FORMATION OF FE(III)FE(IV) SPECIES FROM THE REACTION BETWEEN A DIIRON(II) COMPLEX AND DIOXYGEN : RELEVANCE TO RIBONUCLEOTIDE REDUCTASE INTERMEDIATE X

The reaction of m-terphenyl-based carboxylic acids with ferrous salts produces novel tetracarboxylate dinuclear clusters through an extraordinarily efficient self-assembly process. Recently, we reported the preparation of one such compound, [Fe2(μ-O2CAr)2(O2CAr)2(C5H5N)2] (1), and highlighted its unique reactivity with dioxygen.1 Efforts to explore this chemistry further were hampered by the poor solubility of 1 in nonpolar solvents, thus prompting the synthesis of new ArCO2-derived adducts. Here we describe a tetracarboxylate-bridged complex, [Fe2(μ-O2CAr)4(4-BuC5H4N)2] (2), that reacts with O2 to furnish [Fe2(μ-OH)2(μ-O2CAr)2(O2CAr)2(4-BuC5H4N)2] (4). This process parallels the oxygenation reaction of 1, proceeding through a metastable green intermediate 3 that decays to afford 4 in isolated yields exceeding 75%. Studies with 2 have made possible the characterization and assignment of 3 as a mixture containing equimolar quantities of mixed-valent species FeIIIFeIV and FeIIFeIII. The formation of a high-valent diiron cluster from 2 and dioxygen mimics closely the purported mechanistic chemistry of certain diiron metalloenzymes.2 Additionally, the FeIIIFeIV component in 3 represents a putative model for a key intermediate, X, in the reaction cycle of the R2 subunit of ribonucleotide reductase (RNR-R2).3,4 To the best of our knowledge, the formation of 3 is the first example of a process that utilizes dioxygen to access the FeIV oxidation state in synthetic model complexes.5 Neutral tetracarboxylate complex 2 was prepared upon treatment of [Fe2(μ-O2CAr)2(O2CAr)2(THF)2] with 2 equiv of 4-tert-butylpyridine (Scheme 1). The structure of 2 reveals a dinuclear adduct in which four ArCO2 groups span the two metal centers (Figure S1). Each iron in 2 is square pyramidal and has a coordination geometry that closely resembles those of previously reported diiron(II) paddlewheel complexes.6 Oxygenation of a CH2Cl2 solution of 2 at -78 °C resulted in the irreversible generation of a deep green solution 3 with a broad visible absorption centered at ∼670 nm ( ) 1700 M-1 cm-1). At low temperature, 3 is stable for >12 h, but it slowly decays upon warming above -65 °C to afford a yellow material (Figure S2). X-ray analysis established this product to be the bis(μhydroxo)diiron(III) complex 4 (Scheme 1 and Figures S3, S4). Compound 4 is structurally analogous to that obtained from oxidation of 1,1 having a short Fe‚‚‚Fe separation of ∼2.84 A due to the presence of four bridging ligands. A weak ferromagnetic interaction, J ) 0.63(5) cm-1 with g ) 2.00(1),7 was observed by SQUID susceptometry on solid samples of 4. X-Band EPR spectra collected on frozen CH2Cl2 samples of 3 exhibited a strong isotropic g ) 2 signal and a less intense absorption at g ) 10 (Figure 1). Quantitation of these two species accounted for 70% of the total iron, the former signal contributing 40% and the latter 30%. The X-band signal at g ) 2, with a width of ∼28 G, originates from an S ) /2 species which shows resolved g-anisotropy at Q-band (see inset). The two simulations overlaid on the data use the same parameter set of g ) 1.986, 1.997, and 2.011. This g-anisotropy is similar