Synthetic, Structural, and Spectroscopic Characterization of a Novel Family of High-Spin Iron(II) [(β-Diketiminate)(phosphanylphosphido)] Complexes

Article abstract of DOI:10.1021/acs.inorgchem.7b01374

This work describes a series of iron(II) phosphanylphosphido complexes. These compounds were obtained by reacting lithiated diphosphanes R₂PP(SiMe₃)Li (R = t-Bu, i-Pr) with an iron(II) β-diketiminate complex, [LFe(μ₂-Cl)₂Li(DME)₂] (1), where DME = 1,2-dimethoxyethane and L = Dippnacnac (β-diketiminate). While the reaction of 1 with t-Bu₂PP(SiMe₃)Li yields [LFe(η¹-Me₃SiPP-t-Bu₂)] (2), that of 1 with equimolar amounts of i-Pr₂PP(SiMe₃)Li, in DME, leads to [LFe(η²-i-Pr₂PPSiMe₃)] (3). In contrast, the reaction of 1 with (i-Pr₂N)₂PP(SiMe₃)Li provides not an iron-containing complex but 1-[(diisopropylamino)phosphine]-2,4-bis(diisopropylamino)-3-(trimethylsilyl)tetraphosphetane (4). The structures of 2-4 were determined using diffractometry. Thus, 2 exhibits a three-coordinate iron site and 3 a four-coordinate iron site. The increase in the coordination number is induced by the change from an anticlinal to a synclinal conformation of the phoshpanylphosphido ligands. The electronic structures of 2 and 3 were assessed through a combined field-dependent ⁵⁷Fe M?ssbauer and high-frequency and -field electron paramagnetic resonance spectroscopic investigation in conjunction with analysis of their magnetic susceptibility and magnetization data. These studies revealed two high-spin iron(II) sites with S = 2 ground states that have different properties. While 2 exhibits a zero-field splitting described by a positive D parameter (D = +17.4 cm^-1 E/D = 0.11) for 3, this parameter is negative [D = ?25(5) cm^-1 E/D = 0.15(5)]. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations provide insights into the origin of these differences and allow us to rationalize the fine and hyperfine structure parameters of 2 and 3. Thus, for 2, the spin-orbit coupling mixes a z²-type ground state with two low-lying {xz/yz} orbital states. These interactions lead to an easy plane of magnetization, which is essentially parallel to the plane defined by the N-Fe-N atoms. For 3, we find a yz-type ground state that is strongly mixed with a low-lying z²-type orbital state. In this case, the spin-orbit interaction leads to a partial unquenching of the orbital momentum along the x axis, that is, to an easy axis of magnetization oriented roughly along the Fe-P bond of the phosphido moiety.

Full text of DOI:10.1021/acs.inorgchem.7b01374

Article
pubs.acs.org/IC
Synthetic, Structural, and Spectroscopic Characterization of a Novel
Family of High-Spin Iron(II) [(β-Diketiminate)(phosphanylphosphido)]
Complexes
Rafał Grubba,*,† Kinga Kaniewska,† Łukasz Ponikiewski,† Beata Cristovao,‡ Wiesława Ferenc,‡
́
̃
Alina Dragulescu-Andrasi,§ J. Krzystek,∥ Sebastian A. Stoian,*,§,∥,⊥ and Jerzy Pikies†
†Department of Inorganic Chemistry, Chemical Faculty, Gdan
PL-80-233, Poland
́ ́
sk University of Technology, G. Narutowicza St. 11/12, Gdansk
‡Department of General and Coordination Chemistry, Faculty of Chemistry, Maria Curie-Skłodowska University, Maria
Curie-Skłodowska Sq. 2, Lublin PL-20-031, Poland
§Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States
∥National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States
⊥Department of Chemistry, University of Idaho, Moscow, Idaho 83844, United States
S
* Supporting Information
ABSTRACT: This work describes a series of iron(II)
phosphanylphosphido complexes. These compounds were
obtained by reacting lithiated diphosphanes R2PP(SiMe3)Li
(R = t-Bu, i-Pr) with an iron(II) β-diketiminate complex,
[LFe(μ2-Cl)2Li(DME)2] (1), where DME = 1,2-dimethoxy-
ethane and L = Dippnacnac (β-diketiminate). While the
reaction of 1 with t-Bu2PP(SiMe3)Li yields [LFe(η1-Me3SiPP-
t-Bu2)] (2), that of 1 with equimolar amounts of i-
Pr2PP(SiMe3)Li, in DME, leads to [LFe(η2-i-Pr2PPSiMe3)]
(3). In contrast, the reaction of 1 with (i-Pr2N)2PP(SiMe3)Li
provides not an iron-containing complex but 1-
[(diisopropylamino)phosphine]-2,4-bis(diisopropylamino)-3-
(trimethylsilyl)tetraphosphetane (4). The structures of 2−4
were determined using diffractometry. Thus, 2 exhibits a three-coordinate iron site and 3 a four-coordinate iron site. The increase
in the coordination number is induced by the change from an anticlinal to a synclinal conformation of the phoshpanylphosphido
ligands. The electronic structures of 2 and 3 were assessed through a combined field-dependent 57Fe Mossbauer and high-
̈
frequency and -field electron paramagnetic resonance spectroscopic investigation in conjunction with analysis of their magnetic
susceptibility and magnetization data. These studies revealed two high-spin iron(II) sites with S = 2 ground states that have
different properties. While 2 exhibits a zero-field splitting described by a positive D parameter (D = +17.4 cm−1; E/D = 0.11) for
3, this parameter is negative [D = −25(5) cm−1; E/D = 0.15(5)]. Density functional theory (DFT) and time-dependent DFT
(TDDFT) calculations provide insights into the origin of these differences and allow us to rationalize the fine and hyperfine
structure parameters of 2 and 3. Thus, for 2, the spin−orbit coupling mixes a z2-type ground state with two low-lying {xz/yz}
orbital states. These interactions lead to an easy plane of magnetization, which is essentially parallel to the plane defined by the
N−Fe−N atoms. For 3, we find a yz-type ground state that is strongly mixed with a low-lying z2-type orbital state. In this case,
the spin−orbit interaction leads to a partial unquenching of the orbital momentum along the x axis, that is, to an easy axis of
magnetization oriented roughly along the Fe−P bond of the phosphido moiety.
iron(II) complexes have been successfully stabilized by
employing sterically encumbered β-diketiminate (Dippnacnac)
ligands.3,4 For some of these compounds, detailed field-
1. INTRODUCTION
Low-coordinate, high-spin iron(II) compounds exhibit fascinat-
ing chemical reactivity and unusual electronic structures,
highlighting their importance from both a fundamental and a
practical perspective. These compounds have attracted
increased research interest for their potential as catalysts, as
well as for their importance as structural and functional
synthetic models of key enzymes, including nitrogenases and
hydrogenases.1,2 Heteroleptic, three- and four-coordinate
dependent 57Fe Mossbauer and X-band electron paramagnetic
̈
resonance (EPR) spectroscopic studies have revealed the
presence of large unquenched orbital momenta originating
Received: May 29, 2017
© XXXX American Chemical Society
A
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from the strong spin−orbit coupling acting on quasi-degenerate
orbital ground states.5 Consequently, such compounds show an
increased potential for exhibiting single-molecule-magnet-like
behavior.6
light of recent reports, it is worth emphasizing the need for
further studies of β-diketiminate iron complexes with
phosphido coligands to expand this important class of iron
complexes and their application as effective catalysts.
