Exchange of isonitrile ligands in the complex [Mo(O)Cl(CNMe) 4]+ by the tetraphos ligand prP4: Stereochemical influences on the reaction course

Article abstract of DOI:10.1016/j.ica.2011.02.044

Reaction of [Mo(O)Cl(CNMe)₄]⁺ with the linear tetraphos ligand meso and rac prP₄ leads to a mixture of [Mo(O)Cl(κ⁴-meso-prP₄)]⁺ and [Mo(O)Cl(CNMe)(κ³-rac-prP₄)]⁺ which are identified by X-ray structural analysis and/or ³¹P NMR spectroscopy. In the meso κ⁴-product both of the phenyl groups of the central phosphorus atoms are oriented towards the oxo ligand whereas in the rac κ³-product one of these phenyl groups is oriented to the oxo and the other to the chloro ligand. The origin of the different coordination modes lies in the different steric demands of the oxo and chloro ligands. The influences of the steric interactions are enhanced by the fact that exchange of the fourth isonitrile is difficult. This hypothesis is supported by the preparation of the complex [Mo(O)Cl(CNMe)(dpepp)]PF₆ whose isonitrile ligand is inert towards exchange by monophosphines, even under drastic conditions.

Full text of DOI:10.1016/j.ica.2011.02.044

Inorganica Chimica Acta 374 (2011) 472–479
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Exchange of isonitrile ligands in the complex [Mo(O)Cl(CNMe)4]+ by the
tetraphos ligand prP4: Stereochemical influences on the reaction course
⇑
Ludger Söncksen, René Römer, Christian Näther, Gerhard Peters, Felix Tuczek
Institut für Anorganische Chemie, Christian Albrechts Universität Kiel, Max-Eyth-Straße 2, D-24098 Kiel, Germany
a r t i c l e i n f o
a b s t r a c t
Reaction of [Mo(O)Cl(CNMe)4]+ with the linear tetraphos ligand meso and rac prP4 leads to a mixture of
Article history:
Available online 1 March 2011
[Mo(O)Cl(
analysis and/or 31P NMR spectroscopy. In the meso
phosphorus atoms are oriented towards the oxo ligand whereas in the rac
j j
4-meso-prP4)]+ and [Mo(O)Cl(CNMe)( 3-rac-prP4)]+ which are identified by X-ray structural
j
4-product both of the phenyl groups of the central
3-product one of these phenyl
Dedicated to Prof. W. Kaim on the occasion
of his 60th anniversary
j
groups is oriented to the oxo and the other to the chloro ligand. The origin of the different coordination
modes lies in the different steric demands of the oxo and chloro ligands. The influences of the steric inter-
actions are enhanced by the fact that exchange of the fourth isonitrile is difficult. This hypothesis is sup-
ported by the preparation of the complex [Mo(O)Cl(CNMe)(dpepp)]PF6 whose isonitrile ligand is inert
towards exchange by monophosphines, even under drastic conditions.
Keywords:
Tetraphos ligands
Molybdenum oxo complexes
Isonitrile ligands
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
establishing a link between M(IV)-oxo and M(0) dinitrogen
chemistries (M = Mo, W) [15]. We applied this methodology to
the synthesis of the complex [Mo(N2)2(prP4)] where prP4 is a tetra-
dentate phosphine ligand containing a central C3 and two C2
bridges [16]. The coordination of the linear tetraphos prP4-ligand
to mononuclear metal centers has been a goal of our group for a
number of years [16–18]. In a first study, the prP4 ligand was coor-
dinated to cis- and trans-Fe(II)(NCS)2 units and the stereochemistry
of the resulting complexes was elucidated [17]. Preparation of the
mentioned molybdenum bis(dinitrogen) complex [Mo(N2)2(prP4)]
was achieved by electrochemical reduction of the oxo–iodo com-
plex [Mo(O)I(prP4)], which in turn was synthesized starting from
[Mo(O)I2(PMe3)2] [19]. The latter complex was obtained from
[Mo(O)Cl2(PMe3)2] by halide exchange [16]. As this synthetic route
involved several steps, we were seeking simpler ways to obtain a
tetraphosphine-coordinated molybdenum oxo–halo complex.
