Molybdenum dinitrogen complexes supported by a silicon-centred tripod ligand and dppm or dmpm: Tuning the activation of N2

Article abstract of DOI:10.1039/c3dt52965g

Two molybdenum dinitrogen complexes with a novel pentaphosphine environment have been synthesised and structurally characterised. A combination of the tripodal alkylphosphine ligand tris(dimethylphosphinomethyl)methylsilane (SiP₃) and the coligands dppm/dmpm has been employed to tune the activation of the N₂ ligand.

Full text of DOI:10.1039/c3dt52965g

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Molybdenum dinitrogen complexes supported by
a silicon-centred tripod ligand and dppm or
dmpm: tuning the activation of N2†
Cite this: Dalton Trans., 2014, 43,
2007
Henning Broda, Svea Hinrichsen, Jan Krahmer, Christian Näther and Felix Tuczek*
Received 21st October 2013,
Accepted 5th November 2013
Two molybdenum dinitrogen complexes with a novel pentaphosphine environment have been syn-
thesised and structurally characterised. A combination of the tripodal alkylphosphine ligand tris-
(dimethylphosphinomethyl)methylsilane (SiP3) and the coligands dppm/dmpm has been employed to
tune the activation of the N2 ligand.
DOI: 10.1039/c3dt52965g
www.rsc.org/dalton
of the derived N2 complexes and the activation of the N2
ligand. It could be deduced that for the coordination of such
Introduction
The bonding and activation of the highly inert dinitrogen ligands in a κ3-fashion the presence of an apical methyl group
molecule is still one of the most challenging topics in bio- is necessary. In the investigated complexes the activation
inorganic and organometallic chemistry.1–6 In synthetic nitro- of the N2 ligand turned out to be significantly increased
gen fixation the conversion of atmospheric N2 to ammonia when replacing diphenylphosphine by diisopropylphosphine
under ambient conditions is of particular interest.7 Chatt et al. donors. However, this increase in activation came with an
developed the first mechanistic scenario for this process based intrinsic steric hindrance. Thus, it emerged obligatory in this
on molybdenum and tungsten bis(dinitrogen) phosphine com- concept to use the smallest alkylphosphine donor available,
plexes comprising several protonation and reduction steps.8 namely dimethylphosphine.
We have performed extensive spectroscopic and theoretical
investigations on the elementary steps of this cycle.9–17 Impor-
tantly, there exist some side reactions which prevent the con-
version of N2 from being conducted in a catalytic manner. For
Results and discussion
instance, the intermediate Mo(I) complex coordinated by the
conjugate base X of the employed acid undergoes a dispropor-
bones through substitution of alkyl chlorides by LiPMe2 due to
As PMe2 donors cannot easily be introduced into ligand back-
tionation reaction, which implies a 50% loss of catalyst at each
cycle.18,19 Another issue is the instability of Mo–P bonds at
higher oxidation states of the molybdenum centre.11
the difficulty of handling HPMe2, we decided to employ the
silicon-centred tripod ligand tris(dimethylphosphinomethyl)-
methylsilane (SiP3) in this work. SiP3 is readily obtained in
perfect purity through threefold substitution of the chlorides
in MeSiCl3 by LiCH2PMe2 (Scheme 1, top left) and subsequent
distillation, as described by Karsch et al. and Boncella
et al.23,24
Metalation of the tridentate phosphine ligand was achieved
by adding the ligand to an equimolar amount of [MoCl3(thf)3]
suspended in tetrahydrofuran and stirring at room tempera-
ture for several days yielding [MoCl3(SiP3)] (1, Scheme 1, top
right). Note that to form the complex [MoCl3(SiP3)] with the
tripod ligand bound facially a change of geometry must occur
from the meridional coordination of the metal precursor. The
synthesis of molybdenum N2 complexes 2 and 3 could be per-
formed by the well-established sodium amalgam reduction
(Scheme 1, reaction (a)). [Mo(N2)(SiP3)(dppm)] (2) (dppm = bis-
(diphenylphosphino)methane) and [Mo(N2)(SiP3)(dmpm)] (3)
(dmpm = bis(dimethylphosphino)methane) were obtained by
To overcome these drawbacks, our group investigated
tripod ligands which occupy the pivotal position trans to N2,
thus avoiding the formation of [MoI(N2)(X)L4] species prone
to disproportionation.20,21 Along these lines, we recently
designed tripod ligands with mixed aryl- and alkylphosphine
donors based on neopentyl and isobutyl backbones.22 Employ-
ing this set of ligands it could be shown that the apical group
(H or CH3) as well as the substituents of the phosphine donors
(PiPr2 and PPh2) had a crucial impact on both the constitution
Christian-Albrechts-Universität zu Kiel, Max-Eyth-Straße 2, 24118 Kiel, Germany.
E-mail: ftuczek@ac.uni-kiel.de; Fax: +0049 (0)431 1520; Tel: +0049 (0)431 1410
†Electronic supplementary information (ESI) available: Crystallographic details
for 2; tables with EPR and NMR assignments and (hyperfine) coupling con-
stants. CCDC 960643. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt52965g
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Scheme 1 Synthesis and metalation of SiP3 (top); generation of N2 com-
plexes 2, 3 (bottom) by (a) classic amalgam reduction and (b) triflate route.
Fig. 2 IR (black) spectra of 2 (top) and 3 (bottom). The N–N stretches
are indicated in bold. For 2 a Raman spectrum could not be obtained
due to luminescence.
coordination environment. The facially bound SiP3 ligand is
complemented by dppm and the N2 ligand. The N–N distance
is found to be only 0.938 Å, which is shorter than in free N2
(1.097 Å). This is counterintuitive as weakening of the triple
bond is supposed to increase the bond length rather than
shorten it. However, this feature can be found in other metal
N2 complexes as well.25–27 To explain the short N–N bond,
other effects might also be taken into account like, e.g., a libra-
tion of the terminally bonded N2 ligand. This consideration is
endorsed by the fact that the Mo–N bond is found to be
slightly longer than in comparable structures. The Mo–P dis-
tances, in contrast, are in the usual range (cf. Mo–P distances
in [Mo(N2)(tdppme)(dmpm)],21 [Mo(N2)(PMe3)5]28 or [Mo(N2)2-
(depe)2]29).
The fact that the N2 ligands of both 2 and 3 are strongly
activated is inferred from the position of the N–N stretching
vibration at 1943 and 1952 cm−1, respectively (Fig. 2). The
Raman spectrum of 2 shows the N–N stretch at the same posi-
tion as in the IR spectrum, in agreement with the presence of
only one N2 ligand in these systems.
Fig. 1 Solid-state structure of [Mo(N2)(SiP3)(dppm)] (2); hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and angles (°): Mo–
N1 2.099(3), N1–N2 0.938(4), Mo–P1 2.4293(6), Mo–P2 2.4596(6), Mo–
P3 2.4741(7), Mo–P4 2.4236(6), Mo–P5 2.4443(6), N1–Mo–P3 171.15(6),
P1–Mo–P2 68.83(2), P4–Mo–P5 90.35(2), P2–Mo–P5 101.83(2), P1–
Mo–P4 98.81(2), P1–Mo–P3 95.52(2), P2–Mo–P3 97.69(2), P3–Mo–P5
88.60(2), P4–Mo–P5 90.35(2).
adding a slurry of the Mo(III) precursor in tetrahydrofuran to
The phosphine coordination of the molybdenum centres in
NaxHg under a nitrogen atmosphere in the presence of a 2 and 3 is conveniently analysed by 31P NMR spectroscopy. The
slightly substoichiometric amount of dmpm or dppm. The N2 geometry of the five phosphine donors gives rise to an AA′XX′M
complexes turned out to be relatively sensitive to vacuum and coupling pattern.22 The recorded spectra each comprise two
additionally split AA′XX′ subspectra of second order and a
unpolar solvents, so mild work-up conditions had to be
applied. The products were characterised by IR, Raman and triplet of triplets arising from the M nucleus which is
NMR spectroscopy. For the more stable 2 an X-ray crystal represented by the PMe2 donor trans to the N2 ligand. For both
2 and 3 all resonances could be assigned (Fig. 3 and 4). Coup-
ling constants were calculated and double-checked by com-
structure could be obtained, showing a distorted octahedral
environment of the molybdenum centre (Fig. 1).
