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catalysts

Article Reaction Mechanisms of CO2 Reduction to Catalyzed by Hourglass Ru, Fe, and Os Complexes: A Functional Theory Study

Chunhua Dong 1,2,3, Mingsong Ji 1, Xinzheng Yang 1,*, Jiannian Yao 2 and Hui Chen 2,*

1 Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; [email protected] (C.D.); [email protected] (M.J.) 2 Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; [email protected] 3 College of Chemistry, Chemical Engineering and Material, Handan Key Laboratory of Organic Small Materials, Handan University, Handan 056005, China * Correspondence: [email protected] (X.Y.); [email protected] (H.C.); Tel.: +86-10-8236-2928 (X.Y.); +86-10-6256-1749 (H.C.)

Academic Editors: Albert Demonceau, Ileana Dragutan and Valerian Dragutan Received: 23 September 2016; Accepted: 21 December 2016; Published: 27 December 2016

Abstract: The reaction mechanisms for the reduction of to formaldehyde catalyzed by bis(tricyclopentylphosphine) metal complexes, [RuH2(H2)(PCyp3)2](1Ru), [FeH2(H2)(PCyp3)2] (1Fe) and [OsH4(PCyp3)2](1Os), were studied computationally by using the density functional theory (DFT). 1Ru is a recently reported highly efficient catalyst for this reaction. 1Fe and 1Os are two analogues of 1Ru with the Ru atom replaced by Fe and Os, respectively. The total free energy barriers of the reactions catalyzed by 1Ru, 1Fe and 1Os are 24.2, 24.0 and 29.0 kcal/mol, respectively. With a barrier close to the experimentally observed Ru complex, the newly proposed iron complex is a potential low-cost catalyst for the reduction of carbon dioxide to formaldehyde under mild conditions. The electronic structures of intermediates and transition states in these reactions were analyzed by using the natural bond orbital theory.

Keywords: reaction mechanism; carbon dioxide; formaldehyde; homogeneous ; ruthenium; iron; osmium

1. Introduction

As an abundant and non-toxic C1-building block, carbon dioxide can be reduced to various chemicals, such as [1], [2], formaldehyde [3], [4,5], [5], formate [6,7], [8], [8], formamidines [8], imines [3] and [9]. Recently, Beller and co-workers [10] reported the methylation of aromatic C–H bonds using CO2 and H2 with the assistance of a ruthenium triphos catalyst. In addition to the experimental studies, there are some theoretical studies on catalytic reduction of CO2 in recent years. Pidko [11] used a bis-N-heterocyclic carbene ruthenium CNC-pincer as catalyst and studied the mechanism of CO2 hydrogenation to formates by DFT method. Haunschild [12] reported the catalytic reduction of carbon dioxide to methanol by using (Triphos)Ru(TMM) as catalyst. Musashi and Sakaki [13] reported a theoretical study of cis-RuH2(PH3)4-catalyzed hydrogenation of CO2 into formic acid. As mild reducing agents, boron-containing compounds have been widely used in carbon dioxide reduction [14,15]. Hazari and co-workers [16] reported the allene carboxylation with CO2 using a PSiP pincer ligand supported palladium complex as the catalyst in the hydroboration of CO2. Maron and co-workers [17,18] reported the –borane-mediated hydroboration of CO2 to methanol using

Catalysts 2017, 7, 5; doi:10.3390/catal7010005 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 5 2 of 12

an organocatalyst 1-Bcat-2-PPh2–C6H4 (Bcat = catecholboryl). Their mechanistic study showed that the simultaneous activation of both the reducing agent and CO2 plays an important role in the catalytic reaction. Bouhadir and co-workers [19] reported the hydroboration of CO2 to methoxyboranes with the ambiphilic phosphine–borane derivatives as catalysts. They also demonstrated that the formaldehyde adducts are indeed the actual catalyst in the reaction. Wegner and co-workers [20] reported a selective reduction of CO2 to methanol using Li2[1,2-C6H4(BH3)2] as the catalyst with the presence of pinacolborane. Their work shows a novel transition-metal-free mode in which the aromaticity plays an important role in the bidentate activation process. Given the above progress, we can see that the reduction of carbon dioxide to formic acid and its derivatives has been well studied. However, as a key step in the production of methanol from CO2 and H2, the catalytic reduction of CO2 to formaldehyde is rarely reported. Huang et al. [21] studied the mechanistic details of nickel pincer-catalyzed reduction of CO2 to a methanol derivative with catecholborane (HBcat), and found that formaldehyde is an inevitable intermediate, although it was not observed in the experiment. Hazari and co-workers [22] reported a CO2 reduction reaction using a nickel η3-cyclooctenyl complex supported by tridentate PSiP pincer ligand as the precatalyst, and detected the characteristic peak (8.72 ppm) of formaldehyde in a 1H-NMR spectrum. However, the formaldehyde was mixed with miscellaneous unverifiable products without a clear yield. Hill and co-workers [23] reported a selective reductive hydroboration of CO2 to a methanol equivalent, CH3OBpin, using B(C6F5)3-activated alkaline earth compounds as catalysts. They also proposed a mechanism with the formation of formaldehyde as a byproduct. Tzschucke and co-workers [24] reported an electrocatalytic reduction of CO2 to formic acid and found an occasional formation of formaldehyde using bipyridine iridium complexes with pentamethylcyclopentadienyl ligands as the catalysts. In the above reactions, formaldehyde was only observed as byproducts with extremely low yields. In 2014, Bontemps and co-workers [3] reported the first unambiguous detection of formaldehyde from the pinacolborane reduction of CO2 with a yield of 22% using the dihydride bis(dihydrogen) bis(tricyclopentylphosphine) hourglass ruthenium complex [RuH2(H2)2(PCyp3)2](Ru-1cyp) as the catalyst precursor at room temperature in 24 h. Although the selectivity and yield of formaldehyde are not ideal enough, the controllable generation of formaldehyde by reducing CO2 is still a significant breakthrough. However, detailed mechanism of the above reaction, such as the structures of rate-limiting states, is still missing. In this paper, we report a density functional theory (DFT) study of the reaction mechanisms of the reduction of CO2 to formaldehyde catalyzed by [RuH2(H2)(PCyp3)2] (1Ru) and its Fe and Os analogues, [FeH2(H2)(PCyp3)2](1Fe) and [OsH4(PCyp3)2](1Os). The potentials of 1Fe and 1Os as catalysts for the reaction were predicted accordingly. The relations between the electronic structures and catalytic properties were analyzed using the natural bond orbital (NBO) theory [25].

