One-electron oxidation chemistry and subsequent reactivity of diiron imido complexes

Article abstract of DOI:10.1021/ic403039x

The chemical oxidation and subsequent group transfer activity of the unusual diiron imido complexes Fe(ⁱPrNPPh₂)₃Fe NR (R = tert-butyl (^tBu), 1; adamantyl, 2) was examined. Bulk chemical oxidation of 1 and 2 with Fc[PF₆] (Fc = ferrocene) is accompanied by fluoride ion abstraction from PF₆^- by the iron center trans to the Fe NR functionality, forming F-Fe( ⁱPrNPPh₂)₃Fe NR (ⁱPr = isopropyl) (R = ^tBu, 3; adamantyl, 4). Axial halide ligation in 3 and 4 significantly disrupts the Fe-Fe interaction in these complexes, as is evident by the >0.3 A increase in the intermetallic distance in 3 and 4 compared to 1 and 2. Moessbauer spectroscopy suggests that each of the two pseudotetrahedral iron centers in 3 and 4 is best described as Fe^III and that one-electron oxidation has occurred at the tris(amido)-ligated iron center. The absence of electron delocalization across the Fe-Fe NR chain in 3 and 4 allows these complexes to readily react with CO and ^tBuNC to generate the Fe^IIIFe^I complexes F-Fe( ⁱPrNPPh₂)₃Fe(CO)₂ (5) and F-Fe( ⁱPrNPPh₂)₃Fe(^tBuNC)₂ (6), respectively. Computational methods are utilized to better understand the electronic structure and reactivity of oxidized complexes 3 and 4.

Full text of DOI:10.1021/ic403039x

Article
pubs.acs.org/IC
One-Electron Oxidation Chemistry and Subsequent Reactivity of
Diiron Imido Complexes
Subramaniam Kuppuswamy,† Tamara M. Powers,‡ Bruce M. Johnson,§ Carl K. Brozek,∥
Jeremy P. Krogman,† Mark W. Bezpalko,† Louise A. Berben,§ Jason M. Keith,# Bruce M. Foxman,†
and Christine M. Thomas*,†
†Department of Chemistry, Brandeis University, 415 South Street, MS 015, Waltham, Massachusetts 02454, United States
‡Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02139, United
States
§Department of Chemistry, University of California, 1 Shields Avenue, Davis, California 95616, United States
∥Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139,
United States
#Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States
S
* Supporting Information
ABSTRACT: The chemical oxidation and subsequent group transfer
activity of the unusual diiron imido complexes Fe(iPrNPPh2)3FeNR
(R = tert-butyl (tBu), 1; adamantyl, 2) was examined. Bulk chemical
oxidation of 1 and 2 with Fc[PF6] (Fc = ferrocene) is accompanied by
fluoride ion abstraction from PF6− by the iron center trans to the Fe
NR functionality, forming F−Fe(iPrNPPh2)3FeNR (iPr = iso-
t
propyl) (R = Bu, 3; adamantyl, 4). Axial halide ligation in 3 and 4
significantly disrupts the Fe−Fe interaction in these complexes, as is
evident by the >0.3 Å increase in the intermetallic distance in 3 and 4
compared to 1 and 2. Mossbauer spectroscopy suggests that each of
̈
the two pseudotetrahedral iron centers in 3 and 4 is best described as
FeIII and that one-electron oxidation has occurred at the tris(amido)-
ligated iron center. The absence of electron delocalization across the
Fe−FeNR chain in 3 and 4 allows these complexes to readily react with CO and tBuNC to generate the FeIIIFeI complexes F−
Fe(iPrNPPh2)3Fe(CO)2 (5) and F−Fe(iPrNPPh2)3Fe(tBuNC)2 (6), respectively. Computational methods are utilized to better
understand the electronic structure and reactivity of oxidized complexes 3 and 4.
Ru(dPhf)4RuN (dPhf = N,N′-diphenylformamidinate) and
INTRODUCTION
■
the ditungsten oxo complex [W2O(2,2′-dipyridylamide)4]2+ are
more active than their monometallic analogues toward C−H
amination and oxygen atom transfer, respectively.25−27 This
phenomenon has been exploited most widely in the chemistry
of dirhodium complexes such as dirhodium tetraacetate
derivatives, which are active catalysts for C−H functionalization
reactions proposed to proceed through electrophilic dirhodium
carbene and nitrene intermediates.28 In contrast, however,
diiron imido complexes 1 and 2 were shown to be unreactive
toward substrates such as CO and organic isocyanide reagents,
to which monometallic terminal iron imido complexes are well-
known to transfer the nitrene functional group.13,29 In an effort
to better understand the factors that affect the interplay
between metal−metal bonds and metal−ligand multiple bonds
oriented in a trans arrangement, we now turn our attention to
High valent metal imido functionalities have been proposed as
intermediates in many chemical transformations such as
aziridination, hydrocarbon amination, and nitrene group
transfer processes, and iron imidos, in particular, may be
relevant to biological nitrogen fixation.1−9 To date, mono-
metallic iron imido complexes have been isolated and
structurally characterized with a wide range of Fe oxidation
states from +II to +V,9−22 but terminal Fe imido fragments in
multimetallic assemblies remain less common.23 Recently, our
laboratory reported the first examples of diiron terminal imido
complexes, Fe(iPrNPPh2)3FeNR (iPr = isopropyl) (R = tert-
butyl (tBu) (1), adamantyl, (2), and 2,4,6-trimethylphenyl), in
which the imido fragment is situated trans to a direct Fe−Fe
bond.24
A number of reports in recent years have demonstrated
enhanced reactivity of the multiply bonded ligands trans to
metal−metal multiple bonds as the result of delocalized 3-
center-4-electron bonding weakening the metal−ligand multi-
ple bond. For example, the diruthenium nitride complex
Received: December 10, 2013
Published: May 15, 2014
© 2014 American Chemical Society
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oxidized variants of 1 and 2 and their reactivity toward
oxidative group transfer reactions.