In comparison to the structure of [Mo(N2)(tdppme)(dmpm)] parison of the recorded and simulated spectra (cf. ESI†). The
metal-mediated trans-coupling constants turned out to be
(tdppme
= 1,1,1-tris(diphenylphosphinomethyl)ethane) the
P–Mo–P angles within the metal-tripod framework of
2
around 100 Hz, whereas the backbone- and metal-mediated
are closer to 90°,21 indicating a less trigonally elongated cis-coupling constants are significantly smaller, in this case
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Fig. 3 Experimental (bottom) and simulated (top) 31P NMR spectrum of
[Mo(N2)(SiP3)(dppm)] (2). Assignment of the nuclei corresponds to the
colours of the insets.
Fig. 5 Experimental (bottom) and simulated (top) X-band EPR spec-
trum of [Mo(SiP3)(dmpm)(OTf)](OTf)2 (4) in tetrahydrofuran solution.
indicated the reaction of the intermediate molybdenum tri-
flato complex [Mo(OTf)3(SiP3)] with the diphosphine yielding
[Mo(SiP3)(dmpm)(OTf)](OTf)2 (4; Scheme 1, reaction (b1)). Iso-
lation of the latter complex proved difficult due to its extremely
high sensitivity. Nevertheless, an EPR spectrum could be
recorded and reproduced by simulation on the basis of a
[MoP2P′2P] core (Fig. 5; cf. ESI† for derived fitting parameters).
The constitution of the pentaphosphine-ligated triflato
complex 4 was further proven by subjecting it to a sodium
amalgam reaction. Aside from sodium triflate impurities the
IR spectrum of the product emerged as identical to that of
[Mo(N2)(SiP3)(dmpm)] (3) obtained via the classic route (a).
Reactivity towards Lewis acids
Of particular interest in the field of metal dinitrogen com-
plexes is the enabling of the activated N2 ligand to react with
electrophiles, e.g. Brønsted or Lewis acids.31 However, exposure
of 2 and 3 to strong acids did not yield detectable amounts of
Mo–NNH2 derivatives, most likely due to the incompatibility of
the silyl backbone with mineral acids.32 We therefore added
the Lewis acid AlMe3 to a C6H6 solution of [Mo(N2)(SiP3)-
(dppm)] (2) until a colour change occurred.33 In benzene solu-
Fig. 4 Experimental (bottom) and simulated (top) 31P NMR spetrum of
[Mo(N2)(SiP3)(dmpm)] (3). Assignment of the nuclei corresponds to the
colours of the insets.
between 4 and 30 Hz. These values are comparable to those
determined for analogous systems.15,21,22,30 As the features in
the 31P NMR spectrum of [Mo(N2)(SiP3)(dmpm)] (3) are very
similar to those of 2 for which a crystal structure could be
obtained we conclude that the structure of 3 corresponds to
that of 2.
tion the N–N stretching frequency of 2 is found at 1988 cm−1
.
Upon addition of AlMe3 a considerable shift of the N2 stretch
from 1988 to 1875 cm−1 is observed. This most likely orig-
inates from the formation of the Lewis acid–base adduct
[Mo(N2AlMe3)(SiP3)(dppm)] (Fig. 6). The influence of the
Triflate route
In the sodium amalgam route (Scheme 1, (a)) the penta- adduct formation is also visible in the 31P NMR spectrum.
phosphine ligation is completed during the reduction to Mo(0). While the chemical shift of the equatorial P donors is only
Another promising concept is to establish the P5 environment slightly altered in 2-AlMe3 as compared to the parent N2
already at the Mo(III) stage. This would circumvent the fact that complex 2 (ΔδPPh2 = −0.3 ppm and ΔδPMe2 = −1.4 ppm), the
during the classic sodium amalgam reaction three reduction signal of the PMe2 donor trans to N2 is shifted by 11.7 ppm to
steps and three critical ligand exchanges have to occur in a higher field (Fig. 7). In general, however, the relative high-field
concerted fashion without any possibility of influencing the shifts of all P donors upon reaction with AlMe3, in particular
course of this reaction. We explored the accessibility of a with that of the donor trans to N2, are less pronounced in
pentaphosphine coordination to Mo(III) by treating a slurry of 2-AlMe3 than in hydrazido complexes resulting from proto-
[MoCl3(SiP3)] (1) in tetrahydrofuran with an excess of silver nation of their N2 analogs, e.g. [Mo(NNH2)(tdppme)(dmpm)]2+,
triflate to substitute the chloride ligands by labile triflate [Mo(NNH2)(trpd-1)(dmpm)]2+ or [Mo(NNH2)(dpepp)(dppm)]2+
ligands. After separating the silver halide an equimolar (trpd-1 = H3CC(CH2PPh2)2(CH2PiPr2); dpepp = PhP(CH2CH2-
amount of dmpm was added. An immediate colour change PPh2)2).21,22,34
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Fig. 6 Liquid IR spectra of benzene solutions of [Mo(N2)(SiP3)(dppm)]
(blue) and the Lewis acid–base adduct [Mo(N2AlMe3)(SiP3)(dppm)] (red).
