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.

A reduced 2Fe2S cluster probe of sulfur-hydrogen versus sulfur-gold interactions.

The Ph3 PAu(+) cation, renowned as an isolobal analogue of H(+) , was found to serve as a proton surrogate and form a stable Au2 Fe2 complex, [(µ-SAuPPh3 )2 {Fe(CO)3 }2 ], analogous to the highly reactive dihydrosulfide [(µ-SH)2 {Fe(CO)3 }2 ]. Solid-state X-ray diffraction analysis found the two SAuPPh3 and SH bridges in anti configurations. VT NMR studies, supported by DFT computations, confirmed substantial barriers of approximately 25 kcal mol(-1) to intramolecular interconversion between the three stereoisomers of [(µ-SH)2 {Fe(CO)3 }2 ]. In contrast, the largely dative SAu bond in µ-SAuPPh3 facilitates inversion at S and accounts for the facile equilibration of the SAuPPh3 units, with an energy barrier half that of the SH analogue. The reactivity of the gold-protected sulfur atoms of [(µ-SAuPPh3 )2 {Fe(CO)3 }2 ] was accessed by release of the gold ligand with a strong acid to generate the [(µ-SH)2 {Fe(CO)3 }2 ] precursor of the [FeFe]H2 ase-active-site biomimetic [(µ2 -SCH2 (NR)CH2 S){Fe(CO)3 }2 ].

Metallodithiolates as ligands to dinitrosyl iron complexes: toward the understanding of structures, equilibria, and spin coupling.

Metallodithiolate ligands are used to design heterobimetallic complexes by adduct formation through S-based reactivity. Such adducts of dinitrosyl iron were synthesized with two metalloligands, namely, Ni(bme-daco) and V≡O(bme-daco) (bme-daco = bismercaptoethane diazacyclooctane), and, for comparison, an N-heterocyclic carbene, namely, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (Imes), by cleavage of the (µ-I)2[Fe(NO)2]2 dimer of electronic configuration {Fe(NO)2}(9) (Enemark-Feltham notation). With Fe(NO)2I as Lewis acid acceptor, 1:1 adducts resulted for both the IMes·Fe(NO)2I, complex 2, and V≡O(bme-daco)·Fe(NO)2I, complex 4. The NiN2S2 demonstrated binding capability at both thiolates, with two Fe(NO)2I addenda positioned transoid across the NiN2S2 square plane, Ni(bme-daco)·2(Fe(NO)2I), complex 3. Enhanced binding ability was realized for the dianionic vanadyl dithiolate complex, [Et4N]2[V≡O(ema)], (ema = N,N'-ethylenebis(2-mercaptoacetamide)), which, unlike the neutral (V≡O)N2S2, demonstrated reactivity with the labile tungsten carbonyl complex, cis-W(CO)4(pip)2, (pip = piperidine), yielding [Et4N]2[V≡O(ema)W(CO)4], complex 1, whose ν(CO) IR values indicated the dianionic vanadyl metalloligand to be of similar donor ability to the neutral NiN2S2 ligands. The solid-state molecular structures of 1-4 were determined by X-ray diffraction analyses. Electron paramagnetic resonance (EPR) measurements characterize the {Fe(NO)2}(9) complexes in solution, illustrating superhyperfine coupling via the (127)I to the unpaired electron on iron for complex 2. The EPR characterizations of 3 [Ni(bme-daco)·2(Fe(NO)2I)] and 4 [V≡O(bme-daco)·Fe(NO)2I] indicate these complexes are EPR silent, likely due to strong coupling between paramagnetic centers. Within samples of complex 4, individual paramagnetic centers with localized superhyperfine coupling from the (51)V and (127)I are observed in a 3:1 ratio, respectively. However, spin quantitation reveals that these species represent a minor fraction (<10%) of the total complex and thus likely represent disassociated paramagnetic sites. Computational studies corroborated the EPR assignments as well as the experimentally observed stability/instability of the heterobimetallic DNIC complexes.

Carbon monoxide induced reductive elimination of disulfide in an N-heterocyclic carbene (NHC)/thiolate dinitrosyl iron complex (DNIC).

Dinitrosyliron complexes (DNICs) are organometallic-like compounds of biological significance in that they appear in vivo as products of NO degradation of iron-sulfur clusters; synthetic analogues have potential as NO storage and releasing agents. Their reactivity is expected to depend on ancillary ligands and the redox level of the distinctive Fe(NO)2 unit: paramagnetic {Fe(NO)2}(9), diamagnetic dimerized forms of {Fe(NO)2}(9) and diamagnetic {Fe(NO)2}(10) DNICs (Enemark-Feltham notation). The typical biological ligands cysteine and glutathione themselves are subject to thiolate-disulfide redox processes, which when coupled to DNICs may lead to intricate redox processes involving iron, NO, and RS(-)/RS•. Making use of an N-heterocyclic carbene-stabilized DNIC, (NHC)(RS)Fe(NO)2, we have explored the DNIC-promoted RS(-)/RS• oxidation in the presence of added CO wherein oxidized {Fe(NO)2}(9) is reduced to {Fe(NO)2}(10) through carbon monoxide (CO)/RS• ligand substitution. Kinetic studies indicate a bimolecular process, rate = k [Fe(NO)2](1)[CO](1), and activation parameters derived from kobs dependence on temperature similarly indicate an associative mechanism. This mechanism is further defined by density functional theory computations. Computational results indicate a unique role for the delocalized frontier molecular orbitals of the Fe(NO)2 unit, permitting ligand exchange of RS• and CO through an initial side-on approach of CO to the electron-rich N-Fe-N site, ultimately resulting in a 5-coordinate, 19-electron intermediate with elongated Fe-SR bond and with the NO ligands accommodating the excess charge.