Synthesis, characterisation, and molecular and electronic structure of CpMoCl2(R1O≡CR2) (R1, R2 = Ph, Et, Me): A new class of half-sandwich 17-electron molybdenum(in) organometallics

Article abstract of DOI:10.1039/b000948m

Addition of alkyne to [CpMoCl₂]₂ affords compounds CpMoCl₂(η²-alkyne) (alkyne = EtC≡CMe, 1; EtOC≡CEt, 2; PhC≡CMe, 3; PhC≡CPh, 4) in good yields. The compounds have been characterised by C, H analyses, IR, EPR and mass spectroscopies, magnetic susceptibility, and cyclic voltammetry. In addition, a single crystal X-ray diffraction analysis has been carried out for compound 4. The alkyne ligand adopts an almost parallel conformation relative to the Cp ring, essentially identical with that of previously reported Nb, Ta, and W analogues. Geometry optimisations on the CpMCl₂(HC≡CH) (M = Nb, Mo) model compounds show that the total energy is nearly independent of the alkyne orientation. The SOMO for M = Mo is an essentially metal-based orbital with a slight Mo·Cl π* component, in agreement with the observed trends in M-C1 bond lengths on going from Group 5 to Group 6. The cyclic voltammetric behaviour of 1-4 is similar to the analogous diene complexes CpMoCl₂(η⁴-diene). A reaction between 1-4 and excess alkyne takes place only for the dialkylsubstituted alkyne complexes under forcing conditions. Compound CpMoCl₂(η⁴-C₄Et₄H), 5, has been isolated from the reaction between 2 and excess EtC≡CEt and crystallographically characterized. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b000948m

View Article Online / Journal Homepage / Table of Contents for this issue
Synthesis, characterisation, and molecular and electronic structure
1
2
1
2
᎐
of CpMoCl (R C᎐CR ) (R , R ؍ Ph, Et, Me): a new class of
᎐
2
half-sandwich 17-electron molybdenum(III) organometallics
Erwan Le Grognec, Rinaldo Poli* and Philippe Richard
Laboratoire de Synthèse et d’Electrosynthèse Organometallique, Faculté des Sciences
“Gabriel”, Université de Bourgogne, 6 Boulevard Gabriel, 21100 Dijon, France.
E-mail: Rinaldo.Poli@u-bourgogne.fr
Received 3rd February 2000, Accepted 9th March 2000
Published on the Web 3rd April 2000
2
᎐
᎐
᎐
Addition of alkyne to [CpMoCl ] affords compounds CpMoCl (η -alkyne) (alkyne = EtC᎐CMe, 1; EtC᎐CEt, 2;
᎐
2 2
2
᎐
᎐
PhC᎐CMe, 3; PhC᎐CPh, 4) in good yields. The compounds have been characterised by C,H analyses, IR, EPR
᎐
᎐
and mass spectroscopies, magnetic susceptibility, and cyclic voltammetry. In addition, a single crystal X-ray
diffraction analysis has been carried out for compound 4. The alkyne ligand adopts an almost parallel conformation
relative to the Cp ring, essentially identical with that of previously reported Nb, Ta, and W analogues. Geometry
᎐
optimisations on the CpMCl (HC᎐CH) (M = Nb, Mo) model compounds show that the total energy is nearly
᎐
2
independent of the alkyne orientation. The SOMO for M = Mo is an essentially metal-based orbital with a slight
Mo-Cl π* component, in agreement with the observed trends in M–Cl bond lengths on going from Group 5 to
Group 6. The cyclic voltammetric behaviour of 1–4 is similar to the analogous diene complexes CpMoCl2(η4-diene).
A reaction between 1–4 and excess alkyne takes place only for the dialkylsubstituted alkyne complexes under forcing
4
᎐
conditions. Compound CpMoCl (η -C Et H), 5, has been isolated from the reaction between 2 and excess EtC᎐CEt
᎐
2
4
4
and crystallographically characterized.
᎐
been reported that the thermal treatment of CpMoCl(PhC᎐
Introduction
᎐
CPh)2 affords several rearrangement products, including the 17-
electron Mo() cyclobutadiene complex CpMoCl2(η4-
C4Ph4).31,32 No mention was made, however, whether an alkyne
complex of formula CpMoCl2(η2-C2Ph2) also forms. Since the
direct addition of neutral ligands to [CpMoCl2]2 appears to be
a versatile method to obtain half-sandwich Mo() derivatives
under mild conditions,33 we have explored this methodology for
the preparation of alkyne adducts and report here the success-
ful synthesis and studies of four members of the hitherto
unreported CpMoCl2(alkyne) family.
