A One-Step Preparation of Tetradentate Ligands with Nitrogen and Phosphorus Donors by Reductive Amination and Representative Iron Complexes

Article abstract of DOI:10.1021/acs.inorgchem.0c01535

The synthesis and use of the first examples of unsymmetrical, mixed phosphine donor tripodal NPP2′ ligands N(CH2CH2PR2)2(CH2CH2PPh2) are presented. The ligands are synthesized via a convenient, one pot reductive amination using 2-(diphenylphosphino)ethylamine and various substituted phosphonium dimers in order to introduce mixed phosphine donors substituted with P/P′, those being Ph/Cy (2), Ph/iPr (3), Ph/iBu (4), Ph/o-Tol (5), and Ph/p-Tol (6). Additionally, we have developed the first known synthesis of a symmetrical tripodal NP3 ligand N(CH2CH2PiBu2)3 using bench safe ammonium acetate as the lone nitrogen source (7). This new protocol eliminates the use of extremely dangerous nitrogen mustard reagents typically required to synthesize NP3 ligands. Some of these tetradentate ligands and also P2NN′ ligands N(CH2-o-C5H4N)(CH2CH2PR2)2 (P2NN′-Cy, R = Cy; P2NN′-Ph, R = Ph) prepared by reductive amination using 2-picolylamine are used in the synthesis and reactions of iron complexes. FeCl2(P2NN′-Cy) (8) undergoes single halide abstraction with NaBPh4 to give the trigonal bipyramidal complex [FeCl(P2NN′-Cy)][BPh4] (9). Upon exposure to CO(g), complex 9 readily coordinates CO giving [FeCl(P2NN′-Cy)(CO)][BPh4] (10), and further treatment with an excess of NaBH4 results in formation of the hydride complex [Fe(H)(P2NN′-Cy)(CO)][BPh4] (11). Our previously reported complex FeCl2(P2NN′-Ph) undergoes double halide abstraction with NaBPh4 in the presence of the coordinating solvent to give [Fe(NCMe)2(P2NN′-Ph)][BPh4]2 (12). Ligand 3 can be coordinated to FeCl2, and upon sequential halide abstraction, treatment with NaBH4, and exposure to an atmosphere of dinitrogen, the dinitrogen hydride complex [Fe(H)(NPP2′-iPr)(N2)][BPh4] (13) is isolated. Our symmetrical NP3 ligand 7 can also be coordinated to FeCl2 and, upon exposure to an atmosphere of CO(g), selectively forms [FeCl(NP3)(CO)][BPh4] (14) after salt metathesis with NaBPh4. Complex 14 can be treated with an excess of NaBH4 to give the hydride complex [Fe(H)(NP3)(CO)][BPh4] (15), which can further be deprotonated/reduced to the Fe(0) complex Fe(NP3)(CO) (16) upon treatment with an excess of KH.

Full text of DOI:10.1021/acs.inorgchem.0c01535

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Article
A One-Step Preparation of Tetradentate Ligands with Nitrogen and
Phosphorus Donors by Reductive Amination and Representative
Iron Complexes
Matthew V. Gradiski,§ Ali Nemati Kharat,§ Maegan S. E. Ong, Alan J. Lough, Samantha A. M. Smith,
and Robert H. Morris*
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ABSTRACT: The synthesis and use of the first examples of unsym-
metrical, mixed phosphine donor tripodal NPP2′ ligands N-
(CH2CH2PR2)2(CH2CH2PPh2) are presented. The ligands are synthe-
sized via a convenient, one pot reductive amination using 2-
(diphenylphosphino)ethylamine and various substituted phosphonium
dimers in order to introduce mixed phosphine donors substituted with P/
P′, those being Ph/Cy (2), Ph/iPr (3), Ph/iBu (4), Ph/o-Tol (5), and
Ph/p-Tol (6). Additionally, we have developed the first known synthesis
of a symmetrical tripodal NP3 ligand N(CH2CH2PiBu2)3 using bench safe
ammonium acetate as the lone nitrogen source (7). This new protocol
eliminates the use of extremely dangerous nitrogen mustard reagents
typically required to synthesize NP3 ligands. Some of these tetradentate
ligands and also P2NN′ ligands N(CH2-o-C5H4N)(CH2CH2PR2)2 (P2NN′-Cy, R = Cy; P2NN′-Ph, R = Ph) prepared by reductive
amination using 2-picolylamine are used in the synthesis and reactions of iron complexes. FeCl2(P2NN′-Cy) (8) undergoes single
halide abstraction with NaBPh4 to give the trigonal bipyramidal complex [FeCl(P2NN′-Cy)][BPh4] (9). Upon exposure to CO(g),
complex 9 readily coordinates CO giving [FeCl(P2NN′-Cy)(CO)][BPh4] (10), and further treatment with an excess of NaBH4
results in formation of the hydride complex [Fe(H)(P2NN′-Cy)(CO)][BPh4] (11). Our previously reported complex
FeCl2(P2NN′-Ph) undergoes double halide abstraction with NaBPh4 in the presence of the coordinating solvent to give
[Fe(NCMe)2(P2NN′-Ph)][BPh4]2 (12). Ligand 3 can be coordinated to FeCl2, and upon sequential halide abstraction, treatment
with NaBH4, and exposure to an atmosphere of dinitrogen, the dinitrogen hydride complex [Fe(H)(NPP2′-iPr)(N2)][BPh4] (13) is
isolated. Our symmetrical NP3 ligand 7 can also be coordinated to FeCl2 and, upon exposure to an atmosphere of CO(g), selectively
forms [FeCl(NP3)(CO)][BPh4] (14) after salt metathesis with NaBPh4. Complex 14 can be treated with an excess of NaBH4 to give
the hydride complex [Fe(H)(NP3)(CO)][BPh4] (15), which can further be deprotonated/reduced to the Fe(0) complex
Fe(NP3)(CO) (16) upon treatment with an excess of KH.
nitrogen bridgehead does not coordinate to the metal center
in, for example, complexes of molybdenum4,5 and ruthenium.6
In order for the ligand to adopt a tetradentate chelating mode
which involves the nitrogen, more structural flexibility is
needed. The introduction of ethylene spacers results in the
tetradentate ligand, tris(2-diphenylphosphinoethyl)amine
(NP3). Sacconi pioneered the synthesis and coordination
chemistry of this new class of ligands, and his group discovered
various complexes and coordination geometries with chromi-
INTRODUCTION
■
This article reports a convenient, direct route to a variety of
new tetradentate phosphine ligand architectures. Polyphos-
phines are one of the most commonly used classes of chelating
ligands throughout inorganic coordination chemistry. Among
them is the tridentate ligand triphos MeC(CH2PPh2)3, and
since its earliest reported synthesis by Hewertson and Watson
in 1962,1 triphos has shown widespread use as a ligand support
on RuCl2 for catalytic hydrogenation of olefins, aldehydes,
ketones, and nitriles.2 Triphos bearing m-xylyl substituents
MeC(CH2P(m-xyl)2)3 has seen recent use on cobalt for the
hydrogenation of carbonates to alcohols.3 The related triphos
ligand N(CH2PPh2)3 (N-triphos) has a bridgehead nitrogen
atom and is structurally very similar to triphos. Due to the
constraints imposed by the methylene spacers between the
nitrogen atom and phosphine donors, N-triphos chelates in a
similar fashion to triphos as a tridentate ligand, and the
Received: May 25, 2020
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um,7 cobalt,8−11 and nickel.8,12 It was shown that trigonal
bipyramidal CoH(NP3) complexes and tetrahedral Ni0(NP3)
react with formaldehyde to give, respectively, a cobalt hydrido
carbonyl complex CoH(NP3)(CO) and NiI formyl complex
Ni(CHO)(NP3) that readily decomposes to the Ni0 carbonyl
Ni(NP3)(CO).13 Additionally, rhodium and iridium(I) NP3
supported complexes oxidatively add sp2 C−H bonds.14
Although the ligand prefers to coordinate in a tetradentate
fashion, tridentate coordination is observed in the iridium(III)
trihydride complex IrH3(κ3-P,P,P-NP3), which, upon treatment
with HBF4, undergoes oxidation to an iridium(V) tetrahydride
and then subsequent reductive elimination of H2 and
coordination of the central nitrogen to produce the octahedral
dihydride IrH2(κ4-N,P,P,P-NP3).15 An interesting feature of
Scheme 1. Conventional Synthesis of NP3 Ligands via
Nitrogen Mustard and Phosphide Reagents
Recently, our group reported the synthesis of a variety of
novel PNN′ and P2NN′ ligands via a one-step, one-pot
reductive amination of primary amines with in situ generated
primary phosphine acetaldehydes.33 In addition to this recent
study, our group has taken advantage of the use of
phosphonium dimers to synthesize a wide variety of ligands
and transition metal complexes.37,39,54−59 In this work, we have
expanded the use of our phosphonium dimer chemistry and
report the first synthesis of a tetradentate tripodal NP3 ligand
via reductive amination using bench safe ammonium salts as
the nitrogen source. Additionally, via the same synthetic route
using the readily synthesized 2-(diphenylphosphino)-
ethylamine as a precursor, we report the first synthesis of
unsymmetrical mixed-phosphine donor NPP2′ tetradentate
ligands. An overview of the ligands synthesized and used in this
study is summarized and depicted in Scheme 2. The
coordination chemistry of these ligands is explored with
iron(II) resulting in the synthesis of NP3 iron(II) carbonyl
chlorides, carbonyl hydrides, and a mixed-donor NPP2′
dinitrogen hydride complex. Finally, the reactivity toward
CO(g), hydride, and halide abstraction reagents of iron
dichlorides supported by tetradentate P2NN′ ligands are
investigated.
