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Ligating ability of 4-methyl imidazole: Complexes with cobalt(II) and nickel(II)
t broth,. ~u(i. ('hem. 1977, Vol 39, pp. 2167-2t71 Pergamon Press
Printed in Great Britain
LIGATING ABILITY OF 4-METHYL IMIDAZOLE: COMPLEXES WITH COBALT(II) AND NICKEL(II) KAILASH C. DASH* and PANCHANAN PUJARI+ Department of Chemistry, Utkal University, Vani Vihar. Bhubaneswar-751004, India IReceived 29 October 1976; received for publication 4 May 1977)
Abstract--The ligand, 4-methyl imidazo!elL), forms a wide variety of new coordination complexes with cobalt(ll) and nickel(ll) ions. The complexes obtained by the direct reaction of metal salts, containing anions ranging from small and strongly coordinating to those usually believed to be noncoordinating, and the ligand, are of the type [ML6]X2, [ML6]X(CIO4), [NiL6][HgCI4], [NiL4(NCS)2], [NiL4(NCS)(CIO4)]. [NiL2(NCS)2] having 6-cuordinated octahedral or distorted octahedral structures, and [NiLCL] and [Me4N][NiLCL] with a tetrahedral structure. The [ML6](NCS)2 complexes contain ionic thiocyanate group, the [NiL4(NCS)2] and [NiL4(NCS)(CIO4)] contain N-bonded iso-thiocyanate groups, whereas the [NiL2(NCS)2]has bridging thiocyanate groups. Except for nickel(lt) thiocyanate, attempts to prepare bis- or tetrakis-complexes by direct reaction resulted only in an oily mass and no stable compound could be prepared. The stoichiometry and stereochemistry of the complexes have been deduced on the basis of analytical data, electronic and [R spectra, molar conductance and room temperature magnetic susceptibility measurements. Thermal decomposition of the parent hexakis complexes [ML6]+~ at 130"C gave tetrahedral Co(ll) complexes. e.g. [CoL4]X2 (X = CIO4, BF4) and [CoL2X2] (X = CI, Br, I, NCS) and tetragonally distorted trans- octahedral complexes, [NiL4X,] (X = CI, Br, I).
INTRODUCTION
4- or 5- substituted imidazoles containing labile imiaohydrogen are tautomeric systems and react as a tautomeric mixture of the two possible forms[16]. However, when 4(5) substituted imidazole is coordinated to a metal ion, one form of the ligand may be preferred and it ]has been assumed for steric reasons that the molecule will coordinate with the substituent in the 5-position117]. As a part of our investigations[9-12] of imidazole derivatives we report here the complexes formed between 4-methyl imidazole and cobalt(II) and nickel(II) ions. The ligand forms octahedral complexes of the type [ML6]X2 (M = Co(IlL Ni(II)), where X represents a large number of anions ranging from the strongly coordinating halides and pseudohalides to noncoordinating anions like BF4 and CIO4 . Attempts to prepare the bis or tetrakis complexes by direct metathetic reaction of tigand and metal salt were unsuccessful, except in the case of Ni(II) thiocyanate. However, the thermal decomposition of the parent hexakis complexes resulted in the formation of some tetrahedral cobalt(II) complexes of the type [CoL4]X 2 (X = C[O 4, BF4) and [CoL2X2} (X = (71, Br, l, NCS) and tetragonally distorted octahedral nickel(II) compounds with the composition [NiI,nX2] ( X - CI, Br, I). It is seen that 4(5)-methyl imidazole forms hexakis complexes similar to the unsubstituted imidazole[1] and I-substituted imidazole [18, 19].
Imidazoles are believed to be versatile ligands, because the imidazole nitrogen of histidine residues provides one of the primary means by which metal ions may be bound to proteins, lmidazole complexes are becoming increasingly important as analytical, biochemical and anticancer reagents. The coordinating ability of imidazole and its derivatives have been investigated by various workers[l-12] and it is found that the neutral imidazole coordinates via the unsaturated nitrogen atom rather than the imino nitrogen of the heterocyclic ring[13-15]. In complexes involving neutral monodentate imidazole ligands, the metal ion is nearly coplanar with the imidazole ring. Examples in which imidazole acts as rr-type ligand are unknown. On introducing a methyl group into the 5-membered ring, the coordinating power of the ligand increases due to increase in basicity. Thus, 1-methyl imidazole forms stable hexakis complexes like the unsubstituted imidazole. Although 2-methyl and 1,2-dimethyl imidazole are good coordinating ligands, they only form tetrakis, tris and bis complexes. In no case was a hexakis complex obtained, probably due to the steric effect of the methyl group adjacent to the donor site. Hence it is of interest to study the effect of a methyl group at position-4, which is also adjacent to the donor nitrogen atom. 3
H30+ ~-
HC~.~NH 3 [,,,,(~N. ~
X
EXPERIMENTAL
H
5
I
H 3 C ~ - - NH + H20
R~
'
4 ["-,.~N5 2 + H~O+ 3
*Author for correspondence I-B. J. B. College, Bhubaneswar.
Starting materials. 4-methyl imidazole was used as supplied by BASF (West Germany). Hydrated cobalt(ll) and nickel(II) chloride, nitrate, perchlorate, sulphate, formate and acetate (BDH) were used as such without further purification. CoBr2 and NiBr2 were prepared by the reaction of respective carbonates and HBr. Metal iodide, thiocyanate, tetrafluoroborate and azides were prepared by metathetic reactions of hydrated metallic nitrate with KI, KNCS, NaBF4 or NaN 3 respectively. Solutions of MX(CIQ) (X = C1, Br. I) were conveniently prepared in situ
2167
2168
K. C. DASH and P. PUJARI [NiL6][HgCI4]. HgClz (1 mole) in 5 ml EtOH was added to a hot solution of NiL6CI 2 in 5 ml EtOH, a sky blue compound was obtained on vigorous stirring. [CoL4]X2(X = CIO4, BF4). These purple compounds were obtained by heating the corresponding hexakis complexes at 130°C till constant weight. The observed percentage loss was 20.9 (Theoretical 21.8) for [COL4](C104)2 and 23.3 (theoretical 22.7) for [CoLj(BF4)2. [CoL2X 2] (X = CI, Br, I, NCS). These blue complexes were obtained as above from the hexakis complexes. The observed percentage losses were 53.6, 45.1, 39.9 and 48.2 against the theoretical values 52.8, 46.1, 40.7 and 49.2 for chloro, bromo, iodo and thiocyanato complexes respectively. [NiL4Xz] (X = C1, Br, I). Prepared as above. The observed percentage losses were 28.0, 23.0 and 20.4 against the theoretical values 26.4, 22.8 and 22.1 for chloro, bromo and iodo complexes respectively. Analyses. Cobalt and nickel were estimated gravimetrically as described by Vogel[20]. Halogens and thiocyanate were estimated either gravimetrically as the silver salt or volumetrically by Volhard's method, and sulphate was determined as barium sulphate. The perchlorate, nitrate and tetrafluoroborate ions were estimated gravimetrically as their nitron salt[20]. Physical measurement. The spectral measurement (IR and electronic), electrolytic conductivity and magnetic susceptibility of the complexes were measured as described earlier[9].
