Reactions of tetracyclone molybdenum complexes with electrophilic alkynes: Cyclopentadienone-alkyne coupling and alkyne coordination

Article abstract of DOI:10.1021/acs.organomet.7b00300

The reactions of the complexes [Mo(CO)₂(⁴-C₄Ph₄CO)₂] and [Mo(CO)₃(NCMe)(⁴-C₄Ph₄CO)] with the alkynes dimethyl acetylenedicarboxylate (DMAD; RC - CR where R = CO₂Me) and methyl propiolate (RC - CH) have been studied. In the case of DMAD, the initial product is the green carbonyl complex [Mo(CO)(RC - CR)(⁵,σ-C₄Ph₄COCR - CR)] (3), in which two alkyne molecules have been incorporated: one is linked to the carbonyl group of the tetracyclone ligand, whereas the other is -bound to the metal as a four-electron donor. Oxidation of this compound affords yellow [Mo(O)(RC - CR)(⁵,σ-C₄Ph₄COCR - CR)] (8). When the -acceptor carbonyl ligand is replaced by the-donor oxo group, the alkyne ligand changes orientation: it lies parallel to the Mo-CO bond in 3 but perpendicular to the Mo - O group in 8. Analogous complexes (9, 10) were isolated in the case of methyl propiolate; each exists as a mixture of two isomers depending on the orientation of the unsymmetrical alkyne ligand.

Full text of DOI:10.1021/acs.organomet.7b00300

Article
pubs.acs.org/Organometallics
Reactions of Tetracyclone Molybdenum Complexes with
Electrophilic Alkynes: Cyclopentadienone−Alkyne Coupling and
Alkyne Coordination
Harry Adams,† Yvonne K. Booth,† Elizabeth S. Cook,‡ Sarah Riley,† and Michael J. Morris*,†
†Department of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K.
‡Department of Chemistry, University of Manchester, Manchester M13 9PL, U.K.
S
* Supporting Information
ABSTRACT: The reactions of the complexes [Mo(CO)2(η4-
C4Ph4CO)2] and [Mo(CO)3(NCMe)(η4-C4Ph4CO)] with the
alkynes dimethyl acetylenedicarboxylate (DMAD; RCCR
where R = CO2Me) and methyl propiolate (RCCH) have
been studied. In the case of DMAD, the initial product is the
green carbonyl complex [Mo(CO)(RCCR)(η5,σ-
C4Ph4COCRCR)] (3), in which two alkyne molecules
have been incorporated: one is linked to the carbonyl group of
the tetracyclone ligand, whereas the other is π-bound to the
metal as a four-electron donor. Oxidation of this compound
affords yellow [Mo(O)(RCCR)(η5,σ-C4Ph4COCRCR)] (8). When the π-acceptor carbonyl ligand is replaced by the π-
donor oxo group, the alkyne ligand changes orientation: it lies parallel to the Mo−CO bond in 3 but perpendicular to the Mo
O group in 8. Analogous complexes (9, 10) were isolated in the case of methyl propiolate; each exists as a mixture of two isomers
depending on the orientation of the unsymmetrical alkyne ligand.
dppe).6 In these reactions one (and only one) of the
INTRODUCTION
■
tetracyclone ligands of 1 could be displaced by a phosphine.
Originally isolated as low-yield products from the reactions of
Although molybdenum is not traditionally associated with high
metal carbonyls with alkynes,1 complexes containing cyclo-
catalytic activity in hydrogenation reactions, it is interesting to
pentadienone ligands have attracted a substantial amount of
note that Waymouth and co-workers have recently reported
renewed interest in recent years. Among these, ruthenium
that 2 also displays Shvo-type reactivity in transfer hydro-
complexes of tetraphenylcyclopentadienone (commonly re-
genation reactions.7
ferred to as tetracyclone) and in particular Shvo’s catalyst,
Recently we showed that the reaction of 1 with phenyl-
[Ru2(CO)4(μ-H)(μ,η5:η5-C4Ph4COHOCC4Ph4)], have been
acetylene resulted in an unusual cyclotrimerization process to
demonstrated to be active catalysts for a number of important
reactions, such as the transfer hydrogenation of aldehydes,
give an η6-fulvene ligand.8 In this paper we report the reactions
of 1 and 2 with the activated alkynes DMAD (RCCR; R =
ketones, and imines and the dehydrogenation of ammonia−
CO2Me throughout this paper) and methyl propiolate (RC
CH). Part of this work was included in a preliminary
publication.9
borane.2,3 The catalyst functions by reversible dissociation to
the interrelated mononuclear fragments [RuH(CO)2(η5-
C4Ph4COH)] and [Ru(CO)2(η4-C4Ph4CO)], which can act
as hydrogenating and dehydrogenating agents, respectively
(Scheme 1). It is therefore an example of a bifunctional system,
in that one hydrogen is delivered from the metal and the other
from the hydroxy group of the ligand.4 More recently, attempts
have been made to replace ruthenium with cheaper, more
earth-abundant metals such as iron in catalysts developed by
Casey, Beller, Wills, and others.5