iron(II) complexes bearing the NP3 ligand or NPi‑Pr ligand
3
with isopropyl substituents at phosphorus16 is their facile
coordination of dinitrogen at ambient pressure.17 Iron(0)
complexes of the same NP3 ligand can promote dinitrogen
reduction to ammonia and hydrazine.18 The NP3 ligand has
seen recent use with ruthenium for the catalytic dehydrogen-
ation of formic acid,19 as well as cobalt20 and ruthenium21 for
the catalytic dehydrogenation of ammonia-borane. These two
dehydrogenation processes have become more sought-after in
recent years as hydrogen storage molecules become more
important toward the development of sustainable fuels.22−24
Nickel(II) NP3 complexes are highly active for the catalytic
hydrogenation of carbon dioxide to formate,25 while
ruthenium(0) NP3 complexes are useful for dehydrogenative
coupling of alcohols to esters.26 P4 tetraphos ligands have also
been used for the synthesis of metal-hydride and dihydrogen
complexes,27,28 and recently, a tetradentate P4N2 ligand has
been reported for the study of paramagnetic iron(I) hydrides.29
Silicon has also been incorporated into the NP3 framework,
resulting in a tripodal amido-polyphosphine ligand.30 An
unsymmetrical NP2P ligand with two chains linked by
methylene spacers and one chain linked with an ethylene
spacer has been reported with ruthenium for the hydro-
genation of biobased carboxylic acids.31 Increased flexibility
can be achieved by lengthening the ethylene spacers to
propylene in order to accommodate larger atoms like Mo(0).32
Unsymmetrical tetradentate ligands have also been reported
where one phosphorus donor is substituted for an additional
nitrogen donor, resulting in an open-chain P2N2 framework if
the nitrogen donors are geometrically equivalent, or a P2NN′
framework if they are not.33 Chiral, open-chain P2N2
tetradentate ligands have been shown to be very useful ligands
for the asymmetric transfer hydrogenation of polar bonds.34−43
Chiral, macrocyclic P2N2 ligands have also been reported and
are useful for similar asymmetric catalytic hydrogenations.44−53
Although tripodal NP3 ligands have proven to be very
versatile for a variety of applications, the nature of their
synthesis continues to be a drawback in their use. Conven-
tional synthesis of NP3 ligands8,16 requires the use of an
extremely toxic and dangerous nitrogen mustard reagent tris(2-
chloroethyl)amine (HN3). HN3 is a schedule 1 chemical
weapon, the same classification given to sarin (o-isopropyl
methylphosphonofluoridate) and bis(2-chloroethyl)sulfide,
which is more commonly known as mustard gas. In addition,
pyrophoric phosphides are required for the synthesis as well
RESULTS AND DISCUSSION
■
Synthesis of Unsymmetrical NPP2′ and NP3 Ligands.
Our group has previously reported the synthesis of chiral P-
NH-P′55 and achiral PNN′ and P2NN′ ligands33 via reductive
amination of primary amines with phosphine aldehydes
generated in situ via phosphonium dimers. The synthesis of
novel unsymmetrical tripodal ligands NPP2′ adopts a similar
synthetic strategy where the primary amine starting material is
2-(diphenylphosphino)ethylamine (1; Scheme 3). Compound
1 is commercially available in up to 95% purity; as an
alternative, it is readily synthesized in 38% yield through SN2
substitution of 2-bromoethylamine hydrobromide with an
equivalent of diphenylphosphide in refluxing THF. We have
found that before extracting the product with diethyl ether,
stirring the residue in pentane removes almost all residual
diphenylphosphine from the reaction mixture. By 31P NMR
(Figure S2), we observed only ∼2% diphenylphosphine,
superior to commercial samples. Compound 1 is an important
ligand in hydrogenation catalysis.54,56,60−70
For the reductive amination reaction, we use 1 and sodium
trisacetoxyborohydride (STAB) with variously substituted
phosphinoacetaldehydes that are generated from the reaction
of the STAB base with its parent phosphonium dimers. This
gives access to a variety of unsymmetrical NPP2′ tripodal
ligands 2−6 (Scheme 3). After the reaction is complete, air-
free aqueous workup to remove any residual sodium and
borane salts is performed to isolate the ligands in fair to good
yields (56−84%) and high purity. Ligands 2−6 have been
characterized fully by 1H and 31P{1H} NMR spectroscopy and
DART mass spectrometry. The ligands all show a characteristic
−
(Scheme 1). The need for SN2 substitution with PR2
phosphides leads to an inability to vary phosphine substituents
within the same ligand and is a highly limiting factor in the
steric and electronic tunability of these ligands.
B
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Scheme 2. Overview of the Synthesis of the Tetradentate Ligands of this Work
Scheme 3. Reductive Amination Synthesis of NPP2′ Ligands 2−6
Scheme 4. Reductive Amination Synthesis of NP3 Ligand 7
Scheme 5. Synthesis of P2NN′-Cy Complexes 8 and 9
resonance in their 31P{1H} spectrum around −20 ppm
equivalent of phosphonium dimer and four equivalents of
NH4OAc and STAB in THF reacted to give the tripodal NP3
ligand 7 in 91% isolated yield after aqueous workup (Scheme
4). The ligand displays a sharp singlet at −41.6 ppm in the
31P{1H} NMR spectrum owing to the three equivalent
phosphorus atoms; 1H NMR spectroscopy and mass
spectrometry with DART detection confirmed the structure.
Synthesis and Reactivity of P2NN′ Supported Iron
Complexes. Recently, our lab reported the synthesis of
tetradentate P2NN′-Cy and P2NN′-Ph ligands and an iron
complex FeCl2(P2NN′-Ph).33 Here, we report the synthesis of
iron complexes where the ligand bears cyclohexyl substituents
at phosphorus. One equivalent of the P2NN′-Cy ligand reacts
with FeCl2 in THF to give the octahedral complex
FeCl2(P2NN′-Cy) (8; Scheme 5).
corresponding to the phenyl substituted arm and a second
i
i
resonance ranging from 1.8 (R = Pr) to −41 ppm (R = Bu,
oTol).
Symmetrical NP3 tetradentate ligands can also be synthe-
sized using ammonium salts as the lone nitrogen source.
During optimization of the reaction conditions (SI Table S1),
it was observed that the use of NH4Cl, even in 20-fold excess,
yielded only ∼6% of the desired NP3 product. Upon switching
to NH4OAc, a considerable increase in the yield to 63% was
achieved with 1.5 equiv of the salt. The optimum amount was
4 equiv of NH4OAc, producing the ligand in 71% yield; further
increases in the amount of salt did not significantly improve
the yield. Since NP3 ligands with phenyl12 and isopropyl16
substituents at phosphorus have previously been reported, we
opted to use an isobutyl-substituted phosphonium dimer in
order to create a novel NP3 ligand. A similar synthetic strategy
used in the synthesis of ligands 2−6 was adopted: one
Single crystal X-ray diffraction (XRD) of complex 8 confirms
a distorted octahedral geometry with trans phosphorus donors
while the tertiary amine, pyridine, and two chloride ligands all
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Figure 1. X-ray structure of 8 (left) and 9 (right). Ellipsoids are shown at 30% probability. Hydrogen atoms and tetraphenylborate anion have been
omitted for clarity. There were two independent molecules in the asymmetric unit for 8. Molecule A is shown here, and molecule B is provided in
Figure S16. Selected bond lengths (Å) and angles (deg) for 8: Fe(1A)−N(2A) 2.219(3), Fe(1A)−Cl(1A) 2.3452(12), Fe(1A)−N(1A) 2.393(3),
Fe(1A)−Cl(2A) 2.4723(12), Fe(1A)−P(1A) 2.5192(12), Fe(1A)−P(2A) 2.5501(13); N(2A)−Fe(1A)−Cl(1A) 96.19(10), N(2A)−Fe(1A)−
N(1A) 74.03(12), Cl(1A)−Fe(1A)−N(1A) 169.93(9), N(2A)−Fe(1A)−Cl(2A) 158.36(10), P(1A)−Fe(1A)−P(2A) 159.62(5); for 9: Fe(1)−
N(2) 2.1161(19), Fe(1)−Cl(1) 2.2825(7), Fe(1)−P(1) 2.4275(7), Fe(1)−P(2) 2.4629(7); N(1)−Fe(1)−Cl(1) 170.31(5), N(2)−Fe(1)−Cl(1)
101.05(6), N(2)−Fe(1)−P(1) 120.99(5), Cl(1)−Fe(1)−P(1) 98.04(3), N(2)−Fe(1)−P(2) 102.76(6), Cl(1)−Fe(1)−P(2) 111.12(3), P(1)−
Fe(1)−P(2) 120.93(3).
