Five-Coordinate Aluminum Bromides: Synthesis, Structure, Cation Formation, and Cleavage of Phosphate Ester Bonds

The alkane elimination reaction between Salen(tBu)H2 ligands and diethylaluminum bromide was
used to prepare three Salen aluminum bromide compounds salen(tBu)AlBr (1) (salen = N,N‘-ethylenebis(3,5-di-tert-butylsalicylideneimine)), salpen(tBu)AlBr (2) (salpen = N,N‘-propylenebis(3,5-di-tert-butylsalicylideneimine)), and salophen(tBu)AlBr (3) (salophen = N,N‘-o-phenylenenebis(3,5-di-tert-butylsalicylideneimine)). The compounds contain five-coordinate aluminum either in a distorted square pyramidal or a
trigonal bipyramidal environment. The bromide group in these compounds could be displaced by
triphenylphosphine oxide or triphenyl phosphate to produce the six-coordinate cationic aluminum compounds
[salen(tBu)Al(Ph3PO)2]Br (4), [salpen(tBu)Al(Ph3PO)2]Br (5), [salophen(tBu)Al(Ph3PO)2]Br (6), and [salophen(tBu)Al{(PhO)3PO)}2]Br (7). All the compounds were characterized by 1H, 13C, 27Al, and 31P NMR, IR, mass
spectrometry, and melting point. Furthermore, compounds 1−3 and 5−7 were structurally characterized
by single-crystal X-ray diffraction. Compounds 1−3 dealkylated a series of organophosphates in
stoichiometric reactions by breaking the ester C−O bond. Also, they were catalytic in the dealkylation
reaction between trimethyl phosphate and added boron tribromide.