Computational Study on the Mechanisms of Multiple Complexation of

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A Computational Study on the Mechanisms of Multiple Complexation of CO and Isonitrile Ligand to Boron Xuejiao J. Gao, and Xingfa Gao J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b01313 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 21, 2017

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The Journal of Physical Chemistry

A Computational Study on the Mechanisms of Multiple Complexation of CO and Isonitrile Ligand to Boron Xuejiao J. Gao,†,‡ and Xingfa Gao*,† †

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang,

330022, China. ‡

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,

Beijing, 100049, China.

ABSTRACT: The recent experimental realization of compound Tripp-B(CO)2 (denoted as 2a), where Tripp is 2,6-di(2,4,6-triisopropylphenyl)-phenyl), breaks through the conventional knowledge that only transition metals can bind more than one CO to form multicarbonyl adducts. Compound 2a is stable in air but liberates CO under light. The B-CO bonds of 2a are considered to be similar to donor-acceptor bonds of transition metal complexes. To address the formation mechanism and chemical bonding of this novel type of boron compounds, we present a density functional theory study on the formation and photolysis of 2a and the similar compounds. The results suggest that the formation of 2a is facile by three consecutive additions of CO to the terminal borylene metal complex, i.e., the boron source of the synthesis. These CO additions can be practically accomplished via two different paths: CO direct addition and CO migration

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followed by addition. Such mechanisms can be excellently rationalized by the donor-acceptor bonding model of the terminal borylene complex, which in turn suggests that using donoracceptor bonds for 2a is natural for understanding the mechanisms. The liberation of CO from 2a and its similar compounds have higher energy barriers at the ground states than those at the triplet states by 18 kcal/mol. These energy barriers explain the experimentally observed airstability and photolysis of these compounds. The results for the first time provide mechanistic insights for the unprecedented chemical processes; they allow evaluating the applicability of donor-acceptor bonding in main-group compounds from the new perspective of chemical reactions.

INTRODUCTION Transition metals can bind multiple carbonyls and related ligands to yield complexes. This property can be ascribed to that they have vacant d orbitals to accept electrons from the ligands and some of them can also back donate their own d electrons to the ligands. The modest energy separations between the frontier orbitals of transition metal complexes endow these compounds, in the case of having open coordination sites, with ability to activate small molecules. As such, transition metal complexes have been being widely used as homogeneous catalysts in chemical industries; they also naturally exist and play key roles in the active sites of many enzymes. The value of transition metal complexes has stimulated intense interest in seeking their mimics outside the d-block of the periodic table, with the intention to replace the precious metals by less toxic and abundant main-group elements as catalysts. Many main-group species, which were previously thought to be incapable of stable existence, have been experimentally realized. These species include those containing multiple bonds between heavier group 12,1 13,2−10 14,11−20 and


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15 elements,21−29 singlet carbenes30,31 and their heavier element congeners,32 paramagnetic radicals centered on heavier group elements,33,34 singlet diradicals,35−37 and those containing frustrated Lewis pairs.38,39 All these species are featured by relatively small energy separations between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs); they have catalytic activities towards small molecule activation in a way to some extent similar to transition metal complexes.34,40−43 In contrast with the striking advances in finding main-group compounds catalytically behaving like transition metal complexes, achieving multicarbonyl adducts E(CO)n, where E is a maingroup species and n > 1, is not straightforward. The latter goal is of interest because multiple carbonylation is the fundamental reactivity of transition metals. So far, except for a few monocarbonyl adducts like those of carbenes,44−48 boranes,49,50 phosphorus51 and uranium52 that have been experimentally isolated, only borylene dicarbonyl 2a (Tripp-B(CO)2, Figure 1)53— have been synthesized and crystallographically characterized.

Figure 1. Reactions reported in Ref. 53 to yield 2a, 3, and 4; 1c is a prototype model of 1a and 1b. The inset shows two equivalent bonding structures of 2a. RT, room temperature. Noteworthily, the recently reported 2a by Braunschweig and coworkers was prepared by the direct reaction of CO molecules with the corresponding reagent (Figure 1).53 The possible


