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 (Phi = 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 (Phi = 0.22), but not their water-sensitivity. A total of four crystal structures are reported, tBu2PyrPyH, (Me2PyrPy)2Zn, (tBu2PyrPy)2Zn, and (Me2PyrPy)BF2. tBu2PyrPyH crystallizes as a doubly hydrogen bonded dimer with non-coplanar pyridine and pyrrole rings. The solid-state structures of (Me2PyrPy)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 pi-->pi* 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.