A sterically demanding enolate ligand: Tantalum ligation and pyridine coupling

Article abstract of DOI:10.1021/om0303496

Enolate ligands of formula [OC(Ad)Ar]^- (Ad = 2-adamantylidene; Ar = 3,5-C₆H₃Me₂ 1a, Ar = 2,4,6-C₆H₂Me₃ 1b) were designed and prepared as potassium salts to mimic the topology of related N-tert-hydrocarbyl and ketimide ligands. Subsequently, trimethyltantalum bis-enolate complexes TaMe₃(O-C[Ad]Ar)₂ (Ar = 3,5-C₆H₃Me₂ 2a, Ar = 2,4,6-C₆H₂Me₃ 2b) were prepared in good yields from a salt metathesis reaction between TaMe₃Cl₂ and 2 equiv of the corresponding potassium enolate. Complex 2a was structurally characterized and found to be monomeric in the solid state and to exhibit an inner coordination sphere of approximate D_3h symmetry. Reaction between 2a and pyridine in the presence of dihydrogen resulted in an unanticipated pyridine coupling with formation of the bipyridine complex TaMe(O-C[Ad](3,5-C₆H₃Me₂))₂(py)(bpy) , 3a (py = pyridyl, bpy = 2,2′-bipyridyl), which was structurally characterized to confirm its formulation. Labeling experiments using pyridine-d₅ and D₂ suggest that formation of the tantalum(III) complex 3a occurred via C-H activation involving a putative tantalum(III) intermediate, TaMe-(O-C[Ad](3,5-C₆H₃Me₂))₂ (py)₃, 4a.

Full text of DOI:10.1021/om0303496

498
Organometallics 2004, 23, 498-503
A Ster ica lly Dem a n d in g En ola te Liga n d : Ta n ta lu m
Liga tion a n d P yr id in e Cou p lin g
Han Sen Soo, Paula L. Diaconescu, and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Room 2-227, Cambridge, Massachusetts 02139-4307
Received May 9, 2003
Enolate ligands of formula [OC(Ad)Ar]- (Ad
) 2-adamantylidene; Ar ) 3,5-
C6H3Me2 1a , Ar ) 2,4,6-C6H2Me3 1b) were designed and prepared as potassium salts to
mimic the topology of related N-tert-hydrocarbyl and ketimide ligands. Subsequently,
trimethyltantalum bis-enolate complexes TaMe3(O-C[Ad]Ar)2 (Ar ) 3,5-C6H3Me2 2a , Ar )
2,4,6-C6H2Me3 2b) were prepared in good yields from a salt metathesis reaction between
TaMe3Cl2 and 2 equiv of the corresponding potassium enolate. Complex 2a was structurally
characterized and found to be monomeric in the solid state and to exhibit an inner
coordination sphere of approximate D3h symmetry. Reaction between 2a and pyridine in
the presence of dihydrogen resulted in an unanticipated pyridine coupling with formation
of the bipyridine complex TaMe(O-C[Ad](3,5-C6H3Me2))2(py)(bpy), 3a (py ) pyridyl, bpy )
2,2′-bipyridyl), which was structurally characterized to confirm its formulation. Labeling
experiments using pyridine-d5 and D2 suggest that formation of the tantalum(III) complex
3a occurred via C-H activation involving a putative tantalum(III) intermediate, TaMe-
(O-C[Ad](3,5-C6H3Me2))2(py)3, 4a .
In tr od u ction
by the same metal complex. For instance, the coupling
of pyridine with organic functionalities using cationic
zirconocene methyl complexes is known, involving ortho
C-H activation.16,17 A few other methods using late
transition metals have been described for the coupling
of unactivated pyridine molecules, but extant protocols
typically require harsh conditions.18-21
While related to alkoxide, siloxide, and aryloxide
supporting ligands, oxygen-bound enolates constitute an
ancillary ligand class not utilized previously in the
context of stabilizing low-coordinate early-metal com-
plexes.22 We suspected that enolate ligation would
impart certain benefits to early transition metal com-
plexes, particularly minimizing unwanted side-reactions
stemming from ancillary ligand vulnerability.22-28 Ac-
Early transition metal compounds have been exten-
sively studied owing to their ability to mediate a wide
range of processes. In particular, mononuclear tantalum
complexes supported by sterically demanding alkoxide,
siloxide, or aryloxide ligands are active in coupling
reactions,1,2 C-O activation,1,2 C-N activation,3-7 and
C-H activation.8-15 It is especially attractive when two
or more of these processes can be sequentially mediated
* Corresponding author. E-mail: ccummins@mit.edu. Fax: (617)-
258-5700.
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10.1021/om0303496 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 12/24/2003

Tantalum Ligation and Pyridine Coupling
Organometallics, Vol. 23, No. 3, 2004 499
or 2,4,6-C6H2Me3) upon addition of the corresponding
aryl Grignard reagent.33 Oxidation of crude carbinol
samples with J ones’ reagent delivered ketones 2AdC-
(O)Ar in moderate yields (53% for Ar ) 3,5-C6H3Me2,
65% for Ar ) 2,4,6-C6H2Me3 from adamantyl epoxide).
