Molybdenum(0) Dinitrogen Complexes Supported by Pentadentate Tetrapodal Phosphine Ligands: Structure, Synthesis, and Reactivity toward Acids

Article abstract of DOI:10.1021/acs.inorgchem.6b01255

The syntheses of two pentadentate tetrapodal phosphine (pentaPod^P) ligands, P₂^PhPP₂^Ph and P₂^MePP₂^Ph, are reported, which derive from the fusion of a tripod and a trident ligand. Reaction of the ligand P₂^PhPP₂^Ph with [MoCl₃(THF)₃] followed by an amalgam reduction under N₂ does not lead to well-defined products. The same reactions performed with the ligand P₂^MePP₂^Ph afford the mononuclear molybdenum dinitrogen complex [MoN₂(P₂^MePP₂^Ph)]. Because of the unprecedented topology of the pentaphosphine ligand, the Mo-P bond to the phosphine in the trans position to N₂ is significantly shortened, explaining the very strong activation of the dinitrogen ligand (ν_NN = 1929 cm^-1). The reactivity of this complex toward acids is investigated.

Full text of DOI:10.1021/acs.inorgchem.6b01255

Article
pubs.acs.org/IC
Molybdenum(0) Dinitrogen Complexes Supported by Pentadentate
Tetrapodal Phosphine Ligands: Structure, Synthesis, and Reactivity
toward Acids
Svea Hinrichsen,† Andrei Kindjajev,† Sven Adomeit,‡ Jan Krahmer,† Christian Nather,†
̈
and Felix Tuczek*,†
†Christian-Albrechts-Universitat Kiel, Institute of Inorganic Chemistry, Max-Eyth-Straße 2, D-24118 Kiel, Germany
̈
‡Leibniz-Institut fur Katalyse, Albert-Einstein-Straße 29a, D-18059 Rostock, Germany
̈
S
* Supporting Information
ABSTRACT: The syntheses of two pentadentate tetrapodal phosphine (pentaPodP) ligands,
Ph
P2PhPP2 and P2MePP2Ph, are reported, which derive from the fusion of a tripod and a trident
ligand. Reaction of the ligand P2PhPP2Ph with [MoCl3(THF)3] followed by an amalgam reduction
under N2 does not lead to well-defined products. The same reactions performed with the ligand
Ph
P2MePP2 afford the mononuclear molybdenum dinitrogen complex [MoN2(P2MePP2Ph)].
Because of the unprecedented topology of the pentaphosphine ligand, the Mo−P bond to the
phosphine in the trans position to N2 is significantly shortened, explaining the very strong
activation of the dinitrogen ligand (νNN
= 1929 cm−1). The reactivity of this complex toward acids
̃
is investigated.
proach.19,20 On the basis of a tungsten bis(dinitrogen) complex,
INTRODUCTION
■
a cyclic conversion of N2 to NH3 could also be
demonstrated.20,21 In recent years, related systems have been
established that allow a catalytic ammonia synthesis in a
nitrogenase-like fashion with turnover numbers of up to 60 per
mononuclear catalytic metal site.22,23 The earliest of these
systems was discovered by Schrock and co-workers in 2003 and
is based on a molybdenum complex with a sterically shielding
HIPTN3N ligand (HIPTN3N = [{3,5-(2,4,6-iPr3C6H2)2C6H3-
NCH2CH2}3N]3−).24,25 Using decamethylchromocene as a
reductant and lutidinium BArF {BArF = tetrakis[3,5-bis-
(trifluoromethyl)phenyl]borate} as a proton source with this
system, ammonia was generated from N2 in four cycles with an
overall yield of 65%.25 A few years later, Nishibayashi et al.
established a dinuclear molybdenum dinitrogen complex
supported by a pincer ligand.26−28 With a combination of
decamethylcobaltocene and lutidinium triflate, this system
generated 12 equiv of NH3 per molybdenum. More recently,
the original catalyst was modified by applying a tridentate
triphosphine instead of a PNP pincer ligand, which led to a
further increase in catalytic activity (63 equiv of NH3/Mo)
when collidinium triflate was applied as a proton source.29 In
2013, Peters et al. demonstrated that a catalytic synthesis of
ammonia from N2 is also possible based on iron complexes.30
Using KC8 and HBArF at low temperatures, up to 64 equiv of
NH3 are generated per Fe center.23
Nitrogen is substantial for every living organism as it is a
constituent of amino acids (and, thus, proteins), nucleic acids
(DNA and RNA), and nucleotides involved in energy transfer.1
Although nature provides an infinite supply of nitrogen in the
form of dinitrogen in the earth’s atmosphere, this molecule is
not bioavailable because of its highly inert triple bond.2 Only
very few organisms are able to convert dinitrogen into a useful
form, ammonia.3,4 This process, called nitrogen fixation, is
catalyzed by the enzyme nitrogenase. Although the structure of
this enzyme has been fully elucidated, the detailed mechanism
of biological dinitrogen reduction and protonation is still open
to debate.5−12 A fundamental problem in this respect relates to
the question of whether the dinitrogen molecule during its
conversion to NH3 is exclusively protonated at its β-nitrogen
atom [“asymmetric”13,14 or “distal” (D)8 pathway] or whether
protonation occurs symmetrically on both nitrogen atoms of N2
in a (more or less) alternating fashion [“symmetric”13,14 or
“alternating” (A)8 pathway (Figure 1)]. Whereas the former
pathway proceeds via a transition metal nitrido species (which
are absent in an A pathway), the latter involves diazene (HN
NH) and hydrazine (H2N−NH2) intermediates, which are
excluded from a D pathway. There is strong evidence that
nitrogenase functions via an A pathway and not a D
pathway.9,15,16
Dinitrogen can also be converted to ammonia under ambient
conditions by transition metal complexes.17,18 Using molybde-
num (bis)dinitrogen complexes with phosphine coligands,
Chatt and co-workers first successfully applied this ap-
Received: May 24, 2016
© XXXX American Chemical Society
A
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Article
involve a protonation of Nα.34 Unfortunately, an experimental
study of these steps is hampered by dissociation/association
reactions of anionic coligands or solvent molecules that have
been observed during the protonation of Mo−N2 com-
plexes,35−38 the N−N cleavage of Mo−NNH2 (or NNR2)
complexes,39−42 and the reduction/protonation of Mo−imido
complexes.43,44 A detailed mechanistic analysis of these key
reactions of nitrogen fixation in the absence of coligand or
solvent exchange reactions would require an entirely new type
of molybdenum dinitrogen complex in which the trans donor is
strapped to the ligand frame in a fashion similar to that of the
HIPTN3N ligand of Schrock, the P3PCy ligand of Mez
́
ailles
[P3PCy = tris(dicyclohexylphosphinoethyl)phosphine],45 and
the P3E ligands of Peters (E = B, C, or Si). In contrast to the
corresponding Mo(III) triamidoamine and low-valent Fe
phosphine complexes exhibiting trigonal symmetry, Mo(0)
prefers an octahedral coordination. What we were seeking
therefore was a pentadentate tetrapodal phosphine (penta-
PodP) ligand supporting Mo(0) centers in pseudo-4-fold
symmetry and allowing the bonding and reduction of N2 at a
single site in the trans position to the focal P donor. The
topology of these ligands can be regarded as resulting from the
combination of a tripod and a trident ligand that are fused at
one P donor of the tripod and the central donor atom of the
trident ligand (Figure 3, bottom). The synthesis of such ligands
has not been described to date.
