Coordination Compounds of the Polystyrene-Anchored Schiff Base Obtained From the Condensation of Chloromethylated Polystyrene, 3-Formylsalicylic Acid and o-Hydroxyphenylurea

INTRODUCTION

The functionalized polymers and their coordination compounds are known to exhibit the chelating abilities[331], catalytic activity[332,333] selectivity, efficiency, operational flexibility and stability[334,335]. A perusal of the literature indicates that a number of polymer-anchored ligands containing oxygen atom(s) have been reported[162,336], however, there is no report on the coordination compounds of polymer-anchored ligands containing urea moiety. In view of their pronounced coordinating properties, we have synthesized and characterized the polystyrene-anchored Schiff base, PSCH2-LH3 (19) and its coordination compounds with Cu(II), Cd(II), Mn(II), Fe(III) and MoO2(VI) ions. It is expected that the present compounds may find use in some of the fields mentioned above.

The various chemicals and solvents obtained from the sources mentioned in Appendix-2 were used as received for the syntheses.

The estimation of metal, DMF contents, spectral (IR, reflectance, ESR) studies and the magnetic susceptibility measurements were carried out by the methods described in Appendix-3. Synthesis of 19 Polystyrene 3-formylsalicylate (1.0 g) was swollen in DMF (50 mL) for 45 min. To this suspension, o-hydroxyphenylurea (0.711 g, 4.68 mmol) and ethyl acetate (100 mL) were added, while stirring magnetically. The mixture was refluxed for 8 h and the solid product obtained was suction filtered, washed with DMF, ethyl acetate, MeOH and dried as mentioned elsewhere. Yield = 55%. IR bands and ν(C––O)(aldehyde moiety) (1515 cm-1).

1.0 g of 19 was allowed to suspend and swell in DMF (100 mL) for 1 h. A DMF solution of appropriate metal salt/compound (2.34 mmol) was added to the above suspension. The mixture was refluxed on a water bath for 8-10 h and the products obtained were suction filtered, washed several times with ethyl acetate and DMF. The products were then dried as mentioned above. Yield = 55-68%.

Polystyrene-3-formylsalicylate reacts with o-hydroxyphenylurea in DMF in 1:4 molar ratio and forms the polystyrene-anchored Schiff base, PSCH2-LH3 (19) (Scheme 7). The latter reacts with appropriate metal salt/compound and forms the corresponding polystyrene- anchored coordination compounds (Scheme 8). The per cent reaction conversion (PRC) of the compounds lies between 44.8-81.3 (Table 4.1) and the metal binding capacity (MBC) of 19 between 0.32-0.60 mmol of metal per g of the latter. 19 and its coordination compounds (20-24) are insoluble in water and in common organic solvents. DMF has been chosen as the solvent due to its high dielectric constant and more effective in swelling the resin. DMF is lost completely by heating 20, 21, 22 and 23 at 118, 183, 154 and 135 oC respectively in air-oven.

