Breaking Chemistry's Strongest Bond: Can Three-Coordinate [M{N(R)Ar}3] Complexes Cleave Carbon Monoxide?

Nickel-assisted complete cleavage of CO by a silylene/siliconoid hybrid under formation of an Si[double bond, length as m-dash]C enol ether bridge

Reaction of a silylene-functionalized Si6 siliconoid with CO in the presence of catalytic quantities of a nickel(0) complex results in the complete cleavage of the CO triple bond, but preserves the Si6 scaffold with an exohedrally incorporated Si[double bond, length as m-dash]C enol ether bridge. The uncompromised cluster core emphasizes the role of the so-called benzpolarene motif as the energetic silicon pendants of benzene in carbon chemistry.

Molecularly Defined Manganese Catalyst for Low-Temperature Hydrogenation of Carbon Monoxide to Methanol

Methanol synthesis from syngas (CO/H₂ mixtures) is one of the largest manmade chemical processes with annual production reaching 100 million tons. The current industrial method proceeds at high temperatures (200-300 °C) and pressures (50-100 atm) using a copper-zinc-based heterogeneous catalyst. In contrast, here, we report a molecularly defined manganese catalyst that allows for low-temperature/low-pressure (120-150 °C, 50 bar) carbon monoxide hydrogenation to methanol. This new approach was evaluated and optimized by quantum mechanical simulations virtual high-throughput screenings. Crucial for this achievement is the use of amine-based promoters, which capture carbon monoxide to give formamide intermediates, which then undergo manganese-catalyzed hydrogenolysis, regenerating the promoter. Following this conceptually new approach, high selectivity toward methanol and catalyst turnover numbers (up to 3170) was achieved. The proposed general catalytic cycle for methanol synthesis is supported by model studies and detailed spectroscopic investigations.

Selective reduction and homologation of carbon monoxide by organometallic iron complexes

Carbon monoxide is a key C₁ feedstock for the industrial production of hydrocarbons, where it is used to make millions of tonnes of chemicals, fuels, and solvents per annum. Many transition metal complexes can coordinate CO, but the formation of new C−C bonds in well-defined compounds from the scission and subsequent coupling of two or more CO moieties at a transition metal centre remains a challenge. Herein, we report the use of low-coordinate iron(II) complexes for the selective scission and homologation of CO affording unusual squaraines and iron carboxylates at ambient temperature and pressure. A modification of the ligand framework allows for the isolation and structural characterisation of a proposed metallacyclic Fe(II) carbene intermediate. These results indicate that, with the appropriate choice of supporting ligands, it is possible to cleave and homologate carbon monoxide under mild conditions using an abundant and environmentally benign low-coordinate, first row transition metal.

Metal-mediated activation of CO for C-C coupling reactions is a valuable approach to carbon monoxide valorization. Here, the authors use low-coordinate iron(II) complexes for the selective scission and homologation of CO affording unusual squaraines and iron carboxylates under mild conditions.

Fully Borylated Methane and Ethane by Ruthenium-Mediated Cleavage and Coupling of CO

Many transition-metal complexes and some metal-free compounds are able to bind carbon monoxide, a molecule which has the strongest chemical bond in nature. However, very few of them have been shown to induce the cleavage of its C-O bond and even fewer are those that are able to transform CO into organic reagents with potential in organic synthesis. This work shows that bis(pinacolato)diboron, B2pin2, reacts with ruthenium carbonyl to give metallic complexes containing borylmethylidyne (CBpin) and diborylethyne (pinBC≡CBpin) ligands and also metal-free perborylated C1 and C2 products, such as C(Bpin)4 and C2 (Bpin)6, respectively, which have great potential as building blocks for Suzuki-Miyaura cross-coupling and other reactions. The use of (13)CO-enriched ruthenium carbonyl has demonstrated that the boron-bound carbon atoms of all of these reaction products arise from CO ligands.

Reductive Cleavage of Carbon Monoxide by a Disilenide

The complete reductive cleavage of the triple bond in carbon monoxide was achieved using a lithium disilenide at room temperature. The C-C-coupled product can be regarded as a silanone dimer with pending alkyne and silirene moieties and incorporates two equivalents of CO per disilenide unit. A formation mechanism via ketenyl intermediates is proposed on the basis of DFT calculations and elucidated experimentally by employing Group 6 metal carbonyls as both stabilizing entity and source of CO in the reaction with disilenide. The isolation of cyclic silylene complexes with weakly donating ketenyl donor groups further supports the mechanistic scenario.

On the selective cleavage of nitrous oxide by metal–amide complexes

The remarkable selective cleavage of nitrous oxide by metal–amide systems, involving a bimetallic mechanism, has been investigated using computational methodology and rationalized on the basis of the interplay of structural and bonding factors.

Dinitrogen metal complexes with a strongly activated N-N bond: a computational investigation of [(Cy2N)3Nb-(μ-NN)-Nb(NCy2)3] and related [Nb-(μ-NN)-Nb] systems

The structural and bonding properties of the dinitrogen-bridged diniobium [(Cy(2)N)(3)Nb(μ-NN)Nb(NCy(2))(3)] complex experimentally characterized by Berno and Gambarotta, which exhibits a strongly activated N-N bond of 134 pm, have been explored using density functional methods and compared with those of a series of related [(R(2)N)(3)Nb(μ-NN)Nb(NR(2))(3)] (R = H, Me, (i)Pr, (t)Bu, Cy) model species and other experimentally relevant [Nb(μ-NN)Nb] systems, in order to rationalize the unusually long N-N distance. Geometry optimizations of [(Cy(2)N)(3)Nb(μ-NN)Nb(NCy(2))(3)] and three other known systems indicate that the most favourable N-N distance lies within the range of commonly reported results for end-on bound dinitrogen-diniobium complexes, between 123 and 128 pm. However, structures exhibiting appreciably longer N-N distances, close to 134 pm, are found to be only weakly disfavoured, and may represent the preferred geometry in cases where lengthening of the N-N bond counteracts the effects of highly repulsive steric interactions between terminal fragments. Calculations on model complexes, in which small-sized [R = H, Me] terminal groups are involved, support the finding that N-N bond lengths within the 123-128 pm range are most favoured, whereas calculations on larger [R = (i)Pr, (t)Bu] model species indicate that the presence of excessively repulsive intramolecular interactions can lead to substantial changes in the geometric properties of the [Nb-NN-Nb] core, including significant increase in N-N bond length and activation. The preference for N-N distances ranging between 123 and 128 pm, irrespective of ligand size and identity, can be understood on the basis that the principal bonding mechanisms across the central [Nb-NN-Nb] core are largely unaffected by changes in the chemical composition and properties of terminal fragments. However, the balance between repulsive (steric) and attractive (electrostatic plus orbital) bonding contributions can be altered by the presence of geometrically rigid and oversized peripheral groups and, in these cases, the interplay between repulsive and attractive bonding effects is dominated by the former and can result in abnormally elongated N-N distances. The present calculations thus provide a rationale for the observed structural properties of the [(Cy(2)N)(3)Nb(μ-NN)Nb(NCy(2))(3)] system on the basis of the interplay between electronic and steric factors.