Journal of Inorganic and Organometallic Polymers, Vol. 14, No. 4, December 2004 (© 2004)

Coordination and Organometallic Polymers and Oligomers of Upper-Rim Functionalized Calix[4]arenes by Transition Metals Pierre D. Harvey Received May 16, 2004; revised July 15, 2004 This short review focuses on recent advances in the syntheses, characterization and structures of coordination and organometallic complexes of upper-rim functionalized calix[4]arenes that form either small oligomers or polymers. This field is very limited, presumably due to the lack of X-ray data or reliable characterization that demonstrates the presence of oligomers or polymers. Nonetheless, the few published works already clearly demonstrate the immense versatility of the calix[4]arene macrocycle as it forms polymeric materials via coordination bondings with transition metals. KEY WORDS: Polymers; oligomers; coordination; organometallic complexes; calix[4]arene.

INTRODUCTION The calix[4]arene macrocycle in its cone conformation is a very versatile molecule [1–9]. Indeed, due the bowl-shaped geometry (as shown below), it can be used as a host allowing organic and inorganic guests to penetrate its cavity. The possibility of building organic, coordination and organometallic architectures at the lower – (narrow-) and upper-rims (wide-rim) is also very appealing for extending the cavity, or to take advantage of the proximity to promote substituent interactions. Such molecules find

Contribution from the D´epartement de chimie, Universit´e de Sherbrooke, Sherbrooke, P.Q., Canada J1K 2R1, E-mail: [email protected] 211 1053-0495/04/1200-0211/0 © 2004 Springer Science+Business Media, Inc.

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applications in the area of selective ion extractions, receptors, catalysis, optical devices, biomimetics, and luminescence probes, just to name a few [1–9].

upper-rim

lower-rim

R O O OO R R R R = H or alkyl group

The upper-rim functionalization of this platform molecule occurred only recently with respect to the lower-rim, and this is particularly true for the incorporation of metal-containing fragments [3]. This can be explained by the fact the lower-rim functionalization is easier than the upper-rim. Since the recent emergence of this field, more and more examples of transition metal-containing oligomers and polymers of calix[4]arenes have appeared in the literature. Although these examples are relatively rare, there are enough findings to make adequate descriptions of the various types of polymers, as well as their properties and trends. To our knowledge, there is no other review of this kind, and the timing is appropriate as the quest for new materials for nano-technology, supramolecular assemblies, gels and metallo-gels, photonics, semi- and photo-conductivity, for example, is increasing at a rapid pace.

REVIEW Upper-Rim Functionalized Calix[4]arenes The upper-rim functionalization of the macrocycle in its cone conformation can be performed in several ways. Multiple functionalization is possible as one to four groups (G) can be anchored at the upper-rim. Two geometric isomers exist in the case where two groups are anchored. The official IUPAC nomenclature for 25,26,27,28-calix[4]arene is pentacyclo[19.3.1.13,7 .19,13 .115,19 ]octacosa-1(25),3,5,7(28),9,11,13(27),15,17,19(26), 21,23 -dodecaene-25,26,27,28-tetraol. When substituents are present at the upper-rim, the numbers 5, 11, 17, and 23 designate the positions. So, the tetrasubstituted macrocycle is named 5,11,17,23-tetrahydrocalix[4]arene. To ease the reading, this nomenclature is changed for the lighter mono-, di1,2-, di-1,3-, tri-, and tetra-notations.

