Chapter Synthesis, Characterisation and Metal Uptake of

Chapter 4

Synthesis, Characterisation and Metal Uptake of Polymer Supported

Ligand Systems

A s evident from literature a wide variety of polymer supported systems are

currently available. These involves hydrophilic and hydrophobic, aliphatic

and aromatlc systems. There are several arnphiphilic supports also. As the present

study is not aimed at to look at the effect due to the nature of polymer matrix no

attempt was made in the present study to try through the above vaneties of systems.

Instead we have chosen only one variety of support which is styrene-divinyl

benzene based Keeping the nature of support same it is our intention to generate a

varlety of l~gand functions on the support and study their ligation characteristics

and especially the nature of metal complexes generated on the support. The ligand

functions chosen are N, NN, NO, 00 and SS donor species as these contribute the

major part in co-ordination compounds.

4.1 Synthesis of polymer supported ligand systems

4.11 Divinylbenzene (DVB)-crosslinked polystyrene (P*)

To a solution of benzoyl peroxide (300 mg) in styrene (20 ml, 175 mM)

divinyl benzene (2.65 ml, 18.4 mM) was added. This solution was added with

vtgorous stirrmg to polyvinyl alcohol solution (300 mg in 300 ml water) at 80°C

and stirred for 20 h The polymer separated out in granular form was filtered,

bashed and drled in vacuum. The extent of crosslinking in the sample was about

Solo. Attempts were also made to adjust the crosslinking density by varying the

amount of DVB to get variety of samples.

4.1.2 Chloromethylated P (P*CI)

The DVB-crosslinked polystyrene resins could be surface functionalised to

get their chloromethyl derivative. In a typical reaction, chloromethyl methyl ether

(1 8 ml, 237 mM) was added to polystyrene (I 8 g) in 50 ml dichloromethane at O°C.

Stannic chloride (0.91 g, 3.48 mM) was added to the well stirred mixture and

temperature was allowed to rise to room temperature during a period of 1 h.

Stirring continued for 55 h. The functionalised resin which retained the texture of

the original P* was filtered, washed and dried.

4.1.3 Amine functionalised support (PN)

The conversion of the chloromethyl derivative P* CI into the amino

derivative P*N was done by amination reaction. To about 10 g of P* C1 taken in

250 ml round bottomed flask containing 150 ml DMF, 11.2 g of hexamethylene

tetramine and 13.2 g of KI were added and heated at 1 10°C for 8 h. The resin so

obtained was filtered, washed and hydrolysed by stirring with 10% NaOH. The

ammo resin obtained was washed with water and dried.

4.1.4 Diamine functionalised support (PNN)

On treating PtCl w~th diamine in presence of a base could affect

condensation to append the diamine on the polymer surface. About 10 g of P*CI

was suspended in dioxan (20 ml) to which ethylene diamine (I4 ml, 20.1 mM) and

pyridine (3.4 ml, 42 mM) was added. The mixture was heated with stirring at

1 OO°C for 9 h The functionalised polymer was filtered washed and dried.

4. f .5 Schiff base functionalised support (PWO)

To a suspension of 10 g of aminomethyl polystyrene, P*N, in ethanol

(200 ml) 7 ml sallcylaldehyde was added The mlxture was refluxed for 4 h,

cooled, filtered washed and drled in vacuum

4.1.6 Diazo functionalised support (Pi'NO)

Diazoderivatlves derived from resorcinol and aniline was prepared initially

by diazotisation, by reacting about 5 g of aniline (54 mM) and 5.94 g of resorcinol

(54 mM). The orange coloured dye was used after crystallisation for supporting on

P'CI. About 5 g of the chloromethylated polystyrene was added to 9 g of azodye

dissolved in d~oxane (300 ml). The reaction mixture was heated with catalytic

amount of ammonium fluoride at 120°C for 6 h. Resin so obtained was washed

and dried and used as NO chelate through diazo N and orthohydroxyl group.

