Octahedral molybdenum(0) monodinitrogen complexes facially coordinated by the tripodal ligand 1,1,1-tris(diphenylphosphanylmethyl)ethane-influence of diphosphane coligands on the activation of N2

Article abstract of DOI:10.1002/ejic.201100640

The synthesis and physicochemical properties of the new molybdenum dinitrogen complexes [Mo(N₂)(tdppme)(dmpm)] (2) and [Mo(N ₂)(tdppme)(dppm)] (3) are reported. Complexes 2 and 3 are facially coordinated by the tripodal ligand 1,1,1-

Full text of DOI:10.1002/ejic.201100640

FULL PAPER
DOI: 10.1002/ejic.201100640
Octahedral Molybdenum(0) Monodinitrogen Complexes Facially Coordinated
by the Tripodal Ligand 1,1,1-Tris(diphenylphosphanylmethyl)ethane – Influence
of Diphosphane Coligands on the Activation of N2
Jan Krahmer,[a] Henning Broda,[a] Christian Näther,[a] Gerhard Peters,[a] Wulf Thimm,[a] and
Felix Tuczek*[a]
Keywords: Molybdenum / Nitrogen / P ligands / Nitrogen fixation / Tripodal ligands
The synthesis and physicochemical properties of the new
molybdenum dinitrogen complexes [Mo(N2)(tdppme)-
(dmpm)] (2) and [Mo(N2)(tdppme)(dppm)] (3) are reported.
Complexes 2 and 3 are facially coordinated by the tri-
podal ligand 1,1,1-tris(diphenylphosphanylmethyl)ethane
(tdppme) and contain the bidentate coligands bis(dimethyl-
phosphanyl)methane (dmpm) and bis(diphenylphosphanyl)-
(triflic acid, HOTf) leads to the NNH2 complex [Mo(NNH2)-
(tdppme)(dmpm)](OTf)2 (4) with retention of the pentaphos-
phane coordination. The structural, electronic and vi-
brational properties of 2, 3 and 4 have been investigated by
NMR, IR and Raman spectroscopy coupled with DFT calcula-
tions. The crystal structure of 2 has been determined and is
discussed in relation to the calculated structures of 2 and 3.
methane (dppm), respectively. They are accessible by amal- The results of the investigations are compared with those ob-
gam reduction of the MoIII precursor [MoBr3(tdppme)] (1) un-
der nitrogen in the presence of dppm and dmpm, respec-
tively. Protonation of 2 with trifluoromethanesulfonic acid
tained earlier for the pentaphosphane complexes [Mo(N2)-
(dpepp)(dppm)] and [Mo(NNH2)(dpepp)(dppm)](OTf)2
[dpepp = bis(diphenylphosphanylethyl)phenylphosphane].
ammonia being produced per molybdenum atom of the cat-
alyst.[16] Detailed insight into the mechanism of this reac-
tion is, however, difficult due to the presence of five dinitro-
gen ligands (four terminal, one bridging) in the dinuclear
Mo0 complex.[17]
Introduction
One of the fundamental reactions of nature is the fixa-
tion of molecular nitrogen as ammonia in plants and bacte-
ria. This reaction proceeds at a unique iron/molybdenum/
sulfur cluster, the FeMoco, which is located in the MoFe
protein of the enzyme nitrogenase.[1–6] There have been nu-
merous attempts to reproduce biological nitrogen fixation
by small-molecule model systems.[7,8] Two well-established
mechanistic schemes in this regard are represented by the
Schrock and Chatt cycles, which are based on molybdenum
triamidoamine and phosphane complexes, respectively. In
both cases the Mo–dinitrogen complex can be converted to
complexes of NNHx and NHx species by a series of proton-
ation and reduction steps, which ultimately lead to ammo-
nia.[9,10] For the Schrock cycle a catalytic conversion of N2
to ammonia has been demonstrated in 6–8 cycles with an
overall yield of 65%.[11–13] It is a challenge to achieve a
catalytic action with a comparable efficiency for Chatt-type
molybdenum–phosphane complexes.[14,15] Recently, a dimo-
lybdenum–dinitrogen complex bearing two PNP pincer li-
gands [PNP = 2,6-bis(di-tert-butylphosphanylmethyl)pyr-
idine] was found to work as an effective catalyst for the
formation of ammonia from dinitrogen, with 12 equiv. of
Our group has been involved in a detailed experimental
and theoretical investigation of the Chatt cycle for a
number of years.[18] One of the major mechanistic problems
of this cycle is connected with the exchange of one of the
two dinitrogen ligands of the parent Mo0 bis(dinitrogen)
complex by the conjugate base of the acid employed for the
protonation reaction. In particular, the presence of anionic
coligands leads to disproportionation reactions at the MoI
stage and renders the formation of the pivotal Mo0 bis(dini-
trogen) complex difficult. One of the synthetic strategies to
cope with this problem involves the design of ligands occu-
pying the trans position of the dinitrogen ligand in the Mo0
complex. This concept has already been pursued by George
and co-workers through the synthesis of molybdenum dini-
trogen complexes supported by linear tridentate phosphane
ligands of the type Ph2P–(CH2)2–E–(CH2)2–PPh2
(PEP).[19,20] In the spirit of this approach, Mo–N2–PNHP
(E = NH), –PNMeP (E = NMe), –PSP (E = S) and –POP
(E = O) complexes have been prepared and charac-
terized.[21] The majority of data, however, have been ob-
tained from complexes with the dpepp ligand (E = PPh).[22]
These systems have been synthesized with bidentate phos-
phane coligands such as dppe or dppm. We have augmented
this series with the synthesis and characterization of
[a] Institut für Anorganische Chemie, Christian-Albrechts-
Universität zu Kiel
Max-Eyth-Str. 2, 24118 Kiel, Germany
E-mail: ftuczek@ac.uni-kiel.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100640.