by mixing equimolar amounts of M(CIO4)2 6H20 and MX2'6H20 (M = Co, Ni; X = CI, Br, I) in ethanol or acetone medium. [ML6]X2 (M=Co, Ni; X=CI, Br, I, NO 3, CIQ). The appropriate metal salt was dissolved in the minimum amount of EtOH and added to a solution of the ligand in the same solvent in a 1:6 molar ratio. Trietbylorthoformate was used as dehydrating agent for the nitrato and perchlorato complexes. The solvent mixture was reduced to small volume, when slow cooling afforded the desired compounds. They were filtered, washed with petroleum ether and dried in vacuo. [COL6] (NCS)2. Attempts to prepare the complex in ethanol were unsuccessful, an oily mass being formed. However, a stable pink compound was isolated in M%CO on keeping the mixture in the ice-chest for 2 to 3 days. NiL. (NCS)2 (n = 2, 4, 6). These compounds were prepared in EtOH with the metal and ligand in stoichiometric ratio of 1:2, 1:4 and 1:6. Addition of diethyl ether was required to promote the crystallisation. [ML6]X 2 (M = Co, X = ½ SO4, CI-13COO; M = Ni, X = ½ SO4, CH3COO, HCOO). The metal salt (1 mole) in 10 ml MeOH was added to a stirred solution of 4-methyl imidazole (6 moles) in 5 ml MeOH. On reducing the volume, the sulphato complexes appeared, whereas addition of diethyl ether and keeping the solvent mixture in the freeze was required for the formato and acetato complexes. [ML6]Xz (M = Co, X = BF4; M = Ni, X = BF4, N3). The sodium satt(tetrafluoroborate or azide) in the minimum volume of hot water was added to metal nitrate in 5 ml MeOH and the resultant mixture was added to a stirred solution of the ligand (6 mole) in 5 ml MeOH. The mixture was concentrated to small volume and the compounds were extracted with hot 2-propanol. [ML6]XC104 (M = Co, Ni; X = C1, Br, I) and [NiL4(NCS) (C104)]. The MX(CIO4) solution was added to the ligand in MezCO or EtOl-I medium with the desired stoichiometric ratio. The solvent mixture was concentrated to small volume, and cooled to room temperature. The products were collected as usual. [NiLCU. The metal salt and ligand were mixed in EtOH in molar ratio of 1:2 and refluxed for 1 hr. But the complex with 1 : 1 ratio and not the his complex was obtained. [Me4N][NiLCI3]. To the mixed solution of Me4NCI (1 mole) and 4-methyl imidazole (1 mole) in 10ml EtOH was added NiCIr6HzO (1 mole) in 5 ml EtOH and refluxed for 30 min, a yellow crystalline compound was isolated.
RESULTS AND DISCUSSION Data for the new compounds are presented in Tables 1 and 2. Variations of metal-ligand ratio did not alter the stoichiometry of the complexes, the hexakis complexes always being formed except in the case of the nickel(II) thiocyanate complexes. The compounds possess essentially 6-coordinated octahedral (or tetragonal) or 4-coordinated tetrahedral structures. The thermal decomposition products from the hexakis complexes of cobalt(II) have the composition [CoL2X2] for the coordinating anions like C1, Br, I and NCS, or [CoL4]X2 for the non-coordinating anions like ClO4 and BF4, and are presumably tetrahedral in structure whereas the thermal decomposition of [NiL6]X2 resulted in the formation of te-
Table 1. Colour, m.p., analyses, electronic spectra, conductivity and magnetic susceptibility data for cobalt(II) complexes AM'~in MeOH (~3cm 2 mole -I )
Analyses Compound
Colour
M.p. (°C)
Obs
M (%) Th
X (%) Obs
Th
COL6C12
Pink
148
9.5
(9.4)
11.8
(11.4)
CoL6Brz COL612 CoL6(NCS): CoL6(NO3)2
Pink Pink Pink Pink
155 160 75 152
8.0 6.8 8.3 9.0
(8.2) (7.3) (8.8) (8.8)
23.4 29.9 17.1 18.2
(24.4) (31.6) (17.4) (18.3)
CoL6(CIO4)z
Pink Pink Pink Pink Pink Pink Pink Purple Purple Blue Blue Blue Blue
7.8 8.9 8.9 7.8 7.9 7.8 7.2 9.6 9.6 19.3 16.2 11.9 16.7
(7.8) (9.1) (8.8) (8.1) (8.5) (8.0) (7.5) (10.0) (10.5) (20.0) (15.4) (12.3) (17.3)
25.9 14.1 -(23.6) 8.4 11.3 16.6 32.6 34.8 25.0 42.9 52.6 33.6
(26.5) (14.8) -(23.8) (8.1) (10.9) (16.3) (33.9) (35.6) (24.1) (41.8) (53.2) (34.2)
CoL6SO 4
CoL6(CH3COO) z CoL6(BF4h CoL6CI(CIO4) CoL6Br(CIO4) COL6I(CIO4) COL4(C104)2 CoL4(BF4)2 CoL2C12 CoL2Br2 CoLzI2 CoLE(NCS)z
155 >250 95 135 110 115 120 168 Liquid Liquid Liquid Liquid Liquid
)tm,x (cm-l ) 17,860 16,950 16,950 17,860 16,950 19,230 18,350 17,540 18,870 17,860 17,860 17,300 17,540 17,860 15,860 16,050 15,870 16,050 15,380 16,130
/~eff (B.M.)
148
5.03
157 268 114 178
4.91 4.85 4.77 4.97
184 insol. 64 206 150 156 182 186(240) 175(199) 136(5.4) 140(9.5) 142(33.8) 113(29.6)
4.90 4.41 4.82 4.90 4.73 4.72 4.32 4.40
tValues in brackets indicate the observed molar conductivity values in Me2CO (-10-3M).
Ligating ability of 4-methyl imidazole
2169
qable 2. Colour, m.p., analyses, electronic spectra, conductivity and magnetic susceptibility data for nickel(It) complexes
Analyses Compound
Colour
,')ctahedral (or tetragonal) Nil.,,C1. thght blue NiI.,,Br~ Light blue Nil .,,1~ Light blue "qil.,(N(iSb Light blue "
Yellow l,ight yellow
M.p. (°C)
M (%) obs Th
X (%) Obs Th
AM in MeOH (Ucm ~" mole i)
Electronic spectral bands (cm ~)
bc~, fB.M.)
P3
l'~
210
10.0
(9.4)
10.9
(11.4)
128.2
26.320
16,950
?. 12
218
7.8
(8.2)
22.0
(22.4)
135.5
27,030
17,240
g. III
220
7.2
(7.2)
30.5
131.51
190.0
26,320
16,030
L05
100
8.6
(8.7)
17.1
117.31
72.0
16.670
2.94
205
8.6
(8.6)
18.1
(18.31
160.0
25.640
15,750
2.90
185
7.7
(7.8)
26.1
126.41
180.3
26,320
16.131/
~.13
> 250 125
9.2 9.1
(9.0) 19.11
13.9
(14.81
20.7 82.2
26,970 26,320
16.000 16.130
~ 10 ~.03
>250
8.9
(8.7)
111.9
26,320
15.5/11/
2.98
190
7.8
(8.1)
227.7
27,031)
16,03(1
2.t~5
215
9.2
0.2)
269.6
16,1311
"~,05
200
8.3
18.5t
4.5
(5.11
158.3
16,030
~13
210
7.9
(8.0)
10.9
10.9~
175.2
15,620
2.98
205
7.2
(7.5)
16.tt
( 16.21
181.2
16.[30
~.20
215
6.7
(6.5)
14.8
(14.81
149.5
16.130
~.20
220
12.9
112.81
15.6
(15.41
151.6
17,240
L2,4
228
10.5
110.7)
28.3
(29.1~
158.9
16.670
2.9~
230
8.4
(9.1)
37.8
139.6~
166.2
16.030
2.96
195
11.8
(11.81
22.8
(23.4)
96.8
26,320
16.320
~.01
175
10.1
(10.7)
10.2
110.61
82.6
26,320
15,380
3.06
230
16.3
(17.31
34.8
(34.2~
106.3
26.320
15.620
~. t I
PI
I'~
> 250 >250
27.6 18.3
(27.7) 118.21
32.5 32.5
133.5i 133.11
128.8 131.6
25.640 25.640
14,930 16.0011
tragonally distorted trans-octahedral compounds, [NiL4X21 (X = CI, Br, l). The coordination number and the stereochemistry of the complexes have been established on the basis of analytical data, [R spectra and electronic spectra in solution and room temperature magnetic susceptibility measurements. The electrolytic conductivity in methanol ( - 1 0 3M) clearly indicates that the hexakis complexes are 1:2 electrolytes. However, the interpretation of conductivity data is not straight forward. The low conductance for sulphato, thiocyanato and carboxylato complexes may be due to solvolysis or ion-pair formation. The mono-, bisand tetrakis-complexes exhibit conductivity values which are substantially higher than expected for non-electrolytes. It is believed thal the anions might have been displaced by two molecules of solvent giving rise to an equilibrium of the type:
23.4
123.7~
26,320
3.60 4.17
[ML,Xe] + s o l v e n t ~ [ML. (solvent)~] +: + 2X The pink 6-coordinated complexes of cobalt(ll) have magnetic moment ranging from 4.7 to 5.2 B.M. as .expected for high-spin octahedral cobalt(lI) complexes {21}. The [ C o L 6 ] S Q and [CoL6] I(CIO4) complexes have magnetic moments of 4.4 B.M. and 4.3 B.M. respectively, and are believed to be octahedral on the basis of 1:he conductance measurement and electronic spectra in solution and the IR data. This observation of a low magnetic moment is not surprising since octahedral complexes of cobalt(II) can have magnetic moment as low as 4.2 B.M.[22], where the lowering of the magnetic moments may be due to tetragonal distortion[23]. Tetrahedral cobalt(II) complexes with an 4A2 ground state exhibit magnetic moments of 4.2-4.7 B.M.[24] very close to the spin-only value of 3.89 B.M., [CoL4](CIO4),
2170
K.C. DASH and P. PUJARI
has a magnetic moment of 4.4 B.M. as expected for a tetrahedral cobalt(II) complex. The magnetic susceptibility measurements divide the nickel compounds into two categories--octahedral (or tetragonal) and tetrahedral. The complexes [NiL6]X2 (X=C1, Br, I, NCS, N3, NO3, CIO4, BF,, 1/2 SO4, HCOO, CH3COO); [NiL6]X(C104) (X = CI, Br, I) and [NIL6] [HgC14] have magnetic moments ranging from 2.9 to 3.2 B.M. which is in excellent agreement with the values expected for octahedral nickel(II) complexes [25]. The observed value, somewhat above the spin-only value of 2.83 B.M., may be due to spin-orbit coupling and a contribution from the 3A2gand next higher 3T2gstate [26]. The complexes [NiL4(NCS)(C104)] and [NiL4X2] (X = C1, Br, I, NCS) have moments as expected for 6-coordinate octahedral nickel(II) complexes with small tetragonal distortion. The [NiL4X2] complexes have mononuclear 6-coordinated structures in which the anionic ligands are in trans-position, similar to the Nipy4Xz complexes (X = C1, Br, I, NCS)[27]. The compound, [NiL2(NCS)2], possesses a magnetic moment of 3.1B.M. as expected for an octahedrally coordinated nickel(II) ion. The 6-coordination is achieved through bridging of the thiocyanate group, which is clear from the vibrational spectra of the anion showing C---N stretches at 2125cm ' and C-S stretches at 777 cm -1 [28]. For a regular tetrahedral nickel(II) complex the magnetic moment lies in the range 3.5--4.00 B.M. [29-31]. Two tetrahedral nickel(II) complexes have been reported in this work. The compounds [NiLCI2] and [Me4N][NiLC13] exhibit magnetic moments of 3.6 and 4.17 B.M. respectively, in excellent agreement with those expected. In the former case tetrahedral coordination is achieved by bridging by the chloride ion. The electronic spectra in solution of the haxakis complexes of cobalt(II) show a strong and broad absorption band in the visible region at -18,000 cm L due to the 4T~g(P)~4T~g transition, indicating octahedral structures for these complexes. The tetrahedral complexes [CoL4]Xz(X = C104, BF4) and [CoL2X2] (X = CI, Br, I, NCS) show absorption bands in visible region at -16,000 cm -1 due to the 4T~(P)~4A2 transition[32]. The [CoL2X2] complexes reported are thus believed to be very nearly tetrahedrally coordinated as in Co(Im)2Clz (Ira = imidazole) [33]. The octahedral nickel(II) complexes show a low intensity v3 band in the region 25,000-27,000 cm ' due to the 3T~g(P)~A2g transition, a v2 band at 16,00018,000 cm ~ due to the 3T~g~3Azg transition and in some cases a l,t band around 13,000 cm -1 due to the 3T/~3A2g transition. A weak band in the solid around 10,900 cm is assigned to 3Bzg transition arising from the splitting of the 3Tzg(F) in octahedral symmetry on descending to D4r, symmetry[34]. The tetrahedral nickel(II) complexes show weak absorption bands at ~16,000 cm ~ assignable to the 3T~(P)~3T~(F) transition (v3) and a broad and rather weak absorption band at -25,000 cm -~ assignable to the 3T2~3T~(F) transition (v0135). The IR spectra of all the compounds were recorded as Nujol mulls in the region 4000-500 cm -~ and were compared with the spectra of the free ligand. The spectra indicated all compounds to be pure. The bands in the complexes were, in some cases either split or shifted due to lattice effects or due to departures from idealised symmetry[36, 37]. The spectra of the polyatomic anions were explored to determine the bonding of these anions
Table 3. Vibrational spectra of polyatomic anions (cm-~) for Co(II) and Ni(II) complexes with 4-methyl imidazole CIO4 CoL6(CIO4)2 CoL6CI(Clo") CoL6Br(CIO4) CoL6I(C104) NiL6(CIO4)2 NiL6CI(Clo") NiL6Br(C104) NiL6I(Clo") NiLn(NCS)(Clo")
