title: Tuning anisotropy barriers in a family of tetrairon(III) single-molecule magnets with an S = 5 ground state.
abstract: Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6].Et2O (3.Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) have been prepared by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = tBu; Hdpm = dipivaloylmethane). The iron(III) ions exhibit a centered-triangular topology and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably negative, as found in 5 (D = -0.21 cm(-1)) and amounts to -0.445 cm(-1) in 1, -0.432 cm(-1) in 2, -0.42 cm(-1) in 3.Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier Ueff determined by AC magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3.Et2O, 5.95 K in 4, and 3.5 K in 5. Both |D| and U(eff) are found to increase with increasing helical pitch of the Fe(O2Fe)3 core. The fourth-order longitudinal anisotropy parameter B4(0), which affects the shape of the anisotropy barrier, concomitantly changes from positive in 1 ("compressed parabola") to negative in 5 ("stretched parabola"). With the aid of spin Hamiltonian calculations the observed trends have been attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch.
authors.affiliation: INSTM and Department of Chemistry, University of Modena and Reggio Emilia, via G. Campi 183, 41100 Modena, Italy.
authors.forename: Stefania
authors.initials: S
authors.lastname: Accorsi
authors.forename: Anne-Laure
authors.initials: AL
authors.lastname: Barra
authors.forename: Andrea
authors.initials: A
authors.lastname: Caneschi
authors.forename: Guillaume
authors.initials: G
authors.lastname: Chastanet
authors.forename: Andrea
authors.initials: A
authors.lastname: Cornia
authors.forename: Antonio C
authors.initials: AC
authors.lastname: Fabretti
authors.forename: Dante
authors.initials: D
authors.lastname: Gatteschi
authors.forename: Cecilia
authors.initials: C
authors.lastname: Mortalo
authors.forename: Emiliano
authors.initials: E
authors.lastname: Olivieri
authors.forename: Francesca
authors.initials: F
authors.lastname: Parenti
authors.forename: Patrick
authors.initials: P
authors.lastname: Rosa
authors.forename: Roberta
authors.initials: R
authors.lastname: Sessoli
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Sorace
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Wernsdorfer
authors.forename: Laura
authors.initials: L
authors.lastname: Zobbi