RESULTS AND DISCUSSION
■
Reactions with DMAD. Heating a toluene solution of 1
with an excess of DMAD to reflux for 17 h led to the
production of green [Mo(CO)(RCCR)(η5 ,σ-
C4Ph4COCRCR)] (3) in 70% yield as the only organo-
metallic product after separation by column chromatography
(Scheme 2). The organic byproducts included tetracyclone,
hexamethyl mellitate from the cyclotrimerization of DMAD,
and dimethyl tetraphenylphthalate formed by the Diels−Alder
reaction of the liberated tetracyclone with DMAD. Curiously,
Some time ago we reported efficient preparations of several
molybdenum complexes containing tetracyclone ligands,
including [Mo(CO)2(η4-C4Ph4CO)2] (1) and [Mo-
(CO)3(NCMe)(η4-C4Ph4CO)] (2) (Chart 1) and demon-
strated that their reactions with phosphines afforded complexes
of the type [Mo(CO)3L(η4-C4Ph4CO)] (L = PPh3, PPh2Me,
PPh2H) or [Mo(CO)2(L2)(η4-C4Ph4CO)] (L2 = dppm,
Received: April 20, 2017
© XXXX American Chemical Society
A
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Scheme 1. Formation of Catalytically Active Species by Dissociation of Shvo’s Catalyst
Chart 1
Chart 2
in the light of the results obtained below, complex 2 did not
give any tractable products on treatment with DMAD.
Complex 3 was initially characterized by spectroscopic
techniques. The mass spectrum and analytical data established
the loss of one tetracyclone ligand and the incorporation of two
alkyne molecules. The IR spectrum showed a terminal CO
absorption at 2000 cm−1 together with weaker peaks at 1735
and 1720 cm−1 due to the ester groups. The 1H NMR spectrum
revealed the presence of four inequivalent methyl groups in
addition to phenyl peaks due to one tetracyclone ligand. The
main point of interest, however, proved to be the 13C{1H}
NMR spectrum. Two different sets of peaks for the two alkyne
moieties were observed: the CC carbons of the first resonate
at 193.2 and 190.0 ppm and those of the second at 161.1 and
155.2 ppm. Assignment of the spectrum in the 140−200 ppm
for 4-electron-donor alkynes and 115 ppm for 2-electron
donors.10,11 In structure 4 the two alkyne ligands would be
required to donate 6 electrons in total to the Mo(CO)(η4-
C4Ph4CO) unit to achieve an 18-electron configuration, and
even though it would be a Mo(0) complex as drawn, the two
alkynes would probably be equivalent with an average shift of
around 150−180 ppm. The alternative, that one alkyne donates
4 electrons and the other 2, is unlikely, and even then the shift
of the latter would be rather high. In structure 3, however, the
peaks at lower field can be readily assigned to the 4-electron-
donor alkyne, whereas the vinylic carbons would be expected to
appear at the observed value of ca. 150−160 ppm. Second,
previous work has shown that the Shvo complex [Ru2(CO)4(μ-
H)(μ,η5:η5-C4Ph4COHOCC4Ph4)] reacts with alkynes such as
C2Ph2 to afford the related vinyl complex [Ru(CO)2(η5,σ-
C4Ph4COCPhCPh)] (5), in which a similar linking of the
alkyne to the cyclopentadienone has occurred; moreover, the
13C chemical shift of the vinylic carbons in these compounds
was also around 150 ppm.12 Further literature precedent for
such a structure can be found in the products of the reaction
between [Fe2(CO)9] and methyl phenylpropiolate: not only
was cyclopentadienone complex 6 isolated, but so was 7, its
1
region was assisted by recording a H-coupled version; the
alkyne carbons and the carbonyl of the cyclopentadienone ring
remain as singlets, whereas the ester carbonyl groups collapse
to quartets due to coupling with the methyl protons. The
carbon atoms of the cyclopentadienone ring also show coupling
to the ortho hydrogens of their respective phenyl rings (see the
Supporting Information).
In our original communication we proposed structure 4,
[Mo(CO)(RCCR)2(η4-C4Ph4CO)], for the product (Chart
2).9 However, we later realized that structure 3, in which one
alkyne is linked to the cyclopentadienone ring, is more
consistent with the spectroscopic data. For example, it is well
established that in Mo(II) complexes the 13C chemical shifts of
alkyne ligands can be correlated with the number of electrons
donated to the metal; typical values are approximately 200 ppm
Scheme 2. Synthesis of the DMAD Complexes 3 and 8
B
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addition product with further alkyne.13 The nucleophilic nature
of the carbonyl oxygen in cyclopentadienone complexes can be
attributed to the canonical form shown in Chart 3, in which it
bears a negative charge.
