A thallium coated dianion: trigonal bipyramidal [F2Al(OR)3]2- coordinated to three Tl+ cations in the ion pair [Tl3F2Al(OR)3]+[Al(OR)4]- [R = CH(CF3)2]

The reproducible synthesis of the unusual ionic aluminum compd. [Tl3F2Al(OR)3]+[Al(OR)4]- (1) is reported. In the reaction of Li[Al(OR)4] [R = C(H)(CF3)2] with TlF the initially desired Tl[Al(OR)4] only formed with an exact 1:1 stoichiometry, while an excess of TlF led to [Tl3F2Al(OR)3]+[Al(OR)4]- (1). Addnl. the x-ray single crystal structure of the byproduct [(ROH)TlAl(OR)3(m-F)]2 (2) was detd. Compds. 1 and 2 were characterized by x-ray single crystal structure detns. and 1 also by NMR spectroscopy and an elemental anal. In 1 the [Tl3F2Al(OR)3]+ cation forms a trigonal bipyramid with a pentacoordinate aluminum atom. Three Tl+ cations cover the [F2Al(OR)3]2- dianion core and the charge of the resulting [Tl3F2Al(OR)3]+ cation is compensated by a weakly coordinating [Al(OR)4]- anion. Compd. 2 contains a centrosym. [Al(OR)3(m-F)]22- dianion core with pentacoordinate aluminum atoms building a distorted edge sharing double trigonal bipyramid. The [Al(OR)3(m-F)]22- dianion coordinates two [Tl(ROH)]+ cations giving the non charged mol. [(R-OH)TlAl(OR)3(m-F)]2 (2). Based on BP86/SVP (DFT-) and lattice enthalpy calcns. a pathway of the reaction is proposed to rationalize the formation of the [M3F2Al(OR)3]+ cation upon reaction of Li[Al(OR)4] with MF for M = Tl but not for M = Cs (cf Cs+ and Tl+ have very similar ionic radii). Using a suitable Born-Haber cycle and in agreement with the expt., the enthalpies of the reaction of two M[Al(OR)4] with two MF giving [M3F2Al(OR)3]+[Al(OR)4]- and MOR are favorable for M = Tl by 127 kJ/mol but endothermic for the formation of the hypothetical [Cs3F2Al(OR)3]+[Al(OR)4]- by 95 kJ/mol. It is suggested that in the reaction leading to 1 initially Tl[Al(OR)4] is formed, followed by an abstraction of TlOR and Al(OR)3. The latter very strong Lewis acid reacts subsequently with an excess of TlF yielding 1. [on SciFinder (R)]