Iron PCP Pincer Complexes in Three Oxidation States: Reversible Ligand Protonation to Afford an Fe(0) Complex with an Agostic C-H Arene Bond

Article abstract of DOI:10.1021/acs.inorgchem.8b01018

In the current investigation, the reaction of Fe₂(CO)₉ with the ligand precursor 2-chloro-N¹,N³-bis(diisopropylphosphanyl)-N¹,N³-diethylbenzene-1,3-diamine (P(C-Cl)P^NEt-iPr) (1)

Full text of DOI:10.1021/acs.inorgchem.8b01018

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Iron PCP Pincer Complexes in Three Oxidation States: Reversible
Ligand Protonation To Afford an Fe(0) Complex with an Agostic C−
H Arene Bond
†
†
‡
§
Daniel Himmelbauer, Matthias Mastalir, Berthold Stoger, Luis F. Veiros, Marc Pignitter,∥
̈
Veronika Somoza,∥ and Karl Kirchner*,†
†Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
‡X-Ray Center, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
§
́
Centro de Química Estrutural, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa,
Portugal
∥Department of Physiological Chemistry, Faculty of Chemistry, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
S
* Supporting Information
ABSTRACT: In the current investigation, the reaction of
Fe2(CO)9 with the ligand precursor 2-chloro-N1,N3-bis-
(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine
(P(C−Cl)PNEt-iPr) (1) was investigated. When a suspension
of Fe2(CO)9 and 1 in CH3CN was transferred in a sealed
microwave glass vial and stirred for 18 h at 110 °C the complex
[Fe(PCPNEt-iPr)(CO)2Cl] (2) was obtained. In an attempt to
prepare the hydride Fe(II) complex [Fe(PCPNEt-iPr)(CO)2H]
(3), 2 was reacted with 1 equiv of Li[HBEt3] in THF. Instead
of ligand substitution, this complex underwent a one electron
reduction which led to the formation of the low-spin d7 Fe(I)
complex [Fe(PCPNEt-iPr)(CO)2] (4). Exposure of a benzene
solution of 4 to NO gas (1 bar) at room temperature affords
the diamagnetic complex [Fe(PCPNEt-iPr)(CO)(NO)] (5).
This is the first iron PCP nitrosyl complex. Protonation of 5 with HBF4·Et2O affords the cationic Fe(0) complex [Fe(κ3P,CH,P-
P(CH)PNEt-iPr)(CO)(NO)]BF4 (6) which features an η2-Caryl-H agostic bond. Even with relatively weak bases such as NEt3
the agostic C−H bond can be deprotonated with reformation of the starting material 5. Therefore, protonation of 5 is
completely reversible.
attributed to the failure of many simple metal salts to cleave
the C−H bonds of the arene moiety of the pincer ligands and/
or the thermodynamic instability of the resulting hydride
complexes.4 As iron is concerned, to date, there are only three
established systems. Guan and co-workers synthesized the first
Fe(II) PCP pincer complexes via a direct cyclometalation
route by treatment of the Fe(0) precursor Fe(PMe3)4 with
resorcinol-derived bis(phosphinite) PCP ligand pincer ligands
(Chart 1).5−7 The groups of Sortais8 and Milstein9 and co-
workers described a new and efficient method based on the
commercially available Fe(0) reagent Fe(CO)5 for the simple
synthesis of well-defined cyclometalated hydride carbonyl PCP
iron pincer complexes under mild conditions and UV
irradiation.
INTRODUCTION
■
PCP pincer complexes1 where phosphine donors are
connected via CH2, O, or NR spacers to an aromatic benzene
backbone in the two ortho positions have received
considerable attention over the last decades.2 This type of
compounds can be rationally designed in a modular fashion
which permits the preparation of highly active catalysts for a
variety of chemical reactions which proceed with high
efficiency and selectivity.2 An established synthetic approach
is an intramolecular directed C−H bond coordination to
electron rich low-valent metal fragments,3 followed by
oxidative addition of the agostic C−H bond of the P(C−H)
P ligand leading to the generation of (hydride) PCP
complexes.
