Electronic Structure and Bonding in Iron(II) and Iron(I) Complexes Bearing Bisphosphine Ligands of Relevance to Iron-Catalyzed C-C Cross-Coupling

Article abstract of DOI:10.1021/acs.inorgchem.5b02263

Chelating phosphines are effective additives and supporting ligands for a wide array of iron-catalyzed cross-coupling reactions. While recent studies have begun to unravel the nature of the in situ-formed iron species in several of these reactions, including the identification of the active iron species, insight into the origin of the differential effectiveness of bisphosphine ligands in catalysis as a function of their backbone and peripheral steric structures remains elusive. Herein, we report a spectroscopic and computational investigation of well-defined FeCl₂(bisphosphine) complexes (bisphosphine = SciOPP, dpbz, ^tBudppe, or Xantphos) and known iron(I) variants to systematically discern the relative effects of bisphosphine backbone character and steric substitution on the overall electronic structure and bonding within their iron complexes across oxidation states implicated to be relevant in catalysis. Magnetic circular dichroism (MCD) and density functional theory (DFT) studies demonstrate that common o-phenylene and saturated ethyl backbone motifs result in small but non-negligible perturbations to 10Dq(T_d) and iron-bisphosphine bonding character at the iron(II) level within isostructural tetrahedra as well as in five-coordinate iron(I) complexes FeCl(dpbz)₂ and FeCl(dppe)₂. Notably, coordination of Xantphos to FeCl₂ results in a ligand field significantly reduced relative to those of its iron(II) partners, where a large bite angle and consequent reduced iron-phosphorus Mayer bond orders (MBOs) could play a role in fostering the unique ability of Xantphos to be an effective additive in Kumada and Suzuki-Miyaura alkyl-alkyl cross-couplings. Furthermore, it has been found that the peripheral steric bulk of the SciOPP ligand does little to perturb the electronic structure of FeCl₂(SciOPP) relative to that of the analogous FeCl₂(dpbz) complex, potentially suggesting that differences in the steric properties of these ligands might be more important in determining in situ iron speciation and reactivity.

Full text of DOI:10.1021/acs.inorgchem.5b02263

Article
pubs.acs.org/IC
Electronic Structure and Bonding in Iron(II) and Iron(I) Complexes
Bearing Bisphosphine Ligands of Relevance to Iron-Catalyzed C−C
Cross-Coupling
Jared L. Kneebone, Valerie E. Fleischauer, Stephanie L. Daifuku, Ari A. Shaps, Joseph M. Bailey,
Theresa E. Iannuzzi, and Michael L. Neidig*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: Chelating phosphines are effective additives and
supporting ligands for a wide array of iron-catalyzed cross-
coupling reactions. While recent studies have begun to unravel
the nature of the in situ-formed iron species in several of these
reactions, including the identification of the active iron species,
insight into the origin of the differential effectiveness of
bisphosphine ligands in catalysis as a function of their backbone
and peripheral steric structures remains elusive. Herein, we report
a spectroscopic and computational investigation of well-defined
FeCl2(bisphosphine) complexes (bisphosphine = SciOPP, dpbz,
tBudppe, or Xantphos) and known iron(I) variants to systemati-
cally discern the relative effects of bisphosphine backbone
character and steric substitution on the overall electronic structure and bonding within their iron complexes across oxidation
states implicated to be relevant in catalysis. Magnetic circular dichroism (MCD) and density functional theory (DFT) studies
demonstrate that common o-phenylene and saturated ethyl backbone motifs result in small but non-negligible perturbations to
10Dq(Td) and iron−bisphosphine bonding character at the iron(II) level within isostructural tetrahedra as well as in five-
coordinate iron(I) complexes FeCl(dpbz)2 and FeCl(dppe)2. Notably, coordination of Xantphos to FeCl2 results in a ligand field
significantly reduced relative to those of its iron(II) partners, where a large bite angle and consequent reduced iron−phosphorus
Mayer bond orders (MBOs) could play a role in fostering the unique ability of Xantphos to be an effective additive in Kumada
and Suzuki−Miyaura alkyl−alkyl cross-couplings. Furthermore, it has been found that the peripheral steric bulk of the SciOPP
ligand does little to perturb the electronic structure of FeCl2(SciOPP) relative to that of the analogous FeCl2(dpbz) complex,
potentially suggesting that differences in the steric properties of these ligands might be more important in determining in situ
iron speciation and reactivity.
carbon centers using ferric and ferrous salts in conjunction with
the chelating phosphine Xantphos in the cross-coupling of alkyl
borates25 and Grignards24 with primary alkyl halides. Recent
studies have also continued to expand the breadth of available
iron-catalyzed cross-couplings, including the work of Jacobi von
Wangelin and co-workers on iron-catalyzed cross-couplings of
alkynyl acetates,34 allylations of aryl Grignards,35 and reductive
aryl−alkenyl cross-coupling reactions.36
Despite the advances in developing efficient and selective
bisphosphine-supported iron cross-coupling methods, insight
into how specific bisphosphines affect iron electronic structure
and reactivity remains largely undefined. Such considerations
are important because the bisphosphine ligand that is effective
for a particular cross-coupling reaction may be ineffective in
other reactions (Scheme 1). For example, in the Suzuki−
Miyaura coupling of phenyl borates with secondary alkyl
1. INTRODUCTION
The past two decades have witnessed a renaissance in the
development of iron-based methods for catalytic C−C bond
transformations, motivated by iron’s advantageous economic
profile and its rich and tunable redox chemistry.1−6 Following
Kochi’s seminal reports on iron-catalyzed C−C cross-coupling
in the 1970s using simple iron salts,7−11 recent research efforts
have focused on the effects of additives on reaction efficiency
and product distributions, demonstrating the efficacy of
molecules such as TMEDA,12−15 N-heterocyclic carbenes
(NHCs),16−21 and bisphosphines16,22−25 as effective additives
in various coupling reactions (e.g., Suzuki−Miyaura, Kumada,
and Negishi). Recent work has also demonstrated the utility of
