Abstract Transmetalation conventionally entails the transfer of a single ligand (e.g., aryl, hydride, or alkyl) between two centers. Herein, we demonstrate a 3d-to-5d metal-to-metal transmetalation in which the iron center of an organometallic fulvene complex is formally replaced by platinum, ultimately leaving an iron fragment devoid of ancillary ligands. Representative intermediates, including two trimetallic {Fe2Pt} complexes, were characterized by single-crystal X-ray diffraction, providing direct insight into the sequence of metal–carbon bond formation and ligand reorganization underlying this transmetalation.
Abstract N,C-Cyclometalated palladium(II) acetate dimers were synthesized as structurally related families differing by aryl ring substitution Pd1 or azole modification Pd2–5. X-ray study confirmed an acetate-bridged dimeric scaffold, a pronounced structural trans influence, and systematic differences in steric congestion between the two series. Solution NMR studies showed that Pd1 complexes exist predominantly as a single transoid isomer, whereas Pd2–5 exhibits transoid/cisoid mixtures. Analysis of structural parameters and NMR data indicates that steric effects and trans influence differentiation govern the transoid/cisoid ratio. In the cisoid form, the trans influence is concentrated on a single acetate bridge, weakening this trans-OAc ligand. Electrochemical studies (DPV/CV) revealed pronounced sensitivity of the first oxidation process to ligand substitution, particularly within the Pd1 series, where an anodic shift of 0.37 V was observed. Correlation of oxidation potentials with Hammett parameters and DFT-calculated frontier-orbital energies revealed substantial involvement of the cyclometalated aryl ring in oxidation. DFT calculations showed greater aryl contribution to the HOMO in Pd1 than in Pd2–5, providing an electronic basis for the strong substituent dependence of oxidation. Positive correlations between oxidation potential and Pd–Pd distance (or interplanar angle) indicate that steric effects opposing Pd–Pd contraction disfavor oxidation.
Abstract An alternative route for the reductive conversion of formic acid to methanol via transfer hydrogenation (FATH) reaction is demonstrated. The FATH reaction is catalyzed by “N-heterocyclic carbene–pyridine–benzimidazolato”-based CNN pincer ligand-bound Ru complexes employing easily available alkali metal hydroxides and a renewable (bio)alcohol, ethylene glycol as the hydrogen donor. Unlike formic acid disproportionation where only one molecule of methanol is obtained from three molecules of HCOOH, the reported transfer hydrogenation route offers one molecule of methanol from one molecule of HCOOH, thus maintaining a better formic acid economy.
Abstract Porous organic polymers that can be processed using organic solvents are attractive materials for a range of applications. Switching a CC unit for an isoelectronic BN unit is a powerful method to modulate the key properties of organic polymers; however, this approach is underexplored in the field of soluble porous organic polymers. To access isostructural CC/BN porous organic polymers, the synthesis of azabora-spirobifluorenes (BN-SBF) functionalized with methoxy/bromo groups was developed. While the use of 9-bromo-9-borafluorene (1) in pyridine-directed electrophilic borylation enabled the synthesis of a dimethoxy-substituted BN-SBF, this method could not be extended to access a tetramethoxy-BN-SBF derivative. The latter compound, while accessible via a lithiation/transmetalation route, was incompatible with various ether cleavage protocols. A BN-SBF-containing polymer instead was synthesized using a dibromo-functionalized BN-SBF monomer, which also could be synthesized via pyridine-directed electrophilic borylation using compound 1. The BN-SBF-based polymer proved to have comparable porosity to its CC analogue; however, it and the CC analogue did not form robust free-standing membranes. Nevertheless, this work highlights the feasibility of accessing a BN isostere of an established solution-processable porous organic polymer.
Abstract Square-planar triplet cyclobutadiene is intrinsically unstable, given its four π-electron configuration leading to unavoidable antiaromatic destabilization. One effective way to stabilize cyclobutadiene is through its incorporation into the coordination sphere of an element capable of electron donation. Metals (main group metals, transition metals, lanthanides, and actinides) are most frequently used as electron-donating elements, although recently, it was established that the main group elements may also serve as capping donors in cyclobutadiene complexes. The recent advances in the chemistry of the cyclobutadiene complexes of the s-, p-, d-, and f-block elements are briefly summarized in this review.
