Browsing by Author "Oleg V. Ozerov"

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    Computational studies of reactions of insertion of rhodium(I) and iridium(I) into N-H, N-CH3, and NCH2-H bonds of the diarylamine-based PNP pincer ligands
    (2011-05-17) Panida Surawatanawong; Oleg V. Ozerov; Texas A and M University; Mahidol University
    This work presents the investigation by DFT methods of the mechanism of N-Me and N-H oxidative addition in reactions of the secondary amine form of the PNP pincer ligand 4-Me-2-( i Pr 2 P)-C 6 H 3 ) 2 NH (or PN(H)P), its N-methylated derivative 4-Me-2-( i Pr 2 P)-C 6 H 3 ) 2 NMe (or PN- (Me)P), and a version of the latter whose aromatic rings are "tied" with a CH 2 CH 2 linker (or T PN(Me)P) with Rh(I) and Ir(I). Reactions were considered by starting from (Κ 3 -PN- (H)P)MCl, (Κ 3 -PN(Me)P)MCl, and (Κ 3-T PN(Me)P)MCl(M = Rh, Ir). Oxidative addition from (Κ 3 -PN(H)P)MCl to give (PNP)M(H)(Cl) is predicted to proceed with essentially no barrier via direct migration ofHfromNto the metal. The analogous direct migration of Me fromNto the metal is predicted to be the dominant mechanism for both Rh systems, with the calculated barrier for (Κ 3 -PN(Me)P)RhCl of 21.8 kcal/mol being in reasonable agreement with the experimental value of 24.0(18) kcal/mol. For Ir, an alternative pathway that involves initial NCH 2 -H oxidative addition, followed by CH 2 extrusion and C-Hrecombination, is calculated to be competitive with direct Me transfer, especially for the "tied" ligand where it is preferred. This alternative pathway entails prohibitively high barriers for both Rh systems ( > 35 kcal/mol), which can be traced to the high energy of the intermediate in which a CH 2 carbene is bound to a Rh III center. In general, the energies of all barriers and intermediates are lower with the "tied" ligand. DFT calculations also evaluate the energetics of the NCH 2 -H oxidative addition intermediates. These were observed experimentally for only the "tied" ligand system (for both Rh and Ir), and the DFT energies are consistent with these observations. © 2011 American Chemical Society.
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    Experimental and computational exploration of the dynamic behavior of (PNP)BF2, a boron compound supported by an amido/bis(phosphine) pincer ligand
    (2011-11-21) Jessica C. Demott; Panida Surawatanawong; Shoshanna M. Barnett; Chun Hsing Chen; Bruce M. Foxman; Oleg V. Ozerov; Texas A and M University; Brandeis University; Mahidol University
    The diarylamido/bis(phosphine) PNP pincer ligand (2- i Pr 2 P-4-MeC 6 H 3 ) 2 N has been evaluated as a scaffold for supporting a BF 2 fragment. Compound (PNP)BF 2 (6) was prepared by simple metathesis of (PNP)Li (5) with Me 2 SBF 3 . NMR spectra of 6 in solution are of apparent C 2 symmetry, suggestive of a symmetric environment about boron. However, a combination of X-ray structural studies, low-temperature NMR investigations, and DFT calculations consistently establish that the ground state of this molecule contains a classical four-coordinate boron with a PNBF 2 coordination environment, with one phosphine donor in PNP remaining "free". Fortuitous formation of a single crystal of (PNP)BF 2 ·HBF 4 (7), in which the "free" phosphine is protonated, furnished another structure containing the same PNBF 2 environment about boron for comparison and the two PNBF 2 environments in 6 and 7 are virtually identical. DFT studies on several other diarylamido/bis(phosphine) pincer (PNP)BF 2 systems were carried out and all displayed a similar four coordinate PNBF 2 environment in the ground state structures. The symmetric appearance of the room-temperature NMR spectra is explained by the rapid interconversion between two degenerate four-coordinate, C 1 -symmetric ground-state forms. Lineshape analysis of the 1 H and 19 F NMR spectra over a temperature range of 180-243 K yielded the activation parameters ΔH ‡ = 8.1(3) kcal mol -1 and ΔS ‡ = -6.0(15) eu, which are broadly consistent with the calculated values. Calculations indicate that the exchange of phosphine donors at the boron center proceeds by an intrinsically dissociative mechanism. © 2011 The Royal Society of Chemistry.