An alternative potential precursor for the coordination of the
tetraphos ligand prP4 to mononuclear Mo(IV) centers is the tetra-
kis(isonitrile) molybdenum complex [Mo(O)Cl(CNMe)4]+ (1) whose
isonitrile groups are subject to facile exchange by other ligands. In
the first description of this complex by Novotny and Lippard, e.g., a
high reactivity towards amines and thiols was reported [20,21].
Later on the complex was used by Donahue et al. for the syntheses
of bis(dithiolate) complexes [22,23]. Herein exchange reactions of
the complex [Mo(O)Cl(CNMe)4]+ with polydentate phosphines;
i.e., tri- and tetraphosphines, are investigated. Besides characteriz-
ing the resulting products, the respective reaction courses are
analyzed with the help of 31P NMR spectroscopy. Particular empha-
sis is laid on elucidating the presence of mechanisms leading to
Multiply bonded transition-metal complexes with d2 configura-
tion and phosphine coligands are of significant interest due to their
relevance to synthesis, catalysis and biochemistry [1,2]. Against
this background many complexes of the general formulae trans-
ME(PR3)4X or trans-ME2(PR3)4 (M = transition metal, X = halide or
pseudohalide, PR3 = monodentate or ½ bidentate phosphine) have
been prepared and investigated with respect to their electronic
structures, spectroscopic properties and chemical reactivities [3–
5]. A particularly attractive feature of the tetraphosphine platform
present in these systems is the fact that it allows to accommodate
and investigate a broad range of multiply bonded, axial ligands E
such as oxide [2], nitride [2,6–8], imide [6–8], etc., under compara-
ble geometric and electronic-structural conditions.
Due to the much better knowledge of the metal–O unit as com-
pared to other metal–E moieties transition-metal oxo complexes
doubtless constitute the prototypical examples for this class of
compounds. With respect to Mo(IV) and W(IV) oxo complexes,
several cationic complexes of the type trans-[M(O)(X)(dppe)2]+
(M = Mo and W) have been reported [9,10]: trans-[Mo(O)(Cl)-
(dppe)2]+ [11], trans-[W(O)Cl(dppe)2]+ [12], trans-[Mo(O)(F)-
(dppe)2]+ [13], and trans-[Mo(O)(OCH3)(dppe)2]+ [14]. Remarkably,
[W(O)Cl(dppe)2]+ and its molybdenum analog could be reduced by
Nax/Hg in the presence of phenol and N2 to the dinitrogen com-
plexes [W(N2)2(dppe)2] and [Mo(N2)2(dppe)2], respectively, thus
⇑
Corresponding author. Tel.: +49 (0)431 880 1410; fax: +49 (0)431 880 1520.
E-mail address: ftuczek@ac.uni-kiel.de (F. Tuczek).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.044

L. Söncksen et al. / Inorganica Chimica Acta 374 (2011) 472–479
473
Scheme 1.
Fig. 1. 31P{1H} NMR spectrum of the prP4-ligand.
stereoselectivity. The general implications of the results with
respect to the preparation of Mo complexes with tetraphosphine
ligands are discussed.
2. Results and analysis
2.1. Synthesis of the precursors [21]
The preparation of [Mo(O)Cl(CNMe)4](PF6) (1a) and [Mo(O)Cl-
(CNMe)4](BPh4) (1b) was performed by dissolving MoCl5 in
Fig. 2. Single crystal structure [Mo(O)Cl(j
4-meso-prP4)]BPh4 (2b).
Scheme 2.

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L. Söncksen et al. / Inorganica Chimica Acta 374 (2011) 472–479
31
Fig. 3. P{1H} NMR AA0XX0-coupling scheme of [Mo(O)Cl(
j
4-meso-prP4)]+ (2) measured (bottom) and simulated (top).
methanol and adding methyl isonitrile. The chloride cation was ex-
changed against PF6 and BPh4 by addition of a methanolic solu-
tion of KPF6 and NaBPh4, respectively.
at ꢁꢀ21 ppm should belong to the asymmetric phosphorus atoms
and the signals at ꢀ12.1 ppm to the terminal ones. This assignment
is supported by the fact that the signals of the latter atoms have
identical shifts in both isomers and therefore are superposed
whereas the doublets of the asymmetric P atoms exhibit slightly
different chemical shifts (ꢀ21.25 and ꢀ21.28 ppm), resulting in a
four-line group. The pair of signals at lower field (higher intensity)
belongs to the meso isomer of prP4.
