Abstract
Reactions of N-methylglycine (HMeGly), N-ethylglycine-hydrochloride (H2EtGlyCl) and N-propylglycine-hydrochloride (H2PrGlyCl) with cobalt(ii), nickel(ii) and copper(ii) ions in aqueous solutions resulted in ten new coordination compounds [Co(MeGly)2(H2O)2] (1), [{Co(MeGly)2}2(μ-OH)2]·2H2O (1d), [Cu(MeGly)2(H2O)2] (2α), [Co(EtGly)2(H2O)2] (3), [Ni(EtGly)2(H2O)2] (4), [Cu(μ-EtGly)2]n (5p), [Co(PrGly)2(H2O)2] (6), [Ni(PrGly)2(H2O)2] (7), and two polymorphs of [Cu(PrGly)2(H2O)2] (8α and 8β). Compounds were characterized by single-crystal and powder X-ray diffraction, infrared spectroscopy, thermal analysis and X-band electron spin resonance (ESR) spectroscopy. These studies revealed a wide range of structural types including monomeric, dimeric and polymeric architectures, as well as different polymorphs. In all monomeric compounds, except 2α, and in the coordination polymer 5p hydrogen bonds interconnect the molecules into 2D layers with the alkyl chain pointing outward of the layer. In 2α and in the dimeric compound 1d hydrogen bonds link the molecules into 3D structures. 1d with cobalt(iii), and 4 and 7 with nickel(ii) are ESR silent. The ESR spectra of 1, 3 and 6 are characteristic for paramagnetic high-spin cobalt(ii). The ESR spectra of all copper(ii) coordination compounds show that the unpaired copper electron is located in the dx2−y2 orbital, being in agreement with the elongated octahedral geometry.
Highlights
N-Alkylated-a-amino acids are present in nature and their biocatalytic properties, as well as chemical syntheses, are widely investigated.[1,2,3] They are useful building blocks in peptide science and have found application in structure–activity relationship studies.[4,5,6,7] The simplest modi cation of an amino acid is by N-methylation, so probably the most intensively investigated N-alkylated-a-amino acids are N-methyl-amino acids, especially N-methylglycine, N,N-dimethylglycine and N,N,N-trimethylglycine.[8]
As a part of our ongoing research on synthesis, structural and magnetic characterization as well as biological activity of molybdenum, cobalt, nickel and copper coordination compounds with amino acids, amides and their derivatives[27,28,29,30,31,32,33,34,35,36,37,38,39] we report synthesis, structural and magnetic characterization of
Cobalt(II) and nickel(II) gave analogous monomeric coordination compounds of the general formula [M(RGly)2(H2O)2] (M 1⁄4 Co, Ni; R 1⁄4 methyl, ethyl, or propyl) with yields increasing with the elongation of the hydrocarbon chain R, probably due to their lower solubility
Summary
N-Alkylated-a-amino acids are present in nature and their biocatalytic properties, as well as chemical syntheses, are widely investigated.[1,2,3] They are useful building blocks in peptide science and have found application in structure–activity relationship studies.[4,5,6,7] The simplest modi cation of an amino acid is by N-methylation, so probably the most intensively investigated N-alkylated-a-amino acids are N-methyl-amino acids, especially N-methylglycine (sarcosine), N,N-dimethylglycine and N,N,N-trimethylglycine (betaine).[8] N-Methylation can be useful for conformational studies since introduction of N-methyl groups promotes conformational constraints and can improve pharmacokinetic properties of some peptides.[9,10,11] NMethylglycine is currently used as a dietary supplement, as Structurally characterized coordination compounds with Nalkylated amino acids, especially those containing longer hydrocarbon chains at the amino nitrogen atom are quite rare.[15] To the best of our knowledge, no coordination compounds with N-ethylglycine and N-propylglycine have been structurally characterized up to now. It was shown that the N-methylglycine moiety can occur in different forms: as an anion, a zwitterion or a cation. In the anionic form, the N-methylglycinato moiety acts as a bidentate ligand with O and N atoms involved in metal coordination.[16,17,18,19,20] In the case of the cationic and zwitterion forms, it acts as a monodentate, bidentate or bridging ligand.[21,22,23,24,25,26] Such coordination compounds may show different magnetic properties, depending on the metal oxidation state, local geometry around the metal center, metal-to-metal separation, bridging ligands, dimensionality of the complexes and non-covalent interacations
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