Scheme 6. Synthesis of P2NN′-Cy Complexes 10 and 11
Figure 2. Structure of the cations of 10 (left) and 11 (right). Ellipsoids are shown at 30% probability. Hydrogen atoms (except hydride ligand) and
tetraphenylborate anions have been omitted for clarity. Selected bond lengths (Å) and angles (deg) for 10: Fe(1)−C(17) 1.796(3), Fe(1)−N(2)
1.969(2), Fe(1)−N(1) 2.086(2), Fe(1)−P(2) 2.2886(9), Fe(1)−P(1) 2.2953(8), Fe(1)−Cl(1) 2.3222(8); C(17)−Fe(1)−N(1) 179.11(11),
N(2)−Fe(1)−N(1) 83.82(9), N(2)−Fe(1)−P(1) 90.48(7), P(2)−Fe(1)−P(1) 170.36(3); for 11: Fe(1)−C(11) 1.720(3), Fe(1)−N(2)
2.004(3), Fe(1)−N(1) 2.097(3), Fe(1)−P(1) 2.2021(9), Fe(1)−P(2) 2.2227(9), Fe(1)−H(1FE) 1.67(5); C(11)−Fe(1)−N(1) 176.53(12),
N(2)−Fe(1)−N(1) 83.06(10), N(2)−Fe(1)−P(1) 98.14(8), P(1)−Fe(1)−P(2) 167.16(4).
D
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lie in a plane (Figure 1). Complex 8 is paramagnetic as
evidenced by its broad and paramagnetically shifted H NMR
ligands.81−85 All but one85 of these structures have the
common features like those in 11 of a hydride trans to nitrogen
and carbonyl trans to nitrogen. The resonance of the hydride is
observed in the 1H NMR spectrum as a triplet shifted
1
spectrum (Figure S15). The solution magnetic susceptibility is
5.49 μB as measured via Evan’s method and is consistent with a
high spin Fe(II) center with four unpaired electrons.
Cyclohexyl substituents do not have a significant impact on
the P−Fe−P bond angle of 159.62(5)°, which is similar to
160.57(3)° in FeCl2(P2NN′-Ph); however, one of the Fe−P
bonds lengths shortens from 2.6110(9) Å in FeCl2(P2NN′-Ph)
to 2.550(1) Å in 8 owing to the stronger phosphine donor with
alkyl substituents.
Treatment of 8 with one equivalent of NaBPh4 in methanol
cleanly yields the halide abstracted, five-coordinate cationic
complex [FeCl(P2NN′-Cy)][BPh4] (9; Scheme 5). Single
crystal XRD of 9 (Figure 1) shows a distorted trigonal
bipyramidal geometry with a τ5 geometry index value of 0.82
with amine nitrogen approximately trans to chloride (Cl(1A)−
Fe(1A)−N(1A) 169.93(9)°) and a P−Fe−P bond angle of
120.93(3)°. There are five five-coordinate iron complexes
Fe(N-donor)2(P-donor)2Cl in the Cambridge Structural
Database (CSD), but they are all square pyramidal and
neutral.71−75 There are two structures of trigonal bipyramidal
[FeCl(P(CH2CH2PR2)3)]+ in the literature: R = Cy76 and R =
iPr.77 Surprisingly, the normally higher trans influence of
phosphorus over nitrogen is not seen in these complexes since
shorter Fe−Cl bond lengths are found for the PP3 complexes
with R = Cy (2.235(4) Å) and R = iPr (2.2371(3)) versus the
P2NN′-Cy complex 9 (2.2825(7)).
2
significantly upfield to −18.75 ppm with a JHP of 53.0 Hz. A
change in the 31P{1H} NMR is also observed most notably by
the splitting of the resonance into a doublet owing to coupling
with the hydride ligand (2JPH = 14.7 Hz) along with a
downfield shift from the chloride complex to 82.5 ppm. The
CO stretch of 11 also experiences a shift of ∼60 cm−1 to 1900
cm−1 in the FT-IR spectrum. Usually, monocationic iron
complexes with hydride trans to pyridine are expected to have
an Fe−H stretch near 1940 cm−1, and there is a weak
absorption in this region.86
The poor solubility of the reported complex FeCl2(P2NN′-
Ph) in MeOH did not allow a similar halide abstraction
reaction to that of Scheme 5. Instead, we now find that the use
of acetonitrile allows a rapid double chloride substitution by
acetonitrile to occur to give the octahedral dication [Fe-
(NCMe)2(P2NN′-Ph)][BPh4]2 (12; Scheme 7). The Fe−P
Scheme 7. Synthesis of Dicationic Complex 12
Complex 9 is readily soluble in dichloromethane; however it
is practically insoluble in chloroform, possibly due to the
cationic nature of the complex. Complex 9 has a solution
magnetic susceptibility of 5.31 μB owing to a high spin Fe(II)
center with four unpaired electrons. As in the case with other
paramagnetic transition metal phosphine complexes, 8 and 9
are 31P NMR silent.
Upon exposure of 9 to CO(g), a significant color change
from light yellow to dark green is observed, suggesting
coordination of CO to the empty coordination site (Scheme
6).
Single crystal XRD of the newly isolated yellow-green
compound confirms the formation of the cationic octahedral
complex [FeCl(P2NN′-Cy)(CO)][BPh4] (10) with carbonyl
trans to the apical nitrogen and the two bulky PCy2 groups in
trans positions (Figure 2). The FT-IR spectrum of 10 also
reveals a sharp CO stretch at 1960 cm−1. Complex 10 has less
distortion to its octahedral geometry than 8 as observed by a
P−Fe−P bond angle of 170.36(3)°, and it has significantly
shorter Fe−P bond lengths of 2.2886(9) and 2.2953(8) Å than
8. Short Fe−P bond lengths coincide with a low spin Fe(II)
center, and the diamagnetic nature of 10 is confirmed via
31P{1H} NMR spectroscopy where a sharp singlet at 66.5 ppm
is observed in the 31P{1H} NMR spectrum. The five other
Fe(CO)Cl(N-donor)2(P-donor)2 complexes with tetradentate
ligand complexes in the CSD have linear PNNP ligands with
carbonyl trans to chloride and are also diamagnetic.36,78−80
The addition of a large excess (15 equiv) of NaBH4 to
complex 10 in THF gives access to the hydride complex
[Fe(H)(P2NN′-Cy)(CO)][BPh4] (11; Scheme 6). Single
crystal XRD confirms the substitution of the chloride ligand
with a hydride (Figure 2). While there is only one other
structurally characterized tetradentate iron complex with the
Fe(CO)(H)(P)2(N)2 donor set,78 there are a variety of
structures of hydridocarbonyl complexes with PNP pincer
Figure 3. X-ray structure of the dication of 12. Ellipsoids are shown at
30% probability. Hydrogen atoms and tetraphenylborate anions have
been omitted for clarity. Selected bond lengths (Å) and angles (deg):
Fe(1)−N(3) 1.9028(19), Fe(1)−N(4) 1.9325(19), Fe(1)−N(2)
1.9661(19), Fe(1)−N(1) 2.0597(19), Fe(1)−P(1) 2.2811(7),
Fe(1)−P(2) 2.2859(7); P(1)−Fe(1)−P(2) 169.58(3), N(1)−
Fe(1)−P(2) 85.99(6), N(3)−Fe(1)−N(2) 175.80(8), N(3)−
Fe(1)−N(4) 91.80(8), N(2)−Fe(1)−N(1) 83.92(8).
