Self-assembly of a supramolecular spin-crossover tetrahedron.

A new mononuclear iron(II) SCO compound featuring H-bonding donor and acceptor units has been synthesized and exploited to produce a purely supramolecular switchable [Fe4] tetrahedron. Magnetic and crystallographic measurements evidence a singular magnetic behavior for each of the four Fe(II) centers of the generated architecture and underscore the potential of this strategy to develop novel SCO materials.

Dynamics and local ordering of pentachloronitrobenzene: a molecular-dynamics investigation.

Plastic phases are constituted by molecules whose centers of mass form a long range ordered crystalline lattice, but rotate in a more or less constrained way. Pentachloronitrobenzene (PCNB) is a quasi-planar hexa-substituted benzene formed by a benzene ring decorated with a -NO2 group and five chlorine atoms that displays below the melting point a layered structure of rhombohedral (R3̄) planes in which the molecules can rotate around a six-fold-like axis. Dielectric spectroscopy [Romanini et al., The Journal of Physical Chemistry C, 2016, 120, 10614] of this highly anisotropic phase revealed a complex relaxation dynamics with two coupled primary α processes, initially ascribed to the in-plane and out-of-plane components of the molecular dipole. In this work, we perform a series of molecular dynamics simulations together with single crystal X-ray synchrotron diffraction experiments to investigate the puzzling dynamics of PCNB. We conclude that the molecule undergoes very fast movements due to the high flexibility of the -NO2 group, and two slower movements in which only the in-plane rotation of the whole ring is involved. These two movements are related to fast attempts to perform a 60° in-plane rotation, and a diffusive motion that involves the rotation of the molecule completely decorrelating the dipole orientation. We have also investigated whether a homogeneous or a heterogeneous scenario is better suited to describe the restricted orientational disorder of this anisotropic phase both from a structural and dynamical point of view.

Cytotoxicity of osmium(II) and cycloosmated half-sandwich complexes from 1-pyrenyl-containing phosphane ligands.

Five metal-arene complexes of formula [MX2(η6-p-cymene)(diR(1-pyrenyl)phosphane)] (M = Os or Ru, X = Cl or I, R = isopropyl or phenyl) and symbolized as MRX2 were synthesized and fully characterized, namely OsiPrCl2, OsiPrI2, OsPhCl2, OsPhI2 and RuPhI2. Furthermore, nine cyclometalated half-sandwich complexes of formula [MX-(η6-p-cymene)(k2C-diR(1-pyrenyl)phosphane)] (M = Os or Ru, X = Cl or I, R = isopropyl or phenyl) or [M(η6-p-cymene)(kS-dmso)(k2C-diR(1-pyrenyl)phosphane)]PF6 (M = Os or Ru, R = isopropyl or phenyl) and symbolized as c-MRX were prepared; hence, c-OsiPrCl, c-OsiPrI, c-OsiPrdmso, c-OsPhCl, c-OsPhI, c-OsPhdmso, c-RuPhCl, c-RuPhI and c-RuPhdmso were obtained and fully characterized. The crystal structures of ten out of the fourteen complexes were solved. All complexes exhibit notable cytotoxic properties against A549 (Lung Adenocarcinoma) human cells, with IC50 values ranging from 48 to 1.42 µM. In addition, complex c-OsiPrdmso shows remarkable toxic behaviours agains other cell lines, namely MCF7 (breast carcinoma), MCF10A (non-tumorigenic epithelial breast) and MDA-MB-435 (melanoma) human cells, as illustrated by IC50 values of 4.36, 4.71 and 2.32 µM, respectively. Finally, it has been found that OsiPrI2 affects the cell cycle of A549 cells, impeding their replication (i.e., the cell cycle is blocked), whereas OsPhI2 (namely with phenyl groups instead of isopropyl ones) does not induce this effect.

Steric hindrance, ligand ejection and associated photocytotoxic properties of ruthenium(II) polypyridyl complexes.

