Heteroatomic structures

As well as sulfur, macrocycles containing other large donors such as tertiary phosphorus or arsenic atoms are also known, although the metal-ion chemistry of such ligands has been somewhat less explored. In part, this reflects the synthetic difficulties often encountered in the preparation of ligands containing these heteroatoms structures (49) (Horner, Walach Kunz, 1978), (50) (Kauffmann Ennen, 1981), and (51) (Mealli etal., 1985) provide three representative examples of such macrocycles. 

In polysilane polymers, the polymer backbone is made up entirely of silicon atoms. Therefore these materials differ from other important inorganic polymers, the siloxanes and phosphazenes, in which the polymer chain is heteroatomic. Structurally, they are more closely related to homoatomic organic polymers such as the polyolefins. However, because the units in the main chain are all silicon atoms, the polysilanes exhibit quite unusual properties. The cumulated silicon-silicon bonds in the polymer chain allow extensive electron delocalization to take place, and this delocalization of the sigma electrons in the Si-Si bonds gives the polysilanes unique optical and electronic properties. Many of the potential technical uses, as well as the remarkable properties, of polysilanes result from this unusual mobility of the sigma electrons. 

Although less abundant there is also a significant number of ions which have four heteroatoms, one sulfur and three oxygens. The main series of peaks are at HD = 9 and 14. The first group is quite typical while the second starts at a relatively high carbon count which may be an example of a combination of aromatics as previously described. Finally, a small number of ions were observed to have one of each heteroatom. Structural assignments would be very speculative at this time. 

Figure 23. Chemical structure of basic functional groups. Both heteroatomic structures are required to explain the neutralization reactivity.

In an analysis of analogous data for pyridazine derivatives, i.e., 3,6-disubstituted pyridazines (63), where Z is a reaction site, both situations with respect to each nitrogen heteroatom (structures 64 and 65) can be considered separately and their effects can be regarded as additive. 

From the investigation into asphaltene chemistry, it is obvious that paraffin side chains can be readily cracked during the thermal treatment of pure bitumen at a relatively low temperature. Aromatic and heteroatom structures are inclined to polycondensation reactions and finally to coke formation. Especially interesting are bridge-ring structures since these structural elements will only be cracked either at high temperature or by the addition of plastics to the feedstock. 

The simplest crystal structure built of four-bonded atoms is that of diamond. Each carbon atom has four shortest distances to its neighbors. Each atom has four valence electrons, which allows interpretation of all the short contacts as two center-two electron (2c-2e) bonds. The simplest heteroatomic structure built the same way is the... 

The question of which resonance structure is the principal contributor has been a point of considerable discussion. Since the nonpolar ylene resonance structures have 10 electrons at the phosphorus of sulfur atom, these structures imply participation of d orbitals on the heteroatoms. Structural studies indicate that the dipolar ylide structure is probably the main contributor. Molecular orbital calculations confirm the stabilizing effect that the second-row elements phosphorus and sulfur have in ylides, relative to the corresponding first-row elements nitrogen and oxygen. ... 

One Other Heteroatom.—Structure (112) and larger-ring homologues have been synthesized by acyloin reaction of the diesters (113), and the phosphepin (115) has been reported as the unexpected major product of thermolysis of (114) in an interesting cyclodephosphonation reaction the expected product (116) was formed in 10 % yield. Several interconversions at Si in the system (117) have been examined and the borepanes (118) are suggested intermediates in the conversion outlined in Scheme 1. ... 

Figure 22. Equivalence of hydrocarbon with heteroatomic structures.

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