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Dramatic Increase in the Rate of Olefin Insertion by Coordination of Lewis Acids to the Oxo Ligand in Oxorhenium(V) Hydrides Nikola S. Lambic, Caleb A. Brown, Roger D. Sommer, and Elon A. Ison* Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: In this work we show that classic coordination of the oxo group in an oxorhenium hydride complex to M(C6F5)3 (M = Al, B) leads to dramatic increases in the rate of migratory olefin insertion. Combined experimental and computational studies have been utilized to understand the reasons for the rate enhancement upon coordination of the oxo group to the Lewis acid. The mechanism for migratory insertion involves coordination of the olefin to rhenium in the equatorial plane. This induces mixing of the rhenium−hydride σ bond with a rhenium−oxygen π* orbital. This results in an accumulation of electron density on the oxo ligand. The Lewis acid lowers the barrier for migratory insertion by diminishing the electron density on the oxo ligand in the transition state.

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INTRODUCTION Lewis acid−base interactions in the secondary coordination sphere of a metal complex have been shown to modulate the electronic structure of the metal center.1 This has been especially true in biomimetic systems, where a common approach has been to take advantage of noncovalent interactions.1c However, in synthetic systems there have only been a few examples where Lewis acid−Lewis base interactions have been utilized to systematically tune the reactivity of a complex by modulating the electronic properties of a supporting ligand.2 For recent examples, Bergman and Tilley recently showed that interaction of B(C6F5)3 with the external nitrogen atoms of bipyrazine (bpz) in Pt(bpz)(4-CF3-Ph)2 induced a 64000-fold increase in the rate of biaryl reductive elimination.3 Schrock and co-workers showed that coordination of B(C 6F5)3 to an oxo ligand in tungsten alkylidene bis(aryloxide) complexes accelerates the formation of metallacyclobutanes from alkylidenes, as well as the metallacyclobutane rearrangement.4 In catalysis, Lewis acid additives enhanced the reactivity of a number of SHOP-type nickel catalysts and related derivatives.5 In recent years, our group has taken advantage of the Lewis basic nature of the oxo group in rhenium complexes to develop frustrated Lewis pair catalysts where the oxorhenium complex functions as the Lewis base component.6 These FLPs are stabilized primarily by noncovalent interactions, and activation of the substrate occurs in the secondary coordination sphere (Figure 1).6 In this work we show that classic coordination of the oxo group in an oxorhenium hydride complex to M(C6F5)3 (M = Al, B) leads to dramatic increases in the rate of migratory olefin insertion. © XXXX American Chemical Society

Figure 1. Covalent and noncovalent interactions with oxorhenium complexes and Lewis acids.

The migratory insertion of olefins into transition-metal− hydride bonds is a key elementary transformation in a variety of catalytic reactions, including olefin hydrogenation, hydroformylation, isomerization, polymerization, and oligomerization.7 Work by Bercaw and co-workers on olefin insertion into Received: April 15, 2017

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DOI: 10.1021/acs.organomet.7b00291 Organometallics XXXX, XXX, XXX−XXX

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Organometallics early-transition-metal hydrides have allowed a clear picture of the mechanism for this process to emerge.8 Further, the chemistry of transition-metal hydride complexes is important for many fundamental steps in catalytic reactions.9 Thus, the ability to tune the electronics of a metal−hydride bond in a transition-metal complex has important implications for the design of new catalytic systems. This is the first study, to the best of our knowledge, to show that tuning the electronics of an oxo ligand by coordination to a Lewis acid can have a dramatic effect on the rate of migratory insertion into a transition-metal hydride. Combined experimental and computational studies have been utilized to understand the reasons for the rate enhancement upon coordination of the oxo group to the Lewis acid.

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RESULTS AND DISCUSSION Reaction of Oxorhenium Hydrides with Olefins in the Presence of B(C6F5)3 and Al(C6F5)3. Oxorhenium hydrides of the form (DAP)Re(O)H (1; DAP = 2,6-bis((arylamino)methyl)pyridine, where aryl = Mes, Dipp) interact with the Lewis acids B(C6F5)3 and Al(C6F5)3 to form adducts of the form (C6F5)M·(DAP)Re(O)(H) (2; M = B, Al). In the presence of 1-octene, olefin insertion proceeds efficiently at room temperature to afford the octyl complex (C6F5)M· (DAP)Re(O)(octyl) (3) according to Scheme 1. In the absence of the Lewis acid, no insertion occurs. Further, the formation of 3 is not reversible at room temperature.

Figure 2. Thermal ellipsoid plot (50% ellipsoids) for 3. Selected bond lengths (Å) and angles (deg): Re1−O1, 1.7780(19); Re1−N1, 1.981(2); Re1−N3, 1.967(2); Re1−N2, 2.033(2); Re1−C8, 2.119(3); O1−B1, 1.548(4); Re1−O1−B1, 167.18(18), 109.87(10). The diisopropylphenyl substituent is shown in wireframe format for clarity.

of 1-octene into the rhenium−hydride bond in 1 in the presence of B(C6F5)3 and Al(C6F5)3 are depicted in Figure 3.

