Tetracarbonylmetallates 3- of Manganese and Rhenium

A variety of strong reducing agents, including solutions of alkali metals in liquid ammonia, sodium solubilized by crown ethers or cryptands in tetrahydrofuran (THF), and alkali metal naphthalenides in THF, have been found to reduce M2(CO)10 and/or [M(CO)5] (M = Mn and Re) to the respective [M(CO)4]3- however, [Re(CO)5] has often been observed to be 

Sodium naphthalenide in THF has also been shown to reduce Na-[M(CO)5] (M = Mn, Re). Initially it was believed that solutions of Na3-[M(CO)4] were obtained, but a more careful examination established that only very fine suspensions of insoluble and impure brown trisodium salts had formed. For example, treatment of Re2(CO)10 with 10 equivalents of NaC10H8 in THF provided a complete conversion to finely divided brown insoluble Na3[Re(CO)4] within an hour at room temperature (19). Although Na3[Re(CO)4] prepared with excess Na[C10H8] was impure, we discovered that these HMPA-free suspensions were useful in reactions in which removal of HMPA was very difficult. Attempts to find practical routes to relatively pure salts of [M(CO)4]3 , which do not involve the use of the toxic and high-boiling HMPA, have not been entirely successful to date. A summary of routes to insoluble (and impure) forms of A3[M(CO)4], where A = Na and K and M = Mn and Re, is shown in Eqs. (2) and (3). 

Indeed, the Na-HMPA route consistently provided the cleanest products and has been the only synthesis to provide solutions of Na3[M(CO)4]. It is often important to use solutions rather than slurries of trianion salts to minimize the formation of side products during the reactions of these materials with electrophiles. Until recently, product separation from the viscous and high-boiling HMPA has always been a problem (and remains so in some cases). For example, addition of excess THF to solutions of Na3[M(CO)4] in HMPA invariably resulted in the formation of sticky solids that contained HMPA and did not analyze satisfactorily (14). But recently, it was discovered that addition of these HMPA solutions to excess liquid ammonia resulted in practically quantitative precipitation of tan to pale yellow brown solids, which provided satisfactory elemental analyses of unsolvated Na3[M(CO)4] (M = Mn, Re). Virtually all impurities remained in the HMPA-NHj filtrate [Eqs. (4) and (5)]. 

Unlike the related Na3[M (CO)5], where M = V, Nb, and Ta, which undergoes thermolysis below O0C (vide infra), these materials possess remarkable thermal stabilities for metal carbonyls and briefly survive without melting at temperatures as high as 300°C. By comparison, K2[Fe(CO)4], another metal carbonyl salt of high thermal stability, has been reported to melt at 270-273°C with decomposition (20). The related K[Co(CO)4] melts at about 203°C with decomposition (21). 

Lower yields of Na3[Mn(CO)4] were obtained from the direct reduction of Mn2(CO)10 in Na-HMPA, because the neutral dimer underwent slow disproportionation in this medium to form [Mn(HMPA)Jt][Mn(CO)5]2, in contrast to Re2(CO)10, which showed no tendency to react with HMPA at room temperature. Of all neutral binary carbonyl dimers known, Re2(CO)10 appears to be the most resistant toward Lewis base-promoted disproportionation reactions. The slightly lower yields of Na3[Re(CO)4], compared to those of Na3[Mn(CO)4], may have arisen from the fact that Re2(CO)10 does not cleanly reduce to [Re(CO)5] in HMPA or other solvents (22). It should 

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