Activation and Photoinduced Release of Alkynes on a Biomimetic Tungsten Center: The Photochemical Behavior of the W?S-Phoz System

Article abstract of DOI:10.1002/chem.201805665

The synthesis and structural determination of four tungsten alkyne complexes coordinated by the bio-inspired S,N-donor ligand 2-(4′,4′-dimethyloxazoline-2′-yl)thiophenolate (S-Phoz) is presented. A previously established protocol that involved the reaction of the respective alkyne with the bis-carbonyl precursor [W(CO)₂(S-Phoz)₂] was used for the complexes [W(CO)(C₂R₂)(S-Phoz)₂] (R=H, 1 a; Me, 1 b; Ph, 1 c). Oxidation with pyridine-N-oxide gave the corresponding W-oxo species [WO(C₂R₂)(S-Phoz)₂] (R=H, 2 a; Me, 2 b; Ph, 2 c). All W-oxo-alkyne complexes (2 a, b, c) were found to be capable of alkyne release upon light irradiation to afford five-coordinate [WO(S-Phoz)₂] (3). The photoinduced release of the alkyne ligand was studied in detail by in situ ¹H NMR measurements, which revealed correlation of the photodissociation rate constant (2 b>2 a>2 c) with the elongation of the alkyne C≡C bond in the molecular structures. Oxidation of [WO(S-Phoz)₂] (3) with pyridine-N-oxide yielded [WO₂(S-Phoz)₂] (4), which shows highly fluxional behavior in solution. Variable-temperature ¹H NMR spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W₄O₄(μ-O)₆(S-Phoz)₄] (5) and dinuclear [{WO(μ-O)(S-Phoz)}₂] (6) over time. The latter two were identified by single-crystal X-ray diffraction analyses.

Full text of DOI:10.1002/chem.201805665

DOI: 10.1002/chem.201805665
Full Paper
&
Bioinorganic Chemistry
Activation and Photoinduced Release of Alkynes on a Biomimetic
Tungsten Center: The Photochemical Behavior of the WꢀS-Phoz
System
+ [a]
[a]
[b]
ˇ+ [a]
Lydia M. Peschel , Carina Vidovic , Ferdinand Belaj, Dmytro Neshchadin,* and
Nadia C. Mçsch-Zanetti*[a]
Abstract: The synthesis and structural determination of four
tungsten alkyne complexes coordinated by the bio-inspired
S,N-donor ligand 2-(4’,4’-dimethyloxazoline-2’-yl)thiopheno-
late (S-Phoz) is presented. A previously established protocol
that involved the reaction of the respective alkyne with the
bis-carbonyl precursor [W(CO)2(S-Phoz)2] was used for the
complexes [W(CO)(C2R2)(S-Phoz)2] (R=H, 1a; Me, 1b; Ph,
1c). Oxidation with pyridine-N-oxide gave the corresponding
W-oxo species [WO(C2R2)(S-Phoz)2] (R=H, 2a; Me, 2b; Ph,
2c). All W-oxo-alkyne complexes (2a, b, c) were found to be
capable of alkyne release upon light irradiation to afford
five-coordinate [WO(S-Phoz)2] (3). The photoinduced release
measurements, which revealed correlation of the photodis-
sociation rate constant (2b>2a>2c) with the elongation
of the alkyne CꢁC bond in the molecular structures. Oxida-
tion of [WO(S-Phoz)2] (3) with pyridine-N-oxide yielded
[WO2(S-Phoz)2] (4), which shows highly fluxional behavior in
solution. Variable-temperature 1H NMR spectroscopy re-
vealed three isomeric forms with respect to the ligand ar-
rangement versus each other. Furthermore, compound 4 re-
arranges to tetranuclear oxo compound [W4O4(m-O)6(S-
Phoz)4] (5) and dinuclear [{WO(m-O)(S-Phoz)}2] (6) over time.
The latter two were identified by single-crystal X-ray diffrac-
tion analyses.
1
of the alkyne ligand was studied in detail by in situ H NMR
Introduction
Tungsten is the only third-row transition metal that occurs in
native enzymes. It is used by the unique W-dependent enzyme
acetylene hydratase (AH) for the catalysis of the net hydration
of acetylene (ethyne, C2H2) to acetaldehyde (CH3CHO).[1,2] The
molecular structure of AH has been solved and shows a WIV
center in a sulfur-rich environment created by two molybdo-
pterin moieties, a cysteine, and an oxygen ligand, which is be-
lieved to be water (Figure 1).[3,4] Several proposals on how the
Figure 1. Active site of acetylene hydratase.[4]
two substrates, water and acetylene, react with each other at
the active site of AH have been discussed. While the debate is
ongoing, DFT and QM/MM calculations favor a first-shell mech-
anism, in which the C2H2 substrate is activated by the WIV
center by formation of a side-on (h2) adduct.[4,5]
ˇ +
[a] Dr. L. M. Peschel,+ C. Vidovic, Dr. F. Belaj, Prof. N. C. Mçsch-Zanetti
Biomimetic model chemistry of Mo- and W-dependent en-
zymes has been extensively explored.[6,7] As the metal center is
held in its active site by coordination to the dithiolene moiety
of the molybdopterin cofactor, complexes with dithiolene-type
ligands (e.g., malonitrile) have been developed.[7,8] Neverthe-
less, non-dithiolene ligands have likewise been introduced,
such as the N3-donor hydridotris(3,5-dimethyl-1-pyrazolyl)bo-
rate) (Tp’),[9] the S2,N-donor 2,6-bis(2,2-diphenly-2-thioethyl)pyr-
idinate,[10] the S,N-donor 2-pyridyldiphenyl-methanethiolate,[11]
or the S2,N2-donor N,N’-dimethyl-N,N’-bis(2-thiolatophenyl)eth-
ylenediamine.[12]
Institute of Chemistry, University of Graz
Schubertstrasse 1, 8010 Graz (Austria)
E-mail: nadia.moesch@uni-graz.at
[b] Dr. D. Neshchadin
Institute of Physical and Theoretical Chemistry
Graz University of Technology
Stremayrgasse 9, 8010 Graz (Austria)
E-mail: neshchadin@tugraz.at
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201805665.
ꢀ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons At-
tribution Non-Commercial NoDerivs License, which permits use and distri-
bution in any medium, provided the original work is properly cited, the use
is non-commercial, and no modifications or adaptations are made.
On the other hand, biomimetic model chemistry specifically
for AH is virtually unexplored. The only structural-functional
model [Et4N]2[WO(mnt)2] (mnt=malonitrile), originally de-
signed to mimic W-dependent aldehyde ferredoxin oxidore-
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ductase, was reported to perform approximately nine turn-
overs of C2H2 hydration.[13] However, a recent report ques-
tioned the reproducibility of catalytic acetaldehyde formation
by [Et4N]2[WO(mnt)2].[14] Furthermore, the malonitrile system
did not allow isolation of WꢀC2H2 adducts, although it was
suggested by theoretical calculations on [WO(mnt)2]2ꢀ.[13,18]
In general, tungsten compounds with a coordinated mole-
cule of acetylene are extremely rare with only three systems
being considered models for AH. Although they were not de-
signed as models, the tungsten dithiocarbamate complexes
[W(CO)(C2H2)(dtc)2] and [WO(C2H2)(dtc)2] (dtc=S2CNMe2,
S2CNEt2) were used to study the coordination of alkynes in
[W(CO)(RC2R’)L2X2][26] (X=Cl, Br, I) with appropriate ligands Ln
or b) by introducing the ligands first to form intermediate
[W(CO)mLn] complexes, and subsequent substitution of CO by
the alkyne. Strategy a) has been described for alkynes with R¼
R’¼H,[27] but as precursors with unsubstituted acetylene
groups are not accessible, only method b) is applicable for the
preparation of C2H2 complexes.
