Solar-to-Chemical Conversion. Группа авторов
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The same type of oxyl–oxo coupling can take place starting with the S3B,W isomer. An alternative to the above mechanism was proposed by Shoji et al. [290] and also assumes S3A,W to be the active species, but involves initiation of O—O bond formation at the S3YZ• intermediate coupled with intramolecular proton transfer. This circumvents the need to invoke an actual S4 intermediate because the O—O bond is created with concomitant Mn(IV) reduction to Mn(III) before the tyrosyl radical of the S3YZ• intermediate is reduced by the inorganic core [290]. It is noted that formation of a terminal oxyl radical [291–293] in the hypothetical S4 state is connected to the high‐spin octahedral Mn(IV) ion in the S3 state, since formation of a genuine Mn(V) ion is unfavorable in such ligand field [289]. However, if water binding is not required for advancement of the OEC from the S3 to the S4 state, then the S3B model of Figure 3.11 can also be a candidate for catalytic progression. In this case, Krewald et al. [289] demonstrated that Mn4 remains five‐coordinate and forms a genuine Mn4(V)‐oxo species, with two unpaired electrons localized on the high‐spin Mn4(V) ion and no spin density on the equatorial oxo group [289]. This allows O—O bond formation to occur via genuine nucleophilic coupling that might occur synchronously with water binding to Mn4 (Figure 3.12b) [289], while the formation of three Jahn–Teller axes pointing simultaneously toward the newly formed O2 unit would contribute to its irreversible expulsion from the active site. The mechanism of Krewald et al. provides access to thermodynamically favorable even‐electron water oxidation [294, 295], potentially to a genuine single‐step 4‐electron transformation, and entirely avoids formation of potentially harmful radical intermediates [289]. Finally, it is worth mentioning an idea proposed by Zhang and Sun, as yet unsupported by quantum chemical calculations, according to which a redox isomerization in the highest state of the cycle could create a cluster with a highly oxidized Mn(VII) center and two terminal oxo groups that would couple to yield dioxygen [296]. Detailed discussions of these and other alternative hypotheses for the mechanism of biological O—O bond formation are available in recent literature [7, 255, 274–277, 287, 288, 290, 297–301].
Figure 3.12 Two selected scenarios for the nature of the S4 state and O—O bond formation from the computational literature: (a) formation of a Mn1(IV)‐oxyl group in the S4 state is followed by odd‐electron radical oxyl–oxo coupling [285], and (b) formation of a five‐coordinate high‐spin Mn4(V)‐oxo is followed by intramolecular nucleophilic coupling with concerted water binding [289]. Thick lines indicate direction of Jahn–Teller axes of Mn(III) ions.
It should be clear from the above that despite enormous strides, several aspects of the biological system, including its exact atomistic structure and crucial mechanistic details, remain incompletely understood for the later steps of the catalytic cycle. In the effort to better understand the natural water oxidation catalyst, a major target has been the synthesis of molecular mimics that reproduce structural and electronic properties of the OEC [302]. Following a long history in the development of oligonuclear manganese model complexes [303–305], the past decade has witnessed seminal achievements with the synthesis of manganese–calcium clusters that closely mimic the stoichiometry, metal oxidation states, and bonding topology of the OEC [306–318]. Landmark reports by the groups of Agapie [307] and Christou [308] established access to Mn(IV)3CaO4 cubanes, whose magnetic properties (ferromagnetic coupling to a total S = 9/2 state) mirror those of the cuboidal subunit of the OEC in specific states [198]. Zhang et al. [314] subsequently achieved the synthesis of a complex with a Mn4CaO4 core that reproduces the arrangement of metal ions of the OEC and has oxidation states equivalent to the S1 state of the OEC, Mn(III)2Mn(IV)2. Moreover, the complex can be oxidized and produces spectroscopic signatures similar to those of the natural system [237, 314]. Extensions of this work include variants of the original complex with exchangeable solvent molecules [316].
Molecular biomimetic complexes are indispensable for elucidating structure–property correlations of relevance to the OEC, and they are a valuable source of insight into how specific geometric or electronic features of a polynuclear manganese cluster affect its overall properties and function [319–323]. At the same time, it should be acknowledged that structural mimics of the OEC have not been linked so far to appreciable water oxidizing activity. Water oxidation has been known for heterogeneous manganese oxides [324–328], but as far as molecular systems are concerned, manganese complexes reported to catalyze oxygen evolution are typically not direct mimics of the OEC, while their performance lags far behind noble metal molecular or solid‐state catalysts [324–328]. There is undoubtedly vast unexplored potential for the development of biomimetic manganese‐based molecular water oxidation catalysts. However, our current understanding of the biological system strongly indicates that its catalytic ability is not simply encoded in the structure of the inorganic cluster of the OEC, but depends critically on the protein matrix that both fine‐tunes the properties of the cluster and performs crucial functions in terms of managing proton‐coupled electron transfer and regulating the flow of substrate and product. Therefore, it is conceivable that any small‐molecule mimic of the OEC, although useful as structural and electronic analog of the biological active site, is destined to fail as a practical water oxidation catalyst because it will not be able to reproduce the functionality that is taken care of by the PS‐II enzyme as a whole. Promising approaches that would be arguably more suitable for large‐scale realization of artificial photosynthesis are discussed in subsequent chapters.
3.6 Carbon Fixation
Capture of CO2 and reduction to products such as CO, HCOOC, H2CO, CH3OH, or CH4 are a principal target for artificial photosynthesis in the quest for solar fuels, as discussed in detail elsewhere in this book. A strictly biomimetic approach does not seem ideal in this case compared to photocatalytic and (photo)electrochemical reduction of CO2. However, there are still lessons to be learned from biology, and here we will briefly cover the CO2 fixation process in natural photosynthesis, which is carried out by the enzyme RuBisCO (ribulose‐1,5‐bisphosphate carboxylase/oxygenase) [4]. RuBisCO is considered the most abundant protein on earth [347, 348] and uses CO2 to convert the five‐carbon molecule ribulose‐1,5‐bisphosphate (RuBP) to two three‐carbon 3‐phosphoglycerate (3PGA) molecules, one of which incorporates the CO2‐derived carbon atom. This carboxylation reaction provides the substrate for subsequent reactions in the Calvin–Benson cycle that phosphorylate and reduce 3PGA using ATP and NADPH to produce glyceraldehyde 3‐phosphate (G3P), the precursor molecule of glucose and other carbohydrates. In most photosynthetic organisms RuBisCO is present as a complex composed of eight copies of large (L) proteins and eight copies of small (S) proteins, L8S8 [349]. The active form of the enzyme is generated by carbamylation of an active site lysine residue [350] via reaction with CO2 and subsequent binding of Mg2+ at the carbamate. This is the form that binds RuBP, at the Mg2+ ion [351]. The carboxylation reaction is thought to proceed by initial creation of the enediol form of RuBP, which reacts with CO2 and is hydrated before C—C bond cleavage and release of the two 3PGA molecules (Figure 3.13).