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