Coordination and Organometallic Polymers and Oligomers

G

G

R O O OO R R R

di-1,2G

G

G

G

G

R O O OO R R R

R O O OO R R R monoG

213

di-1,3G

G G

G

R O O OO R R R tetra-

R O O OO R R R tri-

The syntheses of numerous multidentate calix[4]arene macrocycles that contains donor atoms at the upper-rim have recently been reviewed [3]. Typical donor groups include −C ≡ N, −N ≡ C, −PR2 , −CH2 PR2 , −OCH2 P(OR)2 , −CH2 CH = CH2 , −SR, and −SO− 3 (R = alkyl or aryl). Most of them are softer donors, while the sulfonyl (−SO− 3 ) is a rather hard Lewis base. The complete series of functionalization from (mono- to tetra-) by phosphine groups was recently reported by Gagnon et al. [10]. The monodentate ligand is not expected to act as an assembling molecule, but the presence of π-electrons [11] and ether groups at the lower-rim do not preclude the possibility of multidentate behavior either. However, such properties have not been observed in this series. The most frequently encountered species are the tetra- and di-1,3-substituted calix[4]arene ligands. The di-1,2-type is rare while the tridentate ligand is almost entirely absent from the literature. This observation is directly related to the number of synthetic steps required to obtain these derivatives. One important property that is often encountered is the presence of other conformers. These include cone, partial-cone, 1,2- and 1,3-alternate, which have been reviewed on several occasions [1–9].

O R O O OO R R R cone

R OO R

R

R

R

O O R

partial-cone

R

O

R O O

R 1,2-alternate

R O

O

O O

R R 1,3-alternate

Often, equilibrium between these species occurs in solution. Generally, the cone conformation is secured by anchoring larger groups at the

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lower-rim. Normally, a 3-carbon-containing group such as i- and n-propyl is just enough to obtain the cone conformation and avoid inter-conversion between the various conformations. This “rule” is fairly rigorous but exceptions may occur, such as the example provided.

Characterization of the Oligomers and Polymers The presence of coordination and organometallic oligomers and polymers can be demonstrated in several ways. One of them is crystallography; a technique that is typical and highly desired in coordination chemistry. However, as the metal-ligand bond becomes more covalent, ligand dissociation in solution becomes less extensive and the size of the oligomers or polymers becomes large. As the size of the molecules increases, the material has a tendency to be more amorphous in the solid state, rendering the possibility of obtaining crystals suitable for X-ray crystallography very slim. The normal alternative to demonstrate and characterize the material is the use of molecular weight determination techniques. This alternative is not problem-free. For multi-charged oligomers and polymers, the oftenencountered limited solubility prevents the application of common techniques such as osmometry, light scattering, measurements of the intrinsic viscosity, and others. In addition, the use of gel permeability chromatography methods is inadequate as the charged species get blocked in the column due to strong ionic interactions, despite the fact that precautions are taken. Recently, this group reported in detail, an NMR methodology useful to characterize oligomers in solution based on the pulsed techniques spinlattice relaxation time (T1 ) and nuclear Overhauser enhancement (NOE) measurements [12]. The method consists of measuring the correlation time (τc ) of the tumbling molecules, which is directly related to its size. To do so, the knowledge of a T1 that is directly related to molecular motions (via τc ) in solution is required. This T1 is T1DD (a mechanism related to dipole-dipole interactions). Its accurate evaluation is performed by the measurements of the NOE constants as well as the T1 ’s. Since this method is based on motions in solution, the dragging of solvent molecules must be taken into account as well. The best way to do so is to use a welldefined standard; a molecule for which the structure is known and closely related to the sample molecules, where the solvent-solute interactions are identical (ideally) or very similar. Again, this method is fully described in reference 12, and presents the advantage that it is not limited to the quantity of material available, since it is based on the accumulation of data. However, this method presents limitations if the structure exhibits

Coordination and Organometallic Polymers and Oligomers

215

a high degree of flexibility, like metal-containing units mono-bridged by single-bond-containing ligands [13]. If this is the case, the method becomes inaccurate.

Coordination and Organometallic Polymers Among the earlier polymers reported in the literature, the use of nitrile-containing calix[4]arenes was made. In 1998, Hosseini and collaborators reported the synthesis and X-ray structure of a coordination polymer based on a tetranitrilecalix[4]arene shown below (counter anion = AsF− 6 ) [14].

