Synthesis, redox, and amphiphilic properties of responsive salycilaldimine-copper(II) soft materials

Article abstract of DOI:10.1021/ic702233n

Hydrolysis of the asymmetric pyridine- and phenol-containing ligand HL ¹ (2-hydroxy-4-6-di-tert-butylbenzyl-2-pyridylmethyl)imine) led to the use of bis-(3,5-di-tert-butyl-2-phenolato-benzaldehyde)copper(II), [Cu ^II(L^SAL)₂] (1) as a precursor for bis-(2,4-di-tert-butyl-6-octadecyliminomethyl-phenolato)copper(II), [Cu ^II(L²)₂] (3), bis-(2,4-di-tert-butyl-6- octadecyl aminomethyl-phenolato)copper(II), [Cu^II(L ^2A)₂] (3′), and bis-(2,4-di-tert-butyl-6-[(3,4,5- tris-dodecyloxy-phenylimino)-methyl]-phenolato)copper(II), [Cu ^II(L³)₂] (4). These complexes exhibit hydrophilic copper-containing head groups, hydrophobic alkyl and alkoxo tails, and present potential as precursors for redox-responsive Langmuir-Blodgett films. All systems were characterized by means of elemental, spectrometric, spectroscopic, and electrochemical techniques, and their amphiphilic properties were probed by means of compression isotherms and Brewster angle microscopy. Good redox activity was observed for 3 with two phenoxyl radical processes between 0.5 and 0.8 V vs Fc⁺/Fc, but this complex lacks amphiphilic behavior. To attain good balance between redox response and amphiphilicity, increased core flexibility in 3′ and incorporation of alkoxy chains in 4 were attempted. Film formation with collapse at 14 mN·m^-1 was observed for the alkoxy-derivative but redox-response was seriously compromised. Core flexibility improved Langmuir film formation with a higher formal collapse and showed excellent cyclability of the ligand-based processes.

Full text of DOI:10.1021/ic702233n

Inorg. Chem. 2008, 47, 3119-3127
Synthesis, Redox, and Amphiphilic Properties of Responsive
Salycilaldimine-Copper(II) Soft Materials
Sarmad Sahiel Hindo,† Rajendra Shakya,† N. S. Rannulu,† Marco M. Allard,† Mary Jane Heeg,†
M. T. Rodgers,† Sandro R. P. da Rocha,‡ and Claudio N. Verani*,†
Department of Chemistry and Department of Chemical Engineering, Wayne State UniVersity,
Detroit, Michigan 48202
Received November 13, 2007
Hydrolysis of the asymmetric pyridine- and phenol-containing ligand HL1 (2-hydroxy-4-6-di-tert-butylbenzyl-2-
pyridylmethyl)imine) led to the use of bis-(3,5-di-tert-butyl-2-phenolato-benzaldehyde)copper(II), [CuII(LSAL)2] (1) as
a precursor for bis-(2,4-di-tert-butyl-6-octadecyliminomethyl-phenolato)copper(II), [CuII(L2)2] (3), bis-(2,4-di-tert-butyl-
6-octadecyl aminomethyl-phenolato)copper(II), [CuII(L2A)2] (3′), and bis-(2,4-di-tert-butyl-6-[(3,4,5-tris-dodecyloxy-
phenylimino)-methyl]-phenolato)copper(II), [CuII(L3)2] (4). These complexes exhibit hydrophilic copper-containing head
groups, hydrophobic alkyl and alkoxo tails, and present potential as precursors for redox-responsive Langmuir–Blodgett
films. All systems were characterized by means of elemental, spectrometric, spectroscopic, and electrochemical
techniques, and their amphiphilic properties were probed by means of compression isotherms and Brewster angle
microscopy. Good redox activity was observed for 3 with two phenoxyl radical processes between 0.5 and 0.8 V
vs Fc+/Fc, but this complex lacks amphiphilic behavior. To attain good balance between redox response and
amphiphilicity, increased core flexibility in 3′ and incorporation of alkoxy chains in 4 were attempted. Film formation
with collapse at 14 mN · m-1 was observed for the alkoxy-derivative but redox-response was seriously compromised.
Core flexibility improved Langmuir film formation with a higher formal collapse and showed excellent cyclability of
the ligand-based processes.
Introduction
of such materials to the growing field of molecular electron-
ics3 is increasing considerably because it has been demon-
strated that self-assembled films of triple-decker porphyrin-
lanthanide precursors can be used for information storage.4
In spite of the large footprint, these systems use multiple
oxidations for writing of information, whereas the subsequent
reductions are used for reading of stored data.5
Consequently, impending relevance can be envisioned for
organized stimulus-responsive Langmuir–Blodgett films com-
posed of monometallic precursors that present multiple redox
states and smaller footprints. The stabilization of organic
radicals in such precursors will play a vital role in attaining
Metal-containing soft materials merge the intrinsic proper-
ties of transition metals with those of functional organic
scaffolds to build up organized architectures.1,2 The relevance
* To whom correspondence should be addressed. E-mail: cnverani@
chem.wayne.edu. Phone: 313 577 1076. Fax: 313 577 8022.
†
Department of Chemistry.
Department of Chemical Engineering.
‡
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10.1021/ic702233n CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 8, 2008 3119
Published on Web 03/13/2008

Hindo et al.
Scheme 1. Complexes and Their tert-Butyl Substituted Ligands
redox states beyond those expected for the metallic ion.
Ligands based on tert-butyl substituted phenolates6,7 ensure
the required phenolate/phenoxyl bistability8 needed to render
their complexes as viable prototypical molecular switches
that enable multiple read/write cycles.
