One-step synthesis of Mo(0) and W(0) bis(dinitrogen) complexes with the linear tetraphosphine ligand prP4: Stereoselective formation of cis-[M(N2)2(rac-prP4)] and trans-[M(N 2)2(meso-prP4)]; M = Mo, W

Article abstract of DOI:10.1039/c0dt01646b

A new synthetic pathway to Chatt-type Mo(0) and W(0) bis(dinitrogen) complexes with the ligand prP₄ is presented (prP₄ is a linear tetraphos ligand with two ethylene bridges and a central propylene bridge). The synthesis starts from MoCl₅ and WCl₆, respectively, employing Mg as reductant. Whereas the electrochemical reduction of the oxido-iodido-molybdenum(iv) complex [Mo(O)I(meso-prP₄] ⁺ (1) only gave trans-[Mo(N₂)₂(meso-prP ₄)] (2a; Roemer et al., Eur. J. Inorg. Chem.2008, 3258), the direct synthesis under normal conditions affords both trans and cis complexes 2a and 2b. The reaction products are characterised by vibrational and NMR spectroscopy. Moreover, a single-crystal X-ray structure determination of cis-α-[Mo(N₂)₂(rac-prP₄)] (2b) is performed. In contrast to the trans bis(dinitrogen)molybdenum(0) complex 2a supported by the meso prP₄ ligand the corresponding cis-complex is exclusively coordinated by the rac isomer of prP₄. The reactivity of 2 with acids is investigated as well, leading to the NNH₂ complex [MoF(NNH₂)(meso-prP₄)]BF₄ (15). Analogous results are obtained with the tungsten complexes.

Full text of DOI:10.1039/c0dt01646b

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PAPER
One-step synthesis of Mo(0) and W(0) bis(dinitrogen) complexes with the
linear tetraphosphine ligand prP4: stereoselective formation of
cis-[M(N2)2(rac-prP4)] and trans-[M(N2)2(meso-prP4)]; M = Mo, W†‡
Rene´ Ro¨mer, Christian Gradert, Alexander Bannwarth, Gerhard Peters, Christian Na¨ther and Felix Tuczek*
Received 26th November 2010, Accepted 12th January 2011
DOI: 10.1039/c0dt01646b
A new synthetic pathway to Chatt-type Mo(0) and W(0) bis(dinitrogen) complexes with the ligand prP4
is presented (prP4 is a linear tetraphos ligand with two ethylene bridges and a central propylene bridge).
The synthesis starts from MoCl5 and WCl6, respectively, employing Mg as reductant. Whereas the
electrochemical reduction of the oxido–iodido–molybdenum(IV) complex [Mo(O)I(meso-prP4]+ (1) only
gave trans-[Mo(N2)2(meso-prP4)] (2a; Ro¨mer et al., Eur. J. Inorg. Chem. 2008, 3258), the direct synthesis
under normal conditions affords both trans and cis complexes 2a and 2b. The reaction products are
characterised by vibrational and NMR spectroscopy. Moreover, a single-crystal X-ray structure
determination of cis-a-[Mo(N2)2(rac-prP4)] (2b) is performed. In contrast to the trans
bis(dinitrogen)molybdenum(0) complex 2a supported by the meso prP4 ligand the corresponding
cis-complex is exclusively coordinated by the rac isomer of prP4. The reactivity of 2 with acids is
investigated as well, leading to the NNH2 complex [MoF(NNH2)(meso-prP4)]BF4 (15). Analogous
results are obtained with the tungsten complexes.
An obvious strategy to increase the thermodynamic and kinetic
stability of Mo and W phosphine complexes is to replace mono-
One of the fundamental biochemical processes of nature is the
or bidentate by multidentate phosphine ligands.
Against this background we have prepared the tetraden-
Mechanistic information with respect to this reaction can be
tate phosphine ligand prP4 (1,1,4,8,11,11-hexaphenyl-1,4,8,11-
Introduction
bonding and activation of N2 and its reduction to ammonia.1
obtained from the study of small-molecule model systems. Two
tetraphosphaundecane; 3) and explored its coordination proper-
ties in mononuclear complexes.7,8 The preparation of prP4 leads to
a mixture of the meso and the rac isomer, 3a and 3b, respectively.
The meso isomer 3a has the two phenyl rings of the central
phosphorus atoms oriented to the same side whereas in the rac
isomer 3b the two phenyl rings are located on opposite sides of
the ligand. Moreover, octahedral complexes supported by this
ligand can in principle exist in six different configurations: meso-
trans, rac-trans, meso-cis-a, rac-cis-a, meso-cis-b and rac-cis-b
(Scheme 1). For steric reasons, the meso-cis-a configuration has
been excluded.7,9,10
well-established reactive schemes exist in this respect, the Schrock
cycle2 and the Chatt cycle3 While the Schrock cycle is based
on a Mo(III) triamidoamine complex and has been performed
catalytically, giving ammonia from N2 in 6 cycles with an overall
yield of 65%,4 the Chatt cycle involves Mo and W complexes with
phosphine (mono- or diphosphine) ligands; so far a cyclic, but
no truly catalytic action of the Chatt cycle has been achieved.5
The underlying problems of the Chatt cycle have been analyzed
in detail;3,5,6 one of them is the fact that the phosphine ligands
become labile at the Mo(IV) stage.
