)
1.895(3)
2.005(4)
2.083(4)
1.909(4)
1.377(5)
1.499(6)
1.404(6)
1.401(6)
1.507(6)
N11–Cu1–O11
N12–Cu1–O11
O12–Cu1–O11
O13–Cu1–O11
N12–Cu1–N11
O12–Cu1–N11
O13–Cu1–N11
O12–Cu1–N12
N12–Cu1–O13
O12–Cu1–O13
86.77(13)
117.71(13)
90.11(13)
87.11(15)
84.64(15)
174.53(14)
96.25(16)
92.85(15)
155.14(17)
88.07(15)
C1–C2#1
C2–C11
Infrared Spectroscopy
The IR spectra of heteroradialene ligands exhibit charac-
teristic bands at approximately 1610 cm–1, which are as-
signed to a coupled vibration of the C=C and C–N
stretches of the exocyclic double bonds of the heteroradia-
lene backbone, and a broad band at approximately
1545 cm–1, which is assigned to the C=O stretching vi-
brations of the keto functions.[38] Figure 3a exemplarily
shows the IR spectrum of H3felden[21], an extended phloro-
glucinol ligand with three 2-(dimethylamino)ethylimine
groups. In contrast, in this region, Me32 only exhibits a
band at 1581 cm–1, which is assigned to the ring stretching
vibrations of the central phloroglucinol backbone. The li-
Cu2–O21
Cu2–O22
Cu2–O23
Cu2–N21
Cu2–N22
O21–C3
N21-C21
C3–C4
2.397(3)
1.893(3)
2.003(3)
2.086(4)
1.913(4)
1.397(5)
1.488(6)
1.383(6)
1.385(6)
1.528(6)
N21–Cu2–O21
N22–Cu2–O21
O22–Cu2–O21
O23–Cu2–O21
N22–Cu2–N21
O22–Cu2–N22
O23–Cu2–N21
O22–Cu2–N21
N22–Cu2–O23
O22–Cu2–O23
85.90(13)
116.28(14)
89.79(13)
87.17(13)
84.48(16)
93.05(16)
95.96(15)
173.46(14)
156.47(16)
88.73(15)
C3–C4#3
C4–C21
Cu3–O31
Cu3–O32
Cu3–O33
Cu3–N31
Cu3–N32
O31–C5
N31-C31
C5–C6
2.363(3)
1.890(3)
2.006(4)
2.097(3)
1.914(4)
1.382(5)
1.493(6)
1.401(6)
1.399(6)
1.496(6)
N31–Cu3–O31
N32–Cu3–O31
O32–Cu3–O31
O33–Cu3–O31
N32–Cu3–N31
O32–Cu3–N32
O33–Cu3–N31
O32–Cu3–N31
N32–Cu3–O33
O32–Cu3–O33
86.61(13)
118.30(13)
89.53(13)
88.26(15)
84.88(15)
93.07(16)
96.29(15)
107.3(3)
C5–C6#5
C6–C31
153.41(17)
88.01(15)
Trinuclear complexes of the triplesalen ligands exhibit
overall bowl-shaped molecular structures, in which the
NNOO planes are bent relative to the central phloroglu-
cinol ring.[3,10] The different binding situation in
[(Me3talalentBu ){Cu (H2O)}3] leads to a different ligand
folding with an overall calix-like structure (Figure 2c and
d) caused by the almost perpendicular orientation of the
terminal salen subunits with respect to the central ring.
II
3+
2
The complex [(Me3talalentBu ){Cu (H2O)}3]
crys-
II
3+
2
tallizes in the chiral space group P3 and contains chiral
molecules. Each molecule contains three centers of chirality,
which are a result of the coordination of the asymmetrically
substituted tertiary amines of the ligand to the CuII ions.
The two coordinated enantiomeric amines may be named
“R” and “S” and are defined in Figure 2e. As all starting
materials are optically inactive, a spontaneous resolution of
the racemic mixture must have occurred during the
crystallization.
