**1. Introduction**

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Applications

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Schiff base is one of the most popular ligands in the field of coordination chemistry [1-5]. Conventionally, transition metal complexes having Schiff base ligands have been investigat‐ ed about stereochemistry and corresponding electronic properties mainly. For example, sol‐ ution paramagnetism of Ni(II) complexes, structural phase transition of Cu(II) complexes, chiral catalysts, and some types of molecule-based magnets and other interesting facts about correlation between structures and properties are known and these facts are cooperative ef‐ fect involving intermolecular interactions and molecular recognition. Because of developing importance as functional chiral materials, many researchers have investigated crystal struc‐ tures (including thermally-induced structural phase transition and polymorphism by sol‐ vents) of *trans*-type chiral Schiff base metal complexes and extract important features of chiral molecular recognition in the solid states.

As mentioned in Abstract section, we have tested observation of some novel phenomena as‐ sociated with chirality or CD spectroscopy based on intermolecular interactions. Induced CD on various nano-scaled (inorganic) materials from chiral Schiff base metal complexes is one of them and not only electronic and magnetic dipole moments but also molecular recog‐ nition between chiral compounds and nano-scaled materials are important factors for these phenomena [6, 7]. For example, we have observed induced CD peaks from chiral Schiff base Ni(II) complexes at d-d region for achiral or chiral Schiff base Cu(II) complexes (without ex‐ changing ligands) [8], at d-d and CT regions for Cu(II)-coordinated metallodendrimers (PA‐ MAM), and surface plasmon region for Cu-clusters prepared in PAMAM by irradiation of UV light for the first time [9, 10]. In this way, we have also reported on induced CD peaks of metal complexes (both achiral and chiral ones), organometallics (ferrocene) [11], metalloden‐ drimers, metal nano-clusters, and nano-particles [9, 10] of metal-semiconductors [12]. Addi‐

© 2013 Akitsu and Kominato; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Akitsu and Kominato; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

tionally, we have successfully observed size-dependence of wavelengths of induced CD peaks from chiral Schiff base Zn(II) complexes involving azo-groups at surface plasmon re‐ gion on colloidal gold particles [13].

As for the induced CD between chiral Schiff base Ni(II) or Zn(II) complexes and Cu-clusters prepared in PAMAM, we have also investigated the role of chiral ligands for molecular rec‐ ognition. For example, naphtylgroups are appropriate for induced CD, while more flexible groups are not [14] (Figure 1). Therefore, several examples indicated that supramolecular or molecular recognition must be a key reason for specific intermolecular interactions. In this review article, we have summarized several examples of crystal structures and optimized structures (as a model of them in solutions) of *trans*-type chiral Schiff base Ni(II), Cu(II), and Zn(II) complexes. In order to derive important steric factors for molecular recognition, we will point out characteristic features of molecular shapes or their conformational changes *in silico*.

**Figure 1.** Examples of suitable [left] and unsuitable [center] ligands for induced CD based on experiments [9, 10, 14]. [Right]Important (bold circles) and unimportant (broken circle)moieties of ligands for induced CD.
