DFT Studies on Reactions of CO2 with Niobium and Vanadium Nitride

Oct 18, 2011 - using density functional theory. We mainly calculated the energetics for the interesting reaction sequence, reported recently by Cummin...
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DFT Studies on Reactions of CO2 with Niobium and Vanadium Nitride Complexes Congjie Zhang,†,‡ Meiyan Wang,† Liqin Xue,† Ting Fan,† and Zhenyang Lin*,† †

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, China

‡

S Supporting Information *

ABSTRACT: We have investigated the reduction of CO2 to CO mediated by the anionic niobium nitride complex [NNb(NtBuMe)3]−, a model complex of [NNb(NtBuAr)3]−, using density functional theory. We mainly calculated the energetics for the interesting reaction sequence, reported recently by Cummins and his co-workers, involving reactions of the anionic niobium nitride complex [NNb(NtBuAr)3]− with CO2 to first give a carbamate complex. The carbamate complex then reacts with MeC(O)Cl to give an isocyanate−acetate, which can be reduced by SmI2 to give the isocyanate complex (OCN)Nb(NtBuAr)3. Further reduction of the isocyanate complex by Na/Hg extrudes CO and regenerates [NNb(NtBuAr)3]−. In addition, we compare the reaction pathways for the reduction reaction of the isocyanate complexes (OCN)M(N tBuMe)3 (M = Nb, V), model complexes of (OCN)M(NtBuAr)3, with sodium and explain why these two reduction reactions give remarkably different products.

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INTRODUCTION

Scheme 1

Carbon dioxide (CO2) is an easily available, renewable carbon resource. Therefore, there is significant interest in developing efficient transformation of CO 2 into useful chemicals. 1 However, cleavage of the CO bond(s) in carbon dioxide is not easy to achieve because of its high thermodynamic stability. A number of approaches to the transformation have been discussed in detail in a recent review article.2 Of these approaches, many transition-metal complexes were found to be efficient in converting CO2 to other chemicals.3−10 In particular, (PNP)IrH2,11 Li2[W(CO)5], [WCl2(PMePh2)4],12,13 (NHC)Ni (NHC = N-heterocyclic carbene),14 Lt‑BuFe−N2− FeL t ‑ B u (L t ‑ B u = 2,2,6,6-tetramethyl-3,5-bis-[(2,6diisopropylphenyl)imino]hept-4-yl),15 [Cp2TiCl]2, Cp2Zr(II)(CO)2 ,16 Ir2 (CO) 3(dmpm) 2 ,17 [(Ph 2 P(CH 2 ) 2 PPh 2 ) 2Rh][MgCl],18 and a coordinatively unsaturated tris-aryloxide uranium(III) complex [(L)U] (L = 1,4,7-tris(3,5-di-tert-butyl2-hydroxybenzylate)-1,4,7-triazacyclononane) 19 have been found to be capable of abstracting CO or O from CO2 to form strong metal−carbonyl or metal−oxygen bonds. However, formation of strong metal−carbonyl or metal−oxygen bonds from these abstraction reactions represents a challenge to catalytic turnover. Recent studies showed that a sequence of reactions constituted a cycle for CO2 reduction to CO when Zr(III)−O− Cu(II)20 or [NNb(NtBuAr)3]− (1Nb)21−23 was employed. These metal complexes are believed to be promising for a catalytic reduction of CO2 to CO. In this work, we are interested in the reaction sequence elegantly reported recently by Cummins and his co-workers © 2011 American Chemical Society

(Scheme 1) involving the anionic niobium nitride complex 1Nb.21 Scheme 1 shows that an initial binding of CO2 to the terminal nitride nitrogen atom of 1Nb rather than the niobium metal center gives the carbamate complex 2Nb. Reaction of the Received: August 12, 2011 Published: October 18, 2011 5911

