Zr-N systems

If the solid solution of nitrogen in Zr, o -Zr(N), which exists up to a composition Zr2N , is not considered as a nitride phase, the Zr N system is characterized by the presence of only one nitride phase, 5-ZrNi c up to a composition of approximately [N]/[Zr] = 1. It should be mentioned that none of the subnitride phases that exist in the neighboring systems Ti-N and Hf-N occur here. In Massalski s compilation ... 

In a related type of reaction, an alkane RH is activated by addition across a metal-heteroatom bond, as in the Zr=N system of Wolczanski. This could be considered a metathesis-like process where the RH proton is taken up by the basic heteroatom. 

The Zr-N system is characterized by the presence of only one nitride phase, 5ZrNi , up to a composition of [N]/[Zr] = 1. It should be mentioned that none... 

Isothermal section in the Th-Zr-N system at 1773 K, suggested on the basis of cited literature. 

Similarly, other families of metallophosphates oxynitrides have been studied with catalytic objectives in different M-P-O-N systems (M = Ga, AlGa, AlCr, In, Ti, Zr,. ..) (235). 

Some of the earliest examples of modulated structures to be unraveled were the fluorite-related vernier structures. These structures occur in a number of anion-excess fluorite-related phases and use a modulation to accommodate composition variation. They can be illustrated by the orthorhombic phases formed when the oxyfluoride YOF reacts with small amounts of YF3 to give composition YOxF3 with x in the range 0.78-0.87, but similar phases occur in the Zr(N, O, F) system with x taking values of 2.12-2.25 and other systems in which the Zr is replaced by a variety of lanthanides. 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

In the Y O iF + 2 system, compounds n = 4,5,6,7,8 have been confirmed, the structures of which can be understood in terms of the vernier structure, similar to structure model (II) (Fig. 2.36). Closely related compounds have also been observed in the Nb-Zr-O and Zr-N-F systems. Figure 2.37 shows the structure of ZriogNggFijg drawn in the same way as YjOgFg in Fig. 2.35. This structure is also an example of a vernier structure based on a Fluorite-type structure. A vernier structure with layer type [a] (as in Fig. 2.32) has been critically and systematically reviewed by Makovicky and Hyde. °... 

Figure 6 Phase diagrams of group 4 transition metal-nitrogen systems, (a) Zr-N. (Ref. 11. Reproduced by permission of Materials Information Society/ASM International), (b) Hf-N. (Reprinted from Ref 28. 1991, with permission from Elsevier)...

It is obvious that the stability of the NaCl-type phase in these solid-solution systems increases with the content of NbN or ZrN, whose number of valence electrons is one or two less than that of MoN. That is to say, the stability of the NaCl-type phase in these systems decreases with increasing the number of valence electrons, Ny. In the higher Ny regions, the NaCl-type phase in the MoN-NbN system transforms to the WC-type phase in NH3 gas and to y-Mo2N-type phase in N2 gas. These phase transformations take place in the samples with x < 0.5, ie., Ny > 10, of the Moj Zr N and in the samples with X < 0.38, ie., Ny > 10.38, of the Moj jjNb,(N system. The annealed Nb-rich films showed a small reduction in lattice parameter, although the NaCl-type structure was remained until 1173K. 

The spectra of (4(V-E2)-electron charged perimeter n systems are appreciably different from uncharged perimeter zr systems, irrespective of whether the perimeter is even or odd—that is, of whether or not the ion is derived... 

As illustrated in Eqs. (2b) and (2c), the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene-alkene system affords the branched isomer [Eq. (2b)], whereas reaction from the less substituted end of the (EBTHI)Zr-olefin system leads to the formation of the straight chain product [Eq. (2c)]. The results shown in Table 2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacy-clopentane formation can be competitive. 

In the Zr-catalyzed enantioselective alkylation reactions discussed above, we discussed transformations that involve the addition of alkylmagnesium halides and alkylaluminum reagents to olefins. With the exception of studies carried out by Negishi and coworkers, all other processes involve the reaction of a C-C n system that is adjacent to a C-0 bond. Also with the exception of the Negishi study [Eqs. (6) and (7)], where direct olefin carbometallation occurs, all enantioselective alkylations involve the intermediacy of a metallacyclopentane (cf. Scheme 3). In this segment of our discussion, we will examine the Ni-catalyzed addition of hard nucleophiles (e.g., alkylmagnesium halides) to olefins that bear a neighboring C-0 unit. These reactions transpire by neither of the above two mechanistic manifolds (metallacyclopentane intermediacy or direct carbometallation). Rather, these processes take place via a Ni-Ti-allyl complex. 

Another structure is the simple hexagonal structure (hex) such as that of tungsten mononitride (8WN) where the metal atom layers form a sequence of layers AA or BB. Such structures are not close-packed and do not form octahedral sites the available interstitial sites are trigonal prisms (see Fig, 3.13 of Ch. 3). This structure cannot form if the ratio of the nitrogen/ metal atomic radii is small, as is the case in the Zr-N and Hf-N systems. 

The strong attraction between a cation and a n system is denoted as a cation/TT interaction. The size of the interaction energy for a cation/zr interaction is considerably larger than that for interactions between aromatic molecules and neutral molecules (tt/tt, OH/tt, NH/tt and CH/n interactions), as shown in Table 7. The experimental interaction energies of benzene complexes with Li, Na and K" " shown in Fig. 18 are -38.5, - 22.1 and - 17.5kcal/mol, respectively [24]. The importance of the cation/ 7r interaction in biological structures and molecular recognition processes has been... 

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