Copper(II) salicylaldehydes have been used as synthetic
precursors in the self-assembly of catalysts,9,10 in smectic
mesogens to avoid bimetallic cores,11 as well as in modified
electrodes.12 The seminal work of Nagel et al.,13 entailing
the condensation of hexadecylamine with 4-hydroxysalicy-
laldehyde applied this concept to metallosurfactants while
pointing to the importance of polar substituents attached to
the headgroup to achieve organized monolayers. Therefore,
the main challenge in the development of precursors based
on tert-butyl substituted phenolates is the fact that while these
groups can enhance the redox properties, their bulkiness and
apolar nature may compromise the amphiphilic behavior of
the resulting materials.
balance between redox and amphiphilic behavior observed
in these species and offers a rationale for improving these
properties, the advantages, and the limitations of this
approach. These findings are expected to have a positive
impact in the future design of responsive films.
We have developed and studied iron(III)- and cobalt(II/
III)-containing soft materials based on the asymmetric
pyridine and phenol pendant-arms ligand HL1, depicted in
its deprotonated form in Scheme 1.14 Owing to this interest,
the synthesis and characterization of copper(II) complexes
was attempted, but ligand hydrolysis yielded [CuII(LSAL)2]
(1) where LSAL- is the deprotonated 2,4-tert-butylsalycilal-
Results and Discussion
Ligand Hydrolysis. Treatment of HL1 with hydrated
copper(II) perchlorate in methanol produced [CuII(L1)2] (2)
in poor yields. The electrospray ionization (ESI) mass
spectrum (positive ion mode) of the isolated compound
presented m/z ) 710 for [2 + H+]+. However, the spectrum
of the reaction mixture contained peaks at m/z ) 530 and
655 suggesting the presence of additional products along with
2. In wet methanol, the peak at m/z ) 530 became dominant
and was identified as related to the main hydrolysis product
[CuII(LSAL)2] (1) as [1 + H+]+. Slow evaporation of the
dehyde as the main product and [(LSAL)2CuII (µ-OCH3)2] (5)
2
as a minor component. Because free copper(II) ions mediate
ligand hydrolysis,15 1 was used as a precursor for the
formation of [CuII(L1)2] (2), as well as for the metallosur-
factants [CuII(L2)2] (3) and [CuII(L3)2] (4). Complex 3 allows
for in situ reduction yielding the equivalent and more flexible
amine complex [CuII(L2A)2] (3′). Species 3, 3′, and 4 are can-
didates as amphiphilic precursors for redox-responsive Lang-
muir–Blodgett films. This paper describes the delicate
solvent yielded a few crystals of [(LSAL)2CuII (µCH3O)2] (5)
2
that presented m/z ) 655 for [5 + H+]+. Infrared spectros-
copy of the isolated solids revealed strong peaks at 1619
and 1424 cm-1, attributed to the stretching and bending
modes of carbonyl groups, respectively. Precursor 1 was later
obtained synthetically in gram amounts and yielded X-ray
quality crystals.
X-ray Structures. Both 1 and 5 were analyzed by single
crystal diffraction methods and their molecular structures are
displayed in Figure 1. Complex 1 shows two deprotonated
ligands coordinated to a copper(II) center in a square-planar
environment where the O-Cu-O angles deviate slightly
from the expected 90.0°. The ligands are positioned trans to
one another, and the molecule adopts an approximate D2h
local symmetry. The bond lengths of the Cu-O4 coordination
sphere are in good agreement with similar species reported
in the literature,16 where the Cu-Ocarbonyl bonds are longer
than the Cu-Ophenolate bonds. The entire molecule minus the
tert-butyl substituents is planar (mean deviation 0.02 Å).
There are no close axial contacts nor ligands to Cu and no
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1997, 13, 4693.
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Hratchian, H. P.; McGarvey, B. R.; da Rocha, S.; Verani, C. N. Inorg.
Chem. 2007, 46, 9808. (b) Shakya, R.; Imbert, C.; Hratchian, H. P.;
Lanznaster, M.; Heeg, M. J.; McGarvey, B. R.; Allard, M.; Schlegel,
H. B.; Verani, C. N. Dalton Trans. 2006, 2517. (c) Imbert, C.;
Hratchian, H. P.; Lanznaster, M.; Heeg, M. J.; Hryhorczuk, L. M.;
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Abdul Halim, S. N.; Ng, S. W. Acta Crystallogr. 2005, E61, 1429.
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78, 2688.
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ResponsiWe Salycilaldimine-Copper(II) Soft Materials
Scheme 2. Structures for 3, 3′, and 4 and Their ESI(Pos) Peak
a
Clusters for [M + H+]+
a
The relative abundance axis of each complex is omitted for clarity.
CdN stretch of imine groups. The absence of strong peaks
around 1090 cm-1 for all isolated complexes indicates lack
of perchlorate counterions and supports the presence of two
coordinated phenolates in a neutral molecule. To test the
effect of a decreased framework rigidity, reduction of the
imine sCdNs bonds was attempted by treating 3 and 4
with sodium borohydride and aiming at the equivalent but
more flexible amine sHCsNHs counterparts. These at-
tempts failed for 4 but worked well for the octadecylamine-
substituted ligand in 3 converting it to 3′ in quantitave yields.
The identity of the compound was confirmed by infrared
spectroscopy and mass spectrometry: disappearance of the
CdN stretch peak along with a corresponding 4 mass-unit
increase, related to the addition of two hydrogen atoms per
ligand, was observed. The elemental analyses of 3, 3′, and
4 are in excellent agreement with these formulations. block
Electronic Spectra. The electronic spectra of 1, 3, and 4
were measured in dichloromethane as a characterization tool
for the new amphiphiles. Figure 2 displays intense bands
observed in the ultraviolet and early visible range, whereas
much less intense d-d bands (not shown) are observed
between 600 and 1000 nm. The bands around 250–280 and
310–325 nm are attributed to intraligand σ f π* and/or π
f π* transitions. The nature of the bands in the 380–420
nm range has been previously assigned to intraligand
processes, metal-to-ligand, and ligand-to-metal charge trans-
fers.18–21 These bands are assigned as having a predominant
ligand-to-metal charge transfer character. The presence of
Nimine f CuII and Ophenolate f CuII charge transfers is sug-
gested because of the presence of high extinction coefficients.
