Organometallic zwitterions

In summary, the organometallic zwitterionic system [Co (t -C5H4COOH)-(t -C5H4COO)] behaves as a reversible amphoteric sink towards a variety of hydrated vapours of acids and bases. 

The same organometallic zwitterion [Co" (ri -C5H4COOH)(tx -C5H4COO)] used successfully in the solid-solid reactions described above has also been used in solid-sohd reactions with crystalhne alkaU salts MX (M=K+, Rb+, Cs+, NHJ X=C1", Br", T, PFg though not in all permutations of cations and anions) yielding supramolecular complexes of formula [Co (ri -C5H4COOH)-(tx=-C5H4COO)]2 M+X- [67]. 

Fig. 9 The organometallic zwitterion [Co (r -C5H4COOH)(ri -C5H4COO)] reacts quantitatively as a solid polycrystalline phase with a number of crystalline alkali salts MX (M= K+, Rb+, Cs+, NHJ X=Ch, Br, h, PF ) forming supramolecular complexes of formula [Co -(q -C5H4COOH)(q -C5H4COO)]2 M+X. The example shows the adduct obtained from the reaction with KBr...

IV-Methylpyrrole with (Cp IrH3)2 and 3,3-dimethyl-1-butene gives a couple of unique organometallic products, 86 and 87 (990M134). In 86, the C—H bond in position 2 is activated and a rare tiVC) ti (C=C) coordination mode is realized. Species 87 is a zwitterionic compound containing a triple bond between the iridium atoms. 

Organometallic chemistry of pyrrole is characterized by a delicate balance of the ti N)- and -coordination modes. Azacymantrene is an illustration of the considerable nucleophilicity of the heteroatom. However, azaferrocene can be alkylated at C2 and C3 sites. Ruthenium and osmium, rhodium, and iridium chemistry revealed the bridging function of pyrroles, including zwitterionic and pyrrolyne complex formation. The ti (CC) coordination of osmium(2- -) allows versatile derivatizations of the heteroring. 

Many zwitterionic organometallic compounds (betaines) are known in which positively and negatively charged centers of different nature are separated by various spacer groups. Substances of this type always exhibit the pronounced specific physical properties and high and unusual reactivity. Several types of organometallic betaines have been reviewed recently.1 4... 

Figure 4 Crystal structure of a zwitterionic chlorozirconacarborane sandwich complex. Reproduced by permission of the American Chemical Society from Organometallics 1995, 14, 1365. 

Contrary to all other reactions with acids the behaviour of the zwitterion towards vapours of formic acid is intriguing, since no proton transfer is observed. The formic acid vapour uptake generates a material composed of pairs of zwitterion molecules, linked by O-H- - O bonds between the protonated -COOH and the deprotonated -COO() groups [0---0 separation 2.526(4) A] residing on the organometallic moiety. The zwitterion dimers interact with two formic acid molecules via 0-H---0 and C-H---0 hydrogen bonds [0---0 distance 2.541(4),... 

Synthetic routes include anionic, cationic, zwitterionic, and coordination polymerization. A wide range of organometallic compounds has been proven as effective initiators/catalysts for ROP of lactones Lewis acids (e.g., A1C13, BF3, and ZnCl2) [150], alkali metal compounds [160], organozinc compounds [161], tin compounds of which stannous octoate [also referred to as stannous-2-ethylhexanoate or tin(II) octoate] is the most well known [162-164], organo-acid rare earth compounds such as lanthanide complexes [165-168], and aluminum alkoxides [169]. Stannous-2-ethylhexanoate is one of the most extensively used initiators for the coordination polymerization of biomaterials, thanks to the ease of polymerization and because it has been approved by the FDA [170]. 

Two basic approaches have been taken. The first consists in grafting organometallic donor and acceptor groups, such as ferrocene [as in [84] (Calabrese et al, 1991)] ruthenium derivatives [as in [83] (Whittall et al, 1996)] and tungsten carbonyl, instead of their organic counterparts on tt-conjugated chains. Quite successful in this respect, although not truly org nometallic, are zwitterions based on borate donors and ammonium acceptors [86] (Lambert et ah, 1996) and Lewis acid complexation as in [85] (Kammler et al, 1996). 

Ray reviews some recent developments concerning the design of novel materials with large NLO effects. He considers a series of organic salts and various organometallic derivatives where it was found that metal-to-ligand charge-transfer has a dominant contribution to the second-order NLO response. He also discusses the first hyperpolarizability of several retinal derivatives, ionic octupolar molecules and zwitterionic derivativatives. Solvent effects on NLO properties are also reviewed. 

Therefore, some conclusions have been generally accepted and have been summarized as follows the cycloaddition reaction is a stepwise reaction rather than a concerted one the reaction is initiated by nucleophihc attack of an imine to a ketene, giving rise to a zwitterionic intermediate a conrotatory eleclrocyclic ring-closure of the zwitterionic intermediate produces the final 2-azetidone product [85], As the stereochemistry of the structure of the P-lactams strongly affects their biological activity, the stereoselectivity of the process must be carefully considered. Uncatalysed as well as catalysed processes have been reported organometallic and organic catalysts have been utilized in procedures oriented to the syntheses of enantiopure P-lactams [90-92],... 

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