Olefination and group transfer reactions of an electron deficient tantalum methylidene complex

Article abstract of DOI:10.1039/b512741f

The reactivity of an electronically unsaturated tantalum methylidene complex [TolC(NSiMe₃)₂]₂Ta(CH ₂)CH₃ (1) supported by [TolC(NSiMe₃) ₂] amidinate ligands is described. Electro

Full text of DOI:10.1039/b512741f

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Olefination and group transfer reactions of an electron deficient tantalum
methylidene complex†
Sarah M. Mullins, Robert G. Bergman* and John Arnold*
Received 9th September 2005, Accepted 19th October 2005
First published as an Advance Article on the web 14th November 2005
DOI: 10.1039/b512741f
The reactivity of an electronically unsaturated tantalum methylidene complex [TolC(NSiMe3)2]2-
Ta(CH2)CH3 (1) supported by [TolC(NSiMe3)2] amidinate ligands is described. Electrophilic addition
=
and olefination reactions of the Ta CH2 functionality are reported. Alkylidene 1 participates in
group-transfer reactions not observed in sterically similar, but electronically saturated, analogues.
=
Reactions with substrates containing unsaturated C–X (X = C, N, O) bonds yield [Ta] X compounds
and vinylated organic products; carbon–sulfur cleavage reactions to produce tantalum
thioformaldehyde and tantalum sulfido complexes.
saturated complexes, the chemistry of isolable electronically un-
saturated transition-metal methylidene complexes remains largely
unexplored.14–19
Introduction
Transition-metal methylidene complexes are important in both in-
dustrial and synthetic organic applications. For example, surface-
bound methylene groups are proposed as intermediates in the
Fischer–Tropsch reaction.1,2 Heteroatom (O, S, NR) abstraction
reactions by methylidene complexes model hydrodesulfurization
and hydrodeamination of petroleum feedstocks as well as the
poisoning of catalyst surfaces.3 Molecular compounds that are
analogous to the industrially important methylene-functionalized
metal and metal oxide surface species provide insight into the
structure and reaction mechanisms of these complicated heteroge-
neous systems.4 Finally, transition-metal methylidene complexes
are useful reagents for organic transformations.5 Insertion of the
carbene fragment into C–H bonds is one method for the function-
alization of organic molecules.6 As a result of these applications,
the chemistry of transition-metal methylidene complexes is now
well established.
The a-carbon atoms of transition-metal methylidene complexes
have been reported to exhibit either nucleophilic or electrophilic
behavior. The electronic properties of these unsaturated ligands
are highly sensitive to electronegativity of the metal, its oxidation
state, and the overall charge of the complex.5 Generally, high
oxidation state early metal carbenes exhibit nucleophilic behavior,
while low oxidation state late metal carbenes generally act as
electrophiles.20 However, these observations are far from rigid,
and methylidenes are especially sensitive to the electron density
at the metal center. For example, the late metal methylidene,
Cp*(PMe3)Ir(CH2), exhibits nucleophilic behavior.21 The nucle-
ophilic methylidene in Cp2Ta(CH2)CH3 stands in contrast to
the electrophilic nature of the isoelectronic, cationic tungsten
analogue, [Cp2W(CH2)CH3]+.22 Some methylidene complexes even
=
exhibit amphiphilic behavior: an example is Cl(NO)PPh3Os CH2,
which reacts with both electrophiles (H+) and nucleophiles (CO or
CNR).23
While early metal methylidene complexes are unquestionably
of great industrial and academic significance, their synthesis is
often a challenge. In most early metal alkylidene complexes,
Given the potential applications of electronically unsatu-
rated transition-metal methylidenes, we were interested in ex-
ploring the reactivity afforded by the bis(amidinate)tantalum
methylidene complex, [TolC(NSiMe3)2]2Ta(CH2)CH3 (1), the
synthesis of which we have previously reported.24 The
bis(trimethylsilyl)tolylamidinate ligand set enforces a cis geometry
of the methylene and methyl ligands, which is important for
potential migration processes, and provides steric bulk interme-
diate to that of bis(Cp) and bis(Cp*) ligand sets.25 The anionic
bis(trimethylsilyl)tolylamidinate ligand is generally considered
to be a four electron donor; this generates an electronically
unsaturated (14e−) metal center in 1.
Our previous mechanistic work on the reaction of tantalum
methylidene 1 with pyridine N-oxides demonstrates that the
electron deficient metal center is critical to opening new reaction
pathways (Scheme 1).26 The selective reactivity of 1 with pyridine
N-oxides stands in contrast to the 18e− tantalocene analog,
Cp2Ta(CH2)CH3, which is unreactive. Additional support for our
belief that the electron count is important in this reaction comes
from a recent report in which the 16e− “Cp2Ti(CH2)” was used
=
the M CR2 moiety tends to be quite unstable to decomposition
and dimerization. Accordingly, the isolation of such compounds
often requires steric bulk at the metal, a high formal electron count,
or substitution at the a-carbon atom. These strategies have been
employed by Schrock and co-workers,7 who used electronic sat-
8,9
uration to synthesize Cp2Ta(CH2)CH3 and a bulky substituent
on the alkylidene carbon atom to prepare CpCl2Ta(CHCMe3).10
While the 18-electron Cp2Ta(CH2)CH3 reacts with some organic
electrophiles,8 the less electron rich tantalum alkylidenes (e.g. 14e−
CpCl2Ta(CHCMe3)) exhibit the most reactivity.7,11 Electronically
unsaturated titanium methylidenes, which also exhibit a broad
spectrum of reactivity with organic substrates, may be accessed
from “masked” precursors, such as the aluminium alkyl-stabilized
Tebbe reagent12 or titanacyclobutanes.13 Despite the potential
increased reactivity of these derivatives compared to that of the
Department of Chemistry, University of California, Berkeley, CA, USA
94720-1460
† In memory of our friend and colleague Professor Ian Rothwell.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 203–212 | 2 0 3
©

View Article Online
Table 1 Crystallographic data and refinement details for 1, 13, 14 and 15
1
13
14
15
Empirical formula
Mr
C30H55N4Si4Ta
765.08
C39H64N5Si4Ta
896.26
C30H55N4Si4STa
829.20
C29H53N4Si4TaS
783.11
T/◦C
−118
−103
−105
−107
Crystal system
Space group
Monoclinic
C2/c (no. 15)
28.2182(1)
8.7227(1)
19.8359(1)
3814.35(6)
128.625(1)
4
Monoclinic
P21/c (no. 14)
17.9014(4)
11.6015(2)
23.0038(5)
4459.0(2)
111.041(1)
4
Monoclinic
C2/c (no. 15)
12.9195(3)
18.8911(4)
16.7243(2)
3970.3(1)
103.423(1)
4
Monoclinic
C2/c (no. 15)
28.2627(4)
8.7781(2)
19.8801(3)
3819.2(1)
129.253(1)
4
˚
a/A
˚
b/A
˚
c/A
3
˚
V/A
b/◦
Z
Dc/g cm−3
1.332
x, 0.3
1.335
x, 0.3
10
3–52.0
1.387
x, 0.3
1.362
x, 0.3
10
3–49.4
Scan type/◦
Frame collection time/s
2h range/◦
3–52.3
30.26
3–51.9
30.14
l/mm−1
26.00
30.77
T
max, Tmin
0.640, 0.450
0.636, 0.517
0.12 × 0.25 × 0.30
21557
8314
4578
0.817, 0.580
0.496, 0.223
0.20 × 0.15 × 0.25
8561
3355
2646
Crystal dimensions/mm
No. reflections measured
No. unique reflections
No. observations (I > 3r)
No. variables
0.20 × 0.20 × 0.30
0.10 × 0.12 × 0.12
10633
3656
2920
177
9490
3582
2562
173
442
181
Data/parameter
Rint (%)
16.5
2.7
3.0
3.7
1.42
1.59, −0.94
10.4
4.6
2.7
3.0
0.93
0.76, −0.73
14.8
6.2
4.2
5.2
1.47
1.52, −1.13
14.6
4.6
3.4
3.9
1.4
0.72, −2.39
R (%)
Rw (%)
GOF
−3
˚
Largest diff. peak, hole/e A
Reflections measured: hemisphere; refinement method: full-matrix least-squares on F2; hydrogen atoms: idealized positions.
