Synthesis and NMR Studies of [(C5Me5)Os(L)H2(H2)+] Complexes. Evidence of the Adoption of Different Structures by a Dihydrogen Complex in Solution and the Solid State

Protonation of the osmium(IV) trihydrides (C5Me5)OsH3(L) with HBF4 in diethyl ether affords the molecular dihydrogen complexes [(C5Me5)Os(H2)H2(L)][BF4], where L is PPh3 (1), AsPh3 (2), or PCy3 (3). Ruthenium analogues of these species are not stable and instead lose H2 readily. These compounds adopt four-legged piano-stool geometries in which the phosphine ligand is “trans” to an elongated dihydrogen ligand. For 1 and 2, the coordinated H2 ligand is oriented with its H−H vector nearly parallel with the Os−Ct vector, where Ct is the centroid of the C5Me5 ring; in contrast, in 3 the H2 ligand is oriented with its H−H vector perpendicular to the Os−Ct vector. In the 1H NMR spectra, exchange between the Os−H and Os−H2 environments can be slowed at low temperatures for the arsine complex 2 (but not for 1 or 3), and separate resonances could be observed for the hydride and dihydrogen sites; the barrier for exchange is approximately 6.0 kcal/mol. Partially deuterated samples were prepared, and H−H distances within the bound H2 ligands were deduced from the observed 1JHD(av) coupling constants. In addition, H−H distances were deduced from the T1(min) values for the osmium-bound hydrogen atoms, after correction for exchange and ligand-induced dipolar relaxation effects. In all cases, the two solution measurements were in agreement but differed from that deduced from neutron diffraction data. Specifically, for 1 the solution data gave a distance of ca. 1.07 vs 1.01 Å in the solid state; similarly, for 2 the solution value of ca. 1.15 Å was longer than the 1.08 Å value seen in the solid state. In both cases, the ∼0.06 Å lengthening in solution, if real, is the result most likely of one or both of two factors: the effect of removing the BF4 counterion from the vicinity of the cation and the effect of librational motion that tends to shorten artificially H−H distances deduced from neutron diffraction data. In contrast, for 3 the solution H−H distance of ca. 1.12 Å is significantly shorter than the 1.31 Å distance determined from the neutron diffraction data. DFT calculations support the hypothesis that different structures are adopted by 3 in solution and in the solid state and that in solution an equilibrium is established between two dihydrogen−dihydride structures, one with a considerably shorter H−H bond than is seen in the solid state.