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, which bring Mn into the family of earth-abundant EncouragedEncouraged byby thethe results’results’ novelty,novelty, whichwhich bringbring MnMn intointo thethe familyfamily ofof earth-abun-earth-abun- metals catalyzing FA dehydrogenation [31], a detailed mechanistic study on the catalysis dantdant metalsmetals catalyzingcatalyzing FAFA dehydrogenationdehydrogenation [31],[31], aa detailedetailedd mechanisticmechanistic studystudy onon thethe ca-ca- mediated by 2 in the framework of density functional theory (DFT) has been performed, talysistalysis mediatedmediated byby 22 inin thethe frameworkframework ofof densitydensity functionalfunctional theorytheory (DFT)(DFT) hashas beenbeen per-per- with the aim of shedding light on the possible reaction mechanism and to characterize
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the intermediates and transitions states intercepted along the related potential energy surface (PES). The adducts with formic acid of two other manganese ligands (complex 4, buPNNOP and complex 5, BuPNNNP of Scheme2) have been considered. In addition, different solvents have been taken into account to evaluate their effect on the rate limiting step. We hope that our results on the detailed catalytic mechanism of 2 can increase the knowledge of such a promising complex, helping the rationalization of experiments design of more efficient catalysts.
2. Results and Discussion Before proceeding with the mechanistic study, it was mandatory to determine the spin multiplicity. For this purpose, optimizations of 2 catalyst in different spin states (2S + 1 = 1, 3, 5) have been performed. The singlet state (2S + 1 = 1) resulted in being the most stable one in agreement with d6 configuration of Mn(I) species, as analogously determined in a recent study concerning the Mn(I)-mediated catalysis of organic nitriles to amides [34], while the triplet and quintet lay at 12.1 kcal/mol and 18.7 kcal/mol, respectively, with respect to the singlet spin state. Small spin contamination (<6%, see Table S1), monitored as described in other works [35,36], has been observed with
tbu + − tbu [( PNNOPMn(CO)2] + HCOO → [( PNNOPMn(H)(CO)2] + CO2 (1)
tbu + tbu + [( PNNOPMn(H)(CO)2] + Me3NH → [( PNNOPMn(CO)2] + H2 + Me3N (2)
In the first (Equation (1)), the CO2 (the first product) is released with concerted formation of H-Mn bond. In the second phase (Equation (2)), due to the presence of + Me3NH species, the H2 (the second product) is formed and subsequently released. Every phase consisted of a multistep process. The final mechanism included the formic acid in its deprotonated form (FA) produced by the trimethylamine acting as proton acceptor, since all the attempts performed on neutral species failed. The first part of the study is related to the formation of adduct between complex 2 and formate (FA). Two different conformers have been characterized with the formate bound to the Mn(I) in both Mn-O− (2:FA) and Mn-H (2:FA’) fashions as shown in the optimized structures illustrated in Figure2. The 2:FA’ is higher in energy (8.3 kcal/mol) with respect to 2:FA, but, in spite of this, it resulted in being more reactive with the C-H distance of 1.29 Å and the OCOˆ angle of 140◦, if compared to the corresponding ones in 2:FA (1.11 Å and 129◦, respectively). Catalysts 2021, 11, 141 4 of 10 Catalysts 2021, 11, x FOR PEER REVIEW 4 of 10
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SchemeScheme 3. 3.The The investigatedinvestigated mechanism mechanism for the for FA the to FA CO to2 conversion CO2 conversion catalyzed catalyzed by 2. by 2.