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validity of mixing the two types of ECPs in the calculations. This, however, should not be a problem because there is no mixing of the two ECPs in calculating each species present in the reduction reaction. Frequency calculations at the same level of theory were also performed to confirm all stationary points as the transition states or minima. Intrinsic reaction coordinate (IRC)29 calculations were also carried out to verify that transition structures calculated indeed connect relevant reactants, products, and/or intermediates. We employed a continuum medium to do single-point solvation energy calculations for all of the species calculated, using UAKS radii on the conductor polarizable continuum model (CPCM).30 In the CPCM calculations, THF was used as the solvent, corresponding to the experimental conditions.21 In this paper, solvation-corrected relative free energies are used to analyze the reaction pathways calculated. Wiberg bond indices (WBI) were obtained by natural bond orbital (NBO) analysis.31 The minimum-energy crossing points (MECPs) between the singlet and triplet states for some pertinent niobium and vanadium complexes were calculated using the MECP location program developed by Harvey and his co-workers. 32 All calculations were performed with the Gaussian 03 program.33

carbamate complex 2Nb with MeC(O)Cl gives an isocyanate− acetate (3Nb). Reduction of 3Nb by SmI2 leads to breaking of the Nb−O bond in 3Nb to give the isocyanate complex (OCN)Nb(NtBuAr)3 (4Nb). Further reduction of 4Nb by Na/Hg extrudes CO and regenerates 1Nb. Reaction of the vanadium analogue [NV(NtBuAr)3]− (1V)24 with CO2 also gives an analogous carbamate complex [O2CNV(NtBuAr)3]− (2V) in which CO2 also binds to the terminal nitride nitrogen of 1V.22 However, when the analogous vanadium isocyanate complex (OCN)V(NtBuAr)3 (4V) was treated with Na/Hg, no CO extrusion was observed, and the analogous vanadium nitride complex was not regenerated. Instead, the isocyanate ligand dissociated, and the vanadium metal center in 4V was reduced to give a threecoordinate vanadium(III) complex (5Vtriplet) having a triplet ground state (Scheme 2).23 Scheme 2

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RESULTS AND DISCUSSION Reduction of CO 2 to CO Mediated by the Anionic Niobium Nitride Complex [NNb(NtBuAr)3]−. 1Nb + CO2 → 2Nb. Experimentally, [Na]1Nb reacts with CO2 to give the carbamate complex [Na]2Nb.21 We optimized the structures of [NNb(NtBuMe)3]− (1-Nb) and [O2CNNb(NtBuMe)3]− (2-Nb) in which the Ar substituents at N of the amide ligands were modeled with Me and displayed the calculated structures and relative energies in Figure 1. Here, one may argue that it is inappropriate to model the Ar substituents at N of the amide ligands with Me, considering the possible π-interaction between the aryl substituents and N within the amide ligands. We examined the experimentally determined structures21,23 of 2Nb, 3Nb, and 4V and found that in each of the amide ligands, due to the steric hindrance, the Ar plane is nearly perpendicular to the planar N center, suggesting that the π-interaction mentioned above does not exist and justifying the choice of the Me models. Indeed, examination of the calculated energy barrier and reaction energy for 1Nb + CO2 → 2Nb showed that the results are similar using the Me models and the Ar models.25 1-Nb and 2-Nb are model complexes for 1Nb and 2Nb, respectively. We added a hyphen in each of the compound labels to distinguish the model complexes from the real complexes. We have located the van der Waals complex [O2C···NNb(NtBuMe)3]− (2A-Nb) and the transition state (TS(2A‑2)‑Nb) that connects 2A-Nb and 2-Nb. The most stable structure of 1-Nb has a C3 point group. As seen from Figure 1, the bond distance of Nb−N(terminal) in 1-Nb is 1.718 Å, and the Wiberg bond index (WBI) of the Nb−N(terminal) bond is 2.58, suggesting a Nb−N(terminal) triple bond. In 2-Nb, the bond length of Nb−N(triply bonded) and the average C−O bond distance are 1.755 and 1.245 Å, respectively, and the bond angles of O−C−O and C−N(triply bonded)−Nb are 133.1 and 171.9°, respectively. These calculated structural parameters in the model complex 2-Nb are in good agreement with the experimental values in the real complex 2Nb: 1.764 Å (Nb− N(triply bonded)), 1.236 Å (the average CO2 C−O bond distance), 127.3° (O−C−O), and 175.8° (C−N(triply bonded)−Nb).21 The free-energy barrier for the formation of 2-Nb from 1-Nb + CO2 is calculated to be 6.2 kcal/mol, indicating that 1-Nb easily traps CO2. 2Nb + MeC(O)Cl → 3Nb + Cl−. Experimentally, 2Nb reacts with MeC(O)Cl to form 3Nb.21 It was proposed that 2Nb first