E
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Scheme 8. Synthesis of the Dinitrogen Complex 13
bond lengths in 12 confirmed by single crystal XRD (Figure 3)
of 2.2811(7) and 2.2859(7) Å, respectively, are very similar to
those in 10, which is also low spin. Dicationic iron complexes
[FeN4P2]2+ with at least two MeCN ligands are common with
44 structures in the CSD. The complex is completely insoluble
in nearly all common organic and chlorinated solvents except
acetonitrile. 12 displays a sharp singlet in its 31P{1H} NMR
spectrum at 56.4 ppm, further confirming its diamagnetic
nature. Replacement of the chloride ligands appears to occur
stepwise, albeit very quickly. After the addition of NaBPh4, the
mixture experiences a color change to dark green, which
persists for only a few seconds before changing to orange and
forming the isolated product. The driving force for double
halide abstraction may be the preference for formation of a
diamagnetic low spin six-coordinate complex as opposed to a
paramagnetic five-coordinate complex like 9.
Synthesis and Reactivity of NPP2′ and NP3 Tripodal Iron
Complexes. In order to demonstrate the utility of our mixed
phosphine donor NPP2′ ligands, we synthesized a dinitrogen
hydride complex similar to the two complexes [Fe(H)(N-
i
(CH2CH2PR2)3(N2)]+, R = Ph and R = Pr, that have
previously been reported in the literature.16,17 We opted to use
ligand 3 with mixed phenyl and isopropyl phosphine donors in
order to observe the effect of asymmetry on the complex
structure and reactivity. Ligand 3 was reacted with one
equivalent of FeCl2 and NaBPh4 in THF to give a pale yellow
solid. Suitable crystals for XRD could not be obtained.
Nonetheless, its silent 31P{1H} NMR spectrum supports the
formation of a paramagnetic complex [FeCl(NPP2′-iPr)]+, by
analogy to the reaction in Scheme 5. The isolated product
from this reaction was then treated with six equivalents of
NaBH4 at −30 °C under an argon atmosphere resulting in an
orange solution (Scheme 8).
Figure 4. Structure of the cation of 13. Ellipsoids are shown at 30%
probability. Hydrogen atoms (except the hydride ligand) and the
tetraphenylborate anion have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): Fe(1)−N(3) 1.7901(14), Fe(1)−N(1)
2.1106(12), Fe(1)−P(1) 2.2406(4), Fe(1)−P(3) 2.2478(4), Fe(1)−
P(2) 2.2532(4), Fe(1)−H(1FE) 1.441(19), N(2)−N(3) 1.124(2);
N(3)−Fe(1)−N(1) 176.12(6), N(1)−Fe(1)−P(1) 85.26(4), N(3)−
Fe(1)−P(2) 96.96(5), P(1)−Fe(1)−P(2) 140.459(17).
spectrum at 1909 and 2080 cm−1, respectively. The ν(N2) of
2080 cm−1 is lower than that of [Fe(H)(NP3-Ph)(N2)][BPh4]
and [Fe(H)(NP3-iPr)(N2)][PF6], which have a ν(N2) of 2090
and 2087 cm−1, respectively. In complex 13, the Fe−N
distance trans to the N2 ligand of 2.111(1) Å is the same as the
NP3-Ph complex of 2.119(8) Å and shorter than that in the
NP3-iPr complex of 2.147(3) Å. Additionally, the Fe−P
distance trans to the hydride ligand is shorter in 13 at
2.2478(4) Å than in either symmetrical complex: 2.257(4) Å
for NP3-Ph and 2.286(1) Å for NP3-iPr. Therefore, the
unsymmetrical ligand binds stronger and results in more
donation into the metal center as well as into the N2 π* orbital,
giving rise to a lower ν(N2).
Complex 13 is extremely sensitive toward air and moisture
and decomposes in the presence of dichloromethane vapor.
Reactions attempting to synthesize carbonyl compounds were
carried out on starting complexes bearing unsymmetrical
NPP2′ ligands of the assumed structure [Fe(Cl)(NPP2′)][Cl]
of Scheme 8. Upon exposure to CO(g) at liquid nitrogen
temperature, the solution first turned dark blue and then upon
slow warming turned brown. Unfortunately, we could not
characterize the mixture that was produced.
Upon redissolving the residue of this reaction in benzene
and exposing it to a nitrogen atmosphere by way of the
glovebox, a color change to green was observed. Single crystal
XRD (Figure 4) confirmed the uptake of dinitrogen by the
iron complex as well as the presence of a hydride, giving
[Fe(H)(NPP2′-iPr)(N2)][BPh4] (13). This octahedral com-
plex has the dinitrogen ligand trans to the apical N and the
bulky PiPr2 groups trans to each other. The N2 ligand in the
other known complexes [Fe(H)(NP3-iPr)(N2)][PF6] and
[Fe(H)(NP3-Ph)(N2)][BPh4] is also positioned trans to the
bridgehead nitrogen. The N−N distance of 13 measured at
1.124(2) Å is slightly longer than those in NP3-iPr and NP3-Ph
complexes of 1.114(4)16 and 1.102(12)18 Å, respectively. The
complex is diamagnetic as evidenced by the two resonances
observed in the 31P{1H} NMR at 74.5 and 47.6 ppm,
1
respectively. The hydride resonance in the H NMR spectrum
at −9.30 ppm is a doublet of triplets with the trans 2JHP = 62.6
Hz coupling with the PPh2 P nucleus slightly larger than the cis
coupling of 44.6 Hz with the PiPr2 P nuclei. Both the metal-
hydride and dinitrogen stretches are observed in the FT-IR
F
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Scheme 9. Synthesis of Complexes 14 and 15
Figure 5. Structures of the cations of 14 (left) and 15 (right). Ellipsoids are shown at 30% probability. Hydrogen atoms (except the hydride ligand)
and tetraphenylborate anions have been omitted for clarity. There were two independent ion pairs in the asymmetric unit for 14. The cation of A is
shown here, and B is shown in the Supporting Information (Figure S32). Selected bond lengths (Å) and angles (deg) for 14: Fe(1A)−C(31A)
1.737(5), Fe(1A)−N(1A) 2.147(4), Fe(1A)−P(3A) 2.2618(12), Fe(1A)−P(1A) 2.2867(13), Fe(1A)−P(2A) 2.3250(13), Fe(1A)−Cl(1A)
2.3526(12); C(31A)−Fe(1A)−N(1A) 177.2(2), C(31A)−Fe(1A)−P(3A) 96.82(16), N(1A)−Fe(1A)−P(1A) 86.18(12), P(1A)−Fe(1A)−
P(2A) 165.31(5); for 15: Fe(1)−C(31) 1.7303(17), Fe(1)−N(1) 2.1093(14), Fe(1)−P(3) 2.2136(5), Fe(1)−P(2) 2.2192(5), Fe(1)−P(1)
2.2200(5), Fe(1)−H(1FE) 1.460(19); C(31)−Fe(1)−N(1) 175.75(7), C(31)−Fe(1)−P(3) 91.39(6), P(3)−Fe(1)−P(2) 138.617(19), N(1)−
Fe(1)−P(1) 85.88(4).
We are interested in estimating the acidity of transition
metal hydride complexes87 and found that iron(II) carbonyl
phosphine complexes show a linear increase in pKa in THF as
carbonyl ligands are replaced by trialkyl phosphine donors.88
The carbonyl hydride [FeH(CO)(NP3)]+ prepared using our
NP3 ligand 7 was predicted using the LAC method87 to be a
weak acid (pKaLAC 21). This complex was prepared according
to Scheme 9. First 7 and FeCl2 are heated in THF, presumably
forming an octahedral dichloride complex that was not
isolated. Exposure of the reaction mixture to an atmosphere
of CO(g) results in a color change from beige to brown with
the selective substitution of chloride for CO. Salt metathesis
with NaBPh4 in alcohol solvent allows for the precipitation of
[FeCl(NP3)(CO)][BPh4] (14) as the tetraphenylborate salt.
