Unexpectedly high barriers to M–P rotation in tertiary phobane complexes : PhobPR behavior that is commensurate with tBu2PR

The four isomers of 9-butylphosphabicyclo[3.3.1]nonane, s-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl, have been prepared. Seven isomers of 9-butylphosphabicyclo[4.2.1]nonane (a5-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl; a7-PhobPBu, where Bu = n-butyl, isobutyl, tert-butyl) have been identified in solution; isomerically pure a5-PhobPBu and a7-PhobPBu, where Bu = n-butyl, isobutyl, have been isolated. The σ-donor properties of the PhobPBu ligands have been compared using the JPSe values for the PhobP(═Se)Bu derivatives. The following complexes have been prepared: trans-[PtCl2(s-PhobPR)2] (R = nBu (1a), iBu (1b), sBu (1c), tBu (1d)); trans-[PtCl2(a5-PhobPR)2] (R = nBu (2a), iBu (2b)); trans-[PtCl2(a7-PhobPR)2] (R = nBu (3a), iBu (3b)); trans-[PdCl2(s-PhobPR)2] (R = nBu (4a), iBu (4b)); trans-[PdCl2(a5-PhobPR)2] (R = nBu (5a), iBu (5b)); trans-[PdCl2(a7-PhobPR)2] (R = nBu (6a), iBu (6b)). The crystal structures of 1a–4a and 1b–6b have been determined, and of the ten structures, eight show an anti conformation with respect to the position of the ligand R groups and two show a syn conformation. Solution variable-temperature 31P NMR studies reveal that all of the Pt and Pd complexes are fluxional on the NMR time scale. In each case, two species are present (assigned to be the syn and anti conformers) which interconvert with kinetic barriers in the range 9 to >19 kcal mol–1. The observed trend is that, the greater the bulk, the higher the barrier. The magnitudes of the barriers to M–P bond rotation for the PhobPR complexes are of the same order as those previously reported for tBu2PR complexes. Rotational profiles have been calculated for the model anionic complexes [PhobPR-PdCl3]− using DFT, and these faithfully reproduce the trends seen in the NMR studies of trans-[MCl2(PhobPR)2]. Rotational profiles have also been calculated for [tBu2PR-PdCl3]−, and these show that the greater the bulk of the R group, the lower the rotational barrier: i.e., the opposite of the trend for [PhobPR-PdCl3]−. Calculated structures for the species at the maxima and minima in the M–P rotation energy curves indicate the origin of the restricted rotation. In the case of the PhobPR complexes, it is the rigidity of the bicycle that enforces unfavorable H···Cl clashes involving the Pd–Cl groups with H atoms on the α- or β-carbon in the R substituent and H atoms in 1,3-axial sites within the phosphabicycle

Effect of preparation mode on the properties of Mn-Na-W/ Sio2 catalysts for oxidative coupling of methane: conventional methods vs. POSS nanotechnology

Using XPS, BET, XRD, TG-DTA, HRTEM-EDX, TPR and UV-Vis Diffuse Reflectance spectroscopic methods the electronic, redox and structural properties of Mn-Na-W/ SiO2 catalysts prepared by the incipient wetness impregnation method and mixture slurry method were studied in detail. Since POSS nanotechnology (POSS = polyhedral oligomeric silsesquioxanes) has attracted attention as tooling for synthesis of catalysts with novel properties and functionalities, we expanded this method for the preparation of Mn-Na-W/ SiO2 catalyst. The physicochemical and catalytic properties of Mn-Na-W/ SiO2 catalysts prepared by conventional methods and POSS nanotechnology were examined comparatively. In all studied Mn-Na-W/ SiO2 catalysts both individual oxides (MnOx, WO3) and bimetal oxide phases (Na2WO4, MnWO4) are found in addition to oxide particles of high dispersion. The UV-Vis Diffuse Reflectance indicates that Na+ cations facilitates stabilization of octahedrally coordinated Mn3+ Oh cations in the isolated state, while Mn3+ Oh promote the disordering of W6+ cations in the supported system. The Mn-Na-W/ SiO2 prepared using metal-POSS precursors marks out presence of unglobular SiO2 particles, higher dispersion of MnOx and MnWO4 particles and more easily reducible metal-oxide species. The catalysts prepared by incipient impregnation method and mixture slurry method have practically similar catalytic performance while the catalyst prepared by POSS nanotechnology method shows lower activity and selectivity. At 800−850 °C the increase of C2 hydrocarbons yield from 4 to 15% and the rise of molar ratio C2H4/C2H6 from 0.2 to 1 are observed when impregnation or mixture slurry method are used for catalyst preparation instead of POSS nanotechnology method

