Density Functional Study on the Photopolymerization of Styrene Using Dinuclear Ru-Pd and Ir-Pd Complexes with Naphthyl-Substituted Ligands

A density functional study was performed to investigate the mechanism of the photocatalytic reactivity of styrene polymerization using dinuclear Ru-Pd and Ir-Pd catalytic complexes. In previous experiments with these catalysts, the reactivity increased, and more polymer products were yielded compared to dimers under visible light irradiation. The best catalytic reactivity was obtained using an Ir-Pd complex containing naphthyl substituents at the phenyl ligands coordinated to Ir (Ir-Pd1). In contrast, Ir-Pd2, an isomer of Ir-Pd1, containing naphthyl substituents at the pyridine ligands, did not show good reactivity, which may be related to the stability of the excited state of the catalytic complexes. In this study, we calculated the radiative lifetimes of these catalytic complexes and Ir-Pd1 had the longest lifetime; this result was consistent with the experimental results. The longest lifetime of the Ir-Pd1 was attributed to the destabilization of the highest occupied molecular orbital (HOMO) energy by π*-π* interactions between the naphthyl and phenyl ligands. Further, this destabilization of the HOMO energy afforded a small energy gap between the HOMO and lowest unoccupied molecular orbital, enhancing the metal-to-ligand charge transfer to the bridging ligand between Ir and Pd. Additionally, we focused on the reaction of the second insertion of styrene, which was identified as the rate-determining step of the polymerization cycle in a previous study. The singlet-triplet crossing points of the intermediates were estimated, and the barrier heights of the intersystem crossing were much lower than those in the thermal paths, which explained the efficiency of the photocatalytic reactivity in the experiment.

Tethered photocatalyst-directed palladium-catalysed C-H allenylation of N-aryl tetrahydroisoquinolines

Harnessing radical intermediates in regioselective reactions presents a substantial challenge. Here, we report a novel control strategy through engineering covalently tethered transition metal-photocatalysts that conjoin Pd-phosphine and Ru/Ir photoredox units. This strategy allows us to override the innate regioselectivity of the Pd-catalysed C-H allenylation of N-aryl tetrahydroisoquinolines.

Syntheses and structure of dinuclear metal complexes containing naphthyl-Ir bichromophore

A series of novel metal complexes were synthesized containing an Ir-cyclometalated bichromophore as a visible-light sensitizer. A new bichromophoric unit containing a naphthyl substituent and methyl substituents on the 2-phenylpyridine chelating ligand was synthesized and characterized for the first time. According to the increased crystallinity of the bichromophoric unit, novel Ir-M metal complexes (M = Pd, Mn, and Ir) were synthesized and fully characterized. The novel Ir-Pd complex maintained photocatalytic activity toward styrenes under visible-light irradiation, and polymerization with p-chlorostyrene, copolymerization with styrene and p-chlorostyrene furnished corresponding polymers.

Reversible redox controlled acids for cationic ring-opening polymerization

Advancements in externally controlled polymerization methodologies have enabled the synthesis of novel polymeric structures and architectures, and they have been pivotal to the development of new photocontrolled lithographic and 3D printing technologies. In particular, the development of externally controlled ring-opening polymerization (ROP) methodologies is of great interest, as these methods provide access to novel biocompatible and biodegradable block polymer structures. Although ROPs mediated by photoacid generators have made significant contributions to the fields of lithography and microelectronics development, these methodologies rely upon catalysts with poor stability and thus poor temporal control. Herein, we report a class of ferrocene-derived acid catalysts whose acidity can be altered through reversible oxidation and reduction of the ferrocenyl moiety to chemically and electrochemically control the ROP of cyclic esters.

Photoaccelerated energy transfer catalysis of the Suzuki-Miyaura coupling through ligand regulation on Ir(iii)-Pd(ii) bimetallic complexes

Three bimetallic Ir(iii)-Pd(ii) complexes [Ir(ppy)2(bpm)PdCl2](PF6) (ppy = 2-phenylpyridine, 1), [Ir(dfppy)2(bpm)PdCl2](PF6) (dfppy = (4,6-difluorophenyl)pyridine, 2), and [Ir(pq)2(bpm)PdCl2](PF6) (pq = 2-phenylquinoline, 3) were synthesized by using 2,2'-bipyrimidine (bpm) as a bridging ligand. The influences of the cyclometalated ligand at the Ir(iii) center on the photophysical and electrochemical properties as well as photocatalytic activity for the Suzuki-Miyaura coupling reaction under mild conditions were evaluated. The results revealed that complex 3 enables dramatically accelerating the Suzuki-Miyaura coupling reaction under visible light irradiation at room temperature, due to the effective absorption of visible light and appropriate locus of the excited chromophore. Mechanism studies showed that the chromophore [Ir(pq)2(bpm)] fragment absorbs visible light to produce the triplet excited state centering on the bridging ligand which boosts the formation of electron rich Pd(ii) units and facilitates the oxidative addition step of the catalytic cycle. Simultaneously, the excited chromophore undergoes energy transfer efficiently to the Pd(ii) reaction site to form the excited Pd(ii) species, resulting in enhancement of Pd(ii) reduction steps of the Suzuki-Miyaura coupling reaction and increasing the reactivity of the catalyst. This provides a new strategy for designing photocatalysts for coupling reaction through altering the cyclometalated ligand to modulate the photophysical properties and the cooperation between two metal units.

