Bonding and Activation of N2 in Molybdenum(0) Complexes Supported by Tripod Ligands with Phospholane End Groups

Article abstract of DOI:10.1002/zaac.202100166

Tripod ligands with a neopentyl backbone and one (trpd-1 pln), two (trpd-2 pln) or three (trpd-3 pln) phospholane end groups are synthesized and used for the synthesis of MoX₃ (X=Cl, Br, I) complexes supported by these ligands. Na_xHg

Full text of DOI:10.1002/zaac.202100166

Journal of Inorganic and General Chemistry
doi.org/10.1002/zaac.202100166
ARTICLE
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Bonding and Activation of N2 in Molybdenum(0)
Complexes Supported by Tripod Ligands with Phospholane
End Groups
Mareike Pfeil,[a] Tobias A. Engesser,*[a] Jan Krahmer,[a] Christian Näther,[a] and Felix Tuczek*[a]
Dedicated to Prof. Hansgeorg Schnöckel on the Occasion of his 80th Birthday
Tripod ligands with a neopentyl backbone and one (trpd-1pln),
two (trpd-2pln) or three (trpd-3pln) phospholane end groups
are synthesized and used for the synthesis of MoX3 (X=Cl, Br, I)
complexes supported by these ligands. NaxHg reduction of
[MoBr3(trpd-3pln)], in the presence of N2 and the bidentate
coligand dppm leads to the molybdenum(0) mono(dinitrogen)
complex [Mo(N2)(trpd-3pln)(dppm)] with a moderately acti-
vated N2 ligand. An attempt to further increase the activation
by using dmpm as coligand leads to a mixture containing
desired [Mo(N2)(trpd-3pln)(dmpm)], but also the bis(dinitrogen)
complex [Mo(N2)2(k2-trpd-3pln)2] and the homoleptic complex
[Mo(trpd-3pln)2]. Protonation of [Mo(N2)(trpd-3pln)(dppm)]
with [H(OEt2)2][Al{OC(CF3)3}4] generates the hydrazido(2-) com-
plex [Mo(NNH2)(trpd-3pln)(dppm)][Al{OC(CF3)3}4]2.
Introduction
As part of these studies, our working group has investigated
the effect of different oligodentate phosphine ligands in Chatt-
type molybdenum complexes on the activation of coordinated
N2 during the last 15 years. There are several parameters
affecting the ability of these complexes for synthetic nitrogen
fixation such as the donor properties of the ligands, mostly
determined by the electronic influence of their residues (e.g.
À PMe2 vs. À PPh2), their denticity (mono-, bi-, tridentate…) and
topology (i.e.; linear tridentate, tripodal, tetrapodal…), and
steric influences imparted by the residues on the coordinating
donor atoms.[21] For a better understanding of these effects and
the underlying correlations, we employed several linear
tridentate (e.g. prPP(R)P, R=H, Ph)[22] or tripodal (e.g. tdppme,
trpd-1, SiP3),[23–27] phosphine ligands in combination with
bidentate coligands for N2 activation (Figure 1). The properties
of the corresponding dinitrogen complexes were analyzed by
comparison of the, e.g., highly diagnostic NÀ N stretching
frequency and by the protonability of the coordinated N2
The activation and stoichiometric or catalytic conversion of
highly inert dinitrogen into ammonia has been one of the most
demanding tasks in bioinorganic and organometallic
chemistry.[1] In nature, this reaction is catalyzed by the enzyme
nitrogenase with the FeMo cofactor as active site.[2] As part of
synthetic nitrogen fixation, transition metal compounds were
developed to mimic this process; among these, molybdenum
phosphine complexes have been particularly successful.[3–8] With
[M(N2)2(PR3)4] (M=W or Mo, PR3=PMe2Ph or PMePh2), the first
complexes showing stoichiometric conversion of dinitrogen
into ammonia were presented by Chatt et al. in 1975.[8] After
the first catalytic conversion discovered by Schrock et al. in
2003 using a triamidoamine molybdenum complex,[7] further
active catalysts based on molybdenum centers[5,9–15] and
phosphine ligands have been described,[4,5,10–19] but also other
metals and ligands have been explored in this context until
today.[3,4,7,16,18–20]
[a] Dr. M. Pfeil, Dr. T. A. Engesser, Dr. J. Krahmer, Prof. Dr. C. Näther,
Prof. Dr. F. Tuczek
Institut für Anorganische Chemie
Christian-Albrechts-Universität zu Kiel
Max-Eyth-Straße 2
D-24118 Kiel
Fax: +49 (0) 431-880-1520
E-mail: tengesser@ac.uni-kiel.de
ftuczek@ac.uni-kiel.de
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/zaac.202100166
© 2021 The Authors. Zeitschrift für anorganische und allgemeine
Chemie published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-
Commercial NoDerivs License, which permits use and distribution
in any medium, provided the original work is properly cited, the use
is non-commercial and no modifications or adaptations are made.
Figure 1. Selected molybdenum(0) mono(dinitrogen) complexes
synthesized by our working group and corresponding NÀ N
stretching frequencies.[22–25,28,29,33]
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ligand. By combining tridentate and tripodal ligand design, we
recently developed the pentadentate tetrapodal (pentaPod)
ligand P2MePP2Ph, leading to the first catalytically active Chatt
complex with a single coordination site, [MoN2(P2MePP2Ph)]
(Figure 1).[28,29] The design of this complex was the result of an
evolution during which clear trends had been identified.
Molybdenum dinitrogen complexes supported by (linear)
tridentate ligands, e.g., suffered from isomerization[30,31] or
coordination of the conjugate base upon protonation,[31,32]
which affects the stability of the systems and/or their catalytic
activity.[28] This was the reason to include tripod ligands in our
investigations. In order to increase the activation of the
coordinated N2, we raised the σ donor strength of the coligands
by replacement of aryl by alkyl phosphines (tdppme vs. trpd-1
vs. SiP3, Figure 1).[22–26,29] In doing so, we noticed that the steric
demand of the substituents can be a limiting factor.[24,26]
Oligodentate ligand systems with diisopropylphosphino
end groups, for instance, are sterically too demanding for
certain applications,[24] and therefore, smaller substituents such
as methyl groups were employed.[25,26,29] On the other hand, a
certain degree of shielding can be beneficial to enhance the
stability of the N2 ligand and its protonated derivatives and
exclude unwanted reactive pathways.[7,34] This led to the idea of
using ligands with phospholane end groups in our MoÀ N2
chemistry.
