Copper(II) Complexes of 2,2'-Bisdipyrrins: Synthesis, Characterization, Cell Uptake, and Radiolabeling with Copper-64

Complexes prepared with positron-emitting copper-64 are of interest as imaging agents for positron emission tomography (PET). This work investigates the potential of using acyclic tetrapyrrolic 2,2'-bisdipyrrins as ligands to prepare charge-neutral, lipophilic, cell-permeable, redox active complexes with positron-emitting copper-64. The synthesis and characterization of a series of tetrapyrrolic 2,2'-bisdipyrrin copper(II) complexes are reported. Four 2,2'-bisdipyrrin copper(II) complexes were prepared with different functional groups in the meso-position of the ligands. Two of the new copper(II) complexes, one palladium(II) complex, and one nickel(II) complex were characterized by X-ray crystallography, which demonstrated that the copper(II) is in a distorted square planar environment. An investigation of the electrochemical properties of the complexes by cyclic voltammetry revealed that the complexes undergo multiple quasi-reversible processes. A comparison of the cyclic voltammetry of the copper complexes with their palladium(II) analogues suggests that these redox processes are ligand-based and not metal-based. The copper(II) complexes are cell-permeable in A431 mammalian cells and are nontoxic at concentrations of 50 μM. The ligands can be radiolabeled with copper-64 at room temperature.

Stereochemistry and chiroptical properties of bimetallic single helicates of hexapyrrole-α, ω-dicarbaldimines with high diastereoselectivity

Bimetallic complexes of hexapyrrole-[Formula: see text],[Formula: see text]-dicarbaldimines consisting of a pair of four-coordinate metal sites adopt a helical closed [Formula: see text]-symmetric form or sigmoidal open forms depending on whether the 2,2[Formula: see text]-bipyrrole subunit at the center of the hexapyrrole chain takes cis- or trans-conformation. X-ray crystallography of a bisNi complex having N-[([Formula: see text]-1-cyclohexylethyl]carbaldimine units at both ends of the hexapyrrole chain revealed a non-symmetric heterohelical open form where the metal coordination sites of opposite helical sense sit on opposite sides of the central 2,2[Formula: see text]-bipyrrole subunit. BisPd complexes preferred a closed [Formula: see text] form and a steric bulk at the 3,3[Formula: see text]-position of the 2,2[Formula: see text]-bipyrrole subunit improved the helical sense bias. A bisPd complex with N-[([Formula: see text]-1-cyclohexylethyl]carbaldimine units adopts a helical closed [Formula: see text] form exclusively with full bias for a [Formula: see text]-helical sense. These bimetallic single stranded helicates were reversibly oxidized to [Formula: see text]-cation radicals at 0.1[Formula: see text]0.3 V vs. a ferrocene/ferrocenium couple and spectroelectrochemistry revealed remarkable absorption and CD spectral changes in the Vis-NIR region.

Conformational Changes and Redox Properties of Bimetallic Single Helicates of Hexapyrrole-α,ω-dicarbaldehydes

The hexapyrrole-α,ω-dicarbaldehydes 1 a and 1 b were metallated with Cu^II , Ni^II , and Pd^II to give bimetallic complexes where a pair of 3 N+O four-coordinate metal planes are helically distorted and the central 2,2'-bipyrrole subunit adopts a cis or trans conformation. X-ray crystallographic analysis of the bisCu complex revealed a closed form with a cis-2,2'-bipyrrole subunit and an open form with a trans-2,2'-bipyrrole subunit. The bisPd complexes took a closed form both in the solid state and in solution. They are regarded as single helicates of two turns and the energy barrier for the interchange between an M helix and a P helix was remarkably influenced by the bulky 3,3'-substituent of the central 2,2'-bipyrrole subunit. Although the bisNi complexes adopt a closed form in the solid state, they exist as a homohelical open C₂ -symmetric form or a heterohelical open C_i -symmetric form in solution. A theoretical study suggested that the closed form of 1 aPd was stabilized by the Pd-Pd interaction. Compound 1 aPd was reversibly oxidized by one electron at 0.14 V versus ferrocene/ferrocenium (Fc/Fc⁺ ) and this oxidized species showed Vis/NIR absorption bands at λ=767 and 1408 nm.

Electrochemical, Spectroscopic, and 1O2 Sensitization Characteristics of Synthetically Accessible Linear Tetrapyrrole Complexes of Palladium and Platinum

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene (DMBil1) linear tetrapyrrole supporting Pd^II or Pt^II centers is presented. Both of these nonmacrocyclic tetrapyrrole platforms are robust and easily prepared via modular routes. X-ray diffraction experiments reveal that the Pd[DMBil1] and Pt[DMBil1] complexes adopt similar structures and incorporate a single Pd^II and Pt^II center, respectively. Additionally, electrochemical experiments revealed that both Pd[DMBil1] and Pt[DMBil1] can undergo two discrete oxidation and reduction processes. Spectroscopic experiments carried out for Pd[DMBil1] and Pt[DMBil1] provide further understanding of the electronic structure of these systems. Both complexes strongly absorb light in the UV-visible region, especially in the 350-600 nm range. Both Pd[DMBil1] and Pt[DMBil1] are luminescent under a nitrogen atmosphere. Upon photoexcitation of Pd[DMBil1], two emission bands are observed; fluorescence is detected from ∼500-700 nm and phosphorescence from ∼700-875 nm. Photoexcitation of Pt[DMBil1] leads only to phosphorescence, presumably due to enhanced intersystem crossing imparted by the heavier Pt^II center. Phosphorescence from both complexes is quenched under air due to energy transfer from the excited triplet state to ground state oxygen. Accordingly, irradiation with light of λ ≥ 500 nm prompts Pd[DMBil1] and Pt[DMBil1] to photosensitize the generation of ¹O₂ (singlet oxygen) with impressive quantum yields of 80% and 78%, respectively. The synthetic accessibility of these complexes coupled with their ability to efficiently photosensitize ¹O₂ may make them attractive platforms for development of new agents for photodynamic therapy.

Electrochemical, spectroscopic, and (1)O2 sensitization characteristics of 10,10-dimethylbiladiene complexes of zinc and copper

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethylbiladiene tetrapyrrole bearing ancillary pentafluorophenyl groups at the 5- and 15-meso positions (DMBil1) is presented. This nonmacrocyclic tetrapyrrole platform is robust and can serve as an excellent ligand scaffold for Zn²⁺ and Cu²⁺ centers. X-ray diffraction studies conducted for DMBil1 along with the corresponding Zn[DMBil1] and Cu[DMBil1] complexes show that this ligand scaffold binds a single metal ion within the tetrapyrrole core. Additionally, electrochemical experiments revealed that all three of the aforementioned compounds display an interesting redox chemistry as the DMBil1 framework can be both oxidized and reduced by two electrons. Spectroscopic and photophysical experiments carried out for DMBil1, Zn[DMBil1], and Cu[DMBil1] provide a basic picture of the electronic properties of these platforms. All three biladiene derivatives strongly absorb light in the visible region and are weakly emissive. The ability of these compounds to sensitize the formation of ¹O₂ at wavelengths longer than 500 nm was probed. Both the free base and Zn²⁺ 10,10-dimethylbiladiene architectures show modest efficiencies for ¹O₂ sensitization. The combination of structural, electrochemical, and photophysical data detailed herein provides a basis for the design of additional biladiene constructs for the activation of O₂ and other small molecules.