Artificial Metalloproteins Containing Co₄O₄ Cubane Active Sites

Abstract

Artificial metalloproteins (ArMs) containing Co4O4 cubane active sites were constructed via biotin-streptavidin technology. Stabilized by hydrogen bonds (H-bonds), terminal and cofacial CoIII–OH2 moieties are observed crystallographically in a series of immobilized cubane sites. Solution electrochemistry provided correlations of oxidation potential and pH. For variants containing Ser and Phe adjacent to the metallocofactor, 1e−/1H+ chemistry predominates until pH 8, above which the oxidation becomes pH-independent. Installation of Tyr proximal to the Co4O4 active site provided a single H-bond to one of a set of cofacial CoIII–OH2 groups. With this variant, multi-e−/multi-H+ chemistry is observed, along with a change in mechanism at pH 9.5 that is consistent with Tyr deprotonation. With structural similarities to both the oxygen-evolving complex of photosystem II (H-bonded Tyr) and to amorphous water oxidation catalysts (Co4O4 core), these findings bridge synthetic and biological systems for water oxidation, highlighting the importance of secondary sphere interactions in mediating multi-e−/multi-H+ reactivity.

Stabilization of reactive Co 4 O 4 cubane oxygen-evolution catalysts within porous frameworks

“…An alternative method found to provide material of similar purity was Soxhlet extraction of the materials with boiling methanol over 16 h. This method is significantly faster than the chelation method and also produces material that is free of Co(II), as evidenced by the absence of any pink color in a chelating membrane when the Soxhlet-treated solids were stirred for 5 d together with the membrane. (17,19,22), which has a characteristic electron paramagnetic resonance (EPR) signal at g = 2.33 (43). After oxidation with aqueous ceric ammonium nitrate for 1 h (SI Appendix, Scheme S1), each of the networks exhibited an EPR spectrum that is consistent with that reported for 1 + (SI Appendix, Fig.…”

“…This arrangement gives a total of one hydroxide and one water ligating each cobalt(III) center. While molecular cationic dicobalt complexes containing cofacial [Co III 2 (H 2 O) 2 ] units in a napthyridine ligand platform (62) exhibit pK a values of 5.08 and 6.75, measurements of a [Co 4 O 4 ] artificial metalloprotein (22) determined that Co III -OH 2 sites in a more electron-rich cubane environment had a pK a value of 8.0. The latter pK a value is consistent with the observed 1:1 ratio of Co III -OH 2 :Co III -OH moieties at pH 8 in Co 4 -TPT-OH, Co 4 -TPP-OH, and Co 4 -TPB-OH.…”

“…for new generations of catalysts that meet the stringent demands of practical AP. These themes have recently been applied in the incorporation of a [Co 4 O 4 ] cluster into the mutated pocket of a metalloprotein, which allowed stabilization of this active site against condensation, and manipulation of secondary sphere interactions in mediating multielectron, multiproton reactivity (22). Here, we report the greatly improved stability of a [Co 4 O 4 ] molecular cluster by covalent immobilization in a porous metal-organic framework.…”

Mechanistically Driven Control over Cubane Oxo Cluster Catalysts

“…They are quite versatile catalytic model systems -both for nature's cuboidal and heterometallic {CaMn4O5} oxygen evolving complex (OEC) of photosystem II [52][53][54][55] and for crucial surface motifs of oxide-based water oxidation catalysts (WOCs). [56][57][58] While much emphasis has been placed on elucidating the oxygen evolution pathways of synthetic cubane clusters, considerably less is known about their own formation processes. [59][60][61][62][63] Heterometallic cubanes provide particularly excellent opportunities to study synergistic effects in oxide-related as well as in bio-inspired molecular catalysts, 64 but their targeted construction remains demanding.…”

Bi‐Microporous Metal–Organic Frameworks with Cubane [M4(OH)4] (M=Ni, Co) Clusters and Pore‐Space Partition for Electrocatalytic Methanol Oxidation Reaction

“…The classical square-planar M 4 units [16] and Zn 4 O-type clusters [17] are very common. In comparison, MOFs based on cubane [M 4 (OH) 4 ]c lusters are rare.R ecently,c lusters such as [Co 4 O 4 ] , [18] [MnCo 3 O 4 ], [19] [Mn 4 CaO 4 ], [20] [(Co/Fe) 4 O 4 ] [21] have been used as catalysts for photosynthesis and electrocatalytic water splitting. Although various structural mimics of the electrocatalysts have been reported, [22] 4 ]c lusters and a À3/0 ligand combination.…”