Scheme 1. Complexes supported by tridentate prPP(Ph)P-pIn
ligand, containing phospholane end groups and phosphine
coligands (dppm, PMePh2, PMe2Ph), a) leading to isomerization (fac
and mer), b) to coordination of the conjugate base (X) in trans
position of N2 during protonation or c) decomposition.[31]
In an earlier publication, we employed the tridentate ligand
prPP(Ph)P-pln in combination with monodentate- and biden-
tate phosphine coligands for the synthesis of molybdenum(0)
mono- and bis(dinitrogen) complexes.[31] However, as expected,
the flexible structure of prPP(Ph)P-pln led in the presence of
bidentate phosphine coligands to isomerization, and fac- and
mer isomers were obtained (Scheme 1, a). Nevertheless, it was
possible to protonate the bis(dinitrogen) complex [Mo-
(N2)2{prPP(Ph)P-pln}(PMePh2)] to the corresponding hydrazido
(2-)
complex
[Mo(NNH2)(OTf){prPP(Ph)P-pln}(PMePh2)]+
Figure 2. Tripodal P donor ligands with phospholane end groups
(Scheme 1, b), demonstrating the ability of such systems to
activate N2. On the other hand, the mono(dinitrogen) complex
[Mo(N2){prPP(Ph)P-pln}(PMe2Ph)2)] decomposed under acidic
conditions (Scheme 1, c). This and the mechanistically unfav-
ourable coordination of the conjugate base of the employed
acid (HOTf) to the hydrazido(2-) complex indicate that further
optimization of the ligand design is necessary to achieve a
catalytic N2-to-NH3 conversion on the basis of such systems.
Still, the complexes shown in Scheme 1 represent rare examples
of molybdenum complexes coordinated by phospholane
groups.[31,35]
To avoid isomerization and decoordination of the ligand in
the pivotal position trans to N2, we decided to combine the
phospholane end groups with a tripodal topology based on a
neopentyl backbone (Figure 2).
For the introduction of phospholane end groups into the
linear tridentate ligand prPP(Ph)P-pln, we recently employed
lithium phospholanide (LiÀ Pln) as suitable P nucleophile.[31]
Herein, we describe the synthesis of three different tripod
ligands with neopentyl backbone and one (trpd-1pln), two
(trpd-2pln), or three (trpd-3pln) phospholane end groups,
using the same strategy. Due to the rigidity of the neopentyl
employed in this study.
backbone only a facial coordination for this type of tripod
ligands to a metal center is possible. Moreover, the tripodal
geometry ensures that the pivotal trans position of coordinated
N2 is occupied by a part of the supporting tripod ligand.
The coordination behavior of the tripod ligands trpd-Xpln
(X=1–3) to molybdenum(III) precursors is investigated. Aiming
at a molybdenum(0) dinitrogen complex with a highly activated
dinitrogen ligand, the molybdenum(III) complex supported by
trpd-3pln is further subjected to a sodium amalgam reduction,
and the protonation of the resulting molybdenum(0) dinitrogen
complex is examined. Finally, the results are compared to
similar systems supported by tripodal phosphine ligands.
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Molybdenum(III) complexes
Results and Discussion
In order to obtain information about the coordination behav-
iour of the prepared tripod ligands trpd-1pln, trpd-2pln and
trpd-3pln and to investigate their suitability for N2 activation,
molybdenum(III) complexes were prepared first (Scheme 3).
Reaction of trpd-1pln and trpd-2pln with the precursor
[MoI3(thf)3] in THF afforded the corresponding molybdenum(III)
complexes [MoI3(trpd-1pln)] (1) and [MoI3(trpd-2pln)] (2) in
reasonable yields (60 and 61%, resp.) after stirring at room
temperature for 62 h and 42 h, respectively. In contrast to that,
it was not possible to obtain the analogous [MoI3(trpd-3pln)]
complex by using [MoI3(thf)3]. Therefore, [MoBr3(thf)3] was
employed. Stirring with trpd-3pln in THF for 18 h at room
Syntheses of tripod ligands with phospholane end groups
(trpd-1pln, trpd-2pln, trpd-3pln)
The tripod ligands trpd-1pln, trpd-2pln and tripd-3pln were
synthesized by chlorine substitution in the corresponding
ligand precursors by LiÀ Pln in the presence of n-butyllithium in
tetrahydrofuran (Scheme 2).
While the ligands trpd-1pln and trpd-2pln could be
synthesized in pure form and in excellent yields (trpd-1pln:
90%, trpd-2pln: 99%, Figure S1–9) synthesis of the most
demanding – and potentially most interesting – tripod ligand
trpd-3pln is accompanied by the formation of side products
(compare NMR spectra, Figure S10–S13). Possible side reactions
involve a lithium halogen exchange caused by LiÀ Pln or n-
butyllithium, as known from the literature.[36] This assumption is
supported by traces of 2,2-bis(phospholanomethyl)propane
which could be observed in the 13C NMR spectrum (Figure S12).
To minimize the amount of side products it is important to
add the ligand precursor 1,3-dichloro-2-(chloromethyl)-2-meth-
ylpropane to a solution of LiÀ Pln and n-butyllithium in THF at
temperature
afforded
the
molybdenum(III)
complex
[MoBr3(trpd-3pln)] (3) in satisfying purity and yield (71%). Due
to the presence of side products in the ligand (see above), it
was necessary to apply an excess of trpd-3pln. All three
complexes were characterized by elemental analysis and IR
spectroscopy (see Experimental Section).
°
0 C. Furthermore, the amount of n-butyllithium has a huge
Synthesis of dinitrogen complexes supported by the
tridentate tripod ligand trpd-3pln
impact on the side product formation. The best results were
obtained with 1/6 equivalent of n-butyllithium with regard to
LiÀ Pln. As we have shown before,[31] it is important to break up
the LiÀ Pln aggregates present in solution, which reduce the
reactivity of LiÀ Pln by addition of n-butyllithium. After a
reaction time of 4 d the reaction mixture was reduced in vacuo.
The residue was redissolved in n-pentane, and the suspension
was filtered over Celite®. A large amount of lithium species
could be separated by this filtration, which was followed by an
aqueous work up. Further purification was achieved by an
additional filtration over neutral aluminium oxide. After these
steps, trpd-3pln was obtained in a sufficient quality for further
reactions (see below).
To examine the effect of phospholane end groups on the ability
of transition metal complexes to bind and activate dinitrogen,
molybdenum(0) complexes of the type [MoN2(tripod)(diphos)],
tripod=trpd-Xpln (X=1–3) and diphos=dppm (1,1-bis-
(diphenylphosphino)methane)
or
dmpm
(1,1-bis
(dimethylphosphino)methane) were synthesized. Analogous
complexes with other tripod ligands are known to mediate N2
activation and functionalization.[23,24,37] In this context,
[MoBr3(trpd-3pln)] (3) supported by the tripod ligand trpd-
3pln was chosen as the most promising candidate for an
amalgam reduction to the corresponding molybdenum(0)
mono(dinitrogen) complex as the presence of three phospho-
Scheme 2. Synthesis of the tripod ligands trpd-1pln, trpd-2pln and
trpd-3pln.
Scheme 3. Syntheses of the molybdenum(III) complexes [MoI3(trpd-
1pln)] (1), [MoI3(trpd-2pln)] (2) and [MoI3(trpd-3pln)] (3).
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lane donors should lead to the most activated N2 ligand.
The 31P NMR spectrum, however, shows additional signals. The
Moreover, using a symmetric tripodal PPP ligand the formation
of only one complex isomer was expected which should
simplify the characterization of the product.
singlet at 16.0 ppm can be assigned to the homoleptic complex
[Mo(dppm)3],[31,38] and the singlet at À 21.5 ppm is due to
uncoordinated dppm.
