Vanadium Bisimide Bonding Investigated by X-ray Crystallography,⁵¹V and¹³C Nuclear Magnetic Resonance Spectroscopy, and V L_3,2-Edge X-ray Absorption Near-Edge Structure Spectroscopy

Abstract

Syntheses of neutral halide and aryl vanadium bisimides are described. Treatment of VCl2(NtBu)[NTMS(N(t)Bu)], 2, with PMe3, PEt3, PMe2Ph, or pyridine gave vanadium bisimides via TMSCl elimination in good yield: VCl(PMe3)2(N(t)Bu)2 3, VCl(PEt3)2(N(t)Bu)2 4, VCl(PMe2Ph)2(N(t)Bu)2 5, and VCl(Py)2(N(t)Bu)2 6. The halide series (Cl-I) was synthesized by use of TMSBr and TMSI to give VBr(PMe3)2(N(t)Bu)2 7 and VI(PMe3)2(N(t)Bu)2 8. The phenyl derivative was obtained by reaction of 3 with MgPh2 to give VPh(PMe3)2(N(t)Bu)2 9. These neutral complexes are compared to the previously reported cationic bisimides [V(PMe3)3(N(t)Bu)2][Al(PFTB)4] 10, [V(PEt3)2(N(t)Bu)2][Al(PFTB)4] 11, and [V(DMAP)(PEt3)2(N(t)Bu)2][Al(PFTB)4] 12 (DMAP = dimethylaminopyridine, PFTB = perfluoro-tert-butoxide). Characterization of the complexes by X-ray diffraction, (13)C NMR, (51)V NMR, and V L(3,2)-edge X-ray absorption near-edge structure (XANES) spectroscopy provides a description of the electronic structure in comparison to group 6 bisimides and the bent metallocene analogues. The electronic structure is dominated by π bonding to the imides, and localization of electron density at the nitrogen atoms of the imides is dictated by the cone angle and donating ability of the axial neutral supporting ligands. This phenomenon is clearly seen in the sensitivity of (51)V NMR shift, (13)C NMR Δδ(αβ), and L3-edge energy to the nature of the supporting phosphine ligand, which defines the parameters for designing cationic group 5 bisimides that would be capable of breaking stronger σ bonds. Conversely, all three methods show little dependence on the variable equatorial halide ligand. Furthermore, this analysis allows for quantification of the electronic differences between vanadium bisimides and the structurally analogous mixed Cp/imide system CpV(N(t)Bu)X2 (Cp = C5H5(1-)).

Electron localization in a mixed-valence diniobium benzene complex

“…Transmission measurements made using STXM provide accurate peak intensities in a non destructive manner and are compatible with air-sensitive inorganic materials. [37][38][39][40] In previous work, this method was employed to assign (4d 2 )(4d 2 ) electronic congurations to both Nb atoms in 1, which required a formally neutral bridging arene ligand. 24 The backgroundsubtracted Nb L 3,2 -edge spectra for 2 and reference materials Nb metal, neutral 1, and [Nb(BDI)N t BuCl 2 py], 7 (ref.…”

Engendering reactivity at group 5-heteroatom multiple bondsviaπ-loading

“…Bis(imido) complexes are typically accessed from mono(imido) precursors via one of several routes, as outlined in Fig. 4: (1) oxidation of a d 2 imido precursor; 46,48,[55][56][57][58] (2) base-induced aelimination from an imido-amido precursor; 39,41,59 and (3) ahydrogen elimination from an imido-amino-halide precursor. 29,32,53,60 The rst route necessitates generating a lowvalent imido precursor; d 2 group 5 transition metals are known to be highly reactive, so if the precursor is to be isolated, bulky supporting ligands are required to enforce kinetic stability.…”

“…47 V bis(imido) complexes are relatively rare, and there are only a handful of other cationic V bis(imido) complex reported to date. 40,41 When a solution of 1 was placed under an atmosphere of H 2 , the authors observed spectroscopic evidence of equilibrium between 1 and hydridoimido-amido intermediate IM1 (Scheme 1), the product of 1,2addition of H 2 across the V-imido bond. In the presence of alkynes and H 2 , 1 readily and selectively converted alkynes to Zalkenes, with 44-100% conversion rates observed for internal aryl, alkyl, and silyl alkynes.…”

Bis(imido)vanadium(V)-Catalyzed [2+2+1] Coupling of Alkynes and Azobenzenes Giving Multisubstituted Pyrroles

“…The molecular structure of 7 was confirmed by NMR and single-crystal X-ray diffraction analysis (Figure ). Complex 7 catalyzed the pyrrole formation in the presence of B­(C 6 F 5 ) 3 as a phosphine scavenger in 88% yield (eq ), clearly indicating that the bis­(imido)­vanadium­(V) species is a catalytically relevant species. We also found that complex 7 was generated by treating VCl 3 (THF) 3 with 1a in the presence of 3a in C 6 D 5 Br at 145 °C followed by addition of PMe 3…”