Application of an indenyl molybdenum dicarbonyl complex in the isomerisation of α-pinene oxide to campholenic aldehyde

Article abstract of DOI:10.1039/c4nj00371c

The complex [{(η⁵-Ind)Mo(CO)₂(μ-Cl)} ₂] (1) has been tested for the industrially relevant catalytic isomerisation of α-pinene oxide (PinOx) to campholenic aldehyde (CPA) in the liquid phase. PinOx conversion and CPA selectivity are strongly influenced by the solvent employed. Complete conversion of PinOx was achieved within 1 min at 55 °C or 30 min at 35 °C using 1,2-dichloroethane as solvent, giving CPA in 68% yield. Other products included trans-carveol, iso-pinocamphone and trans-pinocarveol. The stability of 1 under the reaction conditions used was investigated by using FT-IR spectroscopy and electrospray ionisation mass spectrometry (ESI-MS) to characterise recovered solids. In the presence of air/moisture 1 undergoes oxidative decarbonylation upon dissolution to give oxomolybdenum species that are proposed to include a tetranuclear oxomolybdenum(v) complex. Conversely, ESI-MS studies of 1 dissolved in dry acetonitrile show mononuclear species of the type [IndMo(CO)₂(CH ₃CN)_n]⁺. The crystal structure of the ring-slipped dicarbonyl complex [(η³-Ind)Mo(CO) ₂Cl(CH₃CN)₂] (2) (obtained after dissolution of 1 in acetonitrile) is reported. the Partner Organisations 2014.

Full text of DOI:10.1039/c4nj00371c

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Application of an indenyl molybdenum dicarbonyl
complex in the isomerisation of a-pinene oxide to
campholenic aldehyde†
Cite this:DOI: 10.1039/c4nj00371c
Sofia M. Bruno,a Ana C. Gomes,a Carla A. Gamelas,bc Marta Abrantes,d
M. Conceiçao Oliveira,d Anabela A. Valente,*a Filipe A. Almeida Paz,a
˜
Martyn Pillinger,a Carlos C. Romaob and Isabel S. Gonçalves*a
˜
The complex [{(Z5-Ind)Mo(CO)2(m-Cl)}2] (1) has been tested for the industrially relevant catalytic isomerisation
of a-pinene oxide (PinOx) to campholenic aldehyde (CPA) in the liquid phase. PinOx conversion and CPA
selectivity are strongly influenced by the solvent employed. Complete conversion of PinOx was achieved
within 1 min at 55 1C or 30 min at 35 1C using 1,2-dichloroethane as solvent, giving CPA in 68% yield.
Other products included trans-carveol, iso-pinocamphone and trans-pinocarveol. The stability of 1 under
the reaction conditions used was investigated by using FT-IR spectroscopy and electrospray ionisation
mass spectrometry (ESI-MS) to characterise recovered solids. In the presence of air/moisture 1 undergoes
oxidative decarbonylation upon dissolution to give oxomolybdenum species that are proposed to include
a tetranuclear oxomolybdenum(V) complex. Conversely, ESI-MS studies of 1 dissolved in dry acetonitrile
show mononuclear species of the type [IndMo(CO)2(CH3CN)n]+. The crystal structure of the ring-slipped
dicarbonyl complex [(Z3-Ind)Mo(CO)2Cl(CH3CN)2] (2) (obtained after dissolution of 1 in acetonitrile) is
reported.
Received (in Porto Alegre, Brazil)
12th March 2014,
Accepted 15th April 2014
DOI: 10.1039/c4nj00371c
www.rsc.org/njc
Different types of catalysts have been tested for this reaction,
including transition metal salts and Keggin-type heteropoly-
Introduction
Food, flavor, fragrance, cosmetics and pharmaceutical indus- acids.2,4,9–12 We recently explored organometallic complexes
tries make use of terpenes as feedstocks.1 In particular, for the catalytic isomerisation of PinOx to CPA, and interesting
a-pinene is a monoterpene present in turpentine oil extracts results were obtained in the case of [(Z5-Ind)Mo(Z3-C3H5)-
of some coniferous trees, e.g. pine tree.2 The epoxidation of (CO)2] (Ind = indenyl, C9H7; 56% CPA yield at 97% PinOx
a-pinene (Pin) gives a-pinene oxide (PinOx) which in turn can conversion, 3 h reaction at 35 1C).13 Indenyl molybdenum
undergo acid-catalysed rearrangements to give a variety of compounds are of special interest due to the tendency of the
value-added products of commercial interest,3 e.g. campholenic indenyl ligand to undergo ring-slippage induced by electro-
aldehyde (CPA), trans-carveol, trans-sobrerol, pinocarveol, iso- chemical reduction or ligand addition. This hapto-flexibility
pinocamphol, iso-pinocamphone,4–6 among others.2–7 In particular allows enhancement of the reaction rates of substitution
CPA is used in the manufacture of commercial sandalwood-like reactions of electronically saturated indenyl metal complexes,
fragrances, e.g. Sandalores and Javanols (Givaudan), Bacdanols relative to their cyclopentadienyl congeners, finding applica-
(IFF), Brahmanols (Dragoso) and Polysantols (Firmenich).8 tion in some important organic reactions.14 To the best of our
knowledge, the use of indenyl molybdenum dicarbonyl com-
plexes for homogeneous catalytic reactions has so far been
a Department of Chemistry, CICECO, University of Aveiro,
restricted to the aforementioned study.13 In contrast, cyclo-
´
Campus Universitario de Santiago, 3810-193 Aveiro, Portugal.
pentadienyl molybdenum dicarbonyl complexes are far more
explored, particularly as precatalysts for the liquid-phase
epoxidation of olefins.15–19
Herein we report the preparation of the complex
[{(Z5-Ind)Mo(CO)2(m-Cl)}2] (1) and its use in the catalytic iso-
merisation of PinOx. Attention has been drawn to the identifi-
cation of the metal species formed from 1 under different
reaction conditions.
E-mail: atav@ua.pt, igoncalves@ua.pt
b
c
´
´
Instituto de Tecnologia Quımica e Biologica da Universidade Nova de Lisboa,
´
˜
´
Av. da Republica, Estaçao Agronomica Nacional, 2780-157 Oeiras, Portugal
´
´
´
Escola Superior de Tecnologia, Instituto Politecnico de Setubal, 2910-761 Setubal,
Portugal
d
´
Centro de Quımica Estrutural, Complexo Interdisciplinar,
´
Instituto Superior Tecnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal
† CCDC 980993. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4nj00371c
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Experimental
Materials and methods
Single crystal X-ray diffraction
Single crystals of [(Z3-Ind)Mo(CO)2Cl(CH3CN)2] (2) were manually
harvested from the crystallisation vial and immediately immersed
in highly viscous FOMBLIN Y perfluoropolyether vacuum oil
(LVAC 140/13, Sigma-Aldrich) to avoid degradation caused by the
evaporation of the solvent.23 Crystals were mounted on Hampton
Research CryoLoops with the help of a Stemi 2000 stereomicro-
scope equipped with Carl Zeiss lenses. Data were collected on a
Bruker X8 Kappa APEX II CCD area-detector diffractometer (Mo Ka
graphite-monochromated radiation, l = 0.71073 Å) controlled
using the APEX2 software package24 and equipped with an Oxford
Cryosystems Series 700 cryostream monitored remotely using the
software interface Cryopad.25 Images were processed using the
software package SAINT+,26 and data were corrected for absorption
by the multiscan semi-empirical method implemented in
SADABS.27 The structure was solved using the Patterson synthesis
algorithm implemented in SHELXS-97,28 which allowed the
immediate location of the crystallographically independent MoII
centre and most of the heaviest atoms. The remaining non-
hydrogen atoms were located from difference Fourier maps calcu-
lated from successive full-matrix least-squares refinement cycles
on F2 using SHELXL-97.28a,29 All non-hydrogen atoms were suc-
cessfully refined using anisotropic displacement parameters.
Hydrogen atoms bound to carbon were placed at their
idealised positions using appropriate HFIX instructions in
SHELXL: 133 for the methyl groups of the coordinated aceto-
nitrile solvent molecules and 43 for the CH groups of the
aromatic rings. All hydrogen atoms were included in sub-
sequent refinement cycles with isotropic thermal displacement
parameters (Uiso) fixed at 1.2 ꢂ Ueq (CH groups) or 1.5 ꢂ Ueq
(methyl groups) of the parent carbon atoms.
Microanalyses (CHN) were carried out at the University of
Aveiro by M. Marques using a Truspec instrument. FT-IR
spectra were recorded using a Bruker Tensor 27 spectrophoto-
meter in a 4000–350 cmꢀ1 range with 256 scans and 4 cmꢀ1
resolution. Attenuated total reflectance (ATR) FT-IR spectra
were recorded on the same instrument equipped with a Specac
Golden Gate Mk II ATR accessory having a diamond top-plate
and KRS-5 focussing lenses. FT-Raman spectra were recorded
on a Bruker RFS 100 spectrometer with a Nd:YAG coherent laser
(l = 1064 nm). Solid-state magic-angle-spinning (MAS) 13C NMR
spectra were recorded at 100 MHz on a Bruker Avance 400
spectrometer with 3 ms 1H 901 pulses, 2 ms contact time, a
spinning rate of 10 kHz and 3 s recycle delays. Chemical shifts
are quoted in parts per million from tetramethylsilane.
ESI mass spectra were obtained on a 500-MS quadrupole ion
trap mass spectrometer (Varian Inc., Palo Alto, CA, USA). The
sample was introduced into the ESI ion source using a syringe
pump set to a flow rate of 20 mL minꢀ1. The ion spray voltage
was set at ꢁ5 kV; capillary voltage: 20–80 V and RF loading
90–80%. Nitrogen was used as nebulising and drying gas at
pressures of 35 psi and 10 psi, respectively; drying gas temperature,
350 1C. The spectra were recorded in the range 100–1000 Da.
Spectra typically correspond to the average of 20–35 scans. Theo-
retical isotope patterns were calculated using CHEMCALC.20
Air-sensitive operations were carried out by using standard
Schlenk techniques under a nitrogen atmosphere. 1 M HCl in
diethyl ether, pentane (99%), hexane (99%), 1,2-dichloroethane
(DCE, 99%), anhydrous a,a,a-trifluorotoluene (TFT, 499%)
and anhydrous acetonitrile (499.8%) were purchased from
Sigma-Aldrich. Where appropriate solvents were dried follow-
ing standard procedures (THF, diethyl ether and hexane over
Na/benzophenone ketyl; CH2Cl2 and DCE over CaH2) and
distilled under nitrogen. [(Z5-Ind)Mo(Z3-C3H5)(CO)2] was pre-
pared as previously reported.21,22
The Flack parameter refined to ꢀ0.05(3).30 The last difference
Fourier map synthesis showed the highest peak (0.304 e Åꢀ3
)
located at 0.85 Å from C14 (one of the coordinated carbonyl groups)
and the deepest hole (ꢀ0.355) at 0.23 Å from H1. Information
concerning crystallographic data collection and structure refine-
ment details is summarised in Table 1. Structural drawings were
created using Crystal Impact Diamond.31
Synthesis of [{(g5-Ind)Mo(CO)2(l-Cl)}2] (1)
1 M HCl in diethyl ether (6.0 mL, 6.00 mmol) was added dropwise
to a solution of [(Z5-Ind)Mo(Z3-C3H5)(CO)2] (0.51 g, 1.67 mmol) in
Catalytic tests
CH2Cl2 and the reaction mixture was stirred for 2 h at room The catalytic reactions were carried out under air in a magne-
temperature. The resultant red brown precipitate was filtered off, tically stirred, closed borosilicate reaction vessel (10 mL capa-
washed with diethyl ether and vacuum-dried. Yield: 0.49 g (98%). city) which was immersed in an oil bath thermostatted at 35 1C.