Alkyne organometallic chemistry has always attracted much
attention because of the versatility of alkynes in metal-
mediated organic synthesis and catalysis1–5 and more recently
also for their application in materials chemistry.6–9 Group 6
alkyne chemistry is quite extensively studied, the vast majority
of the investigations concerning compounds where the metal
has the formal oxidation state II.10–12 The interactions of alkynes
with Group 6 metals in the oxidation state III are mostly related
to dinuclear M2X6-type (M = Mo, W)1,3,13 and [(C5H4R)WCl2]2
(R = Me, Pri)14–18 compounds. A few mononuclear M(III)-
alkyne complexes, however, have been described both for Mo
19
᎐
᎐
and for W, e.g. WCl (PMe ) (PhC᎐CPh), [Tp*WX(CO)(MeC᎐
Results and discussion
The addition of alkynes R C᎐CR to a THF suspension of
᎐
᎐
3
3 2
CMe)][BF4]
[Tp* = hydrotris(3,5-dimethylpyrazolyl)borate;
1
2
᎐
᎐
X = F, Cl, Br or I],20 [MoCl (THT) (PhC᎐CR)] (R = Me, Et,
᎐
᎐
3
2
[CpMoCl2]2 at the THF reflux temperature yields the soluble
THT = tetrahydrothiophene),21 and Mo(NPr Ar) (PhC᎐CPh)
i
᎐
᎐
1
2
3
᎐
adducts CpMoCl (R C᎐CR ) according to eqn. (1). Thermal
᎐
2
(Ar = 3,5-C6H3Me2).22
Paramagnetic half-sandwich compounds of general formula
(η5-C5R5)MCl2(alkyne) (M = Group 6 metal) appear limited to
two related examples for tungsten. The reaction between
1
2
1
2
᎐
᎐
᎐
[CpMoCl ] ϩ 2 R C᎐CR → 2 CpMoCl (R C᎐CR ) (1)
᎐
2 2
2
(1: R1=Me, R2=Et; 2: R1=R2=Et; 3: R1=Me, R2=Ph;
4: R1=R2=Ph)
t
5
t
᎐
᎐
W(᎐CBu )Cl (dme) and excess RC᎐CR yields (η -C R Bu )-
᎐
᎐
3
5
4
23
᎐
WCl (RC᎐CR) (R = Me, Et), the structure of the 2-butyne
᎐
2
adduct being confirmed by an X-ray analysis.24 These reactions
are not selective and do not appear of potentially general syn-
thetic utility. In addition, the reduction of Cp*WX4 (X = OPh,
Cl) with sodium amalgam in the presence of alkynes was
reported to afford the corresponding W() species Cp*WX-
(alkyne)2 with no mention of W() intermediates.25
decomposition of the products does not take place under these
conditions and all products were recovered in good to excellent
yields. The rate of the reaction decreases in the order
1,2 > 3 > 4, compound 1 being also obtained within a reason-
able time (4 h) when the reaction is conducted at room temper-
ature. For compound 4, an excess of alkyne may be used with
no detriment to the yield, and no reaction occurs upon pro-
longed treatment of isolated 4 with a second equivalent of
diphenylacetylene in refluxing THF. For compounds 1 and 2,
on the other hand, the stoichiometric amount of alkyne must
be used in order to avoid the formation of by-products (vide
infra).
For molybdenum, it has been shown that the treatment of
᎐
CpMoCl(CF C᎐CCF ) with dienes involves a disproportion-
᎐
3
3 2
ation process resulting in the isolation of the Mo()-diene
products CpMoCl2(diene).26–28 Whether this transformation
involves Mo()-alkyne or Mo()-diene intermediates does not
appear to have been the subject of investigations. We have more
recently prepared the same diene compounds by the high-yield,
straightforward addition of diene to [CpMoCl2]2.29,30 It has also
It is interesting to observe that while the alkyne addition to
the corresponding tungsten starting materials, [(C5H4R)WCl2]2
DOI: 10.1039/b000948m
J. Chem. Soc., Dalton Trans., 2000, 1499–1506
This journal is © The Royal Society of Chemistry 2000
1499

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(R = Me or Pri), gave only the dinuclear, metal–metal bonded
products (C5H4R)2W2Cl4(µ-C2RЈ2) or (C5H4R)2W2Cl4(µ-C4RЈ4)
depending on stoichiometry and conditions,14–16 reaction (1)
produces only mononuclear paramagnetic products, in agree-
ment with the reduced strength of the Mo–Mo bond relative to
the W–W bond in the starting material. The mononuclear (η5-
expected for the 17-electron configuration of the complexes
(when considering the alkyne as a σ ϩ π 4-electron donor). In
this respect, compounds 1–4 are perfectly isoelectronic with
the growing class of CpMoX2L2 compounds.35 Molybdenum
satellites due to 95Mo(15.9%) and 97Mo(9.5%), both having
I = 5/2, are visible for both the isotropic spectrum and the
perpendicular component of the frozen solution spectrum (see
t
23
᎐
C R Bu )WCl (RC᎐CR) (R = Me, Et) complexes, analogous
᎐
5
4
2
to our compounds 1–4, were obtained by a different synthetic
strategy starting from a mononuclear carbyne precursor. It is
also of relevance that the reaction between [(C5H4Pri)MoCl2]2
and 2-butyne was mentioned to afford intractable products.34
Compounds 1–4 are air sensitive but can be stored in the
solid state under argon without significant decomposition.
They are readily soluble in THF and CH2Cl2, only partially
soluble in diethyl ether and completely insoluble in pentane.
Satisfactory C, H elemental analyses for 3 and 4 could only be
obtained in the presence of V2O5 as a combustion catalyst. The
IR spectra confirm the presence of coordinated alkyne ligands,
Table 1). A nearly tetragonal g tensor was also observed for
᎐
the related alkyne complexes of W(), [Tp*WX(CO)(MeC᎐
᎐
CMe)][BF4] (X = F, Cl, Br, I).20
A mononuclear formulation is finally confirmed by the X-ray
structure of 4, which is illustrated in Fig. 1. The most relevant
feature of the geometry is the three-legged piano stool dis-
position, with the alkyne ligand adopting a conformation
almost parallel to the Cp plane. The angle θ between the Mo–
C(6)–C(7) plane and the plane defined by Mo, CNT (center of
gravity of the Cp ring), and the midpoint of the C(6)–C(7)
vector is 85.61Њ. This tilting away from the ideal parallel con-
formation (θ = 90Њ) is less pronounced relative to the previously
Ϫ1
᎐
a weak ν(C᎐C) absorption being observed at 1730 cm for 1,
᎐
1732 for 2, 1717 cmϪ1 for 3 and 1700 cmϪ1 for 4, i.e. >400 cmϪ1
lower relative to the free alkynes. Although the quantitative
relationship between stretching frequency and C–C bond order
does not appear to have been carefully explored, larger shifts
are usually associated with the 4-electron (σ ϩ π) coordination
mode, the largest observed shifts being around 450 cmϪ1.2 The
mass spectra show the molecular ion for each compound, in
agreement with a mononuclear formulation. The mononuclear
nature of the products is also consistent with the paramagnet-
ism, as indicated by the EPR spectra in solution at room
temperature and confirmed by the magnetic susceptibility
measurement for 1, 3 and 4, yielding a magnetic moment close
to the spin-only value for one unpaired electron.
