Silylimido Complexes of Niobium and Tantalum at the Limit of π‐Bond Saturation

Abstract

Convenient strategies towards the synthesis of silylimido niobium and -tantalum complexes of the metallocene and half-sandwich type are reported. Bent metallocenes [(η5-C5H5)2M(NSiMe3)Cl] (M = Nb: 1a; Ta: 1b) and [(η5-C5H5)2M(NSiMe3)(η1-C5H5)] (M = Nb: 2a; Ta: 2b) are synthesized by reaction of [M(NSiMe3)Cl3(py)2] with 2 or 3 equiv. NaCp, respectively. Half-sandwich type silylimido complexes [(η5-C5Me5)M(NSiMe3)Cl2] (M = Nb: 3a; Ta: 3b) cannot selectively be obtained from the (pyridine)(silylimido) precursor complexes. However, their synthesis is accomplished by reaction of [(η5-C5Me5)MCl4] with LiN(SiMe3)2. On the other hand pyridine silylimido complexes are perfect starting materials for the synthesis of the related hydridotris(pyrazolyl)borato complexes [Tp*M(NSiMe3)Cl2] [M = Nb: 4a; Ta: 4b; Tp* = κ3-HB(3,5-Me2pz)3]. A comparative study of the σ,π-donor capability of isoelectronically related tert-butyl- and trimethylsilylimido ligands in complexes [(η5-C5H5)2Ta(NR)(η1-C5H5)] (R = SiMe3: 2b; tBu: 2c) is presented. With respect to solid-state structural investigations and solution variable-temperature NMR-spectroscopic studies on the dynamic exchange process of η5- and η1-bonded cyclopentadienyl rings in 2b,c, tert-butlyimido ligands are better donors than silylimido ligands, and both are better donors towards a d0 metal center than an η5-C5H5 ligand.