XRD of a crystal of 14 (Figure 5) confirms the octahedral
geometry where the NP3 ligand coordinates in a tetradentate
fashion along with the CO ligand trans to the nitrogen. The
complex is diamagnetic, and two resonances are observed in
the 31P{1H} NMR spectrum corresponding to the two
chemically inequivalent phosphorus environments. A doublet
corresponding to the trans phosphine donors is observed at
crystal of 15 (Figure 5) confirms the presence of a hydride
with the CO ligand remaining trans to the nitrogen as in
compounds 10 and 11. Two similar complexes [Fe(H)(CO)-
(N(CH2CH2PR2)3)]+ have been structurally characterized
with R = Pr16 and R = Ph.18 The hydride resonance of 15 is
i
1
seen in the H NMR spectrum at −11.2 ppm as a doublet of
triplets with the trans 2JHP coupling of 68 Hz being larger than
the cis 2JHP coupling of 34 Hz. The 31P{1H} NMR spectrum is
an A2B pattern with cis 2JPP of 20 Hz. A shift in the CO stretch
is also observed with 15 in the FT-IR to 1903 cm−1. Complex
15 is significantly more stable (less reducing) than the
dinitrogen complex 13, and NMR spectroscopy can be
conducted in CDCl3; however it is still sensitive to air and
moisture. Treatment of complex 15 with an excess of the
strong base KH at room temperature in THF results in the
slow deprotonation of the hydride, a two electron reductive
elimination process, to give trigonal bipyramidal Fe0(NP3)-
(CO) (16) accompanied by the loss of H2 and precipitation of
KBPh4 from solution (Scheme 10).
Single crystal XRD (Figure 6) of 16 confirms the trigonal
bipyramidal geometry which is nearly ideal with a calculated τ5
value of 0.96. Reduction to Fe(0) is also evidenced by the Fe−
P bond lengths which are significantly shorter in 16 than in 15
by ∼0.1 Å and are measured at 2.1514(4) and 2.1633(4) Å,
respectively. The shortening is thought to be a result of less
interligand repulsion around the equatorial plane of the
trigonal bipyramid compared to that in the octahedral complex
15.
2
49.5 ppm with JPP = 42.5 Hz, and a triplet corresponding to
the phosphine donor trans to chloride is observed at 69.8 ppm
with the same 2JPP. A strong CO stretch is observed in the FT-
IR spectrum centered at 1940 cm−1. The treatment of 14 with
an excess of sodium borohydride results in the substitution of
the chloride ligand for a hydride to give the complex
[Fe(H)(NP3)(CO)][BPh4] (15; Scheme 9). XRD of a single
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(10, 14), [FeH(N2)L]+ (13), and [FeH(CO)L]+ (11, 15).
Evidence is provided that the mixed donor ligand NPP2′-iPr
(3) in the complex [FeH(N2)(NPP2′-iPr)]+ (13) is more
tightly bound than either of the symmetrical ligands NP3-Ph or
NP′3-iPr, resulting in the lowest N2 stretch of the three
dinitrogen complexes. The hydride complex [FeH(CO)-
(NP3)]BPh4 (15) is a very weak acid but undergoes
deprotonation/reduction by the strong base KH to produce
the Fe(0) complex Fe(CO)(NP3) (16). Notable features of
the iron complexes include the first structures of a trigonal
Scheme 10. Deprotonation/Reduction of 15 to Complex 16
with KH
+
bipyramidal five coordinate FeClN2P2 donor set in 9 with an
unexpectedly long Fe−Cl bond, a rare hydridocarbonyl
complex with the Fe(CO)(H)(P)2(N)2 donor set in 11, and
the first trigonal bipyramidal structure of the type Fe0(NP3)-
(CO) in 16.
EXPERIMENTAL SECTION
■
General Considerations. All experimental procedures and
manipulations were conducted under a dinitrogen or argon
atmosphere using standard Schlenk-line and glovebox procedures
unless otherwise stated. All solvents were degassed and dried using
standard procedures prior to all procedures. 2-Bromoethylamine
hydrobromide, ammonium acetate, and sodium trisacetoxyborohy-
dride (STAB) were purchased from Sigma-Aldrich and used without
further purification. Anhydrous FeCl2 (99.5%) was purchased from
Alfa Aesar and used without further purification. Phosphonium dimers
were synthesized according to the literature procedure.90 Bis[N-(2-
(dicyclohexylphosphino)ethyl)-N-(pyridin-2-ylmethyl)amine
(P2NN′-Cy) and FeCl2(P2NN′-Ph) were prepared according to their
literature procedure.33 NMR spectra were obtained at ambient
temperature and pressure using a 400 MHz Varian MercuryPlus
Figure 6. Molecular structure of 16. Ellipsoids are shown at 30%
probability. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (deg): Fe(1)−C(31) 1.7171(15),
Fe(1)−N(1) 2.1325(13), Fe(1)−P(2) 2.1514(4), Fe(1)−P(1)
2.1633(4), Fe(1)−P(3) 2.1646(4); C(31)−Fe(1)−N(1) 179.04(6),
N(1)−Fe(1)−P(1) 85.78(4), P(2)−Fe(1)−P(1) 119.321(17),
P(2)−Fe(1)−P(3) 121.161(17).
NMR spectrometer (400.00 MHz for H, 161.98 MHz for 31P, 128.4
1
MHz for 11B, and 376.1 Hz for 19F), an Agilent 500 MHz
1
spectrometer with a OneNMR H/F{X} Probe (500 MHz for H,
126 MHz for 13C, 202 MHz for 31P, and 160 MHz for 11B), or an
Agilent DD2-600 MHz spectrometer (600 MHz for 1H, 151 MHz for
13C, and 242 MHz for 31P) unless stated otherwise. The H and 13C
1
NMR were measured relative to partially deuterated solvent peaks. 31P
chemical shifts were measured relative to 85% phosphoric acid as an
external reference. The elemental analyses were performed on a
PerkinElmer 2400 CHN elemental analyzer. Solution magnetic
susceptibilities were measured at 25 °C by a method originally
described by Evans.91 A solution of the compound in CDCl3/
cyclohexane (95/5 v/v) or CD2Cl2/cyclohexane (95/5 v/v) was
prepared. The external standard consisted of a flame-sealed capillary
containing CDCl3/cyclohexane (95/5 v/v) or CD2Cl2/cyclohexane
(95/5 v/v). The chemical shift difference of cyclohexane between the
inset and the solution was used to determine the magnetic moment
μeff.
Additional evidence for the reduction can be seen in the FT-
IR spectrum of 16 where the CO stretch at 1808 cm−1 is
significantly lower than that of the Fe(II) complex 15. This
significant change in the CO stretch is also consistent with the
C−O bond lengths in 15 and 16. Complex 15 with a higher
ν(CO) has a shorter C−O bond length of 1.160(2) Å, whereas
complex 16, with a lower ν(CO), has a slightly longer one of
1.180(2) Å. While no NP3 hydride complexes of iron have
been deprotonated in this way, the related PP3 complex
[FeH(CO)(P(CH2CH2PMe2)3]+ has been treated with KOtBu
in THF to produce Fe(CO)(PP3) with ν(CO) 1855 cm−1.89
2-(Diphenylphosphino)ethylamine (1). This compound was
synthesized with a modified literature procedure.68 In a Schlenk flask,
diphenylphosphine (8.8 mL, 52 mmol) was dissolved in THF, and
potassium tert-butoxide (11.6 g, 104 mmol) was added in portions
giving rise to a bright orange solution. This was left to stir for 1 h. 2-
Bromoethylamine hydrobromide (10.64 g, 52 mmol) was added to
the mixture, and it was left to stir overnight at room temperature. The
formation of a milky white solution was observed. The solvent was
removed completely under a vacuum, and the resulting solid was
stirred in pentane to remove leftover HPPh2 and decanted. The
product was triturated with ether and filtered through alumina, and
the solvent was removed under a vacuum to give a yellow oil was used
without further purification (3.97 g, 38%). 1H NMR (400 MHz,
CDCl3): δ 7.43−7.30 (m, 10H), 2.84 (tt, J = 7.5 Hz, 9.4 Hz, 2H),
2.24 (t, J = 7.57 Hz, 2H), 1.67 (s,br, 2H). 31P{1H}NMR (162 MHz,
CDCl3): δ −21.85 (s).
