The chemistry of vanadium-carbon bond functionalities over an oxo-surface defined by the calix[4]arene skeleton: The redox relationship between vanadium(III) and vanadium(IV) assisted by carbon-oxygen bond cleavage

Article abstract of DOI:10.1021/om9800389

This report deals with the chemistry of V-C functionalities anchored to a quasi-planar O₄ matrix represented here by the [p-Bu^t-calix[4]-(OMe)₂(O)₂]^2- macrocycle. The starting material [p-Bu^t-calix[4]-(OMe)₂(O)₂V-Cl], 2, has been converted into the corresponding alkyl and aryl derivatives [p-Bu^t-Calix[4]-(OMe)₂(O)₂V-R] (R = Me, 3; R = CH₂Ph, 4; R = p-MeC₆H₄, 5) by conventional procedures. The structure of 5 reveals the self-assembling of the monomeric unit into a columnar arrangement with the p-tolyl substituent functioning as a guest moiety in a neighboring unit. Complexes 3-5 undergo migratory insertion reactions with CO and Bu^tNC to the corresponding η²-acyls [p-Bu^t-calix[4]-(OME)₂(O)₂V(η ²-COR)] (R = Me, 6; R = CH₂Ph, 7) and η²-immoacyls [p-Bu^t-calix[4]-(OMe)₂(O)₂V{η ²-C(NBU^t)R}] (R = Me, 8; R = CH₂Ph, 9; R = p-MeC₆H₄, 10), the reaction with CO being strongly dependent on the R substituent. The structural model for the hexacoordinate intermediate leading to the inserted product is exemplified by the d² high-spin [p-Bu^t-calix-[4]-(OMe)₂(O)₂V(Cl)(CNBu ^t], 11, and proves the presence of four available orbitals at the metal for driving the reactivity in the [p-Bu^t-calix[4]-(OMe)₂(O)₂V] fragment. The bonding mode of the organic functionalities at the metal and the conversion of the alkyl substituent into the η²-acyl have been inspected by extended Hu?ckel calculations. The oxidative demethylation of 3 and 5 by a controlled amount of iodine opened an interesting synthetic route to the corresponding vanadium(IV) organometallic derivatives [p-Bu^t-calix[4]-(OMe)-(O)₃V-X] (X = Cl, 12; X = p-MeC₆H₄, 13). A structural report on 2, 5, and 9 is included.

Full text of DOI:10.1021/om9800389

2328
Organometallics 1998, 17, 2328-2336
Th e Ch em istr y of Va n a d iu m -Ca r bon Bon d
F u n ction a lities over a n Oxo-Su r fa ce Defin ed by th e
Ca lix[4]a r en e Sk eleton : Th e Red ox Rela tion sh ip
betw een Va n a d iu m (III) a n d Va n a d iu m (IV) Assisted by
Ca r bon -Oxygen Bon d Clea va ge
Barbara Castellano, Euro Solari, and Carlo Floriani*
Institut de Chimie Mine´rale et Analytique, BCH, Universite´ de Lausanne,
CH-1015 Lausanne, Switzerland
Nazzareno Re
Dipartimento di Chimica, Universita` di Perugia, I-06100 Perugia, Italy
Angiola Chiesi-Villa and Corrado Rizzoli
Dipartimento di Chimica, Universita` di Parma, I-43100 Parma, Italy
Received J anuary 22, 1998
This report deals with the chemistry of V-C functionalities anchored to a quasi-planar
O4 matrix represented here by the [p-But-calix[4]-(OMe)2(O)2]2- macrocycle. The starting
material [p-But-calix[4]-(OMe)2(O)2V-Cl], 2, has been converted into the corresponding alkyl
and aryl derivatives [p-But-calix[4]-(OMe)2(O)2V-R] (R ) Me, 3; R ) CH2Ph, 4; R )
p-MeC6H4, 5) by conventional procedures. The structure of 5 reveals the self-assembling of
the monomeric unit into a columnar arrangement with the p-tolyl substituent functioning
as a guest moiety in a neighboring unit. Complexes 3-5 undergo migratory insertion
reactions with CO and ButNC to the corresponding η2-acyls [p-But-calix[4]-(OMe)2(O)2V(η2-
COR)] (R ) Me, 6; R ) CH2Ph, 7) and η2-iminoacyls [p-But-calix[4]-(OMe)2(O)2V{η2-
C(NBut)R}] (R ) Me, 8; R ) CH2Ph, 9; R ) p-MeC6H4, 10), the reaction with CO being
strongly dependent on the R substituent. The structural model for the hexacoordinate
intermediate leading to the inserted product is exemplified by the d2 high-spin [p-But-calix-
[4]-(OMe)2(O)2V(Cl)(CNBut], 11, and proves the presence of four available orbitals at the
metal for driving the reactivity in the [p-But-calix[4]-(OMe)2(O)2V] fragment. The bonding
mode of the organic functionalities at the metal and the conversion of the alkyl substituent
into the η2-acyl have been inspected by extended Hu¨ckel calculations. The oxidative
demethylation of 3 and 5 by a controlled amount of iodine opened an interesting synthetic
route to the corresponding vanadium(IV) organometallic derivatives [p-But-calix[4]-(OMe)-
(O)3V-X] (X ) Cl, 12; X ) p-MeC6H4, 13). A structural report on 2, 5, and 9 is included.
In tr od u ction
the recent impact that silica-supported vanadium cata-
lysts have had in olefin polymerization6 and to the
The calix[4]arene skeleton1 provides a unique preor-
ganized O4 quasi-planar environment for organometallic
functionalities2 which can, in turn, be adapted to the
oxidation state and the required degree of functional-
ization of the metal via the appropriate alkylation of
the phenoxo oxygens. Although the oxygen atom envi-
ronment is common in vanadium chemistry,3 it is rarely
used to support the V-C functionality.4 It is also
surprising that the rich organometallic chemistry of
niobium and tantalum,5 based on alkoxo and phenoxo
ligands, is not mirrored for vanadium. In light of these
comments, it is appropriate that we draw attention to
(2) (a) Giannini, L.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli,
C. J . Am. Chem. Soc. 1998, 120, 823-824. (b) Zanotti-Gerosa, A.;
Solari, E.; Giannini, L.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J . Am.
Chem. Soc. 1998, 120, 437. (c) Giannini, L.; Caselli, A.; Solari, E.;
Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.; Re, N.; Sgamellotti, A. J . Am.
Chem. Soc. 1997, 119, 9198. (d) Ibid. 1997, 119, 9709. (e) Caselli, A.;
Giannini, L.; Solari, E.; Floriani, C.; Re, N.; Chiesi-Villa, A.; Rizzoli,
C. Organometallics 1997, 16, 5457. (f) Giannini, L.; Solari, E.; Zanotti-
Gerosa, A.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Angew. Chem., Int.
