HIGH-VALENT TRANSITION-METAL CHEMISTRY - MOSSBAUER AND EPR STUDIES OF HIGH-SPIN (S=2) IRON(IV) AND INTERMEDIATE-SPIN (S=3/2) IRON(III) COMPLEXES WITH A MACROCYCLIC TETRAAMIDO-N LIGAND

Synthetic iron(IV) complexes are attracting interest both as models for putative intermediates of biochemical reactions as well as for proposed catalytic entities. We have previously reported the X-ray structure of [Et4N][Fe(IV)Cl(eta4-MAC)] (H-4[MAC] = 1,4,8,11-tetraaza-13,13-diethyl-2,2,5,5,7,7,10,10-octamethyl-3,6,9,12,14-pentaoxocy-clotetradecane), where iron is coordinated to a plane of four amide nitrogen anions of a macrocyclic ligand and to one axial chloride (Collins, T. J.; Kostka, K. L.; Munck, E.; Uffelman, E. J. Am. Chem. Soc. 1990,112, 5637-5639). In zero magnetic field, the 4.2 K Mossbauer spectrum of [Et4N] [FeIVCl(eta4-MAC)] was a single quadrupole doublet with DELTAE(Q) = 0.89 mm/s and delta = -0.02 mm/s, consistent with an iron(IV) assignment. Here we present full synthetic and characterization results together with detailed Mossbauer and integer spin EPR studies of [FeIVCl(eta4-MAC)]- prepared with [Ph4P]+, [Et4N]+, [n-Bu4N]+, and [PPN]+ counterions. In strong applied fields, the Mossbauer spectra exhibit magnetic hyperfine patterns typical of complexes with integer electronic spin. The zero-field splitting parameters (D = -2.6 cm-1 and E/D = 0. 1 3) are such that the two lowest spin levels of the ground multiplet are nearly degenerate (DELTA = 0.16 cm-1). Correspondingly, [FeIVCl(eta4-MAC)]- exhibits an integer spin EPR resonance at X-band with g(eff) = 8.0, indicative of a high-spin (S = 2) ground configuration. Quantitative analysis of the integer spin EPR spectra observed from both frozen CH3CN solution and from polycrystalline samples shows that the principal g-values are less than 2, as expected for high-spin iron(IV), and that the spin concentration of the S = 2 species agrees within 12% with the concentration determined by optical spectroscopy. We also report synthetic details and the X-ray structure of [Et4N]2[FeIIICl(eta4-MAC)].CH2Cl2.H2O. Crystals of [Et4N]2[FeIIICl(eta4-MAC)].CH2Cl2.H2O at 293 K are monoclinic, space group P2(1)/n, with a = 11.797(9) angstrom, b = 18.662(6) angstrom, c = 21.984(8) angstrom, beta = 102.75(6)degrees, V = 4708.6 angstrom3, Z = 4 (d(calcd) = 1.423 g cm-3), mu(alpha)(Mo Kalpha) = 3.92 cm-1, and R1 (unweighted, based on F) = 0.086 for 3298 observed reflections [I > 3sigma(I)]. Mossbauer and EPR studies show that this complex has an intermediate-spin (S = 3/2) ground state with hyperfine parameters similar to those reported for the porphyrin complex [FeIII(TPP)(FSbF5)] (Gupta, G. P.; Lang, G.; Reed, C. A.; Shelly, K.; Scheidt, W. R. J. Chem. Phys. 1987,86,5288-5293). However, the zero-field splitting tensor of [Et4N]2[FeIIICl(eta4-MAC)] (D = -3.7 cm-1, E/D = 0.05) is rotated by 90-degrees relative to the magnetic hyperfine and electric field gradient tensors. Analysis of the high-field Mossbauer spectra for both [FeIIICl(eta4-MAC)]2- and [FeIVCl(eta4-MAC)]- reveals magnetic hyperfine interactions substantially smaller than expected for an ionic complex, suggesting substantial covalency for both redox states.