Synthesis and characterization of a trigallacycle
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abstract
The reaction of equimolar quantities of 1,8-bis(trimethylstannyl)naphthalene (1) and GaCl3 in toluene at -25C yields the known stannagallacycle bis(-1,8-naphthalenediyl)(-chloride)-methyltin(IV)chlorogallium(III) (2) and a new product which has been identified as the cyclic adduct formed between two molecules of bis(-1,8-naphthalenediyl)bis(gallium(III)chloride) and two molecules of trimethyltin chloride (3). Interestingly, when carried out at 25C 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 H = 43 3 kJmol-1 and S = -36 9 JK-1mol-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.