reported W() complex (η -C Me Bu )WCl (MeC᎐CMe) (θ =
5
t
᎐
᎐
5
4
2
77.16Њ)24 and slightly more relative to Cp*TaCl (PhC᎐CPh)
᎐
᎐
2
(θ = 87.27Њ)36 and (η -C H Me)NbCl (ArC᎐CAr) (Ar = C -
5
᎐
᎐
5
4
2
6
H4CH3-4, θ = 86.40Њ).37
Table 2 collects relevant bond distances and angles, in com-
parison with those of the above mentioned Nb analogue (η5-
38
᎐
᎐
C H Me)NbCl (ArC᎐CAr) and of geometry optimised HC᎐
᎐
᎐
5
4
2
CH model compounds (vide infra). It can be seen from Table 2
that while the M–CNT and M–C(alkyne) distances are signifi-
cantly shorter for Mo relative to Nb, the M–Cl distances are
The EPR spectra are characterised by a sharp isotropic
signal at room temperature, while frozen solutions at low temp-
eratures show an essentially tetragonal g tensor. This is as
Table 1 EPR parameters for compounds 1–4 in CH2Cl2 :THF (1:2)
T = 298 K
T = 100 K
Mo
giso
|Aiso
|
g||
g⊥
|A⊥ |
1
2
3
4
1.987
1.987
1.988
1.989
37.5
37.5
38.6
38.9
2.043
2.043
2.040
2.038
1.970
1.969
1.971
1.971
63.0
63.2
61.5
61.7
Fig. 1 An ORTEP57 view of compound 4 with thermal ellipsoids
drawn at the 30% probability level. Hydrogen atoms are omitted for
clarity.
Table 2 Comparison between experimental and calculated bond distances and angles for compounds CpMCl2(alkyne) (M = Nb, Mo)
M = Nb
M = Mo
Experimentala
Optimisedb
Experimentalc
Optimisedb
M–CNT
M–Cl
M–Cl
M–C
2.113
2.365(2)
2.353(2)
2.079(5)
2.063(5)
1.307(7)
115.7
115.2
111.9
114.1
99.4(1)
36.8(2)
72.3(3)
70.9(3)
142.3
2.180
2.426
2.426
2.096
2.096
2.010(5)
2.375(9)
2.370(9)
2.034(3)
2.021(3)
1.303(4)
120.6(3)
116.0(4)
113.3(3)
116.0(4)
89.12(4)
37.47(11)
71.8(2)
2.087
2.423
2.423
2.056
2.056
M–C
᎐
C᎐C
1.328
1.324
᎐
CNT–M–Cl
CNT–M–Cl
CNT–M–C
CNT–M–C
Cl–M–Cl
114.38
114.38
114.88
114.88
101.69
36.93
71.54
71.54
145.97
145.97
117.29
117.29
116.09
116.09
91.93
37.56
71.22
71.22
143.53
143.53
C–M–C
᎐
M–C᎐C
᎐
᎐
M–C᎐C
70.7(2)
145.1(3)
145.8(3)
᎐
᎐
C–C᎐C
᎐
᎐
C–C᎐C
144.3
᎐
a
5
᎐
For compound (η -C5H4Me)NbCl2(ArC᎐CAr) (Ar = C6H4CH3-4) (ref. 37); the figures without standard deviations were calculated from the
᎐
published crystal data. b For the model compounds CpMCl2(HC᎐CH) with a parallel orientation of the ethyne ligand. c For compound 4.
᎐
᎐
1500
J. Chem. Soc., Dalton Trans., 2000, 1499–1506

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the minimisation of the steric repulsion between the cyclo-
pentadienyl and alkyne ligands. The optimised parameters
reported in Table 2 are those of the parallel geometry, which is
closest to the experimentally determined geometry.
Analogous calculations have been carried out on the niobium
᎐
model system CpNbCl (HC᎐CH), the results being also shown
᎐
2
in Fig. 2. In this case, a full optimisation leads to an essentially
Cs-symmetric geometry with a perpendicular ethyne ligand.
The Cs-imposed parallel orientation is found, like for Mo, to
have an energy very close to that of the perpendicular conform-
ation. The optimised distances and angles reported in Table 2
are those relating to the parallel geometry, which is closest to
the experimental structure. A comparison between calculated
and experimental parameters shows again the adequacy of the
computational method. It seems, therefore, that the choice of
geometry for Group 5 CpMCl2(alkyne) compounds, like for
the analogous Group 6 compounds, is dictated by the steric
requirements.
Additional insights are given by the orbital analysis. The
energy and shape of relevant Kohn–Sham orbitals retrieved
from the DFT calculations are shown in Fig. 3 for both Nb and
Mo systems with the ethyne ligand both in the perpendicular
and in the parallel conformation. For the Mo complex, only the
α spin orbitals are shown (the shapes of the corresponding
β spin orbitals are essentially identical), while the energy
reported is the average of α and β. Orbital no. 38 is the HOMO
for the Nb system while the Mo system has an additional
unpaired electron in orbital no. 39. The latter is essentially a
metal-based orbital. However, there is a significant partici-
pation of Cl lone pairs for the Mo system, giving rise to a Mo–
Cl π* combination. This rationalises why, on going from Nb to
Mo, the M–Cl bond length does not significantly shorten: the
effect of the metal radius reduction is compensated by the Mo–
Cl π* contribution of the additional electron. The alkyne-to-M
π interaction (π⊥) gives a bonding combination which is evident
in orbital no. 28, and the corresponding antibonding compon-
ent in orbital no. 40. The alkyne-to-M σ donation is in a deeper
energy orbital, while the orbitals no. 29–37 (not shown) contain
the M–Cl σ bonds, the Cl p lone pairs, and the M–Cp σ and
π interactions. There is heavy symmetry-allowed mixing of
various bonding components in individual orbitals. Orbital
no. 38 contains the M–alkyne π back-bonding interaction (π||).
Its lower energy and greater percentage contribution of metal
and alkyne orbitals for M = Nb suggests a greater degree of
back-bonding for this metal, in agreement with the observed
structural differences (vide supra).
Fig. 3 allows one to appreciate the reason for the small
dependence of the total energy on the alkyne configuration.
