The Substituent-Effect in NHCP-Plumbylenes with a Phosphaplumbene Character

This work explores the use of plumbylene-phosphinidenes to address the challenge of isolating P=Pb bonds. Herein, we report the synthesis of three N-heterocyclic carbene phosphinidene (NHCP) substituted chlorotetrylene dimers [(IDipp)PECl]₂ (E=Ge, Sn, Pb; IDipp=C([N-(2,6-ⁱPr₂-C₆H₃)CH]₂)). Substituent attachment via salt metathesis (E=Sn, Pb) enables the isolation of NHCP-silyl-substituted stannylene (IDipp)PSn(SiTms₂SiTol₃) as well as NHCP-silyl- and NHCP-aryl-substituted plumbylenes (IDipp)PPb(SiTms₂SiTol₃) and (IDipp)PPb(^mTer) (^mTer=2,6-Mes₂C₆H₃, Tms=Trimethylsilyl). Access to these structures not only expands this chemistry to the heaviest of group 14 elements but also provides a deeper understanding of the substituent effect on NHCP-tetrylene bonding. Computational studies demonstrate that both silyl- and aryl-substituted species exhibit partial multiple bond character, with the silyl-substituted species showing higher bond orders and reduced polarization. This is further supported by the short P−E bond lengths observed in their solid-state structures. Finally, the formal [2+2] cycloaddition of diphenylketene to (IDipp)PPb(^mTer) provides experimental evidence for the ability of NHCP-plumbylenes to serve as synthetic equivalents of P=Pb double bonds.