Indian Journal of Chemistry Vol. 398, June 2000, pp. 414 - 418

Studies on polymer-appended free base porphyrins with ether linkage and their copper complexes

Tessymol Mathew

Post-Graduate Department of Chemistry, SI. George's College, Aruvithura 686 122, Kerala, India

and

M Padmanabhan

Schoo l of Chemical Sciences, Mahatma Gandhi Uni versity, Kottayam, Kerala

and

Sunny Kuriakose*

Research Centre and Post-Graduate Department of Chemistry, SI. Thomas College, Pala 686 574, Kerala, India

Received 30 April 1997, accepted (revised) 9 September 1999

Monohydroxy- and tetrahydroxyporphyrins have been anchored on crosslinked polystyrene matrices through ether linkage. Ort/1O, meta and para substitu ted polymeri c porphyrin s have been synthesised from chl oromethyl polystyrene and suitably substituted porphyrins and the products characterised by spectral studies. The interesting features of the electronic spectra of polymer anchored porphyrin s like split soret bands are explain ed by th e ori entational varia ti on in or/it o-, me/n­and para-isomers and the effect of polymer-porphyrin interacti on on the electron ic leve ls of porphyrin macrocycle. The copper complexes of these polymer-bound porphyrins are synthes ised and analysed spectroscop ica lly. Electronic and ESR spectra show interesting restl lts and these are explained based on the electronie modification of porphyri n systems on a ri gid macromol ecu lar backbone.

The highly de localised n-electron framework of planar porphyrin mo lecules make them a finel y tunable mo lecular system in terms of the ir electronic characteristics'. Among various factors which modulate these key properties the maj or ones are known to be the ty pe of substituents on the porphyrin periphery, the nature of the central metal atoms of meta lloporphyrins and the coordination number of the metal ion concerned. Studies on innumerable van etles of polymeric porphyrins and the ir meta llo derivatives were carried out by several workers24

. Cross linked polymers obtained by the polymeri sation of suitably des igned monomers find extensive use as support mater.ia ls for a w ide variety of cata lysts, reagents and functional fragments to carry out novel reactions by extremely simple synthetic strategy5,6 The idea of porphyrin supported on crosslinked polymer offers innumerable probabi I ities for the structural and e lectronic modification of the porphyrin system by varying the porphyrin , polymer and the nature and position of polymer-porphyrin linkage. The present study covers the synthes is of polymeric porphyrins with ether linkage, metallation with copper(Jl) ions and the chemical and spectral analysis of these polymeric systems.

Materials and Methods

2% Di viny lbenzene (DVB )-cross linked po ly­styrene was prepared by the suspens ion co­po lymeri satio n of sty rene and DVB in th e required rati o. Chloromethy l polystyrene [A], 5, I 0, 15 ,20-tetrakis(hydroxyphenyl)porphyrins 1 and 5-(hydroxy­

phenyl)-I 0, 15,20-triphenylporphy rins I ' were synthe­sised fo ll owing the lite rature procedure6

.8

. The chlorine capacity of the po lymers was estimated by the modified Volhards method . IR spectra were recorded on a Shimadzu I R-4 70 spectrophotometer operating in the range 4000-400 cm- 1 us ing KBr di sks . So lid state e lectronic spectra were recorded on a Carry-2390 UV -vis-near IR spectrophotometer and ES R spectra on a Varian E-12 spectrophotometer.

Polymer-bound porphyrins [A 1 and A 1'1. General procedure. DVB-cross linked chlo romethyl po lystyrene (2%, I g) was suspended in the mixture of chloroform-methanol (3: I). Hydroxypo rphyrin (1 or I') (500 mg) was dissolved in methano l and was added to the above mixture . Triethyl am ine ( I mL) was then added and the mixture refluxed for 8 hr. After cooling, the reslll was filtered , washed

MATHEW et at. : STUDIES ON POLYMER-APPENDED FREE BASE PORPHYRINS WITH ETHER LINKAGE 415

repeatedly with hot methanol, water, acetone and chloroform until a colourless filtrate was obtained. The resin was then dried in an air oven.

