Influence of alkyl chain length in N-alkyl imidazoles upon the complex formation with transition-metal salts

J. inorg, nut1 Chem., 1978, Vol. 40, pp. 143-147. Pergamon Press. Printed in Great Britain

Bio-Inorganic Section INFLUENCE OF ALKYL CHAIN LENGTH IN N-ALKYL IMIDAZOLES UPON THE COMPLEX FORMATION WITH TRANSITION-METAL SALTS J. A. WELLEMAN, F. B. HULSBERGEN, J. VERBIEST and J. REEDIJK Department of Chemistry, Delft University of Technology, Delft 2208, The Netherlands (Received I 1 January 1977; received for publication 2 February, 1977) Abstract--P reparation and properties of a large number of compounds with the general formula M(NRIz), (anion)2 are described; in this formula M = Mn, Co, Ni, Cu, Zn, Cd; n = 1, 2, 3, 4 and 6; NRIz stands for N-alkyl imidazole ligands (alkyl = Me, Et, Pr, Bu); the anions are C104-, BF4-, NOr, Br and CV. The compounds are characterized with the aid of far-lR ligand-field and ESR spectra and X-ray powder diagrams. The stoichiometry of the compounds depends upon the particular cation and anion, but also on the length of the alkyl substituent. This effect appears more pronounced in the m.ps of classes of similar compounds. Generally the m.ps follow three lines: (a) cations: the Irving-Williams sequence is found; (b) the anions occur in a sequence according to the ionic size; the larger the anion, the higher the m.p.; (c) the ligands produce a sequence which follows the ligand size, i.e. the larger the substituent, the lower the m.p. Similar sequences are found in ligand-field and ESR spectra. Only in a few cases deviations from these lines occur in the sequence of stoichiometry, m.ps and spectral data. This may be due to different structures, probably caused by steric effects of the ligands. INTRODUCTION

EXPERIMENTAL

Complex formation between imidazole groups and metal ions plays an important role in both molecular biology and technology[I-5]. Previous studies have shown that several complexes exist between metal ions and imidazoles[6-12]. It was found that substituents placed in the imidazole ring can have both a steric and an electronic effect upon the complex formation[9-12]. With substituents placed directly besides the donornitrogen, steric interactions seldomly allow more than four ligands to be coordinated to a transition-metal ion. Substituents placed at N(I) or C(5), on the other hand, appeared to have mainly electronic influences[9, 10].

Starting materials. N-ethyl and N-n-propyl imidazole were obtained from BASF Nederland BV, Arnhem and were used without purification. The hydrated metal(lI) tetrafluoroborates were prepared from the metal(II) carbonates and hydrogen tetrafiuoroborate. The other hydrated metal(It) salts were commercially available. Preparation of the compounds. All compounds were prepared by basicly the same procedure. An amount of metal(II) salt (usually 0.01 mole) was dissolved in ethanol, upon which sufficient triethylorthoformate was added for dehydration[18.19]. The appropriate amount of ligand was dissolved in ethanol upon which the above metal salt solution was added. Crystals of the compounds separated on standing, either in the cold, or after addition of dry diethyl ether. The crystals were washed with sodium-dried diethyl ether, and finally dried in vacuo (1 mm Hg). A few compounds with N-methyl and N-n-butyl imidazole as ligand were prepared, which were not reported earlier. Analyses. Metal(It) determinations of the complexes were carried out by standard complexometric titrations. Of some compounds also halogen analyses were carried out. In addition carbon, hydrogen and nitrogen analyses of some compounds were determined by the Organisch Chemisch Instituut TNO Utrecht. Physical measurements. IR spectra were recorded as Nujol mulls between sodium chloride plates on a Infrascan Hilger and Watts spectrometer in the 4000-650cm ' region. For a few compounds far-lR spectra were taken on a Beckman 1R-720 interferometer (400-100cm-r), as pressed discs within polythene; ligand-field spectra were obtained using a Beckman DK-2 ratio recording spectrometer fitted with a reflectance attachment (35,000-4000cm '). X-ray powder diagrams of the compounds were performed on a Guinier type powder camera using CuK~ radiation. The samples were protected from atmosphere by adhesive tape. ESR powder spectra were recorded on a Varian E3 instrument at X-band frequencies.

