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

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 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)} units in stabilizing the low-spin Fe(ii) center. Reaction of the 'apo-ligand' 6-(2-(methylthio)phenyl)pyridin-2-amine (NS) with [Fe(CO)(Br)] affords the organometallic complex [(CNS)Fe(CO)(Br)] (1). Facile substitutions of the halide with L-type ligands such as MeCN, PR (R = CH, OEt, Me), pyridine and BuNC afford diamagnetic cations of the type [(CNS)Fe(CO)(L)] (2a-f). Treatment of 1 with Na[S(2,6-MeCH)] affords the neutral complex [(CNS)Fe(CO)(S(2,6-MeCH))] (2g). Substitution for CO ligand(s) was achieved with trimethylamine-N-oxide (TMAO), and in the presence of PPh or pyridine it afforded the six-coordinate monocarbonyl complexes [(CNS)Fe(CO)(Br)(PPh)] (3a), (CNS)Fe(CO)(PPh) (3b), and (CNS)Fe(CO)(py) (3c). Interestingly the stable low-spin Fe(ii), 5-coordinate complex of the formula (CNS)Fe(CO) (4) was accessed by treating 1 with TlBAr in non-coordinating solvents (DCE, FPh); notably, 4 does not react with H in the presence (or absence) of a 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 that 4+ maintains a square-pyramidal geometry, and the Hessian calculation accurately simulates the ν(C[triple bond, length as m-dash]O) in 4+. The electronic structure of 4+ predicts that the HOMO (comprised of Fe|N) 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 H reactivity with this equatorial CNS donor set, despite many other structural similarities to the endogenous active site. Based on a related work from our lab, we conclude that a facial C,N,S coordination mode is necessary to promote H activation and cleavage.