Synthesis and characterization of molybdenum oxo complexes of two tripodal ligands: Reactivity studies of a functional model for molybdenum oxotransferases

Article abstract of DOI:10.1039/b505180k

Reaction of the tetradentate ligand N-(2-hydroxybenzyl)-N,N-bis(2- pyridylmethyl)amine (L-OH) with MoO₂Cl₂ in methanol in the presence of NaOMe and PF₆^- results in the formation of [MoO₂(L-O)]PF₆. Similarly, the reaction of N-(2-mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine (L-SH) with MoO ₂(acac)₂ leads to the formation of [MoO ₂(L-S)]⁺. The dioxo-molybdenum complex [MoO ₂(L-O)]⁺ reacts with phosphines in methanol to afford phosphine oxides and an air-sensitive molybdenum complex, tentatively identified as [Mo(IV)O(L-O)(OCH₃)]. The latter complex is capable of reducing biological oxygen donors such as DMSO or nitrate, thereby mimicking the activity of DMSO reductase and nitrate reductase. Reaction of [MoO₂(L-O)] PF₆ with PPh₃ in other solvents than methanol leads to the formation of the Mo(V) dimer [(L-O)OMo(μ-O)MoO(L-O)](PF₆) ₂. The crystal structures of [MoO₂(L-O)]PF₆ and the μ-oxo bridged dimer are presented. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b505180k

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F U L L P A P E R
Synthesis and characterization of molybdenum oxo complexes of two
tripodal ligands: reactivity studies of a functional model for
molybdenum oxotransferases†‡
Anders Thapper,a Axel Behrens,a,b Jacob Fryxelius,a Maria H. Johansson,a Fabio Prestopino,a
Miklo´s Czaun,a Dieter Rehderb and Ebbe Nordlander*a
a Inorganic Chemistry, Center for Chemistry and Chemical Engineering, Lund University, Box
124, SE-221 00, Lund, Sweden. E-mail: Ebbe.Nordlander@inorg.lu.se; Fax: +46 46 222 44 39
b Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King
Platz 6, D-20146, Hamburg, Germany
Received 12th April 2005, Accepted 22nd June 2005
First published as an Advance Article on the web 19th September 2005
Reaction of the tetradentate ligand N-(2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (L–OH) with MoO2Cl2 in
methanol in the presence of NaOMe and PF6− results in the formation of [MoO2(L–O)]PF6. Similarly, the reaction of
N-(2-mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine (L–SH) with MoO2(acac)2 leads to the formation of
[MoO2(L–S)]+. The dioxo-molybdenum complex [MoO2(L–O)]+ reacts with phosphines in methanol to afford
phosphine oxides and an air-sensitive molybdenum complex, tentatively identified as [Mo(IV)O(L–O)(OCH3)]. The
latter complex is capable of reducing biological oxygen donors such as DMSO or nitrate, thereby mimicking the
activity of DMSO reductase and nitrate reductase. Reaction of [MoO2(L–O)]PF6 with PPh3 in other solvents than
methanol leads to the formation of the Mo(V) dimer [(L–O)OMo(l-O)MoO(L–O)](PF6)2. The crystal structures of
[MoO2(L–O)]PF6 and the l-oxo bridged dimer are presented.
general role for enzymes from the latter group is to catalyze
Introduction
oxygen atom transfer to or from a biological oxygen acceptor or
The mononuclear molybdenum enzymes constitute a diverse set
donor (eqn. (1)).
of enzymes that have been found in all types of living systems
from bacteria to plants and mammals, including man. These
2+
[MoVIOn]2+ + X ꢀ [MoIVOn−1
]
+ XO (n = 1, 2)
(1)
enzymes catalyze reactions that are part of the global carbon,
nitrogen and sulfur metabolisms1,2 and their active sites contain
a cofactor (Moco) which consists of a molybdenum atom
and one or two organic pterin derivatives (pyranopterindithi-
olates) called molybdopterins3 (Fig. 1). The molybdopterin is
coordinated to the molybdenum atom via the dithiolene unit.
Additional ligands that are found in the oxidized Mo(VI) state
of the cofactor includes one or two oxo ligands and/or a sulfido
ligand and/or an amino acid residue such as serine or cysteine.
Corresponding tungsten enzymes containing similar cofactors
are known,2,4 such enzymes have been found in acetate- and
methane-producing bacteria as well as in hyperthermophilic
archaea.
