Regioselectivity in ligand substitution reactions on diiron complexes governed by nucleophilic and electrophilic ligand properties.

The discovery of a diiron organometallic site in nature within the diiron hydrogenase, [FeFe]-H2ase, active site has prompted revisits of the classic organometallic chemistry involving the Fe-Fe bond and bridging ligands, particularly of the (μ-SCH2XCH2S)[Fe(CO)3]2 and (μ-SCH2XCH2S)[Fe(CO)2L]2 (X = CH2, NH; L = PMe3, CN(-), and NHC's (NHC = N-heterocyclic carbene)), derived from CO/L exchange reactions. Through the synergy of synthetic chemistry and density functional theory computations, the regioselectivity of nucleophilic (PMe3 or CN(-)) and electrophilic (nitrosonium, NO(+)) ligand substitution on the diiron dithiolate framework of the (μ-pdt)[Fe(CO)2NHC][Fe(CO)3] complex (pdt = propanedithiolate) reveals the electron density shifts in the diiron core of such complexes that mimic the [FeFe]-H2ase active site. While CO substitution by PMe3, followed by reaction with NO(+), produces (μ-pdt)(μ-CO)[Fe(NHC)(NO)][Fe(CO)2PMe3](+), the alternate order of reagent addition produces the structural isomer (μ-pdt)[Fe(NHC)(NO)PMe3][Fe(CO)3](+), illustrating how the nucleophile and electrophile choose the electron-poor metal and the electron-rich metal, respectively. Theoretical explorations of simpler analogues, (μ-pdt)[Fe(CO)2CN][Fe(CO)3](-), (μ-pdt)[Fe(CO)3]2, and (μ-pdt)[Fe(CO)2NO][Fe(CO)3](+), provide an explanation for the role that the electron-rich iron moiety plays in inducing the rotation of the electron-poor iron moiety to produce a bridging CO ligand, a key factor in stabilizing the electron-rich iron moiety and for support of the rotated structure as found in the enzyme active site.