4.1.7 Canbawyl funciionalised support ( m O )

The following reaction sequence was employed to effect the synthesis of

carbonylated polymeric systems. P*Cl was converted into the aldehyde analog by

treating it with NaHCOz in DMSO and then oxidising to the -COOH derivative by

KzCr207 in acid medium. A suspension of P*CI (10 g) in DMSO (300 ml) was

heated with NaHCO, for 10 h at 140°C. The functionalised resin was filtered,

washed and dried. Aldehyde resin thus obtained (I0 g) was added to a mixture of

conc H2S04 (5 ml) conc. acetlc acid (300 ml) and K~Cr207 and heated at 70°C for

96 h. Carbox~lated resin formed was washed with water and dried.

4.1.8 Acetyl acetone functionalised support (PltOO)

The resln was prepared from chloromethylated polystyrene by treating it

w~th acetyl acetone in presence of triethyl m i n e catalyst. A suspension of P*CI

I I0 g ) In THF (300 ml) was refluxed with acetyl acetone (20 ml) and triethyl amine

15.6 ml) for 48 h The acetyl acetone functionalised resin was collected by

filtrat~on, washed with THF and methanol and dried.

4.1.9 Dithiocarbamate functionalised support (P'SS)

The dlth~ocarbamate functlon was Introduced on to the polymer support by

react~ng CS? - wlth the amino resln, P*N, developed as earher Amno methyl

polystyrene P*N (10 g) was made to react wlth CS2 (19 ml) and NaOH (10 g in

I00 ml water) by stlrrlng for 24 h The dithlocarbamate resln obtiuned was

collected by filtration, washed wrth ethanol and dr~ed In vacuum

4.2 Characterisation

The character~satron of the core polymer supports and their functional

derivatives were done by chemical and spectral analysis. In most of the cases the

core support employed was commercially available Merrifield resin. But to check

and prevent the possible interaction of metal salt in free state into the cavitylpores

of the resin, trials were also carried out with samples prepared in the laboratory.

The DVB-crosslinked polystyrene was obtained in bead form and depending upon

the extent of crosslinking and reaction conditions the texture of the samples was

also different. The samples were prepared with varying crosslink density. The

reactron lnvolved in the radical polymerisation of styrene and DVB could be

represented as follows.

Scheme 4.1

The crosslink denslty of the solid polymer was varied by varying the amount

of DVB crossl~nker Different samples were prepared with crosslink density varied

from 2 to 10% Reasonably good solid granular resin was obtained when the

crosslink dens~ty was 2% or above.

The chloromethyl functionalisation of the above polymer P* was necessary

to synthesise varlous other surface functionalised polymers employed in the present

study The chloromethylation introduces -CH2CI function on phenyl groups

espec~ally on the surface phenyl groups. The reaction involved is given below.

Scheme 4.2

The chlorornethylat~on proceeds slowly and hence the reaction was kept for

sufficiently long time. By about 50 h moderately functionalised (XHzCI) resin

could be obtained. The functionalisation was confirmed by chemical analysis and

b) lnfrared spectroscopy (VC..CI : 700 cm-I). The -CH*CI content was estimated by

modified Volhards method (Section lll). The chlorine content varied from 2 to 5

meqig of the resin

The greater the extent of crosslinking the less was the CI content. The

commercially available (Fluka) sample had CI content about 4 meqlg of resin. For

developing most of the other functionalised resins, 2% DVB-crosslinked

polystyrene contrunlng about 3-4 meq Cllg of the resin or the commercially

available samples were employed.

The amination reactlon of P*CI by treating with hexamethylene tetramine

and KI in DMF followed by hydrolysis with NaOH (see Scheme below) to get

P*NH2 was not found to be fully quantitative.

1 ) HMTADMFIKI 1 10°C

2) Hydrolysis P*CI P*N

Scheme 4.3

Residual amount of CH2CI was present in the resin even after prolonged amination

reaction. The res~dual amount could be detected by pyridine method. The

conversion of -CH2CI to -CH2NH2 was confirmed by chemical analysis and IR

spectroscopy (N-H: 3440 cm-I). A simple and elegant method was also developed

In the present study to estimate the amount of -NH2 present in the polymer resin

(see Section 4.3). No attempt was made to convert -CH*CI quantitatively to

-CH*NH2 as the CI function is inert towards co-ordination.

Unlike P*N which can act as a simple N donor species towards metal ions,

the diamine analog P*NN could be potential N,N-bidentale species. The reaction

of ethylene diam~ne (en) with P*CI involved a simple condensation reaction

(Scheme 4.4).

Scheme 4.4

As in the case of PtNH2 (P'N) the conversion in this case to P*NN was not found

to be quantitative. Res~dual chlorine was detected by pyridine method. However

~ u s t as in the case of P*N, the unreacted -CH2CI function did not cause any

problem with regard to its ability of complexation towards metal ions as the

chlorofunct~on cannot l~gate. So no attempt was made to effect any quantitative

conversion. The possibility of ethylene diamine getting condensed at both the

amlne ends by two -CHZCI groups resulting in cyclic products on the support is

ruled out as the P*CI resin chosen has -CH2CI functions too widely separated.

Further the condensat~on was carried out with a large excess of ethylene diamine

taken In solut~on. The grafting of en on the support was confirmed by chemical

analysis and IR spectroscopy (C-Np,,: 1570 cm" and (C-N)(,,: 1170 cm-I). By

using HCI gas method an attempt was made to estimate the diamine function on the

polymer support (see Sect~on 4.3) .

Schiff bases (SB) are widely studied ligands and their strong co-ordinating

tendency has resulted in rich varlety of metal complexes. Both experimentalists and

theoretical chem~sts have explored M(SB)2 type complexes in detail and this has

contributed substantially to the field of co-ordination chemistry. The Schiff base

@led polymer resin with potential N and 0 donor properties and labelled as

P'NO was developed from P*N which contained -CH2NH2 functions on the

surface of the polymer support. Though a wide variety of orthohydroxy-aryl

aldehydes could be employed for condensation only salicylaldehyde was used in the

present study. The reactlon lnvolved is given in the scheme below. With the

format~on of SB funct~ons on the polymer support there was characteristic colour

change for the resin. Chemical characterisation of the P*NO resin was done by

chernlcal analysis and IR v(C=N) 1630 cm-', v(C-0) 1 150 cm-'.

Scheme 4.5

Attempt was made to graft a different variety of N and 0 donor ligands on

the P*Cl by uslng a suitably selected diazo compound. The diazo compound was

synthesised by coupling resorcinol with the diazonium salt derived from aniline.

The reaction ylelded essentially one isomer because of the ortho- and para-orienting

tendency of -OH group. The orange coloured dye bearing two -OH groups (at the

onho and para positions with respect to -N=N-group) on reacting with P*CI

preferentially attached through the parahydroxyl group because of steric reasons.

The reactions are illustrated in scheme below. .

Scheme 4.6

The grafted dye could be N and 0 donor chelating ligand through the diazo

N and ortho-hydroxyl group. Consequent to the grafting of the dye on P*CI there

was charactenstlc colour change on the polymer. The chlorine estimation by

pyr~dine method showed that there was residual C1 present (3.6 meqlg of resin) in

the dye- grafted resln. No attempt was however made to effect the quantitative

grafting as -CH*CI cannot Interfere w ~ t h complexation to metal Ions Chemical

analysls and 1R spectra [v(C-N)(,, 1290 cm'l, v(0-H)(,, 1470 cm-', V(C-O-)~

1 130 cm I] confirmed the grafting of the azo dye on the support The supported

Iigand system 1s denoted as PI*NO to dlstlngulsh it from the Schlff base analogue

The simplest 00 donor ligand is a carboxylate group. The partially

cOnjLIgated COO- anton can co-ordinate in a variety of forms. The carboxylate

group supported resln was developed from P*CI by first converting into the

aldehyde and then ox~d~slng it to the acid form. The reaction involved is given in

the scheme below

Scheme 4.7

The Conversion of -CH2CI to -COOH function was not found to be quantitative.

