Metal-carbon and carbon-oxygen related reactivity in tantalum p-tert-butylcalix[4]arene complexes undergoing organic functionalization

Article abstract of DOI:10.1021/om960666v

[(Calix[4]OMe)TaCl₂] (2), formed from [calix-[4](OMe)₂] (1) and TaCl₅ via a demethylation reaction, undergoes exhaustive alkylation to [(calix[4]OMe)TaR₂] (3-5), which give photochemically or thermally induced concerted dealkylation at the metal and at the methoxy group, to give [calix[4]TaR] (6-8). Double migration of alkyl groups has been observed in the reaction of 3-5 with CO and Bu^tNC to form the corresponding η²-ketones [(calix[4]OMe)Ta(η²-COR₂)] (9-11) and η²-imines [(calix[4]OMe)Ta{η²-N(Bu^t)CR2}] (12-14).

Full text of DOI:10.1021/om960666v

4894
Organometallics 1996, 15, 4894-4896
Meta l-Ca r bon a n d Ca r bon -Oxygen Rela ted Rea ctivity
in Ta n ta lu m p-ter t-Bu tylca lix[4]a r en e Com p lexes
Un d er goin g Or ga n ic F u n ction a liza tion
Barbara Castellano, Antonio Zanotti-Gerosa, Euro Solari, and Carlo Floriani*
Institut de Chimie Mine´rale et Analytique, BCH, Universite´ de Lausanne,
CH-1015 Lausanne, Switzerland
Angiola Chiesi-Villa and Corrado Rizzoli
Dipartimento di Chimica, Universita` di Parma, I-43100 Parma, Italy
Received August 7, 1996X
Summary: [(Calix[4]OMe)TaCl2] (2), formed from [calix-
[4](OMe)2] (1) and TaCl5 via a demethylation reaction,
undergoes exhaustive alkylation to [(calix[4]OMe)TaR2]
(3-5), which give photochemically or thermally induced
concerted dealkylation at the metal and at the methoxy
group, to give [calix[4]TaR] (6-8). Double migration of
alkyl groups has been observed in the reaction of 3-5
with CO and ButNC to form the corresponding η2-
ketones [(calix[4]OMe)Ta(η2-COR2)] (9-11) and η2-imi-
nes [(calix[4]OMe)Ta{η2-N(But)CR2}] (12-14).
The use of calix[4]arenes1 as ancillary ligands2,3 in
organometallic chemistry, though so far extremely
limited,4 is of growing interest in the area of early
transition metals due to the nature of the donor atoms
and their almost planar arrangement provided by the
calix[4]arene skeleton. Such an arrangement mimics
a planar oxo surface and leaves open a face to the metal
for chemical reactivity. This report deals with the
preliminary results in tantalum organometallic chem-
istry5 based on the p-tert-butylcalix[4]arene skeleton.6
It is particularly relevant that we mention the contribu-
tions of the Rothwell7 and Wolczanski8 groups on the
use of alkoxotantalum organometallic chemistry.
The use of calix[4]arene derivatives adds significant
novelties to alkoxotantalum chemistry, namely (i) forc-
ing the metal into a quasi-planar O4 environment, which
results in a significantly different set of orbitals, and
(ii) tuning the charge of the O4 unit, via the methylation
of the oxo groups, thus adapting it to the oxidation state
and the functionalization degree desired by the metal.
