Ethylene and alkyne carbon-carbon bond cleavage across tungsten-tungsten multiple bonds

Article abstract of DOI:10.1021/om020037n

Treatment of [(silox)₂WH]₂ (1) with RC=CRm′ (R = R′ = H, CH₃; R = H, R′ = Ph) afforded thermally unstable [(silox)₂W]₂(μ:η²,η²-RCCR ′) (μ-H)₂ (R = R′ = H, 2a; CH₃, 2b; R = H, R′ = Ph, 2c), which lose H₂ and convert to [(silox)₂W]₂(μ-CR)(μ-CR′) (R = R′ = H, 4a; CH₃, 4b; R = H, R′ = Ph, 4c). An X-ray structural study of 4b revealed a nearly square W₂C₂ core and a d(WW) of 2.720(2) A?. Thermal degradation of [(silox)₂W(CH₂CH₃)]₂ (5) also produced 4b, and with 2 equiv of C₂H₄, its formation is nearly quantitative with 2 equiv of EtH as a byproduct. Na/Hg reduction of (silox)₂ClW=WCl(silox)₂ (3) in the presence of excess 2-butyne afforded [(silox)₂W]₂(μ:η²,η²-MeC ₂Me) (8), which could be treated with H₂ to give 2b or thermolyzed to 4b. A similar reduction of 3 with excess ethylene present afforded 4a via [(silox)₂W]₂(μ-CH)(μ-CH₂)(H) (9, -78 °C) followed by H₂ loss; ethylene cleavage does not proceed via 2a or 8. Related cleavage chemistry was not observed for [(silox)₂TaH₂]₂ (10) and excess ethylene, which formed [(silox)₂HTaEt]₂ (11) and, ultimately, [(silox)₂EtTa](μ-CHCH₂)(μ-H)₂[Ta(silox) ₂] (12) and EtH. An X-ray structural study of 12 confirmed its configuration. Spectroscopic features of the molecules are addressed, and plausible mechanisms of carbon-carbon bond cleavage-whose thermodynamic impetus is the formation of μ-CR bridges-are discussed.

Full text of DOI:10.1021/om020037n

2724
Organometallics 2002, 21, 2724-2735
Eth ylen e a n d Alk yn e Ca r bon -Ca r bon Bon d Clea va ge
a cr oss Tu n gsten -Tu n gsten Mu ltip le Bon d s
Rebecca L. M. Chamberlin,† Devon C. Rosenfeld, Peter T. Wolczanski,* and
Emil B. Lobkovsky
Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell University,
Ithaca, New York 14853
Received J anuary 17, 2002
Treatment of [(silox)2WH]2 (1) with RCtCR′ (R ) R′ ) H, CH3; R ) H, R′ ) Ph) afforded
thermally unstable [(silox)2W]2(µ:η2,η2-RCCR′)(µ-H)2 (R ) R′ ) H, 2a ; CH3, 2b; R ) H, R′ )
Ph, 2c), which lose H2 and convert to [(silox)2W]2(µ-CR)(µ-CR′) (R ) R′ ) H, 4a ; CH3, 4b; R
) H, R′ ) Ph, 4c). An X-ray structural study of 4b revealed a nearly square W2C2 core and
a d(WW) of 2.720(2) Å. Thermal degradation of [(silox)2W(CH2CH3)]2 (5) also produced 4b,
and with 2 equiv of C2H4, its formation is nearly quantitative with 2 equiv of EtH as a
byproduct. Na/Hg reduction of (silox)2ClWtWCl(silox)2 (3) in the presence of excess 2-butyne
afforded [(silox)2W]2(µ:η2,η2-MeC2Me) (8), which could be treated with H2 to give 2b or
thermolyzed to 4b. A similar reduction of 3 with excess ethylene present afforded 4a via
[(silox)2W]2(µ-CH)(µ-CH2)(H) (9, -78 °C) followed by H2 loss; ethylene cleavage does not
proceed via 2a or 8. Related cleavage chemistry was not observed for [(silox)2TaH2]2 (10)
and excess ethylene, which formed [(silox)2HTaEt]2 (11) and, ultimately, [(silox)2EtTa](µ-
CHCH2)(µ-H)2[Ta(silox)2] (12) and EtH. An X-ray structural study of 12 confirmed its
configuration. Spectroscopic features of the molecules are addressed, and plausible mech-
anisms of carbon-carbon bond cleavageswhose thermodynamic impetus is the formation
of µ-CR bridgessare discussed.
In tr od u ction
cleavage products.9,10 A fascinating array of alkyne
adducts10-21 and alkyne-coupling reactions13-21 were
discovered as extensive ligand and substrate surveys
were conducted to determine the scope of the RCCR′
chemistry. In several instances16-18 and in related
studies,19-28 formation of stable W2(µ-CR) functionalities
was unavoidable and apparently irreversible.
The remarkable reactivity of the d3-d3 triple bond
toward small-molecule substrates1 is readily apparent
in the interactions of alkynes with (WtW)6+-based
complexes. Alkyne cleavage was originally2 observed in
the treatment of (tBuO)3WtW(OtBu)3 with a variety of
alkynes,3 and alkylidyne products were isolated as
pseudo-tetrahedral species or as Lewis-base adducts.2-6
The immediate application of alkylidynes toward alkyne
metathesis7,8 was exploited, and the mechanistic evalu-
ation of the alkyne scission process provided the con-
nection between µ:η2,η2-alkyne precursors and various
In entering the area via the synthesis of (silox)2XWt
t
WX(silox)2 (X ) Cl, H, Me, Et; silox ) Bu3SiO),29,30
t
steric features of the Bu3Si group31 were considered as
potentially aiding alkyne or nitrile2,32-34 and metal-
metal bond cleavage to afford (silox)2XWtCR. As re-
ported below, no alkyne reactivity was evident for X )
Cl, Me, or Et, and hydride proved too small to avert the
generation of dinuclear complexes. Nonetheless, subtle
differences in reactivity were observed as a consequence
of the silox ligand, including CH and CC bond activa-
tions and ethylene cleavage by the ditungsten linkages.
Olefin cleavages and degradations of related alkyl
† Current address: Los Alamos National Laboratory, C-INC, MS
J 514, Los Alamos, NM 87545.
(1) (a) Chisholm, M. H. J . Chem. Soc., Dalton Trans. 1996, 1781-
1791. (b) Chisholm, M. H. J . Organomet. Chem. 1990, 400, 235-253.
(c) Chisholm, M. H. Acc. Chem. Res. 1990, 23, 419-425. (d) Buhro, W.
E.; Chisholm, M. H. Adv. Organomet. Chem. 1987, 27, 311-369. (e)
Chisholm, M. H.; Conroy, B. K.; Eichhorn, B. W.; Folting, K.; Hoffman,
D. M.; Huffman, J . C.; Marchant, N. S. Polyhedron 1987, 6, 783-792.
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1985, 14, 69-91.
(9) Chisholm, M. H.; Folting, K.; Hoffman, D. M.; Huffman, J . C. J .
Am. Chem. Soc. 1984, 106, 6794-6805.
(2) Schrock, R. R.; Listemann, M. L.; Sturgeoff, L. G. J . Am. Chem.
Soc. 1982, 104, 4291-4293.
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(10) Chisholm, M. H.; Conroy, B. K.; Huffman, J . C.; Marchant, N.
S. Angew. Chem., Int. Ed. Engl. 1986, 25, 446-447.
(11) (a) Chisholm, M. H.; Folting, K.; Huffman, J . C.; Rothwell, I.
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1983, 22, 2903-2906.
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1984, 3, 1770-1771.
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Soc. 1981, 103, 3932-3934. (b) Sancho, J .; Schrock, R. R. J . Mol. Catal.
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Soc. 1984, 106, 6806-6815.
10.1021/om020037n CCC: $22.00 © 2002 American Chemical Society
Publication on Web 05/24/2002

