Synthesis, characterization and catalytic activity of addition compounds of dioxomolybdenum(VI) pyridine-2,6-dicarboxylate. Crystal structure of MoO2(dipic)(L) (L = DMF, DMSO, OPPh3)

Article abstract of DOI:10.1016/S0277-5387(00)00513-1

MoO₂(dipic)(L) (dipic = pyridine 2,6-dicarboxylate (dipicolinate); L = dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), hexamethyl phosphorotriamide (HMPA), triphenylphosphine oxide (OPPh₃), triethylamine, tripropylamine, tributy

Full text of DOI:10.1016/S0277-5387(00)00513-1

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 2141–2147
Synthesis, characterization and catalytic activity of addition
compounds of dioxomolybdenum(VI) pyridine-2,6-dicarboxylate.
Crystal structure of MoO2(dipic)(L) (L=DMF, DMSO, OPPh3)
Francisco J. Arna´iz a,*, Rafael Aguado a, Mar´ıa R. Pedrosa a, Andre´ De Cian b,
Jean Fischer b
a Laboratorio de Qu´ımica Inorga´nica, Uni6ersidad de Burgos, Pz Misael Ban˜uelos s/n, 09001 Burgos, Spain
b Laboratoire de Cristallochimie et Chimie Structurale (UMR 7513), Uni6ersite´ Louis Pasteur, Institut Le Bel, 4 rue Blaise Pascal,
67000 Strasbourg, France
Received 11 April 2000; accepted 20 July 2000
Abstract
MoO2(dipic)(L) (dipic=pyridine 2,6-dicarboxylate (dipicolinate); L=dimethyl formamide (DMF), dimethyl sulfoxide
(DMSO), hexamethyl phosphorotriamide (HMPA), triphenylphosphine oxide (OPPh3), triethylamine, tripropylamine, tributyl-
amine, pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine) have been prepared by reacting equimolar amounts of
MoO2Cl2(L)2 (L=DMF, DMSO) with Na2dipic, followed by addition of the appropriate ligand to the resulting solutions. The
molecular structures of the DMF, DMSO and HMPA derivatives have been established by X-ray diffraction analysis. The ability
of MoO2(dipic)(L) (L=DMF, DMSO, HMPA, OPPh3) to work as oxotransfer catalysts has been examined and compared with
that of the prototypical MoO2(Et2dtc)2 (Et2dtc=diethyl dithiocarbamate). Dipicolinate complexes proved to be more efficient
catalysts than dithiocarbamate for the deoxygenation of azoxybenzene with triphenylphosphine in boiling toluene. © 2000
Elsevier Science B.V. All rights reserved.
Keywords: Dioxomolybdenum dipicolinate; Oxygen-atom transfer; Azoxybenzene deoxygenation
[9–14]. One of the most relevant is the anionic
[MoO2(NCS)4]2− species, which contains N-coordi-
nated thiocyanate and has a catalytic activity greater
than that of the prototypical MoO2(Et2dtc)2 in the
oxidation of PPh3 with DMSO [9]. More recently the
catalytic activity of MoO2(CH3)2(L) (L=bipyridine,
alkyl substituted bipyridine) [15], and that of MoO2[2,6-
bis(mentyl)pyridine)] in oxotransfer processes has been
mentioned [16]. In our laboratory we found that simple,
air-stable complexes derived from MoO2Cl2 and
MoO2Br2 are also superior to dithiocarbamates in the
oxygen atom transfer from DMSO to PPh3 at room
temperature [13,14,17]. This prompted us to search for
new dioxomolybdenum(VI) compounds in order to test
their potentiality as oxotransfer catalysts. Here we re-
port on the synthesis, characterization and catalytic
activity of a number of dipicolinate complexes.
