Complexation of Halocarbons

Clearly, the fact that the smaller cryptophane-C is selective for CH2CI2 is not a qnestion of a match between the host s small cavity and the guest s size after all. The enthalpy of the interaction of CHCI3 with this host is much more significant, despite its small size. However, CHCI3 binding 

5 per cent enantiomeric excess and, in conjunction with circular dichroism data, gave the optical rotation (Box 6.2) of the guest [ ] = 1.6 0.5°, a piece of information that had been sought for a century. 

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Complexation of halocarbons with natural substances can enhance the rates of photoreactions that provide sinks. Ionizable halocarbons, such as hal-ogenated organic carboxylic acids, potentially could form complexes with pho-toreactive transition metals, such as iron. In addition, dissolved NOM and sediments are known to sorb or bind ionic and nonionic halocarbons, and sorbed halocarbons may photoreact more efficiently (eq 7). 

The addition of halocarbons (RX) across alkene double bonds in a radical chain process, the Kharasch reaction (Scheme 9.29),261 has been known to organic chemistry since 1932. The overall process can be catalyzed by transition metal complexes (Mt"-X) it is then called Atom Transfer Radical Addition (ATRA) (Scheme 9.30).262... 

In contrast to the dihalogens, there are only a few spectral studies of complex formation of halocarbon acceptors in solution. Indeed, the appearance of new absorption bands is observed in the tetrabromomethane solutions with diazabicyclooctene [49,50] and with halide anions [5]. The formation of tetrachloromethane complexes with aromatic donors has been suggested without definitive spectral characterization [51,52]. Moreover, recent spectral measurements of the intermolecular interactions of CBr4 or CHBr3 with alkyl-, amino- and methoxy-substituted benzenes and polycyclic aromatic donors reveal the appearance of new absorption bands only in the case of the strongest donors, viz. Act = 380 nm with tetramethyl-p-phenylendiamine (TMPD) and Act = 300 nm with 9,10-dimethoxy-l,4 5,8-... 

Water-soluble transition-metal complexes have been used recently for transfer hydrogenolysis of halocarbons. Paetzold and Oehme [110] have realized the reductive dehaiogenation of allyl or benzyl halides in the presence of [(phosphine) 2PdCl2] complexes with sulfonated phosphines as ligands (e.g., Ph2P(CH2)3S03K) by... 

Electrochemical methods are available for the direct dehalogenation of organic halides to a limited extent fluorides and monochlorides are generally not reducible [1], In the presence of transition-metal complexes as mediators (Med), however, the electrolysis of halocarbons (RX) can be performed more effectively and selectively under various conditions [155-158]. Mediated electroreduction is most efficient when the electron transfer step E° (Med/Med -) is more negative than E° (RX/RX -) [157] (cf. Section 18.4.1). 

Coordinatively unsaturated complexes and those giving easily such species by ligand dissociation favor pathways related to that described in Eqs. (10) and (13). Coordinatively saturated complexes reduce halocarbons via outer-sphere ET [193, 194]. In cases of electrochemical dehalogenations, the species formed by one-electron reduction of the mediators on the cathode often react in this way [156, 157, 198], For example (Eq. (14)) [157, 166] ... 

This binuclear photooxidative addition reaction is general for a number of halocarbons (Figure 3). While DCE and 1,2-dibromoethane react cleanly to give the dihalide metal dimers and ethylene, substrates such as bromobenzene or methylene chloride react through an alkyl or aryl intermediate. This intermediate reacts further to yield the dihalide d2-d2 metal complexes. 

The study of pyridinophanes resulted in joint publications on the complexation of water and the encapsulation of halocarbons within pyridino-crown hosts. Additional professional exchanges occurred over the years in which both groups were pursuing mutual synthetic interests in heterocyclic chemistry, stereochemistry, and cyclophanes. Central to those interests was a better understanding of molecular inclusion and recognition phenomena, now more uniquely defined as an aspect of supramolecular chemistry. 

