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.

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.

Elucidating the exchange interactions in a {GdIIICuII4} propellor.

The multinucleating ligand 2,2'-(propane-1,3-diyldiimino)bis[2-(hydroxymethyl)-propane-1,3-diol] (bis-tris propane, H6L) is used in the design of a new family of 3d-4f complexes that display an unusual {LnCu4} four-blade propeller topology. We report the synthesis, structure and magnetic characterisation of [LnCu4(H4L)4](Cl)2(ClO4)·6CH3OH, where Ln = Gd (1), Tb (2), Dy (3), La (4). Previously we have used CH3COO- and NO3- as co-ligands with bis-tris propane, but here the use of Cl- and ClO4- leads to coordination of four {Cu(H4L)} units around the central Ln ion. A magneto-structural analysis reveals that the geometrical arrangement of the Cu(II) centres defined by the H4L2- ligands controls the magnetic communication between the different metal centres. DFT calculations performed on the isotropic (Gd) and diamagnetic (La) systems 1 and 4 help to unravel the intriguing exchange interactions.

Constructing "Closed" and "Open" {Mn8} Clusters.

Use of the 1,3,5-tri(2-hydroxyethyl)-1,3,5-triazacyclohexane ligand, LH3, in manganese chemistry affords access to two structurally related {Mn8} clusters: a "closed" {MnIII 6MnII 2} puckered square wheel of formula [Mn8L2(LH)O3(OH)2(MeO)2Br(imH)(H2O)3](Br)3 (1; imH = imidazole) and an "open" {MnIII 8} rod of formula [MnΙΙΙ 8L2O4(aibH)2(aib)2(MeO)6(MeOH)2](NO3)2 (2, aibH = 2-amino-isobutyric acid). In each case the triaza ligands, L/LH, direct the formation of {Mn3} triangles with their N atoms preferentially bonding to the Jahn-Teller axes of the MnIII ions. Subsequent self-assembly is dependent on the anion of the Mn salt and the identity of the organic coligand employed-the terminally bonded imidazole and the chelating/bridging amino acid. The {Mn3} triangles fold up on themselves in 1, forming a wheel. However, the syn, syn-bridging carboxylates in 2 prevent this from happening, instead directing the formation of a linear rod. Magnetic susceptibility and magnetization measurements reveal competing ferro- and antiferromagnetic interactions in both complexes, the exchange being somewhat weaker in 1 due to the presence of MnII ions.

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.

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.

From tetranuclear to pentanuclear [Co-Ln] (Ln = Gd, Tb, Dy, Ho) complexes across the lanthanide series: effect of varying sequence of ligand addition.

Two new families of cobalt(ii/iii)-lanthanide(iii) coordination aggregates have been reported: tetranuclear [LnCoL2(N-BuDEA)2(O2CCMe3)4(H2O)2]·(MeOH)n·(H2O)m (Ln = Gd, 1; Tb, 2; Dy, 3; n = 2, m = 10 for 1 and 2; n = 6, m = 2 for 3) and pentanuclear LnCoIICoL2(N-BuDEA)2(O2CCMe3)6(MeOH)2 (Ln = Dy, 4; Ho, 5) formed from the reaction of two aggregation assisting ligands H2L (o-vanillin oxime) and N-BuDEAH2 (N-butyldiethanolamine). A change in preference from a lower to higher nuclearity structure was observed on going across the lanthanide series brought about by the variation in the size of the LnIII ions. An interesting observation was made for the varying sequence of addition of the ligands into the reaction medium paving the way to access both structural types for Ln = Dy. HRMS (+ve) of solutions gave further insight into the formation of the aggregates via different pathways. The tetranuclear complexes adopt a modified butterfly structure with a more complex bridging network while trapping of an extra CoII ion in the pentanuclear complexes destroys this arrangement putting the Co-Co-Co axis above the Ln-Ln axis. Direct current (dc) magnetic susceptibility measurements reveal weak antiferromagnetic coupling in 1. Complexes 2 and 5 display no slow magnetic relaxation, whereas complexes 3 and 4 display out-of-phase signals at low temperature in ac susceptibility measurements. All compounds were analyzed with DFT and CASSCF calculations and informations about the single-ion anisotropies and mutual 4f-4f/4f-3d magnetic interactions were derived.