Phosphido ligands can be considered as heavier congeners of
amido groups. The essential difference between them lies in the
stronger propensity of the phosphido to function as a bridging
ligand. This dissimilarity arises from the stronger π-donor
ability of amido ligands7 and the steric demands of phosphido
groups.8 It was shown that, in terms of the structure and
stability, the P(SiPh3)2 moiety closely resembles the N(SiMe3)2
We have recently reported the synthesis and reactivity of
several low-valent phosphorus-based compounds, in particular
of nucleophilic phosphanylphosphinidene complexes (R2PP)
and phosphanylphosphido complexes (R2PPSiMe3).20−32 The
diphosphorus counterparts of the complexes with R2P and RP
ligands have been explored to a much lesser extent. Up to now,
only a few examples of titanium,27 zirconium,22,26 and
hafnium23 complexes having R2PPSiMe3 ligands have been
isolated. In this study, we describe a novel class of β-
diketiminate-supported high-spin ferrous complexes that
incorporate phosphanylphosphido coligands. The structural
and spectroscopic characterization of these compounds
revealed that their electronic structures and magnetic properties
are tightly coupled to conformation of the phosphanylphos-
phido moieties. The change from an anticlinal to a synclinal
conformation of the R2PPSiMe3 moiety leads not only to an
increase in the coordination number (CN) but also to a change
in the nature of the electronic ground state and the character of
spin−orbit interactions. Thus, the four-coordinate complex 3
exhibits an overall behavior that is rather similar to those
observed for analogous low-coordinate, iron(II) β-diketiminate
complexes, in particular, of [LFe(μ2-Cl)2Li(THF)2] (THF =
tetrahydrofuran).33 In contrast, the properties of the three-
coordinate iron complex 2 are similar not with those of
analogous iron(II) β-diketiminate complexes but rather with
those of homoleptic, planar-trigonal iron(II) sulfides,
[FeII(SR)3]− (R = C6H2-2,4,6-t-Bu3).34,35 Our computational
investigation and theoretical analysis of the fine and hyperfine
structures of 2 and 3 provides a clear rational for these
observations vide infra.
group.9 In the early 1980s, Schafer reported the formation of an
̈
iron-based complex, [Cp(CO)2FeP(SiMe3)2], enclosing a
terminal phosphido ligand, via [Cp(CO)2FeCl] and LiP-
(SiMe3)2 metathesis.10 In a related complex [Cp(CO)2FeP-
(CF3)2], the phosphorus-based ligand, P(CF3)2, adopts a
terminal geometry as well.11 Unlike the P(SiMe3)2 and P(CF3)2
moieties, t-Bu2P and smaller phosphido ligands adopt a
bridging geometry upon coordination of the iron centers.
Thus, [Cp(CO)2FeCl] reacts with PhP(SiMe3)2, yielding
[Cp(CO)2Fe{μ2-PPh(SiMe3)}]2.12 Similarly, [(Me3P)2FeCl2]
reacts with t-Bu2PLi, yielding [(Me3P)FeCl(μ2-P-t-Bu2)]2.13
There are only a few other reports of iron β-diketiminate
complexes incorporating phosphorus-donor coligands. One
example is the LFeIPPh3 complex bearing a phosphine ligand,
which was obtained by reacting the dinitrogen complex
LFeNNFeL [L = DippNC(Me)CHC(Me)NDipp] with
PPh3.14 Stephan et al. described the reaction of the iron(I) β-
diketiminate compound, [LFe(η2-CH2CPh2)] [L = DippNC-
(Me)CHC(Me)NDipp], with primary and secondary phos-
phines, Ph2PH or PhPH2, that resulted in P−H and P−C bond
activation with the formation of phosphinidene-incorporating
iron(III) dimers, [LFe(μ-PPh)]2.15 Alternative routes to iron β-
diketiminate complexes supported by phosphorus-based ligands
were presented by Driess et al.16 and Scheer et al.17 The
reaction under mild conditions of an iron β-diketiminate
complex, [LFe(toluene)] (L = DippNC3H3NDipp), with white
phosphorus furnished a dinuclear iron(III) complex,
[(LFe)2(μ-η2:η2-P2)2], featuring two bridging [P2]2− ligands.16
The reduction of [(LFe)2(μ-η2:η2-P2)2] with potassium metal
resulted in the formation of a mixed-valent iron(II,III) complex,
which is isostructural with its neutral precursor.16 The reactivity
of [LFe(toluene)] toward P4 was found to be sensitive to minor
alterations of the β-diketiminate ligands.17 Thus, the steric
properties of nacnac ligands exert a profound influence on the
final iron(II) complex formed, which include [P8]4− or cyclo-
[P4]2− structural motifs for β-diketiminate ligands dmpNC-
(Me)CHC(Me)Ndmp and DippNC(Me)CHC(Me)NDipp,
respectively.17 Recently, iron β-diketiminate complexes, [LFe-
(CH2SiMe3)] [L = dmpNC(Me)CHC(Me)Ndmp and
DippNC(Me)CHC(Me)NDipp], were used as effective pre-
catalysts in the dehydrocoupling of primary and secondary
phosphines18 and in the hydrophosphination of alkenes.18,19
The phosphido complexes of iron, [LFe(PR2)] and [LFe-
(PRH)], were postulated to be active catalysts in these
transformations. More recently, β-diketiminate phoshido
complexes were found to play a crucial role in the
hydrophosphination of nonactivated unsaturated C−C
bonds.19 Utilization of the β-diketiminate complex [LFe-
(CH2SiMe3)] as a precatalyst in the cyclization of primary
phosphines led to the isolation of a dimeric phosphido
complex, [LFe(PRH(CH2)3CHCH2]2 [L = DippNC(Me)-
CHC(Me)NDipp], as a catalytic intermediate.19 Thus, in the
2. METHODS AND MATERIALS
2.1. Synthesis. The 1,2-dimethoxyethane (DME) solvent was
dried over potassium/benzophenone and distilled under argon.
Pentane was dried over sodium/benzophenone/diglyme and distilled
under argon. All manipulations were performed in flame-dried
Schlenk-type glassware on a vacuum line or in an argon-filled
glovebox. 31P{1H} NMR (external standard 85% H3PO4) and 1H
NMR (internal standard tetramethylsilane) spectra were recorded on
Bruker AV300 MHz, Bruker AV400 MHz, and Varian Unity 500 MHz
spectrometers at room temperature. Literature methods were used to
prepare R2PP(SiMe3)Li·nTHF36 and [(Dippnacnac)FeCl2Li(DME)2]
(1).37 All synthesized compounds are extremely moisture- and air-
sensitive.
Synthesis of [(Dippnacnac)Fe(η1-Me3SiPP-t-Bu2)] (2). A solution of
t-Bu2PP(SiMe3)Li·2.2THF (0.207 g, 0.500 mmol) in DME (2.5 mL)
was slowly added to 1 (0.366 g, 0.500 mmol) in DME (2 mL) at −30
°C. The temperature of the reaction was maintained between −30 and
−20 °C for 0.5 h. After warming to room temperature, the solvent was
evaporated under vacuum. The residue was extracted with pentane (10
mL), and LiCl was filtered off. The filtrate was concentrated to 3.5 mL
and stored at +4 and −20 °C. After a few days, dark-red crystals of 2
were formed (0.127 g, 0.176 mmol; yield 35%).
Anal. Calcd for C40H68FeN2P2Si: C, 66.46; H, 9.48; N, 3.88. Found:
C, 65.94; H, 9.51; N, 3.87.
1H NMR (400 MHz, toluene-d8, 25 °C): 97.59 (CH nacnac, 1H),
10.03 (Me nacnac, 6H), 7.88 (SiMe3, 9H), 7.14 (m-H, 4H), 3.08 (t-
Bu, 18H), 2.35 (i-Pr methyl, 6H), 1.19 (i-Pr methyl, 6H), 0.33 (p-H,
2H), −16.75 (i-Pr methyl, 6H), −33.39 (i-Pr methyl, 6H), −39.37 (i-
Pr methine, 1H), −43.61 (i-Pr methine, 1H), −66.13 (i-Pr methine,
2H).
B
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Synthesis of [(Dippnacnac)Fe(η2-i-Pr2PPSiMe3)] (3). A solution of
i-Pr2PP(SiMe3)Li·3THF (0.222 g, 0.500 mmol) in DME (2.5 mL) was
slowly added to 1 (0.366 g, 0.500 mmol) in DME (2 mL) at −30 °C.