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existence of 2a had been predicted by Frenking and coworkers based on first principle calculations prior to its synthesis. The prediction further suggested strong σ-donation bonding from CO to B and strong π backbonding from B to CO in this class of compounds.54 Such interaction is similar to the Dewar-Chatt-Duncanson model previously proposed for an alkene and a metal in certain organometallic compounds,55−59 which was later supported by the experimentally observed short B-C bonds (1.151 Å) and considerably long C-O bonds (1.474 Å) in 2a.53 Similar bonding was later found in boron dicarbonyl and tricarbonyl complexes.60,61 The experiments further demonstrated that 2a is air-stable but liberates CO under light.53 Because π backbonding and photolysis are characteristic for transition metal complexes, 2a was claimed as an example of a main-group species forming multiple donor-acceptor bonds with CO.62 Despite the breakthrough in realizing the peculiar bis(CO) adduct of boron,53 the atomistic level mechanisms of its synthesis and photolysis, which substantialize these unprecedented chemical processes, have not yet been reported. As donor-acceptor bonding is a concept that is more commonly encountered within transition metal compounds, it remains as an open question whether the B-CO bonds of main-group compounds like 2a are appropriate to be considered as donor-acceptor bonds63 or common covalent bonds similar to those of cumulenes64,65 (see the inset of Figure 1). Furthermore, as 2a is shown to have chemical bonding akin to those of transition metal complexes,53,62 a question of immediate interest is whether 2a catalytically also behaves like the latter. Here we present a density functional theory (DFT) study on the formation and photolysis of 2a and 3, which has been also synthesized and characterized by Braunschweig and coworkers.53 The results will for the first time provide mechanistic insights for these novel boron compounds, which explain the experimentally observed reactivity of them, e.g., their facile synthesis, remarkable stability in ambient air and instability under light. Particularly, we


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will analyze the evolution of bonding situation between B and CO during the formation of 2a, which allows evaluating the applicability of donor-acceptor bonding in 2a from the new perspective of chemical reactions. Therefore, the results will be also of fundamental interest concerning chemical bonding of main-group compounds. METHODS All geometries were optimized using the M062X functional66 with the 6-31G* basis sets67,68 for nonmetals, and the SDD basis set and pseudopotential69,70 for metals, as implemented in the GAUSSIAN 09 package.71 The density functional used here has the best balance between efficiency and accuracy, which has been successfully employed before to model the borylene compounds.54 Frequency analyses were done for all the structures at the same level of theory to obtain Gibbs free energies. The solvent effect of benzene, if any, was considered with the PCM model72 during the calculations. The setting of solvent and temperature was according to the experiments.53 To investigate the photolysis mechanism of the borylene dicarbonyl compounds, their dissociation reactions were calculated at closed-shell singlet, triplet, and excited singlet states, respectively. Energies for the closed-shell singlet and triplet structures were calculated by full geometry optimization using the spin-restricted and unrestricted M062X methods, respectively. Energies for the excited singlet structures were obtained by singlet-point energy calculations using the time-dependent DFT (TD-DFT) method.73−75 These single point calculations were based on the corresponding triplet structures, except for 13* that was based on the closed-shell singlet structure. The Wiberg bond76 orders were calculated based on Natural Bond Orbital (NBO) analysis77−79 using the Multiwfn package.80 The ETS-NOCV analysis81 was performed to investigate the orbital interactions between two fragments in a molecule, so as to estimate which orbital interactions primarily contribute to the


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bonding of these fragments. These calculations were carried out using the Becke-Perdew exchange-correlation functional BP8682−84 with the TZ2P basis sets, as implemented in the program package ADF2014.10.85 RESULTS AND DISCUSSION Bonding Model of 1 For terminal borylene metal complexes such as 1, the M and BR are usually linked by double bonds when drawing their molecular structures (e.g., see 1 of Figure 1). However, previous theoretical studies have established that the M-BR bond of 1, especially in the case that the R substituent is a weak π-donor, is a typical donor-acceptor bond. It consists of a σ-donation from B’s σ lone pair to M’s vacant d orbital and two π-back-donations from M’s filled d orbitals to B’s empty p orbitals.86−94 Thus, the M-BR bonding can be qualitatively described using three arrows, as has been previously proposed for the Fe(CO)4-complexed borylenes and their aluminum and gallium congeners.88 Here, we report that the three-arrow bonding model indeed better describes the electronic structures and reactivity of 1. Taking 1c as an example, its three-arrow model is shown in Figure 2a. The B atom adopts sp hybridization. The sp hybrid orbital directing to Mo is the σ lone pair; the 2px and 2py are two orthogonal vacant orbitals.The atomic orbital of Mo directing to B is vacant; the 5dxz, 5dyz and 5dxy are lone pairs. Hence, the donation B(sp)→Mo and back donations Mo(5dxz)→B(2px) and M(5dyz)→B(2py) exist between the BH and Mo(CO)5 fragments, as illustrated by the three arrows (Figure 2a). The 5dxy orbital, which is not shown in Figure 2a for clarity, is present as a nonbonding orbital. Themodel of Figure 2a is in line with the molecular orbitals (MOs) of 1c. Figure 2b shows the relevant MOs of 1c and their relation with those of BH and Mo(CO)5 fragments. Because the


