Mechanochemical Syntheses of Ln(hfac)3(H2O)x (Ln = La-Sm, Tb): Isolation of 10-, 9-, and 8-Coordinate Ln(hfac)n Complexes.

Volatile lanthanide coordination complexes are critical to the generation of new optical and magnetic materials. One of the most common precursors for preparing volatile lanthanide complexes is the hydrate with the general formula Ln(hfac)3(H2O)x (x = 3 for La-Nd, x = 2 for Sm) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato). We have investigated the synthesis of Ln(hfac)3(H2O)x using more environmentally sustainable mechanochemical approaches. Characterization of the products using Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, elemental analysis, and powder X-ray diffraction shows substantial differences in product distribution between methods. The mechanochemical synthesis of the hydrate complexes leads to a variety of coordination compounds including the expected hydrate product, the known retro-Claisen impurity, and hydrated protonated Hhfac ligand depending on the technique employed. Surprisingly, 10-coordinate complexes of the form Na2Ln(hfac)5·3H2O for Ln = La-Nd were also isolated from reactions using a mortar and pestle. The electrostatic bonding of lanthanide coordination complexes is a challenge for obtaining reproducible reactions and clean products. The reproducibility issues are most acute for the large, early lanthanides whereas for the mid to late lanthanides, reproducibility in terms of product distribution and yield is less of an issue because of their smaller size and greater charge to radius ratio. Ball milling increases reproducibility in terms of generating the desired Ln(hfac)3(H2O)x along with hydrated Hhfac (tetraol) and free Hhfac products. The results illustrate the dynamic behavior of lanthanide complexes in solution and the solid state as well as the structural diversity available to the early lanthanides.

Anion-Selective Electrodes Based On a CH-Hydrogen Bonding Bis-macrocyclic Ionophore with a Clamshell Architecture.

CH-hydrogen bonding provides access to new building blocks for making macrocyclic ionophores with high degrees of preorganization and selective anion recognition. In this study, an anion-binding ionophore in the shape of a clamshell (ClS) was employed that is composed of two cyanostar (CNstar) macrocycles with preorganized cavities linked with a 12-carbon chain. This ionophore allows for anion complexation by CH-hydrogen bonding. The potentiometric performance of membrane-based ion-selective electrodes incorporating this ionophore was evaluated. Different membrane compositions were prepared to determine the optimum concentrations of the ionophore and lipophilic additive in the membrane. The optimized electrode had a slope of -58.2 mV/decade and demonstrated an anti-Hofmeister selectivity pattern toward iodide with a nanomolar detection limit. Electrospray ionization mass spectrometry was employed to study the relative association strengths of ClS with various anions. The observed mass peaks of the ion-ionophore complexes were found to be consistent with the potentiometric selectivity pattern of the corresponding electrodes. Overall, the selectivity of the electrode could be altered by using an ionophore in which the two CNstar macrocycles are linked together with a flexible 12-carbon chain to control the molecularity of the binding event.

Inchworm movement of two rings switching onto a thread by biased Brownian diffusion represent a three-body problem.

The coordinated motion of many individual components underpins the operation of all machines. However, despite generations of experience in engineering, understanding the motion of three or more coupled components remains a challenge, known since the time of Newton as the "three-body problem." Here, we describe, quantify, and simulate a molecular three-body problem of threading two molecular rings onto a linear molecular thread. Specifically, we use voltage-triggered reduction of a tetrazine-based thread to capture two cyanostar macrocycles and form a [3]pseudorotaxane product. As a consequence of the noncovalent coupling between the cyanostar rings, we find the threading occurs by an unexpected and rare inchworm-like motion where one ring follows the other. The mechanism was derived from controls, analysis of cyclic voltammetry (CV) traces, and Brownian dynamics simulations. CVs from two noncovalently interacting rings match that of two covalently linked rings designed to thread via the inchworm pathway, and they deviate considerably from the CV of a macrocycle designed to thread via a stepwise pathway. Time-dependent electrochemistry provides estimates of rate constants for threading. Experimentally derived parameters (energy wells, barriers, diffusion coefficients) helped determine likely pathways of motion with rate-kinetics and Brownian dynamics simulations. Simulations verified intercomponent coupling could be separated into ring-thread interactions for kinetics, and ring-ring interactions for thermodynamics to reduce the three-body problem to a two-body one. Our findings provide a basis for high-throughput design of molecular machinery with multiple components undergoing coupled motion.