618 m 615 st 620 st 612 st 618st 620 st 615st 618 st 618 st
955 st 950 st 955 st 953 st 950st 957 st 953 st 955 st 955 st
1095br 1090br 1095br 1110br 1092br 1110br 1095br 1090br 1070br lll0st
972m 977 s
lll0br, st 1155br, st
827st 825 st
1390st
SO. CoL6804 NiL6SO"
NO3 CoL6(NO3)z NiL6(NO3)2 NCS CoL6(NCS)2 NiL6(NCS)2 NiL4(NCS)2
720m 715m
805 s
NiL4(NCS)(CIO4)
805 s
NiL2(NCS)~
777 st
2050s 2050 s 2105vs 2050 sh 2110vs 2060 sh 2125st
as well as the coordination number around the metal cation. The potential ambidentate thiocyanate group offers interesting bonding possibilities[38] with the transition metal ions. The mode of bonding is indicated by the vibrational modes of the thiocyanate group, yeN, Vcs and 6scN[39]. The complexes [ML6](NCS)2 (M=Co(II), Ni(II)) exhibit the C-=N stretching frequency at 2050 cm -~ indicating ionic thiocyanate groups. This is an example where the thiocyanate is ionic[40]. The C=N stretching frequency at -2105 cm ~ and C-S stretching frequency at 805 cm-'suggest terminal M-NCS bonding for [NiL4(NCS)2] and [NiL4(NCS)(CIO4)]. The shoulder at - 2 0 5 0 c m - ' in the mull spectrum of these two thiocyanato complexes may be attributed to crystal effects. In the case of [NiL2(NCS)z], a higher v (C-=N) at 2125cm -j and a relatively lower v (C-S) at 777cm 1 indicate bridging thiocyanate groups. The bridging-NCS group leads to an octahedral arrangement around the nickel(II) ion in NiL2(NCS)2 which is further supported by the magnetic susceptibility data. The nitrato complexes, [ML6](NO3)2 show a strong N-O stretching mode at 1390 cm -~ (v3) and two IR active deformation modes at 827(vz) and 720(v4)cm -~, indicating non-coordinating NO3 ions[41] possessing D3h symmetry. The observed single v3 and absence of the Vl band indicates ionic character of the nitrate group. The perchlorato complexes [ML6](CIO4)2 and [ML6]X(CIO4) are found to be ionic. Strong and broad absorption bands were obtained at - l l 0 0 c m -~ (v3) for C10 stretch, medium band at - 9 4 0 c m -1 (v4) for OCIO bend. The v3 and ~'4 bands show no sign of splitting[42] thus indicating uncoordinated ionic perchlorate. When the perchlorate ion is involved in covalent bonding through the O-atom of the C104 ion it lowers the symmetry from Ta to C~v[43, 44]. The splitting of the v3 band into two components at 1070cm l and l l l 0 c m -l indicates weak coordination of C104 ion in [NiL4(NCS)(CIO4)] thus giving a tetragonal arrangement of ligands around the central nickel(II) ion, where the two trans-
Ligating ability of 4-methyl imidazole axial positions are occupied by the NCS and C104 groups. The compounds [ML~](BF4),_ ( M - C o , Ni) show a strong and broad absorption band at 1050cm ~, corresponding 1o the J,, vibration of the BF4 ion. The ~ibrations ~', and ~,~ are |R inactive and v4 usually observed around 520 cm ' could not be observed since it was beyond the range of the instrument used. An unsplit ,,~ hand is indicative of T~ symmetry of the BF4 ion[451 and thus the BF4 ion is not involved in covalent bondrag. The compounds [ML,,ISO4 exhibit S-O stretching modes, broad and strong band at 972cm ~ (~,~), indicating a T~ symmetry and ionic character of the sulphato group. The only azido complex prepared in this work is iNiL,,][Nd> The NN stretching frequencies in the symmetrical N~ ion are obtained [46] at 1340cm ~ and 2040 cm ' with a wide separation of 700 cm ~. It is clear from the stoichiometry as well as the anion vibration that ~,he N~ group is not involved in covalent bonding with the metal. The IR spectra also clearly reveals that in the carboxylate complexes, e.g. the formate and acetate, the ,:arboxvlate ions are ionic [471. CONC LUSION The ligand 4-methyl imidazole which is a much stronger base than imidazole forms a series of stable, octahedral [ML~]X, complexes where X varies from coordinating anions to those usually believed to be noncoordinating. Attempts to prepare lower coordination number complexes by direct metathetic reactions did not meet with success and no stable compound could be isolated except for [NiL4(NCS)21, [NiLa(NCS)(CIO4)] and [NiL2(NCS)2]. Fhe thermal dissociation of the hexakis complexes resulted in tetrahedral [CoI,~]X2 complexes where X is a non-coordinating anion like CIO4 or BF4 and pseudoletrahedral [CoL2Xe] complexes where X is a coordinating anion like CI, Br, I, NCS and 6-coordinate tetragonal {NiL~Xd (C = CI, Br, I) complexes, where the anions are believed to occupy the axial positions. Nickel(II) formed only two tetrahedral complexes with this ligand demonstrating the fact that nickel(lI) shows a marked preference for an octahedral rather than a tetrahedral environment. In all the complexes it is believed that the methyl group is placed at position 5 of the 5-membered heterocyclic imidazole ring as in Cu(5-methyl imidazoleh (N()~):~ complex[481. A~kmm'ledgement--Thanks arc due to M/s BASF, Ludwigshafen. West t~ernmny for a sample ol 4-methyl imidazole. REFERENCES
I W ,I. Eilbeck. F. Holmes ~nd A. E. Underhilh J. Chem. Soe, (At, 757 ~19671. 2. C, F-_ I'ay,lor and A. E. 1 nderhill, J. Chenl. Soc. (A), 368 i 19691. 3 W J Davis and J. Smith ,I. Chem. Soc. (A), 317 119711. ,4, l) M. I,. Goodgame, M. Goodgame and G. W. RaynerCanham, brorg,. Chim. Acta 3, 399,406 (19691. J. Reedijk, Reel. Trur. Chim. Pays-Bas 90, 1249 119711: 91, S()7 ( 19721. 6. J. Reedijk, J. lnorg. Nucf. Chem. 35, 239 119731. 7 M. Massacesi and G. Ponticelli, J. lnorg. Nucl. Chem. 36, 22(t9 (19741,
2171
8. P. Pujari and K. C. Dash, lnor,~. Nucl. Chem. Lett, It. 165 (19751. 9. K. C. Dash and P. Pujari, J. lnorg. Nucl. Chem. 37, 21161 11975). 10. P. Pujari and K, C. Dash. J. Inorg. Nucl. Chem, 38, 1891 ( 19761. 11. P. Pujari and K. C. Dash. J, lnorx,. NucL Chem. 38, 2183 119761. 12. Ch. K. C. Mohapatra and K. C. Dash, J. Im~rg. Nm't. Chem. 39, 1253 119771. 13. L. Hunter and J. A. Marriot, J, Chem. Soc. 777 119411. 1,4,, T. J. Lane and K. P. Quinalam J. Am. Chem. Soc. 82. 299-4` (19601. 15. H. C. Freeman and J. T. Szymanski, A(ta Crystullov,r. 22, 406 ( 19671. 16, M. Charton, J. Org. Chem. 30, 3346 119651. 17. Y. Nozaki, F. R, N. Gurd, R. F. Chen and J. T. Edsall. J. rm. Chem. Soc. 79, 2123 (19571. 18. J. Reedijk, lnorg. Chim. Acta 3, 517 119691. 19. J. Reedijk, J. lno¢'4. Nucl. Chem. 33, 179 09711, 21). A. I. Vogel, Quantitative lnorv,anic Analysis. Longmans, London (19591. 21, J. Lewis, Sci. Prog. 51,452 119621. 22. C. Stoufer, D. W. Smith, E. A. Clevenger and T. E, Norris, lnoti~. Chem. 5, 1167 119661. 23. R. A. Krause, C. Guy and M. L, Hooker, blorg. Chem 51, 1825 (19661. 