Pentabenzyltantalum: Straightforward synthesis, x-ray structure, and application in the synthesis of [O2N]TaBn3-type and [O3N]TaBn2-type complexes

Article abstract of DOI:10.1021/om034066p

The direct reaction between tantalum pentachloride and benzylmagnesium chloride leads to pentabenzyltantalum, previously prepared by a two-step synthesis. The X-ray structure of pentabenzyltantalum reveals a square-pyramidal geometry. Pentabenzyltantalum was found to be an efficient starting material for the preparation of amine-phenolate complexes via toluene elimination reactions. Thus, its reactions with amine bis(phenolate) and amine tris(phenolate) ligand precursors gave the tribenzyltantalum and dibenzyltantalum complexes, respectively, in quantitative yields.

Full text of DOI:10.1021/om034066p

© Copyright 2003
Volume 22, Number 19, September 15, 2003
American Chemical Society
Communications
P en ta ben zylta n ta lu m : Str a igh tfor w a r d Syn th esis, X-r a y
Str u ctu r e, a n d Ap p lica tion in th e Syn th esis of
[O2N]Ta Bn 3-Typ e a n d [O3N]Ta Bn 2-Typ e Com p lexes
Stanislav Groysman, Israel Goldberg, and Moshe Kol*
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences,
Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel
Zeev Goldschmidt*
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
Received J uly 24, 2003
Summary: The direct reaction between tantalum pen-
tachloride and benzylmagnesium chloride leads to pen-
tabenzyltantalum, previously prepared by a two-step
synthesis. The X-ray structure of pentabenzyltantalum
reveals a square-pyramidal geometry. Pentabenzyltan-
talum was found to be an efficient starting material for
the preparation of amine-phenolate complexes via tolu-
ene elimination reactions. Thus, its reactions with amine
bis(phenolate) and amine tris(phenolate) ligand precur-
sors gave the tribenzyltantalum and dibenzyltantalum
complexes, respectively, in quantitative yields.
BnMgCl) to produce TaCl2(benzyl)3, which is further
reacted with dibenzylmagnesium (also prepared from
BnMgCl), giving the homoleptic compound in an overall
yield of 51%.1b All subsequent preparations of this
compound1c,e (as well as that of the related Ta(p-tolyl)5)3
followed this route.4 Recently, we found that the analo-
gous group IV homoleptic compounds, i.e., M(benzyl)4
(M ) Ti, Zr, Hf), serve as convenient entries for making
metal-alkyl compounds of various amine-phenolate
ligands by direct toluene elimination reactions with the
ligand precursors.5 The latter are synthesized by the
straightforward action of 4 equiv of benzylmagnesium
chloride on the corresponding metal tetrachloride.6 It
would thus seem plausible that pentabenzyltantalum
Pentabenzyltantalum was introduced by Schrock in
1976 alongside related compounds that played a revo-
lutionary role in the development of organometallic
chemistry, having led to the discovery of the metal-
alkylidene bond and to the development of the well-
defined olefin metathesis catalysts.1,2 Its synthesis relied
on a two-step reaction sequence: a reaction between
tantalum pentachloride and dibenzylzinc (prepared from
(3) Piersol, C. J .; Profilet, R. D.; Fanvick, F. E.; Rothwell, I. P.
Polyhedron 1993, 12, 1779.
(4) Notably, Thiele reported in the same year that the action of
benzylmagnesium chloride on tantalum pentachloride led to a mixture
of compounds, none being the desired pentabenzyl: Von Ko¨hler, E.;
J acob, K.; Thiele, K.-H. Z. Anorg. Allg. Chem. 1976, 421, 129.
(5) (a) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Orga-
nometallics 2001, 20, 3017. (b) Tshuva, E. Y.; Groysman, S.; Goldberg,
I.; Kol, M.; Goldschmidt, Z. Organometallics 2002, 21, 662. (c) Tshuva,
E. Y.; Goldberg, I.; Kol, M. J . Am. Chem. Soc. 2000, 122, 10706. (d)
Groysman, S.; Goldberg, I.; Kol, M.; Genizi, E.; Goldschmidt, Z. Inorg.
Chim. Acta 2003, 345, 137.
(1) (a) Schrock, R. R. J . Am. Chem. Soc. 1974, 96, 6796. (b) Schrock,
R. R. J . Organomet. Chem. 1976, 122, 209. (c) Malatesta, V.; Ungold,
K. U.; Schrock, R. R. J . Organomet. Chem. 1978, 152, C53. (d) Schrock,
R. R. Acc. Chem. Res. 1979, 12, 98. (e) Messerle, L. W.; J ennische, P.;
Schrock, R. R.; Stucky, G. J . Am. Chem. Soc. 1980, 102, 6744.
(2) For a recent review see: Fu¨rstner, A Angew. Chem., Int. Ed.
2000, 39, 3012.
(6) Zucchini, U.; Alizzati, E.; Giannini, U. J . Organomet. Chem.
1971, 26, 357.
10.1021/om034066p CCC: $25.00 © 2003 American Chemical Society
Publication on Web 08/20/2003