Scheme 1
to carry out an analogous pyridine N-oxide to methylpyridine
transformation. This report also described the application of this
methodology to a natural product synthesis.27,28
Fig. 1 ORTEP diagram of 1, showing 50% probability ellipsoids.
Hydrogen atoms are omitted for clarity.
These reactions encouraged us to determine whether the
thermodynamic stability of early metal–heteroatom (O, NR,
S) multiple bonds can be coupled with carbene transfer to
selectively functionalize organic substrates. This report describes
how the tantalum methylidene 1 participates not only in familiar
electrophilic addition and olefination reactions, but also group
transfer reactions not observed in sterically similar, but electroni-
cally saturated, analogues.
1 are given in Table 2. The dominance of the bis(trimethylsilyl)-
tolylamidinate ligands in the tantalum coordination sphere is
not only evident visually, but is also reflected by the inability to
crystallographically distinguish between the CH2 and CH3 ligands.
Molecules of 1 have packed in a mixture of two enantiomers, result-
ing in a crystallographic C2 axis which bisects the (CH3)Ta(CH2)
vector (despite the lack of molecular C2 symmetry). The observed
◦
˚
Results and discussion
Table 2 Selected bond distances (A) and angles ( ) for 1
Solid-state structure of 1
Ta(1)–N(1)
Ta(1)–N(2)
Ta(1)–N(5)
Ta(1)–C(1)
N(1)–C(8)
N(2)–C(8)
2.260(3)
2.191(3)
1.996(4)
2.049(5)
1.313(5)
1.347(5)
N(1)–Ta(1)–N(2)
N(1)–Ta(1)–C(1)
N(2)–Ta(1)–C(1)
N(1)–C(8)–N(2)
60.4(1)
92.7(2)
102.6(2)
114.7(3)
The X-ray structure of 1 provides a starting point for analysis
of the steric characteristics of the bis(trimethylsilyl)tolylamidinate
ligands. An ORTEP diagram of 1 is shown in Fig. 1. Selected crys-
tal and refinement data are in Table 1; bond lengths and angles for
2 0 4 | Dalton Trans., 2006, 203–212
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
˚
Differences in the reactions of methyl iodide between the
analogous 14e− (1) and the 18e− (Cp2Ta(CH2)CH3) complexes
provide evidence for the relatively electrophilic nature of the
tolylamidinate-supported tantalum complex (Scheme 2). In the
tantalocene example, iodide may dissociate from the ethyl methyl
iodido intermediate (5–6). b-Hydride elimination followed by
reductive elimination of methane forms the ethylene iodide tanta-
locene complex (7).8 No such dissociation or elimination reactions
were observed for 2; no methane or ethylene was observed, even
upon decomposition of 2. Presumably, the highly electrophilic
tantalum favors more rapid attack of I− after the initial alkylation,
and subsequently disfavors the iodide dissociation required for the
conversion of 2 to 3.
Ta1–C1 bond length, 2.05 A, is an average of the normal metal–
carbon single and double bond lengths.5
A space-filling model of 1 shows that the bis(trimethylsilyl)-
tolylamidinate ligand set has a roughly rectangular shape and that
the majority of the ligands’ steric bulk is relatively distant from
the methyl and methylidene groups (Fig. 2). Using CPK models,
Teuben and co-workers determined the coordination aperture29 to
be 84◦ for [PhC(NSiMe3)2]2M, compared to 103 and 72◦ for the
metallocene analogues Cp2M and Cp*2M (M = Y).25 This result
indicates that the steric bulk of the bis(tolylamidinate) ligand set
is more similar to the bis(Cp*) set. A qualitative comparison of
1 and Cp2Ta(CH2)CH3 (Fig. 2) reveals that the open faces of
the complexes are more similar than might be expected for two
ancillary ligand sets of such different size and shape.
Olefination reactions
Perhaps the most well-known reactions of early metal carbenes
are those that yield organic molecules that contain unsaturated
C–C bonds.30 Not surprisingly, the methylidene complex 1 also
participates in a variety of these reactions. For example, 1
reacts with nitriles (8a,b) to form vinylimido tantalum complexes
1
(11a,b) (Scheme 3). The vinyl group is clearly evident in the H
NMR (d 5.27 (d), 5.34 (d) ppm) and DEPT-135 (d 105.3 ppm)
experiments.
Fig.
2
Side and top views of space filling models for 1 and
Cp2Ta(CH2)CH3. Carbon labels identify the CH2 and CH3 ligands.
Reactions of 1 with electrophiles
The nucleophilic nature of the methylidene moiety of 1 is evident
from its reactions with electrophilic alkylating reagents. The
addition of 1 equiv. of methyl iodide to 1 results in the formation
of the ethyl methyl iodo tantalum complex 2, presumably by initial
attack of the Me+ fragment at the carbene carbon atom (Scheme 2).
No reaction was observed between 1 and tert-butyl chloride or
trimethylsilyl chloride.
Scheme 3
The reaction is proposed to proceed first with nitrile coordi-
nation (9a,b), followed by formation of an azatantalacyclobutene
(10a,b). Cycloreversion to the thermodynamically stable Ta NR
=
compound yields the product (11a,b) (Scheme 3). No intermedi-
ates were observed when the reaction was monitored by 1H NMR
spectroscopy; however, similar mechanisms have been suggested
for the reactions of other tantalum alkylidenes with nitriles.
Schrock and co-workers found that the 14e− CpCl2Ta(CHtBu)10
and 10e− (tBuCH2)3Ta(CHtBu)31 undergo this reaction with
8
nitriles, whereas electronically saturated Cp2Ta(CH2)CH3 and
Cp2Ta(CHtBu)Cl32 do not react at all. This suggests that nitrile
coordination may be the first step in all of these reactions.
Complex 1 reacts with certain aldehydes to yield olefinated
organic products. For example, the addition of acetaldehyde to
a benzene-d6 solution of 1 yields the tantalum oxo complex
1226 and propylene. However, no reaction is observed between
Scheme 2
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 203–212 | 2 0 5
©

View Article Online
◦
˚
1 and carbonyl-containing substrates with larger substituents,
such as tolualdehyde or benzophenone (Scheme 4). These [2 + 2]
cycloaddition/reversion transformations are well-established for
tantalum carbenes and phosphorus ylide complexes.33–35
Table 3 Selected bond distances (A) and angles ( ) for 13
Ta(1)–N(1)
Ta(1)–N(3)
Ta(1)–N(5)
Ta(1)–C(39)
N(5)–C(29)
C(29)–C(30)
Ta(1)–N(2)
Ta(1)–N(4)
Ta(1)–C(29)
N(1)–C(1)
2.123(4)
2.164(4)
1.996(4)
2.188(6)
1.379(7)
1.364(7)
2.296(4)
2.284(4)
2.106(5)
1.347(6)
1.329(6)
1.359(6)
N(1)–Ta(1)–N(2)
N(2)–Ta(1)–N(4)
N(3)–Ta(1)–N(4)
N(4)–Ta(1)–N(5)
N(1)–Ta(1)–N(5)
N(3)–Ta(1)–C(29)
N(5)–C(29)–C(30)
Ta(1)–C(29)–N(5)
Ta(1)–N(5)–C(29)
N(1)–C(1)–N(2)
N(3)–C(15)–N(4)
61.1(2)
91.0(2)
60.6(2)
87.9(2)
119.6(2)
92.3(2)
130.4(5)
66.1(3)
74.7(3)
114.5(4)
114.0(5)
N(2)–C(1)
N(3)–C(15)
Scheme 4
When a benzene-d6 solution of 1 was exposed to small alkenes
(ethylene or propylene, 1 equiv. to 1 atm), complex mixtures of
organic and organometallic products were obtained. Propylene
1
and 1-butene were among the organic products identified by H
NMR and GC-MS. Attempts to isolate the tantalum-containing
product(s) were unsuccessful, even when the reaction was carried
out in the presence of added Lewis base. Carbene fragment
insertion into small alkenes was previously observed in pincer27
and cyclopentadienyl tantalum neopentylidene complexes.36
Reaction with isocyanide: Formation of a metal-bound ketenimine
Metal-ketenimine complexes are versatile building blocks for the
synthesis of heterocyclic complexes. While carbene addition to an
isocyanide is an established method for the synthesis of metal–
ketenimine complexes, examples of this reaction with early metal
carbenes have not been forthcoming.37 Consequently, we tested
the behavior of 1 with isocyanides.
Fig. 3 1H–13C HMQC spectrum of 13 at room temperature.
The reaction of 1 with tert-butyl isocyanide yields multiple
intractable products. In contrast, addition of 1 equiv. of 2,6-
2
dimethylphenyl isocyanide to 1 yields the g -ketenimine complex
1
13 (eqn (1)). The H NMR spectrum of complex 13 reveals two
singlets, at d 4.78 ppm (1H) and d 5.99 ppm (1H). A 13C–1H
HMQC experiment revealed that both protons correlate to a single
resonance in the 13C NMR spectrum (d 77.2 ppm) (Fig. 3). A
DEPT-135 experiment confirmed that this carbon resonance is
the CH2 group of the ketenimine. We presume that the two-bond
coupling of these protons is too small to be observed.
Fig. 4 ORTEP diagram of 13, showing 50% probability ellipsoids.
Hydrogen atoms are omitted for clarity.
prismatic geometry (Fig. 5). C39 is the capping ligand; three of
the amidinate nitrogen atoms (N1, N2, N4) and the ketenimine
nitrogen (N5) form the rectangular face. The tantalum atom sits
(1)
˚
0.09 A below the plane defined by these four nitrogen atoms
˚
The X-ray structure of 13 establishes the regiochemistry of
the ketenimine in the solid state and helps to explain the 1H
NMR spectrum. An ORTEP diagram of 13 is presented in Fig. 4.
Metrical information is in Table 3; selected crystal and refinement
data are in Table 1. The complex has a distorted capped trigonal
(average nitrogen deviation from least squares plane = 0.03 A).
The remaining amidinate nitrogen atom, N3, and the ketenimine
carbon, C29, form the third vertices of the triangular faces. The
C29–N5 bond distance (1.380(7) A) is consistent with the presence
of a single bond (lit. C(sp )–N(sp ) = 1.355 A).38 The very different
˚
2
2
˚
2 0 6 | Dalton Trans., 2006, 203–212
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
the N1, N2 amidinate is unencumbered and can rotate freely.40
Hindered rotation of the 2,6-dimethylphenyl group is also ob-
served at low temperature, leading to two methyl resonances
(h, i), and is consistent with the slow exchange-limit structure.
A
1H NMR spectrum at the slow exchange limit is shown in
Fig. 6.
Sulfur transfer reactions
As part of an exploration of carbon-sulfur bond cleavage reac-
tions, we investigated the reactivity of 1 towards several sulfur-
containing organic compounds. Tantalum methylidene 1 abstracts
2
sulfur from propylene sulfide to yield the g -thioformaldehyde
compound 14 and propylene (eqn (2)). The methylene protons
of 14 appear as a singlet resonance at d 2.87 ppm (2H), in the
Fig. 5 ORTEP diagram of the immediate coordination sphere of 13.
Trimethylsilyl and aryl groups omitted for clarity.
1
1H NMR spectrum. A resonance at d 71.1 ppm in the 13C{ H}
NMR spectrum was identified as the methylene carbon of the
thioformaldehyde ligand by a DEPT-135 experiment.
chemical environments for the two methylene protons of the
ketenimine moiety are apparent from this structure. Proton a is
clearly oriented toward the trimethylsilyl group of the amidinate,
whereas proton b points toward the aryl group of the ketenimine
(Fig. 5).
1
Variable-temperature H NMR spectra of 13 reveal that while
multiple intramolecular fluxional processes occur, the methylene
group orientation is preserved in solution. At room temperature,
fast exchange of the bis(trimethylsilyl)tolylamidinate ligands is
evident from the presence of a single trimethylsilyl resonance
and a single tolyl methyl group resonance. At −70 ◦C, two
tolyl methyl group resonances (c, d; see Fig. 6) are observed,
suggesting slow exchange between amidinate ligands. Three
trimethylsilyl resonances (e, f, g) are observed in a 18H : 9H :
9H integration ratio. This implies that rotation about the internal
C2 axis39 of one amidinate ligand is slow, while for the second
amidinate the rotation barrier is more rapid. The line drawing
of 13, based on the X-ray structure (Fig. 6), shows the two
different chemical environments of the amidinate ligands. The
large 2,6-dimethylphenyl substituent on the ketenimine hinders
the rotation of the amidinate containing N3 and N4, whereas
(2)
Complex 14 crystallizes in the centrosymmetric space group
C2/c. The g -thioformaldehyde ligand is bound as a metallathia-
2
cyclopropane, with the sulfur in the inside position. An ORTEP
diagram is shown in Fig. 7. Selected crystal and refinement data
are in Table 1; metrical parameters are given in Table 4. As
in the X-ray structure of 1, the superposition of enantiomers
produces a crystallographic-C2 axis, which leads to disorder in
refinement of the –CH2S and –CH3 groups. The sulfur atom of the
thioformaldehyde (S1), which was disordered over two positions,
was refined at half-occupancy. The carbon atoms of the methylene
and the methyl group were refined as one full occupancy carbon
(C12); see X-ray details for further information.
Fig. 6 1H NMR spectrum of 13 (toluene-d8) at 203 K (* = residual diethyl ether, ** = solvent).
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 203–212 | 2 0 7
©

View Article Online
◦
◦
˚
˚
Table 5 Selected bond lengths (A) and angles ( ) for 15
Table 4 Selected bond lengths (A) and angles ( ) for 14
Ta(1)–S(1)
Ta(1)–N(1)
Ta(1)–N(2)
Ta(1)–C(15)
S(1)–C(15)
N(1)–C(1)
N(2)–C(1)
2.295(5)
2.281(6)
2.156(6)
2.23(1)
1.93(1)
1.31(1)
1.355(9)
S(1)–Ta(1)–N(1)
S(1)–Ta(1)–N(2)
S(1)–Ta(1)–C(15)
N(1)–Ta(1)–N(2)
N(1)–Ta(1)–C(15)
N(2)Ta(1)–C(15)
Ta(1)–S(1)–C(15)
Ta(1)–C(15)–S(1)
129.5(2)
104.8(2)
77.0(3)
61.0(2)
148.0(3)
98.4(3)
63.0(3)
66.6(4)
Ta(1)–S(1)
Ta(1)–N(1)
Ta(1)–N(2)
Ta(1)–C(15)
N(1)–C(1)
N(2)–C(1)
2.222(10)
2.251(4)
2.144(3)
2.27(4)
1.320(5)
1.337(6)
S(1)–Ta(1)–N(1)
S(1)–Ta(1)–N(2)
S(1)–Ta(1)–C(15)
N(1)–Ta(1)–N(2)
N(1)–C(1)–N(2)
153.7(2)
92.8(2)
95.4(7)
61.1(1)
114.6(4)
Fig. 8 ORTEP diagram of 15, showing 50% thermal probability ellip-
soids. Hydrogen atoms, C15* and S1* are omitted for clarity.
Several possible mechanisms for the formation of 15 and triph-
enylethane are outlined in Scheme 5. In mechanism A, the methyli-
dene deprotonates the thiol to form the symmetrical intermediate
16. b-Elimination reaction yields 15 and triphenylethane. In
mechanism B, Ph3CSH reacts as an electrophile (Ph3C+), attacking
the nucleophilic methylidene to form intermediate 17. b-Hydride
transfer from the thiolate group yields 15 and triphenylethane.
For pathway C, Ph3C+ abstracts the methide group to yield
Fig. 7 ORTEP diagram of 14, showing 50% thermal probability el-
lipsoids. Hydrogen, S1, C16/C16*, C17/C17*, and C18/C18* atoms
(disordered sulfur atom and trimethylsilyl group) are omitted for clarity.
2
Compound 14 is one of only a few g -thioformaldehyde
complexes known for early metals (Ta,41 Zr42 and Ti43). In addition,
14 is thermally robust; no changes were observed upon prolonged
heating at 135 ◦C. This behavior contrasts to that of other
tantalum thioaldehydes, Cp2Ta(SCHR)H, which undergo facile
migrations to the more thermodynamically stable terminal sulfido
alkyl complexes, Cp2Ta(S)CH2R.44
Carbon–sulfur bond cleavage of a different kind occurs in the
reaction of 1 with triphenylmethanethiol, to form the terminal
1
sulfido complex 15 and triphenylethane (eqn (3)). The H and
1
13C{ H} NMR spectra of the tantalum product are similar to those
for the terminal tantalum oxo analog, 12. The electron impactmass
spectrum shows the molecular ion at m/z 782.
(3)
The structure of the terminal tantalum–sulfido complex was
confirmed by X-ray crystallography. An ORTEP diagram is shown
in Fig. 8. Selected crystal and refinement data are given in Table 1;
metrical parameters are in Table 5. Like 1 and 14, the sulfido
complex 15 crystallizes in the centrosymmetric space group,
C2/c. Again, the superposition of the two enantiomers produces
a crystallographic C2 axis. The sulfido and methyl groups are
disordered over two positions; S1 and C15 were each refined at
˚
half-occupancy. The Ta1–S1 bond length is 2.22 A, similar to
other reported terminal Ta–S multiple bonds.45,46
Scheme 5
2 0 8 | Dalton Trans., 2006, 203–212
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
triphenylethane and a tantalum methylidene thiolate intermediate
(18). In the final step, 18 rearranges to the thermodynamically
preferred tautomer (15).
with a nitrogen purge.50 C6D6 was vacuum transferred from
Na/benzophenone. Propylene sulfide (Aldrich) was first dried
over molecular sieves, transferred under static vacuum to a
Teflon-sealed flask and stored at 5 ◦C. Pyridine N-oxide and
2-methylpyridine N-oxide (Aldrich) were dried over molecular
sieves in toluene and the crude solid sublimed before use. 2-
Ethylpyridine N-oxide was synthesized starting from ethylpyridine
via the literature procedure.51 2,2ꢀ-Dipyridyl N-oxide52 was pre-
pared according to literature procedures. Methyl iodide was stored
over Cu wire. 1,3,5-Trimethoxybenzene (Aldrich) was sublimed
before use. Ph3CSH (Aldrich), 2,6-dimethylphenyl isocyanide
(Fluka), and CH3OTf (Aldrich) were used as received. Acetonitrile
and p-tolylnitrile were distilled from CaH2. Melting points were
determined in sealed capillaries and are uncorrected. 1H and
While no intermediates were observed in this reaction, the
three proposed mechanisms proceed through distinct intermedi-
ates that are distinguishable by performing a deuterium tracer
experiment. The addition of 1 equiv. of triphenylmethanethiol
(73%-d1) to 1 yields a mixture of deuterium-incorporated prod-
ucts [TolC(SiMe3)2]2Ta(S)CH2D (0.442 H), Ph3CCH3 (1.09 H),
[TolC(SiMe3)2]2Ta(S)CH3 (1.15 H) and Ph3CCH2D (0.438 H).47
When the relative integration values of these products are
corrected to account for the incompletely deuterated starting
material, a nearly statistical ratio of products is revealed (TaCH3 :
TaCH2D = 1.07 : 1.00, Ph3CCH3 : Ph3CCH2D = 1.02 : 1.00). These
data are consistent with the intermediacy of 16 in mechanism A,
which is also the only pathway expected to give a mixture of deuter-
ated products. Furthermore, the observed isotopic distribution (i.e.
lack of d2 or d3 products) suggests that the protonation reaction
which forms 16 is irreversible.
1
13C{ H} NMR spectra were recorded at ambient temperatures,
unless otherwise noted. IR samples were prepared as mineral
oil mulls and taken between KBr plates. Elemental analysis
and mass spectral data were determined by the College of
Chemistry, University of California, Berkeley, CA. Single-crystal
X-ray determinations were performed at CHEXRAY, University
of California, Berkeley, CA.
Elimination reactions from tantalum(V) thiolate complexes have
been reported. When b-hydrogens are present on the thiolate
ligand, b-hydride elimination to thioformaldehyde is favored.41
In the case of a tertiary substituted thiolate group, carbon-sulfur
cleavage is preferred.48,49 The latter is consistent with the proposed
mechanism for the reaction between 1 and Ph3CSH.
[TolC(NSiMe3)2]2Ta(CH2CH3)(CH3)I (3). [TolC(NSiMe3)2]2-
Ta(CH2)CH3 (0.350 g, 0.458 mmol) was dissolved in toluene
(45 mL) in a 100 mL round bottomed flask. Methyl iodide (85 lL,
1.4 mmol) was added using a microliter syringe. (From this point
forward the reaction and workup were performed in the absence
of light). The reaction mixture was stirred at room temperature
for 7 h, during which time the solution turned green. The volatile
materials were removed under reduced pressure and the product
extracted with hexanes (50 mL). Concentration to 3 mL and
cooling of the filtrate to −30 ◦C afforded green crystals (0.150 g,
Oxygen abstraction reactions
Carbene complex 1 does not react with the oxygen analogs of the
sulfur reagents discussed above (propylene oxide, Ph3COH). We
previously reported on the reactions of 1 with pyridine N-oxides
and nitrones.26
1
36%). H NMR (C6D6, 300 MHz): d 0.268 (s, 36H, Si(CH3)3),
1.97 (s, 6H, ArCH3), 2.12 (s, 3H, TaCH3), 2.51 (q, J = 7 Hz, 2H,
TaCH2CH3), 3.04 (t, J = 7 Hz, 3H, TaCH2CH3), 6.82 (d, J =
8 Hz, 4H, Ar), 7.39 (d, J = 8 Hz, 4H, Ar). Thermal instability
prevented further characterization.
Summary and conclusion
The bis(amidinate) ancillary ligands on the tantalum methyl
methylidene complex 1 produce an electronically unsaturated
(14e−) tantalum center that has open space similar to that of
early metal bent metallocenes. Tantalum methylidene 1 is a stable,
isolable complex, but is reactive toward a range of electrophilic
substrates. Reactions with substrates containing unsaturated C–X
Reaction of [TolC(NSiMe3)2]2Ta(CH2)CH3 with CH3C(O)H.
In the glove box, a tared vial was charged with 1 (10.0 mg,
0.0131 mmol) and 1,3,5-trimethoxybenzene (2.0 mg, 0.012 mmol).
The solids were dissolved in C6D6 (0.5 mL) and transferred to an
NMR tube equipped with a J. Young valve. Acetaldehyde (2.0 lL,
0.036 mmol) was added by syringe. The 1H NMR spectrum that
was obtained after 90 min contained peaks matching an authentic
sample of [TolC(NSiMe3)2]2Ta(O)CH3, 12 (50% conversion vs.
1,3,5-trimethoxybenzene internal standard) and propylene. See
later for full characterization of 12.
=
(X = C, N, O) bonds yield [Ta] X compounds and vinylated
organic products. Methylidene 1 participates in carbon-sulfur
cleavage reactions to produce tantalum thioformaldehyde and
tantalum sulfido complexes. The electrophilic character of the 14e−
tantalum center is important in this chemistry. The range of reac-
tions reported herein for 1 expands the scope of transformations
known for early metal methylidene complexes.
[TolC(NSiMe3)2]2Ta[NC(CH2)(C6H4CH3)]CH3 (11a). A 100 mL
round-bottomed flask was charged with [TolC(NSiMe3)2]2-
Ta(CH2)CH3 (0.130 g, 0.170 mmol) and p-tolylnitrile (0.021 g,
0.18 mmol). Toluene (25 mL) was added and the orange
solution was stirred overnight at room temperature. The
volatile components were removed under reduced pressure and
the product extracted into pentane (30 mL). The solution was
filtered and concentrated to 2 mL. After cooling to −30 ◦C,
off-white microcrystals were isolated by filtration (0.070 g, 46%).
1H NMR (C6D6, 500 MHz): d 0.179 (s, 36H, Si(CH3)3), 1.29 (s,
Experimental
General considerations
Standard Schlenk line and glove box techniques were used
throughout. Hexanes was distilled from purple Na/benzophenone
under nitrogen. Toluene and hexamethyldisiloxane (HMDSO)
were distilled from Na under nitrogen. Benzene and pentane were
dried by passage through activated alumina and then degassed
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 203–212 | 2 0 9
©

View Article Online
3H, TaCH3), 1.96 (s, 6H, ArCH3), 2.20 (s, 3H, ArCH3), 5.27
(d, J = 1 Hz, 1H, TaNC(CHaHb)ArCH3), 5.34 (d, J = 1 Hz,
1H, TaNC(CHbHa)ArCH3), 6.84 (d, J = 8 Hz, 4H, Ar), 7.23 (d,
J = 8 Hz, 2H, Ar), 7.25 (d, J = 8 Hz, 4H, Ar), 8.16 (d, J =
concentrated to 10 mL and cooled to −30 ◦C. Yellow needles were
isolated by filtration (0.120 g, 55%). 1H NMR (C6D6, 500 MHz):
d 0.137 (s, 36 H, Si(CH3)3), 1.11 (s, 3H, TaCH3), 1.96 (s, 6H,
ArCH3), 2.87 (s, 2H, SCH2), 6.80 (d, 4H, J = 8 Hz, Ar), 7.14 (d,
1
1
8 Hz, 2H, Ar). 13C{ H} (C6D6, 125 MHz): d 3.0 (s, Si(CH3)3),
4H, J = 8 Hz, Ar). 13C{ H} (C6D6, 125 MHz): d 3.1 (s, Si(CH3)3),
21.6 (s, ArCH3), 21.7 (s, ArCH3), 49.6 (s, TaCH3), 105.3 (s,
TaNC(CH2)ArCH3), 126.7 (s, Ar), 126.9 (s, Ar), 128.7 (s, Ar),
129.4 (s, Ar), 137.3 (s, Ar), 139.4 (s, Ar), 139.7 (s, Ar), 139.9 (s,
Ar), 159.3 (s, TaNC(CH2)ArCH3), 180.2 (NCN). IR (cm−1): 1612
(m), 1584 (m), 1543 (s), 1503 (s), 1344 (s), 1246 (s), 1184 (m), 1165
(m), 1150 (m), 1110 (m), 1026 (m), 1003 (s), 986 (s), 839 (s), 759 (s),
722 (s), 646 (m), 489 (m), 465 (m). Anal. Calc. for C38H62N5Si4Ta:
C 51.73, H 7.08, N 7.94. Found C 52.02, H 7.18, N 7.83%.
21.6 (s, ArCH3), 57.3 (s, TaCH3), 71.1 (s, SCH2), 126.9 (s, Ar),
129.3 (s, Ar), 138.9 (s, Ar), 139.5 (s, Ar), 180.2 (s, NCN). DEPT-
135 71.1 (inverted). IR (cm−1): 1612 (m), 1524 (m), 1346 (m), 1246
(s), 1178 (w), 1158 (w), 1026 (w), 1007 (m), 985 (s), 839 (s), 761
(s), 716 (s), 644 (m), 462 (m). Anal. Calc. For C30H55N4Si4STa: C
45.20, H 6.95, N 7.03. Found C 44.88, H 6.82, N 6.94%.
[TolC(NSiMe3)2]2Ta(S)CH3 (15). A 100 mL round-bottomed
flask was charged with 1 (0.100 g, 0.131 mmol) and Ph3CSH
(0.037 g, 0.13 mmol). Toluene (20 mL) was added to the solids
at −30 ◦C. The yellow solution was slowly warmed to room
temperature overnight. The volatile materials were removed under
reduced pressure and the yellow solid was extracted into diethyl
ether (15 mL). The◦solution was filtered, concentrated to 1 mL
and cooled to −30 C. Yellow microcrystals of 15 were isolated
NMR-scale reaction of 1 with CH3CN. Complex 1 (7.5 mg,
0.0098 mmol) and 1,3,5-trimethoxybenzene (1.6 mg, 0.0095 mmol)
were weighed into a tared vial. The solids were dissolved in approx.
0.5 mL C6D6 and the solution was transferred to an NMR tube
equipped with a J. Young valve. Acetonitrile (1.0 lL, 0.019 mmol)
was added to the NMR tube using a microliter syringe. After
24 h at room temperature, the 1H NMR spectrum showed
peaks corresponding to [TolC(NSiMe3)2]2Ta[NC(CH2)(CH3)]CH3
(11b) (conversion >99% vs. 1,3,5-trimethoxybenzene internal
standard). 1H NMR (C6D6, 300 MHz): d 0.188 (s, 36H,
Si(CH3)3), 1.16 (s, 3H, TaCH3), 1.98 (s, 6H, ArCH3), 2.13 (s, 3H,
TaNC(CH2)CH3), 4.42 (br s, 1H, TaNC(CHaHb)CH3), 4.75 (br s,
1H, TaNC(CHbHa)CH3), 6.83 (d, J = 8 Hz, 4H, Ar), 7.14 (d, J =
8 Hz, 4H, Ar).
1
by filtration (0.60 g, 60%). H NMR (C6D6, 500 MHz): d 0.256
(s, 36H, Si(CH3)3), 1.67 (s, 3H, TaCH3), 1.97 (s, 6H, ArCH3),
1
6.80 (d, J = 8 Hz, 4H, Ar), 7.16 (d, J = 8 Hz, 4H, Ar). 13C{ H}
NMR (C6D6, 125 MHz): d 3.0 (s, Si(CH3)3), 21.6 (s, ArCH3),
56.4 (s, TaCH3), 126.6 (s, Ar), 129.4 (s, Ar), 138.9 (s, Ar), 139.7
(s, Ar), 181.3 (NCN). EI-MS (m/z): 782 (M+). Anal. Calc. for
C29H53N4Si4STa: C 44.48, H 6.82, N 7.15. Found C 44.32, H 6.58,
N 6.85%.
[TolC(NSiMe3)2]2Ta[g2-N(2,6-dimethylphenyl)C(CH2)]CH3 (13).
A 50 mL round bottomed flask was charged with 1 (0.200 g,
0.261 mmol) and 2,6-dimethylphenyl isocyanide (0.035 g,
0.27 mmol). The solids were cooled to −20 ◦C and toluene (20 mL)
was added to dissolve them. The yellow solution was stirred first
Preparation of Ph3CSD. The following procedure is a modifi-
cation of that reported for n-hexanethiol-d1.53 A 50 mL Schlenk
round bottom flask was charged with Ph3CSH (0.500 g, 1.81
mmol). Under a purge of N2, dry diethyl ether (20 mL) was added
to the flask. The thiol dissolved with gentle heating. NaH (0.045 g,
1.9 mmol) was added and the reaction mixture was stirred until
the bubbling ceased (35 min). D2O (1.0 mL) was added by pipette,
forming a biphasic solution which was stirred vigorously for
30 min. The ether layer was removed using a separatory funnel and
dried over MgSO4. The ether solution was concentrated to yield
white crystals (0.420 g, 84%). Isotopic enrichment as determined
by 1H NMR = 73% d1.
◦
1
at −20 C and then at room temperature for 3 h. The volatile
2
materials were removed under reduced pressure and the residue
was extracted into diethyl ether (20 mL). The solution was filtered,
concentrated to 5 mL, and cooled to −30 ◦C. Orange crystals were
isolated by filtration (0.120 g, 51%). 1H NMR (C6D6, 500 MHz):
d 0.143 (s, 36 H, Si(CH3)3), 1.22 (s, 3H, TaCH3), 1.97 (s, 6H,
ArCH3), 2.57 (s, 6H, NAr(CH3)2), 4.78 (s, 1H, CHaHb), 5.99 (s,
1H, CHaHb), 6.81 (d, 4H, J = 8 Hz, Ar), 7.07 (t, 1H, J = 8 Hz,
Ar), 7.20 (d, 2H, J = 8 Hz, Ar), 7.25 (d, 4H, J = 8 Hz, Ar).
NMR-scale reaction of 1 with Ph3CSD. In the glove box,
a tared vial was charged with 1 (0.0101 g, 0.0132 mmol) and
1,3,5-trimethoxybenzene (0.0024 g, 0.014 mmol). C6D6 (approx.
0.5 mL) was added to dissolve the solids and the solution was
transferred to a NMR tube equipped with a J. Young valve. An
1H NMR spectrum was acquired. In the glove box, Ph3CSD
(0.0037 g, 0.013 mmol) was added to the C6D6 solution. After
3.5 h, during which time the reaction solution turned yellow, the
1H NMR spectrum showed >99% conversion to the products, 7
and triphenylethane. Isotopic incorporation was observed in the
methyl groups of both the metal-containing and organic products
(see text for further details). Relevant 1H NMR (300 MHz)
resonances are as follows: d 1.62 (br t, 0.442 H, TaCH2D), 1.65 (s,
1.15 H, TaCH3), 2.01 (br t, 0.0438 H, Ph3CCH2D), 2.03 (s, 1.09
H, Ph3CCH3). Resonances for Ph3CCH3 matching the literature
1
13C{ H} (C6D6, 125 MHz): d 3.4 (s, Si(CH3)3), 21.0 (s, ArCH3),
21.6 (s, NAr(CH3)2), 63.3 (s, TaCH3), 77.2 (s, CH2), 125.5 (s, Ar),
127.3 (s, Ar), 129.3 (s, Ar), 133.3 (s, Ar), 139.0 (s, Ar), 139.8 (s,
Ar), 152.1 (s, Ar), 182.2 (s, NCN), 206.0 (s, TaCNAr). DEPT-135
77.2 (inverted). IR (cm−1): 2854 (s), 1611 (w), 1581 (w), 1518 (w),
1406 (m), 1358 (s), 1265 (m), 1246 (s), 1163 (w), 1004 (m), 985 (s),
836 (s), 764 (m), 710 (m) 643 (w). EI-MS (m/z): 895.8 (M+). Anal.
Calc. For C39H64N5Si4Ta: C 52.21, H 7.30, N 7.81. Found C 51.98,
H 7.62, N 7.70%.
[TolC(NSiMe3)2]2Ta(SCH2)CH3 (14). Compound 1 (0.207 g,
0.271 mmol) was loaded into a 100 mL round-bottomed flask.
Toluene (13 mL) was added to dissolve the solid. Propylene
sulfide (0.100 mL, 1.28 mmol) was added by syringe to the orange
solution at room temperature. After stirring for 20 h, the volatile
materials were removed under reduced pressure. The yellow solid
was extracted into pentane (50 mL). The solution was filtered,
1
were observed in the 13C{ H} NMR, but peaks for the Ph3CCH2D
were not definitely resolvable from noise.
2 1 0 | Dalton Trans., 2006, 203–212
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
Characterization of methylpyridine products from the reaction of
1 with substituted pyridine N-oxides.
methyl groups of 1 are disordered; C1 was refined anisotropically
at full occupancy to account for both of these ligands.
As in 1, 14 also resides on a crystallographic C2 axis that passes
through Ta1; C15 represents the disordered Ta–(CH2S) and Ta–
CH3 carbon atoms; it was refined isotropically at full occupancy.
The sulfur atom, S1, was disordered over two sites; it was refined
isotropically at half-occupancy. Carbon atoms C2–C4 and C16–
C18 of 14 are part of a disordered trimethylsilyl group; each atom
was refined isotropically at half-occupancy.
Like in 1 and 14, complex 15, resides on a crystallographic
C2 axis that passes through Ta1. The Ta–CH3 carbon atom,
C15, is also positionally disordered; it was refined isotropically at
half-occupancy. The sulfur atom, S1, was disordered in a similar
manner; it was refined anisotropically at half-occupancy.
CCDC reference numbers 283319–283322.
(a) Typical procedure: Reaction with pyridine N-oxide. In the
glove-box, [TolC(NSiMe3)2]2Ta(CH2)CH3 (0.075 g, 0.098 mmol)
and pyridine N-oxide (0.0093 g, 0.098 mmol) were weighed into
a tared vial. C6D6 (0.5 mL) was added to dissolve the reagents.
1
After 14 h, 1H and 13C{ H} NMR spectra were obtained of
this reaction mixture. These spectra contained peaks matching
authentic samples of 12 and 2-methylpyridine.26 Conversion by
1H NMR: 99%.
1
(b) Reaction with 2-methylpyridine N-oxide. 1H and 13C{ H}
NMR spectra matched those for an authentic sample of 2,6-
dimethylpyridine. Conversion by 1H NMR: 99%.26
(c) Reaction with 2-ethylpyridine N-oxide. The 1H NMR spec-
trum contained peaks consistent with 6-ethyl-2-methylpyridine.
Conversion by 1H NMR: 90%. Relevant 1H NMR (300 MHz): d
1.26 (t, J = 7 Hz, 2H, CH2CH3), 2.42 (s, 3H, CH3), 2.79 (q, J =
7 Hz, 3H, CH2CH3), 6.19 (m, 1H, Ar), 6.27 (m, 1H, Ar), 6.60 (m,
1H, Ar).
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b512741f
Acknowledgements
(d) Reaction with 2,2ꢀ-dipyridyl N-oxide. The 1H NMR spec-
This work was supported by grants from the National Science
Foundation (CHE-0345488 to R. G. B.) and (CHE-0072819
to J. A.). We thank Dr Fred Hollander for expert assistance with
the X-ray solution and refinement.
trum contained peaks consistent with 6-methyl-2,2ꢀ-dipyridyl.
1
1
Conversion by H NMR: 95%. Relevant H NMR (300 MHz):
d 2.45 (s, 3H, CH3), 6.67 (m, 2H, Ar), 7.24 (m, 2H, Ar), 8.53 (m,
1H, Ar), 8.64 (d, J = 8 Hz, 1H, Ar), 8.75 (d, J = 8 Hz, 1H, Ar).
References
General experimental details for X-ray structure determinations
1 P. M. Maitlis, H. C. Long, R. Quyoum, M. L. Turner and Z.-Q. Wang,
Chem. Commun., 1996, 1.
2 M. J. Overett, R. O. Hill and J. R. Moss, Coord. Chem. Rev., 2000,
206–207, 581.
3 J. P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke, in Principles
and Applications of Organotransition Metal Chemistry, Mill Valley, CA,
1987.
4 J. Toyir, M. Leconte, G. P. Niccolai and J. M. Basset, J. Catal., 1995,
152, 306.
5 W. A. Nugent and J. M. Mayer, Metal–Ligand Multiple Bonds, John
Wiley & Sons, New York, 1988.
6 M. P. Doyle and M. N. Protopopova, Tetrahedron, 1998, 54, 7919.
7 R. R. Schrock, Acc. Chem. Res., 1979, 12, 98.
8 R. R. Schrock and P. R. Sharp, J. Am. Chem. Soc., 1978, 100, 2389.
9 R. R. Schrock, J. Am. Chem. Soc., 1975, 97, 6577.
10 C. D. Wood, S. J. McLain and R. R. Schrock, J. Am. Chem. Soc., 1979,
101, 3210.
11 R. R. Schrock, Science, 1983, 219, 13.
12 F. N. Tebbe, G. W. Parshall and G. S. Reddy, J. Am. Chem. Soc., 1978,
100, 3611.
13 K. A. Brown-Wensley, S. L. Buchwald, L. Cannizzo, L. Clawson, S.
Ho, D. Meinhardt, J. R. Stille and R. H. Grubbs, Pure Appl. Chem.,
1983, 55, 1733.
14 M. D. Fryzuk, X. Gao, K. Joshi, P. A. MacNeil and R. L. Massey,
J. Am. Chem. Soc., 1993, 115, 10581.
15 M. D. Fryzuk, S. A. Johnson and S. J. Rettig, Organometallics, 1999,
18, 4059.
16 A. H. Lui, R. C. Murray, J. C. Dewn, B. D. Santarsiero and R. R.
Schrock, J. Am. Chem. Soc., 1987, 109, 4282.
Crystals were mounted onto glass fibers using Paratone N
hydrocarbon oil and were transferred to a Siemens SMART
diffractometer/CCD area detector,54 centered in the beam, and
cooled by a nitrogen-flow low-temperature apparatus. Prelimi-
nary orientation matrix and cell constants were determined by
collection of 60 10 s frames, followed by spot integration and
least squares refinement. A hemisphere of data was collected
using x scans of 0.3◦ counted for a total of 10 s per frame.
The raw data were integrated by the program SAINT.55 Data
analysis was performed using XPREP.56 For 13, and 14, an
absorption correction was applied using XPREP; for 1 and 15, the
correction was applied using SADABS.57 The unit cell parameters
and statistical analysis of the intensity distribution were used
for space group determination.58 The data were corrected for
Lorentz and polarization effects, but no correction for crystal
decay was applied. Unique equivalent reflections were merged. The
structures were solved by direct methods59 and expanded using
Fourier techniques.60 Except as noted, all non-hydrogen atoms
were refined anisotropically and hydrogen atoms were included
as fixed contributions but not refined. The quantity minimized
by the least-squares program was Rw(|Fo| − |Fc|),2 where w
is the weight of a given observation. The weighting scheme was
based on counting statistics and included a factor (p = 0.030) to
downweight the intense reflections. The analytical forms of the
scattering factor tables for the neutral atoms were used,61 and all
scattering factors were corrected for both the real and imaginary
components of anomalous dispersion.62 All calculations were
performed using the teXsan63 crystallographic software package
of Molecular Structure Corp. Detailed information regarding
disordered atoms in individual structure solutions is given below.
