Synthesis of Mo(IV) para-Substituted Styrene Complexes and an Exploration of Their Conversion to 1-Phenethylidene Complexes

Molybdenum(IV) styrene complexes, Mo(NAr)(p-X-styrene)(OSiPh3)2 (1-X; X = t-Bu, Me, F, Br, CF3; Ar = 2,6-i-Pr2C6H3), were synthesized and their conversion into 1-phenethylidene complexes was investigated. The two isomers of 1-X (Ph-up and Ph-down) interconvert through the rotation of the styrene about its central Mo(olefin) axis, with Delta H double dagger in the range of 13-16 kcal/mol and Delta S double dagger in the range of -7 to -16 cal/mol center dot K. All 1-X compounds except 1-CF3 react slowly with a catalytic amount of PhNMe2H+ to give an equilibrium mixture of 1-X and Mo(NAr)[C(Me)(p-XC6H4)](OSiPh3)2 (2-X) complexes. The equilibrium constants for the formation of 2-X from 1-X increase in the order of 1-Br < 1-F similar to 1-H < 1 -t-Bu < 1-Me with the range in Keq from 0.25 for 1-Br to 0.51 for 1-Me. It is proposed that the rate-limiting step is the protonation of the styrene's CH2 group, either directly or via the addition of a proton to the Mo-CH2 bond. The rates of protonation increase in the order 1-Br < 1-H < 1 -t-Bu < 1-Me, with that for 1-Me being similar to 30x faster than that for 1-Br. All 1-X compounds react immediately and irreversibly with [(Et2O)2H]+ to form cationic phenethyl complexes (3-X), the intermediates in the conversion of 1-X to 2-X. X-ray structural studies were carried out for 1-X, where X = t Bu, Me, F, Br, or CF3.

Automated summary

Molybdenum(IV) styrene complexes were synthesized and their conversion was investigated. It was found that the two isomers of styrene can interconvert through protonation, and the rate-limiting step is the protonation of styrene. The rate of protonation was found to be dependent on the substituent.