Half-sandwich complexes of molybdenum-(ni), -(iv) and -(v) with P-O and P-N bifunctional ligands Ph2PCH2X (X = 2-oxazolinyl, or C(O)NPh2)

Article abstract of DOI:10.1039/b002403l

The reaction of the ligands Ph₂PCH₂X (X = 2-oxazolinyl, I; or C(O)NPh₂, II) with the half-sandwich molybdenum(III) precursors [Mo(η-C₅R₅)(μ-Cl)₂]₂ (R = H or Me) has been investigated. Ligand I reacts with both complexes to form the corresponding adducts [Mo(η-C₅R₅)Cl₂(Ph₂PCH ₂C₃H₄NO)] (R = H, 1; or Me, 2). The reaction between I and [MoCp*Cl₄] (Cp* = η-C₅Me₅) affords [MoCp*Cl₄(Ph₂PCH₂C₃H ₄NO-K¹P)] as a kinetic isomer, which then transforms quantitatively to [MoCp*Cl₃(Ph₂PCH₂C₃H ₄NO-K²P,N)]⁺Cl^-, 3. Ligand II reacts with [MoCp(μ-Cl)₂]₂ (Cp = η-C₅H₅) to afford the adduct [CpMoCl₂{Ph₂PCH₂C(O)NPh_2-K ²P,O}], 4, as an equilibrium mixture of two isomers. Longer reaction times in CH₂Cl₂ lead to the molybdenum(iv) by-product [MoCpCl₃{Ph₂PCH₂C(O)NPh₂-K ²P,O}], 5. While ligand II does not react with [MoCp*(μ-Cl)₂]₂, it does so in the presence of 1 equivalent of AgBF₄ to afford [MoCp*Cl₂{Ph₂PCH₂C(O)NPh ₂-K²P,O}][BF₄], 6. Further reaction with CH₂Cl₂ leads to [MoCp*Cl₃{Ph₂PCH₂C(O)NPh ₂-K²P,O}] [BF4], 7. The direct reaction between [MoCp*Cl₄] and ligand II affords [MoCp*Cl₄{Ph₂PCH₂C(O)NPh ₂-K¹P}] 8 as a kinetic isomer which then slowly establishes a solvent-dependent equilibrium with the ionic isomer [MoCp*Cl₃{Ph₂PCH²C(O)NPh ²-K²P,0}r Cl . The solid state structure of compounds 5 and 7 has been determined by single crystal X-ray diffraction. The chemical, spectroscopic, and electrochemical data indicate that ligand II has a greater hemilabile character than ligand I. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b002403l

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Half-sandwich complexes of molybdenum-(III), -(IV) and -(V) with
P–O and P–N bifunctional ligands Ph2PCH2X (X ؍ 2-oxazolinyl,
or C(O)NPh2)
Jean-Michel Camus,a Dolores Morales,a Jacques Andrieu,a Philippe Richard,a Rinaldo Poli,*a
Pierre Braunsteinb and Frédéric Naudb
a Laboratoire de Synthèse et d’Electrosynthèse Organométalliques (UMR 5632 CNRS),
Faculté des Sciences “Gabriel”, Université de Bourgogne, 6 Boulevard Gabriel, F-21000 Dijon,
France
b Laboratoire de Chimie de Coordination (UMR 7513 CNRS), Université Louis Pasteur,
4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France
Received 27th March 2000, Accepted 9th June 2000
Published on the Web 11th July 2000
The reaction of the ligands Ph2PCH2X (X = 2-oxazolinyl, I; or C(O)NPh2, II) with the half-sandwich molybdenum()
precursors [Mo(η-C5R5)(µ-Cl)2]2 (R = H or Me) has been investigated. Ligand I reacts with both complexes to form
the corresponding adducts [Mo(η-C5R5)Cl2(Ph2PCH2C3H4NO)] (R = H, 1; or Me, 2). The reaction between I and
[MoCp*Cl4] (Cp* = η-C5Me5) affords [MoCp*Cl4(Ph2PCH2C3H4NO-κ1P)] as a kinetic isomer, which then transforms
quantitatively to [MoCp*Cl3(Ph2PCH2C3H4NO-κ2P,N)]ϩClϪ, 3. Ligand II reacts with [MoCp(µ-Cl)2]2 (Cp = η-C5H5)
to afford the adduct [CpMoCl2{Ph2PCH2C(O)NPh2-κ2P,O}], 4, as an equilibrium mixture of two isomers. Longer
reaction times in CH2Cl2 lead to the molybdenum() by-product [MoCpCl3{Ph2PCH2C(O)NPh2-κ2P,O}], 5.
While ligand II does not react with [MoCp*(µ-Cl)2]2, it does so in the presence of 1 equivalent of AgBF4 to afford
[MoCp*Cl2{Ph2PCH2C(O)NPh2-κ2P,O}][BF4], 6. Further reaction with CH2Cl2 leads to [MoCp*Cl3{Ph2PCH2C(O)-
NPh2-κ2P,O}] [BF4], 7. The direct reaction between [MoCp*Cl4] and ligand II affords [MoCp*Cl4{Ph2PCH2C(O)-
NPh2-κ1P}] 8 as a kinetic isomer which then slowly establishes a solvent-dependent equilibrium with the ionic isomer
[MoCp*Cl3{Ph2PCH2C(O)NPh2-κ2P,O}]ϩ ClϪ. The solid state structure of compounds 5 and 7 has been determined
by single crystal X-ray diffraction. The chemical, spectroscopic, and electrochemical data indicate that ligand II has a
greater hemilabile character than ligand I.
Introduction
Molybdenum() half-sandwich complexes are known as
mononuclear [MoCpX2L2], [MoCpXL3]ϩ and [MoCpL4]2ϩ and
as dinuclear [Mo2Cp2X4(µ-X)]Ϫ, [MoCp(µ-X)2]2, [Mo2Cp2L2-
2ϩ
(µ-X)3]ϩ and [{MoCpL2(µ-X)}2]
and have shown compati-
Experimental
bility with a large array of ligands. Examples with X = halide,1–5
hydrosulfido, alkane- and aryenethiolato,6–9 amido,10 dialkyl-
phosphido,11 phenyldiazo,12 isocyanato,13 azido,14 alkyl,15 and
hydroborate units,16 L = phosphine,17–21 CO22–26 isocyanides27,28
and MeCN,27,29,30 L2 = diphosphines,31–33 phosphine sulfides34
and dienes,35–38 and XL = phosphine–thiolato34 and allyl37–40
have been reported. Our own work in this area has been sum-
marized in review articles.41–44 Interest in these compounds
ranges from modelling the activity of nitrogenase enzymes
and hydrodesulfurization catalysts to the quest for new poly-
merization catalysts. The possibility to link bifunctional
ligands having a soft and a hard donor atom may open new
opportunities in terms of chemical reactivity and catalysis by
virtue of the hemilability concept and in terms of establishing
new building blocks for the assembly of heterobimetallic
architectures.45,46 We have extended the chemistry of this class of
compounds to the bifunctional ligands 2-(diphenylphosphino-
General
All reactions involving air- and moisture-sensitive organo-
metallic compounds were carried out in a Jacomex glove-box or
by the use of standard Schlenk techniques under an argon
atmosphere. The solvents were dried by conventional methods
(THF, toluene and pentane from sodium–benzophenone
and CH2Cl2 from P4O10) and distilled under argon prior to use.
1H NMR measurements were carried out on a Bruker AC200
spectrometer. The peak positions are reported with positive
shifts in ppm downfield of TMS as calculated from the residual
solvent peaks. The IR spectra were recorded on a Bruker IFS
66V spectrophotometer with NaCl optics. EPR measurements
were carried out at the X-band microwave frequency on a
Bruker ESP300 spectrometer. The spectrometer frequency was
calibrated with diphenylpicrylhydrazyl (DPPH, g = 2.0037).
Cyclic voltamograms were recorded with an EG&G 362 poten-
tiostat connected to a Macintosh computer through MacLab
hardware/software. The electrochemical cell was fitted with
a Ag–AgCl reference electrode, a platinum disk working
electrode and a platinum wire counterelectrode. [Bu4N]PF6
(ca. 0.1 M) was used as supporting electrolyte. All potentials are
reported relative to the ferrocene standard, which was added to
methyl)oxazoline
I and diphenylphosphino-N,N-diphenyl-
acetamide II, and report here our results, which also involve the
generation of a few related species in the oxidation states  and
. The complexes obtained are the first examples of this class
where co-ordination by O- and N-based (except for the above
mentioned MeCN complexes) L ligands is demonstrated.
DOI: 10.1039/b002403l
J. Chem. Soc., Dalton Trans., 2000, 2577–2585
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each solution and measured at the end of the experiments. The
magnetic susceptibility measurements were carried out at room
temperature with a Johnson Matthey magnetic balance which
operates with a modified Gouy method, whereby the effect
of the sample on the weight of the magnet, rather than the
effect of the magnet on the weight of the sample, is used to
retrieve the susceptibility information. The elemental analyses
were carried out by the analytical service of the Laboratoire
de Synthèse et d’Electrosynthèse Organométallique with a
Fisons EA 1108 apparatus. Compounds [MoCp*Cl4],47
[MoCp*(µ-Cl)2]2,48 [MoCp(µ-Cl)2]2,49,50 and the ligands 2-(di-
phenylphosphinomethyl)oxazoline45 and diphenylphosphino-
N,N-diphenylacetamide51 were prepared as described in the
literature.
(d, with molybdenum satellites, aMo = 45.7, aP = 25.5 G);
(THF or MeCN) g = 1.962 (broad d, aP = 27.4 G). Cyclic
voltammetry (CH2Cl2): reversible reduction at E1/2 = Ϫ1.096 V
(vs. ferrocene). Compound 3 is insoluble in pentane and in
Et2O.
[MoCpCl2{Ph2PCH2C(O)NPh2-ꢀ2P,O}], 4. To a solution of
II (0.418 g, 1.06 mmol) in 5 mL of CH2Cl2 was added a sus-
pension of [MoCp(µ-Cl)2]2 (0.463 g, 1.00 mmol). After stirring
for 1 h the resulting red brown solution was filtered through
Celite. The filtrate was evaporated to dryness and the residue
extracted with toluene (20 mL) and filtered. Addition of 2 ×
10 mL of pentane gave complex 4 as a brick-red powder, which
was filtered off, washed with pentane (2 × 10 mL) and dried
in vacuo. Yield: 0.225 g, 32%. Calc. for C31H27Cl2MoNOP:
C, 59.35; H, 4.34; N, 2.23. Found: C, 58.89; H, 4.34; N, 2.27%.
IR (CH2Cl2): ν(CO) 1554 cmϪ1. EPR (CH2Cl2): overlap of
two doublets with molybdenum satellites; 1st species (55.1%),
g = 1.977, aP = 11.7, aMo = 38.7 G; 2nd species (44.9%), g =
1.971, aP = 12.2 G, aMo = 37.8 G. Cyclic voltammetry (CH2Cl2):
reversible oxidation at E1/2 = 0.43 V. The voltammogram also
showed a smaller reversible oxidation wave with E1/2 = 0.19 V,
due to the by-product 5 (see below).