A molecule with three centers of chirality, which can
either have R or S configuration, may exist in four different
isomers, the enantiomeric pairs RRR–SSS and RRS–SSR
(note that, e.g., RRS, RSR, and SRR are equivalent by sym-
metry), which are diasteromeric to each other. The C3 sym-
metry of the molecules reduces the number of possible iso-
mers from four to the two enantiomers RRR = R and SSS
= S.
Figure 3. FTIR spectra of (a) Me32, Me3H3talalentBu , Me3H3-
2
talantBu , and H3felden and (b) H2salen , [(Me3talalen )-
tBu2
tBu2
2
{CuII(H2O)}3I]I(ClO4)3,
[(felden){CuII(bpy)}3](ClO4)3,
and
[(salentBu )Cu ].
2
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tBu2
tBu2
gands H2salentBu , Me3H3talalen , and Me3H3talan
also show bands at 1594, 1586, and 1584 cm–1, respectively,
which correspond to these ring stretching vibrations. Ad-
2
ditional bands in Me3H3talalentBu at 1633 cm–1 and in
2
H2salentBu at 1628 cm–1, which are absent in Me3H3-
2
talantBu , may be assigned to the typical C=N stretching
2
mode of the phenol–imine unit.
After coordination to CuII the bands of the phenol–
imine unit shift only slightly in both [(Me3talalentBu ){Cu -
II
2
[(salentBu )Cu ]
II
(1629 cm–1)
and
2
(H2O)}3](ClO4)3
(1630 cm–1). The ring stretches of the central backbone of
[(Me3talalentBu ){Cu (H2O)}3](ClO4)3 can be detected at
II
2
1593 cm–1. A band at 1532 cm–1, which is absent in the li-
gands, is also observed in [(salentBu )Cu ] at 1530 cm and
was assigned to C–C ring stretches of the coordinating
phenolate.[39] It may thus be assigned to the terminal
II
–1
2
phenolate in [(Me3talalentBu ){Cu (H2O)}3](ClO4)3. In
II
2
comparison, the spectrum of the heteroradialene CuII com-
plex [(felden){CuII(bpy)}3](ClO4)3 exhibits broad bands at
1598 and 1499 cm–1, which can be assigned to the ligand
backbone with strong contributions of the heteroradialene
resonance structures. As these bands are absent in [(Me3-
talalentBu ){Cu (H2O)}3](ClO4)3, the heteroradialene for-
II
2
mation was successfully precluded.
Electronic Absorption Spectroscopy
Figure 4. Electronic absorption (a) spectra of Me32 (CH3CN),
2
Me3H3talalentBu (CH2Cl2), Me3H3talantBu (CH2Cl2), H2salentBu
The electronic absorption spectra of the ligands Me3H3-
2
2
talalentBu and Me3H3talantBu and of the model Me32 are (CH2Cl2), H3felden (CH3CN), and H6talentBu (CH2Cl2). Elec-
2
2
2
tronic absorption spectra (b) of the CuII complexes
displayed in Figure 4a. For comparison, the ligands H2sa-
[(Me3talalentBu ){Cu (H2O)}3](ClO4)3 (CH3CN), [(salentBu )Cu ]
II
II
2
2
lentBu
,
H3felden,[21] and H6talentBu
are included. In
[40]
2[3]
2
(CH2Cl2), and [(talentBu )Cu 3] (CH2Cl2). The inset shows the
d–d transitions.
II
2
the inset, the d–d-transitions are displayed. The spectra of
the heteroradialene ligands H3felden and H6talentBu are
2
dominated by two strong absorptions in the region 26000–
35000 cm–1, which are a typical signature of the central het-
eroradialene backbone.[17,22] The spectrum of the model
Me32 exhibits no absorptions of significant intensities be-
tween 10000 and 40000 cm–1, whereas the spectrum of the
sorptions at 25500 cm–1 in [(Me3talalentBu ){Cu -
II
2
(H2O)}3](ClO4)3 and [(salentBu )Cu ] are the typical π–π*
II
2
transitions of the conventional imine unit.
