2-Phenoxypyridyl Dinucleating Ligands for Assembly of Diiron(II) Complexes: Efficient Reactivity with O[subscript 2] to Form (μ-Oxo)diiron(III) Units

A series of 2-phenoxypyridyl and 2-phenoxyimino ligands, H[subscript 2]L[superscript R,R′] [2,2′-(5,5′-(1,2-phenylenebis(ethyne-2,1-diyl))bis(pyridine-5,2-diyl))diphenol, where R = H, Me, or t-Bu, and R′ = H or Ph] and H[subscript 2]BIPS[superscript Me,Ph] [(3,3′-(1E,1′E)-(3,3′-sulfonylbis(3,1-phenylene)bis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)bis(5-methylbiphenyl-2-ol)], were synthesized as platforms for nonheme diiron(II) protein model complexes. UV−vis spectrophotometric studies and preparative-scale reactions of L[superscript R,R′] or BIPS[superscript Me,Ph], where L[superscript R,R′] and BIPS[superscript Me,Ph] are the deprotonated forms of H[subscript 2]L[superscript R,R′] and H[subscript 2]BIPS[superscript Me,Ph], respectively, with iron(II) revealed that the presence of sterically protective o-phenol substituents is necessary to obtain discrete dinuclear species. The reaction of L[superscript Me,Ph] with iron(II) in tetrahydrofuran (THF) afforded the doubly bridged compound [Fe[subscript 2](L[superscript Me,Ph])[subscript 2](THF)[subscript 3]] (1), which was characterized in the solid state by X-ray crystallography. A large internal cavity in this complex facilitates its rapid reaction with dioxygen, even at −50 °C, to produce the thermodynamically stable [Fe[subscript 2](μ-O)(L[superscript Me,Ph])[subscript 2]] (2) species. Reaction of [superscript 18]O[subscript 2] instead of [superscript 16]O[subscript 2] with 1 led to a shift in the Fe−O−Fe vibrational frequency from 833 to 798 cm[superscript −1], confirming the presence of the (μ-oxo)diiron(III) core and molecular oxygen as the source of the bridging oxo group. The L[superscript Me,Ph] ligand is robust toward oxidative decomposition and does not display any reversible redox activity.National Institute of General Medical Sciences (U.S.) (Grant GM032134

Toward Functional Carboxylate-Bridged Diiron Protein Mimics: Achieving Stability and Conformational Flexibility Using a Macrocylic Ligand

A dinucleating macrocycle, H[subscript 2]PIM, containing phenoxylimine metal-binding units has been prepared. Reaction of H[subscript 2]PIM with [Fe[subscript 2](Mes)[subscript 4]] (Mes = 2,4,6-trimethylphenyl) and sterically hindered carboxylic acids, Ph[subscript 3]CCO[subscript 2]H or ArTolCO[subscript 2]H (2,6-bis(p-tolyl)benzoic acid), afforded complexes [Fe[subscript 2](PIM)(Ph[subscript 3]CCO[subscript 2])[subscript 2]] (1) and [Fe[subscript 2](PIM)(Ar[subscript Tol]CO[subscript 2])[subscript 2]] (2), respectively. X-ray diffraction studies revealed that these diiron(II) complexes closely mimic the active site structures of the hydroxylase components of bacterial multicomponent monooxygenases (BMMs), particularly the syn disposition of the nitrogen donor atoms and the bridging μ-η[superscript 1]η[superscript 2] and μ-η[superscript 1]η[superscript 1] modes of the carboxylate ligands at the diiron(II) centers. Cyclic voltammograms of 1 and 2 displayed quasi-reversible redox couples at +16 and +108 mV vs ferrocene/ferrocenium, respectively. Treatment of 2 with silver perchlorate afforded a silver(I)/iron(III) heterodimetallic complex, [Fe[subscript 2](μ-OH)[subscript 2](ClO[subscript 4])[subscript 2](PIM)(Ar[superscript Tol]CO[subscript 2])Ag] (3), which was structurally and spectroscopically characterized. Complexes 1 and 2 both react rapidly with dioxygen. Oxygenation of 1 afforded a (μ-hydroxo)diiron(III) complex [Fe[subscript 2](μ-OH)(PIM)(Ph[subscript 3]CCO[subscript 2])[subscript 3]] (4), a hexa(μ-hydroxo)tetrairon(III) complex [Fe[subscript 4](μ-OH)[subscript 6](PIM)[subscript 2](Ph[subscript 3]CCO[subscript 2])[subscript 2]] (5), and an unidentified iron(III) species. Oxygenation of 2 exclusively formed di(carboxylato)diiron(III) compounds, a testimony to the role of the macrocylic ligand in preserving the dinuclear iron center under oxidizing conditions. X-ray crystallographic and [superscript 57]Fe Mössbauer spectroscopic investigations indicated that 2 reacts with dioxygen to give a mixture of (μ-oxo)diiron(III) [Fe[subscript 2](μ-O)(PIM)(Ar[superscript Tol]CO[subscript 2])[subscript 2]] (6) and di(μ-hydroxo)diiron(III) [Fe[subscript 2](μ-OH)[subscript 2](PIM)(Ar[superscript Tol]CO[subscript 2])[subscript 2]] (7) units in the same crystal lattice. Compounds 6 and 7 spontaneously convert to a tetrairon(III) complex, [Fe[subscript 4](μ-OH)[subscript 6](PIM)[subscript 2](Ar[superscript Tol]CO[subscript 2])[subscript 2]] (8), when treated with excess H[subscript 2]O.National Institute of General Medical Sciences (U.S.) (Grant GM032134

Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [(Mn6OsIII)-Os-III](3+): Promoting SMM Behavior through 3d-5d Transition Metal Substitution

Höke V, Stammler A, Bögge H, Schnack J, Glaser T. Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [(Mn6OsIII)-Os-III](3+): Promoting SMM Behavior through 3d-5d Transition Metal Substitution. Inorganic Chemistry. 2014;53(1):257-268.The reaction of the in situ generated trinuclear triplesalen complex [(talen(t-Bu2))Mn-3(III)(solv)(n)](3+) with (Ph4P)(3)[Os-III(CN)(6)] and NaClO4 center dot H2O affords [(Mn6OsIII)-Os-III] (ClO4)(3) (= [{(talen(t-Bu2))Mn-3(III)}(2){Os-III(CN)(6)}](ClO4)(3)) in the presence of the oxidizing agent [(tacn)(2)Ni-III] (ClO4)(3) (tacn =1,4,7-triazacyclononane), while the reaction of [(talen(t-Bu2))-Mn-3(III)(solv)(n)](3+) with K-4[Os-II(CN)(6)] and NaClO4 center dot H2O yields [(Mn6OsII)-Os-III](ClO4)(2) under an argon atmosphere. The molecular structure of [(Mn6OsIII)-Os-III](3+) as determined by single-crystal X-ray diffraction is closely related to the already published [(Mn6Mc)-M-III](3+) complexes (M-c = Cr-III, Fe-III, Co-III, Mn-III). The half-wave potential of the Os-III/Os-II couple is E-1/2 = 0.07 V vs Fc(+)/Fc. The FT-IR and electronic absorption spectra of [(Mn6OsII)-Os-III](2+) and [(Mn6OsIII)-Os-III](3+) exhibit distinct features of dicationic and tricationic [(Mn6Mc)-M-III](n+) complexes, respectively. The dc magnetic data (mu(eff) vs T, M vs B, and VTVH) of [(Mn6OsII)-Os-III](2+) are successfully simulated by a full-matrix diagonalization of a spin-Hamiltonian including isotropic exchange, zero-field splitting with full consideration of the relative orientation of the D-tensors, and Zeeman interaction, indicating antiferromagnetic Mn-III-Mn-III interactions within the trinuclear triplesalen subunits (J(Mn-Mn)((1)) = -(0.53 +/- 0.01) cm(-1), (H) over cap (ex) = -2 Sigma(i(i)center dot(S) over cap (j)) as well as across the central Os-II ion (J(Mn-Mn)((2,cis)) = -(0.06 +/- 0.01) cm(-1), (J(Mn-Mn)((2,trans)) = -(0.15 +/- 0.01) cm(-1)), while D-Mn = -(3.9 +/- 0.1) cm(-1). The mu(eff) vs T data of [(Mn6OsIII)-Os-III](3+) are excellently reproduced assuming an anisotropic Ising-like Os-III-Mn-III superexchange with a nonzero component J(Os-Mn)((aniso)) = -(11.0 +/- 1.0) cm(-1) along the Os-Mn direction, while J(Mn-Mn) = -(0.9 +/- 0.1) cm(-1) and D-Mn = -(3.0 +/- 1.0) cm(-1). Alternating current measurements indicate a slower relaxation of the magnetization in the SMM [(Mn6OsIII)-Os-III](3+) compared to the 3d analogue [(Mn6FeIII)-Fe-III](3+) due to the stronger and anisotropic M-c-Mn-III exchange interaction