Insights into the Schrock 'chop-chop' reaction gained from density functional theory and preparation and structure of W2(μ-PhCCPh)(SC6H4-2-Me)6

Article abstract of DOI:10.1039/b208819c

Computations employing density functional theory on the reactions between ethyne and the model compounds (HE)₃M≡M(EH)₃, where M = Mo and W and E = O and S, predict that the alkyne adducts M₂(μ-C₂H₂)(E

Full text of DOI:10.1039/b208819c

Insights into the Schrock ‘chop-chop’ reaction gained from density
functional theory and preparation and structure of
W2(m-PhCCPh)(SC6H4-2-Me)6†
Malcolm H. Chisholm,*a Ernest R. Davidson,*b Maren Pinkc and Kristine B. Quinlanc
a Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210, USA.
E-mail: chisholm@chemistry.ohio-state.edu; Fax: 614 292 0368; Tel: 614 292 7216
b Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195, USA.
E-mail: erdavid@u.washington.edu; Fax: 206 543 8621; Tel: 206 685 1617
c
Department of Chemistry, Indiana University, 800 E. Kirkwood Avenue, Bloomington, Indiana 47405, USA
Received (in Purdue, IN, USA) 10th September 2002, Accepted 14th October 2002
First published as an Advance Article on the web 31st October 2002
Computations employing density functional theory on the
reactions between ethyne and the model compounds
(HE)3M·M(EH)3, where M = Mo and W and E = O and S,
predict that the alkyne adducts M2(m-C2H2)(EH)6 are
thermodynamically favored with respect to the metathesis
products HC·M(EH)3 except when M = W and E = O; the
reaction between (tBuO)3W·CPh and 2-MeC6H4SH ( > 3
equiv.) yields W2(m-PhCCPh)(SC6H4-2-Me)6 consistent with
expectations based on the calculations.
K the DGo is slightly positive. See Table 1. While we do not
wish to place undue emphasis on the absolute numerical
Nearly twenty years ago Schrock reported the ‘chop-chop’
predictions for the reaction, we do note that experimentally the
reaction wherein C·C or C·N bonds undergo a metathesis with
W·W bonds, eqn. 1 and eqn. 2.1
compounds M2(OtBu)6 react with ethyne at 25 °C to form
ethyne adducts reversibly in the case of molybdenum and
irreversibly in the case of tungsten.6,7 Moreover, only for M =
W do we observe the formation of the methylidyne complex
(tBuO)3W·CH.7
W2(OtBu)6 + RC·CR ? 2(tBuO)3W·CR,
(1)
where R = Me, Et, Pr
W2(OtBu)6 + RC·N ? (tBuO)3W·N + (tBuO)3W·CR,
We proceeded to calculate the thermodynamics for the
reaction interconverting the alkyne adduct, A, to the alkylidyne,
B. Only in the case when M = W and E = O is the reaction
product B favored (Table 1).
(2)
where R = alkyl or aryl
Schrock and coworkers extended these reactions in the
preparation of (RO)3M·CRA compounds where M = Mo and W
and R, RA = alkyl or aryl and investigated their reactivity
towards alkyne metathesis and other reactions.2 The ‘chop-
chop’ reaction is, however, very sensitive to the nature of the
metal, Mo versus W, and the attendant ligands. For example,
Mo2(OtBu)6 appears quite inert to MeC·N and PhC·N even
though the products of the metathesis are known: (tBuO-
)3Mo·N3 and (tBuO)3Mo·CR.4 Similarly, we have found that
the introduction of StBu groups at a (W·W)6+ center shuts down
the reactivity with alkynes and organic nitriles.5 These results
lend themselves to speculation concerning thermodynamics and
kinetics. In an attempt to evaluate some of those factors, we
have turned to the use of electronic structure calculations
employing density functional theory on the model compounds
M2(EH)6, where M = Mo and W and E = O and S, and their
reactions with ethyne. We report here some findings from these
computational studies together with an experimental observa-
tion that was prompted by the theoretical work.
In addition for the reaction where M = Mo and W and E =
O we have calculated a reaction pathway for the interconversion
of A and B. The highest lying transition state involves an
asymmetric structure with one bridging and one terminal
alkylidyne group as shown in C for molybdenum. At 298 K, the
DG≠ for the forward reaction is 34 kcal mol21 for M = Mo and
19 kcal mol21 for M = W. Again these calculated values find
relevance to the observation that the Schrock ‘chop-chop’
reaction proceeds rapidly for tungsten at ambient temperatures,
but only occurs for Mo2(OtBu)6 with terminal alkynes in low
yield and under more forcing conditions as in the preparation of
(tBuO)3Mo·CPh in the reaction between Mo2(OtBu)6 and
PhC·CH.2
In the first instance we have examined the thermodynamics
involved in the reaction between ethyne and the ethane-like
model compounds which yield ethyne adducts of the type
shown in A.
In all instances the ethyne adduct is predicted to be
enthalpically favored, although for M = Mo and E = O, at 298
Perhaps what is most striking from these calculations is the
prediction of the stability of the W2-thiolate alkyne adduct.
Surprisingly, no such compound had been made. This im-
plicates a significant kinetic effect both in the reactions
involving M2(SAr)6 and M2(OtBu)2(StBu)4 compounds with
† Dedicated to Roald Hoffmann on the occasion of his 65th birthday.
Table 1 DGo (at 298 K, kcal mol21) for the reactions involved in acetylene cleavage by dinuclear M2(EH)6 complexes, where M = Mo and W and E = O,
S
Mo, O
Mo, S
W, O
W, S
M2(EH)6 + HC·CH ? M2(EH)6(m-C2H2)
M2(EH)6(m-C2H2) ? [(HE)3M·CH]2
M2(EH)6 + HC·CH ? [(HE)3M·CH]2
4.5
5.0
9.5
21.0
21.0
20.0
20.8
26.3
27.1
213.3
18.3
5.0
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CHEM. COMMUN., 2002, 2770–2771
This journal is © The Royal Society of Chemistry 2002