Anal. calcd for C22H14Cl2Mo2O4 (605.13): C, 43.66; H, 2.33%. Typically, the reactor was loaded with a metal complex (17 mmol
Found: C, 43.39; H, 2.32%. FT-IR (ATR, cmꢀ1): n = 3097 (w), of molybdenum), a-pinene oxide (PinOx, 170 mmol) and a
2059 (w), 1954 (vs, n(CO)), 1851 (vs, n(CO)), 1602 (w), 1542 (m), solvent (0.5 mL). The PinOx was heated at the reaction tem-
1477 (m), 1460 (m), 1376 (m), 1328 (m), 1240 (w), 1218 (w), perature (10 min) prior to addition to a second heated reactor
1153 (w), 1096 (w), 1039 (m), 984 (m), 902 (m), 832 (s), 745 (vs), containing a solvent and a catalyst (instant taken as zero time).
601 (m), 592 (m), 563 (m), 517 (m), 496 (m), 470 (w), 453 (m), The course of the reaction was monitored using a Varian 3800
443 (s), 385 (w). FT-Raman (cmꢀ1): n = 3095 (w), 3070 (w), 3049 (w), GC equipped with a BR-5 (Bruker) capillary column (30 m ꢂ
1951 (w), 1842 (m), 1545 (w), 1447 (w), 1376 (w), 1328 (vs), 0.25 mm; 0.25 mm) and a flame ionisation detector, using H2 as
1240 (w), 1160 (w), 1038 (w), 1006 (w), 908 (w), 738 (w), 594 (w), the carrier gas. Cyclododecane epoxide was used as an internal
562 (w), 520 (w), 496 (m), 458 (w), 390 (m), 284 (m), 262 (w), 184 (w), standard added after the reaction. The products were identified
165 (s), 122 (vs), 106 (vs). 13C CP MAS NMR: d = 265.7 (CO), 259.2 by GC-MS (Trace GC 2000 Series (Thermo Quest CE Instruments) –
(CO), 130.2 (C8,9), 129.0 (C5,6), 123.9 (C4,7), 89.9 (C2), 76.4 (C1,3).
DSQ II (Thermo Scientific)), equipped with a capillary DB-5
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Table 1 Crystal and structure refinement data for compound [(Z3-Ind)-
Mo(CO)2Cl(CH3CN)2] (2)
Table 2 Isomerisation of a-pinene oxide in the presence of 1a
Temp.
(1C)
Reaction time
(min)
PinOx conv.
(%)
CPA selectivity
(%)
Solvent
DCE
Formula
C15H13ClMoN2O2
384.66
Formula weight
Crystal system
Space group
a/Å
55
35
35
1
1
100
90
68
58
67
Orthorhombic
P212121
30
100
8.0476(7)
b/Å
11.4511(10)
16.8556(14)
1553.3(2)
4
1.645
1.020
TFT
55
1
100
65
c/Å
Volume/Å3
Z
Hexane
55
55
55
1
30
60
20
70
100
53
52
54
Dc/g cmꢀ3
m(Mo-Ka)/mmꢀ1
Crystal size/mm
Crystal type
y range
0.07 ꢂ 0.05 ꢂ 0.02
CH3CN
55
55
55
1
300
1440
1
94
100
56
52
53
Red blocks
3.56 to 29.13
ꢀ10 r h r 11, ꢀ14 r k r 15,
ꢀ23 r l r 22
30 905
Index ranges
a
Reaction conditions: 17 mmol Mo, 170 mmol PinOx, 0.5 mL solvent.
Reflections collected
Independent reflections
Completeness to y = 29.131
Final R indices [I 4 2s(I)]a,b
Final R indices (all data)a,b
Weighting schemec
4143 [Rint = 0.0361]
99.7%
(35 1C, Table 2). Campholenic aldehyde (CPA) was the main
product formed in ca. 68% selectivity at 100% conversion, in
the reaction temperature range 35–55 1C (Scheme 1). Increasing
the reaction time to 24 h did not lead to significant differences
in product selectivity, suggesting that CPA is relatively stable
under the reaction conditions used. Other products included
the menthyl derivative trans-carveol (TCV), and the derivatives
with a pinane skeleton, iso-pinocamphone (IPC) and trans-
pinocarveol (PCV). The reaction did not take place to a measur-
able extent when carried out in the absence of a catalyst. These
results are considerably superior to those reported recently by
some of us for the first molybdenum-based homogeneous catalysts
tested in the isomerisation of PinOx (Table 3).13 The complexes
[(Z5-Ind)Mo(Z3-C3H5)(CO)2], [(Z5-IndCH3)Mo(Z3-C3H5)(CO)2] and
R1 = 0.0233, wR2 = 0.0446
R1 = 0.0271, wR2 = 0.0457
m = 0.0215, n = 0.2428
0.304 and ꢀ0.355 e Åꢀ3
Largest diff. peak and hole
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
h
i.
h
i
P
P
P
P
2
2
2
a
c
b
2
R1 = ||Fo| ꢀ |Fc||/ |Fo|. wR2 ¼
wðFo ꢀ Fc2Þ
wðFo
Þ
.
w = 1/[s2(Fo2) + (mP)2 + nP] where P = (Fo + 2Fc2)/3.
2
type column (30 m ꢂ 0.25 mm; 0.25 mm) and using He as
carrier gas.
