THERMODYNAMIC AND KINETIC STABILITY OF BIS(ETA-3-ALLYL)BIS(MU-2-ETA-3-ALLYL)DIMOLYBDENUM(II) AND BIS(ETA-3-METHALLYL)BIS(MU-2-ETA-3-METHALLYL)DIMOLYBDENUM(II) ISOMERS AND EVIDENCE FOR THEIR LEWIS BASE-CATALYZED ISOMERIZATION

The reactivity of Mo2(mu-2-eta-3-allyl)2(eta 3-allyl)2 (1) is dependent on the conformation of the bridging allyls. The green Mo2(endo-mu-2-eta-3-allyl)(exo-mu-2-eta-3-allyl)(endo-eta-3-allyl)2 (1(b = en,ex)(t = en2) reacts much more rapidly than the violet Mo2(endo-mu-2-eta-3-allyl)2(endo-eta-3-allyl)2 (1(b = en2)(t = en2)) with a variety of reagents including methanol, acetylacetone, and carbon monoxide. In fact, the violet isomer can be isolated from an isomeric mixture of Mo2(mu-2-eta-3-allyl)2(eta-3-allyl)2 via titration with methanol. 1(b = en2)(t = en2) crystallizes in the orthorhombic space group Pbcn with cell constants a = 8.275 (11) angstrom, b = 11.832 (7) angstrom, c = 12.862 (9) angstrom, V = 1259 (2) angstrom 3, Z = 8, and R(F) = 3.01%. A comparison of crystal structures for 1(b = en,ex)(t = en2) and 1(b = en2)(t = en2) shows significantly shorter Mo-C bonds involving the terminal allyls of the latter isomer. 1(b = en2)(t = en2) is isomerized to an equilibrated mixture of isomers upon the addition of a Lewis base catalyst. A comparison of COSY data for various isomers of 1 and Mo2(mu-2-eta-3-methallyl)2(eta-3-methallyl)2 (2) suggests that 2 exists primarily as the 2(b = en,ex)(t = en,ex) isomer (88%) with minor amounts of 1(b = en,ex)(t = ex2) also present (12%).