Photochemistry of chiral pentacoordinated Al salen complexes. Chiral recognition in the quenching of photogenerated tetracoordinated Al salen transient by alkenes

A series of four chiral pentacoordinated aluminium salen complexes differing in the apical ligand have been prepared. The complexes exhibit an intense emission (ø from 0.271 to 0.35) arising from the LMCT band excitation. The complexes were submitted to laser flash photolysis. The same transient was observed in the four cases (ι under N2 between 1.1 and 3.5 µs) and the corresponding UV-visible transient spectrum exhibits two absorption bands at 320 and 500 nm. This transient is quenched by oxygen, hydrogen donors (1,4-cyclohexadiene and 1-octanethiol) and alkenes and it was assigned to the neutral Al(ii) salen complex generated by homolytic bond cleavage of the apical ligand. This transient exhibits different degrees of asymmetric recognition for its quenching with enantiomerically pure pinene kq(R)/kq(S) varying between 1.68 up to 12.26 depending on the nature of the apical ligand.