In the course of our investigations of the prP4 ligand 3, the
stereospecific synthesis of the molybdenum dinitrogen complex
trans-[Mo(N2)2(meso-prP4)] 2a has been achieved (Scheme 2).8
The intermediates and the product of the synthesis have been
Institut fu¨r Anorganische Chemie, Christian-Albrechts-Universita¨t Kiel,
Max Eyth, Straße 2, D-24098, Kiel, Germany. E-mail: ftuczek@ac.uni-
kiel.de
1
† Electronic supplementary information (ESI) available: 31P{ H}NMR
1
characterized by 31P{ H}-NMR, vibrational and UV/Vis spec-
1
spectra of pure trans-[Mo(N2)2(meso-prP4)] 2a, 31P-31P{ H}-COSY–NMR
troscopy as well as X-ray structure analysis. As a matter of
fact 2a was the first molybdenum bis(dinitrogen) complex with
a tetraphosphine ligand. The synthetic problem of coordinating
a tetraphos ligand to a mononuclear molybdenum precursor was
solved by applying the ligand exchange chemistry of molybde-
num(IV) oxo–halide complexes with PMe3 co-ligands established
by Carmona, Galindo et al.11 As reported by these authors
1
of the mixture 2a and 2b, simulated 31P{ H}NMR spectra of 2b
and 2a, trans-[W(N2)2(meso-prP4)] 12a and cis-a-[W(N2)2(rac-prP4)] 12b.
1
31P{ H}NMR spectrum of the prP4 ligand 3, Single-crystal structure
informations for cis-a-[Mo(N2)2(rac-prP4)] 2b. IR Spectrum of trans-
[W(N2)2(meso-prP4)] 12a and cis-a-[W(N2)2(rac-prP4)] 12b. CCDC refer-
ence numbers 798377–798379, 803783. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0dt01646b
‡ Dedicated to Prof. Dr Ju¨rgen Heck on the occasion of his 60th birthday.
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reflected the selection rules applying to Mo(0) bis(dinitrogen)
complexes with a trans configuration.6a,15 Complex 2a therefore
could unambiguously be assigned as a bis(dinitrogen) complex
exhibiting a trans-configuration, as already inferred from NMR
evidence.
In the present paper, an alternative, one-step synthesis of the
complex [Mo(N2)2(prP4)] (2) is presented. In contrast to the
electrochemical route developed earlier, which only gave the trans-
complex 2a, the direct synthesis leads to a mixture of the trans-
and the cis-a-bis(dinitrogen)-complex 2a and 2b, respectively. The
1
products of the synthesis are characterized by 31P{ H}NMR and
vibrational spectroscopy as well as X-ray structure analysis.
Results and discussion
The synthesis of the ligand 3 is achieved in the following manner.7,8
In a first step (2-chloroethyl)diphenylphosphine 7 is prepared by
reaction of lithium diphenylphosphide 8 with dichloroethane 9.16
This precursor is reacted with 1,3-dilithium diphenylphosphide
(10, Lippp) obtained from 1,3-bis(diphenylphosphino)propane
(11, dppp) by treatment with lithium in THF to give a mixture of
meso and rac prP4, 3a and 3b (Scheme 3).7,8 Correspondingly the
Scheme 1 Possible isomers for octahedral prP4 complexes.
1
31P{ H}NMR spectra of the reaction product 3 (cf. ESI†) exhibit
two (AX)2 schemes with equal chemical shifts for the terminal,
and slightly different chemical shifts for the central phosphorus
atoms. The 3JAX coupling constants for both isomers are 28.5 Hz.
No coupling is observed between the central P-atoms, indicating
that spin–spin interaction via the propylene bridge is negligible.
Scheme 2 Stereoselective synthesis of trans-[Mo(N2)2(meso-prP4)] (2a).
the neutral Mo(IV) complex [Mo(O)Cl2(PMe3)3] (4) is able to
exchange three PMe3 groups and one chloro ligand for two
1,2-bis(dimethylphosphino)ethane ligands (dmpe 5), giving trans-
[Mo(O)Cl(dmpe)2]Cl (6).12 We followed this strategy, but, in order
to make the Mo(IV) oxo precursor 4 more reactive towards the
coordination of prP4 (3), replaced the chloro ligands of 4 by iodide.
This leads to the complex [Mo(O)I2(PMe3)3] (5) which had been
prepared and characterized by Poli et al. recently.13 Complex 5
reacts cleanly with the tetradentate prP4 ligand 3 in THF, affording
crystalline [Mo(O)I(prP4)]BPh4 (1) after addition of NaBPh4 to
1
the reaction mixture. Importantly, the 31P{ H}NMR spectra of 1
exhibited a single AA¢XX¢ pattern, showing that it either contains
the meso or the rac ligand but not a mixture of both. Later on, a
single-crystal X-ray structure of 1 showed that the reaction product
contains the meso ligand. This finding was attributed to the steric
influence of the bulky iodo ligand which prevents formation of the
rac complex in which one of the phenyl substituents on the central
phosphorus atoms would point towards this ligand.
The [Mo(O)I(meso-prP4)]BPh4 precursor 1 was finally converted
to the corresponding dinitrogen complex 2a by electrochemical
reduction with a Hg pool electrode in the presence of dinitrogen
and phenol (Scheme 2). This synthetic route had been established
Scheme 3 One-step synthesis of trans- and cis-bis(dinitrogen) complexes.
