Borane-Protected
Cyanides as Surrogates of H‑Bonded
Cyanides in [FeFe]-Hydrogenase Active Site Models
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Abstract
Triarylborane Lewis acids bind [Fe<sub>2</sub>(pdt)(CO)<sub>4</sub>(CN)<sub>2</sub>]<sup>2–</sup> [<b>1</b>]<sup>2–</sup> (pdt<sup>2–</sup> =
1,3-propanedithiolate)
and [Fe<sub>2</sub>(adt)(CO)<sub>4</sub>(CN)<sub>2</sub>]<sup>2–</sup> [<b>3</b>]<sup>2–</sup> (adt<sup>2–</sup> = 1,3-azadithiolate, HN(CH<sub>2</sub>S<sup>–</sup>)<sub>2</sub>) to give the 2:1 adducts [Fe<sub>2</sub>(xdt)(CO)<sub>4</sub>(CNBAr<sub>3</sub>)<sub>2</sub>]<sup>2–</sup>. Attempts
to prepare the 1:1 adducts [<b>1</b>(BAr<sub>3</sub>)]<sup>2–</sup> (Ar = Ph, C<sub>6</sub>F<sub>5</sub>) were unsuccessful, but related
1:1 adducts were obtained using the bulky borane B(C<sub>6</sub>F<sub>4</sub>-<i>o</i>-C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> (BAr<sup>F</sup>*<sub>3</sub>). By virtue of the N-protection by
the borane, salts of [Fe<sub>2</sub>(pdt)(CO)<sub>4</sub>(CNBAr<sub>3</sub>)<sub>2</sub>]<sup>2–</sup> sustain protonation to
give hydrides that are stable (in contrast to [H<b>1</b>]<sup>−</sup>). The hydrides [H<b>1</b>(BAr<sub>3</sub>)<sub>2</sub>]<sup>−</sup> are 2.5–5 p<i>K</i><sub>a</sub> units more acidic than the parent [H<b>1</b>]<sup>−</sup>. The adducts [<b>1</b>(BAr<sub>3</sub>)<sub>2</sub>]<sup>2–</sup> oxidize quasi-reversibly around −0.3 V versus Fc<sup>0/+</sup> in contrast to ca. −0.8 V observed for the [<b>1</b>]<sup>2–/–</sup> couple. A simplified synthesis of
[<b>1</b>]<sup>2–</sup>, [<b>3</b>]<sup>2–</sup>, and [Fe<sub>2</sub>(pdt)(CO)<sub>5</sub>(CN)]<sup>−</sup> ([<b>2</b>]<sup>−</sup>) was developed, entailing reaction
of the diiron hexacarbonyl complexes with KCN in MeCN
