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.

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.

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.

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.

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.

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.

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.

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.

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.

Switchable Fe/Co Prussian blue networks and molecular analogues.

With the long term objective to build the next generation of devices from the molecular scale, scientists have explored extensively in the past two decades the Prussian blue derivatives and their remarkable physico-chemical properties. In particular, the exquisite Fe/Co system displays tuneable optical and magnetic behaviours associated with thermally and photo-induced metal-to-metal electron transfer processes. Recently, numerous research groups have been involved in the transfer of these electronic properties to new Fe/Co coordination networks of lower dimensionality as well as soluble molecular analogues in order to facilitate their manipulation and integration into devices. In this review, the most representative examples of tridimensional Fe/Co Prussian blue compounds are described, focusing on the techniques used to understand their photomagnetic properties. Subsequently, the different strategies employed toward the design of new low dimensional Prussian blue analogues based on a rational molecular building block approach are discussed emphasizing the advantages of these functional molecular systems.

Heterodimetallic [LnLn'] lanthanide complexes: toward a chemical design of two-qubit molecular spin quantum gates.

A major challenge for realizing quantum computation is finding suitable systems to embody quantum bits (qubits) and quantum gates (qugates) in a robust and scalable architecture. An emerging bottom-up approach uses the electronic spins of lanthanides. Universal qugates may then be engineered by arranging in a molecule two interacting and different lanthanide ions. Preparing heterometallic lanthanide species is, however, extremely challenging. We have discovered a method to obtain [LnLn'] complexes with the appropriate requirements. Compound [CeEr] is deemed to represent an ideal situation. Both ions have a doubly degenerate magnetic ground state and can be addressed individually. Their isotopes have mainly zero nuclear spin, which enhances the electronic spin coherence. The analogues [Ce2], [Er2], [CeY], and [LaEr] have also been prepared to assist in showing that [CeEr] meets the qugate requirements, as revealed through magnetic susceptibility, specific heat, and EPR. Molecules could now be used for quantum information processing.

Linear or cyclic clusters of Cu(II) with a hierarchical relationship.

The polydentate ligand 2,6-bis(5-(2-hydroxyphenyl)-pyrazol-3-yl)-pyridine, H4L, exhibits a series of coordination pockets favoring the establishment of metal sequences with predetermined motifs, together with a degree of flexibility for the formation of clusters with various overall topologies. With Cu(II) under strong basic conditions it has a marked tendency to stabilize a cyclic [Cu16L8] cluster. The sequential formation of this compound via [Cu7L8](2-) intermediates, recognized in its structure, is suggested by crystallographic evidence, which shows the persistent formation of the complex salt (NBu4)2[Cu7L8] in the presence of the organic cation. Also, the crystallographic identification of the related cluster [Cu11L5(OH)2(py)12] from similar reaction conditions underscores the rich multiplicity of species attainable from this simple reaction system.

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.

Lanthanide contraction within a series of asymmetric dinuclear [Ln2] complexes.

A complete isostructural series of dinuclear asymmetric lanthanide complexes has been synthesized by using the ligand 6-[3-oxo-3-(2-hydroxyphenyl)propionyl]pyridine-2-carboxylic acid (H3L). All complexes have the formula [Ln2(HL)2(H2L)(NO3)(py)(H2O)] (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14), Y (15); py = pyridine). Complexes of La to Yb and Y have been crystallographically characterized to reveal that the two metal ions are encapsulated within two distinct coordination environments of differing size. Whereas one site maintains the coordination number (nine) through the whole series, the other one increases from nine to ten owing to a change in the coordination mode of an NO3(-) ligand. This series offers a unique opportunity to study in detail the lanthanide contraction within complexes of more than one metal. This analysis shows that various representative parameters proportional to this contraction follow a quadratic decay as a function of the number n of f electrons. Slater's model for the atomic radii has been used to extract, from these decays, the shielding constant of 4f electrons. The average of O⋅⋅⋅O distances within the coordination polyhedra shared by both metals and of the Ln⋅⋅⋅Ln separations follow also a quadratic decay, therefore showing that such dependence holds also for parameters that receive the contribution of two lanthanide ions simultaneously. The magnetic behavior has been studied for all nondiamagnetic complexes. It reveals the effect of the spin-orbit coupling and a weak antiferromagnetic interaction between both metals. Photoluminescent studies of all the complexes in the series reveal a single broad emission band in the visible region, which is related to the coordinated ligand. On the other hand, the Nd, Er, and Yb complexes show features in the near-IR region due to metal-based transitions.

Design of magnetic coordination complexes for quantum computing.

A very exciting prospect in coordination chemistry is to manipulate spins within magnetic complexes for the realization of quantum logic operations. An introduction to the requirements for a paramagnetic molecule to act as a 2-qubit quantum gate is provided in this tutorial review. We propose synthetic methods aimed at accessing such type of functional molecules, based on ligand design and inorganic synthesis. Two strategies are presented: (i) the first consists in targeting molecules containing a pair of well-defined and weakly coupled paramagnetic metal aggregates, each acting as a carrier of one potential qubit, (ii) the second is the design of dinuclear complexes of anisotropic metal ions, exhibiting dissimilar environments and feeble magnetic coupling. The first systems obtained from this synthetic program are presented here and their properties are discussed.

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.

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.

Synthesis and properties of a family of unsymmetric dinuclear complexes of Ln(III) (Ln = Eu, Gd, Tb).

A new ligand has been synthesized with the aim of favoring distinct coordination environments within lanthanide polynuclear complexes. It has led to the formation of three unsymmetrical [Ln(III)(2)] (Ln = Gd, Tb, Eu) complexes, exhibiting weak antiferromagnetic coupling and, for Eu and Tb, high single-ion magnetic anisotropy. All of these attributes are necessary for these clusters to behave as possible 2qubit quantum gates.