1
by Pickett et al.14 The 31P{ H}NMR spectrum of the reaction
Molybdenum bis(dinitrogen)complexes
product 2a again exhibited a single AA¢XX¢ pattern, indicating
a trans-geometry of the complex and the presence of only one
isomer. On the basis of the structural information obtained on the
precursor 1, it was inferred that the dinitrogen complex 2a contains
the meso ligand 3a. A second, independent spectroscopic proof
of the identity of 2a as trans-[Mo(N2)2(meso-prP4)] was provided
by vibrational spectroscopy. The comparison of the infrared and
Raman spectrum showed the existence of two N–N vibrations,
a symmetric combination at 2044 cm-1 and an antisymmetric
combination at 1964 cm-1, indicating the coordination of two
dinitrogen ligands. The observed intensities of ns and nas clearly
In contrast to the previous electrochemical synthesis of
the bis(dinitrogen)-molybdenum(0) complex 2a involving two
stages (vide supra), the direct synthesis proceeds in one step
(Scheme 3). Magnesium metal or sodium amalgam reduction of
MoC13(THF)3 under N2 in the presence of phosphines has already
been described by Chatt et al.17 A direct (one-step) synthesis
of dinitrogenmolybdenum(0) complexes by sodium amalgam
reduction of MoCl5 under N2 in the presence of phosphines has
also been performed by George et al.18 For the preparation of
the bis(dinitrogen) complex 2, the tetraphos ligand prP4 (3) was
3230 | Dalton Trans., 2011, 40, 3229–3236
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˚
Table 1 Selected bond lengths [A] and angles [degree] for 2b
dissolved in dry THF and 25 eq. of magnesium turnings were
added. An equal amount of molybdenum(V)chloride was added
to the reaction mixture, which was stirred for 24 h. The red solution
was separated from the magnesium, evaporated and extracted with
benzene to give a red–orange solid.
Mo(1)–N(1)
Mo(1)–N(3)
N(1)–N(2)
N(3)–N(4)
2.037(7)
2.050(6)
1.119(10)
1.106(9)
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–P(3)
Mo(1)–P(4)
2.469(2)
2.4566(19)
2.420(2)
2.4463(19)
Whereas the electrochemical synthesis of the bis(dinitrogen)–
molybdenum(0) complex 2a leads to only one product, the
N(1)–Mo(1)–N(3)
N(1)–Mo(1)–P(1)
N(1)–Mo(1)–P(2)
N(3)–Mo(1)–P(2)
N(3)–Mo(1)–P(3)
P(2)–Mo(1)–P(1)
P(3)–Mo(1)–P(1)
P(4)–Mo(1)–P(1)
86.2(3)
N(1)–Mo(1)–P(3)
N(1)–Mo(1)–P(4)
N(3)–Mo(1)–P(1)
N(3)–Mo(1)–P(4)
P(2)–Mo(1)–P(3)
P(2)–Mo(1)–P(4)
P(3)–Mo(1)–P(4)
172.02(17)
91.53(18)
91.51(17)
90.14(17)
89.21(7)
98.60(7)
80.52(7)
94.70(17)
92.73(18)
171.22(17)
93.08(18)
79.89(6)
93.27(7)
173.65(7)
1
31P{ H}-NMR spectrum belonging to the product 2 of the direct
synthesis shows the presence of two isomers, trans-[Mo(N2)2(meso-
prP4)] (2a) and cis-a-[Mo(N2)2(rac-prP4)] (2b) (Fig. 1). Specifically,
the spectrum exhibits an AA¢XX¢ pattern (A = central, X =
terminal P) at 71.8 ppm and 63.5 ppm and two double doublets
at 66.3 ppm and 49.4 ppm in an intensity ratio of 4 : 1. The
AA¢XX¢ pattern corresponds to the trans-complex 2a. In contrast
to the free tetraphos ligand 3, a coupling between the central
phosphorus atoms is observed now. The cis-interaction between
the terminal phosphorus atoms PX and PX¢ is only transmitted
by the metal centre, the corresponding coupling constant having
Table 2 Crystal and structure refinement data for 2b at 170 K
Complex
cis-a- [Mo(N2)2(rac-prP4)] (2b)
Empirical formula
Formula weight
Crystal system
Space group
C43H44N4MoP4
836.64
Triclinic
P-1
9.4885(8)
11.4201(11)
19.3015(17)
92.688(11)
90.529(10)
112.629(8)
1927.5(3)
2
1.442
0.544
2.185 – 26.02
14373
0.0449
2
a value of JXX¢ = -15.3 Hz. The coupling constant between the
internal phosphorus atoms PA and PA¢ is due to two interactions,
˚
a (A)
one transmitted by the metal centre and one by the propylene
˚
b (A)
2/4
bridge, and has a value of
J
= -29.9 Hz. The terminal (PX
˚
AA¢
c (A)
a (◦)
b (◦)
g (◦)
and PX¢) and bridging phosphorus atoms (PA and PA¢) are coupled
via the ethylene bridge and the metal, leading to a small coupling
constant |2/3
JA¢X¢/AX| = 3.8 Hz. The trans interaction, finally, is only
3
˚
Volume (A )
Z
transmitted by the metal centre and leads to the largest coupling
constant, 2JA¢X/AX¢ = 105.1 Hz.19 Importantly, all coupling constants
agree with those derived for the product of the stereoselective
synthesis, i.e. the complex trans-[Mo(N2)2(meso-prP4)] (2a).
dcalc (Mg m-3)
m (mm-1)
Theta range (◦)
Refl. collected
Rint
Independent refl.
Parameters
Godness-of-fit
R1 [I > 2s(I)]
wR2 (all data)
7286
474
1.141
0.0725
0.2007
2.497/-1.133
-3
˚
dF (e A )
perform an X-ray structure determination. The structure of the
complex, shown in Fig. 2, corresponds to the R,R form of cis-a-
[Mo(N2)2(rac-prP4)] (2b); the S,S form also existing in the lattice
is related to the R,R form by a center of inversion. Selected bond
lengths and angles and crystal structure data are collected in
Tables 1 and 2.
1
Fig.
1
31P{ H}-NMR spectra in C6D6 at 300
K
of
trans-[Mo(N2)2(meso-prP4)] (2a) and cis-a-[Mo(N2)2(rac-prP4)] (2b).
The other spectrum can be attributed to a cis-bis(dinitrogen)
complex. Specifically, in the cis-a-isomer 2b the terminal phospho-
rus atoms are coordinated in trans position to each other and are
magnetically equivalent (cf. Scheme 1). This leads to two doublets
that can approximately be interpreted in terms of an A2X2 system.
The derived coupling constants of 3.8 Hz and 4.8 Hz correspond
to the coupling between the central and terminal phosphorus
atoms. No other species besides 2a and 2b is detected in the NMR
spectrum.
Fig. 2 Structure of cis-a-[Mo(N2)2(rac-prP4)] (2b), R,R form.
Moreover, single crystals of 2 could be grown by slow evap-
oration of a concentrated solution in benzene, allowing us to
The X-ray structure thus confirms that the cis-isomer 2b carries
the rac-(3b) and not the meso-prP4 ligand (3a). Correspondingly,
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˚
Table 3 Calculated vs. experimental bond lengths [A] of 2b and 2a
the phenyl substituents on the bridging phosphine groups of the
tetraphos ligand are oriented in opposite directions with a torsion
angle of 157.2◦. In the cis-a configuration the terminal phosphorus
atoms are located in a trans position to each other resulting in
a P(1)–Mo–P(4) angle of 173.7◦. The central phosphorus atoms
are located in trans positions to the dinitrogen ligands, the angle
between the terminal and central P atoms P(1)–Mo–P(2) and P(3)–
Mo–P(4) being about 80◦. The N–Mo–N angle is 86.2◦. The N–N
rac-cis-a (2b) expt. rac-cis-a calc. meso-trans calc.
Mo(1)–N(a1) 2.037
Mo(1)–N(a2) 2.050
N(a1)–N(b1) 1.119
N(a2)–N(b2) 1.106
2.070
2.070
1.118
1.117
2.505
2.521
2.495
2.518
2.045
2.037
1.137
1.139
2.508
2.520
2.445
2.451
Mo–P(1)in
Mo–P(2)in
Mo–P(3)ter
Mo–P(4)ter
2.469
2.446
2.420
2.446
˚
˚
distances are slightly elongated to 1.106 A and 1.119 A with respect
to free nitrogen which has an N–N bond length of 1.097 A.20 This
˚
corresponds to a moderate activation of the complex (see below).
˚
˚
The two Mo–N distances are 2.037 A (Mo–N(a1)) and 2.050 A
(Mo–N(a2)), considerably longer than the corresponding Mo–N
distances in trans-[Mo(N2)2(dppe)2]21 (Mo–N = 2.014 A; N(1)–
N(2) = 1.118 A) and cis-[Mo(N2)2(PMe3)4] (Mo–N (average) =
˚
22
˚
˚
˚
˚
1.970 A; N(1)–N(2) = 1.15 A and N(3)–N(4) = 1.14 A).
The combined results from NMR spectroscopy and X-ray
structure analysis thus indicate that the direct synthesis of
the bis(dinitrogen) complex 2 leads to a mixture of trans-
[Mo(N2)2(meso-prP4)] 2a and cis-[Mo(N2)2(rac-prP4)] 2b. For the
trans isomer 2a, single crystals suitable for an X-ray struc-
ture determination have not been obtained yet. Based on the
structural characterization8 of [Mo(O)I(meso-prP4)]BPh4 (1), a
DFT geometry optimization23 of trans-[Mo(N2)2(meso-prP4)] 2a
was performed. The resulting structure (Fig. 3, left) shows the
octahedral trans bis(dinitrogen) complex 2a carrying the meso prP4
ligand 3a.
Fig.
4
Infrared (top) and Raman spectrum (bottom) of
trans-/cis-a-[Mo(N2)2(meso-/rac-prP4)] (2a/2b).
[Mo(N2)2(rac-prP4)] 2b. In particular, the vibrational spectra
show the existence of at least three N–N vibrations, confirming
the presence of two isomers. Comparison with the vibrational
spectra of 2a resulting from the stereoselective synthesis (Fig. 5)
identifies the symmetric combination of trans-[Mo(N2)2(meso-
prP4)] in the reaction product 2 at 2044 cm-1 and the antisymmetric
Fig. 3 DFT optimized structures of trans-[Mo (N2)2(meso-prP4)] (2a, left)
and cis-a-[Mo(N2)2(rac-prP4)] (2b, right).
