Exploring routes to tantalum(V) alkylidene complexes supported by amine tris(phenolate) ligands

Article abstract of DOI:10.1002/adsc.200404273

Two basic organometallic species, tris(neopentyl)mono(neopenylidene) tantalum and pentabenzyltantalum, were evaluated as starting materials on the route to alkyl and alkylidene complexes with amine tris(phenolate) ligands. The reaction of the former with the ligand precursor proceeded via the addition of the O-H functionality to the Ta=C double bond at the first step, forming a sterically congested tetraneopentyl complex, that was characterized by X-ray crystallography. In contrast, the reaction of the less bulky pentabenzyltantalum with a variety of the ligand precursors led directly and in a high yield to the dibenzyl Ta(V) complexes. These species were surprisingly stable toward thermolysis, decomposing only at 120-130°C after prolonged periods of time. The unexpected product of this decomposition was found to be a dinuclear μ-benzylidene Ta(V) complex.

Full text of DOI:10.1002/adsc.200404273

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Exploring Routes to Tantalum(V) Alkylidene Complexes
Supported by Amine Tris(phenolate) Ligands
Stanislav Groysman,a Israel Goldberg,a Moshe Kol,a,* Elisheva Genizi,b
Zeev Goldschmidtb,*
a
School of Chemistry, Raymond and Beverly Sackler of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel
Fax: (þ972)-3-640-9293, e-mail: moshekol@post.tau.ac.il
b
Department of Chemistry, Bar Ilan University, Ramat Gan 52900, Israel
Fax: (þ972)-3-535-1250, e-mail: goldz@mail.biu.ac.il
Received: August 31, 2004; Accepted: November 22, 2004
Dedicated to Prof. R. R. Schrock on the occasion of his 60th birthday.
Abstract: Two basic organometallic species, tris(neo-
pentyl)mono(neopenylidene)tantalum and penta-
benzyltantalum, were evaluated as starting materials
on the route to alkyl and alkylidene complexes with
amine tris(phenolate) ligands. The reaction of the
former with the ligand precursor proceeded via the
addition of the O H functionality to the Ta C dou-
ble bond at the first step, forming a sterically con-
gested tetraneopentyl complex, that was character-
ized by X-ray crystallography. In contrast, the reac-
tion of the less bulky pentabenzyltantalum with a va-
riety of the ligand precursors led directly and in a
high yield to the dibenzyl Ta(V) complexes. These
species were surprisingly stable toward thermolysis,
decomposing only at 120–1308C after prolonged pe-
riods of time. The unexpected product of this decom-
position was found to be a dinuclear m-benzylidene
Ta(V) complex.
benzyltantalum complexes of amine bis(phenolate) li-
gands may be conveniently synthesized by direct reac-
tion between the ligand precursor and pentabenzyltan-
talum, and that these may be thermally converted into
benzyl-benzylidene complexes, depending on the pres-
ence of an extra donor on a side arm.[4b] The related tri-
anionic amine tris(phenolate) ligands are especially in-
teresting since they were found to support both trigonal
bipyramidal and octahedral geometries on binding to
group IV and V metals, and due to their similarity to
the trianionoic triamidoamine ligands (see Figure 1).