3-Formylsalicylic acid exhibits the ν(C==O)(carboxylic) stretch[9] at 1660 cm-1. The appearance of a new band at 1725 cm-1 due to the ν(C==O)(ester) stretch in polystyrene-3- with PSCH2––Cl also suggests the covalent bond formation involving –OH group of carboxylic moiety and ––CH2Cl group of chloromethylated polystyrene moiety[188,338]. The band at 1250 cm-1 in PSCH2––Cl due to the ν(C––Cl) stretch, disappears in polystyrene-3-formylsalicylate, which further supports the covalent bond formation[339]. The resulting polystyrene-anchored Schiff base (19) shows prominent bands at 1735, 1725, 1630, 1528 and 1515 cm-1 due to the ν(C==O)(carbonyl), ν(C==O)(ester), ν(C==N)(azomethine), ν(C–– O)(amino moiety) and ν(C––O)(aldehyde moiety) stretches respectively. The band due to the ν(C–– O) (aldehyde moiety) undergoes a positive shift by ≤10 cm-1 in 20-24 which indicates the involvement of phenolic O atom of the aldehydic moiety[9] towards coordination. The data rule out the adoption of the bimetallic structure in 20-24. The ν(C––O)(amino moiety) stretch remains almost unaltered in 19 and 20-24 indicating the non-involvement of phenolic O atom of amino moiety towards coordination. The shifting of the ν(C==N)(azomethine) stretch towards lower energy by 10-25 cm-1 in 20-24 suggests the participation of azomethine N towards coordination[9]. The ν(C==O)(ester) stretch remains at the same energy in 20-24 indicating the non-participation of the ester O atom(s) on coordination. The ν(C==O)(carbonyl) stretch occurring at 1735 cm-1 also remains unaltered in 20-24 ruling out the possible enolisation and coordination of O atom upon coordination. Although 19 is potentially a pentadentate ligand, but it acts as a monobasic bidentate ON donor ligand coordinating through its phenolic O and azomethine N atoms. The presence of the new non-ligand bands between 521-550 cm-1 and 430-470 cm-1 due to the ν(M––O) and ν(M––N) stretches support the coordination through O and N atoms of 19 respectively[296]. The νas(OAc) and νs(OAc) stretches of free acetate ions occur at 1560 and 1416 cm-1 respectively[148]. The appearance of the bands in 20-22 between 1587-1595 cm-1 and 1355-1362 cm-1 due to the νas(OAc) and νs(OAc) stretches respectively indicates the presence of the coordinated acetato group in these compounds. The energy separation (225-240 cm-1) between νas(OAc) and νs(OAc) stretches is >144 cm-1 which indicates the monodentate nature of the acetato ligand. DMF shows a band at 1680 cm-1 due to the ν(C==O) stretch[327]. This band shifts to lower energy by 12-27 cm-1 in 20-23 and this indicates the involvement of O atom of DMF towards coordination[327]. The presence of bands at 935 and 905 cm-1 due to the νs(O==Mo==O) and νas(O==Mo==O) stretches respectively in 24 suggests the presence of a cis-MoO2 configuration in it[262]. Acetylacetone is coordinated as a monobasic bidentate OO donor ligand in 24 as evident by the presence of two new bands one at

Reflectance spectral studies

The X-band ESR spectrum of 20 has been recorded in DMSO at 77K using DPPH as marker. The spectrum shows usual anisotropic pattern with two g values, which are characteristic of axial symmetry. The spin-Hamiltonian parameters are: A|| = 1.70 × 10-2 cm-1, A┴ = 3.2 × 10-3 cm-1, g|| = 2.24, g = 2.08, gav = 2.1, G = 3.15, Pd = 1.76 × 10-2 cm-1, κ = 0.48, g||/A|| = 132, α2 = 0.79. The pattern g||  g > 2 suggests a tetragonal distortion around Cu(II) ion corresponding to the elongation along four-fold symmetry z axis with dx2-y2 as the ground state[340]. The value of g|| is less than 2.3 suggesting the covalent character of 19. The value of G (3.15) is less than 4, which is indicative of strong field nature of 19 in 20[340]. The value of covalent parameter, 2 (in-plane  bonding) is 0.79 which suggests the considerable covalent bonding[341]. The lower value of Pd (1.76 × 10-2 cm-1) in comparison to the free ion value (3.5 × 10-2) indicates the presence of covalent character between metal-ligand bonding. The absence of a band at ~1500 gauss due to the ∆Ms = 2 transition in the present case precludes the presence of Cu––Cu interaction. This results a magnetically dilute environment around the metal atom as the pathway for dimer formation is blocked.

The room temperature magnetic susceptibilities and magnetic moments of 20, 22 and 23 are presented in the Table 4.2. The magnetic moment of 20 is 1.87 B.M. which is in the expected range reported for magnetically dilute Cu(II) compounds[112]. The magnetic moment of 22 is 5.84 B.M., expected for the high-spin, magnetically dilute Mn(II) compounds[107]. The magnetic moment of 23 is 5.82 B.M. which is closer to the spin-only value reported

The polystyrene anchored Schiff base, PSCH2––LH3 (19) behaves as a monobasic bidentate ON donor ligand in magnetically dilute 1:1 coordination compounds (20-24). The coordination numbers of respective metal in 20, 21, 22, 23 and 24 are 4, 4, 6, 6 and 6 respectively. A square-planar structure to 20, a tetrahedral structure to 21 and the octahedral structures to 22-24 are proposed. 22 and 23 are paramagnetic, while others are diamagnetic. A tetragonal distortion around Cu(II) ion in 20 corresponds to the elongation along four fold symmetry z-axis with dx2-y2 as the ground state.

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