alkyl

C \Nf~%N \~

41

C~-C

The use of N-alkyl imidazoles as corrosion inhibitors and as axial ligands in hemoglobine model compounds[5,13-15] rather than unsubstituted imidazole, has raised the question of a possible steric and/or electronic influence of N-alkyl groups in the imidazole complexation with transition-metal ions. Therefore we decided to start a detailed study on the system metal salts/N-alkyl imidazole for alkyl=methyl, ethyl, npropyl and n-butyl. As metal salts the perchlorates, tetrafluoroborates, nitrates, chlorides and bromides of Mn, Co, Ni, Cu, Zn and Cd(II) were selected, in order to cover a broad range of ionic radii, preferential coordination numbers and different degrees of anion coordination tendency. Several N-methyl and N-n-butyl imidazole complexes were reported[9, 16] previously by us. During the preparation of the manuscript, a few complexes of N-ethyl and N-propyl imidazole similar to ours were reported in this journal [17]. Therefore we will restrict ourselves only to the new compounds and to a general discussion of the steric effects upon stoichiometry, m.ps, ligand-field spectra, far-IR spectra and ESR spectra.

RESULTS AND DISCUSSION

General A listing of all newly prepared compounds, with their colours, m.ps and ligand-field spectra is available upon request. All compounds had satisfactory analyses. During the course of this investigation several of these 143

J. A. WELLEMAN et al.

144

compounds were also reported by Massacesi et a/.[17]. In our study a few compounds were found, that were not found by Massacesi[17]. This may be due to the use of the superior dehydrating agent in our case. In Table 1 a listing is made of stoichiometries, X-ray types and m.ps for all compounds found thus far in the system metal(II) salt/alkyl imidazoles. Compounds reported for the first time are underlined in this Table. Most compounds appeared to be quite stable in the atmosphere and only a few appeared to be hygroscopic. During attempts to prepare tetrafluoroborate compounds with a metal: ligand ratio less than six, we found that Co(II), Mn(II) and Cd(II) in combination with N-ethyl imidazole and Co(II) in combination with N-n-propyl imidazole yielded products, for which chemical analysis indicated a stoichiometry (ML3F)BF4. In a separate paper the characterization and proposed structure of these compounds will be described[20]. Considering the stoichiometries in Table 1 of the new compounds and those published earlier[9, 10, 16, 17] it is seen that the molar metal:ligand ratio's 1:1; 1:2; 1:3; 1:4; and 1:6 occur. For several metal(II) salts more than one complex exists, depending upon the used molar metal:ligand ratio. No complex of Mn(BF4)2 in combination with six N-n-propyl imidazole molecules could be obtained in our case. The division into X,ray types was made according to very similar Guinier photographs with regard to line spacings and intensities. It was seen that all compounds which have similar X-ray patterns, have also nearly identical IR-spectra. This has been observed frequently in the literature for many other ligands [11] and strongly suggest the same coordination for the metal ions within one group having the same stoichiometry. Compounds without the indication A to GG were

found to be neither X-ray powder isomorphous with any of the other compounds in Table 1, nor with those reported in the literature[9, 10, 16,171. For a few compounds the line patterns are not exactly similar; they are indicated by ' in Table 1. Considering the isomorphisms in Table 1, it is seen that for the hexakis solvates the corresponding perchlorates and tetrafluoroborates are not isomorphous for all metals. Although isomorphism between these two classes have been found frequently, sometimes non-isomorphism has been found, e.g. for 5-methyl pyrazole, for N-ethyl pyridone [21, 22]. The origin of this difference in our compounds may be a crystal packing effect, due to a small difference in size between the anions. It is seen that the perchlorates, in combination with Melz and Etlz, have a higher m.p. than the corresponding tetrafluoroborates; this is known for other ligands[23, 24]. For the perchlorates in combination with Prlz and Bulz the m.p. are generally of the same magnitude as the corresponding tetralluoroborates. Regarding the isomorphisms in Table 1, it is remarked that all perchlorate solvates of Cd and Mn, with the same ligand, are isomorphous. The same holds for Co and Ni and the tetrat]uoroborate series. Considering the m.ps of the compounds of different metal(II) salts with the same ligand and stoichiometry, it is seen that the Irving-Williams sequence Cd < Mn < Co < Ni is followed in many cases, even when a class of compounds is not completely isomorphous. With respect to the m.ps of a given metal salt combined with a different ligand but having the same number of iigands, it is remarked that the m.p. is lowered significantly by increasing the number of carbon atoms of the aikyl substituent in the ligand in almost all cases. Some further

