Ji-Complexes

Antimony(V) chloride forms colored Ji-complexes with double bond systems. 

Studies of the relative rates of the zinc chloride-catalysed bromination of alkyl-and halogeno-benzenes in nitromethane at 25 °C have lead to the suggestion that the rate-determining step of the reaction is formation of Ji-complex, since low substrate selectivity was found to be coupled with high (i.e. normal) positional selectivity323. Under some conditions (column 1 in Table 75) the low selectivity... 

On the basis of all these experiments various mechanisms have at some stage been advanced for the Fries rearrangement involving the free acylium ion or as a tightly bound ion pair, Ji-complexes and cyclic intermediates. It is clearly impossible to reconcile all the experimental data by one reaction mechanism. It is probable that many such mechanisms are possible, each one operative under a certain set of conditions. 

Quantitative calculations of the IMECs of the C=C stretching and C-H bending bands confirmed this trend (Table 1). Furthermore, for Ca2+ and Mg2+ cations the IMEC values exceed those for the free molecules, while for the Na+ ions there is little effect. This indicates a stronger polarization of the C=C bond in ji-complexes of propene with bivalent than with monovalent cations. As follows from Table 1 the IMECs of the C-H stretching vibrations of propene adsorbed by different cations are strongly decreased in comparison with the free molecule. The ratio of the IMECs for C=C and C-H stretching bands is increased for propene adsorbed by Mg2+ cations in comparison with the ratio obtained for the free molecule. 

Scheme 1.5. Frontier orbital interactions in ji-complexation, hydrometallation, and carbometallation.

The first instance involves the Pt(II) fragment "TpPdMe," used not as a catalyst, but rather as a protecting function for alkynes during catalytic, and indeed stoichiometric, processes, a role that followed from the noted stability of TpPtMe(r 2-RC=CR) complexes, and their capacity to release the alkyne by carbonylation.56 63 Thus, ji-complexes with a series of bis (amide)acetylenes (144-149, Scheme 12, Section III.B.l), formed from the polymeric TpPtMe (126), could be subjected to conditions of catalytic hydrogenation, or basic hydrolysis of the pendant functions, without... 

The Ji-complexes formed between chromium(O), vanadium(O) or other transition metals, and mono- or poly-fluorobenzene show extreme sensitivity to heat and are explosive [1,2], Hexafluorobenzenenickel(O) exploded at 70°C [3], and presence of two or more fluorine substituents leads to unstable, very explosive chromium(O) complexes [1]. Apparently, the aryl fluorine atoms are quite labile, and on decomposition M—F bonds are formed very exothermically. Laboratory workers should be wary of such behaviour in any haloarenemetal Ji-complex of this type [1]. However, in later work, no indications of explosivity, or indeed of any complex formation, were seen [4]. Individually indexed compounds are ... 

To prepare other 7i-complexes (Sec. 2.2.4.1), the methods of ligand exchange and especially direct gas-phase synthesis are widely used, which will be discussed in Secs. 3.2.1 and 3.4.1 below. For ji-complexes of the radioactive elements, see Section 5.3. 

The ligand-exchange method is the most used method in the synthesis of ji-complexes. The examples of such compounds, obtained from metal carbonyls and cyclic unsaturated hydrocarbons, are represented in a classic issue [11], Among these ligand-exchange syntheses, we mention here the transformations (3.119)—(3.121) ... 

This synthetic method is applied for obtaining aromatic [11] and heteroaromatic [5,292] Ji-complexes. Thus, the replacement of carboxyl groups leads to the following ri6-jr-Ar-metal carbonyl complexes ... 

The first representatives of r 5(ji)-complexes of azoles (compare with Ref. 5) - mixed ligands coordination compounds of the type 711 - were obtained via potassium salts of C-substituted pyrazoles (3.147) with high yields ( 70%) [313] ... 

