The two commercially available zirconium complexes tetrakis(dimethylamido)zirconium,
Zr(NMe2)4, and tetrabenzylzirconium, ZrBn4, 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-1H-pyrrol-2-yl)pyridine,
H2MePDPPh, resulted in the isolation
and structural characterization of the complexes (MePDPPh)Zr(NMe2)2thf and (MePDPPh)ZrBn2, which could be converted to the desired
photosensitizer Zr(MePDPPh)2upon
addition of a second equivalent of H2MePDPPh. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MesPDPPh, only ZrBn4yielded the desired bis-ligand complex
Zr(MesPDPPh)2. Careful monitoring
of the reaction at different temperatures revealed the importance
of the organometallic intermediate (cyclo-MesPDPPh)ZrBn, which was characterized by X-ray diffraction analysis and1H NMR spectroscopy and shown to contain a cyclometalatedMesPDPPhunit. Taking inspiration from the results
for zirconium, syntheses for two hafnium photosensitizers, Hf(MePDPPh)2and Hf(MesPDPPh)2, were established and shown to proceed through
similar intermediates starting from tetrabenzylhafnium, HfBn4. Initial studies of the photophysical properties of the photoluminescent
hafnium complexes indicate similar optical properties compared to
their zirconium analogues.