catalysts Article
Reaction Mechanisms of CO2 Reduction to Formaldehyde Catalyzed by Hourglass Ru, Fe, and Os Complexes: A Density Functional Theory Study Chunhua Dong 1,2,3 , Mingsong Ji 1 , Xinzheng Yang 1, *, Jiannian Yao 2 and Hui Chen 2, * 1
2 3
*
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.) Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China;
[email protected] College of Chemistry, Chemical Engineering and Material, Handan Key Laboratory of Organic Small Molecule 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 carbon dioxide 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 catalysis; ruthenium; iron; osmium
1. Introduction As an abundant and non-toxic C1 -building block, carbon dioxide can be reduced to various chemicals, such as carbon monoxide [1], methanol [2], formaldehyde [3], acetals [4,5], formic acid [5], formate [6,7], formamides [8], methylamines [8], formamidines [8], imines [3] and methane [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 phosphine–borane-mediated hydroboration of CO2 to methanol using Catalysts 2017, 7, 5; doi:10.3390/catal7010005
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an organocatalyst 1-Bcat-2-PPh2 –C6 H4 (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-C6 H4 (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 η 3 -cyclooctenyl complex supported by tridentate PSiP pincer ligand as the precatalyst, and detected the characteristic peak (8.72 ppm) of formaldehyde in a 1 H-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, CH3 OBpin, using B(C6 F5 )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 Scheme 1 shows the whole catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru . Figure 1 shows the corresponding free energy profile. Figure 2 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
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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 which Catalysts 2017, 7, 5 is 12.2 kcal/mol higher than 7Ru in free energy. The dissociation of pinBOCHO3 of 12 from 8Ru for theis formed with the cleavage of the Ru–O bond through transition state TS regeneration of 1Ru is 1.1 kcal/mol downhill. Then the dissociated pinBOCHO molecule 7,8‐Ru, which is 12.2 kcal/mol is formed with the cleavage of the Ru–O bond through transition state TS 7,8‐Ru, which is 12.2 kcal/mol higher than 7 in free energy. The dissociation of pinBOCHO from 8 for the regeneration of 1 Ru is can come back to Ru 1Ru and take a hydride from Ru to its carbonyl Ru carbon for the formation of 10Ru higher than 7 Ru in free energy. The dissociation of pinBOCHO from 8 Ru for the regeneration of 1 Ru is ◦ 1.1 kcal/mol downhill. Then the dissociated pinBOCHO molecule can come back to 1 Ru and take a (∆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 1 Ru and take a hydride Ru to its carbonyl carbon for the formation of 10Ru (ΔG° = −8.3 kcal/mol). After a intermediate 11from aits formaldehyde molecule forms withof the formation of the Ru–OBpin bond and Ru . Then hydride from Ru to carbonyl carbon for the formation 10 Ru (ΔG° = −8.3 kcal/mol). After a structural relaxation, 10Ru transforms to a 5.9 kcal/mol more stable intermediate 11Ru. Then a the breaking of the Ru–OCH and C–OBpin bonds through transition state TS , which is 2 OBpin structural relaxation, 10Ru transforms to a 5.9 kcal/mol more stable intermediate 1111,12-Ru Ru. Then a formaldehyde molecule forms with the formation of the Ru–OBpin bond and the breaking of the Ru– 3.8 kcal/mol higher than 11 in free energy. The release of formaldehyde from 12 for the formation formaldehyde molecule forms with the formation of the Ru–OBpin