Defluorination of HFCs by a magnesium reagent.

Reaction of a series of HFCs with a main group reagent containing a Mg-Mg bond results in defluorination to form the corresponding magnesium fluoride complex. In the case of 1,1,1,2-tetrafluoroethane (HFC-134a) generation of the fluoride occurs alongside selective formation of 1,1-difluoroethene. DFT calculations have been carried out to better understand the selectivity and compare the barriers for sp3 C-F bond activation with sp3 C-H bond activation in this system.

Carbon-carbon bond activation by Mg, Al, and Zn complexes.

Examples of carbon-carbon bond activation reactions at Mg, Al, and Zn are described in this review. Several distinct mechanisms for C-C bond activation at these metals have been proposed, with the key C-C bond activation step occurring by (i) α-alkyl elimination, (ii) ß-alkyl elimination, (iii) oxidative addition, or (iv) an electrocyclic reaction. Many of the known pathways involve an overall 2-electron redox process. Despite this, the direct oxidative addition of C-C bonds to these metals is relatively rare, instead most reactions occur through initial installation of the metal on a hydrocarbon scaffold (e.g. by a cycloaddition reaction or hydrometallation) followed by an α-alkyl or ß-alkyl elimination step. Emerging applications of Mg, Al, and Zn complexes as catalysts for the functionalisation of C-C bonds are also discussed.

Calcium-stabilised transition metal bis(formyl) complexes: structure and bonding.

Reaction of a molecular calcium hydride with a series of group 9 dicarbonyl complexes [M(η5-C5Me5)(CO)2] (M = Co, Rh, Ir) led to the formation of both mono(formyl) and bis(formyl) complexes. The bis(formyl) complexes are unique. They have been characterised by multinuclear NMR spectroscopy and examples have been crystallographically characterised for the first time.

Room Temperature Defluorination of Poly(tetrafluoroethylene) by a Magnesium Reagent.

Perfluoroalkyl substances (PFAS) are pervasive in the environment. The largest single use material within the PFAS compound class is poly(tetrafluoroethylene) (PTFE), a robust and chemically resistant polymer. Despite their widespread use and serious concerns about their role as pollutants, methods for repurposing PFAS are rare. Here we show that a nucleophilic magnesium reagent reacts with PTFE at room temperature, generating a molecular magnesium fluoride which is easily separated from the surface-modified polymer. The fluoride in turn can be used to transfer the fluorine atoms to a small array of compounds. This proof-of-concept study demonstrates that the atomic fluorine content of PTFE can be harvested and reused in chemical synthesis.

Understanding the role of ring strain in ß-alkyl migration at Mg and Zn centres.

The activation of C-C σ-bonds within strained three- and four-membered hydrocarbons at electrophilic Mg and Zn centres is reported. This was achieved in a two-step process involving (i) hydrometallation of a methylidene cycloalkane followed by (ii) intramolecular C-C bond activation. While hydrometallation of methylidene cyclopropane, cyclobutane, cyclopentane and cyclohexane occurs for both Mg and Zn reagents, the C-C bond activation step is sensitive to ring size. For Mg, both cyclopropane and cyclobutane rings participate in C-C bond activation. For Zn, only the smallest cyclopropane ring reacts. These findings were used to expand the scope of catalytic hydrosilylation of C-C σ-bonds to include cyclobutane rings. The mechanism of C-C σ-bond activation was investigated through kinetic analysis (Eyring), spectroscopic observation of intermediates, and a comprehensive series of DFT calculations, including activation strain analysis. Based on our current understanding, C-C bond activation is proposed to occur by a ß-alkyl migration step. ß-Alkyl migration is more facile for more strained rings and occurs with lower barriers for Mg compared to Zn. Relief of ring strain is a key factor in determining the thermodynamics of C-C bond activation, but not in stabilising the transition state for ß-alkyl migration. Rather, we ascribe the differences in reactivity to the stabilising interaction between the metal centre and the hydrocarbon ring-system, with the smaller rings and more electropositive metal (Mg) leading to a smaller destabilisation interaction energy as the transition state is approached. Our findings represent the first example of C-C bond activation at Zn and provide detailed new insight into the factors at play in ß-alkyl migration at main group centres.