An analogous preparative procedure was carried out from II
(0.590 g, 1.49 mmol) and [MoCp(µ-Cl)2]2 (0.346 g, 0.75 mmol)
in 25 mL of CH2Cl2, a difference being that the mixture was
stirred overnight. After filtration through Celite, the filtrate
was evaporated to dryness and the residue extracted with hot
toluene (30 mL) and filtered. The precipitate which formed
upon cooling was separated by decanting off the mother-liquor,
dried and redissolved in CH2Cl2 (10 mL). Diffusion of a
pentane layer (20 mL) afforded crystals of complex 5ؒCH2Cl2
(ca. 0.030 g) over 1 week, one of which was used for the X-ray
analysis. Calc. for C32H29Cl5MoNOP: C, 51.40; H, 3.91;
N, 1.87. Found: C, 51.17; H, 3.76; N, 2.09%. 1H NMR (CDCl3,
297 K): δ 7.6–7.2 (m, 20 H, Ph), 5.35 (br s, 5 H, w1/2 = 50, Cp),
5.28 (s, 2 H, CH2Cl2) and 4.21 (br s, 2 H, w1/2 = 25 Hz, CH2).
Upon warming, the Cp peak broadened and disappeared at
309 K, while the CH2 peak shifted and broadened (w1/2 = 40 Hz)
to δ 4.06. Upon cooling to 270 K, sharp Cp (s) and CH2
(d, JPH = 9 Hz) resonances were observed at δ 5.31 and 4.25,
respectively. 31P-{1H} NMR (CDCl3, 297 K): δ 57.1 (w1/2 = 70
Hz). Cyclic voltammetry (CH2Cl2): reversible oxidation at
E1/2 = 0.43 V.
Preparations
[MoCpCl2(Ph2PCH2C3H4NO)], 1. A solution of ligand I
(0.160 g, 0.59 mmol) in 5 mL of CH2Cl2 was added to a sus-
pension of [MoCp(µ-Cl)2]2 (0.137 g, 0.29 mmol) in 10 mL of
CH2Cl2 at room temperature. The insoluble starting material
disappeared after stirring for 30 min and the resulting brown
solution was filtered through Celite. Addition of 20 mL of
pentane gave complex 1 as a microcrystalline grey solid, which
was filtered off, washed with pentane (3 × 5 mL) and dried
in vacuo. Yield: 0.112 g, 38%. Calc. for C21H21Cl2MoNOP:
C, 50.32; H, 4.22; N, 2.79. Found: C, 50.44; H, 4.28; N,
2.63%. IR (Nujol mull): 1617 cmϪ1. EPR (CH2Cl2): g = 1.972
(d with molybdenum satellites, aMo = 40.2, aP = 21.3 G). Cyclic
voltammetry (CH2Cl2): reversible oxidation at E1/2 = 0.17 V.
Compound 1 is insoluble in all common hydrocarbon solvents
and only slightly soluble in THF. After isolation as a solid
from CH2Cl2 solution, it also exhibited sparing solubility in this
solvent. It is quite air sensitive, becoming immediately brown
upon exposure to air.
Carrying out the reaction between [MoCp(µ-Cl)2]2 (0.370 g,
0.80 mmol) and ligand I (0.370 g, 1.59 mmol) in THF as solvent
(20 mL), led to the precipitation of compound 1 from the
reaction mixture. Yield: 0.492 g, 61%. Dichloromethane
solutions of this product exhibited an EPR spectrum identical
to that of the product obtained directly from CH2Cl2.
[MoCp*Cl2(Ph2PCH2C3H4NO)], 2. To a solution of [MoCp*-
(µ-Cl)2]2 (0.319 g, 0.52 mmol) in 10 mL of CH2Cl2 was added
a solution of I (0.284 g, 1.059 mmol) in 5 mL of CH2Cl2 at
room temperature, followed by stirring for 3 h. The resulting
green-brown solution was evaporated to dryness, the solid
extracted with warm toluene, and the solution was filtered
through Celite and cooled to room temperature to give complex
2 as a microcrystalline green solid. The product was isolated by
filtration, washed with pentane (3 × 5 mL) and dried in vacuo.
Yield: 0.209 g, 35%. Calc. for C26H31Cl2MoNOP: C, 54.66;
H, 5.47; N, 2.45. Found: C, 54.44; H, 5.33; N, 2.78%. IR (Nujol
mull): 1616 cmϪ1. EPR (CH2Cl2): g = 1.982 (d, with molyb-
denum satellites, aMo = 39.8, aP = 19.3 G). Cyclic voltammetry
(CH2Cl2): reversible oxidation at E1/2 = Ϫ0.74 V (vs ferrocene).
[MoCp*Cl2{Ph2PCH2C(O)NPh2-ꢀ2P,O}]BF4, 6. A solution
of II (0.327 g, 0.82 mmol) in 5 mL of CH2Cl2 was added to a
solution of [MoCp*(µ-Cl)2]2 (0.250 g, 0.41 mmol) in 15 mL of
CH2Cl2 at room temperature, followed by stirring for 3 h. NMR
and EPR monitoring showed the absence of any interaction
between the two reagents. The solution was then added to
AgBF4 (0.161 g, 0.82 mmol) resulting in an immediate color
change and the precipitation of a dark solid. After 30 min of
stirring at room temperature the resulting mixture was filtered
through Celite and the red-brown filtrate evaporated to dryness.