salts containing [BF4]− anions.30 Oxidation of 1 with Fc[BF4]
also leads to halide abstraction and formation of complex 3
(Supporting Information, Figure S3). The H NMR spectra of
complexes 3 and 4 display five and seven resonances in the δ
15.0 to δ −7.0 range, respectively, indicating C3-symmetric
structures in solution. Cyclic voltammetry of complexes 3 and 4
reveals a reversible reduction at E1/2 = −1.3 V, and treatment of
3 and 4 with relatively mild reductants such as Mg or Zn
quantitatively regenerates 1 and 2.
1
RESULTS AND DISCUSSION
■
One-Electron Oxidation of 1 and 2. As previously
reported, cyclic voltammetry of complexes 1 and 2 revealed
reversible oxidative events at −0.32 and −0.35 V (vs Fc/Fc+).24
Likewise, the chemical oxidation of these two complexes was
explored. Stirring complex 1 with Fc[PF6] in tetrahydrofuran
(THF) proceeds smoothly to generate the one-electron
oxidized reddish-brown complex 3 in moderate yield. Rather
than the ionic FeIV imido complex [Fe(μ-iPrNPPh2)3Fe
NtBu][PF6] that might have been expected based on
monometallic analogues,10,13 complex 3 was identified as F−
Fe(μ-iPrNPPh2)3FeNtBu, the product of fluoride ion
abstraction by the one-electron oxidized complex (Scheme
1). Monitoring the reaction by NMR spectroscopy reveals the
X-ray quality single crystals of 3 and 4 were obtained by slow
evaporation of concentrated diethyl ether solutions at room
temperature. X-ray crystallography confirms the formulations of
these complexes, and the molecular structures of 3 (left) and 4
(right) are depicted in Figure 1. Selected structural parameters
of these complexes are given in Table 1. Both iron centers in 3
and 4 adopt a distorted tetrahedral geometry with the fluoride
and imido functionalities occupying the apical positions at
either end of the molecule. The most notable structural feature
of these oxidized complexes is that their Fe−Fe distances
(2.9330(3) Å (3) and 2.8796(4) Å (4)) have increased
substantially compared to their precursors 1 and 2 (2.5444(3)
Å (1) and 2.5443(3) Å (2)24). The bond metrics associated
with the Fe imido fragments of 3 and 4 are otherwise in line
with 1 and 2 and with previously reported low-spin
tris(phosphine) Fe(III) imide complexes.11,13,15 For example,
the Fe−P distances in 3 (Fe−Pavg = 2.22 Å) (avg = average) are
nearly identical to those in 1 (Fe−Pavg = 2.23 Å).24 In contrast,
dramatic structural changes are observed at the tris(amido) Fe
center in complexes 3 and 4. As the Fe center moves out of the
plane of the three amide nitrogen atoms and its geometry
becomes more tetrahedral, the Fe−N distances are significantly
elongated (Fe−Navg = 1.97 Å) compared to the starting
materials 1 and 2 (Fe−Navg = 1.94 Å). These structural changes
suggest that the tris(amido)Fe center has been oxidized from
Scheme 1
1
formation of PF5 as a byproduct by 19F (δ −71.0 ppm, JF−P
=
1
710 Hz) and 31P NMR (δ −84.0 ppm, JP−F = 710 Hz)
spectroscopy (see Supporting Information). The analogous
complex F−Fe(μ-iPrNPPh2)3FeNAd (4) was also formed
upon oxidation of complex 2 with Fc[PF6] under similar
reaction conditions. We note that the oxidative feature in the
cyclic voltammogram of complex 1 is reversible and does not
vary when the supporting electrolyte is changed from
[nBu4N][PF6] to [nBu4N][ClO4], suggesting that halide
abstraction is a relatively slow process and does not occur on
the time scale of electrochemical experiments (Supporting
Information, Figure S5). A similar fluoride ion abstraction
reaction was observed by Deng and co-workers upon oxidation
of the diiron(II) bis(μ-imido) complex [(NHC)Fe(μ2-
NDipp)]2 (Dipp = 2,6-diisopropylphenyl, NHC = 2,5-
diisopropyl-3,4-dimethylimidazol-1-ylidene) with ferrocenium
−
FeII to FeIII, leading to halide ion abstraction from PF6 .