Ed. Engl. 1997, 36, 753. (g) Giannini, L.; Solari, E.; Zanotti-Gerosa,
A.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Angew. Chem., Int. Ed. Engl.
1996, 35, 2825. (h) Castellano, B.; Zanotti-Gerosa, A.; Solari, E.;
Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1996, 15, 4894.
(i) Giannini, L.; Solari, E.; Zanotti-Gerosa, A.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. Angew. Chem., Int. Ed. Engl. 1996, 35, 85. (j) Gardiner,
M. G.; Lawrence, S. M.; Raston, C. L.; Skelton, B. W.; White, A. H.
Chem. Commun. 1996, 2491. (k) Gardiner, M. G.; Koutsantonis, G.
A.; Lawrence, S. M.; Nichols, P. J .; Raston, C. L. Chem. Commun. 1996,
2035. (l) Gibson, V. C.; Redshaw, C.; Clegg, W.; Elsegood, M. R. J . J .
Chem. Soc., Chem. Commun. 1995, 2371. (m) Acho, J . A.; Doerrer, L.
H.; Lippard, S. J . Inorg. Chem. 1995, 34, 2542.
* To whom correspondence should be addressed.
(1) (a) Gutsche, C. D. Calixarenes; The 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.
S0276-7333(98)00038-7 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 05/02/1998

Redox Relationship between V(III) and V(IV)
Organometallics, Vol. 17, No. 11, 1998 2329
MS analyses were carried out using a HP 5890 Series II system
and a HP 5890A GC system, respectively. The magnetic
measurements were carried out using a SQUID Quantum
Design magnetometer. The syntheses of [p-But-calix[4]-(OMe)2-
(OH)2], 1,9 and VCl3‚THF310 were performed as reported in the
literature.
Syn th esis of 2. A n-hexane solution of BuLi (48.8 mL, 90.7
mmol) was added dropwise to a THF suspension (260 mL) of
1 (30.8 g, 45.4 mmol). The orange solution was then added
dropwise to a violet THF suspension (260 mL) of VCl3(THF)3
(17.2 g, 46.0 mmol). After the mixture was stirred for 8 h,
the suspension was evaporated to dryness and the green solid
residue extracted with benzene (50 mL) and n-hexane (250
mL). The product was then collected (30.2 g, 87%). Anal.
Calcd for 2, C46H58ClO4V: C, 72.55; H, 7.68. Found: C, 72.91;
H, 7.69. Crystals were grown in toluene. µeff ) 2.69 µB at 300
K.
pioneering work of Feher who used silasesquioxanes as
ancillary ligands to better understand the nature of the
active silica-supported vanadium species.7 An interest-
ing relationship between the use of calix[4]arene as an
ancillary ligand and other oxygen-supported organome-
tallic functionalities has been found.8 The purpose of
this paper is, therefore, to report the genesis and
reactivity of the V-C functionality anchored to the
dimethoxy-p-tert-butylcalix[4]arene dianion (A) and the
oxidative demethylation of the ligand as an entry into
the analogous vanadium(IV) organometallic chemistry.
Syn th esis of 3. A diethyl ether solution of MeLi (6.50 mL,
11.52 mmol) was added dropwise to a benzene suspension (200
mL) of 2 (8.77 g, 11.52 mmol). After the mixture was stirred
for 12 h, a dark suspension formed, which was then evaporated
to dryness. The solid residue was extracted with diethyl ether.
The solvent was removed in vacuo, and the residue was kept
in n-hexane (60 mL) at 9 °C for 24 h and then collected as a
violet powder (4.80 g, 51%). Anal. Calcd for 3‚Et2O,
The extended Hu¨ckel calculations allowed us to define
the frontier orbitals of the [calix[4]-(OMe)2(O)2V] frag-
ment engaged in the transformation of the V-C bonds
and into the conversion of vanadium(III) to vanadium-
(IV) organometallics.
C
51H71O5V: C, 75.14; H, 8.80. Found: C, 74.77; H, 8.48.
Exp er im en ta l Section
Syn th esis of 4. A THF solution of (PhCH2)2Mg (18.65 mL,
5.0 mmol) was added dropwise to a benzene suspension (200
mL) of 2 (7.67 g, 10.0 mmol). After the mixture was stirred
for 4 h, a green solution formed. Dioxane (3 mL) was added,
and after 2 h the suspension was filtered. The solvent was
removed in vacuo, and the residue was kept in hexane (60 mL)
at 9 °C for 24 h and finally collected (5.37 g, 66%). Anal. Calcd
for 4, C53H65O4V: C, 77.90; H, 8.03. Found: C, 77.48; H, 8.13.
µeff ) 2.60 µB at 300 K.
Syn th esis of 5. To a stirred benzene suspension (200 mL)
of 2 (7.80 g, 10.24 mmol) was added dropwise a THF solution
of p-MeC6H4MgBr (21.3 mL, 10.24 mmol). Stirring for 4 h
resulted in a green solution. Dioxane (3 mL) was added, and
after 2 h, the suspension was filtered. The solvent was
removed in vacuo, and the residue was kept in hexane (60 mL)
at 9 °C for 24 h and then collected (6.45 g, 77%). Anal. Calcd
for 5, C53H65O4V: C, 77.90; H, 8.03. Found: C, 77.55; H, 8.10.
Gen er a l P r oced u r e. All reactions were carried out under
an atmosphere of purified nitrogen. Solvents were dried and
distilled before use by standard methods. 1H NMR and IR
spectra were recorded on Bruker AC-200 and DPX-400 and
Perkin-Elmer FT 1600 instruments, respectively. GC and GC-
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Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J .
A., Eds.; Pergamon: Oxford, 1987; Vol. 3, Chapter 33, p 453. (b)
Vanadium in Biological Systems; Chasteen, N. D. Ed.; Kluwer:
Dordrecht, The Netherlands, 1990. (c) Mazzanti, M.; Floriani, C.;
Chiesi-Villa, A.; Guastini, C. J . Chem. Soc., Dalton Trans. 1989, 1793.
(d) Mazzanti, M.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Angew.
Chem., Int. Ed. Engl. 1988, 27, 576.
(4) Seidel, W.; Kreisel, G. Z. Chem. 1981, 21, 295. Preuss, F.; Ogger,
L. Z. Naturforsch. 1982, 37b, 957. Seangprasertkij, R.; Riechel, T. L.
Inorg. Chem. 1986, 25, 3121. Preuss, F.; Becker H. Z. Naturforsch.
1986, 41b, 185.
(5) 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. 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. Wigley,
D. E.; Gray, S. D. In Comprehensive Organometallic Chemistry II; Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995;
Vol. 5, Chapter 2, pp 57-153.
(6) Karol, F. J .; Cann, K. J .; Wagner, B. E. In Transition Metals
and Organometallics as Catalysts for Olefin Polymerization; Kaminsky,
W., Sinn, H., Eds.; Springer: New York, 1988; pp 149-161.
(7) Feher, F. J .; Newman, D. A.; Walzer, J . F. J . Am. Chem. Soc.
1989, 111, 1741. Feher, F. J .; Walzer, J . F. Inorg. Chem. 1991, 30,
1689. Feher, F. J .; Walzer, J . F.; Blanski, R. L. J . Am. Chem. Soc. 1991,
113, 3618. Feher, F. J .; Blanski, R. L. J . Am. Chem. Soc. 1992, 114,
5886. Feher, F. J .; Blanski, R. L. Organometallics 1993, 12, 958. Feher,
F. J .; Budzichowski, T. A. Polyhedron 1995, 14, 3239 and references
therein.
(8) For related molecular approaches to oxo-surfaces binding orga-
nometallic functionalities see: Chisholm, M. H. Chemtracts-Inorg.
Chem. 1992, 4, 273. Kla¨ui, W. Angew. Chem., Int. Ed. Engl. 1990×e229,
627. Feher, F. J .; Budzichowski, T. A. Polyhedron 1995, 14, 3239.
Nagata, T.; Pohl, M.; Weiner, H.; Finke, R. G. Inorg. Chem. 1997, 36,
1366. Pohl, M.; Lyon, D. K.; Mizuno, N.; Nomiya, K.; Finke, R. G. Inorg.
Chem. 1995, 34, 1413. Corker, J .; Lefebvre, F.; Lecuyer, C.; Dufaud,
V.; Quignard, F.; Choplin, A.; Evans, J .; Basset, J .-M. Science 1996,
271, 966. Niccolai, G. P.; Basset, J .-M. Appl. Catal., A 1996, 146, 145.
Vidal, V.; Theolier, A.; Thivolle-Cazat, J .; Basset, J .-M.; Corker, J . J .
Am. Chem. Soc. 1996, 118, 4595. Mechanisms of Reactions of Orga-
nometallic Compounds with Surfaces; Cole-Hamilton, D. J ., Williams,
J . O., Eds.; Plenum: New York, 1989.
Green crystals were grown in
solution at room temperature.
a saturated THF/benzene
Syn t h esis of 6. A benzene (200 mL) solution of 3‚Et2O
(2.86 g, 3.51 mmol) was saturated with CO, resulting in the
solution turning red within seconds. After the mixture was
stirred for 24 h, the solvent was removed and the residue
washed with n-hexane (60 mL) and then collected as a red
microcrystalline solid (1.60 g, 59%). Anal. Calcd for 6,
C48H61O5V: C, 74.98; H, 7.99. Found: C, 74.96; H, 8.01. IR
(Nujol, νmax/cm-1): 1599.3 (w), 1581.8 (m).
Syn th esis of 7. A green benzene (100 mL) solution of 4
(2.78 g, 3.40 mmol) was saturated with CO at room temper-
ature. The solution turned red within 1 h and was then kept
stirring overnight. The solvent was removed, and the remain-
ing solid residue was kept in hexane (60 mL) at 9 °C for 24 h
and then collected as a dark red powder (2.0 g, 70%). Anal.
Calcd for 7, C54H65O5V: C, 76.74; H, 7.78. Found: C, 76.72;
H, 8.70. IR (Nujol, νmax/cm-1): 1599.7 (m), 1571.9 (s). µeff
2.68 µB at 300 K.
)
Syn th esis of 8. A benzene (30 mL) solution of ButNC
(0.221 g, 2.66 mmol) was added dropwise to a green benzene
solution (80 mL) of 3‚Et2O (2.17 g, 2.66 mmol) and the
resulting dark yellow solution was stirred overnight. The
solvent was evaporated to dryness and the residue was kept
(9) Arduini, A.; Casnati, A. In Macrocycle Synthesis; Parker, O., Ed.;
Oxford University Press: New York, 1996; Chapter 7.
(10) Manzer, L. E. Inorg. Synth. 1982, 21, 135.

2330 Organometallics, Vol. 17, No. 11, 1998
Castellano et al.
in n-hexane (60 mL) at 9 °C for 24 h and finally collected as a
yellow microcrystalline solid (1.42 g, 53%). Anal. Calcd for
8‚C6H14, C52H70NO4V‚C6H14, C58H84NO4V: C, 76.52; H, 9.32;
(0.65 g, 74%). Anal. Calcd for 14‚2C4H8O, C96H120O10V2: C,
75.02; H, 7.89. Found: C, 75.28; H, 8.67. A sample of the
crude reaction product was analyzed by CG-MS: CH3I and
p-iodotoluene were identified.
N, 1.54. Found: C, 76.36; H, 9.00; N, 1.30. IR (Nujol, νmax
/
cm-1): 1613.7 (w), 1599.1 (m).
X-r a y Cr ysta llogr a p h y for Com p lexes 2, 5, a n d 9.
Suitable crystals were mounted in glass capillaries and sealed
under nitrogen. The reduced cells were obtained with the use
of TRACER.11 Crystal data and details associated with data
collection are given in Tables 1 and S1 (Supporting Informa-
tion). Data were collected on a single-crystal diffractometer
(Siemens AED for 2 and Rigaku AFC6S for 5 and 9) at 295 K
for 2 and 5 and 133 K for 9. For intensities and background,
the individual reflection profiles were analyzed.12 The struc-
ture amplitudes were obtained after the usual Lorentz and
polarization corrections, and the absolute scale was established
by the Wilson method.13 The crystal quality was tested by ψ
scans, showing that crystal absorption effects could not be
neglected. Data for all complexes were then corrected for
absorption using ABSORB14 for 2 and a semiempirical method15
for 5 and 9. The function minimized during the full-matrix
least-squares refinements was ∑w(∆F2)2. Anomalous scatter-
ing corrections were included in all structure factor calcu-
lations.16b Scattering factors for neutral atoms were taken
from ref 16a for non-hydrogen atoms and from ref 17 for H.
Structure solutions18 were based on the observed reflections
[I > 2σ(I)], while the refinements were based on the unique
reflections having I > 0.19 The structures were solved by the
heavy-atom method starting from a three-dimensional Patter-
son map. Refinements were done by full-matrix least-squares,
first isotropically and then anisotropically for all non-H atoms
except for the disordered atoms. For all complexes, the
hydrogen atoms were located in a difference Fourier map and
introduced in the refinements as fixed atom contributions (Uiso
) 0.10 Å2 for 2 and 5 and 0.05 Å2 for 9). The H atoms
associated to the disordered carbon atoms were ignored as well
as the H atoms of the C55 methyl group in complex 5, which
are required to be statistically distributed over two positions.
