Synthesis and spectroscopy of μ-oxo (O2-)-bridged heme/non-heme diiron complexes:: Models for the active site of nitric oxide reductase

In this paper, we describe the synthesis and study of a series of heme/non-heme Fe-O-Fe' complexes supported by a porphyrin and the tripodal nitrogen ligand TMPA [TMPA = tris(2-pyridylmethyl)amine]. The complete synthesis of [(L-6)Fe-O-Fe(X)](+) (1) (X = OMe- or Cl-, 69:31 ratio), where L-6 is the dianion of 5-(o-O-[(NN-bis(2-pyridylmethyl)-2-(6-methoxyl)pyridinemethanamine)phenyl]-10,15,20-tris(2,6-difluorophenyl)porphine, is reported. The crystal structure for 1(.)PF(6) reveals an intramolecular heme/non-heme diferric complex bridged by an Fe-O-Fe' moiety; <(Fe-O-Fe') = 166.7(3)degrees, and d((FeFe)-Fe-...') = 3.556 Angstrom. Crystal data for C70H57CIF12Fe2N8O3P (1(.)PF(6)): triclinic, P1, a = 13.185(3) Angstrom, b = 14.590 (3) Angstrom, c = 16.885(4) Angstrom, alpha = 104.219(4)degrees, beta = 91.572(4)degrees, gamma = 107.907(4)degrees, V = 2977.3(11) Angstrom(3), Z = 2, T = 150(2) K. Complex 1 (where X = Cl-) is further characterized by UV-vis (lambda(max) = 328, 416 (Soret), 569 nm), H-1 NMR (delta 27-24 [TMPA -CH2-], 16.1 [pyrrole-H], 15.2-10.5 [PY-3H, PY-5H], 7.9-7.2 [m- and p-phenyl-H], 6.9-5.8 [PY-4H] ppm), resonance Raman (upsilon(as)(Fe-O-Fe') 844 cm(-1)), and Mossbauer (delta(Fe) = 0.47, 0.41 mm/s; DeltaE(A) = 1.59, 0.55 mm/s; 80 K) spectroscopies, MALDI-TOF mass spectrometry (m/z 1202), and SQUID susceptometry (J = - 114.82 cm(-1), S = 0). We have also synthesized a series of 3-, 4-, and 5-methyl-substituted as well as selectively deuterated TMPA(Fe') complexes and condensed these with the hydroxo complex (F-8)FeOH or (F-8-d(8))FeOH to yield untethered Fe-O-Fe' analogues. Along with selective deuteration of the methylene hydrogens in TMPA, complete H-1 NMR spectroscopic assignments for 1 have been accomplished. The magnetic properties of several of the untethered complexes and a comparison to those of I are also presented. Complex 1 and related species represent good structural and spectroscopic models for the heme/non-heme diiron active site in the enzyme nitric oxide reductase.