Engineering Clock Transitions in Molecular Lanthanide Complexes.

Molecular lanthanide (Ln) complexes are promising candidates for the development of next-generation quantum technologies. High-symmetry structures incorporating integer spin Ln ions can give rise to well-isolated crystal field quasi-doublet ground states, i.e., quantum two-level systems that may serve as the basis for magnetic qubits. Recent work has shown that symmetry lowering of the coordination environment around the Ln ion can produce an avoided crossing or clock transition within the ground doublet, leading to significantly enhanced coherence. Here, we employ single-crystal high-frequency electron paramagnetic resonance spectroscopy and high-level ab initio calculations to carry out a detailed investigation of the nine-coordinate complexes, [HoIIIL1L2], where L1 = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane and L2 = F- (1) or [MeCN]0 (2). The pseudo-4-fold symmetry imposed by the neutral organic ligand scaffold (L1) and the apical anionic fluoride ion generates a strong axial anisotropy with an mJ = ±8 ground-state quasi-doublet in 1, where mJ denotes the projection of the J = 8 spin-orbital moment onto the ∼C4 axis. Meanwhile, off-diagonal crystal field interactions give rise to a giant 116.4 ± 1.0 GHz clock transition within this doublet. We then demonstrate targeted crystal field engineering of the clock transition by replacing F- with neutral MeCN (2), resulting in an increase in the clock transition frequency by a factor of 2.2. The experimental results are in broad agreement with quantum chemical calculations. This tunability is highly desirable because decoherence caused by second-order sensitivity to magnetic noise scales inversely with the clock transition frequency.

Stellated cuboctahedron of FeIII.

The solvothermal reaction of FeCl2·4H2O and H4TBC[4] in a basic dmf/EtOH solution affords an [FeIII18] Keplerate conforming to a stellated cuboctahedron. Magnetic measurements reveal spin frustration effects arising from the high symmetry.

Importance of an Axial LnIII-F Bond across the Lanthanide Series and Single-Molecule Magnet Behavior in the Ce and Nd Analogues.

The recently reported compound [DyIIILF](CF3SO3)2·H2O (L = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane) displays a strong axial magnetic anisotropy, due to the short axial Dy-F bond, and single-molecule magnet (SMM) behavior. Following our earlier [DyIIILF]2+ work, herein we report the systematic structural and magnetic study of a family of [LnIIILF](CF3SO3)2·H2O compounds (Ln(III) = 1-Ce, 2-Pr, 3-Nd, 4-Eu, 5-Tb, 6-Ho, 7-Er, 8-Tm, and 9-Yb). From this series, the Ce(III) and Nd(III) analogues show slow relaxation of the magnetization under an applied direct current magnetic field, which is modeled using a Raman process. Complete active space self-consistent field theoretical calculations are employed to understand the relaxation pathways in 1-Ce and 3-Nd and also reveal a large tunnel splitting for 5-Tb. Additional computational studies on model compounds where we remove the axial F- ligand, or replace F- with I-, highlight the importance of the F- ligand in creating a strong axial crystal field for 1-Ce and 3-Nd and for promoting the SMM behavior. Importantly, this systematic study provides insight into the magnetic properties of these lighter lanthanide ions.

Insight into D6h Symmetry: Targeting Strong Axiality in Stable Dysprosium(III) Hexagonal Bipyramidal Single-Ion Magnets.

Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph3 SiO- (Ph3 SiO- =anion of triphenylsilanol) and the 2,4-di-t Bu-PhO- (2,4-di-t Bu-PhO- =anion of 2,4-di-tertbutylphenol) ligands combined with the weak equatorial field of the ligand LN6 , leads to [DyIII (LN6 )(2,4-di-t Bu-PhO)2 ](PF6 ) (1), [DyIII (LN6 )(Ph3 SiO)2 ](PF6 ) (2) and [DyIII (LN6 )(Ph3 SiO)2 ](BPh4 ) (3) hexagonal bipyramidal dysprosium(III) single-molecule magnets (SMMs) with high anisotropy barriers of Ueff =973 K for 1, Ueff =1080 K for 2 and Ueff =1124 K for 3 under zero applied dc field. Ab initio calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher-order symmetry SMMs.

