Synthesis and solid-state NMR structural characterization of polysiloxane-immobilized amine ligands and their metal complexes

Journal of Non-Crystalline Solids 209 Ž1997. 19–39

Synthesis and solid-state NMR structural characterization of polysiloxane-immobilized amine ligands and their metal complexes Jane Jie Yang, Issa M. El-Nahhal 1, I-Ssuer Chuang, Gary E. Maciel

)

Department of Chemistry, Colorado State UniÕersity, Fort Collins, CO 80523, USA Received 15 September 1995; revised 7 May 1996

Abstract Polysiloxane-immobilized amine and diamine ligand systems have been made by hydrolytic condensation of SiŽOEt.4 with ŽEtO. 3 SiŽCH 2 . 3 NH 2 or ŽMeO. 3 SiŽCH 2 . 3 NHŽCH 2 . 2 NH 2 . The corresponding triamine ligand was made from the reaction of 3-chloropropylpolysiloxane with diethylenetriamine ŽH 2 NCH 2 CH 2 NHCH 2 CH 2 NH 2 .; solid-state 13 C and 15 N nuclear magnetic resonance ŽNMR. spectra can identify the structure of the product of this reaction. 29 Si, 15 N, 13 C and 1 H NMR spectra are in general valuable for characterizing these polysiloxane structures, and provide evidence for the involvement of the amine ligands in hydrogen bonding with surface silanols. Together with results of classical elemental analysis, the NMR data indicate that a portion of the ligand groups are inaccessible to metal ions and remain uncoordinated when these materials are treated with aqueous solutions of divalent metal ions. 29 Si NMR spectra and elemental analysis reveal some details of the leaching of these polysiloxane materials, when they are treated with acids or metal ion solutions. Relaxation times, T1H , were measured, based on 29 Si and 13C detection, on the polysiloxane-immobilized ligand systems. The T1H results suggest that the organofunctionalities in these systems are evenly distributed in the polysiloxane network.

1. Introduction Much work has been carried out with ligands grafted onto silica surfaces or other metal oxides w1–5x, and substantial research on functionalized polysiloxane ligands has been published w6–10x. These immobilized ligand systems have many potential applications, including preconcentration and separation of metal ions from aqueous solutions w5,11–

) Corresponding author. Tel.: q1-970 491 6480; fax: q1-970 491 1801; e-mail: [email protected]. 1 On leave from Gaza University, Gaza City, Palestine.

14x. A number of useful spectroscopic and chemical tools have been employed to study ligand-modified silica systems w11–19x, and high-resolution solid-state NMR techniques have been used to characterize structures of organosilane-modified silica surfaces w20–39x. In contrast, comparatively little detailed chemical attention has been paid to the structures of analogous polysiloxane-immobilized ligands that have been made through the sol–gel process. This paper describes the preparation and solid-state 29 Si, 13 C, 15 N and 1 H nuclear magnetic resonance ŽNMR. characterization of a variety of polysiloxane-immobilized amine ligands, including mono-, bi-, and tri-amine ligand systems, and of the products of

0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 3 4 - 0

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J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

treatments with aqueous divalent metal ion solutions or aqueous acid solutions at various pH values. Natural-abundance solid-state NMR techniques w20–35,39x based on cross-polarization and magicangle spinning ŽCP-MAS. for 13 C, 29 Si and 15 N, as well as corresponding experiments with 15 N-enriched samples w36x, and CRAMPS Žcombined rotation and multiple-pulse spectroscopy. experiments for 1 H w37,38x, have served as the basis for structure characterization in this study. NMR spectra can provide several types of information; for example, chemical shifts reveal details of chemical form or structure, intensities can provide the populations of particular structures, and relaxation parameters relate to motional dynamics and internuclear distances. Often the information obtained from NMR spectra of one nuclide can be reinforced Žor challenged. by information obtained by observation of another nuclide. Although strict quantitation cannot be claimed for the populations of structural moieties detected in CP-MAS experiments, unless the relevant CP dynamics are characterized in detail Žnot done in this study., as a consistent set of experimental CP parameters was used for each nuclide in this investigation, the CP-MAS intensities obtained for each type of material are certainly valid qualitatively, and in some cases semi-quantitatively. For two samples, the issue of 1 H ™29 Si CP-MAS dynamics was avoided by replacing 1 H ™29 Si cross polarization by direct polarization ŽDP, based on 29 Si spin–lattice relaxation.. In general, solid-state 13 C NMR is very useful in characterizing the organic structure in polysiloxaneimmobilized ligands and degree of hydrolysis of the ethoxy andror methoxy groups. 15 N and 1 H NMR spectra are also useful for learning the primary structure of an immobilized ligand group, and some aspects of its detailed status Že.g., hydrogen bonding., including information on its local dynamical properties. 29 Si NMR can be expected to provide information on the polysiloxane network, such as the extent of attachment of organic moieties to the network, the degree of crosslinking of the whole network and especially on any leaching of these materials when treated with acid or metal ion solutions. Owing to its low natural abundance Ž0.37%. and low g Žabout one tenth that of 1 H., 15 N is in many, if not most, cases not suitable for CP-MAS studies,

unless 15 N-enriched samples are employed. Even with a large-volume MAS rotor Ža 2.5 cm3 rotor developed in this laboratory w40,41x was used in this investigation., the signal-to-noise ratios of 15 N CPMAS spectra presented in this article are far from excellent. Nevertheless, we still can extract useful information from the 15 N NMR spectra.

2. Experimental 2.1. Chemicals and reagents The 3-aminopropyltriethoxysilane, 3-Ž2-aminoethylimino .propyltrimethoxysilane, 3-chloropropyltrimethoxysilane, tetraethylorthosilicate and ŽnBu. 2 SnŽO 2 CCH 3 . 2 were purchased from Aldrich Chemical and used as received. Diethylenetriamine was obtained from Aldrich and distilled before use. All organic solvents were analytical reagent grade materials and used without purification. MetalŽII. chlorides were obtained from Aldrich. 2.2. Preparations The 3-chloropropyl-functionalized siloxane polymer was made as described previously w7x by the reaction of SiŽOEt.4 and ŽMeO. 3 SiŽCH 2 . 3 Cl, catalyzed by HCl or Žn-Bu. 2 SnŽOCOCH 3 . 2 . The immobilized monoamine and diamine ligand systems were prepared as described previously w6,9x from SiŽOEt.4 and ŽRX O. 3 SiŽCH 2 . 3 X ŽX s NH 2 , RX s Et; X s NHŽCH 2 . 2 NH 2 , RX s Me. with various molar ratios, as summarized in Table 1 along with elemental analysis data. The polysiloxane-immobilized triamine ligand system was prepared by refluxing an excess of 100 cm3 of diethylenetriamine Ž926 mmol. with 5.0 g of the 3-chloropropylpolysiloxane Žsample 6, 20.5 mmol chlorine, see Table 1. under N2 for 24 h. The mixture was cooled, and the solid phase filtered off. The solid was washed successively with 50 cm3 portions of 0.050 M aqueous NaOH, water, methanol and diethyl ether. The solid product was dried at a pressure of 0.1 Torr at 808C overnight. Elemental analyses ŽTable 1. were performed by Huffman Laboratories, Golden, CO.