Table 1. Comparison of stoichiometries, m.ps (°C) and X-ray powder isomorphism of a selected number of N-alkyl imidazole compounds. Compounds prepared for the first time are underlined Metal-ion number of ligands 2 anion ligand CIO4 Melz Etlz Prlz Bulz BF,

NO3

Br

CI

Cd

Mn 6

2

Co 6

2

Ni 6

4

Cu 6

A-263 FF-160 D-101 F-145

A-281 FF-180 D-120 F-152

A-258¶ 145, GG-173 E-140 G-138

A-293¶ GG-214 E-174 G-150

Melz Etlz Prlz Bulz

A'-245 139 1-114 K-150

A'-241 1-114 K-148

A'230¶ H-162 J-144 L-150

A'-280¶ H-198 J-178 L-152

Melz Etlz Prlz Bulz

M-131~ N-I18~ P-116§ 79§ S-131

M-136§ N-f17§ P-9.__II§ R-109 S-105

11.__.O0 O-135 7..ft.0 R-126 S-132

Melz Etlz Prlz Bulz

T-181 110 113

Melz Etlz Prlz Bulz

T-220 208 98 CC-105§ CC-105§ U-125, DD-122 100 92, Z-115 AA-124 67§ 62* 77 EE-77¶

169§

5..~3 105

2

B-257 148 165 113

U-184 W-170 165 AA-84 EE-124

2

4

A-177t C-172 169 17__.6.6 I1__~5

B-236¶ A'-157t C-141 15..88 83 144 138 182 117 85 129

_8~ 83 66. 64

169 172 Y-163 125 134 123 118

190 120 106 83

O-147 118 R-137 ._~ S-137

169, 190 155 202 U-146, V-113 W-150 V-127t W-201 119, Z-93 164 AA-213 94* 80 BB-126¶ BB-172

Zn 4

128

159 138 209 117 Y-128 DD-122 130 108 Z-I14 98 68 83

Number of ligandsfor t = 6; * = 4; § = 3;11= 2; ¶ = meltingwith decomposition.Melz = N-methylimidazole;Etlz = N-ethylimidazole; Prlz = N-n-propyl imidazole;Bulz = N-n-butyl imidazole.In addition new compounds of formula CdCI2L(L = Etlz, Prlz), NiCl2(Etlz), and Co(Etlz)~Cl2 have been isolated.

145

Influence of alkyl chain length in N-alkyl imidazoles comments with respect to Table 1 concern the stoichiometries of the compounds M(L),X2. For M = Co, Ni in combination with X = C104, BF4, NO~, Br and CI, hexakis solvates could be prepared in almost all cases. The same holds for M = Mn, Cd and X = 004, BF4. Only a few compounds M(L)6(NO3)2 with M = Cd, Mn exist; for these two metals the compounds M(L)3(NO3)2 are preferably formed. This may be due to the coordination tendency of the nitrate anion. The tendency of the nitrate anion to coordinate with the metal ion is also seen in the compounds M(L)2(NO3)2 with M = Co, Ni, Cu and Zn. This tendency even prevents the formation of Zn(L)4(NO3)2 compounds. For M= Cu and X = CIO4, BF4, NO3, Br and CI, all expected compounds M(L)4X2 could be prepared. The deviations in stoichiometries for the remaining compounds listed in Table i may be caused by the chain length of the substituent in the N-alkyl imidazoles, as well as the coordination tendency of the anions and crystal packing effects. Vibrational spectra IR spectra of all compounds were recorded to obtain information about the coordinated ligands and about the possible presence of water. It was also possible to obtain information about the coordination of the anions. None of the compounds showed the presence of vibrations other than those assigned to the ligands and the anions, indicating the absence of water and solvent molecules. Regarding the anion vibrations of BF4-, C104- and NO3it is remarked that for all compounds with six ligands, as expected from stoichiometry, the anions appeared to be not associated with the metal ion. Coordinated BF4 and C104- anions are easily detected by split v3 and v4 absorptions and the observation of the forbidden ~'t and v, bands[25]. However, we observed rather sharp s,3 and ~'4 bands and ~,1 and v2 bands are absent. The Zn(II) perchlorate compounds with four ligands contain ClO4- ions, that are not coordinated to the Zn(II), as concluded from the unsplit Cl-O stretching (~'3) and CI-O bending (v4) frequency. The same holds for the Cu(II) tetrafluoroborate and perchlorate compounds with four ligands. Coordinated NO3- anions can be detected by split v3 absorptions and a "forbidden" v~ band in the IR. For the nitrates ML6(NO3).~ we observed a single ~'3band whereas ~,~was not observed. In all other compounds the nitrate ions appear to be coordinated to the metal ions, although the mode of coordination, i.e. monodentate or bidentate, cannot be deduced from the available spectral data. For the Cu(ll) compounds the v~ bands are very weak. This indicates at best a weak coordination of the anion to the Cu(II) cation[26, 27]. Most bands due to ligand vibrations are hardly shifted with respect to the corresponding free ligand values in the 4000-650 cm ~ region. Since it is known that the ring deformation of MeIz and Bulz[9, 16] at about 910 cm-~ is shifted to higher wave numbers upon coordination, we have investigated this shift in some detail. As a class of compounds with similar stoichiometry occurs with perchlorates and tetrafluoroborates, we have listed some results with these anions in Table 2. From this Table it is seen that the metal-ion dependence is similar for all four ligands and goes parallel with the well-known IrvingWilliams sequence of metal ions. The alkyl substituents in the imidazole group appear to have no influence upon the shift to higher wave numbers. This indicates that the shift to the ring vibration is exclusively dependent upon JINC Yol. 40. No. ]--J

