Alkali Metal Variation and Twisting of the FeNNFe Core in Bridging Diiron Dinitrogen Complexes

Article abstract of DOI:10.1021/acs.inorgchem.5b02841

Alkali metal cations can interact with Fe-N₂ complexes, potentially enhancing back-bonding or influencing the geometry of the iron atom. These influences are relevant to large-scale N₂ reduction by iron, such as in the FeMoco of nitr

Full text of DOI:10.1021/acs.inorgchem.5b02841

Article
pubs.acs.org/IC
Alkali Metal Variation and Twisting of the FeNNFe Core in Bridging
Diiron Dinitrogen Complexes
Sean F. McWilliams,† Kenton R. Rodgers,‡ Gudrun Lukat-Rodgers,‡ Brandon Q. Mercado,†
Katarzyna Grubel,†,§ and Patrick L. Holland*,†
†Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States
‡Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58105, United States
S
* Supporting Information
ABSTRACT: Alkali metal cations can interact with Fe−N2 complexes, potentially
enhancing back-bonding or influencing the geometry of the iron atom. These
influences are relevant to large-scale N2 reduction by iron, such as in the FeMoco of
nitrogenase and the alkali-promoted Haber−Bosch process. However, to our
knowledge there have been no systematic studies of a large range of alkali metals
regarding their influence on transition metal−dinitrogen complexes. In this work, we
varied the alkali metal in [alkali cation]2[LFeNNFeL] complexes (L = bulky β-
diketiminate ligand) through the size range from Na+ to K+, Rb+, and Cs+. The
FeNNFe cores have similar Fe−N and N−N distances and N−N stretching
frequencies despite the drastic change in alkali metal cation size. The two
diketiminates twist relative to one another, with larger dihedral angles accommodating
the larger cations. In order to explain why the twisting has so little influence on the
core, we performed density functional theory calculations on a simplified LFeNNFeL
model, which show that the two metals surprisingly do not compete for back-bonding to the same π* orbital of N2, even when
the ligand planes are parallel. This diiron system can tolerate distortion of the ligand planes through compensating orbital energy
changes, and thus, a range of ligand orientations can give very similar energies.
examples of iron−potassium−N2 complexes, which would test
this idea and explore it in atomic detail, are rare.9,12,13
INTRODUCTION
■
Nitrogen is an essential element for life. However, its most
abundant form, N2, contains a strong N−N triple bond (226
kcal/mol) that must be cleaved to provide biologically available
nitrogen compounds such as ammonia.1 Nitrogen fixation, the
process of converting dinitrogen to ammonia, occurs mainly in
two ways: a natural process in nitrogenase enzymes at a
predominantly iron cofactor and a high-temperature, high-
pressure process (Haber−Bosch) that most commonly uses a
reduced heterogeneous iron catalyst with aluminum and
potassium dopants.2−4 Atomic resolution of the mechanism
for N−N bond cleavage and subsequent ammonia formation
remains elusive for each process.5
Studies on iron surfaces have led to an understanding of the
microstructure of the iron catalyst for the Haber−Bosch
process.3 Experiments on single crystals of Fe with known
crystal facets have shown that the Fe(111) surface is the most
active for catalysis under high-pressure conditions.6 Experi-
ments on these idealized Fe surfaces show that small
concentrations of potassium on the surface decrease the work
function of the solid (i.e., raise the energy of the highest-lying
electrons), implying that electron donation from the surface to
bound dinitrogen species would be easier.7 In molecular terms,
this corresponds to stronger back-bonding from higher-energy
Fe d orbitals into N2, resulting in a weakened N−N bond.
Adsorption of potassium on these promoted iron surfaces
enhances the binding of N2 to the surface.8 However, molecular
Hundreds of examples of transition metal−N2 complexes
have been reported in the literature, and they are known with
most transition metals.1,10 Only a small percentage of these also
contain an interaction between the bound N2 moiety and an
alkali cation, limiting our understanding of how these positively
charged ions effect N−N bond activation.11−13 Most of the
complexes with interactions between the alkali cation and N2
have an end-on (η1) interaction.11 In several examples, the end-
on coordination by the alkali cation is disrupted by
sequestration of the cation by multiple crown ethers (Figure
1) or by cation exchange to a noncoordinating ion, which
increases the energy of the N−N stretching vibration by 20−40
cm−1.11 This difference in stretching frequency implies that
positively charged redox-inactive alkali cations activate the N2
unit when bound in an end-on fashion.