Further signals in the 31P-NMR spectrum of the product are
located at 31.5 ppm, 27.7 ppm and À 38.8 ppm (6), whereas the
signal at 31.5 ppm is hidden by the M signal of compound 4. To
determine the identity of this side product, the filtrate
remaining after precipitation of the product with methanol was
investigated by NMR and IR spectroscopy. Analysis of the 31P
NMR and the 1H,31P HMBC and 31P,31P COSY NMR spectra (see SI,
Figure S14–S18) along with the IR spectrum (see SI, Figure S19)
indicate that the side product must be cis-[Mo(N2)2(k2-trpd-
3pln)2] (6) The triplets at 31.5 ppm and 27.7 ppm belong to the
coordinating phospholane groups and the singlet at
À 38.8 ppm to the uncoordinated ones (Figure 3). Overall the
desired product 4 was obtained in a ratio of 26:1:1:1 (4:[Mo-
(dppm)3]:6:dppm).
NaxHg reduction of [MoBr3(trpd-3pln)] (3) in the presence of
dppm
For the synthesis of a molybdenum(0) dinitrogen complex
supported by trpd-3pln the corresponding molybdenum(III)
precursor, [MoBr3(trpd-3pln)] (3), was stirred with sodium
amalgam and one equivalent of dppm under nitrogen atmos-
phere in THF for 22 h (Scheme 4). The product was precipitated
as a red solid by addition of methanol and characterized by
NMR, IR and Raman spectroscopy.
In the 31P NMR spectrum, the AA’XX’M pattern of the desired
mono(dinitrogen) complex [Mo(N2)(trpd-3pln)(dppm)] (4) is
observed (Figure 3). The AA’ part (δ=36.7–35.3 ppm) is as-
signed to the equatorially coordinated phospholane donors of
trpd-3pln, the signal at 32.1–31.4 ppm (M signal) belongs to
the axially coordinated phospholane donor and the XX’
subspectrum, located at 13.3–12.0 ppm, is attributed to the P
atoms of the diphenylphosphino groups of the coligand dppm.
The complex [Mo(N2)(trpd-3pln)(dppm)] (4) was also ana-
lyzed by vibrational spectroscopy. In the IR spectrum an intense
NN stretch appears at 1960 cmÀ 1, along with a weak NN-stretch
at 2005 cmÀ 1 (Figure 4, top). The corresponding Raman spec-
trum shows a NN stretching vibration at 1970 cmÀ 1 (Figure 4,
bottom). The band at 1960 cmÀ 1 (IR) and the peak at 1970 cmÀ 1
(Raman) are attributed to the NÀ N stretch of [Mo(N2)(trpd-
3pln)(dppm)] (4). Comparison with analogous systems sup-
ported by tripod ligands in combination with dppm shows that
the activation induced by the phospholane groups is compara-
ble to that of À PMe2, stronger than À PEt2 and much stronger
than À PPh2 (Figure 5).[23,25,39] It should be noted, however, that in
some cases the electron-donating properties of À PEt2 are higher
than of À PMe2 groups.[40] A general comparison of the electron-
donating properties of phosphines containing À PEt2, À PMe2 and
À Pln endgroups thus appears problematic and will not be
attempted here.
Scheme 4. NaxHg reduction of [MoBr3(trpd-3pln)] (3) in presence of
dppm.
Figure 3. 31P{1H} NMR spectrum of the product of the sodium
amalgam reduction of [MoBr3(trpd-3pln)] (3) in the presence of
one equivalent dppm measured in [D8]-THF. Furthermore, enlarged
signals are shown (top).
Figure 4. IR and Raman spectra of the product of the sodium
amalgam reduction of [MoBr3(trpd-3pln)] (3) in the presence of
one equivalent dppm which was precipitated by methanol.
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of the coligand dmpm having two stronger electron-donating
dimethylphosphino groups instead of diphenylphosphino
groups in dppm.
NaxHg reduction of [MoBr3(trpd-3pln)] (3) in the presence of
dmpm
For the synthesis of a second molybdenum(0) dinitrogen
complex supported by trpd-3pln the corresponding precursor,
[MoBr3(trpd-3pln)] (3), was stirred with sodium amalgam and
one equivalent of dmpm under N2 in THF for 18 h (Scheme 5),
and a yellow-orange oil was obtained. This reduction did not
result in a single product. Characterization with IR and NMR
spectroscopy showed that besides the desired mono
(dinitrogen) complex [Mo(N2)(trpd-3pln)(dmpm)] (5), a cis-bis
(dinitrogen) complex, probably cis-[Mo(N2)2(k2-trpd-3pln)2] (6),
and the homoleptic complex [Mo(trpd-3pln)2] (7) were formed.
The IR spectrum exhibits two NN stretching bands at
2005 cmÀ 1 and 1944 cmÀ 1 (Figure 7). The intensity of these
bands is relatively weak compared to other similar molybde-
num(0) dinitrogen complexes such as [Mo(N2)(SiP3)(dmpm)][25]
or [Mo(N2)(trpd-3pln)(dppm)] (4), which indicates that the
mixture contains only smaller amounts of molybdenum
dinitrogen complexes.
Figure 5. Comparison of [Mo(N2)(tripod)(dppm)] systems
(tripod=tdppme,[23] tdepme,[39] trpd-3pln (4, this study), SiP3,[25]
tdmpme[39]) containing differently activated N2 ligands.
To check whether the small IR-band at 2005 cmÀ 1 is caused
by a side product, e.g. a bis(dinitrogen) complex, the filtrate
remaining from the precipitation with methanol (compare
above) was also examined by IR spectroscopy. The correspond-
ing IR spectrum (Figure S19, top) shows two NN stretching
vibrations at 2004 cmÀ 1 and 1942 cmÀ 1 with nearly the same
intensity. This is a typical NN-vibrational pattern for a cis-bis
(dinitrogen) complex. Along with the corresponding NMR
spectra (see above and Figure S14–S18) this feature therefore
can be assigned to cis-[Mo(N2)2(k2-trpd-3pln)2] (6), which was
also present in the precipitated product to a small extent (cf
Figure 3 and Figure 4). By slow diffusion of n-pentane into a
solution of 4 in THF crystals suitable for single crystal diffraction
could be obtained. [Mo(N2)(trpd-3pln)(dppm)] (4) crystallizes in
the monoclinic space group P21/c with two crystallographically
independent complexes exhibiting slightly different bond
lengths (Figure 6). The phospholane rings have the envelope
conformation. Interestingly, the crystal contains 5% of a
bromido instead of the dinitrogen complex (see SI, Figure S26,
Table S1 and S2). This [MoBr(trpd-3pln)(dppm)] complex is
most likely an intermediate of the reduction from [MoBr3(trpd-
3pln)] (3) to [Mo(N2)(trpd-3pln)(dppm)] (4) (Figure 6).
The band at 1944 cmÀ 1 can be assigned to the NN
stretching vibration of [Mo(N2)(trpd-3pln)(dmpm)] (5) (see
Scheme 5. NaxHg reduction of [MoBr3(trpd-3pln)] (3) in the pres-
ence of dmpm.
To further increase the activation of the N2 ligand the
sodium amalgam reduction was also performed in the presence
Figure 6. IR (ATR) spectrum of the product of the sodium amalgam
reduction of [MoBr3(trpd-3pln)] (3) in the presence of one
equivalent dmpm.
