Computation Revealed Mechanistic Complexity of Low-Valent Cobalt

Article Cite This: J. Org. Chem. 2018, 83, 14646−14657

pubs.acs.org/joc

Computation Revealed Mechanistic Complexity of Low-Valent Cobalt-Catalyzed Markovnikov Hydrosilylation Yumiao Ma* and Zongchang Han Department of Chemistry, Tsinghua University, Haidian, Beijing, 100084, People’s Republic of China

J. Org. Chem. 2018.83:14646-14657. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 12/18/18. For personal use only.

S Supporting Information *

ABSTRACT: We explored the mechanism of Markovnikov-selective hydrosilylation of phenylacetylene catalyzed by N−N−N Pincer−cobalt complex with density functional theory (DFT) calculations. In contrast to the previously proposed Co(I) mechanism, computational results suggest a Co(0) pathway, which is further supported by experimental studies. At the same time, our study reveals unexpected complexity in terms of the origin of regioselectivity. First, different orientations between the phenyl group in the substrate and the ligand plane lead to two possible transition states responsible for the branched product. However, the favored one varies according to ligand substitution pattern. Second, both entropy and solvation effects (rather than the conventional approach that considers electronic energies) have to be considered to explain regioselectivity, where the dominant factor also varies from case to case. Despite this complexity, computations predict a general overall ligand structure−regioselectivity relationship. In addition to increasing steric hindrance, introduction of an electron-withdrawing group to the ligands will also increase regioselectivity, which unveils a new dimension of ligand design.

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INTRODUCTION Vinylsilane is a versatile intermediate in organic synthesis. Due to electron-rich character, vinylsilanes are widely used in the cationic formation of a C−C bond.1,2 Additionally, organosilanes can also participate in Hiyama coupling.3 Hydrosilylation of alkynes provides an important access to vinylsilanes.4 Precious metal complexes have been known to catalyze such a process. In 1991, Kartstedt’s catalyst, a platinum-based catalyst, was reported to catalyze the reaction of triphenylsilane and a series of 1-alkynes to give linear antiMarkovnikov products.5 Since then, several ruthenium- and rhodium-based catalysts have been used for anti-Markovnikov hydrosilylation of terminal alkynes.6−8 On the other hand, branch-selective Markovnikov hydrosilylation has also been achieved with some ruthenium complexes, as reported by Trost,9 Yamamoto,10 and others. Despite the fruitful efforts to develop precious metal-based catalysts, first-row transition metal Pincer complexes stand out as an economical catalyst. With the pioneering work of Chirik and others, low-valent diiminopyridine (1 in Figure 1) cobalt complexes have been known to catalyze several transformations including hydrogenation,11−13 anti-Markovnikov hydroboration,14,15 and hydrosilylation16 of alkenes and alkynes. Most of them made use of well-defined formal Co(I) complexes. In 2015, Chirik reported Markovnikov hydroboration of alkynes enabled by an “asymmetric” diiminopyridine−cobalt(I) complex and proposed a mechanism involving alkynylboration followed by formal hydrogenation.17 On the other hand, hydrosilylation of alkynes enabled by Pincer−cobalt complexes has also been developed in recent years. In 2016, Lu utilized a cobalt complex with a © 2018 American Chemical Society

Figure 1. (a) Several important NNN Pincer ligands and (b) previously established well-defined Co(I)-catalyzed alkene and alkyne transformation versus mechanistically unclear in situ-reduced catalystpromoted reactions.

“combinational ligand” (2 in Figure 1) to achieve Markovnikov-selective hydrosilylation of phenylacetylene by an “in situ reduction procedure”. In the presence of less than 2 mol % of “LCoBr2” type complex and 3 equiv (compared to cobalt complex) of NaHBEt3 as reductant, the reaction showed rather high efficiency (complete transformation within 5 min at room temperature) and a moderate branched/linear ratio of 9:1.18 Received: October 1, 2018 Published: November 12, 2018 14646

DOI: 10.1021/acs.joc.8b02455 J. Org. Chem. 2018, 83, 14646−14657

Article

The Journal of Organic Chemistry

Figure 2. (a) Model reaction for computations. (b) Reaction for experiment validating a Co(0) pathway.

