Molybdenum complexes carbyne formation

A closely related formation of >/3-cydopropenyl molybdenum complex Cp P(OMe)3)2Mo( 3-C3Ph2Me), possibly involving an interamolecular ligand cycloaddition within a carbyne-acetylene intermediate complex, has been accomplished by treating the acetylenic cation complex [Cp f P(OMe)3 2Mo( 2-PhC=CPh)]BF4 with vinylmagne-sium bromide in THF (equation 253)321. 

A recent computational investigation of the [2+2] cycloaddition between a hypothetical molybdenum carbyne complex, CbMo CI I, and ethyne suggests that the formation of the molybdacyclobutadiene, CI3Mo(7/2-C3l l (), is allowed by orbital symmetry. In contrast, the direct formation of the isomeric molybdatetrahedrane (i.e., the if -cyclopropene complex), ChMoft -CsID, is symmetry forbidden <19930M1289> such complexes presumably arise by isomerization subsequent to initial molybdacyclobutadiene formation (Section 2.12.5). 

The metallacyclobutadiene formation from acetylene and molybdenum carbyne complex CI3M0CH was studied [45] along a postulated least-motion pathway as shown in Fig. 12. The CI3M0CH was brought together with the acetylene molecule such that the three carbon atoms and the metal center remained coplanar throughout the course of the reaction. It has been found... 

The acyl formation reaction is suggested to proceed via initial attack at the carbyne carbon, but evidence is not conclusive. Reaction of the analogous Cp complexes with cyclohexene sulfide provides exclusively the dithiocarboxylate derivatives. Presumably the second sulfur addition is considerably more rapid than for the Tp complexes due to lowered steric crowding around the molybdenum... 

Calculations on alkyne metathesis catalyzed by high oxidation state molybdenum alkylidynes show that the formation of a metallacyclobutadiene is symmetry allowed with an energy barrier of just 10 kJ mor. Direct formation of a metallatetrahedrane is formally disallowed with an energy barrier of 40 kJ mol". The metallatetrahedrane is 52kJ mol" more stable than the corresponding metallacyclobutadiene and 122 kJ mol more stable than the free alkyne and the carbyne complex." 2o... 

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