Yttrium hydrides

Very recently, the yttrium hydride [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -di-methylbiphenyl]YH(THF) 2 (36), conveniently generated in situ from [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -dimethylbiphenyl]YMe(THF)2 (35) demonstrated its high catalytic activity in olefin hydrosilylation. This system represents the first use of a d° metal complex with non-Cp ligands for the catalytic hydrosilylation of olefins. Hydrosilylation of norbornene with PhSiHs gave the corresponding product (37) of 90% ee (Scheme 3-15) [43]. 

Cerium and yttrium hydrides were successfully tried as neutron moderators because of their temperature stability. 

Yttrium-catalyzed enyne cyclization/hydrosilylation was proposed to occur via cr-bond metathesis of the Y-G bond of pre-catalyst Cp 2YMe(THF) with the Si-H bond of the silane to form the yttrium hydride complex Ig (Scheme 8). Hydrometallation of the C=G bond of the enyne coupled with complexation of the pendant G=G bond could form the alkenylyttrium alkyl complex Ilg. Subsequent / -migratory insertion of the alkene moiety into the Y-C bond of Ilg could form cyclopentylmethyl complex Illg. Silylation of the resulting Y-C bond via cr-bond metathesis could release the silylated cycloalkane and regenerate the active yttrium hydride catalyst. Predominant formation of the /ra //j--cyclopentane presumably results from preferential orientation of the allylic substituent in a pseudo-equatorial position in a chairlike transition state for intramolecular carbometallation (Ilg —IHg). 

Hydrogenation of 38b under mild conditions yields the dimeric yttrium hydrides 44b (Eq. 17). The benzyl derivatives 39b can also be used as starting materials, but the rate of hydrogenolysis is considerably slower due to the presence of coordinated THF. The dimeric hydrides are extremely sensitive towards oxygen and moisture but thermally quite stable [27, 55, 65, 66]. 

The catalytic hydrocyclization of 1,5- and 1,6-dienes has been achieved by using (C5Me5)2YMe(THF) as precatalyst. The catalytic species is bis(pentamethyl-cyclopentadienyl)yttrium hydride, which is formed by C-H-activation and elimination of methane (Scheme 10) [74],... 

Figure 3.11 Single-component catalyst, dimeric homochirotopic rac.-(S, S)-dimethyl-silylenebis[l -(2-trimethylsilyl-4-r-butylcyclopentadienyl)]yttrium hydride [rac.-Me2Si (Me3Si,t-BuCp)2YH]2, for obtaining highly isotactic poly(a-olefins)s. Side view. Reproduced by permission from Ref. 31. Copyright 1992 American Chemical Society...

K-absorption spectrum of yttrium in YH3, which was mechanically treated in a ball planetary mill during 20 min. with angular rate of 1630 rot./min. is shown in Fig. 6 (c). This absorption spectrum, as seen from the picture, is shifted relatively to the spectrum of untreated YH3 hydride to the side of lower energies, which in accordance with the found out correlation has to testify to thermal stability of reduction of treated yttrium hydride. Actually, we determined by the method of hydrogen thermal desorption, that as a result of mechanic dispersing of this hydride its temperature of decomposition decreased more than on 300 °C (Fig. 7), i.e. the... 

Preliminary experiments with yttrium hydride showed increased speed and sensitivity in a simple sensor configuration. Optimization of this design, including the use of SPR, may provide the needed combination necessary for the intended safety application. Research is continuing to determine the ruggedness and durability of the hydride films. 

Figure 6. Initial response measurements for a yttrium-hydride film (17 nm Ag/100 nm Y/20 nm Pd) to 0.45% H2 in air. The first reduction in the signal corresponds to conversion of Y metal to YH2, and subsequent cycles to increases in hydrogen content toward the tri-hydride, YH2. The response was measured in reflection (no SPR) at 700 nm. The time constant of the hydrogen response is about 5.7 seconds... 

Skvortsov, GG, Fukin, GK., Trifonov, A.A. et al. (2007) Intramolecular (sp -hybridized) C-H activation yttrium alkyls versus transient yttrium hydrides. Organometallics, 26, 5770. 

Voskoboynikov, A.Z., Parshina, I.N., Shestakova, A.K. et al. (1997) Reactivity of lanthanide and yttrium hydrides and hydrocarbyls toward organosilicon hydrides and related compounds. Organometallics, 16, 4041. 

The atomic density of hydrogen in many metal hydrides is greater than that in liquid H2 or in H20. Metal hydrides are efficient moderators (Fig. 1) and neutron shielding materials, and help to minimize the core shield volume. Metal-clad yttrium hydride moderators capable of operation at 1000°C in air, uranium-zirconium hydride rods as a combination fuel-moderator element are examples, and metal-clad zirconium hydride units as moderator elements for operation up to 600°C° °. The hydrogen atom density in hydrides, Ah, the number of hydrogen atoms per cubic centimeter of hydride X 10 , is calculated from the hydrogen-to-metal atom ratio, H/M, the density of the hydride p, and the molecular weight W by ... 

Synthesis and reactivity of the yttrium hydride dimer [Cp Y(OAr)(/i-Fl)]2 (Ar = 2,6-Bu2C6H3) have been investigated in detail.294 295 Synthetic routes leading to tetranuclear yttrium and lutetium hydride clusters have been worked out. The complexes are accessible by reacting precursors of the type (CsMe4SiMe2R)Ln(CFI2SiMe3)2(TF[F)... 

A dimeric mono(pentamethylcyclopentadienyl)yttrium hydride has been synthesized through the reaction sequence illustrated in Scheme 115. The hydrogenation of Cp Y(OAr)CFI(SiMe3)2 (Ar= 2,6-BuC2C6H3) in hexane under 10 bar of H2 yielded [Cp Y(OAr)(/i-H)]2.295,548... 

The dissociation of the dimer [Cp 2Y(/i-H)]2 to the Cp 2YH monomer is an important process in the reactions of the dimer with alkenes. The kinetics and formation of yttrium alkyl complexes from [Cp 2Y(//-I I)]2 and alkenes have been investigated.587 In situ prepared dimeric bis(pentamethylcyclopentadienyl)yttrium hydride reacted rapidly with 3,3-dimethyl-l,4-pentadiene in methylcylohexane-r/ at — 78 °C and formed a bright yellow solution of the t/°-yttrium(m)pentenyl chelate complex Cp 2Y[7/,7/2-CI I2CI I2CMc2CI I=CH2] in 98% yield (Scheme 130). This pentenyl chelate complex was also prepared in toluene-// in 84% yield. The chelate complex was stable for about 2 weeks at — 78 °C but decomposed after a few hours at —50 °C. The complex was characterized without isolation by 1H and 13C NMR spectroscopy at —100 °C such complexes can be regarded as models for the coordination of alkenes to lanthanide and isoelectronic grouop 4 polymerization catalysts.588 Agostic interactions in yttrium alkyls of the type... 

Yttrium hydride reacts with methylenecyclobutane to form pentenyl chelate complex 67 [81]. P-Carbon elimination occurs to open the ring of cyclobutylmethyl intermediate 66. 

FIGURE 54 Crystal structure of a tetranuclear yttrium hydride cluster (left) as determined by neutron diffraction with its core structure (right) (redrawn after Yousufuddin et al., 2008). 

FIGURE 59 Synthesis of a tetranuclear yttrium hydride cluster featuring mono-Cp ligands fitted with a silicon-linked, metal-coordinating phosphine group (redrawn after Hou, 2003 Tardif et al., 2003b). 

A recent exhaustive review [1] presents the known and well characterised lanthanide hydrides. Recently, a few additional original complexes have been synthesised. These include phospholyl [7] and indenyl derivatives [8], a bimetallic metallocenic structure (Scheme 1) [4], a samarium hydride supported by a calix-tetrapyrrolyl ring, with a triply bonded hydride ligand (Scheme 2) [9], metallocenes with chiral groups which has been used for the asymmetric hydrogenation [10], a tris(pyrazolyl)borate ytterbium(II) hydride [11], Cp amido-[12] and bis(amido)yttrium hydrides [13]. The latter was not isolated but obtained in situ from the methyl derivative (Scheme 2). 

Insoluble MH hydrides react very slowly, they must be used in large excess and rearrangements often occur. In some cases, however, the addition of the hydride ion H- was observed, and the formation of anionic complexes of poor stability, CpjLnH-, was mentioned [14]. Nevertheless, an yttrium hydride could be structurally characterised [15]. Owing to the high electropositive character of the lanthanide atoms, the use of borohydride (or aluminohydrides) reagents leads to stable borohydrides (or to bimetallic Ln-Al complexes) [1]. The spontaneous elimination of the borane (or alane), which is commonly observed for the late transition metals, does not occur. 

A unique example of non metallocenic catalyst [(DADMB)YMe], [(DADMB) = 2,2 -bis(/er -butyldimethylsilylamido)-6,6 -dimethyldiphenyl], was recently reported [13]. It was assumed that the methyl complex first reacts with the silane to give a small concentration of a monomeric highly reactive hydride (Scheme 2). The yttrium hydride was not detected by NMR spectroscopy, probably because its concentration is very low. Nevertheless, precipitation of the insoluble yttrium dimer [(DADMB)YH]2 occurred at the end of the reaction. The bulky ligand... 

A very important observation was that the MI transition in YH occurs within a single structural phase and is not associated with a trivial first-order phase transition. This can be concluded from the fact that for >c> 2.7 yttrium hydride is in a single phase (y -phase) [119] and as we have seen the MI transition occurs at x = 2.86. 

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