research papers 510 DOI: 10.1107/S0108767304017131 Acta Cryst. (2004). A60, 510±516 Acta Crystallographica Section A Foundations of Crystallography ISSN 0108-7673 Received 31 January 2004 Accepted 13 July 2004 # 2004 International Union of Crystallography Printed in Great Britain ± all rights reserved Electronic structure of the nickel(II) complex of the Schiff base of (S)-N-(2-benzoylphenyl)-1-benzyl- prolinamide and glycine Jozef Koz Ïõ Âs Ïek, a * Marek Fronc, a Pavol Skuba Âk, a Alexander Popkov, b Martin Breza, a Hartmut Fuess c and Carsten Paulmann d a Department of Physical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-812 37 Bratislava, Slovak Republic, b Department of Health Physics and Biophysics, Faculty of Health and Social Studies, University of South Bohemia, CZ-370 05 C Ï eske  Budejovice, Czech Republic, c Materials Science, Darmstadt University of Technology, D-642 87 Darmstadt, Germany, and d Mineralogisch-Petrographisches Institut, Universita Èt Hamburg, Grindelallee 48, D-20146 Hamburg, Germany. Correspondence e-mail: kozisek@cvt.stuba.sk The experimental charge density of the Ni II complex of the Schiff base of (S)-N- (2-benzoylphenyl)-1-benzylprolinamide and glycine was derived from high- resolution single-crystal X-ray diffraction data ( = 0.5604 A Ê ) at low tempera- ture (100 K) with synchrotron radiation at the beamline F1 using a CCD area detector. The central Ni atom is pseudo-square-planar coordinated by three N atoms [1.9414 (3), 1.8559 (3) and 1.8533 (3) A Ê ] and by one O atom [1.8620 (4) A Ê ]. The N(1) atom is 0.359 A Ê above the plane de®ned by the atoms Ni(1), N(2) and N(3). The d-orbital population analysis reveals an oxidation state for the Ni atom of +2 with the con®guration d 8 and a hole mainly in the d x 2 y 2 orbital, located in the plane of the four ligating atoms. The prochiral reaction centre was examined by topological analysis. 1. Introduction In the pharmaceutical industry, enantiomerically pure -methyl amino acids are used as building blocks for pep- tidomimetic drug design. Owing to environmental constraints, catalytic approaches are favoured as compared to their stoi- chiometric alternatives. Recent achievements in catalytic asymmetric synthesis of -methyl amino acids make enan- tiospeci®c processes accessible in many cases (Ager, 2002). Less attention is now paid to the development of chiral stoi- chiometric synthons of -methyl amino acids for non-indus- trial purposes. Preparation of 11 C-labelled amino acids for positron emission tomography (PET) is an important example (Vaalburg et al., 1976; Plenevaux et al., 1994; Fasth et al., 1995; La Ê ngstro Èm et al., 1999). Reliable syntheses of compounds labelled with carbon-11 (half-life 20.4 min) are performed on a submicromolar scale in specially designed remote-controlled robotic devices. Limitations brought by very small amounts of a starting compound signi®cantly restrict the applicable procedures. In PET, -methyl amino acids play a dual role: (i) as precursors of non-metabolized neurotransmitters (analogues of serotonin, dopamine, tyramine etc.) for the study of neurodegenerative diseases; (ii) as non-metabolized analogues of proteinogenic amino acids for the study of amino acid uptake in normal and cancer cells. Difference in the uptake rates during a PET scan could visualize cancer metastases in a human body. Clinical applications of such amino acids are very limited due to their poor availability. For the synthesis of the only enantiomerically pure 11 C-labelled -methyl amino acid, -[ 11 C]methyltryptophan, an industrial procedure was adopted (Crich & Davies, 1989; Bourne et al., 1991; Plenevaux et al., 1994). All attempts to prepare enantiomerically pure -[ 11 C]methylated tyrosine failed (Gee & La Ê ngstro È m, 1991; Rajagopal et al., 1992). Our approach to the desired amino acids is based on an improvement of stereodifferentiating the properties of known nickel-based amino acids synthons (Fig. 1) (Belokon, Bakh- mutov et al. , 1988; Fasth & La Ê ngstro È m, 1990; Popkov et al. ,