Ketone purification consisted simply of recrystallization
from cold methanol. Ligand deprotonation was ef-
ficiently accomplished by KH in THF, the resulting salts
[K(O-C[Ad]Ar)]x (Ad ) 2-adamantylidene rather than
adamantyl; Ar ) 3,5-C6H3Me2 K[1a ], Ar ) 2,4,6-C6H2-
Me3 K[1b]) being obtained as white powders convenient
for delivery of the enolate ligand to metal halides via
transmetalation.
F igu r e 1. Enolate ligands designed to support electro-
philic early transition metals.
Sch em e 1. Liga n d Syn th esis P a th w a y
Trimethyltantalum bis-enolate complexes TaMe3(O-
C[Ad]Ar)2 (Ar ) 3,5-C6H3Me2 2a , Ar ) 2,4,6-C6H2Me2
2b) were prepared quantitatively as white crystalline
solids by treatment of 2 equiv of [K(O-C[Ad]Ar)] with
TaMe3Cl2.34 Associated with the single enolate ligand
environment in the tantalum complexes is a diagnostic
1
pair of broad H NMR signals at 3.58 and 2.88 ppm for
2a and 3.58 and 2.22 ppm for 2b, attributable to the
two different allylic protons. The anticipated 3J coupling
of each allylic proton with the four neighboring ada-
mantyl protons cannot be resolved in the NMR spec-
trum, resulting in the observed broadening. Besides
these pairs of allylic protons, the remaining adamantyl
signals are unremarkable multiplets due to overlapping
peaks. The signals corresponding to the tantalum-bound
methyls are located at 0.69 and 0.43 ppm for these
complexes, respectively.
Single crystals of 2a , grown from a saturated n-
pentane solution at -40 °C, were examined by X-ray
crystallography to confirm the monomeric nature and
preferred coordination geometry of the trimethyltanta-
lum(V) bis-enolate motif (Figure 2). As has been en-
countered previously for trimethyltantalum bis-alkoxide
systems,22,23,26 a regular trigonal bipyramidal geometry
sporting exclusively equatorial methyl ligands is ob-
served, and thus is realized an inner coordination sphere
of approximate D3h symmetry. The three C-Ta-C
angles are 120.1(3)°, 119.3(3)°, and 120.5(2)°, while
the O(1)-Ta-O(2) angle is 175.76(10)°. The near-lin-
ear Ta-O(1)-C(11) (178.5(2)°) and Ta-O(2)-C(21)
(161.7(2)°) angles taken together with relatively short
Ta-O(1) (1.874(2) Å) and Ta-O(2) (1.883(2) Å) inter-
atomic distances indicate significant Ta-O multiple-
bond character, as has been invoked for related tanta-
lum complexes supported by siloxide1,2,6,35,36 and aryl-
oxide3-5,22-28,37,38 ligands.
cordingly, we designed an enolate ligand intended to be
particularly robust. The design elements included (i) an
enolate essentially incapable (for steric reasons) of
coordination to a metal through the enolate carbon, and
(ii) one round adamantylidene cage and one flat aryl
substituent. The latter requirement produces a class of
enolate ligands “O-C[Ad]Ar” (Ad ) 2-adamantylidene;
Ar ) 3,5-C6H3Me2 1a , Ar ) 2,4,6-C6H2Me3 1b), as
illustrated in Figure 1, that is reminiscent topologically
of the N-tert-hydrocarbyl anilide ligands, which have
been demonstrated to be versatile ligands for a variety
of metals.29-31
This work involves the synthesis of a trimethyltan-
talum complex supported by the new robust enolate
ligands. We further describe a new method for dehy-
drogenative pyridine C-C coupling at the ortho position,
mediated by the trimethyltantalum bis-enolate complex.
The resulting 2,2′-bipyridyl complex of tantalum(III) is
characterized in detail, and the mechanism of its
formation is considered with insight gleaned from
deuterium labeling studies.
Resu lts a n d Discu ssion
Ketones corresponding to the desired enolate ligands
were synthesized as white microcrystalline powders in
three steps from commercially available 2-adaman-
tanone (Scheme 1). Smooth methylene transfer from
Me2S(CH2)(O) (generated in situ by dehydrohalogena-
tion of trimethylsulfoxonium iodide using sodium hy-
dride) to 2-adamantanone provides 2-adamantyl ep-
oxide,32 which can be converted to the secondary carbinol
2AdCH(OH)Ar (2Ad ) 2-adamantyl; Ar ) 3,5-C6H3Me2
Symmetry analysis of the molecular orbitals of 2a
reveals that the nine tantalum valence orbitals can
interact favorably with all possible symmetry-adapted
linear combinations of ligand donor functions. These
(33) This reaction is presumably initiated by 1,2-hydride migration
to give a complexed aldehyde prior to C-C bond formation. For a list
of reagents that will induce this rearrangement, see: Larock, R. C. In
Comprehensive Organic Transformations; VCH: New York, 1989; p
628.
(34) Schrock, R. R.; Sharp, P. R. J . Am. Chem. Soc. 1978, 100, 2389-
(28) Steffey, B. D.; Chamberlain, L. R.; Chesnut, R. W.; Chebi, D.
E.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1989, 8, 1419-
1423.
(29) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861-863.
(30) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685-836, and
references therein.
(31) Cummins, C. C. Chem. Commun. 1998, 1777-1786, and
references therein.
(32) Farcasiu, D. Synthesis 1972, 615-616.
2399.
