Molecular assemblies based on strong axial coordination in metal complexes of saddle-distorted dodecaphenylporphyrins

In this mini-review, we have highlighted our works on metal complexes having saddledistorted dodecaphenylporphyrin (DPP) and its derivative as ligands in the light of enhancement of the Lewis acidity of a metal center coordinated by the porphyrin. The important point through this minireview is ill-overlap of the out-of-plane lone pairs of pyrrole nitrogen atoms with σ-orbitals of the metal center bound to the saddle-distorted porphyrin core. The enhanced Lewis acidity of the central metal ions enabled us to construct stable molecular complexes through axial coordination using metal-DPP (M(DPP)) moieties (M = Mo^V or Sn^IV) and molecular or ionic entities with Lewis-basic coordination sites, including Keggin-type polyoxometallates (POM), which are known to have weak Lewis basicity and thus hard to coordinate to metal ions. A discrete 1:2 complex with a Ru-substituted POM performs catalytic substrate oxidation reactions in organic solvents. A 1:1 complex between Sn^IV(DPP) and a Keggin-type POM exhibited photoinduced electron transfer, in which the Sn^IV(DPP) moiety acts as an electron donor and the POM as an electron acceptor. Besides POM, other electron acceptors, including μ₃-oxo trinuclear Ru^III clusters and anthraquinone, having carboxyl groups as a linker unit also formed stable complexes with DPP-metal complexes as axial ligands to perform photoinduced electron transfer. Successful photoreactions of the M(DPP)-acceptor complexes are mainly enabled by the enhanced Lewis acidity of the DPP-metal complexes for the stabilization of the assemblies and also by lowering the oxidation potential of the porphyrin ligand to gain larger driving force of electron transfer to form an electron-transfer state with avoiding intersystem crossing. The stability and photochemical behavior are in sharp contrast to those for metal complexes with planar porphyrins as ligands.