CONCLUSION
■
A variety of novel unsymmetrical mixed phosphine donor
NPP2′ tripodal ligands (2−6) have been synthesized via
reductive amination of tertiary phosphine acetaldehydes with
2-aminoethyldiphenylphosphine. An improvement in the
synthesis of the latter compound, an important precursor to
ligands and catalysts, has also been described. Additionally, a
symmetrical NP3 tripodal ligand (7) can be synthesized via the
same methods using ammonium salts as the nitrogen source,
eliminating the need for dangerous reagents. The previously
reported ligand P2NN′-Cy is now used for coordination
chemistry. These tetradentate ligands (collectively labeled L)
provide a direct route to Fe(II) complexes of the types FeCl2L
(8), [FeCl(L)]+ (9), [Fe(MeCN)2L]2+ (12), [FeCl(CO)L]+
N,N-bis(2-(Dicyclohexylphosphanyl)ethyl)-N-(2-
(diphenylphosphanyl)ethyl)ethanamine (2)-NPP2′-Cy. To a
Schlenk flask was added 1 (500 mg, 2.18 mmol), cyclohexyl
H
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phosphonium dimer (1.4 g, 2.18 mmol), STAB (1.85 g, 8.73 mmol),
3 Å molecular sieves, and 25 mL of THF. The mixture was stirred
overnight, and the resulting cloudy white solution was dried under a
vacuum. The residue was extracted into 20 mL of DCM. The extract
was washed air free on the Schlenk line with degassed NH4Cl (3 × 15
mL) and degassed H2O (3 × 15 mL). The organic layer was dried
over Na2SO4 and then filtered through a short alumina plug. The
solvent was removed under a vacuum to give a colorless oil was used
FeCl2(P2NN′-Cy) (8). A 20 mL vial was charged with FeCl2 (30
mg, 0.237 mmol) and THF (6 mL) and stirred for 1 h at room
temperature. P2NN′-Cy (132 mg, 0.237 mmol) was dissolved in 1 mL
of THF and added dropwise to the stirring FeCl2 solution. A color
change to orange was observed, and after continued stirring (∼2 h) a
yellow solid began to precipitate out of solution. The vial was stirred
overnight, then 8 mL of pentane was added and stirring continued for
3 h. The precipitated solid was collected on a filter frit, washed with
additional pentane, and dried under a vacuum, giving a yellow solid
(141 mg, 87%). Crystallization attempts via pentane diffusion into a
saturated DCM solution always resulted in the formation of fibrous
needles unsuitable for X-ray diffraction. Crystals suitable for X-ray
diffraction were grown by layering a saturated MeOH solution with
1
without further purification (973 mg, 67%). H NMR (400 MHz,
CDCl3): δ 7.45 (m, 4H), 7.37 (m, 6H), 1.84 (m, 6H), 1.72 (dt, J =
10.38, 19.80 Hz, 16H), 1.58 (t, J = 10.38 Hz, 4H), 1.31−1.20 (m,
16H), 1.17−1.11 (m, 8H), 0.88 (t, J = 7.33 Hz, 6H). 31P{1H} NMR
(162 MHz, CDCl3): δ −6.98 (s), −19.58 (s). MS-DART m/z
calculated for [C42H67NP3]+: 678.4481. Found: 678.4484.
N,N-bis(2-(Diisopropylphosphanyl)ethyl)-N-(2-
(diphenylphosphanyl)ethyl)ethanamine (3)-NPP2′-iPr. The
same general procedure was followed as in the synthesis of 2 but
using the isopropyl-substituted phosphonium dimer. Colorless oil was
used without further purification (733 mg, 84%). 1H NMR (400
MHz, CDCl3): δ 7.45−7.41 (m, 4H), 7.35−7.33 (m, 6H), 2.87, (s, br
4H), 1.84 (m, 2H), 1.72 (dq, 4H), 1.57 (s, br, 2H), 1.25−1.14 (m,
2H), 1.08−1.1 (m, 24H). 31P{1H} NMR (162 MHz, CDCl3): δ 1.85
(s), −19.97 (s). MS-DART m/z calculated for [C30H51NP3]+:
518.3229. Found: 518.3232.
N, N-bis(2-(Diisobutylphosphanyl)ethyl)-N-(2-
(diphenylphosphanyl)ethyl)ethanamine (4)-NPP2′-iBu. The
same general procedure was followed as in the synthesis of 2 but
using the isobutyl-substituted phosphonium dimer. Colorless oil was
used without further purification (838 mg, 68%). 1H NMR (400
MHz, CDCl3): δ 7.47−7.41 (m, 4H) 7.37−7.32 (m, 6H) 1.65 (septet,
br, J = 6.43 Hz, 4H), 1.43−1.36 (m, 6H), 1.27 (d, J = 6.10 Hz, 8H),
0.98 (d, J = 6.43 Hz, 24H), 0.88 (m, 6H). 31P{1H} NMR (162 MHz,
CDCl3): δ −19.63 (s), −41.64 (s). MS-DART m/z calculated for
[C34H59NP3]+: 574.3855. Found: 574.3853
N , N - b i s ( 2 - ( D i - o - t o l y l p h o s p h a n y l ) e t h y l ) - N - ( 2 -
(diphenylphosphanyl)ethyl)ethanamine (5)-NPP2′-oTol. The
same general procedure was followed as in the synthesis of 2 but
using the o-tolyl-substituted phosphonium dimer. Colorless oil was
used without further purification (1.18 g, 76%). 1H NMR (400 MHz,
CDCl3): δ 7.44−7.29 (m, 10H), 7.19−7.10 (m, 16H) 2.37 (s, 12H),
2.01 (t, J = 6.83 Hz, 4H), 1.84 (t, J = 7.26 Hz, 2H), 1.21 (t, J = 6.83
Hz, 4H), 0.88 (t, J = 7.26 Hz, 2H). 31P{1H} NMR (162 MHz,
CDCl3): δ −21.13 (s), −41.84 (s). MS-DART m/z calculated for
[C46H51NP3]+: 710.3229. Found: 710.3231
1
Et2O. The complex is paramagnetic and 31P NMR silent. H NMR
(500 MHz, CDCl3): δ −21.66, −13.22, 12.19, 17.02, 42.82, 44.04,
47.23, 47.87, 49.66, 50.79, 78.04, 83.09, 138.58. Anal. calcd for
C34H58Cl2FeN2P2: C, 59.74; H, 8.55; N, 4.10. Found: C, 59.46; H,
8.44; N, 3.69. μeff (Evans method, CDCl3) = 5.49 μB
[FeCl(P2NN′-Cy)][BPh4] (9). A 20 mL vial was charged with 8
(105 mg, 0.15 mmol) and MeOH (6 mL) to give a light yellow
homogeneous solution. NaBPh4 (61 mg, 0.18 mmol) was dissolved in
1 mL of MeOH and added dropwise. A light yellow solid immediately
precipitated out of solution, and the mixture was stirred overnight.
The solid was collected on a filter frit, washed with MeOH and
pentane, and dried under a vacuum to give a pale yellow solid (100
mg, 69%). Crystals suitable for X-ray diffraction were grown by slow
diffusion of pentane into a saturated DCM solution. The complex is
1
paramagnetic and 31P NMR silent. H NMR (500 MHz, CD2Cl2): δ
85.81, 58.86, 52.53, 50.22, 33.28, 9.98, 8.68, 7.77, 7.38, 6.78, 4.48,
2.12, 1.14, 0.26, −0.41, −0.55, −0.68, −0.94, −1.34, −3.29, −4.67,
−5.18, −6.58, −7.70, −10.85, −11.38, −17.43. Anal. calcd for
C58H78BClFeN2P2: C, 72.02; H, 8.13; N, 2.90. Found: C, 72.33; H,
7.98; N, 2.83. τ5 = 0.82. μeff (Evans method CD2Cl2) = 5.31 μB
[FeCl(P2NN′-Cy)(CO)][BPh4] (10). In a Schlenk flask, 9 (164 mg,
0.24 mmol) was dissolved in 15 mL of THF and reacted with a slow
stream of carbon monoxide for 8 h. The color changed from yellow to
light green and then to dark green. The solvent was removed under a
vacuum, and the residue was dissolved in ethanol and precipitated by
the addition of a solution of NaBPh4 (85 mg, 0.25 mmol) in 5 mL of
ethanol. The light yellow-green precipitate was collected on a filter
frit, washed with benzene, and then dissolved in DCM and
precipitated by the addition of Et2O. The precipitate was collected
on a filter frit and dried under a vacuum to give a light yellow-green
solid (110 mg, 45%). Crystals suitable for X-ray diffraction were
1
grown by slow evaporation of a DCM/Et2O solution. H NMR (400
N , N - b i s ( 2 - ( D i - p - t o l y l p h o s p h a n y l ) e t h y l ) - N - ( 2 -
(diphenylphosphanyl)ethyl)ethanamine (6)-NPP2′-pTol. The
same general procedure was followed as in the synthesis of 2 but
using the p-tolyl-substituted phosphonium dimer. A total of 100 mg of
2 was used and the other reagents were scaled down accordingly.
