Dihydrogen with frequency of motion near the 1H Larmor frequency. Solid-state structures and solution NMR spectroscopy of osmium complexes trans-[Os(H··H)X(PPh2CH2CH 2PPh2)2]+ (X = Cl, Br)

Article abstract of DOI:10.1021/ja9529044

A high-yield route to the new complexes OsBr₂(dppe)₂ and trans-OsHBr(dppe)₂ starting from (NH₄)₂-[OsBr₆] is described. The new 5-coordinate complexes [OsX(dppe)₂]PF₆ (X = Cl (80s) and X = Br (90s)) are prepared by reaction of cis-OsX₂(dppe)₂ with NaPF₆. Complexes 80s and 90s consist of distorted trigonal bipyramidal cations with Y -shaped equatorial planes. They react in CH₂Cl₂ with H₂ or HD (1 atm) to give complexes trans-[Os(H··H)X(dppe)₂]PF₆ (X = Cl (10sPF₆), X = Br (20sPF₆)) or trans-[Os(H··D)X(dppe)₂]PF₆, respectively. The last complexes have J(H,D) = 13.9 and 13.7 Hz, respectively. The BF₄^- salts of these complexes, 10sBF₄ and 20sBF₄, respectively, are prepared by reacting trans-OsHX(dppe)₂ with HBF₄·Et₂O or DBF₄·Et₂O. These complexes are characterized by NMR, IR, and FAB MS. The single-crystal X-Yay and neutron diffraction studies of 10sPF₆ revealed an elongated H··H ligand with d_HH = 1.11(6) (X-ray) or 1.22(3) A?(neutron) occupying one site in an octahedral complex. The X-ray diffraction study of 20sBF₄ produced a similar structure with d_HH = 1.13(8) A?. One fluorine of the anion in each structure is positioned near the acidic H₂ ligand. A linear relationship between d_HH and J(H,D) for many dihydrogen complexes is used to indicate that complexes 10s and 20s have H-H distances of about 1.2 A? in solution. Plots of ln(T_l) versus inverse temperature for 10s and 20s are distorted from the usual V shape, suggesting that the rotational frequency of the H₂ ligand is near that of the Larmor frequency. Therefore the d_HH for 10s is between the values of 1.04 and 1.31 A? calculated from the T_l (min) for fast and slow spinning H₂, respectively. The chloride ligand in trans-[Os(H··H)Cl(L)₂]⁺ buffers the effect of changing the cis ligands L from dppe to depe to dcpe so that there is little change in the H-H distance. Complexes 10s and 20s have pK_a values of 7.4 and 5.4, respectively, while trans-[Os(H₂)H(dppe)₂]⁺ is much less acidic with a pK_a of 13.6. These pK_a values and some E_1/2 values are used to show that 10s and 20s are dihydride-like even though they have relatively short H-H distances. Properties of trans-[Os(H··H)Cl(depe)₂]BF₄ (30s) are also reported.