2018, Zhu and Peng conducted computational research on iron-catalyzed Markovnikov hydrosilylation of alkenes and also considered Fe(I) hydride to be responsible for the catalyst.34 However, to the best of our knowledge, there is no detailed mechanism research on cobalt-catalyzed Markovnikov hydrosilylation of alkynes. In this work, we studied low-valent cobalt-catalyzed hydrosilylation of phenylacetylene with DFT computation and found that Co(I) species actually show opposite selectivity toward experimental results and revealed a more energetically accessible Co(0) process. Based on the result, more detailed understanding of the mechanism was obtained.

Following this research, Lu developed a sequential hydrosilylation−hydrogenation reaction between silane and phenylacetylene, providing chiral alkylsilane.19 Ge also reported antiMarkovnikov hydrosilylation of terminal alkynes with a Pincer−Co(II) precatalyst. Diiminopyridine, combinational ligands, and Pybox (3 in Figure 1) were tried, and all of them gave high yield, but the branched/linear ratio varied.20 In 2018, Huang’s group also achieved Markovnikov hydrosilylation of terminal alkynes catalyzed by in situ-reduced Pybox−Co complex.21 Also a similar “in situ reduction” method has been used to achieve ene-yne coupling toward cyclobutanes,22 as well as hydrosilylation of alkenes,23−27 in which formal Co(I) are proposed as active species. Most mechanism investigations about transition metalcatalyzed hydrosilylation focused on precious-metal-catalyzed reaction. A Chalk−Harrod type mechanism involving hydrogen migration into the substrate after oxidative addition of metal to silane followed by reductive elimination has been proposed for olefin hydrosilylation catalyzed by group(VIII) metals.28,29 The group of Wu and Trost studied hydrosilylation of alkynes catalyzed by cationic ruthenium complexes by DFT calculations and proposed a Chalk−Harrod type mechanism for the Markovnikov-selective reaction.30 In contrast, despite alot of experimental observations, mechanism research on low-valent first-row metal-catalyzed hydrosilylation is scarce. Experimental and computational research on cobalt-catalyzed anti-Markovnikov hydrosilylation with a monodentate phosphine ligand has been conducted,31,32 but these ligands differ significantly from tridendate redoxactive Pincer ligands. Based on the work by Chirik using welldefined Co(I) compounds as a catalyst, as well as the difficulties in the characterization of active species in many cases, all the work involving in situ-reduced cobalt catalysts mentioned above proposed Co(I) compounds as active species. Deng synthesized a well-defined Co(I)−IAd complex and achieved linear-selective hydrosilylation of terminal alkynes with diphenylsilane33 in 2014, in which isolation of a silylcobalt(I) intermediate supported a Co(I) catalytic cycle. In

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COMPUTATIONAL METHODS Gaussian 0935 was used for calculations. For each of the following calculations, stability of wave function was checked so that stable wave function could be used all the time. All the geometries were optimized with PBE0 functional36 combined with D3BJ dispersion correction37 and basis set BS1 (BS1 = 631G*38,39 for main group elements and LanL2DZ40,41 for cobalt) in the gas phase. Frequency calculations were performed at the same level, and thermodynamic corrections were obtained at 298.15 K. For each transition state, intrinsic reaction coordination (IRC) was examined. Single point energies were calculated under PBE0-D3BJ/BS2 (BS2 = 6311+G**42,43 for main group elements and def2-TZVP44 for cobalt). SMD solvation free energies45 with diethyl ether as solvent were obtained with M05-2X46/BS1. Gibbs free energy in this article is defined as the sum of single point energy at PBE0-D3BJ/BS2, thermodynamic correction, and solvation free energy. Minimum energy crossing points (MECP) were located by Harvey’s MECP program.47 Wave function analysis was completed with Multiwfn program.48 Molecular 3D structures were generated by CYLView49 and VMD50 program. Electrostatic potential (ESP) mapped molecular surfaces were generated by GaussView51 from fchk files produced by Gaussian calculations, with an isovalue of total density at 0.0004. 14647


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Figure 3. continued

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Figure 3. Potential energy surface involved: Co(I) (a) and Co(0) (b) species. Energies are in kcal/mol. MECP(CoSi) and MECP(Int1l) means minimum energy crossing point (MECP) for interconversion between different spin states.