The latter have been extensively studied in recent
years,[6] and were shown to stabilize an axial tanta-
lum(V) alkylidene functionality in a trigonal bipyrami-
dal complex following a smooth a-hydrogen abstraction
from a dialkyl intermediate.[7]
Several synthetic methodologies are known to lead to
alkylidene complexes, including the employment of pre-
cursors that already include the alkylidene function, or
by an a-elimination reaction sequence from dialkyl pre-
cursors.[3] The amine phenolate ligands are relatively
acidic, and may be bound to tantalum via an alkane elim-
ination starting from a suitable precursor.[4] Ta(CH-t-
Bu)(CH2-t-Bu)3 is an attractive candidate for reaction
with an amine tris(phenolate) ligand precursor since it
already contains an alkylidene function. Two possible
ꢀ
¼
Keywords: alkane elimination reaction; alkylidene
complex; amine tris(phenolate); ligand design; N,O
ligands; tantalum
The first Schrock-type alkylidene complex, Ta(CH-t- reactions may be envisioned to result from the two types
Bu)(CH2-t-Bu)3, was prepared in 1974.[1] Since then, of metal-carbon bonds of this complex. The O H bond
the significance of the alkylidene functionality as the may add across the Ta C double bond, generating a tet-
ꢀ
¼
mediator of the olefin metathesis reaction has been raalkyl species, that can react further, to generate com-
¼
widely established. Although not showing a high reac- plexes of the [O3N]Ta(CH2-t-Bu)2- or [O3N]Ta C(H)-
tivity in metathesis, Ta(V) alkylidene complexes were
extensively studied as model compounds for the forma-
tion and reactivity of metal-carbon multiple bonds.[2]
Various monodentate and polydentate ligand systems
that support this functionality have been introduced in
recent years.[3] Recently, we began to explore the syn-
thesis and reactivity of organometallic tantalum com-
Figure 1. A schematic representation of the amine tris(phe-
plexes of amine-phenolate ligands.[4,5] We found that tri- nolate) and triamidoamine ligand systems.
Adv. Synth. Catal. 2005, 347, 409–415
DOI: 10.1002/adsc.200404273
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Stanislav Groysman et al.
t-Bu-type. Alternatively, the acidic OH functions of the
ligand may react with the neopentyl ligands, eliminating
the corresponding alkanes, and preserving the alkyli-
¼
dene function in the resulting [O3N]Ta C(H)-t-Bu-
type complex.
Ta(CH-t-Bu)(CH2-t-Bu)3, prepared by Schrockꢁs pro-
cedure,[1b] was reacted with the amine tris(phenolate) li-
gand precursor featuring 2,4-dimethyl substitution
1
(Lig1H3) in ether at room temperature for 1 h. The H
NMR spectrum of the crude light-yellow solid product
indicated that no starting materials remained. The solid
was extracted with pentane, and cooled to ꢀ35 8C. Af-
ter prolonged standing at this temperature, yellow crys-
1
tals had formed whose H NMR spectrum supported
their assignment as the major component in the crude
reaction mixture. According to the 1H NMR spectrum,
this complex(1) was neither a dineopentyl nor aneopen-
tylidene complex. The solid state structure of 1 revealed
a rather unexpected product of the [H2O3N]Ta(CH2-t-
Bu)4 formula (see Figure 2), in which the strong chelate
(Lig1) binds to the metal via only one phenolate oxygen,
while the other two are still protonated. In addition to
the phenolate ligand, four neopentyl ligands are bound
to the Ta(V) metal center whose geometry may be de-
scribed as a distorted square-pyramidal, with one of
the neopentyl groups occupying the axial position. To
our knowledge, this is the first crystallographically char-
acterized tantalum complex featuring four neopentyl
groups. Notably, the nitrogen donor is not coordinating
to the tantalum, signifying a highly crowded metal cen-
ter.
Figure 2. ORTEP representation of the molecular structure
of 1 (30% probability ellipsoids). The asymmetric unit con-
tains two crystallographically independent molecules of sim-
ilar structure, only one of which is presented here. The elon-
gated ellipsoids of the carbon atoms of the neopentyl groups
The reactions between Ta(CH-t-Bu)(CH2-t-Bu)3 and
ligand precursors bearing an acidic proton were report-
ed to follow different routes. A reaction with HCl led to reflect their orientational disorder. In addition, one of the
phenolate rings (of the molecule not shown here) is partially
disordered.
