Indian Journal of Chemistry Vol. 24A. August 1985, pp.696-698 PMR & 13CMR Spectra of Some Imidazole Complexes of Cobaloximes JAYANTA K DAS & KAILASH C DASH· Department of Chemistry. Utkal University. Vani Vihar, Bhubaneswar 751004 Received 22 August 1984; rerised and accepted II March 1985 Imidazole (Irn) and its 1- and 2-substituted derivatives react with cobalt(lJ) chloride and dimethylglyoxime (DH z ) in presence of air to form cobaloxime complexes of the type trans-[Co(DH)z(JmXCI)]. The 'H NMR (PMR) and the 'JC NMR (CMR) spectra of four of these complexes and of imidazole and free DHz ligand have been studied in DMF medium by Fourier Transform technique. Cobaloximes are complexes containing the Co(DH); moiety, where DH is the monoanion of dimethylgly- oxime.'. They are known to simulate closely the reactions of vitamin B 12 (refs. 2, 3) and have become leading models in the field of vitamin BI2 chemistry. The five-membered heterocyclic ligand, imidazole, is present in many biological systems (e.g. in the histidyl residues of proteins) and provides a potential binding site for metal ions 4 • 5 . The biological significance of cobaloximes and imidazoles prompted us to synthesise imidazole complexes of cobaloximes of the type trans- [Co(DHh(ImXCl)] and study their stereochemical and biochemical characteristics. Since the NMR technique is capable of providing more detailed information about the various ligands, it may prove valuable in deciphering their role in vital biochemical processes. Keeping this in view, we have synthesised the cobaloximes and studied their 1Hand 13C NMR spectra. Synthesis of complexes The complexes were synthesised from CoCi 2 .6H 2 0, dimethyl glyoxime(DH 2 ) and the imidazole ligand and characterised by elemental analyses, electronic and IR spectra as well as conductivity measurements in non- aqueous medium". 1H NMR (PMR) spectra The 1H NMR spectra of imidazole, dimethylgly- oxime and the complexes were recorded in DMF with TMS as the external reference on a Varian FT-80A spectrometer operating on Fourier mode using 5 mm insert tubes at a spin rate of 20 rps. i3C NMR (CMR) spectra Full spectral width (6000 Hz) random-noise 1H- decoupled 25 MHz i3C NMR spectra were recorded 696 on the JEOL FX-IOO instrument using the Fourier Transform technique which allows measurement of samples with 13C in natural abundance (1.1 %). The solvent was DMF and the spectra were recorded at ambient temperature. TMS served as the internal standard. The assignment of ligand signals was made considering the literature values, the difference in intensity based on the Nuclear Overhauser Effect (NOE) and trends in the shifts. We have earlier reported" the synthesis, con- ductivity data, electronic and IR spectra and thermoanalytical data of a series of six-coordinated, diamagnetic and pseudo-octahedral cobalt(III) complexes of the type trans-[Co(DH)iImXCl)]. The NMR spectra offer great help in elucidating the stereochemistry 7, as the two possible conformations, i.e. the cis and trans are expected to show different resonances, and as the effect of variation of one ligand on the NMR spectra of other ligands can be studied". The PMR shifts (values in b, ppm) of the chlorocobaloxime complexes (structure III) clearly show (Table I) that on coordination the signal due to CH 3 group of DH2 (structure I) at 1.98 undergoes a clear downfield shift by about 0.4 to 2.36-2.38. In all the complexes this signal appears as a very sharp singlet indicating the equivalence of four methyl groups in the complexes and presence of only one chemical entity. Our own work 9 and the work by others!" show that in some thiocyanste cobaloximes there are two distinct CH 3 signals indicating the presence of two separate chemical species in solution. CH3 HON~~/ \ /~~NOH H~a (OH2l (J) (lml (II) L a l-Etlm, 1-Allm. 2-Melm 82-iPrlm (III)
Transcript
Indian Journal of ChemistryVol. 24A. August 1985, pp.696-698
PMR & 13CMR Spectra of Some ImidazoleComplexes of Cobaloximes
JAYANTA K DAS & KAILASH C DASH·Department of Chemistry.
Utkal University. Vani Vihar, Bhubaneswar 751004Received 22 August 1984; rerised and accepted II March 1985
Imidazole (Irn) and its 1- and 2-substituted derivatives react withcobalt(lJ) chloride and dimethylglyoxime (DHz) in presence of air toform cobaloxime complexes of the type trans-[Co(DH)z(JmXCI)].The 'H NMR (PMR) and the 'JC NMR (CMR) spectra of four ofthese complexes and of imidazole and free DHz ligand have beenstudied in DMF medium by Fourier Transform technique.
Cobaloximes are complexes containing the Co(DH);moiety, where DH is the monoanion of dimethylgly-oxime.'. They are known to simulate closely thereactions of vitamin B12 (refs. 2, 3) and have becomeleading models in the field of vitamin BI2 chemistry.The five-membered heterocyclic ligand, imidazole, ispresent in many biological systems (e.g. in the histidylresidues of proteins) and provides a potential bindingsite for metal ions4
•5. The biological significance of
cobaloximes and imidazoles prompted us to synthesiseimidazole complexes of cobaloximes of the type trans-[Co(DHh(ImXCl)] and study their stereochemical andbiochemical characteristics. Since the NMR techniqueis capable of providing more detailed informationabout the various ligands, it may prove valuable indeciphering their role in vital biochemical processes.Keeping this in view, we have synthesised thecobaloximes and studied their 1Hand 13C NMRspectra.