In the first (Equation (1)), the CO2 (the first product) is released with concerted for- Figure 1. Free energy profile of the FA to CO2 conversion catalyzed by complex 2 at the B3LYP- mation of H-Mn bond. In the second phase (Equation (2)), due to the presence of Me3NH+ D3/6-311+G(2d,2p)/SDD/Chlorobenzene level of theory. Energy barriers are depicted in red. species, the H2 (the second product) is formed and subsequently released. Every phase consisted of a multistep process. The final mechanism included the formic acid in its In the intermediate 2(H) the H-Mn bond resulted in being definitely formed, as con- deprotonated form (FA) produced by the trimethylamine acting as proton acceptor, since firmed by the distance of 1.60 Å (see Figure S2). From this species starts the second phase all the attempts performed on neutral species failed. of the dehydrogenation of formic acid leading to the formation of H2. To do this, a proto- The first part of the study is related to the formation of adduct between complex 2 nated species, most likely the conjugated acid of the base involved in the catalysis, must and formate (FA). Two different conformers have been characterized with the formate come into play acting as proton donor. As confirmed by experimental evidences [31], its bound to the Mn(I) in both Mn-O− (2:FA) and Mn-H (2:FA’) fashions as shown in the op- presence is fundamental for the advancing of the reaction, being deputed to the deproto- timized structures illustrated in Figure 2. The 2:FA’ is higher in energy (8.3 kcal/mol) with nation of the formic acid. Moreover, Tondreau et al. in their kinetic studies advised that respect to 2:FA, but, in spite of this, it resulted in being more reactive with the C-H dis- “the identity of the base does not significantly alter catalytic results” [31], so in our investigation tance of 1.29 Å and the OĈO angle of 140°, if compared to the corresponding ones in 2:FA the protonated triethylamine used in the experiment has been replaced by the protonated (1.11 Å and 129°, respectively). trimethylamine (Me3NH+ or TMA) in order to save computational cost. The interconversion between the Mn-O− and Mn-H takes place with an energy barrier The 2(H):Me3NH+ species lying at 4.4 kcal/mol below 2:FA- is the result of the addi- of 15.5 kcal/mol across the transition state TS rot (see Figure 1). In the corresponding op- tion of the protonated base and represents the starting point of the next H2 release step. FigureFigure 1. 1.FreeFreetimized energy energy geometry profile profile of (seethe of FA theFigure to FA CO to2),2 COconversionthe2 formateconversion catalyzed acts as catalyzed a by bidentate complex by complexligand 2 at the owing B3LYP-2 atthe to the B3LYP-D3/6- form- The presence of the Me3NH+ induces a small elongation of the Mn-H bond (1.65 Å) if com- D3/6-311+G(2d,2p)/SDD/Chlorobenzeneing Mn-H 2.22 Å and the breaking level of thMn-O-bondseory. Energy 2.81 barriers Å, thus are offeringdepicted an in epta-coordinatedred. 311+G(2d,2p)/SDD/Chlorobenzenepared with the value of 1.60 Å in 2(H) level species of theory. (see Figure Energy S2) barriers indicating are its depicted early activation. in red. manganese. DFT calculations on the dehydrogenation of formic acid promoted by a pin- In the cer-supportedintermediate 2(H)iron catalystthe H-Mn evidenced bond resulted analogou ins beingbehavior definitely proposing formed, an energy as con- barrier of firmed by the21.4 distance kcal/mol of for 1.60 the Å switching (see Figure from S2). the From Fe-O this− to the species Fe-H startsconformer the second [29]. phase of the dehydrogenationStarting fromof formic 2:FA’ acid, the leading CO2 release to the takes formation place byof Hovercoming2. To do this, the a barrierproto- of 8.4 nated species,kcal/mol most representedlikely the conjugated by TS1 in Figure acid of 3. the In particular,base involved the H in lies the at catalysis,1.76 Å from must the Mn(I) come into playand atacting 1.67 Åas from proton the donor.C, while As the confirmed value of O byĈO experimental angle (152°) increased evidences preparing [31], its for the −1 presence is nextfundamental carbon dioxide for the release. advancing The related of the reaction,imaginary being frequency deputed (236 ito cm the) deproto-concerns the C- nation of theH stretching.formic acid. Moreover, Tondreau et al. in their kinetic studies advised that “the identity of the base does not significantly alter catalytic results” [31], so in our investigation the protonated triethylamine used in the experiment has been replaced by the protonated + trimethylamine (Me3NH or TMA) in order to save computational cost. The 2(H):Me3NH+ species lying at 4.4 kcal/mol below 2:FA- is the result of the addi- tion of the protonated base and represents the starting point of the next H2 release step. + Figure 2.FigureB3LYP-D3/6-31+G(d,p) 2. B3LYP-D3/6-31+G(d,p)The presence optimized optimized of the geometries Megeometries3NH induces of 2:FA,2:FA, TSa TSsmall rot rot and elongationand 2:FA’2:FA’. Distances. Distancesof the implicated Mn-H implicated bond in chemical (1.65 in Å) chemical event if com- event (black) are in Å. Imaginarypared freq withuencies the value are also of reported.1.60 Å in For 2(H) clarity, species hydrogens (see Figure are not S2) shown. indicating its early activation. (black) are in Å. Imaginary frequencies are also reported. For clarity, hydrogens are not shown.
