Theoretical analysis of apical bonding in copper(II) chelates with amino acids

Abstract

The interdependence between the side of apical coordination (H2O, >C=O, dimethyl sulfoxide and Cl−) and steric crowding at the apical positions was investigated for a set of 20 copper(II) bis complexes with naturally occurring amino acids. As a measure of steric crowding the overlapping volumes, V*, were used, calculated by the modified overlapping spheres method. It was found that the apically coordinated side of a complex is more hindered by the side‐chain atoms. In addition, if both sides are apically coordinated, the apical bond is shorter at the more crowded side. The interdependence between the length of the apical bond and distortion of the coordination polyhedron was also studied. The apical bond length shows sigmoidal dependence on the magnitude of distortion, and is also dependent on the kind of distortion (square‐pyramidal or tetrahedral).

Theoretical analysis of apical bonding in copper(II) chelates withN-substituted amino acids

“…by using the method of reaction paths (Muetterties & Guggenburger, 1974), by measuring distortion along the Berry coordinate (Berry, 1960;Holmes & Deiters, 1977;Raos, 1996), and by using distortion indices (Baur, 1974;Addison et al, 1984), distortion vector (Murray-Rust et al, 1978a,b, 1979, tensors (Petruševski & Trenčevski, 1990) and various functions of the coordination polyhedron volume and of the volume of its circumscribed sphere (Makovicky & Balić-Ž unić, 1998). In contrast to our previous finding that apical length negatively correlates with various measures of distortion (Raos, 1995), we have recently found that the length of the apical bond shows sigmoidal dependence on the total distortion of the coordination polyhedron (Miličević & Raos, 2009).…”

“…derivatives; see Table 1). This was done by calculating the overlapping volumes, V*, using the modified OS method (Miličević & Raos, 2009). The central cylinder was placed along the apical bond(s) (Fig.…”

“…Steric crowding in the apical regions of a complex was calculated using the modified OS method (Miličević & Raos, 2009). Generally, the OS method is based on the evaluation of the function (Hopfinger, 1973…”

“…where V* is the overlapping volume of the central sphere S v (with radius R v ) and the volumes of the van der Waals spheres, s j , of the surrounding atoms [the details of calculations along with the set of van der Waals radii are given elsewhere (Raos, 2000)]. Instead of the central sphere (Raos, 2000;Miličević & Raos, 2005, 2006, in the modified OS method (Miličević & Raos, 2009) we applied a central cylinder (Fig. 1).…”

“…While these descriptors deal with the steric properties of ligands, we are interested in steric crowding at the apical positions of a complex. We have therefore developed a new model (Miličević & Raos, 2009) based on the overlapping spheres (OS) method, also developed in our laboratory (Raos, 2000;Miličević & Raos, 2006). The new model was recently checked on copper(II) complexes with naturally occurring amino acids (Miličević & Raos, 2009) and led to an unexpected finding that the apical ligand is bound to the sterically more hindered side of the chelate complex.…”