Mono- and Bimetallic Aluminum Alkyl, Aryl, and Hydride Complexes of a Bulky Dipyrromethene Ligand

Reactions of an appropriate organoaluminum reagent with a sterically demanding dipyrromethene (2) produced the monomeric complexes (N,N)­AlR2 (where (N,N) is deprotonated dipyrromethene; R = Me (3), tBu (4), Ph (5), H (6)). The alkyl and aryl derivatives 3–5 are surprisingly stable and tolerant to exposure to air, moisture, and silica gel. Dihydride 6 is less robust and reacts with water to form the oxo-bridged aluminoxane [(N,N)­AlH]2O (7). The presence of terminal hydrides in 6 and 7 was confirmed by NMR and IR spectroscopy. Furthermore, observed Al–H stretching frequencies agree well with those predicted by DFT calculations. We also report the addition of tBuLi to the meso position of 2, affording 5-tert-butyl-5-phenyl-2,2′-dimesityldipyrromethane (8). In addition to NMR (1H, 13C) and IR spectroscopy, compounds 2–8 were further characterized by X-ray crystallography. Solid-state structures show that the aluminum and dipyrromethene ligand are nearly planar when the coligands at aluminum are slender.