On the different coordination of Ni II , Zn II and Cd II cations in their model Schiff base complexes – Single crystal X-ray and solid state NMR studies Anna A. Hoser a , Wojciech Schilf b,⇑ , Anna Szady Chełmieniecka c , Beata Kołodziej c , Bohdan Kamien ´ ski b,d , Eugeniusz Grech c , Krzysztof Woz ´ niak a a Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland b Institute of Organic Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warszawa, Poland c Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology, Piastów 42, 71-065 Szczecin, Poland d Institute of Physical Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warszawa, Poland article info Article history: Received 4 July 2011 Accepted 13 September 2011 Available online 22 September 2011 Keywords: Schiff bases Metal complexes Structure Coordination Solid state NMR X-ray diffraction abstract Three model metal complexes: Ni(NCS)L, Zn 2 (NCS) 2 L 2 and Cd 2 (NCS) 2 L 2 , consisting of the SCN À anion(s) and L = 2-[(2-dimethylaminoethylimino)-methyl]-phenolate, have been studied by X-ray diffraction and solid state NMR spectroscopy. The metal cations in these complexes have different coordination modes: Ni 2+ is almost square-planar, the Zn 2+ cation in Zn 2 (NCS) 2 L 2 is pentacoordinated, whereas Cd 2+ is penta- and hexacoordinated in [Cd 2 (NCS) 2 L 2 ]. The different coordination of the metal cations influences the chemical shifts of the metal cations and also the nitrogen atoms. These chemical shifts can be correlated with the M–N and M–O bond lengths (M = Ni 2+ , Zn 2+ , Cd 2+ ). Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The condensation reaction of aromatic aldehydes and primary amines leads to imines known as Schiff bases. The Schiff bases obtained from different substituted salicylaldehydes can form intra- and intermolecular hydrogen bonds. These bonds mostly determine the chemical and physicochemical properties of the Schiff bases [1–4]. Enantiomeric Schiff bases and their optically active metal complexes have found many applications in biology, clinical and analytical studies [5–12]. The Schiff base metal com- plexes are also very useful model systems in chelate chemistry. The metal ion in such complexes can be coordinated by the imine nitrogen atom and also by the other active centres present in the molecule. This can lead to many interesting catalytic properties [7–9]. The coordination sphere around the metal ion and its conformation ability are the major factors in specific biological functions such as, for example, metalloenzymes active in many oxygen related processes [13–27]. The thermochromic and photo- chromic properties of some Schiff bases have been subject of both theoretical and practical studies [5,10–12]. Specially, thermochro- mic salicylaldehyde derivatives are very promising candidates in material engineering to obtain novel conductive materials with proton motion coupled with electron conductivity. Previously, we investigated several Schiff bases with neutral and ionic hydrogen bonds [28–40]. It was found, on the basis of NMR and X-ray measurements, that the proton position in the hydrogen bond bridge depends on both the character and position of substituents in the aromatic ring and on the character of the base used. The proton transfer process, from the oxygen atom to the nitrogen site was investigated [35,36]. To identify the major differences between both types of hydrogen bonds, we performed systematic experimental charge density analyses of several ionic and neutral Schiff bases in the solid state [28]. In the present study we have focused on three model complexes of Ni 2+ , Zn 2+ and Cd 2+ with (SCN) À and a Schiff base ligand (Scheme 1) [HL, L = 2-[(2-dimethylaminoethylimino)-methyl]- phenolate] resulting in ML(NCS) type complexes where M = Ni 2+ , Zn 2+ , Cd 2+ . In this work we would like to demonstrate the struc- tural and spectroscopic properties of different metal coordination modes. To determine the structures of the complexes we have ap- plied X-ray diffraction and 13 C, 15 N and 113 Cd CPMAS NMR spec- troscopy. A very important aspect of this study is the comparison of the analytical potential of both methods in structural elucida- tion. In such a comparison we have to remember that NMR results are obtained from easy accessible powder samples, whereas the X- ray method requires single crystals. The X-ray structure of the 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.09.020 ⇑ Corresponding author. Tel.: +48 223433318; fax: +48 226326681. E-mail address: wschilf@icho.edu.pl (W. Schilf). Polyhedron 31 (2012) 241–248 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly