8 Search Results

The synthesis and characterization of pentamethylcyclopentadienyl iron complexes bearing the redox non-innocent α-diimine (N,N'-dimesitylbutane-2,3-diimine, ^MesDI) and α-iminopyridine (N-mesityl(pyridin-2-yl)ethanimine ^MesPI) ligands were explored. One-pot reduction and complexation of the cyclopentadienyl ring was accomplished by treatment of (κ²-N,N')FeCl₂ (N,N'= ^MesDI or ^MesPI) pre-cursors with [C₅Me₅]Li. The resulting iron compounds were characterized by paramagnetic ¹H NMR spectroscopy, magnetic susceptibility measurements, zero-field ⁵⁷Fe Mössbauer spectroscopy, low-temperature EPR spectroscopy, and cyclic voltammetry. The combined spectroscopic, structural and DFT computational data supported low-spin iron(III) compounds (S_Fe = ½) with π-acidic, formally doubly-reduced chelating ligands.

Although metal complexes are known to split dinitrogen at ambient temperature and pressure, the synthesis of ammonia from these compounds and using H₂ as the terminal reductant has been rarely achieved. Here, we report photocatalytic ammonia synthesis from a N₂-derived terminal molybdenum nitride and by using H₂ as the terminal reductant. An iridium hydride photocatalyst mediates the reaction upon irradiation with blue light. A molybdenum pentahydride was identified as the principal metal product arising following ammonia release. Conversion of the molybdenum pentahydride back to the terminal molybdenum nitride was accomplished in three-steps and completes a synthetic cycle for NH3 formationmore » from N₂ and H₂. Mechanistic investigations support a pathway involving photoexcitation of the iridium hydride and subsequent energy transfer rather than electron transfer. Deuterium labelling confirmed H₂ as the source of the N–H bonds. This photodriven, proton coupled electron transfer allows the use of H₂ as the terminal reductant for catalytic formation of NH₃ from N₂ using metal catalysts.« less

Abstract A four‐coordinate cobalt–imido complex, ( ^{t Bu} mPNP)Co=NMes ( ^{t Bu} mPNP=modified PNP pincer ligand) has been synthesized from addition of 2,4,6‐trimethylphenylazide (Mes–N ₃ ) to the corresponding dinitrogen complex. The solid‐state structure determined by X‐ray diffraction established a rare, idealized planar geometry with a Co=N bond distance of 1.716(2) Å. Magnetic measurements revealed an S =1 ground state with CAS‐SCF calculations supporting radical character on the imide nitrogen. Thermolysis of the cobalt–imido compound induced selective insertion of the imido group into a Co−P bond and yielded a three‐coordinate cobalt complex with a distorted T‐shaped geometry.more » Transition state analysis conducted with DFT calculations established the thermodynamic stability of the P–N coupled product and provided insight into the exclusive selectivity.« less

The synthesis of weak chemical bonds at or near thermodynamic potential is a fundamental challenge in chemistry, with applications ranging from catalysis to biology to energy science. Proton-coupled electron transfer using molecular hydrogen is an attractive strategy for synthesizing weak element–hydrogen bonds, but the intrinsic thermodynamics presents a challenge for reactivity. Here we describe the direct photocatalytic synthesis of extremely weak element–hydrogen bonds of metal amido and metal imido complexes, as well as organic compounds with bond dissociation free energies as low as 31 kcal mol^–1. Key to this approach is the bifunctional behaviour of the chromophoric iridium hydride photocatalyst.more » Activation of molecular hydrogen occurs in the ground state and the resulting iridium hydride harvests visible light to enable spontaneous formation of weak chemical bonds near thermodynamic potential with no by-products. In conclusion, photophysical and mechanistic studies corroborate radical-based reaction pathways and highlight the uniqueness of this photodriven approach in promoting new catalytic chemistr« less

A four-coordinate cobalt imido complex, (^tBumPNP)Co=NMes (^tBumPNP = modified PNP pincer ligand) has been synthesized from addition of 2,4,6-trimethylphenylazide (Mes–N₃) to the corresponding dinitrogen complex. The solid-state structure determined by X-ray diffraction established a rare, idealized planar geometry with a Co=N bond distance of 1.716(2) Å. Magnetic measurements revealed an S = 1 ground state with CAS-SCF calculations supporting radical character on the imide nitrogen. Here, thermolysis of the cobalt-imido compound induced selective insertion of the imido group into a Co–P bond and yielded a three-coordinate cobalt complex with a distorted T-shaped geometry. Transition state analysis conducted with DFT calculationsmore » established the thermodynamic stability of the P–N coupled product and provided insight into the exclusive selectivity.« less

The catalytic hydrogenation activity of the readily-prepared, coordinatively saturated cobalt(I) precatalyst, (R,R)-(^iPrDuPhos)Co(CO)₂H ((R,R)-^iPrDuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene) is described. While efficient turnover was observed with a range of alkenes upon heating to 100 ºC, the catalytic performance of the cobalt catalyst was markedly enhanced upon irradiation with blue light at 35 °C. Here, this improved reactivity enabled hydrogenation of terminal, di- and trisubstituted alkenes, alkynes, and carbonyl compounds. A combination of deuterium labeling studies, hydrogenation of alkenes containing radical clocks and experiments probing relative rates support a hydrogen atom transfer pathway under thermal conditions that is enabled by a relatively weak cobalt–hydrogenmore » bond of 56 kcal/mol. In contrast, data for the photocatalytic reactions support light-induced dissociation of a carbonyl ligand followed by a coordination-insertion sequence where the product is released by combination of a cobalt alkyl intermediate with the starting hy-dride, (R,R)-(^iPrDuPhos)Co(CO)₂H. These results demonstrate the versatility with catalysis with Earth-abundant metals as pathways involving open- versus closed-shell intermediates can be switched by the energy source.« less

The catalytic hydrogenation of a metal nitride to make free ammonia using a rhodium hydride catalyst that promotes H₂ activation and hydrogen atom transfer is described. The phenylimine-substituted rhodium complex, (η⁵-C₅Me₅)Rh(^MePhI)H (^MePhI = N-methyl-1-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (η⁵-C₅Me₅)Rh(ppy)H (ppy = 2-phenylpyridine). DFT calculations established that the two rhodium complexes have comparable Rh–H bond dissociation free energies of 51.8 kcal mol-1 for (η⁵-C₅Me₅)Rh(MePhI)H and 51.1 kcal mol^-1 for (η⁵-C₅Me₅)Rh(ppy)H. In the presence of 10 mol% of the phe-nylimine rhodium precatalyst and 4 atm of H₂ in THF, the manganese nitride, (^tBuSalen)Mn≡N underwent hydrogenation to liberatemore » free ammonia with up to 6 total turnovers of NH₃ or 18 turnovers of H·. The phenylpyridine analogue proved inactive for ammonia synthesis under identical conditions owing to competing deleterious hydride transfer chemistry. Subsequent research showed that the use of a non-polar solvent such as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthesis by proton coupled electron transfer.« less