A new C,N,S pincer complex has been derived for structural modeling of the organometallic active site of mono-[Fe] hydrogenase (HMD). The tethered C,N,S ligand set allows for systematic investigation of the exact roles of the Fe–carbamoyl and {Fe(CO)2}2+ units in stabilizing the low-spin Fe(II) as found in the enzyme. Reaction of the base ligand 6-(2-(methylthio)phenyl)pyridin-2-amine (H2NNpySMe) with the iron(II) carbonyl source [Fe(CO)4(Br)2] affords the organometallic complex [(O=CNHNpySMe)Fe(CO)2(Br)] (1), wherein a ferracyclic carbamoyl unit has been formed by nucleophilic addition of the ortho-NH2 with one carbonyl ligand. Complex 1 is diamagnetic, and exhibits IR features (2034, 1974, and 1620 cm–1) derived from the two carbonyl ligands and carbamoyl unit, respectively. In order to study the property a putative iron-hydride species, the hydride complex [(O=CHNNpySMe)Fe(H)(CO)2] (2) is derived from reaction with NaHBEt3. The iron-hydride intermediate is stable at –80°C, evidenced by 1H NMR (δFe-H = –5.08 ppm). Above –40 °C, the hydride species extrudes CH3S– via an intramolecular hydride transfer, which is stoichiometrically trapped in the structurally characterized dimer µ2-(CH3S)2-[(O=CHNCNPh)Fe(CO)2]2 (3). Alternately, when activated by base (tBuOK), 1 undergoes desulfurization to form a cyclometalated species, [(O=CNHNCPh)Fe(CO)2] (4); derivatization of 4 with PPh3 affords the structurally characterized species [(O=CNHNC)Fe(CO)(PPh3)2] (5). In another vein, facile substitution of the halide with L-type ligands such as MeCN, PR3 (R = -C6H5, OEt, Me), pyridine and tBuNC affords the stable diamagnetic di-carbonyl cationic species [(O=CNHNpySMe)Fe(CO)2(L)]+ (6-11) in each case. Starting from [(O=CNHNpySMe)Fe(CO)2(PPh3)]+ (6) and [(O=CNHNpySMe)Fe(CO)2(Py)]+ (9), substitutions of one CO (using Me3NO) with PPh3 or pyridine produce stable diamagnetic mono-carbonyl complexes [(O=CNHNpySMe)Fe(CO)(PPh3)2] (12) and [(O=CNHNpySMe)Fe(CO)(Py)2] (13). Interestingly, reaction of 1 directly with excess of Me3NO and 1 equiv. of PPh3 affords another diamagnetic mono-carbonyl species [(O=CNHNpySMe)Fe(CO)(PPh3)(Br)] (14), which promotes C–H activation of the trimethylamine-N-oxide reagent(s) to generate the N-alkylated species [(O=CNCH2N(Me)2NpySMe)Fe(CO)(PPh3)(Br)] (15). The structures, spin-states and reactivity patterns found in these complexes will be discussed in light of the native enzyme.

Synthetic modeling of mono-iron hydrogenase: CNS Chelates supporting an iron-hydride species, substitution reactions, and C–H activation of TMAO

251st National Meeting (2016)

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