The temperature of the reaction was maintained between −30 and
−20 °C for 0.5 h. After warming to room temperature, the solvent was
evaporated under vacuum. Then the residue was extracted with
pentane (10 mL), and LiCl was filtered off. The filtrate was
concentrated to 3.5 mL, and a dark precipitate was separated after
storage at +4 °C overnight. The resulting clear solution was further
stored at −20 °C. After a few days, bright-red crystals of 3 were
deposited (0.183 g, 0.263 mmol; yield 53%).
filled glovebox ([O2(g)] < 1 ppm), where the crystalline material was
transferred into custom-made Delrin cups appropriately designed for
Mossbauer (∼30 mg) and for high-frequency and -field EPR (HFEPR;
̈
∼50 mg) measurements. Subsequently, the sample cups were stored
and handled under LN2. Variable-field, variable-temperature spectra
were recorded using a constant-acceleration spectrometer. The
spectrometer was fitted with a flow-type Janis Super-Varitemp 8DT
cryostat that was cooled with liquid helium and equipped with an 8 T
superconducting magnet. This setup allowed for the generation of
magnetic fields parallel to the 14.4 keV γ-radiation used to observe the
Mossbauer effect. The source consisted of 100 mCi 57Co dispersed in
̈
Anal. Calcd for C38H65N2P2SiFe: C, 65.59; H, 9.42; N, 4.03. Found:
C, 65.31; H, 9.23; N, 3.89.
rhodium foil and was incorporated in a vertical velocity transducer
system. The 4.2 K spectra were recorded, while the sample space of
the cryostat was flooded with liquid helium. Variable-temperature
spectra were recorded in a flow of helium gas that was controlled using
a needle-type valve. The sample temperature was determined using a
Cernox sensor and maintained using a 50 Ω heater powered by a
Cryocon 32B temperature controller. The isomer shifts are reported
against the centroid of a spectrum of α-iron metal foil recorded at
room temperature. The spectra were analyzed in the framework of a
standard S = 2 spin Hamiltonian, as implemented by the SpinHam
option of the WMOSS spectral analysis software (formerly Web
Research Co., Edina, MN).39
1H NMR (400 MHz, toluene-d8, 25 °C): 122.38 (CH nacnac, 1H),
44.95 (SiMe3, 9H), 23.17 (Me nacnac, 6H), 8.19 (m-H, 4H), 5.09 (i-
Pr2P methyl, 6H), 0.72 (i-Pr methyl, 12H), 0.34 (p-H, 2H), −13.67 (i-
Pr2P methyl, 6H), −38.16 (i-Pr methyl, 12H), −43.70 (i-Pr methine,
2H), −59.93 (i-Pr methine, 2H). The signals of the methine protons
of the i-Pr2P groups are not observed because of their proximity to the
metal center.
Synthesis of 1-[(Diisopropylamino)phosphine]-2,4-bis-
(diisopropylamino)-3-(trimethylsilyl)tetraphosphetane (4). A solu-
tion of (i-Pr2N)2PP(SiMe3)Li·1.5THF (0.450 g; 1.00 mmol) in DME
(2.5 mL) was slowly added to 1 (0.320 g, 0.440 mmol) in DME (2
mL) at −30 °C. After warming to room temperature, the solution was
concentrated to 3.5 mL and stored at −30 °C. After a few days,
colorless crystals of 4 were deposited (0.230 g, 0.366 mmol; yield
91%).
2.4. HFEPR. HFEPR measurements were carried out on a custom-
built spectrometer that was previously described.40 However, in this
case, microwaves of the desired frequency were obtained using a
subterahertz wave generator from Virginia Diodes operating at 13
1
GHz and equipped with a cascade of frequency multipliers. HFEPR
spectra were interpreted in terms of a standard spin Hamiltonian for S
= 2 and simulated using the program SPIN (from A. Ozarowski).
2.5. Magnetometry. Magnetic susceptibilities for polycrystalline
samples of 2 and 3 were measured over the temperature range from
1.8 to 300 K at a magnetic field of 0.1 T using a Quantum Design
SQUID-VSM magnetometer. Field dependencies of magnetization
were measured in the 2−6 K range in applied fields of up to 7 T. The
obtained data were corrected for the sample-holder signal and for
diamagnetism using Pascal’s constants.41 While fits of the susceptibility
data were obtained using the JulX program written by Prof. Eckhard
Bill, the reduced magnetization data were simulated using an in-house-
developed program.42
1H NMR (see Scheme 1 for atom labeling): 3.96 (br m, i-Pr
methine, 2H), 3.96 (br m, i-Pr methine, 2H), 3.62 (hpt, 6.4 Hz, i-Pr
Scheme 1. Structure of the Tetraphosphetane 4
2.6. Density Functional Theory (DFT) Calculations. The DFT
calculations were performed using the Gaussian09 (revisions A02 and
C01) suite of quantum-chemical programs and the B3LYP/6-311G,
B3LYP/6-311++G*, and BPW91/cc-pvtz functional/basis set combi-
nations.43 Self-consistent-field (SCF) and geometry optimizations
were performed using tight and default convergence criteria,
respectively. The stability of the ground state was assessed on the
basis of time-dependent SCF calculations and used as the reference
state when all single-electron excitations were found to be positive.
Population analyses were performed using Mulliken population
analysis and natural population analysis (NPA), as implemented in
the NBOpro v.6 program suite.44 The predicted ΔEQ and η values
describing the electric-field gradient (EFG) and the predicted 57Fe
hyperfine coupling constants were estimated using the standard prop
keyword of the Gaussian code. The predicted isomer shift values were
determined using the calibration given by Vrajmasu et al.45 The
potential energy change as a function of a given structural parameter
was evaluated by performing a series of relaxed scans for which one
internal coordinate was kept fixed and all others were optimized.
Finally, the van der Waals volume and the perimeter of selected
substituents were calculated considering the volume encompassed by
the electron density isosurface with a value of 0.001 e/bohr3 using the
Multiwfn software.46
methine, 2H), 3.59 (hpt, 6.4 Hz, i-Pr methine, 2H), 1.37 (m, (i-Pr
methyl, 12H), 1.26 (m, i-Pr methyl, 12H), 0.60 (d, 3.37 Hz, PSiMe3,
9H). 31P{1H} NMR (spin system AEM2X): 62.65 (A, (i-Pr2N)2P, m),
8.50 (E, P (unsubstituted), m), −20.55 (M, i-Pr2NP, m), −84.74 (X,
Me3SiP, m). 1JEM = −169. 85 Hz, 2JEX = 36.86 Hz, 1JAE = −185.58 Hz,
1JMX = −177.51 Hz, JAM = 192.59 Hz, and JAX = 25.78 Hz. 31P{1H}
NMR inspection of the reaction solution: 81.5 (s, (i-Pr2N)2PP(Ni-
Pr2)2), 59.84 (s, unknown compound), −254.0 (s, P(SiMe3)3).
2.2. X-ray Crystallography. X-ray-quality single crystals of 2−4
were selected for the X-ray diffraction experiments at 150 K.
Diffraction data of 4 were collected on a diffractometer equipped
with a STOE IPDS 2 imaging-plate detector system using Mo Kα
radiation with a graphite monochromator (λ = 0.71073 Å). The
diffraction intensities of 2 and 3 were collected on a Stadi Vari
diffractometer equipment with a Pilatus 300K detector with a graphite
monochromator (λ = 1.54186 Å for 2; λ = 0.71073 Å for 3). The
structures were solved by direct methods and refined against F2 least-
squares techniques using the SHELXS-97 and SHELXL-97 programs.38
Non-hydrogen atoms were refined with anisotropic displacement
parameters; hydrogen atoms were usually refined using the isotropic
model with Uiso(H) values fixed as 1.5Ueq of carbon atoms for −CH3
groups or 1.2Ueq for −CH groups and aromatic H atoms.
