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)2}2+ units in stabilizing the low-spin Fe(ii) center. Reaction of the 'apo-ligand' 6-(2-(methylthio)phenyl)pyridin-2-amine (H2NNpySMe) with [Fe(CO)4(Br)2] affords the organometallic complex [(O[double bond, length as m-dash]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 the type [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)2(L)]+ (2a-f). Treatment of 1 with Na[S(2,6-Me2C6H3)] affords the neutral complex [(O[double bond, length as m-dash]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 it afforded the six-coordinate monocarbonyl complexes [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)(Br)(PPh3)] (3a), [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)(PPh3)2](BArF4) (3b), and [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)(py)2](BArF4) (3c). Interestingly the stable low-spin Fe(ii), 5-coordinate complex of the formula [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)2](BArF4) (4) was accessed by treating 1 with TlBArF4 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(C[triple bond, length as m-dash]O) in 4+. The electronic structure of 4+ predicts 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 a related work from our lab, we conclude that a facial C,N,S coordination mode is necessary to promote H2 activation and cleavage.

Structures, Interconversions, and Spectroscopy of Iron Carbonyl Clusters with an Interstitial Carbide: Localized Metal Center Reduction by Overall Cluster Oxidation.

The syntheses, interconversions, and spectroscopic properties of a set of iron carbonyl clusters containing an interstitial carbide are reported. This includes the low temperature X-ray structures of the six-iron clusters (Y)2[Fe6(µ6-C)(µ2-CO)4(CO)12] (1a-c; where Y = NMe4, NEt4, PPh4); the five-iron cluster [Fe5(µ5-C)(CO)15] (3); and the novel formulation of the five-iron cluster (NMe4)2[Fe5(µ5-C)(µ2-CO)(CO)13] (4). Also included in this set is the novel charge-neutral cluster, [Fe6(µ6-C)(CO)18] (2), for which we were unable to obtain a crystallographic structure. As synthetic proof for the identity of 2, we performed a closed loop of interconversions within a family of crystallographically defined species (1, 3, and 4): [Fe6]2- → [Fe6]0 → [Fe5]0 → [Fe5]2- → [Fe6]2-. The structural, spectroscopic, and electronic properties of this "missing link" cluster 2 were investigated by IR, Raman, XPS, and Mössbauer spectroscopies-as well as by DFT calculations. A single νCO feature (1965 cm-1) in the IR spectrum of 2, as well as a prominent Raman feature (νsymm = 1550 cm-1), are consistent with the presence of terminal carbonyls and a {(µ6-C)Fe6} arrangement of iron centers around the central carbide. The XPS of 2 exhibits a higher energy Fe 2p3/2 feature (707.4 eV) as compared to that of 1 (705.5 eV), consistent with the two-electron oxidation induced by treatment of 1 with two equivalents of [Fc](PF6) under CO atmosphere (for the two added CO ligands). DFT calculations indicate two axial and four equatorial Fe sites in 1, all of which have the same or similar oxidation states, for example, two Fe(0) and four Fe(+0.5). These assignments are supported by Mössbauer spectra for 1, which exhibit two closely spaced quadrupole doublets with δ = 0.076 and 0.064 mm s-1. The high-field Mössbauer spectrum of 2 (4.2 K) exhibits three prominent quadrupole doublets with δ = -0.18, -0.11, and +0.41 mm s-1. This indicates three pairs of chemically equivalent Fe sites. The first two pairs arise from irons of a similar oxidation state, while the last pair arises from irons in a different oxidation state, indicating a mixed-valent cluster. Variable field Mössbauer spectra for 2 were simulated assuming these two groups and a diamagnetic ground state. Taken together, the Mössbauer results and DFT calculations for 2 indicate two axial Fe(II) sites and four equatorial sites of lower valence, probably Fe(0). In the DFT optimized pentagonal bipyramidal structure for 2, the Fe(II)-Ccarbide distances are compressed (∼1.84 Å), while the Fe(0)-Ccarbide distances are elongated (∼2.05 Å). Analysis of the formulations for 1 (closo-square bipyramid) and 2 (nido-pentagonal bipyramid) is considered in the context of the textbook electron-counting rules of 14n+2 and 14n+4 for closo and nido clusters, respectively. This redox-dependent intracluster disproportionation of Fe oxidation states is concluded to arise from changes in bonding to the central carbide. A similar phenomenon may be promoted by the central carbide of the FeMoco cluster of nitrogenase, which may in turn stimulate N2 reduction.