3d single-ion magnets

One of the determining factors in whether single-molecule magnets (SMMs) may be used as the smallest component of data storage, is the size of the barrier to reversal of the magnetisation, Ueff. This physical quantity depends on the magnitude of the magnetic anisotropy of a complex and the size of its spin ground state. In recent years, there has been a growing focus on maximising the anisotropy generated for a single 3d transition metal (TM) ion, by an appropriate ligand field, as a means of achieving higher barriers. Because the magnetic properties of these compounds arise from a single ion in a ligand field, they are often referred to as single-ion magnets (SIMs). Here, the synthetic chemist has a significant role to play, both in the design of ligands to enforce propitious splitting of the 3d orbitals and in the judicious choice of TM ion. Since the publication of the first 3d-based SIM, which was based on Fe(II), many other contributions have been made to this field, using different first row TM ions, and exploring varied coordination environments for the paramagnetic ions

Bis-tris propane as a flexible ligand for high-nuclearity complexes

Polymetallic complexes can be assembled using a wide array of polydentate ligands that give an almost unlimited toolbox to prepare new molecular architectures with fascinating structures and interesting magnetic properties. Bis-tris propane is one such a polydentate ligand that has been used to prepare homo- (3d or 4f) and heterometallic (3d/3d’ or 3d/4f) complexes, ranging from simple complexes such as {Ni4} to spectacular 3d/3d’ {Cu8Zn8} or {Mn18Cu6} complexes. It shows a flexibility in binding mode, utilizing up to six of its potential ligand donor atoms and displaying multiple levels of deprotonation, able to bridge up to six metal ions. The ligand has a particular affinity for binding 3d ions such as Cu(II) or Co(III) in heterometallic syntheses and this can provide a flexible structure-directing effect. This concept has been exploited to prepare new heterometallic 3d/3d’ complexes that display interesting levels of complexity; 3d/4f complexes such as {Cu3Tb2} that show single-molecule magnet behavior where superexchange interactions quench quantum tunneling of the magnetization, or {Co3Gd3} where the magnetocaloric properties arise by using Bis-tris propane to separate the Gd(III) ions and weaken Gd(III)...Gd(III) interactions

Pushing the limits of magnetic anisotropy in trigonal bipyramidal Ni(II)

Monometallic complexes based on 3d transition metal ions in certain axial coordination environments can exhibit appreciably enhanced magnetic anisotropy, important for memory applications, due to stabilisation of an unquenched orbital moment. For high-spin trigonal bipyramidal Ni(II), if competing structural distortions can be minimised, this may result in an axial anisotropy that is at least an order of magnitude stronger than found for orbitally non-degenerate octahedral complexes. Broadband, high-field EPR studies of [Ni(MDABCO)2Cl3]ClO4 (1) confirm an unprecedented axial magnetic anisotropy, which pushes the limits of the familiar spin-only description. Crucially, compared to complexes with multidentate ligands that encapsulate the metal ion, we see only a very small degree of axial symmetry breaking. 1 displays field-induced slow magnetic relaxation, which is rare for monometallic Ni(II) complexes due to efficient spin–lattice and quantum tunnelling relaxation pathways

Strategic synthesis of [Cu2], [Cu4] and [Cu5] complexes: inhibition and triggering of ligand arm hydrolysis and self-aggregation by chosen ancillary bridges

The Schiff base ligand HL1 ({2,6-bis(allylimino)methyl}-4-methylphenol) having no coordinating donor arm has been examined for its reaction medium and ancillary bridge dependent reactivity for hierarchical family of CuII complexes. The ligand showed unique reactivity pattern toward CuII in solution. The bridging nature of in situ generated HO− ions in absence and presence of externally added carboxylates (RCOO−; R= CF3, C6H5 and CH3) has been utilized to produce complexes {[Cu2(µ–L2)2(H2O)]2[Cu2(µ–L2)2(H2O)2](ClO4)6} (1) (HL2 = 3-{(allylimino)methyl}-2-hydroxy-5-methylbenzaldehyde), [Cu4(µ4–O)(µ–L1)2(µ1,3–O2CCF3)4] (2), [Cu4(µ4–O)(µ–L1)2(µ1,3–O2CC6H5)4]∙H2O (3), [Cu5(µ3–OH)2(µ–L1)2(µ1,3–OAc)2(OAc)2(H2O)4][Cu5(µ3–OH)2(µ–L1)2(µ1,3–OAc)2(OAc)3(H2O)](ClO4)3∙2C2H5OH (4). Absence of carboxylate anions did not yield HO− ions in situ and triggered single ligand arm hydrolysis. The formation of tetra- and pentanuclear aggregates as well as ligand hydrolyzed dinulcear products has been rationalized to identify the possible roles of carboxylate anions in solution. Detailed characterization of the complexes in the solid state and in solution have been carried out using spectroscopic measurements, X-ray crystallography, variable temperature magnetic measurements and functional behavior. In MeOH solutions at 298 K, the complexes 1-4 showed catalytic oxidation of 3,5-di-tert-butyl catechol (3,5-DTBCH2) saturated with O2 of air

Dangling and hydrolyzed ligand arms in [Mn3] and [Mn6] coordination assemblies: synthesis, characterization, and functional activity