Scheme 1. Olefin Insertion into Oxorhenium Hydrides in the Presence of M(C6F5)3

Figure 3. Time profiles for the reaction of 1-octene with 1 (R = Dipp) in the presence of the Lewis acids M(C6F5)3 (M = Al, B).

X-ray-quality crystals of (C6F5)B·(DAP)Re(O)(octyl) (3; aryl = Dipp) were obtained by cooling a concentrated hexanes solution of the complex to −20 °C. The thermal ellipsoid plot for (C6F5)B·(DAP)Re(O)(octyl) is depicted in Figure 2. The structure is similar to that of many of the previously reported Lewis acid/oxorhenium adducts from our laboratory.10 Notably, the rhenium−oxo bond is significantly lengthened (1.778 Å), the boron−oxo bond length (1.548 Å) is typical for other oxo boron adducts, and the rhenium−oxo−boron angle is approximately linear (173.2°). Kinetics and Mechanism of Olefin Insertion. Since coordination of the Lewis acid to the oxo group enhances the rate of olefin insertion in these complexes, we examined the mechanism for this reaction. The time profiles for the insertion

As shown in this figure, there was a significant enhancement of the rate of insertion of 1-octene when Al(C6F5)3 was utilized as the Lewis acid in comparison to the corresponding reaction with B(C6F5)3. Reactions of 1 with 1-octene were first order in both 1 and 1-octene (eq 1). d[3] d[1] =− = kobs[1][1‐octene] dt dt

(1)

In order to investigate possible isotope effects, the corresponding deuterated analogue of 3 was synthesized from Bu3SnD and the kinetics of olefin insertion was examined under B


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Organometallics identical conditions. As shown in Figure 4, a kinetic isotope effect of 1.19(6) was obtained from these experiments. The

Figure 5. Hammett plot for the reaction of p-X-C6H4CHCH2 (X = H, Me, CF3) with 1 (R = Dipp). Observed rate constants (min−1): X = H, 0.0161; X = CH3, 0.0229; X = CF3, 0.0180. Observed rate constants are the average for the disappearance of 1 and the formation of 3.

Figure 4. Kinetic isotope effect for the reaction of 1-H/D with 1octene. Each data point is the average of three observed rate constants obtained from pseudo-first-order plots. Error bars represent the standard deviation.

Chart 1. Molecules Calculated at the B3PW91-D3 Level in This Study

observed KIE is consistent with a mechanism that involves a pre-equilibrium involving rapid coordination and dissociation of olefin, followed by rate-determining hydride transfer (vide infra).8b,11 The effects on the rates of insertion of the olefinic substrate were examined by performing reactions with styrene derivatives and tridecafluorooct-1-ene. Reactions with styrene did not proceed at room temperature and required heating to 60 °C before the formation of product was observed. To determine whether the observed difference in reactivity was the result of steric or electronic effects, we performed reactions with parasubstituted styrene derivatives (p-X-C6H4CHCH2, X = H, CH3, CF3). As shown in Figure 5, no correlation was observed with the observed rate constant and the Hammett electronic parameter σ. The styrene derivatives examined differ significantly in their steric profile from 1-octene; thus, it is unclear whether the observed differences in reactivity are the result of electronic or steric factors. To address this question, we examined reactions with tridecafluorooct-1-ene that should exhibit a steric profile similar to that of 1-octene. However, no reaction was observed with tridecafluorooct-1-ene even at elevated temperatures. Computational Studies. The experimental studies described above suggest that, upon coordination of a Lewis acid to the oxo group, there is significant enhancement in the rate of migratory insertion. In order to understand these effects, computational studies were performed at the DFT (B3PW91D3) level of theory. Molecules and complexes used in these studies are depicted in Chart 1. The calculated mechanism for olefin insertion into Lewis acid base adducts of oxorhenium hydrides is summarized in Scheme

2. This mechanism is consistent with the experimentally observed rate equation: i.e., under steady state conditions the rate law is k1k 2 d[3] d[1] [1][olefin] =− = dt dt k −1 + k 2 C

(2)


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Organometallics

ethylene adduct, followed by rate-determining hydride transfer to produce the ethyl complex (C6F5)3M·(O)Re(DAP)(Et). Consistent with experimental results, the energetic barriers for the insertion of ethylene follow the order (O)Re(DAP)(H) > (C6F5)3B·(O)Re(DAP)(H) > (C6F5)3Al·(O)Re(DAP)(H) with activation barriers of 35.7, 26.0, and 20.2 kcal/mol, respectively. In order to examine the electronic effects of the olefinic substrate on the insertion reaction, in addition to ethylene, pathways were also calculated for propene and trifluoropropene. In addition, to probe the electronic effects of the ancillary ligand, calculations were performed with 2e,f, where the para position of the arene ring attached to the amide group is substituted with a H atom and a CF3 group, respectively. Finally, steric effects in this reaction were examined by performing calculations with 2c, which contains isopropyl groups in the 2,6-positions and 2d, which contains ethyl groups in the 2,6-positions. Activation barriers for all calculated model complexes are shown in Table 1. The dramatic effect on the rate of olefin insertion is evident, as for all adducts of Al(C6F5)3 and B(C6F5)3 the relative rate of insertion (krel = kins(2)/kins(1)) is significantly larger than that for the unactivated oxorhenium hydride 1. As an illustration of the significance of the observed rate increase, for adduct 2b, the increase in rate is more than 11 orders of magnitude (entry 4). In order to understand the origins of these rate enhancements, structures of the calculated intermediates were inspected. Optimized structures of 1, 2, INT1, and TS1 are depicted in Figure 7. Selected bond lengths and angles are shown in Table