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39 Table 1 Polysiloxane-immobilized systems studied, S–CH 2 CH 2 CH 2 X Sample X No. 1 2 3 4 5 6 7 a

NH 2 NH 2 NHŽCH 2 . 2 NH 2 NHŽCH 2 . 2 NH 2 NHŽCH 2 . 2 NHŽCH 2 . 2 NH 2 Cl h Cl i

Molar %C ratio b 2:1 1:1 2:1 1:1

13.3 15.0 18.0 20.3

c

%N

5.2 6.0 8.4 9.4

c

mol mol Cr N 3.0 2.9 2.5 2.5

21

a

d

Hq mol Hqr % ligand mol Nir mol Cur mol Cdr mol Hgr uptake mol depleted f mol mol mol mol Žmmolrg. N e N g N g N g N g 3.6 4.1 5.5 6.1

0.97 0.95 0.92 0.91

5.5 12.3

0.24

0.13

0.21

0.22

0.30

0.26

0.25

0.21

1:1 1:1

The symbol S– stands for

, where

represents a silica-like framework. b mol SiŽOEt.4 :mol ŽRO. 3 SiŽCH 2 . 3 X in the reaction mixture of the preparation. c From elemental analysis. Estimated error: - 1%. d Calculated from % carbon and % nitrogen. Estimated error: - 1%. e Calculated from Hq uptake and nitrogen content. Estimated error: - 1%. f Calculated from gravimetric data and from % carbon data before and after HCl extraction. Estimated error: - 1%. g Metal ion uptake, from elemental analysis. Estimated error: - 1%. h Žn-Bu. 2 SnŽOCOCH 3 . 2-catalyzed preparation. The chlorine content is 4.1 mmolrg, calculated from chlorine elemental analysis. Estimated error: - 1%. i HCl-catalyzed preparation.

2.3. Proton uptake A 0.50 g portion of each immobilized ligand system was shaken with a 50 cm3 portion of aqueous 0.11 M hydrochloric acid solution for 24 h at 258C. The unneutralized excess HCl was then titrated with an aqueous 0.10 M sodium hydroxide solution. The results are summarized in Table 1. 2.4. Metal ion uptake A 0.50 g portion of each immobilized ligand system was shaken with an excess Ž50 cm3 . of an aqueous 0.10 M solution of metalŽII. salt for 24 h at 258C. The reagents used were nickelŽII. chloride, copperŽII. chloride, cadmiumŽII. chloride and mer-

curyŽII. chloride. The mixture was filtered, and the residue washed with successive 50 cm3 portions of water, methanol, and diethylether, and dried in vacuo at 808C. The resulting solids were analyzed for metal content by elemental analysis. The results are summarized in Table 1. 2.5. NMR experiments Solid-state 13 C NMR experiments were carried out at 25.1 MHz on a home-built spectrometer, using cross-polarization ŽCP. and magic-angle spinning ŽMAS. with high-power 1 H decoupling w42x. The 13 C CP-MAS spectra were obtained with a 1 ms CP contact time and a 1 s recycle time. 15 N CP-MAS experiments were carried out at 20.3 MHz on a

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J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

severely modified 200 MHz spectrometer ŽNicolet NT-200., using a 2.5 cm3 Chemagnetics Pencil MAS probe system. The optimized parameters for 15 N CP-MAS NMR experiments were a 0.6 ms CP contact time and a 0.6 s recycle time. 29 Si CP-MAS and DP-MAS NMR experiments were carried out at 51.2 MHz on a Chemagnetics M-260S spectrometer. All the 29 Si CP-MAS spectra shown in this article were taken with 2 ms CP contact time and 1 s recycle time. 29 Si-detected T1H measurements were carried out via 1 H inversion-recovery CP-MAS experiments w43x at 200 MHz Ž1 H frequency. on the modified NT-200 spectrometer and at 260 MHz Ž1 H frequency. on the Chemagnetics M-260S spectrometer. 13 C-detected T1H measurements were carried out via 1 H inversion-recovery CP-MAS experiments w43x at 260 MHz Ž1 H frequency. on the Chemagnetics M260S spectrometer. High-resolution solid-state 1 H NMR experiments were carried out at 360 MHz on a modified Nicolet NT-360 spectrometer by the combined rotation and multiple-pulse spectroscopy ŽCRAMPS. technique w44,45x. Chemical shifts are expressed in ppm with respect to the 29 Si, 13 C or 1 H resonance of liquid tetramethylsilane ŽTMS., and with respect to the 15 N resonance of liquid ammonia at 258C. Higher numbers correspond to smaller shieldings.

3. Results 3.1. Synthesis In the preparation of polysiloxane-immobilized monoamine or diamine ligands by hydrolytic condensation between SiŽOEt.4 and ŽRX O. 3 SiŽCH 2 . 3 X, the amine groups act as a basic catalyst, promoting rapid gelation. In the preparation of the immobilized 3-chloropropyl system, either Žn-Bu. 2 SnŽO 2 CCH 3 . 2 or HCl catalysts were used; in both cases, hydrolysis was incomplete and the gelation was very slow, resulting in the presence of some residual ethoxy and methoxy groups, as indicated by 13 C NMR spectra Žvide infra.. In the preparation of the polysiloxane-immobilized triamine ligand system by the reaction between the immobilized 3-chloropropyl material and an excess of diethylenetriamine, there are two types of

amine sites for reaction. However, as shown by the C and 15 N NMR evidence Žvide infra., only one form was obtained, that in which a terminal amino group serves as the reaction site. 13

3.2. ReactiÕity Results of experiments on uptake of acid from aqueous H 3 Oq solution by the immobilized amine ligand systems are summarized in Table 1. Based on nitrogen elemental analysis data, one can compute the amine content Žmmolrg. of each of these systems, and compare it with the Hq uptake Žmmolrg.; these results ŽTable 1. show that most of the amine groups are accessible to the acid. These polysiloxane-immobilized ligand systems show reactivity toward aqueous metal ion solutions ŽNi 2q, Cu2q, Cd 2q and Hg 2q .. The white solid polysiloxane-immobilized amine system changed into a green solid with Ni 2q, a blue solid with Cu2q and pale yellow solids with Cd 2q and Hg 2q. It has been reported w6,9x that these immobilized amine ligands form 1:1 and 1:2 metalŽII.rchelate complexes with Co 2q, Ni 2q, Cu2q and Zn2q. The elemental analysis results summarized in Table 1 show that less than one metal ion is complexed per amino group in the samples isolated from the treatments with aqueous metalŽII. solutions; these results provide no direct evidence on the stoichiometry of each metal–ligand complexation. We found from elemental analyses that the polysiloxane-immobilized monoamine ligand systems undergo some leaching, presumably of small oligomeric groups, when they are treated with HCl solution ŽpH s 1. for 24 h Ž5–13% leaching., and when the ligand is treated with 0.10 M copperŽII. chloride solution for 154 h Ž; 53% leaching for sample 1.. These results are summarized in Table 1. More details of the leaching are revealed by 29 Si NMR results presented below. 3.3. 13

13

C NMR results

C CP-MAS NMR spectra of the polysiloxaneimmobilized monoamine and diamine ligand systems made from the hydrolytic condensation between SiŽOEt.4 and ŽRX O. 3 SiŽCH 2 . 3 X reagents are shown in Figs. 1 and 2A and B. The 13 C spectrum of the

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

23

corresponding case of the Ni 2q-treated monoamine sample Žspectrum not shown here., there is no obvious line broadening due to the presence of the paramagnetic metal ion center. The 13 C chemical shifts of the immobilized monoamine systems are summarized in Table 2. The 13 C NMR spectrum of the untreated polysiloxaneimmobilized monoamine ligand ŽFig. 1A. shows three signals, at 10.7, 27.4 and 44.9 ppm, whereas the spectrum of the corresponding protonated system Žwashed with 0.1 M aqueous HCl. of Fig. 1B displays three signals, at 10.5, 21.3 and 43.1 ppm. By analogy with literature reports on the corresponding

Fig. 1. 13 C CP-MAS NMR spectra of the polysiloxane-immobilized monoamine ligand Žsample 1.. ŽA. Untreated; ŽB. protonated ŽpH s1.; ŽC. Cu2q-treated; ŽD. Cd 2q-treated; ŽE. Hg 2q-treated.

polysiloxane-immobilized triamine ligand system is shown in Fig. 2C. Fig. 3 shows 13 C NMR spectra of 3-chloropropylpolysiloxane samples. 13 C NMR spectra of the polysiloxane-immobilized amine ligands ŽFigs. 1 and 2. show the absence of residual ethoxy or methoxy signals, whereas 13 C spectra of the polysiloxane-immobilized 3-chloropropyl system ŽFig. 3. show strong signals from residual ethoxy Ž18 and 60 ppm. and methoxy Ž51 ppm. groups. The 13 C CP-MAS spectrum of the Cu2q-treated monoamine sample ŽFig. 1C. appears very similar to that of the HCl-treated monoamine sample ŽFig. 1B., except with an ; 5 ppm shift to smaller shielding Žperhaps due to magnetic susceptibility effects.; as in the

Fig. 2. 13 C CP-MAS NMR spectra of the untreated ŽA. and protonated ŽpH s1. ŽB. polysiloxane-immobilized diamine Žsample 3. system and untreated polysiloxane-immobilized triamine sample ŽC..