Electronically Governed ROMP: Expanding Sequence Control for Donor–Acceptor Conjugated Polymers

Controlling the primary sequence of synthetic polymers remains a grand challenge in chemistry. A variety of methods that exert control over monomer sequence have been realized wherein differential reactivity, pre-organization, and stimuli-response have been key factors in programming sequence. Whereas much has been established in nonconjugated systems, π-extended frameworks remain systems wherein subtle structural changes influence bulk properties. The recent introduction of electronically biased ring-opening metathesis polymerization (ROMP) extends the repertoire of feasible approaches to prescribe donor–acceptor sequences in conjugated polymers, by enabling a system to achieve both low dispersity and controlled polymer sequences. Herein, we discuss recent advances in obtaining well-defined (i.e., low dispersity) polymers featuring donor–acceptor sequence control, and present our design of an electronically ambiguous (4-methoxy-1-(2-ethylhexyloxy) and benzothiadiazole-(donor–acceptor-)based [2.2]paracyclophanediene monomer that undergoes electronically dictated ROMP. The resultant donor–acceptor polymers were well-defined (Đ = 1.2, Mn > 20 k) and exhibited lower energy excitation and emission in comparison to ‘sequence-ill-defined’ polymers. Electronically driven ROMP expands on prior synthetic methods to attain sequence control, while providing a promising platform for further interrogation of polymer sequence and resultant properties.

1 Introduction to Sequence Control

2 Sequence Control in Polymers

3 Multistep-Synthesis-Driven Sequence Control

4 Catalyst-Dictated Sequence Control

5 Electronically Governed Sequence Control

6 Conclusions

Trinuclear ruthenium core-containing polyoxometalate-based hybrids: preparation, characterization and catalytic behavior

Two ruthenium-containing polyoxometalate-based hybrids (Ru-POMs), KH8[Ru3O(Trz)6Cl3]2[(NaO6)W6(H2O)6(AsW9O33)4]·28H2O (1) and H3[Ru3O(Trz)6Cl3]2[(WO)2W(OH)(AsW9O33)2]·6H2O (2), (Trz = 1,2,4-triazole), were successfully isolated by a one-step hydrothermal method under different acidic conditions and further characterized by single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), IR spectroscopy, electrospray ionization and mass spectrometry (ESI-MS), thermogravimetric (TG) analyses and elemental analyses. Single-crystal X-ray diffraction analyses reveal two polyanions comprised of four/two identical trivacant Keggin-type polyanion building blocks, leading to the formation of tetrameric/dimeric assembly stabilized by additional metal-oxo cores, respectively. ESI-MS shows that the polyanion units [(NaO6)W6(H2O)6(AsW9O33)4]11- and [(WO)2W(OH)(AsW9O33)2]5- are intact in mixed solvent. Moreover, the two heterogeneous catalysts with trinuclear ruthenium cations were further investigated, and 1 was found to exhibit much higher yield (94.1%)/conversion (95.1%) and better selectivity (>99%) towards the oxidation of thioanisole to sulfoxide than 2.

Light-mediated olefin coordination polymerization and photoswitches

This review outlines photoswitchable, transition metal-based olefin coordination polymerization catalysts ranging from homogeneous to heterogeneous, and monometallic to bimetallic regimes.

Reactive Heterobimetallic Complex Combining Divalent Ytterbium and Dimethyl Nickel Fragments

This article presented the synthesis and characterization of original heterobimetallic species combining a divalent lanthanide fragment and a divalent nickel center bridged by the bipyrimidine ligand, a redox-active ligand. X-ray crystal structures were obtained for the Ni monomer (bipym)NiMe₂, 1, as well as the heterobimetallic dimer compounds, Cp*₂Yb(bipym)NiMe₂, 2, along with ¹H solution NMR, solid-state magnetic data, and DFT calculations only for 1. The reactivity with CO was investigated on both compounds and the stoichiometric acetone formation is discussed based on kinetic and mechanistic studies. The key role of the lanthanide fragment was demonstrated by the relatively slow CO migratory insertion step, which indicated the stability of the intermediate.