ligand Me3H3talalentBu almost coincides with the spectrum
2
Electrochemistry
of H2salentBu . Thus, the spectral features in Me3H3-
2
talalentBu are assigned to the terminal phenol–imine chro-
The cyclic voltammogram of [(Me3talalentBu ){Cu -
II
2
2
mophore. This assignment is confirmed by the spectrum of (H2O)}3](ClO4)3 (Figure 5) exhibits a reversible oxidation
Me3H3talantBu , which possesses no terminal phenol–imine wave at E1/2 = +0.72 V and two irreversible waves at Ep =
2
chromophore, and exhibits only one weaker band at –1.16 and –1.69 V with back currents at Ep = –0.65 and
35000 cm–1.
–0.50 V. Analogous reduction waves are also present in
The spectra of the heteroradialene complexes [(talentBu )- [(talen)CuII
]
3
(H6talen = Figure 1a, R1 = R2 = H)[4] at
[9,10]
2
CuII3] and [(felden){CuII(bpy)}3](ClO4)3 (Figure 4b) both –1.84 V, whereas [(talentBu )Cu 3] shows no reductions, and
II
2
exhibit strong absorption features in the region 27000– they might be attributed to the CuII/CuI redox couple.[41]
37000 cm–1, which are absent in the spectra of the mononu- Interestingly, the voltammogram of [(talentBu )Cu
]
[10] also
3
II
2
clear complex [(salentBu )Cu ] and are thus assigned to the reveals a reversible oxidation at +0.81 V, which is irrevers-
heteroradialene backbone.[17,22,23] These intense absorp- ible in [(talen)CuII3].[10] Although in the study of the trinu-
tions are absent in the reduced triplesalen complex clear CuII triplesalen complexes a definitive assignment of
II
2
[(Me3talalentBu ){Cu (H2O)}3](ClO4)3, which indicates that this oxidation wave to an oxidation of the central phloro-
II
2
the heteroradialene formation is successfully suppressed. As glucinol backbone or the terminal phenolates was not pos-
in [(Me3talalentBu ){Cu (H2O)}3](ClO4)3, absorptions at sible, the occurrence of almost the same oxidative wave in
II
2
36000 cm–1 are also observed in [(salentBu )Cu ] and [(tal- [(talentBu )Cu
]
3
and in [(Me3talalentBu ){Cu (H2O)}]-
II
II
II
2
2
2
entBu )Cu 3] and can therefore be mainly assigned to π–π* (ClO4)3 precludes the assignment to the central ring system.
II
2
transitions of the terminal phenolate chromophore. The ab- The consequential assignment of the oxidation to the ter-
Eur. J. Inorg. Chem. 2013, 388–397
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FULL PAPER
minal phenolates is corroborated by the well-known revers- ions. We tried to simulate the temperature dependence of
ibility of the oxidation of coordinated 2,4-di-tert-butyl- μeff with the appropriate spin Hamiltonian [Equation (1)]
phenolates to the corresponding phenoxyl radicals.[42]
for an equilateral CuII (Si = 1/2) triangle, including a Hei-
3
senberg–Dirac–van Vleck (HDvV) exchange and Zeeman
interactions, by using the program package JulX,[47] which
also takes into account saturation effects. Fitted values of
χTIP (TIP = temperature-independent paramagnetism) are
subtracted from the simulated and experimental data.
3
H = –2J(S1S2 + S2S3 + S1S3) +
[giμBSiB]
(1)
͚
i = 1
The best agreement between the experimental and simu-
lated data was obtained for J = –0.02 cm–1, g = 2.04, and
χTIP = 110ϫ 10–6 cm3 mol–1. The orthogonality of the
dx2–y2 magnetic orbital, which is of δ symmetry with respect
to the CuII–O bond, and the O p-orbitals (none of δ sym-
metry) combined with the long and therefore weak Cu–
OCH bond precludes a significant delocalization of the spin
3
from CuII to the oxygen atom. Therefore, this system is best
described as consisting of three uncoupled CuII ions.