ꢀ
ꢀ
2.2. Synthesis and characterization of the tetradentate prP4 ligand
prP4 (1,1,4,8,11,11-hexaphenyl-1,4,8,11-tetraphosphaundecane)
was synthesized by reaction of two equivalents of (2-chloroethyl)-
diphenylphosphane with one equivalent of 1,3-bis(lithiumphenyl-
phosphido)propane (Scheme 1) [16]. Due to the existence of two
asymmetric phosphorus atoms in the ligand two diastomeric forms
of the product are possible. In meso-prP4 both phenyl rings of the
central phosphorus atoms are oriented to the same side whereas
in rac-prP4 one phenyl ring is oriented to the opposite side. As
evident from 31P NMR spectroscopy the preparation of the ligand
generally leads to a mixture of meso and rac with varying propor-
tions of both diastereomers [17].
Table 2
Comparison of 31P{1H} NMR coupling constants.
[Mo(O)I(prP4)]+ [16]
[Mo(O)Cl(j
4-meso-prP4)]+ (2b)
2/4
J
(Hz)
ꢀ28.2
ꢀ18.4
110.8
4.9
ꢀ27.4
ꢀ18.5
123.8
4.9
ab
2Jcd (Hz)
2Jac/bd (Hz)
2/3
|
J
| (Hz)
ad/bc
[Mo(O)Cl(CNMe) (
j
3-rac-prP4)]+ (3)
The 31P NMR spectrum of the free prP4 ligand shows three dou-
blets corresponding to the two isomers (Fig. 1). Each isomer gives a
four-line spectrum consisting of two doublets. Due to the higher
shielding of the alkyl as compared to the phenyl groups, the signals
2/4
J
(Hz)
ꢀ30.5
–
128.1
8.3
ab
2Jcd
2Jac (Hz)
2/3
|
J | (Hz)
bc
3Jad (Hz)
ꢀ28.2
Table 1
Selected bond distances (Å) and angles (°) of [Mo(O)Cl(
j
4-meso- prP4)]BPh4 (2b).
Bond distance (Å)
Mo–O(1)
Mo–P(1)
Mo–P(2)
1.6771(14)
2.5432(5)
2.4934(5)
Mo–Cl(1)
Mo–P(3)
Mo–P(4)
2.5162(5)
2.4898(6))
2.5201(5)
Bond angle (°)
P(1)–Mo–P(2)
P(1)–Mo–P(4)
O(1)–Mo–P(1)
O(1)–Mo–P(2)
Cl(1)–Mo–P(1)
Cl(1)–Mo–P(2)
O(1)–Mo–Cl(1)
80.391(18)
102.683(17)
96.73(5)
102.67(5)
81.564(19)
77.099(18)
178.29(5)
P(2)–Mo–P(3)
P(3)–Mo–P(4)
O(1)–Mo–P(3)
O(1)–Mo–P(4)
Cl(1)–Mo–P(3)
Cl(1)–Mo–P(4)
90.446(19)
78.871(18)
101.47(5)
101.60(5)
80.229(19)
78.822(18)
Scheme 3.

L. Söncksen et al. / Inorganica Chimica Acta 374 (2011) 472–479
475
Scheme 4.
2.3. Synthesis and characterization of [Mo(O)Cl(
j
4-prP4)]BPh4 (2b)
exhibits a similar structure [16]. The main differences to the struc-
ture of 2b are the longer Mo–I bond length of 2.95 Å as compared
to the Mo–Cl bond of 2.52 Å and an increased angle between the
terminal P-atoms (108° versus 102°).
