Halobenzene Adducts of a Dysprosocenium Single-Molecule Magnet

Dysprosium complexes with strong axial crystal fields are promising candidates for single-molecule magnets (SMMs), which could be used for high-density data storage. Isolated dysprosocenium cations, [Dy(CpR)2]+ (CpR = substituted cyclopentadienyl), have recently shown magnetic hysteresis (a memory effect) above the temperature of liquid nitrogen. Synthetic efforts have focused on reducing strong transverse ligand fields in these systems as they are known to enhance magnetic relaxation by spin-phonon mechanisms. Here we show that equatorial coordination of the halobenzenes PhX (X = F, Cl, Br) and o-C6H4F2 to the cation of a recently reported dysprosocenium complex [Dy(Cpttt)(Cp*)][Al{OC(CF3)3}4] (Cpttt = C5H2tBu3-1,2,4; Cp* = C5Me5) reduces magnetic hysteresis temperatures compared to that of the parent cation. We find that this is due to increased effectiveness of both one- (Orbach) and two-phonon (Raman) relaxation mechanisms, which correlate with the electronegativity and number of interactions with the halide despite κ1-coordination of a single halobenzene having a minimal effect on the metrical parameters of [Dy(Cpttt)(Cp*)(PhX-κ1-X)]+ cations vs the isolated [Dy(Cpttt)(Cp*)]+ cation. We observe unusual divergent behavior of relaxation rates at low temperatures in [Dy(Cpttt)(Cp*)(PhX)][Al{OC(CF3)3}4], which we attribute to a phonon bottleneck effect. We find that, despite the transverse fields introduced by the monohalobenzenes in these cations, the interactions are sufficiently weak that the effective barriers to magnetization reversal remain above 1000 cm-1, being only ca. 100 cm-1 lower than for the parent complex, [Dy(Cpttt)(Cp*)][Al{OC(CF3)3}4].

Influence of the phonon-bottleneck effect and low-energy vibrational modes on the slow spin-phonon relaxation in Kramers-ions-based Cu(II) and Co(II) complexes with 4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole and dicyanamide

Two new complexes, bis-[4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole-κ2N2,N6]bis-(dicyanamide-κN8)copper(II), [Cu(abpt)2(dca)2] (1) and bis-[4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole-κ2N2,N6]bis-(dicyanamide-κN8)cobalt(II), [Co(abpt)2(dca)2] (2), have been prepared and magneto-structurally characterised. Single crystal studies of both complexes have shown that their crystal structures are molecular, in which the central atoms are six-coordinated in the form of a distorted octahedron by two bidentate abpt and two monodentate dca ligands. Even if both complexes have the same composition and crystallize in the same P1̄ space group, they are not isostructural. Both structures contain strong intermolecular N-H⋯N hydrogen bonds and π-π stacking interactions. IR spectra are consistent with the solved structures of both complexes and confirmed the terminal character of the dca ligands and the bidentate coordination of the abpt ligands. The analysis of the magnetic properties showed that both complexes exhibit field-induced slow spin-phonon relaxation. In both complexes, the slow spin-phonon relaxation is influenced by a severe phonon-bottleneck effect that affects the direct process, a dominant relaxation mechanism at low temperatures in both complexes. The phonon-bottleneck effect in 1 was suppressed by simply reducing the crystallite size, and further analysis of the field dependence of the relaxation time yielded the characteristic energy of vibrational modes of 11 cm-1 participating in the Raman process at low magnetic fields. The analysis of magnetic properties and ab initio calculations confirmed that 2 represents a system with a moderate uniaxial anisotropy yielding an average energy barrier of 82 cm-1 (from all four nonequivalent Co(II) sites in the structure of 2).

Slow Magnetic Relaxation in {[CoCxAPy)] 2.15 H2O}n MOF Built from Ladder-Structured 2D Layers with Dimeric SMM Rungs

We present the magnetic properties of the metal-organic framework {[CoCxAPy]·2.15 H2O}n (Cx = bis(carboxypropyl)tetramethyldisiloxane; APy = 4,4`-azopyridine) (1) that builds up from the stacking of 2D coordination polymers. The 2D-coordination polymer in the bc plane is formed by the adjacent bonding of [CoCxAPy] 1D two-leg ladders with Co dimer rungs, running parallel to the c-axis. The crystal packing of 2D layers shows the presence of infinite channels running along the c crystallographic axis, which accommodate the disordered solvate molecules. The Co(II) is six-coordinated in a distorted octahedral geometry, where the equatorial plane is occupied by four carboxylate oxygen atoms. Two nitrogen atoms from APy ligands are coordinated in apical positions. The single-ion magnetic anisotropy has been determined by low temperature EPR and magnetization measurements on an isostructural compound {[Zn0.8Co0.2CxAPy]·1.5 CH3OH}n (2). The results show that the Co(II) ion has orthorhombic anisotropy with the hard-axis direction in the C2V main axis, lying the easy axis in the distorted octahedron equatorial plane, as predicted by the ab initio calculations of the g-tensor. Magnetic and heat capacity properties at very low temperatures are rationalized within a S* = 1/2 magnetic dimer model with anisotropic antiferromagnetic interaction. The magnetic dimer exhibits slow relaxation of the magnetization (SMM) below 6 K in applied field, with a tlf ≈ 2 s direct process at low frequencies, and an Orbach process at higher frequencies with U/kB = 6.7 ± 0.5 K. This compound represents a singular SMM MOF built-up of Co-dimers with an anisotropic exchange interaction.

