Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex Page: 9,798
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Journal of the American Chemical Society
Table 1. Values of Keq for Binding of Various Para-Substi-
tuted Pyridines to the Imido Complex la
"a Data obtained in C7D8 with [Fe] = 18 mM.
3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6
Although pyridine-free 1 is stable for several weeks at -45 OC,
1 -tBupy is significantly less stable and decomposes to the
amidoiron(III) complex (LMe*)Fe(NHAd) (tBupy) (3 tBupy)
within a few hours at room temperature.37 In this reaction, the
LMe ligand undergoes intramolecular C-C coupling to form an
asymmetric dianionic ligand (designated LMe* here), presumably
by way of HAT (Scheme 2). A solution of 1 - tBupy also rapidly
reacts with 1,4-cyclohexadiene (CHD) to afford the amidoiron-
(II) complex LMeFe(NHAd)(tBupy) (2*tBupy) and benzene
(Scheme 3).37 Kinetic studies presented below are consistent
with the mechanisms in Schemes 2 and 3.
Thermodynamics of Pyridine Coordination to 1. A neces-
sary first step in the mechanistic inquiry is to quantify the
equilibrium between 1 and 1 tBupy. The equilibrium constants
for binding of tBupy to compound 1 were explored by evaluating
the 1H NMR signals of mixtures of 1 and tBupy as a function of
[tBupy]. Plotting the change in chemical shift versus [tBupy]o
and fitting the data to a standard weak-binding equation41 gave
values for Keq in the temperature range -51 to +40 OC (Table 1).
The van't Hoff plot (Figure 1) gives values of AHeq =
-7.0(2) kcal/mol and ASeq = -20.6(6) cal/mol K, which
are reasonable values for the proposed equilibrium: the exother-
mic AHeq value is consistent with a new bond being formed, and
the negative ASeq value is consistent with combining two mol-
ecules into one. At 298 K, these values correspond to AGeq =
-0.9(3) kcal/mol, a value that indicates very weak coordination.
Keq values for 4-phenylpyridine, 4-(dimethylamino)pyridine, and
4-(trifluoromethyl)pyridine were also determined at 40 OC. Table 1
shows that more electron-donating pyridines bind more strongly to
1, as expected from their greater basicity.
Kinetic Studies of the Intramolecular HAT Reaction. The
intramolecular hydrogen atom abstraction reaction (conversion
of 1* tBupy to 3 tBupy, Scheme 2) was studied by 1H NMR
spectroscopy in C6D6 at 40 OC with  = 29 mM. The progress
of the reaction was followed by monitoring the disappearance of
1 relative to a capillary integration standard.42 The concentration
of 1 decreased over time, and the relative integrations were fit to a
first-order exponential curve43 to obtain the observed pseudo-
first-order rate constant kobs. A plot of kobs versus [tBupy] is
shown in Figure 2. At low [tBupy], the observed rate has a near
linear dependence on [tBupy], but the observed rate begins to
saturate at [tBupy] 0.2 M. The observed saturation kinetics
indicate a rapid pre-equilibrium involving tBupy association prior
to the rate determining step. The rapid equilibrium is consistent
with the dynamic averaging of signals in the 1H NMR spectrum
of 1 and 1- tBupy.
103 T-1(103. K1)
Figure 1. van't Hoff plot for tBupy binding to 1 over a temperature
range of -51(1) to +40(1) C. Data obtained with [Fe] = 19 mM and
[tBupy] = 0-0.53 M in toluene-d8.
0.0 0.20 0.40 0.60 0.80 1.0
Figure 2. Observed rate of intramolecular HAT as a function of tBupy
concentration. Data obtained in C6D6 at 40(1) OC with [Fe] = 29 mM.
The dashed line represents the fit to eq 3.
The intercept near the origin is consistent with the observation
that decomposition of 1 proceeds at a greatly decreased rate at
40 oC without pyridine. When combined with the dependence of
kobs on [tBupy], this indicates that 1 . tBupy, and not 1, is the active
species in the HAT reaction. These data imply the rate law in eq 2,
where kintra is the first-order rate constant of the elementary HAT
kintra [1 tBupy]
Since tBupy coordinates weakly to 1 at 40 OC (Keq = 2.5(1) M-1,
see above), we can use the weak-binding approximation
[1 tBupy]eq ' (Keqo[tBupy]o)/(1 + Keq[tBupy]o) to obtain
the rate law in eq 3, which expresses the rate in terms of the known
dx.doi.org/10.1021/ja2005303 IJ. Am. Chem. Soc. 2011, 133, 9796-9811
+ tBupy 1 tBupy
AH = -7.0(2) kcal/mol
AS = -20.6(6) cal/mol-K
AG = -0.9(3) kcal/mol
1 + tBupy < 1-tBupy
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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/m1/3/: accessed March 28, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.