In this paper, we first present a synthetic access to the
envisioned pentaPodP ligands. Then, we describe a strategy that
allows, via precoordination to a Mo(III) precursor and
subsequent amalgam reduction, the rational synthesis of
mononuclear molybdenum(0) dinitrogen complexes supported
by this novel type of ligand. The X-ray crystal structure of one
of these complexes is shown; its spectroscopic properties are
described, and its protonation to the corresponding NNH2
complex is demonstrated. These results build on our previous
experience in the synthesis of molybdenum dinitrogen
complexes supported by multidentate phosphine ligands.46−51
In these studies, a pentaphosphine environment has been
created through a combination of a tripod or a trident ligand
with a diphos ligand (or two monophosphines). It, however,
transpired that the phosphine donor in the axial position to the
N2 ligand was also prone to dissociation, again leading to
coordination of an external ligand such as the conjugated base
of the acid employed for protonation or a solvent molecule.
These reactions should be absent in a complex supported by a
pentaPodP ligand. In such a complex, the pivotal phosphine in
the trans position to N2 is strapped to the molybdenum center
by four linkages to the equatorial phosphine donors (Figure 3),
thus precluding its dissociation.
Figure 1. Distal (D) and alternating (A) pathways of nitrogenase (see
the text).8,12
Importantly, these catalytic model systems of nitrogenase
differ with respect to the mechanism of N2 reduction (distal, D,
or alternating, A). Further distinctions relate to the presence or
absence of a dissociable anionic ligand in the trans position to
N2 or its protonated/reduced derivatives NxHy (x = 1 or 2, and
y = 1−3). Moreover, the complex can function as a single-site
(i.e., mononuclear) or dual-site (i.e., dinuclear) catalyst (cf.
Table 1). The Schrock system clearly involves no dissociable
Table 1. Molybdenum and Iron Systems Active in the
Conversion of N2 to NH3 under Ambient Conditions (see
the text)
single
site
exchange of trans
system
Schrock
mechanism
distal (D)
catalytic
ligand
yes
yes
no
yes
yes
yes
no
no
no
Peters
distal and proximal
distal (D)
Nishibayashi
yes
yes
Classic Chatt predominantly D
(?)
yes
Chatt P5
distal and proximal
(?)
yes
no
no
coligand and follows a D pathway (Figure 2).31 In the iron
dinitrogen complexes developed by Peters, the trans donor is
strapped to the equatorial donors, as well, thus precluding its
dissociation. However, the corresponding reduction pathway is
probably not strictly D but also involves protonation steps at
the proximal nitrogen atom, indicating a mixed D/A pathway.23
The Nishibayshi system, finally, does contain a dissociable
ligand and appears to follow a strictly distal pathway, in analogy
to classic Chatt-type complexes that contain dissociable anionic
coligands and predominantly follow a D reduction pathway.
However, this pathway has not yet fully been elucidated
experimentally.32
Herein, the syntheses of two of such pentaPodP ligands are
described. The first, “symmetrically” substituted ligand, [2-
({bis[3-(diphenylphosphino)propyl]phosphino}methyl)-2-
methylpropane-1,3-diyl]bis(diphenylphosphine) (P2PhPP2Ph),
exhibits four diphenylphosphine end groups. The second,
“asymmetrically” substituted version, [2-({bis[3-
(diphenylphosphino)propyl]phosphino}methyl)-2-methylpro-
pane-1,3-diyl]bis(dimethylphosphine) (P2MePP2Ph), exhibits
PMe2 end groups in the tripod and PPh2 end groups in the
trident part. Coordination of these ligands to [MoCl3(THF)3]
is described. Sodium amalgam reduction of the resulting
Mo(III) precursors yields Mo(0) dinitrogen complexes that are
characterized by single-crystal X-ray crystallography and/or
spectroscopy. The steric and electronic properties of these
In spite of the apparent preference of the classic Chatt
systems for a D pathway, a full quantum chemical analysis of
the corresponding mechanistic cycle also reveals steps that
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Figure 2. Model systems for synthetic nitrogen fixation.23,25,26,33
with AIBN and diphenylphosphine afforded ethyl-bis[3-
(diphenylphosphino)propyl]phosphinate (4), which could be
53
reduced with LiAlH4 to produce 5 (Scheme 1).
Scheme 1. Synthetic Route for the Secondary Phosphine
prPPHP (5)52,53
Figure 3. Schematic illustration of the pentaPodP concept. To avoid
decoordination of the P donor trans to the N2 ligand, it is strapped to
the molybdenum center. Color code: blue for nitrogen, turquoise for
molybdenum, orange for the phosphine donor, and green for the
external ligand (conjugate base of an acid used for protonation or
solvent).
complexes are compared with those of similar Mo(0)−N2
complexes with a triphos/diphos coordination sphere. Finally,
the reactivity of a mononuclear Mo(N2)pentaPodP complex
toward acids is investigated, providing evidence of the
protonation to the NNH2 complex under retention of the
ligand environment.
Synthesis of the asymmetrically substituted pentaPodP ligand,
P2MePP2Ph, starts from 3,3′-dichloropivalic acid (6), which can
be reduced with LiAlH4 to yield 3-chloro-2-(chloromethyl)-2-
methylpropan-1-ol (ClClOH, 7). After reaction with trifluoro-
methanesulfonic anhydride,54 3-chloro-2-(chloromethyl)-2-
methylpropyl-trifluoro-methanesulfonate (ClClOTf, 8) could
be obtained. Adding lithiated prPPHP (5-Li) in THF to
ClClOTf in THF afforded (3,3′-{[3-chloro-2-(chloromethyl)-
2-methylpropyl]phosphinediyl}bis(propane-3,1-diyl))bis-
(diphenylphosphine) (ClClprPPHP, 9) (Scheme 2). In the last
step, LiPMe2 was reacted with 9 in THF, giving the
RESULTS AND DISCUSSION
■
Ligand Synthesis. For both pentaPodP ligands, the
secondary phosphine bis[3-(diphenylphosphino)propyl]-
phosphine (prPPHP, 5) is needed. The preparation of 5 starts
with ethyldiallylphosphinate (3), which was synthesized
according to the route established by Bujard et al.52 Reaction
asymmetrically substituted pentaPodP ligand P2MePP2 (10).
Ph
C
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[MoCl3(P2MePP2Ph)] (trpd = tripod) (14) (Scheme 4, top).
This hypothesis is supported by EPR spectroscopy. In fact, the
Scheme 2. Synthesis of the PentaPodP Ligand P2MePP2
(10)
Ph
a
Scheme 4. Coordination of 10 to [MoCl3(THF)3] Yields 14
a
For the synthesis of the symmetrically substituted pentaPodP
ligand 13, 1,3-dichloro-2-(chloromethyl)propane (11) was
reacted with 2 equiv of KPPh2, affording 12.55 Upon addition
Sodium amalgam reduction under N2 affords the molybdenum
dinitrogen complex 15.
of 5-Li, the pentaPodP ligand P2PhPP2 (13) was obtained
Ph
EPR spectrum of 14 qualitatively corresponds to that of the
Mo(III) complex [MoCl3(SiP3)] that is facially coordinated by
the SiP3 tripod ligand [SiP3 = tris(dimethylphosphinomethyl)-
methylsilane] (cf. Figure S5).
(Scheme 3).
Scheme 3. Synthesis of the PentaPodP Ligand P2PhPP2Ph (13)
Reaction of the symmetrically substituted ligand 13 with
[MoCl3(THF)3] under the same conditions that were used for
10 led to a product that has an elemental composition closely
corresponding to the formula [MoCl3(P2PhPP2Ph)] and an EPR
spectrum similar to that of the complex mer-[MoCl3(dpepp)]
(cf. Figure S6). Amalgam reduction of this Mo(III) precursor in
the presence of N2 gave a product that showed several IR
absorption bands (cf. Figure S8) in a spectral region where the
N−N stretch of the mononuclear complex 15 derived from
amalgam reduction of [MoCl3(P2MePP2Ph)] (14) is observed
(see below). We thus conclude that several N2-containing
species have formed and that the symmetrically substituted
Ph
ligand P2PhPP2 is unsuitable for the targeted synthesis of a
mononuclear Mo(0)−dinitrogen complex supported in a κ5
fashion by a pentaPodP ligand.
Synthesis of [MoN2(P2MePP2Ph)]. Sodium amalgam reduc-
tion of the Mo(III) precursor [MoCl3(P2MePP2Ph)] (14)
Ph
coordinated by the asymmetrically substituted ligand P2MePP2
Coordination to Mo(III) Precursors. Reaction of the
Ph
asymmetrically substituted pentaPodP ligand P2MePP2 (10)
cleanly leads to the mononuclear molybdenum dinitrogen
complex [MoN2(P2MePP2Ph)] (15) (Scheme 4, bottom).
Importantly, both IR (ATR) and Raman spectra show a N−
N stretching vibration of 1929 cm−1 (Figure 4), which
corresponds to the lowest value of the N−N stretch ever
found for a molybdenum monodinitrogen complex with a
pentaphosphine ligation (cf. ref 48 for a compilation of N−N
stretching frequencies). The reduction was also performed with
15N2 gas, affording [Mo15N2(P2MePP2Ph)] ([15N]-15), which
exhibits the N−N stretch at 1868 cm−1 (cf. Figure 4, red). The
isotopic shift of Δexp = 61 cm−1 closely conforms to the value
with Mo(III) precursors was achieved by stirring a mixture of
10 and [MoCl3(THF)3] in THF and dichloromethane.56,57
Coordination to the Mo(III) center can in principle occur with
the tripod or the trident part of 10. While the former
necessarily leads to a fac geometry, the latter is known to result
in mer complexes.49,58
Because of the stronger σ donor strength and the smaller
steric demand of the dimethylphosphine as compared to that of
the diphenylphosphine groups, the tripod part should
coordinate selectively to [MoCl3(THF)3], yielding fac(trpd)-
D
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The metal-mediated trans coupling constant is ∼83 Hz, while
the cis coupling constants vary from approximately −19 to 29
Hz. In the 31P{1H} NMR spectrum of [15N]-15, all multiplets
exhibit an additional splitting due to the 15N2 ligand, which is
most significant for the axial phosphine group (cf. Figure S9).
The 15N NMR spectrum of [15N]-15 exhibits two resonances at
−28.75 and −18.64 ppm, corresponding to Nα and Nβ of the
coordinated N2 ligand, respectively (cf. Figure S10). As there
are only very few examples for 15N-substituted molybdenum
dinitrogen complexes with a P5 coordination, an assessment of
this result regarding the possible influence of the trans-
phosphine donor on the 15N chemical shift is not yet possible.
Crystal Structure of 15. Compound 15 crystallizes in
space group P21/n with all atoms located in general positions.
In agreement with the information from NMR spectroscopy,
Ph
the pentaPodP ligand P2MePP2 coordinates to the Mo center
in a pentadentate tetrapodal fashion with the N2 ligand being
bound to position 6. The Mo−Nα distance is 2.033(5) Å, and
the Nα−Nβ distance amounts to 1.099(5) Å, which is in fact
slightly longer compared to that of elemental N2 (1.0975 Å).60
The Mo−Peq distances in the equatorial plane are all very
similar, ranging from 2.4433 to 2.4536 Å, whereas the Mo−Pax
bond trans to the N2 ligand is distinctively shorter [2.3868 Å
(Table S1)].
For comparison, crystal structure data from other molybde-
num dinitrogen complexes supported by a combination of
triphos and diphos ligands46,49−51 are listed in Table 2. These
systems exhibit Mo−Peq distances in the range of 2.44−2.46 Å
and Mo−Pax distances in the range of 2.42−2.47 Å; in contrast
to 15, no significant shortening of the axial Mo−P bond is
observed. In the complex [MoN2(SiP3)(dppm)] (16), the
Figure 4. IR (ATR) (top) and Raman (bottom) spectra of
[MoN2(P2MePP2Ph)] (15, black) and [Mo15N2(P2MePP2Ph)] (red).
predicted for an isolated N−N stretch in the harmonic
oscillator approximation (Δtheo = 65 cm−1).
As expected, the 31P{1H} NMR spectrum of 15 exhibits an
AA′XX′M pattern. All coupling constants could be derived
from an analysis of the AA′ and XX′ half-spectra according to
the method described in ref 59 and were double-checked by
comparison of the resulting simulation and the measured
spectrum (Figure 5).
Figure 5. 31P{1H} NMR spectrum of 15 in benzene-d6. The resulting AA′XX′M spectrum (bottom) and enlargements of the measured signals
(middle) as well as the simulated spectrum (top) are shown.
E
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Table 2. Comparison of Selected Geometrical Parameters Retrieved from Single-Crystal Structures of Molybdenum Dinitrogen
Complexes with a P3/Diphos Coordination Sphere and the PentaPodP Ligand (P2 = dmpm; P2′ = dppm)
Mo−Pax bond (2.4741 Å) is even longer than the average value
of the Mo−Peq distance {2.4392 Å; SiP3 = tris-
[(dimethylphosphino)methyl]methylsilane}. This complex is
closely related to 15 as it is supported by a tripod ligand with
three dimethylphosphine end groups and a diphos ligand with
two diphenylphosphine donors. With respect to this complex
that exhibits Mo−Peq distances comparable to those of 15, the
Mo−Pax bond length is reduced by 0.0873 Å in the
[Mo(N2)(pentaPodP)] system.
The axial phosphine in 15 thus is strongly bound to the
Figure 6. Single-crystal structure of [MoN2(P2MePP2Ph)] (15). The
hydrogen atoms have been omitted for the sake of clarity. An ORTEP
plot of 15 can be found in Figure S9. Selected bond lengths
(angstroms) and angles (degrees): Mo(1)−N(1), 2.033(5); N(1)−
N(2), 1.099(5); Mo(1)−P(2), 2.3868(12); Mo(1)−P(5), 2.4440(13);
Mo(1)−P(1), 2.4518(13); Mo(1)−P(4), 2.4433(14); N(1)−Mo(1)−
P(2), 170.93(11); N(2)−N(1)−Mo(1), 174.1(4).
molybdenum center by its central trialkylphosphine donor.
Remarkably, this does not cause an elongation of the Mo−N
bond in the trans position as the Mo−N distance of 15 (2.033
Å) stays in the lower range of Mo−N distances observed for
the Mo−N2 complexes with triphos/diphos ligation [2.025−
2.099 Å (cf. Table 2)]. The shortening of the Mo−Pax bond
enforced by the topology of the pentaPodP ligand thus is
responsible for the observed increase in the level of activation
of 15 with respect to 16 (νNN
= 1952 cm−1), which has a
̃
phosphine donor set similar to that of 15 (see above) but lacks
the strapping of the phosphine donor in the axial position to
the N2 ligand.
ligand but in the presence of other, sterically less demanding
groups also gives rise to the observed tilt.
A similar tilt of the N2 ligand is observed in the complex
The Mo−N−N bond angle of 174.15° and the Pax−Mo−N
angle (170.93°) show that the N2 ligand of 15 is tilted from the
diphenylphosphine toward the smaller dimethylphosphine
groups. This effect is stronger than in related systems having
a diphosphine coligand terminated by diphenylphosphine
groups in the equatorial plane (cf. Table 2). In these systems,
two phenyl rings above and two below the equatorial plane
point at the N2 ligand; i.e., only two phenyl rings point in the
direction of the N2 ligand. By contrast, three of the four phenyl
rings are directed toward the dinitrogen ligand in 15 (cf. Figure
6), which is due to the fact that these groups are connected to
the axial position of the complex via alkyl bridges. The
corresponding rotation of the PPh2 groups around the
equatorial Mo−P axes leads to a better shielding of the N2
[ M oN 2 ( d p e p p ) ( d p p m ) ] ( 1 9 ) { d p e p p
= b i s -
[(diphenylphosphino)ethyl]phenylphosphine},61−64where, in
contrast to the C3 bridges present in 15, the diphenylphosphine
end groups of the trident ligand are connected via C2 bridges to
the axial P donor. In this case, the Peq−Mo−Pax angles are
reduced to an average value of 80.15°, whereas in 15, both of
these angles are close to 90°. This demonstrates that the C3
bridges better match the octahedral geometry of 15 than the C2
bridges in the dpepp complex. Moreover, the equatorial P−
Mo−P angles of 15 are closer to 90° than in the systems
supported by a triphos/diphos ligand combination. The
distortion of octahedral geometry induced by the tripod part
of 15, on the other hand, is comparable to that observed for
other tripod-supported systems.
F
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Figure 7. NH region (left) of solution-phase IR spectra of 15 in dichloromethane (top, black) and 15-NNH2 generated with HBArF (bottom, red) at
300 K. In situ IR spectroscopic monitoring (right) of the addition of a CH2Cl2 HBArF solution (75 μL = 1 equiv) to 15 at 250 K.
Reactivity of 15 toward Acids. To investigate the first
critical steps of the Chatt cycle, namely the two protonations
yielding the NNH2 complex, the reactivity of [15N]-15 toward
different acids has been examined. For these studies, two acids
known from protonation reactions50 or catalytic experiments23
(see above) have been employed, i.e., trifluoromethanesulfonic
acid (HOTf) and HBArF. 15N substitution thereby allows
monitoring of the protonation reactions by 15N NMR
spectroscopy. When [15N]-15 is exposed to HBArF in
dichloromethane at −20 °C, the corresponding NNH2 complex
is formed. The signals in the 31P{1H} NMR spectrum undergo
a high-field shift compared to the signals in that of [15N]-15,
which is most significant for the trans-phosphine [Δ = 45.9
ppm (cf. Figure S11)]. Importantly, this signal still shows the
additional splitting derived from coupling with the 15N nuclei of
the N2 ligand. In addition, 15N INEPT as well as 15N−1H
HSQC experiments have been performed with HBArF in
CD2Cl2 (cf. Figures S12 and S13). The measured 15N NMR
shift of −234 ppm indicates the presence of a NNH2 complex
exhibiting a doubly protonated Nβ atom.65,66 The NNH2
complex is thermally unstable at 270 K as it starts decaying
during the NMR measurements, leading to the formation of
paramagnetic species as evidenced by very broad signals and the
Surprisingly, if HOTf is added to [15N]-15 in THF-d8, no
15N coupling can be found, either in the 31P{1H} spectrum or
in the 15N NMR spectrum. Mock et al. made the same
observation with the chromium dinitrogen complex mentioned
above.67 To determine whether this effect is due to the
employed acid (HOTf) or ligand exchange with THF, the
protonation was performed with HOTf in the weakly
coordinating solvent CD2Cl2. Under these conditions, the 15N
coupling with the trans-phosphine donor was again observable
(cf. Figure S11), suggesting that in THF solution ligand
exchange has in fact occurred. Nevertheless, the resulting
NNH2 complex with OTf− as a counterion is also thermally
unstable as paramagnetic species are formed during the NMR
measurement.
SUMMARY AND CONCLUSION
■
In the preceding sections, novel pentadentate tetrapodal
phosphine ligands and the first molybdenum dinitrogen
complex supported by such a ligand, [MoN2(P2MePP2Ph)],
have been presented. These pentaPodP ligands derive from a
combination of a tripod and a linear trident ligand and were
synthesized in two versions, a “symmetrically” substituted one
terminated by four diphenylphosphine groups (P2PhPP2Ph) and
an “asymmetrically” substituted one terminated by two
diphenylphosphine and two dimethylphospine groups
(P2MePP2Ph). Because coordination of P2PhPP2Ph to the Mo(III)
precursor [MoCl3(THF)3] followed by amalgam reduction
under N2 did not lead to well-defined products, we conclude
that the differential nucleophilicities of the donor groups
1
paramagnetically shifted protons in the H NMR spectrum (cf.
Figure S15).
