PGSE-NMR Study of a Series of New Silver–Carbosilane Dendrimers

ARTICLE pubs.acs.org/Organometallics

PGSE-NMR Study of a Series of New Silver
Carbosilane Dendrimers Displaying Different Coordination Modes Inmaculada Angurell,* Lara-Isabel Rodríguez,* Oriol Rossell, and Miquel Seco Departament de Química Inorg
anica, Universitat de Barcelona, Martí i Franqu
es, 1-11, 08028 Barcelona, Spain

bS Supporting Information ABSTRACT: The first series of peripherally silver
phosphorus functionalized dendrimers have been obtained selectively using mono- or diphosphine-terminated carbosilane dendrimers and silver triflate. With the first kind of functionalized dendrimers, simple phosphine
Ag(OTf) terminal groups are formed, whereas in the second case, only one silver ion appears bonded to each diphosphine moiety (chelate form).

’ INTRODUCTION A considerable number of dendrimers containing transition metals at their core, branches, or periphery have been reported in the last years.1 Much of these species have been successfully used in areas, such as catalysis,2 supramolecular chemistry,3 nanosciences,4 biology, and medicine,5 among others. In the past decade, we described a good number of peripherally metalfunctionalized carbosilane dendrimers, including Au,6 Pd,7 Rh,8 Ir,8 Ru,9 and Re,10 and also a new class of dendrimers containing up to four metal layers.11 It is really surprising that the number of metallodendrimers containing silver is extremely scarce in the literature. In fact, the vast majority of publications deals with dendrimers displaying nitrogen donor atoms, mainly PAMAM (polyamidoamine) dendrimers.12 Their goal consisted of binding silver ions to the inner nitrogen atoms of tertiary amines in order to obtain, after chemical or photochemical reduction, silver nanoparticles, which are interesting for their potential antibacterial effects.13 The lack of studies describing the formation of stable dendrimers with silver ions grafted at their surface prompted us to extend our research to this type of metallodendrimers. To this aim, we had in our hands a series of carbosilane dendrimers functionalized with mono- or diphosphine fragments at the ends of the arms.6a We expected that both classes of dendrimers would facilitate the synthesis of two different series of silver dendrimers that permitted us to compare them in terms of silver
phosphorus binding modes, self-diffusion coefficients, and hydrodynamic radius. ’ RESULTS AND DISCUSSION Figure 1 shows the carbosilane dendrimers functionalized with diphenylphosphine groups used in this work. The reaction of 1
3 with silver triflate (AgOTf) in dichloromethane at room temperature (rt) proceeded rapidly, and all compounds were obtained as white, light-sensitive solids. Depending on the phosphine/silver molar ratio employed, two r 2011 American Chemical Society

different coordination modes have been observed. Thus, when 1 equiv of silver triflate was reacted with 2 equiv of the
PPh2 moieties located at the surface of the dendrimers, the chelate derivatives 1a, 2a, and 3a were formed, in which only one silver atom is bound to two
PPh2 groups (Scheme 1). It is worth noting that, in chelate dendrimer 2a, AgOTf units can be bound at the surface in two different ways: through two P atoms belonging to the same branch or, alternatively, to different branches, yielding a 6- or a 12-membered chelate ring, respectively. Assuming a linear coordination for the silver(I) metal ion in 2a, the formation of a six-membered chelate ring would induce high tension in the compound so that we tentatively propose that the stereoisomer formed for 2a is the one shown in Scheme 1. The same analysis can be done for dendrimer 3a, where two isomers (12- or 24-membered chelate ring) are possible in this case. On the other hand, if a 1:1 silver(I)/
PPh2 molar ratio is employed, dendrimers 1b, 2b, and 3b are easily obtained (Scheme 1) where one Ag+ ion is bonded to one diphenylphosphine group. Whereas first-generation dendrimers 1b and 2b could be isolated, 3b decomposed in solution after a few minutes, precluding fully spectroscopic characterization. It is remarkable that, in all reactions, one unique coordination mode was observed, despite that it is known that silver(I)
phosphine complexes usually exhibit dynamic behavior in solution and that a mixture of different species is very often obtained.14 The selectivity of this reaction on the basis of the Ag(I)/phosphine molar ratio has important implications in this study since it enables isolating separately both series of silver dendrimers and comparing them from spectroscopic and structural points of view. Confirmation of the proposed coordination mode of the metal center in these metallodendrimers was obtained from the phosphorus
silver coupling constants in 31P{1H} NMR spectra. Received: July 11, 2011 Published: October 12, 2011 5771

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Figure 1

Scheme 1. Silver Metallodendrimers

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Table 1

Figure 2.

