The enzyme mono-iron hydrogenase (mono-[Fe] H2ase) catalyzes a hydride transfer directly from dihydrogen (H2) to the organic substrate tetrahydromethenylmethanopterin (H4MPT+), which acts as a C1 carrier in the methanogenic CO2→CH4 metabolic cyle. This hydride transfer is just one step in the Fe-dependent (i.e. Ni-free) pathway of CO2 utilization. The active site contains several unique ligands, including an iron-acyl unit, a pyridone-N donor (part of a GMP-based cofactor), and a cysteine thiolate. In this work, we have assembled a chelating ligand that facilitates iron-carbon bond formation, as well as a bound pyridine-N and sulfur donor (thioether). This results in a highly stable platform to study substitution reactions, as well as base-activation of the ligand framework (carbamoyl NH). In the resulting complex [(CNS)Fe(CO)2(Br)], facile substitution of bromide leads to binding of various L (PPh3, tBuiNC, pyr, MeCN) in the diamagnetic, structurally characterized cations of type [(CNS)Fe(CO)2(L)][X] [X = BF4, BArF]. Substitution of a single CO ligand using Me3NO, however, affords paramagnetic complexes of type [(NCS)Fe(CO)(L)]0/+ (L = Br, PPh3). Investigations into ligand-metal cooperativity via deprotonation of the carbamoyl NH unit (tBuOK, THF) result in desulfurization of the ligand, forming the cyclometalated product [(CNC)Fe(CO)(PPh3)2]. This pincer-type complex is one of very few mononuclear, cyclometalated Fe(II) reported in the literature, and the only one that exhibits the iron-acyl motif. The implications of these reactivities for understanding the role of this unique set of donors in the mono-[Fe] hydrogenase active site will be discussed.

Tethered C,N,S ligands containing an iron-carbamoyl motif as synthetic models of mono-iron hydrogenase

249th National Meeting (2015)

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