We have prepared supramolecular systems of chiral Schiff base Ni(II), Cu(II), Zn(II) complexes and colloidal gold nanoparticles (AuNP) of 10 nm diameters. They demonstrated that direct adsorption of chiral Schiff base metal complex on the surface of AuNP owing to observation of clear induced CD spectra for the first time. We observed and discussed induced CD bands on AuNP from chiral Schiff base Ni(II), Cu(II), Zn(II) complexes.
Metal nanoparticles including gold nanoparticles have been a subject of research and applications in various fields because it has unique properties of surface plasmon resonance (SPR). There are many types of plasmonic concepts and applications. For example, the plasmon concept of the refractive index change is applied to the refractive index sensing [
Recently, induced CD on metal nanoparticles from chiral additive molecules including biomolecules [
To date we have reported induced CD due to chiral Schiff base metal complexes onto various materials, for example, achiral Schiff base metal complexes [
Practically, chiral AuNP catalysts will be expected as candidate materials for organic silanol reaction that has been used as an attractive nucleophile agent mainly. There may be three reaction acts as nucleophile agent of silanol. The first is used as a bond formation or base by nucleophilic reaction of silanol oxygen. The second is a specific reaction of the neighboring functional groups that use the interaction between the hydroxyl group and a metal compound. And the third is a molecule conversion utilizing specific reactivity of the silicon-bonded carbon one based on the presence of silanol hydroxyl groups. In this way, silanol
Scheme 1. Concept of chiral catalysis including AuNP: [Left] heterogeneous type having chiral ligands [
might be attractive as a nucleophile agent. Moreover, the expected co-product must be hydrogen gas in the synthesis process, which is ideal in terms of the environment.
Herein we have successfully observed considerably intense induced CD demonstrated onto AuNP in plasmon region due to chiral Schiff base Ni(II), Cu(II), and Zn(II) complexes (compound 1a-1c) for the first time. With a different ligand complex of by synthesizing eleven compounds (compound 2a-5c) including compound 1a-1c (Scheme 2), we compared the differences of the induced CD. Using a TD-DFT we predicted adsorption structure.
We prepared eleven compounds has various ligands. Compounds 1a, 1b and 1c were prepared according to the literature procedure [
Preparation of 2c: To a solution of salicylaldehyde (0.244 g, 2.00 mmol) dissolved in methanol (40 mL), (1R,2R)-(+)-1,2-Diphenylethylenediamine (0.212 g, 1.00 mmol) was added and stirred at 313 K for 2 h to give yellow solution of ligand. Zinc(II) acetate dehydrate (0.220 g, 1.00 mmol) was added to the resulting solution and stirring at 313 K for 2 h to give yellow solution of the complex. After cooling the solution, this yellow compound was filtered. Yield 0.302 g (61.4%). Anal. Found: C, 56.27; H, 5.25; N, 54.53%. Calc. for C28H22N2O2Zn: C, 59.15; H, 5.42; N, 4.18%. IR (KBr (cm-1)): 1627 (C=N). UV-Vis (diffuse reflectance) peak/nm; 391. CD (KBr) peak/nm; 383.
Preparation of 3: To a solution of 2,3-Dihydroxybenzaldehyde (0.276 g, 2.00 mmol) dissolved in methanol(40 mL), (1R,2R)-(+)-1,2-Diphenylethylenedia- mine (0.212 g, 1.00 mmol) was added and stirred at 313 K for 2 h to give brown solution of ligand. Copper(II) acetate monohydrate(0.200 g, 1.00 mmol) was added to the resulting solution and stirring at 313 K for 2 h to give brown solution of the complex. After cooling the solution, this brown compound was fil
Scheme 2. Chemical Structures of eleven compounds.
tered. Yield 0.412 g (77.7%). Anal. Found: C, 65.59; H, 4.61; N, 5.37%. Calc. for C28H22N2O4Cu: C, 65.42; H, 4.31; N, 5.45%. IR (KBr(cm−1)): 1617 (C=N),3417 (OH). UV-Vis (diffuse reflectance) peak/nm; 446, 574. CD (KBr) peak/nm ; 434, 610.