Differently substituted polymer-bound porphyrins were prepared by using ortho-, meta- and para­substituted hydroxyporphyrins . IR(KBr): 1260 cm- I

(Ar-O-C), 1150 cm- (-CHr-O-). (

Preparation of copper complexes of Al and AI: General procedure. The copper complex was prepared by the reaction of the polymer-bound porphyrin (1 g) swelled in methanol and chloroform (50 mL, I: 1) with a solution of copper(II) acetate (20 mg/20 mL water). Acetic acid (0.1 mL) and sodium acetate (50 mg) were added. The mixture was refluxed for 2 hr, cooled, filtered and washed well with water and methanol.

Results and Discussion Substituents on the phenyl groups of porphyrin

system lie in a plane orthogonal to the porphyrin frame9

. It is possible to synthesise porphyrins with functional groups attached to various positions (ortho, meta and para) on the mesophenyl groups by reacting with appropriate starting materials. In analogy with the observation on low molecular systems it was proposed that by appending such position isomers of functionalised porphyrins on polymer supports, the orientation of porphyrin 1t-frame with respect to polymer support can be controlled at varying angles depending on the position of the linkage. Thus . . on grafting ortho-substituted H2TPP's the bulky polymer matrix would feel the porphyrin plane quite close and face-to-face leading to intense steric interaction. As a result, there can be expected significant distortion (puckering) on the porphyrin moiety to ease the steric action. In the case of species appended through meta­substituted porphyrin, the proximity of porphyrin

-(A)

QQ - The Porphym skeletoo

frame and the polymer support would not be close enough and hence only moderate puckering is expected. On the other hand, the system with para­substituted porphyrin attached to the polymer support is expected to have only the minimum puckering. In the -O-linked polymeric porphyrins there can be some amount of non-rigidity or flexibility especially with regard to the rotation of porphyrin frame about the connecting single bond or bending about the intervening bonds. These ' flexible ' bonds are likely to cause distortion on porphyrin by steric interaction (especially on the ortho- and meta-isomers).

Polymers with monohydroxy- and tetrahydroxy­tetraphenylporphyrins having different phenyl substituent positions were prepared (Scheme I) and the number of functional groups on the porphyrin ring appended on to the polymer matrix were determined by estimating the residual chlorine capacity of the polymer. It was estimated that 0.6 to 0.95 meq of chlorine/g of the polymer was replaced by the porphyrin groups for the compounds Al,mho, AI /llela

and Alpara .

Characterisation of the polymer-bound porphyrins were done by spectral analys is. IR spectroscopy provides a clear evidence for the bonding pattern in the compounds. The IR spectrum of compound Al shows a broad band in the region 3380-3450 cm- I

corresponding to the O-H stretching vibrations. This is due to the free hydroxyl groups of the porphyrin moiety. AI' compounds do not show this peak indicating the absence of free hydroxyl groups. The peaks observed at 1260 and I 150 cm - I are corresponding to the aryl ether and CHrO vibrations, respectively. The latter indicate the formation of ether linkage between the polymer support and the porphyrin system.

Electronic spectra of the polymeric porphyrins

OH

Scheme 1- Prepration of Al compounds

416 INDIAN J CHEM, SEC B, JUNE 2000

AlartllO' Almeta, Alpara, AI'artho, AI'meta and AI'para exhibit characteristic etio type spectra with the appearance of Q-bands and soret band. The presence of polymer matrix in these derivatives impart some remarkable change on the electronic properties of the porphyrin macrocycIe. On appending the porphyrin through their hydroxy substituents to the polymer backbone, some well-distinguishable observations were obtained for the soret band of the spectra of Al compounds. Here the soret bands get splitted and appear as an intense band with a shoulder. These spectral changes are depicted in Figure 1. The split soret band is more pronounced for the para­substituent.

Apart from the inestimable backbone effect of the polymer, the absorption spectra helps to give us a deeper insight into the electronic and steric interactions in the porphyrin systems. Tetrahydroxy­tetraphenylporphyrins are bonded to the support material through an ether linkage and in this process, a considerable extent of hydroxyl groups remain free . Though the hyperentropic factor is operative in functional polymers, the interaction of the porphyrin core unit and unreacted phenolic groups is quite possible due to the coiling effect of the backbone 10. In such situations, the porphyrins can be protonated by the presence of a close, suitably oriented phenolic group. This will perturb the electronic ordering in the

porphyrin core. The presence of protonated and non­protonated porphyrin systems is responsible for the split soret bands.