Carbon-Carbon Bond Formation from Carbon Monoxide and Hydride: The Role of Metal Formyl Intermediates.

Current examples of carbon chain production from metal formyl intermediates with homogeneous metal complexes are described in this Minireview. Mechanistic aspects of these reactions as well as the challenges and opportunities in using this understanding to develop new reactions of CO and H2 are also discussed.

Charge Transport Across Dynamic Covalent Chemical Bridges.

Relationships between chemical structure and conductivity in ordered polymers (OPs) are difficult to probe using bulk samples. We propose that conductance measurements of appropriate molecular-scale models can reveal trends in electronic coupling(s) between repeat units that may help inform OP design. Here, we apply the scanning tunneling microscope-based break-junction (STM-BJ) method to study transport through single-molecules comprising OP-relevant imine, imidazole, diazaborole, and boronate ester dynamic covalent chemical bridges. Notably, solution-stable boron-based compounds dissociate in situ unless measured under a rigorously inert glovebox atmosphere. We find that junction conductance negatively correlates with the electronegativity difference between bridge atoms, and corroborative first-principles calculations further reveal a different nodal structure in the transmission eigenchannels of boronate ester junctions. This work reaffirms expectations that highly polarized bridge motifs represent poor choices for the construction of OPs with high through-bond conductivity and underscores the utility of glovebox STM-BJ instrumentation for studies of air-sensitive materials.

Magnesium-stabilised transition metal formyl complexes: structures, bonding, and ethenediolate formation.

Herein we report the first comprehensive series of crystallographically characterised transition metal formyl complexes. In these complexes, the formyl ligand is trapped as part of a chelating structure between a transition metal (Cr, Mn, Fe, Co, Rh, W, and Ir) and a magnesium (Mg) cation. Calculations suggest that this bonding mode results in significant oxycarbene-character of the formyl ligand. Further reaction of a heterometallic Cr-Mg formyl complex results in a rare example of C-C coupling and formation of an ethenediolate complex. DFT calculations support a key role for the formyl-intermediate in ethenediolate formation. These results show that well-defined transition metal formyl complexes are potential intermediates in the homologation of carbon monoxide.

Pushing steric limits in osmium(IV) tetraaryl complexes.

Investigations into the reactivity, properties, and applications of osmium(IV) tetraaryl complexes have been hampered by their low yielding syntheses from volatile and toxic OsO4 (typically ≤34%). Here we show that known air-stable M(aryl)4 compounds (M = Os, Ru; aryl = 2-tolyl, 2,5-xylyl) can be prepared in ≤73% yields using new, less hazardous (Oct4N)2[MX6] precursors (M = Os, Ru; X = Cl, Br). This approach also facilitates the preparation of Os(mesityl)4 (Os3) for the first time, a complex comprising bulky 2,6-dimethyl substituted aryl ligands, albeit in low yield (5%). To better understand these yield extremes, we track, by synthesizing two additional new complexes with different 2-substituted σ-aryl ligands, a clear relationship between the yields of Os(aryl)4 and ligand steric bulk. Single-crystal X-ray structures of these compounds indicate that the observed yield trend reflects the ease of accommodating aryl substituents into an open pocket that lies directly opposite each M-aryl coordination site. We perform variable-temperature 1H NMR studies of Os3, utilize a "tetrahedricity" metric to assess geometric distortion in Ru(aryl)4 and Os(aryl)4 materials, and calculate cone angle and percentage buried volume metrics to further illustrate and help quantify σ-aryl ligand steric properties. Solution cyclic voltammograms of Os(aryl)4 show that the potentials of their reversible 1-/0 and 0/1+ redox features can be fine-tuned by varying aryl substituents, and that Os3 exhibits an additional 1+/2+ redox event not previously observed in this class of compounds. Taken together, this work helps to advance the potential application of these relatively underexplored organometallic complexes in established and emerging areas of molecular materials science, such as extended molecular frameworks and self-assembled monolayers, where analogous tetraphenylmethane and silane species (M = C, Si) have been frequently targeted.