Formation of the Group 4B Metallocene Dicarbonyls

Preparation/Formation of Cp2Ti(CO)2 via the Reductive Carbonylation of Cp2TiCl2/ (Cp2TiCl)2 

Due to the commercial availability of Cp2TiCl2, its reduction and subsequent treatment with CO represents one of the most direct routes to Cp2Ti(CO)2 (1). An early example of this method was reported by Calderazzo et al. whereby Cp2TiCl2 was first reduced by sodium naphthalene at 25°C in THF for 24 hours (16). The resulting green titanocene, p.-(rf i75-fulvalene)-di-/i-hydrido-bis(cyclopentadienyltitanium) (17-22), was then treated with 1 atm of CO in toluene for 4 hours at 20°C. However, this procedure proved inefficient for the production of 1 since only 10% of the required amount of CO for complete conversion was consumed. 

In 1972 Van Tamelen and co-workers reported that the reduction of a toluene solution of Cp2TiCl2 by sodium under an argon atmosphere over a period of 6-10 days led to the formation of an active titanocene postulated as (Cp2Ti)1 2 based on its visible and infrared spectrum. Exposure of (Cp2Ti)t 2 in toluene to CO converted it to Cp2Ti(CO)2 (1) (23). 

Bis(ij-cyclopentadienyl)dicarbonyl has also been prepared electrochemi-cally. When DME solutions of Cp2TiCl2 were electrolytically reduced in an atmosphere of CO, high yields of Cp2Ti(CO)2 (1) were obtained (24). 

In 1975 Demerseman and co-workers reported two new preparations for Cp2Ti(CO)2 via the reductive carbonylation of Cp2TiCl2. The first of these involved the reaction of either Cp2TiCl2 or (Cp2TiCl)2 with AlEt3 in a CO atmosphere. After these heptane suspensions or benzene solutions were stirred for 20 hours at 20°C, Cp2Ti(CO)2 (1) could be isolated in 30% yield (26). No speculation as to the mechanism of this reduction was discussed however, alkylation and CO insertion steps are probably involved. 

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