An update on WII and MoII carbonyl precursors and their application in the synthesis of potentially bio-inspired thiophenolate-oxazoline complexes

Article abstract of DOI:10.1002/zaac.201300036

The synthesis of two new low-valent dicarbonyl complexes of tungsten [W(CO)₂(SPh-oz)₂] (3b) and molybdenum [Mo(CO) ₂(SPh-oz)₂] (4b) coordinated by a bidentate thiolate-oxazoline ligand SPh-oz is reported. The thereby created coordination via two anionic sulfur atoms can be seen as biologically inspired as it reflects the first coordination sphere of the pterin cofactor found in molybdenum and tungsten enzymes. The in-situ preparation of the lithium salt Li(SPh-oz) (1) was described previously in literature, but analytical data was not available. A similar situation was found for the published syntheses of the low-valent metal precursors [W(CO)₃(PPh₃)₂Cl₂] (2a), [W₂(CO)₇Br₄] (3a) and [Mo(CO) ₄Br₂] where experimental procedures were unreliable and experimental data was inadequate or missing.To the best of our knowledge this is the first report of a full characterization of the literature known compounds Li(SPh-oz) (1), [W(CO)₃(PPh₃)₂Cl₂] (2a) and [W₂(CO)₇Br₄] (3a). Furthermore the novel tetranuclear Mo-precursor [Mo₄(CO)₇Br₁₀] (4a) was synthesized. The symmetric and asymmetric IR stretching frequencies of its seven carbonyl ligands were calculated using DFT and related to the experimental values. The molecular structures of the novel precursor compounds [W(CO)₂(PPh₃)₂Cl₂] (2a'), [PPh ₃Cl]₂[WCl₆] (2b), [Mo₄(CO) ₇Br₁₀] (4a), the two complexes with the SPh-oz ligand [W(CO)₂(SPh-oz)₂] (3b), [Mo(CO)₂(SPh-oz) ₂] (4b) and a new modification of [W₂(CO) ₇Br₄] (3a) were determined by single-crystal X-ray diffraction analysis. Copyright

Full text of DOI:10.1002/zaac.201300036

ARTICLE
DOI: 10.1002/zaac.201300036
An Update on WII and MoII Carbonyl Precursors and Their Application in
the Synthesis of Potentially Bio-Inspired Thiophenolate-Oxazoline
Complexes
Lydia M. Peschel,[a] Jörg A. Schachner,[a] Chris H. Sala,[a] Ferdinand Belaj,[a] and
Nadia C. Mösch-Zanetti*[a]
Keywords: WII precursors; MoII precursors; Oxazoline; Bidentate S,N-donor
Abstract. The synthesis of two new low-valent dicarbonyl
complexes of tungsten [W(CO)2(SPh-oz)2] (3b) and molybdenum
[Mo(CO)2(SPh-oz)2] (4b) coordinated by a bidentate thiolate-oxazol-
ine ligand SPh-oz is reported. The thereby created coordination via
two anionic sulfur atoms can be seen as biologically inspired as it
reflects the first coordination sphere of the pterin cofactor found in
molybdenum and tungsten enzymes. The in-situ preparation of the
lithium salt Li(SPh-oz) (1) was described previously in literature, but
analytical data was not available. A similar situation was found
for the published syntheses of the low-valent metal precursors
[W(CO)3(PPh3)2Cl2] (2a), [W2(CO)7Br4] (3a) and [Mo(CO)4Br2]
where experimental procedures were unreliable and experimental data
was inadequate or missing.
To the best of our knowledge this is the first report of a full
characterization of the literature known compounds Li(SPh-oz) (1),
[W(CO)3(PPh3)2Cl2] (2a) and [W2(CO)7Br4] (3a). Furthermore the
novel tetranuclear Mo-precursor [Mo4(CO)7Br10] (4a) was synthe-
sized. The symmetric and asymmetric IR stretching frequencies of its
seven carbonyl ligands were calculated using DFT and related to the
experimental values. The molecular structures of the novel precursor
compounds [W(CO)2(PPh3)2Cl2] (2a’), [PPh3Cl]2[WCl6] (2b),
[Mo4(CO)7Br10] (4a), the two complexes with the SPh-oz ligand
[W(CO)2(SPh-oz)2] (3b), [Mo(CO)2(SPh-oz)2] (4b) and a new modifi-
cation of [W2(CO)7Br4] (3a) were determined by single-crystal X-ray
diffraction analysis.
site as potentially bio-inspired models. We found such a ligand
in the literature known lithium thiophenolate oxazoline
Li(SPh-oz) (1).
Introduction
Molybdenum and tungsten dependent enzymes are abundant
and the molecular structure of many of them has been re-
solved. With the exception of nitrogenase, they all transfer
oxygen to or from the substrate. Common to all known oxygen
transferring enzymes is the coordination of the metal ion by
one or two dithiolene moieties of the molybdopterin (MPT)
cofactor. The central Mo/W atom has oxidation numbers rang-
ing from +IV to +VI.[1–3] Dithiolene ligands have been widely
used as ligand environment analogues to mimic the immediate
coordination sphere of the MPT cofactor.[3,4] A variety of other
ligands were investigated for the development of functional
biomimetic models for OAT enzymes.[2,5] Furthermore, the ni-
trogen rich environment found in the active sites of many other
enzymes may in part be mimicked by functional groups such
as imidazole or oxazoline. Therefore, we decided to develop
molybdenum and tungsten complexes with a ligand that con-
tains both an oxazoline heterocycle and a sulfur coordinating
Few examples of transition metal complexes containing the
ligand SPh-oz have been reported previously, none of them
with molybdenum or tungsten. The cadmium and zinc com-
plexes [Cd(SPh-oz)2] and [Zn(SPh-oz)2] have been obtained
from a metathesis reaction of anhydrous MCl2 with two equiv-
alents of 1.[6] [Hg(SPh-oz)2] has been realized by oxidative
addition of the disulfide S2(Ph-oz)2 to elemental mercury.[6]
Furthermore, the preparation of the iron complexes
[Fe(SPh-oz)2] and [(SPh-oz)Fe-(μS)2-Fe(SPh-oz)] has been
described recently.[7] However, we were surprised to find that
in all of these publications no analytical data for the ligand
Li(SPh-oz) (1) was reported.