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24

Table 3 C chemical shifts of polysiloxane-immobilized amine samples, their protonated forms and their metal complexes Žppm. a 13

Fig. 3. 13 C CP-MAS NMR spectra of 3-chloropropylpolysiloxane. ŽA. HCl-catalyzed preparation; ŽB. Žn-Bu. 2 SnŽO 2 CCH 3 . 2-catalyzed preparation.

X b

Sample

C1

C2

C3

Sample 1 1, HCl d 1, NaOH d 1, Cd 2q d 1, Hg 2q d 1, Cu2q d

10.7 10.5 11.3 11.0 11.2 10.5

27.4 21.3 27.6 26 br e 25.0 br 21.9

44.9 43.1 45.4 44.5 44.8 43.0

Sample 3 3, HCl d 3, Cd 2q d 3, Hg 2q d

11.7 10.5 11.6 11.5

24.0 20.8 23.1 22.7

42.9 39.2 41.2 43.4 sh f

53.2 51.5 51.5 51.8

Sample 5 5, Cd 2q 5, Hg 2q

12.0 12.1 11.6

24.3 22.8 22.5

43.0 masked 42.1 sh f

51.9 51.0 50.1

Sample 6

10.9

27.3

47.8

e

OR

18.0, 51.5, 60.0

C 4 –C 7

c

—

a

Estimated chemical shift errors: "0.5 ppm. X R represents residual, unreacted alkoxy carbons from ŽRX O. 3 SiCH 2 CH 2 CH 2 X or ŽRX O.4 Si reagents. c C 4 –C 7 positions are defined in Fig. 2. d A symbol such as 1, HCl stands for a sample 1 treated with 0.1 M HClŽaq.. e Broad peak. f Shoulder. b

modified silicas w30–35,39x, these signals are assigned to C 1 , C 2 and C 3 carbon atoms, respectively ŽFig. 1A.. Carbon atom C 2 manifests a substantial increase in shielding from 27.4 ppm to 21.3 ppm upon protonation ŽFig. 1A and B., whereas C 1 and C 3 display only minor shifts Ž1–2 ppm. on protonation ŽTable 3.. This behavior is in agreement with previous results reported in the literature on aminopropylsilaneŽAPS.-modified silica w22,31,33x. A low-shielding chemical shift of about 27.4 ppm for C 2 was found for the polysiloxane-immobilized amine ligand upon washing the acid-treated material with aqueous 0.1 M NaOH. The 21 ppm C 2 chemical shift is Table 2 13 C chemical shifts of polysiloxane-immobilized monoamine Žsample 1., polymerized APS and APS-modified silica Žin ppm. a Sample

C1

b

C2

b

Immobilized monoamine 10.7 27.4 ligand Polymerized APS 11.7 28.4 APS-modified silica 8.7 25.0 Ž20.7. a

C3

b

Ref.

44.9 this work

c

46.0 w22x 43.0 w22x

Estimated chemical shift errors: "0.5 ppm. C 1 , C 2 and C 3 are defined in term s of S – C Ž1. H 2 C Ž2. H 2 C Ž3. H 2 NH 2 . c Value in parentheses represents intensity as a small shoulder. b

consistent with what has been observed for the 3-Žtrimethylammonium.propylpolysiloxane system w10x and the 27 ppm chemical shift is consistent with the C 2 value reported for an APS-modified silica in which most of the residual silanol groups were capped by a silylation reaction, eliminating them from participation in hydrogen bonding or proton transfer w31x. Caravajal et al. w33x have indicated, on the basis of 13 C chemical shift arguments, that the amino groups of APS grafted onto silica are involved in hydrogen bonding andror Bronsted protonation by ¨ acidic silanols of the silica surface and the large increase in shielding of C 2 is due to protonation of the amino group. 13 C NMR spectra of the CdŽII.and HgŽII.-complexed polysiloxane-immobilized monoamine system ŽFig. 1D and E. show slight broadening of the C 2 and C 3 signals, compared with the spectrum ŽFig. 1A. of the corresponding uncomplexed polysiloxane-immobilized amine system. The 13 C NMR spectrum of the untreated

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

polysiloxane-immobilized diamine system ŽFig. 2A. shows four signals, at 11.7, 24.0, 42.9 and 53.2 ppm. Fig. 2B shows the spectrum of the protonated form of this diamine ligand; there are four signals, at 10.5, 20.8, 39.2 and 51.5 ppm. These are assigned, as shown in Fig. 2, on the basis of spectral data taken from the literature w22,31x. The 13 C chemical shifts derived from the spectra of Fig. 2 are summarized in Table 3. The 13 C spectrum of the polysiloxane-immobilized triamine system ŽFig. 2C. shows four signals, at 12.0, 24.3, 43.0 and 51.9 ppm, a pattern similar to that of the diamine system. The signal at 51.9 ppm is very intense, because it involves the four carbon atoms, C 4 , C 5 , C 6 , and C 7 , as identified in Fig. 2C. The signal at 43.0 ppm is weak and results from one type of carbon, C 3 . This interpretation implies that the chemical form, S–CH 2 CH 2 CH 2NHCH 2CH 2NHCH 2CH 2 NH 2 , shown in Fig. 2C is in fact the form that was obtained from the synthesis employed. Had the other form, S–CH 2 CH 2 CH 2 NŽCH 2 CH 2NH 2 . 2 , been produced in the synthesis, then one would have expected a 13 C spectral pattern different from that observed for the diamine ligand, i.e., a pattern in which a signal due to carbon atoms attached to the tertiary amine nitrogen should appear at ; 58 ppm w46x. Fig. 3 shows the 13 C CP-MAS spectra of 3-chloropropylpolysiloxane samples prepared with HCl or Žn-Bu. 2 SnŽOCOCH 3 . 2 as catalyst. These 13 C NMR spectra indicate that the proportion of the residual –OMe Ž51 ppm. and –OEt Ž18 and 60 ppm. moieties on the 3-chloropropylpolysiloxane prepared by HCl catalysis are higher than those on the sample prepared by Žn-Bu. 2 SnŽOCOCH 3 . 2 catalysis. Dipolar-dephasing experiments w47–49x were carried out on all of the samples for three dipolar-dephasing periods Ž2.0, 20 and 50 ms.. In a dipolar-dephasing experiment, after the cross-polarization step, the 1 H Ždecoupling. power is turned off for a period of time, with a p pulse in the middle, before initiation of the acquisition of 13 C signals under 1 H high-power decoupling. For those 13 C spins with directly-bonded hydrogens in a rigid segment, the 13 C magnetization will decay dramatically before data acquisition, whereas for 13 C spins without any directly-bonded hydrogens or those 13 C spins with directly-bonded hydrogens in a rather mobile moiety,

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Fig. 4. Dipolar-dephasing 13 C CP-MAS NMR spectra of the untreated ŽA. and Hg 2q-treated ŽB. polysiloxane-immobilized monoamine system Žsample 1. and untreated 3-chloropropylpolysiloxane prepared with HCl as catalyst ŽC.. Dephasing period shown in ms.

the 13 C magnetization will substantially survive the dipolar-dephasing period. Representative dipolar-dephasing spectra of the polysiloxane-immobilized monoamine system and its HgŽII. complex and of the 3-chloropropylpolysiloxane prepared with HCl as catalyst are shown in Fig. 4. The dephasing rate of 13 C m agnetization of the uncom plexed polysiloxane-immobilized monoamine system de-

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

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Table 4 Relaxation results on polysiloxane-immobilized systems 13

Sample

C-detected

T1H Žs. a

29

Si-detected Ž29 Si chemical shifts.

