The synthesis, X-ray crystal structure, and electronic and vibrational characterisation of the mixed-valence, iodine-bridged complex [PtII(dien)I][PtIV(dien)I3]I2

Abstract

The complex [Pt(dien)I][Pt(dien)I₃]I₂(dien = diethylenetriamine) can be obtained as bronze crystals by partial oxidation of [Pt(dien)Cl]Cl with iodine in dilute hydroiodic acid. X-Ray structure analysis of the complex shows it to belong to the orthorhombic space group Pmcn, with a= 8.797(1), b= 12.028(2), c= 22.863(3)Å, and Z= 4. Refinement on 1 635 reflections yielded final discrepancy indices R= 0.043 and R′= 0.060. The structure consists of ordered I–[Pt^IV(dien)I]–I ⋯[Pt^II(dien)I]⋯ chains, with significant bending (ca. 21°) about the bridging iodine atom. Pairs of equatorial Pt^IV(dien) I and Pt^II(dien)I units are oppositely oriented when viewed along the chain direction. The equatorial Pt–N bonds cis to the equatorial Pt–I bond in the Pt^II unit have lengths essentially equal to those of the corresponding bonds in the Pt^II unit (ca. 2.09 Å), but the Pt–N bond trans to the equatorial Pt–I bond is significantly shorter in the Pt^II unit [1.97(1)Å] than in the Pt^IV unit [2.03(1)Å]. Likewise, the equatorial Pt^II–I bond length [2.596(1)Å] is less than the equatorial Pt^IV-I bond length [2.650(1)Å]. The electronic spectrum of the complex is dominated by an intense, broad Pt^IV(d_z²)â†� Pt^II(d_z²) intervalence band, centred at 15 500 cm^–1, together with other bands at higher energies. The resonance Raman spectrum of the complex displays a long overtone progression, v₁ν₁ to v₁= 7, and a combination band progression (v₁ν₁+ν₂) to v₁= 4, where ν₁ and ν₂ are the symmetric ν(Pt–I)_ax. and ν(Pt–I)_eq. modes, respectively. The complex represents the first halogen-bridged mixed-valence chain complex to be fully characterised in which each unit carries a +1 charge only, and the first to be characterised in which each platinum ion is co-ordinated to a tridentate ligand.