TEMPO-Mediated Catalysis of the Sterically Hindered Hydrogen Atom Transfer Reaction between (C5Ph5)Cr(CO)3H and a Trityl Radical

Article abstract of DOI:10.1021/jacs.8b12892

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.

Full text of DOI:10.1021/jacs.8b12892

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Communication
TEMPO–Mediated Catalysis of the Sterically Hindered Hydrogen
5
5
3
Atom Transfer Reaction between (CPh)Cr(CO)H and a Trityl Radical
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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TEMPO–Mediated Catalysis of the Sterically Hindered Hydrogen
Atom Transfer Reaction between (C5Ph5)Cr(CO)3H and a Trityl
Radical
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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
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
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.
+
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.
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.
Scheme 1
Uncatalyzed reaction:
(C5Ph5)Cr(CO)3H + Ar3C
(C5Ph5)Cr(CO)3 + Ar3C-H
(1)
Cr-H
Catalysis via H shuttling by TEMPO:
(2)
(3)
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 + TEMPO
(C5Ph5)Cr(CO)3 + TEMPO-H
TEMPO + Ar3C-H
TEMPO-H + Ar3C
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.
(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 (C5H5)Cr(CO)3H and Ar3C•, and have found that the
former reaction is, indeed, a slow H• transfer.
tBu
tBu
N
O
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
Cr
H
CO
OC
tBu
CO
Cr-H
Ar3C
TEMPO
Figure 1. Structures of (a) Cr–H, (b) Ar3C•, and (c) TEMPO.
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
1
2
ΔG ‡
ΔG
Δ푆
arrows indicate (
) and grey arrows indicate ( ), assuming that ( ) = 0, at
3
25 ℃.
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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.
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 ℃.
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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
ΔH ‡
Δ푆 ‡
–1
activation (
) of 8.9(1) kcal mol , an entropy of activation (
) of –
ΔG ‡
–1 –1
37.3(3) cal mol K , and a free energy of activation (
) 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
1
(confirmed by H 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
ΔH ‡
–1
constants from 5 ℃ to 35 ℃ gave us an Ea of 2.1(1) kcal mol , a
of
Δ푆 ‡
Δ퐺 ‡
–1
–1 –1
1.5(1) kcal mol , a
of –43.4(3) cal mol K , and a
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
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•.
at 25 ℃ was found. Determination of the rate constants from 15 ℃ to
훥H ‡
훥푆 ‡
–1
–1
55 ℃ gave us an Ea of 7.3(1) kcal mol , a
of 6.6(1) kcal mol , a
of –
훥퐺 ‡
–1 –1
42.7(5) cal mol K , and a
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.
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
Exp. Reaction
Exp. Activation
Calc.
Difference in
Electrophilicity
퐸푎)
훥H
Energy (
Enthalpy (
)
(kcal mol–1)
(kcal mol–1)
(|∆ω|)
(eV)
eq 1
eq 2
eq 3
–21.4
–10.0
–11.4
9.5
2.1
7.3
1.6
2.1
0.43
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Polar effects: The spin density in the three radicals involved in Scheme 1 is
ASSOCIATED CONTENT
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
1
2
3
4
5
6
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.
than with an electrophilic H• donor (and vice versa) – the basis of Polarity
Reversal Catalysis.
7
8
9
The best measure of electrophilicity seems to be the Global Electrophilicity
AUTHOR INFORMATION
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
Corresponding Author
*E–mail: jrn11@columbia.edu
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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).
*E–mail: grimme@thch.uni–bonn.de
§Current address: Corporate R&D, The Dow Chemical Company, 400 Arcola
Road, Collegeville, PA 19426
ORCID
휇2
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
휔 =
(4)
2휂
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.
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.
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.
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