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The combination of lanthanoid nitrates with 18-crown-6 (18-c-6) and tetrahalocatecholate (X 4 Cat 2- , X=Cl, Br) ligands has afforded two compound series [Ln(18-c-6)(X 4 Cat)(NO 3 )]⋅MeCN (X=Cl, 1-Ln; X=Br, 2-Ln; Ln=La, Ce, Nd, Gd, Tb, Dy). The 18-c-6 ligands occupy equatorial positions of a distorted sphenocorona geometry, whereas the charged ligands occupy the axial positions. The analogues of both series with Ln=Ce, Nd, Tb and Dy exhibit out-of-phase ac magnetic susceptibility signals in the presence of an applied magnetic field, indicative of slow magnetization relaxation. When diluted into a diamagnetic La host to reduce dipolar interactions, the Dy analogue exhibits slow relaxation up to 20 K in the absence of an applied dc field. Concerted magnetic measurements, EPR spectroscopy, and ab initio calculations have allowed elucidation of the mechanisms responsible for slow magnetic relaxation. A consistent approach has been applied to quantitatively model the relaxation data for different lanthanoid analogues, suggesting that the spin dynamics are governed by Raman processes at higher temperatures, transitioning to a dominant phonon bottleneck process as the temperature is decreased, with an observed T -6 rather than the usual T -2 dependence (T is temperature). This anomalous thermal dependence of the phonon bottleneck relaxation is consistent with anharmonic effects in the lattice dynamics, which was predicted by Van Vleck more than 70 years ago., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The first members of a promising new family of hybrid amino acid-polyoxometalates have emerged from a search for modular functional molecules. Incorporation of glycine (Gly) or norleucine (Nle) ligands into an yttrium-tungstoarsenate structural backbone, followed by crystallization with p-methylbenzylammonium (p-MeBzNH3(+)) cations, affords (p-MeBzNH3)6K2(GlyH)[As(III)4(Y(III)W(VI)3)W(VI)44Y(III)4O159(Gly)8(H2O)14]⋅47 H2O (1) and enantiomorphs (p-MeBzNH3)15(NleH)3[As(III)4(Mo(V)2Mo(VI)2)W(VI)44Y(III)4O160(Nle)9(H2O)11][As(III)4(Mo(VI)2W(VI)2)W(VI)44Y(III)4O160(Nle)9(H2O)11] (generically designated 2: L-Nle, 2 a; D-Nle, 2 b). An intensive structural, spectroscopic, electrochemical, magnetochemical and theoretical investigation has allowed the elucidation of site-selective metal substitution and photoreduction of the tetranuclear core of the hybrid polyanions. In the solid state, markedly different crystal packing is evident for the compounds, which indicates the role of noncovalent interactions involving the amino acid ligands. In solution, mass spectrometric and small-angle X-ray scattering studies confirm maintenance of the structure of the polyanions of 2, while circular dichroism demonstrates that the chirality is also maintained. The combination of all of these features in a single modular family emphasizes the potential of such hybrid polyoxometalates to provide nanoscale molecular materials with tunable properties., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The mechanism of stabilization of positive charge on carbon by sulfur, selenium, or tellurium at the beta-position has been investigated kinetically, by measurement of rate enhancements, and structurally, by variation of the bond strength to the leaving group. Stabilization can occur either nonvertically with formation of a bridged intermediate or vertically through hyperconjugation within an open carbocation. We observed large rate enhancements (10(5) for S, 10(6) for Se) in 97% aqueous trifluoroethanol with trifluoroacetate as the leaving group. These enhancements are consistent with either mechanism. Product structures also are consistent with either mechanism. Nine crystal structures revealed that the bond to the leaving group (C-O) is lengthened by the presence of S or Se at the beta-position, in proportion to the basicity of the leaving group. This lengthening is not accompanied by angle distortions expected for the bridging mechanism. The crystallographic data support vertical (hyperconjugative) character along the reaction coordinate, more so for selenium than sulfur.