Cyanostar: C-H Hydrogen Bonding Neutral Carrier Scaffold for Anion-Selective Sensors.

Cyanostar, a pentagonal macrocyclic compound with an electropositive cavity, binds anions with CH-based hydrogen bonding. The large size of the cyanostar's cavity along with its planarity favor formation of 2:1 sandwich complexes with larger anions, like perchlorate, ClO4-, relative to the smaller chloride. We also show that cyanostar is selective for ClO4- over the bulky salicylate anions by using NMR titration studies to measure affinity. The performance of this novel macrocycle as an anion ionophore in membrane ion sensors was evaluated. The cyanostar-based electrodes demonstrated a Nernstian response toward perchlorate with selectivity patterns distinctly different from the normal Hofmeister series. Different membrane compositions were explored to identify the optimum concentrations of the ionophore, plasticizer, and lipophilic additive that give rise to the best perchlorate selectivity. Changing the concentration of the lipophilic additive tridodecylmethylammonium chloride was found to impact the selectivity pattern and the analytical dynamic range of the electrodes. The high selectivity of the cyanostar sensors and their detection limit could enable the determination of ClO4- in contaminated environmental samples. This novel class of macrocycle provides a suitable scaffold for designing various anion-selective ionophores by altering the size of the central cavity and its functionalization.

Host-Host Interactions Control Self-assembly and Switching of Triple and Double Decker Stacks of Tricarbazole Macrocycles Co-assembled with anti-Electrostatic Bisulfate Dimers.

Hierarchical assembly provides a route to complex architectures when using building blocks with strong and structurally well-defined recognition elements. These rules are traditionally expressed using cationic templates with reliable metal-ligand bonding but use of anions is rare on account of weak anion-host contacts. We investigate an approach that relies on host-host interactions to fortify assemblies formed between bisulfate anion dimers, [HSO4⋅⋅⋅HSO4]2- , and shape-persistent macrocycles called tricarbazole triazolophanes. These macrocycles have significant self-association. In chloroform, they form high fidelity, triple-decker stacks with bisulfate dimers. The strength of host-host interactions allows for preferential formation of the 3:2 tricarb:bisulfate architecture over an ion-paired architecture seen with analogous macrocycles with much weaker self-association. Solvent was expected and found to tune host-host contacts enabling formation of a 2:2 complex and solvent-driven switching between triple- and double-stacked structures. Crystallography of the 2:2:2 complex supports the idea that significant host-host interactions with tricarb arises from dipole-stabilized π-stacking. Computational studies were also conducted further highlighting the importance of host-host interactions in stacked complexes of tricarb. These findings unambiguously verify the importance of host-host interactions in the assembly and stability of discrete, responsive anion-templated architectures.

Ion Pairing and Co-facial Stacking Drive High-Fidelity Bisulfate Assembly with Cyanostar Macrocyclic Hosts.

Hydroxyanions pair up inside CH H-bonding cyanostar macrocycles against Coulombic repulsions and solvation forces acting to separate them. The driving forces responsible for assembly of bisulfate (HSO4- ) dimers are unclear. We investigated them using solvent quality to tune the contributing forces and we take advantage of characteristic NMR signatures to follow the species distributions. We show that apolar solvents enhance ion pairing to stabilize formation of a 2:2:2 complex composed of π-stacked cyanostars encapsulating the [HSO4 ⋅⋅⋅HSO4 ]2- dimer and endcapped by tetrabutylammonium cations. Without cations engaged, a third macrocycle can be recruited with the aid of solvophobic forces in more polar solvents. The third macrocycle generates a more potent electropositive pocket in which to stabilize the anti-electrostatic anion dimer as a 3:2 assembly. We also see unprecedented evidence for a water molecule bound to the complex in the acetonitrile solution. In methanol, OH H-bonding leads to formation of 2:1 complexes by bisulfate solvation inside the macrocycles inhibiting anion dimers. Knowledge of the driving forces for stabilization (strong OH⋅⋅⋅O H-bonding, CH H-bonding, ion pairs, π-stacking) competing with destabilization (Coulomb repulsion, solvation) allows high-fidelity selection of the assemblies. Thermodynamic stabilization of hydroxyanion dimers also demonstrates the ability to use macrocycles to control ion speciation and stoichiometry of the overall assemblies.

Extreme Stabilization and Redox Switching of Organic Anions and Radical Anions by Large-Cavity, CH Hydrogen-Bonding Cyanostar Macrocycles.