1. Introduction
It has been assumed for years that the presence of
sulfur atoms coordinated to molybdenum is a requisite
for complexes of this metal to have oxotransfer activity,
and many model compounds that mimic oxotrans-
ferases have been studied [1–7]. The first oxotransfer
reaction involving a molybdenum complex without co-
ordinated sulfur atoms was reported in 1990 [8]. This
observation proved that the presence of sulfur atoms
coordinated to molybdenum is not essential for oxo-
transfer activity in mild conditions, and since then a
number of complexes of this kind have been reported
* Corresponding author..
E-mail address: farnaiz@ubu.es (F.J. Arna´iz).
0277-5387/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0277-5387(00)00513-1

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F.J. Arna´iz et al. / Polyhedron 19 (2000) 2141–2147
2. Experimental
1H, dipic), 8.46 (d, J=7.63, 2H, dipic), 8.19 (s, 1H,
HꢁCO), 3.06 (s, 3H, CH3), 2.62 (s, 3H, CH3). — Anal.
Calc. for C10H10MoN2O7 (367.96): Mo 26.20, C 32.81,
H 2.75, N 7.65. Found: Mo 26.12, C 31.67, H 2.87, N
7.56%.
2.1. General procedures and measurements
All operations were carried out in a dry oxygen-free
nitrogen atmosphere using standard Schlenck tech-
niques, except the synthesis of the compounds with
DMF, DMSO, HMPA and OPPh3 which were con-
ducted in air. Diethyl ether and toluene were distilled
from sodium under nitrogen prior to use. N,N-
Dimethylformamide was dried with barium oxide and
distilled under reduced pressure. Acetone and amines
for the preparation of amine complexes were distilled
immediately before use. Triphenylphosphine was re-
crystallized prior to use and checked by 31P NMR and
melting point for purity. All other reagents were com-
mercial products and were used without further purifi-
cation. MoO2Cl2(DMSO)2 and MoO2Cl2(DMF)2 were
prepared as reported [13]. Na2dipic was prepared by
reacting equimolar amounts of Na2CO3 and H2dipic in
water, crystallizing the resulting solution and heating
the solid at 100°C under vacuum for 5 h. Melting
points were determined with a Buchi ‘Tottoli’ with
variable warm speed. Elemental analyses (C, H, N)
were performed on a Perkin–Elmer 2400 CHN ana-
lyzer and molybdenum was determined by titration
with lead(II) nitrate [18]. IR spectra were recorded on a
Perkin–Elmer 843 spectrometer with Heyden and Son
Spectrafile-IR version 2.2 operating software, and UV–
Vis spectra on a Milton Roy Spectronic 3000 Arrays
spectrometer equipped with a thermostated cell. NMR
spectra were recorded on a Bruker AC-300 and a
Bruker DPX 300 spectrometer (1H, 300 MHz; 31P,
121.5 MHz). Chemical shifts are relative to TMS (1H),
or external 85% H3PO4 (31P), with downfield values
reported as positive. Coupling constants J are given in
Hz.
2.2.2. MoO2(dipic)(DMSO)
Yield 3.40 g (91.6%). — M.p. (dec.) 185–187°C. —
IR (KBr) cm−1: w(MoO2)=942 s, 909 s; w(CO2)=1707
s, 1470 s; w(SꢀO)=1081 s. — 1H NMR (300 MHz,
acetone-d6, 25°C, TMS): l=8.81 (t, J=7.73, 1H,
dipic), 8.45 (d, J=7.73, 2H, dipic), 2.69 (s, 6H,
CH3). — Anal. Calc. for C9H9MoNO7S (372.92): Mo
25.85, C 29.12, H 2.44, N 3.77. Found: Mo 25.73, C
29.24, H 2.39, N 3.79%.