In the last stage of refinement, the weighting scheme w ) 1/[σ2-
Syn th esis of 9. A benzene (20 mL) solution of ButNC (0.30
g, 3.72 mmol) was added dropwise to a benzene solution (80
mL) of 4 (3.04 g, 3.72 mmol) and the resulting dark orange
solution was stirred overnight. The solvent was evaporated
to dryness, and the residue was washed with n-hexane (60
mL), kept in n-hexane at 9 °C for 24 h, and finally collected
as an orange microcrystalline solid (2.65 g, 79%). Crystals
were grown in a mixture (1/1) of hexane-benzene. Anal.
Calcd for 9, C58H74NO4V: C, 77.37; H, 8.30; N, 1.55. Found:
C, 77.14; H, 8.24; N, 1.27. IR (Nujol, νmax/cm-1): 1625.6 (s),
1598.1 (s). µeff ) 2.75 µB at 300 K.
Syn t h esis of 10. A benzene (30 mL) solution of ButNC
(0.20 g, 2.45 mmol) was added dropwise to a benzene solution
(80 mL) of 5 (2.00 g, 2.45 mmol), and the resulting dark green
solution was stirred overnight. The solvent was removed, and
the remaining residue was kept in n-hexane (60 mL) at 9 °C
for 24 h and finally collected as a green powder (1.76 g, 73%).
Anal. Calcd for 10‚C6H14, C64H88NO4V: C, 77.93; H, 9.01; N,
1.42. Found: C, 78.07; H, 8.87; N, 1.24. IR (Nujol, νmax/cm-1):
1614.5 (s), 1597.6 (s).
Syn th esis of 11. ButNC (0.248 g, 2.98 mmol) was added
to a stirred red THF (120 mL) solution of 2 (2.26 g, 2.98 mmol).
After 12 h, the solvent was removed and the remaining residue
washed with pentane (60 mL) and collected as a green powder
(2.0 g, 73%). Anal. Calcd for 11‚THF, C55H75ClNO5V: C,
72.06; H, 8.26; N, 1.53. Found: C, 72.04; H, 8.69; N, 1.17. IR
(Nujol, νmax/cm-1): 2194.3 (m), 1589.9 (w). µeff ) 2.67 µB at
300 K.
Syn th esis of 12. A THF (20 mL) solution of I2 (0.24 g, 1.0
mmol) was added dropwise to a cooled (-30 °C) THF red
solution (120 mL) of 2 (1.60 g, 2.10 mmol). After the mixture
was stirred for 12 h at room temperature, the solvent was
removed in vacuo and the red residue washed with pentane
(60 mL) and collected (1.2 g, 70%). Anal. Calcd for 12‚THF,
C49H63ClO5V: C, 71.90; H, 7.76. Found: C, 71.51; H, 7.33. A
sample of 12 in C6D6 was hydrolyzed by diffusion of aqueous
HCl under N2. p-But-calix[4]-(OMe)(OH)3 was identified in the
1H NMR spectrum. A sample of the crude reaction product
was analyzed by CG-MS: CH3I was identified. 1H NMR of
the hydrolyzed product (C6D6, 298 K, ppm): δ 9.93 (s, 1H, OH),
9.69 (s, 2H, OH), 7.00 (m, 8H, arom), 4.50 (d, 2H, J ) 13.8
Hz, CH2-calix), 4.38 (d, 2H, J ) 13.8 Hz, CH2-calix), 3.54 (s,
3H, OCH3-calix), 3.42 (d, 2H, J ) 13.8 Hz, CH2-calix), 3.31 (d,
2H, J ) 13.8 Hz, CH2-calix), 1.27 (s, 18H, But-calix), 0.97 (s,
9H, But-calix), 0.89 (s, 9H, But-calix).
2
(Fo2) + (aP)2] (with P ) (Fo2 + 2Fc )/3 was applied with a having
values of 0.1204, 0.1220, and 0.1169 for 2, 5, and 9, respec-
tively. All calculations were performed using SHELX92. The
final difference maps showed no unusual features, with no
significant peaks above the general background.
In complex 2, all of the methyl carbon atoms of the tert-
butyl groups of calixarene showed high thermal parameters,
indicating the presence of disorder. The atoms were then split
over two positions (called A and B) isotropically refined with
site occupation factors of 0.5. In complexes 5 and 9, the methyl
carbon atoms of the tert-butyl group (C30, C31, C32 for 5; C34,
C35, C36 for 9) were found to be disordered over two positions
(called A and B) and isotropically refined with site occupation
factors of 0.6 and 0.4, respectively. Moreover, the benzene
solvent molecule of crystallization in complex 5 was found to
be distributed over two positions (A and B) and was isotropi-
cally refined with a site occupation factor of 0.5, constraining
the aromatic ring to have a D6h symmetry.
Syn th esis of 13. A THF solution of I2 (0.32 g, 1.265 mmol)
was added dropwise to a cooled (-30 °C) THF suspension (150
mL) of 5 (2.06 g, 2.53 mmol). The solution became red within
seconds. Stirring was continued for 12 h at room temperature.
The red solution was filtered, the solvent removed in vacuo,
and the red residue finally washed with n-hexane (80 mL) and
collected (2.0 g, 94%). Anal. Calcd for 13‚0.5C6H14
,
C
55H69O4V: C, 78.15; H, 8.25. Found: C, 77.60; H, 8.55.
(11) Lawton, S. L.; J acobson, R. A. TRACER (a cell reduction
program); Ames Laboratory, Iowa State University of Science and
Technology: Ames, IA, 1965.
(12) Lehmann, M. S.; Larsen, F. K. Acta Crystallogr., Sect. A: Cryst.
Phys., Diffr., Theor. Gen. Crystallogr. 1974, A30, 580-584.
(13) Wilson, A. J . C. Nature 1942, 150, 151.
Crystals were grown in hexane solution at room temperature.
A sample of 13 in C6D6 was hydrolyzed by diffusion of aqueous
HCl under N2. p-But-calix[4]-(OMe)(OH)3 was identified in the
1H NMR spectrum. A sample of the crude reaction product
was analyzed by CG-MS: CH3I was identified. For the 1H
NMR of the hydrolyzed product, see preceding preparation.
µeff ) 1.65 µB at 300 K.
Syn th esis of 14. A THF (20 mL) solution of I2 (0.29 g, 1.14
mmol) was added dropwise to a stirred THF solution (120 mL)
of 13‚0.5C6H14 (0.968 g, 1.14 mmol) at room temperature.
After the mixture was stirred for 12 h, the solvent was removed
in vacuo and the red residue extracted with n-hexane (150 mL).
