The kinetics and mechanism of interconversion within a system of [Fe2L3]4+ helicates and [Fe4L6]8+ cages.

Nature builds simple molecules into highly complex assemblies, which are involved in all fundamental processes of life. Some of the most intriguing biological assemblies are those that can be precisely reconfigured to achieve different functions using the same building blocks. Understanding the reconfiguration of synthetic self-assembled systems will allow us to better understand the complexity of proteins and design useful artificial chemical systems. Here we have prepared a relatively simple system in which two distinct self-assembled structures, a [Fe2L3]4+ helicate and a [Fe4L6]8+ cage that are formed from the same precursors, coexist at equilibrium. We have measured the rates of interconversion of these two species and propose a mechanism for the transformation.

Stepwise reduction of interlocked viologen-based complexes in the gas phase.

We present the first application of electrochemical reduction in an ion trap mass spectrometer as a dual-function tool to synthesise and probe the reactivity of interlocked viologen-based complexes. Compared with non-complexed archetypes, electron-donating macrocyclic porphyrin ethers retard electron transfer reaction rates and stabilise intact structures in low oxidation states.

Photophysical Activity and Host-Guest Behavior of Ruthenium Polypyridyl Catalysts Encapsulated in Cucurbit[10]uril.

This work outlines a strategy to combine the use of visible light and confined spaces to form a supramolecular photocatalyst system. Polypyridyl ruthenium(II) complexes [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine), [Ru(bpy)2(bpm)]2+ (bpm = 2,2'-bipyrimidine), and [Ru(bpy)2(bpz)]2+ (bpz = 2,2'-bipyrazine) are encapsulated in cucurbit[10]uril to form host-guest systems in aqueous solution. The photophysical properties of the complexes are altered by encapsulation, with improved emissive behavior for the heteroleptic complexes. Oxidative quenching of the photocatalyst's excited state via intermolecular charge transfer to methyl viologen can occur within the internal cavity, which acts to preorganize the reagents. The host-guest system containing [Ru(bpy)3]2+ can bind suitable substrates, and essential criteria for its use as a supramolecular photocatalyst are investigated.

The pKa values of N-aryl imidazolinium salts, their higher homologues, and formamidinium salts in dimethyl sulfoxide.

A series of imidazolinium salts, their six-, seven- and eight-membered homologues, and the related formamidinium salts were prepared, and their pKa values were determined in DMSO at 25 °C using the bracketing indicator method. The effect of each type of structural variation on the acidity of each salt was considered, particularly noting the importance of ring size and the effect of the steric and electronic nature of the N-aryl substituents. The effect of a cyclic structure was also probed through comparing the cyclic systems with the corresponding formamidinium salts, noting the importance of conformational flexibility in the latter cases. Along with allowing choice of appropriate bases for deprotonation of these species, it is anticipated that the data presented will aid in the understanding of the nucleophilicity, and potentially catalytic efficacy, of the corresponding carbenes.

Diastereoselective Control of Tetraphenylethene Reactivity by Metal Template Self-Assembly.

The reaction of 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetraaniline with 2-pyridinecarboxaldehyde and iron(II) chloride resulted, after aqueous workup, in the diastereoselective formation of an [Fe2 L3 ]4+ triple-stranded helicate structure, irrespective of the stoichiometry employed. The helicate structure was characterized in solution by multinuclear NMR spectroscopy, and in the solid state by single-crystal X-ray crystallography. The reaction of iron(II) tetrafluoroborate or iron(II) bistriflimide with the tetraaniline and 2-pyridinecarboxaldehyde allowed the formation of an [Fe8 L6 ]16+ cube when the appropriate stoichiometry was used, but these structures were unstable with respect to hydrolysis. The pendant amine groups on the helicate can be functionalized by reaction with acid chlorides or anhydrides, and the resulting functionalized tetraphenylethene (TPE) units were isolated by the reaction of the helicate with tris(2-aminoethyl)amine. The emission properties of the TPE units were studied in THF/water mixtures, and they were found by dynamic light scattering to self-assemble into large (av. diameter 250 nm) structures.

