Dendronized Copper(I)-Metallopolymers - American Chemical Society

Chapter 1

Dendronized Copper(I)-Metallopolymers Julia Kubasch and Matthias Rehahn*

Downloaded by 91.229.229.44 on October 6, 2016 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch001

Ernst-Berl-Institute for Chemical Engineering and Macromolecular Science, Darmstadt University of Technology, Petersenstrasse 22, D-64287 Darmstadt, Germany

Most metallopolymers based on kinetically labile transition– metal complexes are polyelectrolytes and dissolve in polar, coordinating solvents only. In these solutions, solvent mole cules can displace the metallopolymers' original ligands and thus cause decomposition. Consequently, such systems are very difficult to characterize. Our adopted strategy to over come this problem consists of the attachment of apolar sub stituents which solubilize the metal-containing polymers in non-coordinating solvents. Then, even intrinsically labile chains should behave like inert systems. Based on this strategy, we developed an efficient synthetic access to dendronized metallopolymers. The dendrons were attached very closely to the tetrahedral copper(I) complexes. Positioned in this way, they also increase the materials' solubility in non-coordinating solvents. Moreover, due to their bulkiness, the dendrons should reduce the tendency of the metal-containing polymers to aggregate in solution, and protect the kinetically labile com plexes against the attack of competing ligand molecules.

2

© 2006 American Chemical Society

Schubert et al.; Metal-Containing and Metallosupramolecular Polymers and Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

3

Background In recent years, a variety of fascinating supramolecular architectures were developed. Many of them are based on transition-metal complexes. In addition to well-defined oligonuclear assemblies like helicates, catenanes, dendrimers or grids, some high-molecular-weight, linear-chain species could be created - the so-called "metallopolymers". These latter compounds bear transition-metal com plexes either as lateral substituents of a classic covalent polymer mainchain ("complex polymers" A), or as an integral part within their polymer backbones ("coordination polymers" B). 1,2


3

a

Κ χ

(M)

M x

m

: transition metal

QSLX

Y'

: coordinative bond

: ligand monomer Y

: bridging unit

: chelating ligand

Figure 1: Schematics of (A) complex- and (B) coordination polymers Metallopolymers require - i f characterization is desired not only in the solid state but also in solution - thermodynamically very stable metal complexes. Moreover, kinetically inert complexes are advantageous because then the choice of solvent for polymer characterization is almost unlimited: ligand-exchange processes occurring between metallopolymer and a coordinating solvent can be excluded. This is the main reason why most soluble, well-defined and highmolecular-weight metallopolymers known today contain kinetically inert com plexes. In contrast to this, most kinetically labile systems decompose simultane ously with their dissolution. Characterization is therefore possible only in the solid state, and important molecular parameters like degree of polymerization, chain conformation, and backbone flexibility etc. are not accessible. Consequently, welldefined metallopolymers from kinetically labile transition-metal complexes are almost unknown, and so far, only very few systems based on copper(I) and silver(I) were prepared and characterized in solution. " 4

11


4 Recently, we outlined a number of requirements whose fulfillment is essential for obtaining readily soluble metallopolymers from kinetically labile complexes. " The key assumption was that kinetically labile complex- and coordination polymers decompose exclusively via the displacement of their original ligand moieties by coordinating solvent molecules. Even i f the latter molecules are not chelating ligands, they nevertheless compete successfully for the metal ions because they are present in a very large excess. To avoid this decomposition, steps may be taken to make the metallopolymers soluble also in strictly non-coordinating media: here, kinetically labile multinuclear complexes should behave like "true" polymers. Un fortunately, most metallopolymers are polyelectrolytes and prefer highly polar, coordinating solvents. A n appropriate measure to increase solubility in less polar solvents is attaching apolar alkyl side chains to the metallopolmers. Figure 2 dis plays two polymers developed recently based on these considerations. " 6


6

11

11

Figure 2: Molecular constitution of published copper(I) complex polymers /, and of copper(I) coordination polymers 2

