Metal Element Multiple Bonding and Small Molecule Activation on Transition Metal Complexes Supported by Mono- and...

Multidentate ligands have the potential to provide stable platforms for small molecule activation and transformation at metal centers by utilizing the chelate effect to discourage deactivation pathways involving ligand loss. Pentadentate ligands represent maximal chelation while still retaining one coordination site for binding and activating small molecules. They also enable provision of a framework for stabilizing highly reactive M═E (E = O, NR or CR2) or M≡E (E = O, N or CR) moieties that can couple or undergo atom transfer reactions to external substrates. Here, a key feature of the pentadentate motif is the prevention of facile loss of the ligand trans to the high trans effect ligands E. The majority of pentadentate ligands are based on oxidation resistant, neutral amine or pyridine type donors, rendering them L5-type ligands; the pentapyridyl (PY5) ligand family introduced by Stack and Feringa are archetypical examples. Necessarily, complexes of metal ions supported by these ligand types are positively charged and accessing higher oxidation states higher than M(II) or M(III) are challenging. For the past decade, our group has explored mono- and dianionic analogues of this classic framework obtained by replacing one or both 2,6-dipyridyl methyl arms in PY5 on the anchoring pyridyl donor with dipyrazolyl aryl borate units, converting them to monoanionic L4X (BPz2Py3) or dianionic L3X2 (B2Pz4Py) pentadentate platforms. The incorporation of borates into the pentadentate framework has important consequences that can be exploited for new chemistry. First, the metals that can be accommodated expand to include the early transition metals; L5 type ligands are not effective for preparing groups 3 and 4 metal complexes, so the anionic ligands are truly "pan-Periodic". Second, complexes of these ligands with metals in higher oxidation states are either neutral or monocationic, allowing highly reactive M═E/M≡E species to retain solubility in nonpolar organic solvents, facilitating detailed solution studies that map out speciation and enable mechanistic studies. This Account will first elaborate and expand on the features and limitations of pentadentate platforms in small molecule activation and reactive group stabilization. The advantages of incorporating borate arms into the framework will then be illustrated through examples from our extensive studies on the coordination chemistry of these ligands with elements from across the Periodic Table, focusing on the first row transition metals. A rare example of a terminal scandium imido and implication of a highly reactive iron(III) imidyl illustrate the ability of these ligands to generate six coordinate examples of these reactive functions. Further, the activation of dioxygen by Co(II) precursors incorporating the B2Pz4Py ligand leads to generation of a highly reactive Co(IV) oxyl species. Finally, the use of this dianionic ligand for molybdenum has allowed for extensive examination of coordination-induced bond weakening in aquo and ammine complexes, a phenomenon highly relevant to water and ammonia oxidation catalysis. Together, this body of work shows that mono and dianionic pentadentate ligands anchored by borate functions are effective platforms for studying small molecule transformations.