Colorless oil was used without further purification (175 mg, 56%). 1H
NMR (300 MHz, CD2Cl2): δ 7.41−7.08 (m, 26H), 2.57−5.47 (m
(br), 3H), 2.39−2.29 (m, 16H), 1.98−1.92 (m (br), 3H). 31P{1H}
NMR (121 MHz, CD2Cl2): δ −20.06 (s), −26.33 (s). MS-DART m/
z calculated for [C46H51NP3]+: 710.3229. Found: 710.3225
tris(2-(Diisobutylphosphaneyl)ethyl)amine (7)-NP3. To a
Schlenk flask was added ammonium acetate (0.308 g, 4 mmol),
isobutyl phosphonium dimer (0.538 g, 1 mmol), STAB (0.848 g, 4
mmol), and THF (15 mL). The resulting mixture was stirred over 3 Å
molecular sieves for 20 h and then filtered over Celite. One milliliter
of MeOH was added to quench any residual borohydride, then the
solvent was removed under a vacuum. The residue was extracted with
pentane (15 mL) and then washed air free on the Schlenk line with
degassed water (2 × 10 mL). The organic layer was dried over
Na2SO4 and filtered and the solvent evaporated to give a colorless oil
that was used without further purification (0.32 g, 91% with respect to
phosphonium dimer). 1H NMR (400 MHz, CDCl3): δ 2.60 (m, 6H),
1.67 (m, 6H), 1.48 (m, 6H), 1.31 (m, 12H), 0.91 (d, J = 6.7 Hz,
36H). 31P{1H}NMR (162 MHz, CDCl3): δ −41.6 (s). MS-DART m/
z calculated for [C30H67NP3]+: 534.4481. Found: 534.4492
MHz, DMSO-d6): δ 8.32 (d, J = 5.9 Hz, 1H), 7.74 (t, J = 7.9 Hz, 1H),
7.24−7.18 (m, 10H), 6.92 (t, J = 7.4 Hz, 8H), 6.78 (t, J = 7.2 Hz,
4H), 4.84 (s, 2H), 2.30−1.11 (m, 48H), 0.63 (s (br), 4H). 31P{1H}
NMR (162 MHz, DMSO-d6): δ 66.5 (s). FT-IR (cm−1): 1960
ν(CO). Anal. calcd for C59H78BClFeN2OP2: C, 71.19; H, 7.90; N,
2.81. Found: C, 71.22; H, 7.95; N, 2.74.
[Fe(H)(P2NN′-Cy)(CO)][BPh4] (11). To a solution of 10 (99 mg,
0.1 mmol) in 15 mL of THF was added dropwise a solution of NaBH4
(56 mg, 1.5 mmol) in 3 mL of ethanol. The resulting ruby-red
solution was stirred overnight and then filtered through Celite. The
solvent was removed under a vacuum, and the product was dissolved
in methanol and precipitated by the addition of a solution of NaBPh4
(34 mg, 0.1 mmol) in minimal methanol. The precipitate was
collected on a filter frit and then dissolved in benzene and filtered
through Celite. The golden yellow solution was evaporated to give a
yellow solid (53 mg, 56%). Crystals suitable for X-ray diffraction were
1
grown by slow evaporation of a DCM/Et2O solution. H NMR (400
MHz, CDCl3): δ 8.62 (d, J = 5.6 Hz, 1H), 7.48 (s (br), 8H), 7.35 (t, J
= 7.4 Hz, 1H), 7.06−6.99 (m, 9H), 6.89 (t, J = 7.1 Hz, 4H), 6.40 (d, J
2
= 7.9 Hz, 1H), 2.81 (s, 2H), 1.98−0.92 (m, 48H), −18.75 (t, JHP
=
53.0 Hz, 1H). 31P{1H} NMR (162 MHz, CDCl3): δ 82.5 (s). FT-IR
(cm−1): 1900 ν(CO). Anal. calcd for C59H79BFeN2OP2: C, 73.75; H,
8.28; N, 2.91. Found: C, 73.82; H, 8.31; N, 2.86.
[Fe(P2NN′-Ph)(NCMe)2][BPh4]2 (12). A 20 mL vial was charged
with FeCl2(P2NN′-Ph) (74 mg, 0.11 mmol) and MeCN (10 mL) to
I
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give a yellow solution with suspended solids. NaBPh4 (46 mg, 0.13
mmol) was dissolved in 1 mL of MeCN and added dropwise. The
reaction mixture experienced a gradient color change to dark green
which was short-lived, and then the color changed to orange-brown.
The vial containing NaBPh4 was then rinsed with an additional 1 mL
of MeCN and added to the reaction mixture, and the vial was stirred
overnight. The solution was then filtered through Celite to give a clear
orange solution, which was dried under a vacuum and gave a dark
orange oily film. The residue was triturated with EtOH to remove
excess NaBPh4 and a pink solid developed. The EtOH suspension was
stirred overnight, and then the solid was collected on a filter frit,
washed with EtOH and Et2O, and dried under a vacuum to give a
pink powder (90 mg, 82%). The pink solid redissolved in MeCN
results in an orange solution. Crystals suitable for X-ray diffraction
to give an orange solid (265 mg, 78%). Crystals suitable for X-ray
diffraction were grown by slow evaporation of a DCM/Et2O solution.
1H NMR (400 MHz, CDCl3): δ 7.45 (s(br), 8H), 7.07 (t, J = 7.3 Hz,
8H), 6.93 (t, J = 7.1 Hz, 4H), 2.92−2.83 (m, 2H), 2.48−2.44 (m,
2H), 2.38−2.26 (m, 4H), 2.16−2.11 (m(br), 2H), 2.07−1.87 (m,
10H), 1.74−1.67 (m, 2H), 1.56−1.36 (m, 6H), 1.24−1.06 (m, 32H),
1.00 (d, J = 6.5 Hz, 6H). 31P{1H}NMR (162 MHz, CDCl3): δ 49.4
(d, J = 42.2 Hz), 69.8 (t, J = 42.5 Hz). FT-IR (cm−1): 1940ν(CO).
Anal. calcd for C55H86BClFeNOP3: C, 67.94; H, 8.91; N, 1.44.
Found: C, 67.96; H, 8.93; N, 1.39.
[Fe(H)(NP3)(CO)][BPh4] (15). To a solution of 14 (48 mg, 0.05
mmol) in 5 mL of THF was added a solution of NaBH4(28 mg, 0.75
mmol) in 2 mL of ethanol, and the resulting solution was stirred
overnight. The solution was evaporated under a vacuum, and the
resulting yellow solid was extracted with 5 mL of benzene. The extract
was filtered through Celite and evaporated to give a yellow solid (34
mg, 74%). Crystals suitable for X-ray diffraction were grown by slow
evaporation of a DCM/Et2O solution. 1H NMR (400 MHz, CDCl3):
δ 7.41 (s (br), 8H), 7.06 (t, J = 6.9 Hz, 8H), 6.92 (t, J = 6.7 Hz, 4H),
2.21 (p, J = 6.1 Hz, 2H), 2.03 (d, J = 6.5 Hz, 4H), 2.00−1.86 (m,
6H), 1.75 (dd, J = 20.0, 11.4 Hz, 2H), 1.57 (dq, J = 24.6, 9.2, 8.0 Hz,
6H), 1.17−1.04 (m, 36H), −11.2 (dt, J = 68.1, 33.8 Hz, 1H). 31P{1H}
NMR (162 MHz, CDCl3): δ 63.6 (d, J = 20, Hz), 65.1 (t, J = 20 Hz).
FT-IR (cm−1): 1903 ν(CO). Anal. calcd for C55H87BFeNOP3: C,
70.43; H, 9.35; N, 1.49. Found: C, 70.47; H, 9.38; N, 1.41.
1
were grown by layering a saturated MeCN solution with Et2O. H
NMR (500 MHz, CD3CN): δ 2.53−2.58, (t (br), 2H (CH2)), 2.79−
2.85 (t (br), 2H (CH2)), 2.97−3.08 (m, 2H (CH2)), 3.29−3.34 (m,
2H (CH2)), 3.81 (s, 2H (CH2-py)), 6.43 (t, J = 6.7 Hz, 1H (Ar−
Hpyridine)) [the remaining aromatic protons did not integrate properly
even after acquiring a 1H spectrum with 25 s relaxation delay], 6.83−
6.86 (t, J = 7.2 Hz, Ar−H), 6.98−7.01 (t, J = 7.4 Hz, Ar−H), 7.21−
7.36 (m, Ar−H), 7.56−7.59 (m, Ar−H), 7.82−7.85 (m, Ar−H).
31P{1H} NMR (202 MHz, CD3CN): δ 56.44 (s). 11B{1H} NMR (160
MHz, CD3CN): δ −6.61 (s). Anal. calcd for C86H80B2FeN4P2: C,
78.91; H, 6.16; N, 4.28. Found: C, 79.22; H, 5.88; N, 3.91.