260 MHz T1H Žs. Žy60 ppm.

S–CH 2 CH 2 CH 2 NH 2 Žsample 2. S–CH 2 CH 2 CH 2 NHCH 2 CH 2 NH 2 Žsample 4. S–CH 2 CH 2 CH 2 Cl Žsample 6.

c

0.69 0.67 0.76 c 0.72 Ž10.9 ppm. 0.56 0.59 Ž27.3 ppm. 0.57 Ž47.8 ppm. 0.56 Ž60.0 ppm. 0.55

c

b

b c

b

Žy100 ppm. Žy60 ppm. Žy100 ppm. 0.68 0.75 0.56

S–CH 2 CH 2 CH 2 NHCH 2 CH 2 NH 2 P nHCl Žfrom sample 4. S–CH 2 CH 2 CH 2 NHCH 2 CH 2 CH 2 NH 2 P Cd 2q Žfrom sample 4. a

200 MHz T1H Žs. 0.48 0.43 0.52

0.47 0.44 0.50

0.70 1.0

0.67 1.0

Measured via 13 C CP-MAS, with 1 H inversion-recovery w50x at 260 MHz for 1 H. Estimated error: "5%. Measured via 29 Si CP-MAS, with 1 H inversion-recovery w50x. Estimated error: "5%. Measured from the total intensity, the sum for all individual peaks, because of peak overlaps.

creases in the order, C 1 ) C 2 ) C 3 ŽC 3 attached to the NH 2 group. ŽFig. 4A.. More details of 13 C dipolar-dephasing CP-MAS results are deferred to Section 4. 1 H ™13 C cross polarization provides a means of elucidating 1 H spin–lattice relaxation behavior in a proton inversion-recovery sequence via 13 C detection w43x. This procedure detects the spin–lattice relaxation behavior of those protons responsible for the CP-generated 13 C signal that is detected in a 13 C CP-MAS experiment, offering the possibility for well-resolved 13 C peaks of detecting separate and distinct T1H behavior for individual 13 C peaks. Such experiments were carried out on several samples of this investigation; and results on selected samples are summarized in Table 4. For reasons of peak overlap andror apparently uniform T1H values detected for all peaks in a specific 13 C spectrum, only one 13 Cdetected T1H value is reported for all peaks in the polysiloxane-immobilized monoamine system Žsample 2., or for all peaks in the polysiloxane-immobilized diamine system Žsample 4., as seen in Table 4. Individual Žalbeit essentially the same. T1H values are reported for the individual 13 C peaks of the polysiloxane-immobilized 3-chloropropyl system Žsample 6.. 3.4. 15

15

N CP-MAS NMR results

N NMR studies of amine ligand systems can be expected to provide useful information about the

environments of the amine groups and their interactions with each other or with other proton donors or acceptors, e.g., silanols of silica-like regions of the polysiloxane framework. Accordingly, natural-abundance 15 N CP-MAS experiments were carried out on the samples of this study. Fig. 5 shows 15 N spectra of polysiloxane-immobilized monoamine samples treated with aqueous HCl or NaOH solutions of various pH values. For the pH s 7 case, the spectrum shows a 15 N signal at 25 ppm, with a small shoulder at 34 ppm. For the pH s 1 case, the spectrum shows a peak at 45 ppm, with a weak shoulder at 33 ppm. For the pH s 13 case, the signal is a sharper, more intense peak at 25 ppm, accompanied by a small shoulder at 33 ppm. The 15 N NMR spectrum of the CdŽII.-treated sample derived from the polysiloxane-immobilized monoamine ligand is shown in Fig. 6B. It consists of a broad signal, with most of its intensity at lower shielding than for the corresponding uncomplexed ligand system ŽFig. 6A. Žbetween that of the ammonium cation Ž45 ppm. and hydrogen bonded forms Ž33 ppm., vide infra.. The 15 N NMR spectra of the polysiloxane-immobilized monoamine samples after treatments with aqueous solutions of various pH values or metal ions reflect changes in the amine environment; details are deferred to Section 4. The 15 N NMR spectrum of the polysiloxane-immobilized diamine system ŽFig. 6C. shows two signals, at 22.2 and 37.0 ppm. On the basis of 15 N NMR data taken from the literature w50–53x, these

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27

peaks are assigned to primary and secondary amine groups, respectively. The spectrum of the protonated form of the polysiloxane-immobilized diamine ligand Žwashed with 0.1 M aqueous HCl. is given in Fig. 6D; it shows that both signals move to lower shielding, at 45.0 ppm and a shoulder at 52 ppm, upon protonation. These signals are assigned on the basis of 15 N data taken from the literature w52,53x to the ammonium cation sites of types, –NqH 3 and –NqH 2 –, respectively. The 15 N NMR spectrum of the polysiloxane-immobilized triamine system is presented in Fig. 6E. It shows two signals, at 18.2 and 33.6 ppm, which are expected for the primary and secondary amine groups, respectively. In comparing peak intensities of the diamine system ŽFig. 6C. with those of the triamine system ŽFig. 6E., one sees that there is a significant increase in the secondary amine intensity relative to that of the primary amine peak in the triamine case, compared with the diamine case. This is consistent with the presence of two secondary amine groups for each primary amine group in the triamine ligand system. Furthermore, the absence of a tertiary amine signal, which would be expected to Fig. 6. 15 N CP-MAS NMR spectra of the untreated ŽA. and Cd 2q-treated ŽB. polysiloxane-immobilized monoamine Žsample 2. samples; the non-protonated ŽC. and protonated ŽpH s1. ŽD. polysiloxane-immobilized diamine system Žsample 4.; and the untreated polysiloxane-immobilized triamine system ŽE..

occur at about 50–55 ppm w50–53x and to survive H– 15 N dipolar dephasing very well Žvide infra., confirms the absence of structure II in the product and shows that structure I is the product of the reaction between diethylenetriamine and the 3-chloropropylpolysiloxane precursor; this result is consistent with the 13 C results described above. 1

Fig. 5. 15 N CP-MAS NMR spectra of the polysiloxane-immobilized monoamine system Žsample 2. at various pH’s.

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

28

Except for the polysiloxane-immobilized triamine system and one of the polysiloxane-immobilized monoamine systems, for which several different CP contact times were used to find the optimal CP contact time Žmaximum SrN ratio at 0.6 ms. and repetition delay Žmaximum SrN ratio at 0.6 s. for cross polarization, the generally low signal-to-noise ratios of 15 N CP-MAS spectra prevented us from carrying out detailed VCT experiments, or other 15 N CP-MAS relaxation measurements. Nevertheless, dipolar-dephasing 15 N CP-MAS experiments with a 120 ms dephasing time were carried out on these samples. Fig. 7A shows 15 N CP-MAS dipolar-dephasing spectra obtained on the polysiloxane-immobilized monoamine system treated at pH s 1. The 15 N magnetization is only moderately dephased in 120 ms. In contrast to this partial dephasing behavior shown for the polysiloxane-immobilized monoamine system ŽFig. 7A., analogous experiments on the polysiloxane-immobilized diamine system treated at pH s 1 ŽFig. 7B. show that both the primary and secondary ammonium cation signals are completely dephased at 120 ms. The 15 N dipolar-dephasing spectrum obtained on the polysiloxane-immobilized triamine system treated at pH s 1 is shown in Fig. 7C. The secondary amine Ž a NH. signal at 33 ppm is seen to dephase faster than the primary amine Ž –NH 2 . signal at 18.2 ppm, as shown in Fig. 7C. Since the secondary amine nitrogen has only one hydrogen attached, compared with two for the primary amine, this result implies substantially more mobility for the NH 2 group than for the –NH– group. 3.5.