well-defined, isolable ferrous complexes bearing bisphosphine
supporting ligands of varying steric bulk (e.g., SciOPP,26−29
dpbz,22,30,31 and dppe32,33) as effective precatalysts in the cross-
coupling of aryl nucleophiles with various alkyl and benzyl
electrophilic substrates (Scheme 1). Furthermore, both
Nakamura and Chai reported the successful coupling of sp3
Received: October 6, 2015
© XXXX American Chemical Society
A
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Scheme 1. Examples of Iron-Bisphosphine-Catalyzed Cross-Coupling Reactions
halides developed by Nakamura and co-workers, product yields
of >90% and excellent selectivity can be achieved under
ambient conditions using FeCl2(SciOPP) as a precatalyst
whereas use of the less sterically demanding dpbz ligand within
the well-defined FeCl2(dpbz)2 precatalyst results in a
significantly diminished yield and a high recovery of starting
material.26 By contrast, simpler bisphosphines such as dpbz and
dppe have been shown by Bedford and co-workers to aid aryl−
benzyl Negishi couplings favorably upon stabilization of ferrous
precatalysts.24−26 It should be noted that no examples of
effective iron-catalyzed alkyl−alkyl couplings using one of the
more widely applied SciOPP, dpbz, or dppe ligands and/or
additives exist, suggesting Xantphos may allow the generation
of distinct reactive catalytic intermediates relative to the other
bisphosphine ligands.24,25
These important reactivity differences combined with our
recent work on elucidating the identity, electronic structure,
and reactivity of transmetalated intermediates within iron-
SciOPP-catalyzed aryl−alkyl coupling systems37,38 motivated
the extension of our spectroscopic and theoretical studies to the
evaluation of the effect of catalytically relevant bisphosphine
supporting ligands on electronic structure and bonding present
in nontransmetalated iron(II) and iron(I) species. The
catalytically relevant bisphosphines highlighted in Scheme 1
differ structurally in their backbone linkages and rigidity as well
as peripheral steric substitution. To date, there has been no
systematic investigation of the relative effects of these structural
characteristics on resulting coordination compounds with iron
in these oxidation states, and by extension, there has yet to be
discussion of how these similarities or differences may
ultimately contribute to reactivity. In the study presented
here, we have sought to address this understudied area by
elucidating electronic structure and bonding in a series of well-
defined, monomeric, nontransmetalated four-coordinate iron-
(II) species bearing bisphosphine ligands with catalytically
relevant structural motifs utilizing magnetic circular dichroism
(MCD) and 57Fe Mossbauer spectroscopies combined with
̈
density functional theory (DFT). We further extend this
experimental and theoretical analysis to documented five-
coordinate Fe(I) species complexes of dpbz and dppe, namely,
FeCl(dpbz)2 and FeCl(dppe)2, respectively, to evaluate the
effect of bisphosphine ligands in lower-valent iron species with
structures of relevance to application as precatalysts in C−C
coupling reactions. The results obtained from these studies
provide the first quantitation of the effects of bisphosphine
backbone structure and peripheral steric substitution on ligand
field (LF) strengths and orbital compositions as a function of
oxidation state and coordination number, which provide initial
insight into the possible effects of these properties on reported
differences in cross-coupling activity as a function of
bisphosphine ligand.
2. RESULTS AND ANALYSIS
2 . 1 . S t r u c t u r a l C h a r a c t e r i z a t i o n o f
FeCl2(Bisphosphine) Complexes. The effectiveness of the
chelating phosphines SciOPP, dpbz, dppe, and Xantphos in
promoting C−C cross-coupling catalysis combined with their
varied backbone and substitutional character heightened our
interest in comparing the electronic structure of their iron
compounds within a specific oxidation state. To date, the only
B
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Figure 1. X-ray crystal structures with selected bond length and angle metrics of (A) FeCl2(dpbz), (B) FeCl2(tBudppe), and (C) FeCl2(Xantphos).
Structures are shown with thermal ellipsoids at the 50% probability level, and hydrogen atoms have been omitted for the sake of clarity. For
FeCl2(Xantphos), one of the two independent molecules in the asymmetric unit is pictured and highlighted metrically.
Table 1. Summary of Ligand Field, Spin Hamiltonian, and Solid State 80 K 57Fe Mossbauer Parameters for
̈
FeCl2(Bisphosphine) Complexes
NIR and VTVH MCD
Mossbauer
̈
a
complex
LF bands (cm−1
)
10Dq(Td) (cm−1
)
D (cm−1
)
|E/D|
δ (cm−1
)
g∥
δ (mm/s)
ΔEQ (mm/s)
FeCl2(SciOPP)
FeCl2(dpbz)
7160, 8140
7170, 8230
6710, 7670
5160, 7100
7650
7700
7190
6130
−8
−9
−7
2
0.31
0.29
0.33
0.22
2.1 0.2
2.3 0.2
2.3 0.2
1.4 0.2
8.2 0.2
8.3 0.2
8.3 0.2
8.2 0.2
0.73
0.73
0.73
0.75
2.54
2.49
2.82
2.67
2
FeCl2(tBudppe)
2
2
FeCl2(Xantphos)
−10
a
The error bars for the |E/D| values are 0.02, with a maximum possible value of |E/D| of 0.33.
structurally characterized monomeric 1:1 ferrous adduct
incorporating these ligands is FeCl2(SciOPP), developed by
Nakamura and co-workers. Because structurally characterized
complexes of the type FeCl2(PP) (PP = bidentate phosphine
ligation) have been widely reported,39−44 we envisioned an
isostructural series of four-coordinate ferrous dihalides as a set
of model compounds to begin to systematically discern how
different structural characteristics of the chelating phosphine
affect the overall electronic structure and bonding within their
coordination compounds at the iron(II) level. While the
preparation of FeCl2(dpbz) has been summarized in the
literature using stoichiometric equivalents of dpbz and FeCl2,30
no solid state characterization of this adduct has been reported.
Reaction of dpbz and FeCl2(THF)1.5 in hot toluene followed by
slow cooling to −30 °C resulted in the isolation of crystals
suitable for X-ray diffraction analysis. Extension of these
synthetic efforts to the Xantphos ligand resulted in isolation
of the desired FeCl2(Xantphos) complex, with single crystals
obtained from the slow evaporation of a concentrated THF
solution of the complex. The solid state structures of
FeCl2(dpbz) and FeCl2(Xantphos) are depicted in Figure 1.
Both FeCl2(dpbz) and FeCl2(Xantphos) are characterized by
a distorted tetrahedral iron center, consistent with the observed
difference between the P−Fe−P and Cl−Fe−Cl angles within
each complex. The differences between these angle metrics
within a specific complex are most pronounced for
FeCl2(dpbz) and FeCl2(SciOPP), in which the rigid o-
phenylene linkage of both dpbz and SciOPP constrain the
P−Fe−P angle to 80.38° and 80.63°, respectively. While the
larger Cl−Fe−Cl angles in these two species [124.85° for
FeCl2(dpbz) and 122.16° for FeCl2(SciOPP)] compare well
with that of FeCl2(Xantphos) (122.12°), the additional width
of the xanthene linker in the latter species results in a much
larger P−Fe−P angle (bite angle) of 109.30°. This bite angle
magnitude compares well with those of the known first row
metal adducts CoCl2(Xantphos)45 and NiCl2(Xantphos).46
Isolation of a monomeric four-coordinate ferrous adduct
bearing catalytically relevant dppe was, in contrast, unattainable
because of the increased flexibility of the backbone. Langer and
co-workers recently demonstrated the prevalence of a bridging
dppe coordination motif when combined with equimolar
amounts of FeCl2,47 isolating the coordination polymer [μ-
(dppe)FeCl2]n. Having isolated the same polymeric material
when attempting the equimolar combination of dppe with
FeCl2 under separate reaction conditions,48 we explored a more
sterically encumbered ligand structure to overcome the
preference of such a flexible bisphosphine to support polymeric
structures. Incorporating the bulky 3,5-di-tert-butyl substitution
of Nakamura’s SciOPP ligand into a bisphosphine scaffold
bearing a saturated ethyl backbone, we synthesized 1,2-bis(3,5-
di-tert-butylphenylphosphino)ethane (tBudppe) and investigated
its coordination to FeCl2. The monomeric complex
FeCl2(tBudppe) was synthesized through the reaction of
equimolar amounts of (tBudppe) with FeCl2(THF)1.5 in
refluxing 2-propanol, and X-ray quality single crystals were
isolated from slow evaporation of a concentrated 1,4-dioxane
solution at room temperature. The bond length metrics of the
crystal structure are very similar to those of the analogous
complexes bearing SciOPP, dpbz, and Xantphos ligands with
Fe−P distances of 2.429 and 2.455 Å and Fe−Cl distances of
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2.212 and 2.223 Å (Figure 1). The P−Fe−P angle of 82.85° is
larger than those observed for FeCl2(SciOPP) and
FeCl2(dpbz), and the Cl−Fe−Cl angle of 120.65° is slightly
reduced by comparison.
= 2 complex, the presence of two observed LF transitions
reflects the distorted nature of the tetrahedron, whereby the
difference in transition energy reflects loss of degeneracy from
the 5T2 excited state. From these LF transitions, the magnitude
of the ligand field is determined to be 10Dq(Td) = 7650 cm−1.
Notably, the observed value of 10Dq(Td) in the FeCl2(SciOPP)
ligand is larger than those previously reported for the
monodentate phosphine complexes FeCl2(PPh3)2 [10Dq(Td)
= 6590 cm−1] and FeCl2(PMe3)2 [10Dq(Td) = 6970 cm−1].49
The LF bands exhibit a pseudo-A term, where the intensity of
the transitions derives from two oppositely signed, temper-
ature-dependent C-term absorption features, a spectral profile
consistent with the NIR MCD spectra of other Fe(II)-
phosphine and bisphosphine distorted tetrahedra studied
previously.37,38,49 Saturation magnetization data collected at
6481 cm−1 for FeCl2(SciOPP) (Figure 2A, inset) are described
well by an S = 2 non-Kramers doublet model with negative
zero-field splitting (ZFS) and ground state spin Hamiltonian
parameters of δ = 2.1 0.2 cm−1, g∥ = 8.2 0.2 cm−1, axial
ZFS parameter D = −8 2 cm−1, and rhombicity |E/D| = 0.31
0.02 (Table 1).
2. 2. Spectroscopic Cha ra cteriz atio n o f
FeCl2(Bisphosphine) Complexes. With access to a series
of distorted tetrahedral bisphosphine-supported ferrous diha-
lide complexes, 57Fe Mossbauer and MCD spectroscopic
̈
investigations were performed to probe the effects of
bisphosphine backbone character and peripheral steric
substitution on the overall electronic structure of their adducts
with FeCl2. For FeCl2(SciOPP), solid state 57Fe Mossbauer
̈
analysis yields a single quadrupole doublet with parameters of δ
= 0.73 mm/s and ΔEQ = 2.54 mm/s, consistent with a high-
spin Fe(II) distorted tetrahedron (Table 1 and Figure S1). The
5 K, 7 T near-infrared (NIR) MCD spectrum of
FeCl2(SciOPP) is described by two LF transitions at low
energy, a positive band at 7160 cm−1 and a negative band at
8140 cm−1 (Figure 2A). While LF theory predicts a single spin-
allowed transition in Td symmetry (5E → 5T2) for a high-spin S
The rigid o-phenylene linkage between the phosphorus
atoms in the SciOPP ligand is shared with dpbz, providing the
same conjugated backbone electronic system between the two
species, though it is obvious that the two ligands differ greatly
in their steric bulk. The similar solid state structural metrics
shared by FeCl2(SciOPP) and FeCl2(dpbz) (vide supra) are
corroborated by very similar 57Fe Mossbauer parameters, with
̈
FeCl2(dpbz) characterized by δ = 0.73 mm/s and ΔEQ = 2.49
mm/s in the solid state (Table 1). The 5 K, 7 T NIR MCD
spectrum of FeCl2(dpbz) exhibits LF transitions at 7170 and
8230 cm−1, yielding a value of 10Dq(Td) = 7700 cm−1, nearly
identical to that determined for FeCl2(SciOPP) (Figure 2B). In
addition, saturation magnetization data for FeCl2(dpbz)
(Figure 2B, inset) are described well by an S = 2 non-Kramers
doublet model with ground state spin Hamiltonian parameters
(Table 1) very similar to those obtained for FeCl2(SciOPP).
The combined spectroscopic investigations highlight the fact
that with constant backbone structure, the presence or absence
of 3,5-di-tert-butyl substitution has little effect on the overall
electronic structure of the resulting four-coordinate adducts at
the Fe(II) level. More appreciable differences in 10Dq(Td) are
observed upon varying the ligand to tBudppe and Xantphos. For
FeCl2(tBudppe), observation of LF transitions at 6710 and 7670
cm−1 [10Dq(Td) = 7190 cm−1] by NIR MCD indicates a ligand
field smaller than that previously observed for FeCl2(SciOPP)
and FeCl2(dpbz). Notably, the magnitude of 10Dq(Td)
undergoes an even larger decrease upon ligation of Xantphos
to FeCl2, with LF transitions observed at 5160 and 7100 cm−1
in the NIR MCD spectrum, corresponding to 10Dq(Td) = 6130
cm−1. Despite this large shift in 10Dq(T ), the Mossbauer
̈
d
isomer shift of FeCl2(Xantphos) (δ = 0.75 mm/s) is observed
to remain quite consistent with those of the other complexes in
the series. While Mossbauer spectroscopy can be insightful for
̈
Figure 2. NIR MCD spectra (5 K, 7 T) of (A) FeCl2(SciOPP), (B)
FeCl2(dpbz), (C) FeCl2(tBudppe), and (D) FeCl2(Xantphos).