Figure 1. ORTEP diagrams for 1 and 5. Selected bond lengths (Å) and
angles (°) for 1: Cu(1)-O(2) ) 1.876; Cu(1)-O(1) ) 1.952; C(1)-O(1)
)1.254;C(7)-O(2))1.305,O(2)-Cu(1)-O(2))180.00;O(1)-Cu(1)-O(1)
) 180.00; O(2)-Cu(1)-O(1) ) 92.83; O(2)-Cu(1)-O(1) ) 87.17. For
5: Cu(1)-Cu(1) ) 2.996; Cu(1)-O(1) ) 1.894; Cu(1)-O(3) ) 1.895;
Cu(1)-O(4) ) 1.914; Cu(1)-O(2) ) 1.933; C(1)-O(1) ) 1.301;
C(7)-O(2) ) 1.253; Cu(1)-O(4)-Cu(1) ) 103.02; O(3)-Cu(1)-O(4)
) 76.25; O(1)-Cu(1)-O(3) ) 167.51; O(4)-Cu(1)-O(2) ) 166.41;
O(1)-Cu(1)-O(4))96.80;O(1)-Cu(1)-O(2))94.40;O(3)-Cu(1)-O(2)
) 94.03.
ring stacking nor counterions are present. The binuclear
complex 5 shows two copper(II) ions doubly bridged by two
deprotonated methoxide groups. Each copper center is also
coordinated to a 3,5-di-tert-butyl-2-hydroxybenzaldehyde
ligand in distorted square planar geometries. Interestingly,
the molecule presents idealized C2V symmetry with the two
ligands positioned cis to one another, thus suggesting that
dimer formation may precede hydrolysis. Bond lengths and
angles are in good agreement with similar structures.17
Because few crystals of this dimer were generated, no further
investigations were carried out.
Syntheses. The formation of 1 as the main product of
imine hydrolysis has driven a series of reactivity studies in
which several amine-containing substrates were tested. When
1 was treated with 2-aminomethylpyridine, the initially
targeted 2 was obtained, as indicated by the peak at m/z )
710 in the ESI mass spectrum corresponding to [2 + H+]+
with the proper isotopic distribution. When 2-aminometh-
ylpyridine was replaced by 1-octadecylamine and 1,2,3-
tris(dodecyloxy)-5-methylbenzene, the amphiphiles 3 and 4
were obtained as shown in Scheme 2. These species were
isolated and carefully investigated by several spectroscopic
methods. The most remarkable feature in the infrared
spectrum of 1 is associated with the stretch of the carbonyl
groups (VCdO) at 1619 cm-1. This peak disappears for 2, 3,
and 4 and a new peak is observed around 1588 cm-1 for the
(18) (a) Rigamonti, L.; Demartin, F.; Forni, A.; Righetto, S.; Pasini, A.
Inorg. Chem. 2006, 45, 10976. (b) Iglesias, A. L.; Aguirre, G.;
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Inorg. Chem. 1984, 23, 3124.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3121

Hindo et al.
Figure 3. Cyclic voltammetry of 1.0 × 10-3 M CH2Cl2 solutions of 1, 3,
3′, and 4: First and second ligand-centered processes at 0.82 and 0.98 for
1; 0.54 and 0.83 for 3; 0.29 and 0.48 for 3′; and 0.53 and 0.80 V vs Fc+/Fc
for 4. The amplitude of the current for each compound is omitted for
clarity.
Figure 2. UV–visible spectra for 1, 3, 3′, and 4 in 1.0 × 10-4
M
dichloromethane solutions. λmax, nm (ε, L·mol-1 ·cm-1) for 1: 282 (15,600),
325 (9500), 403 (7700); for 3 276 (27,200), 325 (13,300), 380 (12,500);
for 3′ 229 (18,300), 301 (13,100), 330sh (∼4000), 448 (1900); for 4 250
(∼39,000), 310 (24,500), 420 (16,700). The d-d bands are found at 692
(170) for 1, 662 (260) for 3, 680 (280) for 3′, and 725 (350) for 4.
reaching up to 0.50 V. This is not unexpected for the Cu(II)/
Cu(I) couple because of the considerable geometrical reor-
ganization and charge balance involved in the process. A
preferred tetrahedral geometry for the reduced ion is
anticipated, and the presence of two phenolate ligands confer
a negatively charged core. Because the 3d10 configuration
of Cu(I) increases lability, one of the phenolate ligands might
have its bond weakened or broken to decrease the electronic
density around the metal.6b,24,25 If metal reduction is pre-
vented, one can focus on the most important features
associated with the ligand-based processes. Two reversible
or quasi-reversible processes are expected in these systems
and associated with the oxidation of the tert-butyl phenolate
species into the corresponding phenoxyl radicals. The radical
nature of these species was also observed by spectrophoto-
metric experiments, where addition of (NH4)2[CeIV(NO3)6]26
to dichloromethane solutions of 3 and 3′ at room temperature
yielded a new band respectively at 390 nm (ε ∼ 4500
L · mol-1 · cm-1) and 440 nm (ε ∼ 7000 L · mol-1 · cm-1)
(Supporting Information Figure S1). The considerable dif-
ference in band position is in good agreement with previously
reported values for copper-phenoxyl species conjugated to
imine27 and amine28 groups and therefore similar to 3 and 3′.
As can be seen in Figure 3, compound 1 shows limited
reversibility with processes at 0.82 and 0.98 V vs Fc+/Fc.
The peak separation for the first process reaches 0.15 V and
increases further to 0.16 V for the second process, rendering
The position of this band in 3 is in good agreement with
other alkyl-substituted ligands in similar solvents,22 but its
intensity is considerably decreased in 3′ where the aldimine
chromophore has been reduced. Also noteworthy is the fact
that when comparing 1 and 3, a hypsochromic (blue) shift
of ∼20 nm is observed, but comparison between 1 and 4
reveals a ∼20 nm bathochromic (red) shift. Because all of
these species have tertiary butyl groups attached to the second
and fourth positions of the phenolate rings, the nature of the
groups in the sixth position must be considered, namely
carbaldehyde for 1, octadecyliminomethyl for 3 and 3′, and
tris(dodecyloxy)phenyl-iminomethyl for 4. Assuming some
degree of distortion from square planar to tetrahedral23 in 3
and 4, the hypsochromic effect is attributed to the free motion
of these alkyl groups that leads to distortions on the
iminomethyl moiety diminishing the extent of the π-delo-
calization. The bulky nature of the phenyl groups in 4, as
well as the presence of the trialkoxy auxochromes, is
suggested to stabilize π-delocalization.
Redox Properties. Cyclic voltammograms in dichloro-
methane solutions of 1, 3, 3′, and 4 were measured to assess
the redox potentials of ligand-centered processes. All po-
tentials are reported at 100 mV · s-1 and versus the Fc+/Fc
couple, unless noted otherwise. A study was also performed
to evaluate the reversibility of the phenolate/phenoxyl couples
in 3 and 3′ by cycling 50 times through the wave associated
with the first and second ligand-centered process. This
cycling is intended to simulate switching mechanisms that
will play a key role in responsive films. These results are
shown in Figure 3. For all compounds the metal-centered
reduction is observed between -1.60 and -1.80 V, whereas
the oxidation appears around -0.90 and -1.20 V. All
processes are irreversible with peak separations (∆Ep)
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3122 Inorganic Chemistry, Vol. 47, No. 8, 2008

ResponsiWe Salycilaldimine-Copper(II) Soft Materials
Scheme 3
them irreversible. The |Ipc/Ipa| peak ratio cannot be calcu-
lated accurately, but visual inspection suggests that the
intensity of the anodic peak is greater than that of the
cathodic peak indicating that |Ipc/Ipa| < 1. It implies that a
smaller concentration of the phenolate species is present at
the surface of the electrode in the time scale of the
experiment because of either depletion or chemical trans-
formation. Two well-shaped processes are observed for 3 at
0.53 and 0.83 V versus Fc+/Fc. These seem to be quasi-
reversible because ∆Ep reaches 0.10 V at 100 mV · s-1,
slightly larger than those values observed experimentally for
the Fc+/Fc couple under identical conditions. The quasi-
reversibility is supported by an increase in the peak separation
at higher scan rates with |Ipc/Ipa| values ranging from 1.27
at 100 mV · s-1 to 1.48 at 300 mV · s-1 for the first redox
process. The insertion of bulky groups intended to enhance
the amphiphilic properties in 4 appears to compromise the
redox reversibility of the compound. The first ligand-centered
peak is comparable to that of 3, but a peak separation ∆Ep
of 0.14 V (|Ipc/Ipa| ) 1.65), thus comparable to that of 1,
renders it irreversible. This irreversibility is increased in the
second process. It is possible that the presence of tris(dode-
cyloxy)phenyl substituents increases the distance between
the redox-active core of the molecule and the electrode
surface, decreasing the rate of interfacial electron transfer,
as seen in dendrimeric porphyrins.29 From the point of view
of redox reversibility, 3′ can be considered the best system.
The ligand-centered processes shift to more positive poten-
tials, namely 0.29 and 0.48 V versus Fc+/Fc with separation
peaks ∆Ep ) 0.09 V and |Ipc/Ipa| ) 1.21, thus well within
the limits observed for the Fc+/Fc couple.
The switch-like activity is fundamental for potential uses
in information storage and appears to depend heavily on the
geometry adopted by the central atom. Octahedral systems
tend to show rather limited switch-like cyclability, whereas
five-coordinate systems6 display reversible processes at 0.63
and 0.81 V vs Fc+/Fc with minor decay after 20–50 cycles.
To the best of our knowledge, the cyclability of square planar
copper-phenolate systems is not known. On the basis of the
data presented above, 3 and 3′ display good reversibility in
the ligand-centered processes. Experiments were attempted
cycling fifty times the two waves in a switch-like process.
In spite of the initial results with 3, a shift of up to 0.17 V
can be detected between the first and the fiftieth cycle
(Supporting Information Figure S2). The observed shift
suggests a small but constant decay that might limit the use
of this compound in responsive films. An excellent result
was achieved with 3′ in which no significant changes (0.04
V) were detected after 50 cycles, thus indicating that
decomposition of the generated species in the time scale of
the voltammetry experiment did not take place. These results
suggest that cycling of the phenoxyl species is viable in a
flexible four-coordinate geometry and in noncoordinating
solvents. Pending characterization of the amphiphilic proper-
ties of these two compounds, formation and study of
Langmuir–Blodgett films of 3 and 3′ will be pursued.
