Bonding and activation of N2 in Mo(0) complexes supported by hybrid tripod ligands with mixed dialkylphosphine/diarylphosphine donor groups: Interplay of steric and electronic factors

Article abstract of DOI:10.1021/ic400582v

Molybdenum dinitrogen complexes are presented which are supported by novel hybrid tripod ligands of the type Me-C(CH₂PPh₂) ₂(CH₂PⁱPr₂) (trpd-1) and H-C(CH ₂PPh₂)(CH₂PⁱPr₂) ₂ (trpd-2) having mixed dialkylphosphine/diarylphosphine donor groups. Reaction of the ligand trpd-1 with [MoI₃(thf)₃] followed by sodium amalgam reduction in the presence of the dppm gives the dinitrogen complex [Mo(N₂)(trpd-1)(dmpm)] where trpd-1 is coordinated in a κ³ fashion. The complex exhibits a moderate activation of N₂ which enables its protonation under retention of the pentaphosphine ligation. Replacement of dmpm by the sterically more demanding coligand dppm is found to hamper coordination of N₂ and leads to [Mo(trpd-1)(dppm)], the first structurally characterized five-coordinate Mo(0) complex with a phosphine-only ligand sphere. Employing the ligand trpd-2 along with the diphosphines dmpm and dppm in an analogous synthetic route results in a mixture of the bis(dinitrogen) complexes trans-[Mo(N₂) ₂(κ²-trpd-2)(diphosphine)] and trans-[Mo(N ₂)₂(iso-κ²-trpd-2)(diphosphine)] where the tripod ligand trpd-2 coordinates with two phosphine arms and one phosphine group (PPh₂ or PⁱPr₂, respectively) is free. Similar results are obtained with the pure alkyl-and arylphosphine tripod ligands H-C(CH₂PⁱPr₂)₃ (trpd-3) and H-C(CH₂PPh₂)₃ (tdppmm), leading to trans-[Mo(N₂)₂(κ²-trpd-3)(diphos)] and trans-[Mo(N₂)₂(κ²-tdppmm)(dmpm)], respectively. The electronic and steric reasons for the experimental findings are considered, and the implications of the results for the area of synthetic nitrogen fixation with molybdenum phosphine systems are discussed.

Full text of DOI:10.1021/ic400582v

Article
pubs.acs.org/IC
Bonding and Activation of N2 in Mo(0) Complexes Supported by
Hybrid Tripod Ligands with Mixed Dialkylphosphine/
Diarylphosphine Donor Groups: Interplay of Steric and Electronic
Factors
Ludger Soncksen, Christian Gradert, Jan Krahmer, Christian Nather, and Felix Tuczek*
̈
̈
Institut fur Anorganische Chemie, Christian Albrechts Universitat Kiel, Max-Eyth-Strasse 2, D-24118 Kiel, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Molybdenum dinitrogen complexes are presented
which are supported by novel hybrid tripod ligands of the type Me-
C(CH2PPh2)2(CH2PiPr2) (trpd-1) and H−C(CH2PPh2)-
(CH2PiPr2)2 (trpd-2) having mixed dialkylphosphine/diarylphos-
phine donor groups. Reaction of the ligand trpd-1 with [MoI3(thf)3]
followed by sodium amalgam reduction in the presence of the dppm
gives the dinitrogen complex [Mo(N2)(trpd-1)(dmpm)] where
trpd-1 is coordinated in a κ3 fashion. The complex exhibits a
moderate activation of N2 which enables its protonation under
retention of the pentaphosphine ligation. Replacement of dmpm by
the sterically more demanding coligand dppm is found to hamper
coordination of N2 and leads to [Mo(trpd-1)(dppm)], the first
structurally characterized five-coordinate Mo(0) complex with a
phosphine-only ligand sphere. Employing the ligand trpd-2 along with the diphosphines dmpm and dppm in an analogous
synthetic route results in a mixture of the bis(dinitrogen) complexes trans-[Mo(N2)2(κ2-trpd-2)(diphosphine)] and trans-
[Mo(N2)2(iso-κ2-trpd-2)(diphosphine)] where the tripod ligand trpd-2 coordinates with two phosphine arms and one phosphine
group (PPh2 or PiPr2, respectively) is free. Similar results are obtained with the pure alkyl- and arylphosphine tripod ligands H−
C(CH2PiPr2)3 (trpd-3) and H−C(CH2PPh2)3 (tdppmm), leading to trans-[Mo(N2)2(κ2-trpd-3)(diphos)] and trans-
[Mo(N2)2(κ2-tdppmm)(dmpm)], respectively. The electronic and steric reasons for the experimental findings are considered,
and the implications of the results for the area of synthetic nitrogen fixation with molybdenum phosphine systems are discussed.
One possible strategy to avoid these problems is to occupy
the position trans to the dinitrogen ligand in the Mo0 complex
by a phosphine group. To achieve this goal tridentate
phosphine ligands like dpepp (bis(diphenylphosphinoethyl)-
phenylphosphine) have already been applied, leading to
complexes of the type [Mo(N2)(dpepp)(diphosphine)]
(diphosphine = dppm (bis(diphenylphosphino)methane),
dmpm (bis(dimethylphosphino)methane), dppe (1,2-bis-
(dimethylphosphino)ethane), depe (1,2-bis(ethylphosphino)-
ethane), and 1,2-dppp (R-(+)-1,2-bis(diphenylphosphino)-
propane)).23−27 Because of the linear topology of the dpepp
ligand, however, different isomeric forms of the mentioned
complexes with varying degrees of activation exhibited by the
dinitrogen ligand were obtained.23,27 As we have shown
recently, this complication can be avoided by use of tridentate,
symmetric tripod phosphine ligands like 1,1,1-tris-
(diphenylphosphinomethyl)ethane (tdppme) along with bi-
dentate coligands like dmpm or dppm, leading to complexes of
the type [Mo(N2)(tdppme)(diphosphine)].28 The tripodal
INTRODUCTION
■
Coordination and activation of the highly inert, nonpolar
molecule dinitrogen to enable its reduction to ammonia or its
functionalization to high-value nitrogen-containing compounds
are topics of fundamental interest in inorganic and organo-
metallic chemistry.1−6 Synthetic nitrogen fixation in particular
refers to the conversion of dinitrogen to ammonia in
homogeneous solution and under ambient conditions.7 The
first complete mechanistic scenario for this reaction was the
Chatt cycle which starts by protonation of bis(dinitrogen)
phosphine-Mo/W complexes carrying diphosphine ligands like
dppe or depe.8 Our group has been involved in synthetic,
spectroscopic, and theoretical investigations of Chatt type
complexes over the past years.9−21 The classic Chatt cycle,
however, suffers from a couple of drawbacks that limit its
performance in a catalytic reaction mode. In particular, the
conjugate base of the acid applied for protonation displaces one
of the dinitrogen ligands in the parent bis(dinitrogen) complex,
leading to a loss of 50% of bound substrate. Moreover, the
presence of anionic coligands causes disproportionation at the
MoI stage which corresponds to 50% loss of reactive complex.22
Received: March 8, 2013
© XXXX American Chemical Society
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geometry of the ligand tdppme was in fact found to enforce the
desired facial coordination, and formation of isomeric
complexes was eliminated.
RESULTS AND ANALYSIS
■
1. Synthesis and Constitution of the Complexes.
A. Ligand Synthesis. To prepare ligands trpd-1 and trpd-2
with mixed diisopropyl-/diphenylphosphine end groups a
preparative method was applied that is based on mono- or
disubstituted proligands obtained by stoichiometric addition of
the respective lithium phosphide to the respective trichloro
precursor.30
The complex [Mo(N2)(tdppme)(dppm)] exhibited a weak
activation of N2. By choosing dmpm as coligand, however, the
activation of the dinitrogen ligand could be increased
significantly, allowing its protonation to [Mo(NNH2)-
(tdppme)(dmpm)](OTf)2. This was ascribed to the fact that
alkyl phosphine donor groups provide more electron density to
the metal center for the activation of dinitrogen than their
arylphosphine counterparts.28 Replacement of aryl- by
alkylphosphine donor groups within the tripod ligand tdppme
would, of course, have a similar effect. Moreover, it would
principally allow examining the variation of the substituents at
the phosphine group in the trans-position to the N2 ligand,
which is not possible with the equatorially coordinating,
bidentate coligands dppm and dmpm. We therefore wanted
to prepare a set of tripod ligands in which the diphenylphos-
phine groups are replaced by diisopropylphosphine groups in a
stepwise fashion, anticipating that in one of the resulting
molybdenum dinitrogen complexes one of these groups would
be positioned trans to the N2 ligand. Following this idea, the
new ligands trpd-1 (Me-C(CH2PPh2)2(CH2PiPr2)) with one
diisopropylphosphine group, trpd-2 (H−C(CH2PPh2)-
(CH2PiPr2)2) with two and trpd-3 (H−C(CH2PiPr2)3) with
three diisopropylphosphine groups were synthesized (Scheme
1). For preparative reasons the methyl group on the apical
Me-C(CH2PPh2)2(CH2PiPr2) (trpd-1). The reaction route
starts with substitution of one chlorine group of 1,1,1-
tris(chloromethyl)ethane (8) by borane-protected lithium
diisopropyl phosphide LiPiPr2−BH3, giving 1,1-(bis-
(chloromethyl))-1-(diisopropylphosphinoboranemethyl)-
ethane, Me-C(CH2Cl2)2(CH2PiPr2−BH3) (9; Scheme 2).
Scheme 2
Addition of 2 equiv of lithium diphenyl phosphide results in
compound 10, and subsequent deprotection by morpholine
leads to the mixed diisopropyl-/diphenylphosphine tripod 1,1-
(bis(diisopropylphosphinomethyl))-1-(diphenylphosphino-
methyl)ethane, Me-C-(CH2PPh2)2- (CH2PiPr2) (trpd-1).
The synthesis of mixed alkyl aryl tripod ligands on this route
is limited by two factors, the large steric demand of borane-
protected diisopropyl phosphide and the little space being
available in the neopentyl backbone. Use of the borane
protection group, on the other hand, is required for the
purification procedures by column chromatography. The
described problem is illustrated by the fact that addition of 2
equiv of Li−PiPr2−BH3 to 8 primarily results in Me-
C(CH2Cl)2(CH2PiPr2−BH3) (9) as well. Consequently, it
was not possible to introduce two of the borane-protected
diisopropyl phosphine groups into the neopentyl backbone.
H−C(CH2PPh2)(CH2PiPr2)2 (trpd-2). To substitute two
chlorine groups of the tris(chloromethyl) precursor by
borane-protected lithium diisopropylphosphide we had to
switch to a tripod with an isobutyl backbone (Scheme 3).