Figure 7. 31P{1H} NMR spectrum of the product of the sodium
amalgam reduction of [MoBr3(trpd-3pln)] (3) in the presence of
one equivalent dmpm measured in [D8]-THF.
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below) and the asymmetric NN stretch of the cis-bis(dinitrogen)
using dmpm as coligand is not particularly favourable. The
complex cis-[Mo(N2)2(k2-trpd-3pln)2] (6), whereas the band at
2005 cmÀ 1 belongs to the symmetric NN stretching vibration of
cis-[Mo(N2)2(k2-trpd-3pln)2] (6). Notably, complex 6 has also
been formed as side product of the sodium amalgam reduction
in the presence of the other coligand dppm, which was
described above (Figure 4). With a NN stretching vibration of
1944 cmÀ 1, the dinitrogen ligand in [Mo(N2)(trpd-3pln)(dmpm)]
(5) is more activated than in [Mo(N2)(trpd-3pln)(dppm)] (4)
exhibiting a ν(NN) with 1960 cmÀ 1. This is due to the stronger
σ-donation of dmpm as compared to dppm.
signal of the homoleptic complex [Mo(trpd-3pln)2] (7) also
appeared in the 31P NMR spectrum of the reaction solution of
the sodium amalgam reduction of 3 in the presence of dppm
(Figure S18, a and b). Furthermore, crystals suitable for single-
crystal X-ray structure could be obtained by diffusion of n-
pentane in a solution of the product obtained from the sodium
amalgam reduction in [D8]-THF (Figure 9). The complex [Mo-
(trpd-3pln)2] (7) crystallizes in the space group R-3 with Z=24.
The crystal structure exhibits two crystallographic independent
complexes in which each Mo center is in a slightly distorted
octahedral coordination 9 geometry, with only small differences
in the bond lengths and angles (Figure S27, Table S3). Interest-
ingly, in one of these complexes all PÀ Mo(2)À P angles of the
The 31P NMR spectrum of the reaction mixture shows the
formation of several products. The AAXX’M pattern of the mono
(dinitrogen) complex [Mo(N2)(trpd-3pln)(dmpm)] (5) is clearly
visible (Figure 8). The AA’ part (δ=39.3 ppm–38.2 ppm) can be
assigned to the equatorially coordinated phospholane donors
of trpd-3pln whereas the M-signal for the axial coordinated
phospholane donor appears at 34.8 ppm. Moreover the P atoms
of the dimethylphosphane groups of the coligand dmpm lead
to the XX’ subspectra at À 20.8 to À 21.8 ppm. At 31.3 and
27.4 ppm appear two triplets which can be assigned to the
coordinated P atoms of cis-[Mo(N2)2(k2-trpd-3pln)2] (6). The
corresponding uncoordinated phospholane groups cause the
singlet at À 39.1 ppm. The complex 6 could also be identified as
side product of the sodium amalgam reduction of [MoBr3(trpd-
3pln)] (3) in the presence of the coligand dppm (see above and
Figure 3).
°
trans phosphines are identical (P(41)À Mo(2)À P(51)=174.01 )
whereas the corresponding PÀ Mo(1)À P angles vary in a range
°
°
between 171.80 and 175.04 .
Protonation of [Mo(N2)(trpd-3pln)(dppm)] (4)
With 1960 cmÀ 1 the NN-stretching frequency of [Mo(N2)(trpd-
3pln)(dppm)] (4) is low enough (i.e., below 2000 cmÀ 1) to allow
protonation, which represents a first step towards dinitrogen
fixation and functionalization with electrophiles.[41] [H(OEt2)2][Al
{OC(CF3)3}4] turned out to be a suitable acid for this application
as it combines protonated ether as strong acid with the
stabilizing effect of the weakly coordinating anion [Al{OC-
(CF3)3}4]À .[42] Furthermore, in case of pentaphosphine complexes,
the aluminate led to better results and purer products than the
commonly used [BArF4]À anion.[43] Therefore, [Mo(N2)(trpd-
3pln)(dppm)] (4) was treated with 3 equivalents of [H(OEt2)2][Al
{OC(CF3)3}4] (Scheme 6).
Finally, the singlet at 36.4 ppm is attributed to the six P
atoms of the tripod ligand trpd-3pln in the homoleptic
complex [Mo(trpd-3pln)2] (7). This assignment is supported by
1
the fact that in the H, 31P-HMBC NMR spectrum a coupling of
these 31P nuclei (δ=36.4 ppm) to only aliphatic protons was
visible (Figure S20). The product also contained a large amount
of uncoordinated trpd-3pln (δ=À 36.6 ppm, Figure 8), which
reveals that formation of molybdenum dinitrogen complexes
The 31P NMR spectrum of the product solution (Figure 10,
top) shows the expected AA’XX’M pattern of a pentaphosphine
environment, and the high-field shift of all three signals indicate
the formation of a hydrazido complex.[23,24,28,29] The XX‘ signal of
the equatorial dppm phosphine donor atoms is at 5.2–4.4 ppm,
which corresponds to a high-field shift of 7.9 ppm compared to
4.
Figure 9. Comparison of the 31P{1H} NMR spectrum of [Mo(N2)(trpd-
3pln)(dppm)] (4) (bottom) and the products of the protonation 4
with [H(OEt2)2][Al{OC(CF3)3}4] (top), [Mo(NNH2)(trpd-3pln)(dppm)]
[Al{OC(CF3)3}4]2 (8) and side products [Mo(trpd-3pln)(dppm)] (9)
and protonated dppm (*). Both spectra were measured in [D8]-THF.
Figure 8. View of the two crystallographically independent com-
plexes in the crystal structure of [Mo(trpd-3pln)2] (7). Hydrogen
atoms are omitted for clarity.
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Summary and Conclusion
Three novel tripod ligands trpd-1pln, trpd-2pln and trpd-3pln,
were prepared by using LiÀ Pln as synthon for phospholane end
groups. This shows not only the versatility of LiÀ Pln for the
synthesis of new oligodentate phosphine ligand systems but
also highlights the fact that entire series of ligands with
differing residues can be obtained, which opens up many
possibilities for fine-tuning of their electronic and steric proper-
ties. The accessibility of metal complexes supported by the
above ligands was demonstrated by synthesis of the molybde-
num(III) complexes [MoI3(trpd-1pln)] (1), [MoI3(trpd-2pln)] (2)
and [MoBr3(trpd-3pln)] (3) containing all three ligands. One of
them, 3, was selected for further reactions aiming at N2
activation and functionalization. Reduction of 3 by sodium
amalgam in the presence of dppm led to molybdenum(0)
mono(dinitrogen) complex [Mo(N2)(trpd-3pln)(dppm)] (4) con-
taining a N2 ligand with an NN stretching frequency of
1960 cmÀ 1. This value indicates an activation high enough for
protonation.
Scheme 6. Protonation of [Mo(N2)(trpd-3pln)(dppm)] (4) with [H-
(OEt2)2][Al{OC(CF3)3}4] leading to hydrazido complex [Mo-
(NNH2)(trpd-3pln)(dppm)][Al{OC(CF3)3}4]2 (8) (anion omitted for
clarity).
Replacement of the coligand by the stronger ligand dmpm,
which should further increase activation of the N2 ligand, led to
a mixture of products containing the desired complex [Mo-
(N2)(trpd-3pln)(dmpm)] (5), but also cis-[Mo(N2)2(k2-trpd-3pln)2]
(6) and homoleptic [Mo(trpd-3pln)2] (7). Because of its purity, 4
was chosen for further functionalization, and protonation with
[H(OEt2)2][Al{OC(CF3)3}4] was found to lead to the hydrazido(2-)
complex [Mo(NNH2)(trpd-3pln)(dppm)][Al{OC(CF3)3}4]2. 31P NMR
Figure 10. View of the two crystallographic independent complexes
in the crystal structure of the mono(dinitrogen) complex
[MoN2(trpd-3pln)(dppm)] (4). Hydrogen atoms are omitted for
clarity.
~
showed the expected changes of the chemical shifts ( δ) under
protonation in [Mo(N2)(P5)]/[Mo(NNH2)(P5)]2+ systems with P5 =
pentaphosphine environment. In conclusion, this paper along
with its predecessor, ref.[31], shows that multidentate
phosphine ligands with terminal phospholane groups can be
prepared along a user-friendly route and successfully employed
for the synthesis of a wide range of molybdenum dinitrogen
complexes suitable for nitrogen fixation. Further exploration
and application of this methodology is under way.
The AA‘ signal of the equatorial phospholane groups can be
found at 18.6–17.8 ppm, which is a shift of around 18 ppm to
higher field compared to 4. The signal of the axial phospholane
group (M) is most strongly affected by changes at the sixth
coordination site due to its trans position; i.e., the signal
~
between À 12.1 and À 13.0 ppm exhibits the largest shift ( δ=
44.4 ppm). Similar values were also found in comparable
systems [Mo(N2)(P5)]/[Mo(NNH2)(P5)]2+ with P5=PMe2PPPh ( δ= Experimental Section
~
2
[23]
45.5 ppm),[28,29] tdppme and dmpm ( δ=44.7 ppm) or trpd-1
~
General Information. Commercially available starting materials
[24]
~
and dmpm, δ=46.8 ppm.
Furthermore, in the 1H NMR
were used as received. Water and oxygen-sensitive reagents were
handled in an M. Braun Labmaster 130 Glovebox under N2. All
syntheses were performed under a N2 atmosphere by using Schlenk
techniques. The solvents were dried and freshly distilled under
argon prior to use. Solvents were dried with CaH2 (dichloro-
methane, n-hexane, n-pentane), LiAlH4 (THF, diethyl ether) or
magnesium methanolate (methanol). [MoBr3(thf)3][45] as well as
[MoI3(thf)3][46] were prepared as described in the literature and
Lithiumphospholanide (LiÀ Pln) as described in our previous
study.[31] The ligand precursors 3-Chloro-2-(chloromethyl)-2-meth-
spectrum a singlet at 8.94.ppm is visible (Figure S22) which ist
in line with the proposed doubly protonated hydrazido
complex [Mo(NNH2)(trpd-3pln)(dppm)][Al{OC(CF3)3}4]2 (8).[44] Ad-
ditionally, signals of small amounts of protonated dppm and
[Mo(trpd-3pln)(dppm)] (9) as product of the decoordination of
N2 are visible at 33.1 and 30.0 ppm in the 31P NMR, 31P, 31P COSY
and 1H, 31P HMBC NMR (Figure 10 and Figure S23–25). This
observation is in analogy with the formation of [Mo(trpd-
1)(dppm)] by reduction of [MoI3(trpd-1)] we described earlier[24]
and these pentacoordinated Mo(0) phosphine complexes seem
to be common side products under harsh conditions.
ylpropyl)diphenylphosphane
and
(2-(Chloromethyl)-2-meth-
ylpropane-1,3-diyl)bis(diphenylphosphane)
are
literature
known.,[46,47] but the syntheses were modified as already decribed
by H. Broda[48] (modified syntheses see SI).
Elemental analyses were performed using a EuroVector CHNSO-
element analyzer (Euro EA 3000) or a vario MICRO cube (Co.
Elementar Analysensysteme). Samples were burned in sealed tin
Z. Anorg. Allg. Chem. 2021, 1–12
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4
containers in a stream of oxygen. NMR spectra were recorded with
Bruker Avance 400 Pulse Fourier Transform spectrometer
31P{1H} NMR: (161.98 MHz, CD2Cl2, 300 K): δ=À 25.1 (d, JPP =3.1 Hz,
a
2 P, PPh2), À 36.5 (t, 4JP P =3.1 Hz, 1 P, Ppln ppm.
operating at frequencies of 400.13 MHz (1H), 161.98 MHz (31P) and
100.62 MHz (13C). Referencing was performed using the solvent
residue signal (7.26 ppm for CDCl3, 5.32 ppm for CD2Cl2, 3.58 ppm
for THF-d8). 31P NMR spectra were referenced to H3PO4 85%
[δ(31P)=0 ppm] as substitutive standard. Infrared spectra were
recorded at room temperature on a Bruker Alpha FT-IR Spectrum
with Platinum ATR setup or on a Bruker Vertex70 FT-IR spectrom-
eter using a broadband spectral range extension VERTEX FM for full
mid and far IR in the range of 6.000–80 cmÀ 1. Raman spectra were
recorded at RT on a Bruker RAM II FT-Raman spectrometer using a
liquid nitrogen cooled, highly sensitive Ge detector, 1064 nm
radiation and 3 cmÀ 1 resolution.
1H-NMR (400.13 MHz, CD2Cl2, 300 K): δ=7.47–7.41 (m, 8 H, CHphenyl),
7.33–7.28 (m, 12 H, CHphenyl), 2.55–2.45 (m, 4 H, CÀ CH2À PPh2), 1.69–
1.54 (m, 6 H, CH2 (pln)), 1.60 (d, 2JHP =4.0 Hz, 2 H, CqÀ CH2À Ppln), 1.36–
1.30 (m, 2 H, CH2 (pln)), 1.01 (s, 3 H, CH3) ppm.
13C{1H}-NMR (101 MHz, CD2Cl2, 300 K): δ=140.7 (m, 4 C, Ci), 133.7
(d, JCP =2.0 Hz, 4 C, CHphenyl), 133.5 (d, JCP =2.0 Hz, 4 C, CHphenyl),
128.9–128.8 (m, 12 C, CHphenyl), 46.1–45.7 (dt, 1JCP =21.2 Hz, JCP
=
3
1
8.0 Hz, 1 C, CqÀ CH2À Ppln), 43.2–42.9 (pseudo-„dt“ (ddd), JCP
=
16.6 Hz, 3JCP =9.1 Hz, 2 C, CqÀ CH2À PPh2), 39.3–38.9 (dt, 2JCP =12.7 Hz,
3
2JCP =13.7 Hz, 1 C, CqÀ CH3), 29.7–29.4 (pseudo-„q“ (dt), JCP =9.1 Hz,
1 C, CH3), 28.5 (d, JCP =11.2 Hz, 2 C, CH2 (pln)), 28.4 (d, JCP =3.5 Hz,
2 C, CH2 (pln)) ppm.
IR (ATR): v˜=3069 (w), 3050 (w), 3027 (w), 3013 (w), 2999 (w), 2948
(m), 2925 (m), 2875 (sh), 2857 (m), 2819 (sh), 1951 (vw), 1884 (vw),
1806 (vw), 1585 (w), 1571 (w), 1480 (m), 1454 (br m), 1433 (s), 1400
(m), 1370 (m), 1328 (w), 1303 (w), 1259 (m), 1183 (w), 1155 (w),
1108 (m), 1091 (m), 1067 (m), 1024 (m), 998 (m), 970 (w), 949 (w),
912 (br w), 866 (sh), 846 (w), 816 (br m), 738 (s), 694 (s), 673 (sh),
653 (vw), 618 (vw), 597 (vw), 580 (vw), 560 (vw), 511 (s), 479 (m),
Single Crystal Structure Analysis of 4and 7
Data collections were performed using an Imaging Plate Diffraction
System (IPDS-2 from STOE using MoÀ Kα radiation. The structures
were solved with SHELXT[49] and refinement was performed against
F2 using SHELXL-2018.[50] For all compounds a numerical absorption
correction was performed using X-Red and X-Shape of the software
package X-Area.[51] All non-hydrogen atoms were refined with
anisotropic displacement parameters. The CÀ H hydrogen atoms
were positioned with idealized geometry (methyl H atoms in 7
allowed to rotate but not to tip) and were refined isotropically with
Uiso (H)=1.2 Ueq (C) (1.5 for methyl H atoms) using a riding model.
The structure of compound 4 contains about 5% of bromine that
might originate from the reactant that occupy the same position as
N2. Therefore, the structure was refined using a split model with
restraints. If this bromide ligand is not considered a much too short
NÀ N distance is observed. The structure is additionally pseudo-
443 (vw), 426 (w) cmÀ 1
.
Synthesis of trpd-2pln
The ligand precursor 1,3-dichloro-2-(diphenylphosphinomethyl)-2-
methylpropane (205 mg, 630 μmol) was dissolved in 5 mL. THF. To
a solution of lithium phospholanide (142 mg, 1.51 mmol) in THF
(10 mL) a solution of n-butyllithium in n-hexane (2.5 m, 0.60 mL,
°
1.50 mmol) was added at 0 C. Within 30 min the lithium
phospholanide/n-butyllithium solution was added to the precursor
°
merohedral twinned with β close to 90 indicating an orthorhombic
°
solution at 0 C. The solution was stirred for 20 h at room
crystal system. Therefore, a twin refinement was performed leading
to a BASF parameter of 0.0331 (4). After structure refinement of
compound 7 there is one peak on the difference map around the
À 3 axis, indicating for some disordered solvent of unknown
identity. Therefore, the data were corrected for disordered solvent
using Squeeze in Platon.[52] Selected crystal data and details of the
structure refinements are given in Table S1–S3.
temperature. The solvent was removed in vacuo. The residue was
redissolved in n-pentane (10 mL), filtered over Celite® and washed
with n-pentane (40 mL). Degassed water (10 mL) was added. The
organic layer was separated with a syringe and the aqueous layer
was extracted three times with diethyl ether (5 mL). The solvent of
the organic layers was removed in vacuo. The residue was
redissolved in dichloromethane (5 mL), filtered over basic aluminum
oxide and washed with dichloromethane (30 mL) After removing
the solvent in vacuo the product was obtained as colorless oil
(270 mg, 630 μmol, 99%).
CCDC 2081802 (4), and CCDC 2081803 (7) contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
4
31P{1H} NMR: (161.98 MHz, CD2Cl2, 300 K): δ=À 24.3 (t, JPP =3.6 Hz,
1 P, PPh2), À 36.6 (d, 4JPP =3.6 Hz, 2 P, Ppln) ppm.
Synthesis of trpd-1pln
1H-NMR (400.13 MHz, CD2Cl2, 300 K): δ=7.51–7.46 (m, 4 H, CHphenyl),
2
7.34–7.28 (m, 6 H, CHphenyl), 2.50 (d, JHP =3.7 Hz, 2 H, CqÀ CH2À PPh2),
The ligand precursor 1-chloro-2,2-bis(diphenylphosphinomethyl)-
propane) (421 mg, 886 μmol) was dissolved in 5 mL. THF. To a
solution of lithium phospholanide (100 mg, 1.06 mmol) in THF
(10 mL) a solution of n-butyllithium in n-hexane (2.5 m, 0.20 mL,
1.78–1.68 (m, 5 H, CH2 (pln)), 1.67–1.57 (m, 7 H, CH2 (pln)), 1.63 (d,
2JHP =1.4 Hz, 2 H, CqÀ CH2À Ppln), 1.62 (d, 2JHP =1.4 Hz, 2 H,
CqÀ CH2À Ppln), 1.45–1.37 (m, 4 H, CH2 (pln)), 1.04 (s, 3 H, CH3) ppm.
°
0.50 mmol) was added at 0 C. Within 10 min the lithium
phospholanide/n-butyllithium solution was added to the precursor
13C{1H}-NMR (101 MHz, CD2Cl2, 300 K): δ=140.7 (d, 1JCP =13.0 Hz,
2 C, Ci), 133.5 (d, JCP =19.6 Hz, 4 C, CHphenyl), 128.7 (s, 2 C, Cp), 128.7
°
solution at 0 C. The solution was stirred for 44 h at room
1
(d, JCP =2.0 Hz, 4 C, CHphenyl), 45.8–45.4 (pseudo-„dt“ (ddd), JCP
=
temperature. The solvent was removed in vacuo. The residue was
redissolved in n-pentane (10 mL), filtered over Celite® and washed
with n-pentane (30 mL). After removing the solvent in vacuo the
product was obtained as colorless oil (425 mg, 807 μmol, 91%). The
chemical shifts here are differing from these ones in the discussion
part due to the use of different NMR solvents. The resolution of the
signals in the experimental part is better so that coupling constants
can be determined. Furthermore, the NMR spectra in the discussion
belong to a batch with higher purity.
3
1
21.3 Hz, JCP =8.2 Hz, 2 C, CqÀ CH2À Ppln), 42.5–42.2 (dt, JCP =15.8 Hz,
3JCP =8.8 Hz, 1 C, CqÀ CH2À PPh2), 38.8–38.5 (dt, 2JCP =12.4 Hz, JCP
=
2
3
13.9 Hz, 1 C, CqÀ CH3), 29.3–29.0 (pseudo-„q“ (dt), JCP =9.1 Hz, 1 C,
CH3), 28.5–28.3 (m, 8 C, CH2 (pln)) ppm.
