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

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Abstract

The article deals with the newly designed mononuclear and asymmetric dinuclear osmium(II) complexes Os-II(bpy)(2)(HL2-) (1) and [(bpy)(2)Os-II(mu-HL2-)Os-II(bpy)(2)](Cl)(2) ([2](Cl)(2))/[(bpy)(2)Os-II(mu-HL2-)Os-II(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 (similar to 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 (g(1) = 2.127, g(2) = 2.096, g(3) = 2.046; <g> = 2.089; Delta g = 0.08). H-1-NMR of the dinuclear 2(2+) 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 (K-c) value of 8.2 x 10(4). The class I mixed valent state of 2(3+) (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 2(3+) or 2(4+) (S = 1, Os1: 0.915, Os2: 0.811 and HL2-: 0.275) implies a mixed electronic structural form of [(bpy)(2)Os-III(mu-HL2-)Os-II(bpy)(2)](3+)(major)/[(bpy)(2)Os-II(mu-HL center dot-)Os-II(bpy)(2)](3+)(minor) or [(bpy)(2)Os-III(mu-HL2-) Os-III(bpy)(2)](4+)(major)/[(bpy)(2)Os-III(mu-HL center dot-)Os-II(bpy)(2)](4+)(minor), respectively. The mixed valent {Os-III(mu-HL2-)Os-II} state in 2(3+), 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 2(2+) 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 2(2+) are equally efficient in recognising the F-ion selectively, with log K values of 6.83 and 5.89, respectively.

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