Nitric oxide reactivity of copper(II) complexes of tris-(2- aminoethyl)amine and ethylenediamine
3.2 Results and discussions
3.2.1 Nitric oxide reactivity in acetonitrile
The NO reactivity of the complexes 3.1 and 3.2 were studied in acetonitrile. The spectral changes were monitored by UV-visible and X-Band EPR spectroscopy. In acetonitrile solution, the complexes, 3.1 and 3.2, exhibited d-d transition at λmax, 812 nm and 550 nm respectively. The deep blue solutions of the both the complexes, in dry and degassed acetonitrile, changed into transient green on exposure to NO gas and finally became colorless. The change was monitored by UV-visible spectroscopy (Figure 3.5). It was found that though there were no significant changes in the d-d transition band, the higher energy band was shifted from 274 nm to 291 nm
immediately after purging NO to the acetonitrilic solution of complex 3.1. In case of complex 3.2, also, there were no significant change in the d-d transition band, but change was observed in the UV region, only. The green intermediates were thermally unstable and we were unable to isolate or characterize it properly. This transient intermediate was found to be EPR silent in frozen state at 77 K. In FT-IR spectrum, the green intermediate, in case of 3.1, exhibited a sharp and strong NO frequency at 1650 cm-1, in acetonitrile (Figure 3.6).20 This frequency was found to diminish gradually and finally disappeared. Presumably, it was a [CuII-NO•] complex which was formed prior to the reduction of CuII.12-15, 18
The intensity of d-d band of the green intermediate was found to decrease with time and finally the disappearance of the d-d band indicated the complete reduction of CuII center of complex 3.1 to CuI by NO (Figure 3.7). The reduction process was monitored with stoichiometric amount of NO and it had been found that the reaction was 1:1 with respect to CuII and NO (Figure 3.7, inset). In case of complex 3.2, also, the reduction was confirmed by the disappearance of the d-d transition band at 550 nm (Figure 3.8).
The complexes 3.1 and 3.2 showed four line EPR spectra in acetonitrile solution.
After NO purging, the signals were found to be disappeared (Figures 3.9a and 3.9b for 3.1 and 3.2, respectively). This is, again, consistent with the reduction of CuII to CuI by NO.21, 22
The reduction of CuII centre had been further authenticated in case of complexes 3.1 and 3.2 by the single crystal structure determination of the reduced complex, [Cu(CH3CN)4]ClO4 (Figure 3.10).§
Figure 3.5: UV-visible spectra of complexes (a) 3.1 and (b) 3.2 in acetonitrile. (Blue trace represents the spectrum before purging NO and the red trace represents that immediately after purging NO).
Figure 3.6: Solution FT-IR spectrum of the green intermediate formed in the case of 3.1 in
Figure 3.7: UV-visible spectra of (a) complex 3.1 and (b) after reaction with NO in acetonitrile solvent. Inset: The enlarged visible region showing the gradual decrease in intensity of the d-d band with increasing NO concentration: (i) complex 3.1; (ii) with 0.1 equivalent; (iii) with 0.2 equivalent; (iv) with 0.3 equivalent; (v) with 0.4 equivalent ; (vi) with 0.5 equivalent; (vii) with 0.6 equivalent; (viii) with 0.7 equivalent and (ix) with 1.1 equivalent NO.
Figure 3.8: UV-visible spectra of complex 3.2 (blue trace) and after purging NO (red trace) showing the reduction of CuII to CuI.
Figure 3.9: X-band EPR spectra of the complexes (a) 3.1 and (b) 3.2 (blue trace represents EPR spectrum of the complex and red trace represents that of the complex after it’s reaction with NO).
Figure 3.10: ORTEP diagram of [Cu(CH3CN)4]ClO4 (50% thermal ellipsoid plot).
Cao et. al. reported the reduction of a series of copper(II) dithiocarbamates with NO in aqueous solution which resulted into the formation of air stable copper nitrosyl and dinitrosyl species.20 Detailed kinetics studies of the CuII/NO reactions are not much.
12-15. 18, 23 In this regard, Tran et. al. studied the NO reduction of the CuII complex, [Cu(dmp)2(H2O)]2+ (dmp, 2,9-dimethyl-1,10-phenanthroline), in aqueous solution and various mixed solvents.12-15 In methanol, the product of the [Cu(dmp)2(H2O)]2+
oxidation of NO was CH3ONO; in water, it was NO2-. The reaction did not occur in CH2Cl2 unless methanol was added, and in such solutions the reaction rate was linearly dependent on the concentration of alcohol added.
It is interesting to note that the reduction of CuII ion in complex 3.1 by NO in dry and degassed acetonitrile was accompanied with a concomitant nitrosation followed by diazotization at the terminal primary amine group of the ligand, L2 which resulted into the cyclization product, L2/ along with tri-protonated ligand, L2// as its perchlorate salt (Scheme 3.2). This can be attributed to the fact that the NO+ formed in the reduction process, reacted with the primary amine center of the ligand which resulted into diazotization followed by ring formation.
Scheme 3.2
The formation of L2/ was characterized by its microanalytical data, FT-IR, 1H-NMR,
13C-NMR and ESI-Mass spectroscopy. All the spectroscopic data were in well agreement with its structure (Appendix II, Figure A2.1- A2.4).
The formation of L2//-perchlorate was further been confirmed by the X-ray single crystal structure determination. The X-ray quality crystals were obtained from the reaction mixture itself on keeping it in freezer for two days. The ORTEP diagram of L2//-perchlorate is shown in figure 3.11. The crystallographic data is listed in appendix II, table A2.1. In the tri-protonated symmetrical L2//-perchlorate, central nitrogen is in tetrahedral geometry with a C-N-C bond angle of 109.2(3)° and N1-C1 bond length of 1.476(5) Å. The tables of selected bond angles and distances are given in appendix II (Table A2.2). The formation of L2//-perchlorate was confirmed by various spectroscopic studies, also (Appendix II, Figure A2.5- A2.7).
Figure 3.11: ORTEP diagram of L2//-perchlorate (50% thermal ellipsoid plot, hydrogen atoms are removed for clarity).
In case of complex 3.2, the reaction of NO afforded similar diazotization at the amine site resulting into the formation of product, L3/ (Scheme 3.3).
Scheme 3.3
The formation of L3/ was confirmed by the FT-IR, 1H-NMR, 13C-NMR, and ESI- Mass spectra (Appendix II, Figure A2.8- A2.11).