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Binding of some first-row transition metal ions by a poly(iminoethylene)dithiocarbamate copolymer
s277-S3s7~non, Pugmnoa Pmr
PolykdmnVol. 1. No. 3. pp. 225431.1982
Printed inGreatBritain.
Ltd.
BINDING OF SOME FIRST-ROW TRANSITION METAL IONS BY A POLY(IMINOETHYLENE)DITHIOCARBAMATE COPOLYMER PHILIP C. H. MITCHELL* and MARINA G. TAYLORt Department of Chemistry, The University, Whiteknights, Reading RG6 2AD, England (Receiued 31 My 1981) Abstra&-The biding of the transition metal ions VO’+, Fe*‘, Fe’+, Co*+, Co3’, Ni*’ and Cu*’ by a poly(iminoethylene)dithiocarbamate copolymer has been investigated by uptake studies and physical measurements (electronic, IR, and ESR spectra and magnetic susceptibility). Metal ions may be bound by both the diiocarbamato and amino groups of the co-polymer. Binding to nitrogen (in addition to bindii to sulphur) increases in the order Fe(D) < Ni(II) < Cu(II) and accounted for increasing metal ion uptake by the copolymer in the same order. Factors which determine the relative uptake of the metal ions by the copolymer are discussed. INTRODUCTION
copolymer (1) (abbreviated “copolymer” in the text and PIED in formulae) is a zwitterion with both nitrogen and sulphur sites. Poly(iioethylene)dithiocarbamate
-CH,CH,NH-CH,CHTNH+-CHZCH,NHI CH,CH,NHCS;.
(1)
The copolymer is amorphous and is insoluble in water and organic solvents. It is of practical use as a scavenger of metal ions from solution. In this paper we report a study of the reactions of the copolymer with aqueous solutions of the ions VO”, Fe”, Co”, Ni” and Cu2’ following our earlier work with molybdate? The object was to determine the uptake of metal ions, the binding sites and the stereochemistry of the copolymer complexes by comparisons with well characterised monomeric dithiocarbamato
complexes.>’
EXPERIMENTAL Materials. Poly(iminoethylene) was obtained from B.D.H. Ltd. as a 50% aqueous solution. Iron(D) chloride was purilied by heating under reflux with iron powder and hydrochloric acid.6 Other chemicals were analytical or reagent grades. Preparations. Poly(iminocthylene)dithiocclrbamate copolymer. Carbon disulphide (10.2g) in ethanol (50 cm3 was added to a stirred solution of poly(immoethylene) (30g) in ethanol (75 cm? at co. 20°C. The white precipitate, which formed immediately in quantitative yield, was l&e-d off, washed with ethanol and d&d in uacuo. (Found: C, 40.6; H, 8.1; N, 20.4; S, 24.1. Calc. for CwH&&.SsO~, i.e. C&N$2~ H20,0.25C2HsOH:C, 41.2; H, 8.1; N, 20.3; S, 23.1%). Me&copolymer complexes. The copolymer (I g, 3.6X 10m3mol dithiocarbamate), which is insoluble in water, was stirred with aq. solutions of the compounds VOS4.2H20, IWl2 * 2Hz0, CoCl2* 6H,O, NiC12.6H2O,CUSO~.5H20 (GIL7.5 X 10;’ mol) in water (100cm’) under air or nitrogen at 20°C for 0.5-l hr. The metal-copolymer complexes were filtered off, washed with water and dried in uacuo. Analyses are given in Table 1. Physical measurements. Electronic spectra were recorded in a Unicam SPMNI spectrophotometer. Diffuse-reflectance spectra of powdered solids were obtained relative to a magnesium oxide *Author to whom correspondence should be addressed. t&sent address: Department of Zoology, The University, Whiteknights, Reading RG6 2AD, England.
standard or the uncomplexed polymer on a Unicam SI7OOC spectrophotometer. IR spectra of compounds in KBr discs or Nuiol mulls were recorded on a P&in-Elmer 557 spectrophoton&r. ESR spectra were obtained on a Varian -E-3 s&ctrometer calibrate-dwith l,ldiphenyl-2-picrylhydraxyl (g 2.0037. Magnetic susceptibilities were measured on a Newport Instruments balance calibrated GouY [Ni(H2NCH2CH2NH3,]$&. Diamagnetic corrections z: taken from Selwood and determined by measurement for the copolymer. Analyses. Carbon, hydrogen and nitrogen were determined by the microanalytical service of this Department and sulphur by Butterworth Microanalytical Consultancy Ltd. Metals were determined by standard gravimetric procedures and by atomic absorption after decomposing the complexes with 1.1 nitricsulphuric acid. BJfSULTSAND DISCUSSION
The copolymer Structure of the copolymer. The dithiocarbamate copolymer (I) is preparedby CS, substitution at primary and secondary amino groups of poly(immoethylene). The latter has an average molar mass of 40,000g mol-‘. Chain branching occurs at nitrogen given a 1: 2: 1 ratio of primary: secondary: tertiary amino groups.’ The maximum number of amino groups which undergo sub stitution with CS, is ca. 3546,the actual number depending on the CSJpoly(iiinoethylene) ratio in the preparation. In the copolymer used in the present work ca., 25% of the amino groups were substituted. Thermal analysis showed substitution was predominantly at the primary amino sites and some water and ethanol was usually retained. UV and IR spectra of the copolymer. Peak positions and assignments are given in Table 2. The UV spectrum of the copolymer was similar to that of Et,NCS;Na” in agreement with the zwitterion formulation (I). In the IR spectrum, by analogy with simple dithiocarbamate R,NCS;, we assign a strong broad band centred at 1460cm-’ as v(NCS,,)+ S(CH3.‘“*‘1The strong band at 965 cm-’ is assigned as v&NC!) + v.,(C!S~).‘~For simple dithiocarbamates, R,NCS;, this band moves to lower wavenumbers as the munber and size of the R groups increase. For our copolymers, the shift of the band to lower wavenumbers was greatest in copolymers with more than 25% substitution of amino groups, i.e. when secondary as well as primary amino groups had reacted with CS2.” The presence of the band at 965 cm-’ is 225
PHILIP C. H. MITCHELL and MARINA G. TAYLOR
226
Table 1. Formulatioo of metal copolymer complexes@ Formulations
Analysis/wt.-$
c
C
II
N
M
VOSO4.1.24 PIED.3.5H20
25.1t25.3)
5.20(5.33)
12.7c13.1)
9.80(9.61)
FeC13.5.0 PIED.30H20
27.5(27.8)
7.00(8.24)
15.1C14.4)
2.80(2.88)
FeC12.3.0 PIED.3.0H20
30.6(30.8)
5.00(6.66)
15.6c16.0)
9.0(9.03)
CoC13.5.0 PIED.5.7H20
36.3(35.6)
7.00(7.39)
16.4(18.6)
3.90(3.90)
CoC13.4.6 PIlID.27H20
27.9(27.7)
6.90(8.15)
14.6(14.4)
3.30(3.29)
NiC12.1.67 PIED 3.0H20
30.3(30.2)
6.20(6.59)
15.6(15.6)
lO.O(g.86)
NiC12.1.55 PIED.3.9H20
26.8(28.7)
6.40(6.63)
14.3(14.8)
10.2(10.1)
CuS04.1.10 PIED.4.5H20
23.5(23.1)
4.70(6.04)
ll.S(l2.0)
12.6(12.4)
8 Prepared as outlined in Table 3. k
PIED is the ideal copolymer repeating unit, CgB20N4S2.
