Optically Active 4- and 5-Coordinate Transition Metal Complexes of Bifurcated Dipeptide Schiff Bases

Abstract

Symmetrical and unsymmetrical benzophenone Schiff bases of bifurcated dipeptides [e.g., Ar2C=N-CHR1- CONH-HNCO-CHR2-N=CAr2] have been synthesized using Boc methodology. These ligands may be regarded as chiral porphyrin mimics because of the alpha-carbons of the amino acids. The Schiff bases function as effective ligands for transition metals, particularly the late transition metals Ni(II), Cu(II), and Zn. Upon metal insertion, there is loss of the amide protons, resulting in N4 chelating ligands that retain the amino acid based chirality as well as newly generated metal-centered chirality, which for the Ni(II) complexes have been shown by X-ray analysis to be lambda (left-handed helix) if the amino acids are S. For Ni(II) and Cu(II), metal insertion results in highly colored complexes and is easily followed by UV-vis spectrophotometry. Several Ni(II) complexes were also characterized by 1H NMR. Co(II) and Mn(II) complexes were characterized by CW EPR. Two Cu(II) complexes, 7f.CuII and 7k.CuII, were characterized by EPR (ENDOR and ESEEM), which clearly showed the pentacoordinate nature of 7k.CuII.

Bis{1-[3-(dimethylamino)propyliminomethyl]naphtholato}nickel(II) bis(perchlorate)

“…However, structures of complexes of the Schiff base ligand 1- [(3-dimethylaminopropylimino)methyl]naphthol (DMPN), a similar ligand to CHPP and CHPN, have never been reported. The prime interest in these complexes is their ability to afford functional solid materials with potentially controllable properties and novel molecular structures (Polt et al, 2003;Kim et al, 2005;Yamada, 1999).…”

{2-[2-(Ethylamino)ethyliminomethyl]-5-methoxyphenolato}thiocyanatonickel(II)

“…For general background to nickel(II) complexes with Schiff bases, see: Campbell & Urbach (1973); Wallis & Cummings (1974); Polt et al (2003); Mukhopadhyay et al (2003). For the structures of related complexes, see: Montazerozohori et al (2009); Zhu et al (2004Zhu et al ( , 2006.…”

“…Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1998); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL. Nickel(II) complexes with Schiff bases have been extensively studied (Campbell & Urbach, 1973;Wallis & Cummings, 1974;Polt et al, 2003;Mukhopadhyay et al, 2003). In the title compound, the Ni atom is four-coordinate by the phenolate O atom, imine N atom, and amine N atom of the Schiff base ligand, and by the N atom of a thiocyanate ligand, forming a tetrahedrally distorted square-planar geometry ( Fig.…”

{5-Methoxy-2-[(2-morpholinoethyl)iminomethyl]phenolato}(thiocyanato-κN)nickel(II)

“…For general background to nickel(II) complexes with Schiff bases, see: Campbell & Urbach (1973); Wallis & Cummings (1974); Polt et al (2003); Mukhopadhyay et al (2003). For related structures, see: Liu (2010); Montazerozohori et al (2009); Zhu et al (2004Zhu et al ( , 2006.…”