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[Mo(N2)(dpepp)(depe)] and [Mo(N2)(dpepp)(1,2-dppp)] the noncentrosymmetric space group Pna21 with Z = 4 mo-
[1,2-dppp = Ph2PCH(CH3)CH2PPh2].[23] As first demon- lecules in the unit cell (Table 1). The tripodal ligand tdppme
strated by George et al. and supported by our own experi- coordinates facially to the molybdenum centre, and the
ments, [Mo(N2)(dpepp)(dppm)] can be protonated by triflic three remaining coordination sites are occupied by one bi-
acid with retention of the pentaphosphane ligation, thus dentate dmpm ligand and one molecule of dinitrogen (Fig-
making this system a candidate for the catalytic conversion ure 1). The coordination sphere of 2 is octahedral with a
of N2 to ammonia on the basis of Mo–phosphane systems. trigonal distortion. The Mo–P bond lengths of the phos-
A corresponding mechanistic cycle has been evaluated by phane ligands vary between 2.3986(16) and 2.4454(16) Å
DFT.[24] The stoichiometric production of ammonia by pro- (Table 2) and thus are slightly shorter than those of
tonation of [Mo(N2)(dpepp)(diphosphane)] complexes has [Mo(N2)(dpepp)(dppm)], which lie between 2.418(2) and
extensively been investigated by George et al.[25]
2.497(2) Å (Table 3).[22] The Mo–N bond length of 2 is
A fundamental problem of Mo(dpepp)(diphosphane)–di- 2.066(6) Å, which is slightly longer than that in [Mo(N2)-
nitrogen complexes is the existence of two isomers. Specifi- (dpepp)(dppm)] [2.025(6) Å]. On the other hand, the N–N
cally, the central P donor of the dpepp ligand can coordi- bond length of 2 [1.069(8) Å] is somewhat shorter than that
nate in the trans or cis position to the dinitrogen ligand, in [Mo(N2)(dpepp)(dppm)] [1.119(8) Å].[22]
which leads to the ortho- and iso forms of the complexes,
respectively.[23] Both isomers can be distinguished by the
Table 1. Summary of crystallographic data for 2.
degree of activation imparted to the dinitrogen ligand. In
order to avoid this complication, we decided to employ trip-
odal ligands, which can only bind in one configuration. So
far, this strategy has not been applied to molybdenum dini-
trogen complexes, and it is the purpose of this paper to fill
this gap. To this end, the synthesis and characterization of c [Å]
[Mo(N2)(tdppme)(dmpm)] (2) and [Mo(N2)(tdppme)-
Formula
C46H53MoN2P5
884.69
orthorhombic
Pna21
21.763(3)
13.972(2)
14.253(2)
4333.8(7)
170
Mw [gmol–1]
Crystal system
Space group
a [Å]
b [Å]
V [Å3]
T [K]
(dppm)] (3) are reported. Both complexes can be prepared
Z
4
by Nax/Hg reduction of the MoIII compound
[MoBr3(tdppme)] (1) under N2. A new synthesis for this
important precursor is also presented. The structural, elec-
tronic and vibrational properties of 2 and 3 are determined,
and their reactivities towards acids are evaluated. The re-
sults are compared with those obtained earlier in the inves-
Dcalcd. [gcm–3]
1.356
0.522
25.00
25417
6746
0.0981
5561
μ [mm–1]
θmax [º]
Measured reflections
Unique reflections
Rint
Reflections [F0 Ͼ 4s(F0)]
tigation of [Mo(N2)(dpepp)(diphosphane)] systems, and the Parameters
488
R1 [F0 Ͼ4s(F0)]
0.0482
0.1167
1.003
implications on the reduction and protonation of N2 in
Chatt-type systems are discussed.
wR2 [all data]
GOF
Δrmax, Δrmin [eÅ–3]
0.576, –0.839
Results and Discussion
[MoBr3(tdppme)] (1)
In order to obtain dinitrogen complexes supported by
the tripodal ligand 1,1,1-tris(diphenylphosphanylmethyl)
ethane (tdppme), we started from the molybdenum(III) pre-
cursor [MoBr3(THF)3]. The published synthesis of 1 by
bromination of [Mo(CO)3(tdppme)][29] gave us unsatisfac-
tory results. Instead, by heating a mixture of tdppme and
[MoBr3(THF)3] in toluene to reflux, [MoBr3(tdppme)] (1)
was obtained in high yield and purity.
[Mo(N2)(tdppme)(dmpm)] (2) and [Mo(15N2)(tdppme)
(dmpm)] (2a)
Figure 1. Molecular structure of [Mo(N2)(tdppme)(dmpm)] (2) (hy-
drogen atoms omitted, phenyl groups drawn as wireframes).
By reduction of 1 with sodium amalgam in THF under
N2 in the presence of bis(dimethylphosphanyl)methane
The P–Mo–P angles of 2 lie between 82.30(5) and
(dmpm), the dinitrogen complex [Mo(N2)(tdppme)(dmpm)] 86.00(6)° and thus are significantly smaller than 90°, which
(2) was formed. Single crystals suitable for an X-ray struc- indicates a trigonal elongation of the octahedral coordina-
ture determination were grown by recrystallization from a tion sphere. Their values closely agree with the P–Mo–P
toluene/n-hexane mixture. Orthorhombic 2 crystallizes in angles in [Mo(tdppme)(CO)3].[30] The coordinated tripodal
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Molybdenum(0) Monodinitrogen Complexes with a Tripodal Ligand
Table 2. Selected bond lengths [Å] and angles [°] for [Mo(N2)- it was not possible to determine the coupling constants. The
(tdppme)(dmpm)] (2).
multiplet at 39.90–39.11 ppm is assigned to the AAЈ atoms
Pc and Pd, and the multiplet at 40.48–40.08 ppm is assigned
to the phosphorus atom Pe (B) of the tripodal ligand. The
corresponding shifts are comparable to those of the tripo-
dal ligand in 3 (see below). The multiplet at –22.54 to
–23.54 ppm belongs to the XXЈ atoms Pa/Pb of the dmpm
ligand. The negative shift of the diphos signal is compatible
with the higher shielding of the phosphorus nuclei by the
methyl groups of dmpm relative to that of the diphenyl-
phosphane groups of the tripodal ligand. The assignment of
Bond lengths
Angles
Mo(1)–N(1) 2.066(6)
Mo(1)–P(2) 2.3986(16)
Mo(1)–P(1) 2.4363(15)
Mo(1)–P(3) 2.4454(16)
Mo(1)–P(4) 2.5033(16)
Mo(1)–P(5) 2.5101(16)
N(1)–Mo(1)–P(2)
N(1)–Mo(1)–P(1)
P(2)–Mo(1)–P(1)
N(1)–Mo(1)–P(3)
P(2)–Mo(1)–P(3)
P(1)–Mo(1)–P(3)
N(1)–Mo(1)–P(4)
P(2)–Mo(1)–P(4)
P(1)–Mo(1)–P(4)
P(3)–Mo(1)–P(4)
N(1)–Mo(1)–P(5)
P(2)–Mo(1)–P(5)
P(1)–Mo(1)–P(5)
P(3)–Mo(1)–P(5)
P(4)–Mo(1)–P(5)
N(2)–N(1)–Mo(1)
C(1)–C(2)–P(1)–Mo(1)
C(1)–C(3)–P(2)–Mo(1)
C(1)–C(4)–P(3)–Mo(1)
91.54(17)
93.04(15)
82.30(5)
177.37(17)
86.00(6)
85.73(5)
N(1)–N(2)
1.069(8)
84.32(15)
105.73(5)
171.57(6)
97.22(6)
85.16(17)
172.11(5)
104.99(6)
97.40(6)
66.86(6)
176.7(6)
38.309(7)
25.883(7)
32.453(7)
1
the signals was supported by the H-31P HMBC spectrum
(Figure S2). In particular, the coupling of the phenyl pro-
tons with the tripodal ligand P atoms, Pc, Pd and Pe, is
observed. The coupling between the P atoms Pa and Pb of
the dmpm ligand with its methyl protons is also visible.
Specific information with respect to the activation of N2
in dinitrogen complexes can be obtained from vibrational
spectroscopy.[32] Complex 2 exhibits a N–N stretch at
1980 cm–1 (Tables 4 and 5 and Figure 2). Therefore, the ac-
tivation of the N2 ligand is stronger than that in 3 (see be-
low) and comparable to that in [Mo(N2)(dpepp)(dppm)]
(Table 4).[22] The stronger activation of 2 relative to that of
3 results from the electron-donating effect of the dimethyl-
phosphanyl groups of the dmpm ligand in relation to the
π-acidic diphenylphosphanyl groups of dppm. The higher
activation of N2 also renders 2 susceptible to protonation
by strong acids such as triflic acid (see below), in contrast
to 3.
ligand undergoes a helical distortion by rotation of the
C(1)–(CH2)3 moiety around an axis connecting the Mo0
centre and the central C atom C(1) of the ligand, in analogy
to that for [Mo(tdppme)(CO)3].[30] This distortion can be
expressed by the dihedral C(1)–C–P–Mo angles of
38.309(7) for C(1)–C(2)–P(1)–Mo(1), 25.883(7) for C(1)–
C(3)–P(2)–Mo(1) and 32.453(7)° for C(1)–C(4)–P(3)–
Mo(1). The bite angle of the bidentate dmpm ligand is
In order to obtain more detailed vibrational spectro-
about 66.86(6)°, which matches the bite angle of dmpm in scopic information about 2, [Mo(15N2)(tdppme)(dmpm)]
[MoBr(dpepp)(dmpm)] [66.7(1)°].[31] It is somewhat smaller (2a) was also prepared, by using 15N2 instead of 14N2 gas.