Light-driven catalytic hydrogenation of carbon dioxide at low-pressure by a trinuclear iridium polyhydride complex

Under irradiation conditions, low-pressure and room-temperature hydrogenation of carbon dioxide (CO₂) has been achieved using a trinuclear iridium hexahydride complex 1.

Bis(bipyridine) ruthenium(ii) bis(phosphido) metalloligand: synthesis of heterometallic complexes and application to catalytic (E)-selective alkyne semi-hydrogenation

The first (bpy)₂Ru(ii) phosphido complex serves as an effective bidentate metallophosphine ligand to generate catalytically active heterobimetallic species.

On Demand Switching of Polymerization Mechanism and Monomer Selectivity with Orthogonal Stimuli

The development of next-generation materials is coupled with the ability to predictably and precisely synthesize polymers with well-defined structures and architectures. In this regard, the discovery of synthetic strategies that allow on demand control over monomer connectivity during polymerization would provide access to complex structures in a modular fashion and remains a grand challenge in polymer chemistry. In this Article, we report a method where monomer selectivity is controlled during the polymerization by the application of two orthogonal stimuli. Specifically, we developed a cationic polymerization where polymer chain growth is controlled by a chemical stimulus and paired it with a compatible photocontrolled radical polymerization. By alternating the application of the chemical and photochemical stimuli the incorporation of vinyl ethers and acrylates could be dictated by switching between cationic and radical polymerization mechanisms, respectively. This enables the synthesis of multiblock copolymers where each block length is governed by the amount of time a stimulus is applied, and the quantity of blocks is determined by the number of times the two stimuli are toggled. This new method allows on demand control over polymer structure with external influences and highlights the potential for using stimuli-controlled polymerizations to access novel materials.

Synthesis of di- and trinuclear iridium polyhydride complexes surrounded by light-absorbing ligands

New di- and trinuclear iridium (Ir) penta- and hexahydride complexes containing light-absorbing diphosphine ligands have been developed. The trinuclear Ir complex possessed a higher absorption coefficient (standardized per Ir) than that of the parent mononuclear Ir complex, indicating the assembly effect. The trinuclear complex showed high reactivity toward catalytic hydrogenation of diphenylacetylene under mild conditions, and the reaction was considerably accelerated under photoirradiation.

Synthesis of Mono-, Di-, and Trinuclear Rhodium Diphosphine Complexes Containing Light-Harvesting Fluorene Backbones

Syntheses and structural determination of rhodium complexes ligated by diphosphines containing light-harvesting fluorene backbones are presented. In these rhodium complexes, the rhodium center is surrounded by a light-absorbing diphosphine unit. The presence of Rh-Rh bonding interactions is suggested by density functional theory studies.

Photoinduced Electron Transfer Reactions for Macromolecular Syntheses

Photochemical reactions, particularly those involving photoinduced electron transfer processes, establish a substantial contribution to the modern synthetic chemistry, and the polymer community has been increasingly interested in exploiting and developing novel photochemical strategies. These reactions are efficiently utilized in almost every aspect of macromolecular architecture synthesis, involving initiation, control of the reaction kinetics and molecular structures, functionalization, and decoration, etc. Merging with polymerization techniques, photochemistry has opened up new intriguing and powerful avenues for macromolecular synthesis. Construction of various polymers with incredibly complex structures and specific control over the chain topology, as well as providing the opportunity to manipulate the reaction course through spatiotemporal control, are one of the unique abilities of such photochemical reactions. This review paper provides a comprehensive account of the fundamentals and applications of photoinduced electron transfer reactions in polymer synthesis. Besides traditional photopolymerization methods, namely free radical and cationic polymerizations, step-growth polymerizations involving electron transfer processes are included. In addition, controlled radical polymerization and "Click Chemistry" methods have significantly evolved over the last few decades allowing access to narrow molecular weight distributions, efficient regulation of the molecular weight and the monomer sequence and incredibly complex architectures, and polymer modifications and surface patterning are covered. Potential applications including synthesis of block and graft copolymers, polymer-metal nanocomposites, various hybrid materials and bioconjugates, and sequence defined polymers through photoinduced electron transfer reactions are also investigated in detail.

Synthesis and photocatalytic activity of a naphthyl-substituted photosensitizing BINAP–palladium complex

A novel light-active Pd complex having a BINAP ligand with 2-naphthyl substituents on the phosphines was synthesized and characterized, which is active in the catalytic light-controlled polymerization of 4-methoxystyrene.