Two ruthenium(II) polypyridyl complexes were prepared with the {Ru(phen)2}2+ moiety and a third sterically non-hindering bidentate ligand, namely 2,2'-dipyridylamine (dpa) and N-benzyl-2,2'-dipyridylamine (Bndpa). Hence, complexes [Ru(phen)2(dpa)](PF6)2 (1) and [Ru(phen)2(Bndpa)](PF6)2 (2) were characterized and their photochemical behaviour in solution (acetonitrile and water) was subsequently investigated. Compounds 1 and 2, which do not exhibit notably distorted octahedral coordination environments, contrarily to the homoleptic "parent" compound [Ru(phen)3](PF6)2, experience two-step photoejection of the dpa and Bndpa ligand upon irradiation (1050-430 nm) for several hours. DNA-binding studies revealed that compounds 1 and 2 affect the biomolecule differently upon irradiation; while 2 solely modifies its electrophoretic mobility, complex 1 is also capable of cleaving it. In vitro cytotoxicity studies with two cancer-cell lines, namely A549 (lung adenocarcinoma) and A375 (melanoma), showed that both 1 and 2 are not toxic in the dark, while only 1 is significantly cytotoxic if irradiated, 2 remaining non-toxic under these conditions. Light irradiation of the complex cation [Ru(phen)2(dpa)]2+ leads to the generation of transient Ru species that is present in the solution medium for several hours, and that is significantly cytotoxic, ultimately producing non-toxic free dpa and [Ru(phen)(OH2)2]2+.

Distributive Nd-to-Yb Energy Transfer within Pure [YbNdYb] Heterometallic Molecules.

Facile access to site-selective hetero-lanthanide molecules will open new avenues in the search of novel photophysical phenomena based on Ln-to-Ln' energy transfer (ET). This challenge demands strategies to segregate efficiently different Ln metal ions among different positions in a molecule. We report here the one-step synthesis and structure of a pure [YbNdYb] (1) coordination complex featuring short Yb···Nd distances, ideal to investigate a potential distributive (i.e., from one donor to two acceptors) intramolecular ET from one Nd3+ ion to two Yb3+ centers within a well-characterized molecule. The difference in ionic radius is the mechanism allowing to allocate selectively both types of metal ion within the molecular structure, exploited with the simultaneous use of two ß-diketone-type ligands. To assist the photophysical investigation of this heterometallic species, the analogues [YbLaYb] (2) and [LuNdLu] (3) have also been prepared. Sensitization of Yb3+ and Nd3+ in the last two complexes, respectively, was observed, with remarkably long decay times, facilitating the determination of the Nd-to-Yb ET within the [YbNdYb] composite. This ET was demonstrated by comparing the emission of iso-absorbant solutions of 1, 2, and 3 and through lifetime determinations in solution and solid state. The comparatively high efficiency of this process corroborates the facilitating effect of having two acceptors for the nonradiative decay of Nd3+ created within the [YbNdYb] molecule.

Unparalleled selectivity and electronic structure of heterometallic [LnLn'Ln] molecules as 3-qubit quantum gates.

Heterometallic lanthanide [LnLn'] coordination complexes that are accessible thermodynamically are very scarce because the metals of this series have very similar chemical behaviour. Trinuclear systems of this category have not been reported. A coordination chemistry scaffold has been shown to produce molecules of type [LnLn'Ln] of high purity, i.e. exhibiting high metal distribution ability, based on their differences in ionic radius. Through a detailed analysis of density functional theory (DFT) based calculations, we discern the energy contributions that lead to the unparalleled chemical selectivity of this molecular system. Some of the previously reported examples are compared here with the newly prepared member of this exotic list, [Er2Pr(LA)2(LB)2(py)(H2O)2](NO3) (1) (H2LA and H2LB are two ß-diketone ligands). A magnetic analysis extracted from magnetization and calorimetry determinations identifies the necessary attributes for it to act as an addressable, conditional multiqubit spin-based quantum gate. Complementary ab initio calculations confirm the feasibility of these complexes as composite quantum gates, since they present well-isolated ground states with highly anisotropic and distinct g-tensors. The electronic structure of 1 has also been analyzed by EPR. Pulsed experiments have allowed the establishment of the quantum coherence of the transitions within the relevant spin states, as well as the feasibility of a coherent control of these states via nutation experiments.