Preparation of 5a: To a solution of 3,5-Dibromosalicylaldehyde (0.560 g, 2.00 mmol) dissolved in methanol (40 mL), (1R,2R)-(+)-1,2-Diphenylethylenedia- mine (0.2122 g, 1.00 mmol) was added and stirred at 313 K for 2 h to give yellow solution of ligand. Nickel (II) acetate tetrahydrate (0.249 g, 1.00 mmol) was added to the resulting solution and stirring at 313 K for 2 h to give orange solution of the complex. After cooling the solution, this orange compound was filtered. Yield 0.695 g (86.8%). Anal. Found: C, 42.76; H, 2.36; N, 3.44%. Calc. for C28H18Br4N2O2Ni: C, 42.42; H, 2.29; N, 3.53%. IR (KBr (cm-1)):1620 (C=N). UV-Vis (diffuse reflectance)/nm ; 430, 573. CD (KBr) peak/nm ; 409, 572.
Preparation of 5b: To a solution of 3,5-Dibromosalicylaldehyde (0.560 g, 2.00 mmol) dissolved in methanol (40 mL), (1R,2R)-(+)-1,2-Diphenylethyle- nediamine (0.212 g, 1.00 mmol) was added and stirred at 313 K for 2 h to give yellow solution of ligand. Copper(II) acetate monohydrate(0.200 g, 1.00 mmol) was added to the resulting solution and stirring at 313 K for 2 h to give green solution of the complex. After cooling the solution, this green compound was filtered. Yield 0.725 g (90.9%). Anal. Found: C, 42.06; H, 2.27; N, 3.43%. Calc. for C28H18Br4N2O2Cu: C, 42.16; H, 2.27; N, 3.51%. IR (KBr (cm-1)):1623 (C=N). UV-Vis (diffuse reflectance) peak/nm; 399, 582. CD(KBr) peak/nm ; 379, 575.
Preparation of 5c: To a solution of 3,5-Dibromosalicylaldehyde (0.560 g, 2.00 mmol dissolved in methanol (40 mL), (1R,2R)-(+)-1,2-Diphenylethylenediamine (0.212 g, 1.00 mmol) was added and stirred at 313 K for 2 h to give yellow solution of ligand. Zinc(II) acetate dihydrate (0.220 g, 1.00 mmol) was added to the resulting solution and stirring at 313 K for 2 h to give yellow solution of the complex. After cooling the solution, this yellow compound was filtered. Yield 0.692 g (86.6%). Anal. Found: C, 42.01; H, 2.02; N, 3.33%. Calc. for C28H18Br4N2O2Zn: C, 42.07; H, 2.27; N, 3.50%. IR (KBr (cm-1)): 1629 (C=N). UV- Vis (diffuse reflectance) peak/nm; 404. CD (KBr) peak/nm ; 401.
AuNP (10 nm) were purchased from Funakoshi (BMGC10) and used without further purification. Spectral measurements were performed as 0.005 mM-0.1 mM solutions for chiral metal complexes and the most suitable 1:1 (v/v) solutions for mixed solutions of chiral metal complexes and AuNP. The experimental conditions were investigated widely.
Electronic spectra were measured on a JASCO V-570 spectrophotometer in the range of 900-200 nm at 298 K. Circular dichroism (CD) spectra were measured on a JASCO J-725 spectropolarimeter in the range of 800-200 nm at 298 K.
