Interfacial and Intrafacial Linkage Isomerizations of Rhenium Complexes with Aromatic Molecules

Abstract

The mechanisms for the interconversion of facial diastereomers of a variety of TpRe(CO)(L)(eta(2)-L(Ar)) complexes [L = (t)BuNC, pyridine (py), PMe(3), or 1-methylimidazole (MeIm); L(Ar) = benzene, anisole, naphthalene, 1-methylpyrrole, furan, or thiophene; Tp = hydridotris(pyrazolyl)borate] have been investigated by (1)H NMR spin saturation experiments. In addition, the rates and free energies of activation for these processes were calculated from spin saturation experiments and T(1) measurements. The operative mechanisms for interconversion of the pi diastereomers were found to be nondissociative, undergoing either an interfacial or intrafacial linkage isomerization. A comparison of the kinetic parameters for isomerization of related eta(2)-olefin complexes of the [TpRe(CO)(PMe(3))] and [CpRe(NO)(PPh(3))](+) fragments is also presented.

Experiments and Direct Dynamics Simulations That Probe η2-Arene/Aryl Hydride Equilibria of Tungsten Benzene Complexes

“…This is in contrast to that observed for the complex ReTp(CO)(MeIm)(η 2 -anisole) (MeIm = N-methylimidazole), which undergoes intrafacial isomerization (ring-slip) roughly four times faster than interfacial isomerization (face-flip; Scheme 5). 41 Possible mechanisms for both face-flip and ring-walk isomerization for the anisole complex were examined with DFT, and an abbreviated energy surface with πand W−H complexes is shown in Figure 6. Consistent with experiment, the meta aryl hydride m-3H is 1.9 kcal/mol lower in Gibbs free energy than the ortho counterpart o-3H.…”

“…For both the molybdenum and rhenium systems mentioned above, any purported C−H insertion would likely result in a face-flip isomerization, which is not observed in the corresponding NOESY data. 41,46 Thus, C−H insertion is significantly slower than the ring-walk for both the Mo and Re complexes.…”

Charge donation to and dearomatization of benzene attending complexation: DFT estimates of binding energies of TpMXO(L) with benzene, for Tp = hydridotris(pyrazolyl) borate, MXO = MoNO, ReCO, and WNO, and L = ammonia, N‐methylimidazole, pyridine, phosphine, methyl isocyanide, and carbon monoxide

“…Brooks et al 16 observe linear correlations between the measured free energy of activation for interfacial migration (i.e., changes in 2 coordination site) and changes in the stretching frequency of CO in tpRe(CO)L complexes with naphthalene. Strong donor L ligands would force charge into the antibonding level of the CO ligand, shifting the CO stretch to the red.…”

Stereochemical Nonrigidity of Organometallic Complexes

“…Nevertheless, SST experiments showed that the lowest energy path for interfacial exchange occurred without passing through an η 1 -S intermediate. 109 The interfacial (face flip) for the thiophene complex (R=H) occurred with a G ‡ = 79.4 kJ mol…”