Jakub Ujma

Luminescent, enantiopure, phenylatopyridine iridium-based coordination capsules

The first molecular capsule based on an [Ir(ppy)(2)](+) unit (ppy = 2-phenylatopyridine) has been prepared. Following the development of a method to resolve rac-[(Ir(ppy)(2)Cl)(2)] into its enantiopure forms, homochiral Ir(6)L(4) octahedra where obtained with the tritopic 1,3,5-tricyanobenzene. Solution studies and X-ray diffraction show that these capsules encapsulate four of the six associated counteranions and that these can be exchanged for other anionic guests. Initial photophysical studies have shown that an ensemble of weakly coordinating ligands can lead to luminescence not present in comparable mononuclear systems.

Shapes of supramolecular cages by ion mobility mass spectrometry

Mass spectrometry and drift tube ion mobility mass spectrometry have been used to analyse several isobaric, multicomponent cages yielding information on three dimensional structure, interactions and dynamics of assembly in the gas phase.

Sequential, Kinetically Controlled Synthesis of Multicomponent Stereoisomeric Assemblies†

The reversibility of noncovalent and metal–ligand interactions has widely been exploited to synthesize a plethora of supramolecular and coordination-based assemblies under thermodynamic control. Occasionally, entrapment in local energy minima leads to the formation of metastable products, which are often converted into lower-energy products upon prolonged reaction times. In contrast, self-assembled products in nature almost always arise according to the most expedient reaction pathway, that is, the kinetically selected. Herein we demonstrate a kinetically controlled approach to self-assembly, in which the sequence of addition of molecular structural units leads to the stereoselective formation of metallosupramolecular isomers. The use of platinum(II) (and other third-row transition metals) is particularly well-suited to a kinetic approach to self-assembly, not just because Pt–ligand bonds can be kinetically inert, but also because the metal ion can be conveniently tuned to produce a vast range of different ligand-exchange labilities. For instance, assemblies that utilize bis(phosphine) ligands as corner protecting groups often readily assemble at room temperature, while those that exploit neutral N-donor bidentate ligands, such as ethylene diamine, typically require several hours at elevated temperature to reach equilibrium. Furthermore, the mechanism of labilization, that is, the trans effect, is such that it is possible for a single metal center to possess cis exchangeable sites with dramatically different kinetic properties. We have recently prepared a metallosupramolecular trigonal prism that possesses an unsymmetrical cyclometalated C _ N corner protecting group, which was assembled in two steps by treating [LPt(dmso)] (where H2L 1 = 2,6-diphenylpyridine) sequentially with 4,4’-bipy and tpt.3CSA (Scheme 1, steps a and b). The isolation of a single isomeric product from a possible fourteen products, as indicated by the H NMR spectrum of the hexa-PF6 salt (see the Supporting Information, Figure S1a), led us to ask, was this selectivity a result of each Pt center possessing one labile and one inert exchangeable site, or was the selectivity thermodynamic in origin? To answer this question, the sequence in which 4,4’-bipy and tpt were added to [LPt(dmso)] was reversed. [LPt(dmso)] was first treated with a third of an equivalent of tpt at room temperature in CH2Cl2 to give [(L Pt)3(tpt)] (Scheme 1, step c), which was then treated with 4,4’-bipy·2CSA, and after metathesis with NH4PF6, the hexa-PF6 salt was isolated in 99% yield (Scheme 1, step d). The H NMR spectrum of this product (see the Supporting Information, Figure S1b) also indicated the formation of a single species, yet there were clear differences between the spectra of the two isomers, in particular, for resonances HA-E. The product from route 1 was assigned as cis-[(HLPt)6(4,4’-bipy)3(tpt)2](PF6)6, in which the tpt ligand is coordinated cis to the nitrogen of the 2,6diphenylpyridine ligand, and the product from route 2 was assigned as trans-[(HLPt)6(4,4’-bipy)3(tpt)2](PF6)6, in which the tpt ligand is coordinated trans to the nitrogen of the 2,6diphenylpyridine ligand. This absolute assignment was made on the basis that a) the resonance of the ortho proton of tpt (HC) is more deshielded in the trans-to-nitrogen coordination site in comparison to the ortho proton 4,4’-bipy (HB), and b) the large and small relative separations between the resonances of HA and HB, and between the resonances of HC and HD. Scheme 1. Sequence-specific control over the formation of metallosupramolecular stereochemical isomers. a) 4,4’-bipy, CH2Cl2, RT, 18 h, 77%; b) (i) tpt·3CSA, CH2Cl2, RT, 1 h; (ii) NH4PF6, 97%; c) tpt, CH2Cl2, 18 h, 85%; d) (i) 4,4’-bipy·2CSA, CH2Cl2, RT, 3 h; (ii) NH4PF6, 99 %. bipy= bipyridine, CSA = camphorsulfonic acid, dmso = dimethylsulfoxide, tpt = tris(4-pyridyl)triazine.

