Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex Page: 9,803
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Journal of the American Chemical Society
Table 6. Comparison of Calculated (QM/MM) and Experimental Bond Lengths and Angles for 2 and 2 tBupy
Fe-Namido (A) 1.88 1.860(2)
Fe-Ndiket. (A) 1.97; 1.99 1.974(2); 1.986(2)
Ndiket.-Fe-Namido (deg) 121; 145 121.04(8); 145.86(8)
Ndiket.-Fe-Ndiket. (deg) 94.4 93.09(3)
Fe-Namido- CAd (deg) 136 134.7(2)
a From ref 38(CSD refcode RUXBIW). b From ref 37 (CSD refcode XEPHEG).
LMeFe(NHAd)(tBupy) (2. tBupy)
For quintet 2, the N-H homolytic BDE is calculated to be
89 kcal/mol relative to the quartet ground state of 1. For quintet
2- tBupy, the N-H bond enthalpy is calculated to be 85 kcal/
mol relative to the sextet ground state of 1 tBupy.
Computations: HAT Transition States. Transition states for
C-H activation of CHD by 1 and by 1-tBupy were evaluated
with hybrid DFT/MM methods for both S = 3/2 and S = 5/2
spin states. Figure 8 depicts the quartet transition state for C-H
activation of CHD by 1-tBupy, which was slightly lower in
enthalpy (6.4 kcal/mol) than the corresponding sextet transition
state. In each transition state, the Fe-Nimido bond has length-
ened to 1.83-1.90 A, and the Fe- * *N-C linkage has become
more bent at 139-1430. The key three-center N*.. H*.. C
interaction shows a similar geometry regardless of spin state or Fe
coordination number, with a linear N* * H* * *C angle (175-
1770) and a slightly longer N* *. H (1.38-1.48 A) distance than
the C*- *H (1.23-1.33 A) distance.
Interestingly, the barriers for HAT are calculated to be
similar for the quartet and sextet transition states. So, even
though sextet 1- tBupy has greater spin density on the imido
nitrogen atom in the ground state than quartet 1 - tBupy, it does
not show a marked improvement in the kinetics of hydrogen
atom abstraction. Additional evaluation of the barrier using
other functionals and basis sets (BP86, M06, B2LYP, and
MPWPW91 with the 6-311++G(d,p) basis set and B3LYP
with 6-31+G(d) and the 6-311++G(d,p) basis sets) gave no
clear preference, with the sign and magnitude of the relative
quartet versus sextet barrier both being very sensitive to these
details of the computation.
C-H Bond Dissociation Energies: Homolytic and Hetero-
lytic. For a complete analysis of the HAT reaction, C-H BDEs
for each substrate are needed. However, to our knowledge there
are no literature values for the substituted cyclohexadienes used
in the experiments above. Typically, there is only a small effect in
the derived C-H BDE from peripheral substitution by hydro-
carbyl groups, so the BDE values for methyl-substituted CHD are
likely near the value for CHD. For example, xanthene (75.5) and
9-phenylxanthene (76.2) have nearly identical derived BDE
values, 3 as do 9,10-dihydroanthracene (83.0) and 1,4,5,8-tetra-
methyl-9,10-dihydroanthracene (83.3).54 Notwithstanding, multiple
values for a single BDE are often encountered in the literature
since various methods are used in their determinations.ss Thus,
we chose to calculate the BDE values for each substrate in this
study using the B3LYP/6-311++G(d,p) level of theory (gas-
phase simulations; enthalpies determined at 298.15 K and 1 atm)
in order to have self-consistent and comparable numbers for all
substrates. The computed C-H BDE values for each substrate
are given in Table 3 above.
Figure 8. Quartet QM/MM-calculated transition state for C-H acti-
vation of CHD by 1* tBupy.
We also sought the pKa values for the substrates used in this
study, especially to compare indene to the cyclohexadiene series.
Empirical pKa values are available for indene (20)56 and DHA
(30)57 in DMSO, but the corresponding values for the substi-
tuted cyclohexadienes are, to our knowledge, unknown. There-
fore, we computed the gas-phase enthalpy of heterolytic bond
cleavage to give R and H+ using the same B3LYP/6-311++G-
(d,p) level of theory employed for homolytic BDEs. The
computed heterolytic C-H BDE values for each substrate are
given alongside the homolytic BDEs in Table 3 above.
The calculated heterolytic BDEs are in excellent agreement
with the available compiled enthalpic data.58 The heterolytic
BDE for 1,3-cyclohexadiene is given as 373.3 4.1 kcal/mol, 9,60
which compares favorably with the calculated value of 375 kcal/mol
for CHD. Two experimental evaluations of indene place its hetero-
lytic BDE at 354.3 2.5 kcal/mol61 and 351.9 2.1 kcal/mol,62
both of which agree well with the B3LYP/6-31 1++G(d,p) value
of 351 kcal/mol. Hence, the agreement between computations
and the extant data is excellent with respect to both trends and
Mechanism of C-H Activation by An Iron(lll) Imido Complex.
Iron(IV) oxo species have been established to abstract hydrogen
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/m1/8/: accessed February 25, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.