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.