Hydride precursors

To mitigate the problem, a diffusion barrier is incorporated between the aluminum and the silicon (see Sec. 5 below). It is also possible to replace aluminum by alloys of aluminum and copper or aluminum and silicon, which have less tendency to electromigration. These alloys are usually deposited by bias sputtering. However, they offer only a temporary solution as electromigration will still occur as greater densities of circuit elements are introduced. It was recently determined that improvements in the deposition of aluminum by MOCVD at low temperature with a dimethyl aluminum hydride precursor may reduce the problem.bl... 

The chemistry has also been extended to related formamidinato metal hydrides of aluminum and gallium. In the case of aluminum, LiAlH4 and AlH3(NMe3) served as metal hydride precursors. Scheme 39 summarizes the synthetic routes. The complexes thus formed are thermally very stable. °... 

Cp 2Sm(jU-H)]2, (188), affords very high-molecular-weight PMMA with very low polydispersities (typically < 1.05).453-456 At — 95 °C the polymer formed is highly syndiotactic (95% rr triad). Isolation and X-ray analysis of (189), the 1 2 complex of (188) and MMA, provides strong support for the participation of a metal-enolate as the active site. (189) behaves in an identical manner to the hydride precursor, converting 100 equivalents MMA to polymer with Mn= 11,000 and Mw/Mn= 1.03.457 The successful structural characterization of (189) provides support for intermediates proposed earlier.458,459... 

Bimolecular decomposition of a non-metal hydride organometallic complex, such as metal formyl complexes, which may involve metal hydride precursors MC( = 0)H + H20 <-> M(CO)+ + OH + H2 N/A 36... 

The direct fluorination of almost all categories of inorganic hydrides leads to fluorine-containing analogues of the hydride precursors. The fluorination of simple inorganic hydrides (47, 48) is an elementary example of such syntheses (see Fig. 9). 

Hiickel MO calculations have not revealed any intrinsic kinetic barrier to hydride migration to coordinated CO (93). Thus it is worthwhile to consider possibilities that might mask the occurrence of a metal hydride carbonylation reaction. For instance, metal hydrides have been observed to react rapidly with metal acyls reduction products such as aldehydes or bridging —CHRO— species form (94-96). Therefore, it is possible that a formyl complex might react with a metal hydride precursor at a rate competitive with its formation. Such a reaction could also complicate the decomposition chemistry of formyl complexes. Preliminary studies have in fact shown that metal hydrides can react with formyl complexes (35, 57), but a complete product analysis has not yet been done. 

Catalytic procedures (introduced by Kuivila and Menapace92) are easier to conduct and the tin hydride concentration is more easily controlled. A catalytic amount of tributyltin hydride or tributyltin chloride is mixed with the radical precursor, the alkene acceptor and a stoichiometric quantity of a coreductant such as sodium borohydride93 or sodium cyanoborohydride.29 Over the course of the reaction, the borohydride continuously converts the tin halide to tin hydride. The use of the catalytic procedure is probably restricted to halide precursors (tin products derived from other precursors may not be reduced to tin hydrides). This method has several advantages over the standard procedures (i) it is simple to conduct (ii) most functional groups are stable to the coreductants (especially sodium cyanoborohydride) (iii) the tin hydride concentration is known, is stationary (assuming that the tin halide is rapidly reduced to tin hydride), and can be varied by either changing the concentration of the reaction or the quantity of the tin reagent (10% is a typical value, but lower quantities can be used) and finally, (iv) the amount of tin hydride precursor that is added limits the amount of tin by-product that must be removed at the end of the reaction. 

Treatment of [(CsMe4SiMe2N-t-Bu)Sc(PMe3)(/i-H)]2 with two equivalents of propylene at low temperature yielded the structurally characterized phosphine-free di-p-propyl complex [(C5Me4SiMe2N-t-Bu)Sc(/i-nPr)]2. This dimeric organoscandium alkyl was found to be an even more active a-olefin polymerization catalyst than the hydride precursor [52],... 

NMR measurements of 1H, 13C, and 31P may be very informative about the structure of the complex. The hydride precursor of the catalyst has a trigonal bipyramidal structure in which the two phosphorus ligands present can occupy either two equatorial sites or one axial and one equatorial site (Fig. 6.6). 

A solution of the hydride precursor of a carbosilane core (A, B or C) (0.128 mmol of SiH) in CH Cl (5ml) was prepared. 0.1ml of 1.34M solution of Br in CH Cl was added dropwise at room temperature. The reaction is exothermic and discolouration of Br solution occurs immediately (at the end of reaction the solution is pale orange). The mixture was stirred for an additional 1 hour at room temperature. Subsequently, the volatiles were pumped off under vacuum to leave white solids (quantitative yields), that were used immediately for siloxane arm grafting. Samples were taken to confirm the complete conversion of Si-H to Si-Br by H NMR. 