Less common are coordination compounds with a side-on
(η2) interaction between alkali cation(s) and a transition-metal-
bound N2 unit.12−14 These may be relevant to the cleavage of
N2 on iron surfaces because the precursor to N−N cleavage on
the surface has been shown to have the N−N bond roughly
parallel to the surface,15 and in this geometry side-on
interactions of surface atoms with N2 are likely. In many of
Received: December 10, 2015
© XXXX American Chemical Society
A
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For all three transition metals (Fe, Co, Ni), both sodium and
potassium analogues have been synthesized using the larger β-
17
diketiminate ligand LtBu
.
In the Co system, the Na+ and K+
analogues had similar N−N bonds, differing by less than 0.01 Å
in the observed N2 bond length and only 1 cm−1 in the
observed N−N stretching frequency.12b In contrast, varying the
alkali metal from Na+ to K+ in the Fe and Ni systems caused
increases in the observed N−N stretching frequency by 6 and
14 cm−1, respectively.12a,c,d,13 A mixed-alkali species with both
Na+ and K+ was isolated in the Ni system and exhibited an N−
N stretching frequency that lies between those of the K+ and
Na+ analogues.12d However, in each of these systems there were
only two data points: Na+ and K+. Studies of the Haber−Bosch
process have shown that the promotion effect increases in the
order Na < K < Rb < Cs,2,18 which indicates that the heavier
alkali metals could be beneficial.
Here, for the first time, we explore the alkali metal trend in a
series of analogous M−N2 complexes with each alkali metal
from Na+ to Cs+. We show that there is an interesting interplay
between the size of the alkali metal ion and the dihedral angle
between the diketiminate ligands on the two metals of the
FeNNFe core. We evaluate the effects of twisting the core on
the frontier orbitals of the FeNNFe core using density
functional theory (DFT) calculations. Interestingly, despite
the drastic change in the cation size and the twisting of the
core, the extent of N−N bond weakening is very similar
throughout the series.
Figure 1. An example of the effect of sequestration of a terminally
bound alkali metal cation on the N−N stretching frequency.11f
the molecular compounds with side-on alkali interactions, the
alkali cations are labile, which impedes the study of the
influence of coordination of these cations on N−N bond
activation.14
β-Diketiminate-supported bridging N2 complexes are partic-
ularly amenable to such investigations because they have been
studied systematically.16 In the Fe, Co, and Ni systems, binding
of N2 occurs upon reduction of MII halide precursors to the MI
oxidation state, yielding end-on/end-on bridging dinitrogen
complexes (Figure 2a).12,13 Further reduction by two electrons
RESULTS AND DISCUSSION
■
Synthesis and Characterization. In order to better
understand the role of the cations in FeNNFe complexes, a
series of compounds were synthesized from iron(II) halide
complexes of LMe using the alkali metal reductants Na, KC8,
RbC8, and CsC8 (Scheme 1). Synthesis of the K+ analogue
a
Scheme 1. Synthesis of 2-AM (AM = Na, K, Rb, Cs)
Figure 2. (a) Formally MI β-diketiminate-supported dinitrogen
complexes. (b) Formally M0 β-diketiminate-supported dinitrogen
complexes with alkali metal (AM) cations side-on to the N2 unit.12,13
yields formally M0 complexes with sodium or potassium bound
side-on to the N2 and to the aryl rings of the β-diketiminate
ligands (Figure 2b).12,13 The alkali metal cations in these
complexes are not coordinated by solvent in the solid-state
structures, and there is no evidence for cation lability in
noncoordinating or weakly coordinating solvents. The lack of
lability of these cations and their close proximity to the N2 unit
make these systems ideal for studying the influence of alkali
metal cations bound side-on to dinitrogen in iron complexes.
Complexes having each of these three transition metals in
both the +0 and +1 oxidation states have been isolated and
exhibit weakening of the N−N bond compared with free
dinitrogen, as observed by crystallography and vibrational
spectroscopy.12,13 In each case, the formally M0 complex
contains a substantially more activated N2 unit (200−500 cm−1
lower stretching frequency) than the formally MI complex.12b,d
a
Reaction conditions: (i) 4AMC8 (AM = K, Rb, Cs), pentane or
diethyl ether; (ii) excess Na0, diethyl ether.
