Intermolecular Proton-Coupled Electron Transfer Reactivity from a Persistent Charge-Transfer State for Reductive Photoelectrocatalysis.

Interest in applying proton-coupled electron transfer (PCET) reagents in reductive electro- and photocatalysis requires strategies that mitigate the competing hydrogen evolution reaction. Photoexcitation of a PCET donor to a charge-separated state (CSS) can produce a powerful H-atom donor capable of being electrochemically recycled at a comparatively anodic potential corresponding to its ground state. However, the challenge is designing a mediator with a sufficiently long-lived excited state for bimolecular reactivity. Here, we describe a powerful ferrocene-derived photoelectrochemical PCET mediator exhibiting an unusually long-lived CSS (τ ∼ 0.9 µs). In addition to detailed photophysical studies, proof-of-concept stoichiometric and catalytic proton-coupled reductive transformations are presented, which illustrate the promise of this approach.

Low-Valent Metal Ions as MOF Pillars: A New Route Toward Stable and Multifunctional MOFs.

PCM-102 is a new organophosphine metal-organic framework (MOF) featuring diphosphine pockets that consist of pairs of offset trans-oriented P(III) donors. Postsynthetic addition of M(I) salts (M = Cu, Ag, Au) to PCM-102 induces single-crystal to single-crystal transformations and the formation of trans-[P2M]+ solid-state complexes (where P = framework-based triarylphosphines). While the unmetalated PCM-102 has low porosity, the addition of secondary Lewis acids to install rigid P-M-P pillars is shown to dramatically increase both stability and selective gas uptake properties, with N2 Brunauer-Emmett-Teller surface areas >1500 m2 g-1. The Ag(I) analogue can also be obtained via a simple, one-pot peri-synthetic route and is an ideal sacrificial precursor for materials with mixed bimetallic MA/MB pillars via postsynthetic, solvent-assisted metal exchange. Notably, the M-PCM-102 family of MOFs contain periodic trans-[P2M]+ sites that are free of counter anions, unlike traditional analogous molecular complexes, since the precursor PCM-102 MOF is monoanionic, enabling access to charge-neutral metal-pillared materials. Four M-PCM-102 materials were evaluated for the separation of C2 hydrocarbons. The separation performance was found to be tunable based on the metal(s) incorporated, and density functional theory was employed to elucidate the nature of the unusual observed sorption preference, C2H2 > C2H6 > C2H4.

Hierarchical nanosheets built from superatomic clusters: properties, exfoliation and single-crystal-to-single-crystal intercalation.

Tuning the properties of atomic crystals in the two-dimensional (2D) limit is synthetically challenging, but critical to unlock their potential in fundamental research and nanotechnology alike. 2D crystals assembled using superatomic blocks could provide a route to encrypt desirable functionality, yet strategies to link the inorganic blocks together in predetermined dimensionality or symmetry are scarce. Here, we describe the synthesis of anisotropic van der Waals crystalline frameworks using the designer superatomic nanocluster Co3(py)3Co6Se8L6 (py = pyridine, L = Ph2PN(Tol)), and ditopic linkers. Post-synthetically, the 3D crystals can be mechanically exfoliated into ultrathin flakes (8 to 60 nm), or intercalated with the redox-active guest tetracyanoethylene in a single-crystal-to-single-crystal transformation. Extensive characterization, including by single crystal X-ray diffraction, reveals how intrinsic features of the nanocluster, such as its structure, chirality, redox-activity and magnetic profile, predetermine key properties of the emerging 2D structures. Within the nanosheets, the strict and unusual stereoselectivity of the nanocluster's Co edges for the low symmetry (α,α,ß) isomer gives rise to in-plane structural anisotropy, while the helically chiral nanoclusters self-organize into alternating Δ- and Λ-homochiral rows. The nanocluster's high-spin Co edges, and its rich redox profile make the nanosheets both magnetically and electrochemically active, as revealed by solid state magnetic and cyclic voltammetry studies. The length and flexibility of the ditopic linker was varied, and found to have a secondary effect on the structure and stacking of the nanosheets within the 3D crystals. With these results we introduce a deterministic and versatile synthetic entry to programmable functionality and symmetry in 2D superatomic crystals.