A Bispidine Iron(IV)-Oxo Complex in the Entatic State.

For a series of Fe(IV) =O complexes with tetra- and pentadentate bispidine ligands, the correlation of their redox potentials with reactivity, involving a variety of substrates for alkane hydroxylation (HAT), alkene epoxidation, and phosphine and thioether oxidation (OAT) are reported. The redox potentials span approximately 350 mV and the reaction rates over 8 orders of magnitude. From the experimental data and in comparison with published studies it emerges that electron transfer and the driving force are of major importance, and this is also supported by the DFT-based computational analysis. The striking difference of reactivity of two isomeric systems with pentadentate bispidines is found to be due to a destabilization of the S=1 ground state of one of the ferryl isomers, and this is supported by the experimentally determined redox potentials and published stability constants with a series of first-row transition metal ions with these two isomeric ligands.

Structure, bonding, and catecholase mechanism of copper bispidine complexes.

Oxygen activation by copper(I) complexes with tetra- or pentadentate mono- or dinucleating bispidine ligands is known to lead to unusually stable end-on-[{(bispidine)Cu}(2)(O(2))](2+) complexes (bispidines are methyl-2,4-bis(2-pyridin-yl)-3,7-diazabicyclo-[3.3.1]-nonane-9-diol-1,5-dicarboxylates); catecholase activity of these dinuclear Cu(II/I) systems has been demonstrated experimentally, and the mechanism has been thoroughly analyzed. The present density functional theory (DFT) based study provides an analysis of the electronic structure and catalytic activity of [{(bispidine)Cu}(2)(O(2))](2+). As a result of the unique square pyramidal coordination geometry, the d(x(2)-y(2)) ground state leads to an unusual σ/π bonding pattern, responsible for the stability of the peroxo complex and the observed catecholase activity with a unique mechanistic pathway. The oxidation of catechol to ortho-quinone (one molecule per catalytic cycle and concomitant formation of one equivalent of H(2)O(2)) is shown to occur via an associative, stepwise pathway. The unusual stability of the end-on-peroxo-dicopper(II) complex and isomerization to copper(II) complexes with chelating catecholate ligands, which inhibit the catalytic cycle, are shown to be responsible for an only moderate catalytic activity.

Origin and effect of thermocapillary convection in subcooled boiling: observations and conclusions from experiments performed at microgravity.

During the past two decades we have performed pool boiling experiments under microgravity conditions with saturated and subcooled liquids using various fluids, mostly fluorinated hydrocarbons, and various shapes of heaters. The common observation at subcooled boiling is the development of thermocapillary convection around bubbles. This convection forms jet streams above the head of the bubbles that carry the heat from the bubbles into the ambient liquid. Heat transfer measurements demonstrate that this flow does not directly contribute to the overall heat transfer itself, but that it is an important transport mechanism in subcooled boiling, not only at microgravity, but also under terrestrial conditions. The development of a thermocapillary flow is surprising, because it is well known that for this convection a temperature gradient along the interface of a bubble is necessary. However, it is also well known that the interfacial kinetics of evaporation and condensation is very strong. Thus, small temperature differences generate strong mass flow rates across the interface, coupled with high heat transfer rates that immediately equalize even moderate temperature gradients appearing along the bubble interface. This is confirmed by the observation that, during boiling in saturated liquids under microgravity conditions, thermocapillary convection was never observed, thus indicating uniformity of the temperature along the interface. From this we must conclude that the temperature difference of wall superheat, which generates boiling, cannot be the driving force for the observed thermocapillary flow even in subcooled liquids. Therefore, the question about the origin of this flow arises and is discussed in this paper.