The introduction of a thiophenolate ligand into a high-oxi-
dation state metal complex is often hindered by the easy oxi-
dation of the thio ligand to a disulfide under concomitant re-
duction of the metal. Hence, we searched for low oxidation
state tungsten and molybdenum precursors which can poten-
tially be oxidized in a subsequent step. Two possible precur-
sors of the general type [MII(CO)nX2] (M = Mo, W; n = 2, 3, 4,
X = Cl, Br, I) have been described in literature. The phosphine-
stabilized, 18-electron species [M(CO)3(PPh3)2X2] (M = Mo,
W; X = Cl, Br, I)[8–13] and the less stable 16-electron species
[M(CO)4X2] (M = Mo, W; X = Cl, Br, I).[10,12–17] In
particular, the WII dichloride precursor [W(CO)3(PPh3)2Cl2]
* Prof. Dr. N. C. Mösch-Zanetti
Fax: +43-316-380-9835
E-Mail: nadia.moesch@uni-graz.at
[a] Institut für Chemie
Karl-Franzens-Universität Graz
Stremayrgasse 16
8010 Graz, Austria
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300036 or from the au-
thor.
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(2a)[8–10] and the WII dibromide [W(CO)4Br2][14,16,18] were
Step b, the insertion of sulfur into the Li–C bond in diethyl
identified as suitable starting materials. Such precursors lead ether has been described to take one to three hours,[6,24] or
to tungsten carbonyl complexes which are generally stable and overnight.[7] We found that stirring overnight has the advan-
relatively easy to handle. Furthermore such MII carbonyl com- tage of Li(SPh-oz) (1) precipitating from the reaction mixture
plexes can be oxidized under relative mild conditions to yield and thereby simplifying work-up. 1 could be obtained in 49%
the biological relevant MIV species. Tertiary amine oxides, like isolated yield after centrifugation and removal of the superna-
pyridine N-oxide and trimethyl N-oxide, have been shown to tant liquid. The obtained microcrystalline pale yellow powder
transfer their oxygen atom quickly and relatively cleanly to 1 exhibited good purity as assessed by 1H and 13C NMR spec-
metal centres.[19,20] Other compounds that have been shown to troscopy. In the H NMR spectrum the four aromatic protons
1
transfer oxygen include [Mo2O3(S2P(OEt)2)4], Me2SO, Ph3PO, are baseline separated, resonating between 7.7–6.6 ppm. The
Ph3AsO, Ph2SeO, PhNN(O)Ph and more recently meta-chloro- ortho substitution is reflected in the coupling pattern of dd-
peroxybenzoic acid.[19,21]
ddd-ddd-dd which is clearly visible. The CH2- and CH3- group
resonances show, as expected, sharp singlets in a ratio of 1:3.
In contrast the in-situ product obtained from a one batch reac-
tion in diethyl ether was impure, containing, amongst other
compounds unreacted Ph-oz and S2(Ph-oz)2. In the direct inlet
EI mass spectrum not only the [M+] signal of the lithiated
species 1 (m/z = 213.2), but also the signals for the dimeric,
trimeric and tetrameric modifications (m/z = 426.4, 639.6,
852.8) could be detected, pointing towards cluster formation.
1 is sparingly soluble in chlorinated solvents and moderately
soluble in acetonitrile. Solutions of 1 in acetonitrile are of a
blue-green to dark green color, even though being pale yellow
as a solid. 1 can be stored for prolonged times under inert
conditions, and only decomposes slowly under ambient condi-
tions.
Results and Discussion
Ligand Synthesis
The thiophenolate oxazoline ligand Li(SPh-oz) (1) was pre-
pared according to a modified literature procedure.[6,7,22] Iso-
lated 1 has been used for metathesis reactions previously, but
a full analytical characterization was unavailable so far. The
preparation of 1 occurs in two steps, as shown in Scheme 1.
Commercially available Ph-oz is first ortho-lithiated and then
sulfur is inserted into the Li–C bond.
Precursor Synthesis
Even though several routes to WII-dihalide precursors
are published, we found their experimental realization
posing a number of difficulties. During the synthesis of
[W(CO)3(PPh3)2Cl2] (2a) according to the literature pro-
cedure[9] it was observed that over-oxidation to WIV or WVI
with concomitant carbonyl loss poses a serious problem, as the
correct stoichiometry of Cl2 is difficult to control when using
condensed Cl2 at –78 °C. Thus by following the literature pro-
tocol[9], an orange, microcrystalline, carbonyl-free, diamag-
netic compound was obtained in large quantities, which could
be identified as [PPh3Cl]2[WCl6] (2b) by single-crystal X-ray
diffraction analysis (see SI). Presumably, excess of Cl2 is re-
sponsible for the almost quantitative formation of 2b, instead
of 2a (Scheme 2). Therefore the rigorous removal of even trace
amounts of chlorine seems vital. Yet, even after vacuum drying
the intermediate [W(CO)4Cl2] for several hours Cl2/HCl fumes
leaving the Schlenk flask could be observed when purged with
argon. Alternatively, the stoichiometric, dropwise addition of
liquefied Cl2 to a solution of [W(CO)6] has been described.[8]
However this method was rejected due to the extensive experi-
mental setup.