2.2.3. MoO2(dipic)(HMPA)
Yield 3.62 g (76.7%). — M.p. (dec.) 234–236°C. —
IR (KBr) cm−1: w(MoO2)=934 s, 912 s; w(CO2)=1707
s, 1466 s; w(PꢀO)=1192 s; w(HMPA)=1303, 993 and
756. — 1H NMR (300 MHz, acetone-d6, 25°C, TMS):
l=8.76 (t, J=7.76, 1H, dipic), 8.43 (d, J=7.76, 2H,
dipic), 2.46 (d, J HP=9.63, 18H, CH3). — 31P NMR
(121.5 MHz, acetone-d6, 25°C, H3PO4): l=29.60. —
Anal. Calc. for C13H21MoN4O7P (474.02): Mo 20.32,
C 33.06, H 4.48, N 11.86. Found: Mo 20.22, C 32.18, H
4.30, N 11.68%.
2.2.4. MoO2(dipic)(OPPh3)
Yield 4.20
g
(73.5%). — IR (KBr) cm−1
:
w(MoO2)=940 s, 910 s; w(CO 2)=1710 s, 1585 s;
w(PꢀO)=1154 s. — 1H NMR (300 MHz, acetone-d6,
25°C, TMS): l=8.71 (t, J=7.76, 1H, dipic), 8.31 (d,
J=7.93, 2H, dipic), 7.7–7.5 (m, 15H, C6H5). — 31P
NMR (121.5 MHz, acetone-d6, 25°C, H3PO4): l=
33.07. — Anal. Calc. for C25H18MoNO7P (572.99):
Mo 16.79, C 52.56, H 3.27, N 2.45. Found: Mo 16.62,
C 52.42, H 3.23, N 2.50%.
2.2. General procedure for the preparation of
MoO2(dipic)(L) (L=DMF, DMSO, HMPA, OPPh3)
2.3. General procedure for the preparation of
MoO2(dipic)(amine) (amine=pyridine (Py),
2-methylpyridine (2-MePy), 3-methylpyridine
(3-MePy), 4-methylpyridine (4-MePy), triethylamine
(NEt3), tripropylamine (NPr3), tributylamine (NBu3))
A
mixture of Na2dipic (2.11 g, 10.5 mmol),
MoO2Cl2(DMF)2 (3.45 g, 10 mmol) and L (12 mmol) in
acetone (60 ml) was refluxed with stirring for 1 h. The
white precipitate was separated by filtration and
washed with acetone (10 ml). The combined filtrate and
washing was concentrated to 10 ml and then diethyl
ether (50 ml) was added. The resulting light yellow
microcrystalline precipitate was filtered, washed with
diethyl ether (2×10 ml) and dried under vacuum.
To a solution of MoO2(dipic)(DMF) (1.46 g, 4
mmol) in acetone (20 ml) a solution of amine (4.2
mmol) in acetone (5 ml) is added. After stirring the
resulting mixture for 5 min, the white precipitate is
filtered, washed with acetone (2×5 ml) and diethyl
ether (2×10 ml), and dried under vacuum.
2.2.1. MoO2(dipic)(DMF)
2.3.1. MoO2(dipic)(Py)
Yield 3.07 g (83.8%). — M.p. (dec.) 200–202°C. —
IR (KBr) cm−1: w(MoO2)=942 s, 905 s; w(CO2)=1693
s, 1489 s; w(CꢀO, DMF)=1637 s. — 1H NMR (300
MHz, acetone-d6, 25°C, TMS): l=8.85 (t, J=7.63,
Yield 1.41 g (95%). — IR (KBr) cm−1: w(MoO2)=
946 s, 919 s; w(CO2)=1730 s, 1484 s. — Anal. Calc.
for C12H8MoN2O6 (372.14): Mo 25.78, C 38.73, H 2.17,
N 7.53. Found: Mo 25.35, C 38.52, H 2.32, N 7.25%.

F.J. Arna´iz et al. / Polyhedron 19 (2000) 2141–2147
2143
2.3.2. MoO2(dipic)(2-MePy)
diethyl ether (30 ml). The dark brown precipitate is
filtered, washed with diethyl ether and dried under
vacuum (0.63 g, 89.5%).
Yield 1.03 g, (67%). — IR(KBr) cm−1: w(MoO2)=
946 s and 918 s; w(CO2)=1726 s, 1431 s. — Anal.