Self-Assembled Heterometallic Complexes by Incorporation of Calcium or Strontium Ion into a Manganese(II) 12-Metallacrown-3 Framework Supported by a Tripodal Ligand with Pyridine-Carboxylate Motifs: Stability in Their Manganese(III) Oxidized Form

We report on the isolation of a new family of μ-carboxylato-bridged metallocrown (MC) compounds by self-assembly of the recently isolated hexadentate tris(2-pyridylmethyl)amine ligand tpada^2- incorporating two carboxylate units with metal cations. Twelve-membered MCs of manganese of the type 12-MC-3, namely, [{Mn^II(tpada)}₃(M)(H₂O)_n]²⁺ (Mn₃M) (M = Mn²⁺ (n = 0), Ca²⁺ (n = 1), or Sr²⁺ (n = 2)), were structurally characterized. The metallamacrocycles connectivity consisting in three -[Mn-O-C-O]- repeating units is provided by one carboxylate unit of the three tpada^2- ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn²⁺ or an alkaline earth metal, Ca²⁺ or Sr²⁺. Mn₃Ca and {Mn₃Sr}₂ join the small family of heterometallic manganese-calcium complexes and even rarer manganese-strontium complexes as models of the OEC of photosystem II (PSII). A 8-MC-4 of strontium of the molecular wheel type with four -[Sr-O]- repeating unit was also isolated by self-assembly of the tpada^2- ligand with Sr²⁺. This complex, namely, [Sr(tpada)(OH₂)]₄ (Sr₄), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr²⁺. Electrochemical investigations coupled to UV-visible absorption and EPR spectroscopies as well as electrospray mass spectrometry reveal the stability of the 12-MC-3 tetranuclear structures in solution, both in the initial oxidation state, Mn^II₃M, as well as in the three-electrons oxidized state, Mn^III₃M. Indeed, the cyclic voltammogram of all these complexes exhibits three-successive reversible oxidation waves between +0.5 and +0.9 V corresponding to the successive one-electron oxidation of the Mn(II) ion into Mn(III) of the three {Mn(tpada)} units constituting the ring, which are fully maintained after bulk electrolysis.

Syntheses, structures and magnetic properties of two isostructural dicyanamide-bridged 2D polymers

Complexes [Co(biq)(μ_1,5-dca)₂]_n (1) and [Ni(biq)(μ_1,5-dca)₂]_n (2) (biq is 2,2′-biquinoline, dca is dicyanamide anion, N(CN)₂⁻) have been characterized by crystal structure analysis, and spectral and magnetic measurements.

1D coordination polymer (OPD)2CoIISO4 showing SMM behaviour and multiple relaxation modes

A novel one-dimensional Co^II coordination polymer (OPD)₂Co^IISO₄ was formed from alternating tetrahedral sulphate anions, Co centers and molecules of 1,2-phenylenediamine (OPD). The thermal stability of the structure was confirmed up to ∼230 °C using thermogravimetry and non-ambient powder X-ray diffraction in air. The distorted pseudo-octahedral coordination sphere of Co^II ions promotes strong magnetic anisotropy. Therefore (OPD)₂Co^IISO₄ was subjected to thorough characterization with ab initio and DFT calculations along with dc magnetic measurements both of which point to strong easy-axis anisotropy (D/k_B≈-87 K, E/k_B≈ 23 K). The analysis of ac susceptibility revealed field assisted magnetic relaxation in two field regimes: a low field one with one relaxation time and a high field one where three relaxation times were distinguished. The main role in relaxations of the fastest process (τ₁) was attributed to the direct and Raman processes.

The effect of the orientation of the Jahn–Teller distortion on the magnetic interactions of trinuclear mixed-valence Mn(ii)/Mn(iii) complexes

The effect of the orientation of the Jahn–Teller distortion on the magnetic interactions in two new mixed-valence trinuclear Mn(iii)–Mn(ii)–Mn(iii) complexes has been investigated.