Electrochemical and spectroelectrochemical studies of C-benzodiazaborolyl-ortho-carboranes

Abstract

. (2013) 'Electrochemical and spectroelectrochemical studies of C-benzodiazaborolyl-ortho-carboranes.', Dalton transactions., 42 (6). pp. 2266-2281.Further information on publisher's website:http://dx.doi.org/10.1039/c2dt32378hPublisher's copyright statement: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractFifteen C-diazaborolyl-ortho-carboranes, 1-R ′ -2-R ″ -1,2-C 2 B 10 H 10 , where R′ represents the groups 2-(1,3-Et 2 -1,3,2-N 2 BC 6 H 4 )-, 2-(1,3-Ph 2 -1,3,2-N 2 BC 6 H 4 )-, 2-(1,3-Ph 2 -5,6-Me 2 -1,3,2-N 2 BC 6 H 2 )-, 2-(1,3-i Pr 2 -1,3,2-N 2 BC 6 H 4 )-, and 2-(1,3,2-N 2 BC 6 H 6 )-and where R ″ equals to H, Me, Ph, t Bu or SiMe 3 were synthesized. Cyclovoltammetric studies of the compounds showed irreversible oxidation waves which are caused by the oxidation of the heterocycle. Those C-diazaborolyl-ortho-carboranes with Ph, tBu SiMe 3 substituents at the adjacent C-atom of the cage displayed two one-electron reduction waves reflecting the formation of stable radical monoanions with unusual (2n+3) skeletal electron counts. The geometries of these anions were determined by combinations of infrared, UV-visible spectroelectrochemical and computational studies. Additionally the structures of seven new C-diazaborolyl-ortho-carboranes and one new 2-bromo-1,3,2-benzodiazaborole were determined by X-ray crystallography and compared with previously obtained structures.

Substituent Effects on the Fluorescence Properties of ortho‐Carbor­anes: Unusual Emission Behaviour in C‐(2′‐Pyridyl)‐ortho‐carboranes

“…[9,15,16,17] No thin-film emissions were observed at room temperatures for the parent carboranes 1-3. Solid-state emissions at ambient temperatures were reported elsewhere for 1 (powder form: 395 nm; [53] silica gel form: 356 nm [5] ), 2 (silica gel: 380 nm [5] ) and 3 (silica gel: 345 nm [5] ).…”

“…(Figure 3) is solvent-dependent with an emission maximum at 329 nm in cyclohexane and at 442 nm in dichloromethane. Based on reported photophysical data of other ortho-carboranes, [12,13,[15][16][17][18][19] the emission at 329 nm is from local transitions at one or both aryl groups, whereas the emission at 442 nm is a charge transfer involving the carborane cluster. Low-energy emissions of 17 are also observed in toluene (440 nm), chloroform (408 nm) and acetonitrile (426 nm).…”

“…Such low-energy emissions have been reported in other diaryl-ortho-carboranes and arise from charge transfer involving both the ring and the carborane cluster. [9,10,13,15,16,18,20,[22][23][24][25][26] The solid-state emission of 1,2-diphenylortho-carborane 16 here is expected as many derivatives containing the diphenyl-ortho-carborane unit emit in the solid state. …”

“…[33] Also other highly luminescent metal complexes have been reported with the pyridylcarborane unit present. [46] Given that there have been many articles on the observed luminescence of 1-monoalkyl-, 1-monoaryl-and 1,2-diaryl-ortho-carborane derivatives, [7][8][9][10][11][12][13][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] by contrast 1-monomethyl-ortho-carborane 4, 1-monophenyl-ortho-carborane 7 and 1,2-diphenyl-ortho-carborane 16 were quoted to be non-emissive. [20,30] …”

“…Photoexcitation of such dyads induced a charge transfer from an organic scaffold to the carborane cluster, which either led to luminescence quenching or to charge-transfer (CT) emissions or both -depending on the solvents used and whether the materials were investigated as solids. [7][8][9][10][11][12][13][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Dual fluorescence with CT and local transitions has been observed in some orthocarboranes. [12,13,[15][16][17][18][19][20]28] The ortho-carborane unit plays a role similar to meta-and para-carborane clusters in the photophysical process of a compound if there is a stronger electron acceptor than the cluster in these systems, [5,6,29,30] and if the ortho-carborane group is connected to a donor at one or more boron cluster atoms.…”