A new Ru₂ azido complex, [Ru₂(chp)₄N₃] (4, chp = 2-chloro-6-hydroxypyridinate), was investigated under photolytic conditions to study the chemical reactivity of the corresponding Ru₂ nitride species, [Ru₂(chp)₄N] (6), towards intermolecular N atom transfer to triphenylphosphane (PPh₃). Photolysis of a dichloromethane solution of 4 at λ > 350 nm leads to a characteristic color change from purple to magenta. Upon acidic workup, triphenylphosphanamine chloride ([H₂NPPh₃]Cl) is produced and [Ru₂(chp)₄Cl] (5), the precursor to 4, is regenerated. The first stoichiometric cycle for intermolecular N atom transfer from a Ru₂ nitride is thus presented.

We report two new analogues of the well-known C–H amination catalyst [Rh₂(esp)₂] (1) (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate) that bear redox-active supporting ligands that are structurally similar to esp. The redox-active ligands are 2-[3-(1-carboxy-1-methylethoxy)phenoxy]-2-methylpropanoic acid (H₂L1) and (3-methoxycarbonyl-2,5-di-tert-butylphenoxy)ethanoic acid (H₂L2), which react with Rh₂(OAc)₄ to form the catalysts [Rh₂(L1)₂] (2) and [Rh₂(L2)₂] (3). Both 2 and 3 have been characterized by X-ray crystallography and cyclic voltammetry, inter alia. Compounds 2 and 3 are structurally similar to 1 but show more complex electrochemical features. Whereas 1 has a single reversible redox wave that corresponds to the Rh₂^II,II/Rh₂^II,III couple, 2 and 3 show multiple oxidations that are characteristic of ligand-centered oxidation. Catalysts 1, 2, and 3 perform well in a model intramolecular C–H amination reaction, and all three catalysts perform equally well during the first four hours of a model intermolecular reaction. After this point, 2 and 3 cease to function, whereas 1 continues to be active. These results support the hypothesis that intermolecular C–H amination utilizes two distinct mechanisms: (1) a nitrene interception/insertion mechanism that is fast but ceases to be operative after four hours, and (2) a one-electron mechanism that is more robust over extended time periods, but requires the catalyst to be able to undergo Rh₂-centered oxidation.

To gain insights into the trends in metal–metal multiple bonding among the Group 6 elements, density functional theory has been employed in combination with multiconfigurational methods (CASSCF and CASPT2) to investigate a selection of bimetallic, multiply bonded compounds. For the compound [Ar-MM-Ar] (Ar=2,6-(C₆H₅)₂-C₆H₃, M=Cr, Mo, W) the effect of the Ar ligand on the M₂ core has been compared with the analogous [Ph-MM-Ph] (Ph=phenyl, M=Cr, Mo, W) compounds. A set of [M₂(dpa)₄] (dpa=2,2′-dipyridylamide, M=Cr, Mo, W, U) compounds has also been investigated. All of the compounds studied here show important multiconfigurational behavior. For the Mo₂ and W₂ compounds, the σ²π⁴δ² configuration dominates the ground-state wavefunction, contributing at least 75 %. The Cr₂ compounds show a more nuanced electronic structure, with many configurations contributing to the ground state. For the Cr, Mo, and W compounds the electronic absorption spectra have been studied, combining density functional theory and multireference methods to make absorption feature assignments. In all cases, the main features observed in the visible spectra may be assigned as charge-transfer bands. For all compounds investigated the Mayer bond order (MBO) and the effective bond order (EBO) were calculated by density functional theory and CASSCF methods, respectively. The MBO and EBO values share a similar trend toward higher values at shorter normalized metal–metal bond lengths.

Swift and energy efficient conversion of chemical feedstocks to pharmaceuticals and agrochemicals requires the development of new methods to add nitrogen functionality to unfunctionalized organic substrates. Dirhodium-catalyzed insertion of nitrene species into CH bonds is a promising new method, the main drawback of which is the currently limited understanding of the catalytic mechanism. Herein, cyclic voltammetry and controlled potential electrolysis measurements have enabled us to solve many of the mechanistic mysteries of intermolecular CH amination catalyzed by [Rh₂(esp)₂] (esp=α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate). The primary result is that, in addition to a simple nitrene-transfer mechanism that dominates the early stages of the reaction, another mechanism is available that relies on sequential proton-coupled electron transfer steps. Whereas the nitrene-transfer mechanism requires the use of expensive, atom-inefficient oxidants, we show that simple one-electron oxidants such as Ce⁴⁺ may be used to achieve catalytic CH amination via the one-electron mechanistic regime.

Reported here are two new compounds containing either a CrCr···Co [1, CrCrCo(dpa)₄Cl₂, dpa = 2,2′-dipyridylamide] or a MoMo···Co [2, MoMoCo(dpa)₄Cl₂] framework both having a multiply-bonded unit (CrCr in 1, MoMo in 2) in close proximity to the Co²⁺ ion and trans to a Co–Cl bond. Variable temperature magnetic susceptibility measurements reveal 1 to have a temperature-dependent spin equilibrium between a low-spin (S = 1/2) and high-spin (S = 3/2) state, whereas the Co²⁺ ion in 2 exists solely in its high-spin state. The crystal structures of 1 and 2 were determined. Variable temperature crystallographic data of 1 at 100 K and at room temperature reveal that the spin-transition affects not only the Co–ligand bond lengths but also the terminal Cr–ligand bond lengths. Whereas the Cr···Co distance becomes shorter by 0.13 Å in the low-spin form, the Co–Cldistance becomes longer by 0.2 Å. These observations,along with the crystal structure of 2, suggest that the multiply-bonded MM group has a trans influence on the Co²⁺ ion.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)