In order to compare the calculated structure of 2a with the
structure of the cis complex 2b, a DFT calculation for the latter
complex was performed as well; the resulting geometry is shown
in Fig. 3, right. Table 3 lists selected calculated vs. experimental
bond lengths of 2b in comparison to 2a. Interestingly, the cis-a
complex exhibits larger Mo–P distances as compared to the trans
complex which is due to the trans-influence of the coordinated
phosphines (vide supra). On the other hand, the N–N distance
appears to be larger in the trans-complex than in the cis-complex,
which can be traced back to the electron-withdrawing effect of
the diphenylphosphine groups in the cis complex. Comparing the
calculated with the experimental structure of the cis complex it is
found that the metal–ligand distances are slightly too large in the
calculation, in agreement with prior results.24
Fig. 5 Infrared spectrum of trans-/cis-a-[Mo(N2)2(meso-/rac-prP4)]
(2) and trans-[Mo(N2)2(meso-prP4)] (2a, dotted line) in a range of
1700–2300 cm-1, indicating the positions of the symmetric and antisym-
metric N–N stretching vibrations.
IR and Raman spectroscopic data of the reaction product
(Fig. 4) are in agreement with the proposed formulation of
2 as a mixture of trans-[Mo(N2)2(meso-prP4)] 2a and cis-a-
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1
Fig. 6 31P{ H}-NMR spectra in THF-d8 at 300 K of trans-[W(N2)2(meso-prP4)] (12a) and cis-a-[W(N2)2(rac-prP4)] (12b).
combination at 1965 cm-1. The symmetric combination for the cis-
a-isomer 2b is observed at 2022 cm-1 and thus lowered by 22 cm-1
in energy compared to the trans isomer 2a. The antisymmetric N–
N stretching combination of 2b can be determined to 1972 cm-1
by subtracting the IR spectrum of the trans product 2a from
the isomeric mixture 2 (Fig. 5, top). Due to the more electron
withdrawing phosphine groups, the vibrations are slightly shifted
to higher energy in comparison to the more activated cis-
[Mo(N2)2(PMe3)4] (ns = 2010 cm-1, nas = 1965 cm-1).22
In the IR spectra of the bis(dinitrogen)–tungsten(0) complexes
12a and 12b (cf. ESI†) two N–N vibrations are found at 2008 cm-1
and 1937 cm-1, corresponding to the symmetric and antisymmetric
combination of the trans isomer 12a. Based on the position
of the N–N vibrations, the trans complex 12a is slightly more
highly activated than trans-[W(N2)2(dppe)2] (ns = 2009 cm-1, nas
=
1948 cm-1).6a Moreover, due to the more diffuse d-functions in
W as compared to Mo, the activation of the dinitrogen ligand is
slightly stronger in trans-[W(N2)2(meso-prP4)] 12a in relation to
its molybdenum analogue 2a. For the cis-a-isomer 12b no N–N
vibrations can be resolved as they are of low intensity due to the
isomeric ratio and probably are masked by the broad signals of
the trans complex 12a.
Tungsten bis(dinitrogen)complexes
In addition to the prepared molybdenum–bis(dinitrogen) com-
plexes 2a and 2b the tungsten complexes trans-[W(N2)2(meso-
prP4)] 12a and cis-a-[W(N2)2(rac-prP4)] (12b) could be synthesised
by an analogous procedure, using tungsten(VI) chloride as a pre-
cursor and magnesium turnings as the reducing agent (Scheme 3).
In summary, the observations made in the preceding sections
lead to the result that magnesium reduction of MoCl5 and
WCl6 with a mixture of the meso- and the rac-prP4 ligand in a
stereoselective fashion leads to formation of the trans-meso and
rac-cis-a Mo(0) and W(0) bis(dinitrogen) prP4 complexes. Other
configurations than these two appear to be excluded. Related
observations have been made with the ligand tetraphos–I (tet-
1) which contains a central ethylene group. The propensity for the
meso-tet-1 ligand to give trans osmium complexes and rac-tet-1
to give osmium complexes with a cis-a coordination geometry
has been reported.9 However, meso-tet-1 (L) does not only form
octahedral complexes in the trans (RuCl2L, MH(Cl)L, M =
Os, Ru, and M(H)2L, M = Ru, Fe) but also in the cis-b
(OsCl2L, Os(H)2L, and RuCl2L) configuration.9 A study of the
stereochemistry of a similar tetradentate ligand containing four
arsenic donor groups (tetars) indicates that for the meso ligand,
the trans complex is preferred over the cis-b complex, whereas
the rac-ligand forms the trans, cis-a and cis-b complex.10 By
reaction of prP4 with FeCl2 and 2 eq. of KSCN the complexes
1
The 31P{ H}-NMR spectrum of the reaction product 12 (Fig. 6)
shows an AA¢XX¢ pattern at 56.9 ppm and 41.4 ppm correspond-
ing to the trans isomer 12a and a double doublet at 45.8 ppm and
25.8 ppm according to the cis-a-isomer 12b in an intensity ratio
of about 3 : 1. For the trans product 12a the terminal and central
phosphorus atoms couple to each other with constants of 2JXX¢
-8.5 Hz, 2/4JAA¢ = -18.4 Hz, 2/3JAX¢/A¢X = 107.2 Hz and 2/3JAX/A¢X¢
=
=
10.2 Hz; in addition 183W–31P couplings can be determined to be
1JXW/X¢W = 322 Hz and 1JAW/A¢W = 302 Hz. According to the double
doublet pattern the second component has to be the cis-a-isomer
12b with the terminal phosphorous atoms coordinated in trans-
position to each other. Couplings between central and terminal
phosphorus atoms of 7.5 Hz and 8.6 Hz are observed. Moreover,
the coupling between the tungsten and the phosphorus atoms are
determined to be 1JWPterm = 313 Hz and 1JWPinner = 304 Hz.