Synthesis of complexesThe complexes were synthesised from CoCi2.6H20,
dimethyl glyoxime(DH2) and the imidazole ligand andcharacterised by elemental analyses, electronic and IRspectra as well as conductivity measurements in non-aqueous medium".
1H NMR (PMR) spectraThe 1H NMR spectra of imidazole, dimethylgly-
oxime and the complexes were recorded in DMF withTMS as the external reference on a Varian FT-80Aspectrometer operating on Fourier mode using 5 mminsert tubes at a spin rate of 20 rps.
on the JEOL FX-IOO instrument using the FourierTransform technique which allows measurement ofsamples with 13C in natural abundance (1.1 %). Thesolvent was DMF and the spectra were recorded atambient temperature. TMS served as the internalstandard.
The assignment of ligand signals was madeconsidering the literature values, the difference inintensity based on the Nuclear Overhauser Effect(NOE) and trends in the shifts.
We have earlier reported" the synthesis, con-ductivity data, electronic and IR spectra andthermoanalytical data of a series of six-coordinated,diamagnetic and pseudo-octahedral cobalt(III)complexes of the type trans-[Co(DH)iImXCl)]. TheNMR spectra offer great help in elucidating thestereochemistry 7, as the two possible conformations,i.e. the cis and trans are expected to show differentresonances, and as the effect of variation of one ligandon the NMR spectra of other ligands can be studied".
The PMR shifts (values in b, ppm) of thechlorocobaloxime complexes (structure III) clearlyshow (Table I) that on coordination the signal due toCH3 group of DH2 (structure I) at 1.98 undergoes aclear downfield shift by about 0.4 to 2.36-2.38. In allthe complexes this signal appears as a very sharpsinglet indicating the equivalence of four methylgroups in the complexes and presence of only onechemical entity. Our own work9 and the work byothers!" show that in some thiocyanste cobaloximesthere are two distinct CH3 signals indicating thepresence of two separate chemical species in solution.
CH3HON~~/
\/~~NOH
H~a(OH2l
(J)
(lml
(II)
Lal-Etlm, 1-Allm. 2-Melm 82-iPrlm
(III)
NOTES
Table 1- 1H NMR Shifts of [Co(DH}z(LXCl)] Complexes?"
(a) Measurements in DMF at ambient temperature. All values in ppm downfield to Me4Si.(P) Present work.
The imidazole protons also undergo a considerabledownfield shift. On coordination to cobalt(III) centrethe equivalence of H-4 and H-5 (structure II) inimidazole is lost and separate signals appear, andfurther, the electron withdrawal by central cobalt(III)leads to a marked deshielding of the protons adjacentto the pyridine nitrogen (N-WI. The situation isindeed parallel to that observed on protonation of aneutral, unsubstituted imidazole species relative to anon-protonated species I2.J 3. The H-2 signal is seen inI-substituted imidazoles at 7.4, shifted downfield byabout 0.4 from the free ligand value. The H-4 and H-5in complexes appear around 7.3 and 6.9 respectively.The signals due to various 1- and 2-substituents havebeen also assigned.
The CMR shifts of the chlorocobaloxime complexescontaining imidazole as an axial ligand are presentedin Table 2. The 13CNMR is much more sensitive to
structural variation in molecules than the IH NMR.As observed in the IH NMR, coordination to themetal centre does create some striking changes in the13C NMR spectra also. The oxime CH3 carbon isconsiderably shifted downfield on coordination andappears invariably as a very sharp signal at 12.4 in allthe complexes. The imine carbon appears around 152uniformly, which is lightly upfield compared to thevalue for free DH2. The 13C NMR signals of theimidazole ligands also undergo a downfield shift oncomplexation and C-4 and C-5 now resonate atdifferent fields. The C-2, C-4 and C-5 of the imidazolering resonate at 138, 127 and 122 respectively. Thesubstituents at 1- and 2-positions of the imidazole ringhave also been appropriately assigned.
The results of this investigation establish a cleartrans-structure for the imidazole complexes of thechlorocobaloximes [Co(DHMlmXCI)]. The coordi-
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INDIAN J. CHEM., VOL. 24A, AUGUST 1985
DMFCH3(DH) TMSDMF
-CoN(DH)
R~~~t
U'> U'> cS'" " 0
N N6
U'>
"N C-4 R(CH'P (1m) C-5 (1m)
(Im)
C-2. M
~m) ~ N M
2 M
~~ 'r,-
I I
175 150 125 I 50 25 0
Fig. I - IJC NMR of [Co(DH)2(2-iPrIm) (C!)] in DMF.
nation of both Iigands has been firmly established andfurther it is shown that the imidazole derivatives arebonded to the metal atom through the pyridinenitrogen (N-3). The probability of the presence of anyother distinct chemical species in solution is also ruledout. Since cobalt(III) can be introduced into a varietyof metalloprotein, the results are expected to provide amodel not only for the cobaIt(III) derivatives, but alsofor the closely related zinc(II) systems 14.15.
698
The authors are indebted to Prof. A. Schmidpeter,Munich University, Munich for his ungrudging help ingetting the FT NMR recorded. One of the authors(JKD) thanks the UGc, New Delhi for a TeacherFellowship and the Government of Orissa for leave ofabsence.
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