31

P{1H} NMR of 1a, 298 K, CD2Cl2.

dendrimer

Dt (m2 s
1)

rH (Å)

1

1.91  10
9

6.11

1a

1.86  10
9

6.18

1b

1.93  10
9

5.93

2

1.83  10
9

6.10

2a

1.59  10
9

6.84

2b

1.83  10
9

6.46

3

1.45  10
9

7.55

3a

1.42  10
9

7.77

Silver metallodendrimers have also been characterized by MALDI-TOF mass spectrometry, and in all cases, the signal corresponding to the loss of one triflate counterion ([M
OTf]+) could be observed with the expected isotopic distributions. Additional signals assigned to subsequent loss of silver triflate units could be identified. The mass spectra of all metallodendrimers showed a similar fragmentation pattern. To know how metal fragment coordination to the surface of carbosilane dendrimers affects dendrimer size and diffusion in solution, PGSE-NMR experiments (pulsed field gradient spin echo-nuclear magnetic resonance)16 were performed. With this method, it is possible to estimate the self-diffusion coefficients (Dt) of the dendrimers and evaluate their relative size through the hydrodynamic radius (rH). The experiments were carried out in CD2Cl2 samples of dendrimers 1, 2, and 3 and all metallodendrimers (1a, 1b, 2a, 2b, and 3a) at room temperature. Table 1 lists the self-diffusion coefficients (Dt) and the hydrodynamic radii (rH) calculated using the approach of Wilkins et al.,17 where Dt were standardized to a reference molecule in the sample solution (dichloromethane, in our case) to determine the hydrodynamic radius using the empirical relationship r H dend ¼ ðDt dichloromethane =Dt dend Þr H dichloromethane

Figure 3.

31

P{1H} VT-NMR of 1b, CD2Cl2.

For example, the 31P{1H} NMR spectrum of the chelate silver metallodendrimer 1a at room temperature showed two doublets at δ = 0.5 ppm with 1JPAg‑109 = 576 Hz and 1JPAg‑107 = 498 Hz (Figure 2), in agreement with those reported for other molecular species exhibiting a P
Ag(I)
P coordination mode.14,15 Analogous spectra were obtained for metallodendrimers 2a and 3a. In contrast, the 31P{1H} NMR spectrum of dendrimer 1b (Figure 3) exhibited only one broad signal at room temperature and no Ag
P coupling constants were present, suggesting that a rapid site exchange of the metal atom occurs at room temperature. This dynamic behavior indicates greater mobility of the arms of this dendrimer in comparison with that of the chelated silver dendrimer series 1a
3a. On cooling the sample at 210 K, the 31P{1H} NMR spectrum showed two doublets with 1JPAg‑109 = 882 Hz and 1JPAg‑107 = 765 Hz. Note that these values are larger than the ones obtained for chelate 1a and that they are in agreement with a terminal P
Ag(I) coordination mode.14 It is relevant that no mixture of metallodendrimers has been observed in any case.

From Table 1, several conclusions can be drawn. Dt coefficients for 1 and 2 are similar and clearly larger than the ones obtained for dendrimer 3, in agreement with the bigger size of the last family of dendrimers. The Dt coefficient for starting dendrimer 1 is similar to the ones obtained for metallodendrimers 1a and 1b, indicating that no cross-linking phenomena occurs in solution. The same behavior is observed for 2 and 3 in comparison to their related derivatives. Dt coefficients decrease in going from the starting dendrimers 1, 2, and 3 to the chelate dendrimers 1a, 2a, and 3a, indicating that dendritic size in metallodendrimers increases, as expected. By contrast, dendrimers 1b and 2b have very similar diffusion coefficients than the precursors 1 and 2, respectively. This observation suggests that appreciable folding of the metal units at the surface takes place. In conclusion, two different series of silver metallodendrimers have been selectively obtained by varying the phosphine/metal molar ratio employed during the synthesis. Phosphorus silver coupling constants have permitted us to establish the coordination bonding mode of the silver metal units to the phosphineterminated dendrimers. PGSE-NMR experiments have confirmed that no cross-linking processes occur during the synthesis of such kinds of species. Besides, these experiments have pointed out that, although the chelate metallodendrimers 1a
3a show shorter Dt coefficients than their precursors 1
3, the Dt 5773

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Organometallics coefficients found for 1b and 2b are slightly larger than those of 1 and 2, respectively, probably due to partial folding of the surface of the dendrimers.

