Low-Coordinate and Neutral Nitrido Complexes of Vanadium

Abstract

Two neutral and four-coordinate vanadium(V)-nitrido complexes have been prepared via the thermolysis of metastable vanadium(III)-azido precursors. All complexes have been fully characterized by multinuclear NMR, FT-IR, isotopic labeling, and, in most instances, single crystal X-ray diffraction. On the basis of activation parameters, N(2) extrusion to form the V[triple bond]N moiety is proposed to occur via an ordered and early transition state having three- or four-triazametallacycle frameworks. In addition, we demonstrate the nitrido ligand to undergo incomplete N-atom transfer to CO and CN{2,6-Me(2)-C(6)H(3)) to form the bent V-N=C=X (X = O, N{2,6-Me(2)-C(6)H(3)}) ligands with concomitant 2e(-) reduction at the vanadium center.

Formation and Reactivity of the Terminal Vanadium Nitride Functionality

“…The decay of 1 was monitored by UV/Vis absorption spectroscopy at variable temperatures, ranging from 60 to 100°C, which enabled the calculation of the activation parameters ΔH ‡ = 4.4(1) kJ/mol and ΔS ‡ = -6.9(1.2) J/mol K from the Eyring plot. [18] The reaction rates decelerated significantly with increasing polarity of the reaction medium. For example, the rate of conversion for 1 to its corresponding nitride in toluene is 16 times faster than that of the reaction performed in THF under otherwise identical experimental conditions.…”

“…In an earlier communication, [18] we reported the thermal elimination of N 2 from 1 to form the nitride [(nacnac)-VϵN(Ntol 2 )] (3). The decay of 1 was monitored by UV/Vis absorption spectroscopy at variable temperatures, ranging from 60 to 100°C, which enabled the calculation of the activation parameters ΔH ‡ = 4.4(1) kJ/mol and ΔS ‡ = -6.9(1.2) J/mol K from the Eyring plot.…”

“…, which eliminates dinitrogen upon thermolysis. [18] This feature is remarkable for two reasons: the only other example of a neutral complex having a terminal vanadium nitride ligand species has been reported to be obtained from a vanadium(V) silylimide in the presence of a Lewis base [Reaction (9), Scheme 1] [41] or from N 2 elimination by using an azide reagent [Reaction (10), Scheme 1], [47] and the mechanism of nitride formation from 3d early transition metal azide precursors has not been explored to date. In our case, the isolation of an azidovanadium(III) compound, [(nacnac)V(N 3 )(Ntol 2 )], provides an opportunity to elucidate the mechanism behind vanadiumnitride bond formation.…”

“…The complete physical characterization and solid-state structure of 1 has been reported in a recent communication. [18] Unlike 1, which is a monomer in the solid state, singlecrystal X-ray diffraction (XRD) studies of 2 unambiguously revealed two azide ligands bridging two vanadium(III) centers to form the dinuclear complex (Figure 1), with a nearly coplanar V(N 3 ) 2 V asymmetric dinuclear core, and each vanadium center adopting a trigonal bipyramidal (TBP) geometry (τ = 0.74). The two vanadium ions are related by an inversion center, thus making the two azide ligands in complex 2 equivalent, as discerned from a single strong azide stretch at 2081 cm -1 in the FTIR spectrum (recorded in Nujol).…”

“…The characterization of 3 has been reported in an earlier communication, and the closely related analogue [(nacnac)VϵN(N[Mes]tol)] (Mes = 2,4,6-Me 3 C 6 H 2 ) has been structurally characterized. [18] Unlike 1, which is stable at room temperature for days in solution, complex 2 converts to the corresponding nitride, [(nacnac)VϵN-(OAr)] (4), over 10 h in solution, while thermolysis at 60°C results in more rapid conversion to 4. After work-up, nitride complex 4 can be isolated as a yellow solid in 70 % yield (Scheme 2).…”