Another interesting and important observation was derived by comparing the absorption spectra of ortho-, meta- and para-isomeric systems Alartha, Almeta and Alpara. No systematic variation could be observed in these cases though it was expected due to the systematic steric participation of polymer in ortho, meta and para-isomers. The meta-isomer showed a blue shift compared to the ortho (Table I). Protonation of the porphyrin system by phenolic groups and the electron redistribution of the resultant phenoxide ion with its porphyrin core is again clearly evident from these results. In ortho- and para­isomers, higher extent of electron delocalisation is possible due to conjugation and hence the absorption band is shifted to longer wavelength region.

On the other hand, electronic spectra of AI' compounds show some systematic change on going from AI'"rtho to AI'para (420, 421 and 423 cm- I for ortho, meta and para compound respectively). This is due to the pure steric interaction of the polymer

400 soo 600 700

1. hUl)

Figure I - Electronic spectra of Al compounds

Table I - Electronic spectral data of Al compounds

Compd Soret band Q-bands (nm) (nm)

A Iorfho 420, 450 510, 542, 586, 648

A Im.,a 418, 448 509, 540, 586, 648

AI1X!r<I 426, 456 520, 558, 596, 694

backbone on the electronic levels of porphyrin system. Split soret bands observed in Al compounds were absent in AI' compounds. This is because of the absence of free or unreacted hydroxyl groups which facilitate protonation in the case of Al compounds.

The polymer-bound porphyrins, Al and AI' were metallated and the formation of the metal complexes of Al is represented in Scheme II (where M=Cu).

The formation of the copper complexes were confirmed by spectral measurements. The optical absorption spectra of these polymer-bound copper complexes gave the characteristic etio type spectra for metalloporphyrins. The absorption spectra of Al (Cu) complexes show an intense soret band and two Q­bands (Table II).

For the free base Al compounds a split soret band was observed. This splitting was absent in their copper complexes. Metallated porphyrins cannot be protonated and as a result, the phenomenon of soret band splitting was not observed in Al (Cu) complexes. Absorption spectra of Al"rtho (eu), Almeta (Cu) and Alpara (Cu) were analysed and a systematic red shift was observed (Table II). The varying degree

MATH EW et at. : STUDIES ON POLYMER-APPENDED FREE BASE PORPHYRINS WITH ETHER LINKAGE 417

Al+M_ ~

OH

(AIM)

Scheme 11 - Prepration of A 1 (Cu) complexes

of steric participation in ortho-, meta- and para­isomers is responsible for this systematic spectral change.

The ortho, meta and para derivatives of AI' (Cu) complexes were also synthesised and analysed spectroscopically. The nature of polymer-porphyrin interaction was deducible from the spectral results . A systematic change was observed in the A.max values of Alort1w (Cu), AtmelO (Cu) and Alpara (Cu) complexes (4 18.5 , 420 and 421 cm- I for ortho, meta and para respective ly) which is a measure of the strain experienced by the 1t-electron framework . AI' (Cu) complexes showed strong red shift when compared to their At (Cu) counterparts. This is because of the higher degree of ste ric effect created in Al (Cu) complexes.

The e lectronic mod ifi cation brought about by the po lymer support can a lso be studi ed by ES R spectroscopy. Copper tetraphenylporphyrin in powder form , which is a magnet ically concentrated system is capable of prov iding only isotropic X-band ES R

Table II - Electronic spectral data of A I (Cu) complexes

Compd

A 1orrho

Al mdll

~"!!lI

Soret band (nm)

41 7

4 19

422

Q-bands (nm)

538, 582

540, 584

542, 584

spectra II. This is because, the neighbouring copper ions in this sample interact well leading to the dipolar broadening of the resonance lines. Two methods have already been suggested to minimise the dipolar interaction between the neighbouring copper ions. The magnetic dilution can be achieved either by using suitable solvent or by a diamagnetic solid solvent l2 In At (Cu) and AI' (Cu) complexes, magnetic dilution is attained by covalently attaching the porphyrin systems to an insoluble polymer support. The copper complexes of Al (Cu) and AI ' (Cu) gave well­defined ESR spectra even at room temperature in the solid state. The spin Hamiltonian parameters showed a systematic variation as we go from orlho to para (Tables III and IV).

