Monodentate Alkoxy/Aryloxy Anchored Yttrium Dihydrides; Anticipated Outcomes and Surprises

Thesynthesis, characterization, and solid-state structureof bulkyalkoxy- and aryloxy-supported yttrium polynuclear hydrides are reported.Hydrogenolysis of the supertrityl alkoxy anchored yttrium dialkyl,Y(OTr*)(CH2SiMe3)(2)(THF)(2) (1) (Tr* = tris(3,5-di-tert-butylphenyl)methyl),resulted in the clean conversion to the tetranuclear dihydride, [Y(OTr*)H-2(THF)](4) (1a). X-ray analysis revealeda highly symmetrical structure (4 site symmetry)with the four Y atoms located on the corners of a compressed tetrahedron,each bonded to an OTr* and tetrahydrofuran (THF) ligand and the clusterheld together by four face-capping, & mu;(3)-H, and fouredge-bridging, & mu;(2)-H, hydrides. DFT calculations onthe full system with and without THF, but also on model systems, clearlyshow that the structural preference for complex 1a iscontrolled by the presence and coordination of THF molecules. Contraryto the exclusive formation of the tetranuclear dihydride, hydrogenolysisof the bulky aryloxy yttrium dialkyl, Y(OAr*)(CH2SiMe3)(2)(THF)(2) (2) (Ar* = 3,5-di-tert-butylphenyl) gave a mixture of the analogous tetranuclear 2a and trinuclear, [Y-3(OAr*)(4)H-5(THF)(4)], polyhydride, 2b. Similar results,i.e., a mixture of tetra-/tri-nuclear products, were obtained fromhydrogenolysis of the even bulkier Y(OArAd2,Me)(CH2SiMe3)(2)(THF)(2) compound. Experimentalconditions were established to optimize the production of either thetetra- or trinuclear products. X-ray structure of 2b revealeda triangular array of three yttrium atoms with two face-capping & mu;(3)-H and three edge-bridging & mu;(2)-H hydrides,with one yttrium bonded to two aryloxy ligands while the other twohave a complement of one aryloxy and two THF ligands; the solid-statestructure is close to being C-2 symmetric, with the C-2 axis running through the unique Y and unique & mu;(2)-H hydride. As opposed to 2a, which shows distinct H-1 NMR resonances for & mu;(3)/ & mu;(2)-H (& delta; = 5.83/6.35 ppm, respectively), no hydride signals for 2b were observed at room temperature, indicating hydride exchangeon the NMR time scale. Their presence and assignment were securedat -40 & DEG;C from H-1 SST (spin saturation) experiment. Hydrogenolysis of the supertrityl yttriumdialkyl, Y(OTr*)(CH2SiMe3)(2)(THF)(2), gave exclusivelythe tetranuclear dihydride, [Y(OTr*)H-2(THF)](4), with four face-capping, & mu;(3)-H, and four edge-bridging,& mu;(2)-H, hydrides. DFT calculations on the full andmodel systems, with and without THF, revealed that the observed structuralpreference is controlled by the presence and coordination of the THFligands. Contrary to this, bulky aryloxy yttrium dialkyls gave mixturesof the analogous tetra- and trinuclear [Y-3(OAr*)(4)H-5(THF)(4)], polyhydrides; optimization allowedthe isolation of each component.