24. B. N. Figgis. lntrodmtion to Ligand Fields. Interscience. Ne~ York (19661. 25. F. A. Cotton and G. Wilkinson, Advanced hlorganic Chemistry. 2nd Edn, p. 882. lnterscience, New York ( 19641. 26. A. B. P. Lever, lnorg, Chem. 4, 763 (19651. 27. M. A. Porai-Kojie, A. S. Antzishkina, L. M. I)ickareva and E. K. Jukhov, Attn. Cryst, 10, 784 119571. 28. R. A. Bailey. S. L. Kozak, T. W. Michdson and W. N, Mills. Coord. Chem. Rer. 6, 1411711971). 29. Z. Dori and H. B. Gray, Im)rg. Chem. 7, 889 ( 19681. 30. N. S. Gill and R. S. Nyholm, J. Chem. Soc. 3997 ( 19591. 31. F. A. Cotton and D. M. L. Goodgame, J. Anl. Chem. Soc. 82, 5771 119601. 32. A. B. P. Lew,~r, Inorganic Electronic Speetlvscopv. Elsevier, Amsterdam 119681. 33. C. J. Antti and B. K. S, Lundeberg, Aeta, Chem. Stand. 26, 3995 (19721. 34. W. J. Eilbeck, F, Holmes, E. C. Taylor and A. E. Underhilh J. Chem. So¢. (A), 1189 (1968). 35. S. Buffagni and T. M. Dunn, Nature 188, 937 (19611). 36. N. S. Gill, R. H. Nuttall, H. C. Scaife and D. W. A. Sharp, J. lnor~. Nuel. Chem. 18, 79 (1961). 37. M. Goldstein. E. F. Mooney. A. Anderson and H. A. Gebbie, Spectrochim. Acta 21, 105 119651. 38. J. k. Burmeister, Coord. Chem. Ret. 1,205 119661. 39. S. N. Das, S. N. Moharana and K. C. Dash, ,1. blm:v. Nucl. Chem. 33, 3739 119711. 40. J. Reedijk and J. A. Smit, Rec. Trar. Chim. 90, 1139 (197!i). 41. N. F. Curtis and Y. M. Curtis, lnorg. Chem. 4. 804 119651 42. B. J. Hathaway. D. G. Holah and M. Hudson, .L Chem, Soc. -4`586 {19631. 43. M. R. Rosenlhal, J. Chem. Educ. 50, 331 11973k 44. B. J, Hathaway and A. E. Underhilh J. Chem, Noc. 3091 (19611. 45. L. E. Moore, R. B. Gayhart and W. E. Bull, J. hlor~. Nucl. Chem. 26. 896 ( 19641. 46. F. A. Cotton, Modern Coordination Chemistry (Edited by J. Lewis and R. G. Wilkins), lnterscience, New York (1960L 47. K. Nakamoto, J. Fujita, S. Tanaka and M. Kobayashi. J. Am. Chem. Soc. 79, 4504 119571. 48, H. Montgomery and E. C. Lingafelter, J. Phys. Chem. 64.831 11960).
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LIGATING ABILITY OF 4-METHYL IMIDAZOLE: COMPLEXES WITH COBALT(II) AND NICKEL(II) KAILASH C. DASH* and PANCHANAN PUJARI+ Department of Chemistry, Utkal University, Vani Vihar. Bhubaneswar-751004, India IReceived 29 October 1976; received for publication 4 May 1977)
Abstract--The ligand, 4-methyl imidazo!elL), forms a wide variety of new coordination complexes with cobalt(ll) and nickel(ll) ions. The complexes obtained by the direct reaction of metal salts, containing anions ranging from small and strongly coordinating to those usually believed to be noncoordinating, and the ligand, are of the type [ML6]X2, [ML6]X(CIO4), [NiL6][HgCI4], [NiL4(NCS)2], [NiL4(NCS)(CIO4)]. [NiL2(NCS)2] having 6-cuordinated octahedral or distorted octahedral structures, and [NiLCL] and [Me4N][NiLCL] with a tetrahedral structure. The [ML6](NCS)2 complexes contain ionic thiocyanate group, the [NiL4(NCS)2] and [NiL4(NCS)(CIO4)] contain N-bonded iso-thiocyanate groups, whereas the [NiL2(NCS)2]has bridging thiocyanate groups. Except for nickel(lt) thiocyanate, attempts to prepare bis- or tetrakis-complexes by direct reaction resulted only in an oily mass and no stable compound could be prepared. The stoichiometry and stereochemistry of the complexes have been deduced on the basis of analytical data, electronic and [R spectra, molar conductance and room temperature magnetic susceptibility measurements. Thermal decomposition of the parent hexakis complexes [ML6]+~ at 130"C gave tetrahedral Co(ll) complexes. e.g. [CoL4]X2 (X = CIO4, BF4) and [CoL2X2] (X = CI, Br, I, NCS) and tetragonally distorted trans- octahedral complexes, [NiL4X,] (X = CI, Br, I).
INTRODUCTION
4- or 5- substituted imidazoles containing labile imiaohydrogen are tautomeric systems and react as a tautomeric mixture of the two possible forms[16]. However, when 4(5) substituted imidazole is coordinated to a metal ion, one form of the ligand may be preferred and it ]has been assumed for steric reasons that the molecule will coordinate with the substituent in the 5-position117]. As a part of our investigations[9-12] of imidazole derivatives we report here the complexes formed between 4-methyl imidazole and cobalt(II) and nickel(II) ions. The ligand forms octahedral complexes of the type [ML6]X2 (M = Co(IlL Ni(II)), where X represents a large number of anions ranging from the strongly coordinating halides and pseudohalides to noncoordinating anions like BF4 and CIO4 . Attempts to prepare the bis or tetrakis complexes by direct metathetic reaction of tigand and metal salt were unsuccessful, except in the case of Ni(II) thiocyanate. However, the thermal decomposition of the parent hexakis complexes resulted in the formation of some tetrahedral cobalt(II) complexes of the type [CoL4]X 2 (X = C[O 4, BF4) and [CoL2X2} (X = (71, Br, l, NCS) and tetragonally distorted octahedral nickel(II) compounds with the composition [NiI,nX2] ( X - CI, Br, I). It is seen that 4(5)-methyl imidazole forms hexakis complexes similar to the unsubstituted imidazole[1] and I-substituted imidazole [18, 19].
Imidazoles are believed to be versatile ligands, because the imidazole nitrogen of histidine residues provides one of the primary means by which metal ions may be bound to proteins, lmidazole complexes are becoming increasingly important as analytical, biochemical and anticancer reagents. The coordinating ability of imidazole and its derivatives have been investigated by various workers[l-12] and it is found that the neutral imidazole coordinates via the unsaturated nitrogen atom rather than the imino nitrogen of the heterocyclic ring[13-15]. In complexes involving neutral monodentate imidazole ligands, the metal ion is nearly coplanar with the imidazole ring. Examples in which imidazole acts as rr-type ligand are unknown. On introducing a methyl group into the 5-membered ring, the coordinating power of the ligand increases due to increase in basicity. Thus, 1-methyl imidazole forms stable hexakis complexes like the unsubstituted imidazole. Although 2-methyl and 1,2-dimethyl imidazole are good coordinating ligands, they only form tetrakis, tris and bis complexes. In no case was a hexakis complex obtained, probably due to the steric effect of the methyl group adjacent to the donor site. Hence it is of interest to study the effect of a methyl group at position-4, which is also adjacent to the donor nitrogen atom. 3
H30+ ~-
HC~.~NH 3 [,,,,(~N. ~
X
EXPERIMENTAL
H
5
I
H 3 C ~ - - NH + H20
R~
'
4 ["-,.~N5 2 + H~O+ 3
*Author for correspondence I-B. J. B. College, Bhubaneswar.
