The behavior of potassium follows that of calcium in heterobimetallic ‘K–H–Al’ hydrides on a picolyl-derived chemically non-innocent triaza ligand

Abstract

Heterobimetallic hydrides pairing s- and p-block metals on a single ligand are rare, but likely to be promising for synergistic chemistry. We report a series of K–H–Al heterobimetallic hydrides on a chemically non-innocent picolyl-based ethylenediamine ligand generated through a metal–ligand interplay mechanism of hydride shuttling. The proligands ^Ar3NH (Ar = Mes, Ph) and AlH₃(NMe₂Et) give the AlH₂ complexes [(^Ar3N)AlH₂] (Ar = Mes (1), Ph (6)). Both 1 and 6 react with KN(SiMe₃)₂ (KHMDS) at room temperature to form 1 : 1 adducts [(^Mes3N)Al(HMDS)(H)₂K] (7) and [(^Ph3N)Al(HMDS)(H)₂K]₂ (8), respectively. Heating 7 and 8 at 70 °C deprotonates a picolyl C–H to give [(^Mes3N^−H)Al(HMDS)(H)K]₄ (9) and [(^Ph3N^−H)Al(HMDS)(H)K]₂ (10), respectively. Monitoring the thermolysis of 7 to 9 at 50 °C reveals an intermediate [(^Mes3N^2H)Al(HMDS)(H)K]₂ (11), where the ligand pyridyl is reduced by a 2-H insertion. DFT analysis suggests the inserted hydride moves onto the potassium before the deprotonation happens. The overall reaction profile is similar to that in the case of heterobimetallic Ca–H–Al for the same ligand framework, except that an adduct like 7 or 8 was previously not observed. A Li–Al combination on the same framework differs from the K–Al and Ca–Al cases, but follows the pattern of Mg–Al.