Interconverting WW Triple Bonds and W4 Clusters: Structures of W4(OPrn)16 and [Li2W2(OPrn)8(DME)]2*

Alcoholysis of W₂(NMe₂)₆ with excess n-propanol in hexane yields the tetranuclear cluster, W₄(OPrⁿ, I. Reduction of I with two equivalents of Li₂COT in THF gives a small yield of Li₂W₂(OPrⁿ)₈. Single crystals were isolated by cooling the product mixture in DME and were shown to be [Li₂W₂(OPrⁿ)₈(DME)]₂, II, which consists of a unique “dimer of dimers” structure. In this reaction sequence, W₄¹⁶⁺ cluster formation is followed by four electron reduction to reform the (W≡W)⁶⁺ unit. Better yields of the lithium salt can be obtained by the addition of LiOPrⁿ/HOPrⁿ solutions to W₂(OBu^t)₆ in which case Li₂W₂(OPrⁿ)₈ has been obtained as a 1:1 adduct with LiOPr. This identity of this salt was confirmed by solution NMR spectroscopy. In the alternative reaction, the (W≡W)⁶⁺ center remains intact from reactant to product. No attempt has been made to separate the product from excess LiOPr. DFT (ADF 2004.01) molecular orbital calculations on the model cluster W₄(OH)₁₆ are used to help elucidate the disruption of the W₄ cluster upon four electron reduction. The molecular structures of compounds I and II are reported.*Dedicated to Professor F. A Cotton on the occasion of his 75th birthday.