Coordination of Terpyridine to Li⁺ in Two Different Ionic Liquids

On the basis of ⁷Li NMR experiments, the complex-formation reaction between Li⁺ and the tridentate N-donor ligand terpyridine was studied in the ionic liquids [emim]­[NTf₂] and [emim]­[ClO₄] as solvents. For both ionic liquids, the NMR data implicate the formation of [Li(terpy)₂]⁺. Density functional theory calculations show that partial coordination of terpyridine involving the coordination of a solvent anion can be excluded. In contrast to the studies in solution, X-ray diffraction measurements led to completely different results. In the case of [emim]­[NTf₂], the polymeric lithium species [Li­(terpy)­(NTf₂)]<i>_n</i> was found to control the stacking of this complex, whereas crystals grown from [emim]­[ClO₄] exhibit the discrete dimeric species [Li­(terpy)­(ClO₄)]₂. However, both structures indicate that each lithium ion is formally coordinated by one terpy molecule and one solvent anion in the solid state, suggesting that charge neutralization and π stacking mainly control the crystallization process

Coordination of Terpyridine to Li⁺ in Two Different Ionic Liquids

On the basis of ⁷Li NMR experiments, the complex-formation reaction between Li⁺ and the tridentate N-donor ligand terpyridine was studied in the ionic liquids [emim]­[NTf₂] and [emim]­[ClO₄] as solvents. For both ionic liquids, the NMR data implicate the formation of [Li(terpy)₂]⁺. Density functional theory calculations show that partial coordination of terpyridine involving the coordination of a solvent anion can be excluded. In contrast to the studies in solution, X-ray diffraction measurements led to completely different results. In the case of [emim]­[NTf₂], the polymeric lithium species [Li­(terpy)­(NTf₂)]<i>_n</i> was found to control the stacking of this complex, whereas crystals grown from [emim]­[ClO₄] exhibit the discrete dimeric species [Li­(terpy)­(ClO₄)]₂. However, both structures indicate that each lithium ion is formally coordinated by one terpy molecule and one solvent anion in the solid state, suggesting that charge neutralization and π stacking mainly control the crystallization process

Solvent and Pressure Effects on the Motions of Encapsulated Guests: Tuning the Flexibility of a Supramolecular Host

The supramolecular host assembly [Ga₄L₆]^12‑ [<b>1</b>; L = 1,5-bis­(2,3-dihydroxybenzamido)­naphthalene] contains a flexible, hydrophobic interior cavity that can encapsulate cationic guest molecules and catalyze a variety of chemical transformations. The Ar–CH₂ bond rotational barrier for encapsulated ortho-substituted benzyl phosphonium guest molecules is sensitive to the size and shape of the host interior space. Here we examine how changes in bulk solvent (water, methanol, or DMF) or applied pressure (up to 150 MPa) affect the rotational dynamics of encapsulated benzyl phosphonium guests, as a way to probe changes in host cavity size or flexibility. When host <b>1</b> is dissolved in organic solvents with large solvent internal pressures (∂<i>U</i>/∂<i>V</i>)_<i>T</i>, we find that the free energy barrier to Ar–CH₂ bond rotation increases by 1–2 kcal/mol, compared with that in aqueous solution. Likewise, when external pressure is applied to the host–guest complex in solution, the bond rotational rates for the encapsulated guests decrease. The magnitude of these rate changes and the volumes of activation obtained using either solvent internal pressure or applied external pressure are very similar. NOE distance measurements reveal shorter average host–guest distances (∼0.3 Å) in organic versus aqueous solution. These experiments demonstrate that increasing solvent internal pressure or applied external pressure reduces the host cavity size or flexibility, resulting in more restricted motions for encapsulated guest molecules. Changing bulk solvent or external pressure might therefore be used to tune the physical properties or reactivity of guest molecules encapsulated in a flexible supramolecular host

Dinuclear Seven-Coordinate Mn(II) Complexes: Effect of Manganese(II)-Hydroxo Species on Water Exchange and Superoxide Dismutase Activity

Two dinuclear seven-coordinate manganese­(II) complexes containing two pentaazamacrocyclic subunits, with imine or amine functionalities, respectively, have been synthesized and characterized in the solid state as well as in aqueous solutions of different pH, by performing X-ray structure analyses, SQUID, potentiometric, electron spray ionization-mass spectrometry (ESI-MS), electrochemical, and ¹⁷O NMR water exchange measurements (varying temperature and pressure), and by determination of SOD activity. The two manganese­(II) centers within the dinuclear structures behave independently from each other and similarly to the manganese centers in the corresponding mononuclear complexes. However, the dinuclear amine complex possesses increased complex stability and acidity of the coordinated water molecules (p<i>K</i>_a2 = 8.92) in comparison to the corresponding mononuclear analogue. This allowed us to observe a stable <i>trans</i>-aqua-hydroxo-Mn­(II) species in an aqueous solution and to study for the first time the <i>trans</i>-effect of the hydroxo group on the water lability on any divalent metal center in general. The observed <i>trans</i>-labilizing effect of the hydroxo ligand is much smaller than in the case of aqua-hydroxo-M­(III) trivalent metal species. Whether this is a general property of <i>trans</i>-aqua-hydroxo-M­(II) species, or if it is specific for Mn­(II) and/or to the seven-coordinate structures, remains to be seen and motivates future studies. In addition, an influence of the hydroxo ligand on the SOD activity of manganese­(II) complexes could be evaluated for the first time as well. Compared with the mononuclear analogue, which is not able to form stable hydroxo species, our pH dependent studies on the SOD activity of the dinuclear amine complex have indicated that the hydroxo ligand may promote protonation and release of the product H₂O₂, especially in solutions of higher pH values, by increasing its p<i>K</i>_a value