Results and discussion
Catalyst preparation
[(Z5-IndSi(CH3)3)Mo(Z3-C3H5)(CO)2] (IndCH3
=
C9H6CH3;
The indenyl complex [{(Z5-Ind)Mo(CO)2(m-Cl)}2] (1) was synthesised
in nearly quantitative yield by reaction of [(Z5-Ind)Mo(Z3-C3H5)(CO)2]
with HCl (1 M in diethyl ether) in dichloromethane at room
temperature. The method is particularly attractive since the
allyl precursor is conveniently available from [Mo(Z3-C3H5)-
Cl(CO)2(CH3CN)2] and KInd in almost quantitative yield.21
IndSi(CH3)3 = C9H6Si(CH3)3) led to 64%, 66% and 61% PinOx
conversion, respectively, and CPA yields in the range of
´ˇ
Recently, Honzıcek et al. isolated dimeric 1 after bubbling
gaseous HCl (Sigma-Aldrich) through a solution of [(Z5-Ind)Mo-
(Z3-C3H5)(CO)2].32 Under similar reaction conditions, bubbling
gaseous HCl (Air Liquide) through the solution at a high flow
rate instead of HCl (1 M in diethyl ether) in a more controlled
reaction produces [(Z5-Ind)Mo(CO)2Cl3] in excellent yield.33
In the solid state, 1 is stable to air and moisture for several
hours. The solid-state 13C CP MAS NMR spectrum presents
the indenyl signals with chemical shifts characteristic of the
Z5-coordination mode, being similar to those observed for
2
other pentahapto indenyl ligands in [(Z5-Ind)Mo{K -S2P(OEt)2}-
(CO)2]34 and [(Z5-Ind)Mo(Z3-C3H5)(CO)2].35
Catalytic isomerisation of a-pinene oxide
Complex 1 was investigated in the acid-catalysed isomerisation
of a-pinene oxide (PinOx), using 1,2-dichloroethane (DCE) as a
solvent. Outstandingly high catalytic activity was observed,
leading to 100% conversion within 1 min of reaction at
55 1C, or within 30 min at approximately room temperature Scheme 1 Isomerisation of PinOx in the presence of 1.
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Table 3 Comparison of the catalytic results with 1 with those reported in the literature for molybdenum dicarbonyl complexes tested in the catalytic
isomerisation of a-pinene oxidea
Catalyst
Time (min)
1
PinOx conv. (%)
100
CPA yield (%)
68
Other product yields (%)
Ref.
1
8-IPC
This work
9-TCV
12-TCV
13-TCV
12-TCV
[(Z5-Ind)Mo(Z3-C3H5)(CO)2]
60
60
60
64
66
61
33
38
33
13
13
13
[(Z5-IndCH3)Mo(Z3-C3H5)(CO)2]
[(Z5-IndSi(CH3)3)Mo(Z3-C3H5)(CO)2]
a
Reaction conditions: 17 mmol catalyst, 170 mmol PinOx, 0.5 mL DCE, 55 1C.
33–38%, at 1 h reaction, at 55 1C (a molar ratio of Mo/PinOx = and solid phases were blue-green in color, for hexane the solids
1 : 10, which is similar to that used in the present work, using were purple. For each solvent used, the species isolated from
DCE as a solvent).13
the solid and liquid phases exhibited similar FT-IR spectra
The catalytic results of 1 were similarly outstanding when (Fig. 1). However, for each type of solvent, considerable differ-
using a,a,a-trifluorotoluene (TFT) as a solvent instead of DCE, ences between the spectra of the recovered solids and that of 1
at 55 1C (65% CPA selectivity at 100% conversion reached at were observed. These results indicate that 1 is converted into
1 min reaction, Table 2). The relatively low toxicity and high different types of metal species under the reaction conditions
boiling point (102 1C) of TFT make it a potentially interesting used. While for DCE and TFT the complete disappearance (or
substitute for chlorinated solvents such as CH2Cl2 and DCE nearly) of the bands characteristic of the carbonyl ligands was
in organic reactions.36 When hexane was used as a solvent, observed, these bands remained evident when hexane was used
the reaction of PinOx was considerably slower (20% conversion as a solvent. For DCE and TFT, the solids exhibited somewhat
at 1 min), and CPA selectivity was lower (54% CPA selectivity similar spectral features, whereas those for hexane exhibited a
at 100% conversion). Using acetonitrile as a solvent led to a mixture of features between those of 1 and those of the solids
much slower reaction of PinOx isomerisation (1% conversion recovered for the halogenated solvents. The similar spectra of
at 1 min), and CPA selectivity at 100% conversion was the solids for DCE and TFT as solvents are consistent with the
53% (reached at 24 h reaction, Table 2). The initial reaction similar catalytic results obtained, and the similar colors of the
rates correlate with the dipole moments of the noncoordinating
solvents which are higher for TFT and DCE (2.86 D and
1.36 D, respectively) than for hexane (0 D).37 Possibly, the active
species are more soluble in more polar solvents, leading to
higher reaction rates. This correlation does not apply for the
coordinating solvent CH3CN (dipole moment of 3.95 D37),
which gave the lowest reaction rate. The CH3CN molecules
are relatively basic which may level off the acidity of the metal
species and consequently their activity in the target reaction.
On the other hand, different metal species may be formed,
which may be stabilised by the coordinating solvent molecules
(ascertained by ESI-MS and single crystal X-ray diffraction
studies, discussed below).
Characterisation studies
For DCE, TFT and hexane, the reaction mixtures were biphasic
solid–liquid, while for CH3CN only a residual amount of
undissolved solid was observed. To get insights into the type
of active species formed, the solid phases were separated from
the reaction mixtures by centrifugation after 1 h reaction to give
the solids denoted as DCE-s(cat), TFT-s(cat) and Hex-s(cat).