Recently, a closely related theoretical work was reported. 25 In this theoretical work, the authors reported their detailed DFT studies on the mechanism for the transformation 1Nb → 2Nb → 3Nb (Scheme 1). The authors also examined how different group 5 metals affect the transformation and found that the Ta analogue has the highest reactivity for the transformation. In studying the step 2M + MeC(O)Cl → 3M (M = V, Nb, and Ta), the authors used MeC(O)+ in their calculations rather than MeC(O)Cl. In addition, the important step, 4Nb + Na → [Na]1Nb + CO, in which the CN double bond of the isocyanate ligand in 4Nb is cleaved, has not been theoretically investigated. More recently, Mosconi and de Angelis further studied the step 2Nb + MeC(O)Cl → 3Nb and emphasized the role of the Na+ counterion in the choice of reaction pathways.26 In this paper, we carried out DFT calculations on the reaction sequence shown in Scheme 1 to obtain insight into the energetics associated with all of the reactions involved. Through the DFT calculations, we will gain a better understanding on why the analogous niobium and vanadium isocyanate complexes 4Nb and 4V have remarkably different reactivities when they are reduced by sodium metal.

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COMPUTATIONAL DETAILS

Density functional theory calculations at the B3LYP level were carried out to optimize geometries for all of the species studied here. In the DFT calculations, we used Me to model the Ar (3,5-C 6H3Me2) substituent at N of the amide ligands. The effective core potentials (ECPs) of Hay and Wadt with the double-ζ valence basis set (LanL2DZ)27 were employed to describe Nb and V atoms, while the 6-31G* basis set was used for H, C, N, O, Na, and Cl atoms. To investigate the step 3Nb + SmI2 → 4Nb + AcOSmI2, a reduction reaction, we only calculated its thermochemistry as the relevant mechanism is expected to be complicated. Because there is no LanL2DZ ECP basis set for samarium, we instead employed the Stuttgart ECP basis set (SDD)28 for samarium and iodine atoms in calculations of the thermochemistry. Here, one may question the 5912

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Figure 1. The potential energy profile calculated for 1-Nb + CO2 → 2-Nb. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively.

Figure 2. Potential energy profile calculated for 2-Nb + CH3COCl → 3-Nb + Cl−. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively.

via the transition state TS(3B‑3A)‑Nb to give the intermediate 3A-Nb with a free-energy barrier of 10.5 kcal/mol. Clearly, the substitution step 2-Nb + MeC(O)Cl → 3A-Nb + Cl− is both kinetically and thermodynamically favorable. Figure 2 also shows that the C−O cleavage via the six-membered-ring transition state TS(3A‑3)‑Nb leading to the formation of 3-Nb is also a very facile process. These results support the proposal that the reaction of

acts as a nucleophile to substitute the chloride in MeC(O)Cl to give the intermediate AcO(O)CNNb(N tBuAr)3 (3ANb) followed by cleavage of a C−O bond to finally afford 3Nb. The energy profile calculated for the above transformation is given in Figure 2. Our calculations based on the model complexes indicate that 2-Nb first forms a van der Waals complex (3B-Nb) with MeC(O)Cl. Then, the 2-Nb-for-chloride substitution occurs 5913

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2Nb with MeC(O)Cl giving 3Nb is a two-step process.21 The 2-Nb-for-chloride substitution with a free-energy barrier of 10.5 kcal/mol is found to be rate-determining. 3Nb + SmI2 → 4Nb + AcOSmI2. Experimentally, reduction of 3Nb by SmI2 leads to the Nb−O bond cleavage in 3Nb to give the isocyanate complex (OCN)Nb(NtBuAr)3 4Nb having a doublet electronic structure.21 Computationally, it is difficult to calculate the reaction barrier for this reduction reaction. Therefore, we simply evaluated the thermochemistry and found that the conversion 3-Nb + SmI2 → 4-Nb + AcOSmI2 is thermodynamically favorable with a reaction free energy of −13.0 kcal/mol (Figure 3).