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RESULTS AND DISCUSSION

kcal/mol. Exothermic complexation with phenylacetylene gives a closed-shell singlet Int1, followed by a regioselectivitydetermining silyl migration step. Surprisingly, the transition state toward linear product is favored by as large as 5.7 kcal/ mol. Efforts to find another conformation was in vain, and such a large energy difference cannot be rationalized by computational error. This step is irreversible and will be followed by rate-determining σ-metathesis type release of the linear product 6. The transition state TS4 of hydrogen migration is high in free energy. Replacing silane with diphenylsilane did not change the preference for linear product (5.3 kcal/mol for

Although most reported experimental works used substituted silanes, our computations are carried out using SiH4 as model reagent for computational simplicity. In the reaction model reaction shown in Figure 2, we initially considered CoH and CoSi as possible intermediates (Figure 3a). Both compounds are at triplet ground state with an open-shell singlet state close in energy, which is consistent with the report from Budzelaar’s group about the electronic structure of a series of (diiminopyridine)cobalt(I) complexes.52 In the presence of silane, CoH can readily convert to CoSi with a barrier of 4.1 14649


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The Journal of Organic Chemistry Table 1. Free Energy of 4LCo, Apparent Barrier for the First Irreversible Step, and Difference in Free Energies of Regioselectivity-Determining Transition States in Co(0) and CoSi Pathwaysa ligand 2a 2d 2e 2b 2c 2f 1a 2d 1e 1b 1c 1f 3a 3b 3c

LCo

ΔG⧧(Co(0))

ΔG⧧(Co(0))b

ΔG⧧(CoSi)

ΔΔG⧧(Co(0))

ΔΔG⧧(CoSi)

7.91 8.84 7.75 5.01 9.21 8.73 10.04 1.88 11.70 6.34 3.47 1.40 6.81 6.30 8.79

2.32 2.20 4.82 4.15 1.98 2.61 0.43 2.11 0.38 1.09 0.65 3.38 0.99 6.97 −4.59

3.29 4.77 5.62 3.38 3.64 2.89 0.40 3.44 −0.56 4.60 4.12 4.78 2.51 4.19 −3.59

7.81 8.46 6.96 6.45 4.59 5.92 10.49 8.69 8.59 7.06 8.60 9.10 6.92 5.45 8.25

1.25 1.20 4.72 3.01 3.80 0.57 1.72 3.04 1.23 2.36 2.51 3.03 2.31 3.33 1.20

6.64 6.76 6.89 7.24 6.77 9.31 5.98 6.99 7.17 7.78 6.01 7.39 6.40 6.98 6.11

4

Free energies are in kcal/mol, and 3CoH is considered as 0.0. ΔG⧧(Co0) is defined as the Gibbs free energy difference between lowest-energy conformations of TS5 and Int4. ΔG⧧(CoSi) is defined as the Gibbs free energy difference between 1TS2l and 1Int1l. ΔΔG⧧(Co0) is defined as G(2TS5l) − G(lowest-energy conformation of TS5b). ΔΔG⧧(CoSi) is defined as G(1TS2l) − G(1TS2b). See Table S2 for detailed relative free energies of each structure involved. bDefined as the sum of ΔG⧧(Co0) and relative free energy of lowest spin state of CoHSi. a

Scheme 1. Catalytic Cycle

TS2b versus −4.7 kcal/mol for 1TS2l with 3CoH as reference), which was then examined in an experiment. We also examined several “unclassical” reaction pathways (Figure S1). The possibility that N−N−N type Pincer ligands act as a “group acceptor” is previously unknown; with Int3 as a model, several thermodynamically accessible isomers with hydrogen attached to the ligand have been located (Figure S1d), but we failed to find an energy-accessible transition state responsible for their formation. Possible carbene and silene 1

intermediates are also high in energy (Figure S1b). These results excluded the possibility that potential ligand participation is responsible for the selectivity. In addition, σmetathesis of CoSi with alkyne to afford alkynylcobalt(I) compound Int6 is kinetically unfavorable compared to silyl migration discussed above. Upon failure of Co(I) pathways to explain reported experimental results, we turned to other possible pathways. In 2010, well-defined Co(0) and Fe(0) compounds with 14650


