Mixed Valence {Ni²⁺Ni¹⁺} Clusters as Models of Acetyl Coenzyme A Synthase Intermediates

Acetyl coenzyme A synthase (ACS) catalyzes the formation and deconstruction of the key biological metabolite, acetyl coenzyme A (acetyl-CoA). The active site of ACS features a {NiNi} cluster bridged to a [Fe₄S₄]ⁿ⁺ cubane known as the A-cluster. The mechanism by which the A-cluster functions is debated, with few model complexes able to replicate the oxidation states, coordination features, or reactivity proposed in the catalytic cycle. In this work, we isolate the first bimetallic models of two hypothesized intermediates on the paramagnetic pathway of the ACS function. The heteroligated {Ni²⁺Ni¹⁺} cluster, [K(12-crown-4)₂][1], effectively replicates the coordination number and oxidation state of the proposed "A_red" state of the A-cluster. Addition of carbon monoxide to [1]^- allows for isolation of a dinuclear {Ni²⁺Ni¹⁺(CO)} complex, [K(12-crown-2)_n][2] (n = 1-2), which bears similarity to the "A_NiFeC" enzyme intermediate. Structural and electronic properties of each cluster are elucidated by X-ray diffraction, nuclear magnetic resonance, cyclic voltammetry, and UV/vis and electron paramagnetic resonance spectroscopies, which are supplemented by density functional theory (DFT) calculations. Calculations indicate that the pseudo-T-shaped geometry of the three-coordinate nickel in [1]^- is more stable than the Y-conformation by 22 kcal mol^-1, and that binding of CO to Ni¹⁺ is barrierless and exergonic by 6 kcal mol^-1. UV/vis absorption spectroscopy on [2]^- in conjunction with time-dependent DFT calculations indicates that the square-planar nickel site is involved in electron transfer to the CO π*-orbital. Further, we demonstrate that [2]^- promotes thioester synthesis in a reaction analogous to the production of acetyl coenzyme A by ACS.