bond and the breaking of the Ru– Ru Ru OCH2OBpin and C–OBpin bonds through transition state TS11,12‐Ru, which is 3.8 kcal/mol higher than OCH 2OBpin and C–OBpin bonds through transition state TS 11,12‐Ru, which is 3.8 kcal/mol higher than of 13Ru11 isRu in free energy. The release of formaldehyde from 12 3.0 kcal/mol downhill. Next, another HBpin molecule approaches 13Ru and forms a much Ru Ru for the formation of 13 is 3.0 kcal/mol 11 Ru in free energy. The release of formaldehyde from 12Ru for the formation of 13Ru is 3.0 kcal/mol downhill. Next, another HBpin molecule approaches 13 more stable intermediate 15Ru with the formation of theRu and forms a much more stable intermediate B–O bond through TS14,15-Ru . The Ru–O and downhill. Next, another HBpin molecule approaches 13 Ru and forms a much more stable intermediate 15 Ru with the formation of the B–O bond through TS 14,15‐Ru Ru could H–B bonds in 15Ru could break easily with the formation of. The Ru–O and H–B bonds in 15 a Ru–H bond through TS15,16-Ru , which is 15Ru with the formation of the B–O bond through TS14,15‐Ru. The Ru–O and H–B bonds in 15 Ru could break easily with the formation of a Ru–H bond through TS 15,16‐Ru, which is only 4.5 kcal/mol higher only 4.5 kcal/mol higher than 15Ru in free energy. Then the dissociation of pinBOBpin from 16Ru break easily with the formation of a Ru–H bond through TS 15,16‐Ru, which is only 4.5 kcal/mol higher than 15 Ru in free energy. Then the dissociation of pinBOBpin from 16Ru regenerates 1Ru and completes regenerates 1RuRu and completes the catalytic cycle. It is worth noting that HBpinRucan also attack 10Ru than 15 in free energy. Then the dissociation of pinBOBpin from 16 regenerates 1 and completes the catalytic cycle. It is worth noting that HBpin can also attack Ru10 Ru and form a stable acetal and form a stable acetal compound (pinBO) CH , which was observed in the experiment. the catalytic cycle. 2CH It is worth noting that can also attack 10Ru and form a stable acetal 2 HBpin 2 2, which was observed in the experiment. compound (pinBO) compound (pinBO)2CH2, which was observed in the experiment. H
H P
P H Ru H HH Ru P H H a = CO2; bP= HBpin; c = pinBOCHO; d = (pinBO)2CH2; e = HCHO; f = pinBOBpin
1Ru =
1Ru =
5 5 0 0 -5 -5 -10 -10 -15 -15 -20 -20 -25 -25 -30 -30 -35 -35
a = CO2; b = HBpin; c = pinBOCHO; d = (pinBO)2CH2; e = HCHO; f = pinBOBpin TS2,3-Ru (4.5) TS 2,3-Ru TS7,8-Ru TS9,10-Ru (4.5) (0.4) +4b TS3,4-Ru TS (-1.7) 7,8-Ru TS9,10-Ru TS TS 4,5-Ru 6,7-Ru (-3.2) (0.4) 1Ru +4b TS 3,4-Ru (-4.8) (-1.7) +3b (-4.8) +3b 2 TS6,7-Ru Ru 9Ru TS11,12-Ru (-3.2) TS4,5-Ru (0.0) +4b 1Ru +3b (-4.8) (-4.8) 3Ru (-1.0) +3b (-2.2) (-10.4) +4b +3b +a 2Ru 9 TS11,12-Ru 4 (0.0) Ru 6 Ru Ru +4b +4b (-4.6) 8 10 +4b (-1.0) +3b Ru Ru 5 3Ru (-2.2) (-10.4) +4b Ru (-5.8)+3b (-6.0) 1 +a Ru 4Ru +3b 6Ru (-8.3) (-9.0) 7Ru +4b (-8.4) (-4.6) 8Ru 10Ru +4b +4b 5Ru +3b (-10.1) +3b TS14,15-Ru +4b (-6.0) (-5.8) +3b 11Ru+3b +3b 1Ru (-11.8) (-8.3) (-9.0) 7 +4b +4b Ru (-8.4) (-21.0) 1Ru-HBpin +3b (-14.2) 12 (-10.1) TS14,15-Ru +3b +4b +3b Ru +3b 11Ru +3b +c (-11.8) +4b (-15.8) +3b (-18.1) (-21.0) +2b 1Ru-HBpin +3b (-14.2) 12Ru 13Ru 14 +3b +a Ru +e +c TS15,16-Ru (-15.8) +3b (-18.1) +3b (-21.1) +2b +3b 13 (-22.6) Ru (-30.5) 14 +a Ru +e 1Ru-HBpin TS15,16-Ru +3b +3b +2b +3b (-21.1) (-25.9) (-30.5) 1Ru-HBpin +e (-22.6) +2b +3b +e 1Ru +2b +e 16Ru 1Ru +2b (-25.9) +e 15Ru +2b(-31.1) (-31.8) (-31.9) +c +e 1Ru +2b +e 16Ru 1Ru (-35.0) +2b +2b (-31.8) (-31.1) 15Ru (-31.9) +c +2b +d +e +2b +e (-35.0) +2b +2b +2b +e +f +d +e +2b +e +e +f
Reaction Coordinate Reaction Coordinate
Figure 1. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru.