2. Results and Discussion

2.1. The Mechanism for the Reduction of Carbon Dioxide to Formaldehyde Catalyzed by the Ru Complex Scheme1 shows the whole catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru. Figure1 shows the corresponding free energy profile. Figure2 shows the optimized structures of transition states in the catalytic cycle. At the beginning of the reaction, the dissociation of H2 from Ru-1cyp forms the 10.5 kcal/mol less stable catalyst [RuH2(H2)(PCyp3)2](1Ru), which could attract a HBpin molecule and form a 15.8 kcal/mol more stable intermediate 1Ru-HBpin. The exchange of CO2 with HBpin in 1Ru-HBpin for the formation of 2Ru is 14.8 kcal/mol uphill. Next, 3Ru is formed through transition state TS2,3-Ru with a 5.5 kcal/mol barrier for the transfer of a hydride from Ru to the carbon in CO2. The newly formed formate group in 3Ru could easily rearrange through TS3,4-Ru and TS4,5-Ru for the formation of a more stable intermediate 5Ru. Then, a HBpin molecule attacks 5Ru and forms a 2.6 kcal/mol less stable intermediate 6Ru with a weak interaction between O and B. Next, 7Ru is formed quickly Catalysts 2017, 7, 5 3 of 12

through TS6,7-Ru with the stretching of the B–H bond and the formation of the O–B and Ru–H bonds. A pinBOCHO molecule is formed with the cleavage of the Ru–O bond through transition state TS7,8-Ru, Catalysts 2017, 7, 5 3 of 12 whichCatalysts is 12.2 2017 kcal/mol, 7, 5 higher than 7Ru in free energy. The dissociation of pinBOCHO3 of from 12 8Ru for theis regeneration formed with the of cleavage1Ru is of 1.1 the kcal/mol Ru–O bond downhill. through transition Then the state dissociated TS7,8‐Ru, which pinBOCHO is 12.2 kcal/mol molecule is formed with the cleavage of the Ru–O bond through transition state TS7,8‐Ru, which is 12.2 kcal/mol can comehigher back than to 7Ru1Ru in freeand energy. take a The hydride dissociation from of Ru pinBOCHO to its carbonyl from 8Ru carbon for the for regeneration the formation of 1Ru is of 10Ru Ru Ru Ru ◦ 1.1higher kcal/mol than 7downhill. in free energy.Then the The dissociated dissociation pinBOCHO of pinBOCHO molecule from can 8 comefor the back regeneration to 1Ru and of take 1 isa (∆G = −8.3 kcal/mol). After a structural relaxation, 10Ru transforms to a 5.9 kcal/mol more stable 1.1 kcal/mol downhill. Then the dissociated pinBOCHO molecule can come back to 1Ru and take a intermediatehydride11 from. ThenRu to a its formaldehyde carbonyl carbon molecule for the formsformation with of the 10 formationRu (ΔG° = − of8.3 the kcal/mol). Ru–OBpin After bond a and Ru Ru structuralhydride from relaxation, Ru to its10 carbonylRu transforms carbon to fora 5.9the kcal/molformation more of 10 stable (ΔG intermediate° = −8.3 kcal/mol). 11Ru. ThenAfter a the breaking of the Ru–OCH OBpin and C–OBpin bonds through transition state TS , which is formaldehydestructural relaxation, molecule 10 formsRu2 transforms with the formationto a 5.9 kcal/molof the Ru–OBpin more stable bond andintermediate the breaking 1111,12-RuRu of. theThen Ru– a 3.8 kcal/mol higher than 11 in free energy. The release of formaldehyde from 12 for the formation OCHformaldehyde2OBpin and molecule C–OBpin Ruforms bonds with through the formation transition of thestate Ru–OBpin TS11,12‐Ru, bondwhich and is 3.8 the kcal/mol breakingRu higher of the than Ru– 2 11,12‐Ru of 13Ru11OCHisRu 3.0inOBpin free kcal/mol energy. and C–OBpin downhill. The release bonds Next, of through formaldehyde another transition HBpin from state 12 molecule RuTS for the, approacheswhichformation is 3.8 of kcal/mol 13RuRu isand 3.0 higher kcal/mol forms than a much 11Ru in free energy. The release of formaldehyde from 12Ru for the formation of 13Ru is 3.0 kcal/mol more stabledownhill. intermediate Next, another15 HBpinRu with molecule the formation approaches of the13Ru B–Oand forms bond a through much moreTS stable14,15-Ru intermediate. The Ru–O and downhill. Next, another HBpin molecule approaches 13Ru and forms a much more stable intermediate H–B bonds15Ru with in 15 theRu formationcould break of the easily B–O withbond thethrough formation TS14,15‐Ru of. The a Ru–H Ru–O bond and H–B through bondsTS in 15,16-Ru15Ru could, which is Ru 14,15‐Ru Ru break15 with easily the with formation the formation of the B–O of a bondRu–H through bond through TS TS. The15,16‐ RuRu–O, which and is H–Bonly bonds4.5 kcal/mol in 15 higher could only 4.5 kcal/mol higher than 15Ru in free energy. Then the dissociation of pinBOBpin from 16Ru 15,16‐Ru thanbreak 15 easilyRu in freewith energy. the formation Then the of dissociation a Ru–H bond of pinBOBpinthrough TS from 16, whichRu regenerates is only 4.5 1Ru kcal/mol and completes higher regenerates 1RuRu and completes the catalytic cycle. It is worth notingRu that HBpinRu can also attack 10Ru thethan catalytic 15 in free cycle. energy. It is Thenworth the noting dissociation that HBpin of pinBOBpin can also from attack 16 10 regeneratesRu and form 1 aand stable completes and form a stable acetal compound (pinBO)2CH2, which was observedRu in the experiment. compoundthe catalytic (pinBO) cycle. 2CHIt is2, whichworth wasnoting observed that HBpin in the experiment.can also attack 10 and form a stable acetal compound (pinBO)2CH2, which was observed in the experiment.

