Metal complexes cationic pentadienyl

Nucleophiles with Cationic Pentadienyl-Metal Complexes... 

Pentadienyl carbanions are analogous to allyl anions with an extended delocalization of charge. Reaction of 1,3- or 1,4-pentadienes and alkali metals in THF in the presence of a base, such as NMes or TMEDA, affords crystalline pentadienylalkali metal complexes. A contact ion pair structure is predicted for these compounds by theoretical calculations and is consistent with solution structural data obtained by NMR. The pentadienyl anion usually interacts with the cation as an rj - or ) -ligand depending on the structural orientation of the backbone carbon atoms of the pentadienyl anion (W-, S-, or U-shaped skeletal structures). A contact ion pair structure having a W-shaped pentadienyl ligand is shown (16). 2,4-Disubstituted... 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

This chapter includes those transition metal-pentadienyl cationic complexes that are quite stable, can be stored and handled easily, and are therefore useful as stoichiometric intermediates for organic synthesis. The dienyliron systems, which are readily available and inexpensive, have dominated this area of chemistry, and will occupy the larger part of the discussion. The chemistry of more expensive and less easily prepared dienylmetal complexes, such as those of manganese and cobalt, will be dealt with at the end of the chapter. 

Ernst has recently reviewed structural and spectroscopic features of pentadienyl compounds. He discusses experimental and theoretical studies of pentadienyl anions, radicals, and cations and also the structures of transition metal pentadienyl complexes (21). He has also written an account of his own work in this field (22). 

Pentadienyltricarbonyliron cations are readily attacked by nucleophiles. They are more reactive in this respect than the corresponding cyclic complexes (160). Cyclohexadienyltricarbonyliron tetrafluoroborate can be recrystallized from water, while the 1-methylpentadienyl derivative is rapidly converted to the alcohol. In reactions of 1-alkylpentadienyltricarbonyliron cations with water or methanol the nucleophile adds preferentially to the carbon atom bearing the substituent, even when this is bulky. It is suggested that the alkyl substituent stabilizes the S-ion intermediate. Some authors consider that this intermediate could have an f/3-pentadienyl structure, the 18-electron configuration at the metal center being maintained by coordination of a molecule of water or of solvent (173). 

Variations in Zr-B bond distances for complexes of the type (CjHjB-R ZrCfe indicate that the nature of R affects the bonding relationship between zirconium and boratabenzene (Table 1). These data imply that for R = Ph, OEt and Me, the interactions between Zr and B are strong. Boratabenzene with R = NPr2 binds in an r -pentadienyl fashion with little overlap between B and Zr. Unfortunately, these metrical parameters do not give sufficient insight into differences in the electron density at the metal. Furthermore, they are measurements of precatalyst structures and it is unclear how they relate to the putative cationic catalytic species. 

Protonation of (norbornadiene)cobalt complex 43 induces C-C bond cleavage of the norbornenyl ring to form cationic complex 44 [63,64]. Re-protonation of the reduced complex 45 induces a second cleavage of a non-strained cyclopen-tene ring to give an open r)5-pentadienyl complex 46. It is postulated that a three-center interaction of the highly electrophilic metal center with the a-electrons of the adjacent C-C bond is involved. 

Since many organometallics behave as Lewis bases due to electron-rich metals, protonation is a common reaction. For example, the tungsten hydride 3 undergoes reversible protonation at the metal, forming the water-soluble cationic hydride 4 (Eq. 5). In nickelocene 5, a 20e complex, protonation occurs at the jr-bonded cyclo-pentadienyl ligand the intermediate 6 has a stable, isolable counterpart in the fully methylated derivative. Consecutive loss of cyclopentadiene forms the cation 7, which adds to unchanged nickelocene forming the tripledecker sandwich 8 (Scheme 3). 

---