Scheme 2. Calculated (B3PW91-D3) Mechanism for Olefin Insertion into Oxorhenium Hydrides in the Presence of the Lewis Acids M(C6F5)3 (M = Al, B)

The calculated pathways for the reaction of (C6F5)3M· (O)Re(DAP)(H) (M = Al, B) with ethylene are depicted in Figure 6. From the infinitely separated oxorhenium complex and ethylene, the reaction proceeds as expected through an

Figure 6. Calculated (B3PW91-D3) pathways for olefin insertion with oxorhenium hydrides in the presence and absence of M(C6F5)3 (M = Al, B) with Al(C6F5)3 (blue), B(C6F5)3 (red), and without Lewis acid (black). Calculations were performed at 298 K and include solvation energies with the PCM method in methylene chloride. D


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Organometallics

idine ligand and the hydride ligand occupying the equatorial plane (oxo−Re−H angle 105°). In INT1, coordination of ethylene occurs between the oxo ligand and the hydride ligand. This forces the hydride ligand out of the equatorial plane (oxo−Re−H angle 152°). Kohn−Sham orbitals for INT1 are depicted in Figure 8. The HOMO reflects the expected π backbonding of the dxy orbital on Re to the π* orbital of the alkene.

Table 1. Activation Barriers and Relative Rate Constants for Olefin Insertion into Oxorhenium−Hydride Bonds in the Presence of M(C6F5)3 (M = Al, B) entry

complex

olefin

ΔG⧧ (kcal/mol)

1 2 3 4 5 6 7 8 9

2a 2a 2a 2b 2c 2d 2e 2f 1

4a 4b 4c 4a 4a 4a 4a 4a 4a

26.0 23.0 28.4 20.2 26.2 32.1 25.4 25.7 35.7

krela 1 2 2 2 9 4 4 2 1

× × × × × × × ×

107 109 105 1011 106 102 107 107

a

krel = kins(2)/kins(1); kins is calculated from the energy of TS1 and the Eyring equation.

Figure 8. Kohn−Sham orbitals (isocontour 0.05) for the HOMO and HOMO-2 orbitals in INT1.

The HOMO-2 orbital involves donation of the alkene π electrons into an orbital that is antibonding between rhenium and oxygen, and bonding between the hydride ligand and rhenium, i.e. movement of the hydride ligand out of the equatorial plane, induces mixing of the rhenium−hydride σ bond with the rhenium−oxygen π* orbital. As shown in Figure 8, a consequence of σ → π mixing is that there is a significant accumulation of electron density on the oxo ligand and a substantial decrease in the rhenium−oxygen bond order upon coordination of the Lewis acid (Re−oxo bond lengths 1.69 and 1.74 Å for 2 and 1, respectively) and the olefin (Re−oxo bond lengths 1.82 and 1.83 Å in INT1 and TS1, respectively). The transition state, TS1, involves the concerted formation of a bond from rhenium to carbon C(α) and a bond from the hydride ligand to carbon C(β). As a consequence, the bond to C(β) is lengthened significantly (Re−C(β) = 2.45 Å) relative to C(α) (Re−C(α) = 2.32 Å). The C(β)−H bond in TS1 is 1.70 Å, which suggests significant C−H bond formation. The small lengthening of the rhenium−hydride bond in the transition state (Re−H 1.64 Å in 2 and 1.67 Å in TS1) for migratory insertion is consistent with the small experimentally observed kinetic isotope effect of 1.19(6). In TS1, the electron pair in the HOMO in Figure 8 moves directly to C(α), leaving C(β) with a partial positive charge. At the same time, C−H bond formation involves donation of the electrons in the Re−H σ bond in HOMO-2 to C(β). The charge distribution in the transition state for migration is shown in Scheme 3 and is consistent with similar models proposed by Doherty and Bercaw for migratory insertion in niobocene hydrides.8a,c−e Activation barriers for the insertion of ethylene, propene, and trifluoropropene (Table 1) follow the order trifluoropropene > ethylene > propene. This is consistent with the transition state

Figure 7. Optimized structures (B3PW91-D3) for 1, 2, INT1, and TS1. Legend: H, white; C, gray; N, blue; O, red; B, pink; F, green. The mestiyl substituents are shown in wireframe for clarity. Selected bond lengths and angles are shown in Table 2.