29

Si CP-MAS NMR results

3.5.1. Silicon attachments to the polysiloxane framework The 29 Si NMR peak assignments discussed below are based on solid-state NMR data taken from the literature w24–35x. The 29 Si NMR spectra of the polysiloxane-immobilized monoamine systems and the polysiloxane-immobilized 3-chloropropyl systems ŽFig. 8A–D. show two regions of major intensity centered at about y105 and y60 ppm. These two peak positions correspond to SiŽ –O– .4 and RSiŽ –O– . 3 units, respectively, where R is an organic group containing the ligand donor atom. The y105

Fig. 7. Dipolar-dephasing 15 N CP-MAS spectra of the protonated ŽpH s1. polysiloxane-immobilized monoamine system Žsample 2. ŽA.; protonated ŽpH s1. polysiloxane-immobilized diamine system Žsample 4. ŽB.; and protonated ŽpH s1. polysiloxane-immobilized triamine system ŽC.. Dephasing period shown in ms.

ppm pattern is composed of at least three contributions, at about y91, y100 and y108 ppm, due to the following types of species: Ž cSiO. 2 SiŽORX . 2 , Ž cSiO. 3 SiORX , and siloxane bridges Ž cSiO.4 Si, respectively, where RX s Et or H. These types are often referred to as Q 2 , Q 3 and Q 4 sites. The relative importance of Et and H for RX in the Q 2 and Q 3 sites can be inferred from the 13 C NMR spectra Žvide supra., which show that there are few remaining ethoxy groups in systems with amino groups present, and more residual ethoxy groups in the 3-chloropro-

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

Fig. 8. 29 Si CP-MAS NMR spectra: of the polysiloxane-immobilized monoamine system, sample 1 ŽA. and sample 2 ŽB.; of the 3-chloropropyl systems, prepared via catalysis by 0.1 M HCl ŽC. and by Žn-Bu. 2 SnŽO 2 CCH 3 . 2 ŽD.; of sample 4 of the polysiloxane-immobilized diamine system ŽE.; of the polysiloxane-immobilized triamine system ŽF.; and of the polysiloxane-immobilized monoamine system Žsample 2. after treatment with 0.10 M HCl for 24 h ŽG..

pylpolysiloxane system. The lower-shielding pattern is composed of at least one peak at about y64 and a shoulder at y57 ppm, due to species containing ligand groups, R SiŽOSid . 3 and RSiŽOSid . 2 ORX , respectively, where RX s H, Me or Et. These assignments are in agreement with previous results on modified silica systems w30–35x. The 29 Si NMR spectrum of the polysiloxane-im-

29

mobilized monoamine system made from a mixture with the high Ž2:1. SiŽOEt.4-to-ŽEtO. 3 SiŽCH 2 . 3 NH 2 molar ratio ŽFig. 8A. shows a lower intensity of the R SiŽOSid . 3 signal at y64 ppm, relative to the SiŽOd .4 signal, in comparison to the ratio seen in the spectrum of the polysiloxane-immobilized monoamine sample prepared from a mixture with a 1:1 molar ratio ŽFig. 8B.. This suggests that the material prepared from a reaction mixture with a lower relative SiŽOEt.4 content is less cross-linked, i.e., has fewer Si ŽOSi d . 4 sites, but more R SiŽOSid . 3 sites. The 29 Si NMR spectra of the polysiloxane-immobilized 3-chloropropyl systems made by using HCl or organotin catalysts are shown in Fig. 8C and D. The sample prepared with a 0.1 M HCl catalyst ŽFig. 8C. shows stronger peaks at y100 ppm and y57 ppm than in the spectrum of the polysiloxaneimmobilized 3-chloropropyl sample prepared with the organotin catalyst ŽFig. 8D.. This indicates that with HCl as catalyst the crosslinking of both organosilane-containing moieties and silica-like moieties was less complete; i.e., there are more R SiŽOSid . 2 ORX and Ž cSiO. 3 SiORX sites. The 13 C spectra of the 3-chloropropylpolysiloxane samples ŽFig. 3. also show substantial amounts of residual ethoxy and methoxy groups. The 29 Si spectrum of 3-chloropropylpolysiloxane prepared using the organotin catalyst also shows an extra peak at y82 ppm, perhaps due to cSi–O–SiŽORX . 3 sites ŽRX s H, Et.. These observations are discussed in Section 4. The 29 Si CP-MAS NMR spectrum of the polysiloxane-immobilized diamine system Žsample 4., shown in Fig. 8E, displays features similar to those of the spectrum of the polysiloxane-immobilized monoamine ligand system ŽFig. 8B., in terms of relative intensities of 29 Si signals in the y60 ppm region and y105 ppm region, because the same m o la r r a tio s Ž 1 :1 . o f S i Ž O E t . 4 to ŽORX . 3 SiCH 2 CH 2 CH 2 X ŽX s NH 2 , RX s Et; X s NHCH 2 CH 2 NH 2 , RX s Me. were used for both preparations. However, in both the y60 ppm and y105 ppm regions, there is evidence of more crosslinking Žhigher relative intensities at y64 ppm versus y57 ppm, and y108 ppm versus y100 ppm. in the diamine ligand system than in the monoamine ligand system; the diamine ligand may serve as a better catalyst for crosslinking than the monoamine

30

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

ligand does. These results indicate that the hydrolysisrpolymerization processes are very similar, with the amine groups acting catalytically and yielding analogous polysiloxane frameworks. The 29 Si CP-MAS spectrum of the polysiloxaneimmobilized triamine system is shown in Fig. 8F. This spectrum displays features that are similar to those of the spectrum of its precursor, 3-chloropropylpolysiloxane prepared via Žn-Bu. 2 SnŽOCOCH 3 . 2 catalysis ŽFig. 8D.. The 29 Si spectrum of the polysiloxane-immobilized triamine system ŽFig. 8F. shows higher intensities at y64 ppm and y108 ppm, relative to those at y57 ppm and y100 ppm, respectively. These results indicate that under the reaction conditions of preparing the polysiloxane-immobilized triamine system, the polysiloxane framework is basically well-formed and has a rather high degree of crosslinking in both the organosiloxane and silica-like regions, even though the polysiloxane-immobilized triamine system is prepared from the 3-chloropropylpolysiloxane, which we have found to contain a substantial quantity of unhydrolyzed alkoxy groups Žhence, uncrosslinked silicons; see Fig. 3.. This result is an indication that the triamine ligand acts as a good catalyst for hydrolysis of cSi–OEt or cSi–OMe and for the crosslinking in both organosiloxane and silica-like regions. 3.5.2. Stability of attachments of polysiloxane-immobilized monoamine systems Ža. In aqueous acid solution. Fig. 8G shows the 29 Si NMR spectrum of the polysiloxane-immobilized monoamine system Žsample 2. after treatment with 0.1 M aqueous HCl solution. Compared to the spectrum of the untreated sample ŽFig. 8B., the spectrum of the sample treated with HCl ŽFig. 8G. has higher relative intensity in the y105 ppm region in comparison to the y60 ppm region, i.e., is a much more ‘silica like’ spectrum, and shows increases in the relative intensities of peaks at y57 ppm, y90 ppm and y100 ppm and decreases of relative intensities at y64 ppm and at y108 ppm, i.e., much less crosslinking in both organosilane-containing moieties and silica-like moieties. Žb. In metal ion solution. 29 Si NMR spectra of the residue of the polysiloxane-immobilized monoamine system Žsample 1. after its treatment with Cu2q Žaq.