Ethylene and Alkyne C-C Bond Cleavage
Organometallics, Vol. 21, No. 13, 2002 2725
complexes support the contention of extremely stable
W2(µ-CR) linkages.
such as diphenylacetylene and tert-butylacetylene failed
to bind to 1 at 25 °C; higher temperatures gave
complicated reaction mixtures.
The spectral features of the parent acetylene adduct,
2a , are typical of the series (Table 1). Diastereotopic
silox ligands, a single µ-acetylene CH resonance at δ
11.73, and equivalent hydride ligands resonating at δ
4.83 reflect molecular C2 symmetry. Asymmetric bridg-
ing hydrides are implicated by two sets of 183W satel-
lites; J WH ) 39 and 176 Hz. The µ:η2,η2-acetylene
carbons resonate as a doublet at δ 181.40 in the 13C
NMR spectrum, with a large J CH value of 182 Hz, which
is typical of structurally characterized, perpendicular
acetylene adducts.9 As expected, no tungsten coupling
to the acetylene proton is observed (J WH ≈ 0), yet the
corresponding J CW of 115 Hz is relatively large. 1H and
13C shifts of 2a appear downfield compared to related
hexaalkoxide complexes9 that contain additional ligands,
perhaps reflecting the lower coordination number and
corresponding lesser electron density at the tungsten
centers (Table 2).
Resu lts
Syn th esis, Ch ar acter ization , an d Reactivity Stu d-
ies. 1. Alk yn e Ad d u cts of [(silox)2WH]2. Treatment
of [(silox)2WH]2 (1)30 with RCtCR′ (R ) R′ ) H, CH3;
R ) H, R′ ) Ph) afforded thermally unstable µ:η2,η2-
adducts, [(silox)2W]2(µ:η2,η2-RCCR′)(µ-H)2 (R ) R′ ) H,
2a (-78 °C, minutes, hexanes); CH3, 2b (25 °C, 4 h,
C6D6); R ) H, R′ ) Ph, 2c (25 °C, 8h, C6D6)) in >90%
1
yield as monitored by H NMR spectroscopy (eq 1). In
The 2-butyne adduct, 2b, possesses spectral features
remarkably similar to the parent species, with hydrides
1
again resonating at δ 4.83 (J WH ) 42, 174 Hz) in the H
NMR spectrum. Diastereotopic silox ligands and a single
methyl resonance at δ 3.41 indicate a close structural
parallel with 2a . Deuterium substitution into the hy-
dride positions of 2b permitted identification of a
contrast, (silox)2XWtWX(silox)2 (X ) Cl (3), Me, Et)
failed to react with acetylene or 2-butyne at room
temperature. After extended thermolysis (100 °C) with
excess acetylene, most of the starting material remained
along with a copious blue-black precipitate assumed to
be polyacetylene.13 The formation of ditungsten alkyne
adducts of 1 in the absence of additional donor ligands,
and without further aggregation, is a testament to the
steric bulk of the silox ligand. Steric factors are also
reflected in the above reaction times, and larger alkynes
bridging W2(H/D) stretch at 1522/1145 cm-1
.
The terminal phenyl acetylene derivative, 2c, exhibits
inequivalent silox and hydride resonances in the 1H
NMR spectrum as the symmetry is lowered from C2.
One hydride at δ 5.27 has tungsten satellites of 39 and
174 Hz, and its small J HH of 3 Hz to the µ-acetylenic
proton at δ 11.95 suggests the two groups are proximate.
The other hydride at δ 5.22 is coupled only to tungsten
(J WH ) 40, 165 Hz) and is presumed to lie on the side
of the phenyl substituent. The average J WH for each
hydride in 2a -c is 106.7(21) Hz, a value remarkably
close to that of D2d [(silox)2W]2(µ-H)4, whose J WH ) 108
Hz was tentatively assigned to four symmetric, bridging
hydrides.30
(16) Chisholm, M. H.; Eichhorn, B. W.; Folting, K.; Huffman, J . C.
Organometallics 1989, 8, 49-66.
(17) Chisholm, M. H.; Eichhorn, B. W.; Huffman, J . C. Organome-
tallics 1989, 8, 67-79.
(18) Chisholm, M. H.; Eichhorn, B. W.; Huffman, J . C. Organome-
tallics 1989, 8, 80-89.
2. Alk yn e Scission . Thermolysis of [(silox)2W]2(µ:
η2,η2-C2H2)(µ-H)2 (2a ) at 80 °C for ∼5 h liberated
dihydrogen and produced [(silox)2W]2(µ-CH)2 (4a , eq 2),
which was isolated as a fluffy yellow solid in 66% yield.
(19) Chisholm, M. H.; Hoffman, D. M.; Northius, J . M.; Huffman,
J . C. Polyhedron 1997, 16, 839-847.
(20) Chisholm, M. H.; Folting, K.; Lynn, M. A.; Streib, W. E.;
Tiedtke, D. B. Angew. Chem., Int. Ed. Engl. 1997, 36, 52-54.
(21) Chisholm, M. H.; Cook, C. M.; Huffman, J . C.; Streib, W. E.
Organometallics 1993, 12, 2677-2685.
(22) Anderson, R. A.; Gayler, A. L.; Wilkinson, G. Angew. Chem.,
Int. Ed. Engl. 1976, 15, 609.
(23) Chisholm, M. H.; Cotton, F. A.; Extine, M.; Stults, B. R. Inorg.
Chem. 1978, 17, 696-698.
(24) Liu, A. H.; Murray, R. C.; Dewan, J . C.; Santarsiero, B. D.;
Schrock, R. R. J . Am. Chem. Soc. 1987, 109, 4282-4291.
(25) Chisholm, M. H.; Heppert, J . A.; Kober, E. M.; Lichtenberger,
D. L. Organometallics 1987, 6, 1065-1073.
(26) Chisholm, M. H.; Heppert, J . A.; Huffman, J . C.; Thornton, P.
J . Chem. Soc., Chem. Commun. 1985, 1466-1467.
(27) Chisholm, M. H.; Heppert, J . A. Adv. Organomet. Chem. 1986,
26, 97-124.
(28) Chisholm, M. H.; J ansen, R. M.; Huffman, J . C. Organometallics
1992, 11, 2305-2307.
(29) Miller, R. L.; Wolczanski, P. T.; Rheingold, A. L. J . Am. Chem.
Soc. 1993, 115, 10422-10423.
(30) Miller, R. L.; Lawler, K. A.; Bennett, J . L.; Wolczanski, P. T.
Inorg. Chem. 1996, 35, 3242-3253.
(31) Wolczanski, P. T. Polyhedron 1995, 14, 3335-3362.
(32) Chisholm, M. H.; Folting, K.; Lynn, M. L.; Tiedtke, D. B.;
Lemoigno, F.; Eisenstein, O. Chem. Eur. J . 1999, 5, 2318-2326.
(33) Chisholm, M. H.; Folting-Streib, K.; Tiedtke, D. B.; Lemoigno,
F.; Eisenstein, O. Angew. Chem., Int. Ed. Engl. 1995, 34, 110-112.
(34) Chisholm, M. H.; Folting, K.; Huffman, J . C.; Lucas, E. A.
Organometallics 1991, 10, 535-537.
Its solubility in hydrocarbon and ethereal solvents was
low at 25 °C but significant at 80 °C. The 1H NMR

2726 Organometallics, Vol. 21, No. 13, 2002
Chamberlin et al.
Ta ble 1. 1H, 13C, a n d 13C{1H} NMR Sp ectr a l Da ta (δ, a ssgm t, m u lt, J (Hz))a for Ditu n gsten a n d Dita n ta lu m
Der iva tives in C6D6 u n less Oth er w ise Noted
1H NMR (δ, mult, assgmt, J (Hz))
13C{1H} NMR (δ, assgmt, J (Hz))
compound
((H3C)3C)3
H/R
C(CH3)3
C(CH3)3
R
[(silox)2W]2(µ:η2,η2-C2H2)(µ-H)2
(2a )
1.08
4.83, J WH ) 39, 176
30.49
23.62
181.40, J CH ) 182, J WC ) 115b
1.36
1.10
11.73 (C2H2)
4.83, J WH ) 42, 174
30.79
30.64
23.98
23.21
[(silox)2W]2(µ:η2,η2-C2Me2)(µ-H)2
(2b)
22.39, CH3, J CH ) 129
1.36
0.96
3.41, (CH3)2
5.22, J WH ) 40, 165
31.14
24.12
194.85, CMe, J WC ) 117b
[(silox)2W]2(µ:η2,η2-HC2Ph)(µ-H)2
(2c)
0.97
1.40
1.42
5.27, d, 3, J WH ) 39, 184
7.08, Ph
7.67, Ph
7.90, Ph
11.95, d, CCH, 3
16.14, J WH ) 29b
5.44, (CH3)2
[(silox)2W]2(µ-CH)2 (4a )
1.20
1.18
30.74
31.05
23.86
23.26
303.44, J CH ) 165, J WC ) 163b
42.61, CH3
[(silox)2W]2(µ-CCH3)2 (4b)
319.06, CMe, J WC ) 160b
124.96, p-CH
[(silox)2W]2(µ-CH)(µ-CPh) (4c)
1.14
6.93, Ph
7.46, Ph
31.61
23.55
135.09, m-CH
7.73, Ph
159.06, o-CH
303.06, µ-CPh
311.46, µ-CH
16.87, CH, J WH ) 28b
[(silox)2W]2(µ:η2,η2-MeC2Me) (8)
[(silox)2W]2(µ-CH)(µ-CH2)(H) (9)c
1.10
1.35
1.04
1.12
1.21
1.27
3.53, (CH3)2
6.76, dd, J HH ) 8, 10,
J WH ) 105
2.55, “t”, CHH, 11
10.78, dd, CHH, 7, 12
21.89, CH, J WH ) 18, 26
10.66, TaH
138.88, ddd, µ-CHH′, J CH ) 153,
126, 9, J WC ) 97, 50
327.53, d, µ-CH, J CH ) 166,
J WC ) 152b
[(silox)2HTaEt]2 (11)
1.25
1.34
2.08, dq, CHH, 12, 6
2.50, “t”, CH3, 7
2.56, dq, CHH, 12, 7
8.37, br s, TaH
[(silox)2TaEt](µ-H)2(µ-CHCH2)-
[Ta(silox)2] (12)
1.09
30.80
23.46
20.68, CH3
1.29
1.35
1.39
8.39, br d, TaH, 4
1.54, br m, CHHMe, 8
2.16, t, CH3, 8
30.98
31.03
31.32
23.91
23.94
24.48
53.70, CH2Me
72.58, CH2CH
144.27, CH2CH
2.28, br dq, CHHMe, 6, 8
3.90, dd, CHHCH, 11, 8
4.02, dd, CHHCH, 12, 8
5.02, m, CH2CH, 11
a
b
J WH given if unambiguous. Coupling to two equivalent tungstens according to integrated or estimated intensities (∼24.7%). c In
THF-d8 at -85 °C; 13C NMR resonances obtained via 9-13C2.
Ta ble 2. Selected 1H a n d 13C NMR Da ta (J in Hz) for Rela ted W2(µ:η2,η2-C2H2) a n d W2(µ-CH) Der iva tives
1H NMR
13C NMR
compound
δ(CH)
J WH
δ(CH)
J WC
J CH
ref
[(silox)2W]2(µ:η2,η2-C2H2)(µ-H)2 (2a )
W2(OtBu)6(py)(µ:η2,η2-C2H2)
W2(OCH2tBu)6(py)2(µ:η2,η2-C2H2)
W2(OiPr)6(py)2(µ:η2,η2-C2H2)
W2(OSiMe2tBu)6(py)(µ:η2,η2-C2H2)
[(silox)2W]2(µ-CH)2 (4a )
[(silox)2W]2(µ-CH)(µ-O)(µ-H)
[Cp*MeW]2(µ-CH)(µ-CMe)
[Cp*MeW][Cp*EtW](µ-CH)2
11.73
5.92
6.51
8.63
6.78
16.42
19.62
19.10
18.37
0
0
0
0
0
29
20
16
16
181.4
120.1
133.2
166.3
145.5
303.4
319.5
340.3
345.7
115
44
33, 45
42
40
163
150, 178
142
182
184
190
192
189
165
168
154
152
9
15
9
13
29
24
24
145
spectrum of 4a is characterized by a single silox peak
and a single µ-methylidyne resonance at δ 16.14, with
a two-bond tungsten coupling of J WH ) 29 Hz whose
satellite abundances of 25% indicate a symmetric µ-CH
bridge. The methylidyne appears as a doublet at δ
303.44 in the 13C NMR, with a J CH of 165 Hz for the
carbon-hydrogen bond, and strong, symmetric coupling
to the two tungsten atoms; J WC ) 163 Hz. The µ-CH of
4a is not as deshielded as comparable Cp-based struc-
tures (Table 2), but the W2(µ-CH)2 group is easily
distinguished from terminal WtCH ligands (e.g.,
(tBuO)3WtCH; δ 252.4, J CH ) 150 Hz, J WC ) 289 Hz).9
Thermolyses of the 2-butyne (2b) and phenylacetylene
(2c) µ:η2,η2-adducts produced the corresponding substi-
tuted bis-µ-alkylidynes [(silox)2W]2(µ-CMe)2 (4b) and
[(silox)2W]2(µ-CH)(µ-CPh) (4c) as yellow and orange
crystals, respectively, but more strenuous conditions
were required to cleave the C-C bond (∼8 h, 120 °C) of
these species (eq 2). Equivalent silox and methyl (δ 5.44)
groups are observed in the 1H NMR spectrum of 4b,
while an alkylidyne carbon is observed at δ 319.06 in
the 13C{1H} NMR spectrum, accompanied by tungsten
satellites (J WC ) 160 Hz (25%)). The benzylidyne
derivative, 4c, has equivalent silox groups, a µ-CH

