Subvalent germanium and tin complexes supported by a dianionic calixarene ligand: Structural characterization of exo and endo isomers of [Butcalix(TMS)2]Ge

Article abstract of DOI:10.1039/a705937j

The 1,3-bis(trimethylsilyl) ether of p-tert-butylcalix[4]arene, [Bu^tcalix^(TMS)2]H₂, has been synthesized and used as a dianionic ligand for Ge and Sn; notably, the Ge complex [Bu^tcalix^(TMS)2]Ge exhibits exolendo isomerism, and provides the first calixarene system for which both isomers have been structurally characterized.

Full text of DOI:10.1039/a705937j

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Subvalent germanium and tin complexes supported by a dianionic calixarene
ligand: structural characterization of exo and endo isomers of
[Butcalix(TMS) ]Ge
2
Tony Hascall,a Arnold L. Rheingold,b Ilia Guzeib and Gerard Parkin*a
a Department of Chemistry, Columbia University, New York, New York 10027, USA
b Department of Chemistry and Biochemistry, Univesity of Delaware, Newark, Delaware 19716, USA
SiMe3
OH
OH
OH
The 1,3-bis(trimethylsilyl) ether of p-tert-butylcalix[4]arene,
Me3Si
O
OH
O
OH
OH
[Butcalix(TMS) ]H2, has been synthesized and used as a
Me3SiI
2
dianionic ligand for Ge and Sn; notably, the Ge complex
pyridine
[Butcalix(TMS) ]Ge exhibits exo/endo isomerism, and pro-
2
vides the first calixarene system for which both isomers have
been structurally characterized.
[Butcalix]H4
M[N(SiMe3)2]
M
The calix[n]arenes are a class of ‘chalice-like’ macrocyclic
molecules that have recently attracted widespread attention.1 In
particular, these molecules have been extensively studied as
receptors for small organic molecules and simple cations.1 By
comparison, however, calix[n]arenes have been infrequently
used as ancilliary ligands for studying the chemistry of
transition and main group metals.2,3 Moreover, for those
situations where calix[n]arenes have been employed as support-
ing ligands, their application has been mainly restricted to the
use of p-tert-butylcalix[4]arene in its tetraanionic form.†
1,3-Dialkyl ethers of calix[4]arenes, [calixR2]H2, on the other
hand, offer the potential for providing dianionic macrocyclic
ligands which may be considered as oxygen relatives of well
known N-containing macrocycles, such as porphyrins, phthalo-
cyanines and tetraazaannulenes. Despite such analogies, the
application of 1,3-diethers of calix[4]arenes as ligands for main
group metals has been limited to recent reports concerned with
the 1,3-dimethyl ether of p-tert-butylcalix[4]arene.3a–c Here, we
report the synthesis of a more sterically demanding, 1,3-di-
substituted calix[4]arene, namely bis(trimethylsilyl)-p-tert-
butylcalix[4]arene, its use as a ligand for divalent Ge and Sn,
and the first structural characterization of a pair of exo and
endo isomers of a calix[n]arene derivative.
SiMe3
Me3Si
O
SiMe3
O
O
O
Me3Si
O
O
O
O
M
endo
(M = Ge)
exo
(M = Ge, Sn)
Scheme 1
system, the reaction yields sequentially two isomers, namely
exo- and endo-[Butcalix(TMS) ]Ge, which differ in the location
2
of the Ge atom with respect to the calixarene cavity (Figs. 1 and
2).¶ Exo and endo isomerism of this type has previously been
suggested for the aluminium hydride complex [ButcalixMe ]-
2
AlH, for which the Al–H moiety may be directed into, or away
from, the calixarene cavity.3b However, only the structure of
exo-[ButcalixMe ]AlH has been determined by X-ray diffrac-
2
tion, with the nature of the endo isomer having been inferred by
NMR spectroscopy. The Ge complexes described here, there-
fore, constitute the first pair of exo and endo isomers to be
structurally characterized.
The coordination geometries about Ge in exo- and endo-
[Butcalix(TMS) ]Ge are summarized in Table 1, from which it is
2
Our particular interest in the use of 1,3-dialkylcalix[4]arenes
as O-donor macrocyclic ligands derives from recent studies
concerned with Ge and Sn complexes supported by the
octamethyldibenzotetraaza[14]annulene dianion, e.g. the sub-
evident that both complexes exhibit Ge–O bond lengths to the
phenoxide moieties which are comparable to, but slightly
shorter than, the sum of their covalent radii (1.96 Å).∑ For the
endo isomer, the Ge center is located ca. 3.5 Å from the O atoms
of the trimethylsilylether groups, so that it is appropriately
4
valent and terminal chalcogenido complexes [h -Me8taa]M and
4
[h -Me8taa]ME (M = Ge, Sn; E = S, Se, Te).4–6 In order to
extend this chemistry, we sought to synthesize analogous
complexes derived from related macrocycles which contain O
(rather than N) donor functionalities. For this purpose, 1,3-di-
alkyl ether calix[4]arene derivatives incorporating bulky sub-
stituents (e.g. SiMe3) appeared ideal. Although 1,3-diethers of
p-tert-butylcalix[4]arene incorporating primary alkyl substi-
tuents have been synthesized previously by the reaction of the
calix[4]arene with 2 equiv. of alkyl halides in the presence of a
weak base,1c the bis(trimethylsilyl) derivative [Butca-
Si1
Si2
Ge
O2
O4
O1
O3
lix(TMS) ]H2 could not be isolated using such procedures.‡8
2
Nevertheless, we have found that the reaction of p-tert-
butylcalix[4]arene with Me3SiI (2 equiv.) in the presence of
pyridine (2 equiv.) affords a useful synthesis of [Butca-
lix(TMS) ]H2 (Scheme 1).
2
Divalent Ge and Sn complexes [Butcalix(TMS) ]M are readily
2
obtained by the reaction of [Butcalix(TMS) ]H2 with
2
M[N(SiMe3)2]2 (M = Ge, Sn) as illustrated in Scheme 1. Each
of the products has been structurally characterized by X-ray
diffraction, as shown in Fig. 1 and 2.§ Interestingly, for the Ge
Fig. 1 The molecular structure of exo-[Butcalix(TMS) ]Ge (the Sn analogue
is similar)
2
Chem. Commun., 1998
101

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Table 1 Metrical data for [Butcalix(TMS) ]M (M = Ge, Sn)
2
d(M–O1)/Å
d(M–O3)/Å
d(M···O2)/Å
d(M···O4)/Å
O–M–O/°
exo-[Butcalix(TMS) ]Ge
1.765(6)
1.841(5)
1.840(5)
1.956(7)
1.842(6)
1.853(5)
1.849(5)
2.011(9)
2.421(5)
3.460(5)
3.454(6)
2.521(6)
2.486(5)
3.565(6)
2.458(5)
2.532(6)
100.2(3)
92.81(22)
92.02(23)
96.3(4)
2
endo-[Butcalix(TMS) ]Ge
2
exo-[Butcalix(TMS) ]Sn
2
‡ Specifically, only recovered p-tert-butylcalix[4]arene was obtained from
the reaction of (i) p-tert-butylcalix[4]arene with Me3SiCl and (Me3Si)2NH,
and (ii) p-tert-butylcalix[4]arene with Me3SiCl and Li2S. The latter
reaction, however, did provide evidence for formation of the bis-
(trimethylsilyl) ether derivative, but the product could not be isolated.7a
Tetrakis(trimethylsilyl)-p-tert-butylcalix[4]arene has, however, been ob-
tained by the reaction of the calix[4]arene with N,O-tris(trimethylsilyl)-
acetamide.7b
Si1
O4
Si2
O2
O3
O1
§ Crystallography. exo-[Butcalix(TMS) ]Ge·C6H5Me is monoclinic, P21/c
2
(no. 14), a = 23.275(3), b = 18.829(2), c = 13.245(1) Å, b = 105.059(7)°,
Ge1
U = 5605(1) Å3, Z = 4, R [I > 2s(I)] = 0.0738. endo-[Butcalix(TMS) ]Ge
2
is monoclinic, P21/c (no. 14),
a = 28.2718(3), b = 18.4591(1),
c = 20.5583(1) Å, b = 111.245(1)°, U = 9999.7(1) Å3, Z = 8, R [I >
2s(I)] = 0.1027. exo-[Butcalix(TMS) ]Sn·C6H6 is monoclinic, P21/c (no.