Examples of well-studied oxotransferases include sulfite
oxidase5–8 and dimethyl sulfoxide (DMSO) reductase.9–16 In
the case of DMSO reductase, isotope labelling experiments17
indicate that the oxygen atom that is removed from the substrate
coordinates to the molybdenum atom in the oxidized state of the
=
enzyme. The presence of a Mo(VI) O functional moiety is thus
an essential feature of this group of enzymes. Several model
systems for oxotransferases have been developed in order to
better understand the reactivity of the molybdenum cofactors
and the mechanisms of oxygen atom transfer.18–25 The general
mechanism proposed for the oxygen atom transfer includes a
transition state where the molybdenum atom and the substrate
are bridged by the oxygen atom that is either entering or leaving
the coordination sphere of the molybdenum atom.26,27
The redox potentials, and thus potential reactivity/catalytic
activity as well as substrate specificity of the enzymes (and model
systems) are expected to be considerably influenced by the direct
coordination environment of the molybdenum atom.28,29 The
presence of a molybdopterin (dithiolene) ligand is invariant in
all mononuclear molybdenum enzymes and great advances in the
preparation of molybdenum dithiolene complexes that replicate
important structural, electronic25 and functional features of
molybdenum hydroxylases and oxotransferases have been made
in recent years.19–21,24–26,30 In order to probe the effects of
the coordination environment, we have developed a synthetic
strategy to obtain a number of tripodal ligands with N,O- and
N,S-donor sets that can be varied in a systematic fashion.31 Here,
we would like to describe the preparation and oxygen atom
transfer chemistry of molybdenum oxo complexes, in which the
functional {Mo(VI)O2}/{Mo(IV)O} system, bearing a relation-
ship to the active site of oxotransferase enzymes, is stabilized
by the tetradentate ligands N-(2-hydroxybenzyl)-N,N-bis(2-
pyridylmethyl)amine) (L–OH) and N-(2-mercaptobenzyl)-N,N-
bis(2-pyridylmethyl)amine (L–SH).
Fig. 1 The structure of molybdopterin. In several enzymes, nucleotides
(AMP, GMP, IMP) are coordinated to the phosphate side chain of the
pyran (pyrene) ring. The “open form” of molybdopterin,3 a bicyclic
dihydropterin rather than the tricyclic pyranopterin, has recently been
detected in the crystal structure of membrane-bound nitrate reductase
from E. coli.55–57
The mononuclear molybdenum enzymes can be divided into
two functional groups, hydroxylases and oxotransferases.1,2 The
† Based on the presentation given at Dalton Discussion No. 8, 7–9th
September 2005, University of Nottingham, UK.
‡ Electronic supplementary information (ESI) available: Synthesis of
bis(pyridin-2-ylmethyl)amine (DPA); COSY spectrum of complex 1. See
http://dx.doi.org/10.1039/b505180k
3 5 6 6
D a l t o n T r a n s . , 2 0 0 5 , 3 5 6 6 – 3 5 7 1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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Complex 1 has been characterized by 1H NMR, UV-Vis and
IR spectroscopy as well as mass spectrometry. In principle, there
are two possible coordination modes for the ligand L–O− to a
cis-dioxo molybdenum group - the methylpyridyl arms may be
coordinated cis or trans to each other. Thus, two coordination
isomers (one of which consists of a pair of enantiomers) are
possible for 1; these are shown in schematic form in Fig. 2. The
crystal structures of mononuclear iron complexes containing L–
O−, [FeCl2(L–O)], and a derivative of L–O−, [FeCl2(NO2–
L–O)] (NO2–L–O = N-(2-hydroxy-5-nitrobenzyl)-N,N-bis(2-
pyridylmethyl)amine)),39,40 reveal that the ligand coordinates
with the pyridyl arms cis to each other (Fig. 2, isomer A). It
is reasonable to expect that isomer A should be favoured, as it
positions two relatively basic amine moieties in trans position to
the oxo ligands while in isomer B, the phenolate moiety is trans
to one oxo ligand. Molecular mechanics minimizations of the
two coordination isomers using the SYBYL force field within
the Spartan 4.1 program suite, indicate that isomer A is indeed
favoured. The one-dimensional 1H NMR spectrum of 1, as well
as 1H–1H COSY measurements (see ESI‡), support the existence
of isomer A (exclusively) in solution. There are twelve resonances
in the aromatic region (6.5–9.3 ppm) which may be attributed to
the phenoxyl ring protons and the protons of two inequivalent
pyridine rings (Fig. 3). Furthermore, the characteristic doublets
of the two pyridine rings at 9.22 and 8.78 ppm are well separated
and it is possible to detect six methylene resonances in the 4–
6 ppm region, which is consistent with all three arms of the L–O−
ligand being inequivalent.
