Selective Transformation of Nickel-Bound Formate to CO or C-C Coupling Products Triggered by Deprotonation and Steered by Alkali-Metal Ions

Abstract

The complexes [L^tBu Ni(OCO-κ² O,C)]M₃ [N(SiMe₃ )₂ ]₂ (M=Li, Na, K), synthesized by deprotonation of a nickel formate complex [L^tBu NiOOCH] with the corresponding amides M[N(SiMe₃ )₂ ], feature a Ni^II -CO₂ ^2- core surrounded by Lewis-acidic cations (M⁺ ) and the influence of the latter on the behavior and reactivity was studied. The results point to a decrease of CO₂ activation within the series Li, Na, and K, which is also reflected in the reactivity with Me₃ SiOTf leading to the liberation of CO and formation of a Ni-OSiMe₃ complex. Furthermore, in case of K⁺ , the {[K₃ [N(SiMe₃ )₂ ]₂ }⁺ shell around the Ni-CO₂ ^2- entity was shown to have a large impact on its stabilization and behavior. If the number of K[N(SiMe₃ )₂ ] equivalents used in the reaction with [L^tBu NiOOCH] is decreased from 3 to 0.5, the deprotonated part of the precursor enters a complex reaction sequence with formation of [L^tBu Ni^I (μ-OOCH)Ni^I L^tBu ]K and [L^tBu Ni(C₂ O₄ )NiL^tBu ]. The same reaction at higher concentrations additionally led to the formation of a unique hexanuclear Ni^II complex containing both oxalate and mesoxalate ([O₂ C-CO₂ -CO₂ ]^4- ) ligands.

Keywords: Lewis acid; formate; mesoxalate; nickel; oxalate.