Further information regarding the protonation of coordi-
nated dinitrogen can be obtained from vibrational spectrosco-
py. As attempts to isolate the NNH2 complex as a solid were
unsuccessful, a liquid IR experiment was performed in
dichloromethane at room temperature. After addition of
HBArF to 15, the N−N stretch disappears and N−H stretching
vibrations appear in the expected spectral region, i.e., at ∼3200
cm−1 (Figure 7, left).49 To obtain further information about
this reaction, the protonation was additionally conducted at a
low temperature (250 K) (Figure 7, right) and monitored in
situ by IR spectroscopy. After addition of 2 equiv of HBArF
(=150 μL), two N−H stretches can clearly be assigned at 3198
and 3179 cm−1. Interestingly, the spectrum after protonation
also shows a band at 2008 cm−1, which is a typical value for the
N−N stretch in a hydrido-N2 complex (cf. Figure S14), but
further work will be required to verify that this band in fact
corresponds to a hydrido-N2 species similar to the chromium
hydrido complex recently reported by Mock et al.67
Ph
present in the P2MePP2 ligand are crucial with respect to a
targeted synthesis of a Mo dinitrogen complex supported by a
pentaPodP ligand. As the tripod part provides the more
nucleophilic and less sterically demanding dimethylphosphine
groups compared to the diphenylphosphine moieties in the
trident part, we anticipated that the mononuclear complex
fac(trpd)-[MoCl3(P2MePP2Ph)] (trpd = tripod) (14) is
selectively obtained at the Mo(III) stage (Scheme 4 top).
This assumption is supported by EPR spectroscopy.
Amalgam reduction of 14 under N2 leads to coordination of
the formerly unbound diphenylphosphine groups to the metal
center. This generates the κ5-pentaphosphine Mo(0) mono-
dinitrogen complex [MoN2(P2MePP2Ph)] (15), which exhibits
the lowest N−N stretch ever found for a molybdenum
G
DOI: 10.1021/acs.inorgchem.6b01255
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
could not be removed because of the extreme viscosity of 4. Therefore,
the analysis has been recalculated for nine molecules of product and
one molecule of EE. Anal. Calcd for C292H341O20P27: C, 70.0; H, 6.9.
dinitrogen complex with phosphine coligands (νNN = 1929
̃
cm−1). First, this can be ascribed to the combination of an
alkylphosphine donor in the trans position with two
dimethylphosphine donors in the equatorial plane. However,
there is an additional activation of the N2 ligand in 15 with
̃
Found: C, 69.7; H, 6.9. IR (ATR): ν 3050 (m, CH arom.), 2979, 2933,
2898 (m, CH aliph.), 1584 (m, C−C), 1480, 1453, 1433 (P−C) cm−1.
1H NMR (400 MHz, C6D6, 300 K, TMS): δ 7.40−7.27 (m, 20H, H
arom.), 3.90 (quintet, 2H, 3J = 7.06 Hz, P-O-CH2-CH3), 2.10 [m, 4H,
P-(CH2-CH2-CH2)2], 1.80−1.59 [m, 8H, P-(CH2-CH2-CH2-PPh2)2],
respect to the complex [MoN2(SiP3)(dppm)] (νNN = 1952
̃
cm−1), which is also supported by three alkylphosphines in the
tripod part and two arylphosphines in the diphos part.46
Importantly, this activation enhancement can be traced back to
the unprecedented ligand architecture of 15, which enforces a
shortening of the Mo−P bond in the trans position of the N2
ligand by strapping this P atom to the four equatorial
phosphine donors. In contrast, the Mo−P bond to the trans-
phosphine donor is elongated in the complex [Mo(N2)(SiP3)-
(dppm)] supported by the tripodal SiP3 ligand, which also
exhibits three alkylphosphine donors. In this complex, the trans-
phosphine is connected to the ligand framework by only one
linkage.
3
1.16 (t, 3H, J = 7.04 Hz, P-O-CH2-CH3). 31P{1H} NMR (161.98
MHz, CDCl3, 300 K, H3PO4): δ 49.79 (t, 4JPP = 2.0 Hz, 1P, EtO-P
4
O), −16.90 (d, JPP = 2.0 Hz, 2P, PPh2). 13C{1H} NMR (100 MHz,
CDCl3, 300 K): δ 138.09 (m, Cq), 132.72 (m, Carom.), 128.52 (m,
Carom.), 60.12 (d, J = 6.88 Hz, OEt), 29.32 [m, P-(CH2-CH2-CH2-
PPh2)2], 18.78 [m, P-(CH2-CH2-CH2-PPh2)2], 16.62 (m, CH3).
prPPHP (5). Ethyl bis[3-(diphenylphosphino)propyl]phosphinate
(4, 6.30 g, 11.5 mmol) was dissolved in 40 mL of diethyl ether and the
mixture slowly added to a suspension of 5.22 g (138 mmol) of
lithiumaluminumhydride in 80 mL of diethyl ether under ice cooling.
The reaction mixture was stirred for 48 h followed by very slow
addition of 10 mL of degassed water, 20 mL of a 15% degassed sodium
hydroxide solution, and again 10 mL of degassed water under ice
cooling. The white precipitate was filtered off and washed with 2 × 50
mL of diethyl ether. The filtrate was concentrated in vacuo yielding a
colorless oil. Yield: 3.99 g (8.12 mmol, 72%). Anal. Calcd for
Protonation experiments employing HBArF and HOTf as
acids indicate that 15 can be transformed to the corresponding
[Mo(NNH2)(P2MePP2Ph)]2+ complex with retention of penta-
phosphine ligation, demonstrating that this system is able to
mediate the first reactions of the Chatt cycle. The next steps in
our investigation of this system will involve the generation of
further intermediates based on the new Mo(pentaPodP)
platform and the investigation of this system regarding the
formation of ammonia from N2.
̃
C30H33P3: C, 74.1; H, 6.8. Found: C, 74.0; H, 7.1. IR (ATR): ν 3050
(m, CH arom.), 2925 (m, CH aliph.), 2267 (m, PH), 1480, 1432 (s, P-
aryl, P-alkyl) cm−1. 1H NMR (400 MHz, C6D6, 300 K, TMS): δ 7.58−
7.17 (m, 20H, R-PPh2), 3.56−2.59 (td, 1H, P-H), 2.08 [t, 4H, PH-
(CH2-CH2-CH2-PPh2)2], 1.81−1.39 [m, 8H, 2 PH-(CH2-CH2-CH2-
PPh2)2]. 31P{1H} NMR (161.98 MHz, C6D6, H3PO4, 300 K): δ
−16.93 (s, 2P, PPh2), −72.96 (s, 1P, PH). 13C{1H} NMR (100 MHz,
C6D6, 300 K): δ 138.68 (d, JC−P = 12.97 Hz, P-C-CH-CH-CH),
132.72 (d, JC−P = 18.45 Hz, P-C-CH-CH-CH), 128.56 (s, P-C-CH-
CH-CH), 128.44 (d, JC−P = 6.64 Hz, P-C-CH-CH-CH), 29.38 [dd,
JC−P = 8.27 Hz, JC−P = 12.15 Hz, PH-(CH2-CH2-CH2-PPh2)2], 24.69
[dd, JC−P = 9.92 Hz, JC−P = 17.33 Hz, PH-(CH2-CH2-CH2-PPh2)2],
21.76 [dd, JC−P = 10.27 Hz, JC−P = 12.98 Hz, PH-(CH2-CH2-CH2-
PPh2)2].