CN double bond in the isocyanate ligand) and generates the starting complex [Na]1Nb.21 When 4Nb reacts with Na, it is reasonable to assume that the first elementary step of this reduction reaction is the formation of the intermediate [Na](OCN)Nb(NtBuAr)3, a salt formed simply from a transfer of the Na valence electron to the complex 4Nb. 4Nb has a doublet electronic structure, and we expect that the intermediate would have either a singlet or a triplet ground state. We first considered species having a singlet ground state. Figure 4 shows the potential energy profiles calculated for the transformation 4-Nb + Na → [Na]1-Nb + CO. The first intermediate immediately derived from the electron transfer is 6A-Nb. 6A-Nb has an end-on isocyanate ligand and is relatively unstable. It easily undergoes a structural rearrangement to a more stable species (6B-Nb) having a side-on isocyanate ligand. A migration of the sodium cation in 6B-Nb gives an even more stable intermediate (6C-Nb), which still has a sideon isocyanate ligand. From 6A-Nb to 6B-Nb and then to 6C-Nb, the C−N bond of the isocyanate ligand changes from a “formally double” bond to a “formally single” bond, with an increase in the bond distance of ∼0.1 Å. Two pathways have been found for the extrusion of CO from 6C-Nb to give [Na]1Nb. One pathway extrudes CO directly, while the other pathway is a stepwise process. Both pathways have similar overall barriers (∼26 kcal/mol). In the one-step pathway, the transition state TS(6C‑1)‑Nb shows simultaneous breaking of the Nb−C and C−N bonds. In the stepwise pathway, an intermediate (6D-Nb) is formed in which the triply bonded N acts as a nucleophile

Figure 3. Thermochemistry calculated for 3-Nb + SmI2 → 4-Nb + AcOSmI2. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively.

4Nb + Na → [Na]1Nb + CO. Experimental results indicate that reduction of 4Nb by Na extrudes CO (cleavage of the

Figure 4. The potential energy profiles calculated for the transformation 4-Nb + Na → [Na]1-Nb + CO. Singlet states are considered here. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively. 5914

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with its bonding orbital accommodating the lone pair of electrons interacting with one of the two CO π* orbitals. Interestingly, we also located a transition state (TS(6A‑6D)‑Nb) linking 6A-Nb and 6D-Nb. This pathway corresponds to a process leading to formation of the CO unit from 6A-Nb having an η 1-NCO ligand to give the 6D-Nb intermediate described above. In the process, the CN double bond is being broken while the NbN triple bond is being formed. The pathways described above give the products Na[1-Nb] + CO. Interestingly, Na[1-Nb] + CO lies higher in the energy profile than 6C-Nb. A dimerization of Na[1-Nb] occurs to give the dimer {Na[1-Nb]}2, stabilizing the species Na[1-Nb], which has a NbN triple bond. The results are consistent with the experimental observation that a dimer was obtained and structurally characterized.34 Summarizing the results presented in Figure 4 when only the singlet states are considered, we can see that 6C-Nb is the species would be initially formed. Because the forward reaction from 6C-Nb to [Na]1-Nb + CO (or via 6D-Nb) and then the dimer {[Na]1-Nb}2 has a higher overall barrier than the reverse reaction from 6C-Nb to 6B-Nb and then 6A-Nb, the favorable pathway is 6C-Nb → 6B-Nb → 6A-Nb → 6D-Nb (via TS(6A‑6D)‑Nb) → [Na]1-Nb + CO → {[Na]1-Nb}2 + CO. Therefore, the overall barrier for the CO extrusion is 20.8 kcal/ mol, the energy difference between 6C-Nb and TS(6A‑6D)‑Nb. We then considered species having a triplet ground state for the reaction of 4-Nb with sodium. On the basis of the structures calculated for the singlet species 6A-Nb, 6B-Nb, and 6C-Nb, we attempted to see if we could locate the corresponding triplet species. All of the optimizations led to the species 6A-Nbtriplet, the corresponding species of 6A-Nb. In other words, we could not locate the corresponding triplet species of 6B-Nb and 6C-Nb. The results are understandable. 6B-Nb and 6C-Nb each can be viewed as formally having a Nb(V) center with five metal−ligand σ bonds. Achieving the electronic structure of a Nb(V) center with five metal−ligand σ bonds is possible only in a singlet electronic state. A triplet electronic state would give a d2 electron configuration and does not allow the formation of five metal−ligand σ bonds. Starting from 6A-Nbtriplet, we calculated two relevant pathways. One pathway considers a direct CN bond cleavage to release a CO molecule. The other pathway is related to the isocyanate ligand dissociation, a pathway observed experimentally in the vanadium analogue only (vide infra).23 The energy profiles calculated for these two pathways are shown in Figure 5. Clearly, the CO extrusion from the triplet species 6ANbtriplet is energetically inaccessible. Instead, the isocyanate ligand dissociation was found to be very facile. As seen from the energy profiles shown in Figures 4 and 5, we can find that the isocyanate ligand dissociation is kinetically favorable and the formation of {[Na]1-Nb}2 is kinetically less favorable but thermodynamically more favorable. 6C-Nb and (5-Nbtriplet + NaOCN) have comparable stability (Figures 4 and 5), and the overall free-energy barrier for their interconversion is 17.9 kcal/mol (the energy difference between (5-Nbtriplet + NaOCN) and TS(6A‑6B)‑Nb). Therefore, we can conclude that 6C-Nb and (5-Nbtriplet + NaOCN) are in equilibrium and the formation of the experimentally observed product {[Na]1-Nb}2, which is a thermodynamic product, requires a overall free-energy barrier of 20.8 kcal/mol from 6C-Nb to TS(6A‑6D)‑Nb. Experimental results indicated that 4Nb and Na were allowed to stir about 7−8 h to give [Na]1Nb and

