Energy Barrier Enhancement by Ligand Substitution in Tetrairon(III) Single Molecule Magnets

A dramatic increase of the energy barrier (Ueff) in tetrairon(III) single-mol. magnets can be achieved by simple chem. modification. Site-specific replacement of the six methoxide bridges in [Fe4(OMe)6(dpm)6] (Hdpm = dipivaloylmethane; Ueff/kB = 3.5 K) with two tripodal 1,1,1-tris(hydroxymethyl)ethane (H3thme) ligands affords [Fe4(thme)2(dpm)6] with Ueff/kB = 15.6(2) K and a magnetic relaxation time exceeding 1000 s at T <0.2 K. The prepd. complex is trigonal, space group R-3c, Z = 6, R1 = 0.0370, R2 = 0.1089

Self-assembly of High-nuclearity Metal Clusters: Programmed Expansion of a Metallasiloxane Cage to an Octacopper(II) Cluster

The novel octanuclear Cu(II) cluster [Cu6{(PhSiO2)6}2{NCCu(Me6tren)}2(MeOH)4]2+ (1) was isolated as a perchlorate salt by reacting the hexacopper(II) metallasiloxane cage [Cu6{(PhSiO2)6}2(nBuOH)x] (x = 4, 6) with [Cu(Me6tren)CN]ClO4 in a MeOH/CHCl3 mixt. (Me6tren = tris(2-(dimethylamino)ethyl)amine). Crystal data for 1(ClO4)2·MeOH: monoclinic, space group P21/n, a = 16.8490(3), b = 22.2966(4), c = 17.2508(3) Å, β = 94.7658(5)°, Z = 2. The structure comprises a highly distorted hexagonal Cu6 array linked to two [Cu(Me6tren)] units via cyanide bridges. Magnetic measurements reveal that the addn. of the Cu cyanide complexes dramatically affects the magnetism of the Cu6 unit, whose ground spin state changes from S = 3 to S = 0

Tuneable Energy Barriers in Tetrairon(III) Single–molecule Magnets

The tetrairon(III) clusters Fe4(L)2(dpm)6 where Hdpm = dipivaloylmethane and H3L = MeC(CH2OH)3 or PhC(CH2OH)3 were obtained by site-specific replacement of the six methoxide bridges in Fe4(OMe)6(dpm)6. As compared with the parent compd., the new clusters show a much larger anisotropy in the S = 5 ground spin state (D/kB∼-0.6 K vs. -0.3 K) and a higher energy barrier to the reversal of the magnetization

New Cyclosiloxanolate Cluster Complexes of Transition Metals

New cyclosiloxanolate transition metal cluster complex derivs. were prepd. PhSiO2K reacted with NiX2 (X2 = Cl2 or acac) to give K2{[η6-(PhSiO2)6]2[μ3-(OH)]2Ni4K4}, a mixed Group 1-group 10 metal complex. PhSiO2Na reacted with Ni(NH3)6I2 to give Na{[η6-(PhSiO2)6]2Ni6(μ6-I)} as the 1st example of encapsulated I- ion in siloxanolate complexes. The macrocyclic Na4{[η12-(PhSiO2)12]Cu4} complex reacted with η6-(1,3,5-C7H8)Cr(CO)3 to give the heterobimetallic adduct Na4{[η12-(PhSiO2)12]Cu4}[Cr(CO)3]3 as one of the rare examples of heterobimetallic complexes with different oxidn. nos. of the metals. The Cu deriv. {[η6-(PhSiO2)6]2Cu6(BuOH)5} reacted in MeOH/CHCl3 (1:6) with Et4NCN to give hexanuclear {[η6-(PhSiO2)6]2Cu6(η2-C3H5N2O2)2}, contg. 2-amino-2-oxoethanimidic acid Me ester monoanion ligands, product of an unexpected C-C coupling reaction. This latter complex was characterized also by x-ray diffraction crystal and mol. structure detn

Tuning Anisotropy Barriers in a Family of Tetrairon(III) Single-molecule Magnets with an S=5 Ground State

Tetrairon(III) Single-Mol. 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) were prepd. 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 topol. 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 neg., as found in 5 (D = -0.21 cm-1) and amts. 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 detd. by a.c. 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 Ueff increase with increasing helical pitch of the Fe(O2Fe)3 core. The 4th-order longitudinal anisotropy parameter B40, which affects the shape of the anisotropy barrier, concomitantly changes from pos. in 1 (compressed parabola) to neg. in 5 (stretched parabola). With the aid of spin Hamiltonian calcns. the obsd. trends were attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch