Reactions of a triazacyclononane-supported tantalum-lithium bridging alkylidene with organic substrates

Article abstract of DOI:10.1021/om020175c

The reactivity of a range of electrophilic organic substrates with a heterobimetallic tantalum-lithium bridging alkylidene [(Me₃SiCH₂)(ArN=)Ta(μ-CHSiMe₃)(μ-η ₁:η₃-ⁱPr₂-tacn)Li, 3] is presented. Proton sources of widely varying acidity react to protonate the alkylidene ligand, leading to an interesting tantalum-lithium bridging hydride complex in the case of H₂. The alkylidene 3 undergoes a series of insertion reactions with unsaturated substrates, such as acetonitrile, carbon monoxide, and carbon disulfide; it also reacts with an acid chloride to yield a tantalum enolate species featuring return of the ⁱPr₂-tacn^- ligand to a tridentate coordination mode. The incorporated lithium in 3 played an important role, at least structurally, in the chemistry observed.

Full text of DOI:10.1021/om020175c

3426
Organometallics 2002, 21, 3426-3433
Rea ction s of a Tr ia za cyclon on a n e-Su p p or ted
Ta n ta lu m -Lith iu m Br id gin g Alk ylid en e w ith Or ga n ic
Su bstr a tes
J oseph A. R. Schmidt and J ohn Arnold*
Department of Chemistry, University of California at Berkeley and Chemical Sciences
Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460
Received February 28, 2002
The reactivity of a range of electrophilic organic substrates with a heterobimetallic
tantalum-lithium bridging alkylidene [(Me3SiCH2)(ArNd)Ta(µ-CHSiMe3)(µ-η1:η3-iPr2-tacn)-
Li, 3] is presented. Proton sources of widely varying acidity react to protonate the alkylidene
ligand, leading to an interesting tantalum-lithium bridging hydride complex in the case of
H2. The alkylidene 3 undergoes a series of insertion reactions with unsaturated substrates,
such as acetonitrile, carbon monoxide, and carbon disulfide; it also reacts with an acid chloride
i
to yield a tantalum enolate species featuring return of the Pr2-tacn- ligand to a tridentate
coordination mode. The incorporated lithium in 3 played an important role, at least
structurally, in the chemistry observed.
In tr od u ction
The use of 1,4,7-triazacyclononane (tacn) as a ligand
in coordination chemistry has been well explored over
the past few decades, with numerous reports appearing
recently.1-15 Two main features make this nine-mem-
bered macrocycle a useful ligand. First, its three nitro-
gen atoms provide for tridentate, facial coordination to
a metal center. Second, the choice of substituents on
each nitrogen atom allows for tuning of the steric bulk
and electronic properties of these ligands. Despite the
preponderance of complexes displaying the former fea-
ture, there are examples of species where tacn ligands
exhibit fluxionality in solution due to partial dissociation
F igu r e 1. Fluxionality in tacn compounds caused by
release and recoordination of nitrogen atoms of the tacn.
of the ligand. In this way, tacn can transiently function
as a bidentate or a monodentate ligand by dissociation
of one or two nitrogen atoms, respectively (Figure 1).
In fact, in rare cases, complexes of this type have been
isolated and characterized crystallographically, showing
the existence of monodentate12 and bidentate16-24 tacn
coordination in the solid state. In general, it appears
that the coordination mode of tacn is strongly influenced
by the electronic environment of the metal center. The
possibility that tacn may function as a hemilabile
ligand25 in catalytic reactivity is, therefore, a further
inducement to its study.
* Corresponding author. E-mail: arnold@socs.berkeley.edu.
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10.1021/om020175c CCC: $22.00 © 2002 American Chemical Society
Publication on Web 07/02/2002

Tantalum-Lithium Bridging Alkylidene
Organometallics, Vol. 21, No. 16, 2002 3427
Sch em e 1
alkylidenes are well studied, and these compounds react
with a wide variety of electrophilic sources, ranging from
reactive acid chlorides to relatively inert acetylenes.29-31
Some useful catalytic applications of alkylidenes include
ring-opening metathetical polymerization (ROMP), acety-
lene polymerization, and olefin metathesis. These cata-
lytic reactions, along with an array of useful stoichio-
metric reactions, allow alkylidene complexes to be
employed to great effect in organic synthesis.
A number of monometallic tantalum alkylidenes have
been synthesized, and their reactivity has been explored
in detail.32-51 Recently, we reported the synthesis of a
tantalum-lithium bridging alkylidene supported by an
anionic triazacyclononane ligand (3, Scheme 1).52 In our
initial studies of its reactivity, we found that insertion
of CO yielded a tantalum-lithium ketene, while double-
bond metathesis led to the generation of a tantalum-
lithium oxo complex.53 Additionally, through salt me-
tathesis routes, the lithium ion in 3 could be replaced
by other transition metals, such as rhodium and iron.53
Here we describe the reactivity of 3 with a wide range
of organic substrates. To understand the reactivity of
this heterobimetallic tantalum-lithium alkylidene spe-
cies, stoichiometric reactions with a number of electro-
philic organic substrates were investigated as a means
of gauging the interaction between the two metal
centers in this complex. The results are presented in
comparison to similar reactions with monometallic
tantalum alkylidenes.
Exp er im en ta l Section
(26) Yamamoto, A. Organotransition Metal Chemistry; J ohn Wiley
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Gen er al Con sider ation s. Standard Schlenk-line and glove-
box techniques were used throughout. Pentane, diethyl ether,
and toluene were passed through a column of activated
alumina and degassed with argon. Octane was distilled from
sodium/benzophenone ketyl under nitrogen. Acetonitrile was
refluxed over sodium and distilled under nitrogen. Pyridinium
chloride, 2,4,6-trimethylphenol, p-toluoyl chloride, and carbon
disulfide were purchased from Aldrich and used as received.
Hydrogen (99.99%) and carbon monoxide (99.50%) were pur-
chased from Airgas and Matheson, respectively, and used as
received. The bridging alkyl 2 and alkylidene 3 were synthe-
sized as detailed previously.52 C6D6 and C7D8 were vacuum
transferred from sodium/benzophenone. Melting points were
determined in sealed capillary tubes under nitrogen and are
uncorrected. 1H, 7Li, and 13C{1H} NMR spectra were recorded
(27) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, 1987.
(28) Nugent, W. A.; Mayer, J . M. Metal-Ligand Multiple Bonds;
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(30) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98-104.
