Controlling the Reactivity of Bifunctional Ligands:
Carboxylate-Bridged Nonheme Diiron(II) Complexes Bearing Free Thiol
Groups
- Publication date
- Publisher
Abstract
Carboxylate-bridged
nonheme diiron(II) complexes, bearing free functional groups in general,
and free thiol groups in particular, were sought. While the addition
of sodium γ-hydroxybutyrate into a mixture of Fe(BF<sub>4</sub>)<sub>2</sub>·6H<sub>2</sub>O, H<i>N</i>-Et-HPTB,
and Et<sub>3</sub>N afforded the complex [Fe<sub>2</sub>(<i>N</i>-Et-HPTB)(μ-O<sub>2</sub>C–(CH<sub>2</sub>)<sub>3</sub>–OH)](BF<sub>4</sub>)<sub>2</sub> (<b>2</b>) (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),
a similar, straightforward process could not be used for the synthesis
of diiron(II) complexes with free thiol groups. In order to circumvent
this problem, a new class of thiolate bridged diiron(II) complexes,
[Fe<sub>2</sub>(<i>N</i>-Et-HPTB)(μ-SR<sup>1</sup>)](BF<sub>4</sub>)<sub>2</sub> (R<sup>1</sup> = Me (<b>1a</b>), Et (<b>1b</b>), <sup><i>t</i></sup>Bu (<b>1c</b>), Ph (<b>1d</b>)) was synthesized. Selective proton exchange
reactions of one representative compound, <b>1b</b>, with reagents
of the type HS–R<sup>2</sup>–COOH yielded the target
compounds, [Fe<sub>2</sub>(<i>N</i>-Et-HPTB)(μ-O<sub>2</sub>C–R<sup>2</sup>–SH)](BF<sub>4</sub>)<sub>2</sub> (R<sup>2</sup> = C<sub>6</sub>H<sub>4</sub> (<b>3a</b>), CH<sub>2</sub>CH<sub>2</sub> (<b>3b</b>), CH<sub>2</sub>(CH<sub>2</sub>)<sub>5</sub>CH<sub>2</sub> (<b>3c</b>)). Redox properties
of the complexes <b>3a</b>–<b>3c</b> were studied
in comparison with the complex, [Fe<sub>2</sub>(<i>N</i>-Et-HPTB)(μ-O<sub>2</sub>CMe)](BF<sub>4</sub>)<sub>2</sub> (<b>5</b>). Reaction of (Cp<sub>2</sub>Fe)(BF<sub>4</sub>) with <b>1b</b> yielded [Fe<sup>II</sup><sub>2</sub>(<i>N</i>-Et-HPTB)(DMF)<sub>3</sub>](BF<sub>4</sub>)<sub>3</sub>·DMF
(<b>4</b>) (when crystallized from DMF/diethyl ether), which
might indicate the formation of a transient ethanethiolate bridged
{Fe<sup>II</sup>Fe<sup>III</sup>} species, followed by a rapid internal
redox reaction to generate diethyldisulfide and the solvent coordinated
diiron(II) complex, <b>4</b>. This possibility was supported
by a comparative cyclic voltammetric study of <b>1a</b>–<b>1c</b> and <b>4</b>. Prospects of the complexes of the type <b>3a</b>–<b>3c</b> as potential building blocks for
the synthesis of nonheme diiron(II) complexes covalently attached
with other redox active metals has been substantiated by the synthesis
of the complexes, [Fe<sub>2</sub>(<i>N</i>-EtHPTB)(μ-O<sub>2</sub>C–R<sup>2</sup>–S)Cu(Me<sub>3</sub>TACN)](BF<sub>4</sub>)<sub>2</sub> (R = <i>p</i>-C<sub>6</sub>H<sub>4</sub> (<b>7a</b>), CH<sub>2</sub>CH<sub>2</sub> (<b>7b</b>)). All the compounds were characterized by a combination of single-crystal
X-ray structure determinations and/or elemental analysis
