Imidazole ! imidazolidine. Preparation by reduction of ½OsðHÞðCOÞðPPh 3 Þ 2 ðPyaiRÞ þ=0 with NaBH 4 and characterisation of the products (PyaiR = 1-alkyl-2-{3 0 -(pyridylazo)}imidazole) T.K. Mondal a , T. Mathur a , A.M.Z. Slawin b , J.D. Woollins b , C. Sinha a, * a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India b Department of Chemistry, University of St-Andrews, St-Andrews KY16 9ST, UK Received 22 July 2006; received in revised form 15 October 2006; accepted 27 November 2006 Available online 6 December 2006 Abstract The reaction of 2-(3 0 -pyridylazo)imidazole (PyaiH) or 1-alkyl-2-(3 0 -pyridylazo)imidazole (PyaiR) with ½OsðHÞðClÞðCOÞðPPh 3 Þ 3  in THF or MeCN has synthesized stable red coloured ½OsðHÞðCOÞðPPh 3 Þ 2 ðPyaiÞ (2a) or ½OsðHÞðCOÞðPPh 3 Þ 2 ðPyaiRÞðPF 6 Þ (2b–2d). While the reaction of PyaiR and ½OsðHÞðClÞðCOÞðPPh 3 Þ 3  in dry n-heptane has separated shining green compound and has been identified azo- anion radical, [Os(Cl)(CO)(PPh 3 ) 2 (PyaiR Å )] (4b–4d). The reaction performance in n-hexane or toluene is very poor. Upon addition of Cl 2 in MeCN to CH 2 Cl 2 solution of 2 or 4 the colour has changed sharply to red-violet and has isolated ½OsðClÞðCOÞðPPh 3 Þ 2 ðPyaiRÞ þ (3). The reaction of 2 in ethanol with NaBH 4 suspension under dinitrogen has reduced coordinated imidazole group to imidazolidine (5). One of the structures of azo-imidazolidinate-hydrido-osmium–carbonyl, [Os(H)(CO)(PPh 3 ) 2 (PyaiH 2 )] (5a), is characterized by X-ray dif- fraction study. Other spectroscopic studies (IR, UV–Vis, NMR) are used to identify the structure of all the complexes. Electrochemistry shows two consecutive anodic peaks (E pa ) and suggest irreversible oxidations Os(III)/Os(II), Os(IV)/Os(III) along with azo reductions. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Pyridylazoimidazole; Azo-anion radical; Imidazolidine; X-ray structure; Electrochemistry 1. Introduction Progress in polypyridine metal complexes [1,2] has encouraged syntheses of complexes of other N-heterocycles [3]. Imidazole and imidazole containing ligands are chosen because of their chemical and biological ubiquity [4]. We have been engaged in the design of azo-functionalised imid- azole ligands those carry azoimine, –N@N–C@N–, function [5]. Azo group is p-acidic, responsible for photochromism, pH-responsive and shows redox activity to the molecules. For the last several years we have been engaged in the transition metal chemistry of arylazoimidazoles [5–9]. Ruthenium–carbonyl complexes of azoheterocycles have attracted recent interest [10,11]. Osmium–carbonyl com- plexes are relatively less explored [12] than ruthenium. In this context we are interested to synthesize azoheterocyclic osmium complexes. Metal–carbonyls are efficient system to stabilize M–H bonds. Transition metal hydrides are good candidates to catalyze hydrogenations. The strength of L n M–H bonds depends on the electronegativity of the metal center and the nature of ligand [13]. Hydridic starting mate- rials [M(H)(Cl)(CO)(PPh 3 ) 3 ] (M = Ru, Os) has inherent reducing equivalent and has been used to generate radical anion by M–H bond cleavage [11,12] (Eq. (1)). Arylazohet- erocycle (L) is reduced to ½MðXÞðCOÞðPPh 3 Þ 2 ðL  Þ under extremely dry oxygen free condition. The hydrido com- plexes are turned to dihydrogen complexes (g 2 -H 2 ) upon addition of acid and capable to synthesize different substi- tuted complexes as well as some interesting compounds by inserting unsaturated molecules (like RC„CH, CO 2 , CS 2 , ArN þ 2 ) [13,14]. In this work we account on the reaction of [Os(H)(Cl)- (CO)(PPh 3 ) 3 ]with 2-(3 0 -pyridylazo)imidazoles (PyaiH) and 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2006.11.044 * Corresponding author. Fax: +91 33 24146584. E-mail address: c_r_sinha@yahoo.com (C. Sinha). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 692 (2007) 1472–1481