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

Abstract

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)Al^I by reduction of (^tBuDPM)AlI₂ with KC₈ failed. However, reduction of (^tBuDPM)GaI₂ (1) with K/KI led to successful isolation of (^tBuDPM)Ga^I (3). The Ga^II 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)Ga^I 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.