For the synthesis of the complex [Mo(O)Cl(
j
4-prP4)]BPh4 (2b)
on a preparative scale equimolar amounts of the tetrakis(isonitrile)
complex [Mo(O)Cl(CNMe)4]BPh4 (1b) and the ligand prP4 were sus-
pended in acetone and reacted for 24 h at room temperature and
about 14 h at 60 °C (cf. Section 4 and Scheme 2). The reaction leads
The 31P NMR spectrum of 2b is shown in Fig. 3, bottom. In con-
trast to the free prP4 ligand which shows no coupling between the
central phosphorus atoms, the spectrum now exhibits an AA0XX0
pattern (A = central, X = terminal P). The signal consists of two sets
containing 10 lines each. The set at 52 ppm can be assigned to the
terminal P atoms c, d and the set at 49 ppm to the internal P atoms
a, b. The trans 2JAX coupling constant between the central P and the
terminal P atoms is 123.8 Hz (cf. Table 2). The 2/4JA coupling con-
stant among the inner P atoms is ꢀ27.5 Hz and the 2JX coupling
constant between the terminal atoms is ꢀ18.5 Hz. Atoms b and c
or a and d couple via the ethylene bridge and the metal which leads
to a small resulting coupling constant of |2/3J0| = 4.9 Hz (Scheme 4,
left). The AA0XX0 pattern of 2b is related to the 31P NMR spectrum
of [Mo(O)I(prP4)]BPh4 [16]. The coupling constants in this complex
to two products: the four times coordinated [Mo(O)Cl(
(2) and the three times coordinated complex [Mo(O)Cl(CN-
Me)(
3-prP4)]+ (3), which is considered in Section 2.5. After reduc-
j
4-prP4)]+
j
tion of the volume of the solution and cooling to ꢀ40 °C 2b could
be isolated as blue-violet solid. Violet crystals, suitable for an
X-ray crystallographic characterization, could be obtained by slow
evaporation of a CH2Cl2 solution.
The structure of the cation of 2b shown in Fig. 2 reveals that the
complex contains the meso and not the rac prP4 ligand. The Mo–P
distances are within the usual range (2.49–2.54 Å); the Mo@O and
Mo–Cl bond lengths are 1.68 and 2.52 Å, respectively (cf. Table 1).
The P–Mo–P angle between the terminal P-atoms is increased from
90° to 102° whereas the angle between the central P-atoms is
about 90°. The angles between the central and the terminal
P-atoms are ꢁ80°, as known from Mo(0) complexes with dppe or
depe ligands [24]. All O–Mo–P angles are larger than 90°, rendering
the axial Mo@O/equatorial P coordination slightly pyramidal. With
178° the O–Mo–Cl angle is almost linear. The [Mo(O)I(prP4)]+
cation which was crystallographically characterized as well
were determined to 2/4JA = ꢀ27.4 Hz, |2/3J0| = 4.9 Hz, trans 2JAX
=
110.8 Hz and 2JX = ꢀ18.4 Hz. Comparison with 2b only reveals a
major difference (13 Hz) for the trans 2JAX coupling constant. More-
over, due to the presence of different halide ligands in the two
complexes different chemical shifts of the signals are observed.
The signals of the chloro complex are centered at 50.6 ppm; for
the iodo-complex they are shifted by 7.1–43.5 ppm. A similar
chemical shift difference of 7.5 ppm was found by Bendix and
Bgevig by comparing the complexes [Mo(O)I(dppe)2]BF4 and
[Mo(O)Cl(dppe)2]BF4 [9].
Complex 2b was also characterized by vibrational spectroscopy.
The infrared and Raman spectra are given in Supplementary
Fig. S1. The Mo@O stretch is found at 955 cmꢀ1, the typical signals
for aromatic phosphine ligands are located at 1443 and 1483 cmꢀ1
.
2.4. Synthesis and characterization of [Mo(O)Cl(CNMe)(dpepp)]PF6 (4)
For the preparation of the complex [Mo(O)Cl(CNMe)-
(dpepp)]PF6 (4) equimolar amounts of the complex [Mo(O)Cl(CN-
Me)4]PF6 (1a) and the ligand bis(2-diphenylphosphinoethyl)
phenylphosphane (dpepp) were suspended in acetone and reacted
for 12 h at room temperature and 8 h at 60 °C (cf. Section 4 and
Scheme 3). After reduction of the volume of the solution, extraction
with THF and cooling of the THF solution to ꢀ40 °C a pale violet
solid was isolated. Violet crystals, suitable for an X-ray crystallo-
graphic characterization, could be obtained by slow vapor diffusion
of diethyl ether into a CH2Cl2 solution.
The structure of the complex cation of 4 is shown in Fig. 4. The
dpepp ligand is bound in a meridional fashion in the equatorial
plane, which in addition contains an isonitrile as a fourth ligand.
The phenyl group of the central P-donor is oriented towards the
Fig. 4. Single crystal structure [Mo(O)Cl(CNMe)(dpepp)]PF6 (4).