We should anticipate that the methoxy group not only
is a spectator fragment determining the overall charge
of the O4 set but also influences the metal reactivity
patterns, via various demethylation pathways. This has
been observed even in the synthesis of the parent
compound 29 obtained from the reaction of 110 with
TaCl5 (Scheme 1), occurring with the demethylation of
one of the two methoxy groups. The metal is bonded to
the trianionic methoxy-p-tert-butylcalix[4]arene (calix-
[4]-OMe) residue. Complex 2 undergoes easy alkylation
with either organomagnesium or organolithium deriva-
tives, occurring with negligible concurrent reduction of
the metal, to give the corresponding dialkyl derivatives
(R ) Me, 3; R ) PhCH2,11 4; R ) p-MeC6H4, 5). The
synthesis is reported in detail for 4. The dialkyl
derivatives 3-5 are rather stable, but under different
conditions, depending on the nature of the alkyl, they
undergo photochemically (R ) PhCH2, 7;12
R )
p-MeC6H4, 8) or, in pyridine, thermally (R ) Me, 6; R
) PhCH2, 7; R ) p-MeC6H4, 8) concerted dealkylation
at one of the methoxy groups, as shown by NMR
experiments (Scheme 1). The transformation of com-
pound 4 to 7 was also achieved by reacting 4 with H2.12
The experimental conditions of this reaction are re-
ported. Although all the demethylations accompanying
both the synthesis of 2 and the conversion of 3-5 to
6-8 are mechanistically difficult to define at present,
they are synthetically very important, since one of the
methoxy substituents is masking an anionic site. This
will intervene, when necessary, during the reaction
X Abstract published in Advance ACS Abstracts, October 15, 1996.
(1) (a) Gutsche, C. D. Calixarenes; Royal Society of Chemistry:
Cambridge, U.K., 1989. (b) Calixarenes, A Versatile Class of Macrocyclic
Compounds; Vicens, J ., Bo¨hmer, V., Eds.; Kluwer: Dordrecht, The
Netherlands, 1991.
(2) Olmstead, M. M.; Sigel, G.; Hope, H.; Xu, X.; Power, P. P. J . Am.
Chem. Soc. 1985, 107, 8087.
(3) (a) Corazza, F.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J .
Chem. Soc., Chem. Commun. 1990, 640. (b) Corazza, F.; Floriani, C.;
Chiesi-Villa, A.; Guastini, C. J . Chem. Soc., Chem. Commun. 1990,
1083. (c) Corazza, F.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Inorg.
Chem. 1991, 30, 4465. (d) Acho, J . A.; Ren, T.; Yun, J . W.; Lippard, S.
J . Inorg. Chem. 1995, 34, 5226. (e) Acho, J . A.; Lippard, S. J . Inorg.
Chim. Acta 1995, 229, 5. (f) Zanotti-Gerosa, A.; Solari, E.; Giannini,
L.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J . Chem. Soc., Chem.
Commun. 1996, 119. (g) Gibson, V. C.; Redshaw, C.; Clegg, W.;
Elsegood, M. R. J . J . Chem. Soc., Chem. Commun. 1995, 2371.
(4) A single case has been reported of M-C reactive functionalities
over a calix[4]arene moiety: Giannini, L.; Solari, E.; Zanotti-Gerosa,
A.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Angew. Chem., Int. Ed. Engl.
1996, 35, 85.
(8) Bonanno, J . B.; Lobkovsky, E. B.; Wolczanski, P. T. J . Am. Chem.
Soc. 1994, 116, 11159. Miller, R. L.; Toreki, R.; LaPointe, R. E.;
Wolczanski, P. T.; Van Duyne, G. D.; Roe, D. C. J . Am. Chem. Soc.
1993, 115, 5570.
(9) Procedure for 2: TaCl5 (20.95 g, 58.5 mmol) was added to a
toluene (400 mL) solution of 1 (39.2 g, 58.0 mmol), and the yellow
mixture was refluxed for 36 h. The solvent was removed in vacuo, and
the yellow residue was washed with hexane (200 mL) and collected
(43.2 g, 74%). Crystals suitable for X-ray analysis were grown in a
saturated benzene-hexane (1/1) solution. Anal. Calcd for 2‚C7H8,
C45H55Cl2O4Ta‚C7H8: C, 62.20; H, 6.34. Found: C, 61.78; H, 6.12. 1H
NMR (C6D6, 200 MHz): δ 0.67 (s, But, 9H), 0.73 (s, But, 9H), 1.34 (s,
But, 18H), 2.1 (s, C7H8, 3H), 3.15 (d, CH2-calix, 2H, J ) 13.3 Hz), 3.3
(d, CH2-calix, 2H, J ) 13.8 Hz), 3.95 (s, OCH3, 3H), 4.36 (d, CH2-calix,
2H, J ) 13.3 Hz), 4.97 (d, CH2-calix, 2H, J ) 13.8 Hz), 7.00 (m, H
arom, 13H).