Calc. for C13H10MoN2O6 (386.17): Mo 24.85, C 40.43,
H 2.61, N 7.25. Found: Mo 24.85, C 40.13, H 2.38, N
7.07%.
IR(KBr) cm−1: w(MoO2)=953 s, 914 s; w(CO2)=
1688 s, 1438 s; w(PꢀO)=1160 s. — 1H NMR (80
MHz, CDCl3, 25°C, TMS): l=8.0–8.4 (m, J=7.76
Hz, 3H, dipic), 7.4–7.6 (m, 15H, C6H5). — Anal.
Calc. for C50H36Mo2N2O13P2 (1129.99): Mo 17.03, C
53.30, H 3.22, N 2.49. Found: Mo 17.23, C 53.12, H
3.06, N 2.32%.
2.3.3. MoO2(dipic)(3-MePy)
Yield 1.24 g, (80%). — IR(KBr) cm−1: w(MoO2)=
947 s, 914 s; w(CO2)=1726 s, 1455 s. — Anal. Calc.
for C13H10MoN2O6 (386.17): Mo 24.85, C 40.43, H
2.61, N 7.25. Found: Mo 23.96, C 39.62, H 2.57, N
6.95%.
2.5. Catalytic acti6ity
The measurement of the catalytic activity of
MoO2(dipic)(DMSO) in the oxidation of triphenyl
phosphine with dimethyl sulfoxide was followed by 31P
NMR spectroscopy, and compared with that of
MoO2(Et2dtc)2, as previously reported [13]. Two 5 ml
DMSO (20% 6/6 in DMSO-d6) solutions, at [Mo]=
0.02 mol dm−3 and [PPh3]=0.40 mol dm−3, were
used. At 25°C the oxidation of 50% PPh3 required 175
min in the dipicolinate solution and 85 min in that of
the dithiocarbamate. PPh3 became undetectable with
the former after 7 h. Solid PPh3 (50 mg) was added to
the dipicolinate solution that was examined after 4 h
showing only the presence of OPPh3. The same result
was obtained when this operation was repeated after 2,
4 and 7 days.
2.3.4. MoO2(dipic)(4-MePy)
Yield 1.32 g, (85%). — IR(KBr) cm−1: w(MoO2)=
942 s, 908 s; w(CO2)=1730 s, 1433 s. — Anal. Calc.
for C13H10MoN2O6 (386.17): Mo 24.85, C 40.43, H
2.61, N 7.25. Found: Mo 24.98, C 39.86, H 2.77, N
6.96%.
2.3.5. MoO2(dipic)(NEt3)
Yield 1.17 g, (74%). — IR(KBr) cm−1: w(MoO2)=
942 s, 912 s; w(CO2)=1609 s, 1447s. — Anal. Calc.
for C13H18MoN2O6 (394.24): Mo 24.34, C 39.61, H
4.60, N 7.11. Found: Mo 24.03, C 39.35, H 4.72, N
7.25%.
2.3.6. MoO2(dipic)(NPr3)
Yield 1.08 g, (62%). — IR(KBr) cm−1: w(MoO2)=
937 s, 909 s; w(CO2)=1690 s, 1470 s. — Anal. Calc.
for C16H24MoN2O6 (436.32): Mo 21.99, C 44.04, H
5.54, N 6.42. Found: Mo 22.54, C 45.04, H 5.70, N
6.22%.
2.6. Deoxygenation of azoxybenzene
The deoxygenation of azoxybenzene was followed in
toluene at 105°C monitoring the reaction by visible
spectroscopy. Samples of 2 ml were collected off by
syringe from the reacting system at regular intervals,
and immediately transferred to graduated rubber-
capped tubes, containing 2 ml of 4 M NaOH, immersed
into an ice bath. After vigorous shaking of the tubes to
remove the molybdenum complexes from the toluene
phase the absorbance was measured at 446 nm, wave-
length at which azobenzene is the only absorbing spe-
cies present in toluene.