The second alkyne is bound to the molybdenum as an η2
ligand; the compound therefore belongs to the well-established
CpM(RCCR)LX class.15,16 Both the CC bond length of
1.312(6) Å and the Mo−C bond lengths (2.031(4) and
2.059(4) Å) are commensurate with this ligand acting as a 4-
electron donor, as required by electron-counting consider-
ations.11 It is also noteworthy that the alkyne lies parallel to the
CO ligand, a point discussed further below.
Chart 3
Although complex 3 is relatively air stable in the solid state,
exposure of a dichloromethane solution to air overnight
resulted in a color change from green to yellow accompanied
by the disappearance of the carbonyl peak in the IR spectrum.
The same transformation can be brought about instantaneously
by dissolving the compound in THF that has been deliberately
exposed to air and sunlight (the active agent presumably being
a peroxide). From the yellow solutions, the corresponding oxo
complex [Mo(O)(RCCR)(η5,σ-C4Ph4COCRCR)] (8)
can be isolated in excellent yield (Scheme 2). The presence
of the MoO group was confirmed by observation of a peak at
937 cm−1 in the solid-state IR spectrum. The 1H NMR
spectrum of 8 is very similar to that of 3, with four inequivalent
methyl groups, but distinct differences are apparent in the 13C
NMR spectrum, where all four alkyne carbons now appear in
the region between 167.2 and 149.7 ppm: i.e., there is an
upfield shift of approximately 40 ppm in the metal-bonded
alkyne. We attribute this to the replacement of the π-acceptor
carbonyl by the π-donor oxo ligand which can compete with
the alkyne ligand for available metal orbitals, as discussed
extensively by Templeton and others.17−20 For example, the
After many years we succeeded in growing crystals of 3
suitable for X-ray diffraction; the structure is shown in Figure 1
with important bond lengths given in its caption. There are two
independent molecules in the unit cell, one of which exhibits
disorder of the carbonyl oxygen of one of the CO2Me groups;
the molecule depicted here (B) is the nondisordered one. The
structure determination confirms the linking of the tetracyclone
carbonyl oxygen to one of the alkyne molecules, creating a
cyclopentadienyl ligand tethered through the vinyl group. The
five-membered ring is bonded to the metal in a slightly tilted η5
manner: the Mo−C bond lengths lie between 2.302(4) and
2.392(4) Å, with those to C(4) and C(5) being the shortest. In
complex 5, the ligand was significantly tilted the opposite way,
with the oxygen-bearing carbon being closest to the Ru. The
bond lengths in the vinylic portion of the ligand are the same,
within experimental error, as those in complex 5.12,14
Figure 1. Single-crystal X-ray structure of [Mo(CO)(RCCR)(η5,σ-C4Ph4COCRCR)] (3). Selected bond lengths (Å): Mo(1B)−C(1B),
2.014(5); Mo(1B)−C(32B), 2.151(4); Mo(1B)−C(35B), 2.031(4); Mo(1B)−C(36B), 2.059(4); C(31B)−C(32B), 1.345(6); C(35B)−C(36B),
1.312(6); O(10B)−C(2B), 1.374(5); O(10B)−C(31B), 1.396(5).
C
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average chemical shift of the alkyne carbons in [W(CO)(HC
CH)(S2CNEt2)2] is 206 ppm, typical for a 4-electron-donor
alkyne, whereas in the analogous oxo complex [W(O)(HC
CH)(S2CNEt2)2] it is 150 ppm: i.e., the alkyne is a much less
effective π donor in the latter.
The X-ray crystal structure of 8 is shown in Figure 2, with
important bond lengths detailed in the caption. The MoO
1.293(11) Å in 8). A more obvious difference is the
reorientation of the alkyne ligand so that it is now
perpendicular to the MoO bond: i.e., it has rotated by 90°
in comparison to its position in the carbonyl complex 3, due to
competition for the available metal d orbitals with the strong π-
donor oxo ligand. The same reorientation of the alkyne ligand
was shown to have taken place in the X-ray crystal structures of
two related complexes, [Mo(CO)(SC6F5)(F3CCCCF3)(η-
C5H5)] and [Mo(O)(SC6F5)(F3CCCCF3)(η-C5H5)],21 and
the alkyne in [W(O)(Ph)(PhCCPh)(η-C5H5)] also adopts
the same geometry.22
Reactions with Methyl Propiolate. Our initial problems
in obtaining crystals for structural characterization from the
complexes derived from DMAD prompted us to explore the
reactions of 1 and 2 with methyl propiolate, because of the
additional information that the CH groups of the alkyne might
1
provide in the H and 13C NMR spectra, especially since the
CH and CR carbons can be readily distinguished in the latter
by use of an attached proton test. This indeed proved to be the
case, though the results were complicated by the presence of
two isomers.
