Niobium hydride complex

The heterocumulene compound p-tolyl isothiocyanate and the thioke-tenes, S=C=CR2 (R2 = Me2(CH2)3CMe2, Bu 2), react with niobium hydride complexes, [NbHCp2(CO)] and [NbH3Cp2], to give insertion products 146 (/ O) and 147 (/2/). 

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. 

Thus, the niobium hydrides in fact do exhibit the desired hydridic character they can reduce acetone but cannot react with CO reduction products. From the color, the absence of any NMR, and the formation of 1 mol of free cyclo-pentadiene, the niobium-containing product appears to be a monocyclopenta-dienyl Nb(IV) complex. The rather facile removal of one of the Cp groups appears somewhat surprising but has been observed in related systems. For example, alcoholysis of Cp2TiCl2 gives CpTiC OEt) plus cyclopentadiene, even at room temperature (26). 

Monomeric, paramagnetic chlorohydrides snch as MH2CI2 (PR3)4 derive from facile oxidative addition of H2 to MCl2(PR3)4 complexes. Of these 17e dodecahedral complexes, (32) is the most interesting (eqnation 17). Its thermolysis in solution offers a qnadruply bridged hydride (33) and it can also be rednced to a monomeric Ta 16e species (34). Ta hydrides display a hydridic character (see Hydride Complexes of the Transition Metals), as shown by the reduction of CO by cyclopentadienyl derivatives. (see Niobium Tantalum Organometallic Chemistry)... 

In the 8-phase of niobium hydride, NbHo.ei, the additional structure observed in both the A and E modes remained in the spectrum of NbHo.03 Do.57 and the coupling model was dismissed [54].An explanation in terms of minor structural differences between the neighbouring sites in this phase was favoured. The degree of detail that can be extracted from more complex materials than the simple metals is limited and the spectra of TbNiAI H, 4 and UNiAl H2.0 are cases in point [55]. 

Bercaw has extended these studies of alkyl formation from niobium olefin-hydride complexes to include additional examples of meta-substituted styrene complexes of permethylniobocene. As noted in earlier work , electron-donating substituents on the j8-carbon accelerate this insertion reaction, whereas electron-withdrawing substituents retard the rate based on a correlation with a p of —1.2... 

Kinetic studies have also been carried out with the sterically less constrained endo-( ACp)2Nb(CH2=CHR)H system. Again, a correlation of insertion rate with (7pj is seen p= -1.1). However, unlike the ( -Cp)2Nb(CH2=CHR)H systems studied earlier, the ground-state energies of the niobium olefin-hydride complexes also vary with the electronic nature of the substituent. 

Complexes XVII are labile, in particular in halogenated solvents, and must be prepared at —10 to — 40°C . Butadiene, isoprene, cyclohexadiene, etc., may be used. In contrast, the niobium hydride XVIH reacts smoothly with butadiene to give XK ... 

Niobium fulvene hydride complex 479 reacted with elemental sulfur with formation of disulfur complex 480 and chelate 481 ° (Scheme 91). 

The niobium ketene-hydride complex (Cp 2Nb(H)(ii -C,0-0C CPh2)], Cp - n -CsH4SiMe3 reacts with nitriles or isonitriles to give isocyanate and the complex Cp 2NbH( C CPli2)]. Further work has been carried out on the chiral... 

The A-trimethylsilylimines 68 (R = t-Bu, Ph, 2-MeCgH4 or 2-BrC6H4), which are prepared by the reaction of non-enolizable aldehydes with lithium bis(trimethylsilyl)amide, followed by trimethylsilyl chloride, undergo pinacolic coupling induced by NbCLt 2THF to yield the vicinal diamines 69 as mixtures of dl- and meso-isomers, in which the former predominate. Another method for the preparation of 1,2-diamines is by the combined action of the niobium tetrachloride/tetrahydrofuran complex and tributyltin hydride on cyanides RCN (R = /-Hu. Ph, cyclopentyl or pcnt-4-en-l-yl) (equation 32)82. 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

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