The alkyne ligand is able to establish both π interactions with
the metal center no matter what the relative orientation of
alkyne and M–Cp axis. In a reference system where z coincides
with the M–CNT vector and the alkyne midpoint lies on the xz
plane, the perpendicular conformation involves a π⊥ interaction
with an essentially xy metal orbital and a π|| interaction with a
(x2 Ϫ z2) orbital, whereas the same two metal orbitals trade
places on going to the parallel conformation. Thus, these two
orbitals may generate linear combinations suitable to establish
equally efficient π⊥ and π|| interactions for any alkyne conform-
ation, effectively following the rotation of the alkyne ligand
around the Mo–alkyne axis. For this reason, little or no elec-
tronic barrier is expected against alkyne rotation and the
observed conformation is largely determined, for both Group 5
and 6 metals, by the intramolecular steric interactions.
᎐
B3LYP/LANL2DZ total energies for CpMCl2(HC᎐CH)
᎐
Fig.
2
(M = Nb, Mo) at different ethyne conformations.
essentially identical. The same trend can be noticed by com-
᎐
paring the previously described Cp*TaCl (PhC᎐CPh) and
᎐
2
5
t
24,36
᎐
(η -C Me Bu )WCl (MeC᎐CMe).
A general shortening is
᎐
5
4
2
expected as a result of the smaller atomic radius of a Group 6
metal relative to the corresponding Group 5 metal in the same
row and formal oxidation state. The alkyne C–C distance is
essentially identical with that of the Nb complex (Table 2) and
of the W complex (1.312(10) Å),24 while this is longer in the Ta
compound (1.337(8) Å).36 In other structurally characterised
᎐
Mo()-alkyne complexes, e.g. MoCl (THT) (PhC᎐CR) (THT =
᎐
3
2
tetrahydrothiophene; R = Me, Et),21 the alkyne C–C distance is
less accurately determined but does not appear to be signifi-
cantly different than for compound 4 (i.e. 1.27(1), 1.31(1) and
᎐
᎐
1.32(1) Å). The C–C᎐C–C framework is quite bent (C–C᎐C
᎐
᎐
angles around 145Њ), only slightly less so than in the above men-
tioned Ta, W and Nb compounds (averages of 139.4(9),
᎐
140.3(7)Њ and 143.3(10)Њ, respectively). Together with the C᎐C
᎐
distance, this parameter confirms that the alkyne ligand acts as
a four-electron donor.
The bonding features of this class of alkyne complexes has
been further investigated at the theoretical level. The electronic
structure for CpMCl2(alkyne) systems has only been previously
explored by Curtis et al. for M = Nb at the extended Huckel
level, at the fixed geometry determined by X-ray diffraction.37
We have carried out geometry optimisations at the B3LYP/
᎐
LANL2DZ level on the model compound CpMoCl (HC᎐CH).
᎐
2
As is now well established, this level of theory affords satisfac-
tory geometries and energies for medium-size organometallic
compounds. A full optimisation indeed yields a geometry with
bond distances and bond angles close to the experimental
values. As is typical for this computational technique, distances
are slightly overestimated (up to 0.07 Å), whereas bond angles
are reproduced to within 3Њ. However, the orientation of the
ethyne ligand is quite different from experiment, θ being only
30.78Њ.
Subsequent optimisations have been carried out with the eth-
yne ligand forced to adopt a parallel and a perpendicular orien-
tation by imposing a Cs symmetry to the molecule. The results
(Fig. 2) show that the total energy is almost independent on the
alkyne orientation and suggest that the adoption of a close to
parallel geometry for compound 4 [and by extension also by the
The electrochemical investigation of compounds 1–4 by
cyclic voltammetry reveals both a one-electron reduction and a
one-electron oxidation process. The reduction wave appears at
relatively large negative potentials vs. the ferrocene standard
(see Table 3) and is reversible (ia/ic = 1, ∆Ep ca. 60 mV) for com-
pounds 3 and 4 in the 100–5000 mV sϪ1 range of scan speeds.
For compounds 1 and 2, on the other hand, the reduction wave
5
t
᎐
tungsten analogue (η -C Me Bu )WCl (MeC᎐CMe)] is due to
᎐
5
4
2
J. Chem. Soc., Dalton Trans., 2000, 1499–1506
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becomes reversible only above 2000 mV sϪ1 and, at lower scan
speeds, the ia/ic becomes smaller and smaller. Since no new wave
appears in the voltammogram at low scan rate, the reduction
products [1]Ϫ and [2]Ϫ must decompose to redox-inactive
species which remain so far uncharacterised. The oxidation
process, on the other hand, is completely irreversible up to 5 V
sϪ1 for all derivatives. In this respect, these compounds behave
similarly to the previously described diene complexes, CpMoCl2-
(η4-diene),26–28,30 while derivatives with more electron donating
ligands (phosphines, phosphine-thioethers, phosphine-oxazo-
lines, and phosphine-amides)35,39,40 show a reversible oxidation
process at much lower potentials and no reduction process or, at
the most, an irreversible reduction before the solvent discharge.
A closer comparison of the reduction potentials between the
isoelectronic complexes with the 4-electron donors alkyne
(σ ϩ π) and diene (2σ) ligands in Table 3 indicates that the
alkynes transfer about the same amount of electron density to
the metal center. The negative shift of the reduction potential
on going from the less alkyl substituted alkyne complex 4 to
the more alkyl substituted complexes 1 and 2 is in agreement
with the greater electron-donating properties of alkyls. The
same trend is observed for the oxidation potentials (1,2 < 3 < 4).
However, the potentials of the oxidation processes will also
reflect the kinetics of the follow-up chemical processes and
cannot be used as a thermodynamic gauge of the metal electron
density.
When reaction (1) was carried out in the presence of excess
diphenylacetylene, no detriment to the yields of 4 and no addi-
tional product were observed. However, a second resonance
grows in the EPR spectrum for the 2-pentyne and 3-hexyne
reactions with an excess of the alkyne under forcing conditions.