EXPERIMENTAL SECTION
■
General Information. All syntheses were conducted under a N2
atmosphere, and the solvents were dried and freshly distilled under
argon prior to use. All other reagents that were used were
commercially available. [MoCl3(THF)3],68 LiPMe2·0.5Et2O,69 Me-
(PPh2)2Cl,55 and ethyldiallylphosphinate52 were prepared as described
in the literature. Elemental analyses were performed using a Euro
Vector CHNSO-element analyzer (Euro EA 3000). Samples were
burned in sealed tin containers in a stream of oxygen. NMR spectra
were recorded with a Bruker AVANCE III HD Pulse Fourier
Transform spectrometer operating at frequencies of 400.13 MHz
(1H), 161.98 MHz (31P), 100.62 MHz (13C), 376.50 MHz (19F), and
40.56 MHz (15N). Referencing was performed either using the solvent
residue signal (5.32 ppm for CD2Cl2, 7.16 ppm for C6D6, and 1.72 and
3.58 ppm for THF-d8) or TMS (δ1H = 0 ppm), 85% H3PO4 (δ31P = 0
ppm), and CH3NO2 (δ15N = 0 ppm) serving as substitutive standards.
FT Raman spectra were recorded with a Bruker IFS 666/CS NIR FT-
Raman spectrometer. Infrared spectra were recorded on a Bruker
ALPHA FT-IR Spectrum with Platinum ATR setup. Solution-phase IR
spectroscopy was performed with a Bruker Vertex 70 spectrometer at a
resolution of 2 cm−1. In situ IR spectroscopy was performed with a
Nicolet iS10 instrument from Thermo Scientific using a modified
Gateway-ATR-IR reaction system from specac. Room-temperature
continuous wave X-band EPR spectra were recorded with an EMXplus
spectrometer with a PremiumX microwave bridge and an HQ X-band
cavity controlled by a computer running the Bruker Xenon Software.
Samples were transferred and measured in a 0.3 mm quartz flat cell
under an inert atmosphere.
ClClOH (7). A solution of 10.1 g (59.4 mmol) of 3,3′-
dichloropivalic acid (6) in 60 mL of diethyl ether was added at 0
°C to a suspension of 4.06 g (107 mmol) of lithiumaluminumhydride
in 60 mL of diethyl ether. The reaction mixture was stirred for 2 h at
room temperature and then poured carefully into 500 mL of 0.5 M
HCl. The layers were separated, and the organic layer was extracted
with 3 × 50 mL of diethyl ether and washed with 3 × 50 mL of water.
The combined organic layers were dried over magnesium sulfate, and
the solvent was removed in vacuo. Purification was achieved by
distillation (10−1 bar, 58 °C), yielding a colorless liquid. Yield: 7.78 g
(49.5 mmol, 83%). Anal. Calcd for C5H10Cl2O: C, 38.2; H, 6.4.
̃
Found: C, 37.9; H, 6.3. IR (ATR): ν 3371 (OH), 2958, 2933, 2875
1
(CH) cm−1. H NMR (400 MHz, CD2Cl2, 300 K, TMS): δ 3.49 (m,
4H, CH2Cl), 3.48 (d, 2H, 3J = 5.4 Hz, CH2OH), 1.83 (t, 1H, 3J = 5.5
Hz, CH2OH), 0.99 (s, 3H, CH3). 13C{1H} NMR (100 MHz, CD2Cl2,
300 K): δ 65.28 (s, CH2OH), 48.65 (s, CH2Cl), 42.10 (s, quart. C),
18.15 (s, CH3).
ClClOTf (8). To a solution of 5.00 g (31.8 mmol) of 3-chloro-2-
(chloromethyl)-2-methylpropan-1-ol (7) in 300 mL of dichloro-
methane was added 15 mL (186 mmol) of pyridine. The reaction
mixture was cooled to −15 °C, and a solution of 10.0 g (35.4 mmol)
of trifluoromethanesulfonic anhydride in 30 mL of dichloromethane
was added dropwise. After being stirred for 4 h at −15 °C, the reaction
mixture was poured into 600 mL of an ice-cooled saturated sodium
bicarbonate solution. The aqueous layer was extracted three times with
100 mL of dichloromethane. The combined organic layers were dried
over magnesium sulfate, and the solvent was removed in vacuo. The
product was purified by distillation (10−2 mbar, 58 °C). Yield: 4.20 g
(14.5 mmol, 46%). Anal. Calcd for C6H9Cl2F3O3S: C, 24.9; H, 3.1.
Ethyl Bis[3-(diphenylphosphino)propyl]phosphinate (4).
Ethyldiallylphosphinate (3, 10.0 g, 53.8 mmol) and diphenylphosphine
(25.0 mL) were mixed with one tip of a spatula AIBN and irradiated
with a construction site spotlight for 7 days. A tip of a spatula AIBN
was added every day. Excess diphenylphosphine was removed in vacuo
at 170 °C, and the residue was purified by column chromatography
(3:1 ethyl acetate/cyclohexanes with 2% methanol; Rf = 0.5) giving a
slightly yellowish viscous oil. Yield: 11.1 g (20.3 mmol, 38%). Anal.