2.3.7. MoO2(dipic)(NBu3)
Yield 1.72 g, (90%). — IR(KBr) cm−1: w(MoO2)=
939 s, 910 s; w(CO2)=1684 s, 1467 s. — Anal. Calc.
for C19H30MoN2O6 (478.40): Mo 20.26, C 47.70, H
6.32, N 5.86. Found: Mo 19.50, C 46.96, H 6.10, N
5.68%.
2.4. Preparation of Mo2O3(dipic)2(OPPh3)2
2.7. Crystal-structure determinations
2.4.1. Method A
A mixture of MoO2(dipic)(OPPh3) (0.71 g, 1.25
mmol) and PPh3 (0.33 g, 1.25 mmol) in toluene (30 ml)
was refluxed for 1 h under nitrogen. The dark brown
powdered precipitate is filtered, washed with diethyl
ether and dried under vacuum (0.61 g, 87%).
Suitable single crystals were grown at room tempera-
ture by diffusion of diethyl ether on acetone solutions
of the complexes in a dry atmosphere. Crystal data
MoO2(dipic)(DMF) were collected using an Enraf–No-
nius MACH3 diffractometer with graphite-monochro-
mated Mo Ka radiation. Crystal data for the complexes
MoO2(dipic)(DMSO) and MoO2(dipic)(OPPh3) were
collected using a KappaCCD diffractometer with gra-
phite-monochromated Mo Ka radiation. Hydrogen atom
positions were located from difference Fourier maps.
All three structures were solved using OPENMOLEN
2.4.2. Method B
A mixture of MoO2(dipic)(OPPh3) (0.71 g, 1.25
mmol) and PPh3 (0.33 g, 1.25 mmol) in acetone (30 ml)
was refluxed for 2 h under nitrogen. The resulting
solution is concentrated to 10 ml and then treated with

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F.J. Arna´iz et al. / Polyhedron 19 (2000) 2141–2147
2.2. Table 1 provides crystallographic details for these
compounds.
removing most of the solvent under vacuum followed
by precipitation with diethyl ether.
MoO2(dipic)(L) (L=HMPA, OPPh3, tertiary amine)
were prepared by treating acetone solutions of
MoO2(dipic)(DMF) or MoO2(dipic)(DMSO) with the
corresponding ligand. MoO2(dipic)(HMPA) and
MoO2(dipic)(OPPh3) are obtained by addition of di-
ethyl ether to the resulting solutions, while the amine
derivatives separated as fine white precipitates; further
studies for the complete characterization of the latter
are in progress. MoO2(dipic)(HMPA) and MoO2-
(dipic)(OPPh3) can also be prepared by reacting
MoO2Cl2(L)2 (L=DMF, DMSO) with Na2dipic in
presence of the corresponding ligand, followed by filtra-
tion of NaCl and addition of diethyl ether to the
resulting solution. In this regard the DMF complex is
superior to that of DMSO since DMF is miscible with
diethyl ether. The presence of minor amounts of water
either in Na2dipic or in the solvent has not significant
effect on the preparation of the derivatives with DMF,
DMSO, HMPA and OPPh3 since these ligands are able
to displace water from the coordination sphere of
molybdenum. Also they have a low affinity for protons,
thus precluding the formation of polymolybdates. By
the contrary, the high proton basicity of tertiary amines
requires the use of rigorously anhydrous conditions to
obtain pure products.
3. Results and discussion
3.1. Synthesis and characterization of the complexes
To the best of our knowledge only a marginal refer-
ence to the synthesis and characterization by melting
point and infrared spectroscopy of MoO2(dipic)-
(HMPA) has been reported [19]. More recently, in
studies on the reactivity of molybdenaoxaziridine com-
plexes, the formation of Mo2O3(dipic)2(HMPA)2 was
postulated [20]. MoO2(dipic)(HMPA) was prepared by
reacting MoO2(acac)2 (acac=acetylacetonate) with
dipicolinic acid and HMPA in CH2Cl2. In these condi-
tions a green solution was obtained which suggests that
a redox process also occurs, probably involving the
acacH displaced.
