Novel vanadium complexes supported by a bulky tris(pyrazolyl)borate ligand

Tale of Three Molecular Nitrides: Mononuclear Vanadium (V) and (IV) Nitrides As Well As a Mixed-Valence Trivanadium Nitride Having a V3N4 Double-Diamond Core

Transmetallation of [VCl₃(THF)₃] and [TlTp^tBu,Me] afforded [(Tp^tBu,Me)VCl₂] (1, Tp^tBu,Me = hydro-tris(3-tert-butyl-5-methylpyrazol-1-yl)borate), which was reduced with KC₈ to form a C_3v symmetric V^II complex, [(Tp^tBu,Me)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^tBu,Me)V≡N(N₃)] (3). A likely V^III azide intermediate en route to 3, [(Tp^tBu,Me)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^tBu,Me)V(N₃)₂] (A), leading to 3. Reduction of 3 with KC₈ furnishes a trinuclear and mixed-valent nitride, [{(Tp^tBu,Me)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^tBu,Me)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^tBu,Me)₂V] upon photolysis of 6 in arene solvents. The putative {V^IV≡N} intermediate, [(Tp^tBu,Me)V≡N] (B), was intercepted by photolyzing 6 in a coordinating solvent, such as tetrahydrofuran (THF), yielding [(Tp^tBu,Me)V≡N(THF)] (B-THF). In arene solvents, B-THF expels THF to afford 5 and [(κ₂-Tp^tBu,Me)₂V]. A more stable adduct (B-OPPh₃) 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.

A Mononuclear and High-Spin Tetrahedral TiII Complex

A high-spin, mononuclear TiII complex, [(TptBu,Me)TiCl] [TptBu,Me- = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(TptBu,Me)TiCl2] with KC8. Complex [(TptBu,Me)TiCl] has a 3A2 ground state (assuming C3v symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined TiII complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(TptBu,Me)TiCl(THF)], which is impervious to N2 binding. However, in the absence of THF, the TiII complex captures N2 to produce the diamagnetic complex [(TptBu,Me)TiCl]2(η1,η1;μ2-N2), with a linear Ti═N═N═Ti topology, established by single-crystal X-ray diffraction. The N2 complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two TiIII centers covalently bridged by an N22- unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(TptBu,Me)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate TiII complex, namely, [(TptBu,Me)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated TiII-containing [(ΤptBu,Me)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear TiIV complexes of the type [(TptBu,Me)Ti═E(Cl)] (with E2- = NSiMe3, N2CPh2, O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N3SiMe3, N2CPh2, N2O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.