Designed polynuclear lanthanide complexes for quantum information processing.

The design of dissymmetric organic ligands featuring combinations of 1,3-diketone and 2,6-diacetylpyridine coordination pockets has been exploited to produce dinuclear and trinuclear lanthanide-based coordination compounds. These molecules exhibit two or more non-equivalent Ln ions, most remarkably enabling the access to well-defined heterolanthanide compositions. The site-selective disposition of each metal ion within the molecular entities allows the study of each centre individually as a spin-based quantum bit, affording unparalleled versatility for quantum gate design. The inherent weak interaction between the Ln ions permits the performance of multi-qubit quantum logical operations realized through their derived magnetic states, or implementing quantum-error correction protocols. The different studies performed to date on these systems are revised, showing their vast potential within spin-based quantum information processing.

Piano-Stool Ruthenium(II) Complexes with Delayed Cytotoxic Activity: Origin of the Lag Time.

We have recently reported a series of piano-stool ruthenium(II) complexes of the general formula [RuCl2(η6-arene)(P(1-pyrenyl)R2R3)] showing excellent cytotoxic activities (particularly when R2 = R3 = methyl). In the present study, new members of this family of compounds have been prepared with the objective to investigate the effect of the steric hindrance of a bulky phosphane ligand, namely diisopropyl(1-pyrenyl)phosphane (L), on exchange reactions involving the coordinated halides (X = Cl, I). Two η6-arene rings were used, i.e. η6-methyl benzoate (mba) and η6-p-cymene (p-cym), and four complexes were synthesized, namely [RuCl2(mba)(L)] (1Cl2iPr), [RuI2(mba)(L)] (1I2iPr), [RuCl2(p-cym)(L)] (2Cl2iPr), and [RuI2(p-cym)(L)] (2I2iPr). Unexpectedly, all of the complexes exhibited poor cytotoxic activities after 24 h of incubation with cells, in contrast to the related compounds previously reported. However, it was observed that aged DMSO solutions of 2I2iPr (from 2 to 7 days) exhibited better activities in comparison to freshly prepared solutions and that the activity improved over "aging" time. Thorough studies were therefore performed to uncover the origin of this lag time in the cytotoxicity efficiency. The data achieved clearly demonstrated that compounds 2I2iPr and 2Cl2iPr were undergoing a series of transformation reactions in DMSO (with higher rates for the iodido complex 2I2iPr), ultimately generating cyclometalated species through a mechanism involving DMSO as a coordinated proton abstractor. The cyclometalated complexes detected in solution were subsequently prepared; hence, pure [RuCl(p-cym)(κ2C-diisopropyl(1-pyrenyl)phosphane)] (3CliPr), [RuI(p-cym)(κ2C-diisopropyl(1-pyrenyl)phosphane)] (3IiPr), and [Ru(p-cym)(κS-dmso)(κ2C-diisopropyl(1-pyrenyl)phosphane)]PF6 (3dmsoiPr) were synthesized and fully characterized. Remarkably, 3CliPr, 3IiPr, and 3dmsoiPr are all very efficient cytotoxic agents, exhibiting slightly better activities in comparison to the chlorido noncyclometalated complexes [RuCl2(η6-arene)(P(1-pyrenyl)R2R3)] described in an earlier report. For comparison purposes, the iodido compounds [RuI2(mba)(dimethyl(1-pyrenyl)phosphane)] (1I2Me) and [RuI2(p-cym)(dimethyl(1-pyrenyl)phosphane)] (2I2Me), bearing the less hindered dimethyl(1-pyrenyl)phosphane ligand, have also been prepared. The cytotoxic and chemical behaviors of 1I2Me and 1I2Me were comparable to those of their chlorido counterparts reported previously.

