Inverse sandwich complexes based on low‐valent group 13 elements and cyclobutadiene: A theoretical investigation on E‐C 4 H 4 ‐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‐C 4 H 4 ‐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‐C 4 H 4 ‐E dissociates via loss of the neutral atom E with the bond strengths of Al > Ga > In > Tl. Moreover, E‐C 4 H 4 ‐E are more stable in dissociation than [E‐Cp‐E] + cations. By comparing with other various isomers, we found that the inverted E‐C 4 H 4 ‐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 SiH 3 and Si(CH 3 ) 3 could significantly stabilize the inverted form in thermodynamics. Possible synthetic routes are proposed for E‐C 4 H 4 ‐E. With no need of counterions, the newly designed neutral complexes E‐C 4 H 4 ‐E welcome future synthesis. © 2012 Wiley Periodicals, Inc.