Mo- and W-N2 and -CO complexes with novel mixed P/N ligands: Structural properties and implications to synthetic nitrogen fixation

Article abstract of DOI:10.1016/j.ica.2007.06.046

Four new ligands containing a pyridine or thiazole group and one or more N-(diphenylphosphinomethyl)amine functions have been prepared and employed for the synthesis of Mo(0) and W(0) carbonyl and dinitrogen complexes. For comparison coordination of the literature-known ligand N,N-bis(diphenylphosphinomethyl)-methylamine (PNP, 1) to such systems has been investigated as well. Two new ligands are N,N-bis(diphenylphosphinomethyl)-2-aminopyridine (pyNP₂, 2) and N,N′-bis(diphenylphosphinomethyl)-2,6-diaminopyridine (PpyP, 3). In a third new ligand, N-diphenylphosphinomethyl-2-aminothiazole (thiazNP, 4), the pyridine group is replaced by thiazol. Finally, the pentadentate ligand N,N,N′,N′-tetrakis(diphenylphosphinomethyl)-2,6-diaminopyridine (pyN₂P₄, 5) has been synthesized. Coordination of ligands 2, 3 and 4 to low-valent metal centers is investigated on the basis of the three molybdenum carbonyl complexes [Mo(CO)₃(NCCH₃)(pyNP₂)] (6), [Mo(CO)₄(PpyP)] (7) and [Mo(CO)₄(thiazNP)] (8), respectively, all of which are structurally characterized. Moreover, employing ligands 1 and 2 the two dinitrogen complexes [W(N₂)₂(dppe)(PNP)] (9) and [Mo(N₂)₂(dppe)(pyNP₂) (10), respectively, are prepared. Both systems are investigated by vibrational and NMR spectroscopy; in addition, complex 10 is structurally characterized.

Full text of DOI:10.1016/j.ica.2007.06.046

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 1008–1019
www.elsevier.com/locate/ica
Mo– and W–N2 and –CO complexes with novel mixed P/N
ligands: Structural properties and implications
to synthetic nitrogen fixation
*
Gerald C. Stephan, Christian Nather, Chinnappan Sivasankar, Felix Tuczek
¨
Institut fu¨r Anorganische Chemie, Christian Albrechts Universita¨t zu Kiel, Olshausenstraße 40, 24098 Kiel, Germany
Received 10 April 2007; received in revised form 21 June 2007; accepted 30 June 2007
Available online 20 July 2007
Dedicated to Edward Solomon on the occasion of his 60th anniversary.
Abstract
Four new ligands containing a pyridine or thiazole group and one or more N-(diphenylphosphinomethyl)amine functions have been
prepared and employed for the synthesis of Mo(0) and W(0) carbonyl and dinitrogen complexes. For comparison coordination of the
literature-known ligand N,N-bis(diphenylphosphinomethyl)-methylamine (PNP, 1) to such systems has been investigated as well. Two
new ligands are N,N-bis(diphenylphosphinomethyl)-2-aminopyridine (pyNP2, 2) and N,N0-bis(diphenylphosphinomethyl)-2,6-diamino-
pyridine (PpyP, 3). In a third new ligand, N-diphenylphosphinomethyl-2-aminothiazole (thiazNP, 4), the pyridine group is replaced by
thiazol. Finally, the pentadentate ligand N,N,N0,N0-tetrakis(diphenylphosphinomethyl)-2,6-diaminopyridine (pyN2P4, 5) has been syn-
thesized. Coordination of ligands 2, 3 and 4 to low-valent metal centers is investigated on the basis of the three molybdenum carbonyl
complexes [Mo(CO)3(NCCH3)(pyNP2)] (6), [Mo(CO)4(PpyP)] (7) and [Mo(CO)4(thiazNP)] (8), respectively, all of which are structurally
characterized. Moreover, employing ligands 1 and 2 the two dinitrogen complexes [W(N2)2(dppe)(PNP)] (9) and [Mo(N2)2(dppe)(pyNP2)
(10), respectively, are prepared. Both systems are investigated by vibrational and NMR spectroscopy; in addition, complex 10 is struc-
turally characterized.
ꢀ 2007 Published by Elsevier B.V.
Keywords: Nitrogen fixation; Dinitrogen complex; Carbonyl complex; Phosphine ligands; Mo complex
1. Introduction
coworkers in 2003 (Schrock cycle). It is based on Mo(III)
complexes with triamidoamine ligands carrying sterically
demanding substituents. For the Mo(III) HIPT complex
(HIPT = hexaisopropylterphenyl-triamidoamine) the cata-
lytic generation of ammonia has been demonstrated in 6
cycles with an overall yield of 65% [3]. Experimental and
theoretical results demonstrate that the reaction proceeds
in a series of alternating protonation and reduction steps
[3,4]. A potential disadvantage of this system is the fact
that the coordinating amide groups are strongly Brønsted
basic. Protonation of these groups renders the complex
unstable with respect to metal-ligand bond cleavage.
Protonation of the ligand amido groups may, on the
other hand, help to mediate protonation of the bound N2
ligand [4b], in analogy to the iron-molybdenum cofactor
For the transition-metal mediated synthesis of ammonia
from N2 two major scenarios have been established. The
first one (Chatt cycle) is based on molybdenum and tung-
sten phosphine complexes that are able to bind dinitrogen.
In a series of protonation and reduction steps, the N2
ligand is transformed to NH3 via well-defined NNHx and
NHx (x = 0, 1, 2 or 3) intermediates [1]. This chemistry
has also been performed in a cyclic fashion, however with
small yields [2]. The second complete scheme of synthetic
nitrogen fixation has been discovered by Schrock and
*
Corresponding author. Tel.: +49 431 880 1410; fax: +49 431 880 1520.
E-mail address: ftuczek@ac.uni-kiel.de (F. Tuczek).
0020-1693/$ - see front matter ꢀ 2007 Published by Elsevier B.V.
doi:10.1016/j.ica.2007.06.046

G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
1009
(FeMoco) of nitrogenase where protonation of the bridg-
ing sulfides is thought to play an important role in the cat-
alytic process [5]. In order to combine the advantage of
phosphine ligands – stability versus acids – with the pres-
ence of Brønsted-basic groups which may assist the proton-
ation of the N2 ligand, we decided to synthesize a series of
mixed phosphine/amine ligands which we wanted to coor-
dinate to low-valent Mo centers in order to bind, activate
and reduce dinitrogen. An important side-effect of the pres-
ence of basic groups in the ligand framework would also be
the fact that the ‘‘secondary’’ (=ancillary ligand) proton-
ation should shift the reduction potentials of these systems
towards less negative values and thus make the NNHx and
NHx intermediates of the Chatt cycle easier to reduce [6].