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L. Söncksen et al. / Inorganica Chimica Acta 374 (2011) 472–479
oxo ligand. The Mo–P distances are within the usual range (2.47–
2.50 Å); the Mo@O and Mo–Cl bond lengths are 1.68 and 2.59 Å,
respectively (cf. Table 3). Both P–Mo–P angles are around 80°,
equal to those of the ethylene bridged P-atoms of the prP4 com-
plexes (vide supra). The Mo–C bond length of the isonitrile ligand
is 2.150(3) Å, and its C–N bond length is 1.143(4) Å. This corre-
sponds to a small shortening of the C–N bond as compared to
the cation [Mo(O)Cl(CNMe)4]+ (C–N bond length 1.158(11) Å) [20].
Complex 4 was also characterized by vibrational spectroscopy.
The infrared and Raman spectra are given in Supplementary
Fig. S2. The spectrum shows the signal for the M@O vibration at
953 cmꢀ1 and the isonitrile vibration at 2218 cmꢀ1. In comparison
to the precursor 1 the isonitrile vibration is shifted about 17 cmꢀ1
to smaller wavenumbers.
equimolar amounts in dry acetone. After a short period of time
the suspension cleared up to a violet solution. This happened much
faster with the PF6 salt 1a than with the BPh4 salt 1b, probably due
to the lower solubility of 1b. The 31P NMR spectrum of the reaction
mixture (Fig. 5) showed several signals which could be assigned to
two species (Scheme 2). As evident from a comparison with the
spectrum of 2b (vide supra) the AA0XX0 pattern at 52.8–48.5 ppm
belongs to the j4 coordinated [Mo(O)Cl(meso-prP4)]+ complex 2.
The other main signal set in the 31P NMR spectrum consists of
the signals at 56.8 ppm (c), 46.2 ppm (b), 21.6 ppm (a) and
ꢀ12.3 ppm (d). This signal set can be attributed to a
j
3 coordinated
complex [Mo(O)Cl(CNMe)(
j
3-prP4)]+ (3). The detailed assignment
of the signals to the individual P atoms is shown in Scheme 4
(right). Atom a (ddd-signal) couples with atoms b, c and d; atoms
b (dd-signal) and c (dd-signal) couple with each other and with
atom a, and atom d (d-signal) only couples with atom a. Analysis
of the signals leads to the following parameters: The trans 2J cou-
2.5. Exchange reaction of [Mo(O)Cl(CNMe)4]+ with prP4: 31P NMR
monitoring
pling constant among atom
a and the terminal atom c is
128.1 Hz. The coupling constant |2/3J0| among atoms b and c is
8.3 Hz; this coupling constant includes the cis coupling over the
metal and the 3J coupling over the ethylene bridge. The coupling
constant 2/4JA among the two central P atoms is ꢀ30.5 Hz. The 3J
coupling among atom a and the uncoordinated atom d is 28.2 Hz,
just as in the free ligand. The chemical shift of signal d is
ꢀ12.3 ppm which is equal to the shift in the free prP4 ligand as
well.
For an NMR-spectroscopic investigation of the exchange
reaction of 1 with prP4 the two components were suspended in
Table 3
Selected bond distances (Å) and angles (°) of [Mo(O)Cl(CNMe)(dpepp)]PF6 (4).
Bond distance (Å)
Mo–O(1)
Mo–P(1)
Mo–P(2)
C(61)–N(1)
1.679(2)
2.5048(8)
2.4745(8)
1.143(4)
Mo–Cl(1)
Mo–P(3)
Mo–C(61)
2.5852(7)
2.4980(8)
2.150(3)
Integrating the signals of the two components in the 31P NMR
spectrum shows that the j3 and j4 products are formed in about
equal amounts. Importantly, the intensity ratio was found to be
independent of the reaction conditions or the reaction time. It thus
became clear that two reaction channels must exist, one leading to
Bond angle (°)
P(1)–Mo–P(2)
P(1)–Mo–C(61)
O(1)–Mo–P(1)
O(1)–Mo–P(2)
Cl(1)–Mo–P(1)
Cl(1)–Mo–P(2)
O(1)–Mo–Cl(1)
79.52(3)
95.59(8)
105.17(8)
102.25(8)
76.43(2)
78.21(3)
178.38(8)
P(2)–Mo–P(3)
P(3)–Mo–C(61)
O(1)–Mo–P(3)
O(1)–Mo–C(61)
Cl(1)–Mo–P(3)
Cl(1)–Mo–C(61)
80.31(3)
94.02(8)
101.38(8)
100.81(11)
77.14(3)
78.71(8)
the
j
3- and the other to the
j
4-product. With respect to a structural
assignment of the
j
3 complex, two possibilities exist; i.e., this com-
plex can derive from the meso or the rac ligand. We believe that the
second possibility applies, for the following reasons: (i) meso and
rac prP4 always form in comparable amounts. If the j4 as well as
Fig. 5. 31P{1H} NMR spectrum of reaction mixture between 1 and prP4.

