Subvalent Group 4B metal alkyls and amides. Part 5. The synthesis and physical properties of thermally stable amides of germanium(II), tin(II), and lead(II)

Abstract

A series of subvalent Group 4B metal amides of general formula M(NR¹R²)₂[(i) R¹= SiMe₃, R²= Bu^t; M = Ge, Sn, or Pb; (ii) R¹= R²= SiMe₃; M = Ge, Sn, or Pb; and (iii) R¹= R²= GeMe₃, SiEt₃, or GePh₃; M = Ge or Sn] has been prepared from the appropriate lithium amide and metal(II) halide. Under ambient conditions, the amides are pale yellow to red, thermochromic, diamagnetic, low-melting solids or liquids, and are soluble in hydrocarbons (C₆H₆ or C₆H₁₂) in which they are diamagnetic monomers. The lower homologues give parent molecular ions as the highest m/e species. Infrared spectra show a band at 380–430 cm^–1[ν_asym(MN₂)], and ¹H or ¹³C n.m.r. spectra are consistent with the bent-singlet formulation. In the visible region the compounds exhibit a band (364–495 nm) of moderate intensity (ε= 600–2 050 dm³ mol^–1 cm^–1 in n-C₆H₁₄) indicative of an allowed electronic transition. Photolysis of each diamide in n-hexane in the cavity of an e.s.r. spectrometer affords (a) the persistent (t_½ 5 min—3 months at 25° C) metal-centred radical Ṁ(NR¹R²)₃[(i) R¹= SiMe₃, R²= Bu^t, M = Ge or Sn; (ii) R¹= R²= SiMe₃ or GeMe₃, M = Ge or Sn; or (iii) R¹= R²= GeEt₃, M = Sn], (b) a lead mirror (for the lead amides), or (c) no sign of reaction (for the more bulky diamides). E.s.r. parameters have been derived from the isotropic spectra.