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Communication
TEMPO–Mediated Catalysis of the Sterically Hindered Hydrogen Atom Transfer Reaction between (CPh)Cr(CO)H and a Trityl Radical 5
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Thilina Gunasekara, Graham P Abramo, Andreas Hansen, Hagen Neugebauer, Markus Bursch, Stefan Grimme, and Jack R. Norton J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019
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Journal of the American Chemical Society
TEMPO–Mediated Catalysis of the Sterically Hindered Hydrogen Atom Transfer Reaction between (C5Ph5)Cr(CO)3H and a Trityl Radical Thilina Gunasekara,† Graham P. Abramo,§,† Andreas Hansen,‡ Hagen Neugebauer,‡ Markus Bursch,‡ Stefan Grimme,*,‡ Jack R. Norton*,† † Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States ‡ Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich–Wilhelms Universität Bonn,
Beringstraße 4, 53115 Bonn, Germany
Supporting Information Placeholder ABSTRACT: We have demonstrated the ability of TEMPO to catalyze H•
transfer from (C5Ph5)Cr(CO)3H to a trityl radical (tris(p–tert–butylphenyl)methyl radical). We have measured the rate constant and activation parameters for the direct reaction, and for each step in the catalytic process: H• transfer from (C5Ph5)Cr(CO)3H to TEMPO and H• transfer from TEMPO–H to the trityl radical. We have compared the measured rate constants with the differences in bond strength, and with the changes in the Global Electrophilicity Index determined with high accuracy for each radical using state of the art quantum chemical methods. We conclude that neither is a major factor in determining the rates of these H• transfer reactions and that the effectiveness of TEMPO as a catalyst is largely the result of its relative lack of steric congestion compared to the trityl radical.
It is known that H• transfer reactions are more sensitive to steric effects than H+ transfer reactions.1 For example, the rate constant of the H• transfer reaction between the congested Tp*Mo(CO)3H and Tp*Mo(CO)3 (Tp*=hydridotris(3,5– dimethylpyrazolyl)borate) is at least eight orders of magnitude slower than that between the less crowded CpW(CO)3H and CpW(CO)3,1b, 2 whereas the rate constants for the corresponding H+ transfer reactions differ by only two orders of magnitude. The Norton group showed some years ago that, with a constant acceptor (Ar3C•, with Ar = tris(p–t–butylphenyl)), the rate constants for H• transfer from transition–metal hydrides vary by over five orders of magnitude – a result that appeared to be due more to steric effects than to bond strength differences.1a We have now tested this hypothesis by comparing the rate constant for the reaction of (C5Ph5)Cr(CO)3H (Cr–H) and Ar3C• (see Figure 1) with that (3.35(2) × 102 M–1s– 1 at 25 ℃ in toluene) for (C H )Cr(CO) H and Ar C•, and have found that the 5 5 3 3 former reaction is, indeed, a slow H• transfer. t
t
Bu
Bu N
Cr
H CO
OC
CO
Cr-H
t
Bu
Ar3C
Figure 1. Structures of (a) Cr–H, (b) Ar3C•, and (c) TEMPO.
TEMPO
O
We wanted to see whether a less congested radical could catalyze such a slow H• transfer. Thus, we have examined the reaction of Cr–H with TEMPO. Comparison of the bond strengths suggests that the TEMPO reaction should be slower: the O–H bond of TEMPO–H is only 69.6 kcal mol–1, whereas the C–H bond of triphenylmethane (and presumably of Ar3C•) is about 81 kcal/mol.1a, 3 However, the Cr–H/TEMPO reaction has proven to be much faster. A similar result has been reported by Darensbourg and co–workers for H+ transfers: they have found that F– removes H+ from HMo(CO)2(L)2+ (L=dppe, dmpe, and depe) more rapidly than pyridine does.4 In this Communication, we report the rate constants and activation parameters for the reactions between Cr–H and Ar3C• (eq 1), between Cr–H and TEMPO (eq 2), and between TEMPO–H and Ar3C• (eq 3) and show that eq 1 is catalyzed by relatively small amounts of TEMPO (see Scheme 1). We have then examined the contribution of thermodynamic driving force, polar effects, and steric effects to the catalytic ability of TEMPO.