Exploiting host-guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages.

Reaction of Ni(OTf)2 with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [NiII 4 L 6]8+. By selectively exchanging the bound triflate from [OTf⊂NiII 4 L 6](OTf)7 (1), we have been able to prepare a series of host-guest complexes that feature an encapsulated paramagnetic tetrahalometallate ion inside this paramagnetic host giving [MIIX4⊂NiII 4 L 6](OTf)6, where MIIX4 2- = MnCl4 2- (2), CoCl4 2- (5), CoBr4 2- (6), NiCl4 2- (7), and CuBr4 2- (8) or [MIIIX4⊂NiII 4 L 6](OTf)7, where MIIIX4 - = FeCl4 - (3) and FeBr4 - (4). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host-guest complexes 1-8 all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN6} vertexes within a single Ni4L6 unit possess the same Δ or Λ stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [NiII 4] complex, and between the host and the MX4 n- guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the magnetic exchange are in close agreement with experiment, revealing that large spin densities on the electronegative X-atoms of particular MX4 n- guest molecules lead to stronger host-guest magnetic exchange interactions.

Solvent-induced structural transformation from heptanuclear to decanuclear [Co-Ln] coordination clusters: trapping of unique counteranion and understanding of aggregation pathways.

Five new cobalt(ii/iii)-lanthanide(iii)-based coordination aggregates, [LnIII3CoII2CoIII2(L1)2(O2CCMe3)8(OH)4(OMe)2(H2O)4]·Ln(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2·2MeOH·2H2O (where Ln = Tb (1), Ho (3), and H2L1 = N-(2-hydroxyethyl)-salicylaldimine), TbIII3CoII3CoIII4(L1)4(O2CCMe3)9(OH)10(H2O) (4) and LnIII3CoII2CoIII5(L1)4(O2CCMe3)10(OH)10 (Ln = Dy (5), Ho (6)) have been synthesized and characterized, including structural analysis via single-crystal X-ray diffraction. The dysprosium analogue (2) of 1 and 3 was previously reported by us. The heptanuclear monocationic clusters in 1 and 3 were formed by placement of seven metal ions (4 Co and 3 Ln) in a vertex shared dicubane structure from the control of two Schiff base anions and crystallized in the presence of in situ generated and literature unknown counter anions Tb(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2- and Ho(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2-. Interesting solvent-induced cluster structure transformation was observed on dissolving the heptanuclear aggregates in MeCN for the formation of decanuclear clusters 4-6. These high nuclearity clusters consist of a vertex shared heptanuclear dicubane part and a curved trinuclear chain linking the two cubic halves. The dicubane unit differs from that of the heptanuclear precursors in the presence of CoII/III at the shared vertex as opposed to LnIII and the absence of OMe- bridges. HRMS (+ve) analysis shed light on the pathway of formation of these heptanuclear molecules, while at the same time revealing a different aggregation process for the decanuclear clusters.

Hydroxido supported and differently networked octanuclear Ni6Ln2 [Ln = GdIII and DyIII] complexes: structural variation, magnetic properties and theoretical insights.

The design and synthesis of a Schiff base H2L, (2-{[(2-hydroxy-3-methoxybenzyl)imino]methyl}phenol), bearing an ONOO donor set has been explored for its reactivity pattern with LnCl3·6H2O (Ln = GdIII and DyIII) and Ni(CH3CO2)2·4H2O in the presence of NEt3 for molecular aggregation. Coordination driven spontaneous self-assembly reactions provide [Gd2Ni6L4(µ3-OH)6(µ-OH)2(CH3CO2)2(µ-H2O)2(H2O)3(MeOH)]·8H2O (1) having a 'butterfly-shaped' core and [Dy2Ni6L4(µ3-OH)4(µ-Cl)2Cl4(H2O)2(MeOH)2]·2MeOH·4H2O (2) with a 'candy-shaped' core. Fusion of six partial cubane units led to a mineral-like core in 1, developed around the central Ni2(OH)2 moiety, whereas in the case of 2, four such partial cubanes collapsed on Dy2(OH)2. Direct-current (dc) magnetic susceptibility measurements revealed that predominant ferromagnetic interactions lead to a high-spin S = 13 ground state for 1. Complex 2 exhibits slow relaxation of magnetization under an applied small dc field with an energy barrier to reorientation of the magnetization, Ueff = 19.3 K. The static and dynamic magnetic data are analysed and corroborated by density functional theory (DFT) and detailed CASSCF based calculations.