’ EXPERIMENTAL SECTION General Procedures. All manipulations were performed under an atmosphere of dry nitrogen using standard Schlenk techniques, and silver complexes were synthesized light-protected to avoid metal reduction. All solvents were distilled from appropriate drying agents. The 1H, 13 C{1H}, 31P{1H}, and 19F NMR spectra were recorded on Bruker DXR 250, Varian Mercury 400, and Bruker DMX 500 spectrometers. Chemical shifts are reported in parts per million relative to external standards (SiMe4 for 1H and 13C{1H}, 85% H3PO4 for 31P{1H}, CF3COOH for 19 F), and coupling constants are given in hertz. MALDI-TOF spectra were recorded on a Voyager DE-RP (Perspective Biosystems) time-offlight (TOF) spectrometer. The starting dendrimers 1, 2, and 3 were prepared as previously described.6a Other reagents were purchased from commercial suppliers and used as received. PGSE-NMR Experiments. PGSE-NMR experiments were recorded on a Bruker DMX 500 spectrometer at 298 K in CD2Cl2 as an internal standard (see the Supporting Information for further details). Dendrimer solutions (0.01 M) in CD2Cl2 were used in the experiments. Using the values of diffusion coefficients of the sample and the internal standard, it was possible to estimate the hydrodynamic radii (rH) using the approach of Wilkins et al.,17 where dendrimer Dt were standardized to dichloromethane using the empirical relationship r H dend ¼ ðDt dichloromethane =Dt dend Þr H dichloromethane where the rH of dichloromethane is 2.49 Å.16a It is worth noting that different approaches can be followed to estimate the hydrodynamic radius from diffusion coefficient values. We have chosen Wilkins et al.'s approach because this method, despite calculations using the Stokes
Einstein equation, has the advantage that it does not require knowledge of solution viscosity and does not rely on interpretation of absolute values of diffusion coefficients. Synthesis of Dendrimer 1a. To a solution of dendrimer 1 (0.174 g, 0.149 mmol) in 20 mL of CH2Cl2 was added AgOTf (0.076 g, 0.296 mmol), and the mixture reaction was stirred at rt for 20 min lightprotected. The solvent was removed under reduced pressure, yielding 1a as a white solid in quantitative yield (0.250 g, 0.148 mmol). 1H NMR (500.1 MHz, CD2Cl2, 298 K): δ 7.57 (m, 16H, Ph), 7.40 (m, 24H, Ph), 1.91 (m, 8H, CH2P), 0.80 (m, 8H, CH2Si), 0.50 (m, 8H, CH2Si),
0.12 (s, 24H, CH3Si). 31P{1H} NMR (101.3 MHz, CDCl3, 298 K): δ 0.5 (d, 1 JPAg‑109 = 576 Hz; d, 1JPAg‑107 = 498 Hz). 19F NMR (376.4 MHz, CDCl3, 298 K): δ
78.0 (s, OTf). 13C{1H} NMR (62.9 MHz, CDCl3, 298 K): δ 133.2 (sbr, C-P), 131.9 (s, CH), 129.7 (s, CH), 128.1 (s, CH), 120.5 (q, 1JCF = 320 Hz, OTf), 13.0 (sbr, CH2P), 7.8 (s, CH2Si), 2.8 (s, CH2Si),
3.1 (s, CH3Si). MS MALDI-TOF (DTH, thf): 1790.7 [M + Ag]+, 1533.0 [M
OTf]+, 1277.3 [M
Ag
2(OTf)]+. Synthesis of Dendrimer 1b. To a solution of dendrimer 1 (0.174 g, 0.149 mmol) in 20 mL of CH2Cl2 was added AgOTf (0.153 g, 0.596 mmol), and the mixture reaction was stirred at rt for 20 min light-protected. The solvent was removed under vacuum, and compound 1b was obtained as a white solid in quantitative yield (0.327 g, 0.149 mmol). 1H NMR (500.1 MHz, CD2Cl2, 298 K): δ 7.60 (m, 16H, Ph), 7.43 (m, 24H, Ph), 1.81 (sbr, 8H, CH2P), 0.41 (sbr, 16H, CH2Si),
0.07 (s, 24H, CH3Si). 31P{1H} NMR (101.3 MHz, CDCl3, 298 K): δ 0.9 (m, PAg). 31P{1H} NMR (101.3 MHz, CD2Cl2, 210 K): δ 1.2 (d, 1 JPAg‑109 = 882 Hz; d, 1JPAg‑107 = 765 Hz). 19F NMR (376.4 MHz, CDCl3, 298 K): δ
77.1 (s, OTf). MS MALDI-TOF (DTH, thf): 2049 [M
OTf]+, 1791.4 [M
Ag
2(OTf)]+, 1533.6 [M
2Ag
3(OTf)]+, 1277.8 [M
3Ag
4(OTf)]+.

ARTICLE

Synthesis of Dendrimer 2a. This dendrimer was prepared following an analogous method to the one used for the synthesis of 1a, but using dendrimer 2 (0.150 g, 0.079 mmol) as starting material. Dendrimer 2a was obtained as a white solid in quantitative yield (0.230 g, 0.079 mmol). 1H NMR (500.1 MHz, CD2Cl2, 298 K): δ 7.43 (m, 32H, Ph), 7.31 (m, 48H, Ph), 1.20
1.70 (m, 16H, CH2P, CH2Si),
0.45 (s, 12H, CH3Si). 31 1 P{ H} NMR (101.3 MHz, CDCl3, 298 K): δ
0.6 (d, 1JPAg‑109 = 581 Hz; d, 1JPAg‑107 = 503 Hz). 19F NMR (376.4 MHz, CDCl3, 298 K): δ
78.1 (s, OTf). 13C{1H} NMR (62.9 MHz, CD2Cl2, 298 K): δ 132.9 (CH), 130.7 (CH), 129.1 (CH), 120.5 (q, 1JCF = 320 Hz, OTf), 12.4 (CH2P), 9.2 (CH2Si), 5.1 (CH2Si),
3.0 (s, CH3Si). MS MALDI-TOF (DTH, thf): 3042.2 [M + Ag]+, 2784.6 [M
OTf]+, 2528.9 [M
Ag
2(OTf)]+. Synthesis of Dendrimer 2b. This dendrimer was prepared following an analogous method to the one used for the synthesis of 1b, but using dendrimer 2 (0.150 g, 0.079 mmol) as starting material. Dendrimer 2b was obtained as a white solid in quantitative yield (0.311 g, 0.079 mmol). 1H NMR (500.1 MHz, CD2Cl2, 298 K): δ 7.47
7.36 (m, 80H, Ph), 2.54
1.55 (m, 16H, CH2P), 1.20
1.06 (m, 16H, CH2Si),
0.49 (sbr, 12H, CH3Si). 31P{1H} NMR (101.3 MHz, CD2Cl2, 298 K): δ
1.1 (sbr, PAg). 31P{1H} NMR (101.3 MHz, CD2Cl2, 200 K): δ 1.7 (sbr, PAg),
6.1 (sbr, PAg). 19F NMR (376.4 MHz, CD2Cl2, 298 K): δ
78.1 (s, OTf). 13C{1H} NMR (62.9 MHz, CD2Cl2, 298 K): δ 132.9
19.0 (C6H5), 120.3 (q, 1JCF = 319 Hz, OTf), 12.2 (CH2P), 9.0 (CH2Si), 4.4 (CH2Si),
2.2 (CH3Si). MS MALDITOF (DTH, thf): 3302 [M
2Ag
3(OTf)]+, 3042 [M
3Ag
4(OTf)]+, 2785 [M
4Ag
5(OTf)]+, 2014 [M
7Ag
8(OTf)]+. Synthesis of Dendrimer 3a. This dendrimer was prepared following an analogous method to the one used for the synthesis of 1a, but using dendrimer 3 (0.115 g, 0.039 mmol) as starting material. Dendrimer 3a was obtained as a white solid in quantitative yield (0.155 g, 0.039 mmol). 1H NMR (500.1 MHz, CD2Cl2, 298 K): δ 7.59
7.41 (m, 80H, Ph), 1.89 (sbr, 16H, CH2P), 0.31 (m, 64H, CH2Si),
0.09 (sbr, 84H, CH3Si). 31P{1H} NMR (101.3 MHz, CD2Cl2, 298 K): δ 1.5 (d, 1JPAg‑109 = 578 Hz; d, 1JPAg‑107 = 503 Hz). 19F NMR (376.4 MHz, CD2Cl2, 298 K): δ
78.0 (s, OTf). MS MALDI-TOF (DTH, thf): 4073.8 [M + Ag]+, 3818.1 [M
OTf]+, 3562.7 [M
Ag
2(OTf)]+, 3049.2 [M
3Ag
4(OTf)]+.

’ ASSOCIATED CONTENT

bS

Supporting Information. PGSE-NMR results and MALDI-TOF mass spectra of metallodendrimers. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (I.A.), lara.rodriguez@qi. ub.es (L.-I.R.).

’ ACKNOWLEDGMENT Financial support for this work was provided by the Ministerio de Ciencia e Innovacion (Project CTQ2009-08795). We acknowledge M. A. Molins for the PGSE-NMR measurements and discussions. ’ REFERENCES (1) See, for example: (a) V€ogtle, F.; Richardt, G.; Werner, N. Dendrimer Chemistry: Concepts, Syntheses, Properties, Applications; Wiley-VCH: Weinheim, Germany, 2009. (b) Newkome, G. R.; V€ogtle, F.; Moorefield, C. N. Dendrimers and Dendrons; Wiley: New York, 2001. 5774

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