EPR spectroscopic characterization of persistent germyl-substituted Pb(iii)- and Sn(iii)-radicals

Abstract

In this report we present the synthesis and the detailed electron paramagnetic resonance (EPR) spectroscopic characterization of novel trivalent lead- and tin-based radicals comprising sterically demanding germyl substituents. The investigated radicals are derived from the recently reported trihypersilyl-substituted tetryl radicals ˙PbHyp₃ and ˙SnHyp₃. The tetryl radicals ˙Pb(Ge(SiMe₃)₃)₃ (8), ˙Pb(Ge(SiMe₃)₃)₂Si(SiMe₃)₃ (9), ˙PbGe(SiMe₃)₃(Si(SiMe₃)₃)₂ (10), and ˙Sn(Ge(SiMe₃)₃)₃ (11) show substitution patterns derived from stepwise (9, 10) or complete (8, 11) substitution of hypersilyl groups (Hyp = Si(SiMe₃)₃) in ˙PbHyp₃ and ˙SnHyp₃ by homologous hypergermyl groups (Hge = Ge(SiMe₃)₃). They were generated through oxidation of the corresponding potassium tetranides KPbR₃ and KSnR₃ (R = Hyp, Hge), which in turn had been synthesized employing nucleophilic addition of KHyp or KHge to PbHyp₂ or to the novel tetrylenes PbHge₂ (1) and SnHge₂ (12). The gained EPR spectroscopic data give insights into the influence of the substitution pattern on the geometric and electronic properties of the lead-centered species. With an increasing number of germyl substituents, the spin–orbit coupling to the central atom increases resulting in larger g_iso-values and larger g-anisotropies while the ²⁰⁹Pb hyperfine splitting constants A_iso decrease. These decreasing splitting constants are indicative of a diminished s-character of the singly occupied molecular orbital (SOMO) and to a molecular geometry running from slightly pyramidal to almost planar. Interestingly, already one germyl substituent (for radical 10) dominates most of the mentioned properties. For stannyl radical 11 a similar trend is seen for the spin–orbit coupling, while the ^117/119Sn hyperfine splittings unexpectedly increase from SnHyp₃ to SnHge₃.