Permethyltitanocene(III) diacetylide magnesium tweezer complexes, intermediates in the catalysis of linear head-to-tail dimerization of terminal acetylenes

The (C5Me5)(2)TiCl2/i-PrMgCl/Et2O (Mg:Ti = 4-10) catalytic systems selectively catalyze the dimerization of terminal acetylenes RC=CH to 2,4-disubstituted 1-buten-3-ynes for R = Me, Et, n-Pr, n-Bu, cyclohexyl, Ph, and SiMe3 with a turnover number 500-700 after 5 days. The ESR investigation of these systems has revealed that permethyltitanocene tweezer complexes with embedded Mg ion between the acetylide arms [(C5Me5)(2)Ti(eta(1)-C=CR)(2)](-)[Mg(OEt2)X](+) (X = Cl or C=CR) (1A-1G for R = Me, Et, n-Pr, n-Bu, cyclohexyl, Ph, and SiMe3) are intermediates in the formation of a dimerization catalytic center. Their ESR spectra are characterized by g-values in the range 1.990-1.993, coupling constant to the proton at a-carbon atom of the acetylene substituent 2.0-2.5 G and by coupling constants to Ti-47 and Ti-49 isotopes in the range 7.6-8.7 G. The tweezer complex [(C5Me5)(2)Ti(eta(1)-C=CSiMe3)(2)](-)[Mg(THF)Cl](+) (2G) has been synthesized. It catalyzes the head-to-tail dimerizations with the same selectivity and similar turnover numbers as the above systems.