H P H RuP 1Ru = H HH H 1 = RuP Ru H H a=CO2; bP = HBpin; c = pinBOCHO; d = (pinBO)2CH2;e=HCHO;f = pinBOBpin

a=CO2; b = HBpin; c = pinBOCHO; d = (pinBO)2CH2;e=HCHO;f = pinBOBpin TS2,3-Ru TS(4.5) 5 2,3-Ru TS7,8-Ru TS9,10-Ru (4.5) (0.4) 5 +4b TS3,4-Ru TS (-1.7) 7,8-Ru TS 0 TS4,5-Ru TS6,7-Ru 9,10-Ru 1 +4b TS(-3.2) (0.4) Ru 3,4-Ru (-4.8) (-4.8) +3b (-1.7)+3b 0 2Ru TS4,5-Ru TS6,7-Ru 9 TS -5 (0.0)1 +4b(-3.2) Ru 11,12-Ru Ru (-4.8) (-4.8) +3b (-1.0)2 3Ru +4b +3b (-2.2)+3b (-10.4) (0.0)+a Ru 4 6 9Ru TS11,12-Ru -10-5 +4b +4b Ru Ru +4b (-1.0) (-4.6)3 5 8Ru +3b 10Ru Ru (-6.0)+4b Ru +3b 1 (-2.2) (-10.4) +a 4Ru (-5.8)6 Ru +3b +4b +4b Ru 7Ru (-9.0) (-8.3) -10 +4b (-4.6) (-8.4)5 8Ru +3b 10Ru -15 (-6.0)+4b Ru (-5.8)+3b (-10.1)1 11 TS14,15-Ru +4b +4b (-11.8)7 (-9.0)+3b Ru (-8.3)+3b Ru+3b 1 (-8.4) Ru +3b (-21.0) -15 Ru-HBpin +4b +3b (-10.1) (-14.2) 12 TS14,15-Ru -20 (-11.8)+3b +3b +3b 11Ru Ru (-15.8) +4b +c +3b 1Ru-HBpin +3b (-14.2)(-18.1) (-21.0)+2b +3b 12Ru 13Ru 14 -20 +a +c Ru +e TS -25 (-15.8) +3b (-18.1)+3b +2b 15,16-Ru +3b (-21.1)13 +a Ru (-22.6)14Ru +e (-30.5) -25 1Ru-HBpin TS15,16-Ru -30 +3b +3b +3b (-25.9) (-21.1)(-22.6)+2b (-30.5) 1Ru-HBpin +e +2b -30 1 +3b +e 16 -35 (-25.9)+2b Ru +2b +e Ru 1Ru +c (-31.9) +e 15 +2b(-31.1) 1 +e Ru 16 (-31.8) +2b Ru +e Ru 1Ru -35 +2b (-35.0) +c (-31.9) 15Ru (-31.1)+2b (-31.8)+2b +d +2b +e +e +2b (-35.0) +2b +e +f+2b +d +2b +e +e +e +f Reaction Coordinate Reaction Coordinate Figure 1. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru. Figure 1. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1 . Figure 1. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru. Ru

Figure 2. Optimized structures of TS2,3‐Ru (513i cm−1), TS3,4‐Ru (170i cm−1), TS4,5‐Ru (176i cm−1), TS6,7‐Ru Figure 2.−1 Optimized structures−1 of TS2,3‐Ru (513i− 1cm−1),− TS1 3,4‐Ru (170i −cm1 −1), TS4,5‐−Ru1 (176i −cm1 −1), TS6,7‐Ru −1 Figure(127 2. iOptimized cm ), TS7,8‐Ru structures (209i cm ), ofTS9,10TS‐Ru2,3-Ru (363i (513cm ),i cmTS11,12‐),Ru TS(1593,4-Rui cm ),(170 TS14,15i cm‐Ru (67),i cmTS4,5-Ru) and TS(17615,16i ‐cm ), −1 −1 −1 −1 −1 Ru(127 (97i icm cm−),1). TS −Bond17,8‐Ru lengths(209i cm are), TSin Å.9,10 ‐TheRu −(363 1cyclopentyli cm ), TS and11,12 ‐pinacolRu (159i − cmgroups1 ), TS are14,15 not‐Ru (67showni cm for) and clarity.−1 TS15,16 ‐ TS6,7-Ru (127i cm ), TS7,8-Ru (209i cm ),TS9,10-Ru (363i cm ), TS11,12-Ru (159i cm ), TS14,15-Ru Ru (97i cm−1). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity. (67 i cm−1) and TS (97i cm−1). Bond lengths are in Å. The cyclopentyl and pinacol groups are 15,16-Ru not shown for clarity. Catalysts 2017, 7, 5 4 of 12

