Dimeric structures compounds

At first, the dimeric nature of the base isolated from 3-ethyl-2-methyl-4-phenylthiazolium was postulated via a chemical route. Indeed the adduct of ICH, on a similar 2-ethylidene base is a 2-isopropylthiazolium salt in the case of methylene base it is an anilinovinyl compound identified by its absorption spectrum and chemical reactivity (45-47). This dimeric structure of the molecule has been definitively established by its NMR spectrum. It is very similar to the base issued from 2.3-dimethyl-benzo thiazolium (48). It corresponds to 2-(3 -ethyl-4 -phenyl-2 -methylenethiazolinilydene)2-methyl-3-ethyl-4-phenylthiazoline (13). There is only one methyl signal (62 = 2.59), and two series of signals (63= 1.36-3.90, 63= 1.12-3.78) correspond to ethyl groups. Three protons attributed to positions T,5,5 are shifted to a lower field 5.93, 6.58, and 8.36 ppm. The bulk of the ten phenyl protons is at 7.3 ppm (Scheme 22). 

In the case of phenyllithium, it has been possible to demonstrate by NMR studies that the compound is tetrameric in 1 2 ether-cyclohexane but dimeric in 1 9 TMEDA-cyclohexane. X-ray crystal structure determinations have been done on both dimeric and tetrameric structures. A dimeric structure crystallizes from hexane containing TMEDA. This structure is shown in Fig. 7.1 A. A tetrameric structure incorporating four ether molecules forms from ether-hexane solution. This structure is shown in Fig. 7.IB. There is a good correspondence between the structures that crystallize and those indicated by the NMR studies. 

In solution the compounds exhibit solvent-dependent colours in dilute solution in non-polar solvents, planar monomers are present but in more concentrated solutions oligomerization occurs. In the solid state a dimeric structure has been identified (X-ray, Figure 2.30) with R = Ph there is a staggered configuration (Rh-Rh 3.193 A) but with other isocyanides (R = 4-FC6H4) the configuration is eclipsed. 

Consequently, due to preferred cis-cis orientation a dimeric structure is observed for the indium complex and an unprecedented cis-trans arrangement in the thallium structure leads to a polymeric aggregate. Further N-NMR spectroscopic studies show that the aluminum and gallium complexes are stable contact ion pairs even in solution whereas the indium and thallium compounds are solvent-separated ion pairs in THE solution. 

Trimethylsilyl ylide 95 reacts with PCI3 or AsClj to give respectively the corresponding halophosphorus yhde 96 and the first example known of haloarsanyl ylide 97 [116]. In various stoichiometric conditions the 1 1 condensation of 95 with ASCI3 is realized to give haloarsanyl ylides with either a dimeric structure (diarsenate 98) or trimeric and tetrameric cationic compounds 99 and 100 (Scheme 29). 

Figure 6 Molecular structure of a dimeric zinc compound with a bridging phosphodiester formed with...

A wide variety of activated olefins (126) undergo reductive electrochemical dimerization to compounds of structure 127 (electrolytic hydrodimerization) 129 i. While the product 127 is capable of existing in either dl or meso modifications, relatively little attention has been paid to the stereochemistry of hydrodimers... 

Figure 24 Dimeric structure of dialkyldichalcogenocarbamate compounds [M(E2CNR2)2] (M = Zn, Cd ...

To investigate this dendritic effect, a dimeric model compound was synthesized which mimics the tethered relationship of two catalytic units within one branch of the PAMAM dendrimer. All dendritic catalysts were more active in the HKR than the parent complex. Furthermore, the dendritic catalysts also displayed significantly higher activity than the dimeric model compound. The authors proposed that this positive dendritic effect arises from restricted conformation imposed by the dendrimer structure, thereby creating a bigger effective molarity of [Co(salen)] units. Alternatively, the multimeric nature of the dendrimer, may lead to higher order in productive cooperative interactions between the catalytic units. 

R = Me, Bu. Compounds of the series with 244 X = / -Me, j9-NH2, / -Br are monomeric. Compounds of the series with X groups in orffro-positions show dimeric structure and have heptacoordinated Sn atoms. The great variation of bond strength in these compounds can be appreciated from the bond lengths in the dimeric case with R = Bu and X = 0-Cl ... 

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