# The Studies of Conditions for Inducing Chirality to Cu(II) Complexes by Chiral Zn(II) and Ni(II) Complexes with Schiff Base

Authors: Takashiro Akitsu; Jun Yamaguchi; Naoki Uchida; Yoshikazu Aritake
Journal: Research Letters in Materials Science
 (2009)

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## Abstract

Recently, we have discovered that some chiral Schiff-base nickel(II) complexes induced d-d bands of CD spectra of some achiral copper(II) complexes. However, the novel phenomenon could be observed only a few systems of hybrid materials or limited conditions so far. In order to test conditions about copper(II) ions, we investigated model systems (1) metal-dendrimer (Cu-PAMAM; G4-NH2 terminal) containing relatively small amount of copper(II) ions (4.5 equivalent to PAMAM) for modeling separated systems of achiral copper(II) complex from chiral Schiff-base nickel(II) or zinc(II) complexes, Bis(N-R-1-naphtylethyl-3,5-dichlorosalicydenaminato)nickel(II) or zinc(II) by polymer matrix. (2) equilibrium of copper(II) N-ethylethylenediamine complexes to measure absorption spectra of d-d band, pH, and electron conductivity during titration of copper(II) ions. The results showed that (1) 4.5Cu-PAMAM could not be induced their d-d bands by the chiral nickel(II) or zinc(II) complexes, which suggested that separation by polymers prevented from inducing CD peaks. (2) Although 36Cu-PAMAM was known, uncoordinated copper(II) ions excess to ligands mainly attributed to increase electron conductivity by remained ions in methanol solutions, which was not associated with intermolecular interaction or dipole moments being effective for the induced CD mechanism by using molecular recognition between neutral molecules of metal complexes.


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## Body

## 1. Introduction

In recent years, various types of organic/inorganic
hybrid materials have been studied, for example, organic part of photochromic compounds
working as photoswitching moiety and inorganic part of metal compounds working
as functional moiety for electronic properties (e.g., magnetic and redox catalytic) or molecular shapes (e.g., stereochemical and conformational).
By employing transition metal complexes as inorganic parts in such systems, we
have developed supramolecular hybrid materials containing photochromic
compounds and chiral Schiff-base metal complexes [1–3]. Accompanying with
photoisomerization of azobenzene or its derivatives, chiral Schiff-base ligands
of nickel(II), copper(II), and zinc(II) complexes changed their conformation in
solutions of organic solvents (methanol, chloroform, acetone, N,N'-dimethylformamide, and toluene),
which could be detected by CD spectroscopy as well as absorption electronic
spectra. Moreover, their spectral features (solvatochromism) and degree of smooth
conformational changes are also different depending on solvents. Suitable groups
of azobenzenes or Schiff-base ligands [4] suggest that the mechanism of
photocontrolling of molecular conformation of supramolecular hybrid systems
may be related to a certain intermolecular interactions between solutes. Therefore,
we are also interested in other methods to evaluate intermolecular interactions
by (absorption electronic, CD, or related) spectroscopy.In this context, we have discovered that some chiral Schiff-base
nickel(II) complexes induced d-d bands of CD spectra at 450, 550, and 650 nm assigned to be d-d
transitions of several achiral copper(II) Schiff-base complexes [5]. However,
the novel phenomenon could be observed only a few systems of hybrid materials
or limited conditions at present. In previous attempt [5], bis(N-R-1-phenylethyl-3,5-dichlorosalicydenaminato)nickel(II) was only effective chiral complex, while bis(N-R-1-naphtylethyl-3,5-dichlorosalicydenaminato)nickel(II)
was inactive for inducing CD peaks. In view of dipole moment of molecules,
large (naphtyl) groups or charged metal ions having unpaired electrons seemed
to have advantage for inducing CD, albeit actually molecular recognition was
also necessary as one of the conditions. Secondly, as employing labile
copper(II) complexes, the effects of uncoordinated (solvated) copper(II) ions by
dissociation can be expected to exhibit d-d bands in the region corresponding to
copper(II) complexes. To confirm the
possibility, we should know some properties of copper(II) ions during stepwise titration
and formation of complexes being appropriate for inducing such CD bands. Finally,
it was observed only in solutions of some organic solvents containing hybrid
solutes, while even bis(N-R-1-naphtylethyl-3,5-dichlorosalicydenaminato)nickel(II)
exhibiting larger dipole moment could not induce such CD peaks of the achiral copper(II)
complexes in polymer matrix. That inducted CD peaks also occur in separated
systems via long-range force may be important for understanding the mechanism.
To reveal this question, we intended to examine other model systems.In this paper, we investigated
by using model systems (1) metal-dendrimer (Cu-PAMAM, G4-NH2 terminal)
containing relatively small amount of copper(II) ions (4.5 equivalent to PAMAM,
abbreviated as 4.5Cu-PAMAM, etc.) for modeling separated systems of achiral
copper(II) complex from chiral Schiff-base nickel(II) or zinc(II) complexes, bis(N-R-1-naphtylethyl-3,5-dichlorosalicydenaminato)nickel(II), or
zinc(II) by polymer matrix. (2) Equilibrium of
copper(II) N-ethylethylenediamine
complexes to measure absorption spectra of d-d band around 800 nm, pH, and
electron conductivity. The main aim of this study is accumulating new knowledge
about this kind of supramolecular
hybrid materials.