Electronic Structure Calculations. A series of compu-
tational calculations were run to correlate the experimental
data observed for the ligand-centered redox properties of
complexes 1, 3, 3′, and 4 with their electronic nature. The
B3LYP/6–311+G(d) level of theory30 was employed to
handle negatively charged species using the GAUSSIAN
suite.31 Geometries were fully minimized, without symmetry
constraints, using standard methods.32 It is assumed that all
phenolate f phenoxyl processes described above will depend
solely or mainly upon the relative energies of the HOMO
orbitals of the ligand, from where electrons are withdrawn
in an oxidative process, and that the higher the energy of
the orbital, the more energetically affordable is the oxidation.
Therefore, the models are nonmetallated and simplified
versions of the deprotonated phenolate ligands as shown in
Scheme 3. Considering that 1 shows the more positive
potential, the HOMO orbital of the equivalent model system
should be the lowest in energy and will display the lowest
comparative energy. The model for 3 (also related to 4) yields
a higher comparative energy and that of 3′ exhibits the
highest energy—therefore the most affordable oxidation
process—in excellent agreement with the observed experi-
mental trends. For 1 and 3, π-delocalization seems to play
an important role. One can conclude that energetically
favorable oxidations can be achieved by replacing the sCdO
by sCdNR groups attached to the phenolate ring. Similarly,
(30) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Becke, A. D.
J. Chem. Phys. 1993, 98, 5648. (c) Lee, T.; Yang, W. T.; Parr, R. G.
Phys. ReV. B 1988, 37, 785. (d) Ditchfield, R.; Hehre, W. R.; Pople,
J. A. J. Chem. Phys. 1971, 54, 724. (e) Gordon, M. S. Chem. Phys.
Lett. 1980, 76, 163. (f) Hariharan, P. C.; Pople, J. A. Theo. Chim.
Acta. 1973, 28, 213. (g) Hariharan, P. C.; Pople, J. A. Mol. Phys.
1974, 27, 209. (h) Hehre, W. R.; Ditchfield, R.; Pople, J. A. J. Chem.
Phys. 1972, 56, 225.
(31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. GAUSSIAN 03;
Gaussian, Inc.: Wallingford, CT, 2003.
(29) Pollak, K. W.; Leon, J. W.; Frechet, J. M. J.; Maskus, M.; Abruna,
H. D. Chem. Mater. 1998, 10, 30.
(32) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3123

Hindo et al.
areas as small as 15 Å2 · molecule-1 and pressures have no
clear physical meaning. The presence of tert-butyl groups
attached to the headgroup of 3 is key to the observed redox
behavior of the compound but precludes the formation of
organized films. Moreover, careful analysis of the disposition
of the salicylaldehyde groups in 1, as revealed by X-ray
diffraction, suggests that the octadecylamine chains in 3 (and
4) will be oriented trans to each other in a rigid imine-like
structure. Exploratory BAM at several surface pressures
distinctively shows the presence of oval-shaped do-
mains (Supporting Information Figure S3) that resemble
vesicles1c,35,36 along with concentric domains, generally
referred to as Newton rings.37 These rings are multilayer
granules formed from the ejection of matter from the
compressed monolayer when localized oscillations are
present. Thus, in addition to the headgroups, the rigidity of
the framework appears to prevent this molecule from
exhibiting amphiphilic properties.
Amphiphilic activity depends on an optimal balance
between the hydrophobic and hydrophilic properties of a
given surfactant. As such, ways of improvement were
considered in the design of surfactants based on tert-
butylsalicyl-copper head groups. Two possible approaches
were examined, involving a decrease of the framework
rigidity or an increase in the number of alkyl chains present.
Imine reduction provided the amine surfactant 3′ with a more
flexible framework, whereas 4 was used to increase the
number of alkyl, or more formally alkoxy, chains present.
Reduced 3′ was initially dissolved in dichloromethane and
subsequently spread on the water surface. As the barriers of
the trough were compressed, the tension (γ) of the air–water
interface in the presence of the amphiphilic species decreased
as compared to that of the bare air–water interface (γ0 ) 72
mN · m-1 at 23 °C), resulting in an increase in Π () γ0 -
γ). Indeed, the isotherm (red trace) shown in Figure 4
exhibits considerable surface activity. Individual molecules
begin interacting at very large areas of 220 Å2, grow into a
plateau-like region at low pressures, and increase consistently
up to about 22 mN ·m-1, when an inflection takes place. The
isotherm of 4 is also shown in Figure 4 and reveals this
species as interfacially active. No inflections indicative of
phase transitions are observed, and a moderate pressure of
14 mN · m-1 is reached.
Figure 4. Compression isotherms for 3′ (black trace) and 4 (red trace).
it can be suggested that the nature of the group R attached
to the Schiff base will have some impact in the electronic
delocalization. This was observed for protonated models in
which R ) sCH3 or saryl, but the minimization of the
deprotonated 4 seemed dependent on the orientation of that
additional aryl ring. Interestingly, 3′ presents a localized
π-frame limited to the aromatic phenolate ring. It can be
concluded that disruption of the delocalization fosters
energetically affordable oxidations and should be used as a
guideline in the design of future amphiphilic precursors.
Amphiphilic Properties. The synthesis of 3 and 4 targets
the attachment of nonpolar amine-containing organic frag-
ments to the tert-butyl-containing precursor 1 by Schiff base
condensation. These organic fragments have played an
important role in the redox properties of these compounds
and are expected to confer amphiphilic properties acting as
hydrophobic counterparts of the copper-containing head-
group. In-situ reduction of 3 yielded 3′. The amphiphilic
properties of 3, 3′, and 4 were studied by means of
compression isotherms plotting surface pressure (Π, mN/
m) versus area per molecule (A, Å2) and Brewster angle
microscopy (BAM). The compression isotherms give infor-
mation about the 2D behavior of the resulting Langmuir film
at the air/water interface. The compression isotherms indicate
the presence of mono or multilayers, collapse pressures, and
average areas per molecule. BAM uses polarized light
passing through media with dissimilar refractive indexes and
is the most powerful method to identify structures such as
agglomerates and domains in films at the air/water interface.