2. The geometry around the rhenium center in 1 is best described as distorted square pyramidal with the diamidopyrTable 2. Selected Bond Lengths (Å) and Angles (deg) for 1, 2, INT1, and TS1 entry

bond (Å)/angle (deg)

2

1

INT1

TS1

1 2 3 4 5 6

Re−O Re−H Re−C(α) Re−C(β) O−Re−H O−Re−C(α)−C(β)

1.74 1.64

1.69 1.66

106

105

1.82 1.66 2.27 2.28 151 85.9

1.83 1.67 2.32 2.45 152 141 E


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Organometallics

para substituent on the styrene ring was varied suggests that the magnitude of the charge on C(β) in the transition state may be small. As shown in Table 3, the charge on the oxo ligand consistently becomes more negative in the transition state (compare entries 1−5 with entries 9−15). However, for Lewis acid base adducts the excess negative charge on the oxo group is stabilized by the presence of an adjacent positively charged atom (B or Al). The charge on the adjacent Lewis acid is significantly greater for Al (entries 2 and 10) in comparison to the corresponding B analogues (entries 1 and 9). The stabilization of the charge in the transition states decreases the activation barrier for migratory insertion. Thus, the computed and experimental rates of insertion for 1 and 2a,b follow the order 1 < 2a < 2b, with calculated activation barriers of 35.7, 26.0, and 20.2 kcal/mol, respectively. Experimentally, the insertion of 1-octene was not observed for 1a, while the reaction was complete in 1600 min for 2a and 10 min for 2b. To evaluate the effect of the sterics on the diamidopyridine ligand on the rate of migration, calculations were performed with 2,6-diisopropylphenyl (2c; DIPP) 2,6-diethylphenyl (2d; DEP), and 2,6-dimethylphenyl (2e; Xyl). As shown in Table 1, activation barriers for 2c−e were 26.2, 32.1, and 25.4 kcal/mol, respectively. Thus, these calculations suggest that, while the rates of migratory insertion are within 1 order of magnitude for the DIPP (2c)- and Xyl (2e)-substituted diamidopyridine ligands, the activation barrier for the DEP ligand is significantly larger. Experimentally, second-order rate constants for the insertion of 1-octene with 2a (Mes) and 2c were 0.0071(9) and 0.046(2) mM−1 min−1, respectively (see the Supporting Information), which again suggests that methyl and isopropyl substituents in the ortho position result in similar rates of insertion. In order to understand the anomalous behavior of the ethylsubstituted ligand, the transition state structures for 2d and 2c were compared (Figure 9). In contrast to 2c, the transition state for migratory insertion in 2d involves an almost perpendicular orientation of the olefin (O−Re−C(α)−C(β) dihedral angle 89.6°). In comparison, the corresponding dihedral angle in 2c is 142°, which is comparable to those for all other calculated transition states in this study. In addition, the migrating hydride ligand in 2d occurs approximately in the

Scheme 3. Charge Distribution in the Transition State for Migratory Insertion

for migratory insertion being stabilized by electron-donating substituents on the β-carbon. Natural partial charge analysis (Table 3) is also consistent with this picture. From Table 3, the charge on C(α) becomes more negative in the transition state, and the charge on C(β) becomes less negative (compare entry 6 with entry 9 for example). An electron-donating substituent on C(β) stabilizes the partial positive charge (entry 11). In contrast, for trifluoroethylene where the CF3 group is electron withdrawing, there is no net change in the charge from the free olefin to the transition state (compare entries 8 and 12). The diminished charge stabilization for trifluoroethylene results in a higher barrier for migration for this substrate. The calculated barriers for olefin insertion in propene, ethylene, and trifluoroethylene (Table 1) are 26.0, 23.0, and 28.4 kcal/mol, respectively, consistent with the increased stabilization of C(β) by an electron-donating substituent. Experimentally, insertion of 1-octene occurred readily at room temperature; however, insertion of styrene required heating to 60 °C for the reaction to proceed, while there was no reaction with tridecafluorooct-1-ene. Since the steric profiles for 1-octene and tridecafluoroocte-1-ene should be similar, it is clear that the reason for the difference in reactivity is electronic. The slower reactivity of styrene most likely reflects that the styrene substituent is a poorer donor than the hexyl group in 1octene. However, the lack of an observed correlation when the

Table 3. Natural Partial Charges for Oxorhenium Complexes, Adducts with M(C6F5)3 Lewis Acids, Olefinic Substrates, and Transition States for Migratory Insertion entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

complex/substrate 2a 2b 2c 2d 1 4a 4b 4c TS1 TS1 TS1 TS1 TS1 TS1 TS1

(2a + 4a) (2b + 4a) (2a + 4b) (2a + 4c) (2c + 3a) (2d + 4a) (1 + 4a)

Re

O

B/Al

H

0.80 0.80 0.84 0.81 0.72

−0.57 −0.74 −0.59 −0.58 −0.46

0.65 1.66 0.65 0.65

0.090 0.096 0.66 0.084 0.04

0.54 0.61 0.58 0.57 0.54 0.59 0.51

−0.63 −0.84 −0.64 −0.62 −0.64 −0.67 −0.52

0.66 1.67 0.66 0.66 0.66 0.66

F

0.14 0.11 0.12 0.17 0.16 0.29 0.08

C(α)

C(β)

−0.45 −0.46 −0.40 −0.52 −0.49 −0.55 −0.52 −0.53 −0.46 −0.54

−0.45 −0.22 −0.36 −0.40 −0.39 −0.14 −0.36 −0.42 −0.57 −0.41


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Organometallics

In contrast, the hydrogen atom of the isopropyl group is oriented in the O−Re−H plane, while the CH3 groups are all oriented away from this plane (see space-filling depictions in Figure 9). As a result, rotation of the olefin in 2c in less restricted, while these steric interactions in 2d are more severe. The transition state in 2d is thus destabilized, resulting in a high barrier for migratory insertion with the DEP ligand.