Results and discussion
Ligand synthesis
Preparative routes to L–OH32 and L–SH33,34 have been described
in the literature, but we have developed new, convenient and
relatively high yield syntheses of these ligands. The starting ma-
terial dipicolylamine (bis(pyridin-2-ylmethyl)amine, DPA) was
preparedaccordingtoaliteraturemethod35 andisolatedasitshy-
drochloride salt. The ligand L–OH was synthesized by a method
used to prepare the closely related ligand N-(2-hydroxybenzyl)-
N,N-bis(2-pyridylethyl)amine36 (Scheme 1). Dipicolylamine was
condensed with 2-bromomethylphenyl acetate;37 subsequent
deacylation afforded L–OH in better than 60% yield.
Scheme 1 Synthetic route to N-(2-hydroxybenzyl)-N,N-bis(2-pyridyl-
methyl)amine) (L–OH).
The synthetic route to L–SH is outlined in Scheme 2. Reduc-
tion of the carboxylic acid of thiosalicylic acid with LiAlH438 and
subsequent protection of the thiol with a propionitrile group34
led to the isolation of 3-(2-hydroxymethylphenylsulfanyl)-
propionitrile. Bromination of 3-(2-hydroxymethylphenyl-
sulfanyl)propionitrile with PBr3 gave 3-(2-bromomethylphenyl-
sulfanyl)propionitrile in 80% yield, and this was reacted
with an equimolar amount of DPA to give L–SCH2CH2CN
(yield: 59%). Removal of the protection group by refluxing in
methanol with NaOMe, or by addition of KOBut at ambient
temperature,34 gave L–SH. The deprotected ligand was isolated
as a sticky solid by the removal of the solvent in vacuo.
Fig. 2 Two possible coordination isomers for the coordination of L–O−
to a MoO2 group (see text).
Fig. 3 The aromatic region of the 1H NMR-spectrum of [MoO2(L–O)]+
1 (* = impurity). The resonances have been assigned on the basis of
COSY spectroscopy (see text and ESI†).
Scheme 2 Synthetic route to N-(2-mercaptobenzyl)-N,N-bis(2-pyridyl-
methyl)amine (L–SH).
Synthesis and characterization of [MoO2(L–O)]PF6 1
Crystal and molecular structure of 1
In order to prepare two isostructural oxomolybdenum com-
plexes with different ligand donor properties, L–OH and L–SH
(vide supra) were reacted with suitable Mo(VI) starting materials.
The dioxo-molybdenum complex of L–OH, [MoO2(L–O)]PF6 1
was easily afforded by reaction of [MoO2Cl2] or [MoO2(acac)2]
with the ligand in methanol or dichloromethane in the presence
of NaOMe and KPF6. Removal of the solvent and subsequent
recrystallization of the product from a diethyl ether–hexane
solution gave 1 as an orange microcrystalline powder.
Crystals of 1 suitable for X-ray diffraction were grown from a
concentrated methanol solution and the crystal structure of 1
was determined in order to confirm its structure and determine
its conformation in the solid state. The molecular structure of
1 (Fig. 4) shows that the molybdenum atom is in a distorted
octahedral environment with the three oxygen donors arranged
in a fac configuration. The structure confirms that the pyridyls
are coordinated cis to each other with one nitrogen trans to the
phenolate and the other trans to one oxo ligand; the central
D a l t o n T r a n s . , 2 0 0 5 , 3 5 6 6 – 3 5 7 1
3 5 6 7

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dark purple solid. It appears that 2 is a lightly stabilized complex
that is present only in solution.
Addition of OAsPh3 to a methanol solution of 2 leads
to reduction of the arsine oxide and (re)oxidation of the
molybdenum complex to form 1 in a slow reaction. In methanol
solutions, the red complex also reduces three biological oxygen
donors, viz. dimethyl sulfoxide (DMSO), nitrate and diphenyl
sulfoxide as evidenced by the (re)formation of 1 (confirmed
by UV-Vis spectroscopy), but the addition of Me3NO leads
to decomposition of the molybdenum complex. If PPh3 and
1 are reacted with DMSO as solvent, a red complex is initially
formed and, in a slower reaction, 1 is reformed, indicating that
the solvent is reduced. Addition of more PPh3 starts the cycle
over again, indicating that molybdenum-mediated oxo transfer
from DMSO to PPh3 is taking place.
Fig.
4
An ORTEP-358 diagram of the molecular structure of
[MoO2(L–O)]+ 1 showing the atom numbering scheme. Thermal ellip-
soids are plotted at the 30% probability level. Hydrogen atoms have been
omitted for clarity.