The complex was obtained as black crystals from this solution
(14) Ugozzoli, F. Comput. Chem. 1987, 11, 109.
(15) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1968, A24, 351.
(16) (a) International Tables for X-ray Crystallography; Kynoch
Press: Birmingham, England, 1974; Vol. IV, p 99. (b) Ibid. p 149.
(17) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J . Chem. Phys.
1965, 42, 3175.
(18) Sheldrick, G. M. SHELX76. Program for crystal structure
determination. University of Cambridge: England, 1976.
(19) Sheldrick, G. M. SHELXL92. Program for Crystal Structure
Refinement. University of Go¨ttingen: Go¨ttingen, Germany, 1992.

Redox Relationship between V(III) and V(IV)
Organometallics, Vol. 17, No. 11, 1998 2331
Ta ble 1. Exp er im en ta l Da ta for th e X-r a y Diffr a ction Stu d ies on Com p lexes 2, 5, a n d 9
2
5
9
formula
a, Å
b, Å
C
46H58ClO4V‚C7H8
C53H65O4V‚2C6H6
24.019(5)
10.837(2)
24.130(4)
90
110.74(1)
90
5873.9(19)
4
C58H74NO4V‚3.5C6H6
14.587(3)
17.769(4)
14.329(4)
103.24(2)
106.44(2)
72.59(2)
3356.4(15)
2
17.523(3)
13.479(2)
21.676(3)
90
108.10(2)
90
c, Å
R, °
â, °
γ, °
V, Å3
Z
4866.4(14)
4
fw
853.5
973.3
1173.6
space group
T, °C
λ, Å
P21/c (No. 14)
295
1.54178
1.165
25.43
0.827-1.000
7386
C2/c (No. 15)
295
1.54178
1.101
17.38
0.742-1.000
4223
P1h (No. 2)
143
1.54178
1.161
16.02
0.456-1.000
6302
F
calc, g cm-3
µ, cm-1
transmission coeff
no. of unique data used
in refinement [I > 0] (NO)
no. of unique obsd data [I > 2σ(I)]
no. of params refined (NV)
overdetermn ratio (NO/NV)
Ra
3248
520
14.2
0.068
0.189
1654
285
14.8
0.074
0.243
3772
758
8.3
0.064
0.191
wR2b
a
b
2 2
R ) ∑|∆F| / ∑|Fo| calculated on the unique observed data [I > 2σ(I)]. wR2 ) [∑w|∆F2|2/∑w|Fo | ]1/2 calculated on the unique data
with I > 0.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Com p lexes 2, 5, a n d 9
Ta ble 3. Com p a r ison of Releva n t Con for m a tion a l
P a r a m eter s w ith in Ca lixa r en e for Com p lexes 2, 5,
a n d 9
2
5
9
2
5
9
V(1)-Xa
2.265(2)
2.042(10)
2.047(8)
2.050(7)
1.883(5)
2.179(4)
1.860(4)
2.161(3)
1.314(8)
1.395(9)
1.450(6)
1.309(6)
1.419(8)
1.452(9)
1.266(8)
1.501(10)
1.495(10)
(a) Dihedral Angles (deg) between Planar Moietiesa
V(1)-N(1)
V(1)-O(1)
V(1)-O(2)
V(1)-O(3)
V(1)-O(4)
O(1)-C(1)
O(2)-C(13)
O(2)-C(45)
O(3)-C(20)
O(4)-C(27)
O(4)-C(46)
N(1)-C(51)
N(1)-C(59)
C(51)-C(52)
1.809(4)
2.156(4)
1.805(4)
2.128(4)
1.329(6)
1.419(6)
1.443(6)
1.353(7)
1.417(5)
1.455(6)
1.841(5)
2.137(4)
E ∧ A
E ∧ B
E ∧ C
E ∧ D
A ∧ C
B ∧ D
139.0(1)
109.7(1)
138.5(1)
117.8(1)
97.5(2)
132.4(2)
112.9(2)
132.4(2)
112.9(2)
95.2(2)
133.9(1)
111.5(1)
148.9(2)
113.5(2)
95.9(2)
1.344(9)
1.425(6)
1.447(8)
132.5(1)
134.3(2)
134.9(2)
(b) Contact Distances (Å) between para-Carbon
Atoms of Opposite Aromatic Ringsb
C4...C17
C10...C24
9.453(9)
7.473(6)
9.057(11)
7.446(9)
9.357(9)
7.386(9)
a
E (reference plane) refers to the least-squares mean plane
defined by the C(7), C(14), C(21), C(28) bridging methylenic
carbons. A, B, C, and D refer to the least-squares mean planes
defined by the aromatic rings bonded to O(1), O(2), O(3), and O(4),
respectively. In complex 5, O(3), O(4), C(21), and C(28) should be
read O(1)′, O(2)′, C(7)′, and C(14)′, respectively (prime (′) ) -x, y,
O(2)-V(1)-O(4)b
O(1)-V(1)-O(3)
X-V(1)-O(3)
X-V(1)-O(1)
X-V(1)-N(1)
167.5(1)
127.3(2)
117.8(1)
114.9(1)
167.6(1)
139.0(2)
110.5(1)
110.5(1)
164.1(2)
113.9(2)
136.0(2)
110.0(2)
36.0(3)
159.9(4)
130.3(4)
116.8(4)
112.7(5)
169.1(4)
127.0(4)
119.1(3)
113.6(5)
154.7(5)
71.9(4)
b
0.5 - z). In complex 5, C(17) and C(24) should be read C(4)′ and
C(10)′, respectively (prime (′) ) -x, y, 0.5 - z).
V(1)-O(1)-C(1)
V(1)-O(2)-C(45)
V(1)-O(2)-C(13)
C(13)-O(2)-C(45)
V(1)-O(3)-C(20)
V(1)-O(4)-C(46)
V(1)-O(4)-C(27)
C(27)-O(4)-C(46)
V(1)-N(1)-C(59)
V(1)-N(1)-X
X-N(1)-C(59)
V(1)-X-N(1)
N(1)-X-C(52)
V(1)-X-C(52)
167.3(4)
131.3(3)
114.6(3)
114.1(4)
161.7(4)
128.6(3)
118.7(3)
112.7(3)
153.1(4)
128.5(4)
117.4(4)
114.1(4)
bond distances and angles are given in in Tables S11-S13
(Supporting Information).20
Resu lts a n d Discu ssion
The parent compound 2 can be considered a useful
entry point into the organometallic chemistry of vana-
dium bonded to the oxo-matrix defined by the calix[4]-
arene skeleton. It was prepared by reacting 1, pretreat-
ed with BuLi, with VCl3‚THF3. Complex 2 is a yellow-
green crystalline solid with the paramagnetism expected
for a d2 high-spin complex (µeff ) 2.69 µB at 300 K).
