Hydrogen binding in coordination compounds of 3(5)-(4-methoxyphenyl)pyrazole Susann Bergner, Gotthelf Wolmersha ¨user, Harald Kelm, Werner R. Thiel * FB Chemie, Technische Universita ¨ t Kaiserslautern, Erwin-Schro ¨ dinger-Strasse, Geb. 54, 67633 Kaiserslautern, Germany Received 18 May 2007; received in revised form 10 October 2007; accepted 21 October 2007 Available online 26 October 2007 Abstract The solid state structures of 3(5)-(4-methoxyphenyl)pyrazole and its coordination compounds with a series of two valent transition metals have been investigated. Since pyrazoles provide not only a nitrogen donor site for the coordination to metal ions, but also an additional N–H function, they are ideal ligands for the formation of hydrogen bound coordination polymers or for the implementation of secondary interactions with other ligands bound to the same central ion, resulting in a rigid ligand environment at the central metal. We chose cobalt, nickel, palladium, copper and zinc as twofold positively charged Lewis acids preferring coordination numbers of four and six to prove the capability of pyrazole to undergo intramolecular hydrogen bonds. In the four-coordinate mode, either tetrahedral (Zn 2+ ) or square planar coordination geometries (Pd 2+ ) are possible, providing different geometric restrictions for hydrogen bonding. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Pyrazole ligands; Cobalt; Nickel; Palladium; Zinc; Copper; Hydrogen bonds; Crystal structure 1. Introduction At a first glance, the geometry of coordination com- pounds is determined by the nature of the central ion and the number and the molecular structure of the ligands. However, this definition results from a static view on the shape of these systems, disregarding dynamic processes going on in the ligand sphere. Ligand exchange reactions, which are of severe importance especially for the coordina- tion compounds of main group and f-block elements, lead to a permanent change of the shape. To overcome this and to create complexes of high stability, a multitude of chelat- ing ligands have been introduced into coordination chemis- try. But even chelating ligands may undergo rotations of side chains or inversions of the chelating ring system. One further disadvantage of chelating ligands in this con- text is the reduced solubility of the derived complexes, which is closely related to the reduced mobility. Addition- ally, more complex procedures are required for the synthe- sis of chelating compared to monodentate ligands. A different strategy for the suppression of ligand fluxio- nality without using chelating ligands is to use, in addition to the metal–ligand interaction, secondary, non-covalent bonds in the ligand sphere of the complex. This will fix the ligands in defined positions at the metal site. One way to do this is to introduce hydrogen bonds in close proxim- ity to the central ion, which can be realized by applying ligands possessing highly polar X–H groups in addition to the donor site. Pyrazole represents a very simple exam- ple for such a ligand system. Scheme 1 shows the secondary H–X interaction freezing the rotation around the M–N bond in comparison to the open system with the isomeric imidazole ligand. Monodentate pyrazoles provide a plethora of different substitution patterns, most of them are not realizable in chelating ligands. Additionally, coordination to a Lewis- acidic metal ion will increase the polarization of the N–H bond of the pyrazole and, therefore, increase its capability to be involved in hydrogen binding. 0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.10.022 * Corresponding author. Tel.: +49 631 2052752; fax: +49 631 2054676. E-mail address: thiel@chemie.uni-kl.de (W.R. Thiel). www.elsevier.com/locate/ica Available online at www.sciencedirect.com Inorganica Chimica Acta 361 (2008) 2059–2069