Pyridylpyrrolides as alternatives to cyclometalated phenylpyridine ligands: Synthesis and characterization of luminescent zinc and boron pyridylpyrrolide complexes

The synthesis, structure, and properties of six luminescent pyridylpyrrolide complexes and the first structural characterization of pyridylpyrrolide metal complexes are reported. A series of new zinc complexes, bis(pyridylpyrrolyl)-zinc, (R₂PyrPy)₂Zn (R = Me, Et, ⁱPr, ^tBu, and Ph), that vary in their substituents on the pyrrole ring (Me, Et, ⁱPr, ^tBu, and Ph), were prepared. Pyrrole substitution produced small structural changes in the complexes and affected the fluorescence properties very little. The zinc complexes were found to be luminescent, emitting at 495 nm (Φ = 0.32, 0.32 0.31, 0.19 and 0.57, respectively). A boron analog, (Me₂PyrPy)BF₂, was prepared and was found to share the luminescent properties with the zinc complexes, emitting at 505 nm (Φ = 0.22), but not their water-sensitivity. A total of four crystal structures are reported, ^tBu₂PyrPyH, (Me ₂PyrPy)₂Zn, (^tBu₂PyrPy) ₂Zn, and (Me₂PyrPy)BF₂. ^tBu ₂PyrPyH crystallizes as a doubly hydrogen bonded dimer with non-coplanar pyridine and pyrrole rings. The solid-state structures of (Me ₂PyrPy)₂Zn and (^tBu₂PyrPy) ₂Zn revealed that despite the large change in steric bulk, the two compounds have very similar structures. The structure of (Me₂PyrPy) BF₂ showed changes that are expected with the interaction between a smaller atom (B as compared to Zn). Molecular orbital calculations were performed on Me₂PyrPyH, (Me₂PyrPy)BF₂, and (Me₂PyrPy)₂Zn using Gaussian 98 methods. It was found that the main transition (HOMO-LUMO) for all three molecules is a π → π* transition and that in the inorganic complexes, the metal atom (zinc or boron) present has very little effect on transition, evidence that the optical properties are largely ligand based and that the B or Zn atom's main effect is lowering of the LUMO relative energy.