protonation of the nucleophilic alkylidene carbon, gen-
erating the tetraalkyl monochloro species.[1b] Similarly,
the first step in the reaction of Ta(CH-t-Bu)(CH2-t-
Bu)3 with silica was proposed to be an addition of the si-
lanol OH bond across the Ta C bond. In contrast, Ta(CH3)5,[11] since it leads to the same product that
Wolczanski et al. reported the whole spectrum of reac- would be expected to result from “Ta(CH2-t-Bu)5”. Ap-
tivities for different OH bearing ligands: the “silox” li- parently, the significant steric crowding at the metal cen-
gand precursor led to protonolysis of a Ta C single ter provides a sufficient kinetic stability to 1. However, 1
bond rather than addition to a Ta C double bond, is unstable in solution at room temperature, and reacts
[8]
¼
ꢀ
¼
whereas the 9-hydroxytriptycene reacted with the alky- further, releasing neopentane. When longer reaction
lidene function. When the ligand precursor was too bul- times were employed (24–72 h), a complex product
ꢀ
ky (“tritox”), no reaction between the O H functional- mixture of various Ta(V) alkyl complexes formed.
ity and this metal complex took place.[9] Compound 1, These products possessed a similar solubility in organic
being the primary product in the reaction between the solvents, and therefore their separation by the regular
amine tris(phenolate) ligand precursor and Ta(CH-t- extraction/recrystallization techniques was not success-
Bu)(CH2-t-Bu)3, undoubtedly results from the addition ful. According to 13C NMR spectroscopy, no alkylidene
¼
of one of the ligand OH groups across the Ta C double complexes were present among the products.
bond. Thus, in Ta(CH-t-Bu)(CH2-t-Bu)3, the alkylidene
Since the Ta(CH-t-Bu)(CH2-t-Bu)3-route proved un-
function appears to be more reactive than the alkyl func- successful, we turned to a less bulky metal precursor:
tion toward the non-bulky phenol function. According pentabenzyltantalum. We have previously shown that
to this reactivity, Ta(CH-t-Bu)(CH2-t-Bu)3 may be pentabenzyltantalum may be prepared directly from
viewed as a masked form of “Ta(CH2-t-Bu)5”,[10] a steri- TaCl5 and BnMgCl in a reasonable yield. Pentabenzyl-
cally hindered analogue of Ta(CH2Ph)5 (vide supra) or tantalum was shown to react quantitatively with a varie-
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Routes to Tantalum(V) Alkylidene Complexes
Scheme 1. The initial product of the reaction between
Ta(CH-t-Bu)(CH2-t-Bu)3 and the amine tris(phenolate) li-
gand precursor Lig1H3.
ty of amine bis(phenolate) ligands, yielding tribenzyl
Ta(V) complexes, that led to formation of alkyl-alkyli-
dene complexes or to ligand b-CH activation upon ther-
molysis.[4b] The amine tris(phenolate) ligand precursor
Lig1H3 reacted with pentabenzyltantalum as well, lead-
ing to the dibenzyl complex 2.[4a] To estimate the gener-
Scheme 2. The reactions of Ta(CH2Ph)5 with the amine
tris(phenolate) ligands precursors.
ality of this approach, we reacted three additional amine the amine bis(phenolate) ligands that led to the corre-
tris(phenolate) ligand precursors, carrying chloro sponding alkyl-alkylidene complexes by thermolytic a-
(Lig2H3), phenyl (Lig3H3), or tert-butyl (Lig4H3) ortho abstraction reaction, complexes 2–4 were found to be
substituents, with pentabenzyltantalum. The reaction relatively stable toward thermolysis. Thus, heating of
of the relatively non-bulky ligands (Lig2H3 and Lig3H3) 2–4 in toluene to ca. 1008C for 1–2 h did not affect
with pentabenzyltantalum was fast and clean, forming them much [in comparison, a fast decomposition of
the corresponding dibenzyl complexes 3, and 4 after the tribenzyl amine bis(phenolate) complexes was ob-
2 h in a high yields. Noteworthy, the bulky amine tris- served at ca. 808C].[4b] When harsher conditions were
(phenolate) ligand precursor, Lig4H3, did not react employed, i.e., heating to 1308C for ca. 2 h in chloroben-
with pentabenzyltantalum under the same conditions zene, a slow color change from yellow-brown to red-pur-
(room temperature, toluene), even after 6 h, after which ple was observed. An inspection of the 1H NMR spectra
only the starting materials were observed.
