title: 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.
authors.affiliation: The Cherry L. Emerson Center for Scientific Computation, Emory University , 1515 Dickey Drive, Atlanta, Georgia 30322, United States.
authors.forename: Adrián
authors.initials: A
authors.lastname: Varela-Álvarez
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
authors.forename: Tzuhsiung
authors.initials: T
authors.lastname: Yang
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
authors.forename: Heather
authors.initials: H
authors.lastname: Jennings
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
authors.forename: Katherine P
authors.initials: KP
authors.lastname: Kornecki
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Samantha N
authors.initials: SN
authors.lastname: Macmillan
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Kyle M
authors.initials: KM
authors.lastname: Lancaster
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.
authors.forename: James B C
authors.initials: JB
authors.lastname: Mack
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.
authors.forename: J
authors.initials: J
authors.lastname: Du Bois
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
authors.forename: John F
authors.initials: JF
authors.lastname: Berry
authors.affiliation: The Cherry L. Emerson Center for Scientific Computation, Emory University , 1515 Dickey Drive, Atlanta, Georgia 30322, United States.
authors.forename: Djamaladdin G
authors.initials: DG
authors.lastname: Musaev
mesh_terms: D000586:Amination; D001384:Azetidines / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D002384:Catalysis; D002614:Chelating Agents / Q000737:chemistry*; D004583:Electrons; D007097:Imines / Q000737:chemistry*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010084:Oxidation-Reduction; D012238:Rhodium / Q000737:chemistry*
chemical_list: D001384:Azetidines; D002264:Carboxylic Acids; D002614:Chelating Agents; D007097:Imines; D009942:Organometallic Compounds; C017621:phenylnitrene; D012238:Rhodium