Photochemistry of fac-Pt(NO 2) 3Cl 32− complexes in water solutions

Abstract

The photochemical transformation of fac-Pt(NO 2) 3Cl 3 2− complex in water solutions and frozen matrices has been studied by laser flash photolysis, optical, ESR, and NMR spectroscopies. Upon steady-state photolysis of liquid water solutions, the fac-Pt(NO 2) 3Cl 3 2− absorption bands disappear in the optical spectrum with both the preservation of isosbestic point at 43 620 cm −1 and the appearance of a new absorption band with a maximum at 48 000 cm −1, belonging to Pt(II) complexes. Applying the NMR method to nuclei 195 Pt and 14 N , we have shown that the final photolysis products are mainly complexes [PtCl 3(NO 2)] 2−, Pt(NO 2) 2(H 2O) 2, and [Pt(NO 2) 6] 2−. The photolysis of frozen (H 2O+LiCl or H 2O+MgClO 4) matrices containing the fac-Pt(NO 2) 3Cl 3 2− complex yields a new wide absorption band with a maximum in a region of 30 000 cm −1 which belongs to the [PtCl 3(NO 2) 2(ONO)] 2− nitrito-isomer of the initial complex. A longer irradiation causes photodissociation of nitrito-isomer into a complex of trivalent platinum [PtCl 3(NO 2) 2] 2− and free NO 2 radical whose characteristic lines are manifested in the ESR spectrum. The laser flash photolysis shows that in solution at room temperature, the [PtCl 3(NO 2) 2(ONO)] 2− nitrito-isomer thermally dissociates in about 300 ns with an activation energy of 48.6±1.7 kJ mol −1. The [PtCl 3(NO 2) 2] 2− complex dissociates in about 7 μs giving the second free NO 2 radical and a [PtCl 3(NO 2)] 2− complex. This reaction has an activation energy of 37.6±1.5 kJ mol −1. It is shown that dissociation of the [PtCl 3(NO 2) 2] 2− complex is reversible and the NO 2 radical coordinates again with the [PtCl 3(NO 2)] 2− ion with a rate constant 4.2×10 9 M −1 s −1. In addition to the repeated coordination, the NO 2 radical also disappears in a reversible reaction of recombination (2 k rec=1.8×10 9 M −1 s −1) to give N 2O 4 molecule.