Systematic Manipulation of Nitrogen Subvalency in Metal Nitrenoid Complexes
Introduction. Transition metal complexes featuring metal-ligand multiple bond (MLMB) motifs are powerful intermediates through which heteroatom functionality is transferred to unreactive substrates. Understanding the electronic structure of these crucial intermediates through isolation and detailed electronic structure analysis is paramount for acquiring insight into MLMB-facilitated mechanisms and for tailoring catalysts to conduct specific transformations, including C–H activation of inert hydrocarbons.
Results. We have prepared a series of dipyrrin-supported 3d transition metal nitrenoid complexes and have conducted spectroscopic and computational analyses to elucidate (i) design principles to manipulate the distribution of electrons about the metal–nitrogen bond and (ii) the corresponding proclivity for intramolecular/intermolecular reactivity. We observe the weak-field dipyrrin results in high-spin metal ground state nitrenoids for Fe and low-lying open-shell excited state for Co, resulting in intrinsic instability and a propensity for nitrene transfer reactivity. We further observe substantial degree of MLMB attenuation for Ni (iminyl) and Cu (nitrene) due to diminished energy gap between the N 2p and M 3d valence shells, reflected in highly electrophilic NR motifs prone for the intramolecular and intermolecular insertion into inert C–H bonds, respectively. Kinetic analysis suggests a radical-type, stepwise hydrogen-atom abstract/radical recombination process.
Future Directions. We are currently extending this methodology for C–H functionalization with carbon and heteroatom (O, P, S) functionalities
Fe: Iovan, D. A.; Betley, T. A. J. Am Chem. Soc., 2016, 138, 1983; Co: Baek, Y.; Hennessy, E. T.; Betley, T.A. J. Am. Chem. Soc., 2019, 141, 16944; Ni: Dong, Y.; Lukens, J.; Clarke, R. M.; Zheng, S-L.; Lancaster, K.; Betley, T. A. Chem. Sci., 2020, 11, 1260-1268; Cu: Carsch, K. M.; DiMucci, I. M.; Iovan, D. A.; Li, A.; Zheng, S.-L.; Titus, C. J.; Lee, S. J.; Irwin, K. D.; Nordlund, D.; Lancaster, K. M.; Betley, T.A. Science., 2019, 365, 1138