Indeed, the proton shift, which leads to the formation of the H2 product, is described in the TS2, where the forming Hbase-H bond assumes a value of 1.05 Å vs. that of 1.45 Å found in 2(H):Me3NH+ of Figure S2 while the Mn-H results to be 1.71 Å. These findings evidence the important role of base also in the formation of the second product, since its nature as a proton-donor species. TS2 lies at 11.8 kcal/mol related to the 2(H):Me3NH+ intermediate but at 7.4 kcal/mol with respect to 2:FA (see Figure 1). In the 2(H2) species,
Figure 2. B3LYP-D3/6-31+G(d,p) optimized geometries of 2:FA, TS rot and 2:FA’. Distances implicated in chemical event (black) are in Å. Imaginary frequencies are also reported. For clarity, hydrogens are not shown.
Indeed, the proton shift, which leads to the formation of the H2 product, is described in the TS2, where the forming Hbase-H bond assumes a value of 1.05 Å vs. that of 1.45 Å found in 2(H):Me3NH+ of Figure S2 while the Mn-H results to be 1.71 Å. These findings evidence the important role of base also in the formation of the second product, since its nature as a proton-donor species. TS2 lies at 11.8 kcal/mol related to the 2(H):Me3NH+ intermediate but at 7.4 kcal/mol with respect to 2:FA (see Figure 1). In the 2(H2) species,
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The interconversion between the Mn-O− and Mn-H takes place with an energy barrier of 15.5 kcal/mol across the transition state TS rot (see Figure1). In the corresponding opti- Catalysts 2021, 11, x FOR PEER REVIEW 6 of 10 mized geometry (see Figure2), the formate acts as a bidentate ligand owing to the forming Mn-H 2.22 Å and the breaking Mn-O-bonds 2.81 Å, thus offering an epta-coordinated manganese. DFT calculations on the dehydrogenation of formic acid promoted by a pincer- thesupported molecular iron hydrogen catalyst is evidenced directly linked analogous to the Mn(I). behavior The proposing H2 σ bond an length energy is 0.77 barrier Å and of − the21.4 distance kcal/mol of forH2 from the switching metallic center from the is equal Fe-O toto 1.91 the Å, Fe-H see conformerFigure S2. [29]. TheStarting TS1 describing from 2:FA’ ,the the release CO2 release of CO takes2 represents place by the overcoming highest relative the barrier barrier, of present- 8.4 kcal/ ingmol an represented energy 1.2 byandTS1 9.3in kcal/mol Figure3 .higher In particular, than those the of H liesthe TSrot at 1.76 and Å from TS2 the, respectively. Mn(I) and Theat 1.67 value Å fromof 16.7 the kcal/mol, C, while in the addition, value of is OinCOˆ good angle agreement (152◦) increased with the preparingavailable kinetic for the −1 datanext proposing carbon dioxide a kcat value release. of 2.4 The s− related1, converted imaginary to a Δ frequencyG# = 17 kcal/mol (236i cm adopting) concerns Eyring’s the equationC-H stretching. [31].