Starting materials. 4-methyl imidazole was used as supplied by BASF (West Germany). Hydrated cobalt(ll) and nickel(II) chloride, nitrate, perchlorate, sulphate, formate and acetate (BDH) were used as such without further purification. CoBr2 and NiBr2 were prepared by the reaction of respective carbonates and HBr. Metal iodide, thiocyanate, tetrafluoroborate and azides were prepared by metathetic reactions of hydrated metallic nitrate with KI, KNCS, NaBF4 or NaN 3 respectively. Solutions of MX(CIQ) (X = C1, Br. I) were conveniently prepared in situ
2167
2168
K. C. DASH and P. PUJARI [NiL6][HgCI4]. HgClz (1 mole) in 5 ml EtOH was added to a hot solution of NiL6CI 2 in 5 ml EtOH, a sky blue compound was obtained on vigorous stirring. [CoL4]X2(X = CIO4, BF4). These purple compounds were obtained by heating the corresponding hexakis complexes at 130°C till constant weight. The observed percentage loss was 20.9 (Theoretical 21.8) for [COL4](C104)2 and 23.3 (theoretical 22.7) for [CoLj(BF4)2. [CoL2X 2] (X = CI, Br, I, NCS). These blue complexes were obtained as above from the hexakis complexes. The observed percentage losses were 53.6, 45.1, 39.9 and 48.2 against the theoretical values 52.8, 46.1, 40.7 and 49.2 for chloro, bromo, iodo and thiocyanato complexes respectively. [NiL4Xz] (X = C1, Br, I). Prepared as above. The observed percentage losses were 28.0, 23.0 and 20.4 against the theoretical values 26.4, 22.8 and 22.1 for chloro, bromo and iodo complexes respectively. Analyses. Cobalt and nickel were estimated gravimetrically as described by Vogel[20]. Halogens and thiocyanate were estimated either gravimetrically as the silver salt or volumetrically by Volhard's method, and sulphate was determined as barium sulphate. The perchlorate, nitrate and tetrafluoroborate ions were estimated gravimetrically as their nitron salt[20]. Physical measurement. The spectral measurement (IR and electronic), electrolytic conductivity and magnetic susceptibility of the complexes were measured as described earlier[9].
by mixing equimolar amounts of M(CIO4)2 6H20 and MX2'6H20 (M = Co, Ni; X = CI, Br, I) in ethanol or acetone medium. [ML6]X2 (M=Co, Ni; X=CI, Br, I, NO 3, CIQ). The appropriate metal salt was dissolved in the minimum amount of EtOH and added to a solution of the ligand in the same solvent in a 1:6 molar ratio. Trietbylorthoformate was used as dehydrating agent for the nitrato and perchlorato complexes. The solvent mixture was reduced to small volume, when slow cooling afforded the desired compounds. They were filtered, washed with petroleum ether and dried in vacuo. [COL6] (NCS)2. Attempts to prepare the complex in ethanol were unsuccessful, an oily mass being formed. However, a stable pink compound was isolated in M%CO on keeping the mixture in the ice-chest for 2 to 3 days. NiL. (NCS)2 (n = 2, 4, 6). These compounds were prepared in EtOH with the metal and ligand in stoichiometric ratio of 1:2, 1:4 and 1:6. Addition of diethyl ether was required to promote the crystallisation. [ML6]X 2 (M = Co, X = ½ SO4, CI-13COO; M = Ni, X = ½ SO4, CH3COO, HCOO). The metal salt (1 mole) in 10 ml MeOH was added to a stirred solution of 4-methyl imidazole (6 moles) in 5 ml MeOH. On reducing the volume, the sulphato complexes appeared, whereas addition of diethyl ether and keeping the solvent mixture in the freeze was required for the formato and acetato complexes. [ML6]Xz (M = Co, X = BF4; M = Ni, X = BF4, N3). The sodium satt(tetrafluoroborate or azide) in the minimum volume of hot water was added to metal nitrate in 5 ml MeOH and the resultant mixture was added to a stirred solution of the ligand (6 mole) in 5 ml MeOH. The mixture was concentrated to small volume and the compounds were extracted with hot 2-propanol. [ML6]XC104 (M = Co, Ni; X = C1, Br, I) and [NiL4(NCS) (C104)]. The MX(CIO4) solution was added to the ligand in MezCO or EtOl-I medium with the desired stoichiometric ratio. The solvent mixture was concentrated to small volume, and cooled to room temperature. The products were collected as usual. [NiLCU. The metal salt and ligand were mixed in EtOH in molar ratio of 1:2 and refluxed for 1 hr. But the complex with 1 : 1 ratio and not the his complex was obtained. [Me4N][NiLCI3]. To the mixed solution of Me4NCI (1 mole) and 4-methyl imidazole (1 mole) in 10ml EtOH was added NiCIr6HzO (1 mole) in 5 ml EtOH and refluxed for 30 min, a yellow crystalline compound was isolated.
RESULTS AND DISCUSSION Data for the new compounds are presented in Tables 1 and 2. Variations of metal-ligand ratio did not alter the stoichiometry of the complexes, the hexakis complexes always being formed except in the case of the nickel(II) thiocyanate complexes. The compounds possess essentially 6-coordinated octahedral (or tetragonal) or 4-coordinated tetrahedral structures. The thermal decomposition products from the hexakis complexes of cobalt(II) have the composition [CoL2X2] for the coordinating anions like C1, Br, I and NCS, or [CoL4]X2 for the non-coordinating anions like ClO4 and BF4, and are presumably tetrahedral in structure whereas the thermal decomposition of [NiL6]X2 resulted in the formation of te-
Table 1. Colour, m.p., analyses, electronic spectra, conductivity and magnetic susceptibility data for cobalt(II) complexes AM'~in MeOH (~3cm 2 mole -I )
Analyses Compound
Colour
M.p. (°C)
Obs
M (%) Th
X (%) Obs
Th
COL6C12
Pink
148
9.5
(9.4)
11.8
(11.4)
CoL6Brz COL612 CoL6(NCS): CoL6(NO3)2
Pink Pink Pink Pink
155 160 75 152
8.0 6.8 8.3 9.0
(8.2) (7.3) (8.8) (8.8)
23.4 29.9 17.1 18.2
(24.4) (31.6) (17.4) (18.3)
CoL6(CIO4)z
Pink Pink Pink Pink Pink Pink Pink Purple Purple Blue Blue Blue Blue
7.8 8.9 8.9 7.8 7.9 7.8 7.2 9.6 9.6 19.3 16.2 11.9 16.7
(7.8) (9.1) (8.8) (8.1) (8.5) (8.0) (7.5) (10.0) (10.5) (20.0) (15.4) (12.3) (17.3)
25.9 14.1 -(23.6) 8.4 11.3 16.6 32.6 34.8 25.0 42.9 52.6 33.6
(26.5) (14.8) -(23.8) (8.1) (10.9) (16.3) (33.9) (35.6) (24.1) (41.8) (53.2) (34.2)
CoL6SO 4
CoL6(CH3COO) z CoL6(BF4h CoL6CI(CIO4) CoL6Br(CIO4) COL6I(CIO4) COL4(C104)2 CoL4(BF4)2 CoL2C12 CoL2Br2 CoLzI2 CoLE(NCS)z
155 >250 95 135 110 115 120 168 Liquid Liquid Liquid Liquid Liquid
)tm,x (cm-l ) 17,860 16,950 16,950 17,860 16,950 19,230 18,350 17,540 18,870 17,860 17,860 17,300 17,540 17,860 15,860 16,050 15,870 16,050 15,380 16,130
/~eff (B.M.)
