Solution, NMR spectroscopic study

Over the past few years, a series of zwitterionic spirocyclic bis[l,2-benzenediolato(2-)]organosilicates and related zwitterions with 2,3-naphthalenediolato(2-) and symmetrically substituted 1,2-benzene-diolato(2 ) ligands have been synthesized and structurally characterized [1]. The zwitterionic X, Si-silicates 1-3 are typical examples of this particular type of compound (Fig. 1). In contrast to the well established solid-state structures of these zwitterions, only little is known about their structure in solution. NMR spectroscopic studies have clearly demonstrated that these species exist in solution ([D J-OMSO)... 

The aqua ions (M = Al, Ga, In, Tl) are acidic (see equation 6.34) and the acidity increases down the group. Solutions of their salts are appreciably hydrolysed and salts of weak acids (e.g. carbonates and cyanides) cannot exist in aqueous solution. Solution NMR spectroscopic studies show that in acidic media, Al(III) is present as octahedral [Al(H20)g], but raising the pH leads to the formation of polymeric species such as hydrated [Al2(OH)2] and [Al7(OH)ig] +. Further increase in pH causes Al(OH)3 to precipitate, and in alkaline solution, the aluminate anions [A1(0H)4] (tetrahedral) and [Al(OH)g] (octahedral) and polymeric species such as [(H0)3Al(p-0)Al(0H)3] are present. The aqueous solution chemistry of Ga(III) resembles that of Al(III), but the later metals are not amphoteric (see Section 12.7). 

The aqua ions (M = Al, Ga, In, Tl) are acidic (see eq. 7.34) and the acidity increases down the group. Solutions of their salts are appreciably hydrolysed and salts of weak acids (e.g. carbonates and cyanides) cannot exist in aqueous solution. Solution NMR spectroscopic studies show that in acidic media, Al(III) is present as octahedral [A1(0H2)6], but raising the pH leads to the formation of... 

The computational results (93JA2465) are consistent with the experimental findings of NMR spectroscopic studies (82JOC5132 97MRC35), which showed the presence of only the H tautomer of tetrazole in DMSO-dfi e = 49) solution. The content of H tautomer 27a in dioxane e = 22) at 30°C was estimated as 78% (82JST283) and 85% (75BSF1675) from its dipole moment 4.88 D and those of 1,5- and 2,5-dimethyltetrazoles as models for the H and 2H tautomers respectively. 

Amides are derivatives of carboxylic acids, so that their coordination behavior to boranes might be similar to that of their parent compounds. B-NMR spectroscopic studies have shown that compounds 31 and 32 are monomeric species in solution, while compounds 33 and 34 with the more Lewis acidic 9-borabicyclo[3.3.1]nonyl unit form aggregates that may be dimeric, oligomeric, or polymeric. The grade of association could not be determined by mass spectrometric analyses, because in all cases only the monomer is liberated into the gas phase [65]. 

Consequently, due to preferred cis-cis orientation a dimeric structure is observed for the indium complex and an unprecedented cis-trans arrangement in the thallium structure leads to a polymeric aggregate. Further N-NMR spectroscopic studies show that the aluminum and gallium complexes are stable contact ion pairs even in solution whereas the indium and thallium compounds are solvent-separated ion pairs in THE solution. 

The structure of the decavanadate ion [Vi0O28]6 is well established. Vanadium-51 NMR spectroscopic studies have shown that the structure in solution corresponds to that in the solid state (2). Numerous X-ray studies have shown that it consists of an arrangement of 10 edge-shared V06 octahedra with approximate D2h symmetry (Fig. 7). All vanadium atoms have distorted octahedral geometry and the oxygen atoms fall into seven categories, ranging from terminal to six coordinate. 

Making use of the same procedure, the ether-functionalized diorganomagnesium compounds 76a-76f were prepared from 3-butenyl ethers and MgH2 (equation 11)"". It should be noted that the addition reaction of allyl ethers to MgHi failed because in that case ether cleavage by MgH2 becomes a competing reaction. Also, for these compounds intramolecular O-Mg coordination in solution was established by NMR spectroscopic studies. 

H and NMR spectroscopic studies of 64 showed that the structure present in the solid state is retained in solution. That the reactivity of 64 is different compared to that of normal organozinc compounds became evident from a hydrolysis reaction of 64 with a carboxylic acid affording the corresponding zinc salt of the carboxylic acid and metallic zinc in a 1 1 ratio and pentamethylcyclopentadiene. This product formation suggests the... 

Experimental mechanistic studies of such reactions are in their early days. However, it is clear that proper understanding will rely on the prior isolation of the organolithium reagent, rather than on its production, then use, in situ. Thereafter, the reagent needs to be identified (e.g., RLi or RLi xL ) and characterized structurally, in the solid by X-ray diffraction and, perhaps more importantly, in solutions (by colli-gative molecular mass measurements and multinuclear NMR spectroscopic studies) in which it will be used. 

This chapter deals with silyl-substituted carbocations. In Section II results of quantum chemical ab initio calculations of energies and structures of silyl-substituted carbocations are summarized1. Throughout the whole chapter results of ab initio calculations which relate directly to the experimental observation of silyl-substituted carbocations and their reactions are reviewed. Section m reports on gas phase studies and Section IV on solvolytic investigations of reactions which involve silyl-substituted carbocation intermediates and transition states. Section V summarizes the structure elucidation studies on stable silyl-substituted carbocations. It includes ultra-fast optical spectroscopic methods for the detection of transient intermediates in solution, NMR spectroscopic investigations of silyl-substituted carbocations in superacids and non-nucleophilic solvents, concomitant computational studies of model cation and X-ray crystallography of some silyl-substituted carbocations which can be prepared as crystals of salts. 

Variable-temperature NMR spectroscopic studies indicate that the crown structure is retained in solution with the compounds existing as two distinct inter-converting conformers with W and Z shapes in benzene solution. 

Adducts ofB(C6F5)3 that have been studied in detail either in solution or the solid state are collected in Table I along with selected solution NMR spectroscopic data and metrical parameters. In particular, both the nB NMR chemical shift35 and the separation between the resonances for the meta and para fluorine atoms in the 19F NMR spectrum109 are quite sensitive to the environment about the boron center and the strength of the LB-B(C6F5)3 interaction. Indeed, as shown in Fig. 1, a rough empirical correlation between these two NMR parameters is observed. Anomalies arise for two classes of LB more linear bases like nitriles or isonitriles that do not pyramidalize the boron center as severely and the RM1 adducts (M = Al, Ga). 

Kreiter (67) first demonstrated by XH NMR spectroscopic studies the acidity of hydrogen atoms that are bonded at the a-C atom of alkoxy-(alkyl) carbene complexes. In CH3OD solution, in the presence of catalytic amounts of sodium methanolate, pen tacarbonyl[methoxy (methyl) car-bene cliromiumfO) exchanges for deuterium all hydrogen atoms of the methyl group that is situated next to the carbene carbon ... 

---