One of the most interesting aspects of complexes 1 and 2 is
the presence of two Fe centers in different oxidation and spin
states, high-spin FeII and low-spin FeIII, within the same
molecule due to different coordination environments.24 In
complexes 3 and 4, a similar phenomenon is to be expected
with the two Fe centers in the same oxidation state (FeIII) but
different spin states. Measurement of the solution magnetic
moments of 3 and 4 using the Evans method reveals μeff values
of 4.83 and 4.62 μB, respectively, consistent with the S = 2
Figure 1. Displacement ellipsoid (50%) representations of 3 (left) and 4 (right). Hydrogen atoms are omitted for clarity.
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Table 1. Relevant Interatomic Distances (Å) and Angles (degrees) in Complexes 3−6
3
4
5
6
Fe1−Fe2
Fe1−N1
Fe1−N2
Fe1−N3
Fe1−F1
Fe2−P1
Fe2−P2
Fe2−P3
Fe2−N4
2.9330(3)
1.9690(15)
1.9753(15)
1.9661(16)
1.8347(11)
2.2290(5)
2.2205(5)
2.2149(5)
1.6251(15)
114.46(7)
114.49(6)
115.47(7)
2.8796(4)
1.9681(14)
1.9619(13)
1.9718(14)
1.8328(10)
2.2255(5)
2.2238(5)
2.2206(5)
1.6268(15)
116.89(6)
116.69(6)
112.59(6)
3.2856(4)
1.9587(15)
1.9533(15)
1.9563(15)
1.8188(10)
2.3189(5)
2.3128(5)
2.2394(5)
3.2792(8)
1.966(3)
1.949(3)
1.955(3)
1.832(2)
2.2877(11)
2.2739(11)
2.2764(11)
N1−Fe1−N2
N1−Fe1−N3
N2−Fe1−N3
115.43(6)
108.19(6)
112.96(6)
109.05(12)
113.47(12)
114.17(12)
ground states that are expected upon removal of an electron
from the S = 5/2 precursors 1 and 2. An S = 2 ground state
might be expected to result from antiferromagnetic coupling of
the single electron on the low-spin FeIII center with one of the
five unpaired electrons on a high-spin FeIII center. Solid-state
magnetic susceptibility measurements determined using super-
conducting quantum interference device (SQUID) magneto-
metry for 3 support this assignment (Figure 2). At 300 K the
Figure 3. 57Fe Mossbauer spectrum of a solid sample of 3 at 90 K
̈
(black dotted lines). The spectrum was adequately fit (red) as a
combination of two signals (δ = 0.26 mm/s, |ΔEQ| = 1.28 mm/s, Γ =
0.13 mm/s, (58%, green); δ = −0.12 mm/s, |ΔEQ| = 2.70 mm/s, Γ =
0.25 mm/s (32%, blue)), along with a ferrocene impurity (δ = 0.52
mm/s, |ΔEQ| = 2.40 mm/s, Γ = 0.16 mm/s (10%, orange)).
Figure 2. Variable-temperature magnetic susceptibility data for
compound 3 in the solid state in an applied field of 0.1 T determined
using SQUID magnetometry.
observed magnetic moment is 4.64 μB, which is in good
agreement with both the solution measurement and the
predicted magnetic moment of 4.83 μB for the S = 2 system
(assuming g = 2.0). The drop in the measured magnetic
moment to 4.21 μB at 5 K could be attributed to several
possibilities, such as intermolecular antiferromagnetic inter-
actions, zero-field splitting, or a very slow spin crossover
process. The proposal that the iron centers are interacting
antiferromagnetically is also in agreement with the predictions
made using density functional theory (DFT) calculations (vide
infra).
center based on comparison to the isomer shift of the imido−
FeIII center in the precursor complex 1 (δ = −0.06 mm/s).24
This value is also consistent with the Mossbauer isomer shift
̈
reported for the low-spin FeIII complex PhB(MesIm)3FeNAd
(δ = −0.11 mm/s)32 and is higher than the isomer shifts
reported for low-spin C3-symmetric FeIV complexes (e.g.,
PhB(MesIm)3FeN, δ = −0.28 mm/s;32 PhB(tBuIm)3Fe
N, δ = −0.28 mm/s;32,33 PhB(CH2PiPr2)3FeN, δ = −0.34
mm/s;34 (TIMENMes)FeN, δ = −0.27 mm/s35). Notably,
the quadrupole splitting observed for this signal is significantly
larger than that observed for 1 (0.54 mm/s), a phenomenon
that may be attributed to the absence of metal−metal bonding
in 3. The isomer shift at δ = 0.26 mm/s, attributed to the
tris(amido)Fe center, is considerably lower than that for the
tris(amido)FeII centers in the similarly ligated complexes
[Fe(iPrNPPh2)3Fe(iPrNPPh2)] (δ = 0.56 mm/s)36 and [Fe-
(iPrNPPh2)3Cu2(iPrNPPh2)] (δ = 0.65 mm/s),37 consistent
with oxidation from FeII to FeIII.
