Planar three-coordinate iron sulfide in a synthetic [4Fe-3S] cluster with biomimetic reactivity.

Iron-sulfur clusters are emerging as reactive sites for the reduction of small-molecule substrates. However, the four-coordinate iron sites of typical iron-sulfur clusters rarely react with substrates, implicating three-coordinate iron. This idea is untested because fully sulfide-coordinated three-coordinate iron is unprecedented. Here we report a new type of [4Fe-3S] cluster that features an iron centre with three bonds to sulfides, and characterize examples of the cluster in three oxidation levels using crystallography, spectroscopy, and ab initio calculations. Although a high-spin electronic configuration is characteristic of other iron-sulfur clusters, the three-coordinate iron centre in these clusters has a surprising low-spin electronic configuration due to the planar geometry and short Fe-S bonds. In a demonstration of biomimetic reactivity, the [4Fe-3S] cluster reduces hydrazine, a natural substrate of nitrogenase. The product is the first example of NH2 bound to an iron-sulfur cluster. Our results demonstrate that three-coordinate iron supported by sulfide donors is a plausible precursor to reactivity in iron-sulfur clusters like the FeMoco of nitrogenase.

A [2Fe-1S] Complex That Affords Access to Bimetallic and Higher-Nuclearity Iron-Sulfur Clusters.

Small, coordinatively unsaturated iron-sulfur clusters are conceived as building blocks for the diverse set of shapes of iron-sulfur clusters in biological and synthetic chemistry. Here we describe a synthetic method for preparing [2Fe-1S] clusters containing two iron(II) ions, which are supported by a relatively unhindered ß-diketiminate supporting ligand. The [2Fe-1S] cluster can be isolated in the presence of trimethylphosphine, and the compound with one PMe3 on each iron(II) ion has been crystallographically characterized. The PMe3 ligands may be removed with B(C6F5)3 to give a spectroscopically characterized species with solvent ligands. This species is a versatile synthon for [2Fe-2S], [4Fe-3S], and [10Fe-8S] clusters. Crystallographic characterization of the 10Fe cluster shows that it has all iron(II) ions, and the core has two [4Fe-4S] cubes that share a face in a novel arrangement. This cluster also has two iron sites that are coordinated to solvent donors, suggesting the potential for using this type of cluster for reactivity in the future.

Reduction of CO2 by a masked two-coordinate cobalt(i) complex and characterization of a proposed oxodicobalt(ii) intermediate.

Fixation and chemical reduction of CO2 are important for utilization of this abundant resource, and understanding the detailed mechanism of C-O cleavage is needed for rational development of CO2 reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(i) complex, L tBuCo (where L tBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO2, which yields two products of C-O cleavage, the cobalt(i) monocarbonyl complex L tBuCo(CO) and the dicobalt(ii) carbonate complex (L tBuCo)2(µ-CO3). Kinetic studies and computations show that the κN,η6-arene isomer of L tBuCo rearranges to the κ2 N,N' binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to L tBuCo. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO2 proceed through bimetallic mechanisms, and DFT and highly correlated domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations reveal the cooperative effects of the two metal centers during facile C-O bond rupture. A plausible intermediate in the reaction of CO2 with L tBuCo is the oxodicobalt(ii) complex L tBuCoOCoL tBu, which has been independently synthesized through the reaction of L tBuCo with N2O. The rapid reaction of L tBuCoOCoL tBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by L tBuCo. L tBuCoOCoL tBu is a novel example of a thoroughly characterized molecular cobalt-oxo complex where the cobalt ions are clearly in the +2 oxidation state. Its nucleophilic reactivity is a consequence of high charge localization on the µ-oxo ligand between two antiferromagnetically coupled high-spin cobalt(ii) centers, as characterized by DFT and multireference complete active space self-consistent field (CASSCF) calculations.

Iron and Cobalt Diazoalkane Complexes Supported by ß-Diketiminate Ligands: A Synthetic, Spectroscopic, and Computational Investigation.

Diazoalkanes are interesting redox-active ligands and also precursors to carbene fragments. We describe a systematic study of the binding and electronic structure of diphenyldiazomethane complexes of ß-diketiminate supported iron and cobalt, which span a range of formal d-electron counts of 7-9. In end-on diazoalkane complexes of formally monovalent three-coordinate transition metals, the electronic structures are best described as having the metal in the +2 oxidation state with an antiferromagnetically coupled radical anion diazoalkane as shown by crystallography, spectroscopy, and computations. A formally zerovalent cobalt complex has different structures depending on whether potassium binds; potassium binding gives transfer of two electrons into the η2-diazoalkane, but the removal of the potassium with crown ether leads to a form with only one electron transferred into an η1-diazoalkane. These results demonstrate the influence of potassium binding and metal oxidation state on the charge localization in the diazoalkane complexes. Interestingly, none of these reduced complexes yield carbene fragments, but the new cobalt(II) complex LtBuCoPF6 (LtBu = bulky ß-diketiminate) does catalyze the formation of an azine from its cognate diazoalkane, suggesting N2 loss and transient carbene formation.