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trans-[Fe(NCS)2(meso-prP4)], trans-[Fe(NCS)2(rac-prP4)] and cis-
1
a-[Fe(NCS)2(rac-prP4)] were obtained.7 Time-dependent 31P{ H}-
NMR spectroscopy of the pure cis-a-compound indicated that it
slowly (with a half-life of 2 days) converts to the corresponding
trans complex in solution. Formation of the meso-cis-b and rac-
cis-b isomers was not observed.
Reactivity of 2 and 13 with acids
The reactivity of the prepared bis(dinitrogen) complexes
[Mo(N2)2(prP4)] 2 and [W(N2)2(prP4)] 12 towards acids was
investigated as well. In order to minimize a possible exchange
of phosphine donors by anionic ligands, an acid with a weakly
coordinating conjugated base (trifluoromethane sulfonic acid)
was employed first. Upon protonation of the molybdenum com-
plex 2 with HOTf poorly resolved N–H bands in the region
of 3500 cm-1 appear, indicating formation of the Mo–NNH2
complex [Mo(NNH2)(prP4)(OTf)]+ (13). Dissolving the product
in deuterated methanol and stirring for 7 days leads to partial
deuteration, as is evident from the appearance of new bands
in the region of 2500 cm-1. Identification of the N–N stretch
is not possible as this signal is masked by vibrations of the
counterion. Similar observations are made upon protonation of
the W dinitrogen complex 12 with HOTf, leading to the NNH2
complex 14. The poor definition of the protonated products 13
and 14 possibly stems from the fact that both molybdenum and
tungsten dinitrogen complex (2 and 12) exist as a mixture of
cis and trans isomers; apparently only the trans isomer allows
a protonation to the corresponding NNH2 complex whereas the
cis complex decays irreversibly.
1
Fig.
7
31P{ H}-NMR spectra in CD2Cl2 at 300
K
of
[MoF(NNH2)(meso-prP4)]BF4 (15).
In order to check this hypothesis the Mo bis(dinitrogen)
complex trans-[Mo(N2)2(meso-prP4)] 2a obtained from the electro-
chemical route was subjected to protonation, this time with HBF4.
1
31P{ H}-NMR spectroscopic characterization of the reaction
product 15 shows the presence of an AA¢XX¢ pattern split by
31P–19F-couplings. This indicates a planar arrangement of the four
phosphorus donors and a trans F–NNH2 geometry (Fig. 7), with
Fig. 8 Infrared spectrum of [MoF(NNH2)(meso-prP4)]BF4 (15, top) and
[MoF(NND2)(meso-prP4)]BF4 (16, bottom).
-
the fluorine ligand having been transferred from BF4 . The signals
appear in the N–H stretching region (at 3316 cm-1, 3240 cm-1 and
3159 cm-1) which upon deuteration shift to 2496 cm-1, 2380 cm-1
and 2328 cm-1. These findings are also similar to those obtained
for [WF(NNH2)(dppe)2]BF4, where three bands at 3335 cm-1,
3258 cm-1 and 3175 cm-1 were observed as well, shifting to
2500 cm-1, 2389 cm-1 and 2345 cm-1 upon deuteration.6a
In the latter case the presence of three N–H stretches (which is
unexpected for an NNH2 complex) was ascribed to the presence
of H-bonding effects between the hydrogen atoms of the NNH2
group and the BF4 counter anion; we assume that this also applies
to the Mo–NNH2 complex 15.
of the central phosphorus atoms are observed at 46.7 ppm, while
those of the terminal phosphorus atoms are found at 41.9 ppm.
Due to the change of the oxidation state of the metal, both signal
patterns are shifted to higher field as compared to the parent
bis(dinitrogen) complex 2a. Moreover, coupling constants could
be determined which are collected and compared with those of the
trans bis(dinitrogen) complexes 2a and 12a in Table 4.
IR spectra of the protonation product 15 (Fig. 8) exhibit a
vibration at 1384 cm-1 which shifts upon deuteration to lower
energy, in analogy to the N N stretch of the NNH2 complex
[WF(NNH2)(dppe)2]BF4 at 1387 cm-1.6a Moreover, three bands
With respect to the molecular structure of 15, two possible
isomers exist; i.e., the NNH2 group can be oriented along the
central phenyl rings of the meso-prP4 ligand or in the opposite
direction. Based on the available spectroscopic information, we
cannot answer this question. However, the N2 ligand on the
opposite side of the phenyl groups is sterically less shielded whence
protonation of this ligand should be kinetically favoured. As in
the formation of NNH2 complexes from trans bis(dinitrogen)
complexes protonation of one N2 ligand precedes dissociation
and ligand exchange of the other N2 ligand,25 this would lead
to a hydrazido(2-) complex where the fluoro ligand is oriented
Table 4 31P–31P-coupling constants of 15 in comparison to 2a and 12a
trans-[MoF(NNH2)-
(prP4)]+ (15)
trans-[Mo(N2)2-
(prP4)] (2a)
trans-[W(N2)2-
(prP4)] (12a)
2/4
J
-27.0 Hz
-16.5 Hz
188.1 Hz
4.6 Hz
-22.0 Hz
-37.0 Hz
-29.9 Hz
-15.3 Hz
105.1 Hz
3.8 Hz
—
-18.4 Hz
-8.5 Hz
107.2 Hz
10.2 Hz
—
AA¢
2JXX¢
2JAX¢/A¢X
2/3
J
AX/A¢X¢
2JAF/A¢F
2JXF/X¢F
—
—
3234 | Dalton Trans., 2011, 40, 3229–3236
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along and the NNH2 ligand opposite to the central phenyl groups
of the prP4 ligand.