Negative absorption is caused by displacement of benzene by the
analyte.
Fig. 8 N2 stretch in [Mo(N2)(tdppme)(dppm)], [Mo(N2)(tdppme)(dmpm)],
[Mo(N2)(SiP3)(dppm)] (2) and [Mo(N2)(SiP3)(dmpm)] (3; see text).
Table 1 N–N stretching vibrations of aryl- and alkylphosphine (mono)-
dinitrogen complexes with a pentaphosphine environment
Compound
νNN/cm−1
[Mo(N2)(tdppme)(dppm)]21
[Mo(N2)(tdppme)(dmpm)]21
[Mo(N2)(dpepp)(dppm)]36
[Mo(N2)(dpepp)(PMe2Ph)]36
[Mo(N2)(dpepp)(dmpm)]36
[Mo(N2)(trpd-1)(dmpm)]22
[Mo(N2)(PMe3)5]28
2035
1980
1978
1978
1966
1965
1950
1950
[Mo(N2)(dmpm)(PMe3)3]37
Fig. 7 Experimental (bottom) and simulated (top) 31P NMR spetrum of
[Mo(N2AlMe3)(SiP3)(dppm)] (2-AlMe3). Assignment of the nuclei corres-
ponds to the colours of the insets.
[Mo(N2)(PMe3)5]. This indicates that besides electronic influ-
ences the activation of N2 in transition-metal complexes also
sensitively depends upon steric factors.22 En route to low-valent
molybdenum complexes an Mo(III) precursor with P5 ligation
has been generated whose structure could be supported by
EPR spectroscopy for the first time. Finally, it has been demon-
strated that the N2 ligand in 2 is enabled to form an adduct
with the Lewis acid AlMe3. In molybdenum chemistry, this
type of functionalisation so far had only been evidenced for
bis(dinitrogen) complexes.33,35
Conclusions
A new and versatile concept has been developed to synthesise
mono(dinitrogen) complexes with a pentaphosphine environ-
ment. In the new complexes 2 and 3 this particular ligation
has been realised by a combination of a tripod ligand with
dppm and dmpm, respectively. This way the activation of the
dinitrogen ligands has been significantly increased with
respect to existing systems having this or a similar ligation
(Fig. 8 and Table 1); i.e., starting from the complex [Mo(N2)-
(tdppme)(dppm)] having five diphenylphosphine groups and
Experimental section
All reactions were performed under a N2-atmosphere using
Schlenk techniques. Sample preparation was carried out in a
MBRAUN glovebox. Solvents were dried and freshly distilled
under argon prior to use. Tris(dimethylphosphinomethyl)-
methylsilane23 (SiP3) and [MoCl3(thf)3]38 were prepared accord-
ing to the published procedures.
an N2 stretch of 2040 cm−1 21
the activation of N2 (as inferred
,
from the N–N stretching frequency) could be tuned in a sys-
tematic and stepwise manner exchanging two, three and five
diphenylphosphine groups with dimethylphosphine moieties.
In the highest activated system, [Mo(N2)(SiP3)(dmpm)], the
N–N stretch (1943 cm−1) is even lower than in [Mo(N2)(PMe3)5]
(1950 cm−1).28 While both complexes comprise an almost
identical coordination environment with methyl phosphine
donors, the tripod/diphos combination introduces additional
steric constraints as compared to the highly symmetric
Elemental analysis
CHNS analyses were performed using a Euro Vector CHNS-O
element analyzer (Euro EA 3000). Samples were burned in
sealed tin containers in a stream of oxygen.