Here we utilize the oxidative addition of 2-chloro-N1,N3-
bis(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine
With respect to nonprecious metals, the chemistry of nickel
PCP complexes is already quite comprehensive, while studies
of cobalt, iron, and molybdenum PCP pincer complexes are
exceedingly rare or virtually nonexistent, such as in the case of
copper, manganese, chromium, or vanadium.2p This may be
Received: April 14, 2018
© XXXX American Chemical Society
A
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Chart 1. Examples of Iron PCP Complexes
Figure 1. Structural view of [Fe(PCPEt-iPr)(CO)2Cl] (2) showing
50% thermal ellipsoids (H atoms omitted for clarity). Selected bond
lengths (Å) and bond angles (deg): Fe1−C1 2.002(2), Fe1−P2
2.2561(7), Fe1−P1 2.270(2), Fe1−Cl1 2.334(2), Fe1−C24 1.713(8),
Fe1−C23 1.818(3), P2−Fe1−P1 162.99(5).
Scheme 2. Synthesis of Complex 4
(P(C−Cl)PNEt-iPr) (1) to the Fe(0) complex [Fe2(CO)9] as a
synthetic approach for the synthesis of the Fe(II) PCP
complex [Fe(PCPEt-iPr)(CO)2Cl]. This complex is a valuable
precursor for iron PCP pincer complexes in oxidation states +I
and 0. X-ray structures of representative complexes are
presented.
(CyPNP)] reacted with Na[HBEt3] in the presence of CO to
yield the low-spin Fe(I) dicarbonyl complex [Fe-
(CO)2(CyPNP)].10 Complex 4 was isolated in 93% yield as
an air-sensitive dark violet solid. As judged by electron
paramagnetic resonance (EPR) studies and solution magnetic
susceptibility measurements (benzene, Evans method), this
compound is a low-spin complex. The solution effective
magnetic moment of 1.8(1) μB is in agreement with a low-spin
d7 center (one unpaired electron).
RESULTS AND DISCUSSION
■
When a suspension of [Fe2(CO)9] and P(C−Cl)PNEt-iPr (1)
in acetonitrile was placed in a sealed microwave glass vial and
stirred for 18 h at 110 °C, the analytically pure complex
[Fe(PCPNEt-iPr)(CO)2Cl] (2) was obtained in 47% isolated
yield (Scheme 1). This complex was fully characterized by a
Scheme 1. Synthesis of Complex 2
In the X-band EPR spectrum, an isotropic triplet at giso
=
2.038 with a well-resolved hyperfine coupling to the
phosphorus atoms A = 19.5 G is observed (Figure 2). Most
reported low-spin Fe(I) compounds feature EPR spectra with g
values close to 2.0.11
The frontier orbitals of complex 4 are also typical of a low
spin d7 species with a square pyramidal geometry (Figure 3).12
The HOMO (single occupied) is based on a metal z2 orbital
with a strong component pointing to the empty sixth
coordination position, and the LUMO is the Fe−L σ* orbital
involving the x2−y2 metal orbital, antibonding to all four
combination of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy,
IR spectroscopy, and HR-MS analysis. In the 13C{1H} NMR
spectrum this complex exhibits a characteristic triplet
resonance at 137.5 ppm (JPC = 14.4 Hz) assigned to the ipso
carbon of the benzene moiety and two low-field triplet
resonances at 215.6 and 212.7 ppm assignable to the carbonyl
carbon atoms trans and cis to the ipso carbon. Two strong
absorption bands typical of a cis CO arrangement observed at
2002 and 1934 cm−1, respectively, are detected in the IR
spectrum. In addition, the solid-state structure of 2 was
established by single-crystal X-ray crystallography. A molecular
view is depicted in Figure 1 with selected bond distances given
in the captions.
We considered this complex as an ideal candidate for the
synthesis of a hydride PCP complex. Thus, in an effort to
prepare the hydride Fe(II) complex [Fe(PCPNEt-iPr)(CO)2H]
(3), complex 2 was reacted with 1 equiv of Li[HBEt3] in THF.
