Triple bonds of niobium with silicon, germaniun and tin: the tetrylidyne complexes [(κ3-tmps)(CO)2NbE–R] (E = Si, Ge, Sn; tmps = MeSi(CH2PMe2)3; R = aryl)

Abstract

A systematic, efficient approach to first complexes containing a triple bond between niobium and the elements silicon, germanium or tin is reported. The approach involves a metathetical exchange of the niobium-centered nucleophile (NMe₄)[Nb(CO)₄(κ²-tmps)] (1) (tmps = MeSi(CH₂PMe₂)₃) with a suitable organotetrel(II)halide. Compound 1 was obtained from (NMe₄)[Nb(CO)₆] and the triphosphane tmps by photodecarbonylation. Reaction of 1 with the disilene E-Tbb(Br)SiSi(Br)Tbb in the presence of 4-dimethylaminopyridine afforded selectively the red-brown silylidyne complex [(κ³-tmps)(CO)₂NbSi–Tbb] (2-Si, Tbb = 4-tert-butyl-2,6-bis(bis(trimethylsilyl)methyl)phenyl). Similarly, treatment of 1 with E(Ar^Mes)Cl (E = Ge, Sn; Ar^Mes = 2,6-mesitylphenyl) afforded after elimination of (NMe₄)Cl and two CO ligands the deep magenta colored germylidyne complex [(κ³-tmps)(CO)₂NbGe–Ar^Mes] (3-Ge), and the deep violet, light-sensitive stannylidyne complex [(κ³-tmps)(CO)₂NbSn–Ar^Mes] (3-Sn), respectively. Formation of 3-Sn proceeds via the niobiastannylene [(κ³-tmps)(CO)₃Nb–SnAr^Mes] (4-Sn), which was detected by IR and NMR spectroscopy. The niobium tetrylidyne complexes 2-Si, 3-Ge and 3-Sn were fully characterized and their solid-state structures determined by single-crystal X-ray diffraction studies. All complexes feature an almost linear tetrel coordination and the shortest Nb–E bond lengths (d(Nb–Si) = 232.7(2) pm; d(Nb–Ge) = 235.79(4) pm; d(Nb–Sn) = 253.3(1) pm) reported to date. Reaction of 3-Ge with a large excess of H₂O afforded upon cleavage of the Nb–Ge triple bond the hydridogermanediol Ge(Ar^Mes)H(OH)₂. Photodecarbonylation of [CpNb(CO)₄] (Cp = η⁵-C₅H₅) in the presence of Ge(Ar^Mes)Cl afforded the red-orange chlorogermylidene complex [Cp(CO)₃NbGe(Ar^Mes)Cl] (5-Ge). The molecular structure of 5-Ge features an upright conformation of the germylidene ligand, a trigonal–planar coordinated Ge atom, and a Nb–Ge double bond length of 251.78(6) pm, which lies in-between the Nb–Ge triple bond length of 3-Ge (235.79(4) pm) and a Nb–Ge single bond length (267.3 pm). Cyclic voltammetric studies of 2-Si, 3-Ge, and 3-Sn reveal several electron-transfer steps. One-electron oxidation and reduction of the germylidyne complex of 3-Ge in THF are electrochemically reversible suggesting that both the radical cation and radical anion of 3-Ge are accessible species in solution.