(35) Bonanno, J . B.; Wolczanski, P. T.; Lobkovsky, E. B. J . Am.
Chem. Soc. 1994, 116, 11159-11160.
(36) Bonanno, J . B.; Henry, T. P.; Neithamer, D. R.; Wolczanski, P.
T. J . Am. Chem. Soc. 1996, 118, 5132-5133.
(37) Weller, K. J .; Gray, S. D.; Briggs, P. M.; Wigley, D. E.
Organometallics 1995, 14, 5588-5597.
(38) Castellano, B.; Solari, E.; Floriani, C.; Re, N.; Chiesi-Villa, A.;
Rizzoli, C. Chem. Eur. J . 1999, 5, 722-737.

500 Organometallics, Vol. 23, No. 3, 2004
Soo et al.
F igu r e 3. Tantalum(V) bis-pyridyl and tantalum(III) 2,2′-
bipyridine formulations consistent with NMR data for the
product obtained by hydrogenolysis of 2a in pyridine
solution.
F igu r e 2. ORTEP diagram of TaMe3(O-C[Ad](3,5-C6H3-
Me2))2 (2a ) with thermal ellipsoids at the 35% probability
level. Selected bond distances (Å): Ta-O(1), 1.874(2);
Ta-O(2), 1.883(2); Ta-C(1), 2.137(5); Ta-C(2), 2.139(4);
Ta-C(3), 2.150(5). Selected bond angles (deg): O(1)-Ta-
O(2), 175.76(10); C(1)-Ta-C(2), 120.1(3); C(1)-Ta-C(3),
119.3(3); C(2)-Ta-C(3), 120.5(2); Ta-O(1)-C(11), 178.5-
(2); Ta-O(2)-C(21), 161.7(2).
interactions give rise to five σ bonds transforming as
2A1′ + A2′′ + E′ and four π interactions transforming
as E′ + E′′. Thus, 2a may be considered an 18e complex.
In accord with this analysis, it was found that the
complex underwent no reaction in toluene (0.22 M) upon
heating at 85 °C for 24 h. Complex 2a also resisted
adduct formation in the presence of up to 3 equiv of
trimethylphosphine under the same reaction conditions.
Furthermore, the robust trimethyltantalum complex
was found not to react with stoichiometric amounts of
protic reagents including 3,5-dimethylaniline and phen-
ylphosphine in toluene solution at elevated tempera-
tures, although free ketone ligand was formed upon
hydrolysis with water.
F igu r e 4. ORTEP diagram of TaMe(O-C[Ad](3,5-C6H3-
Me2))2(py)(bpy) (3a ) with thermal ellipsoids at the 35%
probability level. Selected bond distances (Å): Ta-O(1),
1.903(6); Ta-O(2), 1.903(7); Ta-C(6), 2.184(11); Ta-N(3),
2.370(9); Ta-N(4), 2.110(7); Ta-N(5), 2.093(8); C(41)-
C(51), 1.386(15). Selected bond angles (deg): Ta-
O(1)-C(1), 170.5(6); Ta-O(2)-C(2), 170.3(6); O(1)-Ta-
N(5), 158.1(3); O(2)-Ta-N(4), 163.6(3); C(6)-Ta-N(3),
170.5(4).
uct in C6D6 is consistent with these two formulations
but does not distinguish the tantalum(V) bis-pyridyl
complex, 3a ′, from the tantalum(III) 2,2′-bipyridine
complex, 3a .
Complex 2a exhibited no reaction with dihydrogen in
toluene (1 atm, 90-100 °C, 24 h). In contrast, a solution
of the complex in pyridine under a dihydrogen atmo-
sphere turned from pale yellow to red within 15 min
when heated at 70 °C in a sealed vessel. The color
change suggests that 2a was reduced, possibly to a
tantalum(III) intermediate (vide infra). By monitoring
the reaction in an NMR tube sealed with a Teflon
stopcock, it was revealed that 2a was fully consumed
only after 5 days. Workup of the reaction mixture after
5 days of heating led to the isolation (in 29-45% yield
depending on the reaction scale, see below for formula-
tion) of a red-brown solid that was only sparingly soluble
in aliphatic solvents. The 1H NMR spectrum of the
n-pentane filtrate after removal of the red-brown solid
and concentration indicated the presence of multiple
compounds, from which no clean product was further
isolated. On the other hand, a 1H NMR spectroscopic
study of the red-brown solid in C6D6 revealed a single
enolate ligand environment and a singlet for one
remaining TaCH3 group with respect to two enolate
residues. Furthermore, the aromatic region was com-
prised of well-resolved multiplets (7 distinct signals in
a 2:2:2:2:2:2:1 ratio), consistent with one of two possible
An X-ray structural analysis was undertaken on
single crystals grown from a diethyl ether solution of
the red-brown material layered with n-pentane. The
study revealed the tantalum(III) 2,2′-bipyridine formu-
lation, 3a , to be correct (Figure 4). The six-coordinate
pseudo-octahedral tantalum(III) complex exhibits near-
linear Ta-O(1)-C(1) (170.5(6)°) and Ta-O(2)-C(2)
(170.3(6)°) angles, as seen for its synthetic precursor,
and the C(41)-C(51) bond length in the bipyridine
ligand is 1.386(15) Å. However, the Ta-O(1) (1.903(6)
Å) and Ta-O(2) (1.903(7) Å) bond lengths of TaMe(O-
C[Ad](3,5-C6H3Me2))2(py)(bpy), 3a , are slightly longer
(by ca. 0.02 Å) than the corresponding tantalum-oxygen
bond lengths in 2a .