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metallics 2000, 19, 2243-2245.
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1999, 18, 4465-4467.
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1993, 126, 1565-1569.
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1993, 126, 1571-1577.
in C6D6 at ambient temperature on
a Bruker DRX-500
spectrometer, unless otherwise specified. 1H NMR chemical
(38) Abbenhuis, H. C. L.; Feiken, N.; Grove, D. M.; J astrzebski, J .;
Kooijman, H.; Vandersluis, P.; Smeets, W. J . J .; Spek, A. L.; Vankoten,
G. J . Am. Chem. Soc. 1992, 114, 9773-9781.
shifts are given relative to C6D5H (δ 7.16) and C6D5CD2H (δ
7
2.09). Li NMR spectra are relative to an external LiCl (3 M
(39) Berry, D. H.; Koloski, T. S.; Carroll, P. J . Organometallics 1990,
9, 2952-2962.
in D2O) standard (0.00 ppm). 13C NMR chemical shifts are
relative to C6D6 (δ 128.39) and C7D8 (δ 20.4). IR samples were
prepared as Nujol mulls and taken between KBr plates.
Elemental analyses were determined by the Microanalytical
Laboratory of the College of Chemistry, University of Califor-
nia, Berkeley. Single-crystal X-ray structure determinations
were performed at CHEXRAY, University of California, Ber-
keley.
(40) Wallace, K. C.; Liu, A. H.; Dewan, J . C.; Schrock, R. R. J . Am.
Chem. Soc. 1988, 110, 4964-4977.
(41) Rocklage, S. M.; Turner, H. W.; Fellmann, J . D.; Schrock, R. R.
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3077-3081.
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L. W.; Schrock, R. R. J . Am. Chem. Soc. 1981, 103, 1440-1447.
(44) Rupprecht, G. A.; Messerle, L. W.; Fellmann, J . D.; Schrock,
R. R. J . Am. Chem. Soc. 1980, 102, 6236-6244.
(45) Antonelli, D. M.; Schaefer, W. P.; Parkin, G.; Bercaw, J . E. J .
Organomet. Chem. 1993, 462, 213-220.
(Me3SiCH 2)2(Ar Nd)Ta (µ-2,4,6-Me3C6H 2O)(µ-η1:η3-iP r 2-
ta cn )Li (4). A solution of 2,4,6-trimethylphenol (55 mg, 0.40
mmol) in toluene (20 mL) was added to a solution of 3 (300
mg, 0.400 mmol) in toluene (30 mL), and the mixture was
stirred for 1 h at ambient temperature. After the removal of
solvent under vacuum, pentane was added (30 mL) and the
solvent was again removed in vacuo, eliminating residual
(46) Belmonte, P. A.; Schrock, R. R.; Day, C. S. J . Am. Chem. Soc.
1982, 104, 3082-3089.
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485-487.
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1981, 20, 978-980.
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183-185.
(52) Schmidt, J . A. R.; Chmura, S. A.; Arnold, J . Organometallics
2001, 20, 1062-1064.
(53) Schmidt, J . A. R.; Arnold, J . J . Am. Chem. Soc. 2001, 123,
8424-8425.
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(51) Vanasselt, A.; Burger, B. J .; Gibson, V. C.; Bercaw, J . E. J . Am.
Chem. Soc. 1986, 108, 5347-5349.

3428 Organometallics, Vol. 21, No. 16, 2002
Schmidt and Arnold
toluene. The solid was then extracted into diethyl ether (20
mL) and filtered, and the solvent was removed under vacuum
to give a pale yellow solid (276 mg, 78%): mp 155-157 °C; 1H
NMR δ 7.258 (d, 2H, 8 Hz), 6.980 (t, 1H, 8 Hz), 6.840 (s, 2H),
4.486 (sept, 2H, 7 Hz), 4.03 (mult, 2H), 2.93 (mult, 2H), 2.780
(sept, 2H, 7 Hz), 2.496 (s, 6H), 2.43 (mult, 2H), 2.192 (s, 3H),
2.14 (mult, 4H), 1.85 (mult, 2H), 1.501 (d, 12H, 7 Hz), 0.519
(d, 2H, 13 Hz), 0.471 (d, 6H, 7 Hz), 0.450 (d, 6H, 7 Hz), 0.375
syringe. The system was closed and heated to 90 °C for 36 h.
The toluene was removed under vacuum, and pentane was
added (40 mL). Again, all volatile materials were removed in
vacuo, yielding an orange solid. The solid was extracted with
diethyl ether (40 mL), and the solution was filtered to another
flask. The removal of solvents under vacuum yielded the
desired product (281 mg, 76%): mp 78-79 °C; 1H NMR δ 7.302
(d, 2H, 7.5 Hz), 7.011 (t, 1H, 7.5 Hz), 4.506 (sept, 2H, 7 Hz),
3.93 (mult, 1H), 3.854 (s, 1H), 3.33 (mult, 1H), 2.83 (mult, 2H),
2.564 (sept, 1H, 7 Hz), 2.512 (sept, 1H, 7 Hz), 2.43 (mult, 1H),
2.189 (s, 3H), 1.95 (mult, 3H), 1.82 (mult, 2H), 1.71 (mult, 2H),
1.577 (d, 6H, 7 Hz), 1.509 (d, 6H, 7 Hz), 0.874 (d, 3H, 7 Hz),
0.802 (d, 3H, 7 Hz), 0.739 (d, 3H, 7 Hz), 0.550 (d, 3H, 7 Hz),
0.425 (d, 1H, 11 Hz), 0.385 (s, 9H), 0.301 (s, 9H), 0.284 (d, 1H,
11 Hz); 7Li NMR δ 2.699; 13C NMR δ 173.6, 142.2, 121.7, 120.0,
95.9, 63.1, 56.5, 54.8, 54.4, 51.0, 50.2, 47.5, 45.6, 44.2, 44.1,
27.7, 27.3, 24.4, 23.8, 20.7, 18.8, 17.6, 16.2, 2.9, 0.9; IR 1632
(w), 1585 (w), 1525 (m), 1428 (s), 1354 (s), 1315 (s), 1291 (s),
1255 (m), 1242 (s), 1148 (w), 1124 (w), 1114 (w), 1099 (w), 1082
(w), 1071 (w), 1043 (m), 1021 (w), 977 (w), 964 (w), 950 (w),
918 (w), 894 (w), 880 (w), 848 (s), 832 (s), 750 (m), 712 (m),
681 (w), 632 (w). Anal. Calcd for C34H67N5Si2TaLi: C, 51.69;
H, 8.55; N, 8.87. Found: C, 51.96; H, 8.82; N, 8.54.
