Jeff Espinas

Small changes have consequences: lessons from tetrabenzyltitanium and -zirconium surface organometallic chemistry.

Homoleptic benzyl derivatives of titanium and zirconium have been grafted onto silica that was dehydroxylated at 200 and 700 °C, thereby affording bi-grafted and mono-grafted single-site species, respectively, as shown by a combination of experimental techniques (IR, MAS NMR, EXAFS, and elemental analysis) and theoretical calculations. Marked differences between these compounds and their neopentyl analogues are discussed and rationalized by using DFT. These differences were assigned to the selectivity of the grafting process, which, depending on the structure of the molecular precursors, led to different outcomes in terms of the mono- versus bi-grafted species for the same surface concentration of silanol species. The benzylzirconium derivatives were active towards ethylene polymerization in the absence of an activator and the bi-grafted species displayed higher activity than their mono-grafted analogues. In contrast, the benzyltitanium and neopentylzirconium counterparts were not active under similar reaction conditions.

Design and Application of a Hybrid Material Featuring Well-Defined, Tuneable Grafting Sites for Supported Catalysis.

A new material based on amorphous silica, and featuring a well‐defined phenolic functionality was prepared in two simple steps by using commercially available, cheap reagents. Silica was first reacted with aluminum isobutyl etherate to yield an aluminum isobutyl site [(≡SiO)2Al(iBu)(Et2O)], which then selectively reacted with a hydroquinone spacer to yield [(≡SiO)2(AlOC6X4OH)(Et2O)] (X=H or F). This support material was then used to tether the organometallic tungstenocarbyne complex [W(≡CtBu)(CH2tBu)3] to yield the surface species [(≡SiO)2(AlOC6X4O‐W(≡CtBu)(CH2tBu)2(Et2O)] (X=H or F). Both H‐ and F‐containing species were fully characterized, and their activities in the self‐metathesis reaction of propene to ethylene and 2‐butenes were found to be two and three times higher, respectively, than the activity of the corresponding tungstenocarbyne complex directly grafted onto silica.

Easy and selective method for the synthesis of various mono-O-functionalized calix[4]arenes: de-O-functionalization using TiCl4.

An efficient and selective method for the monofunctionalization of p-tert-butylcalix[4]arene is described. A mono-de-O-functionalization of disubstituted p-tert-butylcalix[4]arenes using titanium tetrachloride was developed to synthesize a series of monosubstituted p-tert-butylcalix[4]arenes with the pendant functions being ethoxycarbonylmethyloxy, 3-ethoxycarbonylpropyloxy, cyanomethyloxy, 3-cyanopropyloxy, 4-bromobutyloxy, 3-hydroxypropyloxy, propyloxy, 2-methylpropyloxy, 3-butynyloxy, and 3-cyanopropyloxy groups. The reaction mechanism of the formation of 5,11,17,23-tetra-tert-butyl-26,27,28-trihydroxy-25-(3-ethoxycarbonylpropyloxy) calix[4]arene was studied by (1)H NMR and GC/mass spectroscopy monitoring. Reaction of TiCl4 with the disubstituted p-tert-butylcalix[4]arene produced the corresponding dioxocalix[4]arene titanium dichloride complex, which undergoes elimination of ethyl 4-chlorobutyrate, leading to a trioxocalix[4]arene titanium dichloride complex and to monosubstituted calix[4]arene after hydrolysis. These two complexes were also synthesized, isolated, and fully characterized.

Stereochemistry effect of dicyclohexano-18-crown-6 on zinc isotope separation

Zinc isotope fractionation is reported with the pure cis-syn-cis and cis-anti-cis stereoisomers of dicyclohexano-18-crown-6. These structures contribute differently to the enrichment of the Zn isotopes, and the best fractionation was obtained with the cis-syn-cis derivative.