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fragment MOs are mainly located on B and Mo atoms, whose shapes are similar to B and Mo atomic orbitals, they are simply labelled as the corresponding atomic orbitals. Figure 2b shows that the bonding MO 56A and anti-bonding MO 74A are originated from the B(sp)→M interaction. Likewise, the bonding MO 57A (58A) and antibonding MO 66A (67A) are from the Mo(5dxz)→B(2px) (Mo(5dyz)→B(2py)) interaction. The vacant B(2px) and B(2py) in the model of Figure 2a are consistent with that the LUMOs 66A and 67A are indeed mainly located on the B atom.

(−66.3) (a)

(−30.6)

(−30.6) (b)

(c)

Figure 2. Bonding model and electronic structure of 1c. a) The three-arrow model of 1c. b) Diagrams of frontier MOs of 1c and their relation with those of the HB and Mo(CO)5 fragments. (c) NOCV differential densities (isovalue = 0.003 a.u.) for the three strongest orbital interactions between the HB and Mo(CO)5 fragments in 1c. In each row of (c), the left diagram shows the interacting orbitals of the two fragments; the right one shows the NOCV differential densities of


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the orbital interaction, in which the blue and red colors mean negative and positive electron densities, respectively, and the value in parenthesis is orbital interaction energy in kcal/mol. The model also excellently agrees with the results from energy decomposition analysis using the extended transition state (ETS) method in conjunction with the natural orbitals for chemical valence (NOCV) theory. The orbital interactions between BH and Mo(CO)5 fragments of 1c can be decomposed into three main components. As shown in Figure 2c, the top, medium and bottom panels correspond to the B(sp)→Mo, Mo(5dxz)→B(2px), and Mo(5dyz)→B(2py) interactions, respectively, which contribute to orbital interaction energies by −66.3, −30.6, and −30.6 kcal/mol, respectively. The σ donation and two π back donations contribute nearly equally to the total bonding energy (−66.3 vs. −61.2 kcal/mol). Themodel of 1c is based on the interaction between the BH and Mo(CO)5 fragments at singlet states. Alternative bonding models consisting of one double bond and one arrow are given in Figure S1 of the ESI. Differently, these double-bond-plus-arrow models are based on the triplet BH and M(CO) fragments. The triplet BH and Mo(CO)5 are energetically higher than the corresponding singlets by 28.0 and 22.4 kcal/mol. Therefore, the model of Figure 2a better describes the ground-state electronic structure of 1c and thus will be used to predict the reactivity of 1c and the simialr compounds. Formation Mechanisms of 2a and 3 Experimentally, borylene compound 2a (3) can be synthesized by reacting a benzene solution of 1a (1b) in the presence of CO (CNDipp) molecules.53 To explore the mechanism by which 2a is formed, we firstly evaluated the reaction of 1 and CO on the basis of a three-arrow model. Figure 3a shows the prediction for 1a and CO: because CO is also a σ-donator despite its strong π


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accepting character,95,96 it will consecutively attack the vacant B(2px) and B(2py) to give int11a and int21a, and then the vacant Mo orbital to afford 2a and Mo(CO)6. To confirm this prediction, we investigated the reactions of CO and 1a with DFT calculations. The DFT results confirm that int11a and int21a are indeed involved. Int11a, int21a, and 2a can be formed via two different pathways: CO direct addition and CO migration followed by CO addition.

Figure 3. The formation mechanisms of 2a. (a) Consecutive CO addition mechanisms predicted using the three-arrow model of 1a. (b) Reaction energy profiles obtained by DFT calculations for the CO direct addition pathways, which confirm the prediction of (a). (c) Diagrams of the MOs


