Binuclear Platinum(II) Triazolopyrimidine Bridged Complexes. Preparation, Crystal Structure, NMR Spectroscopy, and ab Initio MO Investigation on the Bonding Nature of the Pt(II)···Pt(II) Interaction in the Model Compound {Pt₂[NHCHN(C(CH₂)(CH₃))]₄}

A condensation reaction between two cis-[PtCl2(Hmtpo)2] (where Hmtpo= 4,7-H-5-methyl-7-oxo[1,2,4]triazolo[1,5a]pyrimidine) molecules takes place in neutral aqueous media, giving [Pt2(μ-mtpo)4]·4H2O (1). The X-ray structure of the recrystallization product of 1 in DMSO:EtOH (1:1) (DMSO = dimethyl sulfoxide), namely, [Pt2(μ-mtpo)4]·2DMSO (2) has been determined. Compound 2 crystallizes in the orthorhombic space group Pbcn with unit cell dimensions a = 14.207(3) Å, b = 15.187(3) Å, c = 17.165(3) Å, and Z = 4. The molecular structure shows that the two Pt atoms are bridged by four mtpo ligands. Thus, it presents two face to face PtN4 units with a Pt···Pt 2.744(2) Å separation. Compound 1 has also been characterized by 1H and 195Pt NMR. The very short Pt(II)···Pt(II) contact suggests an interaction between the two metal centers, supported by the great deshielding observed for the platinum nuclei in the 195Pt NMR spectrum (δ = −2005 ppm) compared to a Pt(II) in a typical N4 environment. In order to make an approach to the possible bonding nature of the Pt(II)···Pt(II) interaction, a theoretical analysis has been performed on the basis of the properties of the electronic charge density distribution, derived from ab initio MO calculations for the model compound {Pt2[NHCHN(C(CH2)(CH3))]4} using RHF/LANL2DZ and B3LYP/LANL2DZ wave functions; both take into account relativistic effects and the second electronic correlation also. A significant directional interaction between the two metal centers has been found. Thus, a bond critical point appears between the two platinum nuclei, with a density of charge ρb = 0.056 e·bohr-3, which is half of that found for the platinum nitrogen bond. Moreover, a value of the energy density, Ed(rb) < 0 (Ed(r) = −0.0175 hartree·bohr-3), at this point, shows the bonding nature of the interaction.