The functional used was B3LYP27 with the Ahlrichs-TZV basis-set
plus the TZV/J auxiliary basis28 in conjunction with the built-in
density-fitting and chain-of-spheres-approximation routines in the
program-package.29
Conclusions
[Mo(N2)2(prP4)] (2)
In the preceding sections a direct synthetic route leading to
the bis(dinitrogen) complex [Mo(N2)2(prP4)] 2 was presented. In
contrast to the electrochemical route developed earlier, giving
only the trans-complex 2a, the direct route leads to a mixture of
the trans-complex 2a and the cis-a-complex 2b. Importantly, the
trans-complex 2a is supported by the meso-prP4 3a ligand whereas
the cis-a-complex 2b contains the rac ligand 3b. Starting from a
mixture of the meso- and rac-prP4 ligand (3a and 3b), the direct
route thus in a stereoselective fashion leads to a mixture of trans-
[Mo(N2)2(meso-prP4)] (2a) and cis-a-[Mo(N2)2(rac-prP4)] (2b).
Other configurations are excluded, probably for steric reasons.
Furthermore, it is shown that the trans complex 2a can be
protonated with HBF4, leading to the NNH2 complex trans-
[MoF(NNH2)(meso-prP4)]BF4 (15) whereas the cis complex 2b
appears to be unstable towards protonation. Similar observations
are made with tungsten bis(dinitrogen) complexes 13a and 13b
which can be prepared and protonated to NNH2 complexes in
analogy to their molybdenum counterparts.
To a mixture of 255 mg (0.37 mmol) prP4 3 and 250 mg (10.4
mmol) magnesium turnings in 15 mL THF 105 mg (0.38 mmol)
MoCl5 were added slowly. The suspension was stirred at room
temperature for 24 h. The solution was separated from the
magnesium turnings and the solvent was removed in vacuo. The
residue was extracted with 15 mL of benzene and filtered. The
solution was reduced in vacuo to 5 mL and 15 mL n-hexane were
added. The formed precipitate was filtered off, washed with 4
¥ 4 mL n-hexane and dried in vacuo. Single crystals for X-ray
diffraction were obtained by slow evaporation over several days of
a concentrated solution of the product in benzene. Anal. calc. for
C43H46N2MoP4: C, 61.7; H, 5.3; N, 6.6, found: C, 61.2; H, 6.4; N,
1
5.3. 31P{ H}-NMR (161.975 MHz, C6D6, 300 K) (2a): d = 71.82
(m, |2/3
JAX/A¢X¢| = 3.8 Hz, JAX¢/A¢X = 105.2 Hz, JXX¢ = -15.3 Hz,
2
2
2/4
J
= -29.8 Hz, PA and PA¢), 63.59 (m, |2/3
JAX/A¢X¢| = 3.8 Hz,
2 2/4
AA¢
2JAX¢/A¢X = 105.2 Hz, JXX¢ = -15.3 Hz,
J
= 29.8 Hz, PX and
1
AA¢
PX¢). IR (KBr): vs(NN) = 2044 cm-1, vas(NN) = 1964 cm-1, 31P{ H}-
2
NMR (161.975 MHz, C6D6, 300 K) (2b): d = 66.3 (dd, JAD/BC
=
2
2
Experimental
3.8 Hz, JBD/AC = 4.8 Hz, PA and PB), 49.3 (dd, JAD/BC = 3.8 Hz,
2JBD/AC = 4.8 Hz, PA and PB) ppm. IR (KBr): vs(NN) = 2022 cm-1,
vas(NN) = 1972 cm-1.
General
All reactions were performed under an inert gas atmosphere using
Schlenk techniques. The solvents were dried and freshly distilled
under inert gas. The ligand prP4 was prepared by the reaction
of (2-chloroethyl)diphenylphosphine with Lippp.7,8 IR spectra
were obtained from KBr pellets using a Bruker IFS v66/S FT-
IR-Spectrometer. Resonance Raman spectra were recorded with
a DILOR XY-multichannel Raman spectrometer with a triple
monochromator and a CCD detector. Excitation wavelengths
between 454.5 nm and 647.1 nm were generated by an Ar+/Kr+
laser. NMR spectra were recorded on a Bruker Avance 400 pulse
[W(N2)2(prP4)] (12)
To a mixture of 255 mg (0.37 mmol) prP4 and 250 mg (10.4 mmol)
magnesium turnings in 15 mL THF was slowly added 153 mg (0.38
mmol) WCl6. The suspension was then stirred at room temperature
for 24 h. The solution was separated from the magnesium turnings
and the solvent was removed in vacuo. The residue was extracted
with 15 mL of benzene and filtered. The solution was reduced
in vacuo to 5 mL and 15 mL n-hexane were added. The formed
precipitate was filtered off, washed with 4 ¥ 4 mL n-hexane and
dried in vacuo. Anal. calc. for C43H46N2WP4: C, 55.8; H, 4.7; N,
1
Fourier transform spectrometer operating at a H frequency of
400.13 MHz (31P: 161.975 MHz) using a 5 mm inverse triple-
resonance probe head with z-gradient. Reference as substitutive
standard: H3PO4 85% pure, d(31P) = 0 ppm. X-ray structure
analysis: intensity data were collected using an STOE imaging
plate diffraction system (IPDS-1) with Mo-Ka radiation. The
structure was solved with direct methods using SHELXS-97, and
refinement was performed against F2 using SHELXL-97.26 All
non-hydrogen were refined anisotropically. The C–H H atoms were
positioned with idealized geometry and were refined using a riding
model. The crystal investigated was slightly twinned with a pseudo
2-fold axis as twinning element. The low reliability factors can be
attributed to the low crystallinity and the twinning of the crystals.