The same synthetic approach was used in classic syntheses of r (ji)-complexes of various metals [11]. Such syntheses are carried out in two steps preparation, for example, of cyclopentadienyl derivatives of alkali metals with their further application in the syntheses (3.148), (3.149) of sandwich ri5-complexes of mainly transition metals ... 

Among the electrochemical syntheses related to the change of metal oxidation number, we emphasize obtaining acetylacetonates of divalent iron, cobalt, and nickel [551,623]. The method of alternating-current electrochemical synthesis was applied to isolate Ji-complexes of monovalent copper with allylamines, allylimines, and ally-lurea from the salts of divalent copper [624-628], We note that the same method was used for preparation of analogous ji-complexes with copper(II) halides (X = Cl, Br) [629a]. Other electrochemical syntheses with participation of metal salts and complexes are described in monographs [201,202] and literature cited therein. 

We note that Ji-complexes with blockading methylsubstituents and electron-donor R fragments can also be obtained by ligand-exchange reactions with the use of metal carbonyls, for instance (4.14) [15,72-74] ... 

M. Herberhold, Metal-Ji-Complexes, Vol. II, Complexes with Monoolefinic Ligands, Part I. Elsevier, Amsterdam, London, New York, 1972. 

Positively-charged fragments such as [ML,]+, CH , and H + are all strong electrophiles ( superelectrophiles in the extreme sense (13)) towards the Lewis basic H2, but transition metals can uniquely stabilize H2 and other cr-bond coordination by back donation from d-orbitals that main group analogues cannot do. This bonding is then remarkably analogous (14) to the Dewar-Chatt- Duncanson model (15) for ji-complexes (6). 

Complex formation between olefins and Lewis acids has been demonstrated in a number of cases, e.g., isobutene and titanium tetrachloride (66), butene-2 and boron trifluoride (67—69), propylene and aluminum bromide (70), stflbene and various Lewis acids (71), styrene and stannic chloride (72), and in similar systems (73). Monomer-catalyst Ji-complex formation occurs during the polymerization of styrene or a-methyl styrene with chloroacetic acids (74,75). All these complexes are usually very weak and only stable at low temperatures. Evans contends (76) that isobutene and boron trifluoride do not interact because no polymerization occurs in the absence of moisture and therefore he postulates that BF3 HaO is the primary species. This does not rule out the possibility of a weak interaction between isobutene and the Lewis acid. Indeed, Nakana et al. (77) found direct evidence for the existence of boron trifluoride-propene complexes at low temperatures. 

It reacts rapidly with oxygen forming a white polymeric solid with trinitro-fluorenone and trinitrobenzene, very weak Ji-complexes were formed, which appear to be stable only in the solid phase. Tetracyanoethylene does not add at the furan moiety but reacts with the cyclooctatetraene part of the molecule to give 355. The stability of 353 is such that prolonged heating with tributylphosphine has no effect. 

Figure 36 A molecular model of ji-ji complex of a rigid conjugated poly(aryleneethynylene) with SWNT (6,6) and H NMR spectra the complex with HiPco SWNT. (Reprinted with permission from Ref 242. 2002 American Chemical Society)...

A plausible mechanism for the Lewis acid-catalyzed fran -vinylsilylation is shown in Sch. 74. The coordination of a Lewis acid to the triple bond of 112 would form ji-complex 114 and the a-carbon of the vinylsilane would attack the electron-deficient triple bond from the side opposite to the Lewis acid to produce an aluminum ate complex 115 stereoselectively. The migration of the trimethylsilyl group to the aluminate center would afford 113 and regenerate the Lewis acid catalyst. 

Transition Metal Arene Ji-Complexes in Organic Synthesis and Catalysis... 

The alkyl migrates (rearranges) such that an open coordination site moves to a crystallite edge position. Coordination of an ethylene monomer occurs to create a Ji-complex as in eq 3.5. Subsequent addition across ethylene results in the propagating species ... 

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