We expected that because of the absence of the central methyl
group the precursor 1,1,1-tris(chloromethyl)methane (11)
should exhibit a significantly lower steric hindrance regarding
substitution of the chlorine atoms. In fact, addition of 2 equiv of
LiPiPr2−BH3 to 11 led to doubly borane-protected 1-
(chloromethyl)-1,1-(bis(diisopropylphosphinomethyl))-
methane (12) (Scheme 3, right). Subsequent substitution of
the remaining chlorine atom by LiPPh2 and deprotection in
morpholine afforded 1,1-(bis(diisopropylphosphinomethyl))-1-
(diphenylphosphinomethyl)methane, H−C(CH2PPh2)-
(CH2PiPr2)2 (trpd-2), which exhibits an inverted distribution
of phosphine donor groups as compared to trpd-1.
Scheme 1
atom of the tripod was replaced by a hydrogen atom in the
ligands trpd-2 and trpd-3, leading to an isobutyl backbone in
these systems. To allow a direct comparison between the two
different tripod architectures the isobutyl version of tdppme,
that is, the symmetric arylphosphine tripod 1,1,1-tris-
(diphenylphosphinomethyl)methane (H−C(CH2PPh2)3;
tdppmm),29 has been included in this study as well (Scheme
1; double arrows denote substitutions of one group at the
phosphine donors or at the backbone atoms).
In the present study this set of tripod ligands is employed in
combination with the diphosphine coligands dmpm and dppm
to the synthesis of molybdenum dinitrogen complexes. The
resulting compounds are characterized by X-ray crystallography
as well as vibrational and NMR spectroscopy, and their
reactivity with respect to protonation is investigated. The
implications of the results for the area of synthetic nitrogen
fixation with molybdenum phosphine complexes are discussed.
H−C(CH2PiPr2)3 (trpd-3) and H−C(CH2PPh2)3 (tdppmm).
Addition of 3 equiv of LiPiPr2 or LiPPh2 to 11 (Scheme 3,
middle, left) gave the respective symmetric tripod ligands 1,1,1-
tris(diisopropylphosphinomethyl)methane (trpd-3) and 1,1,1-
tris(diphenylphosphinomethyl)methane (tdppmm), respec-
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It turned out that the best results for the ligands employed in
this study are obtained by stirring [MoI3(thf)3]32,38 with the
respective tripod overnight at room temperature in THF
(Scheme 4). The resulting red solution was concentrated, and
Scheme 3
Scheme 4
the [MoI3(trpd-X)] complex was isolated after precipitation
upon addition of diethyl ether or hexane. The tripod
complexation occurs reasonably fast and can be performed in
a practical time range. Furthermore, the synthesis is generally
applicable and allows the preparation of the different
molybdenum(III) tripod complexes in good purities and high
yields.
After the subsequent sodium amalgam reduction (see below)
the ligands trpd-X (X = 2, 3) and tdppmm are found to be
coordinated in a κ2-fashion. This raises the question about the
coordination mode on the Mo(III) stage. Elemental analysis
and DTA-TG measurements show no evidence for remaining
coordinated thf ligands in the Mo(III) complexes with the
neopentyl ligand trpd-1 and the isobutyl ligands trpd-2 and
trpd-3 (Supporting Information, Figures S1−S3). In contrast,
the Mo(III) complex with isobutyl tripod tdppmm seems to be
coordinated in a κ2-fashion, corresponding to the complex
[MoI3(thf)κ2-(tdppmm)]. This is indicated by the fact that the
DTA-TG measurement shows a mass loss at 137 °C that
corresponds to one coordinated thf-ligand (−6%, MID 72 m/z)
(Supporting Information, Figure S4).
Mo(0) Dinitrogen Complexes. To obtain molybdenum
dinitrogen complexes supported by our mixed tripod ligands,
the respective precursor was stirred with sodium amalgam (7−
11 equiv) in THF under a nitrogen atmosphere with addition
of 1 equiv of dmpm or dppm as coligands. After partial
evaporation of the solvent THF, addition of methanol and
further concentration of the solution, the dinitrogen complexes
could be obtained as red precipitates or red viscous oils,
depending on the tripod ligand and the employed coligand.
[Mo(N2)(trpd-1)(dmpm)] (1). Reduction of [MoI3(trpd-1)]
with dmpm resulted in the octahedral Mo(0) pentaphosphine
complex [Mo(N2)(trpd-1)(dmpm)] (1; Scheme 5). As inferred
from X-ray structure analysis and 31P NMR spectroscopy (see
below), the diisopropyl phosphine group is coordinated in the
equatorial plane of this complex, in cis position to the axial
dinitrogen ligand. The other possible coordination isomer with
the diisopropyl phosphine group in trans position to the N2
ligand is not formed.
tively. The ligand trpd-3 is the first tripodal ligand with three
diisopropylphosphine end groups on the basis of a carbon
backbone. The tripod tdppmm has already been synthesized by
Janssen et al.29
B. Synthesis and Constitution of Mo(III) and Mo(0)
Complexes. Molybdenum dinitrogen complexes with the set
of tripod ligands described above were prepared employing the
Mo(III) precursors [MoX3(tripod)] X = Cl, Br, I, which in turn
can be synthesized by reacting [MoX3(thf)3] with the
respective tripod ligand in THF or toluene.28,31−33 Subsequent
sodium amalgam reduction under a nitrogen atmosphere with
addition of the respective diphosphine coligand then afforded
the corresponding dinitrogen complex.
Mo(III) Complexes. Because of facile substitution of the three
tetrahydrofuran (thf, THF) ligands, the Mo(III) precursors
[MoX3(thf)3] (X = Cl, Br, I) are well suited for the
coordination of tridentate ligands.34 A barrier for the
coordination of tripod ligands lies in the fact that the thf
ligands in all [MoX3(thf)3] complexes are coordinated in
meridional geometry,35−37 which has to change to facial in the
target compounds. For [MoI3(thf)3] such a change has, for
example, been observed in the conversion to fac-[MoI3(dppe)-
(PMe3)].32 The reactions of [MoBr3(thf)3] and [MoCl3(thf)3]
with tdppme provided further evidence for the transition from a
meridional to a facial coordination28,33 However, refluxing a
solution of the tripod ligand trpd-1 in toluene with
[MoBr3(thf)3], as successfully applied for the coordination of
the tdppme ligand to Mo(III),28 turned out to be too harsh for
our mixed dialkylphosphine/diarylphosphine tripods. Only low
quantities of product could be obtained which in addition were
contaminated by black impurities, probably because of a
possible oxidation of the alkyl phosphines. Similar degradation
reactions have been observed upon refluxing [MoI3(PEt3)3]
and [MoCl3(PMe3)3] in toluene.36 More gentle reaction
conditions, that is, stirring at ambient temperature in THF,
which successfully have been applied by Dahlenburg et al. to
the coordination of linear multidentate alkylphosphane ligands
to [MoCl3(thf)3],31 gave unsatisfyingly low yields, too.
Replacement of the solvent THF by a mixture of THF and
dichloromethane neither lead to improvements regarding the
yield and the purity of the products.33
Scheme 5
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Scheme 6
[Mo(trpd-1)(dppm)] (2). Reduction of [MoI3(trpd-1)] in the
presence of dppm did not lead to a dinitrogen complex. The
reaction instead resulted in a red powder which according to
elemental analysis and vibrational spectroscopy does not
contain dinitrogen. This result was supported by 31P NMR
spectroscopy (see below) which indicates that [Mo(trpd-
1)(dppm)] (2) is a Mo(0) complex that contains only five
coordinated phosphine groups (Scheme 5). As for the related
complexes [Mo(N2)(tdppme)(dmpm)] and [Mo(N2)(trpd-
1)(dmpm)] (vide supra) conventional octahedral coordination
geometries have been evidenced, this rare coordination mode
obviously results from replacement of one PPh2 by a sterically
more demanding PiPr2 group in the tdppme ligand along with
replacement of dmpm by the bigger coligand dppm.
NMR spectrum shows that both isomers 4a and 4b are formed
in about equal quantities (Supporting Information, Figure S6).
[Mo(N2)2(κ2-trpd-3)(dppm)] (5) and [Mo(N2)2(κ2-trpd-3)-
(dmpm)] (6). Sodium amalgam reductions of [MoI3(trpd-3)]
under N2 in the presence of dppm or dmpm both resulted in
trans-bis(dinitrogen) complexes (5 and 6) with one tripod arm
being free (Scheme 7).
Scheme 7
Possible limiting geometries for complex 2 are square-
pyramidal or trigonal-bipyramidal. The latter has been observed
for the five coordinate complex [Mo(C2H2) (dppe)2] by Ishino
et al.39 A trigonal-bipyrimdal structure is also indicated by the
crystal structure analysis of 2 (see below). The 31P NMR
spectrum of 2, however, is not compatible with this geometry
and rather indicates a trigonal-pyramidal structure with the
dppm ligand rotating around an axis that is approximately
perpendicular to the plane of the three tripod P atoms. To the
best of our knowledge, no other structurally characterized
example for such a five-coordinate Mo(0) complex exists in the
literature.
[Mo(N2)2(κ2-tdppmm)(dmpm)] (7). Sodium amalgam reduc-
tion of the complex [MoI3(thf)κ2-(tdppmm)] in the presence
of the diphosphine ligand dmpm under N2 was also found to
lead to a trans-bis(dinitrogen) complex, trans-[Mo(N2)2(κ2-
tdppmm)(dmpm)] (7), with one PPh2 group of the tripod
ligand being free (Scheme 7). Note that the analogous
neopentyl tripod ligand tdppme along with coligands dmpm
and dppm afforded the κ3-complexes [Mo(N2)(tdppme)-
(diphos)].28 This difference is considered in more detail below.
2. Structural and Spectroscopic Characterization. A. X-
ray Structure Determination. Information on the solid-state
structures of compounds 1 and 2 was obtained from X-ray
crystallography. Selected crystal structure data are collected in
Table 1.
[Mo(N2)(trpd-1)(dmpm)] (1). Single crystals of complex 1
were grown by evaporation of a THF/pentane solution. The
compound crystallizes in the space group P21/n. Figure 1 shows
the structure of 1. The octahedral complex has five coordinated
phosphine groups. The two dimethylphosphine donors of
dmpm as well as one PPh2 and one PiPr group of trpd-1 are
coordinated in the equatorial plane. The second PPh2 group is
bound in trans position to the N2 ligand. The Mo−Nα distance
(2.047(2) Å) and the Nα-Nβ distance (1.055(3) Å) are both
slightly shorter than in [Mo(N2)(tdppme)(dmpm)] (2.066(6)
Å and 1.069(8) Å, respectively; cf. Table 2);28 as in the latter
complex the N−N distance of 1 is also shorter than in
molecular dinitrogen (1.098 Å).40
[Mo(N2)2(κ2-trpd-2)(dppm)]/[Mo(N2)2(iso-κ2-trpd-2)-
(dppm)] (3a,b). Sodium amalgam reduction of [MoI3(trpd-2)]
under N2 in the presence of dppm results in a mixture of two
trans-bis(dinitrogen) complexes (Scheme 6) with only four
coordinated P groups, one P donor of the tripod phosphine
arms being free. In the isomer denoted as [Mo(N2)2(κ2-trpd-
2)(dppm)] (3a) two different P donor groups (PiPr2 and PPh2)
of the tripod are coordinated in the equatorial plane whereas in
the other one two PiPr2 groups are coordinated at these
positions (3b). By integration of the 31P NMR spectrum isomer
3a with coordinated PPh2 and PiPr2 groups can be identified as
the main product (ratio 3a: 3b ∼ 6: 1; Supporting Information,
Figure S5). A possible reason for this finding is the fact that
coordination of one aromatic phosphine group enables π-
backbonding which results in a lower energy as compared to
coordination of two alkylphosphine groups.