L. Söncksen et al. / Inorganica Chimica Acta 374 (2011) 472–479
477
the
j
3 complex derive from the meso ligand, the question arises as
phosphine is oriented towards the oxo ligand, the meso prP4 ligand
forms the
4 complex without problems (Scheme 5, left).
to where the rac ligand has gone. Moreover (ii), with increasing
reaction time the j3 meso complex should convert to the j4 com-
plex, which is not observed (vide supra). (iii) There is the possibility
that the j3 species derives from a meso complex where one of the
central phenyl groups is oriented to the chlorine ligand. This is ste-
j
In order to check the above-mentioned sterical selection rule
(which provides a consistent explanation of all experimental find-
ings), the complex [Mo(O)Cl(CNMe)(dpepp)]PF6 (4) was prepared
by reaction of the tridentate phosphane dpepp (bis(2-diphenyl-
phosphinoethyl)phenylphosphane) with the precursor 1a (Scheme
3). In fact, the phenyl substituent of the central phosphine group
was found to be oriented towards the oxo and not towards the
chlorine ligand (vide supra). The C–N stretch of this complex was
observed at 2218 cmꢀ1 (Supplementary Fig. S2). In order to explore
its reactivity compound 4 was treated with an excess of the mono-
phosphine ligand dimethylphenylphosphine in CH2Cl2 under re-
flux. Moreover, this mixture was irradiated with UV light. In
spite of these fairly drastic reaction conditions, only a very limited
exchange of the isonitrile ligand was observed; i.e., the product still
exhibited the CN-vibration of compound 4 at 2218 cmꢀ1 (Supple-
mentary Fig. S2). These findings demonstrate the general difficulty
to exchange the fourth isonitrile ligand. Only if the steric
conditions are favorable, i.e., only if the fourth P atom of the tetra-
dentate prP4 ligand is suitably arranged to coordinate in the posi-
tion of the fourth isonitrile ligand, a successful exchange reaction
(promoted by the chelate effect) occurs. If this is not the case, only
three isonitrile ligands are exchanged, leading to the j3
complex.
rically unfavorable and would imply that for a j
4 coordination the
second phenyl group is also oriented to the chlorine ligand (which
is even more unfavorable). So this meso complex would ‘‘get stuck’’
in the j3 configuration. However, there is strong evidence that for
steric reasons in a stable j3 Mo(IV) oxo–chloro tri- or tetraphos
intermediate not even one phenyl group of a coordinated phosphine
bridging two other coordinated phosphines can be oriented to-
wards the chlorine ligand (see below). (iv) We thus conclude that
the j
3 complex does not derive from the meso, but from the rac li-
gand. Again, it is noteworthy that only one isomer is observed. From
the rule established in (iii) that the complex does not tolerate a con-
figuration in which one of the phenyl groups of the bridging, coor-
dinated phosphines is oriented towards the chlorine ligand, we
conclude that only the j3 complex with the rac ligand is formed
where the central phenyl group is oriented towards the oxo ligand
(Scheme 5, right). Starting from this complex the j4 complex can-
not form as in this case one of the phenyl groups of the bridging
phosphines would have to be oriented towards the chlorine ligand.
If, in contrast, the phenyl group of the first bridging, coordinated
Scheme 5.

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L. Söncksen et al. / Inorganica Chimica Acta 374 (2011) 472–479
and the content of solvent was considered in the calculation of
the molecular weight.