of the crude products revealed the presence of complex
The NMR spectra of 2–4 at room temperature were mixtures. It was not clear whether alkylidene formation
consistent with C3v-symmetrical species, exhibiting had taken place.
three identical ligand arms and two identical benzyl
To determine the nature of the main product(s), we at-
groups. However, the X-ray structure of 2 revealed a tempted recrystallization of a “model” compound 5, a
hexacoordinate complex, in which the three phenolate decomposition product of 2. Slow evaporation of ben-
oxygens, the two benzyl carbons, and the central (weakly zene solution of 5 resulted in formation of dark-red crys-
bound) nitrogen donor create an octahedral environ- tals, along with some non-crystalline beige material. The
ment around the metal center.[4a] According to VT- X-ray structure determination disclosed a dinuclear Ta
NMR experiments, these segments (three phenolate complex, possessing bridging benzylidene ligands in
arms and two benzyl ligands) are equilibrated by some which the geometry around each metal atom is nearly
low-energy process(es): Upon cooling, all benzyl peaks octahedral (Figure 3). Thus, the coordination mode of
flatten until completely disappearing at ca. ꢀ738C.[4a] the amine tris(phenolate) ligand does not change, as
In contrast to the related tribenzyl Ta(V) complexes of would occur if a mononuclear benzylidene product
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Figure 3. ORTEP representation of 5, 50% probability. Left:
the dinuclear complex core including both conformations of
the bridging benzylidene groups. Right: the whole structure
except for the crystallization solvent, that is omitted for clari-
ty.
ꢀ
ꢀ
would have formed. Moreover, the Ta O and the Ta N
bond lengths are similar to those in 2.[4a] The Ta C bond
ꢀ
lengths are not identical, being 2.159(9) and 2.207(9) ꢂ.
As expected for a d metal, the Ta Ta distance (3.19 ꢂ)
0
ꢀ
seems to be too long to support a metal-metal bond.
Compound 5 may form either through an intramolec-
ular a-abstraction reaction leading to a terminal alkyli-
dene group followed by dimerization of two such units
to give the bridging dinuclear species, or by an intermo-
lecular pathway leading to it directly. The latter pathway
may explain the harsh conditions required for formation
of 5. Still, the former pathway cannot be ruled out as the
high activation energy may result from the instability of
a pentacoordinate terminal alkylidene species. These
findings are especially intriguing when compared with
the facile formation of benzylidene complexes from
Figure 4. Two possible isomers of the dinuclear m-benzyli-
dene complex, 5.
the proposed unstable [N3N]Ta(CH2Ph)2 complexes. or two different orientations of the Ci-symmetrical iso-
Thus, the amine tris(phenolate) ligand, in contrast to mer, at the same sites in the crystal. Yet, these isomers,
the triamidoamine ligand, is not inclined to support a if they exist, are of a diastereoisomeric relationship
metal-carbon multiple bond in the axial position of a that should be manifested in their different spectroscop-
[12]
¼
“tripodal” complex of the [O3N]M R-type.
ic data. Therefore, we attempted the separation of the
The structure of 5 contains two overlapping orienta- isomers by extraction and recrystallization from differ-
tions of the benzylidene ligand (Figure 3, right) that ent solvents.
may arise from two different isomers of 5 (5a and 5b,
The crude product was extracted consecutively with
see Figure 4). A “cis” disposition of the benzylidene pentane, ether, and toluene. Red-purple crystals formed
phenyl groups leads to a C2-symmetrical complex, while in the ethereal phase within several days at ꢀ358C. The
a “trans” disposition of these groups leads to a Ci-sym- 1H NMR spectrum of the crystalline solid showed a sin-
metrical complex.[13] In this crystal structure, 5 co-crys- gle alkylidene species (5a), displaying the benzylidene
tallizes with a C6D6 molecule, occupying two variable signal at 8.20 ppm. In addition, the spectrum featured
sites in the crystal grid, in accordance with the orienta- six different Me groups, six different aromatic protons,
tion of the benzylidene group: In one of its sites, deuter- and three AB systems for the ligand methylene protons.