Catalysts 2017, 7, 5 4 of 12 By comparing all relative free energies shown in Figure1, we can conclude that 1Ru-HBpin and TS are the rate-determining states of the catalytic reaction with a total free energy barrier of 9,10-Ru By comparing all relative free energies shown in Figure 1, we can conclude that 1Ru‐HBpin and 24.2 kcal/mol,TS9,10‐Ru are which the rate agrees‐determining well with states the of observedthe catalytic experimental reaction with reactiona total free rate. energy barrier of 24.2 Comparedkcal/mol, which with agrees the mechanismwell with the proposedobserved experimental by Huang etreaction al. for rate. the reduction reaction of CO2 to a methanolCompared derivative with the with mechanism catecholborane proposed by (HBcat) Huang catalyzedet al. for the by reduction a pincer reaction nickel of complex CO2 to a [21], the catalyticmethanol reduction derivative of with carbon catecholborane dioxide to formaldehyde (HBcat) catalyzed reactions by a pincer catalyzed nickel by complex hourglass [21], ruthenium the complexcatalytic and reduction pincer nickel of carbon complex dioxide have to formaldehyde similar but not reactions exactly catalyzed the same by pathways. hourglass ruthenium There are two oxygencomplex atoms and as pincer possible nickel bonding complex sites have for similar HBpin but innot the exactly nickel the formate. same pathways. However, There there are two is only atoms as possible bonding sites for HBpin in the nickel formate. However, there is only one one oxygen atom as the bonding site for HBpin in the ruthenium formate (5Ru to 6Ru in Scheme1). oxygen atom as the bonding site for HBpin in the ruthenium formate (5Ru to 6Ru in Scheme 1). Our Our calculations indicate that the six-membered ring structure is less likely to be formed in the calculations indicate that the six‐membered ring structure is less likely to be formed in the hourglass hourglassruthenium ruthenium complexes. complexes.

u -R 2 ,1 1 1

Scheme 1. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru. Scheme 1. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru.

Catalysts 2017, 7, 5 5 of 12

Catalysts 2017, 7, 5 5 of 12 2.2. The Mechanism for the Reduction of Carbon Dioxide to Formaldehyde Catalyzed by the Fe Complex 2.2. The Mechanism for the Reduction of Carbon Dioxide to Formaldehyde Catalyzed by the Fe Complex Scheme2 shows the whole catalytic cycle for the reduction of carbon dioxide to formaldehyde Scheme 2 shows the whole catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by the newly proposed Fe complex 1Fe. Figures3 and4 show the corresponding free energy catalyzed by the newly proposed Fe complex 1Fe. Figures 3 and 4 show the corresponding free energy profile and optimized structures of key transition states, respectively. The reactions catalyzed by 1 profile and optimized structures of key transition states, respectively. The reactions catalyzed by 1Fe Fe andand1Ru 1Ruhave have similar similar pathways pathways butbut differentdifferent rate rate-determining‐determining states. states.

O P P H H C H H Fe O Fe O O in H p H H C B P TS H P 3 H ,4 5 -F Fe 4Fe e P O P H C H H O P = P(cyp)3 Fe O Fe C O H H H O H H P H Bpin P

Bpin = B e T 6 Fe F 3 S Fe O 3- , 6 2 , 7 S - F T e O H P H P H H C C H O O O Fe O OCH Fe n pinB i CO H p 2 H H P H B P H H P H P P H H H HBpin H H 2 7 H 2 H Fe Fe Fe Fe Fe Bpin H H H H H H P P P P pi P 1 Fe-1cyp H H 1Fe nBO Fe-HBpin in C H O H H Bpin Bp H Fe BO O Fe C pin H H O HBpin H H P Bpin P O

12Fe 8Fe Bpin

(pinBO)2CH2 HBpin P H P O H H H Fe Fe C H H O H Bpin H O P P P H O H Bpin 11Fe H C 9Fe Fe 0- Fe H ,1 HCHO S 9 H O T P Bpin 10Fe

Scheme 2. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Fe. Scheme 2. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Fe.