to that of the RNRR2 X (g ) 1.994, 1.999, and 2.007) signal,8,9 which arises from the antiferromagnetically coupled FeIIIFeIV core in this enzyme intermediate. The signals at g ) 10 (Xand Q-band) and g ) 4.3 and 2.8 (Q-band) originate from an S ) /2 species, and the simulations overlaid on the data were obtained with D ) 1.2 cm-1, E/D ) 0.013, and g ) 2.00.10 Both the S ) /2 and S ) /2 signals display Curie law behavior up to 150 K, indicating exchange interactions of |2J| > 200 cm-1 for the former and > 50 cm-1 for the latter.7 The similarity of the S ) /2 EPR signal with that of a complex prepared by one-electron chemical oxidation of 211 leads us to assign it as the corresponding FeIIFeIII cation. The S ) /2 spin state can result either from a ferromagnetic interaction between iron centers or by electron delocalization for which a double-exchange mechanism12 is dominant. Figure 2 displays the Mossbauer spectra of a solid powder sample of 3 recorded at 4.2 K with a 50-mT magnetic field applied parallel (A) and perpendicular (B) to the γ-rays. The spectra may be deconvoluted into three major components. A central quadrupole doublet (marked by brackets) with apparent Mossbauer parameters of ∆EQ ) 1.13 mm/s and δ ) 0.54 mm/s is assigned † Massachusetts Institute of Technology. ‡ Carnegie Mellon University. § Emory University. (1) Lee, D.; Lippard, S. J. J. Am. Chem. Soc. 1998, 120, 12153-12154. (2) (a) Feig, A. L.; Lippard, S. J. Chem. ReV. 1994, 94, 759-805. (b) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625-2657. (c)Valentine, A. M.; Lippard, S. J. J. Chem. Soc., Dalton Trans. 1997, 3925-3931. (d) Que, L., Jr. J. Chem. Soc., Dalton Trans. 1997, 3933-3940. (3) Riggs-Gelasco, P. J.; Shu, L.; Chen, S.; Burdi, D.; Huynh, B. H.; Que, L., Jr.; Stubbe, J. J. Am. Chem. Soc. 1998, 120, 849-860. (4) Stubbe, J.; van der Donk, W. A. Chem. ReV. 1998, 98, 705-762. (5) Bis(μ-oxo)diiron(III, IV) complexes were prepared by oxidizing diiron(III) complexes with H2O2 at low temperature: (a) Dong, Y.; Fujii, H.; Hendrich, M. P.; Leising, R. A.; Pan, G.; Randall, C. R.; Wilkinson, E. C.; Zang, Y.; Que, L., Jr.; Fox, B. G.; Kauffmann, K.; Munck, E. J. Am. Chem. Soc. 1995, 117, 2778-2792. (b) Dong, Y.; Que, L., Jr.; Kauffmann, K.; Munck, E. J. Am. Chem. Soc. 1995, 117, 11377-11378. (c) Hsu, H.-F.; Dong, Y.; Shu, L.; Young, V. G., Jr.; Que, L., Jr. J. Am. Chem. Soc. 1999, 121, 52305237. (6) Randall, C. R.; Shu, L.; Chiou, Y.-M.; Hagen, K. S.; Ito, M.; Kitajima, N.; Lachicotte, R. J.; Zang, Y.; Que, L., Jr. Inorg. Chem. 1995, 34, 10361039. (7) H ) -2JS1‚S2. (8) Burdi, D.; Sturgeon, B. E.; Tong, W. H.; Stubbe, J.; Hoffman, B. M. J. Am. Chem. Soc. 1996, 118, 281-282. (9) Sturgeon, B. E.; Burdi, D.; Chen, S.; Huynh, B. H.; Edmondson, D. E.; Stubbe, J.; Hoffman, B. M. J. Am. Chem. Soc. 1996, 118, 7551-7557. (10) The resonances near g ) 4 in Q-band for both parallel and perpendicular modes are from an intradoublet transition and are not included in the simulation. The g ) 19 (X-band) and possibly the g ) 29 (Q-band) signals are from the diiron(III) species, whereas the g ) 16 signal (Q-band) and its simulation are from unreacted 2, which account for 20 and 10% of the total iron, respectively. (11) Lee, D.; Lippard, S. J., manuscript in preparation. Scheme 1 9893 J. Am. Chem. Soc. 1999, 121, 9893-9894