Gaussian fits are given for each spectrum (---). Saturation magnet-
ization data (dots) and best fits (lines) are given in the insets for each
species, collected at 6481, 6536, 6061, and 5482 cm−1 for
FeCl2(SciOPP), FeCl2(dpbz), FeCl2(tBudppe), and FeCl2(Xantphos),
respectively. Saturation magnetization data were collected at 2, 3, 5,
7.5, 10, 15, 25, and 35 K. All spectra were collected in a 6:1 toluene-
d8/benzene-d6 mixture except for FeCl2(Xantphos), which was
collected on a solid mull sample.
determining differences in oxidation or spin state among iron
species, the results presented here clearly demonstrate that
MCD spectroscopy is a higher-resolution method for probing
differences in the ligand field of iron species in the same
oxidation and spin state. Lastly, the 5 K, 7 T UV−vis MCD
spectra of each of the FeCl2(bisphosphine) species contain
multiple high-energy charge transfer (CT) transitions (Figure
S2) that are assigned and summarized in the Supporting
Information using TD-DFT analysis.
D
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Table 2. Comparison of Experimental and Calculated Structural Parameters for FeCl2(Bisphosphine) Complexes
FeCl2(SciOPP)
exp
FeCl2(dpbz)
exp
FeCl2(Xantphos)
FeCl2(tBudppe)
calc
calc
exp
calc
exp
calc
Fe−P1 (Å)
Fe−P2 (Å)
Fe−Cl1 (Å)
Fe−Cl2 (Å)
P−Fe−P (deg)
Cl−Fe−Cl (deg)
2.463(1)
2.441(2)
2.219(1)
2.217(2)
80.63(4)
122.16(6)
2.42
2.41
2.25
2.23
83.0
121.7
2.439(1)
2.433(1)
2.219(1)
2.213(1)
80.38(3)
124.85(4)
2.41
2.41
2.24
2.23
81.9
123.6
2.456(2)
2.457(2)
2.214(1)
2.255(1)
109.30(4)
122.12(5)
2.45
2.44
2.23
2.23
108.8
128.6
2.429(1)
2.455(1)
2.212(1)
2.223(2)
82.85(3)
120.65(4)
2.45
2.44
2.23
2.24
84.7
119.1
Figure 3. Calculated FMO energy diagrams for FeCl2(bisphosphine) complexes and selected orbital depictions for FeCl2(SciOPP).
2.3. Electronic Structure Calculations of
FeCl2(Bisphosphine) Complexes. Spin-unrestricted DFT
calculations were used in conjunction with MCD spectroscopy
to gain further insight into the effects of bisphosphine backbone
and steric substitution on electronic structure and bonding in
this series of iron(II) bisphosphine complexes. Geometry
optimizations using PBEPBE/TZVP were performed on the
crystal coordinates, demonstrating good agreement between
experiment and theory (Table 2) with minor contractions of
the Fe−P bond distances and slightly elongated Fe−Cl bonds
observed in the calculated structures (in solvent models)
relative to the crystal structures. Additionally, the order of
increasing P−Fe−P angle in the crystal structures is preserved
in the optimized geometries: FeCl2(dpbz) < FeCl2(SciOPP) <
FeCl2(tBudppe) < FeCl2(Xantphos).
Evaluations of molecular orbital character and energies were
subsequently conducted from the optimized geometries using
spin-unrestricted B3LYP/TZVP, placing emphasis on the
occupied and unoccupied frontier molecular orbitals (FMOs)
of the β manifold to describe the major contributions to
bonding. A cumulative β FMO energy level diagram containing
all four complexes is shown in Figure 3, accompanied by
selected FMO depictions for FeCl2(SciOPP). The FMOs of
FeCl2(SciOPP) bear dominant Fe d character in the HOMO
2
2
(β273, 85% dx −y ) as well as in β274 (LUMO), β275 (LUMO
+1), β276 (LUMO+2), and β279 (LUMO+5), assigned as dxz,
E
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2
dyz, dz , and dxy, respectively (also see Figure S5). The highest
Table 3. Calculated Mayer Bond Orders for
FeCl2(Bisphosphine) Complexes
2
2
occupied ligand-based MO lies directly below the dx −y HOMO
in energy (β272), containing Fe dxy/phosphorus p σ bonding
interaction character. Orbital contributions in addition to iron d
character in the LUMO, LUMO+1, and LUMO+2 derive from
mixing with p orbital character of both the chloride ligands and
the phosphorus atoms of the SciOPP ligand. Increased orbital
covalency is found in the LUMO (57% Fe dxz) due to an
increased level of mixing with phosphorus p character and an
elevated Fe−P overlap population relative to the other Fe d-
based MOs. An increased level of mixing in the dxy-based β279
(LUMO+5, 26% dxy) originates from elevated intra-aryl π
character, present as the major component of the orbital
description. Notably, a consecutive energy ordering of
unoccupied d-based FMOs is disrupted by low-lying ligand-
based orbitals β277 and β278. In these orbitals, high degrees of
intraligand π bonding and antibonding interactions in the
conjugated o-phenylene backbone are observed, and thus,
ligand-based character dominates the orbital descriptions. The
stabilization of these ligand-based acceptor MOs is consistent
with the FMO description of the analogous o-phenylene-linked
FeCl2(dpbz) (vide infra) and has also been observed in the MO
compositions of mono- and bis-mesitylated Fe(II)-SciOPP
species.37
Mayer bond orders
Fe−P Fe−Cl
complex
FeCl2(SciOPP)
FeCl2(dpbz)
FeCl2(tBudppe)
0.707, 0.711
0.706, 0.707
0.693, 0.680
0.670, 0.669
0.823, 0.854
0.831, 0.852
0.864, 0.833
0.867, 0.854
FeCl2(Xantphos)
reduced Fe−P bond covalency, and, hence, lower values of
10Dq(Td). Additionally, increased Fe−P bond orders are found
to be accompanied by a general decrease in the overall Fe−Cl
bond orders of the complexes. Overall, the FMO descriptions
and MBO analysis support the spectroscopic observations that
FeCl2(SciOPP) and FeCl2(dpbz) are very similar with respect
to their electronic structure and bonding, despite the peripheral
steric bulk present in the SciOPP ligand structure. In contrast,
FeCl2(Xantphos) is observed to have the most distinct
electronic structure and bonding characteristics of the
complexes present in this iron(II) bisphosphine series.
