The structural, spectroscopic, and magnetochemical characteristics of a new tetranuclear iron-oxo complex are reported. [Fe402(02CCH3)7(bpy)2](C104>y4CH2Cl2-H20 (l) crystallizes in the monoclinic space group C2/c with a = 27.261 (10) A, b = 11.789 (4) Á, c = 16.439 (5) A, ß = 118.27 (2)°, V = 4653.19 Á3, and Z = 4. The structure was refined with 2646 reflections having F > 2.33a(F), giving final R factors of 0.0644 and 0.0688 for R and Rw, respectively. The [Fe402]8+ core of the cation is structurally similar to other [M402]8+ (M = Mn, Fe) complexes which have been previously reported.The core structure consists of a tetranuclear bis^-O) cluster disposed in a "butterfly" arrangement. Two different Fe-O(oxide) bridge distances of 1.819 ( 5) Á (wing-body) and 1.926 ( 5) Á (body-body) are observed. These differences are reflected in the Mossbauer spectrum of the complex, which analyzes as two quadrupole-split doublets in the range of 100-300 K. Each of the doublets has parameters characteristic of high-spin Fe(III) ions.NMR spectra are reported for two [Fe402]8+ complexes. Assignments for all resonances were made on the basis of chemical shift data for two related complexes and one deuterated complex as well as measurements of spin-lattice (T¡) relaxation times. The magnetic susceptibility of complex 1 was measured in the range of 5.01-277.4 K. The effective moment per molecule decreases gradually from 4.20 µ at 277.4 K to 0.82 µ at 5.01 K, indicating a diamagnetic 5 = 0 ground state. A detailed theoretical analysis of the susceptibility data using a spin Hamiltonian approach gives a value for the "wing-body" Fe-Fe magnetic exchange interaction parameter of Jwb = -45 cm'1. It was interesting to find that the "body-body" interaction Jbb is indeterminate and can only be described as being more positive than -15 cm"1. The lack of definition of Jbb is due to spin frustration, where the relative magnitudes of the antiferromagnetic Jwb and 7bb interactions result in a net alignment of the spin vectors on the two body dioxo bridge core Fem ions. The significance of these results as they pertain to exchange coupling in iron-oxo proteins is discussed.
Joints of polystyrene (PS) and poly(methyl methacrylate) (PMMA) modified with ∼50 nm of poly(styrene-co-methyl methacrylate) random copolymer [P(S-ran-MMA)] were investigated. Copolymers having styrene compositions of fS ) 0.48 and fS ) 0.73 were used. Transmission electron microscopy reveals that the copolymers phase separate to form a distinct layer at the joint such that there is an interface with each homopolymer. Interfacial fracture toughness measurements, using the asymmetric double cantilever beam geometry, show a strong effect of the PS to PMMA sheet thickness ratio; that is, the phase angle influences the measured interfacial toughness. Reflection infrared spectroscopy on fracture surfaces indicates that the crack propagates at or near the PS/copolymer interface for all thickness ratios and for both copolymers. In-plane crazing was not observed in front of the crack tip for these systems. Rather, strengthening appears to be exclusively a consequence of oblique crazes in the more compliant PS sheet which form at 45°or 135°relative to the crack direction. Joints modified with P(S0.73-ran-MMA) exhibit denser oblique crazes than those modified with P(S0.48-ran-MMA), resulting in a higher measured fracture toughness at all sheet thickness ratios or phase angles.
3815by N M R spectroscopy due to rapid back reaction with the CO p r o d u d in the photolysis. Studies on Fe(0) complexes of dfepe suggest that the bulky bidentate ligand is able to prevent clustering reactions of unsaturated species.30 Thus, given the substantial Cralkane binding energy in (dfepe)Cr(CO)3(alkane), it seems plausible that this alkane complex could be observed in N M R experiments if it could be generated in the absence of CO or other (30) Brookhart, M.; Chandler, W. A.; Pfister, A. C.; Santini, C. C.; White, P. S. Abstract:The nature of valence trapping is studied for a series of six pphenoxo-bis(N-carboxylate)-bridged FeI1Fe1I1 complexes. The septadentate ligand bimp-binds the two metal ions and provides a phenoxide bridge as well as four imidazole moieties. The X-ray structure of [Fe11Fe111(bimp)(p02CPh)2] (BPh4)2.3/2CH3CN at 295 K is reported. This mixed-valence complex crystallizes in the Pi space group with unit cell parameters of a = 15.995 (4) A, b = 23.475 (4) A, c = 11.464 (4) A, a = 97.57 (l)", @ = 101.74 ( l ) ' , y = 85.22 ( l ) ' , and Z = 2. A total of 11334 unique data with I > 3 4 were refined to values of R = 0.045 and R, = 0.051. The structure shows distinct octahedral (N303) high-spin FelI1 and Fe" ions, with an Fe-Fe distance of 3.440 A. The phenoxide FeIW-Fe"' bridging angle is 115.17 (8)'. The BPh, ions are not symmetrically distributed about the FeI1Fe1I1 cation; they are closer to the FeI1 ion than the FelI1 ion. Sharp 'H NMR signals spanning a chemical shift range of -350 ppm are seen for CD3CN solutions of the Fe"Fe"' complexes. Relative to the 'H NMR timescale these mixed-valence complexes are rapidly transferring electrons in solution. This is due to either rapid intramolecular electron transfer or rapid electron transfer between binuclear complexes. A weak intervalence transfer (IT) electronic absorption band is seen at -1300 nm (e = -200 M-l cm-I) for all six FeI1Fe1I1 complexes in solution. Two quasireversible one-electron wavescorresponding to the FellFell/FellFelll and FellFelll/FelllFelll couples are seen for each complex by cyclic voltammetry. The two waves observed for the Fe"FelI1(bimp) complexes occur at potentials 100-300 mV more negative than those reported for the analogous polypyridyl and benzimidazole complexes. Variable-temperature (5-300 K) magnetic susceptibility data are presented for two of the FellFelll complexes. These data^ were least-squares fit in a matrix diagonalization approac! including the effects of Heisenberg magnetic exchange ( H = -USI.S2), isotropic Zeeman and axial single-ion zero-field (DSz2 -I/3S(S + l)] for both Fe" and Fe"' interactions. Fitting of the data for one sample of [Fe11Fe111(bimp)(fi-02CCH3)2] (C104)2.2H,0, for example, gave parameters of J = -3.4 cm-l, D(Fe") = 0.63 cm-', and g(Fe") = 1.86. There is a S = ground state; however, the S = 3/2, 7/2, and 9/2 excited states are near to the ground state. 57Fe Miissbauer spectroscopy was used to thoroughly examine the temperature dependence of valence detrapping seen for the ...
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