To overcome this difficulty we have used the easily
available MoO2Cl2(L)2 (L=DMF, DMSO) [17] com-
plexes instead of MoO2(acac)2. Metathetic reactions
between the disodium salt of dipicolinic acid and the
corresponding chlorocomplex in acetone proved to be
appropriate for preparing MoO2(dipic)(DMF) and
MoO2(dipic)(DMSO). The reaction requires about 24 h
at room temperature for completion, probably because
Na2dipic is scarcely soluble in acetone, but less than 1
h when carried out to reflux. The compounds are
recovered in good yield from the resulting solutions by
No relevant features are observed in the IR spectra
of the compounds. The presence of two bands in the
region 900–950 cm−1 indicates that, as usual for dioxo-
molybdenum(VI) species, all the complexes contain
Table 1
Crystal data and summary of intensity data collection and structure refinement of complexes MoO2(dipic)(DMF), MoO2(dipic)(DMSO) and
MoO2(dipic)(OPPh3)
MoO2(dipic)(DMF)
MoO2(dipic)(DMSO)
MoO2(dipic)(OPPh3)
Empirical formula
Molecular weight
Crystal system
Space group
Unit cell dimensions
C10H10MoN2O7
366.14
triclinic
C9H9MoNO7S
371.18
triclinic
C25H18MoNO7P
571.34
monoclinic
P121/n1
(
(
P1
P1
,
a (A)
7.1627(9)
7.991(1)
13.486(4)
72.21(2)
79.49(2)
62.19(1)
649.4(2)
2
1.87
1.043
2737
2320
6.627(1)
7.619(1)
12.793(1)
97.164(4)
101.912(4)
93.063(4)
625.0(2)
2
1.97
1.244
8656
2586
9.5225(3)
17.9542(6)
14.5104(4)
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
107.537(2)
3
,
2365.5(2)
4
1.60
0.669
18322
5009
Z
Dcalc (g cm−3
)
v (mm−1
)
Number of data measured
Number of data with I\3|(I)
Scan mode
T (K)
q/2q
294
ƒ and ꢀ scans
294
phi scans
173
R
wR
0.025
0.041
1.012
0.027
0.043
1.094
0.040
0.056
1.043
Goodness-of-fit (GOF)

F.J. Arna´iz et al. / Polyhedron 19 (2000) 2141–2147
2145
Table 2
Selected bond lengths (A) and angles (°) for MoO2(dipic)(DMF), MoO2(dipic)(DMSO) and MoO2(dipic)(OPPh3)
,
MoO2(dipic)(DMF)
MoO2(dipic)(DMSO)
MoO2(dipic)(OPPh3)
Mo(1)ꢁO(5)
Mo(1)ꢁO(6)
Mo(1)ꢁO(1)
Mo(1)ꢁO(4)
Mo(1)ꢁN(1)
Mo(1)ꢁO(7)
O(4)ꢁC(7)
O(1)ꢁC(1)
O(7)ꢁX
1.674(2)
1.702(2)
2.009(2)
2.010(2)
2.190(2)
2.308(2)
1.320(4)
1.325(4)
1.233(4)
1.696(2)
1.703(2)
2.008(2)
2.017(2)
2.198(2)
2.312(2)
1.335(3)
1.334(3)
1.544(2)
1.687(2)
1.700(2)
2.003(2)
2.000(2)
2.205(2)
2.247(2)
1.326(3)
1.325(3)
1.502(2)
O(5)ꢁMo(1)ꢁO(6)
O(1)ꢁMo(1)ꢁO(4)
O(6)ꢁMo(1)ꢁO(4)
O(6)ꢁMo(1)ꢁO(1)
N(1)ꢁMo(1)ꢁO(4)
N(1)ꢁMo(1)ꢁO(1)
O(5)ꢁMo(1)ꢁO(4)
O(5)ꢁMo(1)ꢁO(1)
O(6)ꢁMo(1)ꢁO(7)
N(1)ꢁMo(1)ꢁO(7)
N(1)ꢁMo(1)ꢁO(5)
O(5)ꢁMo(1)ꢁO(7)
N(1)ꢁMo(1)ꢁO(6)
105.5(1)
144.68(9)
106.5(1)
101.8(1)
72.48(8)
72.79(8)
95.8(1)
96.6(1)
82.3(1)
71.76(8)
100.5(1)
172.0(1)
153.9(1)
104.7(1)
144.81(7)
101.82(8)
106.90(9)
72.56(7)
73.12(7)
94.49(9)
97.40(8)
83.86(8)
73.97(6)
97.40(8)