A slight amount of the chlorofunction remained unreacted even after prolonged

reaction. No attempt was further made to convert this into the -COOH form. The

characterisat~on of the resin (labelled as P*OO) was done by chemical analysis and

1R spectra [v(C=O)(,,: 1700 cm", v(C-0)(,,: 1305 cm-'I. Another popular 00

donor ligand is acetyl acetone (acac) which in its enolate form acts as a strong

chelating ligand. The supporting of acac on the polymer was done on P*CI. A

fGr amount of acac function could be grafted through the central C of the ligand

moiety. The reaction involved is given in the scheme below.

Scheme 4.8

The grafting of the lryand moiety was confirmed by chemical analysis and

IR spectra [C=O,,,: 1720 cm-', vC-O(,,: 1395 cm.'].

.4n extens~vely studied SS donor ligand system is dithiocarbamate. Formed

hv the reactlon of CS2 on amines in alkaline medium they posses strongly

co-ordinating CS2- funct~on This group cannot be considered as the sulphur

analogue of carboxylate group slnce CS2 group is attached to N through C which

permlts extended conjugation. While rich chemistry is reported on dithiocarbamates

derlved from secondary amines (which are also known as dialkyl or diary1

dith~ocarbamates) their primary m i n e analogues (which are also known as

monoalkyl or monoaryl d~thiocarbamates) are scantly reported owing, possibly, to

their relative ~nstabillty and metal ion mediated decomposition. In the present work

the dith~ocarbamate funct~on was developed on polymer matrix, P*N which posses

prlmary m i n e functions Besides studying the complexation tendencies of the

supported monoalkyl dithiocarbamates it was also our intention to look at their

reported instability aspects on the polymer backbone. The scheme below illustrates

reaction involved in the dithiocarbamate formation.

H I

CS2 c m C H 2 - N - C S i NaOH

Scheme 4.9

Care was taken to cany out the reaction with excess of CS2 to convert all

-NH2 to -NHCSZ funct~on We, however, do not rule out the presence of mlnute

amount of res~dual NH2 wh~ch could be remalnlng unreacted w ~ t h ~ n the pores of

the sol~d polymer Thls was checked in the case of some ESR actlve metal

complexes generated on thls supported l~gand system (see Chapter 5) The

characterisation of the dithiocarbamate function was carried out by chemical

analysis and 1R spectra. (vC-N(,,: 1450 cm-I, vC-St,,: 1020 cm.').

For the sake of clarity seven of the polymer supported ligand systems mahe

use of in the present work are tabulated in Table 4.1

Table 4.1. Polymer supported ligand (P*SL) systems Employed in the present work.

4.3 A simple method for the estimationof active amine fundions on crosslinked solid polymer matrix

There are several reactions in which the active functional groups attached to

solid polymer surface play significant role directly or indirectly. Unlike in solution

state where the functional entities would be uniformly distributed, in the solid

polymer supported form the functional groups get located within the network

system. There are advantages and disadvantages like diffusion-related problems in

reacttons tnvolvtng such systems. Equally significant are several reaction related

aspects whereln the concentratton of the acttve functtons on the solid polymer play

Important role Whtle P*OO, P1*00, P*NO, P,*NO and P*SS species are

chelating anionic ligand systems, P*N and P*NN are simple neutral Lewis base

liyands, which unlike the former category of systems, could play vital role in many

of the metal mediated reductive elimination and insertion reactions where co-

ordination unsaturated species become part of rate determining step. An estimation

of these Lewis base functions on the solid polymer system assume yeat

significance in this context. Rather than the quantity of the entire m i n e function

present in the solid polymer system, an estimate of the active (or 'available') m i n e

functions is more significant. Described below is our attempt to estimate the active

functionality of the amino resin P*N, diamine function P*NN, along with a

pyridine bearing polymer supported system derived from polyvinyl pyridine (PVP).