Dinuclear Seven-Coordinate Mn(II) Complexes: Effect of Manganese(II)-Hydroxo Species on Water Exchange and Superoxide Dismutase Activity

Two dinuclear seven-coordinate manganese­(II) complexes containing two pentaazamacrocyclic subunits, with imine or amine functionalities, respectively, have been synthesized and characterized in the solid state as well as in aqueous solutions of different pH, by performing X-ray structure analyses, SQUID, potentiometric, electron spray ionization-mass spectrometry (ESI-MS), electrochemical, and ¹⁷O NMR water exchange measurements (varying temperature and pressure), and by determination of SOD activity. The two manganese­(II) centers within the dinuclear structures behave independently from each other and similarly to the manganese centers in the corresponding mononuclear complexes. However, the dinuclear amine complex possesses increased complex stability and acidity of the coordinated water molecules (p<i>K</i>_a2 = 8.92) in comparison to the corresponding mononuclear analogue. This allowed us to observe a stable <i>trans</i>-aqua-hydroxo-Mn­(II) species in an aqueous solution and to study for the first time the <i>trans</i>-effect of the hydroxo group on the water lability on any divalent metal center in general. The observed <i>trans</i>-labilizing effect of the hydroxo ligand is much smaller than in the case of aqua-hydroxo-M­(III) trivalent metal species. Whether this is a general property of <i>trans</i>-aqua-hydroxo-M­(II) species, or if it is specific for Mn­(II) and/or to the seven-coordinate structures, remains to be seen and motivates future studies. In addition, an influence of the hydroxo ligand on the SOD activity of manganese­(II) complexes could be evaluated for the first time as well. Compared with the mononuclear analogue, which is not able to form stable hydroxo species, our pH dependent studies on the SOD activity of the dinuclear amine complex have indicated that the hydroxo ligand may promote protonation and release of the product H₂O₂, especially in solutions of higher pH values, by increasing its p<i>K</i>_a value

Does Perthionitrite (SSNO^–) Account for Sustained Bioactivity of NO? A (Bio)chemical Characterization

Hydrogen sulfide (H₂S) and nitric oxide (NO) are important signaling molecules that regulate several physiological functions. Understanding the chemistry behind their interplay is important for explaining these functions. The reaction of H₂S with <i>S</i>-nitrosothiols to form the smallest <i>S</i>-nitrosothiol, thionitrous acid (HSNO), is one example of physiologically relevant cross-talk between H₂S and nitrogen species. Perthionitrite (SSNO^–) has recently been considered as an important biological source of NO that is far more stable and longer living than HSNO. In order to experimentally address this issue here, we prepared SSNO^– by two different approaches, which lead to two distinct species: SSNO^– and dithionitric acid [HON­(S)­S/HSN­(O)­S]. (H)­S₂NO species and their reactivity were studied by ¹⁵N NMR, IR, electron paramagnetic resonance and high-resolution electrospray ionization time-of-flight mass spectrometry, as well as by X-ray structure analysis and cyclic voltammetry. The obtained results pointed toward the inherent instability of SSNO^– in water solutions. SSNO^– decomposed readily in the presence of light, water, or acid, with concomitant formation of elemental sulfur and HNO. Furthermore, SSNO⁻ reacted with H₂S to generate HSNO. Computational studies on (H)­SSNO provided additional explanations for its instability. Thus, on the basis of our data, it seems to be less probable that SSNO^– can serve as a signaling molecule and biological source of NO. SSNO^– salts could, however, be used as fast generators of HNO in water solutions

Switching between Inner- and Outer-Sphere PCET Mechanisms of Small-Molecule Activation: Superoxide Dismutation and Oxygen/Superoxide Reduction Reactivity Deriving from the Same Manganese Complex

Readily exchangeable water molecules are commonly found in the active sites of oxidoreductases, yet the overwhelming majority of studies on small-molecule mimics of these enzymes entirely ignores the contribution of water to the reactivity. Studies of how these enzymes can continue to function in spite of the presence of highly oxidizing species are likewise limited. The mononuclear Mn^II complex with the potentially hexadentate ligand <i>N</i>-(2-hydroxy-5-methylbenzyl)-<i>N</i>,<i>N</i>′,<i>N</i>′-tris­(2-pyridinylmethyl)-1,2-ethanediamine (L^OH) was previously found to act as both a H₂O₂-responsive MRI contrast agent and a mimic of superoxide dismutase (SOD). Here, we studied this complex in aqueous solutions at different pH values in order to determine its (i) acid–base equilibria, (ii) coordination equilibria, (iii) substitution lability and operative mechanisms for water exchange, (iv) redox behavior and ability to participate in proton-coupled electron transfer (PCET) reactions, (v) SOD activity and reductive activity toward both oxygen and superoxide, and (vi) mechanism for its transformation into the binuclear Mn^II complex with ^(H)OL–L^OH and its hydroxylated derivatives. The conclusions drawn from potentiometric titrations, low-temperature mass spectrometry, temperature- and pressure-dependent ¹⁷O NMR spectroscopy, electrochemistry, stopped-flow kinetic analyses, and EPR measurements were supported by the structural characterization and quantum chemical analysis of proposed intermediate species. These comprehensive studies enabled us to determine how transiently bound water molecules impact the rate and mechanism of SOD catalysis. Metal-bound water molecules facilitate the PCET necessary for outer-sphere SOD activity. The absence of the water ligand, conversely, enables the inner-sphere reduction of both superoxide and dioxygen. The L^OH complex maintains its SOD activity in the presence of ^•OH and Mn^IV-oxo species by channeling these oxidants toward the synthesis of a functionally equivalent binuclear Mn^II species