Rh2(II,III) Catalysts with Chelating Carboxylate and Carboxamidate Supports: Electronic Structure and Nitrene Transfer Reactivity

Dirhodium-catalyzed C-H amination is hypothesized to proceed via Rh-2-nitrene intermediates in either the Rh-2(II,II) or Rh-2(II,III) redox state. Herein, we report joint theoretical and experimental studies of the ground electronic state (GES), redox potentials, and C-H amination of [Rh2,(II,III)(O2CCH3)(4)(L)(n)](+) (1_L) (L = none, Cl-, and H2O), [Rh-2(esp),] (2), and Rh-2(espn),Cl (3) (esp = a,a,d,a'-tetramethy1-1,3-benzenedipropanoate and espn = a,a,a',a' tetramethy1-1,3-benzenedipropanamidate). CASSCF calculations on 1_L yield a wave function with two closely weighted configurations, (5492(941,92(71.2:91 and, (5*)2(irmi(7.1.2*.2) consistent with reported EPR g values [Chem. Phys. Lett. 1986, 130, 20-23]. In contrast, EPR spectra of 2 show g values consistent with the DFT-computed (pi*)(delta*)(1) GES. EPR spectra and Cl K-edge XAS for 3 are consistent with a (pi)4(delta*)2 GES, as supported by DFT. Nitrene intermediates 2N_L and 3N_L are also examined by DFT (the nitrene is an NSO3R species). DFT calculations suggest a doublet GES for 2N_L and a quartet GES for 3N_L. CASSCF calculations describe the GES of 2N as Rh,(II,II) with a coordinated nitrene radical cation, (pi*)(4)(delta*)(2)(pi(nitrene))(1)(pi(nitrene2))(0). Conversely, the GES of 3N is Rh-2(II,III) with a coordinated triplet nitrene, (pi*)(4)(delta*)1(pi(nitrene,1))1(pi(nitrene,2))1. Quartet transition states ((4)TSs) are found to react via a stepwise radical mechanism, whereas 2TSs are found to react via a concerted mechanism that is lower in energy compared to 4TSs for both 2N_L and 3N_L. The experimental (determined by intramolecular competition) and 'TS-calculated kinetic isotopic effect (KIE) shows a KIE 3 for both 2N and 3N, which is consistent with a concerted mechanism.