Geometry optimization and normal vibration analysis are computed using DFT with B3LYP functional, Lanl2dz for Cu, Ni, and Zn and 6-31+G(d) basis set for H, C, N, O and Br in the GAUSSIAN 03 program package [
The powder X-ray diffraction (XRD) pattern of complexes 2c (
Single crystals were glued on top of a glass fiber and coated with a thin layer of epoxy resin to measure the diffraction data. Intensity data were collected on a Bruker APEX2 CCD diffractometer with graphite monochromated Mo Kα radiation (k = 0.71073 Å). Data analysis was conducted using the SAINT program package. The structures were solved by direct methods with SHELXS-97 [
Figures 2-4 depict electronic and CD spectra of hybrid system of AuNP and 1a, 1b and 1c, respectively. Pure AuNP (10 nm) showed surface plasmon band at around 530 nm without distinct CD bands. On the other hand, Pure compound 1a, 1b and 1c exhibited predominant absorption bands at 400, 380, and 380 nm, respectively, and predominant CD peaks at 400 nm. It should be noted that mixed solutions (namely hybrid systems) of each metal complex and AuNP (10 nm) exhibited predominantly induced CD bands at about 600 nm. The bands are close to surface plasmon band of pure AuNP and no CD peak could be observed as neither pure AuNP nor pure 1a, 1b and 1c. It should also be noted that the intensity of induced CD peaks were clearly increased for the first time among our studies using chiral Schiff base additives, which is distinctly different from weakly induced or negatively induced ones [
Induced CD was observed in the same manner about 4, 5a, 5b and 5c. Induced CD was not observed about 2a, 2b, 2c and 3. From the existing theory [
The crystallographic data and selected bond lengths for 2c are listed in
planar ligands not to overlap (
The crystallographic data and selected bond lengths for 3 are listed in
Empirical formula | C28H22N2O2Zn |
---|---|
Crystal system | orthorhombic |
Space group | P 21 21 21 |
Z | 4 |
a (Å) | 11.616 (3) |
b (Å) | 23.198 (8) |
c (Å) | 9.379 (3) |
V (Å3) | 2527.4 (13) |
ρcalc (g/cm3) | 1.270 |
μ (mm−1) | 1.5413 |
F(000) | 1000.0 |
Rwp (%) | 8.37 |
Zn1-N1 | 2.102(4) | O1-Zn1-O2 | 107.72 (2) |
---|---|---|---|
Zn1-O1 | 1.951(5) | O1-Zn1-N1 | 153.22 (4) |
Zn1-N2 | 2.102(3) | O1-Zn1-N2 | 90.79 (2) |
Zn1-O2 | 1.951(4) | O2-Zn1-N1 | 90.79 (2) |
O2-Zn1-N2 | 153.22 (5) | ||
N1-Zn1-N2 | 79.55 (19) |
Empirical formula | C28H22N2O4Cu |
---|---|
Crystal system | monoclinic |
Space group | C2 |
Z | 4 |
a (Å) | 16.703 (8) |
b (Å) | 18.783 (8) |
c (Å) | 8.056 (4) |
β (˚) | 93.856 (7) |
V (Å3) | 2521.7 (19) |
ρcalc (g/cm3) | 1.396 |
μ (mm−1) | 0.907 |
F(000) | 1092.0 |
Goodness of fit | 1.786 |
R1[I > 2σ(I)] | 0.0863 |
wR2 | 0.2462 |
and
We have observed significantly enhanced induced CD bands on AuNP for chiral Schiff base complexes and estimated their molecular arrangement based on parallel transition dipole moment. We have observed significantly enhanced induced CD bands on AuNP due to chiral Schiff base complexes and estimated their molecular arrangement based on parallel transition dipole moment. From
Cu1-N1 | 1.918 (13) | O1-Cu1-O1a | 91.8 (6) |
---|---|---|---|
Cu1-O1 | 1.899 (10) | O1-Cu1-N1 | 92.1 (4) |
Cu1-N1a | 1.918 (13) | O1-Cu1-N1a | 174.0 (5) |
Cu1-O1a | 1.899 (10) | O1a-Cu1-N1 | 174.0 (5) |
Cu2-N2 | 1.955 (12) | O1a-Cu1-N1a | 92.1 (4) |
Cu2-O2 | 1.907 (10) | N1-Cu1-N1a | 84.3 (7) |
Cu2-N2a | 1.955 (12) | O2-Cu2-O2b | 87.0 (6) |
Cu2-O2a | 1.907 (10) | O2-Cu2-N2 | 95.4 (4) |
O2b-Cu2-N2 | 167.4 (4) | ||
O2-Cu2-N2b | 167.4 (4) | ||
O2b-Cu2-N2b | 95.4 (4) | ||
N2-Cu2-N2b | 85.0 (7) |
XPS measurement, we found the complex is present in the Au substrate surface in the form of a complex. From the STM measurement, the complex which has been observed induced CD is adsorbed standing on the Au surface. On the other hand, the complex which has not been observed induced CD is adsorbed on the Au surface in the state complex which doesn’t have central metal. The proposed supramolecular structures have potential application as new type of chiral AuNP catalysts possessing merits of both heterogenous (AuNP with chiral ligands) and homogenous (AuNP supported chiral metal complexes) ones on the surface (Scheme 1). Further investigation about chiral catalytic functions is in progress now. However, it should be noted here that merely small amount of bubbles was confirmed from the Au substrate surface during the preliminary reaction aiming at applications.