Encapsulation of unstable guests is a powerful way to enhance their stability. The lifetimes of organic anions and their radicals produced by reduction are typically short on account of reactivity with oxygen while their larger sizes preclude use of traditional anion receptors. Here we demonstrate the encapsulation and noncovalent stabilization of organic radical anions by C-H hydrogen bonding in π-stacked pairs of cyanostar macrocycles having large cavities. Using electrogenerated tetrazine radical anions, we observe significant extension of their lifetimes, facile molecular switching, and extremely large stabilization energies. The guests form threaded pseudorotaxanes. Complexation extends the radical lifetimes from 2 h to over 20 days without altering its electronic structure. Electrochemical studies show tetrazines thread inside a pair of cyanostar macrocycles following voltage-driven reduction (+e-) of the tetrazine at -1.00 V and that the complex disassembles after reoxidation (-e-) at -0.05 V. This reoxidation is shifted 830 mV relative to the free tetrazine radical indicating it is stabilized by an unexpectedly large -80 kJ mol-1. The stabilization is general as shown using a dithiadiazolyl anion. This finding opens up a new approach to capturing and studying unstable anions and a radical anions when encapsulated by size-complementary anion receptors.

Anions Stabilize Each Other inside Macrocyclic Hosts.

Contrary to the simple expectations from Coulomb's law, Weinhold proposed that anions can stabilize each other as metastable dimers, yet experimental evidence for these species and their mutual stabilization is missing. We show that two bisulfate anions can form such dimers, which stabilize each other with self-complementary hydrogen bonds, by encapsulation inside a pair of cyanostar macrocycles. The resulting 2:2 complex of the bisulfate homodimer persists across all states of matter, including in solution. The bisulfate dimer's OH⋅⋅⋅O hydrogen bonding is seen in a 1 H NMR peak at 13.75 ppm, which is consistent with borderline-strong hydrogen bonds.

Fine-tuning the single-molecule magnet properties of a [Dy(III)-radical]2 pair.

A supramolecular species composed of a pair of nonequivalent Dy(III)-radical complexes exhibits single-molecule magnet (SMM) properties. The weak effective antiferromagnetic coupling between the Dy(III) ions can be compensated by application of a small (700 Oe) dc field, revealing the relaxation mode of the two distinct SMMs. These unique results illustrate how the dynamics of a supramolecular [Dy-Radical]2 SMM can be fine-tuned by the exchange-bias and an applied magnetic field.

High-spin ribbons and antiferromagnetic ordering of a Mn(II)-biradical-Mn(II) complex.

A binuclear metal coordination complex of the first thiazyl-based biradical ligand 1 is reported (1 = 4,6-bis(1,2,3,5-dithiadiazolyl)pyrimidine; hfac =1,1,1,5,5,5,-hexafluoroacetylacetonato-). The Mn(hfac)2-biradical-Mn(hfac)2 complex 2 is a rare example of a discrete, molecular species employing a neutral bridging biradical ligand. It is soluble in common organic solvents and can be easily sublimed as a crystalline solid. Complex 2 has a spin ground state of S(T) = 4 resulting from antiferromagnetic coupling between the S(birad) = 1 biradical bridging ligand and two S(Mn) = 5/2 Mn(II) ions. Electrostatic contacts between atoms with large spin density promote a ferromagnetic arrangement of the moments of neighboring complexes in ribbon-like arrays. Weak antiferromagnetic coupling between these high-spin ribbons stabilizes an ordered antiferromagnetic ground state below 4.5 K. This is an unusual example of magnetic ordering in a molecular metal-radical complex, wherein the electrostatic contacts that direct the crystal packing are also responsible for providing an efficient exchange coupling pathway between molecules.

(µ-1,2-dimethoxyethane-κ2O:O')bis[(1,2-dimethoxyethane-κ2O,O')tris(1,1,1,5,5,5-hexafluoro-4-oxopent-2-en-2-olato-κ2O,O')cerium(III)].

A previous analysis [Fatila et al. (2012). Dalton Trans. 41, 1352-1362] of the title complex, [Ce(2)(C(5)HF(6)O(2))(6)(C(4)H(10)O(2))(3)], had identified it as Ce(hfac)(3)(dme)(1.5) according to the (1)H NMR integration [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate (1,1,1,5,5,5-hexafluoro-4-oxopent-2-en-2-olate) and dme = 1,2-dimethoxyethane]; however, it was not possible to determine the coordination environment unambiguously. The structural data presented here reveal that the complex is a binuclear species located on a crystallographic inversion center. Each Ce(III) ion is coordinated to three hfac ligands, one bidentate dme ligand and one monodentate (bridging) dme ligand, thus giving a coordination number of nine (CN = 9) to each Ce(III) ion. The atoms of the bridging dme ligand are unequally disordered over two sets of sites. In addition, in two of the -CF(3) groups, the F atoms are rotationally disordered over two sets of sites. This is the first crystal structure of a binuclear lanthanide ß-diketonate with a bridging dme ligand.

