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)GaI2 (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)GaN4(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.