The “Hidden” Reductive [2+2+1]‐Cycloaddition Chemistry of 2‐Phosphaethynolate Revealed by Reduction of a Th‐OCP Linkage

Abstract

The reduction chemistry of the newly emerging 2-phosphaethynolate (OCP) À is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(Tren TIPS )(OCP)] ( 2, Tren TIPS = [N(CH 2 CH 2 NSiPr i 3 )] 3À ), with RbC 8 via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(Tren TIPS )} 6 (m-OC 2 P 3 ) 2 (m-OC 2 P 3 H) 2 Rb 4 ] (5) featuring four five-membered [C 2 P 3 ] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(Tren TIPS )-(m-ORb)} 2 ] (6) and the known cyclometallated complex [Th{N(CH 2 CH 2 NSiPr i 3 ) 2 (CH 2 CH 2 SiPr i 2 CHMeCH 2 )}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2-phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP) À , rather than cycloaddition products that then decompose with [Th(Tren TIPS )O] À essentially acting as a protecting then leaving group. In contrast, when KC 8 or CsC 8 were used the phosphinidiide C À H bond activation product [{Th(Tren TIPS )}Th{N(CH 2 CH 2 NSiPr i 3 ) 2 [CH 2 CH 2 -SiPr i 2 CH(Me)CH 2 C(O)m-P]}] (3) and the oxo complex [{Th(Tren TIPS )(m-OCs)} 2 ] (7) were isolated.