2.4. Electronic Structure and Bonding in Iron(I)
Bisphosphine Complexes. Recent reports by Bedford and
co-workers have highlighted the use of well-defined low-spin
five-coordinate iron(I) bisphosphine complexes supported by
dpbz and dppe ligands as precatalysts in the Negishi coupling of
benzyl substrates with aryl nucleophiles.31,32 The implication of
iron(I) as a potentially catalytically relevant oxidation state
within these C−C cross-coupling reactions motivated the
extension of the approach described above to understanding
the effects of a supporting bisphosphine ligand on the
electronic structure and bonding at iron in the +1 oxidation
state. While well-defined iron(I) bisphosphines with ligands
relevant to cross-coupling are rare, FeCl(dppe)2 and FeCl-
(dpbz)2 provide a firm starting point for evaluating the effects
of bisphosphine backbone structure on electronic structure and
bonding at the iron(I) oxidation state. NIR MCD analyses of
both species yield rich LF spectra in which six d−d transitions
can be resolved (Figure 4), consistent with their low-spin
nature and the resulting spin-allowedness of both α and β
transitions for these S = 1/2 species. TD-DFT analysis of the LF
spectra predicts high degrees of mixing within the transitions,
FMO analysis of the remainder of the series of iron(II)
bisphosphine complexes results in a very similar description of
Fe d-based MOs, with FeCl2(dpbz), FeCl2(tBudppe), and
FeCl2(Xantphos) each possessing a HOMO of dominant Fe
2
2
dx −y character and high-lying dxy-based MOs (Figure 3 and
Figures S6−S8). Notably, the dxy-derived MOs are mixed to a
lesser degree for FeCl2(dpbz) (β152) and FeCl2(tBudppe)
(β265) than in FeCl2(Xantphos), the latter bearing two
unoccupied FMOs of very similar dxy character (β188, 22%;
β196, 25%). While FeCl2(SciOPP), FeCl2(dpbz), and
FeCl2(tBudppe) contain the same cumulative d orbital energy
ordering, FeCl2(Xantphos) possesses a more destabilized dxz
orbital, pushing it to an energy higher than that of dyz.
Analogous to FeCl2(SciOPP), each additional complex in the
series bears high-lying occupied ligand-based MOs charac-
terized by Fe−P σ bonding interactions, where these MOs are
slightly more stabilized in FeCl2(dpbz) and FeCl2(Xantphos).
The most notable deviations in FMO descriptions across the
iron(II) complexes occur in the nature of their low-lying ligand-
based acceptor orbitals, an effect that is a direct consequence of
the nature of the ligand backbone structure. As seen in Figure 3
and Figures S6−S8, MOs bearing high degrees of π character
within the conjugated o-phenylene backbone of FeCl2(SciOPP)
(β277, β278, and β280) and FeCl2(dpbz) (β149, β150, and
β151) are more energetically stabilized than the unoccupied
ligand-based FMOs of FeCl2(tBudppe) in which π MO character
resides only in the conjugated systems of the aryl substituents.
Intermediate stabilization of unoccupied ligand-based MOs is
observed in FeCl2(Xantphos), in which increased π density in
the extended xanthene system combined with phenyl π
character imparts sufficient stability to push these MOs closer
in energy to Fe dz (β183) than is observed in FeCl2(tBudppe)
2
(Figure S8).
Mayer bond order (MBO) analyses were also conducted
across the series of Fe(II) bisphosphine complexes, the results
of which are shown in Table 3. The calculated magnitudes of
Fe−P MBOs trend with the experimental magnitude of
10Dq(Td) obtained from NIR MCD (vide supra), where
reduced Fe−P bond orders indicate a weaker Fe−P interaction,
Figure 4. NIR MCD spectra (5 K, 7 T) of (A) FeCl(dpbz)2 and (B)
FeCl(dppe)2. Spectra were collected in a 6:1 toluene-d8/benzene-d6
mixture.
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Figure 5. Calculated FMO energy diagrams for FeCl(dppe)2 and FeCl(dpbz)2 and selected orbital depictions for FeCl(dpbz)2.
and the experimentally observed shift of the LF transitions to
slightly higher energies in the case of FeCl(dppe)2 relative to
FeCl(dpbz)2 demonstrates a slightly increased ligand field in
the case of the former. UV−vis MCD analysis of both
complexes results in similar CT profiles, with TD-DFT used to
assign Fe d → phenyl/o-phen π MLCT as the dominant
transition character between 16000 and 32000 cm−1 (Figure
S3). The experimental CT regions are consistent with the
presence of a large number of low-lying ligand-based π acceptor
FMOs (Figure 5).
3. DISCUSSION
The application of bisphosphines as additives and supporting
ligands in iron-catalyzed C−C cross-coupling catalysis has
resulted in numerous reports of efficient and selective reaction
methodologies. An attractive feature of these molecules is the
ability to tune their backbone electronic properties and
peripheral steric structure to access new ligand architectures.
Despite the reported differences in reactivity of catalytic
systems as a function of bisphosphine ligand, no systematic
investigation of the effects of bisphosphine ligand structural
variations on electronic structure and bonding of well-defined
iron complexes has been reported. In this study, we have
utilized a combination of MCD spectroscopic investigations
and DFT studies to obtain insight into electronic structure and
bonding in iron(II) and iron(I) species containing bi-
sphosphine ligand structures relevant to iron-catalyzed cross-
coupling, thus focusing on analysis of iron oxidation states
proposed to be relevant as on-cycle active species in catalysis.
From this study, the following effects of bisphosphine
structural variations on resulting iron-bisphosphine electronic
structure and bonding have been determined. (1) The presence
of o-phenylene (SciOPP and dpbz) versus ethyl (tBudppe and
dppe) backbones in bisphosphine ligands has a minor but non-
negligible effect on electronic structure and bonding in both
iron(II) and iron(I) bisphosphine complexes. At the iron(II)
level, the presence of an ethyl linker in FeCl2(tBudppe) results
in a small decrease in 10Dq(Td) relative to those of the
corresponding SciOPP and dpbz complexes. The most notable
effects of backbone saturation exist in the characterization of
excited states, where the lack of a conjugated ligand backbone
in FeCl2(tBudppe) results in higher-lying ligand-based acceptor
MOs as characterized by MO calculations and experimental CT
analysis. Similarly, at the iron(I) level, varying the saturation of
the bisphosphine backbone in FeCl(dppe)2 and FeCl(dpbz)2
results in minor differences in electronic structure and bonding.
