From Persistent Triplet Carbenes to Persistent High-Spin Polycarbenes

Abstract

Abstract Triplet carbenes are regarded as one of the most effective spin sources for organic ferromagnetic materials since the magnitude of the exchange coupling between the neighboring centers is large. However, those systems have two disadvantages that prevent their use as practical magnetic materials. First, a triplet carbene unit is highly unstable and lacks the stability for practical applications under ambient conditions. Second, diazo groups, which are precursors for triplet carbenes, are generally labile and, hence, cannot be used as building blocks to prepare a more complicated poly(diazo) compound. After great efforts to stabilize the triplet carbenes, we have succeeded in preparing fairly stable triplet carbenes that survive for days. We found that a diazo precursor for a persistent triplet carbene was also persistent and, hence, can be further modified into a more complicated diazo compound that can generate persistent high-spin polycarbenes. In this account, we would like to summarize our efforts along this line. We employed the following three approaches to prepare poly(diazo) compounds; (1) synthesis of dendritic molecules having peripheral diazo groups starting from diazo compounds, (2) preparation of a poly(phenylacetylene) with a diazo unit using Rh-complex-catalyzed polymerization of diazo compounds with p-ethynyl substituents, and (3) preparation of a polymer chain as a result of complexation of the pyridine ligand on diazo compounds with coordinatively unsaturated metal ions. We also characterized the magnetic properties of photoproducts obtained by photolysis of those poly(diazo) compounds. Although spin-states observed for those photoproducts were not so high, the results suggest that our approach using persistent triplet carbenes will eventually lead us to a persistent high-spin polycarbenes by taking advantage of the stability of our diazo compounds.