(Me 3 S iC H 2 )(Ar N d)T a (µ-η1 :η3 -i P r 2 -t a c n )[η2 -C,O :η1 -
O-C(O)dCHSiMe3] (8). A solution of 3 (410 mg, 0.547 mmol)
in toluene (40 mL) was frozen with liquid nitrogen, and
dissolved gases were removed by evacuating the flask for 15
min. The flask was refilled with CO (1 atm) and allowed to
warm to room temperature. After stirring for 1 h, the volatiles
were removed in vacuo to give an orange oil. Addition and
subsequent removal of pentane (40 mL) and residual toluene
under vacuum yielded a yellow-orange solid. The product was
extracted with octane (20 mL), filtered, and concentrated to 6
mL, whereupon crystal growth commenced at room temper-
ature. Cooling to -40 °C maximized crystallization, and the
product (pale orange needles) was isolated by filtration and
dried under vacuum (180 mg, 42%): mp 127 °C (dec); 1H NMR
δ 7.216 (d, 2H, 7.5 Hz), 6.973 (t, 1H, 7.5 Hz), 5.517 (s, 1H),
4.089 (sept, 2H, 7 Hz), 3.98 (mult, 1H), 2.88 (mult, 1H), 2.80
(mult, 1H), 2.70 (mult, 1H), 2.426 (sept, 1H, 7 Hz), 2.41 (mult,
1H), 2.254 (sept, 1H, 7 Hz), 1.91 (mult, 1H), 1.81 (mult, 4H),
1.74 (mult, 2H), 1.481 (d, 6H, 7 Hz), 1.474 (d, 6H, 7 Hz), 0.796
(d, 1H, 12 Hz), 0.795 (d, 3H, 7 Hz), 0.755 (d, 3H, 7 Hz), 0.591
(d, 1H, 12 Hz), 0.583 (d, 3H, 7 Hz), 0.517 (d, 3H, 7 Hz), 0.505
(s, 9H), 0.475 (s, 9H); 7Li NMR δ 1.874; 13C NMR δ 153.5,
143.0, 122.3, 121.2, 93.5, 54.9, 54.5, 54.1, 53.8, 51.2, 49.4, 47.8,
44.6, 44.4, 28.0, 25.1, 24.1, 19.3, 18.7, 18.0, 17.6, 3.8, 1.7; IR:
1577 (m), 1433 (s), 1368 (s), 1355 (s), 1297 (m), 1254 (w), 1237
(m), 1157 (w), 1124 (w), 1106 (w), 1080 (w), 1043 (w), 1023
(w), 990 (w), 961 (m), 952 (m), 925 (m), 906 (w), 877 (w), 848
(s), 831 (s), 800 (w), 772 (w), 756 (m), 732 (w), 716 (w), 692
(m), 681 (w). Anal. Calcd for: C33H64LiN4OSi2Ta: C, 51.01;
H, 8.30; N, 7.21. Found: C, 50.98; H, 8.29; N, 7.22.
7
(s, 18H), 0.175 (d, 2H, 13 Hz); Li NMR δ 1.775; 13C NMR δ
158.7, 153.2, 144.4, 129.7, 128.5, 125.9, 122.9, 122.4, 56.8, 54.1,
51.6, 44.5, 27.6, 25.5, 22.5, 21.9, 20.6, 18.8, 13.8, 4.5; IR 1587
(w), 1432 (s), 1346 (s), 1309 (m), 1294 (m), 1253 (m), 1240 (s),
1226 (s), 1158 (m), 1147 (w), 1115 (w), 1104 (w), 1081 (w), 1065
(w), 1046 (w), 1020 (w), 989 (w), 958 (m), 935 (m), 876 (w),
848 (s), 813 (s), 753 (m), 727 (m), 672 (w), 605 (w). Anal. Calcd
for C41H76LiN4OSi2Ta: C, 55.64; H, 8.65; N, 6.33. Found: C,
55.10; H, 8.12; N, 6.15.
(Me3SiCH2)2(Ar Nd)(η1-iP r 2-ta cn )Ta (5). A reaction flask
was charged with pyridinium chloride (69 mg, 0.60 mmol) and
toluene (60 mL), forming a white suspension. A solution of 2
(500 mg, 0.597 mmol) in toluene (20 mL) was added, and the
mixture was stirred for 14 h at ambient temperature. After
removal of solvent under vacuum, pentane (50 mL) was added,
and after stirring, the volatile materials were again removed
in vacuo. The yellow oil was then extracted with pentane (60
mL) and filtered to remove lithium chloride. The clear yellow
extract was pumped down to an oil under vacuum (373 mg,
84%): 1H NMR δ 7.239 (d, 2H, 8 Hz), 7.030 (t, 1H, 8 Hz), 4.465
(br, 4H), 4.236 (sept, 2H, 7 Hz), 2.87 (mult, 4H), 2.802 (sept,
2H, 7 Hz), 2.224 (s, 4H), 1.446 (d, 12H, 7 Hz), 0.871 (d, 12H,
7 Hz), 0.768 (d, 2H, 12 Hz), 0.579 (d, 2H, 12 Hz), 0.219 (s,
18H); 13C NMR δ 153.5, 142.5, 122.7, 122.6, 64.8, 58.3, 55.4,
52.2, 28.4, 24.5, 18.3, 3.2; IR 1432 (s), 1351 (s), 1296 (w), 1257
(m), 1245 (m), 1167 (w), 1114 (w), 1094 (w), 1046 (w), 1006
(w), 989 (w), 931 (w), 917 (w), 848 (s), 828 (m), 797 (w), 751
(m), 702 (w), 686 (w). Anal. Calcd for C32H65N4Si2Ta: C, 51.73;
H, 8.82; N, 7.54. Found: C, 51.77; H, 8.85; N, 7.43.
(Me3SiCH2)2(Ar Nd)Ta (µ-H)(µ-η1:η3-iP r 2-ta cn )Li (6). A
250 mL reaction flask containing 3 (452 mg, 0.603 mmol)
dissolved in toluene (50 mL) was frozen solid in a liquid
nitrogen bath. The flask was evacuated for 15 min and then
refilled with H2. The taps were closed, and after warming to
room temperature, the flask was heated to 65 °C in an oil bath
for 23 h. The flask was cooled to room temperature, and the
solvent was removed under vacuum over the course of 14 h.