The simplest route to ligands with the desired properties
is based on diphosphines with a central amine group. These
ligands are in particular the literature-known (mono-) aza
analogues of dppm (1,2-bis(diphenylphosphino)methane)
and dppp (1,2-bis(diphenylphosphino)propane). In the for-
mer ligand, Ph2PNRPPh2, the two terminal phosphine
groups are directly bridged by an amino function whereas
in the latter ligand, Ph2PCH2N(Me)CH2PPh2 (‘‘PNP’’),
the central carbon atom of the propylene group is substi-
tuted by amine. Bidentate coordination of both ligand
types to transition-metals, in analogy to dppm and dppp,
respectively, has been observed for all 6th and 12th group
metals as well as in Rh, Ir, Pt and Pd complexes which were
investigated for catalytic applications [7]. Moreover, the
Brønsted basicity of the central amino group of PNP has
been exploited in model systems of hydrogenase, where
proton transfer from the PNP amine to a hydrido ligand
coordinated to a Ni or Fe center was evidenced [8].
the corresponding free phosphines and formaldehyde, can
equally be used as phosphine components. This procedure
provides a standard route to linear, branched and cyclic
aminophosphines [7,9,10].
Another goal of our studies in nitrogen fixation has been
to synthesize pentadentate phosphine ligands that occupy
the equatorial positions in octahedral mono-dinitrogen
complexes as well as the trans-position of the bound N2
ligand. Such ligand systems would act to avoid the unfa-
vourable axial ligand exchange reactions which are one of
the reasons that catalytic action has not been achieved with
Mo and W diphosphine complexes so far. Combining this
second goal with the first one – the introduction of proton-
atable groups in multidentate phosphine ligands – we came
up with a ligand architecture that is based on a central 2,6-
diaminopyridine group carrying four diphenylphosphi-
nomethyl groups. As this ligand has one pyridine group,
two amino functions and four phosphine endgroups, it is
referred to as pyN2P4 (5, Scheme 2). This ligand can be
considered as the diaza analogue of the 2,6-bis{(2-
diphenylphosphonanyl)-1-[(diphenylphosphanyl)methyl]-
1-methylethyl}pyridine
Grohmann et al. [11].
(‘‘py{PPh2}4’’)
ligand
of
In the present paper, the synthesis and the coordination
properties of diphosphine and tetraphosphine ligands
based on 2-aminopyridine and 2,6-diaminopyridine are
described. Besides N,N,N0,N0-tetrakis(diphenylphosphi-
nomethyl)-2,6-diaminopyridine (pyN2P4; 5) two simpler
analogues such as N,N-bis(diphenylphosphinomethyl)-2-
aminopyridine (pyNP2; 2) and N,N0-bis(diphenylphosphi-
nomethyl)-2,6-diaminopyridine (PpyP; 3) have been pre-
pared (Scheme 2). Moreover, we synthesized the ligand
N-diphenylphosphinomethyl-2-aminothiazole (thiazNP; 4)
in which the pyridine group is replaced by thiazol. For
comparison, finally, the PNP ligand (1) is considered which
corresponds to the N,N-bis(diphosphinomethyl)amine
group that is contained in the more complex ligand systems
pyNP2 and pyN2P4.
In order to obtain information on the coordination of
these ligands to low-valent transition-metal centers, Mo
carbonyl complexes were prepared with ligands 2, 3 and
4. All of these systems are structurally characterized by
X-ray crystallography. In addition, Mo- and W–N2 com-
plexes of PNP (1) and pyNP2 (2) were synthesized and
structurally as well as spectroscopically characterized.
The spectroscopic properties of these complexes are com-
pared to those of their counterparts with diphosphine col-
igands. The implications of the structural and
spectroscopic results with respect to the synthesis of an effi-
cient catalyst for synthetic nitrogen fixation are discussed.
The PNP ligands are easily prepared by the phosphorus-
analogous Mannich reaction. By means of this transforma-
tion primary or secondary amines can be substituted by
methylenephosphine residues –CH2PRR0 (R,R0 = alkyl,
aryl) through reaction with a secondary phosphine and
formaldehyde (Scheme 1). The mechanism of this reaction
is comparable to the standard Mannich reaction and starts
with the addition of amine to formaldehyde [9]. After
release of water the cationic intermediate reacts with the
phosphine to form –N–CH2-PRR0 units. In practice
hydroxymethylphosphines, which are in equilibrium with
2. Results and analysis
2.1. Synthesis and structures of ligands 1–5
All of the ligands investigated in this paper have been
prepared by the phosphorus-analogous Mannich reaction
Scheme 1. Phosphorus-analogous Mannich reaction (R1ꢀ4 = H, alkyl,
aryl).

1010
G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
Scheme 2. PNP-ligands 1, 2, 3, 4, and 5.
(vide supra). Thus, reaction of diphenylphosphine with
HCHO and CH3NH2 Æ HCl in the presence of EtOH and
water afforded the PNP ligand (1) in good yield [12]. In
contrast to aliphatic and aromatic amines, aminopyridines
react sluggishly. Moreover, while aniline and its derivatives
give bidentate PNP-ligands in high yields [7c], aminopyri-
dines prefer a monosubstitution of the amine, leading to
pyNHCH2PR2 derivatives (Scheme 3) [13]. This may be
due to the deactivating effect of the pyridine ring, consider-
ably reducing the nucleophilicity of the amino group. By
introducing electron-withdrawing groups in ortho-position,
a further reduction in reactivity is observed. In case of
2-amino-3-nitropyridine, e.g., the amine function is so
unreactive that no product could be isolated under the
investigated conditions.
Reaction of 2-aminopyridine with 2 equivalents of
hydroxymethyldiphenylphosphine in toluene was found
to lead to a mixture of the mono- and disubstituted amines,
N-diphenylphosphinomethyl-2-aminopyridine and N,N-
bis(diphenylphosphinomethyl)-2-aminopyridine (pyNP2, 2).
The disubstituted amine 2 can be isolated by crystallisation
in about 35% yield. Yield and purity can be improved by
using an ethanol/toluene mixture as a more polar solvent
and by adding NEt3HCl as acid. Under these conditions
the yield of 2 can be raised to more than 70% and the prod-
uct is free of the monosubstituted analogue. Single crystals
suitable for structural analysis were obtained from cooling
a dilute solution of 2 in a mixture of CH2Cl2 and ethanol
(Fig. 1, Table 1). Selected bond lengths and angles are sum-
marised in Table 2.
The reactivity of 2,6-diaminopyridine was found to
be similar. Reaction of the diamine with 4 equivalents of
hydroxymethylphosphine in mixed toluene/alcohol
solvents leads to an oily mixture of the mono-, di-, tri-
and tetrasubstituted coumpounds in which N,N0-bis(diphe-
nylphosphinomethyl)-2,6-diaminopyridine PpyP (3) is
identified as major product by NMR. The differently
substituted compounds cannot be separated by crystallisa-
tion. Ligand 3 can also be prepared in pure form with a
yield of 30–40% by reaction of 2,6-diaminopyridine with
two equivalents of Ph2PCH2OH. The addition of NEt3HCl
to the reaction mixture gives the tri- and tetrasubstituted
compounds as major products. The tetrasubstituted com-
pound N,N,N0,N0-tetrakis(diphenylphosphinomethyl)-2,6-
diaminopyridine (pyN2P4; 5) can be isolated as the only
Scheme 3. Preferred reaction products of aminopyridines and
hydroxymethylphosphines.
Fig. 1. Crystal structure of pyNP2 (2).