Figure 5. The potential energy profiles calculated for the transformations 4-Nb + Na → [Na]1-Nbtriplet + CO and 4-Nb + Na → 5-Nbtriplet + NaOCN. Triplet states are considered here. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively.

CO, in which CO was trapped and monitored by Cp*RuCl(PCy3).21,23 The equilibrium mentioned above requires interconversion between 6C-Nb and (5-Nbtriplet + NaOCN). The interconversion between the two requires an intersystem crossing between the singlet and triplet potential energy surfaces. The estimation of a free-energy barrier of 17.9 kcal/mol for the interconversion (vide supra) assumes that the structure of the MECP does not lie higher in energy than TS(6A‑6B)‑Nb (the highest point in the path from 6C-Nb (Figure 4) to (5-Nbtriplet + NaOCN) (Figure 5)). Indeed, an MECP structure, which is similar and closely related to both 6A-Nb and 6A-Nbtriplet, was located, and its energy is very close to that of 6A-Nb (Figure 6). Reduction of the Vanadium Isocyanate Complex (OCN)V(NtBuAr)3 (4V) by Na. The vanadium isocyanate complex (OCN)V(NtBuAr)3 (4V) is an analogue of 4Nb. As we discussed above, reduction of 4Nb by Na leads to a CN bond cleavage in the isocyanate ligand. Interestingly, reduction of 4V simply results in dissociation of the isocyanate ligand, that is, (OCN)V(NtBuAr)3 (4V) + Na → V(NtBuAr)3 (5 Vtriplet) + NaOCN, and a reduction at the vanadium metal center from V(IV) to V(III). To understand why 4V shows different reactivity toward Na, we considered the model complex (OCN)V(NtBuMe)3 (4-V) together with its reaction with Na and calculated the pathways leading to the CN bond cleavage in the isocyanate ligand and the dissociation of the isocyanate ligand as discussed above for the niobium analogue 4-Nb (Figures 4 and 5). Figure 7 shows the energy profiles calculated for the pathways leading to the CN bond cleavage in the isocyanate ligand for species having a singlet ground state. We located the 5915

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occurs to give [Na]1-V + CO via the transition state TS(6C‑1)‑V. We also located a transition state (TS(6A‑1)‑V) directly linking 6A-V and [Na]1-V + CO. On the basis of Figure 7, we can see that the overall free-energy barrier for the favorable bond cleavage process was calculated to be 23.3 kcal/mol relative to 6A-V. The CN bond cleavage process 6A-V → 1/2 {[Na]1-V}2 + CO is only slightly exergonic. Figure 8 shows the energy profile calculated for the pathway leading to dissociation of the isocyanate ligand for species having a triplet ground state. Remarkably, the ligand dissociation process shown in Figure 8 is both kinetically and thermodynamically much more favorable than the CN bond cleavage process shown in Figure 7. These calculation results are in good agreement with the experimental observation of (OCN)V(NtBuAr)3 (4V) + Na → V(NtBuAr)3 (5Vtriplet) + NaOCN.23 In Figure 7, we see that [Na]1-V + CO lies higher in energy than 6A-V. This calculation result explains the experimental observation that reaction of [Na]1V with CO gives V(NtBuAr)3 (5Vtriplet) + NaOCN.23 A reaction sequence for this reaction can be postulated as follows based on the energy profiles shown in Figures 7 and 8. [Na]1-V + CO easily gives 6A-V, and via an intersystem crossing, 6A-V changes its spin state to 6A-Vtriplet. 6A-Vtriplet undergoes ligand dissociation via TS(6A‑5)‑Vtriplet to give 5-Vtriplet + NaOCN. The MECP for the intersystem crossing from 6A-V to 6A-Vtriplet was also located and is shown in Figure 9.

