Ten-Membered
Cyclodecatetraene Derivatives Including
Two Gallium Atoms: Experimental and Theoretical Studies on Synthesis,
Structures, and Their Transformations to Nine- and Five-Membered Gallacycles
The chemistry of medium-sized ring compounds, including
group 13
elements, has scarcely been investigated. Herein, we report that 5,10-digallacyclodeca-1,3,6,8-tetraene
derivatives Cl2Ga2C8R8 (R = Me (5a), Et (5b)) can be obtained
by the reaction of GaCl3 and zirconacyclopentadiene Cp2ZrC4R4 (R = Me (2a), Et
(2b), Cp: η5-C5H5) in toluene. X-ray crystal structure analysis revealed that compound 5b has a 10-membered cyclodecatetraene structure composed
of two butadiene skeletons and two GaCl fragments. X-ray crystal structure
analysis and IR measurement confirmed that compound 5b forms Ga2Cl2 four-membered rings via intermolecular
Cl to Ga donor–acceptor interactions, which result in the formation
of oligomeric chain structures in the solid state and toluene solution.
The treatment of compound 5b with 2 equivalents of 4-(dimethylamino)pyridine
(DMAP) proceeded via the coordination of DMAP to the Ga center and
the ring contraction to give DMAP-coordinated five-membered gallole
Cl(DMAP)GaC4Et4 (4bDMAP). The reaction
of compound 5a with 2 equivalents of MesLi (Mes: 2,4,6-Me3C6H2) in toluene to form a 9,10-digallabicyclo[4.3.1]decatriene
derivative (MesGaC8Me8)GaMes (6a) caused ring contraction. Theoretical investigation revealed that
the 10-membered ring derivative, Cl2Ga2C8H8 (GaH-10), is formed by the dimerization
of two gallole molecules, ClGaC4H4 (GaH-5), via the addition of two Ga–C bonds without an energy barrier.
The immediate formation of GaH-10 was attributed to the
transannular electronic interaction between the expanded empty p-orbital
on the Ga atom and the electron-rich sp2 carbons of the CC double bond in GaH-5. Moreover, GaH-10 and one of the isomer, nine-membered ring derivative
(ClGaC8H8)GaCl, were found to be thermodynamically
more stable, as compared to GaH-5.