Scheme 1 Uncatalyzed reaction: (C5Ph5)Cr(CO)3H + Ar3C Cr-H
(C5Ph5)Cr(CO)3 + Ar3C-H
(1)
(C5Ph5)Cr(CO)3 + TEMPO-H
(2)
Catalysis via H shuttling by TEMPO: (C5Ph5)Cr(CO)3H + TEMPO TEMPO-H + Ar3C
TEMPO + Ar3C-H
(3)
Both the metalloradical (C5Ph5)Cr(CO)3• and Ar3C• are monomeric in solution,5,6 and we do not observe any coupling between them. Ingold and co– workers have reported that there is no measurable trapping of trityl radical by TEMPO, consistent with earlier reports,7 and we have not observed any coupling of these radicals. Finally, we have seen no evidence for coupling between (C5Ph5)Cr(CO)3• and TEMPO. Results. A light green solution of Cr–H reacts with a golden yellow solution of Ar3C• (eq 1) to form a dark blue solution of (C5Ph5)Cr(CO)3• and Ar3CH (identified by 1H NMR). Pseudo–first–order conditions (at different excess Cr–H) allowed the observation over time of the spectra in Figure 2. Monitoring the absorbance of Ar3C• at 526 nm and that of (C5Ph5)Cr(CO)3• at 611 nm by UV– Vis Spectroscopy gave a second–order rate constant of 1.342(6) × 10–2 M–1s–1 at 25 ℃.
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Figure 3. The experimentally derived reaction profile diagram illustrating the ability of TEMPO to catalyze the reaction between Cr–H and Ar3C•. Black arrows indicate (ΔG ‡ ) and grey arrows indicate (ΔG), assuming that (Δ𝑆) = 0, at 25 ℃.
Figure 2. Time profile of the UV–Vis spectrum for the reaction between 11.4 mM of Cr–H and 0.78 mM of Ar3C• in Toluene at 25 ℃.
We then monitored the reaction between Cr–H and Ar3C• (eq 1) in the presence of a catalytic amount of TEMPO. Figure 4 shows the disappearance over time of Ar3C• in the catalyzed (blue symbols) and uncatalyzed (red symbols) reactions. The rate constants for eqs 1, 2, and 3 enable us to calculate (using KINSIM10) the disappearance of Ar3C• over time in both cases, and to conclude that the addition of a catalytic amount of TEMPO increases the rate at which Ar3C• is consumed.
The existence of an isosbestic point in Figure 2 confirms that there is no side reaction, such as a coupling between the (C5Ph5)Cr(CO)3• and Ar3C•. Determination of the rate constants from 15 ℃ to 55 ℃ (see Supporting Information) gave us an activation energy (Ea) of 9.5(1) kcal mol–1, an enthalpy of activation (ΔH ‡ ) of 8.9(1) kcal mol–1, an entropy of activation (Δ𝑆 ‡ ) of – 37.3(3) cal mol–1 K–1, and a free energy of activation (ΔG ‡ ) of 20.017(2) kcal mol–1 at 25 ℃. We found that the reaction of Cr–H with TEMPO (eq 2) produces (C5Ph5)Cr(CO)3• (confirmed by UV–Vis spectroscopy) and TEMPO–H (confirmed by 1H NMR3c, 8,9). The reaction rate was studied under pseudo–first– order conditions (at different excess TEMPO) using a stopped–flow apparatus. Monitoring the absorbance of (C5Ph5)Cr(CO)3• at 611 nm over time using a rapid–scanning UV–Vis spectrophotometer gave a second–order rate constant of 1.68(2) × 102 M–1s–1. Determination of the rate constants from 5 ℃ to 35 ℃ gave us an Ea of 2.1(1) kcal mol–1, a ΔH ‡ of 1.5(1) kcal mol–1, a Δ𝑆 ‡ of –43.4(3) cal mol–1 K–1, and a Δ𝐺 ‡ of 14.410(2) kcal mol–1 at 25 ℃. The second step in the H• shuttling process is the transfer of H• from TEMPO– H to Ar3C• (eq 3). The absorbance of Ar3C• at 526 nm over time was observed using a UV–Vis spectrophotometer under pseudo–first–order conditions (at different excess TEMPO–H). A second–order rate constant of 4.1(1) × 10–2 M–1s–1 at 25 ℃ was found. Determination