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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(pz2Bpz2) and (tpy)2Ir(pz2BEt2) require the chloride abstraction by CF3SO3Ag before pyrazolyl-borate is added to the reaction solution. (tpy)2Ir(pz2H), (tpy)2Ir(pzH)2(CF3SO3), (tpy)2Ir(pz2Bpz2) and (tpy)2Ir(pz2BEt2) 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–H⋯N hydrogen bond (N⋯N distance of 2.56 A), 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(pz2BR2) 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., pz2Bpz2 −, decrease the electron density on the iridium, stabilizing the metal-centered HOMOs. |
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