Recent studies on the C16-containing copper surfactants
described by Nagel et al.13 have shown that longer octadecyl
(C18) chains tend to lead to an increase of collapse pres-
sures.33 Similarly, it has been demonstrated that replacement
of the hydroxy groups by methoxy groups34 yields complex
behavior at the air/water interface. Therefore, considering
the presence of bulky tert-butyl groups on 3, longer octadecyl
chains were employed. Nonetheless, the isotherm of 3 was
marked by an erratic profile (Figure 4, inset) that confirms
lack of organization and suggests random molecular ag-
gregation. Loss of matter to the subphase was supported by
Both approaches improve surface activity suggesting that
a better equilibrium between hydrophilic and hydrophobic
properties can be obtained in the design of redox-active
surfactants. Interestingly, for both cases the compression
barriers of the trough touch each other without the event of
formal constant-area collapse characterized by a sudden
decrease in the surface pressure.38 We have suggested that
metallosurfactants can exibit distinct collapse mechanisms,14a
(35) Dominguez-Gutierrez, D.; Surtchev, M.; Eiser, E.; Elsevier, C. J. Nano
Lett. 2006, 06, 145.
(36) Svenson, S. Curr. Opin. Colloid Interface Sci. 2004, 9, 201.
(37) Galvan-Miyoshi, J.; Ramos, S.; Ruiz-Garcia, J.; Castillo, R. J. Chem.
Phys. 2001, 115, 8178.
(33) Pang, S.; Li, C.; Huang, J.; Liang, Y. Colloids Surf. 2001, A178, 143.
(34) Hemakanthi, G.; Unni Nair, B.; Dhathathreyan, A. Chem. Phys. Lett.
2001, 341, 407.
(38) (a) Vaknin, D.; Bu, W.; Satija, S. K.; Travesset, A. Langmuir 2007,
23, 1888. (b) Ybert, C.; Lu, W.; Moeller, G.; Knobler, C. M. J. Phys.:
Condens. Matter 2002, 14, 4753.
3124 Inorganic Chemistry, Vol. 47, No. 8, 2008

ResponsiWe Salycilaldimine-Copper(II) Soft Materials
interface, promoting a better equilibrium between the hy-
drophobic and hydrophilic portions of 4 and allowing for
film formation. Because repulsion forces are also present, it
is unlikely that all six chains can be aligned, accounting for
the formation of multilayers.
Summary and Conclusions
In this Article we have used precursor 1 to develop a series
of amphiphilic and redox-responsive complexes as candidates
for the development of responsive films. We have demon-
strated that the design of systems exhibiting a good balance
between amphiphilicity and redox response is far from trivial.
In 3 an apparent redox reversibility is obfuscated by a
complete lack of amphiphilic behavior that prevents orga-
nization and ultimately the formation of films. The inclusion
of three alkoxy groups per ligand in 4 appears to be key to
the enhancement of the amphiphilic properties and to an
improved response in the formation of relatively organized
Langmuir films that show formal collapse at about 14
mN · m-1. Nonetheless, the redox-response was seriously
compromised by an increased separation between cathodic
and anodic peaks in the first phenolate/phenoxyl process. This
behavior is a signature for a quasi-reversible process that is
unlikely to withstand switch-like activities and is related to
the nature and the bulkiness of the group attached to the
imine function. Finally, an increased flexibility of the core
was attained for 3′ by an in situ reduction of the CdN groups
present in 3. Improved amphiphilicity and consequent
Langmuir film formation with a higher formal collapse at
27 mN ·m-1 were observed. Moreover, excellent reversibility
of the ligand-based processes and marginal decay upon
switch-like cycling were also observed. The reversibility and
cyclability seem to be associated with the disruption of the
electronic delocalization between the phenolate and the CdN
group and will be incorporated in the design of amphiphilic
precursors for future responsive films.
On the basis of these results, it can be concluded that an
increased flexibility of the core and electronic localization
are fundamental to achieving a good balance between the
desired redox and the amphiphilic properties. The replace-
ment of the tert-butyl groups by other substituents that
simultaneously foster phenolate/phenoxyl cycling and en-
hance the polarity of the headgroups—thus the amphiphilic
behavior of the precursors—would be advantageous to the
design of desired responsive Langmuir–Blodgett films.
Recent advances14b suggest that chloro-groups might be able
to deliver such properties. These results will pave the way
to a new series of amine surfactants based on chloro-
substituted phenolates coordinated to bivalent copper, nickel,
and cadmium. The use of the latter cations should improve
the thermodynamic stability of the resulting complexes
leaving the ligand-centered redox activity unaltered. These
topics are currently under development in our laboratories.
Figure 5. Brewster angle micrographs. For 3′: (a) before compression,
(b) below 2 mN ·m-1, (c) at 6 mN ·m-1, (d) at 21 mN ·m-1. For 4: (e)
before compression, (f) at 3 mN ·m-1, (g) at 5 mN ·m-1, and (h) at 14
mN·m-1
.
and this fact led to the consideration that, in spite of their
shape, the isotherms describe the formation of multilayers.
BAM was used to monitor the compression of 3′ and 4 in
an attempt to investigate the nature of the resulting films.
Figure 5 shows a series of selected micrographs gathered
along the compression process for each of the amphiphiles.
Multiple domains are present before compression of 3′.