[Mo(N2)2(κ2-trpd-2)(dmpm)]/[Mo(N2)2(iso-κ2-trpd-2)-
(dmpm)] (4a,b). To reduce the steric demand of the
diphosphine coligand and obtain a dinitrogen complex with a
κ3-coordinated tripod the sodium amalgam reduction was
repeated with the smaller coligand dmpm (Scheme 6).
However, the reaction again led to a mixture of two
coordination isomers, [Mo(N2)2(κ2-trpd-2)(dmpm)] (4a)
and [Mo(N2)2(iso-κ2-trpd-2)(dmpm)] (4b), with one phos-
In comparing to the symmetric tdppme complex, we observe
a deformation of the facial coordination of the trpd-1 ligand. In
complex [Mo(N2)(tdppme)(dmpm)] the two P−Mo−P angles
from the equatorial tripod P atoms to the P atom in trans
position to N2 are almost equal (85.7° and 86.0°), and all
phine arm of the tripod ligand being free. Integration of the 31
P
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Table 1. Selected Crystal Data and Details on the Structure
Determinations from Single Crystal Data of Compound 1
and Compound 2
Table 2. Selected Bond Distances and Angles of Compound
1
́
Bond Distances [Å]
1
2
Mo−N(1)
Mo−P(a)
Mo−P(c)
Mo−P(e)
2.047(2)
N(1)−N(2)
Mo−P(b)
Mo−P(d)
1.055(3)
2.4749(5)
2.4428(5)
2.4653(6)
2.4130(6)
2.4439(5)
formula
MW/g mol−1
crystal system
space group
a/Å
C40H57MoN2P5
816.67
C60H65MoP5
1036.91
monoclinic
P21/c
monoclinic
P21/n
Bond Angles [deg]
N(1)−Mo−P(a)
N(1)−Mo−P(c)
N(1)−Mo−P(e)
P(e)−Mo−P(a)
P(e)−Mo−P(c)
P(b)−Mo−P(a)
P(d)−Mo−P(a)
87.87(5)
N(1)−Mo−P(b)
N(1)−Mo−P(d)
87.25(5)
89.64(5)
12.3102(9)
18.5529(9)
20.8163(18)
106.03 (1)
4569.3(6)
200(2)
12.3476(7)
25.3230(19)
18.3494(12)
95.107(7)
5714.7(7)
200(2)
91.43(5)
b/Å
174.35(5)
97.319(18)
83.593(2)
67.444(2)
99.836(3)
c/Å
P(e)−Mo−P(b)
P(e)−Mo−P(d)
P(c)−Mo−P(b)
P(c)−Mo−P(d)
96.780(18)
87.283(5)
107.532(3)
85.158(2)
β/deg
V/Å3
T/K
Z
4
4
D
calc/g cm−3
1.187
1.205
μ/mm−1
0.489
0.405
solution of the complex. Figure 2 shows the structure of 2. The
compound crystallizes in the space group P21/c. Selected bond
lengths and angles are collected in Table 3.
min/max transmission
θmax/deg
0.8215/0.9564
25.95
27.00
measured reflections
unique reflections
Rint
36756
59603
12112
0.0612
10505
596
8862
0.0540
reflections [Fo > 4σ(Fo)]
parameter
7566
457
a
R1 [Fo > 4σ(Fo)]
0.0332
0.0819
0.1838
1.252
b
wR2 [all data]
0.0837
GOF
Δρmax, Δρmin/e Å−3
1.045
0.358, −0.702
b
1.388, −0.746
2
a
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑[w(Fo − Fc )2]/
2
∑[w(Fo )2]]1/2
.
Figure 2. Single crystal structure of [Mo(trpd-1)(dppm)] (2);
hydrogen atoms are omitted (displacement ellipsoids are drawn at
the 50% probability level). No indication of an agostic interaction
between the Mo center and the C−H of one of the iPr groups in P(e)
could be found.
Table 3. Selected Bond Distances and Angles of Compound
2
́
Bond Distances [Å]
Mo−P(a)
Mo−P(c)
Mo−P(e)
2.4782(14)
2.3979(15)
2.5304(14)
Mo−P(b)
Mo−P(d)
2.3986(13)
2.4511(16)
Figure 1. Single crystal structure of [Mo(N2)(trpd-1)(dmpm)] (1);
hydrogen atoms are omitted (displacement ellipsoids are drawn at the
50% probability level).
Bond Angles [deg]
P(c)−Mo−C(60)
P(e)−Mo−C(60)
P(e)−Mo−P(a)
P(c)−Mo−P(a)
P(d)−Mo−P(a)
P(e)−Mo−P(c)
P(c)−Mo−P(d)
131.72
P(d)−Mo−C(60)
134.30
120.10
105.81(5)
166.48(5)
104.24(5)
84.51(5)
85.32(5)
P(e)−Mo−P(b)
P(c)−Mo−P(b)
P(d)−Mo−P(b)
P(e)−Mo−P(d)
P(b)−Mo−P(a)
141.08(6)
99.68(5)
135.27(5)
83.50(5)
66.80(5)
tripod bond lengths are in the range of Mo−P = 2.39−2.44 Å.
The distortion in compound 1 is reflected by a difference of the
angle between the diisopropyl phosphine donor Pd and the
diphenylphosphine donor Pe in trans position to N2 (87.3°)
and the angle between the two arylphosphine donors Pc and Pe
(83.6°). Moreover, the Mo−Pc bond (2.4130(6) Å) is shorter
than the Mo−Pd bond (2.4428(5) Å). The angle between Pc
and Pd in the equatorial plane is 85.2°, which is slightly larger
than in the tdppme complex (82.3°). All of these observations
indicate a larger steric demand of PiPr2 as compared to PPh2.
The bite angle 67.4° of the coligand dmpm is almost similar to
the angle in [Mo(N2)(tdppme)(dmpm)] (66.9°).28
The structure determination proves that compound 2 only
contains five coordinated phosphine donor groups and no sixth
ligand. The geometry shown in Figure 2 is best described as
distorted trigonal bipyramidal with atoms P(b), P(d), and P(e)
forming the trigonal plane and atoms P(a) and P(c) being in
axial positions. Within the trigonal plane the angles [P(e)−
Mo−P(b)] and [P(d)−Mo−P(b)] are 141° and 135° whereas
the angle [P(e)−Mo−P(d)] is 83°, giving a sum of 359°. In
[Mo(trpd-1)(dppm)] (2). Single crystals of compound 2
could be obtained by evaporation of a diethylether into a THF
E
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a
comparison to complex 1 the angles [P−Mo−P] within the
tripod (83.5−85.3°) are more equal to each other, showing less
distortion of the tripod coordination. The dppm bite angle
(66.8°) is similar to the dmpm angle in complex 1.
Table 4. Infrared and Raman (Middle) N−N Stretching
b
Wavenumbers
ν(N−N) IR/
ν(N−N)
number of alkyl
phosphines
compound
cm−1
RA/cm−1
B. Vibrational Spectroscopy. Vibrational spectroscopy is
ideally suited to monitor the activation of coordinated
N2.10,12,14,41−44 The IR and Raman spectra of [Mo(N2)(trpd-
1)(dmpm)] (1) (Figure 3) show the N−N stretch at ν = 1965
1 [Mo(N2)(trpd-1)
1965
1966
1980
2040
3
(dmpm)]
[Mo(N2)(tdppme)
1979
2
(dmpm)]28
[Mo(N2)(tdppme)
(dppm)]28
2035
0
4a, b [Mo(N2)2
2009 s/1933 as
2019 s/1936 as
2005 s/1925 as
2006 s/1935 as
2018 s/1937 as
4/3
2/1
4
(κ2-trpd-2)(dmpm)]
3a, b [Mo(N2)2
(κ2-trpd-2)(dppm)]
6 [Mo(N2)2(κ2-trpd-3)
2006
2005
2018
(dmpm)]
5 [Mo(N2)2(κ2-trpd-3)
2
(dppm)]
7 [Mo(N2)2
(κ2-tdppmm)
(dmpm)]
2
a
Symmetric and antisymmetric stretching for bis(dinitrogen) com-
b
plexes) are given on the left. The number of alkyl phosphine groups
coordinated to Mo is given on the right.
complexes is summarized in the Supporting Information, Table
S1. The coupling constants derived from analysis of the spectra
were checked by simulation of the 31P NMR spectra.
[Mo(N2)(trpd-1)(dmpm)] (1). The 31P NMR spectrum of 1
contains a set of five dddd-signals located at 47.3, 41.1, 40.3,
−23.9, and −25.0 ppm, which are associated with the five P
nuclei coupling among each other (cf. Figure 4). The signals
could be assigned by comparison of the corresponding coupling
constants. The resulting assignment was supported by the
1H-31P HMBC spectrum which allowed identifying the signal of
the PiPr2-group (Supporting Information, Figure S12). The
signals at −23.9 ppm and −25.0 ppm are attributed to the two
atoms Pa and Pb of the dmpm ligand which couple among each
other with a constant of |2Jab| = 3.8 Hz. The coupling consists of
a metal- and a ligand-mediated contribution. The negative shift
is in accordance with the high shielding of the P nuclei by the
methyl groups, which was also observed for [Mo(N2)-
(tdppme)(dmpm)] exhibiting signals in the range between
−22.5 to −23.5 ppm.28 The signal at 40.3 ppm can be assigned
to the aromatic P atom Pe in trans position to N2 which couples
via the metal to all other P atoms in cis-position with constants
in the range between 2J(cis) = −20.8 to −27.3 Hz, resulting in a
strongly overlapped dddd signal. The signal of the second
aromatic P atom Pc is located at 41.1 ppm, and the signal of the
diisopropyl Pd atom is found at 47.29 ppm. Both atoms couple
with four other P atoms, corresponding to three coupling
Figure 3. IR (top) and Raman (bottom) of [Mo(N2)(trpd-
1)(dmpm)] (1).
cm−1 which indicates a moderately activated dinitrogen ligand.