3794 Organometallics, Vol. 22, No. 19, 2003
Communications
Sch em e 1
F igu r e 1. Molecular structure (ORTEP drawing, 50%
probability ellipsoids) of 1. Selected bond lengths (Å) and
angles (deg): Ta-C5 ) 2.167(4), Ta-C4 ) 2.181(4), Ta-
C2 ) 2.205(4), Ta-C6 ) 2.218(4), Ta-C3 ) 2.221(4); C5-
Ta-C4 ) 104.32(14), C5-Ta-C3 ) 113.29(14), C4-Ta-
C2 ) 81.85(15), C4-Ta-C3 ) 82.15(14), C4-Ta-C6 )
138.13(14), C2-Ta-C3 ) 138.59(14).
cursor bearing a dimethylamino donor on the sidearm,
and Lig2H3 is an [O3N]-type trianionic ligand precursor
(Scheme 1). Both ligands feature methyl substituents
in the 2,4-positions of the phenolate rings.
may be a useful starting material for the synthesis of
various benzyl complexes via the toluene elimination
reaction; however, to our knowledge, it has never been
employed as such before. In this paper we report a
straightforward synthesis of Ta(benzyl)5, its X-ray
structure, and its successful application as a starting
material for the synthesis of [O2N]Ta(benzyl)3-type and
[O3N]Ta(benzyl)2-type compounds.
Lig1H2 reacted smoothly with 1 to afford the tribenzyl
complex Lig1Ta(CH2Ph)3 (2) as a yellow solid in quan-
titative yield. The 1H NMR spectrum supports a Cs
symmetrical binding of the [ONO] core to the metal, as
evident from the AB system for the N-CH2 protons, and
a single set of peaks for the two substituted phenolate
groups. Somewhat surprisingly, the three benzyl groups
are nearly equivalent on the NMR time scale at room
temperature, giving rise to two broad absorptions of the
methylene protons that coalesce to a broad singlet upon
heating to 45 °C. The 13C NMR spectrum exhibits the
same pattern: only one flat peak is detected at room
temperature for the benzylic carbon, which sharpens as
the temperature increases. Cooling a toluene-d8 solution
of 2 to ca. -50 °C shows three distinct resonances for
the Ta-CH2 protons. It thus seems that a fluxional
process equilibrates the benzyl groups, without affecting
the mode of binding of the amine bis(phenolate) core.
Single crystals of Lig1Ta(CH2Ph)3 were obtained from
cold ether, and the solid-state structure was determined
by X-ray crystallography (see Figure 2 and the Sup-
porting Information). The geometry at the Ta center is
Our wish for a quicker access to Ta(benzyl)5 prompted
us to retry the Grignard reagent route,4 which, gratify-
ingly, worked out very well. Thus, a slow addition of
5.0 equiv of commercial 1.0 M benzylmagnesium chlo-
ride in ether to a stirred, precooled suspension of
tantalum pentachloride in ether resulted in an immedi-
ate yellow to red-brown color change, and the reaction
mixture was stirred at room temperature for 2 h. NMR
characterization of the crude product indicated that it
consisted of ca. 85% of the desired product and several
impurities. Employing less than 5.0 equiv of BnMgCl
simplifies the purification of Ta(CH2Ph)5 (1), which is
isolated in pure form after workup in ca. 60% yield (see
the Supporting Information). The 1H and 13C NMR
spectra of 1 are identical with those reported by
Schrock.1b Interestingly, the X-ray structure of this
fundamental compound had not been reported before.
Thus, we grew deep red crystals of 1 from a saturated
pentane solution at -35 °C and solved its structure (see
Figure 1 and the Supporting Information). The bond
distances and angles of Ta(CH2Ph)5 were found to
3
closely resemble those of the related Ta(p-tolyl)5, and
the overall geometry around the metal is that of a
distorted square pyramid.
Pentabenzyltantalum was reported to start decom-
posing within a few minutes of heating to 40 °C.1c
However, we found that when it is stored at -35 °C
under a nitrogen atmosphere it does not exhibit ap-
preciable decomposition for weeks and may be employed
successfully in toluene elimination reactions with amine-
phenolate ligand precursors. Two prototypical ligand
skeletons were chosen for the preliminary investiga-
tion: Lig1H2 is an [ONNO]-type dianionic ligand pre-
F igu r e 2. Molecular structure of 2.