FigureFigure 1. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1 1. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzedRuby . 1Ru .
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 −1 −1), TS6,7‐Ru Figure 2. Optimized structures of TS (513i cm 4,5‐Ru −1), TS7,8‐Ru (209i cm−1),TS9,10‐Ru2,3‐Ru −1), TS −1 −1), TS −1 ),3,4‐Ru − 1 (176i cm (363i cm 11,12‐Ru (159i cm (67i cm ) and TS 15,16‐ cm−1 ), Figure(127i cm 2. Optimized structures of TS2,3-Ru (513i cm), TS TS (170i cm (170i14,15‐Ru cm ), TS−14,5-Ru (176i 3,4-Ru ), TS −1 −1 −1 −1 −1 ), TS 7,8‐Ru (209i cm ),TS9,10‐Ru (363i cm ), TS11,12‐Ru (159i cm ), TS14,15‐Ru (67i cm ) and TS 15,16‐ (127i cm −1 − 1 − 1 − 1 − 1 (97i cm ). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity. Ru 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. −1 −1
(67i cm ) and TS15,16-Ru (97i cm not shown for clarity.
). Bond lengths are in Å. The cyclopentyl and pinacol groups are
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11 ,
12 -R u
By comparing all relative free energies shown in Figure 1, we can conclude that 1Ru-HBpin and TS9,10-Ru By arecomparing the rate-determining states of the catalytic reaction with a total free energy all relative free energies shown in Figure 1, we can conclude that 1Ru‐HBpinbarrier and of 24.2 kcal/mol, which agrees well with the observed experimental reaction rate. TS9,10‐Ru are the rate‐determining states of the catalytic reaction with a total free energy barrier of 24.2 Compared with the mechanism proposed by Huang et al. for the reduction reaction of CO2 kcal/mol, which agrees well with the observed experimental reaction rate. Compared with the mechanism proposed by Huang et al. for the reduction reaction of CO 2 to a [21], to a methanol derivative with catecholborane (HBcat) catalyzed by a pincer nickel complex methanol derivative with catecholborane (HBcat) catalyzed by a pincer nickel complex [21], the the catalytic reduction of carbon dioxide to formaldehyde reactions catalyzed by hourglass ruthenium catalytic reduction of carbon dioxide to formaldehyde reactions catalyzed by hourglass ruthenium complex and pincer nickel complex have similar but not exactly the same pathways. There are two complex and pincer nickel complex have similar but not exactly the same pathways. There are two oxygen atoms as possible bonding sites for HBpin in the nickel formate. However, there is only oxygen 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 Scheme 1). 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 hourglass ruthenium complexes. ruthenium complexes.
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 .
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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 Scheme 2 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 . Figures 3 and 4 show the corresponding free energy catalyzed by the newly proposed Fe complex 1 Fe. Figures 3 and 4 show the corresponding free energy profile and optimized structures of key transition states, respectively. The reactions catalyzed byFe1 Fe profile and optimized structures of key transition states, respectively. The reactions catalyzed by 1 and 1 similar pathways but different rate-determining states. Ru and 1Ruhave have similar pathways but different rate‐determining states. H
H
C H
Fe n H i p H P HB 5Fe O P
O
Fe H
H H
P 7Fe
H2
B O
H
H
P H
H P Fe H P
H
H
P
P 3Fe O C
H
H P
O
H P 2Fe
HBpin
P 1Fe H in Bpin Bp BO n i p O
H Fe
Bpin H
P
pin BO H CH O HBpin H
P Fe
H
1Fe-HBpin
H
O C
H P 8Fe
O Bpin
P H
O C
Fe H
Bpin H
HCHO
P
(pinBO)2CH2
HBpin
11Fe
H H
Bpin
O
H
H
H Fe
O
C H
Fe
12Fe
H
H
P
Fe H
O Fe
H
CHO pinBO CO2 H P
H Fe-1cyp
-F e
H
Bp inO OC H
-Fe
H
3 ,4
H
O
Bpin =
TS 6,7
H
P
P 4Fe
H
H P
O TS
C
P
O
Fe H
O
P = P(cyp)3
H P H Bpin 6Fe
H
P Fe
H
C H Fe
H
H
TS2,3Fe
H
O
P
H
P Fe
H P
O O
H
H
C H
Bpin
P 9Fe
O
H
Bpin
Fe 0,1 S9
T
10Fe
Scheme 2. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1 Scheme 2. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed byFe1. Fe .