A selection of bond distances and angles for complexes
2, 5, and 9 is given in Table 2. Structural parameters
concerning the conformation of the [calix[4]arene-
(OMe2)V] units are compared in Table 3. The aromatic
rings bonded to the O(1), O(2), O(3), and O(4) oxygen
atoms are labeled A, B, C, and D, respectively.
133.1(7)
72.1(5)
129.6(7)
157.9(5)
a
X should be read Cl(1) for complex 2 and C(51) for complexes
b
5 and 9. In complex 5, O(3) and O(4) should be read O(1)′ and
O(2)′, respectively (prime (′) ) -x, y, 0.5 - z).
Final atomic coordinates are listed in Tables S2-S4 (Sup-
porting Information) for non-H atoms and in Tables S5-S7
(Supporting Information) for hydrogens. Thermal parameters
are given in Tables S8-S10 (Supporting Information), and
The structure of complex 2 is shown in Figure 1.
Vanadium has a pseudo-trigonal-bipyramidal coordina-
(20) See paragraph at the end regarding Supporting Information.

2332 Organometallics, Vol. 17, No. 11, 1998
Castellano et al.
Sch em e 1
F igu r e 1. ORTEP drawing of complex 2 (30% probability
ellipsoids). Disorder has been omitted for clarity.
Complex 2 undergoes alkylation to the corresponding
paramagnetic vanadium(III) derivatives 3-5 using
either a lithium or a Grignard reagent as described in
the Experimental Section (Scheme 1). They have been
isolated as thermally stable organometallic derivatives,
although they undergo facile oxidative transformations
(see below). The proposed structure for 3-5 is sup-
ported by the X-ray analysis of 5, which additionally
showed a particularly interesting solid-state self-as-
sembling of the organometallic monomeric unit.
Complex 5 possesses a crystallographically imposed
C2 symmetry, the 2-fold axis running through the
molecular axis (see Figure 2). The main feature of the
solid-state structure consists of the packing of the
molecules along the 2-fold axis: each complex molecule
includes the p-tolyl ligand of an adjacent molecule
translated along the b crystallographic axis and posi-
tioned with the methyl group (C(55)) directly below the
vanadium atom at a separation of 4.380(10) Å. This
results in a head-to-tail columnar chain arrangement
of the molecules running along the 2-fold axis (Figure
3). The arrangement is very close to that observed in
an analogous Ti derivative.21 The host-guest interac-
tions could be interpreted in terms of CH3/π interactions
between the protons of the C(55) methyl group of the
guest and the symmetry-related C(8)‚‚‚C(13) and
C(8′)‚‚‚C(13′) aromatic rings (′ ) -x, y, 0.5 - z). The
rather narrow range of the Cmethyl‚‚‚Cring separations
F igu r e 2. ORTEP drawing of complex 5 (30% probability
ellipsoids). Prime denotes a transformation of -x, y, 0.5 -
z. Disorder has been omitted for clarity.
tion environment with the equatorial plane being de-
fined by V, Cl, O(1), O(3). The V-O bond distances vary
from 1.807(2) (mean value for V-O(1) and V-O(3)) to
2.142(14) Å (mean value for V-O(2) and V-O(4)), the
latter being related to the methoxy groups. The V-O(1)
and V-O(3) bond distances along with the values of the
V-O(1)-C(1) (167.3(4)°) and V-O(3)-C(21) (161.7(4)°)
bond angles are in agreement with a V-O multiple
bond.3c,d Because of the presence of the methoxy groups,
the O4 core is tetrahedrally distorted (distortion ranging
from -0.349(4) to 0.224(3) Å; vanadium protrudes by
0.453(1) Å), resulting in an elliptical cross section
conformation of the macrocycle elongated along the
O(1)‚‚‚O(3) direction as is observed from the dihedral
angles and the contact distances quoted in Table 3. This
conformation allows the aromatic ring of a toluene
molecule of crystallization to enter as a guest nearly
parallel to the O(1),V,O(3) direction (dihedral angle 10.2-
(2)°) and perpendicular to the C(1)‚‚‚C(6) and C(15)‚‚‚C-
(20) rings (dihedral angles 87.7(2)° and 82.5(2)°, respec-
tively) with the C(55)-H(55) bond directly below the
vanadium atom (V‚‚‚C(55) 4.614(8) Å; V‚‚‚H(55)-C(55)
166.5°).
(21) Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.; Re, N.;
Chiesi-Villa, A.; Rizzoli, C. Inorg. Chim. Acta 1998, 270, 298.

Redox Relationship between V(III) and V(IV)
Organometallics, Vol. 17, No. 11, 1998 2333
F igu r e 4. ORTEP drawing of complex 9 (50% probability
ellipsoids). Disorder has been omitted for clarity.
distances parallel those observed in complex 2, while
the V-C(51) distance (2.042(10) Å) is in the usual
range.22
The V-C bond in complexes 3-5 can be engaged in
migratory insertion reactions both with carbon monox-
ide and ButNC (Scheme 1). The reaction with CO is
strongly dependent on the alkyl substituent. In the case
of methyl and benzyl substituents, we were able to
isolate the corresponding η2-acyls, 6 and 7. In the case
of p-tolyl, the absorption of CO was limited and we were
unable to identify a single characterizable compound.
The η2-bonding mode of the acyl group has been inferred
from the absence in the IR spectrum of any CO band
higher than 1600 cm-1 and by analogy with the corre-
sponding η2-iminoacyl compound (see below).23 The
reaction of 3-5 with ButNC was carried out in benzene
at room temperature and led (see Scheme 1) to the
isolation of 8-10. The IR spectra showed the CdN
band of the η2-iminoacyl in the 1614-1625 cm-1 region.
The migratory insertion of alkyl and aryl groups to
isocyanides in vanadium(III) chemistry has some sig-
nificant precedents.22,23
F igu r e 3. SCHAKAL view of complex 5 showing the
chaining along the [010] axis. Disorder has been omitted
for clarity. Prime denotes a transformation of -x, y, 0.5 -
z.
