The importance of being exchanged: [Gd III 4M II 8(OH) 8(L) 8(O 2CR) 8] 4+ clusters for magnetic refrigeration

El pdf del artículo es la versión post-print.-- et al.Playing it cool: Almost all of the constituent parts of the complex [Ln III 4M II 8(OH) 8(L) 8(O 2CR) 8]X 4, namely the lanthanide ions Ln 3+, the transition-metal ions M 2+, the bridging ligand L, the carboxylates, and the counterions X can be exchanged, thus allowing a thorough understanding of the individual contributions to the magnetocaloric effect. Example in picture: Gd purple, Cu green, O red, N blue. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.E.K.B. wishes to thank the EPSRC for funding. M.E. acknowledges contracts MAT2009-13977-C03 and CSD2007-00010. S.P. thanks the Danish Natural Science Research Council for a Sapere Aude Fellowship (10-081659).Peer Reviewe

Cryogenic magnetocaloric effect in the Fe17 molecular nanomagnet

Trabajo presentado al "The 23rd IIR International Congress of Refrigeration" celebrado del 21 al 26 de Agosto del 2011 en Praga.-- Dedicated to Alfred Werner on the 100th Anniversary of his Nobel prize in Chemistry in 1913.We study the magnetothermal properties of magnetically isotropic high-spin molecular nanomagnets containing 17 Fe3+ ions per molecule linked via oxide and hydroxide ions, packed in a crystallographic cubic symmetry. Low-temperature magnetization and heat capacity experiments reveal that each molecular unit carries a net spin ground state as large as S = 35/2 and a magnetic anisotropy as small as D = −0.023 K, while no magnetic order, purely driven by dipolar interactions, is to be expected down to very-low temperatures. These characteristics suggest that the Fe17 molecular nanomagnet can potentially be employed as a sub-Kelvin magnetic refrigerant.This work has been partially supported by Spanish MINECO through grants MAT2009-13977-C03 and PIE201060I012, the EPSRC and The Leverhulme Trust (UK).Peer Reviewe

Equatorial restriction of the photoinduced Jahn–Teller switch in Mn(iii)-cyclam complexes

Ultrafast transient absorption spectra were recorded for solutions of [MnIII(cyclam)(H2O)(OTf)][OTf]2 (cyclam = 1,4,8,11-tetraazacyclotetradecane and OTf = trifluoromethanesulfonate) in water to explore the possibility to restrict the equatorial expansion following photoexcitation of the dxy ← dz2 electronic transition, often resulting in a switch from axial to equatorial Jahn–Teller distortion in MnIII complexes. Strong oscillations were observed in the excited state absorption signal and were attributed to an excited state wavepacket. The structural rigidity of the cyclam ligand causes a complex reaction coordinate with frequencies of 333, 368, 454 and 517 cm−1, and a significantly shorter compressed-state lifetime compared to other MnIII complexes with less restricted equatorial ligands. Complementary density functional theory quantum chemistry calculations indicate a switch from an axially elongated to a compressed structure in the first excited quintet state Q1, which is accompanied by a modulation of the axial tilt angle. Computed harmonic frequencies for the axial stretching mode (∼379 cm−1) and the equatorial expansions (∼410 and 503 cm−1) of the Q1 state agree well with the observed coherences and indicate that the axial bond length contraction is significantly larger than the equatorial expansion, which implies a successful restriction of the wavepacket motion. The weak oscillation observed around 517 cm−1 is assigned to a see-saw motion of the axial tilt (predicted ∼610 cm−1). The results provide insights into the structural perturbations to the molecular evolution along excited state potential energy surfaces of MnIII octahedral complexes and can be used to guide the synthesis of optically controlled MnIII-based single-molecule magnets

Importance of Steric Influences in the Construction of Multicomponent Hybrid Polymetallic Clusters

The straightforward room temperature synthesis of hybrid polymetallic manganese clusters is investigated, exploiting complementary ligand combinations of <i>p</i>-<i>tert</i>-butylcalix­[4]­arene and salicylaldoximes. Eight new [Mn^III₇Mn^II] clusters have been prepared wherein the simple substitution of alkyl or aryl groups at well-defined positions of the salicylaldoxime scaffold leads to two distinct structure types that, while exhibiting the same general topology, contain the unique Mn^II ion in different positions. Incorporation of a methyl, ethyl, or isopropyl group at the 3-position of the aromatic skeleton or a phenyl group at the oximic carbon gives structure type A that displays competing weak ferromagnetic and antiferromagnetic interactions. Substitution of a methyl or ethyl group at the oximic carbon atom invokes structure type B, incorporating an additional bulky chloride or nitrate into the metallic core due to the smaller steric imposition and position of the methyl or ethyl group. The distortion of the cluster core is consequently enhanced, switching the magnetic properties and resulting in single-molecule magnet behavior. The presence of <i>tert</i>-butyl groups at the 3- and 5-positions of the salicylaldoxime skeleton leads to a new [Mn^IV₂Mn^III₂] cluster that is found to be a single-molecule magnet. The bulky <i>tert</i>-butyl group in the 3-position is too large to facilitate Mn₈ cluster formation, and thus assembly occurs by an alternative pathway. Characteristic bonding modes of the constituent ligands are retained in every case, and the results presented here give insight into the potential of ligand combinations in future studies, highlighting the importance of steric factors in evaluating their relevant compatibilities