M.Purificación Sánchez

Metal complexes of [1,2,4]triazolo-[1,5-a]pyrimidine derivatives

Abstract This article covers all the research work published until September 1998 about the coordination chemistry of 1,2,4-triazolo-[1,5-a]pyrimidine derivatives. The revision is mainly focused in the structural features of the compounds, obtained from single crystal X-ray diffraction. The references are classified according to the present metal atoms. These kind of ligands display a broad versatility when binding metal ions, monodentate binding through the endocyclic nitrogen atom at position 3 being however their main coordinating behaviour. Dimeric and polymeric structures are observed when a bidentate binding through N3–N4 or N3–N1, respectively, takes place. N1–O7 chelating binding mode has also been observed in a few cases. In some cases, dimers or polymers are also generated when the present auxiliary ligands are the ones that bridge the metal centres.

Structural and ab initio studies of AgAg bonding in the silver(I) dimer bis-μ-(5,7-dimethyl[1,2,4]triazolo[1,5-a]pyrimidine)dinitrato disilver(I)

Abstract This work is an attempt to address the issue of metal-metal bonding in d10 systems. The compound [Ag(dmtp)(NO3)]2, where dmtp represents the ligand 5,7-dimethyl[1,2,4]triazolo[1,5-a]pyrimidine, has been prepared, characterized by thermal analysis and IR spectroscopy and its structure has been solved by X-ray diffraction. The compound is triclinic, space group P 1 , a = 5.1295(4), b = 9.6009(7), c = 10.332(2) A , α = 103.73(1), β = 99.14(1), γ = 97.67(1)°, Z = 2, Dcal = 2.200 g cm−3, R = 0.029 for 4144 observed reflections. The compound exhibits a [Ag2(dmtp)2]2+ core structure with two silver atoms bridged by the dmtp ligands via N3 and N4 to give eight-membered Ag2N4C2 rings. The nitrato groups are coordinated in a semibridging and semichelating way (distances AgO, 2.816(2) A for non-bridging and 2.653(2) and 2.749(2) A for bridging oxygens). The AgAg separation is 3.058(1) A, only 0.17 A greater than in metallic silver. In order to make checking of the possible AgAg bonding, a theoretical analysis has been performed, based on the properties of the charge density distribution derived from ab initio MO calculations using STO-3G∗ and 3-21G(∗) wavefunctions, a significant direction interaction between the two metal centres having been found. Similar conclusions have been obtained for the model compound [Ag(NHCHNH)]2 after performing a geometry optimization and using improved LANL2DZ wavefunctions that take into account relativistic effect.

Molecular orbital calculations on 6-amino-1,3 dimethyl-5-nitrosouracil. Crystal structure of its copper(II) complex

Abstract Molecular orbital calculations have been performed for 6-amino-1,3-dimethyl-5-nitrosouracil (HADNU) and its monoanion with the MOPAC program, discussing the role of the MOs of the latter in coordination. The copper complex of this derivative, [Cu(ADNU)2(H2O)2], has been synthesized and its structure has been determined by X-ray diffraction. The metal atom lies on a crystallographic inversion centre that relates the two bidentate [through N(5) and N(6)] pyrimidine moieties and the two water molecules that complete the octahedral environment. The characterization of the compound has been completed by the study of its vis-NIR diffuse reflectance and IR spectra, thermal behaviour and magnetic susceptibility measurements.

Palladium( II)-purine nucleoside complexes: synthesis, characterization and X-ray determination

Twelve Pd(I1) complexes containing the purine nucleosides adenosine (Ado), guanosine (Guo), inosine (Ino) and xanthosine (Xao) and chloride, bromide or thiocyanate as auxiliary ligands have been isolated and characterized by spectroscopic techniques. Moreover, crystal structures of trun.s-[Pd(Ino)&l,] .5HZ0 and trans-[Pd(Ino)zBrl] .3H20 have been solved. Both compounds are almost symmetry-centred, which introduces important problems into the solving process. The structure is frans square planar with N(7) coordinated inosine.

