Synthesis and Spectroscopy of μ-Oxo (O^2-)-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 [(6L)Fe−O−Fe(X)]+ (1) (X = OMe- or Cl-, 69:31 ratio), where 6L is the dianion of 5-(o-O-[(N,N-bis(2-pyridylmethyl)-2-(6-methoxyl)pyridinemethanamine)phenyl]-10,15,20-tris(2,6-difluorophenyl)porphine, is reported. The crystal structure for 1·PF6 reveals an intramolecular heme/non-heme diferric complex bridged by an Fe−O−Fe‘ moiety; ∠(Fe−O−Fe‘) = 166.7(3)°, and d(Fe···Fe‘) = 3.556 Å. Crystal data for C70H57ClF12Fe2N8O3P (1·PF6): triclinic, P1̄, a = 13.185(3) Å, b = 14.590 (3) Å, c = 16.885(4) Å, α = 104.219(4)°, β = 91.572(4)°, γ = 107.907(4)°, V = 2977.3(11) Å3, Z = 2, T = 150(2) K. Complex 1 (where X = Cl-) is further characterized by UV−vis (λmax = 328, 416 (Soret), 569 nm), 1H NMR (δ 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 (νas(Fe−O−Fe‘) 844 cm-1), and Mössbauer (δFe = 0.47, 0.41 mm/s; ΔEA = 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 (F8)FeOH or (F8-d8)FeOH to yield “untethered” Fe−O−Fe‘ analogues. Along with selective deuteration of the methylene hydrogens in TMPA, complete 1H NMR spectroscopic assignments for 1 have been accomplished. The magnetic properties of several of the untethered complexes and a comparison to those of 1 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.