To better understand this multistep reaction, three
deuterium-labeling studies were conducted: (i) a reac-
tion was carried out using C5D5N under H2; (ii) a
reaction was carried out using C6H5N under D2; and (iii)
a reaction was carried out using C5D5N under D2. In
each case, the gases evolved upon complete reaction
were vacuum-transferred simultaneously for 1H NMR
and infrared (IR) spectroscopic analysis. The following
is a summary of observations that bear on the mecha-
1
isomers 3a or 3a ′ (Figure 3). A 2D H-1H correlation
spectroscopic (COSY) study of the intriguing new prod-

Tantalum Ligation and Pyridine Coupling
Organometallics, Vol. 23, No. 3, 2004 501
Sch em e 2. Mech a n istic Sk eleton Con sisten t w ith
th e Da ta In volvin g â-Elim in a tion fr om th e
Ta n ta lu m (III) In ter m ed ia te 4a a n d Su bsequ en t
Red u ctive C-C Cou p lin g of Tw o Coor d in a ted
P yr id in es a t th e or th o P osition to Give 3a
alternatively, intramolecular pyridyl nucleophilic attack
on coordinated pyridine.
Con clu sion s a n d F u tu r e Wor k
A new class of original enolate ligands has been
conceived and synthesized to explore their applications
as ancillary ligands for stabilizing low-coordinate early
transition metals. The outcome of the design was a pair
of enolate ligands that are topologically reminiscent of
related anilide and ketimide ligands.29-31,40 Trimeth-
yltantalum bis-enolate complexes were subsequently
prepared in high yield, and then converted in the
presence of both dihydrogen and pyridine to an interest-
ing complex, 3a , containing both a coordinated pyridine
and bipyridine. Preliminary mechanistic investigations
suggest the intermediacy of monomethyl tantalum bis-
enolate complexes in a complex multistep pathway.
Further investigations of interest include attempts to
access isolable tantalum(III) species by reduction of
halogenated tantalum(V) bis- and tris-enolates (since
the d2 tantalum moiety portends fascinating and prom-
ising small molecule activation properties), as well as
synthetic routes to prepare other early transition metal
complexes supported by the enolate ligand system.
nism of TaMe3(O-C[Ad](3,5-C6H3Me2))2 hydrogenolysis
and pyridine C-C coupling:
1. The sole gaseous product of the reaction was
methane (C6D6, 0.16 ppm, singlet); no ethane was
formed.
Exp er im en ta l Section
Gen er a l P r oced u r es. Unless otherwise stated, all ma-
nipulations were performed in a Vacuum Atmospheres drybox
under an atmosphere of purified nitrogen or using Schlenk
techniques under an argon atmosphere. Adamantyl epoxide32
2. Use of C5D5N as solvent in conjunction with H2
produced CH4 as the sole methane isotopomer.
3. Use of D2 gas with either C5D5N or C6H5N as
solvent produced 60% CH3D (C6D6, 0.15 ppm, triplet,
J HD ) 2.0 Hz) and 40% CH4.
34
and TaMe3Cl2 were prepared according to published proce-
dures. 2-Admantanone, 5-bromo-m-xylene, and bromomesity-
lene were obtained from Aldrich and used as received. Diethyl
ether, n-pentane, n-hexane, and toluene were dried and
deoxygenated by the method of Grubbs.41 Pyridine was dis-
tilled from calcium hydride and collected under vacuum. C6D6
and C5D5N were degassed and dried over 4 Å molecular sieves.
Celite, alumina, and 4 Å molecular sieves were dried in vacuo
overnight above 200 °C. 1H NMR (300 or 500 MHz) and 13C
NMR (75 MHz) spectra were recorded on Varian Unity 300,
Mercury 300, and Inova-501 spectrometers at room temper-
ature unless otherwise stated. 1H and 13C NMR chemical shifts
are reported with respect to internal solvent (7.16 ppm for
C6D5H and 128.39 (t) ppm for C6D6). IR spectra were recorded
on a Bio-Rad FT-IR spectrometer using a solution KBr cell.
C, H, and N elemental analyses were performed by H. Kolbe
Mikroanalytisches Laboratorium, Mu¨lheim an der Ruhr,
Germany.
Syn th esis. OC(2Ad )Ar (1-H) (2Ad ) 2-a d a m a n tyl; Ar )
3,5-C6H3Me2, Ar ) 2,4,6-C6H2Me3). Since the preparations
of both ligands were carried out in essentially the same way,
only the synthesis of OC(2Ad)Ar (2Ad ) 2-adamantyl; Ar )
2,4,6-C6H2Me3, H[1b]) is given in detail.
P r ep a r a tion of th e Gr ign a r d Rea gen t. A 2 L, two-neck,
round-bottom flask was charged with 5.5 g (0.23 mol) of Mg
powder and 600 mL of diethyl ether and capped with a reflux
condenser and a septum. After the Mg was activated with 2.0
mL (10 mol %) of 1,2-dibromoethane, as observed by the
decolorization of a grain of I2, 34.4 g (0.17 mol) of bromomesi-
4. In none of the deuterium labeling studies was
observed either of the following: (i) incorporation of D
into the Ta-CH3 group of the isolated and purified
product or (ii) formation of HD gas.
