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)?

Dalton Transactions Pub Date: 2014-07-24 DOI: 10.1039/C4DT01821D

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?)OsII(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 (HL2?)-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, g2 = 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 22+ 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-HL2?)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 {OsIII(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 22+ 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 H2PO4?. This, however, establishes that both 1 and 22+ are equally efficient in recognising the F? ion selectively, with log?K values of 6.83 and 5.89, respectively.

Graphical abstract: 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)
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