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for structures involved in (b). In (b), Gibbs free energies (in kcal/mol) calculated in benzene at 80˚C are marked; stationary point structures with some marked bond lengths are also given; in the structures of (b), some phenyl groups of the Tripp are not shown for clarity Figure 3b shows the CO direct addition mechanism: the additions of the first, second and third CO respectively give int11a, int21a, and the final products 2a and Mo(CO)6. The first two CO are directly added to the B atom, giving rise to two new B-CO bonds (see path 1 and 2 of Figure 3b). The third CO is added on Mo, forming the Mo-CO bond and simultaneously causing the separation of 2a from Mo(CO)6 (see path 3 of Figure 3b). Marked in Figure 3b are also the bond lengths key to the reactions. The Mo-B bond is 2.014 Å in 1a. It is elongated to 1.519 Å and 2.687 Å in int11a and int21a, respectively. It is clear that the consecutive addition of CO ligands gradually weakens 1a’s Mo-B bond and results in its cleavage, in agreement with the mechanism of Figure 3a. The feasibility of the mechanisms of Figure 3b is supported by the energetics. From 1a + 3CO to 2a + Mo(CO)6, the Gibbs free energy change of the reaction (Gr) is −33.1 kcal/mol, which is highly exothermic. Regarding the individual reaction steps, the Gr’s are 12.6, −17.7 and −28.0 kcal/mol, respectively. The Gibbs free energies of activation (G≠’s) of the first, second, and third CO additions are 20.5 and 14.0 and 16.4 kcal/mol, respectively (Figure 3b). The low G≠’s for the second and third CO additions as well as their largely negative Gr’s suggest that the latter two CO additions are facile. The first CO addition is thus the rate-limiting step; its relatively high G≠ (20.5 kcal/mol) and positive Gr (12.6 kcal/mol) are in line with the experiment that a pretty long time (i.e., 18 h) was taken for the synthesis of 2a.53 Figure 4 of shows the CO migration and addition mechanism, i.e., paths 1’, 2’, and 3’. To form int11a (int21a) via path 1’ (2’), firstly, a Mo-CO donor-acceptor bond of 1a breaks to yield


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int41a (int71a), in which the CO turns parallel to the Mo-B bond. Then, the CO in int41a (int71a) rearranges to form int51a (int81a). The subsequent addition of CO to Mo of int51a (int81a) gives rise to int11a (int21a), the same intermediate as that of path 1 (2). In int41a, int51a, int71a, and int81a, there is remarkable contribution of ŋ2 coordination between CO and Mo, in which the CO donate π-electrons to the vacant d orbitals of Mo. As shown in Figure 4a (b), the rate-limiting step of path 1’ (2’) is the breaking of the Mo-CO donor-acceptor bond through transition state ts41a (ts71a), which has a G≠ of 25.1 (24.0) kcal/mol. This energy barrier is higher than that of path 1 (2), which is 20.5 (14.0) kcal/mol. Therefore, CO direct addition mechanism is kinetically more favorable than the CO migration and addition mechanism for the formation of int11a and int21a. As for path 3’, int21a firstly transforms to int101a, in which the terminal O of CO makes a bond with Mo. Then, an additional CO attacks Mo, leading to of 2a and Mo(CO)6. Path 3’ has a G≠ of 15.0 (i.e., 17.1 − 2.1) kcal/mol, which is comparable to that of path 3 (i.e., 16.4 kcal/mol). Therefore, both path 3 and path 3’ are competitive to yield 2a from int21a.

51 2.1

14 2.1

96 1.9

8 98 1.

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00 2.0

0 2.

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Figure 4. CO migration and addition mechanism. Some CO ligands are denoted by the filled circles (●) and some donor-acceptor bonds are denoted by lines for simplicity, except for those


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key to the reactions. The optimized bond lengths (in Å) for selected bonds are labeled. Gibbs free energies (in kcal/mol) calculated in benzene at 80˚C are marked for all involved structures. Expectedly, the mechanisms of Figure 3b can be rationalized by the electronic structures of the corresponding CO addition substrates. The most probable sites for CO additions are where the LUMOs of substrates are located. Figure 3c shows the LUMOs of 1a, int11a, and int21a. For int21a, the vacant MO next to LUMO (i.e., LUMO+1) is also displayed. In 1a (int11a), the LUMO is mainly distributed on B atom. This is in excellent agreement with transition structure ts11a (ts21a), in which the CO indeed attacks the B atom. In terms of int21a, although the LUMO is mainly distributed on B, the addition of CO to B is unfavorable because the LUMO has positive and negative phases aligned closely together and will hardly have an effective orbital overlap with the HOMO (i.e., σ lone pair) of CO. Instead, the LUMO+1 of int21a is mainly distributed on Mo, in agreement with transition state ts31a where the CO attacks the metal. The results of Figure 3 suggest that the formation mechanisms of 2a can be predicted and rationalized by the three-arrow model of 1a, which in turn suggests that using donor-acceptor bonding model for 2a is natural and more convenient for understanding its formation mechanisms than using the cumulene-like bonding model. As for the formation of 3, mechanisms similar to those of Figure 3a can be predicted. However, because CNDipp is a bulky ligand, only one CNDipp is sterically allowed to add to B of 1b; CO migration from Cr to B followed by CNDipp addition to Cr must be involved. This is in agreement with the experimental structure of 3, which has both CO and CNDipp ligands.53 Evolution of Bonding during Formation of 2 Essentially, the formation of 2a from 1a and CO is the formation of two B-CO bonds (of 2a) and one Mo-CO bond (of Mo(CO)6), accompanied by the breaking of one Mo-B bond (of 1a). It is