Careful N M R analysis showed that metallopolymers 1 and 2 behave like real macromolecules when dissolved in non-coordinating solvents, but like open solution aggregates in the presence of even very small amounts of coordinating molecules. Unfortunately, further characterization in non-coordinating media via, for example, viscosimetry and light scattering was still affected by aggrega tion. Therefore, the effect of the solubilizing substituents had to be intensified: additional substituents had to be introduced into the metallopolymers which (/) improve solubility even more efficiently, (ii) reduce the aggregation tendency of the ionic complexes, (Hi) protect the labile complexes via steric shielding but (iv) do not affect the complex formation process itself. 12


5

Objective It is the objective of this contribution to present a further step towards solu ble, well-defined metallopolymers based on kinetically labile copper(I) com plexes. We attach sterically demanding dendrons close to the metallopolymers' coordinative centers. The synthesis of the dendrons is described as well as their introduction into the kinetically labile copper(I)-based metallopolymers 1 and 2.

Experimental N M R spectra were recorded on a B R U K E R D R X 5 0 0 spectrometer (500 M H z for H and 125 M H z for C ) . Acetone-d and 1,1,2,2-tetrachloroethane-d were used as the solvents. Signal assignment was done based on gs-COSYDF, N O E S Y , gs-HSQC, gs-HMBC and D E P T measurements and is given according to the numbering shown in Schemes 2 and 3. Mass spectra were recorded on a V A R I A N M A T 311 A and a V A R I A N M A T 212 mass spectrome ter. Ionization was done by field ionization (FI), field desorption (FD), fast atom bombardment (FAB) and electrospray ionization (ESI) technique, respectively. M A L D I - T O F mass spectra were recorded on a K R A T O S A N A L Y T I C A L Kompact M A L D I 4 mass spectrometer. Dithranol was used as the matrix, and L i C l or CuCl as salt. !

, 3


6

2

A l l chemicals were from F L U K A , A L D R I C H and A C R O S . oPhenanthroline derivatives 5[OH] and 5[OCH J, ligand monomer 6 [ O C H J , ' precursor polymer 1 and metallopolymer 1[L 1 were prepared according to the literature. Dendrons G (l->2) were prepared in generations χ = 1 - 3 according to the literature. Dendrons G ^ l - ^ 3 ) were prepared according to the procedure given here for Gi(l->3) (Scheme 2). A l l reactions were carried out under nitro gen. 13

6

3

7

3

8

2

x

14

G i ( l - » 3 ) C O O C H : Compound 4 (5.00 g, 27.15 mmol), 18-crown-6 (2.87 g, 10.86 mmol), K C 0 (15.01 g, 108.61 mmol) and dry acetone (500 mL) were stirred and refluxed. Benzyl bromide (11.29 mL, 16.25 g, 95.03 mmol) was added slowly, and refluxing is continued for 2 d. The mixture is cooled down, and the solvent is removed. The residue is dissolved in C H C I , washed with water and dried (MgS0 ). After evaporation of the solvent, the residue is crystal lized from acetone. The yield was 12.09 g (26.61 mmol, 98%). 3

2

3

2

2

4

G , ( l - » 3 ) O H : Compound G , ( l - > 3 ) C O O C H (10.00 g, 22.00 mmol), L i B H (0.96 g, 44.00 mmol) and dry T H F (250 mL) were stirred for 1 h at 0 °C, for 1 h at room temperature, and for 2 d under reflux. At room temperature, satu3

4


6 rated aqueous NaCl (150 mL) and ter/-butyl methyl ether (150 mL) were added. The organic layer was separated, washed (4 χ saturated NaCl, 1 χ water) and dried ( M g S 0 ) . The solvent was removed, and the pure product was obtained in yields of9.10g(97%). 4


Gi(l->3)Br: Compound G i ( l - * 3 ) O H (5.00 g, 11.72 mmol) and tetrabromomethane (11.66 g, 35.16 mmol) were dissolved in T H F (40 mL). A t 0 °C, triphenyl phosphine (9.22 g, 35.16 mmol) in T H F (40 mL) was added. The mix ture was stirred for 20 min at room temperature and filtered over a column of silica gel (toluene as the eluent). The oil obtained after removal of the solvent was again purified by column chromatography (silica gel, toluene). A crystalline product was obtained in yields of 3.50 g (61%). 5 ( G i ( l - » 3 ) ] : Compound 5 [ O H l (0.20 g, 0.55 mmol), G ( l - > 3 ) B r (0.65 g, 1.32 mmol), 18-crown-6 (0.07 g, 0.28 mmol), K C 0 (0.30 g, 2.20 mmol) and dry D M F (40 mL) were stirred and refluxed for 15 h. After cooling down to room temperature, saturated aqueous NaCl (100 mL) and C H C 1 (100 mL) were added. The organic layer is separated off, the aqueous one extrated with CH C1 . The combined organic layers were dried (MgS0 ). Removal of the solvent and recrystallization of the residue from acetone gave the pure product in yields of 0.43 g (65%). t

2

3

2

2

2

2

4

6[Gi(l->3)]: Compound 6[OHl (0.40 g, 0.43 mmol), Gj(l-»3)Br (0.50 g, 1.02 mmol), 18-crown-6 (0.04 g, 0.15 mmol), K C 0 (0.25 g, 1.81 mmol) and dry D M F (20 mL) were stirred and refluxed for 3 d. The work-up procedure was as described for 5[Gi(l->3)j, recrystallization was done from chloroform. The yield was 0.19 g (25%). 2

3

Model complexes 7[G (l->2/3)J: 0.003 mmol of the respective o-phenanthroline derivative 5, 0.0015 mmol [Cu(CH CN) ]PF and 1,1,2,2-tetrachloroethane-d (0.6 mL) were mixed in an N M R tube. A n N M R spectrum is recorded to control the 2:1-equivalence of ligand and metal species. If necessary, adequate amounts of the minor component were added. Precipitation of the pure com plexes was achieved by pouring the solution into w-hexane. x

3

4

6

2

Complex polymer l[G (l-»2/3)]: In an N M R tube, poly(2,9-o-phenanthroline-α/^2^5'-dί-Λ-hexyl-4,4"-p-teφhenylene) 1 (13 mg, 0.023 mol-equiv.) was dissolved in l,l,2,2-tetrachloroethane-d (0.5 mL). [Cu(CH CN) ]PF (8.4 mg, 0.023 mmol) was added. After shaking, a suspension of the respective phenanthroline derivative 5[G (l->2/3)J (0.023 mmol) in 1,1,2,2-tetrachloroethane-d (0.1 mL) was added. Isolation o f the product is possible via pouring the solution into an excess of w-hexane. B

2

x

2


3

4

6

7 Coordination polymer 2[G (l->2/3)]: Equimolar amounts of tigand monomer 6 l G ( l - » 2 / 3 ) l and [Cu(CH CN) ]PF (0.011 mmol for experiments in N M R tubes) were polymerized in l,l,2,2-tetrachloroethane-d (0.6 mL). After N M R analysis, the product is isolated via pouring the solution into w-hexane. x

x

3

4

6

2

Results & Discussion The first step towards dendronized copper(I)-metallopolymers was the preparation of appropriately functionalized bulky groups. Dendrons G ( l - » 2 ) B r were prepared for χ = 1 - 3 according to the literature (Scheme l ) . x


1 4

λ

+

K CQ ^ 2

G (1-2)Br

3

3

t

G (1-*2)OH

X

^

2

CBr /PPrt 4

g

i

2

* -* >

o h

•

K

X J

a C O i

18-CIOWTV6

\X

Τ

,0

3

Τ

G (1-»*2)Br 2

Scheme 1: Synthesis of G (l->2)Br dendrons x

The synthetic protocol includes two steps. The first step is ether formation: two equivalents of dendron G . t ( l - » 2 ) B r are treated with dihydroxybenzyl alco hol 3. In the second step, the formed G,(l->2)OH dendron is then converted into the bromo derivative G ( l - » 2 ) B r via treatment with C B r and PPh . The reaction cycle is then repeated. x