Ethylene and Alkyne C-C Bond Cleavage
Organometallics, Vol. 21, No. 13, 2002 2727
Ta ble 3. Cr ysta llogr a p h ic Da ta for
[(silox)2W]2(µ-CCH3)2 (4b) a n d
[(silox)2EtTa ](µ-CHCH2)(µ-H)2[Ta (silox)2 (12)
formula
fw
W2Si4O4C52H114
1283.5
Ta2Si4O4C52H118
1281.8
space group
a (Å)
b (Å)
Pbca
Pbca
26.178(11)
17.083(4)
28.360(15)
12682(9)
0.71073 (Mo KR)
3.813
26.083(3)
16.808(2)
28.764(3)
12610(3)
0.71073 (Mo KR)
3.581
c (Å)
V (Å3)
λ (Å)
µ (mm-1
T (K)
)
298
1.344
173(2)
1.350
F
calc (g/cm3)
Z
8
8
R
Rw
GOF
0.0546a
0.0679b
0.94d
0.0572a
0.1307c
1.117d
a
b
2
1/2
R1 ) ∑||Fo| - |Fc||/∑||Fo|. wR2 ) {∑[w(Fo - Fc)2/∑[wFo
}
,
,
where w-1 ) σ2F + 0.0026F2. c wR2 ) {∑[w(Fo2 - Fc2)2/∑w(Fo2)2]1/2
d
where w-1 ) σ2F. GOF ) {∑w(Fo2 - Fc2)2/(n - p)}1/2, n ) number
of independent reflections, p ) number of parameters.
resonance at δ 16.87 (J WH ) 28 Hz, 24%) in the 1H NMR
spectrum, and alkylidyne carbons resonating at δ 311.46
(µ-CH) and δ 303.06 (µ-CPh) in the 13C{1H} NMR
spectrum.
With 1 atm H2 present, the conversion of [(silox)2W]2-
(µ:η2,η2-C2Me2)(µ-H)2 (2b) to [(silox)2W]2(µ-CMe)2 (4b)
was severely inhibited, reaching only ∼25% conversion
after 20 h at 120 °C. Furthermore, when the thermolysis
was conducted under 1 atm of D2 in a sealed NMR tube,
loss of the hydride signal and formation of [(silox)2W]2-
(µ:η2,η2-C2Me2)(µ-D)2 (2b -d2) occurred prior to alkyne
cleavage.
3. Str u ctu r e of [(silox)2W]2(µ-CMe)2 (4b). A single-
crystal X-ray structure (Table 3, Table 4) of [(silox)2W]2-
(µ-CMe)2 (4b) was obtained, and its molecular and
skeletal structures are illustrated in Figure 1. The bis-
µ-ethylidene (4b) possesses idealized D2 symmetry and
a planar W2C2 core that is approximately square, with
a W-W single bond distance of 2.720(2) Å and average
WCW and CWC angles of 88.5(5)° and 91.5(5)°, respec-
tively. The alkylidyne methyl groups lie in the W2C2
plane and occupy large open pockets amidst the silox
ligands, but tip slightly toward one side by ∼2.6(9)°. As
predicted by extended Hu¨ckel molecular orbital calcula-
tions on related bis-µ-alkylidynes,25 delocalized bonding
is manifested by nearly equivalent W-C distances
(1.950(12) Å av).
F igu r e 1. Molecular structure of [(silox)2W]2(µ-CMe)2
(4b): (a) view along the alkylidyne vector, emphasizing the
steric interactions among the silox ligands; (b) inner
coordination sphere.
nuclear steric repulsions may also contribute to the
rather small O-W-O angles of 114.7(35)°, but a more
dramatic result of the steric crowding is the twisting of
the O-W-O planes away from their ideal eclipsed
geometry, resulting in a dihedral angle of ∼10°. Despite
these core distortions, the silox ligands have normal
bond lengths (W-Oav ) 1.895(8) Å) and angles (W-O-
Siav ) 167.8(6)°). The interlocking of the silox tert-butyl
groups may account for the low, room-temperature
solubility of 4a -c. Assuming the solution structure of
[(silox)2W]2(µ-CH)(µ-CPh) (4c) resembles that of 4b, its
likely C2 symmetry is contradicted by the observation
of a single silox resonance. Access to an eclipsed
conformation or transition state is energetically feasible;
hence a fluxional process between diasterotopic silox
groups in the µ-alkylidynes 4a -c is likely. A related
The periphery of the complex reveals strong steric
interactions between the eclipsed silox ligands, which
may account for the stretching of the W-W bond, as
compared to electronically similar [(tBuO)2W]2(µ-CPh)2
(d(W-W) ) 2.665(1) Å; C-W-C ) 93.1°);6 the C-W-C
angles in 4b are correspondingly more acute. Inter-
Ta ble 4. Selected In ter a tom ic Dista n ces (Å) a n d An gles (d eg) for [(silox)2W]2(µ-CCH3)2 (4b)
W1-W2
W2-C1
C2-C4
W2-O3
Si2-O2
2.720(2)
1.959(12)
1.523(18)
1.895(8)
1.660(9)
W1-C1
W2-C2
W1-O1
W2-O4
Si2-O3
1.954(12)
1.940(11)
1.902(8)
1.876(7)
1.661(9)
W1-C2
C1-C3
W2-O2
Si1-O1
Si4-O4
1.945(11)
1.470(18)
1.906(8)
1.663(9)
1.680(8)
W1-C1-W2
C1-W2-C2
W1-C2-C4
W2-W1-O2
O1-W1-O2
W1-O2-Si2
88.1(5)
91.5(5)
133.0(8)
122.6(2)
117.1(3)
169.8(5)
W1-C2-W2
W1-C1-C3
W2-C2-C4
W1-W2-O3
O3-W2-O4
W2-O3-Si3
88.9(5)
133.6(10)
138.1(9)
124.0(2)
112.2(3)
166.6(6)
C1-W1-C2
W2-C1-C3
W2-W1-O1
W1-W2-O4
W1-O1-Si1
W2-O4-Si4
91.5(5)
138.4(10)
120.4(2)
123.9(2)
168.0(5)
166.7(6)

