Metal-Ligand Cooperativity Enables Zero-Valent Metal Transfer

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.