0
Found and, in parenthesis,
calculated.
Table 2. IR and UV spectra of the copolymer and related compounds. (a) IR spectra (ficm-‘)’ Copolymer
Assignment
v(OH)
3400 vs,b
v(NH)
3240 vs
u(CH)
2930 vs, 2830 "8
A(Nh)
1615
Et2NCS2-Na+.3H20
3300 vs.b
2915 "8, 2860 vs
mb,1505 sh
v(N-CS2) + 6(CH2)
1460 vs
1478 vs
u[(C)CNl
1215 w
1204 vs
v(CNC+CS2)
965 vs,b
5 In KSr discs. Peak maxima, relative intensities
988 vs,b
(~8, very strong;
8, strong; m, medium; w, weak; b, broad, sh, shoulder) (v, stretching; 6, bending). (b) U.V. spectra (?/lo' cm -l)
Phase
Copolymer
Et2NCS2-Na+.3H20
Solid a
44.5 37.6 33.4 28.4
45.0 37.0 33.4 28.4
aqueous solution
47.4 38.8 34.7
48.5 38.9 35.5
a Reflectance
relative to MgO.
221
Bindingof some M-row transitionmetal ions Table 3. Uptake of metal ions by the copolymer@ QJnd.itiam~
10n
Bormdmetal
Initialcalcentrati& 102[Mlo/mDlL-l
10~~1,bl(~
Fracticaal
PO~YDX)-~ 0wa9ge(e) a
"G2+
e
0.5h, air
8.272
3.09
0.81
*e2+
f
0.5h, air
7.603
0.675
0.177
0.25h, N2
6.753
2.10
0.551
0.5h, N2
7.634
0.732
0.192
0.5h, N2
7.616 B
0.782
0.205
l.Oh, air
7.603
2.22
0.583
l.Oh, N2
7.575
2.47
0.648
0.75h, air
7.605
3.40
0.892
Co2+
Ni2+
Cu2+
f
f
e
a Copolymer
(l.Og, 3.813 x 10-3 mol CS2- Groups, 14.6 x 10s3 mol N) stirred
with metal ion solution
(100 cm3) at ca. 2o"c.
h Contact time, whether under air or nitrogen. 5 In deionised water unless otherwise 4 [Ml,,/[Ll where
indicated.
[Ll is the molar concentration
of CS2- groups per g
polymer (see text). 5 As sulphate. f As chloride. B In hydrochloric
acid (0.4 mol 8-l)
consistent with the RNHCS; struchue formed by reaction of CS, at primary amine sites.”
Metal-copolymer complexes. Aqueous solutions of metal salts, at concentrations twice that of the dithiocarbamato groups in the copolymer, were stirred with the copolymer until reaction was complete (0.5-l hr). The fractional coverage (13)of dithiocarbamato binding sites by the metal ions is given in Table 3. For 1: 1 coordination of metal ions by dithiocarbamato groups 0 should be unity but this value was never attained. If bisand tristomplexes were formed 0 would take the value 0.5 and 0.33. With Fe(U) in air and with Co(H) in air and under nitrogen, 0 10.2 and the products consisted mainly of copolymer complexes of Fe(III) and Co@) which are therefore, as in the monomeric complexes, stabiised by dithiocarbamate. For the other metal ions, and the anaerobic reaction with FeO, 0.5 < B< 1 and 0 increased Fe” < Ni*+ < VO” < Cu’+. The capacity of the copolymer for metal ions depends therefore not only on the stoichiometry of the complexes formed but also on the accessibility of the ligating groups. Binding with the amino groups by individual metal ions is discussed below. For nickel the quantity taken up by the solid copolymer was determined as a function of the concentration of nickel in solution. The resulting adsorption
isotherm is shown in Fig. 1. The horizontal portion, which occurs when the surface is saturated with nickel, corresponds to a ratio of nickel to copolymer repeating unit of 0.65 : 1, i.e. 65% utilisation of binding groups. The inllexion in the isotherm shows that adsorption of the nickel is stepwise. Since, in general, Sdonor ligands bind Ni(II) more strongly then N-donor ligands we expect the initial, steeper part of the isotherm to represent Ni(II)dithiocarbamato binding and the second, shallower part, Ni(II)-N binding. If, as indicated by the horizontal portion of the isotherm, a fraction 0.65 of the potential binding sites is available, then the first inflexion corresponds to a Ni: dithiocarbamate ratio of 1:2. The presence of Ni bound to S and N is consistent with the physical measurements (see below). ZR spectra of the metal-copolymer complexes. Peak positions and assignments are given in Table 4. Coordination of dithiocarbamate causes v(N-Cs;) to move to higher wavenumber? owing to an increased contribution of the canonical form R,N’=CS:- with the precise value for each metal depending on the R group (e.g. R=Et, 1478; L-proline, 1458cm-‘).” In the IR spectrum of the copolymer a strong, broad band centred at MOcm-’ is assigned v(N-CSJ t S(CH2). In the metal-copolymer complexes the v(NCSd component moved to higher wavenumbers leaving,
PHILIP C. H. MITCHELL and MARINA G. TAYLOR
228
Table 4. Spectroscopic and magnetic properties of the metal-copolymer complexes” vo2+
Fe2+
Fe3+
co3+ b
Ni2+ a
Cu2+
1500
1495
1490
1480
1495
1495
v(CNC+CS2)
970
990
975-980
970
980
960
V(MS)
370
352
365-360
384
355
others
985 fi
Metal ion I.r.spectra(&n-l) 2 v(N-CS2)
1105 f
875 2
613 f
1105 f 613 f 460 g w-visible spectra ($/lo3cm-') h 28.5
25.0
25.0
25.4
23.0
24.0sh
23.0
19.0
19.0
20.6
21.0
16.0sh
10.6
17.0sh 17.0sh
15.6
15.8
12.0sh
10.0
7.0sh lO.Osh 5.0 Esr
7.OOsh
9.4sh
spectra (Jpalues) 2.008
4.250
2.041
1.993
2.186
2.010
2.899
2.140
1.870
2.111
2.069
1.982 Magnetic moments (u[uS) 1.39
5.40
5.19
0.87
1.86
2.26
*Complexes in Table I. b Identical data for the two cobalt and nickel complexes respectively of Table I. c In KBr discs, positions of gtructurallyesignificantfpeaks and assignments (cf. Table I ). - v(V=O). - V(VOV). - Sulphate. g v (V-O) (cf. 480 cm- I for VO(acetylacetonate) I and 463 cm-l for its pyridine adduct). a Reflectance spectrZ relative to MgO (sh. shoulder).
