Synthesis and chemical oxidation of 3-ferrocenylpyrrole and ferrocenyl-substituted triazoles: Iron versus ligand based oxidation Michael Verschoor-Kirss a , Joszef Kreisz a , William Feighery b , William M. Reiff a, * , Christoph M. Frommen a , Rein U. Kirss a, * a Department of Chemistry and Chemical Biology, Northeastern University, Boston, MA 02115, United States b Department of Chemistry, Indiana University-South Bend, South Bend, IN 46634, United States article info Article history: Received 6 January 2009 Received in revised form 8 June 2009 Accepted 10 June 2009 Available online 13 June 2009 Keywords: Copper catalyzed [3+2] cycloaddition reactions Ferrocenyl-1,2,3-triazoles 3-Ferrocenylpyrrole 57 Fe Mössbauer spectroscopy Oxidation of ferrocenes abstract Copper catalyzed [3+2] cycloaddition reactions between ethynylferrocene and benzylazides yields 1-ben- zyl-4-ferrocenyl-1,2,3-triazoles (2–5). Reaction between phenylacetylene and azidoferrocene yields 1- ferrocenyl-4-phenyl-1,2,3-triazole (6). Anodic electrochemistry of 2–6 suggests reversible oxidation at potentials more positive than ferrocene. Chemical oxidation of 2 and 3-ferrocenylpyrrole (1) with dichlo- rodicyanoquinone (DDQ) yields the salts [2 +Å ][DDQ Å ] and [1 +Å ][DDQ Å ], respectively. 57 Fe Mössbauer spectroscopy reveals the presence of low-spin Fe II in [1 +Å ][DDQ Å ] while Fe II is oxidized to low-spin Fe III in [2 +Å ][DDQ Å ]. Magnetization measurements indicate that [1 +Å ][DDQ Å ] is paramagnetic and cannot be viewed as a simple neutral charge transfer complex reminiscent of the mixed stack diamagnetic [ferrocene] 0 [TCNE] 0 . Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Copper catalyzed [3+2] cycloaddition (‘‘click”) reactions have been used to functionalize dendrimers [1], modify graphite surfaces [2] and self-assembled monolayers at electrodes [3] with 4-ferrocenyl-1,2,3-triazoles. The functionalized electrodes allow for studies of electron transfer rates in a variety of redox processes. A fundamental assumption in the latter studies has been that oxidation occurs at the Fe II of the ferrocene moiety. Evidence sup- porting this assumption is found in the spectroelectrochemistry of 1,1 0 -bis(4-pyridyl) ferrocene where oxidation leads to a UV spectrum consistent with formation of a ferrocenium ion [4]. On the other hand, the nitrogen containing ferrocene derivative 3-ferrocenylpyrrole (1) undergoes a reversible, electrochemical oxidation at 0.25 V versus SCE in acetonitrile solution, however, electrochemistry alone could not distinguish between oxidation at the Fe II center and oxidation of the pyrrole substituent [5]. In related work, the premise that oxidation of ferrocenes with phosphorus-containing substituents occurs at iron has recently been explored by several groups [6–9]. Studies of the chemical and electrochemical oxidation of ferrocenyl phosphines indicate that oxidation and/or intramolecular electron transfer results in the apparent loss of electrons from the phosphorus rather than the iron(II) center. For example, electrochemical oxidation of 1,1 0 -bis(diphenylphosphino) ferrocene (dppf) and 1,1 0 -bis(dii- sopropylphosphino) ferrocene (dippf) leads to the dimerization of phosphorus-centered radicals [7]. Similarly, the anodic electro- chemistry of [CpFe(C 5 H 4 )] 3 P, [CpFe(C 5 H 4 )] 2 PPh, [CpFe(C 5 H 4 )] 2 P@ O(Ph) and [CpFe(C 5 H 4 )] 3 P@Se, is consistent with loss of an electron from a largely ferrocenyl-based HOMO with significant phosphine- or phosphine chalcogenide character [8]. In these ferrocenylphos- phines, electrochemical oxidation also appears to lead to products derived from dimerization of phosphorus-centered radicals [9]. Anodic oxidation of 1,1 0 -bis(di-tert-butylpropylphosphino)ferro- cene (dtbpf), however, is reversible and does not lead to dimeriza- tion, reflecting the larger substituents on phosphorus [10]. Chemical oxidation of [CpFe(C 5 H 4 )] 3 P and [CpFe(C 5 H 4 )] 2 PPh with DDQ yields products containing the DDQ Å anion and low- spin Fe II [11]. The possibility that oxidation of ferrocenylpyrrole and related ferrocene-substituted nitrogen heterocycles might lead to loss of an electron from a HOMO with significant nitrogen char- acter rather than iron, led to the present study of the chemical oxi- dation of 1 and 1-benzyl-4-ferrocenyl-1,2,3-triazole (2). In this paper, we report on the synthesis and characterization of five ferrocene-substituted triazoles, 1-benzyl-4-ferrocenyl-1,2,3- triazole (2), 1-(p-methylbenzyl)-4-ferrocenyl-1,2,3-triazole (3), 1-(m-bromobenzyl)-4-ferrocenyl-1,2,3-triazole (4), 1-(p-bromo- benzyl)-4-ferrocenyl-1,2,3-triazole (5) and 1-ferrocenyl-4-phenyl- 1,2,3-triazole (6). We also describe the differences in the chemical oxidation products of 1 and 2. 0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2009.06.011 * Corresponding author. Tel.: +1 617 373 4513; fax: +1 617 373 8795. E-mail address: rkirss@neu.edu (R.U. Kirss). Journal of Organometallic Chemistry 694 (2009) 3262–3269 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem