Metal-Ligand Cooperativity Enables Zero-Valent Metal Transfer

Abstract

Group 13 TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) complexes of the form (L)E(TEMPO)₃ (L=THF (tetrahydrofuran) or Py (pyridine); E = Al, Ga, In) were prepared and structurally characterized. The complexes (THF)Ga(TEMPO)₃ (1·THF) and (Py)In(TEMPO)₃ (2·Py) are shown to heterolytically cleave H₂ under mild conditions (3 atm, 20 ◦C, t ≤ 1 h). 1·THF reacts reversibly with H₂ to form a formal H₂-adduct that bears a Ga(III) hydride center and a protonated TEMPO ligand with concomitant loss of THF, consistent with Ga(III) and TEMPO functioning as Lewis acid and base, respectively. Conversely, 2·Py is reduced by H₂ to form an intermediate dimer complex of monovalent {In(TEMPO)}₂, which undergoes further reactivity with H₂ to form elemental indium as determined by powder X-ray diffraction. Treatment of 2·Py with H₂ and Ph₃PSe forms binary InSe, in addition to Ph₃P and TEMPOH, demonstrating that 2·Py functions as a molecular source of zerovalent indium under mildly reducing conditions. Computational studies support an intramolecular metal-ligand cooperativity pathway in the heterolytic cleavage of H₂.