POLYNUCLEAR METAL HYDRIDE ALKOXIDES - PREPARATION AOD CHARACTERIZATION OF MO-4(MU-H)(3)(OBUT)(7)(HNME2) AND [K(18-CROWN-6)][MO-4(MU(4)-H)(OR)(12)] (R = PR-I OR CH2BUT)

The compounds Mo-2(OR)(6) and KH [or KHB(Bu-s)(3)] react in tetrahydro furan in the presence of 18-crown-6 to yield the charge separated clus ters, [Mo-4(mu-H)(OR)(12)](-) (R = Pr-i or CH2But) supported by the po tassium ions [K+(eta(6)-18-crown-6)]. 2THF. The cluster anions contain a Mo-4 butterfly Mo-Mo 2.50 Angstrom (average) supported by a hydride ligand that bridges the wingtip Mo atoms. Each edge of the Mo-4 butte rfly is bridged by an OR group and the two faces are capped by mu(3)-O R ligands. The backbone Mo atoms have one terminal bridge and the wing tip Mo atoms have two. Hydrogenation of the mixture of compounds forme d in the reaction between 1,2-Mo-2(tol)(2)(NMe2)(4) (tol = p-tolyl) an d (BuOH)-O-t (3.5 equivalents) in hydrocarbon solvents yields Mo-4(mu- H)(3)(OBut)(7)(HNMe2) which contains a Mo, butterfly asymmetrically su pported by alkoxide and hydride ligands. The hydride ligands are propo sed to be located on one triangle of Mo atoms, one capping the face an d the other two bridging between the wingtip and backbone Mo atoms. Th e Mo-Mo distances associated with the hydride bridges are notably shor ter (2.38 Angstrom) than those having OR bridges (2.50 Angstrom averag e). The H-1 NMR spectra of [Mo-4(mu-H)(OR)(12)](-) and Mo-4(mu-H)(3)(O But)(7)(HNMe2) reveal the presence of the hydride ligands and furtherm ore that these clusters are static on the NMR timescale. These results are compared with the synthesis of related tungsten hydride alkoxides .