Before the first inflection in the isotherm, as well as at low
surface pressures, an apparently homogeneous film is ob-
tained. After this inflection the presence of Newton rings
suggests instability of the film. At 20–21 mN · m-1 the film
ceases to behave as a monolayer. Before compression of 4,
a film composed of several bidimensional oval-shaped
domains is present. At about 3 mN ·m-1 these domains seem
to be replaced by a homogeneous monolayer, but the
presence of several Newton rings suggest considerable
instability. At 5 mN · m-1, rings and oval-shaped domains
similar to those observed for 3 are present. Comparison of
the domains present at 3 and 5 mN · m-1 suggest that only
the former are flat, reinforcing the notion of a vesicular
nature. Future dynamic light scattering experiments would
be necessary to confirm this hypothesis. From 6 to 14
mN · m-1, a considerable increase in the roughness of the
surface and the appearance of multiple rings takes place; this
is interpreted as indicative of multilayers. Similar to the
octadecyl chains, 4 has a rigid structure with trans-oriented
trialkoxy groups. However, the three oxidodecyl (sOC12H26)
chains present per amine can rotate almost freely. Contrary
to 3, these chains can point outward from the air/water
Experimental Section
Materials and Methods. All the reagents were obtained from
commercial sources and were used without further purification.
Dichloromethane was purified using an I.T. solvent purification
Inorganic Chemistry, Vol. 47, No. 8, 2008 3125

Hindo et al.
Table 1. Crystal Data for Complexes 1 and 5a
a resistivity of 17.5–18 MΩ·cm-1. Adventitious impurities at the
surface of freshly poured aqueous subphases were removed by
vacuum after barrier compression. Spreading of a known quantity
(∼30 µL) of 1.0 mg ·mL-1 chloroform solutions on the aqueous
subphase was followed by about 20 min equilibrium time before
monolayer compression. The isotherms were obtained at a compres-
sion rate of 5 mm ·min-1. The pressure was measured using the
Wilhelmy plate method (platinum plate, 40 mm perimeter). At least
three independent measurements were carried out per sample with
excellent reproducibility.
Brewster Angle Microscopy. A KSV-Optrel BAM 300 with a
HeNe laser (10 mW, 632.8 nm) and a CCD detector was used in
all micrographs. The compression rate was 5 mm/min, the field of
view was 800 × 600 µm, and the lateral resolution was about 2
µm.
complex
1
5
empirical formula
fw
C30H42Cu O4
530.18
C32H48Cu2 O6
655.78
100(2) K
temperature
wavelength
100(2) K
0.71073 Å
0.71073 Å
crystal system, space group orthorhombic, Pbca monoclinic, C2/c
unit cell dimensions
a ) 7.8735(5) Å
a ) 30.5691(9) Å
b ) 16.6446(11) Å b ) 9.5625(3) Å
c ) 20.9754(15) Å c ) 10.9862(3) Å
ꢀ ) 90.632(3) deg
volume
2748.9(3) Å3
4, 1.281 Mg/m3
0.827 mm-1
3211.26(16) Å3
4, 1.356 Mg/m3
1.364 mm-1
R1 ) 0.0506,
wR2 ) 0.1254
R1 ) 0.0729,
wR2 ) 0.1301
Z, calculated density
absorption coefficient
final R indices [I > 2σ(I)]
R1 ) 0.0428,
wR2 ) 0.1079
R1 ) 0.0764,
wR2 ) 0.1201
R indices (all data)
Syntheses. Caution! Perchlorate salts are potentially explosiVe.
Although the final products do not contain perchlorates, proper
precautions should be taken for the synthesis.
a R(F) ) Σ|Fo| - |Fc|/Σ|Fo|; wR(F) ) [Σw(Fo2 - Fc )2/Σw(Fo )2]1/2 for
I > 2σ(I).
2
2
[Cu(LSAL)2] (1). Bis-(3,5-di-tert-butyl-2-phenolato-benzalde-
hyde)copper(II). To a stirring solution of 3,5-di-tert-butyl-2-
hydroxy-benzaldehyde (0.515 g, 2.2 mmol) and 1.5 eq. triethyl-
amine in 20 mL methanol was added a methanol solution (10 mL)
of Cu(ClO4)2 ·6H2O (0.375 g, 1 mmol), dropwise. After warming
gently for 30 min, a brown microcrystalline solid was isolated by
filtration. The precipitate was washed with 3 × 20 mL portions of
cold methanol and 2 × 10 mL portions of cold ether and then dried
under vacuum. The crude product was crystallized from 40 mL of
methanol. After 24 h, a brown microcrystalline solid was isolated
Yield ) 95%. Anal. Calcd for [C30H42O4Cu]: C, 67.96; H, 7.98.
Found C, 67.77; H, 8.16%. IR (KBr, cm-1) 1333 (VCsO), 1619
(VCdO), 2868–957 (VCsH from tert-butyl groups). ESI pos. in MeOH:
m/z ) 530.1 for [1 + H+]+.
system. IR spectra were measured from 4000 to 400 cm-1 as KBr
pellets on a Tensor 27 FTIR-spectrophotometer. ESI spectra were
measured on a Micromass QuattroLC triple quadrupole mass
spectrometer with an electrospray/APCI source and Walters Alliance
2695 LC, autosampler and photodiode array UV detector. Experi-
mental assignments were simulated based on peak position and
isotopic distributions. Elemental analyses were performed by
Midwest Microlab, Indianapolis, Indiana. UV–visible spectra of 1.0
× 10-4 M dichloromethane solutions were recorded in the range
200 to 1000 nm on a Cary 50 spectrophotometer. Electrochemical
experiments were performed in dichloromethane on a BAS 50W
voltammetric analyzer. A standard three-electrode-cell was em-
ployed with a glassy-carbon working electrode, a Pt-wire auxiliary
electrode, and an Ag/AgCl reference electrode under an inert
atmosphere at room temperature. TBAPF6 was used as the
supporting electrolyte. The |Ipa/Ipc| values were obtained using
available software within the BAS package. All potentials are given
versus Fc+/Fc.39
X-ray Structural Determination for 1 and 5. Diffraction data
were collected on a Bruker X8 APEX-II diffractometer equipped with
Mo KR radiation and a graphite monochromator at 100 K. Frames
were collected at 10 s/frame (1) or 5 s/frame (5) and 0.3 degree between
frames at a detector distance of 40 mm. The frame data were indexed
and integrated with the manufacturer’s SMART, SAINT, and SAD-
ABS software.40 Both structures were refined and reported using
Sheldrick’s SHELX-97 software.41 Crystals of 1 formed as brown
plates and yielded 16135 reflections, of which 3652 were independent.