Extraction of the residue with warm toluene (20 mL), followed
by filtration and cooling to room temperature, gave complex
6 as red crystals (yield 0.17 g, 26%). Calc. for C36H37BCl2F4-
MoNOP: C, 55.13; H, 4.75. Found: C, 55.19; H, 4.42%. IR
(CH2Cl2): ν(CO) 1540; ν(BF4) 1061 cmϪ1. 1H NMR (CDCl3, 297
K): δ 16.0 (br s, w1/2 = 70, Ph), 12.3 (br s, w1/2 = 130, Ph), ca. 9.8
(sh, w1/2 = 100, Ph), 9.7 (br s, w1/2 = 35, Ph), 9.3 (br s, w1/2 = 30,
Ph), 8.3–8.1 (br m, total w1/2 = 75, Ph), 7.4 (s, overlaps with
solvent peak, Ph), 6.8 (br s, w1/2 = 30, Ph), 6.3 (br s, w1/2 = 110,
Ph), 3.6 (br s, 15 H, w1/2 = 290, Cp*), Ϫ9.0 (br s, 1 H, w1/2 = 300,
CH2) and Ϫ65.8 (br s, 1 H, w1/2 = 400 Hz, CH2).
[MoCp*Cl4(Ph2PCH2C3H4NO)], 3. A solution of I (0.160 g,
0.59 mmol) in 5 mL of CH2Cl2 was added to a suspension of
[MoCp*Cl4] (0.223 g, 0.59 mmol) in 20 mL of CH2Cl2 at 0 ЊC.
The solution was warmed to room temperature and an aliquot
immediately withdrawn for an EPR investigation: doublet
with molybdenum satellites, g = 1.986, aMo = 44.2, aP = 26.1 G.
The solution was stirred overnight and then filtered through
Celite. The filtrate was concentrated under reduced pressure to
ca. 1 mL. Addition of 20 mL of pentane gave complex 3 as
a red microcrystalline solid, which was filtered off, washed
with pentane (4 × 5 mL) and dried in vacuo. Yield: 0.220 g,
57%. Calc. for C26H31Cl4MoNOP: C, 48.62; H, 4.86; N, 2.18.
Found: C, 48.23; H, 4.80; N, 2.09%. EPR: (CH2Cl2) g = 1.978
Recrystallization of complex 6 (0.070 g) by diffusion of
a toluene layer (25 mL) into a CH2Cl2 solution (10 mL) over
1 week afforded crystals of [MoCp*Cl3{Ph2PCH2C(O)NPh2-
κ2P,O}]BF4, 7, which were used for the X-ray analysis. EPR
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(CH2Cl2): doublet with molybdenum satellites, g = 1.979,
aMo = 44.0, aP = 25.2 G.
voltammetry. All data are consistent with the mononuclear
formulation shown with a chelating (P,N) ligand.
The IR spectrum shows strong C᎐N stretching vibrations for
᎐
the oxazoline ring at 1617 and 1616 cmϪ1 for complexes 1 and 2,
respectively. These are 43–44 cmϪ1 red-shifted relative to the
“free” ligand (1660 cmϪ1). Other N-co-ordinated oxazoline
complexes also display red-shifted absorptions, for instance
from 1661 to 1648 and 1649 cmϪ1 upon co-ordination of
the bis(oxazolinyl)phosphine ligand PhP(CH2C3H4NO-2)2 to
ruthenium() centres.54 Oxygen co-ordination (to our know-
ledge yet unreported), on the other hand, would be expected to
[MoCp*Cl4{Ph2PCH2C(O)NPh2}], 8. A solution of II (0.045
g, 0.11 mmol) in 5 mL of THF was added at room temperature
to a suspension of MoCp*Cl4 (0.043 g, 0.11 mmol) in 5 mL
of THF. The solid immediately dissolved to yield an orange-
brown solution. An EPR spectrum was recorded immediately,
showing a main doublet with molybdenum satellites (g =
1.986, aMo = 43.0, aP = 26.7 G) and a smaller doublet at
g = 1.976, aP = 26.0 Hz. The solution was stirred overnight
and an EPR spectrum recorded again. The weaker doublet
at g = 1.976 had increased (see Results section). The EPR
properties of this solution did not further evolve upon more
prolonged stirring. An IR spectrum of the equilibrium THF
solution was also recorded (see Results section).
result in a smaller or even opposite shift of the C᎐N absorption.
᎐
The IR evidence, therefore, agrees with N-co-ordination for the
phosphinomethyloxazoline ligand.
The X-band EPR spectrum for each compound shows a dis-
tinctive central doublet resonance (g = 1.972 for 1, 1.982 for 2)
showing coupling to a single phosphorus atom (aP = 21.3 G for
1, 19.3 G for 2) flanked with satellites generated by the I = 5/2
molybdenum isotopes (25% abundance; aMo = 40.2 G for 1,
39.8 G for 2). The EPR spectra therefore establish phosphorus
co-ordination for the ligand. The similarity of the coupling
constants indicates that the two compounds adopt the same
stereochemistry. It was previously reported for analogous
Ph2PCH2CH2PPh2 (dppe) derivatives that a relative cis con-
figuration in complexes [MoCpX2(dppe)] (as verified by X-ray
crystallography for X = Br)32 gave aP hyperfine constants in the
18–26 G range (26 G for the dichloride), while the structurally
characterized trans dichloride derivatives [MoCpCl2(PR3)2]
(PR3 = PMe3, PMePh2, PPh3 or PEt3)19,20,55 have aP in the 9–16
G range20 and complex [Mo{η-C5H3-(SiMe2CH2PPri2-1,3)2}-
Cl2], also showing a trans relative arrangement in the solid state,
has aP = 15.2 G.21 For these reasons, we prefer to assign a
cis relative arrangement to both compounds 1 and 2, although
an unambiguous assignment can only be provided by a single
crystal X-ray study. Unfortunately, suitable crystals for such a
determination could not be obtained for either compound.