To further probe the electronic structure of complexes 3 and
4, a zero-field 57Fe Mossbauer spectrum of complex 3 was
̈
collected (Figure 3). The Mossbauer spectrum features two
̈
quadrupole doublets centered at δ = 0.26 mm/s (|ΔEQ| = 1.28
mm/s) and δ = −0.12 mm/s (|ΔEQ| = 2.70 mm/s), further
confirming the disparity between the two different iron centers
in this complex. A third signal of lesser intensity (10%, δ = 0.52
mm/s (|ΔEQ| = 2.40 mm/s)) is also observed as the result of a
residual ferrocene impurity.31 The isomer shift centered at δ =
−0.12 mm/s is assigned to the imido-bound low-spin FeIII
To corroborate our assignment of each quadrupole doublet
in the Mossbauer spectrum of 3 to the two respective Fe
̈
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centers, computational methods were used to predict the
isomer shift and quadrupole splitting of each Fe center. The
also reactive toward isocyanide reagents: treatment of complex
t
3 with 3 equiv of BuNC at room temperature gives rise to
̈
Fe(iPrNPPh2)3Fe(CNtBu)2 (6) in 75% isolated yield, eliminat-
ing tBuNCNtBu. The isocyanate and carbodiimide
byproducts extruded during the reactions to form 5 and 6
were identified by GC/MS and by characteristic IR stretches at
2280 and 2271 cm−1. The four broad paramagnetically shifted
calculated Mossbauer parameters are in good agreement with
our intuitive assignment: the imido-bound Fe center is
predicted to have an isomer shift of δ = −0.28 mm/s and a
|ΔEQ| = 2.57 mm/s, while the computed parameters for the
tris(amido) Fe center are δ = 0.18 mm/s and |ΔEQ| = 1.48
mm/s.
1
resonances observed in the H NMR spectrum of 5 between δ
Reactivity of 3 and 4 toward Nitrene Transfer.
Complexes 1 and 2 were previously reported to be unreactive
toward nitrene transfer to carbon monoxide or isocyanide
reagents,24 in contrast to monometallic Fe(III) imido
analogues.13 This difference was attributed to both steric
(diminished accessibility to the FeNR fragment) and
electronic factors (stabilization of Fe−N π symmetry orbitals),
each of which results from the wider P−Fe−P angles that occur
as a result of the Fe−Fe interaction. As the Fe−Fe interaction is
disrupted upon one-electron oxidation and fluoride ion
abstraction, complexes 3 and 4 might be expected to show
enhanced reactivity toward nitrene transfer. Indeed, complex 3
quantitatively reacts with CO (1 atm) at room temperature to
generate two-electron reduced F−Fe(iPrNPPh2)3Fe(CO)2 (5)
as a red microcrystalline solid in 80% isolated yield, with
concomitant elimination of tBuNCO (Scheme 2). Complex 3 is
16.0 and 1.7 ppm suggest a relatively symmetric geometry in
solution. A similar H NMR spectrum is observed for complex
1
6 with an additional resonance observed at 0.8 ppm assigned to
t
t
the Bu groups on the BuNC ligands. In contrast to
t
monometallic systems, addition of BuN3 to complex 5 does
not regenerate imido complex 3, precluding catalytic nitrene
transfer.29
The chemical formulations of 5 and 6 were further confirmed
by X-ray crystallography; their solid-state structures are shown
in Figure 4, and their bond metrics are tabulated in Table 1.
The tris(phosphine)Fe(L)2 fragment in both complexes adopts
a distorted square pyramidal geometry (τ = 0.03 for 5 and 0.02
for 6),38 suggesting that the equivalence of the phosphinoamide
ligands by 1H NMR spectroscopy is the result of fluxionality in
solution. The Fe−Fe interatomic distance in 5 and 6 is
considerably longer than that in 3 and 4 (3.2856(4) Å (5) and
3.2792(8) Å (6) vs 2.9330(3) Å (3) and 2.8796(4) Å (4)), but
since there is no metal−metal interaction in either pair of
complexes, this can be attributed to the Fe center moving
further out of the plane of the three phosphorus atoms to
accommodate a fifth ligand. Consistent with reduction of the
tris(phosphine)Fe center, the Fe−P distances are elongated in
FeIIIFeI 5 and 6 with respect to FeIIIFeIII precursors 3 and 4
(Table 1). The geometry at the tris(amido)Fe center remains
pseudotetrahedral, with geometric parameters largely similar to
those of 3 and 4.
Scheme 2
Complexes 5 and 6 present another unusual scenario where
each metal in the bimetallic complex is expected to adopt a
different spin state. While the tris(amido)FeIII−F center
remains high-spin and in a weak ligand field, the strong-field
CO ligands on the tris(phosphine)FeI fragment promote a low-
spin configuration. Interestingly, the solution magnetic mo-
ments of 5 and 6 are 4.86 and 4.89 μB, indicative of the S = 2
electronic configuration similar to that observed for complexes
3 and 4. Antiferromagnetic coupling of the single unpaired
Figure 4. Displacement ellipsoid (40%) representations of 5 (left) and 6 (right). Hydrogen atoms are omitted for clarity.
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electron on the low-spin FeI(CO)2 fragment with one of the
five unpaired electrons on the high-spin tris(amido)FeIII−F
fragment, would again account for the observed S = 2 ground
states of 5 and 6. The variable-temperature magnetic
susceptibility of complex 5 was further investigated using
SQUID magnetometry; the observed magnetic moment at 300
K is 4.93 μB, and this is consistent with both the solution
measurement and an electronic structure that has four unpaired
electrons (Figure 5). As in complex 3, the measured magnetic
mm/s). On the basis of the similarity of both the isomer shift
and quadrupole splitting of the signal at 0.20 mm/s to that
observed in 3, this doublet is assigned to the tris(amido)Fe
center, which remains in a high-spin FeIII state. The doublet
centered at 0.08 mm/s is, therefore, attributed to the Fe center
in the tris(phosphine)Fe(CO)2 fragment. There are few low-
spin FeI complexes with Mossbauer parameters available for
̈
comparison; however, we note that the 0.08 mm/s isomer shift
is relatively low compared to other low-spin FeI carbonyl
complexes reported in the literature, including Riordan’s
PhB(CH2StBu)3Fe(CO)2 complex (δ = 0.21 mm/s, |ΔEQ| =
0.47 mm/s)39 and Chirik’s [(iPrPDI)Fe(CO)2]+ complex (δ =
0.17 mm/s, |ΔEQ| = 0.62 mm/s).40 Again, we turned to
computational methods to validate our assignment of each set
of Mossbauer parameters to the two Fe centers. The calculated
̈
Mossbauer parameters are consistent with our assignment. The
̈
Fe(CO)2 center in 5 is predicted to have an isomer shift of δ =
−0.08 mm/s and a |ΔEQ| = 0.46 mm/s, while the computed
parameters for the tris(amido) Fe center are δ = 0.21 mm/s
and a |ΔEQ| = 1.37 mm/s.
The best evidence for the low-spin FeI oxidation state
assignment of the tris(phosphine)-ligated Fe(CO)2 fragment is
the IR carbonyl stretching frequencies. The IR spectrum of
complex 5 exhibits two diagnostic νCO stretches at 1982 and
1921 cm−1, which are consistent with those reported for the
similarly ligated FeI(CO)2 fragment in PhB(CH2PPh2)3Fe-
(CO)2 (νCO = 1979 and 1914 cm−1)13 or the tris(thioether)
complex PhB(CH2StBu)3Fe(CO)2 ((νCO = 1984 and 1911
cm−1).39 The ultraviolet−visible−near-infrared (UV−vis−NIR)
spectrum of mixed-valence complex 5 also has a low-intensity
broad absorption at 878 nm that is not present in complex 3.
This feature may be assigned to intervalence charge transfer
(Supporting Information, Figure S12).
Figure 5. Variable-temperature magnetic susceptibility data for
compound 5 in the solid state in an applied field of 0.1 T determined
using SQUID magnetometry.
moment of 5 falls slightly at 5 K, in this case to 4.03 μB. Once
again this behavior suggests the presence of weak intermo-
lecular antiferromagnetic coupling or possibly a zero-field
splitting effect.
To further probe the reduced oxidation state assignment of
complexes 5 and 6, complex 5 was examined by 57Fe
Mossbauer spectroscopy. As shown in Figure 6, the Mossbauer
̈
̈
Computational Studies. To better understand the
electronic structure of the oxidized imido complexes 3 and 4,
a computational investigation was undertaken using DFT. We
first investigated the oxidized S = 2 diiron imido cation
[Fe(iPrNPPh2)3FeNtBu]+ (7) that would be formed upon
oxidation of 3 prior to halide abstraction. The molecular orbital
diagram of 7 is largely similar to that reported for 3,24 with an
electron removed from the highest-energy singly occupied
molecular orbital (SOMO): the metal−metal σ* orbital (Figure
7 and Supporting Information). Consequently, the DFT-
optimized geometry of 7 reveals a shorter Fe−Fe distance
(2.41 Å in 7 compared to 2.5443(3) Å in 3).24 The lowest
unoccupied molecular orbital (LUMO) of this electrophilic
cation (shown in Figure 7) is Fe−Fe σ* in character, with
spectrum of 5 shows two quadrupole doublets centered at δ =
0.20 mm/s (|ΔEQ| = 1.24 mm/s) and 0.08 mm/s (|ΔEQ| = 0.37
2
2
repulsion of the Fe dz orbitals polarizing the empty dz lobe of
the tris(amido)Fe center in the direction of the open
coordination site and promoting F− abstraction.
