We report the synthesis, structure, and reactivity of [Fe(T1Et4iPrIP)(OTf)2] [1; T1Et4iPrIP = tris(1-ethyl-4-isopropylimidazolyl)phosphine]. Compound 1 reacts reversibly with nitric oxide to afford [Fe(T1Et4iPrIP)(NO)(THF)(OTf)](OTf) (2), which is the first example of a 6-coordinate {FeNO}(7) S = 3/2 complex containing a linear Fe-N-O group. 2 exhibits the highest ν(NO) for compounds in this class. Density functional theory studies reveal an enhanced degree of β-electron transfer from π*(NO) to the Fe d orbitals accounting for the large stretching frequency.

We present the synthesis, properties, and characterization of [Fe(T1Et4iPrIP)(NO)(H2O)2](OTf)2 (1) {T1Et4iPrIP = Tris(1‐ethyl‐4‐isopropyl‐imidazolyl)phosphine} as a model for the nitrosyl adduct of gentisate 1,2‐dioxygenase (GDO). The further characterization of [Fe(T1Et4iPrIP)(THF)(NO)(OTf)](OTf) (2) which was previously communicated (Inorg. Chem. 2014, 53, 5414) is also presented. The weighted average Fe–N–O angle of 162° for 1 is very close to linear (≥ 165°) for these types of complexes. The coordinated water ligands participate in hydrogen bonding interactions. The spectral properties (EPR, UV/Vis, FTIR) for 1 are compared with 2 and found to be quite comparable. Complex 1 closely follows the relationship between the Fe–N–O angle and NO vibrational frequency which was previously identified for six‐coordinate {FeNO}7 complexes. Liquid FTIR studies on 2 indicate that the ν(NO) vibration position is sensitive to solvent shifting to lower energy (relative to the solid) in donor solvent THF and shifting to higher energy in dichloromethane. The basis for this behavior is discussed. The Keq for NO binding in 2 was calculated in THF and found to be 470 m–1. Density functional theory (DFT) studies on 1 indicate donation of electron density to the iron center from the π* orbitals of formally NO–. Such a donation accounts for the near linearity of the Fe–N–O bond and the large ν(NO) value of 1791 cm–1.

Two new binuclear metal complexes supported by 1,4,8-triazacycloundecane (tacud) are reported. [Fe(2)(tacud)(2)(μ-Cl)(2)Cl(2)] (1) and [Mn(2)(tacud)(2)(μ-Cl)(2)Cl(2)] (2) are isomorphs consisting of bis(μ-chloro) bridged metal centers along with terminal chloro groups and tacud ligands. Both compounds 1 and 2 crystallize in the P1 space group. For 1, a = 7.7321(12) Å, b = 7.8896(12) Å, c = 11.4945(17) Å, α = 107.832(2)°, β = 107.827(2)°, γ = 92.642(2)°, V = 627.85(17) Å(3) and Z = 1. For 2, a = 7.7607(12) Å, b = 7.9068(12) Å, c = 11.6111(18) Å, α = 108.201(2)°, β = 108.041(2)°, γ = 92.118(3)°, V = 636.47(17) Å(3) and Z = 1. Variable-temperature and variable-field magnetic susceptibility studies on 1 indicate the presence of weak ferromagnetic interactions between the high-spin iron(ii) centers in the dimer (J = + 1.6 cm(-1)) and the crystalline field anisotropy of the ferrous ion (D = - 2.8, E = - 0.1 cm(-1)). Variable temperature magnetic susceptometry studies on 2 indicate that weak antiferromagnetic coupling exists between the manganese(ii) centers (J = - 1.8 cm(-1)). Compounds 1 and 2 retain their dinuclearity in weakly coordinating or low polarity solvents, while both become mononuclear in solvents such as methanol.

The synthesis, structure and aqueous solution behavior of [FeL 1 Cl 2 ] (1, L 1 ϭ 1-benzyl-4-acetato-1,4,7-triazacyclononane) is reported. The X-ray structure of 1 reveals iron(III) in distorted octahedral geometry bonded to three amine nitrogen atoms and one carboxylate oxygen atom along with two cis chloride atoms. Electrochemical measurements of 1 in acetonitrile indicate that E 1/2 ϭ Ϫ200 mV vs Ag/AgCl. The stability of the ϩ3 oxidation state is attributed to the hard N amine and carboxylate ligand donors. In acetonitrile, 1 is mononuclear and has an S ϭ 5/2 spin state. The 1087 presence of a μ-oxo bridge upon dissolution of 1 in water is supported by EPR and solution magnetic susceptibility NMR studies. pH-metric titrations and UV-vis studies indicate the presence of a total of four acidic protons. At low pH, L 1 (OH 2 )Fe-O-Fe(OH 2 )L 1 is proposed to be stabilized while at high pH, L 1 (OH)Fe-O-Fe(OH)L 1 is suggested to persist.

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