REACTIONS OF METAL-TO-METAL MULTIPLE BONDS .6. REVERSIBLE CARBONYLATION OF HEXAKIS(TERT-BUTOXY)DIMOLYBDENUM (M=M) - CARBONYL-BRIDGED METAL-TO-METAL DOUBLE-BOND, MO2(OBU-T)6(CO) (M=M)

Hydrocarbon solutions of Mo2(OBu-t)6 (M≡M) react with carbon monoxide at room temperature and 1 atm according to the stoichiometric equation 2Mo2(OBu-t)6 + 6CO = Mo(CO)6 + 3Mo(OBu-t)4 The reaction proceeds via a slow and reversible formation of a monocarbonyl, Mo2(OBU-t)6(µ-CO), that has been isolated as a crystalline compound and characterized by a number of spectroscopic techniques. The compound shows a strong infrared absorption at 1670 cm-1 in the solid state and at 1690 cm-1 in hydrocarbon solutions assignable to V(CO) of a carbonyl group bridging two molybdenum atoms. An X-ray study shows that in the crystalline state the molecule has virtual C2LJ symmetry. The coordination polyhedron about each metal atom is a distorted square pyramid with the carbonyl carbon atom at the apex. The Mo-to-Mo distance is 2.489 (1)Å which, together with the observed diamagnetism, is consistent with the existence of a metal-to-metal double bond. Crystal data for Mo2(OBU-t)6(CO) follow : a = 17.827 (3) k, b = 9.335 (2)Å c = 19.447 (4)Å α = β = γ = 90.00°, V= 3236 (1) Å3, Z = 4, space group Cmc2. In solution both IR and NMR data support the view that the carbonyl group remains bridging. Low-temperature 13C NMR spectra indicate that the molecule is fluxional : only one type of OBu-t group is found on the NMR time scale. Carbon monoxide reacts readily under similar conditions with Mo2(OBU-t)6 (M≡M) to give Mo(CO)66 and as yet unidentified oxidized molybdenum alkoxides. A black, paramagnetic, crystalline compound of empirical formula Mo(OPrO3CO has been isolated from this reaction. Crystals of this compound repeatedly decomposed in the X-ray beam but based on limited X-ray data and mass spectral analysis this compound is formulated as a tetranuclear alkoxide Mo4(OPr-i)12(CO)4. The difference in the course of the reactions between Mo2(OBU-t)6 and Mo2(OBU-t)6 with CO is believed to be dominated by the different steric properties of the alkyl ligands. The reaction of CO with Mo2(OPr-i)8 (M̿M) produces a compound formulated as (pri-i-O4)4Mo(µ-Pri-i)2Mo(OPR-i)2(CO)2 on the basis of analytical and spectroscopic data. These reactions of CO with dinuclear molybdenum alkoxides are compared with other carbonylation reactions involving compounds containing metal-to-metal triple bonds. © 1979, American Chemical Society. All rights reserved.