Because of the additional alkylphosphine donor, the electron
density at the molybdenum center is higher as compared with
[Mo(N2)(tdppme)(dmpm)], which exhibits a N−N stretch of
ν = 1979 cm−1. The shift of the vibration at lower wavenumbers
in compound 1 is in contrast to the shorter N−N bond length
determined in the crystal structure of 1 (N−N = 1.055(3) Å) as
compared with [Mo(N2)(tdppme)(dmpm)] (N−N = 1.069(8)
Å) (vide supra).
Complex 2 does not show any vibrations of a bound N2
ligand, in agreement with the structural and NMR data. The
bis(dinitrogen) complexes 2 to 7 are coordinated by the
respective tripod ligands in a κ2 fashion and therefore exhibit
symmetric and antisymmetric N−N stretches; the correspond-
ing frequencies are collected in Table 4 (vibrational spectra are
given in Supporting Information, Figures S7−S11). The
intensity distribution observed in the infrared spectra (higher
intensities for the antisymmetric stretches and lower intensities
for the symmetric stretches; Raman spectra vice versa)
qualitatively agree with the data obtained on trans-bis-
(dinitrogen) complexes with two diphosphine ligands.14,44 In
general it is observed that a larger number of alkyl phosphine
groups results in a higher activation (lower frequency),
although the frequency changes are smaller for the bis-
(dinitrogen) complexes than for their mono(dinitrogen)
counterparts. This is particularly evident for complexes 5 and
6 which only differ by the coligand, that is, in [Mo(N2)2(κ2-
trpd-3)(dmpm)] (6) the asymmetric stretching frequency is
only 10 cm−1 lower than in complex 5, [Mo(N2)2(κ2-trpd-
3)(dppm)].
2
constants J(cis) in the range between −20.8 to −27.3 Hz and
2
two larger trans coupling constants J(trans) of 93.5 and 86.8
Hz, respectively.
[Mo(trpd-1)(dppm)] (2). The 31P NMR spectrum of 2
contains three signals located at 48.9 ppm (dt), 40.6 ppm (t)
and 33.8 ppm (br s) (Figure 5). The signal at 48.9 ppm can be
assigned to the two aromatic P donors Pc and Pd of the tripod
ligand. It is split through coupling with the signal at 40.6 ppm
deriving from the PiPr2 group (Pe; J = −41.3 Hz) and
furthermore by weak coupling (|J| = 6.7 Hz) with the signal at
33.8 ppm being associated with the two P atoms Pa and Pb of
dppm.
C. 31P NMR Spectroscopy. Information on the solution
structures of the synthesized Mo(0) complexes was obtained
from 31P NMR spectroscopy. NMR spectroscopic data for all
The signal of dppm appears as a broad singlet exhibiting
three shoulders, from which again the coupling constant |J| =
F
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Figure 4. 31P NMR spectrum of [Mo(N2)(trpd-1)(dmpm)] (1), detailed zoom and simulation with the derived coupling constants on top.
Assignments of the atoms are given in Scheme 5.
Figure 5. 31P NMR spectrum of [Mo(trpd-1)(dppm)] (2). Assignments of the atoms are given in Scheme 5.
6.7 Hz can be derived. Interestingly, the signals of Pe at 40.6
ppm and Pa,b at 33.8 ppm exhibit no coupling among each
other. Furthermore, no trans coupling constant in the usual
range of J = 100 Hz is observed in the 31P NMR spectrum of 2.
This unusual set of coupling constants, which is not observed in
any other molybdenum(0) complex investigated in this work
and in our earlier studies,26−28 can be attributed to the
geometry shown in Figure 7. Assuming a rapid turnstile
rotation of the dppm ligand,45 the signals of the two P atoms Pa
and Pb get averaged.
dependent with respect to their positions and their linewidths.
The strongest dependence is observed for the signal of Pa,b at
33.8 ppm (300 K) which around 200 K becomes strongly
broadened and disappears at lower temperatures. The signals
Pc,d,e of the tripod ligand, in contrast, are still visible. Because of
the freezing point of the solvent thf-d8 (164.8 K) we were not
able to further cool down the sample until a new, static
spectrum appears.
Referring to the structural model of Figure 7, it can be stated
that the coupling between Pa,b and Pe vanishes, and the
coupling between Pa,b and Pc,d is weak (|J| = 6.7 Hz). For
further discussion of this finding, a vector is defined from the
To validate this model, variable-temperature 31P NMR
spectra were recorded (Figure 6). All signals are temperature
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Figure 6. Variable-temperature 31P NMR spectra of [Mo(trpd-1)(dppm)] (2).
uncoordinated third arms can be found in the negative range
of the spectrum. The bis(dinitrogen) complexes 5 and 6 of
ligand trpd-3 and the bis(dinitrogen) complex 7 of ligand
tdppmm exhibit AA′XX′ patterns in the 31P NMR spectrum as
well. The assignments for one set of ddd signals deriving from
3a and one AA′XX′ set deriving from 3b are given in detail as
examples; all other data is summarized in Supporting
Information, Table S1.
Figure 7. Turnstile rotation of the dppm ligand in [Mo(trpd-
1)(dppm)] (2) around an axis approximately perpendicular to the P3
plane of trpd-1.
Complex [Mo(N2)2(κ2-trpd-2)(dppm)] (3a) exhibits a set of
four ddd-signals. The two signals at 9.2 ppm and 10.9 ppm can
be assigned to the dppm atoms Pa,b while the signal at 40.2 ppm
belongs to the diphenylphosphine donor Pc and the signal at
46.2 ppm to the diisopropyl phosphine donor Pd. Compound
[Mo(N2)2(iso-κ2-trpd-2)(dppm)] (3b), in contrast, exhibits an
AA′XX′ spectrum with multiplets at 44.3 ppm and 9.4 ppm.
Again the signals at 9.4 ppm can be assigned to the dppm
atoms Pa,b while the signal set at 44.3 ppm can be attributed to
the Pc,d PiPr2 groups of trpd-2. The signal of the respective
uncoordinated third arm can again be found in the negative
range at −3.6 ppm for 3a (free PiPr2) and at −20.9 ppm for 3b
(free PPh2). All spectra are displayed in the Supporting
Information, Figures S5, S6, and S13−S15.
3. Reactivity with Acids. Of particular interest with respect
to the application of the synthesized dinitrogen complexes in
nitrogen fixation is their reactivity toward acids. Whereas the κ2
tripod-coordinated Mo(0) complexes 2−7 are bis(dinitrogen)
complexes which react with acids under exchange of one
dinitrogen ligand against the conjugate base of the acid
employed for protonation,12,16 the monodinitrogen complex
[Mo(N2)(trpd-1)(dmpm)] (1) has the potential of being
protonated under retention of its ligand sphere, in analogy to
the complex [Mo(N2)(tdppme)(dmpm)] investigated earlier.28
Protonation of 1 with HOTf, H2SO4, or HCl, however, did not
lead to well-defined products. The only acid allowing
generation and detection of the NNH2 complex was HBArF
(BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate).47 The
main advantage of this reagent is the low coordinating ability of
the [BArF]− anion. The good solubility of [Mo(NNH2)(trpd-
central C atom of the dppm ligand to the molybdenum atom
representing the rotation axis of dppm. The angles α and β
denote the positions of Pc,d and Pe, respectively, with regard to
this vector. Under the assumption that all [P−Mo−P] tripod
angles are 90° and the rotation axis goes through the center of
the tripod ligand the angles α and β are 125.25°. However,
because of the larger steric demand of the PiPr2 group as
compared to the PPh2 groups (vide supra), the angle β is
supposed to be somewhat larger and the angle α somewhat
smaller than 125°.
It is known that the sign of the P−Mo−P coupling constants
depends on the corresponding bond angle,27 that is, the trans
coupling constants (for [P−Mo−P] angles around 180°) are
large and positive, whereas the cis coupling constants (for [P−
Mo−P] angles around 90°) are smaller and negative. It thus
follows that there has to be an angle between 180° and 90° at
which the P−Mo−P coupling constant vanishes. Obviously this
situation is encountered in complex 2 for angle β, explaining
the lack of coupling between atoms Pa,b and Pe. The angles α, in
contrast, have values that allow a weak, but nonvanishing
coupling between Pa,b and Pc,d.
Bis(dinitrogen) Complexes of the Tripod Ligands trpd-2,
trpd-3, and tdppmm. For a bidentate coordination of the
ligand trpd-2 two different types of signal sets are expected.
Correspondingly, the complexes 3a and 4a exhibit sets of four
ddd signals while the more symmetric complexes 3b and 4b
exhibit AA′XX′ patterns.46 The signals of the respective
H
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1)(dmpm)]BArF (13), however, also precluded isolation of
this compound as a solid. The protonation reaction was
therefore performed as an NMR experiment.
To this end, about 2.5 equiv of [H(OEt2)2]+[BArF]−47 were
added to a solution of 1 in CD2Cl2, and the changes in the 31
P
NMR spectrum were monitored. The presence of the five
phosphine donor groups in the NNH2 complex 13 is evident
from retention of the AA′XX′M pattern (Supporting
Information, Figure S16). Because of oxidation of the
molybdenum center the signals of 13 are shifted to high field
with respect to the N2 complex 1. Comparison of the 31P NMR
shifts between the latter complex and the NNH2 complex 13
indicates a difference of Δδ = −5.0 ppm and −7.2 ppm for the
signals of Pa and Pb (Scheme 8) and Δδ = −17.5 ppm and
Scheme 8
Figure 8. Optimized geometry of [Mo(NNH2)(trpd-1)(dmpm)]2+
(13).
compared with those obtained for a model of the parent
dinitrogen complex 1 as well as the related NNH2 complex
[Mo(NNH2)(tdppme)(dmpm)]2+ in the Supporting Informa-
tion, Tables S2−S4. The bond distances of the NNH2 unit
calculated for 13 are very similar to those obtained for
26
[Mo(NNH2)(dpepp)(dppm)]2+ and [Mo(NNH2)(tdppme)-
(dmpm)]2+ (Supporting Information, Table S5).28 Because of
protonation, the N−N bond is lengthened to 1.32 Å in 13 with
respect to 1 (N−N expt. 1.06 Å, calc. 1.16 Å, cf. Table 2 and
Supporting Information, Table S5); simultaneously, the Mo−N
bond is shortened to 1.77 Å (Mo−N of 1 expt. 2.05 Å, calc.
1.97 Å).
