Hydrogen bonding of 18-crown-6 ether to ruthenium–ammine complexes at second sphere

Solid-State End-On to Side-On Isomerization of (N═N)2- in {[(R2N)3Nd]2N2}2- (R = SiMe3) Connects In Situ LnIII(NR2)3/K and Isolated [LnII(NR2)3]1- Dinitrogen Reduction

Examination of the reduction chemistry of Nd(NR₂)₃ (R = SiMe₃) under N₂ has provided connections between the in situ Ln(III)-based Ln^III(NR₂)₃/K reductions of N₂ that form side-on bound neutral (N=N)^2- complexes, [(R₂N)₂(THF)Ln]₂[μ-η²:η²-N₂], and the Ln(II)-based [Ln^II(NR₂)₃]^1- reductions by Sc, Gd, and Tb that form end-on bound (N=N)^2- complexes, {[(R₂N)₃Ln]₂[μ-η¹:η¹-N₂]}^2-, which are dianions. The reduction of Nd(NR₂)₃ by KC₈ under dinitrogen in Et₂O in the presence of 18-crown-6 (18-c-6) forms dark yellow solutions of [K₂(18-c-6)₃]{[(R₂N)₃Nd]₂N₂} at low temperatures that become green as they warm up to -35 °C in a glovebox freezer. Green crystals obtained from the solution turn yellow-brown when cooled below -100 °C, and the yellow-brown compound has an end-on Nd₂(μ-η¹:η¹-N₂) structure. The yellow-brown crystals isomerize in the solid state on the diffractometer upon warming, and at -25 °C, the crystals are green and have a side-on Nd₂(μ-η²:η²-N₂) structure. Collection of X-ray diffraction data at 10 °C intervals from -50 to -90 °C revealed that the isomerization occurs at temperatures below -100 °C. In the presence of tetrahydrofuran (THF), the dianionic {[(R₂N)₃Nd]₂N₂}^2- system can lose an amide ligand to provide the monoanionic [(R₂N)₃Nd^III(μ-η²:η²-N₂)Nd^III(NR₂)₂(THF)]^1-, characterized by X-ray crystallography. These data suggest a connection between the in situ Ln(III)/K reductions and Ln(II) reductions that depends on solvent, temperature, the presence of a chelate, and the specific rare-earth metal.

Reaction and reorganization free energies of electron-transfer reactions under restricted geometry conditions

Electron-transfer reactions between iron and cobalt complexes were studied in beta-cyclodextrin (betaCD), 2-hydroxypropyl-beta-cyclodextrin (HbetaCD), and 18-crown-6 ether (18C6) solutions. The results were rationalized taking as the basis the Marcus-Hush formalism. We employed two different approaches, depending on the kind of receptor and solvent, to obtain the reorganization and reaction free energies that determine the reaction rate constant. The opposite trends in reactivity observed in betaCD and HbetaCD solutions and the behavior in solutions of 18C6 are explained.

Two-state model based on electron-transfer reactivity changes to quantify the noncovalent interaction between Co(NH3)5Cl2+ and 18-crown-6 ether: the effect of second-sphere coordination on electron-transfer processes

The electron-transfer reaction between [Fe(CN)6]4- and [CoCl(NH3)5]2+ was studied in the presence of 18-crown-6 ether (18C6) in different reaction media constituted by water and acetonitrile as organic cosolvent at 298.2 K. The results corresponding to this reaction show a clear influence of 18C6 on the kinetics: a positive catalytic effect. Trends in the observed reactivity are explained by a change in the degree of association of one of the reactants (the cobalt complex) with the 18C6. This association is governed by an equilibrium constant that depends on the dielectric constant of the medium. The results show an increase of the rate constants for the electron-transfer process as the 18-crown-ether concentration increases and an increase of the binding free energy of the cobalt complex to the 18C6 when the electrostatic field of the medium becomes weaker. An analysis of the experimental data allows not only the reactivity changes associated with adducts formation processes for an electron-transfer reaction but also information on the binding free energy of the cobalt complex to the 18C6 to be obtained, which can be quantified by using a two-state model. We have found a good correlation between the energy of binding and the Kosower's Z-value. The influence of the 18C6 in the intramolecular electron transfer in the binuclear complex [Fe(CN)5pzCo(NH3)5] has been also investigated.