Preparation of [(l-O){MoO(L–O)}2](PF6)2 3
Upon reaction of 1 with PPh3 in CH2Cl2, CHCl3, pyridine,
THF or acetonitrile, the solution initially turns red but rapidly
changes color to dark purple and, in the case of dichloromethane
or chloroform, leads to the formation of a dark purple solid.
amine is coordinated trans to the second oxo ligand. There are
two optical isomers of the cation of 1 and both are present
in the crystal. The Ooxo–Mo–Ooxo angle (105.0◦) and the Mo–
˚
˚
Ooxo distances (Mo–O1 1.667 A, Mo–O2 1.676 A) are similar
to those reported in earlier studies of octahedral molybdenum
dioxo complexes.18,28,29,41–43 Due to the larger trans influence of
the oxo ligand compared to the phenolate, the Mo–N1 bond
1
The H NMR spectrum of the complex exhibited only very
broad signals; this observation indicates that the product is a
paramagnetic Mo(V) complex, and very likely a dimer, since the
formation of oxo-bridged Mo(V) dimers is prevalent in this type
of chemistry. Mass spectrometry of the dark purple complex
indicated that it is indeed [(l-O){MoO(L–O)}2](PF6)2 3.
˚
distance is 0.16 A longer than the Mo–N2 bond [2.286(3) and
˚
2.128(4) A, respectively] but almost identical to the Mo–N3
˚
bond distance [2.290(4) A]. All Mo–N distances are shorter than
those observed in the closely related complexes [MO2(L–O2)]
Crystal and molecular structure of 3
(M = Mo,41 W28), where L–O2 is the corresponding tripodal
2−
In order to confirm the dimeric nature of 3, single crystals
were grown from a THF solution of 3 with added excess of
NaBPh4. The crystals were needle-shaped and very thin in one
dimension. The low volume of the crystals made them weakly
diffracting and a good quality data set could not be obtained
with a conventional rotating anode X-ray source. However, a
high quality data set could be obtained on the Lund University
synchrotron (MAX-lab) and a satisfactory structure solution
was obtained (cf. Experimental section).
ligand with one pyridyl and two phenolate arms. The planes of
the two pyridyl rings in 1 are almost perpendicular to each other
and point straight out from the metal. The phenolate ring is
coordinated so that the angle between this ring and the closest
pyridyl ring is 40◦. As has been observed in related complexes,41
the Mo–O(3)–C(31) angle is unusually large (134.5◦), suggesting
partial sp2 (re)hybridization to maximize orbital overlap with the
metal.42 Selected distances and angles are summarized in Table 1.
The molecular structure of 3, which is shown in Fig. 5,
reveals that the cation is a dinuclear molybdenum complex
with an Mo1–Mo2 distance of 3.717(24) A. Selected distances
and angles are summarized in Table 2. The two molybdenum
atoms are bridged by an oxygen atom with an Mo1–O3–Mo2
angle of 169.7◦. There is one terminal oxo ligand on each
molybdenum and the two oxo ligands are in a trans configuration
with an O1–Mo1 · · · Mo2–O2 torsion angle of 179.8◦; such
trans configuration of the oxo groups is a feature that has
been observed for several oxo-bridged Mo dimers.44–49 On both
Reactivity studies of [MoO2(L–O)]PF6 1
˚
Reaction of equimolar amounts of 1 and PPh3 in methanol at low
concentrations (0.4 mM) is fast and affords Ph3PO (as identified
by 31P NMR) and a complex that is tentatively formulated as
[MoIVO(L–O)]+. However, the reduced molybdenum complex is
1
not stable and decomposes rapidly, H NMR of the resultant
solution shows only signals from the free ligand. At higher
concentrations (>4 mM), reactions of 1 in methanol solution
with an equimolar amount of PPh3, or an excess of PPh3 or
PEt3, affords a red complex and phosphine oxide. The red
complex is tentatively identified as [MoO(L–O)(OCH3)] 2 on
the basis of its FAB+ mass spectrum which exhibits an M+ peak
with a characteristic molybdenum pattern corresponding to the
formulation. Although 2 is air-sensitive, it is stable for days in
anaerobic solutions at room temperature but decomposes slowly
at elevated temperatures. Unfortunately, all attempts of isolation
by evaporation of solvent or addition of diethyl ether or other
solvents have been unfruitful, leading only to the isolation of a
◦
˚
Table 1 Selected distances (A) and angles ( ) for [MoO2(L–O)](PF6) 1
Mo1–O1
Mo1–O2
Mo1–O3
Mo1–N1
1.697(3)
1.690(3)
1.899(3)
2.292(3)
Mo1–N2
Mo1–N3
O3–C36
2.194(3)
2.312(3)
1.372(4)
O1–Mo1–O2
O1–Mo1–N1
O2–Mo1–N3
105.01(14)
163.31(13)
155.31(13)
O3–Mo1–N2
N1–Mo1–N2
Mo1–O3–C36
155.21(12)
82.66(12)
133.0(2)
Fig. 5 A diagram of the molecular structure of [(l-O){MoO(L–O)}2]2+
3 showing the atom numbering scheme. Thermal ellipsoids are plotted at
the 30% probability level. Hydrogen atoms have been omitted for clarity.