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consensus that the newly formed Mo-CO bond is a donor-acceptor bond. However, it is an open question whether the newly formed B-CO bonds are necessary to be classified as donor-acceptor bonds.63−65 To explore the possible difference between the formation of the B-CO and Mo-CO bonds, we analyzed bond orders for structures in the reaction trajectories. Shown in Figure S2 of the ESI are the reaction energy profiles for the prototype reaction 1c + 3CO = 2c + Mo(CO)6 (2c = HB(CO)2), which are similar to those of Figure 3b.53 Figure 5 shows the variation of bond orders along the reaction trajectory for this prototype reaction, in which all bond orders were calculated based on structures taken from the intrinsic reaction coordinates.

Figure 5. Wiberg bond orders for structures in the reaction trajectories of the prototype reaction 1c + 3CO = 2c + Mo(CO)6. For each panel, the energies relative to those of the reactants (Erel) refer to the total energies


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As shown by the green, pink, and brown curves of Figures 5a, 5b, and 5c, respectively, the three CO additions form two B-CO bonds and one Mo-CO bond, whose bond orders are all about 1.2. In the meantime, the bond orders of the attacking CO are all reduced from about 3 in the pristine CO to about 2.5 in the adducts (see the blue, purple, and orange curves in Figures 5a, 5b, and 5c, respectively). Such similarity is indicative of the similar bonding situation of the BCO bond in 2c to the Mo-CO in Mo(CO)6. Figure 5 provides valuable insights on how the CO additions consecutively break the Mo-B bond and influence the bonds that are already formed. As shown by the red curves of Figures 5a, 5b, and 5c, the three CO additions respectively reduce the Mo-B bond order from about 1.8 (in 1c) to about 1.3 (in int11c), then to about 0.5 (in int21c), and finally to zero. The subsequent CO additions also influence the bond orders of the already formed B-CO bonds. For example, the green curve of Figure 5c suggests that the B-CO bond order is increased from about 1.2 (in int21c) to about 1.4 (in 2c) by the third CO addition. Such increase of B-CO bond orders is in agreement with shortening of the B-CO bond lengths, which were calculated to 1.511 Å, 1.484 Å, and 1.464 Å in int11c, int21c, and 2c, respectively. In contrast, the subsequent CO additions little influence the bond orders of CO that are already attached to B. This is reflected by the almost flat blue lines in Figures 5b and 5c, and is also in line with the almost constant CO bond lengths in int11c, int21c, and 2c (1.151 Å, 1.146 Å, and 1.150 Å, respectively). This probably means the subsequent CO additions enhance the σ donation from CO to B but minimal change in π-backdonation from B to CO.. To get a deeper insight into the electron density variation during the formation processes, we performed ETS-NOCV analyses for int11c, int21c, and 2c—the three structures involved in the prototype reaction 1c + 3CO = 2c + Mo(CO)6. Figure 6 shows the orbital interactions between


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the CO ligands and the corresponding residues. There are two orbital interactions contributing markedly to the binding, which are shown in the medium and bottom panels of Figure 6. For int11c (int21c) of Figure 6, the medium and bottom panels can be ascribed to electron density transfer from between Mo and B to CO (i.e., B→CO π back donation) and that from CO to B (i.e., OC→B σ donation), respectively. These interactions weaken the Mo-B bond and form the new B-CO bonds. As for 2c of Figure 6, the medium and bottom panels correspond to the B→CO π back donation and OC→B σ donation, respectively. Comparing int11c, int21c, and 2c, the OC→B σ donation increases from −44.1 over −48.1 to −93.8 kcal/mol; the remarkably enhanced σ donation in 2c is consistent with the increased B-CO bond order as suggested by Figure 5c.

(−153.1)

(−169.5)

(−52.6)

(−44.1)

(−48.5)

(−93.8)

int11c

int21c

2c

Figure 6. Orbital interactions contributing the most to the bonding between CO and the rest of the structures. For each structure, the top panel defines the two interacting fragments; the medium and bottom panels are the NOCV differential densities (isovalue = 0.005 a.u.) for the two strongest orbital interactions between the two fragments; the blue and red colors mean


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negative and positive electron densities, respectively; the value in parenthesis is orbital interaction energy in kcal/mol. According to the above results, the following scenario can be summarized for the formation of 2 by consecutive additions of CO to 1: as the first two CO ligands approach 1, they donate σ lone pairs to the vacant 2p orbitals of B in 1, forming the OC→B σ -bonds; simultaneously, the π* anti-bonding orbitals of CO accept electrons from between the M and B of 1, weakening the MB bond and contributing to the B→CO π backbonding; the addition of the third CO completely breaks M-B bond and gives rise to the final products. This formation scenario supports the necessity of using the donor-acceptor bonding models for 2.

