Hypercoordinate structure

Siehl and co-workers949 950 have used the matrix co-condensation technique to generate the l-(trimethylsilyl)bicyclobutonium ion 528 (Scheme 3.23). The 1H and 13 C NMR spectra of ion 528 show averaged methylene signals, which is in accord with a fast threefold degenerate rearrangement and a puckered hypercoordinate structure. 

The isotopic perturbation method is very useful for studying fractional bonding and hypercoordination in coordinatively unsaturated and electron deficient compounds such as transition metal complexes or carbocations. The 2-norbornyl cation and the bicyclobutonium cation are the most prominent examples of carbocations whose structures have led to much controversial discussion (the so-called classical-nonclassical ion controversy). The application of the isotopic perturbation methed is likely to be the most decisive piece of nmr evidence for the hypercoordinated structure of these two cations in solution. 

The protracted and controversial discussions on the structure and dynamics of the 2-norbornyl cation have been recently summarised (Walling, 1983 Olah et al 1983 Brown, 1983 Grob, 1983). The question has been whether the 2-norbornyl cation has a symmetrically bridged hypercoordinated structure [99] with a pentacoordinated carbon atom or whether it is a rapidly equilibrating pair of trivalent cations [100], In the context of this review only the evidence from isotope effects on the nmr-spectra of the 2-norbomyl and related cations is discussed. 

This is true even if a slow 6,2-hydride shift in secondary cations converts part of [103] (i.e. 3,3-D2-[100a,bj) into 5,5-D2-[101]. Cation 5,5-D2-[101] would lack an equilibrium isotope effect because of the symmetry of an assumed Wagner-Meerwein rearrangement between D2-[101a] and D2-[101b], This does not vitiate the validity of the isotopic perturbation test, which shows that a double energy minimum of secondary cations [103] is not present. A static symmetrical hypercoordinated structure [102] is in accord with the observed results. If the 6,2,1-hydride shift in [102] was not frozen out on the timescale of the experiment, the C spectrum of [102] measured below — 150 C is still conclusive, although it results from an equal mixture of isomers of [102] labelled at C-3, C-5 and C-7. 

The isotope effect in the trishomocyclopropenyl cation [119] has been investigated (Olah et al 1985b). The virtual identity of the C nmr spectrum of 3-deuteriobicyclo[3.1.0]hexyl cation D,-[119] with that of the nonlabelled cation, except for a small (0.2 ppm — 80°C) upheld shift of the deuteriated methine triplet, is in accord with the symmetrical single minimum hypercoordinated structure of cation [119]. 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

For the CH with a Cs symmetrical structure 1 a 13C NMR chemical shift of 11.5 ppm is calculated. This is 7.1 ppm more shielded than that calculated for the hydrocarbon CH4 at the GIAO-MP2 level.19 The shielding effect in 1 is as expected for a hypercoordinated carbocation carbon.20 The calculated minimum energy structure for diprotonated methane (CHi+) 2 has C2v symmetry with two 3c-2e... 

The structure of the 2-norbornyl cation has been a focal point of controversy in physical organic chemistry. Experimental NMR spectroscopy and computational methods have been the decisive tools, favoring the hypercoordinated symmetric bridged structure 30, a protonated nortricyclane.79 The tricoordinated 2-norbornyl cation 31 is not a local minimum (MP2/6-31G(d)) on the energy surface.80... 

It should also be mentioned that OH hypercoordination is not observed in concentrated solutions of NaOH and KOH. In contrast to acidic solutions, where structure diffusion is suppressed with increasing concentration (see above and Figure 2) the transference number of OH (e.g., in aqueous... 

For the analogous 1-methyl-substituted cation [C gCFb]"1", recent experimental investigations and quantum chemical ab initio calculations agree that the dynamics of this cation system can be adequately described by considering only one degenerate set of cations, which have the hypercoordinated puckered methylbicyclobutonium ion structure 420 without contributions from a degenerate set of (l -methylcyclopropyl)methyl cation structures 4217,166,167. 

A new cation-anionic complex of hypercoordinate tin (593) was obtained by treatment of (L5)2SnCl2 (L5 is a bidentate (2-oxopyrrolidino)methyl C,0-chelating ligand) with AgBF41250. As in the case of (L7)2GeCl2 (Section VIII.A), the reaction occurs with replacement of both Cl atoms. As shown by X-ray crystallography, the reaction product, however, has a dimeric distannoxane structure, in which both tin atoms are stannacenium... 

The hypercoordination of silicon can be forced by intramolecular coordination, as shown in structures 1 and 2121. 

In order to study the effect of hybridization of the donor atom (nitrogen) on donor strength, a complex with the isopropylideneimino donor group (7) was prepared (Eq. 4) and compared with the dimethylamino complexes 4. Compound 7 was obtained by the same exchange reaction (Eq. 1) as other hypercoordinate complexes from the isopropylideneimine 6a and 2. A crystal structure of 7 was determined, and is depicted in Fig. 6, and selected data are compared with those of the dimethylamino analog in Table IV. 

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