Synthesis and Reactivity of an Early-Transition-Metal Alkynyl Cubane Mn4 C4 Cluster

Chemoselective Hydrogenation of α,β-Unsaturated Ketones Catalyzed by a Manganese(I) Hydride Complex

Here, we report the chemoselective hydrogenation of α,β-unsaturated ketones catalyzed by a well-defined Mn(I) PCNHCP pincer complex [(PCNHCP)Mn(CO)2H] (1). The reaction is compatible with a wide variety of functional groups that include halides, esters, amides, nitriles, nitro, alkynes, and alkenes, and for most substrates occurs readily at ambient hydrogen pressure (1-2 bar). Mechanistic studies and deuterium labeling experiments reveal a non-cooperative mechanism, which is further discussed in this report.

Synthesis, Characterization, and Catalytic Application of Colloidal and Supported Manganese Nanoparticles

Colloidal and supported manganese nanoparticles were synthesized following an organometallic approach and applied in the catalytic transfer hydrogenation (CTH) of aldehydes and ketones. Reaction parameters for the preparation of colloidal nanoparticles (NPs) were optimized to yield small (2-2.5 nm) and well-dispersed NPs. Manganese NPs were further immobilized on an imidazolium-based supported ionic phase (SILP) and characterized to evaluate NP size, metal loading, and oxidation states. Oxidation of the Mn NPs by the support was observed resulting in an average formal oxidation state of +2.5. The MnOx@SILP material showed promising performance in the CTH of aldehydes and ketones using 2-propanol as a hydrogen donor, outperforming previously reported Mn NPs-based CTH catalysts in terms of metal loading-normalized turnover numbers. Interestingly, MnOx@SILP were found to lose activity upon air exposure, which correlates with an additional increase in the average oxidation state of Mn as revealed by X-ray absorption spectroscopic studies.

Hydrogenation of Terminal Alkenes Catalyzed by Air-Stable Mn(I) Complexes Bearing an N-Heterocyclic Carbene-Based PCP Pincer Ligand

Efficient hydrogenations of terminal alkenes with molecular hydrogen catalyzed by well-defined bench stable Mn(I) complexes containing an N-heterocyclic carbene-based PCP pincer ligand are described. These reactions are environmentally benign and atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. A range of aromatic and aliphatic alkenes were efficiently converted into alkanes in good to excellent yields. The hydrogenation proceeds at 100 °C with catalyst loadings of 0.25-0.5 mol %, 2.5-5 mol % base (KOt Bu) and a hydrogen pressure of 20 bar. Mechanistic insight into the catalytic reaction is provided by means of DFT calculations.

Manganese-Catalyzed Hydrogenative Desulfurization of Thioamides

Earth-abundant transition metal catalysis has emerged as an important alternative to noble transition metal catalysis in hydrogenation reactions. However, there has been no Earth-abundant transition metal catalyzed hydrogenation of thioamides reported so far, presumably due to the poisoning of catalysts by sulfur-containing molecules. Herein, we described the first manganese-catalyzed hydrogenative desulfurization of thioamides to amines or imines. The key to success is the use of MnBr(CO)₅ instead of commonly-employed pincer-manganese catalysts, together with simple NEt₃ and CuBr. This protocol features excellent selectivity on sole cleavage of the C=S bond of thioamides, in contrast to the only known Ru-catalyzed hydrogenation of thioamides, and unprecedented chemo-selectivity tolerating vulnerable functional groups such as nitrile, ketone, aldehyde, ester, sulfone, nitro, olefin, alkyne and heterocycle, which are usually susceptible to common hydride-type reductive protocols.

Polynuclear Iron(II) Pyridonates: Synthesis and Reactivity of Fe4 and Fe5 Clusters

The combination of pyridonate ligands with transition metal ions enables the synthesis of an especially rich set of diverse coordination compounds involving various κ- and μ-bonding modes and higher nuclearities. With iron(II) ions, this chemical space is rather poorly explored beyond some biomimetic models of the pyridone iron-containing hydrogenase. Here, the topologically new Fe₅ and Fe₄ clusters, Fe₅(L^H)₆[N(SiMe₃)₂]₄ (1) and Fe₄(L^Me)₆[N(SiMe₃)₂]₂ (2), were synthesized (L^H = 2-pyridonate; L^Me = 6-methyl-2-pyridonate). Complex 1 contained an unprecedented diamondoid Fe@Fe₄ tetrahedron with a central-to-peripheral Fe-Fe distance of ∼3.1 Å. The crystal structure of complex 2 displayed an Fe₄O₆ butterfly motif containing a planar Fe₄ arrangement. Mössbauer spectroscopy confirmed the high-spin ferrous character of all iron ions. SQUID magnetometry reveals that the Fe(II) ions are involved in weak magnetic exchange coupling across the pyridonate bridges that results in antiferromagnetic interactions. The Fe₄ cluster exhibits slow relaxation of magnetization under an applied magnetic field with an effective energy barrier of 38.5 K, rarely observed among the very rare examples of Fe(II) cluster-based single-molecule magnets. Studies of protolytic substitution of the amido ligands demonstrated the lability of the diamondoid Fe₅ core in 1 and the stability of the Fe₄ rhomboid in 2.

Selective Benzylic CH-Borylations by Tandem Cobalt Catalysis

Metal-catalyzed C-H activations are environmentally and economically attractive synthetic strategies for the construction of functional molecules as they obviate the need for pre-functionalized substrates and minimize waste generation. Great challenges reside in the control of selectivities, the utilization of unbiased hydrocarbons, and the operation of atom-economical dehydrocoupling mechanisms. An especially mild borylation of benzylic CH bonds was developed with the ligand-free pre-catalyst Co[N(SiMe3 )2 ]2 and the bench-stable and inexpensive borylation reagent B2 pin2 that produces H2 as the only by-product. A full set of kinetic, spectroscopic, and preparative mechanistic studies are indicative of a tandem catalysis mechanism of CH-borylation and dehydrocoupling via molecular CoI catalysts.

Polymeric tungsten carbide nanoclusters: structural evolution, ligand modulation, and assembled nanomaterials

Seeking novel superatoms with tunable electronic and magnetic properties has attracted much interest due to their potential application in cluster assembly nanomaterials. By employing density functional theory (DFT) calculations, the recently observed superatomic WC cluster was adopted as the basic unit to construct larger polymeric clusters, namely (WC)_n (n = 2-7), and their structural evolution was explored to understand the growth pattern of these superatomic clusters into nanoscale materials. An unusual odd-even pattern in structural evolution was disclosed, in which the (WC)₂ unit is considered as the basic building block. Moreover, W₄C₄ is found to possess a cubic structure, based on which the CO and PH₃ ligands were attached to examine their ligation effects on W₄C₄. Theoretical results show that the electronic properties of W₄C₄ can be dramatically altered during the ligation process. Intriguingly, the continuous attachment of CO and PH₃ ligands strongly increases and decreases the electron affinities (EA) and ionization potentials (IP) of the ligated W₄C₄ clusters, respectively, leading to the formation of superhalogen and superalkali species with high magnetic moments. The observed ligand induced strategy highlighted here could serve as an effective way to tune the electronic and magnetic properties of clusters resulting in the formation of novel superatoms. Finally, studies on the geometrical and electronic structures of the W₄C₄ cluster solid unveil its special 3-D cubic honeycomb geometry and metallic properties with predominant contribution from the 5d of W, which may have potential applications in electro-catalysis.