Accessing Lanthanide-to-Lanthanide Energy Transfer in a Family of Site-Resolved [LnIII LnIII '] Heterodimetallic Complexes.

The ligand H3 L (6-[3-oxo-3-(2-hydroxyphenyl)propionyl]pyridine-2-carboxylic acid), which exhibits two different coordination pockets, has been exploited to engender and study energy transfer (ET) in two dinuclear [LnIII LnIII '] analogues of interest, [EuYb] and [NdYb]. Their structural and physical properties have been compared with newly synthesised analogues featuring no possible ET ([EuLu], [NdLu], and [GdYb]) and with the corresponding homometallic [EuEu] and [NdNd] analogues, which have been previously reported. Photophysical data suggest that ET between EuIII and YbIII does not occur to a significant extent, whereas emission from YbIII originates from sensitisation of the ligand. In contrast, energy migration seems to be occurring between the two NdIII centres in [NdNd], as well as in [NdYb], in which YbIII luminescence is thus, in part, sensitised by ET from Nd. This study shows the versatility of this molecular platform to further the investigation of lanthanide-to-lanthanide ET phenomena in defined molecular systems.

A heterometallic [LnLn'Ln] lanthanide complex as a qubit with embedded quantum error correction.

We show that a [Er-Ce-Er] molecular trinuclear coordination compound is a promising platform to implement the three-qubit quantum error correction code protecting against pure dephasing, the most important error in magnetic molecules. We characterize it by preparing the [Lu-Ce-Lu] and [Er-La-Er] analogues, which contain only one of the two types of qubit, and by combining magnetometry, low-temperature specific heat and electron paramagnetic resonance measurements on both the elementary constituents and the trimer. Using the resulting parameters, we demonstrate by numerical simulations that the proposed molecular device can efficiently suppress pure dephasing of the spin qubits.

A dissymmetric [Gd2] coordination molecular dimer hosting six addressable spin qubits.

Artificial magnetic molecules can host several spin qubits, which could then implement small-scale algorithms. In order to become of practical use, such molecular spin processors need to increase the available computational space and warrant universal operations. Here, we design, synthesize and fully characterize dissymetric molecular dimers hosting either one or two Gadolinium(III) ions. The strong sensitivity of Gadolinium magnetic anisotropy to its local coordination gives rise to different zero-field splittings at each metal site. As a result, the [LaGd] and [GdLu] complexes provide realizations of distinct spin qudits with eight unequally spaced levels. In the [Gd2] dimer, these properties are combined with a Gd-Gd magnetic interaction, sufficiently strong to lift all level degeneracies, yet sufficiently weak to keep all levels within an experimentally accessible energy window. The spin Hamiltonian of this dimer allows a complete set of operations to act as a 64-dimensional all-electron spin qudit, or, equivalently, as six addressable qubits. Electron paramagnetic resonance experiments show that resonant transitions between different spin states can be coherently controlled, with coherence times TM of the order of 1 µs limited by hyperfine interactions. Coordination complexes with embedded quantum functionalities are promising building blocks for quantum computation and simulation hybrid platforms.

Designed asymmetric coordination helicates with bis-ß-diketonate ligands.

A new bis-(ß-diketone) ligand featuring built-up structural asymmetry yields non-symmetric Fe(iii) and Ga(iii) dinuclear, triple-stranded helicates by design. Their structural properties have been studied, both in solid state and in solution, and compared with their corresponding symmetric analogues. The robustness observed shows the potential of this synthetic strategy to develop non-symmetric helicoidal motifs with specific functional groups.

Controlled Heterometallic Composition in Linear Trinuclear [LnCeLn] Lanthanide Molecular Assemblies.

