Changing Mechanical Strength in Cr(III)- Metallosupramolecular Polymers with Ligand Groups and Light Irradiation

We have demonstrated the ability to control the mechanical properties of metallosupramolecular materials via choice of ligand binding group, as well as with external light irradiation. These photoresponsive Cr­(III)-based materials were prepared from a series of modified hydrogenated poly­(ethylene-<i>co</i>-butylene) polymers linked through metal–ligand interactions between a Cr­(III) metal center and pyridyl ligand termini of the polymers. The introduction of these Cr­(III)-pyridine bonds gave rise to new mechanical and optical properties of the polymer materials. Depending on the type of pyridyl ligand, density functional theory calculations revealed changes in coordination to the Cr­(III), which ultimately led to materials with significantly different mechanical properties. Electronic excitation of the Cr­(III) materials with 450 and 655 nm CW lasers (800 mW/cm²) resulted in generation of excited state photophysical processes which led to temporary softening of the materials up to 143 kPa (41.5%) in storage modulus (<i>G</i>′) magnitude. The initial mechanical strength of the materials was recovered when the light stimulus was removed, and no change in mechanical properties was observed with light irradiation where there was no absorbance by the Cr­(III) moiety. These materials demonstrate that introduction of metal–ligand bonding interactions into polymers enables the design and synthesis of photoresponsive materials with tunable optical-mechanical properties not seen in traditional polymeric materials

Dinitrosyl Iron Complexes with Cysteine. Kinetics Studies of the Formation and Reactions of DNICs in Aqueous Solution

Kinetics studies provide mechanistic insight regarding the formation of dinitrosyl iron complexes (DNICs) now viewed as playing important roles in the mammalian chemical biology of the ubiquitous bioregulator nitric oxide (NO). Reactions in deaerated aqueous solutions containing FeSO₄, cysteine (CysSH), and NO demonstrate that both the rates and the outcomes are markedly pH dependent. The dinuclear DNIC Fe₂(μ-CysS)₂(NO)₄, a Roussin’s red salt ester (<b>Cys-RSE</b>), is formed at pH 5.0 as well as at lower concentrations of cysteine in neutral pH solutions. The mononuclear DNIC Fe­(NO)₂(CysS)₂^– (<b>Cys-DNIC</b>) is produced from the same three components at pH 10.0 and at higher cysteine concentrations at neutral pH. The kinetics studies suggest that both <b>Cys-RSE</b> and <b>Cys-DNIC</b> are formed via a common intermediate Fe­(NO)­(CysS)₂^–. <b>Cys-DNIC</b> and <b>Cys-RSE</b> interconvert, and the rates of this process depend on the cysteine concentration and on the pH. Flash photolysis of the <b>Cys-RSE</b> formed from Fe­(II)/NO/cysteine mixtures in anaerobic pH 5.0 solution led to reversible NO dissociation and a rapid, second-order back reaction with a rate constant <i>k</i>_NO = 6.9 × 10⁷ M^–1 s^–1. In contrast, photolysis of the mononuclear-DNIC species <b>Cys-DNIC</b> formed from Fe­(II)/NO/cysteine mixtures in anaerobic pH 10.0 solution did not labilize NO but instead apparently led to release of the CysS^• radical. These studies illustrate the complicated reaction dynamics interconnecting the DNIC species and offer a mechanistic model for the key steps leading to these non-heme iron nitrosyl complexes

Dinuclear PhotoCORMs: Dioxygen-Assisted Carbon Monoxide Uncaging from Long-Wavelength-Absorbing Metal–Metal-Bonded Carbonyl Complexes

We describe a new strategy for triggering the photochemical release of caged carbon monoxide (CO) in aerobic media using long-wavelength visible and near-infrared (NIR) light. The dinuclear rhenium–manganese carbonyl complexes (CO)₅ReMn­(CO)₃(L), where L = phenanthroline (<b>1</b>), bipyridine (<b>2</b>), biquinoline (<b>3</b>), or phenanthrolinecarboxaldehyde (<b>4</b>), each show a strong metal–metal-bond-to-ligand (σ_MM → π_L*) charge-transfer absorption band at longer wavelengths. Photolysis with deep-red (<b>1</b> and <b>2</b>) or NIR (<b>3</b> and <b>4</b>) light leads to homolytic cleavage of the Re–Mn bonds to give mononuclear metal radicals. In the absence of trapping agents, these radicals primarily recombine to reform dinuclear complexes. In oxygenated media, however, the radicals react with dioxygen to form species much more labile toward CO release via secondary thermal and/or photochemical reactions. Conjugation of <b>4</b>, with an amine-terminated poly­(ethylene glycol) oligomer, gives a water-soluble derivative with similar photochemistry. In this context, we discuss the potential applications of these dinuclear complexes as visible/NIR-light-photoactivated CO-releasing moieties (photoCORMs)

Dinuclear PhotoCORMs: Dioxygen-Assisted Carbon Monoxide Uncaging from Long-Wavelength-Absorbing Metal–Metal-Bonded Carbonyl Complexes

We describe a new strategy for triggering the photochemical release of caged carbon monoxide (CO) in aerobic media using long-wavelength visible and near-infrared (NIR) light. The dinuclear rhenium–manganese carbonyl complexes (CO)₅ReMn­(CO)₃(L), where L = phenanthroline (<b>1</b>), bipyridine (<b>2</b>), biquinoline (<b>3</b>), or phenanthrolinecarboxaldehyde (<b>4</b>), each show a strong metal–metal-bond-to-ligand (σ_MM → π_L*) charge-transfer absorption band at longer wavelengths. Photolysis with deep-red (<b>1</b> and <b>2</b>) or NIR (<b>3</b> and <b>4</b>) light leads to homolytic cleavage of the Re–Mn bonds to give mononuclear metal radicals. In the absence of trapping agents, these radicals primarily recombine to reform dinuclear complexes. In oxygenated media, however, the radicals react with dioxygen to form species much more labile toward CO release via secondary thermal and/or photochemical reactions. Conjugation of <b>4</b>, with an amine-terminated poly­(ethylene glycol) oligomer, gives a water-soluble derivative with similar photochemistry. In this context, we discuss the potential applications of these dinuclear complexes as visible/NIR-light-photoactivated CO-releasing moieties (photoCORMs)