3. Summary and conclusions
Reaction of the tetraisonitrile complex [Mo(O)Cl(CNMe)4]+ (1)
with the tetraphos ligand prP4 leads to a 1:1 mixture of two
complexes, [Mo(O)Cl(
3-rac-prP4)]+ (3). The
of the central phosphorus atoms oriented towards the oxo ligand
whereas in the
3 product 3 one of these phenyl groups is oriented
j
4-meso-prP4)]+ (2) and [Mo(O)Cl(CNMe)
4.2. [Mo(O)Cl(j
4-prP4)]BPh4 (2b)
(j
j
4-product 2 has the two phenyl groups
0.40 g (0.70 mmol) [Mo(O)Cl(CNMe)4]BPh4 and 0.48 g
(0.70 mmol) prP4 were suspended in 70 ml of acetone. After 3 h
the cloudy suspension turned into a clear violet solution. The solu-
tion was refluxed for 14 h and stirred at room temperature for fur-
ther 24 h. Afterwards the solution was reduced in vacuum to 5 ml
and was allowed to stand at ꢀ40° C. After 4 days a solid blue-violet
precipitate was collected, washed with toluene and dried in
vacuum.
Violet crystals, suitable for X-ray diffraction studies, could be
obtained by slow evaporation of a CH2Cl2 solution.
Yield: 100 mg (12%). Elemental Anal. Calc. for C67H64BClMoOP4:
C, 69.90; H, 5.60. Found: C, 69.38; H, 5.55%.
j
towards the oxo and the other towards the chlorine ligand. The ori-
gin of the different coordination modes lies in the different steric
demands of the oxo and chloro ligands. The influences of the steric
interactions are enhanced by the fact that exchange of the fourth
isonitrile ligand is difficult. These hypotheses are supported by
the preparation of the complex [Mo(O)Cl(CNMe)(dpepp)]PF6 (4),
where the phenyl group of the central P-atom is oriented towards
the oxo ligand. Moreover, the isonitrile ligand of 4 is inert towards
exchange by monophosphines, even under drastic conditions.
With the described isonitrile exchange reactions a general
method for the coordination of tetraphos ligands to mononuclear
molybdenum centers has been found. This complements our ear-
lier studies on the coordination of the tetraphos ligand prP4 to
the molybdenum oxo complex [Mo(O)I2(PMe3)2]+, leading to the
complex [Mo(O)I(prP4)]+ [16]. It has been shown that the steric de-
mand of the axial halide ligand plays an important role in the reac-
tion course, in particular with respect to a successful completion of
the equatorial j4 coordination and a stereoselective formation of
ꢀ1
~
IR (KBr):
m
= 1483 (P-CH2), 1435 (P-Ph), 955 (Mo@O) cm
.
J
31P{1H} NMR (161.975 MHz, CD2Cl2): d = 52.25
(
=
ab
2/4
ꢀ27.4 Hz, Jcd = ꢀ18.5 Hz, Jac/bd = 123.8 Hz, |2/3Jad/bc| = 4.9 Hz, 2P,
2
2
2
2
P
|
c,d), 49.03 (2/4Jab = ꢀ27.4 Hz, Jcd = ꢀ18.5 Hz, Jac/bd = 123.8 Hz,
2/3
J
| = 4.9 Hz, 2P, Pa,b) ppm.
ad/bc
the j
4-meso complex.
4.3. Reaction mixture of [Mo(O)Cl(
prP4)]PF6
j -
3-prP4)]PF6 and [Mo(O)Cl(j4
4. Experimental
An equimolar mixture of the precursor [Mo(O)Cl(CNMe)4]
BPh4/PF6 and the prP4-ligand was refluxed in acetone for different
periods of time from 6 h to several days. The solvent was removed
in vacuum and the residue was characterized by NMR and vibra-
tional spectroscopy.
4.1. Material and methods
All synthetic reactions were performed under an inert gas
atmosphere using Schlenk techniques. The solvents were dried
and distilled under argon atmosphere. The precursor
[Mo(O)Cl(CNMe)4]+ (1) and the required methyl isonitrile [25]
were prepared using the literature procedure [21]. The ligand
prP4 was prepared the reaction of (C6H5)2P–(CH2)2Cl [26] with
1,3-bis(lithiumphenylphosphido)propane [17].
Table 4
Crystallographic data of compounds 2b and 4.