Figure 5. Electrochemical measurements of [(Me3talalentBu ){Cu -
(H2O)}3](ClO4)3 in a CH3CN solution [0.10 m (NBu4)PF6] at 20 °C,
recorded with a Pt working electrode at a scan rate 200 mVs–1.
II
2
Conclusions
To overcome the heteroradialene formation in extended
phloroglucinol ligands, we intended to formally substitute
the three imine groups by amine groups. Herein, we could
show that a simple reduction of the imine groups to amine
groups is not feasible, as no real imine groups are present
in heteroradialene ligands. As an alternative synthetic path-
way, we investigated the triple nucleophilic substitution of
2,4,6-tris(bromomethyl)-1,3,5-trimethoxybenzene (1) with
secondary amines, which resulted in Me32 and Me3H3-
Magnetochemistry
Temperature-dependent measurements of the magnetic
susceptibility (SQUID, 2–290 K) at different magnetic fields
of samples of [(Me3talalentBu ){Cu (H2O)}3](ClO4)3 were
II
2
performed (Figure 6). The effective magnetic moment μeff is
3.06 μB at 290 K, which is close to the expected μeff value
of 3.17 μB for three non-interacting CuII ions (g = 2.11).
When the temperature is lowered, μeff remains constant and
exhibits a rapid drop below 10 K to a value of 1.92 μB at
2 K (Figure 5). This temperature behavior indicates that
there is no significant interaction between the three CuII
talalentBu . Interestingly, in contrast to our efforts to reduce
2
the triplesalen ligands, the remaining terminal imine in this
triplesalalen ligand could easily be reduced to afford
Me3H3talantBu . In analogy to literature reports, we tried to
2
deprotect Me3H3talalentBu by reacting it with Lewis acids,
2
which was not successful. Even the use of transition metal
ions did not result in a demethylation, but we could obtain
the trinuclear complex [(Me3talalentBu ){Cu (H2O)}3]-
II
2
(ClO4)3.
The characterization of the newly synthesized com-
pounds by FTIR, UV/Vis, and NMR spectroscopy and in
the case of [(Me3talalentBu ){Cu (H2O)}3](ClO4)3 by X-ray
II
2
diffraction and electrochemistry clearly demonstrates the
suppression of heteroradialene formation. This arises from
the substitution of imine by amine groups and from the
methyl protection of the phenolate. As this reaction path-
way does not afford the methyl deprotected triplesalalen
and triplesalan ligands and complexes, we are currently in-
vestigating other synthetic pathways to the anticipated li-
gands and complexes.
Figure 6. Temperature dependence of the effective magnetic mo-
Experimental Section
ment μeff of [(Me3talalentBu ){Cu (H2O)}3](ClO4)3. The solid line
II
2
corresponds to the best fits: J = –0.02 cm–1, g = 2.04, and χTIP
110ϫ 10–6 cm3 mol–1.
=
General: Solvents and starting materials were of the highest com-
mercially available purity and used as received. 2,4,6-Tris(bromo-
Eur. J. Inorg. Chem. 2013, 388–397
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FULL PAPER
methyl)-1,3,5-trimethoxybenzene (1)[43] and the half-unit 3[44] were
prepared according to reported procedures.