The cyclic voltammograms of compounds 1 and 2 show a
reversible oxidation wave at 0.17 and Ϫ0.74 V vs. ferrocene,
respectively. All compounds of the type [MoCpX2L2] show a
one-electron oxidation process of which the potential and
reversibility depend on the nature of X and L. For instance, a
reversible process is observed in the Ϫ0.4 to Ϫ0.6 V range for
[MoCpCl2(PR3)2]41 and at Ϫ0.33 V for [MoCpCl2(dppe)],32
while the same process is shifted to Ϫ1.1 V for [MoCpCl2-
(η4-C4H6)]35 and to 0.12 V for [MoCpCl2(Ph2PCH2CH2SMe)].34
Therefore, according to the cyclic voltammetric data, the
relative ability of the N-co-ordinated oxazoline moiety in the
chelate to stabilize the molybdenum() and -() species
involved in the oxidation process is similar to that of the S-co-
ordinated ϪCH2SMe moiety. The negative shift observed on
going from Cp to Cp* is consistent with the greater electron
donating power of the latter. The magnitude of this shift (0.91
V), however, is quite large when compared to other Cp and Cp*
bis(phosphine) complexes [0.32 V for [Mo(η-C5R5)Cl2(PMe3)2]
and 0.25 V for [Mo(η-C5R5)Cl2(dppe)] on going from R = H to
Me].26,56 A possible rationalization of this effect is to invoke a
greater ability of the π-acidic phosphorus donors to buffer the
electron density variations caused by the different cyclopenta-
dienyl substitution.
X-Ray analysis
Compound 5ؒCH2Cl2. A red crystal (0.20 × 0.16 × 0.12 mm)
suitable for an X-ray analysis was obtained by layering n-
pentane onto a saturated methylene chloride solution at room
temperature. The crystal was mounted on an Enraf-Nonius
KappaCCD diffractometer equipped with Mo-Kα radiation.
Absorption corrections were applied to the data during inte-
gration by the SCALEPACK52 algorithm. The structure was
solved via a Patterson search program53 and refined (space
53
¯
group P1) with full-matrix least-squares methods based on
|F2|. All non hydrogen atoms were refined with anisotropic
thermal parameters. The hydrogen atoms of the complex were
included in their calculated positions and refined with a riding
model. Crystal data are collected in Table 1 and selected bond
distances and angles are listed in Table 2.
Compound 7. A small brown crystal (0.22 × 0.10 × 0.05 mm)
suitable for an X-ray analysis was obtained by layering toluene
onto a saturated methylene chloride solution at room tem-
perature. The crystal was mounted on an Enraf-Nonius
KappaCCD using MoKα radiation. Although the final Fourier
difference map exhibited strong electron density close to the
molybdenum atom (1.98 and Ϫ1.02 e ÅϪ3), any tentative to
apply absorption corrections failed to fix the problem, thus no
correction was applied. The structure was solved and refined
(space group P21/n) as above.
CCDC reference number 186/2025.
See http://www.rsc.org/suppdata/dt/b0/b002403l/ for crystal-
lographic files in .cif format.
Results and discussion
(a) Phosphinomethyloxazoline complexes
Compounds 1 and 2 are obtained by addition of ligand I
to the dinuclear complexes [Mo(η-C5R5)(µ-Cl)2]2 (R = H or Me)
as shown in eqn. (1). The products have been characterized by
C,H,N analysis, IR and EPR spectroscopic methods and cyclic
(1)
In an attempt to find a more convenient synthetic method for
compound 2, ligand I was added to [MoCp*Cl4], yielding the
corresponding adduct 3, eqn. (2). Monitoring this reaction by
EPR spectroscopy (Fig. 1) revealed the immediate formation of
an intermediate, followed by the slow rearrangement (12 h)
to the isolated product. Both compounds show similar hyper-
fine splittings for Mo and P. It seems reasonable to propose that
the compound which forms immediately corresponds to an
adduct with the P-co-ordinated monodentate ligand, A, with a
structure related to those determined for [M(η-C5R5)Cl4(PR3)]
(M = Mo or W), namely with the phosphine ligand located
J. Chem. Soc., Dalton Trans., 2000, 2577–2585
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Fig. 2 Room temperature X-band EPR spectrum of compound 4 in
CH2Cl2 solution.
by-product. The latter corresponds to the neutral trichloride
derivative [MoCpCl3{Ph2PCH2C(O)NPh2}], 5 (see below). The
formation of this by-product is more pronounced when the
reaction time in CH2Cl2 is longer, in agreement with a slow
Cl atom abstraction from the solvent by the first reaction
product 4. When the reaction was stopped soon after the
complete comsumption of the [MoCp(µ-Cl)2]2 starting material
(1 h) elemental analysis and cyclic voltammetry (see below) of
the isolated product indicated only a minor contamination by
5. Compound 4 was also obtained by carrying out the reaction
in THF, as confirmed by EPR spectroscopy and cyclic
voltammetry, but the analytical purity of this sample was not
superior to that of the reaction product from CH2Cl2.
An additional complication is that the EPR spectrum of
the molybdenum() product 4 appears as a distorted triplet
(Fig. 2). However, this is interpreted as the overlap of two
doublets that are assigned to two different isomers of the
adduct [MoCpCl2{Ph2PCH2C(O)NPh2-κ2P,O}], 4 (see eqn. 3).
Both materials obtained from CH2Cl2 or THF show identical
EPR properties. Variable temperature EPR measurements
did not improve the resolution. However, a satisfactory
digital fitting of the spectrum confirms the co-ordination
of just one P donor atom for both isomers and provides the
spectral parameters as well as the relative proportion of the two
species.
