Abstract:We report a survey of the reactivity of the first isolable iron-hydride complexes with a coordination number less than 5. The high-spin iron(II) complexes [( -diketiminate)Fe(µ-H)] 2 react rapidly with representative cyanide, isocyanide, alkyne, N 2, alkene, diazene, azide, CO2, carbodiimide, and Brønsted acid containing substrates. The reaction outcomes fall into three categories: (1) addition of Fe-H across a multiple bond of the substrate, (2) reductive elimination of H 2 to form iron(I) products, and (3) protonation of the hydride to form iron(II) products. The products include imide, isocyanide, vinyl, alkyl, azide, triazenido, benzo[c]cinnoline, amidinate, formate, and hydroxo complexes. These results expand the range of known bond transformations at iron complexes. Additionally, they give insight into the elementary transformations that may be possible at the iron-molybdenum cofactor of nitrogenases, which may have hydride ligands on high-spin, low-coordinate metal atoms.

The isomerization of simple terminal alkenes to internal isomers with Z-stereochemistry is rare, because the more stable E-isomers are typically formed. We show here that cobalt(II) catalysts supported by bulky β-diketiminate ligands have the appropriate kinetic selectivity to catalyze the isomerization of some simple 1-alkenes specifically to the 2-alkene as the less stable Z-isomer. The catalysis proceeds via an "alkyl" mechanism, with a three-coordinate cobalt(II) alkyl complex as the resting state. β-Hydride elimination and [1,2]-insertion steps are both rapid, as shown by isotopic labeling experiments. A steric model explains the selectivity through a square-planar geometry at cobalt(II) in the transition state for β-hydride elimination. The catalyst works not only with simple alkenes, but also with homoallyl silanes, ketals, and silyl ethers. Isolation of cobalt(I) or cobalt(II) products from reactions with poor substrates suggests that the key catalyst decomposition pathways are bimolecular, and lowering the catalyst concentration often improves the selectivity. In addition to a potentially useful, selective transformation, these studies provide a mechanistic understanding for catalytic alkene isomerization by high-spin cobalt complexes, and demonstrate the effectiveness of steric bulk in controlling the stereoselectivity of alkene formation.

We report the isolation, characterization, and reactions of the unsaturated complex L tBu Co (L tBu = bulky β-diketiminate ligand). The unusual slipped kN,η 6 -arene binding mode in L tBu Co interconverts rapidly and reversibly with the traditional k 2 N,N 0 ligation mode upon binding of Lewis bases, making it a "masked" two-coordinate complex. The mechanism of this isomerization is demonstrated using kinetic studies. L tBu Co is a stable yet reactive synthon for low-coordinate cobalt(I) complexes and is capable of cleaving the CÀF bond in fluorobenzene.H emilabile ligands, which contain a Lewis basic moiety that can reversibly dissociate from a metal, create transient coordinative unsaturation that can be used for bond activation and catalysis. 1 Notable examples include catalysts for olefin metathesis and cross-coupling reactions. 2 In this communication, we report that a bulky β-diketiminate ligand on cobalt(I) undergoes a novel isomerization that allows it to behave as a hemilabile ligand. Though β-diketiminates are represented in thousands of metal complexes, 3 this is the first example of this new binding mode. We show that the ligand isomerization is rapidly reversible and that it provides a "masked two-coordinate" cobalt(I) center for ligand binding and for activation of a strong CÀF bond.The addition of tetrahydrofuran (THF) to a solution of L tBu CoNNCoL tBu [L tBu = 2,2,6,6-tetramethyl-3,5-bis(2,4,6-triisopropylphenylimido)hept-4-yl] 4 in C 6 D 6 gives an immediate color change from brown to dark-green. This color change corresponds to the appearance of a set of signals in the 1 H NMR spectrum for a new species, L tBu Co(THF) (1), in quantitative yield. On a preparative scale, 1 can be produced from the reaction of L tBu CoCl with KC 8 in THF under Ar, which gives 1 as a dark-green crystalline solid in 64% yield. Complex 1 (Figure 1 left) has pseudo-C 2 symmetry in the solid state. The threecoordinate cobalt atom is planar, as the sum of the NÀCoÀN and NÀCoÀO bond angles is 359.7(2)°. The THF ligand is bent slightly toward one aryl arm of the β-diketiminate ligand, with NÀCoÀO angles of 135. Dissolution of 1 in C 6 D 6 under Ar gives a dark-orange solution whose 1 H NMR spectrum shows peaks from 1 plus a number of additional resonances. Evaporation of the volatile materials and redissolution in fresh C 6 D 6 leads to an 1 H NMR spectrum that contains very little 1. The displacement of THF with aromatic solvents can be used for the synthesis of the unknown species on a gram scale. Thus, 1 was dissolved in toluene, and two cycles of solvent removal and addition of more toluene removed all of the THF. Crystallization from a concentrated pentane solution at À45°C under Ar gave L tBu Co (2) in 72% yield as brown crystals.The molecular structure of 2 (Figure 1 right) shows a cobalt atom and a single β-diketiminate ligand with no additional donors. In contrast to the typical k 2 N,N 0 binding mode for L tBu , the β-diketiminate ligand in 2 is bound to cobalt in a kN,η 6 -arene mode in which the co...

This manuscript describes the formally iron(I) complexes L(Me)Fe(Py-R)(2) (L(Me) = bulky β-diketiminate; R = H, 4-tBu), in which the basal pyridine ligands preferentially accept significant unpaired spin density. Structural, spectroscopic, and computational studies on the complex with 4-tert-butylpyridine ((tBu)py) indicate that the S = 3/2 species is a resonance hybrid between descriptions as (a) high-spin iron(II) with antiferromagnetic coupling to a pyridine anion radical and (b) high-spin iron(I). When the pyridine lacks the protection of the tert-butyl group, it rapidly and reversibly undergoes radical coupling reactions that form new C-C bonds. In one reaction, the coordinated pyridine couples to triphenylmethyl radical, and in another, it dimerizes to give a pyridine-derived dianion that bridges two iron(II) ions. The rapid, reversible C-C bond formation in the dimer stores electrons from the formally reduced metal as a C-C bond in the ligands, as demonstrated by using the coupled diiron(II) complex to generate products that are known to come from iron(I) precursors.

The cooperative binding of N2 by late transition metals and main-group metals is a promising strategy for N-N bond weakening and activation. We report the use of activated Rieke magnesium for reduction of iron and cobalt complexes supported by bulky β-diketiminate ligands. Binding of N2 is accompanied by assembly of a linear M-NN-Mg-NN-M (M = Co, Fe) core with N-N bonds that are weakened, as judged by infrared spectroscopy. Both the cobalt and iron complexes require THF solvent, because of Mg-THF binding. The cobalt complex can be isolated as a pure solid, but the iron complex is stable only in solution. These results demonstrate the correlation between the binding mode and N-N weakening in heterobimetallic N2 complexes.