Table 2. Comparisonof metal-iondependenceof a characteristic ring vibration of the N-alkyl imidazoles Metal ion

Numberof ligands

MeIz

Ligand]' Etlz Prlz

Bulz

Cd Mn Co Ni Cu Zn

Freeligand value 6 6 6 6 4 4

910 932 934 937 939 956 %0

908 932 935¢ 937 939 947 958

908 932 934 939 940 953 95%

908 933 933 939 939 952 959~

]'Bands are in cm-' and accurate to 2cm-'. :~Forthese compounds the C104 anion was taken because the compounds with BF,- as anion are not known. the used metal ion and the coordination geometry. Similar results are obtained when other anions are compared; observed deviations are within experimental error. Additional structural information could be obtained from far-IR spectra (400-100cm-1). The ligands have only a few weak absorptions in this region, so all strong absorptions bands, that appear in the complexes have to be assigned to M-N and M-anion vibrations. The assignment of metal-ligand modes into metal-nitrogen and metal-anion stretchings is made according to the literature [28] and previous work [11, 12, 29]. For the compounds from Table 1 with general formula M(L)6X2 only one strong absorption band in the 160265 cm -I region was found. Because of the fact that this region is the M-N stretching region[30] and the results of the ligand-field spectra for the coloured compounds indicate octahedral geometry (see below) we assign these absorptions to the metal-ligand stretches. It is found that again the Irving-Williams sequence for the metal ions is followed. The influence of the different anions upon the value for the M-N stretching is beyond experimental error. Differences due to the N-alkyl substituents in these compounds, are also of the order of the experimental error. A listing of the observed spectra of the M(L)2X2 compounds is made in Table 3 together with some tentative assignments. The assignments in these metal halogen compounds are based upon findings with the hexakis solvates and agree with the picture obtained from ligandfield spectra for the Co(II) compounds (see below) and X-ray isomorphisms. The spectral data of the compounds M(L)2C12 in Table 3 agree with metal(II) ions tetrahedrally coordinated by two chloride ions and two ligands [16, 28]. The u(M-Br) bands for the M(L)2Br2 compounds are not as easily identified as the M-C1 bands, because of the fact that they occur in the same region as the metal-nitrogen stretchings. Therefore the u(M-Br) bands in Table 3 must possess u(M-N) character. In case of M = Zn only one single band assignable to u(Zn-N) and v(Zn-Br) is observed. On going from N-methyl imidazole to N-n-butyl imidazole, we see only a small difference in frequency for the metal-ligand stretchings, indicating that the M-N vibrations are mainly metal-nitrogen in character. The alkyl substituent in the imidazole group appears to have hardly any influence upon the metal-nitrogen vibrations; this is known for several other classes of ligands, such as sulfoxides and N-oxides [31,32].