Pentane (30 mL) was added, and the volatile materials were
again removed under vacuum. The sticky yellow solid was
extracted with pentane and filtered to a new flask, removing
a white solid. The yellow solution was pumped down to a foamy
yellow solid (281 mg, 62%): mp 123-125 °C; 1H NMR (in
1
toluene-d8, -28 °C) δ 11.03 (q (1:1:1:1), 1H, J LiH ) 14.7 Hz),
7.177 (d, 2H, 8 Hz), 6.933 (t, 1H, 8 Hz), 4.202 (sept, 2H, 7 Hz),
3.64 (mult, 1H), 3.11 (mult, 1H), 2.95 (mult, 1H), 2.72 (mult,
1H), 2.688 (sept, 1H, 7 Hz), 2.40 (mult, 1H), 2.023 (sept, 1H,
7 Hz), 1.84 (mult, 2H), 1.66 (mult, 1H), 1.57 (mult, 2H), 1.474
(d, 6H, 7 Hz), 1.446 (d, 6H, 7 Hz), 1.35 (mult, 2H), 0.896 (d,
1H, 12 Hz), 0.840 (d, 3H, 7 Hz), 0.582 (s, 9H), 0.544 (d, 3H, 7
Hz), 0.524 (d, 1H, 12 Hz), 0.517 (s, 9H), 0.505 (d, 1H, 12 Hz),
0.472 (d, 3H, 7 Hz), 0.372 (d, 3H, 7 Hz), 0.232 (d, 1H, 12 Hz);
7Li NMR δ 3.627 (d, 1J LiH ) 14.7 Hz); 13C NMR (in toluene-d8,
-28 °C) δ 153.9, 142.9, 122.3, 121.1, 53.5, 53.0, 52.7, 51.7, 51.2,
49.1, 46.1, 45.9, 45.3, 45.1, 27.8, 25.8, 24.0, 19.3, 17.1, 15.1,
4.5, 4.2; IR 1430 (s), 1350 (s), 1294 (m), 1260 (m), 1238 (m),
1164 (w), 1147 (w), 1128 (w), 1113 (m), 1005 (m), 1100 (m),
1089 (m), 1078 (m), 1045 (m), 988 (w), 964 (w), 924 (w), 897
(w), 876 (w), 849 (s), 819 (m), 775 (w), 750 (m), 672 (w), 638
(w), 620 (w). Anal. Calcd for C32H66LiN4Si2Ta: C, 51.18; H,
8.86; N, 7.46. Found: C, 51.57; H, 8.51; N, 7.16.
(Me3SiCH2)(Ar Nd)Ta (η1:η1-Me3SiCHdCS2)(µ-η1:η3-iP r 2-
ta cn )Li (9). A flask was charged with 3 (500 mg, 0.667 mmol)
and toluene (40 mL). CS2 (40 µL, 51 mg, 0.66 mmol) was added
via syringe. The system was sealed and heated to 85 °C for a
period of 14 h, resulting in a red solution. After cooling to room
temperature, the solution was filtered and pumped down to a
red-orange foamy solid. Pentane (10 mL) was added, and the
volatile materials were again removed under vacuum. The
solid was then washed with cold pentane (5 mL, -40 °C) and
dried under vacuum (236 mg, 43%): mp 158 °C (dec); 1H NMR
δ 7.253 (t, 2H, 7.5 Hz), 6.967 (d, 1H, 7.5 Hz), 5.754 (s, 1H),
4.414 (sept, 2H, 7 Hz), 4.09 (mult, 1H), 3.98 (mult, 1H), 2.85
(mult, 2H), 2.660 (sept, 1H, 6.5 Hz), 2.506 (sept, 1H, 6.5 Hz),
2.46 (mult, 1H), 1.97 (mult, 3H), 1.81 (mult, 2H), 1.60 (mult,
2H), 1.543 (d, 6H, 7 Hz), 1.482 (d, 6H, 7 Hz), 0.981 (d, 3H, 6.5
Hz), 0.882 (d, 1H, 8 Hz), 0.868 (d, 1H, 8 Hz), 0.691 (d, 3H, 6.5
Hz), 0.684 (d, 3H, 6.5 Hz), 0.612 (d, 3H, 6.5 Hz), 0.457 (s, 9H),
0.406 (s, 9H); 7Li NMR δ 4.224; 13C NMR δ 154.3, 152.7, 144.8,
(Me3SiCH2)(Ar Nd)Ta [µ-Me3SiCHdC(Me)N](µ-η1:η3-iP r 2-
ta cn )Li (7). A flask was charged with 3 (350 mg, 0.467 mmol)
and toluene (40 mL). Acetonitrile (25 µL) was added via

Tantalum-Lithium Bridging Alkylidene
Organometallics, Vol. 21, No. 16, 2002 3429
Ta ble 1. Cr ysta l Da ta a n d Collection P a r a m eter s
122.8, 122.5, 63.2, 56.8, 54.8, 53.7, 53.5, 49.6, 48.5, 47.4, 43.8,
28.5, 25.1, 24.8, 22.7, 20.3, 16.4, 16.3, 14.2, 3.5, 0.6; IR 1586
(w), 1519 (s), 1483 (m), 1429 (s), 1388 (m), 1359 (s), 1345 (s),
1293 (s), 1254 (m), 1240 (s), 1166 (w), 1145 (w), 1125 (w), 1109
(w), 1078 (w), 1044 (w), 1019 (w), 983 (w), 965 (w), 953 (w),
937 (w), 914 (w), 898 (m), 869 (s), 850 (s), 835 (s), 798 (w), 771
(w), 754 (s), 731 (w), 719 (w), 685 (w), 639 (w), 616 (w), 566
(w), 555 (w). Anal. Calcd for C33H64N4Si2S2TaLi: C, 48.04; H,
7.82; N, 6.79. Found: C, 47.80; H, 7.86; N, 6.75.
