Kinetics and Thermodynamics of H^–/H•/H⁺ Transfer from a Rhodium(III) Hydride

Abstract

The thermodynamics and kinetics of all three cleavage modes for Rh-H, the transfer of H(-), H(+), or H•, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, ΔG°H(-), for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H2 with Et3N in CH3CN. The transfer of H(-) from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (kH(-) = 3.5(1) × 10(5) M(-1) s(-1)), making 1a a very efficient catalyst for the ionic hydrogenation of iminium cations. The pKa of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H(+) transfer from 1a to 12 has been estimated (kH(+) = 5(1) × 10(-4) M(-1) s(-1)) from the half-life of the equilibration. Thus, 1a is a poor H(+) donor both thermodynamically and kinetically. However, 1a transfers H• to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl)• (14a) that can activate H2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at -1.85 V vs Fc(+)/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh-H bond of 1a of 58.2(3) kcal/mol--among the weakest Rh(III)-H bonds reported. The transfer of H• from 1a to Ar3C• (Ar = p-(t)BuC6H4) is fast, with kH• = 1.17(3) × 10(3) M(-1) s(-1). Thus, 1a is a good H(-) and H• donor but a poor H(+) donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)](-).

Diversity and Mefenoxam Sensitivity of Phytophthora spp. Associated with the Ornamental Horticulture Industry in the Southeastern United States

“…Although the Phytopiithora isolates examined primarily came from established collections maintained by the authors and did not come from surveys conducted specifically for this study, most of the predominant species associated with ornamental plants are well-established pathogens of ornamental crops (10,23,26,27,29,32,37,45,47,51,52). Although the Phytopiithora isolates examined primarily came from established collections maintained by the authors and did not come from surveys conducted specifically for this study, most of the predominant species associated with ornamental plants are well-established pathogens of ornamental crops (10,23,26,27,29,32,37,45,47,51,52).…”

“…To ensure that results from the 48-well culture plate assay were directly comparable with results from the standard petri plate assay, 43 isolates of P. nicotianae from ornamental plants and irrigation water (12 from North Carolina and 31 from Virginia) were screened with the 48well method, and these results were compared with those from the petri plate assay that were published previously (24,26,36,37). To ensure that results from the 48-well culture plate assay were directly comparable with results from the standard petri plate assay, 43 isolates of P. nicotianae from ornamental plants and irrigation water (12 from North Carolina and 31 from Virginia) were screened with the 48well method, and these results were compared with those from the petri plate assay that were published previously (24,26,36,37).…”

“…Likewise, in a study by Hu et al(26), mefenoxam did not manage disease on geranium plants inoculated with isolates of P. nicotianae insensitive to mefenoxam at 100 \ig a.i./ml. *• States of isolate origin: GA = Georgia and VA = Virginia.Fig.…”

“…In the past 10 years, surveys of nurseries and greenhouses for species of Phytophthora have been conducted in several states in the United States (10,23,26,27,29,32,37,45,47,51,52). These statewide surveys have been extremely important in establishing baseline information on the impact of Phytophthora spp.…”

NH3 Synthesis in the N2/H2 Reaction System using Cooperative Molecular Tungsten/Rhodium Catalysis in Ionic Hydrogenation: A DFT Study

“…[8] The authors reported that H 2 can replace the weakly bound acetonitrile ligand of the rhodiumc omplex, leading to H 2 coordination at the metal center.I nt he presence of an appropriate Lewis base, the H 2 molecule is cleaved heterolytically.I nt he approach chosen in this work, the Lewis base is aN 2 -containing tungsten pincer complex (see below). Norton and co-workersr eported the use of this rhodiumc omplex to cleave H 2 andh ydrogenateo rganic substrates.…”