Synthesis and Reactivity of Cyclometalated Triamidophosphine Complexes of Niobium and Tantalum

The
triamidophosphine protioligand 1 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-N,N,P,C,N]­M­(NMe2) (2-M). 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 2-M can
be converted into the corresponding triflates [N2PCN-κ5-N,N,P,C,N]­M­(OTf) (3-M) and alkyl
complexes [N2PCN-κ5-N,N,P,C,N]­M­(CH2SiMe3) (4-M) 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-N,P,C,N,C,N]­M­(PMe3) (5-M). In the case of tantalum, the formation
of a dinuclear hydrido complex (6) is observed upon hydrogenation
of 4-Ta. In the case of niobium, the metallaziridine
substructure in 4-Nb is prone to ring opening via protonation
with triphenylsilylamine (Ph3SiNH2), resulting
in formation of the corresponding imido complex [PN3-κ4-P,N,N,N]­NbNSiPh3 (7).