−12.0 ppm for the signals of Pc and Pd, respectively (cf Table
5). The largest chemical shift difference (Δδ = −46.7 ppm) is
Table 5. 31P-NMR Chemical Shifts of N2- and
Corresponding NNH2 Complexes with Pentaphosphine
a
Ligation
In agreement with the results from 31P NMR spectroscopy
(vide supra), the coordination environment of 1 is retained in
13. According to the calculation the two protons of the NNH2
group are oriented toward the PiPr2 and one of the PMe2
1
groups, respectively (cf Figure.8). The H NMR chemical shift
difference observed experimentally for these protons indicates
that the terminal NH2 group of the NNH2 ligand does not
rotate. This supports the NN double bond obtained for the
NNH2 ligand of 13, similar to complex-bound diazene,48 and
agrees with our earlier formulation of Mo-NNH2 systems as
complexes of isodiazene.14,15
DISCUSSION, SUMMARY, AND CONCLUSION
■
In the preceding sections novel Mo(0) complexes supported by
hybrid tripod ligands with mixed diarylphosphine/dialkylphos-
phine donor groups have been presented. Employing the
hybrid tripod ligand trpd-1 containing one diisopropyl
phosphine group and two diphenyl phosphine groups, the
molybdenum dinitrogen complex [Mo(N2)(trpd-1)(dmpm)]
a
Proton shifts are also given for the latter .
observed for Pe in trans position to N2/NNH2. Very similar
values were determined for the relative shifts of the two
complexes [Mo(N2)(tdppme)(dmpm)] and [Mo(NNH2)-
(tdppme)(dmpm)]2+ (Pa,b Δδ = −4.5 ppm, Pc,d Δδ = −18.0
ppm, Pe Δδ = −44.4 ppm).28 The two protons of the NNH2
(1) has been synthesized. With a NN stretch of ν = 1965 cm −1
,
the complex exhibits a moderate activation of the dinitrogen
ligand. In comparison to the complex [Mo(N2)(tdppme)-
(dmpm)] (ν = 1979 cm−1) presented earlier the activation is a
little higher, which is due to the presence of one additional alkyl
phosphine donor group. Note that the diisopropylphosphine
group of the trpd-1 ligand is coordinated in cis- and not in
trans-position to the N2 ligand. This is obviously due to the
trans-effect, that is, the fact that diisopropylphosphine is a
stronger σ-donor than diphenylphosphine. Importantly, dini-
trogen complex 1 could be converted by protonation with
[H(OEt2)2]BArF to the corresponding NNH2 complex 13 for
which retention of the five phosphine donor groups of 1 was
demonstrated by 31P NMR.
1
group of 13 appear in the H NMR spectrum at 9.4 and 10.8
ppm (cf Table 5). These positions are also very similar to those
28
found for the complexes [Mo(NNH2)(tdppme)(dmpm)]2+
26
and [Mo(NNH2)(dpepp)(dppm)]2+ where the NNH2
proton signals are located at 9.2 and 9.9 ppm, respectively. In
contrast to these systems where the proton resonances appear
as singlets, however, they are split in 13 because of the C1
symmetry of this complex.
To obtain further information on the constitution of 13 a
geometry optimization was performed (cf Figure 8). Cartesian
coordinates of the resulting structure are collected and
I
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As found in our previous studies, diphosphines with a C1
bridge are particularly well suited to stabilize dinitrogen
complexes supported by tridentate or tripod ligands.26,28 For
the tripod ligand trpd-1, replacement of dmpm with the bigger
coligand dppm led to an N2 free complex, which was identified
as the five-coordinate complex [Mo(trpd-1)(dppm)] (2).
Apparently the steric demand of the PiPr2 in trpd-1 in
combination with the bigger dppm does not allow the
coordination of a sixth ligand. It is remarkable that in the
complex [Mo(N2)(tdppme)(dppm)],28 which bears 10 phenyl
rings, generation of the octahedral dinitrogen complex is still
possible, whereas exchange of one diphenylphosphine for a
bulkier diisopropyphosphine group prevents formation of the
six-coordinate complex. The larger steric demand of PiPr2 as
compared to PPh2 is compatible with the Tolman cone angles
of the respective monophosphines which are 160° for PiPr3 and
145° for PPh3.49 To the best of our knowledge complex 2 is the
first structurally characterized molybdenum(0) complex coor-
dinated by five phosphine donor groups only.50
the trpd-1 ligands are of paramount importance to guarantee a
facial coordination, although the presence of this group renders
the synthesis of tripod ligands with bulky substituents much
more demanding.
EXPERIMENTAL SECTION
■
General Information. All reactions were carried out under N2-
atmosphere using standard Schlenk-techniques or preparation in a
MBRAUN glovebox. All solvents were dried and freshly distilled under
Argon. All other reagents were used as commercially available without
further purification. Silica gel (0.04−0.063 mm) from Merck was used
for column chromatography. The compounds 1,1,1-tris-
(chloromethyl)methane29,52,53 (11), diisopropyl phosphine,54,55 bor-
ane protected disopropyl phosphine,56 [MoBr3(thf)3],32
[MoCl3(thf)3],57 [MoI3(thf)3],38 [H(OEt2)2]BArF47,58,59 were pre-
pared according to the literature known procedures. 1,1,1-Tris-
(diphenylphosphinomethyl)methane29 (tdppmm) was prepared ac-
cording to the literature procedure slightly varied by using n-
butyllithium for deprotonation. For all deprotonation reactions a
solution of 2.5 mol/L n-butyllithium in hexanes was used. The
synthesis of trpd-3 was performed with a 1.6 mol/L solution of n-
butyllithium in hexanes. Elemental analyses were performed by 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 400 Pulse Fourier
Transform spectrometer operating at frequencies of 400.13 MHz
(1H), 161.98 MHz (31P) 100.62 MHz (13C). Referencing was
performed either on the solvent residue signal (CH2Cl2 = 5.32 ppm,
C6D6 = 7.16 ppm) or TMS (δ1H = 0 ppm) and 85% H3PO4 (δ31P = 0
ppm) serving as substitutive standards. Spectral simulations were
performed by using the MestReC program package (MestreLab
Research, Santiago de Compostela, Spain). Vibrational spectroscopy:
The FT-Raman spectra were recorded with a Bruker IFS 666/CS NIR
FT-Raman spectrometer. Resonance-Raman-spectra were obtained by
a XY-multichannel-Raman-Spectrometer with triple monochromator
and CCD-detector by DILOR, Lille, France. A Ar+/Kr+-laser by
Spectra Physics, Darmstadt, with excitation wavelength from 454.5 to
647.1 nm was used as a light source. The resolution was between 2.5
and 0.8 cm−1, depending on the excitation wavelength. Infrared spectra
were recorded on a Bruker ALPHA FT-IR Spectrum with Platinum
ATR setup. The DTA-TG measurements were performed in a
nitrogen atmosphere (purity: 5.0) in Al2O3 crucibles using a STA-
409CD thermobalance from Netzsch. All measurements were
performed with a flow rate of 75 mL min−1 and were corrected for
buoyancy and current effects. The instrument was calibrated using
standard reference materials. Density functional theory (DFT)
calculations were performed by using Gaussian 09. B3LYP was used
as approximation functional. LANL2DZ for molybdenum and 6-311G
for other atoms were used as basis sets.60
The tripod ligands trpd-2 (two PiPr2), trpd-3 (three PiPr2),
and tdppmm (three PPh2) with an isobutyl backbone were
found to be coordinated in a κ2-fashion, forming trans-
bis(dinitrogen) complexes. The steric demand of the respective
phosphine groups seems to be without any influence on these
results because all isobutyl tripods trpd-2, trpd-3, and tdppmm
were found to coordinate in a κ2 mode. This finding rather
appears to be due to the missing methyl group in comparison
to the neopentyl based tripods which leads to a higher flexibility
of the ligand whereby its capability to enforce a tridentate, facial
coordination is lowered. This conclusion is supported by the
fact that the all-diphenylphosphine tripod ligand tdppmm does
not coordinate in a κ3 fashion either, in contrast to its
counterpart with a neopentyl-backbone, tdppme. Similar effects
of an alkyl substitution within the backbone of the tripod ligand
on its coordination mode (κ2 vs κ3) have been observed by
Rosenberg and co-workers and attributed to steric influences of
the side groups on the “kappticity” of the ligand.51 In contrast
to the Mo(0) complexes, however, it appears that the κ3
coordination of the isobutyl tripods trpd-2 and trpd-3 is well
possible at the molybdenum(III) stage. This could be due to a
higher electrophilicity of the Mo(III) as compared to the
Mo(0) center, in particular leading to strong bonding of the
highly nucleophilic diisopropylphosphine groups, and would be
further supported by the fact that the isobutyl ligand tdppmm,
with less electron-donating diphenylphosphine groups, is only
coordinated in a κ2-fashion to Mo(III).
In summary, this study has provided important insights into
the formation of molybdenum dinitrogen complexes supported
by tripod ligands. In cases where κ3-complexes are formed
protonation of the N2 ligand is possible under retention of the
pentaphosphine ligation, which represents the first step in the
Chatt cycle and is a necessary condition for a dinitrogen
complex to serve as a catalyst in the reduction and protonation
of N2 to ammonia. As shown in this paper, however, most
tripod ligands only coordinate in a κ2 fashion to the parent
Mo(0) dinitrogen complex, which can be traced back to a
combination of three factors, (i) steric demand of the
phosphine substituents, (ii) electronic properties of the
phosphine donors and, most importantly, (iii) flexibility of
the ligand backbone. The fact that employing tripod ligands
with an isobutyl backbone did not lead to a single Mo(0)
complex exhibiting a κ3 coordination in this study impressively
demonstrates that the central methyl groups of the tdppme and
Single Crystal Structure Analysis. Data collections for both
compounds were performed with an imaging plate diffraction system
(IPDS-1) from STOE & CIE using Mo−Kα-radiation (λ = 0.71073 Å).
The structure solutions were done with direct methods using
SHELXS-97 and structure refinements were performed against |F|2
using SHELXL-97.61 For compound 1 a numerical absorption
correction was applied using X-Red Version 1.31 and X-Shape Version
2.11 of the Program Package X-Area.62 All non-hydrogen atoms except
the disordered C atoms of lower occupancy in 1 were refined with
anisotropic displacement parameters. All hydrogen atoms were
positioned with idealized geometry (in compound 1 methyl H
atoms allowed to rotate but not to tip) and were refined with fixed
isotropic displacement parameters [Ueq(H) = −1.2·Ueq(C)] (1.5 for
methyl H atoms) using a riding model. In compound 1 one of the
diisopropyl groups is disordered over two sites and was refined using a
split model (sof: 0.80:0.20). The structure of both compounds
contains some additional amount of THF, which is disordered and for
which no reasonable structure model was found.63 Therefore, the data
were corrected for disordered solvent using the SQUEEZE option in
Platon. The electron count is in accordance with about half a THF
J
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Article
molecule per complex in 1 and about one-quarter of THF in 2. All
crystals investigated for 2 were always slightly nonmerohedral twinned
but both individuals cannot be separated successfully and therefore,
the twinning was not considered in the structure determination.
However, this is the reason for the relatively high reliability factors and
the highest residual peak in the electron density map which does not
originate from absorption effects. Details of the structure determi-
nation are given in Table 1.
CCDC-940376 (1) and CCDC-940377 (2) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/.