The reaction of complex 1 with an excess of methyl
propiolate in refluxing toluene proceeded in a way similar to the
DMAD reaction above; a green zone could be separated by
chromatography which was shown to contain two isomers of
[Mo(CO)(RCCH)(η5,σ-C4Ph4COCHCR)] (9a,b) to-
gether with a small amount of the corresponding oxo species
[Mo(O)(RCCH)(η5,σ-C4Ph4COCHCR)] (10a,b), which
also exists as two isomers (Scheme 3). Also present in the crude
material was methyl tetraphenylbenzoate derived from Diels−
Alder addition of tetracyclone with methyl propiolate.
In the hope of obtaining a product free from organic
contaminants, the reaction of the alkyne with 2, which contains
a labile MeCN ligand, was explored under milder conditions.
Stirring a THF solution of 2 with methyl propiolate (5 equiv)
at room temperature for 1 h caused a change from orange to
green. Column chromatography gave some free tetracyclone
due to decomposition, a small amount of an organometallic
byproduct, [Mo2(H2O)(CO)5(μ-C4Ph4CO)(η-C4Ph4CO)],
which is presumably formed by the presence of adventitious
water23,24 and a green zone consisting of 9. It is not necessary
to isolate pure 2 as a starting material; a one-pot synthesis
directly from [Mo(CO)6] gave 9 in an improved yield of 76%.
There are four possible isomers of complex 9, depending on
the orientation of the alkyne ligand and the regiochemistry of
the alkyne−cyclopentadienone linkage, but only two of these
are observed, in a ratio of 1.3:1. The 1H NMR spectrum of the
mixture of isomers shows peaks for the CH of the π-bound
alkyne ligand at δ 10.86 for the major isomer 9a and at δ 11.49
Figure 2. Single-crystal X-ray structure of [Mo(O)(RCCR)(η5,σ-
C4Ph4COCRCR)] (8). Selected bond lengths (Å): Mo(1)−O(1),
1.693(5); Mo(1)−C(1), 2.173(8); Mo(1)−C(36), 2.100(8); Mo(1)−
C(37), 2.101(7); C(1)−C(2), 1.333(11); C(36)−C(37), 1.293(11);
C(2)−O(2), 1.378(8); C(7)−O(2), 1.363(9).
distance of the terminal oxo ligand is 1.693(5) Å, which is
typical for Mo(IV) oxo complexes of this type.21 Whereas the
bonds between Mo and the four former diene carbons C(8)−
C(11) are equal within experimental error, that to the oxygen-
bearing carbon C(7) is now significantly shorter than in
complex 3. The remaining distances within the vinylic portion
of the complex are unchanged, however.
The main structural change concerns the alkyne ligand. The
Mo−C distances have increased significantly, in accordance
with the idea that the alkyne is less strongly bound in the oxo
complex. Although the CC bond length might have been
expected to decrease, this does not appear to be the case within
experimental error (1.312(6) Å in 3 in comparison to
a
Scheme 3. Synthesis of the Methyl Propiolate Complexes 9 and 10
a
The major isomer 9a is depicted; in the minor isomer 9b the η2-methyl propiolate ligand is rotated by 180°.
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1
Figure 3. (a) Expansion of 13C{1H} spectrum of 9 in the carbonyl region. (b) H-coupled spectrum showing J(CH) = 10 Hz in isomer 9b.
for the minor isomer 9b.23 The corresponding peaks for the
vinylic CH groups appear at δ 7.58 and 7.64, respectively. In
the reaction of Shvo’s complex with PhCCH to give
[Ru(CO)2(η5,σ-C4Ph4COCPhCH)] (the analogue of 5),
the 1H NMR signal for the vinylic proton was observed at 5.81
ppm, on which basis the coupling of the cyclopentadienone
oxygen was proposed to occur exclusively to the CPh terminus
of the alkyne; only one isomer was present.12 This shift is far
removed from those in 9, implying the opposite regiochemistry
in our case, a deduction confirmed crystallographically.25 Given
that the linkage with the tetracyclone ligand is likely to be
regiospecific, attacking the CH terminus of the alkyne, we
attribute the presence of two isomers to the two possible
orientations of the unsymmetrical alkyne ligand. This is clearly
confirmed by the 1H-coupled 13C NMR spectrum: as shown in
Figure 3, the signal due to the CO ligand of the minor isomer
exhibits a coupling of 10 Hz to the proton of the alkyne ligand.
By analogy with the structure of 3, we assume that this isomer
has the CH terminus located closer to the CO ligand.
