American Chemical Society
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Synthesis of Oxorhenium(V) Complexes with Diamido Amine Ancillary Ligands and Their Role in Oxygen Atom Transfer Catalysis

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posted on 2009-12-07, 00:00 authored by Yuee Feng, Joel Aponte, Paul J. Houseworth, Paul D. Boyle, Elon A. Ison
The detailed syntheses of complexes of the form [Re(O)(X)(RNCH2CH2)2N(Me)] (X = Me, Cl, I, R = mesityl, C6F5), 13, incorporating diamidoamine ancillary ligands are described. X-ray crystal structures for the complexes [Re(O)(Me)((C6F5)NCH2CH2)2N(Me)], 1a, [Re(O)(I)((C6F5)N CH2CH2)2N(Me)], 3a, and [Re(O)(I)((Mes)NCH2)2N(Me)], 3b, are reported. The geometry about the metal center in 1a is best described as a severely distorted square pyramid with the oxo ligand in the apical position. In contrast, the geometry about the metal center in 3a is best described as a severely distorted trigonal bipyramid, with the iodo ligand occupying the apical position and the diamido nitrogens and the oxo ligand occupying the equatorial plane. The catalytic activities of these complexes for oxygen atom transfer, OAT, from pyridine-N-oxides, PyO, to PPh3 were also examined. The reactions exhibited a clear dependence on the diamido ligand substituent and the X ligand (Me, I, Cl) attached to the metal, with the combined effect that electron-withdrawing substituents on the diamido ligand and poor σ donors directly attached to the metal center increases the rate of catalytic activity. The kinetics of OAT from pyridine-N-oxides to Re were also investigated. The reactions displayed clean first order kinetics in Re and saturation kinetics for the dependence on PyO. Changing the PyO substrate had no effect on the saturation value, ksat, suggesting that the OAT reaction in these five-coordinate complexes appears to be governed by isomerization of the starting complex. Attempts to isolate a postulated Re(VII) intermediate were not successful because of hydrolytic degradation. The product of hydrolytic degradation [((C6F5)N(H)CH2CH2))2NH(Me)][X], (X = ReO4, or I), 4 can be isolated, and its X-ray crystal structure is reported. Although the Re(VII) intermediate could not be isolated, its activity in OAT reactions was probed by competition experiments with PPh3 and four para-substituted triarylphosphines (p-X-Ph)3P (X = OMe, Me, Cl, CF3). These experiments led to a Hammett that yielded a reaction constant of ρ = −0.30 ± 0.01. This data suggests a positive charge buildup on phosphorus for the OAT reaction and is consistent with the nucleophilic attack of phosphorus on an electrophillic metal oxo.