Tables III and IV show that the greater steric effect on the ortho compound is ev idenced by the increase in a 2 value compared to meta and para. Compared to Al (Cu) compounds, a 2 values are higher for AI' (Cu) complexes. Again, it was fo und that on moving from Alnrthn (Cu) to Alpara (Cu) or AI'orlho (Cu) to AI'pora (Cu) the metal nitrogen bond strength increases .

Conclusion The macromolecular structural features of the

polymeric support material have been proved to contribute s ignificantly to the reactivity of the

Table 111 - Sp in Hamiltonian parameters of Al (Cu) complexes

Compd

A I "rlh" (Cu) A Imelu (C u)

A I,,(lru (Cu)

2. 1902

2.1865

2.1814

2.0245

2.0264

2.0249

AU Cu (G)

192.0

192.5

194.0

A.LCu

(G)

3 1.0

32.5

34 .0

AgN A.LN a 2 G (G) (G)

12.0 15 .5 0.768 1 7.76

12.5 16.0 0.7666 7.06

13.5 17.5 0.7650 7.28

418 INDIAN J CHEM, SEC B, JUNE 2000

Table IV - Spin Hamiltonian parameters of AI' (Cu) complexes

Compd gl gl A,Cu (G)

AI' orlho (Cu) 2.1844 20278 194.5

AI'mela(Cu) 2.1826 2.0281 195.0

AI 'C!!!ru {Cu2 2.1821 2.0258 195.5

attached functions. The nature of the interaction between the polymeric backbone and the functional species is either physical or chemical in nature. Electronic properties of porphyrins and the catalytic behaviour of metalloporphyrins are controlled by the sensitive 1t-electron framework of the porphyrin core unit. The polymer support of polymeric porphyrins is used to regulate the electron distribution and thereby impart specific characteristics to the system. By varying the position and nature of polymer-porphyrin linkage, any desired situation can be created in the porphyrin system. The synthesis and electronic studies of polymer-bound porphyrins with flexible ether linkage were presented here.

Acknowledgements The authors thank UGC, New Delhi, for the award

of JRF and SRF to TM.

Al.Cu (G) AIN (G) Al.N (G) 0.2 G

34.0 12.5 17.0 0.7706 6.63

36.0 13.0 17.5 0.7703 6.49

39.0 14.0 18.5 0.7701 7.05

References I Dolphin D (Ed.), The porphyrins, Vol. I-VII, (Academic

Press, New York), 1979. 2 Collman J P & Reed C A, JAm Chern Soc, 95, 1973,2048.

3 Meunier B, Chern Rev, 92, 1992, 1411. 4 Pratviel G, Bemadou J & Meunier B, Angew Chern Int Ed

Engl, 34, 1995, 746. 5 Sherrington D C & Hodge P, Synthesis and Separation using

Functional Polymers, (John Wiley, New York), 1988.

6 Hodge P & Sherrington D C, Polymer Supported Reactions in Organic Synthesis , (John Wiley, New York), 1980.

7 Haridasan V K, Ajayaghosh A & Pillai V N R, J Org Chern , 52, 1987, 2662.

8 Alder A D, Longo, F R, Kamps F & Kin J, J lnorg Nucl Chern, 32, 1970, 2443 .

9 Smiih K M (Ed.), The Porphyrins and Metal/oporphyrins, (E lsevier, Amsterdam), 1975.

10 Crowley J I & Rapoport H, Acc Chern Res, 9, 1976, 135. II Mathew T, Padmanabhan M & Kuriakose S, J Appl Polyrn

Sci, 59, 1996, 23. 12 Assour J M, J Chern Phys, 43 , 1965, 2477.