Borane-Protected Cyanides as Surrogates of H‑Bonded Cyanides in [FeFe]-Hydrogenase Active Site Models

Triarylborane Lewis acids bind [Fe2(pdt)­(CO)4(CN)2]2– [1]2– (pdt2– =
1,3-propanedithiolate)
and [Fe2(adt)­(CO)4(CN)2]2– [3]2– (adt2– = 1,3-azadithiolate, HN­(CH2S–)2) to give the 2:1 adducts [Fe2(xdt)­(CO)4(CNBAr3)2]2–. Attempts
to prepare the 1:1 adducts [1(BAr3)]2– (Ar = Ph, C6F5) were unsuccessful, but related
1:1 adducts were obtained using the bulky borane B­(C6F4-o-C6F5)3 (BArF*3). By virtue of the N-protection by
the borane, salts of [Fe2(pdt)­(CO)4(CNBAr3)2]2– sustain protonation to
give hydrides that are stable (in contrast to [H1]−). The hydrides [H1(BAr3)2]− are 2.5–5 pKa units more acidic than the parent [H1]−. The adducts [1(BAr3)2]2– oxidize quasi-reversibly around −0.3 V versus Fc0/+ in contrast to ca. −0.8 V observed for the [1]2–/– couple. A simplified synthesis of
[1]2–, [3]2–, and [Fe2(pdt)­(CO)5(CN)]− ([2]−) was developed, entailing reaction
of the diiron hexacarbonyl complexes with KCN in MeCN.