J. Indian Chern. Soc., Vol. 77, November-December 2000, pp. S47-SSI Metal bound azo anion radicals t Prasanta Ghosh, Kausikisankar Pramanik, Maya Shivakumar and Animesh Chakravorty* Department oflnorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta-700 032, India E-mail : icac@mahendra.iacs.res.in Manuscnpt rece1ved 17 July 2000 Using azopyridine ligands (L) the first anion radical complexes of ruthenium( H) have been synthcsiLed using hydrhllc t- ing mate.-ials which supply both the metal and the reducing equivalent. Species incorporating Ru 11 (L Ru 11 (1.'-) 2 and Ru 11 (L)(L fragments have been isolated, the coligands being PPh 3 , Cl-, CO and u-. Weak acids including y,ater readil) convert the radicals to nonradical species. The X-ray structure, spectra, EHMO frontier orbitals. magnetism and electro- chemistry of the species have been scrutinized and correlated. Among catenated nitrogen compounds, N-N bond orders of l and 2 occur as in hydrazines and azo compounds, res- pectively. The intermediate bond order 1.5 corresponds to the anion radical state formed by addition of an electron to the azo-n* orbital as in eq. (1). Reductive generation ofazo anion radicals in solution has been known for long 1 -4 but their isolation in the free state (in metal coordinated form) was achieved only very recently in this laboratoryS-8 and elsewhere 9 • 10 . In this work we summarize some of our findings. -N = N- + e ----7- N .,_,_, N -- ( a2 n2) ( a2n2n* I) Azo ligands, metals and synthetic strategy (I) The azopyridines 1 and 2, generally abbreviated as L, were explored as ligands because oftheir documented facile reducibility in solution to anion radical species 4 • 11 - 13 . These ligands have excellent affinities for the 1t-basic metal ions specially ruthenium(ll) and osmium(II). In this work we shall limit ourselves to the case ofruthenium(II). R=H,pap R=CI,Cipap abp 2 Since the synthesis of crystalline anion radical species could not be achieved via the reduction of preformed com- plexes of L, we searched for a qualitatively different syn- thetic route. We looked for a starting material in which the obligatory reducing equivalent as well as certain radt.;al sta- bilizing features are already built-in. It was hoped that the reaction of such a material with L might afford Ru(L'-) di- rectly. Here L'- is the anion radical ligand, (2) In practice, hydrides 14 3 and 4 turned out to be excellent starting materials, the reducing equivalent being supplied by the hydridic ligand, eq. (2). 3 4 Selected radical chelates isolated by this route are col- lected in Table 1. Table 1 Representative radtcal and nonradtcal complexes (PPh- ) 2 ]. Sa [Ru 11 (Clpap" )(Cl)(CO) (PPh 3 ) 2 ], Sb (PPh 3 h]. Sc [Ru 11 (abp"-h(CO)(PPh 3 )], 8 [Ru 11 (abp}(abp'-)(C0)(PPh 3 )] PF 6 . s+PF6 [Ru 11 (abp'-)(H)(CO)(PPh 3 ) 2 ], 9 [Ru 11 (pap)( Cl )(CO )(PPh3 l2J PF 6 . Sa 'Pf 6 JRu 11 (Clpap)(Cl)(C0)(PPhl)2j PF 6 . Sb'PF 6 [Ru 11 (abp)(Cl)(C0)(PPh 3 l 2 ] PF 6 . Sc+Pf6 [Ru 11 (pal)(H)(Cl)(C0)(PPh 3 )], 7 [Ru 11 (abp)(l{)(CO)(PPh 3 ) 2 ] PF 6 • 9+PF6 [Ru 11 (Clpap)(H)(CO)(PPh 1 J 2 ] PF6 10TPF6 t Acharya J C Memonal Lecture ( 1997) delivered under the ausptces of Indtan Chemtcal Soctety on December II. 1999 at Calcutta 547