The methylamine analogues of Wolffram's red and Reihlen's green: syntheses, infrared, electronic, Raman, and resonance-Raman spectroscopy, and X-ray crystal structures of chain chlorine- and bromine-bridged platinum(II,IV) complexes of methylamine

Abstract

The syntheses and the electronic, i.r., Raman, and resonance-Raman spectra of the mixed-valence chain complexes [Pt(CH₃NH₂)₄][Pt(CH₃NH₂)₄X₂][ClO₄]₄, where X = CI or Br, are reported. The electronic spectra are characterised by intense, broad intervalence bands whose maxima are at 24 100 and 18 200 cm^–1, respectively. The i.r. spectra are near superpositions of those of the constituentions. The resonance-Raman spectra are dominated by long overtone progressions in ν₁, the symmetric X–Pt^IV–X stretch, of up to nine members for X = Cl and seven for X = Br. The ω₁ and x₁₁ values for each complex are 324.8 ± 0.5 and –0.91 ± 0.07 cm^–1, respectively, for X = Cl and 186.3 ± 0.4 and –0.14 ± 0.1 cm^–1 for X = Br. The Pt^IV–X stretching force constants are 2.08 and 1.55 mdyn Å^–1 for X = Cl and Br, respectively, and the X–Pt^IV–X interaction constants are 0.074 and 0.057 mdyn Å^–1, respectively. The excitation profiles for the ν, bands maximise 1500–2800 cm^–1 to the red of the Pt^II→ Pt^II intervalence band, as determined by transmission measurements. The crystal structures of the two complexes have been determined by single-crystal X-ray methods. In the bromide the structure contains ordered [⋯ Pt^II(CH₃NH₂)₄⋯ Br–Pt^II(CH₃NH₂)_4^–Br ⋯] chains; the Pt^IV–Br and Pt^II–Br distances are 2.468(3) and 3.219(4)Å, respectively, and the Pt^II–Br–Pt^IV angle is 165.4(3)°. The chloride structure contains an antiparallel disordering of the type of chain found in the bromide. The Pt^IV–Cl and Pt^II–Cl distances are 2.334(12) and 3.203(18)Å, respectively, and the Pt^II–Cl–Pt^IV angle is 170.5(6)°. Both the spectroscopic and the crystallographic evidence indicate that the platinum-ion valences are localised in these complexes.