as compared to that of the dppm [bis(diphenylphosphanyl)- In 2a, the N–N stretch shifts to 1915 cm–1 (1980 cm–1 in 2,
methane] ligand in [Mo(N2)(dpepp)(dppm)], which exhibits Tables 4 and 5, Figure 2). In the far-IR region, some iso-
a bite angle of 68.3(1)°, most likely because of steric tope sensitive signals are also found (Figure 3); in particu-
reasons.[22]
lar, one band at 497 shifting to 490 cm–1 and two bands at
On the basis of its crystal structure, 2 was investigated 486/475 shifting to 483/472 cm–1 are observed. These fea-
by DFT. As evident from Table 3, the structural parameters tures can be assigned on the basis of DFT calculations
derived from geometry optimization match those derived (Table 5). Importantly, these calculations place the Mo–N–
from the crystal structure fairly well. Mo–N and Mo–P dis- N bending vibrations at higher energy than the Mo–N
tances are about 0.1 Å too large, which has been observed stretch. The band at 497 cm–1 that exhibits a shift to
before in this type of calculation.[22,23] The geometry of 2 490 cm–1 is thus assigned to one of the Mo–N–N bending
determined by single-crystal structure analysis and DFT ge- vibrations, and the two bands at 486 and 475 cm–1 that shift
ometry optimization is also preserved in solution, as in- to 483 and 472 cm–1, respectively, are assigned to the Mo–
ferred from NMR spectroscopy. The 31P NMR spectrum of N stretch, distributed over two vibrations. These fre-
[Mo(N2)(tdppme)(dmpm)] exhibits an AAЈXXЈB structure quencies are comparable to those observed for [Mo(N2)-
(Figure S1). Because of an overlap of the signals, however, (dpepp)(dppm)].[22]
Table 3. Selected bond lengths [Å] and angles [°] for the crystal structure of [Mo(N2)(tdppme)(dmpm)] (2) and the calculated structures
of [Mo(N2)(tdppme)(dmpm)] (2) and [Mo(N2)(tdppme)(dppm)] (3), in comparison with those of [Mo(N2)(dpepp)(dppm)].[22]
2 (expt.)
2 (calcd.)
3 (calcd.)
[Mo(N2)(dpepp)(dppm)] (expt.)
Nα–Nβ
Mo–Nα
1.069(8)
2.066(6)
1.1579
1.9823
1.1589
1.9770
1.119(8)
2.025(6)
Mo–Pa,b,c,d,e
Nα–Nβ–Mo
Nβ–Mo–Pe
Pa–Mo–Pb
Pc–Mo–Pd
2.3986(16)–2.4454(16)
177.372(6)
177.372(4)
66.856(6)
2.4972–2.5942
177.242
175.500
69.123
2.5274–2.6597
179.157
169.742
67.583
2.418(2)–2.497(2)
179.2(6)
167.0(2)
68.3(1)
97.9(1)
82.301(5)
83.901
86.119
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Table 4. Comparison of the measured IR and Raman (R) frequencies of [Mo(N2)(tdppme)(dmpm)] (2), [Mo(N2)(tdppme)-
(dppm)] (3) and [Mo(N2)(dpepp)(dmpm)].[22]
2
3
[Mo(N2)(dpepp)(dppm)]
14N [cm–1]
14N [cm–1]
15N [cm–1]
14N [cm–1]
15N [cm–1]
ν(N–N)
1980 (IR)
1979 (R)
497 (IR)
1915 (IR)
1914 (R)
490 (IR)
2035 (IR)
2040 (R)
1979 (IR)
1984/2003 (R)
505/497 (IR)
453 (IR)
1914 (IR)
1918/1935 (R)
503/493 (IR)
448 (IR)
ν(Mo–N–N)
ν(Mo–N)
486/475 (IR)
483/472 (IR)
454 (R)
449 (R)
Table 5. Comparison of calculated and measured IR and Raman frequencies of [Mo(N2)(tdppme)(dmpm)] (2).
14N [cm–1] (meas.)
15N [cm–1] (meas.)
14N [cm–1] (calcd.)
15N [cm–1] (calcd.)
1859
ν(N–N)
1979 (IR)
1980 (R)
497 (IR)
486/475
(IR)
1914 (IR)
1915 (R)
490 (IR)
483/472
(IR)
1924
ν(Mo–N–N)
ν(Mo–N)
512
461
500
449
Complex 2a was also investigated by 15N NMR spec-
troscopy (Figure 4). The doublet at –25.61 ppm is attrib-
uted to the terminal nitrogen atom Nβ (1JNN = 5.90 Hz),
and the multiplet at –33.09 to –33.63 ppm to the coordi-
nated nitrogen atom Nα of the dinitrogen ligand. The split-
ting of the latter signal is induced by coupling with the mo-
lybdenum centre and the phosphane ligands. The N–N cou-
pling constant and the shift for Nα approximately agree
with the values found for [Mo(15N2)(dpepp)(dppm)]
1
[–30.355 to –30.712 (Nα, m), JNN = 5.7 Hz], whereas the
signal of Nβ is shifted to higher field relative to that of the
latter system [–15.7 ppm (Nβ)].[22]
Figure 2. IR (top) and Raman (bottom) spectra of [Mo(14N2)-
(tdppme)(dmpm)] (2) (black) and [Mo(15N2)(tdppme)(dmpm)] (2a)
(grey).
Figure 4. 15N{1H CPD} NMR spectrum of [Mo(15N2)-
(tdppme)(dmpm)] (2a) (CPD = composite pulse decoupling).
