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Four new tetranuclear iron(lll) complexes of formula [(Fe(L)[sub2])[sub3]Fe], 1-4, have been prepared by reacting [Fe(ClO[sub4])[sub3]]∙6H[sub2]O with H[sub2]L in methanol. Here, L[sup2-] is the deprotonated form of N-(2-hyrdoxybenzyl)-L-valinol (H[sub2]L[sup1]), N-(2-hyrdoxybenzyl)-L-leucinol (H[sub2]L[sup2]), N-(5-chloro-2-hyrdoxybenzyl)-L-leucinol (H[sub2]L[sup3]), and N-(2-hyrdoxybenzyl)-L-phenylalaninol (H[sub2]L[sup4]). The complexes are prepared in an enantiomeric pure form. The complexes have been characterized with the help of lR, UV-vis, circular dichroism (CD), 1H, and elemental analyses. The complex [(Fe(L[sup2])[sub2])[sub3]Fe]-CH[sub3]OH-2H[sub2]O, 2∙CH[sub3]OH-2H[sub2]O, crystallizes in enantiomeric pure form containing a propeller-like Fe[sub4]O[sub6] core. [sup1]H and CD spectral studies of the four species are consistent with the structural similarities of the complexes in solution. Variable-temperature magnetic susceptibility of one case shows an intramolecular antiferromagnetic coupling between the Fe(lll) ions. Magnetic measurements are in accord with the S = 5 ground state and suggest single molecular magnet behavior. The magnetic exchange coupling constant between the iron centers within the molecule is interpreted using broken-symmetry density functional theory calculation. [ABSTRACT FROM AUTHOR]

The reactivity of six Mn IV -oxo complexes in C-H bond oxidation has been examined using a combination of kinetic experiments and computational methods. Variable-temperature studies of the oxidation of 9,10-dihydroanthracene (DHA) and ethylbenzene by these Mn IV -oxo complexes yielded activation parameters suitable for evaluating electronic structure computations. Complementary kinetic experiments of the oxidation of deuterated DHA provided evidence for hydrogen-atom tunneling in C-H bond oxidation for all Mn IV -oxo complexes. These results are in accordance with the Bell model, where tunneling occurs near the top of the transition-state barrier. Density functional theory (DFT) and DLPNO-CCSD(T 1 ) computations were performed for three of the six Mn IV -oxo complexes to probe a previously predicted multistate reactivity model. The DFT computations predicted a thermal crossing from the 4 B 1 ground state to a 4 E state along the C-H bond oxidation reaction coordinate. DLPNO-CCSD(T 1 ) calculations further confirm that the 4 E transition state offers a lower energy barrier, reinforcing the multistate reactivity model for these complexes. We discuss how this multistate model can be reconciled with recent computations that revealed that the kinetics of C-H bond oxidation by this set of Mn IV -oxo complexes can be well-predicted on the basis of the thermodynamic driving force for these reactions.