Making hybrid [n]?-?rotaxanes as supramolecular arrays of molecular electron spin qubits

Quantum information processing (QIP) would require that the individual units involved—qubits—communicate to other qubits while retaining their identity. In many ways this resembles the way supramolecular chemistry brings together individual molecules into interlocked structures, where the assembly has one identity but where the individual components are still recognizable. Here a fully modular supramolecular strategy has been to link hybrid organic–inorganic [2]- and [3]-rotaxanes into still larger [4]-, [5]- and [7]-rotaxanes. The ring components are heterometallic octanuclear [Cr7NiF8(O2CtBu)16]– coordination cages and the thread components template the formation of the ring about the organic axle, and are further functionalized to act as a ligand, which leads to large supramolecular arrays of these heterometallic rings. As the rings have been proposed as qubits for QIP, the strategy provides a possible route towards scalable molecular electron spin devices for QIP. Double electron–electron resonance experiments demonstrate inter-qubit interactions suitable for mediating two-qubit quantum logic gates.

Sizing and Discovery of Nanosized Polyoxometalate Clusters by Mass Spectrometry

Ion mobility-mass spectrometry (IM-MS) is a powerful technique for structural characterization, e.g., sizing and conformation, particularly when combined with quantitative modeling and comparison to theoretical values. Traveling wave IM-MS (TW-IM-MS) has recently become commercially available to nonspecialist groups and has been exploited in the structural study of large biomolecules, however reliable calibrants for large anions have not been available. Polyoxometalate (POM) species—nanoscale inorganic anions—share many of the facets of large biomolecules, however, the full potential of IM-MS in their study has yet to be realized due to a lack of suitable calibration data or validated theoretical models. Herein we address these limitations by reporting DT-IM (drift tube) data for a set of POM clusters {M12} Keggin 1, {M18} Dawson 2, and two {M7} Anderson derivatives 3 and 4 which demonstrate their use as a TW-IM-MS calibrant set to facilitate characterization of very large (ca. 1–4 nm) anionic species. The data was also used to assess the validity of standard techniques to model the collision cross sections of large inorganic anions using the nanoscale family of compounds based upon the {Se2W29} unit including the trimer, {Se8W86O299} A, tetramer, {Se8W116O408} B, and hexamer {Se12W174O612} C, including their relative sizing in solution. Furthermore, using this data set, we demonstrated how IM-MS can be used to conveniently characterize and identify the synthesis of two new, i.e., previously unreported POM species, {P8W116}, unknown D, and {Te8W116}, unknown E, which are not amenable to analysis by other means with the approximate formulation of [H34W118X8M2O416]44–, where X = P and M = Co for D and X = Te and M = Mn for E. This work establishes a new type of inorganic calibrant for IM-MS allowing sizing, structural analysis, and discovery of molecular nanostructures directly from solution.

UV photodissociation of trapped ions following ion mobility separation in a Q-ToF mass spectrometer

An ion mobility mass spectrometer has been modified to allow optical interrogation of ions with different mass-to-charge (m/z) ratios and/or mobilities (K). An ion gating and trapping procedure has been developed which allows us to store ions for several seconds enabling UV photodissociation (UVPD).

Initial steps of amyloidogenic peptide assembly revealed by cold ion spectroscopy

The early stages of fibril formation are difficult to capture in solution. We use cold-ion spectroscopy to examine an 11-residue peptide derived from the protein transthyretin and clusters of this fibre-forming peptide containing up to five units in the gas phase. For each oligomer, the UV spectra exhibit distinct changes in the electronic environment of aromatic residues in this peptide compared to that of the monomer and in the bulk solution. The UV spectra of the tetra- and pentamer are superimposable but differ significantly from the spectra of the monomer and trimer. Such a spectral evolution suggests that a common structural motif is formed as early as the tetramer. The presence of this stable motif is further supported by the low conformational heterogeneity of the tetra- and pentamer, revealed from their IR spectra. From comparison of the IR-spectra in the gas and condensed phases, we propose putative assignments for the dominant motif in the oligomers.