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.