Mixed-Sandwich Cp*Cr Complexes
Containing Poly(methimazolyl)borates
(Cp* = C<sub>5</sub>Me<sub>5</sub>): Syntheses and Structural and
Electrochemical Studies
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Abstract
Reaction of the scorpionate salts K[HB(mt)<sub>3</sub>], Na[H<sub>2</sub>B(mt)<sub>2</sub>], and Li[HB(mt)<sub>2</sub>(pz)]
with [Cp*CrBr<sub>2</sub>]<sub>2</sub> (<b>1</b>) gave the 15-electron
Cr(III)
complexes [Cp*Cr{κ<sup>3</sup><i>-S</i>,<i>S′</i>,<i>S″</i>-HB(mt)<sub>3</sub>}]Br (<b>2</b>), [Cp*Cr{κ<sup>2</sup><i>-S</i>,<i>S′</i>-H<sub>2</sub>B(mt)<sub>2</sub>}Br] (<b>3</b>), and [Cp*Cr{κ<sup>2</sup><i>-S,S′</i>-HB(mt)<sub>2</sub>(pz)}Br] (<b>4</b>) in moderate to high yields. These are the first examples
of mixed-sandwich chromium(III) complexes containing poly(methimazolylborate)
ligands. The tridentate coordination of the monoanionic ligands [H<sub>2</sub>B(mt)<sub>2</sub>] in <b>3</b> and [HB(mt)<sub>2</sub>(pz)] in <b>4</b> can be effected by using a silver salt to
remove the Br coligand, thus yielding the complexes [Cp*Cr{κ<sup>3</sup><i>-H</i>,<i>S</i>,<i>S′</i>-H<sub>2</sub>B(mt)<sub>2</sub>}]PF<sub>6</sub> (<b>6</b>)
and [Cp*Cr{κ<sup>3</sup><i>-N</i>,<i>S</i>,<i>S′</i>-HB(mt)<sub>2</sub>(pz)}]PF<sub>6</sub> (<b>7</b>). It was also found that, in the presence of acetonitrile,
the reaction of <b>3</b> with AgPF<sub>6</sub> afforded [Cp*Cr{κ<sup>2</sup><i>-S</i>,<i>S′</i>-H<sub>2</sub>B(mt)<sub>2</sub>}(NCMe)]PF<sub>6</sub> (<b>5</b>). The coordination
geometries of all the complexes have been determined by X-ray diffraction
analyses. Cyclic voltammetric studies of complexes <b>2</b>,<b> 4</b>, and <b>7</b> showed that the oxidation and reduction
processes are chemically reversible and that the reduced and oxidized
states of complexes <b>3</b>, <b>5</b>, and <b>6</b> are very short-lived
