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] (E=Ge, Sn, Pb; IDipp=C([N-(2,6-Pr-CH)CH])). Substituent attachment via salt metathesis (E=Sn, Pb) enables the isolation of NHCP-silyl-substituted stannylene (IDipp)PSn(SiTmsSiTol) as well as NHCP-silyl- and NHCP-aryl-substituted plumbylenes (IDipp)PPb(SiTmsSiTol) and (IDipp)PPb(Ter) (Ter=2,6-MesCH, 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(Ter) provides experimental evidence for the ability of NHCP-plumbylenes to serve as synthetic equivalents of P=Pb double bonds.