4,5,6,7-tetrahydro-5,7-dioxo-[1,2,4]triazolo-[1,5-a]pyrimidine: characterisation and theoretical study

Abstract 4,5,6,7-tetrahydro-5,7-dioxo-[1,2,4]triazolo-[1,5-a]pyrimidine (H 2 tpO 2 ) has been synthesized as well as its monosodic salt, the crystal structure of the latter having been determined by X-ray diffraction. H 2 tpO 2 possesses a very acidic active methylene group, as indicated by its dissociation constant (p K al = 2.9). One of the hydrogen atoms in this group could then migrate to a basic position of the molecule, generating a zwitterionic or enolic form, nevertheless molecular orbital calculations point to the tautomer that keeps the CH 2 group as the most stable one. The anionic form HtpO − 2 is generated by the elimination of one of the protons at C6 rather than that attached to N4, as indicated by NMR data and also by the X-ray diffraction data of Na(HtpO 2 )·2H 2 O, this being consistent with theoretical calculations. The role of the molecular orbitals of HtpO − 2 in possible coordination to metal ions is discussed.

Molecular orbital study of 8-azaxanthine derivatives and crystal structure of 1,3-dimethyl-8-azaxanthine monohydrate

Abstract The crystal and molecular structure of 1,3-dimethyl-8-azaxanthine (HDAX) monohydrate has been determined by X-ray diffraction. The compound exhibits the N8-H tautomer in the solid state and hydrogen-bonded dimers are formed. Molecular orbital calculations have been performed for this compound, its analogues 1,3-dimethyl-2-thio-8-azaxanthine, 3-methyl-8-azaxanthine and 3-methyl-2-thio-8-azaxanthine and their anionic forms with the mopac program. The most stable tautomer is shown to be N7-H for the neutral compounds. Calculations have also been performed for the hydrogen-bonded dimer (HDAX·H2O)2, a stabilization energy of −19.76 kcal mol−1 having been found, which may explain why the N8-H tautomer is found in the solid state. The role of the molecular orbitals in possible coordination to metal ions is discussed for all the species.

First transition row metal complexes with 4,5,6,7-tetrahydro-5,7-dioxo-[1,2,4] triazolo- [1,5-a] pyrimidine

Abstract Complexes of 4,5,6,7-tetrahydro-5,7-dioxo- 1 , 2 , 4 triazolo- 1 , 5 pyrimidine (H2tpO2) with the divalent transition metals of the first transition series from Mn to Zn have been synthesized and characterized, their general formula being M (HtpO2) 2 · nH2O. The crystal structure of the manganese compound, [Mn (HtpO2) 2 (H2O) 2] , has been solved by single crystal X-ray diffraction, showing that the ligand binds bidentately through the exocyclic oxygen O7 and the endocyclic nitrogen N1. The deprotonation of the ligand takes place at the active methylene group at position 6, as it happened in Na (HtpO2) · 2H2O, and not at the N — H group at position 4. Infrared data suggest that the ligand binds in a similar fashion in the Fe, Co, Ni and Zn compounds but not in the copper one. This has been corroborated by powder X-ray diffraction, which indicates that all compounds except Cu are isostructural.

X-ray crystal structure of tetraaquabis (xanthosinato)nickel(II) hexahydrate

The complex [Ni(XsH−1)2(H2O)4]·6H2O (XsH−1: xanthosinate) was isolated from the reaction of nickel carbonate with xanthosine in aqueous solution. The octahedral coordination is supported by the reflectance measurements. Single-crystal X-ray diffraction studies show that the crystals belong to the triclinic P1 space group, with a = 7.152(1), b = 8.830(1),c = 13.783(1) Å, α = 82.024(1)°, β = 86.155(1)°, γ = 70.900(1)°, and D = 1.643 mg/m3, forZ = 1. Nickel is six coordinate with four water molecules, and two N7 atoms from xanthosinato ligands. A complicated hydrogen-bonding network is present, all possible donors taking part in it.

Metal complexes of theophylline-7-acetic acid. Crystal structure of a nickel(II) compound containing non-coordinated theophylline-7-acetate ion

Abstract The solids obtained by interaction of theophylline-7-acetic acid with the metal ions nickel(II), cobalt(II) and palladium(II) have been studied by standard thermal and spectroscopic methods. The structure of the compound formed with nickel(II), hexaaqua nickel(II) theophylline-7-acetate tetrahydrate, has been solved by X-ray diffraction, this being the first example of a metal salt of an anionic purine derivative in which there is not a direct metal-purine bond. Powder X-ray diffraction demonstrates that the cobalt compound is isostructural to the nickel one. A trans-square planar structure with the ligand bound to the palladium via N(9) is proposed for the palladium compound [PdCl2(theophylline-7-acetic acid)2].