Molybdenum-Containing Metalloenzymes and Synthetic Catalysts for Conversion of Small Molecules

catalysts Review Molybdenum-Containing Metalloenzymes and Synthetic Catalysts for Conversion of Small Molecules Donghyeon Kim, Jaeheon Lee and Junhyeok Seo * Department of Chemistry, Gwangju Institute of Science and Technology, Gwangju 61005, Korea; [email protected] (D.K.); [email protected] (J.L.) * Correspondence: [email protected] Abstract: The energy deficiency and environmental problems have motivated researchers to develop energy conversion systems into a sustainable pathway, and the development of catalysts holds the center of the research endeavors. Natural catalysts such as metalloenzymes have maintained energy cycles on Earth, thus proving themselves the optimal catalysts. In the previous research results, the structural and functional analogs of enzymes and nano-sized electrocatalysts have shown promising activities in energy conversion reactions. Mo ion plays essential roles in natural and artificial catalysts, and the unique electrochemical properties render its versatile utilization as an electrocatalyst. In this review paper, we show the current understandings of the Mo-enzyme active sites and the recent advances in the synthesis of Mo-catalysts aiming for high-performing catalysts. Keywords: molybdenum; metalloenzymes; FDH; CODH; nitrogenase; homogeneous; heterogeneous electrocatalysts; HER; OER Citation: Kim, D.; Lee, J.; Seo, J. 1. Introduction Molybdenum-Containing Fixation of atmospheric nitrogen to ammonia has been promoted by the Haber–Bosch Metalloenzymes and Synthetic process, and the capacitated mass production of foods accelerated the increase of the world Catalysts for Conversion of Small population [1,2]. The rapid growth of fossil fuel-based industry unavoidably increased Molecules. Catalysts 2021, 11, 217. atmospheric CO2, and resultantly aggravated the global warming and climate crisis. To https://doi.org/10.3390/ suggest solutions for those anthropogenic issues and mitigate the current environmental catal11020217 problems, scientific progress to convert fossil fuel-based energy systems toward sustainable ways is keenly desired along with other social and economic efforts [3]. In the line of Academic Editor: Carla D. Nunes scientific cognition, we review catalysts for energy conversion reactions including small Received: 2 January 2021 molecules such as CO ,N ,H , and O as reactants or products. Numerous catalysts Accepted: 3 February 2021 2 2 2 2 Published: 6 February 2021 have been developed in recent decades owing to the high interest in energy conversion reactions [4–8]. Mo ion is found to be an essential component in biological catalysis [9], as Publisher’s Note: MDPI stays neutral well as a very promising metal candidate among Earth-abundant metals for applications to with regard to jurisdictional claims in electrocatalysts and photocatalysts [4,10,11]. Herein, we review versatile utilization of Mo published maps and institutional affil- ions in natural and synthetic catalysts. iations. Mo-containing homogeneous catalysts utilize both low- and high-valent Mo ions, whereas metalloenzymes and their model complexes have high-valent Mo centers. Except for FeMo-nitrogenase [12], Mo-enzyme active sites always use dithiolene ligation of the molybdopterin (MPT) moiety to stabilize a Mo(IV/VI) ion during the redox reaction and to assist electron transfer between metal and ligand [13]. Moreover, the chalcogenide Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. ligand and nearby amino acid play essential roles to determine the selective reactivity This article is an open access article of Mo active sites. Although enzyme mechanisms remain unclear despite significant distributed under the terms and interests, ligand identity and coordination structures seem to be critical factors to deter- conditions of the Creative Commons mine catalytic activity. We can propose synthetic routes for the design of new Mo-based Attribution (CC BY) license (https:// catalysts from comparing structures, properties, and reactivities of metalloenzymes and creativecommons.org/licenses/by/ artificial catalysts [14]. Although relatively less studied, Mo-clusters have shown promising 4.0/). electrocatalytic activities due to their intermediate behaviors between metal-coordination Catalysts 2021, 11, 217. https://doi.org/10.3390/catal11020217 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 35 Catalysts 2021, 11, 217 2 of 34 from comparing structures, properties, and reactivities of metalloenzymes and artificial catalysts [14]. Although relatively less studied, Mo-clusters have shown promising elec- trocatalyticcomplexes activities and nanoparticle due to catalyststheir interm [15].ediate In the behaviors perspective betwee of developingn