Taken collectively, and bearing in mind that the
isolated TaMe(O-C[Ad](3,5-C6H3Me2))2(py)(bpy) product
is not formed quantitatively, the above observations
place some limits on the possible reaction mechanisms.
Clearly, the lack of CH3D formation using a H2/C5D5N
combination rules out pyridine as the source of H for
the two departing methyl groups, implying that both
departing Ta-Me groups have reacted prior to pyridine
ortho C-D bond activation. A minimalist mechanistic
skeleton consistent with the data is presented in Scheme
2. The first step is reductive hydrogenolysis (by any of
several possible mechanisms) of both Ta-Me groups to
produce a tantalum(III) intermediate (not observed)
where coordination of pyridine seems likely since no
reaction is observed in toluene. As illustrated in Scheme
2 for the reaction between 2a and C5D5N under H2, the
postulated six-coordinate intermediate, TaMe(O-C[Ad]-
(3,5-C6H3Me2))2(py)3 (4a ), exhibits a fac arrangement of
the three pyridine ligands reminiscent of the py/bpy
arrangement in the structurally characterized final
product. Pyridine dissociation leads to the formation of
TaMe(O-C[Ad](3,5-C6H3Me2))2(py)n (n ) 1 or 2), from
which C-H activation occurs to give a pyridyl-hydride/
deuteride complex.39 Conversion of this logical tanta-
lum(V) intermediate to the observed and isolated final
product is not mechanistically well-defined. The carbon-
carbon bond forming step might involve either C-C
reductive elimination from a bis-pyridyl species or,
(39) Another possibility suggested by a reviewer involves direct
coupling of two coordinated pyridines followed by subsequent loss of
dihydrogen, obviating the requirement for C-H activation at Ta
occurring and an intermediate such as 5a being formed.
(40) Diaconescu, P. L.; Cummins, C. C. J . Am. Chem. Soc. 2002,
124, 7660-7661.
(41) Pangborn, A. M.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518-1520.

502 Organometallics, Vol. 23, No. 3, 2004
Soo et al.
tylene was syringed slowly into the vigorously stirring slurry
containing Mg over a period of 0.5 h. A greenish-gray slurry
developed within 12 h, after which the Grignard reagent was
cooled in an ice-bath.
After 16 h of stirring at room temperature, the reaction
mixture was filtered through Celite and the THF was subse-
quently removed in vacuo from the brown filtrate. The residue
was then washed with n-pentane and dried to give 14.87 g
(93%) of solvent-free K[1a ] as a white powder. 1H NMR
(C6D6): δ 1.58-2.20 (m, 12 H), 2.28 (s, 6 H, Me), 2.75 (s, 1 H,
allylic R-H), 2.93 (s, 1 H, allylic R-H), 6.70 (s, 1 H, para-H),
7.06 (s, 2 H, ortho-H). 13C{1H} NMR (C6D6) could not be
obtained due to the poor solubility of the salt. IR (C6D6, cm-1):
2904 (C-H, s), 2849 (C-H, m), 2840 (C-H, m), 1680 (m),
P r ep a r a tion of 2-Ad a m a n tyl(2,4,6-C6H2Me3)m eth a n ol.
A 2 L, two-neck, round-bottom flask, charged with 25.1 g (0.15
mol) of adamantyl epoxide dissolved in 800 mL of diethyl ether,
was sealed with two septa and cooled to -78 °C. The cooled
Grignard reagent was then transferred via cannula to the
epoxide solution, and the reaction mixture warmed and
maintained at -42 °C in a liquid N2/acetonitrile bath for 2 h,
after which it was stirred at ambient temperature for 22 h.
The white slurry was quenched with 500 mL of 1 M HCl, and
the two colorless phases separated. After extracting the
aqueous phase with hexanes, the combined organic phases
were neutralized with several portions of dilute sodium
bicarbonate solution and dried over anhydrous MgSO4. After
removal of the solvent, 41.6 g (0.14 mol, 93%) of white solid
was collected (90% pure by 1H NMR spectroscopy). The product
is readily recrystallized from cold n-pentane but can be used
1587 (m), 1300 (m), 1242 (m), 1197 (m). Anal. Calcd for C19H23
OK: C, 74.46; H, 7.56. Found: C, 74.18; H, 7.68.
-
K(O-C[Ad ](2,4,6-C6H 2Me3)) (Ad ) 2-a d a m a n t ylid en e,
K[1b]). A 2.71 g (0.068 mol) sample of KH was added to a 200
mL THF solution of 18.18 g (0.068 mol) of OC(2Ad)(2,4,6-C6H2-
Me3) (H[1b]) at room temperature. Slow effervescence oc-
curred, the slurry changing from colorless to yellow-brown
within 5 min. After 10 h of stirring at room temperature, the
reaction mixture was filtered through Celite and the THF was
subsequently removed in vacuo. The salt was then dissolved
in n-pentane twice and dried to give 20.69 g (94%) of solvent-
1
without purification in the next step. H NMR (C6D6): δ 1.07
1
(s, 1 H), 1.42 (s, 2 H), 1.58-2.09 (m, 12 H), 2.14 (s, 3 H, Me),
2.27 (s, 3 H, Me), 2.46-2.66 (m, 4 H), 5.18 (d, J ) 9.9 Hz, 1 H,
CH(OH)), 6.72 (s, 1 H, meta-H), 6.78 (s, 1 H, meta-H). 13C{1H}
NMR (C6D6): δ 21.19 (meta-Me), 21.60 (para-Me), 28.56, 28.97,
29.14, 30.01, 32.62, 33.57, 38.80, 39.91, 40.07, 49.50, 70.96 (C-
O), 129.70, 132.22, 136.50, 136.63 (meta-C), 137.46 (meta-C),
138.31. IR (C6D6, cm-1): 3582 (O-H, m), 2903 (C-H, s), 2848
(C-H, m). Anal. Calcd for C20H28O: C, 84.45; H, 9.92. Found:
C, 84.40; H, 9.80.