FigureFigure 3. 3. B3LYP-D3/6-31+G(d,p)B3LYP-D3/6-31+G(d,p) optimized optimized geometries geometries of TS1 of TS1 andand TS2TS2. Relevant. Relevant distances distances (in (in black)black) are are in in Å. Imaginary frequenciesfrequencies areare alsoalso indicated. indicated. For For clarity, clarity, hydrogens hydrogens were were not not represented. repre- sented. In the intermediate 2(H) the H-Mn bond resulted in being definitely formed, as con- firmedAs bycan the be distance also noted of 1.60from Å the (see analysis Figure S2). of th Frome energy this speciesprofile, starts the kinetic the second of reversible phase of reaction,the dehydrogenation proceeding from of formic 2(H2) acid → 2:FA leading is slower. to the formationIndeed, for of this H2. reactionTo do this, an aenergy protonated bar- rierspecies, of 21.1 most kcal/mol likely resulted the conjugated for the step acid 2(H):Me of the base3NH involved+-TS1, 4.4 in kcal/mol the catalysis, higher must than come that calculatedinto play acting for the as 2:FA proton → donor.2(H2) pathway. As confirmed by experimental evidences [31], its presence is fundamentalWe also attempted for the advancing to rationalize of the the reaction, effect of being the pincer deputed ligands to the 4 deprotonation and 5 of Scheme of the 2 formic acid. Moreover, Tondreau et al. in their kinetic studies advised that “the identity of on the catalysis, focusing on their affinity (ΔHaff) for the substrate and to the fundamental formationthe base does of notcomplex:substrate significantly alter adduct catalytic that results represents”[31], so a in crucial our investigation step for the theproceeding protonated of triethylamine used in the experiment has been replaced by the protonated trimethylamine catalysis, +as follows: (Me3NH or TMA) in order to save computational cost. + − + - tbu tbu tbu- [XMn(CO)2] + HCOOThe 2(H):Me → [(XMn(HCOO3NH species)(CO) lying2] atX = 4.4 PNNOP, kcal/mol belowPONOP,2:FA isPNNNP the result of the addi-(3) tion of the protonated base and represents the starting point of the next H2 release step. The + presence ofΔH theaff Me= H3[(XMn(HCOONH induces−)(CO)2] a− smallH[XMn(CO)2]+ elongation − HHCOO of− the Mn-H bond (1.65 Å) if compared(4) with the value of 1.60 Å in 2(H) species (see Figure S2) indicating its early activation. In the case of the complex 2 the adduct 2:FA is more stabilized in energy (−35.1 Indeed, the proton shift, which leads to the formation of the H product, is described kcal/mol respect to the separated reactants), contrarily to the complexes2 4:FA and 5:FA in the TS2, where the forming Hbase-H bond assumes a value of 1.05 Å vs. that of 1.45 Å (16.6 and 22.0 kcal/mol).+ Based on these results, the increasing energy request for the for- found in 2(H):Me3NH of Figure S2 while the Mn-H results to be 1.71 Å. These findings mation of adduct (complex 2 > complex 4 > complex 5) well reproduces the experimental evidence the important role of base also in the formation of the second product, since its observations [31]. nature as a proton-donor species. TS2 lies at 11.8 kcal/mol related to the 2(H):Me NH+ A possible explanation for this behavior arises from the analysis of the charge distri-3 intermediate but at 7.4 kcal/mol with respect to 2:FA (see Figure1). In the 2(H2) species, bution in the three 2:FA, 4:FA and 5:FA adducts, reported in Figure S3. the molecular hydrogen is directly linked to the Mn(I). The H2 σ bond length is 0.77 Å and In particular, an evident effect concerns the Mn(I) center that exhibits a more negative the distance of H2 from metallic center is equal to 1.91 Å, see Figure S2. charge in presence of tbuPNNNP (−0.08 |e|) and of tbuPONOP (−0.05 |e|) than in presence The TS1 describing the release of CO2 represents the highest relative barrier, pre- tbu ofsenting the anPNNOP. energy This 1.2 and could 9.3 kcal/molconsequently higher affect than the those coordination of the TSrot ofand theTS2 negatively, respec- − chargedtively. The HCOO value with of 16.7 a reduced kcal/mol, catalytic in addition, activity. is in good agreement with the available The ligand tbuPNNOP proposes the right balance between electron withdrawing and electron donating effect, represented respectively by the O and N atoms, to the pyridine ring. Moreover, from the maps of the electrostatic potential of the three ligands depicted in Figure S4 emerges as the presence of two oxygens in tbuPONOP leads to a marked de- crease of negative charge on the aromatic ring contrarily to what occurs in tbuPNNNP, this behavior can have repercussions on their catalytic activity.
