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|Title:||Synthesis and characterization of redox-active tris(pyrazolyl)borate cobalt complexes|
|Authors:||David J. Harding|
|Abstract:||The reaction of CoX<inf>2</inf> (X = Cl, Br, NO<inf>3</inf>) with KTp <sup>Ph2</sup> in tetrahydrofuran (THF) yields the half-sandwich compounds [Tp<sup>Ph2</sup>CoX] (X = Cl 1, Br 2, NO<inf>3</inf>3). The reaction of [Tp<sup>Ph2</sup>CoBr] with NaX (X = N<inf>3</inf>, NO<inf>2</inf>) or potassium thiocyanate (KNCS) permits isolation of [Tp<sup>Ph2</sup>CoX] (X = N <inf>3</inf>4, NCS 5, NO<inf>2</inf>6). In contrast, the reaction of cobalt(ii) acetate with KTp<sup>Ph2</sup> yields [Tp<sup>Ph2</sup>Co(OAc)(Hpz <sup>Ph2</sup>)] 7 as a result of B-N bond cleavage. Subsequent reaction of 7 with a range of β-diketones in the presence of NaOMe produces the β-diketonate complexes, [Tp<sup>Ph2</sup>Co(β-diketonate)] (β-diketonate = acac 8, hfac 9, dbm 10, tmhd 11). IR spectroscopy suggests that the Tp<sup>Ph2</sup> ligands are κ<sup>3</sup>-coordinated and that the β-diketonate ligands adopt a bidentate coordination mode. Electronic spectra are consistent with four- or five-coordinate species in solution. X-Ray crystallographic studies of 7 reveal an intermediate five-coordinate cobalt centre with a hydrogen bonding interaction between the pyrazole hydrogen and the acetate carbonyl oxygen. The molecular structures of 9 and 10 show cobalt centres with square pyramidal coordination geometries and κ<sup>2</sup>- coordinated β-diketonate ligands. Cyclic voltammetric studies of 6 reveal irreversible one-electron reduction to Co(i). However, the β-diketonate complexes, 8, 10 and 11 undergo irreversible one-electron oxidation. The redox potential and reversibility increases as the steric bulk of the substituent on the β-diketonate ligand increases. © 2009 The Royal Society of Chemistry.|
|Appears in Collections:||CMUL: Journal Articles|
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