Fig. 9. 29 Si CP-MAS NMR spectra of the polysiloxane-immobilized monoamine system Žsample 1.. ŽA. untreated; ŽB. residue from treatment with 0.10 M CuCl 2 Žaq. for 24 h at 258C; ŽC. residue from treatment with 0.10 M CuCl 2 Žaq. for 154 h at 258C; ŽD. evaporated filtrate from treatment with 0.10 M CuCl 2 Žaq. for 154 h at 258C.

for 24 and 154 h, and of the filtrate from treatment with Cu2q Žaq. for 154 h Žafter evaporation to dryness., are shown in Fig. 9A–D. The changes upon treatment with Cu2q Žaq. are similar to those observed when the immobilized ligand system Žsample 1. is treated with aqueous acid. 29 Si NMR spectra of the residue of copperŽII.-treated samples ŽFig. 9B and C. show an increase in the relative intensities of Q 3 and Q 2 silanol peaks at y100 and y91 ppm and a decrease in the relative intensity of the whole y64 ppm, region, i.e., a decrease in crosslinking at silicalike moieties and a decrease in organosiloxane-containing moieties. The 29 Si spectrum of the filtrate from treatment with Cu2q Žaq. after evaporation ŽFig. 9D. shows high intensity in a broad peak at y55 ppm, which is due to structures of the type R SiŽOSid . 2 ORX . Gravimetric data on this system show that 52.7% of the initial polysiloxane-immobilized amine system is extracted by this 154 hour

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39 Table 5 Relative populations of structural moieties determined from DP-MAS Sample

%RSiŽ –O– . 3

a b c

Si spectra

a

elemental analysis S–CH 2 CH 2 CH 2 NH 2 sample 1 S–CH 2 CH 2 CH 2 Cl sample 6

29

31

Fraction R SiŽOSid . 3 b , DP-MAS 29 Si NMR

Fraction SiŽOSid .4 c , DP-MAS 29 Si NMR

DP-MAS 29 Si NMR

33

28 52

25 39

41 20

% of silicon with organoligand group attached. Fraction of R SiŽ –O– . 3 moieties that are R SiŽOSid . 3 . Fraction of SiŽ –O– .4 moieties that are SiŽOSid .4 .

Cu2q Žaq. treatment, and elemental analysis shows that the extracted material is 4.7% carbon and 5.3% silicon. The CP-detected 1 H-inversion–recovery approach w43x was also used to determine T1H values measured via CP-generated 29 Si signals of the y60 ppm re-

gion and of the y105 ppm region of the 29 Si CP-MAS spectra of several of the samples of this study. These measurements were made at 1 H resonance frequencies of both 200 MHz and 260 MHz and some of the results are summarized in Table 4. 3.6.

29

Si DP-MAS NMR results

In order to avoid issues of cross polarization dynamics, and their impact on analytical quantitation w54–57x, we have also carried out, on samples 1 and 6, 29 Si MAS NMR experiments that employed direct polarization ŽDP. of the 29 Si nuclei via 29 Si spin– lattice relaxation. The resulting DP-MAS 29 Si spectra, obtained with a repetition delay of 300 s to avoid intensity distortions, are given in Fig. 10. It can be noted that relative intensities in the DP-MAS spectrum of the 3-chloropolysiloxane ŽFig. 10C. are qualitatively similar to those of the corresponding CP-MAS spectrum shown in Fig. 8D. The DP-MAS

Table 6 1 H chemical shifts Žppm. of polysiloxane-immobilized amine ligand Žsample 1. and its metal complexes and polysiloxane-immobilized 3-chloropropyl system a

Fig. 10. 29 Si DP-MAS spectra of polysiloxane-immobilized ligand systems. ŽA. Experimental and ŽB. deconvoluted spectra of polysiloxane-immobilized monoamine system Žsample 1.. ŽC. Experimental and ŽD. deconvoluted spectra of polysiloxane-immobilized 3-chloropropyl system Žsample 6..

Sample

H1

Monoamine Monoamine, Hq Monoamine, Cd 2q Monoamine, Hg 2q 3-chloropropyl

0.8 1.1 0.9 0.9 1.1

a

b

H2 1.7 2.0 1.9 1.9 2.0

b

H3 2.7 3.1 2.9 3.0 3.6

b

H4

b

5.1 br 7.8 7.3 7.8

c

Estimated chemical shift errors: "0.1 ppm. H 1 , H 2 , H 3 and H 4 are defined in terms of S– Ž2. Ž3. Ž4. Ž1. Ž2. Ž3. CH Ž1. 2 CH 2 CH 2 NH 2 or S–CH 2 CH 2 CH 2 Cl. c br s broad. b

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

32

spectrum of the polysiloxane-im m obilized monoamine system ŽFig. 10A. shows higher intensity in the y105 ppm region relative to that of the y60 ppm region, in comparison with the corresponding CP-MAS spectrum ŽFig. 8A.. The populations of various structural moieties obtained from the intensities derived from the DP-MAS 29 Si spectra shown in Fig. 10 are summarized in Table 5. 3.7. 1H NMR results 1

H CRAMPS spectra of the polysiloxane-immobilized monoamine system, its protonated form

and two of its metal complexes, and of the polysiloxane-immobilized 3-chloropropyl system are shown in Fig. 11. The results are summarized in Table 6. The spectra of the protonated ligand ŽFig. 11B. and products of treatment with aqueous metal ion solutions ŽFig. 11C and D. show strong, broad signals for the –NHq 3 resonance at about 7–8 ppm. The methylene proton signals are also broad and partially overlapped, and probably obscure the weak silanols signals. In contrast to these spectra, the 1 H NMR spectrum of the polysiloxane-immobilized 3-chloropropyl system ŽFig. 11E. shows sharp signals. Besides the methylene proton signals at about 1.1, 2.0 and 3.6 ppm, this spectrum also shows the presence of residual unhydrolyzed methoxy and ethoxy groups at about 1.4 and 3.6–4.1 ppm. The 1 H CRAMPS spectra of the various polysiloxane-immobilized monoamine samples display broader peaks than those in the 1 H NMR spectrum of the polysiloxane-immobilized propylchloride system.

4. Discussion

Fig. 11. 1 H CRAMPS NMR spectra of the untreated ŽA.; protonated ŽpH s1. ŽB.; Cd 2q-treated ŽC.; and Hg 2q-treated ŽD. polysiloxane-immobilized monoamine system Žsample 1.; and untreated 3-chloropropylpolysiloxane Žsample 6. ŽE..

Information obtained from T1H and dipolar-dephasing experiments carried out on the samples of this study via 13 C and 29 Si experiments is discussed here in terms of the effect of specific ligand moieties on the polysiloxane-immobilized system. For each individual sample, the value of T1H obtained from 29 Si CP-MAS experiments are essentially the same Žwithin experimental error, "5%. for the signals of the y60 ppm region and the signals of y105 ppm region Žsee Table 4.. These results indicate that those protons that transfer polarization to 29 Si nuclei of the organosiloxane moieties Žy60 ppm region. and of the silica-like moieties Žy105 ppm region. have essentially the same T1H values. This correspondence between T1H values indicates that 1 H– 1 H spin diffusion between organosiloxane moieties and silica-like moieties is sufficiently rapid to render a uniform T1H value within each sample. The fact that the same T1H values Žwithin experimental error, "5%, Table 4. were obtained from 29 Si-detection and 13 C-detection at 260 MHz Ž1 H frequency. indicates that the organic ligand groups are not phase-separated from the silica-like framework. The fact that, for each of the samples represented