Table 1
Selected crystallographic data and results of the structure refinement for pyNP2 (2), thiazNP (4), [Mo(CO)3(NCCH3)(pyNP2)] (6), [Mo(CO)4(PpyP)] (7), [Mo(CO)4(thiazNP)] (8) and
[Mo(N2)2(dppe)(pyNP2)] (10)
Compound
2
4
6
7
8
10
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C
31H28N2P2
C16H15N2PS
298.33
200(2)
monoclinic
C2/c
18.404(1)
11.643(1)
15.573(1)
90
115.833(6)
90
3003.4(3)
8
1.320
0.313
C72H62Mo2N6O6P4
1423.04
220(2)
C43H45MoN3O6P2
857.70
220(2)
orthorhombic
Pbca
C28H31MoN2O6PS
650.52
220(2)
monoclinic
P21/n
15.528(1)
10.299(1)
19.694(1)
90
108.864(6)
90
2980.4(3)
4
1.450
0.606
C66H61MoN6P4
1158.03
220(2)
490.49
293(2)
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
˚
a (A)
10.490(2)
10.830(2)
13.321(2)
72.2(2)
80.71(2)
63.20(2)
1285.4(4)
2
12.890(1)
14.885(2)
21.474(2)
69.76(1)
88.99(1)
79.22(1)
3792.7(5)
2
14.961(1)
20.287(1)
27.162(2)
90
90
90
8243.6(8)
8
1.382
11.533(1)
13.847(1)
19.000(2)
89.13(1)
78.77(1)
74.01(1)
2858.4(3)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z
dcalc (mg/m3)
1.267
0.192
1.246
0.465
1.345
0.389
l (mmꢀ1
)
0.446
Theta range
Index ranges
2.18–25.02
0 6 h 6 12,
ꢀ15 6 k 6 12,
ꢀ15 6 l 6 15
4789
2.26–25.98ꢁ.
ꢀ20 6 h 6 22,
ꢀ14 6 k 6 14,
ꢀ19 6 l 6 19
11602
2.41–26.02
ꢀ15 6 h 6 15,
ꢀ18 6 k 6 18,
ꢀ26 6 l 6 26
26311
2.26–24.98
ꢀ15 6 h 6 17,
ꢀ22 6 k 6 24,
ꢀ32 6 l 6 26
23245
2.26–28.11
ꢀ20 6 h 6 1,
ꢀ13 6 k 6 13,
22 6 l 6 26
22274
2.48–25.03
ꢀ12 6 h 6 13,
ꢀ16 6 k 6 16,
ꢀ22 6 l 6 22
17018
Reflection collected
Rint
0.0230
0.0594
0.0415
0.0352
0.0418
0.0562
Independent reflection
Refl. with I > 2r(I)
Parameters
4517
3393
316
2816
2466
185
14482
10981
870
7167
5770
501
7087
5606
350
9768
6650
695
Goodness-of-fit
R1 [I > 2r(I)]
1.021
0.0345
1.085
0.0335
0.939
0.0376
1.014
0.0290
1.002
0.0469
0.937
0.0444
wR2 (all data)
dF (e/A )
0.0995
0.0927
0.191/ꢀ0.276
0.0974
0.392/ꢀ0.741
0.0751
0.351/ꢀ0.485
0.1297
0.705/ꢀ0.659
0.1062
0.455/ꢀ0.745
3
˚

1012
G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
Table 2
Table 3
Selected bond lengths (A) and angles (ꢁ) for thiazNP (4)
˚
˚
Selected bond lengths (A) and angles (ꢁ) for pyNP2 (1)
N(1)–C(27)
N(2)–C(27)
N(1)–C(14)
N(2)–C(29)
1.378(2)
1.354(3)
1.459(2)
1.360(3)
P(2)–C(14)
1.8664(19)
120.02(15)
116.96(17)
120.56(17)
S(1)–C(2)
S(1)–C(1)
C(1)–N(1)
C(1)–N(2)
N(1)–C(3)
1.728(2)
1.746(2)
1.314(2)
1.344(2)
1.373(2)
N(2)–C(4)
1.449(2)
123.6(2)
114.3(2)
110.0(2)
117.1(2)
C(27)–N(1)–C(14)
N(2)–C(27)–N(1)
N(1)–C(27)–C(28)
N(1)–C(1)–N(2)
N(1)–C(1)–S(1)
C(1)–N(1)–C(3)
C(2)–C(3)–N(1)
product after a reaction time of 50 h, using an excess of
phosphine. Highest yields and purities were obtained when
the formed water was removed by a molsieve placed in a
Soxhlet-Extractor.
bonyl complexes were synthesized. All of these complexes
were prepared by refluxing mixtures of the respective
ligand and Mo(CO)6 in acetonitrile [14]. By recrystallisa-
tion in THF crystals suitable for X-ray analysis could be
isolated. Application of this protocol to ligand 2, pyNP2,
was found to lead to complex [Mo(CO)3(NCCH3)(pyNP2)]
(6) which was crystallographically characterized. The com-
pound crystallizes in a triclinic crystal system containing
two complex and two THF molecules in the unit cell.
The crystal structure reveals that pyNP2 acts as a bidentate
ligand coordinating in a cis geometry while the pyridine
group remains uncoordinated (Fig. 3). Two of the three
carbonyl groups are bound within the MoP2 plane whereas
the axial positions are occupied by the third carbonyl and
the acetonitrile ligand. Due to the fact that acetonitrile is
a weaker r-donor and p-acceptor than phosphine the CO
ligand in trans-position to acetonitrile is more activated;
i.e., exhibits a shorter Mo–C and a longer C–O bond length
than the carbonyl ligands in trans-position to the phos-
phine groups. Selected bond lengths and angles are given
in Table 4. Crystallographic data are collected in Table 1.
In an analogous fashion the PpyP ligand (3) was con-
verted to the tetracarbonyl complex [Mo(CO)4(PpyP)] (7)
which could be structurally characterized as well (Fig. 4).
The compound crystallises in the space group Pbca with
eight molecules per unit cell. Selected bond lengths and
angles are collected in Table 5; crystallographic data are
given in Table 1. Just like pyNP2, PpyP acts as a bidentate,
cis-coordinating ligand with the pyridine ring remaining
uncoordinated. In this case, however, the bound phosphine
2-Aminothiazole reacts with hydroxymethyldiphenyl-
phosphine exclusively to the monosubstituted product N-
diphenylphosphinomethyl-2-aminothiazole (thiazNP; 4).
The disubstituted amine could not be observed in the reac-
tion mixture, even after a reaction time of 50 h. This might
be caused by a tautomeric equilibrium between the amine
and the imine form of the aminothiazole (Scheme 4), inhib-
iting further reaction of the monosubstituted amine to the
N,N-disubstituted product. Single crystals suitable for
structural analysis could be obtained by cooling an ethanol-
ic solution of 4. In agreement with Scheme 4, the structure of
4 (Fig. 2, Table 1) exhibits an equalisation of the C–N bond
˚
˚
lengths C1–N1 (1.314(2) A) and C1–N2 (1.344(2) A). Both
C–N bond distances are shorter than the N2–C4
˚
˚
(1.449(2) A) and the N1–C3 bond (1.373(2) A; Table 3).
2.2. Preparation and structural characterization of Mo(0)
carbonyl complexes with ligands 2, 3 and 4
In order to investigate the coordination geometries of
ligands 2, 3 and 4 to low-valent metal centers, Mo(0) car-
Scheme 4. Tautomeric forms of thiazNP (4).