Instead of ligand substitution, this complex underwent a formal
one electron reduction which afforded the Fe(I) complex
[Fe(PCPNEt-iPr)(CO)2] (4) (Scheme 2). It has to be noted
that the related pyrrole-based Fe(II) complex [FeCl(py)-
Figure 2. X-band EPR spectrum of [Fe(PCPEt-iPr)(CO)2] (4) in
toluene at 293 K at a microwave frequency of 9.86 GHz. The red line
represents a simulation with parameters giso = 2.038 and A (31P) =
19.5 G.
B
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spectroscopy, IR spectroscopy, and HR-MS analysis. The
13C{1H} NMR spectrum gives rise to low-field triplet
resonances at 228.3 ppm (t, J = 29.0 Hz) assignable to the
carbonyl carbon atom and a triplet resonance at 137.5 ppm
(JPC = 14.4.Hz) assigned to the ipso carbon of the benzene
moiety. In the IR spectrum two strong absorption bands
assignable to the CO and NO stretching frequencies,
respectively, are observed at 1890 and 1663 cm−1 (cf. a related
cationic complex [Fe(PNPO-iPr)(CO)(NO)]+ exhibits the
respective stretching frequencies at 1948 and 1732 cm−1).13
The oxidation state of Fe in complex 5 is not straightforward
because the two electrons of the Fe−NO bond seem to be
almost equally distributed among the metal and the ligand.
However, the frontier orbitals (d splitting) obtained indicate a
d6 species (see ESI) and the charge of the metal (CFe = −0.10)
is about half the value calculated for complex 4 (CFe = −0.19)
where the metal is unquestionably Fe(I). Those results suggest
a metal oxidation state closer to Fe(II) than to Fe(0), in
complex 5. Interestingly, the Fe−N bond is very strong with a
Wiberg index (WI) of 1.17.
Figure 3. Frontier orbitals (d-splitting) and spin density for
[Fe(PCPEt-iPr)(CO)2] (4).
coordinating atoms. The other three metal d orbitals, xy, xz,
and yz are involved in filled molecular orbitals, HOMO−5,
HOMO−4, and HOMO−3, respectively. Accordingly, the spin
density of the molecule is located on the metal atom.
The solid-state structure of 4 was determined by X-ray
diffraction. A view of the molecular structure is depicted in
Figure 4 with selected bond distances and angles reported in
Protonation of [Fe(PCPNEt-iPr)(CO)(NO)] (5) with
HBF4·Et2O leads to the formation of the cationic Fe(0)
complex [Fe(κ3P,CH,P-P(CH)PNEt-iPr)(CO)(NO)]BF4 (6)
in 95% isolated yield (Scheme 3). This complex features an
agostic arene C−H bond. There was no evidence for the
formation of the Fe(II) hydride complex [Fe(PCPNMe
-
iPr)(CO)(NO)(H)]+. DFT calculations reveal that such a
hydride complex is thermodynamically less favorable by 10
kcal/mol. The agostic proton is comparatively acidic and thus
readily deprotonated even with weak bases, such as NEt3,
thereby reforming the starting material 5. Accordingly, this
process is fully reversible (Scheme 3).
1
Complex 6 was again characterized by H, 13C{1H}, and
31P{1H} NMR spectroscopy, IR, HR-MS, and X-ray
crystallography. A characteristic feature of the 1H NMR
spectrum is the high-field shift of the proton attached to the
ipso-carbon giving rise to a triplet at 2.90 ppm (4JHP = 6.1 Hz).
In the 13C{1H} NMR spectrum the ipso-carbon atom shows a
signal at 66.5 ppm, and the CO ligand gives rise to a low-field
resonance as triplet centered at 216.6 ppm (JCP = 61.0 Hz).
Figure 4. Structural view of [Fe(PCPEt-iPr)(CO)2] (4) showing 50%
thermal ellipsoids (H atoms omitted for clarity). Selected bond
lengths (Å) and bond angles (deg): Fe1−C1 2.015(3), Fe1−P1
2.1791(8), Fe1−P2 2.1868(8), Fe1−C24 1.772(3), Fe1−C23
1.774(3), P1−Fe1−P2 153.25(3), C24−Fe1−C23 97.5(1), C24−
Fe1−C1 172.4(1).
1
the caption. The complex adopts square pyramidal geometry.
The Cipso−Fe1 bond distance is 2.015(3) Å The P−Fe−P,
CCO−Fe−CCO, and C1−Fe1−CCO angles are 153.25(3),
97.5(1), and 172.4(1)°, respectively.
