One-dimensional anionic radical chains based on M$_2$P$_2$ rings (M = Ti, Zr, Hf)

This work deals with the synthesis of metallocene-phosphorus based radical coordination polymers. The radical polymeric chains were synthesized by a single step reaction between commercially available group 4 metallocene dichlorides [Cp$_2$MCl$_2$] (M = Ti, Zr, Hf; Cp = $\eta$$^5$-C5 H$_5$), pentaphosphaferrocene [Cp*Fe($\eta^5$ -P$_5$ )] (Cp* = $\eta^5$-C$_5$Me$_5$) and potassium metal. These reactions selectively gave polymeric chains of radical, [K{Cp$_2$M}$_2${($\eta^4$ -P$_5$)FeCp*}2 ]n (M = Ti, Zr, Hf). The four membered P$_2$M$_2$ core is the first example of such a phosphorus-based ring with a radical anionic nature. Moreover, it is also a rare example of a main group element based radical polymeric chain. The radical nature of the metallopolymer is further established by EPR, XANES and quantum chemical calculations. The potassium ion acts as a linker ion between two [{Cp$_2$M}{($\eta^4$ -P$_5$)FeCp*}] fragments. The reaction of [K{Cp$_2$M}$_2${($\eta^4$-P$_5$)FeCp*}$_2$]$_n$ with [2.2.2]-cryptand in THF resulted in chain cleavage into charge separated monomeric complexes [(K[2.2.2]-cryptand){Cp$_2$M}$_2$ {($\eta^4$-P$_5$)FeCp*}$_2$] (M = Ti, Zr, Hf).