J. A. WELLEMAN et al.

146

Table 3. Far-IR spectra (400-100cm-~) of N-alkyl imidazole compounds of formula: M(L)2X2 (M=Co, Zn; L = Melz, Etlz, Prlz, Bulz; X = CI, Br) with a few assignments Compound Co(Melz)2Cl2 Co(Etlz)2C12 Co(Prlz)~Cl2 Co(Bulz)zCl2 Zn(Melz)2Cl~ Zn(Etlz)2Cl Zn(Prlz)2Cl2 Zn(Bulz)2C12 (2o(Melz)2Br2 Co(Etlz)2Br2 Co(Prlz)2Br2 Co(Bulz)2Br2 Zn(Melz)Br2 Zn(Etlz)2Br Zn(Prlz)2Br2 Zu(Bulz)2Br2

Ligand'~,~t vibration

M-X stretch

370m, 2 2 4 w 380w 380w 372ms 368w, 218m

328vs,299vs 271s 322s, 302s 263s 324s, 302s 267m 328s,303s 266s 308vs, 288vs 239ms 305s, 294s 242m 303s,291ms 244m 310vs,292vs 239s 265vs,253s 276ms 264 ms, 248ms 2 7 2 s h 258s, 247sh 271ms 252s 271s 239s 235s 236s 239s

372m 372mw,227wsh 365mw 371m,228sh

M-N stretch

Otherbands 244w, 187w, 157row, 142s, 108ms. 192w, 171w, 129row. 288sh, 220wbr, 132sh, 11lm. 188w, 165mw, 133mbr, 103s. 326wsh,186row, 160m, 144ms, l12m. 226m, 198w, 174mw, 133mw 321wsh,212wbr, l13w 198m, 174w, 132ms, 100s 185br, 159w, 142mw 218wsh,196m 183row 177w 189m, 160m, 138m 191m 194m 205w, 180w

%, strong; m, medium; w, weak; br, broad; sh, shoulder; v, very. ~Free ligand values: Melz: 353s, 222s; Etlz: 335vbr,w; Prlz: 378w, 368w; Bulz: 255s, 326w, 220vbr,vw.

Ligand-lield and ESR spectra To obtain information concerning the coordination around the metal ions and to determine spectrochemical parameters, ligand-field spectra were recorded of the coloured compounds and ESR spectra were taken of the Cu(II) and Mn(II) compounds. The spectral parameters for the new compounds containing the octahedral ions ML6z÷ are in agreement with those reported for the corresponding compounds with N-methyl imidazole and N-n-butyl imidazole as ligands[9, 10,16] within experimental error. No clear distinction can be made between the ligand-field strength of the present ligands, all with respect to the same anion, indicating that an electronic influence of the substituent is hardly felt by the metal-ion. The absorption bands for the Co(II) compounds with less than six ligands and the Ni(Etlz)zBr2 complex could be assigned to the usual transitions found in pseudo-tetrahedral configurations. The compounds Ni(Etlz)4(NO3(: and Ni(Prlz)4Cl2 have a ligand-field spectrum in agreement with a distorted octahedral geometry. However, the distortion in the compounds is in such a way that the calculation of Dq and B has no significance. The spectral properties of the Ni(Etlz),C12 complex are very similar to those of their imidazole[7] and N-methyl imidazole[10] analogs and indicate, a chloro-bridged polymeric compound. To find out any possible relation between ESR parameters, ligand-field maxima, m.ps and the nature of the anions for the compounds Cu(L)~X2, Table 4 has been set up. The reflectance spectra of the Cu(L)~X2 compounds each show a broad asymmetrical band with its maximum in the 15,900-18,900cm -1 region. Such spectra are commonly observed for distorted octahedral Cu(II) and have been found for many compounds with nitrogen-donor ligands and weakly coordinated anions [27, 33]. The electronic band energies and shapes varied with X = NO3, Br and Cl, suggesting that these anions are weakly coordinated to the Cu(lI) ion, in agreement with the results of the vibrational spectra. From Table 4 it is further seen that, with respect to the same anion, the ligand-field maximum increases from N-methyl imidazole to N-n-butyl imidazole. Only the chlorides deriviate from this trend.