Amines and pyridines being Bronsted bases are capable of getting

protonated by acids. However, acidimetric titration of these bases in aqueous

solution is not possible because the acid is capable of protonating both the bases

and Hz0 with only moderate discrimination. In (A) the protonation of the 'bases-l '

by H@+ is a thermodynamically favoured reaction because the R3N: is a stronger

base than water R3NHf ion which is the conjugate acid of R3N: is a weak acid

but the backward reaction of protonating Hz0 is, however, operative in

R3N: + H ~ O + R ~ N H + + Hz0 h l ad i Acid 2 -2

to the extent of keeping the reaction in a dynamic equilibrium. A comparatively

high equilibrium constant for (A) indicates that R3N- can be more or less

quantitatively converted into the aminium salt by using high concentration of an

acid. However, quantitative estimation of the mines by adding high concentration

of an acid of known strength and then titrating excess acid by alkali, is not possible

in aqueous condition as it disturbs the equilibrium in (A) in that process.

Literature reports, however, give a few methods of estimation of free

aminelpyridine content which involves titration of the base in non-aqueous solvents

like dioxane, nitromethane, etc Evidently these methods are generally expensive

and in the solid polymer systems are liable to be erroneous

The amlne or pyrldine bearing unit of our interest being a solid particulate

polymer, it is possible, in principle, to estimate the surface (active) aminelpyndine

content in aqueous condition itself. We tried to closely monitor this method which

appears to be in use with some workers. To a known amount of solid aminel

pyridine bearing polymer was added a measured volume of 5N (standard)

hydrochlor~c acid. The sample after shaking gently and keeping for about 20 min

was filtered to collect the filtrate but without washing the solid polymer with water.

All the available amindpyridine functions supported on the polymer reacted with

equivalent amount of the acid to form aminiurnlpyridinium chloride on the support.

The procedure followed is to attempt to collect all the unreacted acid (but without

washing) and titrate it to estimate the acid that has reacted with the polymer sample

which would be equivalent to the available aminelpyridine functions in the polymer

sample. Since it is not possible to collect the unreacted acid without washing and

since washing with water upset the equilibrium in (A) the estimation is bound to be

significantly incorrect.

We have modified the above method slightly to make it more suitable for

the investigations. The method involved is treating the solid aminelpyridine

bearing polymer with dry I4CI. A weighed amount of the solid polymer was

loosely packed into a tube of 1.5 cm diameter and 20 cm length with both ends

open. Cotton plugs were inserted at both the ends to prevent the sample from being

flown off. Perfectly dry HCI gas was allowed to flow through the sample gently for

about 20 min. the direction of flow was reversed by changing the tube-end for

another 20 min. The sample was then carefully and completely transferred into a

dry beaker and heated in a vacuum oven at 80°C for about 1 h remove the

unreacted HCI trapped in the solid. Water was added to the sample to hydrolyse

the pyridinium chloride and then titrated it against standard alkali to estimate the

acid equivalent of the available aminelpyridine groups in the sample.

The expenrnent was repeated with varying amount of sample, the results of

wh~ch are tabulated in Table 4 2

Table 4.2 Available h i n e b i d i n e content in the polymer supported ligand systems.

PSL system Available Amine/Pyridine content Total mmol mrnolhz

PVP*

P*N

The quantitative conversion of available pyridine to pyndinium chloride has

P*NNa

-

been verified by taking pure liquid pyridine and passing dly HCI thro~lgh the

1.0

2.0

3.0

1 .O

2.0

3.0

sample in a moisture free atmosphere to convert it into its salt. It was found that

"In h s case it was considered that both the mine hnctions of the diamine moiety get quantitatively protonated

1.0

2.0

3.0

the entire pyridine got converted to solid pyridinium chloride. M e r removing the

I -

unreacted HCI by heating the sample in a vacuum oven, the salt was hydrolysed

w~th water and titrated against standard alkali. The acid equivalent was calculated

6.20

12.50

18.65

3.10

6.22

9.30

2.9

5.82

8.71

which agreed well with the theoretically expected value for the amount of pyridine

taken. The method was followed in the case of aniline and ethylene diamine, by

6.20

6.25 >

6.22

3.10

3.11

3.10 ~-

2.9 .