Similar to the reaction catalyzed by 1Ru, a −22.0 kcal/mol more stable intermediate 1Fe‐HBpin is Similar to the reaction catalyzed by 1Ru, a −22.0 kcal/mol more stable intermediate 1Fe-HBpin formed at the beginning of the reaction catalyzed by 1Fe. The exchange of CO2 with HBpin in 1Fe‐HBpin is formed at the beginning of the reaction catalyzed by 1Fe. The exchange of CO2 with HBpin in and the formation of 2Fe is 16.4 kcal/mol uphill. Next, 3Fe is formed quickly through transition state 1Fe-HBpin and the formation of 2Fe is 16.4 kcal/mol uphill. Next, 3Fe is formed quickly through TS2,3‐Fe for the formation of C–H bond and the rearrangement of atoms. The rearrangement transition state TS2,3-Fe for the formation of C–H bond and the rearrangement of hydrogen atoms. of the newly formed formate group and hydrogen atoms in 3Fe forms a slightly more stable The rearrangement of the newly formed formate group and hydrogen atoms in 3Fe forms a slightly intermediate 4Fe. After a structural relaxation, 4Fe transforms to a 2.0 kcal/mol more stable more stable intermediate 4 . After a structural relaxation, 4 transforms to a 2.0 kcal/mol more stable intermediate 5Fe, which attractsFe a HBpin molecule for theFe formation of a 3.9 kcal/mol more stable intermediateintermediate5 Fe6Fe, whichwith strong attracts Fe–H a HBpinand O–B molecule interactions. for theThen, formation 7Fe is formed of a through 3.9 kcal/mol a transition more state stable intermediateTS6,7‐Fe with 6aFe freewith energy strong barrier Fe–H of and 20.8 O–Bkcal/mol interactions. with the breaking Then, 7Fe of isthe formed Fe–O and through B–H bonds a transition and statetheTS formation6,7-Fe with of the a free O–B energy bond. barrier 7Fe is 12.3 of kcal/mol 20.8 kcal/mol less stable with than the breaking6Fe with a ofweak the interaction Fe–O and B–Hbetween bonds andH theand formation B. The dissociation of the O–B of pinBOCHO bond. 7Fe is from 12.3 7 kcal/molFe for the regeneration less stable than of 1Fe6Fe is with3.6 kcal/mol a weak downhill. interaction

Catalysts 2017, 7, 5 6 of 12

between H and B. The dissociation of pinBOCHO from 7Fe for the regeneration of 1Fe is 3.6 kcal/mol Catalysts 2017, 7, 5 6 of 12 downhill.Catalysts Next, 2017 the, 7, 5 dissociated pinBOCHO molecule comes back to 1Fe and forms a Fe–O bond6 of 12 for the formation of a 7.3 kcal/mol more stable intermediate 8 . After a structural relaxation, 8 transforms Next, the dissociated pinBOCHO molecule comes backFe to 1Fe and forms a Fe–O bond Fefor the Next, the dissociated pinBOCHO molecule comes back to 1Fe and forms a Fe–O bond for the to a 3.1formation kcal/mol of a more 7.3 kcal/mol stable more intermediate stable intermediate9Fe. Then 8Fe a. After formaldehyde a structural moleculerelaxation, 8 isFe formedtransforms with the formation of a 7.3 kcal/mol more stable intermediate 8Fe. After a structural relaxation, 8Fe transforms formationto a of3.1 the kcal/mol Fe–OBpin more bondstable andintermediate the breaking 9Fe. Then of the a formaldehyde Fe–OCH2OBpin molecule and C–OBpinis formed bondswith the through to a 3.1 kcal/mol more stable intermediate 9Fe. Then a formaldehyde molecule is formed with the transitionformation state TSof the9,10-Fe Fe–OBpin. The dissociation bond and the of breaking formaldehyde of the Fe–OCH from 102OBpinFe leaves and an C–OBpin 8.6 kcal/mol bonds more 2 throughformation transition of the Fe–OBpinstate TS9,10 ‐bondFe. The and dissociation the breaking of formaldehyde of the Fe–OCH fromOBpin 10Fe leaves and C–OBpinan 8.6 kcal/mol bonds stable intermediate 11Fe. Then, another HBpin molecule approaches 11Fe and forms a 10.9 kcal/mol through transition state TS9,10‐Fe. The dissociation of formaldehyde from 10Fe leaves an 8.6 kcal/mol more stable intermediate12 11Fe. Then, another HBpin molecule approaches 11Fe and forms a 10.9 more stablemore stable intermediate intermediateFe 11withFe. Then, the formation another HBpin of a B–Omolecule bond. approaches The dissociation 11Fe and forms of newly a 10.9 formed kcal/mol more stable intermediate 12Fe with the formation of a B–O bond. The dissociation of newly pinBOBpinkcal/mol molecule more stable from intermediate12Fe for the 12 regenerationFe with the formation of the of catalyst a B–O bond.1Fe is The 9.5 kcal/moldissociation uphill. of newly formed pinBOBpin molecule from 12Fe for the regeneration of the catalyst 1Fe is 9.5 kcal/mol uphill. formed pinBOBpin molecule from 12Fe for the regeneration of the catalyst 1Fe is 9.5 kcal/mol uphill.

Figure 3. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Fe. Figure 3. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1 . Figure 3. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Fe. Fe

Figure 4. Optimized structures of TS2,3‐Fe (460i cm−1), TS3,4‐Fe (78i cm−1), TS6,7‐Fe (191i cm−1) and TS9,10‐Fe −1 −1 −1 Figure− 14. Optimized structures of TS2,3‐Fe (460i cm ),− TS1 3,4‐Fe (78i cm ), TS6,7−‐Fe1 (191i cm ) and TS9,10‐−Fe1 Figure(86 4.i Optimizedcm ). Bond lengths structures are in of Å.TS The2,3-Fe cyclopentyl(460i cm and pinacol), TS3,4-Fe groups(78 arei cm not shown), TS6,7-Fe for clarity.(191i cm ) and (86i cm−1). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity. TS (86i cm−1). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown 9,10-Fe for clarity. Catalysts 2017, 7, 5 7 of 12

Catalysts 2017, 7, 5 7 of 12 By comparing all relative free energies shown in Figure3, we can conclude that 1Fe-HBpin and

