The field of Surface Organometallic Chemistry (SOMC) aims to blend the positive attributes of homogeneous and heterogeneous catalysis. Significant insight into heterogeneous systems has been gained over the years through the synthesis, characterization, and application of well-defined surface organometallic catalysts, predominantly supported on silica and alumina. Considerable research efforts have focused on the application of homogeneous methods to the synthesis and characterization of these systems. Homogeneous catalysis has thrived on its ability to electronically and sterically tune ligands to yield desired reactivity and selectivity. Efforts in SOMC, however, have only recently turned to harnessing the stereoelectronic diversity of potential inorganic support materials beyond silica and alumina in order to exert similar control on the reactivity of the organometallic active site. The support material is intrinsically linked to electronic structure and reactivity of heterogeneous organometallic systems in the same way that ligands exert control over homogeneous catalyst systems. The ability to tune the reactivity of heterogeneous catalysts by changing the support is of great value, and it is anticipated that this will represent an area of significant growth in the field. In this Perspective, the use and future of nontraditional catalyst supports, such as sulfated metal oxides, modified silicas, and redox active supports are discussed.

The development of general strategies for the electronic tuning of a catalyst's active site is an ongoing challenge in heterogeneous catalysis. To this end, herein, we describe the application of Li-ion battery cathode and anode materials as redox non-innocent catalyst supports that can be continuously modulated as a function of lithium intercalation. A zero-valent nickel complex was oxidatively grafted onto the surface of lithium manganese oxide (Li x Mn 2 O 4 ) to yield isolated Ni 2+ occupying the vacant interstitial octahedral site in the Li diffusion channel on the surface and subsurface of the spinel structure (Ni/Li x Mn 2 O 4 ). The activity of Ni/Li x Mn 2 O 4 for olefin hydrogenation, as a representative probe reaction, was found to increase monotonically as a function of support reductive lithiation. Simulation of Ni/Li x Mn 2 O 4 reveals the dramatic impact of surface redox states on the viability of the homolytic oxidative addition mechanism for H 2 activation. Catalyst control through support lithiation was extended to an organotantalum complex on Li x TiO 2 , demonstrating the generality of this phenomenon.

A general synthetic strategy for the construction of large, nitrogen-containing polycyclic aromatic hydrocarbons (PAHs) is reported. The strategy involves two key steps: (1) a titanocene-mediated reductive cyclization of an oligo(dinitrile) precursor to form a PAH appended with di(aza)titanacyclopentadiene functionality; (2) a divergent titanocene transfer reaction, which allows final-step installation of one or more o-quinone, diazole, or pyrazine units into the PAH framework. The new methodology enables rational, late-stage control of HOMO and LUMO energy levels and thus photophysical and electrochemical properties, as revealed by UV/vis and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations. More generally, this contribution presents the first productive use of di(aza)metallacyclopentadiene intermediates in organic synthesis, including the first formal [2 + 2 + 2] reaction to form a pyrazine ring.

Pentacene's extraordinary photophysical and electronic properties are highly dependent on intermolecular, through-space interactions. Macrocyclic arrangements of chromophores have been shown to provide a high level of control over these interactions, but few examples exist for pentacene due to inherent synthetic challenges. In this work, zirconocene-mediated alkyne coupling was used as a dynamic covalent CC bond forming reaction to synthesize two geometrically distinct, pentacene-containing macrocycles on a gram scale and in four or fewer steps. Both macrocycles undergo singlet fission in solution, with rates that differ by an order of magnitude while the rate of triplet recombination is approximately the same. This independent modulation of singlet and triplet decay rates is highly desirable for the design of efficient singlet fission materials. The dimeric macrocycle adopts a columnar packing motif in the solid state, with large void spaces between pentacene units of the crystal lattice. File list (7) download file view on ChemRxiv 2020_09_18 Pentacene Paper-Final.pdf (5.28 MiB) download file view on ChemRxiv 2020_09_18 Pentacene Paper-Final.docx (9.61 MiB) download file view on ChemRxiv 2020_09_18 Pentacene Paper SI-Final.pdf (3.65 MiB) download file view on ChemRxiv 2020_09_18 Pentacene Paper SI-Final.docx (60.83 MiB) download file view on ChemRxiv Compound 1b.cif (430.53 KiB) download file view on ChemRxiv Compound 8.cif (1.64 MiB) download file view on ChemRxiv Compound S1.cif (1.03 MiB)

The two-coordinate compound (IPr)­Fe­[N­(SiMe3)­DIPP] (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene; DIPP = 2,6-diisopropylphenyl) catalyzes the cyclotrimerization of alkynes to arenes. Treatment of the Fe complex with 1 equiv of diphenylacetylene results in the formation of a bimetallic bridging alkyne complex, along with dissociation of IPr from Fe. At elevated temperatures, the bridging alkyne complex undergoes oxidative coupling to form a dimetallacyclopentadiene complex, formally by a one-electron oxidation at each metal center. Each complex catalyzes the cyclotrimerization of diphenylacetylene. Kinetic studies exhibit first-order dependence on the bimetallic complexes, providing further support for the presence of these species in the catalytic cycle. DFT calculations support the experimental mechanistic data and suggest that the catalytic cycle is completed by binding of an alkyne to the diene complex, followed by insertion to form a hexatriene species that then undergoes ring closure to form an inverse sandwich complex, [DIPP­(Me3Si)­N]­Fe­(η6-arene)­Fe­[N­(SiMe3)­DIPP]. The arene product is then displaced by alkyne to close the catalytic cycle.