CN Di pp

CN Di pp

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Figure 7. Reaction energy profiles for the liberation of CO from 3 to yield 4 at singlet (S0), excited singlet (S1), and triplet (T1) states, calculated in gas phase at room temperature. Gibbs free energies (in kcal/mol) with respect to the reactants and some key atomic distances are marked. The insets show the donor-acceptor bonding models for selected structures, in which the pink arrow shows the electron transition associated with the next reaction step. Photolysis Mechanisms of 2a and 3 Experimentally, the borylene compounds 2a and 3 are stable towards ambient air but liberate CO under light. As for 3, the photolysis product has been characterized to be 4 (Figure 1).53 The synthesis of analogue of 4 has been reported previously.97 To understand the underlying mechanisms and especially the essential role of photoexcitation in their decomposition reactions, we studied the CO liberation reactions for 2a and 3 at singlet, triplet, and excited singlet states, respectively. As shown in Figure 7, borylene compound 3 can be excited from the ground state (13) to the first excited singlet state 13*, which subsequently decay to the lowest-energy triplet state 33. The energy difference between 13’* and 33 is only 5.3 kcal/mol. Comparing the B-C-O bond angles in 13* and 13’* (180° vs. 136.6°), we deduce that the spin-surface crossing from excited singlet to the triplet is possible when the B-C-O bond angle properly vibrates. Then, 33 liberates a CO to give the triplet borylene radical 3int13. This CO liberation step has a G≠ of only 14.3 kcal/mol and is very facile. Such preference of heterolytic dissociation is exactly one of the traits of donor-acceptor bonds, in agreement with the ETS-NOCV result.98 In 3int13, the B can add to the phenyl to give int23, encountering a G≠ of 17.6 kcal/mol. Because the energies of int23 are close at singlet and triplet states, intersystem crossing from triplet 3int23 to singlet 1int23 will occur. The subsequent migration of isopropyl from phenyl to borylene in 1int23 gives rise to 4. This


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isopropyl migration step is fast because of its small G≠ of only 13.5 kcal/mol. The overall reaction is exothermic with a Gr of −11.2 kcal/mol; the rate-determining step is from 3int13 to 3

int23 with a moderate G≠ of 17.6 kcal/mol. This suggests the CO liberation from 3 to give 4 is

feasible in case of an initial excitation of 3 from ground state to the triplet state. For the above reactions to occur at ground state, the structure 1int23 is the unavoidable intermediate. Considering the high energy of 1int23 (37.2 kcal/mol), the ground state reaction will have a G≠ higher than 37.2 kcal/mol and thus is very slow at room temperatures. Therefore, the results of Figure 7 explain the high stability of 3 in ambient air and the liberation of CO to give 4 under light.53 It should be noted that the photolysis of 3 at singlet excited state, which is not explored in this work, may be also possible. In 4, the CNDipp makes a σ-donation bond with the boron. Similar complexes with CO ligands have been reported earlier.51 In contrast with the exothermic CO liberation of 3, our calculations suggested that the liberation of CNDipp from 3 is thermodynamically disfavored with a Gr of 2.4 kcal/mol. This explains why CO rather than CNDipp liberation product was detected for the photolysis of 3.53 As for 2a, the similar CO liberation reaction has a Gr of −8.0 kcal/mol and the ratedetermining G≠ of 18.9 kcal/mol at the triplet state. In contrast, the reaction at ground state has a G≠ > 39.6 kcal/mol (Figure S3 of the ESI). This suggests 2a is similar to 3, being stable in ambient air but subject to decomposition under light, in agreement with the experiment.53 The above photolysis mechanisms can be quantitatively rationalized by the donor-acceptor bonding models as shown in the insets of Figure 7. The bottom, left inset shows the B-CO bonding of 13, in which the lone pair of B represents the HOMO and the pink arrow represents the electron excitation from B to O, i.e., the most electronegative atom in the vicinity. Such