Interconversion of the two isomers (e.g., by alkyne rotation)
was not observed even at elevated temperatures (1H NMR, 353
K in d8-toluene).
Table 1. 13C Chemical Shifts and C−H Coupling Constants
for the Alkyne-Derived Carbon Atoms in Complexes 9 and
10
δ of alkyne
CCO2Me
(2J(CH) in
Hz)
δ of vinylic
CCO2Me
(2J(CH) in
Hz)
δ of alkyne
CH (1J(CH)
in Hz)
δ of vinylic
CH (1J(CH)
in Hz)
compound
9a
189.9 (213)
191.2 (224)
143.9 (217)
151.5 (223)
190.2 (9)
190.15 (8)
147.0 (9)
140.0 (12)
170.7 (194)
171.1 (193)
178.6 (192)
177.6 (192)
151.5 (8)
151.6 (8)
155.8 (9)
156.4 (9)
9b
10a
10b
the vinylic carbon atoms remain relatively unchanged by the
oxidation process, whereas the chemical shifts of the alkyne
carbons are reduced by over 40 ppm (Table 1).
Confirmation that the cyclopentadienone oxygen is linked to
the CH terminus of the alkyne was obtained from an X-ray
crystal structure determination of one isomer of 10 (see the
Supporting Information).25 The gross features of the structure
are similar to those of complex 8, with the π-bound methyl
propiolate ligand oriented perpendicular to the metal−oxo
bond.
1
A combination of attached proton test, H-coupled spectra,
and 2D-NMR techniques allowed the complete assignment of
the 13C NMR spectra of both isomers of 9 (see the Supporting
Information for full details). The alkyne carbons appear at
approximately 190 ppm, with the vinylic carbons at about 170
ppm (CH) and 151 ppm (CCO2Me), respectively (see Table 1
below), consistent again with the alkyne acting as a 4-electron
donor.
Exposure of the isomeric mixture of 9 to air results in
complete conversion to the corresponding oxo complex 10,
which again exists as two isomers, again in a ratio of 1.3:1.26
The 1H NMR spectrum of 10 contains peaks at δ 9.00 and 8.22
due to the CH protons of 10a and at δ 8.80 and 8.20 for its
isomer 10b. The alkyne protons have shifted significantly to
higher field in comparison to those in the carbonyl analogue,
consistent with the reduced π-donor capability of the alkyne
ligand in the oxo complex, whereas the vinylic protons have
moved slightly in the opposite direction, presumably as a
consequence of the higher oxidation state of the Mo atom.
Detailed examination of the 13C NMR spectrum revealed that
CONCLUSIONS
■
In this paper we have shown that the reaction of [Mo(CO)2(η4-
C4Ph4CO)2] with electrophilic alkynes results in the incorpo-
ration of two alkyne molecules: one of these becomes linked to
the oxygen of the cyclopentadienone ligand, whereas the other
is coordinated to the metal as a 4-electron π-bound ligand. In
the case of methyl propiolate the same complex can be
prepared from [Mo(CO)6] in a one-pot reaction via the
acetonitrile complex [Mo(CO)3(NCMe)(η4-C4Ph4CO)].
Clearly the coupling of cyclopentadienone ligand and alkyne
parallels the reaction of Shvo’s complex with alkynes. However,
the ability of the molybdenum complexes to lose additional
ligands (tetracyclone ring and CO in 1, acetonitrile and CO in
2) allows the coordination of a second alkyne molecule.
Oxidation of the complex leads to the replacement of the Mo−
CO group by a MoO unit and provides a further example of
the reorientation of an alkyne ligand depending on the π-
acceptor/π-donor properties of the coligand in two analogous
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argon atmosphere and stirred for 18 h. The products were separated
by column chromatography. Elution with a mixture of light petroleum
and CH2Cl2 (1/1) afforded a small amount of tetracyclone. Elution
with a dichloromethane/acetone mixture (99/1) produced a yellow
band which yielded [Mo(O)(RCCR)(η5,σ-C4Ph4COCRCR)]
(213.5 mg, 273.7 mmol, 78%) after removal of the solvent.
Data for 8: mp darkens above 68 °C, melts at 110−114 °C; IR
(CH2Cl2) 1772, 1719 cm−1; IR (KBr) 937 cm−1 (MoO); 1H NMR
δ 7.73−6.91 (m, 20H, Ph), 3.77, 3.75, 3.65, 3.58 (all s, 3H, Me);
13C{1H) NMR δ 170.3, 167.9 (both CO2Me), 167.2 (CCO2Me),
complexes that have both been crystallographically charac-
terized.
