Synthesis and NMR characterization of dinuclear Fe(II) organometallic complexes containing a non-equivalently bridging 5-aryl tetrazolate ligand Antonio Palazzi * , Stefano Stagni Dipartimento di Chimica Fisica e Inorganica, University of Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy Received 10 December 2004; accepted 10 December 2004 Available online 7 february 2005 Abstract The synthetic routes to the formation of a wide range of dinuclear Fe(II) organometallic complexes of the general formula [Cp(CO)(L)Fe–N 4 C–C 6 H 4 –CN–Fe(L)(CO)Cp][SO 3 CF 3 ], in which the 4-cyanophenyl-tetrazolate anion N 4 C–C 6 H 4 –CN acts as bridging ligand, are described. 1 H and 13 C NMR characterization of the product complexes indicate the presence of a significant interannular conjugation effect involving the aromatic rings of the organic spacer, the extent of which can be chemically modified by addition of electrophiles such as CH þ 3 and H + . Furthermore, the reversibility of protonation reaction entails the opportunity of modulating the conjugative properties of the title compounds by a proton addition–elimination mechanism. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Dinuclear complexes; 5-Aryl tetrazolate ligand; Interannular conjugation; Molecular switches; NMR spectroscopy 1. Introduction Over the past two decades, the opportunity of repro- ducing devices (wires, switches, etc.) at the molecular le- vel [1a–1c], as well as the chance to transfer the potentially applicable redox and optical properties of coordination compounds to a macroscopic context [2], have prompted the rapid development of di- and poly- nuclear arrays in which two or more metal centres are connected by organic linkers [2,3]. In all cases, the p-conjugated character of the bridging ligand plays a crucial role on determining both the presence of intra- molecular electron transfer processes [4a,4b] and the enhancement of NLO response [5], as it has been shown for organometallic di- and polynuclear complexes [6a– 6c]. Relative to these research areas, aromatic N-hetero- cyclic compounds represent one of the most important class of ligands for the design and the construction of molecular devices [7a,7b] and NLO molecules [8]. How- ever, although a wide variety of bridging ligands have been introduced in recent years, the study of dinuclear complexes bridged by ligands comprised of two non- equivalent binding sites is less common [9]. Therefore, in this paper we report our results about the synthesis and the characterisation of dinuclear systems in which two Fe(II) organometallic fragments are connected by a non-equivalently bridging 5-aryl tetrazolate ligand. Such research can be viewed as the natural extension of our recent studies [10] in which we reported some of the first examples of mononuclear 5-aryl tetrazolate Fe(II) organometallic complexes of the type [Cp(CO)- (L)Fe(N 4 C–C 6 H 4 –CN)] [L = CO; PPh 3 ; P(OCH 3 ) 3 ; CN–2,6-Me 2 C 6 H 3 ], the characterisation of which indi- cated the presence of an extended p-delocalized system involving the transition metal and the aromatic rings of the 5-substituted tetrazolate ligand. The presence of 0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2004.12.007 * Corresponding author. Tel.: +390516446496; fax: +390512093690. E-mail address: palazzi@ms.fci.unibo.it (A. Palazzi). Journal of Organometallic Chemistry 690 (2005) 2052–2061 www.elsevier.com/locate/jorganchem