[Fe(H)(NPP2′-iPr)(N2)][BPh4] (13). In a Schlenk flask, 3 (77 mg,
0.15 mmol), FeCl2 (19 mg, 0.15 mmol), and NaBPh4 (51 mg, 0.15
mmol) were dissolved in THF. The resulting mixture was stirred at
room temperature overnight under argon, giving rise to a cloudy
colorless solution, then filtered over Celite and dried under a vacuum.
The resulting solid was dissolved in a minimal amount of DCM and
then precipitated by addition to a rapidly stirring vial of Et2O (10
mL). The precipitate was collected on a filter frit and dried under a
vacuum, giving rise to a pale-yellow solid (80 mg). The full structural
identity of the resulting solid was unable to be determined; however
the complex was confirmed to be paramagnetic as it was 31P NMR
silent. The solid was used in the next step under an assumed structural
identity without any additional purification or characterization. To a
vial of the pale-yellow solid (102 mg, 0.11 mmol) under an argon
atmosphere was added MeOH (8 mL), and the resulting slurry was
placed in a cold well and cooled to −30 °C. NaBH4 (25 mg, 0.66
mmol) was dissolved in a minimal amount of MeOH and added
dropwise to the slurry, which immediately experienced a color change
to orange and was left to stir overnight. The mixture was then
transferred to a N2 filled glovebox and dried fully under a vacuum.
The residue was redissolved in benzene, resulting in a color change to
yellow-green. The solution was then dried under a vacuum resulting
in a yellow-green solid (65 mg, 64%). Crystals suitable for X-ray
diffraction were grown over a period of four months by cooling a
saturated benzene solution to −30 °C with occasional thawing for
inspection. 1H NMR (400 MHz, C6D6): δ 7.49 (t, J = 7.18 Hz, 4H),
7.35−7.30 (m, 6H), 2.87−2.69 (m, 4H), 1.60−1.51 (m, 6H) 1.34−
1.28 (m, 6H), 1.06−1.02 (m, 24H), −9.30 (dt, J = 62.6, 44.6 Hz,
1H). 31P{1H} NMR (161.98 MHz, C6D6): δ 73.9 (d, J = 15 Hz), 46.7
(t, J = 15 Hz). FT-IR (cm−1): 2080 ν(N2), 1907 ν(Fe−H). Elemental
analysis was attempted multiple times; however due to the high
sensitivity of the complex, the results did not accurately reflect the X-
ray confirmed structural identity.
Fe(NP3)(CO) (16). To a solution of 15 (47 mg, 0.05 mmol) in 5
mL of THF was added solid KH (20 mg, 0.5 mmol), and the resulting
mixture was stirred overnight. After filtration over Celite and
evaporation of the solvent under a vacuum, the residue was extracted
with 5 mL of benzene. The extract was filtered through Celite and
evaporated to give a red solid (25 mg, 82%). Crystals suitable for X-
ray diffraction were grown by slow evaporation of a benzene/hexane
1
solution. H NMR (400 MHz, C6D6): δ 2.78 (m, 1H), 2.43 (s(br),
4H), 1.83−1.62 (m, 20H), 1.38−1.23 (m, 5H), 1.17−1.05 (m, 36H).
31P{1H} NMR (162 MHz, C6D6): δ 64.05 (s). FT-IR (cm−1): 1808
ν(CO). Anal. calcd for C31H66FeNOP3: C, 60.28; H, 10.77; N, 2.26.
Found: C, 60.31; H, 10.80; N, 2.21. τ5 = 0.96.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01535.
■
sı
NMR spectra and IR spectra of CO compounds and
crystallographic data tables (PDF)
Accession Codes
CCDC 2004808−2004816 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
■
Robert H. Morris − Department of Chemistry, University of
Toronto, Toronto, Ontario, Canada M5S 3H6; orcid.org/
0000-0002-7574-9388; Email: rmorris@chem.utoronto.ca
[FeCl(NP3)(CO)][BPh4] (14). In a Schlenk flask, 7 (180 mg, 0.35
mmol) and FeCl2 (44 mg, 0.35 mmol) were dissolved in 10 mL of
THF under an argon atmosphere and stirred at 60 °C overnight
giving rise to a beige solution. The flask was cooled to room
temperature and then placed under a slow stream of carbon monoxide
for 6 h. The solution immediately turned dark blue, and then
gradually changed color to brown. The solution was evaporated under
a vacuum, and the resulting brown solid was extracted with 3 mL of
ethanol and precipitated by the addition of solid NaBPh4 (0.120 g,
0.35 mmol). The precipitate was filtered, washed with MeOH, and
dried under a vacuum, and then the resulting orange solids were
dissolved in benzene, filtered over Celite, and dried under a vacuum
Authors
Matthew V. Gradiski − Department of Chemistry, University of
Toronto, Toronto, Ontario, Canada M5S 3H6; orcid.org/
0000-0001-7374-5554
Ali Nemati Kharat − School of Chemistry, College of Science,
University of Tehran, Tehran, Iran
Maegan S. E. Ong − Department of Chemistry, University of
Toronto, Toronto, Ontario, Canada M5S 3H6
J
https://dx.doi.org/10.1021/acs.inorgchem.0c01535
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
(13) Bianchini, C.; Meli, A. Reactivity of Formaldehyde and
Acetaldehyde Toward [N(CH2CH2PPh2)3]Ni and [N-
(CH2CH2PPh2)3]CoH. Organometallics 1985, 4, 1537−1542.
(14) Bianchini, C.; Masi, D.; Meli, A.; Peruzzini, M.; Sabat, M.;
Zanobini, F. Intra- and Intermolecular Activation of Sp2 Carbon-
Hydrogen Bonds at Rhodium(I) and Iridium(I) Metal Centers. X-Ray
Structure of the Cis σ-Cyclooctadienyl Hydride [{N-
(CH2CH2PPh2)3}IrH(σ-C8H11)]BPh4·CH3COCH3. Organometallics
1986, 5, 2557−2559.
Alan J. Lough − Department of Chemistry, University of
Toronto, Toronto, Ontario, Canada M5S 3H6; orcid.org/
0000-0001-6012-0379
Samantha A. M. Smith − Department of Chemistry, University
of Toronto, Toronto, Ontario, Canada M5S 3H6;
orcid.org/0000-0003-0807-8315
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01535
(15) Rossin, A.; Gutsul, E. I.; Belkova, N. V.; Epstein, L. M.;
́
Gonsalvi, L.; Lledos, A.; Lyssenko, K. A.; Peruzzini, M.; Shubina, E.
Author Contributions
S.; Zanobini, F. Mechanistic Studies on the Interaction of [(K3-P,P,P-
NP3)IrH3] [NP3 = N(CH2CH2PPh2)3] with HBF4 and Fluorinated
Alcohols by Combined NMR, IR, and DFT Techniques. Inorg. Chem.
2010, 49, 4343−4354.
§These authors contributed equally.
Notes
The authors declare no competing financial interest.
(16) MacBeth, C. E.; Harkins, S. B.; Peters, J. C. Synthesis and
Characterization of Cationic Iron Complexes Supported by the
Neutral Ligands NPi-Pr3, NArPi-Pr3, and NSt-Bu3. Can. J. Chem.
2005, 83, 332−340.
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Dinitrogen-Complexes of Iron(II) with Tritertiary Phosphines. Inorg.
Chim. Acta 1974, 11, 227−230.
ACKNOWLEDGMENTS
■
NSERC is acknowledged for a Discovery Grant to R.H.M. and
the Strategic Grant “Catalytic Synthesis of Specialty Chemicals
from Sustainable Resources.” The Ontario Government is
acknowledged for an Ontario Graduate Scholarship for M.V.G.
(18) George, T. A.; Rose, D. J.; Chang, Y.; Chen, Q.; Zubieta, J.
Reduction of Dinitrogen to Ammonia and Hydrazine in Iron(0) and
Molybdenum(0) Complexes Containing the N(CH2CH2PPh2)3
Ligand. Crystal Structures of [FeH(L)(N(CH2CH2PPh2)3)][BPh4]
(L = N2, CO). Inorg. Chem. 1995, 34, 1295−1298.
(19) Mellone, I.; Peruzzini, M.; Rosi, L.; Mellmann, D.; Junge, H.;
Beller, M.; Gonsalvi, L. Formic Acid Dehydrogenation Catalysed by
Ruthenium Complexes Bearing the Tripodal Ligands Triphos and
NP3. Dalton Trans. 2013, 42, 2495−2501.
DEDICATION
■
This article is dedicated to the memory of Professor Deborah
Zamble, a dear mentor and colleague and an extraordinary
bioinorganic chemist.
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