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The relative energy of “naked” LCo from CoH is lowered by more electron-withdrawing groups, and surprisingly the diiminopyridine ligand provides less stabilization toward LCo, despite seeming to be more electron-deficient. To evaluate the donor ability of each ligand, carbonyl vibration frequency in compounds LCoCO were calculated (Table S1). Again, vibration wavenumber decreased in the order of Pybox > combinational > diiminopyridine, implying that diiminopyridine is a weaker acceptor. Cyano-substituted 1d and 1f exhibit extremely large stabilization toward LCo, which is caused by the solvation free energy (see Supporting Information for the solvation energy of each structure). In attempt to explain the trend, geometries of relevant compounds were examined. Interestingly, the Co−N bond length is significantly longer for the oxazoline nitrogen compared to the imine nitrogen in almost every compound (Figure 4a with 4LCo as an example). In the 4LCo case, two typical bond lengths are 1.98 and 2.04 Å, respectively. In the case of TS5, the Co−N distance is even longer, and reduced density gradient (RDG) analysis54 shows that, while there is still strong interaction between cobalt and imine nitrogen, interaction between cobalt and the oxazoline nitrogen has shifted into electrostatic attraction (Figure 6b−d). The shape of the frontier orbital in the ligand can account for this phenomenon: both HOMO and LUMO of the combinational ligand are mostly involved with the pyridine and imine N but not the oxazoline N (Figure 4b). A longer Co−N distance implies both less electron donation from ligand to metal and less back-donation. Further insight into the electronic structural difference between Co(I) and Co(0) was obtained by other wave function analysis methods. Co(I) compounds are known to be best considered as Co(II) coordinated by a ligand radical anion,52 and localized orbital bonding analysis (LOBA),55 a method for evaluating oxidation state, indeed gives a +2 oxidation state for cobalt in all intermediates involved in the formal Co(I) pathway. However, 0, +1, and +2 oxidation states can all be obtained from LOBA analysis of formal Co(0) species according to different parameter settings in the algorithm. The sums of CM5 atomic charges56 over ligand atoms of CoH and LCo are −0.18 and −0.33, showing more reduction of ligand in the formal Co(0) species. Charge decomposition analysis (CDA)57,58 can give donor (d) and back-donation (b) values as indications of electron transfer from ligand to metal and metal to ligand, respectively. For 3 CoH, d is 0.2448 and b is 0.0145 but 0.3502 and 0.0522 for 4 LCo, which indicated both more σ-donation and more backdonation in the Co(0) case, which is consistent with the more stabilized LCo having an EWG-substituted ligand, as well as a stabilization order of Pybox > combinational > diiminopyridine. We next examined regioselectivity. The orientation of the phenyl group in phenylacetylene perpendicular or parallel to the ligand plane gives two possible conformations of TS5b (Figure 5). We failed to find a general trend for which conformation is favored, but if the lowest energy TS is used to calculate ΔΔG⧧ for branched/linear selectivity, it can be found that aromatic R1 increases regioselectivity. Interestingly, substitution of polar groups on the pyridine ring can also significantly influence ΔΔG⧧: a cyano group increased ΔΔG⧧ to 4.72 kcal/mol in the case of a combination ligand, whereas electron-donating methyl slightly decreased regioselectivity. The same trend can be seen in ligands 1a, 1d, 1e, and 3a−c.

dinitrogen were isolated by Chirik et al. by reaction of (diiminopyridine)cobalt dichloride and NaBEt3H.53 Thus, we examined the potential surface involving a Co(0) compound (Figure 3b). LCo without further complexation is 7.9 kcal/mol higher in energy than that of CoH and is stabilized after complexation with silane by 6.9 kcal/mol, giving Int3. Int3 reacts with phenylacetylene to form preactivation complex Int4 and then undergoes rapid and irreversible hydrogen insertion with a barrier less than 3 kcal/mol. In this step, the transition state toward branched product TS5b is favored by 1.2 kcal/mol, leading to Int5b, which is prior to reductive elimination, and finally gave branched product. Despite the high energy of LCo, because it is unnecessary to be in equilibrium with Co(I) species, as well as energetic accessibility of CoHSi, we consider the Co(0) pathway to be feasible. Although the calculations above were accomplished with combinational ligands, the potential energy surface with diiminopyridine 1a shown in Figure 1 is similar (as shown in Table 1 and discussed below). With this insight, an experimental validation was carried out (Figure 2b). Extremely oxygen-sensitive, dark teal dinitrogen complexes of Co(0) with diiminopyridine ligands 1c and 1g were prepared as reported by Chirik and used as a catalyst in parallel with the dark brown LCoCl2/NaBEt3H system. While reaction with both 7g and 8g did not occur in 2 days, the reaction using 7c and 8c results in full conversion in 15 min. Pregenerated Co(0) catalyst 8c and standard in situ reducing conditions gave branched/linear ratios of 19:1 and 17:1, respectively, providing support to the Co(0) pathway. Furthermore, on reducing the amount of reductant to 1 equiv, the branched/linear ratio under in situ reduction conditions using 7c shifted to 0.7:1. In view of the calculations and experimental observations, we suggest the catalytic cycle as shown in Scheme 1. The relationship between ligand structure and reactivity was studied. Ligands in Scheme 2 were examined, and the free Scheme 2. Ligands Considered in This Research