of the rate constants from 15 ℃ to 55 ℃ gave us an Ea of 7.3(1) kcal mol–1, a 𝛥H ‡ of 6.6(1) kcal mol–1, a 𝛥𝑆 ‡ of – 42.7(5) cal mol–1 K–1, and a 𝛥𝐺 ‡ of 19.371(5) kcal mol–1 at 25 ℃. The reaction profile diagram in Figure 3 predicts that TEMPO should be able to catalyze the reaction between Cr–H and Ar3C•. The path with the larger activation barrier (in red) is the uncatalyzed H• transfer from Cr–H to Ar3C•. The two steps with smaller activation barriers (in blue) are the elementary steps of the reaction between Cr–H and Ar3C• catalyzed by TEMPO. The reaction between TEMPO–H and Ar3C• (eq 3) is the slower step.
Figure 4. The consumption time profile of Ar3C• for the reaction between 1.0 mM of Cr–H and 1.0 mM of Ar3C• in toluene at 25 ℃. Red: no catalyst; Blue: 25 mol% of TEMPO. The effectiveness of TEMPO as a catalyst may depend on three factors:11 1) the effect of reaction enthalpy on the activation energy of each reaction; 2) polar effects that may favor eqs 2 and 3 over eq 1; and 3) steric effects that may favor TEMPO over Ar3C•. Reaction enthalpy: The reaction enthalpies (∆H) calculated from the known bond energies of Cr–H of Cr–H12, C–H of triphenylmethane1a, 3b, and O–H of TEMPO–H3a are listed in Table 1, along with the activation energy for each reaction in Scheme 1. The Bell–Evans–Polanyi relationship is routinely used to compare the activation energy of analogous H• transfer reactions with the reaction enthalpy.13 Beckwith has shown that the order of the rate of reaction of Bu3Sn• with various substrates, namely RI > RBr > RSeAr > RCl > RSAr > RSMe, is roughly the same as the order of the exothermicities.14 However, the data in Table 1 clearly show that there is no relationship between the reaction enthalpies and the activation energies for the three reactions in Scheme 1.
Table 1. Reaction Enthalpy (∆H), Activation Energy (Ea), and Difference in Electrophilicity (|∆ω|) for reactions in Scheme 1 Reaction
Enthalpy (𝛥H)
Exp. Reaction
Exp. Activation Energy (𝐸𝑎)
(kcal mol–1)
(kcal mol–1)
Calc. Difference in Electrophilicity (|∆ω|) (eV)
eq 1
–21.4
9.5
1.6
eq 2
–10.0
2.1
2.1
eq 3
–11.4
7.3
0.43
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Journal of the American Chemical Society Polar effects: The spin density in the three radicals involved in Scheme 1 is centered on different atoms, i.e. Cr in (C5Ph5)Cr(CO)3•, C in Ar3C•, N and O in TEMPO, so we expect the three radicals to have distinct polarities. Roberts has observed that the relative electrophilicities of the two radicals involved in an H• transfer reaction seem to influence the energy of the transition state.15 Generally, an electrophilic H• acceptor will react more readily with a nucleophilic H• donor than with an electrophilic H• donor (and vice versa) – the basis of Polarity Reversal Catalysis. The best measure of electrophilicity seems to be the Global Electrophilicity Index (ω), first suggested by Maynard and co–workers16 and later validated by Parr and co–workers17, defined by eq 4. The electronic chemical potential, µ, is the average of the ionization potential IP and the electron affinity EA of the radical; the chemical hardness, η, is the difference between the ionization potential and the electron affinity of the radical. The most suitable way to determine these quantities is given by accurate quantum chemical calculations of IPs and EAs (see the SI for details on the computational part).