Scheme 1. Synthesis of 1; a) nBuLi, pentane, –20 °C to room temp.;
b) S8, diethyl ether, 16 h.
The mechanism of ortho-lithiation of Ph-oz (step a) has
been previously studied in great detail, however a detailed ex-
perimental protocol, including yields, was not published.[23]
Therefore we determined the isolated yields after step a. We
found that the initial ortho-lithiation of Ph-oz is critical and
does not proceed quantitatively with nBuLi, but instead crude
yields of 70 to 80% were obtained. An increase in the base
strength of the organolithium reagent resulted in a significant
decrease of crude Li(Ph-oz). Roughly 44% crude product was
obtained with sec-BuLi and the use of tert-BuLi yielded ap-
proximately 31% crude Li(Ph-oz) (cp. Table 1). This is attrib-
uted to a higher tendency of side reactions under the applied
conditions, due to the increased reactivity of the organolithium
reagent.
a)
Table 1. Comparison of organolithium reagents
.
The targeted precursor 2a could finally be synthesized
under modified conditions, albeit in poor yield. Compared to
McDonald and co-workers[9] the volume of Cl2 was reduced
to half, the reaction time was shortened, residual Cl2 was
evaporated extremely carefully and residual [W(CO)6] was re-
moved by sublimation giving a yellow microcrystalline mate-
rial. IR spectra were recorded before and after sublimation,
b)
Organolithium reagent
Molarity
in
Yield
nBuLi
sec-BuLi
tert-BuLi
1.6
1.4
1.7
hexanes
cyclohexane
pentane
80%
44%
31%
a) Conditions: 1 mL Ph-oz in 20 mL pentane, addition of 1.05 equiv.
LiR at –20 °C, stirring at room temp. for 1 hr., filtration, washing with
cold pentane (2 ϫ 10 mL) b) crude yield Li(Ph-oz).
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Update on WII and MoII Carbonyl Precursors
Scheme 2. Synthesis of 2a and 2b; a) 2 equiv.PPh3, –78 °C b) excess Cl2, 2 equiv. PPh3, –78 °C.
both are in good agreement with literature, showing three CO Table 2. Selected bond lengths /Å and angles /° for 2a’a).
stretches [IR (cm–1): 2011 (s), 1930 (s), 1889 (s)].[8,9,25] From
this material single crystals suitable for X-ray diffraction
analysis could be obtained from a saturated CH2Cl2 solution W1–P1 2.4816(7) P1–W1–P1
at –35 °C. Over the course of three months, the solution pro-
i
W1–C1 1.962(2) C1–W1–Cl1 146.23(7) Cl1–W1–Cl1 80.11(3)
i
W1–Cl1 2.4140(6) Cl1–W1–P1 138.01(2) C1–W1–P1
77.82(7)
82.82(2)
73.48(7)
176.3(2)
i
132.36(3) Cl1–W1–P1
104.36(14) C1–W1–P1
i
i
C1–O1 1.159(3) C1–W1–C1
i
C1–W1–Cl1 96.17(7)
O1–C1–W1
gressively turned red and dark red needles crystallized. Deter-
mination of the molecular structure revealed the 16-electron
species [W(CO)2(PPh3)2Cl2] (2a’, Figure 1) with only two
carbonyl ligands coordinated to tungsten compared to the three
in 2a. The lower coordination number in 2a’ is equal
to the one found in the analogous bromine compound
[W(CO)2(PPh3)2Br2].[26] However, the X-ray structure of the
latter shows an octahedral geometry in contrast to that of 2a’,
in which the tungsten atom is found in an unusual trigonal
prismatic surrounding. All bond lengths in 2a’ are similar to
those in the analogue bromine compound [W(CO)2(PPh3)2Br2]
(Table 2). It is interesting to note that a compound with the
formula [W(CO)2(PPh3)2Cl2] has been described previously.
On the basis of IR data and its diamagnetic behavior it was
suggested to be the dimer [W(CO)2(PPh3)2Cl2]2, which was
obtained by heating 2a in CH2Cl2 under reflux for four days
and was described as light blue solid.[8] The crystals of 2a’ on
the other hand were dark red.
a) Symmetry transformations used to generate equivalent atoms:
1–x, y, 3/2–z.
i)
viously reported by refluxing in 1,2-dichloroethane or dichlo-
romethane until a color change is observed.[26,27] However,
this reactivity has as yet not been mentioned at lowered tem-
peratures.
[W(CO)3(PPh3)2Cl2] (2a) is a relatively air-stable precursor
with a long shelf life, but its synthesis proved to be time-con-
suming and inefficient. Moreover, in subsequent reactions the
coordinated PPh3 has to be removed leading to additional steps
in the work-up protocol. Therefore our focus was moved
towards the phosphine and chlorine-free species [W(CO)4Br2],
which is obtainable by a straightforward oxidation reaction of
[W(CO)6] with Br2 in CH2Cl2.[14,16] Compared to 2a, by using
bromine the stoichiometry of the reagents can be controlled
much easier and therefore the risk of over-oxidation can be
minimized (cp. Scheme 3). However, in our hands, the synthe-
sis according to literature methods[14,16] did not give the ex-
pected monomeric product, but yielded the dimeric species
[W2(CO)7Br4] (3a).[18]
The identity of 3a could be confirmed by NMR, IR spec-
troscopy and X-ray crystallography of single crystals grown
from a saturated CH2Cl2 solution at –35 °C. In contrast to the
published orthorhombic modification Pnma[18], 3a crystallized
in the new monoclinic modification P21/n (see SI). The 13C
NMR spectrum of 3a revealed two broad, weak signals in the
ranges of 208.5–205.8 and 194.9–192.9 ppm (CD2Cl2,
75 MHz) equaling roughly one set of four carbons and one of
three carbons. By raising the reaction temperature from
–78 °C[14,16] or –60 °C[18] respectively, to –20 °C the yield
could be improved significantly (see Exp. Sect.). 3a is moder-
ately soluble in dichloromethane and can be dissolved in coor-
dinating solvents like THF or acetonitrile with concomitant
Figure 1. ORTEP plot of 2a’ showing the atomic numbering scheme.