metal-coordination efficient elec- complexestrocatalysts, and Mo-nanomaterials, nanoparticle catalysts which [15]. have In the shown perspective remarkable of developing electrocatalytic efficient activities, electrocatalysts,should be considered Mo-nanomaterials, together. Preparationwhich have methodsshown remarkable of Mo-nanomaterials electrocatalytic determine activities, their shouldcatalytic be considered activities, and together. proximal Preparation atoms and methods electronic of Mo-nanomaterials conditions around determine Mo ions their are catalyticclosely relatedactivities, to reactivitiesand proximal of surficial atoms Moand ions electronic [16,17]. conditions around Mo ions are closely related to reactivities of surficial Mo ions [16,17]. 2. CO2 Electrocatalysts 2. CO2Considering Electrocatalysts current energy systems, a drastic diminution of fossil fuel consumption seemsConsidering unrealistic. current However, energy continuous systems, researcha drastic for diminution recycling of anthropogenic fossil fuel consumption CO2 will be seemsable tounrealistic. provide feasible However, solutions continuous to current research energy for recycling and climate anthropogenic issues. Among CO2 will various be ableCO 2totransformation provide feasible methods, solutions electrochemical to current energy conversion and climate of CO2 mayissues. be anAmong attractive various way to be conjugated with solar/electric energy conversion reactions [18]. Mo-enzymes of the CO2 transformation methods, electrochemical conversion of CO2 may be an attractive way toformate be conjugated dehydrogenases with solar/electric [19] (Mo-FDH) energy and conversion the carbon reactions monoxide [18]. dehydrogenasesMo-enzymes of the [20 ] formate(MoCu-CODH) dehydrogenases are themost [19] (Mo-FDH) efficient CO and2-catalysts the carbon on Earth, monoxide and theirdehydrogenases synthetic models [20] are known, albeit short of the enzyme activity [21]. Moreover, many homogeneous and (MoCu-CODH) are the most efficient CO2-catalysts on Earth, and their synthetic models heterogeneous catalysts have been developed for CO conversion reactions. are known, albeit short of the enzyme activity [21]. Moreover,2 many homogeneous and heterogeneous catalysts have been developed for CO2 conversion reactions. 2.1. Metalloenzymes with CO2 Reactivity 2.1.1. Formate Dehydrogenases 2.1. Metalloenzymes with CO2 Reactivity Formate dehydrogenases (FDHs) is a group of metalloenzymes that catalyze a re- 2.1.1. Formate Dehydrogenases versible conversion of formate to CO2. The CO2 reactivity of the FDH active site has obtainedFormate research dehydrogenases interests because (FDHs) the is enzyme a group functions of metalloenzymes near the potential that ofcatalyze−420 mVa re- vs. versibleNHE (Normal conversion hydrogen of formate electrode) to CO scarcely2. The CO requiring2 reactivity overpotential. of the FDH It isactive known site that has the ob- re- tainedaction research rate is varied interests by bacteria. because Rhodobacterthe enzyme capsulatus functionsFDH nearfacilitates the potential oxidation of −420 of mV formate vs. −1 −1 NHEto CO (Normal2 at a rate hydrogen of 36.5 s electrode)and the scarcely reverse directionrequiring atoverpotential. a rate of 1.6 sIt is[ 22known]. Desulfovibrio that the −1 reactiondesulfuricans rate isFDH varied oxidized by bacteria. formate Rhodobacter much more capsulatus quickly FDH at 543 facilitates s and oxidation reduced of CO for-2 at −1 −1 −1 mate46.6 to s CO[232 at]. Thea rate FDH of 36.5 reactivity s and isthe directly reverse related direction to the at a active rate of site 1.6 structure, s [22]. Desulfovibrio which has a desulfuricansMo center surrounded FDH oxidized by twoformate pyranopterin-conjugated much more quickly at dithiolene 543 s−1 and ligand, reduced a chalcogenide CO2 at 46.6 s−(hydroxide1 [23]. The FDH or sulfide) reactivity ligand, is anddirectly SeCys related amino to acid the asactive a sixth site ligand structure, (Figure which1 ). In has addition, a Mo centeradditional surrounded roles of by adjacent two pyranopterin-conjuga amino acids such asted Arg dithiolene and His seem ligand, to be a chalcogenide important to assist(hy- droxidethe FDH or reaction sulfide) byligand, stabilizing and SeCys reaction amino intermediates. acid as a sixth ligand (Figure 1). In addition, additional roles of adjacent amino acids such as Arg and His seem to be important to assist − + − ◦0 the FDH reaction byCO stabilizing2 + 2e reaction+ H intermediates. HCOO ,E = −420 mV Figure 1. Oxidized form of the FDH active site from Escherichia coli (PDB 2IV2). Figure 1. Oxidized form of the FDH active site from Escherichia coli (PDB 2IV2). In 1997, Boyington and coworkers first reported the crystal structure of E. coli FDH [24]. Later,

Load more