iobenzene overlaps with one of the orientations of the The 13C NMR spectrum of 5a included a single peak for
benzylidene group. On its own, the crystal structure can- the alkylidene carbon at 212.9 ppm. The above spectro-
not provide reliable evidence for the existence of these scopic data were consistent with either (C2 and Ci) of the
two isomers, since it may result from the co-crystalliza- isomers of the dinuclear complex, proposed by the solid-
tion of the two different enantiomers of C2-symmetry, state structure. No crystals formed in the toluene frac-
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cooled to ca. 110 K. The structures were solved by a combina-
tion of direct methods and Fourier techniques using mostly
DIRDIF software, and were refined by full-matrix least
squares with SHELXL-97. Elemental analyses of the ligand
precursors and of the tantalum complexes were performed in
the microanalytical laboratories of the Hebrew University in
Jerusalem. The analysis of the complexes consistently showed
a somewhat low carbon value apparently due to air sensitivity.
tion, however, the removal of the solvent revealed a rel-
atively clean mixture of the two alkylidene isomers (5a
and 5b), in ca. 3:1 ratio. The pentane fraction also con-
tained both isomers (in ca. 1:1.5 ratio), though being
contaminated by unknown impurities. According to its
1H NMR spectrum, the second alkylidene species dis-
playing a benzylidene proton at 8.04 ppm, 6 aromatic
protons, and 6 Me groups, fits the structure of the second
possible diastereoisomer of the general formula [Lig-
¼
Ta CH(Ph)]2 (5b). The ability to isolate and handle
Synthesis of Lig2H3 {2,2’,2’’-
[Nitrilotris(methylene)]tris(4,6-dichlorophenol)}
complex 5a in its pure form for at least 24 hours at
room temperature, indicates that these diastereoisom-
ers do not interconvert readily.
2,4-Dichlorophenol (4.0 g, 24.5 mmol), and hexamethylene-
tetramine (1.2 g, 8.5 mmol) were melted and stirred at 1108C
for 2.5 h. Hot ethanol was added to the reaction mixture and
the stirring was continued until the oil was dissolved complete-
ly, and some solid started to form. The stirring was stopped
and the solid was allowed to precipitate overnight. The solid
was filtered, washed with ethanol and dried to give a yellow
powder (1.4 g, 32% yield). This was further purified by recrys-
tallization from 2-ethoxyethanol (or DMF) to give colorless
crystals of 2,2’-[bis(methylene)imino]bis(4,6-dichlorophenol);
mp 2108C–2128C [Lit. mp.[15] 197–1988C]. The filtrate was
evaporated to give an oil, which was triturated with methanol
to give the desired compound, which was further purified by
column chromatography (silica gel, chloroform). Recrystalli-
zation from chloroform gave the pure tris product as colorless
plates; yield: 1.0 g (21%); mp 143–1448C; anal. calcd. for
C21H15Cl6NO3: C 46.53, H 2.79, N 2.58; Cl 39.24; found: C 46.12,
In conclusion, both the tri(alkyl)mono(alkylidene)
and homoleptic pentaalkyl Ta complexes are potential
precursors for the preparation of Ta(V) alkyl complexes
with amine tris(phenolate) ligand precursors. The reac-
tion of tris(neopentyl)mono(neopenylidene)tantalum
with the ligand precursor proceeded via the addition
ꢀ
¼
of the O H functionality to the Ta C double bond, gen-
erating the tetra(alkyl) species, that reacted further to
give a mixture of Ta alkyl species. In contrast, the reac-
tion of pentabenzyltantalum with a variety of the amine
tris(phenolate) ligand precursors led cleanly to the for-
mation of the octahedral dibenzyl Ta(V) complexes,
that were found to be surprisingly stable toward a ther-
molytic decomposition (toluene, 100 8C, several hours).
