Monomeric kinetic stabilization

The heterocycles can be cleaved by reaction with 4-(dimethylamino)pyri-dine, yielding Lewis base-stabilized monomeric compounds of the type dmap—M(R2)E(Tms)2 (M = Al, Ga E = P, As, Sb, Bi). This general reaction now offers the possibility to synthesize electronically rather than kinetically stabilized monomeric group 13/15 compounds. These can be used for further complexation reactions with transition metal complexes, leading to bimetallic complexes of the type dmap—M(Me2)E(Tms)2—M (CO) (M = Al, Ga E = P, As, Sb M = Ni, Gr, Ee). 

The kinetically-stabilized complexes of the cage ligands normally yield redox reagents free of the exchange problems often associated with simple complexes. Indeed, the redox chemistry of the complexes shows a number of unusual features for example, saturated cages of the type mentioned in Chapter 3 are able to stabilize rare (monomeric) octahedral Rh(n) species (d7 electronic configuration) (Harrowfield etal., 1983). In a further study, radiolytical or electrochemical reduction of the Pt(iv) complexes of particular cages has been demonstrated to yield transient complexes of platinum in the unusual 3+ oxidation state (Boucher et al., 1983). 

There are three anions that may loosely claim to be nitrides. Pentazolides (salts of cyclic N ) will all be explosive. Some azides (salts of N3) fall just short of being explosive but all are violently unstable. The true nitrides, nominal derivatives of N3-, are more various. In addition to some ionic structures, there are polymeric covalent examples, and some monomeric covalent ones, while most of those of transition metals are best considered as alloys. Several are endothermic and explosive, almost all are thermodynamically very unstable in air with respect to the oxide. Many are therefore pyrophoric if finely divided and also may react violently with water and, more particularly, acids, especially oxidising acids. A few are of considerable kinetic stability in these circumstances. There is no very clear classification of probable safety by position in the periodic table but polymeric and alloy structures are in general the more stable. Individual nitrides having entries ... 

In 1994, Weidenbruch et al. reported that introduction of two 2,4,6-tri-tert-butylphenyl groups onto the tin atom led to the successful isolation of 167, which is the first example of kinetically stabilized monomeric diarylstannylene characterized in the solid state. Note that Sn NMR spectrum of 167 shows two signals at 961 and 1105 ppm instead of a singlet expected at room temperature in solution. The striking feature of 167 is its partial isomerization into a sterically less encumbered stannylene 168 in solution (Scheme 14.73). The two signals of 167 coalesce... 

Soon after the isolation of 136, Tokitoh et described the synthesis of the first kinetically stabilized diarylstannylene stable in solution, that is, Tbt(Tip)Sn (169), by treatment of TbtLi with stannous chloride followed by addition of TipLi (Scheme 14.74). Under an inert atmosphere, stannylene 169 was found to be quite stable even at 60 °C in solution, and it showed a deep purple color (A,max =561 nm) in hexane. The Sn NMR spectrum of 169 showed only one signal at 2208 ppm, the chemical shift of which is characteristic of a divalent organotin compound as in the case of a monomeric dialkylstannylene (136). The bandwidth and the chemical shift of 169 were almost unchanged between —30 and 60 °C, indicating the absence of a monomer-dimer equilibrium. 

A similar dilithium reagent derived from l,l-bis(trimethyl silyl)ethylene has been used for the protecting ligand of the first monomeric stannylene (equation 8) and later, of various kinetically stabilized species (vide infra). Cyclooctatetraene undergoes l,4-(hsilylation as shown below (equation 9). 

Both monomeric oxo- and thioxosilanes, X2Si=0 and X2Si=S, respectively, contain silicon in a trigonal-planar coordination. Their strongly polarized silicon-chalcogen double bonds, Si =0 and Si =S , render them kinetically unstable unless efficient protecting substituents X are present at silicon. The first kinetically stabilized thioxosilane, stable at ambient temperature, has recently been described. Stable oxosilanes have, however, not been isolated to date, and their synthesis continues to challenge experimentalists. ... 

Individual examples of monomeric silanols such as triethylsilanol [10] and phenylsilanetriol [II] have been prepared under regimens different from those used for surface treatments and they exhibit extended stability. To date, no monomeric silanetriols have been isolated from aqueous hydrolysates of alkoxysilanes. The kinetics of silanetriol condensation have been studied [12]. The conditions which promote the hydrolysis of alkoxysilanes also promote condensation of silanols the persistence of monomeric silanetriols for more than a few hours in typical solutions is unlikely. However, the persistence of silanols in reaction mixtures containing condensed structures have been observed empirically [ 13] and by JVSi-NMR [ 14,15],... 

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