Chelation of dppe leads to a small increase in LF, ligand-based
acceptor MOs of slightly higher energy and slightly higher-
energy MLCT transitions for FeCl(dppe)2 than for FeCl-
(dpbz)2. (2) The introduction of peripheral steric bulk in
bisphosphines with o-phenylene backbones (SciOPP and dpbz)
has minimal effects on iron(II)-bisphosphine electronic
The highest-lying occupied MOs in both the α and β
manifolds for FeCl(dppe)2 and FeCl(dpbz)2 are characterized
by increased Fe d character (Figure 5). The β manifold of each
2
2
complex is described by HOMOs of dx −y character [β231 for
FeCl(dppe)2 and β255 for FeCl(dpbz)2] with lower-lying dyz
and dxz orbitals at β229 and β230, respectively, for FeCl(dppe)2
and β253 and β254, respectively, in the case of FeCl(dpbz)2. A
consecutive ordering of α and β Fe d-based MOs is broken in
the case of both complexes by low-lying ligand-derived π
acceptor orbitals. In the case of FeCl(dpbz)2, π orbital density
located on the o-phenylene backbone of the dpbz ligands
results in α and β LUMO energies lower than those of
FeCl(dppe)2. This is consistent with observed energy differ-
ences between ligand-based acceptor orbitals at the iron(II)
level upon variation of ligand backbone saturation (vide supra).
MBO calculations result in an increased average Fe−P MBO in
FeCl(dppe)2 (0.981) relative to that of FeCl(dpbz)2 (0.968),
consistent with the generally shorter Fe−P bond distances in
both the crystal and DFT-optimized structure in FeCl(dppe)2
and its slightly larger LF magnitude obtained from NIR MCD.
Overall, the electronic structure calculations on these bi-
sphosphine-supported complexes corroborate the experimental
observations that the differences in backbone saturation in dppe
and dpbz result in an observable but small effect on the overall
electronic structure and bonding at the iron(I) level, in turn
indicating that the nature of the bisphosphine ligand in
analogous five-coordinate halides likely has little effect on the
reactivity of the starting precatalyst.
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structure and bonding. Nearly identical LF splittings and MO
compositions and energies are observed for FeCl2(SciOPP) and
FeCl2(dpbz), indicating that the introduction of the 3,5-di-tert-
butyl substitution pattern in SciOPP does not result in
structural distortions that significantly perturb the overall
electronic structure and bonding at iron relative to the less
bulky dpbz ligand. (3) Xantphos ligation at the iron(II) level
results in a distinctly different iron−phosphorus bonding and
electronic structure description compared to those of the other
ferrous bisphosphine complexes employed in this study. The
magnitude of 10Dq(Td) in FeCl2(Xantphos) is significantly
reduced compared to the magnitudes of those of
FeCl2(SciOPP) and FeCl2(dpbz), correlating with the lowest
overall iron−phosphorus MBOs in the series.
reductive elimination of a mononuclear, bisphenylated iron(II)-
SciOPP species.38 By contrast, 2:1 ferrous adducts can be easily
accessed with the less sterically bulky dpbz and used as
precursors to generate bis-chelated FeX(dpbz)2 (X = Cl, Br, or
p-tolyl) iron(I) species through stoichiometric treatment with
Grignard or organozinc nucleophiles.31 In fact, efforts in our lab
to isolate the analogous iron(I) SciOPP species, FeBr-
(SciOPP)2, have resulted in only the monochelated bridged
iron(I) dimer [FeBr(SciOPP)]2 being accessible upon reduc-
tion of FeBr2(SciOPP) with KC8 (Figure 6), consistent with
the fact that no 2:1 adduct of SciOPP to iron has yet been
reported for iron in any oxidation state.
Studies of the effects of bisphosphine ligands on electronic
structure and bonding in transmetalated, active iron-bi-
sphosphine species are required to definitively correlate such
effects to differences in reactivity. However, the observed
similarities and differences in electronic structure and bonding
within nontransmetalated iron complexes coordinating catalyti-
cally relevant bisphosphine scaffolds do provide some
preliminary insight into potential contributions of bisphosphine
ligands to reactivity. For example, Xantphos is the only
bisphosphine ligand shown to date to be effective in promoting
iron-catalyzed Kumada and Suzuki−Miyaura alkyl−alkyl cross-
couplings.18,19 The difference in effects of the molecular
structure of Xantphos on electronic structure and iron−
phosphorus bonding in ferrous Xantphos adducts relative to
isostructural adducts supported by o-phenylene and ethyl
backbone linkers is likely a significant contributor to the unique
cross-coupling reactivities observed, potentially functioning to
govern the extent of reduction accessible in Xantphos-
supported intermediates and helping to mitigate β hydrogen
elimination within in situ-formed species. Furthermore, both
dpbz and dppe have been shown by Bedford and co-workers to
be effective for iron-catalyzed Negishi cross-couplings of aryl
nucleophiles and benzyl halides despite their significant
differences in backbone structure.24−26 While the question of
whether iron(I) or iron(II) active species are functional in these
catalytic systems is still to be answered, the studies herein
demonstrate that small differences in electronic structure and
bonding result from chelation of o-phenylene and ethyl-bridged
bisphosphines at both oxidation state levels. Combined with
the observation of comparable catalytic activity by Bedford and
co-workers for iron(I) precatalysts bearing dpbz and dppe, this
observation appears to suggest that the small electronic
structure and bonding differences resulting from o-phenylene
versus ethyl backbones may not have a significant effect in these
reactions. Lastly, it is interesting to note that disparities in
reactivity reported in the literature using well-defined SciOPP-
and dpbz-supported precatalysts in Kumada and Suzuki−
Miyaura cross-couplings of phenyl nucleophiles and secondary
alkyl halides do exist despite their nearly identical electronic
and orbital descriptions in four-coordinate ferrous dichloride
complexes as determined herein.26,27 Thus, it may be that the
steric variations present in these ligands are more important for
dictating catalytic performance by altering in situ iron
speciation. In fact, evidence of such sterically driven effects
on iron speciation has already been reported on the basis of the
nature of the reduced iron species observed to form in situ with
SciOPP and dpbz ligands. In reactions of FeCl2(SciOPP) with
phenyl nucleophiles, it has been shown that the dominant
reduced iron species is Fe(η6-biphenyl) (SciOPP), formed via
Figure 6. X-ray crystal structure of [FeBr(SciOPP)]2 shown with
thermal ellipsoids at the 50% probability level. Hydrogen atoms and
cocrystallized solvent (Et2O) have been omitted for the sake of clarity.
