Metal distribution and disorder in the crystal structure of [NH2Et2][Cr7MF8(tBuCO2)16] wheel molecules forM= Mn, Fe, Co, Ni, Cu, Zn and Cd

Studying Cation Exchange in {Cr7Co} Pseudorotaxanes: Preparatory Studies for Making Hybrid Molecular Machines

In the design of dynamic supramolecular systems used in molecular machines, it is important to understand the binding preferences between the macrocycle and stations along the thread. Here, we apply 1H NMR spectroscopy to investigate the relative stabilities of a series of linear alkylammonium templated pseudorotaxanes with the general formula [H2NRR'][Cr7CoF8(O2CCH2 tBu)16] by exchanging the cation in solution. Our results show that the pseudorotaxanes are able to exchange threads via a dissociative mechanism. The position of equilibrium is dependent upon the ammonium cation and solvent used. Short chain primary ammonium cations are shown to be far less favourable macrocycle stations than secondary ammonium cations. Collision-induced dissociation mass spectrometry (CID-MS) has been used to look at disassembly of the pseudorotaxanes in a solvent-free environment and stability trends compared to those in acetone-d6. The energy needed to induce 50 % of the precursor ion loss (E50) is used and shows a similar trend to the equilibria measured by NMR. The relative stabilities of these hybrid inorganic-organic pseudo-rotaxanes are different to those of host-guest compounds involving crown ethers and this may be valuable for the design of molecular machines.

Adduct Ions as Diagnostic Probes of Metallosupramolecular Complexes Using Ion Mobility Mass Spectrometry

Following electrospray ionization, it is common for analytes to enter the gas phase accompanied by a charge-carrying ion, and in most cases, this addition is required to enable detection in the mass spectrometer. These small charge carriers may not be influential in solution but can markedly tune the analyte properties in the gas phase. Therefore, measuring their relative influence on the target molecule can assist our understanding of the structure and stability of the analyte. As the formed adducts are usually distinguishable by their mass, differences in the behavior of the analyte resulting from these added species (e.g., structure, stability, and conformational dynamics) can be easily extracted. Here, we use ion mobility mass spectrometry, supported by density functional theory, to investigate how charge carriers (H⁺, Na⁺, K⁺, and Cs⁺) as well as water influence the disassembly, stability, and conformational landscape of the homometallic ring [Cr₈F₈(O₂C^tBu)₁₆] and the heterometallic rotaxanes [NH₂RR'][Cr₇MF₈(O₂C^tBu)₁₆], where M = Mn^II, Fe^II, Co^II, Ni^II, Cu^II, Zn^II, and Cd^II. The results yield new insights on their disassembly mechanisms and support previously reported trends in cavity size and transition metal properties, demonstrating the potential of adduct ion studies for characterizing metallosupramolecular complexes in general.

Disassembly Mechanisms and Energetics of Polymetallic Rings and Rotaxanes

Understanding the fundamental reactivity of polymetallic complexes is challenging due to the complexity of their structures with many possible bond breaking and forming processes. Here, we apply ion mobility mass spectrometry coupled with density functional theory to investigate the disassembly mechanisms and energetics of a family of heterometallic rings and rotaxanes with the general formula [NH2RR'][Cr7MF8(O2CtBu)16] with M = MnII, FeII, CoII, NiII, CuII, ZnII, CdII. Our results show that their stability can be tuned both by altering the d-metal composition in the macrocycle and by the end groups of the secondary ammonium cation [NH2RR']+. Ion mobility probes the conformational landscape of the disassembly process from intact complex to structurally distinct isobaric fragments, providing unique insights to how a given divalent metal tunes the structural dynamics.

Single Isomer Heterometallic {CrIII6MII2} Rings Templated by Tetramethylammonium

A family of heterometallic rings [Me₄N]₂[Cr^III₆M^II₂F₈(O₂C^tBu)₁₆] is reported using tetramethylammonium hydroxide pentahydrate as the source of a template, where M = Zn, Mn, Ni, and Co. The metal cores are octagons with metal-metal edges bridged by one fluoride and two carboxylate ligands. The divalent metal ions are found ordered at positions 1 and 5 in the octagon. The tetramethylammonium cations are above and below the metal plane of the ring in the crystal structure. Magnetic studies show antiferromagnetic coupling between the paramagnetic metal ions present, leading to paramagnetic ground states in each case. ¹H NMR spectroscopy confirms that the structure of the {Cr^III₆Co^II₂} ring exists in solution, and electron paramagnetic resonance spectroscopy confirms the magnetic structure of the other three rings.

Studies of a Large Odd-Numbered Odd-Electron Metal Ring: Inelastic Neutron Scattering and Muon Spin Relaxation Spectroscopy of Cr8 Mn

The spin dynamics of Cr₈Mn, a nine‐membered antiferromagnetic (AF) molecular nanomagnet, are investigated. Cr₈Mn is a rare example of a large odd‐membered AF ring, and has an odd‐number of 3d‐electrons present. Odd‐membered AF rings are unusual and of interest due to the presence of competing exchange interactions that result in frustrated‐spin ground states. The chemical synthesis and structures of two Cr₈Mn variants that differ only in their crystal packing are reported. Evidence of spin frustration is investigated by inelastic neutron scattering (INS) and muon spin relaxation spectroscopy (μSR). From INS studies we accurately determine an appropriate microscopic spin Hamiltonian and we show that μSR is sensitive to the ground‐spin‐state crossing from S=1/2 to S=3/2 in Cr₈Mn. The estimated width of the muon asymmetry resonance is consistent with the presence of an avoided crossing. The investigation of the internal spin structure of the ground state, through the analysis of spin‐pair correlations and scalar‐spin chirality, shows a non‐collinear spin structure that fluctuates between non‐planar states of opposite chiralities.

Heterometallic Rings: Their Physics and use as Supramolecular Building Blocks

An enormous family of heterometallic rings has been made. The first were Cr7 M rings where M = Ni(II), Zn(II), Mn(II), and rings have been made with as many as fourteen metal centers in the cyclic structure. They are bridged externally by carboxylates, and internally by fluorides or a penta-deprotonated polyol. The size of the rings is controlled through templates which have included a range of ammonium or imidazolium ions, alkali metals and coordination compounds. The rings can be functionalized to act as ligands, and incorporated into hybrid organic-inorganic rotaxanes and into molecules containing up to 200 metal centers. Physical studies reported include: magnetic measurements, inelastic neutron scattering (including single crystal measurements), electron paramagnetic resonance spectroscopy (including measurements of phase memory times), NMR spectroscopy (both solution and solid state), and polarized neutron diffraction. The rings are hence ideal for understanding magnetism in elegant exchange-coupled systems.

A hybrid organic–inorganic molecular daisy chain

A hybrid daisy-chain has been made, involving an organic thread for an inorganic ring, where the organic thread for the ring also acts as a ligand for a second ring. The ring contains six chromium(iii) and two zinc(ii) ions, and two isomers of the rings are found in the daisy-chain.