2
14), a = 23.339(7), b = 18.684(4), c = 13.204(3) Å, b = 105.98(1)°,
U = 5535(1) Å3, Z = 4, R [I > 2s(I)] = 0.0744. CCDC 182/655.
¶ In contrast, for the Sn system, only the exo isomer has been isolated so
far.
Fig. 2 The molecular structure of endo-[Butcalix(TMS) ]Ge (only one of the
∑ For comparison, the following metrical data have been reported for two-
coordinate Ge and Sn alkoxide and aryloxide complexes: (i) (ArO)2Ge
(Ar = C6H2MeBut2): (Ge–O) 1.802(8), 1.812(7) Å; O–Ge–O 92.0(4)°;92a
(ii) (ArO)2Sn (Ar = C6H2MeBut2): (Sn–O) 1.995(4), 2.022(4) Å; O–Sn–O
88.8(2)°;9a (iii) (But3CO)2Ge: (Ge–O) 1.896(6), 1.832(11) Å; O–Ge–O
85.9(4)°.9b
2
crystallographically independent molecules is shown)
considered to be two-coordinate, with the dianionic calixarene
acting as a bidentate, rather than tetradentate, ligand. As such,
the coordination environment offered by the dianionic calixar-
ene provides a contrast with the tetracoordination provided by
** The conversion of exo-[Butcalix(TMS) ]Ge to its endo isomer (ca. 95%)
2
has been observed to occur over a 1 h at ca. 80 °C, as monitored by 1H NMR
4
related macrocyclic derivatives, e.g. [h -Me8taa]Ge.4 The
spectroscopy.
†† The endo isomer of [ButcalixMe ]AlH has also been reported to be more
ability of the calixarene to sustain monomeric two-coordinate
GeII centers is of interest since complexes with such coordina-
tion environments are uncommon; for example, only two two-
coordinate GeII alkoxide complexes are listed in the Cambridge
Structural Database (Version 5.13), namely (ArO)2Ge
(Ar = C6H2MeBut2) and (But3CO)2Ge.∑ In contrast to the two-
coordinate endo isomer, the exo isomer does exhibit interactions
with the O atoms of the trimethylsilylether groups (ca. 2.45 Å)
that are shorter than the sum of their van der Waals radii (3.40
Å).∑
2
stable than its exo isomer. See ref. (3b).
‡‡ Other factors, such as the small conformational differences of the
calixarene ligand, may also influence the stability of the endo with respect
to exo isomer.
1 (a) G. D. Gutsche, Calixarenes, Royal Society of Chemistry, Cambridge,
1989; (b) Calixarenes: A Versatile Class of Macrocyclic Compounds, ed.
J. Vincens and V. Bo¨hmer, Kluwer, Dordrecht, 1991; (c) V. Bo¨hmer,
Angew. Chem., Int. Ed. Engl., 1995, 34, 713.
2 D. M. Roundhill, Prog. Inorg. Chem., 1995, 43, 533.
It is interesting to note that, despite the observation that the
dative interaction between Ge and the trimethylsilyl ether
groups is shorter for the exo isomer, the endo isomer is evidently
the more thermodynamically stable, as judged by the observed
exo to endo isomerization.**,†† Thus, the [O?Ge] dative
interactions in the exo isomer are presumably weak and
insufficient to compensate for other structural changes which
accompany the isomerization. For example, one factor which
favors the endo isomer being the more stable is concerned with
the possibility that the calixarene conformation is such that it
furnishes a more appropriate bite angle for Ge in the endo
position than for Ge in the exo position (compare Figs. 1 and 2).