Predictable Substituent Control of CoIII/II Redox Potential and Spin Crossover in Bis(dipyridylpyrrolide)cobalt Complexes.

A family of five easily prepared tridentate monoanionic 2,5-dipyridyl-3-(R1)-4-(R2)-pyrrolide anions (dppR1,R2)-, varying in the nature of the R1 and R2 substituents [R1, R2 = CN, Ph; CO2Et, CO2Et; CO2Me, 4-Py; CO2Me, Me; Me, Me], has been used to generate the analogous family of neutral [CoII(dppR1,R2)2] complexes, two of which are structurally characterized at both 100 and 298 K. Both the oxidation and spin states of these complexes can be switched in response to appropriate external stimuli. All complexes, except [CoII(dppMe,Me)2], exhibit gradual spin crossover (SCO) in the solid state, and SCO activity is observed for three complexes in CDCl3 solution. The cobalt(II) centers in the low spin (LS) complexes are Jahn-Teller tetragonally compressed along the pyrrolide-Co-pyrrolide axis. The complexes in their high spin (HS) states are more distorted than in the LS states, as is also usually the case for SCO active iron(II) complexes. The reversible CoIII/II redox potentials are predictably tuned by choice of substituents R1 and R2, from -0.95 (Me,Me) to -0.45 (CN,Ph) V vs Fc+/Fc, with a linear correlation observed between E1/2(CoIII/II) and the Swain-Lupton parameters of the pyrrolide substituents.

Structure-function relationships of donor-acceptor Stenhouse adduct photochromic switches.

The first in-depth, systematic study of the photoswitching properties of Donor-Acceptor Stenhouse Adducts (DASAs) is reported. Barbituric acid derived DASAs functionalised with 14 different amines ranging from dimethylamine to 4-methoxy-N-methylaniline were structurally characterised in solution using 1H and 13C NMR spectroscopy and, in eight cases, in the solid state by single crystal X-ray diffraction. The distribution of coloured and colourless isomers in the dark, their photostationary states under irradiation, apparent thermal half-lives, and fatigue resistance are systematically compared. A simple kinetic model is used to characterise photoswitching behaviour and reveals that minor structural modifications can significantly improve the photoswitching properties of DASA photochromes. These modifications result in excellent photoswitching properties for '1st generation' DASAs in chloroform, including exceptional fatigue resistance, opening the door for these photochromic molecules to find widespread applications.

Luminescent Tetrahedral Molecular Cages Containing Ruthenium(II) Chromophores.

We have designed linear metalloligands which contain a central photoactive [Ru(N∧N)3]2+ unit bordered by peripheral metal binding sites. The combination of these metalloligands with Zn(II) and Fe(II) ions leads to heterometallic tetrahedral cages, which were studied by NMR spectroscopy, mass spectrometry, and photophysical methods. Like the parent metalloligands, the cages remain emissive in solution. This approach allows direct incorporation of the favorable properties of ruthenium(II) polypyridyl complexes into larger self-assembled structures.

Chiral Ruthenium(II) Complexes as Supramolecular Building Blocks for Heterometallic Self-Assembly.

A series of enantiopure ruthenium(II) polypyridyl complexes are reported that feature pendant pyridyl groups suitable for building larger self-assembled structures. The complexes are characterized in detail in solution using NMR spectroscopy, cyclic voltammetry, and photophysical methods and in the solid state using single-crystal X-ray crystallography. The complexes are luminescent, displaying long excited-state lifetimes that are quenched when the pendant pyridyl groups are protonated. Reaction with cadmium(II) ions results in the formation of a mixed-metal one-dimensional coordination polymer, which was characterized by single-crystal X-ray crystallography.