nitrogenase and carbene

The all-ferrous, carbene-capped Fe(4)S(4) cluster, synthesized by Deng and Holm (DH complex), has been studied with density functional theory (DFT). The geometry of the complex was optimized for several electronic configurations. The lowest energy was obtained for the broken-symmetry (BS) configuration derived from the ferromagnetic state by reversing the spin projection of one of the high spin (S(i) = 2) irons. The optimized geometry of the latter configuration contains one unique and three equivalent iron sites, which are both structurally and electronically clearly distinguishable. For example, a distinctive feature of the unique iron site is the diagonal Fe···S distance, which is 0.3 Å longer than for the equivalent irons. The calculated (57)Fe hyperfine parameters show the same 1:3 pattern as observed in the Mössbauer spectra and are in good agreement with experiment. BS analysis of the exchange interactions in the optimized geometry for the 1:3, M(S) = 4, BS configuration confirms the prediction of an earlier study that the unique site is coupled to the three equivalent ones by strong antiferromagnetic exchange (J > 0 in J Σ(j<4)Ŝ(4)·Ŝ(j)) and that the latter are mutually coupled by ferromagnetic exchange (J' < 0 in J' Σ(i

Topics: Electron Spin Resonance Spectroscopy; Iron; Iron-Sulfur Proteins; Magnetics; Methane; Models, Molecular; Nitrogenase; Quantum Theory; Spectroscopy, Mossbauer; Spin Trapping; Sulfur; Thermodynamics

It is well established that the cysteinate-coordinated [Fe(4)S(4)] cluster of the iron protein of nitrogenase from Azotobacter vinelandii (Av2) can attain the all-ferrous core oxidation state. Mössbauer and electron paramagnetic resonance (EPR) studies have shown that the all-ferrous cluster has a ground-state spin S = 4 and an effective 3:1 site symmetry in the spin structure and (57)Fe quadrupole interactions. Recently, Deng and Holm reported the synthesis of [Fe(4)S(4)(Pr(i)(2)NHCMe(2))(4)],(2) (1; Pr(i)(2)NHCMe(2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and showed that the all-ferrous carbene-coordinated cluster is amenable to physicochemical studies. Mössbauer and EPR studies of 1, reported here, reveal that the electronic structure of this complex is strikingly similar to that of the protein-bound cluster, suggesting that the ground-state spin and the 3:1 site ratio are consequences of spontaneous distortions of the cluster core. To gain insight into the origin of the peculiar ground state of the all-ferrous clusters in 1 and Av2, we have studied a theoretical model that is based on a Heisenberg-Dirac-van Vleck Hamiltonian whose exchange-coupling constants are a function of the Fe-Fe distances. By combining the exchange energies with the elastic deformation energies in the harmonic approximation, we obtain for a T(2) distortion a minimum with spin S = 4 and a C(3v) core structure in which one iron is unique and three irons are equivalent. This minimum has all of the spectroscopic and structural characteristics of the all-ferrous clusters of 1 and Av2. Our analysis maps the unique spectroscopic iron site to a specific site in the X-ray structure of the [Fe(4)S(4)](0) core both in complex 1 and in Av2.

Topics: Azotobacter vinelandii; Electron Spin Resonance Spectroscopy; Iron-Sulfur Proteins; Methane; Models, Chemical; Models, Molecular; Nitrogenase

The all-ferrous [Fe4S4](0) state has been demonstrated in the fully reduced Fe protein of the Azotobacter vinelandii nitrogenase complex. We seek synthetic analogues of this state more tractable than the recently prepared but highly unstable cluster [Fe4S4(CN)4](4-) (Scott, Berlinguette, Holm, and Zhou, Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9741). The N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Pr(i)2NHCMe2) has been found to stabilize the fully reduced clusters [Fe8S8(Pr(i)2NHCMe2)6] (4) and [Fe4S4(Pr(i)2NHCMe2)4] (5), which are prepared by cluster assembly or phosphine substitution of FenSn (n = 8, 16) clusters. Cluster 4 is also obtained by reaction of the carbene with all-ferrous [Fe7S6(PEt3)5Cl2] (3) and cluster 5 by carbene cleavage of 4. Detailed structures of 3 (monocapped prismatic), 4, and 5 are described; the latter two are the first iron-sulfur clusters with Fe-C sigma bonds. Cluster 4 possesses the [Fe8(mu3-S) 6(mu4-S)2] edge-bridged double cubane structure and 5 the cubane-type [Fe4(mu3-S)4] stereochemistry. The all-ferrous formulations of the clusters are confirmed by X-ray structure parameters and (57)Fe isomer shifts. Both clusters are stable under conventional aprotic anaerobic conditions, enabling further study of reactivity. The collective properties of 5 indicate that it is a meaningful synthetic analogue of the core of the fully reduced protein-bound cluster.

Topics: Biomimetic Materials; Ferrous Compounds; Iron-Sulfur Proteins; Methane; Models, Molecular; Nitrogenase; Oxidation-Reduction; Solutions; Spectroscopy, Mossbauer; X-Ray Diffraction