A reaction time of 1 h was chosen partly because 100% conver-
sion was reached at this point for all three solvents. Species
dissolved in the corresponding liquid phases were precipitated
by addition of pentane, which led to the solids denoted as
DCE-l(cat), TFT-l(cat) and Hex-l(cat). All the solids obtained
were washed with pentane–hexane, dried overnight at room
Fig. 1 FT-IR spectra of (a) compound 1, and the recovered solids (b) DCE-
temperature, and characterised by ATR FT-IR spectroscopy.
While for DCE and TFT the solids recovered from the liquid
s(cat), (c) DCE-l(cat), (d) DCE-s(55), (e) DCE-s(rt), (f) TFT-s(cat), (g) TFT-l(cat),
(h) Hex-s(cat), (i) Hex-l(cat), (j) CH3CN-s(55) and (k) CH3CN-s(rt).
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solids DCE-s(cat), DCE-l(cat), TFT-s(cat) and TFT-l(cat). On the present at 950–980 for n(MoQO) and 660–760 cmꢀ1 for
other hand, the spectral differences between the solids n(Mo–O–Mo), similar to those observed in this work for the
obtained using the halogenated solvents and hexane are con- recovered solids. The rhombic tetranuclear core may be regarded
sistent with their different colors, and with the above hypo- as a fusion of two {Mo2O4}2+ units sharing a pair of triply-bridging
thesis that the metal species are less soluble in hexane, leading oxo groups and a pair of doubly-bridging OR groups. The overall
to a lower overall reaction rate of PinOx. It is possible that 1 acts charge of the complex is then dependent on the nature of the 6
as a precatalyst, undergoing decarbonylation upon dissolution remaining peripheral ligands (e.g., 2 L and 4 Cl in Fig. 2). In recent
in the presence of air/water to give active species, and this work we found that aerial oxidation of 1 in DMF gave the
reaction may be slower for hexane than for the halogenated dinuclear oxomolybdenum(V) chloride [Mo2O2(DMF)4(m-O)2Cl2]
solvents. For the three solvents, several new bands appeared, containing the same type of {Mo2O4}2+ unit found in the tetra-
e.g. at ca. 1630, 975 and 735 cmꢀ1, which can be assigned to a nuclear complexes described above.42 Hence it seems plausible
water bending vibration, and stretching vibrations of MoQO that a tetranuclear oxomolybdenum(V) complex could be a species
and Mo–O–Mo bonds, respectively.
formed from 1 under the catalytic reaction conditions used.
In two further experiments carried out in the absence of
As mentioned above, when acetonitrile was used as a solvent
PinOx, complex 1 was added to DCE (undried) and the mixture in the catalytic reaction, only a residual amount of undissolved
was stirred at either 55 1C for 1 h or room temperature for 24 h solid was observed. To gain insight into the stability of 1 in this
under air. The former experiment was carried out using the solvent, the following additional experiments were performed
same quantities as those used in a typical catalytic test, while in the absence of PinOx: (I) in a manner similar to that carried
the latter experiment was carried out on a larger (ca. 10-fold out for DCE-s(55) and DCE-s(rt), 1 (0.05 g, 83 mmol) and
increase) scale (0.05 g 1, 83 mmol; 5 mL DCE). In each experi- acetonitrile (5 mL) were stirred together for either 1 h at
ment a solid (0.02 g for the large-scale experiment) was recov- 55 1C or 48 h at room temperature, and the resultant suspended
ered by filtration, washed with pentane and vacuum-dried. The solid was recovered by filtration, washed with pentane and
FT-IR spectra of the solids (denoted DCE-s(55) and DCE-s(rt)) vacuum-dried, giving CH3CN-s(55) and CH3CN-s(rt); (II) com-
were quite similar to those recorded for the solids recovered pound 1 (0.1 g, 0.17 mmol) was dissolved in dry acetonitrile
after catalytic reactions performed in the presence of PinOx (5 mL) and the solution was immediately filtered and evapo-
using either DCE or TFT as a solvent (Fig. 1).
rated to dryness, giving a brown solid, which was washed with
To gain further insight into the possible nature of this dry hexane (2 ꢂ 10 mL) and diethyl ether (2 ꢂ 10 mL), and
compound, ESI studies were carried out for DCE-s(rt) in acetone. finally vacuum-dried; (III) 1 was dissolved in dry MeCN and the
The negative ion ESI mass spectrum exhibited a group of peaks solution was immediately analysed by ESI mass spectrometry.
centered at m/z 440 with an isotopic pattern characteristic of a
The FT-IR spectrum of CH3CN-s(rt) was very similar to that
polynuclear Mo complex containing Cl, in particular a doubly of DCE-s(rt) (Fig. 1), indicating that in the presence of air
charged tetranuclear complex. For example, the pattern agreed oxidative decarbonylation of 1 takes place upon dissolution to
well with that calculated for [Mo4O8Cl4(H2O)6(C3H6O)2]2ꢀ, where give the same type of oxomolybdenum species. In the case of
C3H6O = acetone. However, no plausible structure can be CH3CN-s(55), the solid presented a spectrum similar to those
proposed for a compound having this formula. On the other recorded for Hex-l(cat) and Hex-s(cat), i.e., indicative of a
hand, numerous compounds containing the tetranuclear mixture of species including unreacted 1. These results may
[Mo4O8(OR)2(ROH)2Cl4]2ꢀ (R = Me, Et) anionic unit are known, partly explain the poor initial catalytic activity observed when
all comprising the rhombic [Mo4O4(m3-O)2(m2-O)2(m2-OR)2]2+ core using CH3CN as a solvent, at 55 1C.