[Mo(N2)(tdppme)(dppm)] (3)
The reduction of 1 with sodium amalgam in THF in the
presence of dppm leads to the formation of the dinitrogen
complex [Mo(N2)(tdppme)(dppm)] (3). Single crystals of 3
suitable for an X-ray structure determination could not be
Figure 3. Far-IR spectrum of [Mo(14N2)(tdppme)(dmpm)] (2)
(black) and [Mo(15N2)(tdppme)(dmpm)] (2a) (grey).
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Molybdenum(0) Monodinitrogen Complexes with a Tripodal Ligand
obtained. The structure of this complex was thus derived spectrum was obtained by analysis of the M- and X-half-
spectra according to the literature.[33] Each half-spectrum
consists of ten lines, which, in turn, are split into doublets
as a result of the interaction with Pe. The coupling con-
stants between (Pc, Pd) and Pe are transmitted by the metal
centre and the C3 bridges of the tripodal ligand and have
values of 2Jed/ec = –24.2 Hz. The same applies to 2Jcd, which
has a somewhat larger value (–32.7 Hz). The cis coupling
between the Pa and Pb donors of the diphos ligand, on the
from a DFT geometry optimization. The topology of the
complex is the same as that of 2, with facial coordination
of the tripodal ligand and bidentate coordination of the di-
phos ligand. A space-filling representation of the structure
of 3 derived from this treatment is presented in Figure 5.
The steric crowding around the metal centre imposed by
the presence of five diphenylphosphane donors bearing ten
phenyl rings is observed. Because of the presence of four
phenyl groups above the equatorial plane, the apical N–N
group is shielded fairly well.
other hand, results from two interactions through the metal
2
and the methylene bridge and has a value of Jab
=
–11.30 Hz. The remaining cis interactions are larger again,
i.e. the coupling between Pe and the phosphorus atoms (Pa,
2
Pb) of the dppm ligand has a magnitude of Jea/eb
=
–22.0 Hz, and the cis interactions between the dppm and
2
tripodal ligands within the equatorial plane are Jac/bd
=
–27.78 Hz. As usual, trans interactions give rise to much
larger splittings (91.13 Hz).
Complex 3 exhibits IR and Raman spectra with sharp
vibrational features. The N–N stretch is located at
2035 cm–1 (Figure 7). The activation of the N2 ligand in 3
is thus weaker than that in the dmpm complex 2, which is
due to the electron-withdrawing effect of the diphenylphos-
phanyl groups (vide supra). This conforms to our observa-
tion that we were not able protonate the N2 ligand with
triflic acid (see below). The far-IR spectrum of 3 (Fig-
ure S3) is qualitatively similar to that of 2 (Figure 3), exhib-
iting two clusters of bands located at about 400 cm–1 and
between 450 and 500 cm–1, respectively. Without further in-
Figure 5. Space-filling structure of [Mo(dppm)(tdppme)N2] (3)
from the top (left) and from the side (right).
The 31P NMR spectrum of 3 exhibits a typical
AMMЈXXЈ pattern (Figure 6), which proves the presence
of the geometry optimized structure in solution. The signal
at 10.58 ppm is assigned to the XXЈ phosphorus atoms of
the dppm ligand, Pa and Pb. The signal at 43.92 ppm is
associated with the equatorial MMЈ atoms, Pc and Pd, of formation from isotope substitution, however, the Mo–N
the tripodal ligand, and the signal at 33.32 ppm is attrib- stretch and Mo–N–N bending vibrations cannot be iden-
uted to its axial phosphane donor, Pe (A). The simulated tified.
Figure 6. Measured (top) and simulated (bottom) 31P{1H CPD} NMR spectrum of [Mo(N2)(tdppme)(dppm)] (3).
Eur. J. Inorg. Chem. 2011, 4377–4386
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FULL PAPER
rialatomsPcandPdandtheaxialatomPeofthetripodalligandare
2Jed/ec = –29.44 Hz; the interaction of Pe with the phospho-
rus atoms (Pa, Pb) of the dppm ligand gives rise to coupling
constants of a similar magnitude (2Jea/eb = –31.03 Hz). Fur-
2
ther cis interactions result in coupling constants of Jab
=
2
2
–9.55, Jac/bd = –24.85 and Jcd = –18.75 Hz. At 83.15 Hz,
the trans interactions have the usual large splitting. With
increasing measuring time, two signals in the form of an
AAЈXXЈ pattern evolve in the 31P NMR spectrum of 4 at
–23.35 and 24.82 ppm (marked by asterisks in Figure 8).
These can be attributed to a decomposition product of 4,
probably arising from substitution of the axial atom, Pe, of
the tripodal ligand by triflate.
1
In the H-15N HMQC NMR spectrum (Figure 9) of 4a,
the coupling of the protons of the 15N15NH2 group
(9.25 ppm) to the terminal 15Nβ atom (–217.84 ppm) can be
observed. The shift for the protons was derived from the
1H NMR spectrum, and the shift for the 15Nβ atom was
derived from the cross-peak in the HMQC NMR spectrum.
We were not able to determine the shifts for the 15Nα atom
and the 15Nβ atom by regular 15N NMR spectroscopy be-
cause of the limited solubility and decomposition of 4a in
solution (vide supra).
Figure 7. IR (top) and Raman (bottom) spectra of [Mo(N2)-
(tdppme)(dppm)] (3).
