Effects of Temperature, Axial Ligand, and Photoexcitation on the Structure and Spin-State of Nickel(II) Complexes with Water-Soluble 5,10,15,20-Tetrakis(1-methylpyridinium-4-yl)porphyrin

Water-soluble metalloporphyrins, depending on the metal center, possess special spectral, coordination, and photochemical features. In nickel(II) porphyrins, the Ni(II) center can occur with low-spin or high-spin electronic configuration. In aqueous solution, the cationic nickel(II) complex (Ni(II)TMPyP4+, where H2TMPyP4+ = 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin), exists in both forms in equilibrium. In this study, an equilibrium system involving the low-spin and high-spin forms of Ni(II)TMPyP4+ was investigated via application of irradiation, temperature change, and various potential axial ligands. Soret band excitation of this aqueous system, in the absence of additional axial ligands, resulted in a shift in the equilibrium toward the low-spin species due to the removal of axial solvent ligands. The kinetics and the thermodynamics of the processes were also studied via determination of the rate and equilibrium constants, as well as the ΔS, ΔH, and ΔG values. Temperature increase had a similar effect. The equilibrium of the spin isomers was also shifted by decreasing the solvent polarity (using n-propanol) as well as by the addition of a stronger coordinating axial ligand (such as ammonia). Since triethanolamine is an efficient electron donor in Ni(II)TMPyP4+-based photocatalytic systems, its interaction with this metalloporphyin was also studied. The results promote the development of efficient photocatalytic systems based on this complex.

Engineering an Oxygen-Binding Protein for Photocatalytic CO2 Reductions in Water

While native CO2 -reducing enzymes display remarkable catalytic efficiency and product selectivity, few artificial biocatalysts have been engineered to allow understanding how the native enzymes work. To address this issue, we report cobalt porphyrin substituted myoglobin (CoMb) as a homogeneous catalyst for photo-driven CO2 to CO conversion in water. The activity and product selectivity were optimized by varying pH and concentrations of the enzyme and the photosensitizer. Up to 2000 TON(CO) was attained at low enzyme concentrations with low product selectivity (15 %), while a product selectivity of 74 % was reached by increasing the enzyme loading but with a compromised TON(CO). The efficiency of CO generation and overall TON(CO) were further improved by introducing positively charged residues (Lys or Arg) near the active stie of CoMb, which demonstrates the value of tuning the enzyme secondary coordination sphere to enhance the CO2 -reducing performance of a protein-based photocatalytic system.

Radical-Based Route to Functionalized Tetralin: Formal Total Synthesis of (±)-Hamigeran B

A formal synthetic route to hamigeran B, an antiviral marine natural product with a unique tricyclic molecular architecture, has been developed. The key chemical transformations in the present route include a novel zinc(II)porphyrin-catalyzed photoredox radical cascade cyclization to access a functionalized tetralin, a catalyst-free benzylic radical bromination with NBS by visible-light irradiation, and a samarium(II)-induced cyclization of brominated tetralone possibly via an orthoquinodimethane-like intermediate.

Bioinspired Applications of Porphyrin Derivatives

Porphyrin derivatives are ubiquitous in nature and have important biological roles, such as in light harvesting, oxygen transport, and catalysis. Owing to their intrinsic π-conjugated structure, porphyrin derivatives exhibit characteristic photophysical and electrochemical properties. In biological systems, porphyrin derivatives are associated with various protein molecules through noncovalent interactions. For example, hemoglobin, which is responsible for oxygen transport in most vertebrates, consists of four subunits of a globular protein with an iron porphyrin derivative prosthetic group. Furthermore, noncovalently arranged porphyrin derivatives are the fundamental chromophores in light-harvesting systems for photosynthesis in plants and algae. These biologically important roles originate from the functional versatility of porphyrin derivatives. Specifically, porphyrins are excellent host compounds, forming coordination complexes with various metal ions that adds functionality to the porphyrin unit, such as redox activity and additional ligand binding at the central metal ion. In addition, porphyrins are useful building blocks for functional supramolecular assemblies because of their flat and symmetrical molecular architectures, and their excellent photophysical properties are typically utilized for the fabrication of bioactive functional materials. In this Account, we summarize our endeavors over the past decade to develop functional materials based on porphyrin derivatives using bioinspired approaches. In the first section, we discuss several synthetic receptors that act as artificial allosteric host systems and can be used for the selective detection of various chemicals, such as cyanide, chloride, and amino acids. In the second section, we introduce multiporphyrin arrays as mimics of natural light-harvesting complexes. The active control of energy transfer processes by additional guest binding and the fabrication of organic photovoltaic devices using porphyrin derivatives are also introduced. In the third section, we introduce several types of porphyrin-based supramolecular assemblies. Through noncovalent interactions such as metal-ligand interaction, hydrogen bonding, and π-π interaction, porphyrin derivatives were constructed as supramolecular polymers with formation of fiber or toroidal assembly. In the last section, the application of porphyrin derivatives for biomedical nanodevice fabrication is introduced. Even though porphyrins were good candidates as photosensitizers for photodynamic therapy, they have limitations for biomedical application owing to aggregation in aqueous media. We suggested ionic dendrimer porphyrins and they showed excellent photodynamic therapy (PDT) efficacy.

Light-Driven CO2 Reduction by Co-Cytochrome b 562

The current trend in atmospheric carbon dioxide concentrations is causing increasing concerns for its environmental impacts, and spurring the developments of sustainable methods to reduce CO2 to usable molecules. We report the light-driven CO2 reduction in water in mild conditions by artificial protein catalysts based on cytochrome b 562 and incorporating cobalt protoporphyrin IX as cofactor. Incorporation into the protein scaffolds enhances the intrinsic reactivity of the cobalt porphyrin toward proton reduction and CO generation. Mutations around the binding site modulate the activity of the enzyme, pointing to the possibility of further improving catalytic activity through rational design or directed evolution.

A biradical oxo-molybdenum complex containing semiquinone and o-aminophenol benzoxazole-based ligands

We report a new mononuclear molybdenum(iv) complex, MoOL^BISL^SQ, in which L^SQ (2,4-di-tert-butyl o-semibenzoquinone ligand) has been prepared from the reaction of the o-iminosemibenzoquinone form of a tridentate non-innocent benzoxazole ligand, L^BIS, and MoO₂(acac)₂. The complex was characterized by X-ray crystallography, elemental analysis, IR and UV-vis spectroscopy and magnetic susceptibility measurements. The crystal structure of MoOL^BISL^SQ revealed a distorted octahedral geometry around the metal centre, surrounded by one O and two N atoms of L^BIS and two O atoms of L^SQ. The effective magnetic moment (μ_eff) of MoOL^BISL^SQ decreased from 2.36 to 0.2 μ_B in the temperature range of 290 to 2 K, indicating a singlet ground state caused by antiferromagnetic coupling between the metal and ligand centred unpaired electrons. Also, the latter led to the EPR silence of the complex. Cyclic voltammetry (CV) studies indicate both ligand and metal-centered redox processes. MoOL^BISL^SQ was applied as a catalyst for the oxidative cleavage of cyclohexene to adipic acid and selective oxidation of sulfides to sulfones with aqueous hydrogen peroxide.

Biradical molybdenum(iv) complex, MoOL^BISL^SQ, has been prepared from the reaction of the o-iminosemibenzoquinone form of a tridentate non-innocent benzoxazole ligand, L^BIS, and MoO₂(acac)₂ and used as catalyst in oxidation reaction..