l=tI-Fii. 1. Isotherm of concentration change at 20°C for the binding of Ni” ions by a poly@minoethylene) dithiocarbamate copolymer: bound nickel [lO’x/mol copolymer)-‘] vs equilibrium concentration of Ni + ions [l Jmol I-‘] in water. The copolymer (0.2s) was equilibrated with aqueous Ni(II) chloride (20 cm?, the suspension filtered, and Ni determined in the tiltrate by EIYTAtitration.
if any dithiocarbamato groups remained free, a medium intensity band at 146Ocm-’ [v(NCS3] of free dithiocarbamate and a band at 1430cm-’ [S(CH,)]. Therefore, for the metal copolymer complexes assignments may be complicated by the presence of bound and free dithiocarbamato groups. For the copolymer v(N-CS2) (MOcm-‘) was 18cm-’ lower than for Et,NCS;Na’ (1478cm-‘) (see p. 3). For our metal-copolymer complexes we found additional strong bands at wavenumbers consistently co. 15cm-’ less than for the Et,NCS, complexes and so we assign these bands as v(NCS~) and they are evidence for the coordination of the dithiocarbamato groups of the copolymer. The dithiocarbamato group can be bidentate or, less commonly, monodentate. In the case of monodentate binding the 950-1050 cm-’ band [v..(CNC) + v.,(C!$)l is split by about 25-30cm-‘. Smaller splitting (<20cm-‘) or broadening of the band may be observed for bidentate, unsymmetrical dithiocarbamates.‘O For our copolymer complexes, the band was fairly broad but was split only in the Cu complex and then by less than 20cm-‘. Therefore binding of the dithiocarbamate is bidentate. Further evidence for metal coordination to sulphur is the weak to medium bands at 352-384cm-’ assigned u(M-S) by comparison with the low molecular weight complexes.
Bindingof some first-rowtransitionmetal ions
229
netic moment (5.40B.M.) was in the range expected for high spin Fe(II) with a small orbital contribution and greater than the moments observed for simple iron (II) dithiocarbamates {see [Fe(R,NCSJJ dimers, 4% tris-complex [Fe(Et2NCSJ2 4.2B.M., and the S2C2(CCF3)J, 2.25 B.M.}. Complex (II) was ESR inactive as expected for Fe(H). Both the simple iron (II) complexes and our iron (II) copolymer complex readily oxidise in air. The question arises as to the number of dithiocarbamato groups bound to iron in complexes (I) and (II). The spectroscopic properties of the copolymer complexes are similar but not identical with those of simple iron dithiocarbamato complexes and the magnetic moments are greater. The NCS, vibrations in the IR spectra gave broadbands which can be atrributed to the presence of both bound and free NCS2 groups. In complex (I) Fe(II1) probably has distorted octahedral geometry (see the ESR spectrum) and is ligated only by NC!% groups. In complex (II) the number of NCS; groups bound to Fe(II) is one or two and the coordination sphere is completed by Cl-, H20, or amino groups of the copolymer. Cobalt. Co(I1) chloride and the copolymer in the presence and absence of air gave green complexes which contained Co(II1). Co(III) is strongly stabilised by dithiocarbamate; reaction of Co(II) with Et*NCS; even in the absence of air is reported to give a Co(II1) complexa3The IR spectrum of our cocopolymer complex was similar to that of the Fe(III) complex indicating the presence of both bound and free dithiocarbamate groups.
Vanadium. With Et,NCS; the vanadium (IV) air sensitive complexes [VO(Et,NCS&l (grey)“b*‘4 and [V(Et,NCSJ4] (red-brown)” are known. However, our vanadium-copolymer complex was green with a V : PIED ratio (0.81) indicative of 1: 1 coordination. The electronic spectrum of the V-copolymer complex (Table 4) was different from that of [VO(EtzNCS&l (peaks at 17,200, 18,600, 23,500 and 27,4OOcm-‘) and the magnetic moment (p = 1.39) was in the range for VO*+ complexes having interactions between adjacent VO*’ groups (e.g. carboxylato and tridentate S&#‘s base complexes). MJ’ The ESR spectrum was complex (gay 1.99) and was not well enough resolved to establish whether there were fifteen lines, as would be expected for a complex with two interacting V(lV) atoms, or a monomer with very low symmetry. The IR spectrum showed the presence of some uncoordinated CS; groups [v(NCSJ, MOcm-‘I. Our data on the vanadium copolymer complex are, therefore, best interpreted by the structure 2 with dithiocarbamate bound to interacting vanadyl groups. Since v(V=O)is in the range observed for the monomeric VO*‘ complexes, the interaction does not involve terminal oxide. The stoichiometry (0.81: 1 PIED) indicates predominantly 1: 1 coordination of VO*+ and dithiocarbamate. The coordination sphere could be completed by Hz0 or OH- ligands, and in agreement with this proposal we can assign medium intensity IR bands (Table 4) to v(V-0) and u(VOV).‘* The relatively high uptake of vanadium by the copolymer is therefore to be attributed not to a high binding affinity but rather to the formation of a 1: 1 complex.