The probability ellipsoids are drawn at 50% probability level.
The IR spectrum of crystalline of 2a’ displays two formation of W(CO)3(Br)2-solvent adducts. 3a can be synthe-
CO-stretches [IR (cm–1): 1864 (s), 1843 (s)], in contrast sized efficiently and can be stored under inert atmospheres for
to the three found for 2a and the lowered frequencies prolonged times without decomposition. Therefore, 3a was
demonstrate an increased double-bond character of the two re- used subsequently as the starting material of choice.
maining carbonyls. In the bromine analogous compound
A similar procedure was employed for the preparation
[W(CO)3(PPh3)2Br2] the removal of a carbonyl has been pre- of a convenient molybdenum precursor. The oxidation of
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ARTICLE
Scheme 3. Synthesis of 3a; a) CH2Cl2, –78 °C, b) in-situ, –78 °C.
Scheme 4. Synthesis of 4a; a) CH2Cl2, –20 °C, b) in-situ, –20 °C.
[Mo(CO)6] with one equivalent of Br2 at –78 °C yielding
orange [Mo(CO)4Br2] has been published previously.[14,16,17]
It is described to be soluble in dichloromethane and to exhibit,
depending on the medium, three or four carbonyl stretches.[14]
The published methods suggest a monomeric form with two
equatorial bromine atoms in cis-configuration, albeit no X-ray
structure is available. We, however, carried out the synthesis
under conditions as optimized for [W2(CO)7Br4] (3a). The re-
action at –20 °C yielded a brown to purple microcrystalline
solid, which readily precipitated from CH2Cl2. In analogy to
the W compound 3a we expected the formation of a dimeric
form with three bridging bromine atoms.
The identity of the reaction product could be elucidated by
X-ray crystallography of single crystals grown from a saturated
CH2Cl2 solution at –35 °C (Scheme 4). The structure refine-
ment revealed the compound to be a linear, tetrameric molybd-
enum compound of the general formula [Mo4(CO)7Br10] (4a)
(see Figure 2). Selected bond lengths and angles can be found
in Table 3. 4a is the first example of a neutral nonakis(μ2-
halo)-tetra-metal molecule. The arrangement shows approxi-
Figure 2. ORTEP plot of 4a showing the atomic numbering scheme.
The probability ellipsoids are drawn at 50% probability level.
Table 3. Selected bond lengths /Å and angles /° for 4a.
Mo3–Mo4 2.7842(3) Mo3–Br6 2.5643(4) C2–O2
Mo1–C5 1.991(3) Mo3–Br4 2.5882(3) C5–O5
Mo1–Br1 2.6372(4) Mo4–Br5 2.5434(4)
Mo1–Br4 2.7030(4) Mo4–Br7 2.5465(3) Mo3–Br4–Mo1 82.140(11)
Mo2–C4 2.031(3) Mo4–Br6 2.5522(4) Mo4–Br7–Mo3 66.043(10)
Mo2–Br8 2.6607(3) Mo4–Br9 2.6011(4) Mo4–Br9–Mo2 81.299(11)
1.130(4)
1.141(3)
mately Cs symmetry. The central Mo–Mo distance of to exhibit diamagnetic behavior. Despite being a tetramer
Mo3–Mo4 2.7842(3) Å is much shorter than the two other [Mo4(CO)7Br10] (4a) displays similar stability and reactivity
Mo–Mo distances [Mo1–Mo3 3.4772(3) Å, Mo2–Mo4 to 3a. The formation of the tetramer, instead of the dimer is
3.4579(3) Å]. These in turn are distinctly shorter than the attributed to a higher reactivity of [Mo(CO)6] compared to
W1–W2 distance in 3a [3.6799(6) Å] (see SI). The inner part [W(CO)6]. Studies are currently underway to develop milder
of the molecule is composed of face-sharing MoBr6 octahedra. reaction conditions that favor the formation of the MoII dimer.
The two terminal molybdenum atoms have coordination num-
IR spectroscopy of compound 4a revealed three major CO
bers of seven (see SI). The pattern found resembles the molec- frequencies with several smaller shoulders as shown in Fig-
ular structure of [MoBr3] which crystallizes in chains of dis- ure 3. For a better understanding, theoretical calculations using
torted octahedra.[28,29] In [MoBr3] bridging also occurs via DFT methods were performed. The experimentally found CO
three bromine atoms and every second Mo–Mo distance is sig- frequencies of 4a were compared to those calculated for the
nificantly shorter (2.92 vs. 3.14 Å). This and the rather low published monomeric structure [Mo(CO)4Br2],[14,16,17] the tar-
magnetic susceptibility (μexptl. = 1.2 BM, μtheor. = 3.87 BM) of geted dimeric [Mo2(CO)7Br4] and the obtained tetrameric
[MoBr3] hint at the presence of a Mo–Mo bond at the shorter [Mo4(CO)7Br10] (4a) form. Figure 3 shows the CO region of
bridges.[29]
the measured spectrum and the calculated, scaled IR fre-
The resolved X-ray structure points to an oxidation number quencies of the monomeric, dimeric and tetrameric form.
of +II for Mo1 and Mo2 (chain end) and of +III for the inner Table 4 presents the analysis of the given IR signals. The mo-
molybdenum atoms. Due to the short Mo3–Mo4 distance a nomeric form displaying a distorted octahedral geometry
Mo–Mo bond was assigned and thus 4a was expected shows four IR signals matching two of the three measured IR
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Update on WII and MoII Carbonyl Precursors
Figure 3. Carbonyl region of the measured and calculated, scaled IR spectra, of the possible monomeric [Mo(CO)4Br2], dimeric [Mo2(CO)7Br4]
and tetrameric [Mo4(CO)7Br10] (4a) products.
