Electronic structures and selective fluoride sensing features of Os(bpy)2(HL2-) and [{Os(bpy)2} 2(μ-HL2-)]2+ (H3L: 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid)

Abstract

The article deals with the newly designed mononuclear and asymmetric dinuclear osmium(ii) complexes OsII(bpy)2(HL2-) (1) and [(bpy)2OsII(μ-HL2-)Os II(bpy)2](Cl)2 ([2](Cl)2)/[(bpy) 2OsII(μ-HL2-)OsII(bpy) 2](ClO4)2 ([2](ClO4)2), respectively, (H3L = 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4- carboxylic acid and bpy = 2,2′-bipyridine). The identity of 1 has been established by its single crystal X-ray structure. The ligand (HL 2-)-based primary oxidation process (E°298, 0.23 V versus SCE) along with the partial metal contribution (∼20%) in 1 has been revealed by the ligand-dominated HOMO of 1 (HL2-: 88%, Os: 8%), as well as by the Mulliken spin density distribution of 1+ (HL2-: 0.878, Os: 0.220). Accordingly, 1+ exhibits a free radical type EPR at 77 K with a partial metal-based anisotropic feature (g1 = 2.127, g 2 = 2.096, g3 = 2.046; <g> = 2.089; Δg = 0.08). 1H-NMR of the dinuclear 22+ in CDCl3 suggests an intimate mixture of two diastereomeric forms in a 1:1 ratio. The DFT-supported predominantly Os(ii)/Os(iii)-based couples of asymmetric 2 2+ at 0.24 V and 0.50 V versus SCE result in a comproportionation constant (Kc) value of 8.2 × 104. The class I mixed valent state of 23+ (S = 1/2) has, however, been corroborated by the Mulliken spin density distribution of Os1: 0.887, Os2: 0.005, HL2-: 0.117, as well as by the absence of a low-energy IVCT (intervalence charge transfer) band in the near-IR region (up to 2000 nm). The appreciable spin accumulation on the bridge in 23+ or 24+ (S = 1, Os1: 0.915, Os2: 0.811 and HL2-: 0.275) implies a mixed electronic structural form of [(bpy)2OsIII(μ-HL 2-)OsII(bpy)2]3+(major)/[(bpy) 2OsII(μ-HL-)OsII(bpy) 2]3+(minor) or [(bpy)2OsIII(μ- HL2-)OsIII(bpy)2]4+(major)/[(bpy) 2OsIII(μ-HL-)OsII(bpy) 2]4+ (minor), respectively. The mixed valent {Os III(μ-HL2-)OsII} state in 23+, however, fails to show EPR at 77 K due to the rapid spin relaxation process. The DFT-supported bpy-based two reductions for both 1+ and 2 2+ appear in the potential range of -1.5 V to -1.8 V versus SCE. The electronic transitions in 1n and 2n are assigned by the TD-DFT calculations. Furthermore, the potential anion sensing features of 1 and 22+via the involvement of the available N-H proton in the framework of coordinated HL2- have been evaluated by different experimental investigations, in conjunction with the DFT calculations, using a wide variety of anions such as F-, Cl-, Br-, I-, OAc-, SCN-, HSO4- and H 2PO4-. This, however, establishes that both 1 and 22+ are equally efficient in recognising the F- ion selectively, with logK values of 6.83 and 5.89, respectively. © 2014 the Partner Organisations.