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

The reduction chemistry of the newly emerging 2‐phosphaethynolate (OCP)(−) 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))](3−)), with RbC(8) via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(Tren(TIPS))}(6)(μ‐OC(2)P(3))(2)(μ‐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))(μ‐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)(−), rather than cycloaddition products that then decompose with [Th(Tren(TIPS))O](−) essentially acting as a protecting then leaving group. In contrast, when KC(8) or CsC(8) were used the phosphinidiide C−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)μ‐P]}] (3) and the oxo complex [{Th(Tren(TIPS))(μ‐OCs)}(2)] (7) were isolated.