Enneanuclear [Ni6Ln3] Cages: [Ln(III)3] Triangles Capping [Ni(II)6] Trigonal Prisms Including a [Ni6Dy3] Single-Molecule Magnet.

The use of (2-(ß-naphthalideneamino)-2-hydroxymethyl-1-propanol) ligand, H3L, in Ni/Ln chemistry has led to the isolation of three new isostructural [Ni(II)6Ln(III)3] metallic cages. More specifically, the reaction of Ni(ClO4)2·6H2O, the corresponding lanthanide nitrate salt, and H3L in MeCN, under solvothermal conditions in the presence of NEt3, led to the isolation of three complexes with the formulas [Ni6Gd3(OH)6(HL)6(NO3)3]·5.75MeCN·2Et2O·1.5H2O (1·5.75MeCN·2Et2O·1.5H2O), [Ni6Dy3(OH)6(HL)6(NO3)3]·2MeCN·2.7Et2O·2.4H2O (2·2MeCN·2.7Et2O·2.4H2O), and [Ni6Er3(OH)6(HL)6(NO3)3]·5.75MeCN·2Et2O·1.5H2O (3·5.75MeCN·2Et2O·1.5H2O). The structure of all three clusters describes a [Ln(III)3] triangle capping a [Ni(II)6] trigonal prism. Direct current magnetic susceptibility studies in the 5-300 K range for complexes 1-3 reveal the different nature of the magnetic interactions within the clusters: dominant antiferromagnetic exchange interactions for the Dy(III) and Er(III) analogues and dominant ferromagnetic interactions for the Gd(III) example. Alternating current magnetic susceptibility measurements under zero external dc field displayed fully formed temperature- and frequency-dependent out-of-phase peaks for the [Ni(II)6Dy(III)3] analogue, establishing its single molecule magnetism behavior with Ueff = 24 K.

Reaction of Ph2C(X)(CO2H) (X = OH, NH2) with [VO(OR)3] (R = Et, nPr): structure, magnetic susceptibility and ROP capability.

Reaction of [VO(OR)3] (R = Et, nPr) with 2,2'-diphenylglycine afforded the alkoxide-bridged dimers {[VO(OR)(µ-OR)][Ph2C(NH2)(CO2)]}2, whereas use of benzilic acid, in the presence of alkali metals, afforded 16-membered metallocycles {V8(O)4M(OR)8[Ph2C(OH)(CO2)]12} (M = <1 Na, K). For the ring systems, magnetic susceptibility data is consistent with mixed-valence vanadium with an average oxidation state of 3.5. The dimer and ring systems are capable of the ring opening polymerisation (ROP) of ε-caprolactone under N2, air, or as melts affording mostly low to medium molecular weight cyclic and linear products.

Tandem templating strategies facilitate the assembly of calix[8]arene-supported Ln18 clusters.

Calix[n]arenes offer ideal chemical functionality through the polyphenolic lower rim to construct nano-sized coordination clusters with lanthanide (Ln) metal ions (e.g., NdIII10, GdIII8). However, the number of metal centers they can accommodate is still limited compared to that achievable with smaller ligands (e.g., GdIII140, GdIII104). Here, we exploit a combination of the "anion template strategy" and "templating ligands" to synthesise three highly symmetric (D3h, trigonal planar) LnIII18 (Ln = La, Nd, and Gd) systems, representing the largest calix[n]arene-based coordination clusters yet. The LnIII18 fragment is templated by a chloride anion located at the center of the cluster, wherefrom twelve µ3-OH- ligands bind 'internally' to the eighteen LnIII ions. 'Externally' the metallic skeleton is connected by p-tert-butylcalix[8]arene, oxo, chloro and carbonate ligands. The crystal packing in the lattice reveals large cylindrical channels of ∼26 Å in diameter, whose pore volume corresponds to ∼50% of the unit cell volume (using a 1.2 Å spherical probe radius). Magnetic measurements reveal the predominance of weak antiferromagnetic exchange in the Gd analog. Heat capacity data of GdIII18 reveal a high magnetic entropy with -ΔSm = 23.7 J K-1 kg-1, indicating potential for engineering magnetic refrigerant materials with calix[8]arenes.

