Polynuclear metal hydrido alkoxides. Preparation and characterization of Mo4(µ-H)3(OBut)7(HNMe2) and [K(18-crown-6)][Mo4(µ4-H)(OR)12] (R = Pri or CH2But)

Abstract

The compounds Mo₂(OR)₆ and KH [or KHB(Bu^s)₃] react in tetrahydrofuran in the presence of 18-crown-6 to yield the charge separated clusters, [Mo₄(µ-H)(OR)₁₂]^– (R = Prⁱ or CH₂Bu^t) supported by the potassium ions [K⁺(η⁶-18-crown-6)]·2THF. The cluster anions contain a Mo₄ butterfly, Mo–Mo 2.50 Å (average) supported by a hydride ligand that bridges the wingtip Mo atoms. Each edge of the Mo₄ butterfly is bridged by an OR group and the two faces are capped by µ₃-OR ligands. The backbone Mo atoms have one terminal bridge and the wingtip Mo atoms have two. Hydrogenation of the mixture of compounds formed in the reaction between 1,2-Mo₂(tol)₂(NMe₂)₄ (tol = p-tolyl) and Bu^tOH (3.5 equivalents) in hydrocarbon solvents yields Mo₄(µ-H)₃(OBu^t)₇(HNMe₂) which contains a Mo₄ butterfly asymmetrically supported by alkoxide and hydride ligands. The hydride ligands are proposed to be located on one triangle of Mo atoms, one capping the face and the other two bridging between the wingtip and backbone Mo atoms. The Mo–Mo distances associated with the hydride bridges are notably shorter (2.38 Å) than those having OR bridges (2.50 Å, average). The ¹H NMR spectra of [Mo₄(µ-H)(OR)₁₂]^– and Mo₄(µ-H)₃(OBu^t)₇(HNMe₂) reveal the presence of the hydride ligands and furthermore that these clusters are static on the NMR timescale. These results are compared with the synthesis of related tungsten hydrido alkoxides.