Effect of preparation mode on the properties of Mn-Na-W/ Sio₂ catalysts for oxidative coupling of methane:conventional methods vs. POSS nanotechnology

Using XPS, BET, XRD, TG-DTA, HRTEM-EDX, TPR and UV-Vis Diffuse Reflectance spectroscopic methods the electronic, redox and structural properties of Mn-Na-W/ SiO2 catalysts prepared by the incipient wetness impregnation method and mixture slurry method were studied in detail. Since POSS nanotechnology (POSS = polyhedral oligomeric silsesquioxanes) has attracted attention as tooling for synthesis of catalysts with novel properties and functionalities, we expanded this method for the preparation of Mn-Na-W/ SiO2 catalyst. The physicochemical and catalytic properties of Mn-Na-W/ SiO2 catalysts prepared by conventional methods and POSS nanotechnology were examined comparatively. In all studied Mn-Na-W/ SiO2 catalysts both individual oxides (MnOx, WO3) and bimetal oxide phases (Na2WO4, MnWO4) are found in addition to oxide particles of high dispersion. The UV-Vis Diffuse Reflectance indicates that Na+ cations facilitates stabilization of octahedrally coordinated Mn3+ Oh cations in the isolated state, while Mn3+ Oh promote the disordering of W6+ cations in the supported system. The Mn-Na-W/ SiO2 prepared using metal-POSS precursors marks out presence of unglobular SiO2 particles, higher dispersion of MnOx and MnWO4 particles and more easily reducible metal-oxide species. The catalysts prepared by incipient impregnation method and mixture slurry method have practically similar catalytic performance while the catalyst prepared by POSS nanotechnology method shows lower activity and selectivity. At 800−850 °C the increase of C2 hydrocarbons yield from 4 to 15% and the rise of molar ratio C2H4/C2H6 from 0.2 to 1 are observed when impregnation or mixture slurry method are used for catalyst preparation instead of POSS nanotechnology method.</p

Effect of preparation mode on the properties of Mn-Na-W/ Sio₂ catalysts for oxidative coupling of methane:conventional methods vs. POSS nanotechnology

Using XPS, BET, XRD, TG-DTA, HRTEM-EDX, TPR and UV-Vis Diffuse Reflectance spectroscopic methods the electronic, redox and structural properties of Mn-Na-W/ SiO2 catalysts prepared by the incipient wetness impregnation method and mixture slurry method were studied in detail. Since POSS nanotechnology (POSS = polyhedral oligomeric silsesquioxanes) has attracted attention as tooling for synthesis of catalysts with novel properties and functionalities, we expanded this method for the preparation of Mn-Na-W/ SiO2 catalyst. The physicochemical and catalytic properties of Mn-Na-W/ SiO2 catalysts prepared by conventional methods and POSS nanotechnology were examined comparatively. In all studied Mn-Na-W/ SiO2 catalysts both individual oxides (MnOx, WO3) and bimetal oxide phases (Na2WO4, MnWO4) are found in addition to oxide particles of high dispersion. The UV-Vis Diffuse Reflectance indicates that Na+ cations facilitates stabilization of octahedrally coordinated Mn3+ Oh cations in the isolated state, while Mn3+ Oh promote the disordering of W6+ cations in the supported system. The Mn-Na-W/ SiO2 prepared using metal-POSS precursors marks out presence of unglobular SiO2 particles, higher dispersion of MnOx and MnWO4 particles and more easily reducible metal-oxide species. The catalysts prepared by incipient impregnation method and mixture slurry method have practically similar catalytic performance while the catalyst prepared by POSS nanotechnology method shows lower activity and selectivity. At 800−850 °C the increase of C2 hydrocarbons yield from 4 to 15% and the rise of molar ratio C2H4/C2H6 from 0.2 to 1 are observed when impregnation or mixture slurry method are used for catalyst preparation instead of POSS nanotechnology method.</p