Figure 6. The MECP structure together with its closely related structures 6A-Nb and 6A-Nbtriplet. The calculated relative electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively.

intermediates 6A-V and 6C-V, which are analogous to 6A-Nb and 6C-Nb, respectively. We did not find 6B-V and 6D-V, the analogues of 6B-Nb and 6D-Nb. The transition state structure TS(6A‑6C)‑V was located connecting 6A-V and 6C-V. The freeenergy barrier calculated for the rearrangement from 6A-V to 6C-V is 15.8 kcal/mol. From 6C-V, the CN bond cleavage

Figure 7. The potential energy profiles calculated for 4-V + Na → [Na]1-V + CO. Singlet states are considered here. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively. 5916

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Figure 8. The potential energy profile calculated for the transformation 4-V + Na → 5-Vtriplet + NaOCN. Triplet states are considered here. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively.

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bonded to one of the CO2 oxygens in the anionic carbamate complex. The free-energy barrier for this step was calculated to be 10.5 kcal/mol. The intermediate easily isomerizes to an isocyanate−acetate complex (3-Nb) with a free-energy barrier of only 4.9 kcal/mol. The reaction is highly exergonic by 37.0 kcal/mol. (3) Reduction of the isocyanate−acetate complex (3-Nb) by SmI2 leads to dissociation of the acetate ligand and formation of an isocyanate complex ((OCN)Nb(NtBuMe)3, 4-Nb). This reduction reaction is exergonic by 13.0 kcal/mol. (4) Further reduction of the isocyanate complex (4-Nb) by sodium leads to cleavage of the CN double bond in the isocyanate ligand and regenerates the anionic niobium nitride complex (1-Nb) in the form of a dimer. Our calculations indicate that the reduction initially generates a triplet Nb(III) center that then goes to a singlet Nb(III) center via an intersystem crossing. The singlet Nb(III) center is capable of binding the isocyanate ligand and facilitates the CN bond cleavage of the isocyanate ligand by forming a strong NbN triple bond. The reduction leading to the CN bond cleavage of the isocyanate ligand is exergonic by 16.6 kcal/mol, and the overall free-energy barrier for the bond cleavage was calculated to be 20.8 kcal/mol. Compared to that of the niobium isocyanate complex (4-Nb), reduction of the analogous vanadium isocyanate complex ((OCN)V(NtBuMe)3, 4-V) by sodium simply results in the dissociation of the isocyanate ligand, that is, (OCN)V(NtBuAr)3 (4V) + Na → V(NtBuAr)3 (5 Vtriplet) + NaOCN. Our calculation results indicate that a vanadium(III) metal center very much prefers a high-spin electron configuration and that the vanadium(V) complex with a VN triple bond is not favored thermodynamically. Therefore, the CN bond cleavage of the isocyanate ligand cannot be realized. Instead, upon the addition of Na, the ligand dissociation is very facile, with a free-energy barrier of only 13.7 kcal/mol, and the reduction by sodium leading to dissociation of the isocyanate ligand is highly exergonic by 37.2 kcal/mol.

SUMMARY The detailed reaction mechanism for the reduction of CO2 to CO mediated by the anionic niobium nitride complex [NNb(NtBuMe)3]− (1-Nb) has been investigated with the aid of DFT calculations. (1) The initial binding of CO2 to the terminal nitride nitrogen atom to give an anionic carbamate complex (2-Nb) is facile and exergonic by around 10 kcal/mol. (2) Reaction of the anionic carbamate complex (2-Nb) with MeC(O)Cl resembles a substitution of the anionic complex for chloride to give an intermediate formally having MeC(O)+

Figure 9. The MECP structure together with its closely related structures 6A-V and 6A-Vtriplet. The calculated relative electronic energies (in parentheses) are given in kcal/mol. The bond distances and angles in the calculated structures shown are given in angstroms and degrees, respectively. 5917

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ASSOCIATED CONTENT S Supporting Information * Complete ref 33 and tables giving Cartesian coordinates and electronic energies for all of the calculated structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by the Research Grants Council of Hong Kong (SBI09/10.SC4 and HKU1/CRF/08) and the Fundamental Research Funds for the Central Universities (GK201002013).

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REFERENCES

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dx.doi.org/10.1021/om2007584 | Organometallics 2011, 30, 5911−5918