Tetra-, tetradeca- and octadecametallic clusters of Mn.

Reaction of equimolar amounts of MnBr2·4H2O and HL1 ((3,5-dimethyl-1H-pyrazol-1-yl)methanol) in a basic MeCN solution leads to the formation of [MnII4(L1)4Br4(H2O)4] (1), whose metallic skeleton is a [MnII4] tetrahedron and cluster core a [MnII4(µ3-O)4] cubane. Replacing MnBr2·4H2O with Mn(O2CMe)2·4H2O affords [MnIII2MnII12O2(L1)4(OAc)22] (2) which is best described as a series of edge-sharing [Mn4] tetrahedra that have self-assembled into a linear array in which each [Mn2] pair is 'twisted' with respect to its neighbours in a corkscrew-like manner. Employment of the triangle [MnIII3O(OAc)6(py)3](ClO4) as a reactant instead of a MnII salt results in the formation of [MnIII14MnII4O14(L1)4(HL1)2(OAc)18(H2O)2] (3) whose core is comprised of three vertex-sharing [MnIII4] butterflies flanked on either side by one [MnIII4] cubane and one [MnIII2MnII2] tetrahedron. Dc magnetic susceptibility and magnetisation measurements of polycrystalline samples of 1-3 reveal the predominance of antiferromagnetic exchange interactions. For [Mn4] (1) this leads to a diamagnetic ground state, while for [Mn18] (3) competing exchange interactions result in Single-Molecule Magnet (SMM) behaviour with Ueff = 22 K.

Synthesis and characterization of two self-assembled [Cu6Gd3] and [Cu5Dy2] complexes exhibiting the magnetocaloric effect, slow relaxation of magnetization, and anticancer activity.

Two new paths for coordination driven self-assembly reactions under the binding support of 2-((1-hydroxy-2-methylpropan-2-ylimino)methyl)-6-methoxyphenol (H2L) have been discovered from the reactions of Cu(ClO4)2·6H2O, NEt3 and GdCl3/DyCl3·6H2O in MeOH/CHCl3 (2 : 1) medium. A similar synthetic protocol is useful to provide two different types of self-aggregated molecular clusters [Cu6Gd3(L)3(HL)3(µ3-Cl)3(µ3-OH)6(OH)2]ClO4·4H2O (1) and [Cu5Dy2(L)2(HL)2(µ-Cl)2(µ3-OH)4(ClO4)2(H2O)6](ClO4)2·2NHEt3Cl·21H2O (2). The adopted reaction procedure established the importance of the HO- and Cl- ions in the mineral-like growth of the complexes, derived from solvents and metal ion salts. In the case of complex 1, one GdIII center has been trapped at the central position of the core upheld by six µ3-OH and three µ3-Cl groups, whereas for complex 2 one CuII center was trapped using four µ3-hydroxo and two µ-chlorido groups. The magnetothermal behavior of 1 has been examined for a magnetocaloric effect of -ΔSm = 11.3 J kg-1 K-1 at 2 K for ΔH = 7 T, whereas the magnetic susceptibility measurements of 2 showed slow magnetic relaxation with Ueff = 15.8 K and τ0 = 9.8 × 10-7 s in zero external dc field. Cancer cell growth inhibition studies proved the potential of both the complexes with interestingly high activity for the Cu6Gd3 complex against human lung cancer cells. Both complexes 1 and 2 also exhibited DNA and human serum albumin (HSA) binding abilities in relation to the involved binding sites and thermodynamics.

Odd and Even Numbered Ferric Wheels.

The structurally related odd and even numbered wheels [FeIII 11 ZnII 4 (tea)10 (teaH)1 (OMe)Cl8 ] (1) and [FeIII 12 ZnII 4 (tea)12 Cl8 ] (2) can be synthesized under ambient conditions by reacting FeIII and ZnII salts with triethanolamine (teaH3 ), the change in nuclearity being dictated by the solvents employed. An antiferromagnetic exchange between nearest neighbors, J = -10.0 cm-1 for 1 and J = -12.0 cm-1 for 2, leads to a frustrated S = 1/2 ground state in the former and an S = 0 ground state in the latter.