TS6,7-FeByare comparing the rate-determining all relative free states energies of the shown catalytic in Figure reaction 3, we with can a conclude total free that energy 1Fe‐HBpin barrier and of 24.0TS kcal/mol,6,7‐Fe are the which rate‐determining is 0.2 kcal/mol states lower of the than catalytic the free reaction energy with barrier a total of free the energy reaction barrier catalyzed of by24.01Ru .kcal/mol, Therefore, which1Fe isis 0.2 a potential kcal/mol low-costlower than and the high free efficiencyenergy barrier catalyst of the for reaction the reduction catalyzed of by CO 2 to1 formaldehyde.Ru. Therefore, 1Fe is a potential low‐cost and high efficiency catalyst for the reduction of CO2 to formaldehyde.Sabo-Etienne and Bontemps [26] recently reported the catalytic reduction of CO2 to bis(boryl)acetalSabo‐Etienne and methoxyborane,and Bontemps with[26] Fe(H) recently2(dmpe) reported2 used asthe catalyst catalytic precursor, reduction and pinacolboraneof CO2 to 2 2 usedbis(boryl)acetal as the reductant. and Formethoxyborane,1Ru, only one oxygenwith Fe(H) atom(dmpe) of CO2 isused abstracted, as catalyst and CO precursor,2 transforms and into pinacolborane used as the reductant. For 1Ru, only one oxygen atom of CO2 is abstracted, and CO2 HCHO. For Fe(H)2(dmpe)2, one oxygen atom of CO2 is abstracted at the reduction step, then the transforms into HCHO. For Fe(H)2(dmpe)2, one oxygen atom of CO2 is abstracted at the reduction second oxygen atom is removed and CO2 is transformed to . This illustrates that iron step, then the second oxygen atom is removed and CO2 is transformed to methylene. This illustrates complexes can efficiently catalyze the reduction of CO2. that iron complexes can efficiently catalyze the reduction of CO2. 2.3. The Mechanism for the Reduction of Carbon Dioxide to Formaldehyde Catalyzed by the Os Complex 2.3. The Mechanism for the Reduction of Carbon Dioxide to Formaldehyde Catalyzed by the Os Complex The reaction catalyzed by 1Os also has similar pathways to the reaction catalyzed by 1Ru The reaction catalyzed by 1Os also has similar pathways to the reaction catalyzed by 1Ru (see (see Scheme S1 on catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed Scheme S1 on catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Os). by 1Os). Figure5 shows the calculated relative free energies in the reaction catalyzed by 1Os. Figure6 Figure 5 shows the calculated relative free energies in the reaction catalyzed by 1Os. Figure 6 shows shows the optimized structures of Os-1cyp, 1Os, 4Os, TS7,8-Os and 8Os, in which the distances between the optimized structures of Os‐1cyp, 1Os, 4Os, TS7,8‐Os and 8Os, in which the distances between the two the two hydrogen atoms (marked in Figure6) are longer than those in corresponding Ru complexes. hydrogen atoms (marked in Figure 6) are longer than those in corresponding Ru complexes. As As shown in Figure5, the rate-determining states in the reaction catalyzed by the osmium complex are shown in Figure 5, the rate‐determining states in the reaction catalyzed by the osmium complex are 1 and TS with a free energy barrier of 29.0 kcal/mol, which is 4.8 kcal/mol higher than Os-HBpin1Os‐HBpin and TS2,3-Os2,3‐Os with a free energy barrier of 29.0 kcal/mol, which is 4.8 kcal/mol higher than the 1 1 thefree free energy energy barrier barrier of of the the reaction reaction catalyzed catalyzed by by 1Ru.Therefore,Ru.Therefore, 1Os couldOs could also catalyze also catalyze the reduction the reduction of ofcarbon dioxide dioxide to to formaldehyde formaldehyde under under harsher harsher conditions. conditions.

P H H 1Os = Os H P H

a=CO2; b = HBpin; c = pinBOCHO; d = (pinBO)2CH2; e = HCHO; f = pinBOBpin

15 TS2,3-Os (9.8) 10 +4b TS3,4-Os 5 TS7,8-Os (0.5) TS4,5-Os TS 6,7-Os (-0.3) TS9,10-Os 0 (-3.4) (-2.7) 1 2Os 3 +4b (-4.2) Os Os +3b TS -5 (0.5) 11,12-Os (0.0) (-0.3) 4Os +4b +3b +3b +4b 6 (-9.4) +a (-3.5) 5 Os 9Os +4b Os 8Os -10 +4b (-4.6) (-5.2) 10Os +4b (-6.0) 7Os (-7.2)1 +3b (kcal/mol) +3b (-7.5) Os +3b G -15 +4b (-10.3) +3b (-10.1) +3b 11Os TS14,15-Os +3b -20 +3b (-14.1) (-22.4) +c 12 1Os-HBpin +3b Os (-18.7)13Os +2b -25 (-19.2) 14Os (-21.1) +e +3b (-22.3) TS15,16-Os +a +3b -30 +3b +2b (-32.6) 1 +e Os-HBpin +e 1Os -35 (-29.3) 1Os +2b 16Os (-31.8) +2b (-31.9) +e 15Os (-33.6) +2b -40 +c +2b (-36.3) +2b +e +d +2b +e +f +e Reaction Coordinate

Figure 5. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Os. Figure 5. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Os.

Catalysts 2017, 7, 5 8 of 12 Catalysts 2017, 7, 5 8 of 12

−1 −1 Figure 6. Optimized structures ofof Os-1cypOs‐1cyp,, 11OsOs, 4Os, ,TSTS7,87,8-Os‐Os (239(239i cmi cm) and) and 8Os8. OsBond. Bond lengths lengths are are in Å. in Å.The The cyclopentyl cyclopentyl and and pinacol pinacol groups groups are are not not shown shown for for clarity. clarity.

