Pincer Nickel Metalloligands for the Stabilization of Alkali Metal Alkoxide Aggregates

The structural chemistry of alkali metal alkoxides remains challenging to elucidate because of their high reactivity and tendency to form ill-defined aggregates. Herein, we report two complementary pincer nickel platforms that serve as metalloligands to stabilize well-defined multinuclear aggregates of sodium and potassium alkoxides. The reaction of a POCOP–NiBr pincer complex with MeLi or NaOtBu led to unexpected double C–O bond cleavage of the pincer framework, generating anionic oxygen-rich fragments that assembled into a zwitterionic trinuclear lithium complex {(η3-allyl)Ni}2(μ,κP:κO-iPr2PO)4[Li3(Br)(κO-iPr2P(O)Me)] (1) and a hexanuclear sodium cluster {(η3-allyl)Ni}2(μ,κP:κO-iPr2PO)4[Na6(μ4-OtBu)2(μ-OtBu)2(μ-OtBuH)2] (2). In stark contrast, the unsymmetrical complex (κN,κC,κN′-NCN)NiBr (3) reacts with KOtBu to form a centrosymmetric trinickel species {(κN,κC,κN′-NCN:η4-Ar,η4-Ar)Ni(μ3-O)K2}2Ni(μ3-OtBu)4 (4) bridged by four KOtBu moieties, in which the NCN ligand framework remains intact. In 4, the central Ni adopts a distorted tetrahedral geometry coordinated by four oxygen atoms, while each potassium center completes its coordination sphere through three oxygen atoms and an η4-interaction with a pendant phenyl group, revealing a rare κK,κO bridging mode for KOtBu. Comprehensive single-crystal X-ray diffraction analyses confirm all structures. These results demonstrate the capacity of pincer nickel halides to stabilize otherwise elusive alkali metal alkoxide aggregates and provide new insights into the ligand-directed structural diversity of these important reagents.