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 (pdt(2-) = 1,3-propanedithiolate) and Fe2(adt)(CO)4(CN)2 3 (adt(2-) = 1,3-azadithiolate, HN(CH2S(-))2) to give the 2:1 adducts Fe2(xdt)(CO)4(CNBAr3)2. Attempts to prepare the 1:1 adducts 1(BAr3) (Ar = Ph, C6F5) were unsuccessful, but related 1:1 adducts were obtained using the bulky borane B(C6F4-o-C6F5)3 (BAr(F)*3). By virtue of the N-protection by the borane, salts of Fe2(pdt)(CO)4(CNBAr3)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 oxidize quasi-reversibly around -0.3 V versus Fc(0/+) in contrast to ca. -0.8 V observed for the 1 couple. A simplified synthesis of 1, 3, and Fe2(pdt)(CO)5(CN) (2) was developed, entailing reaction of the diiron hexacarbonyl complexes with KCN in MeCN.