K2[LMeFeNNFeLMe] (2-K) was previously reported through
reduction of [LMeFe(μ-Cl)]2 with 4 equiv of potassium/
graphite (KC8) in pentane under N2.13 Here we synthesized 2-
K from [LMeFe(μ-Br)]2 (1) instead by a procedure adapted
from those previously reported methods.13,19 Reduction of
[LMeFe(μ-Br)]2 with 4 equiv of rubidium/graphite (RbC8) in
pentane under N2 and cesium/graphite (CsC8) in diethyl ether
under N2 led to the formation of the dark-green complexes
B
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Rb2[LMeFeNNFeLMe] (2-Rb) and Cs2[LMeFeNNFeLMe] (2-
Cs), respectively. Stirring [LMeFe(μ-Br)]2 in diethyl ether with
excess sodium metal yielded the dark-green complex
Na2[LMeFeNNFeLMe] (2-Na). Samples of 2-Rb and 2-Na
gave satisfactory microanalytical results, while 2-Cs was
1
consistently low in N; however, the H NMR spectra were all
comprehensible using averaged D2h or D2d symmetry. Solid-
state molecular structures were obtained from crystals of each
compound that were grown from diethyl ether or pentane, and
thermal ellipsoid plots are shown in Figures 3 and 4. Sonication
Figure 4. Molecular structure of 2-Na. Two different conformations
are present in the asymmetric unit: one with the sodium cations
centered between the aryl rings of the supporting ligands (2-Na
(centered), top) and one with the sodium cations off-center between
the aryl rings of the supporting ligand (2-Na (not centered), bottom).
Thermal ellipsoids are displayed at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg) for 2-Na (centered): Fe(2)−N(2) 1.725(3),
Fe(2)−N(31) 1.918(3), Fe(2)−N(41) 1.923(3), N(2)−N(2)*
1.253(6), Na(4)−N(2) 2.524(3), Na(4)−N(2)* 2.512(3), N(31)−
Fe(2)−N(41) 94.76(11), N(2)−Fe(2)−N(31) 131.74(13), N(2)−
Fe(2)−N(41) 133.41(13). For 2-Na (not centered): Fe(1)−N(1)
1.728(3), Fe(1)−N(11) 1.936(3), Fe(1)−N(21) 1.936(3), N(1)−
N(1)* 1.254(6), Na(3)−N(1) 2.566(3), Na(3)−N(1)* 2.454(3),
N(11)−Fe(1)−N(21) 93.15(11), N(1)−Fe(1)−N(11) 127.48(13),
N(1)−Fe(1)−N(21) 139.14(13).
Figure 3. Molecular structures of (top) 2-Rb and (bottom) 2-Cs.
Thermal ellipsoids are displayed at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg) for 2-Rb: Fe(1)−N(1) 1.740(7), Fe(1)−N(11)
1.928(6), Fe(1)−N(21) 1.938(6), Fe(2)−N(2) 1.741(7), Fe(2)−
N(31) 1.933(6), Fe(2)−N(41) 1.931(6), N(1)−N(2) 1.257(8),
Rb(1)−N(1) 2.908(7), Rb(1)−N(2) 2.869(6), Rb(2)−N(1)
2.918(7), Rb(2)−N(2) 2.931(7), N(11)−Fe(1)−N(21) 96.0(2),
N(1)−Fe(1)−N(11) 131.8(3), N(1)−Fe(1)−N(21) 133.2(3),
N(31)−Fe(2)−N(41) 95.0(2), N(2)−Fe(2)−N(31) 131.6(3),
N(2)−Fe(2)−N(41) 133.2(3). For 2-Cs: Fe(1)−N(1) 1.713(12),
Fe(1)−N(11) 1.933(11), Fe(1)−N(21) 1.930(10), N(1)−N(1)*
1.33(2), Cs(1)−N(1) 3.087(10), Cs(1)−N(1)* 3.093(11), N(11)−
Fe(1)−N(21) 95.6(4), N(1)−Fe(1)−N(11) 131.1(5), N(1)−Fe(1)−
N(21) 133.2(4).
state molecular structures range from 1.215(6) to 1.33(2) Å
and are substantially longer than that of free N2 (1.098 Å)
(Table 1).20 The N−N bond lengths from the crystal structures
are similar, except that the N−N bond in 2-Cs is about 0.1 Å
longer than that in 2-K. Unfortunately, the precision (and
probably the accuracy; see below) of the N−N bond length in
2-Cs is poor because of either libration in the core or
unavoidable Fourier truncation as a result of the large Cs atoms.
In order to gain a more reliable view of the N−N bond
weakening, we used resonance Raman spectroscopy with 406.7
nm laser excitation, as reported previously for 2-K.13 All four
compounds showed vibrations between 1612 and 1625 cm−1
that shifted to lower frequency in 15N2-enriched samples (Table
1). The low frequencies indicate significant N−N bond
weakening relative to N2 (2359 cm−1) (Table 1).21 The bond
lengths and stretching frequencies are similar to those in
diazene, HNNH (1.25 Å and 1583 cm−1, respectively),
suggesting that each complex has a N−N double bond.21a 2-Cs
or stirring of [LMeFe(μ-Br)]2 or [LMeFeNNFeLMe] with excess
Li0 under N2 yielded unidentifiable mixtures of compounds,
none of which are spectroscopically analogous to the other 2-
AM compounds.
