A C,N,S pincer complex has been synthesized for structural modeling of the organometallic active site of mono-[Fe] hydrogenase (HMD). The C,N,S chelate set allows for systematic investigation of the substitution reactions of CO and other exogenous X/L-type ligands, as well as examination of the exact roles of the Fe–carbamoyl and {Fe(CO)2}2+ units in stabilizing the low-spin Fe(II) center. Reaction of 6-(2-(methylthio)phenyl)pyridin-2-amine (H2NNpySMe) with the [Fe(CO)4(Br)2] affords the organometallic complex [(O=CNHNpySMe)Fe(CO)2(Br)] (1). Facile substitutions of the halide with L-type ligands such as MeCN, PR3 (R = C6H5, OEt, Me), pyridine and tBuNC afford diamagnetic cations of type [(O=CNHNpySMe)Fe(CO)2(L)]+ (2a-f). Treatment of 1 with [Na][S(2,6-Me2C6H3)] affords the neutral complex [(O=CNHNpySMe)Fe(CO)2(S(2,6-Me2C6H3))] (2g). Substitution for CO ligand(s) was achieved with trimethylamine-N-oxide (TMAO), and in the presence of PPh3 or pyridine afforded the six-coordinate monocarbonyl complexes [(O=CNHNpySMe)Fe(CO)(Br)(PPh3)] (3a), [(O=CNHNpySMe)Fe(CO)(PPh3)2](BArF4) (3b), [(O=CNHNpySMe)Fe(CO)(py)2](BArF4) (3c). Interestingly the stable low-spin Fe(II), 5-coordinate complex of formula [(O=CNHNpySMe)Fe(CO)2](BArF4) (4) was accessed by treating 1 with TlBArF4 in non-coordinating solvent (DCE, FPh); notably, 4 does not react with H2 in the presence (or absence) of base. To elucidate the electronic structure differences between the five-coordinate versus six-coordinate complexes, DFT calculations for 4 and 1 were performed. Geometry optimization indicates 4+ maintains a square-pyramidal geometry, and the Hessian calculation accurately simulates the ν(C≡O) in 4+, but a large error (&gt;100 cm–1) occurs in the predicted carbamoyl ν(C=O) frequency. The electronic structure of 4+ shows that the HOMO (comprised of Fe|Ncarb) and LUMO (Fe only) orbitals in 4+ are properly oriented to interact with an incoming ligand. However, we postulate that codirectional orientation of the HOMO and LUMO orbitals explains the lack of H2 reactivity with this equatorial CNS donor set, despite many other structural similarities to the endogenous active site. Based on related work from our lab, we conclude that a facial C,N,S coordination mode is deemed necessary to promote H2 cleavage.

Substitution reactions of iron(II) carbamoyl-thioether complexes related to mono-iron hydrogenase

254th National Meeting (2017)

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