NMR (100.62 MHz, CD2Cl2, 300 K): δ = 140.3 (m, 4C, Cq arom.),
2
133.9, 133.0 (2d, 8C, CHo arom., JCP = 20.2 Hz), 128.4−128.3 (m,
12C, CHm,p arom.), 42.9−42.6 (m, 2C, CH2−PPh2), 37.9−37.7 (m,
1C, Cq-CH3), 37.0−36.7 (m, 1C, CH2−PiPr2), 28.2 (q, 1C, Cq-CH3,
1
3JCP = 9.6 Hz), 23.9 (d, 2C, CH−CH3, JCP = 12.4 Hz), 19.9 (d, 2C,
CH-CH3, 2JCP = 14.5 Hz), 19.1 (d, 2C, CH-CH3, 2JCP = 10.6 Hz) ppm.
H−C(CH2Cl)(CH2PiPr2−BH3)2 (12). A 4.91 g portion of (37.2
mmol) H−PiPr2−BH3 has been deprotonated by 14.9 mL of n-
butyllithium (1 equiv) in 20 mL of THF at 0 °C. The mixture was
stirred for 2 h, warmed to room temperature and added to 3.0 g of
(18.6 mmol) H−C(CH2−Cl)3 (11) in 10 mL of THF by dropping
funnel. The mixture was stirred for 16 h at room temperature and
hydrolyzed with 10 mL of H2O, afterward the water was removed by
syringe and extracted twice with diethyl ether. The solvent was
removed in vacuo, and the crude product was purified by column
chromatography (toluene:cyclohexane/1:1/Rf = 0.34). Yield: 4.80 g
(13.6 mmol, 73%) Anal. Calc. for C16H41B2ClP2: C, 54.5; H, 11.7;
found: C, 55.0; H, 12.3. IR (ATR): 2960−2860, 2370−2348(BH),
1458 (P−C), 1432 (P−C) cm−1. 1H NMR (400.13 MHz, CD2Cl2, 300
K): δ = 3.90 (d, 2H, CH2-Cl, 3J = 4.1 Hz), 2.54 (m, 1H, CH-(CH2)3),
2.07−1.76 (m, 8H, 2 × CH2-PiPr2, 4 × CH-CH3), 1.20−1.14 (m q,
24H, CH−CH3), 0.36 (br q, 6H, -BH3) ppm. 31P NMR (161.98 MHz,
CD2Cl2, 300 K, H3PO4) δ = 30.9 (br q, 2P, PiPr2) ppm. 13C NMR
(100.62 MHz, CD2Cl2, 300 K): δ = 50.8 (tr, 1C, CH2−Cl), 33.3 (tr,
Me-C(CH2Cl)2(CH2PiPr2−BH3) (9). A 3.24 g portion (24.5 mmol)
of H−PiPr2−BH3 has been deprotonated by 9.8 mL of n-butyllithium
(1 equiv) in 20 mL of THF at 0 °C. The mixture was stirred for 2 h,
warmed to room temperature and added to 4.47 g (24.5 mmol) of Me-
C(CH2Cl)3 (8) in 20 mL of THF. The mixture was stirred at ambient
temperature for 12 h, afterward the reaction was quenched with 10 mL
of H2O. The water was removed syringe and extracted twice with 10
mL of diethyl ether. Evaporation of the solvent in vacuo gave a
colorless liquid which was purified by column chromatography
(toluene:cyclohexane/1:0.7/Rf = 0.54). Yield: 4.28 g (15.8 mmol,
64%) Anal. Calc. for C11H26BCl2P: C, 48.8; H, 9.7; found: C, 49.0; H,
10.0. IR (ATR): 2970−2870, 2380−2347 (BH), 1460 (P−C), 1432
(P−C) cm−1. 1H NMR (400.13 MHz, CD2Cl2, 300 K, {31P,11B CPD}:
δ = 3.71 (dd, 4H, CH2-Cl, J = 11.1 Hz), 1.98 (sep, 2H, CH-(CH3)2, 3J
= 7.1 Hz), 1.80 (s, 2H, CH2- PiPr2), 1.27 (s, 3H, Cq-CH3), 1.18 (dd,
12H, CH−CH3, J = 3.8, 7.1 Hz), 0.49 (s, 3H, -BH3) ppm. 31P NMR
(161.98 MHz, CD2Cl2, 300 K, H3PO4) δ = 27.9 (br q, 1P, PiPr2−BH3,
J = 50.5 Hz, 108.3 Hz) ppm. 13C NMR (100.62 MHz, CD2Cl2, 300
K): δ = 51.5 (d, 2C, CH2−Cl, J = 6.3 Hz), 40.4 (s, 1C, C-quart.), 24.0
(d, 2C, CH−CH3, J = 24.9 Hz), 23.8 (d, 1C, CH2−P, J = 34.1 Hz),
21.6 (d, 1C, Cq-CH3), 16.7 (d, 4C, CH-CH3) ppm.
1
1C, CH-(CH2)3), 23.1, 23.2 (2d, 2C, CH2−PiPr2, JCP = 28.2 Hz),
23.1, 23.2 (2d, 4C, CH−CH3, 1JCP = 33.8 Hz), 16.8−16.4 (m, 8C, CH-
CH3) ppm.
H−C(CH2PiPr2)2(CH2PPh2) (trpd-2). A 1.45 g portion of (7.80
mmol) H-PPh2 was deprotonated with 3.12 mL of n-butyllithium (1
equiv) in 20 mL of THF and added to 2.50 g of (7.09 mmol) H−
C(CH2Cl)(CH2PiPr2−BH3)2 (11) in 40 mL of THF by dropping
funnel. The reaction mixture was stirred for 16 h, hydrolyzed by
addition of 20 mL of H2O, afterward the water was removed by
syringe and extracted twice with dichloromethane. The solvent was
removed in vacuo and the crude product was purified by column
chromatography (cyclohexane:ethylaceate/3:1/Rf = 0.68). Afterward
the viscous product was stirred in morpholine for 9 h at 100 °C. The
morpholine was removed, and the product was dried in vacuo for 24 h.
Yield: 1.7 g (3.60 mmol, 46%) Anal. Calc. for C28H45P3: C, 70.9; H,
9.6; found: C, 70.6; H, 9.8. IR (ATR): 3063, 3045, 2960−2860, 1480
Me-C(CH2PPh2)2(CH2PiPr2−BH3) (10). A 4.25 g portion of (22.9
mmol) H-PPh2 was deprotonated with 9.20 mL of n-butyllithium (1
equiv) in 20 mL of THF and added to 2.45 g of (9.04 mmol) Me-
C(CH2Cl)2(CH2PiPr2−BH3) (9) in 40 mL of THF by dropping
funnel. The reaction mixture was stirred for 18 h, hydrolyzed by
addition of 20 mL of H2O, afterward the water was removed by
syringe and extracted twice with diethyl ether. The solvent was
removed in vacuo, and the crude product was purified by column
chromatography (cyclohexane: ethylaceate/12:1/Rf = 0.71). Yield:
2.08 g (3.65 mmol, 40%) Anal. Calc. for C35H46BP3: C, 73.7; H, 8.1;
found: C, 73.9; H, 8.4. IR (ATR): 3070, 3050, 2954−2880, 2370−
2340 (BH), 1480 (P−C), 1432 (P−C) cm−1. 1H NMR (400.13 MHz,
CD2Cl2, 300 K): δ = 7.29 (m, 20H, CH phenyl), 2.51 (ddd, 4H,
diastereotope H CH2PPh2, J = 14.2 Hz, 4JHP = 2.8 Hz), 1.74 (sep, 2H,
CH-(CH3)2), 1.70 (d, 2H, CH2- PiPr2, J = 11.4 Hz), 1.10 (s, 3H, Cq-
CH3), 1.19 (m, 12H, CH−CH3), 0.49 (br m, 3H, -BH3) ppm. 31P
NMR (161.98 MHz, CD2Cl2, 300 K, H3PO4) δ = 26.9 (br d, 1P,
PiPr2), −24.5 (s, 2P, PPh2) ppm. 13C NMR (100.62 MHz, CD2Cl2,
1
(P−C), 1458 (P−C), 1431 (P−C) cm−1. H NMR (400.13 MHz,
CD2Cl2, 300 K): δ = 7.51−7.28 (m, 10H, CH phenyl), 2.40 (d, 2H,
3
CH2-PPh2, J = 6.7 Hz), 1.77−1.60 (m, 8H, 2 × CH2-PiPr2, 4 × CH-
CH3), 1.58−1.48 (m, 1H, CH-(CH2)3), 1.04−0.95 (m, 24H, CH−
CH3) ppm. 31P NMR (161.98 MHz, CD2Cl2, 300 K, H3PO4) δ =
−3.6(s, 2P, PiPr2), −20.8 (s, 1P, PPh2) ppm.
H−C(CH2PiPr2)3 (trpd-3). A 2.66 g portion of (22.55 mmol) H−
PiPr2−BH3 have been deprotonated by 14.1 mL of n-butyllithium (1
eq., 1.6 mol/L) in 20 mL of THF at 0 °C. The mixture was stirred for
2 h, warmed to room temperature and added to 1.17 g of (7.72 mmol)
H−C(CH2−Cl)3 (11) in 20 mL of THF by dropping funnel. The
mixture was stirred for 14 h at room temperature and hydrolyzed with
20 mL of H2O, afterward the water was removed by syringe and
extracted twice with diethyl ether. The solvent was removed, and the
slightly green liquid was dried in vacuo. Yield: 2.60 g (6.40 mmol,
88%) Anal. Calc. for C22H49P3: C, 65.0; H, 12.2; found: C, 64.7; H,
12.1. IR (ATR): 2947, 2920, 2865, 1460 (P−C) cm−1. 1H NMR
(400.13 MHz, CD2Cl2, 300 K): δ = 1.75−1.53 (m, 6H, 6 × CH-CH3),
1.48−1.38 (br m, 1H CH-(CH2)3), 1.45 (br d, 6H, CH2-PiPr2), 1.06−
1.00 (m, 36H, CH−CH3) ppm. 31P NMR (161.98 MHz, CD2Cl2, 300
K, H3PO4) δ = −3.27 (s, 3P, PiPr2) ppm.
300 K): δ = 139.8 (m, 4C, Cq. arom.), 133.9 (d, 8C, CHo arom., 2JCP
=
20.2 Hz), 128.6−128.3 (m, 12C, CHm,p arom.), 42.9−42.6 (m, 2C,
CH2−PPh2), 38.7 (m, 1C, Cq-CH3), 32.2−31.8 (m, 1C, CH2−PiPr2),
1
28.2 (m, 1C, Cq-CH3), 24.1 (d, 2C, CH−CH3, JCP = 34.0 Hz), 16.8
2
(d, 4C, CH-CH3, JCP = 30.5 Hz) ppm.
