Niobium complexes amido

The corresponding chemistry of analogous niobium complexes was inhibited by the requirement of a more complicated synthetic approach for the isolation of the niobaziridine hydride. The use of the isopropyl substitued —N(Pr )Ar amido ligand proved unsuitable for the stabilization of [Nb(H) N(P )Ar 2(ri -Me2CNAr)] because of insertion into the Nb—H bond. °2 These difficulties were overcome with use of the N(CH2Bu )Ar substituent and a synthetic approach based upon [Nb(O) N(CH2Bu )Ar 3]3 which enabled the isolation of [Nb(H) N(CH2Bu )Ar 2( ri -CH(Bu )=NAr)] via reduction. The synthesis of this species has opened routes to some unusual chemistry as shown in Scheme 6.5. ... 

Hydroamination is an atom-economical process for the synthesis of industrially and pharmaceutically valuable amines. The hydroamination reaction has been studied intensively, including asymmetric reactions, and a variety of catalytic systems based on early and late transition metals as well as main-group metals have been developed." However, Group 5 metal-catalysed hydroaminations of alkenes had not been reported until Hultzsch s work in 2011. Hultzsch discovered that 3,3 -silylated binaphtho-late niobium complex 69 was an efficient catalyst for the enantioselective hydroaminoalkylation of iV-methyl amine derivatives 70 with simple alkenes 71, giving enantioselectivities up to 80% (Scheme 9.30). Enantiomerically pure (l )-binaphtholate niobium amido complex 69 was readily prepared at room temperature in 5 min via rapid amine elimination reactions between Nb(NMe2)5 and l,l-binaphthyl-2-ol possessing bullqr 3,3 -silyl substituents. Since the complex prepared in situ showed reactivity and selectivity identical... 

In contrast to the tantalum compounds considered above, the niobium complexes can exist as conformers of different stability at low temperatures. For example, in the H NMR spectrum of complex 4, the intensity ratio between the signals of the conformers is about 3 1 [ 32 ]. The crystal structures of the bis-amido compound 5 and the methylamido compound 6 all show that the lone pair of the amido ligand lies preferentially in the equatorial binding plane of the cyclopentadienyl and imido ligands, a conformation due to the ir bonding interaction of the lone pair with the niobium centre. If rotation about the Nb-Namido bond becomes slow on the NMR time scale then two conformers will be observed, one with the amido proton oriented toward the cyclopentadienyl ring and one with it oriented away. It is supposed that the major conformer is that with the amido proton oriented toward the cyclopentadienyl ring, which minimizes the steric interactions present [32]. 

Cyclopentadienones, in molybdenum carbonyls, 5, 451 Cyclopentadienyl alkenes, with niobium, 5, 76 Cyclopentadienyl-alkoxo complexes, with Ti(IV), 4, 495 Cyclopentadienyl alkyl complexes, with niobium, 5, 66 Cyclopentadienyl-amido complexes, with Ti(IV), via dehalosilylation, 4, 448... 

When NbCls, methane, and ammonia were premixed prior to injection into the CVD reactor, (i.e., approach (i)), a parasitic side reaction occured which led to the formation of soot [69]. This soot formation was explained as the spontaneous precipitation of a niobium amido complex, NbC1.3(NH2)2 as shown in Eq. 2.5. This snowing effect critically limits this approach, yielding only powdery deposits. 

As the formation of the niobium amido complex was deemed unavoidable at atmospheric pressure, attempts were made to synthesize fine-grained coatings of NbCl3(NH2)2 on carbon fibers in the first step and then to convert the film into the desired NbCi yNy layers by subsequent heat treatment in a methane/ammonia atmosphere, (i.e., approach (ii)) (Eq. 2.6) [69]. Although fine-grained deposits were... 

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