EXPERIMENTAL SECTION
■
General experimental techniques were as detailed in other papers from
this laboratory.6,8,27 Infrared spectra were recorded in CH2Cl2 solution
(0.5 mm NaCl cells) over the range 2200−1550 cm−1, as KBr disks or
neat with a diamond ATR device over the range 4000−400 cm−1, on a
1
PerkinElmer Spectrum Two instrument. The H (400 or 500 MHz)
and 13C (100 or 125.8 MHz) NMR spectra were obtained in CDCl3
solution (unless otherwise stated) on Bruker Avance AV400 and
AV500 machines, the first of these having an automated sample
changer. Chemical shifts are given on the δ scale relative to SiMe4 at
0.0 ppm. The 13C{1H} NMR spectra were routinely recorded using an
attached proton test technique (DEPT pulse sequence). Mass spectra
were recorded on a Kratos MS 80 instrument operating in fast atom
bombardment mode with 3-nitrobenzyl alcohol as matrix or on a VG
AutoSpec instrument operating in electron impact mode and are given
for the most abundant isotope (98Mo). Elemental analyses were
carried out by the Microanalytical Service of the Department of
Chemistry.
165.4, 158.4 (both CO2Me), 153.5, 150.1, 149.7 (all CCO2Me) 142.4
(ring CO), 131.9−126.6 (m, Ph), 120.3, 116.0, 114.8, 109.9 (all CPh),
53.0, 52.8, 52.5, 52.2 (all Me); mass spectrum m/z 783 (M + H)+.
Anal. Found: C, 63.07; H, 4.34. Calcd for C41H32O10Mo: C, 63.08; H,
4.10..
Synthesis of [Mo(CO)(RCCH)(η5,σ-C4Ph4COCHCR)] (9)
from [Mo(CO)3(NCMe)(η4-C4Ph4CO)]. Five equivalents of methyl
propiolate (0.7 cm3, 8.25 mmol) was added to a solution of
[Mo(CO)3(NCMe)(η4-C4Ph4CO)] (1.0102 g, 1.65 mmol) in THF
(175 cm3). The resulting mixture was stirred at room temperature with
periodic TLC monitoring. After 1 h the product mixture was absorbed
onto silica and chromatographed. Tetracyclone (0.1568 g) was eluted
with a mixture of light petroleum and CH2Cl2 (1/1). Use of a 1/3
mixture of the same solvents caused the elution of a small orange band
identified as [Mo2(H2O)(CO)5(μ-η5,σ-C4Ph4CO)(η4-C4Ph4CO)]
(48.4 mg, 0.043 mmol, 5.2%). The green band of [Mo(CO)(RC
CH)(η5,σ-C4Ph4COCHCR)] (402 mg, 36%) was eluted with
CH2Cl2 and recrystallized by diffusion from toluene and diethyl ether.
The product consists of an inseparable mixture of the two isomers of
9.
Tetracyclone and the complexes [Mo(CO)2(η4-C4Ph4CO)2] and
[Mo(CO)3(NCMe)(η4-C4Ph4CO)] were prepared by literature
procedures.6,28 Alkynes were obtained from Aldrich and used as
received. Light petroleum refers to the fraction boiling in the range
40−60 °C. THF for oxidation reactions was prepared by allowing a
stoppered clear glass flask of the solvent to stand on a sunny
windowsill for several weeks. Warning! All peroxides should be treated
as potentially explosive, and under no circumstances should THF
treated in this way be subsequently distilled.
In a separate experiment the synthesis was conducted as a one-pot
procedure. Distilled acetonitrile (200 cm3) was added to [Mo(CO)6]
(5.0 g, 18.9 mmol). The solution was stirred and heated to reflux for
4.75 h. The solvent was removed in vacuo, and the resulting yellow-
green [Me(CO)3(MeCN)3] was redissolved in THF. Tetracyclone
(7.5 g, 19.5 mmol) was added and the reaction mixture was stirred
overnight. Methyl propiolate (8.6 cm3, 94.5 mmol) was then added,
and stirring was continued for a further 3 h. Column chromatography
as above afforded orange [Mo2(H2O)(CO)5((μ-η5,σ-C4Ph4CO)(η4-
C4Ph4CO)] (430 mg, 0.385 mmol, 4%) and a green band (9.7 g).
Crystallization by diffusion of light petroleum into a chloroform
solution produced a blue solid which consists of a 1.3/1 mixture of
isomers 9a,b and which dissolves to give a blue-green solution.
Mp: 190−192 °C. IR (CH2Cl2): 1963, 1728, 1716, 1688, 1602
cm−1. Mass spectrum: m/z 689 (M+). Anal. Found: C, 67.27; H, 4.18.
Calcd for C38H28O6Mo: C, 67.46; H, 4.14.
Synthesis of [Mo(CO)(RCCR)(η5,σ-C4Ph4COCRCR)] (3).