The N−N distances in all of these complexes show
significant weakening of the bound dinitrogen, as previously
demonstrated in 2-K.13 The N−N bond lengths in the solid-
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Table 1. Fe−N and N−N Bond Lengths, N−N Bond Stretching Frequencies, and Mossbauer Parameters for Bridging Fe−N
̈
2
Complexes with Intercalated Alkali Cations
b
c
compound
Fe−N (Å)
N−N (Å)
νNN (cm−1
)
ν
(cm−1
15N15
N
)
δ (mm/s)
|ΔEQ| (mm/s)
ref
2-Na
2-K
1.728(3), 1.725(3)
1.750(4), 1.755(5)
1.740(7), 1741(7)
1.254(6), 1.253(6)
1.215(6)
1612
1625
1621
1559 [1557]
1569 [1570]
1567 [1566]
1566 [1558]
0.44
0.47
0.46
0.48
2.52
2.48
2.34
this work
13
2-Rb
1.257(6)
this work
a
a
2-Cs
1.711(12)
1.33(2)
1613
2.20
this work
a
b
c
These values are likely influenced by a systematic error (see the text). Experimentally observed 14N14N frequencies. Experimentally observed
15N15N frequencies; the values in brackets are the 15N15N stretching frequencies expected on the basis of the harmonic oscillator approximation.
does not have a lower frequency than the others, indicating that
the long N−N distance in the crystal structure is unlikely to be
accurate.
(1613 cm−1). These small changes and the indistinguishable
isomer shifts of the three complexes suggest that there is very
little change in back-bonding from Fe to the dinitrogen and
only small changes in N−N bond activation. On the basis of a
correlation between the N−N stretching frequency and the N−
N distance in transition metal−N2 complexes,21a the expected
N−N distance for 2-Cs is 1.202 Å. Thus, it seems most likely
that the crystallographic N−N distance suffers from a
systematic error, a conclusion that is consistent with the large
thermal ellipsoids for the N atoms that are visible in Figure 3.
We have reported that the diiron(I) complex
[LMeFeNNFeLMe] readily binds tetrahydrofuran (THF) to
form four-coordinate iron species and reacts with benzene to
give the iron(I) η6-benzene complex LMeFe(η6-C6H6).13 The
iron(0) analogues with the larger cations K+, Rb+, and Cs+ do
not bind THF and are stable in C6D6 with no change in the 1H
NMR spectrum over the course of 12 h. The reactions of 2-K
and 2-Rb with 2 equiv of 18-crown-6 result in a mixture of
products, none of which is similar spectroscopically to the
starting bimetallic species. Identification of these products is
ongoing and will be reported separately. Since the alkali metal
cations do not appear to influence the electronics or geometry
of the Fe−N2−Fe core and upon extraction of the cations the
bimetallic species is no longer present, we attribute the stability
of the complexes 2-AM to the four cation−π interactions
between the alkali cations and the arene rings. We have noted
similar stabilizing effects of alkali metal cations in highly
reduced Fe3(μ-N2)3 cores and FeSFe cores.23
The β-diketiminate supporting ligands can distort through
rotation to optimize the size of the binding pocket to
comfortably accommodate large cations, which provides
stability to the complexes. Intercalation of smaller cations
such as Na+ and Li+ led to the less stable analogue 2-Na and in
the case of Li+ gave an unidentifiable mixture of compounds,
none of which were spectroscopically similar to the 2-AM
compounds. The presence of two crystallographically in-
dependent molecules in the asymmetric unit of 2-Na offered
insight as to why the smaller-cation analogues are less stable. In
one molecule, 2-Na (centered), the Na+ ion is centered
between the aryl rings with an average Na−Caryl distance of
3.146 Å and the Na−N(N2) distances differ by less than 0.02 Å.
In the other molecule, 2-Na (not centered), the Na+ ion is
closer to one of the aryl rings with an average Na−Cclose
distance of 3.017 Å to the close arene ring and an average Na−
Cfar distance of 3.282 Å to the other arene ring. The Na−
N(N2) distances are asymmetric as well, differing by more than
0.1 Å. (It should be noted that in solution 2-Na exhibits seven
1H NMR resonances, which is consistent with averaged D2
symmetry resulting from exchange of the Na+ between the
positions observed in the crystal structure that is fast on the
NMR time scale.)
Each complex exhibits one quadrupole doublet in the
Mossbauer spectrum, indicating that the two iron centers
̈
have equivalent (or nearly equivalent) environments. The
isomer shifts (δ) are 0.44−0.48 mm/s, and the quadrupole
splitting values (|ΔEQ|) are 2.20−2.52 mm/s.
