Zr system

Production of NH ia these Mo and W systems has certain features ia common with the Ti and Zr systems. Each has mote than one bound N2 ligand ia the metal s coordiaatioa sphere, but only one is reduced ia a process ioitiated by acid additioa. Protoaatioa of N2 occurs as the phosphine or... 

Fig. 7. Time-of-flight spectra for Zr and Nb+C2H4 at con = 14, 9, and 6 kcal/mol from top to bottom, respectively. Note that at con = 6 kcal/mol, the slower peak disappears in the Zr system. Reprinted with permission from Ref. 121.

Several other supports have been used in order to generate more-electrophilic Zr systems, including sulfated zirconia [186], sulfated alumina [187], or other sulfated oxide supports [188], though the surface species are quite complex for these sys-... 

The data in Table 6.7 illustrate that when the non-racemic (ebthi)Zr system is used to catalyze the hydrogenation of prochiral alkenes, moderate levels of enantiofacial differentiation are observed (23—65% ee). Enantioselective deuteration of pentene occurs in low yield but shows noticeable enantioselection (23% ee). The same reaction with styrene proceeds in 61% yield and with moderate enantioselectivity (65% ee). Hydrogenation of 2-phenyl-l-pentene proceeds in excellent yield but with poor control of stereochemistry (95% yield, 36% ee). 

Chain transfer was observed to occur for A1( Bu)3 in olefin-polymerization systems with CpTiCl368 and LZrCl2 (L = rac-dimethylsilylenebis(indenyl)).69 There was no effect of increase in A1( Bu)3 concentration on the molecular weight of the polymer produced for the Zr system, but for the Ti system, there was a sharp decrease in molecular weight. The use of AlMe3 instead of A1( Bu)3 led to decrease in both catalytic activity and polymer molecular weight with increasing concentration. 

There have been some electrochemical studies of monocyclopentadienyl Ti and Zr systems including [Ti or Zr () -C5H5)R2],... 

The first synthesis of a crystalline phosphasilene 832 was achieved by a two-step reaction of t-Bu SiCI with four equivalents of SupPHLi (Sup = 2,4,6-t-Bu3CgH2) and Ph2PCl384 385 via the diphosphasila-allyl salt 833386, a remarkable conjugated sila-zr system (equation 288). The Si=P bond length in 832 is 2.094 A and the silicon atom is pyramidalized (E <= 356.7°). The 1 Jp=si of 832 is remarkably high with 203 Hz. 

A recent summary of the history and dynamics of the theoretical models of benzene39 cites a view that even though the current molecular orbital (MO) view of benzene seems complete and ultimate while the valence bond (VB) view seems obsolete, the recent findings about zr-distortivity in benzene indicate that the benzene story is likely to take additional twists and turns that will revive the VB viewpoint (see footnote 96 in ref 39). What the present review will show is that the notion of delocalized zr-systems in Scheme 1 is an outcome of both VB and MO theories, and the chemical manifestations are reproduced at all levels. The use of VB theory leads, however, to a more natural appreciation of the zr-distortivity, while the manifestations of this ground state s zr-distortivity in the excited state of delocalized species provides for the first time a physical probe of a Kekule structure .3... 

The methodology for obtaining the zr-and (-/-energies for zr-systems is reviewed elsewhere,2-4 6418 and hence, this section presents some of the computational evidence without going into the computational details. The interested reader is advised to consult the above references. The evidence comes from two types of computational experiments , shown in Schemes... 

Fig. 19. Phase diagrams of the Co-Ti and Cr-Zr systems. a-TiCo (C15 type) with higher melting-temperature and p-TiCo2 (Cl6 type) with lower melting-temperature is noted. ZrCr (H) high temperature form again has Cl5 type structure whereas ZrCr(L) low temperature form has a C16 type structure.

Comparison of cyclic zr-systems with their isoelec-tronic linear analogues has always played an important role in zr-bond theory. Interestingly, this concept has been extended to the description of conjugated... 

Palladium-catalyzed carbon-nitrogen bond formation (for reviews see [86— 91]) has been considerably developed in the past decade and, due to the omnipresence of nitrogen-containing compounds in natural products such as alkaloids or non-natural functional targets such as electron-rich zr-systems,... 