Communications
Organometallics, Vol. 22, No. 19, 2003 3795
TaX2-type complexes (i.e. Lig2TaCl2, Lig2Ta(OEt)2, and
Lig2Ta(NMe2)2, etc.), feature Cs symmetry on the NMR
time scale, indicating that no fluxional process occurs
at room temperature.7,9 Upon cooling to as low as
-80 °C, only the disappearance of the benzyl CH2
signals is observed, supporting a slowing but not
complete arresting of the fluxional process. The crystal
structure of Lig2Ta(CH2Ph)2 (Figure 3), however, is
closely related to those of the previously reported [O3N]-
TaX2-type complexes, featuring an overall octahedral
geometry and an approximate Cs symmetry.7,9 Obvi-
ously, the barrier for the fluxional process in the
complexes of the [O3N]TaX2 type is reduced substan-
tially for X ) benzyl.
In conclusion, this convenient one-step synthesis of
Ta(CH2Ph)5 and its efficient transformations into two
novel types of amine phenolate complexes via toluene
elimination reactions demonstrate that, much like its
group IV analogues, Ta(CH2Ph)5 may serve as a useful
entry for the synthesis of tantalum-alkyl complexes.
We are currently investigating the reactivity of the
newly prepared complexes.
F igu r e 3. Molecular structure of Lig2Ta(CH2Ph)2 (3).
that of a distorted octahedron. The amine bis(phenolate)
ligand wraps around the metal in a meridional fashion,
with the sidearm dimethylamino donor unbound. The
central nitrogen-to-metal bond is very weak, it being the
longest (2.513 Å) amid analogous distances in all
previously reported metal complexes of this family. The
Ta-C bond distances and Ta-C-C bond angles lie in
the ranges of 2.248-2.279 Å and 103.60-122.88°,
respectively, and clearly indicate η1 binding of all benzyl
groups.
Ack n ow led gm en t. This research was supported by
the Ministry of Science, Culture and Sport and was also
supported in part by the Israel Science Foundation
founded by the Israel Academy of Sciences and Hu-
manities. S.G. thanks Adi Yeori for technical assistance.
The reaction of the amine tris(phenolate) ligand
precursor Lig2H3 with 1 generated the corresponding
dibenzyl complex Lig2Ta(CH2Ph)2 (3) quantitatively. 3
may also be synthesized (albeit in a lower yield) by
reacting Lig2TaCl2 with 2 equiv of BnMgCl; however,
this approach is less advantageous, as the synthesis of
Lig2TaCl2 is somewhat tedious.7,8 The 1H NMR spec-
trum of 3 displays a singlet for the N-CH2 protons and
a singlet for the benzyl CH2 protons, thereby denoting
a highly fluxional complex of C3v symmetry on the NMR
time scale. In contrast, all previously studied [O3N]-
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures and spectroscopic and crystallographic data
for 1-3; data for the crystal structures of 1-3 are also
available as CIF files. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM034066P
(8) Interestingly, the related [N3N]TaCl2 complex featuring the
tripodal triamidoamine ligand leads to a benzylidene rather than to a
dibenzyl complex upon reaction with benzylmagnesium chloride:
Freundlich, J . S.; Schrock, R. R.; Davis, W. M. J . Am. Chem. Soc. 1996,
118, 3643.
(9) Kim, Y.; Kapoor, P. N.; Verkade, J . G. Inorg. Chem., 2002, 41,
4834.
(7) Groysman, S.; Segal, S.; Shamis, M.; Goldberg, I.; Kol, M.;
Goldcshmidt, Z.; Hayut-Salant, E. J . Chem. Soc., Dalton Trans. 2002,
3425.