8
10
formula
fw
C
33H64LiN4OSi2Ta
C33H71N4OSi2Ta
861.15
776.95
space group
temperature (°C)
a (Å)
b (Å)
c (Å)
Pbca (#61)
-107
17.1859(3)
19.2859(3)
24.3778(4)
90
P1h (#2)
-117
10.345(2)
10.809(2)
21.829(5)
79.567(3)
80.290(4)
67.108(3)
2198.3(8)
2
(Me 3SiCH 2)(Ar Nd)(η3-iP r 2-t a cn )[Me 3SiCH dC(4-Me -
C6H4)O]Ta (10). A small flask was charged with 3 (312 mg,
0.416 mmol) and toluene (40 mL), and p-toluoyl chloride (55
µL, 0.416 mmol) was added via syringe, causing the solution
to darken to a deep orange color. After stirring for 2 h at
ambient temperature, the solution was filtered in order to
remove liberated lithium chloride. The toluene was removed
under vacuum, and the solution was thrice treated with
pentane (30 mL) and pumped down to a solid, yielding a pale
orange solid. The solid was extracted with pentane (25 mL)
and was filtered to another flask. The solution was concen-
trated to 3 mL in vacuo and slowly cooled to -40 °C, resulting
in the formation of pale yellow crystals. The product was
isolated by filtration and dried under vacuum (244 mg, 68%):
mp 159-162 °C; 1H NMR δ 7.444 (d, 2H, 8 Hz), 7.307 (d, 2H,
7.5 Hz), 6.985 (d, 2H, 7.5 Hz), 6.979 (t, 1H, 8 Hz), 5.079 (s,
1H), 4.62 (br, 2H), 4.29 (mult, 1H), 3.81 (mult, 1H), 3.162 (sept,
1H, 6 Hz), 2.982 (sept, 1H, 6 Hz), 2.86 (mult, 1H), 2.65 (mult,
2H), 2.54 (mult, 1H), 2.25 (mult, 1H), 2.15 (mult, 1H), 2.10 (s,
3H), 1.88 (mult, 1H), 1.72 (mult, 2H), 1.59 (mult, 1H), 1.550
(d, 12H, 6 Hz), 1.005 (d, 3H, 6 Hz), 0.92 (br, 6H), 0.695 (s, 1H,
7.5 Hz), 0.610 (s, 1H, 7.5 Hz), 0.604 (d, 3H, 6 Hz), 0.370 (s,
9H), 0.113 (s, 9H); 13C NMR δ 171.5 (C), 152.0 (C), 146.6 (C),
140.4 (C), 137.3 (C), 129.1 (CH), 128.3 (CH), 122.9 (CH), 122.8
(CH), 100.4 (CH), 58.7 (CH2), 55.6 (CH), 55.5 (CH2), 55.4 (CH),
55.3 (CH2), 53.4 (CH), 52.5 (CH2), 50.7 (CH2), 50.4 (CH2), 49.1
(CH2), 27.0 (CH3), 26.6 (CH3), 21.1 (CH3), 19.8 (CH3), 17.7
(CH3), 17.3 (CH3), 3.6 (CH3), 1.2 (CH3); IR 1904 (w), 1611 (w),
1578 (s), 1506 (w), 1489 (w), 1431 (s), 1344 (s), 1306 (s), 1292
(s), 1245 (m), 1173 (s), 1156 (m), 1107 (s), 1073 (m), 1044 (m),
1021 (w), 982 (w), 953 (m), 906 (w), 881 (s), 841 (s), 830 (s),
755 (m), 738 (m), 717 (m), 660 (m), 608 (w). Anal. Calcd for
R (deg)
â (deg)
90
90
γ (deg)
V (Å3)
8079.9(2)
8
1.277
Siemens SMART
Mo KR
(λ ) 0.71069 Å)
graphite
CCD area detector
ω, 0.3°
Z
densitycalc (g/cm3)
diffractometer
radiation
1.301
Siemens SMART
Mo KR
(λ ) 0.71069 Å)
graphite
CCD area detector
ω, 0.3°
monochromator
detector
scan type, width
scan speed
10.0 s/frame
hemisphere
3-46.4
20.0 s/frame
hemisphere
3-46.5
no. of reflns measd
2θ range (deg)
µ (cm-1
)
28.03
25.83
T
max, Tmin
0.72, 0.51
0.17 × 0.17 × 0.16
33 010
6371
3179
0.907, 0.447
0.17 × 0.11 × 0.04
10 049
6151
4439
cryst dimens (mm)
no. of reflns measd
no. of unique reflns
no. of observations
no. of params
R, Rw, Rall
378
220
0.025; 0.028; 0.060
0.79
0.109; 0.154; 0.139
4.07
GOF
software package. Hydrogen atoms were included as fixed
atoms but not refined. The weighting schemes were based on
counting statistics and included a factor (p ) 0.030) to reduce
the weight of 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
Com p ou n d 8. X-ray quality crystals were grown from a
saturated octane solution that was cooled to -40 °C. One
isopropyl group exhibited rotational disorder with a methyl
group occupying each of two positions in a 2:1 ratio. The final
cycle of full-matrix least squares refinement [minimizing the
quantity ∑w(|Fo| - |Fc|)2, where w is the weight of a given
observation] was based on 3179 observed reflections [I >
3.00σ(I)] and 378 variable parameters and converged with final
residuals:27 R ) 0.025, Rw ) 0.028, and GOF ) 0.79.
Com p ou n d 10. X-ray quality crystals were grown by slow
cooling of pentane to -40 °C. One trimethylsilyl group was
rotationally disordered in the typical fashion and modeled as
six carbon atoms of 50% occupancy. The final cycle of full-
matrix least squares refinement [minimizing the quantity
∑w(|Fo| - |Fc|)2, where w is the weight of a given observation]
was based on 4439 observed reflections [I > 3.00σ(I)] and 220
variable parameters and converged yielding final residuals:27
R ) 0.109, Rw ) 0.154, and GOF ) 4.07.
Crystallographic data for the structural analyses have been
deposited with the Cambridge Crystallographic Data Centre,
CCDC. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cam-
C
40H71N4OSi2Ta: C, 55.79; H, 8.31; N, 6.51. Found: C, 55.54;
H, 8.40; N, 6.27.
X-r a y Cr ysta llogr a p h y. A summary of crystal data and
collection parameters for the crystal structures of 8 and 10
are given in Table 1. Details of individual data collection and
solution are given below. ORTEP diagrams were created using
the ORTEP-3 software package.54 For each sample, a crystal
was mounted on a glass capillary using Paratone-N hydrocar-
bon oil. The crystal was transferred to a Siemens SMART55
diffractometer/CCD area detector, centered in the X-ray beam,
and cooled using a nitrogen-flow low-temperature apparatus
that had been previously calibrated by a thermocouple placed
at the same position as the crystal. A least-squares refinement
on data from 60 sample frames allowed determination of cell
constants and the orientation matrix. An arbitrary hemisphere
of data was collected using 0.3° ω-scans, and the data were
integrated by the program SAINT.56 The final unit cell
parameters were determined by least-squares analysis of the
reflections with I > 10σ(I). Data analysis using Siemens
XPREP57 determined the space group. The data were corrected
for Lorentz and polarization effects, but no correction for
crystal decay was applied. Equivalent reflections were aver-
aged, and the structure was solved by direct methods58 and
expanded using Fourier techniques,59 all within the teXsan60
(58) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J .
