Imidazole -> imidazolidine. Preparation by reduction of [Os(H)(CO)(PPh3)(2)(PyaiR)](+/0) with NaBH4 and characterisation of the products (PyaiR=1-alkyl-2-{3 '-(pyridylazo)}imidazole)

The reaction of 2-(3'-pyridylazo)imidazole (PyaiH) or 1-alkyl-2-(3'-pyridylazo)imidazole (PyaiR) with [Os(H)(Cl)(CO)(PPh3)(3)] in THF or MeCN has synthesized stable red coloured [Os(H) (CO) (PPh3)(2) (Pyai)] (2a) or [Os(H) (CO) (PPh3)(2) (PyaiR)] (PF6) (2b-2d). While the reaction of PyaiR and [Os(H)(Cl)(CO)(PPh3)(3)] in dry n-heptane has separated shining green compound and has been identified azo-anion radical, [Os(Cl)(CO)(PPh3)(2)(PyaiR(-.))] (4b-4d). The reaction performance in n-hexane or toluene is very poor. Upon addition of Cl-2 in MeCN to CH2Cl2 solution of 2 or 4 the colour has changed sharply to red-violet and has isolated [Os(Cl)(CO)(PPh3)(2)(PyaiR)](+) (3). The reaction of 2 in ethanol with NaBH4 Suspension under dinitrogen has reduced coordinated imidazole group to imidazolidine (5). One of the structures of azo-imidazolidinate-hydrido-osmium-carbonyl, [Os(H)(CO)(PPh3)(2)(PyaiH(2))](5a), is characterized by X-ray diffraction 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. (c) 2006 Elsevier B.V. All rights reserved.