6 H, CH2NMe), 2.17 (s, 9 H, NCH3), 1.42 (s, 27 H, tBu), 1.29 (s,
27 H, tBu) ppm. 13C{1H} NMR (125.75 MHz, CDCl3, 25 °C): δ =
160.0 (s, CArOMe), 155.0 (s, CArOH), 140.2 (s, CArtBu), 135.7 (s,
CArtBu), 123.1 (s, CArH), 122.8 (s, CArH), 122.1 (s, CArCH2NMe),
63.1 (s, OCH3), 56.1 (s, CH2NMe), 53.4 (s, CArCH2NH), 50.7 (s,
CH2NMe), 46.0 (s, CH2CH2NH), 41.9 (s, NCH3), 35.0 (s, CMe3),
34.2 (s, CMe3), 31.8 [s, C(CH3)3], 29.7 [s, C(CH3)3] ppm. ESI-MS:
2,4,6-Trimethoxy-1,3,5-tris[(methylbutylamino)methyl]benzene
(Me32): 2,4,6-Tris(bromomethyl)-1,3,5-trimethoxybenzene (1)
(82 mg, 0.183 mmol), N-methylbutylamine (56 mg, 0.642 mmol),
and KOH (56 mg) were dissolved in toluene (20 mL) and stirred
for 2 h at 80 °C. The mixture was filtered, and the volatiles were
removed in vacuo, which yielded the product as a colorless oil
(yield: 75 mg, 0.161 mmol, 88 %). 1H NMR (300 MHz, CDCl3,
25 °C): δ = 3.81 (s, 9 H, OCH3), 3.44 (s, 6 H, Ar-CH2), 2.37 (t,
3JH,H = 7.4 Hz, 6 H, NCH2CH2), 2.14 (s, 9 H, NCH3), 1.46 (m, 6
H, NCH2CH2), 1.26 (sextet, 3JH,H = 7.4 Hz, 6 H, CH2CH3), 0.85 (t,
3JH,H = 7.4 Hz, 6 H, CH2CH3) ppm. 13C{1H} NMR (125.75 MHz,
CDCl3, 25 °C): δ = 159.9 (s, COMe), 122.4 (s, CArC), 63.0 (s,
OCH3), 57.6 (s, NCH2CH2), 50.8 (s, CArCH2), 41.9 (s, NCH3), 29.8
(s, NCH2CH2), 20.8 (s, CH2CH3), 14.2 (s, CH2CH3) ppm. ESI-MS:
m/z = 1081.7 [M + H]+. IR (KBr): ν = 3304 (w), 2955 (s), 2907 (s),
˜
2868 (s), 2799 (s), 1584 (m), 1479 (s), 1458 (s), 1412 (m), 1391 (m),
1362 (m), 1302 (m), 1238 (m), 1202 (m), 1165 (m), 1126 (m), 1101
(s), 1057 (w), 1036 (w), 1013 (w), 972 (w), 910 (w), 878 (w), 822
(w), 799 (w), 762 (w), 733 (m) cm–1. Me3H3talantBu2·2.5H2O
(C66H113N6O8.5): calcd. C 70.36, H 10.11, N 7.46; found C 70.46,
H 9.84, N 7.13.
[(Me3talalentBu ){Cu (H2O)}3](ClO4)3: To a suspension of Me3H3-
II
2
talalentBu (263 mg, 0.244 mmol) in methanol (25 mL) was added a
2
solution of CuII(ClO4)2·6H2O (285 mg, 0.769 mmol) in methanol
(10 mL), and the resulting mixture was stirred for 16 h at 50 °C.
The green solution was filtered, and slow evaporation of the solvent
yielded green crystal needles, which were washed with ethyl ether
and dried in vacuo to yield the product as a green solid (yield:
m/z = 466.4 [M + H]+, 488.3 [M + Na]+. IR (KBr): ν = 2957 (s),
˜
2934 (s), 2872 (s), 2861 (s), 2843 (s), 2789 (s), 1582 (s), 1462 (s),
1452 (s), 1410 (s), 1366 (m), 1306 (m), 1283 (m), 1261 (w), 1244
(w), 1204 (m), 1169 (m), 1103 (s), 1063 (w), 1034 (m), 1015 (m),
997 (m), 972 (m), 891 (w), 858 (w), 816 (w), 733 (w) cm–1. Me32
(C27H51N3O3): calcd. C 69.63, H 11.04, N 9.02; found C 69.86, H
11.17, N 8.63.