Hydrogen atoms in 1 were placed in observed positions, and the
asymmetric unit contains one-half complex with the Cu atom occupy-
ing an inversion center in the lattice. Crystals of 5 appeared as pale
green plates, and 27592 reflections were harvested of which 3985 were
independent. Hydrogen positions in 5 were observed or calculated.
The asymmetric unit in 5 consists of one-half dimeric complex with
the bridging methoxy groups occupying a 2-fold axis. The hydrogen
atoms on these methoxy groups are disordered in two sets of partial
positions.
[CuII(L2)2] (3). Bis-(2,4-di-tert-butyl-6-octadecyliminomethyl-
phenolato)copper(II). To a stirring solution of octadecylamine (0.270
g, 1.0 mmol) in 20 mL ethanol was added the precursor 1 (0.265
g, 0.5 mmol) in 10 mL methanol, dropwise. The solution was
refluxed for 1 h. The solvent was removed to obtain an olive colored
powder. The power was dissolved in dichloromethane:methanol
mixture (1:1) and yielded a homogeneous powder that was dried
undervacuumovernight.Yield)72%.Anal.Calcdfor[C66H116N2O2Cu]:
C, 76.72; H, 11.32; N, 2.71. Found C, 76.32; H, 11.44; N, 2.77%.
IR (KBr, cm-1) 1590 (VCdN), 2859–2961 (VCsH from tert-butyl
groups). ESI pos. in MeOH: m/z ) 1033.2 for [3 + H+]+.
[CuII(L2A)2] (3′). Bis-(2,4-di-tert-butyl-6-octadecylaminomethyl-
phenolato)copper(II). To a stirring solution of 3 (0.270 g, 1.0 mmol)
in 20 mL of a 1:1 methanol:dichloromethane mixture at 0 °C small
portions of NaBH4 (0.12 g, 3.0 mmol) were added. The solution
was kept in an ice bath until gas evolution ceased. The solvent
was removed, and the crude product was dissolved in dichlo-
romethane and washed with 10 × 100 mL aliquots of a saturated
aqueous solution of sodium chloride. The resulting solution was
dried on NaSO4, and the dichloromethane evaporated to yield a
pale green powder.
Yield ) 60%. Anal. Calcd for [C66H120N2O2Cu]: C, 76.43, H,
11.66, N, 2.70. Found C, 76.63, H, 10.95, N, 2.68%. IR (KBr, cm-1
)
Table 1 lists the experimental parameters.
2860–2963 (VC-H from tert-butyl groups). ESI pos. in MeOH: m/z
Compression Isotherms. The Π-A isotherms were measured
in an automated KSV 2000 minitrough at 23 ( 0.5 °C. Ultrapure
water (Barnstead NANO pure) was used for all experiments with
) 1036.9 for [3′ + H+]+.
[CuII(L3)2] (4). Bis-(2,4-di-tert-butyl-6-[(3,4,5-tris-dodecyloxy-
phenylimino)-methyl]-phenolato)copper(II). Precursor 1 (0.265 g,
0.5 mmol) was added to a stirring 20 mL methanol solution of
3,4,5-tris-dodecyloxy-phenylamine42(0.646 g, 1.0 mmol) dropwise.
After a mild reflux of 2 h, two-thirds of the solvent was removed
to precipitate a brown powder. The powder was isolated and washed
(39) Gagne, R.; Koval, C.; Licenski, G. Inorg. Chem. 1980, 19, 2854.
(40) APEX II, SMART, SAINT and SADABS collection and processing
programs are distributed by the manufacturer.; Bruker AXS Inc.:
Madison, WI, U.S.A.
(41) Sheldrick, G. SHELX-97; University of Gottingen: Germany, 1997.
3126 Inorganic Chemistry, Vol. 47, No. 8, 2008

ResponsiWe Salycilaldimine-Copper(II) Soft Materials
with cold methanol and dried under vacuum. Yield ) 61%. Anal.
Calcd for [C114H196N2O8Cu]: C, 76.65; H, 11.06; N, 1.57. Found
C, 76.38; H, 11.16; N, 1.71%. IR (KBr, cm-1) 1588 (VCdN),
2852–2956 (VCsH from tert-butyl groups). ESI pos. in MeOH: m/z
) 1784.9 for [4 + H+]+.
tive (Fund-11E420), and the National Science Foundation
(Grant CHE-0718470), M.T.R. acknowledges the National
Science Foundation (Grant CHE-0518262), and S.R.P.R.
acknowledges the National Science Foundation (Grant
CBET-0553537) for financial support.
Acknowledgment. C.N.V. thankfully acknowledges the
Wayne State University, the Donors of the ACS-Petroleum
Research Fund (Grant 42575-G3), the Nano@Wayne initia-
Supporting Information Available: Three figures and a table
with data for compounds 1, 3, and 3′ (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(42) Percec, V.; Ahn, C.-H.; Bera, T. K.; Ungar, G.; Yeardley, D. J. P.
Chem.sEur. J. 1999, 5, 1070.
IC702233N
Inorganic Chemistry, Vol. 47, No. 8, 2008 3127