Dihydrogen with Frequency of Motion Near the¹H Larmor Frequency. Solid-State Structures and Solution NMR Spectroscopy of Osmium Complexes<i>trans</i>-[Os(H<b>··</b>H)X(PPh₂CH₂CH₂PPh₂)₂]⁺(X = Cl, Br)

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 (8Os) and X = Br (9Os)) are prepared by reaction of cis-OsX2(dppe)2 with NaPF6. Complexes 8Os and 9Os 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 (1OsPF6), X = Br (2OsPF6)) 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, 1OsBF4 and 2OsBF4, 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-ray and neutron diffraction studies of 1OsPF6 revealed an elongated H··H ligand with dHH = 1.11(6) (X-ray) or 1.22(3) Å (neutron) occupying one site in an octahedral complex. The X-ray diffraction study of 2OsBF4 produced a similar structure with dHH = 1.13(8) Å. One fluorine of the anion in each structure is positioned near the acidic H2 ligand. A linear relationship between dHH and J(H,D) for many dihydrogen complexes is used to indicate that complexes 1Os and 2Os have H−H distances of about 1.2 Å in solution. Plots of ln(T1) versus inverse temperature for 1Os and 2Os 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 dHH for 1Os is between the values of 1.04 and 1.31 Å calculated from the T1(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 1Os and 2Os have pKa values of 7.4 and 5.4, respectively, while trans-[Os(H2)H(dppe)2]+ is much less acidic with a pKa of 13.6. These pKa values and some E1/2 values are used to show that 1Os and 2Os are dihydride-like even though they have relatively short H−H distances. Properties of trans-[Os(H··H)Cl(depe)2]BF4 (3Os) are also reported.