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CONCLUSIONS In this paper, a dramatic increase in the rate of olefin insertion has been demonstrated with oxorhenium hydride complexes upon adduct formation with the Lewis acids M(C6F5)3 (M = Al, B). Computational studies (DFT/B3PW91-D3) suggest that this enhancement in rate is as much as 2 × 1011 relative to the Lewis acid free oxorhenium complex. Data suggest that the transition state for insertion involves the transfer of electron density from a Re−H σ bond to the partially positive β-carbon on the olefin. The evidence for this includes the following: (1) There is a small kinetic isotope effect of 1.19(6), which is consistent with a mechanism that involves prior coordination of the olefin to form an intermediate, followed by transfer of a hydride ligand to the coordinated olefin. This picture is similar to the mechanistic picture proposed by Bercaw and co-workers for olefin insertion into niobocene hydrides.8,12 (2) Experimental and computational observations show that olefins with electron-donating groups on the β-carbon of the olefin are more reactive. (3) Experimental and computational observations show that the rates of insertion with Al adducts are significantly faster than those for the corresponding B analogues. From kinetic and computational data, the rate-determining step for olefin insertion involves the expected addition of the olefinic substrate in a plane orthogonal to the Re−oxo multiple bond. This forces the hydride ligand into the plane that contains Re and the oxo group. This bonding situation induces mixing of the Re−H σ bond with the Re−oxo π* orbital and results in an accumulation of electron density on the oxo ligand. A Lewis acid adjacent to the oxo group in the transition state for olefin insertion in oxorhenium/Lewis acid adducts stabilizes the excess electron density that accumulates on the oxo ligand as a result of σ−π mixing. This effect is larger for adducts of Al in comparison to adducts of B because of the greater charge on the Al atom. It was previously shown that the protonation of the oxo group in oxorhenium isopropyl complexes results in a lowering of the barrier for β-hydrogen elimination.13 We show here for the first time that migratory insertion, the microscopic reverse of β-hydrogen elimination, is accelerated by the coordination of a Lewis acid to the the oxo ligand in oxorhenium alkyls. Given the importance of this elementary step in many catalytic reactions, the results reported here could have significant implications for the design of new catalysts. Schrock and co-workers have shown the addition of B(C6F5)3 to the tungsten alkylidene catalyst W(O)(CH-tBu)(OHMT)(Me2Pyr)(PMe2Ph) (where OHMT = O-2,6dimesitylphenoxide) leads to increased activity in olefin metathesis.14 These authors later showed that the coordination of the Lewis acid to the oxo group in these catalysts accelerates metallacyclobutane formation, which lies on the catalytic cycle.15 The results presented above may also be used to rationalize the observed increase in catalytic activity. As shown in Scheme 5, the preferred geometry for the metallacyclobutane intermediate is trigonal bypyramidal, with the alkoxide ligand

Figure 9. Optimized structures for transition states for the insertion of ethylene into the rhenium−hydride bond in 2c,d. Space-filling depictions showing the relevant ethylene, ethyl (2d), and isopropyl (2c) fragments are shown on the right. Selected bond lengths (Å) and angles (deg) are as follows. 2d: Re−H, 1.71; Re−C(α), 2.21; Re− C(β), 2.32; C(β)−H, 1.33; O−Re−H, 115; O−Re−C(α)−C(β), 89.6. 2c: Re−H, 1.67; Re−C(α), 2.27; Re−C(β), 2.40; C(β)−H, 1.70; O− Re−H, 153; O−Re−C(α)−C(β), 142.

same plane as the olefin (O−Re−H, 115°), as opposed to 153° in 2c. The nature of the transition state in 2d appears to occur later than the transition state in 2c, as the Re−H bond in 2d (1.71 Å) is lengthened relative to that in 2c (1.67 Å). In addition, the C(β)−H bond (1.33 Å) in 2d is shorter than the corresponding bond in 2c (1.70 Å). These results suggest a greater degree of Re−H bond cleavage and C−H bond formation in the transition state for 2c. As described above, the transition state for migratory insertion involves the concerted cleavage of the Re−C(β) bond and formation of a new C(β)−H bond. For maximum overlap the olefin ligand rotates out of the equatorial plane so the C(β) p orbital can overlap with the orthogonal Re−H σ bond. As shown in Scheme 4, in the DEP ligand, rotation of the olefin ligand is prevented as the CH3 group from the ethyl fragment is oriented in the plane that contains the oxo group, the Re atom, and the hydride ligand. Scheme 4. Transition State Steric Interactions in 2c,d

