Tale of Three Molecular Nitrides: Mononuclear Vanadium (V) and (IV) Nitrides As Well As a Mixed-Valence Trivanadium...

Transmetallation of [VCl₃(THF)₃] and [TlTp] afforded [(Tp)VCl₂] (1, Tp = hydro-tris(3-tert-butyl-5-methylpyrazol-1-yl)borate), which was reduced with KC₈ to form a C symmetric V^II complex, [(Tp)VCl] (2). Complex 1 has a high-spin (S = 1) ground state and displays rhombic high-frequency and -field electron paramagnetic resonance (HFEPR) spectra, while complex 2 has an S = 3/2 ⁴A₂ ground state observable by conventional EPR spectroscopy. Complex 1 reacts with NaN₃ to form the V^V nitride-azide complex [(Tp)V≡N(N₃)] (3). A likely V^III azide intermediate en route to 3, [(Tp)VCl(N₃)] (4), was isolated by reacting 1 with N₃SiMe₃. Complex 4 is thermally stable but reacts with NaN₃ to form 3, implying a bis-azide intermediate, [(Tp)V(N₃)₂] (A), leading to 3. Reduction of 3 with KC₈ furnishes a trinuclear and mixed-valent nitride, [{(Tp)V}₂(μ₄-VN₄)] (5), conforming to a Robin-Day class I description. Complex 5 features a central vanadium ion supported only by bridging nitride ligands. Contrary to 1, complex 2 reacts with NaN₃ to produce an azide-bridged dimer, [{(Tp)V}₂(1,3-μ₂-N₃)₂] (6), with two antiferromagnetically coupled high-spin V^II ions. Complex 5 could be independently produced along with [(κ₂-Tp)₂V] upon photolysis of 6 in arene solvents. The putative {V^IV≡N} intermediate, [(Tp)V≡N] (B), was intercepted by photolyzing 6 in a coordinating solvent, such as tetrahydrofuran (THF), yielding [(Tp)V≡N(THF)] (B-THF). In arene solvents, B-THF expels THF to afford 5 and [(κ₂-Tp)₂V]. A more stable adduct (B-OPPh) was prepared by reacting B-THF with OPPh₃. These adducts of B are the first neutral and mononuclear V^IV nitride complexes to be isolated.