Group 13 element containing conformationally rigid "N-E-N" heteroatomic bridged [3.3](2,6)pyridinophanes (E = B, Al)

Aluminum containing molecular bowls and pyridinophanes: use of pyridine modules to access different molecular topologies

Molecular topologies varying from simple complexes to pyridinophanes (neutral and cationic) and to bicyclic pyridinophane containing organoaluminum (Al-Me) species were synthesized by varying the relative stoichiometry of bis(trimethylsilyl)-N,N'-2,6-diaminopyridine (bap) and the reactive partner (AlMe₃). The ultimate goal of these reactions was to systematically design cyclic structures containing group 13 elements. To highlight the reaction potential of these shapes, the bowl-shaped pyridinophane was reacted with the Lewis acid, B(C₆F₅)₃, to generate a stable cationic derivative. An unprecedented bicyclic pyridinophane, [2,6-(Me₃SiN)₂C₅H₃N]₃Al₂, was obtained from the reaction of bap with AlH₃·NMe₂Et. The formation of [2,6-(Me₃SiN)₂C₅H₃N]₃Al₂ is in contrast to the known reaction between BH₃·SMe₂ and bap that afforded the syn-tetraazadibora[3.3](2,6)pyridinophane. Quantum chemical calculations have been performed to rationalize the preference for the formation of B-pyridinophane and Al-bicyclic pyridinophane and can be attributed to the nature of B-N and Al-N bonds.