Me-C(CH2PPh2)2(CH2PiPr2) (trpd-1). A 1.60 g portion of (2.8
mmol) Me-C(CH2PPh2)2(CH2PiPr2−BH3) (10) was stirred in 15 mL
of degassed morpholine at 100 °C for 6 h, afterward the morpholine
was removed in vacuo at 50 °C. The crude product was refluxed with
10 mL of methanol leading to a white precipitate which was filtered off
and dried in vacuo. Yield: 1.25 g (2.25 mmol, 80%) Anal. Calc. for
C35H43P3: C, 75.5; H, 7.8, found: C, 75.8; H, 7.8. IR (ATR): 3070,
3045, 2960−2860, 1480 (P−C), 1432 (P−C) cm−1. 1H NMR (400.13
MHz, CD2Cl2, 300 K): δ = 7.44−7.27 (m, 20H, CH phenyl), 2.53
[MoI3(trpd-X)]. A equimolar suspension of tripod ligand and
[MoI3(thf)3] (typical amount of [MoI3(thf)3]: 0.5−1 g) in 20 mL of
THF was stirred for 18 h at room temperature. The dark red solution
was reduced in vacuo to a quantity of 5 mL and a dark red precipitate
was formed by adding 40 mL of diethylether. In case of trpd-3
precipitation was effected by addition of 40 mL of hexanes. The
precipitate was filtered and dried in vacuo.
4
(ddd, 4H, diastereotope CH2-PPh22, J = 14.2 Hz, JHP = 3.1 Hz), 1.67
(d sep, 2H, CH-CH3, J = 7.1 Hz, JHP = 3.1 Hz), 1.70 (d, 2H, CH2-
2
PiPr2, JHP = 5.5 Hz), 1.08 (s, 3H, CH2−CH3, 0.99 (dd, 12H, CH−
3
CH3, J = 3.8, 7.1 Hz, JHP = 14.0 Hz) ppm. 31P NMR (161.98 MHz,
[MoI3(trpd-1)]: 620 mg of (900 μmol) [MoI3(thf)3] was reacted
with 500 mg of (900 μmol) trpd-1. Yield: 700 mg (0.677 mmol, 75%)
CD2Cl2, 300 K,): δ = −7.6 (s, 1P, PiPr2) −25.1 (s, 2P, PPh2) ppm. 13
C
K
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Anal. Calc. for C35H43I3MoP3: C, 40.7; H, 4.2; I, 36.8; found: C, 40.2;
H, 4.2; I, 36.3. IR (ATR): 3050, 2970, 2860, 1480 (P−C), 1460 (P−
C), 1432 (P−C) cm−1.
[MoI3(trpd-2)]: 990 mg of (1.43 mmol) [MoI3(thf)3] was reacted
with 680 mg of (1.43 mmol) trpd-2. Yield: 950 mg (0.340 mmol,
69%) Anal. Calc. for C28H45I3MoP3: C, 35.4; H, 4.8; I, 40.0; found: C,
35.1; H, 5.1; I, 40.4. IR (ATR): 3050, 2960, 2869, 1620, 1480 (P−C),
1460 (P−C), 1432 (P−C) cm−1.
[MoI3(trpd-3)]: 1.36 g of (1.97 mmol) [MoI3(thf)3] was reacted
with 800 mg of (1.97 mmol) trpd-3. Yield: 670 mg (0.758 mmol,
39%) Anal. Calc. for C22H49I3MoP3: C, 29.9; H, 5.6; I, 43.1; found: C,
29.7; H, 5.7; I, 43.1. IR (ATR): 2970−2869, 1455 (P−C) cm−1.
[MoI3(thf)(tdppmm)]: 680 mg of (980 μmol) [MoI3(thf)3] was
reacted with 600 mg of (980 μmol) tdppmm. Yield: 740 mg (0.640
mmol, 65%) Anal. Calc. for C44H45I3MoOP3: C, 45.6; H, 3.9; I, 32.8;
found: C, 45.3; H, 3.9; I, 33.2. IR (ATR): 3050, 2960−2855, 1480
(P−C), 1434 (P−C) cm−1.
Reduction under N2. All reductions to dinitrogen complexes were
performed in similar fashion. A suspension of the respective
[MoI3(trpd)] complex (typical 500 mg) and an equimolar amount
of dppm or dmpm as coligand in 30 mL of THF were added to
sodium amalgam (300 mg Na, 3 mL Hg) and stirred under nitrogen
for 24 h at room temperature. The solution was decanted from
amalgam, filtrated and reduced to 5 mL. After addition of 5 mL of
methanol and further reduction of the volume to 5 mL (repeated
twice) a red precipitate or viscous product could be obtained.
[Mo(N2)(trpd-1)(dmpm)] (1). A 500 mg portion of (480 μmol)
[MoI3(trpd-1)] was reacted with 65.8 mg of (0.48 mmol) dmpm as
described above. Yield: 105 mg (0.129 mmol, 26%). Anal. Calc. for
C40H57MoN2P5: C, 58.8; H, 7.0; N, 3.4 found: C, 58.2; H, 7.0; N, 3.0.
IR (ATR): 3056, 2952−2862, 1965 (NN), 1480 (P−C), 1433 (P−C)
cm−1. 1H NMR (400.13 MHz, C6D6, 300 K): δ = 7.86−6.78 (m, 20H,
CH arom.), 3.76 (m, 1H, CH2 dmpm), 3.41 (m, 1H, CH2 dmpm),
2.78−2.73 (br t, 1H, CH-CH3), 2.48−2.44 (br t, 1H, CH2-PPh2),
2.25−2.20 (br d, 1H, CH2-PPh2), 2.00 (d, 3H, CH3 dmpm, J = 4.8
Hz), 1.89 (br t, 2H, CH2-PPh2), 1.80−1.77 (m, 2H, CH2-PiPr2), 1.72
(d, 3H, CH3 dmpm, J = 5.0 Hz), 1.45 (m, 1H, CH-CH3), 1.41 (d, 3H,
CH3 dmpm, J = 4.8 Hz), 1.36−1.25 (m, 12H, CH−CH3, CqCH3),
1.10−1.08 (m, 3H, CHCH3), 0.61 (d, 3H, CH3 dmpm, J = 5.0 Hz)
ppm. 31P NMR (161.98 MHz, C6D6, 300 K) δ = 47.3 (dddd, 1P, Pd, J
= −12.6, −22.4, −30.5, 86.8 Hz), 41.1 (dddd, 1P, Pc, J = −12.6, −27.3,
−27.3, 93.5 Hz), 40.3 (dddd, 1P, Pe, J = −20.8, −22.4, −24.4, −27.3
Hz), −23.9 (dddd, 1P, Pa, J = 3.8, −20.8, 30.6, 93.5 Hz), −24.9 (dddd,
1P, Pb, J = 3.8, −24.4, −27.9, 86.8 Hz) ppm.
1.15 (m, 24H, CH−CH3) ppm. 31P NMR (161.98 MHz, C6D6, 300
K) 3a δ = 46.2 (ddd, 1P, Pd, J = 15.1, −27.1, 102.8 Hz), 40.2 (ddd, 1P,
Pc, J = 10.6, −24.7, 104.5 Hz), 10.9 (ddd, 1P, Pb, J = 15.1, −24.7, 102.8
Hz), 9.2 (ddd, 1P, Pa, J = 15.1, −27.1, 104.5 Hz), −3.6 (s, 1P, Pe) ppm.
3b δ = 44.3 (m, 2P, Pc,d, J = 12.9, 15.2, −25.9, 102.9 Hz), 9.4 (m, 2P,
Pa,b, J = 12.9, 15.2, −25.9, 102.9 Hz), −20.9 (s, 1P, Pe) ppm.
trans-[Mo(N2)2(κ2-trpd-2)(dmpm)] (4a, b). A 600 mg portion of
(0.63 mmol) [MoI3(trpd-2)] was reacted with 85.8 mg of (0.63
mmol) dmpm as described. A dark red oily precipitate could be
obtained which was extracted by 20 mL of benzene which was
evaporated in vacuo. A mixture of two coordination isomers could be
determined by 31P NMR spectroscopy. Yield: 80 mg (0.11 mmol,
17%)
IR (ATR): 3053, 2960−2865, 2009 (NN), 1933 (NN), 1460 (P−
C), 1430 (P−C) cm−1. 1H NMR (400.13 MHz, C6D6, 300 K): 4a δ =
7.70−7.08 (m, 10H, CH arom), 3.67−3.53 (m, 2H, CH2 dmpm), 2.89
(br d, 2H, CH2-PPh2), 2.16 (m, 1H, CH-(CH2)3), 1.73−1.65 (m, 4H,
CH-CH3), 1.48−1.33 (br d, 4H, CH2-PiPr2), 1.31−1.03 (m, 36H, 8 ×
CH−CH3, 4 × CH3 dmpm) ppm. 4a δ = 7.70−7.08 (m, 10H, CH
arom), 3.67−3.53 (m, 2H, CH2 dmpm), 2.16 (m, 1H, CH-(CH2)3),
1.85 (br d, 2H, CH2-PPh2), 1.73−1.65 (m, 4H, CH-CH3), 1.48−1.33
(br d, 4H, CH2-PiPr2), 1.31−1.03 (m, 36H, 8 × CH−CH3, 4 × CH3
dmpm) ppm. 31P NMR (161.98 MHz, C6D6, 300 K) 4a δ = 51.5 (ddd,
1P, Pd, J = 11.5, −26.4, 110.2 Hz), 42.7 (ddd, 1P, Pc, J = 11.5, −28.3,
106.0 Hz), −17.9 (ddd, 1P, Pb, J = 4.1, −28.3, 107.7 Hz), −20.8 (ddd,
1P, Pa, J = 4.2, −26.4, 106.0 Hz), −5.6 (s, 1P, Pe) ppm. 4b δ = 50.2
(m, 2P, Pc,d, J = 7.6, 10.8, −27.1, 107.7 Hz), −21.7 (m, 2P, Pa,b, J = 7.6,
10.8, −27.1, 107.7 Hz), −22.8 (s, 1P, Pe) ppm.
trans-[Mo(N2)2(κ2-trpd-3)(dppm)] (5). A 370 mg portion of
(0.42 mmol) [MoI3(trpd-3)] was reacted with 161.4 mg of (0.42
mmol) dppm as described above. Yield: 80 mg (0.085 mmol, 20%)
Anal. Calc. for C47H71MoN4P5: C, 59.9; H, 7.6; N, 5.9 found: C,
59.8; H, 7.2; N, 5.8.