Table 4. Analysis of the IR signals presented in Figure 3.
Wave number /cm–1
IR Intensity /km·mol–1
Vibrational mode[a]
Monomer
Dimer
2020
2028
2043
2109
530.4
851.0
1438
126.7
AS stretch of equatorial CO
AS stretch of axial CO
AS stretch all four CO
S stretch all four CO
1977
1987
2023
2033
2038
2049
2100
1004
390.3
812.2
958.6
182.3
1179
AS stretch of C6 & C7
AS stretch of C5 & C6/C7
AS stretch of C2 & C3
AS stretch of C1/C3 & C4
AS stretch of C1/C3 & C2/C4
S stretch of C5, C6, C7
S stretch of C1, C2, C3, C4
720.3
Tetramer
1979
1986
2031
2034
2042
2050
2103
1000
454.0
760.6
923.9
183.2
1451
1182
AS stretch of C6 & C7
AS stretch of C5 & C6/C7
AS stretch of C2 & C3
AS stretch of C1/C3 & C4
AS stretch of C1/C3 & C2/C4
S stretch of C5, C6, C7
S stretch of C1, C2, C3, C4
a) Bold carbonyls: main contribution to respective vibration; AS: asymmetrical, S: symmetrical, /: same motion in vibration.
bands. The calculated spectra of the dimeric and tetrameric low carbon value (found: C 5.28%; calcd.: C 6.10%) excludes
form are almost identical. The IR modes of the dimeric and the material to be the dimeric compound [Mo2(CO)7Br4]
tetrameric geometry fit to all three experimental bands. Hence (calcd.: C 11.88%).
the computational results support the asymmetric structure of
4a similar to the corresponding tungsten complex 3a. Further-
more, the M–CO bond lengths were calculated and found to
Coordination Compounds
be in good agreement with those of the X-ray crystal structure
(max. rel. deviation: 1.5%; cp. SI).
The novel complexes [W(CO)2(SPh-oz)2] (3b) and
[Mo(CO)2(SPh-oz)2] (4b) could be synthesized from the re-
The microcrystalline material of 4a which readily precipi- spective MII bromine precursor 3a/4a and Li(SPh-oz) (1) in a
tated from the reaction mixture was analyzed by 13C NMR straightforward metathesis reaction (see Scheme 5). A solution
spectroscopy. We were not able to detect 13C NMR resonances of the ligand in acetonitrile was slowly added to the dissolved
after measuring 53 hours on a 500 MHz spectrometer, which precursor at room temp. The respective precursors were dis-
is in contrast to the tungsten species 3a. The elemental analysis solved in acetonitrile in order to break up the μ-bromo bridges
of the bulk material indicates it to be mainly the tetrametic prior to the reaction, as evidenced by CO evolution. Extraction
compound [Mo4(CO)7Br10] (4a), although the determined val- with toluene, followed by recrystallization from a concentrated
ues are slightly too low indicating the presence of some impu- toluene solution yielded orange (3b) or purple (4b) crystals.
rities which we were as yet unable to remove. However, the Satisfying yields (47%) could be achieved for 3b, whereas 4b
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ARTICLE
could only be obtained in 23% yield. The novel compounds Table 6. Both compounds exhibit very similar structural prop-
were characterized by NMR and IR spectroscopy, elemental erties. The metal atoms are coordinated in a distorted octahe-
analysis and X-ray crystallography. Interestingly, the 13C NMR dral arrangement by two, symmetry-equivalent SPh-oz ligands.
spectra reveal rather deshielded carbonyl carbons, resonating The two CO ligands are cis orientated towards one another, as
at δ = 244.7 and 246.5 ppm, respectively. The IR resonance expected due to the strong trans-effect of carbonyls. The
frequencies of 1917, 1807 and 1935, 1831 cm–1 indicate re- weaker coordinating nitrogen lone pair takes up the positions
duced triple bond character of the carbonyl ligands. The tung- trans to the CO ligands. A Cambridge Structure Database se-
sten species exhibits slightly lower triple bond character and arch revealed that there are only two other published six-coor-
slightly less deshielded carbonyl carbons hinting that tungsten dinate, dicarbonyl compounds with two sulfur and two nitro-
is capable of stronger π-back bonding than molybdenum (cp. gen atoms coordinated to tungsten. However neither has a bi-
Table 5). On the other hand the C=N stretching frequencies of dentate S,N-donor ligand like 3b. One structure shows octahe-
3b and 4b are virtually identical, resonating at 1593 and dral coordination as well, and the other exhibits an unusual
1592 cm–1 respectively. This indicates a reduction of the trigonal prismatic coordination.[30,31] In the case of molybd-
double bond character compared to the free ligand where the enum, 4b is the first example of a dicarbonyl complex coordi-
C=N resonance can be found at 1627 cm–1. Both compounds nated by two bidentate S and/or N donor ligands. There are
are soluble in polar organic solvents, solutions are relatively three other six-coordinate dicarbonyl compounds displaying a
air stable, but slowly decompose after several days. Microcrys- similar first coordination sphere, but using mono- and biden-
talline powders of 3b and 4b can be exposed to air for several tate ligands.[32,33] The deviation from ideal octahedral geome-
weeks without decomposition.
try is reflected, amongst other, by the angle of the two cis
oriented carbonyl ligands, which is C1i)– W1–C1 = 77.46(8)°
and C1–Mo1–C1i) = 77.97(6) respectively. In 3b and 4b all
M–C, M–S and M–N distances lie within the expected range
of other published structures.[30–33] However, the Mo–C,
Mo–S and Mo–N bonds are somewhat longer than the corre-
sponding W–X bonds.
