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Treatment of (η5-C5H4C2H4NR)V(N-t-Bu)Me (R = Me, i-Pr) and CpV(N-p-Tol)(N-i-Pr2)Me (Cp = η5-C5H5) with B(C6F5)3 or [Ph3C][B(C6F5)4] results in formation of the corresponding cations, [(η5-C5H4C2H4NR)V(N-t-Bu)]+ and [CpV(N-p-Tol)(N-i-Pr2)]+. The latter could also be generated as its N,N-dimethylaniline adduct by treatment of the methyl complex with [PhNMe2H][BAr4] (Ar = Ph, C6F5). Instead, the analogous reaction with the linked Cp-amido precursor results in protonation of the imido-nitrogen atom. Sequential cyclometalation of the amide substituents gave cationic imine complexes [(η5-C5H4C2H4NCR′2)V(NH-t-Bu)]+ (R′ = H, Me) and methane. Reaction of cationic [(η5-C5H4C2H4NR)V(N-t-Bu)]+ with olefins affords the corresponding olefin adducts, whereas treatment with 1 or 2 equiv of 2-butyne results in insertion of the alkyne into the vanadium−nitrogen single bond, affording the mono- and bis-insertion products [(η5-C5H4C2H4N(i-Pr)C2Me2)V(N-t-Bu)]+ and [(η5-C5H4C2H4N(i-Pr)C4Me4)V(N-t-Bu)]+. The same reaction with the half-sandwich compound [CpV(N-p-Tol)(N-i-Pr2)]+ results in a paramagnetic compound that, upon alcoholysis, affords sec-butylidene-p-tolylamine, suggesting an initial [2+2] cycloaddition reaction. The difference in reactivity between the V−N bond versus the VN bond was further studied using computational methods. Results were compared to the isoelectronic titanium system CpTi(NH)(NH2). These studies indicate that the kinetic product in each system is derived from a [2+2] cycloaddition reaction. For titanium, this was found as the thermodynamic product as well, whereas the insertion reaction was found to be thermodynamically more favorable in the case of vanadium.

An X-ray crystallographic study of 'meso-amavadin' revealed that in the crystal the negatively charged anionic species of the title compound join into infinite hydrogen-bonded chains, counterbalanced by cationic hydronium species. Along with water of crystallization a three-dimensional hydrogen-bonded network is formed. Based on NMR-and X-ray data of amavadin and 'meso-amavadin', a model was developed that accounts for the structure of amavadin-type complexes, i.e. vanadium(IV) non-oxo complexes that contain two ligands with a tridentate N-hydroxyiminodiacetate backbone. The model describes the different arrangements of the two ligands around the vanadium and it accounts for eventual symmetry in the complex. The model was used for the interpretation of NMR-data of an amavadin analogue with a benzyl group at the ligand backbone.

Keywords: Amavadin / Vanadium / Reductive hydroxylamination / Hydroxylamines / Nitrones / Mass spectrometry Six secondary N-hydroxy amino acids (amavadin-based ligands) have been prepared through a strategy with nitrone reduction as its key step. Two of these amino acids are the new amavadin ligand analogues 4a and 4b containing either a phenyl or a benzyl group in combination with a small backbone substituent. In addition, the monoester 5 of the amavadin ligand as well as three N-alkylated N-hydroxy amino acids 6 were prepared. Complexation studies with the new tri-and bidentate ligands using HRMS revealed that only