A strongly blue-emitting heptametallic [Dy(III)7] centered-octahedral single-molecule magnet.

The employment of 2-(ß-naphthalideneamino)-2-(hydroxymethyl)-1-propanol and 2-aminoisobutyric acid in dysprosium chemistry has led to the isolation of a novel heptanuclear [Dy(III)(7)] cluster displaying single-molecule-magnetism behavior and blue-emitting properties.

Employment of a new tripodal ligand for the synthesis of cobalt(II/III), nickel(II), and copper(II) clusters: magnetic, optical, and thermal properties.

The employment of 2-(ß-naphthalideneamino)-2-(hydroxymethyl)-1-propanol (LH(3)) in cobalt, nickel, and copper chemistry has led to the isolation of five new metallic complexes with interesting magnetic properties. More specifically, the reaction of Co(OAc)(2)·4H(2)O with LH(3) in MeOH in the presence of NEt(3) under solvothermal conditions forms the complex [Co(III)(2)Co(II)(3)(L)(2)(LH)(2)(L')(OAc)]·8.5MeOH (1·8.5MeOH; L' = monoanion of 2-hydroxy-1-naphthaldehyde), while in nickel chemistry, a similar reaction of Ni(OAc)(2)·6H(2)O with LH(3) in MeCN in the presence of NEt(3) under high pressure/temperature forms the complex [Ni(II)(LH(2))(2)]·2MeCN (2·2MeCN). Repeating the same reaction in MeOH and switching from Ni(OAc)(2)·4H(2)O to NiSO(4)·4H(2)O produces the complex [Ni(II)(4)(HL)(3)(OMe)(MeOH)(3)](SO(4))(0.5)·2MeOH (3·2MeOH) under solvothermal conditions. Furthermore, in copper chemistry, the reaction of Cu(2)(OAc)(4)·2H(2)O with LH(3) in the presence of NEt(3) in MeOH under solvothermal conditions affords the complex [Cu(II)(4)(LH)(4)] (4), while the same reaction under ambient temperature and pressure conditions forms [Cu(II)(4)(LH)(4)] ·3.5MeOH·2.25H(2)O (5·3.5MeOH·2.25H(2)O). Complex 1 is a mixed-valent [Co(III)(2)Co(II)(3)] complex, consisting of three edge-sharing [Co(3)] triangles. Complex 2 is a nickel(II) monomer in which the central metal is found in an octahedral geometry, while complex 3 describes a [Ni(II)(4)] cubane. Complexes 4 and 5 may be considered as structural isomers because they possess the same formulas but different topologies: 4 describes a highly distorted [Cu(II)(4)(OR)(4)](4+) eight-membered ring, while 5 consists of a distorted [Cu(II)(4)(µ(3)-OR)(4)](4+) cubane. In addition, 5 can be converted to 4 in excellent yield under solvothermal conditions. Direct-current magnetic susceptibility studies have been carried out in the 5-300 K range for complexes 1 and 3-5, revealing the possibility of a high-spin ground state for 1, an S = 4 ground state for 2, and diamagnetic ground states for 4 and 5.

Heptanuclear heterometallic [Cu6Ln] clusters: trapping lanthanides into copper cages with artificial amino acids.

Employment of the artificial amino acid 2-amino-isobutyric acid, aibH, in Cu(II) and Cu(II)/Ln(III) chemistry led to the isolation and characterization of 12 new heterometallic heptanuclear [Cu(6)Ln(aib)(6)(OH)(3)(OAc)(3)(NO(3))(3)] complexes consisting of trivalent lanthanide centers within a hexanuclear copper trigonal prism (aibH = 2-amino-butyric acid; Ln = Ce (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), and Yb (12)). Direct curent magnetic susceptibility studies have been carried out in the 5-300 K range for all complexes, revealing the different nature of the magnetic interactions between the 3d-4f metallic pairs: dominant antiferromagnetic interactions for the majority of the pairs and dominant ferromagnetic interactions for when the lanthanide center is Gd(III) and Dy(III). Furthermore, alternating current magnetic susceptibility studies reveal the possibility of single-molecule magnetism behavior for complexes 7 and 8. Finally, complexes 2, 5-8, 10, and 12 were analyzed using positive ion electrospray mass spectrometry (ES-MS), establishing the structural integrity of the heterometallic heptanuclear cage structure in acetonitrile.

