Synthesis and Reactivity of Cyclometalated Triamidophosphine Complexes of Niobium and Tantalum

The triamidophosphine protioligand <b>1</b> reacts with the homoleptic pentakis­(dimethylamido) precursors of niobium and tantalum [M­(NMe₂)₅, where M = Nb, Ta] to form cyclometalated complexes of the type [N₂PCN-κ⁵-<i>N</i>,<i>N</i>,<i>P</i>,<i>C</i>,<i>N</i>]­M­(NMe₂) (<b>2-M</b>). Apart from the three amido donors, one benzylic position of the ligand backbone is deprotonated over the course of this reaction, resulting in the formation of a new M–C bond. As a consequence, a metallaziridine substructure is formed, and the triamidophosphine moiety thus serves as a tetraanionic pentadentate ligand. The dimethylamido complexes <b>2-M</b> can be converted into the corresponding triflates [N₂PCN-κ⁵-<i>N</i>,<i>N</i>,<i>P</i>,<i>C</i>,<i>N</i>]­M­(OTf) (<b>3-M</b>) and alkyl complexes [N₂PCN-κ⁵-<i>N</i>,<i>N</i>,<i>P</i>,<i>C</i>,<i>N</i>]­M­(CH₂SiMe₃) (<b>4-M</b>) by treatment with triethylsilyl triflate (Et₃SiO₃SCF₃) followed by (trimethylsilyl)­methyllithium (LiCH₂SiMe₃). The alkyl complexes exhibit interesting reactivities, including a second cyclometalative backbone activation affording the trimethylphosphine-stabilized complexes [NP­(CN)₂-κ⁶-<i>N</i>,<i>P</i>,<i>C</i>,<i>N,C</i>,<i>N</i>]­M­(PMe₃) (<b>5-M</b>). In the case of tantalum, the formation of a dinuclear hydrido complex (<b>6</b>) is observed upon hydrogenation of <b>4-Ta</b>. In the case of niobium, the metallaziridine substructure in <b>4-Nb</b> is prone to ring opening via protonation with triphenylsilylamine (Ph₃SiNH₂), resulting in formation of the corresponding imido complex [PN₃<i>-κ</i>⁴-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>]­NbNSiPh₃ (<b>7</b>)