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oxygen molecule on the iron(iii) catecholate moiety.

COMMUNICATIONS[(I] Scveriil comp~)tind\ of thc type PhSeSiR, (R, = Me,, Et,. Me,Ph. Me,rBu. Ph,iBu) Mere Acreened for this purpose; however, compound 1 was hest suited fool the radicnl chain group transfer reaction because ofits stability to hydrolysis [J] 1351. _I. H. Byen. T G. Gleason, K S. Knight. J. Clirm. So<,. C/iriii. Cmiiiiuii. 19YI. 354 356. 1x1 T. H:iinnd,i. A Nishida. 0. Yonemitsu. J Ain. Cheni So< 1986. 108, 140-145: 1' . Hamada. .A Nishida. 0 Yonemitsu. ihid. 1980. /02. 3978-3Y80. [O] 1). Rchin. A Wcllei.. 1 ,~. J C/im. 1970. 8. 259 [7] M. Nchcomb. D J. Marquardt. M. U. Kumar. 7?7rtro/itvhii 19Y0, 46. 2345 [lo] The ouidatioii potential (€(";I = 1.2X eV;SCE) and energy orexcitation (E,,<> = 367 37 k J ) value\ for DMN wcre taken from ref. [8]. whereas the E\T',"]of 1 wiis rmined by cyclic voltammetry. by using ii PAR 173 potentiostat coupled with a PAR 175 universal progammer, and the data werc recorded on :I RE0091 X-Y recorder: Pt UME (Ultra micro electrode) working electrode. Pt foil counter electrode; tetrabutylammonium hexafluurohorate ITBABF,) in dry CH ,CN as supporting electrolyte. The CH,CN rolution wi\ degassed hy argon bubbling for 10 min before measurement. The sciiii rille wii\ 500 iiiVb-' and values :ire relative to SCE. [12] TIIS G i b b frcc cnergy was determined with theequation AG,, = E[,r;l -Ei';d'. i n ~. h i c h the El'';] value of H,A estimated by cyclic \oltanimetry [lo] w a y 1.OX4cV'SCI~ and the E\'Sdl value for DMN" was 1.28 eV:SCE [XI.

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The reaction of iron(ii) perchlorate with the tetraazamacrocyclic ligand N,N'-dimethyl-2,11-diaza[3.3]-(2,6)pyridinophane (L-N 4 Me 2 ) and 3,5-di-tert-butyl-1,2-benzoquinone in 96 % ethanol yields the blue compound Me 2 )(dbsq)](ClO 4 ) 2´2 .5 H 2 O (3 a, dbsq À 3,5-di-tert-butyl-1,2-benzosemiquinonate). On the basis of structural, Mössbauer spectroscopic, and magnetic evidence, this compound was identified as a low-spin iron(iii) semiquinonate complex, the first of its kind, in which the unpaired electron of the coordinated semiquinonate radical is strongly antiferromagnetically coupled with the unpaired electron of the lowspin iron(iii) ion. In acetonitrile solution, Me 2 )(dbsq)] 2 (3) is in equilibrium with uncoordinated dbq and with the low-spin iron(ii) complex ions. Solutions of complex 3 in acetonitrile are found to be stable towards molecular oxygen. In addition, the reaction of the iron(iii) semiquinonate complex 3 with superoxide quantitatively yields the corresponding iron(iii) catecholate complex Me 2 )(dbc)](2). Therefore, the reactivity of 3 with molecular oxygen and with superoxide demonstrates that the correct oxidation states of both the metal ion and the coordinated dioxolene unit are required for the occurrence of the well-established cleavage of the intradiol C ± C bond of 3,5-di-tert-butylcatecholate ligand coordinated to the iron(iii) ion in 2 by molecular oxygen and that the cleavage reaction does not occur through an initial electron-transfer step, resulting in the formation of an iron(iii) semiquinonate as intermediate, but instead by the direct attack of the oxygen molecule on the iron(iii) catecholate moiety.

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