(21
II
Iron. A solution of Fe(I1) chloride with the copolymer gave a Fe(III) complex (I) in air and a Fe(II) complex (II) under nitrogen (Table 1). The Fe(III)-copolymer complex (I) was black; its electronic spectrum (Table 4) was similar to that of the tris-complexes,‘9 e.g. [Fe(Et2NCSd3] (absorption maxima at 7000, 17,100, 19,700,25,700 and 28,900cm-‘). These complexes, which have distorted prismatic structures,m show the phenomenon of spin cross-over;‘9 the “Z’& state lies below 6A,, with a separation of the order of thermal energies and at room temperature the magnetic moment is less than the spin only value for high spin Fe(m) (5.9B.M.). The magnetic moment of our Fe(M) copolymer complex (Table 4) was similarly less than the high spin value. The Fe(III) complex also gave an ESR spectrum at room temperature which compared with that of other dithiocarbamate Fe(III) complexes.*’ There were signals at g = 4.25, g = 2 and a five line signal at g = 1.98. The UV-visible spectrum of our Fe(II) complex was similar to that of complex (I) except for a shoulder at co. 10,000cm-’ which corresponds with a peak in the spectra of Fe(II) dithiocarbamato complexes. ‘The mag-
The electronic spectrum was similar to that of [Co(Et2NCS3J (peaks at 15,500; 20,600; 25,ooO and 27,000cm-‘)3*9 but there were additional shoulders at 9400 and 12,000cm-‘. The magnetic moment was greater than expected for diamagnetic low spin Co(III). Broad ESR signals (g - 2) observed at room temperature sharpened on cooling to 77 K. Such signals could arise from distorted octahedral Co(II). We conclude that the copolymer complexes contain mainly cobalt(III) bound to dithiocarbamate and some Co(II) in a distorted coordination environment. Nickel. Ni(II) chloride and the copolymer gave green complexes in the presence and absence of air. Their IR spectra showed the absence of free NCS; groups (no MOcm-’ band). The monomeric Ni(II) bis-(dithiocarbamates) are square planar= and, therefore, if Ni(II) is similarly coordinated in the Ni-copolymer complex (Ni : copolymer 0.63 : 1) some nickel must be bound to the amino groups. The electronic spectra of the monomeric Ni(II) dithiocarbamate and our copolymer complexes were similar. A peak at ca. 30,OOOcm-’has been assigned to charge transfer in monomeric Ni(II) dithiocarbamates.= A peak at lO,OOOcm- is characteristic of
230
PHILIP C. H. MITCHELL and MARINA G. TAYLOR
octahedral Ni(II) coordinated to amino groups. The copolymer complex was paramagnetic (1.86B.M.) and gave a broad ESR signal. From the observed magnetic moment (1.86B.M.) we calculate the ratio of planar yl= 0) to octahedral (typically p = 3.2 B.M.) species as : . Copper. Copper (II) sulphate and the copolymer gave a khaki complex with $e highest metal content of all the complexes prepared. The IR spectrum showed that not all the dithiocarbamate groups had reacted. The main feature of the electronic spectrum was a peak at 16,OOOcm-’ similar to a peak found for [Cu(Et,NCS&$39 Absorbance extended into the near IR as in copper (II) amino complexes. The magnetic moment was close to the spin only value for one unpaired electron showing that the Cu was present as Cu(II). The ESR spectrum was isotropic with a g value (2.089) slightly higher than found for the analogous monomeric complex (2.052).= The ESR spectrum was quite similar to that of [Cu(dien),(N0,)].26 Low molecular weight copper dithiocarbamato complexes occur in +l, t2, t3 and mixed oxidation states, and the complexes often have interacting copper atoms in dimeric or polymeric species.’ There was no evidence of reduction or magnetic interactions in the copolymer complexes. We conclude that copper (II) is bound to NCS; and also to amino groups of the copolymer rather like Ni(II). CONCLUSIONS Au the metals combined with the dithiocarbamato groups
of the copolymer as deduced from IR spectra, i.e. the shift in the v(NC$.) band and the metal sulphur vibrations. The electronic spectra, magnetic moments and ESR spectra, however, show that the structures were not always the same as found in the analogous low molecular weight complexes. Not all the dithiocarbamato groups of the copolymer were coordinated and the degree of reaction appears to depend on the number of dithiocarbamates bound in the low molecular weight species. Thus Co(III) and Fe(III) dithiocarbamates are coordinated to six sulphurs and so the degree of reaction depends on the accessibility of three dithiocarbamato groups for coordination whereas the requirements for VO’+, Fe”, N?’ and Cu*’ are two dithiocarbamates. The evidence for the binding of amino groups is limited. The electronic spectra of the nickel and copper complexes both have bands characteristic of metal amino complexes. This was supported in the case of copper by the ESR spectrum. The structure of the copolymer and the binding of metal ions The shape of the polyethylene molecule has been described as “elliptical’* similar to an American football.8 The rapid and extensive reaction of the amino groups with CS2 suggests that the reactive groups are on the outside of the polymer molecule and that the resulting dithiocarbamato groups are also on the outside. Consistent with this is our conclusion that ca. 80% of the dithiocarbamato groups are accessible to metal ions. The acid form of the copolymer prepared by us is insoluble in water but the copolymer dissolves readily in alkaline solutions. The insolubility of the polymer suggests an interaction between molecules (or particles) possibly through NH: and CS; groups [see cl)]. This interaction is destroyed by deprotonation in alkali and so the polymer dissolves. Transition metal complexes of the
copolymer were prepared in our work by reaction of an aqueous suspension of the acid form of the copolymer with solutions of metal salts. Copolymer complexes may also be prepared by reaction of aqueous solutions of the sodium salt of the polymer and metal ions. The complexes are insoluble and precipitate.z Therefore, the transition metal ions, like the proton, are effective in binding together polymer molecules or particles by forming bis and tris complexes with particles by binding dithiocarbamato groups and amino groups from diflerent polymer molecules or particles. Such interactions must also occur when the complexes are prepared by interaction with suspensions of the polymer. The ligating groups on the copolymer are charged and distributed over the polymer surface. The initial reaction is electrostatic, cations binding to the negative sulphur and anions to the positive nitrogen. Davydova% has proposed a mechanism for metal binding with polymers whereby initial coordination of the metal ion is to one ligand on the polymer. Further coordination of the metal will depend on the proximity of neighbouring groups and the flexibility of the copolymer to change conformation. This treatment has usually been confined to soluble polymers but we have shown that our insoluble copolymer behaves similarly. None of the copolymer complexes had properties identical with those of analogous low molecular weight compounds. Studies of binding of metal ions with proteins have shown that conformational changes in the protein occur following binding of the metal ion and the consequent redistribution of charge. Structural studies of metalloproteins have revealed that at the binding site the metal may have a distorted stereochemistry not found in complexes of simple ligands.29 Our work has shown that in reactions of metal ions with a copolymer complexes with properties and probably structures different from those of analogous complexes with simple ligands are formed.
Acknowledgements-MGT thanks the Science Research Council for a research studentship. We have refined our script following helpful comments from the referee. REFERENCES
‘J. H. Barnes and G. F. Esselmoat,Die Makromol Gem. 1976, 177,307; C. 0. Giwa and M. J. Hudson,to be wblished. *P. C. H. Mitchell and M. G. Taylor, i Less common Met& 1977.54.111: Proc. Climax 2nd Int. Conf. on the Chemistrv and Uses oj Molybdenum (Edited by P. e. H. Mitchell), d. 55. London (1976). ‘D. Coucouvanis, Prog. Inorg. Chem. 1970, 11, 234 and references therein. ‘3. Willemse, J. A. Gras, J. J. Steggerda and C. P. Keijzers, Structure and Bonding 1976,28,83 and references therein. ‘D. Coucouvanis, Prog. Inorg. Chem. 1979,26,301. 6D. D. Perrin, W. L. F. Armarego and D. R. Perrin, Purification of Laboratory Chemicals. PergamonPress, Oxford (1966). ‘P. W. Selwood, Magnetochemistry, 2nd Edn. Interscience, London (1956). sL. E. Davis in R. L. Davidson and M. Sitha (Editors), Watersoluble Resins. Reinhold, New York (1968). Q. G. Taylor, Ph.D. Thesis. University of Reading (1978). ‘9. A. Brown, W. K. Glass and M. A. Burke, Spectrochim. Acra 1976.32a. 137. ““S. %aj& and K. Drabent, Bull. Acad. Polon Sci. Ser. Sci. Chim. 1977,25, %3. bB. J. McCormick, Jnorg. Chem. 1968,7, 1965. ‘S. Vigoe and J. Selbin, J. Inorg. Nucl. Chem. 1969,31, 3187.