Scheme 5. Synthesis of 3b and 4b; a) acetonitrile, room temp.
Conclusions
Within this paper we report on the synthesis of two new
tungsten and molybdenum complexes 3b and 4b coordinated
by a thiolate-oxazoline ligand. This ligand creates an anionic
sulfur coordination sphere around the metal atom, which ren-
ders 3a and 4b starting points for bio-inspired model com-
plexes of enzymes like W-dependent Acetylene Hydratase or
Mo-dependent Oxotransferases. In further studies the oxi-
dation of the MII atoms to the biologically relevant MIV state
Table 5. Comparison of the carbonyl ligands in 3b and 4b.
13C NMR /ppma) IR /cm–1 M1–C1 /Å C1–O1 /Å M1–C1–O1 /°b)
3b 244.7
4b 246.5
1917,
1807
1935,
1831
1.9718(15) 1.1589(19) 4.24
1.9811(11) 1.1538(14) 4.91
a) CD2Cl2; b) Deviation from ideal linear geometry.
Single crystals of 3b and 4b suitable for structural analysis under CO loss will be addressed. We re-investigated the pub-
by X-ray crystallography were obtained from a saturated tolu- lished syntheses for ligand 1 and the metal precursors 2a and
ene solution at –25 °C. Molecular views of 3b and 4b are 3a and added full analytical details where this information was
shown in Figure 4 and selected bond lengths are given in missing so far. The synthesis of the novel tetranuclear molybd-
Figure 4. ORTEP plot of 3b and 4b showing the atomic numbering scheme. The probability ellipsoids are drawn at 50% probability level.
1564
www.zaac.wiley-vch.de
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 1559–1567

Update on WII and MoII Carbonyl Precursors
Table 6. Selected bond lengths /Å and angles /° for 3b and 4ba).
M = W
M = Mo
M = W
M = Mo
M = W
M = Mo
i
i
M1–C1
M1–N13 2.2190(12)
M1–S1
C1–O1
1.9718(15)
1.9811(11)
2.233–3(9)
2.3669(3)
1.1538(14)
C1–M1–N13
172.47(5)
170.645(17)
77.46(8)
97.12(5)
88.73(6)
172.37(4)
171.795(14)
77.97(6)
96.46(4)
89.54(4)
C1–M1–S1
C1–M1–S1
92.48(4)
94.82(4)
82.69(3)
90.61(3)
175.76(13)
92.18(3)
94.20(3)
82.95(2)
91.21(2)
175.09(10)
i
S1–M1–S1
C1–M1–C1
i
2.3636(4)
1.1589(19)
N13–M1–S1
N13–M1–S1
O1–C1–M1
i
C1–M1–N13
i
N13–M1–N13
i)
a) Symmetry transformations used to generate equivalent atoms: 1–x, y, 3/2–z.
structure refinement program. Full-matrix least-squares on F2 was em-
ployed as refinement method. Further details on the solution of the
structures can be found in the SI. Crystallographic data (excluding
structure factors) for the structures of the compounds reported herein
have been deposited with the Cambridge Crystallographic Data Center
as supplementary publication numbers. The following identification
numbers have been assigned: CCDC-916659 (2a‘), CCDC -916658
(2b), CCDC -916660 (3a), CCDC -916661 (3b), CCDC -918776 (4a)
and CCDC -916662 (4b). Copies of the data can be obtained free of
charge on application to The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, U. K. [Fax: (international)þ44-1223/336-033;
E-mail: deposit@ccdc.cam.ac.uk].
enum precursor 4a could be achieved. Further studies are un-
derway towards monomeric or dimeric modifications of 4a in
order to increase the atom efficiency of subsequent reactions.
Experimental Section
General Procedures. If not stated otherwise, all manipulations were
performed under an atmosphere of Ar employing standard Schlenk and
glovebox techniques. Chlorine 2.8 was dried with P4O10 and H2SO4.
Ph-oz (4,4-dimethyl-2-phenyl-2-oxazoline) was degassed prior to use
and sulfur was recrystallized from boiling toluene. All other chemicals
were purchased from commercial sources and were used without fur-
ther purification. Solvents were purified via a Pure Solv Solvent Purifi-
cation System. All NMR spectra were measured on a Bruker Avance
III 300 MHz spectrometer. 1H and 13C NMR spectroscopy chemical
shifts are given in ppm and are referenced to residual protons in the
solvent. Spectra were obtained at 25 °C. Solid state IR spectra were
measured with a Bruker ALPHA-P Diamant ATR-FTIR spectrometer
at a resolution of 2 cm–1. Solution IR spectra were measured using a
Bruker ALPHA-T probe head at a resolution of 2 cm–1. Signal inten-
sities are assigned according to their relative intensities as strong (s),
medium (m). Weak or very weak resonances are omitted. All IR-bands
are listed in cm–1. Mass spectra were recorded with an Agilent Tech-
nologies 5975C inert XL MSD instrument using the direct insertion
technique.
Li(SPh-oz) (1)[7], [W(CO)3(PPh3)2Cl2] (2a)[9] and [W2(CO)7Br4]
(3a)[14,16,18] were prepared by modifications of the respective literature
procedures.