[Mo(NNH2)(tdppme)(dmpm)](OTf)2 (4) and
[Mo(15N15NH2)(tdppme)(dmpm)](OTf)2 (4a)
As a result of moderate activation of the N2 ligand, 2
could be protonated by using triflic acid, which led to the
NNH2 complex [Mo(NNH2)(tdppme)(dmpm)](OTf)2 (4).
Similarly, the 15N-substituted complex [Mo(15N15NH2)-
(tdppme)(dmpm)](OTf)2 (4a) was obtained by protonation
of the 15N-isotopomer of 2, 2a.
The 31P NMR spectrum of 4 again exhibits a typical
AMMЈXXЈ pattern (Figure 8), which was analyzed in anal-
ogy to that of 3 (vide supra). The signal at –27.50 ppm be-
longs to the XXЈ atoms, Pa and Pb, of the dmpm ligand;
the signals at 21.50 and –4.40 ppm are assigned to the MMЈ
phosphorus atoms (Pc, Pd) and the axial A atom, Pe, of
the tripodal ligand, respectively. With respect to the parent
dinitrogen complex 2, all 31P resonances are shifted to
higher field by 20–40 ppm. The simulated spectrum was ob-
tained by analysis of the M- and X-half-spectra as de-
scribed earlier. The coupling constants between the equato-
Figure 9. 1H15N HMQC NMR spectrum of [Mo(15N15NH2)-
(tdppme)(dmpm)](OTf)2 (4a).
Important structural information can again be inferred
from vibrational spectroscopy. The symmetric and antisym-
metric NH2 stretches of 4 are located at 3238 and 3301 cm–1
(Figure 10), which is at about the same frequency as that
observed in [Mo(NNH2)(dpepp)(dppm)] (Table 6).[22] In 4a,
the symmetric and antisymmetric NH2 stretches are shifted
to slightly lower energy (3230 and 3289 cm–1, Figure 10).
The N–N stretch of 4 is observed at 1410 cm–1. However,
the corresponding N–N stretch of 4a is masked by the in-
tense absorption of triflate. Again, these vibrational fre-
quencies agree with results from DFT (Table 6). In particu-
lar, these calculations predict the symmetric NH2 stretching
vibration to be at lower frequency than the antisymmetric
one.
Figure 8. 31P{1H CPD} NMR spectrum of [Mo(NNH2)(tdppme)-
(dmpm)](OTf)2 (4).
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Molybdenum(0) Monodinitrogen Complexes with a Tripodal Ligand
Table 6. Comparison of calculated and measured IR and Raman frequencies of [Mo(NNH2)(tdppme)(dmpm)](OTf)2 (4) with those of
[Mo(NNH2)(dpepp)(dppm)](OTf)2.[22]
[Mo(NNH2)(tdppme)(dmpm)](OTf)2
[Mo(NNH2)(dpepp)(dppm)](OTf)2
14N [cm–1] (meas.)
14N [cm–1] (meas.) 15N [cm–1] (meas.)
14N [cm–1] (calcd.)
15N [cm–1] (calcd.)
ν(NH2) symm.
ν(NH2) antisymm. 3301 (IR)
ν(N–N)
ν(M–N)
3238 (IR)
3230 (IR)
3289 (IR)
masked
3461
3629
1442
628
3458
3618
1397
613
3244
not resolved
not resolved
not resolved
1410 (IR)
616 (R)
596 (R)
shifted to higher energy with respect to those of 3 because
of the lower effective nuclear charge of the Mo0 centre in 2
than that in 3. This conforms to the higher activation of N2
encountered in 2 than in 3.
Figure 10. IR (top) and Raman (bottom) spectra of [Mo(NNH2)-
(tdppme)(dmpm)](OTf)2 (4) (black) and [Mo(15N15NH2)(tdppme)-
(dmpm)](OTf)2 (4a) (grey).
Figure 11. MO schemes of [Mo(dmpm)(tdppme)N2] (2) (left) and
[Mo(dppm)(tdppme)N2] (3) (right) and representation of the three
doubly occupied metal-type orbitals.
Raman spectra of the NNH2 complexes 4 and 4a show
several isotope sensitive peaks in the 850 to 450 cm–1 region
(Figure S4). In particular, a new vibration appears at
596 cm–1 in the spectrum of 4a as compared to the spec-
trum of 4, which is attributed to the 15N-shifted Mo–N
stretch. This feature appears to be at somewhat higher en-
ergy than those observed in Mo– and W–NNH2 complexes
with anionic trans ligands.[34]
Conclusion
We were able to synthesize the first molybdenum dinitro-
gen complexes facially coordinated by a tripodal ligand,
[Mo(N2)(tdppme)(dmpm)] (2) and [Mo(N2)(tdppme)-
(dppm)] (3), by sodium amalgam reduction of the molybde-
num(III) precursor [MoBr3(tdppme)] (1) under N2 in the
DFT Investigations
Figure 11 shows the highest occupied (HOMO) and low- presence of dppm and dppm, respectively. Compound 1 was
est unoccupied molecular orbitals (LUMO) of 2 and 3. prepared in high yield and purity from [MoBr3(THF)3].