For 1, the complex resides on a crystallographic C2 axis that
passes through Ta1. Therefore, the tantalum-bound methylene and
17 L. Andrews, H. G. Cho and X. Wang, Inorg. Chem., 2005, 44, 4834.
18 J. J. Eisch and A. A. Adeosun, Eur. J. Org. Chem., 2005, 993.
19 A. E. Enriquez, P. S. White and J. L. Templeton, J. Am. Chem. Soc.,
2001, 123, 4992.
20 R. H. Crabtree, The Organometallic Chemistry of the Transition Metals,
John Wiley and Sons, New York, 1994.
21 D. P. Klein and R. G. Bergman, J. Am. Chem. Soc., 1989, 111, 3079.
22 J. C. Hayes, P. Jernakoff, G. A. Miller and N. J. Cooper, Pure Appl.
Chem., 1984, 56, 25.
23 A. F. Hill, W. R. Roper, J. M. Waters and A. H. Wright, J. Am. Chem.
Soc., 1983, 105, 5939.
24 D. Y. Dawson and J. Arnold, Organometallics, 1997, 16, 1111.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 203–212 | 2 1 1
©

View Article Online
25 R. Duchateau, C. Van Wee, A. Meetsma, P. van Duijen and J. Teuben,
Organometallics, 1996, 15, 3539.
26 S. M. Mullins, R. G. Bergman and J. Arnold, Organometallics, 1999,
18, 4465.
27 K. C. Nicolaou, A. E. Koumbis, S. A. Snyder and K. B. Simonsen,
Angew. Chem., Int. Ed., 2000, 39, 2529.
28 K. C. Nicolaou, S. A. Snyder, X. H. Huang, K. B. Simonsen, A. E.
Koumbis and A. Bigot, J. Am. Chem. Soc., 2004, 126, 10162.
29 The coordination aperture is defined as the angle between the two
planes through the metal center that touch the inner van der Waals
surface of the ligand system.
46 G. Parkin, Prog. Inorg. Chem., 1998, 47, 1.
47 The sums of TaCH3 and TaCH2 integrations relative to trimethoxy-
benzene internal standard are: 3.35
(finish).
H (start) and 3.12 H
48 H. Kawaguchi and K. Tatsumi, Organometallics, 1997, 16, 307.
49 K. Tatsumi, A. Tahara and A. Nakamura, J. Organomet. Chem., 1994,
471, 111.
50 P. J. Alaimo, D. W. Peters, J. Arnold and R. G. Bergman, J. Chem.
Educ., 2001, 78, 64.
51 L. Kaczmarek, R. Balicki and P. Nantka-Namirski, Chem. Ber., 1992,
125, 1965.
30 T. Takeda, Bull. Chem. Soc. Jpn., 2005, 78, 195.
31 R. R. Schrock and J. D. Fellmann, J. Am. Chem. Soc., 1978, 100, 3359.
32 R. R. Schrock, L. W. Messerle, C. D. Wood and L. J. Guggenberger,
J. Am. Chem. Soc., 1978, 100, 3793.
33 R. R. Schrock, J. Am. Chem. Soc., 1976, 98, 5399.
34 G. Wittig, J. Organomet. Chem., 1975, 100, 279.
35 G. Wittig and G. Geissler, Justus Liebigs Ann. Chem., 1954, 580, 44.
36 S. J. McLain, C. D. Wood and R. R. Schrock, J. Am. Chem. Soc., 1976,
98, 3519.
37 R. Aumann, Angew. Chem., Int. Ed. Engl., 1988, 27, 1456.
38 D. R. Lide, in Handbook of Chemistry and Physics, CRC Press, Boca
Raton, FL, 1996.
39 Rotation around the internal C2 axis has been observed in amidinate
complexes. See: L. A. Koterwas, J. C. Fettinger and L. R. Sita,
Organometallics, 1999, 18, 4183.
52 D. Wenkert and R. B. Woodward, J. Org. Chem., 1983, 48, 283.
53 D. Plant, D. S. Tarbell and C. Whiteman, J. Am. Chem. Soc., 1955, 77,
1572.
54 SMART: Area-Detector Software Package, Siemens Industrial Au-
tomation, Inc., Madison, WI, 1995.
55 SAINT: SAX Area-Dectector Integration Program, V4.024, Siemens
Industrial Automation, Inc., Madison, WI, 1995.
56 XPREP: (v 5.03) Part of the SHELXTL Crystal Structure Determina-
tion Siemens Industrial Automation, Inc., Madison, WI, 1995.
57 G. M. Sheldrick, SADABS: Siemens Area Detector ABSorption correc-
tion program, 1996. Advance copy, private communication.
58 D. T. Cromer and J. T. Waber, International Tables for X-ray Crystallog-
raphy, The Kynoch Press, Birmingham, England, 1974, vol. IV, table
2.2 A.
59 SIR92: A. Altomare, M. C. Burla, Camalli, M. Cascarano, C.
Giacovazzo, A. Guagliardi and G. Polidori, J. Appl. Crystallogr., 1993,
26, 343.
60 DIRDIF92: P. T. Beurskens, G. Admiraal, G. Beurskens, W. P.
Bosman, S. Garcia-Granda, R. O. Gould, J. M. M. Smits and C.
Smykalla, The DIRDIF program system, Technical Report of the
Crystallography Laboratory, University of Nijmegen, The Netherlands,
1992.
40 The assignments of the individual tolyl methyl (c, d) and trimethylsilyl
groups (e, f, g) are not conclusively known, but are instead based on
the X-ray structure.
41 J. E. Nelson, G. Parkin and J. E. Bercaw, Organometallics, 1992, 11,
2181.
42 S. L. Buchwald, R. B. Nielson and J. C. Dewan, J. Am. Chem. Soc.,
1987, 109, 1590.
43 J. W. Park, L. M. Henling, W. P. Schaefer and R. H. Grubbs,
Organometallics, 1990, 9, 1650.
44 G. Parkin, E. Bunel, B. J. Burger, M. S. Trimmer, A. van Asselt and
J. E. Bercaw, J. Mol. Catal., 1987, 41, 21.
45 M. G. B. Drew, D. A. Rice and D. M. Williams, J. Chem. Soc., Dalton
Trans., 1984, 845.
61 D. C. Creagh and J. H. Hubbell, International Tables for Crystallogra-
phy, Vol C, ed. A. J. C. Wilson, Kluwer Academic Publishers, Boston,
MA, 1992, Table 4.2.4.3, pp. 200–206.
62 J. A. Ibers and W. C. Hamilton, Acta Crystallogr., 1964, 17, 781.
63 teXsan: Crystal Structure Analysis Package, Molecular Structure
Corporation, The Woodlands, TX, 1985 and 1992.
2 1 2 | Dalton Trans., 2006, 203–212
This journal is
The Royal Society of Chemistry 2006
©