Binding of some first-row transition metal ions “K. A. Jensen, B. M. Dahl, P. H. Nielsen and G. Borch, Acta Chem. Scand. 1971.X 20.29: Ibid 1971.25.2038: 1972.26, $?imtt L. A. Duncanson and L. M. Venanzi, Nature 1956, m, 104i. “K. Hemick, C. L. Raston and A. H. White, J. Chem. Sot. (Dplton) 1976,26. “‘D. C. Bradley and M. H. Gitlitz, J. Chem. Sot. (A), 1%9,1152. ‘D. C. Bradley, I. P. Rendall and K. D. Sales, Ibid 1973,2228. “A. P. Ginsberg, E. Koubeck and H. J. Wiis, Inorg. Chem. 1966,5, 1656. “A. Syamal, Co-ord Chem. Reu. 1975,16,309. “A. Aaagnostopoulos, D. Nichols and M. E. Pettifer, 1. Chem. !3oc. IMton 1974,569. ‘“R. L. Martin and A. H. White, Trans. Met. Chem. 1968,4, 113. bS. A. Cotton, Coord Chem. Reu. 1972,8. ‘C. A. Tsipis, C. C. Hadjikostas and G. E. Manoussakis, Inorg. Chim. Ada 1977, 23, 163.
231
%. F. Hoskins and B. P. Kelly, I. Chem. Sot. (Gem. Comm.) 1968,1517. *‘C. Flick and E. Gelerinter, Chem. Phys. Lett. 1973,23,422. **L.F. Larkworthy, B. W. Fitzsimmons and R. R. Pate], .I. C/tern. Sec. (Chem. Comm.) 1973,902. uG. Peyronel and A. Pignedati, Acta Cryst. Sect. A. W66, 21, 156. 9. Oktare, B. Siles, J. Stefanec, E. Korgara and J. Garoj, Coil. Czech. Chem. Comm. 1980,4!t,791. *VT.R. Reddy and R. Srinivasan, J. Chem. Phys. 1%5,43,1404. a6B.J. Hathaway, M. J. Bew and D. E. Billing, 1. Chem. Sot. (A) 1970,1090. ?& Okawara and T. Nakai, Bull. Tokyo Inst. Technol. 1966,78, 1. 28S.L. Davydova and N. A. Plate, Co-ord. Chem. Rev. 1975, 16, 195. 29B. L. Vallee and R. J. P. Williams, Proc. Natl. Acad. Sci. U.S.A. I%& 59.4%.
PolykdmnVol. 1. No. 3. pp. 225431.1982
Printed inGreatBritain.
Ltd.
BINDING OF SOME FIRST-ROW TRANSITION METAL IONS BY A POLY(IMINOETHYLENE)DITHIOCARBAMATE COPOLYMER PHILIP C. H. MITCHELL* and MARINA G. TAYLORt Department of Chemistry, The University, Whiteknights, Reading RG6 2AD, England (Receiued 31 My 1981) Abstra&-The biding of the transition metal ions VO’+, Fe*‘, Fe’+, Co*+, Co3’, Ni*’ and Cu*’ by a poly(iminoethylene)dithiocarbamate copolymer has been investigated by uptake studies and physical measurements (electronic, IR, and ESR spectra and magnetic susceptibility). Metal ions may be bound by both the diiocarbamato and amino groups of the co-polymer. Binding to nitrogen (in addition to bindii to sulphur) increases in the order Fe(D) < Ni(II) < Cu(II) and accounted for increasing metal ion uptake by the copolymer in the same order. Factors which determine the relative uptake of the metal ions by the copolymer are discussed. INTRODUCTION
copolymer (1) (abbreviated “copolymer” in the text and PIED in formulae) is a zwitterion with both nitrogen and sulphur sites. Poly(iioethylene)dithiocarbamate
-CH,CH,NH-CH,CHTNH+-CHZCH,NHI CH,CH,NHCS;.
(1)
The copolymer is amorphous and is insoluble in water and organic solvents. It is of practical use as a scavenger of metal ions from solution. In this paper we report a study of the reactions of the copolymer with aqueous solutions of the ions VO”, Fe”, Co”, Ni” and Cu2’ following our earlier work with molybdate? The object was to determine the uptake of metal ions, the binding sites and the stereochemistry of the copolymer complexes by comparisons with well characterised monomeric dithiocarbamato
complexes.>’
EXPERIMENTAL Materials. Poly(iminoethylene) was obtained from B.D.H. Ltd. as a 50% aqueous solution. Iron(D) chloride was purilied by heating under reflux with iron powder and hydrochloric acid.6 Other chemicals were analytical or reagent grades. Preparations. Poly(iminocthylene)dithiocclrbamate copolymer. Carbon disulphide (10.2g) in ethanol (50 cm3 was added to a stirred solution of poly(immoethylene) (30g) in ethanol (75 cm? at co. 20°C. The white precipitate, which formed immediately in quantitative yield, was l&e-d off, washed with ethanol and d&d in uacuo. (Found: C, 40.6; H, 8.1; N, 20.4; S, 24.1. Calc. for CwH&&.SsO~, i.e. C&N$2~ H20,0.25C2HsOH:C, 41.2; H, 8.1; N, 20.3; S, 23.1%). Me&copolymer complexes. The copolymer (I g, 3.6X 10m3mol dithiocarbamate), which is insoluble in water, was stirred with aq. solutions of the compounds VOS4.2H20, IWl2 * 2Hz0, CoCl2* 6H,O, NiC12.6H2O,CUSO~.5H20 (GIL7.5 X 10;’ mol) in water (100cm’) under air or nitrogen at 20°C for 0.5-l hr. The metal-copolymer complexes were filtered off, washed with water and dried in uacuo. Analyses are given in Table 1. Physical measurements. Electronic spectra were recorded in a Unicam SPMNI spectrophotometer. Diffuse-reflectance spectra of powdered solids were obtained relative to a magnesium oxide *Author to whom correspondence should be addressed. t&sent address: Department of Zoology, The University, Whiteknights, Reading RG6 2AD, England.