free K[1b]. H NMR (C6D6): δ 1.65-2.20 (m, 13 H), 2.23 (br
s, 9 H, all Me), 2.85 (s, 1 H, allylic R-H), 6.75 (s, 2 H, meta-H).
13C{1H} NMR (C6D6): δ 20.64 (ortho-Me), 21.47 (para-Me),
30.09 (C-H), 30.41 (C-H), 34.33 (C-H), 38.81 (C-H2), 40.32
(C-H2), 40.58 (C-H2), 107.04, 128.78 (meta-C), 134.21 (ortho-
C), 134.40, 144.06, 149.00. IR (C6D6, cm-1): 2897 (C-H, s),
2840 (C-H, s), 1631 (w), 1605 (w), 1364 (m), 1351 (m), 1299
(C-O, s), 1265 (s), 1202 (m). Anal. Calcd for C20H25OK: C,
74.95; H, 7.86. Found: C, 75.08; H, 7.78.
OC(2Ad )(2,4,6-C6H2Me3) (2Ad ) 2-a d a m a n tyl, H[1b]). A
14.70 g (0.147 mol, 1.5 equiv) sample of CrO3 dissolved in 50
mL of 6 M H2SO4 was slowly added to a vigorously stirring
acetone solution of 41.55 g (0.146 mol) of the alcohol obtained
above. The reaction is highly exothermic, with the acetone
refluxing occasionally during the addition; however, the exo-
thermicity is well-controlled if the acid is added slowly. After
the dark green solution had cooled to ambient temperature,
the two phases were separated, and the aqueous phase
extracted with petroleum ether. The organic phases were
combined, neutralized with dilute KOH, dried over anhydrous
MgSO4, and concentrated to give 36.55 g (0.129 mol, 88%) of
Ta Me3(O-C[Ad ](3,5-C6H 3Me2))2 (Ad ) 2-a d a m a n t yl-
id en e, 2a ). A thawing solution of 2.00 g (6.73 mmol) of
TaMe3Cl2 dissolved in 6 mL of toluene was added to a thawing
slurry of 4.131 g (13.48 mmol, 2.00 equiv) of K[OC[2-Ad](3,5-
C6H3Me2)] (K[1a ]) in 60 mL of toluene. The slurry turned clear
and pale yellow within 1 h with concomitant formation of
colorless crystals (KCl), as the reaction mixture warmed to
ambient temperature. After removal of toluene from the
viscous mixture, the gelatinous residue was extracted with 200
mL of n-pentane and filtered. On removal of all volatiles, 4.910
g (6.45 mmol, 96%) of white crystalline plates of pure 2a
remained. The plates were recrystallized from n-pentane at
1
1
white solid (95% pure by H NMR spectroscopy). The product
-40 °C to give X-ray quality crystals. H NMR (C6D6): δ 0.69
was purified by recrystallization from cold methanol (28.18 g,
(s, 9 H, Ta-Me), 1.68-1.81 (m, 8 H), 1.88-1.97 (m, 12 H),
2.03-2.12 (m, 4 H), 2.14 (s, 12 H, meta-Me), 2.88 (s, 2 H, allylic
R-H), 3.58 (s, 2 H, allylic R-H), 6.76 (s, 2 H, para-H), 7.22 (s,
2 H, ortho-H). 13C{1H} NMR (C6D6): δ 21.77 (meta-Me), 29.25
(C-H), 31.72 (C-H), 33.26 (C-H), 37.87 (C-H2), 39.93
(C-H2), 40.13 (C-H2), 55.86 (Ta-Me), 127.55 (ortho-C),
129.01, 129.81 (para-C), 137.92 (meta-C), 138.26, 147.82. IR
(C6D6, cm-1): 2898 (C-H, s), 2848 (C-H, m), 1656 (w), 1598
(m), 1293 (C-O, s), 1247 (m), 1189 (s), 1123 (m). Anal. Calcd
for C41H55O2Ta: C, 64.72; H, 7.29. Found: C, 64.90; H, 7.40.
Syn th esis of Ta Me3(O-C[Ad ](2,4,6-C6H2Me3))2 (Ad )
2-a d a m a n tylid en e, 2b). A thawing diethyl ether solution
containing 0.544 g (1.70 mmol, 2.20 equiv) of K[OC[2-Ad](2,4,6-
C6H2Me3)] (K[1b]) was added to a thawing solution of 0.220 g
(0.743 mmol) of TaMe3Cl2 dissolved in 5 mL of n-pentane.