The relatively low JHC coupling constant of 125.6 Hz, as
compared to 160.8 and 165.2 Hz for the other two aromatic
C−H bonds, is also a typical feature of a strong C−H metal
bond.4,14−22 Complex 6 exhibits two bands at 1932 and 1720
cm−1 in the IR spectrum assignable to the CO and NO
stretching frequencies, respectively (cf. 1890 and 1663 cm−1 in
complex 4 which is more electron rich).
We then began to investigate the reactivity of complex 4.
Exposure of a benzene solution of 4 to NO (1 bar) at room
temperature affords the diamagnetic complex [Fe(PCPNEt
-
iPr)(CO)(NO)] (5) in 71% isolated yield (Scheme 3).
According to our knowledge, this is the first iron PCP nitrosyl
complex. This reaction could be viewed as a formal one
electron reduction of the metal center by the NO radical from
Fe(I) to Fe(0), if NO is counted as NO+, or as an oxidation to
Fe(II) with a negative ligand (NO−). This complex was again
fully characterized by standard methods including NMR
Contrarily to what was found for complex 5, the frontier
orbitals obtained for complex 6 (see Supporting Information
and Figure 5) indicate a d8 complex and, thus, suggest an
Fe(0) species. This conclusion is reinforced by the calculated
Scheme 3. Addition of NO to 4 and Reversible Protonation
of 5
Figure 5. HOMO−2 of complex 6 showing π-backdonation from
metal d to the C−H σ* orbital.
C
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charges. The NO ligand has a charge of CNO = −0.06, being
clearly electron poorer than in 5 (CNO = −0.17), while the
metal is rather rich, in terms of electronic density, considering
the cationic nature of the molecule (CFe = −0.06).
The agostic Fe−(C−H) interaction is also reflected in the
complex HOMO−2 that represents the characteristic π-
backdonation from metal d to the C−H σ* orbital (Figure
6). The weakening of the C−H bond due to the agostic
[Fe(PCPNEt-iPr)(CO)2] (4). Exposure of a benzene solution
of complex 4 to NO gas (1 bar) at room temperature resulted
in the substitution of a CO ligand with a concomitant one-
electron reduction of the metal center to afford the
diamagnetic complex [Fe(PCPNEt-iPr)(CO)(NO)] (5). DFT
calculations suggest a metal oxidation state closer to Fe(II)
than to Fe(0). This is the first iron PCP nitrosyl complex.
Protonation of 5 with HBF4·Et2O leads to the formation of the
cationic Fe(0) complex [Fe(κ3P,CH,P-P(CH)PNEt-iPr)(CO)-
(NO)]BF4 (6), which features an η2-Caryl-H agostic bond. The
agostic proton is comparatively acidic and thus readily
deprotonated even with weak bases, such as NEt3, thereby
reforming the starting material 5. Accordingly, this process is
fully reversible.
EXPERIMENTAL SECTION
■
General Information. All reactions were performed under an
inert atmosphere of argon by using Schlenk techniques or in a
MBraun inert-gas glovebox. The solvents were purified according to
standard procedures.23 The deuterated solvents were purchased from
Aldrich and dried over 3 Å molecular sieves. The synthesis of 2-
chloro-N1,N3-bis(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-
diamine (P(C−Cl)PNEt-iPr) (1) is described in the Supporting
Information. All starting materials are known compounds and were
Figure 6. Structural view of [Fe(κ3P,CH,P-P(CH)PEt-iPr)(CO)-
(NO)]BF4 (6) showing 50% thermal ellipsoids (most H atoms and
−
BF4 counterion omitted for clarity). Selected bond lengths (Å) and
bond angles (deg): Fe1−C1 2.158(1), Fe1−N3 1.672(1), Fe1−C23
1.761(1), Fe1−P2 2.2596(4), Fe1−P1 2.2690(4), Fe1−H1 1.94(2),
C1−H1 1.0(2), N3−Fe1−C23 113.17(5), C23−Fe1−C1 136.24(5),
P2−Fe1−P1 154.74(2).
used as obtained from commercial resources. H and 13C{1H} and
1
31P{1H} NMR spectra were recorded on Bruker AVANCE-250,
AVANCE-400, and AVANCE-600 spectrometers. H and 13C{1H}
1
NMR spectra were referenced internally to residual protio-solvent and
solvent resonances, respectively, and are reported relative to
tetramethylsilane (δ = 0 ppm). 31P{1H} NMR spectra were referenced
externally to H3PO4 (85%) (δ = 0 ppm).
