Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex Page: 9,802
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Journal of the American Chemical Society
Table 5. Bond Lengths and Angles of the QM/MM-Optimized
Geometries for S = 3/2 and S = 5/2 Spin States of 1*tBupy
Fe=Nimido- C (deg)
Figure 7. QM/MM-optimized geometries of (a) quartet 1 and (b)
sextet ltBupy. Hydrogen atoms are removed for clarity. Key: iron, light
pink; nitrogen, dark pink; carbon, gray.
Table 4. Comparison of Bond Lengths and Angles between
the Crystal Structure of 1 and the QM/MM-Optimized
Geometries for S = 3/2 and S = 5/2 Spin States
S = 3/2 S = 5/2
a Ref 38.
of only 0.2 kcal/mol, slightly in favor of the quartet. Because
these are close to the same energy, both must be considered as
potential ground-state candidates. The optimized HS model of
1-tBupy is shown in Figure 7b, and the optimized IS model is
geometrically similar. Both spin states of 1-tBupy have an iron
coordination geometry that is distorted toward trigonal pyramidal.5s
The metrical parameters are given in Table 5. It is notable that
regardless of spin state, the Fe=N bonds are significantly longer in
the four-coordinate imido models (1.76 and 1.77 A) than the
quartet three-coordinate imido model (1.70 A). These metrical
parameters reflect weaker iron-nitrogen tr-bonding in the four-
Spin density was observed on the imido nitrogen in each
complex. The nitrogen spin density is much greater in the S = 5/2
states (1.08 e for 1- tBupy; 1.07 e for 1) than in the S = 3/2
states (0.44 e for 1 - tBupy; 0.26 e for 1). The spin localization
on the imido ligand suggests that the sextet states of both 1 and
1- tBupy may be alternatively described as iron(II) complexes
with an imidyl radical anion ligands, (Fe2+)/(NAd'-).27c
Computations: Iron(ll) Amido Complexes and N-H Bond
Enthalpies. The amido complexes 2 and 2- tBupy were modeled
with QM/MM methods in both triplet and quintet spin states.
Geometry optimizations of amido complexes utilized the same
QM/MM partitioning scheme applied to the full imido models,
with the amido proton also modeled quantum mechanically. Smaller
basis set QM/MM calculations (i.e., ONIOM(B3LYP/6-31+G-
(d):UFF)) indicated that the quintet amido product is more
stable than the triplet amido product by 12 kcal/mol for 2 and by
18 kcal/mol for 2* tBupy, which is consistent with their experi-
mental room-temperature magnetic moments (~5.5 psB)37'38 that
suggest quintet ground states. Hence, simulations of 2 and 2- tBupy
with a larger QM basis set, ONIOM(B3LYP/6-311++G(d,p):
UFF), were performed only on the quintet spin state.
The geometries of LRFe-NHR' have been found by crystal-
lography to be quite sensitive to the particular amido (R' = tBu,
Ph, tolyl, 2,6-Me2Ph, 2,6-iPr2Ph) and /3-diketiminate (R = Me,
tBu) substituents, as well as the presence or absence of a fourth
ligand (THF, tBupy),S2 which provides an opportunity to test the
geometric predictions of the computations on 2 and 2- tBupy.
For the optimized structures of 2 and 2- tBupy, there is asym-
metry in the Ndiket.-Fe-Namido angles, large Fe-Namido -CAd
angles (~136-1370), and corresponding small Fe-N-H an-
gles ( ~1130). Table 6 compares the calculated bond lengths and
angles of 2 and 2-tBupy to those determined experimentally
from their crystal structures.3738
The computed N-H homolytic BDEs are derived from the
enthalpies of the amido and imido complexes and the calculated
enthalpy of H' (eq 8).
LMeFe(NAd)(tBupy)0o/1 + H' - LMeFe(NHAd) (tBupy)0/1
dx.doi.org/10.1021/ja2005303 IJ. Am. Chem. Soc. 2011, 133, 9796-9811
S = 3/2 model
S = 5/2 model
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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/m1/7/: accessed June 22, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.