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

in Table 4, the T1H value measured at 260 MHz is greater than T1H measured at 200 MHz indicates that the motional correlation time tc associated with the motions responsible for proton spin–lattice relaxation is larger than Ž260 = 10 6 Hz.y1 , i.e., ) 3.8 = 10y9 s w58x. The fact that T1Hr measurements Žnot shown here. indicate that T1Hr < T1H for each case represented in Table 4 shows that tc 4 Ž200 = 10 6 Hz.y1 , i.e., 4 5 = 10y9 s. For each sample represented in Table 4, both T1H and T1Hr are increased by treatment with HClŽaq., or Cd 2q Žaq.. If one assumes that both treatments reduce mobility in these polysiloxane-immobilized amine systems Žby introducing ionic interactions and complexation., the increases imply that tc in treated samples is larger than about Ž40 = 10 3 Hz.y1 , i.e., ) 2.5 = 10y5 s. The absence of residual ethoxy carbon signals Ž18 and 60 ppm. and residual methoxy carbon signals Ž51 ppm. in the 13 C spectra of the immobilized polysiloxane amine systems ŽFigs. 1 and 2. indicate that complete hydrolysis andror condensation of Si–OEt and Si–OMe groups has occurred, reflecting the fact that the amine group can serve as a hydrolysis and condensation catalyst. This behavior contrasts with 13 C NMR signals that demonstrate the presence of these residual groups in the spectrum ŽFig. 3. of the polysiloxane-immobilized 3-chloropropyl system, for which the hydrolysisrcondensation process was catalyzed by either hydrochloric acid or ŽnBu. 2 SnŽOCOCH 3 . 2 . With both catalysts the 13 C peaks due to residual methoxy and ethoxy groups show that hydrolysisrcondensation was not complete for the chloropropyl system. The degrees of hydrolysisrcondensation in the polysiloxane-immobilized amine ligand systems and the polysiloxane-immobilized chloride system are also reflected in the 29 Si CP-MAS spectra. Because there are no residual methoxy or ethoxy groups in the polysiloxane-immobilized amine systems, as indicated by 13 C NMR spectra ŽFigs. 1 and 2., the y57 and y100 ppm peaks of the 29 Si NMR spectra in Fig. 8A and B represent R SiŽOSid . 2 OH and SiŽOSid . 3 OH moieties, respectively. 29 Si NMR spectra also indicate that there are substantial amounts of crosslinking in the polysiloxane-immobilized amine systems in both the organosilane-containing moieties Žconsistent with the R SiŽOSid . 3 peak at y64 ppm. and silica-like moieties Žconsistent with

33

the SiŽOSid .4 peak at y108 ppm.. The rather high relative intensities of the y57 and y100 ppm peaks in Fig. 8C and D reveal the presence of substantial amounts of R SiŽOSid . 2 ORX and SiŽOSid . 3 ORX , with RX as H, Me or Et, in the two 3-chloropropylpolysiloxane samples; i.e., the degree of crosslinking in both the organosilane-containing and silica-like moieties is less in the 3-chloropropylpolysiloxane samples than in polysiloxane-immobilized amine systems. 29 Si NMR spectra ŽFig. 8C and D. and 13 C NMR spectra ŽFig. 3. show that ŽnBu. 2 SnŽOCOCH 3 . 2 is a more effective catalyst than HClŽaq. for condensation of the 3-chloropropylpolysiloxane system, and 13 C NMR spectra ŽFig. 3. show that Žn-Bu. 2 SnŽOCOCH 3 . 2 is a better catalyst than HCl for the hydrolysis of both SiŽOEt.4 and ClŽCH 2 . 3 SiŽOMe. 3 . Compared to the 29 Si spectra of polysilxoane-immobilized 3-chloropropyl systems ŽFig. 8C and D., the 29 Si spectra of polysiloxane-immobilized monoamine ligand systems ŽFig. 8A and B. show much stronger Q 4 intensity relative to Q 3 , indicating that the condensation in the latter case is more complete, and the material has more crosslinking. Of course, since the signal intensities in CP-MAS spectra are affected by CP dynamics, the results obtained from 29 Si CP-MAS spectra are only qualitative. The 13 C NMR chemical shifts summarized in Table 2 show that the polysiloxane-immobilized monoamine systems prepared in this study display chemical shifts that are similar to those of the bulk self-polymerized 3-aminopropyltriethoxysilane ŽAPS. polysiloxane system Ži.e., with no SiŽOEt.4 employed. w22x; but the chemical shifts measured in this study differ from those of APS-modified silica w22x. The 13 C NMR spectra of the polysiloxane-immobilized amine systems show apparent small shielding shifts of all 13 C peaks, compared with those of APS-modified silica ŽTable 2.. The relatively broad peaks in the 13 C NMR spectra of the immobilized amine ligands ŽFig. 1A and Fig. 2., compared with those of the 3-chloropropylpolysiloxane spectrum ŽFig. 3. and of the 3-Žtrimethylammonium.propyl–polysiloxane w10x suggest that one source of line broadening in spectra of the nonprotonated amine ligand may involve hydrogen bonding between the amine groups and surface silanols or with other ligand groups, as shown below:

34

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

Possible reasons why hydrogen bonding Žor related proton transfers, leading to hydrogen-bonded structures like VI or VII. could lead to line broadening include the following: Ž1. Proton-transfer at rates comparable to the inverse of the relevant 13 C

chemical shift differences due to hydrogen bonding or protonation of amines w31–33x. Ž2. Hydrogen

bonding and proton transfer enhance the possible number of chemically different structures, which can result in inhomogeneous line broadening due to the dispersion of isotropic chemical shifts. Ž3. Hydrogen bonding networks can impart a kind of three-dimensional rigidity to an otherwise flexible system, thereby ‘freezing-in’ a distribution of configurations Že.g., conformations. and preventing or attenuating the line-narrowing that could otherwise result from motional averaging of different isotropic chemical shifts Žsuch as the case for 3-chloropropyl siloxane system.. Another likely source of line broadening in the spectra of amine systems is the effect of the 14 N quadrupole interaction in interfering with MAS averaging of the 14 N– 13 C dipolar interaction w48,59–63x, an effect that is especially strong when unprotonated amine groups, which have large 14 N quadrupolar couplings, are present; apparently this effect is attenuated in quaternary ammonium forms, in which the quadrupole coupling constant is reduced. The quadrupolar effect of 35 Cl or 37Cl on 13 C is absent due to the rapid motion of the 3-chloropropyl moiety. The broadening of peaks in the 1 H NMR spectra of the various polysiloxane-immobilized monoamine samples ŽFig. 10. are presumably caused by some combination of effects of the types discussed above for 13 C spectra of these systems. In the present case, the effects of metal ion complexation Že.g., ‘freezing in’ a variety, or range, of structures and isotropic chemical shifts. can be analogous to what were discussed above for hydrogen-bonding effects. This explanation that hydrogen bonding may be one source of linebroadening in the spectra of polysiloxane-immobilized amine ligands is supported by the 15 N NMR results. In Fig. 5 one can see that the intensity of the shoulder at 34 ppm in the 15 N CP-MAS spectrum of the polysiloxane-immobilized monoamine system increases as the pH is changed from 13 to 3.5. This increase is consistent with the idea that there is a simple analogy between hydrogen bonding and protonation w51x; the presence of the shoulder at 34 ppm between the free amine peak at 25 ppm and the ammonium cation peak at 45 ppm may indicate the involvement of the NH 2 group in hydrogen bonding with acidic surface silanols and perhaps with other amine groups. According to 15 N NMR data in the literature

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

w39,50–53x, one can assign the signals at 45, 34 and 25 ppm in the spectra of Fig. 5 to the ammonium cation form ŽVI, VII or VIII., hydrogen bonded amine forms ŽIII or IV., and the non-hydrogenbonded amine forms ŽIX and X., respectively.