Upon fluoride ion abstraction to generate complex 3, the
geometry at the tris(amido)Fe center changes to pseudote-
2
2
trahedral, leading to a weaker ligand field as the dx −y - and dxy-
derived orbitals lower in energy, while the dxz and dyz orbitals
are destabilized. This is evident in the overall molecular orbital
picture of 3 (Figure 7), but the most important change
resulting from fluoride ion abstraction is the destabilization of
2
the dz orbital of the fluoride-bound Fe center as it becomes
Figure 6. 57Fe Mossbauer spectrum of a solid sample of 5 at 90 K
̈
antibonding with respect to F−. This serves to substantially
weaken the Fe−Fe interaction in 3, likely in combination with
geometric constraints that impose an elongated intermetallic
distance between the two tetrahedral Fe centers.
(black dotted lines) fitted (red) as two signals δ = 0.20 mm/s; |ΔEQ| =
1.24 mm/s, Γ = 0.23 mm/s (58%, blue) and δ = 0.08 mm/s; |ΔEQ| =
0.37 mm/s, Γ = 0.12 mm/s (30%, green) along with an impurity at δ =
1.1 mm/s; |ΔEQ| = 2.07 mm/s, Γ = 0.34 mm/s (12%, orange).
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Figure 7. Qualitative frontier molecular orbital diagrams of precursor 1, oxidized intermediate 7, and fluoride abstraction product 3 predicted
computationally. Representative pictorial representations of the LUMO of complex 7 and the Fe−Fe σ bonding orbital of 3 are shown.
To explore the magnetic interaction between the two Fe
centers in complex 3 as well as in the dicarbonyl complex 5,
additional computational studies were carried out. In both
cases, the antiferromagnetically coupled quintet state was lower
in energy than the ferromagnetically coupled septet state, and
the optimized Fe−Fe distance for the S = 2 state matched best
with the X-ray derived experimental Fe−Fe interatomic
distance. The computed spin densities shown in Figure 8 and
CONCLUSION
■
In summary, we have shown that the diiron imido complexes 1
and 2 can be readily oxidized, but the oxidized products are
unstable and undergo fluoride ion abstraction from PF6 to
−
form the neutral FeIIIFeIII imido complexes 3 and 4. The
metal−metal interaction in complexes 3 and 4 is disrupted
significantly, and this appears to facilitate reactivity toward
nitrene transfer. Indeed, complexes 3 and 4 readily undergo
nitrene transfer to CO and tBuNC at room temperature similar
to monometallic FeIII imido complexes,13,29 while their FeIIFeIII
precursors 1 and 2 are unreactive toward these substrates even
at elevated temperature. The enhanced reactivity of FeIII imido
complexes upon removal of the Fe−Fe interaction is perhaps
surprising in light of other bimetallic systems in which metal−
metal interactions serve to promote reactivity.28 However, in
contrast to the previously studied examples of M−M=E
delocalized systems that show enhanced electrophilicity, the
reactivity examined here hinges on the nucleophilicity of the
imido fragment. The delocalization of electron density through
the metal−metal interaction in complexes 1 and 2 serves to
attenuate the nucleophilicity of the imido fragment, preventing
Figure 8. Computed spin density surfaces for the antiferromagnetically
coupled S = 2 complexes 3 and 5 with α-spin density in yellow and β-
spin density in blue.
t
reactivity with CO or BuNC. Upon elimination of the metal−
metal interaction in complexes 3 and 4, however, the FeNR
fragment becomes more nucleophilic and reactive toward
t
nitrene transfer for CO and BuNC.
tabulated in Table 2 reveal a single unpaired electron on the
tris(phosphine)Fe center antiferromagnetically coupled to ∼5
unpaired electrons distributed throughout the tris(amido) Fe
center and the ligand atoms bound to it, consistent with the
experimental S = 2 ground state.
EXPERIMENTAL SECTION
■
General Considerations. Unless specified otherwise, all manip-
ulations were performed under an inert atmosphere using standard
Schlenk or glovebox techniques. Glassware was oven-dried before use.
