RESUMEN
Treatment of the bulky metallocene hydride Cp*2Zr(H)OMes (Cp* = pentamethylcyclopentadienyl, Mes = mesityl) with Piers' borane [HB(C6F5)2] and carbon monoxide (CO) gave the formylhydridoborate complex [Zr]-OâCH-BH(C6F5)2 ([Zr] = Cp*2Zr-OMes). From the dynamic NMR behavior, its endergonic equilibration with the [Zr]-O-CH2-B(C6F5)2 isomer was deduced, which showed typical reactions of an oxygen/boron frustrated Lewis pair. It was trapped with CO to give an O-[Zr] bonded borata-ß-lactone. Trapping with carbon dioxide (CO2) gave the respective O-[Zr] bonded cyclic boratacarbonate product. These reaction pathways were analyzed by density functional theory calculation. The formylhydridoborate complex was further reduced by dihydrogen via two steps; it reacted rapidly with H2 to give Cp*2Zr(OH)OMes and H3C-B(C6F5)2, which then slowly reacted further with H2 to eventually give [Zr]-O(H)-B(H)(C6F5)2 and methane (CH4). Most complexes were characterized by X-ray diffraction.
RESUMEN
The dimesitylphosphinocyclopentene/HB(C6 F5 )2 -derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane-borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO2 . It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C6 F5 )2 ] to generate a FLP-η2 -formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.
RESUMEN
Hydroboration of the conjugated enynes 1 a and 1 b with Piers' borane [HB(C6F5)2] gave the respective dienylboranes trans-2 c and trans-2 d. Their photolysis resulted in the formation of the dihydroborole products 3 c and 3 d. Both were converted to their pyridine adducts 5 c and 5 d, respectively. Compoundsâ 3 c and 5 c,d were characterized by X-ray diffraction. The reaction of the bis(enynyl)boranes 6 a and 6 b with B(C6F5)3 resulted in the formation of the dihydroboroles 7 a and 7 b, respectively. This reaction is thought to proceed by 1,1-carboboration of one of the enynyl substituents at boron to generate the dienylborane intermediates 8 a/8 b, followed by thermally induced bora-Nazarov ring-closure and subsequent stabilizing 1,2-pentafluorophenyl group migration from boron to carbon. Compoundâ 7 a was characterized by X-ray diffraction and solid-state (11)Bâ NMR spectroscopy.
RESUMEN
A solid state NMR method is described for measuring the angle Θ specifying the orientation of the principal component of the (11)B electric field gradient tensor relative to the (11)B( 31)P internuclear vector of (11)B-(31)P spin pairs. It is based on the anisotropic dephasing of (11)B spins in the dipolar field of (31)P nuclei via (11)B{(31)P} Rotational Echo DOuble Resonance (REDOR) experiments. The method is applied to four solid borane-phosphane compounds related to Frustrated Lewis Pair (FLP) chemistry. Results determined by numerical line shape simulations are found in excellent agreement with theoretically calculated values using advanced DFT methods. The angle Θ, which can be measured with an estimated precision of ±5°, offers a clear spectroscopic distinction between classical Lewis-acid/base adducts and active Frustrated Lewis pairs (FLPs).