3 5 6 8
D a l t o n T r a n s . , 2 0 0 5 , 3 5 6 6 – 3 5 7 1

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◦
˚
acceptors. The (unstable) Mo(IV) mono-oxo species [MoO(L–
O)(OCH3)] 2 that is the product of such oxo transfer in methanol,
may in turn reduce (deoxygenate) the biological substrates
nitrate, DMSO and diphenyl sulfoxide, thus emulating the re-
ductive half reactions of nitrate reductase and DMSO reductase.
However, in non-protic solvents, reaction of 1 with phosphines
leads to the formation of the Mo(V) dimer [(l-O){MoO(L–
O)}2](PF6)2 3. The molybdenum(VI) dioxo complex of the
thiolate analogue of L–OH, i.e. [MoO2(L–S)]+ 4 was found to
be unable to effect oxygen atom transfer to triphenylphosphine.
Table 2 Selected distances (A) and angles ( ) and [(l-O){MoO(L–
O)}2](BPh4)2 3
Mo1–Mo2
Mo1–O1
Mo1–O3
Mo1–O4
Mo1–N1
Mo1–N2
Mo1–N3
3.717(24)
1.675(1)
1.865(7)
1.950(5)
2.191(5)
2.195(11)
2.300(1)
Mo2–O2
Mo2–O3
Mo2–O5
Mo2–N4
Mo2–N5
Mo2–N6
1.681(2)
1.868(16)
1.956(17)
2.194(16)
2.211(24)
2.326(0)
Mo2–O3–Mo1
O1–Mo1–O3
O2–Mo2–O3
O3–Mo1–N2
O3–Mo2–N5
O1–Mo1–N3
169.64(2)
105.36(1)
104.74(1)
165.05(2)
163.51(2)
155.60(1)
O2–Mo2–N6
O4–Mo1–N1
O5–Mo2–N4
N1–Mo1–N2
N4–Mo2–N5
158.02(1)
157.24(2)
157.06(2)
91.92(1)
93.59(1)
Experimental
General methods
All synthetic procedures were performed under an atmosphere
of dry nitrogen using standard Schlenk and vacuum-line tech-
niques or in a glove box. All solvents were dried by distillation
over appropriate drying agents and degassed prior to use. All
other chemicals were used as received. NMR Spectra were
recorded using Varian Unity 300 MHz and Bruker Avance
400 MHz spectrometers using the solvent resonance as an
molybdenum atoms, the amine nitrogen is coordinated trans to
the oxo group, one pyridyl nitrogen is trans to the l-oxo bridge,
and the other pyridyl nitrogen is trans to the phenolate oxygen.
The geometrical configuration of the ligand donor atoms around
each molybdenum in 3 is thus the same as the configuration in 1,
with one of the oxo ligands in 1 (the one trans to the pyridyl
nitrogen) being replaced by the l-oxo bridge, but it should
be noted that the two molybdenum atoms in 3 have opposite
stereochemical octahedral configurations (corresponding to the
enantiomeric forms of 1), probably due to steric reasons. Since
there is no inversion center in the position of O3, there are two
possible diastereomers of the whole cation of 3 and both of these
are found in the unit cell. The Mo1–O1 and Mo2–O2 distances
1
internal standard for the H NMR spectra while H3PO4 was
used as an external standard for the 31P spectra. IR Spectra
were recorded in KBr pellets on Nicolet Avatar 360 and Perkin
Elmer 1720 FT-IR instruments. UV-vis spectra were obtained
on Varian Cary 50 Scan and Cary 300 Bio spectrometers. FAB-
MS spectra were obtained on a JEOL SX-102 instrument;
3-nitrobenzyl alcohol was used as a matrix and CsI as the
calibrant. The syntheses of bis(pyridin-2-ylmethyl)amine35 via
its hydrochloride salt (cf. ESI), 2-bromomethylphenyl acetate37
and (2-mercaptophenyl)methanol38 were performed according
to literature procedures.
˚
are 1.675(1) and 1.681(2) A, respectively, while the Mo1–O3
˚
and Mo2–O3 distances are longer, 1.865(7) and 1.868(16) A,
respectively. All four Mo–Npy distances in the structure are
˚
close to 2.2 A indicating that the trans influences exerted by
the phenolate substituent of the ligand and the oxo bridge are
approximately equal.