A graceful break-up: serendipitous self-assembly of a ferromagnetically coupled [NiII14] wheel.

The complex [NiII14(HL2)12(HCOO)14Cl14(MeOH)(H2O)] describes an aesthetically pleasing wheel displaying ferromagnetic nearest neighbour exchange.

An [FeIII8] molecular oxyhydroxide.

An [FeIII8] hexagonal bipyramid displays antiferromagnetic exchange between the two capping tetrahedral ions and the six ring octahedral ions resulting in a spin ground state of S = 10.

A ferromagnetically coupled pseudo-calixarene [Co16] wheel that self-assembles as a tubular network of capsules.

Reaction of Co(OAc)2·4H2O and Hsal in a basic MeCN solution affords the hexadecanuclear wheel [Co16(sal)16(OAc)16]·16MeCN (1·16MeCN) that displays ferromagnetic nearest neighbour exchange and has pseudo-calixarene character. Symmetry equivalent wheels self-assemble to form remarkable tubular networks of capsules in the extended structure.

Magnetic anisotropies of Ho(III) and Dy(III) single-molecule magnets experimentally determined via polarized neutron diffraction.

We present the magnetic anisotropy of two isostructural pentagonal-bipyramidal complexes, [Ln(H2O)5(HMPA)2]I3·2HMPA (HMPA = hexamethylphosphoramide, Ln = Dy, Ho). Using ac magnetic susceptibility measurements, we find magnetic relaxation barriers of 600 K and 270 K for the Dy- and Ho-compounds, respectively. This difference is supported by polarized neutron diffraction (PND) measured at 5 K and 1 T which provides the first experimental evidence that the transverse elements in the magnetic anisotropy of the Ho-analogue are significant, whereas the Dy-analogue has a near-axial magnetic anisotropy with vanishing transverse contributions. The coordination geometries of the two complexes are highly similar, and we attribute the loss of strong magnetic axiality as expressed in the atomic susceptibility tensors from PND, as well as the smaller relaxation barrier in the Ho-complex compared to the Dy-complex, to the less favorable interaction of the pentagonal bipyramidal crystal field with the characteristics of the Ho(III) 4f-charge distribution.

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.

There is nothing wrong with being soft: using sulfur ligands to increase axiality in a Dy(iii) single-ion magnet.

A new air-stable sulfur-ligated Dy(iii) single-ion magnet has been successfully isolated with Ueff = 638 K and hysteresis loops open up to 7 K. In silico studies show that the S co-ligands significantly boost the axiality and that Te- and Se-donors have the potential to further enhance the magnetic properties.

One class classification as a practical approach for accelerating π-π co-crystal discovery.

The implementation of machine learning models has brought major changes in the decision-making process for materials design. One matter of concern for the data-driven approaches is the lack of negative data from unsuccessful synthetic attempts, which might generate inherently imbalanced datasets. We propose the application of the one-class classification methodology as an effective tool for tackling these limitations on the materials design problems. This is a concept of learning based only on a well-defined class without counter examples. An extensive study on the different one-class classification algorithms is performed until the most appropriate workflow is identified for guiding the discovery of emerging materials belonging to a relatively small class, that being the weakly bound polyaromatic hydrocarbon co-crystals. The two-step approach presented in this study first trains the model using all the known molecular combinations that form this class of co-crystals extracted from the Cambridge Structural Database (1722 molecular combinations), followed by scoring possible yet unknown pairs from the ZINC15 database (21 736 possible molecular combinations). Focusing on the highest-ranking pairs predicted to have higher probability of forming co-crystals, materials discovery can be accelerated by reducing the vast molecular space and directing the synthetic efforts of chemists. Further on, using interpretability techniques a more detailed understanding of the molecular properties causing co-crystallization is sought after. The applicability of the current methodology is demonstrated with the discovery of two novel co-crystals, namely pyrene-6H-benzo[c]chromen-6-one (1) and pyrene-9,10-dicyanoanthracene (2).