(5) Wigley, D. E.; Gray, S. D. In Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: Oxford, U.K., 1995; Vol. 5, Chapter 2.
(6) A [(η5-C5Me5)Ta(calix[4]arene)] species which does not contain
any reactive Ta-C bond has been reported: Acho, J . A.; Doerrer, L.
H.; Lippard, S. J . Inorg. Chem. 1995, 34, 2542.
(7) Parkin, B. C.; Clark, J . R.; Visciglio, V. M.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1995, 14, 3002. Clark, J . R.; Fanwick,
P. E.; Rothwell, I. P. J . Chem. Soc., Chem. Commun. 1995, 553. Yu, J .
S.; Ankianiec, B. C.; Rothwell, I. P.; Nguyen, M. T. J . Am. Chem. Soc.
1992, 114, 1927. Chesnut, R. W.; J acob, G. G.; Yu, J . S.; Fanwick, P.
E.; Rothwell, I. P. Organometallics 1991, 10, 321.
(10) Casnati, A.; Arduini, A.; Ghidini, E.; Pochini, A.; Ungaro, R.
Tetrahedron 1991, 47, 2221 and references therein.
S0276-7333(96)00666-8 CCC: $12.00 © 1996 American Chemical Society

Communications
Organometallics, Vol. 15, No. 23, 1996 4895
Sch em e 1
F igu r e 1. ORTEP drawing of complex 10. Selected bond
distances (Å) and angles (deg) are as follows: Ta1-O1,
1.895(5); Ta1-O2, 2.297(6); Ta1-O3, 1.900(5); Ta1-O4,
1.971(5); Ta1-O5, 1.942(5); Ta1-C46, 2.100(9); O5-C46,
1.405(11); C46-C47, 1.503(12); C46-C48, 1.553(11). O5-
Ta1-C46, 40.4(3); O3-Ta1-O4, 90.0(3); O2-Ta1-O4,
157.8(3); O2-Ta1-O3, 81.1(2); O1-Ta1-O4, 91.0(2); O1-
Ta1-O3, 136.0(3); O1-Ta1-O2, 81.8(2); Ta1-O5-C46,
75.8(4); Ta1-C46-O5, 63.7(4). For the disordered atoms
only the A position is given for clarity.
Details are reported on the synthesis and X-ray
structure of 10, shown in Figure 1, along with selected
structural parameters.16 The calix[4]arene skeleton
assumes an elliptical cone conformation, as determined
by the distances between para carbon atoms: C4‚‚‚C17,
9.181(11) Å; C10‚‚‚C24, 7.306(11) Å. This is the conse-
pathway of the metal. Among the preliminary explora-
tions we chose to engage 3-5 into migratory insertion
reactions with CO and RNC5,13 (Scheme 1). In both
cases we observed the migration of both alkyls to the
inserting ligand, to give the corresponding η2-ketones
(9-11)14 and η2-imines (12-14) via the expected inter-
mediacy of the corresponding η2-acyl and η2-iminoacyl,
respectively.13,15 Complexes 9-11 and 12-14 do not
undergo, even under forced conditions, thermally, pho-
tochemically, or base (i.e. pyridine) induced demethy-
lation of the methoxy group with concomitant migration
of the methyl to the η2-imino or η2-ketone functionality,
which allows us to make a direct comparison with
reactions occurring on metal-oxo surfaces.
