SYNTHESIS, STRUCTURE, AND ELECTRONIC CHARACTERIZATION OF REACTIVE DIIRON(II) 1,2-BIS(2-HYDROXYBENZAMIDO)BENZENE COMPLEXES AS MODELS FOR METHANE MONOOXYGENASE

The dinuclear iron(II) compound [Fe2(H2Hbab)2(N-MeIm)2], 10, and its DMF solvated form, [Fe2(H2Hbab)2-(DMF)2(N-MeIm)], 11 (H4Hbab = 1,2-bis(2-hydroxybenzamido)benzene), were synthesized, and their structures were determined by X-ray crystallography. Compound 10 crystallizes in triclinic space group PT with a = 9.969 (4) angstrom, b = 12.754 (5) angstrom, c = 9.919 (7) angstrom, alpha = 100.60 (5)degrees, beta = 18.40 (4)degrees, gamma = 77.42 (3)degrees, and Z = 2. The compound is centrosymmetric with two equivalent FeNO4 trigonal-bipyramidal coordination spheres composed of amide oxygen atoms, terminal and bridging phenolate oxygen atoms, and a single nitrogen from N-MeIm. Compound 11 crystallizes in triclinic space group P(I) with a = 13.198 (4) angstrom, b = 20.294 (6) angstrom, c = 9.651 (5) angstrom, a = 90.29 (3)degrees, beta = 98.70 (3)degrees, gamma = 101.54 (2)degrees, and Z = 2. The dinuclear compound 11 contains one FeNO4 and one FeO6 coordination sphere in which one N-MeIm has been displaced by two solvent DMF molecules. The asymmetric Fe2O2 rhombs contain d(Fe-Fe) of 3.165 (7) angstrom (10) and 3.190 (4) angstrom (11), with Fe-O-Fe angles of 98.9 (3)degrees (10) and 97.7 (5)degrees and 100.7 (5)degrees (11). Spectroscopic properties of 10 and 11 are compared to those of fully reduced methane monooxygenase (MMO) and the B2 subunit of ribonucleotide reductase (RRB2). The optical spectrum (270-280 nm, 312 nm, epsilon-M = 30000 vs 280-290 nm for MMO), magnetic exchange properties (spin exchange coupling constant J almost-equal-to +2.5 cm-1 vs weak ferromagnetic coupling for MMO), and Mossbauer spectroscopic parameters (delta = 1.1 8 mm s-1, DELTA-E(Q) = 3.26 mm s-1 for 10 and delta-1 = 1.24 mm s-1, DELTA-E(Q1), = 3.08 MM S-1, delta-2 = 1.29 mm s-1 and DELTA-E(Q2) = 2.54 mm s-1 for frozen DMF solutions of 10 vs delta = 1.30 mm s-1 and DELTA-E(Q) = 3.14 mm s-1 for MMO and delta = 1.26 mm s-1 and DELTA-E(Q) = 3.13 mm s-1 for RRB2) are presented. Cyclic and square-wave voltammetric studies of 11 in DMF revEaled two quasi-reversible one-electron oxidation steps at -500 and -250 mV versus SCE corresponding to [Fe2+,Fe2+] --> [Fe2+,Fe3+] and [Fe2+,Fe3+] --> [Fe3+,Fe3+] core oxidation state transformations. Thus, the mixed-valence species is thermodynamically stable with a comproportionation constant K(com) = 1.7 X 10(4). A comparison of these results with those reported for MMO (-135 and +48 mV vs NHE) is presented. The proton NMR spectrum of DMSO solutions of 11 is reported as are electron spin resonance studies which reveal integer spin features at g almost-equal-to 12 (vs g almost-equal-to 16 for MMO and RRB2) in frozen DMF glasses at 4.2 K.