Niobium complexes imido

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]. 

The niobium imines can also be prepared by reaction of a niobium halo hydride with a nitrile. Thus a niobium reagent prepared from 1 and Bu3SnH reacts with nitriles to form a niobium-imido complex, which is hydrolyzed to a v/c-diamine. 

A half-sandwich imido complex of niobium (132), which is both isoelec-tronic and isolobal with the bent metallocene Cp2ZrCl2, reacts with two equiv PhMgBr in the presence of PMe3 to give the 18-e complex 6 [Eq. (21)].29 In contrast to the zirconocene analogue, trimethylphosphine is required to stabilize this tr-complex. 

The lack of stability of terminal oxo compounds of the heavier Group 5 metals led us naturally to the door of the isoelectronic imido (NR) ligand, where the availability of a substituent attached to the multiply-bonded group would allow both steric and electronic modulation of the products stability and reactivity. There had been a handful of half-sandwich imido complexes of the Group 5 metals synthesised by other workers, especially for vanadium and tantalum, but at that time none were known for niobium. A half-sandwich imido compound of niobium we considered, therefore, a prime target. 

Ta-N phases include Ta(NtBu)(NEt2)3, Ta(NEt)(NEt2)3, and Ta(NEt2)s. A niobium imido complex has been used for the growth of superconducting NbN films. ... 

A 2000 study from Buffon [69] examined the reaction of Schrock-type, alkoxy-Mo-alkylidenes with the surface OH groups of silica, silica-alumina, and niobium oxide. The specific mode of attachment of the Mo-complex was found to be highly dependent upon the acid-base properties of the support. For silica, it appears to be a Lewis acid-base interaction between the Mo center and surface silanols, whereas in the case of silica-alumina, the attachment appears to occur via the addition of a surface OH group across the Mo-imido bond. In the case of niobium oxide, it is possible that the OH adds across the Mo-alkylidene, deactivating the complex, as the resulting material was completely inactive for metathesis. The activities of both immobilized catalysts were less than the parent homogeneous... 

Galajov M, Garcia C, Gomez M, Gomez-Sal P. Alkyl chlorido hydridotris(3,5-dimethylpyrazolyl)borate imido niobium and tantalum(V) complexes synthesis, conformational states of alkyl groups in soHd and solution. X-ray diffraction and multinuclear magnetic resonance spectroscopy studies. Dalton Trans. 2014 43 5747-5758. 

Very recently, the groups of Bergman and Arnold reported that a niobium-imido complex catalyzes an efficient and selective partial hydrogenation of 1-phenyl-1-propyne to Z-p-methylstyrene under H2/CO mixtures (Fig. 14) [36], An Nb(V) metallacyclopropene complex similar to the previous Ti system was proposed, which was followed by a-bond metathesis with H2 and subsequent reductive elimination to yield the Z-alkene. An excess of CO is required not only for catalyst stability but also for achieving catalyst turnover by replacing the product from the Nb complex. However, only one substrate was included in this report. 

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