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 investigated about stereochemistry and corresponding electronic properties mainly. For example, solution 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 effect involving intermolecular interactions and molecular recognition. Because of developing importance as functional chiral materials, many researchers have investigated crystal structures (including thermally-induced structural phase transition and polymorphism by solvents) 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 associated 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 recognition 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 exchanging ligands) , at d-d and CT regions for Cu(II)-coordinated metallodendrimers (PAMAM), 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) , metallodendrimers, metal nano-clusters, and nano-particles [9, 10] of metal-semiconductors . Additionally, 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 region on colloidal gold particles .
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 recognition. For example, naphtylgroups are appropriate for induced CD, while more flexible groups are not  (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.
According to a CCDC database , we selected some crystal structures of trans-type Schiff base metal complexes. As modeling conformational changes in solutions, we obtain optimized structures and their heat of formation by using MM2. We will search appropriate features of molecular shapesfor induced CD.
3. Results and discussion
12 examples of trans-type Schiff base complexes investigated are mentioned below, molecular structures [top], crystal structures [middle], and optimized structures [bottom] as space-filling models with comments.
In principle, induced CD is caused by non-contact interactions between (electric) dipole moments of chiral additives and achiral materials. Because it is an electromagnetic phenomenon essentially, contact intermolecular interactions, in other words molecular recognition, may not be an important factor for it. However, the experimental facts that only complexes with specific ligands or metal ions (which determine their coordination geometries) suggested that induced CD appears under appropriate steric (as well as stereochemical) conditions for metal complexes. One of the important factors of steric factors for metal complexes may be distance between (electric) dipole moments at the surface achiral materials which keep their shapes rigidly. The reason for this assumption is that both metallodendrimers metal and nanoparticles have approximately spherical shapes essentially even surrounded in softmaters.
As for biomolecules such as proteins, however, CD spectra are used for monitoring folding or unfolding of peptide chains after binding small molecules of metal complexes . This different phenomenon is not classified into the induced CD mentioned in this article. By including small molecules into proteins with weakly supramolecular forces, molecules of proteins change their molecular conformation, which attributed to shift of strong π−π* bands of C=O moieties electronic or CD spectra. This docking mechanism is directly molecular recognition accompanying with conformational changes of proteins as well as small molecules, which is also confirmed by quenching of fluorescence intensity due to energy transfer.
In contrast, non-contact interactions of (electric) dipole moments for CD spectra have complicated problems. Our preliminary results of CD spectra of chiral Schiff base metal complexes in viscous solutions dissolved a certain protein exhibited concentration dependence of so-called artifact peaks of solid-state CD spectra . The artifact CD peaks are attributed to anisotropic molecular orientation and removed in matrix environment which permits molecular rotation isotropically accompanying with (magnetic) dipole moments of chiral molecules . Therefore,not only CD spectra of chiral molecules in anisotropically oriented matrix such as biomolecules but also induced CD bands involving softmaters is still an open question.
As summarized in Figure 1[right], according to chemical structures, Zn(II) center and naphtylgroups are suitable factors for induced CD, while 3,5-dichlorosalycilaldehyde moieties are not regardless of common factors. Previous study  revealed that in optimized structure, naphtylgroups act as largely spread planar parts outside of a molecular face, which plays an important role in induced CD for this case. In the present study, compounds having identical features were also investigated in view of optimized structures. According to not only3,5-dichlorosalycilaldehyde moieties (IBHBCU01 and MAJCUW) but also tert-Bu- groups (MIMTOS01 and YUBLAJ), EtO- groups (MAJNIV), and NO2- groups (METSUZ) gave significantly large steric hindrance resulting in steric repulsion between ligands. However, specific geometry could not be induced by bulky groups. Generally, Zn(II) complexes afford a tetrahedral coordination geometry, which prevents from forming flatten planar molecular shapes in view of ligands. Therefore, these two factors may not be definitive factors solely. On the other hand, besides in amine parts (Figure 1), naphtylgroups in aldehyde parts (KUPBIH and KUPBON) are also keeping appropriate conditions, namely largely spread planar parts outside of a molecular face. As far as in the sense of molecular recognition, it has advantage for penetrating into inside of dendrimer as well as contacting to the surface of metal nano-particles. Further experimental and/or theoretical investigation including electric factors will be necessary to understand deeply.