Synthesis of Li(SPh-oz) (1): 4,4-dimethyl-2-phenyl-2-oxazoline (Ph-
oz, 1 mL, 98 mmol) was dissolved in pentane (20 mL), the solution
was cooled to –20 °C (NaCl/ ice) and nBuLi (1.6 m in hexanes,
3.7 mL, 1.0 mmol) was added dropwise. After stirring the reaction
mixture for 1 h at room temp. the supernatant liquid was removed. The
white precipitate was washed with cold pentane (2 ϫ 10 mL) to yield
crude Li(Ph-oz) (0.809 g, 4.5 mmol, 80%). Li(Ph-oz) was suspended
in diethyl ether and S8 (0.143 g, 4.5 mmol) was added. Stirring over-
night, centrifugation and removal of the supernatant liquid yielded 1 as
pale yellow, microcrystalline solid. Yield (overall): 0.583 g (2.7 mmol,
1
49%) H NMR (300 MHz, CD3CN): δ = 7.64 (dd, J = 8.0, 1.7 Hz, 1
Elemental analyses were carried out at the Graz University of Tech-
nology using a Heraeus Vario Elementar automatic analyzer (2a/b, 3b,
4a/b) and at the Microanalytical Laboratory at the University of Vi-
enna using a EuroVector EA3000 (4a). The Br contents of 3a and 4a
were measured at the Microanalytical Laboratory at the Universtity of
Vienna by digestion and subsequent titration against a 0.1 n AgNO3
solution. The tungsten and molybdenum content of 3a and 4a was
determined with an Agilent 7500ce ICP-MS instrument in helium col-
lision mode using external calibration. Further details are described in
the supporting information.
H, Ar), 7.48 (dd, J = 8.1, 1.0 Hz, 1 H, Ar), 6.88 (ddd, J = 8.1, 7.0,
1.7 Hz, 1 H, Ar), 6.69 (ddd, J = 8.3, 7.0, 1.4 Hz, 1 H, Ar), 3.99 (s, 2
H, CH2), 1.36 (s, 6 H, 2 CH3). 13C NMR (75 MHz, CD3CN): δ =
165.80, 160.31, 138.83, 131.34, 129.25, 126.98, 118.94, 77.86, 68.71,
28.41. IR: 2966 (m), 1627 (s), 1463 (m), 1355 (m), 1316 (m), 1249
(m), 1206 (m), 1133 (m), 1088 (m), 1039 (s), 969 (m), 768 (m), 728
(s),690 (s),648 (m), 512 (m), 432 (m). EI-MS: m/z 213.2 [M+], 426.4
[M+]2, 639.6 [M+]3, 852.8 [M+]4.
Synthesis of [W(CO)3(PPh3)2Cl2] (2a): [W(CO)6] (2.512 g,
7.1 mmol) was added to a Schlenk flask containing condensed chlorine
(10 mL, acetone/ dry ice). The yellow slurry was stirred for 20 min at
–78 °C. Keeping the reaction vessel at –78 °C, the chlorine was evapo-
rated in vacuo into a trap cooled to –196 °C. PPh3 (3.755 g,
14.3 mmol), dissolved in 50 mL CH2Cl2, was added dropwise over
90 min to the bright yellow residue. After stirring at –78 °C for 15 min,
the reaction mixture was allowed to warm to room temperature, where-
upon a bright yellow solid precipitated. Removal of the solvent in
vacuo yielded crude 2a as yellow microcrystalline solid. Residual
[W(CO)6] was removed by sublimation under vacuum at 40 °C. Yield:
1.504 g (1.7 mmol, 24%) 1H NMR (300 MHz, CD2Cl2): δ = 7.68–
7.27 (m, 30 H). 13C NMR (75 MHz, CD2Cl2): δ = Cq obscured,
136.06–125.89. 31P NMR (121 MHz, CD2Cl2): δ = 14.81, 9.13. IR
(CO): 2011, 1930, 1889. . C39H30Cl2O3P2W (863.35): calcd. C 54.26,
Computational Details: Input geometries were obtained using Avoga-
dro.[34] DFT-calculations were performed in the gas phase using the
B3-LYP functional as implemented in TURBOMOLE.[35] Structures
were pre-optimized using the standard double-ζ quality basis set def2-
SVP.[36,37] Vibrational analysis was performed on structures optimized
with a triple-ζ quality basis set def2-TZVP.[37,38] Effective core poten-
tials (ecp-28-mwb) and corresponding basis sets were applied to the
molybdenum central atom(s). Infrared signals were scaled by 0.9654
for comparison to experimental data.[39] Plots were obtained using
QtiPlot.[40]
Crystallographic Details: X–ray data collection was performed with a
Bruker AXS SMART APEX 2 CCD diffractometer by using graphite-
monochromated Mo-Kα radiation (0.71073 Å) from a fine-focus sealed
tube at 100 K. SHELXS-97[41] was used as structure solution and H 3.50%; found: C 52.85, H 3.22%.
Z. Anorg. Allg. Chem. 2013, 1559–1567
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1565

L. M. Peschel, J. A. Schachner, C. H. Sala, F. Belaj, N. C. Mösch-Zanetti
ARTICLE
Single crystals of 2a’ suitable for structure determination by X-ray lography were obtained from a saturated solution in CH2Cl2 at –35 °C.
crystallography were obtained from a supersaturated CH2Cl2 solution 13C NMR: No signal could be detected after 53 h on a 500 MHz NMR
at –35 °C. IR 1864 (s), 1843 (s).
spectrometer. IR (CO): 2119, 2054, 1987. C7Br10O7Mo4 (1378.95):
calcd. C 6.10, Br 57.95, Mo 27.84%; found: C 5.28. Br 55.61, Mo
25.61%.
