Molecular Constraint and Flexibility - American Chemical Society

Chapter 17

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Main Group Element Calixarenes: Molecular Constraint and Flexibility Michael Lattman Chemistry Department, Southern Methodist University, Dallas, TX 75275

The class of compounds commonly referred to as calixarenes possess a combination of constraint and flexibility. This combination of features allows the calixarene backbone to conform to the geometrical needs of a central atom or atoms while at the same time stabilizing the geometry. The smaller calix[4]arene is ideally suited for inserting a single main group element and adapting to geometry changes of that central atom. The larger calix[5]arene can accommodate two atoms within the cavity and provide sufficient constraint, in certain cases, to control the interaction between the two atoms.

© 2006 American Chemical Society

Lattman and Kemp; Modern Aspects of Main Group Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Calixarenes (J) are a unique class of macrocyclic compounds possessing a combination of constraint and flexibility. The constraint is due to the size of the calixarene cavity, while the flexibility arises from the conformational mobility of the phenolic rings. This is particularly true for the smaller members of the series, calix[4]- and calix[5]arenes. The calix[4]arenes are ideally suited to supporting a single main group element in its central cavity and adapting to various coordination changes of that central atom. The larger ealix[5]arene can accommodate two atoms within the cavity and, in certain cases, allows control of the interaction between the two atoms. This review describes our work, as well as related work by others, with main group element calix[4]- and [5]arenes.

Calix[4] arènes The ability of the calix[4]arene framework to support a variety of coordination geometries around a single main group element is best illustrated with phosphorus (2, 3, 4, 5). A single phosphorus atom can be inserted into [4] (See Figure 1 for abbreviations, etc.) via treatment with hexamethylphosphorustriamide (Figure 2). The reaction proceeds via elimination of two moles of dimethylamine yielding the zwitterionic sixcoordinate phosphorus derivative 1. Removal of the last mole of dimethylamine can be accomplished via treatment of 1 with acid or by heating. This results in cleavage of one of the P - 0 bonds yielding the three-coordinate phosphite, 2, containing a "dangling" phenolic group. The phosphorus in 2 can be alkylated to give the four-coordinate phosphonium salt 3. Finally, deprotonation produces thefive-coordinatephosphorane 4. The x-ray crystal structures of 1 (R = H), 3, and 4 (Figure 3) are illustrative of the conformational changes that the calixarene undergoes to support these different geometries. We have not obtained the x-ray structure of 2; however, the geometrical requirements of this compound should be similar to 3. The structure of 1 shows the geometry around the phosphorus atom to be a slightly distorted octahedron with the phosphorus lying just above the plane of the four oxygens and the hydrogen directed "inside" the basket. The calixarene backbone is in the cone conformation (see Figure 1 for definitions). The structure of the four-coordinate phosphonium cation shows that the phosphorus is a distorted tetrahedron. The free phenolic ring [0(4)] is flipped "up" leading to an approximate partial cone conformation. In addition, one of the P-O bonds (fP(l)-0(l)] shows an "upward twist" relative to the other two bonds. This twist places the actual conformation of the calix[4]arene backbone between the partial cone and 1,2-alternate conformations. Finally, the structure of the pentacoordinate phosphorus is very close an ideal trigonal bipyramid: the 0(l)-P-0(3) angle is 177°, all of the axial/equatorial bonds are within 2° of 90°, and the sum of the three equatorial angles is 360°. The calix[4]arene backbone is in an approximate partial cone conformation. tBu



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The symbols [w] will be used throughout with η and R defined in the calix[n]arene structure at left. R

In most cases R = t-Bu, unless otherwise indicated. calix[n]arene Weaitod çQnfQrmatipp^ pf cajjxKlarenes

OH

OH

cone

partial cone

OH

1,2-alternate

OH

1,3-alternate

Figure 1. Abbreviations and idealized conformations of calix[4]arenes. Note that the four idealized conformations of calix[5]arenes are the same as for th calix[4]arenes.