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−1 # kinetic data proposing a kcat value of 2.4 s , converted to a ∆G = 17 kcal/mol adopting Eyring’s equation [31]. As can be also noted from the analysis of the energy profile, the kinetic of reversible reaction, proceeding from 2(H2) → 2:FA is slower. Indeed, for this reaction an energy + barrier of 21.1 kcal/mol resulted for the step 2(H):Me3NH -TS1, 4.4 kcal/mol higher than that calculated for the 2:FA → 2(H2) pathway. We also attempted to rationalize the effect of the pincer ligands 4 and 5 of Scheme2 on the catalysis, focusing on their affinity (∆Haff) for the substrate and to the fundamental formation of complex:substrate adduct that represents a crucial step for the proceeding of catalysis, as follows: + − - tbu tbu tbu [XMn(CO)2] + HCOO → [(XMn(HCOO )(CO)2] X = PNNOP, PONOP, PNNNP (3)
∆Haff = H[(XMn(HCOO−)(CO)2] − H[XMn(CO)2]+ − HHCOO− (4) In the case of the complex 2 the adduct 2:FA is more stabilized in energy (−35.1 kcal/mol respect to the separated reactants), contrarily to the complexes 4:FA and 5:FA (16.6 and Catalysts 2021, 11, x FOR PEER REVIEW 7 of 10 22.0 kcal/mol). Based on these results, the increasing energy request for the formation of adduct (complex 2 > complex 4 > complex 5) well reproduces the experimental observations [31]. A possible explanation for this behavior arises from the analysis of the charge distri- butionGiven in thethe threeimportant2:FA, 4:FArole playedand 5:FA in adducts,homogeneous reported catalysis in Figure by the S3. solvent, with the aim ofIn investigating particular, an its evident effect on effect the concernsdehydrogenation the Mn(I) of center formic that acid exhibits assisted a more by the negative 2 cat- alyst,charge different in presence implicit of tbusolvenPNNNPts have (− 0.08been |e|) tested and on of thetbu COPONOP2 release (− 0.05step |e|)(TS1 than), consider- in pres- ingence dielectric of the tbu constantPNNOP. with This gradually could consequently increasing values affect theto simulate coordination apolar, of thepolar negatively aprotic andcharged polar HCOOprotic solvents− with a extensively reduced catalytic used in activity. organometallic reactions [37]. The results are collectedThe in ligand Figuretbu 4.PNNOP proposes the right balance between electron withdrawing and electronThe use donating of polar effect, aprotic represented solvent with respectively lower dielectric by the constant, O and N atoms,respect toto thechloroben- pyridine zenering. (ε Moreover, = 5.7), like from chloroform the maps (εof = the4.7), electrostatic results in a potentialdestabilization of the threeof the ligands energy depictedbarrier (+0.9in Figure kcal/mol), S4 emerges while an as increased the presence ε, like of in two the oxygens case of tetrahydrofuran in tbuPONOP leads (ε = 7.4) to a and marked di- methylformammidedecrease of negative (ε charge = 37.2) on gives the aromaticrise to lower ring contrarilyenergy barriers to what (0.3 occurs kcal/mol in tbu andPNNNP, 0.5 kcal/mol,this behavior respectively). can have repercussions on their catalytic activity. AnGiven interesting the important behavior role has played been inobserved homogeneous in the case catalysis of protic by the polar solvent, solvents. with theIn the aim caseof investigating of ethanol (ε its= 24.8), effect methanol on the dehydrogenation (ε = 32.6) and water of formic (ε = 78.3), acid assisteda remarkable by the increment2 catalyst, ofdifferent TS1 barrier implicit resulted, solvents with havevalues been of 3.2 tested kcal/m onol, the 4.2 CO kcal/mol2 release and step 3.8 ( TS1kcal/mol,), considering respec- tively,dielectric indicating constant that with the gradually choice of increasingoperative conditions values to simulate can affect apolar, the efficiency polar aprotic of cata- and lyticpolar mechanism. protic solvents Finally, extensively for the apolar used solvent in organometallic cyclohexane reactions (ε = 2.0), [37 no]. relevant The results varia- are tionscollected occurred. in Figure 4.
