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 (<b>2</b>) produced the monomeric complexes (<i>N</i>,<i>N</i>)­AlR₂ (where (<i>N</i>,<i>N</i>) is deprotonated dipyrromethene; R = Me (<b>3</b>), ^tBu (<b>4</b>), Ph (<b>5</b>), H (<b>6</b>)). The alkyl and aryl derivatives <b>3</b>–<b>5</b> are surprisingly stable and tolerant to exposure to air, moisture, and silica gel. Dihydride <b>6</b> is less robust and reacts with water to form the oxo-bridged aluminoxane [(<i>N</i>,<i>N</i>)­AlH]₂O (<b>7</b>). The presence of terminal hydrides in <b>6</b> and <b>7</b> 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 <b>2</b>, affording 5-<i>tert</i>-butyl-5-phenyl-2,2′-dimesityldipyrromethane (<b>8</b>). In addition to NMR (¹H, ¹³C) and IR spectroscopy, compounds <b>2</b>–<b>8</b> 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