Fig. 1 EPR monitoring of the room temperature reaction between
[MoCp*Cl44] and ligand I in CH2Cl2. (a) After 15 minutes; (b) after
1.5 h; (c) after 12 h.
(2)
(3)
trans relative to the Cp* ligand.57,58 The final product, on the
other hand, probably involves chelation of the ligand with dis-
placement of a chloride ligand from the co-ordination sphere,
as indicated in B. The structure of the cation would be identical
to that determined for the cation of compound 7 containing
ligand II, see below, whose EPR parameters are essentially
identical to those of B and significantly different from those
of the reaction intermediate A. Another related structure is that
of [MoCp*Cl3{S2P(OEt)2}],59 where the bidentate dithiophos-
phate ligand occupies the axial and one of the equatorial
positions. The reduction of compound 3 (thermodynamic iso-
mer B) with two equivalents of sodium failed to yield 2 and the
characterization of the reduction product was therefore not
pursued.
(b) Phosphine amide complexes
The addition reactions of the phosphine amide ligand II are
more complex than those of ligand I. With [MoCp(µ-Cl)2]2
the reaction leads to the formation of the desired ligand
adduct [MoCpCl2{Ph2PCH2C(O)NPh2-κ2P,O}], 4, which is
however accompanied by the formation of a molybdenum()
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The IR spectrum contains a single and strong ν(C᎐O) band at
᎐
1554 cmϪ1, showing that both isomers have a similar environ-
ment for the amide function. The “free” ligand51 exhibits this
absorption at 1658 cmϪ1, while previously described co-
ordination compounds with analogous phosphine amide
ligands display absorptions around 1650 cmϪ1 when N-co-
ordinated [e.g. for (Ph2P)2CHC(O)NPh2]60 and around 1560
cmϪ1 when O-co-ordinated.51 Thus, the IR results document
O-co-ordination for the ligand in both isomers of compound 4.
Given that both species appear to contain the phosphine amide
ligand in the same co-ordination mode (κ2P,O), we propose
that these are stereoisomers differing by the relative configur-
ation at the metal center. Since both cisoid and transoid
arrangements of 4-atom chelating ligands have been reported
for [Mo(η-C5R5)X2L2] complexes, for instance cisoid for
[MoCpBr2(dppe)]32 and transoid for the more sterically
encumbered [MoCp*Cl2(dppe)],61 it seems reasonable to pro-
pose that these two alternative arrangements, shown as C
and D in eqn. (3), are responsible for the isomerism of 4.
The bulkier nature of ligand II would presumably force the
structure to access the sterically less congested, but orbitally
disfavored, transoid structure.
Fig. 3 Room temperature 1H NMR spectrum of compound 6 in
CDCl3 solution.
{Ph2PCH2C(O)NPh2-κ2P,O}]BF4, 6, may be recovered from
the solution, eqn. (4).
(4)
The cyclic voltammogram of compound 4 shows only one
oxidation wave at E1/2 = 0.43 V. This observation indicates that
the rate of interconversion of the two isomers of 4 is fast on the
cyclic voltammetric timescale, while it is slow on the EPR time-
scale. The same phenomenon has been observed for other
MoCpIII derivatives.20 The E1/2 value is slightly higher than that
of compound 1, suggesting that the amide function in II is a
poorer donor than the oxazoline function in I.
As stated above, the molybdenum() by-product 5 was also
obtained, in addition to 4, when ligand II was treated
with [MoCp(µ-Cl)2]2 in CH2Cl2 and crystallized selectively after
long contact times in this solvent. Compound 5 has been
Complex 6 has been characterized by elemental analysis
and by IR and 1H NMR spectroscopy. The IR spectrum shows
1
evidence for O-co-ordination, with a ν(CO) band at 1540 cmϪ1
.
characterized by H NMR spectroscopy, cyclic voltammetry,
1
elemental analysis (C, H, N), and by X-ray crystallography (see
below). The cyclic voltammogram shows a reversible oxidation
wave at E1/2 = 0.19 V, i.e. 0.24 V less positive than the oxidation
of 4. This signifies that donation from one additional ClϪ
ligand more than compensates for the electron density loss
associated to the one-electron oxidation. This situation is
analogous to that of the corresponding [MoCpCln(dppe)] com-
plexes (E1/2 = Ϫ0.33 V for n = 2 and Ϫ0.39 for the ax-eq isomer,
The H NMR spectrum (see Fig. 3) indicates paramagnetism.
Related spin triplet 16-electron half-sandwich molybdenum()
complexes, e.g. [MoCpCl2(PMe3)2]ϩ and [MoCp*Cl3-
1
(PMe3)],19,62 exhibit similar shifts for the H NMR resonances.
On the basis of these precedents, we are able to assign the
upfield shifts at δ Ϫ9.0 and Ϫ65.8 to the two inequivalent
methylene protons and the strong resonance at δ 3.6 to the Cp*
protons, while the 4 inequivalent phenyl groups yield 12 signals
(11 of which are distinctly observed) that are all more or less
shifted downfield by the electron spin density.