A third polymorph of 4-(2,6-difluorophenyl)-1,2,3,5-dithiadiazolyl.

The crystal structure of a third polymorphic form of the known 4-(2,6-difluorophenyl)-1,2,3,5-dithiadiazolyl radical, C(7)H(3)F(2)N(2)S(2), is reported. This new polymorph represents a unique crystal-packing motif never before observed for 1,2,3,5-dithiadiazolyl (DTDA) radicals. In the two known polymorphic forms of the title compound, all of the molecules form cis-cofacial dimers, such that two molecules are pi-stacked with like atoms one on top of the other, a common arrangement for DTDA species. By contrast, the third polymorph, reported herein, contains two crystallographically unique molecules organized such that only 50% are dimerized, while the other 50% remain monomeric radicals. The dimerized molecules are arranged in the trans-antarafacial mode. This less common dimer motif for DTDA species is characterized by pi-pi interactions between the S atoms [S...S = 3.208 (1) A at 110 K], such that the two molecules of the dimer are related by a centre of inversion. The most remarkable aspect of this third polymorph is that the DTDA dimers are co-packed with monomers. The monomeric radicals are arranged in one-dimensional chains directed by close lateral intermolecular contacts between the two S atoms of one DTDA heterocycle and an N atom of a neighbouring coplanar DTDA heterocycle [S...N = 2.857 (2) and 3.147 (2) A at 110 K].

Ni(II) and hs-Fe(II) complexes of a paramagnetic thiazyl ligand, and decomposition products of the iron complex, including an Fe(III) tetramer.

Synthesis and structural, magnetic and electrochemical characterization of the Ni(hfac) 2(pyDTDA) and the Fe(hfac) 2(pyDTDA) complexes are reported (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-; pyDTDA = 4-(2'-pyridyl)-1,2,3,5-dithiadiazolyl). Unlike the previously reported Mn(II) and Cu(II) complexes, but similar to the Co(II) complex, the Ni(II) and Fe(II) complexes are not dimerized in the solid state, allowing for magnetic coupling between the metal ion and paramagnetic ligand to be readily obtained from solid state magnetic measurements: Ni complex, J/k B = +132(1) K, using H = -2 J{ S Ni. S Rad} and g Ni = 2.04(2) and g Rad = 1.99(2); Fe complex, J/k B = -60.3(3) K, using H = -2 J{ S Fe. S Rad} and g av = 2.11(2). The iron complex is unusually unstable. A thermal decomposition product is isolated wherein the coordinated pyDTDA ligand appears to have been transformed into a coordinated 2-(2'-pyridyl)-4,6-bis(trifluoromethyl)pyrimidine. The iron complex also yields a solution decomposition product in the presence of air that is best described as an oxygen bridged iron(III) tetramer with two hfac ligands on each of three iron atoms and two oxidized pyDTDA ligands chelated on the fourth.

Phosphate-phosphate oligomerization drives higher order co-assemblies with stacks of cyanostar macrocycles.

The importance of phosphate in biology and chemistry has long motivated investigation of its recognition. Despite this interest, phosphate's facile oligomerization is only now being examined following the discovery of complexes of anion-anion dimers of hydroxyanions. Here we address how oligomerization dictates phosphate's recognition properties when engaged with planar cyanostar macrocycles that can also oligomerize by stacking. The crystal structure of cyanostar with phosphate shows an unprecedented tetrameric stack of cyanostar macrocycles threaded by a phosphate trimer, [H2PO4···H2PO4···H2PO4]3-. The solution behaviour, studied as a function of solvent quality, highlights how dimers and trimers of phosphate drive formation of higher order stacks of cyanostar into dimer, trimer and tetramer co-assemblies. Solution behaviors differ significantly from simpler complexes of bisulfate hydroxyanion dimers. Phosphate oligomerization is: (1) preferred over ion pairing with tetrabutylammonium cations, (2) inhibits disassembly of the complexes upon dilution, and (3) resists interference from competitive anion solvation. The phosphate oligomers also appear critical for stability; complexation of just one phosphate with cyanostars is unfavored. The cyanostar's ability to self-assemble is found to create a tubular, highly electropositive cavity that complements the size and shape of the phosphate oligomers as well as their higher charge. When given the opportunity, phosphate will cooperate with the receptor to form co-assembled architectures.