Fig. 1 ORTEP drawings of W2(m-C2Ph2)(m-SAr)2(SAr)4 with atoms at 50% probability.
Fig. 2 Optimized structure of W2(m-C2H2)(m-SH)2(SH)4 with selected bond
lengths (Å) and angles (deg).
alkynes and in the back reaction involving thiolato metal
alkylidynes such as (ArS)3M·CtBu,8 where Ar
=
2,4,6-Me3C6H2 and 2,4,6-iPr3C6H2, and (tBuS)3M·CtBu.9 Of
course, computations involving the model compounds with SH
ligands and CH for the alkylidyne negate what are obviously
significant steric considerations. No tBuCCtBu alkyne adduct of
a M2(OR)6 compound, for example, has ever been seen. We
reasoned, however, that the use of a benzylidyne ligand in
combination with 2-MeC6H4S ligands should allow steric
access to the alkyne adduct in W2(m-PhCCPh)(SC6H4-2-Me)6.
Use of the aryl thiolate ligand also obviates facile C–S bond
cleavage reactions which commonly occur for alkyl thiolates.
The reaction between 2-MeC6H4SH ( > 3 equiv.) and
(tBuO)3W·CPh proceeds at room temperature in toluene to give
a green solution from which green crystals are obtained of the
alkyne adduct W2(m-PhCCPh)(SC6H4-2-Me)6 in 60% isolated
yield. The molecular structure seen in the solid state is shown in
Fig. 1.‡ There are two bridging thiolate ligands and a twisted
bridging alkyne ligand. The W–W distance 2.662(1) Å and C–C
distance 1.420(8) Å are comparable to those seen in the alkyne
adducts of W2(OR)6 compounds, though the skewed bridge is
rather exceptional. The W–C distances of 1.980(8) Å (ave) and
2.559(8) Å (ave) are approaching W–C double and non-bonding
distances, respectively. The C–C/W–W twist angle is 43.0(3)°,
where 90° represents a m-perpendicular and 0° a m-parallel
alkyne adduct.10 The calculated structure for W2(m-C2H2)(SH)6
is shown in Fig. 2 which can be seen to closely represent that
observed for the arylthiolate with the m-PhCCPh bridge. Most
notably, the skewed orientation of the C–C bridge of the alkyne
is reproduced, despite the lack of steric bulk in the model
compound.
We thank the National Science Foundation for support of this
work.
Notes and references
‡ B3LYP DFT calculations were done using the Gaussian 98 program.11
6-31G* was used for O, S, C and H and LANL2DZ was used for Mo and W.
CCDC 192708. See http://www.rsc.org/suppdata/cc/b2/b208819c/ for crys-
tallographic data in CIF or other electronic format.
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The 1H NMR spectrum of the W2(m-PhCCPh)(SC6H4-
2-Me)6 in benzene-d6 reveals three methyl signals in the ratio
1+1+1, indicative of the maintenance of the skewed-C2 structure
in solution.
In conclusion, we believe that the DFT calculations have
provided insight into the Schrock ‘chop-chop’ reaction and can
be useful in the design of new experiments involving these types
of reagents as we have shown here in the synthesis of the first
alkyne adduct of a W2(SAr)6 compound.
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