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)

Abstract

A high-yield route to the new complexes OsBr 2 (dppe) 2 and trans -OsHBr(dppe) 2 starting from (NH 4 ) 2 [OsBr 6 ] is described. The new 5-coordinate complexes [OsX(dppe) 2 ]PF 6 (X = Cl ( 8Os ) and X = Br ( 9Os )) are prepared by reaction of cis -OsX 2 (dppe) 2 with NaPF 6 . Complexes 8Os and 9Os consist of distorted trigonal bipyramidal cations with “Y”-shaped equatorial planes. They react in CH 2 Cl 2 with H 2 or HD (1 atm) to give complexes trans -[Os(H··H)X(dppe) 2 ]PF 6 (X = Cl ( 1Os PF 6 ), X = Br ( 2Os PF 6 )) or trans -[Os(H··D)X(dppe) 2 ]PF 6, respectively. The last complexes have J (H,D) = 13.9 and 13.7 Hz, respectively. The BF 4 - salts of these complexes, 1Os BF 4 and 2Os BF 4, respectively, are prepared by reacting trans -OsHX(dppe) 2 with HBF 4 ·Et 2 O or DBF 4 ·Et 2 O. These complexes are characterized by NMR, IR, and FAB MS. The single-crystal X-ray and neutron diffraction studies of 1Os PF 6 revealed an elongated H··H ligand with d HH = 1.11(6) (X-ray) or 1.22(3) Å (neutron) occupying one site in an octahedral complex. The X-ray diffraction study of 2Os BF 4 produced a similar structure with d HH = 1.13(8) Å. One fluorine of the anion in each structure is positioned near the acidic H 2 ligand. A linear relationship between d HH 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( T 1 ) versus inverse temperature for 1Os and 2Os are distorted from the usual “V” shape, suggesting that the rotational frequency of the H 2 ligand is near that of the Larmor frequency. Therefore the d HH for 1Os is between the values of 1.04 and 1.31 Å calculated from the T 1 (min) for fast and slow spinning H 2, 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 p K a values of 7.4 and 5.4, respectively, while trans -[Os(H 2 )H(dppe) 2 ] + is much less acidic with a p K a of 13.6. These p K a values and some E 1/2 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 ]BF 4 ( 3Os ) are also reported.