(Fig. 2).38 This rhomboid disposition of metal atoms is a
Recrystallisation of the brown solid obtained in experiment
frequently encountered structural motif in molybdenum(V) II by slow diffusion of diethyl ether into a concentrated solution
coordination chemistry, appearing, for example, in the of the compound in dry acetonitrile under nitrogen gave red
hydroxyl-bridged derivatives [Mo4O8(OH)2(HpzMe2)6]I2 (HpzMe2
=
crystals, which were identified by X-ray diffraction as the ring-
3,5-dimethylpyrazole)39 and K2[Mo4O8(OH)2(H2O)2(C2O4)2],40 and slipped dicarbonyl complex [(Z3-Ind)Mo(CO)2Cl(CH3CN)2] (2)
the MoV–glycolate complex (PyH)4[Mo4O8Cl4(glyc)2] (Py = pyridine).41 (Scheme 2; please see below for a full structural description).
In the FT-IR spectra of these compounds, strong bands are usually
Fig. 2 Schematic representation of the rhombic [MoV4O4(m3-O)2(m2-O)2-
(m2-OR)2(L)2Cl4]2ꢀ anion.
Scheme 2 Formation of 2 after dissolution of 1 in dry CH3CN.
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Fig. 3 Positive mode ESI spectrum of 1 in dry CH3CN. Insets (a) and (b)
illustrate the calculated isotopic distributions for the species [IndMo(CO)2-
(CH3CN)2]+ and [(Ind)2Mo2Cl2]+, respectively.
These results somewhat support the above hypothesis of the
conversion of 1 to different metal species which may be
stabilised by the CH3CN (base) molecules. Nevertheless, on
the basis of the FT-IR studies, complex 1 is also converted to
other metal species which are possibly common to the different
solvents tested.
Fig. 4 Negative mode ESI spectra of 1 in dry CH3CN at different infusion
times: (A) immediately after infusion; (B) after 2 min; (C) after 10 min.
Insets: calculated isotopic patterns matching the main species. In inset d,
the isotopic separation of 0.5 m/z units is indicative of a doubly charged ion.
The positive mode ESI spectrum recorded after infusion of a
dry CH3CN solution of 1 exhibited two major isotopic patterns,
Table 5 Molybdenum species observed in the negative mode ESI mass
corresponding to mono- and dinuclear species, respectively. spectrum of 1 (solvent = CH3CN)
The mononuclear region showed a group of peaks at m/z 351
Calculated
monoisotopic mass Probable species
m/za
and 310 each of which displayed a seven-peak isotopic pattern
characteristic of a single Mo atom that agreed well with that
calculated for [IndMo(CO)2(CH3CN)2]+ (Fig. 3, inset a) and
181
256
180.89
255.83
310.81
294.82
327.26
367.23
440.67
[MoCl(CH3O)(OH)]ꢀ
[MoCl3(H2O)2(OH)]ꢀ
[IndMo(CO)2(CH3CN)]+, respectively. The major patterns at 311
[MoCl3(CO)2(H2O)(OH)2]ꢀ
[Mo2Cl8(CH3CN)2CH3OH]2ꢀ
[Mo3Cl8(CH3OH)2(OH)]2ꢀ
[Mo3Cl10(CH3CN)(CH3OH)(H2O)]2ꢀ
[Mo4Cl11(CH3CN)(CH3OH)2]2ꢀ
[Mo2Cl6(CO)4(CH3CN)(CH3OH)2(OH)]ꢀ
[Mo4Cl11(CH3OH)(OH)]ꢀ
295
326
m/z 230–290 correspond to adduct species formed in the gas-
phase due to the presence of residual methanol and water
367
molecules. Two groups of major peaks appear in the dimolyb- 440
638/640 639.68
821/823 825.30
877/879 881.35
denum region, fitting the calculated isotopic distributions for
species containing two Mo centers and two Cl atoms (Fig. 3,
inset b). The presence of these species was confirmed by MS/MS
analysis. The most abundant species are listed in Table 4.
The negative mode ESI spectrum recorded immediately after
infusion of a dry CH3CN solution of 1 displayed two peaks
[Mo4Cl11(CH3CN)(CH3OH)2]ꢀ
a
See footnote a in Table 4.
centered at m/z 310.8 and 637.7/639.7, corresponding to
[MoCl3(CO)2(H2O)(OH)2]ꢀ and [Mo2Cl6(CO)4(CH3CN)(CH3-OH)2-
(OH)]ꢀ, respectively (Fig. 4A). These species were not stable under
the MS conditions used, and an oxidation process rapidly
occurred leading to [MoCl3(H2O)2(OH)]ꢀ with m/z 256 and loss
of CO2 (Fig. 4B). After 10 minutes of infusion, the mass spectrum
(Fig. 4C) exhibited a substantial amount of mono- and doubly
charged molybdenum species, indicating that rearrangement
processes occurred in the gas-phase. The most abundant species
are listed in Table 5.
Table 4 Molybdenum species observed in the positive mode ESI mass
spectrum of 1 (solvent = CH3CN)
Calculated
monoisotopic mass
m/za
Probable species
245
263
286
310
244.98
262.99
286.01
309.97
351.00
460.88
495.86
530.83
551.84
565.79
586.81
607.83
[(C9H7)Mo(CH3OH)]+
[(C9H7)Mo(H2O)(CH3OH)]+
[(C9H7)Mo(CH3CN)(CH3OH)]+
[(C9H7)Mo(CO)2(CH3CN)]+
[(C9H7)Mo(CO)2(CH3CN)2]+
[(C9H7)2Mo2Cl]+
351
457/459
492/494
527/529
548/550
564/566
583/585
604/606
[(C9H7)2Mo2Cl2]+
Crystal structure of [(g3-Ind)Mo(CO)2Cl(CH3CN)2] (2)
[(C9H7)2Mo2Cl3]+
[(C9H7)2Mo2Cl2(CO)2]+
[(C9H7)2Mo2Cl4]+
Compound 2 crystallises in the non-centrosymmetric ortho-
rhombic space group P212121 with the asymmetric unit being
composed of a whole [(Z3-Ind)Mo(CO)2Cl(CH3CN)2] molecular
unit (Fig. 5). The crystallographically independent MoII metal
center is coordinated to an indenyl ligand, a chloride anion,
two carbonyl groups and two acetonitrile solvent molecules,
[(C9H7)2Mo2Cl3(CO)2]+
[(C9H7)2Mo2Cl2(CO)4]+
a
More abundant peaks in the envelope isotope patterns. Species were
assigned based on matches between calculated and observed isotopic
patterns.