The 3-hexyne reaction has been subjected to more detailed
investigations. A new compound (characterised by an EPR
resonance at g = 1.996; aMo = 34.6 G) forms only quite slowly
and a complete consumption of 2 was achieved only upon
prolonged reflux in neat 3-hexyne (bp 82 ЊC). The reaction,
however, is not selective and the Mo-containing, EPR active
product could not be obtained in a pure form. However, a mass
spectrum of the crude product mixture exhibits, in addition to
the peaks of 2, peaks attributable to a compound of formula
CpMoCl2(C4Et4). On the basis of the existence of the cyclo-
butadiene complex CpMoCl2(η4-C4Ph4)31,32 and on the EPR
properties which indicate a 17-electron species, we tentatively
suggest a cyclobutadiene structure also for this compound.
Crystallization of the crude product from this reaction afforded
instead a small amount of the previously reported41 CpMoCl2-
(η4-C4Et4H), 5.
The NMR properties for the isolated crystals of 5 match with
those previously reported41 and a single crystal X-ray analysis
(see Fig. 4 and Table 4) confirms the structural assignment. In
particular, the hydrogen atom of the η4(5e)-butadienyl ligand
was directly located from the difference Fourier map and
freely refined. While the quality of the data affords metric
parameters of greater precision, the structural parameters of
5 compare quite closely with those previously reported for the
analogous CpMoBr2(η4-C4Et4H).41 No further discussion of
the structure of 5 is therefore warranted here, except for
pointing out that the structural data and the NMR data con-
cur in interpreting the metal–butadienyl bonding mode as
mostly I (Fig. 5).
Table 3 Cyclic voltammetric data for compounds 1–4 in THF at room
temperature and comparison with related η4-diene complexes
E1/2(MoII/
Ep.a(MoIII
/
a
Compound
MoIII
)
MoIV
)
Ref.
2
᎐
CpMoCl2(η -EtC᎐CMe), 1
Ϫ1.32
Ϫ1.33
Ϫ1.20
Ϫ1.14
Ϫ1.11
Ϫ1.19
Ϫ1.24
0.14
0.16
0.22
0.32
0.73
0.75
0.66
This work
This work
This work
This work
28,56
56
56
᎐
2
᎐
CpMoCl2(η -EtC᎐CEt), 2
᎐
We presume that, under the forcing conditions used for the
reaction with excess alkyne, a hydrogen atom abstraction from
the medium by either the cyclobutadiene product or by a reac-
tion intermediate may be responsible for the formation of 5.
This compound was previously obtained starting from a bis-
alkyne Mo() precursor, where the C–C coupling process was
presumed to follow the protonation of one alkyne ligand to
2
᎐
CpMoCl2(η -PhC᎐CMe), 3
᎐
2
᎐
CpMoCl2(η -PhC᎐CPh), 4
᎐
CpMoCl2(η4-C4H6)
CpMoCl2(η4-C4H5Me-2)
CpMoCl2(η4-C4H4Me2-2,3)
a All potentials are measured at a scan rate of 200 mV sϪ1
.
᎐
Fig. 3 Energy and shape of relevant orbitals for CpMCl2(HC᎐CH) (M = Nb, Mo).
᎐
1502
J. Chem. Soc., Dalton Trans., 2000, 1499–1506

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Table 4 Selected bond distances (Å) and angles (Њ) for compound 5
whereas for related tungsten chemistry the same process occurs
on a dinuclear CpW() species.14–16
Mo–CNT
Mo–Cl(1)
Mo–Cl(2)
Mo–C(6)
Mo–C(7)
1.986(3)
2.4652(7)
2.4874(6)
1.934(2)
2.329(2)
Mo–C(8)
Mo–C(9)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
2.444(2)
2.303(3)
1.438(4)
1.404(4)
1.440(4)
Conclusions
The alkyne addition to [CpMoCl2]2 provides a facile entry to
hitherto unknown half-sandwich Mo() alkyne complexes
under mild conditions. The electronic structure of these com-
plexes is quite similar to that of the corresponding Group 5
derivatives, the additional electron residing in an essentially
pure metal orbital which is not engaged in the metal–alkyne
interaction. As a consequence, the nature of the metal–alkyne
interaction is rather similar for Group 5 and Group 6 com-
pounds, including the slight dependence of the energy on the
alkyne conformation. On the other hand, the oxidation to
afford isoelectronic species with the Group 5 complexes is
irreversible, while the reduction to anionic 18-electron com-
plexes is reversible on the cyclic voltammetric time scale. The
electrochemical investigations further indicate that the
CpMoCl2(η2-alkyne) system is about as electron-rich as the
CpMoCl2(η4-diene) system, while the reactivity study shows
that a 2 ϩ 2 cycloaddition to afford a cyclobutadiene system
has a high activation barrier for this system.
CNT–Mo–Cl(1) 112.6(2)
CNT–Mo–Cl(2) 111.7(2)
C(9)–Mo–Cl(1)
C(7)–Mo–Cl(1)
C(6)–Mo–Cl(2)
C(9)–Mo–Cl(2)
C(7)–C(6)–Mo
C(8)–C(9)–Mo
C(10)–C(6)–Mo
C(16)–C(9)–Mo
C(8)–C(9)–C(16)
C(7)–C(6)–C(10)
C(8)–C(7)–C(6)
C(7)–C(8)–C(9)
137.51(7)
84.55(7)
140.61(8)
80.85(7)
86.0(2)
CNT–Mo–C(6)
CNT–Mo–C(9)
Cl(1)–Mo–Cl(2)
C(6)–Mo–C(9)
C(6)–Mo–C(7)
C(9)–Mo–C(7)
C(3)–Mo–C(7)
C(6)–Mo–C(8)
C(9)–Mo–C(8)
C(7)–Mo–C(8)
C(6)–Mo–Cl(1)
106.5(2)
109.7(2)
80.36(2)
77.52(10)
38.02(10)
63.06(9)
131.65(9)
65.26(9)
35.16(9)
34.11(9)
93.97(8)
77.8(2)
147.9(2)
126.2(2)
122.9(2)
126.1(2)
115.2(2)
116.8(2)
Experimental
All reactions involving air- and moisture-sensitive organo-
metallic compounds were carried out in a Jacomex glove box or
by the use of standard Schlenk techniques under an argon
atmosphere. The solvents were dried by conventional methods
(THF, Et2O, toluene and pentane from sodium benzophenone
ketyl and CH2Cl2 from P4O10) and distilled under argon prior to
use. EPR measurements were carried out at the X-band micro-
wave frequency on a Bruker ESP300 spectrometer. All EPR
spectral parameters for the new compounds are collected in
Table 1. The spectrometer frequency was calibrated with DPPH
(diphenylpicrylhydrazyl, g = 2.0037). Cyclic voltamograms
were recorded with an EG&G 362 potentiostat connected to a
Macintosh computer through MacLab hardware/software.