In the solid-state structures of these complexes, the
backbones of the β-diketiminate supporting ligands rotate
with respect to one another as the size of the intercalated cation
increases (Figure 5). The aryl groups also rotate to create a
Figure 5. End-on views of 2-AM, illustrating the twisting of the
FeNNFe core as the cation size changes. Torsion angles between the
Fe−N−C−C−C−N planes are shown. Isopropyl groups and hydro-
gens have been omitted for clarity. Thermal ellipsoids are displayed at
the 50% probability level.
larger distance between the centroids of the aryl rings, with an
increase of 1.1 Å from 2-Na to 2-Cs (Table 2). The larger size
of the alkali binding pocket is consistent with the larger ionic
radii of the alkali metals. The alkali metal−N2 distance also
lengthens through the series from 2-Na to 2-Cs because of the
increase in the ionic radius. However, these various changes in
the periphery of the complex do not correlate with the extent of
back-bonding from iron to N2, as judged by the N−N distances,
the N−N stretching frequency, and the Mossbauer isomer shift.
̈
The crystal structures of 2-K, 2-Rb, and 2-Cs demonstrate
the ability of the β-diketiminate-ligand-supported Fe2N2AM2
core to accommodate larger cations through rotation of the
ligand backbones and aryl groups to create a larger binding
pocket for the alkali metal cation. In all three complexes, the
alkali cations are centered between the aryl rings of the
supporting ligands and also centered above the N2 unit.
Though the N−N distance appears to be longer in 2-Cs,
resonance Raman spectra show only a small decrease in the N−
N stretching frequency in going from 2-K (1625 cm−1) to 2-Cs
D
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Table 2. Fe−Fe Distances, Distances from the Alkali Cations to N2 and the Aryl Rings, and Torsion Angles between the β-
Diketiminate Planes
Fe−Fe
alkali ion−arene centroid
centroid−centroid
torsion angle
alkali ion radius
a
c
complex
(Å)
N−N (Å) AM−N (Å)
(Å)
(Å)
(deg)
(Å)
b
2-Na (not centered)
4.709(1)
4.702(1)
4.715(2)
1.254(6)
1.253(6)
1.215(6)
2.566(3)
2.454(3)
2.524(3)
2.511(3)
2.753(6)
2.782(6)
2.792(6)
2.795(6)
2.918(7)
2.931(7)
2.908(7)
2.869(6)
3.097(11)
3.085(10)
2.675(2)
2.975(2)
2.824(2)
2.837(2)
2.886(3)
2.870(3)
2.944(3)
2.957(3)
2.953(3)
2.984(3)
3.041(3)
3.042(3)
3.141(6)
3.160(6)
4.864(2)
4.830(2)
0.0(2)
0.0(2)
1.02
1.02
1.38
b
2-Na (centered)
2-K
5.229(4)
5.400(4)
34.5(3)
2-Rb
4.736(2)
4.751(4)
1.257(6)
5.469(5)
5.675(5)
35.8(4)
50.6(6)
1.49
b
d
2-Cs
1.33(2)
5.975(8)
1.67
a
b
The torsion angle was determined by measuring the angle between planes formed from the Fe−N−C−C−C−N atoms of each ligand. Half of the
c
molecule is related by crystallographic symmetry to the other half, and therefore, only one set of distances is given. Values obtained from ref 22 for
hexacoordinate alkali cations are given. This value is likely influenced by a systematic error (see the text).
d
In the analogues with larger cations, the alkali metal ions
occupy a single position in the crystal structure centered
between the aryl rings and over the N2 unit. To better
understand why the Na+ compound was not similar to the
other three 2-AM complexes, we compared 2-Na with other
reported complexes with similar η6 interactions of two arenes
with a Na+ ion. A search of the Cambridge Structural Database
(CSD)24 of reported structures containing such Na+ species
revealed an average Na−Caryl distance of 3.060 Å to the aryl
rings. By comparison to the two molecules of 2-Na, the
noncentered molecule has a distance 0.04 Å shorter than the
average literature value, while the centered molecule has an
average distance 0.1 Å longer than the average literature value.
The presence of an off-center molecule in the crystal structure
and the unusually long cation−π interaction are consistent with
the idea that the Na+ cations are too small to fit ideally between
the aryl rings. This also explains the relatively low stability of 2-
Na.
MM′). Thus, 2-K, 2-Rb, and 2-Cs appear to be comparable in
stability. Attempts to exchange the cations in 2-Na were
unsuccessful because of the low stability of 2-Na in THF, so its
ability to exchange is unknown.