IR (ATR): 3052, 2960−2866, 2006 (NN), 1935 (NN), 1477 (P−
1
C), 1431 (P−C) cm−1. H NMR (400.13 MHz, C6D6, 300 K): δ =
7.52−7.08 (m, 20H, CH arom), 2.40 (br t, 2H, CH2 dppm), 2.19 (m,
1H, CH-(CH2)3), 1.84−1.72 (m, 6H, CH-CH3), 1.45−1.34 (m, 6H,
CH2-PiPr2), 1.23−1.15 (m, 24H, CH−CH3 coord.), 1.10−1.04 (m,
12H, CH−CH3 uncoord.) ppm. 31P NMR (161.98 MHz, C6D6, 300
K) δ = 44.1 (m, 2P, Pc,d, J = 9.5, 12.8, −25.3, 102.1 Hz), 9.5 (m, 2P,
Pa,b, J = 9.5, 12.8, −25.3, 102.1 Hz), −6.1 (s, 1P, Pe) ppm.
trans-[Mo(N2)2(κ2-trpd-3)(dmpm)] (6). A 500 mg portion of
(0.57 mmol) [MoI3(trpd-3)] with 77.1 mg of (0.57 mmol) dmpm was
reacted as described above. The reaction mixture was decanted and
filtrated and afterward the solvent was evaporated by nitrogen steam.
The achieved dark red viscous product was extracted by 20 mL of
benzene which was evaporated in vacuo. Yield: 100 mg (0.144 mmol,
25%)
[Mo(trpd-1)(dppm)] (2). A 400 mg portion of (390 μmol)
[MoI3(trpd-1)] was reacted with 148 mg of (390 μmol) dppm as
described above. Yield: 200 mg (0.129 mmol, 45%). Anal. Calc. for
C60H65MoP5: C, 69.5; H, 6.3; found: C, 69.0; H, 7.0. IR (ATR): 3050,
2950−2860, 1481 (P−C), 1430 (P−C) cm−1. 1H NMR (400.13 MHz,
C6D6, 300 K): δ = 7.73−7.60 (m, 20H, CH arom.), 6.88 (m, 20H, CH
arom.), 5.28 (br t, 2H, CH2 dppm), 2.58−2.22 (dd, 4H, CH2-PPh2),
1.62 (d, 2H, CH2-PiPr2), 1.58 (m, 2H, CH-CH3, 1.26 (s, 3H, CqCH3),
1.26−1.19 (dd, 6H, CH−CH3), 0.81−0.77(dd, 6H, CH−CH3) ppm.
31P NMR (161.98 MHz, C6D6, 300 K) δ = 48.9 (dt, 2P, Pc,d, |J| = 6.7, J
= −41.3 Hz), 40.6 (t, 1P, Pe, J = −41.3 Hz), 33.8 (br s, 2P, Pa,b, |J| =
6.7 Hz) ppm.
Anal. Calc. for C27H63MoN4P5: C, 46.7; H, 9.1; N, 8.1 found: C,
47.1; H, 9.2; N, 7.9.
IR (ATR): 2948−2866, 2005 (NN), 1925 (NN), 1460 (P−C),
1
1410 (P−C) cm−1. H NMR (400.13 MHz, C6D6, 300 K): δ = 2.37
(br t, 2H, CH2 dmpm), 2.13 (m, 1H, CH-(CH2)3), 1.79−1.65 (m, 6H,
CH-CH3), 1.46−1.44 (br d, 6H, CH2-PiPr2), 1.44−1.08 (m, 48H, 12 ×
CH−CH3, 4 × CH3 dmpm) ppm. 31P NMR (161.98 MHz, C6D6, 300
K) δ = 49.8 (m, 2P, Pc,d, J = 8.4, 10.2, −26.9, 107.6 Hz), −21.75 (m,
2P, Pa,b, J = 8.4, 10.2, −26.9, 107.6 Hz), −5.7 (s, 1P, Pe) ppm.
trans-[Mo(N2)2(κ2-tdppmm)(dmpm)] (7). A 500 mg portion of
(0.43 mmol) [MoI3(thf)(tdppmm)] with 62.6 mg of (0.46 mmol)
dmpm was reacted as described above. Yield: 100 mg (0.111 mmol,
26%)
trans-[Mo(N2)2(κ2-trpd-2)(dppm)] (3a, b). A 500 mg portion of
(0.49 mmol) [MoI3(trpd-2)] was reacted with 189 mg of (0.49 mmol)
dppm as described. A red precipitate could be obtained which was
determined as a mixture of two coordination isomers by 31P NMR
spectroscopy. Yield: 100 mg (0.098 mmol, 20%)
IR (ATR): 3049, 2960, 2012 (NN), 1936 (NN), 1480 (P−C), 1430
1
(P−C) cm−1. H NMR (400.13 MHz, C6D6, 300 K): 3a δ = 7.85−
Anal. Calc. for C45H51MoN4P5: C, 60.1; H, 5.7; N, 6.2 found: C,
59.7; H, 6.1; N, 5.9.
7.48 (m, 10H, CH arom), 7.34−6.90 (m, 20H, CH arom), 2.66 (br d,
2H, CH2-PPh2), 2.58−2.44 (br t, 2H, CH2 dppm), 2.05 (m, 1H, CH-
(CH2)3), 1.84−1.74 (m, 4H, CH-CH3), 1.72−1.57 (m, 4H, CH2-
PiPr2), 1.23−1.15 (m, 24H, CH−CH3) ppm. 3b δ = 7.85−7.48 (m,
10H, CH arom), 7.34−6.90 (m, 20H, CH arom), 2.58−2.44 (br t, 2H,
CH2 dppm), 2.05 (m, 1H, CH-(CH2)3), 1.85 (br d, 2H, CH2-PPh2),
1.84−1.74 (m, 4H, CH-CH3), 1.72−1.57 (m, 4H, CH2-PiPr2), 1.23−
IR (ATR): 3033, 2955−2900, 2020 (NN), 1940 (NN), 1477 (P−
1
C), 1430 (P−C) cm−1. H NMR (400.13 MHz, C6D6, 300 K): δ =
7.64−7.05 (m, 30H, CH arom.), 3.21 (m, 2H, CH2 dmpm), 2.82 (br
d, 4H, CH2-PPh2 coord.), 2.62 (d, 2H, CH2-PPh2 free), 2.34 (m, 1H,
CH-(CH2)3), 1.12 (d, 6H, CH3 dmpm), 0.90 (d, 6H, CH3 dmpm)
ppm 31P NMR (161.98 MHz, C6D6, 300 K) δ = 44.2 (m, 2P, Pc,d, J =
L
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1.5, 11.5, −27.9, 110.2 Hz), −18.6 (m, 2P, Pa,b, J = 1.5, 11.5, −27.9,
110.2 H), −22.9 (s, 1P, Pe) ppm.
(13) Soncksen, L.; Romer, R.; Nather, C.; Peters, G.; Tuczek, F.
̈ ̈ ̈
Inorg. Chim. Acta 2011, 374, 472−479.
[Mo(NNH2)(trpd-1)(dmpm)](BArF)2 (13). A 65 mg portion of
(0.064 mmol) of [H(OEt2)2]BArF dissolved in 3 mL of CD2Cl2 was
cooled to −10 °C and was added to 26 mg (0.032 mmol) of 1 in 3 mL
of CD2Cl2. The light red mixture turned into dark red and was
(14) Lehnert, N.; Tuczek, F. Inorg. Chem. 1999, 38, 1659−1670.
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1076−1086.
1
transferred into an NMR tube immediately. H NMR (400.13 MHz,
(17) Horn, K. H.; Bores, N.; Lehnert, N.; Mersmann, K.; Nather, C.;
̈
̈
C6D6, 300 K): δ = 10.84 (br s, 1H, NNH2), 9.43 (br s, 1H, NNH2),
7.75 (br t, 16H, CHo arom BArF), 7.58 (s, 8H, CHp arom
BArF),7.56−6.83 (m, 20H, CH arom.), 3.75−3.67 (m, 2H, CH2
dmpm), 3.50 (q, 16H, O(−CH2−CH3)2 BArF), 2.52 (br t, 1H, CH-
CH3), 2.52−2.35 (m, 2H, CH2-PPh2), 2.20 (d, 3H, CH3 dmpm), 1.89
(m, 2H, CH2-PPh2), 1.84 (d, 3H, CH3 dmpm), 1.74 (m, 2H, CH2-
PiPr2), 1.47 (m, 1H, CH-CH3), 1.51 (d, 3H, CH3 dmpm), 1.29 (m,
3H, Cq-CH3), 1.18 (t, 24H, O(−CH2−CH3)2 BArF), 1.13−0.98 (m,
12H, CHCH3), 0.56 (d, 3H, CH3 dmpm) ppm. 31P NMR (161.98
MHz, CD2Cl2, 300 K) δ = 35.2 (dddd, 1P, Pd, J = −18.6, −26.8,
−26.5, 74.6 Hz), 24.5 (dddd, 1P, Pc, J = −18.6, −23.3, −34.6, 85.8
Hz), −6.53 (dddd, 1P, Pe, J = −26.5, −26.7, −34.6, −35.9 Hz), −28.9
(ddd, 1P, Pa, J = −26.7, −26.8, 85.8 Hz), −32.2 (ddd, 1P, Pb, J =
−23.3, −35.9, 74.4 Hz) ppm.
Peters, G.; Tuczek, F. Inorg. Chem. 2005, 44, 3016−3030.
(18) Mersmann, K.; Horn, K. H.; Bores, N.; Lehnert, N.; Studt, F.;
̈
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ASSOCIATED CONTENT
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(25) George, T. A.; Ma, L.; Shailh, S. N.; Tisdale, R. C.; Zubieta, J.
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■
S
* Supporting Information
Additional DTA-TG Data (Figures S1−S4), 31P NMR Spectra
(Figures S5−S6, S12−S15), detailed coupling constant values
of all 31P NMR spectra (Table S1), and IR/Raman spectra
(Figures S7−S11). The results of DFT geometry optimizations
(Cartesian coordinates) for the complexes 1, 13 and [Mo-
(NNH2)(tdppme)(dmpm)]2+ are given in Tables S2−S5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(26) Stephan, G. C.; Peters, G.; Lehnert, N.; Habeck, C. M.; Nather,
̈
C.; Tuczek, F. Can. J. Chem. 2005, 83, 385−402.
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AUTHOR INFORMATION
■
Corresponding Author
(32) Owens, B. E.; Poli, R.; Rheingold, A. L. Inorg. Chem. 1989, 28,
*E-mail: ftuczek@ac.uni-kiel.de. Tel: ++49 (0)431 880 1410.
Fax: ++49 (0)431 880 1520.
1456−1462.
(33) Walter, O.; Huttner, G.; Zsolnai, L. Z. Naturforsch. B 1993, 48,
636−640.
Notes
The authors declare no competing financial interest.
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(35) Hofacker, P.; Friebel, C.; Dehnicke, K.; Bauml, P.; Hiller, W.;
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ACKNOWLEDGMENTS
̈
■
(36) Cotton, F. A.; Poli, R. Inorg. Chem. 1987, 26, 1514−1518.
F.T. thanks CAU Kiel for support of this research.
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J. Chem. Soc., Dalton Trans. 1993, 655−658.
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