2728 Organometallics, Vol. 21, No. 13, 2002
Chamberlin et al.
interchange is apparently responsible for silox signal
averaging in 1H NMR spectra of [(silox)2W]2(µ-CH)(µ-
O)(µ-H).29
(silox)2HWtW(OSitBu2CMe2CH2)(silox) (6)30 and an-
other species with a singlet at δ 3.53 in addition to
diastereotopic silox groups, tentatively formulated as
[(silox)2W]2(µ:η2,η2-MeC2Me) (8). Addition of 1 atm of H2
(or D2) to the crude mixture containing 8 afforded
[(silox)2W]2(µ:η2,η2-MeCCMe)(µ-H)2 (2b, eq 6) (or 2b-d2)
and 1 (or 1-d2), presumably derived from 6.30
The low solubility of 4a -c in comparison to 2a -c
t
correlates with the restricted motion of their silox Bu
groups, which suggests steric destabilization in accord
with the structural study above. Despite apparently
severe steric interactions, no reversibility was observed
when 4a and 4b were exposed to 3 atm H2, even at high
temperatures (>100 °C) and extended reaction times
(>1 day). These C-C bond cleavage reactions are
favorable due to the formation of the bis-µ-alkylidyne
core, which is a proven thermodynamic “sink” in diverse
tungsten systems.19-28
4. µ-Eth ylid yn e fr om [(silox)2W(CH2CH3)]2 (5).
The thermodynamic driving force associated with bis-
µ-alkylidyne formation is further evidenced by the
appearance of [(silox)2W]2(µ-CMe)2 (4b) as a product of
a seemingly unrelated reaction, the thermal degradation
of [(silox)2W(CH2CH3)]2 (5, eq 3).30 When 5 is heated to
Similarly, thermolysis (80 °C, 5 h) of the mixture
converted 8 to [(silox)2W]2(µ-CMe)2 (4b), and the relative
concentration of cyclometalated product 6 remained
constant.
100 °C
[(silox)2W(CH2CH3)]2
8
30 min
5
1/2 [(silox)2W]2(µ-CMe)2 1/2 [(silox) WH]
+
+ EtH
2
2
4b
1
5 h, 80 °C
[(silox) W] (µ-CMe)
8
8
(7)
2
2
4b
2
(3)
C6D6
100 °C for 30 min in the presence of an additional 2
equiv of ethylene, 4b is quantitatively generated along
with 2 equiv of C2H6 (eq 4). If additional ethylene is not
6. Eth ylen e Deh yd r ogen a tion a n d Clea va ge by
P u ta tive [(silox)2W]2. Given the success of 2-butyne
trapping of the reduction product(s) of [(silox)2WCl]2 (3),
scavenging with ethylene was attempted. Na/Hg reduc-
tion of [(silox)2WCl]2 (3) in DME under 1 atm C2H4 at
25 °C produced a red solution within 5 min. Dissipation
of the color over 15-20 min occurred concurrently with
the precipitation of a yellow solid, [(silox)2W]2(µ-CH)2
(4a ), which was isolated in 68% yield from hexanes (eq
8). Toepler analysis of the volatiles revealed the evolu-
100 °C
5 + 2 C2H4
8 4b + 2 EtH
(4)
30 min
present, the products are a 1:1:2 mixture of 4b ,
[(silox)2WH]2 (1), and ethane. Formation of 4b is con-
sidered to be promoted by ethylene released upon
successive â-H-elimination reactions of the diethyl di-
nuclear 5 that afford dihydride 1, a pathway previously
established.30 The reversibility of the â-H-elimination
process was also presumed to be responsible for the
inability to selectively label 5 via 1 and C2D4 or 1-d2
and C2H4; rapid total label scrambling was noted.
5. 2-Bu tyn e Tr a p p in g of P u ta tive [(silox)2W]2.
The conversion of the µ-alkyne dihydrides 2a -c to
bis-µ-alkylidynes 4a -c prompted consideration of
[(silox)2W]2(µ:η2,η2-RC2R′) as an intermediate. Previous
studies30,35,36 suggested that the putative quadruple-
bonded complex [(silox)2W]2 (7) could be trapped by
added ligands; hence (silox)2ClWtWCl(silox)2 (3) was
reduced with a >200-fold excess of 2-butyne present
according to eq 5. An assay of the products revealed a
tion of 0.79 equiv of H2 as a byproduct of the reaction.
The generation of H2 is further corroborated by the
formation of [(silox)2WH]2 (1) as a spurious product (10-
20% yield) when the reduction is carried out under low
(300 Torr) pressures of ethylene. The reduced inter-
mediatespresumably [(silox)2W]2 (7)sis competitively
scavenged by the H2 evolved from the dehydrogenation
of ethylene.
The AA′XX′ spin system of the W2(µ-CH)2 core (J HH
) 9, 1J CH ) 165, 3J CH ) 3, J CC ≈ 0 Hz)37 was evident in
the NMR spectra of a sample of 4a -13C2 prepared by
reduction of 3 under 13C2H4. A crossover experiment,
Na/Hg, DME
+ MeCtCMe 25 °C, 1.5 h, -2 NaCl8
[(silox)2WCl]2
3
t
+
(silox)2HWtW(OSi Bu2CMe2CH2)(silox)
6
[(silox)2W]2(µ:η2,η2-MeC2Me) (5)
8
∼3:7 ratio of the cyclometalated triple-bonded complex
(35) Miller, R. L. Ph.D. Thesis, Cornell University, 1993.
(36) Mayol, A.-R. Ph.D. Thesis, Cornell University, 1999.
(37) Leydon, D. E.; Cox, R. H. Analytical Applications of NMR; J ohn
Wiley & Sons: New York, 1977.

Ethylene and Alkyne C-C Bond Cleavage
Organometallics, Vol. 21, No. 13, 2002 2729
in which a mixture of C2H4 and 13C2H4 was admitted to
the reaction vessel, showed only 4a and 4a -13C2 as
products, confirming that the conversion proceeds with-
out the involvement of monomeric intermediates or
other ethylene-scrambling pathways. Cyclometalated
sion, methylene rotation, and reconnection within a
framework effectively locked by the typical, pseudo-D2d
arrangement of the siloxides. Note that the isoelectronic
µ-oxo derivative [(silox)2W]2(µ-CH)(µ-O)(H) exhibited
coalescence of its four silox resonances at 80 °C (∆Gq )
17 (1) kcal/mol);29 hence interchange of silox ligands is
probably difficult due to steric interactions.
7. µ-Vin yl F or m a t ion fr om [(silox)2Ta H 2]2 a n d
Eth ylen e. Given the propensity of [(silox)2W(CH2CH3)]2
(5) to form the bis-µ-ethylidyne [(silox)2W]2(µ-CMe)2 (4b)
and the unusual carbon-hydrogen and carbon-carbon
bond scission of ethylene by putative [(silox)2W]2 (7), the
reactivity of ethylene with [(silox)2TaH2]2 (10)38 was
explored in order to seek related chemistry. Treatment
of 10 with excess C2H4 in C6D6 incurred the immediate
formation of [(silox)2HTaEt]2 (11, eq 10), whose probable
C2 symmetry was signified by diastereotopic methylene
protons at δ 2.08 (dq, J HH ) 12, 6) and δ 2.56 (dq, J HH
) 12, 7) in addition to inequivalent silox resonances.
(silox)2HWtW(OSitBu2CMe2CH2)(silox) (6) was ruled
out as the source of reactivity, since it is unreactive
toward C2H4 and other substrates (2-butyne, CO, etc.).
The red intermediate persisted in solution for >3 h
at -78 °C and was monitored by low-temperature NMR
spectroscopy. Solutions of [(silox)2WCl]2 (3) in THF-d8
were reduced under C2H4 and 13C2H4 for 1H and 13C
NMR spectral analysis, respectively. Upon development
of the red color, each solution was cooled to -78 °C,
degassed to remove excess ethylene, and decanted into
an NMR tube that was sealed for monitoring. The
conversion to [(silox)2W]2(µ-CH)2 (4a ) from the red
intermediate was clean in the absence of additional
C2H4 or reducing agent, confirming that reduction and
ethylene uptake are complete at that stage.
At -85 °C, four proton signals were assigned to
ethylene-derived ligands, and four silox groups were
observed, corresponding to the intermediate [(silox)2W]2-
(µ-CH)(µ-CH2)(H) (9) shown in eq 9. In the 1H NMR
spectrum, a µ-methylidyne singlet at δ 21.89 displayed
couplings to 183W of ∼18 and 26 Hz, and in the 13C NMR
spectrum, the µ-methylidyne carbon was identified at
δ 327.53 (J CH ) 166 Hz; J WC ≈ J W′C ) 152 Hz, 24%); no
carbon-carbon coupling was discerned for 9-13C2. A
hydride ligand at δ 6.76 had tungsten satellites sug-
gestive of bridging coordination (J WH ) 105 Hz), but the
assignment is ambiguous because the intensities did not
reliably integrate for two tungstens. The hydride was
coupled to inequivalent methylene protons (dd, J HH
)
8, 10 Hz), and a weak coupling to the methylene carbon
was evident from the broadening and loss of definition
in the hydride multiplet in 9-13C2.
Although 11 can be identified by 1H NMR spectroscopy,
with excess ethylene conversion to the final product,
orange [(silox)2EtTa](µ-CHCH2)(µ-H)2[Ta(silox)2] (12), is
swift and complete at 23 °C within 5 h and ethane is a
A methylene carbon resonating at δ 138.88 in the 13C
NMR spectrum was found to bridge the W centers (J WC
) 50, 97 Hz), and at -85 °C it showed coupling to two
1
byproduct. In the H NMR spectrum, four inequivalent
disparate protons: one was found at δ 2.55 (“t”, J HH
)
silox resonances are observed and two broad hydride
resonances appear at d 8.37 and δ 8.39, with the latter
exhibiting coupling (4 Hz) tentatively assigned to one
of the diasterotopic CHHMe protons (δ 1.54, 2.28; J )
8), whose resonances are also broadened. The IR spec-
trum of 12 manifests a broad hydride absorption at 1603
cm-1 that is assigned to bridging hydrides. Since this
frequency is on the high end for Ta2(µ-H) derivatives,38
asymmetric bridges are tentatively suggested. The vinyl
11 Hz) in the 1H NMR spectrum with J CH ) 126 Hz
and the other at δ 10.78 (dd, J HH ) 7, 12) with a larger
coupling constant of J CH ) 150 Hz. As the sample
temperature was raised, the two methylene proton
signals exchange-broadened and coalesced at ca. 5 °C,
via a process whose ∆Gq ) 11(1) kcal/mol. The 13C signal
at δ 138.88 simultaneously merged to a sharp triplet
with J CH ) 137 Hz. Indications of silox resonance
coalescence or hydride participation, etc., are absent;
consequently, the simplest path to exchange of the
methylene protons involves a µ-CH2 to η1-CH2 conver-
(38) Miller, R. L.; Toreki, R.; LaPointe, R. E.; Wolczanski, P. T.; Van
Duyne, G. D.; Roe, D. C. J . Am. Chem. Soc. 1993, 115, 5570-5588.