Within 10 min, a white precipitate formed in the colorless
solution. After 1 h, the precipitate was removed by filtration
through a bed of Celite, and the filtrate concentrated in vacuo
to give 0.517 g (0.655 mmol, 88%) of spectroscopically pure
plates of 2b. The plates were recrystallized from diethyl ether
at -40 °C. 1H NMR (C6D6): δ 0.43 (s, 9 H, Ta-Me), 1.64-
2.02 (m, 24 H), 2.13 (s, 6 H, para-Me), 2.22 (s, 2 H, allylic R-H),
2.41 (s, 12 H, ortho-Me), 3.58 (s, 2 H, allylic R-H), 6.81 (s, 4 H,
meta-H). 13C{1H} NMR (C6D6): δ 20.48 (ortho-Me), 21.55 (para-
Me), 29.30 (C-H), 31.19 (C-H), 33.21 (C-H), 37.82 (C-H2),
39.78 (C-H2), 39.81 (C-H2), 53.71 (Ta-Me), 128.63 (meta-C),
128.77, 134.05, 136.97 (ortho-C), 137.34, 144.04. IR (C6D6,
cm-1): 2909 (C-H, s), 2849 (C-H, m), 1666 (w), 1268 (C-O,
1
0.100 mol, 65% from adamantyl epoxide). H NMR (C6D6): δ
1.52-1.88 (m, 10 H), 2.07 (s, 3 H, para-Me), 2.18 (s, 6 H, ortho-
Me), 2.29-2.40 (m, 4 H), 2.98 (s, 1 H, OCCH), 6.63 (s, 2 H,
meta-H). 13C{1H} NMR (C6D6): δ 20.56 (meta-Me), 21.31 (para-
Me), 28.25 (C-H), 28.62 (C-H), 30.00 (C-H), 33.70 (CH2),
38.01 (CH2), 39.53 (CH2), 58.29 (OCCH), 129.60 (meta-C),
134.32 (ortho-C), 138.36 (para-C), 139.88 (ipso-C), 211.43
(OdC). IR (C6D6, cm-1): 2908 (C-H, s), 2851 (C-H, m), 1719
(m), 1690 (CdO, s). Anal. Calcd for C20H26O: C, 84.99; H, 9.28.
Found: C, 84.76; H, 9.44.
OC(2Ad )(3,5-C6H3Me2) (2Ad ) 2-a d a m a n t yl, H[1a ]). A
total of 25.85 g (0.096 mol, 53% from 30.0 g of adamantyl
1
epoxide) was obtained. H NMR (C6D6): δ 1.49-1.58 (d, 2 H),
1.64 (s, 2 H), 1.73-1.86 (m, 6 H), 2.08 (s, 6 H, Me), 2.30-2.42
(m, 4 H), 3.25 (s, 1 H, OCCH), 6.83 (s, 1 H, para-H), 7.53 (s, 2
H, ortho-H). 13C{1H} NMR (C6D6): δ 21.73 (Me), 28.59 (C-
H), 28.92 (C-H), 31.22 (C-H), 33.62 (C-H2), 38.25 (C-H2),
39.47 (C-H2), 52.75 (OCCH), 126.70 (ortho-C), 134.09 (para-
C), 138.38 (meta-C), 138.50 (ipso-C), 203.47 (OdC). IR (C6D6,
cm-1): 2906 (C-H, s), 2851 (C-H, m), 1723 (w), 1679 (CdO,
s). Anal. Calcd for C20H24O: C, 85.03; H, 9.01. Found: C, 84.93;
H, 8.92.
K(O-C[Ad ](3,5-C6H 3Me2)) (Ad ) 2-a d a m a n t ylid en e,
K[1a ]). A 2.16 g (0.054 mol) sample of KH was added to a 200
mL THF solution of 13.95 g (0.052 mol) of OC(2Ad)(C6H3Me2)
(H[1a ]) at room temperature. Slow effervescence occurred, the
slurry changing from colorless to reddish-brown within 5 min.

Tantalum Ligation and Pyridine Coupling
Organometallics, Vol. 23, No. 3, 2004 503
s), 1227 (m), 1208 (s), 1092 (m). Anal. Calcd for C43H59O2Ta:
C, 65.47; H, 7.54. Found: C, 65.70; H, 7.72.
(ii) Exp er im en t w ith Deu ter iu m (D2) On ly. The protocol
described above for the preparation of 3a was repeated using
0.347 g (0.456 mmol) of 2a and 5 mL of pyridine under an
atmosphere of D2. After 5 days, the gases in the reaction bomb
flask were vacuum-transferred simultaneously into a gas IR
cell and a NMR tube containing frozen C6D6. IR (gas-phase,
cm-1, only CH3D stretches described, see above for CH4
stretches): 2820 (e C-H stretch), 2200 (a1 C-D stretch), 1156
Ta Me(O-C[Ad ](3,5-C6H3Me2))2(p y)(bp y) (Ad ) 2-a d a -
m a n tylid en e, p y ) N-bou n d p yr id in e, bp y ) K2-2,2′-
bip yr id in e), 3a . A 5 mL pyridine solution of 2a (0.500 g, 0.657
mmol) in a 50 mL reaction vessel was degassed twice before
being sealed with a Teflon valve under an H2 atmosphere at
-196 °C. After the pale yellow solution had warmed to ambient
temperature, the reaction vessel was heated to 70 °C for 5
days. Within 15 min of heating, a red solution formed. After 5
days, all volatiles were removed in vacuo from the dark brown
solution. The oily brown residue was then dissolved in diethyl
ether, concentrated, and triturated twice with n-pentane to
yield 0.284 g (0.294 mmol, 45%) of dark brown 3a . Recrystal-
lization from a diethyl ether solution gave 0.094 g (0.097 mmol,
1
(e C-H bend), 780 (a1 bend). H NMR (C6D6): δ 4.47 (s, H2),
0.15 (t, J HD ) 2.0 Hz, CH3D, 61%), 0.16 (s, CH4, 39%). The
brown solution was worked up as above to give 0.199 g (0.210
mmol, 45%) of 3a as verified by comparison of its 1H NMR
spectrum with that of an independently prepared sample.
