Neutral molecules complexation

Complexes are also formed in certain instances between neutral molecules and macrocyclic receptors. Neutral molecules which form such complexes for the most part contain polar O—H, N—H or C—H bonds, and hydrogen bonding interactions are responsible for the solid state structural characteristics of these complexes. For many of these complexes, stoichiometries range considerably, from 1 1 to 1 6 host guest, and include a variety of odd ratios such as 3 2, 2 7, etc. Structural results for these complexes indicate them not to be of the inclusion type in a majority of cases. Thus, discussion in this subsection will be limited to a general overview. A more complete review of neutral molecule complexation can be obtained elsewhere.21... 

In this chapter the scope and limitation of crownophanes are reviewed, focusing on supramolecular functions as ionophores and receptors. Most are designed for the recognition of metal cations and some organic cations, and done partly for neutral organic materials. Their structures are classified into five groups from benzene nuclei to heteroaromatic nuclei, hi the individual section their supramolecular properties are summarized and the properties are mainly focused on metal ion-binding and chiral and/or achiral neutral molecule complexation. Rotaxanes and catenanes are also included in this chapter. Their preparation almost always accompanies some supramolecular complexation. Some typical and important crownophanes are summarized in Table 1. 

It is advantageous if the laser system pemiits rotation of the optical polarization. Detached electrons correlated witii different final electronic states of the neutral molecule will generally be emitted with different angular distributions about the direction of polarization. Measurement of the angular distribution helps in the interpretation of complex photoelectron spectra. The angular distribution/(0) of photoelectrons is [50]... 

The existence of half-salt complex cations formed by the association of an ammonium cation B", with a neutral molecule, B, has also been postulated. 

A chelate compound may be either a neutral molecule or a complex ion associated with the appropriate counterions to produce electroneutraUty. 

It should probably also be noted before closing this section that 18-crown-6 and some of its aliphatic relatives have been found to form quite a variety of complexes or solvates with neutral molecules. We have noted the formation of an acetonitrile sol-vate . Although the other complexes are generally beyond the scope of the present work, we wish to call attention to their existence so that the worker attempting to improve purification procedures will check the literature for his particular method. This is especially important since a number of the solvates are found, to our knowledge, only in the patent literature. 

Tin(II) chlorides are similarly complex (Fig. 10.5). In the gas phase, SnCh forms bent molecules, but the crystalline material (mp 246°, bp 623°) has a layer structure with chains of comer-shared trigonal pyramidal SnClsl groups. The dihydrate also has a 3-coordinated structure with only I of the H2O molecules directly bonded to the Sn (Fig. I0.5c) the neutral aquo complexes are arranged in double layers with the second H2O molecules interleaved between them to form a two-dimensional H-bonded network... 

An X-ray study of this latter complex reveals a C2v anion as in the chlorine analogue (Fig. 17.28a). This is closely related to the Cs structure of the neutral molecule F3IO (Fig. 17.28b). 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

Here the ligand (L) can be either a neutral molecule or a charged ion, and successive replacement of water molecules by other ligand groups can occur until the complex ML, is formed n is the coordination number of the metal ion and represents the maximum number of monodentate ligands that can be bound to it. 

Outer sphere coordination of organic molecules to electrically neutral metal complexes. V. M. Nek-ipelov and K. I. Zamarawu, Coord. Chem. Rev., 1985,61,185 (136). 

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