2730 Organometallics, Vol. 21, No. 13, 2002
Chamberlin et al.
Ta ble 5. Selected In ter a tom ic Dista n ces (Å) a n d An gles (d eg) for [(silox)2EtTa ](µ-CHCH2)(µ-H)2[Ta (silox)2]
(12)
Ta1-Ta2
Ta1-O1
Ta1-O2
Ta2-O3
Ta2-O4
Si-Cav
2.9991(6)
1.874(7)
1.880(7)
1.895(7)
1.887(7)
1.918(20)
Ta1-C1
Ta1-C2
Ta2-C1
Ta2-C3
C1-C2
C-Cav
2.302(18)
2.218(16)
2.096(18)
2.191(14)
1.62(2)
C3-C4
Si1-O1
Si2-O2
Si3-O3
Si4-O4
1.33(2)
1.680(7)
1.673(7)
1.651(7)
1.672(7)
1.546(32)
O1-Ta1-O2
O1-Ta1-C2
O1-Ta1-C1
O2-Ta1-C2
O2-Ta1-C1
C1-Ta1-C2
O3-Ta2-O4
O3-Ta2-C1
110.1(3)
101.3(4)
106.8(5)
104.2(5)
134.5(5)
41.9(6)
O3-Ta2-C3
O4-Ta2-C1
O4-Ta2-C3
C1-Ta2-C3
Ta1-C1-Ta2
Ta2-C1-C2
Ta2-C3-C4
Ta1-C1-C2
97.0(6)
111.0(5)
101.3(4)
136.8(6)
85.9(7)
137.8(12)
127(2)
Ta1-C2-C1
Ta1-O1-Si1
Ta1-O2-Si2
Ta2-O3-Si3
Ta2-O4-Si4
O-Si-Cav
71.8(9)
173.9(5)
175.7(4)
171.2(5)
173.9(5)
106.5(10)
112.3(12)
111.1(21)
107.7(17)
111.1(3)
97.3(5)
C-Si-Cav
66.3(8)
Si-C-Cav
C-C-Cav
is observed by coupled 1H NMR spectroscopic reso-
nances at δ 3.90, 4.02, and 5.02 and 13C{1H} NMR
resonances at δ 72.58 (CH2) and δ 144.27 (CH).
A third equivalent of ethylene is needed for the
transformation from 11 to 12, since ethane is formed.
Without ethylene, 11 slowly disproportionates to 10 and
12 (∼1:2), and with only one additional equivalent, 11
was converted to 12 over the course of 4 days. Rough
rate measurements conducted with 10-75 equiv (2-19
solution equiv) revealed an order in [C2H4] that was
significantly less than 1 (0.1-0.2). Unfortunately, the
mechanstic details of the 11 to 12 conversion have been
difficult to determine, because the addition of C2D4 to
10 or the combination of C2H4 and 10-d4 resulted in
rapid label scramblingspresumably via â-H-elimina-
tion/insertion pathssthroughout the ethyl groups of 11
and in all positions of 12. When treated with excess
C2D4, 12 exhibited a minor amount (<10%) of H/D
exchange in its hydride and ethyl positions within 40
h. Less than 50% exchange in these positions was
evident after 9 days, and after 60 days additional H/D
exchange into the vinyl positions was observed. A rate
comparison between 10 and C2H4 (4 equiv) versus 10-
d4 and C2D4 (4 equiv) provided a phenomenological kH/
kD for the 11/11-d10 to 12/12-d10 conversions of 9(1).
8. Str u ctu r e of [(silox)2EtTa ](µ-CHCH2)(µ-H)2-
[Ta (silox)2] (12). An X-ray crystal structure (Table 3,
Table 5) of [(silox)2EtTa](µ-CHCH2)(µ-H)2[Ta(silox)2]
(12) confirmed the configuration based on NMR spec-
troscopy. Figure 2. reveals a D2 arrangement of (silox)2-
Ta cores connected by the bridging vinyl group. Al-
though the structure clearly supports the spectral data,
some geometric parameters of the bridge are imprecise
(e.g., d(C1-C2) ) 1.62(2) Å, Ta2-C1-C2 ) 137.8-
(12)°), as is often the case for relatively light atoms
positioned between two heavy atoms. The pseudo-sp2
vinyl tantalum-carbon bond (d(Ta2-C1) ) 2.096(18)
Å) is shorter than that of its sp3 ethyl counterpart
(d(Ta2-C3) ) 2.191(14) Å), and both are shorter than
the “tantalum-olefin” distances (d(Ta1-C1) ) 2.302-
(18) Å, d(Ta1-C2) ) 2.218(16) Å). The Ta2-C3-C4
) 127(2)° angle may indicate a modest agostic interac-
tion, but it is also consistent with the steric demands
of the Ta2 core.
F igu r e 2. Molecular structure of [(silox)2TaEt](µ-H)2(µ-
CHCH2)[Ta(silox)2] (12). The hydrides were not located and
have been placed in appropriate bridging positions.
intersilox angle O3-Ta2-O4 ) 111.1(3)° and remain-
ing core angles (i.e., C1-Ta2-O3 ) 97.3(5)°, C1-
Ta2-O4 ) 111.0(5)°, C3-Ta2-O3 ) 97.0(6)°, C3-
Ta2-O4 ) 101.3 (4)°) reveal a splay of the ligands
( C1-Ta2-C3 ) 136.8(6)°) that can accommodate a
pair of bridging hydrides in the O4-Ta2-O3 plane
essentially opposite O4. Likewise, the O1-Ta1-O1
angle of 110.1(3)° and the other Ta1 core angles (i.e.,
C1-Ta1-O1 ) 106.8(5)°, C1-Ta1-O2 ) 134.5(5)°,
C2-Ta1-O1 ) 101.3(4)°, C2-Ta1-O2 ) 104.2(5)°)
leave a related pocket for the µ-H ligands above the C1-
O1-O2 plane opposite C2.
Discu ssion
Str u ctu r es a n d Sp ectr oscop y. Interligand repul-
sions within the tetrasilox dimetal cores dominate the
structural features manifested by the spectroscopy and
diffraction studies. In the thermally unstable µ:η2,η2-
acetylene adducts, [(silox)2W]2(µ:η2,η2-RCCR′)(µ-H)2 (R
) R′ ) H, 2a ; CH3, 2b; R ) H, R′ ) Ph, 2c), a plausible
symmetric C2v environment would dictate equivalent
silox groups, yet a distortion to C2 renders the hydrides
asymmetric and two different silox ligands for 2a and
2b; the phenylacetylene derivatives exhibit two in-
equivalent hydrides and four different silox groups.
Symmetrical bridging hydrides were tentatively pro-
The hydrides could not be detected with confidence
and have been placed in bridging positions commensu-
rate with the methine bridging angle of Ta1-C1-Ta2
) 85.9(7)° (d(Ta-Ta) ) 2.9996(6) Å, Ta1-C1-C2 )
66.3(8)°) and the geometries of each tantalum’s core. The

