Modeling solid-state structures

Single crystal X-ray structure analyses of analogously substituted adducts confirmed the applicability of this model. Adducts of the type EtaAl—E(Tms)3 and f-BuaAl—E(i-Pr)3 (E = P, As, Sb, Bi) were structurally characterized [50], allowing detailed comparisons of their solid state structural parameters. The trends observed for the average Al—C bond lengths and the C— Al—C bond angular sums are summarized in Figs. 5 and 6. 

Boche and coworkers drew attention to the solid state structure of mixed amine-metal amide99 and silylamide-nitrile complexes100 as models of reaction intermediates. 

In closely related experiments it was shown that sp C—H activation takes place reversibly within the coordinahon sphere of the electron-rich Ir(I)-diphosphine complex 58 (Scheme 6.9) to form an alkyl-amino-hydrido derivative 57 reminiscent of the CCM intermediate 24 the solid-state structure of 57 is shown in Figure 6.13 [40]. It appears that C—H activation only takes place after coordination of the amine function to the Ir(I) center (complex 58, NMR characterized). Amine coordination allows to break the chloro bridge of 59 and to augment the electron density of the metal center, thus favoring oxidative addihon of the C—H bond. Most importantly, the microscopic reverse of this C—H activation process (i.e. C—H reductive elimination) models the final step of the CCM cycle (see Scheme 6.1) indeed, the reaction of Scheme 6.10 is cleanly reversible at 373 K. 

Numerous X-ray investigations have unravelled the solid state structure of contact and solvent-separated ion pairs. It was therefore considered to be of interest to evaluate also the potential of solid state NMR as a tool for the investigation of this structural problem. In addition to the study of chemical shifts discussed above (Section II.B), the quadrupole coupling constant of the nuclide Li, x( Li), was expected to be an ideal sensor for the bonding situation around the lithium cation because, due to its dependence on the electric field gradient, the quadrupolar interaction for this spin-3/2 nucleus is strongly influenced by local symmetry, as exemplified in Section II.C.3. This is also shown with some model calculations in Section ILF. 

FIGURE 113. Solid state structure of 250. Space filling model of 251. Solid state structure of 252... 

The different responses of the NXL and XL bands would not occur if the NXL material were present as defects within tire XL phase. These data support a two-phase model of solid state structure in semicrystalline PTLE. Of course, the precise location of the two phases is difficult to specify, but for the XL phase we have good data on its initial morphology and subsequent changes based upon microscopy and X-ray diffraction. 

Thus, solution and solid-state structural studies of such fluoroolefin compounds have helped to formulate and refine theories of metal-carbon bonding. Studies of the reactivity of metal-fluoroolefin compounds have also provided useful models and predictions for hydrocarbon systems. For example, the oxidative cyclization of fluoroolefins within the coordination sphere of a metal to give metallacyclopentane compounds was discovered many years before the importance of the corresponding reaction of hydrocarbon olefins was realized (3). 

Molecular modeling of aliphatic polyesters and polyamides suggested [61] that both classes of polymers may be capable of forming these IC s. For example, it was suggested that poly(e- caprolactone) (PEC) chains in either the all - trans or kink (g + tg+) conformations are slim enough to fit in these narrow IC channels (D = 5.5 A). Preliminary studies of its stability, stoichiometry, and structure, both the three - dimensional, solid - state structure of the PEC - U- IC and the conformation adopted by the included PEC chains have been reported [58],... 

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