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Figure 2. The calix[4]areneframeworkcan support a six-,five-,four-, and three-coordinate phosphorus atom. Similar reactivity is observedfor R


241 It is interesting to postulate how 4 is formedfrom3. For example, after deprotonation, P-O(l) might twist down and the phenoxy group simply rotates to form the fourth P-0 bond. Alternatively, the phenoxy group might flip and then form the fourth P-0 bond. The difference between these two pathways is that 0(1) and 0(3) in 3 become axial groups in 4 in the former route, whereas they end up at equatorial positions in the latter. The asymmetry created by the upward twist of the P - 0 bonds in 3 and 4 is not observed in the solution Ή NMR spectra of these compounds: the phosphonium cation exhibits C symmetry while the phosphorane shows C symmetry. Similar, but more limited, coordination changes are observed for siliconcontaining calix[4]arenes (Figure 4) (5, 6). When the calixarene oxygens are not all bound to the central atom, for example in 3 and 5, there exists the possibility of conformational isomerism. In fact, we do find that both of these derivatives exist as partial cone and cone isomers. These can be isolated and separately characterized. We measured the interconversion barrier for the silicon dervative and found an activation energy of about 120 kJ/mol with a negligible enthalpy difference between the conformers (5). This is about twice the value of the inversion barrier of the free calix[4]arene, 14J . For the R = H derivative, this isomerism even occurs in the solid state: the partial cone conformer of 5 (R = H) undergoes an irreversible phase change to the cone conformer at 230 °C, well below the melting point. No such conversion is observed for the R = t-Bu derivative.


s

4v

R

u

Me

5-cone

Ô

\le

R

5-partiaI cone

The observation and isolation of these isomers also illustrates the power of *H NMR spectroscopy to help determine conformation. Both conformers of 5 have C symmetry, so their spectra have the same number and multiplicities of resonances. However, the orientation of the free phenolic group has a marked effect on the position of the (silyl) methyl resonance. In the cone conformation (R = H), this resonance appears at 0.72 ppm, while in the partial cone, this same resonance is at - 0.29 ppm, a full ppm upfield. The large upfield shift is due to the shielding of the methyl group in the partial cone conformation, since this group lies "above" the aromatic ring in this orientation. Insertion of the larger arsenic into calix[4]arene proceeds smoothly (7). However, in this case, all three moles of dimethylamine are lost, and the product obtained is the arsenite, 7. No hexacoordinate compound analogous to 1 is observed. While it is tempting to assume that the heavier congener is too large s



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Lattman and Kemp; Modern Aspects of Main Group Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2005. C(48)

Figure 3. X-ray crystal structures ofl(R= H), 3, and 4 illustrating the conformational changes that the calix[4]arene framework undergoes due to the coordination at the central main group atom. (Adaptedfrom references 4 and 5. Copyright 1994, 2000 American Chemical Society.)

C1301



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to fit into the base of the basket when bound to all four oxygens, the fact that transition metals can be inserted and bind to all four oxygens argues against this. Perhaps the more significant factor in the inability to observe the hexacoordinate arsenic is the A s - H bond energy which is substantially lower than that of the P-H, by about 75 kJ/mol.

Attempts to use some of our silicon-containing calix[4]arenes to direct substitution at specific oxygen groups has met with limited success. We have succeeded in using the silicon in 5 as a protecting group for the generation of monosubstituted calix[4]arenes according to Figure 5 (6). Unfortunately, attempts to synthesize "1,2-disubstituted" calixarenes using the dimethylsilyl protecting group led to an apparent disproportionation (Figure 6) (5). However, Miyano and coworkers, reasoning that such a disproportionation might arise due to "...the lability of the dimethylsilyl moiety per se as well as by the ring strain..." did succeed in synthesizing 1,2-disbustituted calixarenes by use of "...a silyl bridge of proper chain length...", specifically tetraisopropyldisiloxane (TIPDS) according to Figure 7 (9).



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Figure 5. Synthesis of monosubstituted calix[4]arènes via 5.

Figure 6, Apparent disproportionation attempting to synthesize 1,2disubstituted calix[4]arenes.

Y m /-Pr SiOSi/-Pr 2

2

Figure 7. Successful use of the \-Pr SiOSi\-Pr protecting group to synthesi 1,2-disubstituted calix[4]arenes. 2

2


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Several groups have used some of our silyl and phosphorus calix[4]arene compounds (as well as derivatives synthesized from them) in catalytic applications. For example, Ladipo and coworkers have used 16, obtained by the treatment of 10 with TiCLt, for the highly regioselective

eyclotrimerization of alkynes (10). We have used a series of silyl-substituted calix[4]arenes as external electron donors in Ziegler-Natta polypropylene catalysts (11). Pringle and coworkers have synthesized a variety of metal complexes of 2 (R = /-Bu, H). In addition, they reported a catalytic study of rhodium complexes of these ligands in the hydroformylation of 1-hexene (12, 13). Concurrently, van Leeuwen and coworkers reported the use of derivatives of 2 in the rhodium-catalyzed hydroformylation of 1-octene (14). The complexes of 2 mentioned in the previous paragraph utilize this phosphorus calixarene as a simple monodentate phosphorus ligand. However, the free phenolic group might allow for binding of both the phosphorus and oxygen to a single metal center. Moreover, the constraint provided by the calixarene backbone might allow steric control of the ligand-metal interaction. In an effort to synthesize such a derivative, we treated 2 with butyllithium followed by addition of CVTiCp. This does lead to 17, a species containing a T i - 0 bond (15). However, the x-ray crystal structure of this complex (Figure 8) reveals that the calix[4]arene is in an approximate partial cone conformation placing the phosphorus and metal on "opposite" sides of the calix[4]arene, thus prohibiting any phosphorus/titanium interaction. If the reason for this orientation is the steric crowding that would occur if both the phosphorus and metal were on the same side of the small calix[4]arene cavity, then moving to a slightly larger calixarene might lead to complexes where both the phosphorus and oxygen might bind to a single metal.



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Figure 8. Complex 17 and its x-ray crystal structure. (Adapted with permis from reference 15. Copyright 1999 Taylor and Francis.)

Figure 9. Insertion of one and two bridging silyl groups into calixfSJarene


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CalixfS] arènes Examining models of the larger calixarenes, we felt that the calix[6]arene would be too large and floppy to provide any significant constraint. The calix[5]arene, on the other hand, appeared to have the right combination of constraint and flexibility to allow insertion of both a (phosphorus) ligand and metal inside and to be able to study and control their interaction. In a study to determine substitution patterns in the calix[5]arene (16), we were able to insert one and two bridging silyl groups (Figure 9). The insertion of the single silyl group is best accomplished by using a large excess of the calixarene (0.35:1 silanexalixarene). Otherwise, significant amounts of the disubstituted 19 are formed which leads to mixtures that are difficult to separate. Tricoordinate phosphorus can also be inserted into calix[5]arene (Figure 10). However, control of substitution is more difficult. We have only succeeded in isolating a monophosphorus deriviative with the Me NP group, not withPhP(/7). 2

R

21b(R' = Ph) Figure 10. Insertion of one and two bridging phosphorus groups into calixfSJarene.

Calixarene 20 seemed to be the ideal candidate to test our hypothesis that the calix[5]areneframeworkis ideal to control the interaction between a ligand and metal. Tungsten can be inserted via the mixed imido/amido W(VI) reagent according to Figure 11 (18). The P---W distance in 22 is 3.15 Â, outside the range of a P-W bond. The x-ray crystal structure (18) shows the phosphorus lone pair to be pointing directly at the vacant coordination site of the tungsten. Several reasons could account for the long distance: the calix[5]arene framework is holding the two atoms apart; the soft phosphorus lone pair and the hard tungsten (VI) center are a poor match; the amido and imido ligands place a lot of electron density back onto the tungsten, so no extra density is needed. To test the importance of the latter reason, we treated 22 with triflic acid to replace the excellent π-backbonding amido ligand with a much weaker binder, triflate. The resulting product, 23, has a Ρ—W distance of 2.74 Â, on the order of a normal P-W bond, demonstrating that the calix[5]arene backbone is indeed the right framework to study and control the interaction between a ligand and metal.



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Figure 11. Insertion of tungsten and control of the phosphorus/metal interaction. We have expanded the range of phosphorus-containing calix[5]arene ligands, two examples of which are illustrated in Figure 12(17).