Ethylene and Alkyne C-C Bond Cleavage
Organometallics, Vol. 21, No. 13, 2002 2731
Sch em e 1
posed for the related tetrahydride, D2d [(silox)2W]2(µ-
H)4,30 and a symmetrical acetylene and hydride envi-
ronment for 2a and 2b would be expected were it not
for the eclipsing of the siloxides in C2v. In the solid state
structure of [(silox)2W]2(µ-CMe)2 (4b), a subtle rotation
of a [(silox)2W] unit by 10° lowers the symmetry from
D2h to D2, and it is likely that the same sterically derived
distortion is responsible for the lower symmetries of 2a ,
2b, and 2c. It is difficult to know whether the hydrides
would be symmetrically bridged in the absence of the
silox steric constraints, but it is notable that the average
J WH in 2a , 2b, and 2c is similar to that of [(silox)2W]2-
(µ-H)4, as previously stated.
may occur first (B) followed by dehydrogenation. In B,
the initial C-C cleavage step is analogous to a micro-
scopic step in the alkyne scission reactions promoted
by W2(tBuO)6.2-6,9 J ustification for dihydrogen elimina-
tion is provided by observation of Cl2, Br2, and ROOR
oxidative additions to the W-W bond in bis-µ-alkyli-
dynes,26,27 reactions that constitute the microscopic
reverse.
Recall that D2 was observed to wash into the hydride
positions of 2b when 1 atm D2 was present, forming 2b-
d2 prior to conversion to 4b. While associative H2/D2
exchange cannot be ruled out (HD was not observed),
it is likely that reversible addition of dihydrogen, as in
path A, permits [(silox)2W]2(µ:η2,η2-RCCR′) to form and
cleavage occurs via this intermediate. H2/D2 exchange
may not rule out B, but inhibition of cleavage by the
presence of excess dihydrogen (or D2) and the indepen-
dent entry (C) to [(silox)2W]2(µ:η2,η2-MeCCMe) (8) via
2-butyne trapping of putative [(silox)2W]2 (7, eq 5)
strongly support A. The cleavage of 8 to 4b occurs under
conditions milder than those of 2b to 4b, consistent with
the dihydrogen preequilibrium evident in the latter
path. Path B would show H2 inhibition and H2/D2
exchange only if the dihydrogen loss step is reversible,
but none of the bis-µ-alkylidynes (i.e., 4a , 4b, nor 4c)
could be hydrogenated back to 2a , 2b, or 2c.
Mech a n istic Eva lu a tion s. 1. Alk yn e Scission .
While the initial alkyne adducts [(silox)2W]2(µ:η2,η2-
RCCR′)(µ-H)2 (R ) R′ ) H, 2a ; CH3, 2b; R ) H, R′ )
Ph, 2c) appear to be structurally related to numerous
others,9-21 further reactions with RCCR′ to form C4-
containing products15-21 or direct cleavages2-6,9 to (silox)2-
(H)WtCR/R′ or (silox)2WdCHR/R′39,40 species were not
observed. Instead, cleavage occurs with reductive elimi-
nation of H2. One ostensibly analogous reaction is the
high-temperature reaction of (tBuO)6W2 with dipheny-
lacetylene, which proceeds with apparent decomposition
or transfer of two tBuO• groups to form a product
mixture including a bis-µ-alkylidyne, [(tBuO)2W]2(µ-
CPh)2.6 No intermediates were detected, and it was
surmised that alkoxide loss preceded alkyne binding. It
is also notable that another siloxide-based alkyne ad-
duct, (tBuMe2SiO)6W2(µ:η2-C2H2)(py), does not promote
2. Eth ylen e Scission . The generation of [(silox)2W]2-
(µ-CH)2 (4a ) upon reduction of (silox)2ClWtWCl(silox)2
(3) in the presence of C2H4 represents a formal dehy-
drogenation and C-C cleavage of ethylene, a reaction
unprecedented in tungsten chemistry. A plausible sce-
nario for ethylene cleavage is indicated in Scheme 2.
In it, the putative quadruply bonded intermediate,
[(silox)2W] (7), is invoked as the C-H bond activating
species. Dichloride 3 did not interact with any olefin,
ethylene or alkyne, even at high temperatures (>100
t
C-C cleavage but instead eliminates BuMe2SiOH in
hydrocarbon solutions, forming a µ-ethynyl complex, (t-
BuMe2SiO)5W2(µ:η2-CCH)(py).13
Two mechanisms for the dehydrogenation and al-
kyne scission of [(silox)2W]2(µ:η2,η2-RCCR′)(µ-H)2 (R )
R′ ) H, 2a ; CH3, 2b; R ) H, R′ ) Ph, 2c) to give
[(silox)2W]2(µ-CR)(µ-CR′) (R ) R′ ) H, 4a ; CH3, 4b; R
) H, R′ ) Ph, 4c) are presented in Scheme 1. In A,
reversible dihydrogen elimination is followed by C-C
bond cleavage of a [(silox)2W]2(µ:η2,η2-RCCR′) (8 if R )
R′ ) Me) intermediate; alternatively, alkyne cracking
°C). Cyclometalation product (silox)2HWtW(OSitBu2-
CMe2CH2)(silox) (6) can also be eliminated as the source
of reactivity since it responded only sluggishly to eth-
ylene in NMR tube reactions. Some product of the
reduction of 3 must therefore be responsible for the
activation of C2H4, given the mild conditions (e25 °C),
and a low coordinate transient such as 7 has tremen-
(39) Holmes, S. J .; Clark, D. N.; Turner, H. W.; Schrock, R. R. J .
Am. Chem. Soc. 1982, 104, 6322-6329.
(40) Chacon, S. T.; Chisholm, M. H.; Eisenstein, O.; Huffman, J . C.
J . Am. Chem. Soc. 1992, 109, 8497-8509.

2732 Organometallics, Vol. 21, No. 13, 2002
Chamberlin et al.
Sch em e 2
dous appeal on the basis of the favorable sterics and
electronics of its D2d structure.30
3. µ-Eth ylid en e F or m a tion . Since the thermal
conversion of [(silox)2W(CH2CH3)]2 (5) to [(silox)2W]2(µ-
CMe)2 (4b) proceeds cleanly, even under conditions of
low ethylene concentration, any steps occurring before
uptake of the first C2H4 molecule may be reversible. In
addition, treatment of 5 with C2D4 revealed H/D scram-
bling into all ethyl groups and the added ethylene,
consistent with rapid and reversible â-H/D-eliminations
prior to conversion to 5. As a consequence, mechanistic
information for the 5 f 4b transformation is extremely
limited.
Ignoring stereochemical aspects of the mechanism,
Scheme 3 illustrates a plausible sequence featuring
oxidative addition of an R-methylene unit across the
metal-metal bond and hydrogen migration as the initial
step. Ethylene insertion into the hydride affords the
means to conduct an R-abstraction17,18,44-48 that gener-
ates the first µ-CH bridge and ethane. A repeat of this
sequence involving a hydrogenation of a second equiva-
lent of C2H4 provides a pathway to 5 as the second
µ-methine is formed. Scheme 3 accounts for the uptake
of 2 equiv of free ethylene and the failure to observe
quantitative conversion in its absence.
Oxidative addition of a ethylene carbon-hydrogen
bond across the W&W bond of putative [(silox)2W]2 (7)
leads to a µ-vinyl hydride. A similar vinylic C-H
activation by a metal-metal multiple bond is implicated
when (Cp*Ta)2(µ-X)4 (X ) Cl, Br) reacts with ethylene
to form a Ta2(µ:η1,η2-CHCH2)(µ-H) complex.41,42 Fur-
thermore, generation of [(silox)2EtTa](µ-CHCH2)(µ-H)2-
[Ta(silox)2] (12) via [(silox)2HTaEt]2 (11) upon exposure
of [(silox)2TaH2]2 (10) to C2H4 lends credence to such
an intermediate in a tetrasiloxdimetallic system. An R-
or â-H elimination from the µ-vinyl ligand should lead
to [(silox)2W]2(µ:η2,η2-HCCH)(µ-H)2 (2a ) as path E in-
dicates, but the conditions for ethylene cleavage are
markedly milder than those for the thermal µ-alkyne
scission of 2a ; mechanism E is therefore discounted.
Furthermore, exposure of 2a to C2H4 and THF did not
induce formation of 4a , thereby supporting exclusion of
the E pathway.
Cleavage of the carbon-carbon bond in the proposed
[(silox)2W]2(µ:η1,η2-CHCH2)(µ-H) transient would gener-
ate the observed intermediate [(silox)2W]2(µ-CH)(µ-
CH2)(H) (9). The microscopic reverse of this reaction has
been inferred in the oxidatively induced coupling of
µ-methylene and µ-methylidyne ligands in a dirhodium
complex; because of subsequent reactivity, the µ-vinyl
product was not observed, but its intermediacy was
implied by a series of labeling experiments.43 Note that
the asymmetry of the µ-methylene group in 9 may play
a role in allowing a low kinetic barrier for the 1,2-H2-
elimination to form [(silox)2W]2(µ-CH)2 (4a ).17,18,44-48
(43) Maitlis, P. M.; Saez, I. M.; Meanwell, N. J .; Isobe, K.; Nutton,
A.; de Miguel, A. V.; Bruce, D. W.; Okeya, S.; Bailey, P. M.; Andrews,
D. G.; Ashton, P. R.; J ohnstone, I. R. New J . Chem. 1989, 13, 419-
425.
(44) Schrock, R. R.; Seidel, S. W.; Mo¨sch-Zanetti, N. C.; Shih, K.-Y.;
O’Donohue, M. B.; Davis, W. M.; Reiff, W. M. J . Am. Chem. Soc. 1997,
119, 11876-11893
(45) Schrock, R. R.; Seidel, S. W.; Mo¨sch-Zanetti, N. C.; Dobbs, D.
A.; Shih, K.-Y.; Davis, W. M. Organometallics 1997, 16, 5195-5208.
(46) Davies, D. L.; Gracey, B. P.; Guerchais, V.; Knox, S. A. R.;
Orpen, A. G. J . Chem. Soc., Chem. Commun. 1984, 841-843.
(47) Connelly, N. G.; Forrow, N. J .; Gracey, B. P.; Knox, S. A. R.;
Orpen, A. G. J . Chem. Soc., Chem. Commun. 1985, 14-16.
(48) Blau, R. J .; Chisholm, M. H.; Eichhorn, B. W.; Huffman, J . C.;
Kramer, K. S.; Lobkovsky, E. B.; Streib, W. E. Organometallics 1995,
14, 1855-1869.
(41) Ting, C.; Messerle, L. J . Am. Chem. Soc. 1987, 109, 6506-6508.
(42) For a reversible CH activation in related metal-metal bonded
systems, see: Chisholm, M. H.; Huang, J . H.; Huffman, J . C.; Parkin,
I. P. Inorg. Chem. 1997, 36, 1642-1651.