Calcd for C32H37P3O2: C, 70.3; H, 6.8. Found: C, 69.7; H, 6.9. The
product still contains ethyl acetate in a 9:1 ratio (product:EE) that
Found: C, 25.0; H, 3.0. IR: ν 2978, 2963 (m, CH-valence aliph.), 1466
̃
1
(CH), 1413, 1245, 1203, 1142 (s, SO) cm−1. H NMR (400 MHz,
CD2Cl2, 300 K, TMS): δ 4.43 (d, 4J = 0.4 Hz, 2H, CH2OTf), 3.50 (m,
H
DOI: 10.1021/acs.inorgchem.6b01255
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Article
4H, CH2Cl), 1.14 (s, 3H, CH3). 13C{1H} NMR (100 MHz, CD2Cl2,
300 K): δ 136.37 (s, CF3), 77.63 (s, CH2OTf), 47.06 (s, CH2Cl), 41.62
(s, quart. C), 18.06 (s, CH3).19F NMR (376.50 Hz, CD2Cl2, CFCl3,
300 K): δ −74.74 (s, 3F).
Calcd for C58H59P5: C, 76.6; H, 6.7. Found: C, 77.0; H, 7.1. IR (ATR):
̃
ν 3067, 3049 (m, CH arom.), 2922, 2801 (m, CH aliph.), 1570 (m,
1
CC), 1479, 1451, 1432 (s, PC) cm−1. H NMR (400 MHz, CD2Cl2,
300 K, TMS): δ 7.47−7.21 (m, 40H, arom.), 2.41 (dd, 4H, 1JHH = 14.3
ClClprPPHP (9). To a solution of 2.00 g (4.11 mmol) of prPPHP
(5) in 20 mL of THF was added 1.64 mL (4.11 mmol) of n-
butyllithium (2.5 M in hexanes) at 0 °C. The reaction mixture was
stirred for 30 min at 0 °C and slowly added at −78 °C to a solution of
1.19 g (4.11 mmol) of 8 in 20 mL of THF. The cold bath was
removed and the reaction mixture stirred for 90 min. Twenty milliliters
of degassed water and 20 mL of diethyl ether were added, and the
mixture was stirred for an additional 30 min. The aqueous layer was
removed with a syringe, and the solvent was removed in vacuo. The
residue was resolved in toluene and filtered over silica gel to give the
product as a colorless viscous oil. Yield: 2.23 g (3.56 mmol, 87%).
Anal. Calcd for C35H41Cl2P3: C, 67.2; H, 6.6. Found: C, 67.2; H, 6.9.
2
Hz, JPH = 3.1 Hz, CH2PPh2 trpd), 2.03 (m, 4H, CH2Pcentr trident),
2
1.62 (d, 2H, JPH = 4.5 Hz, CH2Pcentr trpd), 1.49−1.33 (m, 8H, 2 ×
CH2-CH2-PPh2), 0.91 (s, 3H, CH3). 31P{1H} NMR (161.98 MHz,
CD2Cl2, H3PO4, 300 K): δ −17.24 (s, 2P, PPh2 trident), −25.91 (d,
4
2P, JP−P = 2.5 Hz, PPh2 tripod), −43.00 (m, 1P, central P). 13C{1H}
NMR (100 MHz, CD2Cl2, 300 K): δ 140.03−140.11 (m, Cq arom.),
139.29−139.06 (m, Cq arom.), 133.23−132.61 (m, C PPh2 meta),
128.79−128.26 (m, PPh2 ortho, para), 42.93−42.43 (m, CH2PPh2
tripod), 42.09−41.86 (m, CH2Pcentr.), 38.36 (m, Cq tripod), 29.98−
29.37 (m, 2 × CH2-CH2-CH2-PPh2), 29.06 (m, CH3), 22.50 (m, 2 ×
CH2-CH2-CH2-PPh2).
̃
IR (ATR): ν 3051 (m, CH arom.), 2930, 2891 (m, CH aliph.), 1586
MoCl3(P2MePP2Ph) (14). A mixture of 1.30 g (1.92 mmol) of
1
(w, CC), 1481, 1433 (m, PC) cm−1. H NMR (400 MHz, CD2Cl2,
300 K, TMS): δ 7.34−7.20 (m, 20H, H arom.), 3.41 (m, 4H, CH2-Cl),
2.02 (m, 4H, CH2-Pcentr.), 1.50−1.35 (m, 8H, 2 × PPh2-CH2-CH2),
Ph
P2MePP2 (10) and 887 mg (1.92 mmol) of [MoCl3(THF)3] was
dissolved in 20 mL of THF and 10 mL of dichloromethane and stirred
overnight. The solution was concentrated in vacuo, and 20 mL of
diethyl ether was added. The yellow precipitate was filtered off and
washed with 5 mL of THF, diethyl ether, and n-hexane. Yield: 1.43 g
(1.63 mmol, 85%). Anal. Calcd for C39H53P5MoCl3: C, 53.3; H, 6.1.
1
1.32 (d, JPH = 4.24 Hz, 2H, CH2-P tripod), 0.93 (s, 3H, CH3).
31P{1H} NMR (161.98 MHz, CD2Cl2, H3PO4, 300 K): δ −16.79 (s,
2P, PPh2), −42.46 (s, 1P, Pcentr.). 13C{1H} NMR (100 MHz, CD2Cl2,
300 K): δ 139.03 (dd, Cq arom.), 132.77 (dd, Cortho), 128.56 (m,
3
2
Cmeta,para), 51.32 (d, JCP = 10.6 Hz), 40.23 (d, JCP = 13.9 Hz, Cq
̃
Found: C, 53.0; H, 6.0. IR (ATR): ν 3052 (m, CH arom.), 2909 (m,
1
CH aliph.), 1587 (w, CC), 1484, 1434 (s, PC) cm−1.
tripod), 34.37 (d, JCP = 19.0 Hz, CH2-Pcentr. tripod), 29.67 (m, P-
CH2CH2CH2PPh2), 22.41 (dd, JCP = 14.2, 17.2 Hz, P-
2
MoN2(P2MePP2Ph) (15). A portion of 300 mg (0.34 mmol) of
[MoCl3(P2MePP2Ph)] (14) was dissolved in 20 mL of THF and the
mixture added to sodium amalgam prepared from 2 mL of Hg and 200
mg of sodium. The reaction mixture was stirred overnight under a
nitrogen atmosphere. Stirring was stopped, and the remaining mercury
was collected in a neck of the flask. The red solution was carefully
filtered over a frit to avoid any residue of the mercury, and the solvent
of the filtrate was removed in vacuo. The residue was resolved in
diethyl ether and filtered over neutral alumina. The bright yellow
solution was concentrated to dryness in vacuo, yielding an orange
solid. Single crystals could be grown from benzene-d6 by slow
evaporation. Yield: 118 mg (0.147 mmol, 43%). Anal. Calcd for
C39H53P5MoN2: C, 58.5; H, 6.7; N, 3.5. Found: C, 58.6; H, 7.0; N, 1.3.
The nitrogen value is too low because of the thermal instability of the
3
CH2CH2CH2PPh2) 21.73 (d, JCP = 10.0 Hz, CH3).
P2MePP2 (10). A portion of 520 mg (4.74 mmol) of LiPMe2·
Ph
0.5Et2O was dissolved in 20 mL of THF at −78 °C, and 1.90 mL (4.74
mmol) of n-butyllithium (2.5 M in hexanes) was added. After the
mixture had been stirred for 5 min, a solution of 1.20 g (1.92 mmol) of
ClClprPPHP (9) in 15 mL of THF was added and the reaction
mixture was stirred for 15 min at −78 °C. Afterward, cooling was
stopped, and the mixture was stirred for an additional 90 min followed
by the addition of 10 mL of degassed water and 10 mL of diethyl
ether. After the mixture had been stirred for 1 h, the aqueous layer was
removed with a syringe and the solvent removed in vacuo. The residue
was resolved in n-pentane and filtered over Celite, yielding a colorless
viscous oil. Yield: 1.40 g (1.83 mmol, 95%) Anal. Calcd for C39H53P5:
̃
C, 69.2; H, 7.9. Found: C, 69.4; H, 8.3. IR (ATR): ν 3068, 3050, 3000
(m, CH arom.), 2950, 2925, 2892, 2811 (m, CH aliph.), 1584 (w,
product. IR (ATR): ν
aliph.), 1929 (s, NN), 1584 (w, CC), 1480, 1451, 1430 (m, PC) cm−1.
