STRUCTURE AND ELECTROCHEMICAL STUDIES OF [(TRISPICMEEN)CLFEIIIOFEIIICL(TRISPICMEEN)](2- SPECTROSCOPIC CHARACTERIZATION OF THE MIXED-VALENCEFEIIIOFEII FORM - RELEVANCE TO THE ACTIVE-SITE OF DINUCLEAR IRON-OXO PROTEINS() )

The dinuclear species lFeOFeCl)-O-III-Cl-III(trispicMeen)]Cl(OH)(H2O)( 7) (1) (trispicMeen = N,N,N'-tris-(2-pyridylmethyl)-N'-methylethane- 1 ,2-diamine) was synthesized. It crystallizes in the monoclinic space g roup C2/c with a = 33.87(2) Angstrom, b = 17.42(2) Angstrom, c = 23.41 (5) Angstrom, beta = 132.88(5)degrees, V = 10 121(25) Angstrom(3), and Z = 8. It contains an almost linear unit (Fe-O-Fe angle = 177.4(7)deg rees). The potentially pentadentate Ligand is in fact only tetracoordi nated with one pyridine not bound to the metal ion. The octahedral coo rdination of Fe(Ln) is completed by one chloride ion. The structure of [(bispicMeen)ClFeOFeCl(bispicMeen)]Cl-2 . CH3COCH3 . 2H(2)O (2) (bisp icMeen = -bis(2-pyridylmethyl)-N'-methylethane-1,2-diamine) was also d etermined. It crystallizes in the monoclinic space group C2/c with a 1 1.124(4) Angstrom, b = 22.769(9) Angstrom, c = 15.874(6) Angstrom, bet a = 97.79(4)degrees, V = 3984(3) Angstrom(3), and Z = 4. The main diff erence from 1 is that, in 2, the Fe-O-Fe unit is bent with an FeOFe an gle 152.3(3)degrees. In cyclic voltammetry, 1 exhibits two reduction p eaks at -0.230 and -0.960 V/SCE. They correspond respectively to the r eduction to the (FeFeIII)-Fe-II and (FeFeIII)-Fe-II states. Cyclic vol tammetry shows that the mixed-valent form ispicMeen)(ClFeOFeCl)-O-II-C l-III(trispicMeen)](+) (E degrees = -0.175 V/SCE) is in equilibrium wi th another species (E degrees = +0.065 V/SCE) proposed to be rispicMee n)(FeOFeCl)-O-II-Cl-III(trispicMeen)](2+) in which a chloride ion has been displaced by the originally unbound pyridine. The equilibrium con stant was estimated to be 90 M(-1), and the rate of the recombination of chloride to the ispicMeen)(FeOFeCl)-O-III-Cl-III(trispicMeen)](3+) complex was found equal to 3 x 10(5) M(-1) s(-1). Controlled potential electrolysis of an acetonitrile solution of 1 allowed the preparation of the mixed-valent Fe(II)-O-Fe(III) form which gives an almost isotr opic EPR signal similar to that already observed with ore-bridged mode l compounds (Holt, et al. Inorg. Chem. 1993, 32, 5844. Hartman, et al. J. Am. Chem. Sec. 1987, 109, 7387) but different from the rhombic one observed in the mixed valent form of MMO. The mixed-valent forms slow ly disproportionate to a mixture of (FeFeIII)-Fe-III and Fe(II) forms. The mixed-valent forms could be generated by radiolysis at 77 K, and an EPR study of the mixed-valent forms obtained by this procedure demo nstrated that these species could not be protonated. Radiolysis of 1 a t 77 K afforded the EPR spectrum of spicMeen)(ClFeOFeCl)-O-II-Cl-III(t rispicMeen)](+); upon annealing at 200 K, the solution gave an EPR spe ctrum very similar to that observed on the electrochemically reduced s olution. This is in agreement with the observation of substitution of a chloride ligand. The mixed-valent form was not detected with the ana logous complexes of the bispicen family: [(bispicMeen)ClFeOFeCl(bispic Meen)]Cl-2 . CH3COCH3 . 2H(2)O (2) and [(bispicMe(2)en)ClFeOFeCl(bispi cMe(2)en)]Cl-2 (3). Upon reduction, these complexes quickly (at CV tim e scale) decompose to mononuclear species.