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

Abstract

Dirhodium-catalyzed C-H amination is hypothesized to proceed via Rh2-nitrene intermediates in either the Rh2(II,II) or Rh2(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), [Rh2(esp)2](+) (2), and Rh2(espn)2Cl (3) (esp = α,α,α',α'-tetramethyl-1,3-benzenedipropanoate and espn = α,α,α',α'-tetramethyl-1,3-benzenedipropanamidate). CASSCF calculations on 1_L yield a wave function with two closely weighted configurations, (δ*)(2)(π1*)(2)(π2*)(1) and (δ*)(2)(π1*)(1)(π2*)(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 (π*)(4)(δ*)(1) GES. EPR spectra and Cl K-edge XAS for 3 are consistent with a (π*)(4)(δ*)(1) 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 Rh2(II,II) with a coordinated nitrene radical cation, (π*)(4)(δ*)(2)(π(nitrene,1))(1)(π(nitrene,2))(0). Conversely, the GES of 3N is Rh2(II,III) with a coordinated triplet nitrene, (π*)(4)(δ*)(1)(π(nitrene,1))(1)(π(nitrene,2))(1). Quartet transition states ((4)TSs) are found to react via a stepwise radical mechanism, whereas (2)TSs are found to react via a concerted mechanism that is lower in energy compared to (4)TSs for both 2N_L and 3N_L. The experimental (determined by intramolecular competition) and (2)TS-calculated kinetic isotopic effect (KIE) shows a KIE ∼ 3 for both 2N and 3N, which is consistent with a concerted mechanism.

Inverting Steric Effects: Using “Attractive” Noncovalent Interactions To Direct Silver-Catalyzed Nitrene Transfer

“…The effect of the counteranion on selectivity was minimal, suggesting it is not bound to Ag during the key bond-forming event. 17e,i,21 Counteranion-free Ag-nitrene species were also supported by computational studies (vide infra). 17e,i Concentration had little effect on selectivity, while decreasing the temperature from +25 to −20 °C improved the 2:3 ratio.…”

“…13 However, computational studies carried out by Berry and Musaev showed that a requirement for concerted nitrene transfer is the presence of an empty N-centered orbital on the metal−nitrene intermediate. 21 The triplet ground states of Ag−nitrene intermediates do not permit this condition to be met; thus, reactions catalyzed by Ag(tpa)OTf are electronically prohibited from occurring via concerted pathways. Further computational modeling resolved this discrepancy by showing that Ag-catalyzed NT can proceed via a mechanism we have termed “elementary hydrogen transfer/radical recombination” (E-HT:RR).…”

Catalyst-Controlled and Tunable, Chemoselective Silver-Catalyzed Intermolecular Nitrene Transfer: Experimental and Computational Studies

“…In concerted insertions of nitrenes into C‒H bonds, as is seen in Rh 2 -catalyzed reactions, the transition state involves simultaneous (though asynchronous) C-N and N-H bond formation and C‒H cleavage. 18 These three interactions take place within a triangular, three-membered ring transition state (left, Scheme 6). Stepwise reactions proceed first through a hydrogen atom transfer (HAT) transition state leading to radical intermediates.…”

DFT calculations of the defect structures, electronic structures, and EPR parameters for three Rh2+ centers in AgCl

“…For example, the electric charge (about 1.04 e) of central Rh 2+ is remarkably underestimated, whereas covalence and the strength of local crystal fields around Rh are overestimated, leading to the calculation errors for the resultant g tensor . Meanwhile, the two‐fold orbital degenerated ground 2 E g state for octahedral 4d 7 (Rh 2+ ) centers can induce extra errors in the DFT calculations . Second, LDA is found to be the best choice in all functionals for both g and A tensors.…”