4. CONCLUSIONS
In this study, the combination of MCD and DFT studies has
provided direct insight into the effects of bisphosphine ligand
structural variations in ligands utilized in iron-catalyzed cross-
coupling on iron-bisphosphine electronic structure and bonding
in both iron(II) and iron(I) complexes. Interestingly, the
unique cross-coupling reactivity observed employing Xantphos
is found to correlate with distinct differences in electronic
structure and bonding with this ligand compared to other
bisphosphines employed in iron-catalyzed cross-coupling
bearing o-phenylene or saturated ethyl backbone linkages.
The analogous reactivities in Negishi cross-couplings with dppe
and dpbz combined with the small but non-negligible
differences in electronic structure and bonding observed herein
in both their iron(II) and iron(I) complexes appear to suggest
that electronic structure and bonding effects of o-phenylene
versus ethyl backbones may not have a significant effect in these
reactions. Furthermore, it has been found that the peripheral
steric bulk of the SciOPP ligand does little to perturb the
electronic structure of FeCl2(SciOPP) relative to the analogous
FeCl2(dpbz) complex, potentially suggesting that differences in
the steric properties of these ligands might be more important
in determining catalytic performance in these systems.
Extension of these electronic structure studies to additional
iron-bisphosphine species, including series of transmetalated
species as a function of bisphosphine ligand, should continue to
expand our understanding of ligand structural variations on
reactivity in cross-couplings utilizing iron-bisphosphines.
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plotted with the ChemCraft program. TD-DFT was used to calculate
electronic transition energies for the 80−100 lowest-energy states.
5.4.1. 1,2-Bis(3,5-di-tert-butylphenylphosphino)ethane (tBudppe).
3,5-Di-tert-butylphenylmagnesium bromide was prepared by stirring
4.34 g of 3,5-di-tert-butylbromobenzene (16.1 mmol) and 502 mg of
Mg0 (20.7 mmol, 1.3 equiv) in 50 mL of THF under gentle heat for 12
h. The mixture was filtered to remove excess Mg0, and the filtrate was
added dropwise to a stirring, prechilled (−65 °C) solution of 1,2-
bis(dichlorophosphino)ethane (852 mg, 3.7 mmol) in 25 mL of THF.
After completion of addition, the reaction mixture was stirred at −65
°C for 1 h and then allowed to warm to room temperature while being
stirred for an additional 4 h. The solvent was then removed under
reduced pressure and the off-white crude solid redissolved in
dichloromethane (50 mL) and washed with saturated (NH4)2SO4
(50 mL). The aqueous layer was then separated and extracted with
dichloromethane (3 × 50 mL portions). The combined organic layers
were then washed with brine (150 mL). The organic phase was
separated, dried over MgSO4, and filtered, and the solvent was
removed under reduced pressure. The resulting solid was sonicated in
50 mL of MeOH and isolated by filtration as a colorless powder: yield
5. EXPERIMENTAL SECTION
5.1. General Considerations. All reagents were purchased from
commercial sources. All air and moisture sensitive synthetic
manipulations were performed using an MBraun inert atmosphere
(N2) drybox or by standard Schlenk techniques. All preparations of
spectroscopy samples were conducted in an MBraun inert atmosphere
(N2) drybox equipped with a direct liquid nitrogen inlet line.
Anhydrous solvents were further dried using activated alumina, 4 Å
molecular sieves and stored under an inert atmosphere over molecular
sieves. SciOPP ligand was prepared according to the literature
method,26 and dpbz, dppe, and Xantphos were purchased from
Strem and used as received. FeCl2(SciOPP), FeCl(dpbz)2, and
FeCl(dppe)2 were prepared using literature procedures.26,31 All
iron(II) bisphosphine complexes in this study were moderately air
sensitive, whereas all iron(I) bisphosphines were highly air sensitive.
̈
5.2. Mossbauer Spectroscopy. Solid state samples were
prepared in an inert atmosphere glovebox equipped with a liquid
nitrogen fill port to allow sample freezing to 77 K within the glovebox.
Each sample was loaded into a Delrin Mossbauer sample cup for
̈
measurements and loaded under liquid nitrogen. 57Fe Mossbauer
2.70 g (87%); H NMR (400 MHz, THF-d8, 298 K) δ 1.26 (s, 72H),
1
̈
measurements were performed using a SeeCo MS4 Mossbauer
spectrometer integrated with a Janis SVT-400T He/N2 cryostat for
measurements at 80 K with a 0.07 T applied magnetic field. Isomer
shifts were determined relative to α-Fe at 298 K. All Mossbauer
2.11 (s, 4H), 7.24 (s, 8H), 7.38 (s, 4H); 31P{1H} NMR (162 MHz,
THF-d8, 298 K) δ −8.84 (relative to external 85% H3PO4). Elemental
Anal. Calcd: C, 82.22; H, 10.47. Found: C, 81.55; H, 10.55. Slow
cooling of a hot 2-propanol solution of tBudppe to room temperature
afforded colorless crystals suitable for X-ray analysis.
̈
̈
spectra were fit using the program WMoss (SeeCo).
5.4.2. FeCl2(dpbz). A 100 mL round-bottom flask was charged with
80 mg of FeCl2·THF1.5 (0.3 mmol) along with 25 mL of toluene. To a
separate scintillation vial were added 152 mg of dpbz (0.3 mmol) and
10 mL of toluene. Both vessels were stirred while being heated to 85
°C, followed by the dropwise addition of the hot solution of ligand to
the hot stirring iron suspension. The resulting mixture was allowed to
stir for 4 h at 85 °C, by which time the reaction mixture had become a
pale yellow solution. The hot solution was filtered through a pad of
Celite, and the filtrate was allowed to cool to room temperature before
being divided into two equal fractions and stored at −30 °C. After 3
days, colorless block-shaped X-ray quality crystals had precipitated
from solution and were isolated by filtration from both fractions: yield
0.100 g (51%). Elemental Anal. Calcd for FeCl2(dpbz) and 0.5 equiv
of toluene: C, 64.97; H, 4.56. Found: C, 64.91; H, 4.48. The slightly
higher analyzed C value coincides with observation of cocrystallized
5.3. Magnetic Circular Dichroism Spectroscopy. All samples
for MCD spectroscopy were prepared in an inert atmosphere glovebox
equipped with a liquid nitrogen fill port to allow sample freezing to 77
K within the glovebox. Frozen solution MCD samples were prepared
in a 6:1 (v:v) toluene-d8/benzene-d6 mixture (mixtures used to afford
low-temperature optical glasses) in copper cells fitted with quartz disks
and a 3 mm gasket. For FeCl2(Xantphos), solid state mulls were
prepared using ground polycrystalline sample and paratone oil as a
mulling agent. NIR MCD experiments were conducted using a Jasco J-
730 spectropolarimeter and a liquid nitrogen-cooled InSb detector.