(iii) Exp er im en t w ith Deu ter iu m (D2) a n d P yr id in e-
1
d
5 in NMR Tu be Exp er im en t. The protocol described above
15%) of brown crystals. H NMR and COSY (C6D6, 500 MHz):
for the preparation of 3a was repeated using 0.035 g (0.046
mmol) of 2a and approximately 0.7 mL of pyridine-d5 under
an atmosphere of D2 in a J -Young NMR tube with a Teflon
valve. The reaction was monitored regularly over 5 days. The
1H NMR spectrum before heating showed only 2a and H2 with
no indication of any pyridine adducts. Both CH3D and CH4
were formed in a ratio of ca. 59% to 41%, respectively. The
ratio did not change over 5 days, and HD formation was not
observed. In addition, more than one tantalum compound was
formed, as indicated by the complex 1H NMR spectrum. 1H
NMR (C6D6, methane species only): δ 0.11 (t, J HD ) 2.0 Hz,
CH3D, 59%), 0.12 (s, CH4, 41%).
δ 1.03 (s, 3 H, Ta-Me), 1.65-2.20 (m, 24 H), 2.06 (s, 12 H,
meta-Me), 2.73 (s, 2 H, allylic R-H), 3.47 (s, 2 H, allylic R-H),
4.96 (t, J ) 5.9 Hz, 2 H, 2,2′-bpy 5,5′-H), 5.56 (dd, J ) 6.3 and
8.4 Hz, 2 H, 2,2′-bpy 4,4′-H), 6.29 (t, J ) 6.8 Hz, 2 H, py meta-
H), 6.62 (m, 3 H, enolate para-H and py para-H), 6.93 (d, J )
9.3 Hz, 2 H, 2,2′-bpy 3,3′-H), 7.00 (s, 4 H, enolate ortho-H),
8.11 (d, J ) 4.5 Hz, 2 H, py ortho-H), 8.31 (d, J ) 7.2 Hz, 2 H,
2,2′-bpy 6,6′-H). 13C{1H} NMR (C6D6): δ 21.79 (meta-Me), 29.24
(C-H), 29.27 (C-H), 30.59 (C-H), 33.50 (C-H), 37.89 (C-
H2), 39.76 (C-H2), 39.84 (C-H2), 40.11 (C-H2), 40.25 (C-H2),
50.58 (Ta-Me), 111.67 (C-H), 119.37 (C-H), 120.70 (C-H),
124.19 (C-H), 127.41 (enolate ortho-C), 127.50, 129.34 (enolate
para-C), 131.43, 137.19 (py para-C), 137.69 (enolate meta-C),
137.80 (C-H), 138.78 (enolate ipso-C), 146.73, 146.90 (C-H).
IR (C6D6, cm-1): 2903 (C-H, s), 2845 (C-H, m), 1646 (w), 1599
(m), 1481 (m), 1293 (s), 1249 (m), 1213 (m), 1191 (s). Anal.
Calcd for C54H62O2N3Ta: C, 67.14; H, 6.47; N, 4.35. Found:
C, 66.86; H, 6.34; N, 4.18.
Ack n ow led gm en t. For support of this work, we are
grateful to the National Science Foundation (CHE-
0316823). H.S.S. thanks the Public Service Commission
(Singapore) for an OMS scholarship. The authors thank
J oshua S. Figueroa and Dr. William M. Davis for
assistance with the X-ray crystal structure of 3a .
Isotop e-La belin g Stu d ies. (i) Exp er im en t w ith P yr i-
d in e-d 5 On ly. The protocol described above for the prepara-
tion of 3a was repeated using 0.276 g (0.363 mmol) of 2a and
4 mL of pyridine-d5 as the solvent. After 5 days, the gases in
the reaction vessel were vacuum-transferred simultaneously
into a gas IR cell and a NMR tube containing frozen C6D6. IR
(gas-phase, cm-1): 3017 (CH4 t2 stretch), 1306 (CH4 t2 bend).
1H NMR (C6D6): δ 4.47 (s, H2), trace at 4.43 (t, J HD ) 42.5 Hz,
HD), 0.16 (s, CH4), trace at 0.15 (t, J HD ) 2.0 Hz, CH3D). The
brown solution was worked up as above to give 0.104 g (0.106
Su p p or tin g In for m a tion Ava ila ble: Full X-ray crystal-
lographic experimental details are provided including tables
with data for the X-ray crystallographic studies of 2a and 3a .
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
mmol, 29%) of 3a -d13 as verified by H NMR.
OM0303496