2.4. Structures and Reactivity

According to our mechanistic study, the catalyticcatalytic reaction ofof 2HBpin2HBpin ++ CO2 → HCHO + pinBOBpin has fivefive stages, CO2 insertion (S1), σ‐σ-BondBond metathesis (S2), (S2), pinBOCHO insertion insertion (S3), (S3), HCHO elimination (S4) and σ‐σ-BondBond metathesis (S5). (S1) CO2 Insertion. The The reaction reaction in in S1 S1 is [M]–H + CO2 → [M]–OOCH. The most significantsignificant structural change in S1 is the cleavage of M–H bond. Table 11 lists lists the the natural natural population population analysis analysis (NPA) chargescharges of metal atoms (e), the Wiberg bond indicesindices (WBIs) of the bonds, and the relative free energies between intermediatesintermediates andand transition transition states/intermediates states/intermediates ((∆ΔGG‡‡/Δ∆GG°)◦ )in in stages. stages. We We can see that 2Fe hashas the the most most negative negative NPA NPA charge, charge, the smallest WBI of the M–H bond and the lowest barrier for the cleavage of M–H bond among these three intermediatesintermediates inin S1.S1. (S2) σ‐σ-BondBond Metathesis Metathesis.. The The reaction reaction in in S2 S2 is [M]–OOCH is [M]–OOCH + HBpin + HBpin → [M]–H→ [M]–H + pinBOCHO. + pinBOCHO. The Themost most important important structural structural change change in S2 in is S2 the is cleavage the cleavage of M–O of M–O bond. bond. We can We see can TS see7,8TS‐Os 7,8-Oshas thehas least the NPAleast NPAcharge, charge, the smallest the smallest WBI of WBI the of M–O the M–Obond bondand the and lowest the lowest barrier barrier for the for cleavage the cleavage of M–O of bond M–O amongbond among these thesethree transition three transition states statesin S2. in S2. (S3) pinBOCHO pinBOCHO Insertion. Insertion. The The reaction reaction in in S3 S3 is [M]–H is [M]–H + pinBOCHO + pinBOCHO → [M]–O–CH→ [M]–O–CH2–OBpin.2–OBpin. The Themost most important important structural structural change change in S3 in S3is the is the M–H M–H bond bond cleavage. cleavage. 9Ru9Ru hashas a amore more negative negative NPA NPA charge, a smaller WBI WBI of of the the M–H M–H bond bond and and a a lower lower barrier barrier for for the the cleavage cleavage of of M–H M–H bond bond than than 9Os9 Osin S3.in S3. The The Fe Fecomplexes complexes cannot cannot be be compared compared with with the the Ru Ru and and Os Os complexes complexes because because they they have a different pathway in in S3. S3. Scheme Scheme 11 shows shows that that 1Ru1 Ruandand pinBOCHO pinBOCHO first first generate generate the theintermediate intermediate 9Ru, then9Ru, thenform form the intermediate the intermediate 10Ru10 throughRu through TS9,10TS‐Ru9,10-Ru, in which, in which a Ru–O a Ru–O bond bond is formed. is formed. However, However, as shownas shown in inScheme Scheme 2, 21,Fe1 combinesFe combines directly directly with with pinBOCHO pinBOCHO and and transforms transforms to tointermediate intermediate 8Fe,8 Fein, whichin which an anFe–O Fe–O bond bond is formed. is formed. (S4) HCHO HCHO Elimination. Elimination. The The reaction reaction in inS4 S4is [M]–O–CH is [M]–O–CH2–OBpin2–OBpin → HCHO→ HCHO + [M]–OBpin. + [M]–OBpin. The Themost most important important structural structural change change in S4 in is S4the is cleavage the cleavage of the of M–O the M–Obond. bond. TS9,10‐FeTS has9,10-Fe the haslowest the barrier lowest andbarrier the and smallest the smallest WBI of the WBI M–O of the bond M–O among bond these among three these transition three transition states in statesS4. There in S4. is Thereno obvious is no relationobvious between relation betweenthe energy the barriers energy and barriers the NPA and the charges NPA chargesof metal of atoms metal in atoms S4. in S4. (S5) σ‐σ-BondBond Metathesis. Metathesis. The The reaction reaction in inS5 S5 is [M]–OBpin is [M]–OBpin + HBpin + HBpin → [M]–H→ [M]–H + pinBOBpin. + pinBOBpin. The Themost most important important structural structural change change in S5 in is S5 the is thecleavage cleavage of the of theH–B H–B bond. bond. 15Os15 hasOs ahas smaller a smaller WBI WBI of the of H–Bthe H–B bond bond and and a lower a lower barrier barrier for for the the cleavage cleavage of of H–B H–B bond bond than than 1515RuRu inin S5. S5. There There is is no no obvious relation between the energy barriers and the NPA chargescharges of metalmetal atomsatoms inin S5.S5. The Fe complexes cannot be compared with the Ru and Os complexes because theythey havehave aa differentdifferent pathwaypathway inin S5.S5.

Catalysts 2017, 7, 5 9 of 12

Table 1. The natural population analysis (NPA) charges of metal atoms (e), the Wiberg bond indices (WBIs) of the bonds, the relative free energies between intermediates and transition states/intermediates (∆G‡/∆G◦).

