Halide anion acceptors, 57-58

The tendency of the halogens to form chain-like polyanions that are stabilized by delocalization of the negative charge [15,34] is a basic chemical principle. Donor-acceptor interactions between Lewis-acidic Br2 and halide anions, but also with polyhalides acting as Lewis bases, give rise to the formation of a variety of homo and heteroatomic adducts. The maximum number of atoms in these chains increases with the atomic weights... 

When one, or both, the interactive modules are tridentate, bidimensional (2D) architectures can be formed. A frequently recurring pattern is the (6,3) network (honeycomb structure), which is sometimes formed when onium halides self-assemble with dihalocarbons. Halide anions work as tridentate XB acceptors and occupy the nodes while the dihalocarbons work as bidentate XB donors and form the sides that space the nodes. Such architectures are present in the co-crystals l,4-DITFB/Ph4P+Br , l,4-DITFB/Me4N+r [155], and a,oo-diiodoperfluoroalkanes/K.2.2.2.cKI [128,189]. The less planar the trigonal arrangement around the nodes, the more corrugated the honeycomb structure (Fig. 9). 

Fig. 9 Honeycomb-like architectures formed on self-assembly of halide anions (which work as tridentate XB acceptors and sit at the networks nodes) with 1,4-DITFB (which works as bidentate donor and forms network sides) (A). The angles formed by the XBs around the halide anions determine the corrugation of the honeycomb architecture, a more planar arrangement around the halide anions (as is the case of the iodide anions in adduct l,4-DITFB/Me4P+r (B) with respect to the bromide anions in adduct l,4-DITFB/Ph4P+Br (C)) results in a less corrugated honeycomb architecture...

Complexes of Halide Anions with Aromatic and Olefinic n-Acceptors. . . 142... 

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-... 

However, X-ray analysis of these salts reveals the overall ratio of the acceptor to donor to vary (depending on the counterion and halide) from 4 1 to 1 1 (Table 3), and they show halide anions in close contact with two to four acceptor moieties, as illustrated in Fig. 10. 

In related complexes of bromide and iodide anions with tetracyanoben-zene, the halide anions are also surrounded by four acceptor molecules [24], The coordination of the halides in two of these moieties is similar to that observed in TCP complexes, i.e., the anion is arranged above (or slightly outside) of the ring and forms close contacts with the cyano-bearing carbons. On the other hand, coordination with the third TCB occurs via the unsubstituted carbon, and the halide is positioned far outside the ring in this case The fourth acceptor moiety is hydrogen-bonded to the halide (Fig. 11). 

Fig. 13 a Location of the halide anions above the tetracyanopyrazine (o) and tetracyano-benzene ( ) ir-systems (adapted from [24]) in comparison with b the LUMO shape of the TCP acceptor... 

The lanthanide phthalocyanine complexes, obtained by conventional methods starting from metal salts at 170-290°C and phthalonitrile (Example 26), contain one or two macrocycles for each metal atom [5,6,8,63,82,84-98]. Thus, according to Refs. 6,63, and 85, the complexes having compositions LnPc2H, XLnPc (X- is halide anion), and Ln2Pc3 (a super-complex ) were prepared from phthalonitrile as a precursor the ratio of the reaction products depends on the synthesis conditions and the metal nature. The ionic structure Nd(Pc)+Nd(Pc)2 was suggested [85] and refuted [63] for the neodymium super-complex Nd2Pc3 the covalent character of the donor-acceptor bonds in this compound and other lanthanide triple-decker phthalocyanines was proved by the study of dissociation conditions of these compounds [63]. 

The use of a Lewis acid to promote the abstraction of a halide anion from a metal halide complex, in the presence of a neutral ligand, has been widely employed to produce both substituted and totally unsubstituted metal carbonyl cations. The halide acceptor most generally used is aluminum trichloride, but other Lewis acids have been employed, as indicated below. The requirements for the halide acceptor include that the anion product should be sufficiently large to stabilize the salt formed (15). 

This reaction may be considered analogous to those of Section A,2, in which the acid is the anion acceptor, this time for the hydride rather than the halide anion. 

Allerhand A, Schleyer PvR (1963) Halide anions as proton acceptors in hydrogen bonding. J Am Chem Soc 85 1233-1237... 

Sulfur dioxide is a r-electron-pair acceptor. The standard explanation for the strong ionizing power of SO2 is the formation of an EPD—EPA complex between the halide anion and the sulfur dioxide molecules [148], Table 2-11 summarizes some of the available data concerning the comparative efficiencies of various solvents in promoting the ionization of chloro-triphenylmethane [150],... 

A number of years ago G. N. Lewis extended our understanding of acid-base behavior to include reactions other than proton transfers. According to Lewis, an acid is an electron-pair acceptor and a base is an electron-pair donor. Thus, carbocations are electron-pair acceptors and are Lewis acids. Halide anions are electron-pair donors and are Lewis bases. It is generally true that electrophiles are Lewis acids, and nucleophiles are Lewis bases. 

Processes taking place in ionic melt-solvents are considerably affected by impurities contained in the initial components of the melt or formed during preparation (mainly, melting) of solvents due to the high-temperature hydrolysis of melts or their interactions with container materials (AI2O3, SiOj, etc.) or active components of atmosphere (O2, CO2, etc.). The list of these impurities is wide enough and includes multivalent cations of transition metals, different complex anions (0x0- or halide anions). The effect of the mentioned admixtures on the processes in ionic melts depends mainly on the degree of their donor-acceptor interactions with constituent parts of the melt. 

One of the earliest known organomercury macrocycles was the trimer of oriho-phenylene mercury, (o-C6H4Hg)3, known from X-ray diffraction studies to have a planar cyclic structure, 1 [1, 2]. It has recently been found, with the aid of Hg NMR spectroscopy, that this compound forms complexes with hahde anions (Cl, Br and I ) in dichloromethane solution, but the complexes could not be isolated in solid state [3]. The fluorinated analog (o-C6F4Hg)3, 2, is a much better electron acceptor and forms stable complexes with the same halide anions these could be... 

Tin-containing macrocycles 14-16 use the capacity of the metal atom acceptor to capture halide anions in the cavity, acting like inverse crown ethers [49-51]. Thus,... 

The base, as proton acceptor, deprotonates alkyl halide causing the removal of a halide anion. The reaction proceeds via the transition state in which both C-H and C-Br bonds break simultaneously. Such reactions are called concerted. The reaction is bimolecular because its rate depends on the concentrations of two molecular species, alkyl-bromide and the ethoxy anion. This mechanism is called the E2 elimination mechanism. 

The NARXs possess four spatially-fixed halogen-bond acceptor sites (the four anions) with deep cavities for guest binding. The resorcinarene skeleton and the strong hydrogen bond interactions from the ammonium ions shield the halide anions, so that in the presence of suitable halogen bond donors, the latter interactions can only form either parallel or perpendicular to the upper rim of the NARXs. 

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