171.37(8)
157.58(8)
104.6(1)
145.43(8)
103.86(9)
104.73(8)
72.95(8)
73.03(7)
72.95(8)
95.79(9)
85.04(8)
71.31(7)
99.06(9)
170.36(9)
156.35(9)
the cis-MoO2 moiety [21,22]. A relationship between the
basicity of the O-donor neutral ligand and the separation
of the w(MoO2) bands (the higher the basicity the lower
the separation) is also appreciated, but a definite expla-
nation for this effect is not attempted since the differences
in bond lengths and angles for the group cis-MoO2 are
not significant (see Table 2). The stretching bands for
OꢁC, OꢁS and OꢁP of the coordinated ligands in
complexes with DMF, DMSO, HMPA and OPPh3 occur
at 1637, 1081, 1154 and 1192 cm−1 respectively; they are
shifted 20–25 cm−1 to lower wavenumbers with respect
to those of the free ligands, which is indicative of
coordination through the oxygen atom [23].
Selected bond lengths and angles are listed in Table 2.
The bond lengths for MoꢁO(5) and MoꢁO(6) are not
identical, the former being shorter. While the MoꢁO(6)
distance, involving the oxygen trans to N(1), is nearly
identical for all the complexes, the MoꢁO(5) distance
increases with the basicity of the trans ligand.
1
The H NMR spectra of MoO2(dipic)(L) (L=DMF,
DMSO, HMPA and OPPh3) are in agreement with those
expected for octahedral complexes having a reflection
plane and free rotation around the molybdenum-neutral
ligand bond. The presence of two singlets for the methyl
groups in MoO2(dipic)(DMF) indicates that no free
rotation exists around the CꢁN bond.
3.2. Molecular structures
Single-crystal X-ray diffraction studies on MoO2-
(dipic)(L) (L=DMF, DMSO, OPPh3) revealed that the
three complexes have a closely related structure, similar
to that of MoO2(dipic)(DMF) shown in Fig. 1.
The molybdenum atom adopts the expected distorted
octahedral geometry, which is determined by the usual
way of coordination for the dipicolinate ligand (planar
tridentate) to the angular cis-MoO2 moiety.
Fig. 1. Molecular structure and atom-labeling for complex
MoO2(dipic)(DMF); the hydrogen atoms are shown with small arbi-
trary radii.

2146
F.J. Arna´iz et al. / Polyhedron 19 (2000) 2141–2147
Fig. 2. Reduction of PhN(O)ꢀNPh to PhNꢀNPh using PPh3 as a funtion of time at [Mo]=1.25 10−3 mol dm−3 and [PPh3]0/[PhN(O)ꢀNPh]0/
[MoVI]0=10/1/0.05.
The angles for the cis-MoO2 group, O(5)ꢁMoꢁO(6),
are very similar in the three compounds (ca. 105°), and
in good agreement with those found for analogous
complexes with N- and O-coordinated ligands [1,24].
resulting, soft ligand dimethyl sulfide is easily displaced
from the coordination sphere of molybdenum. After this,
it becomes evident that at least MoO2(dipic)(DMSO)
displays catalytic activity in oxotransfer reactions.