Under more severe conditions, the reaction led to the
formation of unusual dinuclear Ta(V) complex, possess-
ing bridging benzylidene ligands. Two diastereoisomers
of this compound (“cis” and “trans”) were observed, one
of which was obtained in a pure state by recrystallization
from ether. While the stability of the dibenzyl species
seems to support an intermolecular a-hydrogen abstrac-
tionpathway for formation of the bridging alkylidene com-
plexes, an intramolecular pathway cannot be ruled out.
1
H 2.83, N 2.54, Cl 40.01%; H NMR (200 MHz, CDCl3): d¼
7.23 (d, J¼2 Hz, 3H), 7.02 (d, J¼2 Hz, 3H), 5.60 (br s, 3H), 3.72
(s, 6H); 13C NMR (CDCl3) d¼149.81 (3 C),129.10 (3 C), 128.55
(3 C), 124.80 (3 CH), 124.73 (3 CH), 121.09 (3 C), 55.33 (3 CH2);
HRMS (DCI-I-Bu): m/z¼538.917527 (C21H15Cl6NO3) (Mþ).
Synthesis of Lig3H3 {2,2’,2’’-
[Nitrilotris(methylene)]tris[4-(2,2-dimethylethyl)-6-
phenyl-phenol]}
A mixture of 4-t-butyl-2-phenylphenol (4.53 g, 20.0 mmol),
hexamethylenetetramine (0.23 g, 1.65 mmol) and 36% aque-
ous formaldehyde (0.84 mL, 10.0 mmol) was stirred and heat-
ed to 1258C for 48 h. The mixture was cooled to room temper-
ature, triturated with methanol, filtered, washed with cold
methanol and dried to give the product as a pale yellow powder
(2.7 g, 55% yield) which could be further purified by recrystal-
lization from methanol; mp 1748C; anal. calcd. for C51H57NO3:
Experimental Section
General Remarks
All manipulations of the metal complexes were performed un-
der a dry nitrogen atmosphere in a nitrogen-filled glove-box.
Ta(CH-t-Bu)(CH2-t-Bu)3,[1b] Ta(CH2Ph)5,[4a] Lig1H3,[14] and
1
[14]
Lig4H3 were prepared according to previously published
C 83.58, H 7.85, N 1.91; found: C 83.39, H 7.96, N 1.89%; H
NMR (CDCl3, 200 MHz): d¼7.5–7.2 (21H, m), 3.89 (6H, s),
1.28 (27H, s); 13C NMR (50.38 MHz, CDCl3): d¼150.14 (3
C), 142.6 (3 C), 138.19 (3C), 129.41 (6 CH), 128.65 (6 CH),
127.95 (3 C), 127.24 (6 CH), 126.77 (3 CH), 122.76 (3 C),
55.65 (3 CH2), 34.06 (3 C), 31.55 (9 CH3); HRMS (DCI-CH4):
m/z¼731.430384 (C51H57NO3) (Mþ).