High resolution-accurate mass data mass spectra were recorded on
a hybrid Maxis Qq-aoTOF mass spectrometer (Bruker Daltonics,
Bremen, Germany) fitted with an ESI-source. Measured accurate mass
data of the [M]+ ions for confirming calculated elemental
compositions were typically within 5 ppm accuracy. The mass
calibration was done with a commercial mixture of perfluorinated
trialkyl-triazines (ES Tuning Mix, Agilent Technologies, Santa Clara,
CA, USA).
CW-EPR spectroscopic measurements were performed on an X-
band Bruker Elexsys-II E500 EPR spectrometer (Bruker Biospin
GmbH, Rheinstetten, Germany) in solution at 293 K. A high
sensitivity cavity (SHQE1119) was used for measurements setting the
microwave frequency to 9.86 GHz, the modulation frequency to 100
kHz, the center field to 6000 G, the sweep width to 12000 G, the
sweep time to 30.0 s, the modulation amplitude to 6 G, the
microwave power to 15.9 mW, the conversion time to 7.33 ms, and
the resolution to 4096 points. The spectra were analyzed using the
Bruker Xepr software.
interaction is evident from the longer bond distance (dC−H
=
1.11 Å) and weaker WI of 0.80 when compared to the other
Caryl−H bonds (d = 1.08 Å, WI = 0.91).
An ORTEP plot of complex 6 is presented in Figure 6. This
compound exhibits essentially a trigonal bipyramidal geometry
with the CO and NO ligands and the agostic C−H bond in the
equatorial plane and the phosphine donors in the axial
positions. The bond distance of the ipso-carbon and the Fe
atom is rather long (2.258(1) Å) when compared to simple
Fe−C σ-bonds. For example, in the Fe(II) and Fe(I) PCP
complexes [Fe(PCPNEt-iPr)(CO)2Cl] (2) and [Fe(PCPNEt
-
iPr)(CO)2] (4) the Fe−Cipso bond distances are significantly
shorter, being 2.002(2) and 2.015(3) Å, respectively. The
H(1) atom could be located in difference Fourier maps and
was refined freely. This hydrogen atom clearly interacts with
the Fe center as evident from a Fe−H bond length of 1.94(2)
1
Å and is also supported by the H NMR spectrum of 6 as
discussed above. Moreover, this hydrogen is severely bent away
from the benzene plane by ca. 29.2° (cf. in similar Ru, Rh, and
Pd pincer complexes this angle is about 14−30°,14−22 while in
Cr, Mo,4 and Co14 it is 32, 28, and 35°, respectively). The
C1−H1 bond distance is 1.0(2) Å, which is comparable to
hydrocarbons such as 1.08 Å in benzene.
Synthesis. [Fe(PCPNEt-iPr)(CO)2Cl] (2). A suspension of [Fe2(CO)9]
(151 mg, 0.42 mmol) and 2 equivs of 1 (200 mg, 0.84 mmol) in
CH3CN (2 mL) were placed in a 20 mL sealed glass tube and stirred
for 18 h at 110 °C. The reaction mixture was allowed to cool to room
temperature without stirring. Insoluble materials were removed by
filtration over a syringe filter (polytetrafluoroethylene 0.2 μm), and
the solvent was removed under vacuum. The product was obtained as
yellow powder and washed three times with pentane. Yield: 118 mg
(47%). Anal. Calcd for C24H41ClFeN2O2P2 (542.85): C, 53.10; H,
CONCLUSION
■
We described here the preparation of the Fe(II) PCP complex
[Fe(PCPNEt-iPr)(CO)2Cl] (2). Under solvothermal condi-
tions, treatment of Fe2(CO)9 with 2-chloro-N1,N3-bis-
(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine
(P(C−Cl)PNEt-iPr) (1) in CH3CN in a sealed microwave glass
vial for 18 h at 110 °C afforded complex 2 in 47% isolated
yield. In an endeavor to obtain the hydride Fe(II) complex
[Fe(PCPNEt-iPr)(CO)2H] (3), complex 2 was treated with
Li[HBEt3] in THF. However, instead of ligand substitution,
this complex was reduced to the low-spin d7 Fe(I) complex
1
7.61; N, 5.16. Found: C, 53.35; H, 7.59; N, 5.20. H NMR (δ, 600
MHz, C6D6, 20 °C): 7.20 (t, J = 8.1 Hz, 1H, CarH), 6.27 (d, J = 8.0
Hz, 2H, CarH), 3.10 (m, 4H, N−CH2), 2.92 (m, 2H, P−CH), 2.34
(m, 2H, P−CH), 1.46 (q, J = 7.7 Hz, 6H, CH−CH3), 1.25 (q, J = 7.1
Hz, 6H, CH−CH3), 1.20 (q, J = 7.5 Hz, 6H, CH−CH3), 1.11 (q, J =
7.1 Hz, 6H, CH−CH3), 1.05 (t, 6H, J = 6.9 Hz, CH2−CH3). 13C{1H}
NMR (δ, 600 MHz, C6D6, 20 °C): 215.6 (t, J = 27.4 Hz, CO), 212.7
(t, J = 11.6 Hz, CO), 156.8 (t, J = 13.7 Hz, C-N), 137.5 (t, J = 14.4
Hz, C-Fe), 126.6 (s, CarH), 103.0 (t, J = 5.4 Hz, CarH), 41.0 (s, CH2),
30.7 (t, J = 11.6 Hz, P-CH), 29.0 (t, J = 11.0 Hz, P-CH), 22.1 (s, CH-
D
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CH3), 19.5 (s, CH-CH3), 19.5 (s, CH-CH3), 18.9 (t, J = 3.4 Hz, CH-
CH3), 14.2 (s, CH2−CH3). 31P{1H} NMR (δ, 250 MHz, C6D6, 20
°C): 151.5. HR-MS (ESI+, CH3CN/MeOH+1%H2O): m/z calcd for
C24H41FeN2O2P2 [M − Cl]+ 507.1993, found 507.1990. IR (ATR,
cm−1): 2002 (νCO), 1934 (νCO).
space approach implemented in SHELXT25 and refined against F2
with SHELXL.26 Non-hydrogen atoms were refined anisotropically.
The H atoms connected to C atoms were placed in calculated
positions and thereafter refined as riding on the parent atoms. The
agostic H was located from difference Fourier maps and refined freely.
The Cl ligands and the CO ligands in the trans-position to Cl in 2
were refined as occupationally disordered (occupancies constrained to
reflect the nominal composition; minor positions realized to
39.4(10)% and 37.9(10)% for both crystallographically independent
complexes, respectively). A part of the PCP ligand was refined as
positionally disordered, with the occupancies constrained to those of
the Cl/CO disorder. The Fe atom in 4 was refined as disordered
about two positions (sum of occupancies constrained to 1; ADPs
constrained to be identical; refined occupancy of the minor position
2.79(8)%). A peak in the difference electron map located ca. 2.4 Å
from the minor Fe position suggests a cocrystallization with the
chloride precursor. Although placing a Cl atom in this position led to
convergence, it did not improve the reliability factors and was omitted
in the final model. In 6, the Fe atom was likewise modeled as
disordered about two positions. Owing to the absence of secondary
species in the NMR spectra, we suppose a different orientation of the
molecule. Since the minor position is only realized in 1.18% of the
molecules, no significant peaks are observed in the difference Fourier
maps and no definite statement can be made. Molecular graphics were
generated with the program MERCURY.27
Computational Details. The computational results presented
have been achieved in part using the Vienna Scientific Cluster (VSC).
Calculations were performed using the GAUSSIAN 09 software
package28 and the B3LYP functional, without symmetry constraints.