The electrochemical cell was fitted with a Ag/AgCl reference
electrode, a platinum disk working electrode and a Pt wire
counterelectrode. Bu4NPF6 (ca. 0.1 M) was used as supporting
electrolyte. All potentials are reported relative to the ferrocene
standard, which was added to each solution and measured at
the end of the experiments. All the voltammetric results
obtained in this study are collected in Table 3. The mass spectra
were obtained by electron impact at 70 eV on a Kratos Concept
32S spectrometer by direct introduction of solid samples. The
magnetic susceptibility measurements were carried out at room
temperature with a Johnson Matthey magnetic balance which
operates with a modified Gouy method, whereby the effect of
the sample on the weight of the magnet, rather than the effect
of the magnet on the weight of the sample, is used to retrieve
the susceptibility information. The elemental analyses were
carried out by the analytical service of the Laboratoire de
Synthèse et d’Electrosynthèse Organométallique with a Fisons
Fig. 4 An ORTEP view of compound 5 with thermal ellipsoids drawn
at the 50% probability level. Hydrogen atoms except for H(9) of the
η4(5e)-butadienyl ligand are omitted for clarity.
Fig. 5 Possible bonding modes for the butadienyl ligand.
afford a reactive alkyne–vinyl intermediate.41 A similar process
has also been observed for related dithiocarbamato derivatives
of Mo() and W().42
᎐
᎐
᎐
᎐
EA 1108 apparatus. EtC᎐CMe, EtC᎐CEt, PhC᎐CMe, PhC᎐
᎐
᎐
᎐
᎐
CPh were purchased from Aldrich Chemical Co. and used as
received. Compound [CpMoCl2]2 was prepared as described in
the literature.43,44
The failure to obtain the previously reported tetraphenyl-
᎐
cyclobutadiene complex from 4 ϩ PhC᎐CPh means that the
᎐
barrier to the 2 ϩ 2 cycloaddition reaction must be pro-
2
᎐
Synthesis of CpMoCl (ꢀ -EtC᎐CMe), 1
᎐
2
᎐
hibitively high, higher than for the 2 ϩ EtC᎐CEt reaction, on
᎐
this metal system. Thus, the reported formation of CpMoCl2-
To a suspension of [CpMoCl2]2 (0.668 g, 1.44 mmol) in 20 mL
of THF was added 2-pentyne (277 µL, 2.88 mmol) via a syringe.
The mixture was then refluxed for 30 minutes, transforming the
initial pale brown suspension to a dark brown solution. This
solution was filtered through Celite and evaporated to dryness
to yield an oily residue. Diethyl ether (10 mL) was added and
the suspension was vigorously stirred to afford 1 as a brown
4
31,32
᎐
(η -C Ph ) by thermolysis of CpMoCl(PhC᎐CPh)
most
᎐
4
4
2
likely does not involve compound 4 as an intermediate, while a
more likely possibility is oxidative coupling of the alkyne
ligands to generate a CpMoCl(η4-C4Ph4) intermediate which
subsequently disproportionates. It is notable that alkyne
dimerization occurs on a mononuclear CpMo() species,
J. Chem. Soc., Dalton Trans., 2000, 1499–1506
1503

View Article Online
powder. The supernatant was decanted off and the solid was
washed with several portions of pentane (total ca. 30 mL).
Yield: 0.604 g, 70%. Anal. Calc. for C10H13Cl2Mo: C, 40.03; H,
4.37. Found: C, 40.32; H, 4.22%. IR (Nujol mull, cmϪ1): 1730
(MoCl pattern, 3.8%). The remainder of the solution was evap-
orated to dryness and the residue was extracted with diethyl
ether (5 mL) followed by filtration through Celite. The solution
was concentrated to ca. half volume and a layer of pentane (5
mL) was left to diffuse slowly at room temperature. Red crystals
of compound CpMoCl2(η4-C4Et4H), 5, formed within 1 week.
ϩ
᎐
[m, ν(C᎐C)], MS (EI, 70 eV): m/z 301, [M] (MoCl2 pattern,
᎐
ϩ
2%); 233 [CpMoCl2]ϩ (MoCl pattern, 6%), 68 [EtC᎐CMe]
᎐
᎐
2
(47%). µeff = 1.60µB (diamagnetic correction = Ϫ148 × 10Ϫ6
One of the crystals was used for the X-ray analysis. H NMR
1
cgsu).
(CDCl3): δ 5.32 (s, 5H, Cp), 3.28 (m, 2H, CH2), 2.67 (m, 2H,
CH2), 2.58 (m, 2H, CH2), 2.34 (m, 2H, CH3CH2CH), 2.00 (dd,
1H, J1 = 3.5 Hz, J2 = 8.0 Hz, C4Et4H), 1.42 (t, 3H, J = 8.0 Hz,
CH3CH2CH), 1.41 (t, 3H, J = 7.5 Hz, CH3), 1.11 (t, 3H, J = 7.5
Hz, CH3), 0.99 (t, 3H, J = 7.5 Hz, CH3). 13C{1H} NMR
(CDCl3): δ 294.8 (Mo=CEt), 139.1 (CEt), 116.6 (CEt), 102.6
(Cp), 76.5 (CHEt), 36.4 (CH2), 24.4 (CH2), 23.9 (CH2), 20.3
(CH2), 17.4 (CH3), 14.5 (CH3), 13.3 (CH3), 9.2 (CH3).