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electron excitation alters the C-O triple bond and causes the sp → sp2 rehybridization of the C. This rehybridization can be verified by the structural changes from 13 to 33. Comparing the DFT optimized structures of 13 and 33, the B-C-O angle bends from 173.3º to 136.6º; the B-CO bond elongates from 1.480 Å to 1.586 Å. The weakening of the B-CO bond makes the CO liberation possible. The up, left inset of Figure 7 shows the bonding pattern of 33, in which the pink arrow indicates the electron transition from O back to B accompanied with the CO liberation. This electron back transition rationalizes the formation of the triplet borylene radical 3int13. Reactions of 2a with O2 For borylene compound 2a to activate small molecules in the way that transition metal complexes normally do, at least one CO ligand should be detached from the central boron, leaving the boron with open coordination sites to bind small molecules.39,98 According to the experiment53 and Figure S3 of the SI, such way is not feasible for 2a, because the borylene radical resulted from CO liberation is subject to quick deactivation via structural rearrangements. Therefore, we studied whether 2a can behave as catalysts without CO detaching. Figure S4 shows the reaction energy profiles for the oxidation of CO with O2 catalyzed by 2a. The G≠’s of the rate-limiting steps are as high as 30.1 and 29.8 kcal/mol at the ground state and the lowestenergy triplet state, respectively, suggesting the poor catalytic performance of 2a. 2a is catalytically inactive is consistent with its large HOMO-LUMO energy gap (5.60 eV at the M062X/6-31G* level of theory), as small energy gaps are usually required for good catalytic performances.42 Noteworthily, the high G≠’s of Figure S4 means that 2a has high resistance against oxidation with O2, in agreement with its remarkable air-stability.53 CONCLUSION


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The bonding between boron and metal atoms in terminal borylene metal complexes 1 can be described using the three-arrow models, in which the bonding is depicted as one σ donation (B(sp)→Mo) and two π back donations (Mo(5dxz)→B(2px) and Mo(5dyz)→B(2py)). On the basis of this model, the synthesis mechanisms of borylene dicarbonyl 2a can be predicted as the consecutive additions of three CO to 1a. DFT calculations have confirmed this prediction and suggested that the CO additions can be accomplished via two different paths: CO direct addition and CO migration followed by addition. The addition of the first two CO forms two OC→B σ donating bonds and simultaneously two MB→CO back-donating bonds; this back-donating abstract electron density from between M and B and weaken the M-B bonding; the addition of the third CO eventually breaks the M-B bond and completes the reaction. The formation mechanisms of 3 are similar to those of 2a. According to these mechanisms, the use of donoracceptor bonding models for 2a and 3 is natural, which is more convenient for understanding the mechanisms than using the cumulene-like bonding models. As for the decomposition reactions, the liberations of CO from 2a and similar have energy barriers greater than 37 kcal/mol at the ground states; the barriers become smaller than 19 kcal/mol at the triplet states. These energy barriers explain the experimentally observed air-stability and photolysis of these compounds. Despite owning donor-acceptor bonds similar to those of transition metal complexes, 2a shows little to no catalytic activity toward oxidation of CO with O2 because of its large HOMO-LUMO energy gap. The results provide mechanistic insights for the unprecedented chemical processes regarding the novel bis(CO) adduct of boron and the similar compounds. They allow evaluating the applicability of donor-acceptor bonding in them from the new perspective of chemical reactions and will be also of fundamental interest concerning the chemical bonding of maingroup compounds.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acs.jpca.xxxxxxx. Figure S1-S4 and full citation for Ref. 71 (PDF) Optimized coordinates for related structures (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC) Project (No. 21373226) and the National Research Foundation of Korea (NRF) grant funded by the Korean government [Ministry of Science, ICT and Future Planning (MSIP)] (No. NRF2014K2A2A2000610). REFERENCES (1) Green, S. P.; Jones, C.; Stasch, A. Stable Magnesium(I) Compounds with Mg-Mg Bonds. Science 2007, 318, 1754-1757.


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time-dependent density functional theory polarizable continuum model. J. Chem. Phys. 2006, 124, 094107. (76) Wiberg, K. B. Application of the pople-santry-segal CNDO method to the cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane. Tetrahedron 1968, 24, 10831096. (77) Foster, J. P.; Weinhold, F. Natural hybrid orbitals. J. Am. Chem. Soc. 1980, 102, 72117218. (78) Reed, A. E.; Weinhold, F. Natural localized molecular orbitals. J. Chem. Phys. 1985, 83, 1736-1740. (79) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural population analysis. J. Chem. Phys. 1985, 83, 735-746. (80) Lu, T.; Chen, F. Multiwfn: A multifunctional wavefunction analyzer. J. Comput. Chem. 2012, 33, 580-592. (81) Mitoraj, M. P.; Michalak, A.; Ziegler, T. A Combined Charge and Energy Decomposition Scheme for Bond Analysis. J. Chem. Theory Comput. 2009, 5, 962-975. (82) Perdew, J. P. Erratum: Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B 1986, 34, 7406-7406. (83) Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B 1986, 33, 8822-8824. (84) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A 1988, 38, 3098-3100. (85) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. Chemistry with ADF. J. Comput. Chem. 2001, 22, 931-967.


