Polyhydrides of Sc, Zr and Hf and Their Proposed Formation.

Hydrogenation at 500 psi of (PNP)Sc(CH3)2 results in formation of a trinuclear polyhydride complex [(PNP)Sc]3(μ2-H)4(μ3-H)2 (1) in 55 % yield. The solid-state structure shows a non-symmetric trinuclear species resulting from one pincer phosphine arm being demetallated, and where two hydrides bridge all three Sc centers, whereas the other four bridge two. Hydrogenation of (PNP)Zr(CH3)3 at 200 psi results instead in formation of a dinclear polyhydride species [(PNP)Zr(H)]2(μ2-H)4 (2). Conducting the hydrogenation at atmospheric pressure, resulted instead in formation of the bridging methylidene complex [(PNP)Zr(CH3)]2(μ2-H)2(μ2-CH2) (3), which cleanly converted to 2, upon hydrogenation at higher pressure. Both 2 and 3 were also structurally characterized. Hydrogenation of (PNP)Hf(CH3)3 at 200 psi resulted in incomplete hydrogenation with some formation of dinuclear mono- and dimethyl-polyhydride complexes [(PNP)Hf(CH3)]2(μ2-H)4 (4) and [(PNP)Hf(CH3)][(PNP)Hf(H)](μ2-H)4 (5), which were identified by solid-state X-ray structural studies. Based on these results, we propose a pathway for the complete hydrogenation of (PNP)Zr(CH3)3 to 2.