Ethylene and Alkyne C-C Bond Cleavage
Organometallics, Vol. 21, No. 13, 2002 2733
Sch em e 3
Sch em e 4
4. Dita n ta lu m µ-Vin yl F or m a tion . Given the stoi-
chiometry requirements indicated by eq 11, the minor
order in added ethylene on the rate of conversion of
[(silox)2HTaEt]2 (11) to [(silox)2EtTa](µ-CHCH2)(µ-H)2-
[Ta(silox)2] (12) is puzzling. The magnitude of kH/kD (∼9)
is most consistent with an R-abstraction49 as the rate-
determining step, but little else can be discerned
mechanistically due to the aforementioned label scram-
bling problem. Scheme 4 illustrates a scenario whereby
the R-abstraction can occur directly from 11 or from a
triethylhydride-ditantalum complex generated via eth-
ylene insertion. The former would not exhibit an order
(n ) 0) in [C2H4]n, while the latter would (n ) 1), thereby
providing a possible explanation for the observations.
â-H-eliminations from the µ-ethylidenes lead to 12,
although some formal rearrangement of the ditantalum
bond and hydride is a necessary accompaniment. Any
post-abstraction involvement of ethylene should have
no consequence on its order, unless the R-abstraction
is reversible, which is improbable. It is somewhat
surprising that further ethane loss to give a ditantalum
analogue of [(silox)2W]2(µ-CH)(µ-CH2)(H) (9), or loss of
both C2H6 and H2 leading to “[(silox)2Ta]2(µ:η2,η2-
HCCH)”, or especially “[(silox)2Ta]2(µ-CH)2”, was not
cleanly observed. The µ-vinyl dihydride 12 does decom-
pose, but the identification of any one of the myriad
products has proven difficult.
Con clu sion s
J ust as the steric bulk of the silox ligand proved
(49) Slaughter, L. M.; Wolczanski, P. T.; Klinckman, T. R.; Cundari,
T. R. J . Am. Chem. Soc. 2000, 122, 7953-7975.
crucial in synthesizing the first isolable (WtW)6+ di-

2734 Organometallics, Vol. 21, No. 13, 2002
Chamberlin et al.
hydride,30 it has also permitted preparation of µ:η2-
alkyne dihydrides, [(silox)2W]2(µ:η2,η2-RCCR′)(µ-H)2 (R
) R′ ) H, 2a ; CH3, 2b; R ) H, R′ ) Ph, 2c), in the
absence of additional donor ligands. While the alkyne
adducts 2a -c resemble known (RO)6W2(µ:η2-C2R′2)(py)x
species,4,9 few analogues of their degradation products,
[(silox)2W]2(µ-CR)(µ-CR′) (R ) R′ ) H, 4a ; CH3, 4b; R
) H, R′ ) Ph, 4c), have been found. (tBuO)4W2(µ-CPh)2
remains the lone related complex,6 presumably due to
its unusual high-temperature formation and the dif-
ficulty of achieving alkoxide eliminations from the most
likely precursors, (RO)6W2.
Laboratories, Houston, TX, Oneida Research Services, Whites-
boro, NY, and Robertson Microlit Laboratories, Madison, NJ .
P r oced u r es. 1. [(silox)2W]2(µ:η2,η2-HCCH)(µ-H)2 (2a ). A
flask containing 1 (100 mg, 0.081 mmol) in 8 mL of hexanes
was cooled to -78 °C, and acetylene (121 Torr, 0.081 mmol)
was admitted via a 12.3 mL calibrated gas bulb. The light
brown solution darkened immediately. After warming to 25
°C and stirring for 2 h, the solvent was removed in vacuo,
giving a red-orange crystalline solid that was used without
further purification. 1H NMR analysis indicated >95% conver-
sion to 2a .
2. [(silox)2W]2(µ:η2,η2-MeCCMe)(µ-H)2 (2b). An NMR tube
charged with 1 (50 mg, 0.041 mmol) was attached to a needle
valve adapter. C6D6 (0.6 mL) was vacuum distilled into the
tube, then 2-butyne (15 Torr, 0.041 mmol) was admitted via a
calibrated 50.1 mL gas bulb. Partial conversion (20%) to 2b
was observed after 15 min at 25 °C. The reaction was complete
after 4 h, yielding a solution of brown 2b in 95% conversion.
The room-temperature dehydrogenation and carbon-
carbon bond cleavage of ethylene, promoted by reduction
of [(silox)2WCl]2 (3), is a unique event whose ultimate
product, [(silox)2W]2(µ-CH)2 (4a ), presumably reflects
the favorable thermodynamics of the W2(µ-CH)2 core
formation. Precedent for the CH activation is clear from
Messerle’s work,41 but the actual C-C cleavage event
leading to [(silox)2W]2(µ-CH)(µ-CH2)(H) (9)scertain from
the absence of a 13C-13C coupling constant when 13C2H4
was usedsis rare in early transition metal chemistry.
While the abstraction events responsible for the
conversion of [(silox)2W(CH2CH3)]2 (5) to [(silox)2W]2(µ-
CMe)2 (4b) have ample precedent17,18,44-48 and the
favorable thermodynamics of bis-µ-alkylidyne formation
are unquestionable,16-28 analogous ditantalum chem-
istry was not found. In the decomposition of [(silox)2EtTa]-
(µ-CHCH2)(µ-H)2[Ta(silox)2] (12), “[(silox)2Ta]2(µ-CH)2”
was anticipated as the most likely product. It is cer-
tainly conceivable in view of previous calculations on
(RO)4W2(µ-CR′)225 that the additional electrons in metal-
metal bonding orbitals play a critical role in the stability
of the M2(µ-CR)2 framework. Two less electrons in the
ditantalum system indicate that prior reduction of one
or both metal centers must occur if the µ-vinyl ligand
of 12 was to be cleaved; alternatively, additional ab-
straction eventsssuch as loss of ethane from 12 to afford
“[(silox)2Ta]2(µ:η2,η2-HCCH)(µ-H)2”smust occur to pro-
vide a ready path for CC bond cleavage. In any case, no
clean version of these events was noted, and the
commonality hoped for in the study of the ditantalum
system proved to be modest.
IR (Nujol): ν(W2H/D) ) 1522/1145 cm-1
.
3. [(silox)2W]2(µ:η2,η2-HCCP h )(µ-H)2 (2c). To a solution
of 1 (50 mg, 0.041 mmol) in C6D6 (0.6 mL) was added
phenylacetylene (4.5 µL, 0.041 mmol) via a microliter syringe.
The solution was transferred into an NMR tube attached to a
needle valve adapter, freeze-pump-thaw degassed, and
flame-sealed under vacuum at 77 K. Conversion to 2c was
1
complete (90% yield by H NMR) after 8 h at 25 °C.
4. [(silox)2W]2(µ-CH)2 (4a ). a . F r om 2a . Into an NMR tube
charged with 2a (50 mg, 0.040 mmol) was vacuum-distilled
0.6 mL of C6D6. The tube was cooled to 77 K and flame-sealed
under vacuum. Thermolysis in an 80 °C bath for 5 h produced
H2 and 4a , which was isolated as a flocculent yellow solid by
vacuum filtration (33 mg, 66%).
b. Red u ction of 3 w ith C2H4. Into a glass bomb reactor
containing [(silox)2ClW]2 (3, 250 mg, 0.192 mmol) and 0.9%
Na/Hg (1.00 g, 0.391 mmol) was vacuum-distilled 5 mL of
DME. The solution was freeze-pump-thaw degassed and
cooled to -78 °C. C2H4 (∼12 mmol) was admitted from a 300
mL vacuum manifold. The solution was warmed to 25 °C and
stirred vigorously for 30 min, as a yellow precipitate formed.
The bomb was degassed via Toepler pump, and the volatiles
were passed through three liquid nitrogen traps and collected
in a 16 mL volume (174 Torr, 0.151 mmol). The gas was
converted quantitatively to H2O by circulation over CuO (300
°C), indicating evolution of 0.79 equiv of H2. The yellow solid
residue was extracted from the salts by repeated washing with
hexanes. Concentration of the resulting yellow solution to 1
mL, filtration, and drying in vacuo yielded 163 mg of yellow
solid 4a (68% yield). Anal. Calcd for W2Si4O4C50H110: C, 47.84;
H, 8.83. Found: C, 47.21; H, 8.60.
Exp er im en ta l Section
5. [(silox)2W]2(µ-CMe)2 (4b). a . F r om 2b. A C6D6 solution
of freshly prepared 2b (54 mg, 0.041 mmol) in a sealed NMR
tube was heated at 120 °C and monitored periodically by H
Gen er a l Con sid er a tion s. All manipulations were per-
formed using either glovebox or high vacuum line techniques.
Hydrocarbon solvents containing 1-2 mL of added tetraglyme
and ethereal solvents were distilled under nitrogen from
purple benzophenone ketyl and vacuum transferred from the
same prior to use. Benzene-d6 was dried over activated 4 Å
molecular sieves, vacuum transferred, and stored under N2;
toluene-d8, methylcyclohexane-d14, and THF-d8 were dried over
sodium benzophenone ketyl. All glassware was oven dried, and
NMR tubes were additionally flamed under dynamic vacuum.
Ethylene, 2-butyne, acetylene (Matheson), and 13C2H4 (Cam-
bridge) were used as received. H2 and D2 were passed over
activated MnO and 4 Å sieves. [(silox)2WH]2 (1), [(silox)2WMe]2,
[(silox)2WCl]2 (3), [(silox)2WEt]2 (5),30 and [(silox)2TaH2]2 (10)38
were prepared according to published procedures.
1
NMR spectroscopy. The solution turned yellow over the course
of 6-7 h as 4b formed in >95% conversion. H2 was detected
in the 1H NMR spectrum. The NMR tube was cracked open
inside an evacuated flask, and the volatiles were passed
through three liquid nitrogen traps and collected (16 mL) via
Toepler pump (21 Torr, 0.018 mmol, 0.45 equiv). The yellow
crystals of 2b were washed with 0.5 mL of benzene, collected,
and dried via vacuum filtration (29 mg, 58% yield).
b. F r om [(silox)2WEt]2 (5). A solution of 5 (50 mg, 0.040
mmol) in 0.6 mL of C6D6 was loaded into an NMR tube. The
solution was freeze-pump-thaw degassed three times, and
ethylene (100 Torr, 0.27 mmol) was condensed into the tube
at 77 K via a 50.1 mL gas bulb. The tube was flame-sealed,
then heated to 100 °C for 2 h. Upon cooling to 25 °C, yellow
crystals precipitated. 1H NMR analysis of the supernatant
showed quantitative formation of 4a and evolution of ethane.
The tube was cracked open, and yellow crystals of 4a were
collected and dried by vacuum filtration (34 mg, 68% yield).
NMR spectra were obtained using Varian XL-200, XL-400,
and VXR-400S and Bruker AF-300 spectrometers. Chemical
shifts are reported relative to TMS. Infrared spectra were
recorded on a Mattson FT-IR interfaced to an AT&T PC7300
computer or on a Perkin-Elmer 299B spectrophotometer.
Elemental analyses were performed by Texas Analytical