148
5.03
157 268 114 178
4.91 4.85 4.77 4.97
184 insol. 64 206 150 156 182 186(240) 175(199) 136(5.4) 140(9.5) 142(33.8) 113(29.6)
4.90 4.41 4.82 4.90 4.73 4.72 4.32 4.40
tValues in brackets indicate the observed molar conductivity values in Me2CO (-10-3M).
Ligating ability of 4-methyl imidazole
2169
qable 2. Colour, m.p., analyses, electronic spectra, conductivity and magnetic susceptibility data for nickel(It) complexes
Analyses Compound
Colour
,')ctahedral (or tetragonal) Nil.,,C1. thght blue NiI.,,Br~ Light blue Nil .,,1~ Light blue "qil.,(N(iSb Light blue "
Yellow l,ight yellow
M.p. (°C)
M (%) obs Th
X (%) Obs Th
AM in MeOH (Ucm ~" mole i)
Electronic spectral bands (cm ~)
bc~, fB.M.)
P3
l'~
210
10.0
(9.4)
10.9
(11.4)
128.2
26.320
16,950
?. 12
218
7.8
(8.2)
22.0
(22.4)
135.5
27,030
17,240
g. III
220
7.2
(7.2)
30.5
131.51
190.0
26,320
16,030
L05
100
8.6
(8.7)
17.1
117.31
72.0
16.670
2.94
205
8.6
(8.6)
18.1
(18.31
160.0
25.640
15,750
2.90
185
7.7
(7.8)
26.1
126.41
180.3
26,320
16.131/
~.13
> 250 125
9.2 9.1
(9.0) 19.11
13.9
(14.81
20.7 82.2
26,970 26,320
16.000 16.130
~ 10 ~.03
>250
8.9
(8.7)
111.9
26,320
15.5/11/
2.98
190
7.8
(8.1)
227.7
27,031)
16,03(1
2.t~5
215
9.2
0.2)
269.6
16,1311
"~,05
200
8.3
18.5t
4.5
(5.11
158.3
16,030
~13
210
7.9
(8.0)
10.9
10.9~
175.2
15,620
2.98
205
7.2
(7.5)
16.tt
( 16.21
181.2
16.[30
~.20
215
6.7
(6.5)
14.8
(14.81
149.5
16.130
~.20
220
12.9
112.81
15.6
(15.41
151.6
17,240
L2,4
228
10.5
110.7)
28.3
(29.1~
158.9
16.670
2.9~
230
8.4
(9.1)
37.8
139.6~
166.2
16.030
2.96
195
11.8
(11.81
22.8
(23.4)
96.8
26,320
16.320
~.01
175
10.1
(10.7)
10.2
110.61
82.6
26,320
15,380
3.06
230
16.3
(17.31
34.8
(34.2~
106.3
26.320
15.620
~. t I
PI
I'~
> 250 >250
27.6 18.3
(27.7) 118.21
32.5 32.5
133.5i 133.11
128.8 131.6
25.640 25.640
14,930 16.0011
tragonally distorted trans-octahedral compounds, [NiL4X21 (X = CI, Br, l). The coordination number and the stereochemistry of the complexes have been established on the basis of analytical data, [R spectra and electronic spectra in solution and room temperature magnetic susceptibility measurements. The electrolytic conductivity in methanol ( - 1 0 3M) clearly indicates that the hexakis complexes are 1:2 electrolytes. However, the interpretation of conductivity data is not straight forward. The low conductance for sulphato, thiocyanato and carboxylato complexes may be due to solvolysis or ion-pair formation. The mono-, bisand tetrakis-complexes exhibit conductivity values which are substantially higher than expected for non-electrolytes. It is believed thal the anions might have been displaced by two molecules of solvent giving rise to an equilibrium of the type:
23.4
123.7~
26,320
3.60 4.17
[ML,Xe] + s o l v e n t ~ [ML. (solvent)~] +: + 2X The pink 6-coordinated complexes of cobalt(ll) have magnetic moment ranging from 4.7 to 5.2 B.M. as .expected for high-spin octahedral cobalt(lI) complexes {21}. The [ C o L 6 ] S Q and [CoL6] I(CIO4) complexes have magnetic moments of 4.4 B.M. and 4.3 B.M. respectively, and are believed to be octahedral on the basis of 1:he conductance measurement and electronic spectra in solution and the IR data. This observation of a low magnetic moment is not surprising since octahedral complexes of cobalt(II) can have magnetic moment as low as 4.2 B.M.[22], where the lowering of the magnetic moments may be due to tetragonal distortion[23]. Tetrahedral cobalt(II) complexes with an 4A2 ground state exhibit magnetic moments of 4.2-4.7 B.M.[24] very close to the spin-only value of 3.89 B.M., [CoL4](CIO4),
2170
K.C. DASH and P. PUJARI
has a magnetic moment of 4.4 B.M. as expected for a tetrahedral cobalt(II) complex. The magnetic susceptibility measurements divide the nickel compounds into two categories--octahedral (or tetragonal) and tetrahedral. The complexes [NiL6]X2 (X=C1, Br, I, NCS, N3, NO3, CIO4, BF,, 1/2 SO4, HCOO, CH3COO); [NiL6]X(C104) (X = CI, Br, I) and [NIL6] [HgC14] have magnetic moments ranging from 2.9 to 3.2 B.M. which is in excellent agreement with the values expected for octahedral nickel(II) complexes [25]. The observed value, somewhat above the spin-only value of 2.83 B.M., may be due to spin-orbit coupling and a contribution from the 3A2gand next higher 3T2gstate [26]. The complexes [NiL4(NCS)(C104)] and [NiL4X2] (X = C1, Br, I, NCS) have moments as expected for 6-coordinate octahedral nickel(II) complexes with small tetragonal distortion. The [NiL4X2] complexes have mononuclear 6-coordinated structures in which the anionic ligands are in trans-position, similar to the Nipy4Xz complexes (X = C1, Br, I, NCS)[27]. The compound, [NiL2(NCS)2], possesses a magnetic moment of 3.1B.M. as expected for an octahedrally coordinated nickel(II) ion. The 6-coordination is achieved through bridging of the thiocyanate group, which is clear from the vibrational spectra of the anion showing C---N stretches at 2125cm ' and C-S stretches at 777 cm -1 [28]. For a regular tetrahedral nickel(II) complex the magnetic moment lies in the range 3.5--4.00 B.M. [29-31]. Two tetrahedral nickel(II) complexes have been reported in this work. The compounds [NiLCI2] and [Me4N][NiLC13] exhibit magnetic moments of 3.6 and 4.17 B.M. respectively, in excellent agreement with those expected. In the former case tetrahedral coordination is achieved by bridging by the chloride ion. The electronic spectra in solution of the haxakis complexes of cobalt(II) show a strong and broad absorption band in the visible region at -18,000 cm L due to the 4T~g(P)~4T~g transition, indicating octahedral structures for these complexes. The tetrahedral complexes [CoL4]Xz(X = C104, BF4) and [CoL2X2] (X = CI, Br, I, NCS) show absorption bands in visible region at -16,000 cm -1 due to the 4T~(P)~4A2 transition[32]. The [CoL2X2] complexes reported are thus believed to be very nearly tetrahedrally coordinated as in Co(Im)2Clz (Ira = imidazole) [33]. The octahedral nickel(II) complexes show a low intensity v3 band in the region 25,000-27,000 cm ' due to the 3T~g(P)~A2g transition, a v2 band at 16,00018,000 cm ~ due to the 3T~g~3Azg transition and in some cases a l,t band around 13,000 cm -1 due to the 3T/~3A2g transition. A weak band in the solid around 10,900 cm is assigned to 3Bzg transition arising from the splitting of the 3Tzg(F) in octahedral symmetry on descending to D4r, symmetry[34]. The tetrahedral nickel(II) complexes show weak absorption bands at ~16,000 cm ~ assignable to the 3T~(P)~3T~(F) transition (v3) and a broad and rather weak absorption band at -25,000 cm -~ assignable to the 3T2~3T~(F) transition (v0135). The IR spectra of all the compounds were recorded as Nujol mulls in the region 4000-500 cm -~ and were compared with the spectra of the free ligand. The spectra indicated all compounds to be pure. The bands in the complexes were, in some cases either split or shifted due to lattice effects or due to departures from idealised symmetry[36, 37]. The spectra of the polyatomic anions were explored to determine the bonding of these anions