Two flexible, branched, and sterically constrained di- and tripodal side arms around a phenol backbone were utilized in ligands H3L1 and H5L2 to isolate {Mn6} and {Mn3} coordination aggregates. 2,6-Bis{(1-hydroxy-2-methylpropan-2-ylimino)methyl}-4-methylphenol (H3L1) gave trinuclear complex [Mn3(μ-H2L1)2(μ1,3-O2CCH3)4(CH3OH)2](ClO4)2·4CH3OH (1), whereas 2,6-bis[{1-hydroxy-2-(hydroxymethyl)butan-2-ylimino}methyl]-4-methylphenol (H5L2) provided hexanuclear complex [Mn6(μ4-H2L2)2(μ-HL3)2(μ3-OH)2(μ1,3-O2CC2H5)4](ClO4)2·2H2O (2). Binding of acetates and coordination of {H2L1}− provided a linear MnIIIMnIIMnIII arrangement in 1. A MnIII6 fused diadamantane-type assembly was obtained in 2 from propionate bridges, coordination of {H2L2}3–, and in situ generated {HL3}2–. The magnetic characterization of 1 and 2 revealed the properties dominated by intramolecular anti-ferromagnetic exchange interactions, and this was confirmed using density functional theory calculations. Complex 1 exhibited field-induced slow magnetic relaxation at 2 K due to the axial anisotropy of MnIII centers. Both the complexes show effective solvent-dependent catechol oxidation toward 3,5-di-tert-butylcatechol in air. The catechol oxidation abilities are comparable from two complexes of different nuclearity and structure

Enhancement of TbIII-CuII single-molecule magnet performance through structural modification

We report a series of 3d–4f complexes {Ln2Cu3(H3L)2Xn} (X=OAc−, Ln=Gd, Tb or X=NO3−, Ln=Gd, Tb, Dy, Ho, Er) using the 2,2′-(propane-1,3-diyldiimino)bis[2-(hydroxylmethyl)propane-1,3-diol] (H6L) pro-ligand. All complexes, except that in which Ln=Gd, show slow magnetic relaxation in zero applied dc field. A remarkable improvement of the energy barrier to reorientation of the magnetisation in the {Tb2Cu3(H3L)2Xn} complexes is seen by changing the auxiliary ligands (X=OAc− for NO3−). This leads to the largest reported relaxation barrier in zero applied dc field for a Tb/Cu-based single-molecule magnet. Ab initio CASSCF calculations performed on mononuclear TbIII models are employed to understand the increase in energy barrier and the calculations suggest that the difference stems from a change in the TbIII coordination environment (C4v versus Cs)

Chemical and in silico tuning of the magnetisation reversal barrier in pentagonal bipyramidal Dy(III) single-ion magnets

Two new air-stable axial Dy(III) Single-Ion Magnets, [Dy(H2O)5(HMPA)2]Cl3·HMPA·H2O (1) and [Dy(H2O)5(HMPA)2]I3 ·2HMPA (2) (HMPA = hexamethylphosphoramide), exhibit magnetic hysteresis which remains open up to 9 K for 1@Y and 10 K for 2@Y, respectively. Ab initio calculations, using step-by-step elimination of fragments to generate a series of model complexes, reveal that the secondary coordination sphere plays a key role in controlling the magnetisation reversal barrier and predict that the removal of outer-sphere molecules and anions will enhance the barrier further

Trigonal to pentagonal bipyramidal coordination switching in a Co(II) single-ion magnet

In molecular magnetism and single-ion magnets in particular, the observation of slow relaxation of the magnetization is intimately linked to the coordination environment of the metal center. Such systems typically have blocking temperatures well below that of liquid nitrogen, and therefore detailed magnetic characterization is usually carried out at very low temperatures. Despite this, there has been little advantage taken of ultralow temperature single-crystal X-ray diffraction techniques that could provide a full understanding of the crystal structure in the same temperature regime where slow magnetic relaxation occurs. Here, we present a systematic variable temperature single crystal X-ray diffraction study of [CoII(NO3)3(H2O)(HDABCO)] (1) {DABCO = 1,4-diazabicyclo[2.2.2]octane} conducted between 295 to 4 K. A reversible and robust disorder-to-order, single-crystal to single-crystal phase transition was identified, which accompanied a switching of the coordination geometry around the central Co(II) from 5- to 7-coordinate below 140 K. The magnetic properties were investigated, revealing slow relaxation of the magnetization arising from a large easy-plane magnetic anisotropy (+D) in the Co(II) pentagonal bipyramidal environment observed at low temperatures. This study highlights the importance of conducting thorough low temperature crystallographic studies, particularly where magnetic characterization is carried out at such low temperatures

Magnetic properties of a family of [MnIII4LnIII4] wheel complexes: an experimental and theoretical study

The chelating ligand 1,3-bis(tris(hydroxymethyl)methylamino)propane (H6L) has been used to synthesize a family of octanuclear heterometallic complexes with the formula (NMe4)3[Mn4Ln4(H2L)3(H3L)(NO3)12] (Ln = La (1), Ce (2), Pr (3), Nd (4)). Encapsulation by the ligand causes the Mn(III) centers to lie in an unusually distorted (∼C2v) environment, which is shown by density functional theory and complete active space self-consistent field calculations to impact on the magnetic anisotropy of the Mn(III) ion. The theoretical study also supports the experimental observation of a ferromagnetic superexchange interaction between the Mn(III) ions in 1, despite the ions being separated by the diamagnetic La(III) ion. The optical properties of the compounds show that the distortion of the Mn(III) ions leads to three broad absorption bands originating from the transition metal ion, while the Nd(III) containing complex also displays some weak sharp features arising from the lanthanide f–f transitions

Engineering macrocyclic high performance pentagonal bipyramidal Dy(iii) single-ion magnets

We generate a new air-stable pseudo-D5h Dy(III) Single-Molecule Magnet (Ueff = 1108 K, TB = 14 K) by combining a weak equatorial ligand field from a macrocyclic LN5 ligand with a strong axial ligand field. Based on our synthetic blueprint, we use ab initio calculations to show the vast scope for macrocyclic engineering of magnetic anisotropy