(14) Procedure for 10: a toluene (120 mL) suspension of 4 (2.34 g,
2.3 mmol) was saturated with CO. Stirring at room temperature for 2
h resulted in a yellow solution. The solvent was removed in vacuo and
the yellow residue was dissolved in n-hexane (50 mL). The solution
was filtered and kept at -30 °C for 12 h, yielding a white precipitate
which was then collected and dried in vacuo (1.37 g, 57%). Crystals
suitable for X-ray analysis were obtained by cooling a saturated
solution of 10 in hexane/benzene (10/1) from room temperature to 2
°C. Anal. Calcd for 10, C60H69O5Ta: C, 68.54; H, 6.63. Found: C, 68.62;
H, 7.33. 1H NMR (C6D6, 200 MHz): δ 0.67 (s, But, 9H), 0.83 (s, But,
9H), 1.38 (s, But, 18H), 3.1 (d, CH2-calix, 2H, J ) 12.5 Hz), 3.26 (d,
CH2-calix, 2H, J ) 12.5 Hz), 4.06 (d, CH2-calix, 2H, J ) 12.5 Hz), 4.15
(d, CH2Ph, 2H, J ) 14.7 Hz), 4.32 (s, OCH3, 3H), 4.42 (d, CH2Ph, 2H,
J ) 14.7 Hz), 4.94 (d, CH2-calix, 2H, J ) 12.5 Hz), 7.02 (m, H arom,
14H), 7.70 (d, H arom, 4H, J ) 8.28 Hz).
(11) Procedure for 4: a THF solution (10.96 mL) of (PhCH2)2Mg (0.34
M, 3.73 mmol) was added dropwise to a yellow toluene (150 mL)
solution of 2 (3.74 g, 3.73 mmol) at room temperature. The mixture
turned orange in seconds, and after 15 min dioxane (6 mL) was added.
The reaction mixture was stirred for 1 h, salts were filtered off, and
volatiles were removed in vacuo. The yellow residue was washed with
n-hexane (100 mL), collected, and dried (1.6 g, 42%). Anal. Calcd for
4, C59H69O4Ta: C, 69.25; H, 6.81. Found: C, 69.35; H, 7.10. 1H NMR
(C6D6, 200 MHz): δ 0.67 (s, But, 9H), 0.72 (s, But, 9H), 1.39 (s, But,
18H), 3.12 (d, CH2-calix, 2H, J ) 13.3 Hz), 3.19 (s, OCH3, 3H), 3.23 (d,
CH2-calix, 2H, J ) 12.6 Hz), 3.6 (brd s, CH2Ph, 4H), 4.31 (d, CH2-
calix, 2H, J ) 13.3 Hz), 4.44 (d, CH2-calix, 2H, J ) 12.6 Hz), 7.16 (m,
H arom, 18H). Solutions of the product are thermally stable (benzene,
100 °C, 24 h) but photosensitive. A sample in C6D6 exposed to light
for 24 h gave, as shown by the NMR spectrum, a mixture of 7 (>80%)
and another unidentified product (1H NMR singlet at δ 9.63). A sample
heated for 2 h at 80 °C in CD5N gave quantitatively compound 7.
(12) Procedure for 7: a benzene (250 mL) suspension of 4 (5.1 g,
5.0 mmol) was saturated with H2. Stirring at room temperature for 6
days resulted in the yellow mixture turning orange. The solvent was
removed in vacuo, and the light yellow-brown residue was dissolved
in n-hexane (60 mL). The solution was then kept at -30 °C for 36 h,
yielding a light yellow-brown precipitate of 7, which was collected and
dried. Anal. Calcd for C51H59O4Ta: C, 66.79; H, 6.49. Found: C, 66.66;
H, 6.67. 1H NMR (C6D5, 200 MHz): δ 1.09 (s, 36H, But), 3.28 (d, J )
12.2 Hz, 4H, CH2-calix), 3.62 (bs, 2H, CH2Ph), 4.98 (d, J ) 12.2 Hz,
4H, CH2-calix), 7.06 (s, 8H, H arom-calix), 7.37 (m, 5H, arom-CH2Ph).
(13) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059. The
review contains references also to the related Zr-alkoxo-based orga-
nometallic chemistry.
(15) Chamberlain, L. R.; Steffey, B. D.; Rothwell, I. P.; Huffman, J .