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Table 2. Computed Mulliken Spin Densities for Selected Atoms in Complexes 3 and 5 in Both the Antiferromagnetically
Coupled S = 2 State and the Ferromagnetically Coupled S = 3 State
3
5
S = 2 (Fe−Fe = 2.81 Å)
S = 3 (Fe−Fe = 3.32 Å)
S = 2 (Fe−Fe = 3.38 Å)
S = 3 (Fe−Fe = 3.42 Å)
Fe(PR2)3
−0.67
3.81
0.18
0.16
0.16
0.16
1.18
4.02
0.19
0.18
0.18
0.19
−0.98
4.05
0.20
0.15
0.17
0.19
0.97
4.07
0.21
0.18
0.20
0.17
Fe(NR)3
F
N
N
N
Benzene, pentane, diethyl ether (Et2O), THF, and toluene were dried
using a Glass Contours solvent purification system. All solvents were
stored over 3 Å molecular sieves prior to use. Benzene-d6 (Cambridge
Isotopes) was degassed via repeated freeze−pump−thaw cycles and
dried over 3 Å molecular sieves. Complexes [Fe(iPrNPPh2)3Fe
NtBu] (1) and [Fe(iPrNPPh2)3FeNAd] (2) were synthesized using
literature procedures.24 NMR spectra were recorded at ambient
temperature on a Varian Inova 400 MHz instrument. Chemical shifts
are reported in δ (ppm). For 1H and 13C{1H} NMR spectra, the
solvent resonance was used as an internal reference, and for 31P{1H}
NMR spectra, 85% H3PO4 was referenced as an external standard (0
ppm). 19F NMR spectra were reported with respect to external
CF3CO2H (−76.5 ppm). IR spectra were recorded on a Varian 640-IR
spectrometer controlled by Resolutions Pro software. UV−vis spectra
were recorded on a Cary 50 UV−vis spectrophotometer using Cary
WinUV software. Gas chromatography−mass spectrometry (GC-MS)
data were collected on Agilent 7890A GC System and 5975C VL MSD
with Triple-Axis Detector, and yields were reported via integration
versus an internal standard (tetradecane). Elemental analyses were
performed at Complete Analysis Laboratory Inc., Parsippany, NJ.
Solution magnetic moments were measured using the Evans
method.41,42
absorption corrections, were carried out using the Bruker Apex2
software.43 Preliminary cell constants were obtained from three sets of
12 frames. Data collection and refinement details are presented in
Table S1, and fully labeled diagrams and data collection and
refinement details are included in the Supporting Information file.
Computational Details. All calculations were performed using
Gaussian09-E.0144 for the Linux operating system. DFT calculations
were carried out using a combination of Becke’s 1988 gradient-
corrected exchange functional45 and Perdew’s 1986 electron-
correlation functional46 (BP86). For open-shell systems, unrestricted
wavefunctions were used in energy calculations. A mixed-basis set was
employed, using the LANL2TZ(f) triple-ζ basis set with effective core
potentials for iron,47−49 Gaussian09’s internal 6-311+G(d) for atoms
bonded directly to the metal centers (nitrogen and phosphorus), and
Gaussian09’s internal LANL2DZ basis set (equivalent to D95V50) for
carbon and hydrogen. Starting with crystallographically determined
geometries as a starting point, when available, the geometries were
optimized to a minimum, followed by analytical frequency calculations
to confirm that no imaginary frequencies were present.
The unrestricted antiferromagnetically coupled ground state
solutions were located through a two-step process: (1) High-spin (S
= 3) ferromagnetically coupled optimizations were performed; (2) the
ferromagnetically coupled solution was used as an initial guess for the
intermediate-spin (S = 2) antiferromagnetically coupled system.
Mulliken spin densities and spin density plots were used to verify
that the correct antiferromagnetically coupled state was obtained.
The ORCA 2.9.151 computational package was used to optimize the
Electrochemistry. Cyclic voltammetry measurements were carried
out in a glovebox under a dinitrogen atmosphere in a one-
compartment cell using a CH Instruments electrochemical analyzer.
A glassy carbon electrode and platinum wire were used as the working
and auxiliary electrodes, respectively. The reference electrode was Ag/
AgNO3 in THF. Solutions of electrolyte (0.40 M [nBu4N][PF6] in
THF) and analyte (2 mM) were also prepared in the glovebox. All
potentials are reported versus an internal ferrocene/ferrocenium
reference.
geometry and simulate Mossbauer parameters of a model of
̈
compounds 3 and 5. We employed the pure-DFT functional
BP8645,46 and the Ahlrichs TZV basis sets52 and polarization functions
for all calculations. The self-consistent field equations were converged
tightly to 10−8 Hartree in the total energy, and the open-shell systems
were treated with spin-unrestricted Kohn−Sham determinants. The
Broken Symmetry feature was used to specify one unpaired electron
on the imido-bound iron site, while four unpaired electrons were
imposed on the second site.
Mossbauer Spectroscopy. 57Fe Mossbauer spectra were
̈
̈
measured on a constant acceleration spectrometer (SEE Co,
Minneapolis, MN) with a Janis SVT-100 cryostat. Isomer shifts are
quoted relative to α-Fe foil (<25 μm thick) at room temperature. The
Fe foil standard spectrum has line widths Γ (full width at half-
maximum) of 0.292 and 0.326 mm/s for the doublets within the
4
We constructed the model compound by starting from the crystal
structure and simplifying all −R to methyl groups. The entire structure
was optimized with a tolerance of 5 × 10−6 Hartree for each energy
change. The electron paramagnetic resonance/NMR module was used
mm/s window when measured outside the cryostat at room
temperature. Samples of 3 and 5 were prepared using approximately
60 mg of sample suspended in paratone-N oil. Data were analyzed
using a package written by E. R. King and modified by E. V. Eames in
Igor Pro (Wavemetrics) using a simple model consisting of Lorentzian
lineshapes with optional asymmetry. Percent components of the
to simulate the Mossbauer parameters. Both the spin−spin and spin−
̈
orbit operators were evaluated, using the coupled-perturbed method to
solve the spin−orbit component. This calculation provided values of η,
the asymmetry parameter, and ρ, the s-orbital electron density at the
absorbing nucleus. The quadrupole splittings of both iron nuclei were
calculated from the values of η. To correlate the computed ρ to an
isomer shift value, we calculated ρ for a series of compounds with
known isomer shifts and, using their linear relation, determined isomer
shifts from a best-fit line. The known compounds were simple iron
salts covering a range of oxidation states and were treated in a manner
identical to 3 and 5 by starting from reported crystal structures.