Five equivalents of DMAD (1.0 cm3, 8.15 mmol) was added to a
solution of [Mo(CO)2(η4-C4Ph4CO)2] (1.528 g, 1.59 mmol) in
toluene (175 cm3). The yellow solution was heated to reflux for 17 h,
changing first to purple then to green-brown. Monitoring by TLC
showed that the purple color was due to released tetracyclone. The
reaction mixture was cooled to room temperature, a small amount of
silica was added, and the toluene was removed on the vacuum line.
The resulting solid was loaded onto a chromatography column.
Elution with a mixture of light petroleum and dichloromethane (1/1)
produced a faint yellow band that was not collected, followed by the
recovered tetracyclone. Elution with dichloromethane afforded a
yellow band of organic material (IR 1741 cm−1), identified as a
mixture of dimethyl tetraphenylphthalate (by comparison with an
authentic sample prepared from DMAD and tetracyclone in refluxing
toluene)29 and hexamethyl mellitate.30 Elution with a mixture of
CH2Cl2 and acetone (99/1) separated a small yellow-brown band, and
changing to a 95/5 ratio of the same solvents produced the dark green
zone due to [Mo(CO)(RCCR)(η5,σ-C4Ph4COCRCR)]. Yield:
922.3 mg, 70%. On some occasions an unidentified dark red band
could subsequently be eluted with acetone: IR (KBr) 1736 cm−1.
Data for 3: mp 135−138 °C; IR (CH2Cl2) 2000, 1735, 1720 cm−1;
1H NMR δ 7.79−6.49 (m, 20H, Ph), 3.98, 3.62, 3.58, 3.50 (all s, 3H,
Me); 13C{1H} NMR δ 230.4 (CO), 193.2, 190.0 (both CCO2Me),
170.6, 168.7, 166.6, (all CO2Me), 161.1 (CCO2Me) 156.2 (CO2Me),
155.2 (CCO2Me), 146.0 (ring CO), 132.6−127.6 (m, Ph), 114.4,
102.8, 101.9, 100.6 (all CPh), 53.3, 52.8, 52.1, 51.7 (all Me); mass
spectrum m/z 766 (M − CO)+. Anal. Found: C, 61.02; H, 4.36. Calcd
for C42H32O10Mo·0.5CH2Cl2: C, 61.11; H, 3.95.
1
Isomer 9a (major): H NMR δ 10.83 (s, 1H, CH), 7.57 (s, 1H,
CH), 7.80−6.30 (m, 20H, Ph), 4.05, 3.40 (both s, 3H, CO2Me);
13C{1H} NMR: δ 235.7 (CO), 190.2 (CCO2Me), 189.9 (CH), 170.7
(CH), 170.2 (CO2Me), 169.7 (CO2Me), 151.5 (CCO2Me), 139.0
(ring CO), 133.1−127.7 (m, Ph), 112.4, 102.9, 98.9, 96.5 (all CPh),
52.9, 51.5 (both Me).
1
Isomer 9b (minor): H NMR δ 11.48 (s, 1H, CH), 7.63 (s, 1H,
CH), 7.80−6.30 (m, 20H, Ph), 3.57, 3.42 (both s, 3H, CO2Me);
13C{1H} NMR: δ 238.1 (CO), 191.2 (CH), 190.15 (CCO2Me), 171.4
(CO2Me), 171.1 (CH), 169.55 (CO2Me), 151.6 (CCO2Me), 136.7
(ring CO), 133.1−127.7 (m, Ph), 112.9, 101.2, 100.9, 99.7 (all CPh),
52.3, 51.5 (both Me).
Synthesis of [Mo(CO)(RCCH)(η5,σ-C4Ph4COCHCR)] (9)
from [Mo(CO)2(η-C4Ph4CO)2]. Five equivalents of methyl propiolate
(1.5 cm3, 16.4 mmol) was added to a solution of [Mo(CO)2(η4-
C4Ph4CO)2] (3.02 g, 3.28 mmol) in toluene (175 cm3), and the
reaction mixture was refluxed for 17 h with TLC monitoring. The
solution was then absorbed onto a small amount of silica and
chromatographed. A mixture of light petroleum and CH2Cl2 (1/1)
eluted tetracyclone followed by two narrow yellow bands consisting of
organic byproducts, which were not collected. A green band (1.47 g)
Synthesis of [Mo(O)(RCCR)(η5,σ-C4Ph4COCRCR)] (8).
Complex 3 (519.4 mg) was dissolved in 50 cm3 of THF containing
peroxides and stirred under argon for 1 h. The solution rapidly
changed color from green to yellow. The solvent was removed at room
temperature on a rotary evaporator and the residue triturated with
light petroleum to remove a small amount of tetracyclone. The yield
was virtually quantitative.
