catalysts Article Dehydrogenation of Formic Acid to CO2 and H2 by Manganese(I)–Complex: Theoretical Insights for Green and Sustainable Route Tiziana Marino * and Mario Prejanò Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, Ponte P. Bucci cubo 14 C, Arcavacata di Rende, CAP 87036 Cosenza, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-0984492085 Abstract: In this work, a detailed computational study on a recently synthetized Mn(I)-dependent tBu + complex [( PNNOP)Mn(CO)2] is reported. This species promotes the dehydrogenation of formic acid to carbon dioxide and hydrogen. The here proposed catalytic cycle proceeds through the tBu tBu + formation of stabilized adduct between [( PNNOP )Mn(CO)2] and formate and the progressive release of CO2 and H2, mediated by the presence of trimethylamine. In order to evaluate the influence of the environment on the catalytic activity, different solvents have been taken into account. The computed barriers and the geometrical parameters account well for the available experimental data, confirming the robustness of the complex and reproducing its good catalytic performance. Outcomes from the present investigation can stimulate further experimental works in the design of new more efficient catalysts devoted to H2 production. Keywords: CO ;H ; formic acid; transition state; DFT 2 2 Citation: Marino, T.; Prejanò, M. Dehydrogenation of Formic Acid to 1. Introduction CO2 and H2 by Manganese(I)–Complex: Theoretical The increasing need for new and sustainable energetic resources represents one of Insights for Green and Sustainable the most important challenges characterizing the current century [1,2]. Indeed, fossil fuels, Route. Catalysts 2021, 11, 141. gas, coal and nuclear energy are still widely used, but environmentally dangerous, energy https://doi.org/10.3390/catal11 sources [2–5]. The intensive use of fossil fuels, for example, has been directly linked to 010141 the increasing level of CO2 and greenhouse gas emissions, dramatically influencing the climate changes [3]. Received: 4 January 2021 For these reasons, in the last fifty years, the interest devoted to possible “green” Accepted: 16 January 2021 alternatives, like the use of sunlight-, wind- and water-based energies [6], have been Published: 19 January 2021 increased, but, despite their promising efficiency, different technical issues are related to them and in particular to the storage of energy vectors on large scale [7]. Publisher’s Note: MDPI stays neutral One of the possible solutions is represented by the so-called sustainable hydrogen with regard to jurisdictional claims in economy [8–11]. In this route, indeed, the electricity is converted in a secondary chemical published maps and institutional affil- energy carrier that can be used on demand [12–16]. The combustion of H2 in the presence iations. of O2, in devices like fuel cells, formally produces electricity and H2O, a green product. On the other hand, the H2 is not present on earth and it can be obtained/stored from/in organic compounds, like methanol and formic acid (FA) [17–19]. In particular, since its chemical-physical properties and its involvement in chemical industries and biomass Copyright: © 2021 by the authors. production, the FA is believed a promising species for the hydrogen economy [1,2,20,21]. Licensee MDPI, Basel, Switzerland. The dehydrogenation of FA, which generates CO2 and H2, (Scheme1) is usually mediated This article is an open access article by metal-containing catalysts. distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Catalysts 2021, 11, 141. https://doi.org/10.3390/catal11010141 https://www.mdpi.com/journal/catalysts CatalystsCatalystsCatalysts2021 20212021,,11, 1111,, 141, xx FORFOR PEERPEER REVIEWREVIEW 222 of ofof 10 1010 Scheme 1. Conversion of formic acid in CO2 and H2. SchemeScheme 1.1. ConversionConversion ofof formicformic acidacid inin COCO22 and H2.. InInIn detail, detail,detail, the thethe most mostmost widely widelywidely adopted adoptedadopted species speciesspecies contain cocontainntain Pd, Pd,Pd, Pt, Pt,Pt, Ir, RhIr,Ir, RhRh and anan Rudd Ru (seeRu (see(seerefs refsrefs [22 – [22–[22–26] 2 6− −1 as26]26] recent asas recentrecent examples), examples),examples), with withwith turnover turnoverturnover frequencies frequenciesfrequencies (TOFs) (TOFs)(TOFs) variating variatingvariating from fromfrom 10 21010to2 toto 10 10106 h6 hh−11 [ [27].