Siamese-Twin Porphyrin Goes Platinum: Group 10 Monometallic, Homobimetallic, and Heterobimetallic Complexes.

A series of PtII-based monometallic (H2PtL), homobimetallic (Pt2L), and heterobimetallic (NiPtL and PdPtL) group 10 complexes of the previously established expanded twin porphyrin (H4L) were prepared. Structural characterization of the bimetallic PtII series (Pt2L, NiPtL, and PdPtL) revealed their similar general structures, with slight differences correlated to the ion size. An improvement of the metal-ion insertion process also allowed efficient preparation of the known Pd2L complex, and the novel heterobimetallic NiPdL complex was also structurally characterized. UV-vis spectroscopy, NMR spectroscopy, magnetic circular dichroism (MCD), and (spectro)electrochemistry were used to characterize the complexes; the electronic properties followed largely established lines for metal complexes of the twin porphyrin, except that the PtII-based systems exhibited more complex UV-vis spectral signatures. MCD spectra accompanied by density functional theory (DFT)/time-dependent DFT computations (TDDFT) rationalize the origins of the optical features of the twin porphyrin. The presence of the nonplanar, nonaromatic macrocyclic π system with conjugation pathways confined to each half of the molecule could be visualized. Significant pyrazole(π) → pyrrole(π*) charge-transfer character was predicted for several transitions in the visible region. This study adds to our fundamental understanding of the formation, structure, and electronic structure of bimetallic complexes of this class of expanded metalloporphyrins containing nonpyrrolic moieties.

A stable covalent organic framework for photocatalytic carbon dioxide reduction.

Photocatalytic conversion of CO2 into fuels is an important challenge for clean energy research and has attracted considerable interest. Here we show that tethering molecular catalysts-a rhenium complex, [Re(bpy)(CO)3Cl]-together in the form of a crystalline covalent organic framework (COF) affords a heterogeneous photocatalyst with a strong visible light absorption, a high CO2 binding affinity, and ultimately an improved catalytic performance over its homogeneous Re counterpart. The COF incorporates bipyridine sites, allowing for ligation of the Re complex, into a fully π-conjugated backbone that is chemically robust and promotes light-harvesting. A maximum rate of 1040 µmol g-1 h-1 for CO production with 81% selectivity was measured. CO production rates were further increased up to 1400 µmol g-1 h-1, with an improved selectivity of 86%, when a photosensitizer was added. Addition of platinum resulted in production of syngas, hence, the co-formation of H2 and CO, the chemical composition of which could be adjusted by varying the ratio of COF to platinum. An amorphous analog of the COF showed significantly lower CO production rates, suggesting that crystallinity of the COF is beneficial to its photocatalytic performance in CO2 reduction.

Photocatalytically active ladder polymers.

Conjugated ladder polymers (cLaPs) are introduced as organic semiconductors for photocatalytic hydrogen evolution from water under sacrificial conditions. Starting from a linear conjugated polymer (cLiP1), two ladder polymers are synthesized via post-polymerization annulation and oxidation techniques to generate rigidified, planarized materials bearing dibenzo[b,d]thiophene (cLaP1) and dibenzo[b,d]thiophene sulfone subunits (cLaP2). The high photocatalytic activity of cLaP1 (1307 µmol h-1 g-1) in comparison to that of cLaP2 (18 µmol h-1 g-1) under broadband illumination (λ > 295 nm) in the presence of a hole-scavenger is attributed to a higher yield of long-lived charges (µs to ms timescale), as evidenced by transient absorption spectroscopy. Additionally, cLaP1 has a larger overpotential for proton reduction and thus an increased driving force for the evolution of hydrogen under sacrificial conditions.

Reaching across the Divide: How Monometalation of One Binding Pocket Affects the Empty Binding Pocket in a Siamese-Twin Porphyrin Palladium Complex.

Siamese-twin porphyrin is a pyrazole-containing expanded porphyrin incorporating two porphyrin-like binding pockets. The macrocycle, however, does not possess an aromatic π system but rather two separated conjugation pathways that are isolated by the pyrazole junctions. Mono- and bimetallic complexes of the Siamese-twin porphyrin are known. This work addresses in detail the electronic consequences that monometalation (with PdII) has on the electronic properties of the nonmetalated binding pocket by studying the solid-state structure, acid/base, and electrochemical properties of the monopalladium twin-porphyrin complex. Specifically, metalation leads to a switch of the protonation sites of the free-base pocket. The unusual location of the protons at adjacent pyrrolic nitrogen atoms was revealed using X-ray diffraction and 1D/2D NMR spectroscopy. The one-electron oxidation and reduction events are both ligand-centered, as derived by spectroelectrochemical and electron paramagnetic resonance measurements, but are located on different halves of the molecule. Single-electron oxidation (-0.32 V vs Fc/Fc+) generated an organic radical centered on the metal-coordinating side of the ligand, while single-electron reduction (-1.59 V vs Fc/Fc+) led to the formation of an organic radical on the free-base side of the macrocycle. Density functional theory calculations corroborated the redox chemistry observed. The possibility of selectively preparing the monometallic complexes carrying two distinct redox sites-a metal-containing oxidation site and a metal-free reduction site-further expands the potential of Siamese-twin porphyrins to serve as an adjustable platform for multielectron redox processes in chemical catalysis or molecular electronics applications.

Siamese-Twin Porphyrin Origami: Oxidative Fusing and Folding.

Oxidation of a nonaromatic Siamese-twin porphyrin, a pyrazole-containing expanded porphyrin with two porphyrinlike binding pockets, with a stoichiometric amount of the two-electron, two-proton oxidizing agent 2,3-dichloro-5,6-dicyano-1,4-benzochinone led to the formation of a single N(pz) -C(o-Ph) linkage between the pyrazole unit with a neighboring meso-phenyl group, forming a pyrazolo- [1,5-a]indole moiety. Repeated treatment with a second equivalent of the oxidant yielded a doubly N-fused species, involving the second pyrazole moiety. The conversion products were characterized by variable-temperature and multinuclear 1D and 2D NMR spectroscopy. The fusions strongly alter the conformation of the macrocycles, as shown by X-ray diffraction analyses of all three compounds, eventually leading to a folded structure. UV/Vis and NMR-spectroscopic investigations indicated the presence of highly delocalized but nonmacrocycle-aromatic π systems. This behavior of the Siamese-twin porphyrin in response to oxidation is in contrast to the behavior of related all-pyrrole-based expanded macrocycles that switch, by redox processes and protonation, between Hückel and Möbius aromatic states.