Figure 12. Further examples ofphosphorus-containing calix[5]arene ligands The derivatives containing two trivalent phosphorus atoms, such as 21a, 21b, and 25 should be ideal as bidentate ligands toward transition metals. However, the specifics of the binding may be significantly different. This is illustrated in Figure 13 which shows the x-ray crystal structures of 21a and 25. In 21a, the calix[5]arene is in an approximate cone conformation, while 25 is in an approximate 1,2-alternate conformation. These compounds are expected to be excellent ligands toward transition metals, particularly in light of work on metal binding of larger calixarenes (19). Both phosphorus lone pairs are oriented to bind to a single metal in 21a and 25 with, perhaps, the metal "outside" the calixarene cavity with 21a and "inside" the cavity with 25. In addition, the free phenolic oxygen may also interact. The monophosphorus compound 24 may serve a similar function as 20, where both the phosphorus and oxygens bind to the metal.



Figure 13. X-ray crystal structures of21a (top) and 25 (bottom). (Rep from reference 17. Copyright 2004 American Chemical Society.)


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Acknowledgment This work was supported in part by funding from the the National Science Foundation (CHE-9522606) and the Robert A. Welch Foundation. This paper is dedicated to my postdoctoral advisor, Alan H. Cowley, F. R. S., on the occasion of his 70 birthday, for his continuing inspiration and support throughout my career. th


References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

See, for example, Calixarenes 2001; Asfari, Z.; Böhmer, V.; Harrowfield, J.; Vicens, J., Eds.; Kluwer: Dordrecht, 2001., and references cited therein. Khasnis, D. V.; Lattman, M . ; Gutsche, D. J. Am. Chem. Soc. 1990, 112, 9422. Khasnis, D. V.; Burton, J. M . ; Lattman, M . ; Zhang, H. J. Chem. Soc., Chem. Commun. 1991, 562. Khasnis, D. V.; Burton, J. M . ; McNeil, J. D.; Santini, C. J.; Zhang, H.; Lattman, M . Inorg. Chem., 1994, 33, 2657. Fan, M . ; Shevchenko, I. V.; Voorhies, R. H.; Eckert, S. F.; Zhang, H.; Lattman, M . Inorg. Chem., 2000, 39, 4704-4712. Shang, S.; Khasnis, D. V.; Burton, J. M . ; Santini, C. J.; Fan, M . ; Small, A. C.; Lattman, M . Organometallics, 1994, 13, 5157. Shang, S.; Khasnis, D. V.; Zhang, H.; Small, A. C.; Fan, M . ; Lattman, M . Inorg. Chem., 1995, 34, 3610. Fan, M . , Zhang, H.; Lattman, M . Organometallics, 1996, 15, 5216. Narumi, F.; Morohashi, N . ; Matsumura, N . ; Iki, N . ; Kameyama, H.; Miyano, S. Tetrahedron Letters 2002, 43, 621. Ozerov, Ο. V.; Ladipo, F. T.; Patrick, B. O. J. Am. Chem. Soc. 1999, 121, 7941. Kemp, R. Α.; Brown, D. S.; Lattman, M . ; Li, J. J. Mol. Catal. A: Chem., 1999, 149, 125. Cobley, C. J.; Ellis, D. D.; Orpen, A. G.; Pringle, P. G. J. Chem. Soc. Dalton Trans. 2000, 1101. Cobley, C. J.; Ellis, D. D.; Orpen, A. G.; Pringle, P. G. J. Chem. Soc. Dalton Trans. 2000, 1109. Parlevliet, F. J.; Kiener, C.; Fraanje, J.; Lutz, M . ; Spek, A. L.; Kamer, P. C.; van Leuwen, P. W. Ν. M . J. Chem. Soc. Dalton Trans. 2000, 1113. Fan, M . , Zhang, H.; Lattman, M . Phosphorus Sulfur and Silicon, 1999, 1446, 257. Sood, P.; Zhang, H.; Lattman, M . Organometallics, 2002, 21, 4442. Sood, P.; Koutha, M . ; Fan, M . ; Klichko, Y.; Zhang, H.; Lattman, M . Inorg. Chem. 2004, 43, 2975. Fan, M . , Zhang, H.; Lattman, M . J. Chem. Soc., Chem. Commun., 1998, 99. Redshaw, C. Coord. Chem. Rev. 2003, 244, 45.


Molecular Constraint and Flexibility - American Chemical Society

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