Figure 4. Variation of TS1 barrier as function of implicit solvents: Cyclohexane (CHX) ε = 2.0; Figure 4. Variation of TS1 barrier as function of implicit solvents: Cyclohexane (CHX) ε = 2.0; ε ε ε ChloroformChloroform (CLF) (CLF) ε == 4.7; 4.7; CLB CLB (Chlorobenzene) (Chlorobenzene) ε = 5.7; THF (Tetrahydrofuran) ε= =7.4; 7.4;ETH ETH (Ethanol) (Etha- nol)ε = ε 24.8; = 24.8; MET MET (Methanol) (Methanol)ε = ε 32.6; = 32.6; DMF DMF (Dimethylformamide) (Dimethylformamide)ε = ε 37.2; = 37.2; WAT WAT (Water) (Water)ε = ε 78.3. = 78.3. The use of polar aprotic solvent with lower dielectric constant, respect to chloroben- 3. Materials and Methods zene (ε = 5.7), like chloroform (ε = 4.7), results in a destabilization of the energy barrier All the calculations have been carried out with the Gaussian09 D.01 software package [38]. The available x-ray data (CCDC 1848774) has been selected as starting structure for the mechanistic study [31]. The geometries have been fully optimized without any physi- cal constraints, adopting B3LYP functional [39,40] and 6-31+G(d,p) basis set for C, H, N, O and P atoms. For the Mn, instead, the SDD pseudopotential [41] and its related basis set has been chosen. In addition, the dispersions according to Grimme’s scheme (D3-BJ) have been included in all the computations [42]. The strategy here presented has been success- fully described and adopted in previous works on catalytic systems containing first row transition metals [34,43–47]. Given the lack of information on the multiplicity of the cata- lytic system, preliminary investigation has been carried out considering different spin states, (2S + 1 = 1, 3, 5). The potential energy surfaces have been explored via relaxed linear transit scan and the nature of intercepted minima and maxima has been confirmed through thermochem- ical analysis (one and no imaginary frequency for intermediates and transition states, re- spectively). To include the effect of solvent, the SMD implicit solvation method [48] has been assumed, selecting the chlorobenzene as medium (ε = 5.7) to better reproduce the experimental conditions [31]. In order to investigate the effect to the catalysis, cyclohexane (ε = 2.0) chloroform (ε = 4.7), chlorobenzene (ε = 5.7), Tetrahydrofuran (ε = 7.4), ethanol (ε
Catalysts 2021, 11, 141 7 of 10
(+0.9 kcal/mol), while an increased ε, like in the case of tetrahydrofuran (ε = 7.4) and dimethylformammide (ε = 37.2) gives rise to lower energy barriers (0.3 kcal/mol and 0.5 kcal/mol, respectively). An interesting behavior has been observed in the case of protic polar solvents. In the case of ethanol (ε = 24.8), methanol (ε = 32.6) and water (ε = 78.3), a remarkable increment of TS1 barrier resulted, with values of 3.2 kcal/mol, 4.2 kcal/mol and 3.8 kcal/mol, respectively, indicating that the choice of operative conditions can affect the efficiency of catalytic mechanism. Finally, for the apolar solvent cyclohexane (ε = 2.0), no relevant variations occurred.
