Heavy metal pi-complexes .2. Synthesis, structure and thermal behavior of bis-(1,2,4-trimethylbenzene)thallium(I) tetrachloroaluminate and -gallate - Influence of counterion on structure and stability of a cationic bis(arene) complexes

Bis(1,2,4-trimethylbenzene)thallium( + 1) tetrachloroaluminate( + 3) (1) has been prepared from 1,2,4-trimethylbenzene, thallium(I) chloride and aluminium trichloride. In an analogous reaction with gallium trichloride, bis(1,2,4-trimethylbenzene)thdlium( + 1) tetrachlorogallate( + 3) (2) was obtained. The colourless compounds crystallize isotypically in the triclinic space group <P(1)over bar> (1: a = 9.525(2) Angstrom, b = 10.903(2) Angstrom, c = 11.972(2) Angstrom, alpha = 102.39(2)degrees, beta = 90.29(1)degrees, gamma = 107.96(1)degrees at T = -90 +/- 1 degrees C; 2: a = 9.585(2) Angstrom, b = 10.923(2) Angstrom, c = 11.966(2) Angstrom, alpha = 102.18(2)degrees, beta = 90.66(2)degrees, gamma = 107.74(2)degrees at T = - 90 +/- 1 degrees C; Z = 2). In the solid state the compounds can be described as dimeric thallium(I) tetrachlorometallates with a skeleton similar to that of (TeI4)(4), shielded by four arenes coordinated in pairs at the thallium atoms. The complete 'molecules' have point group symmetry (1) over bar. According to DTA measurements, the compounds loose all the coordinated arenes simultaneously, 1 at 59 degrees C and 2 at 51 degrees C, yielding pure thallium( + 1) tetrachlorometallates. The different thermal stability, the mean Tl-C(arene) bond lengths and indirectly the corresponding Tl-Cl bond lengths show the thallium-arene pi-bonding to be significantly weaker in the gallate than in the aluminate 1.