(3.731(7)-3.864(9) Å) suggests the methyl protons point
toward the centroid of the rings. The macrocycle
assumes an elliptical cross section conformation elon-
gated along the O1‚‚‚O1′ direction, very close to that
observed in complex 2. The aromatic ring of the guest
p-tolyl ligand is oriented nearly parallel to the
O(1),V,O(1)′ direction (dihedral angle 5.1(2)°) and per-
pendicular to the C(1)‚‚‚C(6) and C(1)′‚‚‚C(6)′ rings
(dihedral angle 86.5(2)°). The O4 core shows a remark-
able tetrahedral distortion (displacements ranging from
-0.150(3) to 0.264(4) Å), with vanadium protruding by
0.381(1) Å from the mean plane. The coordination
environment around vanadium can be described as a
trigonal bipyramid, the equatorial plane being defined
by the O(1), O(1)′, C(51), and V atoms, which are planar
from symmetry requirements. O(2) and O(2)′ occupy the
apexes (O(2)-V-O(2)′ 167.6(1)°). The distortion from
an ideal trigonal symmetry is mainly indicated by the
value of the O(1)-V-O(1)′ bond angle (139.0(2)°). The
V-O(2) (2.137(4) Å) and V-O(1) (1.841(5) Å) bond
The proposed structure for the η2-iminoacyl function-
ality in 8-10 is supported by the X-ray analysis of 9
(see Figure 4). The O4 core shows a remarkable
tetrahedral distortion (displacements ranging from
-0.379(5) to 0.366(5) Å), with vanadium protruding by
0.658(2) Å from the mean plane. The plane of the η2-
bonded atoms (V, N(1), C(51)) is perpendicular to the
O4 mean plane (dihedral angle 87.3(2)°). The V-O bond
distances (V-O(2) 2.179(4) Å; V-O(4) 2.161(3) Å; V-O(1)
1.883(5) Å; V-O(3), 1.860(4) Å) are significantly longer
than in 2 and 5, while the approximate linearity of the
V-O(1)-C(1) (159.9(4)°) and V-O(3)-C(20) (169.1(4)°)
bond angles is maintained. The η2-bonded N,C frag-
ment shows a symmetric bonding mode to the metal (V-
N(1) 2.050(7) Å; V-C(51) 2.047(8) Å) and a N(1)-C(51)
distance of 1.266(8) Å.22 The calixarene ligand assumes
a conformation of elliptical cross section, similar to that
observed in 2 and 5, allowing the inclusion of a benzene
molecule which enters the cavity nearly parallel to the
O(1),V,O(3) plane (dihedral angle 5.6(2)°) and perpen-
(22) Vivanco, M.; Ruiz, J .; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1993, 12, 1802 and 1794. Ruiz, J .; Vivanco, M.;
Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 1811.
J acoby, D.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J . Chem. Soc.,
Chem. Commun. 1991, 790. Rosset, J .-M.; Floriani, C.; Mazzanti, M.;
Chiesi-Villa, A.; Guastini, C. Inorg. Chem. 1990, 29, 3991.
(23) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059 and
references therein.

2334 Organometallics, Vol. 17, No. 11, 1998
Castellano et al.
distinguish two low-lying almost degenerate orbitals,
i.e., 1b1(dxz), pointing in the plane of the two methoxy
2
ligands, and 1a1(dz ). Due to the stronger σ and π
interactions with the phenoxo ligands in the yz plane,
1b2(dyz) is almost 1 eV higher in energy than 1b1(dxz).
Due to the in-plane π interaction with the nonmethy-
lated O atom, the 1a2(dxy) orbital is destabilized and lies
only 0.3 eV lower in energy. On the extreme right of
Figure 5 is the singly occupied σ orbital of Me. Figure
5 shows that the main bonding interaction for this
2
system is between 1a1(dz ) of the metal fragment and
the σ orbital of CH3. The remaining low-lying orbitals
of the metal fragment remain essentially unaltered. It
follows that for such a d2 V(III) complex we would expect
a triplet ground state with the closely spaced 1b1(dxz)
and 1a2(dxy) orbitals singly occupied, which is in agree-
ment with the observed magnetic behavior.
A similar situation is found in complex 2 for the
π-donor ligand Cl- where the interaction with the π
donor orbitals of Cl- leads to a destabilization of 1b1-
(dxz) (the only orbital which does not show π interactions
with O atoms) but not of 1a2(dxy), which in turn inverts
the order of the two singly occupied orbitals. Complex
2 reacts with Lewis bases, illustrated here by ButNC,
to give complex 11, which has been characterized by
elemental analysis, IR, and magnetism studies (see
Experimental Section). The observed magnetic moment
(µeff ) 2.67 µB at 300 K) corresponds to a d2 high-spin
system and emphasizes the difference with the corre-
sponding vanadium(III) [Cp2V(X)(Y)] derivative (Cp )
η5-C5H5).26 In the latter case, the metal is diamagnetic
due to the availability of three orbitals at the metal only.
In the case of 11, the high-spin state is allowed with
the availability of four orbitals at an accessible energy,
with the two unpaired electrons in 1a2 and 1b1 (see
Figure 6). Complex 11 can be considered as a model
for the incoming inserting ligand preceding the insertion
reaction. The migratory insertion reactions leading to
6-10 have been exemplified by the reaction of 3 with
carbon monoxide. Since the extended Hu¨ckel method
is not very well suited to describe the energetics of bond
breaking and bond formation, we examined only the
initial approach of CO to vanadium. The most favorable
approach of the CO or ButNC nucleophilic species is
determined by the spatial extension of the 1b1(dyz)
LUMO. The attack is, therefore, expected to occur along
a line in the yz plane forming an angle of approximately
45° with the z axis. We first simulated the initial stages
of the CO attack by performing extended Hu¨ckel
calculations on the [calix[4](OMe)2(O)2VMe]-CO system
with different (CO)-to-metal distances, from 5.0 to 2.5
Å, along a line in the yz plane by relaxing the axial
position of the Me ligand and the angular position of
CO to the z axis at each point along the attack pathway.
The total energy profile is given in Figure 7 and shows
a very small energy barrier followed by a significant
stabilization, in harmony with the experimental evi-
dence for a facile CO insertion. It is worth noting that
for the shortest considered (CO)-to-metal distance there
is a significant back-donation from the occupied vana-
dium 1b1(dxz) orbitals to the π* orbital of CO. This is
confirmed by the fragment orbital populations calcu-
lated for V-CO ) 2.5 Å which show a significant
F igu r e 5. Interaction diagram for the valence orbitals of
3.
dicular to the C(1)‚‚‚C(6) and C(15)‚‚‚C(20) rings (dihe-
dral angles 87.0(2)° and 83.4(2)°, respectively) with the
C(71)-H(71) bond directly below the vanadium atom
(V‚‚‚C(71) 4.692(9) Å; V‚‚‚H(71)-C(71) 176.1°). In this
orientation, the H(72) and H(76) atoms of the guest
point toward the A and C rings, respectively, as indi-
cated by the narrow range of the H(72)‚‚‚Cring C and
H(76)‚‚‚Cring A contact distances, which vary from 2.95
to 3.06 Å and from 3.05 to 3.25 Å, respectively. At the
same time, a CH3/π host-guest interaction seems to
take place involving the C(44) methyl group.24 The
contact distances involving the H(441) methyl hydrogen
are in the range from 2.92 to 3.29 Å for the C(72)‚‚‚C(75)
group of atoms, while the H(441)-C(71) and H(441)-
C(76) distances are greater than 3.5 Å.21,24
Extended Hu¨ckel calculations25 have been performed
on the calix[4]arene-vanadium derivatives with the
purpose of understanding the frontier orbitals of the
[calix[4]-(OMe)2(O)2V] fragment, the bonding of the
organometallic functionalities in 3-10, and the conver-
sion of the V-alkyl and V-aryl bonds (3-5) into the
corresponding η2-acyl and iminoacyl derivatives (6-10).
