Vibrational and structural mapping of [Os(bpy)3]3+/2+ and [Os(phen)3]3+/2+

Structural changes between [OsIIL3]2+ and [OsIIIL3]3+ (L: 2,2′-bipyridine; 1,10-phenanthroline) and molecular and electronic structures of the OsIII complexes [OsIII(bpy)3]3+ and [OsIII(phen)3]3+ are discussed in this paper. Mid-infrared spectra in the ν(bpy) and ν(phen) ring stretching region for [OsII(bpy)3](PF6)2, [OsIII(bpy)3](PF6)3, [OsII(phen)3](PF6)2, and [OsIII(phen)3](PF6)3 are compared, as are X-ray crystal structures. Absorption spectra in the UV region for [OsIII(bpy)3](PF6)3 and [OsIII(phen)3](PF6)3 are dominated by very intense absorptions (ε = 40 000–50 000 M−1 cm−1) due to bpy and phen intra-ligand π → π∗ transitions. In the visible region, relatively narrow bands with vibronic progressions of ∼1500 cm−1 appear, and have been assigned to bpy or phen-based, spin–orbit coupling enhanced, 1π → 3π∗ electronic transitions. Also present in the visible region are ligand-to-metal charge transfer bands (LMCT) arising from π(bpy) → t2g(OsIII) or π(phen) → t2g(OsIII) transitions. In the near infrared, two broad absorption features appear for oxidized forms [OsIII(bpy)3](PF6)3 and [OsIII(phen)3](PF6)3 arising from dπ–dπ interconfigurational bands characteristic of dπ5OsIII. They are observed at 4580 and 5090 cm−1 for [OsIII(bpy)3](PF6)3 and at 4400 and 4990 cm−1 for [OsIII(phen)3](PF6)3. The bpy and phen infrared vibrational bands shift to higher energy upon oxidation of Os(II) to Os(III). In the cation structure in [OsIII(bpy)3](PF6)3, the OsIII atom resides at a distorted octahedral site, as judged by ∠N–Os–N, which varies from 78.78(22)° to 96.61(22)°. Os–N bond lengths are also in general longer for [OsIII(bpy)3](PF6)3 compared to [OsII(bpy)3](PF6)2 (0.010 Å), and for [OsIII(phen)3](PF6)3 compared to [OsII(phen)3](PF6)2 (0.014 Å). Structural changes in the ligands between oxidation states are discussed as originating from a combination of dπ(OsII) → π∗ (bpy or phen) backbonding and charge redistribution on the ligands as calculated by natural population analysis.

Graphical abstract[Os(bpy)3]3+/2+ and [Os(phen)3]3+/2+ structural changes (bpy is 2,2′-bipyridine; phen is 1,10-phenanthroline) and electronic structures of the OsIII complexes [OsIII(bpy)3]3+ and [OsIII(phen)3]3+ are discussed. Average Os–N bond distances increase for OsIII compared to OsII because of it loss in backbonding. Structural changes in the ligands between oxidation states are discussed as originating from a combination of dπ(OsII) → π∗ (bpy or phen) backbonding and charge redistribution on the ligands.Figure optionsDownload full-size imageDownload as PowerPoint slide