Inorganica Chimica Acta, 56 (1981) L9-LlO @Elsevier Sequoia %A., Lausanne - Printed in Switzerland L9 A Synthetic Iron-Sulfur Cluster with Phenoxide Terminal Ligands WALTER E. CLELAND and BRUCE A. AVERILL* Department zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Chemistry, Michigan State University, East Lansing, Mich. 48824, U.S.A. Received February 24, 1981 In addition to the iron-molybdenum cofactor [II* a novel molybdenum-iron-sulfur cluster of as yet unknown structure [2], the molybdenum- iron protein of nitrogenase contains additionaliron- sulfur centers released as 4Fe-4S clusters upon treatment with thiols in denaturing organic sol- vents [3]. The only technique thus far used to examine these centers in situ is “Fe Miissbauer spectroscopy, which has shown that they contain iron in two distinct sites in a I:3 ratio. Three iron atoms termed D (with a small quadrupole split- ting) and one iron atom termed Fe*+ (with Mdss- bauer parameters typical of high-spin Fe*+ in a tetrahedral sulfur environment) comprise a dia- magnetic (S = 0) unit, called a ‘P-cluster’ [4]. This and the fact that oxidation by one electron per P- cluster yields a paramagnetic state (S > 5/2) has led to the conclusion that the P-clusters are a variant of normal 4Fe-4S clusters in the fully reduced ([4Fe+S]“) oxidation state. The most likely means of differentiating the iron atoms D from Fe*’ is coordination of the former by non-sulfur protein ligands, with oxygen (phenoxide or carboxylate) being most likely. Holm [5] has generated in solu- tion and measured some physical properties of a carboxylate-substituted iron-sulfur tetramer, but to date no iron-sulfur clusters with oxygen ligands have been prepared in pure form. We report herein the preparation and some of the properties of the first such synthetic complex, (EtQN)z [Fe$&(OPh),] (0. Reaction of 40 equivalents of anhydrous, sublim- ed phenol with (Et4N)2[Fe4S4(SEt)4] in aceto- nitrile establishes an equilibrium in which a small fraction of bound ethanethiolate is displaced by phenol [6]. Repeated removal of solvent in vacua produces (Et4N)* [Fe&(OPh),] (I), isolated as well- formed red-black plates upon recrystallization from dry acetonitrile/isopropanol. Anal. : Calcd. for &- H60Fe4N204S4: C, 48.80; H, 6.14; Fe, 22.69; N, 2.84; 0, 6.50; S, 13.03. Found: C, 48.49; H, 6.26; Fe, 22.08; N, 2.87; 0,6.93; S, 13.14. *Author to whom correspondence should be addressed. The optical spectrum of I in acetonitrile solution has maxima at 410 nm and 239 nm (E =15,700 and 44,000 i@ cm-‘), respectively, with shoulders at 650, 320, and 272 nm. These bands are thus blue- shifted by 38 and 21 mn, respectively, compared to similar features observed [7] for (Et4N)2[Fe&- (SPh),] (II), as expected upon replacing a sulfur donor by more electronegative oxygen. The magnetic moment per iron, &e, is 1 .19 c(~ for Z in the solid state at 22 “c, essentially unchanged from that report- ed [8] for ZZ (1.09 pg). In zero applied field, the “Fe MCissbauer sp c e trum of I shows only a simple quadrupole doublet with 6 = 0.50 and AEQ = 1.21 mm/set at 4.2 K (vs. metallic Fe at room tempera- ture), compared to 0.35 and 1.10 mm/set for ZZ [9]. More interesting are ‘H NMR spectra and electrochemical measurements, which give unexpect- ed results. Isotropically shifted ‘H NMR spectra are observed for Z in CD&N solution; the observed shifts, (a-, m- , p-H at t2.28, -2.25, and +2.83 ppm vs. free PhOH at 22 “c) show a pattern typical of dominant contact interaction. At any temperature, however, the magnitude of the shifts for Z is approximately twice that observed [lo] for II, suggesting signifi- cantly greater delocalization of spin into the phenyl rings of the former, a result of the greater covalent character of the Fe-O bond. All isotropic shifts increase in magnitude with increasing temperature, as expected for an antiferromagnetically coupled system [lo]. Electrochemical measurements (DC polarography at DME; cyclic voltammetry at plati- num flag) show that Z is reduced sequentially in reversible one-electron steps at -1.08 and -1.80 vs. SCE, compared to -0.98 and -1.66 V for ZZ. Phen- oxide ligation thus makes the [4Fe-%]*+ core more difficult to reduce (by -100 mV) than for thio- phenoxide; for comparison, substitution by chloride [l l] (in the [Fe4S4C14]*- ion) causes a positive shift in E, of about 200 mV, while carboxylate substitution (in [Fe4S4 (OAc),] “3 causes a positive shift of -100 mV [ 51. The seemingly anomalous electrochemical data may be explained by the high affinity of phenoxide ligands for ferric ion. Reduc- tion of the [4FedS]*+ core will decrease the ferric character of the iron (if a delocalized description [ 121 is still correct in this case), and is therefore more difficult with phenoxide ligands. Our results show that simple iron-sulfur clusters with phenoxide ligands are stable chemical species capable of existing in several net oxidation states. The most notable effect of substituting phenoxide for thiophenoxide is a negative shift of both first and second reduction potentials, indicating that Fe,& centers coordinated to a protein via tyrosi- nate residues will have reduction potentials either comparable to or slightly more negative than normal