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 hydrogen 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 the newly formedrelaxation, formate group and hydrogen in 3Fe forms slightly intermediate 4Fe. of After a structural 4Fe transforms to a atoms 2.0 kcal/mol more a stable more stable intermediate structural relaxation, 4Feformation transforms a 2.0 kcal/mol more stable Fe . Aftera aHBpin intermediate 5Fe, which 4attracts molecule for the of to a 3.9 kcal/mol more stable intermediate 5 , which attracts a HBpin molecule for the formation of a 3.9 kcal/mol more stable Fe intermediate 6Fe with strong Fe–H and O–B interactions. Then, 7Fe is formed through a transition state intermediate 6Fe with strong Fe–H and O–B interactions. Then, 7Fe is formed through a transition TS6,7‐Fe with a free energy barrier of 20.8 kcal/mol with the breaking of the Fe–O and B–H bonds and state TS6,7-Fe with a free energy barrier of 20.8 kcal/mol with the breaking of the Fe–O and B–H bonds the formation of the O–B bond. 7 Fe is 12.3 kcal/mol less stable than 6 Fe with a weak interaction between and the formation of the O–B bond. 7Fe is 12.3 kcal/mol less stable than Fe 6Fe with a weak interaction H and B. The dissociation of pinBOCHO from 7 Fe for the regeneration of 1 is 3.6 kcal/mol downhill.
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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. Next, the dissociated pinBOCHO molecule comes back to 1Fe and forms a Fe–O bond for the Catalysts 2017, 7, 5 6 of 12 formation ofthe a 7.3 kcal/molpinBOCHO more stable intermediate structural Fe . After Next, dissociated molecule comes 8 back to 1Fe aand forms a relaxation, Fe–O bond 8Fe for transforms the Next, the dissociated pinBOCHO molecule to 1 Fe and forms a Fe–O bond for the to a 3.1formation of a 7.3 kcal/mol more stable intermediate 8 kcal/mol more stable intermediate 9Fecomes . Thenback a formaldehyde molecule is formed with the Fe. After a structural relaxation, 8Fe transforms formation of a 7.3 kcal/mol more stable intermediate 8 . After a structural relaxation, 8 Fe transforms formation Fe–OBpin the breaking of the Fe–OCH2 OBpin and C–OBpin bonds through to a of 3.1 the kcal/mol more bond stable and intermediate 9Fe. Then a Feformaldehyde molecule is formed with the to a 3.1 kcal/mol more stable intermediate 9Fe. Then of a formaldehyde molecule is C–OBpin formed with the formation of the Fe–OBpin bond and the breaking the Fe–OCH 2 OBpin and bonds transition state TS9,10-Fe . The dissociation of formaldehyde from 10Fe leaves an 8.6 kcal/mol more formation of the Fe–OBpin bond and the breaking of the Fe–OCH2OBpin and C–OBpin bonds . The dissociation of formaldehyde from 10 stable through transition state TS intermediate 11Fe . Then,9,10‐Fe another HBpin molecule approaches 11FeFe leaves an 8.6 kcal/mol and forms a 10.9 kcal/mol through transition state TS . The dissociation of formaldehyde from 10 Fe leaves an 8.6 kcal/mol more stable intermediate 119,10‐Fe Fe. Then, another HBpin molecule approaches 11Fe and forms a 10.9 more stable intermediate 12Fe 11 with the formation of a B–O bond. The dissociation of newly formed more stable intermediate Fe. Then, another HBpin molecule approaches 11Fe and forms a 10.9 kcal/mol more stable intermediate 12 Fe with the formation of a B–O bond. The dissociation of newly pinBOBpin molecule from 12 for the regeneration of the catalyst 1Fe is Fe9.5 kcal/mol uphill. kcal/mol more stable intermediate 12 with the formation of a B–O bond. The dissociation of newly Fe formed pinBOBpin molecule from 12FeFe for the regeneration of the catalyst 1 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 Figure 3. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1 3. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzedFeby . 1Fe .