2
3
3. RESULTS
̈
2.3. Mossbauer Spectroscopy. Samples of 2 and 3 investigated
by 57Fe Mossbauer spectroscopy contained unenriched, natural-
3.1. Synthesis and Reactivity. The series of phosphanyl-
phosphidoiron complexes were synthesized via reactions of
R2PP(SiMe3)Li (R = t-Bu, i-Pr) with the iron(II) β-
diketiminate complex [LFeCl2Li(DME)2] (1), where L =
̈
abundance iron. For each complex, about 80 mg polycrystalline
powder samples were shipped from Gdan
FL, in sealed Schlenk flasks. These flasks were opened in an argon-
́
sk, Poland, to Tallahassee,
C
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Scheme 2. Synthesis of the Terminal Phosphanylphosphido Complex 2
Scheme 3. Synthesis of the Side-On Phosphanylphosphido Complex 3
Scheme 4. Synthesis of Tetraphosphetane 4
Dippnacnac. Treatment of a DME solution of 1 with an
equimolar amount of t-Bu2PP(SiMe3)Li yielded a terminal
phosphanylphosphido complex, [LFe(η1-Me3SiPP-t-Bu2)] (2;
Scheme 2). Complex 2 was isolated as dark-red crystals from
crystallization in pentane at +4 °C (yield 35%). This compound
is thermally stable and can be stored under argon at room
temperature for a long time.
We studied the influence of steric effects exerted by the R
group of diphosphane on the structure of the products. We
found that the sterically less demanding precursor i-Pr2PP-
(SiMe3)Li reacts with 1, yielding [LFe(η2-i-Pr2PPSiMe3)] (3),
featuring a side-on-bonded i-Pr2PPSiMe3 moiety (Scheme 3).
Complex 3 formed bright-red crystals and was isolated in a
manner similar to that used for compound 2; however,
compared to 2, the yield is significantly higher (53%). Complex
3 is stable at room temperature but easily reacts with moisture
and oxygen.
It is worth emphasizing that, in solvents such as THF and
DME, complex 1 does not react with lithium derivatives of
diphosphanes to yield phosphanylphosphinidene (R2PP)-
supported iron complexes. In contrast, we observed the
formation of such compounds in reactions with platinum,
zirconium, molybdenum, and tungsten dichloride complexes
when the SiMe3 group of the phosphanylphosphido complex
was lithiated by 1 equiv of R2PP(SiMe3)Li with subsequent
elimination of LiCl and R2PP(SiMe3)2.21−31 Unlike for these
results, complexes 2 and 3 show no tendency for lithiation of
the SiMe3 group and to form related anionic phosphinidene
complexes.
Reactions of 1 with the lithium salt of diphosphane-
containing diisopropylamino substituents led to the formation
of polyphosphorus compounds. The mixing of DME solutions
of 1 with ca. 2-fold molar excess of (i-Pr2N)2PP(SiMe3)Li·
1.5THF in DME at −30 °C produced dark-brown solutions,
which yielded colorless crystals of 1-[(diisopropylamino)-
phosphine]-2,4-bis(diisopropylamino)-3-(trimethylsilyl)-
tetraphosphetane (4), after workup at ambient temperature.
Presumably the starting iron(II) complex is reduced to an
iron(I) species by the lithium salt of diphosphane with the
simultaneous formation of polyphosphorous compounds
(Scheme 4). NMR examination of the reaction mixture
revealed the formation of tetraphosphetane 4, (i-Pr2N)2PP-
(Ni-Pr2)2, and P(SiMe3)3 in a 1:2:4 ratio. Consideration of (i-
Pr2N)2PP(Ni-Pr2)2 as a product of a reaction leading to 4 is
however doubtful. This compound is often present in reactions
involving (i-Pr2N)2PP(SiMe3)Li, most likely because of the
relatively high stability of the (i-Pr2N)2P radical.
D
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Compound 4 was first reported as a minor, not isolated,
reaction product of (i-Pr2N)2PP(SiMe3)Li with [Cp2ZrCl2]. In
this case, the main reaction product was 1,3-bis(trimethylsilyl)-
2,4-bis(diisopropylamino)tetraphosphetane.22 The THF solu-
tion of (i-Pr2N)2PP(SiMe3)Li is not very stable, and after
prolonged storage at ambient temperature, it converts to 4 and
P(SiMe3)2Li.
3.2. X-ray Crystallography. 3.2.1. Crystal Structure of 2.
Complex 2 crystallizes in the P21/c space group. The
representation of its X-ray structure is shown in Figure 1. In
Figure 2. Molecular structure of 3 showing the atom numbering
scheme. Hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and bond angles (deg) for 3: Fe1−N1 2.003(2), Fe1−N2
2.0218(19), Fe1−P1 2.3638(9), Fe1−P2 2.5070(10), P1−P2
2.1490(10); N1−Fe1−N2 93.02(8), P1−Fe1−P2 52.26(3), Fe1−
P1−P2 67.30(4).
iron center adopts an intermediate mode between terminal and
side-on. The P1−P2 distance of 2.1490(10) Å is shorter than
that in 2 and clearly indicates a partial double-bond character.
Thus, the side-on bonding of a R2PPSiMe3 ligand clearly
enhanced the multiple-bond character of the P−P bond.
Our recent investigation of the phosphanylphosphido
complexes supported by transition-metal ions suggests that
coligands such as the cyclopentadienyl anion in the case of early
transition metals stabilize terminal coordination of the
R2PPSiMe3 ligand to the metal center,22,23 whereas nitrogen
ligands like imido or nacnac stabilize side-on coordination
mode.26,27,29 To our surprise, the phosphido ligand t-
Bu2PPSiMe3 adopts in 2 a terminal geometry, although the
similar i-Pr2PPSiMe3 moiety adopts the expected side-on
coordination in sterically less demanding 3.
Figure 1. Molecular structure of 2 showing the atom numbering
scheme. Hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and bond angles (deg) for 2: Fe1−N1 1.999(4), Fe1−N2
1.987(4), Fe1−P1 2.3144(17), P1−P2 2.168(2); N1−Fe1−N2
94.57(18), N1−Fe1−P1 124.27(13), N2−Fe1−P1 140.50(15), Fe1−
P1−P2 127.40(10).
this coordinatively unsaturated compound (12e), the iron atom
exhibits a planar, trigonal coordination environment (ΣFe
359.34°). Furthermore, the phosphido atom P1 is also nearly
planar (ΣP1 354.03°). The planes defined by N1−Fe1−N2 and
Si1−P1−P2 are nearly orthogonal to each other. The geometry
at the phosphanyl P2 atom is pyramidal (ΣP2 318.8°). The bite
angle N1−Fe1−N2 of 94.57° and the N−Fe bond lengths are
typical of three-coordinate iron diketiminate compounds.3 The
Fe1−P1 distance of 2.3144(17) Å is long and has a clear single-
bond character.47 One striking structural feature of 2 is
embodied by the short P1−P2 distance of 2.168(2) Å,
suggesting a partial double-bond character of the P−P bond.
Similar bond lengths were previously linked by us to η2-
coordination mode of the t-Bu2PPP-t-Bu2 moiety in [(η2-
R2PPPR2)M(Cl)L] (2.12−2.16 Å; L = tertiary phosphine; M
= Ni, Pd, Pt)48 and in [(η2-t-Bu2PPP-t-Bu2)M(2,6-i-
Pr2C6H3N)2Cl] (2.15−2.16 Å; M = Mo, W).49 We have
discussed the geometries of these nonclassical phosphido
ligands in terms of a η2-bonded diphosphenium cation.48
3.2.2. Crystal Structure of 3. Compound 3 crystallizes in the
3.2.3. Crystal Structure of 4. The ORTEP diagram of
complex 4, which crystallizes in the P1 space group, is shown in
̅
Figure 3. The P−P distances in 4 are typical for single P−P
P1 space group. The ORTEP diagram of complex 3 is shown in
̅
Figure 2. The Fe1 atom displays a distorted tetrahedral
geometry. The planes defined by N1−Fe1−N2 and Fe1−P1−
P2 are nearly perpendicular to each other. The phosphanyl P2
atom exhibits a pyramidal geometry (ΣP2 318.8°). The bite
angle N1−Fe1−N2 of 93.02° and the N−Fe bond lengths are
typical for three-coordinate iron diketiminate compounds.3
While the Fe1−P1 distance of 2.3638(9) Å is long and almost
the same as that in 2, the distance Fe1−P2 of 2.5070(10) Å is
substantially longer. Thus, the bonding of i-Pr2PPSiMe3 to the
Figure 3. Molecular structure of 4 showing the atom numbering
scheme. Hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and bond angles (deg) for 4: P1−P2 2.2353(7), P1−P3
2.2446(7), P2−P4 2.2336(7), P3−P4 2.2294(7), P4−P5 2.2460(6);
P2−P1−P3 89.82(2), P4−P3−P1 87.02(2), P2−P4−P3 90.28(2),
P2−P4−P5 97.97(2).
