Jason A. Denny

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 NHCs (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 ].

CCC-NHC Pincer Complexes

Abstract The synthesis of organometallic CCC-NHC pincer complexes is discussed along with their applications. Such complexes span the transition metals of the periodic table with examples including group 4, 5, 8, 9, and 10 metals. Lanthanide, group 3, Ir, and Pd complexes of C^C^C-NHC pincer ligands are also reported. Such a variety highlights the broadly applicable nature of the CCC-NHC ligand set. Organometallic CCC-NHC pincer complexes are employed as efficient catalysts and emitters. Such complexes are achieved via three major routes: metalation with amido regents, transmetalation from early to late metals, or by direct metalation of a suitable proligand. Yields of these syntheses vary greatly, depending upon the metal and the reaction conditions. Researchers have used the modular nature of the ligand precursors to engineer around unwanted side products.