The syntheses and structures of a series of (N,C2′-(2-para- tolylpyridine))2Ir(LX) [(tpy)2Ir(LX)] and (N,C2′-(4′,6′-difluorophenylpyridine))2Ir(LX) [(dfppy)2Ir(LX)] are reported, where LX are pyrazolyl and pyrazolyl-borate ligands. Reaction of the dichloro-bridged dimer [(tpy) 2Ir(μ-Cl)]2 with excess pyrazolyl-borate ligands forms a protonated-dipyrazolyl Ir complex, (tpy)2Ir(pz2H). The Ir bound chloride in [(tpy)2Ir(μ-Cl)]2 presumably facilitates the hydrolysis of pyrazolyl-borate. Thus, the syntheses of Ir complexes with bis(pyrazolyl)borates, e.g., (tpy)2Ir(pz 2Bpz2) and (tpy)2Ir(pz2BEt 2) require the chloride abstraction by CF3SO 3Ag before pyrazolyl-borate is added to the reaction solution. (tpy)2Ir(pz2H), (tpy)2Ir(pzH) 2(CF3SO3), (tpy)2Ir(pz 2Bpz2) and (tpy)2Ir(pz2BEt 2) have been structurally characterized by X-ray crystallography. The two pyrazolyl rings of (tpy)2Ir(pz2H) are nearly coplanar (dihedral angle, 12°), due to the presence of N-HN hydrogen bond (NN distance of 2.56 Å), while the two pyrazolyl rings of (tpy) 2Ir(pzH)2(CF3SO3) are not coplanar (pz-pz dihedral angle, 47°). Unlike other η2-pyrazolyl- borate complexes, the (tpy)2Ir(pz2BEt2) exhibits a rare quasi-chair conformation adopted by the ring, rather than the more common boat conformations which have been observed previously. Similarly, the cycle of (tpy)2Ir(pz2Bpz2) adopts a half-chair conformation, more flattened than the typical boat conformation. While the choice of pyrazolyl-borate ligand does not change the structural properties of the "(tpy)2Ir" fragment, it markedly affects the photophysical properties of both (tpy)2Ir(pz2BR 2) and (dfppy)2Ir(pz2BR2) complexes. The pyrazolyl-borate ancillary ligands affect the absorption and emission energies of these complexes by tuning the HOMO (highest occupied molecular orbital) energies. The electron-withdrawing ancillary ligands, e.g., pz 2Bpz2-, decrease the electron density on the iridium, stabilizing the metal-centered HOMOs.
- Boat conformations
- Cyclometalated Ir(II) complexes
- Pyrazolyl ancillary ligands
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry