Ternary Porphyrinato HfIV and ZrIV Polyoxometalate Complexes

We report a facile, high-yield synthesis and characterization of discrete, ternary porphyrin–metal–polyoxometalate (por–M–POM) complexes where a group IV transition-metal ion is bound both to the porphyrin core and to the lacunary site of a Keggin POM, PW11O39–7. The remarkably robust complexes exploit the fact that HfIV and ZrIV are 7–8 coordinate and reside outside the plane of the porphyrin macrocycle, thus enabling the simultaneous coordination to meso-tetraphenylporphyrin (tpp) or meso-tetra(4-pyridyl)porphyrin (tpyp) and to the defect site in the Keggin framework. The physical properties of the (tpp)Hf(PW11O39)[tba]5, (tpyp)Hf(PW11O39)[tba]5, and (tpp)Zr(PW11O39)[tba]5 complexes are similar because the metal ions have similar oxidation states and coordination chemistry. This architecture couples the photonic properties of the porphyrin to the POM because the metal ion is incorporated into both frameworks. Thus, the ternary complexes can serve as a basis for the characterization of HfIV and ZrIV porphyrins bound to oxide surfaces through the group IV metal ions. The Hf(por) and Zr(por) bind strongly to TiO2 nanoparticles and indium tin oxide (ITO) surfaces, but significantly less binds to crystalline SiO2 or TiO2 surfaces. Together, the strong binding of the metalloporphyrins to the POM, nanoparticles, and the ITO surfaces, and paucity of binding to crystalline surfaces, suggests that the three-to-four open coordination sites on the Hf(por) and Zr(por) are predominantly bound at surface defect sites. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)