| Abstrakt: |
The reaction of equimolar quantities of 1,8-bis(trimethylstannyl)naphthalene (1) and GaCl3 in toluene at −25 °C yields the known stannagallacycle bis(μ-1,8-naphthalenediyl)(μ-chloride)methyltin(IV)chlorogallium(III) (2) and a new product which has been identified as the cyclic adduct formed between two molecules of bis(μ-1,8-naphthalenediyl)bis(gallium(III)chloride) and two molecules of trimethyltin chloride (3). Interestingly, when carried out at 25 °C and in the presence of water, this reaction leads to the formation of an additional product (4) that is a 12-membered macrocycle containing three gallium atoms linked by 1,8-naphthalenediyl ligands and arranged about a central oxygen atom. The charge balance of 4 is achieved by the presence of a chloride atom that occupies a bridging position between two of the gallium centers. Compounds 3 and 4 have been characterized by NMR, elemental analysis, and single-crystal X-ray analysis. In the crystal, 4 exists as a dimer wherein the monomers are bridged via a Ga−O−Ga−O four-membered ring. The solution structure of 4 in THF-d8 has been investigated by pulse field gradient spin−echo (PGSE) methods and VT 1H NMR spectroscopy. The PGSE measurements suggest that, in solution, 4 undergoes a rapid monomer−dimer equilibrium. VT 1H NMR spectroscopy confirms the existence of a fluxional system. The high-temperature spectrum exhibits three sharp signals. Upon cooling, each of the three sharp signals decoalesces into three signals. Simulation of the spectra followed by an Eyring analysis afforded ΔH&thermod; = 43 ± 3 kJ·mol-1 and ΔS&thermod; = −36 ± 9 J·K-1·mol-1 for this process. As a whole, these results support the existence of an exchange between the monomeric and dimeric form of 4. The high-temperature data suggest that upon dissociation into the monomer, the chloride anion undergoes rapid exchange leading to an apparent three-fold molecular symmetry. |