N

OR

C R

+

Ag

RO N C

O

C N C N

OR

C

N

R +

Ag

RO

N C

OR

O

C N C N OR

n

R = CH2CH2OMe

1

The particularity of this polymer (1) is that the calix[4]arene macrocycle exhibits the 1,3-alternate conformation despite the size of the ether group of the lower-rim, and induces the formation of an interesting 1-D polymer. The Ag(I) cations are tetravalent with a distorted tetrahedral geometry and coordinate with two nitrile groups coming from each neighboring calix[4]arenes, forming an alternating Ag-calix[4]arene chain. Atwood and collaborators reported another example of coordination polymer based on a tetranitrilecalix[4]arene (2) [15]. In this example, benzyl groups were used at the lower-rim and the calix[4]arene macrocycles exhibit the pinched cone conformation. The Ag(I) ions were also tetracoordinated with a distorted tetrahedral geometry (the counter-ion is PF− 6 ). Each Ag metal coordinates two cofacial nitrile groups coming from the same calix[4]arene, acting as a chelating fragment. The two other ligands are two nitrile groups coming from two neighboring calix[4]arenes completing the coordination sphere. The overall structure is also 1-D, somewhat resembling a flattened stick. Large cavities were present, which were occupied by dichloromethane and methanol solvate molecules.

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Harvey OR OR OR RO

OR OR OR RO

C N Ag+ N C

OR

N C

C C N N

Ag+

Ag+

N N C C

C N

C N

C C N N

Ag+

N N C C

N C

ORRO OR

OR

C N

N C

OR RO OR

n

R = CH2Ph 2

The corresponding tetraisocyanide ligands were investigated as well (3). The first report for the syntheses of the ligand was made by Gagnon and collaborators [16]. For comparison purposes, the monodentate ligand (4) was synthesized as well. C N

C N

C N

C N

C N

R O O OO R R R

R O O OO R R R

3

4 Cl Au C N

R O O OO R R R

5

Cl Au

Cl Cl Au Au C C N N

Cl Au C N C N

R O O OO R R R

6

These ligands bind soft metals such as Au(I) and allow the construction of multinuclear architectures at the upper-rim such as in 5 and 6. In these examples, the MALDI-TOF data for the tetragold(I) species exhibit

Coordination and Organometallic Polymers and Oligomers

217

a fragment peak at 1584, which correspond to the molecular ion minus a Cl atom [17]. The spectra also exhibit weaker signals at 2738, 2967, 3128, 3197, 3273, and 3678, which correspond to (calix)2 Au4 Cl, (calix)2 Au5 Cl2 , (calix)2 Au6 Cl, (calix)2 Au6 Cl3 , (calix)2 Au6 Cl5 , (calix)2 Au8 Cl5 (calix = 5,11, 17,23-tetraisocyano-25,26,27,28-tetrapropoxycalix[4]arene), respectively, and suggest the presence of a dimer structure in the solid state. No X-ray structure was available and 1 H NMR observed no evidence for agglomeration in solution. No further investigation was pursued, but the presence of Au· · · Au interactions in the solid state was strongy suspected in order to explain the data. A polymeric network built upon multiple Au· · · Au interactions can also be suspected. Both 5 and 6 exhibit luminescence properties in the solid state and in solution arising from the π –π * triplet level of the aryl-NC fragment. The use of the di-1,3- (7) and 1,2-diisocyanide (8) ligands (with four n-propyl groups at the lower-rim) was also made in order to form the corresponding coordination polymers 9 and 10 with Ag(I) ions [18]. C N

C N

C N

C N

R O O OO R R R

R O O OO R R R

7

8 Ag+

C N

Ag+ C N

C N

R O O OO R R R

9

n

R O O OO R R R

C N

n

10

The presence of a single IR absorption in the 2203–2210 cm−1 range (associated with coordinated Ag-CNAryl species) and the 1:1 stoichiometry obtained from the elemental analyses, strongly suggests the presence of a polymer in the solid state. The fact that the materials are amorphous precludes the possibility of obtaining single crystals suitable for X-ray analysis. The light sensitivity of the materials suggests that the Ag atoms are unsaturated in these cases. Pehaps the presence of inter calix[4]arene steric hindrance prevents the formation of polymers with a

218

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1:2 stoichiometry where the Ag(I) atom would be tetravalent such as in {M(dmb)+ 2 }n (M = Cu(I), Ag(I); dmb = 1,8-diisocyano-p-methane) [12]. On the other hand, when the tetraisocyanide ligand 3 was used with Ag(I), an insoluble white material, that was particularly robust to heat (up to 300◦ C), was obtained. Ag+

BF4 -

C

C

C

CN

N

N

N

OPr OPr

PrO

n

OPr

11

The IR data suggest the presence of small oligomers, as absorption associated with uncoordinated −NC groups are observed. The material exhibits a glass transition at 56◦ C (Cp = 0.08 J o−1 g−1 ). Motions of the counterions are thought to be responsible for the thermal behavior. The ligand 1,3-bis(methylenediphenylphosphino)tetrapropoxycalix[4] arene was prepared and characterized by X-ray crystallography by Kubas and collaborators [19]. Its reaction with Pd(COD)MeCl and (COD)PtCl2 leads to the formation of polymers as suggested by the presence of broad peaks in the 1 H NMR spectra. X Ph2 P M

P Ph2

Cl

OPr

OPr PrO OPr

n

12; M = Pd; X = Me 13; M = Pt; X = Cl

When this ligand was reacted with [(COD)RhCl]2 , a mono-calix[4]arene species exhibiting two pendant Rh(COD)Cl groups was obtained. By removing the Cl− ligand with an halogen abstractor such as Tl+ , an unsaturated site was created at the Rh(I) center, available for coordination with a bridging ligand. Indeed, Plourde et al. [20] and Mongrain [21]

Coordination and Organometallic Polymers and Oligomers

219

reacted [(COD)RhCl]2 , with the bi- (14–16) and tetradentate ligands (17) shown below and obtained dimer species in solution.

R2 P

R2P

Pr

R2 P

O O O O Pr Pr Pr

Pr

14 a,b,c

PR2

O O O O Pr Pr Pr

Ph2P

Ph2P Ph2P

Ph2P

OO Pr

O O Pr Pr Pr

Ph2P

OO Pr

O O Pr Pr Pr

17

16

15 a,b,c

PPh2

R = Me (a), i-Pr (b), Ph (c)

The unambiguous determination of the species in solution was made on the basis of 31 P NMR T1 and NOE measurements using the ligand themselves as standards. The molecular modeling was used to propose structures, and so dimers 18–21 were computed at the MMX level (PCModel). The optimized structures proved plausible on the basis of the absence of deviation from normal bond distances and angles, and absence of steric hindrance where no abnormal close contact was noticed. Drawings of potential structures are shown below. Other conformations may exist. These oligomers proved to be very efficient catalysts for the homogeneous hydroformylation of terminal alkynes, as the turnover frequencies are among the best reported so far. On the other hand, the regioselectivity described by the n/i ratio (normal vs. inverse; i.e., terminal vs. branched) turned out to be good but fell short with respect to the best Rh-catalysts [20].

Pr Pr Pr Pr O O O O

PR2

Pr Pr Pr Pr O O O O

R2P +

Rh +

Rh

PR2

P PR2 R2 + Rh Rh +

R2P

Pr O O O O Pr Pr Pr

PR2

PR2

Pr O O O O Pr Pr Pr

19 a, b, c

18 a, b,c R = Me (a), i-Pr (b), Ph (c)

220

Harvey Pr Pr Pr Pr O O O O

Pr Pr Pr Pr O O O O

PR2 PR2 +

+

Rh

Rh

R2P

PR2 P R2

PR2 PR2

R2P

Rh

PR2 PR2

PR2

Pr O O O O Pr Pr Pr

PR2

+

+

Rh

Rh+

Rh+

Pr O O O O Pr Pr

R =Ph

Pr

20

21

The formation of dimer species with square planar metallic fragments is not unprecedented. Indeed, Takenaka et al. recently reported the synthesis and the X-ray structure of a dimer complex, 22, formulated as [dichloro(5,17-bis(diphenylphosphino)-25,26,27,28- benzoxycalix[4]arene)palladium (II)] [22]. This result contrasts the findings for other square planar complexes of 1,3-diphosphinated calix[4]arenes, including the dicholoplatinum(II) analogue, which is a monomeric species [23, 24].