Computational Analysis of the Dihedral Angle
between Fe Planes. One of the surprising aspects of the
complexes 2-AM is that the extent of π back-bonding is similar
in all of the complexes despite substantial changes in the
dihedral angle between the two diketiminate planes (from 0 to
50.6°). One might have guessed that back-bonding would be
maximized when the two ligand planes are perpendicular,
because this relative orientation would place the highest-lying
half-occupied d orbital (the one in the plane of the diketiminate
N donors)25 of each Fe atom in a position where it does not
need to compete for the same π* orbital of N2. On the basis of
this reasoning, the orientation with coplanar diketiminates
should give back-bonding primarily into only one of the two π*
orbitals, leading to less N−N bond activation. However, no
correlation between core twisting and back-bonding is evident.
This surprising situation also holds for the previously
reported formally diiron(I) complexes LFeNNFeL (L = LMe,
LtBu). Three X-ray crystal structures of LFeNNFeL complexes
have been reported (Figure 6): one in which the LtBu ligands
are perpendicular,12c one in which the LMe ligands are
coplanar,13 and yet another where the LMe ligands are coplanar
but the iron atom is distorted from a Y shape to a T shape29
(the latter two structures are for the same compound, but the
crystals were grown at different temperatures and yielded
different unit cells and different solvents of crystallization).
Despite these marked structural differences, there is no sign
that the geometry causes substantial differences in N−N back-
bonding (Table 3). Though the N−N stretching frequency is
lower in the LtBu compound, this is primarily due to an
electronic effect, as shown by comparing the difference in the
stretching frequencies of [LtBuFeNNFeLtBu] and
[LMeFeNNFeLMe] having different orientations (Δν = 32
cm−1) with the difference in the stretching frequencies of
K2[LtBuFeNNFeLtBu] and K2[LMeFeNNFeLMe] with the same
orientation (Δν = 36 cm−1).
We were unable to isolate a Li+ analogue of 2-AM, which is
also consistent with the idea that smaller cations are unable to
interact strongly with both aryl rings profitably. By the same
approach as for Na+, a CSD search for η6-arene−(μ-Li+)−η6-
arene interactions revealed the average Li−C distance to be
2.497 Å in the reported structures. This value is 0.5 Å shorter
than the interactions observed in 2-Na. We hypothesize that Li+
is too small to hold the aryl rings together, and without
bridging cation−π interactions, the species is unstable (or at
least difficult to isolate).
To further understand the stabilizing effect of the cations and
discern the relative energies of the complexes, we tested the
ability to exchange the alkali cations. This was done by adding
triflate salts of other alkali metal ions to solutions of 2-K, 2-Rb,
1
and 2-Cs in THF and monitoring the mixtures by H NMR
spectroscopy. Each reaction yielded a mixture containing the
two previously characterized bimetallic complexes as well as a
new product with lower symmetry, as indicated by the number
1
of peaks in the H NMR spectrum. Each lower-symmetry
product exhibited 13 resonances and chemical shifts between
those for the two single-cation complexes, indicating the
formation of mixed alkali cation species (which we label 2-
E
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these deviations are the result of crystal packing influences or
difficulty in computationally reproducing the interaction
between the alkali metal ions and the π systems is uncertain,
but they prevented us from computationally querying the
influence of the alkali metal ion on the diiron(0) core through
validated computations.
On the other hand, geometry optimization of the neutral
compound LMe,MeFeNNFeLMe,Me (where LMe,Me is a model of
LMe in which the isopropyl groups are truncated to methyl
groups) at the BP86/TZVP(Fe,N,C)/SVP(H) level gave
excellent metrical agreement with the core of the crystallo-
graphic structure of LtBuFeNNFeLtBu (Figure 7). In addition,
Figure 7. Comparisons of the crystal structure of [LtBuFeNNFeLtBu
]
(black) to the BP86/TZVP(Fe,N,C)/SVP(H) geometry-optimized
structure of [LMe,MeFeNNFeLMe,Me] (red): (top) comparison of bond
lengths (Å); (bottom) comparison of bond angles (deg).
single-point calculations on the optimized septet geometry
(using the calibrated TPSSh functional)26 predicted Mossbauer
̈
parameters of δ = 0.56 mm/s and |ΔEQ| = 1.27 mm/s, in
excellent agreement with the experimentally observed values of
δ = 0.62 mm/s and |ΔEQ| = 1.41 mm/s. Therefore, more in-
depth computational studies on the dependence of the
electronic structure on the dihedral angle were pursued
through systematic variation of diiron(I) models.
Figure 6. (a) Crystal structure of [LtBuFeNNFeLtBu].12c (b) Y-shaped
crystal structure of [LMeFeNNFeLMe].13 Pentane of crystallization is
not shown. (c) T-shaped crystal structure of [LMeFeNNFeLMe].29 In
each structure, the thermal ellipsoids are displayed at the 50%
probability level. Hydrogen atoms have been omitted for clarity.