Herein, we report a detailed description of the WꢀS-Phoz
system and its reactivity towards alkynes and oxygen atom
transfer agents and show that the alkyne scope can easily be
extended from ethyne to 2-butyne (dimethylacetylene, C2Me2)
and 1,2-diphenylethyne (diphenylacetylene, C2Ph2). This brings
further insights into the photochemistry of alkyne complexes
and helps to reveal the basic underlying principles.
general
(Figure 2,
left).[17,19]
Scorpionate
complexes
[W(CO)(C2H2)Tp’I], [WO(C2H2)Tp’I], and [WO(C2H2)(H2O)Tp’][OTf]
were found to form tungsten-vinyl and -vinylidene compounds
(Figure 2, middle).[16,20,21]
Results and Discussion
Synthesis of alkyne complexes
[W(CO)(C2Me2)(S-Phoz)2] (1b) and [W(CO)(C2Ph2)(S-Phoz)2] (1c)
were synthesized from [W(CO)2(S-Phoz)2][15,23] in yields of 76
and 67%, respectively (Scheme 1). Formation of the product is
accompanied by a gradual color change from deep orange to
green (1b) or brownish-green (1c). The oily crude products
were purified by dissolving them in dichloromethane and stir-
ring with silica. After filtration and precipitation with heptane,
the products were obtained as microcrystalline materials.
Figure 2. Structural models of AH.[15–17]
Recently, we have reported a model system specifically de-
signed for AH by using the bio-inspired S-Phoz ligand (S-
Phoz=2-(4’,4’-dimethyloxazoline-2’-yl)thiophenolate, Figure 2,
right).[15,22,23] The thiophenolate donors are considered to re-
semble the anionic S-donors of the natural pterin cofactor. Ox-
azoline moieties are naturally occurring[24] and have been
shown to be versatile in transition-metal coordination chemis-
try and catalysis.[25] This fruitful combination of the biologically
relevant S-donor and the versatile oxazoline allowed the devel-
opment of WꢀC2H2 adducts to include the WIV-oxo-ethyne
compound with the metal being in the biologically relevant
+IV oxidation state, which was established for the first time
by X-ray diffraction analysis. Furthermore, with [WO(C2H2)(S-
Phoz)2], we could demonstrate reversible activation of acety-
lene by light,[15] a process which has not been reported previ-
ously.
Under the exclusion of light, oxidation with pyridine-N-oxide
(PyNO) yielded the desired oxo compounds [WO(C2Me2)(S-
Phoz)2] (2b) and [WO(C2Ph2)(S-Phoz)2] (2c) in a fast reaction,
which was evidenced by CO evolution, a color change to
yellow, and a characteristic pyridine odor. The microcrystalline,
pale yellow products were obtained after recrystallization in 65
and 62% yield, respectively. Compounds [W(CO)(C2H2)(S-
Phoz)2] (1a) and [WO(C2H2)(S-Phoz)2] (2a) were both prepared
as previously reported.[15] Under an inert atmosphere, all com-
pounds are stable for several months without decomposition.
The CO compounds 1a–c are highly soluble in polar solvents,
especially in dichloromethane, and in toluene. The solubility of
the oxo compounds 2a–c is significantly lowered. Additionally,
they were found to be only sparingly soluble in acetonitrile,
which can be exploited during workup. As previously observed
for [W(CO)(C2H2)(S-Phoz)2] (1a),[15] NMR spectroscopy in CD2Cl2
revealed the presence of two isomers of [W(CO)(C2Me2)(S-
Phoz)2] (1b, 5:95) and [W(CO)(C2Ph2)(S-Phoz)2] (1c, 45:55) with
respect to the relative orientation of the S,N-ligand (Scheme 1).
For complex 1b, a clear preference for the N,N-trans configura-
In general, mononuclear compounds of the type
[W(CO)(RC2R’)Ln] can be obtained through two synthetic
routes: a) prevailingly, by reacting precursors of the type
[W(CO)(RC2R’)2(MeCN)X2],
[W(CO)(RC2R’)2(2,2’-bipy)X]X,
or
Scheme 1. Syntheses: i) 5.0 equiv. C2Me2/C2Ph2 or >2.0 equiv. C2H2, CH2Cl2, heated at reflux, isomeric mixtures in solution; ii) 1.1 equiv. PyNO, CH2Cl2, RT.[15]
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tion was observed, whereas for complex 1c, a highly dynamic
behavior was demonstrated (see below). In the H NMR spectra
intact complex 2a. Also, complex 2b was treated with the
aqueous stock solution, and the reaction was monitored by
1H NMR spectroscopy, but as for complex 2a, hydrolysis pre-
dominantly occurred and no typical multiplet peaks of the
water-addition product could be observed. Although the hy-
dration might occur, the observation of it is hampered by the
fast hydrolysis. Nevertheless, the synthesis of our structural
model is important as it provides much-needed information on
basic tungsten acetylene chemistry in higher oxidation states
and will help the development of future systems that are more
resistant towards the aqueous enzyme conditions.
1
of all WꢀS-Phoz complexes, the oxazoline CH2 resonances are
particularly indicative. In the bis-CO starting material
[W(CO)2(S-Phoz)2], they are found at 3.9 and 2.8 ppm, whereas
in complexes 1b and 1c, a loss of symmetry and significant
deshielding are observed, which shifts the resonances in the
region between 4.3 and 3.8 ppm. The introduction of the oxo
group (2b,c) causes the resonances to spread between 3.9 and
1.9 ppm. In complex 2b, the methyl resonances of 2-butyne
are found at 3.1 and 3.0 ppm, whereas in complex 1b, they
are at 2.9 and 2.5 ppm, which indicates less electron density
on its methyl groups in the oxo compound. The coordination
of the alkyne ligand to the W center hinders rotation around
the Wꢀalkyne bond, and thus, the compounds lose their C2
symmetry. Two-dimensional NMR spectroscopy and 13Cꢀ183W
couplings in the 13C NMR spectra were used to identify the
CꢁC resonances (Table 1). These resonances have previously
been used to estimate the formal alkyne electron donation
number.[27] According to this concept the shifts in the carbonyl
complexes 1b and 1c (211 to 192 ppm) indicate a four-elec-
tron donation. In the case of the oxo species 2b and 2c, the
resonances (148 to 159 ppm) are in the typical region for
three-electron donation in bis-alkyne systems. In rare cases,
shifts as low as 137 ppm have been previously attributed to
four-electron donation;[28] hence, four-electron donation is pro-
posed for these complexes. The drastic shifts are believed to
originate from a reduced ability for p-backdonation from the
W center upon oxidation, which causes a weaker coordination,
and thus, leaves more electron density on the alkyne carbon
atoms.
Dynamic behavior of [W(CO)(C2Ph2)(S-Phoz)2] (1c)
As mentioned above, NMR spectroscopy revealed the presence
of two isomers of [W(CO)(C2Ph2)(S-Phoz)2] (1c) in solution
(45:55 in CD2Cl2). To date, all obtained solid-state structures of
WꢀS-Phoz complexes revealed the S,S-trans or N,N-trans iso-
mers (see below). The conceivable S,N-trans isomer has never
been observed. Several crystals of complex 1c were picked,
and their molecular structure or unit cells and IR spectra were
determined (IR: n˜(CO)=1927 cmꢀ1). All showed that only the
S,S-trans isomer crystallizes as single crystals. Microcrystalline
products, on the other hand, were identified as isomeric mix-
tures (IR: n˜(CO)=1927, 1894 cmꢀ1). Redissolving single crystals
afforded the initially observed 45:55 S,S-/N,N-trans mixture in
CD2Cl2, whereas in other solvents, such as CDCl3, [D6]DMSO,
[D8]THF, C6D6, and [D3]MeCN, isomeric ratios ranging from
20:80 to 47:53 were observed. This suggests a dynamic equilib-
rium in solution that is dependent on the solvent.
Solid-state IR spectra exhibit a single CꢁO stretch for com-
plex 1b (bulk material) and single crystals of complex 1c and
two resonances for complex 1c as bulk material (Table 1, see
below). Oxidation is evidenced by distinct W=O bands at 934
(2b) and 936 cmꢀ1 (2c). The alkyne CꢁC stretches are found
between 1620 and 1500 cmꢀ1 and were assigned only tenta-
tively as the C=N stretch of S-Phoz is found in the same
region.
This behavior highlights the flexibility and adaptability of
the S-Phoz system. The required hemi-lability and capability to
coordinate in k1- and k2-fashion have been demonstrated pre-
viously.[29] Similar steps have been proposed to explain the dy-
namic behavior of dithiocarbamate and dithiophosphinate Wꢀ
alkyne complexes.[17,30] The dynamic behavior seen in complex
1c is also likely to occur in complexes 1a and 1b, although
less pronounced, because a clear preference for one isomer,
most likely N,N-trans, is observed. Nevertheless, it explains why
both isomeric forms of the Wꢀcarbonyl species converge to
the isomerically pure Wꢀoxo species [WO(C2R2)(S-Phoz)2] (2a–
c).
Treatment of the model complex [WO(C2H2)(S-Phoz)2] (2a)
with 1 equivalent of degassed water in CDCl3 (10 mL of a stock
solution of 14 mL H2O in 200 mL CDCl3) resulted in decomposi-
tion of the complex. The added water quantitatively protonat-
ed the ligand to lead to a 2:1 ratio of free ligand HS-Phoz and
Table 1. Selected 13C NMR (CD2Cl2) and IR data of [W(CO)(C2R2)(S-Phoz)2] and [WO(C2R2)(S-Phoz)2] complexes.
13C NMR
IR
Ref.
CꢁC
JWꢀC
CꢁO
JWꢀC
CꢁO
CꢁC
W=O
1a
1b
1c
2a
2b
2c
[W(CO)(C2H2)L2]
[W(CO)(C2Me2)L2]
[W(CO)(C2Ph2)L2]
[WO(C2H2)L2]
[WO(C2Me2)L2]
[WO(C2Ph2)L2]
200.0, 193.1
208.7, 200.2
210.6, 208.9, 203.4, 192.5
152.9, 151.9
152.8, 148.5
53, 2
57, 12
53, 53, 16, 16
28, 36
12, 9
n.d.[a]
235.3
235.5
236.8, 235.7
–
–
–
145
145
140, 140
–
–
–
1889
1880
1927, 1894
–
–
–
1588, 1566
1590, 1570
1588, 1574, 1565
1604, 1597
1599
–
–
–
939
934
936
[15]
–
–
[15]
–
–
151.7, 148.5
1611, 1595
[a] n.d.=not detected after 16000 scans due to low solubility.
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Photoinduced alkyne release
We assume that photodissociation of the alkynes from the
W center is a monomolecular process and depends only on
concentration of the complexes itself. Thus, first-order kinetics
Previously, we have demonstrated the reversible, photoin-
duced release of the C2H2 molecule from [WO(C2H2)(S-Phoz)2]
(2a) to yield [WO(S-Phoz)2] (3) (Scheme 2).[15] Similarly,
[WO(C2Me2)(S-Phoz)2] (2b) and [WO(C2Ph2)(S-Phoz)2] (2c) were
also found to be capable of an analogous alkyne release to
afford complex 3 and C2R2. In all cases, reactivity is evidenced
by a characteristic color change from pale yellow to deep
purple when exposing their solutions to sunlight for 2 min
(Scheme 2). The color change was most pronounced in com-
plex 2b and barely visible in complex 2c. These qualitative ob-
servations suggest a faster alkyne release from complex 2b
than from complex 2a and a very slow release from complex
2c. In agreement with the qualitatively observed alkyne re-
lease rates, [WO(S-Phoz)2] (3) is best synthesized by 2-butyne
release from complex 2b. Yields could be increased from
<20% from [WO(C2H2)(S-Phoz)2][15] to up to 75% by exposing
solutions of complex 2b to light while constantly removing
the liberated 2-butyne by bubbling N2 through the reaction
mixture. As the 14-electron species 3 forms in light but also
decomposes after prolonged exposure, a fast reaction is criti-
cal. The observed difference in alkyne release from the
[WO(C2R2)(S-Phoz)2] (2a–c) complexes was quantitatively as-
1
were applied to describe changes in H NMR spectra during UV
irradiation. Pseudo-first-order photodissociation rate constants
for complexes 2a–c were extracted from the least-square fit of
1
the time-resolved H NMR data (see the Supporting Informa-
tion, Figures S4–S6). In all reactions, a maximum product for-
mation was reached after, at most, 3000 ms of UV irradiation.
This is probably due to the equilibrium between the highly lo-
calized alkyne release (only part of the NMR-active volume was
irradiated) and diffusion along the NMR tube, which continu-
ously supplies new unreacted alkyne adduct. From integration
results, the following rate constants of 1.37ꢁ108 sꢀ1 (2a),
1.97ꢁ108 sꢀ1 (2b), and 4.6ꢁ107 sꢀ1 (2c) were extracted. These
1
in situ H NMR results confirm the qualitatively observed effi-
ciency of 2b>2a>2c. To shed light on the microsecond pho-
toinduced behavior of the Wꢀoxoꢀalkyne complexes, we used
transient absorbance spectroscopy to explore all three com-
pounds at micromolar concentrations. The time profiles of the
alkyne release of complexes 2a–c show an instant change in
absorbance (see the Supporting Information, Figure S7) on the
timescale of our experiment. This indicates a submicrosecond
(few ns) release of the alkyne from the complex upon laser ir-
radiation (355 nm). Additionally, these changes in the absorb-
ance of complexes 2a–c after equienergetic laser pulses corre-
spond to the rates of alkyne release determined earlier by
1H NMR spectroscopy.
1
sessed by in situ H NMR measurements (Figure 3). Samples of
the oxo-alkyne complexes (3.3ꢁ10ꢀ3 molLꢀ1 in CD2Cl2) were ir-
radiated with monochromatic UV light (362 nm) from a high-
pressure mercury lamp. The sample was exposed to the irradia-
1
1
tion for 150 ms and H NMR spectra were recorded immediate-
These H NMR spectroscopy data suggest a correlation be-
ly after the lamp flash. The efficiency of the alkyne release was
monitored by integration of the CH2 resonances.
tween the rate of the photoinduced release and the electronic
properties of the alkyne. The electron-withdrawing properties
Scheme 2. Reversible behavior of the WꢀS-Phoz system towards light; S-Phoz ligand abbreviated for clarity. Left: solutions of complexes 2a, b, and c after
2 min exposure to sunlight; Right: isolated complex 3 (CH2Cl2, 1 mgmLꢀ1).
1
Figure 3. In situ H NMR measurements. Comparison of the efficiency of the alkyne release from [WO(C2R2)(S-Phoz)2] (2a–c) complexes at 362 nm. S-Phoz
ligand abbreviated for clarity.
