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]1 [1]2- (pdt2- = 1,3-propanedithiolate) and [Fe2(adt)CO4(CN)2]2- [3]2- (xdt2- = 1,3-azadithiolate, HN(CH2S-)2) to give the 2:1 adducts [Fe2(pdt)(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(BAr32]- 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. [ABSTRACT FROM AUTHOR]