Generation and Reactivity of Late Transition Metal Hydr(oxo) Complexes Supported by Tripodal Carboxamidate Ligand

Abstract

Metal oxo complexes (M=O) are important mediators of oxidative transformations in biological systems.1 Their high reactivity and transient nature have limited their study, with only a handful of well characterized oxo complexes with d-counts >4 existing in the literature.2, 3 Formation of a stable terminal metal-oxo bond may be derived from simple ligand field theory, where a metal, double bonded to oxygen (M=O), is made possible by low coordinate ligands.4 Among these ligands are four coordinate deprotonated tripodial amidate systems that enforce a unique trigonal bipyramidal (TBP, C3v) geometry, capable of supporting a terminal M=O species. The highly sigma-donating nitrilotriacetamide ligand derivative (LPh) was utilized in the generation of TBP symmetric Fe(II) and Ni(II) complexes. These low valent metal precursors have proven to be a promising platform towards the generation of high valent oxo and hydroxo complexes. The tripodal M(II) precursor complexes in conjunction with peroxyacetic acids allowed us to trap and characterize an electrophilic FeIII(O) complex as well as a NiIII(OH) intermediate at low temperature. Both entities displayed reactivity towards phosphine and hydrocarbon substrates. We observed intramolecular ligand oxidation of a putative NiIV intermediate that underwent oxidation of the pendant aryl ring via electrophilic aromatic substitution that was intercepted upon adding hydrocarbons bearing medium to strong C-H bonds. The use of protons along with peroxy acids resulted in the formation of a protonated FeIV(O) intermediate showing a greatly enhanced oxygen atom transfer reactivity with triphenylphosphine. Results yielding of this project shed light on the reactivity patterns observed for TPB symmetric d5 and d6 metal oxo species, guiding the development of more efficient oxidation catalysts. References 1. X. Huang and J. T. Groves, Chem. Rev., 2017, 118, 2491-2553. 2. E. Spaltenstein, R. R. Conry, S. C. Critchlow and J. M. Mayer, J. Am. Chem. Soc., 1989, 111, 8741-8742. 3. M. K. Goetz, E. A. Hill, A. S. Filatov and J. S. Anderson, J. Am. Chem. Soc., 2018, 140(41), 13176-13180. 4. K. Ray, F. Heims and F. F. Pfaff, Eur. J. Inorg. Chem., 2013, 2013, 3784-3807.