Studies on low-coordinate phosphorus centers and sterically encumbered ligands: Structure, reactivity, materials, and catalysis.

摘要

The current study focuses on the development of appropriate synthetic protocol and starting materials for use in the high-yield preparation of thermally stable materials featuring an extended pi-conjugation system incorporating phosphaalkene and/or diphosphene linkages. Previous studies revealed the high yield preparation of phosphaalkenes by reaction of phospha-Wittig reagents (ArP = PMe3) with aldehydes, while the current work also capitalizes on the quantitative formation of diphosphenes upon thermolysis of select phospha-Wittig reagents. Phospha-Wittig reagents thus serve as a convenient common starting material for high-yield preparation of either P=C or P=P bonds. The phospha-Wittig methodology was utilized with a set of newly-developed m-terphenyls with pi-conjugated substituents to prepare PPV (poly(p-phenylenevinylene)) oligomers with P=C or P=P bonds in place of olefinic linkages. Spectroscopic studies on these oligomers show a progressive red shift in lambdamax as olefinic carbons are replaced by phosphorus. Bifunctional sterically-encumbered monomers appropriate for pi-conjugated polymer synthesis were also prepared. Phospha-PPV's utilizing such monomers were prepared with C=P or P=P linkages along the main chain, including the first examples of polymers incorporating a heavier main group element-element multiple bond. The phospha-PPV's exhibit red shifted absorption onset and lower photoluminescence versus all-carbon analogues. A second area of work involved the development of novel phosphines for utility in palladium-catalyzed C-C bond forming reactions. In this vein, a series of novel trans-spanning diphosphines based on a m-terphenyl scaffold, and bulky (m-terphenyl)dialkylphosphines were prepared, and palladium complexes of these phosphines have been structurally characterized. Both of these classes of phosphines show promise for utility in C-C coupling reaction of aryl halides with arylboronic acids (Suzuki reactions) or styrenes (Heck reactions).