Dipyrromethene as a Ligand for the Stabilization of Low-Valent Gallium Complexes

We investigated the low-valent chemistry of Al and Ga with the ligand tBuDPM, a dipyrromethene ligand scaffold with two large tBu groups in the flanking 1- and 9-positions and a mesityl group in the backbone 5-position. Attempted synthesis of ( tBuDPM)­AlI by reduction of ( tBuDPM)­AlI2 with KC8 failed. However, reduction of ( tBuDPM)­GaI2 (1) with K/KI led to successful isolation of ( tBuDPM)­GaI (3). The GaII intermediate in the reduction process crystallized as a digallane: [( tBuDPM)­GaI]2 (2). Also, 3 crystallized as a dinuclear complex with a Ga–Ga bond. However, in a benzene solution, the 3 dissociates into two mononuclear complexes. Reaction of a benzene solution of ( tBuDPM)­GaI with excess Me3SiN3 gave the tetrazagallole product ( tBuDPM)­Ga­[N4(SiMe3)2] (4) and not the alternative azide-amide product ( tBuDPM)­Ga­(N3)­[N­(SiMe3)2], which according to calculations is thermodynamically considerably more stable. Theoretical investigations on the nature of the Ga–Ga bonds in 2 and 3 and the mechanism for selective formation of 4 have been included.