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Table 6 Bond distances (in Å) and angles (in degrees) for the crystallo-
graphically independent MoII metal center present in 2
Mo1–C14
Mo1–C15
Mo1–Cl1
Mo1–N1
1.968(2)
1.962(2)
2.4687(6)
2.2283(16)
2.2276(18)
153.00(5)
35.54(7)
35.93(8)
160.20(6)
83.39(7)
83.10(7)
56.38(7)
150.30(5)
106.74(7)
106.09(8)
70.85(8)
88.02(6)
170.50(8)
98.18(7)
94.68(11)
72.34(8)
Mo1–C1
Mo1–C2
Mo1–C3
Mo1–Cg
2.4101(19)
2.1865(19)
2.393(2)
2.102(3)
C1–C2–C3
Mo1–N2
C1–Mo1–Cl1
C2–Mo1–C1
C2–Mo1–C3
C2–Mo1–Cl1
C2–Mo1–N1
C2–Mo1–N2
C3–Mo1–C1
C3–Mo1–Cl1
C14–Mo1–C1
C14–Mo1–C2
C14–Mo1–C3
C14–Mo1–Cl1
C14–Mo1–N1
C14–Mo1–N2
C14–Mo1–Cg
C15–Mo1–C2
C15–Mo1–C3
C15–Mo1–C14
C15–Mo1–Cl1
C15–Mo1–N1
C15–Mo1–N2
C15–Mo1–Cg
N1–Mo1–C1
N1–Mo1–C3
N1–Mo1–Cl1
N1–Mo1–Cg
N2–Mo1–C1
N2–Mo1–C3
N2–Mo1–Cl1
N2–Mo1–N1
N2–Mo1–Cg
107.28(8)
108.06(8)
81.20(8)
88.27(6)
96.73(7)
169.41(8)
96.00(11)
81.24(6)
118.47(7)
82.63(5)
94.75(10)
117.68(6)
81.50(7)
81.14(5)
82.14(6)
94.59(10)
175.24(9)
C1–C2–C3
C1–C2–C3
Fig. 5 Schematic representation of the asymmetric unit of 2. Non-
hydrogen atoms are represented as thermal ellipsoids drawn at the 50%
probability level, and hydrogen atoms as small spheres with arbitrary radii.
Cg stands for the centroid of the coordinating C1–C2–C3 atoms of the Z3-
indenyl ligand. For selected bond lengths and angle ranges referring to the MoII
coordination sphere see Table 6.
C1–C2–C3
C15–Mo1–C1
C1–C2–C3
Cl1–Mo1–C
gC1–C2–C3
with each pair of identical ligands being cis to each other within environment around the metal center is best envisaged as a
the coordination polyhedron of the metal center. slightly distorted octahedron in which the apical positions are
A search in the literature and in the Cambridge Structural occupied by the charged ligands (the Z3-indenyl and the
Database (CSD, Version 5.34, November 2012 with 3 updates)43 chloride anion) and the equatorial plane is composed of
reveals that this organometallic complex is very unique among the neutral ones: while the internal octahedral distances cover
molybdenum complexes bearing derivatives of indenyl mole- the wide range from 1.962(2) to 2.4687(6) Å, the cis and trans
cules and coordinated to halogen anions. Indeed, partial struc- octahedral angles are found in the narrow ranges of 81.14(5)–
tural resemblances can solely be found in the complexes 98.18(7) and 169.41(8)–175.24(9)1, respectively (Table 6). To
reported by Mawby & Pringle44 and Schumann et al.45 bearing further help in the evaluation of the variation of the degree of
indenyl and 2-menthylindenyl ligands, respectively. Neverthe- distortion of the MoII octahedron, an adaptation of a simple
less, in these two already known structures the metal center has method proposed by Baur47 to calculate a distortion index (DI)
a strikingly distinct coordination environment from that found for bonds and angles can be employed:
in 2 (see below): they resemble a four-legged piano stool with an
overall coordination number of five.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
n
P
dðMo ꢀ LÞ ꢀ dðMo ꢀ LÞ
i
average
i¼1
The most striking feature of the [(Z3-Ind)Mo(CO)2Cl-
(CH3CN)2] complex is the presence of the indenyl group which,
as shown in Fig. 5, loses planarity upon coordination to the
metal center. The Hinge Angle (HA), the Fold Angle (FA) and the
slip parameter (D) are numerical values that can be determined
from the crystallographic data of 2 that ultimately permit an
unequivocal characterisation of the coordination mode of the
indenyl ligand to the MoII metal center and the determination
of the hapticity of the ligand:46
DIBond
¼
(4)
n
ꢀ
ꢀ
P
ꢀ
ꢀ
dðMo ꢀ LÞi
i¼1
n
ꢀ
ꢀ
ꢀ
ꢀ
P
ffðL ꢀ Mo ꢀ LÞi ꢀ ffðL ꢀ Mo ꢀ LÞideal
i¼1
DIAngle
¼
(5)
n
ꢀ
ꢀ
P
ꢀ
ꢀ
ffðL ꢀ Mo ꢀ LÞi
i¼1
where L corresponds to any coordination site, and subscripts i
and average/ideal indicate individual and average/ideal bond
lengths and angles (see Table 6 for tabulated data). For an
octahedron the value of DIAngle should be calculated indepen-
dently of the cis and trans angles, and always compared to the
ideal values (i.e., 90 and 1801, respectively). In this way, the
DIBond value is calculated as 0.069, while the DIAngle parameters
D ¼ d½Mo ꢀ ðC4; C9Þꢃ ꢀ d½Mo ꢀ ðC1; C3Þꢃ
(1)
HA = +[(C1, C2, C3), (C1, C3, C4, C9)]
(2)
(3)
FA = +[(C1, C2, C3), (C4, C5, C6, C7, C8, C9)]
From the crystallographic determination the values are for the cis and trans angles are 0.066 and 0.028. These values
calculated as HA = 22.9(2)1, FA = 23.03(19)1 and D = 0.753 Å, unequivocally indicate that the dispersion of bond lengths and
which is a clear indication of the Z3 hapticity of the indenyl cis octahedral angles is at the genesis of the distortion of the
ligand (coordinated via the C1, C2 and C3 atoms, Fig. 5).