These schemes conform to the prototypical MO scheme de- NMR, IR and Raman spectroscopy provided valuable spec-
termined for fac-[Mo(N2)(PH3)5] by Stephan et al.[22] The troscopic information with regard to the pentaphosphane
two LUMOs consist of the antibonding π* orbitals of the coordination and the activation of the N2 ligands in 2 and
dinitrogen ligand, which lie energetically close to each 3. In the dppm complex 3, the dinitrogen ligand is not acti-
other. Three energetically closely spaced MOs lie beneath. vated for protonation. Nevertheless, it is remarkable that
In 2 and 3 the nonbonding dxy orbital of molybdenum is this complex tolerates ten phenyl groups on the coordinat-
the highest occupied molecular orbital (HOMO). The back- ing phosphorus atoms. The space-filling structure derived
bonding dxz–π*x and dyz–π*y orbitals between molybdenum by DFT calculations shows the impressive steric crowding
and nitrogen follow close by. The ligand orbitals lie below around the molybdenum–N2 moiety.
these; their positions and number differ because of the dif-
ferent substituents on the diphos ligands. The metal–ligand dppm ligand of
In order to increase the N2 activation, we exchanged the
with dmpm, which led to
3
antibonding orbitals could only be identified if the alkyl/ [Mo(N2)(tdppme)(dmpm)] (2). IR and Raman spectroscopy
aryl substituents on the phosphane ligands were exchanged indicate a moderate activation of the N2 ligand. The struc-
with hydrogen atoms. Importantly, all orbitals of 2 are ture of this complex was investigated by X-ray crystallogra-
Eur. J. Inorg. Chem. 2011, 4377–4386
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J. Krahmer, H. Broda, C. Näther, G. Peters, W. Thimm, F. Tuczek
FULL PAPER
phy and NMR spectroscopy. The facial coordination of the
tripodal ligand to the metal centre potentially resolves a
fundamental problem of the Chatt cycle: the ligand ex-
change in trans position to the dinitrogen ligand. In fact,
protonation of 2 with triflic acid led to the NNH2 complex
[Mo(NNH2)(tdppme)(dmpm)](OTf)2 (4) with retention of
the pentaphosphane coordination. The NNH2 complex 4
was again characterized by vibrational and NMR spec-
troscopy, coupled with DFT.
Complexes in pentaphosphane environments similar to 2
and 3, i.e. exhibiting the combination of a tripodal and a
diphos ligand, are known for other metals such as ruthe-
nium, chromium, rhodium and cobalt.[35,36] Dinitrogen
complexes containing a combination of tripodal and diphos
ligands are, however, unknown to date. In the ruthenium(II)
complex [RuCl(tdppme)(dmpm)], the bidentate phosphane
ligand dmpm, which is less bulky than its diphenylphos-
phane-substituted counterpart dppm, has been chosen as a
partner for the tripodal ligand tdppme.[35] The pentaphos-
phane coordination sphere in the chromium and rhodium
complexes consists of the sterically less-demanding
tripodal ligand 1,1,1-tris(dimethylphosphanylmethyl)ethane
(tdmpme) and the bidentate phosphane ligand dmpm. The
capability of 3 to accommodate ten phenyl groups bound
to the coordinating phosphorus atoms can be attributed to
the larger size of the molybdenum(0) central atom in com-
parison to ruthenium, chromium and rhodium centres.
The lower activation of the dinitrogen ligand in 3 in com-
parison to the literature-known complex [Mo(N2)(dpepp)-
Scheme 1. Conversion of [MoBr3(THF)3] to [MoBr3(tdppme)] (1)
and to [Mo(N2)(tdppme)(dmpm)] (2 and 2a), respectively, and con-
version of 1 to [Mo(N2)(tdppme)(dppm)] (3). Protonation of 2 and
2a to [Mo(NNH2)(tdppme)(dmpm)](OTf)2 (4 and 4a). Labelling of
the phosphanes is shown for 3.
(dppm)] can be explained by the fact that, in 3, a diphenyl- (960.33): calcd. C 51.28, H 4.09, Br 24.96; found C 51.14, H 4.08,
Br 24.87.
phosphane donor atom is bound trans to the dinitrogen li-
gand. In contrast, in [Mo(N2)(dpepp)(dppm)], the central P
atom in the dpepp ligand is coordinated trans to the N2
[Mo(N2)(tdppme)(dmpm)] (2) and [Mo(15N2)(tdppme)(dmpm)] (2a):
A suspension of 1 (500 mg, 0.52 mmol) and dmpm (71 mg,
ligand. This P atom binds two alkyl groups and one phenyl 0.52 mmol) in THF (20 mL) was added to sodium amalgam
(200 mg Na, 30.0 g Hg) and stirred under nitrogen for 3 h at 0 °C
and overnight at ambient temperature to obtain 2. For the prepara-
tion of 2a, the reaction was carried out under a 15N2 atmosphere.
The solution was decanted, filtered and reduced in vacuo to 6 mL.
Methanol (6 mL) was added, and the volume of the solution was
reduced again. After another addition of methanol (6 mL), a pre-
cipitate formed, which was collected by filtration, washed four
times with methanol (4 mL) and dried in vacuo. Yield of 2: 286 mg
(0.323 mmol, 62%). Yield of 2a: 304 mg (0.343 mmol, 66%).
