Heteropolyhedral silver compounds containing the polydentate ligandN,N,O-E-[6-(hydroxyimino)ethyl]-1,3,7-trimethyllumazine. Preparation, spectral and XRD structural study and AIM calculations

Abstract

The oxime derived from 6-acetyl-1,3,7-trimethyllumazine (1) ((E-6-(hydroxyimino)ethyl)-1,3,7-trimethylpteridine-2,4(1H,3H)-dione, DLMAceMox) has been prepared and its molecular and crystal structure determined from spectral and XRD data. The oxime ligand was reacted with silver nitrate, perchlorate, thiocyanate, trifluoromethylsulfonate and tetrafluoroborate to give complexes with formulas [Ag₂(DLMAceMox)₂(NO₃)₂]_n (2), [Ag₂(DLMAceMox)₂(ClO₄)₂]_n (3), [Ag₂(DLMAceMox)₂(SCN)₂] (4), [Ag₂(DLMAceMox)₂(CF₃SO₃)₂(CH₃CH₂OH)]·CH₃CH₂OH (5) and [Ag(DLMAceMox)₂]BF₄ (6). Single-crystal XRD studies show that the asymmetrical residual unit of complexes 2, 3 and 5 contains two quite different but connected silver centers (Ag1–Ag2, 2.9–3.2 Å). In addition to this, the Ag1 ion displays coordination with the N5 and O4 atoms from both lumazine moieties and a ligand (nitrato, perchlorato or ethanol) bridging to another disilver unit. The Ag2 ion is coordinated to the N61 oxime nitrogens, a monodentate and a (O,O)-bridging nitrato/perchlorato or two monodentate O-trifluoromethylsulfonato anions. The coordination polyhedra can be best described as a strongly distorted octahedron (around Ag1) and a square-based pyramid (around Ag2). The Ag–N and Ag–O bond lengths range between 2.22–2.41 and 2.40–2.67 Å, respectively. Although the structure of 4 cannot be resolved by XRD, it is likely to be similar to those described for 2, 3 and 5, containing Ag–Ag units with S-thiocyanato terminal ligands. Finally, the structure of the tetrafluoroborate compound 6 is mononuclear with a strongly distorted tetrahedral AgN₄ core (Ag–N, 2.27–2.43 Å). Always, the different Ag–N distances found clearly point to the more basic character of the oxime N61 nitrogen atom when compared with the pyrazine N5 one. A topological analysis of the electron density within the framework provided by the quantum theory of atoms in molecules (QTAIM) using DFT(M06L) levels of theory has been performed. Every Ag–Ag and Ag–ligand interaction has been characterized in terms of Laplacian of the electron density, ∇²ρ(r), and the total energy density, H(r).