2.91

2.90

taking care that no moisture gets entered wh~le passing HCI gas. The conversion to

their salts was quantitative and the titration gave theoretically expected results

(Table 4.3).

The samples taken were vacuum distilled and the manipulations were done,

as far as poss~ble, In a motsture free atmosphere. There was found to be excess

HCI present in the sample which was removed by heating in vacuum oven at about

'1O0C.

Table 4 3 Aminelpyridine content of the bases aniline, ethylene diamine and pyTidine.

4.4 Metal ion uptake capacity of polymer supported ligand systems

The affinity of these polymer supported ligand systems towards a variety of

transition and non transition metal ions was tried by solid-solution phase reactions.

The metal ions investigated are coZL, ~ i " , CU'~, VO~', 2n2+, cdZ+ and H ~ ~ ' . The

structural characterisation of the metal complexes formed on the resin is discussed

In Section 5. Before d~scussing the characterisation of metal complex, an attempt

was made to determine the metal uptake capacity of each of the resins. Studies

were generally restricted to neutral pH but in a few cases pH dependent uptake

capacity was also monitored. The metal uptake was determined by stirring the

polymeric ligand system with the discussed metal ion solution of known strength

for sufficient tune and estimating the unreacting ions by appropriate methods (see

Sect~on 3). The uptake capacity was found to be varying with the ligand system

employing. The metal uptake capacity of the dithiocarbamate resin and ethylene

dlamino resin for various transition metal ions are given in Table 4.4.

Table 4.4. Metal uptake capacity of dithiocarbamate resin and ethylene diamino resln

The trend emerged for PtSS (dithiocarbamate resin) was ~ g ~ ' > cuZ+ >

r d 2 > zn2 ' 3 CO' ~ i ~ ~ , while for the ethylene diamine resin the intake capacity

varied as H ~ ~ ' > Cu2- > 2n2' > cdZi = co2'. The trend emerged for various metal

ions is related to the stability constants of complexes on the support. Factors like

the acid base characters of the donor and the acceptor, ionic nature of the species

involved, types of counter ions associated etc. can influence the stability of metal

complex of these polymeric ligands. Anion dependent study showed that there is a

significant influence of the type of anion of the metal salt employed on the metal

uptake capacity. The amount of CU", complexed by the dithiocarbamate resin,

when different copper salts were employed, were estimated to investigate the effect

of anions on complexation Nitrate, sulphate and chloride were the anions used in

the study and the results are given in Table 4.5.

Metal lon used

c u 2 .

co2 '

N I ~ -

zn2 '

~ d '

H ~ ' .

Uptake capacity of

P*SS (mmollg)

0.43

0.17

0.15

0.18

0.19

2.0

P*NN (mmollg) -

1.65

0.39

0.50

0.639

0.55

2.6

Table 4.5. C u 2 uptake of dithiocarbamate resin in presence of different anions

Sulphate might form rather strong chelates with copper, while nitrate and

chloride form weak chelates. The difference in the uptake of CU'. in presence of

these antons can be explained in terms of this difference in stability of the anion

metal chelates

Anlon

C I

SO,'

NO3

The Schlff-base resln showed only marg~nal aff~nity for complexatlon and

the trend noted was CU' > c o 2 > H ~ ~ - > zn2* > cd2' All other reslns exh~blted

reduced affinity towards the metal ions studled under normal cond~t~ons For the

diazo resin and carboxylate resin the metal uptake decreases in the order co2' >

C d' , ~ g ' , c u 2 and Co2 > cd2 > CU'*, respectively For Sch~ff-base restn,

carboxylate resln and acetyl acetonate resln it was found that the metal uptake can

be Increased by increastng the p1-I of the medlum of cornplexat~on

CU' uptake (mmoltg)

43

3 3

52