procedures. TaCl5, PhCH2MgCl (1.0 M in ether), and t-
BuCH2MgCl (1.0 M in ether) were obtained from Aldrich
Inc. Solvents were purified and dried following standard proce-
dures. NMR data were collected on BrukerAC-200 and Bruker
AC-400 spectrometers, and referenced to protio impurities in
benzene-d6 (d¼7.15 ppm) and to the 13C chemical shift of ben-
zene (d¼128.70 ppm). Routine characterization of metal com-
1
plexes consisted of H, and BB 13C NMR experiments, per-
formed in C6D6 at 298 K. The X-ray diffraction measurements
were performed on a Nonius Kappa CCD diffractometer sys-
tem, using MoKa (l¼0.7107 ꢂ) radiation. The analyzed crys-
tals were embedded within a drop of viscous oil and freeze-
Synthesis of 1
22 mg (0.053 mmol) of Lig1H3 were dissolved in ether (1 mL)
and added dropwise to a stirred yellow-orange solution of
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Ta(CH-t-Bu)(CH2-t-Bu)3 (24 mg, 1 equiv.) in ether (1 mL). Af-
ter stirring at room temperature for 1 h, a pale yellow solution
was obtained. The solvent was removed under vacuum, and the
Synthesis of 5
A solution of 124 mg (0.159 mmol) of 2 in chlorobenzene
(1 mL) was heated to 1308C for 1.5 h. After cooling, a deep-
red solution was observed. The solvent was removed, and the
resulting deep-red solid was extracted with pentane, ether,
and toluene (ca. 2 mL each). Red-purple crystals (22 mg)
that had formed in the ethereal phase were separated from
the solution and dried under vacuum, leading to the single iso-
mer 5a in ca. 20% yield. 1H NMR of 5a (400 MHz, C6D6): d¼
8.20 [s, 2H, PhC(H)(Ta)2], 7.24 (m, 6H), 6.93 (br s, 2H), 6.67
(br, 2H), 6.62 (br s, 2H), 6.50 (t, J¼7 Hz, 4H), 6.39 (br s, 2H),
6.17 (br s, 2H), 5.74 (br s, 2H), 4.37 (d, J¼13.3 Hz, 2H), 3.36
(d, J¼13.1 Hz, 2H), 3.14 (d, J¼12.9 Hz, 2H), 2.87 (d, J¼
13.2 Hz, 2H), 2.59 (s, 3H, Ar-Me), 2.52 (d, J¼12.9 Hz, 2H,
Ar-Me), ca. 2.42 [m (sþd), 5H], 2.25 (s, 3H, Ar-Me), 2.05 (s,
3H, Ar-Me), 1.85 (s, 3H, Ar), 1.66 (s, 3H, Ar-Me); 13C NMR
(100.76 MHz, C6D6): d¼212.91 [PhC(H)-(Ta)2], 155.66,
155.55, 155.12, 150.26, 145.05, 132.50, 132.34, 131.19, 130.81,
130.48, 129.34, 129.22, 128.29, 128.22, 128.16, 128.05, 127.35,
126.28, 126.14, 125.61, 124.08, 129.34, 65.46, 65.05, 60.62,
21.47, 21.17, 20.95, 19.63, 17.19, 16.35. The remaining ethereal
phase has shown no traces of additional alkylidene products.
The non-crystalline solid material that was found in the pen-
tane phase has shown the presence of both alkylidene products,
according to the benzylidene signals at 8.20 and 8.04 ppm, in ca.
1:1.5 (5a:5b) ratio, in addition to some impurities. Evapora-
tion of the toluene led to a purple solid, containing a relatively
clean mixture of two alkylidene isomers, in 3:1 (5a:5b) ratio.
The relative amount of 5b in the mixture of 5a and 5b was esti-
mated according to the following resonances: d¼8.04 [s, 2H,
PhC(H)(Ta)2], 6.88 (br s, 2H, Ar-H), 6.76 (br s, 2H, Ar-H),
6.57 (br s, 2H, Ar-H), 6.26 (br s, 2H, Ar-H), 5.78 (br s, 2H,
Ar-H), 7.54 5.78 (br s, 2H, Ar-H), 2.66 (s, 3H, Ar-Me), 2.61 (s,
3H, Ar-Me), 2.20 (s, 3H, Ar-Me), 2.09 (s, 3H, Ar-Me), 1.87 (s,
3H, Ar-Me), 1.70 (s, 3H, Ar-Me). X-ray quality crystals were
obtained upon slow evaporation of a C6D6 solution of crude 5.
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supplementa-
ry publication no. CCDC-248942 (complex 5) and 248943
(complex 1). Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [Fax: int. codeþ44(1223)336-033; E-mail: depos-
it@ccdc.cam.ac.uk].