## 2. Experimental Section

### 2.1. Materials

Chemicals and solvents of the highest commercial grade available
(3,5-dichlorosalicylaldehyde from TCI; other ones from Wako) were used as received
without further purification.


### 2.2. Sample Preparation

Bis(N-R-1-naphtylethyl-3,5-dichlorosalicydenaminato) nickel(II) (1)
or zinc(II) (2) (Figure 1) were prepared according to literature [1].Figure 1
Molecular
structure of chiral complexes (M = Ni for1, M = Zn for 2). Reference [1].To 10 mL of 10 wt% methanol
solution of G4-NH2 terminal PAMAM dendrimer (0.029 mM), 2.14 mL of methanol
solution of CuSO4 (0.60 mM) was added according to literature [6]
and monitoring by UV-Vis absorption spectra. The 4.5 equivalent amount of
copper(II) ions into the interior of the PAMAM dendrimer (4.5Cu-PAPAM) was used
for the experiments of induced CD mixed (about 0.1 mM) methanol solution with chiral
complexes 1 or 2.To 10 mL methanol solution
ofN-Etylethylenediamine (10 mM),
methanol solution of Cu(NO3)22H2O
(2.5 mM) was added by 1.0 mL dropwise up to 20.0 mL during the titration, we
measured UV-vis absorption spectra, pH, and electron conductivity every 1.0 mL
addition.


### 2.3. Physical Measurements

Absorption spectra were measured on a JASCO V-570 UV/VIS/NIR spectrophotometer
in the range of 900–200 nm at 298 K. CD (circular dichroism) spectra were
measured on a JASCO J-820 spectrophotometer in the range of 900–200 nm at 298 K. The pH was measured with a Mettler Toledo MI229. The electron conductivity
was measured  with a TOA DK CM-30G.


## 2.1. Materials

Chemicals and solvents of the highest commercial grade available
(3,5-dichlorosalicylaldehyde from TCI; other ones from Wako) were used as received
without further purification.


## 2.2. Sample Preparation

Bis(N-R-1-naphtylethyl-3,5-dichlorosalicydenaminato) nickel(II) (1)
or zinc(II) (2) (Figure 1) were prepared according to literature [1].Figure 1
Molecular
structure of chiral complexes (M = Ni for1, M = Zn for 2). Reference [1].To 10 mL of 10 wt% methanol
solution of G4-NH2 terminal PAMAM dendrimer (0.029 mM), 2.14 mL of methanol
solution of CuSO4 (0.60 mM) was added according to literature [6]
and monitoring by UV-Vis absorption spectra. The 4.5 equivalent amount of
copper(II) ions into the interior of the PAMAM dendrimer (4.5Cu-PAPAM) was used
for the experiments of induced CD mixed (about 0.1 mM) methanol solution with chiral
complexes 1 or 2.To 10 mL methanol solution
ofN-Etylethylenediamine (10 mM),
methanol solution of Cu(NO3)22H2O
(2.5 mM) was added by 1.0 mL dropwise up to 20.0 mL during the titration, we
measured UV-vis absorption spectra, pH, and electron conductivity every 1.0 mL
addition.


## 2.3. Physical Measurements

Absorption spectra were measured on a JASCO V-570 UV/VIS/NIR spectrophotometer
in the range of 900–200 nm at 298 K. CD (circular dichroism) spectra were
measured on a JASCO J-820 spectrophotometer in the range of 900–200 nm at 298 K. The pH was measured with a Mettler Toledo MI229. The electron conductivity
was measured  with a TOA DK CM-30G.