The frontier molecular orbital (MO) energies of LFeNNFeL
are illustrated in Figure 8. Mossbauer and solution magnetic
̈
studies have shown this molecule to have a septet (S = 3)
ground state,29 and this is the computed spin state that gave
Table 3. N−N Bond Distances and Stretching Frequencies
for Different Geometries of β-Diketiminate Iron(I)
Dinitrogen Complexes
excellent metrical and Mossbauer agreement. This suggests that
̈
there are six unpaired electrons in nonbonding d orbitals, which
come in pairs that have roughly the same energy and differ only
in the relationship between the orbitals on the two metal ions.
(Plots showing one orbital of each pair are presented in Figure
8.) The frontier orbitals with N2 character also arise in a pair
that differs by rotation by 90° around the Fe−Fe axis. However,
in this pair, the α and β electrons have an overlap of only 89%,
which is characteristic of “spin polarization”.27 (We note that a
broken-symmetry (8,2) calculation gave the same ground state.
More details on the computations are given in the Supporting
Information.) The α orbitals are more localized on the Fe and
the β orbitals are more localized on the N2, as visualized in the
“correlated pair” orbitals in Figure 9.28 This is suggestive of an
electronic structure that is intermediate between the diiron(I)−
N2 resonance structure at the top of Figure 10 and the
diiron(II)−N22− resonance structure at the bottom of Figure 10
(the latter has two high-spin iron(II) ions and a triplet
dinitrogen, with strong antiferromagnetic coupling between the
N22− and each of the Fe ions). This electronic structure model
dihedral angle
(deg)
νNN
complex
[LtBuFeNNFeLtBu
[LMeFeNNFeLMe],
Y-shaped
[LMeFeNNFeLMe],
T-shaped
N−N (Å)
(cm−1
)
ref
12
]
87.2(3)
15.7(2)
1.192(6)
1.186(7)
1778
1810
29
9.0(2)
1.172(5)
DFT calculations were utilized to explore the influence of
core distortions on the orbitals and energetics in the FeNNFe
core of simple models (details about the calculations are
provided in the Supporting Information). Efforts to systemati-
cally model the alkali-metal-containing, formally diiron(0)
complexes with a range of functionals and basis sets were
unsuccessful because geometry optimizations of 2-Na and 2-Cs
gave diketiminate−diketiminate dihedral angles that did not
agree with the experimentally observed structures. Whether
F
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the two FeN3 planes. Interestingly, the BP86-optimized model
had very similar N−N distances (ΔN−N = 0.002 Å) and Fe−N
distances (ΔFe−N = 0.006 Å) that are close enough to be
experimentally indistinguishable. The predicted Mossbauer
̈
parameters for the coplanar structure are δ = 0.54 mm/s and
|ΔEQ| = 1.54 mm/s, which are also within the expected
uncertainty limits of the experimentally observed values. In
addition, the energy of the optimized structure was nearly the
same after planarization of the core (+4 kcal/mol using BP86; 0
kcal/mol using TPSSh). These results suggest that twisting
around the FeNNFe axis has little energetic influence on the
core, consistent with the observation that both parallel and
perpendicular structures have been observed for the LFeNN-
FeL cores. In addition, the geometry optimization of the
coplanar model gave a substantial lateral twist at each Fe atom
to give asymmetric N(diketiminate)−Fe−N(N2) angles of 115°
and 148°, which are close to the angles of 110° and 155°
observed in the T-shaped crystal form of LMeFeNNFeLMe
(Figure 6c). Thus, the change from a Y shape to a T shape
at iron is also inconsequential with respect to the energy of the
molecule.
In order to explore the influence of rotation in more detail,
the dihedral angle between the two FeN3 planes was varied
between 0° and 90° in 10° steps using a simplified model where
the diketiminates were further truncated to N2C3H5 (Ltrunc).
The BP86-calculated energies vary smoothly over a range of
only 2 kcal/mol (Figure 11). Why are these changes so small?
Figure 8. Electronic structure of [LMe,MeFeNNFeLMe,Me] computed at
the BP86/TZVP(Fe,N,C)/SVP(H) level in the experimentally
demonstrated septet (S = 3) ground state. “Correlated pair” α and β
orbitals are shown in Figure 9.
Figure 9. “Correlated pair” from Figure 8, showing an α orbital
localized on Fe and a β orbital localized on the N2 unit with an overlap
of 0.89. The other pair of correlated α and β orbitals have the same
appearance but are rotated 90° from these.