The catalytic activity of MoO2(dipic)(DMSO) in the
oxotransfer from DMSO to PPh3 has been measured and
compared with that of the prototypical MoO2(Et2dtc)2 by
following the formation of OPPh3 by 31P NMR as
previously reported [13]. At 25°C the dithiocarbamate
complex showed to be considerably more active than
MoO2(dipic)(DMSO). However, while MoO2(Et2dtc)2
decomposes slowly in DMSO at room temperature,
MoO2(dipic)(DMSO) retains its catalytic activity for
several days, as shown by addition of new batches of
PPh3 to the original solution.
The oxotransfer from azoxybenzene to PPh3 has also
been examined. Azoxybenzene derivatives are difficult to
deoxygenate [25], and because of the low basicity of the
azoxy group a weak coordinating solvent is advisable to
avoid competition for the molybdenum center.
Fig. 2 shows the transformation of azoxybenzene into
azobenzene with a large excess of PPh3 in hot toluene in
presence of MoO2(Et2dtc)2 and MoO2(dipic)(DMSO).
The behavior of MoO2(dipic)(DMF), MoO2(dipic)-
(HMPA) and MoO2(dipic)(OPPh3) is closely similar to
that of MoO2(dipic)(DMSO) and the results are omitted
for clarity.
3.3. Oxotransfer reactions
The oxygen atom transfer from MoO2(dipic)(L) (L=
DMF, DMSO, HMPA, OPPh3) to PPh3 has been exam-
ined. By treating acetone solutions of the complexes with
PPh3 a brown solution results at the same time that
OPPh3 forms. No significant difference in the behavior
of the complexes MoO2(dipic)(DMF), MoO2-
(dipic)(DMSO) and MoO2(dipic)(OPPh3) was observed,
except for the evolution of the foul-smelling dimethyl
sulfide (DMS) when the DMSO adduct was used. The
only molybdenum product we could characterize, even
in a large excess of PPh3, is MoV2 O3(dipic)2(OPPh3)2. At-
tempts to grow crystals suitable for X-ray characteriza-
tion have been unsuccessful to date. Evidence — based
on the 1H NMR of the product — about formation of
MoV2 O3(dipic)2(HMPA)2 was found when using
MoO2(dipic)(HMPA), presumably due to the superior
coordinating ability of HMPA with respect to OPPh3.
These results strongly suggest that: (i) MoIVO(dipic)(L)
comproportionates as it forms with the remaining
MoVIO2(dipic)(L), at the same time that the coordinated
DMF or DMSO are displaced by the OPPh3 produced,
thus leading to the dinuclear complex MoV2 O3(dipic)2-
(OPPh3)2 which is very resistant to reduction; (ii)
MoIVO(dipic)(DMSO) rearranges to MoVIO2(dipic)-
(DMS) (PPh3 is unable to deoxygenate DMSO in absence
of MoO2(dipic)(L) in the same conditions) and the
After completion of the deoxygenation process,
Mo2O3(Et2dtc)4 and MoV2 O3(dipic)2(OPPh3)2 could be
isolated. The fact that we were unable to isolate oxo-
molybdenum(IV) complexes in a large excess of PPh3
suggests that Mo2O3(Et2dtc)4 and MoV2 O3(dipic)2-
(OPPh3)2 are the prevalent species in toluene solution,
and that the equilibrium represented below — as well

F.J. Arna´iz et al. / Polyhedron 19 (2000) 2141–2147
2147
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(i) A number of air stable dioxomolybdenum(VI)
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5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 137435 {for [MoO2(dipic)-
(DMF)]}, 137436 {for [MoO2(dipic)(DMSO)]} and
137437 {for [MoO2(dipic)(OPPh3)]}. Copies of this in-
formation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44-1223-336033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
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Acknowledgements
This work was supported by the Direccion General
de Ensen˜anza Superior (Grant PS95-0832) and Junta de
Castilla y Leon (Grant BU14/98).
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