144 mg, 0.089 mmol, 36%). ESI-MS: m/z = 420.8 [M – 3ClO4
–
3H2O]3+, 638.7 [M – 3ClO4 – 2H2O – H]2+. IR (KBr): ν = 2959
˜
(s), 2907 (m), 2870 (w), 1630 (s), 1533 (m), 1460 (w), 1437 (w), 1414
(w), 1389 (w), 1364 (w), 1323 (w), 1273 (w), 1256 (w), 1231 (w),
1200 (w), 1169 (mm), 1121 (s), 1094 (s), 970 (m), 837 (w), 787 (w),
746 (w), 623 (w) cm–1. [(Me3talalentBu2){CuII(H2O)}3](ClO4)
3·3.5H2O·0.5MeOH (C66.5H114N6O25Cu3Cl3): calcd. C 47.13, H
6.78, N 4.96; found C 47.13, H 6.59, N 5.05.
Me3H3talalentBu : 2,4,6-Tris(bromomethyl)-1,3,5-trimethoxybenz-
2
ene (1) (47 mg, 0.105 mmol), half-unit 3 (90 mg, 0.310 mmol), and
KOH (45 mg) were suspended in toluene (6 mL) and stirred for
10 h at 80 °C. The mixture was filtered, and the volatiles were re-
moved in vacuo, which yielded the product as a yellow oil, which
was redissolved in CH2Cl2, the solvent was evaporated in vacuo,
and the process was repeated two times to yield the product as a
yellow powder (yield: 81 mg, 0.075 mmol, 72 %). 1H NMR
(500 MHz, CDCl3, 25 °C): δ = 13.9 (s, 3 H, OH), 8.31 (s, 3 H,
X-ray Crystallography: Crystal data for [(Me3talalentBu ){Cu -
(H2O)}3](ClO4)3·4H2O·1MeOH: M = 1719.64 gmol–1, C67H117N6-
O26Cu3Cl3, trigonal, space group P3, a = 28.3863(12) Å, c =
9.8085(5) Å, V = 6844.6(5) Å3, Z = 3, ρ = 1.252 g cm–3, μ =
2.199 mm–1, F(000) = 2721, Crystal size: 0.25ϫ0.08ϫ0.06 mm3,
II
2
4
4
N=CH), 7.36 (d, JH,H = 2.3 Hz, 3 H, CArH), 7.04 (d, JH,H
=
2.3 Hz, 3 H, CArH), 3.77 (s, 9 H, OCH3), 3.70 (m, 6 H, CH2N=C),
3.56 (s, 6 H, CArCH2), 2.77 (t, 3JH,H = 6.7 Hz, 6 H, 6H, CH2NMe),
2.28 (s, 9 H, NCH3), 1.45 (s, 27 H, tBu), 1.31 (s, 27 H, tBu) ppm.
13C{1H} NMR (125.75 MHz, CDCl3, 25 °C): δ = 166.4 (s, C=N),
160.0 (s, CArOMe), 158.3 (s, CArOH), 139.8 (s, CAr–tBu), 136.6 (s,
CAr–tBu), 126.7 (s, CArH), 125.8 (s, CArH), 122.1 (s,CArCH2), 118.0
(s, CArCH2), 63.3 (s, OCH3), 57.8 (s, CH2NMe), 57.4 (s, CH2N=C),
50.8 (s, CArCH2), 42.1 (s, NCH3), 35.1 (s, CMe3), 34.2 (s, CMe3),
31.6 [s, C(CH3)3], 29.5 [s, C(CH3)3] ppm. ESI-MS: m/z = 1075.7
Flack parameter: 0.036(19). Crystals of [(Me3talalentBu ){Cu -
(H2O)}3](ClO4)3·4H2O·1MeOH were removed from the mother
liquor and immediately cooled to 100(2) K on a Bruker X8 Pros-
pector Ultra diffractometer (three circle goniometer with 4K CCD
detector, Cu-Kα radiation, IμS microfocus tube, multilayer optics).
A total of 90422 reflections (3.11° Ͻ Θ Ͻ 69.96°) were collected, of
which 15149 reflections were unique [R(int) = 0.0437]. An empirical
absorption correction based on equivalent reflections was per-
formed with the program SADABS 2008/1.[45] The structure was
solved with the program SHELXS-97[46] and refined by using
SHELXL-97[33] to R = 0.0582 for 13557 reflections with I Ͼ 2σ(I),
R = 0.0640 for all reflections; the maximal and minimal residual
electron density is 1.086 and –1.126 eÅ–3, respectively.
II
2
[M + H]+, 1097.7 [M + Na]+. IR (KBr): ν = 2955 (s), 2909 (m),
˜
2868 (m), 2793 (m), 1636 (s), 1584 (m), 1460 (m), 1441 (s), 1412
(w), 1391 (w), 1362 (m), 1341 (w), 1275 (m), 1252 (m), 1202 (m),
1175 (m), 1130 (w), 1101 (m), 1036 (w), 999 (w), 970 (w), 878 (w),
827 (w), 773 (w), 729 (w), 644 (w) cm–1. Me3H3talalentBu2
(C66H102N6O6): calcd. C 73.70, H 9.56, N 7.81; found C 73.54, H
9.60, N 7.58.
During the early stages of structure completion one MeOH and
four H2O molecules were found in the asymmetric unit, but except
for 0.5 MeOH molecules, they did not refine properly, which was
due to severe disorder, and they were therefore removed from the
coordinate set. The resulting void was then treated with the
SQUEEZE routine to account for the now missing scattering
power. The removed molecules, however, are included in the given
formula and thus contribute to derived quantities.
tBu2
Me3H3talantBu : Me3H3talalen
(110 mg, 0.102 mmol) was dis-
2
solved in EtOH (10 mL) and cooled to 0 °C. To this solution,
NaBH4 (140 mg, 3.700 mmol) was added in small portions, and the
reaction mixture was warmed up to room temperature overnight.
The volume of the resulting colorless solution was reduced in vacuo
until a colorless solid precipitated, and then water (20 mL) was
added. After extraction with CH2Cl2, the organic layers were col-
lected, dried with Na2SO4, and the volatiles were evaporated in
CCDC-897942 contains 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.ac.uk/
data_request/cif.
Other Physical Measurements: Infrared spectra (400–4000 cm–1) of
solid samples were recorded with a Shimadzu FTIR-8400S spec-
vacuo, which yielded the ligand precursor Me3H3talantBu as a col-
2
orless solid (yield: 80 mg, 0.074 mmol, 73%). 1H NMR (500 MHz,
4
CDCl3, 25 °C): δ = 7.21 (d, JH,H = 2.0 Hz, 3 H, CArH), 6.81 (d,
4JH,H = 2.0 Hz, 3 H, CArH), 3.68 (s, 6 H, CH2NH), 3.80 (s, 9 H,
OCH3), 3.50 (s, 6 H, CH2NMe), 2.74 (m, 6 H, CH2NH), 2.59 (m, trometer and KBr-disk samples. UV/Vis/NIR absorption spectra of
Eur. J. Inorg. Chem. 2013, 388–397
395
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FULL PAPER
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trap mass spectrometer equipped with a standard ESI source. 1H
and 13C NMR spectra were measured either with a Bruker
DRX500 or a Bruker AV300 spectrometer by using the solvent as
an internal standard. The electrochemical experiments were per-
formed with Ar-flushed CH3CN solutions containing 0.1 m [NBu4]-
PF6 in a classical three-electrode cell. The working electrode was a
platinum electrode, the counter electrode was a platinum wire, and
the reference electrode was Ag/0.01 m AgNO3/CH3CN. All poten-
tials are referenced with respect to the ferrocenium/ferrocene (Fc+/
Fc) couple used as an internal standard. The electrochemical cell
was connected to an EG&G potentiostat–galvanostat (model 273
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sured by using a SQUID magnetometer (MPMS-7, Quantum De-
sign) at 1 T (0.2–300 K). To calculate the molar magnetic suscep-
tibilities, χm, the measured susceptibilities were corrected for the
underlying diamagnetism of the sample holder and the sample by
using tabulated Pascal constants. The JulX program package was
used for simulations of the spin Hamiltonian and for the fitting of
the data by a full-matrix diagonalization approach.[47]
[6]
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