Table 4. ESR data, ligand-fleldmaxima and m.p. of compounds Cu(L)4X2 with L = N-methyl imidazole, N-ethyl imidazole, Npropyl imidazole,N-butyl imidazole;X = CIO,, BF4, NO3, Br and CI ligand anion

004

BF,

NO3

Br

CI

Melzt ESR data ~ g'~:~ 2.24 2.25 d II I.As§ 185 190 UV-vis 18.0 18.0 17.2 maximum in kK m.p. in °C 257 236 182

2.28 2.28 175 175 16.7 17.2

{~% 2.24 d It ESR data 180 UV-vis 18.2 18.4 maximum in kK m.p. in *C 148 158

2.30 2.28 180 166 16.8 16.2

Etlz

Prlz

Bulz

d II 17.5 117

169

163

138

128

{,~ 2.25 2.25 2.27 2.28 2.29 ESR data 189 189 177 165 144 UV-vis 18.4 18.5 17.7 17.2 15.9 maximum in kK m.p. in *C 165 144 85 134 108 {~% 2.24 2.25 2.28 2.27 2.29 ESR data 185 184 177 164 162 UV-vis 18.9 18.9 18.2 17.6 16.4 maximum in kK m.p. in °C 113 138 129 118 68

tLigands are abbreviated as in Table 1. $g values are accurate to 0.01. §A values are accurate to 5 G. ~Unresolved,see text. The compounds marked with "d" in Table 4 have an ESR spectrum of one single asymmetric band, characteristic for large line widths with respect to the value of the hyperfine coupling constant A and the difference between ge and &. The only parameter that can be calculated from this spectra is g .... measured at the point with a derivative value of zero. The ESR spectra of the other Cu(L)oXz compounds of Table 4 are similar to each other, although some band shapes are slightly different. These compounds show a typical two g-value spectrum:

Influence of alkyl chain length in N-alkyl imidazotes g~ is even split into four hyperfine components with A~ varying from 190-160 G. Such spectra usually occur with large ligands[34] or anions, that increase the Cu-Cu distances and decrease the line widths. The observed g~ and A,e values agree with tetragonal Cu(II) in an environment of nitrogen-donor ligands [33, 35]. For the perchlorates and tetrafluoroborates the g,~ values are hardly different, indicating that the kind of distortion must be the same in these compounds. Also the nitrates, bromides and chlorides have similar parameters. The larger differences between the A~ values in these compounds must be due to the varying overlap of the Cu orbitals and the ligand orbitals in the axial direction[31]. The different g,e values for the compounds with X = CIO4, BF, and X = NO~, Br, C1 respectively, demonstrate the influence of the axial-ligand on g / a n d confirm the results of the ligand-field and vibrational spectra. The ESR spectra of the compounds Mn(L),Br2 (L = Melz, Etlz and Prlz); Mn(Etlz)4Cl2 and Mn(Etlzh(NO3), are similar to the spectra found for Mn(5-methyl pyrazole)4X2 with respectively X = Br, CI and NO3. For these compounds a distorted-octahedral symmetry is suggested[36]. No attempts were undertaken to determine the zero-field splitting parameters. Comparing the parameters listed in Table 4 for the compounds Cu(LhX2 with the same ligand, but having different anions, learns that no clear connection between these parameters occurs. Generally it can be said that on going from C104 via BF4, NO3 and Br to C1, the value of g/increases while the values of A~, the UV-vis maxima and m.ps decrease. It is surprising that in the whole series only a few exceptions occur in this sequence. Concluding remarks (1) The N-alkyl imidazoles appear to be quite strongly coordinating monodentate ligands, with a position similar to unsubstituted imidazole in both the spectrochemical and nephelauxetic series. (2) The effect of the alkyl chain length in N-alkyl imidazoles appears most pronounced in the m.ps of classes of similar compounds: the larger the substituent, the lower the m.p. (3) The length of the alkyl group at the imidazole ring appears to have hardly any influence upon the maximum coordination number of the metal and the number of coordinating ligands. (4) The main difference between unsubstituted imidazole and N-alkyl substituted imidazoles resulting in different properties of the complexes (see introduction), must originate from the hydrogen-bonding possibilities and acid properties of the hydrogen atom placed at the N(I) atom of the imidazole molecule. Acknowledgements--The authors are indebted to Mr. C. F. Vermeulen for assistance with the metal and halogen analysis. The assistence of Mr. 'tHart with the far-IR spectra and of Mr. N. M. v. d. d. Pers with the X-ray powder diffraction measurements is gratefully acknowledged. Thanks are also due to BASF Nederland B.V., Arnhem for providing the N-alkyl imidazoles.

i47

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