## 3. Results and Discussions

Figures2 and 3 show absorption and CD spectra of 4.5Cu-PAMAM and 1 and 2 in 0.1 mM methanol solutions, respectively. The assignment of
bands for 1 and 2 [1] and absorption spectra of stepwise complexation
of copper(II) ions into PAMAM has been reported elsewhere [6, 7].Figure 2
Absorption and CD
spectra of 4.5Cu-PAMAM and1 in 0.1 mM methanol solutions.Figure 3
Absorption and CD
spectra of 4.5Cu-PAMAM and2 in 0.1 mM methanol solutions.As for a Cu-PAMAM complex, a
broad d-d band was observed around 600 nm, intense while LMCT band was observed
around 300 nm (not shown) [6]. The intensity of d-d band was proportional to
the amount of copper(II) ions coordinated to PAMAM, and dissociated copper(II)
ion, namely, formation of solvated copper(II) complexes, resulted in shift of
peak wavelength. In Figure 2, intense and
broad d-d bands of 4.5Cu-PAMAM and weak d-d bands of 1 appeared around
600 nm, respectively.Of course both achiral PAMAM
dendrimer and achiral Cu-PAMAM complexes are inactive for CD spectroscopy, CD
peaks are attributed to chiral complexes1 and 2 mainly. Because
nickel(II) or zinc(II) complexes show weak or no d-d bands, predominant
absorption and CD peaks below 400 nm are assigned to π–π* bonds of chiral Schiff-base ligands. It should be noted that the
difference of calculated sum of CD spectra for 4.5Cu-PMMA and 1 or 2 and experimental spectra of mixed solution of
4.5Cu-PMMA and 1 or 2 (in addition, wavelength is d-d or
CT band region for the corresponding achiral copper(II) complex) can be ascribed
to induced CD peaks by chiral complexes.In the present case, both1 and 2 showed no or very slight induced d-d bands of 4.5Cu-PAMAM in 0.1 mM methanol solution (considerably weaker than general intensity values of “observed”
ones). Previously, we reported that bis(N-R-1-naphtylethyl-3,5-dichlorosalicydenaminato)
nickel(II) could induce CD peaks of achiral mononuclear Schiff-base copper(II)
complexes [8, 9] in more diluted 0.01 mM methanol solutions [5]. Therefore, the
reasons for the absence of induced CD peaks for the present cases may be
attributed to (1) inappropriate molecular recognition between chiral and
achiral metal complexes, indeed the present systems copper(II) ions may exist deeply
inner region of PAMAM dendrimers (2) copper(II) ions does not coordinate to
PAMAM appropriately, which can be proved to show free copper(II) ions can
enhance such induced CD peaks or not.The possibility of (1) can
be supported by these facts; in previous study, induced CD phenomenon exhibited
concentration dependence, and suitable combinations about molecular recognition
(against expectation by dipole moments of molecules) for several cases.
Moreover, induced CD peaks disappeared by separation of complexes in polymer
matrix. Therefore, if the induction is attributed to long-range intermolecular
interactions, there are suitable conditions for this system about
supramolecular chirality [10, 11].The possibility of (2) can
be confirmed as shown in Figures4 and 5. As known well, Cu-PAMAM of proper
ratio is stable to measure ESR spectra [12, 13] or discuss photochemical
reactions [14] generally.Figure 4
Absorption spectra
of 2.5 mM methanol solution of [Cu(NO3)2] and gradual changes
by copper(II) ion titration added every 1.0 mL in 0.1 mM N-ethylethylenediamine methanol
solutions.Figure 5
Electron
conductivity (right; empty squares) and pH (left; filled circle) of copper(II)titration (the [Cu(NO3)2] solution) in 0.1 mM N-ethylethylenediamine methanol
solutions.Figure4 shows absorption
spectra of copper(II) ion titration in 0.1 mM N-ethylethylenediamine methanol solutions. The peak of d-d bands of
pure Cu(NO3)2 solution (excess copper(II) ion in
methanol) and that of appropriately complexing species of N-ethylethylenediamine (and that of Cu-PAMAM) are apparently
different from each other. However, similar study on equilibrium has been
reported for the analogous ethylenediamine complexes [15] because it is typical and well-known chelating reagent for
copper(II) ion.The corresponding pH and
electron conductivity are shown in Figure5. Regardless of equilibrium
involving protons in solutions [15], the pH values were kept almost constant
values if excess copper(II) ions were not contained (below about 5 mL). On the
other hand, electron conductivity obviously increased when copper(II) ions were
added more than equivalent ratios.The features suggest that excess
or dissociated free copper(II) ions give rise to solvated species, which
exhibit shifted d-d bands Cu-PAMAM at least distinguishable. Free copper(II)
ions contained in solution systems may result in slight changes in pH and
increasing of electron conductivity, which may be increasing polarity of the
solutions. However, the magnitude of increasing polarity may be less than that
of solvatochromism of different organic solvents.


## 4. Conclusions

The present results showed that (1) the 4.5Cu-PAMAM could not be induced
their d-d bands by the chiral Schiff-base nickel(II) or zinc(II) complexes,
which suggested that separation by polymers prevented from inducing CD peaks. (2) Although even 36Cu-PAMAM was known, uncoordinated
copper(II) ions excess to ligands mainly attributed to increasing electron
conductivity by remained ions in methanol solutions, which was not associated
with intermolecular interaction or dipole moments effective for the induced CD
mechanism by using molecular recognition between neutral molecules.



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*Source: 484172-2009-06-01.xml*