[27].27]. SinceSinceSince allall these these species species containcontain contain preciousprecious precious metalsmetals metals,,, nowadaysnowadays nowadays oneone one ofof of thethe the mainmain main goalsgoals goals isis synthe-synthe- is syn- thesizingsizingsizing complexescomplexes complexes containingcontaining containing equallyequally equally efficientefficient efficient metalmetal metal ionsions ions belongingbelonging belonging toto thethe to firstfirst the row firstrow of rowof tran-tran- of transitionsitionsition series,series, series, whichwhich which areare arenotoriouslynotoriously notoriously cheapercheaper cheaper andand and moremore more abundantabundant abundant inin thethe in theEarth’sEarth’s Earth’s crust.crust. crust. PromisingPromisingPromising solutions solutionssolutions can cancan arise arisearise bybyby Fe-basedFe-basedFe-based catalystscatalysts showingshowing turnoverturnover valuesvalues compa-compa- rablerablerable tototo thosethosethose containingcontainingcontaining preciouspreciousprecious metals,metals,metals, butbutbut stillstillstill demandingdemandingdemanding ananan improvementimprovementimprovement ofofof their theirtheir catalyticcatalyticcatalytic efficiency efficiencyefficiency [ [28–30].28[28–30].–30]. In InIn addition, adaddition,dition, these thesethese species speciesspecies require requirerequire the thethe presence presencepresence of ofof a aa general generalgeneral base basebase ++ andandand Lewis LewisLewis acid, acid,acid, like likelike Li LiLi+,, to to explicate explicate the the catalysis catalysis [[29].[29].29]. VeryVeryVery recently,recently, forfor aa a newnew new synthesizedsynthesized synthesized complexcomplex complex ofof Mn(I)-basedMn(I)-based of Mn(I)-based complex,complex, complex, fastfast catalyzedcatalyzed fast cat- alyzeddehydrogenationdehydrogenation dehydrogenation ofof formicformic of acidacid formic hashas acidbeenbeen has describeddescribed been described [31].[31]. TheThe organometallicorganometallic [31]. The organometallic compound,compound, tbu + tbu compound, having the structuretbu [( PNNOPMn(CO)2 + ] (2), withtbu PNNOP = 2,6-(di-tert- havinghaving thethe structurestructure [([(tbuPNNOPMn(CO)PNNOPMn(CO)2]]+ ((22),), 2withwith tbuPNNOPPNNOP == 2,6-(di-tert-bu-2,6-(di-tert-bu- butylphosphinito)(di-tert-butylphosphinamine)pyridine),tylphosphinito)(di-tert-butylphosphinamine)pyridine),tylphosphinito)(di-tert-butylphosphinamine)pyridine), exhibitedexhibited exhibited substantiallysubstantially higherhigherhigher catalyticcatalyticcatalytic activityactivityactivity withwithwith respect respectrespect to toto the thethe unique uniqueunique known knownknown Mn-dependent Mn-dependentMn-dependent complex complexcomplex acting actingacting on onon the thethe −1−1 formicformicformic acidacidacid [ [32],32[32],], with withwith TOFs TOFsTOFs improved improvedimproved to toto the thethe value valuevalue of ofof 8500 85008500 h hh−1 [ [31].[31].31]. TheTheThe reactionreactionreaction requires requiresrequires the presence of a proton-acceptor system, but intriguingly the catalysis was not affected by thethe presencepresence ofof aa proton-acceptorproton-acceptor system,system, bubutt intriguinglyintriguingly thethe catalysiscatalysis waswas notnot affectedaffected the nature of the base [31]. byby thethe naturenature ofof thethe basebase [31].[31]. tbutbu InIn addition,addition, thethe contribution contribution ofof the the hybrid hybrid tbuPNNOPPNNOP ligandligand hashas beenbeen highlighted,highlighted, In addition, thetbu contributiontBu of the hybrid PNNOP ligand has been highlighted, tbu tBu sincesince attemptsattempts with with tbuPONOPPONOP ((tBuPONOPPONOP =2,6-bis(di-tert-butylphosphinito)pyridine,=2,6-bis(di-tert-butylphosphinito)pyridine, 44)) sincetbu attempts withtBu PONOP ( PONOP =2,6-bis(di-tert-butylphosphinito)pyridine, 4) and tbuPNNNP ( tBu PNNNP = 2,6-bis (di-tert-butylphosphinamine)pyridine, 5), two ligands andand tbuPNNNPPNNNP ((tBuPNNNPPNNNP == 2,6-bis2,6-bis (di-tert-butylphosphinamine)pyridine,(di-tert-butylphosphinamine)pyridine, 55),), twotwo ligandsligands tested on a previous Ru-based organometallic complex and depicted in Scheme2[33] , did testedtested onon aa previousprevious Ru-basedRu-based organometallicorganometallic complexcomplex andand depicteddepicted inin SchemeScheme 22 [33],[33], diddid not provide remarkable catalytic activity [31]. notnot provideprovide remarkableremarkable catalyticcatalytic activityactivity [31].[31]. SchemeSchemeScheme 2. 2.2. The TheThe three threethree ligands ligandsligands (on (on(on the thethetop toptop)) and) andand the thethe three threethree related relatedrelated complexes complexescomplexes (on (on(on the thethebottom bottombottom). The).). TheThe name ofnamename the complexesofof thethe complexescomplexes has been hashas retained beenbeen retainedretained according accordingaccording to the experimental toto thethe experimentalexperimental work [ 31 workwork]. [31].[31]. Encouraged by the results’ novelty,