Synthesis of N-(2-hydroxybenzyl)-N,N-
bis(2-pyridylmethyl)amine (L–OH)33,34
Synthesis and reactivity studies of [MoO2(L–S)]+ 4
Bis(pyridin-2-ylmethyl)amine (DPA, 1.70 g, 8.5 mmol) was
dissolved in 40 ml ethyl acetate and 2.78 g of a 70 wt% solution of
2-bromomethylphenylacetate (8.5 mmol) and 5 ml triethylamine
was added. The reaction was stirred at room temperature for 72 h
and a white solid precipitated. The mixture was filtered and the
solvent was removed under reduced pressure, yielding a brown
oil. The oil was treated with a mixture of 40 ml water, 80 ml
ethanol and 40 ml saturated sodium hydrogen carbonate and
stirred for 3 h. Sodium hydroxide (2.0 g) dissolved in 20 ml
water was added and the mixture extracted with 3 × 40 ml
dichloromethane. The organic phase was dried with MgSO4,
filtered and evaporated to give 1.8 g of a brown oil. The oil
was purified by flash column chromatography on silica using a
70 : 30 mixture of ethanol–ethyl acetate as eluent to give 1.6 g
(5.26 mmol, 62% yield) of the product as a light brown oil. 1H
NMR (CDCl3): d 8.56 (d, 2H), 7.64 (t, 2H), 7.35 (d, 2H), 7.14–
7.20 (m, 3H), 7.06 (d, 1H), 6.89 (d, 1 H), 6.76 (t, 1H), 3.89 (s,
4H), 3.80 (s, 2H).
It was found that the preparation of complex 4 is dependent
on the method of addition of the reactants. Thus, dropwise
addition of a solution of L–SH to methanol or THF solutions
of [MoO2Cl2] led to the formation of dark precipitates that were
not analyzed further. The reactions were investigated at ambient
temperature as well as −20 ◦C with the same result; at even lower
temperature no reaction was found to occur. It is possible that
reaction of L-SH with high-valent molybdenum complexes may
lead to ligand oxidation and formation of a disulfide bridge.
Such reactivity is not unprecedented, reaction of Ni(II) acetate
with the protected form of L–SH in the presence of air leads
to the dinuclear complex [Ni2(L–S–S–L)(CH3COO)2](BPh4)2.34
However, the dropwise addition of [MoO2(acac)2] in THF to a
methanol solution of L–SH under the rigorous exclusion of air,
led to the formation of [MoO2(L–S)]+ 4, which was recrystallized
in the presence of KPF6 to form the hexafluorophosphate salt.
1
The H NMR spectrum of 4 reveals that the ligand has the
same coordination mode as observed for 1 (Fig. 2, isomer A),
i.e. the two pyridyl moieties are chemically and magnetically
inequivalent. The oxo transfer capability of 4 was investigated
in NMR tube reactions, where an excess of triphenylphosphine
was added to a CD2Cl2 solution of 4. However, no formation
of the corresponding phosphine oxide could be detected. This
is in contrast to what has been observed for the oxygen
atom transfer capabilities of tungsten dioxo complexes with
tetradentate N2O2 and N2S2 phenolate/thiolate ligands29 where
the thiolate-containing complexes were found to be better oxo-
transfer reagents than the corresponding phenolate complexes;
however, a direct comparison between analogous phenolate and
thiophenolate ligands was not made in this study.
Synthesis of 3-(2-hydroxymethylphenylsulfanyl)propionitrile
A solution of 5.22 g (37.2 mmol) of (2-mercaptophenyl)-
methanol in 20 ml ethanol was added to a mixture of 1.45 g
NaOH (36.3 mmol) in 2 ml water and 6 ml ethanol. To this
solution, 4.98 g (37.2 mmol) 3-bromopropionitrile was added
dropwise and the reaction was stirred for 5 h. The sodium
bromide formed was filtered off and the solution evaporated
to a yellow oil. The oil was dissolved in 40 ml diethyl ether and
washed with 20 ml 5% NaOH and 20 ml water. The ether phase
was dried and evaporated to give 6.16 g (31.9 mmol, 86% yield)
of the product as a light yellow solid. Recrystallization of this
solid from ether–pentane gave 4.48 g white crystalline material.
1H NMR (CDCl3): d 7.28–7.50 (m, 4H), 4.85 (s, 2H), 3.15 (t,
2H), 2.60 (t, 2H).