C46H53MoN2P5 (884.75): calcd. C 62.45, H 6.04, N 3.17; found C
62.39, H 5.86, N 2.97. C46H53Mo15N2P5 (2227.91): calcd. C 62.31,
H 6.02, N 3.38; found C 62.22, H 6.12, N 3.26. 31P{1H CPD}
NMR (161.975 MHz, C6D6): δ = 40.48–40.08 (m, Pe), 39.90–39.07
(m, Pc/d), –22.54 to –23.54 (m, Pa/b) ppm. 15N{1H, 31P CPD} NMR
(40.540 MHz, C6D6): δ = –25.61 (d, 1JNN = 5.77 Hz, Nβ), –33.09 to
–33.63 (m, Nα) ppm. Single crystals of 2 suitable for X-ray structure
determination were grown by recrystallization from a toluene/n-
hexane mixture.
substituent, which results in a stronger activation. In com-
plex 2, the weaker activating potential of the tdppme tripod
is compensated by the use of the bidentate alkylphosphane
ligand dmpm, which allows protonation to the correspond-
ing hydrazido complex 4. Detailed studies investigating the
possible catalytic activity of 2 in a nitrogen-fixing cycle are
in progress.
Experimental Section
General: Schematic structures of the complexes prepared and inves-
tigated in this study are given in Scheme 1.
Synthesis and Sample Preparation: All reactions were carried out
under a N2 atmosphere using Schlenk techniques. All solvents were
dried and freshly distilled under argon. The other reagents were
used in the best available qualities. The phosphane ligands were
obtained from Strem Chemicals or Sigma–Aldrich. 15N2 (98 atom-
[Mo(N2)(tdppme)(dppm)] (3):
A
suspension of
1
(500 mg,
%
15N) was obtained from Sigma–Aldrich. The preparation of
0.52 mmol) and dppm (200 mg, 0.52 mmol) in THF (20 mL) was
added to sodium amalgam (200 mg Na, 30.0 g Hg) and stirred un-
der nitrogen for 3 h at 0 °C and overnight at ambient temperature.
The solution was decanted, filtered and reduced in vacuo to 6 mL.
Methanol (6 mL) was added, and the volume of the solution was
reduced again. After another addition of methanol (6 mL), a pre-
cipitate formed, which was collected by filtration, washed four
[MoBr3(THF)3] was performed according to the literature.[26,27]
[MoBr3(tdppme)] (1): 1,1,1-Tris(diphenylphosphanylmethyl)ethane
(500 mg, 0.800 mmol) and [MoBr3(THF)3] (442 mg, 0.800 mmol)
were heated to reflux in toluene (20 mL) for 3 h. The resulting yel-
low powder was washed twice with toluene (5 mL) and dried in
vacuo. Yield: 736 mg (0.768 mmol, 96%). C41H39Br3MoP3
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Molybdenum(0) Monodinitrogen Complexes with a Tripodal Ligand
times with methanol (4 mL) and dried in vacuo. Yield: 342 mg
(0.302 mmol, 58%). C66H61MoN2P5 (1133.03): calcd. C 69.96, H
5.43, N 2.47; found C 69.93, H 5.50, N 2.36. 31P{1H CPD} NMR
and the LANL2DZ basis set for molybdenum and 6-311G for all
other elements.[28] Geometrical optimization and frequency calcula-
tions were performed with very tight thresholds in the self-consis-
tent field (SCF) and optimization steps.
2
2
(161.975 MHz, C6D6): δ = 43.92 (dddd, Jcd = –32.70, Jac/bd
=
2
2
2
–27.78, Jcb/da = 91.13 Hz; Pc/d), 33.32 (ddd, Jea/eb = –22.0, Jed/ec
Supporting Information (see footnote on the first page of this arti-
2
= –24.20 Hz; Pe), 10.58 (dddd, Jab = –11.30 Hz, Pa/b) ppm.
1
cle): 31P{1H CPD} and H31P-HMBC NMR, Far-IR and Raman
[Mo(NNH2)(tdppme)(dmpm)](OTf)2 (4) and [Mo(15N15NH2)-
(tdppme)(dmpm)](OTf)2 (4a): Complex 2 (200 mg, 0.226 mmol) –
or 2a for the preparation of 4a – was dissolved in THF (7 mL) and
cooled to –78 °C. Triflic acid (0.04 mL, 0.443 mmol) was added.
After 15 min of stirring, the solution was allowed to stand for 2 h
at –78 °C and then warmed up to room temperature. n-Hexane
(3 mL) was added, and the yellow precipitate was collected by fil-
tration, washed three times with n-hexane (2 mL) and dried in
vacuo. Yield of 4: 137 mg (0.115 mmol, 51%). Yield of 4a: 145 mg
(0.122 mmol, 54%). C48H55F6MoN2O6P5S2 (1184.89): calcd. C
48.66, H 4.68, N 2.36, S 5.41; found C 48.52, H 4.71, N 1.9, S 5.51.
C48H55F6Mo15N2O6P5S2 (2528.05): calcd. C 48.57, H 4.67, N 2.53,
S 5.40; found C 49.02, H 4.73, N 2.24, S 5.44. 31P{1H CPD} NMR
spectra are provided for some compounds.
Acknowledgments
Support by the State of Schleswig-Holstein for this research is
gratefully acknowledged.
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Eur. J. Inorg. Chem. 2011, 4377–4386
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4385

J. Krahmer, H. Broda, C. Näther, G. Peters, W. Thimm, F. Tuczek
FULL PAPER
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