Niobium-mediated synthesis of phosphorus-rich molecules

Abstract

The use of a sterically demanding enolate ligand -OC[2Ad]Mes supported by niobium has allowed for the synthesis of (Mes[2Ad]CO) 3Nb=PP7Nb(OC [2Ad]Mes)3 through P4 coupling by a low-valent niobium intermediate. This asymmetric phosphorus-rich cluster harbors a niobium phosphinidene unit bound to a niobium-supported P7 cluster. The phosphinidene terminus was found to react with a range of ketones to give phosphaalkene complexes R2C=PP7Nb(OC[2Ad]Mes) 3. These phosphaalkenes are unstable towards intramolecular rearrangement to give R2CP8Nb(OC[ 2Ad]Mes)3 in which the carbene unit has been internalized into the P8 cage. This rearrangement was studied through both Eyring and Hammett analyses. Structrually the new R2CPsNb(OC[2Ad]Mes)3 are viewed as coordinated-diphosphenes and it was found that the diphosphene unit could be liberated and trapped by reaction with pyridine-N-oxide in the presence of excess 1,3-cyclohexadiene, generating niobium oxo and R2CP8(C 6H8). Searching for new platforms to investigate niobium-phosphorus chemistry led to the synthesis of [Na(THF) 3][P3Nb(ODipp) 3], an anionic cyclo-P3 complex that is accessible in two steps from commercially available reagents. It was discovered that [Na(THF) 3] [P3Nb(ODipp) 3] could function as a source of P3 3-, which has allowed for the synthesis of the tetraatomic molecule AsP 3 as a pure substance for the first time. AsP 3 has been studied by gas-phase electron diffraction, photoelectron spectroscopy, solid-state NMR spectroscopy, raman spectroscopy, as well as high resolution mass spectrometry, and a variety of quantum chemical calculations. Further, a wide array of AsP3 reaction chemistry has been probed including the synthesis and structural characterization of two metal complexes with a coordinated, intact AsP3 ligand. Motivated to explore the chemistry of [Na(THF)3][P3Nb(ODipp) 3] further, a series of investigations were carried out to generate substituted triphosphirene ligands complexed to niobium. In particular Ph3SnP3Nb(ODipp) 3 was prepared and was found to react cleanly and efficiently with pyridine-N-oxide in the presence of an excess of 1,3-cyclohexadiene to generate the niobium oxo complex and Ph3SnP3(C6H8), the product of a Diels-Alder reaction between the liberated triphosphirene and 1,3-cyclohexadiene. This unusual phosphorus-rich molecule was exploited in a number of transformations. One reaction of particular interest was the synthesis of P3Rh(PPh 3)3 from Ph3SnP3(C6Hg) and Wilkinson's catalyst (ClRh(PPh3)3) with loss of 1,3-cyclohexadiene and ClSnPh3- This transformation is an illustrative example of the ability of Ph3SnP 3(C6H8) to act as a P3 transfer agent.