standard or the uncomplexed polymer on a Unicam SI7OOC spectrophotometer. IR spectra of compounds in KBr discs or Nuiol mulls were recorded on a P&in-Elmer 557 spectrophoton&r. ESR spectra were obtained on a Varian -E-3 s&ctrometer calibrate-dwith l,ldiphenyl-2-picrylhydraxyl (g 2.0037. Magnetic susceptibilities were measured on a Newport Instruments balance calibrated GouY [Ni(H2NCH2CH2NH3,]$&. Diamagnetic corrections z: taken from Selwood and determined by measurement for the copolymer. Analyses. Carbon, hydrogen and nitrogen were determined by the microanalytical service of this Department and sulphur by Butterworth Microanalytical Consultancy Ltd. Metals were determined by standard gravimetric procedures and by atomic absorption after decomposing the complexes with 1.1 nitricsulphuric acid. BJfSULTSAND DISCUSSION
The copolymer Structure of the copolymer. The dithiocarbamate copolymer (I) is preparedby CS, substitution at primary and secondary amino groups of poly(immoethylene). The latter has an average molar mass of 40,000g mol-‘. Chain branching occurs at nitrogen given a 1: 2: 1 ratio of primary: secondary: tertiary amino groups.’ The maximum number of amino groups which undergo sub stitution with CS, is ca. 3546,the actual number depending on the CSJpoly(iiinoethylene) ratio in the preparation. In the copolymer used in the present work ca., 25% of the amino groups were substituted. Thermal analysis showed substitution was predominantly at the primary amino sites and some water and ethanol was usually retained. UV and IR spectra of the copolymer. Peak positions and assignments are given in Table 2. The UV spectrum of the copolymer was similar to that of Et,NCS;Na” in agreement with the zwitterion formulation (I). In the IR spectrum, by analogy with simple dithiocarbamate R,NCS;, we assign a strong broad band centred at 1460cm-’ as v(NCS,,)+ S(CH3.‘“*‘1The strong band at 965 cm-’ is assigned as v&NC!) + v.,(C!S~).‘~For simple dithiocarbamates, R,NCS;, this band moves to lower wavenumbers as the munber and size of the R groups increase. For our copolymers, the shift of the band to lower wavenumbers was greatest in copolymers with more than 25% substitution of amino groups, i.e. when secondary as well as primary amino groups had reacted with CS2.” The presence of the band at 965 cm-’ is 225
PHILIP C. H. MITCHELL and MARINA G. TAYLOR
226
Table 1. Formulatioo of metal copolymer complexes@ Formulations
Analysis/wt.-$
c
C
II
N
M
VOSO4.1.24 PIED.3.5H20
25.1t25.3)
5.20(5.33)
12.7c13.1)
9.80(9.61)
FeC13.5.0 PIED.30H20
27.5(27.8)
7.00(8.24)
15.1C14.4)
2.80(2.88)
FeC12.3.0 PIED.3.0H20
30.6(30.8)
5.00(6.66)
15.6c16.0)
9.0(9.03)
CoC13.5.0 PIED.5.7H20
36.3(35.6)
7.00(7.39)
16.4(18.6)
3.90(3.90)
CoC13.4.6 PIlID.27H20
27.9(27.7)
6.90(8.15)
14.6(14.4)
3.30(3.29)
NiC12.1.67 PIED 3.0H20
30.3(30.2)
6.20(6.59)
15.6(15.6)
lO.O(g.86)
NiC12.1.55 PIED.3.9H20
26.8(28.7)
6.40(6.63)
14.3(14.8)
10.2(10.1)
CuS04.1.10 PIED.4.5H20
23.5(23.1)
4.70(6.04)
ll.S(l2.0)
12.6(12.4)
8 Prepared as outlined in Table 3. k
PIED is the ideal copolymer repeating unit, CgB20N4S2.
0
Found and, in parenthesis,
calculated.
Table 2. IR and UV spectra of the copolymer and related compounds. (a) IR spectra (ficm-‘)’ Copolymer
Assignment
v(OH)
3400 vs,b
v(NH)
3240 vs
u(CH)
2930 vs, 2830 "8
A(Nh)
1615
Et2NCS2-Na+.3H20
3300 vs.b
2915 "8, 2860 vs
mb,1505 sh
v(N-CS2) + 6(CH2)
1460 vs
1478 vs
u[(C)CNl
1215 w
1204 vs
v(CNC+CS2)
965 vs,b
5 In KSr discs. Peak maxima, relative intensities
988 vs,b
(~8, very strong;
8, strong; m, medium; w, weak; b, broad, sh, shoulder) (v, stretching; 6, bending). (b) U.V. spectra (?/lo' cm -l)
Phase
Copolymer
Et2NCS2-Na+.3H20
Solid a
44.5 37.6 33.4 28.4
45.0 37.0 33.4 28.4
aqueous solution
47.4 38.8 34.7
48.5 38.9 35.5
a Reflectance
relative to MgO.
221
Bindingof some M-row transitionmetal ions Table 3. Uptake of metal ions by the copolymer@ QJnd.itiam~
10n
Bormdmetal
Initialcalcentrati& 102[Mlo/mDlL-l
10~~1,bl(~
Fracticaal
PO~YDX)-~ 0wa9ge(e) a
"G2+
e
0.5h, air
8.272
3.09
0.81
*e2+
f
0.5h, air
7.603
0.675
0.177
0.25h, N2
6.753
2.10
0.551
0.5h, N2
7.634
0.732
0.192
0.5h, N2
7.616 B
0.782
0.205
l.Oh, air
7.603
2.22
0.583
l.Oh, N2
7.575
2.47
0.648
0.75h, air
7.605
3.40
0.892
Co2+
Ni2+
Cu2+
f
f
e
a Copolymer
(l.Og, 3.813 x 10-3 mol CS2- Groups, 14.6 x 10s3 mol N) stirred
with metal ion solution
(100 cm3) at ca. 2o"c.
h Contact time, whether under air or nitrogen. 5 In deionised water unless otherwise 4 [Ml,,/[Ll where
indicated.
[Ll is the molar concentration
of CS2- groups per g
polymer (see text). 5 As sulphate. f As chloride. B In hydrochloric
acid (0.4 mol 8-l)
consistent with the RNHCS; struchue formed by reaction of CS, at primary amine sites.”
Metal-copolymer complexes. Aqueous solutions of metal salts, at concentrations twice that of the dithiocarbamato groups in the copolymer, were stirred with the copolymer until reaction was complete (0.5-l hr). The fractional coverage (13)of dithiocarbamato binding sites by the metal ions is given in Table 3. For 1: 1 coordination of metal ions by dithiocarbamato groups 0 should be unity but this value was never attained. If bisand tristomplexes were formed 0 would take the value 0.5 and 0.33. With Fe(U) in air and with Co(H) in air and under nitrogen, 0 10.2 and the products consisted mainly of copolymer complexes of Fe(III) and Co@) which are therefore, as in the monomeric complexes, stabiised by dithiocarbamate. For the other metal ions, and the anaerobic reaction with FeO, 0.5 < B< 1 and 0 increased Fe” < Ni*+ < VO” < Cu’+. The capacity of the copolymer for metal ions depends therefore not only on the stoichiometry of the complexes formed but also on the accessibility of the ligating groups. Binding with the amino groups by individual metal ions is discussed below. For nickel the quantity taken up by the solid copolymer was determined as a function of the concentration of nickel in solution. The resulting adsorption
isotherm is shown in Fig. 1. The horizontal portion, which occurs when the surface is saturated with nickel, corresponds to a ratio of nickel to copolymer repeating unit of 0.65 : 1, i.e. 65% utilisation of binding groups. The inllexion in the isotherm shows that adsorption of the nickel is stepwise. Since, in general, Sdonor ligands bind Ni(II) more strongly then N-donor ligands we expect the initial, steeper part of the isotherm to represent Ni(II)dithiocarbamato binding and the second, shallower part, Ni(II)-N binding. If, as indicated by the horizontal portion of the isotherm, a fraction 0.65 of the potential binding sites is available, then the first inflexion corresponds to a Ni: dithiocarbamate ratio of 1:2. The presence of Ni bound to S and N is consistent with the physical measurements (see below). ZR spectra of the metal-copolymer complexes. Peak positions and assignments are given in Table 4. Coordination of dithiocarbamate causes v(N-Cs;) to move to higher wavenumber? owing to an increased contribution of the canonical form R,N’=CS:- with the precise value for each metal depending on the R group (e.g. R=Et, 1478; L-proline, 1458cm-‘).” In the IR spectrum of the copolymer a strong, broad band centred at MOcm-’ is assigned v(N-CSJ t S(CH2). In the metal-copolymer complexes the v(NCSd component moved to higher wavenumbers leaving,
PHILIP C. H. MITCHELL and MARINA G. TAYLOR
228
Table 4. Spectroscopic and magnetic properties of the metal-copolymer complexes” vo2+
Fe2+
Fe3+
co3+ b
Ni2+ a
Cu2+
1500
1495
1490
1480
1495
1495
v(CNC+CS2)
970
990
975-980
970
980
960
V(MS)
370
352
365-360
384
355
others
985 fi
Metal ion I.r.spectra(&n-l) 2 v(N-CS2)
1105 f
875 2
613 f
1105 f 613 f 460 g w-visible spectra ($/lo3cm-') h 28.5
25.0
25.0
25.4
23.0
24.0sh
23.0
19.0
19.0
20.6
21.0
16.0sh
10.6
17.0sh 17.0sh
15.6
15.8
12.0sh
10.0
7.0sh lO.Osh 5.0 Esr
7.OOsh
9.4sh
spectra (Jpalues) 2.008
4.250
2.041
1.993
2.186
2.010
2.899
2.140
1.870
2.111
2.069
1.982 Magnetic moments (u[uS) 1.39
5.40
5.19
0.87
1.86
2.26
*Complexes in Table I. b Identical data for the two cobalt and nickel complexes respectively of Table I. c In KBr discs, positions of gtructurallyesignificantfpeaks and assignments (cf. Table I ). - v(V=O). - V(VOV). - Sulphate. g v (V-O) (cf. 480 cm- I for VO(acetylacetonate) I and 463 cm-l for its pyridine adduct). a Reflectance spectrZ relative to MgO (sh. shoulder).