Compound
[Mo(O)Cl(j4
prP4)]BPh4 (2b)
-
[Mo(O)Cl(CNMe)(dpepp)]PF6
(4)
IR spectra were obtained from KBr pellets using a Bruker IFS
v66/S FT-IR-Spectrometer. Raman spectra were recorded on a Bru-
ker IFS 66 RA 106 (laser wavelength 1064 nm). Elemental analyses
were performed using a Euro Vector CHNS-O-element analyzer
(Euro EA 3000). Samples were burned in sealed tin containers by
a stream of oxygen. NMR spectra were recorded on a Bruker
Avance 400 pulse Fourier transform spectrometer operating at a
1H frequency of 400.13 MHz (31P: 161.975 MHz) using a 5 mm
inverse triple-resonance probe head with z-gradient. Reference as
substitutive standard: H3PO4 85% pure, d(31P) = 0 ppm.
Empirical formula
Formula weight
T (K)
Crystal system
Space group
A (Å)
C
68.50H62BCl4Mo(O)P4 C36H36ClF6MoNOP4
1273.61
293(2)
triclinic
867.93
170(2)
monoclinic
P21/c
ꢀ
P1
13.576(1)
15.084(1)
18.299(1)
88.16(1)
70.49(1)
65.65(1)
3193.7(2)
2
18.647(2)
12.374(1)
18.446(2)
90
92.52(1)
90
4252.1(5)
4
1.356
B (Å)
C (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (mg/m3)
1.324
0.515
l
(mmꢀ1
)
0.575
4.1.1. X-ray single crystal structure analysis
h (°)
Index ranges
1.49–28.00
h: ꢀ17/17
k: ꢀ19/19
l: ꢀ24/24
49 752
2.29–27.94
h: ꢀ24/24
k: ꢀ16/16
l: ꢀ24/24
47 835
0.0689
10 158
Intensity data were collected using an STOE imaging plate dif-
fraction system (IPDS-1) with Mo Ka radiation. Details of the struc-
ture analysis are collected in Table 4. The structure was solved
with direct methods using SHELXS-97 and refinement was performed
against F2 using SHELXL-97 [27]. All non-hydrogens were refined
anisotropic. The C–H H atoms were positioned with idealized
geometry (methyl H atoms allowed to rotate but not to tip) and
were refined using a riding model. For 2b a numerical absorption
correction was performed (Tmin/max: 0.7653/0.9589). The asymmet-
ric unit of 2b and 4 contain additional dichloromethane solvent
molecules which are completely disordered and for which no rea-
sonable split model can be found. Therefore, the data were cor-
rected for disordered solvent using the Squeeze option in Platon
Reflections collected
Rint
Independent
reflections
Reflections with
I > 3r(I)
Parameters
Goodness-of-fit
R1 [I > 2
0.0302
15 247
12 622
7894
676
1.016
0.0363
0.0990
0.326/ꢀ0.456
453
1.021
0.0479
0.1204
0.879/ꢀ1.033
r(I)]
wR2 (all data)
dF (e/Å3)

L. Söncksen et al. / Inorganica Chimica Acta 374 (2011) 472–479
479
4.4. [Mo(O)Cl(CNMe)(dpepp)]PF6 (4)
Appendix A. Supplementary material
A mixture of 300 mg (660 mmol) [Mo(O)Cl(CNMe)4]PF6 and
350 mg (660 mmol) of bis(2-diphenylphosphinoethyl)phenylphos-
phane was suspended in 40 ml acetone. After 10 min the cloudy
suspension turned into a clear violet solution. The solution was re-
fluxed for 8 h in acetone and was stirred for further 12 h at room
temperature. Afterwards the solution was reduced in vacuum
and the residue was extracted with 10 ml THF. A pale violet prod-
uct could be obtained by cooling to ꢀ40 °C for 12 h. The product
was dissolved in CH2Cl2, filtered over celite and again precipitated
by adding THF. A small amount of the solid was solved in CH2Cl2
and was crystallized by slow vapor diffusion of diethyl ether. The
obtained violet crystals were suitable for X-ray diffraction
studies.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.02.044.
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~
IR (KBr):
cmꢀ1
m = 2218 (CN), 1485 (P-CH2), 1436 (P-Ph), 956 (Mo@O)
.
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inner P-Ph), 46.73 (d, J = 2.6 Hz, 2P, outer P-Ph2) ppm.
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ꢀ20 °C a violet resin could be isolated and was dried.
~
IR (KBr):
cmꢀ1
m = 2218 (CN), 1485 (P-CH2), 1436 (P-Ph), 956 (Mo@O)
.
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F.T. acknowledges funding in the initial phase of this project by
DFG. Financial support by the State of Schleswig-Holstein is grate-
fully acknowledged.