Cooperativity Between Low-Valent Iron and Potassium Promoters in Dinitrogen Fixation Page: 7,546
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Cooperativity Between Low-Valent Iron and Potassium Promoters in
Travis M. Figg, Patrick L. I land, '' and Thomas R. Cundari'
Department of Chemistry and Cencter for Advanced Scientific (lComputing and Modeling (CASCa\), University of North T exas,
Denton, Texas 762.0 3-5017, United States
:Ie )partmenOt olfhemistry, University of Rochester Rochester, Rochest Ne York 1.127, United States
'ABSTRAC(F: A density functional theory (DIi I) study was [ e
performed to understand the role of cooperativity between iron- N N ,N
dikettiinate f tra ments aindl potassium prolotcrs in N, Fe Fe
activation. Sequential addition of iron ftragmeints to N, reveals , e NN-F ---- K N Fe
that a iinimium of three L;e centers interact with N, in order to r Fe
break the triple bond. The potassium promoter stabilizes the N
ligand formed upon N, scission, thus making the activated iron I)
nitride complex more energetically accessible. Reduction of the
complex and stabilization of N3 by K* have similar impact on the energetic in the gas phase. However, uLpon inclusion of
continuum T'HI solvent efects, coordination of K+ has a reduced influence upon the overall energetic of dinitrogein fixation;
thus, redLiction of the trimetallic lFe complex becomes more impactfLi] than coordination of K vis a-vis N, activation upon the
inclusion of solvent effects.
Conversion of dinitrogcen (N) into Lseftll materials is desired
fl'or uses s ucll as the production of ammonia (NH0), one of the
most important chemicals used in synthetic terti]izers.1
However. N, is diflticL]t to activate, because of the inherent
strength of the N N triple bon d ( 235 kcal mol)1. The
dominant industrial method for the reductive cleavage ot N,
and formation of N11 is the catalytic reduction of N, with
dihydrogen (H1,) via the H1aber osch process. Because of its
low cost, iron is commonl- used to catalyze the 1-laber Bosch
process.- PotassiLm promoters ilnprove the catalytic activity of
iron surfaces, partially because of an increase in the rate
constant for N, dissociation on the iron surface. in synthetic
compul nds, cooperative binding o N, by iron and akali metal
ions has been shown to weaken the N N bond more than iron
alone, and this t rend has been extended to chromium,
cobalt, '6 and nickel.7 H1ow ever, these systems do not cleave the
N N bond. I urthe r progress in cooperative N, activation
requires better understanding of two key factors: (1) the
reductive cleavage of the N, bond, and (2) the role of
promoters such as potassium.
Recently, Holland and co-workers reported a soluble iron-
dik.etiminate ( Fe ) system that can cleave N, to give a
bis(nitride) intermediate (Figure 1). 1Relatively few fe- nitride
complexes have been reported in the literature that involve
more than two l"c centers interacting with nitride atomns, and
no others are derived f tron N,."' ']le complex in Figure 1
arises from cleavage oft N,, and has three Ie fragments
interacting directly with nitrites and a fourth lhe interacting
indirectly throuLgh a series of Cl and K interactions. Although
this system is not catalytic, the chemistry depicted in Fligure I is
ACS Publications .' 2012 American Chemical Society
A Ar - ,F-e. .-Fe.,
A ' Fe +j Kreductant rArm Ar
Figure 1. Structure of the soluble iron- -dik et]minate-nitride complex
formed upon cleavage of the N, triple bond. A - 2,(:6H-NIMe,.
a potential stepping stone to a better understanding of catalysts
for solution-phase N fixation.
in this study, density tLunctional theory (1i)1i') calculations
are employed to understand the role of cooperativity between
multiple iron- -diketimin ate fragments. lor example, ho
many Fe atoms are needed to cleave the N, bond in the
reduction step. iand what intermediates are potentially involved?
lheoretical calculations have greatly aided in understanding
N N bond cleavage., i lhe research reported here indicates
that interaction of N, with more metal centers increases the
N N activation, and thus N, fixation benefits from cooperation
betwe en metals. lhe present calculations also yield insight into
the effects of K promoters in N, fixation.
ReceivedI: January 19, 2012
Published: June 26, 2012
dx.doi.orq/10.1021ic300150ullInorg. Chem. 2012, 51, 7546 7550
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Figg, Travis M.; Holland, Patrick L. & Cundari, Thomas R., 1964-. Cooperativity Between Low-Valent Iron and Potassium Promoters in Dinitrogen Fixation, article, June 26, 2012; [Washington, D.C.]. (digital.library.unt.edu/ark:/67531/metadc177427/m1/1/: accessed March 27, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.