Bromination of tris(dimethylsilyl)methane yielded HC(SiMe2Br)3 (core A). [12] Hydride precursors of carbosilane cores B and C tris[1,1,1-tri(dimethylsilyl)hexy 1-dimethylsilyl]methane HC[SiMe2(CH2)3C(SiMe2H)3]3 and 1,1,3,3-tetramethyl-2,2,4,4-tetrakis-(dimethylsilyl)-l,3-disilacyclobutane [SiMe2C(SiMe2H)j2 were also prepared utilizing HC(SiMe2H)3. 

The reactive cores B and C were prepared by bromination of their hydride precursors. Due to their high moisture susceptibility, all Si-Br containing products were not isolated, but used directly in the siloxane arms grafting step. To confirm the availability of all Si-Br groups in the core B, which appeared as the most sterically demanding one, a low molecular weight poly(dimethylsiloxane) star with core B was prepared (Mn of an average arm -1000 D) and its structure was analyzed and proved by NMR. 

Cores A and C, as well as the hydride precursor of C, are crystalline. T. -cores of Il-nd generation, in spite of their relatively small molecular weight and spherical shape, exhibit some features characteristic of amorphous phases (the hydride precursor of core B has its glass transition (Tg) at 233 K, and its parent compound HC[SiMe2(CH2)5Br]3 at 195 K). 

Disilyl chalcogen compounds react in a similar manner (eqnation 39), to give both mononuclear complexes and bridged complexes (if excess platinum hydride precursor is used). ... 

In the precatalytic process the rhodium hydride precursor undergoes insertion into the butenyl carbonate to form an alkylrhodium complex. (3-Elimina-tion yields 1-butene and phenylcarbonatorhodium complex. Upon decarboxylation a phenoxorhodium complex is produced that undergoes the SN2 type reaction with 2-butenyl phenyl carbonate to liberate the branched allylic ether, 1-... 

Deprotonation of the zinc alcohol complexes shown in Fig. 12 to produce zinc alkoxide species has not been reported. Instead, mononuclear, tetrahedral zinc alkoxide complexes, supported by hydrotris(pyrazolyl)borate ligands, ([TpBut,Me or Tpph,Me, Scheme 8), have been generated via treatment of zinc hydride precursor complexes with aliphatic alcohols.68-70 A zinc ethoxide complex, [TpBut,Me]Zn-OEt, was also prepared via decarboxylation of the ethyl carbonate complex, [TpBut,Me]Zn-0C(0)0Et.49 X-ray crystallographic studies of [Tpph Me] Zn-OCH3 and [TpBut,Me]Zn-OEt revealed Zn-O bond lengths of 1.874(2) and 1.826(2) A, respectively.68,71 These bond distances are 0.1 A shorter than found for the alcohol complexes shown in Fig. 12. 

Scheme 8 Equilibrium formation of tetrahedral zinc-alkoxide and aryloxide complexes. The alkoxide complexes can be independently prepared via treatment of a zinc hydride precursor complex with the appropriate alcohol. Experimental methods for the determination of Kk (300 K) are given in reference 69.

When 17-electron complexes generated by oxidation decompose by deprotonation, the overall stoichiometry is highly dependent on the nature of the base capturing the proton, on the stability of the proton transfer products, and on the rate of oxidation. Equation 14 shows proton capture by an external base (e.g. pyridine or lutidine, often used for this kind of studies). The resulting 17-electron deprotonated radical may in principle evolve by either dimerization (equation 15), or by reaction with the paramagnetic hydride precursor, (equation 16), or by subsequent oxidation, which is usually assumed to be preceded by solvent coordination (equations 17-18) [86]. The oxidation potential of M(S) may be less positive than that of the MH precursor, resulting in an overall two-electron process for the oxidation of MH. 

Fumed silicas are obtained by high temperature oxydecomposition of SiH4, or other methyl hydride precursors (SiHMe3,SiH2Me2 ) ... 

Various cyclopentadienyl-ligated early metal and actinide alkoxides were prepared by reactions of the alcohols with the metal hydride precursors to eliminate hydrogen gas,266,275,277,278 g j.jy metal alkoxides, as well as their high-valent late-metal analogs, can also be formed by attack of an alkyl group at an oxo ligand, where the alkyl could be added as a carbocation, a radical, or a carbanion (Equation 4.59). ... 

The reaction between Pt(0) or Pt(II) hydride precursors and silane or germanes with E-H bonds (E = Si, Ge) can provide a variety of E-H bond activated products, for example, a series of silyl or germyl bridging diplatinum(I) compounds (Eigure 10.19) [130]. Further conversion of these compounds was possible including E-E bond coupling as well as the formation of Pt(II) silyl/germyl hydride. 

---