Electron-Deficient Solids

We have already mentioned a very strong dyadic association in the formally d5 cobalt complexes such as [Cp Co(dddt)]+ which dimerizes in the solid state to a fully diamagnetic dicationic dyad (Fig. 6a). It represents the extreme situation where the two radicals form a true 2e bond, with the sulfur atom of one dithiolene ligand entering the coordination sphere of the other metal. It should be considered as the consequence of the electron deficiency of these cationic [CpCo(dt)]+ 15-electron complexes. 

The electrostatically favored cation (Li) and anion (RE) arrangement implies the presence of two different E-, Si- and Li sorts, which has been established by solution and solid-state NMR spectroscopy. The electronic structures of the mixed-valent pnictides 10 and 11 have been simply described as electron-deficient clusters with delocalized framework electrons. Formally the latter consist of two low-valent anediyl moieties RE and eight andiides (RE)2- (E = P, As). The relatively large E-E distances of >4 A exclude the occurrence of localized E-E bonds. However, delocalization of the cluster valence electrons is achieved without Li-Li bonds via Li-mediated multiple bonding. Evidence for this has been seen in the NMR spectra (31P, 7Li, 29Si), which are in accordance with the electron delocalization model (see later discussion). 

Natta s bimetallic mechanism stipulates that when the catalyst and cocatalyst components are mixed, the chemisorption of the aluminium alkyl (electropositive in nature) occurs on the titanium chloride solid surface which results in the formation of an electron-deficient bridge complex of the structure shown... 

The electron-deficient alkene (5.2 mmol) in MeCN (5 ml) is added to an intimate mixture of powdered K2C03 (1 g) and NaOH (0.2 g), the (S)-menthone-protected ethyl glycine (1.27 g, 5 mmol), and TBA-Br (0.16 g, 0.5 mmol) in MeCN (20 ml). The mixture is stirred for 1 h at 0°C and then filtered. The solid is washed with MeCN (10 ml) and the combined organic solutions are evaporated and the residue is taken up in Et20. The ethereal solution is washed well with H20, dried (MgS04), and evaporated to produce the alkylated imine, which can be converted into the amino acid upon hydrolysis with aqueous acid. 

As written, the reaction is endothermic by ca 30 kJmoD. Pyrrole and pyridine are both 6-71 nitrogen-containing heterocycles. However, the former is electron-rich while the latter is electron-deficient and so conjugative stabilization mechanisms are different for the two species. Furthermore, the former can form one more hydrogen bond per molecule than the latter, a feature that may account for pyrrole-2-carboxaldehyde being a solid while pyridine-2-carboxaldehyde is a liquid. We wonder if either difference accounts for the profound lack of thermoneutrality for the above reaction. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

Fig. 9 (a) Electron precise chains of group 15 elements in solid state structures with formal charges assigned by valence count (b) electron-deficient chains of group 15 elements all chains are represented in perspective view to convey the helical assemblies... 

A poly(phenylquinoxaline) was prepared for electroluminescence applications <1996SM(76)105>. Crystallization of solution donor-acceptor complexes of 2,3-dimethylquinoxaline 1,4-dioxide or phenazine 5,10-dioxide with TCNE afforded two-component solids containing weakly bound 1-D donor-acceptor arrays <1997TL7665>. A pyrazine ladder polymer was constructed from two different pyrazine units, as an optical device <1999JA8783>. The new electron-deficient macrocycle tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine was prepared from l,2-di(2-pyridyl)ethanedione and 2,3-diaminomaleonitrile for a study of its electrochemical properties <2004IC8626>. 

Ramamoorthy et al. (444) found that a-phenyl-A-(4-methylphenyl)nitrone can be the guest molecule in inclusion complexes with a p-cyclodextrin host in 1 1 and 1 2 ratios (guest/host), and that the latter undergoes a 1,3-dipolar cycloaddition reaction with electron-deficient alkenes. In more recent work, they have formed 1 1 inclusion complexes of the bowl-shaped p-cyclodextrin 383 with (3-nitrostyrene 381 or 1-nitrocyclohexene 382, which leave the alkene moiety exposed (Fig. 1.9) (445). Complexes 381 and 382 undergo cycloaddition reaction with ot-phenyl-A-(4-methylphenyl)nitrone in the solid state after thorough homogenization (60 °C, 3 h) to give the 4-substituted products exclusively in 80 and 85% yield, respectively. 

Gowravaram and Gallop (169) adapted the rhodium-catalyzed generation of isomiinchnones from diazo imides to the solid-phase synthesis of furans, following a 1,3-dipolar cycloaddition reaction with alkynes. A variety of furans were prepared in this fashion. With unsymmetrical electron-deficient alkynes (e.g., methyl... 

---