2
᎐
Synthesis of CpMoCl (ꢀ -EtC᎐CEt), 2
᎐
2
To a suspension of [CpMoCl2]2 (0.905 g, 1.95 mmol) in 20 mL
of THF was added 3-hexyne (440 µL, 3.9 mmol) via a syringe.
The mixture was then refluxed for 2 h, transforming the initial
pale brown suspension to a dark brown solution. This solution
was filtered through Celite and evaporated to dryness to yield
an oily residue. The product 2 was extracted with 40 mL of
refluxing diethyl ether. Following filtration of the hot solution,
crystals formed upon cooling to 4 ЊC. The supernatant was
decanted off and the crystals were dried in vacuo. Yield: 0.612 g,
50%. Anal. Calc. for C11H15Cl2Mo: C, 42.07; H, 4.80. Found : C,
Crystal structure determinations
(a) Compound 4. Crystals for the X-ray structure analysis
were grown from a slow diffusion of pentane in a CH2Cl2 solu-
tion at room temperature. A prism of 0.48 × 0.36 × 0.24 mm3
was cut from a red needle and sealed under argon in a capillary.
A total of 25 reflections was used for an accurate orthorhombic
cell determination and three reflections were selected to check
the Laue group. A total of 1860 reflections were collected at
room temperature up to sin(θ)/λ = 0.623 ÅϪ1 on an Enraf-
Nonius CAD4 diffractometer. The data were corrected for
Lorentz and polarization effects45 and for absorption (psi-scan
method).46 A 3% decay was linearly corrected. The structure
was solved via a Patterson search program and refined in
the polar space group Pna21 with full-matrix least squares
methods47 based on |F2|. All non-hydrogen atoms were refined
with anisotropic thermal parameters. Hydrogen atoms were
included in their calculated positions and refined with a riding
model. At the end of this refinement the agreement indices were
wR2 = 0.0526 for all data and R1 = 0.0196 for 1764 intensities
with I > 2σ(I), the absolute structure parameter48 was
x = 0.00(5) (x = 0.19(5) and wR2 = 0.0539 for the inverted struc-
ture). The final difference electron density is featurless:
∆ρ = 0.256 and Ϫ0.217 e ÅϪ3. Crystal data and final refinement
parameters are reported in Table 5.
Ϫ1
᎐
41.63; H, 4.75%. IR (Nujol mull, cm ): 1732 [m, ν(C᎐C)],
᎐
MS (EI, 70 eV): m/z 315, [M]ϩ (MoCl2 pattern, 13%); 233
ϩ
ϩ
᎐
[CpMoCl ] (MoCl pattern, 67%), 82 [EtC᎐CEt] (56%).
᎐
2
2
2
᎐
Synthesis of CpMoCl (ꢀ -PhC᎐CMe), 3
᎐
2
To a suspension of [CpMoCl2]2 (0.720 g, 1.55 mmol) in THF in
20 mL was added 1-phenyl-1-propyne (388 µL, 3.1 mmol) via a
syringe. The mixture was then refluxed for 2 hours yielding a
red brown solution. The solution was filtered through Celite
and concentrated to ca. 3 mL. Diethyl ether (10 mL) was then
added under a vigorous stirring, precipitating a fine red-brown
powder. The supernatant was cannulated off and the residue
was washed with several portions of pentane (total ca. 50 mL).
Yield: 0.750 g, 70%. Anal. (obtained in the presence of V2O5 as
a combustion catalyst) Calc. for C14H13Cl2Mo: C, 48.31; H, 3.76.
Found: C, 48.16; H, 3.65%. IR (Nujol mull, cmϪ1): 1717 [m,
ϩ
᎐
ν(C᎐C)], MS (EI, 70 eV): m/z 349 [M] (MoCl pattern, 4%), 233
᎐
2
[CpMoCl2]ϩ (MoCl2 pattern, 6%), 116 [PhC᎐CMe] (80%).
ϩ
᎐
᎐
µeff = 1.64µB (diamagnetic correction = Ϫ157 × 10Ϫ6 cgsu).
2
᎐
Synthesis of CpMoCl (ꢀ -PhC᎐CPh), 4
᎐
2
(b) Compound 5. A small red crystal of 5 (0.05 × 0.62 × 0.12
mm3) suitable for an X-ray analysis was obtained by diffusion
of pentane into a diethyl ether solution at 4 ЊC. The crystal was
mounted on an Enraf-Nonius KappaCCD using Mo-Kα radi-
ation and 10243 reflections (3924 unique) were collected up to
sin(θ)/λ = 0.65 at 110 K. Absorption corrections were applied to
the data during integration by the SCALEPACK algorithm.49
The structure was solved via a Patterson search program
To a pale brown suspension of [CpMoCl2]2 (1.438 g, 3.1 mmol)
in 40 mL of THF was added a THF solution of diphenyl-
acetylene (1.3 g, 7.3 mmol). The mixture was heated to 70–
80 ЊC for 2 hours, yielding a red solution. After cooling to room
temperature, the solvent was reduced in volume to ca. 7 mL and
pentane (30 mL) was added to complete the precipitation of the
product. The supernatant was decanted off, and the brick-red
powder was washed with several portions of pentane (total ca.
50 mL). Yield: 2.150 g, 85%. Recrystallisation by slow diffusion
of pentane into a CH2Cl2 solution afforded dark red crystals.
Anal. (obtained in the presence of V2O5 as a combustion
catalyst) Calc. for C19H15Cl2Mo: C, 55.64; H, 3.69. Found: C,
¯
and refined (space group P1) with full-matrix least-squares
methods based on |F2|.47 All non-hydrogen atoms were refined
with anisotropic thermal parameters. Except for H(9), the
hydrogen atoms of the complex were included in their calcu-
lated positions and refined with a riding model. The H(9)
hydrogen atom attached to the C(9) carbon atom was located in
a Fourier difference map and freely refined with an isotropic
temperature factor. Final agreement indices are reported in
Table 5.