At pH s 1 ŽFig. 5A., the cation form ŽVIII. is favored, whereas at pH s 13 ŽFig. 5E., the ionic amine form ŽX. would be favored; the proportion of various hydrogen bonded forms depends on the pH value. Both 13 C and 15 N NMR spectra a priori can provide information on the complexation of the amine ligands with metal ions. The observed broadening of the C 2 and C 3 signals in the 13 C spectra of the CdŽII.- and HgŽII.-complexed polysiloxane-immobilized monoamine system ŽFig. 1D and E., compared with the spectrum ŽFig. 1A. of the corresponding uncomplexed polysiloxane-immobilized amine system, may result from an increase in chemical structural heterogeneity, indicating, along with the elemental analysis data of Table 1, that not all ligand sites are accessible to metal ions, thereby remaining uncoordinated and therefore having somewhat differ-

35

ent chemical shifts from those groups involved in coordination. This inaccessibility of metal ions to some amino sites may be due to a combination of steric hindrance and electrostatic repulsions between metal ions approaching amino groups and metal ions already complexed to the immobilized amino groups. This view, that some of the ligands remain uncoordinated, is in agreement with the suggestions of other workers for other polysiloxane systems w8x. Another possible cause of the increase in chemical structural heterogeneity might be the presence of different complexation forms of the amine groups with the metal ions, e.g., the number of ligands complexed to each metal ion. The broad signal observed in the 15 N NMR spectrum of the CdŽII.-treated sample derived from the polysiloxane-immobilized monoamine ligand ŽFig. 6B. also suggests that a substantial fraction of the immobilized monoamine that has been treated with aqueous CdŽII. solution is involved in coordination to CdŽII.. The breadth of the peak in Fig. 6B suggests that a portion of the ligand groups remain uncoordinated to metal ion in the sample. Of course, there may be a rapid exchange of Cd 2q ions between different amino sites, which could result in another line-broadening mechanism. In general, dipolar-dephasing measurements can provide information on the strengths of dipolar interactions. In turn, the strength of dipolar interactions can provide information on internuclear distances andror mobilities of the ligand Žvide supra.. The fact that the 1 H– 13 C dipolar-dephasing rates displayed in Fig. 4A follow the order, C 1 ) C 2 ) C 3 , suggests that the amino end of the pendant group has some mobility, so that C 3 and C 2 are more mobile than C 1 , which is closer to the point of attachment to the polysiloxane polymer framework. In the 13 C dipolar-dephasing spectra ŽFig. 4B. of the HgŽII. com plex of the polysiloxane-im m obilized monoamine, the 13 C magnetization due to C 3 dephases only slightly more slowly than that of C 1 and C 2 , and all three carbons dephase somewhat faster than in the uncomplexed system. From these observations, it appears that complexation of the polysiloxane-immobilized monoamine system by HgŽII. is through the amine group, as expected. The enhanced dephasing rate of the 13 C signal intensity in the 13 C dipolar-dephasing spectra of the HgŽII.

36

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

complex of the polysiloxane-immobilized amine occurs because amino complexation by Hg 2q has reduced the mobility of the pendant ligand group. Fig. 4C shows 13 C dipolar-dephasing spectra of the 3-chloropropylpolysiloxane prepared with HCl as catalyst; it shows that 13 C magnetization due to C 3 Žattached to Cl. decays only moderately even with a 50 ms dipolar-dephasing time and 13 C magnetizations of all three carbons of the 3-chloropropyl ligand decay more slowly than the 13 C magnetization of the corresponding three carbons of the polysiloxaneimmobilized monoamine ŽFig. 4A.. This result indicates that the 3-chloropropyl ligands have much greater mobilities Žespecially at the chain end. than the monoamine ligand. The 13 C magnetizations of the residual –OMe and the methyl group of residual –OEt decay only slightly with even a 50 ms dipolardephasing time, because of the rapid rotation of CH 3

groups. 15 N dipolar-dephasing experiments on polysilxoane-immobilized monoamine and diamine ligands ŽFig. 7A and B. show that the 15 N intensities in the spectra of the polysiloxane-immobilized diamine system dephase faster than those in the dipolar-dephasing spectra of the polysiloxane-immobilized monoamine system, indicating that the local mobility of the polysiloxane-immobilized monoamine system is greater than that of the corresponding diamine system, a difference that may result from some combination of steric and hydrogen bonding effects. 29 Si spectra are expected to provide useful information about the nature of attachments of silicon atoms of pendant groups to the polysiloxane framework. This information is relevant to the stability of these attachments to treatment with aqueous solutions of acids or metal ions. These kinds of issues

Fig. 12. Representative Žhypothetical. structures of polysiloxane-immobilized systems based on 29 Si DP-MAS spectra of Fig. 10. ŽA. Polysiloxane-immobilized monoamine system Žsample 1.. ŽB. Polysiloxane-immobilized 3-chloropropyl system Žsample 6..

J.J. Yang et al.r Journal of Non-Crystalline Solids 209 (1997) 19–39

should be reflected in the relative intensities of various 29 Si peaks, although the conclusions can be quantitative only in cases for which detailed studies Žnot part of this work. of the relevant spin dynamics are carried out. However, since the same set of experimental parameters was used for all CP-MAS experiments on a particular nuclide, and since all of the samples of this study are physically similar Žamorphous polysiloxane solids with substantial local mobilities., comparisons of relative CP-MAS intensities, especially within a given spectrum, should be at least qualitatively valid. Changes of intensities in the 29 Si CP-MAS spectrum of the polysiloxane-immobilized monoamine system Žsample 2. upon treatment with aqueous HCl Žcompare Fig. 8B and G., together with elemental analysis data showing that 12.3% of the carbon and 10.9% of the silicon of the original polymer are extracted into the aqueous HCl, can be explained by postulating that some leaching of small oligomeric material containing ligand species into the solution is facilitated by aqueous HCl. This leaching presumably results from hydrolysis of some Si–O–Si linkages, especially those near the immobilized ligand. This kind of leaching of small oligomeric species by acidic aqueous solution has been discussed by others for amine ligands grafted onto silica surfaces w64x. Gravimetric data, together with the 29 Si NMR results shown in Fig. 9, indicate that the Cu2q Žaq. solution, which has a pH s 4.5, causes some hydrolysis of siloxane Si– O–Si linkages, including those on or near pendant ligand groups. From peak areas of computer-deconvoluted DPMAS 29 Si spectra ŽFig. 10. one can determine Žexpected error: "5%. the relative amounts of R–SiŽ – O– . 3 and SiŽ –O– .4 moieties of various types, the relative proportions of which can be estimated from the areas of deconvoluted peaks. Reliable elemental analysis data ŽTable 1. can also be used to estimate the relative amounts of R–SiŽ –O– . 3 and SiŽ –O– .4 moieties in the polysiloxane-immobilized monoamine ligand system. Results of these estimates are shown in Table 5. This table shows that, for the samples examined, 28% and 52% of the silicon atoms of the polysiloxane-immobilized monoamine system Žsample 1. and 3-chloropropyl system Žsample 6., respectively, have organoligand groups attached. Fig. 12 shows hypothetical representative structures that are

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consistent with the structural data we have derived on these two samples, as reflected in the DP-MAS 29 Si spectra. As the 13 C CP-MAS spectra show that the polysiloxane-immobilized amine samples contain no unhydrolyzed methoxy or ethoxy groups, none are shown in the structure of Fig. 12A. Such groups are included, in proportion to the corresponding intensities in the 13 C CP-MAS spectrum, in the structure of Fig. 12B, representing the polysiloxane-immobilized 3-chloropropyl system, for which hydrolysisrcondensation of methoxy and ethoxy groups is not complete.

5. Summary and conclusions Polysiloxane-immobilized mono-, bi- and triamine ligand systems have been prepared by the sol–gel method. The amine ligands can form complexes with metal ions such as Hg 2q, Cd 2q, Cu2q and Ni 2q. Solid-state 29 Si, 13 C, 15 N and 1 H NMR spectra provide valuable information on the primary structures of these systems, including possible indications for the involvement of the amine ligands in hydrogen bonding with surface silanols, as well as the intimacy between organosilane-containing and silica-like moieties, and the mobilities of various organic ligands. Together with results of elemental analysis, the NMR data indicate that a portion of the ligand groups are inaccessible to metal ions and remain uncoordinated when treated with aqueous solutions of divalent metal ions. The results from 29 Si NMR and elemental analysis data indicate that these materials undergo some leaching when treated with acids or metal ion solutions, presumably due to the hydrolysis of the polysiloxane networks under these conditions. T1H measurements show uniform proton relaxation behavior within each sample examined. This shows that each of these samples is homogeneous, rather than constituted of regions of different phases, i.e., separate silica-like and wRSiŽOSid . 3 – x n regions.

Acknowledgements This work was supported in part by National Science Foundation Grant No. CHE-9021003. The

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