A carboxylate-bridged non-heme diiron dinitrosyl complex

The reaction of nitric oxide with the carboxylate-bridged diiron(II) complex [Fe-2(Et-HPTB)(O(2)CPh)](BF4)(2) (1a) afforded the dinitrosyl adduct, [Fe-2(NO)(2)(Et-HPTB)(O(2)CPh)](BF4)(2) (1b), where Et-HPTB = N,N,N',N'-tetrakis-(N-ethyl-2-benzimidazolylmethyl)-2-hydroxy- 1,3-diaminopropane, in 69% yield. Compound 1b further reacts with dioxygen to form the bis(nitrato) complex, [Fe-2(Et-HPTB)(NO3)(2)(OH)](BF4)(2) (1c). The structure of 1b was determined by X-ray crystallography (triclinic, <P(1)over bar>, a = 13.5765(8) Angstrom, b = 15.4088(10) Angstrom, c = alpha = 73.656(1)degrees, beta = 73.546(1)degrees, gamma = 73.499(1)degrees, V = 3043.8(7) Angstrom(3), T = -80 degrees C, Z = 2, and R(w) = 0.095 for 5644 independent reflections with I > 3 sigma(I)). The two nitrosyl units are equivalent with an average Fe-N-O angle of 167.4 +/- 0.8 degrees. Spectroscopic characterization of solid 1b revealed an NO stretch at 1785 cm(-1) in the infrared and Mossbauer parameters of delta = 0.67 mm s(-1) and Delta E(Q) 1.44 mm s(-1) at 4.2 K. These data are comparable to those for other {FeNO}(7) systems. An S = 3/2 spin state was assigned from magnetic susceptibility studies to the two individual {FeNO} centers, each of which has a nitrosyl ligand antiferromagnetically coupled to iron. A least-squares fit of the chi vs temperature plots to a theoretical model yielded an exchange coupling constant J of -23 cm(-1), where H = -2JS(1) . S-2, indicating that the two S = 3/2 centers are antiferromagnetically coupled to one another. An extended Huckel calculation on a model complex, [Fe-2(NO)(2)(NH3)(6)(O2CH)(OH)](2+), revealed that the magnitudes of Fe-N-O angles are dictated by pi-bonding interactions between the Fe d(xz) and NO pi* orbitals.