energy of several key species are listed in Table 1. There are some general trends suggested by the data. In each case, zerovalent branched TS5b is favored over TS5l, whereas linear TS2l is favored in the Co(I) pathway. Because one cannot assume that LCo and CoH are in equilibrium in the reaction system, we cannot get the exact ratio of Co(0) and Co(I) species in the reaction mixture, but in all cases the barrier of the first irreversible step in the Co(0) pathway is much lower. If the energy of Int4 is considered as a probe of the amount of Co(0) species in the reaction mixture, it can be added to the Co(0) barrier and would not undermine the conclusion that the Co(0) pathway is dominant. 14651


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Figure 4. (a) Geometries of 4LCo with 1a, 2a, and 3a as ligand. (b) Frontier orbitals of 2a with geometry extracted from optimized structure of 4 LCo.

Figure 5. Geometries of three TS5 isomers with 1a as ligand. (a) TS5b_paralleled, (b) TS5b_perpendicular, and (c) TS5l. Bond lengths are in angstroms.

ΔTC mainly involves the entropy term ΔTS produced by the difference in vibrational entropy between TS5b and TS5l. Vibrational modes of the greatest contribution to entropy are those involving skeleton vibration with low frequencies and mostly involve the movement of the phenyl ring in substrate. We postulate that it is an enthalpy−entropy compensation effect in which stronger dispersion attraction will make relevant vibrations restrained, and the C32−N4 bond length may be a good probe. Indeed a linear relationship is obtained within two substitution modes (substitution group with and without significant ability for dispersion interaction) (Figure 6e). With electron-withdrawing R3 or large aromatic R1, the distance between the phenyl group of the substrate and the ligand plane increases in TS5b, accompanied by more favorable entropy. However, this does not necessarily suggest a causal relationship. Furthermore, the reason for changes in geometry is still unclear. With Pybox and the diiminopyridine ligand, a linear relationship between ΔG and ΔTS no longer holds and the three terms show a similar standard deviation, indicating that all of them must be considered to rationalize ΔG. TS5b_perpendicular is lower in energy and favored in most cases, but its complexity provides great challenge for rationalization. ΔEelec tends to be less positive, as the ligand tends to be a stronger electron donor. This observation can be rationalized by CM5 charge. With TS5b_perpendicular and TS5l with 2a as an example, the sum of CM5 charge over ligand is 0.3547 and 0.3757, respectively. The more positive total charge on the ligand will make TS5l more destabilized in

Although the general trend can be obtained based on the observation of ΔΔG⧧, the origin is rather complex. Relative energy is composed of contributions from the electronic energy, solvation free energy, and thermodynamic terms. The most important term varies from case to case (Table 2). At the very beginning, TS5b_perpendicular and TS5b_paralleled show different origins of relative free energy compared to TS5l. In the case of TS5b_paralleled with a combinational ligand, polar R3 has a minor influence on ΔEelec but changes the thermodynamic correction greatly. Interestingly, the phenyl and mesityl groups significantly reduced ΔEelec; thus, the increase in ΔG comes from solvation and ΔTC. The standard deviation of ΔGsolv is much smaller, and one would expect that ΔTC contributes the most to various regioselectivities. Indeed, the plot of ΔG against ΔTC gives a straight line of slope 1.23 (Figure 6a). The mesityl-substituted ligand 2c is an exception and may be due to increased steric repulsion in TS5b. Furthermore, Table 2 shows the importance of dispersion interaction; qualitatively wrong relative electronic energy will be obtained without D3 correction. Dispersion interaction can be visualized in the RDG isosurface (Figure 6b,c). In TS5b_paralleled there is significant dispersion attraction between the alkyne phenyl group and ligand, whereas such interaction is absent in its linear isomer. The less positive dispersion term with aromatic R1 can be rationalized by dispersion interaction introduced in TS5l, as shown in Figure 6d. 14652