Figure 4. Optimized structures of TS2,3‐Fe (460i cm ), TS3,4‐Fe (78i cm ), TS6,7‐Fe (191i cm−1) and TS9,10‐Fe Figure 4. Optimized structures of TS 2,3‐Fe (460i cm−1), TS3,4‐Fe (78i cm−1), TS6,7‐Fe (191i cm−1) and TS9,10‐Fe −1). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity. Figure(86i cm 4. Optimized structures of TS2,3-Fe (460i cm−1 ), TS3,4-Fe (78i cm−1 ), TS6,7-Fe (191i cm−1 ) and −1 (86i cm ). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity. −1 −1
TS9,10-Fe (86i cm for clarity.
−1
). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown
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By comparing all relative free energies shown in Figure 3, we can conclude that 1Fe-HBpin and TS6,7-FeBy comparing all relative free energies shown in Figure 3, we can conclude that 1 are the rate-determining states of the catalytic reaction with a total free energy barrier Fe‐HBpin and of 24.0 which is 0.2 kcal/mol lower than the free energy barrier of the reaction catalyzed TSkcal/mol, 6,7‐Fe are the rate‐determining states of the catalytic reaction with a total free energy barrier of by24.0 kcal/mol, which is 0.2 kcal/mol lower than the free energy barrier of the reaction catalyzed by 1Ru . Therefore, 1Fe is a potential low-cost and high efficiency catalyst for the reduction of CO2 Ru. Therefore, 1Fe is a potential low‐cost and high efficiency catalyst for the reduction of CO2 to to 1formaldehyde. formaldehyde. Sabo-Etienne and Bontemps [26] recently reported the catalytic reduction of CO2 to Sabo‐Etienne and Bontemps with [26] Fe(H) recently reported the catalytic reduction of CO2 to bis(boryl)acetal and methoxyborane, catalyst precursor, and pinacolborane 2 (dmpe) 2 used as bis(boryl)acetal and methoxyborane, with Fe(H) 2 (dmpe) 2 used as catalyst precursor, and used as the reductant. For 1Ru , only one oxygen atom of CO2 is abstracted, and CO2 transforms into pinacolborane used as the reductant. For 1 Ru, 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 2(dmpe)2, one oxygen atom of CO2 is abstracted at the reduction transforms into HCHO. For Fe(H) second oxygen atom is removed and CO2 is transformed to methylene. This illustrates that iron step, then the second oxygen atom is removed and CO complexes can efficiently catalyze the reduction of CO22. is transformed to methylene. This illustrates 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 ). Figure 5 shows the calculated relative free energies in the reaction catalyzed by 1Os . Figure 6 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 Figure 6) are longer than those in corresponding Ru complexes. hydrogen atoms (marked in Figure 6) are longer than those in corresponding Ru complexes. As Asshown in Figure 5, 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 1Os-HBpin and TS2,3-Os with a free energy barrier of 29.0 kcal/mol, which is 4.8 kcal/mol higher than 1Os‐HBpin and TS 2,3‐Os with a free energy barrier of 29.0 kcal/mol, which is 4.8 kcal/mol higher than the thefree energy barrier of the reaction catalyzed by 1 free energy barrier of the reaction catalyzed byRu1.Therefore, 1 1Os could also catalyze the reduction Ru .Therefore, Os could also catalyze the reduction of of carbon dioxide to formaldehyde under harsher conditions. carbon dioxide to formaldehyde under harsher conditions. P H 1Os = H 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
5
G(kcal/mol)
0 -5 -10
(0.5)
1Os 2Os (0.0) (0.5) +4b +a +4b
-15 -20 -25 -30 -35 -40
TS3,4-Os
1Os-HBpin (-19.2) +a +3b
TS4,5-Os
TS7,8-Os TS6,7-Os
(-0.3) TS9,10-Os (-3.4) (-2.7) (-4.2) 3Os +4b +3b TS11,12-Os (-0.3) 4Os +4b +3b +3b 6Os (-9.4) 9 Os 5Os (-3.5) +4b 8Os (-4.6) (-5.2) 10Os (-6.0) +4b (-7.2) 1 +3b 7Os (-7.5) +3b Os +3b +4b (-10.3) +3b (-10.1) TS14,15-Os +3b 11Os +3b +3b (-14.1) (-22.4) +c +3b 12Os 13 (-18.7) Os 14 +2b Os +e TS 15,16-Os +3b (-21.1)(-22.3) +3b (-32.6) +2b +e 1Os-HBpin +e 1Os 1Os (-29.3) +2b 16 Os (-31.8) (-31.9) +e +2b (-33.6) +2b 15Os +c +2b +2b +e (-36.3) +d +f +e +2b +e Reaction Coordinate
Figure 5. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1 Figure 5. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed byOs1. Os .