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bonds and for amino-substituted phosphetanes.50 The P1−P2−
P3−P4 ring is folded along the P2−P4 vector (Θ = 19.75°).
The nitrogen atoms of the (i-Pr2N)2P group display exact
planar geometry, with nitrogen atoms connected to ring
phosphorus atoms being almost planar.
1
3.3. H NMR Studies and Stability in Solution. The
stabilities of 2 and 3 in solution were investigated by variable-
1
temperature H NMR spectroscopy. Samples were made by
dissolving crystals of complexes in toluene-d8 at −50 °C under
an argon atmosphere and then heated to room temperature
1
inside the spectrometer. H NMR spectra were recorded in
heating mode at −50, −25, 0, and +25 °C. Both complexes
exhibit resonances in a very wide range, from 97.59 to −66.13
ppm for 2 and from 122.38 to −59.93 ppm for 3 (see the
experimental part for a complete list of resonances). This wide
1
range of H NMR resonances is characteristic of paramagnetic,
low-valent, iron(II) β-diketiminate complexes.1 Compound 3
seems to be stable over the range of temperatures from −50 to
+25 °C, with only shifts of the signals due to temperature
changes being observed. Complex 2 is less stable in a toluene-d8
solution compared to compound 3. The 1H NMR spectrum of
2 recorded at +25 °C consists of not only relatively sharp
signals from 2 but also very broad signals, which can be
attributed to a newly formed iron(III) species. These findings
suggest partial disproportionation of 2 in a toluene-d8 solution
at room temperature with the formation of iron(III) and
iron(I) complexes. Compound 3 contains a Fe1−P1−P2 ring
due to additional coordination of the phosphanyl i-Pr2P group
to the metal center, a structural feature that most likely
contributes to the increased stability of 3 in the solution
Figure 4. Field- and temperature-dependent 57Fe Mossbauer spectra
recorded for polycrystalline samples of 2 (left) and 3 (right). The solid
red lines are simulations obtained using the spin Hamiltonian of eq 1
and the parameters listed in Table 1.
̈
K. Both the isomer shift and quadrupole splitting of 2 are
within the range of values observed for previously characterized
high-spin iron(II) complexes that are supported by β-
diketimiate ligands (Table 1). The zero-field, 4.2 K spectrum
of 3 consists of a well-defined, symmetric quadrupole doublet
characterized by δ = 0.82 mm/s, ΔEQ = 1.80 mm/s, and Γ =
0.37 mm/s (Figure 4, right). Furthermore, this doublet
accounts for essentially all of the iron present in the sample.
As the temperature is raised, the quadrupole splitting and the
isomer shift exhibit only a minor decrease in their values such
that at 180 K δ = 0.76 mm/s and ΔEQ = 1.70 mm/s (Figure S6
and Table S2). These values are typical of a high-spin iron(II)
ion and clearly demonstrate a 2+ oxidation state and high-spin
electronic configuration of the metal site.
Inspection of Figures 4 (left) and S5 shows that for 2 the
applied magnetic field leads to the progressive development of
a magnetic hyperfine splitting pattern. At 4.2 K, the extent of
the observed splitting fails to reach a saturation value even at
8.0 T. This behavior suggests that 2 exhibits a zero-field
splitting (ZFS) of the electronic spin sublevels characterized by
a relatively large and positive D parameter. These observations
were corroborated by the spectral simulations of the complete
data set (see the Supporting Information). In particular,
analysis of the 4.2−100 K, 8 T spectra clearly demonstrates
that D = 15 5 cm−1, E/D ∼ 0.1(1), Ax/gnβn = +5(1) T, Ay/
gnβn = +10(1) T, and Az/gnβn = −29(3) T. These spectra could
be rationalized only by considering two positive, relatively small
hyperfine coupling tensor components (Figure S4). This
observation reveals that the A tensor is dominated by large
orbital contributions, vide infra. The ZFS parameters were
further corroborated by analysis of the HFEPR spectra and
magnetic data (Figures 5 and S1). The simulations presented
here were obtained using the numerical values of D, E/D, and g
determined from HFEPR and the values of the A tensor
components (αEFG, βEFG, and γEFG) and η obtained from the
least-squares fitting of selected sets of spectra and spectral
simulations of the complete data set (Table 1). Interestingly,
this table shows that the spectroscopic behavior of 2 is rather
dissimilar from that of other three-coordinate, β-diketiminate-
compared to 2.
3.4. Fe Mossbauer Spectroscopy. The electronic
57
̈
structures of 2 and 3 were determined, in part, by recording
a series of field- and temperature-dependent 57Fe Mossbauer
̈
spectra on polycrystalline samples for temperatures between 4.2
and 220 K in applied fields of up to 8 T. The spectra were
analyzed in the framework of a typical S = 2 spin Hamiltonian
described by eqs 1a−1c.51 The quantities included in these
equations have their conventional meanings and are detailed in
the Supporting Information.
2
E
D
2
2
̂
̂
̂
̂
̂
̂
̃
̂
⃗
H = D Sz − 2 + (Sx − Sy ) + βS·g·B + S·A·I
̃
e
{
}
̂
̂
⃗
− β g I·B + δ + HQ
(1a)
n
n
⎡
⎤
eQVZZ
12
2
15
4
2
2
̂
̂
̂ ̂
+ η(IX − I )
HQ =
3I
−
⎢
⎣
⎥
⎦
Z
Y
(1b)
(1c)
ΔEQ = (eQVZZ/2) 1 + η2/3
At 4.2 K, the zero-field spectrum of 2 consists of a well-
defined, slightly asymmetric quadrupole doublet characterized
by an isomer shift δ = 0.55(1) mm/s, a quadrupole splitting
ΔEQ = 1.48(1) mm/s, and relatively narrow line widths ΓL/R
=
0.31/0.37 mm/s (Figure 4, left). This doublet accounts for
more than 97% of the spectral area, which demonstrates that
the sample under investigation was virtually free of impurities.
The asymmetry of the quadrupole doublets originates, most
likely, from the partial alignment of the polycrystalline material.
Figure S3 and Table S2 show that the quadrupole splitting is
temperature-independent and, as expected, the second-order
Doppler shift induces only a slight change in the isomer shift
such that δ = 0.50(1) mm/s and ΔEQ = 1.49(2) mm/s at 180
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Figure 5. Left: HFEPR spectrum recorded for 2 at 5 K, 406.4 GHz
(black trace). The red and blue traces are powder-pattern simulations
obtained for S = 2 assuming a positive and negative, respectively, ZFS
parameter D where |D| = 17.4 cm−1, E/D = 0.11, gx = 2.18, gy = 2.32,
and gz = 1.92. Right: Field versus frequency dependence of the
observed resonances indicated as black squares. The curves are
simulations using spin-Hamiltonian parameters as above. Red curves
represent turning points with the magnetic field B0 parallel to the x axis
of the ZFS tensor. Blue: B0∥y. Black: B0∥z. The solid curves are
dependencies predicted for the resonances originating from the
ground-state spin sublevel (MS = 0), and the dotted lines are obtained
for those originating from excited-state MS levels.
supported iron(II) complexes. In contrast, the electronic
structure of 2 is comparable to that of a distorted three-
coordinate, homoleptic, sulfido-supported iron(II) complex.
