Nitrogen Lewis acid-base interactions

Substitution of the dimethylsilyl group by bis(tert-butyl)-stannyl does not change the structure in solution, e.g. 33 is found to be monomeric. A very interesting dimer is 26. In contrast to the centrosymmetrical dimer of 1 (C-Symmetry), 26 has a twofold axis (C2, see Fig. 9). This special structure may be due to intramolecular Lewis acid-base interactions between the boron and nitrogen atoms 39). Nevertheless,... 

The participation of the germanium dimers in nucleophilic/electrophilic or Lewis acid/base reactions has been the subject of several investigations on the Ge(100)-2x1 surface [16,49,255,288,294,313-318]. As for the case of silicon, adsorption of amines has provided an excellent system for probing such reactions. Amines contain nitrogen lone pair electrons that can interact with the electrophilic down atom of a tilted Ge dimer to form a dative bond via a Lewis acid/base interaction (illustrated for trimethylamine at the Si(100)-2 x 1 surface in Ligure 5.17). In the dative bond, the lone pair electrons on nitrogen donate charge to the Ge down atom [49]. 

The hypothesized delocalization of lone pair electrons in the above silicon compounds is supported by the lowered basicity of the silyl compounds as compared to the corresponding carbon compounds. This reduced basicity is contrary to that expected on the basis of electronegativity effects operating through the a system since silicon is less electronegative than carbon. It is consistent with an internal Lewis acid-base interaction between the nitrogen and oxygen lone pairs and empty acceptor d orbitals on the silicon. Experimentally this reduced basicity is shown by the absence of disiioxane adducts with BF3 and BO ... 

In copyrolytic reactions of the aminosilylenes with unsaturated ketones or imines (heterodienes) we mainly obtained isomeric mixtures. The chemo- and regioselectivity of main- and byproducts can be explained with multistep-cycloadditions. We assume a primary Lewis acid-base interaction between the lone electron pair of the heteroatom (oxygen or nitrogen) and the electron gap at silylene, which is followed by a [2+l]-cycloaddition and a radical ring-opening ring-closure reaction. 

Polar compounds (water, oxygen-, nitrogen-, and sulfur-containing compoimds). They react with the organoaluminum co-catalyst (water, alcohols, amines, thiols), complex the aluminum center in a Lewis acid/base interaction (ketones, ethers, thioefhers), or coordinate strongly to the nickel catalytic center (amines, sulfur compounds). They reduce the concentration of the active species and may be responsible of the formation of sludges in the reaction section. 

In the case of solvents with a low solvating power, in which protonated or deprotonated species are not free from counterion effects, spectroscopic and thermodynamic parameters associated with hydrogen bonding or Lewis acid-base interactions can be usefully related to PA or GB. Kamlet and coworkers reported linear relationships for five series of bases with sp - and sp -hybridized oxygen and sp-, sp - and sp -hybridized nitrogen. At the same time, Zeegers-Huyskens presented the relationships " reported in Figure 2. 

Transition Metal Complexes Related to the Simon test is a family of color-producing reactions based on transition metal complexes (coordination complexes) and tightly associated ion pairs. Coordination complexes arise from a Lewis acid-base interaction between a metal cation, such as cobalt, and an atom with unshared electrons, such as water or, in the case of drugs, basic nitrogen found in alkaloids and amines. Metals that have been used in these reagents include copper, vanadium, bismuth, and cobalt Cobalt, as part of two common reagents (cobalt thiocyanate and Dilli-Koppanyi) is perhaps the most versatile. Cobalt has an electron structure of 3d 4s, while ttie cation has a 3d (2 ) or 3d (3 ") structure. 

The silatrane molecules with their intramolecular Si N Lewis acid/base bond have already been listed together with other nitrogen compounds (Table 4). For 1,4-bis(trimethylsiloxy)benzene, a crystal structure analysis proves a 60° dihedral angle for the conrotatory twist of the RsSiO substituents around the OC6H4O axis, reducing the ng/jr interaction to 9.24 — 7.96 = 1.28 eV (cf. Figure 15). 

The bonding for oxygen atoms in heteratomic molecules is viewed as essentially covalent (see Covalent Bonds) (e.g. MeOH, Mc2C=0, MeCHO, and MeC(O)OOH) and similar to that for carbon, nitrogen, and chlorine atoms. In contrast, a Lewis acid base formahsm often is used for metal oxygen compounds with ionic interactions by dianionic 0x0 groups (e.g. [Ba2+ q2-], [Fe + (0 -) ] -, [Mn + (Q2-)4]-, and [(Cu+)20 ]). This results from the thermodynamic relations for ionic solution equihbria, and the inference that the combination of ions results in molecules and complexes held together by electrostatic interactions (equations 33 35). 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

The term halogen bonding (XB) [3-8] indicates any D X-Y interaction in which X is the electrophilic halogen (Lewis acid, XB donor), D is a donor of electron density (Lewis base, XB acceptor), and Y is carbon, nitrogen, halogen, etc. (Fig. 1). 

---