Fig. 3. Crystal structure of [Mo(CO)3(NCCH3)(pyNP2)] (6); only one of
Fig. 2. Crystal structure of thiazNP (4).
the two crystallographically independent complexes is shown.

G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
1013
Table 4
given in Table 6; crystallographic data are collected in
Table 1. The crystal structure (Fig. 5) shows that the thia-
zNP ligand coordinates in a bidentate fashion with the
phosphine group and the thiazole group binding in cis-
geometry to the metal center. Interestingly, the thiazole
group is coordinated by the N- and not the S-atom which
probably is due to the possibility of backbonding into the
p* orbitals of the heterocycle in this configuration.
˚
Selected bond lengths (A) and angles (ꢁ) for [Mo(CO)3(NCCH3)(pyNP2)]
(6)
Mo(2)–C(112)
Mo(2)–C(113)
Mo(2)–C(111)
1.959(3) Mo(2)–N(14)
1.982(3) Mo(2)–P(4)
1.989(3) Mo(2)–P(3)
2.207(2)
2.5192(8)
2.5199(8)
C(112)–Mo(2)–C(113)
C(112)–Mo(2)–C(111)
C(113)–Mo(2)–C(111)
C(112)–Mo(2)–N(14)
C(113)–Mo(2)–N(14)
C(111)–Mo(2)–N(14)
C(112)–Mo(2)–P(4)
P(4)–Mo(2)–P(3)
90.18(12) C(113)–Mo(2)–P(4)
88.07(12) C(111)–Mo(2)–P(4)
86.40(13) N(14)–Mo(2)–P(4)
175.34(11) C(112)–Mo(2)–P(3)
94.41(11) C(113)–Mo(2)–P(3)
91.45(11) C(111)–Mo(2)–P(3)
90.50(10)
176.33(9)
90.75(7)
89.74(9)
177.28(10)
96.31(10)
85.71(7)
1.130(4)
1.474(5)
2.3. Synthesis of dinitrogen complexes with ligands 1 and 2
89.98(9)
86.78(2)
N(14)–Mo(2)–P(3)
N(14)–C(114)
For the synthesis of Mo/W-dinitrogen complexes with
different diphosphine coligands it is necessary to already
bind one of the diphosphines to a M(III)/(IV)-halide pre-
cursor which is then reduced in the presence of the second
diphosphine under N2 (Scheme 5, top) [15]. Correspond-
ingly, [W(N2)2(dppe)(PNP)] (9) was prepared by reduction
of [WCl4(dppe)] with Mg under N2 in presence of PNP (1).
The isotope-substituted complex [W(15N2)2(dppe)(PNP)]
was prepared analogously in an 15N2 atmosphere. A spec-
troscopic analysis of this complex is presented in the fol-
lowing section.
O(11)–C(111)
O(12)–C(112)
O(13)–C(113)
1.144(4) C(114)–C(115)
1.160(4) N(14)–C(114)–C(115) 178.0(4)
1.144(4)
Attempts to perform the synthesis of complex
[Mo(N2)2(dppe)(pyNP2)] (10) in a similar fashion by reduc-
tion of [MoCl4(dppe)] with sodium amalgam under N2 in
the presence of pyNP2 (2) did not lead to the target com-
pound in pure form. Instead, the reaction was found to
give a mixture of 10 and [Mo(N2)2(dppe)2], as evidenced
by IR and 31P NMR spectroscopy (Scheme 5, middle). In
order to obtain a pure product, it appeared necessary to
develop a different synthetic route which in particular
allowed to avoid possible ligand exchange reactions at
the level of the Mo(IV) chloro complexes. To this end the
Mo(III) complex [MoBr3(pyNP2)(thf)] (11) was chosen as
precursor which was prepared by reaction of [MoBr3(thf)3]
with pyNP2 in THF/CH2Cl2. The corresponding chloro
complex could also be obtained by a corresponding reac-
tion from [MoCl3(thf)3], but only in very low yields
(<10%); therefore the bromo congener was preferred for
further reactions. Subsequent reduction of 11 with Na/
Hg and dppe under N2 then gave [Mo(N2)2(dppe)(pyNP2)]
(10), which was isolated as an orange, crystalline product
Fig. 4. Crystal structure of [Mo(CO)4(PpyP)] (7).
groups derive from two different amino functions of the
pyridine moiety, giving an eleven-membered metallacycle.
Two carbonyl ligands are bound in a cis-geometry in the
plane of the phosphine ligands whereas the other two are
arranged in a trans-fashion in the axial positions.
A similar arrangement of the CO ligands is found in the
tetracarbonyl complex [Mo(CO)4(thiazNP)] (8) which was
obtained by refluxing a mixture of Mo(CO)6 and thiazNP
(4) in acetonitrile. The compound crystallises in the space
group P21/n containing four complex and two THF mole-
cules in the unit cell. Selected bond lengths and angles are
Table 6
˚
Selected bond lengths (A) and angles (ꢁ) for [Mo(CO)4(thiazNP)] (8)
Mo(1)–C(34)
Mo(1)–C(32)
Mo(1)–C(31)
1.937(3)
1.992(3)
2.034(3)
Mo(1)–C(33)
Mo(1)–N(2)
Mo(1)–P(1)
2.052(3)
2.310(2)
2.4980(8)
Table 5
˚
Selected bond lengths (A) and angles (ꢁ) for [Mo(CO)4(PpyP)] (7)
Mo(1)–C(54)
Mo(1)–C(53)
Mo(1)–C(51)
1.964(2)
1.979(3)
2.039(3)
Mo(1)–C(52)
Mo(1)–P(1)
Mo(1)–P(2)
2.045(2)
2.5778(6)
2.5906(6)
C(34)–Mo(1)–C(32)
C(34)–Mo(1)–C(31)
C(32)–Mo(1)–C(31)
C(34)–Mo(1)–C(33)
C(32)–Mo(1)–C(33)
C(31)–Mo(1)–C(33)
C(34)–Mo(1)–N(2)
C(32)–Mo(1)–N(2)
C(1)–N(1)
86.82(14)
88.34(15)
90.23(14)
88.26(13)
89.12(14)
176.57(14)
177.78(12)
92.36(12)
1.462(4)
C(31)–Mo(1)–N(2)
C(33)–Mo(1)–N(2)
C(34)–Mo(1)–P(1)
C(32)–Mo(1)–P(1)
C(31)–Mo(1)–P(1)
C(33)–Mo(1)–P(1)
N(2)–Mo(1)–P(1)
C(1)–P(1)
89.60(13)
93.79(11)
95.23(10)
177.31(10)
88.10(11)
92.68(10)
85.53(7)
C(54)–Mo(1)–C(53)
C(54)–Mo(1)–C(51)
C(53)–Mo(1)–C(51)
C(54)–Mo(1)–C(52)
C(53)–Mo(1)–C(52)
C(51)–Mo(1)–C(52)
C(54)–Mo(1)–P(1)
84.67(10)
89.18(9)
86.25(10)
89.68(9)
89.92(10)
176.09(9)
171.66(7)
C(51)–Mo(1)–P(1)
C(52)–Mo(1)–P(1)
C(54)–Mo(1)–P(2)
C(53)–Mo(1)–P(2)
C(51)–Mo(1)–P(2)
C(52)–Mo(1)–P(2)
P(1)–Mo(1)–P(2)
87.74(6)
92.88(6)
86.27(7)
170.77(7)
91.84(7)
91.81(7)
1.845(3)
1.369(4)
N(1)–C(2)
101.573(18)
C(2)–N(2)
1.310(4)

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G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
Table 7
Selected bond lengths (A) and angles (ꢁ) for [Mo(N2)2(dppe)(pyNP2)] (10)
˚
Mo(1)–N(3)
Mo(1)–N(5)
Mo(1)–P(1)
2.014(4)
2.032(4)
2.4428(10)
Mo(1)–P(2)
Mo(1)–P(4)
Mo(1)–P(3)
2.4440(10)
2.4838(9)
2.4848(10)
N(3)–Mo(1)–N(5)
N(3)–Mo(1)–P(1)
N(5)–Mo(1)–P(1)
N(3)–Mo(1)–P(2)
N(5)–Mo(1)–P(2)
P(1)–Mo(1)–P(2)
N(3)–Mo(1)–P(4)
N(5)–Mo(1)–P(4)
176.28(12)
81.82(9)
96.56(9)
87.26(9)
89.32(9)
86.89(3)
98.68(8)
83.20(9)
P(1)–Mo(1)–P(4)
P(2)–Mo(1)–P(4)
N(3)–Mo(1)–P(3)
N(5)–Mo(1)–P(3)
P(1)–Mo(1)–P(3)
P(2)–Mo(1)–P(3)
P(4)–Mo(1)–P(3)
175.51(4)
97.59(3)
93.72(9)
89.76(9)
95.89(3)
177.15(4)
79.62(3)
Fig. 5. Crystal structure of [Mo(CO)4(thiazNP)] (8).