Appl. Crystallogr. 1993, 26, 343-350.
(59) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J . M. M.; Smykalla, C.
University of Nijmegen, The Netherlands, 1992.
(60) teXan; Molecular Structure Corporation: 1992.
(61) Cromer, D. T.; Waber, J . T. In International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, England, 1974; Vol.
IV, Table 2.2A.
(54) Farrugia, L. J . J . Appl. Crystallogr. 1997, 30, 565.
(55) SMART; Siemens Industrial Automation, Inc.: Madison, WI,
1995.
(56) SAINT; Siemens Industrial Automation, Inc.: Madison, WI,
1995.
(57) XPREP; Siemens Industrial Automation, Inc.: Madison, WI,
1995.
(62) Ibers, J . A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781-
782.

3430 Organometallics, Vol. 21, No. 16, 2002
Schmidt and Arnold
Sch em e 2
bridge, CB2 1EZ [fax: +44-1223-336033 or e-mail: deposit@
ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk].
4 was assigned as a bridging aryloxide species, as shown
in Scheme 2. The 13C NMR spectrum was rather
7
unremarkable, and the Li NMR showed a resonance
at 1.775 ppm, shifted upfield from its precursor (3, 2.986
ppm), as well as from the corresponding bridging alkyl
species (2, 2.103 ppm). This is likely a result of the
shielding caused by the phenoxy ligand bound to the
lithium.63 Although structural evidence confirming the
proposed connectivity is presently lacking, the bridging
motif has been identified in a number of related
complexes (see below), in which four-coordinate Li is
invariably observed. The reactivity seen here is com-
parable to that of (Me3CCHd)Ta(Me3CCH2)3, which
reacts with 2 equiv of TpOH [Tp ) (C6H4)3C2HOH] to
yield (Me3CCH2)3Ta(OTp)2.64 In both cases, the nucleo-
philic alkylidene moiety is protonated by the weakly
acidic alcohol.
Resu lts a n d Discu ssion
To probe the reactivity of the tantalum-lithium
alkylidene 3, we explored reaction pathways commonly
observed in traditional monometallic alkylidenes.27,28
Specifically, the reactions of 3 with proton sources,
nitriles, and other unsaturated electrophiles have been
investigated as a means of determining the effect of the
lithium ion in this compound.
Rea ction w ith P r oton Sou r ces. The reaction be-
tween 3 and 2,4,6-trimethylphenol proceeded rapidly
upon mixing toluene solutions of the two materials, as
observed by a color change from deep yellow to nearly
colorless. After removal of reaction solvent under vacuum,
compound 4 was isolated by extraction with ether and
was obtained as a slightly sticky solid in excellent yield.
The reaction between 3 and pyridinium chloride
proceeded slowly in toluene due, no doubt, to the low
solubility of the latter in this solvent. After stirring
overnight, a quantitative conversion to a new compound
1
The H NMR spectrum showed six independent reso-
nances (all multiplets) for the three pairs of diaste-
reotopic protons of the ethylene bridges in the Pr2-tacn-
(5) was observed by H NMR spectroscopy (Scheme 2).
i
1
ligand, consistent with mirror symmetry in the mol-
ecule. The two trimethylsilylmethyl ligands were equiva-
lent, showing three resonances (two for the diaste-
reotopic methylene and one for the methyl groups) in a
1:1:9 ratio. On the basis of these data, the structure of
The spectrum of this species was very distinctive,
(63) Akitt, J . W. In Multinuclear NMR, 2nd ed.; Mason, J ., Ed.;
Plenum Press: New York, 1989; pp 189-220.
(64) Lapointe, R. E.; Wolczanski, P. T.; Vanduyne, G. D. Organo-
metallics 1985, 4, 1810-1818.

Tantalum-Lithium Bridging Alkylidene
Organometallics, Vol. 21, No. 16, 2002 3431
Sch em e 3
F igu r e 2. Lithium hydride NMR signals observed at room
temperature for 3.
containing only three resonances for the ethylene bridges
i
of the Pr2-tacn- ligand (two multiplets, one singlet).
This indicated not only the presence of mirror symmetry
but also equilibration of the methylene hydrogens in this
ligand. These data suggest a monodentate bonding motif
i
for Pr2-tacn- (operating as a simple amide ligand) in
this compound. Also, only one isopropyl resonance was
observed for the imido group, demonstrating its rapid
rotation on the NMR time scale. Additionally, the 13C
NMR spectrum showed a set of peaks fully in agreement
with the high symmetry exhibited by 5. Thus, 5 was
formulated as the pseudotetrahedral compound (Me3-
SiCH2)2(ArNd)(η1-iPr2-tacn)Ta. The compound was iso-
lated in analytically pure form from pentane as a yellow
oil. The reaction of 3 with pyHCl can be compared with
that of a traditional alkylidene, as shown by the reaction
of (Me3CCHd)Ta(Me3CCH2)3 with HCl to give (Me3-
CCH2)4TaCl.32 In contrast to 4, the chloride ligand
departs with the lithium ion, due to the strong driving
force provided by the precipitation of lithium chloride.
1
F igu r e 3. Portion of the H NMR spectra for 3 at 25 °C
and -28 °C, showing the enhanced resolution achieved
upon cooling.
been reported in two previous cases,70,71 and the J Li-H
coupling constant for 6 (14.7 Hz) is somewhat larger
than those observed previously (6.4-10.5 Hz).
1
The remainder of the 1H NMR spectrum for 6 is broad
and uninterpretable at room temperature, indicating
fluxionality in this compound. Upon cooling to -28 °C,
the spectrum sharpened considerably (Figure 3), with
the resonances being indicative of an asymmetric spe-
cies with inequivalent trimethylsilylmethyl ligands.