Table 3. Vibrational spectra of polyatomic anions (cm-~) for Co(II) and Ni(II) complexes with 4-methyl imidazole CIO4 CoL6(CIO4)2 CoL6CI(Clo") CoL6Br(CIO4) CoL6I(C104) NiL6(CIO4)2 NiL6CI(Clo") NiL6Br(C104) NiL6I(Clo") NiLn(NCS)(Clo")
618 m 615 st 620 st 612 st 618st 620 st 615st 618 st 618 st
955 st 950 st 955 st 953 st 950st 957 st 953 st 955 st 955 st
1095br 1090br 1095br 1110br 1092br 1110br 1095br 1090br 1070br lll0st
972m 977 s
lll0br, st 1155br, st
827st 825 st
1390st
SO. CoL6804 NiL6SO"
NO3 CoL6(NO3)z NiL6(NO3)2 NCS CoL6(NCS)2 NiL6(NCS)2 NiL4(NCS)2
720m 715m
805 s
NiL4(NCS)(CIO4)
805 s
NiL2(NCS)~
777 st
2050s 2050 s 2105vs 2050 sh 2110vs 2060 sh 2125st
as well as the coordination number around the metal cation. The potential ambidentate thiocyanate group offers interesting bonding possibilities[38] with the transition metal ions. The mode of bonding is indicated by the vibrational modes of the thiocyanate group, yeN, Vcs and 6scN[39]. The complexes [ML6](NCS)2 (M=Co(II), Ni(II)) exhibit the C-=N stretching frequency at 2050 cm -~ indicating ionic thiocyanate groups. This is an example where the thiocyanate is ionic[40]. The C=N stretching frequency at -2105 cm ~ and C-S stretching frequency at 805 cm-'suggest terminal M-NCS bonding for [NiL4(NCS)2] and [NiL4(NCS)(CIO4)]. The shoulder at - 2 0 5 0 c m - ' in the mull spectrum of these two thiocyanato complexes may be attributed to crystal effects. In the case of [NiL2(NCS)z], a higher v (C-=N) at 2125cm -j and a relatively lower v (C-S) at 777cm 1 indicate bridging thiocyanate groups. The bridging-NCS group leads to an octahedral arrangement around the nickel(II) ion in NiL2(NCS)2 which is further supported by the magnetic susceptibility data. The nitrato complexes, [ML6](NO3)2 show a strong N-O stretching mode at 1390 cm -~ (v3) and two IR active deformation modes at 827(vz) and 720(v4)cm -~, indicating non-coordinating NO3 ions[41] possessing D3h symmetry. The observed single v3 and absence of the Vl band indicates ionic character of the nitrate group. The perchlorato complexes [ML6](CIO4)2 and [ML6]X(CIO4) are found to be ionic. Strong and broad absorption bands were obtained at - l l 0 0 c m -~ (v3) for C10 stretch, medium band at - 9 4 0 c m -1 (v4) for OCIO bend. The v3 and ~'4 bands show no sign of splitting[42] thus indicating uncoordinated ionic perchlorate. When the perchlorate ion is involved in covalent bonding through the O-atom of the C104 ion it lowers the symmetry from Ta to C~v[43, 44]. The splitting of the v3 band into two components at 1070cm l and l l l 0 c m -l indicates weak coordination of C104 ion in [NiL4(NCS)(CIO4)] thus giving a tetragonal arrangement of ligands around the central nickel(II) ion, where the two trans-
Ligating ability of 4-methyl imidazole axial positions are occupied by the NCS and C104 groups. The compounds [ML~](BF4),_ ( M - C o , Ni) show a strong and broad absorption band at 1050cm ~, corresponding 1o the J,, vibration of the BF4 ion. The ~ibrations ~', and ~,~ are |R inactive and v4 usually observed around 520 cm ' could not be observed since it was beyond the range of the instrument used. An unsplit ,,~ hand is indicative of T~ symmetry of the BF4 ion[451 and thus the BF4 ion is not involved in covalent bondrag. The compounds [ML,,ISO4 exhibit S-O stretching modes, broad and strong band at 972cm ~ (~,~), indicating a T~ symmetry and ionic character of the sulphato group. The only azido complex prepared in this work is iNiL,,][Nd> The NN stretching frequencies in the symmetrical N~ ion are obtained [46] at 1340cm ~ and 2040 cm ' with a wide separation of 700 cm ~. It is clear from the stoichiometry as well as the anion vibration that ~,he N~ group is not involved in covalent bonding with the metal. The IR spectra also clearly reveals that in the carboxylate complexes, e.g. the formate and acetate, the ,:arboxvlate ions are ionic [471. CONC LUSION The ligand 4-methyl imidazole which is a much stronger base than imidazole forms a series of stable, octahedral [ML~]X, complexes where X varies from coordinating anions to those usually believed to be noncoordinating. Attempts to prepare lower coordination number complexes by direct metathetic reactions did not meet with success and no stable compound could be isolated except for [NiL4(NCS)21, [NiLa(NCS)(CIO4)] and [NiL2(NCS)2]. Fhe thermal dissociation of the hexakis complexes resulted in tetrahedral [CoI,~]X2 complexes where X is a non-coordinating anion like CIO4 or BF4 and pseudoletrahedral [CoL2Xe] complexes where X is a coordinating anion like CI, Br, I, NCS and 6-coordinate tetragonal {NiL~Xd (C = CI, Br, I) complexes, where the anions are believed to occupy the axial positions. Nickel(II) formed only two tetrahedral complexes with this ligand demonstrating the fact that nickel(lI) shows a marked preference for an octahedral rather than a tetrahedral environment. In all the complexes it is believed that the methyl group is placed at position 5 of the 5-membered heterocyclic imidazole ring as in Cu(5-methyl imidazoleh (N()~):~ complex[481. A~kmm'ledgement--Thanks arc due to M/s BASF, Ludwigshafen. West t~ernmny for a sample ol 4-methyl imidazole. REFERENCES
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