with 1.5 eq. of benzene cocrystallised (Scheme 5, bottom).
X-ray crystallographic characterization of this compound
indicates a triclinic crystal system with two complex and
three benzene molecules in the unit cell; one of these ben-
zene molecules is located on a center of inversion. Selected
bond lengths and angles are given in Table 7; crystallo-
graphic data are collected in Table 1. As already evidenced
for the CO complex 6 of pyNP2 (vide supra), this ligand
acts as a bidentate, cis-coordinating ligand with the pyri-
dine group remaining uncoordinated (Fig. 6). Correspond-
ingly, a bis(dinitrogen) complex is formed and there is no
evidence for a mono-N2-complex with a coordinated pyri-
dine group in trans-position to the N2 ligand.
2.4. Vibrational and NMR-spectroscopic characterization of
Mo/W–N2 complexes 9 and 10
As for other molybdenum and tungsten bis(dinitrogen)
complexes, the IR spectra of [W(N2)2(dppe)(PNP)] (9)
and [Mo(N2)2(dppe)(pyNP2)] (10) exhibit an intense band
Fig. 6. Crystal structure of [Mo(N2)2(dppe)(pyNP2)] (10).
Scheme 5. Synthesis of Mo/W–N2-complexes 9, 10 with PNP-coligands.

G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
1015
for the antisymmetric N–N-stretching vibration mas(NN)
and a less intense band for the symmetric N–N-stretching
vibration ms(NN) (Fig. 7) [16]. The Raman spectra show
these vibrations with the intensities interchanged (Supp.
Mat.). The symmetric NN-stretching vibration of complex
(9) appears at 2013 cmꢀ1 in the IR and 2015 cmꢀ1 in the
Raman spectrum while the antisymmetric N–N-vibrations
are located at mas(NN) = 1910 cmꢀ1 (IR) and 1908 cmꢀ1
(Raman). The isotope substituted complex [W(15N2)2-
(dppe)(PNP)] shows an isotopic shift to ms(15N15N) =
1943 cmꢀ1 and mas(15N15N) = 1853 cmꢀ1 in the IR-spec-
trum (Fig. 7). Importantly, the dinitrogen ligand in this
complex is activated more than in the complex with two
dppe ligands, as indicated by the lower N–N stretching fre-
quencies (ms(NN) and mas(NN) for [W(N2)2(dppe)2]:
2009 cmꢀ1 and 1942 cmꢀ1, respectively [16]). Moreover,
the difference between symmetric and antisymmetric
stretching frequency is larger as compared to
[W(N2)2(dppe)2]. The same observations have been made
for the carbon-analogous ligand 1,2-bis(diphenylphosphi-
no)propane (dppp) and attributed to stronger electron
donation of the propylene bridged phosphines [17]: com-
pared with ethylene or methylene bridged ditertiary phos-
phines the propylene bridge exhibits a larger bite angle
(closer to the ideal 90ꢁ) and therefore leads to a stronger
interaction between the phosphine group and the metal.
In addition to vibrational spectroscopy, NMR has been
employed to characterize complex 9. The 15N resonances of
the coordinated N2 ligand are found at Na = ꢀ61.6 ppm
and Nb = ꢀ50.0 ppm (versus nitromethane) and thus are
located in a normal range for W(0)-dinitrogen complexes
[18]. The 31P NMR-spectrum exhibits two signal groups
at 41.2 and 4.3 ppm arising from an AA0XX0 spectrum.
These two resonances appear as doublets due to the strong
trans-phosphorus–phosphorus coupling; the two doublets
0.5
0.0
2000
1900
Wavenumber / cm-1
Fig. 8. NN-stretching vibrations of 10 in KBr (full line) and in CH2Cl2
(dotted line).
further split into doublets due to the weak cis-phospho-
rus–phosphorus interaction.
Similar spectroscopic observations are made for com-
plex 10, [Mo(N2)2(dppe)(pyNP2)]. However, depending
on the crystallinity and the preparation of the sample, the
N–N-vibrations of 10 are split in the IR-spectrum mea-
sured in KBr. Thus, the mas(NN) band has two minima at
1944 cmꢀ1 and at 1968 cmꢀ1 whereas the ms(NN) band
exhibits two minima at 2030 cmꢀ1 and 2042 cmꢀ1
(Fig. 8). In contrast, the IR-spectrum of 10 in CH2Cl2 solu-
tion does not show any splitting (cf. Fig. 8). Therefore, this
finding is assigned to a solid state effect. This is compatible
with the fact that the crystal structure of 10 shows two
bis(dinitrogen)-complexes per unit cell; cf. Table 1. Like
in complex 9 the dinitrogen ligands in 6 are more activated
compared to the Mo-bis(dinitrogen) complex with two
dppe ligands, which again can be attributed to the larger
bite angle of the pyNP2 ligand (ms(NN) and mas(NN) for
[Mo(N2)2(dppe)2]: 2030 cmꢀ1 and 1980 cmꢀ1, respectively
[19]). Because of strong fluorescence the Raman-spectra
of 10 recorded at different excitation wavelengths are less
informative. In analogy to complex 9, the 31P NMR spec-
trum of 10 shows two signal groups from an AA0XX0 spec-
trum (61.5 ppm, 32.3 ppm); the coupling constants are
given in Section 4.
3. Discussion
In the present paper, the synthesis and the coordinative
properties of a number of new multidentate phosphine
ligands for synthetic nitrogen fixation have been reported.
Fig. 7. IR spectrum of [W(N2)2(dppe)(PNP)] (9) (full line) and
[W(15N2)2(dppe)(PNP)] (dotted line).

