Activation of propargylic alcohols by dimolybdenum tris(mu-thiolate) complexes: Influence of the substituents R in HC CCR2(OH)-vinylidene/allenylidene transformation. Reactivity of allenylidene complexes

Reaction of the bis(nitrile) complex [Mo2Cp2(mu-SMe)(3)(NCMe)(2)](BF4) ( 1) with dimethylpropargylic alcohol, HC CCMe2(OH), at room temperature in dichloromethane produced good yields of the mu-alkynol species [Mo2Cp2(mu-SMe)(3){mu-CHCCMe2(OH)}](BF4) (2a) through replacement of the two acetonitrile ligands in 1 by the alkynol. The NMR spectra of 2a indicate a l-g 1: g 1 coordination mode for the alkyne which is thereby incorporated into a dimetallacyclobutene ring like that found here by X-ray diffraction (XRD) analysis of the related complex [Mo2Cp2(mu-SMe)(3)(mu-eta(1):eta(1)-CHCCO2Me)](BPh4) (2b). When 2a was stirred with Et3N at room temperature in dichloromethane, deprotonation gave high yields of the mu-3-hydroxyalkynyl derivative [Mo2Cp2(mu-SMe)(3){mu-eta(1):eta(2)-CCCMe2(OH)}] (3), together with small amounts of the already-known vinylacetylide [Mo2Cp2(mu-SMe)(3){mu-eta(1):eta(2)-CCC(Me)CH2}] (4) resulting from dehydration of 3. Treatment of 3 with 1 equiv. of HBF4 OEt2 in diethyl ether at room temperature gave the 3-hydroxyvinylidene derivative [ Mo2Cp2(mu-SMe)(3){mu-eta(1):eta(2)-CCHCMe2(OH)}](BF4) (5) as the major product, together with other minor products [Mo2Cp2(mu-SMe)(3){mu-eta(1):eta(2)-CCHC(Me) CH2}](BF4) (6), [Mo2Cp2(mu-SMe)(3)(mu-eta(1):eta(2)-CCCMe2)]( BF4) (7), [ Mo2Cp2(mu-SMe)(3)(mu-eta(1):eta(CCH2)-C-2)]( BF4) (8), [Mo2Cp2(mu-SMe)(3){mu-eta(1):eta(2)-CCH(CHMe2)}](BF4) (9) and [Mo2Cp2(mu-SMe)(3)(mu-O)](BF4) (10). The vinylidene (6) and allenylidene (7) species resulted from dehydration of the 3-hydroxyvinylidene complex 5 whereas the vinylidene derivative 8 was formed by deketonisation of 5. When 3 reacted with a large excess of HBF4 center dot OEt2 in dichloromethane, the 3-isopropylvinylidene complex 9 was obtained nearly quantatively via a H-center dot radical process. When left for several days CD2Cl2 solutions of 5 afforded mainly the vinylidene species 8 by deketonisation and the side-oxoproduct [Mo2Cp2(mu-SMe)(3)(mu-O)](BF4) (10) by hydrolysis or reaction with oxygen. Addition of nucleophiles (H-, OMe-, OH-, SMe-) to the allenylidene complex [Mo2Cp2(mu-SMe)(3)(mu-eta(1):eta(2)-CCCPh2)](BF4) (11) resulted in the formation of the corresponding mu-acetylide derivatives [Mo2Cp2(mu-SMe)(3)(mu-eta(1):eta(2)-CCCRPh2)][R = H (12), OMe (16a), OH (17), SMe (16b)], which by further reaction with tetrafluoroboric acid afforded either the vinylidene species [Mo2Cp2(mu-SMe)(3){mu-eta(1):eta(CH)-C-2( CRPh2)}](BF4) when R = H (13), or the starting complex 11 when R is a leaving group (OMe). Reaction of 13 with Na(BH4) gave the mu-alkylidyne complex [Mo2Cp2(mu-SMe)(3)(mu-eta(1)-CCH2CPh2H)] (14) by nucleophilic attack of H- at the C-beta carbon atom of the vinylidene chain. Proton addition at C-alpha in 14 led to the formation of a mu-vinylidene compound 15 containing an agostic C-H bond. New complexes have been characterised by elemental analyses and spectroscopic methods, supplemented for 2b and 3 by X-ray diffraction studies. (C) 2007 Elsevier B.V. All rights reserved.