Ϫ1
᎐
55.29; H, 3.65%. IR (Nujol mull, cm ): 1700 [m, ν(C᎐C)], MS
᎐
(EI, 70 eV): m/z 411 [M]ϩ (MoCl2 pattern, 2%), 233 [CpMoCl2]ϩ
ϩ
᎐
(MoCl pattern, 6.5%), 178 [PhC᎐CPh] (100%). µeff = 1.61µB
᎐
2
(diamagnetic correction = Ϫ178 × 10Ϫ6 cgsu).
CCDC reference number 186/1895.
See http://www.rsc.org/suppdata/dt/b0/b000948m/ for crys-
tallographic files in .cif format.
᎐
Reaction between 2 and excess EtC᎐CEt
᎐
Compound 2 (50 mg, 0.16 mmol) was dissolved in 3-hexyne (2
mL) and the resulting red solution was refluxed for 15 h with
EPR monitoring. The initial EPR signal (g = 1.987, aMo = 37.5)
was slowly replaced by a new signal at g = 1.996, aMo = 34.6 G.
During this time the color changed to dark yellow and a white
precipitate formed. An aliquot of the final solution was evapor-
ated to dryness and the residue was redissolved in the minimum
amount of CH2Cl2 for the mass spectral analysis. MS (EI, 70
eV): m/z 397 [M]ϩ (MoCl2 pattern, 5%), 361 {[M]ϩ Ϫ HCl}
Computational details
All calculations were performed using Gaussian 9450 on an SGI
Origin200 workstation. The LANL2DZ basis set was employed
to perform geometry optimisations with a DFT approach. The
three parameter form of the Becke, Lee, Tang and Parr func-
tional (B3LYP)51 was used in all cases. The LANL2DZ basis set
includes both Dunning and Hay’s D95 sets for H, C, N and O,52
1504
J. Chem. Soc., Dalton Trans., 2000, 1499–1506

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Table 5 Crystal data and structure refinement for compounds 4 and 5
Compound
4
5
Formula
M
C19H15Cl2Mo
410.15
C17H26Cl2Mo
397.22
T/K
Crystal system
Space group
293(2)
Orthorhombic
Pna21
110(2)
Triclinic
¯
P1
a/Å
b/Å
c/Å
18.958(6)
12.485(1)
7.187(1)
7.4272(4)
8.5344(3)
13.7739(6)
α/Њ
β/Њ
89.929(3)
79.954(3)
γ/Њ
86.459(2)
858.01(7)
V/Å3
1701.1(6)
Z
λ/Å
4
2
0.71073
0.71073
Diffractometer
Nonius CAD4
1.078
0.48 × 0.36 × 0.24
1860
Nonius Kappa CCD
1.065
0.050 × 0.062 × 0.118
10243
µ/mmϪ1
Crystal size/mm3
RC = reflections collected
IRC = independent RC
IRCGT = IRC and [I > 2σ(I)]
R for IRCGT
R for IRC
1860
1764
3924 [R(int) = 0.04]
3546
R1a = 0.0198, wR2b = 0.0512
R1a = 0.0312, wR2b = 0.0673
R1a = 0.0378, wR2b = 0.0716
R1a = 0.0231, wR2b = 0.0526
2
2
2
a R1 = Σ(|Fo| Ϫ |Fc|)/Σ|Fo|. b wR2 = [Σw(Fo2 Ϫ Fc2)2/Σ[w(Fo )2]1/2 where w = 1/[σ2(Fo ) ϩ (0.0326P)2 ϩ 0.21P] for 4 and w = 1/[σ2(Fo ) ϩ 1.27P] for 5 and
2
P = (Max(Fo , 0) ϩ 2Fc2)/3.
19 A. J. Nielson, P. D. W. Boyd, G. R. Clark, P. A. Hunt, M. B.
Hursthouse, J. B. Metson, C. E. F. Rickard and P. A. Schwerdtfeger,
J. Chem. Soc., Dalton Trans., 1995, 1153.
20 I. M. Bartlett, S. Carlton, N. G. Connelly, D. J. Harding, O. D.
Hayward, A. G. Orpen, C. D. Ray and P. H. Rieger, Chem.
Commun., 1999, 2403.
21 P. M. Boorman, M. Wang and M. Parvez, J. Chem. Soc., Dalton
Trans., 1996, 4533.
22 Y.-C. Tsai, M. J. A. Johnson, D. J. Mindiola and C. C. Cummins,
J. Am. Chem. Soc., 1999, 121, 10426.
23 S. F. Pedersen, R. R. Schrock, M. R. Churchill and H. J.
Wasserman, J. Am. Chem. Soc., 1982, 104, 6808.
24 M. R. Churchill and H. J. Wasserman, Organometallics, 1983, 2, 755.
25 M. B. O’Regan, M. G. Vale, J. F. Payack and R. R. Schrock, Inorg.
Chem., 1992, 31, 1112.
and the relativistic Electron Core Potential (ECP) sets of Hay
and Wadt for Mo and W.53–55 The energies reported for the open
shell (doublet) systems correspond to unrestricted B3LYP cal-
culations. The mean value of the first-order wavefunction,
which is not an exact eigenstate of S2 for unrestricted calcu-
lations on the doublet systems, was considered suitable for the
unambiguous identification of the spin state. Spin contamin-
ation was carefully monitored and the values of 〈S2〉 for the
unrestricted B3LYP systems at convergence were very close to
the ideal value of 0.75.
Acknowledgements
26 J. L. Davidson, M. Green and F. G. A. Stone, J. Chem. Soc., Dalton
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27 J. L. Davidson, K. Davidson and W. E. Lindsell, J. Chem. Soc.,
Chem. Commun., 1983, 452.
We are grateful to the MENRT, the CNRS and the Conseil
Régional de Bourgogne for support of this work.
28 J. L. Davidson, K. Davidson, W. E. Lindsell, N. W. Murrall and A. J.
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30 E. Le Grognec, R. Poli and L.-S. Wang, Inorg. Chem. Commun.,
1999, 2, 95.
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