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(86) Ehlers, A. W.; Baerends, E. J.; Bickelhaupt, F. M.; Radius, U. Alternatives to the CO Ligand: Coordination of the Isolobal Analogues BF, BNH2, BN(CH3)2, and BO− in Mono- and Binuclear First-Row Transition Metal Complexes. Chem. Eur. J. 1998, 4, 210-221. (87) Radius, U.; Bickelhaupt, F. M.; Ehlers, A. W.; Goldberg, N.; Hoffmann, R. Is CO a Special Ligand in Organometallic Chemistry? Theoretical Investigation of AB, Fe(CO)4AB, and Fe(AB)5 (AB = N2, CO, BF, SiO). Inorg. Chem. 1998, 37, 1080-1090. (88) Macdonald, C. L. B.; Cowley, A. H. A Theoretical Study of Free and Fe(CO)4Complexed Borylenes (Boranediyls) and Heavier Congeners: The Nature of the Iron−Group 13 Element Bonding. J. Am. Chem. Soc. 1999, 121, 12113-12126. (89) Uddin, J.; Boehme, C.; Frenking, G. Nature of the Chemical Bond between a Transition Metal and a Group-13 Element: Structure and Bonding of Transition Metal Complexes with Terminal Group-13 Diyl Ligands ER (E = B to Tl; R = Cp, N(SiH3)2, Ph, Me). Organometallics 2000, 19, 571-582. (90) Chen, Y.; Frenking, G. Theory predicts that the weaker π-accepting ligand diaminoborylene occupies the equatorial position in (OC)4Fe-B(NH2): theoretical study of (OC)4Fe-B(NH2) and (OC)4Fe-BH. J. Chem. Soc., Dalton Trans., 2001, 434-440. (91) Bollwein, T.; Brothers, P. J.; Hermann, H. L.; Schwerdtfeger, P. Theoretical Investigations into Transition Metal−Group 13 Element Bonding: Comparison between Ruthenium Porphyrin and Ruthenium Carbonyl Diyl Compounds. Organometallics 2002, 21, 5236-5242. (92) Pandey, K. K.; Lledós, A.; Maseras, F. The Nature of M−B Versus M═B Bonds in Cationic Terminal Borylene Complexes: Structure and Energy Analysis in the Borylene


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Complexes

[(η5-C5H5)(CO)2M{B(η5-C5Me5)}]+,

[(η5-C5H5)(CO)2M(BMes)]+,

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and

[(η5-

C5H5)(CO)2M(BNMe2)]+ (M = Fe, Ru, Os). Organometallics 2009, 28, 6442-6449. (93) Pandey, K. K.; Braunschweig, H.; Lledós, A. Nature of Bonding in Terminal Borylene, Alylene, and Gallylene Complexes of Vanadium and Niobium [(η5-C5H5)(CO)3M(ENR2)] (M = V, Nb; E = B, Al, Ga; R = CH3, SiH3, CMe3, SiMe3): A DFT Study. Inorg. Chem. 2011, 50, 1402-1410. (94) Braunschweig, H.; Dewhurst, R. D.; Gessner, V. H. Transition metal borylene complexes. Chem. Soc. Rev. 2013, 42, 3197-3208. (95) Doerr, M.; Frenking, G. Why are the Homoleptic Diyl Complexes M(InR)4 with M = Ni and Pt Stable Compounds while only Ni(CO)4 but not Pt(CO)4 can become Isolated? A Theoretical Study of M(EMe)4 and M(CO)4 (M = Ni, Pd, Pt; E = B, Al, Ga, In, Tl). Z. Anorg. Allg. Chem. 2002, 628, 843-850. (96) Frenking, G.; Wichmann, K.; Fröhlich, N.; Loschen, C.; Lein, M.; Frunzke, J.; Rayón, V. c. M. Towards a rigorously defined quantum chemical analysis of the chemical bond in donor– acceptor complexes. Coord. Chem. Rev. 2003, 238–239, 55-82. (97) Grigsby, W. J.; Power, P. P., Isolation and Reduction of Sterically Encumbered Arylboron Dihalides: Novel Boranediyl Insertion into C−C σ-Bonds. J. Am. Chem. Soc. 1996, 118, 7981-7988. (98) Haaland, A. Covalent versus Dative Bonds to Main Group Metals, a Useful Distinction. Angew. Chem. Int. Ed. 1989, 28, 992-1007.


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TOC Graphic


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Computational Study on the Mechanisms of Multiple Complexation of

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