Functional metallodendrimers: From novel ruthenium(II) systems to gadolinium(III) MRI contrasting agents

Since their inception and development in the early 1980s, dendrimers have been utilized in the design and synthesis of an incredibly diverse array of macromolecular compounds. Dendrimers, which are specifically branched and highly ordered spherical polymers created through an iterative synthetic procedure, have been designed to address innumerable synthetic applications and problems. Metallodendrimers are dendrimer derivatives that can possess metal chelates at the core, branching units, periphery, or throughout the macromolecular structure of a dendrimer. Today, by harnessing the remarkable synthetic control that is available over the design and placement of highly specialized functional groups throughout the structure of a dendrimer, synthetic organic and inorganic/organometallic chemists have available to them the tools necessary to construct unique macromolecular compounds that are capable of performing a myriad of functions.; The goal of this project encompasses the design, synthesis, and application of two metallodendrimer systems to demonstrate the wide range of functionality that can be achieved through using dendrimers. To this end, a series of ruthenium(II)-based metallodendrimers and an effective gadolinium(III) magnetic resonance imaging (MRI) contrast agent have been synthesized.; We report the synthesis of terpyridine-functionalized poly(aryl ether) monodendrons, surface-modified organic dendrimers built upon an octafunctionalized polyhedral oligomeric silsesquioxane (POSS) core, and the corresponding metallo-dendrimers with peripheral Ruthenium(II)-based chromophores. Specifically, three generations of metallodendrimers, [((bpy)₂Ru(tpy))₈-G1-Si₈ O₁₂P₈](PF₆)₁₆ (11 ), [((bpy)₂Ru(tpy))₁₆-G2-Si₈O ₁₂P₈](PF₆)₃₂ (12), and [((bpy)₂Ru(tpy₃₂-G3-Si₈O₁₂P ₈](PF₆)₆₄ (13) have been synthesized and characterized, which exhibit interesting photophysical and electrochemical properties, including enhanced extinction coefficients, improved overall quantum yields, long-lived excited-state lifetimes, and simultaneous redox processes for all metal centers on the dendrimers surface.; In addition, we report herein on the first known dendritic Gadolinium(III)-based magnetic resonance imaging (MRI) contrasting agents built upon an octafunctionalized POSS core moiety, (DTPA-Gd)₈-G1-Si₈O₁₂N ₈ (18). The use of an eight-coordinate core yields dendrimers with significant advantages over more traditional three- and four-coordinate dendritic MRI contrasting agents. Dendrimers of our design, which contain multiple Gd(III) chelates along the dendrimer periphery, have shown moderate increases in both the r₁ (longitudinal) and r₂ (transverse) proton relaxation rates over commercially available mono-metallic agents.