Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.

Frustrated Lewis pairs: metal-free hydrogen activation and more.

Monazite and xenotime, the RE(P04) dimorphs, are the most ubiquitous rare earth (RE) minerals, yet accurate structure studies of the natural phases have not been reported. Here we report the results of high-precision structure studies of both the natural phases and the synthetic RE(P04) phases for all individual stable rare earth elements. Monazite is monoclinic, P2/n, and xenotime is isostructural with zircon (space group 14/amd). Both atomic arrangements are based on [001] chains of intervening phosphate tetrahedra and RE polyhedra, with a REOgpolyhedron in xenotime that accommodates the heavy lanthanides (Tb-Lu in the synthetic phases) and a RE09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements (La-Gd). As the structure "transforms" from xenotime to monazite, the crystallographic properties are comparable along the [00I] chains, with structural adjustments to the different sizes of RE atoms occurring principally in (00I). There are distinct similarities between the structures that are evident when their atomic arrangements are projected down [001]. In that projection, the chains exist in (100) planes, with two planes per unit cell. In monazite the planes are offset by 2.2 A along [010], relative to those in xenotime, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phase to bond to an additional 02' atom to complete the RE09 polyhedron.

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Crystal chemistry of the monazite and xenotime structures

Radiation effects in nuclear waste forms for high-level radioactive waste

Frustrierte Lewis‐Paare: metallfreie Wasserstoffaktivierung und mehr

Metal sites play a central role in the vast majority of molecular recognition processes involving biological and synthetic extended systems due to their ability to impart highly selective and specific molecular transformations, transport, and storage. 1-4 Although extensive research efforts worldwide have been devoted to studying the result of chemical reactions at metal centers, definitive structural characterization ofpenmetal (OM) sites, that are coordinatively unsaturated, by single crystal X-ray diffraction have been largely absent. The inherent difficulty in stabilizing such entities in molecular scaffolds arises from their reactive nature and from the lack of structural rigidity around the sites they occupy, which often leads to their aggregation and severe distortion of their local structure features that preclude their characterization as OM sites. In this report, we present a design strategy based on knowledge gained from modular chemistry, 5-14

Donald F. Mullica

Papers

Structural investigations of YPO4, ScPO4, and LuPO4

Ninefold coordination LaPO4: Pentagonal interpenetrating tetrahedral polyhedron

The structure of cerium orthophosphate, a synthetic analogue of monazite

Coordination geometry and structural determinations of SmPO4, EuPO4 and GdPO4

Structural investigations of several LnVO4 compounds