IR (ATR): v˜=3069 (w), 3051 (w), 3029 (vw), 3013 (vw), 3000 (vw),
2946 (sh), 2926 (s), 2873 (sh), 2857 (s), 2822 (sh), 2326 (br vw), 1946
(vw), 1882 (vw), 1808 (vw), 1753 (vw), 1584 (w), 1571 (vw), 1480 (m),
1460 (m), 1446 (m), 1432 (s), 1423 (sh), 1402 (br m), 1378 (sh), 1367
(br m), 1329 (vw), 1317 (vw), 1302 (w), 1260 (br m), 1207 (sh), 1185
(br w), 1157 (w), 1139 (vw), 1109 (m), 1093 (m), 1067 (m), 1053 (sh),
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1027 (m), 999 (w), 970 (w), 951 (w), 939 (vw), 911 (br vw), 867 (sh),
699 (s), 691 (s), 674 (m), 627 (vw), 620 (w), 610 (w), 601 (vw), 596
846 (m), 804 (br m), 739 (vs), 695 (vs), 670 (sh), 652 (m), 619 (sh),
(vw), 592 (vw), 586 (vw), 579 (vw), 574 (vw), 567 (vw), 546 (vw), 540
(vw), 537 (vw), 521 (sh), 515 (s), 502 (s), 489 (m), 477 (m), 461 (br w),
442 (vw), 411 (m), 406 (br m), 375 (vw), 364 (vw), 352 (vw), 336 (w),
328 (vw), 323 (vw), 314 (vw), 312 (vw), 305 (vw), 301 (vw), 290 (vw),
286 (vw), 282 (vw), 272 (br vw), 261 (vw), 253 (vw), 250 (sh), 245
(vw), 239 (vw), 234 (vw), 230 (vw), 225 (sh), 220 (m), 212 (sh), 208
(s), 203 (s), 192 (br s), 183 (sh), 178 (w), 173 (vw), 169 (m), 166 (m),
161 (m), 157 (m), 155 (sh), 147 (sh), 145 (m), 142 (m), 136 (m), 131
(m), 126 (w), 116 (br s), 110 (s), 106 (m), 102 (m), 97 (sh), 95 (s), 89
513 (m), 482 (m), 452 (vw), 422 (w) cmÀ 1
.
Synthesis of trpd-3pln
To a solution of lithium phospholanide (194 mg, 2.06 mmol) in THF
(20 mL) a solution of n-butyllithium in n-hexane (2.5 m, 0.14 mL,
°
0.35 mmol) was added at 0 C. to this solution. The ligand precursor
1,3-Dichloro-2-(chloromethyl)-2-methylpropane (117 mg, 667 μmol)
(s), 85 (m) cmÀ 1
.
°
was added to this solution at 0 C. The solution was stirred for 4 d
at room temperature. The solvent was removed in vacuo. The
residue was redissolved in n-pentane (10 mL), filtered over Celite®
and washed with n-pentane (30 mL). The solvent was removed in
vacuo. After redissolving the oily residue in diethyl ether (5 mL)
degassed water (5 mL) was added and it was stirred for 5 min. The
solvent of the separated organic layer was removed in vacuo. The
oily residue was redissolved in dichloromethane (5 mL), filtered
over basic aluminum oxide and washed with dichloromethane
(30 mL). After removing the solvent in vacuo the product was
obtained as colorless oil (213 mg). The formation of trpd-3pln was
accompanied by side reactions and not all side products, which
were probably caused by a lithium halogen exchange, could be
separated and identified. So that the yield of trpd-3pln could not
be determined. Therefore, a small excess of the product was used
for the synthesis of [MoBr3(trpd-3pln)] (3).
[MoI3(trpd-2pln)] (2)
The ligand trpd-2pln (128 mg, 299 μmol) was dissolved in THF
(5 mL). This solution was added to a suspension of [MoI3(thf)3]
(200 mg, 289 μmol) and THF (15 mL). It was stirred for 42 h at room
temperature. The suspension was reduced in vacuo to a quantity of
5 mL and diethyl ether (20 mL) was added. The precipitate was
filtered, washed with diethyl ether (15 mL) and dried in vacuo. The
product was obtained as red-brown solid (159 mg, 299 μmol, 61%).
Anal. Calcd C25H35I3MoP3 (905.15): C 33.2, H 3.9, IÀ 42.1; C 33.0, H
4.0, IÀ 40.6.
~
MIR-FIR: n=3077 (sh), 3052 (w), 2970 (sh), 2936 (m), 2865 (m), 2658
(vw), 2115 (vw), 2080 (vw), 2051 (vw), 1981 (vw), 1939 (w), 1875 (w),
1825 (sh), 1809 (br w), 1636 (sh), 1616 (sh), 1587 (w), 1573 (w), 1480
(w), 1457 (w), 1435 (s), 1406 (m), 1378 (m), 1337 (w), 1317 (sh), 1303
(w), 1279 (vw), 1253 (w), 1182 (w), 1150 (w), 1139 (sh), 1113 (s),
1094 (sh), 1067 (s), 1028 (m), 999 (w), 974 (sh), 952 (br m), 912 (w),
877 (sh), 856 (s), 836 (sh), 824 (sh), 794 (sh), 740 (br s), 720 (vw), 694
(br s), 664 (sh), 617 (vw), 605 (vw), 574 (w), 553 (m), 513 (s), 498 (s),
428 (br m), 398 (sh), 313 (vw), 303 (vw), 273 (vw), 260 (vw), 248 (sh),
204 (s), 182 (sh), 160 (m), 156 (m), 145 (m), 136 (m), 127 (m), 111
31P{1H} NMR: (161.98 MHz, CDCl3, 300 K): δ=À 36.5 (s, 3 P, Ppln) ppm.
1H-NMR (400.13 MHz, CDCl3, 300 K): δ=1.83–1.59 (2 m, 22 H, CH2
(pln)), 1.64 (br d, 6 H, CqÀ CH2À Ppln), 1.54–1.46 (m, 7 H, CH2 (pln)),
1.07 (s, 3 H, CH3) ppm.
1
13C{1H}-NMR (101 MHz, CDCl3, 300 K): δ=44.0–43.6 (dt, JPC
=
3
2
20.9 Hz, JCP =8.3 Hz, 3 C, CqÀ CH2À Ppln), 37.2–36.8 (q, JCP =11.8 Hz,
1 C, CqÀ CH3), 27.7–27.4 (q, 3JCP =8.9 Hz, 1 C, CH3), 27.0–26.9 (m, 12 C,
CH2 (pln)) ppm.
(m), 98 (m), 93 (vw), 87 (m), 83 (sh) cmÀ 1
.
IR (ATR): v˜=2949 (sh), 2928 (s), 2875 (sh), 2857 (s), 1666 (w), 1640
(w), 1616 (w), 1603 (w), 1460 (s), 1445 (s), 1422 (s), 1405 (s), 1365 (s),
1315 (w), 1300 (m), 1259 (br s), 1203 (m), 1189 (sh), 1166 (s), 1110
(s), 1075 (s), 1049 (s), 1027 (s), 1008 (sh), 974 (w), 952 (w), 917 (w),
875 (sh), 861 (sh), 847 (s), 808 (br s), 792 (br s), 773 (sh), 727 (br m),
720 (br m), 680 (sh), 665 (m), 649 (s), 545 (sh), 512 (br s), 474 (br s)
[MoBr3(trpd-3pln)] (3)
The ligand trpd-3pln (207 mg, 627 μmol) was dissolved in THF
(15 mL). This solution was added to [MoBr3(thf)3] (300 mg,
543 μmol) and it was stirred for 18 h at room temperature. The
suspension was reduced in vacuo to a quantity of 5 mL and diethyl
ether (20 mL) was added. The precipitate was filtered, washed with
diethyl ether (20 mL), n-hexane (10 mL) and again with diethyl
ether (10 mL) and dried in vacuo. The product was obtained as
yellow solid (258 mg, 387 μmol, 71%).
cmÀ 1
.
