Hybrid Organic–Inorganic Rotaxanes,Including a Hetero‐Hybrid [3]Rotaxane Featuring Two Distinct Heterometallic Rings and a Molecular Shuttle

[2] and [3] hybrid rotaxanes are reported based on {Ti₇M} rings (M is a trivalent metal such as Fe^III or Ga^III). NMR studies show that [2]rotaxanes can act as molecular shuttles, while EPR studies of [3]rotaxanes show weak interactions between the paramagnetic components of the supramolecular assemblies.     

Supramolecular Control of Spin Exchange in a Spin‐Labelled [2]Rotaxane Incorporating a Tetrathiafulvalene Unit

The EPR properties of a novel triradical obtained by single‐electron oxidation of a nitroxide‐spin‐labelled rotaxane containing a tetrathiafulvalene unit and cyclobis(paraquat‐p‐phenylene) ring is reported. Rotaxanation is proved to have a dramatic effect on through‐space magnetic interactions between radical fragments. Analysis of the EPR spectra by a three‐jump model, allowed us to obtain structural information on the interlocked structure.     

Synergistic Spin Transition between Spin Crossover and Spin‐Peierls‐like Singlet Formation in the Halogen‐Bonded Molecular Hybrid System: [Fe(Iqsal)2][Ni(dmit)2]⋅CH3CN⋅H2O

To introduce halogen‐bond interactions between a cation and an anion, a novel Fe^III complex from iodine‐substituted ligands involving a paramagnetic nickel dithiolene anion was prepared and characterized. The compound exhibited the synergy between a spin‐crossover transition and a spin‐Peierls‐like singlet formation. The halogen‐bond interactions between the iodine and the sulfur atoms stabilized the paramagnetic state of π‐spins and played a crucial role in the synergistic magnetic transition between d‐ and π‐spins. In addition, the compound showed the light‐induced excited spin state trapping effect.     

Observation of CH⋅⋅⋅π Interactions between Methyl and Carbonyl Groups in Proteins

Protein structure and function is dependent on myriad noncovalent interactions. Direct detection and characterization of these weak interactions in large biomolecules, such as proteins, is experimentally challenging. Herein, we report the first observation and measurement of long‐range “through‐space” scalar couplings between methyl and backbone carbonyl groups in proteins. These J couplings are indicative of the presence of noncovalent C−H⋅⋅⋅π hydrogen‐bond‐like interactions involving the amide π network. Experimentally detected scalar couplings were corroborated by a natural bond orbital analysis, which revealed the orbital nature of the interaction and the origins of the through‐space J couplings. The experimental observation of this type of CH⋅⋅⋅π interaction adds a new dimension to the study of protein structure, function, and dynamics by NMR spectroscopy.     

Preparing Gold(I) for Interactions with Proton Donors: The Elusive [Au]⋅⋅⋅HO Hydrogen Bond

MP2 and DFT calculations with correlation consistent basis sets indicate that isolated linear anionic dialkylgold(I) complexes form moderately strong (ca. 10 kcal mol^?1) Au???H hydrogen bonds with single H₂O molecules as donors in the absence of sterically demanding substituents. Relativistic effects are critically important in the attraction. Such bonds are significantly weaker in neutral, strong σ‐donor N‐heterocyclic carbene (NHC) complexes (ca. 5 kcal mol^?1). The overall association (>11 kcal mol^?1), however, is strengthened by co‐operative, synergistic classical hydrogen bonding when the NHC ligands bear NH units. Further manipulation of the interaction by ligands positioned trans to the carbene, is possible.     

Nuclear Spin Isomers: Engineering a Et4N[DyPc2] Spin Qudit

Two dysprosium isotopic isomers were synthesized: Et₄N[¹⁶³DyPc₂] ( 1 ) with I =5/2 and Et₄N[¹⁶⁴DyPc₂] ( 2 ) with I =0 (where Pc=phthalocyaninato). Both isotopologues are single‐molecule magnets (SMMs); however, their relaxation times as well as their magnetic hystereses differ considerably. Quantum tunneling of the magnetization (QTM) at the energy level crossings is found for both systems via ac‐susceptibility and μ‐SQUID measurements. μ‐SQUID studies of 1 ^{(I =5/2)} reveal several nuclear‐spin‐driven QTM events; hence determination of the hyperfine coupling and the nuclear quadrupole splitting is possible. Compound 2 ^{(I =0)} shows only strongly reduced QTM at zero magnetic field. 1 ^{(I =5/2)} could be used as a multilevel nuclear spin qubit, namely qudit (d =6), for quantum information processing (QIP) schemes and provides an example of novel coordination‐chemistry‐discriminating nuclear spin isotopes. Our results show that the nuclear spin of the lanthanide must be included in the design principles of molecular qubits and SMMs.     

The Nature of the [TTF].+⋅⋅⋅[TTF].+ Interactions in the [TTF]22+ Dimers Embedded in Charged [3]Catenanes: Room‐Temperature Multicenter Long Bonds