The combination of two different ß-diketone ligands facilitates the size-controlled assembly of pure heterometallic [LnLn'Ln] linear compounds thanks to two different coordination sites present in the molecular scaffold. [HoCeHo], [ErCeEr], and [YbCeYb] analogues are presented here and are characterized both in the solid state and in solution, demonstrating the selectivity of this unique method to produce heterometallic 4f molecular entities.

Selective Lanthanide Distribution within a Comprehensive Series of Heterometallic [LnPr] Complexes.

The preparation of heterometallic, lanthanide-only complexes is an extremely difficult synthetic challenge. By a ligand-based strategy, a complete isostructural series of dinuclear heterometallic [LnPr] complexes has been synthesized and structurally characterized. The two different coordination sites featured in this molecular entity allow study of the preferences of the praseodymium ion for a specific position depending on the ionic radii of the accompanying lanthanide partner. The purity of each heterometallic moiety has been evaluated in the solid state and in solution by means of crystallographic and spectrometric methods, respectively, revealing the limits of this strategy for ions with similar sizes. DFT calculations have been carried out to support the experimental results, confirming the nature of the site-selective lanthanide distribution. The predictable selectivity of this system has been exploited to assess the magnetic properties of the [DyPr] and [LuPr] derivatives, showing that the origin of the slow dynamics observed in the former arises from the dysprosium ion.

MnIII-FeIII Heterometallic Compounds within Hydrogen-Bonded Supramolecular Networks Promoted by an [Fe(CN)5(CNH)]2- Building Block: Structural and Magnetic Properties.

The reaction of [Fe(CN)6]3- and [Mn(acacen)]+ (H2acacen = N, N'-bis(acetylacetone)ethylenediamine) building units in the presence of supramolecular cations, [(F-Anil)(18-crown-6)]+ (F-Anil+ = 3-fluoroanilinium) or [(Me-F-Anil)(18-crown-6)]+ (Me-F-Anil+ = 3-fluoro-4-methylanilinium), affords two new bimetallic compounds, [(F-Anil)(18-crown-6)][Mn(acacen)Fe(CN)5(CNH)]·MeOH (1) and [(Me-F-Anil)(18-crown-6)][Mn(acacen)(MeOH)Fe(CN)5(CNH)]·MeOH (2), respectively. Compound 1 exhibits a one-dimensional topology, while compound 2 is a dinuclear discrete system due to the coordination of a MeOH molecule at the axial position of the [Mn(acacen)]- unit. For both systems, the acidity of the corresponding supramolecular cation triggers the protonation of the FeIII moiety as [Fe(CN)5(CNH)]2-. Moreover, the resulting -CNH ligand induces hydrogen bonding interactions connecting the chains for 1 or the molecules for 2 into higher dimensional supramolecular networks. Magnetic properties of compounds incorporating these [Fe(CN)5(CNH)]2- building blocks were, for the first time, thoroughly investigated, indicating a three-dimensional antiferromagnetic order of single-chain magnets for 1 and an antiferromagnetically interacting S = 3/2 spin ground state for 2.

Spin State Chemistry: Modulation of Ligand p Ka by Spin State Switching in a [2×2] Iron(II) Grid-Type Complex.

The iron(II) [2×2] grid complex Fe-8H has been synthesized and characterized. It undergoes spin-crossover (SCO) upon deprotonation of the hydrazine-based terpyridine-like ligand. The deprotonation patterns have been determined by X-ray crystallography and 1H NMR spectroscopy and discussed in relation to the spin state of the iron(II) centers, which influences greatly the p Ka of the ligand. The synthesis of the magnetically silent zinc(II) analogue is also reported, and its (de)protonation behavior has been characterized to serve as a reference for the study of the FeII grid complexes. DFT computations have also been performed in order to investigate how the successive deprotonation of the bridging ligands affects the SCO behavior within the grid.