l=tI-Fii. 1. Isotherm of concentration change at 20°C for the binding of Ni” ions by a poly@minoethylene) dithiocarbamate copolymer: bound nickel [lO’x/mol copolymer)-‘] vs equilibrium concentration of Ni + ions [l Jmol I-‘] in water. The copolymer (0.2s) was equilibrated with aqueous Ni(II) chloride (20 cm?, the suspension filtered, and Ni determined in the tiltrate by EIYTAtitration.
if any dithiocarbamato groups remained free, a medium intensity band at 146Ocm-’ [v(NCS3] of free dithiocarbamate and a band at 1430cm-’ [S(CH,)]. Therefore, for the metal copolymer complexes assignments may be complicated by the presence of bound and free dithiocarbamato groups. For the copolymer v(N-CS2) (MOcm-‘) was 18cm-’ lower than for Et,NCS;Na’ (1478cm-‘) (see p. 3). For our metal-copolymer complexes we found additional strong bands at wavenumbers consistently co. 15cm-’ less than for the Et,NCS, complexes and so we assign these bands as v(NCS~) and they are evidence for the coordination of the dithiocarbamato groups of the copolymer. The dithiocarbamato group can be bidentate or, less commonly, monodentate. In the case of monodentate binding the 950-1050 cm-’ band [v..(CNC) + v.,(C!$)l is split by about 25-30cm-‘. Smaller splitting (<20cm-‘) or broadening of the band may be observed for bidentate, unsymmetrical dithiocarbamates.‘O For our copolymer complexes, the band was fairly broad but was split only in the Cu complex and then by less than 20cm-‘. Therefore binding of the dithiocarbamate is bidentate. Further evidence for metal coordination to sulphur is the weak to medium bands at 352-384cm-’ assigned u(M-S) by comparison with the low molecular weight complexes.
Bindingof some first-rowtransitionmetal ions
229
netic moment (5.40B.M.) was in the range expected for high spin Fe(II) with a small orbital contribution and greater than the moments observed for simple iron (II) dithiocarbamates {see [Fe(R,NCSJJ dimers, 4% tris-complex [Fe(Et2NCSJ2 4.2B.M., and the S2C2(CCF3)J, 2.25 B.M.}. Complex (II) was ESR inactive as expected for Fe(H). Both the simple iron (II) complexes and our iron (II) copolymer complex readily oxidise in air. The question arises as to the number of dithiocarbamato groups bound to iron in complexes (I) and (II). The spectroscopic properties of the copolymer complexes are similar but not identical with those of simple iron dithiocarbamato complexes and the magnetic moments are greater. The NCS, vibrations in the IR spectra gave broadbands which can be atrributed to the presence of both bound and free NCS2 groups. In complex (I) Fe(II1) probably has distorted octahedral geometry (see the ESR spectrum) and is ligated only by NC!% groups. In complex (II) the number of NCS; groups bound to Fe(II) is one or two and the coordination sphere is completed by Cl-, H20, or amino groups of the copolymer. Cobalt. Co(I1) chloride and the copolymer in the presence and absence of air gave green complexes which contained Co(II1). Co(III) is strongly stabilised by dithiocarbamate; reaction of Co(II) with Et*NCS; even in the absence of air is reported to give a Co(II1) complexa3The IR spectrum of our cocopolymer complex was similar to that of the Fe(III) complex indicating the presence of both bound and free dithiocarbamate groups.
Vanadium. With Et,NCS; the vanadium (IV) air sensitive complexes [VO(Et,NCS&l (grey)“b*‘4 and [V(Et,NCSJ4] (red-brown)” are known. However, our vanadium-copolymer complex was green with a V : PIED ratio (0.81) indicative of 1: 1 coordination. The electronic spectrum of the V-copolymer complex (Table 4) was different from that of [VO(EtzNCS&l (peaks at 17,200, 18,600, 23,500 and 27,4OOcm-‘) and the magnetic moment (p = 1.39) was in the range for VO*+ complexes having interactions between adjacent VO*’ groups (e.g. carboxylato and tridentate S&#‘s base complexes). MJ’ The ESR spectrum was complex (gay 1.99) and was not well enough resolved to establish whether there were fifteen lines, as would be expected for a complex with two interacting V(lV) atoms, or a monomer with very low symmetry. The IR spectrum showed the presence of some uncoordinated CS; groups [v(NCSJ, MOcm-‘I. Our data on the vanadium copolymer complex are, therefore, best interpreted by the structure 2 with dithiocarbamate bound to interacting vanadyl groups. Since v(V=O)is in the range observed for the monomeric VO*‘ complexes, the interaction does not involve terminal oxide. The stoichiometry (0.81: 1 PIED) indicates predominantly 1: 1 coordination of VO*+ and dithiocarbamate. The coordination sphere could be completed by Hz0 or OH- ligands, and in agreement with this proposal we can assign medium intensity IR bands (Table 4) to v(V-0) and u(VOV).‘* The relatively high uptake of vanadium by the copolymer is therefore to be attributed not to a high binding affinity but rather to the formation of a 1: 1 complex.
