Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands.

The two commercially available zirconium complexes tetrakis(dimethylamido)zirconium, Zr(NMe), and tetrabenzylzirconium, ZrBn, were investigated for their utility as starting materials in the synthesis of bis(pyridine dipyrrolide)zirconium photosensitizers, Zr(PDP). Reaction with one equivalent of the ligand precursor 2,6-bis(5-methyl-3-phenyl-1-pyrrol-2-yl)pyridine, HPDP, resulted in the isolation and structural characterization of the complexes (PDP)Zr(NMe)thf and (PDP)ZrBn, which could be converted to the desired photosensitizer Zr(PDP) upon addition of a second equivalent of HPDP. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1-pyrrol-2-yl)pyridine, HPDP, only ZrBn yielded the desired bis-ligand complex Zr(PDP). Careful monitoring of the reaction at different temperatures revealed the importance of the organometallic intermediate (cyclo-PDP)ZrBn, which was characterized by X-ray diffraction analysis and H NMR spectroscopy and shown to contain a cyclometalated PDP unit. Taking inspiration from the results for zirconium, syntheses for two hafnium photosensitizers, Hf(PDP) and Hf(PDP), were established and shown to proceed through similar intermediates starting from tetrabenzylhafnium, HfBn. Initial studies of the photophysical properties of the photoluminescent hafnium complexes indicate similar optical properties compared to their zirconium analogues.