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of the phenyl substituents in complex 2c are believed to
cause a more pronounced metallacyclopropene character of
the h2-bound alkyne, thereby hindering its release. On the
other hand, the electron-donating qualities of the methyl
groups in complex 2b might demand less backbonding from
the W center to form a stable adduct, which would in turn fa-
cilitate the alkyne release. In the crystal structure of all Wꢀ
alkyne complexes, the CꢁC bonds are significantly elongated
compared with the corresponding free alkyne bonds (C2H2 =
1.198(4),[31] C2Me2 =1.211,[32] C2Ph2 =1.18(6) ꢂ).[33] Comparison
of the elongation of the CꢁC bonds upon coordination to the
W center (molecular structures, see below) and observed pho-
toinduced release suggests a correlation. Only complexes 2a
and 2b (DCꢁC
=0.088 and 0.074 ꢂ, respectively) with
coordꢀCꢁCfree
significant h2-adduct character are adept to a fast photochemi-
cal alkyne release, whereas increasing metallacyclopropene
character, as observed in complex 2c (DCꢁC =0.099 ꢂ), leads to
a significantly slower release. No photoreactivity was observed
in the carbonyl complexes (1a–c), which exhibit a pronounced
metallacyclopropene character. In the literature, several reports
on the photoinduced release of small substrates (“guests”)
from their parent molecules (“hosts, cages”) exist. This principle
is currently under investigation for a wide array of applications,
including targeted drug delivery.[34] The release of small, bio-
logically relevant molecules, such as CO, H2O2, H2S, and NO, by
application of light has been shown.[35] However, a targeted
alkyne release has not been reported so far, which makes our
WꢀS-Phoz system the first example of a fully characterized
system that is capable of a targeted and reversible release of
acetylene, 2-butyne, and diphenylacetylene.
1
Figure 4. CH2 region of variable-temperature H NMR spectra of [WO2(S-
Phoz)2] (4) in CD2Cl2. See the Supporting Information, Figure S1 for full spec-
tra.
trans in the ratio 1:1:0.6. The dd (doublet of doublets) resonan-
ces of their CH2 groups are shown in Figure 4, and their assign-
ment was confirmed by H,1H COSY measurements at ꢀ558C
1
(see the Supporting Information, Figure S2). The signals of the
symmetric complexes were assigned by comparison of these
CH2 shifts to those of isomerically pure S,S-trans [WO(C2H2)(S-
Phoz)2] (2a) and N,N-trans [W(CO)(C2H2)(S-Phoz)2] (1a) because
of the close structural similarity.[15] Thus, the downfield-shifted
set of signals at 4.50 and 4.26 ppm is believed to belong to
the N,N-trans isomer, whereas the set at 3.87 and 2.44 ppm
remain for the S,S-trans isomer. FTIR and EI-MS measurements
further support the formation of the desired [WO2(S-Phoz)2] (4).
The IR spectrum showed two terminal W=O stretches at 923
and 884 cmꢀ1, and the mass spectrum contained characteristic
m/z peaks for [WO2L2] and [WO2L]. Single-crystal X-ray diffrac-
tion analysis ultimately confirmed the compound to be com-
plex 4. Elemental analysis data were hampered by the pro-
nounced sensitivity. The occurrence of all three possible iso-
meric forms in [WO2L2] complexes strongly supports the as-
signment of a mononuclear, octahedral, cis-dioxo structure for
[WO2(S-Phoz)2] (4) in solution. Furthermore, the VT-NMR experi-
ment demonstrates the highly dynamic behavior of the S-Phoz
ligand system, which has been previously observed for
Oxidation products
We were also interested to determine whether the WꢀS-Phoz
system can be oxidized further to WVI, as such information is
highly valuable for the understanding of nature’s choice of oxi-
dation states. Thus, [WIVO(S-Phoz)2] (3) was reacted in diethyl
ether with one equivalent of PyNO for the formation of
[WO2(S-Phoz)2] (4), as shown in Scheme 3. This resulted in an
immediate color change from purple to deep yellow, and after
workup, 40% of an air- and moisture-sensitive microcrystalline
1
product was obtained. The H NMR spectrum at room temper-
ature exhibited only broad signals so that variable-temperature
1
(VT) H NMR experiments were performed. At ꢀ558C three sets
of resonances are observed, which can be assigned to the
three possible isomers of cis-dioxo [WO2(S-Phoz)2] (4): asym-
metric S,N-trans (Figure 4, left) and symmetric S,S- and N,N-
Scheme 3. Behavior of the WꢀS-Phoz system towards oxidation; S-Phoz ligand abbreviated for clarity.
Chem. Eur. J. 2019, 25, 1 – 11 www.chemeurj.org ꢀ 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[W(CO)(C2Ph2)(S-Phoz)2] (1c, see above) and PdIIꢀS-Phoz com-
plexes.[29]
show the necessary degree of lability for further reactions. Fur-
thermore, compounds 4–6 are interesting from a structural
point of view as only four cis-WO2 compounds with S,N or
S2,N2-donor ligands, no [W4O10]-clusters, or WO(m-O)2OW motifs
with S,N-ligands have been described, which hints at the elu-
siveness and instability of these systems.[36]
Over time, [WO2(S-Phoz)2] (4) reacts to two additional ligand-
containing complexes and disulfide (S-Phoz)2, as shown in
1
Scheme 3 and as monitored by H NMR spectroscopy (see the
Supporting Information, Figure S3). We were able to isolate
them in small quantities (see the Supporting Information for
procedure), which allowed their identification as the oxo com-
pounds [W4O4(m-O)6(S-Phoz)4] (5) and [W2O2(m-O)2(S-Phoz)2] (6),
as evidenced by single-crystal X-ray diffraction analyses (see
Molecular structures
1
Alkyne compounds
below). For both compounds, H NMR spectra reveal symmet-
ric, diamagnetic species evidenced by only one set of ligand
signals. Their IR spectra show tungsten oxo and tungsten m-
oxo stretching frequencies. Also, during the synthesis of
[WO(S-Phoz)2] (3), the formation of compounds 5 and 6 is ob-
served.
Molecular structures of the newly presented alkyne com-
pounds (1b, 1c, 2b, and 2c) were determined by single-crystal
X-ray diffraction analysis. Selected bond lengths and angles are
given in Table 2, and molecular views are presented in Fig-
¯
ures 5 and 6. Compound 1c was crystallized in triclinic (P1; de-
The mechanisms for their formation remain speculative, par-
ticularly the source of the two additional O atoms in complex
5. However, it seems to be independent from O2, as exposure
of compounds 2a, 3, and 4 to O2 with and without light only
resulted in the formation of (S-Phoz)2 as the identifiable de-
composition product observed by NMR spectroscopy. This
points toward complex 3 being the source of the additional
oxygen atoms. Isolation of larger quantities of complexes 4–6
is hampered by the fact that they are all light-sensitive, highly
susceptible to hydrolysis, and rearrange in solution.
picted) and cubic (I23) arrangements. All compounds feature
distorted octahedral environments around the W atom. The
center of the alkyne CꢁC bond occupies the sixth position,
which is always located cis to the carbonyl or oxo groups. A
significant loss of triple bond character and linearity is ob-
served in all h2-bound alkynes, which shows that the actual
bonding situation is between the h2-adduct and metallacyclo-
propene resonance structures. All bond lengths, angles, and
The understanding of these oxidation products is important
in the overall context of the dioxygen sensitivity of the
enzyme. The anaerobic enzyme AH loses its acetylene hydra-
tion activity when exposed to O2.[2] Similar sensitivity is also
observed with our complexes. While enzyme activity is re-
stored by strong reducing agents like TiIIIcitrate,[1] our system
irreversibly forms [WVI4O4(m-O)6(S-Phoz)4] (5) and [WV O2(m-O)2(S-
2
Phoz)2] (6) and the disulfide (S-Phoz)2. The investigation of the
oxidation products also provides possible clues on the require-
ment of AH’s natural oxidation state of +IV. In our system, the
higher +VI oxidation state, which is common in Mo-depen-
dent oxygen atom transfer enzymes, leads to labile species,
not only towards acetylene activation but also towards aque-
ous conditions found in the enzyme. The lower +II state af-
forded too tightly bound metallacyclopropanes, which did not
Figure 5. Molecular views of [W(CO)(C2Me2)(S-Phoz)2] (1b, left) and
[W(CO)(C2Ph2)(S-Phoz)2] (1c, right) that show the atomic numbering scheme.
Probability ellipsoids are drawn at the 50% level.
Table 2. Selected bond lengths [ꢂ] and angles [8] of all presented compounds.
CꢁC
WꢀCꢁC
ꢁCꢀR
RꢀCꢁC
WꢀCO
WCꢁO W=O
Wꢀ(m-O)
Ref.
1a N,N 1.327(3) 2.0548(18), 2.0268(17) 1.031(17), 1.031(17) 146.9(16), 122.7(15)
1b N,N 1.314(3) 2.0210(17), 2.0565(17) 1.485(2), 1.482(2) 136.28(17), 139.03(17) 1.9535(19) 1.164(2) –
1.9623(18) 1.160(2) –
–
–
–
–
–
–
–
–
[15]
–
–
–
[15]
-
–
S,S 1.305(6) 2.036(4), 2.057(4)
S,S 1.309(3) 2.0510(19), 2.078(2)
2a S,S 1.268(6) 2.094(4), 2.109(4)
2b S,S 1.285(3) 2.109(4), 2.120(4)
2c S,S 1.297(5) 2.102(2), 2.107(2)
1.470(6), 1.463(6)
1.463(3), 1.461(3)
0.92(3), 0.92(3)
1.494(3), 1.484(3)
1.465(5), 1.471(5)
–
135.4(4), 137.3(4)
142.8(2), 143.0(2)
143(4), 149(3)
143.8(2), 144.0(2)
141.9(4), 141.1(4)
–
1.966(4)
1.949(2)
1.154(5) –
1.155(3) –
1c
–
–
–
–
–
–
–
–
–
–
1.724(3)
1.7148(14)
1.714(3)
1.733(2), 1.728(2)
1.711(16), 1.7264(16) 1.7713(15), 1.9266(7), –
2.1287(15)[a]
4
5
S,N
S,mO –
–
–
–
–
–
–
6
S,mO –
–
–
–
–
–
1.7085(19)
1.9237(16)
–
[a] Representative selection of the six observed bond lengths.
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Oxidation products
Molecular structures of the oxidation products [WO2(S-Phoz)2]
(4), [W4O4(m-O)6(S-Phoz)4] (5), and [W2O2(m-O)2(S-Phoz)2] (6)
were determined by single-crystal X-ray diffraction analysis.
Molecular views are presented in Figure 7, and selected bond
lengths and angles are given in Table 2. Regarding complex 5,
molecular structures of two different single crystals were deter-
mined, which were found to be virtually identical complexes in
different packings. The dioxo compound [WO2(S-Phoz)2] (4)
was found to crystallize in a distorted octahedral geometry.
The two thiophenolate ligands are coordinated in the asym-
metric S,N-trans form with one nitrogen and one sulfur atom
trans to an oxo group (N33-W1-O2 171.86(8)8, S1-W1-O1
164.21(7)8). The WꢀN33 and WꢀS1 bonds trans to the oxo moi-
eties are typically elongated compared with the WꢀN and WꢀS
bonds trans to each other. However, the WꢀO bond lengths
are only slightly influenced by the different trans donors
(1.733(2) and 1.728(2) ꢂ). This isomer is also observed in solu-
tion, where it represents approximately 38% next to the S,S-
and N,N-trans isomers (Figure 4). The occurrence of the asym-
metric isomer is interesting as it has never been observed
before in the WꢀS-Phoz system.[15] Furthermore, in molybde-
num dioxo complexes with various phenolate-oxazoline li-
gands, only symmetric isomers were observed.[39] In [W4O4(m-
O)6(S-Phoz)4] (5), each of the distorted octahedral WVI centers is
surrounded by one bidentate S-Phoz, one terminal oxygen,
and three bridging O atoms. The complex lies around a two-
fold rotation axis through the atoms O10 and O20, which leads
to equal WꢀO bond lengths to these atoms (W1/W1’ꢀO10
1.9266(7) ꢂ, W2/W2’ꢀO20 1.9122(7) ꢂ). The other WꢀO distan-
ces within the W4O6 cluster are rather different. The short dis-
tances (e.g., W1ꢀO12 1.7713(15) ꢂ) are very similar to those of
the terminal oxo groups (W1ꢀO1 1.711(16) ꢂ). The longer ones
(e.g., W2ꢀO12 2.1287(15) ꢂ) are within the typical range of m-
oxo bridges. Thus, tetrameric 5 can be described as a dimer of
[W2O2(m-O)2(S-Phoz)2] dimers (see the Supporting Information,
Figure S17). One W4O6-cluster has been published previously.
Equivalent dimeric–dimer behavior has been observed, which
results in comparable bond lengths and angles.[40] [W2O2(m-
O)2(S-Phoz)2] (6) features distorted square pyramidal WV centers
with the terminal O atom occupying the apex of the pyramid.
Figure 6. Molecular views of [WO(C2Me2)(S-Phoz)2] (2b, left) and
[WO(C2Ph2)(S-Phoz)2] (2c, right) that show the atomic numbering scheme.
Probability ellipsoids are drawn at the 50% level.
Wꢀalkyne distances in complexes 1b and 1c are within the
range previously observed in scorpionate-based Wꢀcarbonyl
complexes.[37] Oxidation of the WꢀCO species results in an in-
creased Wꢀalkyne distance, which corresponds to a shift to-
wards a more h2-adduct-like structure and a less tightly bound
alkyne. In contrast to the previously published C2H2 adduct 1a,
the introduction of the oxo ligand has little (1b) or no (1c) in-
fluence on the alkyne CꢁC and CꢀR bond lengths.[15] The struc-
ture of complex 2b represents only the second example of a
WꢀoxoꢀC2Me2 adduct, next to [WO(Tp’)(H2O)(C2Me2)][CF3SO3],
for which a marginally shorter CꢁC bond and a comparable
Wꢀalkyne distance (1.278(5) and 2.090(3) ꢂ vs. 1.314(3) and
2.0210(17)/2.0565(17) ꢂ) have been reported.[21] In complex 2c,
all bond lengths, angles, and Wꢀalkyne distances are within
the expected range of monomeric WꢀoxoꢀC2Ph2 adducts.[38]
The first coordination spheres of the carbonyl (1b,c) and oxo
species, (2b,c) show two major differences. In the carbonyl
complexes, the alkyne CꢁC bond is arranged parallel to the
CꢁO bond that is typical for group IV, d4 alkyne complexes.[27]
Upon oxidation, a 908 alkyne rotation is observed, which ena-
bles a perpendicular arrangement of the CꢁC bond and the
W=O group. We have observed this behavior previously in the
C2H2 analogue, and it is consistent with literature.[15,21] Addi-
tionally, S,S-trans (C2Ph2) and N,N-trans (C2Me2) isomers were
obtained. The CO compounds are believed to exist as isomeric
mixtures (Scheme 1, see above), whereas all Wꢀoxo com-
pounds were obtained as pure S,S-trans isomers, with the N
atom trans to the oxo group only weakly coordinated.
Figure 7. Molecular views of [WO2(S-Phoz)2] (4, left), [W4O4(m-O)6(S-Phoz)4] (5, middle), and [W2O2(m-O)2(S-Phoz)2] (6, right) that show the atomic numbering
scheme. For tetrameric 5, the ligand’s backbone was omitted for clarity. Probability ellipsoids are drawn at the 50% level.
Chem. Eur. J. 2019, 25, 1 – 11
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was recrystallized from EtOH. 2-Butyne was dried over molecular
sieves. Silica was washed with NEt3 and dried at 1108C prior to use.
The WO(m-O)2OW motif has been described previously, with
most examples featuring five-coordinate W centers, but distort-
ed octahedral arrangements have also been reported.[41] As ob-
served in complex 6, a syn-arrangement of the two terminal
oxo groups seems to be preferred. Acid-catalyzed isomeriza-
tion to the anti configuration has been reported.[42] The
lengths of the terminal W=O and Wꢀ(m-O) bonds observed in
complex 6 are within the expected range.[41–43]
Synthesis of [W(CO)(C2Me2)(S-Phoz)2] (1b)
[W(CO)2(S-Phoz)2] (1.500 g, 2.30 mmol) was dissolved in CH2Cl2
(15 mL) and 2-butyne (0.901 mL, 11.50 mmol) was added. After stir-
ring the mixture at 408C for 15 h, silica (1.5 g) was added and the
mixture was stirred for 10 min. The solution was filtered, concen-
trated, and heptane was added to precipitate the product. After fil-
tration and washing with heptane, the dark green, microcrystalline
product was obtained in 76% yield (1.193 g, 1.76 mmol). Single
crystals suitable for X-ray diffraction analysis were obtained from
Conclusion
By using the S-Phoz ligand, we have successfully developed a
synthetic protocol for the h2-coordination of the alkynes C2H2,
C2Me2, and C2Ph2 on bio-inspired WII-bis-carbonyl centers to
yield [W(CO)(C2R2)(S-Phoz)2] (1a–c). The introduction of an
oxygen atom to yield [WO(C2R2)(S-Phoz)2] (2a–c) allowed
access to the biological oxidation state of WIV and the oxida-
tion state WVI. Additionally, the oxidation causes a decreased
coordination of the alkynes, whilst leaving them significantly
activated for further reactions. This is best reflected in the ca-
pability of the oxo complexes 2a–c to reversibly release their
alkyne from their metal center upon irradiation with UV light
to afford five-coordinate [WO(S-Phoz)2] (3).
The WIV species 3 was found to be prone to further oxida-
tion to [WO2(S-Phoz)2] (4) and ultimate rearrangement to the
WVI tetramer 5 and the WV dimer 6. This behavior is similar to
the oxygen sensitive W center in AH and a good indication as
to why native AH is only active, when present in the +IV state.
The observed cis-dioxo, oxo-(bis-m-oxo), and oxo-tris-(m-oxo)
motifs have remained largely elusive in high-valent W chemis-
try with bio-inspired S-donor ligands. Moreover, we found that
the rate of the reversible photoinduced alkyne release is close-
ly correlated to the molecular structure of the W complex. This
release, to the best of our knowledge, has never been de-
scribed before and opens new possibilities for the design of
novel photoswitchable enzymatic systems. Although the S-
Phoz system is prone to hydrolysis and can therefore only be
considered as a structural model of AH, complex 2a shows the
degree of lability of the acetylene group that is necessary for
catalysis. Thus, their characterization is expected to further
help in the design of a targeted synthesis of biomimetic WIV
and WVI model complexes.
1
CH2Cl2/heptane solutions at ꢀ358C. H NMR (300 MHz, CD2Cl2): d=
7.76 (dd, J=8.0, 1.0 Hz, 1H, Ar-H), 7.65 (dd, J=7.9, 1.3 Hz, 1H, Ar-
H), 7.61 (dd, J=7.9, 0.9 Hz, 1H, Ar-H), 7.43 (dd, J=8.0, 1.4 Hz, 1H,
Ar-H), 7.31 (td, J=7.6, 1.5 Hz, 1H, Ar-H), 7.17 (ddd, J=8.0, 7.2,
1.6 Hz, 1H, Ar-H), 7.07 (td, J=7.9, 1.3 Hz, 1H, Ar-H), 6.89 (ddd, J=
8.3, 7.2, 1.3 Hz, 1H, Ar-H), 4.17 (d, J=8.3 Hz, 1H, CH2), 4.04–3.88
(m, 3H, CH2), 2.90 (s, 3H, C2Me2), 2.46 (s, 3H, C2Me2), 1.67 (s, 3H,
Me), 1.41 (s, 3H, Me), 0.77 (s, 3H, Me), 0.58 ppm (s, 3H, Me);
13C NMR (75 MHz, CD2Cl2): d=235.45 (JWꢀC =144.9 Hz, CꢁO), 208.68
(JWꢀC =57.0 Hz, C2Me2), 200.18 (JWꢀC =11.4 Hz, C2Me2), 169.44 (Cq),
168.14 (Cq), 157.63 (Cq), 154.54(Cq), 134.41 (CHar), 131.95 (CHar),
131.10 (CHar), 130.99 (CHar), 130.63 (2 CHar), 130.04 (Cq), 128.72
(Cq), 123.22 (CHar), 121.81 (CHar), 79.84 (CH2), 79.27 (CH2), 76.12
(Cq), 74.21 (Cq), 28.50 (d, J=1.9 Hz, Me), 27.72 (Me), 27.06 (Me),
26.50 (Me), 20.38 (C2Me2), 18.68 ppm (d, J=1.5, C2Me2); IR: n˜ =
1880 (s, CꢁO), 1590 (s, CꢁC), 1570 (s, CꢁC), 1540 cmꢀ1 (m, C=N);
MS (EI): m/z: 678.3 [M+], 650.3 [MꢀCO]; elemental analysis calcd
(%) for C27H30N2O3S2W (678.51): C 47.80, H 4.46, N 4.13, S 9.45;
found: C 48.13, H 4.56, N 3.97, S 9.31.
Synthesis of [W(CO)(C2Ph2)(S-Phoz)2] (1c)
[W(CO)2(S-Phoz)2] (1.000 g, 1.53 mmol) and diphenylacetylene
(1.362 g, 7.64 mmol) were suspended in CH2Cl2. After stirring the
mixture at 358C for 32 h, silica (1.1 g) was added, and the mixture
was stirred for another 10 min. The solution was filtered, diluted
with heptane (20 mL), and concentrated to precipitate the product.