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Table 2. Contributions to the Free Energy Difference between Two Transition States Leading to Branched and Linear Productsa Parallel 2a* 2b* 2c 2d 2e 2f* standard deviation 3a* 3c* 3b standard deviation 1a* 1d* 1e 1b 1c 1f standard deviation Perpendicular 2a 2b 2c* 2d* 2e* 2f standard deviation 3a 3b* 3c standard deviation 1a 1d* 1e* 1b* 1c* 1f* standard deviation

nondispersion

dispersion

ΔEelec

ΔEsolv

ΔTC

ΔTS

ΔG

−1.83 −1.08 0.42 −1.98 −2.05 −1.95

3.57 1.90 −0.33 3.83 3.99 3.55 3.54 3.80 3.70

−2.12 −2.10 −1.48 −1.40 −2.60

3.93 3.89 2.13 2.02 4.66

0.53 1.67 0.97 0.22 0.03 0.49 0.54 0.89 0.15 0.12 0.36 0.30 0.32 −0.03 1.38 0.49 −0.73 0.63

−1.02 0.52 1.44 −0.24 −1.63 −1.52 1.11 −0.55 −1.22 −0.71 0.29 0.10 0.53 −0.89 −0.16 0.77 −0.50 0.57

−0.92 0.63 1.51 −0.48 −1.58 −1.34 1.10 −0.52 −1.18 −0.82 0.29 0.18 0.57 −0.81 −0.27 0.74 −0.58 0.57

1.25 3.01 2.50 1.82 0.34 0.57

−1.56 −1.53 −1.78

1.74 0.82 0.09 1.84 1.94 1.60 0.67 1.97 2.27 1.92 0.15 1.45 1.81 1.79 0.65 0.62 2.05 0.57

1.06 1.75 3.91 2.03 0.81 1.03

0.87 0.41 −2.63 0.64 0.94 1.01 0.92 0.83 0.95

0.90 1.75 0.75 1.40 2.04 1.54

0.83 0.59 0.90 0.67 0.47 0.63

−1.03 −0.20 0.35 0.21 −0.72 −1.20 0.59 −1.29 0.02 −0.77 0.54 −0.90 0.17 −0.68 −0.56 −0.75 0.18 0.44

−0.09 −0.11 2.18 0.95 0.17 −1.01 1.00 0.22 0.73 −0.88 0.67 0.89 0.53 0.25 0.85 0.74 0.68 0.22

0.06 0.04 2.23 0.88 0.28 −0.83 0.94 −22.53 0.81 −0.74 10.66 0.89 0.22 0.27 0.97 0.80 0.72 −0.92

0.81 1.86 3.81 3.83 1.20 −0.17

1.16 1.75 0.92

1.93 2.17 1.28 2.67 1.76 2.04 0.42 2.08 2.58 1.87 0.29 1.73 2.34 1.65 2.07 2.52 2.17 0.31

2.31 1.20 1.32 1.85 2.65 0.87 1.87 1.89 0.82

1.02 3.33 0.23 1.72 3.04 1.22 2.37 2.50 3.03

ΔG is defined as G(TS5l) − G(TS5b), with TS5b adopting a conformation either perpendicular or parallel as noted in the table, and there is only one conformation of TS5l. ΔEelec, ΔEsolv, and ΔTC are differences in electronic energy, solvation free energy, and thermodynamic correction, respectively, and the sum of the three terms equals ΔG. The nondispersion part of ΔEelec is defined as ΔEelec without D3BJ correction, and the dispersion part is defined as ΔEelec minus the nondispersion part. ΔTS is the contribution from entropy and is a part of ΔTC. An asterisk means this orientation (parallel or perpendicular) is lower in energy. Energies are in kcal/mol. a