Raman: νNN
1930 cm−1. 31P{1H} NMR (161. 98 MHz, C6D6, H3PO4,
̃
3049 (m, CH arom.), 2955, 2912, 2852 (m, CH
1
CC), 1480, 1432 (s, PC) cm−1. H NMR (400 MHz, CDCl3, 300 K,
TMS): δ 7.43−7.27 (m, 20H, H arom.), 2.10 (m, 4H, CH2Pcentr.),
1.49−1.37 [m, 14H, CH2P(tripod), 2 × CH2CH2PPh2], 0.89−0.86
[m, 15H, 2 × P(CH3)2, CH3 backbone]. 31P{1H} NMR (161.98 MHz,
CDCl3, H3PO4, 300 K): δ −16.43 (m, 2P, PPh2), −41.76 (m, 1P,
Pcentr.), −60.83 (m, 2P, PMe2). 13C{1H} NMR (100 MHz, CDCl3, 300
K): δ 139.59 (m, ArC-1), 133.18 (ddd, J = 18.53, 7.10, 1.8 Hz, ArC-
2,6), 128.98 (d, J = 4.61 Hz, ArC-4), 128.90 (d, J = 6.54 Hz, ArC-3,5),
̃
300 K): δ 25.52 (dddd, J = 83.4, 14.7, −19.8, 20.4 Hz, 2P, PPh2), 12.40
(tt, J = 20.4, 28.7 Hz, 1P, Pcentr.), −3.69 (dddd, J = 83.4, 27.4, −19.8,
28.7 Hz, 2P, PMe2). A sample of [15N]-15 was prepared following an
analogous synthetic procedure using 15N2 instead of 14N2. IR (ATR): ν
̃
3050 (m, CH arom.), 2957, 2925, 2852 (m, CH aliph.), 1868 (s, NN),
1
3
3
48.05 {ddd, JCP = 14.9 Hz, JCP = 8.5 Hz, JCP = 8.4 Hz, C-[CH2-
1585 (w, CC), 1480, 1453, 1432 (m, PC) cm−1. Raman: νNN
1868
̃
1
2
P(CH3)2]2}, 42.67 (dt, J = 16.74 Hz, J = 8.54 Hz, C-CH2-P), 38.11
cm−1. 31P{1H} NMR (161.98 MHz, C6D6, H3PO4, 300 K): δ 25.79
(td, 2JCP = 13.4 Hz, 2JCP = 11.8 Hz, Cq tripod), 30.56 [dd, 1JCP = 12.68
(m, 2P, PPh2), 12.74 (m, 1P, Pax), −3.48 (m, 2P, PMe2). 15N{1H}
Hz, 3JCP = 12.71 Hz, 2C, P-(CH2-CH2-CH2-PPh2)2], 30.13 [dd, 1JCP
=
1
12.14 Hz, 3JCP = 12.12 Hz, P-(CH2-CH2-CH2-PPh2)2], 28.89 (td, JCP
3
NMR (40.56 MHz, C6D6, CH3NO2, 300 K): δ −18.6 (d, 1N, JNN
6.9 Hz, Nβ), −28.8 (m, 1N, Nα).
=
3
2
2
= 8.71 Hz, JCP = 8.69 Hz, CH3), 23.03 [dd, JCP = 17.0 Hz, JCP
=
[Mo15NNH2(P2MePP2Ph)]X2 (X = BArF or OTf) (15NNH2-15).
Protonation was performed as a NMR experiment. In a typical run, 10
mg of [15N]-15 was dissolved in 0.3 mL of the NMR solvent in a glass
vial and cooled to −20 °C. In another vial, the acid was dissolved in
0.2 mL of the NMR solvent and also cooled to −20 °C. The acid
solution was added quickly to the complex solution, resulting in a
change in color. The reaction mixture was transferred to a NMR tube
a n d m e a s u r e d i m m e d i a t e l y a t 2 7 0 K . D a t a f o r
14.2 Hz, P-(CH2-CH2-CH2-PPh2)2], 16.40 [m, 2 × P(CH3)2].
Ph
P2PhPP2 (13). To a solution of 290 mg (0.59 mmol) of prPPHP
(5) in 15 mL of THF was added 0.38 mL (0.94 mL) of n-butyllithium
(2.5 M in hexanes) under ice cooling. The resulting orange solution
was stirred for 60 min at room temperature. The reaction mixture was
slowly added at 0 °C to a solution of 375 mg (0.79 mmol) of
methyl(2-methyldiphenylphosphino)-1-chloro-3-diphenylphosphino-
propane (12) in 20 mL of THF and the mixture stirred overnight
while being warmed to room temperature. After the addition of 10 mL
of degassed water, the solution was stirred for 30 min and 10 mL of
diethyl ether was added. The water was removed with a syringe and
the organic phase dried in vacuo. The product was purified by column
chromatography (3:1 toluene/cyclohexanes; Rf = 0.05), yielding a
colorless highly viscous oil. Yield: 250 mg (0.27 mmol, 34%). Anal.
[Mo15NNH2(P2MePP2Ph)](BArF)2. IR (CH2Cl2, 300 K): νNH
̃
3223,
3177 cm−1. νNN is masked by other vibrations. 31P{1H} NMR (161.98
̃
MHz, CD2Cl2, H3PO4, 270 K): δ 8.95 (m, 2P, PPh2), −15.24 (m, 2P,
PMe2), −33.09 (m, 1P, Paxial). 15N INEPT (40.56 MHz, CD2Cl2,
CH3NO2, 300 K): δ −233.67 (m, 1N, -NβH2).
I
DOI: 10.1021/acs.inorgchem.6b01255
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Inorganic Chemistry
Article
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01255.
■
S
Synthetic attempts regarding the coordination of
P2PhPP2Ph, EPR spectra, spectroscopic details for
[Mo15N(P2PhPP2Ph)] and the protonation experiments,
and an ORTEP plot (PDF)
Selected crystal structure data of 15 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: ftuczek@ac.uni-kiel.de. Telephone: ++49 (0)431 880
1410. Fax: ++49 (0)431 880 1520.
■
Funding
F.T. thanks CAU Kiel for funding of this research.
Notes
The authors declare no competing financial interest.
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Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2014, 5, 3737.
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40.
ACKNOWLEDGMENTS
■
F.T. thanks the Bruckner group of the Likat in Rostock for the
̈
in situ IR measurements.
(34) Stephan, G. C.; Sivasankar, C.; Studt, F.; Tuczek, F. Chem. - Eur.
J. 2008, 14, 644−652.
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