1
bright-yellow product was analyzed by H NMR. The crude
product was extracted with pentane and toluene, and left at
ꢀ35 8C. During several weeks, bright-yellow crystals of 1
were obtained in the pentane phase and were analyzed by
1H NMR and X-ray diffraction analyses. 1H NMR (200 MHz,
C6D6): d¼8.04 (s, 2H, OH), 7.63 (br s, 1H, Ar-H), 6.83 (br s,
2H, Ar-H), 6.75 (m, 3H, Ar-H), 4.10 (s, 2H, Ar-CH2-N), 3.90
(s, 4H, Ar-CH2-N), 2.33 (s, 3H, Ar-CH3), 2.19 (s, 6H, Ar-
CH3), 2.17 (s, 3H, Ar-CH3), 2.09 (s, 6H, Ar- CH3), 1.64 [s, 8H,
Ta-CH2-C(CH3)3], 1.20 [s, 36H, Ta-CH2-C(CH3)3]. These data
were consistent with the major product (ca. 80%) in the crude
reaction mixture. This compound was too unstable in solution
at room temperature to be further characterized.
Synthesis of 2
Compound 2 was prepared as previously described.[4a] Anal.
calcd. for C41H44NO3Ta: C 63.15, H 5.69, N 1.80; found: C
62.49, H 5.82, N 1.48%.
Synthesis of 3
53 mg (0.098 mmol) of Lig2H3 were dissolved in toluene and
added dropwise to a stirred solution of Ta(CH2Ph)5 (66 mg,
0.104 mmol). After 2 h, the yellow-brown solution was concen-
trated under vacuum. The resulting solid was washed with a
small amount of pentane and dried under vacuum, yielding
73 mg (0.081 mmol, 83%) of 3 as yellow solid; 1H NMR
(200 MHz, C6D6): d¼7.21 (m, 8H), 7.06 (d, J¼2.5 Hz, 3H),
6.88 (tt, J1 ¼6.3 Hz, J2 ¼2.4 Hz, 3H), 6.29 (d, J¼2.5 Hz, 3H),
2.88 (s, 4H, Ph-CH2-Ta), 2.62 (s, Ar-CH2-N); 13C NMR
(50.38 MHz, C6D6): d¼153.05, 143.36, 131.55, 130.95, 130.52,
129.99, 129.44, 129.37, 129.00, 128.72, 127.86, 91.25, 61.80;
anal. calcd. for C35H26Cl6NO3Ta: C 46.59, H 2.90, N 1.55; found:
C 45.72, H 3.01, N 1.22%.
Synthesis of 4
Acknowledgements
63 mg (0.087 mmol) of Lig3H3 were dissolved in 2 mL of tol-
uene and added to a stirred solution of Ta(CH2Ph)5 (56 mg,
0.088 mmol) in 1 mL of toluene. The brown reaction mixture
was stirred for 2 h, after which the solvent was removed under
vacuum. The resulting brown solid was washed with pentane
(ca. 3 mL), and dried under vacuum, yielding 80 mg
We thank the Israel Science Foundation administered by the Is-
rael Academy of Sciences and Humanities for financial support.
We thank Sima Alfi for technical assistance.
1
(0.071 mmol, 81%) of 4; H NMR (200 MHz, C6D6): d¼7.69
References and Notes
(d, J¼7.3 Hz, 6H), 7.38 (m, 9H), 7.19 (t, J¼7.2 Hz, 3H), 6.89
(t, J¼7.5 Hz, 4H), 6.75 (d, J¼2.0 Hz, 3H), 6.57 (t, J¼7.2 Hz,
3H), 6.44 (d, J¼7.4 Hz, 4H), 3.31 (s, 6H, Ar-CH2-N), 2.65 (s,
4H, Ph-CH2-Ta), 1.26 [s, 27H, C(CH3)3]; 13C NMR
(50.38 MHz, C6D6): d¼153.81, 149.16, 144.56, 139.97, 131.41,
130.96, 129.26, 128.46, 128.37, 128.04, 126.54, 126.27, 124.12,
79.14, 63.02, 34.89, 32.44.
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