The calix[4]arene ligand has been slightly simplified
by replacing it with four phenoxo groups and sym-
metrizing to a C4v geometry. This simplified model
retains the main features of the whole ligand with the
geometrical constraints on the O4 set of donors atoms
being maintained by fixing the geometry of the four
phenoxo groups to the experimental X-ray values. Let
us first consider complex 3, in which the Me ligand lies
along the z axis. The electronic structure is analyzed
in terms of the interactions between the frontier orbitals
of the [calix[4](OMe)2(O)2V)] metal fragment and the Me
moiety. The frontier orbitals of the [calix[4](OMe)2-
(O)2V] fragment are shown on the left of Figure 5 and
consist of four low-lying metal-based orbitals. We can
(24) Andreetti, G. D.; Ugozzoli, F. In Calixarenes, A Versabile Class
of Macrocyclic Compounds; Vicens, J ., Bo¨hmer, V., Eds.; Kluwer:
Dordrecht, The Netherlands, 1991; pp 88-122.
(25) Hoffmann, R.; Lipscomb, W. N. J . Chem. Phys. 1962, 36, 2179.
Hoffmann, R. J . Chem. Phys. 1963, 39, 1397.
(26) Poli, R. Chem. Rev. 1996, 96, 2135.

Redox Relationship between V(III) and V(IV)
Organometallics, Vol. 17, No. 11, 1998 2335
F igu r e 8. Interaction diagram for the valence orbitals of
6.
F igu r e 6. Interaction diagram for the valence orbitals of
2.
combination of the carbon and the oxygen pz orbitals,
the lower lying n+, which is the corresponding in-phase
combination, and the CO π and π* orbitals. We see that
the vanadium acyl bonding is mainly achieved through
the interaction between the metal dyz and the acyl n-,
2
orbitals with minor σ, dz -n+, and δ, dxy-π*(CO)
contributions (we considered an orientation with the
acyl plane lying in the yz symmetry plane of the Cs
molecule with the CO bond perpendicular to the z axis).
The vanadium(III) complexes so far mentioned dis-
play interesting redox behavior. Compounds 2 and 5
treated with 1 equiv of iodine led to the corresponding
vanadium(IV) derivatives 12 and 13, respectively. Both
compounds have been analytically characterized, in-
cluding magnetic moments, and show a d1 configuration
(13 shows a µeff ) 1.65 µB at 300 K); the corresponding
hydrolysis product was [p-But-calix[4]-(OMe)(OH)3]. The
oxidation with I2 is particularly remarkable in the
reaction of 5 to 13 since the V-C bond is not affected
by the oxidation process. The reaction occurs with the
simultaneous loss of MeI (see Experimental Section).
The MO diagram in Figure 5 suggests that the oxidation
involves the removal of one electron from an essentially
pure d orbital, that is 1a2. In the resulting vanadium-
(IV) cationic complex, the strong nucleophile I- removes
the methyl group from a methoxy substituent activated
by the relatively high metal oxidation state. In several
cases, we observed demethylation of the methoxy groups
in the interaction of methoxy-calix[4]arenes with high-
valent metals.2h The methoxy group not only functions
as a weakly binding group, but it can also undergo an
acid- or redox-assisted conversion into the corresponding
anion. We should also emphasize the synthetic rel-
evance of the reaction of 2 and 5 with iodine, since
complexes 12 and 13 would be hardly accessible from a
different route. In the case an excess of I2 being used
in the oxidation of 5 to 13, the reaction proceeds further
with the cleavage of the V-C bond and the complete
F igu r e 7. Total energy profiles for the CO (O) and NH3
(0) approach toward 3.
population of the π* orbital of CO (0.21 vs 0.00 for
isolated CO) and a simultaneous depopulation of the two
singly occupied metal orbitals (1.77 vs 2.00 for the
isolated fragment).
Let us now consider the η2-acyl complex [calix[4]-
(OMe)2(O)2V(η2-COMe)]. The bonding between the metal
fragment and the acyl moiety is illustrated by the
interaction orbital diagram in Figure 8. The extreme
right of Figure 8 shows the frontier orbitals of the acyl
ligand, i.e., the HOMO n-, consisting of the out-of-phase

2336 Organometallics, Vol. 17, No. 11, 1998
Castellano et al.
demethylation of calix[4]arene. The reaction may pro-
ceed via the intermediacy of the corresponding vana-
dium(IV) monoiodide derivative, which transfers I- to
the methyl of the residual methoxy group. Complex 14
has been obtained as a crystalline solid and structurally
characterized.27 All attempts to convert 6-10 to the
corresponding vanadium(IV) derivatives via the iodine
oxidation failed, at least partially, because the dem-
ethylation parallels the R-C cleavage in the η2-acyls
and η2-iminoacyls.
of the methoxy group allowing interconversion between
two oxidation states of the metal bearing the same
functionality.
(iii) The magnetic properties of hexacoordinate d2
high-spin vanadium(III) showed the availability of four
frontier orbitals for the [p-But-calix[4]-(OMe)2(O)2V]
fragment. This has been confirmed by the extended
Hu¨ckel calculations and gives us a better picture of the
bonding mode of the various organometallic function-
alities and of the migratory insertion reactions.
Con clu sion s
Ack n ow led gm en t. We thank the Fonds National
Suisse de la Recherche Scientifique (Bern, Switzerland,
Grant No. 20-46′590.96), Ciba Specialty Chemicals
(Basle, Switzerland), and Fondation Herbette (Univer-
sity of Lausanne, N. Re) for financial support.
Some relevant results from the present report should
be emphasized:
(i) The synthesis and migratory insertion of V-C
functionalities anchored exclusively to an oxo-matrix
provided by the O4 set from a calix[4]arene.
(ii) The oxidative demethylation of the methoxy-calix-
[4]arene, which converts vanadium(III) into vanadium-
(IV) organometallic functionalities, emphasizes the role
Su p p or tin g In for m a tion Ava ila ble: SCHAKAL draw-
ings and tables of crystal data, atomic coordinates, thermal
parameters, and bond distances and angles for 2, 5, and 9 (19
pages). Ordering information is given on any current mast-
head page.
(27) A preliminary X-ray analysis confirmed the structure proposed
for 14.
OM9800389