Dialkylzinc compounds structure

Table 2 Structural data of selected dialkylzinc compounds from gas electron diffraction (GED) and computational (DFT) studies...

Dialkylzinc compounds have a tendency to form colored, and sometimes paramagnetic, adducts with 7r-acceptor ligands. The potentially bridging nitrogen donor pyrazine (and 4,4 -bipyridine) formed dinuclear complexes, Scheme 31, while the chelating 2,2 - bipyridine formed mononuclear adducts with diisopropylzinc.78 In the bis(diisopropylzinc)-pyrazine adduct 33, the presence of an unpaired electron was detected, but the resolution of the ESR spectra was insufficient for an unambiguous structural assignment. 

The complex of Me2Zn with (5, 5 )-ebpe, 107, has been applied successfully as catalyst in the enantioselective reduction of ketones by polymethylhydrosiloxane and combines excellent product yields with high ee values . Its structure comprises the iV,iV-chelate coordination of the ebpe ligand to the MeiZn unit (Figure 51). It is remarkable that in this case the two secondary amine functionalities are coordinated to zinc and leave the Zn—C bonds unaffected. Indeed, usually secondary amines undergo a fast deprotonation reaction with dialkylzinc compounds. 

The dialkylzinc compounds are extremely moisture sensitive and even react explosively with bulk water. Nevertheless, cryoscopic studies carried out on the controlled hydrolysis of diethylzinc show the formation of organozinc hydroxide EtZnOH as intermediate. This intermediate, which is thought to have the dimeric structure (EtZnOH)2, is very unstable and reacts further to give polymers of the form Et(ZnO) (equation 19). Some of these products exhibit a good catalytic activity for the polymerization of lactones and cyclic ethers. 

Predictably, alkyltrisilanols react with dialkylzincs to yield larger aggregates, such as 133, which are much more inorganic in nature than those formed by mono- and disilanols (Scheme 84). Often these zinc siloxanes are without direct zinc-carbon bonds and resemble the silicates for which they serve as model compounds.192 The specific structures of these products depend heavily on the substituents of the silanetriols and diorganozincs, as well as the reaction stoichiometries.193... 

Development of new methodologies for formation of carbon-carbon bonds has been one of the major tasks in organic chemistry. Obviously, organometallic compounds, particularly zinc derivatives, have found great use in such reactions. During the past several years, there have been several significant reports of nickel- and palladium-catalyzed reactions of dialkylzincs and alkylzinc halides with alkyl halides of diverse structure. A detailed account of most of these studies can be found in a recent review by Knochel et al,246... 

The structural features of the dialkylzinc units in these compounds are closely related, i.e. the zinc atoms are trigonal-planar coordinated, with C—Zn distances as expected. The Zn—N coordination bonds range from 2.078 to 2.184 A and the C—Zn—C bond angles range from 132 to 145 . 

With both of these ligands, dialkylzinc and -cadmium compounds gave tetrahedral bis-ligand complexes with structures analogous to 23. 

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