Synthesis of [PPh3Cl]2[WCl6] (2b): Chlorine (approx. 120 mL)
was condensed (acetone/ dry ice) into a Schlenk flask containing
[W(CO)6] (15.438 g, 43.9 mmol). The resulting dark red solution was
stirred for 1 h at –78 °C. Keeping the reaction vessel at –78 °C, the
chlorine was evaporated in vacuo into a trap cooled to –196 °C. PPh3
(23.312 g, 88.9 mmol) dissolved in CH2Cl2 (80 mL) was added drop-
wise over an hour to the dark red residue. After stirring at –78 °C for
30 min, the reaction mixture was allowed to warm to room tempera-
ture. Gas evolution was observed and an orange solid started to pre-
cipitate, which was isolated by filtration and washed with diethyl ether
(3 ϫ 50 mL) to yield 2b as a bright orange microcrystalline solid.
Yield: 38.360 g (39 mmol, 88%). Single crystals of 2b suitable for
structure determination by X-ray crystallography were obtained by
slow evaporation of CH2Cl2 at room temp. 1H NMR (300 MHz,
CD2Cl2): δ = 7.98–7.74 (m, 30 H). 13C NMR (75 MHz, CD2Cl2): δ =
148.65, 139.25 (d, J = 3.1 Hz), 137.71 (d, J = 13.2 Hz), 136.39, 134.64
(d, J = 14.8 Hz), 131.79, 119.23 (d, J = 93.4 Hz). 31P NMR (121 MHz,
CD2Cl2): δ = 72.55. IR 1436 (s), 1107 (s), 995 (m), 756 (s), 727 (s),
684 (s), 575 (s), 510 (s), 449 (m) . C36H30Cl8P2W (992.03): calcd. C
43.59, H 3.05%; found: C 43.50, H 2.72%
Synthesis of [Mo(CO)2(SPh-oz)2] (4b): Li(SPh-oz) (1) (0.200 g,
0.94 mmol) and [Mo4(CO)7Br10] (4a) (0.168 g, 0.12 mmol) were sepa-
rately dissolved in acetonitrile and the ligand was slowly added to the
precursor. Stirring for 90 min followed by evaporation to dryness
yielded crude 4b, which was further purified by extraction with toluene
and crystallization as its 1:1 toluene adduct at –25 °C. Yield: 0.032 g
(0.06 mmol, 23%) Single crystals of 4b suitable for structure determi-
nation by X-ray crystallography were obtained from a toluene solution
1
at –25 °C. H NMR (300 MHz, CD2Cl2): δ = 7.92 (ddd, J = 11.0, 7.9,
1.4 Hz, 4 H, Ar), 7.49 (td, J = 7.6, 1.6 Hz, 2 H, Ar), 7.34 (td, J = 7.6,
1.3 Hz, 2 H, Ar), 3.85 (d, J = 8.2 Hz, 2 H, CH2), 2.95 (d, J = 8.2 Hz,
2 H, CH2), 0.64 (s, 6 H, CH3), 0.42 (s, 6 H, CH3).b 13C NMR
(75 MHz, CD2Cl2): δ = 246.48, 166.26, 160.19, 131.50, 130.93,
130.11, 125.44, 77.93, 72.21, 27.94, 22.69. IR (neat): ν˜ = 1935,
1831 cm–1 (CO) 1592 (C=N). . C31H33MoN2O4S2 (659.09): calcd. C
56.61, H 5.06, N 4.26%; found C 55.72, H 4.13, N 5.31.
Supporting Information (see footnote on the first page of this article):
Description of the structures, selected bond lengths and angles
and molecular views of compounds [PPh3Cl)[WCl6] (2b) and
[W2(CO)7Br4] (3a). Packing view of [Mo4(CO)7Br10] (4a). Details on
elemental analyses of 3a and 4a. Calculated bond lengths of com-
pounds [Mo(CO)4Br2], [Mo2(CO)7Br4] and 4a.
Synthesis of [W2(CO)7Br4] (3a): At –20 °C (NaCl/ ice) Br2 (0.5 mL,
9.8 mmol) was added dropwise to a suspension of [W(CO)6] (3.130 g,
8.9 mmol) in CH2Cl2 (approx. 50 mL). The solution was stirred at
–20 °C for 2 h after which the resulting dark brown slurry was allowed
to warm to room temp. Removal of the solvent in vacuo yielded crude
3a. Residual [W(CO)6] was removed by sublimation under vacuum at
40 °C. Yield: 3.496 g, (4.0 mmol, 89%). Single crystals of 3a suitable
for structure determination by X-ray crystallography were obtained
from a saturated solution in CH2Cl2 at –35 °C. 13C NMR (75 MHz,
CD2Cl2): δ = 208.5–205.8, 194.9–192.9. IR (CO): 2107, 2022, 2008,
1974, 1966, 1949, 1936. . C7Br4O7W2· 0.1 WBr6 (949.97): calcd. C
8.85, Br 38.70, W 40.65%; found: C 8.67,Br 39.07, W 40.67%.
Acknowledgments
We would like to kindly thank Ass.-Prof. Dr. Georg Raber for the
development of a protocol for the determination of the tungsten and
molybdenum content of 3a /4a using ICP-MS.
Synthesis of [W(CO)2(SPh-oz)2] (3b): Li(SPh-oz) (1) (0.204 g,
0.94 mmol) and [W2(CO)7Br4] (3a) (0.202 g, 0.23 mmol) were dis-
solved in acetonitrile in separate flasks and the ligand solution was
slowly added to the precursor. Stirring for 90 min and evaporation to
dryness yielded crude 3b, which was further purified by extraction
with toluene and crystallization, as its 1:1 toluene adduct, from a super-
saturated toluene solution at –25 °C. Yield: 0.160 g (0.21 mmol, 47%)
Single crystals suitable for structure determination by X-ray crystal-
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