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.
Competing Interests: The authors declare no competing financial interest.
(© 2023 The Authors. Published by American Chemical Society.)