7t-Acceptors

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary TT-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controUable molecular shuttles (6,236). The TT-donors in the shuttles are located in the dumb-beU shaped component of the rotaxane and the 7T-acceptors in the macrocycHc component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

Carbon monoxide [630-08-0] (qv), CO, the most important 7T-acceptor ligand, forms a host of neutral, anionic, and cationic transition-metal complexes. There is at least one known type of carbonyl derivative for every transition metal, as well as evidence supporting the existence of the carbonyls of some lanthanides (qv) and actinides (1) (see AcTINIDES AND THANSACTINIDES COORDINATION COMPOUNDS). 

Eor the selective pre-concentration of deactivated phenols a new silica-based material with the grafted 2,3,5-triphenyltetrazole was proposed. This method is based on the formation of molecular chai ge-transfer comlexes of 2,3,5-triphenyltetrazole (7t-acceptor) with picric acid (7t-donor) in the phase of the sorbent. Proposed SPE is suitable for HPEC analysis of nitrophenols after their desorption by acetonitrile. Test-system for visual monitoring of polynitrophenols under their maximum concentration limits was developed using the proposed adsorbent. 

An example of a 7t-acceptor group is the formyl group as in acrolein. 

Pyrazole Equivalent 1. 7T-Acceptor group prefers 3-position 2. Acceptor group (NH3", CF3) prefers 5-... 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

It can be determined from the higher effect of the p-substitution compared with the 7-substitution and the high donor ability of the stilbene (ECT = 200 kJ mol-1 x(HOMO) = 0.504 qa = qp = 1.000), that an even electron distribution in the n-system of the donor causing a high electron density in the vicinity of the monomer double bond is important for the strength of the EDA interaction between 71-donor and 7t-acceptor. 

Padilha LA, Webster S, Przhonska OV, Hu H, Peceli D, Ensley TR, Bondar MV, Gerasov AO, Kovtun YP, Shandura MP, Kachkovski AD, Hagan DJ, Van Stryland EW (2010) Efficient two-photon absorbing acceptor-7t-acceptor polymethine dyes. J Phys Chem A, Submitted... 

A strong jr-donor therefore stabilizes the tram coordination of 7T-acceptors by a push-pull effect illustrated in transmission path F. Further evidence for such push-pull effects is found in the static or dynamic tram effects observed with metallopor-phyrins that carry dinitrogen, nitrosonium ion, or dioxygen. Table 11 displays (inter alia) some properties of dinitrogen or nitrosylosmium porphyrins. 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

The planarity of the tricoordinate phosphorus and thus the aromaticity of 4 can be influenced by substituents. It turns out from ab initio calculations that 7T-acceptor groups either at phosphorus or at the neighboring carbon have a planarizing effect. The barrier to planarity decreases to 1.56 kcal/mol (MP2/ 6-31G(d)) as a result of —BH2 substitution at phosphorus. The bond length alternation in the planar form of this substituted derivative increases in the... 

For instance, ti-donor substituents R in [(CO)5Cr=(C)xR2] should stabilize complexes with an odd number of carbon atoms x and, conversely, complexes with an even number of carbon atoms should be stabilized by 7t-acceptor substituents [11]. 

In this chapter, we first analyzed the electronic structures of metal vinylidene and allenylidene complexes. The electronic structures allow us to understand the reactivities of these complexes. For metal vinylidene complexes of the Fischer-type, nucleophilic attack usually occurs at the a-carbon and electrophilic attack at the P-carbon. For the corresponding metal allenylidenes, electrophilic attack occurs at the P-carbon and/or the metal center. Then we briefly reviewed the theoretical study of the barriers ofrotation ofvinylidene ligands in various flve-coordinate complexes M (X) C1(=C=CHR)L2 (M = Os, Ru L = phosphine). The study showed that 7t-acceptor ligands (X), electron-withdrawing substituents and lighter metals gave smaller barriers. 

Baker Bakerbond Chiral Ionic DNBPG Brush , 7t-acceptor (R)-DNB-phenylglycine... 

It should be noted that, more recently, two different, also highly efficient template catenane syntheses have been developed one of them, first introduced by Stoddart and coworkers [70], is based on the n donor-7t acceptor gathering effect between aromatic nuclei, whereas in the second, mainly developed by Hunter and Vogtle and coworkers, gathering and orientation of the various subunits of the future catenane are induced by hydrogen bonding [71-73]. 

Molecular nitrogen. Ni- is isoelectronic with both carbon monoxide and the nitrosyl ion but, despite the numerous complexes of CO and NO, for many years it proved to be impossible to form complexes of dinitrogen This difference in behavior was usually ascribed to the lack of polarity of N2 and a resultant inability to behave as a 7T acceptor.49... 

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