octahedral coordination environment. This was expected
Considering the centroid of the Z3-indenyl ligand as a single a priori due to the very distinct chemical nature of the various
coordination site (as depicted in Fig. 5), the coordination ligands. These studies further show that despite the bulky
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nature of some of the ligands, the overall coordination poly-
hedron permits their accommodation around the metal center,
leading to near-to-ideal trans angles (as clearly indicated by the
very low DIAngle parameter for the trans angles).
Individual [(Z3-Ind)Mo(CO)2Cl(CH3CN)2] complexes closely
pack in the solid state to yield the crystal structure of compound
2 (Fig. 6) mediated, on the one hand, by the need to effectively fill
the available space and, on the other hand, by a small number of
weak supramolecular contacts, namely C–HÁÁÁCl and C–HÁÁÁp
interactions (see Table 7 for specific geometrical data and Fig. 7
for a schematic representation of the interactions).
On the basis of these structural features, the dissolution of
compound 1 in CH3CN may be accompanied by the formation of
soluble compounds of type 2. On the other hand, the (non-
coordinating) halogenated solvents possibly facilitate the solvation
of 1 by disrupting relatively weak supramolecular interactions
(accounting for faster overall catalytic reaction). In contrast to that
observed for CH3CN as a solvent, no traces of 1 were observed for
DCE after 1 h at 55 1C (spectra j and d of Fig. 1, respectively).
Fig. 6 Crystal packing of [(Z3-Ind)Mo(CO)2Cl(CH3CN)2] (2) viewed in
perspective along the [100] direction of the unit cell. Supramolecular
contacts have been omitted for clarity.
Table 7 Supramolecular contacts present in [(Z3-Ind)Mo(CO)2Cl(CH3CN)2]
(2)a,b
D–HÁ Á ÁA
d(DÁ Á ÁA) (Å)
o(DHA) (degrees)
C13–H13BÁ Á ÁCl1i
3.461(2)
164
Conclusion
We have shown that the organometallic complex [{(Z5-Ind)-
Mo(CO)2(m-Cl)}2] can be used for the catalytic isomerisation of
a-pinene oxide to campholenic aldehyde, giving the desired
product in good yield. The catalytic performance surpasses that
previously found for related complexes of the type [(IndR)Mo-
(Z3-C3H5)(CO)2]. Under the reaction conditions used, the organo-
metallic complex may function as a precatalyst, undergoing
decarbonylation in the presence of air/water to give active species
(in a manner comparable to that already extensively investigated
for molybdenum carbonyl complexes used as catalyst precursors
for the epoxidation of olefins). On the basis of a comparison of
these results with those previously described for transition-
metal-based homogeneous catalysts, further exploration of organo-
molybdenum complexes as (pre)catalysts for this industrially
relevant reaction is warranted.
C–HÁ Á Áp contacts
d(CÁ Á Áp) (Å)
o(CHp) (degrees)
ii
C8–H8Á Á ÁCg1
3.811(3)
149
a
Symmetry transformations used to generate equivalent atoms: (i) ꢀ1 +
b
x, y, z; (ii) 1/2 + x, 1.5 ꢀ y, 1 ꢀ z. Centroids (Cg) (see Fig. 5): Cg1 = C4,
C5, C6, C7, C8 and C9.
Acknowledgements
This work was partly financed by FEDER (Fundo Europeu de
Desenvolvimento Regional) through COMPETE (Programa
Operacional Factores de Competitividade) and by national
˜
ˆ
funds through the FCT (Fundaçao para a Ciencia e a Tecno-
logia) with the projects CICECO – FCOMP-01-0124-FEDER-037271
(FCT ref. PEst-C/CTM/LA0011/2013), FCOMP-01-0124-FEDER-
029779 (FCT ref. PTDC/QEQ-SUP/1906/2012, including the
research grant with ref. BPD/UI89/4864/2013 to A.C.G), the
´
Centro de Quımica Estrutural (PEst-OE/QUI/UI0100/2013),
Fig. 7 Supramolecular arrangement of individual [(Z3-Ind)Mo(CO)2Cl- and REM 2013. The FCT and the European Union are acknowl-
(CH3CN)2] molecular units in the crystal packing of compound 2. Inter-
edged for a post-doctoral grant to S.M.B. (ref. SFRH/BPD/46473/
2008) cofunded by MCTES and the European Social Fund
through the program POPH of QREN. We further wish to thank
actions are based on weak C–HÁ Á ÁCl and C–HÁ Á Áp contacts (purple green
lines). Cg1 stands for the centroid of the six-membered aromatic ring of the
indenyl group. For geometrical details on the represented supramolecular
the FCT for specific funding towards the purchase of the single-
crystal diffractometer. The NMR spectrometers used are part of
contacts see Table 7. Symmetry transformations used to generate equiva-
lent atoms have been omitted for clarity.
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