Figure 11. Computed relative energies of [LtruncFeNNFeLtrunc] as the
dihedral angle between the diketiminate ligands is varied from 0° to
90°. It should be noted that the change in energy is less than 2 kcal/
mol. The insets show one of the two perpendicular Fe−N2 back-
bonding orbitals (MO 63) at different dihedral angles.
Examination of the key N2 back-bonding orbitals (MOs 63 and
64; Figure 12, bottom left) shows that they are degenerate in
the D2d structure with its 90° dihedral angle (same energy at
the bottom right in Figure 12) because they lie in the mirror
planes that contain the perpendicular C2 (or C2′) axes. As the
molecule twists through intermediate geometries (having D2
symmetry) toward the geometry having D2h symmetry with its
dihedral angle of 0°, the back-bonding orbitals rotate to follow
the C2 axes that are perpendicular to the FeNNFe vector rather
than the ligand planes themselves. Additionally, the energies of
MOs 63 and 64 compensate for one another (blue and green
lines in Figure 12). This indicates that the interaction between
the Fe atomic orbitals and the N2 π* orbitals can change but
that the gain in energy of one MO is balanced by a loss of
energy of the other through the entire range of accessible
dihedral angles.
Figure 10. Resonance structures of [LtBuFeNNFeLtBu]. (top) diiron(I)
with ferromagnetic coupling and a neutral N2 unit, yielding S = 3;
(bottom) diiron(II) antiferromagnetically coupled to a triplet N2
2−
unit, yielding S = 3.
was previously presented on the basis of variable-field
Mossbauer studies of LMeFeNNFeLMe.29
̈
In order to explore the interplay of the electronic and
geometric structures, we constrained the model of
LMe,MeFeNNFeLMe,Me to have a dihedral angle of 0° between
Visualization of the frontier orbitals also shows that one of
the orbitals (MO 72; red box in Figure 12), which is a
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FeNNFe axis. Computational models support the idea that the
rotation does not modify the extent of N−N bond activation
and explain the surprising lack of geometric preferences
through mixing of key d orbitals that gives similar back-
bonding into the π* orbitals of N2 irrespective of the relative
orientation of the supporting ligands.
In previous work with a smaller diketiminate ligand, we
observed that the choice of alkali metal can influence the shape
of trimetallic and tetrametallic iron−N2 clusters and concluded
that the influence was primarily from the size of the cation and
its ability to fit in the appropriate space between the aromatic
rings and N2.23a The current work indicates a similar conclusion
for bimetallic complexes: the main differences arise from size
matching between the cation and the available space near the
Fe−N2 core and the aryl groups. It is thus reasonable to
speculate that there will be other opportunities to vary the size
of the alkali metals to tune the geometry of other low-valent
complexes.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02841.
Crystallographic details (CIF)
Synthetic, spectroscopic, crystallographic, and computa-
tional data (PDF)
Figure 12. Walsh diagrams of high-lying molecular orbitals of
L
truncFeNNFeLtrunc, showing the changes in the energies of the MOs
through a scan of the dihedral angle between the ligands from 0° to
90°. The top part shows singly occupied orbitals (α only), and the
bottom part shows doubly occupied orbitals. Example orbitals (with
dihedral angle = 0°) are pictured on the left side, with the box colors
corresponding to the line colors in the Walsh diagrams.
AUTHOR INFORMATION
Corresponding Author
*E-mail: patrick.holland@yale.edu.
■
Present Address
nonbonding d orbital in the 90° structure, gains Fe−N2
antibonding character upon twisting of the Fe from a Y
shape to a T shape, and its energy increases. However, lowering
of the energy of its partner, MO 71, compensates for this.
Overall, these computational results show that the
surprisingly small influence of distortion on the energy comes
from flexible interactions of Fe with N2, where the in-plane and
out-of-plane orbitals can mix in such a way to maintain similar
back-bonding. Computations on an optimized, all-atom model
of K2[LMeFeNNFeLMe] suggest that the frontier orbitals are
similar to those in LFeNNFeL as described above, except that
the positive charge of the potassium ions causes a lowering of
the energy of the key N2 π* orbitals. The aforementioned
difficulties in reproducing the experimental geometries
prevented us from systematically examining the influence of
the dihedral angle on the formally diiron(0) complexes, but it is
reasonable to hypothesize that similar compensatory factors are
at play.
§K.G.: Pacific Northwest National Laboratory, 902 Battelle
Boulevard, Richland, WA 99352.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health
(GM065313 to P.L.H.; GM116463 to S.F.M.). We thank Frank
Neese and Shengfa Ye (Max Planck Institute for Chemical
Energy Conversion, Mulheim an der Ruhr, Germany) for
̈
assistance and resources for some of the computations, which
were performed during sabbatical work by P.L.H. and funded
by the Germany Fulbright Commission.
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