H2 addition to (Me4PCP)Ir(CO): studies of the isomerization mechanism.

Reduced steric demand of the Me4PCP pincer ligand (PCP = κ3-C6H4-1,3-[CH2PR2]2, R = Me), allows for a more open metal center. This is evident through structure and reactivity comparisons between (Me4PCP)Ir derivatives and other (R4PCP)Ir complexes (R = tBu, iPr, CF3). In particular, isomerization from cis-(R4PCP)Ir(H)2(CO) to trans-(R4PCP)Ir(H)2(CO) is more facile when R = Me than when R = iPr. Deuterium incorporation in the hydride ligands from solvent C6D6 was observed during this isomerization when R = Me. This deuterium exchange has not been observed for other analogous R4PCP iridium complexes. A kinetic study of the cis/trans isomerization combined with computational studies suggests that the cis/trans isomerization proceeds through a migratory-insertion pathway involving a formyl intermediate.

An Uncanny Dehydrogenation Mechanism: Polar Bond Control over Stepwise or Concerted Transition States.

The mechanism of the dehydrogenation of N-heterocycles with the recently established bifunctional catalyst (iPrPNP)Fe(CO)(H) was investigated through experiments and density functional theory calculations (iPrPNP = iPr2PCH2CH2NCH2CH2PiPr2). In this system, the saturated N-heterocyclic substrates are completely dehydrogenated to the aromatic products. Calculations indicate that dehydrogenation barriers of the C-C bonds are very high in energy (ΔG‡ = 37.4-42.2 kcal/mol), and thus dehydrogenation only occurs at the C-N bond (ΔG‡ = 9.6-22.2 kcal/mol). Interestingly, substrates like piperidine with relatively unpolarized C-N bonds are dehydrogenated through a concerted proton/hydride transfer bifunctional transition state involving the nitrogen on the PNP ligand. However, substrates with polarized C-N bonds entail stepwise (proton then hydride) bifunctional dehydrogenation.

Aqueous Hydricity from Calculations of Reduction Potential and Acidity in Water.

Hydricity, or hydride donating ability, is a thermodynamic value that helps define the reactivity of transition metal hydrides. To avoid some of the challenges of experimental hydricity measurements in water, a computational method for the determination of aqueous hydricity values has been developed. With a thermochemical cycle involving deprotonation of the metal hydride (pKa), 2e- oxidation of the metal (E°), and 2e- reduction of the proton, hydricity values are provided along with other valuable thermodynamic information. The impact of empirical corrections (for example, calibrating reduction potentials with 2e- organic versus 1e- inorganic potentials) was assessed in the calculation of the reduction potentials, acidities, and hydricities of a series of iridium hydride complexes. Calculated hydricities are consistent with electronic trends and agree well with experimental values.

The Mechanism of N-N Double Bond Cleavage by an Iron(II) Hydride Complex.

The use of hydride species for substrate reductions avoids strong reductants, and may enable nitrogenase to reduce multiple bonds without unreasonably low redox potentials. In this work, we explore the N═N bond cleaving ability of a high-spin iron(II) hydride dimer with concomitant release of H2. Specifically, this diiron(II) complex reacts with azobenzene (PhN═NPh) to perform a four-electron reduction, where two electrons come from H2 reductive elimination and the other two come from iron oxidation. The rate law of the H2 releasing reaction indicates that diazene binding occurs prior to H2 elimination, and the negative entropy of activation and inverse kinetic isotope effect indicate that H-H bond formation is the rate-limiting step. Thus, substrate binding causes reductive elimination of H2 that formally reduces the metals, and the metals use the additional two electrons to cleave the N-N multiple bond.

Effects of Ligand Halogenation on the Electron Localization, Geometry and Spin State of Low-Coordinate (ß-Diketiminato)iron Complexes.

This contribution explores the influences of incorporating electron-withdrawing CF3 and halide groups into ß-diketiminate iron complexes of tetrazene and isocyanide. The synthesis of a new halogenated ß-diketimine (LCF3,ClH) was accomplished via two different methods, including a novel microwave-assisted synthesis that improves the yield of the difficult condensation. Treatment of an iron(II) complex of this ligand with reductant and azide gives two diiron complexes with novel tetrazenes as bridging ligands. Structural and Mössbauer data show that the bridging tetrazene is a radical anion. The halogenation of the supporting ligand also influences iron(I) complexes of the type LFe(CNtBu)2, which are low-spin and square-planar with alkyl substituents but high-spin and pseudotetrahedral with halogen substituents. DFT calculations suggest that the changes from halogenation come from a combination of steric and electronic effects, and that the electronic influence of ligand halogenation is minor.

Tuning steric and electronic effects in transition-metal ß-diketiminate complexes.

ß-Diketiminates are widely used supporting ligands for building a range of metal complexes with different oxidation states, structures, and reactivities. This Perspective summarizes the steric and electronic influences of ligand substituents on these complexes, with an eye toward informing the design of new complexes with optimized properties. The backbone and N-aryl substituents can give significant steric effects on structure, reactivity and selectivity of reactions. The electron density on the metal can be tuned by installation of electron withdrawing or donating groups on the ß-diketiminate ligand as well. Examples are shown from throughout the transition metal series to demonstrate different types of effects attributable to systematic variation of ß-diketiminate ligands.

Multimetallic Cooperativity in Activation of Dinitrogen at Iron-Potassium Sites.

The reaction of soluble iron-oxygen-potassium assemblies with N2 gives insight into the mechanisms of multimetallic N2 coordination. We report a series of very electron-rich three-coordinate, ß-diketiminate-supported iron(I) phenoxide complexes, which are metastable but have been characterized under Ar by both crystallography and solution methods. Both monomeric and dimeric Fe-OPh-K compounds have been characterized, and their iron environments are very similar in the solid and solution states. In the dimer, potassium ions hold together the phenoxide oxygens and aryl rings of the two halves, to give a flexible diiron core. The reactions of the monomeric and dimeric iron(I) compounds with N2 are surprisingly different: the mononuclear iron(I) complexes give no reaction with N2, but the dimeric Fe2K2 complex reacts rapidly to give a diiron-N2 product. Computational studies show that the key to the rapid N2 reaction of the dimer is the preorganization of the two iron atoms. Thus, cooperation between Fe (which weakens the N-N bond) and K (which orients the Fe atoms) can be used to create a low-energy pathway for N2 reactions.