G


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Organometallics

hygroscopic nature of these molecules, elemental analyses were not attempted. NMR spectra are provided in the Supporting Information. General Procedure for Obtaining Kinetic Data with 1Octene. A 0.0227 M stock solution of the oxorhenium hydride acid base adducts 2 were prepared by combining the Re−H complex 1 (30 mg, 0.0455 mmol) with B(C6F5)3 or Al(C6F5)3 (2 equiv with respect to Re, 0.0911 mmol) and diluting the solution to 2 mL with toluened8. Aliquots of this solution (0.5 mL each, 0.0114 mmol) were transferred into four separate Teflon-capped NMR tubes in the glovebox. Different molar amounts of 1-octene (0.079 mmol, 7 equiv; 0.159 mmol, 14 equiv; 0.239 mmol, 21 equiv) were transferred to each solution via a microsyringe. Reactions were monitored by 1H NMR spectroscopy over a period of 1200 min. Yields were determined with mesitylene as an internal standard. The same conditions were employed to obtain the kinetic isotope effect with 2-d. General Procedure for Obtaining Hammett Data with ParaSubstituted Styrene Derivatives. A 0.0227 M stock solution of the oxorhenium hydride acid base adducts were prepared by combining the Re−H complex 1 (30 mg, 0.0455 mmol) and B(C6F5)3 (46.6 mg, 0.0911 mmol). Aliquots of the solution (0.5 mL each, 0.0114 mmol) were transferred into three separate Teflon-capped NMR tubes in the glovebox. Para-substituted styrene derivatives (10 equiv, 0.12 mmol; 15 μL for p-Me-styrene, 13.0 μL for styrene, 16.8 μL for p-CF3styrene) were transferred to individual solutions via a microsyringe. The reactions were monitored for a period of 200 min at 60 °C by 1H NMR spectroscopy. Yields were determined with mesitylene as an internal standard. Characterization of (C6F5)M·(DAP)Re(O)(R) (DAP = DIPP, Mes; M = Al, B; R = Octyl, CH2CH(C6H4-p-X) (X = H, Me, CF3)). Because of the hygroscopic nature of these complexes, alkyl and phenethyl complexes were not isolated but were analyzed in situ. (C6F5)B·(DAP)Re(O)(octyl) (DAP = DIPP). Quantitative conversion of 2 with excess 1-octene was observed by 1H NMR spectroscopy during kinetic runs. Concentrations were determined with mesitylene as an internal standard. 1H NMR (toluene-d8, 300 MHz, 300 K, ppm): 7.02 (t, 1H, J = 7.8 Hz, pyr-para H), 6.75 (m, 6H, Dipp-aromatics), 6.72 (d, 2H, J = 7.8 Hz, pyr-meta H) 6.02 (d, 2H, J = 20.2 Hz, pyr-CH2-N), 5.31 (d, 2H, J = 20.2 Hz, pyr-CH2-N), 5.18 (m, 2H, Re-CH2-R), 3.65 (sep, 2H, iPr-H), 2.04 (m, 2H, Re-CH2CH2-R), 1.29 (overlapping m, 8H, octyl), 1.22 (d, 6H, J = 6.6 Hz, iPr-CH3) 1.11 (d, 6H, J = 6.6 Hz, iPr-CH3), 0.96 (overlapping m, 11H, octyl and iPr-CH3), 0.79 (d, 6H, J = 6.6 Hz, iPr-CH3). The second isopropyl signal for the DIPP moiety could not be resolved because it is obscured by aliphatic protons in the 1.5−0.8 ppm region. Two iPr-H protons are obscured by aliphatic protons in this region. 19F NMR (toluene-d8, 400 MHz, 300 K, ppm): −132.2 (m, 2F), −157.8 (m, 1F), −165.4 (m, 2F). (C6F5)Al·(DAP)Re(O)(octyl) (DAP = DIPP). Quantitative conversion of 2 with excess 1-octene was observed by 1H NMR spectroscopy during kinetic runs. Concentrations were determined with mesitylene as internal standard. 1H NMR (toluene-d8, 300 MHz, 300 K, ppm): 7.00 (t, 1H, J = 7.8 Hz, pyr-para H), 6.92 (m, 6H, Dipparomatics), 6.53 (d, 2H, J = 7.8 Hz, pyr-meta H), 5.73 (d, 2H, J = 20.2 Hz, pyr-CH2-N), 5.41 (m, 2H, Re-CH2-R), 5.20 (d, 2H, J = 20.2 Hz, pyr-CH2-N), 3.65 (sep, 2H, iPr-H), 1.95 (m, 2H, Re-CH2CH2-R), 1.30 (overlapping m, 8H, octyl), 1.11 (d, 6H, J = 6.6 Hz, iPr-CH3), 0.95 (d, 6H, J = 6.6 Hz, iPr-CH3), 0.90 (overlapping m, 11H, octyl and iPr-CH3), 0.78 (d, 6H, J = 6.6 Hz, iPr-CH3). The second isopropyl signal for the DIPP moiety could not be resolved because it is obscured by aliphatic protons in the 1.5−0.8 ppm region. Two iPr-H protons are obscured by aliphatic protons in this region. 19F NMR (toluene-d8, 400 MHz, 300 K ppm): −122.8 (m, 2F), −154.3 (m, 1F), −162.5 (m, 2F). (C6F5)B·(DAP)Re(O)(octyl) (DAP = Mes). Quantitative conversion of 2 with excess 1-octene was observed by 1H NMR spectroscopy during kinetic runs. Concentrations were determined with mesitylene as internal standard. 1H NMR (toluene-d8, 300 MHz, 300 K, ppm): 6.97 (t, 1H, J = 7.8 Hz, pyr-para H), 6.81 (overlapping, m, 6H, Mesaromatics, pyr-meta H), 5.69 (d, 2H, J = 21.3 Hz, pyr-CH2-N), 4.93 (m, 2H, Re-CH2-R), 4.82 (d, 2H, J = 21.3 Hz, pyr-CH2-N), 2.24 (s,