𝜔=
𝜇2 2𝜂
(4)
We have computed the ω value of these three radicals using dispersion corrected density functional theory18 at X/ma–def2–QZVPP19//B97–3c20 (X = B97–D3(BJ)21, PW6B95–D3ATM(BJ)22, PWPB95–D3ATM(BJ)23) level benchmarked against a high–level local coupled cluster method (DLPNO– CCSD(T)24 /CBS25 (VeryTightPNO26|aug–cc–pVTZ27/TightPNO26|aug–cc– pVQZ27), with the values obtained for the latter being discussed in the following. (C5Ph5)Cr(CO)3• is the most electrophilic (ω = 3.139 ± 0.136 eV), the trityl radical has an intermediate electrophilicity (ω = 1.492 ± 0.035 eV), and TEMPO is the least electrophilic (ω = 1.061 ± 0.021 eV). Table 1 lists |∆ω| for all three reactions, along with the activation energy for each reaction. Plainly, the catalytic effect of TEMPO cannot be explained by polar effects alone. Steric effects: The crystal structure of Cr–H has not been reported, but from the reported structures of (C5Ph5)Cr(CO)3•5a and (C5H5)Cr(CO)3H28 we can infer the distorted piano stool structure for Cr–H shown in Figure 1. A DFT–D3 equilibrium structure for Cr–H (Figure 5) supports the conclusion that the opening for an H• acceptor is small.
Figure 5. Molecular structure plots of the B97–3c equilibrium structure of Cr– H (ball–and–stick, left and space–filling with default Van–der–Waals radii, right). Given that eq 1 is fast (as are many reactions of TEMPO29) and that the reaction enthalpy and the polar effects cannot explain the catalytic effect, the catalytic effect must be due to the relatively less congested nature of TEMPO compared to the trityl radical. We imagine that the singly occupied orbital on the oxygen atom, being at the terminus of the TEMPO molecule, can reach into the pocket containing the hydride ligand (Figure 5). In contrast, it is difficult for the singly occupied orbital on the carbon atom of the trityl radical to reach the hydride ligand because it is at the center of the trityl molecule. Although eq 3 (the slower step in the catalyzed reaction) suffers from both the steric congestion of Ar3C• and the unfavorable polar effect, it still enables us to catalyze eq 1. In summary, we have shown that the slow reaction between a chromium hydride and a trityl radical can be catalyzed by a less congested H• abstractor; in this case TEMPO acts as an H• shuttle between Cr–H and Ar3C•. To the best of our knowledge, this is the first example of catalysis of an H• transfer from a transition–metal hydride to an organic radical. Consideration of the steric effects presented here may lead to more efficient use of transition–metal hydrides in H• transfer systems.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic details, kinetic data, spectra, and details on the calculation of Global Electrophilicity Index including the molecular structures in xyz format and all single point energies.
AUTHOR INFORMATION Corresponding Author *E–mail:
[email protected] *E–mail:
[email protected]–bonn.de §Current address: Corporate R&D, The Dow Chemical Company, 400 Arcola Road, Collegeville, PA 19426
ORCID Markus Bursch: 0000–0001–6711–5804 Stefan Grimme: 0000–0002–5844–4371 Thilina Gunasekara: 0000–0003–0821–7944 Andreas Hansen: 0000–0003–1659–8206 Hagen Neugebauer: 0000–0003–1309–0503 Jack R. Norton: 0000–0003–1563–9555
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT We thank the U.S. Department of Energy (award number DE–FG02–97ER14807) and the German Research Foundation (DFG, Gottfried Wilhelm Leibniz–Prize to S.G.) for financial support.
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