Chelating Lewis acids have not been developed to the same degree as multidentate Lewis bases. Nonetheless, such compounds have attracted interest due to their potential for the enhanced activation of basic substrates and the selective binding of anions. Organodiboryl compounds form a class of bidentate Lewis acids which have a long, but relatively underdeveloped history. Many examples exist where donor groups on boron serve to stabilize the Lewis acid centers. More recently, advances in the chemistry of diboryls with highly Lewis acidic boron centers substituted with perfluoroaryl groups have been made. In particular, compounds of general formula (F5C6)2B−linker−B(C6F5)2 have been prepared and their chemistry examined. In this Microreview, we survey the classes of bifunctional boron Lewis acids known, including their synthesis, properties and anion binding chemistry. Particular emphasis is placed on the role these Lewis acids play in olefin polymerization catalyst generation from simple metallocene precursors.
The crystallization of the complex halide PbI2 in discrete and bundled single-walled carbon nanotubes
(SWNTs), double-walled carbon nanotubes (DWNTs), and thicker walled nanotubes is described. The
nanotubes were produced by either arc synthesis or catalytic chemical vapor deposition. The obtained
crystals could be described in terms of 1D fragments derived principally from the 2H form of PbI2,
although some evidence for the formation of fragments derived from 4H PbI2 in some nanotubes was
observed. The crystallization inside nanotubes was compared to the crystallization behavior of bulk PbI2
as determined by X-ray powder diffraction measurements obtained under comparable heating conditions.
While the 2H to 4H polytype transition is clearly observable in bulk PbI2, stacking behavior correlated
with this type of polytypism was only very occasionally observed within nanotubes, suggesting that
crystallizing PbI2 within nanotubes has a tendency to order the halide into the 2H form. Additionally,
PbI2 apparently does not crystallize in rigid narrow DWNTs with internal diameters of less than 2 nm.
Raman studies performed on the PbI2-filled nanotubes show that the ratio of intensity of the D and G
bands generally increases after filling and that both the RBM peaks and the G band are slightly upshifted.
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