Luminescent Osmium(II) Complexes with Functionalized 2-Phenylpyridine Chelating Ligands: Preparation, Structural Analyses, and Photophysical Properties

Cyclometalated osmium complexes with the formulas [Os(ppy)2(CO)2] (1a,b), [Os(dfppy)2(CO)2] (2a,b), and [Os(btfppy)2(CO)2] (3a,b) have been synthesized, for which the chelating chromophores ppyH, dfppyH, and btfppyH denote 2-phenylpyridine, 2-(2,4-difluorophenyl)pyridine, and 2-(2,4-bis(trifluoromethyl)phenyl)pyridine, respectively. The isomers 1a−3a, possessing an intrinsic C2 rotational axis as determined by single-crystal X-ray diffraction analysis, underwent slow isomerization in solution at elevated temperature, giving the respective thermodynamic products 1b−3b, which showed a distinctive coordination arrangement produced by a 180° rotation of one cyclometalated ligand around the Os(II) metal center. In contrast to the case for 1a,b and 2a, which are inert to substitution, complexes 2b and 3b (or 3a) readily react with PPh2Me to afford the products [Os(dfppy)2(CO)(PPh2Me)] (4) and [Os(btfppy)2)(PPh2Me)] (6), in which the incoming PPh2Me replaced the CO located trans to the carbon atom of one cyclometalated ligand. UV–vis and emission spectra were measured, revealing the lowest excited state for all complexes as a nominally ligand-centered 3ππ* state mixed with certain MLCT character. Introduction of the electron-withdrawing substituents on the cyclometalated chelates or replacement of one CO ligand with phosphine at the metal center increased the MLCT contribution in the first excited state, giving a broad and featureless emission with greatly enhanced quantum yields.

我们合成了通式分别为[Os(ppy)₂(CO)₂]（1a、b）、[Os(dfppy)₂(CO)₂]（2a、b）以及[Os(btfppy)₂(CO)₂]（3a、b）的环金属化锇配合物，其中螯合发色团ppyH、dfppyH和btfppyH分别对应2-苯基吡啶（2-phenylpyridine）、2-(2,4-二氟苯基)吡啶（2-(2,4-difluorophenyl)pyridine）和2-(2,4-双(三氟甲基)苯基)吡啶（2-(2,4-bis(trifluoromethyl)phenyl)pyridine）。经单晶X射线衍射分析确认，异构体1a~3a具有固有C₂旋转轴，其在升温溶液中会发生缓慢异构化，生成对应的热力学产物1b~3b；后者的配位构型独特，源于一个环金属配体围绕Os(II)金属中心发生180°旋转。与对取代反应呈惰性的1a、b及2a不同，配合物2b和3b（或3a）可与PPh₂Me顺利反应，生成产物[Os(dfppy)₂(CO)(PPh₂Me)]（4）和[Os(btfppy)₂)(PPh₂Me)]（6），其中外来配体PPh₂Me取代了与一个环金属配体的碳原子处于反位的CO配体。测试了所有配合物的紫外-可见吸收光谱与发射光谱，结果表明所有配合物的最低激发态均为名义上的配体中心³ππ*态，且混合有一定的金属到配体电荷转移（Metal-to-Ligand Charge Transfer, MLCT）特征。在环金属螯合物上引入吸电子取代基，或是在金属中心用膦配体取代一个CO配体，均会提升第一激发态中的MLCT贡献，使得发射光谱变得宽化且无精细结构，同时量子产率大幅提升。