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G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
All of the ligands contain protonatable groups that could
help to mediate proton transfer and facilitate chemical or
electrochemical reduction of the corresponding transition-
metal N2, NNHx or NHx complexes (x = 0,1,2 or 3).
Our target ligand architecture consists of a central pyridine
group carrying two PNP-like sidearms, as realized in the
ligand N,N,N0,N0-tetrakis(diphenylphosphinomethyl)-2,6-
diaminopyridine (pyN2P4, 5; Scheme 2).
the CO complexes 8 and 9, on the other hand, could be
caused by the applied reaction conditions. To achieve a
complete coordination of the tridentate ligands PpyP and
pyNP2 to these systems the reaction between ligand and
Mo(CO)6 should possibly be performed in non-coordinat-
ing solvents applying longer reaction times.
In summary, a series of new phosphine ligands contain-
ing amine and pyridine groups has been synthesised and
the coordination properties of these ligands to low-valent
metals have been explored. So far, no coordination of pyr-
idine has been achieved to Mo/W-(0)-center. Efforts are
underway to enforce this coordination at the level of
Mo(III) and Mo(IV) precursors, which afterwards are con-
verted to the dinitrogen complexes under retention of the
pyridine–metal bond. The final goal of these studies
remains the synthesis of a dinitrogen complex with the
pyN2P4 ligand 5. Moreover, we want to further evaluate
the concept of protonatable ligands by careful titration of
dinitrogen complexes containing PNP ligands or ligands
containing PNP sidearms with weak acids.
Ligand 5 has successfully been synthesized, but coordi-
nation to low-valent Mo and W centers could not be
achieved yet. Nevertheless, we prepared two simpler
ligands containing a combination of pyridine with one or
two PNP diphosphine or PNH monophosphine side arms
which were found to coordinate to Mo(0) centers. These
are N,N-bis(diphenylphosphinomethyl)-2-aminopyridine
(pyNP2, 2) and N,N0-bis(diphenylphosphinomethyl)-2,6-
diaminopyridine (PpyP, 3). In a fourth new ligand, N-
diphenylphosphinomethyl-2-aminothiazole (thiazNP, 4),
the pyridine group is replaced by thiazol. For comparison,
finally, the literature-known ligand PNP (1) was synthe-
sized which contains to N,N-bis(diphenylphosphinometh-
yl)amine moiety that is also part of the ligands 2 and 5.
The coordination of ligands 2, 3 and 4 to low-valent metal
centers was probed by the synthesis of the three molybde-
num carbonyl complexes [Mo(CO)3(NCCH3)(pyNP2)] (6),
[Mo(CO)4(PpyP)] (7) and [Mo(CO)4(thiazNP)] (8) all of
which were structurally characterized. Moreover, employ-
ing ligands 1 and 2 the two dinitrogen complexes
[W(N2)2(dppe)(PNP)] (9) and [Mo(N2)2(dppe)(pyNP2)
(10), respectively, could be obtained. Both systems were
investigated by vibrational and NMR spectroscopy; in
addition, complex 10 was structurally characterized.
A conventional route to Mo/W dinitrogen complexes
with different phosphine ligands starts from M(III)/(IV)
halide precursors already having one of the phosphines
coordinated (Scheme 5). Importantly, the coordination
chemistry of the [MoCl3(thf)3] or [MoBr3(thf)3] precursors
may influence the nature of the final product. In this
respect, it is interesting to note that these trihalide precur-
sors favor the mer configuration [20]. Consequently, only
linear tridentate ligands like bis(diphenylphosphinoeth-
yl)phenylphosphine (dpepp) or bis(diphenylphosphinoeth-
yl)amines cause a complete substitution of THF [15]. For
a branched ligand like 2, this does not appear to be possi-
ble; pyridine thus will already be uncoordinated in the
Mo(III) precursor (e.g., [MoBr3(thf)(pyNP2)]). The free
pyridine group then competes with a stronger backbonding
ligand like N2 for metal coordination after the subsequent
reduction step. Formation of a mono-N2 complex with
pyNP2 acting as a tridentate ligand therefore probably
requires a complete coordination of this ligand already at
the level of the Mo(III)-precursor. Alternatively, the sec-
ond N2 ligand of a bis(dinitrogen) complex like 10 may
be exchanged against the uncoordinated pyridine group
after the reduction step. Until now, however, we were
unable to achieve such an exchange reaction at the level
of Mo(0) complexes. The lack of pyridine coordination in
4. Experimental
All reactions were performed under an inert gas atmo-
sphere using Schlenk techniques. The solvents were dried
and freshly distilled under inert gas. All other reagents were
used without further purification, except 2,6-diaminopyri-
dine which was recrystallised from toluene. Hydrox-
ymethyldiphenylphosphine was synthesized by stirring
diphenylphosphine and paraformaldehyde for 2 h at 100–
110 ꢁC [21]. [MoBr3(thf)3] [20a], [WCl4(dppe)] [22], and
bis(diphenylphosphinomethyl)-methylamine (PNP) [12]
were prepared using the literature procedures. IR spectra
were obtained from KBr pellets using a Bruker IFS v66/S
FT-IR-Spectrometer. NMR spectra were recorded on a
Bruker Avance 400 pulse Fourier transform spectrometer
operating at
a
1H frequency of 400.13 MHz (31P
161.98 MHz, 13C 106.62, 15N 40.54 MHz) using a 5 mm
inverse triple-resonance probe head. References as substi-
tutive standards: H3PO4 85% pure, d(31P) 0 ppm; CH3NO2
neat, d(15N) 0 ppm. Elemental analyses were performed
with a Euro Vector Euro EA 3000.
4.1. X-ray structure analysis
The structure data for compound 2 were obtained with
Mo Ka radiation using a CAD4 4-circle diffractometer.
For structure analysis of 4, 6, 7, 8 an 10 an Imaging Plate
diffraction system (IPDS) was employed. The structure
solution was performed with direct methods using SHEL-
XS-97 [23] and structure refinement was done with full
matrix least-squares against F2 with SHELXL-97. All non-
hydrogen atoms, except those which are disordered were
refined anisotropic and all hydrogen atoms were positioned
with idealized geometry and were refined isotropic using a
riding model. The N–H H atoms which cannot be posi-
tioned were located in difference map, their bond lengths

G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
1017
were set to ideal values and afterwards they were refined
isotropic using a riding model. In the crystal of 6 two
THF molecules are found, which are completely disor-
dered. Because no reasonable structure model could be
found the data were corrected for disordered solvent using
the Squeeze option in Platon. In 7 one carbon atom of one
solvent molecules is disordered and was refined using a
split model. In the crystal of 8 THF molecules are found,
which are disordered and which were refined using a split
model. Therefore, some of the carbon atoms were refined
only isotropically. The crystals of 10 contain 1.5 molecules
benzene per formula unit, with one of them located on a
center of inversion.
20 ꢁC): 158.7 (py2,6), 139.6 (py4), 138.0 (Ph1), 134.0
(Ph2,6), 129.7 (Ph4), 129.6 (Ph3,5), 96.8 (py3,5), 42.5
(NCH2P); 31P{H} NMR (d, CD2Cl2, 20 ꢁC): ꢀ17.3
4.4. N-diphenylphosphinomethyl-2-aminothiazole (4,
thiazNP)
A mixture of 2.16 g (10 mmol) hydroxymethyldiphenyl-
phosphine and 1.00 g (10 mmol) 2-aminothiazole in 25 mL
toluene was refluxed for 2 h and stirred at 70 ꢁC over night.
The solvent was removed under vacuum and the oily crude
product was taken up in 10 mL warm ethanol. The solution
was cooled over night at ꢀ40 ꢁC and the white product was
filtered from the cold solution, washed with hexane and
dried under vacuum. Yield 1.1 g (37%). Anal. Calc. for
C16H15N2PS: C, 64.41; H, 5.03; N, 9.39; S, 10.73. Found:
C, 64.27; H, 4.93; N, 9.73; S, 10.41%. 1H, NMR (d,
CD2Cl2, 20 ꢁC): 7.6–7.3 (m, 10H, Ph), 7.04 (d,
J = 2.6 Hz, 1H, Thiaz4), 6.50 (d, J = 2.6 Hz, 1H, Thiaz5),
5.78 (broad s, 1H, NH), 4.02 (d,2JPH = 4.5 Hz, 2H,
NCH2P); 13C{H} NMR (d, CD2Cl2, 20 ꢁC): 170.2 (Thiaz2),
139.2 (Thiaz4), 136.0 (Ph1), 132.8 (Ph2,6), 129.2 (Ph4), 128.7
(Ph3,5), 106.9 (Thiaz5), 45.4 (NCH2P); 31P{H} NMR (d,
CD2Cl2, 20 ꢁC): ꢀ17.8.
4.2. N,N-bis(diphenylphosphinomethyl)-2-aminopyridine (2,
pyNP2)
450 mg (4.8 mmol) 2-aminopyridine and 500 mg triethy-
laminehydrochloride in 15 mL ethanol were added to
2.16 g (10 mmol) of hydroxymethyldiphenylphosphine in
35 ml toluene at 70 ꢁC. The mixture was refluxed for 6 h
and stirred at 70 ꢁC over night. The solvent was removed
under vacuum. The oily residue was taken up in 1.5 mL
dichloromethane and 8 mL ethanol were added. The mix-
ture was cooled over night. The white product was filtered
at room temperature, washed with 2 · 4 mL methanol and
dried in vacuo. Yield: 1.74 g (74%) Anal. Calc. for
C31H28N2P2: C, 75.92; H, 5.76; N, 5.71. Found: C, 75.63;
H, 6.00; N, 5.71%. 1H NMR (d, CD2Cl2, 20 ꢁC): 8.12
(ddd, J = 4.9 Hz, J = 2.0 Hz, J = 0.8 Hz, 1H, py6), 7.57–
7.26 (m, 21H, Ph/py4), 6.58 (dt, J = 8.7 Hz, J = 0.8 Hz,
1H, py5), 6.55 (ddd, J = 7.1 Hz, J = 4.9 Hz, J = 0.8 Hz,
4.5. N,N,N0,N0-tetrakis(diphenylphosphinomethyl)-2,6-
diaminopyridine (5, pyN2P4)
560 mg (5 mmol) 2,6-diaminopyridine and 1 g triethy-
laminehydrochloride in 40 mL ethanol were added to
6.48 g (30 mmol) hydroxymethyldiphenylphosphine in
80 mL toluene. The mixture was heated for 50 h. The sol-
2
1H, py3), 4.23 (d, JPH = 3.3 Hz, 4H, NCH2P); 13C{H}
vent was refluxed over activated 4 A mole sieve at 120 ꢁC
˚
NMR (d, CD2Cl2, 20 ꢁC): 158.3 (py2), 148.6 (py6), 138.7
(Ph1), 137.8 (py4), 134.3 (Ph2,6), 129.9 (Ph3), 129.5 (Ph3,5),
113.2 (py5), 108.7 (py3), 51.9 (NCH2P); 31P{H} NMR (d,
CD2Cl2, 20 ꢁC): ꢀ22.4.
for about one day and afterwards stirred at 70 ꢁC over
night. The solvent was removed under vacuum. The oily
residue was washed with 14 mL degassed water and 5 ml
methanol. The resulting crude product was refluxed in
10 mL methanol for 45 min. The mixture was slowly cooled
to room temperature and stirred over night. The white
product was filtered, washed with 3 · 5 mL methanol and
dried under vacuum. Yield: 2.91 g (65%). Anal. Calc. for
C57H51N3P4: C, 75.89; H, 5.71; N, 4.66. Found: C, 75.40;
4.3. N,N0-bis(diphenylphosphinomethyl)-2,6-
diaminopyridine (3, PpyP)
560 mg (5 mmol) of 2,6-diaminopyridine in 12 mL etha-
nol were added to 2.16 g (10 mmol) hydroxymethyldiphe-
nylphosphine in 28 mL toluene. The mixture was refluxed
at 95 ꢁC for 5 h and stirred over night at room temperature.
The solvent was removed under vacuum, the oily residue
was taken up in 2 mL dichloromethane and under stirring
10 mL ethanol were added. The resulting pasty crude prod-
uct was separated from the solvent and stirred in a small
volume of ethanol until a white product formed. The prod-
uct was filtered and dried under vacuum. Yield: 1.2 g
(47%). Anal. Calc. for C31H29N3P2: C, 73.64; H, 5.79; N,
1
H, 5.82; N, 4.40%. H NMR (d, CD2Cl2, 20 ꢁC): 7.35–
7.20 (m, 40H, Ph), 7.19 (t, J = 8.1 Hz, 1H, py4), 5.90 (d,
2
J = 8.1 Hz, 2H, py3,5), 4.14 (d, JPH = 3.6 Hz, 8H,
NCH2P); 13C{H} NMR (d, CD2Cl2, 20 ꢁC): 155.9 (py2,6),
138.2 (py4), 137.8 (Ph1), 132.3 (Ph2,6), 128.7 (Ph4), 128.4
(Ph3,5), 95.5 (py3,5), 50.2 (NCH2P); 31P{H} NMR (d,
CD2Cl2, 20 ꢁC): ꢀ23.4.
4.6. Mo(0) carbonyl complexes: [Mo(CO)3
(NCCH3)(pyNP2)] (6), [Mo(CO)4(PpyP)] (7) and
[Mo(CO)4(thiazNP)] (8)
1
8.31. Found: C, 74.20; H, 5.61; N 8.10%. H NMR (d,
CD2Cl2, 20 ꢁC): 7.50–7.30 (m, 20H, Ph), 7.17 (t,
J = 8.1 Hz, 1H, py4), 5.75 (d, J = 8.1 Hz, 2H, py3,5), 4.31
(broad t, 3J = 6.0 Hz, NH), 4.14 (dd, 2JP = 4.3 Hz,
3JNH = 6.0 Hz, 4H, NCH2P); 13C{H} NMR (d, CD2Cl2,
210 mg (0.8 mmol) Mo(CO)6 was refluxed in 10 mL ace-
tonitrile until the colour changed to pale yellow. 0.8 mmol
of ligand were added and the mixture was refluxed for

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G.C. Stephan et al. / Inorganica Chimica Acta 361 (2008) 1008–1019
another 2 h. The solvent was removed under vacuum and
the residue was dissolved in a small volume of warm
THF. The solution was cooled to room temperature and
afterwards cooled in the refrigerator. The resulting crystals
are only stable in solution and decompose due to loss of
cocrystallised THF.
dried under vacuum. The product was dissolved in ben-
zene, the solution was filtered and layered with n-hexane.
