Elisabeth M. Fatila

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.

Anions Stabilize Each Other inside Macrocyclic Hosts

Contrary to the simple expectations from Coulombs 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 dimers OH⋅⋅⋅O hydrogen bonding is seen in a 1 H NMR peak at 13.75 ppm, which is consistent with borderline-strong hydrogen bonds.

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.

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.

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].

Ruthenium-Catalyzed Nucleophilic Ring-Opening Reactions of 7-Oxabenzonorbornadienes with Methanol

Abstract GRAPHICAL ABSTRACT

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

Significance This article describes a molecular realization of the classical three-body problem, where the motion of three or more bodies is directed by a set of pairwise forces. Surprisingly, motion of the components of the three-body molecular systems is found to be highly choreographed by differences in strength of intercomponent interactions, promoting a rare inchworm-like loading of molecular rings onto a molecular thread. Our work demonstrates the utility of an integrative approach to design and develop functional molecular machines. 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 cyanostars 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.

(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 β-diketonate with a bridging dme ligand.