OBz

OBz BzO OBz

Ph2P Cl

Pd

PPh2 Cl

Cl

Ph2P

Pd

Cl

PPh2

OBz

OBz BzO OBz

22, Bz = CH2Ph

Other examples of dimers built upon Pd(II) and Rh(II) cations and other donor groups, include compounds 23 [25], 24 [26], and 25 [27]. All in all, the dimerization of upper-rim functionalized calix[4]arenes via the coordination of metal fragments, is common.

Coordination and Organometallic Polymers and Oligomers

221 O

OPr

Br

OPr PrO OPr

Pr Pr

OO OO

Pr

O

O

Br

Pr

Ph2 P

O O

OO

O O O N N RhRh O O O N N

OPr

R

PrO

O

O

R

OP

R

N

P Ph2

Pd

N

Ph2 Ph2 P N P Pd

P Ph2N

N

Pd

P N Ph2

P Ph2

OO R R`

=

N

Ph2N P

Pd R`

OPr

O O

O O

O

OO O

Rh Rh Br

O O

R`

Br

O O OO

8+ O

O O

O O

NN RhRh N N

O

O O

O R`

R= n-Pr; R`= p-C6H4OMe 23

25

24

This information is relevant to the possibility of having monomer-dimer and dimer-oligomer equilibria in solution from ring opening polymerization (ROP). Although this process is highly possible, no such observation was made so far for a calix[4]arene species. Xu and collaborators recently reported an interesting stable Pd-containing polymer gel capable of “uptaking” neutral organic molecules [28]. The reaction of tetra-(3-pyridineazo)calix[4]arene (26) with Pd(en)(OH2 )2+ 2 (en = ethylene-diamine leads to the metallogel 27, presumably best described as {calix[Pd(en)]2+ }n . NH2 H2N Pd N

N N N N

N

N N

N

N

N N N

N

N 2+

Pd(en)(OH2)2

N NN N

NH2 H2N Pd N N N NN

DMF H O O HOO H H

26

H

OO OO H H H

n

27

The proposed structure consists of interstitial space of random conformers of the tetradentate ligand 26, cross-linked by Pd(en)2+ fragments. The typical percentage of calix[4]arene molecules in the gel vs. solvent molecules is 0.5 to 2.0 wt.%, corresponding to a molar ratio of about 1:1000.

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By using Pd(COD)Cl2 as a source of Pd(II) ions, the gel formation was also observed but takes a longer time (72 h in DMSO). This extended period of time was probably due to a slower ligand exchange between pyridine and COD. When the gelator was at 0.5 wt.% (gel-to-solvent), the formation of the metallogel was reversible. At a higher ratio (such as 2 wt.%), the process was irreversible, even upon heating. The greater number of cross-links explains this behavior. The gel was stable in most hydrophilic solvents such as THF, DMF, PhCH2 OH, CH3 CO2 H, and DMSO, and hydrophobic solvents such as CHCl3 , CH2 Cl2 , and PhCH3 . Raston and collaborators recently reported another interesting 2-D coordination polymer built with an early transition metal [29]. The reaction of tetra-p-sulfanatocalix[4]arene salts with scandium(III) triflate in the presence of [18]crown-6 in aqueous solution leads to polymer 28, described as {[Sc2 (µ-OH)2 (H2 O)8 ][Sc(H2 O)4 ]-(calix[4]arene(SO3 )4 -H+ )2 ([18] crown6]·16H2 O}n according to an X-ray structure determination. The structure data reveals a linear array of capsules formed by two calix[4]arene macrocycles, associated with an infinite chain of aquated hydroxy-bridged scandium(III) monomers and dimers, to form a 2-D network. The crown ether resides in the cavity created by two adjacent calix[4]arenes placed on different sheets held in place by H-bonds. OH2H OH2 OH2 O Sc Sc OH2 H2O O O H OH2 OH2 OH2 SO2 H2O O SO2 O2S O Sc OH2 OH2 H2O

SO3

OH HO

OH O

n

28

This structure bears a close resemblance to that of 29, {[Cu(OH2 )4 ] [Na]2 -(calix[4]arenesulfonate)·7.5H2 0}n reported by Atwood et al. over 10 years ago [30], where the Cu(OH2 )2+ unit bridges two calix[4]arene 4 macrocycles via sulfanate-to-copper coordination bonds. The interactions between the Sc(III) and Cu(II) centers with sulfanate groups was anticipated to be ionic in nature, so that the coordination polymers were anticipated to strongly dissociate in solution. The use of

Coordination and Organometallic Polymers and Oligomers

223

tetra-p-sulfanatocalix[4]arene salts with other non-transition metals that form coordination polymers in the crystal phase was made extensively [1–8]. The chemistry of the dithiocarmate donor placed at the upper-rim of the calix[4]arene macrocycle has also been explored. Such examples include those reported by Beer and collaborators for compounds 30–32 [31]. The autoassembling was performed using Cu(II) metal for a monoand a 1,3-biscarbamate systems, and Ni(II) cation for the 1,3-biscarbamate ligand. On the other hand, the use of Zn(II) leads to a monomeric species, where an intra-molecular coordination of the metal atom caps the cavity. The reason for this particularity is not known. R

R

R OO R

OO

Bu S

N Bu

N S M

S

M S

S N

Bu N

R

OO R

S

S

S Bu

OO R

R

R OO

R OO R

N Bu S S Cu S S

R = CH2CH2OEt

Bu N

R

OO R

R

30 (M= Cu(II)) 31 (M= Ni(II))

OO R

R

32

Another interesting example of a dimer is complex 33 [32]. The X-ray structure reveals the presence of two rhenium complexes of calix[4]arene bonded together by an O-Re-O bridge. The rhenium atoms were placed just above the macrocycle cavities and each Re=O fragment point towards the pocket. Contrarily to the examples of dimers presented above, ROP was not possible.

PrO PrO PrO

OPr N O N O Re O O O Re O N O N

PrO

33

OPr OPr OPr

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Harvey

Metacyclophane Hosseini and collaborators reported a 1-D coordination polymer built with a tetracyano[1,1,1,1]metacylophane blocked in the 1,3-alternate conformation [33]. This macrocycle bears an important similarity to the calix[4]arene, where the hydroxy (or alkoxy) groups were replaced by methyl residues. The reaction with Ag(I) ion leads to an interesting tubular coordination polymer (34).

+

Ag N C

C N +

C N Ag

N C +

C N Ag N C +

Ag N C

C N

n

34

CONCLUSION The coordination of metallic fragments at the upper-rim of the calix[4]arene macrocycle shows a great tendency to form oligomers (dimers in particular), and on occasion, polymers. While is it relatively easy to demonstrate the nature of the dimers from X-ray structure determination or by mass spectrometry, when applicable, the situation is totally different for the polymers. While about a dozen transition metal-containing polymers exist for upper-rim functionalized calix[4]arene, only four of them have been characterized by X-ray crystallography so far based on the Cambridge Data Bank. The common point is the ionic nature of the metal-ligand bonding, allowing the possibility of extensive dissociative processes in solution, giving a better chance of crystal growth of the polymers. On the other hand, in systems where the bonding is more covalent, the oligomeric and polymeric materials have a greater tendency of being amorphous, which makes the characterization more difficult. In addition to these problems, the lack of solubility and the large charge on the polymer also complicate the characterization. Despite these challenges, this survey demonstrates the great potential for this macrocyle to form stable transition metal-containing polymeric materials. Although this field is only recently emerging, the investigation of the ROP process on various dimer species discovered so far as well other analogues that may be reported in the future, is an important avenue for research that deserve close attention.

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ACKNOWLEDGMENTS The author thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for funding for research in this area. REFERENCES ¨ 1. Z. Asfari, V. Bohmer, J. Harrowfield, and J. Vicens, Calixarenes (Kluwer Academic Publisher, Dordrecht, 2001). 2. C. D. Gutsche, Calixarene Revisited (Royal Society of Chemistry, Cambridge, U.K., 1998). 3. P. D. Harvey, Coord. Chem. Rev. 233–234, 289 (2002). 4. C. D. Gutsche, Calixarenes (Royal Society of Chemistry, Cambridge, U.K., 1989). ¨ 5. Calixarenes: A Versatile Class of Macrocyclic Compounds, J. Vicens and V. Bohmer, eds. (Kluwer Academic Publishers, Dordrecht, 1991). 6. Calixarenes 50th Anniversary: Commemorative Issue, J. Vicens, Z. Asfari, and J. M. Harrowfield, eds. (Kluwer Academic Publishers, Dordrecht, 1994). ¨ 7. V. Bohmer, Angew. Chem. Int. Ed. Engl. 34, 713 (1995). 8. J. Scheerder, M. Fochi, J. F. J. Engbersen, and D. N. Reinhoudt J. Org. Chem. 59, 7815 (1994). 9. C. Wieser, C. B. Dieleman, and D. Matt, Coord. Chem. Rev. 165, 93 (1997). 10. J. Gagnon, M. V´ezina, M. Drouin, and P. D. Harvey, Can. J. Chem. 7, 1439 (2001). 11. H. Iki, T. Kikuchi, H. Tsuzuki, and S. Shinkai, Chem. Lett. 1735 (1993). 12. M. Turcotte and P. D. Harvey, Inorg. Chem. 41, 1739 (2002). 13. P. D. Harvey, Macromol. Symp. 209, 67 (2004). 14. G. Mislin, E. Graf, M. W. Hosseini, A. De Cian, N. Kyritsakas, and J. Fischer, Chem. Commun. 2545 (1998). 15. E. Elisabeth, L. J. Barbour, G. W. Orr, K. T. Holman, and J. L. Atwood, Supramol. Chem. 12, 317 (2000). 16. J. Gagnon, M. Drouin, and P. D. Harvey, Inorg. Chem. 40, 6052 (2001). 17. J. Gagnon, Ph.D. Dissertation (Universit´e de Sherbrooke, 2001). 18. P. Mongrain and P. D. Harvey, Can. J. Chem. 81, 1246 (2003). 19. X. Fang, B. L. Scott, J. G. Watkin, C. A. G. Carter, and G. J. Kubas, Inorg. Chim. Acta 317, 276 (2001). 20. F. Plourde, K. Gilbert, J. Gagnon, and P. D. Harvey, Organometallics 22, 2862 (2003). 21. P. Mongrain, M.Sc. Dissertation (Universit´e de Sherbrooke, 2004). 22. K. Takenaka, Y. Obora, L. H. Jiang, and Y. Tsuji, Organometallics 21, 1158 (2002). 23. C. Wieser-Jeunesse, D. Matt, and A. DeCian, Angew. Chem. Int. Ed. 37, 2861 (1998). 24. M. Lejeune, C. Jeunesse, D. Matt, N. Kyritsakas, R. Wleter, and J. -P. Kintzinger, J. Chem. Soc. Dalton Trans. 1642 (2002). 25. J. Seitz and G. Maas, Chem. Commun. 338 (2002). 26. F. A. Cotton, P. Lei, C. Lin, C. A. Murilo, X. Wang, S.-Y. Yu, and Z.-X. Zhang, J. Am. Chem. Soc. 126, 1518 (2004). 27. Z. Zhong, A. Ikeda, M. Ayabe, S. Shinkai, S. Sakamoto, and K. Yamaguchi, J. Org. Chem. 66, 1002 (2001). 28. B. Xing, M. -F. Choi, and B. Xu, Chem. Commun. 362 (2002). 29. H. R. Webb, M. J. Hardie, and C. L. Raston, Chem. Eur. J. 7, 3616 (2001). 30. J. L. Atwood, G. W. Orr, C. Means, F. Hamada, H. Zhang, S. G. Bott, and K. D. Robinson, Inorg. Chem. 31, 603 (1992).

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31. P. R. A. Webber, M. G. B. Drew, R. Hibbert, and P. D. Beer, Dalton Trans. 1127 (2004). 32. K. J. C. van Bommel, W. Verboom, R. Huslt, H. Kooijman, A. L. Spek, and D. N. Reinhoudt, Inorg. Chem. 39, 4099 (2000). 33. C. Kleina, E. Graf, M. W. Hosseini, A. De Cian, and J. Fischer, Chem. Commun. 239 (2000).