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−1) and 8 −1 ) and Figure 6. Optimized structures of Os‐1cyp, 1 Os, 4 (239i cm Os8 . Bond lengths are in Å. Figure 6. Optimized structures of Os-1cyp, 1Os , 4Os , TS7,8‐Os (239i cm Os, TS 7,8-Os Os . Bond lengths are in The cyclopentyl and pinacol groups are not shown for clarity. Å. The cyclopentyl and pinacol groups are not shown for clarity.
2.4. Structures and Reactivity 2.4. Structures and Reactivity According our mechanistic mechanistic study, study, the the catalytic catalytic reaction reaction of of 2HBpin 2HBpin ++ CO22 → According to to our → HCHO HCHO + + pinBOBpin has five stages, stages, CO22 insertion pinBOBpin has five insertion (S1), (S1), σ‐Bond σ-Bond metathesis metathesis (S2), (S2), pinBOCHO pinBOCHO insertion insertion (S3), (S3), HCHO elimination (S4) and σ‐Bond metathesis (S5). HCHO elimination (S4) and σ-Bond metathesis (S5). (S1) [M]–H + + CO22 → significant (S1) CO22 Insertion. Insertion. The The reaction reaction in in S1 S1 is is [M]–H → [M]–OOCH. [M]–OOCH. The The most most significant structural change in S1 is the cleavage of M–H bond. Table 1 lists the natural population analysis structural change in S1 is the cleavage of M–H bond. Table 1 lists the natural population analysis (NPA) charges of metal atoms (e), the Wiberg bond indices (WBIs) of the bonds, and the relative free (NPA) charges of metal atoms (e), the Wiberg bond indices (WBIs) of the bonds, and the relative free ◦ ) in stages. We can see energies between intermediates and transition states/intermediates (ΔG /ΔG°) in stages. We can see energies between intermediates and transition states/intermediates (∆G‡‡/∆G that 2 that 2Fe has the most negative NPA charge, the smallest WBI of the M–H bond and the lowest barrier Fe has the most negative NPA charge, the smallest WBI of the M–H bond and the lowest barrier for the cleavage of M–H bond among these three intermediates in S1. for the cleavage of M–H bond among these three intermediates in S1. (S2) σ‐Bond Metathesis. The reaction in S2 is [M]–OOCH + HBpin → [M]–H + pinBOCHO. The (S2) σ-Bond Metathesis. The reaction in S2 is [M]–OOCH + HBpin → [M]–H + pinBOCHO. most important structural change in S2 is the cleavage of M–O bond. We can see TS The most important structural change in S2 is the cleavage of M–O bond. We can see7,8‐Os TS has the least 7,8-Os has the NPA charge, the smallest WBI of the M–O bond and the lowest barrier for the cleavage of M–O bond least NPA charge, the smallest WBI of the M–O bond and the lowest barrier for the cleavage of M–O among these three transition states in S2. bond among these three transition states in S2. (S3) pinBOCHO Insertion. The reaction in S3 is [M]–H + pinBOCHO → [M]–O–CH 2–OBpin. The (S3) pinBOCHO Insertion. The reaction in S3 is [M]–H + pinBOCHO → [M]–O–CH 2 –OBpin. most important structural change in S3 is the M–H has The most important structural change in S3 is the M–Hbond bondcleavage. cleavage.9Ru 9Ru hasa amore morenegative negative NPA NPA charge, a smaller WBI of the M–H bond and a lower barrier for the cleavage of M–H bond than 9 charge, a smaller WBI of the M–H bond and a lower barrier for the cleavage of M–H bond thanOs9 in Os S3. The Fe Fe complexes cannot be be compared in S3. The complexes cannot comparedwith withthe theRu Ruand andOs Oscomplexes complexes because because they they have have a a different pathway in S3. Scheme 1 shows that 1 Ru , different pathway in S3. Scheme 1 shows