Photochemical oxidation of square-planar bis(l,2-dithiolene) complexes of Ni, Pd, and Pt is by no means limited to IPCT excitation. Photooxidation also occurs in halocarbon solvents. In 1982, two separate reports addressed the photochemistry of metal bis(l,2-dithiolene) complexes. Vogler and Kunkely (80) investigated complexes of the type M(S2C2R-2)2, where M = Ni, Pd, Pt,... 

ESR spectroscopy provided evidence for the radical ion of the oxidized tdt ligand, but the metal complex cation was not isolated nor were the products of halocarbon reduction identified. Interestingly, the related complex Ni(phen) (S2C2Ph2) was reported to undergo similar photooxidation when irradiated at higher energy, but not when irradiated in the low-energy CT band (118). 

In the earliest authentic halocarbon complex (1982), o-diiodobenzene was found to chelate to cationic Ir(III) as shown in diagram (5)." An earlier proposed example proved to be misidentified when the crystal structure was carried out. To be stable, any such complex must resist oxidative addition, hence the use of an oxidation state, Ir(III), that is only oxidized with difficulty. The normally rather weakly basic halocarbon lone pairs are often reluctant to bind, but chelation and involvement of the least electronegative hahde, iodine, favor binding as does the cationic character of the complex. A series of such complexes was soon found, including complexes of fr(I)" and a series of weakly bound dichloromethane complexes for certain systems." These solvento complexes can be very labile and so find use as precursors for binding of other weakly basic hgands. Even fluorocarbon complexes proved viable." A review of the area is available. It now seems... 

Methylarsines and methylstibines are subject to a number of reactions such as oxidation, quaternization and complex formation, which could facilitate or inhibit their dispersal in the environment . It has been reported that environmentally important concentrations of halocarbons (Mel, MeBr and MeCl) are produced naturally and accumulate in the oceans and the atmosphere. Parris and Brinckman reported quantitative measurements of the rate of quaternization of trimethylstibine and trimethylarsine by alkyl halides in polar solvents. 

U represents the scalar irradiance, ex is the molar absorptivity (or molar extinction coefficient) of the chromophore, l is the light path length, and [P] is the concentration of the chromophore that initiates the photoreaction (e.g., the halocarbon itself, a natural substance, or a complex of the halocarbon with a natural substance). The rate of light absorption depends on the spectral overlap between the light source and the spectrum of the chromophore that initiates the photorcaction. 

Evidence is presented here that sorption to NOM enhances the photoreaction rates of halocarbons. Possible enhancement mechanisms are photoinduced electron transfer involving photoejected electrons or NOM excited states and/or formation of photoreactive complexes with NOM-associated electron donors, such as nitrogen bases. 

The overall photoreaction rate of a given halocarbon in a certain aquatic environment is the sum of the rates of the direct photoreactions of the uncom-plexed halocarbon, indirect photoreactions involving reactive transients that are produced by natural substances, and photoreactions of halocarbon complexes. After first discussing the effects of sorption on photoreaction kinetics, we then discuss these various reaction pathways in more detail. 

Ionic halocarbons, including halogenated carboxylic acids, may form pho-toreactive complexes with transition metals in the aquatic environment. Indeed, complexes of carboxylates with dissolved Fe(III) and iron oxides are very photoreactive under solar radiation (28, 29). Photoreactions of such complexes may help to explain the enhanced photoreactivity of chlorinated acetates in natural water samples. 

The enhanced photoreactivity of sorbed nonionic halocarbons may involve photoreactive complexes with amines and other electron-donating substances. The enhanced photoreactivity of ionic halocarbons (e.g., chloroacetates) may involve complexes with DOM and transition metals. Additional studies are needed to examine the role of complexation in the aquatic photochemistry of halocarbons. 

Natural chromophores that participate in indirect photoreactions or complexation tend to be highest in concentration in aquatic environments that are most biologically productive. Thus, the highest rates of indirect photoreactions of halocarbons are likely to be in fresh waters, coastal waters, and upwelling regions of the sea. Field studies looking for halocarbon sinks are most likely to be rewarded by focusing on these aquatic environments. 

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