Alternatively, dichloromethane (175 cm3) was slowly added to
[Mo(CO)(RCCR)(η5,σ-C4Ph4COCRCR)] (352.9 mg, 425
mmol) with stirring in air. The solution was then placed under an
1
was then eluted with CH2Cl2. The H NMR spectrum showed it to
consist of a mixture of the complexes 9a,b, with small amounts of
F
DOI: 10.1021/acs.organomet.7b00300
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
10a,b and methyl 2,3,4,5-tetraphenylbenzoate (1H NMR δ 7.95 (s, 1H,
CH), 7.34−6.75 (m, 20H, Ph), 3.62 (s, 3H, CO2Me)).
van Meurs for assistance in recording the 13C and variable-
temperature H NMR spectra, and Dr. Craig Robertson for
assistance with the crystal structure determination of complex
8.
1
Synthesis of [Mo(O)(RCCH)(η5,σ-C4Ph4COCHCR)] (10). An
isomeric mixture of 9a and 9b from the above experiments (103.9 mg,
0.154 mmol) was dissolved in toluene (175 cm3). The solution was
briefly exposed to air by removing the stopper from the flask and then
reconnected to the argon supply and stirred for 18 h with TLC
monitoring. Column chromatography gave tetracyclone, eluted with a
mixture of light petroleum and CH2Cl2 (2/5), followed by the yellow
product [Mo(O)(HCCR)(η5,σ-C4Ph4COCHCR)], which was
eluted with CH2Cl2 and recrystallized from ethyl acetate and diethyl
ether. Yield: 73.2 mg, 0.110 mmol, 73%.
REFERENCES
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Alternatively, 9 (303.6 mg, 0.44 mmol) was dissolved in CH2Cl2
(10 cm3) and THF containing peroxides (5 cm3) was added.31 The
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Calcd for C37H28O6Mo·0.5CH2Cl2: C, 63.69; H, 4.10.
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1
Isomer 10a (major): H NMR δ 9.02 (s, 1H, CH), 8.23 (s, 1H,
CH); 7.80−6.60 (m, 20H, Ph), 3.64, 3.62 (both s, 3H, Me); 13C{1H}
NMR (CD2Cl2): δ 178.6 (CH), 170.05, 169.7 (CO2Me), 155.8
(CCO2Me), 147.0 (CCO2Me), 143.9 (CH), 139.9 (ring CO), 132.1−
126.7 (m, Ph), 119.3, 114.0, 112.1, 108.8 (all CPh), 51.9, 51.6 (both
Me).
1
Isomer 10b (minor): H NMR δ 8.83 (s, 1H, CH), 8.22 (s, 1H,
CH), 7.80−6.30 (m, 20H, Ph), 3.70, 3.65 (both s, 3H, Me). 13C{1H}
NMR (CD2Cl2): δ 177.6 (CH), 170.1, 170.0 (CO2Me), 156.4
(CCO2Me), 151.5 (CH), 140.0 (CCO2Me), 139.4 (ring CO), 132.1−
126.7 (m, Ph), 121.2, 113.2, 112.4, 110.1 (all CPh), 52.7, 51.7 (both
Me).
ASSOCIATED CONTENT
* Supporting Information
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1H and 13C NMR spectra of all compounds and full
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Accession Codes
CCDC 1544956−1544957 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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S.; Beddoes, R. L.; Cook, E. S. J. Chem. Soc., Dalton Trans. 1993, 91−
100.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.J.M.: M.Morris@sheffield.ac.uk.
■
(7) (a) Seki, T.; Wu, W.; Waymouth, R. M. Abstracts of Papers, 248th
ACS National Meeting & Exposition, San Francisco, CA, United
States, August 10−14, 2014; American Chemical Society: Washington,
DC, 2014; Abstract INOR-99. (b) Wu, W.; Waymouth, R. M.; Seki,
T.; Solis, D.; Nozaki, K.; Ando, H.; Kusumoto, S. Abstracts of Papers,
253rd ACS National Meeting & Exposition, San Francisco, CA, United
States, April 2−6, 2017; American Chemical Society: Washington, DC,
2017; Abstract INOR 117.
ORCID
Michael J. Morris: 0000-0001-8802-9147
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the University of Manchester (1987−1988) and the
University of Sheffield (1988−2017) for support, Dr. Sandra
■
(8) Adams, H.; Brown, P.; Cook, E. S.; Hanson, R. J.; Morris, M. J.
Organometallics 2012, 31, 7622−7624.
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Organometallics
Article
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give [Ru(CRCHR)(CO)2(η5-C4Ph4COH)].12
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Supporting Information.
(26) Unlike the case for 9, it is not possible in this case to determine
which orientation of the alkyne ligand corresponds to the major
isomer and which to the minor. It was also not possible to determine
whether the isomers of 10 interconvert at high temperatures because
the compound decomposes on heating in toluene.
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H
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