C. Polyhedron 1989, 8, 341. Chamberlain, L. R.; Durfee, L. D.; Fanwick,
P. E.; Kobriger, L. M.; Latesky, S. L.; McMullen, A. K.; Steffey, B. D.;
Rothwell, I. P.; Folting, K.; Huffman, J . C. J . Am. Chem. Soc. 1987,
109, 6068.
(16) Crystal data for 10: C60H69O5Ta‚1.5C6H6‚0.5C6H14, Mr )
1211.4, triclinic, space group P1h, a ) 16.399(8) Å, b ) 17.074(9) Å, c )
11.854(5) Å, R ) 99.10(5)°, â ) 107.92(3)°, γ ) 88.70(4)°, V ) 3117(3)
Å3, Z ) 2, Fcalcd ) 1.291 g cm-3, F(000) ) 1260, λ(Mo KR) ) 0.710 69 Å,
µ(Mo KR) ) 17.88 cm-1: crystal dimensions 0.22 × 0.52 × 0.64 mm.
The structure was solved by the heavy-atom method and anisotropi-
cally refined for all non-hydrogen atoms except for those affected by
disorder. Some But groups were affected by high thermal parameters,
indicating the presence of disorder. The best fit was found by splitting
the C35 and C36 atoms over two positions (A and B) isotropically
refined with site occupation factors of 0.5. All the hydrogen atoms but
those related to the disordered carbon atoms, which were ignored, were
put in geometrically calculated positions and introduced as fixed
contributors in the last stage of refinement (Uiso ) 0.05 Å2). For 8529
unique observed reflections (I > 2σ(I)) collected at T ) 133 K on a
Rigaku AFC6S diffractometer (5 < 2θ < 50°) and corrected for
absorption the final R is 0.061 (wR2 ) 0.165 for the 9825 reflections
having I > 0 used in the refinement). All calculations were carried
out on a Quansan Personal Computer equipped with an Intel Pentium
processor. Atomic coordinates, bond lengths and angles, and thermal
parameters have been deposited at the Cambridge Crystallographic
Data Centre, University Chemical Laboratory, Lensfield Road, Cam-
bridge CB2 1EW, U.K. See the Supporting Information for more
details.

4896 Organometallics, Vol. 15, No. 23, 1996
Communications
quence of the rather irregular trend in the Ta-O bond
distances: they are quite close in the case of Ta-O1
and Ta-O3 (mean value 1.897(5) Å), significantly longer
for Ta-O4 (1.971(5) Å), and much longer for the
methoxy oxygen Ta-O2 (2.297(6) Å). The short Ta-O
bond distances along with the wide Ta-O-C angles
involving O1 and O3 (Ta-O1-C1, 152.4(5)°; Ta-O3-
C20, 154.9(5)°) support a Ta-O multiple-bond interac-
tion.6
The O4 plane shows tetrahedral distortions varying
within (0.143(6) Å, the metal being displaced by
0.550(3) Å from the mean plane toward the η2-ketone.
The plane of the η2-ketone is perpendicular to the O4
mean plane, the dihedral angle being 90.6(3)°. The
asymmetric η2-bonding mode of dibenzyl ketone
(Ta-O5, 1.942(5) Å; Ta-C46, 2.100(9) Å) and the O5-
C46 distance (1.405(11) Å) are in agreement with other
η2-metal-bonded ketones.13 The potential of the Ta-C,
Ta-H bond chemistry over a calix[4]arene skeleton
remains largely unexplored, along with the use of 2 as
a starting material for accessing low-valent tantalum
species. In addition, we can take advantage, in syn-
thesis, of the controlled demethylation of one of the
methoxy groups.
Ack n ow led gm en t. We thank the Fonds National
Suisse de la Recherche Scientifique (Bern, Switzerland,
Grant No. 20-40268.94) and Ciba-Geigy SA (Basel,
Switzerland) for financial support.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, atomic coordinates, thermal parameters, and bond
distances and angles for complex 10 (10 pages). Ordering
information is given on any current masthead page.
OM960666V