This work was partly performed under the Cooperative Research Program of “Network Joint Research Centre for Materials and Devices”. The authors thank Profs. Kenji Hara and Toshikazu Kawaguchi (Hokkaido University) for preliminary discussion for surface and catalysis chemistry (Supplementary File).
Oshima, M., Matsuno, M., Yuki, T., Nobumitsu, S., Haraguchi, T. and Akitsu, T. (2017) Synthesis of Chiral Schiff Base Metal Complex Inducing CD and Elucidation of Structure of Adsorption on Surface of Gold Nanoparticles. International Journal of Organic Chemistry, 7, 153-170. https://doi.org/10.4236/ijoc.2017.72013
CCDC 1480602 and 1480604 contains the supplementary crystallographic data for 2c and 3 respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
The XPS measurement was performed in the gold substrate and complexes. This was confirmed the presence or absence of elements of gold substrate surface in order to examine the state of the adsorption of gold and complex in detail as a model case of gold colloid and complex solution.
We performed XPS measurement for 1a, 1b, 1c, 3 and 4. First we prepared 10 nm × 10 nm square gold substrate, then prepared 0.01 mM methanol solution of compound 1a, 1b and 1c, also prepared 0.2 mM methanol solution of 3 and 4, respectively. The gold substrate put into sample tubes and tried interactions gold surface and complexes by immersing 1 hour complexes solution. Immersed gold substrate washed for three times with chloroform, and then dry surface with Ar gas. We measured for Au, C, O, N and Central metal atoms, respectively.
According to XPS measurement, for example, it shows that the constituent elements of the 1a is present in the gold substrate surface. We observe binding energy that 84 eV for Au4f7/2 and 88 eV for Au4f5/2 peaks of the Au substrate elements, and then 285 eV for C1s, 532 eV for O1s, 398 eV for N1s, 853 eV for Ni2p3/2 and 870 eV for Ni2p1/2 peaks of constituent elements of 1a. Copper atom has been measured as a comparison but it should be appreciated that copper atom was not observed matter of course. In the same way, peak of the complex elements (C, O, N and Central metal atoms) also for 1b, 1c and 4 was observed. Although the C, O and N elements was observed for 3, the central metal has not been observed. This point will be discussed in conjunction with the STM. The supporting information shows the results of 1b, 1c, 3 and 4.
We performed a quantitative analysis of elements by the area calculation for XPS spectrum of previous chapter. Results are shown
Configuration element ratio Nitrogen and the center metal of the complexes is matched from the calculation results. Induced CD compounds is present on gold substrate with keeping its shape. Then, we calculated the percentage of nitrogen and central metal of the complexes on the basis of
STM measurement was performed to investigate the adsorption structure of the gold substrate and the complex visually.
We performed STM measurement for 1a, 1b, 1c, 3 and 4. First we prepared 10 nm × 10 nm square gold substrate, then prepared 0.01 mM methanol solution of 1a, 1b and 1c, also prepared 0.2 mM methanol solution of 3 and 4, respectively. It was performed exima treatment (172 nm) for 3 minutes for activating the Au surface. The gold substrate put into sample tubes and tried interactions Au surface and complexes by immersing 1 hour complexes solution. Immersed gold substrate washed for three times with chloroform, and then dry surface with Ar gas.
In
In
The grain height is 1.194 nm and it is that substantially coincide with the longitudinal length of 1a (1.266 nm). From this it can be said that the complex is adsorbed standing on the Au surface. This fact is consistent with the theoretical study [
In