Controlling the Activation at NiII−CO22− Moieties through Lewis Acid Interactions in the Second Coordination Sphere

In nickel-carbonite complexes the degree of CO₂ activation depends on the nature of the alkali metal counterion, the strength of its interaction with the carbonite unit – and thus its spacial separation – as well as on further polar entities in the second coordination sphere. This is also reflected in the reactivity towards electrophiles which lead to C−O bond cleavage.

Abstract

Nickel complexes with a two-electron reduced CO₂ ligand (CO₂ ²⁻, “carbonite”) are investigated with regard to the influence alkali metal (AM) ions have as Lewis acids on the activation of the CO₂ entity. For this purpose complexes with Ni^II(CO₂)AM (AM=Li, Na, K) moieties were accessed via deprotonation of nickel-formate compounds with (AM)N(ⁱPr)₂. It was found that not only the nature of the AM ions in vicinity to CO₂ affect the activation, but also the number and the ligation of a given AM. To this end the effects of added (AM)N(R)₂, THF, open and closed polyethers as well as cryptands were systematically studied. In 14 cases the products were characterized by X-ray diffraction and correlations with the situation in solution were made. The more the AM ions get detached from the carbonite ligand, the lower is the degree of aggregation. At the same time the extent of CO₂ activation is decreased as indicated by the structural and spectroscopic analysis and reactivity studies. Accompanying DFT studies showed that the coordinating AM Lewis acidic fragment withdraws only a small amount of charge from the carbonite moiety, but it also affects the internal charge equilibration between the L^tBuNi and carbonite moieties.

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