That functional includes a mixture of Hartree−Fock29 exchange with
DFT30 exchange-correlation, given by Becke’s three parameter
functional31 with the Lee, Yang, and Parr correlation functional,
which includes both local and nonlocal terms.32,33 The basis set used
consisted of the Stuttgart/Dresden ECP (SDD) basis set34 to describe
the electrons of Fe and a standard 6-31G(d,p) basis set35 for all other
atoms. The frontier orbitals of complex 4 result from a single point
restricted open shell calculation. A Natural Population Analysis
(NPA)36 and the resulting Wiberg indices37 were used to study the
electronic structure and bonding of the optimized species. The NPA
analysis was performed with the NBO 5.0 program,38 and the three-
dimensional representations of the orbitals were obtained with
Molekel.39
[Fe(PCPNEt-iPr)(CO)2] (4). A solution of 2 (50 mg, 0.092 mmol) in
THF (3 mL) was reacted Li[HBEt3] (0.06 mL, 1.7 M solution in
THF, 0.10 mmol) at room temperature and stirred for 1 h. The color
of the solution instantly changed from orange to dark violet. All
volatiles were removed under vacuum and an oily residue was
obtained. The product was redissolved in pentane and filtered over a
syringe filter (polytetrafluoroethylene 0.2 μm). Crystals could be
obtained by cooling the pentane solution to −30 °C for 24 h. Yield:
43 mg (93%). Anal. Calcd for C24H41FeN2O2P2 (507.40): C, 56.81;
H, 8.15; N, 5.52. Found: C, 56.95; H, 8.39; N, 5.45. HR-MS (ESI+,
CH3CN/MeOH + 1%H2O) m/z calcd for C23H41FeN2OP2 [M −
CO]+ 479.2044, found 479.2038. μeff = 1.8(1) μB (benzene, Evans
method). IR (ATR, cm−1): 1937 (νCO), 1866 (νCO).
[Fe(PCPNEt-iPr)(CO)(NO)] (5). A solution of 4 (80 mg, 0.16 mmol)
in benzene (2 mL) was stirred for 1 h under an NO gas atmosphere
(1 bar) whereupon the solution turned from dark violet to red.
Insoluble materials were removed by filtration through a syringe filter
(polytetrafluoroethylene 0.2 μm). After removal of the solvent under
vacuum, the product was obtained as a red solid. Yield: 57 mg (71%).
Anal. Calcd for C23H41FeN3O2P2 (509.39): C, 54.23; H, 8.11; N,
1
8.25. Found: C, 54.37; H, 8.09; N, 8.10. H NMR (δ, 600 MHz,
C6D6, 20 °C): 7.19 (t, J = 7.8 Hz, 1H, CarH), 6.21 (d, J = 7.8 Hz, 2H,
CarH), 3.15 (m, 2H, N−CH2), 3.03 (m, 2H, N−CH2), 2.39 (m, 2H,
P−CH), 2.30 (h, J = 7.1 Hz, 2H, P−CH), 1.33 (m, 6H, CH−CH3),
1.08 (dt, J = 7.1 Hz, J = 14.2 Hz, 12H, CH−CH3, CH2CH3), 0.99 (q,
J = 7.4 Hz, 6H, CH−CH3), 0.90 (q, J = 7.0 Hz, 6H, CH−CH3).
13C{1H} NMR (δ, 600 MHz, C6D6, 20 °C): 228.3 (t, J = 29.0 Hz,
CO), 157.9 (t, J = 15.4 Hz, C−N), 137.4 (t, J = 24.1 Hz, C−Fe),
126.9 (s, CarH), 101.5 (t, J = 6.2 Hz, CarH), 40.3 (s, CH2), 31.8 (t, J =
11.0 Hz, P−CH), 28.1 (t, J = 13.6 Hz, P−CH), 18.2 (s, CH−CH3),
17.9 (s, CH−CH3), 17.2 (s, CH−CH3), 17.1 (t, J = 4.1 Hz, CH−
CH3), 14.2 (s, CH2−CH3). 31P{1H} NMR (δ, 250 MHz, C6D6, 20
°C): 171.6. HR-MS (ESI+, CH3CN/MeOH + 1%H2O): m/z calcd for
C22H41FeN3OP2 481.2074 [M − CO]+, found 481.2075. IR (ATR,
cm−1): 1890 (νCO), 1663 (νNO).