After filtration and washing with heptane (2ꢁ10 mL) the yellowish
green, microcrystalline product was obtained in 67% yield
(0.822 g, 1.02 mmol). Single crystals suitable for X-ray diffraction
analysis were obtained from CH2Cl2/toluene/heptane solutions at
ꢀ358C or CH2Cl2/heptane solutions at RT (cubic). 1H NMR
(300 MHz, CD2Cl2, two isomers in a 45:55 ratio): d=8.08 (dd, J=
8.2, 1.1 Hz, 1H, Ar-H), 7.88–7.83 (m, 1H, Ar-H), 7.81 (dd, J=8.0,
1.2 Hz, 1H, Ar-H), 7.67 (d, J=6.4 Hz, 1H, Ar-H), 7.59 (d, J=8.0 Hz,
1H, Ar-H), 7.53–6.97 (m, 28H, Ar-H), 6.80–6.71 (m, 1H, Ar-H), 6.53–
6.45 (m, 2H, Ar-H), 4.29 (d, J=15.9 Hz, 1H, CH2), 4.26 (d, J=
15.9 Hz, 1H, CH2), 4.10–3.96 (m, 5H, CH2), 3.85 (d, J=8.3 Hz, 1H,
CH2), 1.84 (s, 3H, Me), 1.72 (s, 3H, Me), 1.45 (s, 6H, Me), 1.34 (s, 3H,
Me), 1.29 (s, 3H, Me), 0.84 (s, 3H, Me), 0.76 ppm (s, 3H, Me);
13C NMR (75 MHz, CD2Cl2): d=236.81 (JWꢀC =139.8 Hz, CꢁO), 235.70
Experimental Section
Information regarding spectral characterization, VT- and in situ
NMR measurements, transient absorbance spectroscopy, and crys-
tallographic data and structure refinement are provided in the Sup-
porting Information. CCDC 1868876 (1b), 1868877 (1c), 1868878
(1c’), 1868879 (2b), 1868880 (2c), 1868881 (4), 1868882 (5),
1868883 (5’), and 1868884 (6) contain the supplementary crystallo-
graphic data. These data can be obtained free of charge by The
Cambridge Crystallographic Data Centre. All manipulations were
performed under an atmosphere of N2 by using standard Schlenk
(JWꢀC =140.4 Hz, CꢁO), 210.58 (JWꢀC =52.4 Hz, C2Ph2), 208.94 (JWꢀC
=
53.0 Hz, C2Ph2), 203.41 (JWꢀC =16.5 Hz, C2Ph2), 192.53 (JWꢀC =15.5 Hz,
C2Ph2), 170.25 (Cq), 169.75 (Cq), 167.50 (Cq), 167.26 (Cq), 158.34
(Cq), 156.02 (Cq), 155.27 (Cq), 154.30 (Cq), 143.36 (Cq), 141.22 (Cq),
141.19 (Cq), 138.56 (Cq), 134.91 (CHar), 132.69 (CHar), 132.41 (CHar),
132.03, 131.75, 131.61, 131.47, 131.36, 131.25 (CHar), 130.97 (CHar),
130.60 (CHar), 130.46, 130.34 (CHar), 129.62, 129.19, 128.98, 128.86,
128.55, 128.49, 128.38, 128.30, 128.05, 127.98, 127.62, 126.71,
126.67, 125.84 (2 CHar), 124.11 (CHar), 123.58 (CHar), 123.17 (CHar),
121.87 (CHar), 80.32 (CH2), 79.83 (CH2), 79.65 (CH2), 78.97 (CH2),
and
glovebox
techniques.
LiS-Phoz,
[W(CO)2(S-Phoz)2],
[W(CO)(C2H2)(S-Phoz)2] (1a) and [WO(C2H2)(S-Phoz)2] (2a) were syn-
thesized according to our previously published protocol.[15,23] Pyri-
dine-N-oxide was sublimated under vacuum. Diphenylacetylene
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76.58 (Cq), 76.10 (Cq), 75.24 (Cq), 74.59 (Cq),28.87 (Me), 28.34 (Me),
28.32 (Me), 27.96 (Me), 27.92 (Me), 27.06 (Me), 26.85 (Me),
24.24 ppm (Me); IR: n˜ =1927 (s, CꢁO), 1894 (s, CꢁO), 1588 (s, CꢁC),
1574 (sh, CꢁC), 1565 (s, CꢁC), 1543 (sh, C=N), 1532 cmꢀ1 (m, C=N);
MS (EI): No assignable signals; elemental analysis calcd (%) for
C37H34N2O3S2W (802.65): C 55.37, H 4.27, N 3.49, S 7.99; found: C
55.47, H 4.43, N 3.27, S 7.66.
O); MS (EI): m/z: 612.3 [MꢀC2Ph2]; elemental analysis calcd (%) for
C36H34N2O3S2W·0.5CH2Cl2 (833.11): C 52.62, H 4.23, N 3.36, S 7.70;
found: C 52.45, H 4.24, N 3.46, S 7.92.
Improved synthesis of [WO(S-Phoz)2] (3)
[WO(C2Me2)(S-Phoz)2] (2b) (0.384 g, 0.58 mmol) was dissolved in
CH2Cl2 (35 mL) in a Schlenk flask equipped with a bubbler, and the
solution was irradiated with light under a continuous N2 flow for
3 h. The reaction mixture was dried, suspended in MeCN (7 mL),
and the insoluble fraction was isolated by filtration. Washing with
MeCN (2ꢁ3 mL) afforded the product in 75% yield (0.252 g,
Dynamic behavior in solution (isomeric ratios): CDCl3 (47:53),
[D6]DMSO (30:70), [D8]THF (20:80), C6D6 (32:68), [D3]MeCN (38:62).
Synthesis of [WO(C2Me2)(S-Phoz)2] (2b)
1
Pyridine-N-oxide (0.048 g, 0.51 mmol) dissolved in CH2Cl2 (1 mL)
was added to a solution of [W(CO)(C2Me2)(S-Phoz)2] (1b) (0.344 g,
0.46 mmol) in CH2Cl2 (3 mL) and stirred at RT. After 1 h, the reaction
mixture was evaporated to dryness and washed with diethyl ether
(3ꢁ5 mL), dissolved in CH2Cl2/heptane, and evaporated to dryness
to yield the product as pale yellow microcrystalline powder
(0.221 g, 0.33 mmol, 65%). Single crystals suitable for X-ray diffrac-
tion analysis were obtained from CH2Cl2/toluene/heptane solutions
0.41 mmol). H NMR (300 MHz, CD2Cl2): d=8.07 (dd, J=7.9, 1.5 Hz,
2H, Ar-H), 7.71 (dd, J=7.7, 1.2 Hz, 2H, Ar-H), 7.11 (dtd, J=19.9, 7.3,
1.4 Hz, 4H, Ar-H), 4.59 (d, J=8.5 Hz, 2H, CH2), 4.39 (d, J=8.5 Hz,
2H, CH2), 1.99 (s, 6H Hz, Me), 1.42 ppm (s, 6H, Me); 13C NMR
(75 MHz, CD2Cl2): d=161.33 (Cq), 159.39 (Cq), 133.57 (2CHarom),
132.79 (CHarom), 123.98 (CHarom), 121.87 (Cq), 78.28 (Cq), 77.85 (CH2),
27.98 (Me), 25.41 ppm (Me); IR: n=1589 (m, C=N), 1569 (m, C=N),
˜
934 cmꢀ1 (s, W=O); MS (EI): m/z: 612.1 [M+]; elemental analysis
calcd (%) for C22H24N2O3S2W·0.3CH2Cl2 (637.88): C 41.99, H 3.89, N
4.39, S 10.05; found: C 41.72, H 3.86, N 4.40, S 10.32.