Inspection of Figures 4 (right) and S7 shows that for 3 at 4.2
K the applied field leads to a hyperfine splitting pattern that is
remarkably different from that observed for 2. Thus, even a
relatively small field of 0.5 T induces a fully developed magnetic
hyperfine splitting. This behavior reveals a ground state
consisting of a nearly degenerate quasi-doublet of spin-orbital
sublevels. Thus, the ZFS of the two lowest spin sublevels is
readily overwhelmed by the applied field through the Zeeman
interaction. In turn, this observation strongly supports the
presence of a negative ZFS parameter D for 3 (see the
Supporting Information). This suggestion is validated by our
spectral simulations, which reveal that, indeed, D = −25(5)
cm−1. Moreover, we find that the Az/gnβn = +9.8(2) tensor
component dominates the field-dependent spectra recorded at
4.2 K. Our simulations also suggest that the fine structure
parameter E/D is distributed (see the Supporting Information).
This behavior is reminiscent of that observed for the four-
coordinate LFe(μ2-Cl)2Li(THF)2 complex (Table 1). The ZFS
parameters were further corroborated by analysis of the
magnetic susceptibility and of the reduced magnetization data
(Figure S2).
3.5. HFEPR. HFEPR experiments on complex 2 resulted in
fairly strong spectra in the frequency range of 146−406 GHz at
liquid-helium temperatures (Figure 5). The spectra could be
effectively interpreted as originating from an S = 2 species
characterized by a positive ZFS parameter D = 17.4 cm−1 and
sizable rhombicity of the ZFS tensor, E/D = 0.11.
HFEPR experiments on complex 3 in the same conditions as
those for 2 yielded no spectra, with the exception of weak
resonances that could be attributed to iron(III) impurity/
impurities (not shown). This is easy to understand in the case
of large negative D. In the latter case, the energy splitting
between the populated ground-state quasi-doublet of |S, mS⟩ ≈ |
2, 2⟩ states and the higher-lying |2, 1⟩ states, on the order of
3|D| ∼ 75 cm−1 (according to the Mossbauer results), is much
̈
too large to be coupled even by the highest available
subterahertz wave frequency and magnetic field.
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conformer of t-Bu2PPSiMe3− and several discontinuities for the
PES of i-Pr2PPSiMe3 (Figure S11, right).
4. DISCUSSION
−
4.1. Stereochemistry and Electronic Structure of the
−
The electronic structures of the phosphanylphosphido anions
were monitored by performing for each minimum an analysis of
the natural bond orbitals (NBOs) and canonical molecular
orbitals (see the Supporting Information. Thus, we found that
(i) the ground-state electronic configurations are minimally
affected by the conformational change (Tables S5 and S6), (ii)
two of the three phosphorus lone pairs are accommodated by
hybrid sp2/sp3 orbitals, while the other is supported by a
nonbonding PSi-localized py orbital (Figure S12), (iii) the
negative charge is localized mainly on the phosphido moiety,
and (iv) the P−P bond is stabilized by non-Lewis-type, donor−
acceptor interactions involving the empty low-lying, phospho-
rus-based d orbitals (Figure S13). Thus, the X-ray crystallo-
graphic investigations of various chemical species that enclose a
phosphanylphosphido moiety including 2 and 3 revealed a
fairly consistent P−P bond length of 2.16 0.2 Å. This value is
somewhat shorter than that of a typical single P−P bond, that
is, 2.20 Å.52,53 This observation led to the inference that for
these ligands the P−P bond exhibits multiple-bond character.36
Our calculations support these arguments; we obtained a
Wiberg bond index of the P−P interaction ranging from 1.03 to
1.17 (Table S3). This finding is further corroborated by the
natural resonance theory, which yielded a natural bond order of
the P−P bond of 1.05(4), as well as the predicted weights of
the dominant resonance structures (Table S4 and Schemes S1
and S2).54
R2PPSiMe3 Anions. The phosphanylphosphido anions are a
relatively new class of ligands. They coordinate metal ions in a
chelating or nonchelating fashion by adopting either a synclinal
or an anticlinal conformation (Schemes S1 and S2). In
particular, inspection of Figures 1 and 2 shows that the
coordination numbers and geometries of the iron(II) sites of 2
and 3 are essentially determined by the conformations of the
R2PPSiMe3 ligands. The electronic structures of these ligands
are virtually unexplored. To assess the steric and electronic
factors responsible for their versatility, we have performed a
series of DFT calculations at the B3LYP/6-311G, B3LYP/6-
311++G*, and BPW91/ccc-pvtz levels of theory. For a detailed
discussion, see the Supporting Information.
−
We have explored the stereochemistry of the R2PPSiMe3
ligands by scanning the potential energy surfaces (PESs) of
selected R2PPSiMe3 anions, as well as of H2PPH− along the
−
∠(CPPSi) dihedral angle or ∠(HPPH) dihedral angle in the
case of H2PPH− (Figures 6 and S11). For the putative H2PPH−
4.2. Ground-State Geometries and Electronic Config-
urations of 2 and 3. Complexes 2 and 3 are differentiated
from one another by the coordination number of the metal ion
that is imposed by the conformation adopted by the
phosphanylphosphido ligand. For 2, the phosphanylphosphido
ligand adopts an anticlinal conformation, which leads to a
monodentate binding mode. For 3, the phosphanylphosphido
adopts a synclinal conformation, which, in turn, allows for a
bidentate, chelating mode. Considering that the synclinal
conformation is destabilized by intraligand steric interactions,
the latter observation indicates that the higher-energy
conformation of the secondary ligand and the higher
coordination number of 3 are stabilized by a strong chelating
effect. Moreover, compared to most of the other previously
characterized β-diketiminate-supported iron(II) complexes, the
geometries of 2 and 3 are differentiated by the fact that the iron
ion is located out-of-the-plane of the diketiminate ligand. This
distinctive structural feature is induced, specifically, by the
shape asymmetry of the phosphanylphosphido anions, which
leads to an increased anisotropy in the steric repulsions among
the two iron-bound ligands (Figures S14−S16).
Figure 6. Dependence of the ground-state PES on the Si(H)PPC(H)
dihedral angle calculated at the BPW91/cc-pvtz level of theory for the
−
−
H2PPH− (a), H2PPSiH3 (b), i-Pr2PPSiMe3− (c), and t-Bu2PPSiMe3
(d) anions. The Newman projection diagrams shown above the graph,
drawn along the P−P bond, indicate the conformations corresponding
to the particular values of the dihedral angles (highlighted by the black
arrow).
−
and H2PPSiH3 anions, which lack bulky substituents, the
predicted minima corresponding to the approximate synclinal
and anticlinal conformations are nearly degenerate and their
relative energies are found to be within 0.5 kcal/mol from each
other. However, for the remaining anions, while the SiMe3
substituent of the phosphido group is kept constant, the
synclinal conformer is destabilized by an increase in the size of
the phosphanyl substituents (Figure S11, left). The difference
in the relative energies increases roughly proportionally with
the increasing size of the substituents (R = H, Me, i-Pr, t-Bu);
Each geometry optimization of 2 and 3 was followed by time-
dependent DFT (TDDFT) calculations. The observation of
only positive excitation energies allowed us to establish the
presence of an electronic ground state. The character of the
electronic configuration was assessed on the basis of the
Mulliken population analysis and NPA. Compared to the
Mulliken population analysis, NPA provides a more unified
picture of the electron distributions of 2 and 3, which are less
strongly dependent on the choice of the computational method
(Tables S11−S14). Consequently, in our analysis of the
electronic structure, we focused on the results provided by
d(ΔE)/dV = 0.075(5) kJ/mol·Å3. Thus, for i-Pr2PPSiMe3 , the
−
synclinal conformer is ∼6.5 kcal/mol higher in energy than the
−
anticlinal conformer. For t-Bu2PPSiMe3 , this energy difference
2
increases to ∼8.0 kcal/mol. Concomitantly, the increase in the
size of the substituents leads to a more complicated PES
reflected by the presence of ltwo ocal minima for the synclinal
NPA. We found that for 2 the 3dz orbital is essentially doubly
occupied so that, in a first approximation, the electronic
configuration of the formal iron(II) ion can be described using
H
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Figure 7. Crystal-field-splitting diagram of the iron 3d orbitals of 2 and 3 derived from analysis of the TDDFT calculations performed using the
BPW91/cc-pvtz functional/basis set combination (Table S7). The coordinate system was selected such that the z axis is perpendicular to the N−
Fe−N plane and FePSi is contained in the xz plane. For comparison, we have also included the crystal-field-splitting diagram of the previously
investigated three-coordinate LFeIIX complexes.
