The crystal structure of solvent-free diphenylzinc has been determined. Crystals of [Ph2Zn] are triclinic, space group P1, with unit cell dimensions a = 9.956 (1) A, b = 10.024 (2) A, c = 12.011 (2) b, a = 113.74(2)O, / 3 = 108.79 (1)O, y = 97.68 (2)O, and Z = 2. The structure refinement converged at RF = 0.042. A remarkable PhZn(p-Ph)zZnPh dimeric structure is found, in which the zinc atoms are tricoordinated. The structure confirms the ability of Zn to take part in multicenter bonding to carbon. It is unique not only in the sense that it is the first example of an arylzinc derivative but also because of the unsymmetrical orientation of the p-phenyl groups: they retain the predominant bonding relation to one zinc atom, as the association of the two monomeric PhzZn units only partially disturbs their original, linear geometry: the average Zn-C bond distances within one PhzZn unit are 1.946 (5) A (terminal) and 2.011 (5) A (bridging), whereas the bridging Zn-C bond to the second PhzZn unit is 2.403 (5) A. In the crystal packing, two dimeric residues are weakly associated by interactions (3.0-3.7 A) between the two zinc atoms of one dimer and one of the terminal phenyl groups of the other.
The reaction between certain aryl bromides RBr having substituents with group 14 elements and magnesium in THF to form the corresponding Grignard reagents RMgBr were found to be accompanied by unusual migrations of the group 14 functionality; it partially exchanges position with that of the original aryl bromide function. This migration is purely intramolecular for organosilicon derivatives but both intra-and intermolecular for the corresponding tin compounds. For example, 1-bromo-2-((trimethylsilyl)methyl)benzene (1b) gives rise to 1-(bromomagnesio)-2-((trimethylsilyl)methyl)benzene (2b) and to its intramolecular rearrangement product 1-((bromomagnesio)methyl)-2-(trimethylsilyl)benzene (3b), whereas the trimethylstannyl analogue 1a forms, in addition to the two corresponding Grignard reagents 2a and 3a formed by the intramolecular pathway, the intermolecular exchange products 1-(trimethylstannyl)-2-((trimethylstannyl)methyl)benzene (4a) and 1-(bromomagnesio)-2-((bromomagnesio)methyl)benzene (5′). These results, together with those from analogous reactions of compounds such as 1-bromo-2-(trimethylstannyl)benzene (18), its 5-methyl derivative 23, 2-bromo-2′-(trimethylstannyl)biphenyl (40), and its silicon analogue 57, can be convincingly explained by invoking the intermediate formation of the highly reactive carbanion R: -, which may in some cases be slightly stabilized by formation of an (intra-or intermolecular) stannate (or silicate) complex. Another serious candidate for such migrations would have been the radical R • , the occurrence of which is well-documented in Grignard reactions. However, this alternative route could be excluded by producing this radical in an unambiguous fashion from 1a and samarium(II) diiodide; under these conditions, hydrogen abstraction by R • strongly predominated, and minor quantities of rearranged products were shown to be due to further reduction of R • to R: -.
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