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.

A large axial magnetic anisotropy in trigonal bipyramidal Fe(ii).

The first trigonal bipyramidal Fe(ii) complex to display slow relaxation of magnetisation has been isolated, with this behaviour found to arise through a combination of a large magnetic anisotropy (D = -27.5 cm-1) and a pseudo-D3h symmetry at the Fe(ii) centre, as investigated through ab initio and magnetic studies.

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.

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.

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.

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.

Investigation of the magnetic anisotropy in a series of trigonal bipyramidal Mn(ii) complexes.

Understanding how the magnetic anisotropy in simple coordination complexes can be manipulated is instrumental to the development of single-molecule magnets (SMMs). Clear strategies can then be designed to control both the axial and transverse contributions to the magnetic anisotropy in such compounds, and allow them to reach their full potential. Here we show a strategy for boosting the magnetic anisotropy in a series of trigonal bipyramidal Mn(ii) complexes - [MnCl3(HDABCO)(DABCO)] (1), [MnCl3(MDABCO)2]·[ClO4] (2), and [MnCl3(H2O)(MDABCO)] (3). These have been successfully synthesised using the monodentate [DABCO] and [MDABCO]+ ligands. Through static (DC) magnetic measurements and detailed theoretical investigation using ab initio methods, the magnetic anisotropy of each system has been studied. The calculations reveal that the rhombic zero-field splitting (ZFS) term (E) can be tuned as the symmetry around the Mn(ii) ion is changed. Furthermore, an in silico investigation reveals a strategy to increase the axial ZFS parameter (D) of trigonal bipyramidal Mn(ii) by an order of magnitude.

In-depth investigation of large axial magnetic anisotropy in monometallic 3d complexes using frequency domain magnetic resonance and ab initio methods: a study of trigonal bipyramidal Co(ii).

The magnetic properties of 3d monometallic complexes can be tuned through geometric control, owing to their synthetic accessibility and relative structural simplicity. Monodentate ligands offer great potential for fine-tuning the coordination environment to engineer both the axial and rhombic zero-field splitting (ZFS) parameters. In [CoCl3(DABCO)(HDABCO)] (1), the trigonal bipyramidal Co(ii) centre has two bulky axial ligands and three equatorial chloride ligands. An in-depth experimental and theoretical study of 1 reveals a large easy-plane magnetic anisotropy (+ve D) with a negligible rhombic zero-field splitting (E) due to the strict axial symmetry imposed by the C 3 symmetric ligand and trigonal space group. The large easy-plane magnetic anisotropy (D = +44.5 cm-1) is directly deduced using high-field EPR and frequency-domain magnetic resonance (FDMR) studies. Ab initio calculations reveal a large positive contribution to the D term arising from ground state/excited state mixing of the 4E'' states at ∼4085 cm-1 and a minor contribution from the spin-flip transition as well. The nature of the slow relaxation in 1 is elucidated through analysis of the rates of relaxation of magnetisation, taking into account Raman and direct spin-lattice relaxation processes and Quantum Tunnelling of the Magnetisation (QTM). The terms relating to the direct process and QTM were found based on the fit of the field-dependence of τ at 2 K. Subsequently, these were used as fixed parameters in the fit of the temperature-dependence of τ to obtain the Raman terms. This experimental-theoretical investigation provides further insight into the power of FDMR and ab initio methods for the thorough investigation of magnetic anisotropy. Thus, these results contribute to design criteria for high magnetic anisotropy systems.

Boosting axiality in stable high-coordinate Dy(iii) single-molecule magnets.

A new nine-coordinate, air-stable Dy(iii) single-ion magnet has been successfully isolated. Our in silico studies demonstrate that through carefully modulating the ligand electronics, the axiality can be boosted to generate Ucal barriers of over 600 K.

Microwave-assisted synthesis: from a mononuclear {CoII} complex to {Co} solvomorphs.

We report a new {CoII9} complex with an unprecedented structure and its solvated analogue, using microwave heating to tune the synthesis and improve the product selectivity.