Monometallic and Bimetallic Complexes with Sulfonamido-Based Tripodal Ligands

Author(s): Lau, Nathanael | Advisor(s): Borovik, Andrew S | Abstract: A central tenet of chemistry is the importance of the local environments that surround molecules. Rules for how such local environments control molecular properties have been developed and form the basis for coordination chemistry, an area of chemistry devoted to the study of molecules containing metal ions. Within this context, the volume of space surrounding metal ions is divided into two regions, referred to as the primary and secondary coordination spheres. The primary coordination sphere involves covalent interactions between atoms on ligands that are directly bound to the metal center. The secondary coordination sphere, which involves non-covalent interactions, is part of the volume of space around the metal center and often interacts with the ligands of the primary coordination sphere. Together, the coordination spheres define the physical properties and reactivity of a metal ion. The importance of modulating both is seen within the active sites of metalloproteins, in which the interplay between the two coordination spheres allow these proteins to catalyze difficult reactions under ambient conditions, with selectivities and efficiencies that are currently unattainable in synthetic systems.One approach towards understanding how the two coordination spheres affect function involves specially designed ligands that account for effects in both coordination spheres. The aim of this dissertation is to study synthetic metal complexes that incorporate these types of ligands, and explore their fundamental physical, structural, and chemical properties. The ligands used are based on the tripodal sulfonamido-based ligand N,N’,N”-[2,2’,2”-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3–). This ligand contains a tris(2-aminoethyl)amine (tren) backbone that allows for the preparation of four- or five-coordinate metal complexes with local C3 symmetry to control the primary coordination sphere. The trigonal environment leads to high-spin metal complexes, and the presence of three anionic nitrogen donors helps to stabilize relatively high oxidation states. Secondary coordination sphere effects are modulated through the sulfonamido moieties. The [MST]3– ligand can support monometallic metal complexes with terminal hydroxido, aqua, or ammine ligands, as the sulfonamido moieties can accept H-bonds from H-atom containing exogenous ligands. Additionally, the sulfonamido O-atoms can serve as a secondary metal binding site, allowing discrete bimetallic complexes to be prepared with [MST]3–.In this dissertation, new monometallic and bimetallic complexes with sulfonamido-based tripodal ligands were prepared, with the goal of understanding how the choice of ligands influences the properties of metal complexes. The first study investigated the effect of ligand modification on the physical properties of a series of FeII–OH2 complexes supported by ligands related to [MST]3–. The aryl groups of the five new N,N',N-[2,2',2-nitrilotris(ethane-2,1-diyl)]-tris-({R-Ph}-sulfonamido)) ([RST]3–) ligands had para-substituents of varying electron-withdrawing and donating strengths. The physical properties of the subsequent FeII–OH2 complexes were probed by various characterization methods, which revealed that the greatest impact of the ligand modification occurred in the metal complexes’ electrochemical properties.Monometallic Ni complexes with [MST]3– and a related urea-based ligand, [H3buea]3–, were then studied. The solid-state structures of these compounds showed that these ligands allowed for the preparation of NiII complexes with terminal aqua or hydroxido ligands in distorted trigonal bipyramidal geometries. Additionally, the oxidation chemistry of both NiII compounds was investigated, allowing for the preparation and characterization of uncommon NiIII complexes.Bimetallic complexes with [MST]3– are prepared by treating a solution of a monometallic [MST]3– complex, secondary metal salt, and secondary multidentate ligand with O2. The secondary ligand serves to “cap” the secondary metal center, resulting in discretely bimetallic units. A new series of bimetallic complexes with FeII(OH)FeIII, CoII(OH)FeIII, and NiII(OH)FeIII cores was prepared, using the bidentate capping ligand tetramethylethylenediamine (TMEDA). Previously, all other capping ligands used in this system had denticities of three and above. The bidentate capping ligand TMEDA allows the previously outer-sphere trifluoromethansulfonate (OTf–) counter anion to become inner-sphere, occupying the sixth coordination site of the second metal center.The ability to form heterobimetallic complexes and the presence of the weakly coordinating OTf– ligand prompted an investigation of the substitution chemistry of the system. In the FeII(OH)FeIII species, the OTf– ligand could be directly substituted for either isothiocyanate (NCS–) or azide (N3–) ligands. Additionally, a Br– ligand can occupy the sixth coordination site of the FeII center if the preparation of the diiron complex was modified. Furthermore, the TMEDA capping ligand can be directly substituted for two ethylenediamine (en) ligands. These results stress the importance of the choice of the capping ligand in these types of compounds, which has important implications on the substitution chemistry of this bimetallic system.