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Stabilization of aliphatic diazonium ions and/or their precursor at room temperature by coordination to [IrCl5]2-. Synthesis, mechanism and characterization
Simposio; IV Simposio Latinoamericano de Química de Coordinación y Organometálica (SILQCOM); 2013
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Primary nitrosamines (RN(H)N=O) and their tautomeric isomers, diazoic acids (RN=NOH), are important intermediates in the deamination of DNA bases[1] and in the formation of diazonium salts [2]. In acidic or neutral medium, these compounds rapidly produce unstable diazonium ions (RNN+) by loss of hydroxide ion [3]. Aromatic diazonium ions are known for a long time, and show multiple applications in synthesis, covering a large area of organic chemistry [4]. On the contrary, the closely related diazonium ions attached to sp3 carbons have been very elusive, and only a few examples observed at low temperature are reported [5]. As an alternative to stabilize those species, nitrosyl complexes have been previously used in our group. The reaction of aromatic amines with the complex [Ru(bipy)2(NO)Cl]2+ (bipy = 2,2'- bipyridine) shows an important enhance in the stability of the aromatic diazonium ions compared to their free form due to the coordination to the metal. When aliphatic amines are used, a relative improvement of their stability is observed but there is no spectroscopic evidence of the formation of the diazonium ions in solution [6]. On the other hand, the addition of nucleophiles to the nitrosyl ligand in [IrCl5(NO)]ˉ, which is probably the most electrophilic known to date [7], became a tool for the facile obtention of coordinated N(O)XR species (X = NH, S, C and R = alkyl, aryl) that are unstable in their free form [8]. The study of the decomposition pathway of the aliphatic primary nitrosamines (X = NH above) coordinated to [IrCl5]2- by acid reaction, accounted for the remarkable stability of the derived diazonium ions (intermediates of the process) due to the absence of rearrangement and elimination products. Such results together with DFT calculations showed us which are the best candidates regarding the aliphatic diazonium isolation. We present in this work spectroscopic evidence of the diazonium ions and/or their precursors derived from primary nitrosamines. Recent experiments suggest that the oxidation product can be either the expected diazonium (RCH2-NN+) or the diazene (RCH2-NN-X, with X = Cl-, Nu-, or solvent). Characteristics signals from those species were observed in the 1H and 13C NMR spectra in DMSO_d6 at room temperature. The species were also characterized by UV-visible, where for example for the n-butyldiazonium the λmax in DMSO is 530 nm. Electrochemical studies of the involved species showed that the oxidation of primary nitrosamines gives place to similar spectroscopic signals. Ciclyc-Voltametry (CV) in DMSO for n-butyl and bencylnitrosamines showed only one reversible oxidation band with E1/2 = 0.30 V and 0.34 V vs. FeCp2/FeCp2+ respectively. 1H NMR in DMSO-d6 after oxidation confirmed the presence of the diazonium ions and/or their precursors and the SEC-UV experiments exhibited the increase of the product band at about 530 nm together with the disappearance of the signal around 325 nm assigned to the nitrosamine. Chemical oxidation of the coordinated nitrosamines was performed with two different oxidation agents: tris(4-bromophenyl)aminiumhexachloridoantimonate usually known as "Magic Blue" and the ion [Ce(NO3)6]2+. In agreement with the other experiments, the identity of the products was concluded to be the same as in the electrochemical experiments by NMR and UV-vis in DMSO. EPR measurements in acetonitrile solution after the addition of the oxidizing agent confirmed the presence of OH* radicals, due to a narrow triplet signal regarded the N coupling of the radical with the added spin trap (PBN = alpha-phenyl N-tertiary-butyl nitron). Such evidence could be an indication of the mechanism associated with the species generated by oxidation of aliphatic primary nitrosamines. We have recently isolated the chemical oxidation product of the benzyl nitrosamine in solid state. We obtained a dark violet solid by adittion of [Ce(NO3)6]2+ to a methanol solution of the benzyl nitrosamine and precipitation through anti-solvent techniques. This will allow us in the near future to conclude with the characterization, and thus elucidate the identity of the compound. References. [1] Caulfield, J.; Wishnok, J.; Tannenbaum, S. J. Biol. Chem. 1998, 273, 12689. [2] (a) Mohrig, J. R.; Keegstra, K. J. Am. Chem. Soc. 1967, 89, 5492. (b) Berner, D.; McGarrity, J. F. J. Am. Chem. Soc. 1979, 101, 3135. [3] (a) Ho, J.; Fishbein, J. C. J. Am. Chem. Soc. 1994, 116, 6611. (b) Finneman, J. I.; Fishbein, J. C. J. Am. Chem. Soc. 1996, 118, 7134. Finneman, J. I.; Ho, J.; Fishbein, J. C. J. Am. Chem. Soc.1993, 115, 3016. [4] Patai, S.The Chemistry of Diazonium and Diazo Groups; Wiley:New York, 1978. [5] Caulfield, J.; Wishnok, J.; Tannenbaum, S. J. Biol. Chem. 1998, 273, 12689. [6] Doctorovich, F.; Di Salvo, F. Acc. Chem. Res. 2007, 40, 985 and references therein [7] Di Salvo, F.; Escola, N.; Scherlis, D. A.; Estrin, D. A.; Bondía, C.; Murgida, D.; Ramallo-López, J. M.; Requejo, F. G.; Shimon, L.; Doctorovich, F. Chem. Eur. J., 2007, 13, 8428. [8] For N-nitrosamines: (a) Di Salvo, F.; Estrin, D. A.; Leitus, G.; Doctorovich, F. Organometallics, 2008, 27, 1985. For C-nitroso compounds: (b) Escola, N.; Llebaría, A.; Leitus, G.; Doctorovich, F. Organometallics, 2006, 25, 3799. For S-nitrosothiols: (c) Perissinotti, L. L.; Leitus, G.; Shimon, L.; Estrin, D.; Doctorovich, F. Inorg. Chem. 2008, 47, 4723