F−Fe(iPrNPPh2)3FeNtBu (3). A solution of ferrocenium hexa-
fluorophosphate (0.13 g, 0.39 mmol) in THF (3 mL) was chilled to
−32 °C, and to this a THF solution of 1 (0.35 g, 0.39 mmol) was
added dropwise over a period of 10 min. The reaction mixture slowly
turned from reddish-brown to brown and was continuously stirred for
quadrupole doublets in the Mossbauer spectra were determined by
̈
integrating the individual components’ fits and using the areas of those
fits to determine the relative area ratios of the quadrupole doublets.
Magnetic Susceptibility Measurements. Solid-state magnetic
measurements were recorded using a Quantum Designs MPMS XL
magnetometer at 0.1 T. The sample was contained under nitrogen in a
gelcap and suspended in the magnetometer in a plastic straw. The
magnetic susceptibility was adjusted for diamagnetic contributions
using Pascal’s constants.
X-ray Crystallography Procedures. All operations were
performed on a Bruker-Nonius Kappa Apex2 diffractometer using
graphite-monochromated Mo Kα radiation. All diffractometer
manipulations, including data collection, integration, scaling, and
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Inorganic Chemistry
Article
2 h to ensure completion of the reaction to form 3. Volatiles were
removed under vacuum. Extraction of the remaining brown material
into Et2O, followed by removal of volatiles in vacuo yielded analytically
pure 3 (0.25 g, 69%). X-ray quality single crystals of 3 were grown
upon slow evaporation of a concentrated Et2O solution of 3 at room
temperature. 1H NMR (400 MHz, C6D6): δ 14.7 (18H, iPr−CH3), 7.2
(6H, Ph), 5.5 (9H, NtBu), 1.3 (12H, Ph), −7.1 (12H, Ph) (isopropyl
methine resonance is not observed). UV−vis−NIR (C6H6) λmax, nm
(ε, L mol−1 cm−1): 495 (1600), 667 (400). Evans’ method (C6D6):
4.83 μB. Anal. Calcd for C49H60FFe2N4P3: C, 63.38; H, 6.51; N, 6.03.
Found: C, 63.46; H, 6.59; N, 5.98%.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: thomasc@brandeis.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by Brandeis
University and, under Award No. DE-SC0004019, the
Department of Energy. C.M.T. and L.A.B. are Alfred P. Sloan
Foundation Fellows. The authors also thank Dr. T. A. Betley
F−Fe(iPrNPPh2)3FeNAd (4). A solution of ferrocenium hexa-
fluorophosphate (0.075 g, 0.23 mmol) in THF (3 mL) was chilled to
−32 °C, and to this a THF solution of 2 (0.23 g, 0.23 mmol) was
added dropwise over a period of 10 min. The reaction mixture slowly
turned from reddish-brown to brown and was continuously stirred for
2 h to ensure completion of the reaction. Volatiles were removed in
vacuo, and the brown crude materials were washed with cold pentane
to remove ferrocene and the soluble impurities. Extraction of the
remaining brown material into Et2O, followed by removal of volatiles
in vacuo yielded analytically pure 4 (0.17 g, 74%). X-ray quality single
crystals of 4 were grown upon slow evaporation of a concentrated
Et2O solution of 4 at room temperature. 1H NMR (400 MHz, C6D6):
δ 14.9 (18H, iPr−CH3), 9.4 (6H, Ad), 7.8 (6H, Ph), 4.6 (3H, Ad), 1.3
(12H, Ph), −1.1 (6H, Ad), −7.1 (12H, Ph) (isopropyl methine
resonance is not observed). UV−vis (C6H6) λmax, nm (ε, L mol−1
cm−1): 503 (1200), 670 (300). Evans’ method (C6D6): 4.62 μB. Anal.
Calcd for C55H66FFe2N4P3: C, 65.62; H, 6.61; N, 5.57. Found: C,
65.67; H, 6.74; N, 5.38%.
(Harvard University) for assistance with Mossbauer spectros-
̈
copy and Dr. M. Dinca
̆
(MIT) for access to computational
resources for Mossbauer calculations. J.M.K would like to thank
̈
Colgate University for startup funding.
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