Scheme 5. Stabilization of Bonding in Tungsten Oxo Alkylidene Intermediates by Lewis Acids

occupying the axial position trans to the oxo group. This geometry results in a three-center−four-electron σ interaction, as well as π donation from the oxygen atom of the OHMT ligand, which results in an accumulation of electron density on the oxo group. Similar to the results described above, this excess electron density is relieved by the presence of B(C6F5)3 next to the oxo ligand. In recent years, the importance of utilizing main-group Lewis acids to mediate transformations with transition-metal hydride complexes has been recognized, and in a few cases, these interactions have been shown to be important in catalysis.2a Most commonly, the role of the Lewis acid is to interact directly with the substrate or to allow the generation of an open coordination site.1a,b,5b,16 A rare example has been demonstrated here where a Lewis acid directly modulates the electronics of an oxo ligand and consequently affects the rate of migratory insertion, which is a key elementary step in many catalytic reactions.

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EXPERIMENTAL SECTION

General Considerations. Complex 217 and Al(C6F5)3 were prepared according to a previously reported procedure.18 B(C6F5)3 was sublimed before use. All other reagents were purchased from commercial sources and used as received. 1H and 19F NMR spectra were obtained on 300 or 400 MHz spectrometers at room temperature. Chemical shifts are listed in parts per million (ppm) and referenced to their residual protons or carbons of the deuterated solvents, respectively. All reactions were run under an inert atmosphere with dry solvents unless otherwise noted. Because of the H