The spectral range accessible with this NIR MCD setup is 2000−600
nm. UV−visible (UV−vis) MCD spectra were collected using a Jasco
J-715 spectropolarimeter and a shielded S-20 photomultiplier tube.
Both instruments utilize a modified sample compartment incorporat-
ing focusing optics and an Oxford Instruments SM4000-7T super-
conducting magnet/cryostat, permitting measurements from 1.6 to
290 K with magnetic fields of up to 7 T. A calibrated Cernox sensor
directly inserted into the copper sample holder is used to measure the
temperature at the sample to 0.001 K. All MCD spectra were
baseline-corrected against zero-field scans. VTVH-MCD spectra were
analyzed using previously reported fitting procedures.50,51 For VTVH-
MCD fitting, both negative and positive zero-field splitting models
were evaluated. The reported error bars were determined via
evaluation of the effects of systematic variations of the fit parameters
on the quality of the overall fit.
5.4. Electronic Structure Calculations. Spin-unrestricted density
functional theory (DFT) calculations were performed with the
Gaussian 09 package.52 All geometry optimization calculations were
performed with the PBEPBE53 exchange-correlation functional with
the TZVP54 basis set on all atoms with the inclusion of solvation
effects using the polarized continuum model (PCM) with toluene as
the solvent55 with the exception of FeCl2(Xantphos) for which no
solvent model was included (to correlate with the obtained solid state
spectroscopy for this complex). The geometries of all complexes were
fully optimized starting from X-ray crystal structures with initial
optimization performed with cep-31g before optimizing at the TZVP
level. All optimized geometries had frequencies found to be real.
Energies given in the Supporting Information include zero-point and
thermal corrections. Further calculations of MOs and TD-DFT
analysis used the spin-unrestricted B3LYP functional with the TZVP
basis set on all atoms. MO compositions, analyzed via Mulliken
population analysis, and calculation of Mayer bond orders were
performed using the AOMix program.56,57 Calculated MOs were
solvent during X-ray analysis. 57Fe Mossbauer values (solid, 80 K): δ =
̈
0.73 mm/s, and ΔEQ = 2.49 mm/s.
5.4.3. FeCl2(tBudppe). A 500 mL round-bottom flask was charged
with 214 mg of FeCl2·THF1.5 (0.9 mmol, 851 mg), tBudppe (1.0 mmol,
1.1 equiv), and 200 mL of 2-propanol. The mixture was stirred at
reflux for 6 h, after which the reaction mixture was allowed to cool to
room temperature and the solvent removed under reduced pressure.
The resulting solid was redissolved in 50 mL of dichloromethane and
the solution filtered through a pad of Celite. Removal of the solvent
under reduced pressure resulted in a white solid that was subsequently
washed with cold hexane (10 mL, three times) and isolated by
filtration: yield 670 mg (76%). Elemental Anal. Calcd: C, 71.52; H,
9.11. Found: C, 71.65; H, 9.09. 57Fe Mossbauer values (solid, 80 K): δ
̈
= 0.73 mm/s, and ΔEQ = 2.82 mm/s. X-ray quality crystals were
grown from slow evaporation of a concentrated 1,4-dioxane solution of
the complex.
5.4.4. FeCl2(Xantphos). A 50 mL round-bottom flask was charged
with 95 mg of FeCl2·THF1.5 (0.40 mmol), 250 mg of Xantphos (1.1
equiv, 0.43 mmol), and 20 mL of toluene. The mixture was stirred for
4 h at 60 °C by which time a white precipitate was produced. The
reaction mixture was cooled to room temperature and filtered, and the
isolated white powder was washed with cold (−30 °C) THF (3 mL).
Concentrating the complex in THF and allowing for slow evaporation
at room temperature afford colorless block crystals suitable for X-ray
analysis: yield 280 mg (100%). Elemental Anal. Calcd: C, 66.41; H,
4.57. Found: C, 66.30; H, 4.63. 57Fe Mossbauer values (solid, 80 K): δ
̈
= 0.75 mm/s, and ΔEQ = 2.67 mm/s.
5.4.5. [FeBr(SciOPP)]2. To a stirring solution of FeBr2(SciOPP) (60
mg, 0.054 mmol) dissolved in 4 mL of diethyl ether cooled to −80 °C
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was added 8.7 mg of KC8 (1.2 equiv, 0.064 mmol) in approximately 2
mg portions. The resulting dark green solution was allowed to stir for
10 min at −80 °C and then filtered through Celite to remove solid
graphite and unreacted KC8. The reaction vial was sealed with Apiezon
N grease and transferred to a −80 °C freezer. After 2 weeks, a few
crystals of red-orange crystalline solid precipitated and the highly
temperature sensitive crystals were analyzed by X-ray crystallography.
Insufficient material combined with the high temperature sensitivity of
this species precluded additional characterization.
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P. M.; Gower, N. J.; Haddow, M. F.; Harvey, J. N.; Kehl, J. A.; Murphy,
D. M.; Neeve, E. C.; Neidig, M. L.; Nunn, J.; Snyder, B. E. R.; Taylor,
J. Organometallics 2014, 33, 5767−5780.
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.5b02263.
(24) Dongol, K. G.; Koh, H.; Sau, M.; Chai, C. L. L. Adv. Synth.
Catal. 2007, 349, 1015−1018.
Supplementary figures and data, including 57Fe Mossba-
̈
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Zenmyo, T.; Seike, H.; Nakamura, M. Angew. Chem., Int. Ed. 2012, 51,
8834−8837.
1
uer, UV−vis MCD, and H and 31P NMR spectra, MO
diagrams, and TD-DFT analyses (PDF)
X-ray crystal structure details (CIF)
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H.; Takaya, H.; Tamada, Y.; Ono, T.; Nakamura, M. J. Am. Chem. Soc.
2010, 132, 10674−10676.
AUTHOR INFORMATION
Corresponding Author
*E-mail: neidig@chem.rochester.edu.
■
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T.; Kawamura, S.; Ogata, K.; Takaya, H.; Nakamura, M. Chem. Lett.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
(30) Bedford, R. B.; Huwe, M.; Wilkinson, M. C. Chem. Commun.
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This work was supported by a grant from the National
Institutes of Health (R01GM111480 to M.L.N.). The authors
thank Dr. William W. Brennessel for assistance in the collection
and analysis of X-ray crystallographic data and Malik Al-
Afyouni for assistance in preliminary studies. Further acknowl-
edgement is extended to the Center for Integrated Research
Computing at the University of Rochester for providing the
resources necessary for performing the computational work
presented in this study.
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