Zn2+ Ion Surface Enrichment in Doped Iron Oxide Nanoparticles Leads to Charge Carrier Density Enhancement.

Here, we report the development of monodisperse Zn-doped iron oxide nanoparticles (NPs) with different amounts of Zn (Zn x Fe3-x O4, 0 < x < 0.43) by thermal decomposition of a mixture of zinc and iron oleates. The as-synthesized NPs show a considerable fraction of wüstite (FeO) which is transformed to spinel upon 2 h oxidation of the NP reaction solutions. At any Zn doping amounts, we observed the enrichment of the NP surface with Zn2+ ions, which is enhanced at higher Zn loadings. Such a distribution of Zn2+ ions is attributed to the different thermal decomposition profiles of Zn and Fe oleates, with Fe oleate decomposing at much lower temperature than that of Zn oleate. The decomposition of Zn oleate is, in turn, catalyzed by a forming iron oxide phase. The magnetic properties were found to be strongly dependent on the Zn doping amounts, showing the saturation magnetization to decrease by 9 and 20% for x = 0.05 and 0.1, respectively. On the other hand, X-ray photoelectron spectroscopy near the Fermi level demonstrates that the Zn0.05Fe2.95O4 sample displays a more metallic character (a higher charge carrier density) than undoped iron oxide NPs, supporting its use as a spintronic material.

Direct crystallographic evidence of the reversible photo-formation and thermo-rupture of a coordination bond inducing spin-crossover phenomenon.

A detailed crystallographic study of [FeII(LN5)(CN)2]·MeOH (LN5 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene) shows the light-induced formation and thermally-assisted rupture of a metal-ligand coordination bond concomitantly with a spin-crossover process occurring at the Fe(ii) center between its low spin (LS) and high spin (HS) states. These results illustrate the remarkable reversibility and non-destructive character of this photo-induced magneto-structural process in a single crystal.

Thermodynamic Stability of Heterodimetallic [LnLn'] Complexes: Synthesis and DFT Studies.

The solid-state and solution configurations of the heterodimetallic complexes (Hpy)[LaEr(HL)3 (NO3 )(py)(H2 O)] (1), (Hpy)[CeEr(HL)3 (NO3 )(py)(H2 O)] (2), (Hpy)[CeGd(HL)3 (NO3 )(py)(H2 O)] (3), (Hpy)[PrSm(HL)3 (NO3 )(py)(H2 O)] (4), and (Hpy)2 [LaYb(HL)3 (NO3 )(H2 O)](NO3 ) (5), in which H3 L is 6-(3-oxo-3-(2-hydroxyphenyl)propionyl)pyridine-2-carboxylic acid and py is pyridine, were analyzed experimentally and by using DFT calculations. Complexes 3, 4, and 5 are described here for the first time, and were analyzed by using single-crystal X-ray diffraction and mass spectrometry. The theoretical study was also extended to the [LaCe] and [LaLu] analogues. The results are consistent with a remarkable selectivity of the metal distribution within the molecule in the solid state, enhanced by the size difference between the different ions. This selectivity was reduced in solution, particularly for ions with the most similar radii. This unique entry into 4f-4f'' heterometallic chemistry establishes for the first time the difference between the selectivity in solution and that in the solid state, as a result of changes to the coordination that follow the dissociation of terminal ligands upon dissolution of the complexes.

Multifaceted magnetization dynamics in the mononuclear complex [ReIVCl4(CN)2]2.

The mononuclear complex (Bu4N)2[ReIVCl4(CN)2]·2DMA (DMA = N,N-dimethylacetamide) displays intricate magnetization dynamics, implying Orbach, direct, and Raman-type relaxation processes. The Orbach relaxation process is characterized by an energy barrier of 39 K (27 cm-1) that is discussed based on high-field electron paramagnetic resonance (EPR), inelastic neutron scattering and frequency-domain THz EPR investigations.