Cooling over night afforded crystalline [Mo(N2)2(dppe)-
(PyNP )] 1.5C H . Yield: 238 mg (41%). Anal. Calc. for
*
2
6
6
C66H61N6P4Mo: C, 68.38; H, 5.32; N, 7.26. Found: C,
1
67.98; H, 5.02; N, 6.85%. H NMR (d, C6D6, 20 ꢁC): 8.04
(6): 1H NMR (d, CD2Cl2, 20 ꢁC): 8.17 (ddd, J = 4.9 Hz,
J = 2.0 Hz, J = 0.9 Hz, 1H, py6), 7.57–7.10 (m, 21H, Ph/
py4), 6.61 (dd, J = 7.0 Hz, J = 5.0 Hz, 1H, py5), 6.10 (d,
(dd, J = 4.9 Hz, J = 1.6 Hz, 1H, py6), 7.57–7.26 (m, 50H,
Ph/py4/benzene), 6.47 (dt, J = 8.7 Hz, J = 0.8 Hz, 1H,
py5), 6.36 (ddd, J = 7.1 Hz, J = 4.9 Hz, J = 0.8 Hz, 1H,
py3), 4.62 (s, 4H, NCH2P), 2.13 (m, 4H, CH2-dppe);
31P{H} NMR (d, C6D6, 20 ꢁC): 61.4 (2 P, dppe/P3P4),
2
J = 8.6 Hz, py3), 4.23 (broad d, JPH = 15.3 Hz, 4H,
NCH2P), 1.96 (s, 3H, CH3CN); 31P{H} NMR (d, CD2Cl2,
20 ꢁC): 22.4; IR (KBr)/cmꢀ1: CN 2265, CO 2018, 1938,
1841, 1810.
32.3
(2
P,
PyNP2/P1P2),
2JP1P2 = ꢀ19.1 Hz,
2
2/
2JP1P3 = 2JP2P4 = ꢀ22.0 Hz, JP2P3 = 2JP1P4 = 119.0 Hz,
1
(7): H NMR (d, CD2Cl2, 20 ꢁC): 7.55–7. 39 (m, 20 H,
3JP3P4 = 4.4 Hz.
Ph), 7.14 (t, J = 7.9 Hz, 1H, py4), 5.69 (d, J = 7.9 Hz,
2
2H, py3,5), 4.34 (d, JPH = 6.3 Hz, 4H, NCH2P), 4.23
4.9. [MoBr3 (pyNP2 )(thf)] (11)
(broad t, 6.2 Hz, 2H, NH), 13C{H} NMR (d, CD2Cl2,
20 ꢁC): 215.0 (COtrans P), 207.9 (COtrans CO) 155.7 (py2,6),
139.2 (Ph1), 138.7 (py4), 134.0 (Ph2,6), 129,3 (Ph4), 128.2
(Ph3,5), 97.3 (py3,5), 47.6 (NCH2P); 31P{H} NMR (d,
CD2Cl2, 20 ꢁC): 19.3; IR (KBr)/cmꢀ1: NH 3433, CO
2021, 1926, 1829, 1879.
988 mg [MoBr3(thf)3] were suspended in a mixture of
4 mL THF and 4 mL dichloromethane. Under stirring
900 mg of pyNP2 (2) in 3 mL THF and 1 mL dichlorometh-
ane were added. The mixture was warmed to 40–50 ꢁC for
10 min and stirred over night at room temperature. The
resulting bright orange solid was filtered, washed with
2 · 5 mL THF and dried under vacuum. Yield: 1.04 g
(63%). Anal. Calc. for C31H28N2P2MoBr3: C, 46.8; H,
3.9; N, 3.1. Found: C, 46.7; H, 4.0; N, 3.0%. IR (KBr)/
cmꢀ1: 3048, 2981, 1596, 1567, 1472, 1223, 746, 696.
1
(8): H NMR (d, CD2Cl2, 20 ꢁC): 7.7–7.4 (m, 10H, Ph),
7.35 (d, J = 4.0 Hz, 1H, Thiaz4), 6.80 (d, J = 4.0 Hz, 1H,
2
3
Thiaz5), 4.02 (dd, JPH = 4.5 Hz, JNH = 1.5 Hz, 2H,
NCH2P); 13C{H} NMR (d, CD2Cl2, 20 ꢁC): 220.6
(COtransN), 215.0 (COtransP), 208.1 (COtransCO), 171.4 (Thi-
az2), 145.9 (Thiaz4), 134.6 (Ph1), 132.1 (Ph2,6), 130.2 (Ph4),
128.7 (Ph3,5), 108.0 (Thiaz5), 44.8 (NCH2P); 31P{H} NMR
(d, CD2Cl2, 20 ꢁC): 30.0. IR (KBr)/cmꢀ1: NH 3246, CO
2019, 1911, 1860, 1833.
Acknowledgements
F.T. thanks DFG Tu58/13-1 (DFG SPP1118 ‘‘Sekun-
da¨re Wechselwirkungen’’) and Christian Albrechts Univer-
sita¨t Kiel for funding of this research. GS thanks J.B.
Scho¨nborn and T. Michalak for synthetic support.
4.7. [W(N2)2 (dppe)(PNP)] (9)
109 mg (0.26 mmol) bis(diphenylphosphinomethyl)-
methylamine and 190 mg (0.26 mmol) [WCl4(dppe)] were
reduced with 200 mg Mg-turnings in 20 mL THF. After
stirring for 18 h, the brown solution was filtered (D4-frit),
concentrated and layered with Et2O. Cooling over night
gives 179 mg (64%) light orange product. Anal. Calc. for
C53H51N5P4W: C, 59.73; H, 4.82; N, 6.51. Found C,
Appendix A. Supplementary material
CCDC 657017, 657018, 657019, 657020, 657021 and
657022 contain the supplementary crystallographic data
for 2, 4, 6, 7, 8 and 10. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplemen-
tary data associated with this article can be found, in the
online version, at doi:10.1016/j.ica.2007.06. 046.
1
59.23; H, 4.21; N, 6.51%. H NMR (d, THF-d8, 20 ꢁC):
6.8–7.2 (m, 40H, phenyl), 3.6 (d, 4H, PCH2N), 1.8 (m,
4H, dppe), 0.11 (s, 3H, NCH3); 31P{H} NMR(d, THF-
d8, 20 ꢁC): 4.3 (m, 2P, PNP), 41.4 (m, 2P, dppe) ppm.
15N NMR: ꢀ50.0 (Nb), ꢀ61.6 (Na).
4.8. [Mo(N2)2 (dppe)(pyNP2)] (10)
References
A suspension of 450 mg (0.5 mmol) [MoBr3(pyNP2)-
(thf)] and 200 mg (0.5 mmol) dppe in 10 mL THF was
added to sodium amalgam (170 mg Na in 30 g Hg). The
mixture was stirred under nitrogen for 3 h at 0 ꢁC and at
room temperature over night. The red-brown solution
was decanted from the amalgam and filtered over celite.
The solution was reduced under vacuum and 8 mL metha-
nol were added. The orange precipitate was filtered and
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