This is consistent with a square pyramidal coordination
sphere in which the imido ligand is axial and the two
alkyl ligands are oriented in a cis fashion. The fluxion-
ality in 6 observed at room temperature can be at-
tributed to equilibrium between this square pyramidal
structure and a trigonal bipyramidal arrangement. The
hydride 6 was isolated as a pale yellow solid and is
stable for weeks as a solid, but slowly decomposes to
an intractable product in solution.
Interestingly, 5 can also be synthesized by the reac-
tion of the bridging alkyl species (2) with 1 equiv of
pyridinium chloride. In this case, the reaction resulted
in the release of both tetramethylsilane and lithium
chloride as the reaction proceeded to yield a product
spectroscopically identical to that described above
(Scheme 3). In accord with this reactivity, addition of
LiCH2SiMe3 to 5 regenerates 2 quantitatively.
The alkylidene ligand in 3 is also converted to an alkyl
group when exposed to hydrogen gas at 65 °C in toluene.
Heterolysis of H2 protonates the alkylidene, and the
counterion (hydride) is retained in a bridging fashion,
bound to the tantalum and lithium atoms (Scheme 2).
At room temperature, the 1H NMR spectrum of this
(65) Mons, H. E.; Gunther, H.; Maercker, A. Chem. Ber. 1993, 126,
2747-2751.
(66) Bauer, W.; Griesinger, C. J . Am. Chem. Soc. 1993, 115, 10871-
10882.
(67) Field, L. D.; Gardiner, M. G.; Kennard, C. H. L.; Messerle, B.
A.; Raston, C. L. Organometallics 1991, 10, 3167-3172.
(68) Bauer, W.; Schleyer, P. V. Magn. Reson. Chem. 1988, 26, 827-
833.
1
compound (6) supports this geometry by showing a H
resonance for the hydride at 11.03 ppm as a 1:1:1:1
quartet (1J ) 14.7 Hz). This splitting, due to a coupling
between H and 7Li (S ) 3/2), is matched by the
appearance of a doublet with an identical 1J LiH at 3.627
(69) Gunther, H.; Moskau, D.; Bast, P.; Schmalz, D. Angew. Chem.,
Int. Edit. Engl. 1987, 26, 1212-1220.
7
(70) Gilbert, T. M.; Bergman, R. G. J . Am. Chem. Soc. 1985, 107,
6391-6393.
ppm in the Li NMR spectrum (Figure 2). Observation
of coupling in lithium hydride species is rare.65-69 One-
bond coupling between lithium and hydrogen nuclei has
(71) Heine, A.; Stalke, D. Angew. Chem., Int. Edit. Engl. 1992, 31,
854-855.

3432 Organometallics, Vol. 21, No. 16, 2002
Schmidt and Arnold
Sch em e 4
F igu r e 4. ORTEP view of 8 with 50% thermal ellipsoids.
Selected bond lengths (Å): Ta1-C1 ) 2.134(7), Tal-C6 )
2.166(7), Ta1-O1 ) 2.067(4), Ta1-N1 ) 1.793(5), Ta1-
N2 ) 2.025(5), Li1-O1 ) 1.82(1), Li1-N2 ) 2.22(1), Li1-
N3 ) 2.07(1), Li1-N4 ) 2.08(1), C1-O1 ) 1.398(8), C1-
C2 ) 1.345(9).
Further evidence for the formation of this product was
obtained by repeating the reaction with D2. Under these
conditions, the resonance at 11.03 ppm in the H NMR
are bound to the metal center.72-74 To further investi-
gate its alkylidene-like reactivity, 3 was exposed to
carbon monoxide (1 atm) at ambient temperature in
toluene,53 and quantitative conversion to a new com-
pound (8) was observed (Scheme 2). The NMR spectra
of this product were qualitatively very similar to that
of 3, except that the alkylidene CH resonance shifted
significantly (from 4.205 ppm (1H) and 57.4 ppm (13C)
in 3 to 5.517 ppm (1H) and 93.5 ppm (13C) in 8).
1
spectrum was absent, and the 7Li NMR signal was
broadened due to coupling from the quadrupolar deu-
teride (S ) 1). In the 2H NMR spectrum of 6-D,
resonances appeared at 11.03 (br, s) and 0.52 (br, d) due
to the bridging deuteride and the CDHSiMe3 group,
respectively (Scheme 4).
Reaction of 6 with chloroform in C6D6 proceeds slowly
to form a product spectroscopically identical to 5, with
concomitant formation of dichloromethane and LiCl
(Scheme 4). This contrasts with typical reactions of
transition metal hydrides with chloroform, which yield
metal chlorides.
In ser tion Rea ction s. Upon reaction of 3 with or-
ganic substrates such as isocyanates (tBuNCO, PhNCO),
carbodiimides (iPrNCNiPr, p-tolNCN-p-tol), aldehydes
(PhCHO), esters (PhCOOPh), or isocyanides (xylNC), a
mixture of products was formed. These mixtures proved
to be inseparable upon workup and were not pursued
further. On the other hand, reactions with acetonitrile,
carbon monoxide, and carbon disulfide cleanly gave a
single product in each case, as detailed below.
7
Additionally, the Li NMR resonance shifted consider-
ably, from 2.986 ppm (3) to 1.874 ppm (8). On the basis
of these data, an insertion of CO at the alkylidene
functionality to form a ketene complex (8) was postu-
lated, which was subsequently confirmed by X-ray
crystallography (Figure 4).53
i
The bridging structure of the Pr2-tacn- is retained
in 8, bound to Ta through the anionic nitrogen and
coordinated to Li through three N-Li dative bonds. The
ketene moiety bridges the Ta and Li atoms, with bond
lengths [Ta1-C1 ) 2.134(7) Å, Ta1-O1 ) 2.067(4) Å]
similar to those in a related η2-Zr ketene complex.75
These lengths also parallel those found in a related η2-
Ta acyl complex,76 while the Li-O distance [1.82(1) Å]
is similar to that observed for related carboxylate
species.77,78 Virtually no changes in the remaining
metal-ligand bond lengths from that of the precursor
3 were observed.
When 3 was exposed to 1 equiv of CH3CN in toluene,
no reaction was observed at room temperature. Upon
warming to 90 °C, however, a clean insertion reaction
proceeded smoothly over a period of 36 h to form a new
compound (7), in which the alkylidene proton is shifted
to higher field (from 4.205 ppm in 3 to 3.854 ppm in 7).
The remaining proton signals, though altered slightly,
showed little change, with the molecule retaining the
As was the case for the insertion of acetonitrile,
reaction between 3 and CS2 required elevated temper-
(72) Antinolo, A.; Otero, A.; Fajardo, M.; Gil-Sanz, R.; Herranz, M.
