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

Abstract

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)₂}²⁺ units in stabilizing the low-spin Fe(II) center. Reaction of the ‘apo-ligand’ 6-(2-(methylthio)phenyl)pyridin-2-amine (^H2NN^pyS^Me) with [Fe(CO)₄(Br)₂] affords the organometallic complex [(^OC^NHN^pyS^Me)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 the type [(^OC^NHN^pyS^Me)Fe(CO)₂(L)]⁺ (2a–f). Treatment of 1 with Na[S(2,6-Me₂C₆H₃)] affords the neutral complex [(^OC^NHN^pyS^Me)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 it afforded the six-coordinate monocarbonyl complexes [(^OC^NHN^pyS^Me)Fe(CO)(Br)(PPh₃)] (3a), [(^OC^NHN^pyS^Me)Fe(CO)(PPh₃)₂](BAr^F₄) (3b), and [(^OC^NHN^pyS^Me)Fe(CO)(py)₂](BAr^F₄) (3c). Interestingly the stable low-spin Fe(II), 5-coordinate complex of the formula [(^OC^NHN^pyS^Me)Fe(CO)₂](BAr^F₄) (4) was accessed by treating 1 with TlBAr^F₄ in non-coordinating solvents (DCE, FPh); notably, 4 does not react with H₂ 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(CO) in 4⁺. The electronic structure of 4⁺ predicts that the HOMO (comprised of Fe|N_carb) 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 a related work from our lab, we conclude that a facial C,N,S coordination mode is necessary to promote H₂ activation and cleavage.