Inverse sandwich complexes based on low-valent group 13 elements and cyclobutadiene: A theoretical investigation on E-C4H4-E (E = Al, Ga, In, Tl)

The chemistry of the low-valent Group 13 elements (E = B, Al, Ga, In, Tl) has formed the recent hot topic. Recently, a series of low-valent Group 13-based compounds have been synthesized, i.e., [E-Cp*-E]+ (E = Al, Ga, In, Tl) cations, which have been termed as the interesting inverse sandwich complexes. To enrich the family of inverse sandwiches, we report our theoretical design of a new type of inverse sandwiches E-C4H4-E (E = Al, Ga, In, Tl) for stabilizing the low-valent Group 13 elements. The calculated dissociation energies indicate that unlike [E-Cp-E]+ that dissociates via loss of the charged atom E+, E-C4H4-E dissociates via loss of the neutral atom E with the bond strengths of Al > Ga > In > Tl. Moreover, E-C4H4-E are more stable in dissociation than [E-Cp-E]+ cations. By comparing with other various isomers, we found that the inverted E-C4H4-E should be kinetically quite stable with the least conversion barriers of 33.5, 33.5, 35.2, and 36.9 kcal/mol for E = Al, Ga, In, and Tl, respectively. Furthermore, replacement of cyclobutadiene-H atoms by the highly electron-positive groups such as SiH3 and Si(CH3)3 could significantly stabilize the inverted form in thermodynamics. Possible synthetic routes are proposed for E-C4H4-E. With no need of counterions, the newly designed neutral complexes E-C4H4-E welcome future synthesis. (c) 2012 Wiley Periodicals, Inc.