(MU-OXO-HYDROXO)BIS(MU-CARBOXYLATO)DIIRON(III) AND DIMANGANESE(III) COMPLEXES WITH CAPPING TRIS(IMIDAZOL-2-YL)PHOSPHINE LIGANDS

Reported are the syntheses and characterizations of the first examples of (mu-oxo/hydroxo)bis(mu-carboxylato)diiron(III) and -dimanganese(III) cores containing capping tridentate imidazolyl ligands. The capping ligand are tris(imidazol-2-yl phosphine (TIP) and tris(N-methylimidazol-2-yl)phosphine (TMIP). [Fe2O(OAc)2(TMIP)2](ClO4)2.3CH3CN.(C2H5)2O (OAc = acetato(1-)) crystallizes in the monoclinic space group P2(1)/c, with Z = 4, a = 12.490 (4) angstrom, b = 25.654 (12) angstrom, c = 16.909 (9) angstrom, beta = 90.11 (4)degrees, V = 5418 (4) angstrom-3, R = 0.057, and R(w) = 0.062. The average Fe-O(oxo) bond distance is 1.800 (5) angstrom, the Fe-O(oxo)-Fe angle is 122.7 (2)degrees, and the intramolecular Fe---Fe distance is 3.158 (2) angstrom. The two Fe-N(Im) bond distances trans to the oxo bridge are an average of 0.03 angstrom longer than the four cis Fe-N(Im) bond distances. The iron atoms in [FeO(O2CC2H5)2-(TMIP)2](PF6)2.CH3CN are antiferromagnetically coupled, H(ex) = -2HS1.S2, s1 = S2 = 5/2 with -J = 120 cm-1. For the hydroxo-bridged complexes, [Fe2(OH)(OAc)2(TMIP)2](Cl4)2BF4 and [Fe2(OH)(O2CC2H5)2(TMIP)2](CIO4)3, -J approximately 17 cm-1 was estimated from H-1 NMR isotropic shifts and a room-temperature magnetic moment. The Fe-57 Mossbauer quadrupole splitting for the latter hydroxo-bridged complex (0.51 mm/s) shows a pronounced decrease compared to that of the oxo-bridged complexes (1.6 mm/s). These results confirm previous interpretations of H-1 NMR spectra of these diiron complexes in terms of their structures and magnetic properties (Wu, F.-J.; Kurtz, D. M., Jr.J.Am.Chem.Soc.1989,111,6563). The isostructural dimanganese (III) complex, [Mn2O(OAc)2(TMIP)2](CiO4)2.2CH3CN.0.5(C2H5)2O, crystallizes in the orthorhombic space group Pnaa with Z = 8, a = 13.545 (4) angstrom, b = 22.285 (15), angstrom, c = 33.47 (3) angstrom, V = 10101 (12) angstrom-3, R = 0.087, and R(w) = 0.089. The average Mn-O(oxo) bond distance is 1.789 (11) angstrom, the Mn-O(oxo)-Mn angle is 124.4 (6)degrees, and the intramolecular Mn---Mn distance is 3.164 (5) angstrom. Bond lengthening along one set of N-Mn-O(OAc) axes on each Mn atom is consistent with unoccupied d(z)2 orbitals along the Mn-O(oxo) axes. These unoccupied orbitals are reflected in extremely weak antiferromagnetic coupling (-J less-than-or-equal-to 0.5 cm-1) and in the pattern of isotropic shifts observed for H-1 NMR resonances of imidazolyl ligands that are cis and trans to the oxo bridge. These complexes provide structural and spectroscopic benchmarks for imidazole-containing oxo/hydroxo-bridged diiron (III) and dimanganese (III) sites in non-heme proteins such as hemerythrin, ribonucleotide reductase, and pseudocatalase.