In summary, the complex [MoO2(L–O)]+ 1 is capable of
effecting oxygen atom transfer reactions with phosphines as oxo
D a l t o n T r a n s . , 2 0 0 5 , 3 5 6 6 – 3 5 7 1
3 5 6 9

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Table 3 Crystallographic data for [MoO2(L–O)](PF6) 1 and [(l-O)-
{MoO(L–O)}2](BPh4)2 3
Synthesis of 3-(2-bromomethylphenylsulfanyl)propionitrile
A suspension of 3.45 g (17.9 mmol) 3-(2-hydroxymethylphenyl-
sulfanyl)propionitrile in 60 ml diethyl ether was cooled on an
ice-bath and 1.61 g (6.0 mmol) PBr3 was added dropwise under
stirring. The ice-bath was removed and the reaction left for
3 h at room temperature. The solution was washed with 20 ml
5% NaOH and 20 ml water, dried with MgSO4, filtered and
evaporated to give 3.66 g (14.3 mmol, 80% yield) of the product
1
3
Formula
C19H18F6Mo1N3O3P1 C86H76B2Mo2N6O5
Mr
˚
577.27
1487.03
k/A
0.71073
1.515
Crystallographic system Triclinic
Monoclinic
P21/n
4
16.499(3)
21.562(4)
20.882(4)
90
91.28(3)
90
7427(3)
1.330
¯
Space group
P1
1
as a light yellow oil. H NMR (CDCl3): d 7.44–7.50 (m, 2H),
Z
a/A
b/A
2
7.29–7.33 (m, 2H), 4.74 (s, 2H), 3.20 (t, 2H), 2.62 (t, 2H).
˚
˚
˚
8.4549(17)
9.2820(19)
14.303(3)
89.61(3)
78.13(3)
76.02(3)
1064.8(4)
1.800
Synthesis of N-(2-propionitrilemercaptobenzyl)-N,N-
c/A
bis(2-pyridylmethyl)amine (L–SCH2CH2CN)34
a/◦
b/◦
c /◦
A mixture of 1.752 g DPA (8.79 mmol), 2.25 g 3-(2-
bromomethylphenylsulfanyl)propionitrile (8.36 mmol), 3 ml
NEt3 and 150 ml ethyl acetate was stirred for 3 days. The reaction
was filtered and evaporated to give 2.34 g of a yellow–brown oil.
The oil was purified by chromatography on a silica column with
a 70 : 30 mixture of ethanol–ethyl acetate as eluent and the
product was obtained as 1.82 g (4.9 mmol, 59% yield) of a light
yellow oil. 1H NMR (CDCl3): d 8.49 (d, 2H), 7.62 (t, 2H), 7.54
(dd, 1H), 7.49 (d, 2H), 7.34–7.30 (m, 1H), 7.22–7.18 (m, 2H),
7.10 (dd, 2H), 3.87 (s, 2H), 3.80 (s, 4H), 3.08 (t, 2H), 2.52 (t, 2H)
3
˚
V/A
Dc/g cm−3
2h Range/◦
1.5–31.7
0.0538, 0.1282
b
R1,a wR2
0.0538, 0.1682
ꢀ
ꢀ
ꢀ
2
a R1
=
ꢀFo|
−
|Fcꢀ/ |Fo|. b wR2
=
{
[w(Fo
−
Fc2)2]/
ꢀ
1/2
[w(Fo2)2]}
.
4 as a dark red microcrystalline product which was dried under
vacuum for more than 2 h. Recrystallization from an acetonitrile
solution in the presence of KPF6 gave [MoO2(L–S)]PF6. Yield:
Preparation of [MoO2(L–O)]PF6 1
1
130 mg (0.3 mmol, 60%). IR (KBr): mMoO 952s, 898m cm−1. H
To a solution of L–OH (100 mg, 0.33 mmol), KPF6 (72 mg,
0.39 mmol) and NaOMe (22 mg, 0.41 mmol) in 20 ml methanol,
67 mg MoO2Cl2 (0.34 mmol) was added. The reaction was stirred
for 4 h and placed in a freezer overnight. The reaction was filtered
to remove a white precipitate. Diethyl ether (5 ml) and hexane
(15 ml) was added and more white solid formed and was removed
by filtration. The yellow solution was kept at −20 ◦C overnight.
Removal of the solvent at reduced pressure gave 122 mg of 1
(0.21 mmol, 64% yield). IR (KBr): mMoO 949, 918 cm−1 1H NMR
(CD3OD): d 9.23 (d, 1H) 8.79 (d, 1H), 8.29 (t, 1H), 7.90 (d, 1H),
7.81 (t, 1H), 7.72 (t, 1H), 7.52 (t, 1H), 7.39 (d, 1H), 7.11 (t, 1H),
7.03 (d, 1H), 6.98 (t, 1 H), 6.51 (d, 1H), 5.32 (s, br, 1H), 5.28 (d,
1H), 4.92 (d, 1H), 4.51 (d, 1H), 4.46 (d, 1H), 4.17 (d, 1H). MS-
FAB+ (m/z): 434 (M+). UV-vis (MeCN), kmax/nm (e/M−1 cm−1):
228, 261 (9400 300), 355 (2350 60).
