Through-Space Spin Coupling in a Silver(II) Porphyrin Dimer upon Stepwise Oxidations: AgII⋅⋅⋅AgII,AgII⋅⋅⋅AgIII,and AgIII⋅⋅⋅AgIII Metallophilic Interactions

Metallophilic interactions between closed-shell metal ions are becoming a popular tool for a variety of applications related to high-end materials. Heavier d⁸ transition-metal ions are also considered to have a closed shell and can be involved in such interactions. There is no systematic investigation so far to estimate the structure and energy characteristics of metallophilic interactions in Ag^II/Ag^II (d⁹/d⁹), Ag^III/Ag^III (d⁸/d⁸), and mixed-valent Ag^II/Ag^III (d⁹/d⁸) complexes, which have been demonstrated in the present study. Both interporphyrinic and intermetallic interactions were investigated on stepwise oxidation by using a rigid ethene-bridged cis silver(II) porphyrin dimer and the results compared with those for highly flexible ethane-bridged analogues. By controlling the nature of chemical oxidants and their stoichiometry, both 1e and 2e oxidations were done stepwise to generate Ag^II/Ag^III mixed-valent and Ag^III/Ag^III porphyrin dimers, respectively. Unlike all other ethene-bridged metalloporphyrin dimers reported earlier, in which 2e oxidation stabilizes only the trans form, such an oxidation of silver(II) porphyrin dimer stabilizes only the cis form because of the metallophilic interaction. Besides silver(II) ⋅⋅⋅ silver(II) interactions in cis silver(II) porphyrin dimer, stepwise oxidations also enabled us to achieve various hitherto-unknown silver(II) ⋅⋅⋅ silver(III) and silver(III) ⋅⋅⋅ silver(III) interactions, which thereby allow significant modulation of their structure and properties. The strength of Ag ⋅⋅⋅ Ag interaction follows the order Ag^II/Ag^II (d⁹/d⁹)<Ag^II/Ag^III (d⁹/d⁸)<Ag^III/Ag^III (d⁸/d⁸). Single-crystal XRD, X-ray photoelectron spectroscopy (XPS), ¹H NMR and EPR spectroscopy, and variable-temperature magnetic investigations revealed various oxidation states of silver and metallophilic interactions, which are also well supported by computational analysis.     

A Cucurbit[8]uril 2:2 Complex with a Negative pKa Shift

A viologen derivative carrying a benzimidazole group ( V-P-I ²⁺; viologen–phenylene–imidazole V-P-I ) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest pK_a, by more than 1 pH unit, contrasting with the positive pK_a shift usually observed for CB-based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V-P- I ³⁺ indeed released protons as monitored by pH-metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V-P-I ³⁺ ⋅ CB[7] and a CB[8] complex of 1-methyl-4-(4-pyridyl)pyridinium. The origin of this negative pK_a shift is proposed to stand in an ideal charge state, and in the position of the two pH-responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.     

Diverse Reactivity of Dienes with Pentaphenylborole and 1-Phenyl-2,3,4,5-Tetramethylborole Dimer

The reactions of a monomeric borole and a dimeric borole with 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene were investigated. The monomeric borole reacted at ambient temperature whereas heat was required to crack the dimer to form the monomer and induce reactivity. 2,3-Dimethyl-1,3-butadiene reacts to give diverse products resulting from a cycloaddition process with the B−C moiety of the boroles acting as a dienophile, followed by rearrangements to furnish bicyclic species. For 1,3-cyclohexadiene, a [4+2] process is observed in which 1,3-cyclohexadiene serves as the dienophile and the boroles as the diene partner. The experimental results are corroborated with mechanistic theoretical calculations that indicate boroles can serve as either a diene or dienophile in cycloaddition reactions with dienes.     

Push–Pull Porphyrins via β-Pyrrole Functionalization: Evidence of Excited State Events Leading to High-Potential Charge-Separated States

A new set of free-base and zinc(II)-metallated, β-pyrrole-functionalized unsymmetrical push–pull porphyrins were designed and synthesized via β-mono- and dibrominated tetraphenylporphyrins using Sonogashira cross-coupling reactions. The ability of donors and acceptors on the push–pull porphyrins to produce high-potential charge separated states was investigated. The porphyrins were functionalized at the opposite β,β′-pyrrole positions of porphyrin ring bearing triphenylamine push groups and naphthalimide pull groups. Systematic studies involving optical absorption, steady-state and time-resolved emission revealed existence of intramolecular type interactions both in the ground and excited states. The push–pull nature of the molecular systems was supported by frontier orbitals generated on optimized structures, wherein delocalization of HOMO over the push group and LUMO over the pull group connecting the porphyrin π-system was witnessed. Electrochemical studies were performed to visualize the effect of push and pull groups on the overall redox potentials of the porphyrins. Spectroelectrochemical studies combined with frontier orbitals helped in characterizing the one-electron oxidized and reduced porphyrins. Finally, by performing transient absorption studies in polar benzonitrile, the ability of push–pull porphyrins to produce charge-separated states upon photoexcitation was confirmed and the measured rates were in the range of 10⁹ s⁻¹. The lifetime of the final charge separated state was around 5 ns. This study ascertains the importance of push–pull porphyrins in solar energy conversion and diverse optoelectronic applications, for which high-potential charge-separated states are warranted.     

Hydrothermal synthesis,thermal decomposition and optical properties of Fe2F5(H2O)(Htaz)(taz)(Hdma)

Crystal structure of Fe₂F₅(H₂O)(Htaz)(taz)(Hdma) which crystallizes in the triclinic system space group $\text{P}\overline{1}$ with unit cell parameters a = 8.8392(5) Å, b = 9.1948(5) Å, c = 9.5877(5) Å, α = 82.070(3)°, β = 63.699(3)°, γ = 89.202(3)°, Z = 2, and V = 690.91(7) ų, was synthesized under hydrothermal conditions at 393 K for 72 h, by a mixture of FeF₂/FeF₃, 1,2,4-triazole molecule (Htaz), and hydrofluoric acid solution (HF 4%) in dimethylformamide solvent (DMF). The main feature of this material is the coexistence of two oxidation states for iron atoms (Fe²⁺, Fe³⁺) in the unit cell, which associate by opposite fluorine corners of FeF₅N and FeF₂N₄ octahedra, and/or triazole molecule which originates the 2D produces material. The structure determination, performed from single crystal X-ray diffraction data, lead to the R₁/_WR₂ reliability factors 0.031/0.087. Thermal stability studies (TG/DTG/DTA) show that the decomposition provides in the temperature range 473–773 K and no mass loss was detected before 473 K. Mass spectrometry (MS) has been used. The optical absorption of the solid was measured at the corresponding λ_max using UV–vis diffuse-reflectance spectrum.     

Ground state solutions for Klein–Gordon–Maxwell system with steep potential well

In this paper, we study the following Klein–Gordon–Maxwell system $\left\{\begin{array}{c}-\Delta u+(\lambda a\left(x\right)+1)u-(2\omega +\varphi )\varphi u=f(x,u),\mathrm{in}{\mathbb{R}}^3,\hfill \\ -\Delta \varphi =-(\omega +\varphi )u^2,\mathrm{in}{\mathbb{R}}^3.\hfill \end{array}\right.$Using variational methods, we obtain the existence of ground state solutions under some appropriate assumptions on $a$ and $f$.