Biomimetic complexes of Co(II), Cu(II) and Ni(II) with 2-aminomethylbenzimidazole. EGA-MS characterization of the thermally induced decomposition
Graphical abstract
Introduction
Complexes that contain the imidazole ring of the histidine residue are useful model compounds of bioinorganic interest [1], [2], [3], [4], [5], [6], [7], [8]. The imidazole ring is an essential metal binding site in metalloproteins (one or more imidazole units are bound to metal ions in almost all copper- and zinc-metalloproteins or e.g., in nickel-containing urease) and thus has profound effects on their biological actions [9], [10]. It is also well known that the amino group acts as the primary anchor site for metal ions and, as such, is able to promote the stepwise deprotonation and subsequent coordination of other successive binding sites, leading to the formation of the hydrolytically stable, fused, five-membered chelate rings with M–N bonds. Thus, complexes formed between metal ions and different types of bioligands, namely heteroaromatic nitrogen bases, may be considered as models for substrate–metal ion–enzyme interactions and other metal ion mediated biochemical interactions [11]. Recently, benzimidazole derived drugs have received much attention owing to the fact that benzimidazole residue is a constituent of vitamin B12 which supports their potential use as therapeutics [12], [13]. The interest in 2-aminomethylbenzimidazole (2-AMBI) is also due to the fact that compounds containing this heterocycle have been shown to exhibit a broad spectrum of pharmacological activities [14], including a variety of antifungal [15], antibacterial [16], [17], antimicrobial [18], [19], antiamoebic [20], antiparasitic [21] and antitumor applications [22], [23]. Clinical examples include mebendazole and albendazole (antihelmintics) [24]. The antiviral activity of some 2-substituted benzimidazole derivatives is considered to be related to their ability to chelate trace metal ions in biological systems [25]. Additionally, 2-AMBI possesses two aromatic rings and one of them contains basic nitrogen (imidazole) and possesses π-accepting properties, which are expected to display a stability-enhancement due to the hydrophobic interaction with the substituted group of the amino acids or involved in the aromatic ring π–π stacking effects with purine and pyrimidine bases.
The scientific experience of the authors showed systematic behaviors of coordination compounds [26], [27], [28], [29], [30], enhanced by mass spectrometry [31], [32], [33], [34] and thermoanalytical techniques [35], [36], [37], [38], [39], [40], [41], [42].
To confirm the supposed general characteristic decomposition mechanism, the aim of this work was to study the thermally induced processes on six coordination compounds of general formula ML2X2 (where M = Co(II), Cu(II) or Ni(II), and X = Cl or Br). Complexes were precipitated following literature references [43], [44], [45] and characterized by elemental analysis and atomic spectroscopy. Evolved Gas Analysis performed by mass spectrometry (EGA-MS) was applied since this hyphenated technique is a very useful tool to solve analytical problems in several different fields [46], [47], [48], [49], [50], [51]. Experimental evidences proved the thermally induced decomposition mechanism.
Section snippets
Materials
2-Aminomethylbenzimidazole and the cobalt, copper and nickel chlorides and bromides were purchased from Aldrich and Merck. All the reagents were of A.R. grade and used without further purification. Several different procedures can be found in the literature to precipitate the complexes of general formula ML2X2, where M = Co(II), Cu(II) or Ni(II), and X = Cl or Br [43], [44], [45].
The generic reaction is shown in Scheme 1.
Several different attempts, following those reported in the literature, gave
Results and discussion
The results from the elemental analysis of the six precipitated complexes are reported in Table 1. Calculated and found element percent are in good agreement.
The thermally induced decomposition steps of the precipitated complexes were comparatively studied by mass spectrometry evolved gas analysis (EGA-MS). In Fig. 1, the thermoanalytical profiles of the chloride complexes are overlapped to compare the releasing steps when the purging flow is air.
The thermal decomposition is based on three main
Conclusions
Cobalt, Copper and Nickel chloride or bromide complexes with the 2-aminomethylbenzimidazole ligand were synthesized and characterized by elemental analysis and emission atomic spectroscopy.
Their thermally induced decomposition mechanism was studied by on-line coupling a mass spectrometer to a thermal analyzer. The resulting m/z traces from the Evolved Gas Analysis (EGA-MS) confirmed the decomposition mechanism.
Results showed that neither the different metal nor the different anion significantly
Acknowledgment
Authors acknowledge the help of Dr. L.W. Wo in the synthesis of the complexes.
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