(21
II
Iron. A solution of Fe(I1) chloride with the copolymer gave a Fe(III) complex (I) in air and a Fe(II) complex (II) under nitrogen (Table 1). The Fe(III)-copolymer complex (I) was black; its electronic spectrum (Table 4) was similar to that of the tris-complexes,‘9 e.g. [Fe(Et2NCSd3] (absorption maxima at 7000, 17,100, 19,700,25,700 and 28,900cm-‘). These complexes, which have distorted prismatic structures,m show the phenomenon of spin cross-over;‘9 the “Z’& state lies below 6A,, with a separation of the order of thermal energies and at room temperature the magnetic moment is less than the spin only value for high spin Fe(m) (5.9B.M.). The magnetic moment of our Fe(M) copolymer complex (Table 4) was similarly less than the high spin value. The Fe(III) complex also gave an ESR spectrum at room temperature which compared with that of other dithiocarbamate Fe(III) complexes.*’ There were signals at g = 4.25, g = 2 and a five line signal at g = 1.98. The UV-visible spectrum of our Fe(II) complex was similar to that of complex (I) except for a shoulder at co. 10,000cm-’ which corresponds with a peak in the spectra of Fe(II) dithiocarbamato complexes. ‘The mag-
The electronic spectrum was similar to that of [Co(Et2NCS3J (peaks at 15,500; 20,600; 25,ooO and 27,000cm-‘)3*9 but there were additional shoulders at 9400 and 12,000cm-‘. The magnetic moment was greater than expected for diamagnetic low spin Co(III). Broad ESR signals (g - 2) observed at room temperature sharpened on cooling to 77 K. Such signals could arise from distorted octahedral Co(II). We conclude that the copolymer complexes contain mainly cobalt(III) bound to dithiocarbamate and some Co(II) in a distorted coordination environment. Nickel. Ni(II) chloride and the copolymer gave green complexes in the presence and absence of air. Their IR spectra showed the absence of free NCS; groups (no MOcm-’ band). The monomeric Ni(II) bis-(dithiocarbamates) are square planar= and, therefore, if Ni(II) is similarly coordinated in the Ni-copolymer complex (Ni : copolymer 0.63 : 1) some nickel must be bound to the amino groups. The electronic spectra of the monomeric Ni(II) dithiocarbamate and our copolymer complexes were similar. A peak at ca. 30,OOOcm-’has been assigned to charge transfer in monomeric Ni(II) dithiocarbamates.= A peak at lO,OOOcm- is characteristic of
230
PHILIP C. H. MITCHELL and MARINA G. TAYLOR
octahedral Ni(II) coordinated to amino groups. The copolymer complex was paramagnetic (1.86B.M.) and gave a broad ESR signal. From the observed magnetic moment (1.86B.M.) we calculate the ratio of planar yl= 0) to octahedral (typically p = 3.2 B.M.) species as : . Copper. Copper (II) sulphate and the copolymer gave a khaki complex with $e highest metal content of all the complexes prepared. The IR spectrum showed that not all the dithiocarbamate groups had reacted. The main feature of the electronic spectrum was a peak at 16,OOOcm-’ similar to a peak found for [Cu(Et,NCS&$39 Absorbance extended into the near IR as in copper (II) amino complexes. The magnetic moment was close to the spin only value for one unpaired electron showing that the Cu was present as Cu(II). The ESR spectrum was isotropic with a g value (2.089) slightly higher than found for the analogous monomeric complex (2.052).= The ESR spectrum was quite similar to that of [Cu(dien),(N0,)].26 Low molecular weight copper dithiocarbamato complexes occur in +l, t2, t3 and mixed oxidation states, and the complexes often have interacting copper atoms in dimeric or polymeric species.’ There was no evidence of reduction or magnetic interactions in the copolymer complexes. We conclude that copper (II) is bound to NCS; and also to amino groups of the copolymer rather like Ni(II). CONCLUSIONS Au the metals combined with the dithiocarbamato groups
of the copolymer as deduced from IR spectra, i.e. the shift in the v(NC$.) band and the metal sulphur vibrations. The electronic spectra, magnetic moments and ESR spectra, however, show that the structures were not always the same as found in the analogous low molecular weight complexes. Not all the dithiocarbamato groups of the copolymer were coordinated and the degree of reaction appears to depend on the number of dithiocarbamates bound in the low molecular weight species. Thus Co(III) and Fe(III) dithiocarbamates are coordinated to six sulphurs and so the degree of reaction depends on the accessibility of three dithiocarbamato groups for coordination whereas the requirements for VO’+, Fe”, N?’ and Cu*’ are two dithiocarbamates. The evidence for the binding of amino groups is limited. The electronic spectra of the nickel and copper complexes both have bands characteristic of metal amino complexes. This was supported in the case of copper by the ESR spectrum. The structure of the copolymer and the binding of metal ions The shape of the polyethylene molecule has been described as “elliptical’* similar to an American football.8 The rapid and extensive reaction of the amino groups with CS2 suggests that the reactive groups are on the outside of the polymer molecule and that the resulting dithiocarbamato groups are also on the outside. Consistent with this is our conclusion that ca. 80% of the dithiocarbamato groups are accessible to metal ions. The acid form of the copolymer prepared by us is insoluble in water but the copolymer dissolves readily in alkaline solutions. The insolubility of the polymer suggests an interaction between molecules (or particles) possibly through NH: and CS; groups [see cl)]. This interaction is destroyed by deprotonation in alkali and so the polymer dissolves. Transition metal complexes of the
copolymer were prepared in our work by reaction of an aqueous suspension of the acid form of the copolymer with solutions of metal salts. Copolymer complexes may also be prepared by reaction of aqueous solutions of the sodium salt of the polymer and metal ions. The complexes are insoluble and precipitate.z Therefore, the transition metal ions, like the proton, are effective in binding together polymer molecules or particles by forming bis and tris complexes with particles by binding dithiocarbamato groups and amino groups from diflerent polymer molecules or particles. Such interactions must also occur when the complexes are prepared by interaction with suspensions of the polymer. The ligating groups on the copolymer are charged and distributed over the polymer surface. The initial reaction is electrostatic, cations binding to the negative sulphur and anions to the positive nitrogen. Davydova% has proposed a mechanism for metal binding with polymers whereby initial coordination of the metal ion is to one ligand on the polymer. Further coordination of the metal will depend on the proximity of neighbouring groups and the flexibility of the copolymer to change conformation. This treatment has usually been confined to soluble polymers but we have shown that our insoluble copolymer behaves similarly. None of the copolymer complexes had properties identical with those of analogous low molecular weight compounds. Studies of binding of metal ions with proteins have shown that conformational changes in the protein occur following binding of the metal ion and the consequent redistribution of charge. Structural studies of metalloproteins have revealed that at the binding site the metal may have a distorted stereochemistry not found in complexes of simple ligands.29 Our work has shown that in reactions of metal ions with a copolymer complexes with properties and probably structures different from those of analogous complexes with simple ligands are formed.
Acknowledgements-MGT thanks the Science Research Council for a research studentship. We have refined our script following helpful comments from the referee. REFERENCES
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