Attempts to grow single crystals of compound 6 from
CH2Cl2–toluene afforded instead crystals of the related oxi-
dized trichloride species, [MoCp*Cl3{Ph2PCH2C(O)NPh2-κ2-
P,O}]BF4, 7, which is characterized by an EPR spectrum
essentially identical with that of the final product of reaction
(2), i.e. B. The cation of compound 7 has also been generated
by direct interaction between [MoCp*Cl4] and ligand II,
eqn. (5). EPR spectroscopy (Fig. 4) shows that the reaction
1
i.e. isostructural with 5, with n = 3).26 The H NMR spectrum
of 5 is consistent with a ground state diamagnetism, in agree-
ment with its 18-electron configuration, but shows abnormally
broad resonances for the Cp and methylene protons. The
31P NMR resonance is also abnormally broad. A variable tem-
perature study shows that these resonances sharpen and shift
slightly upon cooling, consistent with a thermal equilibrium
between the diamagnetic ground state and a higher energy
paramagnetic species. This behaviour is not consistent with a
fast equilibration between two diamagnetic species, because in
that case the resonances would sharpen upon warming in the
fast exchange limit (only one set of resonances), contrary to
the observation. Previous studies have shown that 16-electron
derivatives of formula [MoCpCl2L2]ϩ and [MoCp*Cl3L]
(L = tertiary phosphine) adopt a spin triplet ground state.
Therefore, we propose that the NMR properties result from
a thermal equilibrium between the diamagnetic 18-electron
structure observed in the solid state, E, and a paramagnetic
16-electron structure resulting from deco-ordination of the
amide function, F [eqn. (3)].
(5)
Complex [MoCp*(µ-Cl)2]2 does not show any reactivity with
ligand II. This contrasts with both the reactivity of the Cp
analogue with the same ligand and with that of the same
MoCp* complex with ligand I (see above). However, when one
Ϫ
equivalent of Agϩ per Mo atom is added as the BF4 salt to a
CH2Cl2 solution containing [MoCp*(µ-Cl)2]2 and ligand II
a
redox process takes place and complex [MoCp*Cl2-
J. Chem. Soc., Dalton Trans., 2000, 2577–2585
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Fig. 5 An ORTEP63 view of a molecule of complex 5. Ellipsoids are
shown at the 50% probability level.
Fig. 6 An ORTEP view of the cation of complex 7. Details as
in Fig. 5.
Fig. 4 Equilibrium spectral properties of compound 8 in THF solu-
tion at room temperature: (a) EPR spectrum; (b) IR spectrum in com-
parison with that of the “free” ligand.
pseudo-octahedral geometry when considering the cyclopenta-
dienyl ring as occupying a single co-ordination position. The
three chloride ligands occupy three equatorial positions in
both structures and the amide function is co-ordinated via the
oxygen atom in both cases, confirming the indications obtained
by IR spectroscopy. On the other hand, the orientation of
the phosphine amide ligand is inverted in the two structures.
For 5 the phosphorus atom occupies an equatorial position and
the oxygen is in the axial position, while the reverse holds true
for 7. The reason for this change may have a steric origin.
For the more sterically encumbered 7 the bulkier PPh2 end of
the chelating ligand is located farther from the bulky Cp*
ligand. The less sterically congested 5, however, may have an
electronic preference for the observed structure. An analysis
of the previously reported pseudo-octahedral [MoCpCl3L2]
(L2 = dppe or dmpe) complexes64,65 shows that the axially co-
ordinated P donor atom is less tightly bonded than the corre-
sponding equatorial P atom, a situation that may be associated
with the trans influence of the cyclopentadienyl ligand. Thus,
the more strongly co-ordinated phosphorus donor in ligand II
should have a greater tendency to occupy an equatorial
position, leaving the labile oxygen end of the ligand to provide a
supporting stabilization by co-ordinatively saturating the axial
position.
follows a similar course to the corresponding reaction of
[MoCp*Cl4] with ligand I, namely a molecular adduct G is
formed as immediate, followed by a structural rearrangement to
the ionic isomer H. The EPR parameters of the kinetic product
G are essentially identical with those of A, while those of H are
essentially identical with those of B and superimposable with
those of 7. However, while the transformation of A to B [eqn. (2)]
is quantitative, both the molecular and ionic isomers persist for
compound 8, the neutral isomer remaining the predominant
species at equilibrium. This is in agreement with a weaker
chelating power of ligand II relative to ligand I.
The presence of an isomeric equilibrium between mono-
dentate and chelating forms is further supported by solution
IR studies. A THF solution of complex 8 shows a major
absorption at 1670 cmϪ1 for the unco-ordinated amide function
of G (cf. 1658 cmϪ1 for the “free” ligand) and a smaller absorp-
tion at 1540 cmϪ1 for the O-co-ordinated amide function of
H (see Fig. 4).