[MoI3(trpd-1pln)] (1)
The ligand trpd-3pln (105 mg, 199 μmol) was dissolved in THF
(10 mL). This solution was added to a suspension of [MoI3(thf)3]
(135 mg, 195 μmol) and THF (5 mL). It was stirred for 62 h at room
temperature. The suspension was reduced in vacuo to a quantity of
6 mL and diethyl ether (15 mL) was added. The precipitate was
filtered, washed with diethyl ether (15 mL) and n-hexane (15 mL)
and dried in vacuo. The product was obtained as red-brown solid
(117 mg, 117 μmol, 60%).
Anal. Calcd C17H33Br3MoP3 (666.04): C 30.7, H 5.0, BrÀ 36.0; found C
31.2, H 5.0, BrÀ 35.6.
~
MIR-FIR: n=2978 (sh), 2936 (m), 2865 (m), 1473 (sh), 1464 (sh),
1457 (m), 1446 (m), 1437 (sh), 1419 (sh), 1406 (br m), 1399 (sh),
1388 (vw), 1380 (m), 1363 (sh), 1339(vw), 1320 (vw), 1304 (w), 1262
(br w), 1225 (vw), 1203 (sh), 1180 (w), 1111 (s), 1061 (s), 1029 (m),
950 (m), 914 (vw), 875 (sh), 857 (s), 821 (sh), 791 (vw), 764 (vw), 747
(vw), 717 (w), 675 (m), 543 (sh), 512 (m), 495 (s), 462 (sh), 280 (vw),
252 (sh), 245 (sh), 234 (vs), 228 (sh), 214 (vw), 209 (vw), 203 (w), 197
(vw), 193 (vw), 188 (vw), 182 (vw), 177 (w), 170 (w), 163 (sh), 158 (s),
152 (m), 146 (sh), 140 (w), 132 (w), 127 (vw), 122 (w), 110 (w), 105
Anal. Calcd C33H37I3MoP3 (1003.25): C 39.5, H 3.7, IÀ 38.0; found C
39.3, H 4.1, IÀ 38.1.
~
MIR-FIR: n=3078 (vw), 3055 (vw), 3029 (vw), 2974 (sh), 2955 (w),
(w), 99 (m), 95 (w), 91 (sh), 85 (br m), 80 (m) cmÀ 1
.
2932 (w), 2918 (w), 2857 (br w), 1586 (w), 1574 (w), 1483 (m), 1458
(vw), 1448 (w), 1433 (s), 1416 (w), 1413 (sh), 1402 (w), 1375 (w),
1364 (vw), 1336 (w), 1318 (w), 1305 (w), 1277 (w), 1267 (sh), 1256
(sh), 1234 (sh), 1223 (w), 1212 (vw), 1189 (w), 1158 (w), 1139 (w),
1115 (m), 1089 (br m), 1080 (vw), 1066 (m), 1048 (w), 1027 (w), 999
(w), 985 (vw), 953 (vw), 934 (vw), 914 (br w), 905 (br w), 876 (w), 854
(m), 835 (m), 815 (w), 777 (vw), 769 (vw), 752 (m), 743 (s), 723 (m),
[Mo(N2)(trpd-3pln)(dppm)] (4)
[MoBr3(trpd-3pln)] (3, 320 mg, 480 μmol) and the coligand dppm
(185 mg, 481 μmol) was added to sodium amalgam (200 mg,
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8.70 mmol Na, 2 mL of Hg) and THF (5 mL). After further THF
31P{1H} NMR (161.98 MHz, [D8]THF, 300 K): δ=18.6–17.8 (m, 2 P,
PAA’), 5.2–4.4 (m, 1 P, PXX‘), À 12.1–(À 13.0) (m, 2 P, PM) ppm.
(25 mL) was added the reaction mixture was stirred for 22 h at
room temperature under nitrogen atmosphere. The supernatant
solution was separated from sodium amalgam and the solvent was
reduced in vacuo. The received red oil was redissolved in 5 mL THF
and methanol (10 mL) was added. After reducing the solvent in
vacuo to a quantity of 5 mL, additional methanol (10 mL) was
added. The solvent was reduced in vacuo once more to a quantity
of 5 mL. The precipitate was filtered, washed with methanol
(15 mL) and dried in vacuo to obtain the product as a red solid
(29 mg). The product contains side products which were discussed
in the discussion part.
3
3
Side products: δ=33.1 (q, JPP =6.3 Hz, 2 P, 9), 30.0 (t, JPP =6.3 Hz,
3
3
3P, 9), 25.0 (d, JPP =60.0 Hz, 1 P, [HPh2PCH2PPH2]+), À 28.4 (d, JPP
=
60.0 Hz, 1 P, [HPh2PCH2PPH2]+).
Acknowledgements
The authors thank Prof. Ingo Krossing for providing us with
samples of [H(OEt2)2][Al{OC(CF3)3}4] and the spectroscopic
department of the inorganic chemistry, especially S. Pehlke and
J. Pick for measurements, as well as CAU Kiel for financial
support of this research. Open access funding enabled and
organized by Projekt DEAL.
31P{1H} NMR (161.98 MHz, [D8]THF, 300 K): δ=36.7–35.3 (m, 2 P,
PAA’), 32.1–31.4 (m, 1 P, PM), 13.3–12.0 (m, 2 P, PXX’) ppm.
Side product [Mo(dppm)3]: δ=16.0 ppm, side product 6: δ=
31.5 ppm (t, 2 P), 27.7 ppm (br t, 2 P) and À 38.8 ppm (s, 2 P).
~
MIR: n=3066 (vw), 3050 (w), 3015 (vw), 2999 (vw), 2961 (m), 2933
(m), 2907(m), 2875 (m), 2857 (m), 2005 (w), 1960 (br s), 1583 (w),
1571 (vw), 1478 (w), 1450(w), 1430 (m), 1410 (w), 1398 (sh), 1368
(w), 1318 (vw), 1301 (w), 1259 (s), 1220 (w), 1177 (br w), 1156 (vw),
1104 (sh), 1079 (s), 1063 (sh), 1018 (br s), 967 (m), 952 (sh), 917 (vw),
860 (m), 848 (m), 821 (sh), 798 (br s), 744 (m), 717 (sh), 708 (sh), 694
(s), 659 (s), 617 (m), 522 (sh), 500 (s), 486 (s), 470 (sh), 438 (s), 427 (s)
Keywords: Nitrogen Fixation · Phospholane · tripod ligands ·
Molybdenum Complexes
cmÀ 1
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Manuscript received: May 10, 2021
Revised manuscript received: June 30, 2021
Accepted manuscript online: July 7, 2021
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© 2021 The Authors. Zeitschrift für anorganische und allgemeine Chemie
published by Wiley-VCH GmbH.
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These are not the final page numbers!

ARTICLE
Dr. M. Pfeil, Dr. T. A. Engesser*, Dr. J.
Krahmer, Prof. Dr. C. Näther,
Prof. Dr. F. Tuczek*
1 – 12
Bonding and Activation of N2 in
Molybdenum(0) Complexes
Supported by Tripod Ligands with
Phospholane End Groups