x

4

3

9

In order to increase the dendrons steric demand further, a fourfold branch ing point was introduced in some of the dendrons, leading to the G (l~*3) series. The synthesis is shown in Scheme 2. The higher steric demand is achieved by using 4 as the core molecule, and dendrons G ( l - * 2 ) B r of generation 1 - 3 as bromo counterparts. The conditions used for ether formation were the same as for G ( l - * 2 ) O H . Subsequently, G ( l - * 3 ) C O O C H were converted in almost x

x

x

x

3


8 quantitative yields into G , ( l - > 3 ) O H by treatment with L i B H in T H F . nally, the hydroxyl derivatives were brominated using C B r and PPh .

1 5 , 1 6

4


4

Fi

3

Scheme 2: Synthesis of G (l->3) dendons; the numbering given for Gi(l->3)Br is used for NMR signal assignment x

While yields o f 60% could be realized for G ( l - > 3 ) B r when an improved workup procedure was used, the yields were significantly lower for the higher generations. We assume this is the result of excessive steric demand within the dendron molecules which affects its preparation - at least according to the pro cedures published in the literature. t

, 3

Further proof of structure was carried out for all products using *H and C N M R , 2D N M R experiments and mass spectrometry. As a representative exam ple, Figure 3a shows the H N M R spectrum of G i ( l - » 3 ) B r together with the signal assignment. As one can see, all observed resonances support to the ex pected constitution, and the lack of extraneous resonances proves the high purity of the material. Analogous results were obtained for most other dendrons which were used subsequently to derivatize the hydroxyphenyl-functionalized ophenanthroline-based ligands. !

Dendronized ligands Using G ( l - » 2 ) B r and G ( l - » 3 ) B r , the dendronized auxiliary ligands S[G (l->2/3)] - required to stabilize complex polymer 1 - and the dendronized x

x

s


9 ligand monomers 6[G (1 -»2/3)l - required for the synthesis of coordination polymers 2 - were prepared. Auxiliary ligands 5 were dendronized according to Scheme 3:


%

Scheme 3: Synthesis of dendronized o-phenanthrolines 5; the numbering given for 5fG (l-+3)J is usedfor NMR signal assignment x

Because of the low solubility of starting material 5[OH], the reactions were carried out in D M F . In the presence of potassium carbonate and 18-crown-6, the desired ligands were formed in yields of 30 - 60 % after careful purification: it was obvious that the rate of the conversions as well as the obtained yields were strongly dependent on the size of the dendron used. A l l products were readily soluble in a variety of organic solvents. As a representative example, Figure 3b shows the H N M R of 5[Gi(l-»3)J. Analogously, we dendronized ligand mono mer 6 | O H ] (see Scheme 4 for the G ( l - » 3 ) series). !

K

Scheme 4: Synthesis of G (l-+3)-dendronized ligand monomers 6 x


10 The hydroxyl-functionalized ligand monomer 6[OHJ required was obtained by treating 6 [ O C H ] with pyridinium hydrochloride for 5 h at 230 °C. The pure, yellow material 6[OH] was obtained in 93% yield. Subsequently, dendronization leading to 6(G (l->2/3)] was possible in approx. 30% yield. Full characteriza tion was done using N M R and M A L D I M S techniques. As a representative ex ample, Figure 6a shows the H N M R spectrum of 6IGi(l->3)J. 3

x

!


Mononuclear model complexes In order to analyze the complex formation behavior of the dendronized ligands, systematic series of mononuclear complexes 7 were prepared (Scheme 5). Most of the experiments were carried out in N M R tubes. The evolution of the color and the characteristic chemical shifts in the *H N M R spectra allowed estimat ing the rate and the completeness of the complex formation processes.

5[G (1 X

2/3)]

7[G (1 X

2/3)]

Scheme 5: Synthesis of dendronized mononuclear model complexes 7

As a representative example, Figure 3b shows the *H N M R spectrum of the free ligand 5(Gi(l-»3)], and Figure 3c shows the corresponding spectrum of the resulting copper(I) complex 7[Gi(l-»3)]. Despite of the considerable steric de mand of the dendrons, rapid complex formation was observed in all cases. Obvi ously, the dendrons are flexible enough to allow for complex formation. Moreover, we show that ligand exchange is possible also in the dendronized complexes 7[G (l-»2/3)l. These studies were carried out by adding a second chelating ligand such as 5 | O C H j to solutions of the complexes 7 (Scheme 6) followed by monitoring the changes in the N M R spectra of the mixtures (Fig ure 4). 1

3


π 24

19

21

a) jy«_ I24

21 16

3,8

5

ι

19

6

17


4,7

24 16

c)

L

19

5,6 3,8

4,7 J

21

_

J

U

17

1

JL 7.0 6.5 6.0 Chemical Shift (ppm)

J

I [ r I I 1 1 I I I I ; ι ι ι ι ι I I I I j 1 I I 1 1 I I I I j I I 1 1 1 < r I I J ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι I I I I J I I I 1 1 I > I I ; ι ι ι ι ι '

8.5

8.0

7.5

l

5.5

5.0

4.5

4.0

Figure 3: HNMR spectra of (a) G (l-*3)Br (acetone-dc), (b) 5[Gi(1^3)] (c) 7[Gi(l-*3)J(IJJMetrachloroethane-di) t

and

Scheme 6: Ligand exchange experiments on mononuclear complexes 7

The equilibria found in these ligand-exchange experiments allow the con clusion that there is a slight preference of the sterically less demanding ligands to be incorporated in the complexes.


1

1 1

I *'

1 1

I

1 1

1 1

I


12

I

'

8.5

8.0

'

7.5

τ

7.0

6.5

6.0

. , . , I . . . . I •••.(.. , , ! . . ,

5.5

5.0

4.5

i u u p . n i

4.0

3.5

ppm ι

Figure 4: Η NMR spectra of (a) 7[Gt(l-*3)] of (b) free chelating ligand 5lOCH J and (c) of a 1:1 mixture thereof (tetrachloroethane-d^ t

3

t

Synthesis of complex polymers l[G (l->2/3)] x

Using the previously described copper(l) precursor polymer ÎPUI, the step-by-step introduction of the dendronized ligands 5IG (l-»2/3)J was studied (Scheme 7). HL2I was obtained via dissolving the metal-free precursor polymer 1 in tetrachloroethane and addition of equimolar amounts of [Cu(CH CN) ]PF . In the resulting precursor metallopolymer 1[L J, two coordinating sites of each copper(I) are occupied by acetronitrile (L) as the ligands. Subsequently, the bidentate dendronized auxiliary ligands 5(G (l->2/3)] were added. The progress of these conversions could be monitored using N M R . Figure 5 displays a representative spectrum of die metal-free precursor polymer 1, metallopolymer 1[L ] and the dendronized complex polymer l [ G i ( l - » 3 ) ] . K

3

4

6

2

g

2


13

J


1CG«î1-*2/3)l

n

Scheme 7: Synthesis of complex polymers J bearing dendronized ligands

The observed chemical shifts, and here in particular die considerable shift of the aC H protons (*) of the w-hexyl side chains show clearly that all dendronized ligands are coordinated to the polymer. 2

8

7

6

S 4 3 Chemical Shift (ppm)

2

1

0

Figure 5: *HNMR spectra of (a) the metal-free precursor polymer /, (b) l/LJ, and (c) IfGrfl-ta)/ (! l 2Metrachloroetham-d$ t

t


14 Synthesis of coordination polymers 2[G (l-*2/3)] x

Finally, we broadened the concept of dendronized copper(I)metallopolymers to well-defined coordination polymers 2. For polymer synthe sis, we used ligand monomers 6 [ G ( l - » 2 / 3 ) | . Again, the experiments were car ried out in N M R tubes by stepwise addition of the metal monomer to the solution of the ligand monomer.


x

Scheme 8: Synthesis of copper(l) coordination polymers 2[G (1->2/3)J X

After each addition of [Cu(CH CN) ]PF , N M R spectra were recorded until precise 1:1 equivalence of ligand- and metal monomer was achieved - mani fested by the disappearance of all endgroup absorptions. As an example, Figure 6 displays the H N M R spectra of (a) ligand monomer 6|G|(1->3)J, (b) an oligomeric complex 2[Gi(l->3)], and (c) a high-moleclar-weight polymer 2 [ G ( l - » 3 ) ] . The latter N M R spectrum - where no endgroup resonances can be detected - together with the increased solution viscosity support the formation of a very high-molecular-weight coordination polymer 2. This polymer is formed despite of the sterically very demanding dendritic substituents. On the other hand, the dendrons gave the polymers an improved solubility in conventional organic solvents. This will allow extensive analysis of the chain molecules in solution using various methods in the future - despite their polyelectrolyte char acter. 3

4

6

!

1

Conclusions We showed that dendrons attached to phenanthroline-based chelating ligands are well-appropriate to increase the solubility of kinetically labile copperil) coordination polyelectrolytes in apolar, non-coordinating organic solvents.


15 Moreover, there is no evidence that the complex formation processes are af fected by the sterically demanding dendrons. Therefore, in future, a more de tailed analysis of the chain conformation and the solution properties of these metallopolymers will be possible.

5,6

24

21

19

16 4,7


a )

1711

•

3,8

lu I .

JJJL

J L .

ULl 21

4,7

3,8 5.6 16

19

17 uii

'L. "

•

1

I "

"

' " " I " "

' ' ' ' ' I ' '

5 4 3 Chemical Shift (ppm)

1

' '

' ' Ί

1

1

2

!

Figure 6: HNMR spectra (a) of ligand monomer 6[G (l-*3)], (b) of an oligomeric- and (c) of a high-molecular-weight coordination polymer 2[Gi(l-+3)J (1, l 2 2-tetrachloroethane-d^ t

t

t

Acknowledgments We thank the German Science Foundation (Deutsche Forschungsgemeinschaft, DFG) for financial support of this work.


16

References 1 2 3


4 5 6 7 8 9 10 11 12 13 14 15 16

Sauvage, J.-P.; Dietrich-Buchecker, C . (Eds.), Catenanes, Rotaxanes and Knots, Wiley V C H , Weinheim, 1999. Lehn, J.-M., Supramolecular Chemistry - Concepts and Perspectives, Wiley V C H , Weinheim, 1995 Wöhrle, D.; Pomogaile, A. D., Metal Complexes and Metals in Macromole-cules, Wiley V C H , Weinheim, 2003. Eisenbach, C. D.; Schubert, U . S., Macromolecules 1993, 26, 7372. Eisenbach, C . D.; Göldel, Α.; Terskan-Reinold, M.; Schubert, U . S., Colloid Polym.Sci. 1998, 276, 780. Velten,U.;Rehahn, M., J. Chem. Soc., Chem. Commun. 1996, 2639. Velten,U.;Lahn, B.; Rehahn, M., Macromol. Chem. Phys. 1997, 198, 2789. Velten,U.;Rehahn, M., Macromol. Chem. Phys. 1998, 199, 127. Lahn, B.; Rehahn,M.,Macromol. Symp. 2001, 163, 157. Lahn, B.; Rehahn, M., e-Polymers 2002, no. 001. Kubasch, J.; Rehahn, M., Polym. Preprints 2004, 45(1), 480. [0]Albrecht-Gary, A.-M.; Saab, Z . ; Dietrich-Buchecker, C. O.; Sauvage, J.P., J. Am. Chem. Soc. 1985, 107, 3205. Dietrich-Buchecker, C . O.; Sauvage, J.-P., Tetrahedron Lett. 1983, 24, 5091. Hawker, C. J.; Fréchet, J. M. J., J. Am. Chem. Soc. 112, 7638 (1990). Nystrom, R. F.; Chaikin, S. W.; Brown, W. G., J. Am. Chem. Soc. 1949, 3245. Brown, H . C.; Narasimhan, S.; Choi, Y.M., J. Org.Chem. 1982, 47, 4702.


Dendronized Copper(I)-Metallopolymers - American Chemical Society

Recommend Documents