Both the permethylmetallocene and the cyclopentadienyl metallocene, low-valent titanium and zirconium systems are extremely reactive, primarily because of their electron-deficient carbenoid character (Section I,B). Generally speaking, there are a number of important differences vis-a-vis the chemistry of the [t7-C5(CH3)5]2 M and (77-C5 Hs)2M (M = Ti, Zr) systems. 

The four-membered ring Zr systems (24) can undergo further ring expansion reactions with other chalcogenides, nitriles, and isonitriles (Scheme 9). ... 

The ethylene chemistry of isolable Cp2Ti(CH3)(L) complexes is similar to that of the Zr systems. Complexes with relatively labile ligands (L = THF, Et20, EtOPh) exhibit low polymerization activity in CH2CI2 solution. For the THF complex, Taube reported an activity of about 3 g polyethylene/(mmol Ti-atm-hour) (CH2CI2 solution, 25 °C, 1 atm) (52). The corresponding pyridine, RCN, and PR3 complexes, which are less labile, are inactive under these conditions (39). Surprisingly, the penta-methylcyclopentadienyl systems Cp 2Ti(CH3)(L) (L = THF, THT) do... 

Turner and co-workers and Bochmann el al. have reported that propylene is polymerized by in situ generated base-free cations Cp2M(R) (49,51). In contrast, Cp 2M(CH3)(THT) (M = Zr, Hf) complexes catalyze propylene oligomerization under mild conditions (PhNMe2, 25°C, 1 atm) by an insertion/)J-CH3 elimination process (Scheme 8) (53). The conventional J3-H elimination chain transfer process is disfavored by severe steric interactions between the C)J-substituents and the bulky Cp ligands in the transition state (Scheme 8). The Hf system produces only Cs and C, products whereas the Zr system produces oligomers up to C24 under these conditions. 

A self-consistent thermodynamic description of the Si-Ag-Al-As-Au-B-Bi -C-Ca-Co-Cr Cu Fe-Ga-Ge-In Li Mg Mn Mo N-Na-Ni-O-P-Pb-S-Sb-Sn-Te-Ti-V-W-Zn-Zr system has recently been developed by SINTEF... 

Figure 10 Transition states for primary insertion of propylene (a) with the isospecific Me2Si(1 -lnd)2Zr system and (b) with the syndiospecific Me2C(Cp)(9-Flu)Zr systems.

Figure 11 Favored transition states for the secondary insertion of propylene with (a) the isospecific Me2Si(1 -lnd)2Zr system and with (b) the syndiospecific Me2C(Cp)(9-Flu)Zr system. High-energy transition states for the secondary insertion of propylene with (c) the isospecific Me2Si(1 -lnd)2Zr system and (d) the syndiospecific Me2C(Cp)(9-Flu)Zr system.

There was 10 mole % excess metal present. The calculated ARTs are presented in Table 1 for the Zr system, as well as the Ti and Hf analogs, which are discussed in the next section. The theoretical total pressures generated in all systems are greater than 1 atm. The composition of the gas phase varies from system to system, though it is composed primarily of metal and boron vapor in all cases suggesting that the ART is controlled by the highly endothermic vaporization of the metal and boron (A//vap = 450 to 650 kJ/mol). 

These considerations indicate that the thermodynamic analysis of the Zr system must not be restricted only to the processing temperature of 1900°C. The kinetics of this reaction have been studied using thermo-gravimetic analysis (TGA), interrupted growth experiments, ART calculations, and reaction velocity measurements. The proposed reaction mechanism is as follows [36] ... 

One potential route around this problem is to infiltrate above the melting temperature of TiB, i.e., above 2190°C. In this case, the molten Ti will be in equilibrium with TiB2 and TiC, rather than TiB and TiC. The reaction conditions will then become similar to those in the Zr system. On cooling below 2190°C, though, the residual Ti in the composite should, at least thermodynamically, form TiB, according to a class II ternary reaction ... 

The Hf-B-C system presents a situation that falls somewhat between the Ti and Zr systems [60]. Although the HfB phase is stable in the Hf-B binary system, it melts at 2100°C, below the melting point of the Hf parent metal (2227°C). During a directed metal oxidation reaction of molten Hf with B4C at just above the melting point of Hf, e.g., 2400°C, the Hf-B-C isothermal ternary cross section (Fig. 18) indicates that the molten metal is... 

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