Stages Complexes e Bond WBI ∆G‡/∆G◦ (kcal/mol) 2 −1.892 0.5876 Fe 0.4 TS2,3-Fe −1.782 0.5420

Stage 1 2Ru −1.538 M–H bond 0.6273 5.5 TS2,3-Ru −1.439 0.4674 2 −1.540 0.6887 Os 9.3 TS2,3-Os −1.518 0.5094 6 −1.209 0.3517 Fe 20.8 TS6,7-Fe −0.974 0.0287

Stage 2 7Ru −1.411 M–O bond 0.2704 12.2 TS7,8-Ru −1.104 0.0275 7 −1.427 0.2995 Os 10.0 TS7,8-Os −1.111 0.0271 1 −1.280 0.7154 Fe −7.3 8Fe −1.279 0.2182

Stage 3 9Ru −1.867 M–H bond 0.6661 0.5 TS9,10-Ru −1.850 0.5757 9 −1.832 0.7206 Os 1.0 TS9,10-Os −1.808 0.6002 9 −0.890 0.4354 Fe 3.1 TS9,10-Fe −0.840 0.2609

Stage 4 11Ru −1.190 M–O bond 0.2583 3.8 TS11,12-Ru −1.184 0.2616 11 −1.177 0.4049 Os 4.7 TS11,12-Os −1.193 0.3061 11 −0.778 // Fe −10.9 12Fe −1.290 0.5236

Stage 5 15Ru −1.478 0.5325 H–B bond 4.5 TS15,16-Ru −1.795 0.0863 15 −1.483 0.4898 Os 3.7 TS15,16-Os −1.742 0.0958

Overall, the above comparison of the important elementary steps of the reactions catalyzed by different metal complexes indicate that the smaller corresponding WBIs of bonds, and the lower energy barriers. We believe the reaction energy barriers are influenced by many factors. Due to the structural complexity of the transition metal complexes, the reaction energy barrier is not only decided by the electronic effect, but also relates to the sizes of metals, the binding strengths between metals and ligands, and the steric effects.

3. Computational Details All DFT calculations were performed using the Gaussian 09 suite of programs [27] for the ωB97X-D functional [28,29] with the Stuttgart relativistic effective core potential and associated valence basis sets for Ru (ECP28MWB, (8s7p6d2f1g)/[6s5p3d2f1g]) and Os (ECP60MWB, (8s7p6d2f1g)/[6s5p3d2f1g]) [30,31], all-electron 6-31++G(d,p) basis set for Fe, the atoms coordinated to metal atoms and the atoms in the reactant, and the 6-31G(d) basis set for all other atoms [32–34]. The ωB97X-D functional was also used in the previous theoretical modeling of hydrogenation of Catalysts 2017, 7, 5 10 of 12 carbon dioxide by Fe complex [35] and the hydrogenation of dimethyl carbonate by Ru and Fe complexes [36]. The calculation results in those studies are in agreement with the experimental observations. In addition, high-level ab initio coupled cluster calibration study shows that ωB97X-D performs well in barrier calculation of σ-bond activation promoted by Ru [37] and Fe [38] complexes. All structures reported in this paper were fully optimized with solvent effect corrections using the cavity-dispersion-solvent-structure terms in Truhlar and co-workers’ SMD solvation model for (ε = 2.2706) [39]. Tables and an xyz file giving solvent effect corrected absolute free energies, electronic energies and atomic coordinates of all optimized structures are given in Supporting Information. The thermal corrections were calculated at 298.15 K and 1 atm pressure with harmonic approximation. An ultrafine integration grid (99, 590) was used for numerical integrations. The ground states of intermediates and transition states were confirmed as singlets through a comparison with the optimized high-spin analogues. Calculating the harmonic vibrational frequencies for optimized structures and noting the number of imaginary frequencies (IFs) confirmed the nature of all intermediates (no IF) and transition state structures (only one IF for each transition state). The latter were also confirmed to connect reactants and products by intrinsic reaction coordinate (IRC) calculations. The 3D molecular structure figures displayed in this paper were drawn by using the JIMP2 molecular visualizing and manipulating program [40]. The NPA charges [41] and Wiberg indices [42] were obtained by using the NBO 3.1 program.

4. Conclusions In summary, the mechanistic insights of the reduction of carbon dioxide to formaldehyde catalyzed by Ru, Fe and Os complexes are investigated by using DFT. The formation of C–H and Ru–O bonds (TS9,10-Ru), the cleavage of Fe–O bond and the formation of O–B bond (TS6,7-Fe), and the formation of C–H bond (TS2,3-Os) are the rate-determining steps in the reactions catalyzed by the Ru, Fe, and Os complexes with total free energy barriers of 24.2 (1Ru-HBpin → TS9,10-Ru), 24.0 (1Fe-HBpin → TS6,7-Fe) and 29.0 (1Os-HBpin → TS2,3-Os) kcal/mol, respectively. Such barriers indicate that 1Fe is a potential low-cost catalyst for the reduction of carbon dioxide to formaldehyde under mild conditions. With all of our computational studies, we expect to predict the effects of different metals on the reaction and provide useful information for the development of metal complexes for the conversion and utilization of carbon dioxide.

Supplementary Materials: Additional Supporting Information (Tables and an xyz file giving solvent effect corrected absolute free energies, electronic energies and atomic coordinates of all optimized structures; Scheme on catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Os) are available online at www.mdpi.com/2073-4344/7/1/5/s1. Acknowledgments: X.Y. acknowledges financial support from the 100-Talent Program of the Chinese Academy of Sciences (CAS), and the National Natural Science Foundation of China (NSFC, 21373228, 21673250). H.C. is supported by the NSFC (21290194, 21473215). C.D. is grateful for the financial support from Natural Science Program of Handan University (16217). Author Contributions: Xinzheng Yang and Chunhua Dong conceived and designed the computations; Chunhua Dong performed the computations; Chunhua Dong, Mingsong Ji, Hui Chen and Jiannian Yao analyzed the data; Chunhua Dong and Xinzheng Yang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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