Quantum Exchange Coupling:  A Hypersensitive Indicator of Weak Interactions

Abstract

Os(H) 3 ClL 2 (L ) P i Pr 3 ) forms a 1:1 adduct with L′ ) PEt 3 , NH 3 , MeCN, acetone, methanol, and THF. The case L′ ) PEt 3 permits the clearest identification of adduct structure as pentagonal bipyramidal. For NH 3 and MeCN, the respective kinetics of L′ loss are measured as ∆H q ) 20.7(3) and 17.6(3) kcal/mol and ∆S q ) 16(1) and 14.7(9) cal/(mol K). For acetone, methanol, and THF, the following respective ∆H°and ∆S°values for L′ binding are measured: ∆H°) -10.4(1), -6.66(8), and -5.8(2) kcal/mol; ∆S°) -41.8(5), -25.5(3), and -33(1) cal/(mol K). Decoalesced 1 H NMR spectra are reported for several of these Os(H) 3 ClL 2 L′ species, and they show a variety of examples of quantum exchange coupling among the hydride ligands. The values of J ex are higher when L′ is a more weakly-binding ligand. The quantum exchange coupling constants of Os(H) 3 XL 2 (X ) Cl, Br, I, OCH 2 CF 3 , OCH(CF 3 ) 2 ) in CD 2 Cl 2 , in toluene, and in methylcyclohexane show an unprecedented decrease of J with increasing temperature, which is attributed to weak formation of Os(H) 3 Cl(solvent)L 2 adducts at low temperature. For L′ ) CO, adduct formation leads to liberation of coordinated H 2 . Excess L′ ) MeCN or NH 3 slowly leads to formation of [Os(H) 3 L′ 2 L 2 ]Cl; the X-ray structure for L′ ) NH 3 is reported. Crystal data (-171°C): a ) 11.561(4) Å, b ) 14.215(5) Å, c ) 8.851(3) Å, R ) 97.51(2)°, ) 107.73(2)°, γ ) 104.47(2)°, with Z ) 2 in space group P1 h. The potential energy was calculated for exchange of 2H of OsH 3 X(PH 3 ) 2 L (X ) Cl with L ) no ligand and PH 3 , X ) I with L ) no ligand) using effective core potential ab initio methods at the MP2 level. The site exchange is found to be energetically easier for Cl than for I, in agreement with experiment. The hydride site exchange in the sevencoordinate species OsH 3 Cl(PH 3 ) 3 (a model for coordination of either ligand or solvent to Os) is found to be easier than that in the 16-electron species. No dihydrogen ligand is located on the reaction path for site exchange. The current theory which relates quantum exchange to a tunneling effect was used for calculating J ex as a function of temperature. The dynamic study was done using several sets of coordinates, in particular the rotation angle φ and the internuclear distance r between the exchanging H. The vibrational levels have been calculated and the symmetry of each level assigned within the permutation group in order to determine the nature of the nuclear spin function associated with each level. It is found that the rotation, φ, gives rise to the largest tunneling effect but that r cannot be neglected. The influence of the temperature, J ex (T), was included by a Boltzmann distribution. The results are in qualitative agreement with experiment in that quantum exchange coupling is larger in the case of Cl than in the case of I. Additional ligand L increases the value of the quantum exchange coupling mostly by lowering the activation energy for pairwise exchange.

An Entry to Stable Mixed Phosphine–Osmium–NHC Polyhydrides

“…The distortion partially cancels the electron deficiency at the metal, which receives additional electron density from the π-donor group. [25][26][27][28] 30 and addition of BH 3 to unsaturated hydride compounds. 32 The formation of 7 and 8 was confirmed by means of the Xray structure of 8 (Figure 2), which proves the κ 2 -coordination of the terahydroborate group.…”

Facile C(sp2)/O2CR bond cleavage by Ru or Os

“…Scheme 1 y Electronic supplementary information (ESI) available: 1 H NMR spectra in toluene-d 8 for the reaction of compound IV and vinyl trifluoroacetate mixed at À80 C and 1 H, 31 P( 1 H) and 13 C( 1 H) NMR spectra of the two isomers of compound IX. See http:// www.rsc.org/suppdata/nj/b3/b306111f/ B) Mechanism.…”

Sigma-bond activation reactions induced by unsaturated Os(IV)-hydride complexes

“…23 This distortion partially cancels the electron deficiency at the metal center, which receives electron density through the σ-bonds with the hydride ligands and additionally from a lone pair of the halide via a π bond. 24 [Insert Chart 1] Chart 1 Geometries of unsaturated osmium(IV)-hydride complexes.…”