Article

Organometallics 6H, Mes-CH3), 2.20 (s, 6H, Mes-CH3). 1.29 (m, 8H, octyl), 1.20 (s, 6H, Mes-CH3), 0.97 (overlapping t, 3H, J = 7.3 Hz, Re-R-CH3), 0.76 (m, 2H, octyl), 0.49 (m, 2H, octyl). The signal for the remaining octyl protons could not be resolved because it is obscured by aliphatic protons in the 1.5−0.8 ppm region. 19F NMR (toluene-d8, 300 MHz, 300 K, ppm): −132.6 (m, 2F), −158.9 (m, 1F), −165.3 (m, 2F). (C6F5)Al·(DAP)Re(O)(octyl) (DAP = Mes). Quantitative conversion of 2 with excess 1-octene was observed by 1H NMR spectroscopy during kinetic runs. Concentrations were determined with mesitylene as internal standard. 1H NMR (toluene-d8,, 300 MHz, 300 K, ppm): 6.97 (overlapping t, 1H, J = 7.8 Hz, pyr-para H), 6.84 (m, 4H, Mes-aromatics), 6.77 (d, 2H, J = 7.9 Hz, pyr-meta H), 5.63 (d, 2H, J = 21.7 Hz, pyr-CH2-N), 5.27 (m, 2H, Re-CH2-R), 4.88 (d, 2H, J = 21.7 Hz, pyr-CH2-N), 2.32 (s, 6H, Mes-CH3), 2.25 (s, 6H, MesCH3), 2.12 (m, 2H, Re-CH2CH2-R), 2.16 (m, 2H, octyl), 1.23 (s, 6H, Mes-CH3), 0.97 (overlapping t, 3H, J = 7.3 Hz, Re-R-CH3), 0.86 (m, 2H, octyl), 0.71 (m, 2H, octyl), 0.41 (m, 2H, octyl). The signal for the remaining octyl protons could not be resolved because it is obscured by aliphatic protons in the 1.5−0.8 ppm region. 19F NMR (toluene-d8, 300 MHz, 300 K, ppm): −122.3 (m, 2F), −154.4 (m, 1F), −162.5 (m, 2F). (C6F5)B·(DAP)Re(O)(CHCH2C6H4-p-Me) (DAP = DIPP). NMR yield 74%. 1H NMR (toluene-d8, 300 MHz, 300 K, ppm): 7.15 (d, 2H, J = 7.6 Hz, styrene-aromatics) 7.07 (d, 2H, J = 7.6 Hz, styrenearomatics), 7.11 (t, 1H, J = 7.8 Hz, pyr-para H), 7.00 (t, 1H, J = 7.8 Hz, pyr-para H), 6.63 (m, 6H, Dipp-aromatics), 6.22 (d, J = 7.8 Hz, pyr-meta H), 5.94 (d, 2H, J = 21.7 Hz, pyr-CH2-N), 5.30 (m, 2H, ReCH2CH2-Ar), 5.19 (d, 2H, J = 21.7 Hz, pyr-CH2-N), 3.64 (sep, 2H, J = 6.5 Hz, iPr-H) 2.48 (m, 2H, Re-CH2CH2-Ar), 1.84 (s, 3H, styrene-pCH3), 1.16 (d, 6H, J = 6.5 Hz, iPr-CH3), 1.08 (sep, 2H, J = 6.5 Hz, iPrH), (1.01 (d, 6H, J = 6.5 Hz, iPr-CH3), 0.68 (d, 6H, J = 6.5 Hz, iPrCH3), 0.64 (d, 6H, J = 6.5 Hz, iPr-CH3). (C6F5)B·(DAP)Re(O)(CHCH2C6H5) (DAP = DIPP). NMR yield 87%. 1H NMR (toluene-d8, 300 MHz, 300 K, ppm): 7.07−6.92 (overlapping m, 5H, styrene aromatics), 6.82 (t, 1H, J = 7.8 Hz, pyrpara H), 6.63 (s, 6H, Dipp-aromatics), 6.32 (d, 2H, J = 7.8 Hz, pyrmeta H), 5.93 (d, 2H, J = 21.1 Hz, pyr-CH2-N), 5.27 (m, 2H, ReCH2CH2-Ar), 5.18 (d, 2H, J = 21.1 Hz, pyr-CH2-N), 3.63 (sep, 2H, J = 6.5 Hz, iPr-H), 2.52 (m, 2H, Re-CH2CH2-Ar), 1.14 (d, 6H, J = 6.5 Hz, iPr-CH3), 1.10 (sep, 2H, J = 6.5 Hz, iPr-H), 1.00 (d, 6H, J = 6.5 Hz, iPr-CH3), 0.66 (d, 6H, J = 6.5 Hz, iPr-CH3), 0.64 (d, 6H, J = 6.5 Hz, iPr-CH3). (C6F5)B·(DAP)Re(O)(CHCH2C6H4-p-CF3) (DAP = DIPP). NMR yield 60%. 1H NMR (toluene-d8, 300 MHz, 300 K, ppm): 7.12 (t, 1H, J = 7.8 Hz, pyr-para H), 6.97 (d, 2H, J = 7.0 Hz, styrene aromatics), 6.63 (s, 6H, Dipp-aromatics), 6.59 (d, 2H, J = 7.0 Hz, styrene aromatics), 6.23 (d, 2H, J = 7.8 Hz, pyr-meta H), 5.94 (d, 2H, J = 21.1 Hz, pyr-CH2-N), 5.21 (d, 2H, J = 21.1 Hz, pyr-CH2-N), 5.12 (m, 2H, Re-CH2CH2-Ar) 3.60 (sep, 2H, J = 7.0 Hz, iPr-H), 2.51 (m, 2H, ReCH2CH2-Ar), 1.26 (sep, 2H, J = 7.0 Hz, iPr-H), 1.13 (d, 6H, J = 6.5 Hz, iPr-CH3), 1.00 (d, 6H, J = 7.0 Hz, iPr-CH3), 0.64 (d, 6H, J = 7.0 Hz, iPr-CH3), 0.61 (d, 6H, J = 7.0 Hz, iPr-CH3). Computational Details. Geometry and transition state optimizations were performed with the 6-31G(d,p) basis set19 on light atoms and the SDD basis set20 with an added f polarization function on rhenium.21 Each optimization involved tight optimization criteria implemented in Gaussian 0922 (opt = tight) with an ultrafine integral grid (int = ultrafine) and the B3PW91 functional.23 The use of Grimme’s dispersion correction24 was also employed in all calculations. For comparison, calculations were performed with the same basis set and the M06 functional.25 Similar structures were obtained (see the Supporting Information). All structures were fully optimized, and analytical frequency calculations were performed on all structures to ensure either a zeroth-order saddle point (a local minimum) or a first-order saddle point (a transition state). The minima associated with each transition state were determined by animation of the imaginary frequency. Energetics were calculated at 298 K with the 6-311++G(d,p)26 basis set for C, H, N, O, and P atoms and the SDD basis set with an added f polarization function on Re with the B3PW91 functional. Reported energies utilized analytical

frequencies and the zero-point corrections from the gas-phase optimized geometries and include solvation corrections which were computed using the PCM method,27 with methylene chloride as the solvent as implemented in Gaussian 09. Similar energy profiles were obtained with the M06 functional with benzene as the solvent (see the Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

and XYZ files. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.organomet.7b00291. X-ray experimental data for 3, additional NMR data, kinetic data, computational details, and full Gaussian reference (PDF) Cartesian coordinates for the calculated structures (XYZ) Accession Codes

CCDC 1547421 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for E.A.I.: [email protected]. ORCID

Elon A. Ison: 0000-0002-2902-2671 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge North Carolina State University and the National Science Foundation via the CAREER Award (CHE0955636) for funding.

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REFERENCES

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