C–S Bond Cleavage, Redox Reactions, and Dioxygen
Activation by Nonheme Dicobalt(II) Complexes
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Abstract
Synthesis
and reactivity of a series of thiolate/thiocarboxylate bridged dicobalt(II)
complexes were investigated in comparison with their carboxylate bridged
analogues bearing free thiol/hydroxyl groups. Upon one-electron oxidation,
complexes [Co<sub>2</sub>(<i>N-</i>Et-HPTB)(μ-SR<sup>1</sup>)](BF<sub>4</sub>)<sub>2</sub> (R<sup>1</sup> = Ph, <b>1a</b>; Et, <b>1b</b>; Py, <b>1c</b>) and [Co<sub>2</sub>(<i>N-</i>Et-HPTB)(μ-SCOR<sup>2</sup>)](BF<sub>4</sub>)<sub>2</sub> (R<sup>2</sup> = Ph, <b>2a</b>; Me, <b>2b</b>) yielded [Co<sub>2</sub>(<i>N-</i>Et-HPTB)(DMF)<sub>2</sub>](BF<sub>4</sub>)<sub>3</sub> (<b>6</b>) (DMF = dimethylformamide) along with the corresponding
disulfides (where <i>N</i>-Et-HPTB is the anion of <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane).
Unlike the inertness of carboxylate bridged complexes [Co<sub>2</sub>(<i>N-</i>Et-HPTB)(μ-O<sub>2</sub>C-R<sup>3</sup>-SH)](BF<sub>4</sub>)<sub>2</sub> (R<sup>3</sup> = Ph, <b>3a</b>; CH<sub>2</sub>CH<sub>2</sub>, <b>3b</b>) and [Co<sub>2</sub>(<i>N-</i>Et-HPTB)(μ-O<sub>2</sub>CR<sup>4</sup>)](BF<sub>4</sub>)<sub>2</sub> (R<sup>4</sup> = Ph, <b>4a</b>; Me, <b>4b</b>; CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>OH, <b>5</b>) toward O<sub>2</sub>, the bridging ethanethiolate
in <b>1b</b> was oxidized to yield a sulfinate bridged complex,
[Co<sub>2</sub>(<i>N-</i>Et-HPTB)(μ-O<sub>2</sub>SEt)](BF<sub>4</sub>)<sub>2</sub> (<b>10</b>). Detailed investigation
of the synthetic aspects of <b>1a</b>–<b>1c</b> led to the discovery of a C–S bond cleavage reaction and
yielded the dicobalt(II) complexes [Co<sub>2</sub>(<i>N-</i>Et-HPTB)(SH)(H<sub>2</sub>O)](BF<sub>4</sub>)<sub>2</sub> (<b>8a</b>), [Co<sub>2</sub>(<i>N-</i>CH<sub>2</sub>Py-HPTB)(SH)(H<sub>2</sub>O)](BF<sub>4</sub>)<sub>2</sub> (<b>8b</b>) (where <i>N</i>-CH<sub>2</sub>Py-HPTB is the anion of <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis[2-(1-picolylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane)),
and [Co<sub>2</sub>(<i>N-</i>Et-HPTB)(μ-S)](BF<sub>4</sub>) (<b>9</b>). Both <b>8a</b> and <b>8b</b> feature nonheme dinuclear Co(II) units containing a terminal hydrosulfide.
The present study thus reports comparative redox reactions for a rare
class of 16 dicobalt(II) complexes and introduces a selective synthetic
strategy for the synthesis of unprecedented dicobalt(II) complexes
featuring only one terminal hydrosulfide
