Axial Anion Research Articles

EXAFS studies on Mn(III) tetraphenylporphyrin halides in Nafion

Manganese K-edge EXAFS of [Mn(III)(TPP)X] (X = Cl −, Br −, I −) were recorded in the solid state and when incorporated into the perfluorinated ion-exchange polymer Nation. Refinement of the solid sample data produced interatomic distances typical of Mn(III) porphyrins and Mn-X bond lengths which increase with the size of the axial anion. On incorporation into Nation, the porphyrins become planar with Mn-N av < 2 Å, and six-coordinate with oxygen groups occupying the axial sites. The anion does not appear to accompany the macrocycle into Nafion. However, the solvent used to incorporate the porphyrins into Nafion occupies at least one of the axial coordination sites.

The Influence of the Axial Ligand on the Solid-State Structures of Pentacoordinate Manganese Tetraphenylporphyrin Complexes

The crystal structures of Mn(tpp)(NCO), Mn(tpp)(CH3CO2).0·5C7H8, Mn(tpp)(NCS).0·5C7H8, Mn(tpp)(Br).C7H8, Mn(tpp)(I).C7H8 and the previously reported structures Mn(tpp)(Cl).(CH3)2CO, Mn(tpp)(Cl).C7H8, Mn(tpp)(NO2).C6H6, Mn(tpp)(NO3).2C6H6, Mn(tpp)(OSO3H), Mn(tpp)(H2O).-SO3CF3 and Mn(tpp)(CN).CHCl3 are used to assess the role of the axial anion in the crystal packing of pentacoordinate manganese tetraphenylporphyrin complexes. A comparison of the packing strategies adopted by the toluene solvates suggests that the axial anion can sterically determine the lattice packing motif. Amongst the structures examined, there appears to be a hierarchy of packing strategies led by the elegant ‘slot together’ assembly pattern underpinning the Mn(tpp)(Br).C7H8 and Mn(tpp)(Cl).C7H8 lattices. The metrical parameters that have been used to assess intercomplex interactions in the solid state are shown to be crystal packing sensitive. This is clearly evident in the structure of Mn(tpp)(I).C7H8 which has two crystallographically independent molecules. The displacement of the metal ion from the porphyrin core is determined by the axial ligand field; however, this otherwise intrinsic property is also crystal packing dependent. That is, the iodo ligand field itself is modulated by crystal packing interactions. The isomorphous Mn(tpp)(CH3CO2).0·5C7H8 and Mn(tpp)(NCS).0·5C7H8 structures, which also have two crystallographically independent molecules, indicate that the axial ligand field strength can influence intercomplex interactions in the solid state, by moderating charge donation from the porphyrin to the metal.

Thermal behaviour of some crystal solvates of magnanese(III) complexes

The thermal behaviour of some crystal solvates of square pyramidal Mn(III) complexes has been studied. The nature of interaction of the water of crystallization has been clarified, and a correlation was found between the hardness of the axial anions and the desolvation temperatures. The decomposition pathways of the investigated complexes have also been discussed.

Effects of axial anions and solvent on the redox behaviour of nickel complexes with C-functionalized tetraazamacrocycles

The redox behaviour of Ni III /Ni II complexes with unsubstituted cyclam ( L 1) and with carbon-functionalized analogues ( L 2, L 3) was studied in CH 3CN+0.1 M TEAP. The Ni III /Ni II redox couple was shifted slightly to more negative potentials in C-functionalized cyclams, indicative of an effective, although small, interaction between the side arms and the nickel. Steric effects of the side arms on the central nickel cyclam core can be ruled out as an explanation of this slight potential shift because in pure aqueous solvent all complexes gave the same Ni III/Ni II redox potential. Also, the axial coordination of monodentate Lewis bases — such as chloride — on Ni II and Ni III cyclams was analysed and compared. In CH 3CN, the general trend was that coordination-decoordination of such Lewis bases with Ni III/Ni II had much more dramatic effects on the Ni III/Ni II redox couple than the pendant arm borne by one carbon atom in the cyclam ring. Specifically, chloride anions coordinated more strongly than perchlorate with both Ni II and Ni III. In addition, the coordination constant of chloride was higher with Ni III than with Ni II. These distinct coordination effects faded away when enough water was added to acetonitrile.

Conformation and reactivity of anomeric radicals

Reductive decyanations of 2-cyanotetrahydropyran derivatives with sodium in ammonia show a strong preference for axial protonation. For the 2-cyanotetrahydropyran 6, the selectivity is 119:1, and for the cyanooxadecalin 13, which is sterically biased against axial hydrogen introduction, the ratio is 1.78:1. These stereochemical outcomes and ab initio calculations of the intermediate radical conformations are consistent with the following mechanistic model: reductive decyanation proceeds through the pyramidal, axial radical to give the configurationally stable, contrathermodynamic axial anion which is protonated with retention of configuration

Effect of electrolyte concentration on axial anion ligation in manganese(III) meso-tetraphenylporphyrin chlorides

The ionic strength dependence of axial ligation in manganese(III) 5,10,15,20-tetraarylporphyrin chlorides has been investigated and found to be much more important than has hitherto been recognized. In this study tetrahydrofuran solutions of the metalloporphyrin, 0.0-0.5 M in the electrolyte tetrabutylammonium tetrafluoroborate, were examined by electronic absorption and resonance Raman spectroscopies as well as ultramicroelectrode voltammetry. The metalloporphyrin complexes were found to be significantly dissociated even at moderate electrolytc concentrations.

Effect of axial anions on the thermal stability of square pyramidal manganese(III) complexes

The effect of the axial anion on the thermal behaviour of square pyramidal MnIII complexes of [Mn(SB)X] (where SB=tetradentate dibasic Schiff base; X=OAc−, SCN−, Cl−, Br− or I−) has been studied. A correlation has been made between the thermal stabilities of the complexes and the electronegativities (ξ) of the axial donor atoms. Other solid state phenomena (nature of interaction of the solvent of crystallization and polymorphic transformation) were also studied for the acetato and isothiocyanato complexes.

Solvent Effects on Optical and ESR Spectra of One-Electron Reduced Species of Cobalt(III) and Manganese(III) Tetraphenylporphyrins Produced by γ-Radiolysis at 77 K

Abstract One-electron reduction by γ-ray irradiation of cobalt(III) and manganese(III) tetraphenylporphyrins (ClCo(III)TPP, ClMn(III)TPP, and IMn(III)TPP) in ethanol and MTHF(2-methyltetrahydrofuran) solutions at 77 K results in the reduction of the central metals of these porphyrins. From the optical and ESR measurements, the reduced species are found to retain the axial anion, Cl− or I−, in MTHF but not in ethanol. The absorption spectroscopic studies of these porphyrins in ethanol solutions indicate that the axial halide ion is displaced by an ethanol molecule prior to γ-ray irradiation.

Effective luminal hypotonicity: the driving force for isotonic proximal tubular fluid absorption

This Editorial Review summarizes certain considerations relevant to the mechanism(s) of isotonic fluid absorption by the mammalian proximal nephron. Recent evidence indicates that the paracellular pathway in this epithelium has a low diffusion resistance. Therefore it is possible that lateral intercellular spaces are in diffusion equilibrium with the peritubular medium. For such a circumstance, the driving forces for isotonic fluid absorption may reside in external solutions. But since the hydraulic conductance of mammalian proximal tubules is remarkably high, the effective osmotic pressure gradient between luminal and pertibular solutions required to drive isotonic fluid transport is relatively small. In the mammalian proximal nephron, effective luminal hypotonicity may provide the driving force for isotonic fluid transport. At least two mechanisms could account for the development of effective luminal hypotonicity. First, preferential absorption of bicarbonate results in a rise of luminal Cl- concentration. And because proximal tubules are more permeable to Cl- than to HCO3-, there develops a driving force for isotonic fluid transport. Second, trivial degrees of luminal hypotonicity may develop attendant on active Na+ absorption. We provide evidence that, of these two mechanisms, axial anion asymmetry is the dominant force for isotonic fluid transport.

The self trapped hole in alkaline earth fluorides: II. Hopping motion

For pt. I, see ibid., vol.6, 229 (1973). The activation energies for the hopping motion of a self trapped hole (Vk centre) have been calculated for CaF2, SrF2 and BaF2. The estimates use the well known small polaron model, together with the microscopic model used successfully for the static properties in the previous paper. The results are in reasonable agreement with observation. Unfortunately, the only data available refer to temperatures so low that simple thermal activation is not expected, and the most serious approximation appears to be the relation of the calculated and observed parameters. In CaF2 90 degrees jumps are not observed, in contrast to all simple theories. Possible explanations are discussed; chemical bonding between the Vk ions and the nearest axial anions is not adequate, but a modification of the corresponding repulsive interaction has the correct properties.