DFT calculations were performed employing the ORCA
program-package.23 The geometry of the cis-compound used
the crystal structure as the starting geometry without further
modification. The trans-compound was derived from the crystal
structure of the oxido–iodido–molybdenum(IV) complex, replac-
ing the oxygen and iodine-atom with dinitrogen and subsequent
6.0, found: C, 56.4; H, 5.4; N, 5.1. 31P{ H}-NMR (161.975 MHz,
1
THF-d8, 300 K) (12a): d = 56.95 (m, |2/3
JAX/A¢X¢| = 10.2 Hz,
2JAX¢/A¢X = 107.2 Hz, 2JXX¢ = -8.5 Hz, 2/4JAA¢ = 18.4 Hz, PA and PA¢,
183W-satellite: 1JAW/AW = 302 Hz), 41.44 (m, |2/3
AX/A¢X¢| = 10.2 Hz,
J
2JAX¢/A¢X = 107.2 Hz, 2JXX¢ = -8.5 Hz, 2/4 AA¢ = 18.4 Hz, 183W-satellite:
J
1JXW/X¢W = 322 Hz, PX and PX¢). IR (KBr): vs(NN) = 2008 cm-1,
vas(NN) = 1937 cm-1. 31P{ H}-NMR (161.975 MHz, THF-d8, 300
1
2
2
K) (12b): d = 45.88 (dd, JAD/BC = 7.5 Hz, JBD/AC = 8.6 Hz, PA
and PB, 183W-satellite: JAW/BW = 304 Hz, PA and PB), 25.84 (dd,
1
2JAD/BC = 7.5 Hz, 2JBD/AC = 8.6 Hz, 1JCW/DW = 313 Hz, PC,and PD)
ppm. IR (KBr): vs(NN) = 2008 cm-1, vas(NN) = 1937 cm-1.
[MoF(NNH2)(meso-prP4)]BF4 (15)
0.215 ml HBF4·Et2O were added to 400 mg (0.48 mmol) trans-
[Mo(N2)2(meso-prP4)] 2a in 10 mL dichloromethane at -15 ◦C. The
solution was stirred for 15 min and 15 mL of n-hexane were added.
The solid was filtered off, washed with 4 ¥ 4 mL diethylether
and dried under vacuum. Anal. calc. for BC43H46N2MoF5P4: C,
˚
adjustment of the Mo–N bondlength to 2.025 A. The full
1
optimizations were carried out using the ORCA program-package.
56.3; H, 5.0; N, 3.0, found C, 56.3; H, 5.1; N, 2.3. 31P{ H}-NMR
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2/4
(161.975 MHz, CD2Cl2, 300 K): d = 46.70 (m,
J
= -27 Hz,
7 C. M. Habeck, C. Hoberg, G. Peters, C. Na¨ther and F. Tuczek,
Organometallics, 2004, 23, 3252.
8 R. Ro¨mer, G. C. Stephan, C. Habeck, C. Hoberg, G. Peters, C. Na¨ther
and F. Tuczek, Eur. J. Inorg. Chem., 2008, 3258.
9 (a) M. T. Bautista, K. A. Earl, P. A. Maltby and R. H. Morris,
Can. J. Chem., 1994, 72, 547; (b) G. Jia, A. J. Lough and R. H. Morris,
J. Organomet. Chem., 1993, 461, 147.
AA¢
2
2
|
2/3JAX/A¢X¢| = 4.6 Hz, JAX¢/A¢X = 188.1 Hz, JXX¢ = -16.5 Hz,
2/4
2JXF/X¢F = -37 Hz, PX and PX¢), 41.06 (m,
J
= -27 Hz,
AA¢
2
2
|
2/3JAX/A¢X¢| = 4.6 Hz, JAX¢/A¢X = 188.1 Hz, JXX¢ = -16.5 Hz,
2JAF/A¢F = -22 Hz, PA and PA¢) ppm.
10 (a) B. Bosnich, W. G. Jackson and S. B. Wild, J. Am. Chem. Soc., 1973,
95, 8269; (b) A. L. Gavrilova and B. Bosnich, Chem. Rev., 2004, 104,
349.
Acknowledgements
11 E. Carmona, A. Galindo and L. Sanchez, Polyhedron, 1984, 3, 347.
12 E. Carmona, A. Galindo, C. Guille-Photin and L. Sanchez, Polyhedron,
1988, 7, 1767.
F.T. thanks DFG and the State of Schleswig-Holstein for funding
of this research.
13 J. A. Mata, S. Maria, J. C. Daran and R. Poli, Eur. J. Inorg. Chem.,
2006, 2624.
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©