(c) Crystal structures
The shortening of the Mo–Cl distances on going from
complex 5 to 7 follows the expected decrease of the metal
ionic radius upon increasing the oxidation state. The corre-
sponding lengthening of the Mo-P distance, on the other hand,
is related to the structural change which places the phosphorus
The geometries of compound 5 and of the cation of 7 are
shown in Figs. 5 and 6, respectively, and the relevant bond dis-
tances and angles are compared in Table 2. As is immediately
evident from the figures, both complexes adopt the same basic
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Table 1 Crystal data and structure refinement parameters for compounds 5ؒCH2Cl2 and 7
5ؒCH2Cl2
7
Formula
M
T/K
C31H27Cl3MoNOPؒCH2Cl2
747.72
110(2)
C36H37Cl3MoNOPؒBF4
819.74
110(2)
Crystal system
Space group
Triclinic
P1
Monoclinic
P21/n
¯
a/Å
b/Å
c/Å
α/Њ
11.1707(6)
11.8110(6)
12.2717(8)
88.024(3)
80.438(2)
79.718(3)
1570.9(2)
2
12.354(1)
14.895(1)
19.937(1)
β/Њ
106.961(2)
γ/Њ
V/Å3
3509.1(4)
4
0.70
Z
µ/mmϪ1
0.922
RC = Reflections collected
IRC = Independent RC
IRCGT = IRC and [I >2σ(I)]
R1, wR2 for IRCGT
for IRC
11333
6814 [R(int) = 0.031]
5916
0.030, 0.065
0.039, 0.069
24328
8008 [R(int) = 0.08]
5910
0.064, 0.13
0.096, 0.15
Table 2 Selected bond distances (Å) and angles (Њ) for compounds
5ؒCH2Cl2 and 7a
5ؒCH2Cl2
7
Mo–CNT
Mo–Cl(1)
Mo–Cl(2)
Mo–Cl(3)
Mo–P
Mo–O
C–O
1.977(3)
2.4923(6)
2.5309(6)
2.4583(6)
2.4702(6)
2.143(2)
1.255(3)
1.341(3)
2.082(5)
2.3794(12)
2.3888(13)
2.3955(14)
2.6039(13)
2.134(3)
1.263(5)
1.331(6)
C(O)–N
CNT–Mo–Cl(1)
CNT–Mo–Cl(2)
CNT–Mo–Cl(3)
CNT–Mo–P
CNT–Mo–O
Cl(1)–Mo–Cl(2)
Cl(1)–Mo–Cl(3)
Cl(1)–Mo–P
Cl(1)–Mo–O
Cl(2)–Mo–Cl(3)
Cl(2)–Mo–P
Cl(2)–Mo–O
Cl(3)–Mo–P
Cl(3)–Mo–O
P–Mo–O
106.2(1)
105.4(2)
105.4(2)
101.9(2)
176.6(4)
85.90(2)
84.32(2)
151.77(2)
77.08(4)
149.11(2)
84.79(2)
74.38(4)
90.17(2)
74.89(4)
74.76(4)
119.1(2)
119.8(2)
121.0(2)
104.2(3)
105.3(3)
104.1(3)
174.6(4)
103.0(4)
88.46(4)
91.59(4)
78.89(4)
152.58(9)
149.67(5)
79.12(4)
80.76(9)
71.15(4)
85.50(10)
74.37(9)
119.7(4)
120.4(4)
119.9(4)
Scheme 1
an oxygen lone pair is seen to reinforce the donor power of
the nitrogen function for I and the nitrogen lone pair reinforces
the donor power of the oxygen function for II, as a result
of a similar delocalization mechanism. Thus, in spite of
a greater “oxophilicity” of molybdenum relative to the late
transition metals, the phosphine oxazoline ligand I still binds
preferencially with the nitrogen function and oxygen binding
with this ligand remains undocumented.
C–C–O
C–C–N
O–C–N
a CNT = Cyclopentadienyl ring centroid.
While ligand I adds selectively to [MoCp(µ-Cl)2]2 in CH2Cl2,
II also provokes a chlorine abstraction process from the solvent.
A similar chlorine abstraction was previously observed when
treating [MoCp*Cl2(PMe2Ph)2] with AlCl3 in CH2Cl2.66 It
was proposed that the abstraction of a chloride ion by AlCl3
produces the highly reactive 15-electron species [MoCp*Cl-
(PMe2Ph)2]ϩ, which is capable of abstracting a Cl atom in order
to produce the more stable 16-electron species [MoCp*Cl2-
(PMe2Ph)2]ϩ. A similar mechanism may therefore take place for
the formation of compound 5 (Scheme 2). This observation
may therefore be taken as an indication of a greater hemilabile
character of ligand II relative to I. This is also demonstrated by
the equilibrium between the κ1P (major) and κ2P,O (minor)
forms of compound 8 (eqn. 5), while for ligand I the kinetically
accessed κ1P form of compound 3 transforms quantitatively to
the κ2P,N form (eqn. 2). However, the hemilability of ligand II
in compound 8 is obviously related to the presence of the
strongly co-ordinating ClϪ anion in the ionic form H. For
the analogous salt 7 containing the less strongly co-ordinating
donor in the more tightly bonded equatorial position in 5
and in the less tightly bonded axial position in 7. Other
related Mo–P distances are 2.542(1) and 2.553(1) Å (equatorial)
for the molybdenum() compound [MoCpCl3(PMe2Ph)2],62
and 2.617(1) and 2.554(1) Å (axial) for the molybdenum()
compounds [Mo(C5H4Me)Cl4(PH2Ar)] (Ar = 2,4,6-Pri3C6H2
or C6H5, respectively).58 The Mo-O distance changes in the
opposite direction, as expected.
(d) Comparison of the two ligands
Both ligands I and II contain, in addition to a Ph2P donor
group, a nitrogen and an oxygen-based donor function. How-
ever, I chose binds selectively with the nitrogen function, II with
the oxygen function. This behaviour can easily be understood
on the basis of the resonance forms shown in Scheme 1, where
J. Chem. Soc., Dalton Trans., 2000, 2577–2585
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donating oxazoline function make the former a less tightly
bonded bidentate ligand. The greater hemilability exhibited by
the phosphine amide ligand II is the source of greater reactivity.
Therefore, the molybdenum complexes with the phosphine
amide ligand II have greater potential for use as building blocks
for the construction of more complex architectures and for
catalytic applications.
Acknowledgements
We are grateful to the Ministère de l’Education Nationale, de
la Recherche et de la Technologie (MENRT) for support and
a PhD grant (to J.-M. C. and F. N.), the Centre National de la
Recherche Scientifique and the Conseil Régional de Bourgogne
for support of this work.
Scheme 2
BF4 anion the EPR properties indicate that only the κ2P,O
form is present in solution. The hemilabile properties of ligand
II are also evidenced by a similar κ1–κ2 equilibrium for com-
pound 5 (eqn. 3).
Ϫ
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