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Spectroscopic characterization
[Mo(N2)(SiP3)(dppm)] (2): elemental analysis calcd (%) for
C35H49MoN2P5Si: C, 54.1; H, 6.4; N, 3.6; found: C, 52.4; H, 6.7;
1H, 13C, 29Si and 31P NMR spectra were recorded at 300 K with N, 3.1; IR (ATR): 3054, 2961, 1952, 1582, 1430 cm−1
.
a Bruker AVANCE 400 Pulse FT spectrometer and referenced
either to the solvent residual signal (CH2Cl2 = 5.32 ppm, C6D6 not be obtained due to the oily character of the material; IR
= 7.16 ppm) or to TMS (δ1H = 0 ppm) and 85% H3PO4 (δ31P
(ATR): 2955, 2897, 2810, 1943, 1460, 1414 cm−1
0 ppm) as substitutive standards. Spectral simulations were [Mo(SiP3)(dmpm)(OTf)](OTf)2 (4). 350 mg (1.36 mmol,
For [Mo(N2)(SiP3)(dmpm)] (3) an elemental analysis could
=
.
performed using the MestReC program package (MestreLab 4 equiv.) silver triflate were placed in a flask and dried under
Research, Santiago de Compostella, Spain). Room temperature vacuum at 90 °C for 2 hours. After cooling to room tempera-
cw X-band EPR spectra were recorded with an EMXplus ture a slurry of 0.34 mmol (1 equiv.) [MoCl3(SiP3)] in 10 ml
spectrometer with a PremiumX microwave bridge and an HQ tetrahydrofuran was added and the mixture refluxed for 5 min.
X-band cavity controlled by a computer running the Bruker The mixture was set aside for 10 min to ensure the precipi-
Xenon Software. Samples were transferred and measured in a tation of silver chloride. The green supernatant was passed
0.3 mm quartz flat cell under an inert atmosphere. Spectra through a syringe filter and then 46.3 mg (0.34 mmol, 1 equiv.)
were simulated using MATLAB (MathWorks Inc.) employing dmpm in a minimal amount of tetrahydrofuran were added.
the “garlic” simulation function for fast-motion cw EPR The solution containing the product was subsequently sub-
spectra of the EasySpin toolbox (v. 4.0.0).39 Infrared spectra jected to EPR spectroscopy or sodium amalgam reduction
were recorded on a Bruker Alpha FT-IR Spectrometer with since purification of the triflato complex turned out to be
Platinum ATR setup. Liquid IR spectra were measured with a impractical due to the inherent instability of the sample.
Bruker Vertex 70 FT-IR spectrometer in a CaF2 cell with a
resolution of 4 cm−1
Tris(dimethylphosphinomethyl)methylsilane
[Mo(NNAlMe3)(SiP3)(dppm)]. 20 mg (0.026 mmol) [Mo(N2)-
(SiP3)(dppm)] (2) were dissolved in 1 ml benzene and a 1.6 m
(SiP3). The solution of trimethylaluminium in heptane was added drop-
.
compound was synthesised in 65% yield as described by wise until the color changed to green. The benzene solution
Karsch et al.23 1H NMR (400 MHz, C6D6): δ = 1.00 (d, J = was directly subjected to liquid IR spectroscopy without
3.4 Hz, 18H, PCH3), 0.76 (d, J = 1.6 Hz, 6H, CH2), 0.30 (pseudo further purification.
q, J = 0.7 Hz, 3H, SiCH3) ppm. 13C{1H CPD} NMR (101 MHz,
C6D6): δ = 19.26 (ddd, J = 30.2 Hz, J = 5.5 Hz, J = 4.2 Hz, CH2),
17.99 (m, PCH3), −0.14 (q, J = 4.8 Hz, SiCH3) ppm. 29Si{1H}
(79 MHz, C6D6): −0.3 (SiCH3) ppm. 31P{1H} NMR (162 MHz,
C6D6): δ = −54.82 (s, PMe2) ppm.
Notes and references
[MoCl3(SiP3)] (1). 10 ml tetrahydrofuran were added
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a
flask containing 300 mg (1.12 mmol) SiP3 and
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