Synthesis and photophysical characterization of a bis-pincer osmium complex

Abstract The neutral bis-pincer transition metal complex Os(PCP) 2 (PCP = 2,6-(CH 2 PPh 2 ) 2 C 6 H 3 , Ph = C 6 H 5 ) was prepared via two synthetic routes with up to 42% yield. The lemon-yellow complex is air stable as a neat solid and in solution, and is thermally stable as can be purified by high vacuum zone sublimation at 280-240-200 °C under 1 × 10 −6  torr. The pincer ligands coordinate in a pseudo-octahedral arrangement around the metal center. X-ray crystallography reveals that the complex is in pseudo-D 2 geometry symmetry due to the twist caused by the methylenes. NMR studies at varied temperatures between 223 and 343 K along with low temperature inversion recovery studies show the fluxional behavior and exchanging process between the two enantiomers ( E a  = 8.6 kcal/mol). The Os(PCP) 2 shows no emission in solution at room temperature, but emits at 77 K in 2-MeTHF glass ( Φ  = 60%). At room temperature, the neat solid of Os(PCP) 2 shows yellow emission of 3% quantum yield. TDDFT predicted S 0  → S 1 transition shows that the lowest singlet state (S 1 ) is metal-ligand (the Os-Xylyl fragments) to ligand’ (the PPh 2 fragments) charge transfer (ML-L’CT) in character with a small contribution from the higher lying metal centered ( 1 MC) states. The large spin–orbit coupling constant of osmium leads to effective intersystem crossing to a triplet state with increased MC character. The DFT predicted triplet spin surface of Os(PCP) 2 indicates that the emission is dominated by 3 MC state. Significant Os–P bonds elongation at the lowest triplet excited state also strongly supports the conclusion.