Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex Page: 9,806
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Journal of the American Chemical Society
JH = -7.0(2)
AG = -0.9(3) letBu
1H in kcal/mol
AG in kcal/mol at 298 K _H = x - 7(1)
H = -14. 5(6)
AG = -5.6(9)
Figure 9. Thermodynamic square relating pyridine binding to HAT
enthalpies and free energies.
low-lying high-spin (S = 2) excited state.8s In these systems,
computations suggest that spin crossover is important at the
HAT transition state, since the reaction on the quintet surface
has a lower barrier than the triplet surface.83 Thus, an accessible
high-spin state is thought to facilitate HAT in these complexes.
Magnetic susceptibility, EPR, and computational data show that
the ground state in compound 1 is intermediate spin (S = 3/2),
and the computational model places the high-spin state higher in
energy (+12 kcal/mol). Addition of pyridine to 1 does bring the
S = 3/2 and S = 5/2 spin states to similar energy (computed to be
within 2 kcal/mol); however, in contrast to the octahedral
oxoiron(IV) systems, the calculated energy of the lowest transi-
tion state for HAT by 1 tBupy does not differ significantly by spin
state. Therefore, our computational results do not support the
idea of spin-state-dependent reactivity in this iron system.
Hypothesis 4: HAT by 1 -tBupy is More Exothermic be-
cause tBupy Binds to 2 More Strongly Than to 1. The
Hammond postulate86 suggests that the faster HAT rate for
1 tBupy might follow from a greater driving force for HAT in the
pyridine adducts. This driving force may be quantified by
comparing the thermodynamics of tBupy coordination to 1
and to 2 and subsequently constructing a thermodynamic square
(Figure 9). Using the same method described above for quantifying
the equilibrium of 1 and 1- tBupy, we determined Keq for tBupy
coordination to 2 over the temperature range of 25-110 C
in toluene-d8. The van't Hoff plot (Supporting Information
Figure S-11) gives AHeq = -14.5(6) kcal/mol and ASeq =
-30(1) cal/mol* K for the equilibrium of 2 and 2- tBupy, which
corresponds to AGeq = -5.6(9) kcal/mol at 298 K. Because of
the constraint imposed by the thermodynamic square, HAT by
1- tBupy must therefore be 5(1) kcal/mol more exergonic and
7(1) kcal/mol more exothermic than HAT by 1 at 298 K. Thus,
pyridine binding indeed makes HAT more thermodynamically
We also sought to quantify the effect of tBupy on AH* and
AG* of the HAT reaction, to test whether or not the kinetic effect
of tBupy on the HAT rate follows the thermodynamic effect of
stronger binding to 2 than 1. Thus, we performed HAT reactions
between 1 and CHD without tBupy. An Eyring plot of the second-
order rate constant kinter for this reaction between 25 and 55 C
(Supporting Information Figure S-12) gives the activation para-
meters for the HAT reaction without tBupy of AH* = +12.2(3)
kcal/mol, AS* = -33(2) cal/mol*K, and AG* = +22(1) kcal/
mol at 298 K. Thus, the addition of tBupy results in a drop in
HAT barrier height by AAGt = -5(2) kcal/mol, which is the
similar to the thermodynamic driving force. In summary, the
magnitude of change in thermodynamics for the HAT reactions
with and without pyridine is sufficient to explain the change in
rate, if the thermodynamic stabilization of binding in the product
is already realized in the transition state.
Thus, a combination of experimental data, computational
results, and chemical principles support both hypotheses 1 and 4.
In hypothesis 1, coordination of a fourth ligand gives weakening
of the Fe=NR bond. In this explanation, the basicity of the
pyridine is "relayed" through the iron to the imido group, which
abstracts a hydrogen atom through a transition state with proton
transfer character. It is reasonable that the current system has at
least as much dependence on basicity as the well-studied oxoiron-
(IV) species,84 because of the lower formal oxidation state of
these imido complexes (Fe3 vs Fe4+) and the lesser electro-
negativity of nitrogen as compared to oxygen.87 In hypothesis 4,
HAT becomes faster with an added pyridine donor because
coordination of pyridine to the product amido complex 2 is
stronger than coordination to imido 1, providing a thermody-
namic driving force for HAT. A explanation similar to hypothesis
4 for rate enhancement by added ligands was presented recently
in the context of oxomanganese chemistry.79
Limitations and Implications. How potent is the iron(III)
imido complex for HAT? A clue comes from the observation that
the intramolecular HAT reaction breaks a C-H bond of the
diisopropylphenyl substituent in the f3-diketiminate ligand. As-
suming the BDE of the isopropyl methine C-H is similar to that
in cumene (~86 kcal/mol), 1 tBupy is thermodynamically
capable of breaking C-H bonds significantly stronger than those
in CHD (71 kcal/mol). However, the "effective concentration"
of isopropyl groups in 1 tBupy is huge, since there are always
four methine protons near the metal. Thus, even added sub-
strates with a similar BDE would need to be in a large excess in
solution to compete kinetically with intramolecular ligand activa-
tion. It is also possible that fast trapping of the benzylic radical by
the diketiminate backbone drives the intramolecular HAT reac-
tion. Current efforts are devoted to the design of ligands that will
not be attacked by the reactive imido fragment, in order to
harness the full thermodynamic potential of imidoiron(III)
species for C-H activation reactions.
Although ligand activation limits the substrate scope of inter-
molecular HAT by 1- tBupy, this work is important because it
shows that sterically bulky, weak-field supporting ligands can
produce low-coordinate complexes that are reactive toward
homolytic C-H bond cleavage. It is noteworthy that other late
transition metal oxo and imido complexes that are reactive
toward HAT are also supported by weak-field ligand frame-
works,12,2sd,31 suggesting that the use of weak-field (Jr-donor)
ligands may be a general advantage in the design of ligand
systems for productive HAT processes.
The ability of bulky ligands to stabilize a reactive Fe=NR
species has enabled the first in-depth study of the mechanism of
H-atom transfer (HAT) reactivity of an imidoiron complex. The
imido complex LMeFe=NAd (1) reversibly coordinates tBupy,
forming the highly reactive four-coordinate imido complex
LMeFe=NAd(tBupy) (1-tBupy). Complex 1-tBupy activates
small substrates with weak C-H bonds (e.g., CHD and indene),
as well as the methine C-H on its own supporting ligand. In each
case, the iron-amido complex resulting from HAT was observed.
Significantly, 1- tBupy abstracts hydrogen atoms even at -51 OC.
Mechanistic studies were used to elucidate the details of the
HAT reaction. Large H/D KIEs of > 80 in the intermolecular
dx.doi.org/10.1021/ja2005303 IJ. Am. Chem. Soc. 2011, 133, 9796-9811
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Cowley, Ryan E.; Eckert, Nathan A.; Vaddadi, Sridhar; Figg, Travis M.; Cundari, Thomas R., 1964- & Holland, Patrick L. Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex, article, May 12, 2011; [Washington, D.C.]. (digital.library.unt.edu/ark:/67531/metadc107786/m1/11/: accessed October 18, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.