Supporting this notion, the O–Ge–O bond angle for the exo
isomer [100.2(3)°] is notably larger than is observed for simple
(RO)2Ge complexes (ca. 86–92°)∑ which are not subject to the
constraints of the macrocyclic configuration. In contrast, the
endo isomer does exhibit a O–Ge–O bond angle (ca. 92.4°) that
is comparable to the values for (RO)2Ge derivatives, thereby
suggesting that there is less strain for the endo isomer, so that it
may be more thermodynamically favored.‡‡
3 For recent examples of the use of calix[n]arenes as ligands in main group
metal chemistry, see: (a) M. G. Gardiner, S. M. Lawrence, C. L. Raston,
B. W. Skelton and A. H. White, Chem. Commun., 1996, 2491; (b)
M. G. Gardiner, G. A. Koutsantonis, S. M. Lawrence, P. J. Nichols and
C. L. Raston, Chem. Commun., 1996, 2035; (c) J. L. Atwood,
M. G. Gardiner, C. Jones, C. L. Raston, B. W. Skelton and A. H. White,
Chem. Commun., 1996, 2487; (d) J. L. Atwood, P. C. Junk, S. M.
Lawrence and C. L. Raston, Supramol. Chem., 1996, 7, 15; (e)
J. M. Smith and S. G. Bott, Chem. Commun., 1996, 377; (f) S. G. Bott,
A. W. Coleman and J. L. Atwood, J. Incl. Phenom., 1987, 5, 747.
4 M. C. Kuchta and G. Parkin, J. Chem. Soc., Chem. Commun., 1994,
1351.
5 M. C. Kuchta and G. Parkin, J. Am. Chem. Soc., 1994, 116 8372.
6 M. C. Kuchta and G. Parkin, Chem. Commun., 1996, 1669.
7 (a) C. D. Gutsche, B. Dhawan, K. H. No and R. Muthukrishnan, J. Am.
Chem. Soc., 1981, 103, 3782; (b) C. D. Gutsche, B. Dhawan, J. A. Levine,
K. H. No and L. J. Bauer, Tetrahedron, 1983, 39, 409.
8 For some other examples of O-silylated calixarenes see: (a) M. M.
Olmstead, G. Sigel, H. Hope, X. Xu and P. P. Power, J. Am. Chem. Soc.,
1985, 107, 8087; (b) X. Delaigue, M. W. Hosseini, A. De Cian, J. Fischer,
E. Leize, S. Kieffer and A. Van Dorsselaer, Tetrahedron Lett., 1993, 34,
3285; (c) S. Shang, D. V. Khasnis, J. M. Burton, C. J. Santini, M. Fan,
A. C. Small and M. Lattman, Organometallics, 1994, 13, 5157; (d)
M. Fan, H. Zhang and M. Lattman, Organometallics, 1996, 15, 5216.
9 (a) B. Cetinkaya, I. Gu¨mru¨kcu¨, M. F. Lappert, J. L. Atwood, R. D. Rogers
and M. J. Zaworotko, J. Am. Chem. Soc., 1980, 102, 2088; (b)
T. Fjeldberg, P. B. Hitchcock, M. F. Lappert, S. J. Smith and A. J. Thorne,
J. Chem. Soc., Chem. Commun., 1985, 939.
We thank the National Science Foundation (CHE 96-10497)
for support of this research. G. P. is the recipient of a
Presidential Faculty Fellowship Award (1992–1997).
Footnotes and References
* E-mail: parkin@chem.columbia.edu
† For the use of neutral calixarene ligands, see ref. 3(f).
Received in Bloomington, IN, USA, 13th August 1997; 7/05937J
102
Chem. Commun., 1998