Tripodal carbene and aryloxide ligands for small-molecule activation at electron-rich uranium and transition metal centers

Karsten Meyer; Suzanne C. Bart    University of Erlangen-Nürnberg, Department of Chemistry and Pharmacy, Inorganic Chemistry, Egerlandstr. 1, 91058 Erlangen, Germany

I Introduction

Chelating ligands that induce tripodal configurations at coordinated metal centers are known to provide powerful platforms for small-molecule activation. These ligands consist of an atom or small molecule which acts as an anchor, and holds three pendant arms capable of coordinating to a metal in a trigonal conformation (Fig. 1). Typically, these ligands hold several advantages over monodentate and bidentate ligands. Due to the enhanced chelating effects, tripodal ligands often bind to metal ions very strongly and can stabilize reactive intermediate species with unusual electronic and geometric structures. In addition, the steric bulk of both the anchor and pendant arms is highly modular, providing the electronic and structural flexibility to effectively block undesired side- and decomposition reactions, more effectively controlling reactivity at the metal center. Due to these distinctive benefits, the design and development of new tripodal ligand systems has been an active area in inorganic and organometallic coordination chemistry (1–4).

The chemistry presented herein has been presented on the occasion of the SFB symposium “Redox active metal complexes – Control of Reactivity via Molecular Architecture.”

Commonly used tripodal ligands such as tripodal tris(amido)amine (Fig. 2, A) and tris-phosphine ligands (Fig. 2, B) have drawn much attention recently for supporting metal centers capable of small-molecule activation (1). Metal complexes supported by tris(pyrazolyl)borate (Tp) ligands promote catalytic transformations, including C–H activation (5), C–C (6), C–O (7), and C–N (8) bond formation, assist dioxygen activation (9) and serve as structural mimics of metal-containing enzymes (10). The tetradentate tris(amido)amine framework, composed of three negatively charged “hard”(11) amido donors, coordinates to transition metal (2,12) and main group elements (13) in 3+ or higher oxidation states. The resulting metal complexes have a pair of degenerate π-type frontier orbitals that aid in metal–ligand multi-bond formation (2) and have proven essential for supporting well-defined catalytic reactions such as dinitrogen reduction at a single molybdenum center (14,15). In contrast, tripodal ligands bearing “soft” donor atoms such as sulfur (16) and phosphorus (4,17,18) are more suitable for stabilizing electron-rich, low-valent metal centers. A prime example is the work by Sacconi (19) which shows the development of the tris(phosphino)amine ligand (Fig. 2, C).

Using this rich chemistry as inspiration, the development of tripodal N-heterocyclic carbene (NHC) analogues was explored (Fig. 2, D). These chelators should mimic the properties of monodentate NHC ligands, producing distinct beneficial synthetic, electronic, and steric properties over these previously known tripodal ligand systems, including reduction of oxidative ligand degradation which can occur with air-sensitive phosphine ligands. Recent studies show that the seemingly “soft” NHCs can coordinate to both “soft”, electron-rich metal fragments and “hard”, electron-deficient metal centers (20), resulting in their coordination to virtually every metal in the periodic table with a range of oxidation states (20a). The newly developed tripodal NHC ligands thus are complementary to both the tris(amido)amine and the tris(phosphino)borate ligand systems.

The geometries of tripodal ligand systems are determined by the type of pendant arm and the number of atoms in the linker to the anchor (Fig. 2, Types A and B).

Depending on these variables, the sterically encumbering substituents may be directed away from the metal center, leaving the reactive core wide open or be directed towards the metal center in order to protect it (see arrows in Fig. 2). Protecting the metal center prevents binuclear decomposition of reactive species. For instance, Peters et al. report that the bulky isopropyl derivatized tris(phosphino)-borate ligand does not prevent dimerization of the unique yet highly reactive terminal nitrido complex [(PhBPiPr)3Fe≡N] to form a dinitrogen-bridged dinuclear species [(PhBPiPr3)Fe]2(μ-N2) (Fig. 3, E) (21). To prevent [(ArN3N)Mo≡N] from forming a similar dinuclear dinitrogen complex [{(ArN3N)Mo}2 (μ:η1,η1–N2)], Schrock et al. had to introduce three extremely bulky hexaisopropyl terphenyl substituents at the tris(amido)amine ligand (Fig. 3, D) (14). The synthesis of these sterically bulky ligand derivatives is both challenging and time-consuming. In contrast, the sterics of tripodal NHC ligands are controlled by the alkyl or aryl substituents at the sp2-hybridized ring nitrogen (N3), allowing perpendicular alignment of the steric bulk to the plane of the pendant arms forming a deep (5–6 Å) well-protected cavity for ligand binding to the metal center (Fig. 2, Type C).

Prior to our work, only two tripodal NHC ligand systems were known. The mesitylene-anchored tris(carbene) ligand (Fig. 4, 1) was unique (22), yet metal complexation had not been achieved with this ligand. Thorough investigation by Nakai et al. of the coordination chemistry of 1 and its derivatives showed that the cavity of this ligand system can only host exceptionally large metal ions, such as the thallium(I) cation (23). Attempts to synthesize transition metal complexes of derivatives of 1 have been unsuccessful up to this point. The development of new tripodal NHC ligand systems for stabilization of a single transition metal center in a coordinatively unsaturated ligand environment to allow the binding and activation of small molecules in a controlled manner is discussed here. In particular, we describe the synthesis of two new classes of tripodal NHC ligands TIMER (1,1,1-tris(3-alkylimidazol-2-ylidene)methyl]ethane) (2R) and TIMENR (tris[2-(3-alkylimidazol-2-ylidene)ethyl]amine) (3R) and their coordination chemistry.

The coordination chemistry of uranium centers with a classic Werner-type polyamine tripodal chelator is also being investigated (Fig. 5) with a goal of identifying and isolating uranium complexes with enhanced reactivity towards binding, activation, and functionalization of small molecules. Because of the large size of the uranium center, instead of using a single atom anchor, the small molecule 1,4,7-triazacyclononane is used. Enacting a small, weakly binding molecule in this position protects one side of the uranium center from unwanted side and decomposition reactions. Each nitrogen contains an alkyl-substituted aryloxide pendant arm which coordinates strongly to the uranium center in a distorted trigonal planar fashion. Because the polyamine chelator is a weak ligand for uranium ions, the metal orbitals do not participate in strong metal–ligand interactions, thus creating a more electron-rich uranium center for small-molecule activation trans to the tacn anchor. This coordination geometry places the aliphatic ortho substitutents (R) perpendicular to the plane formed by the aryloxides, making a protective cavity around the uranium ion similar to that for the NHC ligand system. Specifically, we show herein that the introduction of hexadentate tris-anionic 1,4,7-tris(3,5-alkyl-2-hydroxybenzylate)-1,4,7-triazacyclononane derivatives ((RArO)3tacn3− with R = tert-butyl (t-Bu) (24) and 1-adamantyl (Ad) (25)) to redox-active uranium centers results in formation of stable, coordinatively unsaturated core complexes with a single axial coordination site (L) available for ligand binding, substitution reactions, and redox events associated with small-molecule activation and functionalization.

The molecular architecture of the axial binding site is dependant on the...