J .; Lopez-Mardomingo, C.; Martin, A.; Gomez-Sal, P. J . Organomet.
Chem. 1997, 533, 87-96.
7
low symmetry of the starting alkylidene. The Li signal
(73) Fermin, M. C.; Hneihen, A. S.; Maas, J . J .; Bruno, J . W.
Organometallics 1993, 12, 1845-1856.
also changed slightly, from 2.986 ppm in 3 to 2.699 ppm
in 7. These data are consistent with clean insertion of
the acetonitrile into the alkylidene metal bond followed
by a rearrangement to form a bridging eneimido species
(Scheme 2) and are related to reactivity observed in
terminal alkylidenes, which form terminal eneimido
ligands.32
In general, carbon monoxide inserts into MdCR2
double bonds to form the corresponding η2-ketene spe-
cies [M(η2-OCCR2)], in which both carbon and oxygen
(74) Neithamer, D. R.; Lapointe, R. E.; Wheeler, R. A.; Richeson,
D. S.; Vanduyne, G. D.; Wolczanski, P. T. J . Am. Chem. Soc. 1989,
111, 9056-9072.
(75) Fryzuk, M. D.; Duval, P. B.; Mao, S.; Rettig, S. J .; Zaworotko,
M. J .; MacGillivray, L. R. J . Am. Chem. Soc. 1999, 121, 1707-1716.
(76) Meyer, T. Y.; Garner, L. R.; Baenziger, N. C.; Messerle, L. Inorg.
Chem. 1990, 29, 4045-4050.
(77) Hagadorn, J . R.; Que, L.; Tolman, W. B. J . Am. Chem. Soc.
1998, 120, 13531-13532.
(78) Ball, S. C.; Cragg-Hine, I.; Davidson, M. G.; Davies, R. P.;
Edwards, A. J .; Lopez-Solera, I.; Raithby, P. R.; Snaith, R. Angew.
Chem., Int. Ed. Engl. 1995, 34, 921-923.

Tantalum-Lithium Bridging Alkylidene
Organometallics, Vol. 21, No. 16, 2002 3433
atures. After stirring the two compounds in toluene at
85 °C for 14 h, the clean conversion to a new product
(9) was observed. The H NMR spectrum again showed
Even though the compound could be recrystallized
from pentane, the crystals of 10 that were obtained did
not diffract well, preventing the acquisition of a high
quality X-ray structure of this species. A low-resolution
structure was obtained which at least confirmed con-
nectivity.85 The structure shows an anionic tacn bonded
to tantalum in a tridentate fashion. The tantalum
coordination sphere is completed by one alkyl ligand,
one imido group, and the newly formed enolate ligand.
Though the reaction of 3 with the acid chloride follows
a reaction pathway very similar to that of a standard
alkylidene, it is noteworthy that upon departure of the
lithium chloride from this species, the anionic tacn
ligand returns to a tridentate coordination mode. This
indicates that the tacn moiety responds to the electronic
needs of the complexes formed by coordination of an
appropriate number of nitrogen atoms, to stabilize these
compounds.
1
an asymmetric species, with the only equilibration of
protons due to rapid rotation of the phenyl imido unit.
Notably, the CH alkylidene resonance was strongly
downfield shifted, to 5.754 ppm. The six isopropyl
methyl doublets and two SiMe3 resonances were typical
for these low-symmetry compounds, located at chemical
shifts well within the expected ranges. Another impor-
tant spectroscopic change upon reaction was the large
downfield shift in the 7Li NMR resonance to a new value
of 4.224 ppm in 9 (as compared to 2.986 ppm in 3).
Compound 9 was therefore formulated as a bridging
enedithiolato complex, resulting from formal insertion
of each of the double bonds of CS2 into the alkylidene
metal bonds (Scheme 2). A number of enedithiolato
ligands, generated by oxidative cleavage of tetrathiane
moieties,79 deprotonation of enedithiols,80 or deproto-
nation of â-hydroxydithiocarboxylate ligands,81-84 are
known.
Con clu sion s
The tantalum complexes reported here demonstrate
conclusively that the anionic tacn ligand has the ability
to function as a hemilabile ligand which, depending on
the steric and electronic demands of the metal to which
it is bound, can act as a mono- or tridentate donor to a
single metal center. Furthermore, this work has pro-
vided unusual examples of the tacn moiety’s ability to
act as a ligand to two different metals that are also
bridged by an additional ligand, in a µ-η1:η3-fashion. The
presence of lithium in the absence of a good leaving
group (such as chloride, for example), invariably leads
to such a structural arrangement, a finding that pre-
sumably reflects the relative tenacity with which lithium
is bound by tacn. The structural effects of this unusual
type of coordination behavior are apparent from NMR
spectroscopy and X-ray crystallography; our understand-
ing of the electronic contributions to this unusual mode
of bonding is the focus of continuing synthetic and
theoretical work.
Rea ction w ith Acid Ch lor id es. In the case of a
monometallic alkylidene complex, acid chlorides are
known to react at the nucleophilic carbon of the alky-
lidene; keto-enol tautomerization then occurs, resulting
in conversion of the metal alkylidene to the correspond-
ing enolate metal chloride. For example, the reaction
of (Me3CCH2)3Ta- (dCHCMe3) with RCOCl results in
the formation of (Me3CCH2)3ClTa-(OCRdCHCMe3).32
The tantalum-lithium bridging alkylidene 3 was
found to react readily with 1 equiv of p-tolCOCl at room
temperature in toluene. After filtration to remove LiCl,
the product (10) was crystallized from pentane. The
1
most noteworthy aspect of the H NMR spectrum of 10
is a singlet at 5.079 ppm. The remainder of the spectrum
showed the usual lack of symmetry seen throughout the
compounds presented here. On the basis of the loss of
lithium chloride and the presence of the low-field NMR
resonance at 5.079 ppm, 10 was formulated as the
enolate, (Me3SiCH2)(ArNd)(η3-iPr2-tacn)Ta[OC(p-tol)d
CHSiMe3] (Scheme 2).
Ack n ow led gm en t. The authors gratefully acknowl-
edge the NSF for funding, the Department of Defense
Science and Engineering Graduate (NDSEG) Fellowship
Program for fellowship support (J .A.R.S.), and Drs. Fred
Hollander and Allen Oliver for insightful discussions.
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