1
at ꢀ358C. H NMR (300 MHz, CD2Cl2): d=7.83 (d, J=7.8 Hz, 1H, Ar-
H), 7.63–7.50 (m, 2H, Ar-H), 7.45 (dd, J=7.8, 1.2 Hz, 1H, Ar-H), 7.39
(td, J=7.7, 1.5 Hz, 1H, Ar-H), 7.29 (td, J=7.7, 1.6 Hz, 1H, Ar-H), 7.14
(td, J=7.8, 1.1 Hz, 1H, Ar-H), 7.08–7.00 (m, 1H, Ar-H), 3.86 (d, J=
8.1 Hz, 1H, CH2), 3.39 (d, J=8.1 Hz, 1H, CH2), 3.12 (s, 3H, C2Me2),
3.00 (s, 3H, C2Me2), 2.51 (d, J=8.1 Hz, 1H, CH2), 2.00 (d, J=8.1 Hz,
1H, CH2), 1.75 (s, 3H, Me), 1.29 (s, 3H, Me), 0.98 (s, 3H, Me),
0.60 ppm (s, 3H, Me); 13C NMR (75 MHz, CD2Cl2): d=165.62 (Cq),
164.94 (Cq), 154.57 (Cq), 154.46 (Cq), 152.82 (JWꢀC =11.2 Hz, C2Me2),
148.48, (JWꢀC =9.1 Hz, C2Me2), 135.78 (CHar), 134.38 (CHar), 132.02
(CHar), 131.25 (CHar), 129.92 (CHar), 129.64 (CHar), 129.50 (Cq), 128.12
(Cq), 123.72 (CHar), 123.60 (CHar), 77.85 (CH2), 76.66 (CH2), 75.22
(Cq), 71.61 (Cq), 28.73 (Me), 28.16 (Me), 21.84 (Me), 20.95 (Me, d,
J=1.9 Hz), 15.17 (C2Me2), 14.12 ppm (C2Me2); IR: n˜ =1599 (s, CꢁC),
1576 (w, C=N), 1549 (w, C=N), 934 cmꢀ1 (s, W=O); MS (EI): m/z:
Synthesis of [WO2(S-Phoz)2] (4)
Pyridine-N-oxide (0.026 g, 27 mmol) was added to [WO(S-Phoz)2]
(3) (0.035 g, 0.06 mmol) suspended in diethyl ether (4 mL). After
3 min, the yellow suspension was filtered over a pad of Celite. The
yellow product was eluted with CH2Cl2, precipitated with heptane,
and isolated by filtration to afford a yellow microcrystalline powder
in 40% yield. Single crystals suitable for X-ray diffraction analysis
were obtained from CH2Cl2/heptane solutions at ꢀ358C. 1H NMR
(300 MHz, CD2Cl2, RT, three dynamic isomers): d=7.87–6.89 (m, 8H,
Ar-H), 4.38–3.71 (bd, J=30 Hz Hz, 3H, -CH2-), 2.71–2.42 (s, 1H,
-CH2-), 1.87–1.14 ppm (m, 12H, Me); 1H NMR (300 MHz, CD2Cl2,
ꢀ558C, three isomers: S,N-trans/S,S-trans/N,N-trans=1:1:0.6): d=
7.77–7.06 (m, Ar-H), 6.69 (d, J=7.8 Hz, Ar-H), 4.62 (d, J=8.7 Hz,
CH2, S,N-trans), 4.50 (d, J=8.6, CH2, N,N-trans), 4.26 (t, J=9.5 Hz,
-CH2, S,N- and N,N-trans), 3.89 (t, J=9.3 Hz, CH2, S,N- and S,S-trans),
2.76 (d, J=8.1 Hz, CH2, S,N-trans), 2.44 (d, J=8.1, CH2, S,S-trans),
1.88 (s, Me), 1.71 (s, Me), 1.65 (s, Me), 1.40 (s, Me), 1.39 (s, Me), 1.32
(s, Me), 1.27 ppm (s, Me); IR: n˜ =923 (W=O), 884 cmꢀ1 (W=O); MS
(EI): m/z: 628.2 [M+], 612.2 [MꢀO], 422.0 [MꢀL].
612.4
[MꢀC2Me2];
elemental
analysis
calcd
(%)
for
C26H30N2O3S2W·0.4CH2Cl2 (700.48): C 45.27, H 4.43, N 4.00, S 9.15;
found: C 45.35, H 4.42, N 4.08, S 9.31.
Synthesis of [WO(C2Ph2)(S-Phoz)2] (2c)
Pyridine-N-oxide (0.048 g, 0.51 mmol) dissolved in CH2Cl2 (1 mL)
was added to a solution of [W(CO)(C2Ph2)(S-Phoz)2] (1c) (0.407 g,
0.46 mmol) in CH2Cl2 (3 mL) and stirred at RT. After 1 h, the reaction
mixture was evaporated to dryness and washed with diethyl ether
(2ꢁ5 mL). The residue was dissolved in CH2Cl2/heptane and evapo-
rated to dryness to yield the product as yellow microcrystalline
powder (0.225 g, 0.28 mmol, 62%). Single crystals suitable for X-ray
diffraction analysis were obtained from CH2Cl2/toluene/heptane
solutions at ꢀ358C. 1H NMR (300 MHz, CD2Cl2): d=7.86–7.78 (m,
3H, Ar-H), 7.70 (dd, J=8.1, 1.1 Hz, 2H, Ar-H), 7.61–7.37 (m, 9H, Ar-
H), 7.34–7.28 (m, 1H, Ar-H), 7.25 (td, J=7.8, 1.6 Hz, 1H, Ar-H), 7.16
(td, J=7.7, 1.1 Hz, 1H, Ar-H), 7.09–7.02 (m, 1H, Ar-H), 3.93 (d, J=
8.1 Hz, 1H, CH2), 3.43 (d, J=8.1 Hz, 1H, CH2), 2.62 (d, J=8.2 Hz, 1H,
CH2), 2.03 (d, J=8.1 Hz, 1H, CH2), 1.92 (s, 3H, Me), 1.43 (s, 3H, Me),
1.06 (s, 3H, Me), 0.80 ppm (s, 3H, Me); 13C NMR (75 MHz, CD2Cl2):
d=165.85 (Cq), 165.07 (Cq), 163.87 (Cq), 163.13 (Cq), 151.73
(C2Ph2), 148.45 (C2Ph2), 138.52 (Cq), 137.16 (Cq), 135.72 (CHar),
134.20 (CHar), 132.09 (CHar), 131.75 (2 CHar), 131.45 (CHar), 130.84 (2
CHar), 130.06 (CHar), 129.85 (CHar), 129.15 (Cq), 128.85 (2CHar),
128.66 (CHar), 128.49 (2 CHar), 128.24 (CHar), 127.82 (Cq), 123.99
(CHar), 123.79 (CHar), 78.09 (CH2), 76.99 (CH2), 75.67 (Cq), 71.81 (Cq),
29.01 (Me), 28.16 (Me), 22.21 (Me), 20.99 ppm (Me); IR: n˜ =1611 (s,
CꢁC), 1595 (s, CꢁC), 1573 (w, C=N), 1548 (w, C=N), 936 cmꢀ1 (s, W=
Acknowledgements
The authors would also like to thank Bernd Werner for per-
forming VT-NMR spectroscopic measurements. The authors
gratefully acknowledge financial support from the NAWI Graz.
Conflict of interest
The authors declare no conflict of interest.
Keywords: alkyne ligands
·
bioinorganic chemistry
·
photodissociation · sulfur · tungsten
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Manuscript received: November 13, 2018
Revised manuscript received: December 13, 2018
Version of record online: && &&, 0000
&
&
Chem. Eur. J. 2019, 25, 1 – 11
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ꢀ 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Light on/alkyne off: Laser-induced re-
lease of three types of alkynes from
tungsten complexes that contain a bio-
mimetic sulfur ligand is investigated by
various spectroscopic means (see pic-
ture). Kinetic studies support the impor-
tance of the physiological oxidation
state +IV and reveal correlation of the
photodissociation rate constant with
the elongation of the alkyne CꢁC bond.
&
Bioinorganic Chemistry
ˇ
L. M. Peschel, C. Vidovic, F. Belaj,
D. Neshchadin,* N. C. Mçsch-Zanetti*
&& – &&
Activation and Photoinduced Release
of Alkynes on a Biomimetic Tungsten
Center: The Photochemical Behavior
of the WꢀS-Phoz System
Chem. Eur. J. 2019, 25, 1 – 11
www.chemeurj.org
11
ꢀ 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