Four new pentadentate N5-donor ligands, [ N -(1-methyl-2-imidazolyl)methyl- N -(2-pyridyl)-methyl- N -(bis-2-pyridylmethyl)-amine] ( L 1 ), [ N -bis(1-methyl-2-imidazolyl)methyl- N -(bis-2-pyridylmethyl)amine] ( L 2 ), ( N -(isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)- N -(pyridin-2-ylmethyl)methanamine ( L 3 ), and N , N -bis(isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)methanamine ( L 4 ), have been synthesized based on the N4Py ligand framework, where one or two pyridyl arms of the N4Py parent are replaced by ( N -methyl)imidazolyl or N -(isoquinolin-3-ylmethyl) moieties. Using these four pentadentate ligands, the mononuclear complexes [Fe II (CH 3 CN)( L 1 )] 2+ ( 1a ), [Fe II (CH 3 CN)( L 2 )] 2+ ( 2a ), [Fe II (CH 3 CN)( L 3 )] 2+ ( 3a ), and [Fe II (CH 3 CN)( L 4 )] 2+ ( 4a ) have been synthesized and characterized. The half-wave potentials ( E 1/2 ) of the complexes become more positive in the order: 2a < 1a < 4a ≤ 3a ≤ [Fe(N4Py)(CH 3 CN)] 2+ . The order of redox potentials correlates well with the Fe-N amine distances observed by crystallography, which are 2a > 1a ≥ 4a > 3a ≥ [Fe(N4Py)(CH 3 CN)] 2+ . The corresponding ferryl complexes [Fe IV (O)( L 1 )] 2+ ( 1b ), [Fe IV (O)( L 2 )] 2+ ( 2b ), [Fe IV (O)( L 3 )] 2+ ( 3b ), and [Fe IV (O)( L 4 )] 2+ ( 4b ) were prepared by the reaction of the ferrous complexes with isopropyl 2-iodoxybenzoate (IBX ester) in acetonitrile. The greenish complexes 3b and 4b were also isolated in the solid state by the reaction of the ferrous complexes in CH 3 CN with ceric ammonium nitrate in water. Mössbauer spectroscopy and magnetic measurements (using superconducting quantum interference device) show that the four complexes 1b, 2b, 3b , and 4b are low-spin ( S = 1) Fe IV ═O complexes. UV/vis spectra of the four Fe IV ═O complexes in acetonitrile show typical long-wavelength absorptions of around 700 nm, which are expected for Fe IV ═O complexes with N4Py-type ligands. The wavelengths of these absorptions decrease in the following order: 721 nm ( 2b ) > 706 nm ( 1b ) > 696 nm ( 4b ) > 695 nm ( 3b ) = 695 nm ([Fe IV (O) (N4Py)] 2+ ), indicating that the replacement of the pyridyl arms with ( N -methyl) imidazolyl moieties makes L 1 and L 2 exert weaker ligand fields than the parent N4Py ligand, while the ligand field strengths of L 3 and L 4 are similar to the N4Py parent despite the replacement of the pyridyl arms with N -(isoquinolin-3-ylmethyl) moieties. Consequently, complexes 1b and 2b tend to be less stable than the parent [Fe IV (O)(N4Py)] 2+ complex: the half-life sequence at room temperature is 1.67 h ( 2b ) < 16 h ( 1b ) < 45 h ( 4b ) < 63 h ( 3b ) ≈ 60 h ([Fe IV (O)(N4Py)] 2+ ). Compared to the parent complex, 1b and 2b exhibit enhanced reactivity in both the oxidation of thioanisole in the oxygen atom transfer (OAT) reaction and the oxygenation of C-H bonds of aromatic and aliphatic substrates, presumed to occur via an oxygen rebound process. Furthermore, the second-order rate constants for hydrogen atom transfer (HAT) reactions affected by the ferryl complexes can be directly related to the C-H bond dissociation energies of a range of substrates that have been studied. Using either IBX ester or H 2 O 2 as an oxidant, all four new Fe II complexes display good performance in catalytic reactions involving both HAT and OAT reactions.

A series of iron(II) benzilate complexes (1-7) with general formula [(L)Fe II (benzilate)] + have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N 4 ligands (tris(2-pyridylmethyl)amine (TPA in 1), tris(6-methyl-2-pyridylmethyl)amine (6-Me 3 -TPA in 2), N 1 ,N 1 -dimethyl-N 2 ,N 2 -bis(2-pyridylmethyl)ethane-1,2-diamine (iso-BPMEN in 3), N 1 ,N 1 -dimethyl-N 2 ,N 2 -bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me 2 -iso-BPMEN in 4), and tris(2-benzimidazolylmethyl)amine (TBimA in 7)) along with two linear tetradentate amine ligands (N 1 ,N 2 -dimethyl-N 1 ,N 2 -bis(2-pyridylmethyl)ethane-1,2-diamine (BPMEN in 5) and N 1 ,N 2 -dimethyl-N 1 ,N 2 -bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me 2 -BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of 1-3 and 5 contains a mononuclear six-coordinate iron(II) center coordinated by a monoanionic benzilate, whereas complex 7 contains a mononuclear five-coordinate iron(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in 2, 3, and 5. All of the iron(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron-oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes 2 and 4-6 react with alkenes to produce cis-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O 2 -derived iron-oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron(II) hydroperoxides are energetically favorable for the complexes of 6-Me 3 -TPA, 6-Me 2 -iso-BPMEN, BPMEN, and 6-Me 2 -BPMEN ligands, while high spin end-on iron(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.