positively charged electron-deficient ligand; the presence of a cyano group introduces a large amount of negative charge and reverses the dipole. As for the ΔTC term, again the main contributors are the vibration modes involving movement of the phenyl ring in the substrate, but we failed to find a proper geometry probe which can be connected to ΔTC. With a large R1, the phenyl group of phenylacetylene no longer lies above the “center line” of the ligand and may make ΔTC more complicated. Because of the importance of the solvation and entropy terms, the origin of preference for the branched TS is much more complicated than conventional consideration about steric and electronic interaction that only accounts for electronic energy directly. Although the Co(0) pathway is supported by both calculation and experiment, and because the Co(I) pathway

the presence of electron-withdrawing groups. Because Co− oxazoline interaction is weaker than Co−imine bonding, it is expected that there should be stronger interaction between cobalt and the pyridine site in combinational ligands as a compensation, resulting in greater sensitivity toward polar substitution. ΔEelec data are indeed consistent, though electronic energy only contributes a minor part to ΔG herein. In most cases, a branched TS is unfavored in terms of solvation free energy, but all substitution groups decrease this unfavorability. Consideration of dipole fails to give a rationale, and there must be other factors such as polarizability or molecular surface-dominating solvation. Interestingly, the TS of cyano-substituted species shows an inversed dipole compared to others (Figure S2). The major contributor to molecular dipole is the negative hydrogens in SiH3 and 14653


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Figure 6. (a) Plot of ΔG against ΔTS involving TS5b_paralleled with combinational ligands. (b−d) RDG isosurface of TS5b_paralleled with 2a (b), TS5l with 2a (c), and TS5b_paralleled with 2b (d). The green area corresponds to dispersion attraction. The blue solid surface indicates strong electrostatic attraction, whereas the blank area inside a surface corresponds to chemical bonding. Part of the phenyl ring is omitted for clarity in c. (e) Plot of ΔTSagainst C−N bond length.

Figure 7. Geometry of the closed-shell singlet state of TS4b and TS2l, the two critical transistion states in the formal Co(I) pathway.

point between Si and C1, and RDG analysis indicates strong repulsion, suggesting that the short C1−Si distance does not provide any stabilization. The sum of CM5 charge over alkyne atoms of Int1l and Int4b is −0.2987 and −0.3375, implying less back-donation in Int1l, which is consistent with the Lewis acidity of the SiH3 group. The CM5 charge on the cobalt atom in the two species is 0.4998 and 0.4043, respectively, which again shows a more positively charged cobalt center with a Lewis acidic SiH3 substitution. Again, the CM5 charge on the cobalt atom in CoSi is 0.4519, more positive than that in CoH, which is 0.3677. We suggest that a greater affinity of CoSi toward alkyne can be explained by the more electrophilic metal center compared to CoH. In sharp contrast to the Co(0) case, the energy difference between branched and linear transition states is largely contributed by electronic energy. Due to the Lewis acidicity

is also energetically accessible, it is possible that the Co(I) pathway will take a role in other cases. Hence, we also made an effort to understand the Co(I) pathway. Regarding the two pathways involving CoH and CoSi, the CoSi pathway is strongly preferred (Figure 3), but the elementary steps of hydrogen migration have a slightly lower barrier (5.1 kcal/mol for hydrogen migration versus 8.0 kcal/mol for SiH 3 migration). The preference for SiH3 migration comes from the large energetic gain in the formation of Int1l. To investigate the cause, geometries of the closed-shell singlet ground state of Int1l and Int4b were examined (Figure 7). Int1l adopts a slightly asymmetric bonding mode for cobalt and alkyne; the Co−C1 bond is longer than the Co−C2 bond by 0.01 Å. The C1−Si distance in Int1l is 2.77 Å, much smaller than the sum of van der Waals radii. However, electron localization function (ELF)59 analysis does not give any critical 14654