Stoichiometric control: 8- and 10-coordinate Ln(hfac)3(bpy) and Ln(hfac)3(bpy)2 complexes of the early lanthanides La-Sm.

The coordination sphere of early lanthanide(iii) ions is highly versatile, exhibiting the ability to form 8-, 9-, and 10-coordinate complexes with the same ligand set. The ability to isolate 10-coordinate complexes decreases across the period, and the late lanthanides typically cannot support a coordination number higher than eight. Using two common, commercially available ligands, hfac (1,1,1,5,5,5-hexafluoroacetylacetonato-) and bpy (2,2'-bipyridine), the 8- and 10-coordinate series Ln(hfac)3(bpy) and Ln(hfac)3(bpy)2 (Ln = La-Sm) are compiled in a single investigation, demonstrating that the desired coordination number can be targeted through stoichiometry. Solvent-free syntheses of Ln(hfac)3(bpy) and Ln(hfac)3(bpy)2 complexes from Ln(hfac)3(H2O)3 precursors are investigated using a mechanochemical approach. Structural and spectroscopic properties as well as melting point trends are reported for the series.

Ferromagnetic ordering of -[Sm(iii)-radical]n- coordination polymers.

[Sm(hfac)3(boaDTDA)]n is the first coordination compound of a thiazyl-based neutral radical ligand to exhibit ferromagnetic ordering; TC = 3 K. The [Sm(iii)-radical]n species is soluble in common organic solvents and can be sublimed quantitatively. A McConnell I mechanism is implicated in local exchange pathways that contribute to cooperative magnetic properties.

McConnell I mechanism promotes ferromagnetic interactions between π-stacked Ni(II)-thiazyl complexes.

The coordination complex of Ni(hfac)2 (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato) and the 4-(benzoxazol-2'-yl)-1,2,3,5-dithiadiazolyl (boaDTDA) neutral radical π-stacks in a one-dimensional "staircase" arrangement. This particular packing aligns regions of α and ß spin densities on neighbouring Ni(II)(hfac)2(boaDTDA) molecules. This complex exemplifies a McConnell I type mechanism, giving rise to intermolecular ferromagnetic exchange observed for the first time between metal-thiazyl complexes.

Ferromagnetic superexchange in a 1D -[La(III)-radical]- coordination polymer.

The first coordination polymer of a bridging radical 1,2,3,5-dithiadiazolyl ligand is reported. Upon coordination with the La(hfac)3 fragment, the paramagnetic 4-(benzoxazol-2'-yl)-1,2,3,5-dithiadiazolyl (boaDTDA) ligand forms a one-dimensional (1D) alternating -[La(hfac)3-boaDTDA]n- polymer exhibiting ferromagnetic (FM) coupling between the radicals, mediated through the diamagnetic La(III) ion (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato).

Syntheses and crystal structures of anhydrous Ln(hfac)3(monoglyme). Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Er, Tm.

The crystal structures of a broad series of anhydrous Ln(hfac)(3)(monoglyme) complexes, prepared in moderate to high yield, are presented: hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-; Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Er, Tm. This study contradicts the general assumption that monoglyme is too small a polyether to act as a partitioning agent displacing coordinated water on the larger lanthanide(III) ions. The structures of an intermediate La(hfac)(3)(monoglyme)(2) species and the hydrated Ce(hfac)(3)(monoglyme)(H(2)O) species are also included. The crystallographic evidence presented herein is supplemented by other characterization techniques (melting point, IR, etc.) and trends are delineated.

High-spin supramolecular pair of Mn(II)/thiazyl radical complexes.

The Mn(hfac)(2) complex of the paramagnetic 4-(benzoxazol-2'-yl)-1,2,3,5-dithiadiazolyl ligand is reported (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-). The Mn(ii) and radical ligand spins are coupled antiferromagnetically (AF) in the coordination complex. Short sulfur-oxygen contacts between molecules provide an efficient pathway for AF coupling between the radical ligand of one molecule and the Mn(ii) of a neighbouring molecule, resulting in a large total spin ground state (S(T) = 4) for a pair of molecules.