NMR (dmso-d6): d 8.97 (m, 1H) 8.79 (m, 1H), 8.18 (t, 1H), 7.74
(d, 1H), 7.60 (m, 1H), 7.53 (t, 1H), 7.44 (d, 1H), 7.34 (d, 1H),
7.21 (m, 1H), 7.16 (m, 2H), 6.88 (t, 1 H), 5.19 (d, 1H), 4.87
(d, 1H), 4.64 (d, 1H), 4.34 (d, 1H), 4.20 (d, 1H), 3.95 (d, 1H).
MS-FAB+ (m/z): 450 (M+). UV-vis (MeCN), kmax/nm: 253 (sh),
362, 450, 537.
Crystallography
Relevant data about the collections and structure solutions
are summarized in Table 3. Crystals of 1 suitable for X-ray
crystallography were grown by slow evaporation of a methanol
solution of 1. Crystals of 3 were grown from a THF solution of
[(l-O){MoO(L–O)}2](PF6)2 with an added excess of NaBPh4.
The intensity data sets for both 1 and 3 were collected with
a SMART CCD detector at room temperature using x-scans.50
For complex 1, a rotating anode was used as the radiation source
and no decay was detected. The intensity was corrected for
Lorentz, polarisation and absorption effects using SADABS.51
Complex 3 crystallised as needle-shaped crystals that were very
thin in one dimension and therefore weakly diffracting. The
data was therefore collected at the I711 beamline52 at the MAX
laboratory in Lund, Sweden, using synchrotron radiation at a
Preparation of [MoO(OMe)(L–O)] 2
In a typical reaction, 1 (10 mg, 0.02 mmol) was dissolved in 5 ml
of methanol and PPh3 (7 mg, 0.03 mmol) was added under ni-
trogen atmosphere, whereupon the solution immediately turned
red. UV-vis (MeOH), kmax/nm: 220, 255, 290 (sh) MS-FAB+
(m/z): 465 (M+). Removal of solvent under reduced pressure
leads to the isolation of a small amount of complex 3 (vide
infra).
˚
wavelength of 1.515 A. The data was collected with 2 s/frame
where each frame scanned −0.2◦ in x. The whole reflection
sphere was collected twice, with the detector at two different 2h
angles (31 and 75◦). The two sets of data, collected directly after
each other, were merged. The intensity data set was corrected
for decay in the incoming beam current when integrating
the reflection using SAINT.53 The intensity data set was also
corrected for Lorentz, polarisation, and absorption effects using
SADABS.51 The structure of 1 was solved by the Patterson
method and the structure of 3 was solved by direct methods
using the SHELXTL97 program package.54 All non-hydrogen
atoms were refined anisotropically.
Preparation of [(l-O){MoO(L–O)}2](PF6)2 3
To a solution of 40 mg (0.070 mmol) of 1 in 5 ml
dichloromethane, 10 mg PPh3 (0.038 mmol, 0.5 eq.) dissolved
in 2 ml dichloromethane was added. The color changed from
yellow to dark purple within 5 min. After 10 min a purple pre-
cipitate started to form and 10 ml diethyl ether was added. The
product [(l-O){MoO(L–O)}2](PF6)2 was isolated by filtration
after 30 min as 26 mg (0.023 mmol, 65% yield) of dark purple
powder. MS-FAB+ (m/z): 848 (M+). UV-vis (MeCN), kmax/nm
(e/M−1 cm−1): 228, 265, 383 (5000 300), 534 (10700 700).
CCDC reference numbers 268641 (1) and 268642 (3).
See http://dx.doi.org/10.1039/b505180k for crystallographic
data in CIF or other electronic format.
Preparation of [MoO2(L–S)]PF6 4
A total of 163 mg [MoO2(acac)2] (0.5 mmol) in 10 ml THF was
added dropwise to a solution of L–SH (180 mg, 0.5 mmol) in
10 ml methanol, resulting in a yellow solution that slowly turned
dark red. The solution was left stirring at ambient temperature
overnight after which the solvent was removed in vacuo to yield
Acknowledgements
This research has been sponsored by the Swedish Research
Council (VR). We thank Prof. Ake Oskarsson for assistance with
˚
3 5 7 0
D a l t o n T r a n s . , 2 0 0 5 , 3 5 6 6 – 3 5 7 1

View Article Online
the collection of the crystallographic data set for complex 3, and
the MAX laboratory for beam time at beamline I711 of the Lund
University synchrotron. We thank Dr Vladimir M. Trukhan for
valuable advice regarding the synthesis of L–OH.
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D a l t o n T r a n s . , 2 0 0 5 , 3 5 6 6 – 3 5 7 1
3 5 7 1