Oxygen Atom Transfer and Oxidative Water Incorporation in Cuboidal Mn₃MO_n Complexes Based on Synthetic, Isotopic Labeling, and Computational Studies

Abstract

The oxygen-evolving complex (OEC) of photosystem II contains a Mn 4 CaO n catalytic site, in which reactivity of bridging oxidos is fundamental to OEC function. We synthesized structurally relevant cuboidal Mn 3 MO n complexes (M = Mn, Ca, Sc; n = 3,4) to enable mechanistic studies of reactivity and incorporation of μ 3 -oxido moieties. We found that Mn IV 3 CaO 4 and Mn IV 3 ScO 4 were unreactive toward trimethylphosphine (PMe 3 ). In contrast, our Mn III 2 Mn IV 2 O 4 cubane reacts with this phosphine within minutes to generate a novel Mn III 4 O 3 partial cubane plus Me 3 PO. We used quantum mechanics to investigate the reaction paths for oxygen atom transfer to phosphine from Mn III 2 Mn IV 2 O 4 and Mn IV 3 CaO 4 . We found that the most favorable reaction path leads to partial detachment of the CH 3 COO − ligand, which is energetically feasible only when Mn(III) is present. Experimentally, the lability of metal-bound acetates is greatest for Mn III 2 Mn IV 2 O 4 . These results indicate that even with a strong oxygen atom acceptor, such as PMe 3 , the oxygen atom transfer chemistry from Mn 3 MO 4 cubanes is controlled by ligand lability, with the Mn IV 3 CaO 4 OEC model being unreactive. The oxidative oxide incorporation into the partial cubane, Mn III 4 O 3 , was observed experimentally upon treatment with water, base, and oxidizing equivalents. 18 O-labeling experiments provided mechanistic insight into the position of incorporation in the partial cubane structure, consistent with mechanisms involving migration of oxide moieties within the cluster but not consistent with selective incorporation at the site available in the starting species. These results support recent proposals for the mechanism of the OEC, involving oxido migration between distinct positions within the cluster.

Reduction potentials of heterometallic manganese–oxido cubane complexes modulated by redox-inactive metals

“…The strontium compound [LSrMn 3 O 4 (OAc) 3 Fig. 2, DMF, N,N-dimethylformamide) was prepared analogously to 1-Ca from a lowoxidation-state trimanganese precursor supported by a triarylbenzene architecture appended with pyridine and alkoxide donors (31,32 (25).…”

“…With these complexes in hand, in addition to the previously reported compounds 1-Ca, [OTf], and LMn 4 O 4 (OAc) 3 (24,25), the effect of changing the redox-inactive metals in the clusters was studied electrochemically. Cyclic voltammograms (CVs) of (Fig.…”

“…The (24,25) were plotted against the pK a of the metal aqua ions measured in water (38), used here as a measure of the Lewis acidity of cation M (Table S6). As with the related series of trimanganese dioxido complexes (23), a linear correlation is observed (Fig.…”

“…We have previously described a [CaMn IV 3 O 4 ] cubane complex (1-Ca) (24,25) supported by a multinucleating ligand framework (H 3 L, Fig. 2) that is structurally related to the CaMn 3 OEC subsite proposed by crystallographic studies (Fig.…”

Computational study on chain pathways for oxygen atom transfer catalyzed by a methyl(dithiolate) thiorhenium(V) compound

“…Up to my knowledge, this is the first example of d 2 mono oxo complex with nucleophilic properties. To gain further insight into the details mechanism and the nature of the intermediate that involved in the catalytic process of the metal oxo complexes, computational studies on these complexes have been performed and attracted the scientific attention [23][24][25][26][27][28]. The general function of oxomolybdenum enzymes is catalysis of oxygen transfer reaction through the catalytic cycle between Mo(VI)O 2 and Mo(IV)=O [1,29].…”