Synthesis and Mechanism of Formation of Hydride-Sulfide Complexes of Iron.

Iron-sulfide complexes with hydride ligands provide an experimental precedent for spectroscopically detected hydride species on the iron-sulfur MoFe7S9C cofactor of nitrogenase. In this contribution, we expand upon our recent synthesis of the first iron sulfide hydride complex from an iron hydride and a sodium thiolate ( Arnet, N. A.; Dugan, T. R.; Menges, F. S.; Mercado, B. Q.; Brennessel, W. W.; Bill, E.; Johnson, M. A.; Holland, P. L., J. Am. Chem. Soc. 2015 , 137 , 13220 - 13223 ). First, we describe the isolation of an analogous iron sulfide hydride with a smaller diketiminate supporting ligand, which benefits from easier preparation of the hydride precursor and easier isolation of the product. Second, we describe mechanistic studies on the C-S bond cleavage through which the iron sulfide hydride product is formed. In a key experiment, use of cyclopropylmethanethiolate as the sulfur precursor leads to products from cyclopropane ring opening, implicating an alkyl radical as an intermediate. Combined with the results of isotopic labeling studies, the data are consistent with a mechanism in which homolytic C-S bond cleavage is followed by rebound of the alkyl radical to abstract a hydrogen atom from iron to give the observed alkane and iron-sulfide products.

Enhancement of C-H Oxidizing Ability in Co-O2 †Complexes through an Isolated Heterobimetallic Oxo Intermediate.

The characterization of intermediates formed through the reaction of transition-metal complexes with dioxygen (O2 ) is important for understanding oxidation in biological and synthetic processes. Here, the reaction of the diketiminate-supported cobalt(I) complex LtBu Co with O2 gives a rare example of a side-on dioxygen complex of cobalt. Structural, spectroscopic, and computational data are most consistent with its assignment as a cobalt(III)-peroxo complex. Treatment of LtBu Co(O2 ) with low-valent Fe and Co diketiminate complexes affords isolable oxo species with M2 O2 "diamond" cores, including the first example of a crystallographically characterized heterobimetallic bis(µ-oxo) complex of two transition metals. The bimetallic species are capable of cleaving C-H bonds in the supporting ligands, and kinetic studies show that the Fe/Co heterobimetallic species activates C-H bonds much more rapidly than the Co/Co homobimetallic analogue. Thus heterobimetallic oxo intermediates provide a promising route for enhancing the rates of oxidation reactions.

C-H and C-N Activation at Redox-Active Pyridine Complexes of Iron.

Pyridine activation by inexpensive iron catalysts has great utility, but the steps through which iron species can break the strong (105-111 kcal mol-1 ) C-H bonds of pyridine substrates are unknown. In this work, we report the rapid room-temperature cleavage of C-H bonds in pyridine, 4-tert-butylpyridine, and 2-phenylpyridine by an iron(I) species, to give well-characterized iron(II) products. In addition, 4-dimethylaminopyridine (DMAP) undergoes room-temperature C-N bond cleavage, which forms a dimethylamidoiron(II) complex and a pyridyl-bridged tetrairon(II) square. These facile bond-cleaving reactions are proposed to occur through intermediates having a two-electron reduced pyridine that bridges two iron centers. Thus, the redox non-innocence of the pyridine can play a key role in enabling high regioselectivity for difficult reactions.

Spin Isomers and Ligand Isomerization in a Three-Coordinate Cobalt(I) Carbonyl Complex.

Hemilabile ligands, which have one donor that can reversibly bind to a metal, are widely used in transition-metal catalysts to create open coordination sites. This change in coordination at the metal can also cause spin-state changes. Here, we explore a cobalt(I) system that is poised on the brink of hemilability and of a spin-state change and can rapidly interconvert between different spin states with different structures ("spin isomers"). The new cobalt(I) monocarbonyl complex L(tBu)Co(CO) (2) is a singlet ((1)2) in the solid state, with an unprecedented diketiminate binding mode where one of the C═C double bonds of an aromatic ring completes a pseudo-square-planar coordination. Dissolving the compound gives a substantial population of the triplet ((3)2), which has exceptionally large uniaxial zero-field splitting due to strong spin-orbit coupling with a low-lying excited state. The interconversion of the two spin isomers is rapid, even at low temperature, and temperature-dependent NMR and electronic absorption spectroscopy studies show the energy differences quantitatively. Spectroscopically validated computations corroborate the presence of a low minimum-energy crossing point (MECP) between the two potential energy surfaces and elucidate the detailed pathway through which the ß-diketiminate ligand "slips" between bidentate and arene-bound forms: rather than dissociation, the cobalt slides along the aromatic system in a pathway that balances strain energy and cobalt-ligand bonding. These results show that multiple spin states are easily accessible in this hemilabile system and map the thermodynamics and mechanism of the transition.