[Fe(κ3P,CH,P-P(CH)PNEt-iPr)(CO)(NO)]BF4 (6). A solution of 5 (57
mg, 0.11 mmol) in THF (2 mL) was treated with HBF4·Et2O (24 mg,
0.15 mmol) and the reaction mixture was stirred for 15 min at room
temperature. All volatiles were then removed under vacuum, and a red
solid was obtained which was washed with ether (2 × 5 mL) and
dried under vacuum. Yield: 65 mg (95%). Anal. Calcd for
C23H42BF4FeN3O2P2 (597.20): C, 46.26; H, 7.09; N, 7.04. Found:
C, 46.39; H, 7.12; N, 7.13. 1H NMR (δ, 600 MHz, CD3CN, 20 °C):
7.66 (tt, J = 8.2 Hz, J = 1.5 Hz, 1H, CarH), 6.56 (d, J = 8.3 Hz, 2H,
CarH), 3.76 (m, 2H, N−CH2), 3.57 (m, 2H, N−CH2), 2.98 (m, 2H,
P−CH), 2.90 (t, J = 6.1 Hz, 1H, CH−Fe) 2.82 (m, 2H, P−CH), 1.38
(dd, J = 18.0 Hz, J = 7.1 Hz, 6H, CH−CH3), 1.33 (dd, J = 16.1 Hz, J
= 7.1 Hz, 6H, CH−CH3), 1.20 (t, J = 7.1 Hz, 6H, CH2−CH3), 1.16
(dd, J = 20.9 Hz, J = 7.0 Hz, 6H, CH−CH3), 1.03 (dd, J = 17.0 Hz, J
= 7.0 Hz, 6H, CH−CH3). 13C{1H} NMR (δ, 600 MHz, CD3CN, 20
°C): 216.6 (t, J = 61.0 Hz, CO), 167.5 (t, J = 10.2 Hz, C−N), 142.7
(s, CarH), 107.4 (t, J = 4.2 Hz, CarH), 66.5 (t, J = 3.4 Hz, CH−Fe),
41.7 (s, CH2), 30.9 (t, J = 14.0 Hz, P−CH), 30.0 (t, J = 11.5 Hz, P−
CH), 18.2 (s, CH−CH3), 17.7 (s, CH−CH3), 16.9 (s, CH−CH3),
15.2 (t, J = 4.0 Hz, CH−CH3), 13.2 (s, CH2−CH3). 31P{1H} NMR
(δ, 250 MHz, CD3CN, 20 °C): 142.2. HR-MS (ESI+, CH3CN/
MeOH + 1%H2O) m/z: calcd for C22H41FeN3OP2 481.2074 [M − H
− CO]+, found 481.2068. IR (ATR, cm−1): 1932 (νCO), 1720 (νNO).
X-ray Structure Determination. X-ray diffraction data of 2, 4,
and 6 were collected at T = 100 K in a dry stream of nitrogen on a
Bruker Kappa APEX II diffractometer system using graphite-
monochromatized Mo Kα radiation (λ = 0.71073 Å) and fine sliced
φ- and ω-scans. Data were reduced to intensity values with SAINT,
and an absorption correction was applied with the multiscan approach
implemented in SADABS.24 The structure was solved by the dual-
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.8b01018.
Complete crystallographic data, 1H, 13C{1H}, and
31P{1H} NMR spectra of all complexes (PDF)
Accession Codes
CCDC 1836427−1836429 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*K.K.: E-mail, karl.kirchner@tuwien.ac.at; tel, (+43) 1 58801
163611; fax, (+43) 1 58801 16399.
■
ORCID
Luis F. Veiros: 0000-0001-5841-3519
Marc Pignitter: 0000-0002-5793-8572
Veronika Somoza: 0000-0003-2456-9245
E
DOI: 10.1021/acs.inorgchem.8b01018
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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Heterocycles via Rh-Catalyzed C−H Bond Activation. Acc. Chem. Res.
2008, 41, 1013−1025. (f) Gaillard, S.; Cazin, C. S. J.; Nolan, S. P. N-
Heterocyclic Carbene Gold(I) and Copper(I) Complexes in C−H
Bond Activation. Acc. Chem. Res. 2012, 45, 778−787. (g) Li, B.;
Dixneuf, P. H. sp2 C−H bond activation in water and catalytic cross-
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Karl Kirchner: 0000-0003-0872-6159
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the Austrian Science Fund (FWF) is
gratefully acknowledged (Project No. P29584-N28). LFV
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(h) Wencel-Delord, J.; Droge, T.; Liu, F.; Glorius, F. Towards mild
metal-catalyzed C−H bond activation Chem. Chem. Soc. Rev. 2011,
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acknowledges Fundaca̧ o para a Ciencia e Tecnologia, UID/
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QUI/00100/2013.
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