The properties of tetrathiafulvalene dimers ([TTF]₂²⁺) and the functionalized ring‐shaped bispropargyl (BPP)‐functionalized TTF dimers, [BPP–TTF]₂²⁺, found at room temperature in charged [3]catenanes, were evaluated by M06L calculations. The results showed that their isolated [TTF]₂²⁺ and [BPP–TTF]₂²⁺ dimers are energetically unstable towards dissociation. When enclosed in the 4⁺‐charged central cyclophane ring of charged [3]catenanes (CBPQT⁴⁺), [TTF]₂²⁺ and [BPP–TTF]₂²⁺ dimers are also energetically unstable with respect to leaving the CBPQT⁴⁺ ring; since the barrier for the exiting process is only about 3 kcal mol^?1, that is, within the reach of thermal energies at room temperature (neutral [TTF]₂⁰ dimers are stable within the CBPQT⁴⁺ ring). However, the [BPP–TTF]₂²⁺ dimers in charged [3]catenanes cannot exit, because this would imply breaking the covalent bonds of the BPP–TTF⁺ macrocycle. Finally, it was shown that the [TTF]₂²⁺, [BPP–TTF]₂²⁺ dimers, and charged [3]catenanes are energetically stable in solution and in crystals of their salts, in the first case due to the interactions with the solvent, and in the second case mostly due to cation–anion interactions. In these environmental conditions at room temperature the TTF units of the [BPP–TTF]₂²⁺ dimers make short contacts, thus allowing their SOMO orbitals to overlap: a room‐temperature multicenter long bond is formed, similar to those previously found in other [TTF]₂²⁺ salts and their solutions.     

Columnar Stacking of Partially Fluorinated [4]Helicenes: C−H⋅⋅⋅F Interactions Change the Stacking Orientation

The partial fluorination of polycyclic aromatic hydrocarbons often produces a layered crystal packing, where fluorinated aromatic surfaces are stacked over nonfluorinated aromatic surfaces. Herein, we report the synthesis and crystal packing of partially fluorinated [4]helicenes with steric congestion resulting from H and F atoms in the fjord region. F₆‐[4]Helicene forms head‐to‐tail columnar stacks consisting of an alternate arrangement of perfluorinated and nonfluorinated naphthalene moieties. With decreasing fluorine content, aromatic stacking switched from arene?fluoroarene (Ar^H?Ar^F) hetero‐stacking to Ar^H?Ar^H/Ar^F?Ar^F homo‐stacking with the help of intermolecular C?H???F contacts in the fjord region. As a result, head‐to‐head columnar stacks appear. Therefore, the conventional Ar^H?Ar^F stacking motif is not always applicable to F_n‐[4]helicenes with twisted π‐surfaces.     

Solvent‐Dependent Dihydrogen/Dihydride Stability for [Mo(CO)(Cp*)H2(PMe3)2]+[BF4]− Determined by Multiple Solvent⋅⋅⋅Anion⋅⋅⋅Cation Non‐Covalent Interactions

Low‐temperature (200 K) protonation of [Mo(CO)(Cp*)H(PMe₃)₂] ( 1 ) by Et₂O ? HBF₄ gives a different result depending on a subtle solvent change: The dihydrogen complex [Mo(CO)(Cp*)(η²‐H₂)(PMe₃)₂]⁺ ( 2 ) is obtained in THF, whereas the tautomeric classical dihydride [Mo(CO)(Cp*)(H)₂(PMe₃)₂]⁺ ( 3 ) is the only observable product in dichloromethane. Both products were fully characterised (ν_CO IR; ¹H, ³¹P, ¹³C NMR spectroscopies) at low temperature; they lose H₂ upon warming to 230 K at approximately the same rate (ca. 10^?3 s^?1), with no detection of the non‐classical form in CD₂Cl₂, to generate [Mo(CO)(Cp*)(FBF₃)(PMe₃)₂] ( 4 ). The latter also slowly decomposes at ambient temperature. One of the decomposition products was crystallised and identified by X‐ray crystallography as [Mo(CO)(Cp*)(FH???FBF₃)(PMe₃)₂] ( 5 ), which features a neutral HF ligand coordinated to the transition metal through the F atom and to the BF₄^? anion through a hydrogen bond. The reason for the switch in relative stability between 2 and 3 was probed by DFT calculations based on the B3LYP and M05‐2X functionals, with inclusion of anion and solvent effects by the conductor‐like polarisable continuum model and by explicit consideration of the solvent molecules. Calculations at the MP4(SDQ) and CCSD(T) levels were also carried out for calibration. The calculations reveal the key role of non‐covalent anion–solvent interactions, which modulate the anion–cation interaction ultimately altering the energetic balance between the two isomeric forms.     

[N⋅⋅⋅I+⋅⋅⋅N] Halogen‐Bonded Dimeric Capsules from Tetrakis(3‐pyridyl)ethylene Cavitands

Two [N???I⁺???N] halogen‐bonded dimeric capsules using tetrakis(3‐pyridyl)ethylene cavitands with different lower rim alkyl chains are synthesized and analyzed in solution and the gas phase. These first examples of symmetrical dimeric capsules making use of the iodonium ion (I⁺) as the main connecting module are characterized by ¹H NMR spectroscopy, diffusion ordered NMR spectroscopy (DOSY), electrospray ionization mass spectrometry (ESI‐MS), and ion mobility‐mass spectrometry (TW‐IMS) experiments. The synthesis and effective halogen‐bonded dimerization proceeds through analogous dimeric capsules with [N???Ag⁺???N] binding motifs as the intermediates as evidenced by the X‐ray structures of (CH₂Cl₂)₂@[ 3 a ₂?Ag₄?(H₂O)₂?OTs₄] and (CH₂Cl₂)₂@[ 3 a ₂?Ag₄?(H₂O)₄?OTs₄], two structurally different capsules.     

Quantification of CH⋅⋅⋅π Interactions: Implications on How Substituent Effects Influence Aromatic Interactions

Attractive interactions between a substituted benzene ring and an α‐substituted acetate group were determined experimentally by using the triptycene model system. The attractive interaction correlates well with the Hammett constants σ_m (R²=0.90), but correlates much better with the acidity of the α‐protons (R²=0.98).