thatRu1 and pinBOCHO first generate the intermediate 9 and pinBOCHO first generate the intermediate Ru then form form the intermediate 10Ru10 through TS9,10‐Ru , in which a Ru–O bond is formed. However, as 9Ru , then the intermediate TS9,10-Ru , in which a Ru–O bond is formed. However, Ru through shown in in Scheme 2, 12,Fe1 combines directly with pinBOCHO and transforms to to intermediate 8Fe, 8Fe in , as shown Scheme directly with pinBOCHO and transforms intermediate Fe combines which an Fe–O bond is formed. in which an Fe–O bond is formed. 2–OBpin → HCHO + [M]–OBpin. The (S4) HCHO Elimination. The reaction in S4 is [M]–O–CH (S4) HCHO Elimination. The reaction in S4 is [M]–O–CH 2 –OBpin → HCHO + [M]–OBpin. most important structural change in S4 is the cleavage of the M–O bond. TS 9,10‐Fe has the lowest barrier The most important structural change in S4 is the cleavage of the M–O bond. TS 9,10-Fe has the lowest and the smallest WBI of the M–O bond among these three transition states in S4. There is no obvious barrier and the smallest WBI of the M–O bond among these three transition states in S4. There is no relation between the energy barriers and the NPA charges of metal atoms in S4. obvious relation between the energy barriers and the NPA charges of metal atoms in S4. (S5) σ‐Bond Metathesis. The reaction in S5 is [M]–OBpin + HBpin → [M]–H + pinBOBpin. The (S5) σ-Bond Metathesis. The reaction in S5 is [M]–OBpin + HBpin → [M]–H + pinBOBpin. most important structural change in S5 is the cleavage of the H–B bond. 15 has a smaller WBI of the The most important structural change in S5 is the cleavage of the H–B bond.Os15 Os has a smaller WBI of H–B bond and and a lower barrier for for the cleavage in obvious the H–B bond a lower barrier the cleavageof ofH–B H–Bbond bondthan than15 15RuRu inS5. S5.There There is is no no obvious relation between the energy barriers and the NPA charges of metal atoms in S5. The Fe complexes relation between the energy barriers and the NPA charges of metal atoms in S5. The Fe complexes cannot be compared with the Ru and Os complexes because they have a different pathway in S5. cannot be compared with the Ru and Os complexes because they have a different pathway in S5.
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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
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
WBI
∆G‡ /∆G◦ (kcal/mol)
0.5876 0.5420
0.4
0.6273 0.4674
5.5
−1.540 −1.518
0.6887 0.5094
9.3
6Fe TS6,7-Fe
−1.209 −0.974
0.3517 0.0287
20.8
7Ru TS7,8-Ru
−1.411 −1.104
0.2704 0.0275
12.2
7Os TS7,8-Os
−1.427 −1.111
0.2995 0.0271
10.0
1Fe 8Fe
−1.280 −1.279
0.7154 0.2182
−7.3
9Ru TS9,10-Ru
−1.867 −1.850
0.6661 0.5757
0.5
9Os TS9,10-Os
−1.832 −1.808
0.7206 0.6002
1.0
9Fe TS9,10-Fe
−0.890 −0.840
0.4354 0.2609
3.1
11Ru TS11,12-Ru
−1.190 −1.184
0.2583 0.2616
3.8
11Os TS11,12-Os
−1.177 −1.193
0.4049 0.3061
4.7
11Fe 12Fe
−0.778 −1.290
/ 0.5236
−10.9
15Ru TS15,16-Ru
−1.478 −1.795
0.5325 0.0863
4.5
15Os TS15,16-Os
−1.483 −1.742
0.4898 0.0958
3.7
Complexes
e
2Fe TS2,3-Fe
−1.892 −1.782
2Ru TS2,3-Ru
−1.538 −1.439
2Os TS2,3-Os
Bond
M–H bond
M–O bond
M–H bond
M–O bond
/
H–B bond
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
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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 benzene (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 base 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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