Ethylene and Alkyne C-C Bond Cleavage
Organometallics, Vol. 21, No. 13, 2002 2735
6. [(silox)2W]2(µ-CH)(µ-CP h ) (4c). A C6D6 solution of
freshly prepared 2c (54 mg, 0.041 mmol) in a sealed NMR tube
was heated at 120 °C. Periodic H NMR monitoring revealed
quantitative conversion to 4c and release of H2 after 8 h
thermolysis. Cooling the solution to room temperature yielded
orange crystals, which were isolated and dried by vacuum
filtration (23 mg, 43% yield).
vacuum distilled 10 mL of benzene at -78 °C. The solution
was exposed to 1 atm of ethylene, resulting in a dark brown
color that became deep orange upon warming to 25 °C during
the course of 12 h. The benzene was removed, and the dark
orange solid was triturated with three 8 mL portions of
hexanes and filtered. Upon reducing the volume to 2 mL and
cooling to -78 °C, 0.114 g of 12 was isolated by filtration (56%).
1
7. [(silox)2W]2(µ:η2,η2-H3CCtCCH3) (8). a . Syn th esis a n d
Con ver sion to 4b. Into a flask containing 3 (75 mg, 0.058
mmol) and 0.9% Na/Hg amalgam (296 mg, 0.116 mmol Na) at
77 K was distilled 3 mL of DME. 2-Butyne (400 Torr, 13 mmol)
was admitted from a 600 mL vacuum manifold, then the
solution was warmed to 25 °C and stirred for 1.5 h. Volatiles
were removed in vacuo, and the brown residue was triturated
with 1 mL of benzene, then redissolved in 1 mL of C6D6. An
aliquot was loaded into an NMR tube attached to a needle
valve adapter and freeze-pump-thaw degassed, and the tube
X-r a y Cr ysta llogr a p h ic Stu d ies. 11. Sin gle-Cr ysta l
X-r a y Diffr a ction An a lysis of [(silox)2W]2(µ-CCH3)2 (4b).
A yellow block-shaped single crystal of 4b, with dimensions
0.3 × 0.3 × 0.4 mm, was grown by cooling of a supersaturated
benzene solution from 80 °C. The crystal was sealed in a
Lindemann capillary for data collection at 298 K (Siemens
R3m/V). Preliminary data collection and axial photographs
indicated an orthorhombic lattice, and analysis of systematic
absences indicated space group Pbca. Diffraction maxima were
measured using variable speed ω-scans and graphite-mono-
chromated Mo KR radiation. After correction for Lorentz,
polarization, background, and decomposition, 7572 (54.2%) of
the unique data (13 962 out of 15 046 measured) were judged
observed (F > 4.0σ(F)). The structure solution proceeded using
direct methods and successive difference electron density maps
(SHELXTL PLUS), full-matrix least-squares refinement was
conducted with all non-hydrogen atoms anisotropic, and
hydrogen atoms were treated as idealized contributions.
1
flame-sealed under vacuum. NMR analysis indicated a 70%
yield of 8, contaminated with 30% (silox)2HWtW(OSitBu2-
CMe2CH2)(silox) (6). Thermolysis of this solution for 5 h at 80
°C converted 8 quantitatively to 4b.
b. Con ver sion to 2b. The remaining C6D6 solution of 8
(with 6) from above was loaded into an NMR tube attached to
a needle valve adapter. The solution was freeze-pump-thaw
degassed three times and frozen at 77 K. H2 (700 Torr) was
admitted and the tube flame-sealed. 1H NMR analysis showed
a 70:30 ratio of 2b and 1. Thermolysis of this solution for 20
h at 110 °C showed <20% conversion to 4b by 1H NMR
spectroscopy. A similar procedure using D2 gas yielded 2b-d2
for IR analysis.
12. Sin gle-Cr ystal X-r ay Diffr action An alysis of [(silox)2-
EtTa ](µ-CHCH2)(µ-H)2[Ta (silox)2] (12). A crystal (0.30 ×
0.20 × 0.05 mm) of 12 was immersed in polyisobutylene,
extracted out with a glass fiber, and placed on the goniometer
head of a Siemens SMART CCD area detector system equipped
with a fine-focus molybdenum X-ray tube (λ ) 0.71073 Å).
Preliminary diffraction data revealed an orthorhombic crystal
system. A hemisphere routine was used for data collection,
determination of lattice constants, and identification of space
group Pbca. A total of 55 978 reflections were collected, of
which 8398 were symmetry independent (Rint ) 0.0483). The
data were processed with the Bruker SAINT program and
corrected for absorption by SADABS. The structure was solved
by direct methods (SHELXS), completed by subsequent dif-
ference Fourier syntheses, and refined by full-matrix least-
squares procedures (SHELXL). Heavy atoms were refined with
anisotropic displacement parameters, whereas all hydrogen
atoms were included at calculated positions.
8. [(silox)2W](µ-CH)(µ-CH2)(µ-H) (9). A 5 mL flask charged
with 3 (50 mg, 0.038 mmol) and 0.9% Na/Hg (194 mg, 0.076
mmol Na) was attached to a needle valve adapter equipped
with a sidearm leading to an NMR tube. THF-d8 (0.75 mL)
was distilled into the evacuated flask at 77 K, then an excess
of ethylene (150 Torr, 4.9 mmol) was admitted to a 600 mL
manifold and condensed into the flask. The solution was
warmed to 25 °C and stirred vigorously. An intense red color
developed within 5 min, whereupon the solution was rapidly
cooled to -78 °C and stirred under dynamic vacuum for 1 min
to degas. The sidearm and NMR tube surfaces were cooled to
-78 °C, and the solution was rapidly decanted into the NMR
tube. The tube was cooled to 77 K for flame-sealing and kept
1
frozen until just before inserting into a precooled probe for H
NMR analysis. A similar procedure using 13C2H4 generated
9-13C2 for NMR analysis.
Ack n ow led gm en t. We thank the National Science
Foundation (CHE-9528914) and Cornell University for
support of this research.
9. [(silox)2HTa Et]2 (11). A 25 mL flask charged with 0.150
g (0.123 mmol) of [(silox)2TaH2]2 (10) was attached to a gas
bulb and mounted on the vacuum line. At -78 °C, 10 mL of
diethyl ether were distilled in, and 2 equiv of ethylene was
admitted via the gas bulb. The solution turned dark brown
and was stirred for 15 min. The solvent was removed, affording
Su p p or tin g In for m a tion Ava ila ble: Crystal data, data
collection, and solution and refinement details pertaining to
[(silox)2W]2(µ-CCH3)2 (4b); CIF file pertaining to [(silox)2EtTa]-
(µ-CHCH2)(µ-H)2[Ta(silox)2] (12). This material is available free
of charge via the Internet at http://pubs.acs.org.
1
a brown solid. H NMR spectroscopic analysis indicated a 3:1
mixture of 11 and 12.
10. [(silox)2EtTa ](µ-CHCH2)(µ-H)2[Ta (silox)2] (12). To a
25 mL flask charged with 0.200 g (0.163 mmol) of 10 was
OM020037N