3. Materials and Methods All the calculations have been carried out with the Gaussian09 D.01 software pack- age [38]. The available x-ray data (CCDC 1848774) has been selected as starting structure for the mechanistic study [31]. The geometries have been fully optimized without any physical constraints, adopting B3LYP functional [39,40] and 6-31+G(d,p) basis set for C, H, N, O and P atoms. For the Mn, instead, the SDD pseudopotential [41] and its related basis set has been chosen. In addition, the dispersions according to Grimme’s scheme (D3-BJ) have been included in all the computations [42]. The strategy here presented has been successfully described and adopted in previous works on catalytic systems containing first row transition metals [34,43–47]. Given the lack of information on the multiplicity of the catalytic system, preliminary investigation has been carried out considering different spin states, (2S + 1 = 1, 3, 5). The potential energy surfaces have been explored via relaxed linear transit scan and the nature of intercepted minima and maxima has been confirmed through thermochem- ical analysis (one and no imaginary frequency for intermediates and transition states, respectively). To include the effect of solvent, the SMD implicit solvation method [48] has been assumed, selecting the chlorobenzene as medium (ε = 5.7) to better reproduce the experimental conditions [31]. In order to investigate the effect to the catalysis, cyclohexane (ε = 2.0) chloroform (ε = 4.7), chlorobenzene (ε = 5.7), Tetrahydrofuran (ε = 7.4), ethanol (ε = 24.8), methanol (ε = 32.6), dimethylformammide (ε = 37.2) and water (ε = 78.3) implicit solvents have been additionally tested. The final energies (∆G) reported on the PESs additionally include the most accurate electronic energy obtained from single point energy calculations employing the more extended 6-311+G(2d,2p) basis set, and the Gibbs free energy corrections, extrapolated from thermochemical analysis.
4. Conclusions On the basis of a detailed computational investigation performed in solvent and taking into account the experimental information, a catalytic cycle for the dehydrogenation of formic acid promoted by a recently synthetized Mn(I)-dependent complex [(tBuPNNOP) + Mn(CO)2] as catalyst has been proposed. The process consists of two elementary steps: release of CO2 and production of H2. The proposed reaction mechanism well fits the experimental behaviors. The CO2 release represents the rate determining state requiring 16.7 kcal/mol. The pincer ligand with its steric hindrance ensures the occurrence of the catalytic process confirmed by the scaffold stability on all the intercepted stationary points. Our calculations highlight that the presence of a base is fundamental for the dehydro- genation of formic acid, not only in the deprotonation of acid, but also during the formation of the second product, acting as proton-donor species. The behavior of other pincer ligands (with tbuPONOP (tBuPONOP = 2,6-bis(di-tert- butylphosphinito)pyridine 4 and tbuPNNNP (tBuPNNNP = 2,6-bis (di-tert-butylphosphinamine) pyridine, 5) has been rationalized in terms of a different electrostatic distribution induced on the pyridine ring coordinated to Mn(I) by nitrogen. Catalysts 2021, 11, 141 8 of 10
The effect of the solvent on the catalytic process has been evaluated by taking into account apolar, polar aprotic and polar protic solvents simulated by opportune dielectric constant values. We believe that the results from the present investigation can stimulate further experi- mental works in the design of new more efficient catalysts devoted to H2 production.
Supplementary Materials: The following are available online at https://www.mdpi.com/2073-434 4/11/1/141/s1, Figure S1. On the left, superposition of substrate-free complex fixing singlet, triplet (green) and quintet (gray) state. On the right, comparison between optimized and experimental observed relevant geometrical parameters. Figure S2. B3LYP-D3/6-31+G(d,p) optimized geometries + of 2(H), 2(H):Me3NH and 2(H2) stationary points. Relevant distances (in black) are in Å. Figure S3. Charge distribution in NBO population of relevant atoms. Figure S4. Map of electrostatic potential plotted for the three different ligands. Table S1. Calculated spin contamination for triplet and quintet spin states. Cartesian coordinates. Author Contributions: T.M. and M.P. performed the calculations and analyzed the results. T.M. and M.P wrote the manuscript and contributed to the final version. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article. Acknowledgments: We thank the Università degli Studi della Calabria-Dipartimento di Chimica e Tecnologie Chimiche (CTC) for the financial support. Conflicts of Interest: The authors declare no conflict of interest.
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