the |(z2)2(x2 − y2)α(xy)α(xz)α(yz)α| Slater determinant. The
ground-state electronic configuration of 3 exhibits a doubly
occupied 3dyz orbital and thus, to a good degree, can be
approximated as |(z2)α(x2 − y2)α(xy)α(xz)α(yz)2|. However, we
find that the iron sites of 2 (3) have an increased low-valent
character. Accordingly, up to 0.8 (0.9), additional electrons are
donated from the two ligands to the iron mainly into the empty
properties and ground-state electronic configurations. However,
the spectroscopic characterization of 2 revealed a magnetic
behavior that is dramatically different from that of the LFeIIX
complexes. Thus, 2 exhibits a quintet ground spin state with a
positive ZFS parameter, D = +17.4 cm−1, so that the lowest
spin sublevel corresponds to |S, mS⟩ ≅ |2, 0⟩. Inspection of
Figure 7 reveals the origin of this surprising difference: the
interaction of the metal center with the t-Bu2PPSiMe3 ligand, in
particular with the SiMe3 group, leads to destabilization of the
3dyz orbital by an energy that is of the same magnitude as that
2
2
4s, β-3dx −y , and β-3dxy(3dxz) iron orbitals; for details, see the
Supporting Information discussion and Tables S8−S14.
4.3. Excited Orbital States and ZFS Parameters of 2
and 3. Analysis of the TDDFT results shows that the lowest-
energy one-electron excitations correspond to d−d transitions
(Table S7). Consequently, these calculations allow us to build
up the crystal-field-splitting diagram of 2 and 3 (Figure 7). For
comparison, this figure also includes the crystal-field splitting of
2
of the 3dxz orbital. Consequently, for 2 we have an isolated 3dz -
type state separated by ∼2600−3200 cm−1 from the two lowest
excited-state orbitals, 3dyz/3dxz. Interestingly, the DFT-
predicted crystal-field diagram of 2 is analogous to that inferred
for the homoleptic, three-coordinate, planar [FeII(SR)3]− (R =
C6H2-2,4,6-t-Bu3) complexes described by Power and co-
workers.33,34
LFeIIX (X = CH3 , Cl−, NHTol−, NHt-Bu−), the series of β-
−
diketiminate iron(II) complexes that were investigated by
Andres et al.5 In this case, the most important feature of the
crystal-field splitting is given by the near-degeneracy of the
The TDDFT-derived orbital-splitting diagram of Figure 7
provides a clear rationale for the puzzling similarity between the
fine and hyperfine structures of 2 with those of the iron(II)
thiolate complexes, in particular of [Li(THF)2Fe(SR)3 (Table
1). For the latter complex, the bridging by the Li+ ion of two S-
thiolates leads to the lowering of the expected trigonal
symmetry such that one of the ∠(SFeS) is 92°. This value is
similar to ∠(NFeN) ≈ 94° of 2. In both cases, the general
features of the S = 2 ground-state ZFS can be rationalized by
2
{3dz /3dyz} orbitals. The spin−orbit coupling of the two lowest
configurations derived from the highest spin-down, β electron
2
being placed either in a 3dz or in a 3dyz orbital leads to
unquenching of the angular orbital momentum along the x
̂
direction (⟨Lx⟩ ∼
3), that is, along the Fe−X bond.
Ultimately, this interaction yields a ground state consisting of a
magnetically uniaxial spin-orbital quasi-doublet that is quan-
̂
̂
̂
treating the spin−orbit interaction, HSO = λL·S, as a
tized along the x direction, where geff,x ≳ 10 and geff,y ≈ geff,z
≈
2
perturbation of the crystal-field splitting. Considering the 3dz
0.1. The ground-state quasi-doublet is virtually isolated from
the lowest excited states. Although not applicable (with the
spin−orbit interaction being treated as a perturbation of the
crystal-field splitting), in the framework of a quadratic S = 2
spin Hamiltonian, the ground-state quasi-doublet of LFeIIX
would correspond to the |S, ms⟩ ≈ |2, 2⟩ spin sublevels.
Moreover, the ZFS parameter D is unusually large so that,
instead of having a value of only a few wavenumbers, |D| ∼ λ,
where λ ≈ −100 cm−1 is the spin−orbit coupling constant of
the iron(II) ions.
character of the ground state, the important matrix elements are
̂
̂
2
2
⟨3dz |Lx|3dyz⟩ = −⟨3dz |Ly|3dxz⟩ = −i 3. These show that
̂
2
HSO mixes the 3dz ground state with the two lowest excited
states, {3dxz/3dyz}. The spin−orbit contributions to the ZFS
and g tensors are Dx = −3λ2/ε (3dyz), Dy = −3λ2/ε (3dxz), and
Dz = 0 and gx = 2.00 − 3λ/ε (3dyz), gy = 2.00 − 3λ/ε (3dxz),
and gz = 2.00, respectively. The experimental D = +17.4 cm−1
and E/D = 0.11 values yield the DXX = −3.9 cm−1, DYY = −7.7
cm−1, and DZZ = +11.6 cm−1 diagonal components of the
traceless D tensor. In turn, these values were used to determine
the magnitude of the spin−orbit coupling along the x and y
directions, Dx = −15.5 cm−1 and Dy = −23.2 cm−1. Using these
Considering that the metal sites of LFeIIX and LFeII(t-
Bu2PPSiMe3) have identical coordination numbers and similar
geometries, we expected them to have comparable magnetic
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values to minimize the perturbation theory expressions
{15.5ε(3dyz) − 3λ2 = 0; 23.2ε(3dxz) − 3λ2 = 0; 0.18ε(3dyz)
= 3λ; 0.32ε(3dyz) = 3λ} with respect to ε(3dyz), ε(3dxz) and λ
parameters, we obtained ε(3dyz) = 1400 cm−1, ε(3dxz) = 700
cm−1, and λ = −80 cm−1. Although these energies are smaller
than those predicted by TDDFT, they are still within the
margin of error associated with this method.55
components of the valence and ligand contributions, including
the largest components, are virtually collinear and have
opposite signs. Consequently, their sums lead to an overall
EFG tensor with components of diminished magnitude for
which the largest negative component is parallel, rather than
perpendicular, to the N−Fe−N plane. Interestingly, an
analogous large Vligand is also responsible for the nearly
quenched quadrupole splitting of square-planar iron(II)
complexes (ΔEQ = 0.55 mm/s),57 the large ΔEQ = +1 to +2
Upon going from 2 to 3, the conformational change of the
coordinated phosphanylphosphido ligand leads not only to an
increase in the coordination number but also a dramatic
reorganization of the 3d iron orbitals. In particular, the increase
in the Fe−Si distance leads to stabilization of the 3dyz orbital by
such a degree that it becomes even lower in energy than the
mm/s value of some iron(III) porphyrins,58 and the ΔEQ
=
+5.12 mm/s value of FeIII[N(SiMe3)2]3.59 For high-spin
iron(III) compounds, the 6S character of the ground-state
configuration is expected to lead, in an ideal case, to a zero
Vvalence contribution. For 3, the doubly occupied 3dyz orbital
leads to a nearly axial Vvalence for which the large positive
component, Vvalence,max ≅ +3 mm/s, is found along the easy axis
of magnetization, that is, approximatively along the Fe−PSi
bond. Similar to 2, the approximate planar arrangement of the
ligands with increased negative charges of 3 leads to a large
Vligand contribution for which the largest positive component is
essentially orthogonal to the N−Fe−N plane. Together, these
contributions lead to a rhombic, η ∼ 1 EFG tensor that exhibits
a large positive along the easy axis of magnetization (found
approximately along the Fe−PSi bond), a large negative
component roughly orthogonal to the β-diketiminate plane,
and a null component normal to the P−Fe−P plane (Table
S15).
2
3dz orbital. Additionally, displacement of the PSi biting atom
2
2
from the N−Fe−N plane leads to stabilization of the 3dx −y
and destabilization of the 3dxz orbitals such that when
compared to 2 and the LFeIIX complexes, their order is
reversed. Although the latter orbital is also destabilized by the
bonding interaction of the metal ion with PC, its energy is still
lower than that of 3dxy. Similar to the electronic structures of
[LFeIIX] and [LFe(μ2-Cl)2Li(THF)2] (Table 1), the most
important feature of the crystal-field-splitting diagram of 3 is
given by the low-energy separation of the two lowest orbitals,
2
that is, ε(3dz ). Thus, the magnetic properties of 3 are
dominated by the mixing of the two lowest states by the two
lowest orbital states. Thus, perturbation theory yields D = Dx ≈
2
2
2
−3λ /ε(3dz ) and Δgx = gx − 2.00 ≈ −6λ/ε(3dz ).
The magnetic hyperfine splittings of 2 and 3 are described
using the hyperfine coupling tensor A, which results from the
interplay of three distinct interactions Aξ = AFC,ξ + ASD,ξ + AL,ξ.
The individual components represent the Fermi contact term
(AFC,ξ = −κP), the spin dipolar contribution (ASD,ξ = lξP), and
the orbital term (AL,ξ = ΔgξP). The P = gegnβeβn⟨r−3⟩3d scaling
factor of the magnetic hyperfine interaction can be conveniently
expressed in magnetic field units (Tesla) by taking the ratio P/
gnβn. On the basis of Hartree−Fock solutions for the radial 3d
functions, P was estimated at about 64 T for high-spin ferrous
ions. Inspection of Table 1 shows that for 2 Δgx = 0.18, Δgy =
0.32, and Δgz = 0.00 (assuming that the lower than 2.00 value
Consequently, the easy axis of magnetization of 3, that is, the
quantization axis of the spin operator S (z axis of eq 1) is
̂
contained by the N−Fe−N and P−Fe−P planes being found,
roughly, along the Fe−PSi bond. Using the D = −25 cm−1 and
gz = 2.4 values derived from field-dependent Mossbauer
̈
spectroscopy, no EPR signal was observed for 3, and we obtain
−1
a coarse estimate of λ = −125 cm−1 and ε(3dz ) = 1875 cm .
2
The latter value is in good agreement with the TDDFT-
2
predicted energy of the lowest 3dz -type orbital state.
4.4. Hyperfine Structure Parameters of 2 and 3.
Interestingly, both the isomer shift and quadrupole splitting
values of 2 are lower than those expected for a typical high-spin
iron(II) site (δ ≥ 0.6 mm/s and |ΔEQ| ≥ 1.5 mm/s), vide infra.
However, the observed isomer shift value is similar to not only
that of [FeII(SR)3]− complexes, δ = 0.55−0.60 mm/s, but also
those observed for the analogous three-coordinate iron(II) sites
2
of gz is an experimental artifact). Moreover, the 3dz ground-
state configuration leads to the lx,y = 1/14 and lz = −1/7 factors of
the spin dipolar contribution. By introducing these values into
the expressions of the hyperfine coupling tensor components
and solving for κ and P, we obtain for 2 a covalently reduced
2
supported by β-diketiminate complexes that exhibit a 3dz
ground state, i.e., LFeII(μ2-N22−)FeIIL (δ = 0.62 mm/s) and
LFeIICH3 (δ = 0.48 mm/s).56 Consequently, these observations
disclose that the lower than usual δ value of 2 originates from
the combined effects of a low coordination number and of the
value of P = 35 T, κ = 0.68, and a Fermi contact term AFC
=
−23 T. Albeit, the P value is much smaller than that of the free
ion and it is very similar to that observed for Ph4P[Fe(SR)3],
that is, P = 38.7 T. For 3, Δgx = 0.4 along the easy axis of
magnetization (z axis of the spin Hamiltionian) and Δgy,z = 0.0,
together with the lx = 1/7 and lz,y = −1/14 factors, lead us to P =
55 T, κ = 0.36, and a Fermi contact term AFC = −20 T.
2
symmetry-allowed mixing of 3dz and 4s atomic orbitals (Table
S13). The increased population of the 4s atomic orbital, due to
2
the formation of a hybrid 3dz −4s orbital, leads to a higher
electronic density at the nucleus and thus to a lower value of δ.
At the B3LYP/6311G level of theory, the isomer shift values
predicted for 2, δcalc = 0.46 mm/s and δexp = 0.55 mm/s, and 3,
δcalc = 0.69 mm/s and δexp = 0.80 mm/s, are within the error
associated with this method (∼0.15 mm/s).
5. CONCLUSIONS
When the lithiated diphosphanes R2PP(SiMe3)Li (R = t-Bu, i-
Pr) were reacted with [LFeCl2Li(DME)2] (L = Dippnacnac),
they furnished two novel phosphanylphosphido complexes of
iron(II). Their structural characterization revealed that while,
for 2, R2PPSiMe3− adopts an end-on coordination for 3, the P2-
containing moiety adopts an side-on coordination. Interest-
ingly, the reaction of (i-Pr2N)2PP(SiMe3)Li with [LFeCl2Li-
(DME)2] is considerably more complex and yields not another
iron(II) complex but 4. The electronic structures of 2 and 3
were characterized using magnetometry, field-dependent 57Fe
2
The fine structure of 2 fits well with a 3dz -type ground-state
configuration. For such a state, we expected the largest principal
component of the EFG to be negative and to be aligned with
the z axis (see the Supporting Information). However, we
found that the experimental EFG tensor exhibits a small
positive component along the z axis. Analysis of the DFT-
predicted EFG tensors shows that for 2 the individual
J
DOI: 10.1021/acs.inorgchem.7b01374
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Mossbauer spectroscopy, and HFEPR. This investigation
Andrzej Ozarowski. A.D.-A. acknowledges partial support from
the NSF (Grant CHE-1152020). The authors thank A.
Ozarowski for his EPR simulation and fitting program SPIN.
Moreover, the authors thank Karlsruhe Nano Micro Facility,
Karlsruhe Institute of Technology, for X-ray diffraction
measurements.
̈
revealed that 2 and 3 exhibit distinct magnetic properties and
electronic structures. Thus, for 2, the lowest spin sublevel is
given by a |S, mS⟩ = |2, 0⟩ substate. In contrast, the ground state
of 3 consists of an isolated quasi-doublet with a |S, mS⟩ ∼ |2,
2⟩ parentage. Our DFT calculations provide a clear rationale
for the observed fine and hyperfine structure parameters. For
example, 2 exhibits a ground state for which z2 is doubly
occupied. Moreover, its ZFS originates from the strong spin−
orbit coupling of the ground state with two low-lying orbital
states provided by the {z2 → xz} and {z2 → yz} β-electron
excitations. For 3, the high-spin iron(II) site adopts a ground-
state electronic configuration for which the yz orbital is doubly
occupied. In this case, the observed ZFS of the spin ground
state originates from the spin−orbit interaction acting on the
two configurations spanned by the {yz → z2} excitation of the
single β-3d electron.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01374.
■
S
Crystallographic details, magnetic susceptibility and
reduced magnetization measurements, 57Fe Mossbauer
spectroscopy, energy-level diagrams and spin expectation
1
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: grubba@pg.gda.pl.
*E-mail: sebastian.stoian@magnet.fsu.edu.
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Rafał Grubba: 0000-0001-6965-2304
Łukasz Ponikiewski: 0000-0002-5037-1956
Sebastian A. Stoian: 0000-0003-3362-7697
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The authors declare no competing financial interest.
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