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sensitivity, the solid was used as precatalyst without further characterization. Hydrosilylation of Phenylacetylene with 7c as Precatalyst. To 2 mL of dry THF was added 8.3 mg (0.02 mmol) of 7c in a glovebox. Then 184 mg (1 mmol) of diphenylsilane and 60 μL of 1.0 M NaBEt3H solution were added in sequence, and the solution turned to dark brown. Then 110 mg (1 mmol) of phenylacetylene was added. The mixture was stirred for 15 min and quenched with saturated aqueous EDTA solution. After extraction with DCM, the product ratio was determined by NMR. Hydrosilylation of Phenylacetylene with 7c as Precatalyst: Modified Version with 1 Equiv of Reductant. To 2 mL of dry THF was added 8.3 mg (0.02 mmol) of 7c in a glovebox. Then 184 mg (1 mmol) of diphenylsilane and 20 μL of 1.0 M NaBEt3H solution were added in sequence, and the solution turned to dark brown. Then 110 mg (1 mmol) of phenylacetylene was added. The mixture was stirred for 15 min and quenched by saturated aqueous EDTA solution. After extraction with DCM, the product ratio was determined by NMR. Hydrosilylation of Phenylacetylene with 8c as Precatalyst. To 2 mL of dry THF was added 7.4 mg (0.02 mmol) of 8c in a glovebox. Then 184 mg (1 mmol) of diphenylsilane and 110 mg (1 mmol) of phenylacetylene were added in sequence. The dark teal mixture was stirred for 15 min and quenched by saturated EDTA solution in water. After extraction with DCM, the product ratio was determined by NMR.

of cobalt and the negatively charged silicon atom, as well as the minimal open-shell character (S2 = 0.3194 for 1TS2l and zero for 1TS4) and near-zero spin population on the alkyne and silicon atom in TS2, the hydrogen or SiH3 migration step in the formal Co(I) pathway can be considered as an intramolecular Lewis acid-catalyzed nucleophilic attack of silyl (for TS2) and hydride (for TS4) onto the alkyne. In this way the preference for bond formation with C-1 is understandable.

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CONCLUSION

In summary, we explored the detailed mechanism of low-valent cobalt-catalyzed hydrosilylation of phenylacetylene and found that pathways involving Co(I) and Co(0) show opposite regioselectivity. By combining experimental observation and computational result, we consider Co(0) as the actual catalyzing species. The Co(0) pathway is favored among cases where a series of Pybox, combinational, and diiminopyridine ligands are involved. Both more electronic deficiency and aromatic substitution increase regioselectivity in the Co(0) pathway, though the origin varies from case to case. Contrary to conventional ideas about electronic energy, both solvation and entropy must be considered to account for the preference in free energy. Besides providing insight into this reaction and its regioselectivity, this research also implies that polar substitution can be used to regulate regioselectivity without introduction of bulky, rate-decreasing aromatic groups, which provides a new dimension for ligand design. On the other hand, although the Co(0) pathway is preferred if Co(0) species are present in reaction system, it is still possible to reverse the regioselectivity by developing a well-defined Co(I) catalyst. Despite these research studies on cobalt catalysts, other metals also exhibit a similar variation in reactivity according to oxidation state, and further study is in process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b02455. Geometries and energies of all structures, and copies of NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

EXPERIMENTAL SECTION

ORCID

General Information. All commercial reagents were purchased from Sigma-Aldrich, Alfa-Aesar, and Acros. Solvents were dried before use. Product ratios were determined by 400 MHz NMR. Preperation of Precatalysts. Precatalyst 7c and 7g were prepared according to a literature procedure.60

Yumiao Ma: 0000-0002-0628-8864 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Xuetang Program (Tsinghua University) and Tsinghua University Initiative Scientific Research Program for financial support, as well as Shanghai Supercomputer Center for computational resources. Also we thank all students in Department of Chemistry, Tsinghua University, for their encouragement and great love toward the authors.

Reduction of 7 to 8 was performed as in Chirik’s report:53

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REFERENCES

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Under a nitrogen atmosphere, a Schlenk tube was charged with 0.02 mmol of 7c or 7g, and 2 mL of toluene was added and cooled with liquid nitrogen. Twenty microliters of 1.0 M NaBEt3H solution in THF was added. After being warmed to room temperature, the reaction mixture turned purple. The solution was again cooled by liquid nitrogen and another 20 μL of 1.0 M NaBEt3H solution was added. The mixture was warmed to room temperature, and toluene was removed by nitrogen purging. A dark teal solid was obtained and transferred into a glovebox immediately. Due to extreme air14655


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Computation Revealed Mechanistic Complexity of Low-Valent Cobalt

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