Neutral and Cationic Bis(guanidinate) Digallanes and Gallium-Catalyzed Alkene Hydroboration

Abstract

A bis(guanidinate) supported gallium dihydride, (L)GaH2; (1-H) (where L = {(DepHN)(DepN)–C═N–C═(NDep)(NHDep)}; Dep = 2,6-Et2-C6H3), serves as an ideal precursor for accessing neutral and cationic Gall–Gall bonded species. The neutral digallane [(L)GaCl]2 (2) can be accessed through multiple synthetic pathways, including the reactions of (1-H) with Cp2ZrCl2, (L)GaCl2; (1-Cl) with Cp2ZrH2, (1-H) with (1-Cl), and (1-H) with Ph3CCl. These complementary routes demonstrate the facile formation of the Ga–Ga bond from both gallium hydride and chloride precursors. Moreover, treatment of (1-H) with triphenylmethylium tetrakis(pentafluorophenyl)borate, [Ph3C][B(C6F5)4], generates a reactive cationic gallium hydride intermediate that undergoes halogenated solvent activation via C–X bond cleavage to furnish the dicationic digallanes [(HL)GaX]22+·2[B(C6F5)4]− [X = Cl (3-Cl) and X = Br (3-Br)]. Notably, these transformations are accompanied by ligand cooperativity involving conversion of the monoanionic LX (3e– donor) bis(guanidinate) framework of (1-H) into a neutral L2 (4e– donor) bis(guanidine) donor system in compounds (3-Cl) and (3-Br). All compounds were comprehensively characterized using standard spectroscopic techniques, including solid-state structural analysis. Furthermore, we have demonstrated that the in situ-generated cationic gallium hydride species efficiently catalyzes the regioselective anti-Markovnikov hydroboration of a broad range of alkene substrates.